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Controlled Synthesis of Transition Metal Phosphide Nanoparticles to Establish Composition-Dependent Trends in Electrocatalytic Activity

Cite this: Chem. Mater. 2022, 34, 14, 6255–6267
Publication Date (Web):July 15, 2022
https://doi.org/10.1021/acs.chemmater.2c00085

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Transition-metal phosphides (TMPs) are versatile materials with tunable electronic and structural properties that have led to exceptional catalytic performances for important energy applications. Identifying predictive relationships between the catalytic performance and key features such as the composition, morphology, and crystalline structure hinges on the ability to independently tune these variables within a TMP system. Here, we have developed a versatile, low-temperature solution synthesis route to alloyed nickel phosphide (Ni1.6M0.4P, where M = Co, Cu, Mo, Pd, Rh, or Ru) nanoparticles (NPs) that retains the structure of the parent Ni2P NPs, allowing investigation of compositional effects on activity without convoluting factors from differences in morphology and crystalline phase. As a measure of the controlled changes introduced within the isostructural series by the second metal, the binary and alloyed ternary TMP NPs supported on carbon at a nominal 5% weight loading were studied as electrocatalysts for the hydrogen evolution reaction (HER). The resultant activity of the electrocatalyst series spanned a 125 mV range in overpotential, and composition-dependent trends were investigated using density functional theory calculations on flat (0001) and corrugated (101̅0) Ni1.67M0.33P surfaces. Applying the adsorption free energy of atomic H (GH) as a descriptor for HER activity revealed a facet-dependent volcano-shaped correlation between the overpotential and GH, with the activity trend well represented by the corrugated (101̅0) surfaces on which metal–metal bridge sites are available for H adsorption but not the flat (0001) surfaces. The versatility of the rational synthetic methodology allows for the preparation of a wide range of compositionally diverse TMP NPs, enabling the investigation of critical composition–performance relationships for energy applications.

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Introduction

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Identifying predictive relationships between the performance of catalytic materials and key features that impact catalysis, such as the morphology, composition, and associated crystalline structure, hinges on the ability to independently control these variables within a material system. Transition-metal phosphides (TMPs), for example, are structurally complex materials that exhibit unique chemical, physical, and electronic properties due to the metal–phosphorus interactions in the crystal lattice. (1,2) These M–P interactions offer a route to precisely tune the properties of TMPs for diverse catalytic applications. TMPs are known to be highly active hydroprocessing catalysts (1,3) and, more recently, have generated significant interest as electrocatalysts due to their inherent electrical conductivity and stability. (4−6) Specifically, inexpensive and earth-abundant compositions have demonstrated activity for the hydrogen evolution reaction (HER) comparable to that of noble metal catalysts, (7,8) as well as the reduction of CO2 to oxygenated single- and multi-carbon products. (9−14)
The composition and structure of TMPs have distinct effects on the electrocatalytic performance. (13,15−20) The introduction of a second metal to form ternary compositions (MxMyPz), in particular, has led to catalytic performances that can exceed that of the binary parent phase. This enhancement in catalytic performance has been attributed to electronic and geometric effects that strongly influence the properties of the active sites. For example, Brock and co-workers found that Co2–xRhxP (0.1 ≤ x ≤ 0.5) and Ni2–xRuxP (0.25 ≤ x ≤ 1) nanoparticles (NPs) were more active than Co2P and Ni2P NPs, respectively, for the oxygen evolution reaction (OER), despite Rh2P and Ru2P displaying minimal OER activity. (21,22) The introduction of the second metal into the Co2P and Ni2P structures was proposed to electronically activate the Co and Ni active sites, thus improving the OER performance. Similarly, for the HER, the highest activity in a series of Ni2–xCoxP (0 ≤ x ≤ 2) NP electrocatalysts was observed for NiCoP (x = 1). (23) Density functional theory (DFT) calculations revealed that the NiCoP composition had an optimal adsorption free energy of atomic H (GH), a key descriptor of HER activity. Within an Fe2–xCoxP series, Fe0.5Co0.5P displayed the highest HER activity, consistent with its optimal GH. (24−26) Whereas prior studies have demonstrated the effect of changing the relative proportion of two metals in a ternary TMP on the electrocatalytic performance, fewer reports systematically evaluate the effect of the identity of the second metal on performance. Establishing such composition–performance relationships requires synthetic approaches that control the other potential compounding factors, namely, the crystal phase and morphology.
A number of synthetic approaches have been developed to access binary and ternary TMP phases. TMP nanomaterials have traditionally been synthesized using temperature-programmed reduction techniques that rely on high-temperature (>500 °C) reduction of metal and phosphorus precursors and lead to difficulty in controlling the particle morphology, phase impurities, and excess phosphorus species. (2,8) To overcome these challenges, solution-synthesis methods, (1,27−31) including those that utilize metal–phosphine complexes with tunable decomposition temperatures, (12,32−34) have enabled the synthesis of binary TMP NPs with uniform structural features. The synthesis of ternary phases, however, is complicated by differences in reactivity between two metal precursors, which can lead to phase segregation and inhomogeneities in morphology. (2) Brock et al. have developed a solution-phase arrested precipitation technique and applied it to understand the composition dependence of Co2–xRhxP NPs for water splitting reactions. (21,35) The coreduction of Co2+ and Rh3+ ions, followed by phosphidation of the resulting intermediate product at above 300 °C, allowed for investigation of a wide range of elemental ratios within a specific composition, and similar techniques have been applied to modify the elemental ratio within other ternary NP systems. (23,36−39) Alternatively, a general method for synthesizing ternary compositions in which the elemental ratio remains fixed but the identity of the second metal is changed would provide an opportunity to directly compare the impact of the second metal without convoluting factors arising from differences in the NP morphology or phase.
Here, we have developed a rational, low-temperature solution-synthesis approach using molecular precursors to incorporate a second metal into the binary Ni2P structure. We applied this methodology to prepare ternary Ni2–xMxP NPs (M = Co, Cu, Mo, Pd, Rh, or Ru) without altering the morphology and crystal phase of the parent Ni2P NPs. The ability to independently tune the composition without modifying other features of the ternary TMP NPs allows for determination of composition-dependent catalytic activity. To this end, Ni2–xMxP NPs with targeted compositions of 20 mol % M (Ni1.6M0.4P) were dispersed on a high-surface-area carbon support at a nominal loading of 5 wt %. These catalysts were then deposited on glassy carbon electrodes and investigated as electrocatalysts for the HER using a rotating disk electrode (RDE) in a 0.5 M H2SO4 solution. The versatility of the rational synthetic methodology enables the preparation of a wide range of compositionally diverse TMP NPs, more broadly enabling the investigation of critical composition–performance relationships for energy applications.

Experimental Section

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Synthesis and Characterization

General

Synthetic manipulations to prepare metal phosphide NPs were conducted in a N2 atmosphere using standard Schlenk techniques or in a N2-filled Vacuum Atmospheres glovebox, unless otherwise noted. Caution! The metal phosphide precursors have the potential to evolve pyrophoric and/or toxic phosphorus species under reaction conditions. These reactions should only be performed by trained personnel under rigorously air-free conditions. Oleylamine (OAm, 70% technical grade) and 1-octadecene (ODE, 90%) were purchased from Sigma-Aldrich and dried prior to use by heating to 120 and 150 °C, respectively, under vacuum for 5 h and were stored in a N2-filled glovebox prior to use. Triphenylphosphine (PPh3, 99%), [Cu(PPh3)H]6, Mo(CO)4(PPh3)2, RhCl(CO)(PPh3)2, Pd(PPh3)4, and RuCl2(CO)2(PPh3)2 were purchased from Sigma-Aldrich and used as received. Ni(CO)2(PPh3)2 was purchased from Strem Chemicals and used as received. Co(NO)(CO)2(PPh3) was synthesized according to established methods. (40) The silicon powder NIST Standard Reference Material 640f was purchased from Millipore-Sigma. The carbon support (Vulcan XC 72R) was supplied by Cabot and used as received. A commercial 29.0 wt % Pt Vulcan carbon catalyst was purchased from Tanaka Kikinzoku International K.K. (electrocatalyst TEC10V30E, lot no. 1013-6231, 2 g) and was used as received for ink preparation without any thermal reduction. A 5 wt % Nafion perfluorinated resin solution in lower alcohols and water was purchased from Sigma-Aldrich (product number 274704).

Synthesis of Metal Phosphide NPs

Ni2P NPs were prepared based on a previously described procedure. (34) Ni(CO)2(PPh3)2 (0.64 g, 1.0 mmol), PPh3 (1.05 g, 4.00 mmol), dried OAm (6.5 mL, 20 mmol), and dried ODE (6.5 mL) were combined in a three-neck round-bottom flask fitted with a condenser and two septa and heated under N2 with rapid stirring to 250 °C at ca. 10 °C/min. The mixture was held at 250 °C for 15 min to form amorphous Ni–P NPs and then heated to 300 °C at ca. 10 °C/min. The mixture was maintained at 300 °C for 1 h, followed by removal of the heat source and ambient cooling to room temperature. The resulting NPs were recovered in CHCl3 (ca. 0.1 mL), flocculated with isopropanol (ca. 40 mL), and separated by centrifugation at 10 000 rpm for 5 min. The NPs were redispersed in CHCl3 (ca. 0.1 mL), flocculated with isopropanol (ca. 40 mL), separated by centrifugation, and redispersed in CHCl3 (ca. 10 mL).
Ni2–xMxP NPs, where M = Co, Cu, Mo, Pd, Rh, or Ru, were prepared by first forming amorphous Ni–P NPs, as described above in the procedure for the synthesis of Ni2P NPs, by heating the reaction mixture to 250 °C and holding for 15 min. After 15 min, the heat source was removed, and the mixture was allowed to cool to room temperature. Co(NO)(CO)2(PPh3) (0.10 g, 0.25 mmol), [Cu(PPh3)H]6 (0.08 g, 0.04 mmol), Mo(CO)4(PPh3)2 (0.18 g, 0.25 mmol), RuCl2(CO)2(PPh3)2 (0.19 g, 0.25 mmol), RhCl(CO)(PPh3)2 (0.17 g, 0.25 mmol), or Pd(PPh3)4 (0.29 g, 0.25 mmol) was added under flowing N2, and then, the reaction vessel was purged with N2 for 2 min through a needle inserted in one of the septa. The reaction mixture was heated to 300 °C at ca. 10 °C/min and maintained at this temperature for 1 h before cooling. The Ni2–xMxP NPs were recovered as described above for the synthesis of Ni2P NPs.

Preparation of Metal Phosphide Electrocatalysts

The carbon-supported catalysts were prepared by adding a suspension of NPs dropwise to a rapidly stirring suspension of Vulcan XC 72R in CHCl3 (ca. 40 mL) to yield catalysts with nominal loadings of 5 wt % Ni2P and 5 wt % Ni2–xMxP NPs. The mixtures were sonicated for 5 min, stirred overnight, and recovered by centrifugation at 8000 rpm for 10 min. The resulting catalysts were dried under vacuum overnight. The carbon-supported catalysts were stored in an inert atmosphere but were exposed to air for short time periods (<1 h) to prepare for heat treatment and other manipulations. To remove surface organic ligands, each catalyst was thermally reduced in a tube furnace with flowing 5% H2 in N2 (500 sccm). The temperature was increased at 5 °C/min to 450 °C and held for 2 h. After cooling to room temperature, the catalysts were passivated for 2 h in flowing 1% O2/N2 (500 sccm).

Catalyst Characterization

Thermogravimetric analyses (TGAs) of Co(NO)(CO)2(PPh3), RuCl2(CO)2(PPh3)2, and Mo(CO)4(PPh3)2 were performed using a TA Instruments SDT Q600 integrated TGA/DSC analyzer, and the analysis of [Cu(PPh3)H]6 was performed with a SYS Evolution TGA instrument, all with a heating rate of 10 °C/min under flowing N2. Powder X-ray diffraction (XRD) data were collected using a Rigaku Ultima IV diffractometer with a Cu Kα source (40 kV, 44 mA). Diffraction patterns were collected in the 2θ range of 20–80° at a scan rate of 4°/min. The unsupported NPs were drop-cast onto a glass slide from a chloroform suspension. Powder samples (10–20 mg) were supported on a glass sample holder with a 0.5 mm recessed sample area and were pressed into the recess with a glass slide to obtain a uniform z-axis height. The resulting patterns were compared to powder diffraction files (PDF) from the International Centre for Diffraction Data (ICDD). A NIST Si standard was used to calibrate the XRD peak positions. The crystallite sizes were calculated from XRD peak broadening using the Scherrer equation. For transmission electron microscopy (TEM) analysis of Ni2P, the unsupported NPs were drop-cast onto continuous carbon-coated copper grids (Ted Pella part no. 01824) from chloroform suspensions. Imaging was performed using a FEI G2 T20 Tecnai transmission electron microscope operated at 200 kV. For scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) analysis, unsupported Ni2–xMxP NPs (where M = Co, Mo, Pd, Rh, or Ru) were drop-cast onto lacey carbon-coated copper grids (SPI Supplies part no. Z3820C) from chloroform suspensions, whereas the unsupported Ni2–xCuxP NPs were drop-cast onto lacey carbon-coated gold grids (SPI Supplies part no. Z3820G) to avoid additional X-rays from the copper grid. STEM–EDS measurements were performed on a FEI F200X Talos operating at 200 kV, equipped with an extreme field emission gun (X-FEG) electron source, a high-angle annular dark-field (HAADF) detector, and a Super-X EDS system with four silicon-drift detectors (Bruker XFlash 6 series with a detector size of 120 mm2) with a solid angle of 0.9 steradian for chemical analysis. To avoid and/or decrease any potential electron beam damage during spectroscopic analysis while maintaining a high signal-to-noise ratio, the current of the electron beam was set to 480 pA. Part of the high-resolution STEM imaging was performed on an aberration-corrected JEOL JEM-ARM200F TEM/STEM operated at 200 kV with a unique cold field emission gun as well as a next-generation Cs corrector (ASCOR) that compensates for higher order aberrations. High-resolution analysis was performed with a nominal beam current of 25 pA and an associated nominal resolution of 0.07 nm. Additional high-resolution STEM imaging was performed on a JEOL 2200FS TEM/STEM instrument equipped with a CEOS GmbH (Heidelberg, Germany) corrector on the illuminating lenses. The AMAG 5C mode was used to achieve a probe with a nominal 150 pA current and an associated nominal resolution of 0.07 nm. All image analysis was conducted with Gatan Microscopy Suite software, while ImageJ software was used for particle size measurements. (41) Lattice spacings were measured from the fast Fourier transforms (FFTs) of STEM images. Size distributions were determined from a manual diameter measurement of >100 particles. The metal loadings of the reduced and passivated carbon-supported catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) performed by Galbraith Laboratories (Knoxville, TN). The metal phosphide loading was determined from the measured metal content and an assumed total metal/phosphorus stoichiometry of 2:1 (Ni2P or Ni2–xMxP) since the phosphorus content, determined using ICP-OES, could include unincorporated phosphorus species (e.g., surface ligands).

Electrochemical Methods

Equipment

Testing was performed using a Metrohm Autolab PGSTAT302N potentiostat/galvanostat equipped with a FRA32M frequency response analyzer. All measurements were performed under potentiostatic control. RDEs and a modulated speed rotator (MSR) were obtained from Pine Research.

Development of the Electrode Preparation Procedure

To accurately characterize differences in the electrocatalytic activity of the NP materials, we developed a testing procedure that minimizes variability in catalyst deposition and electrode adhesion. Although the electrochemical activity of metal phosphide NPs has been studied previously on stationary Ti foils (42) and pyrolytic graphite, (24) our procedure has several notable differences. First, we based our procedure on carbon-supported NPs. Second, we used an RDE for our HER experiments, which can reduce the measurement error and variability resulting from hydrogen bubble occlusion of the electrode surface. Finally, we optimized a catalyst ink deposition procedure that resisted delamination under RDE HER conditions. In all cases, five separate electrodes were prepared and tested to obtain an estimate of the measurement error in the observed activity.
The reduced NP/carbon materials were deposited on electrodes by formation of a catalyst ink (details below). Information on preparing such inks is extensive in the hydrogen fuel cell literature and describes factors such as the solvent, (43−50) pH, (51) catalyst concentration, (52,53) ionomer concentration, (50,52,53) sonication procedures, (54) and so forth. Notably, Jung et al. developed an ink preparation procedure designed for benchmarking of nanopowder electrocatalysts for the HER and OER. (55) Because the ink formation parameters also depend on the properties of a specific catalyst or catalyst support, we investigated a range of parameters to improve catalyst dispersion and film quality. Our goal was to maximize the uniformity and reproducibility of our inks so that activity differences could be quantified with minimal uncertainty. The resulting ink recipe (10 mg of carbon-supported catalyst powder, 20 μL of a 5% Nafion ionomer mixture, and 0.97 mL of isopropanol) was based primarily on previous studies involving nanoparticulate materials. (55−57) Note that our concentrations of both the catalyst powder and Nafion ionomer in the ink are lower than those commonly used in fuel cell preparation owing to our objective of quantitative catalyst activity comparison rather than practical device assembly or long-term testing. Each ink was drop-cast onto five individual glassy carbon RDEs using an electronically controlled positive displacement pipette, which we found to be essential to reliable deposition. Five sequential cyclic voltammetry (CV) scans were performed on each electrode to assess relative current stability.

Catalyst Ink Preparation

The carbon-supported catalyst was weighed in a glovebox in a 2 mL glass vial. Isopropanol and a 5 wt % Nafion solution were added to the glass vial to achieve an ink with a composition of 0.97 mL of isopropanol, 20 μL of the 5 wt % Nafion solution, and 10 mg of the supported catalyst powder. The ink was homogenized using a Qsonica Misonix S-4000 horn sonicator fitted with a 1/8 in. MicroTip attachment, ensuring that the tip reached the bottom of the vial. The sonicator was configured to its lowest amplitude setting (“1”) and a 10 s continuous dose length, which delivers approximately 67 J per dose to the solution. The sonication procedure consisted of two 10 s doses, each followed by immersing the vial in ice water for 1 min to dissipate the heat generated by sonication. The vial was then allowed to warm to room temperature (ca. 10 min) before the drop-casting procedure.

Deposition of Catalyst Inks on RDEs

The catalyst inks were drop-cast onto five individual RDEs having 5 mm diameter glassy carbon disk working surfaces surrounded by a 15 mm outer diameter poly(tetrafluoroethylene) (PTFE) shroud (Pine Research, AFE5T050GC). The electrodes were prepared by polishing with 0.05 μm alumina and were placed with their working electrode surfaces facing up. The catalyst ink (10 μL) was dropped onto each electrode using an electronic positive-displacement autopipette (Eppendorf Repeater E3), resulting in a mass loading of supported catalyst powder of 0.51 mg/cm2. The electronic autopipette was used because control of the dispensing speed was critical in controlling splatter and spillover from the glassy carbon onto the PTFE shroud (when the dispensing rate was too fast), as well as droplet adhesion to the pipette tip (when the dispensing rate was too slow). When needed, two successive 5 μL drops were used instead for electrodes with a propensity for spillover. The electrodes were allowed to dry with minimal movement and airflow for 2–24 h before measurement.

Electrochemical Hydrogen Evolution Testing via CV

HER measurements were performed in a 0.5 M sulfuric acid solution (prepared from deionized water and EMD Millipore Suprapur 96% sulfuric acid) in a glass H-cell with the counter electrode separated from the working and reference electrodes by a glass frit. The cell was cleaned between measurements using aqua regia and deionized water. The working side of the cell was continuously purged with bubbling hydrogen. A silver/silver chloride reference electrode and carbon rod counter electrode were used. The potential measured using the reference electrode was adjusted to be relative to the reversible hydrogen electrode potential, after measurement, by observation of the open-circuit potential of a Pt wire electrode inserted into the working electrode compartment. The glassy carbon rotating disk working electrodes were rotated at 1600 rpm for all measurements except alternating current (AC) impedance, which was observed in quiescent solution.
For each catalyst material, the cell and solution resistances were first characterized by AC impedance on the first working electrode. The working electrode position was then maintained for the remainder of the measurement. The solution resistance was determined as the lowest-magnitude real-axis intercept on a Nyquist plot of the AC impedance. No iR compensation was used during measurement; potentials were corrected for 100% of the estimated voltage drop due to solution resistance during data processing. After AC impedance measurement, rotation and hydrogen bubbling were resumed and linear sweep voltammetry (LSV) at −20 mV/s was used to determine the scan limits for CV. LSV was stopped after reaching 5 mA (∼25 mA/cm2). Five successive CV scans at 5 mV/s were then performed starting from the open-circuit potential (or, occasionally, from a more negative potential to shorten the measurement time) and reversing at the potential identified above as sufficient to achieve 5 mA. The five-scan CV measurement was then repeated for each of the four remaining electrodes. Voltammograms were plotted giving the cathodic potential and current a negative sign. Cathodic overpotential required to achieve a cathodic current of 10 mA/cm2 is reported as an absolute value (|η|).

Post-HER Characterization of the NP Catalyst

The post-HER properties of a Ni1.6Rh0.4P/C catalyst were studied by preparing a large sheet electrode. A catalyst ink comprising 60 mg of 3.7 wt % Ni1.58Rh0.42P/C, 120 μL of a 5 wt % Nafion solution, and 3 mL of isopropanol (approx. double the concentration used above, for more efficient deposition) was homogenized by sonication as described above. This ink was then hand-painted in its entirety, followed by an additional 1 mL of isopropanol used to rinse the ink container and brush, onto a 6 cm × 5 cm (30 cm2) area on a sheet of Spectracarb 2050A-0850 carbon paper (Fuel Cell Store). During painting, the carbon paper was held by vacuum onto a heated stage at 80 °C. An additional 2 cm at the top of the carbon paper sheet was left unpainted for electrode assembly for a total sheet size of 8 cm × 5 cm. The sheet was weighed before and after hand-painting and had increased in mass by 45.7 mg, as compared to the theoretical value of 66 mg (60 mg catalyst + 6 mg Nafion). The difference between expected and observed loadings is attributed to the catalyst and/or Nafion remaining in the brush used to paint the electrode. The observed loading of the catalyst averaged over the 30 cm2 active area, assuming about 10% of the observed mass is accounted for by Nafion, is 1.37 mg/cm2. The painted sheet was then cut into two 8 × 2.5 cm pieces, one for pre-HER analysis and one for post-HER analysis. The piece to be used for the electrochemical HER was prepared by affixing a coiled copper wire to the uncoated upper area using a two-part silver epoxy. The exposed copper wire and silver epoxy were then completely sealed using a combination of sealant epoxy (3M Scotch-Weld DP460NS) and a glass tube to prevent contact between the electrolyte and these parts of the electrode assembly. The resulting electrode was tested in a H-cell where the working and counter compartments (60 mL of electrolyte in each) were separated by a Nafion 115 membrane sealed with an o-ring. The HER conditioning used a 0.5 M sulfuric acid electrolyte, a 3 M Ag/AgCl reference electrode, and a Pt mesh counter electrode. After determining solution resistance by AC impedance, LSV at −20 mV/s was used to determine scan limits for CV cycling. Five successive CV scans at 5 mV/s were then performed from the open-circuit potential to a potential sufficient to achieve ca. 15–20 mA/cm2 of peak current. After HER conditioning, the working electrode was washed with deionized water and allowed to air dry. Identical coupons were then cut from both the pre- and post-HER electrodes for various characterization techniques, including XRD (ca. 4 cm2), ICP-OES (ca. 5 cm2), and STEM–EDS (ca. 2 cm2). The sample for XRD was used without preparation. The samples for ICP-OES were sent intact to be digested by the analytical lab. For STEM–EDS, the carbon-supported NPs from the electrode sample were redispersed into ethanol using a bath sonicator for 5 min and then dip-coated onto lacey carbon-coated copper grids.

Computational Methods

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The Vienna ab initio simulation package was used to implement DFT calculations. (58,59) The generalized-gradient approximation (GGA) was used via the Perdew–Burke–Ernzerhof formalism to describe the exchange-correlation functional. (60) Projector augmented-wave potentials with valence wavefunctions expanded through a plane-wave basis set with a cutoff energy of 500 eV were used to capture interactions between electrons and ions. (61,62) Spin-polarized adjustments were made to the GGA to describe the magnetic ordering of the slabs considered in this study. Dispersion corrections to the calculated structures and energetics were included through the D3 method by Grimme et al. (63)
To model the ternary Ni2–xMxP catalyst systems (M = Co, Cu, Mo, Ni, Pd, Rh, or Ru), two surface facets were considered, the (101̅0) and the (0001) surfaces, based on a previous theoretical work indicating that these surfaces are two of the dominant exposed facets on Ni2P NPs. (64) The (0001) surface is the Ni2P surface most commonly used in previous computational studies. (23,24,65−67) Vacuum layers at least 10 Å thick were employed to separate each successive slab in the z direction. In all calculations, the bottom half of each studied Ni2–xMxP surface was fixed in its respective bulk-truncated position, while the top half could freely relax. For each (101̅0) and (0001) surface considered in this work, 8 × 8 × 1 and 6 × 6 × 1 Monkhorst–Pack k-point meshes were used to sample the surface Brillouin zone, respectively. (68) Ionic convergence occurred when the forces acting upon each atom fell below 0.02 eV/Å.
To quantify the strength of adsorption of atomic hydrogen, the binding energy (EB) was determined by
(1)
where Etotal, Eclean, and Egas are the energy of the adsorbate + substrate complex, clean surface, and gas-phase adsorbate (1/2H2), respectively. Temperature corrections were applied to the most-stable adsorption structures for each surface to calculate GH as follows
(2)
In eq 2, E is the total energy, ZPE is the zero-point energy, T is the absolute temperature, and S is the entropy. The subscripts “total”, “clean”, and “gas” refer to the adsorbate + surface complex, the clean surface, and the gas-phase reference (1/2H2), respectively. By this definition, more negative EB and GH values correspond to stronger interactions between the surface and adsorbate. The ZPE and S were calculated using vibrational-frequency analyses through a Hessian matrix calculated via a second-order finite-difference approach. In these calculations, adsorbed H was freely relaxed, all slab atoms were fixed, and bonds were oscillated using a step size of 0.015 Å.
The optimized lattice parameters for bulk Ni2P were 5.80 and 3.35 Å for a and c, respectively, which agree well with the experimental values of 5.86 and 3.38 Å, respectively. (69) To calculate the effect of a second metal on the calculated lattice parameters, one of the six Ni atoms in the bulk primitive cell was substituted with each second metal. In the primitive cell, there are two unique metal positions for the second metal to substitute into the NiI and NiII positions (see Figure S2). (64) The preferred position for M substitution and the effect of each second metal on the DFT-calculated lattice parameters is summarized in Table S2.
In both the (101̅0) and the (0001) ternary Ni2–xMxP model surfaces, there are 12 metal-atom positions. Two Ni atoms in each ternary metal–phosphide system were substituted with each M to achieve a ternary metal–phosphide composition of Ni1.67M0.33P, that is, to achieve an x value of 0.33. This x value of 0.33 is close to the target experimental value of 0.40 and within the experimental range of 0.31–0.46 (see Results and Discussion). To determine the optimal metal positions for each second metal atom (M), the positions were optimized for one M atom in the freely relaxed top half of the slab and another M atom in the fixed bottom half of the slab. Ni1.33M0.67P systems (M = Cu and Pd) with a 50:50 ratio of Ni/Cu and Ni/Pd in the freely relaxed surface layers (i.e., M-enriched surfaces) were also considered based on the respective EDS elemental maps discussed in the Results and Discussion section.
For the (0001) surface, there are two bulk terminations, the Ni3P2- and the Ni3P-terminated surfaces. Based on the DFT calculations presented here for the pristine Ni2P(0001) surface and for each ternary Ni1.67M0.33P(0001) slab, the stoichiometric Ni3P-surface termination is most stable (see Table S4), consistent with prior computational studies of Ni2P surface stability at a low P chemical potential (μP). (64,70) At high μP, consistent with Ni2P synthesis conditions, the P-enriched P–Ni3P2(0001) surface termination is predicted to be most stable (i.e., Ni3P2 termination with an adsorbed P adatom). Both the Ni3P- and P-Ni3P2-terminated Ni1.67M0.33P(0001) surfaces were modeled as possible representative catalyst surface models. The (101̅0) surface is composed of two different Ni2P planes, denoted as A and B (they differ in the positions of the two distinct phosphorus atoms in the bulk unit cell, PI and PII), stacked with an −ABB– repetition. Thus, there are three possible (101̅0) surface terminations, denoted ABB–, BAB–, and BBA–. He and co-workers showed that the stoichiometric ABB-terminated and P-enriched P-BAB-terminated (101̅0) surfaces are most stable at low and high μP, respectively; (64) thus, these two surfaces are considered in this analysis.

Results and Discussion

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The Ni2P NPs were prepared via the thermal decomposition of Ni(CO)2(PPh3)2 in OAm and ODE with PPh3, similar to the reported procedure. (34) The Ni2–xMxP (M = Co, Cu, Mo, Pd, Rh, or Ru) NPs were synthesized using a two-step synthesis route. Amorphous Ni–P NPs, which were previously characterized by our group and others as an intermediate to crystalline Ni2P, (34,71−73) were targeted here as precursors to facilitate the incorporation of a second metal and formation of Ni2–xMxP NPs. (37) The amorphous Ni–P NPs were formed by the thermal decomposition of Ni(CO)2(PPh3)2 in OAm and ODE with PPh3 at 250 °C. Molecular precursors for the second metal were identified based on the commercial availability or ease of preparation, air stability, and decomposition temperature. TGA data for Ni(PPh3)2(CO)2, Rh(PPh3)2(CO)Cl, and Pd(PPh3)4 in an inert atmosphere were previously reported (34) and demonstrated high-temperature decomposition events with precipitous weight losses beginning at 204, 274, and 212 °C for the Ni, Rh, and Pd complexes, respectively. The Co(NO)(CO)2(PPh3) and [Cu(PPh3)H]6 complexes, reported here (Figure S1), exhibited a small, gradual weight loss around 120 °C and then more rapid weight loss indicative of full decomposition around 220 and 250 °C, respectively. Mo(CO)4(PPh3)2 demonstrated precipitous weight loss beginning at 160 °C, while RuCl2(CO)2(PPh3)2 decomposed gradually starting at 230 °C. The resulting ceramic yields at 500 °C for the Co, Cu, and Mo complexes were greater than those expected for the common metal products and could contain O and/or P. The Ru complex had a significantly higher ceramic yield of 45 wt % that could indicate incomplete decomposition under these conditions.
After the room-temperature addition of the molecular precursor of the second metal, the reaction mixture was heated to 300 °C to promote precursor decomposition, alloying, and crystallization of the ternary TMP NPs. The Ni2–xMxP NPs retained the hexagonal structure of Ni2P, and all observed powder XRD peaks were attributed to Ni2P (Figure 1). Elemental analysis of the metal composition, determined by ICP-OES, revealed Ni2–xMxP compositions ranging from x = 0.31 to x = 0.46 (Table 1).

Figure 1

Figure 1. XRD patterns of Ni2P and Ni2–xMxP NPs with the corresponding Ni2P reference pattern (PDF 01-089-2742), below.

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Table 1. Compositions Determined by ICP-OES and Particle Sizes Determined by XRD and STEM for Ni2P and Ni2–xMxP NPs
compositionXRD size (nm)STEM size (nm)
Ni2P9.810.0 ± 0.9
Ni1.61Co0.39P10.09.8 ± 1.0
Ni1.69Cu0.31P8.69.5 ± 3.8
Ni1.66Rh0.34P9.79.5 ± 0.8
Ni1.62Ru0.38P9.511.8 ± 1.2
Ni1.69Pd0.31P9.29.2 ± 1.1
Ni1.54Mo0.46P10.411.6 ± 1.1
Upon introduction of ca. 20 mol % M into the Ni2P structure, the reflections in the XRD patterns shifted, as has previously been observed for Ni2–xMxP NPs. (22,38) For all metals except Co, patterns shifted to lower 2θ values, indicative of lattice expansion due to substitution of the smaller-atomic-radius Ni with the larger-atomic-radius metal. The magnitude of this shift was dependent on the atomic radius of the second metal with the largest shift observed for Ni1.54Mo0.46P (Table S1). Evaluation of the preferred Ni-substitution site and optimized lattice constants in bulk Ni1.67M0.33P (Figure S2, Table S2) by DFT calculations supported the observed shifts in the XRD peak positions (Table S1). Additionally, for M = Rh, Ru, and Mo, the (201) and (210) reflections broadened relative to the (111) and (300)/(211) reflections. This suggests that modifications of the local Ni2P structure occurred with the addition of the larger second metal, while the overall hexagonal crystal structure was retained. (21,22)
The Ni2P NPs are solid particles exhibiting a spherical morphology with an average size of 10.0 ± 0.9 nm, as determined by TEM analysis (Figure S4 and Table 1). The spherical morphology was maintained across the series of Ni2–xMxP NPs, as confirmed by bright-field (BF)-STEM analysis (Figures 2a, 3a, and S4a–S7a), and the average sizes ranged from 9.2 to 11.8 nm with narrow distributions for all compositions except Ni1.61Cu0.39P (Figure S5a). We hypothesize that the wide size distribution of Ni1.61Cu0.39P (9.5 ± 3.8 nm) was due to the less well-controlled or incomplete decomposition of the [Cu(PPh3)H]6 precursor. A similar issue was previously observed when using this molecular precursor to synthesize Cu3P NPs. (12) Crystallite sizes were calculated for all compositions by applying the Scherrer equation to the (111) XRD peaks, and the calculated sizes were in good agreement with the particle sizes measured by STEM (Table 1).

Figure 2

Figure 2. (a) BF-STEM image of Ni1.61Co0.39P NPs; (b) high-resolution HAADF-STEM image with the (c) FFT oriented along the [112̅2] direction; and (d) HAADF-STEM image for EDS analysis and associated EDS elemental maps for (e) Ni, (f) P, (g) Co, and (h) Ni and Co overlaid.

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Figure 3

Figure 3. (a) BF-STEM image of Ni1.69Pd0.31P NPs; (b) high-resolution HAADF-STEM image with the (c) FFT oriented along the [123̅3] direction; and (d) HAADF-STEM image for EDS analysis; and associated EDS elemental maps for (e) Ni, (f) P, (g) Pd, and (h) Ni and Pd overlaid.

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The high-resolution HAADF-STEM analysis of Ni1.61Co0.39P NPs, shown in Figure 2b with the corresponding FFT (Figure 2c, Table S5) oriented along the [112̅2] direction, is characterized by ordered lattice fringes confirming the crystallinity and hexagonal phase of the NPs. The structural analysis is consistent for all Ni2–xMxP NPs, as shown in Figures 3a–c and S4c–S7c and described in Tables S6–S10. The EDS elemental mapping of Ni2–xMxP showed relatively uniform distributions of Ni, P, and M (when M = Co, Rh, Ru, or Mo) within the NPs (Figures 2e–h and S5–S7). For Ni1.61Cu0.39P and Ni1.69Pd0.31P NPs, EDS revealed that the distributions of Cu and Pd were more concentrated at the NP surfaces (Figures S5e–h and 3e–h). In the case of Ni1.61Cu0.39P, the less well-controlled decomposition of the [Cu(PPh3)H]6 complex, as discussed above, may play a role in the surface enrichment. For the larger-atomic-radius Pd, surface enrichment may have limited modifications of the local Ni2P structure that led to the relative broadening of the (201) and (210) reflections observed in the XRD patterns of M = Rh, Ru, and Mo.
Electrochemical activity measurements were performed using Ni2–xMxP NPs supported on carbon (Vulcan XC 72R). The NPs were supported with a targeted weight loading of 5% and thermally reduced at 450 °C in 5% H2 before passivation with 1% O2. Electrochemical analysis of colloidal NPs has indicated that the presence of surface ligands reduces the electrochemically active surface area (ECSA), thus inhibiting the catalytic activity. (30,74) Reduction at 450 °C in H2 has been established as a method to remove organic surface ligands from metal phosphide NPs. (7,12,75) Elemental analysis of the reduced and passivated Ni2–xMxP/C revealed weight loadings ranging from 4.1 to 5.9% (Table 2). XRD patterns of the NPs after supporting and thermal reduction confirmed retention of the Ni2P hexagonal structure, with no appreciable alterations (Figures S9–S15). Previous TEM analysis indicated no appreciable changes to the particle size of Ni2P NPs following the supporting procedure and thermal reduction. (75)
Table 2. Weight Loading, Overpotential Required to Reach 10 mA/cm2 (η), Tafel Slope, and Exchange Current Density for Ni2P/C, Ni2–xMxP/C, and Commercial Pt/C, Ordered by Increasing Overpotential
catalystwt % on Caoverpotential to achieve 10 mA/cm2, η (mV)bTafel slope (mV/dec)cexchange current density (mA/cm2)c
Pt/C2942 ± 2525 ± 100.65 ± 0.09
Ni1.66Rh0.34P/C4.1149 ± 458.5 ± 0.40.029 ± 0.005
Ni1.69Pd0.31P/C4.6158 ± 1296 ± 90.26 ± 0.05
Ni1.62Ru0.38P/C4.8168 ± 970 ± 50.04 ± 0.01
Ni2P/C5.4178 ± 264.2 ± 0.40.016 ± 0.001
Ni1.61Co0.39P/C4.9184 ± 1068 ± 40.019 ± 0.002
Ni1.54Mo0.46P/C5.1199 ± 665 ± 30.008 ± 0.002
Ni1.69Cu0.31P/C5.9274 ± 17101 ± 80.020 ± 0.003
a

Weight loading of the metal or metal phosphide on carbon as determined by ICP-OES.

b

Five electrodes were tested for each catalyst. For each electrode, the potentials at 10 mA/cm2 of the forward and reverse sweeps of the third consecutive CV scan were averaged (see Figure S19). The resulting five potentials were averaged to determine the overall average and standard deviation.

c

From a fit to the linear region; values and standard deviations determined by averaging the values for all five electrodes (see Figure S20). Exemplary fits are shown in Figure 4b.

Electrodes were prepared for each catalyst by drop-casting an ink consisting of Ni2–xMxP/C or Ni2P/C, Nafion, and isopropanol onto a glassy carbon RDE, as seen in literature methods for NP- and carbon-supported materials (see the Experimental Section). (7,42,55,76,77) A commercial 29 wt % Pt/C material was also included as a reference. This material did not suspend as well using the ink preparation method optimized for the NP catalysts, which may have caused the greater variability observed for Pt (vide infra). Figure 4a shows the voltammograms for Ni2–xMxP/C, Ni2P/C, and Pt/C. Stable current responses were observed for five consecutive CV scans across the series of NP catalysts (Figure S18). To reduce contributions from surface contaminants and allow for catalyst equilibration, (78) the third scan is presented in Figure 4a.

Figure 4

Figure 4. (a) Voltammograms (cathodic sweep only, 5 mV/s) and (b) Tafel plots for Ni2–xMxP/C (nominally 5 wt %; see Table 2) and Pt/C (29 wt %) catalysts in H2-saturated 0.5 M H2SO4 at room temperature, with both experimental data (dashed) and linear fits (solid; Table 2) shown (catalyst loading = 0.51 mg/cm2; NP loading ≈ 0.025 mg/cm2). The legend for both plots is shown in (b).

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Modulation of the HER activity of Ni2P by controlled doping was quantified by measuring the overpotential required to achieve a cathodic current of 10 mA/cm2 (η; Table 2). The parent Ni2P NPs exhibited an η of 178 ± 2 mV, activity that is consistent with that previously reported for Ni2P NPs. (7) Markedly different HER activities were observed across the Ni2–xMxP NP series, with η ranging from 149 mV for the most active catalyst, Ni1.66Rh0.34P, to 274 mV for the least active, Ni1.61Cu0.39P (Table 2). Because determination of the ECSA via the double-layer capacitance is challenging for carbon-supported materials, catalyst comparisons were performed using a geometric surface area basis. This approach relies on our observations that the particles maintain their size and shape and are well dispersed, upon carbon immobilization, allowing an assumption of a similar ECSA across the series. (75) We also compared the catalysts normalized on per-milligram NP and per-mole metal bases in Figure S16, which revealed no significant changes in activity ranking except that the gap in activity between Pt and the Ni2–xMxP NPs was much smaller on a mass-normalized basis. The observed activity trends for Ni2–xMxP NPs reveal some notable differences from expectations. For example, while Kibsgaard et al. found experimentally that MoP and CoP NPs were substantially more active than Ni2P NPs, the Ni1.54Mo0.46P and Ni1.61Co0.39P NPs in our study appear to be similar to or less active than Ni2P NPs. (24) The activity for Ni2–xMxP follows expectations from the study by Zheng et al. for the corresponding metal surfaces for Rh, Pd, Co, and Mo but does not agree for Cu; (79) instead of having a negligible effect on the Ni2P overpotential as the metal performance might suggest, Cu doping substantially increases the overpotential in our case. In other relevant studies, doping Cu into CoP nanowires (under alkaline conditions) (80) and nanosheets (in buffered phosphate) (81) reduced the HER overpotential. The contrasting increase we observed in the overpotential for Ni1.69Cu0.31P/C may indicate a more subtle relationship between dopant and parent structures than is captured by simple analogy to pure phosphide materials and metals. (21−26) Finally, in a study of a Ni2–xCoxP series, Liu et al. found that activity improved steadily upon doping Co into Ni2P NPs and upon doping Ni into Co2P NPs, reaching a maximum near x = 1; (23) on this basis, an improvement of up to 10–15 mV in overpotential might be expected for our Ni1.61Co0.39P NPs relative to Ni2P. Our observation instead of similar or slightly worse activity for Ni1.61Co0.39P may be attributable to differences in NP preparation methods. Figures 4b and S20 show the corresponding Tafel plots for Ni2–xMxP NPs, Ni2P NPs, and Pt/C, and the Tafel slopes are summarized in Table 2. The estimated exchange current densities are also provided in Table 2, and no clear trend was observed. Generally, lower Tafel slopes indicate more favorable electrochemical kinetics, and Ni1.66Rh0.34P, the most active TMP material investigated here, exhibited the lowest Tafel slope (58.5 ± 0.4 mV/decade) across the Ni2–xMxP series. It is noteworthy that the two materials in which surface enrichment was observed for the second metal, Ni1.61Cu0.39P and Ni1.69Pd0.31P, exhibited higher Tafel slopes of 101 ± 8 and 96 ± 9 mV/decade, whereas the other phosphide materials had lower slopes in the range of 58.5–70 mV/decade. This electrochemical study thereby established composition-dependent activity trends within an isostructural series that we next sought to explain through computations.
To better understand the influence of the various second metals on the experimental HER activity, DFT calculations were performed to compare GH on Ni1.67M0.33P surfaces, as GH is a well-established descriptor for HER activity. (82) Two terminations for both the (101̅0) and the (0001) Ni2P surfaces were considered in this analysis (see Computational Methods; each ternary TMP surface is shown in Figure S3), representing the thermodynamic equilibrium surface composition under a range of μP. The (0001) surface is most commonly studied (23,24,65−67) due to its lower surface energy relative to other low-index Ni2P facets, (64) while more corrugated surfaces such as the (101̅0) surface are also likely exposed and have been identified by Hansen and co-workers to play a key role in HER activity. (83) Furthermore, by assessing the relative ability of the flat (0001) and the corrugated (101̅0) surfaces to describe the experimental HER activity, insights regarding the facet-dependent catalytic properties of these materials can be obtained. Due to the higher surface concentration of Cu and Pd observed by EDS mapping (Figures S5e–h and 3e–h), Ni1.33M0.67P surfaces with a 50:50 ratio of Ni/Cu and Ni/Pd in the surface layers were also considered alongside the Ni1.67M0.33P analogues (see Figure S3). The minimum-energy adsorption structures and corresponding GH values for atomic H on each surface are shown in Figure 5. EB values for the studied high-symmetry adsorption sites on each surface are given in Tables S12–S15. H preferentially binds to threefold metal (M3) hollow sites or twofold bridge (M2) sites on the Ni3P-(0001) surface and to M2 sites on the ABB-(101̅0) surface. In contrast, on the P-enriched surfaces where the M3 and M2 sites are blocked by surface P adatoms, H preferentially binds to the P adatoms. H adsorption is generally weaker on the P-enriched surfaces, though there are some notable exceptions on the (0001) surfaces. These general trends in the H binding site and strength are consistent with previous reports for Ni2P. (65,67)

Figure 5

Figure 5. Top views of the optimal H adsorption structure for the (a) ABB- and (b) P-BAB-terminated Ni1.67M0.33P(101̅0) and (c) Ni3P- and (d) P-Ni3P2-terminated Ni1.67M0.33P(0001) surfaces (M = Co, Cu, Mo, Ni, Pd, Rh, or Ru). Also shown are the H adsorption structures for the Cu- and Pd-enriched Ni1.33M0.67P surfaces with a 1:1 Ni/Cu and Ni/Pd surface-layer composition [Cu(50-50) and Pd(50-50), respectively; see main text for more details]. Calculated adsorption free energies of atomic H (GH, in eV) are provided below each snapshot. Black lines denote the surface unit cell. Atom colors: black, H; yellow, P; green, Ni; cyan, Co; blue, Cu; red, Mo; orange, Pd; purple, Rh; and pink, Ru.

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To compare the DFT-calculated GH values with the experimental HER measurements, η is plotted against GH for the ABB-(101̅0), P-BAB-(101̅0), Ni3P-(0001), and P-Ni3P2-(0001) surfaces. For the P-BAB(101̅0) surface and the two (0001) surface terminations, the calculated GH values did not present a clear trend with the experimental HER activity data (Figure S24). However, a clear volcano-shaped trend was observed for the ABB-(101̅0) surface termination (Figure 6), indicating a good representation of the active site for the HER on Ni1.67M0.33P. There are similarities between this volcano plot and the volcano plot for the HER on the bare metals, but there are also noteworthy differences, including that Mo lies on the opposite side of the volcano peak (i.e., binds H too weakly) in the Ni1.6M0.4P system. (79) These differences illustrate the interplay between strain, electronic, and ensemble effects in the Ni1.6M0.4P materials. Bulk-terminated (101̅0) surfaces are more corrugated and exhibit different preferred adsorption configurations than bulk-terminated (0001) surfaces. For example, H preferentially adsorbs to M2 sites on the ABB-(101̅0) surface, compared to M3 hollow sites on the Ni3P-terminated (0001) surface; thus, surfaces on which H preferentially adsorbs to corrugated M2 bridge sites may provide better HER active-site models for the HER on Ni2P-based ternary metal phosphide materials. Furthermore, the volcano-shaped trend helps explain the enhanced HER activity following the addition of precious metals (Pd, Rh, and Ru) as the hydrogen adsorption energy is modulated more effectively (i.e., slight weakening of H adsorption) with these metals as opposed to the Co-, Cu-, or Mo-modified materials. It should be noted that other low-index facets or the edge sites of Ni2–xMxP NPs may also provide active sites for the HER (64,83) and that these results indicate that multiple surface facets should be considered for these complex multicomponent materials to determine facet dependence in describing experimental properties trends.

Figure 6

Figure 6. Overpotential (η) vs the adsorption free energy of atomic H (GH) on the ABB-terminated Ni1.67M0.33P(101̅0) (solid markers) and Cu- and Pd-enriched Ni1.33M0.67P(101̅0) (open markers) surfaces. Gray lines show the volcano-shaped dependence of η on GH for the Ni1.67M0.33P(101̅0) surfaces (solid) and when Cu- and Pd-enriched surfaces are included (dashed).

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Moreover, to better represent the experimentally observed surface enrichment of the Cu- and Pd-alloyed Ni2P NPs based on the corresponding EDS elemental maps (Figures S5e–h and 3e–h), additional DFT models of the Cu- and Pd-containing (0001) and (101̅0) surfaces were constructed such that the surfaces of these materials were enriched to a 50–50 Ni–M composition. The calculated GH values for the surface-enriched ABB-(101̅0) surfaces were destabilized by 0.15 eV for the Cu-alloyed surface and stabilized by 0.05 eV for the Pd-alloyed surface. The 50-50 Ni–M surface compositions resulted in a decrease in slope for the right-hand side of the volcano plot (Figure 6, dashed line) which gave a better overall fit to the measured electrocatalytic HER activity compared to the fit obtained with only the non-surface-enriched analogues (linear-fit R2 of 0.99 vs 0.88, respectively). Bader charge analyses and d-band center calculations were performed to elucidate the effect of electronic structure changes in the M-alloyed ABB-(101̅0) surfaces on the corresponding GH value (Figure S25). However, neither analysis provided a strong correlation with the GH values, indicating that the effect of the alloyed metal on GH is very likely multifactorial with respect to electronic structure, lattice strain, and M surface versus subsurface substitution effects.
Finally, to provide an initial glimpse into the possible evolution of the Ni2–xMxP NP materials under HER conditions, we compared the properties of pristine and post-HER electrodes for one of the materials, 3.7 wt % Ni1.58Rh0.42P/C (note that because we used a different catalyst batch for this experiment, the composition is slightly different from that used in the other experiments). Because the HER measurements we reported above, designed to determine activity with minimal errors, used a small RDE and a correspondingly small quantity of deposited NPs (0.2 cm2 electrode × ca. 0.025 mg NPs/cm2 = ca. 5 μg NPs per electrode), those electrodes were not conducive to postreaction characterization. As a result, we devised an alternative electrode preparation method that would preserve the HER treatment conditions while making possible the characterization of coated electrode materials via standard techniques. The preparation method is similar to the work we have published previously. (12) Briefly, we hand-painted a catalyst ink containing 3.7 wt % Ni1.58Rh0.42P/C and Nafion onto a large sheet of carbon paper at weight loadings similar to those for the RDE measurements (ca. 0.045 mg NPs/cm2 as determined by ICP-OES; see Table S11). The total surface area was ca. 30 cm2, which was divided into two 15 cm2 pieces (ca. 0.7 mg of NPs per electrode, over 2 orders of magnitude more than on the RDEs). One of the pieces was then tested under HER conditions similar to those already described. The pristine and post-HER electrode pieces were compared using powder XRD, ICP-OES, and STEM. In the powder XRD pattern, the major Ni1.58Rh0.42P peak was preserved without apparent peak shifting or introduction of new peaks (Figure S21). However, ICP-OES and STEM revealed changes in the loaded NPs that indicate some evolution under HER conditions. First, there was an ICP-determined reduction in the loading of Ni (from 0.241 wt % in the pristine electrode to 0.113 wt % in the post-HER electrode), Rh (from 0.0949 to 0.0745%), and P (from 0.083 to 0.071%; Table S11). While both physical delamination and chemical leaching are potential explanations for the reduced loading, the significantly higher reduction in Ni compared to the other elements is more consistent with chemical leaching, for which the 0.5 M H2SO4 solution would be expected to dissolve Ni at a higher rate than that for Rh. (17,35) Furthermore, the HAADF-STEM images of the post-HER electrode revealed a change in morphology from spherical to less well-defined (Figure S22). The STEM–EDS mapping shows that the colocation of Ni, Rh, and P was preserved in post-HER samples, with possible loss of some Ni from the surface that would be consistent with the ICP-OES results (Figure S23). We suggest that future studies that aim to improve the electrochemical performance and durability, which was not the focus of this work, could use unsupported Ni2–xMxP NPs to further investigate the operando evolution of ternary phosphide materials.

Conclusions

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A rational solution synthesis method to access ternary Ni2–xMxP NP compositions was developed using combinations of commercially available or easily prepared metal–phosphine precursors. The particle morphology and crystal phase of the parent Ni2P NPs were retained across the series. This approach allowed us to directly compare the impact of the second metal on electrocatalytic activity without convoluting factors arising from differences in the NP morphology or phase. The NPs exhibited composition-dependent HER activity, spanning a 125 mV range in overpotential, attributed to the introduction of a second metal into the parent Ni2P NPs. Computational analysis identified a volcano-shaped trend when the overpotential was plotted against GH for the stoichiometric ABB-(101̅0) surfaces, suggesting that the HER over Ni2–xMxP NPs is facet-dependent and likely requires the presence of corrugated surface features. The ability to independently tune the composition of the ternary nickel phosphide NPs without modifying other key features was crucial to identifying the composition-dependent electrocatalytic activity and the correlation with the (101̅0) surface. Furthermore, the versatility of the synthetic methodology allows for the preparation of a wide range of compositionally diverse TMP NPs, more broadly enabling the investigation of critical composition–performance relationships for energy applications.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.2c00085.

  • TGA profiles; atomic radii and XRD shifts; computational methods; electron microscopy, powder XRD, electrocatalytic HER, and post-HER catalyst characterizations; and computed surfaces and adsorption energies (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Courtney A. Downes - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0001-8631-1579
    • Kurt M. Van Allsburg - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0003-4553-6709
    • Sean A. Tacey - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0001-7833-6901
    • Kinga A. Unocic - Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
    • Frederick G. Baddour - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0002-5837-5804
    • Daniel A. Ruddy - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttps://orcid.org/0000-0003-2654-3778
    • Nicole J. LiBretto - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States
    • Max M. O’Connor - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Present Address: Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, CO 80401, USA. Department of Chemistry, University of Colorado, Boulder, CO 80309, USA;  Orcidhttps://orcid.org/0000-0001-5446-0799
    • Carrie A. Farberow - Catalytic Carbon Transformation and Scale-Up Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States
  • Author Contributions

    C.A.D. and K.M.V.A. contributed equally to this work.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was authored in part by the National Renewable Energy Laboratory (NREL), operated by Alliance for Sustainable Energy, LLC, and in part by the Oak Ridge National Laboratory, operated by UT-Battelle, LLC, for the U.S. Department of Energy (DOE) under contract nos. DE-AC36-08GO28308 and DE-AC05-00OR22725, respectively. This work was supported by the Laboratory Directed Research and Development (LDRD) program at NREL. Support for this work was also provided by the U.S. DOE Office of Energy Efficiency and Renewable Energy Bioenergy Technologies Office. This research was conducted in collaboration with the Chemical Catalysis for Bioenergy (ChemCatBio) Consortium, a member of the Energy Materials Network (EMN). Part of the microscopy was supported by Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE, Office of Science User Facility. STEM-EDS microscopy research was performed using instrumentation (FEI Talos F200X S/TEM) provided by the U.S. DOE, Office of Nuclear Energy, Fuel Cycle R&D Program and the Nuclear Science User Facilities. Computational modeling was performed using computational resources sponsored by the U.S. DOE Office of Energy Efficiency and Renewable Energy and located at NREL. Authors thank Vassili Vorotnikov for collaboration on computational modeling, Anne K. Starace for performing thermogravimetric analysis, and Shawn K. Reeves for assistance with TEM sample preparation. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

References

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    Carenco, Sophie; Portehault, David; Boissiere, Cedric; Mezailles, Nicolas; Sanchez, Clement
    Chemical Reviews (Washington, DC, United States) (2013), 113 (10), 7981-8065CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The wide variety of nanostructured metal boride and metal phosphide nanoparticles that can be obtained nowadays, and the strategies that researchers successfully developed to reach these nanoscaled phases are summarized. The development of nanoscaled metal borides is clearly less mature essentially for two reasons: the lack of highly reactive boron sources and the resistance to crystn. While mechanistic studies could shed light on the crucial exptl. levers for better compositional, structural, and morphol. control of nanostructured metal phosphides and metal borides, another tantalizing approach is to rethink the existing syntheses in terms of precursors, in an attempt to rationalize the stoichiometry introduced and the reaction temp. The use of single-source precursors provides an interesting soln. by bringing the constituents in the right stoichiometry and already intertwined at the mol. scale. A third path is to use very reactive P or B sources to ensure a quant. incorporation in the final nanomaterials. The two families of materials are not mirrored in this aspect since elemental phosphorus P4 is highly reactive while elemental boron allotropes are very stable. Relying on specific reaction media that can enhance kinetics, like specific solvents (e.g., salt melts) for metal borides, is another tempting soln. The properties of nanoscaled metal borides and metal phosphides are shown to differ, if not outperform in some cases, those of the traditional bulk phases because of increased surface exposure or grain boundary d., and also confinement, like quantum effects in semiconductors. The behavior of these nanoscaled cryst. phases in electronics, optics, magnetism, mechanics, and biomedicine are still to be explored, esp. because most of these compds. have not been obtained yet at the nanoscale in the strict sense: one particle dimension below 100 nm. Future works may then first conc. on a better rationalization of the synthetic procedures, in order to further develop applications, for instance in the field of catalysis.
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  2. 2
    Kolny-Olesiak, J. Recent Advances in the Colloidal Synthesis of Ternary Transition Metal Phosphides. Z. Naturforsch., A: Phys. Sci. 2019, 74, 709– 719,  DOI: 10.1515/zna-2019-0133
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    Recent Advances in the Colloidal Synthesis of Ternary Transition Metal Phosphides
    Kolny-Olesiak, Joanna
    Zeitschrift fuer Naturforschung, A: Physical Sciences (2019), 74 (8), 709-719CODEN: ZNASEI; ISSN:1865-7109. (Walter de Gruyter GmbH)
    A review. Transition metal phosphides are interesting candidates for application as catalysts in electrochem. water splitting or heteroatom removal reactions, energy storage, and magnetic refrigeration. Metal phosphides can adopt a wide range of stoichiometries, and have metallic or semiconducting properties. The electronic and magnetic characteristics of metal phosphides can be further controlled by changing the size and the shape of the crystallites and by combining two different metals. In this review, the wet-chem. methods to synthesize such ternary Fe, Co, and Ni phosphides are summarized. First, an overview of the general synthetic strategies and the crystallog. structure of the related materials is given. Then, examples of the synthesis of several ternary phosphides are described, and the motivation for synthesizing the specific materials is discussed.
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    Prins, R.; Bussell, M. E. Metal Phosphides: Preparation, Characterization and Catalytic Reactivity. Catal. Lett. 2012, 142, 1413– 1436,  DOI: 10.1007/s10562-012-0929-7
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    Metal Phosphides: Preparation, Characterization and Catalytic Reactivity
    Prins, Roel; Bussell, Mark E.
    Catalysis Letters (2012), 142 (12), 1413-1436CODEN: CALEER; ISSN:1011-372X. (Springer)
    A review; the prepn., characterization, and catalytic activity of supported metal phosphides are reviewed. Redn. of metal compds. together with phosphate is a convenient method to prep. metal phosphides, but requires high temp. Redn. with phosphite, hypophosphite, or phosphine and the plasma redn. of phosphate can be carried out at lower temps., which leads to smaller metal phosphide particles and more active catalysts. Organometallic routes allow the sep. synthesis of metal phosphide nanoparticles, which have to be added to the support in a second step. LEED, STM, XPS, and DFT studies have shown that the surfaces of Ni2P reconstruct to P-rich surfaces. The investigation of metal phosphides as catalysts for hydrotreating reactions continues to be a topic of considerable research with recent advances realized in using bimetallic and noble metal phosphides to achieve high activities and tailored selectivities. Finally, hydrodeoxygenation catalysis over metal phosphides is a growing area of research given the need to develop catalysts for upgrading biomass to transportation fuels.
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    Joo, J.; Kim, T.; Lee, J.; Choi, S. I.; Lee, K. Morphology-Controlled Metal Sulfides and Phosphides for Electrochemical Water Splitting. Adv. Mater. 2019, 31, e1806682  DOI: 10.1002/adma.201806682
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    Li, Y.; Dong, Z.; Jiao, L. Multifunctional Transition Metal-Based Phosphides in Energy-Related Electrocatalysis. Adv. Energy Mater. 2020, 10, 1902104,  DOI: 10.1002/aenm.201902104
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    Multifunctional Transition Metal-Based Phosphides in Energy-Related Electrocatalysis
    Li, Yang; Dong, Zihao; Jiao, Lifang
    Advanced Energy Materials (2020), 10 (11), 1902104CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    A review. The exploitation of cheap and efficient electrocatalysts is the key to make energy-related electrocatalytic techniques com. viable. In recent years, transition metal phosphides (TMPs) electrocatalysts have gained a great deal of attention owing to their multifunctional active sites, tunable structure, and compn., as well as unique physicochem. properties. This review summarizes the up-to-date progress on TMPs in energy-related electrocatalysis from diversified synthetic methods, ingenious-modulated strategies, and novel applications. In order to set forth theory-structure-performance relationships upon TMPs, the corresponding reaction mechanisms, electrocatalytsts' structure/compn. designs and desired electrochem. performance are jointly discussed, along with demonstrating their practical electrocatalytic applications in overall water splitting, metal-air batteries, lithium-sulfur batteries, etc. In the end, some underpinning issues and research orientations of TMPs toward efficient energy-related electrocatalysis are briefly proposed.
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    Pei, Y.; Cheng, Y.; Chen, J.; Smith, W.; Dong, P.; Ajayan, P. M.; Ye, M.; Shen, J. Recent Developments of Transition Metal Phosphides as Catalysts in the Energy Conversion field. J. Mater. Chem. A 2018, 6, 23220– 23243,  DOI: 10.1039/c8ta09454c
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    Recent developments of transition metal phosphides as catalysts in the energy conversion field
    Pei, Yu; Cheng, Yu; Chen, Jiyi; Smith, Will; Dong, Pei; Ajayan, Pulickel M.; Ye, Mingxin; Shen, Jianfeng
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (46), 23220-23243CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A review is given. Catalysts play an important role in the energy conversion reaction process. With the continuous development of the energy sector, researchers are constantly exploring and developing new high-performance, inexpensive catalysts to meet the demand. Transition metal phosphides (TMPs) as ever-evolving catalytic materials have received more and more attention. Many studies have suggested that transition metal phosphides could be ideal catalysts due to their low cost, abundance, and high efficiency. In many areas, TMPs can be applied as stable and efficient catalytic materials. Therefore, many kinds of TMPs are designed and synthesized by different methods. This article includes the recent research progress of TMPs, including their prepn. methods and morphologies. The application and catalytic performance of TMPs in different catalytic fields are elaborated, and views on the opportunities and challenges faced by phosphide catalysts are presented.
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    Callejas, J. F.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater. 2016, 28, 6017– 6044,  DOI: 10.1021/acs.chemmater.6b02148
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    Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction
    Callejas, Juan F.; Read, Carlos G.; Roske, Christopher W.; Lewis, Nathan S.; Schaak, Raymond E.
    Chemistry of Materials (2016), 28 (17), 6017-6044CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A review. Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (MxPy) have recently been identified as a promising family of Earth-abundant electrocatalysts for the hydrogen-evolution reaction (HER), and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this review, we highlight the progress that has been made in this field and provide insights into the synthesis, characterization and electrochem. behavior of transition metal phosphides as HER electrocatalysts. We also discuss strategies for the incorporation of metal phosphides into integrated solar-driven water-splitting systems and highlight key considerations involved in the testing and benchmarking of such devices.
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    Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 1529– 1541,  DOI: 10.1039/c5cs00434a
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    Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction
    Shi, Yanmei; Zhang, Bin
    Chemical Society Reviews (2016), 45 (6), 1529-1541CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    The urgent need of clean and renewable energy drives the exploration of effective strategies to produce mol. hydrogen. With the assistance of highly active non-noble metal electrocatalysts, electrolysis of water is becoming a promising candidate to generate pure hydrogen with low cost and high efficiency. Very recently, transition metal phosphides (TMPs) have been proven to be high performance catalysts with high activity, high stability, and nearly ∼100% Faradic efficiency in not only strong acidic solns., but also in strong alk. and neutral media for electrochem. hydrogen evolution. In this tutorial review, an overview of recent development of TMP nanomaterials as catalysts for hydrogen generation with high activity and stability is presented. The effects of phosphorus (P) on HER activity, and their synthetic methods of TMPs are briefly discussed. Then we will demonstrate the specific strategies to further improve the catalytic efficiency and stability of TMPs by structural engineering. Making use of TMPs as cocatalysts and catalysts in photochem. and photoelectrochem. water splitting is also discussed. Finally, some key challenges and issues which should not be ignored during the rapid development of TMPs are pointed out. These strategies and challenges of TMPs are instructive for designing other high-performance non-noble metal catalysts.
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  9. 9
    Calvinho, K. U. D.; Laursen, A. B.; Yap, K. M. K.; Goetjen, T. A.; Hwang, S.; Murali, N.; Mejia-Sosa, B.; Lubarski, A.; Teeluck, K. M.; Hall, E. S.; Garfunkel, E.; Greenblatt, M.; Dismukes, G. C. Selective CO2 Reduction to C3 and C4 Oxyhydrocarbons on Nickel Phosphides at Overpotentials as Low as 10 mV. Energy Environ. Sci. 2018, 11, 2550– 2559,  DOI: 10.1039/c8ee00936h
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    10
    Selective CO2 reduction to C3 and C4 oxyhydrocarbons on nickel phosphides at overpotentials as low as 10 mV
    Calvinho, Karin U. D.; Laursen, Anders B.; Yap, Kyra M. K.; Goetjen, Timothy A.; Hwang, Shinjae; Murali, Nagarajan; Mejia-Sosa, Bryan; Lubarski, Alexander; Teeluck, Krishani M.; Hall, Eugene S.; Garfunkel, Eric; Greenblatt, Martha; Dismukes, G. Charles
    Energy & Environmental Science (2018), 11 (9), 2550-2559CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    We introduce five nickel phosphide compds. as electro-catalysts for the redn. of carbon dioxide in aq. soln., that achieve unprecedented selectivity to C3 and C4 products (the first such report). Three products: formic acid (C1), methylglyoxal (C3), and 2,3-furandiol (C4), are obsd. at potentials as low as +50 mV vs. RHE, and at the highest half-reaction energy efficiencies reported to date for any >C1 product (99%). The max. selectivity for 2,3-furandiol is 71% (faradaic efficiency) at 0.00 V vs. RHE on Ni2P, which is equiv. to an overpotential of 10 mV, with the balance forming methylglyoxal, the proposed reaction intermediate. P content in the series correlates closely with both the total C products and product selectivity, establishing definitive structure-function relationships. We propose a reaction mechanism for the formation of multi-carbon products, involving hydride transfer as the potential-detg. step to oxygen-bound intermediates. This unlocks a new and more energy-efficient redn. route that has only been previously obsd. in nickel-based enzymes. This performance contrasts with simple metallic catalysts that have poor selectivity between multi-carbon products, and which require high overpotentials (>700 mV) to achieve comparable reaction rates.
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  10. 10
    Li, H.; Wen, P.; Itanze, D. S.; Hood, Z. D.; Ma, X.; Kim, M.; Adhikari, S.; Lu, C.; Dun, C.; Chi, M.; Qiu, Y.; Geyer, S. M. Colloidal Silver Diphosphide (AgP2) Nanocrystals as Low Overpotential Catalysts for CO2 Reduction to Tunable Syngas. Nat. Commun. 2019, 10, 5724,  DOI: 10.1038/s41467-019-13388-8
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    11
    Colloidal silver diphosphide (AgP2) nanocrystals as low overpotential catalysts for CO2 reduction to tunable syngas
    Li, Hui; Wen, Peng; Itanze, Dominique S.; Hood, Zachary D.; Ma, Xiao; Kim, Michael; Adhikari, Shiba; Lu, Chang; Dun, Chaochao; Chi, Miaofang; Qiu, Yejun; Geyer, Scott M.
    Nature Communications (2019), 10 (1), 5724CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Prodn. of syngas with tunable CO/H2 ratio from renewable resources is an ideal way to provide a carbon-neutral feedstock for liq. fuel prodn. Ag is a benchmark electrocatalysts for CO2-to-CO conversion but high overpotential limits the efficiency. We synthesize AgP2 nanocrystals (NCs) with a greater than 3-fold redn. in overpotential for electrochem. CO2-to-CO redn. compared to Ag and greatly enhanced stability. D. functional theory calcns. reveal a significant energy barrier decrease in the formate intermediate formation step. In situ X-ray absorption spectroscopy (XAS) shows that a max. Faradaic efficiency is achieved at an av. silver valence state of +1.08 in AgP2 NCs. A photocathode consisting of a n+p-Si wafer coated with ultrathin Al2O3 and AgP2 NCs achieves an onset potential of 0.2 V vs. RHE for CO prodn. and a partial photocurrent d. for CO at -0.11 V vs. RHE (j-0.11,CO) of -3.2 mA cm-2.
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  11. 11
    Ji, L.; Li, L.; Ji, X.; Zhang, Y.; Mou, S.; Wu, T.; Liu, Q.; Li, B.; Zhu, X.; Luo, Y.; Shi, X.; Asiri, A. M.; Sun, X. Highly Selective Electrochemical Reduction of to Alcohols on an FeP Nanoarray. Angew. Chem., Int. Ed. 2020, 59, 758– 762,  DOI: 10.1002/anie.201912836
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    12
    Highly Selective Electrochemical Reduction of CO2 to Alcohols on a FeP Nanoarray
    Ji, Lei; Li, Lei; Ji, Xuqiang; Zhang, Ya; Mou, Shiyong; Wu, Tongwei; Liu, Qian; Li, Baihai; Zhu, Xiaojuan; Luo, Yonglan; Shi, Xifeng; Asiri, Abdullah M.; Sun, Xuping
    Angewandte Chemie, International Edition (2020), 59 (2), 758-762CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Electrochem. redn. of CO2 into various chems. and fuels provides an attractive pathway for environmental and energy sustainability. It is now shown that a FeP nanoarray on Ti mesh (FeP NA/TM) acts as an efficient 3-dimensional catalyst electrode for the CO2 redn. reaction to convert CO2 into alcs. with high selectivity. In 0.5 M KHCO3, such FeP NA/TM is capable of achieving a high faradaic efficiency (FECH3OH) up to 80.2 %, with a total FECH3OH+C2H5OH of 94.3% at -0.20 V vs. reversible H electrode. D. functional theory calcns. reveal that the FeP(211) surface significantly promotes the adsorption and redn. of CO2 toward MeOH owing to the synergistic effect of two adjacent Fe atoms, and the potential-detg. step is the hydrogenation process of *CO.
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  12. 12
    Downes, C. A.; Libretto, N. J.; Harman-Ware, A. E.; Happs, R. M.; Ruddy, D. A.; Baddour, F. G.; Ferrell, J. R., III; Habas, S. E.; Schaidle, J. A. Electrocatalytic CO2 Reduction over Cu3P Nanoparticles Generated Via a Molecular Precursor Route. ACS Appl. Energy Mater. 2020, 3, 10435– 10446,  DOI: 10.1021/acsaem.0c01360
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    13
    Electrocatalytic CO2 reduction over Cu3P nanoparticles generated via molecular precursor route
    Downes, Courtney A.; Libretto, Nicole J.; Harman-Ware, Anne E.; Happs, Renee M.; Ruddy, Daniel A.; Baddour, Frederick G.; Ferrell III, Jack R.; Habas, Susan E.; Schaidle, Joshua A.
    ACS Applied Energy Materials (2020), 3 (11), 10435-10446CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    The design of nanoparticles (NPs) with tailored morphologies and finely tuned electronic and phys. properties has become a key strategy for controlling selectivity and improving conversion efficiency in a variety of important electrocatalytic transformations. Transition metal phosphide NPs, in particular, have emerged as a versatile class of catalytic materials due to their multifunctional active sites and compn.- and phase-dependent properties. Access to targeted transition metal phosphide NPs with controlled features is necessary to tune the catalytic activity. To this end, we have established a soln.-synthesis route utilizing a mol. precursor contg. M-P bonds to generate solid metal phosphide NPs with controlled stoichiometry and morphol. We expand here the application of mol. precursors in metal phosphide NP synthesis to include the prepn. of phase-pure Cu3P NPs from the thermal decompn. of [Cu(H)(PPh3)]6. The mechanism of [Cu(H)(PPh3)]6 decompn. and subsequent formation of Cu3P was investigated through modification of the reaction parameters. Identification and optimization of the crit. reaction parameters (i.e., time, temp., and oleylamine concn.) enabled the synthesis of phase-pure 9-11 nm Cu3P NPs. To probe the multifunctionality of this materials system, Cu3P NPs were investigated as an electrocatalyst for CO2 redn. At low overpotential (-0.30 V vs. RHE) in 0.1 M KHCO3 electrolyte, Cu3P-modified carbon paper electrodes produced formate (HCOO-) at a max. Faradaic efficiency of 8%.
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  13. 13
    Laursen, A. B.; Calvinho, K. U. D.; Goetjen, T. A.; Yap, K. M. K.; Hwang, S.; Yang, H.; Garfunkel, E.; Dismukes, G. C. CO2 Electro-Reduction on Cu3P: Role of Cu(I) Oxidation State and Surface Facet Structure in C1-Formate Production and H2 Selectivity. Electrochim. Acta 2021, 391, 138889,  DOI: 10.1016/j.electacta.2021.138889
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    14
    CO2 electro-reduction on Cu3P: Role of Cu(I) oxidation state and surface facet structure in C1-formate production and H2 selectivity
    Laursen, Anders B.; Calvinho, Karin U. D.; Goetjen, Timothy A.; Yap, Kyra M. K.; Hwang, Shinjae; Yang, Hongbin; Garfunkel, Eric; Dismukes, G. Charles
    Electrochimica Acta (2021), 391 (), 138889CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    We report the catalytic activity and mechanism of copper(I) phosphide, Cu3P, with predominant [001] facet exposure for the electrochem. redn. of CO2 (CO2RR) to formic acid. Cryst. nanosheets of this compd. that show a preferential [001] facet orientation exhibit undiminished CO2RR activity after 16 h with full retention of crystal structure and surface chem. speciation and no detectable corrosion. In contrast to the range of products formed on Cu metal, CuO, and Cu2O, the CO2RR on Cu3P [001] produces mainly hydrogen and formate as the sole carbon product in KHCO3 electrolyte. Anal. of the Cu3P [001] facet by HAADF-STEM was used to det. the surface lattice structure, while both XPS and Auger spectroscopies were used to det. the surface chem. speciation from the kinetic energies of ionized electrons. The presented anal. identifies isolated trigonal CuP3 sites on the Cu3P[001]-Cu3P3 terminated surface and the Cu(I) oxidn. state as precursor to the active catalyst. The CO2RR selectivity to formate and the higher turnover rate for H2 prodn. on the [001] facet allows a structure-activity anal. and chem. mechanism to be proposed. Formation of a surface hydride at isolated *H-CuP3 sites is proposed as the catalytic site in forming both H2 and formate, while the long Cu-Cu sepn. retards forming C-C coupling products. These results disprove previously published claims of Cu(I) oxidn. state as a sufficient criterion to promote CO2RR to C2+ products, show that stronger bonded hydrides, *H-CuP3 on Cu3P, favor prodn. of the C1 product formate over all other carbon products, and predict that stronger formate binding (bidentate) is needed for CO2RR currents to compete with H2 prodn.
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  14. 14
    Calvinho, K. U. D.; Alherz, A. W.; Yap, K. M. K.; Laursen, A. B.; Hwang, S.; Bare, Z. J. L.; Clifford, Z.; Musgrave, C. B.; Dismukes, G. C. Surface Hydrides on Fe2P Electrocatalyst Reduce at Low Overpotential: Steering Selectivity to Ethylene Glycol. J. Am. Chem. Soc. 2021, 143, 21275,  DOI: 10.1021/jacs.1c03428
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    15
    Surface Hydrides on Fe2P Electrocatalyst Reduce CO2 at Low Overpotential: Steering Selectivity to Ethylene Glycol
    Calvinho, Karin U. D.; Alherz, Abdulaziz W.; Yap, Kyra M. K.; Laursen, Anders B.; Hwang, Shinjae; Bare, Zachary J. L.; Clifford, Zachary; Musgrave, Charles B.; Dismukes, G. Charles
    Journal of the American Chemical Society (2021), 143 (50), 21275-21285CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Development of efficient electrocatalysts for the CO2 redn. reaction (CO2RR) to multicarbon products was constrained by high overpotentials and poor selectivity. Here, the authors introduce Fe phosphide (Fe2P) as an earth-abundant catalyst for the CO2RR to mainly C2-C4 products with a total CO2RR faradaic efficiency of 53% at 0 V vs. RHE. C product selectivity is tuned in favor of ethylene glycol formation with increasing neg. bias at the expense of C3-C4 products. Both Grand Canonical-DFT (GC-DFT) calcns. and expts. reveal that *formate, not *CO, is the initial intermediate formed from surface phosphino-hydrides and that the latter form ionic hydrides at both surface P atoms (H@Ps) and P-reconstructed Fe3 hollow sites (H@P*). Binding of these surface hydrides weakens with neg. bias (reactivity increases), which accounts for both the shift to C2 products over higher C-C coupling products and the increase in the H2 evolution reaction (HER) rate. GC-DFT predicts that phosphino-hydrides convert *formate to *formaldehyde, the key intermediate for C-C coupling, whereas H atoms on Fe generate tightly bound *CO via sequential PCET reactions to H2O. GC-DFT predicts the peak in CO2RR c.d. near -0.1 V is due to a local max. in the binding affinity of *formate and *formaldehyde at this bias, which together with the more labile C2 product affinity, accounts for the shift to ethylene glycol and away from C3-C4 products. Consistent with these predictions, addn. of exogenous CO is shown to block all C product formation and lower the HER rate. The formation of ionic hydrides and their binding affinity, as modulated by the applied potential, controls the C product distribution. This knowledge provides new insight into the influence of hydride speciation and applied bias on the chem. reaction mechanism of CO2RR that is relevant to all transition metal phosphides.
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    Kucernak, A. R. J.; Sundaram, V. N. N. Nickel Phosphide: The Effect of Phosphorus Content on Hydrogen Evolution Activity and Corrosion Resistance in Acidic Medium. J. Mater. Chem. A 2014, 2, 17435– 17445,  DOI: 10.1039/c4ta03468f
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    16
    Nickel phosphide: the effect of phosphorus content on hydrogen evolution activity and corrosion resistance in acidic medium
    Kucernak, Anthony R. J.; Naranammalpuram Sundaram, Venkata N.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (41), 17435-17445CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Transition metal phosphides possess novel, structural, phys. and chem. properties and are an emerging new class of materials for various catalytic applications. Electroplated or electrolessly plated Ni phosphide alloy materials with achievable P contents <15 at.% P are known to be more corrosion resistant than Ni alone, and were studied as H evolution catalysts in alk. environments. However, there is significant interest in developing new inexpensive catalysts for solid polymer electrolyte electrolyzers which require acid stable catalysts. By increasing the P content beyond the limit available using electroplating techniques (∼12 at.% P), the Ni based phosphides Ni12P5 and Ni2P with higher levels of P (29 and 33 at.% P) may be used for the H evolution reaction (HER) in acidic medium. Corrosion resistance in acid is directly correlated with P content - those materials with higher P content are more corrosion resistant. H evolution activity in acid is also correlated with P content - Ni2P based catalysts appear to be more active for the H evolution reaction than Ni12P5. Electrochem. kinetic studies of the HER reveal high exchange current densities and little deviation in the Tafel slope esp. in the lower overpotential regime for these Ni phosphide catalysts. The electrochem. impedance spectroscopy response of the resp. system in acidic medium reveals two time consts. assocd. with the HER.
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  16. 16
    Pan, Y.; Liu, Y.; Zhao, J.; Yang, K.; Liang, J.; Liu, D.; Hu, W.; Liu, D.; Liu, Y.; Liu, C. Monodispersed Nickel Phosphide Nanocrystals with Different Phases: Synthesis, Characterization and Electrocatalytic Properties for Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 1656– 1665,  DOI: 10.1039/c4ta04867a
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    17
    Monodispersed nickel phosphide nanocrystals with different phases: synthesis, characterization and electrocatalytic properties for hydrogen evolution
    Pan, Yuan; Liu, Yanru; Zhao, Jinchong; Yang, Kang; Liang, Jilei; Liu, Dandan; Hu, Wenhui; Liu, Dapeng; Liu, Yunqi; Liu, Chenguang
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (4), 1656-1665CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Monodispersed nickel phosphide nanocrystals (NCs) with different phases (Ni12P5, Ni2P and Ni5P4) were prepd. via the thermal decompn. approach using nickel acetylacetonate as the nickel source, trioctylphosphine as the phosphorus source and oleylamine in 1-octadecene as the reductant. The phases of the as-prepd. nickel phosphide NCs could easily be controlled by changing the P:Ni precursor ratio. The structure and morphol. of the nickel phosphide NCs were characterized by x-ray diffraction (XRD), TEM, energy dispersive x-ray anal. (EDX), XPS, FTIR, and N2 adsorption-desorption. A formation mechanism for the nickel phosphide NCs is proposed. The influence of the phase of the nickel phosphide NCs on the electrocatalytic properties for the hydrogen evolution reaction (HER) was studied. All phases showed good catalytic properties, and the Ni5P4 NCs with a solid structure exhibited higher catalytic activity than the Ni12P5 and Ni2P NCs. This superior catalytic activity is attributed to the higher pos. charge of Ni and a stronger ensemble effect of P in Ni5P4 NCs. The cryst. phase is important for affecting the electrocatalytic properties.
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  17. 17
    Laursen, A. B.; Patraju, K. R.; Whitaker, M. J.; Retuerto, M.; Sarkar, T.; Yao, N.; Ramanujachary, K. V.; Greenblatt, M.; Dismukes, G. C. Nanocrystalline Ni5P4: A Hydrogen Evolution Electrocatalyst of Exceptional Efficiency in Both Alkaline and Acidic Media. Energy Environ. Sci. 2015, 8, 1027– 1034,  DOI: 10.1039/c4ee02940b
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    18
    Nanocrystalline Ni5P4: a hydrogen evolution electrocatalyst of exceptional efficiency in both alkaline and acidic media
    Laursen, A. B.; Patraju, K. R.; Whitaker, M. J.; Retuerto, M.; Sarkar, T.; Yao, N.; Ramanujachary, K. V.; Greenblatt, M.; Dismukes, G. C.
    Energy & Environmental Science (2015), 8 (3), 1027-1034CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Producing hydrogen (H2) by splitting water with fossil-free electricity is considered a grand challenge for developing sustainable energy systems and a carbon dioxide free source of renewable H2. Renewable H2 may be produced from water by electrolysis with either low efficiency alk. electrolyzers that suffer 50-65% losses, or by more efficient acidic electrolyzers with rare platinum group metal catalysts (Pt). Consequently, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements. Here, we show that cryst. Ni5P4 evolves H2 with geometric elec. to chem. conversion efficiency on par with Pt in strong acid (33 mV dec-1 Tafel slope and -62 mV overpotential at -100 mA cm-2 in 1 M H2SO4). The cond. of Ni5P4 microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degrdn. is seen in short term studies at current densities of -10 mA cm-2, equiv. to ∼10% solar photoelec. conversion efficiency. The realization of a noble metal-free catalyst performing on par with Pt in both strong acid and base offers a key step towards industrially relevant electrolyzers competing with conventional H2 sources.
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  18. 18
    Callejas, J. F.; Read, C. G.; Popczun, E. J.; McEnaney, J. M.; Schaak, R. E. Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP. Chem. Mater. 2015, 27, 3769– 3774,  DOI: 10.1021/acs.chemmater.5b01284
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    19
    Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP
    Callejas, Juan F.; Read, Carlos G.; Popczun, Eric J.; McEnaney, Joshua M.; Schaak, Raymond E.
    Chemistry of Materials (2015), 27 (10), 3769-3774CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Metal phosphides have emerged as promising Earth-abundant alternatives to platinum for catalyzing the hydrogen evolution reaction (HER) in acidic aq. solns. Here, Co2P nanoparticles having a hollow, multifaceted, cryst. morphol. were evaluated as HER electrocatalysts at a mass loading of 1 mg cm-2 on Ti foil substrates. The Co2P/Ti electrodes required low overpotentials of -95 and -109 mV to produce operationally relevant cathodic current densities of -10 and -20 mA cm-2, resp. These values establish Co2P nanoparticles as highly active Earth-abundant HER catalyst materials. Importantly, the Co2P nanoparticles are morphol. equiv. to previously reported CoP nanoparticle HER catalysts, allowing a direct side-by-side evaluation of their HER activities. Such comparisons of different metal phosphide HER catalysts with the same constituent elements and morphologies are important for identifying the key materials characteristics that lead to high activity.
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  19. 19
    Pan, Y.; Lin, Y.; Chen, Y.; Liu, Y.; Liu, C. Cobalt Phosphide-Based Electrocatalysts: Synthesis and Phase Catalytic Activity Comparison for Hydrogen Evolution. J. Mater. Chem. A 2016, 4, 4745– 4754,  DOI: 10.1039/c6ta00575f
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    20
    Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution
    Pan, Yuan; Lin, Yan; Chen, Yinjuan; Liu, Yunqi; Liu, Chenguang
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (13), 4745-4754CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Cobalt phosphides have been used as promising electrocatalysts for catalyzing the hydrogen evolution reaction (HER) in acidic aq. solns. In order to further explore the influence of phase structure and support effect on the catalytic activity for HER, herein, a series of cobalt phosphide-based electrocatalysts, including Co2P, CoP, Co2P/CNTs, CoP/CNTs, Co2P/NCNTs and CoP/NCNTs (NCNTs are nitrogen-doped carbon nanotubes), were synthesized successfully via a facile thermal decompn. approach. The cryst. phase can be controlled by changing the phosphide source species. When the phosphide source was trioctylphosphine, CoP-based catalysts were obtained. However, Co2P-based catalysts can be obtained by using triphenylphosphine as the phosphide source. Then the phase catalytic activity and stability of the as-synthesized cobalt phosphide-based catalysts for hydrogen evolution were compared. The results show that the catalytic activity followed the order CoP/NCNTs > Co2P/NCNTs > CoP/CNTs > Co2P/CNTs > CoP > Co2P, which can be attributed to the different at. ratios of Co to P, the strong interaction between cobalt phosphide and carbon species and the doping of N atoms into CNTs. Our studies indicate that the HER catalytic efficiency of transition metal phosphide catalysts can be improved significantly by adjusting active phase and carbon species structures.
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  20. 20
    Seo, B.; Baek, D. S.; Sa, Y. J.; Joo, S. H. Shape Effects of Nickel Phosphide Nanocrystals on Hydrogen Evolution Reaction. CrystEngComm 2016, 18, 6083– 6089,  DOI: 10.1039/c6ce00985a
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    21
    Shape effects of nickel phosphide nanocrystals on hydrogen evolution reaction
    Seo, Bora; Baek, Du San; Sa, Young Jin; Joo, Sang Hoon
    CrystEngComm (2016), 18 (32), 6083-6089CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
    The prepn. of size- and shape-controlled nanoparticles has enabled the understanding of important nanoscale catalytic phenomena, resulting in the design of advanced catalysts with enhanced activities and selectivities. Metal phosphides have recently emerged as a promising class of non-precious metal catalysts for hydrogen evolution reaction (HER), which is a cornerstone in clean and environmentally benign hydrogen prodn. Although significant progress has been made in metal phosphide catalysts, the impact of the metal phosphide shape has not yet been explored. Herein, we investigated the shape-dependent electrocatalytic activity of nickel phosphide nanoparticles (Ni2P NPs) for the HER. Spherical Ni2P NPs mainly composed of the Ni2P(001) surface showed higher HER activity than rod-shaped Ni2P NPs with the Ni2P(210) surface in terms of overpotential, Tafel slope, and turnover frequency. The results imply that the Ni2P(001) surface would have preferential interactions with the adsorbent and a lower activation barrier for hydrogen adsorption, promoting the overall rate of HER. This study highlights the importance of morphol. control in electrocatalysts to boost catalytic performances.
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    Mutinda, S. I.; Li, D.; Kay, J.; Brock, S. L. Synthesis and Characterization of Co2–xRhxP Nanoparticles and Their Catalytic Activity Towards the Oxygen Evolution Reaction. J. Mater. Chem. A 2018, 6, 12142– 12152,  DOI: 10.1039/c8ta02016g
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    22
    Synthesis and characterization of Co2-xRhxP nanoparticles and their catalytic activity towards the oxygen evolution reaction
    Mutinda, Samuel I.; Li, Da; Kay, Jacob; Brock, Stephanie L.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (25), 12142-12152CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    We show that Co2-xRhxP ternary phase nanoparticles can be synthesized using the arrested pptn. technique, via a requisite low temp. co-redn. of Co2+ and Rh3+ ions, followed by phosphidation of the resulting intermediate product at above 300 °C. The most unique aspect of our synthesis protocol is the fact that we are able to solubilize both cubic antifluorite and orthorhombic crystal systems, into uniform solid solns. of Co2-xRhxP nanoparticles under mild reaction conditions. These nanoparticles are active towards the oxygen evolution reaction and deliver higher electrolytic activities as compared to those of Co2P and Rh2P binary phases. Cobalt-rich compns. were found to be the most active, with a sweet spot occurring at a target compn. of Co1.75Rh0.25P, where a c.d. of 10 mA cm-2 was achieved at an overpotential of 0.29 V.
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  22. 22
    Liyanage, D. R.; Li, D.; Cheek, Q. B.; Baydoun, H.; Brock, S. L. Synthesis and Oxygen Evolution Reaction (OER) Catalytic Performance of Ni2–xRuxP Nanocrystals: Enhancing Activity by Dilution of the Noble Metal. J. Mater. Chem. A 2017, 5, 17609– 17618,  DOI: 10.1039/c7ta05353c
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    23
    Synthesis and oxygen evolution reaction (OER) catalytic performance of Ni2-xRuxP nanocrystals: enhancing activity by dilution of the noble metal
    Liyanage, D. Ruchira; Li, Da; Cheek, Quintin B.; Baydoun, Habib; Brock, Stephanie L.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (33), 17609-17618CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Aiming to create an efficient, less-expensive catalyst for the oxygen evolution reaction (OER), a synthetic protocol is developed to prep. ternary metal phosphide nanoparticles, Ni2-xRuxP, incorporating Ru, a traditional catalyst for OER, and Ni, a highly active but inexpensive metal. Using soln.-phase arrested-pptn. reactions, cryst. Ni2-xRuxP particles could be realized for compns. up to x ≤ 1, whereas more Ru-rich compns., including Ru2P, were amorphous. For x ≤ 1, particles are spherical, of sizes that vary between 5 and 10 nm in diam. (with a clear decreasing trend as the Ru amt. is increased), and samples exhibit narrow size distributions (polydispersity < 15%). In contrast, amorphous Ru-rich phases exhibit worm-like morphologies. ICP-MS data indicate the actual metal ratio closely follows the target ratio employed in the synthesis. OER electrocatalytic activity was evaluated for selected compns. over the entire synthesis range (0 ≤ x ≤ 2). Intriguingly, Ru2P proved to be the least active phase (overpotential of 0.56 V at 10 mA cm-2 in 1.0 M KOH) with the best performance obsd. for the bimetallic Ni1.25Ru0.75P phase (overpotential of 0.34 V). The augmented activity at x = 0.75 is attributed, at least in part, to electronic activation of Ni by Ru, facilitating Ni oxidn. and thus decreasing the kinetic barrier for OER.
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    Liu, J.; Wang, Z.; David, J.; Llorca, J.; Li, J.; Yu, X.; Shavel, A.; Arbiol, J.; Meyns, M.; Cabot, A. Colloidal Ni2–xCoxP Nanocrystals for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2018, 6, 11453– 11462,  DOI: 10.1039/c8ta03485k
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    24
    Colloidal Ni2-xCoxP nanocrystals for the hydrogen evolution reaction
    Liu, Junfeng; Wang, Zhenxing; David, Jeremy; Llorca, Jordi; Li, Junshan; Yu, Xiaoting; Shavel, Alexey; Arbiol, Jordi; Meyns, Michaela; Cabot, Andreu
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (24), 11453-11462CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A cost-effective and scalable approach was developed to produce monodisperse Ni2-xCoxP nanocrystals (NCs) with compn. tuned over the entire range (0 ≤ x ≤ 2). Ni2-xCoxP NCs were synthesized using low-cost, stable and low-toxicity tri-Ph phosphite (TPP) as a phosphorus source, metal chlorides as metal precursors and hexadecylamine (HDA) as a ligand. The synthesis involved the nucleation of amorphous Ni-P and its posterior crystn. and simultaneous incorporation of Co. The compn., size and morphol. of the Ni2-xCoxP NCs could be controlled simply by varying the ratio of Ni and Co precursors and the amts. of TPP and HDA. Ternary Ni2-xCoxP-based electrocatalysts exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) compared to binary phosphides. In particular, NiCoP electrocatalysts displayed the lowest overpotential of 97 mV at J = 10 mA cm-2 and an excellent long-term stability. DFT calcns. of the Gibbs free energy for hydrogen adsorption at the surface of Ni2-xCoxP NCs showed NiCoP to have the most appropriate compn. to optimize this parameter within the whole Ni2-xCoxP series. However, the hydrogen adsorption energy was demonstrated not to be the only parameter controlling the HER activity in Ni2-xCoxP.
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    Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Nørskov, J. K.; Abild-Pedersen, F.; Jaramillo, T. F. Designing an Improved Transition Metal Phosphide Catalyst for Hydrogen Evolution Using Experimental and Theoretical Trends. Energy Environ. Sci. 2015, 8, 3022– 3029,  DOI: 10.1039/c5ee02179k
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    25
    Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends
    Kibsgaard, Jakob; Tsai, Charlie; Chan, Karen; Benck, Jesse D.; Noerskov, Jens K.; Abild-Pedersen, Frank; Jaramillo, Thomas F.
    Energy & Environmental Science (2015), 8 (10), 3022-3029CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Transition metal phosphides (TMPs) have emerged as highly active catalysts for the hydrogen evolution reaction (HER). However, insights into the trends and limitations in their activity are lacking, and there are presently no guidelines for systematically improving their intrinsic activity. The complexity and variations in their structures further pose challenges in theor. estg. their activity. Herein, we demonstrate a combined exptl.-theor. approach: by synthesizing different TMPs and comparing exptl. detd. HER activities with the hydrogen adsorption free energies, ΔGH, calcd. by d. functional theory, we det. the level of detail needed in the simulations to bring out useful trends in the exptl. data. In particular, we show that the TMPs follow the HER volcano relationship. Using our combined exptl.-theor. model, we predict that the mixed metal TMP, Fe0.5Co0.5P, should have a near-optimal ΔGH. We synthesized several mixts. of Co and Fe phosphides alloys and confirmed that Fe0.5Co0.5P exhibits the highest HER activity of the investigated TMPs. Furthermore, our results suggest that there could be inherent limitations in the intrinsic HER activity of TMPs that prevent them from performing as well as Pt-group metals. Our work demonstrates that it is possible to generate and verify a model of activity trends with predictive capabilities even for new transition metal compds. with varied structures and surface terminations. The identification of an improved mixed metal TMP based on theor. predictions and subsequent synthesis and testing demonstrates the need for an approach that combines theory and expt. to understand and ultimately design advanced catalysts.
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  25. 25
    Tang, C.; Gan, L.; Zhang, R.; Lu, W.; Jiang, X.; Asiri, A. M.; Sun, X.; Wang, J.; Chen, L. Ternary FexCo1–xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-Like Activity: Experimental and Theoretical Insight. Nano Lett. 2016, 16, 6617– 6621,  DOI: 10.1021/acs.nanolett.6b03332
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    26
    Ternary FexCo1-xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-like Activity: Experimental and Theoretical Insight
    Tang, Chun; Gan, Linfeng; Zhang, Rong; Lu, Wenbo; Jiang, Xiue; Asiri, Abdullah M.; Sun, Xuping; Wang, Jin; Chen, Liang
    Nano Letters (2016), 16 (10), 6617-6621CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Replacement of precious Pt with earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) holds great promise for clean energy devices, but the development of low-cost and durable HER catalysts with Pt-like activity is still a huge challenge. In this communication, we report on the development of self-standing ternary FexCo1-xP nanowire array on carbon cloth (FexCo1-xP/CC) as a Pt-free HER catalyst with activities being strongly related to Fe substitution ratio. Electrochem. tests show that Fe0.5Co0.5P/CC not only possesses Pt-like activity with the need of overpotential of only 37 mV to drive 10 mA cm-2, outperforming all non-noble-metal HER catalysts reported to date, but demonstrates superior long-term durability in 0.5 M H2SO4. D. functional theory calcns. further reveal that Fe substitution of Co in CoP leads to more optimal free energy of hydrogen adsorption to the catalyst surface. This study offers us a promising flexible monolithic catalyst for practical applications.
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    Tan, Y.; Wang, H.; Liu, P.; Shen, Y.; Cheng, C.; Hirata, A.; Fujita, T.; Tang, Z.; Chen, M. Versatile Nanoporous Bimetallic Phosphides Towards Electrochemical Water Splitting. Energy Environ. Sci. 2016, 9, 2257– 2261,  DOI: 10.1039/c6ee01109h
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    Versatile nanoporous bimetallic phosphides towards electrochemical water splitting
    Tan, Yongwen; Wang, Hao; Liu, Pan; Shen, Yuhao; Cheng, Chun; Hirata, Akihiko; Fujita, Takeshi; Tang, Zheng; Chen, Mingwei
    Energy & Environmental Science (2016), 9 (7), 2257-2261CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Alloying is an important approach to improving catalytic activities and realizing new functions of heterogeneous catalysts, which has extensively been employed in fabricating noble metal based bimetallic catalysts. However, it is tech. unviable in the synthesis of alloyed transition metal compds., which are emerging as important catalysts for water splitting, in a controllable manner using conventional wet chem. methods. Nanoporous bimetallic (Co1-xFex)2P phosphides with controllable compns. and tuneable porosity, which are fabricated by the combination of metallurgical alloy design and electrochem. etching has been reported. By tailoring the Co/Fe ratios and nanoporosity, the bimetallic phosphides exhibit versatile catalytic activities towards HER and OER in acidic and basic electrolytes. As both the cathode and the anode of an electrolyzer, nanoporous (Co0.52Fe0.48)2P shows an outstanding performance in water electrolysis, comparable to the com. electrolyzer with paired Pt/C and IrO2 catalysts.
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    Brock, S. L.; Perera, S. C.; Stamm, K. L. Chemical Routes for Production of Transition-Metal Phosphides on the Nanoscale: Implications for Advanced Magnetic and Catalytic Materials. Chem.─Eur. J. 2004, 10, 3364– 3371,  DOI: 10.1002/chem.200305775
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    Chemical routes for production of transition-metal phosphides on the nanoscale: Implications for advanced magnetic and catalytic materials
    Brock, Stephanie L.; Perera, Susanthri C.; Stamm, Kimber L.
    Chemistry - A European Journal (2004), 10 (14), 3364-3371CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Nanoparticulate transition-metal phosphides remain an unexplored, though emerging area of interest on the materials landscape, due principally to their promising magnetic and catalytic properties. This review describes synthetic strategies for the formation of both supported and unsupported transition-metal phosphide nanoparticles, provides a summary of their relevant magnetic and catalytic properties, and indicates new directions for exploration.
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    Henkes, A. E.; Vasquez, Y.; Schaak, R. E. Converting Metals into Phosphides: A General Strategy for the Synthesis of Metal Phosphide Nanocrystals. J. Am. Chem. Soc. 2007, 129, 1896– 1897,  DOI: 10.1021/ja068502l
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    30
    Converting Metals into Phosphides: A General Strategy for the Synthesis of Metal Phosphide Nanocrystals
    Henkes, Amanda E.; Vasquez, Yolanda; Schaak, Raymond E.
    Journal of the American Chemical Society (2007), 129 (7), 1896-1897CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Nanocrystals of metal phosphides, which can useful catalytic, electronic, and magnetic properties, are known to be accessible by using trioctylphosphine (TOP) as a highly reactive P source. Here the authors report a general strategy for synthesizing transition metal phosphides, including those with 4d and 5d transition metals that have not previously been reported as unsupported nanocrystals. Unlike previously reported methods that involve direct decompn. of organometallic precursors, the authors' method uses preformed metal nanoparticles as templates for generating metal phosphide nanocrystals. Metal nanoparticles are reacted with TOP in a hot solvent (290-360°) to form transition metal phosphides such as Ni2P, PtP2, Rh2P, PdP2, Pd5P2, and Au2P3. Also, nanostructures such as hollow spheres can be easily made using a Kirkendall-type mechanism, which uses metal nanoparticles as reactive templates.
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    Carenco, S.; Hu, Y.; Florea, I.; Ersen, O.; Boissière, C.; Mézailles, N.; Sanchez, C. Metal-Dependent Interplay between Crystallization and Phosphorus Diffusion During the Synthesis of Metal Phosphide Nanoparticles. Chem. Mater. 2012, 24, 4134– 4145,  DOI: 10.1021/cm3022243
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    31
    Metal-Dependent Interplay between Crystallization and Phosphorus Diffusion during the Synthesis of Metal Phosphide Nanoparticles
    Carenco, S.; Hu, Y.; Florea, I.; Ersen, O.; Boissiere, C.; Mezailles, N.; Sanchez, C.
    Chemistry of Materials (2012), 24 (21), 4134-4145CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The interplay between crystn. and phosphorus diffusion in the versatile synthesis of metal phosphide nanoparticles from well-defined metal nanoparticles is studied by using a favorable P(0) source for mechanistic studies: white phosphorus. In this study, the reaction of Ni, Fe, Pd, and Cu nanoparticles with P4 was quant. even at relatively low temps. thanks to the high reactivity of this sol. P source. Intermediate amorphous alloys could be identified for the first time in the case of Fe and Pd, while the quant. character of the reaction provided a selective and controlled access to Pd5P4 vs. PdP2 and Cu3P vs. CuP2. Morphol. evolution of the nanoparticles with temp. and M/P stoichiometry was also discussed and provided new insights in the kinetics of the reaction in each case. Hollow Ni2P and FeP nanoparticles were finally obtained while the particularly high stability of the amorphous plain Pd3P nanoparticles was uncovered.
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  30. 30
    Mundy, M. E.; Ung, D.; Lai, N. L.; Jahrman, E. P.; Seidler, G. T.; Cossairt, B. M. Aminophosphines as Versatile Precursors for the Synthesis of Metal Phosphide Nanocrystals. Chem. Mater. 2018, 30, 5373– 5379,  DOI: 10.1021/acs.chemmater.8b02206
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    32
    Aminophosphines as Versatile Precursors for the Synthesis of Metal Phosphide Nanocrystals
    Mundy, M. Elizabeth; Ung, David; Lai, Nathan L.; Jahrman, Evan P.; Seidler, Gerald T.; Cossairt, Brandi M.
    Chemistry of Materials (2018), 30 (15), 5373-5379CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    We have broadened the scope of the aminophosphine precursor chem. that has been developed for InP quantum dots to the synthesis of cadmium, zinc, cobalt, and nickel phosphide nanocrystals. The generalized synthetic conditions involve thermolysis of the appropriate MX2 salt with tris-diethylaminophosphine in a long chain primary amine. The resulting Cd3P2 nanocrystals exhibit size tuning effects based on the metal halide reactivity. 31P NMR studies show that the II-V materials form via the previously described mechanism obsd. for InP, demonstrating the invariance of this chem. to the metal valence. We also demonstrate that electrocatalytically active transition metal phosphides, specifically Co2P, CoP, and Ni2P, can be produced using this synthetic method at relatively mild temps. and in high yields.
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    Tappan, B. A.; Chen, K.; Lu, H.; Sharada, S. M.; Brutchey, R. L. Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu3–xP Nanocrystals. ACS Appl. Mater. Interfaces 2020, 12, 16394– 16401,  DOI: 10.1021/acsami.0c00025
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    Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu3-xP Nanocrystals
    Tappan, Bryce A.; Chen, Keying; Lu, Haipeng; Sharada, Shaama Mallikarjun; Brutchey, Richard L.
    ACS Applied Materials & Interfaces (2020), 12 (14), 16394-16401CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    N-heterocyclic carbenes (NHCs) are an important class of ligands capable of making strong carbon-metal bonds. Recently, there has been a growing interest in the study of carbene-ligated nanocrystals, primarily coinage metal nanocrystals, which have found application as catalysts for numerous reactions. The general ability of NHC ligands to pos. affect the catalytic properties of other types of nanocrystal catalysts remains unknown. Herein, we present the first carbene-stabilized Cu3-xP nanocrystals. Inquiries into the mechanism of formation of NHC-ligated Cu3-xP nanocrystals suggest that cryst. Cu3-xP forms directly as a result of a high-temp. metathesis reaction between a tris(trimethylsilyl)phosphine precursor and an NHC-CuBr precursor, the latter of which behaves as a source of both the carbene ligand and Cu+. To study the effect of the NHC surface ligands on the catalytic performance, we tested the electrocatalytic hydrogen evolving ability of the NHC-ligated Cu3-xP nanocrystals and found that they possess superior activity to analogous oleylamine-ligated Cu3-xP nanocrystals. D. functional theory calcns. suggest that the NHC ligands minimize unfavorable electrostatic interactions between the copper phosphide surface and H+ during the first step of the hydrogen evolution reaction, which contributes to the superior performance of NHC-ligated Cu3-xP catalysts as compared to oleylamine-ligated Cu3-xP catalysts.
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    Sun, M.; Liu, H.; Qu, J.; Li, J. Earth-Rich Transition Metal Phosphide for Energy Conversion and Storage. Adv. Energy Mater. 2016, 6, 1600087,  DOI: 10.1002/aenm.201600087
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    Li, S.-H.; Qi, M.-Y.; Tang, Z.-R.; Xu, Y.-J. Nanostructured Metal Phosphides: From Controllable Synthesis to Sustainable Catalysis. Chem. Soc. Rev. 2021, 50, 7539– 7586,  DOI: 10.1039/d1cs00323b
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    Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis
    Li, Shao-Hai; Qi, Ming-Yu; Tang, Zi-Rong; Xu, Yi-Jun
    Chemical Society Reviews (2021), 50 (13), 7539-7586CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Metal phosphides (MPs) with unique and desirable physicochem. properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, esp. for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Addnl., different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.
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    Habas, S. E.; Baddour, F. G.; Ruddy, D. A.; Nash, C. P.; Wang, J.; Pan, M.; Hensley, J. E.; Schaidle, J. A. A Facile Molecular Precursor Route to Metal Phosphide Nanoparticles and Their Evaluation as Hydrodeoxygenation Catalysts. Chem. Mater. 2015, 27, 7580– 7592,  DOI: 10.1021/acs.chemmater.5b02140
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    A Facile Molecular Precursor Route to Metal Phosphide Nanoparticles and Their Evaluation as Hydrodeoxygenation Catalysts
    Habas, Susan E.; Baddour, Frederick G.; Ruddy, Daniel A.; Nash, Connor P.; Wang, Jun; Pan, Ming; Hensley, Jesse E.; Schaidle, Joshua A.
    Chemistry of Materials (2015), 27 (22), 7580-7592CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Metal phosphides have been identified as a promising class of materials for the catalytic upgrading of bio-oils, which are renewable and potentially inexpensive sources for liq. fuels. Herein, we report the facile synthesis of a series of solid, phase-pure metal phosphide nanoparticles (NPs) (Ni2P, Rh2P, and Pd3P) utilizing com. available, air-stable metal-phosphine complexes in a one-pot reaction. This single-source mol. precursor route provides an alternative method to access metal phosphide NPs with controlled phases and without the formation of metal NP intermediates that can lead to hollow particles. The formation of the Ni2P NPs was shown to proceed through an amorphous Ni-P intermediate, leading to the desired NP morphol. and metal-rich phase. This low-temp., rapid route to well-defined metal NPs is expected to have broad applicability to a variety of readily available or easily synthesized metal-phosphine complexes with high decompn. temps. Hydrodeoxygenation of acetic acid, an abundant bio-oil component, was performed to investigate H2 activation and deoxygenation pathways under conditions that are relevant to ex situ catalytic fast pyrolysis (high temps., low pressures, and near-stoichiometric H2 concns.). The catalytic performance of the silica-supported metal phosphide NPs was compared to the analogous incipient wetness (IW) metal and metal phosphide catalysts over the range 200-500 °C. Decarbonylation was the primary pathway for H2 incorporation in the presence of all of the catalysts except NP-Pd3P, which exhibited minimal productive activity, and IW-Ni, which evolved H2. The highly controlled NP-Ni2P and NP-Rh2P catalysts, which were stable under these conditions, behaved comparably to the IW-metal phosphides, with a slight shift to higher product onset temps., likely due to the presence of surface ligands. Most importantly, the NP-Ni2P catalyst exhibited H2 activation and incorporation, in contrast to IW-Ni, indicating that the behavior of the metal phosphide is significantly different from that of the parent metal, and more closely resembles that of noble metal catalysts.
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    Mutinda, S. I.; Batugedara, T. N.; Brown, B.; Adeniran, O.; Liu, Z.-F.; Brock, S. L. Rh2P Activity at a Fraction of the Cost? Co2–xRhxP Nanoparticles as Electrocatalysts for the Hydrogen Evolution Reaction in Acidic Media. ACS Appl. Energy Mater. 2021, 4, 946– 955,  DOI: 10.1021/acsaem.0c02880
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    37
    Rh2P Activity at a Fraction of the Cost? Co2-xRhxP Nanoparticles as Electrocatalysts for the Hydrogen Evolution Reaction in Acidic Media
    Mutinda, Samuel I.; Batugedara, Tharanga N.; Brown, Benjamin; Adeniran, Olugbenga; Liu, Zhen-Fei; Brock, Stephanie L.
    ACS Applied Energy Materials (2021), 4 (1), 946-955CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    The compn.-dependent hydrogen evolution reaction (HER) activity of Co2-xRhxP nanoparticles in acid is reported. The motivation for the current study stems from (1) prior work demonstrating that, although costly, Rh2P nanoparticles are highly active and stable toward the HER process and (2) the expectation that dilg. Rh sites with Co will result in catalytic synergism while also lowering the overall cost of the material. Here, we establish that the HER activity of Co2-xRhxP nanoparticles in acidic media is compn.-dependent, with Rh-rich electrocatalysts showing superior activity as compared to those that are Co-rich. Addnl., compns. of Co2-xRhxP for which x ≥ 1.25, where the materials adopt the cubic antifluorite structure, deliver comparable initial catalytic activities to pure Rh2P, suggesting that the crystal structure of the material may play a more significant role in driving the overall HER activity than the compn. Despite comparable activity to Rh2P, Co2-xRhxP systems do not have the stability assocd. with Rh2P but undergo a drop from 10 to 5 mA/cm2 within the first hour of stability testing, assocd. with Co loss from the surface. In cases where Pt is used as the counter electrode, no such drop in c.d. is obsd., despite the loss of Co, with Pt transfer to the working electrode compensating for the Co depletion. First-principles calcns. based on d. functional theory show that both the hydrogen binding energies and the Gibbs free energies of hydrogen adsorption increase linearly with x, with Co0.75Rh1.25P exhibiting a ΔG value that is closest to zero, suggesting that this compn. is the most active for HER in this series. Double-layer capacitance data, from which electrochem. surface area (ECSA) data for all of the compns. are computed, are used to demonstrate that the quality and quantity of active sites among different compns. of Co2-xRhxP can vary significantly, even when the morphologies and particle sizes are similar.
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    Yoon, K. Y.; Jang, Y.; Park, J.; Hwang, Y.; Koo, B.; Park, J.-G.; Hyeon, T. Synthesis of Uniform-Sized Bimetallic Iron–Nickel Phosphide Nanorods. J. Solid State Chem. 2008, 181, 1609– 1613,  DOI: 10.1016/j.jssc.2008.05.022
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    Synthesis of uniform-sized bimetallic iron-nickel phosphide nanorods
    Yoon, Ki Youl; Jang, Youngjin; Park, Jongnam; Hwang, Yosun; Koo, Bonil; Park, Je-Geun; Hyeon, Taeghwan
    Journal of Solid State Chemistry (2008), 181 (7), 1609-1613CODEN: JSSCBI; ISSN:0022-4596. (Elsevier Inc.)
    We synthesized uniform-sized nanorods of iron-nickel phosphides from the thermal decompn. of metal-phosphine complexes. Uniform-sized (FexNi1-x)2P nanorods (0≤x≤1) of various compns. were synthesized by thermal decompn. of Ni-trioctylphosphine (TOP) complex and Fe-TOP complex. By measuring magnetic properties, we found that blocking temp. and coercive field depend on Ni content in the nanorods. Both parameters were more sensitive to doping compared with bulk samples.
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    Hitihami-Mudiyanselage, A.; Arachchige, M. P.; Seda, T.; Lawes, G.; Brock, S. L. Synthesis and Characterization of Discrete FexNi2–xP Nanocrystals (0 < x < 2): Compositional Effects on Magnetic Properties. Chem. Mater. 2015, 27, 6592– 6600,  DOI: 10.1021/acs.chemmater.5b02149
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    Synthesis and Characterization of Discrete FexNi2-xP Nanocrystals (0 < x < 2): Compositional Effects on Magnetic Properties
    Hitihami-Mudiyanselage, Asha; Arachchige, Maheshika Palihawadana; Seda, Takele; Lawes, Gavin; Brock, Stephanie L.
    Chemistry of Materials (2015), 27 (19), 6592-6600CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Ternary FexNi2-xP (0 < x < 2) phases exhibit a range of useful properties that can be augmented or tuned by confinement to the nanoscale including hydrotreating catalytic activity for small x and near-room temp. ferromagnetism for high x. A soln.-phase arrested-pptn. method was developed for the synthesis of FexNi2-xP over all values of x (0 < x < 2). The synthesis involves prepn. of Ni-P amorphous particles, introduction of the Fe precursor to form amorphous Fe-Ni-P particles, and high-temp. conversion of Fe-Ni-P particles into cryst. ternary phosphide nanocrystals. The ternary FexNi2-xP nanocrystals crystallize in the hexagonal Fe2P-type structure, and the morphol. of the nanocrystals showed a distinct compositional dependence, transitioning from ∼11 nm diam. spheres to rods with aspect ratios approaching 2 as the Fe fraction is increased (x ≥ 1.2). Lattice parameters do not follow Vegard's law, consistent with Mossbauer data showing preferential site occupation by Fe of the tetrahedral over the square pyramidal sites at low Fe concns., and the opposite effect for x > 0.8. Magnetic measurements of FexNi2-xP (x = 1.8, 1.4, and 1.2) nanorods showed a strong compositional dependence of the Curie temp. (TC) that differs from observations in bulk phases, with the highest TC (265 K) obtained for x = 1.4.
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    Liyanage, D. R.; Danforth, S. J.; Liu, Y.; Bussell, M. E.; Brock, S. L. Simultaneous Control of Composition, Size, and Morphology in Discrete Ni2–xCoxP Nanoparticles. Chem. Mater. 2015, 27, 4349– 4357,  DOI: 10.1021/acs.chemmater.5b00958
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    Simultaneous Control of Composition, Size, and Morphology in Discrete Ni2-xCoxP Nanoparticles
    Liyanage, D. Ruchira; Danforth, Samuel J.; Liu, Yi; Bussell, Mark E.; Brock, Stephanie L.
    Chemistry of Materials (2015), 27 (12), 4349-4357CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A synthetic protocol developed to produce phase-pure, nearly monodisperse Ni2-xCoxP nanoparticles (x ≤ 1.7) is described. The Ni2-xCoxP particles vary in size, ranging from 9-14 nm with std. deviations of <20% (based on transmission electron microscopy anal.), and the actual metal ratios obtained from energy-dispersive spectroscopy closely follow the targeted ratios. With increasing Co, samples with larger size distributions are obtained and include particles with voids, attributed to the Kirkendall effect. To probe the mechanism of ternary phosphide particle formation, detailed studies were conducted for Ni:Co = 1:1 as a representative compn. It was revealed that the P:M ratio, heating temp., and heating time have a large impact on the nature of both intermediate and final cryst. particles formed. By tuning these conditions, nanoparticles can be produced with different sizes (from ca. 7-25 nm) and morphologies (hollow vs. dense).
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    Danforth, S. J.; Liyanage, D. R.; Hitihami-Mudiyanselage, A.; Ilic, B.; Brock, S. L.; Bussell, M. E. Probing Hydrodesulfurization over Bimetallic Phosphides Using Monodisperse Ni2-xMxP Nanoparticles Encapsulated in Mesoporous Silica. Surf. Sci. 2016, 648, 126– 135,  DOI: 10.1016/j.susc.2015.10.005
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    Probing hydrodesulfurization over bimetallic phosphides using monodisperse Ni2-xMxP nanoparticles encapsulated in mesoporous silica
    Danforth, Samuel J.; Liyanage, D. Ruchira; Hitihami-Mudiyanselage, Asha; Ilic, Boris; Brock, Stephanie L.; Bussell, Mark E.
    Surface Science (2016), 648 (), 126-135CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    Metal phosphide nanoparticles encapsulated in mesoporous silica provide a well-defined system for probing the fundamental chem. of the hydrodesulfurization (HDS) reaction over this new class of hydrotreating catalysts. To investigate compn. effects in bimetallic phosphides, the HDS of dibenzothiophene (DBT) was carried out over a series of Ni-rich Ni2-xMxP@mSiO2 (M = Co, Fe) nanocatalysts (x ≤ 0.50). The Ni2-xMxP nanoparticles (av. diams.: 11-13 nm) were prepd. by soln.-phase arrested pptn. and encapsulated in mesoporous silica, characterized by a range of techniques (x-ray diffraction, TEM, IR spectroscopy, BET surface area, CO chemisorption) and tested for DBT HDS activity and selectivity. The highest activity was obsd. for a Ni1.92Co0.08P@mSiO2 nanocatalyst, but the overall trend was a decrease in HDS activity with increasing Co or Fe content. In contrast, the highest turnover frequency (TOF) was obsd. for the most Co- and Fe-rich compns. based on sites titrated by CO chemisorption. IR spectral studies of adsorbed CO on the Ni2-xMxP@mSiO2 catalysts indicate that an increase in electron d. occurs on Ni sites as the Co or Fe content is increased, which may be responsible for the increased TOFs of the catalytic sites. The Ni2-xMxP@mSiO2 nanocatalysts exhibit a strong preference for the direct desulfurization pathway (DDS) for DBT HDS that changes only slightly with increasing Co or Fe content.
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    Hegedus, L. S.; Perry, R. J. Phosphinecarbonylnitrosylacylcobaltate Complexes as Acyl Transfer Reagents. Acylation of Allylic Halides, Conjugated Enones, and Quinones. J. Org. Chem. 1985, 50, 4955– 4960,  DOI: 10.1021/jo00224a061
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    Phosphinecarbonylnitrosylacylcobaltate complexes as acyl transfer reagents. Acylation of allylic halides, conjugated enones, and quinones
    Hegedus, Louis S.; Perry, Robert J.
    Journal of Organic Chemistry (1985), 50 (24), 4955-60CODEN: JOCEAH; ISSN:0022-3263.
    The complex Co(NO)(CO)2(PPh3) is prepd. from Co2(CO)8, NaNO2, and Ph3P without isolation of the volatile intermediate Co(NO)(CO)3. Treatment of this complex with organolithium reagent at -40° generated unstable acylate complexes [RCOCo(NO)(CO)(PPh3)]- (R = Bu, Me) which readily transferred the acyl group to allylic halides to produce β,γ-unsatd. ketones, to conjugated ketones to produce 1,4-dicarbonyl compds., and to quinones to form 4-acylcyclohexadienones.
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    Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. Methods 2012, 9, 671,  DOI: 10.1038/nmeth.2089
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    NIH Image to ImageJ: 25 years of image analysis
    Schneider, Caroline A.; Rasband, Wayne S.; Eliceiri, Kevin W.
    Nature Methods (2012), 9 (7_part1), 671-675CODEN: NMAEA3; ISSN:1548-7091. (Nature Publishing Group)
    For the past 25 years NIH Image and ImageJ software have been pioneers as open tools for the anal. of scientific images. We discuss the origins, challenges and solns. of these two programs, and how their history can serve to advise and inform other software projects.
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    Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267– 9270,  DOI: 10.1021/ja403440e
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    Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction
    Popczun, Eric J.; McKone, James R.; Read, Carlos G.; Biacchi, Adam J.; Wiltrout, Alex M.; Lewis, Nathan S.; Schaak, Raymond E.
    Journal of the American Chemical Society (2013), 135 (25), 9267-9270CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Nanoparticles of nickel phosphide (Ni2P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solns., under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni2P nanoparticles were hollow and faceted to expose a high d. of the Ni2P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H2(g) with nearly quant. faradaic yield, while also affording stability in aq. acidic media.
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    Alcaide, F.; Álvarez, G.; Cabot, P. L.; Genova-Koleva, R.; Grande, H.-J.; Miguel, O. Effect of the Solvent in the Catalyst Ink Preparation on the Properties and Performance of Unsupported PtRu Catalyst Layers in Direct Methanol Fuel Cells. Electrochim. Acta 2017, 231, 529– 538,  DOI: 10.1016/j.electacta.2017.02.127
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    Effect of the solvent in the catalyst ink preparation on the properties and performance of unsupported PtRu catalyst layers in direct methanol fuel cells
    Alcaide, Francisco; Alvarez, Garbine; Cabot, Pere L.; Genova-Koleva, Radostina; Grande, Hans-Jurgen; Miguel, Oscar
    Electrochimica Acta (2017), 231 (), 529-538CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    The effect of the org. solvent polarity on the properties of unsupported PtRu catalyst inks and on the performance of the catalyst layers prepd. with them for the methanol electrooxidn., has been studied. The light scattering results indicate that the PtRu-Nafion aggregates in the inks prepd. with Bu acetate (NBA) are larger than those prepd. with 2-propanol (IPA). The lower polarity of the former favors the aggregation of Nafion and nanoparticles. The electron microscopy images and porosimetry measurements of the catalyst layers show that the secondary pore vol. between the agglomerates is larger for NBA. The linear sweep voltammetry and EIS results for the methanol electrooxidn. in the three-electrode cell denote the higher active surface area for NBA and comparable specific oxidn. rates of the intermediates in both catalysts layers. The current densities for PtRu anode catalyst layers in single DMFC are higher when the solvent is NBA, the mass transport limitations being much more apparent with IPA. The adapted transmission line equiv. circuit to interpret the impedance results in single DMFC indicates that the proton resistance for NBA is significantly lower than for IPA, thus suggesting that the greater no. of accessible active sites for methanol oxidn. in the former are well connected to the Nafion ionomers and easier transported to the membrane.
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    Huang, D.-C.; Yu, P.-J.; Liu, F.-J.; Huang, S.-L.; Hsueh, K.-L.; Chen, Y.-C.; Wu, C.-H.; Chang, W.-C.; Tsau, F.-H. Effect of Dispersion Solvent in Catalyst Ink on Proton Exchange Membrane Fuel Cell Performance. Int. J. Electrochem. Sci. 2011, 6, 2551– 2565
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    Effect of dispersion solvent in catalyst ink on proton exchange membrane fuel cell performance
    Huang, De-Chin; Yu, Pei-Jung; Liu, Feng-Jiin; Huang, Shu-Ling; Hsueh, Kan-Lin; Chen, Yen-Cho; Wu, Chun-Hsing; Chang, Wen-Chen; Tsau, Fang-Hei
    International Journal of Electrochemical Science (2011), 6 (7), 2551-2565CODEN: IJESIV; ISSN:1452-3981. (Electrochemical Science Group)
    The effects of dispersion solvents in catalyst ink on performance of the membrane electrode assembly (MEA) for proton exchange membrane fuel cell are studied. Dispersion solvents under examn. are water, ethylene glycol, glycerin, propylene glycols, and methanol. These solvents cover a wide range property of dielec. const., b.ps., and viscosity. We examine the rheol. behaviors of catalyst ink prepd. with these solvents. Both anode and cathode of the MEA are made by using doctor-blade method to spread catalyst ink onto the gas diffusion layers. The single cell with prepd. MEA is measured by linear scanning voltammetry to evaluate the discharge characteristics, by cyclic voltammetry to evaluate catalyst utilization, and electrochem. impedance spectroscopy to evaluate internal resistance. A semi-empirical equation is use to analyze the voltage losses due to activation over-potential and internal resistance. We find that ethylene glycol was the best dispersion agent among org. solvents tested. The power d. of the single cell is able to reach the max. power d. of 1,428 mA (mg Pt)-1 at hot-pressed temp. of 135 °C, and 1200 psi for 90 s.
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  45. 45
    Ngo, T. T.; Yu, T. L.; Lin, H.-L. Influence of the Composition of Isopropyl Alcohol/Water Mixture Solvents in Catalyst Ink Solutions on Proton Exchange Membrane Fuel Cell Performance. J. Power Sources 2013, 225, 293– 303,  DOI: 10.1016/j.jpowsour.2012.10.055
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    49
    Influence of the composition of isopropyl alcohol/water mixture solvents in catalyst ink solutions on proton exchange membrane fuel cell performance
    Ngo, Trung Truc; Yu, T. Leon; Lin, Hsiu-Li
    Journal of Power Sources (2013), 225 (), 293-303CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    We study the morphol. of Nafion in the dil. isopropanol (IPA)/water mixt. solns. contg. 20-100 wt.% of IPA and in the Pt-C/Nafion gas diffusion electrodes (where Pt-C is the carbon powder deposited on its surface with Pt particles), which are prepd. by spraying on the carbon paper surfaces with a layer of Pt-C, Nafion and IPA/water ink soln. The fuel cell performance of the gas diffusion electrodes strongly depends on the Nafion morphol. in the ink solns. A lower IPA content in the Pt-C/Nafion ink solns. results in the formation of larger and higher neg. charged Nafion aggregated particles, which leads to higher steric hindrance of the deposition of Nafion ionomer on the surface of Pt-C particles and thus a thinner Nafion film in contact on the Pt-C particle surfaces. The thinner Nafion film in contact with the Pt particles in the catalyst layer increases the chances of the Pt particles in contact with the H2/O2 gas, leading to a higher fuel cell performance.
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  46. 46
    Ngo, T. T.; Yu, T. L.; Lin, H.-L. Nafion-Based Membrane Electrode Assemblies Prepared from Catalyst Inks Containing Alcohol/Water Solvent Mixtures. J. Power Sources 2013, 238, 1– 10,  DOI: 10.1016/j.jpowsour.2013.03.055
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    Nafion-based membrane fuel cell electrode assemblies prepared from catalyst inks containing alcohol/water solvent mixtures
    Ngo, Trung Truc; Yu, T. Leon; Lin, Hsiu-Li
    Journal of Power Sources (2013), 238 (), 1-10CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Many researchers have studied the influence on fuel cell (FC) performance of the dielec. consts. (ε values) of the dispersion solvents of catalyst ink solns. in the fabrication of Nafion-based gas diffusion electrodes (GDEs). They have reported that Nafion forms colloidal particles in dispersion solvents possessing ε = 3-10, making these solvents suitable for fabricating GDEs with excellent FC performance. In this paper, we study the dependence of both the Nafion ionomer morphol. in the catalyst layers and the FC performance of the GDEs on ε and the soly. parameter δ of the alc. (i.e., methanol and iso-Pr alc.)/water dispersion mixt. The dispersion solvents of the catalyst ink solns. contain 20-100 wt.% alc. with 23.4 > δ > 11.3 and 78.4 > ε > 19.9. By plotting the FC performance against the ε and δ values of the alc./water dispersion solvent mixt., we demonstrate that the FC performance increases with increasing ε and δ values of the catalyst ink dispersion solvents. ε and δ are detd. to be better parameters than the compns. of the solvent mixts. for describing GDE performance.
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    Shimanuki, J.; Takahashi, S.; Tohma, H.; Ohma, A.; Ishihara, A.; Ito, Y.; Nishino, Y.; Miyazawa, A. Microstructural Observation of Fuel Cell Catalyst Inks by Cryo-SEM and Cryo-TEM. Microscopy 2017, 66, 204– 208,  DOI: 10.1093/jmicro/dfx001
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    Microstructural observation of fuel cell catalyst inks by Cryo-SEM and Cryo-TEM
    Shimanuki, Junichi; Takahashi, Shinichi; Tohma, Hajime; Ohma, Atsushi; Ishihara, Ayumi; Ito, Yoshiko; Nishino, Yuri; Miyazawa, Atsuo
    Microscopy (Oxford, United Kingdom) (2017), 66 (3), 204-208CODEN: MICRJM; ISSN:2050-5701. (Oxford University Press)
    In order to improve the electricity generation performance of fuel cell elec. vehicles, it is necessary to optimize the microstructure of the catalyst layer of a polymer electrolyte fuel cell. The catalyst layer is formed by a wet coating process using catalyst inks. Therefore, it is very important to observe the microstructure of the catalyst ink. In this study, the morphol. of carbon-supported platinum (Pt/C) particles in catalyst inks with a different solvent compn. was investigated by cryogenic SEM (cryo-SEM). In addn., the morphol. of the ionomer, which presumably influences the formation of agglomerated Pt/C particles in a catalyst ink, was investigated by cryogenic transmission electron microscopy (cryo-TEM). The results of a cryo-SEM observation revealed that the agglomerated Pt/C particles tended to become coarser with a higher 1-propanol (NPA) wt. fraction. The results of a cryo-TEM observation indicated that the actual ionomer dispersion in a catalyst ink formed a network structure different than that of the ionomer in the solvent.
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    Shukla, S.; Bhattacharjee, S.; Secanell, M. Rationalizing Catalyst Inks for PEMFC Electrodes Based on Colloidal Interactions. ECS Trans. 2013, 58, 1409– 1428,  DOI: 10.1149/05801.1409ecst
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    Rationalizing catalyst inks for PEMFC electrodes based on colloidal interactions
    Shukla, S.; Bhattacharjee, S.; Secanell, M.
    ECS Transactions (2013), 58 (1, Polymer Electrolyte Fuel Cells 13), 1409-1428, 20 pp.CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
    A preliminary kinetic model was developed for polymer electrolyte membrane fuel cell (PEMFC) catalyst inks in order to understand their particle stability. The Derjaguin Landau Verwey Overbeek (DLVO) model contg. van der Waals attractive and electrostatic repulsive interaction energy was applied to the aq. ink dispersions, while a modified DLVO type interaction contg. a Coulombic term instead of the electrostatic term was applied to the non-aq. dispersions. Solvents were compared based on their particle size distribution and stability ratios. Results show that the carbon black particles are stable in a higher dielec. medium whereas they tend to aggregate in a lower dielec. medium. A low ionic concn. for the aq. medium also helped to improve the ink stability by providing a better electrostatic particle repulsion. Expts. conducted with Et acetate, iso-propanol and deionized water agree with the model predictions.
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    Therdthianwong, A.; Ekdharmasuit, P.; Therdthianwong, S. Fabrication and Performance of Membrane Electrode Assembly Prepared by a Catalyst-Coated Membrane Method: Effect of Solvents Used in a Catalyst Ink Mixture. Energy Fuels 2010, 24, 1191– 1196,  DOI: 10.1021/ef901105k
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    Fabrication and Performance of Membrane Electrode Assembly Prepared by a Catalyst-Coated Membrane Method: Effect of Solvents Used in a Catalyst Ink Mixture
    Therdthianwong, Apichai; Ekdharmasuit, Panuwat; Therdthianwong, Supaporn
    Energy & Fuels (2010), 24 (2), 1191-1196CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    The effect of solvent types used to prep. catalyst-coated membrane (CCM) electrodes on the coating ink, applied coating process, interaction of catalyst ink and Nafion membrane and, thus, on the performance and power d. of the proton exchange membrane fuel cells (PEM fuel cells) was investigated. Among seven types of solvents studied, the catalyst ink prepd. using the isopropanol solvent showed the highest cell performance, followed in descending order by ethanol, acetone, water-ethylene glycol, ethylene glycol di-Me ether (EGDME), and ethylene glycol di-Et ether (EGDEE). The superior performance was due to the good attachment of catalyst layer to membrane and the good surface coverage revealed by the results from the scanning electron microscope (SEM) of the CCMs, optical transmittance of the CCMs, and solvent absorption capacity of membrane. In electrode prepn. by the CCM method, the solvent-ionomer mixt. in soln. form was more favorable than that in colloid form, as it gave higher cell performance. Finally, the catalyst-coated membrane on the H+ form Nafion demonstrated better power d. than the CCM on the Na+ form Nafion.
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    Wuttikid, K.; Worayos, N.; Punyawudho, K. Analysis of Catalyst Ink Compositions for Fabricating Membrane Electrode Assemblies in PEM Fuel Cells. Chiang Mai Univ. J. Nat. Sci. 2017, 16, 275– 281,  DOI: 10.12982/cmujns.2017.0022
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    Inaba, M.; Quinson, J.; Arenz, M. Ph Matters: The Influence of the Catalyst Ink on the Oxygen Reduction Activity Determined in Thin Film Rotating Disk Electrode Measurements. J. Power Sources 2017, 353, 19– 27,  DOI: 10.1016/j.jpowsour.2017.03.140
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    pH matters: The influence of the catalyst ink on the oxygen reduction activity determined in thin film rotating disk electrode measurements
    Inaba, Masanori; Quinson, Jonathan; Arenz, Matthias
    Journal of Power Sources (2017), 353 (), 19-27CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    We investigated the influence of the ink properties of proton exchange membrane fuel cell (PEMFC) catalysts on the oxygen redn. reaction (ORR) activity detd. in thin film rotating disk electrode (TF-RDE) measurements. It was found that the adaptation of a previously reported ink recipe to home-made catalysts does not lead to satisfying results, although reported work could be reproduced using com. catalyst samples. It is demonstrated that the pH of the catalyst ink, which has not been addressed in previous TF-RDE studies, is an important parameter that needs to be carefully controlled to det. the intrinsic ORR activity of high surface area catalysts.
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    Jung, C.-Y.; Kim, W.-J.; Yi, S.-C. Optimization of Catalyst Ink Composition for the Preparation of a Membrane Electrode Assembly in a Proton Exchange Membrane Fuel Cell Using the Decal Transfer. Int. J. Hydrogen Energy 2012, 37, 18446– 18454,  DOI: 10.1016/j.ijhydene.2012.09.013
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    Optimization of catalyst ink composition for the preparation of a membrane electrode assembly in a proton exchange membrane fuel cell using the decal transfer
    Jung, Chi-Young; Kim, Wha-Jung; Yi, Sung-Chul
    International Journal of Hydrogen Energy (2012), 37 (23), 18446-18454CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    For low interfacial resistance and feasibility of forming catalyst layer (CL), decal transfer (DT) is considered as one of the most effective methods for prepg. a membrane electrode assembly. However, optimization of the catalyst ink compn. is necessary, because of the complexity of the CL. Here, 1-propanol is adsorbed onto the CL coated onto the decal, as a swelling agent, for complete transfer of the CL onto Nafion membrane. Using this methodol., flat and complete DT is achieved at the hot-pressing conditions of 60 °C and 5 MPa. For optimization, the solvent-to-carbon ratio (SCR) and Nafion-to-carbon ratio (NCR) are controlled to achieve improved cell performance. In this study, by considering the morphol. of CL and the cell performance when CL is annealed at temps. sufficiently below the b.p. of the solvent, optimized SCR and NCR values of approx. 12.0 and 0.65, resp., are obtained. In addn., microstructure, thickness and various electrochem. properties of the CLs are examd. in detail.
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    Shinozaki, K.; Zack, J. W.; Pylypenko, S.; Pivovar, B. S.; Kocha, S. S. Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique. J. Electrochem. Soc. 2015, 162, F1384– F1396,  DOI: 10.1149/2.0551512jes
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    Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique
    Shinozaki, Kazuma; Zack, Jason W.; Pylypenko, Svitlana; Pivovar, Bryan S.; Kocha, Shyam S.
    Journal of the Electrochemical Society (2015), 162 (12), F1384-F1396CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    Platinum electrocatalysts supported on high surface area and Vulcan carbon blacks (Pt/HSC, Pt/V) were characterized in rotating disk electrode (RDE) setups for electrochem. area (ECA) and oxygen redn. reaction (ORR) area specific activity (SA) and mass specific activity (MA) at 0.9 V. Films fabricated using several ink formulations and film-drying techniques were characterized for a statistically significant no. of independent samples. The highest quality Pt/HSC films exhibited MA 870 ± 91 mA/mgPt and SA 864 ± 56 μA/cm2Pt while Pt/V had MA 706 ± 42 mA/mgPt and SA 1120 ± 70 μA/cm2Pt when measured in 0.1 M HClO4, 20 mV/s, 100 kPa O2 and 23 ± 2°C. An enhancement factor of 2.8 in the measured SA was observable on eliminating Nafion ionomer and employing extremely thin, uniform films (∼4.5 μg/cm2Pt) of Pt/HSC. The ECA for Pt/HSC (99 ± 7 m2/gPt) and Pt/V (65 ± 5 m2/gPt) were statistically invariant and insensitive to film uniformity/thickness/fabrication technique; accordingly, enhancements in MA are wholly attributable to increases in SA. Impedance measurements coupled with SEM were used to de-convolute the losses within the catalyst layer and ascribed to the catalyst layer resistance, oxygen diffusion, and sulfonate anion adsorption/blocking. The ramifications of these results for proton exchange membrane fuel cells have also been examd.
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    Pollet, B. G.; Goh, J. T. E. The Importance of Ultrasonic Parameters in the Preparation of Fuel Cell Catalyst Inks. Electrochim. Acta 2014, 128, 292– 303,  DOI: 10.1016/j.electacta.2013.09.160
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    The importance of ultrasonic parameters in the preparation of fuel cell catalyst inks
    Pollet, Bruno G.; Goh, Jonathan T. E.
    Electrochimica Acta (2014), 128 (), 292-303CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    We report here that ultrasound (20 kHz and 40 kHz) affects the catalyst ink compn. when irradiated for longer periods and at high ultrasonic powers. In our study, two com. carbon supported Pt (Pt/C) catalysts were used and dispersed in Nafion ionomer. Catalyst ink samples prepd. from Nafion, IPA and water were either ultrasonicated (20 kHz up to 12.23 W and 40 kHz at 1.82 W) or mech. shear-mixed (19,000 rpm) for various durations (up to 120 min). All catalyst ink samples were characterised by XRD, BET, TEM and electrochem. measurements were performed in liq. electrolytes. It was found that an optimized ultrasonic treatment is required to improve the catalytic ink activity, but longer irradn. is detrimental to its compn. and morphol., mainly due to cavitation and sonolysis phenomena.
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    Jung, S.; McCrory, C. C. L.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Nanoparticulate Metal Oxide Electrocatalysts for the Alkaline Water Oxidation Reaction. J. Mater. Chem. A 2016, 4, 3068– 3076,  DOI: 10.1039/c5ta07586f
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    Benchmarking nanoparticulate metal oxide electrocatalysts for the alkaline water oxidation reaction
    Jung, Suho; McCrory, Charles C. L.; Ferrer, Ivonne M.; Peters, Jonas C.; Jaramillo, Thomas F.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (8), 3068-3076CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Nanoparticulate metal-oxide catalysts are among the most prevalent systems for alk. water oxidn. However, comparisons of the electrochem. performance of these materials have been challenging due to the different methods of attachment, catalyst loadings, and electrochem. test conditions reported in the literature. Herein, we have leveraged a conventional drop-casting method that allows for the successful adhesion of a wide range of nanoparticulate catalysts to glassy-carbon electrode surfaces. We have applied this adhesion method to prep. catalyst films from 16 cryst. metal-oxide nanoparticles with a const. loading of 0.8 mg cm-2, and evaluated the resulting nanoparticulate films for the oxygen evolution reaction under conditions relevant to an integrated solar fuels device. In general, the activities of the adhered nanoparticulate films are similar to those of thin-film catalysts prepd. by electrodeposition or sputtering, achieving 10 mA cm-2 current densities per geometric area at overpotentials of ∼0.35-0.5 V.
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    Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765– 1772,  DOI: 10.1021/cs3003098
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    Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials
    Reier, Tobias; Oezaslan, Mehtap; Strasser, Peter
    ACS Catalysis (2012), 2 (8), 1765-1772CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A comparative study was performed to examine the intrinsic catalytic activity and durability of C supported Ru, Ir, and Pt nanoparticles and corresponding bulk materials for the electrocatalytic O evolution reaction (OER). The electrochem. surface characteristics of nanoparticles and bulk materials were studied by surface-sensitive cyclic voltammetry. Although basically similar voltammetric features were obsd. for nanoparticles and bulk materials of each metal, some differences were uncovered highlighting the changes in oxidn. chem. From the electrochem. results, Ru nanoparticles show lower passivation potentials compared to bulk Ru material. Ir nanoparticles completely lost their voltammetric metallic features during the voltage cycling, in contrast to the corresponding bulk material. Finally, Pt nanoparticles show an increased oxophilic nature compared to bulk Pt. With regard to the OER performance, the most pronounced effects of nanoscaling were identified for Ru and Pt catalysts. In particular, the Ru nanoparticles suffered from strong corrosion at applied OER potentials and were therefore unable to sustain the OER. The Pt nanoparticles exhibited a lower OER activity from the beginning on and were completely deactivated during the applied OER stability protocol, in contrast to the corresponding bulk Pt catalyst. The authors highlight that the OER activity and durability were comparable for Ir nanoparticles and bulk materials. Thus, Ir nanoparticles provide a high potential as nanoscaled OER catalyst.
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    Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452– 8455,  DOI: 10.1021/ja4027715
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    An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation
    Gong, Ming; Li, Yanguang; Wang, Hailiang; Liang, Yongye; Wu, Justin Z.; Zhou, Jigang; Wang, Jian; Regier, Tom; Wei, Fei; Dai, Hongjie
    Journal of the American Chemical Society (2013), 135 (23), 8452-8455CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Highly active, durable, and cost-effective electrocatalysts for H2O oxidn. to evolve O gas hold a key to a range of renewable energy solns., including H2O-splitting and rechargeable metal-air batteries. Here, the authors report the synthesis of ultrathin Ni-Fe layered double hydroxide (NiFe-LDH) nanoplates on mildly oxidized multi-walled C nanotubes (CNTs). Incorporation of Fe into the Ni hydroxide induced the formation of NiFe-LDH. The cryst. NiFe-LDH phase in nanoplate form is highly active for O evolution reaction in alk. solns. For NiFe-LDH grown on a network of CNTs, the resulting NiFe-LDH/CNT complex exhibits higher electrocatalytic activity and stability for O evolution than com. precious metal Ir catalysts.
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    Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186,  DOI: 10.1103/physrevb.54.11169
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    Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
    Kresse, G.; Furthmueller, J.
    Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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    Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15– 50,  DOI: 10.1016/0927-0256(96)00008-0
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    Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
    Kresse, G.; Furthmuller, J.
    Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
    The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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    Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953– 17979,  DOI: 10.1103/physrevb.50.17953
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    Projector augmented-wave method
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    Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
    There is no expanded citation for this reference.
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    Kresse, G.; Joubert, D. From Ultrasoft Pseudoptentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775,  DOI: 10.1103/physrevb.59.1758
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    From ultrasoft pseudopotentials to the projector augmented-wave method
    Kresse, G.; Joubert, D.
    Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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    Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104,  DOI: 10.1063/1.3382344
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    A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
    Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
    Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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    He, J.; Morales-García, Á.; Bludský, O.; Nachtigall, P. The Surface Stability and Equilibrium Crystal Morphology of Ni2P Nanoparticles and Nanowires from an Ab Initio Atomistic Thermodynamic Approach. CrystEngComm 2016, 18, 3808– 3818,  DOI: 10.1039/c6ce00584e
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    The surface stability and equilibrium crystal morphology of Ni2P nanoparticles and nanowires from an ab initio atomistic thermodynamic approach
    He, Junjie; Morales-Garcia, Angel; Bludsky, Ota; Nachtigall, Petr
    CrystEngComm (2016), 18 (21), 3808-3818CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
    Knowledge of the equil. crystal shape and structure of the exposed surfaces of nickel phosphide (Ni2P) nanostructures is essential for understanding and control of their catalytic performance. Ab initio atomistic thermodn. was used to investigate computationally the effects of the exptl. conditions (temp., pressure, and chem. potentials) on the relative stabilities of low-Miller index surfaces and on the equil. crystal morphol. of Ni2P nanoparticles and nanowires. The P-covered (0001)-Ni3P2 (denoted as (0001)-A-P) surface was found to be the most stable surface at a considerably wide range of chem. potentials, whereas the (0001)-A, (10‾11)-Ni/P and (10‾12)-Ni/P surfaces are the thermodynamically most favored phases just in narrow chem. potential regions. The theor. equil. shapes and structures of the Ni2P nanoparticles and nanowires were obtained based on the Wulff construction at various chem. potentials. The morphol. of the surfaces of the Ni2P nanoparticles and nanowires does depend on the chem. potential; thus, it can be tailored for particular applications by a suitable choice of exptl. conditions. The (0001), (10‾10) and (10‾11) side facets dominate the nanoparticle surface in a wide range of chem. potentials but other side facets can also appear at particular ranges of chem. potentials. Results reported herein give new insight into the Ni2P nanoparticle morphol. showing how it depends on the exptl. conditions; this information can help to tailor the surface and shape of Ni2P nanoparticles for specific applications, e.g., in catalysis.
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    Wexler, R. B.; Martirez, J. M. P.; Rappe, A. M. Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces. ACS Catal. 2017, 7, 7718– 7725,  DOI: 10.1021/acscatal.7b02761
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    69
    Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces
    Wexler, Robert B.; Martirez, John Mark P.; Rappe, Andrew M.
    ACS Catalysis (2017), 7 (11), 7718-7725CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Optimizing catalysts for the hydrogen evolution reaction (HER) is a crit. step toward the efficient prodn. of H2(g) fuel from water. It has been demonstrated exptl. that transition-metal phosphides, specifically nickel phosphides Ni2P and Ni5P4, efficiently catalyze the HER at a small fraction of the cost of archetypal Pt-based electrocatalysts. However, the HER mechanism on nickel phosphides remains unclear. We explore, through d. functional theory with thermodn., the aq. reconstructions of Ni2P(0001) and Ni5P4(0001)/(000‾1), and we find that the surface P content on Ni2P(0001) depends on the applied potential, which has not been considered previously. At -0.21 V ≥ U ≥ -0.36 V vs. the std. hydrogen electrode and pH = 0, a PHx-enriched Ni3P2 termination of Ni2P(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni3P2 surface is instead passivated by H at the Ni3-hollow sites. On the other hand, Ni5P4(000‾1) does not favor addnl. P. Instead, the Ni4P3 bulk termination of Ni5P4(000‾1) is passivated by H at both the Ni3 and P3-hollow sites. We also found that the most HER-active surfaces are Ni3P2+P+(7/3)H of Ni2P(0001) and Ni4P3+4H of Ni5P4(000‾1) due to weak H adsorption at P catalytic sites, in contrast with other computational investigations that propose Ni as or part of the active site. By looking at viable catalytic cycles for HER on the stable reconstructed surfaces, and calcg. the reaction free energies of the assocd. elementary steps, we calc. that the overpotential on the Ni4P3+4H surface of Ni5P4(000‾1) (-0.16 V) is lower than that of the Ni3P2+P+(7/3)H surface of Ni2P(0001) (-0.21 V). This is due to the abundance of P3-hollow sites on Ni5P4 and the limited surface stability of the P-enriched Ni2P(0001) surface phase. The trend in the calcd. catalytic overpotentials, and the potential-dependent bulk and surface stabilities explain why the nickel phosphides studied here perform almost as well as Pt, and why Ni5P4 is more active than Ni2P toward HER, as is found in the exptl. literature. This study emphasizes the importance of considering aq. surface stability in predicting the HER-active sites, mechanism, and overpotential, and highlights the primary role of P in HER catalysis on transition-metal phosphides.
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    Partanen, L.; Hakala, M.; Laasonen, K. Hydrogen Adsorption Trends on Various Metal-Doped Ni2P Surfaces for Optimal Catalyst Design. Phys. Chem. Chem. Phys. 2018, 21, 184– 191,  DOI: 10.1039/c8cp06143b
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    Hydrogen adsorption trends on various metal-doped Ni2P surfaces for optimal catalyst design
    Partanen Lauri; Hakala Mikko; Laasonen Kari
    Physical chemistry chemical physics : PCCP (2018), 21 (1), 184-191 ISSN:.
    In this study, we looked at the hydrogen evolution reaction on Mg-, Mo-, Fe-, Co-, V-, and Cu-doped Ni3P2 and Ni3P2 + P terminated Ni2P surfaces. The DFT calculated hydrogen adsorption free energy was employed as a predictor of the materials' catalytic HER activity. Our results indicate that doping can substantially improve the catalytic activity of the Ni3P2 terminated surface. In contrast, the Ni3P2 + P terminated one seems to be catalytically active irrespective of the type of doping, including in the absence of doping. Based on our doping energy and adsorption free energy calculations, the most promising dopants are iron and cobalt, whereas copper is less likely to function well as a doping element.
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    Moon, J.-S.; Jang, J.-H.; Kim, E.-G.; Chung, Y.-H.; Yoo, S. J.; Lee, Y.-K. The Nature of Active Sites of Ni2P Electrocatalyst for Hydrogen Evolution Reaction. J. Catal. 2015, 326, 92– 99,  DOI: 10.1016/j.jcat.2015.03.012
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    The nature of active sites of Ni2P electrocatalyst for hydrogen evolution reaction
    Moon, Ji-Sue; Jang, Jue-Hyuk; Kim, Eung-Gun; Chung, Young-Hoon; Yoo, Sung Jong; Lee, Yong-Kul
    Journal of Catalysis (2015), 326 (), 92-99CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Nano-scaled Ni2P particles were synthesized by ligand stabilization method and applied for hydrogen evolution reaction (HER). X-ray diffraction (XRD), transmission electron microscope (TEM), and X-ray absorption fine structure (XAFS) spectroscopy were employed to examine structural properties of Ni2P nanoparticles. The electrocatalytic HER activity and stability for the Ni2P nanocatalyst were tested in 0.5M H2SO4, and the Ni2P electrocatalyst exhibited a low onset potential for the HER at around -0.02 V vs. RHE, a little more neg. compared to the Pt catalyst which shows almost 0 V vs. reversible hydrogen electrode (RHE), and the Tafel slope of 75 mV per decade, i.e. following Volmer step as a rate-detg. step. D. functional theory (DFT) calcns. for hydrogen adsorption over Ni2P surfaces (0 0 1) and (0 0 2) revealed that the hydrogen adsorption might occur via two reaction pathways: consecutive or simultaneous hydrogen adsorption. The consecutive hydrogen adsorptions on threefold hollow (TFH)-Ni site followed by on P(II) site on a Ni2P (0 0 1) surface led to a lower reaction barrier than simultaneous hydrogen adsorption. These results thus demonstrated that the Volmer step might follow consecutive adsorption mechanism over the Ni2P surface.
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    Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B: Solid State 1976, 13, 5188– 5192,  DOI: 10.1103/physrevb.13.5188
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    Wang, X.; Wan, F.; Gao, Y.; Liu, J.; Jiang, K. Synthesis of High-Quality Ni2P Hollow Sphere Via a Template-Free Surfactant-Assisted Solvothermal Route. J. Cryst. Growth 2008, 310, 2569– 2574,  DOI: 10.1016/j.jcrysgro.2008.01.014
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    Synthesis of high-quality Ni2P hollow sphere via a template-free surfactant-assisted solvothermal route
    Wang, Xinjun; Wan, Fuquan; Gao, Youjun; Liu, Juan; Jiang, Kai
    Journal of Crystal Growth (2008), 310 (10), 2569-2574CODEN: JCRGAE; ISSN:0022-0248. (Elsevier B.V.)
    Ni2P uniform hollow nanospheres were prepd. via a template-free surfactant-assisted solvothermal route using NiCl2·6H2O and elemental yellow P as starting materials and urea as pH regulator in a mixt. soln. of ethylene glycol (EG), EtOH and H2O. X-ray powder diffraction (XRD), XPS, energy dispersive x-ray spectroscopy (EDX), transmission electronic microscopy (TEM), electron diffraction (ED), and SEM studies show that the as-obtained nanocrystal is pure Ni2P with a hexagonal phase and the high-quality spheres are ∼500 nm in diam. Na dodecyl sulfate (SDS) and urea play an important role in controlling the formation of Ni2P hollow nanospheres. The possible growth mechanism is proposed.
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    Wexler, R. B.; Martirez, J. M. P.; Rappe, A. M. Stable Phosphorus-Enriched (0001) Surfaces of Nickel Phosphides. Chem. Mater. 2016, 28, 5365– 5372,  DOI: 10.1021/acs.chemmater.6b01437
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    74
    Stable Phosphorus-Enriched (0001) Surfaces of Nickel Phosphides
    Wexler, Robert B.; Martirez, John Mark P.; Rappe, Andrew M.
    Chemistry of Materials (2016), 28 (15), 5365-5372CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    In heterogeneous catalysis, catalyst synthesis precedes operation and, in most cases, is conducted in an altogether different chem. environment. Thus, detn. of the structure and compn. of the catalyst surface(s) due to fabrication is essential in accurately evaluating their eventual structure(s) during operation, which provides the origin of their catalytic activities and are therefore key to catalyst optimization. We explore the reconstructions of both Ni2P(0001) and Ni5P4(0001)/(000‾1) surfaces with first-principles d. functional theory (DFT). Most of the stable terminations under realistic synthesis conditions are detd. to be P-rich on both materials. A P-covered reconstruction of the Ni3P2 termination of Ni2P(0001) is found to be most stable, consistent with the current literature. By contrast, the most energetically favorable surfaces of Ni5P4 are found to be the Ni3P3 and Ni4P3 bulk-derived terminations with P-adatoms. The preferred excess P binding sites and their energies are identified on each surface. We find that the P3 site, which is present on Ni5P4, and the Ni3 site, which is present on both Ni2P and Ni5P4, strongly bind excess P. Addnl., we predict the presence of stable Pn (n = 2, 4) agglomerates on Ni5P4 at the P3-hollow and Ni-Ni bridge sites. This study highlights the importance of considering the aggregation behavior of nonmetal components in predicting the surface reconstruction of transition metal compds.
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    Wang, J.; Johnston-Peck, A. C.; Tracy, J. B. Nickel Phosphide Nanoparticles with Hollow, Solid, and Amorphous Structures. Chem. Mater. 2009, 21, 4462– 4467,  DOI: 10.1021/cm901073k
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    75
    Nickel Phosphide Nanoparticles with Hollow, Solid, and Amorphous Structures
    Wang, Junwei; Johnston-Peck, Aaron C.; Tracy, Joseph B.
    Chemistry of Materials (2009), 21 (19), 4462-4467CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Conversion of unary metal nanoparticles (NPs) upon exposure to oxygen, sulfur, selenium, and phosphorus precursors usually produces hollow metal oxide, sulfide, selenide, or phosphide NPs through the Kirkendall effect. Here, nanostructural control of mixed-phase Ni2P/Ni12P5 (represented as NixPy) NPs prepd. through the thermolysis of nickel acetylacetonate using trioctylphosphine (TOP) as a ligand and phosphorus precursor is reported. The P:Ni molar ratio controls the NP size and is the key factor in detg. the nanostructure. For P:Ni molar ratios of 1-3, nickel NPs form below 240°C and subsequently convert to cryst.-hollow NixPy NPs at 300°C. For higher P:Ni ratios, a Ni-TOP complex forms that requires higher temps. for NP growth, thus favoring direct formation of NixPy rather than nickel. Consequently, for P:Ni molar ratios of >9, amorphous-solid NixPy NPs form at 240°C and become cryst.-solid NixPy NPs at 300°C. For intermediate P:Ni molar ratios of ∼6, both growth mechanisms result in a mixt. of hollow and solid NixPy NPs. Similar results have been obtained using tributylphosphine or triphenylphosphine as the phosphorus source, but trioctylphosphine oxide cannot serve as a phosphorus source.
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    Muthuswamy, E.; Savithra, G. H. L.; Brock, S. L. Synthetic Levers Enabling Independent Control of Phase, Size, and Morphology in Nickel Phosphide Nanoparticles. ACS Nano 2011, 5, 2402– 2411,  DOI: 10.1021/nn1033357
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    76
    Synthetic levers enabling independent control of phase, size, and morphology in nickel phosphide nanoparticles
    Muthuswamy, Elayaraja; Savithra, Galbokka H. Layan; Brock, Stephanie L.
    ACS Nano (2011), 5 (3), 2402-2411CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Simultaneous control of phase, size, and morphol. in nanoscale nickel phosphides is reported. Phase-pure samples of discrete nanoparticles of Ni12P5 and Ni2P in hollow and solid morphologies can be prepd. in a range of sizes (10-32 nm) by tuning key interdependent synthetic levers (P:Ni precursor ratio, temp., time, oleylamine quantity). Size and morphol. are controlled by the P:Ni ratio in the synthesis of the precursor particles, with large, hollow particles formed at low P:Ni and small, solid particles formed at high P:Ni. The P:Ni ratio also impacts the phase at the crystn. temp. (300-350°), with metal-rich Ni12P5 generated at low P:Ni and Ni2P at high P:Ni. Also, the product phase formed can be decoupled from the initial precursor ratio by the addn. of more P at the crystn. temp. This enables formation of hollow particles (favored by low P:Ni) of Ni2P (favored by high P:Ni). Increasing temp. and time also favor formation of Ni2P, by generating more reactive P and providing sufficient time for conversion to the thermodn. product. Finally, increasing oleylamine concn. allows Ni12P5 to be obtained under high P:Ni precursor ratios that favor solid particle formation. Oleylamine concn. also acts to tune the size of the voids in particles formed at low P:Ni ratios, enabling access to Ni12P5 particles with different void sizes. This approach enables an unprecedented level of control over phase and morphol. of nickel phosphide nanoparticles, paving the way for systematic study of the impact of these parameters on hydrodesulfurization activities of nickel phosphides.
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    Moreau, L. M.; Ha, D.-H.; Zhang, H.; Hovden, R.; Muller, D. A.; Robinson, R. D. Defining Crystalline/Amorphous Phases of Nanoparticles through X-Ray Absorption Spectroscopy and X-Ray Diffraction: The Case of Nickel Phosphide. Chem. Mater. 2013, 25, 2394– 2403,  DOI: 10.1021/cm303490y
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    77
    Defining Crystalline/Amorphous Phases of Nanoparticles through X-ray Absorption Spectroscopy and X-ray Diffraction: The Case of Nickel Phosphide
    Moreau, Liane M.; Ha, Don-Hyung; Zhang, Haitao; Hovden, Robert; Muller, David A.; Robinson, Richard D.
    Chemistry of Materials (2013), 25 (12), 2394-2403CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The authors elucidate the structural distinctions between amorphous and cryst. Ni2P nanoparticles synthesized using tri-n-octylphosphine (TOP), through x-ray absorption spectroscopy (XAS), XRD, and inductively coupled plasma (ICP). The authors det. the differences in their chem. and at. structure, which were not previously reported, yet are essential for understanding their potential as nanocatalysts. These structural characteristics are related to the corresponding nanoparticle magnetic properties analyzed by performing magnetic measurements. XAS results reveal a significant P concn. in the amorphous nanoparticle sample - placing the stoichiometry close to Ni2P - despite XRD results that show only fcc. Ni contributions. By comparing the long-range structural order from XRD to the short-range radial structure from EXAFS both techniques are necessary to obtain a complete structural picture of amorphous and cryst. nanoparticle phases due to the limitations of XRD amorphous characterization. Phases are amorphous with respect to XRD when their offsets (deviations) from bulk interat. distances have a std. deviation as high as ∼4.82. Phases with lower std. deviation (e.g., .ltorsim.1.22), however, are detectable as cryst. through XRD. The possible presence of amorphous phases should be considered when using XRD alone for nanoparticle characterization. This is particularly important when highly reactive reagents such as TOP were used in synthesis. By characterizing amorphous nickel phosphide nanoparticles that have a comparable stoichiometry to Ni2P, TOP serves as a highly effective phosphorus source, even at temps. ≥230°. Unintended amorphous structure domains may significantly affect nanoparticle properties, and in turn, their functionality.
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    Ung, D.; Cossairt, B. M. Effect of Surface Ligands on CoP for the Hydrogen Evolution Reaction. ACS Appl. Energy Mater. 2019, 2, 1642– 1645,  DOI: 10.1021/acsaem.9b00240
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    80
    Effect of Surface Ligands on CoP for the Hydrogen Evolution Reaction
    Ung, David; Cossairt, Brandi M.
    ACS Applied Energy Materials (2019), 2 (3), 1642-1645CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
    The activity of colloidally synthesized CoP for the hydrogen evolution reaction (HER) was studied to det. the impact of surface ligands on catalysis. The as-synthesized CoP nanocrystals were stripped with trialkyloxonium tetrafluoroborate (Meerwein's reagent) to create a "blank template" that was then re-ligated with carboxylates (oleate, octanoate, acetate) and amines (oleylamine, octylamine, dioctylamine, trioctylamine butylamine). Carboxylates and amines were chosen due to their prevalence in colloidal syntheses, and the range of ligands was chosen to study the impact of sterics and hydrophobicity of the surface ligands on catalysis. Long chain carboxylates were found to result in a larger increase in overpotential when compared to the equiv. amine (e.g., oleate vs oleylamine), due to higher ligand d. and stronger coordination with carboxylates. Increased carbon chain length resulted in increased overpotential with carboxylates; however, the range of 1° amines studied had similar overpotentials. This is due to the lower ligand d. and therefore sparse ligand packing for the 1° amines. These results suggest that the mechanism by which surface ligands impede catalysis on CoP for HER is primarily through inhibiting substrate access to surface active sites rather than poisoning the active sites.
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    Kim, K.-Y.; Habas, S. E.; Schaidle, J. A.; Logan, B. E. Application of Phase-Pure Nickel Phosphide Nanoparticles as Cathode Catalysts for Hydrogen Production in Microbial Electrolysis Cells. Bioresour. Technol. 2019, 293, 122067,  DOI: 10.1016/j.biortech.2019.122067
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    Application of phase-pure nickel phosphide nanoparticles as cathode catalysts for hydrogen production in microbial electrolysis cells
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    Transition metal phosphide catalysts such as nickel phosphide (Ni2P) have shown excellent activities for the hydrogen evolution reaction, but they have primarily been studied in strongly acidic or alk. electrolytes. In microbial electrolysis cells (MECs), however, the electrolyte is usually a neutral pH to support the bacteria. Carbon-supported phase-pure Ni2P nanoparticle catalysts (Ni2P/C) were synthesized using soln.-phase methods and their performance was compared to Pt/C and Ni/C catalysts in MECs. The Ni2P/C produced a similar quantity of hydrogen over a 24 h cycle (0.29 ± 0.04 L-H2/L-reactor) as that obtained using Pt/C (0.32 ± 0.03 L-H2/L) or Ni/C (0.29 ± 0.02 L-H2/L). The mass normalized c.d. of the Ni2P/C was 14 times higher than that of the Ni/C, and the Ni2P/C exhibited stable performance over 11 days. Ni2P/C may therefore be a useful alternative to Pt/C or other Ni-based catalysts in MECs due to its chem. stability over time.
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    Objective comparisons of electrocatalyst activity and stability using std. methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, the authors use a std. protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochem. active surface area (ECSA) of 18 electrocatalysts for the H evolution reaction (HER) and 26 electrocatalysts for the O evolution reaction (OER) under conditions relevant to an integrated solar H2O-splitting device in aq. acidic or alk. soln. The primary figure of merit is the overpotential necessary to achieve a magnitude c.d. of 10 mA cm-2 per geometric area, the approx. c.d. expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm-2 with overpotentials <0.1 V in acidic and/or alk. solns. Among OER catalysts in acidic soln., no nonnoble metal based materials showed promising activity and stability, whereas in alk. soln. many OER catalysts performed with similar activity achieving 10 mA cm-2 current densities at overpotentials of ∼0.33-0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alk. solns., while most HER catalysts showed much lower specific activity than Pt in both acidic and alk. solns. For select catalysts, addnl. secondary screening measurements were conducted including faradaic efficiency and extended stability measurements.
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    Active site no., water dissocn., and hydrogen adsorption free energy are the three main parameters for regulating the activity of electrocatalysts for hydrogen evolution reaction (HER) in alk. media. However, at present, simultaneous modulations of these three parameters for alk. HER still remain challenging. In this work, we take CoP as the model material and demonstrate that a metal and nonmetal dual-doping strategy can achieve simultaneous modulation of these three parameters by inducing lattice irregularity and optimizing the electronic configuration in CoP nanomaterials. Benefiting from the oxygen and copper dual-doping collective effect, the optimized O,Cu-CoP nanowire array electrode shows nearly 10-fold enhancement in catalytic activity for alk. HER compared to a pure CoP nanowire electrode. Our work may provide a new concept to boost performance of nonprecious metal electrocatalysts for alk. HER.
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    Trends in the exchange current for hydrogen evolution
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    The authors present insights into the mechanism and the active site for H evolution on Ni phosphide (Ni2P). Ni2P was recently discovered to be a very active nonprecious H evolution catalyst. Current literature attributes the activity of Ni2P to a particular site on the (0001) facet. Using D. Functional Theory (DFT) calcns., several widely available low index crystal facets on Ni2P have better properties for a high catalytic activity. DFT calcns. were used to identify moderately bonding Ni bridge sites and Ni hollow sites for H adsorption and to calc. barriers for the Tafel pathway. The studied surfaces in this study were the (10‾10), (‾1‾120), (11‾20), (11‾21) and (0001) facets of the hexagonal Ni2P crystal. In addn. to the DFT results, the authors present expts. on Ni2P nanowires growing along the 〈0001〉 direction, which are shown as efficient H evolution catalysts. The exptl. results add these nanowires to a variety of different morphologies of Ni2P, which are all active for HER.
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Cited By

This article is cited by 3 publications.

  1. Tharanga N. Batugedara, Stephanie L. Brock. A Little Nickel Goes a Long Way: Ni Incorporation into Rh2P for Stable Bifunctional Electrocatalytic Water Splitting in Acidic Media. ACS Materials Au 2023, 3 (4) , 299-309. https://doi.org/10.1021/acsmaterialsau.2c00080
  2. Ding-Yuan Kuo, Emily Nishiwaki, Ricardo A. Rivera-Maldonado, Brandi M. Cossairt. The Role of Hydrogen Adsorption Site Diversity in Catalysis on Transition-Metal Phosphide Surfaces. ACS Catalysis 2023, 13 (1) , 287-295. https://doi.org/10.1021/acscatal.2c04936
  3. Kousik Bhunia, Moumita Chandra, Sanjeev Kumar Sharma, Debabrata Pradhan, Sang-Jae Kim. A critical review on transition metal phosphide based catalyst for electrochemical hydrogen evolution reaction: Gibbs free energy, composition, stability, and true identity of active site. Coordination Chemistry Reviews 2023, 478 , 214956. https://doi.org/10.1016/j.ccr.2022.214956
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  • Abstract

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    Figure 1

    Figure 1. XRD patterns of Ni2P and Ni2–xMxP NPs with the corresponding Ni2P reference pattern (PDF 01-089-2742), below.

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    Figure 2

    Figure 2. (a) BF-STEM image of Ni1.61Co0.39P NPs; (b) high-resolution HAADF-STEM image with the (c) FFT oriented along the [112̅2] direction; and (d) HAADF-STEM image for EDS analysis and associated EDS elemental maps for (e) Ni, (f) P, (g) Co, and (h) Ni and Co overlaid.

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    Figure 3

    Figure 3. (a) BF-STEM image of Ni1.69Pd0.31P NPs; (b) high-resolution HAADF-STEM image with the (c) FFT oriented along the [123̅3] direction; and (d) HAADF-STEM image for EDS analysis; and associated EDS elemental maps for (e) Ni, (f) P, (g) Pd, and (h) Ni and Pd overlaid.

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    Figure 4

    Figure 4. (a) Voltammograms (cathodic sweep only, 5 mV/s) and (b) Tafel plots for Ni2–xMxP/C (nominally 5 wt %; see Table 2) and Pt/C (29 wt %) catalysts in H2-saturated 0.5 M H2SO4 at room temperature, with both experimental data (dashed) and linear fits (solid; Table 2) shown (catalyst loading = 0.51 mg/cm2; NP loading ≈ 0.025 mg/cm2). The legend for both plots is shown in (b).

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    Figure 5

    Figure 5. Top views of the optimal H adsorption structure for the (a) ABB- and (b) P-BAB-terminated Ni1.67M0.33P(101̅0) and (c) Ni3P- and (d) P-Ni3P2-terminated Ni1.67M0.33P(0001) surfaces (M = Co, Cu, Mo, Ni, Pd, Rh, or Ru). Also shown are the H adsorption structures for the Cu- and Pd-enriched Ni1.33M0.67P surfaces with a 1:1 Ni/Cu and Ni/Pd surface-layer composition [Cu(50-50) and Pd(50-50), respectively; see main text for more details]. Calculated adsorption free energies of atomic H (GH, in eV) are provided below each snapshot. Black lines denote the surface unit cell. Atom colors: black, H; yellow, P; green, Ni; cyan, Co; blue, Cu; red, Mo; orange, Pd; purple, Rh; and pink, Ru.

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    Figure 6

    Figure 6. Overpotential (η) vs the adsorption free energy of atomic H (GH) on the ABB-terminated Ni1.67M0.33P(101̅0) (solid markers) and Cu- and Pd-enriched Ni1.33M0.67P(101̅0) (open markers) surfaces. Gray lines show the volcano-shaped dependence of η on GH for the Ni1.67M0.33P(101̅0) surfaces (solid) and when Cu- and Pd-enriched surfaces are included (dashed).

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  • References

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    This article references 83 other publications.

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      7
      Recent developments of transition metal phosphides as catalysts in the energy conversion field
      Pei, Yu; Cheng, Yu; Chen, Jiyi; Smith, Will; Dong, Pei; Ajayan, Pulickel M.; Ye, Mingxin; Shen, Jianfeng
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (46), 23220-23243CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A review is given. Catalysts play an important role in the energy conversion reaction process. With the continuous development of the energy sector, researchers are constantly exploring and developing new high-performance, inexpensive catalysts to meet the demand. Transition metal phosphides (TMPs) as ever-evolving catalytic materials have received more and more attention. Many studies have suggested that transition metal phosphides could be ideal catalysts due to their low cost, abundance, and high efficiency. In many areas, TMPs can be applied as stable and efficient catalytic materials. Therefore, many kinds of TMPs are designed and synthesized by different methods. This article includes the recent research progress of TMPs, including their prepn. methods and morphologies. The application and catalytic performance of TMPs in different catalytic fields are elaborated, and views on the opportunities and challenges faced by phosphide catalysts are presented.
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    7. 7
      Callejas, J. F.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater. 2016, 28, 6017– 6044,  DOI: 10.1021/acs.chemmater.6b02148
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      8
      Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction
      Callejas, Juan F.; Read, Carlos G.; Roske, Christopher W.; Lewis, Nathan S.; Schaak, Raymond E.
      Chemistry of Materials (2016), 28 (17), 6017-6044CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A review. Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (MxPy) have recently been identified as a promising family of Earth-abundant electrocatalysts for the hydrogen-evolution reaction (HER), and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this review, we highlight the progress that has been made in this field and provide insights into the synthesis, characterization and electrochem. behavior of transition metal phosphides as HER electrocatalysts. We also discuss strategies for the incorporation of metal phosphides into integrated solar-driven water-splitting systems and highlight key considerations involved in the testing and benchmarking of such devices.
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    8. 8
      Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 1529– 1541,  DOI: 10.1039/c5cs00434a
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      9
      Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction
      Shi, Yanmei; Zhang, Bin
      Chemical Society Reviews (2016), 45 (6), 1529-1541CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      The urgent need of clean and renewable energy drives the exploration of effective strategies to produce mol. hydrogen. With the assistance of highly active non-noble metal electrocatalysts, electrolysis of water is becoming a promising candidate to generate pure hydrogen with low cost and high efficiency. Very recently, transition metal phosphides (TMPs) have been proven to be high performance catalysts with high activity, high stability, and nearly ∼100% Faradic efficiency in not only strong acidic solns., but also in strong alk. and neutral media for electrochem. hydrogen evolution. In this tutorial review, an overview of recent development of TMP nanomaterials as catalysts for hydrogen generation with high activity and stability is presented. The effects of phosphorus (P) on HER activity, and their synthetic methods of TMPs are briefly discussed. Then we will demonstrate the specific strategies to further improve the catalytic efficiency and stability of TMPs by structural engineering. Making use of TMPs as cocatalysts and catalysts in photochem. and photoelectrochem. water splitting is also discussed. Finally, some key challenges and issues which should not be ignored during the rapid development of TMPs are pointed out. These strategies and challenges of TMPs are instructive for designing other high-performance non-noble metal catalysts.
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    9. 9
      Calvinho, K. U. D.; Laursen, A. B.; Yap, K. M. K.; Goetjen, T. A.; Hwang, S.; Murali, N.; Mejia-Sosa, B.; Lubarski, A.; Teeluck, K. M.; Hall, E. S.; Garfunkel, E.; Greenblatt, M.; Dismukes, G. C. Selective CO2 Reduction to C3 and C4 Oxyhydrocarbons on Nickel Phosphides at Overpotentials as Low as 10 mV. Energy Environ. Sci. 2018, 11, 2550– 2559,  DOI: 10.1039/c8ee00936h
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      10
      Selective CO2 reduction to C3 and C4 oxyhydrocarbons on nickel phosphides at overpotentials as low as 10 mV
      Calvinho, Karin U. D.; Laursen, Anders B.; Yap, Kyra M. K.; Goetjen, Timothy A.; Hwang, Shinjae; Murali, Nagarajan; Mejia-Sosa, Bryan; Lubarski, Alexander; Teeluck, Krishani M.; Hall, Eugene S.; Garfunkel, Eric; Greenblatt, Martha; Dismukes, G. Charles
      Energy & Environmental Science (2018), 11 (9), 2550-2559CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      We introduce five nickel phosphide compds. as electro-catalysts for the redn. of carbon dioxide in aq. soln., that achieve unprecedented selectivity to C3 and C4 products (the first such report). Three products: formic acid (C1), methylglyoxal (C3), and 2,3-furandiol (C4), are obsd. at potentials as low as +50 mV vs. RHE, and at the highest half-reaction energy efficiencies reported to date for any >C1 product (99%). The max. selectivity for 2,3-furandiol is 71% (faradaic efficiency) at 0.00 V vs. RHE on Ni2P, which is equiv. to an overpotential of 10 mV, with the balance forming methylglyoxal, the proposed reaction intermediate. P content in the series correlates closely with both the total C products and product selectivity, establishing definitive structure-function relationships. We propose a reaction mechanism for the formation of multi-carbon products, involving hydride transfer as the potential-detg. step to oxygen-bound intermediates. This unlocks a new and more energy-efficient redn. route that has only been previously obsd. in nickel-based enzymes. This performance contrasts with simple metallic catalysts that have poor selectivity between multi-carbon products, and which require high overpotentials (>700 mV) to achieve comparable reaction rates.
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    10. 10
      Li, H.; Wen, P.; Itanze, D. S.; Hood, Z. D.; Ma, X.; Kim, M.; Adhikari, S.; Lu, C.; Dun, C.; Chi, M.; Qiu, Y.; Geyer, S. M. Colloidal Silver Diphosphide (AgP2) Nanocrystals as Low Overpotential Catalysts for CO2 Reduction to Tunable Syngas. Nat. Commun. 2019, 10, 5724,  DOI: 10.1038/s41467-019-13388-8
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      11
      Colloidal silver diphosphide (AgP2) nanocrystals as low overpotential catalysts for CO2 reduction to tunable syngas
      Li, Hui; Wen, Peng; Itanze, Dominique S.; Hood, Zachary D.; Ma, Xiao; Kim, Michael; Adhikari, Shiba; Lu, Chang; Dun, Chaochao; Chi, Miaofang; Qiu, Yejun; Geyer, Scott M.
      Nature Communications (2019), 10 (1), 5724CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Prodn. of syngas with tunable CO/H2 ratio from renewable resources is an ideal way to provide a carbon-neutral feedstock for liq. fuel prodn. Ag is a benchmark electrocatalysts for CO2-to-CO conversion but high overpotential limits the efficiency. We synthesize AgP2 nanocrystals (NCs) with a greater than 3-fold redn. in overpotential for electrochem. CO2-to-CO redn. compared to Ag and greatly enhanced stability. D. functional theory calcns. reveal a significant energy barrier decrease in the formate intermediate formation step. In situ X-ray absorption spectroscopy (XAS) shows that a max. Faradaic efficiency is achieved at an av. silver valence state of +1.08 in AgP2 NCs. A photocathode consisting of a n+p-Si wafer coated with ultrathin Al2O3 and AgP2 NCs achieves an onset potential of 0.2 V vs. RHE for CO prodn. and a partial photocurrent d. for CO at -0.11 V vs. RHE (j-0.11,CO) of -3.2 mA cm-2.
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    11. 11
      Ji, L.; Li, L.; Ji, X.; Zhang, Y.; Mou, S.; Wu, T.; Liu, Q.; Li, B.; Zhu, X.; Luo, Y.; Shi, X.; Asiri, A. M.; Sun, X. Highly Selective Electrochemical Reduction of to Alcohols on an FeP Nanoarray. Angew. Chem., Int. Ed. 2020, 59, 758– 762,  DOI: 10.1002/anie.201912836
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      12
      Highly Selective Electrochemical Reduction of CO2 to Alcohols on a FeP Nanoarray
      Ji, Lei; Li, Lei; Ji, Xuqiang; Zhang, Ya; Mou, Shiyong; Wu, Tongwei; Liu, Qian; Li, Baihai; Zhu, Xiaojuan; Luo, Yonglan; Shi, Xifeng; Asiri, Abdullah M.; Sun, Xuping
      Angewandte Chemie, International Edition (2020), 59 (2), 758-762CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Electrochem. redn. of CO2 into various chems. and fuels provides an attractive pathway for environmental and energy sustainability. It is now shown that a FeP nanoarray on Ti mesh (FeP NA/TM) acts as an efficient 3-dimensional catalyst electrode for the CO2 redn. reaction to convert CO2 into alcs. with high selectivity. In 0.5 M KHCO3, such FeP NA/TM is capable of achieving a high faradaic efficiency (FECH3OH) up to 80.2 %, with a total FECH3OH+C2H5OH of 94.3% at -0.20 V vs. reversible H electrode. D. functional theory calcns. reveal that the FeP(211) surface significantly promotes the adsorption and redn. of CO2 toward MeOH owing to the synergistic effect of two adjacent Fe atoms, and the potential-detg. step is the hydrogenation process of *CO.
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    12. 12
      Downes, C. A.; Libretto, N. J.; Harman-Ware, A. E.; Happs, R. M.; Ruddy, D. A.; Baddour, F. G.; Ferrell, J. R., III; Habas, S. E.; Schaidle, J. A. Electrocatalytic CO2 Reduction over Cu3P Nanoparticles Generated Via a Molecular Precursor Route. ACS Appl. Energy Mater. 2020, 3, 10435– 10446,  DOI: 10.1021/acsaem.0c01360
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      13
      Electrocatalytic CO2 reduction over Cu3P nanoparticles generated via molecular precursor route
      Downes, Courtney A.; Libretto, Nicole J.; Harman-Ware, Anne E.; Happs, Renee M.; Ruddy, Daniel A.; Baddour, Frederick G.; Ferrell III, Jack R.; Habas, Susan E.; Schaidle, Joshua A.
      ACS Applied Energy Materials (2020), 3 (11), 10435-10446CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      The design of nanoparticles (NPs) with tailored morphologies and finely tuned electronic and phys. properties has become a key strategy for controlling selectivity and improving conversion efficiency in a variety of important electrocatalytic transformations. Transition metal phosphide NPs, in particular, have emerged as a versatile class of catalytic materials due to their multifunctional active sites and compn.- and phase-dependent properties. Access to targeted transition metal phosphide NPs with controlled features is necessary to tune the catalytic activity. To this end, we have established a soln.-synthesis route utilizing a mol. precursor contg. M-P bonds to generate solid metal phosphide NPs with controlled stoichiometry and morphol. We expand here the application of mol. precursors in metal phosphide NP synthesis to include the prepn. of phase-pure Cu3P NPs from the thermal decompn. of [Cu(H)(PPh3)]6. The mechanism of [Cu(H)(PPh3)]6 decompn. and subsequent formation of Cu3P was investigated through modification of the reaction parameters. Identification and optimization of the crit. reaction parameters (i.e., time, temp., and oleylamine concn.) enabled the synthesis of phase-pure 9-11 nm Cu3P NPs. To probe the multifunctionality of this materials system, Cu3P NPs were investigated as an electrocatalyst for CO2 redn. At low overpotential (-0.30 V vs. RHE) in 0.1 M KHCO3 electrolyte, Cu3P-modified carbon paper electrodes produced formate (HCOO-) at a max. Faradaic efficiency of 8%.
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    13. 13
      Laursen, A. B.; Calvinho, K. U. D.; Goetjen, T. A.; Yap, K. M. K.; Hwang, S.; Yang, H.; Garfunkel, E.; Dismukes, G. C. CO2 Electro-Reduction on Cu3P: Role of Cu(I) Oxidation State and Surface Facet Structure in C1-Formate Production and H2 Selectivity. Electrochim. Acta 2021, 391, 138889,  DOI: 10.1016/j.electacta.2021.138889
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      14
      CO2 electro-reduction on Cu3P: Role of Cu(I) oxidation state and surface facet structure in C1-formate production and H2 selectivity
      Laursen, Anders B.; Calvinho, Karin U. D.; Goetjen, Timothy A.; Yap, Kyra M. K.; Hwang, Shinjae; Yang, Hongbin; Garfunkel, Eric; Dismukes, G. Charles
      Electrochimica Acta (2021), 391 (), 138889CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      We report the catalytic activity and mechanism of copper(I) phosphide, Cu3P, with predominant [001] facet exposure for the electrochem. redn. of CO2 (CO2RR) to formic acid. Cryst. nanosheets of this compd. that show a preferential [001] facet orientation exhibit undiminished CO2RR activity after 16 h with full retention of crystal structure and surface chem. speciation and no detectable corrosion. In contrast to the range of products formed on Cu metal, CuO, and Cu2O, the CO2RR on Cu3P [001] produces mainly hydrogen and formate as the sole carbon product in KHCO3 electrolyte. Anal. of the Cu3P [001] facet by HAADF-STEM was used to det. the surface lattice structure, while both XPS and Auger spectroscopies were used to det. the surface chem. speciation from the kinetic energies of ionized electrons. The presented anal. identifies isolated trigonal CuP3 sites on the Cu3P[001]-Cu3P3 terminated surface and the Cu(I) oxidn. state as precursor to the active catalyst. The CO2RR selectivity to formate and the higher turnover rate for H2 prodn. on the [001] facet allows a structure-activity anal. and chem. mechanism to be proposed. Formation of a surface hydride at isolated *H-CuP3 sites is proposed as the catalytic site in forming both H2 and formate, while the long Cu-Cu sepn. retards forming C-C coupling products. These results disprove previously published claims of Cu(I) oxidn. state as a sufficient criterion to promote CO2RR to C2+ products, show that stronger bonded hydrides, *H-CuP3 on Cu3P, favor prodn. of the C1 product formate over all other carbon products, and predict that stronger formate binding (bidentate) is needed for CO2RR currents to compete with H2 prodn.
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    14. 14
      Calvinho, K. U. D.; Alherz, A. W.; Yap, K. M. K.; Laursen, A. B.; Hwang, S.; Bare, Z. J. L.; Clifford, Z.; Musgrave, C. B.; Dismukes, G. C. Surface Hydrides on Fe2P Electrocatalyst Reduce at Low Overpotential: Steering Selectivity to Ethylene Glycol. J. Am. Chem. Soc. 2021, 143, 21275,  DOI: 10.1021/jacs.1c03428
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      15
      Surface Hydrides on Fe2P Electrocatalyst Reduce CO2 at Low Overpotential: Steering Selectivity to Ethylene Glycol
      Calvinho, Karin U. D.; Alherz, Abdulaziz W.; Yap, Kyra M. K.; Laursen, Anders B.; Hwang, Shinjae; Bare, Zachary J. L.; Clifford, Zachary; Musgrave, Charles B.; Dismukes, G. Charles
      Journal of the American Chemical Society (2021), 143 (50), 21275-21285CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Development of efficient electrocatalysts for the CO2 redn. reaction (CO2RR) to multicarbon products was constrained by high overpotentials and poor selectivity. Here, the authors introduce Fe phosphide (Fe2P) as an earth-abundant catalyst for the CO2RR to mainly C2-C4 products with a total CO2RR faradaic efficiency of 53% at 0 V vs. RHE. C product selectivity is tuned in favor of ethylene glycol formation with increasing neg. bias at the expense of C3-C4 products. Both Grand Canonical-DFT (GC-DFT) calcns. and expts. reveal that *formate, not *CO, is the initial intermediate formed from surface phosphino-hydrides and that the latter form ionic hydrides at both surface P atoms (H@Ps) and P-reconstructed Fe3 hollow sites (H@P*). Binding of these surface hydrides weakens with neg. bias (reactivity increases), which accounts for both the shift to C2 products over higher C-C coupling products and the increase in the H2 evolution reaction (HER) rate. GC-DFT predicts that phosphino-hydrides convert *formate to *formaldehyde, the key intermediate for C-C coupling, whereas H atoms on Fe generate tightly bound *CO via sequential PCET reactions to H2O. GC-DFT predicts the peak in CO2RR c.d. near -0.1 V is due to a local max. in the binding affinity of *formate and *formaldehyde at this bias, which together with the more labile C2 product affinity, accounts for the shift to ethylene glycol and away from C3-C4 products. Consistent with these predictions, addn. of exogenous CO is shown to block all C product formation and lower the HER rate. The formation of ionic hydrides and their binding affinity, as modulated by the applied potential, controls the C product distribution. This knowledge provides new insight into the influence of hydride speciation and applied bias on the chem. reaction mechanism of CO2RR that is relevant to all transition metal phosphides.
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    15. 15
      Kucernak, A. R. J.; Sundaram, V. N. N. Nickel Phosphide: The Effect of Phosphorus Content on Hydrogen Evolution Activity and Corrosion Resistance in Acidic Medium. J. Mater. Chem. A 2014, 2, 17435– 17445,  DOI: 10.1039/c4ta03468f
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      16
      Nickel phosphide: the effect of phosphorus content on hydrogen evolution activity and corrosion resistance in acidic medium
      Kucernak, Anthony R. J.; Naranammalpuram Sundaram, Venkata N.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (41), 17435-17445CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Transition metal phosphides possess novel, structural, phys. and chem. properties and are an emerging new class of materials for various catalytic applications. Electroplated or electrolessly plated Ni phosphide alloy materials with achievable P contents <15 at.% P are known to be more corrosion resistant than Ni alone, and were studied as H evolution catalysts in alk. environments. However, there is significant interest in developing new inexpensive catalysts for solid polymer electrolyte electrolyzers which require acid stable catalysts. By increasing the P content beyond the limit available using electroplating techniques (∼12 at.% P), the Ni based phosphides Ni12P5 and Ni2P with higher levels of P (29 and 33 at.% P) may be used for the H evolution reaction (HER) in acidic medium. Corrosion resistance in acid is directly correlated with P content - those materials with higher P content are more corrosion resistant. H evolution activity in acid is also correlated with P content - Ni2P based catalysts appear to be more active for the H evolution reaction than Ni12P5. Electrochem. kinetic studies of the HER reveal high exchange current densities and little deviation in the Tafel slope esp. in the lower overpotential regime for these Ni phosphide catalysts. The electrochem. impedance spectroscopy response of the resp. system in acidic medium reveals two time consts. assocd. with the HER.
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    16. 16
      Pan, Y.; Liu, Y.; Zhao, J.; Yang, K.; Liang, J.; Liu, D.; Hu, W.; Liu, D.; Liu, Y.; Liu, C. Monodispersed Nickel Phosphide Nanocrystals with Different Phases: Synthesis, Characterization and Electrocatalytic Properties for Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 1656– 1665,  DOI: 10.1039/c4ta04867a
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      17
      Monodispersed nickel phosphide nanocrystals with different phases: synthesis, characterization and electrocatalytic properties for hydrogen evolution
      Pan, Yuan; Liu, Yanru; Zhao, Jinchong; Yang, Kang; Liang, Jilei; Liu, Dandan; Hu, Wenhui; Liu, Dapeng; Liu, Yunqi; Liu, Chenguang
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (4), 1656-1665CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Monodispersed nickel phosphide nanocrystals (NCs) with different phases (Ni12P5, Ni2P and Ni5P4) were prepd. via the thermal decompn. approach using nickel acetylacetonate as the nickel source, trioctylphosphine as the phosphorus source and oleylamine in 1-octadecene as the reductant. The phases of the as-prepd. nickel phosphide NCs could easily be controlled by changing the P:Ni precursor ratio. The structure and morphol. of the nickel phosphide NCs were characterized by x-ray diffraction (XRD), TEM, energy dispersive x-ray anal. (EDX), XPS, FTIR, and N2 adsorption-desorption. A formation mechanism for the nickel phosphide NCs is proposed. The influence of the phase of the nickel phosphide NCs on the electrocatalytic properties for the hydrogen evolution reaction (HER) was studied. All phases showed good catalytic properties, and the Ni5P4 NCs with a solid structure exhibited higher catalytic activity than the Ni12P5 and Ni2P NCs. This superior catalytic activity is attributed to the higher pos. charge of Ni and a stronger ensemble effect of P in Ni5P4 NCs. The cryst. phase is important for affecting the electrocatalytic properties.
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    17. 17
      Laursen, A. B.; Patraju, K. R.; Whitaker, M. J.; Retuerto, M.; Sarkar, T.; Yao, N.; Ramanujachary, K. V.; Greenblatt, M.; Dismukes, G. C. Nanocrystalline Ni5P4: A Hydrogen Evolution Electrocatalyst of Exceptional Efficiency in Both Alkaline and Acidic Media. Energy Environ. Sci. 2015, 8, 1027– 1034,  DOI: 10.1039/c4ee02940b
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      18
      Nanocrystalline Ni5P4: a hydrogen evolution electrocatalyst of exceptional efficiency in both alkaline and acidic media
      Laursen, A. B.; Patraju, K. R.; Whitaker, M. J.; Retuerto, M.; Sarkar, T.; Yao, N.; Ramanujachary, K. V.; Greenblatt, M.; Dismukes, G. C.
      Energy & Environmental Science (2015), 8 (3), 1027-1034CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Producing hydrogen (H2) by splitting water with fossil-free electricity is considered a grand challenge for developing sustainable energy systems and a carbon dioxide free source of renewable H2. Renewable H2 may be produced from water by electrolysis with either low efficiency alk. electrolyzers that suffer 50-65% losses, or by more efficient acidic electrolyzers with rare platinum group metal catalysts (Pt). Consequently, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements. Here, we show that cryst. Ni5P4 evolves H2 with geometric elec. to chem. conversion efficiency on par with Pt in strong acid (33 mV dec-1 Tafel slope and -62 mV overpotential at -100 mA cm-2 in 1 M H2SO4). The cond. of Ni5P4 microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degrdn. is seen in short term studies at current densities of -10 mA cm-2, equiv. to ∼10% solar photoelec. conversion efficiency. The realization of a noble metal-free catalyst performing on par with Pt in both strong acid and base offers a key step towards industrially relevant electrolyzers competing with conventional H2 sources.
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    18. 18
      Callejas, J. F.; Read, C. G.; Popczun, E. J.; McEnaney, J. M.; Schaak, R. E. Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP. Chem. Mater. 2015, 27, 3769– 3774,  DOI: 10.1021/acs.chemmater.5b01284
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      19
      Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP
      Callejas, Juan F.; Read, Carlos G.; Popczun, Eric J.; McEnaney, Joshua M.; Schaak, Raymond E.
      Chemistry of Materials (2015), 27 (10), 3769-3774CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Metal phosphides have emerged as promising Earth-abundant alternatives to platinum for catalyzing the hydrogen evolution reaction (HER) in acidic aq. solns. Here, Co2P nanoparticles having a hollow, multifaceted, cryst. morphol. were evaluated as HER electrocatalysts at a mass loading of 1 mg cm-2 on Ti foil substrates. The Co2P/Ti electrodes required low overpotentials of -95 and -109 mV to produce operationally relevant cathodic current densities of -10 and -20 mA cm-2, resp. These values establish Co2P nanoparticles as highly active Earth-abundant HER catalyst materials. Importantly, the Co2P nanoparticles are morphol. equiv. to previously reported CoP nanoparticle HER catalysts, allowing a direct side-by-side evaluation of their HER activities. Such comparisons of different metal phosphide HER catalysts with the same constituent elements and morphologies are important for identifying the key materials characteristics that lead to high activity.
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    19. 19
      Pan, Y.; Lin, Y.; Chen, Y.; Liu, Y.; Liu, C. Cobalt Phosphide-Based Electrocatalysts: Synthesis and Phase Catalytic Activity Comparison for Hydrogen Evolution. J. Mater. Chem. A 2016, 4, 4745– 4754,  DOI: 10.1039/c6ta00575f
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      20
      Cobalt phosphide-based electrocatalysts: synthesis and phase catalytic activity comparison for hydrogen evolution
      Pan, Yuan; Lin, Yan; Chen, Yinjuan; Liu, Yunqi; Liu, Chenguang
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (13), 4745-4754CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Cobalt phosphides have been used as promising electrocatalysts for catalyzing the hydrogen evolution reaction (HER) in acidic aq. solns. In order to further explore the influence of phase structure and support effect on the catalytic activity for HER, herein, a series of cobalt phosphide-based electrocatalysts, including Co2P, CoP, Co2P/CNTs, CoP/CNTs, Co2P/NCNTs and CoP/NCNTs (NCNTs are nitrogen-doped carbon nanotubes), were synthesized successfully via a facile thermal decompn. approach. The cryst. phase can be controlled by changing the phosphide source species. When the phosphide source was trioctylphosphine, CoP-based catalysts were obtained. However, Co2P-based catalysts can be obtained by using triphenylphosphine as the phosphide source. Then the phase catalytic activity and stability of the as-synthesized cobalt phosphide-based catalysts for hydrogen evolution were compared. The results show that the catalytic activity followed the order CoP/NCNTs > Co2P/NCNTs > CoP/CNTs > Co2P/CNTs > CoP > Co2P, which can be attributed to the different at. ratios of Co to P, the strong interaction between cobalt phosphide and carbon species and the doping of N atoms into CNTs. Our studies indicate that the HER catalytic efficiency of transition metal phosphide catalysts can be improved significantly by adjusting active phase and carbon species structures.
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    20. 20
      Seo, B.; Baek, D. S.; Sa, Y. J.; Joo, S. H. Shape Effects of Nickel Phosphide Nanocrystals on Hydrogen Evolution Reaction. CrystEngComm 2016, 18, 6083– 6089,  DOI: 10.1039/c6ce00985a
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      21
      Shape effects of nickel phosphide nanocrystals on hydrogen evolution reaction
      Seo, Bora; Baek, Du San; Sa, Young Jin; Joo, Sang Hoon
      CrystEngComm (2016), 18 (32), 6083-6089CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
      The prepn. of size- and shape-controlled nanoparticles has enabled the understanding of important nanoscale catalytic phenomena, resulting in the design of advanced catalysts with enhanced activities and selectivities. Metal phosphides have recently emerged as a promising class of non-precious metal catalysts for hydrogen evolution reaction (HER), which is a cornerstone in clean and environmentally benign hydrogen prodn. Although significant progress has been made in metal phosphide catalysts, the impact of the metal phosphide shape has not yet been explored. Herein, we investigated the shape-dependent electrocatalytic activity of nickel phosphide nanoparticles (Ni2P NPs) for the HER. Spherical Ni2P NPs mainly composed of the Ni2P(001) surface showed higher HER activity than rod-shaped Ni2P NPs with the Ni2P(210) surface in terms of overpotential, Tafel slope, and turnover frequency. The results imply that the Ni2P(001) surface would have preferential interactions with the adsorbent and a lower activation barrier for hydrogen adsorption, promoting the overall rate of HER. This study highlights the importance of morphol. control in electrocatalysts to boost catalytic performances.
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    21. 21
      Mutinda, S. I.; Li, D.; Kay, J.; Brock, S. L. Synthesis and Characterization of Co2–xRhxP Nanoparticles and Their Catalytic Activity Towards the Oxygen Evolution Reaction. J. Mater. Chem. A 2018, 6, 12142– 12152,  DOI: 10.1039/c8ta02016g
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      22
      Synthesis and characterization of Co2-xRhxP nanoparticles and their catalytic activity towards the oxygen evolution reaction
      Mutinda, Samuel I.; Li, Da; Kay, Jacob; Brock, Stephanie L.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (25), 12142-12152CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      We show that Co2-xRhxP ternary phase nanoparticles can be synthesized using the arrested pptn. technique, via a requisite low temp. co-redn. of Co2+ and Rh3+ ions, followed by phosphidation of the resulting intermediate product at above 300 °C. The most unique aspect of our synthesis protocol is the fact that we are able to solubilize both cubic antifluorite and orthorhombic crystal systems, into uniform solid solns. of Co2-xRhxP nanoparticles under mild reaction conditions. These nanoparticles are active towards the oxygen evolution reaction and deliver higher electrolytic activities as compared to those of Co2P and Rh2P binary phases. Cobalt-rich compns. were found to be the most active, with a sweet spot occurring at a target compn. of Co1.75Rh0.25P, where a c.d. of 10 mA cm-2 was achieved at an overpotential of 0.29 V.
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    22. 22
      Liyanage, D. R.; Li, D.; Cheek, Q. B.; Baydoun, H.; Brock, S. L. Synthesis and Oxygen Evolution Reaction (OER) Catalytic Performance of Ni2–xRuxP Nanocrystals: Enhancing Activity by Dilution of the Noble Metal. J. Mater. Chem. A 2017, 5, 17609– 17618,  DOI: 10.1039/c7ta05353c
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      23
      Synthesis and oxygen evolution reaction (OER) catalytic performance of Ni2-xRuxP nanocrystals: enhancing activity by dilution of the noble metal
      Liyanage, D. Ruchira; Li, Da; Cheek, Quintin B.; Baydoun, Habib; Brock, Stephanie L.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (33), 17609-17618CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Aiming to create an efficient, less-expensive catalyst for the oxygen evolution reaction (OER), a synthetic protocol is developed to prep. ternary metal phosphide nanoparticles, Ni2-xRuxP, incorporating Ru, a traditional catalyst for OER, and Ni, a highly active but inexpensive metal. Using soln.-phase arrested-pptn. reactions, cryst. Ni2-xRuxP particles could be realized for compns. up to x ≤ 1, whereas more Ru-rich compns., including Ru2P, were amorphous. For x ≤ 1, particles are spherical, of sizes that vary between 5 and 10 nm in diam. (with a clear decreasing trend as the Ru amt. is increased), and samples exhibit narrow size distributions (polydispersity < 15%). In contrast, amorphous Ru-rich phases exhibit worm-like morphologies. ICP-MS data indicate the actual metal ratio closely follows the target ratio employed in the synthesis. OER electrocatalytic activity was evaluated for selected compns. over the entire synthesis range (0 ≤ x ≤ 2). Intriguingly, Ru2P proved to be the least active phase (overpotential of 0.56 V at 10 mA cm-2 in 1.0 M KOH) with the best performance obsd. for the bimetallic Ni1.25Ru0.75P phase (overpotential of 0.34 V). The augmented activity at x = 0.75 is attributed, at least in part, to electronic activation of Ni by Ru, facilitating Ni oxidn. and thus decreasing the kinetic barrier for OER.
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    23. 23
      Liu, J.; Wang, Z.; David, J.; Llorca, J.; Li, J.; Yu, X.; Shavel, A.; Arbiol, J.; Meyns, M.; Cabot, A. Colloidal Ni2–xCoxP Nanocrystals for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2018, 6, 11453– 11462,  DOI: 10.1039/c8ta03485k
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      24
      Colloidal Ni2-xCoxP nanocrystals for the hydrogen evolution reaction
      Liu, Junfeng; Wang, Zhenxing; David, Jeremy; Llorca, Jordi; Li, Junshan; Yu, Xiaoting; Shavel, Alexey; Arbiol, Jordi; Meyns, Michaela; Cabot, Andreu
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (24), 11453-11462CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A cost-effective and scalable approach was developed to produce monodisperse Ni2-xCoxP nanocrystals (NCs) with compn. tuned over the entire range (0 ≤ x ≤ 2). Ni2-xCoxP NCs were synthesized using low-cost, stable and low-toxicity tri-Ph phosphite (TPP) as a phosphorus source, metal chlorides as metal precursors and hexadecylamine (HDA) as a ligand. The synthesis involved the nucleation of amorphous Ni-P and its posterior crystn. and simultaneous incorporation of Co. The compn., size and morphol. of the Ni2-xCoxP NCs could be controlled simply by varying the ratio of Ni and Co precursors and the amts. of TPP and HDA. Ternary Ni2-xCoxP-based electrocatalysts exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) compared to binary phosphides. In particular, NiCoP electrocatalysts displayed the lowest overpotential of 97 mV at J = 10 mA cm-2 and an excellent long-term stability. DFT calcns. of the Gibbs free energy for hydrogen adsorption at the surface of Ni2-xCoxP NCs showed NiCoP to have the most appropriate compn. to optimize this parameter within the whole Ni2-xCoxP series. However, the hydrogen adsorption energy was demonstrated not to be the only parameter controlling the HER activity in Ni2-xCoxP.
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      Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Nørskov, J. K.; Abild-Pedersen, F.; Jaramillo, T. F. Designing an Improved Transition Metal Phosphide Catalyst for Hydrogen Evolution Using Experimental and Theoretical Trends. Energy Environ. Sci. 2015, 8, 3022– 3029,  DOI: 10.1039/c5ee02179k
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      Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends
      Kibsgaard, Jakob; Tsai, Charlie; Chan, Karen; Benck, Jesse D.; Noerskov, Jens K.; Abild-Pedersen, Frank; Jaramillo, Thomas F.
      Energy & Environmental Science (2015), 8 (10), 3022-3029CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Transition metal phosphides (TMPs) have emerged as highly active catalysts for the hydrogen evolution reaction (HER). However, insights into the trends and limitations in their activity are lacking, and there are presently no guidelines for systematically improving their intrinsic activity. The complexity and variations in their structures further pose challenges in theor. estg. their activity. Herein, we demonstrate a combined exptl.-theor. approach: by synthesizing different TMPs and comparing exptl. detd. HER activities with the hydrogen adsorption free energies, ΔGH, calcd. by d. functional theory, we det. the level of detail needed in the simulations to bring out useful trends in the exptl. data. In particular, we show that the TMPs follow the HER volcano relationship. Using our combined exptl.-theor. model, we predict that the mixed metal TMP, Fe0.5Co0.5P, should have a near-optimal ΔGH. We synthesized several mixts. of Co and Fe phosphides alloys and confirmed that Fe0.5Co0.5P exhibits the highest HER activity of the investigated TMPs. Furthermore, our results suggest that there could be inherent limitations in the intrinsic HER activity of TMPs that prevent them from performing as well as Pt-group metals. Our work demonstrates that it is possible to generate and verify a model of activity trends with predictive capabilities even for new transition metal compds. with varied structures and surface terminations. The identification of an improved mixed metal TMP based on theor. predictions and subsequent synthesis and testing demonstrates the need for an approach that combines theory and expt. to understand and ultimately design advanced catalysts.
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    25. 25
      Tang, C.; Gan, L.; Zhang, R.; Lu, W.; Jiang, X.; Asiri, A. M.; Sun, X.; Wang, J.; Chen, L. Ternary FexCo1–xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-Like Activity: Experimental and Theoretical Insight. Nano Lett. 2016, 16, 6617– 6621,  DOI: 10.1021/acs.nanolett.6b03332
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      26
      Ternary FexCo1-xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-like Activity: Experimental and Theoretical Insight
      Tang, Chun; Gan, Linfeng; Zhang, Rong; Lu, Wenbo; Jiang, Xiue; Asiri, Abdullah M.; Sun, Xuping; Wang, Jin; Chen, Liang
      Nano Letters (2016), 16 (10), 6617-6621CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Replacement of precious Pt with earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) holds great promise for clean energy devices, but the development of low-cost and durable HER catalysts with Pt-like activity is still a huge challenge. In this communication, we report on the development of self-standing ternary FexCo1-xP nanowire array on carbon cloth (FexCo1-xP/CC) as a Pt-free HER catalyst with activities being strongly related to Fe substitution ratio. Electrochem. tests show that Fe0.5Co0.5P/CC not only possesses Pt-like activity with the need of overpotential of only 37 mV to drive 10 mA cm-2, outperforming all non-noble-metal HER catalysts reported to date, but demonstrates superior long-term durability in 0.5 M H2SO4. D. functional theory calcns. further reveal that Fe substitution of Co in CoP leads to more optimal free energy of hydrogen adsorption to the catalyst surface. This study offers us a promising flexible monolithic catalyst for practical applications.
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      Tan, Y.; Wang, H.; Liu, P.; Shen, Y.; Cheng, C.; Hirata, A.; Fujita, T.; Tang, Z.; Chen, M. Versatile Nanoporous Bimetallic Phosphides Towards Electrochemical Water Splitting. Energy Environ. Sci. 2016, 9, 2257– 2261,  DOI: 10.1039/c6ee01109h
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      Versatile nanoporous bimetallic phosphides towards electrochemical water splitting
      Tan, Yongwen; Wang, Hao; Liu, Pan; Shen, Yuhao; Cheng, Chun; Hirata, Akihiko; Fujita, Takeshi; Tang, Zheng; Chen, Mingwei
      Energy & Environmental Science (2016), 9 (7), 2257-2261CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Alloying is an important approach to improving catalytic activities and realizing new functions of heterogeneous catalysts, which has extensively been employed in fabricating noble metal based bimetallic catalysts. However, it is tech. unviable in the synthesis of alloyed transition metal compds., which are emerging as important catalysts for water splitting, in a controllable manner using conventional wet chem. methods. Nanoporous bimetallic (Co1-xFex)2P phosphides with controllable compns. and tuneable porosity, which are fabricated by the combination of metallurgical alloy design and electrochem. etching has been reported. By tailoring the Co/Fe ratios and nanoporosity, the bimetallic phosphides exhibit versatile catalytic activities towards HER and OER in acidic and basic electrolytes. As both the cathode and the anode of an electrolyzer, nanoporous (Co0.52Fe0.48)2P shows an outstanding performance in water electrolysis, comparable to the com. electrolyzer with paired Pt/C and IrO2 catalysts.
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      Brock, S. L.; Perera, S. C.; Stamm, K. L. Chemical Routes for Production of Transition-Metal Phosphides on the Nanoscale: Implications for Advanced Magnetic and Catalytic Materials. Chem.─Eur. J. 2004, 10, 3364– 3371,  DOI: 10.1002/chem.200305775
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      Chemical routes for production of transition-metal phosphides on the nanoscale: Implications for advanced magnetic and catalytic materials
      Brock, Stephanie L.; Perera, Susanthri C.; Stamm, Kimber L.
      Chemistry - A European Journal (2004), 10 (14), 3364-3371CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Nanoparticulate transition-metal phosphides remain an unexplored, though emerging area of interest on the materials landscape, due principally to their promising magnetic and catalytic properties. This review describes synthetic strategies for the formation of both supported and unsupported transition-metal phosphide nanoparticles, provides a summary of their relevant magnetic and catalytic properties, and indicates new directions for exploration.
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      Henkes, A. E.; Vasquez, Y.; Schaak, R. E. Converting Metals into Phosphides: A General Strategy for the Synthesis of Metal Phosphide Nanocrystals. J. Am. Chem. Soc. 2007, 129, 1896– 1897,  DOI: 10.1021/ja068502l
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      30
      Converting Metals into Phosphides: A General Strategy for the Synthesis of Metal Phosphide Nanocrystals
      Henkes, Amanda E.; Vasquez, Yolanda; Schaak, Raymond E.
      Journal of the American Chemical Society (2007), 129 (7), 1896-1897CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Nanocrystals of metal phosphides, which can useful catalytic, electronic, and magnetic properties, are known to be accessible by using trioctylphosphine (TOP) as a highly reactive P source. Here the authors report a general strategy for synthesizing transition metal phosphides, including those with 4d and 5d transition metals that have not previously been reported as unsupported nanocrystals. Unlike previously reported methods that involve direct decompn. of organometallic precursors, the authors' method uses preformed metal nanoparticles as templates for generating metal phosphide nanocrystals. Metal nanoparticles are reacted with TOP in a hot solvent (290-360°) to form transition metal phosphides such as Ni2P, PtP2, Rh2P, PdP2, Pd5P2, and Au2P3. Also, nanostructures such as hollow spheres can be easily made using a Kirkendall-type mechanism, which uses metal nanoparticles as reactive templates.
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    29. 29
      Carenco, S.; Hu, Y.; Florea, I.; Ersen, O.; Boissière, C.; Mézailles, N.; Sanchez, C. Metal-Dependent Interplay between Crystallization and Phosphorus Diffusion During the Synthesis of Metal Phosphide Nanoparticles. Chem. Mater. 2012, 24, 4134– 4145,  DOI: 10.1021/cm3022243
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      31
      Metal-Dependent Interplay between Crystallization and Phosphorus Diffusion during the Synthesis of Metal Phosphide Nanoparticles
      Carenco, S.; Hu, Y.; Florea, I.; Ersen, O.; Boissiere, C.; Mezailles, N.; Sanchez, C.
      Chemistry of Materials (2012), 24 (21), 4134-4145CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The interplay between crystn. and phosphorus diffusion in the versatile synthesis of metal phosphide nanoparticles from well-defined metal nanoparticles is studied by using a favorable P(0) source for mechanistic studies: white phosphorus. In this study, the reaction of Ni, Fe, Pd, and Cu nanoparticles with P4 was quant. even at relatively low temps. thanks to the high reactivity of this sol. P source. Intermediate amorphous alloys could be identified for the first time in the case of Fe and Pd, while the quant. character of the reaction provided a selective and controlled access to Pd5P4 vs. PdP2 and Cu3P vs. CuP2. Morphol. evolution of the nanoparticles with temp. and M/P stoichiometry was also discussed and provided new insights in the kinetics of the reaction in each case. Hollow Ni2P and FeP nanoparticles were finally obtained while the particularly high stability of the amorphous plain Pd3P nanoparticles was uncovered.
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    30. 30
      Mundy, M. E.; Ung, D.; Lai, N. L.; Jahrman, E. P.; Seidler, G. T.; Cossairt, B. M. Aminophosphines as Versatile Precursors for the Synthesis of Metal Phosphide Nanocrystals. Chem. Mater. 2018, 30, 5373– 5379,  DOI: 10.1021/acs.chemmater.8b02206
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      32
      Aminophosphines as Versatile Precursors for the Synthesis of Metal Phosphide Nanocrystals
      Mundy, M. Elizabeth; Ung, David; Lai, Nathan L.; Jahrman, Evan P.; Seidler, Gerald T.; Cossairt, Brandi M.
      Chemistry of Materials (2018), 30 (15), 5373-5379CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      We have broadened the scope of the aminophosphine precursor chem. that has been developed for InP quantum dots to the synthesis of cadmium, zinc, cobalt, and nickel phosphide nanocrystals. The generalized synthetic conditions involve thermolysis of the appropriate MX2 salt with tris-diethylaminophosphine in a long chain primary amine. The resulting Cd3P2 nanocrystals exhibit size tuning effects based on the metal halide reactivity. 31P NMR studies show that the II-V materials form via the previously described mechanism obsd. for InP, demonstrating the invariance of this chem. to the metal valence. We also demonstrate that electrocatalytically active transition metal phosphides, specifically Co2P, CoP, and Ni2P, can be produced using this synthetic method at relatively mild temps. and in high yields.
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      Tappan, B. A.; Chen, K.; Lu, H.; Sharada, S. M.; Brutchey, R. L. Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu3–xP Nanocrystals. ACS Appl. Mater. Interfaces 2020, 12, 16394– 16401,  DOI: 10.1021/acsami.0c00025
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      Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu3-xP Nanocrystals
      Tappan, Bryce A.; Chen, Keying; Lu, Haipeng; Sharada, Shaama Mallikarjun; Brutchey, Richard L.
      ACS Applied Materials & Interfaces (2020), 12 (14), 16394-16401CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      N-heterocyclic carbenes (NHCs) are an important class of ligands capable of making strong carbon-metal bonds. Recently, there has been a growing interest in the study of carbene-ligated nanocrystals, primarily coinage metal nanocrystals, which have found application as catalysts for numerous reactions. The general ability of NHC ligands to pos. affect the catalytic properties of other types of nanocrystal catalysts remains unknown. Herein, we present the first carbene-stabilized Cu3-xP nanocrystals. Inquiries into the mechanism of formation of NHC-ligated Cu3-xP nanocrystals suggest that cryst. Cu3-xP forms directly as a result of a high-temp. metathesis reaction between a tris(trimethylsilyl)phosphine precursor and an NHC-CuBr precursor, the latter of which behaves as a source of both the carbene ligand and Cu+. To study the effect of the NHC surface ligands on the catalytic performance, we tested the electrocatalytic hydrogen evolving ability of the NHC-ligated Cu3-xP nanocrystals and found that they possess superior activity to analogous oleylamine-ligated Cu3-xP nanocrystals. D. functional theory calcns. suggest that the NHC ligands minimize unfavorable electrostatic interactions between the copper phosphide surface and H+ during the first step of the hydrogen evolution reaction, which contributes to the superior performance of NHC-ligated Cu3-xP catalysts as compared to oleylamine-ligated Cu3-xP catalysts.
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      Sun, M.; Liu, H.; Qu, J.; Li, J. Earth-Rich Transition Metal Phosphide for Energy Conversion and Storage. Adv. Energy Mater. 2016, 6, 1600087,  DOI: 10.1002/aenm.201600087
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      Li, S.-H.; Qi, M.-Y.; Tang, Z.-R.; Xu, Y.-J. Nanostructured Metal Phosphides: From Controllable Synthesis to Sustainable Catalysis. Chem. Soc. Rev. 2021, 50, 7539– 7586,  DOI: 10.1039/d1cs00323b
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      Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis
      Li, Shao-Hai; Qi, Ming-Yu; Tang, Zi-Rong; Xu, Yi-Jun
      Chemical Society Reviews (2021), 50 (13), 7539-7586CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Metal phosphides (MPs) with unique and desirable physicochem. properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, esp. for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Addnl., different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.
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      Habas, S. E.; Baddour, F. G.; Ruddy, D. A.; Nash, C. P.; Wang, J.; Pan, M.; Hensley, J. E.; Schaidle, J. A. A Facile Molecular Precursor Route to Metal Phosphide Nanoparticles and Their Evaluation as Hydrodeoxygenation Catalysts. Chem. Mater. 2015, 27, 7580– 7592,  DOI: 10.1021/acs.chemmater.5b02140
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      A Facile Molecular Precursor Route to Metal Phosphide Nanoparticles and Their Evaluation as Hydrodeoxygenation Catalysts
      Habas, Susan E.; Baddour, Frederick G.; Ruddy, Daniel A.; Nash, Connor P.; Wang, Jun; Pan, Ming; Hensley, Jesse E.; Schaidle, Joshua A.
      Chemistry of Materials (2015), 27 (22), 7580-7592CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Metal phosphides have been identified as a promising class of materials for the catalytic upgrading of bio-oils, which are renewable and potentially inexpensive sources for liq. fuels. Herein, we report the facile synthesis of a series of solid, phase-pure metal phosphide nanoparticles (NPs) (Ni2P, Rh2P, and Pd3P) utilizing com. available, air-stable metal-phosphine complexes in a one-pot reaction. This single-source mol. precursor route provides an alternative method to access metal phosphide NPs with controlled phases and without the formation of metal NP intermediates that can lead to hollow particles. The formation of the Ni2P NPs was shown to proceed through an amorphous Ni-P intermediate, leading to the desired NP morphol. and metal-rich phase. This low-temp., rapid route to well-defined metal NPs is expected to have broad applicability to a variety of readily available or easily synthesized metal-phosphine complexes with high decompn. temps. Hydrodeoxygenation of acetic acid, an abundant bio-oil component, was performed to investigate H2 activation and deoxygenation pathways under conditions that are relevant to ex situ catalytic fast pyrolysis (high temps., low pressures, and near-stoichiometric H2 concns.). The catalytic performance of the silica-supported metal phosphide NPs was compared to the analogous incipient wetness (IW) metal and metal phosphide catalysts over the range 200-500 °C. Decarbonylation was the primary pathway for H2 incorporation in the presence of all of the catalysts except NP-Pd3P, which exhibited minimal productive activity, and IW-Ni, which evolved H2. The highly controlled NP-Ni2P and NP-Rh2P catalysts, which were stable under these conditions, behaved comparably to the IW-metal phosphides, with a slight shift to higher product onset temps., likely due to the presence of surface ligands. Most importantly, the NP-Ni2P catalyst exhibited H2 activation and incorporation, in contrast to IW-Ni, indicating that the behavior of the metal phosphide is significantly different from that of the parent metal, and more closely resembles that of noble metal catalysts.
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    35. 35
      Mutinda, S. I.; Batugedara, T. N.; Brown, B.; Adeniran, O.; Liu, Z.-F.; Brock, S. L. Rh2P Activity at a Fraction of the Cost? Co2–xRhxP Nanoparticles as Electrocatalysts for the Hydrogen Evolution Reaction in Acidic Media. ACS Appl. Energy Mater. 2021, 4, 946– 955,  DOI: 10.1021/acsaem.0c02880
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      37
      Rh2P Activity at a Fraction of the Cost? Co2-xRhxP Nanoparticles as Electrocatalysts for the Hydrogen Evolution Reaction in Acidic Media
      Mutinda, Samuel I.; Batugedara, Tharanga N.; Brown, Benjamin; Adeniran, Olugbenga; Liu, Zhen-Fei; Brock, Stephanie L.
      ACS Applied Energy Materials (2021), 4 (1), 946-955CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      The compn.-dependent hydrogen evolution reaction (HER) activity of Co2-xRhxP nanoparticles in acid is reported. The motivation for the current study stems from (1) prior work demonstrating that, although costly, Rh2P nanoparticles are highly active and stable toward the HER process and (2) the expectation that dilg. Rh sites with Co will result in catalytic synergism while also lowering the overall cost of the material. Here, we establish that the HER activity of Co2-xRhxP nanoparticles in acidic media is compn.-dependent, with Rh-rich electrocatalysts showing superior activity as compared to those that are Co-rich. Addnl., compns. of Co2-xRhxP for which x ≥ 1.25, where the materials adopt the cubic antifluorite structure, deliver comparable initial catalytic activities to pure Rh2P, suggesting that the crystal structure of the material may play a more significant role in driving the overall HER activity than the compn. Despite comparable activity to Rh2P, Co2-xRhxP systems do not have the stability assocd. with Rh2P but undergo a drop from 10 to 5 mA/cm2 within the first hour of stability testing, assocd. with Co loss from the surface. In cases where Pt is used as the counter electrode, no such drop in c.d. is obsd., despite the loss of Co, with Pt transfer to the working electrode compensating for the Co depletion. First-principles calcns. based on d. functional theory show that both the hydrogen binding energies and the Gibbs free energies of hydrogen adsorption increase linearly with x, with Co0.75Rh1.25P exhibiting a ΔG value that is closest to zero, suggesting that this compn. is the most active for HER in this series. Double-layer capacitance data, from which electrochem. surface area (ECSA) data for all of the compns. are computed, are used to demonstrate that the quality and quantity of active sites among different compns. of Co2-xRhxP can vary significantly, even when the morphologies and particle sizes are similar.
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    36. 36
      Yoon, K. Y.; Jang, Y.; Park, J.; Hwang, Y.; Koo, B.; Park, J.-G.; Hyeon, T. Synthesis of Uniform-Sized Bimetallic Iron–Nickel Phosphide Nanorods. J. Solid State Chem. 2008, 181, 1609– 1613,  DOI: 10.1016/j.jssc.2008.05.022
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      Synthesis of uniform-sized bimetallic iron-nickel phosphide nanorods
      Yoon, Ki Youl; Jang, Youngjin; Park, Jongnam; Hwang, Yosun; Koo, Bonil; Park, Je-Geun; Hyeon, Taeghwan
      Journal of Solid State Chemistry (2008), 181 (7), 1609-1613CODEN: JSSCBI; ISSN:0022-4596. (Elsevier Inc.)
      We synthesized uniform-sized nanorods of iron-nickel phosphides from the thermal decompn. of metal-phosphine complexes. Uniform-sized (FexNi1-x)2P nanorods (0≤x≤1) of various compns. were synthesized by thermal decompn. of Ni-trioctylphosphine (TOP) complex and Fe-TOP complex. By measuring magnetic properties, we found that blocking temp. and coercive field depend on Ni content in the nanorods. Both parameters were more sensitive to doping compared with bulk samples.
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      Hitihami-Mudiyanselage, A.; Arachchige, M. P.; Seda, T.; Lawes, G.; Brock, S. L. Synthesis and Characterization of Discrete FexNi2–xP Nanocrystals (0 < x < 2): Compositional Effects on Magnetic Properties. Chem. Mater. 2015, 27, 6592– 6600,  DOI: 10.1021/acs.chemmater.5b02149
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      Synthesis and Characterization of Discrete FexNi2-xP Nanocrystals (0 < x < 2): Compositional Effects on Magnetic Properties
      Hitihami-Mudiyanselage, Asha; Arachchige, Maheshika Palihawadana; Seda, Takele; Lawes, Gavin; Brock, Stephanie L.
      Chemistry of Materials (2015), 27 (19), 6592-6600CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Ternary FexNi2-xP (0 < x < 2) phases exhibit a range of useful properties that can be augmented or tuned by confinement to the nanoscale including hydrotreating catalytic activity for small x and near-room temp. ferromagnetism for high x. A soln.-phase arrested-pptn. method was developed for the synthesis of FexNi2-xP over all values of x (0 < x < 2). The synthesis involves prepn. of Ni-P amorphous particles, introduction of the Fe precursor to form amorphous Fe-Ni-P particles, and high-temp. conversion of Fe-Ni-P particles into cryst. ternary phosphide nanocrystals. The ternary FexNi2-xP nanocrystals crystallize in the hexagonal Fe2P-type structure, and the morphol. of the nanocrystals showed a distinct compositional dependence, transitioning from ∼11 nm diam. spheres to rods with aspect ratios approaching 2 as the Fe fraction is increased (x ≥ 1.2). Lattice parameters do not follow Vegard's law, consistent with Mossbauer data showing preferential site occupation by Fe of the tetrahedral over the square pyramidal sites at low Fe concns., and the opposite effect for x > 0.8. Magnetic measurements of FexNi2-xP (x = 1.8, 1.4, and 1.2) nanorods showed a strong compositional dependence of the Curie temp. (TC) that differs from observations in bulk phases, with the highest TC (265 K) obtained for x = 1.4.
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      Liyanage, D. R.; Danforth, S. J.; Liu, Y.; Bussell, M. E.; Brock, S. L. Simultaneous Control of Composition, Size, and Morphology in Discrete Ni2–xCoxP Nanoparticles. Chem. Mater. 2015, 27, 4349– 4357,  DOI: 10.1021/acs.chemmater.5b00958
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      Simultaneous Control of Composition, Size, and Morphology in Discrete Ni2-xCoxP Nanoparticles
      Liyanage, D. Ruchira; Danforth, Samuel J.; Liu, Yi; Bussell, Mark E.; Brock, Stephanie L.
      Chemistry of Materials (2015), 27 (12), 4349-4357CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A synthetic protocol developed to produce phase-pure, nearly monodisperse Ni2-xCoxP nanoparticles (x ≤ 1.7) is described. The Ni2-xCoxP particles vary in size, ranging from 9-14 nm with std. deviations of <20% (based on transmission electron microscopy anal.), and the actual metal ratios obtained from energy-dispersive spectroscopy closely follow the targeted ratios. With increasing Co, samples with larger size distributions are obtained and include particles with voids, attributed to the Kirkendall effect. To probe the mechanism of ternary phosphide particle formation, detailed studies were conducted for Ni:Co = 1:1 as a representative compn. It was revealed that the P:M ratio, heating temp., and heating time have a large impact on the nature of both intermediate and final cryst. particles formed. By tuning these conditions, nanoparticles can be produced with different sizes (from ca. 7-25 nm) and morphologies (hollow vs. dense).
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    39. 39
      Danforth, S. J.; Liyanage, D. R.; Hitihami-Mudiyanselage, A.; Ilic, B.; Brock, S. L.; Bussell, M. E. Probing Hydrodesulfurization over Bimetallic Phosphides Using Monodisperse Ni2-xMxP Nanoparticles Encapsulated in Mesoporous Silica. Surf. Sci. 2016, 648, 126– 135,  DOI: 10.1016/j.susc.2015.10.005
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      Probing hydrodesulfurization over bimetallic phosphides using monodisperse Ni2-xMxP nanoparticles encapsulated in mesoporous silica
      Danforth, Samuel J.; Liyanage, D. Ruchira; Hitihami-Mudiyanselage, Asha; Ilic, Boris; Brock, Stephanie L.; Bussell, Mark E.
      Surface Science (2016), 648 (), 126-135CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      Metal phosphide nanoparticles encapsulated in mesoporous silica provide a well-defined system for probing the fundamental chem. of the hydrodesulfurization (HDS) reaction over this new class of hydrotreating catalysts. To investigate compn. effects in bimetallic phosphides, the HDS of dibenzothiophene (DBT) was carried out over a series of Ni-rich Ni2-xMxP@mSiO2 (M = Co, Fe) nanocatalysts (x ≤ 0.50). The Ni2-xMxP nanoparticles (av. diams.: 11-13 nm) were prepd. by soln.-phase arrested pptn. and encapsulated in mesoporous silica, characterized by a range of techniques (x-ray diffraction, TEM, IR spectroscopy, BET surface area, CO chemisorption) and tested for DBT HDS activity and selectivity. The highest activity was obsd. for a Ni1.92Co0.08P@mSiO2 nanocatalyst, but the overall trend was a decrease in HDS activity with increasing Co or Fe content. In contrast, the highest turnover frequency (TOF) was obsd. for the most Co- and Fe-rich compns. based on sites titrated by CO chemisorption. IR spectral studies of adsorbed CO on the Ni2-xMxP@mSiO2 catalysts indicate that an increase in electron d. occurs on Ni sites as the Co or Fe content is increased, which may be responsible for the increased TOFs of the catalytic sites. The Ni2-xMxP@mSiO2 nanocatalysts exhibit a strong preference for the direct desulfurization pathway (DDS) for DBT HDS that changes only slightly with increasing Co or Fe content.
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      Hegedus, L. S.; Perry, R. J. Phosphinecarbonylnitrosylacylcobaltate Complexes as Acyl Transfer Reagents. Acylation of Allylic Halides, Conjugated Enones, and Quinones. J. Org. Chem. 1985, 50, 4955– 4960,  DOI: 10.1021/jo00224a061
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      Phosphinecarbonylnitrosylacylcobaltate complexes as acyl transfer reagents. Acylation of allylic halides, conjugated enones, and quinones
      Hegedus, Louis S.; Perry, Robert J.
      Journal of Organic Chemistry (1985), 50 (24), 4955-60CODEN: JOCEAH; ISSN:0022-3263.
      The complex Co(NO)(CO)2(PPh3) is prepd. from Co2(CO)8, NaNO2, and Ph3P without isolation of the volatile intermediate Co(NO)(CO)3. Treatment of this complex with organolithium reagent at -40° generated unstable acylate complexes [RCOCo(NO)(CO)(PPh3)]- (R = Bu, Me) which readily transferred the acyl group to allylic halides to produce β,γ-unsatd. ketones, to conjugated ketones to produce 1,4-dicarbonyl compds., and to quinones to form 4-acylcyclohexadienones.
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      Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. Methods 2012, 9, 671,  DOI: 10.1038/nmeth.2089
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      NIH Image to ImageJ: 25 years of image analysis
      Schneider, Caroline A.; Rasband, Wayne S.; Eliceiri, Kevin W.
      Nature Methods (2012), 9 (7_part1), 671-675CODEN: NMAEA3; ISSN:1548-7091. (Nature Publishing Group)
      For the past 25 years NIH Image and ImageJ software have been pioneers as open tools for the anal. of scientific images. We discuss the origins, challenges and solns. of these two programs, and how their history can serve to advise and inform other software projects.
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      Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267– 9270,  DOI: 10.1021/ja403440e
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      Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction
      Popczun, Eric J.; McKone, James R.; Read, Carlos G.; Biacchi, Adam J.; Wiltrout, Alex M.; Lewis, Nathan S.; Schaak, Raymond E.
      Journal of the American Chemical Society (2013), 135 (25), 9267-9270CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Nanoparticles of nickel phosphide (Ni2P) have been investigated for electrocatalytic activity and stability for the hydrogen evolution reaction (HER) in acidic solns., under which proton exchange membrane-based electrolysis is operational. The catalytically active Ni2P nanoparticles were hollow and faceted to expose a high d. of the Ni2P(001) surface, which has previously been predicted based on theory to be an active HER catalyst. The Ni2P nanoparticles had among the highest HER activity of any non-noble metal electrocatalyst reported to date, producing H2(g) with nearly quant. faradaic yield, while also affording stability in aq. acidic media.
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    43. 43
      Alcaide, F.; Álvarez, G.; Cabot, P. L.; Genova-Koleva, R.; Grande, H.-J.; Miguel, O. Effect of the Solvent in the Catalyst Ink Preparation on the Properties and Performance of Unsupported PtRu Catalyst Layers in Direct Methanol Fuel Cells. Electrochim. Acta 2017, 231, 529– 538,  DOI: 10.1016/j.electacta.2017.02.127
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      Effect of the solvent in the catalyst ink preparation on the properties and performance of unsupported PtRu catalyst layers in direct methanol fuel cells
      Alcaide, Francisco; Alvarez, Garbine; Cabot, Pere L.; Genova-Koleva, Radostina; Grande, Hans-Jurgen; Miguel, Oscar
      Electrochimica Acta (2017), 231 (), 529-538CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The effect of the org. solvent polarity on the properties of unsupported PtRu catalyst inks and on the performance of the catalyst layers prepd. with them for the methanol electrooxidn., has been studied. The light scattering results indicate that the PtRu-Nafion aggregates in the inks prepd. with Bu acetate (NBA) are larger than those prepd. with 2-propanol (IPA). The lower polarity of the former favors the aggregation of Nafion and nanoparticles. The electron microscopy images and porosimetry measurements of the catalyst layers show that the secondary pore vol. between the agglomerates is larger for NBA. The linear sweep voltammetry and EIS results for the methanol electrooxidn. in the three-electrode cell denote the higher active surface area for NBA and comparable specific oxidn. rates of the intermediates in both catalysts layers. The current densities for PtRu anode catalyst layers in single DMFC are higher when the solvent is NBA, the mass transport limitations being much more apparent with IPA. The adapted transmission line equiv. circuit to interpret the impedance results in single DMFC indicates that the proton resistance for NBA is significantly lower than for IPA, thus suggesting that the greater no. of accessible active sites for methanol oxidn. in the former are well connected to the Nafion ionomers and easier transported to the membrane.
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      Huang, D.-C.; Yu, P.-J.; Liu, F.-J.; Huang, S.-L.; Hsueh, K.-L.; Chen, Y.-C.; Wu, C.-H.; Chang, W.-C.; Tsau, F.-H. Effect of Dispersion Solvent in Catalyst Ink on Proton Exchange Membrane Fuel Cell Performance. Int. J. Electrochem. Sci. 2011, 6, 2551– 2565
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      Effect of dispersion solvent in catalyst ink on proton exchange membrane fuel cell performance
      Huang, De-Chin; Yu, Pei-Jung; Liu, Feng-Jiin; Huang, Shu-Ling; Hsueh, Kan-Lin; Chen, Yen-Cho; Wu, Chun-Hsing; Chang, Wen-Chen; Tsau, Fang-Hei
      International Journal of Electrochemical Science (2011), 6 (7), 2551-2565CODEN: IJESIV; ISSN:1452-3981. (Electrochemical Science Group)
      The effects of dispersion solvents in catalyst ink on performance of the membrane electrode assembly (MEA) for proton exchange membrane fuel cell are studied. Dispersion solvents under examn. are water, ethylene glycol, glycerin, propylene glycols, and methanol. These solvents cover a wide range property of dielec. const., b.ps., and viscosity. We examine the rheol. behaviors of catalyst ink prepd. with these solvents. Both anode and cathode of the MEA are made by using doctor-blade method to spread catalyst ink onto the gas diffusion layers. The single cell with prepd. MEA is measured by linear scanning voltammetry to evaluate the discharge characteristics, by cyclic voltammetry to evaluate catalyst utilization, and electrochem. impedance spectroscopy to evaluate internal resistance. A semi-empirical equation is use to analyze the voltage losses due to activation over-potential and internal resistance. We find that ethylene glycol was the best dispersion agent among org. solvents tested. The power d. of the single cell is able to reach the max. power d. of 1,428 mA (mg Pt)-1 at hot-pressed temp. of 135 °C, and 1200 psi for 90 s.
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      Ngo, T. T.; Yu, T. L.; Lin, H.-L. Influence of the Composition of Isopropyl Alcohol/Water Mixture Solvents in Catalyst Ink Solutions on Proton Exchange Membrane Fuel Cell Performance. J. Power Sources 2013, 225, 293– 303,  DOI: 10.1016/j.jpowsour.2012.10.055
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      Influence of the composition of isopropyl alcohol/water mixture solvents in catalyst ink solutions on proton exchange membrane fuel cell performance
      Ngo, Trung Truc; Yu, T. Leon; Lin, Hsiu-Li
      Journal of Power Sources (2013), 225 (), 293-303CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      We study the morphol. of Nafion in the dil. isopropanol (IPA)/water mixt. solns. contg. 20-100 wt.% of IPA and in the Pt-C/Nafion gas diffusion electrodes (where Pt-C is the carbon powder deposited on its surface with Pt particles), which are prepd. by spraying on the carbon paper surfaces with a layer of Pt-C, Nafion and IPA/water ink soln. The fuel cell performance of the gas diffusion electrodes strongly depends on the Nafion morphol. in the ink solns. A lower IPA content in the Pt-C/Nafion ink solns. results in the formation of larger and higher neg. charged Nafion aggregated particles, which leads to higher steric hindrance of the deposition of Nafion ionomer on the surface of Pt-C particles and thus a thinner Nafion film in contact on the Pt-C particle surfaces. The thinner Nafion film in contact with the Pt particles in the catalyst layer increases the chances of the Pt particles in contact with the H2/O2 gas, leading to a higher fuel cell performance.
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      Ngo, T. T.; Yu, T. L.; Lin, H.-L. Nafion-Based Membrane Electrode Assemblies Prepared from Catalyst Inks Containing Alcohol/Water Solvent Mixtures. J. Power Sources 2013, 238, 1– 10,  DOI: 10.1016/j.jpowsour.2013.03.055
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      Nafion-based membrane fuel cell electrode assemblies prepared from catalyst inks containing alcohol/water solvent mixtures
      Ngo, Trung Truc; Yu, T. Leon; Lin, Hsiu-Li
      Journal of Power Sources (2013), 238 (), 1-10CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Many researchers have studied the influence on fuel cell (FC) performance of the dielec. consts. (ε values) of the dispersion solvents of catalyst ink solns. in the fabrication of Nafion-based gas diffusion electrodes (GDEs). They have reported that Nafion forms colloidal particles in dispersion solvents possessing ε = 3-10, making these solvents suitable for fabricating GDEs with excellent FC performance. In this paper, we study the dependence of both the Nafion ionomer morphol. in the catalyst layers and the FC performance of the GDEs on ε and the soly. parameter δ of the alc. (i.e., methanol and iso-Pr alc.)/water dispersion mixt. The dispersion solvents of the catalyst ink solns. contain 20-100 wt.% alc. with 23.4 > δ > 11.3 and 78.4 > ε > 19.9. By plotting the FC performance against the ε and δ values of the alc./water dispersion solvent mixt., we demonstrate that the FC performance increases with increasing ε and δ values of the catalyst ink dispersion solvents. ε and δ are detd. to be better parameters than the compns. of the solvent mixts. for describing GDE performance.
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      Shimanuki, J.; Takahashi, S.; Tohma, H.; Ohma, A.; Ishihara, A.; Ito, Y.; Nishino, Y.; Miyazawa, A. Microstructural Observation of Fuel Cell Catalyst Inks by Cryo-SEM and Cryo-TEM. Microscopy 2017, 66, 204– 208,  DOI: 10.1093/jmicro/dfx001
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      Microstructural observation of fuel cell catalyst inks by Cryo-SEM and Cryo-TEM
      Shimanuki, Junichi; Takahashi, Shinichi; Tohma, Hajime; Ohma, Atsushi; Ishihara, Ayumi; Ito, Yoshiko; Nishino, Yuri; Miyazawa, Atsuo
      Microscopy (Oxford, United Kingdom) (2017), 66 (3), 204-208CODEN: MICRJM; ISSN:2050-5701. (Oxford University Press)
      In order to improve the electricity generation performance of fuel cell elec. vehicles, it is necessary to optimize the microstructure of the catalyst layer of a polymer electrolyte fuel cell. The catalyst layer is formed by a wet coating process using catalyst inks. Therefore, it is very important to observe the microstructure of the catalyst ink. In this study, the morphol. of carbon-supported platinum (Pt/C) particles in catalyst inks with a different solvent compn. was investigated by cryogenic SEM (cryo-SEM). In addn., the morphol. of the ionomer, which presumably influences the formation of agglomerated Pt/C particles in a catalyst ink, was investigated by cryogenic transmission electron microscopy (cryo-TEM). The results of a cryo-SEM observation revealed that the agglomerated Pt/C particles tended to become coarser with a higher 1-propanol (NPA) wt. fraction. The results of a cryo-TEM observation indicated that the actual ionomer dispersion in a catalyst ink formed a network structure different than that of the ionomer in the solvent.
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      Shukla, S.; Bhattacharjee, S.; Secanell, M. Rationalizing Catalyst Inks for PEMFC Electrodes Based on Colloidal Interactions. ECS Trans. 2013, 58, 1409– 1428,  DOI: 10.1149/05801.1409ecst
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      Rationalizing catalyst inks for PEMFC electrodes based on colloidal interactions
      Shukla, S.; Bhattacharjee, S.; Secanell, M.
      ECS Transactions (2013), 58 (1, Polymer Electrolyte Fuel Cells 13), 1409-1428, 20 pp.CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
      A preliminary kinetic model was developed for polymer electrolyte membrane fuel cell (PEMFC) catalyst inks in order to understand their particle stability. The Derjaguin Landau Verwey Overbeek (DLVO) model contg. van der Waals attractive and electrostatic repulsive interaction energy was applied to the aq. ink dispersions, while a modified DLVO type interaction contg. a Coulombic term instead of the electrostatic term was applied to the non-aq. dispersions. Solvents were compared based on their particle size distribution and stability ratios. Results show that the carbon black particles are stable in a higher dielec. medium whereas they tend to aggregate in a lower dielec. medium. A low ionic concn. for the aq. medium also helped to improve the ink stability by providing a better electrostatic particle repulsion. Expts. conducted with Et acetate, iso-propanol and deionized water agree with the model predictions.
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    49. 49
      Therdthianwong, A.; Ekdharmasuit, P.; Therdthianwong, S. Fabrication and Performance of Membrane Electrode Assembly Prepared by a Catalyst-Coated Membrane Method: Effect of Solvents Used in a Catalyst Ink Mixture. Energy Fuels 2010, 24, 1191– 1196,  DOI: 10.1021/ef901105k
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      53
      Fabrication and Performance of Membrane Electrode Assembly Prepared by a Catalyst-Coated Membrane Method: Effect of Solvents Used in a Catalyst Ink Mixture
      Therdthianwong, Apichai; Ekdharmasuit, Panuwat; Therdthianwong, Supaporn
      Energy & Fuels (2010), 24 (2), 1191-1196CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      The effect of solvent types used to prep. catalyst-coated membrane (CCM) electrodes on the coating ink, applied coating process, interaction of catalyst ink and Nafion membrane and, thus, on the performance and power d. of the proton exchange membrane fuel cells (PEM fuel cells) was investigated. Among seven types of solvents studied, the catalyst ink prepd. using the isopropanol solvent showed the highest cell performance, followed in descending order by ethanol, acetone, water-ethylene glycol, ethylene glycol di-Me ether (EGDME), and ethylene glycol di-Et ether (EGDEE). The superior performance was due to the good attachment of catalyst layer to membrane and the good surface coverage revealed by the results from the scanning electron microscope (SEM) of the CCMs, optical transmittance of the CCMs, and solvent absorption capacity of membrane. In electrode prepn. by the CCM method, the solvent-ionomer mixt. in soln. form was more favorable than that in colloid form, as it gave higher cell performance. Finally, the catalyst-coated membrane on the H+ form Nafion demonstrated better power d. than the CCM on the Na+ form Nafion.
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    50. 50
      Wuttikid, K.; Worayos, N.; Punyawudho, K. Analysis of Catalyst Ink Compositions for Fabricating Membrane Electrode Assemblies in PEM Fuel Cells. Chiang Mai Univ. J. Nat. Sci. 2017, 16, 275– 281,  DOI: 10.12982/cmujns.2017.0022
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      Inaba, M.; Quinson, J.; Arenz, M. Ph Matters: The Influence of the Catalyst Ink on the Oxygen Reduction Activity Determined in Thin Film Rotating Disk Electrode Measurements. J. Power Sources 2017, 353, 19– 27,  DOI: 10.1016/j.jpowsour.2017.03.140
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      pH matters: The influence of the catalyst ink on the oxygen reduction activity determined in thin film rotating disk electrode measurements
      Inaba, Masanori; Quinson, Jonathan; Arenz, Matthias
      Journal of Power Sources (2017), 353 (), 19-27CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      We investigated the influence of the ink properties of proton exchange membrane fuel cell (PEMFC) catalysts on the oxygen redn. reaction (ORR) activity detd. in thin film rotating disk electrode (TF-RDE) measurements. It was found that the adaptation of a previously reported ink recipe to home-made catalysts does not lead to satisfying results, although reported work could be reproduced using com. catalyst samples. It is demonstrated that the pH of the catalyst ink, which has not been addressed in previous TF-RDE studies, is an important parameter that needs to be carefully controlled to det. the intrinsic ORR activity of high surface area catalysts.
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    52. 52
      Jung, C.-Y.; Kim, W.-J.; Yi, S.-C. Optimization of Catalyst Ink Composition for the Preparation of a Membrane Electrode Assembly in a Proton Exchange Membrane Fuel Cell Using the Decal Transfer. Int. J. Hydrogen Energy 2012, 37, 18446– 18454,  DOI: 10.1016/j.ijhydene.2012.09.013
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      Optimization of catalyst ink composition for the preparation of a membrane electrode assembly in a proton exchange membrane fuel cell using the decal transfer
      Jung, Chi-Young; Kim, Wha-Jung; Yi, Sung-Chul
      International Journal of Hydrogen Energy (2012), 37 (23), 18446-18454CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      For low interfacial resistance and feasibility of forming catalyst layer (CL), decal transfer (DT) is considered as one of the most effective methods for prepg. a membrane electrode assembly. However, optimization of the catalyst ink compn. is necessary, because of the complexity of the CL. Here, 1-propanol is adsorbed onto the CL coated onto the decal, as a swelling agent, for complete transfer of the CL onto Nafion membrane. Using this methodol., flat and complete DT is achieved at the hot-pressing conditions of 60 °C and 5 MPa. For optimization, the solvent-to-carbon ratio (SCR) and Nafion-to-carbon ratio (NCR) are controlled to achieve improved cell performance. In this study, by considering the morphol. of CL and the cell performance when CL is annealed at temps. sufficiently below the b.p. of the solvent, optimized SCR and NCR values of approx. 12.0 and 0.65, resp., are obtained. In addn., microstructure, thickness and various electrochem. properties of the CLs are examd. in detail.
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    53. 53
      Shinozaki, K.; Zack, J. W.; Pylypenko, S.; Pivovar, B. S.; Kocha, S. S. Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique. J. Electrochem. Soc. 2015, 162, F1384– F1396,  DOI: 10.1149/2.0551512jes
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      Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique
      Shinozaki, Kazuma; Zack, Jason W.; Pylypenko, Svitlana; Pivovar, Bryan S.; Kocha, Shyam S.
      Journal of the Electrochemical Society (2015), 162 (12), F1384-F1396CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      Platinum electrocatalysts supported on high surface area and Vulcan carbon blacks (Pt/HSC, Pt/V) were characterized in rotating disk electrode (RDE) setups for electrochem. area (ECA) and oxygen redn. reaction (ORR) area specific activity (SA) and mass specific activity (MA) at 0.9 V. Films fabricated using several ink formulations and film-drying techniques were characterized for a statistically significant no. of independent samples. The highest quality Pt/HSC films exhibited MA 870 ± 91 mA/mgPt and SA 864 ± 56 μA/cm2Pt while Pt/V had MA 706 ± 42 mA/mgPt and SA 1120 ± 70 μA/cm2Pt when measured in 0.1 M HClO4, 20 mV/s, 100 kPa O2 and 23 ± 2°C. An enhancement factor of 2.8 in the measured SA was observable on eliminating Nafion ionomer and employing extremely thin, uniform films (∼4.5 μg/cm2Pt) of Pt/HSC. The ECA for Pt/HSC (99 ± 7 m2/gPt) and Pt/V (65 ± 5 m2/gPt) were statistically invariant and insensitive to film uniformity/thickness/fabrication technique; accordingly, enhancements in MA are wholly attributable to increases in SA. Impedance measurements coupled with SEM were used to de-convolute the losses within the catalyst layer and ascribed to the catalyst layer resistance, oxygen diffusion, and sulfonate anion adsorption/blocking. The ramifications of these results for proton exchange membrane fuel cells have also been examd.
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    54. 54
      Pollet, B. G.; Goh, J. T. E. The Importance of Ultrasonic Parameters in the Preparation of Fuel Cell Catalyst Inks. Electrochim. Acta 2014, 128, 292– 303,  DOI: 10.1016/j.electacta.2013.09.160
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      The importance of ultrasonic parameters in the preparation of fuel cell catalyst inks
      Pollet, Bruno G.; Goh, Jonathan T. E.
      Electrochimica Acta (2014), 128 (), 292-303CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      We report here that ultrasound (20 kHz and 40 kHz) affects the catalyst ink compn. when irradiated for longer periods and at high ultrasonic powers. In our study, two com. carbon supported Pt (Pt/C) catalysts were used and dispersed in Nafion ionomer. Catalyst ink samples prepd. from Nafion, IPA and water were either ultrasonicated (20 kHz up to 12.23 W and 40 kHz at 1.82 W) or mech. shear-mixed (19,000 rpm) for various durations (up to 120 min). All catalyst ink samples were characterised by XRD, BET, TEM and electrochem. measurements were performed in liq. electrolytes. It was found that an optimized ultrasonic treatment is required to improve the catalytic ink activity, but longer irradn. is detrimental to its compn. and morphol., mainly due to cavitation and sonolysis phenomena.
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      Jung, S.; McCrory, C. C. L.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Nanoparticulate Metal Oxide Electrocatalysts for the Alkaline Water Oxidation Reaction. J. Mater. Chem. A 2016, 4, 3068– 3076,  DOI: 10.1039/c5ta07586f
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      59
      Benchmarking nanoparticulate metal oxide electrocatalysts for the alkaline water oxidation reaction
      Jung, Suho; McCrory, Charles C. L.; Ferrer, Ivonne M.; Peters, Jonas C.; Jaramillo, Thomas F.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (8), 3068-3076CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Nanoparticulate metal-oxide catalysts are among the most prevalent systems for alk. water oxidn. However, comparisons of the electrochem. performance of these materials have been challenging due to the different methods of attachment, catalyst loadings, and electrochem. test conditions reported in the literature. Herein, we have leveraged a conventional drop-casting method that allows for the successful adhesion of a wide range of nanoparticulate catalysts to glassy-carbon electrode surfaces. We have applied this adhesion method to prep. catalyst films from 16 cryst. metal-oxide nanoparticles with a const. loading of 0.8 mg cm-2, and evaluated the resulting nanoparticulate films for the oxygen evolution reaction under conditions relevant to an integrated solar fuels device. In general, the activities of the adhered nanoparticulate films are similar to those of thin-film catalysts prepd. by electrodeposition or sputtering, achieving 10 mA cm-2 current densities per geometric area at overpotentials of ∼0.35-0.5 V.
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      Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765– 1772,  DOI: 10.1021/cs3003098
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      60
      Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials
      Reier, Tobias; Oezaslan, Mehtap; Strasser, Peter
      ACS Catalysis (2012), 2 (8), 1765-1772CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A comparative study was performed to examine the intrinsic catalytic activity and durability of C supported Ru, Ir, and Pt nanoparticles and corresponding bulk materials for the electrocatalytic O evolution reaction (OER). The electrochem. surface characteristics of nanoparticles and bulk materials were studied by surface-sensitive cyclic voltammetry. Although basically similar voltammetric features were obsd. for nanoparticles and bulk materials of each metal, some differences were uncovered highlighting the changes in oxidn. chem. From the electrochem. results, Ru nanoparticles show lower passivation potentials compared to bulk Ru material. Ir nanoparticles completely lost their voltammetric metallic features during the voltage cycling, in contrast to the corresponding bulk material. Finally, Pt nanoparticles show an increased oxophilic nature compared to bulk Pt. With regard to the OER performance, the most pronounced effects of nanoscaling were identified for Ru and Pt catalysts. In particular, the Ru nanoparticles suffered from strong corrosion at applied OER potentials and were therefore unable to sustain the OER. The Pt nanoparticles exhibited a lower OER activity from the beginning on and were completely deactivated during the applied OER stability protocol, in contrast to the corresponding bulk Pt catalyst. The authors highlight that the OER activity and durability were comparable for Ir nanoparticles and bulk materials. Thus, Ir nanoparticles provide a high potential as nanoscaled OER catalyst.
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    57. 57
      Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452– 8455,  DOI: 10.1021/ja4027715
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      An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation
      Gong, Ming; Li, Yanguang; Wang, Hailiang; Liang, Yongye; Wu, Justin Z.; Zhou, Jigang; Wang, Jian; Regier, Tom; Wei, Fei; Dai, Hongjie
      Journal of the American Chemical Society (2013), 135 (23), 8452-8455CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Highly active, durable, and cost-effective electrocatalysts for H2O oxidn. to evolve O gas hold a key to a range of renewable energy solns., including H2O-splitting and rechargeable metal-air batteries. Here, the authors report the synthesis of ultrathin Ni-Fe layered double hydroxide (NiFe-LDH) nanoplates on mildly oxidized multi-walled C nanotubes (CNTs). Incorporation of Fe into the Ni hydroxide induced the formation of NiFe-LDH. The cryst. NiFe-LDH phase in nanoplate form is highly active for O evolution reaction in alk. solns. For NiFe-LDH grown on a network of CNTs, the resulting NiFe-LDH/CNT complex exhibits higher electrocatalytic activity and stability for O evolution than com. precious metal Ir catalysts.
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      Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169– 11186,  DOI: 10.1103/physrevb.54.11169
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      Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
      Kresse, G.; Furthmueller, J.
      Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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      Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15– 50,  DOI: 10.1016/0927-0256(96)00008-0
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      Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set
      Kresse, G.; Furthmuller, J.
      Computational Materials Science (1996), 6 (1), 15-50CODEN: CMMSEM; ISSN:0927-0256. (Elsevier)
      The authors present a detailed description and comparison of algorithms for performing ab-initio quantum-mech. calcns. using pseudopotentials and a plane-wave basis set. The authors will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temp. d.-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N2atoms scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge d. including a new special preconditioning optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. The authors have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio mol.-dynamics package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
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      Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868,  DOI: 10.1103/physrevlett.77.3865
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      Generalized gradient approximation made simple
      Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
      Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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      Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953– 17979,  DOI: 10.1103/physrevb.50.17953
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      Projector augmented-wave method
      Blochl
      Physical review. B, Condensed matter (1994), 50 (24), 17953-17979 ISSN:0163-1829.
      There is no expanded citation for this reference.
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      Kresse, G.; Joubert, D. From Ultrasoft Pseudoptentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758– 1775,  DOI: 10.1103/physrevb.59.1758
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      From ultrasoft pseudopotentials to the projector augmented-wave method
      Kresse, G.; Joubert, D.
      Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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      Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104,  DOI: 10.1063/1.3382344
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      67
      A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
      Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
      Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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    64. 64
      He, J.; Morales-García, Á.; Bludský, O.; Nachtigall, P. The Surface Stability and Equilibrium Crystal Morphology of Ni2P Nanoparticles and Nanowires from an Ab Initio Atomistic Thermodynamic Approach. CrystEngComm 2016, 18, 3808– 3818,  DOI: 10.1039/c6ce00584e
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      The surface stability and equilibrium crystal morphology of Ni2P nanoparticles and nanowires from an ab initio atomistic thermodynamic approach
      He, Junjie; Morales-Garcia, Angel; Bludsky, Ota; Nachtigall, Petr
      CrystEngComm (2016), 18 (21), 3808-3818CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
      Knowledge of the equil. crystal shape and structure of the exposed surfaces of nickel phosphide (Ni2P) nanostructures is essential for understanding and control of their catalytic performance. Ab initio atomistic thermodn. was used to investigate computationally the effects of the exptl. conditions (temp., pressure, and chem. potentials) on the relative stabilities of low-Miller index surfaces and on the equil. crystal morphol. of Ni2P nanoparticles and nanowires. The P-covered (0001)-Ni3P2 (denoted as (0001)-A-P) surface was found to be the most stable surface at a considerably wide range of chem. potentials, whereas the (0001)-A, (10‾11)-Ni/P and (10‾12)-Ni/P surfaces are the thermodynamically most favored phases just in narrow chem. potential regions. The theor. equil. shapes and structures of the Ni2P nanoparticles and nanowires were obtained based on the Wulff construction at various chem. potentials. The morphol. of the surfaces of the Ni2P nanoparticles and nanowires does depend on the chem. potential; thus, it can be tailored for particular applications by a suitable choice of exptl. conditions. The (0001), (10‾10) and (10‾11) side facets dominate the nanoparticle surface in a wide range of chem. potentials but other side facets can also appear at particular ranges of chem. potentials. Results reported herein give new insight into the Ni2P nanoparticle morphol. showing how it depends on the exptl. conditions; this information can help to tailor the surface and shape of Ni2P nanoparticles for specific applications, e.g., in catalysis.
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    65. 65
      Wexler, R. B.; Martirez, J. M. P.; Rappe, A. M. Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces. ACS Catal. 2017, 7, 7718– 7725,  DOI: 10.1021/acscatal.7b02761
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      69
      Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces
      Wexler, Robert B.; Martirez, John Mark P.; Rappe, Andrew M.
      ACS Catalysis (2017), 7 (11), 7718-7725CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Optimizing catalysts for the hydrogen evolution reaction (HER) is a crit. step toward the efficient prodn. of H2(g) fuel from water. It has been demonstrated exptl. that transition-metal phosphides, specifically nickel phosphides Ni2P and Ni5P4, efficiently catalyze the HER at a small fraction of the cost of archetypal Pt-based electrocatalysts. However, the HER mechanism on nickel phosphides remains unclear. We explore, through d. functional theory with thermodn., the aq. reconstructions of Ni2P(0001) and Ni5P4(0001)/(000‾1), and we find that the surface P content on Ni2P(0001) depends on the applied potential, which has not been considered previously. At -0.21 V ≥ U ≥ -0.36 V vs. the std. hydrogen electrode and pH = 0, a PHx-enriched Ni3P2 termination of Ni2P(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni3P2 surface is instead passivated by H at the Ni3-hollow sites. On the other hand, Ni5P4(000‾1) does not favor addnl. P. Instead, the Ni4P3 bulk termination of Ni5P4(000‾1) is passivated by H at both the Ni3 and P3-hollow sites. We also found that the most HER-active surfaces are Ni3P2+P+(7/3)H of Ni2P(0001) and Ni4P3+4H of Ni5P4(000‾1) due to weak H adsorption at P catalytic sites, in contrast with other computational investigations that propose Ni as or part of the active site. By looking at viable catalytic cycles for HER on the stable reconstructed surfaces, and calcg. the reaction free energies of the assocd. elementary steps, we calc. that the overpotential on the Ni4P3+4H surface of Ni5P4(000‾1) (-0.16 V) is lower than that of the Ni3P2+P+(7/3)H surface of Ni2P(0001) (-0.21 V). This is due to the abundance of P3-hollow sites on Ni5P4 and the limited surface stability of the P-enriched Ni2P(0001) surface phase. The trend in the calcd. catalytic overpotentials, and the potential-dependent bulk and surface stabilities explain why the nickel phosphides studied here perform almost as well as Pt, and why Ni5P4 is more active than Ni2P toward HER, as is found in the exptl. literature. This study emphasizes the importance of considering aq. surface stability in predicting the HER-active sites, mechanism, and overpotential, and highlights the primary role of P in HER catalysis on transition-metal phosphides.
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      Partanen, L.; Hakala, M.; Laasonen, K. Hydrogen Adsorption Trends on Various Metal-Doped Ni2P Surfaces for Optimal Catalyst Design. Phys. Chem. Chem. Phys. 2018, 21, 184– 191,  DOI: 10.1039/c8cp06143b
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      Hydrogen adsorption trends on various metal-doped Ni2P surfaces for optimal catalyst design
      Partanen Lauri; Hakala Mikko; Laasonen Kari
      Physical chemistry chemical physics : PCCP (2018), 21 (1), 184-191 ISSN:.
      In this study, we looked at the hydrogen evolution reaction on Mg-, Mo-, Fe-, Co-, V-, and Cu-doped Ni3P2 and Ni3P2 + P terminated Ni2P surfaces. The DFT calculated hydrogen adsorption free energy was employed as a predictor of the materials' catalytic HER activity. Our results indicate that doping can substantially improve the catalytic activity of the Ni3P2 terminated surface. In contrast, the Ni3P2 + P terminated one seems to be catalytically active irrespective of the type of doping, including in the absence of doping. Based on our doping energy and adsorption free energy calculations, the most promising dopants are iron and cobalt, whereas copper is less likely to function well as a doping element.
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      Moon, J.-S.; Jang, J.-H.; Kim, E.-G.; Chung, Y.-H.; Yoo, S. J.; Lee, Y.-K. The Nature of Active Sites of Ni2P Electrocatalyst for Hydrogen Evolution Reaction. J. Catal. 2015, 326, 92– 99,  DOI: 10.1016/j.jcat.2015.03.012
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      The nature of active sites of Ni2P electrocatalyst for hydrogen evolution reaction
      Moon, Ji-Sue; Jang, Jue-Hyuk; Kim, Eung-Gun; Chung, Young-Hoon; Yoo, Sung Jong; Lee, Yong-Kul
      Journal of Catalysis (2015), 326 (), 92-99CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Nano-scaled Ni2P particles were synthesized by ligand stabilization method and applied for hydrogen evolution reaction (HER). X-ray diffraction (XRD), transmission electron microscope (TEM), and X-ray absorption fine structure (XAFS) spectroscopy were employed to examine structural properties of Ni2P nanoparticles. The electrocatalytic HER activity and stability for the Ni2P nanocatalyst were tested in 0.5M H2SO4, and the Ni2P electrocatalyst exhibited a low onset potential for the HER at around -0.02 V vs. RHE, a little more neg. compared to the Pt catalyst which shows almost 0 V vs. reversible hydrogen electrode (RHE), and the Tafel slope of 75 mV per decade, i.e. following Volmer step as a rate-detg. step. D. functional theory (DFT) calcns. for hydrogen adsorption over Ni2P surfaces (0 0 1) and (0 0 2) revealed that the hydrogen adsorption might occur via two reaction pathways: consecutive or simultaneous hydrogen adsorption. The consecutive hydrogen adsorptions on threefold hollow (TFH)-Ni site followed by on P(II) site on a Ni2P (0 0 1) surface led to a lower reaction barrier than simultaneous hydrogen adsorption. These results thus demonstrated that the Volmer step might follow consecutive adsorption mechanism over the Ni2P surface.
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      Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B: Solid State 1976, 13, 5188– 5192,  DOI: 10.1103/physrevb.13.5188
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      Wang, X.; Wan, F.; Gao, Y.; Liu, J.; Jiang, K. Synthesis of High-Quality Ni2P Hollow Sphere Via a Template-Free Surfactant-Assisted Solvothermal Route. J. Cryst. Growth 2008, 310, 2569– 2574,  DOI: 10.1016/j.jcrysgro.2008.01.014
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      Synthesis of high-quality Ni2P hollow sphere via a template-free surfactant-assisted solvothermal route
      Wang, Xinjun; Wan, Fuquan; Gao, Youjun; Liu, Juan; Jiang, Kai
      Journal of Crystal Growth (2008), 310 (10), 2569-2574CODEN: JCRGAE; ISSN:0022-0248. (Elsevier B.V.)
      Ni2P uniform hollow nanospheres were prepd. via a template-free surfactant-assisted solvothermal route using NiCl2·6H2O and elemental yellow P as starting materials and urea as pH regulator in a mixt. soln. of ethylene glycol (EG), EtOH and H2O. X-ray powder diffraction (XRD), XPS, energy dispersive x-ray spectroscopy (EDX), transmission electronic microscopy (TEM), electron diffraction (ED), and SEM studies show that the as-obtained nanocrystal is pure Ni2P with a hexagonal phase and the high-quality spheres are ∼500 nm in diam. Na dodecyl sulfate (SDS) and urea play an important role in controlling the formation of Ni2P hollow nanospheres. The possible growth mechanism is proposed.
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      Wexler, R. B.; Martirez, J. M. P.; Rappe, A. M. Stable Phosphorus-Enriched (0001) Surfaces of Nickel Phosphides. Chem. Mater. 2016, 28, 5365– 5372,  DOI: 10.1021/acs.chemmater.6b01437
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      74
      Stable Phosphorus-Enriched (0001) Surfaces of Nickel Phosphides
      Wexler, Robert B.; Martirez, John Mark P.; Rappe, Andrew M.
      Chemistry of Materials (2016), 28 (15), 5365-5372CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      In heterogeneous catalysis, catalyst synthesis precedes operation and, in most cases, is conducted in an altogether different chem. environment. Thus, detn. of the structure and compn. of the catalyst surface(s) due to fabrication is essential in accurately evaluating their eventual structure(s) during operation, which provides the origin of their catalytic activities and are therefore key to catalyst optimization. We explore the reconstructions of both Ni2P(0001) and Ni5P4(0001)/(000‾1) surfaces with first-principles d. functional theory (DFT). Most of the stable terminations under realistic synthesis conditions are detd. to be P-rich on both materials. A P-covered reconstruction of the Ni3P2 termination of Ni2P(0001) is found to be most stable, consistent with the current literature. By contrast, the most energetically favorable surfaces of Ni5P4 are found to be the Ni3P3 and Ni4P3 bulk-derived terminations with P-adatoms. The preferred excess P binding sites and their energies are identified on each surface. We find that the P3 site, which is present on Ni5P4, and the Ni3 site, which is present on both Ni2P and Ni5P4, strongly bind excess P. Addnl., we predict the presence of stable Pn (n = 2, 4) agglomerates on Ni5P4 at the P3-hollow and Ni-Ni bridge sites. This study highlights the importance of considering the aggregation behavior of nonmetal components in predicting the surface reconstruction of transition metal compds.
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      Wang, J.; Johnston-Peck, A. C.; Tracy, J. B. Nickel Phosphide Nanoparticles with Hollow, Solid, and Amorphous Structures. Chem. Mater. 2009, 21, 4462– 4467,  DOI: 10.1021/cm901073k
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      Nickel Phosphide Nanoparticles with Hollow, Solid, and Amorphous Structures
      Wang, Junwei; Johnston-Peck, Aaron C.; Tracy, Joseph B.
      Chemistry of Materials (2009), 21 (19), 4462-4467CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Conversion of unary metal nanoparticles (NPs) upon exposure to oxygen, sulfur, selenium, and phosphorus precursors usually produces hollow metal oxide, sulfide, selenide, or phosphide NPs through the Kirkendall effect. Here, nanostructural control of mixed-phase Ni2P/Ni12P5 (represented as NixPy) NPs prepd. through the thermolysis of nickel acetylacetonate using trioctylphosphine (TOP) as a ligand and phosphorus precursor is reported. The P:Ni molar ratio controls the NP size and is the key factor in detg. the nanostructure. For P:Ni molar ratios of 1-3, nickel NPs form below 240°C and subsequently convert to cryst.-hollow NixPy NPs at 300°C. For higher P:Ni ratios, a Ni-TOP complex forms that requires higher temps. for NP growth, thus favoring direct formation of NixPy rather than nickel. Consequently, for P:Ni molar ratios of >9, amorphous-solid NixPy NPs form at 240°C and become cryst.-solid NixPy NPs at 300°C. For intermediate P:Ni molar ratios of ∼6, both growth mechanisms result in a mixt. of hollow and solid NixPy NPs. Similar results have been obtained using tributylphosphine or triphenylphosphine as the phosphorus source, but trioctylphosphine oxide cannot serve as a phosphorus source.
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      Muthuswamy, E.; Savithra, G. H. L.; Brock, S. L. Synthetic Levers Enabling Independent Control of Phase, Size, and Morphology in Nickel Phosphide Nanoparticles. ACS Nano 2011, 5, 2402– 2411,  DOI: 10.1021/nn1033357
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      76
      Synthetic levers enabling independent control of phase, size, and morphology in nickel phosphide nanoparticles
      Muthuswamy, Elayaraja; Savithra, Galbokka H. Layan; Brock, Stephanie L.
      ACS Nano (2011), 5 (3), 2402-2411CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Simultaneous control of phase, size, and morphol. in nanoscale nickel phosphides is reported. Phase-pure samples of discrete nanoparticles of Ni12P5 and Ni2P in hollow and solid morphologies can be prepd. in a range of sizes (10-32 nm) by tuning key interdependent synthetic levers (P:Ni precursor ratio, temp., time, oleylamine quantity). Size and morphol. are controlled by the P:Ni ratio in the synthesis of the precursor particles, with large, hollow particles formed at low P:Ni and small, solid particles formed at high P:Ni. The P:Ni ratio also impacts the phase at the crystn. temp. (300-350°), with metal-rich Ni12P5 generated at low P:Ni and Ni2P at high P:Ni. Also, the product phase formed can be decoupled from the initial precursor ratio by the addn. of more P at the crystn. temp. This enables formation of hollow particles (favored by low P:Ni) of Ni2P (favored by high P:Ni). Increasing temp. and time also favor formation of Ni2P, by generating more reactive P and providing sufficient time for conversion to the thermodn. product. Finally, increasing oleylamine concn. allows Ni12P5 to be obtained under high P:Ni precursor ratios that favor solid particle formation. Oleylamine concn. also acts to tune the size of the voids in particles formed at low P:Ni ratios, enabling access to Ni12P5 particles with different void sizes. This approach enables an unprecedented level of control over phase and morphol. of nickel phosphide nanoparticles, paving the way for systematic study of the impact of these parameters on hydrodesulfurization activities of nickel phosphides.
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    73. 73
      Moreau, L. M.; Ha, D.-H.; Zhang, H.; Hovden, R.; Muller, D. A.; Robinson, R. D. Defining Crystalline/Amorphous Phases of Nanoparticles through X-Ray Absorption Spectroscopy and X-Ray Diffraction: The Case of Nickel Phosphide. Chem. Mater. 2013, 25, 2394– 2403,  DOI: 10.1021/cm303490y
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      77
      Defining Crystalline/Amorphous Phases of Nanoparticles through X-ray Absorption Spectroscopy and X-ray Diffraction: The Case of Nickel Phosphide
      Moreau, Liane M.; Ha, Don-Hyung; Zhang, Haitao; Hovden, Robert; Muller, David A.; Robinson, Richard D.
      Chemistry of Materials (2013), 25 (12), 2394-2403CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The authors elucidate the structural distinctions between amorphous and cryst. Ni2P nanoparticles synthesized using tri-n-octylphosphine (TOP), through x-ray absorption spectroscopy (XAS), XRD, and inductively coupled plasma (ICP). The authors det. the differences in their chem. and at. structure, which were not previously reported, yet are essential for understanding their potential as nanocatalysts. These structural characteristics are related to the corresponding nanoparticle magnetic properties analyzed by performing magnetic measurements. XAS results reveal a significant P concn. in the amorphous nanoparticle sample - placing the stoichiometry close to Ni2P - despite XRD results that show only fcc. Ni contributions. By comparing the long-range structural order from XRD to the short-range radial structure from EXAFS both techniques are necessary to obtain a complete structural picture of amorphous and cryst. nanoparticle phases due to the limitations of XRD amorphous characterization. Phases are amorphous with respect to XRD when their offsets (deviations) from bulk interat. distances have a std. deviation as high as ∼4.82. Phases with lower std. deviation (e.g., .ltorsim.1.22), however, are detectable as cryst. through XRD. The possible presence of amorphous phases should be considered when using XRD alone for nanoparticle characterization. This is particularly important when highly reactive reagents such as TOP were used in synthesis. By characterizing amorphous nickel phosphide nanoparticles that have a comparable stoichiometry to Ni2P, TOP serves as a highly effective phosphorus source, even at temps. ≥230°. Unintended amorphous structure domains may significantly affect nanoparticle properties, and in turn, their functionality.
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    74. 74
      Ung, D.; Cossairt, B. M. Effect of Surface Ligands on CoP for the Hydrogen Evolution Reaction. ACS Appl. Energy Mater. 2019, 2, 1642– 1645,  DOI: 10.1021/acsaem.9b00240
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      Effect of Surface Ligands on CoP for the Hydrogen Evolution Reaction
      Ung, David; Cossairt, Brandi M.
      ACS Applied Energy Materials (2019), 2 (3), 1642-1645CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
      The activity of colloidally synthesized CoP for the hydrogen evolution reaction (HER) was studied to det. the impact of surface ligands on catalysis. The as-synthesized CoP nanocrystals were stripped with trialkyloxonium tetrafluoroborate (Meerwein's reagent) to create a "blank template" that was then re-ligated with carboxylates (oleate, octanoate, acetate) and amines (oleylamine, octylamine, dioctylamine, trioctylamine butylamine). Carboxylates and amines were chosen due to their prevalence in colloidal syntheses, and the range of ligands was chosen to study the impact of sterics and hydrophobicity of the surface ligands on catalysis. Long chain carboxylates were found to result in a larger increase in overpotential when compared to the equiv. amine (e.g., oleate vs oleylamine), due to higher ligand d. and stronger coordination with carboxylates. Increased carbon chain length resulted in increased overpotential with carboxylates; however, the range of 1° amines studied had similar overpotentials. This is due to the lower ligand d. and therefore sparse ligand packing for the 1° amines. These results suggest that the mechanism by which surface ligands impede catalysis on CoP for HER is primarily through inhibiting substrate access to surface active sites rather than poisoning the active sites.
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    75. 75
      Kim, K.-Y.; Habas, S. E.; Schaidle, J. A.; Logan, B. E. Application of Phase-Pure Nickel Phosphide Nanoparticles as Cathode Catalysts for Hydrogen Production in Microbial Electrolysis Cells. Bioresour. Technol. 2019, 293, 122067,  DOI: 10.1016/j.biortech.2019.122067
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      Application of phase-pure nickel phosphide nanoparticles as cathode catalysts for hydrogen production in microbial electrolysis cells
      Kim, Kyoung-Yeol; Habas, Susan E.; Schaidle, Joshua A.; Logan, Bruce E.
      Bioresource Technology (2019), 293 (), 122067CODEN: BIRTEB; ISSN:0960-8524. (Elsevier Ltd.)
      Transition metal phosphide catalysts such as nickel phosphide (Ni2P) have shown excellent activities for the hydrogen evolution reaction, but they have primarily been studied in strongly acidic or alk. electrolytes. In microbial electrolysis cells (MECs), however, the electrolyte is usually a neutral pH to support the bacteria. Carbon-supported phase-pure Ni2P nanoparticle catalysts (Ni2P/C) were synthesized using soln.-phase methods and their performance was compared to Pt/C and Ni/C catalysts in MECs. The Ni2P/C produced a similar quantity of hydrogen over a 24 h cycle (0.29 ± 0.04 L-H2/L-reactor) as that obtained using Pt/C (0.32 ± 0.03 L-H2/L) or Ni/C (0.29 ± 0.02 L-H2/L). The mass normalized c.d. of the Ni2P/C was 14 times higher than that of the Ni/C, and the Ni2P/C exhibited stable performance over 11 days. Ni2P/C may therefore be a useful alternative to Pt/C or other Ni-based catalysts in MECs due to its chem. stability over time.
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      McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347– 4357,  DOI: 10.1021/ja510442p
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      82
      Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices
      McCrory, Charles C. L.; Jung, Suho; Ferrer, Ivonne M.; Chatman, Shawn M.; Peters, Jonas C.; Jaramillo, Thomas F.
      Journal of the American Chemical Society (2015), 137 (13), 4347-4357CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Objective comparisons of electrocatalyst activity and stability using std. methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, the authors use a std. protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochem. active surface area (ECSA) of 18 electrocatalysts for the H evolution reaction (HER) and 26 electrocatalysts for the O evolution reaction (OER) under conditions relevant to an integrated solar H2O-splitting device in aq. acidic or alk. soln. The primary figure of merit is the overpotential necessary to achieve a magnitude c.d. of 10 mA cm-2 per geometric area, the approx. c.d. expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm-2 with overpotentials <0.1 V in acidic and/or alk. solns. Among OER catalysts in acidic soln., no nonnoble metal based materials showed promising activity and stability, whereas in alk. soln. many OER catalysts performed with similar activity achieving 10 mA cm-2 current densities at overpotentials of ∼0.33-0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alk. solns., while most HER catalysts showed much lower specific activity than Pt in both acidic and alk. solns. For select catalysts, addnl. secondary screening measurements were conducted including faradaic efficiency and extended stability measurements.
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      Wang, F.; Zhang, Y.; Liu, Z.; Du, Z.; Zhang, L.; Ren, J.; Qu, X. A Biocompatible Heterogeneous MOF-Cu Catalyst for In Vivo Drug Synthesis in Targeted Subcellular Organelles. Angew. Chem., Int. Ed. 2019, 58, 6987– 6992,  DOI: 10.1002/anie.201901760
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      A Biocompatible Heterogeneous MOF-Cu Catalyst for In Vivo Drug Synthesis in Targeted Subcellular Organelles
      Wang, Faming; Zhang, Yan; Liu, Zhengwei; Du, Zhi; Zhang, Lu; Ren, Jinsong; Qu, Xiaogang
      Angewandte Chemie, International Edition (2019), 58 (21), 6987-6992CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      As a typical bioorthogonal reaction, the copper-catalyzed azide-alkyne cycloaddn. (CuAAC) has been used for drug design and synthesis. However, for localized drug synthesis, it is important to be able to det. where the CuAAC reaction occurs in living cells. In this study, we constructed a heterogeneous copper catalyst on a metal-org. framework that could preferentially accumulate in the mitochondria of living cells. Our system enabled the localized synthesis of drugs through a site-specific CuAAC reaction in mitochondria with good biocompatibility. Importantly, the subcellular catalytic process for localized drug synthesis avoided the problems of the delivery and distribution of toxic mols. In vivo tumor therapy expts. indicated that the localized synthesis of resveratrol-derived drugs led to greater antitumor efficacy and minimized side effects usually assocd. with drug delivery and distribution.
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      Wei, C.; Rao, R. R.; Peng, J.; Huang, B.; Stephens, I. E. L.; Risch, M.; Xu, Z. J.; Shao-Horn, Y. Recommended Practices and Benchmark Activity for Hydrogen and Oxygen Electrocatalysis in Water Splitting and Fuel Cells. Adv. Mater. 2019, 31, e1806296  DOI: 10.1002/adma.201806296
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      Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory. Angew. Chem., Int. Ed. 2015, 54, 52– 65,  DOI: 10.1002/anie.201407031
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      Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory
      Zheng, Yao; Jiao, Yan; Jaroniec, Mietek; Qiao, Shi Zhang
      Angewandte Chemie, International Edition (2015), 54 (1), 52-65CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The electrocatalytic hydrogen-evolution reaction (HER), as the main step of water splitting and the cornerstone of exploring the mechanism of other multi-electron transfer electrochem. processes, is the subject of extensive studies. A large no. of high-performance electrocatalysts have been developed for HER accompanied by recent significant advances in exploring its electrochem. nature. Herein we present a crit. appraisal of both theor. and exptl. studies of HER electrocatalysts with special emphasis on the electronic structure, surface (electro)chem., and mol. design. It addresses the importance of correlating theor. calcns. and electrochem. measurements toward better understanding of HER electrocatalysis at the at. level. Fundamental concepts in the computational quantum chem. and its relation to exptl. electrochem. are also presented along with some featured examples.
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    80. 80
      Xu, K.; Sun, Y.; Sun, Y.; Zhang, Y.; Jia, G.; Zhang, Q.; Gu, L.; Li, S.; Li, Y.; Fan, H. J. Yin-Yang Harmony: Metal and Nonmetal Dual-Doping Boosts Electrocatalytic Activity for Alkaline Hydrogen Evolution. ACS Energy Lett. 2018, 3, 2750– 2756,  DOI: 10.1021/acsenergylett.8b01893
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      Yin-Yang Harmony: Metal and Nonmetal Dual-Doping Boosts Electrocatalytic Activity for Alkaline Hydrogen Evolution
      Xu, Kun; Sun, Yiqiang; Sun, Yuanmiao; Zhang, Yongqi; Jia, Guichong; Zhang, Qinghua; Gu, Lin; Li, Shuzhou; Li, Yue; Fan, Hong Jin
      ACS Energy Letters (2018), 3 (11), 2750-2756CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Active site no., water dissocn., and hydrogen adsorption free energy are the three main parameters for regulating the activity of electrocatalysts for hydrogen evolution reaction (HER) in alk. media. However, at present, simultaneous modulations of these three parameters for alk. HER still remain challenging. In this work, we take CoP as the model material and demonstrate that a metal and nonmetal dual-doping strategy can achieve simultaneous modulation of these three parameters by inducing lattice irregularity and optimizing the electronic configuration in CoP nanomaterials. Benefiting from the oxygen and copper dual-doping collective effect, the optimized O,Cu-CoP nanowire array electrode shows nearly 10-fold enhancement in catalytic activity for alk. HER compared to a pure CoP nanowire electrode. Our work may provide a new concept to boost performance of nonprecious metal electrocatalysts for alk. HER.
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    81. 81
      Yan, L.; Zhang, B.; Zhu, J.; Li, Y.; Tsiakaras, P.; Kang Shen, P. Electronic Modulation of Cobalt Phosphide Nanosheet Arrays Via Copper Doping for Highly Efficient Neutral-pH Overall Water Splitting. Appl. Catal., B 2020, 265, 118555,  DOI: 10.1016/j.apcatb.2019.118555
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      Electronic modulation of cobalt phosphide nanosheet arrays via copper doping for highly efficient neutral-pH overall water splitting
      Yan, Liang; Zhang, Bing; Zhu, Junlu; Li, Yunyong; Tsiakaras, Panagiotis; Pei, Kang Shen
      Applied Catalysis, B: Environmental (2020), 265 (), 118555CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      An effective strategy is reported to optimize the electronic structure of CoP using copper doping, for greatly enhancing the intrinsic activity and cond. of CoP in neutral-pH water splitting. As a result, the as-synthesized 3D self-supported Cu-doped CoP nanosheet arrays on carbon paper (Cu-CoP NAs/CP) exhibits admirable electrocatalytic performance toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with overpotentials of 81 and 411 mV at 10 mA cm-2 in 1.0 M PBS (phosphate buffer soln.), resp. Moreover, a neutral electrolyzer, using Cu-CoP NAs/CP as both the anode and cathode, achieves a low cell voltage of 1.72 V at 10 mA cm-2, superior to that of the typical Pt/C||IrO2 couple (1.81 V) and of most of the state-of-the-art bifunctional electrocatalysts. Impressively, the electrolyzer can be driven by a single AA battery (∼1.5 V), indicating its practicality in neutral water or seawater splitting. Exptl. and d. functional theory (DFT) calcns. results reveal that the incorporation of Cu into CoP can effectively improve the cond. and optimize the electronic structure to facilitate the H* adsorption and desorption and the formation of O* intermediates (generated CoOOH active species), thus yielding superior HER and OER catalytic activities. This study opens up a promising way to rationally design highly efficient and low-cost electrocatalysts for electrocatalysis applications.
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    82. 82
      Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23,  DOI: 10.1149/1.1856988
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      Trends in the exchange current for hydrogen evolution
      Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U.
      Journal of the Electrochemical Society (2005), 152 (3), J23-J26CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      A d. functional theory database of hydrogen chemisorption energies on close packed surfaces of a no. of transition and noble metals is presented. The bond energies are used to understand the trends in the exchange current for hydrogen evolution. A volcano curve is obtained when measured exchange currents are plotted as a function of the calcd. hydrogen adsorption energies and a simple kinetic model is developed to understand the origin of the volcano. The volcano curve is also consistent with Pt being the most efficient electrocatalyst for hydrogen evolution.
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      Hansen, M. H.; Stern, L.-A.; Feng, L.; Rossmeisl, J.; Hu, X. Widely Available Active Sites on Ni2P for Electrochemical Hydrogen Evolution─Insights from First Principles Calculations. Phys. Chem. Chem. Phys. 2015, 17, 10823– 10829,  DOI: 10.1039/c5cp01065a
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      Widely available active sites on Ni2P for electrochemical hydrogen evolution - insights from first principles calculations
      Hansen, Martin H.; Stern, Lucas-Alexandre; Feng, Ligang; Rossmeisl, Jan; Hu, Xile
      Physical Chemistry Chemical Physics (2015), 17 (16), 10823-10829CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The authors present insights into the mechanism and the active site for H evolution on Ni phosphide (Ni2P). Ni2P was recently discovered to be a very active nonprecious H evolution catalyst. Current literature attributes the activity of Ni2P to a particular site on the (0001) facet. Using D. Functional Theory (DFT) calcns., several widely available low index crystal facets on Ni2P have better properties for a high catalytic activity. DFT calcns. were used to identify moderately bonding Ni bridge sites and Ni hollow sites for H adsorption and to calc. barriers for the Tafel pathway. The studied surfaces in this study were the (10‾10), (‾1‾120), (11‾20), (11‾21) and (0001) facets of the hexagonal Ni2P crystal. In addn. to the DFT results, the authors present expts. on Ni2P nanowires growing along the 〈0001〉 direction, which are shown as efficient H evolution catalysts. The exptl. results add these nanowires to a variety of different morphologies of Ni2P, which are all active for HER.
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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.2c00085.

    • TGA profiles; atomic radii and XRD shifts; computational methods; electron microscopy, powder XRD, electrocatalytic HER, and post-HER catalyst characterizations; and computed surfaces and adsorption energies (PDF)


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