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Oxygen Reduction Reaction Activity and Stability of Shaped Metal-Doped PtNi Electrocatalysts Evaluated in Gas Diffusion Electrode Half-Cells
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Energy, Environmental, and Catalysis Applications

Oxygen Reduction Reaction Activity and Stability of Shaped Metal-Doped PtNi Electrocatalysts Evaluated in Gas Diffusion Electrode Half-Cells
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  • Shlomi Polani*
    Shlomi Polani
    Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-5961-0661
  • Raffaele Amitrano
    Raffaele Amitrano
    Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
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  • Adrian Felix Baumunk
    Adrian Felix Baumunk
    Friedrich-Alexander-Universität Erlangen Nürnberg, Power-to-X Technologies, Dr.-Mack-Straße 81, Fürth 90762, Germany
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    Orcidhttps://orcid.org/0009-0007-1401-4859
  • Lujin Pan
    Lujin Pan
    Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
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  • Jiasheng Lu
    Jiasheng Lu
    Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
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  • Nicolai Schmitt
    Nicolai Schmitt
    Ernst-Berl-Institute for Technical Chemistry and Macromolecular Science, Technical University of Darmstadt, Peter-Grünberg-Strasse 8, Darmstadt 64287, Germany
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    Orcidhttps://orcid.org/0000-0001-8668-288X
  • Ulrich Gernert
    Ulrich Gernert
    Center for Electron Microscopy (ZELMI), Technical University of Berlin, Berlin 10623, Germany
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  • Malte Klingenhof
    Malte Klingenhof
    Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
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  • Sören Selve
    Sören Selve
    Center for Electron Microscopy (ZELMI), Technical University of Berlin, Berlin 10623, Germany
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    Christian M. Günther
    Center for Electron Microscopy (ZELMI), Technical University of Berlin, Berlin 10623, Germany
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  • Bastian J. M. Etzold
    Bastian J. M. Etzold
    Friedrich-Alexander-Universität Erlangen Nürnberg, Power-to-X Technologies, Dr.-Mack-Straße 81, Fürth 90762, Germany
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    Orcidhttps://orcid.org/0000-0001-6530-4978
  • Peter Strasser*
    Peter Strasser
    Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-3884-436X
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ACS Applied Materials & Interfaces

Cite this: ACS Appl. Mater. Interfaces 2024, 16, 39, 52406–52413
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https://doi.org/10.1021/acsami.4c11068
Published September 19, 2024

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Abstract

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The synthesis of bimetallic and trimetallic platinum-based octahedral catalysts for the cathode of proton exchange membrane fuel cells (PEMFCs) is a particularly active area aimed at meeting technological requirements in terms of durability and cost. The electrocatalytic activity and stability of these shaped catalysts were tested at relatively high potentials (@0.9 V vs RHE) and at lower current densities using the rotating disk electrode, which is less suitable for assessing their behavior under the operating conditions of PEMFCs. In this work, we use a gas diffusion electrode (GDE) half-cell setup to test the performance of the catalysts under application-oriented conditions, relatively higher current densities, and a square-wave stability test. After the stability test, we analyzed the GDE catalytic layer to study the agglomeration and dissolution of the transition metal under these conditions by using high-resolution scanning electron microscopy and energy-dispersive X-ray spectroscopy. The present results provide valuable guidance for developing next-generation active and durable catalysts for PEMFCs.

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Introduction

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Passenger cars powered by hydrogen fuel cells have reached the early stages of commercialization. Despite significant advantages in terms of refueling convenience and driving range, durability and cost remain the biggest challenges to broader fuel cell vehicle deployment. This is due to the high Pt loading at the fuel cell cathode required to compensate for the kinetically sluggish oxygen reduction reaction (ORR) (still about 0.3 mgPt cm–2). (1) Therefore, current research is focused on identifying active and stable catalysts that can achieve and maintain the same or higher activity with lower Pt loading, preferably less than 0.125 mgPt cm–2 and 5000 h system durability with less than 10% loss of performance, the 2025 target of the United States Department of Energy (DOE). (2,3)
Among pure metals, Pt is the ideal catalyst for oxygen reduction. Alloying Pt with a transition metal not only lowers the cost of Pt-based catalysts but also greatly increases their catalytic activity due to geometric strain and electronic ligand effects. (4,5) As a result of this, the electrocatalytic ORR reactivity of the platinum–nickel (111) single crystal facets (6) is much superior to that of its (100) and (110) counterparts, ranging among the highest catalytic ORR activities ever recorded and reported.
The PtNi single-crystal work led to heightened interest in faceted PtNi nanocatalysts, particularly octahedral (oh-) catalysts with exclusive (111) orientation. (7) Although these catalysts have shown ever increasing Pt mass activity using the liquid-electrolyte electrochemical rotating disk electrode (RDE) screening method, they suffer from poor stability. This is due to the acidic environment (pH < 1) and the very positive electrode potentials (>0.7 V vs RHE), which lead to selective metal corrosion and leaching of the transition metals and make the overall activity of the catalyst gradually approach that of pure Pt. (8) In a membrane electrode assembly (MEA), relative humidity (RH) and the operating temperature have additional impacts on the dissolution of the non-noble metal and Pt mitigation. MEA tests with low RH have shown enhanced stability of a Pt alloy catalyst, whereas increasing operating temperatures led to growing degradation effects. (9,10) To increase the stability of oh-PtNi catalysts, doping with a third metal has been shown to be effective.
While the RDE setup has been convenient and useful to reject catalyst candidates based on their low kinetic catalytic reactivity, (11) this kinetic evaluation is limited to electrode potentials at low current densities commensurate with the oxygen solubility in the aqueous electrolyte. This is why the intrinsic activity of ORR catalysts can only be measured in a narrow potential range (0.85–0.95 V vs RHE) and at lower current densities compared to the operating conditions of a single MEA cell, the basic repeating unit in a fuel cell. (1) Another shortcoming is the successful prediction of catalytic activity and materials transfer from the RDE level to single-cell MEA. Typically, for Pt alloys, such attempts have resulted in a more than 3-fold drop-in activity in MEAs, which has prevented a successful transfer of high RDE-performing, shape-controlled octahedral catalysts to a MEA. (12,13) In addition, compared to the RDE, the fabrication of MEA requires a significant amount of catalyst for ink preparation and the deposition of the catalytic layer, so lab-scale synthesis must be scalable, which is not always straightforward, especially for shaped catalysts. (14)
Over the past years, the gas diffusion electrode (GDE) half-cell setup was introduced as a bridging technology between the RDE level and the MEA setup. (15) Similar to the RDE, the GDE half-cell requires less than 5 mg of catalysts for ink preparation and can serve as a fast method for the electrochemical screening of catalysts. (16) Furthermore, in a GDE flow cell, oxygen flows through one side of the electrode, while the other side is in a concentrated electrolyte. In this way, we can test the intrinsic activity of catalysts in a broader potential range (0.6–0.95 V vs RHE) and higher current densities (∼3000 mA cm–2) than the RDE, which are more relevant to the operation conditions of a proton exchange membrane fuel cell (PEMFC) cathode (0.65–0.8 V vs RHE), but still in a liquid medium. (15,17) In a recent study, it was shown that the GDE half-cell setup is able to correctly predict trends for the MEA activity, which underlines its potential as bridging technology. (18)
To test the stability of the doped oh-PtNi catalysts under appropriate conditions, (19) we used the US DOE recommended accelerated stress test (AST) protocol, (20) which includes 30,000 potential cycles between 0.6 and 0.95 V vs RHE with a hold time of 3 s at each potential limit, which is an AST protocol that corresponds to a predicted system lifetime of 5000 h. (21)
Here, we demonstrate high catalytic ORR reactivity and stability of doped octahedral oh-PtNi catalysts in a GDE half-cell system. We performed a square-wave AST protocol to evaluate and compare the stability of Mo- and MoRh-doped oh-PtNi catalysts with that of Pt/C, the benchmark catalyst. In addition, we analyzed the GDE catalytic layer before and after the stability tests to investigate Ni dissolution and particle agglomeration. Our study identifies new promising Pt alloy ORR catalyst candidates and their ink and layer preparation protocols for improved PEMFC cathodes.

Experimental Section

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Materials

Platinum(II) acetylacetonate [Pt(acac)2] (98%) was purchased from Agros Organics. Nickel(II) acetylacetonate [Ni(acac)2] (≥98%) was purchased from Merck KGaA. Molybdenum hexacarbonyl (≥99.9% trace metals basis) was purchased from Sigma-Aldrich. Rhodium(III) acetylacetonate [Rh(acac)3] (Rh 25.2% min), benzoic acid (99%), and polyvinylpyrrolidone (PVP, M.W. 10,000) were purchased from Alfa Aesar. Benzyl alcohol (BA, ≥99%) was purchased from Carl Roth. Carbon Vulcan (XC72-R) was purchased from Cabot. For the washing, ethanol (VWR, absolute, ≥99.8%), acetone (VWR, absolute, ≥99.5%), and ultrapure water (Milli-Q, 18.2 MΩ) were used. All of the chemicals were used as received without any purification.

Synthesis of PtNi(Mo)/C and PtNi(MoRh)/C Octahedral Nanoparticles

In a typical one-pot synthesis of Ni-rich octahedral-shaped nanoparticles (NPs), 64 mg of Pt(acac)2, 200 mg of Ni(acac)2, 20 mg of Mo(CO)6, in the case of Rh doping 20 mg of Rh(acac)3, 640 mg of PVP (M.W. 10,000), 400 mg of benzoic acid, and BA (60 mL) were added into a 100 mL pressure vessel with a stirring bar. The reaction mixture was heated from room temperature to 60 °C and kept for 1 h under vigorous stirring. At the same time in a glass vial with a magnetic stir bar, a suspension of 75 mg of Vulcan carbon XC72-R in 10 mL of BA was prepared and stirred overnight. The reaction solution was heated to 150 °C and kept for 12 h while stirring the mixture during the whole solvothermal treatment. The solution was allowed to cool to room temperature and the carbon suspension was added. The mixture was stirred overnight, followed by the washing procedure as described in the “Washing Procedure” section below.

Washing Procedure

After the mixture was stirred overnight, the pressure vessel was opened, and the suspension was centrifuged in a 250 mL container. The particles were dispersed in 120 mL of a 2:1 solution of acetone and ethanol, and the centrifuge container was ultrasonicated (5–10 min). After centrifugation (10 min, 7500 rpm) of the suspension, solvents were discarded and the NPs were air-dried overnight.

Characterization

X-ray powder diffraction (XRD) patterns were collected using a D8 ADVANCE diffractometer (Bruker) equipped with a LYNXEYE detector and a KFL Cu 2K X-ray tube. The measurement was carried out at a step size of 0.04°, in a 2θ range between 20 and 85°. The atomic composition of the different NPs was determined by inductively coupled plasma optical emission spectrometry (ICP–OES) using a 715-ES-ICP analysis system (Varian). Transmission electron microscopy (TEM) images were recorded on a FEI Tecnai G2 20 S-TWIN with a LaB6 cathode operating with 200 kV acceleration voltage and a resolution limit of 0.24 nm. Scanning electron microscopy (SEM) images were obtained using a Hitachi SU8030 instrument with a cold field emission gun operating with a 10 kV acceleration voltage. Corresponding energy-dispersive X-ray (EDX) spectroscopy analysis was established by an Ametek Inc., EDAX APEX2.5.

Electrochemical Measurements

For the determination of electrochemical surface areas (ECSAs), activities, and stabilities of the catalysts, a RDE three-electrode cell setup in a liquid electrolyte with a glassy carbon (GC) working electrode (WE) was used. The electrochemical cell consisted of a Pt mesh as the counter electrode (CE), an RHE or MMS reference electrode (RE), and a GC as the WE. The WE was controlled by a rotator from Pine Research Instrumentation. Electrochemical measurements were performed with a BioLogics Science Instruments potentiostat model SP-150 and VSP. Potentials obtained with the MMS RE were converted to the RHE, and the potential difference between the MMS and the RHE was measured in a H2-saturated 0.1 M HClO4 solution. The catalyst ink for the RDE was prepared by ultrasonication of 1.8–3 mg of catalyst in a solution consisting of 79.6 vol % deionized water, 20 vol % isopropanol, and 0.4 vol % Nafion ionomer (5 wt %, Sigma-Aldrich). Then, 10 μL of the homogeneous ink was deposited on a polished GC-RDE, followed by drying at 60 °C. The Pt loading of the catalysts on the electrodes was 10 μgPt cm–2. The catalyst thin film was first activated by cyclic voltammetry (CV) from 0.05 to 0.925 VRHE (50 cycles) at a scan rate of 100 mV s–1 in N2-saturated 0.1 M HClO4. The ECSA of the catalysts was determined by Hupd- and CO-stripping experiments. The catalytic ORR activity was tested by using linear sweep voltammetry (LSV) from 0.05 to 1.0 V vs RHE at a scan rate of 20 mV s–1 in O2-saturated 0.1 M HClO4 with a rotation speed of 1600 rpm. All obtained potentials were corrected by using IR correction. The resistance (R) was determined by potentiostatic electrochemical impedance spectroscopy (PEIS). The AST was conducted by performing square-wave potential cycling from 0.6 to 0.95 V vs RHE for 30,000 cycles with a dwelling time of 3 s for each potential in N2-saturated 0.1 M HClO4. Further details of the RDE measurements are summarized in the Supporting Information.

GDE Preparation and Testing

GDE characterization was performed using a commercial half-cell (FlexCell PTFE, Gaskatel GmbH) operated at room temperature in 2 M HClO4. The used cell consists of a gas chamber and an electrolyte chamber, which includes a separate reservoir for the RE (HydroFlex, Gaskatel GmbH), which is connected to the WE via a Luggin capillary. The utilized half-cell setup is a three-electrode cell that uses a RHE as the RE, a titanium expanded metal coated with Ir/Ta mixed oxide as the CE, and the GDE as the WE. Electrochemical measurements were carried out on an Ivium multichannel potentiostat (OctoStat 5000) which is controlled by IviumSoft software. The catalyst ink for the GDE was prepared by mixing 3 mg of catalyst with a 4:1 solution of deionized water and isopropanol and further mixed with Nafion ionomer (5 wt %, Sigma-Aldrich) to obtain an ionomer to carbon ratio (I/C) of 0.5 g/g. The total volume was 3 mL to get an ink with a concentration of 1 mgcatalyst/mLink. The ink was dispersed 3 times, 1 min each, by an ultrasonic processor (Hielscher, UP200 St). For GDE preparation, the catalysts were deposited on a gas diffusion layer (GDL, Sigracet 25 BC) by using a drop-casting method, resulting in catalyst loadings of 92, 42, and 26 μgPt cm–2 for HiSPEC 3000, PtNi(Mo)/C, and PtNi(MoRh)/C, respectively. Detailed information on the GDE preparation is given in Supporting Information. The GDE was pretreated by CV from 0.05 to 1.2 V vs RHE (200 cycles) at a scan rate of 500 mV s–1 in a N2 atmosphere to activate the catalyst. The ECSA of the catalysts was determined by Hupd- and CO-stripping experiments. The ORR activity was measured by conducting a protocol consisting of galvanostatic steps. For the construction of the polarization curves, the average of the last five points (2.5 s) of each step is used. The uncompensated resistance (IR drop) in the used GDE setup was determined by using EIS and was used for correction of all measured potentials. For each experiment, at least two samples were tested. An overview on the applied measurement protocol is given in Table S2. A square-wave cycling between 0.6 and 0.95 V vs RHE with 3 s holding time was carried out for a total of 30,000 cycles. Before the test and after a total number of 5000, 15,000, and 30,000 cycles, a complete electrochemical characterization was performed according to Table S2. The AST was performed under a N2 atmosphere and 2 M HClO4.

Results and Discussion

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Physicochemical Characterization

A number of different Mo- and RhMo-doped octahedrally shaped oh-PtNi catalysts were synthesized using a solvothermal protocol published elsewhere. (22,23) The syntheses resulted in Ni-rich Mo- or MoRh-doped oh-NPs with mean edge-length particle sizes of 13.1 and 11.3 nm, respectively. Particle sizes were manually measured using acquired TEM (Figure 1). Nominal nickel and platinum ratios as well as the actual resulting Pt/Ni atomic ratios of the NPs are provided in Table S3. The X-ray diffraction patterns of the Ni-rich Mo- or MoRh-doped oh-PtNi NPs are shown in Figure S2, where the reflections shifted to lower 2θ angles with increasing platinum content.

Figure 1

Figure 1. Bright-field TEM images of (a) carbon-supported Ni-rich Mo-doped oh-PtNi NPs (Pt27.4Ni71.8Mo0.8) and (b) carbon-supported Ni-rich MoRh-doped oh-PtNi NPs (Pt22.2Ni75.8Mo1.0Rh1.0); insets: edge length histograms and distribution function and mean edge length (see Table S3 for additional compositional information).

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Electrochemical Characterization Using RDE Half-Cells

We used CV in the RDE setup to obtain electrochemical Hupd- and CO-based stripping charges and the corresponding ECSAs of the oh-Pt alloy catalysts in N2- or CO-saturated electrolytes, respectively (Figure S3). We also determined the catalytic reactivity for the ORR using LSV in the O2-saturated electrolyte (Figure 2a–c). The Hupd-based ECSAs were, as expected and well-documented, lower than the values obtained from CO-stripping measurements. Also, the Hupd- and CO-based ECSA values of the HiSPEC 3000 Pt/C benchmark catalyst were, as expected, larger than those of the oh-Pt alloy catalysts, with the variance between the two getting smaller after the ASTs (Table 1) suggesting gradual non-noble transition metal leaching. Indeed, the catalyst morphology after the AST showed a hexapod structure, which is the result of Ni leaching from the octahedron facets (Figure S4). (24)

Figure 2

Figure 2. Electrochemical polarization curves and MAs of activated electrocatalysts obtained by the RDE half-cell. ORR LSVs of (a) Mo-doped oh-PtNi, (b) MoRh-doped oh-PtNi, (c) Pt/C HiSPEC 3000, and the (d) corresponding MAs at 0.9 V vs RHE evaluated before and after the AST (30,000 cycles between 0.6 and 0.95 V vs RHE), calculated according to the Koutecký–Levich equation.

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Table 1. Overview of the Hupd- and CO-Based ECSA (RDE) before and after the AST (30k)
sampleECSAHupd-based [m2/gPt]ECSACO-based [m2/gPt]
 before ASTafter ASTbefore ASTafter AST
PtNi(Mo)/C33.3327.0242.1633.74
PtNi(MoRh)/C39.5738.0452.4344.54
HiSPEC 300068.3147.4969.4652.32
After a Koutecký–Levich mass transport correction (eq 4, Supporting Information), we normalized the LSV currents obtained at 0.9 V vs RHE with the Pt mass deposited on the WE to obtain the mass activities (MAs) for the different catalysts before and after the AST (Figure 2d). Both the Mo- and MoRh-doped oh-PtNi catalysts exhibited 4.8- and 5.3-fold higher initial MAs compared with the commercial reference Pt/C (0.3 A mgPt–1), respectively. After the AST, the Mo-doped oh-PtNi catalyst retained 73% of its initial MA (1.45 A mgPt–1), while the MoRh-doped catalysts exhibited even increased MA reaching 2.03 A mgPt–1, a unique property for Rh-doped oh-PtNi catalysts. (23,25)

Electrochemical Characterization Using a GDE Half-Cell Setup

Gas diffusion electrons were prepared by spraying catalyst inks onto porous GDLs with microporous layers (Figure S18). To control the active spray-coated electrode surface area to precisely 0.5 cm2 on the GDL, we utilized custom-made PTFE masking schemes. To determine the precise catalyst loading on the GDE with a 0.5 cm2 coated area after removing the PTFE mask, we used the ratio of CO-based ECSAs with and without masking (Table S4). (26) The Hupd- and CO-based ECSAs obtained from the GDE were comparable to values acquired by the RDE and showed the same trends of decreasing ECSA during and after the ASTs (Figures S3, S5, and S6 and Table S5). Consistently, the ECSA values based on CO stripping were higher compared to the values based on Hupd for all catalysts tested. (15,27)
To obtain the polarization curves for the different catalysts with the GDE flow cell setup, we used a galvanostatic method, as this is a widely used electrochemical method for fuel cell testing. Also, this method has been shown to lead to better comparability between different catalyst loadings and thus different catalysts compared to ORR test protocols with potentiostatic control. (26,28,29) Typical geometric Pt mass loadings on the GDE ranged around 0.026–0.092 mgPt cm–2, compared to 0.01 mgPt cm–2 in the RDE. Catalytic ORR activity became apparent for all catalysts cathodic of 0.9 V vs RHE (Figure 3). This onset potential was slightly more cathodic compared to those obtained in RDE setups, likely due to inhibition by the much higher concentrated perchloric acid. (30) Accordingly, the initial MAs of the oh-Pt alloy catalysts obtained from RDE tests at a cathode potential of 0.9 V vs RHE ranged higher than those obtained in the GDE setup @0.85 V vs RHE (0.1 and 2 M perchloric solution, respectively, Figures 2d and 3c). Clearly, the GDE setup was able to achieve ORR current densities in the A cm–2 range and therefore represented a much more relevant and predictive electrode performance for PEM fuel cell membrane electrolyte assemblies. Throughout all electrode potentials, the shaped Pt alloy catalysts exhibited much higher MAs than the Pt/C benchmark catalysts (Figures 2 and 3). In addition, the shaped catalysts outperformed the Pt/C catalysts in terms of stability during and after the ASTs and exceeded the catalytic activity of the Pt/C catalysts at every interval of 5000 stability cycles. For example, the Pt/C MA @0.75 V vs RHE lacked behind that of the shaped catalysts @0.8 V vs RHE. After the AST, the Mo-doped oh-catalyst retained most of its initial MA, while MoRh-doped catalysts lost about 30% of it @0.8 V vs RHE. After the galvanostatic steps in oxygen, we also tested the oh-Pt alloy catalysts with synthetic air feed (see Table S2 for the testing protocol). The catalysts showed the same activity and stability trends but at lower absolute values, as expected for a lower partial pressure oxygen feed (Figure S9). At potentials less than 0.65 V vs RHE, the shaped oh-Pt alloy catalysts began to lose their advantages over the Pt/C catalyst, probably due to mass transport limitations and low Pt loadings. (28)

Figure 3

Figure 3. GDE half-cell measurements: electrochemical polarization curves and extracted Pt MAs of oh-Pt alloy electrocatalysts obtained in the GDE half-cell setup. ORR polarization curves of Mo-doped oh-PtNi, MoRh-doped oh-PtNi, and Pt/C HiSPEC 3000 (a) before the AST, (b) after the AST, and the (c) corresponding calculated after 5k, 15k, and 30k cycles at 0.75, 0.8, and 0.85 V vs RHE.

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Important stability differences between RDE and GDE testing became apparent as well. The RDE results predicted that the MoRh-doped oh-Pt alloy catalyst showed a higher MA at 0.9 V vs RHE both before and after the AST compared to the Mo-doped catalyst (Figure 2d). By contrast, the GDE results at 0.85, 0.8, and 0.75 V vs RHE revealed an opposite trend after a 15,000-cycle AST (Figure 3c). This could indicate that the MoRh-doped catalyst degraded faster than the Mo-doped catalyst under GDE conditions. One reason for that could be distinct Ni-leaching rates due to the higher (2 M) acid concentration in the GDE.
As done for RDE-tested electrodes, we used SEM to investigate the morphology of the catalyzed GDE before and after ASTs. We found that after the AST, the morphology of the initially oh-shaped doped PtNi/C catalysts turned into a hexapod structure, obviously a result of significant Ni leaching from the octahedral facets (Figure 4a,b,d,e). The Ni leaching was also confirmed by the EDX spectroscopy measurements on the catalysts, which showed that the Pt/Ni ratio increased after ASTs (Figures S10–S17). The back-scattered image evidences the characteristic Ni-leached multipod morphology (Figure S19).

Figure 4

Figure 4. SEM images with a magnification of 250k of the as-prepared (a) Ni-rich Mo-doped oh-PtNi NPs, (b) Ni-rich MoRh-doped oh-PtNi, and (c) Pt/C HiSPEC 3000 catalysts collected with the secondary electron detector and of the corresponding tested catalysts (after the AST) in (d–f), respectively. The colored insets in panels (d,e) highlight the zoomed-in views of the aged Mo- (red) and MoRh-doped (blue) oh-PtNi NPs. Bright-field TEM images of Pt/C HiSPEC 3000 NPs (g) before and (h) after the AST and the particle size distribution in the insets.

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Noteworthily, the oh-shaped doped PtNi/c catalysts did not exhibit apparent particle agglomeration (Figures S19 and S20), while the Pt/C catalyst increased its mean particle size from 3.4 to 6.1 nm due to particle agglomeration or Ostwald ripening, as shown in Figure 4c,f–h. These observations are in line with similar reports (31) based on identical location-TEM techniques. The increase in Pt/C particle size is supported by the relatively steep decrease in its ECSA (Figures S5 and S6).
Since testing catalysts for ORR using a GDE half-cell setup are still poorly used or reported, there are only a few comparable reports in the literature, especially for bimetallic catalysts. (11,15,16,26,32) While the Pt/C catalyst had comparable activity values to previously published GDE results, GDE-based stability data of oh-shaped Pt-based alloy catalysts have never been reported before. However, the MEA-based activity of doped octahedral PtNi catalysts was published earlier. Dionigi et al. (33) reported RDE activity more than 7 times higher than MEA activity (3.43 and 0.45 A mgPt–1 @0.9 V vs RHE, respectively). Pan et al. (34) reported a MEA-based MA of 0.35 A mgPt–1 at 0.9 V vs RHE but a significantly high performance at high current density for octahedral PtNi NPs, 1500 mA cm–2 at 0.6 V vs RHE, to our knowledge the highest to date for this class of catalysts.
Given the more realistic testing environments, the larger current densities, and the prevention of oxygen mass transport limitations in the GDE setup, we consider our obtained catalytic performance and stability results as much more reliable and predictive for single PEM fuel cell testing compared to conventional RDE test data.

Conclusions

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We report and discuss a comparative investigation of the catalytic ORR performance and stability of carbon-supported shape-controlled Mo- and MoRh-doped oh-PtNi/C catalysts in conventional RDE half-cells and GDE half-cells. The ORR activity and stability were tested in RDE half-cell and GDE flow half-cell setups in (RDE typical) lower and (GDE typical) higher overpotential (and hence current density) regions, respectively. The shaped doped Pt alloy catalysts showed superior ORR activity compared to the Pt/C benchmark catalyst over a wide range of working potentials from 0.65 to 0.85 V vs RHE. We tested the catalyst stability using a recent DOE-issued square-wave stability test. In doing so, we found that the octahedral catalysts maintained their excellent activity throughout the stability test and after 30,000 stability cycles. While the Pt/C catalyst showed agglomeration accompanied by a sharp decrease in its ECSA, the octahedral catalysts did not show significant particle agglomeration, and only a slight decrease in their ECSAs was observed. Given the larger current densities, the GDE test results feature a more reliable predictive character for their electrode performance in full PEM fuel cells compared to RDE tests, in particular, for catalyst stability under fully humidified conditions, where incorrect RDE-based stability predictions of PEMFC results are common. Our electrochemical performance data of Pt-based oh-Pt alloy catalysts at low Pt loadings are a testament to exceptionally high Pt MAs over a wide range of electrode potentials and current densities (several A cm–2 range) combined with excellent stability. While these results provide new specific catalyst candidates for deployment at cathodes in PEMFCs, our comparative approach highlights the practical value of advanced shaped catalysts in both the low- and high-current-density regimes, the latter of which has been underutilized to date.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c11068.

  • Detailed information on the synthesis, chemical, and structural characterization of the catalyst by ICP–OES, TEM, and XRD; electrochemical performance and stability protocols, as well as the data obtained by RDE and GDE half-cell experiments; and SEM images and EDX data of the GDEs before and after ASTs (PDF)

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  • Corresponding Authors
  • Authors
    • Raffaele Amitrano - Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
    • Adrian Felix Baumunk - Friedrich-Alexander-Universität Erlangen Nürnberg, Power-to-X Technologies, Dr.-Mack-Straße 81, Fürth 90762, Germany;  Orcidhttps://orcid.org/0009-0007-1401-4859
    • Lujin Pan - Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
    • Jiasheng Lu - Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
    • Nicolai Schmitt - Ernst-Berl-Institute for Technical Chemistry and Macromolecular Science, Technical University of Darmstadt, Peter-Grünberg-Strasse 8, Darmstadt 64287, Germany;  Orcidhttps://orcid.org/0000-0001-8668-288X
    • Ulrich Gernert - Center for Electron Microscopy (ZELMI), Technical University of Berlin, Berlin 10623, Germany
    • Malte Klingenhof - Electrochemical Energy, Catalysis and Material Science Laboratory, Department of Chemistry, Technical University Berlin, Berlin 10623, Germany
    • Sören Selve - Center for Electron Microscopy (ZELMI), Technical University of Berlin, Berlin 10623, Germany
    • Christian M. Günther - Center for Electron Microscopy (ZELMI), Technical University of Berlin, Berlin 10623, Germany;  Orcidhttps://orcid.org/0000-0002-3750-7556
    • Bastian J. M. Etzold - Friedrich-Alexander-Universität Erlangen Nürnberg, Power-to-X Technologies, Dr.-Mack-Straße 81, Fürth 90762, Germany;  Orcidhttps://orcid.org/0000-0001-6530-4978
  • Author Contributions

    S.P. and R.A. contributed equally. The manuscript was written by contributions of all authors. All authors have agreed to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors are grateful for financial support from the Deutsche Forschungsgemeinschaft (DFG) under grant number STR 596/18-1. This project has received funding from the European Union under grant number 10110149 and the Clean Hydrogen Partnership under grant agreement no. 101101346. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research. Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the Clean Hydrogen Joint Undertaking can be held responsible for them. B.E. acknowledges the support by the Bavarian State Ministry for Science and Arts through the Distinguished Professorship Program.

References

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This article references 34 other publications.

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    The heterogeneity of polymer electrolyte fuel cell catalyst degrdn. is studied under varied relative humidity and types of feed gas. Accelerated stress tests (ASTs) are performed on four membrane electrode assemblies (MEAs) under wet and dry conditions in an air or nitrogen environment for 30 000 square voltage cycles. The largest electrochem. active area loss is obsd. for MEA under wet conditions in a nitrogen gas environment AST due to const. upper potential limit of 0.95 V and significant water content. The mean Pt particle size is larger for the ASTs under wet conditions compared to dry conditions, and the Pt particle size under land is generally larger than under the channel. Observations from ASTs in both conditions and gas environments indicate that water content promotes Pt particle size growth. ASTs under wet conditions and an air environment show the largest difference in Pt particle size growth for inlet vs. outlet and channel vs. land, which can be attributed to larger water content at outlet and under land compared to inlet and under channel. From X-ray fluorescence expts. Pt particle size increase is a local phenomenon as Pt loading remains relatively uniform across the MEA.
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  22. 22
    Polani, S.; MacArthur, K. E.; Kang, J.; Klingenhof, M.; Wang, X.; Möller, T.; Amitrano, R.; Chattot, R.; Heggen, M.; Dunin-Borkowski, R. E. Highly Active and Stable Large Mo-Doped Pt–Ni Octahedral Catalysts for ORR: Synthesis, Post-treatments, and Electrochemical Performance and Stability. ACS Appl. Mater. Interfaces 2022, 14, 29690– 29702,  DOI: 10.1021/acsami.2c02397
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    22
    Highly Active and Stable Large Mo-Doped Pt-Ni Octahedral Catalysts for ORR: Synthesis, Post-treatments, and Electrochemical Performance and Stability
    Polani, Shlomi; MacArthur, Katherine E.; Kang, Jiaqi; Klingenhof, Malte; Wang, Xingli; Moeller, Tim; Amitrano, Raffaele; Chattot, Raphael; Heggen, Marc; Dunin-Borkowski, Rafal E.; Strasser, Peter
    ACS Applied Materials & Interfaces (2022), 14 (26), 29690-29702CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Over the past decade, advances in the colloidal syntheses of octahedral-shaped Pt-Ni alloy nanocatalysts for use in fuel cell cathodes have raised our at.-scale control of particle morphol. and surface compn., which, in turn, helped raise their catalytic activity far above that of benchmark Pt catalysts. Future fuel cell deployment in heavy-duty vehicles caused the scientific priorities to shift from alloy particle activity to stability. Larger particles generally offer enhanced thermodn. stability, yet synthetic approaches toward larger octahedral Pt-Ni alloy nanoparticles have remained elusive. In this study, we show how a simple manipulation of solvothermal synthesis reaction kinetics involving depressurization of the gas phase at different stages of the reaction allows tuning the size of the resulting octahedral nanocatalysts to previously unachieved scales. We then link the underlying mechanism of our approach to the classical "LaMer" model of nucleation and growth. We focus on large, annealed Mo-doped Pt-Ni octahedra and investigate their synthesis, post-synthesis treatments, and elemental distribution using advanced electron microscopy. We evaluate the electrocatalytic ORR performance and stability and succeed to obtain a deeper understanding of the enhanced stability of a new class of relatively large, active, and long-lived Mo-doped Pt-Ni octahedral catalysts for the cathode of PEMFCs.
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    Hornberger, E.; Klingenhof, M.; Polani, S.; Paciok, P.; Kormányos, A.; Chattot, R.; MacArthur, K. E.; Wang, X.; Pan, L.; Drnec, J. On the electrocatalytical oxygen reduction reaction activity and stability of quaternary RhMo-doped PtNi/C octahedral nanocrystals. Chem. Sci. 2022, 13, 9295– 9304,  DOI: 10.1039/D2SC01585D
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    23
    On the electrocatalytical oxygen reduction reaction activity and stability of quaternary RhMo-doped PtNi/C octahedral nanocrystals
    Hornberger, Elisabeth; Klingenhof, Malte; Polani, Shlomi; Paciok, Paul; Kormanyos, Attila; Chattot, Raphael; MacArthur, Katherine E.; Wang, Xingli; Pan, Lujin; Drnec, Jakub; Cherevko, Serhiy; Heggen, Marc; Dunin-Borkowski, Rafal E.; Strasser, Peter
    Chemical Science (2022), 13 (32), 9295-9304CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Recently proposed bimetallic octahedral Pt-Ni electrocatalysts for the oxygen redn. reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes suffer from particle instabilities in the form of Ni corrosion and shape degrdn. Advanced trimetallic Pt-based electrocatalysts have contributed to their catalytic performance and stability. In this work, we propose and analyze a novel quaternary octahedral (oh-)Pt nanoalloy concept with two distinct metals serving as stabilizing surface dopants. An efficient solvothermal one-pot strategy was developed for the prepn. of shape-controlled oh-PtNi catalysts doped with Rh and Mo in its surface. The as-prepd. quaternary octahedral PtNi(RhMo) catalysts showed exceptionally high ORR performance accompanied by improved activity and shape integrity after stability tests compared to previously reported bi- and tri-metallic systems. Synthesis, performance characteristics and degrdn. behavior are investigated targeting deeper understanding for catalyst system improvement strategies. A no. of different operando and online anal. techniques were employed to monitor the structural and elemental evolution, including identical location scanning transmission electron microscopy and energy dispersive X-ray anal. (IL-STEM-EDX), operando wide angle X-ray spectroscopy (WAXS), and online scanning flow cell inductively coupled plasma mass spectrometry (SFC-ICP-MS). Our studies show that doping PtNi octahedral catalysts with small amts. of Rh and Mo suppresses detrimental Pt diffusion and thus offers an attractive new family of shaped Pt alloy catalysts for deployment in PEMFC cathode layers.
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    Gan, L.; Cui, C.; Heggen, M.; Dionigi, F.; Rudi, S.; Strasser, P. Element-specific anisotropic growth of shaped platinum alloy nanocrystals. Science 2014, 346, 1502– 1506,  DOI: 10.1126/science.1261212
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    Element-specific anisotropic growth of shaped platinum alloy nanocrystals
    Gan, Lin; Cui, Chunhua; Heggen, Marc; Dionigi, Fabio; Rudi, Stefan; Strasser, Peter
    Science (Washington, DC, United States) (2014), 346 (6216), 1502-1506CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Morphol. shape in chem. and biol. owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-cor. scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chem. degrdn. pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties.
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    Beermann, V.; Gocyla, M.; Willinger, E.; Rudi, S.; Heggen, M.; Dunin-Borkowski, R. E.; Willinger, M. G.; Strasser, P. Rh-Doped Pt-Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability. Nano Lett. 2016, 16, 1719– 1725,  DOI: 10.1021/acs.nanolett.5b04636
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    Rh-Doped Pt-Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability
    Beermann, Vera; Gocyla, Martin; Willinger, Elena; Rudi, Stefan; Heggen, Marc; Dunin-Borkowski, Rafal E.; Willinger, Marc-Georg; Strasser, Peter
    Nano Letters (2016), 16 (3), 1719-1725CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Thanks to their remarkably high activity toward oxygen redn. reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt-Ni octahedral nanoparticles with high activities up to 1.14 A mgPt-1 combined with improved performance and shape stability compared to previous bimetallic Pt-Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and at. degrdn. mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphol. particle shape and consequently their performance. Rh surface-doped octahedral Pt-Ni particles were prepd. at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30 000 potential cycles, while undoped bimetallic ref. nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed at. insight in these observations is obtained from aberration-cor. scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) anal. Our anal. shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissoln. of Ni that constitutes the primary origin of the octahedral shape loss for Pt-Ni nanoparticles. Using small amts. of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt-Ni nanoparticles.
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    Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces
    van der Vliet, Dennis F.; Wang, Chao; Li, Dongguo; Paulikas, Arvydas P.; Greeley, Jeffrey; Rankin, Rees B.; Strmcnik, Dusan; Tripkovic, Dusan; Markovic, Nenad M.; Stamenkovic, Vojislav R.
    Angewandte Chemie, International Edition (2012), 51 (13), 3139-3142, S3139/1-S3139/5CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Authors demonstrated that by alloying platinum with a non-noble second metal and inducing a Pt-skin-type structure, the adsorption properties of formed structures are significantly changed. The adsorption of hydrogen and oxide species is shifted in potential and reduced in magnitude compared to platinum. The LEIS data showed that the surface consists only of platinum. The suppression of hydrogen adatoms on platinum underestimate the surface area and overrates the specific activity. The Co stripping can be used alongside with the adsorbed hydrogen charge for the detn. of the electrochem. active surface area of platinum-alloy catalysts.
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    Ehelebe, K.; Seeberger, D.; Paul, M. T. Y.; Thiele, S.; Mayrhofer, K. J. J.; Cherevko, S. Evaluating Electrocatalysts at Relevant Currents in a Half-Cell: The Impact of Pt Loading on Oxygen Reduction Reaction. J. Electrochem. Soc. 2019, 166, F1259– F1268,  DOI: 10.1149/2.0911915jes
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    Evaluating electrocatalysts at relevant currents in a half-cell: the impact of Pt loading on oxygen reduction reaction
    Ehelebe, Konrad; Seeberger, Z. Dominik; Paul, Mike T. Y.; Thiele, Simon; Mayrhofer, Karl J. J.; Cherevko, Serhi
    Journal of the Electrochemical Society (2019), 166 (16), F1259-F1268CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    In this work gas diffusion electrode (GDE) half-cells expts. are proposed as powerful tool in fuel cell catalyst layer evaluation as it is possible to transfer the advantages of fundamental methods like thin-film rotating disk electrode (TF-RDE) such as good comparability of results, dedicated elimination of undesired parameters etc. to relevant potential ranges for fuel cell applications without mass transport limitations. With the developed setup and electrochem. protocol, 1st expts. on different Pt/C loadings confirm excellent reproducibility. Thereby mass-specific current densities up to 30 A mgPt-1 at 0.6 V vs. RHE are achieved. From a methodol. perspective, good comparability to single cell measurements is obtained after theor. corrections for temp. and concn. effects. In comparison to previous studies with GDE half-cells, polarization curves without severe mass transport limitations are recorded in a broad potential window. All these achievements indicate that the proposed method can be an efficient tool to bridge the gap between TF-RDE and single cell expts. by providing fast and dedicated insights into the effects of catalyst layers on O redn. reaction performance. This method will enable straightforward and efficient optimization of catalyst layer compn. and structure, esp. for novel catalysts, thereby contributing to the performance enhancements of fuel cells with reduced Pt loading.
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    Pinaud, B. A.; Bonakdarpour, A.; Daniel, L.; Sharman, J.; Wilkinson, D. P. Key Considerations for High Current Fuel Cell Catalyst Testing in an Electrochemical Half-Cell. J. Electrochem. Soc. 2017, 164, F321– F327,  DOI: 10.1149/2.0891704jes
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    Key Considerations for High Current Fuel Cell Catalyst Testing in an Electrochemical Half-Cell
    Pinaud, Blaise A.; Bonakdarpour, Arman; Daniel, Lius; Sharman, Jonathan; Wilkinson, David P.
    Journal of the Electrochemical Society (2017), 164 (4), F321-F327CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    An economical and novel half-cell approach for quick and precise measurement of oxygen redn. catalysts in various practical electrode formats, including GDE and bonded GDE/membrane layers is demonstrated. Fuel cell current densities (∼1 A/cm2) were achieved with the key design considerations clearly outlined, to highlight the challenges in developing such a test platform. Const. current polarizations with IR drop measurement at each step are found to provide a simple, reproducible method of measuring catalyst performance. An optimal electrolyte concn. of 1.0 M HClO4 balances the requirement of non-limiting H+ transport at high currents while minimizing the effect of electrolyte impurities, which reduce Pt activity. The half-cell used here can accurately provide the ORR activity of com. products showing good agreement with measurements made in fuel cell hardware, but at a significantly lower testing cost. This half-cell approach can be used to characterize various types of GDEs and CCMs with different catalyst layers.
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    Attard, G. A.; Brew, A.; Hunter, K.; Sharman, J.; Wright, E. Specific adsorption of perchlorate anions on Pt{hkl} single crystal electrodes. Phys. Chem. Chem. Phys. 2014, 16, 13689– 13698,  DOI: 10.1039/C4CP00564C
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    Specific adsorption of perchlorate anions on Pt{hkl} single crystal electrodes
    Attard, Gary A.; Brew, Ashley; Hunter, Katherine; Sharman, Jonathan; Wright, Edward
    Physical Chemistry Chemical Physics (2014), 16 (27), 13689-13698CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The voltammetry of Pt{111}, Pt{100}, Pt{110} and Pt{311} single crystal electrodes as a function of perchloric acid concn. (0.05-2.00 M) was studied to test the assertion made in recent reports by Watanabe et al. that perchlorate anions specifically adsorb on polycryst. platinum. Such an assertion would have significant ramifications for the authors' understanding of electrocatalytic processes at platinum surfaces since perchlorate anions at low pH have classically been assumed not to specifically adsorb. For Pt{111}, OHad and electrochem. oxide states are both perturbed significantly as perchloric acid concn. is increased. Probably this is due to specific adsorption of perchlorate anions competing with OHad for adsorption sites. The hydrogen underpotential deposition (H UPD) region of Pt{111} however remains unchanged although evidence for perchlorate anion decompn. to chloride on Pt{111} is reported. In contrast, for Pt{100} no variation in the onset of electrochem. oxide formation is found nor any shift in the potential of the OHad state which normally results from the action of specifically adsorbing anions. Probably perchlorate anions are nonspecifically adsorbed on this plane although strong changes in all H UPD states are obsd. as perchloric acid concn. is increased. This manifests itself as a redistribution of charge from the H UPD state situated at more pos. potential to the one at more neg. potential. For Pt{110} and Pt{311}, marginal changes in the onset of electrochem. oxide formation are recorded, assocd. with specific adsorption of perchlorate. Specific adsorption of perchlorate anions on Pt{111} is deleterious to electrocatalytic activity in relation to the oxygen redn. reaction (ORR) as measured using a rotating disk electrode (RDE) in a hanging meniscus configuration. This study supports previous work suggesting that a large component of the ORR activity on platinum is governed by simple site blocking by specifically adsorbed anions and/or electrosorbed oxide.
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    Alinejad, S.; Inaba, M.; Schröder, J.; Du, J.; Quinson, J.; Zana, A.; Arenz, M. Testing fuel cell catalysts under more realistic reaction conditions: accelerated stress tests in a gas diffusion electrode setup. JPhys Energy 2020, 2, 024003,  DOI: 10.1088/2515-7655/ab67e2
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    Testing fuel cell catalysts under more realistic reaction conditions: accelerated stress tests in a gas diffusion electrode setup
    Alinejad, Shima; Inaba, Masanori; Schroeder, Johanna; Du, Jia; Quinson, Jonathan; Zana, Alessandro; Arenz, Matthias
    JPhys Energy (2020), 2 (2), 024003CODEN: JPEOEY; ISSN:2515-7655. (IOP Publishing Ltd.)
    Gas diffusion electrode (GDE) setups have very recently received increasing attention as a fast and straightforward tool for testing the oxygen redn. reaction (ORR) activity of surface area proton exchange membrane fuel cell (PEMFC) catalysts under more realistic reaction conditions. In the work presented here, we demonstrate that our recently introduced GDE setup is suitable for benchmarking the stability of PEMFC catalysts as well. Based on the obtained results, it is argued that the GDE setup offers inherent advantages for accelerated degrdn. tests (ADT) over classical three-electrode setups using liq. electrolytes. Instead of the solid-liq. electrolyte interface in classical electrochem. cells, in the GDE setup a realistic three-phase boundary of (humidified) reactant gas, proton exchange polymer (e.g.Nafion) and the electrocatalyst is formed. Therefore, the GDE setup not only allows accurate potential control but also independent control over the reactant atm., humidity and temp. In addn., the identical location transmission electronmicroscopy (IL-TEM) technique can easily be adopted into the setup, enabling a combination of benchmarking with mechanistic studies.
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    Inaba, M.; Jensen, A. W.; Sievers, G. W.; Escudero-Escribano, M.; Zana, A.; Arenz, M. Benchmarking high surface area electrocatalysts in a gas diffusion electrode: Measurement of oxygen reduction activities under realistic conditions. Energy Environ. Sci. 2018, 11, 988– 994,  DOI: 10.1039/C8EE00019K
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    Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions
    Inaba, Masanori; Jensen, Anders Westergaard; Sievers, Gustav Wilhelm; Escudero-Escribano, Maria; Zana, Alessandro; Arenz, Matthias
    Energy & Environmental Science (2018), 11 (4), 988-994CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    In this work, we introduce the application of gas diffusion electrodes (GDE) for benchmarking the electrocatalytic performance of high surface area fuel cell catalysts. It is demonstrated that GDEs offer several inherent advantages over the state-of-the-art technique, i.e. thin film rotating disk electrode (TF-RDE) measurements for fast fuel cell catalyst evaluation. The most crit. advantage is reactant mass transport. While in RDE measurements the reactant mass transport is severely limited by the gas soly. of the reactant in the electrolyte, GDEs enable reactant transport rates similar to tech. fuel cell devices. Hence, in contrast to TF-RDE measurements, performance data obtained from GDE measurements can be directly compared to membrane electrode assembly (MEA) tests. Therefore, the application of GDEs for the testing of fuel cell catalysts closes the gap between catalyst research in academia and real applications.
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    Dionigi, F.; Weber, C. C.; Primbs, M.; Gocyla, M.; Bonastre, A. M.; Spöri, C.; Schmies, H.; Hornberger, E.; Kühl, S.; Drnec, J. Controlling Near-Surface Ni Composition in Octahedral PtNi(Mo) Nanoparticles by Mo Doping for a Highly Active Oxygen Reduction Reaction Catalyst. Nano Lett. 2019, 19, 6876– 6885,  DOI: 10.1021/acs.nanolett.9b02116
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    Controlling Near-Surface Ni Composition in Octahedral PtNi(Mo) Nanoparticles by Mo Doping for a Highly Active Oxygen Reduction Reaction Catalyst
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    Nano Letters (2019), 19 (10), 6876-6885CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We report and study the translation of exceptionally high catalytic oxygen electroredn. activities of molybdenum-doped octahedrally shaped PtNi(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 ± 0.35 A mgPt-1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt-1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray anal. (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochem. measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidn. region induced detectable Ni losses and structural changes. Our study shows on an at. level how Mo adatoms on the surface impact the Ni surface compn., which, in turn, gives rise to the exceptionally high exptl. catalytic ORR reactivity and calls for strategies on how to preserve this particular surface compn. to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core-shell catalysts.
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  • Abstract

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    Figure 1

    Figure 1. Bright-field TEM images of (a) carbon-supported Ni-rich Mo-doped oh-PtNi NPs (Pt27.4Ni71.8Mo0.8) and (b) carbon-supported Ni-rich MoRh-doped oh-PtNi NPs (Pt22.2Ni75.8Mo1.0Rh1.0); insets: edge length histograms and distribution function and mean edge length (see Table S3 for additional compositional information).

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    Figure 2

    Figure 2. Electrochemical polarization curves and MAs of activated electrocatalysts obtained by the RDE half-cell. ORR LSVs of (a) Mo-doped oh-PtNi, (b) MoRh-doped oh-PtNi, (c) Pt/C HiSPEC 3000, and the (d) corresponding MAs at 0.9 V vs RHE evaluated before and after the AST (30,000 cycles between 0.6 and 0.95 V vs RHE), calculated according to the Koutecký–Levich equation.

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    Figure 3

    Figure 3. GDE half-cell measurements: electrochemical polarization curves and extracted Pt MAs of oh-Pt alloy electrocatalysts obtained in the GDE half-cell setup. ORR polarization curves of Mo-doped oh-PtNi, MoRh-doped oh-PtNi, and Pt/C HiSPEC 3000 (a) before the AST, (b) after the AST, and the (c) corresponding calculated after 5k, 15k, and 30k cycles at 0.75, 0.8, and 0.85 V vs RHE.

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    Figure 4

    Figure 4. SEM images with a magnification of 250k of the as-prepared (a) Ni-rich Mo-doped oh-PtNi NPs, (b) Ni-rich MoRh-doped oh-PtNi, and (c) Pt/C HiSPEC 3000 catalysts collected with the secondary electron detector and of the corresponding tested catalysts (after the AST) in (d–f), respectively. The colored insets in panels (d,e) highlight the zoomed-in views of the aged Mo- (red) and MoRh-doped (blue) oh-PtNi NPs. Bright-field TEM images of Pt/C HiSPEC 3000 NPs (g) before and (h) after the AST and the particle size distribution in the insets.

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  • References

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      Ramaswamy, N.; Kumaraguru, S.; Kukreja, R. S.; Groom, D.; Jarvis, K.; Ferreira, P. Mitigation of PtCo/C Cathode Catalyst Degradation via Control of Relative Humidity. J. Electrochem. Soc. 2021, 168, 124512,  DOI: 10.1149/1945-7111/ac4374
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      Lochner, T.; Kluge, R. M.; Fichtner, J.; El-Sayed, H. A.; Garlyyev, B.; Bandarenka, A. S. Temperature Effects in Polymer Electrolyte Membrane Fuel Cells. ChemElectroChem 2020, 7, 3545– 3568,  DOI: 10.1002/celc.202000588
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      A review. The behavior of proton exchange membrane fuel cells (PEMFCs) strongly depends on the operational temps. In mobile applications, for instance in fuel cell elec. vehicles, PEMFC stacks are often subjected to temps. as low as -20°C, esp. during cold start periods, and to temps. up to 120°C during regular operation. Therefore, it is important to understand the impact of temp. on the performance and degrdn. of hydrogen fuel cells to ensure a stable system operation. To get a comprehensive understanding of the temp. effects in PEMFCs, this manuscript addresses and summarizes in- situ and ex- situ investigations of fuel cells operated at different temps. Initially, different measurement techniques for thermal monitoring are presented. Afterwards, the temp. effects related to the degrdn. and performance of main membrane electrode assembly components, namely gas diffusion layers, proton exchange membranes and catalyst layers, are analyzed.
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      Sun, Y.; Polani, S.; Luo, F.; Ott, S.; Strasser, P.; Dionigi, F. Advancements in cathode catalyst and cathode layer design for proton exchange membrane fuel cells. Nat. Commun. 2021, 12, 5984– 6014,  DOI: 10.1038/s41467-021-25911-x
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      Advancements in cathode catalyst and cathode layer design for proton exchange membrane fuel cells
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      A review. Proton exchange membrane fuel cells have been recently developed at an increasing pace as clean energy conversion devices for stationary and transport sector applications. High platinum cathode loadings contribute significantly to costs. This is why improved catalyst and support materials as well as catalyst layer design are critically needed. Recent advances in nanotechnologies and material sciences have led to the discoveries of several highly promising families of materials. These include platinum-based alloys with shape-selected nanostructures, platinum-group-metal-free catalysts such as metal-nitrogen-doped carbon materials and modification of the carbon support to control surface properties and ionomer/catalyst interactions. Furthermore, the development of advanced characterization techniques allows a deeper understanding of the catalyst evolution under different conditions. This review focuses on all these recent developments and it closes with a discussion of future research directions in the field.
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      Fan, J.; Chen, M.; Zhao, Z.; Zhang, Z.; Ye, S.; Xu, S.; Wang, H.; Li, H. Bridging the gap between highly active oxygen reduction reaction catalysts and effective catalyst layers for proton exchange membrane fuel cells. Nat. Energy 2021, 6, 475– 486,  DOI: 10.1038/s41560-021-00824-7
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      Bridging the gap between highly active oxygen reduction reaction catalysts and effective catalyst layers for proton exchange membrane fuel cells
      Fan, Jiantao; Chen, Ming; Zhao, Zhiliang; Zhang, Zhen; Ye, Siyu; Xu, Shaoyi; Wang, Haijiang; Li, Hui
      Nature Energy (2021), 6 (5), 475-486CODEN: NEANFD; ISSN:2058-7546. (Nature Portfolio)
      A review. Ultralow platinum loading and high catalytic performance at the membrane electrode assembly (MEA) level are essential for reducing the cost of proton exchange membrane fuel cells. The past decade has seen substantial progress in developing a variety of highly active platinum-based catalysts for the oxygen redn. reaction. However, these high activities are almost exclusively obtained from rotating disk electrode (RDE) measurements and have rarely translated into MEA performance. In this Review, we elucidate the intrinsic limitations that lead to a persistent failure to transfer catalysts' high RDE activities into maximized MEA performance. We discuss catalyst-layer engineering strategies for controlling mass transport resistances at local catalyst sites, in the bulk of the catalyst layer and at the interfaces of the MEA to achieve high performance with ultralow platinum loading. We also examine promising intermediate testing methods for closing the gap between RDE and MEA expts.
      >> More from SciFinder ®
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      Lazaridis, T.; Stühmeier, B. M.; Gasteiger, H. A.; El-Sayed, H. A. Capabilities and limitations of rotating disk electrodes versus membrane electrode assemblies in the investigation of electrocatalysts. Nat. Catal. 2022, 5, 363– 373,  DOI: 10.1038/s41929-022-00776-5
      14
      Capabilities and limitations of rotating disk electrodes versus membrane electrode assemblies in the investigation of electrocatalysts
      Lazaridis, Timon; Stuehmeier, Bjoern M.; Gasteiger, Hubert A.; El-Sayed, Hany A.
      Nature Catalysis (2022), 5 (5), 363-373CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
      A review. Cost-competitive fuel cells and water electrolyzers require highly efficient electrocatalysts for the resp. reactions of hydrogen oxidn. and evolution, and oxygen evolution and redn. Electrocatalyst activity and durability are commonly assessed using rotating disk electrodes (RDEs) or membrane electrode assemblies (MEAs). RDEs provide a quick and widely accessible testing tool, whereas MEA testing is more complex but closely resembles the actual application. Although both exptl. set-ups allow investigation of the same reactions, there are scientific questions that cannot be answered by the RDE technique. In this Perspective, we scrutinize protocols widely used to det. the activity and durability of electrocatalysts, and highlight discrepancies in the results obtained using RDEs and MEAs. We discuss where the use of RDEs is appropriate and, conversely, where it leads to erroneous interpretations. Ultimately, we show that many of the current challenges for hydrogen and oxygen electrocatalysts require MEA testing and advocate for its greater adoption in the early stages of electrocatalyst development.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XhtlCqu73E&md5=572ca360bcc434daba0cf932a4540f4a
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      Ehelebe, K.; Schmitt, N.; Sievers, G.; Jensen, A. W.; Hrnjić, A.; Collantes Jiménez, P.; Kaiser, P.; Geuß, M.; Ku, Y. P.; Jovanovič, P. Benchmarking Fuel Cell Electrocatalysts Using Gas Diffusion Electrodes: Inter-lab Comparison and Best Practices. ACS Energy Lett. 2022, 7, 816– 826,  DOI: 10.1021/acsenergylett.1c02659
      15
      Benchmarking Fuel Cell Electrocatalysts Using Gas Diffusion Electrodes: Inter-lab Comparison and Best Practices
      Ehelebe, Konrad; Schmitt, Nicolai; Sievers, Gustav; Jensen, Anders W.; Hrnjic, Armin; Collantes Jimenez, Pablo; Kaiser, Pascal; Geuss, Moritz; Ku, Yu-Ping; Jovanovic, Primoz; Mayrhofer, Karl J. J.; Etzold, Bastian; Hodnik, Nejc; Escudero-Escribano, Maria; Arenz, Matthias; Cherevko, Serhiy
      ACS Energy Letters (2022), 7 (2), 816-826CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      There is no expanded citation for this reference.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xhs1ylt7s%253D&md5=0fe16ffc2364ab4050e4a6a78e0c7617
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      Schmitt, N.; Schmidt, M.; Mueller, J. E.; Schmidt, L.; Etzold, B. J. M. How to maximize geometric current density in testing of fuel cell catalysts by using gas diffusion electrode half-cell setups. Electrochem. Commun. 2022, 141, 107362,  DOI: 10.1016/j.elecom.2022.107362
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      Zhu, W.; Pei, Y.; Douglin, J. C.; Zhang, J.; Zhao, H.; Xue, J.; Wang, Q.; Li, R.; Qin, Y.; Yin, Y. Multi-scale study on bifunctional Co/Fe–N–C cathode catalyst layers with high active site density for the oxygen reduction reaction. Appl. Catal., B 2021, 299, 120656– 120659,  DOI: 10.1016/j.apcatb.2021.120656
      17
      Multi-scale study on bifunctional Co/Fe-N-C cathode catalyst layers with high active site density for the oxygen reduction reaction
      Zhu, Weikang; Pei, Yabiao; Douglin, John C.; Zhang, Junfeng; Zhao, Haoyang; Xue, Jiandang; Wang, Qingfa; Li, Ran; Qin, Yanzhou; Yin, Yan; Dekel, Dario R.; Guiver, Michael D.
      Applied Catalysis, B: Environmental (2021), 299 (), 120656CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Recently, much work has been devoted to designing catalysts with high porosity and efficient active sites. Although very promising results are achieved using Co/Fe-N-C catalysts based on rotating disk electrode (RDE) tests, actual fuel cell performance is below expectations, probably due to insufficient understanding of the catalyst layer (CL). Therefore, catalyst design should be considered holistically by taking into account CL performance, not only intrinsic activity. Here, Co/Fe-N-C with highly dispersed CoFe nanoalloy in the carbon network is obtained by careful design of Co/Fe-ZIF precursor, resulting in a high oxygen redn. reaction (ORR) site d. with good stability. Concerning RDE test in the kinetic region and single cell test (SCT) with complex influence factors, the half-cell test (HCT) is introduced to more accurately evaluate the quality of the Co/Fe-CL. Multi-scale measurements (RDE, HCT and SCT) in different c.d. ranges allows targeting the key CL influence factors for fuel cell performance.
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    18. 18
      Schmitt, N.; Schmidt, M.; Mueller, J. E.; Schmidt, L.; Trabold, M.; Jeschonek, K.; Etzold, B. J. M. Which insights can gas diffusion electrode half-cell experiments give into activity trends and transport phenomena of membrane electrode assemblies?. Energy Adv. 2023, 2, 854– 863,  DOI: 10.1039/D3YA00055A
      There is no corresponding record for this reference.
    19. 19
      Stariha, S.; Macauley, N.; Sneed, B. T.; Langlois, D.; More, K. L.; Mukundan, R.; Borup, R. L. Recent Advances in Catalyst Accelerated Stress Tests for Polymer Electrolyte Membrane Fuel Cells. J. Electrochem. Soc. 2018, 165, F492– F501,  DOI: 10.1149/2.0881807jes
      19
      Recent Advances in Catalyst Accelerated Stress Tests for Polymer Electrolyte Membrane Fuel Cells
      Stariha, Sarah; Macauley, Natalia; Sneed, Brian T.; Langlois, David; More, Karren L.; Mukundan, Rangachary; Borup, Rodney L.
      Journal of the Electrochemical Society (2018), 165 (7), F492-F501CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The U.S. Department of Energy (DOE) set the 2020 durability target for polymer electrolyte membrane fuel cell transportation applications at 5000 h. Since it is impractical to test every fuel cell for this length of time, there is ever increasing interest in developing accelerated stress tests (ASTs) that can accurately simulate the material component degrdn. in the membrane electrode assembly (MEA) obsd. under automotive operating conditions, but over a much shorter time frame. In this work, a square-wave catalyst AST was examd. that shows a 5X time acceleration factor over the triangle-wave catalyst AST and a 25X time acceleration factor over the modified wet drive-cycle catalyst durability protocol, significantly decreasing the testing time. These acceleration factors were correlated to the platinum (Pt) particle size increase and assocd. decrease in electrochem. surface area (ECSA). This square-wave AST was adopted by the DOE as a std. protocol to evaluate catalyst durability. We also compare three catalyst-durability protocols using state-of-the-art platinum-cobalt catalysts supported on high surface area carbon (SOA Pt-Co/HSAC) in the cathode catalyst layer. The results for each of the three tests showed both catalyst particle size increase and transition metal leaching. Moreover the acceleration factors for the alloy catalysts were smaller due to Co leaching being the predominant mechanism of voltage decay in ∼5 nm PtCo/C catalysts. Finally, an extremely harsh carbon corrosion AST was run using the same SOA Pt-Co/HSAC catalyst. This showed minimal change in particle size and a low percentage Co loss from the cathode catalyst particles, despite a significant loss in catalyst layer thickness and cell performance. The carbon corrosion rates during these various ASTs were directly measured by monitoring the CO2 emission from the cathode, further confirming the ability of the square-wave AST to evaluate the electro-catalyst independently of the support.
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    20. 20
      U.S. Department of Energy. Fuel Cell Technologies Office Multiyear Research, Development and Demonstration Plan; DOE, 2016, Vol. 2015; pp 1– 58.
      There is no corresponding record for this reference.
    21. 21
      Khedekar, K.; Rezaei Talarposhti, M.; Besli, M. M.; Kuppan, S.; Perego, A.; Chen, Y.; Metzger, M.; Stewart, S.; Atanassov, P.; Tamura, N. Probing Heterogeneous Degradation of Catalyst in PEM Fuel Cells under Realistic Automotive Conditions with Multi-Modal Techniques. Adv. Energy Mater. 2021, 11, 2101794,  DOI: 10.1002/aenm.202101794
      21
      Probing Heterogeneous Degradation of Catalyst in PEM Fuel Cells under Realistic Automotive Conditions with Multi-Modal Techniques
      Khedekar, Kaustubh; Rezaei Talarposhti, Morteza; Besli, Muenir M.; Kuppan, Saravanan; Perego, Andrea; Chen, Yechuan; Metzger, Michael; Stewart, Sarah; Atanassov, Plamen; Tamura, Nobumichi; Craig, Nathan; Cheng, Lei; Johnston, Christina M.; Zenyuk, Iryna V.
      Advanced Energy Materials (2021), 11 (35), 2101794CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      The heterogeneity of polymer electrolyte fuel cell catalyst degrdn. is studied under varied relative humidity and types of feed gas. Accelerated stress tests (ASTs) are performed on four membrane electrode assemblies (MEAs) under wet and dry conditions in an air or nitrogen environment for 30 000 square voltage cycles. The largest electrochem. active area loss is obsd. for MEA under wet conditions in a nitrogen gas environment AST due to const. upper potential limit of 0.95 V and significant water content. The mean Pt particle size is larger for the ASTs under wet conditions compared to dry conditions, and the Pt particle size under land is generally larger than under the channel. Observations from ASTs in both conditions and gas environments indicate that water content promotes Pt particle size growth. ASTs under wet conditions and an air environment show the largest difference in Pt particle size growth for inlet vs. outlet and channel vs. land, which can be attributed to larger water content at outlet and under land compared to inlet and under channel. From X-ray fluorescence expts. Pt particle size increase is a local phenomenon as Pt loading remains relatively uniform across the MEA.
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    22. 22
      Polani, S.; MacArthur, K. E.; Kang, J.; Klingenhof, M.; Wang, X.; Möller, T.; Amitrano, R.; Chattot, R.; Heggen, M.; Dunin-Borkowski, R. E. Highly Active and Stable Large Mo-Doped Pt–Ni Octahedral Catalysts for ORR: Synthesis, Post-treatments, and Electrochemical Performance and Stability. ACS Appl. Mater. Interfaces 2022, 14, 29690– 29702,  DOI: 10.1021/acsami.2c02397
      22
      Highly Active and Stable Large Mo-Doped Pt-Ni Octahedral Catalysts for ORR: Synthesis, Post-treatments, and Electrochemical Performance and Stability
      Polani, Shlomi; MacArthur, Katherine E.; Kang, Jiaqi; Klingenhof, Malte; Wang, Xingli; Moeller, Tim; Amitrano, Raffaele; Chattot, Raphael; Heggen, Marc; Dunin-Borkowski, Rafal E.; Strasser, Peter
      ACS Applied Materials & Interfaces (2022), 14 (26), 29690-29702CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Over the past decade, advances in the colloidal syntheses of octahedral-shaped Pt-Ni alloy nanocatalysts for use in fuel cell cathodes have raised our at.-scale control of particle morphol. and surface compn., which, in turn, helped raise their catalytic activity far above that of benchmark Pt catalysts. Future fuel cell deployment in heavy-duty vehicles caused the scientific priorities to shift from alloy particle activity to stability. Larger particles generally offer enhanced thermodn. stability, yet synthetic approaches toward larger octahedral Pt-Ni alloy nanoparticles have remained elusive. In this study, we show how a simple manipulation of solvothermal synthesis reaction kinetics involving depressurization of the gas phase at different stages of the reaction allows tuning the size of the resulting octahedral nanocatalysts to previously unachieved scales. We then link the underlying mechanism of our approach to the classical "LaMer" model of nucleation and growth. We focus on large, annealed Mo-doped Pt-Ni octahedra and investigate their synthesis, post-synthesis treatments, and elemental distribution using advanced electron microscopy. We evaluate the electrocatalytic ORR performance and stability and succeed to obtain a deeper understanding of the enhanced stability of a new class of relatively large, active, and long-lived Mo-doped Pt-Ni octahedral catalysts for the cathode of PEMFCs.
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    23. 23
      Hornberger, E.; Klingenhof, M.; Polani, S.; Paciok, P.; Kormányos, A.; Chattot, R.; MacArthur, K. E.; Wang, X.; Pan, L.; Drnec, J. On the electrocatalytical oxygen reduction reaction activity and stability of quaternary RhMo-doped PtNi/C octahedral nanocrystals. Chem. Sci. 2022, 13, 9295– 9304,  DOI: 10.1039/D2SC01585D
      23
      On the electrocatalytical oxygen reduction reaction activity and stability of quaternary RhMo-doped PtNi/C octahedral nanocrystals
      Hornberger, Elisabeth; Klingenhof, Malte; Polani, Shlomi; Paciok, Paul; Kormanyos, Attila; Chattot, Raphael; MacArthur, Katherine E.; Wang, Xingli; Pan, Lujin; Drnec, Jakub; Cherevko, Serhiy; Heggen, Marc; Dunin-Borkowski, Rafal E.; Strasser, Peter
      Chemical Science (2022), 13 (32), 9295-9304CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Recently proposed bimetallic octahedral Pt-Ni electrocatalysts for the oxygen redn. reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes suffer from particle instabilities in the form of Ni corrosion and shape degrdn. Advanced trimetallic Pt-based electrocatalysts have contributed to their catalytic performance and stability. In this work, we propose and analyze a novel quaternary octahedral (oh-)Pt nanoalloy concept with two distinct metals serving as stabilizing surface dopants. An efficient solvothermal one-pot strategy was developed for the prepn. of shape-controlled oh-PtNi catalysts doped with Rh and Mo in its surface. The as-prepd. quaternary octahedral PtNi(RhMo) catalysts showed exceptionally high ORR performance accompanied by improved activity and shape integrity after stability tests compared to previously reported bi- and tri-metallic systems. Synthesis, performance characteristics and degrdn. behavior are investigated targeting deeper understanding for catalyst system improvement strategies. A no. of different operando and online anal. techniques were employed to monitor the structural and elemental evolution, including identical location scanning transmission electron microscopy and energy dispersive X-ray anal. (IL-STEM-EDX), operando wide angle X-ray spectroscopy (WAXS), and online scanning flow cell inductively coupled plasma mass spectrometry (SFC-ICP-MS). Our studies show that doping PtNi octahedral catalysts with small amts. of Rh and Mo suppresses detrimental Pt diffusion and thus offers an attractive new family of shaped Pt alloy catalysts for deployment in PEMFC cathode layers.
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    24. 24
      Gan, L.; Cui, C.; Heggen, M.; Dionigi, F.; Rudi, S.; Strasser, P. Element-specific anisotropic growth of shaped platinum alloy nanocrystals. Science 2014, 346, 1502– 1506,  DOI: 10.1126/science.1261212
      24
      Element-specific anisotropic growth of shaped platinum alloy nanocrystals
      Gan, Lin; Cui, Chunhua; Heggen, Marc; Dionigi, Fabio; Rudi, Stefan; Strasser, Peter
      Science (Washington, DC, United States) (2014), 346 (6216), 1502-1506CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Morphol. shape in chem. and biol. owes its existence to anisotropic growth and is closely coupled to distinct functionality. Although much is known about the principal growth mechanisms of monometallic shaped nanocrystals, the anisotropic growth of shaped alloy nanocrystals is still poorly understood. Using aberration-cor. scanning transmission electron microscopy, we reveal an element-specific anisotropic growth mechanism of platinum (Pt) bimetallic nano-octahedra where compositional anisotropy couples to geometric anisotropy. A Pt-rich phase evolves into precursor nanohexapods, followed by a slower step-induced deposition of an M-rich (M = Ni, Co, etc.) phase at the concave hexapod surface forming the octahedral facets. Our finding explains earlier reports on unusual compositional segregations and chem. degrdn. pathways of bimetallic polyhedral catalysts and may aid rational synthesis of shaped alloy catalysts with desired compositional patterns and properties.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitV2gtr%252FF&md5=2bf200ee8a5629a5200ac8ebda34be69
    25. 25
      Beermann, V.; Gocyla, M.; Willinger, E.; Rudi, S.; Heggen, M.; Dunin-Borkowski, R. E.; Willinger, M. G.; Strasser, P. Rh-Doped Pt-Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability. Nano Lett. 2016, 16, 1719– 1725,  DOI: 10.1021/acs.nanolett.5b04636
      25
      Rh-Doped Pt-Ni Octahedral Nanoparticles: Understanding the Correlation between Elemental Distribution, Oxygen Reduction Reaction, and Shape Stability
      Beermann, Vera; Gocyla, Martin; Willinger, Elena; Rudi, Stefan; Heggen, Marc; Dunin-Borkowski, Rafal E.; Willinger, Marc-Georg; Strasser, Peter
      Nano Letters (2016), 16 (3), 1719-1725CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Thanks to their remarkably high activity toward oxygen redn. reaction (ORR), platinum-based octahedrally shaped nanoparticles have attracted ever increasing attention in last years. Although high activities for ORR catalysts have been attained, the practical use is still limited by their long-term stability. In this work, we present Rh-doped Pt-Ni octahedral nanoparticles with high activities up to 1.14 A mgPt-1 combined with improved performance and shape stability compared to previous bimetallic Pt-Ni octahedral particles. The synthesis, the electrocatalytic performance of the particles toward ORR, and at. degrdn. mechanisms are investigated with a major focus on a deeper understanding of strategies to stabilize morphol. particle shape and consequently their performance. Rh surface-doped octahedral Pt-Ni particles were prepd. at various Rh levels. At and above about 3 atom %, the nanoparticles maintained their octahedral shape even past 30 000 potential cycles, while undoped bimetallic ref. nanoparticles show a complete loss in octahedral shape already after 8000 cycles in the same potential window. Detailed at. insight in these observations is obtained from aberration-cor. scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) anal. Our anal. shows that it is the migration of Pt surface atoms and not, as commonly thought, the dissoln. of Ni that constitutes the primary origin of the octahedral shape loss for Pt-Ni nanoparticles. Using small amts. of Rh we were able to suppress the migration rate of platinum atoms and consequently suppress the octahedral shape loss of Pt-Ni nanoparticles.
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    26. 26
      Schmitt, N.; Schmidt, M.; Hübner, G.; Etzold, B. J. M. Oxygen reduction reaction measurements on platinum electrocatalysts in gas diffusion electrode half-cells: Influence of electrode preparation, measurement protocols and common pitfalls. J. Power Sources 2022, 539, 231530,  DOI: 10.1016/j.jpowsour.2022.231530
      There is no corresponding record for this reference.
    27. 27
      van der Vliet, D. F. Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces. Angew. Chem., Int. Ed. 2012, 51, 3139,  DOI: 10.1002/anie.201107668
      27
      Unique Electrochemical Adsorption Properties of Pt-Skin Surfaces
      van der Vliet, Dennis F.; Wang, Chao; Li, Dongguo; Paulikas, Arvydas P.; Greeley, Jeffrey; Rankin, Rees B.; Strmcnik, Dusan; Tripkovic, Dusan; Markovic, Nenad M.; Stamenkovic, Vojislav R.
      Angewandte Chemie, International Edition (2012), 51 (13), 3139-3142, S3139/1-S3139/5CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Authors demonstrated that by alloying platinum with a non-noble second metal and inducing a Pt-skin-type structure, the adsorption properties of formed structures are significantly changed. The adsorption of hydrogen and oxide species is shifted in potential and reduced in magnitude compared to platinum. The LEIS data showed that the surface consists only of platinum. The suppression of hydrogen adatoms on platinum underestimate the surface area and overrates the specific activity. The Co stripping can be used alongside with the adsorbed hydrogen charge for the detn. of the electrochem. active surface area of platinum-alloy catalysts.
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    28. 28
      Ehelebe, K.; Seeberger, D.; Paul, M. T. Y.; Thiele, S.; Mayrhofer, K. J. J.; Cherevko, S. Evaluating Electrocatalysts at Relevant Currents in a Half-Cell: The Impact of Pt Loading on Oxygen Reduction Reaction. J. Electrochem. Soc. 2019, 166, F1259– F1268,  DOI: 10.1149/2.0911915jes
      28
      Evaluating electrocatalysts at relevant currents in a half-cell: the impact of Pt loading on oxygen reduction reaction
      Ehelebe, Konrad; Seeberger, Z. Dominik; Paul, Mike T. Y.; Thiele, Simon; Mayrhofer, Karl J. J.; Cherevko, Serhi
      Journal of the Electrochemical Society (2019), 166 (16), F1259-F1268CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      In this work gas diffusion electrode (GDE) half-cells expts. are proposed as powerful tool in fuel cell catalyst layer evaluation as it is possible to transfer the advantages of fundamental methods like thin-film rotating disk electrode (TF-RDE) such as good comparability of results, dedicated elimination of undesired parameters etc. to relevant potential ranges for fuel cell applications without mass transport limitations. With the developed setup and electrochem. protocol, 1st expts. on different Pt/C loadings confirm excellent reproducibility. Thereby mass-specific current densities up to 30 A mgPt-1 at 0.6 V vs. RHE are achieved. From a methodol. perspective, good comparability to single cell measurements is obtained after theor. corrections for temp. and concn. effects. In comparison to previous studies with GDE half-cells, polarization curves without severe mass transport limitations are recorded in a broad potential window. All these achievements indicate that the proposed method can be an efficient tool to bridge the gap between TF-RDE and single cell expts. by providing fast and dedicated insights into the effects of catalyst layers on O redn. reaction performance. This method will enable straightforward and efficient optimization of catalyst layer compn. and structure, esp. for novel catalysts, thereby contributing to the performance enhancements of fuel cells with reduced Pt loading.
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    29. 29
      Pinaud, B. A.; Bonakdarpour, A.; Daniel, L.; Sharman, J.; Wilkinson, D. P. Key Considerations for High Current Fuel Cell Catalyst Testing in an Electrochemical Half-Cell. J. Electrochem. Soc. 2017, 164, F321– F327,  DOI: 10.1149/2.0891704jes
      29
      Key Considerations for High Current Fuel Cell Catalyst Testing in an Electrochemical Half-Cell
      Pinaud, Blaise A.; Bonakdarpour, Arman; Daniel, Lius; Sharman, Jonathan; Wilkinson, David P.
      Journal of the Electrochemical Society (2017), 164 (4), F321-F327CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      An economical and novel half-cell approach for quick and precise measurement of oxygen redn. catalysts in various practical electrode formats, including GDE and bonded GDE/membrane layers is demonstrated. Fuel cell current densities (∼1 A/cm2) were achieved with the key design considerations clearly outlined, to highlight the challenges in developing such a test platform. Const. current polarizations with IR drop measurement at each step are found to provide a simple, reproducible method of measuring catalyst performance. An optimal electrolyte concn. of 1.0 M HClO4 balances the requirement of non-limiting H+ transport at high currents while minimizing the effect of electrolyte impurities, which reduce Pt activity. The half-cell used here can accurately provide the ORR activity of com. products showing good agreement with measurements made in fuel cell hardware, but at a significantly lower testing cost. This half-cell approach can be used to characterize various types of GDEs and CCMs with different catalyst layers.
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    30. 30
      Attard, G. A.; Brew, A.; Hunter, K.; Sharman, J.; Wright, E. Specific adsorption of perchlorate anions on Pt{hkl} single crystal electrodes. Phys. Chem. Chem. Phys. 2014, 16, 13689– 13698,  DOI: 10.1039/C4CP00564C
      30
      Specific adsorption of perchlorate anions on Pt{hkl} single crystal electrodes
      Attard, Gary A.; Brew, Ashley; Hunter, Katherine; Sharman, Jonathan; Wright, Edward
      Physical Chemistry Chemical Physics (2014), 16 (27), 13689-13698CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The voltammetry of Pt{111}, Pt{100}, Pt{110} and Pt{311} single crystal electrodes as a function of perchloric acid concn. (0.05-2.00 M) was studied to test the assertion made in recent reports by Watanabe et al. that perchlorate anions specifically adsorb on polycryst. platinum. Such an assertion would have significant ramifications for the authors' understanding of electrocatalytic processes at platinum surfaces since perchlorate anions at low pH have classically been assumed not to specifically adsorb. For Pt{111}, OHad and electrochem. oxide states are both perturbed significantly as perchloric acid concn. is increased. Probably this is due to specific adsorption of perchlorate anions competing with OHad for adsorption sites. The hydrogen underpotential deposition (H UPD) region of Pt{111} however remains unchanged although evidence for perchlorate anion decompn. to chloride on Pt{111} is reported. In contrast, for Pt{100} no variation in the onset of electrochem. oxide formation is found nor any shift in the potential of the OHad state which normally results from the action of specifically adsorbing anions. Probably perchlorate anions are nonspecifically adsorbed on this plane although strong changes in all H UPD states are obsd. as perchloric acid concn. is increased. This manifests itself as a redistribution of charge from the H UPD state situated at more pos. potential to the one at more neg. potential. For Pt{110} and Pt{311}, marginal changes in the onset of electrochem. oxide formation are recorded, assocd. with specific adsorption of perchlorate. Specific adsorption of perchlorate anions on Pt{111} is deleterious to electrocatalytic activity in relation to the oxygen redn. reaction (ORR) as measured using a rotating disk electrode (RDE) in a hanging meniscus configuration. This study supports previous work suggesting that a large component of the ORR activity on platinum is governed by simple site blocking by specifically adsorbed anions and/or electrosorbed oxide.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVantrzM&md5=e0b3f44d3e967d570e5ab242c4dc00db
    31. 31
      Alinejad, S.; Inaba, M.; Schröder, J.; Du, J.; Quinson, J.; Zana, A.; Arenz, M. Testing fuel cell catalysts under more realistic reaction conditions: accelerated stress tests in a gas diffusion electrode setup. JPhys Energy 2020, 2, 024003,  DOI: 10.1088/2515-7655/ab67e2
      31
      Testing fuel cell catalysts under more realistic reaction conditions: accelerated stress tests in a gas diffusion electrode setup
      Alinejad, Shima; Inaba, Masanori; Schroeder, Johanna; Du, Jia; Quinson, Jonathan; Zana, Alessandro; Arenz, Matthias
      JPhys Energy (2020), 2 (2), 024003CODEN: JPEOEY; ISSN:2515-7655. (IOP Publishing Ltd.)
      Gas diffusion electrode (GDE) setups have very recently received increasing attention as a fast and straightforward tool for testing the oxygen redn. reaction (ORR) activity of surface area proton exchange membrane fuel cell (PEMFC) catalysts under more realistic reaction conditions. In the work presented here, we demonstrate that our recently introduced GDE setup is suitable for benchmarking the stability of PEMFC catalysts as well. Based on the obtained results, it is argued that the GDE setup offers inherent advantages for accelerated degrdn. tests (ADT) over classical three-electrode setups using liq. electrolytes. Instead of the solid-liq. electrolyte interface in classical electrochem. cells, in the GDE setup a realistic three-phase boundary of (humidified) reactant gas, proton exchange polymer (e.g.Nafion) and the electrocatalyst is formed. Therefore, the GDE setup not only allows accurate potential control but also independent control over the reactant atm., humidity and temp. In addn., the identical location transmission electronmicroscopy (IL-TEM) technique can easily be adopted into the setup, enabling a combination of benchmarking with mechanistic studies.
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    32. 32
      Inaba, M.; Jensen, A. W.; Sievers, G. W.; Escudero-Escribano, M.; Zana, A.; Arenz, M. Benchmarking high surface area electrocatalysts in a gas diffusion electrode: Measurement of oxygen reduction activities under realistic conditions. Energy Environ. Sci. 2018, 11, 988– 994,  DOI: 10.1039/C8EE00019K
      32
      Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions
      Inaba, Masanori; Jensen, Anders Westergaard; Sievers, Gustav Wilhelm; Escudero-Escribano, Maria; Zana, Alessandro; Arenz, Matthias
      Energy & Environmental Science (2018), 11 (4), 988-994CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      In this work, we introduce the application of gas diffusion electrodes (GDE) for benchmarking the electrocatalytic performance of high surface area fuel cell catalysts. It is demonstrated that GDEs offer several inherent advantages over the state-of-the-art technique, i.e. thin film rotating disk electrode (TF-RDE) measurements for fast fuel cell catalyst evaluation. The most crit. advantage is reactant mass transport. While in RDE measurements the reactant mass transport is severely limited by the gas soly. of the reactant in the electrolyte, GDEs enable reactant transport rates similar to tech. fuel cell devices. Hence, in contrast to TF-RDE measurements, performance data obtained from GDE measurements can be directly compared to membrane electrode assembly (MEA) tests. Therefore, the application of GDEs for the testing of fuel cell catalysts closes the gap between catalyst research in academia and real applications.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXktFWktb0%253D&md5=e2b689adbf3cd1f924b245fac4c693e3
    33. 33
      Dionigi, F.; Weber, C. C.; Primbs, M.; Gocyla, M.; Bonastre, A. M.; Spöri, C.; Schmies, H.; Hornberger, E.; Kühl, S.; Drnec, J. Controlling Near-Surface Ni Composition in Octahedral PtNi(Mo) Nanoparticles by Mo Doping for a Highly Active Oxygen Reduction Reaction Catalyst. Nano Lett. 2019, 19, 6876– 6885,  DOI: 10.1021/acs.nanolett.9b02116
      33
      Controlling Near-Surface Ni Composition in Octahedral PtNi(Mo) Nanoparticles by Mo Doping for a Highly Active Oxygen Reduction Reaction Catalyst
      Dionigi, F.; Weber, C. Cesar; Primbs, M.; Gocyla, M.; Bonastre, A. Martinez; Spoeri, C.; Schmies, H.; Hornberger, E.; Kuehl, S.; Drnec, J.; Heggen, M.; Sharman, J.; Dunin-Borkowski, R. Edward; Strasser, P.
      Nano Letters (2019), 19 (10), 6876-6885CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We report and study the translation of exceptionally high catalytic oxygen electroredn. activities of molybdenum-doped octahedrally shaped PtNi(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 ± 0.35 A mgPt-1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt-1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray anal. (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochem. measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidn. region induced detectable Ni losses and structural changes. Our study shows on an at. level how Mo adatoms on the surface impact the Ni surface compn., which, in turn, gives rise to the exceptionally high exptl. catalytic ORR reactivity and calls for strategies on how to preserve this particular surface compn. to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core-shell catalysts.
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    34. 34
      Pan, L.; Parnière, A.; Dunseath, O.; Fongalland, D.; Nicolau, G.; Weber, C. C.; Lu, J.; Klingenhof, M.; Arinchtein, A.; Oh, H. S. Enhancing the Performance of Shape-Controlled Octahedral Rhodium-Doped PtNi Nanoalloys inside Hydrogen-Air Fuel Cell Cathodes Using a Rational Design of Catalysts, Supports, and Layering. ACS Catal. 2024, 14, 10– 20,  DOI: 10.1021/acscatal.3c02619
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c11068.

    • Detailed information on the synthesis, chemical, and structural characterization of the catalyst by ICP–OES, TEM, and XRD; electrochemical performance and stability protocols, as well as the data obtained by RDE and GDE half-cell experiments; and SEM images and EDX data of the GDEs before and after ASTs (PDF)


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