ACS Publications. Most Trusted. Most Cited. Most Read
Light-Mediated Electrochemical Synthesis of Manganese Oxide Enhances Its Stability for Water Oxidation
More

OR SEARCH CITATIONS

My Activity
Publications

Figure 1Loading Img
Download Hi-Res ImageDownload to MS-PowerPointCite This:ACS Nanosci. Au 2023, 3, 4, 310-322
  • Open Access
Article

Light-Mediated Electrochemical Synthesis of Manganese Oxide Enhances Its Stability for Water Oxidation
Click to copy article linkArticle link copied!

Open PDFSupporting Information (1)

ACS Nanoscience Au

Cite this: ACS Nanosci. Au 2023, 3, 4, 310–322
Click to copy citationCitation copied!
https://doi.org/10.1021/acsnanoscienceau.3c00002
Published April 23, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY-NC-ND 4.0 .

Abstract

Click to copy section linkSection link copied!
High Resolution Image
Download MS PowerPoint Slide

New methods are needed to increase the activity and stability of earth-abundant catalysts for electrochemical water splitting to produce hydrogen fuel. Electrodeposition has been previously used to synthesize manganese oxide films with a high degree of disorder and a mixture of oxidation states for Mn, which has led to electrocatalysts with high activity but low stability for the oxygen evolution reaction (OER) at high current densities. In this study, we show that multipotential electrodeposition of manganese oxide under illumination produces nanostructured films with significantly higher stability for the OER compared to films grown under otherwise identical conditions in the dark. Manganese oxide films grown by multipotential deposition under illumination sustain a current density of 10 mA/cm2 at 2.2 V versus reversible hydrogen electrode for 18 h (pH 13). Illumination does not enhance the activity or stability of manganese oxide films grown using a constant potential, and films grown by multipotential deposition in the dark undergo a complete loss of activity within 1 h of electrolysis. Electrochemical and structural characterization indicate that photoexcitation of the films during growth reduces Mn ions and changes the content and structure of intercalated potassium ions and water molecules in between the disordered layers of birnessite-like sheets of MnOx, which stabilizes the nanostructured film during electrocatalysis. These results demonstrate that combining multiple external stimuli (i.e., light and an external potential) can induce structural changes not attainable by either stimulus alone to make earth-abundant catalysts more active and stable for important chemical transformations such as water oxidation.

This publication is licensed under

CC-BY-NC-ND 4.0 .
  • cc licence
  • by licence
  • nc licence
  • nd licence
Copyright © 2023 The Authors. Published by American Chemical Society

Subjects

what are subjects

Keywords

what are keywords

Introduction

Click to copy section linkSection link copied!
Hydrogen production through the splitting of water has the potential to provide a clean fuel source to replace petroleum for the transportation industry. In electrochemical water splitting to generate H2 and O2, the oxidative half-reaction, known as the oxygen evolution reaction (OER) is kinetically more challenging to achieve at low overpotentials as it involves the transfer of four protons and four electrons and the formation of an oxygen–oxygen double bond. (1) While oxides of precious metals like Ir and Ru are currently the best-performing electrocatalysts for the OER, (2) their rarity makes them prohibitively expensive in addressing the world’s current and future energy needs. (3,4) First-row transition metal oxides including the oxides of Mn, Fe, Ni, and Co are highly earth-abundant, (3) so they could be incorporated into systems for photo- and electrochemical water splitting to produce hydrogen fuel on a terawatt scale. (2,5−11) However, methods to improve the activity and long-term stability of such first-row transition metal oxides for the OER are needed.
Manganese oxides exhibit a wide variety of different stoichiometries and structures and have provided a testbed for elucidating the structure–activity relationships in transition metal oxide catalysts for water oxidation. (12−36) For example, Mn2O3 with a bixbyite structure exhibits higher activity for photochemical water oxidation relative to other stoichiometries (e.g., Mn3O4 and different phases of MnO2). (17,18) Melder and coworkers have recently compiled a comparison of the activity and stability of different manganese oxides for electrochemical water oxidation. (36) Bixbyite Mn2O3 and disordered birnessite MnOx (containing a mixture of Mn3+ and Mn4+ ions) have demonstrated some of the lowest overpotentials of manganese oxides for electrochemical water oxidation, with both operating at a current density of 10 mA/cm2 in 1 M KOH with overpotentials between 330 and 340 mV. (23,35) Nanoparticles of γ-MnO2 (a mixture of the pyrolusite and ramsdellite phases of MnO2) have exhibited high stability, performing the OER in acid (pH 2) at a current density of 10 mA/cm2 for 8000 h (24) While different methods of measuring the activity for water oxidation (e.g., chemical, photochemical, or electrochemical) and different experimental conditions have led to the development of various structure–activity relationships, two important trends have emerged for electrochemical water oxidation: (1) the presence of manganese in a mixture of oxidation states (e.g., Mn3+ and Mn4+) at potentials where the OER occurs is important for the redox cycling of active sites that takes place during water oxidation (13,17,20,25−27,29−34,37) and (2) relative to highly crystalline forms of manganese oxide, disordered structures can exhibit higher activity. (13,14,22,27,31,32,37)
The electrochemical growth of manganese oxide provides a method to both synthesize films with a mixture of oxidation states for Mn and introduce disorder into the resulting films. Multipotential deposition through alternating or cycling the applied potential to both oxidize Mn2+ ions in solution and reduce Mn4+ ions in the growing film increases the relative amount of Mn3+ incorporated into the films (relative to deposition at a constant potential). MnOx films prepared through multipotential electrodeposition (in which the Mn ions are in a mixture of +3 and +4 oxidation states) possess a structure that resembles birnessite MnO2 but with a high amount of disorder between layers of edge-sharing MnO6 octahedra; these disordered films exhibit higher activity than ordered birnessite MnO2 films electrodeposited at a constant potential. (27,29,31,32) While electrodeposited MnOx films exhibit promising activity for electrochemical water oxidation, they are typically stable for only a few hours at current densities between 0.1 and 1 mA/cm2. (27,31,36) Current densities near 10 mA/cm2 are needed to match the incident flux of sunlight and convert solar energy into hydrogen either through photoelectrochemical water splitting or by coupling a photovoltaic device with an electrolyzer. Thus, electrodeposited MnOx films need to be more stable at higher current densities before they can be incorporated into such systems.
Illumination during the electrochemical growth of metal oxide (e.g., Cu2O) and chalcogenide (e.g., PbSe and Se–Te) semiconductors has been used to alter their composition, structure, and nanoscale morphology. (38−44) Photoexcitation during electrodeposition both enhances the concentration of mobile charge carriers and alters the spatial distribution of charge transfer at the interface between the deposition solution and the growing semiconductor film. (38,39,43) Birnessite MnO2 is a light-absorbing semiconductor, but its efficiency for the photoelectrochemical conversion of incident photons into chemical products has so far been relatively low. (45−47) On the other hand, the photoreduction of Mn4+ ions in birnessite MnO2 by sunlight with the concomitant oxidation of organic matter plays an important role in the biogeochemical cycle of aquatic ecosystems. (13,48−51) Thus, we hypothesized that the photoreduction of Mn4+ may be a way to alter the structure of MnOx films during electrochemical growth. (27)
We show that manganese oxide films synthesized by multipotential electrodeposition under illumination with a 405 nm light-emitting diode (LED) exhibit higher current densities for the OER and significantly better stability compared to films synthesized under otherwise identical conditions in the dark. MnOx films grown under illumination have a Tafel slope of 64.1 mV/decade in alkaline solution (pH 13), and they maintain a stable overpotential (∼2.2 V vs RHE) for 18 h while performing the OER at a current density of 10 mA/cm2. On the other hand, MnOx films grown in the dark lose nearly all their activity for the OER after 1 h of electrolysis at this current density. Importantly, the enhanced stability is only observed when combining multipotential electrodeposition with illumination during growth. Electrochemical and structural characterization indicate that the enhanced catalytic activity and stability for the OER observed in MnOx films deposited under illumination arises from light-induced structural and compositional changes during growth. MnOx films grown by multipotential deposition contain Mn ions in a mixture of the +2, +3, and +4 oxidation states and possess a disordered layered structure. We propose that the interlayer structure of intercalated potassium ions and water molecules in MnOx films grown under illumination enhances their stability during the redox cycling and the transport of interlayer species that occurs during electrochemical water oxidation.

Results and Discussion

Click to copy section linkSection link copied!
MnOx films were electrodeposited based on modifications to a procedure developed by Huynh and coworkers. (31) A square-wave potential waveform is applied to the working electrode (i.e., glass coated with fluorine-doped tin oxide) in a solution containing MnCl2 and KNO3 (see the Experimental Section for details). Electrodeposition of manganese oxide while cycling or alternating the potential has been shown to incorporate a higher fraction of Mn3+ ions as well as induce disorder into the growing manganese oxide film relative to deposition at constant potential (see the Supporting Information for additional discussion of this reaction). (27,29,31,32) We thus refer to the manganese oxide films as MnOx as they contain Mn in different oxidation states (i.e., Mn2+, Mn3+, and Mn4+ as discussed further below). We varied the values of the two potentials used in the square wave and their duration (see Supporting Discussion and Figure S1a,b in the Supporting Information). Although we modified the procedure by increasing the overall deposition time and illuminating the films during growth, we found that the square-wave potential originally developed by Huynh and coworkers (31) produced MnOx films with the highest activity for the OER. This sequence consists of an oxidizing potential of 0.9 V versus Ag/AgCl for 3 s (used to oxidize Mn2+ ions in solution to Mn4+ and induce the crystallization of MnO2) and a reducing potential of −0.6 V Ag/AgCl for 2 s [used to increase the local pH through the reduction of NO3– and induce the formation of Mn(OH)2]. We varied the total deposition time using this pulse sequence (Figure S1c) and found that deposition times between 40 and 60 min gave the best combination of activity and stability when the MnOx films were used as catalysts for the OER.
We next compared the activity and stability of MnOx films grown through multipotential electrodeposition under illumination with those of films grown in the dark. A high-power LED with a peak wavelength of 405 nm was used as the light source to grow the illuminated MnOx films (see the Experimental Section for further details). All subsequent electrochemical testing of the films was performed in the dark. Cyclic voltammetry (CV) was performed in a solution of 0.1 M KOH and 0.9 M KNO3 to characterize the as-synthesized, MnOx films grown in the dark and under illumination, as shown in Figure 1a. The initial current densities of both the illuminated and dark films at potentials where the OER can occur were similar during the first CV scan. The onset potential of the as-synthesized, illuminated MnOx film for the OER was 1.57 V versus the reversible hydrogen electrode (RHE), and the film produced a current density of 9.9 mA/cm2 at 2.15 V versus RHE. Furthermore, the film grown under illumination retained the same high activity during the second CV scan. The onset potential of the as-synthesized MnOx film grown in the dark was 1.65 V versus RHE, and the film produced a current density of 8.8 mA/cm2 at 2.15 V versus RHE during the first CV scan. However, the current density dropped to 7.8 mA/cm2 at 2.15 V versus RHE during the second CV scan. CV scans of the illuminated and dark MnOx films after 1 h of constant-current electrolysis at a current density of 10 mA/cm2 are shown in Figure 1b. The onset potential of the illuminated sample remained at 1.57 V versus RHE, and the current density at 2.15 V remained at 9.9 mA/cm2 after electrolysis. However, the current density of the dark sample decreased significantly to 0.8 mA/cm2 at 2.15 V versus RHE. The onset potential for the dark film after electrolysis was difficult to measure because it was too close to background current density produced by a bare fluorine-doped tin oxide (FTO) substrate. Potential sweeps were also performed for films after electrolysis at a current density of 3 mA/cm2. At a lower current density used for performing electrolysis, MnOx films grown under illumination were also more stable relative to films grown in the dark and retained their activity toward OER (Figure S2). These results indicate that MnOx films grown under illumination exhibit enhanced stability for water oxidation compared to films grown under otherwise identical conditions in the dark.

Figure 1

Figure 1. (a,b) CV scans (using IUPAC convention) of MnOx films measured in 0.1 M KOH and 0.9 M KNO3. (a) CV scans of the as-synthesized films grown under illumination (red trace) and in the dark (blue trace). (b) CV scans of the same illuminated film (orange trace) and dark film (light-blue trace) after 1 h of constant-current electrolysis at a current density of 10 mA/cm2. The arrows indicate the onset potentials for the OER. (c) Applied potential vs time for an illuminated MnOx film (red trace) and a dark film (blue trace) during constant-current electrolysis at a current density of 10 mA/cm2 to test the stability of the films during the OER.

High Resolution Image
Download MS PowerPoint Slide
The stability of the MnOx films during the OER was further quantified by performing electrolysis at a constant current density of 10 mA/cm2 in a solution containing 0.1 M KOH and 0.9 M KNO3, as shown in Figure 1c. The stability of each electrode was evaluated based on the change in the potential needed to maintain a constant current; an active and stable electrocatalyst maintains a high current density at a lower overpotential (i.e., a less positive potential for oxidation). During the 40 min test, the MnOx film grown under illumination operated at a relatively stable potential window near 2.2 V versus RHE (Figure 1c). However, for the dark film, the potential required to maintain 10 mA/cm2 continuously increased over the 40 min period and eventually reached a maximum value of 3.25 V versus RHE. In a control experiment, a bare FTO substrate required a similar potential (∼3.45 V vs RHE) to produce a current density of 10 mA/cm2, indicating a near-complete loss of activity for the MnOx film grown in the dark. Based on these electrochemical tests, MnOx films grown under illumination exhibit better performance than the dark films with higher current density and stability for the OER. A MnOx film grown under illumination was tested at a current density of 10 mA/cm2 for a period of 18 h (Figure S3), and the potential only increased by 0.15 V over the testing period.
Scanning electron microscopy (SEM) images of MnOx films before and after using them as electrocatalysts for the OER are shown in Figure 2. The initial films grown under illumination and in the dark exhibit similar morphologies consisting of aggregates of nanoscale platelets that cover the FTO substrate. After electrolysis for 30 min at a current density of 3 mA/cm2, the MnOx film grown under illumination maintained the nanoplate morphology. However, the individual nanoplates became less distinct for the MnOx film grown in the dark. The electrocatalytic activity of the MnOx film grown under illumination was stable, while the sample grown in the dark dramatically decreased in activity; these observations indicate that the nanoplate morphology is important in determining the surface area that is catalytically active.

Figure 2

Figure 2. SEM images of (a) as-synthesized MnOx film grown under illumination with a 405 nm LED, (b) MnOx film synthesized under illumination after 30 min of electrolysis at 3 mA/cm2, (c) as-synthesized MnOx film grown in the dark, and (d) MnOx film synthesized in the dark after 30 min of electrolysis at 3 mA/cm2. The scale bar of 1 μm applies to all images.

High Resolution Image
Download MS PowerPoint Slide
The electrochemically active surface areas (ECSAs) of different MnOx films were compared by measuring their electrochemical double-layer capacitance, CDL, which is directly proportional to the ECSA. The double-layer capacitance of each film was calculated using CV scans that were performed at different scan rates over a potential window where there is no Faradaic current (see Figure S4). (52) The as-synthesized MnOx film grown in the dark had a CDL of 1.6 ± 0.1 mF/cm2, but this value decreased to 0.25 ± 0.02 mF/cm2 after constant-current electrolysis for 40 min at 5 mA/cm2. The as-synthesized MnOx film grown under illumination had a CDL of 2.8 ± 0.1 mF/cm2, and this value decreased to 2.0 ± 0.2 mF/cm2 after electrolysis. These results indicate that the initial ECSA of the illuminated MnOx film was greater than that of the dark film. The difference in active surface area is consistent with the higher initial current density for OER observed for the illuminated MnOx film compared to the dark film (Figure 1a). Assuming a specific capacitance of 0.04 mF/cm2, a value that is recommended for metal oxide electrodes in alkaline solution when the true specific capacitance is not known, (22,52) the ESCA of the MnOx electrode grown under illumination is 70 cm2 for a geometric area of 1 cm2. Thus, the nanoscale morphology of the electrodeposited MnOx films provides a high surface area for electrocatalysis. Furthermore, the significant decrease in the ECSA of the dark film after electrolysis helps to explain the loss of activity for the OER from a morphological perspective.
Tafel plots for the OER using MnOx films synthesized in the dark and under illumination before and after electrolysis are shown in Figure 3a. The Tafel slope reflects how much the applied potential needs to be increased to increase the current density by a factor of 10. A low Tafel slope is desirable as it means the overpotential needed to drive the reaction is small. When the electrode reaction is not limited by mass transport, the Tafel slope reflects the kinetics of the OER. The potential applied to each MnOx film was scanned from 2.0 to 1.9 V versus RHE under stirring at 500 rpm to minimize mass-transport effects. The Tafel slopes were obtained by linear fitting, and the errors are provided after each Tafel slope. The as-synthesized, MnOx film grown under illumination exhibited a Tafel slope of 64.1 ± 0.4 mV/decade, while the as-synthesized, dark film had a slope of 72.7 ± 0.7 mV/decade. A Tafel slope near 60 mV/decade is consistent with a rapid one-electron, charge-transfer step that is in quasi-equilibrium and followed by a rate-determining chemical process. For the OER using MnOx films in alkaline solution, proton-coupled electron transfer at a surface Mn3+ site followed by the release of O2 from the electrode surface as the rate-determining step has been proposed to account for this Tafel slope. (30,53) Next, both MnOx electrodes were subjected to constant-current electrolysis at 5 mA/cm2 for 40 min in an electrolyte with 0.1 M KOH and 0.9 M KNO3. The Tafel slope for the illuminated MnOx film after electrolysis was 71.4 ± 0.8 mV/decade, while the slope for dark film after electrolysis was 138 ± 2 mV/decade. These results show that the MnOx film grown under illumination was able to maintain a relatively high activity during the OER as its Tafel slope only increased by 7.3 mV/decade. However, the Tafel slope of the dark film increased significantly after OER by 65.9 mV/decade, which indicates that it was not stable during oxygen evolution. The changes in Tafel slopes support characterization by CV and constant-current electrolysis, which indicates that MnOx films synthesized under illumination are more stable and active during the OER.

Figure 3

Figure 3. Electrochemical characterization of electrodeposited MnOx films. (a) Tafel plots of MnOx films synthesized under illumination before (red triangles) and after (orange circles) electrolysis at 5 mA/cm2 for 40 min and films synthesized in the dark before (blue triangles) and after (light-blue squares) electrolysis. The symbols represent the actual data, and the lines connecting the symbols represent linear fits of the data used to calculate Tafel slopes. The errors were determined by linear fitting, and the R2 values of the linear fits were greater than 0.995. (b) Mott–Schottky plots of MnOx films synthesized under illumination before (red triangles) and after (orange circles) electrolysis at 5 mA/cm2 for 1 h, and films synthesized in the dark before (blue triangles) and after (light-blue squares) electrolysis. The lines represent linear fits of the raw data used to calculate the Mott–Schottky slopes. (c) Nyquist plots of MnOx films synthesized under illumination before (red) and after (orange) electrolysis at 3 mA/cm2 for 1.5 h, and films synthesized in the dark before (blue) and after (light blue) electrolysis. The inset shows the equivalent circuit model used to fit the Nyquist plots. R1 represents the uncompensated resistance from the solution, FTO substrate, and MnOx film, C2 represents the double-layer capacitance, R2 represents the polarization resistance, and W2 represents a Warburg impedance.

High Resolution Image
Download MS PowerPoint Slide
We used electrochemical impedance spectroscopy (EIS) to correlate the changes in the activity of the MnOx films for the OER with changes in their resistivity and ECSA. EIS was carried out in an aqueous solution containing 0.1 M KOH and 0.9 M KNO3 in the dark at different applied potentials. Two types of plots were derived from the EIS data: Mott–Schottky plots and Nyquist plots. The Mott–Schottky equation relates the capacitance (C) of a semiconductor electrode to its mobile carrier concentration (Nd) and its flat-band potential (VFB), as shown in eq 1
1C2=(2eεε0A2Nd)[V−VFB−kBTe]
(1)
where e is the charge of an electron, ε0 is the permittivity of vacuum, ε is the relative permittivity of the semiconductor, A is the ESCA, V is the applied potential, kB is the Boltzmann’s constant, and T is the temperature.
Mott–Schottky plots (i.e., 1/C2 versus the applied potential, V vs RHE) measured at a frequency of 10 kHz for both the as-synthesized MnOx films and films after 1 h of electrolysis at a current density of 5 mA/cm2 are shown in Figure 3b. Figure S5 shows a magnified view of the Mott–Schottky plots at 10 kHz near the abscissa to show that the slopes are positive for all four samples, which indicates the MnOx films are n-type semiconductors, similar to previous reports. (46) Mott–Schottky plots made at other frequencies gave similar results and are shown in Figure S6. The slope of the Mott–Schottky plot for the MnOx film grown under illumination was similar before and after electrolysis. On the other hand, the slope of the dark film after electrolysis was much greater compared to the dark film before electrolysis and films grown under illumination both before and after electrolysis. As seen from eq 1, decreases in the surface area, carrier concentration, and relative permittivity of the semiconductor will all increase the Mott–Schottky slope. Based on measurements of the double-layer capacitance (Table S1), the ESCA for the MnOx film grown in the dark decreased by a factor of 6.4 after electrolysis. However, the Mott–Schottky slope increased by a factor of 300 for the dark MnOx film after electrolysis, indicating that the effective carrier concentration and relative permittivity also change during the OER for films grown in the dark. These results show that in addition to the morphological changes observed by SEM after electrolysis, the structural changes that occur during the OER also lead to a much lower conductivity for MnOx films grown in the dark.
Nyquist plots were used to quantify changes in the resistance of the MnOx films after electrolysis (Figure 3c). The x- and y-axes of the Nyquist plots show the real and imaginary components of the impedance, respectively. The inset in Figure 3c shows the equivalent circuit model used to fit each of the Nyquist plots. The resistor, R1, accounts for uncompensated resistance in the electrochemical cell including that from the solution, the FTO substrate, and the MnOx film. While the Nyquist plots were obtained at a potential where the OER does not occur (i.e., 0.2 V vs Ag/AgCl), the second resistor element, R2, accounts for any polarization resistance away from the open-circuit potential. Based on fitting the impedance data to this circuit model, the values of R1 and R2 for the as-synthesized MnOx film grown under illumination were 38 ± 1 Ω and 63 ± 3 Ω, respectively. After electrolysis at 3 mA/cm2 for 1.5 h, the values of R1 and R2 increased to 43 ± 2 Ω and 70 ± 3 Ω. The resistance of the MnOx film grown in the dark increased much more substantially after electrolysis. The values of R1 and R2 for the as-synthesized film grown in the dark were 28 ± 1 Ω and 54 ± 3 Ω, respectively, while they increased to 159 ± 2 Ω and 161 ± 3 Ω after electrolysis at 3 mA/cm2 for 1.5 h. The higher resistance for the dark film after OER as measured from the fit to the Nyquist plot agrees with the large increase in slope seen in the Mott–Schottky plot. Thus, the loss in activity for MnOx films grown in the dark is due to a combination of structural and morphological changes that decrease both the film’s active surface area and conductivity. The cumulative results from electrochemical characterization indicate that illumination of the MnOx films during their growth alters their structure to stabilize them against such changes while they are undergoing the OER.
Cycling the applied potential during electrochemical growth of manganese oxide films has been previously shown to produce disordered MnOx films in which the Mn ions possess a mixture of oxidation states. (27,29,31,32) Separately, illumination has been shown to reduce Mn in both synthetic and naturally occurring manganese oxides. (13,48−51) However, the combined effect of these two stimuli on the distribution of the resulting oxidation states for Mn has not been studied. We used X-ray photoelectron spectroscopy (XPS) to compare the differences in MnOx films grown in the dark and under illumination (see Figure 4 and Table 1). For each sample, we fit the photoelectron peak in the binding energy region for Mn 2p3/2 electrons based on a procedure developed by Nesbitt and Banerjee (see the Experimental Section for details of the fitting procedure). (54) Each oxidation state of Mn was decomposed into five different peaks due to coupling between unpaired valence electrons and unpaired core electrons (once photoionization has taken place). (55−57) This procedure has been applied to manganese oxide powders including MnO, MnOOH, Mn2O3, MnO2, and KMnO4. (20,54,57) More recently, it has been used to characterize the distribution of oxidation states in disordered, birnessite-like MnOx particles (similar in structure to the MnOx films studied here). (37) We first applied this fitting procedure to a series of manganese oxide powders (see Tables 1, S2, and Figure S7). The 2p3/2 peak of a bixbyite Mn2O3 powder could be fit to predominantly Mn3+. The 2p3/2 peak of a hausmannite Mn3O4 powder could be fit to a mixture of Mn3+ and Mn2+ but with an excess of Mn3+ relative to the 2:1 ratio that would be expected for this compound (possibly due to surface oxidation). Birnessite MnO2 synthesized by a hydrothermal method showed a mixture of Mn4+ and Mn3+ as expected based on its X-ray diffraction pattern. An MnOx film electrodeposited at a constant potential with the birnessite structure (based on Raman spectroscopy described below) also showed a mixture of Mn4+ and Mn3+. However, the electrodeposited film exhibited a higher percentage of Mn2+ relative to the birnessite powder prepared by hydrothermal synthesis. Although there is significant overlap in the binding energies of the Mn 2p3/2 peaks in different oxidation states, results obtained from manganese oxide powders show that the fitting procedure can be used to compare the differences in the distribution of the oxidation states for the electrodeposited films.

Figure 4

Figure 4. XPS of electrodeposited MnOx films in the binding energy region of Mn 2p3/2 electrons. (a,b) MnOx films synthesized under illumination before (a) before and (b) after electrolysis at 5 mA/cm2 for 1 h (c,d) MnOx films synthesized in the dark (c) before and (d) after electrolysis. For each spectrum, the shaded regions show the deconvolution of the peak into contributions from Mn4+ (purple), Mn3+ (green), and Mn2+ (light red). The gray circles show the raw data, and the black line shows the sum of fitting the peak to the different oxidation states of Mn. The dashed line shows the residual of the fit. The peak maxima for each peak in the deconvolution are provided in Table S3 of the Supporting Information.

High Resolution Image
Download MS PowerPoint Slide
Table 1. Relative Peak Areas for Mn in Different Oxidation States After Deconvoluting the Mn 2p3/2 Peak in the Photoelectron Spectra of Different Manganese Oxide Samplesa
componentratio of peak areas (%)
sampleilluminated MnOxilluminated MnOx–OERdark MnOxdark MnOx–OERMnOx–constant potentialbirnessite MnO2bixbyite Mn2O3hausmannite Mn3O4
Mn4+13.629.923.025.862.857.64.85.0
Mn3+67.162.770.467.820.140.495.271.8
Mn2+19.37.46.66.417.12.00.023.2
a

Illuminated MnOx = film grown by multipotential deposition under illumination, dark MnOx = film grown by multipotential deposition in the dark. OER indicates that the film underwent electrolysis at a current density of 5 mA/cm2 for 1 h. MnOx – constant potential = film grown at a constant potential of 0.9 V vs Ag/AgCl.

XPS reveals that illumination during multipotential electrodeposition reduces the average oxidation state of Mn relative to MnOx films grown in the dark (Figure 4 and Table 1). MnOx films grown under illumination exhibit a similar percentage of Mn3+ compared with films grown in the dark, but they possess a higher percentage of Mn2+ (and correspondingly a lower percentage of Mn4+). After constant-current electrolysis at 5 mA/cm2 for 1 h, the fraction of Mn4+ increases for both films grown under illumination and in the dark, indicating that the OER also leads to partial oxidation of the MnOx films. Notably, the relative amounts of Mn in different oxidation states were similar after the OER for both films grown under illumination and in the dark. Characterization of the samples by X-ray absorption near-edge spectroscopy (XANES) supports the conclusions drawn from XPS (see Figures S8 and S9). The MnOx film grown under illumination has a lower average oxidation state compared to the film grown in the dark, and both films become more oxidized after the OER.
The structures of the MnOx films were characterized by Raman spectroscopy (Figure 5a). For comparison, the Raman spectrum of a film grown at a constant potential of 0.9 V versus Ag/AgCl is shown in Figure S10. The peaks of this film at 493, 567, and 651 cm–1 are indicative of birnessite MnO2. (58) Birnessite is composed of layers of edge-sharing MnO6 octahedra (Figure 5b). Different types of birnessite (e.g., hexagonal vs triclinic) exhibit different stacking sequences of the layers. While Mn ions are primarily in the +4 oxidation state, a portion of the Mn ions can be in the +3 oxidation state. Metal cations (e.g., Na+ or K+) along with H2O can intercalate between the layers of MnO2 to compensate for Mn ions in a reduced oxidation state and to screen charge between layers. Thus, the interlayer spacing and peak positions in the Raman spectra of birnessite and birnessite-like films are sensitive to the amount and structure of intercalated species. Because the electrolyte solution used for both electrochemical deposition and electrolysis in our studies contained KOH and KNO3, we assume K+ is the primary ion intercalated into our MnOx films, which is supported by XPS results showing the presence of K+ (Figure S11). Typically, the peak at 651 cm–1 (which can vary between ∼620 and 651 cm–1) is assigned to the symmetric stretching of the Mn–O bonds in MnO6 octahedra, while the peak at 567 cm–1 (which can vary between ∼570 and 590 cm–1) is attributed to the stretching of the Mn–O bonds parallel to the basal plane of the layers of MnO2. (58−60) A recent report by Scheitenberger and coworkers, which used density-functional theory to calculate the Raman spectra of birnessite films with different structures of interlayer H2O, has called these traditional assignments into question and provides evidence that the peak at 651 cm–1 arises from the vibrations of interlayer H2O rather than Mn–O bonds. (60)

Figure 5

Figure 5. (a) Raman spectra of electrodeposited MnOx films. Red trace: as-synthesized MnOx film grown under illumination; orange trace: illuminated film after electrolysis at 5 mA/cm2 for 1 h; blue trace: as-synthesized MnOx film grown in the dark; light-blue trace: dark film after electrolysis at 5 mA/cm2 for 1 h. (b) Crystal structure of potassium-intercalated birnessite hydrate, K0.27(Mn0.98O2)(H2O)0.51. Mn atoms are pink, oxygen atoms are gray, and potassium atoms are purple. The oxygen atoms within the layer represent intercalated H2O (hydrogen atoms not shown). The structure was made using collection code no 55407 from the Inorganic Crystal Structure Database.

High Resolution Image
Download MS PowerPoint Slide
The square-wave potential (i.e., alternating between 0.9 V versus Ag/AgCl for 3 s and −0.6 V for 2 s) leads to significant disorder in the resulting MnOx films (both for deposition in the dark and under illumination). Thus, the Raman spectra shown in Figure 5a, exhibit reduced peak intensities compared to the film grown at constant potential. Raman spectra of disordered MnOx particles and films have been previously interpreted based on their deviations from well-ordered birnessite MnO2. (14,32,37) While all peaks observed in the Raman spectra (probing the bulk structure) of our samples are indicative of disordered birnessite, XPS indicates a large fraction of Mn3+ ions and the presence of Mn2+ at the surface of the films. Hausmannite Mn3O4 is composed of Mn3+ and Mn2+ ions, and electrochemical cycling of manganese oxide films have been previously shown to produce a disordered mixture of birnessite MnO2 and hausmannite Mn3O4. (31,32,61,62) As the most intense peak in the Raman spectrum of Mn3O4 occurs at 657 cm–1 (i.e., at a similar frequency of 651 cm–1 for birnessite), we cannot exclude the presence of Mn3O4 in our films.
The as-synthesized MnOx films possess a peak at 651 cm–1 for the film grown under illumination and at 652 cm–1 for the film grown in the dark. After electrolysis at 5 mA/cm2 for 1 h, the intensity of this peak decreases significantly for both films (Figure 5a). SEM images show little change in the nanoplate morphology of the MnOx film grown under illumination after the OER (Figure 2). We attribute the loss of the peak at 651 cm–1 along with the appearance of a peak at 567 cm–1 (described further below) that occurs after electrolysis using the MnOx film grown under illumination to reflect a change in the content and structure of intercalated species (i.e., K+ and H2O) rather than a change in the structure of the MnOx layers. After electrolysis, the Raman spectrum of the MnOx film grown in the dark only shows a broad hump with low intensity between 500 and 700 cm–1; the loss of long-range order observed by Raman spectroscopy agrees with the changes in morphology observed by SEM.
The MnOx film grown under illumination also shows a peak at 611 cm–1 before electrolysis, which shifts to 615 cm–1 after electrolysis (Figure 5a). This peak is not observed for MnOx films grown in the dark. A peak between 606 and 611 cm–1 is occasionally observed in birnessite films, (32,37,59,63,64) but this peak is typically left unassigned. Along with the peak at 651 cm–1, Scheitenberger and coworkers also assigned this peak to vibrations of interlayer H2O in birnessite. (60) Finally, a peak appears at 567 cm–1 after electrolysis for the MnOx film grown under illumination. XPS shows that the amount of K+ in the film increased significantly after electrolysis only for the MnOx film grown under illumination (Figure S11). Chen and coworkers previously saw an increase in the intensity of this peak with an increase in the amount of K+ intercalated into birnessite MnO2 films. (59) Thus, we assign the appearance of the peak at 567 cm–1 to a change in the interlayer spacing induced by the intercalation of K+ during the OER. Together, the Raman spectra of these samples indicate that the content and structure of species intercalated between the layers of MnOx differ significantly between films grown in the dark and under illumination.
The presence of Mn3+ in manganese oxides has been correlated with higher activity for water oxidation. (17,20,25−27,29−34) Different coordination geometries for catalytically active Mn3+ ions have been proposed. Bixbyite Mn2O3, which consists of edge-sharing, distorted MnO6 octahedra (as the Mn3+ ions possess a Jahn–Teller distortion), exhibits high activity for both photochemical and electrochemical water oxidation. (17,18,20,23) Corner-sharing Mn3+O6 octahedra and tetrahedrally coordinated Mn3+ ions have also been proposed to be active sites for water oxidation. (14,20,32) Our initial hypothesis was that photochemical reduction could enhance the amount of Mn3+ incorporated into electrodeposited MnOx films; this hypothesis was based on work by Marafatto and coworkers, who showed that photoexcitation of birnessite MnO2 particles with 400 nm illumination led to the photoreduction of Mn4+ to Mn3+. (51) However, XPS of our samples indicates that illumination during multipotential electrodeposition leads to a higher fraction of Mn2+ incorporation rather than Mn3+. The photoreduction of Mn4+ in biogenic manganese oxides by sunlight normally leads to the dissolution of Mn2+ ions. (48−50) Hocking and coworkers showed that disordered, birnessite-like nanoparticles (containing a mixture of Mn4+ and Mn3+ ions) embedded in Nafion could be reversibly photoreduced to Mn2+ and then electrochemically oxidized over multiple cycles. (13) In a similar manner, photoreduction during multipotential deposition could enhance the concentration of Mn2+ ions trapped in electrodeposited MnOx films. Electrochemical oxidation of Mn during the OER (performed without photoexcitation) then brings the concentration of Mn2+ to a level similar to those of films grown by multipotential deposition in the dark. Interestingly, we found that illumination during constant potential deposition did not improve the activity or stability of the films for the OER. Films grown at constant potential under illumination were inactive for the OER, as has been previously observed for films grown at constant potential in the dark. (27,31,32) Thus, photoexcitation appears to work in concert with the disorder induced by multipotential deposition to incorporate a higher fraction of Mn2+ ions and change the interlayer structure of the MnOx films.
While both XPS and XANES indicate that the photoreduction of MnOx films grown by multipotential electrodeposition leads to films with a lower average oxidation state for Mn, these techniques do not provide information on the coordination geometry of Mn ions. Following electrolysis, the relative amounts of Mn4+, Mn3+, and Mn2+ at the surface of films grown in the dark and under illumination are similar (based on XPS). However, the films grown under illumination remain active for the OER, while films grown in the dark have become inactive. SEM imaging, Raman spectroscopy, EIS, Tafel plots, and measurements of the active surface area all indicate that the OER causes different degrees of structural and morphological changes in these samples. The high amount of disorder for MnOx films synthesized by multipotential electrodeposition makes it difficult to quantify the differences in structure that enhance the stability of films grown under illumination. The films do not show any peaks by X-ray diffraction. While there are differences in the extended X-ray absorption fine structure spectra for MnOx films grown in the dark and under illumination, we have not yet been able to fit a satisfactory model to explain these differences (likely because the Mn ions exhibit a range of different local coordination geometries in the disordered films). While the peaks are relatively weak, the Raman spectra indicate that the major differences in the samples are the spacing of the MnOx layers and the structure of intercalated species between these layers.
A mechanism consistent with the sum of our electrochemical and structural data and with prior literature is that photoreduction of Mn4+ ions during growth of the MnOx films followed by their partial oxidation during the OER induces a different coordination geometry for catalytically active Mn sites compared with films grown in the absence of illumination. The multipotential method we used for electrodepositing MnOx films is based on a procedure developed by Nocera and coworkers. (31,32) They proposed that Mn3+ ions introduced by the comproportionation between MnO2 and Mn(OH)2 were trapped in tetrahedral coordination sites that stabilized them against disproportionation during the OER. On the other hand, Marafatto and coworkers used time-resolved X-ray absorption spectroscopy to follow the photoreduction of Mn4+ ions in birnessite MnO2 particles to Mn3+. (51) Following photoreduction, the Jahn–Teller distortion of Mn3+ ions causes them to move from within the layer of edge-sharing MnO6 octahedra to sit above the layer where they share corner oxygen atoms with three MnO6 octahedra within the layer. Finally, Deibert and coworkers studied the structural changes that occur in a metal–organic framework (MOF) containing Mn2+ ions during photochemical water oxidation. (37) In the initial stage of photoexcitation, Mn2+ ions are oxidized, and the MOF transforms into a disordered birnessite composed of Mn4+, Mn3+, and Mn2+ ions with intercalated H2O and Na+ ions. They proposed that Mn3+ ions occupy sites both within the layers of birnessite and in between them.
The concentration and distribution of Mn ions in a reduced oxidation state will affect the spacing between layers in birnessite-like MnOx; Mn3+ and Mn2+ ions can be charge compensated through the intercalation of cations such as Na+ and K+. The additional peaks at 611 and 615 cm–1 (before and after electrolysis) in the Raman spectra of MnOx films grown under illumination, the appearance of a peak at 567 cm–1 for the illuminated film after electrolysis (Figure 5a), and the increase in the amount of intercalated K+ after electrolysis (Figure S11) support the hypothesis that the interlayer structure is different for these films compared to films grown in the dark. Previous reports have shown that the identity of intercalated cations (e.g., H+, Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, or Sr2+) in layered manganese oxides affects their activity for water oxidation. (15,16,22) Furthermore, there is evidence that water oxidation can occur in between the layers of birnessite MnO2. (16,21) In MnO2 electrodes used for Li+-ion and Na+-ion batteries, the interlayer spacing and structure of interlayer water strongly affect the ability to reversibly cycle between different oxidation states of Mn while intercalating/de-intercalating Li+ or Na+ ions. (65−67) Our results (Figure S11) and previous reports (68,69) both indicate that potassium ion intercalation/de-intercalation occurs during the OER. While we have not yet elucidated the exact structure of MnOx films grown under illumination, it appears that photoreduction of Mn ions during growth and their subsequent oxidation during the OER changes the interlayer structure (possibly through incorporating more corner-sharing, interlayer Mn3+ ions that are active sites for the OER). The resulting structure (not accessible to films grown in the dark or at constant potential) stabilizes the illuminated MnOx films during the OER by facilitating the transport of intercalated species (e.g., H+, H2O, and K+), while interlayer Mn sites catalyze water oxidation in between disordered layers of MnOx.

Conclusions

Click to copy section linkSection link copied!
We demonstrate how the combination of multiple external stimuli (i.e., light and an external potential) can be used to direct the nanoscale structure of manganese oxide films. MnOx films synthesized by multipotential electrodeposition under illumination with a 405 nm LED exhibit higher activity and stability toward OER relative to samples synthesized by multipotential deposition in the dark or at a constant potential under illumination. The nanoplate morphology of the MnOx films only degrades during the OER for the films grown in the dark, which decreases both their active surface area and conductivity. Changes in the interlayer structure of disordered MnOx films induced by illumination during growth appear to facilitate the intercalation and de-intercalation of species (e.g., H+, K+, and H2O) during the OER, which provides a stable electrocatalyst. The interlayer structure could be further modified through introducing other metal cations during synthesis (e.g., Rb+ or Cs+) to change the interlayer spacing. (22) This strategy of photoexcitation during electrochemical cycling could also be applied to other semiconductor metal oxides that are active for water oxidation, such as nickel oxide.

Experimental Section

Click to copy section linkSection link copied!

Materials

All chemicals were used as received. Manganese chloride tetrahydrate (MnCl2·4H2O, ≥99%), manganese(IV) oxide (MnO2, ≥99%), manganese(II, III) oxide (Mn3O4, ≥97%), manganese(III) oxide (Mn2O3, ≥99%), manganese(II) oxide (MnO, ≥99%), sodium hydroxide (NaOH, ≥97%), potassium hydroxide (KOH, ≥85%), potassium nitrate (KNO3, ≥99%), potassium permanganate (KMnO4, ≥99%), manganese carbonate (MnCO3, ≥99.9%, trace metal basis), and manganese nitrate tetrahydrate (Mn(NO3)2·4H2O, ≥97.0%) were purchased from Sigma-Aldrich/Millipore Sigma. Platinum gauze (Pt, 100 mesh, 99.9% metals basis), Pt wire (0.5 mm diameter, 99.95% metals basis), and d-(+)-glucose (anhydrous, ≥99%) were purchased from Alfa Aesar. All aqueous solutions were prepared using purified water from a GenPure Pro water purification system with a resistivity of 18.2 MΩ·cm.

Electrochemical Synthesis of MnOx Films

Preparation of Electrodes

The counter electrode for all electrochemical experiments consisted of a Pt gauze attached to a Pt wire. The Pt wire was soldered to a tinned copper wire, and the soldered joint was sealed in a glass tube. A silver/silver chloride (Ag/AgCl) electrode in 3 M NaCl was used as the reference electrode. Glass slides coated with FTO from MSE Supplies with a resistivity of 15 Ω/sq were cut into smaller pieces to be used as working electrodes with typical lateral dimensions of 1 cm × 2 cm. Each FTO slide was rinsed with purified water and then dried using a stream of nitrogen gas before use. The FTO electrodes were connected to the potentiostat with a copper clip.

Electrodeposition of MnOx Films

A custom cell was used for all electrochemical experiments. The cell was made from borosilicate glass with 14/20 ground-glass joints at the top to insert the electrodes and a flat window on the side for illumination. A BioLogic VSP-300 potentiostat/galvanostat operated using EC-Lab software (V11.33) was used to perform all electrochemical experiments. A photo scanner was used to scan images of the working electrodes, and their geometric areas were measured using ImageJ software. The MnOx films were electrodeposited from an aqueous solution containing 0.5 mM MnCl2 and 0.9 M KNO3 based on a previous report with modifications. (31) The pH of the deposition solution was adjusted to 8 using a diluted solution of KOH before use. The MnOx films were prepared by multipotential deposition, where the working electrode (i.e., FTO-coated glass) was held at 0.9 V versus Ag/AgCl for 3 s, followed by −0.6 V versus Ag/AgCl for 2 s; this cycle was repeated for either 40 min or 60 min. Variations of this deposition protocol are described in the Supporting Information and Figure S1. The electrodeposition was performed either in the dark or under illumination with a high-power, LED with a peak wavelength of 405 nm and a full width at half maximum of ∼30 nm. An aspheric condenser lens (Thor, ACL3026) was used to collimate the light. The FTO electrode faced the window of the electrochemical cell where it was irradiated by the 405 nm LED. The irradiance was measured using a calibrated Si photodiode from ThorLabs (FDS-100 CAL), which was placed at the same position as the working electrode prior to deposition. A typical current reading was 2 mA, which based on the responsivity and area (3.6 × 3.6 mm) of the photodiode gives an irradiance of 173 mW/cm2. For comparison, a MnO2 film was also deposited at a constant potential of 0.9 V versus Ag/AgCl for 20 min.

Electrochemical Measurements

Cyclic Voltammetry

CV was performed to characterize the onset potential and current density for oxygen evolution when using MnOx films as the working electrode. All CV scans started at the open-circuit potential. For each scan, the potential was first swept in the positive direction to 1.2 V versus Ag/AgCl and then swept in the negative direction to 0 V versus Ag/AgCl. The scan rate was 20 mV/s, and two consecutive CV scans were performed for each electrode to observe the change in current density. CV scans are plotted using IUPAC convention. The onset potential for the OER using different MnOx films was calculated by drawing tangent lines in the non-Faradaic and Faradaic regions of the CV scan. The abscissa of the point where the two lines intersect indicates the onset potential. In this work, the CV scans were referenced to the Ag/AgCl half-cell, which can be converted to the RHE scale by the following relation: ERHE = EAg/AgCl + 0.197 V + 0.059pH. The overpotential, η, can be calculated as η = ERHE – 1.23 V.

Electrode Stability

The stabilities of the MnOx films for the OER were evaluated by constant-current electrolysis. The MnOx films were tested at various current densities (e.g., 3, 5, and 10 mA/cm2) in a solution containing 0.1 M KOH and 0.9 M KNO3 as the supporting electrolyte. The solutions were stirred at ∼600 rpm throughout the experiment using a magnetic stir plate and stir bar. The potential was measured every 0.5 s, and the electrolysis times ranged from 40 min to 18 h. The electrode stability was analyzed by tracking the change of potential over time. A CV scan was taken after each stability test.

Electrochemical Impedance Spectroscopy

EIS was performed to determine the impedance and capacitance of the MnOx films before and after constant-current electrolysis. Mott–Schottky plots (i.e., 1/C2 vs the applied potential, where C is the space-charge capacitance) and Nyquist plots were extracted from EIS data and were fit using EC-Lab software (V11.33). Impedance measurements were conducted in a solution containing 0.1 M KOH and 0.9 M KNO3. For Mott–Schottky plots, the AC potential oscillation was 10 mV, and the DC potential range was from 1.0 to 0.2 V versus Ag/AgCl. The frequencies scanned ranged from 1 to 10 kHz. The data obtained at 10 kHz are shown in Figure 3b, and data obtained at other frequencies are shown in Figure S6 in the Supporting Information. No iR compensation was applied during these measurements. For Nyquist plots, the frequencies scanned ranged from 30 Hz to 3 MHz, the AC potential oscillation was 10 mV, and the DC potential range was 0.1 to 0.4 V versus Ag/AgCl. Only the data at a potential of 0.2 V are shown in Figure 3. A modified Randles circuit was used to model the electrochemical cell based on the fitting of Nyquist plots at different applied potentials. This circuit model has been previously used to describe electrodeposited MnOx films. (31) It consists of two resistors (R1 and R2), a capacitor (C2), and a Warburg impedance (W2) element: [R1 + C2/(R2 + W2)], as shown in Figure 3c.

Tafel Plots

To compare the activities of different MnOx films for the OER, we quantified the relationship between the steady-state current density and the applied potential during oxygen evolution by measuring their Tafel slopes. Tafel plots were tested in an aqueous solution containing 0.1 M KOH and 0.9 M KNO3 as the supporting electrolyte following previously published protocols. (30,31) The solutions were stirred at ∼500 rpm with a magnetic stir bar and stir plate to minimize the effect of mass-transport during the measurements. To limit the pseudocapacitance, each electrode was first held at 1.05 V versus Ag/AgCl for 200 s. A series of decreasing potentials, ranging from 1.05 to 0.95 V versus Ag/AgCl with an interval of 0.01 V, was then applied with each potential lasting for 30 s. The potentials were automatically iR compensated during the measurements by the EC-Lab software for the solution and substrate resistances. The applied potential versus the log of the current density was used to calculate the Tafel slope for each sample. Bare FTO slides exhibited Tafel slopes that were over 500 mV/decade and current densities that were at least 2 orders of magnitude lower than those of the MnOx films deposited on FTO, indicating that the substrate by itself is a poor catalyst for the OER.

Electrochemically Active Surface Area

Differences in the ECSAs of the MnOx films were determined from their electrochemical double-layer capacitance. (52) CV scans were measured over a potential window in which there was no Faradaic current in an aqueous solution containing 0.1 M KOH and 0.9 M KNO3. The potential window used was ±0.05 V with respect to the open-circuit potential of the cell using the MnOx film as the working electrode. CV scans were conducted by sweeping the potential within this window at eight different scan rates: 5, 25, 50, 75, 100, 125, 150, and 175 mV/s. At each vertex potential, the working electrode was held for 10 s before reversing the sweep direction.

Structural Characterization

Scanning Electron Microscopy

The morphologies of the MnOx films were characterized by using a ThermoFisher Quattro S environmental SEM operated at an acceleration voltage of 10 kV. The conductive top surface of each substrate was connected to the SEM sample holder with copper tape to avoid charge buildup. To compare films before and after constant-current electrolysis, the as-synthesized MnOx film on FTO-coated glass was cut into two smaller pieces of approximately 1 cm × 1 cm. One piece of the sample was imaged by SEM and the other underwent constant-current electrolysis as described in the manuscript before imaging.

X-ray Absorption Near-Edge Spectroscopy

X-ray absorption spectra at the Mn K-edge were collected in fluorescence mode using a four-element Vortex detector at beamline 10-BM-A,B of the Advanced Photon Source at Argonne National Laboratory. The XANES data were analyzed using Athena software. The first derivative of each spectrum was calculated to determine the edge energy of the peak. Manganese powders purchased from Millipore Sigma (MnO, Mn2O3, and MnO2) were used as references for the edge energies of Mn in different oxidation states. A linear fit was made to the edge energies and oxidation states of the reference manganese oxide powders. The average oxidation states of Mn in the electrodeposited MnOx films were determined through comparison of their edge energies to this linear fit.

X-ray Photoelectron Spectroscopy

XPS was performed by using a Physical Electronics 5000 VersaProbe II Scanning ESCA Microprobe system with a base pressure below 1 × 10–9 Torr. XPS data were acquired using the 1486.6 eV line from a monochromatic Al Kα source at 150 W with a spherical capacitor analyzer set to a pass energy of 23.5 eV for the high-resolution scans. The step size was 0.05 eV. High-resolution spectra in the binding regions for Mn 2p, Mn 3s, Mn 3p, C 1s, and K 2p electrons were measured for the MnOx films before and after constant-current electrolysis. Manganese powders prepared by hydrothermal synthesis or calcination (MnO2, Mn2O3, Mn3O4) were used as references for the peak positions of Mn in different oxidation states.
To monitor changes in the average Mn oxidation state of the MnOx films, we fit the Mn 2p3/2 peak using the method proposed by Nesbitt and Banerjee. (54) The binding energies of the MnOx films were first corrected based on the C 1s peak. A Shirley background was used in the binding energy region for Mn 2p3/2 electrons. Each oxidation state of Mn was decomposed into a multiplet of five different peaks based on calculations by Gupta and Sen, (55,56) who accounted for coupling between unpaired valence electrons in high-spin manganese and unpaired core electrons (once photoionization has taken place). The peak positions and the ratios of peak areas were constrained based on the fitting parameters used in prior work. (20,37,54,57) Each peak was fit by using a 70% Gaussian and 30% Lorentzian peak shape. For the reference manganese oxide powders, the full width at half maximum (fwhm) for each peak in the multiplet was optimized to reduce the residual standard deviation (see Table S2). The fwhm for peaks in the electrodeposited films was set to 1.25 eV to compare the distribution of oxidation states. For each oxidation state of Mn, we allowed the peak position to vary by up to 0.3 eV from the reference value. For example, when fitting the five peaks for Mn3+ multiplet, the binding energy of the peak with the lowest binding energy was 640.7 ± 0.3 eV, and the binding energies for the other four peaks were fixed based on their relative position to the peak with the lowest binding energy. The binding energies of the peaks with the lowest binding energy among the multiplets for Mn2+ and Mn4+ were 640.0 ± 0.3 eV and 641.9 ± 0.3 eV, respectively. The area for the peak with the lowest binding energy was allowed to vary freely to obtain the lowest residual standard deviation. The areas for the other four peaks were constrained based on their relative intensity to the peak with the lowest binding energy.

Raman Spectroscopy

Raman spectroscopy was used to characterize the structure of the MnOx films with an InVia Raman microscope (Renishaw, UK). The excitation source was a 514 nm laser (∼4 mW), and a grating of 1800 lines/mm was used. A 20× Leica objective and a reduced power of 50% were utilized. The integration time for each measurement was 10 s, and the step size was 1.2 cm–1. Raman spectra of MnOx films grown on FTO substrates in the dark and under illumination were measured before and after constant-current electrolysis.

Supporting Information

Click to copy section linkSection link copied!

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.3c00002.

  • Procedures used to synthesize manganese oxide powders and obtain their diffraction patterns, variations of the square-wave potential used to electrodeposit MnOx films, procedure for obtaining the double-layer capacitances and ECSAs, and characterization of electrodeposited MnOx films and reference powders by XANES; summary of electrochemical characterization of MnOx films and the parameters used to fit the X-ray photoelectron spectra of different MnOx films and reference powders; additional characterization by linear sweep voltammetry and constant-current electrolysis to measure the stability of MnOx films, CV used to measure the double-layer capacitances of the MnOx films, Mott–Schottky plots of MnOx films at different frequencies, X-ray absorption near edge spectroscopy and linear fitting of XANES data, Raman spectrum of a MnO2 film grown at constant potential, and XPS of electrodeposited MnOx films in the region for K 2p electrons (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

Click to copy section linkSection link copied!
  • Corresponding Author
    • Bryce Sadtler - Department of Chemistry, Washington University, St. Louis, Missouri 63130, United States;  Institute of Materials Science & Engineering, Washington University, St. Louis, Missouri 63130, United States;  Orcidhttps://orcid.org/0000-0003-4860-501X;  Email: [email protected]
  • Authors
  • Author Contributions

    CRediT: Chu Qin conceptualization (supporting), formal analysis (lead), investigation (lead), writing-original draft (equal); Jiang Luo formal analysis (supporting), investigation (supporting), writing-original draft (supporting); Dongyan Zhang formal analysis (supporting), investigation (supporting); Logan Brennan investigation (supporting); shijun Tian formal analysis (supporting); Ashlynn Berry investigation (supporting); Brandon M. Campbell investigation (supporting); Bryce Sadtler conceptualization (lead), formal analysis (supporting), supervision (lead), writing-original draft (equal).

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

Click to copy section linkSection link copied!

This material is based upon work supported by the National Science Foundation (NSF) under grant no. CHE-1753344 to B.S. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract no DE-AC02-06CH11357. B.M.C. acknowledges support from the MARC U-STAR program at Washington University. Electron microscopy and XPS were performed at the Institute of Materials Science & Engineering at Washington University. X-ray diffraction was performed in the Department of Earth and Planetary Sciences at Washington University. The authors thank S. Singamaneni for use of his Raman spectrometer and M. Warren for assistance in acquiring and interpreting the X-ray absorption spectra.

References

Click to copy section linkSection link copied!

This article references 69 other publications.

  1. 1
    Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729– 15735,  DOI: 10.1073/pnas.0603395103
    Google Scholar
    1
    Powering the planet: chemical challenges in solar energy utilization
    Lewis, Nathan S.; Nocera, Daniel G.
    Proceedings of the National Academy of Sciences of the United States of America (2006), 103 (43), 15729-15735CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A review. Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, ≥2-fold by mid century relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atm. demands that holding atm. CO2 levels to even twice their pre-anthropogenic values by mid century will require invention, development, and deployment of schemes for C-neutral energy prodn. on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 h to the earth than all of the energy consumed by humans in an entire year. In view of the intermittence of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An esp. attractive approach is to store solar-converted energy as chem. bonds, i.e., in a photosynthetic process at a year-round av. efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy as chem. bonds, producing O from H2O and a reduced fuel such as H, methane, MeOH, or other hydrocarbon species.
  2. 2
    Tahir, M.; Pan, L.; Idrees, F.; Zhang, X.; Wang, L.; Zou, J.-J.; Wang, Z. L. Electrocatalytic Oxygen Evolution Reaction for Energy Conversion and Storage: A Comprehensive Review. Nano Energy 2017, 37, 136– 157,  DOI: 10.1016/j.nanoen.2017.05.022
    Google Scholar
    2
    Electrocatalytic oxygen evolution reaction for energy conversion and storage: A comprehensive review
    Tahir, Muhammad; Pan, Lun; Idrees, Faryal; Zhang, Xiangwen; Wang, Li; Zou, Ji-Jun; Wang, Zhong Lin
    Nano Energy (2017), 37 (), 136-157CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
    Water oxidn. or oxygen evolution reaction (OER) electrocatalysis got much attention in the last few years because of its prime role in water splitting, rechargeable metal-air batteries and fuel cells. Therefore, the development of efficient, abundant and economical catalysts for water oxidn. reaction is one of the main subjects of present study in renewable energies. This review article summarizes the very recent efforts in the field of OER electrocatalysis along with the faced challenges. The solns. to these challenges also outline with appropriate examples of scientific literatures. Significantly, the present review will provide the stds. to evaluate the activity and stability for heterogeneous OER catalysts. It will clearly summarize the future directions and applications, esp. the combination of sustainable energy utilization (like triboelec. nanogenerator) with water splitting. The providing study will help to explore and develop better catalysts and units for practical applications and will offer basic understanding of OER process along with the std. parameters to evaluate the performance.
  3. 3
    Vesborg, P. C. K.; Jaramillo, T. F. Addressing the Terawatt Challenge: Scalability in the Supply of Chemical Elements for Renewable Energy. RSC Adv. 2012, 2, 7933– 7947,  DOI: 10.1039/c2ra20839c
    Google Scholar
    3
    Addressing the terawatt challenge: scalability in the supply of chemical elements for renewable energy
    Vesborg, Peter C. K.; Jaramillo, Thomas F.
    RSC Advances (2012), 2 (21), 7933-7947CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The energy infrastructure for fossil fuels is well-established, accounting for approx. 87% of the 16 TW of power consumed globally. For renewable and sustainable energy conversion technologies to play a relevant role at the terrestrial scale, they must be able to scale to the TW level of deployment. This would place a significant demand on the current and future supply of raw materials (chem. elements) used by those technologies. Oftentimes, the av. crustal abundance of a chem. element is cited as a measure of its scalability, however another important metric for scalability is the existence (of lack thereof) of mineable ores with a high concn. of the targeted element. This paper aims to provide an overview of the availability of all elements. This is accomplished via a compilation of data for global primary prodn. rates for each element, as a measure of availability at the present time. This work also addresses the potential future availability based on current and possible future primary sources.
  4. 4
    Gaultois, M. W.; Sparks, T. D.; Borg, C. K. H.; Seshadri, R.; Bonificio, W. D.; Clarke, D. R. Data-Driven Review of Thermoelectric Materials: Performance and Resource Considerations. Chem. Mater. 2013, 25, 2911– 2920,  DOI: 10.1021/cm400893e
    Google Scholar
    4
    Data-Driven Review of Thermoelectric Materials: Performance and Resource Considerations
    Gaultois, Michael W.; Sparks, Taylor D.; Borg, Christopher K. H.; Seshadri, Ram; Bonificio, William D.; Clarke, David R.
    Chemistry of Materials (2013), 25 (15), 2911-2920CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A review. In this review, the authors describe the creation of a large database of thermoelec. materials prepd. by abstracting information from over 100 publications. The database has over 18,000 data points from multiple classes of compds., whose relevant properties were measured at several temps. Appropriate visualization of the data immediately allows certain insights to be gained with regard to the property space of plausible thermoelec. materials. Of particular note is that any candidate material needs to display an elec. resistivity value that is ∼1 mΩ cm at 300 K, i.e., samples should be significantly more conductive than the Mott min. metallic cond. The Herfindahl-Hirschman index, a commonly accepted measure of market concn., was calcd. from geol. data (known elemental reserves) and geopolitical data (elemental prodn.) for much of the periodic table. The visualization strategy employed here allows rapid sorting of thermoelec. compns. with respect to important issues of elemental scarcity and supply risk.
  5. 5
    Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production Via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983– 2002,  DOI: 10.1039/c3ee40831k
    Google Scholar
    5
    Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry
    Pinaud, Blaise A.; Benck, Jesse D.; Seitz, Linsey C.; Forman, Arnold J.; Chen, Zhebo; Deutsch, Todd G.; James, Brian D.; Baum, Kevin N.; Baum, George N.; Ardo, Shane; Wang, Heli; Miller, Eric; Jaramillo, Thomas F.
    Energy & Environmental Science (2013), 6 (7), 1983-2002CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Photoelectrochem. water splitting is a promising route for the renewable prodn. of hydrogen fuel. This work presents the results of a tech. and economic feasibility anal. conducted for four hypothetical, centralized, large-scale hydrogen prodn. plants based on this technol. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calc. the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day prodn. scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from $1.60-$10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O2 and H2 in a single bed system and long mol. transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity anal. revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the prodn. costs are consistent with the Department of Energy's targeted threshold cost of $2.00-$4.00 per kg H2 for dispensed hydrogen, demonstrating that photoelectrochem. water splitting could be a viable route for hydrogen prodn. in the future if material performance targets can be met.
  6. 6
    Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution. J. Am. Chem. Soc. 2012, 134, 17253– 17261,  DOI: 10.1021/ja307507a
    Google Scholar
    6
    Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution
    Trotochaud, Lena; Ranney, James K.; Williams, Kerisha N.; Boettcher, Shannon W.
    Journal of the American Chemical Society (2012), 134 (41), 17253-17261CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Water oxidn. is a crit. step in water splitting to make hydrogen fuel. The authors report the soln. synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) electrocatalytic properties of ∼2-3 nm thick films of NiOx, CoOx, NiyCo1-yOx, Ni0.9Fe0.1Ox, IrOx, MnOx, and FeOx. The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochem. properties of the oxides. Ni0.9Fe0.1Ox is the most active water oxidn. catalyst in basic media, passing 10 mA/cm2 at an overpotential of 336 mV with a Tafel slope of 30 mV/dec with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrOx control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni0.9Fe0.1OOH oxyhydroxide species with nearly every Ni atom electrochem. active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, NiyCo1-yOx thin films showed decreasing activity relative to the pure NiOx films with increasing Co content. This finding is explained by the suppressed in situ formation of the active layered oxyhydroxide with increasing Co. The high OER activity and simple synthesis make these Ni-based catalyst thin films useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting or in high-surface-area electrodes for water electrolysis.
  7. 7
    Sun, K.; Park, N.; Sun, Z.; Zhou, J.; Wang, J.; Pang, X.; Shen, S.; Noh, S. Y.; Jing, Y.; Jin, S.; Yu, P. K. L.; Wang, D. Nickel Oxide Functionalized Silicon for Efficient Photo-Oxidation of Water. Energy Environ. Sci. 2012, 5, 7872– 7877,  DOI: 10.1039/c2ee21708b
    Google Scholar
    7
    Nickel oxide functionalized silicon for efficient photo-oxidation of water
    Sun, Ke; Park, Namseok; Sun, Zhelin; Zhou, Jigang; Wang, Jian; Pang, Xiaolu; Shen, Shaohua; Noh, Sun Young; Jing, Yi; Jin, Sungho; Yu, Paul K. L.; Wang, Deli
    Energy & Environmental Science (2012), 5 (7), 7872-7877CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    We report a nickel oxide (NiOx) thin film, from a cost-effective sol-gel process, coated n-type silicon (n-Si) as a photoanode for efficient photo-oxidn. of water under neutral pH condition. The NiOx thin film has three functions: (a) serves as a protection layer to improve the chem. stability of the Si photoelectrode, (b) acts as an oxygen evolution catalyst, and (c) provides junction photovoltage to further reduce overpotential. The oxygen evolution onset potential is reduced to below the thermodn. water oxidn. level and oxygen evolution was obsd. at low overpotentials. Our results demonstrate the fabrication of robust photoelectrodes from low-cost NiOx and Si, which enable a practical solar water oxidn. with high efficiency.
  8. 8
    Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 342, 836– 840,  DOI: 10.1126/science.1241327
    Google Scholar
    8
    High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation
    Kenney, Michael J.; Gong, Ming; Li, Yanguang; Wu, Justin Z.; Feng, Ju; Lanza, Mario; Dai, Hongjie
    Science (Washington, DC, United States) (2013), 342 (6160), 836-840CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Si's sensitivity to corrosion has hindered its use in photoanode applications. Deposition of a ∼2-nm Ni film on n-type Si (n-Si) with its native oxide affords a high-performance metal-insulator-semiconductor photoanode for photoelectrochem. (PEC) H2O oxidn. in both aq. KOH (KOH, pH = 14) and aq. borate buffer (pH = 9.5) solns. The Ni film acted as a surface protection layer against corrosion and as a nonprecious metal electrocatalyst for O evolution. In 1 M aq. KOH, the Ni/n-Si photoanodes exhibited high PEC activity with a low onset potential (∼1.07 V vs. reversible H electrode), high photocurrent d., and durability. The electrode showed no sign of decay after ∼80 h of continuous PEC H2O oxidn. in a mixed Li borate-K borate electrolyte. The high photovoltage was attributed to a high built-in potential in a metal-insulator-semiconductor-like device with an ultrathin, incomplete screening Ni/NiOx layer from the electrolyte.
  9. 9
    Zou, S.; Burke, M. S.; Kast, M. G.; Fan, J.; Danilovic, N.; Boettcher, S. W. Fe (Oxy)Hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution. Chem. Mater. 2015, 27, 8011– 8020,  DOI: 10.1021/acs.chemmater.5b03404
    Google Scholar
    9
    Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution
    Zou, Shihui; Burke, Michaela S.; Kast, Matthew G.; Fan, Jie; Danilovic, Nemanja; Boettcher, Shannon W.
    Chemistry of Materials (2015), 27 (23), 8011-8020CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Fe cations dramatically enhance oxygen evolution reaction (OER) activity when incorporated substitutionally into Ni or Co (oxy)hydroxides, serving as possible OER active sites. Pure Fe (oxy)hydroxides, however, are typically thought to be poor OER catalysts and are not well-understood. Here, we report a systematic investigation of Fe (oxy)hydroxide OER catalysis in alk. media. At low overpotentials of ∼350 mV, the catalyst dissoln. rate is low, the activity is dramatically enhanced by an AuOx/Au substrate, and the geometric OER c.d. is largely independent of mass loading. At higher overpotentials of ∼450 mV, the dissoln. rate is high, the activity is largely independent of substrate choice, and the geometric c.d. depends linearly on loading. These observations, along with previously reported in situ cond. measurements, suggest a new model for OER catalysis on Fe (oxy)hydroxide. At low overpotentials, only the first monolayer of the electrolyte-permeable Fe (oxy)hydroxide, which is in direct contact with the conductive support, is OER-active due to elec. cond. limitations. On Au substrates, Fe cations interact with AuOx after redox cycling, leading to enhanced intrinsic activity over FeOOH on Pt substrates. At higher overpotentials, the cond. of Fe (oxy)hydroxide increases, leading to a larger fraction of the electrolyte-permeable catalyst film participating in catalysis. Comparing the apparent activity of the putative Fe active sites in/on different hosts/surfaces supports a possible connection between OER activity and local structure.
  10. 10
    Wang, H.; Lee, H.-W.; Deng, Y.; Lu, Z.; Hsu, P.-C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional Non-Noble Metal Oxide Nanoparticle Electrocatalysts through Lithium-Induced Conversion for Overall Water Splitting. Nat. Commun. 2015, 6, 7261,  DOI: 10.1038/ncomms8261
    Google Scholar
    10
    Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting
    Wang, Haotian; Lee, Hyun-Wook; Deng, Yong; Lu, Zhiyi; Hsu, Po-Chun; Liu, Yayuan; Lin, Dingchang; Cui, Yi
    Nature Communications (2015), 6 (), 7261CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Developing earth-abundant, active and stable electrocatalysts which operate in the same electrolyte for water splitting, including oxygen evolution reaction and hydrogen evolution reaction, is important for many renewable energy conversion processes. Here we demonstrate the improvement of catalytic activity when transition metal oxide (iron, cobalt, nickel oxides and their mixed oxides) nanoparticles (∼20 nm) are electrochem. transformed into ultra-small diam. (2-5 nm) nanoparticles through lithium-induced conversion reactions. Different from most traditional chem. syntheses, this method maintains excellent elec. interconnection among nanoparticles and results in large surface areas and many catalytically active sites. We demonstrate that lithium-induced ultra-small NiFeOx nanoparticles are active bifunctional catalysts exhibiting high activity and stability for overall water splitting in base. We achieve 10 mA cm-2 water-splitting current at only 1.51 V for over 200 h without degrdn. in a two-electrode configuration and 1 M KOH, better than the combination of iridium and platinum as benchmark catalysts.
  11. 11
    Hill, J. C.; Landers, A. T.; Switzer, J. A. An Electrodeposited Inhomogeneous Metal–Insulator–Semiconductor Junction for Efficient Photoelectrochemical Water Oxidation. Nat. Mater. 2015, 14, 1150– 1155,  DOI: 10.1038/nmat4408
    Google Scholar
    11
    An electrodeposited inhomogeneous metal-insulator-semiconductor junction for efficient photoelectrochemical water oxidation
    Hill, James C.; Landers, Alan T.; Switzer, Jay A.
    Nature Materials (2015), 14 (11), 1150-1155CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    The photoelectrochem. splitting of water into hydrogen and oxygen requires a semiconductor to absorb light and generate electron-hole pairs, and a catalyst to enhance the kinetics of electron transfer between the semiconductor and soln. A crucial question is how this catalyst affects the band bending in the semiconductor, and, therefore, the photovoltage of the cell. A simple and inexpensive electrodeposition method is introduced to produce an efficient n-Si/SiOx/Co/CoOOH photoanode for the photoelectrochem. oxidn. of water to oxygen. The photoanode functions as a solid-state, metal-insulator-semiconductor photovoltaic cell with spatially non-uniform barrier heights in series with a low overpotential water-splitting electrochem. cell. The barrier height is a function of the Co coverage; it increases from 0.74 eV for a thick, continuous film to 0.91 eV for a thin, inhomogeneous film that has not reached coalescence. The larger barrier height leads to a 360 mV photovoltage enhancement relative to a solid-state Schottky barrier.
  12. 12
    Jiao, F.; Frei, H. Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts. Energy Environ. Sci. 2010, 3, 1018– 1027,  DOI: 10.1039/c002074e
    Google Scholar
    12
    Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts
    Jiao, Feng; Frei, Heinz
    Energy & Environmental Science (2010), 3 (8), 1018-1027CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. Recent development of new methods of prepg. cobalt oxide and manganese oxide clusters has led to oxygen evolving catalysts that operate under mild conditions and modest overpotentials at rates approaching practical utility. Synthesis of nanostructured Co3O4 and Mn oxide clusters in mesoporous silica scaffolds affords catalysts with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites. Stacking of the nanoclusters of these earth abundant, durable oxide catalysts in the scaffold results in turnover frequencies per projected area that are sufficient for keeping up with the photon flux at high solar intensity. Opportunities for expanding the metal oxide/silica interface approach to heterogeneous water oxidn. catalysis to a more general approach for multi-electron catalyst designs based on core/shell constructs are discussed. The results are reviewed in the context of all-inorg. materials for catalytic water oxidn. reported recently from other labs., in particular electrodeposits generated from Co phosphate solns., a mol. water oxidn. catalyst based on a polyoxotungstate featuring a Co oxide core, and Mn oxide materials with incorporated Ca ions.
  13. 13
    Hocking, R. K.; Brimblecombe, R.; Chang, L.-Y.; Singh, A.; Cheah, M. H.; Glover, C.; Casey, W. H.; Spiccia, L. Water-Oxidation Catalysis by Manganese in a Geochemical-Like Cycle. Nat. Chem. 2011, 3, 461– 466,  DOI: 10.1038/nchem.1049
    Google Scholar
    13
    Water-oxidation catalysis by manganese in a geochemical-like cycle
    Hocking, Rosalie K.; Brimblecombe, Robin; Chang, Lan-Yun; Singh, Archana; Cheah, Mun Hon; Glover, Chris; Casey, William H.; Spiccia, Leone
    Nature Chemistry (2011), 3 (6), 461-466CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Water oxidn. in all oxygenic photosynthetic organisms is catalyzed by the Mn4CaO4 cluster of Photosystem II. This cluster has inspired the development of synthetic Mn catalysts for solar energy prodn. A photoelectrochem. device, made by impregnating a synthetic tetranuclear-Mn cluster into a Nafion matrix, was shown to achieve efficient H2O oxidn. catalysis. The authors reported in situ x-ray absorption spectroscopy and TEM studies that demonstrate that this cluster dissocs. into Mn(II) compds. in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the obsd. photochem. H2O-oxidn. catalysis. The original Mn cluster serves only as a precursor to the catalytically active material. The behavior of Mn in Nafion parallels its broader biogeochem., which is also dominated by cycles of oxidn. into solid Mn(III/IV) oxides followed by photoredn. to Mn2+.
  14. 14
    Iyer, A.; Del-Pilar, J.; King’ondu, C. K.; Kissel, E.; Garces, H. F.; Huang, H.; El-Sawy, A. M.; Dutta, P. K.; Suib, S. L. Water Oxidation Catalysis Using Amorphous Manganese Oxides, Octahedral Molecular Sieves (OMS-2), and Octahedral Layered (OL-1) Manganese Oxide Structures. J. Phys. Chem. C 2012, 116, 6474– 6483,  DOI: 10.1021/jp2120737
    Google Scholar
    14
    Water Oxidation Catalysis using Amorphous Manganese Oxides, Octahedral Molecular Sieves (OMS-2), and Octahedral Layered (OL-1) Manganese Oxide Structures
    Iyer, Aparna; Del-Pilar, Joselyn; King'ondu, Cecil K.; Kissel, Edward; Garces, Hector F.; Huang, Hui; El-Sawy, Abdelhamid M.; Dutta, Prabir K.; Suib., Steven L.
    Journal of Physical Chemistry C (2012), 116 (10), 6474-6483CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Water oxidn. is the bottleneck in artificial photosynthetic systems that aim to split water into hydrogen and oxygen. However, water oxidn. occurs readily in plants, catalyzed by the Mn4O4Ca manganese cluster. In addn. to this, manganese minerals are ubiquitous in nature displaying layered and tunnel structures. In this study, mixed valent porous amorphous manganese oxides (AMO), along with cryptomelane type tunnel manganese oxides (OMS-2) and layered birnessite (OL-1) have been used as water oxidn. catalysts. Significantly higher turnovers were obtained with AMO (290 mmol O2/mol Mn) compared to tunnel structure OMS-2 (110 mmol O2/mol Mn) and layered structure OL-1 (27 mmol O2/mol Mn) in water oxidn. tests with Ce4+. Oxygen evolution was also confirmed under photochem. conditions using Ru(bpy)32+ as a photosensitizer and persulfate as a sacrificial agent. The differences in catalytic activity among these catalysts have been probed using X-ray diffraction, transmission electron microscopy, Raman and Fourier transform IR (FTIR) spectroscopy, av. oxidn. state, and compositional analyses. Comparison of AMO against prominent manganese catalysts described in literature shows AMO provided the highest turnover nos. AMO catalyst was also reusable after regeneration. O-18 labeling studies proved that water was the source of dioxygen and IR proved the structural stability of AMO after reaction. AMO is related to hexagonal birnessites such as layered biogenic manganese oxides or H+-birnessite that have cation vacancies in the MnO2 sheets rather than completely filled Mn3+/Mn4+ sheets, and this is influential in catalytic activity.
  15. 15
    Wiechen, M.; Zaharieva, I.; Dau, H.; Kurz, P. Layered Manganese Oxides for Water-Oxidation: Alkaline Earth Cations Influence Catalytic Activity in a Photosystem II-Like Fashion. Chem. Sci. 2012, 3, 2330– 2339,  DOI: 10.1039/c2sc20226c
    Google Scholar
    15
    Layered manganese oxides for water-oxidation: alkaline earth cations influence catalytic activity in a photosystem II-like fashion
    Wiechen, Mathias; Zaharieva, Ivelina; Dau, Holger; Kurz, Philipp
    Chemical Science (2012), 3 (7), 2330-2339CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    In reaction sequences for light driven water-splitting into H2 and O2, water-oxidn. is a crucial reaction step. In vivo, the process is catalyzed within a photoenzyme called photosystem II (PSII) by a μ-oxido CaMn4 cluster, the oxygen-evolving complex (OEC). The OEC is known to be virtually inactive if Ca2+ is removed from its structure. Activity can be restored not only by the addn. of Ca2+ but also Sr2+ ions. We have recently introduced layered calcium manganese oxides of the birnessite mineral family as functional synthetic model compds. for the OEC. Here, we present the syntheses of layered manganese oxides where we varied the interlayer cations, prepg. a series of K-, Ca-, Sr- and Mg-contg. birnessites. Structural motifs within these materials were detd. using X-ray absorption spectroscopy (XAS) showing that all materials have similar at. structures despite their different elemental compns. Water-oxidn. expts. were carried out to elucidate structure-reactivity relations. These expts. demonstrated that the oxides-like the OEC-require the presence of calcium in their structures to reach max. catalytic activity. As another similarity to the OEC, Sr2+ is the "second best choice" for the secondary cation. The results thus support mechanistic proposals which involve an important catalytic role for Ca2+ in biol. water-oxidn. Addnl., they offer valuable hints for the development of synthetic, manganese-based water-oxidn. catalysts for artificial photosynthesis.
  16. 16
    Boppana, V. B. R.; Yusuf, S.; Hutchings, G. S.; Jiao, F. Nanostructured Alkaline-Cation-Containing δ-MnO2 for Photocatalytic Water Oxidation. Adv. Funct. Mater. 2013, 23, 878– 884,  DOI: 10.1002/adfm.201202141
    Google Scholar
    16
    Nanostructured Alkaline-Cation-Containing δ-MnO2 for Photocatalytic Water Oxidation
    Boppana, Venkata Bharat Ram; Yusuf, Seif; Hutchings, Gregory S.; Jiao, Feng
    Advanced Functional Materials (2013), 23 (7), 878-884CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Oxygen evolution from water is one of the key reactions for solar fuel prodn. Here, two nanostructured K-contg. δ-MnO2 are synthesized: K-δ-MnO2 nanosheets and K-δ-MnO2 nanoparticles, both of which exhibit high catalytic activity in visible-light-driven water oxidn. The role of alk. cations in oxygen evolution is first explored by replacing the K+ ions in the δ-MnO2 structure with H+ ions through proton ion exchange. H-δ-MnO2 catalysts with a similar morphol. and crystal structure exhibit activities per surface site approx. one order of magnitude lower than that of K-δ-MnO2, although both nanostructured H-δ-MnO2 catalysts have much larger Brunauer-Emmett-Teller (BET) surface areas. Such a low turnover frequency (TOF) per surface Mn atom might be due to the fact that the Ru2+(bpy)3 sensitizer is too large to access the addnl. surface area created during proton exchange. Also, a prepd. Na-contg. δ-MnO2 material with an identical crystal structure exhibits a TOF similar to that of the K-contg. δ-MnO2, suggesting that the alk. cations are not directly involved in catalytic water oxidn., but instead stabilize the layered structure of the δ-MnO2.
  17. 17
    Robinson, D. M.; Go, Y. B.; Mui, M.; Gardner, G.; Zhang, Z.; Mastrogiovanni, D.; Garfunkel, E.; Li, J.; Greenblatt, M.; Dismukes, G. C. Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis. J. Am. Chem. Soc. 2013, 135, 3494– 3501,  DOI: 10.1021/ja310286h
    Google Scholar
    17
    Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis
    Robinson, David M.; Go, Yong Bok; Mui, Michelle; Gardner, Graeme; Zhang, Zhijuan; Mastrogiovanni, Daniel; Garfunkel, Eric; Li, Jing; Greenblatt, Martha; Dismukes, G. Charles
    Journal of the American Chemical Society (2013), 135 (9), 3494-3501CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Manganese oxides occur naturally as minerals in at least 30 different crystal structures, providing a rigorous test system to explore the significance of at. positions on the catalytic efficiency of water oxidn. In this study, the authors chose to systematically compare eight synthetic oxide structures contg. Mn(III) and Mn(IV) only, with particular emphasis on the five known structural polymorphs of MnO2. The authors have adapted literature synthesis methods to obtain pure polymorphs and validated their homogeneity and crystallinity by powder x-ray diffraction and both transmission and scanning electron microscopies. Measurement of water oxidn. rate by oxygen evolution in aq. soln. was conducted with dispersed nanoparticulate manganese oxides and a std. ruthenium dye photooxidant system. No Ru was absorbed on the catalyst surface as obsd. by XPS and EDX. The post reaction at. structure was completely preserved with no amorphization, as obsd. by HRTEM. Catalytic activities, normalized to surface area (BET), decrease in the series Mn2O3 > Mn3O4 » λ-MnO2, where the latter is derived from spinel LiMn2O4 following partial Li+ removal. No catalytic activity is obsd. from LiMn2O4 and four of the MnO2 polymorphs, in contrast to some literature reports with polydispersed manganese oxides and electro-deposited films. Catalytic activity within the eight examd. Mn oxides was found exclusively for (distorted) cubic phases, Mn2O3 (bixbyite), Mn3O4 (hausmannite), and λ-MnO2 (spinel), all contg. Mn(III) possessing longer Mn-O bonds between edge-sharing MnO6 octahedra. Electronically degenerate Mn(III) has antibonding electronic configuration eg1 which imparts lattice distortions due to the Jahn-Teller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidn. at the surface.
  18. 18
    Pokhrel, R.; Goetz, M. K.; Shaner, S. E.; Wu, X.; Stahl, S. S. The “Best Catalyst” for Water Oxidation Depends on the Oxidation Method Employed: A Case Study of Manganese Oxides. J. Am. Chem. Soc. 2015, 137, 8384– 8387,  DOI: 10.1021/jacs.5b05093
    Google Scholar
    18
    The "Best Catalyst" for Water Oxidation Depends on the Oxidation Method Employed: A Case Study of Manganese Oxides
    Pokhrel, Ravi; Goetz, McKenna K.; Shaner, Sarah E.; Wu, Xiaoxia; Stahl, Shannon S.
    Journal of the American Chemical Society (2015), 137 (26), 8384-8387CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Manganese oxides are a highly promising class of water-oxidn. catalysts (WOCs), but the optimal MnOx formulation or polymorph is not clear from previous reports in the literature. A complication not limited to MnOx-based WOCs is that such catalysts are routinely evaluated by different methods, ranging from the use of a chem. oxidant such as Ce4+, photoactive mediators such as [Ru(bpy)3]2+, or electrochem. techniques. Here, we report a systematic study of nine cryst. MnOx materials as WOCs and show that the identity of the "best" catalyst changes, depending on the oxidn. method used to probe the catalytic activity.
  19. 19
    Frey, C. E.; Kurz, P. Water Oxidation Catalysis by Synthetic Manganese Oxides with Different Structural Motifs: A Comparative Study. Chem.─Eur. J. 2015, 21, 14958– 14968,  DOI: 10.1002/chem.201501367
    Google Scholar
    19
    Water Oxidation Catalysis by Synthetic Manganese Oxides with Different Structural Motifs: A Comparative Study
    Frey, Carolin E.; Kurz, Philipp
    Chemistry - A European Journal (2015), 21 (42), 14958-14968CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Manganese oxides are considered to be very promising materials for water oxidn. catalysis (WOC), but the structural parameters influencing their catalytic activity have so far not been clearly identified. For this study, a dozen manganese oxides (MnOx) with various solid-state structures were synthesized and carefully characterised by various phys. and chem. methods. WOC by the different MnOx was then investigated with Ce4+ as chem. oxidant. Oxides with layered structures (birnessites) and those contg. large tunnels (todorokites) clearly gave the best results with reaction rates exceeding 1250 mmolO2 molMn-1 h-1 or about 50 μmolO2 m-2 h-1. In comparison, catalytic rates per mol of Mn of oxides characterized by well-defined 3D networks were rather low (e.g., ca. 90 mmolO2 molMn-1 h-1 for bixbyite, Mn2O3), but impressive if normalized per unit surface area (>100 μmolO2 m-2 h-1 for marokite, CaMn2O4). Thus, two groups of MnOx emerge from this screening as hot candidates for manganese-based WOC materials: 1) amorphous oxides with tunnelled structures and the well-established layered oxides; 2) cryst. MnIII oxides. However, synthetic methods to increase surface areas must be developed for the latter to obtain good catalysis rates per mol of Mn or per unit catalyst mass.
  20. 20
    Smith, P. F.; Deibert, B. J.; Kaushik, S.; Gardner, G.; Hwang, S.; Wang, H.; Al-Sharab, J. F.; Garfunkel, E.; Fabris, L.; Li, J.; Dismukes, G. C. Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (γ-MnOOH). ACS Catal. 2016, 6, 2089– 2099,  DOI: 10.1021/acscatal.6b00099
    Google Scholar
    20
    Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (γ-MnOOH)
    Smith, Paul F.; Deibert, Benjamin J.; Kaushik, Shivam; Gardner, Graeme; Hwang, Shinjae; Wang, Hao; Al-Sharab, Jafar F.; Garfunkel, Eric; Fabris, Laura; Li, Jing; Dismukes, G. Charles
    ACS Catalysis (2016), 6 (3), 2089-2099CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Surface-directed corner-sharing MnO6 octahedra within numerous manganese oxide compds. contg. Mn3+ or Mn4+ oxidn. states show strikingly different catalytic activities for water oxidn., paradoxically poorest for Mn4+ oxides, regardless of oxidn. assay (photochem. and electrochem.). This is demonstrated herein by comparing cryst. oxides consisting of Mn3+ (manganite, γ-MnOOH; bixbyite, Mn2O3), Mn4+ (pyrolusite, β-MnO2) and multiple monophasic mixed-valence manganese oxides. Like all Mn4+ oxides, pure β-MnO2 has no detectable catalytic activity, while γ-MnOOH (tetragonally distorted Mn3+O6, D4h symmetry) is significantly more active and Mn2O3 (trigonal antiprismatic Mn3+O6, D3d symmetry) is the most active. γ-MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallog. defined corner-sharing Mn3+O6 and the appearance of Mn4+. In a comparison of 2D-layered cryst. birnessites (δ-MnO2), the monovalent Mn4+ form is catalytically inert, while the hexagonal polymorph, contg. few out-of-layer corner-sharing Mn3+O6, has ∼10-fold higher catalytic activity than the triclinic polymorph, contg. in-plane edge-sharing Mn3+O6. These electronic and structural correlations point toward the more flexible (corner-shared) Mn3+O6 sites, over more rigid (edge-shared) sites as substantially more active catalytic centers. Electrochem. measurements show and ligand field theory predicts that, among corner-shared Mn3+O6 sites, those possessing D3d ligand field symmetry have stronger covalent Mn-O bonding to the six equiv. oxygen ligands, which we ascribe as responsible for more efficient and faster electrolytic water oxidn. In contrast, D4h Mn3+O6 sites have weaker Mn-O bonding to the two axial oxygen ligands, have sepd. electrochem. oxidn. waves for Mn and O, and are catalytically less efficient and exhibit slower catalytic turnover. By controlling the ligand field geometry and strength to oxygen ligands, we have identified the key variables for tuning water oxidn. activity by manganese oxides. We apply these findings to propose a mechanism for water oxidn. by the CaMn4O5 catalytic site of natural photosynthesis.
  21. 21
    Thenuwara, A. C.; Cerkez, E. B.; Shumlas, S. L.; Attanayake, N. H.; McKendry, I. G.; Frazer, L.; Borguet, E.; Kang, Q.; Remsing, R. C.; Klein, M. L.; Zdilla, M. J.; Strongin, D. R. Nickel Confined in the Interlayer Region of Birnessite: An Active Electrocatalyst for Water Oxidation. Angew. Chem., Int. Ed. 2016, 55, 10381– 10385,  DOI: 10.1002/anie.201601935
    Google Scholar
    21
    Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation
    Thenuwara, Akila C.; Cerkez, Elizabeth B.; Shumlas, Samantha L.; Attanayake, Nuwan H.; McKendry, Ian G.; Frazer, Laszlo; Borguet, Eric; Kang, Qing; Remsing, Richard C.; Klein, Michael L.; Zdilla, Michael J.; Strongin, Daniel R.
    Angewandte Chemie, International Edition (2016), 55 (35), 10381-10385CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni2+ ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 at. %)-intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA/cm2. This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst β-Ni(OH)2 (η≈550 mV). Mol. dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chem. in the spatially confined interlayer region.
  22. 22
    Kang, Q.; Vernisse, L.; Remsing, R. C.; Thenuwara, A. C.; Shumlas, S. L.; McKendry, I. G.; Klein, M. L.; Borguet, E.; Zdilla, M. J.; Strongin, D. R. Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 1863– 1870,  DOI: 10.1021/jacs.6b09184
    Google Scholar
    22
    Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction
    Kang, Qing; Vernisse, Loranne; Remsing, Richard C.; Thenuwara, Akila C.; Shumlas, Samantha L.; McKendry, Ian G.; Klein, Michael L.; Borguet, Eric; Zdilla, Michael J.; Strongin, Daniel R.
    Journal of the American Chemical Society (2017), 139 (5), 1863-1870CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The dependence is investigated of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to det. the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers pos. correlates with the size of the alkali cation: NS/Cs+/NS > NS/Rb+/NS > NS/K+/NS > NS/Na+/NS > NS/Li+/NS. The thermodn. origins of these bilayer heights were investigated using mol. dynamics simulations. The overpotential (η) for the OER correlates with the interlayer spacing; NS/Cs+/NS has the lowest η (0.45 V), while NS/Li+/NS exhibits the highest η (0.68 V) for OER at a c.d. of 1 mA/cm2. Kinetic parameters (η and Tafel slope) assocd. with NS/Cs+/NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies.
  23. 23
    Kölbach, M.; Fiechter, S.; van de Krol, R.; Bogdanoff, P. Evaluation of Electrodeposited α-Mn2O3 as a Catalyst for the Oxygen Evolution Reaction. Catal. Today 2017, 290, 2– 9,  DOI: 10.1016/j.cattod.2017.03.030
    Google Scholar
    There is no corresponding record for this reference.
  24. 24
    Li, A.; Ooka, H.; Bonnet, N.; Hayashi, T.; Sun, Y.; Jiang, Q.; Li, C.; Han, H.; Nakamura, R. Stable Potential Windows for Long-Term Electrocatalysis by Manganese Oxides under Acidic Conditions. Angew. Chem., Int. Ed. 2019, 58, 5054– 5058,  DOI: 10.1002/anie.201813361
    Google Scholar
    24
    Stable Potential Windows for Long-Term Electrocatalysis by Manganese Oxides Under Acidic Conditions
    Li, Ailong; Ooka, Hideshi; Bonnet, Nadege; Hayashi, Toru; Sun, Yimeng; Jiang, Qike; Li, Can; Han, Hongxian; Nakamura, Ryuhei
    Angewandte Chemie, International Edition (2019), 58 (15), 5054-5058CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Efficient, earth-abundant, and acid-stable catalysts for the oxygen evolution reaction (OER) are missing pieces for the prodn. of hydrogen via water electrolysis. Here, we report how the limitations on the stability of 3d-metal materials can be overcome by the spectroscopic identification of stable potential windows in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. We demonstrate the benefits of this approach using gamma manganese oxide (γ-MnO2), which shows no signs of deactivation even after 8000 h of electrolysis at a pH of 2. This stability is vastly superior to existing acid-stable 3d-metal OER catalysts, but cannot be realized if there is a deviation as small as 50-mV from the stable potential window. A stable voltage efficiency of over 70 % in a polymer-electrolyte membrane (PEM) electrolyzer further verifies the availability of this approach and showcases how materials previously perceived to be unstable may have potential application for water electrolysis in an acidic environment.
  25. 25
    Takashima, T.; Hashimoto, K.; Nakamura, R. Mechanisms of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO2 Electrocatalysts. J. Am. Chem. Soc. 2012, 134, 1519– 1527,  DOI: 10.1021/ja206511w
    Google Scholar
    25
    Mechanisms of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO2 Electrocatalysts
    Takashima, Toshihiro; Hashimoto, Kazuhito; Nakamura, Ryuhei
    Journal of the American Chemical Society (2012), 134 (3), 1519-1527CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Mn oxides function as efficient electrocatalysts for H2O oxidn. to O2 in strongly alk. conditions, but are inefficient at neutral pH. To provide new insight into the mechanism underlying the pH-dependent activity of the electrooxidn. reaction, the authors performed UV-visible spectroelectrochem. detection of the intermediate species for H2O oxidn. by a Mn oxide electrode. Layered Mn oxide nanoparticles, δ-MnO2 (K0.17[Mn4+0.90Mn3+0.07.box.0.03]O2·0.53H2O) deposited on F-doped Sn oxide electrodes catalyze H2O oxidn. at pH from 4 to 13. At this pH range, a sharp rise in absorption at 510 nm was obsd. with a concomitant increase of anodic current for O2 evolution. Using pyrophosphate as a probe mol., the 510 nm absorption was attributable to Mn3+ on the surface of δ-MnO2. The onset potential of the H2O oxidn. current was const. at ∼1.5 V vs. SHE from pH 4 to pH 8, but sharply shifted to neg. at pH > 8. Strikingly, this behavior was well reproduced by the pH dependence of the onset of 510 nm absorption, indicating that Mn3+ acts as the precursor of H2O oxidn. Mn3+ is unstable at pH < 9 due to the disproportionation reaction giving Mn2+ and Mn4+, whereas it is effectively stabilized by the comproportionation of Mn2+ and Mn4+ in alk. conditions. Thus, the low activity of Mn oxides for H2O oxidn. under neutral conditions is most likely due to the inherent instability of Mn3+, whose accumulation at the surface of catalysts requires the electrochem. oxidn. of Mn2+ at a potential of ∼1.4 V. This new model suggests that the control of the disproportionation and comproportionation efficiencies of Mn3+ is essential for the development of Mn catalysts that afford H2O oxidn. with a small overpotential at neutral pH.
  26. 26
    Takashima, T.; Hashimoto, K.; Nakamura, R. Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation under Neutral Conditions. J. Am. Chem. Soc. 2012, 134, 18153– 18156,  DOI: 10.1021/ja306499n
    Google Scholar
    26
    Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation under Neutral Conditions
    Takashima, Toshihiro; Hashimoto, Kazuhito; Nakamura, Ryuhei
    Journal of the American Chemical Society (2012), 134 (44), 18153-18156CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The development of Mn-oxide electrocatalysts for the oxidn. of H2O to O2 was the subject of intensive researches not only for their importance as components of artificial photosynthetic systems, but also as O2-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidn. reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochem. techniques, the stabilization of surface-assocd. intermediate Mn3+ species relative to charge disproportionation is an effective strategy to lower the overpotential for H2O oxidn. by MnO2. The formation of N-Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO2 electrodes effectively stabilized the Mn3+ species, resulting in an ∼500-mV neg. shift of the onset potential for the O2 evolution reaction at neutral pH.
  27. 27
    Zaharieva, I.; Chernev, P.; Risch, M.; Klingan, K.; Kohlhoff, M.; Fischer, A.; Dau, H. Electrosynthesis, Functional, and Structural Characterization of a Water-Oxidizing Manganese Oxide. Energy Environ. Sci. 2012, 5, 7081– 7089,  DOI: 10.1039/c2ee21191b
    Google Scholar
    27
    Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide
    Zaharieva, Ivelina; Chernev, Petko; Risch, Marcel; Klingan, Katharina; Kohlhoff, Mike; Fischer, Anna; Dau, Holger
    Energy & Environmental Science (2012), 5 (5), 7081-7089CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    In the sustainable prodn. of non-fossil fuels, water oxidn. is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochem. water oxidn. at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at const. anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biol. paragon, the water-splitting Mn4Ca complex of photosynthesis, with respect to mean Mn oxidn. state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidn., we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di-μ-oxo bridging between Mn ions is obsd. However in the MnCat, the voltage-cycling protocol resulted in formation of MnIII sites and prevented formation of well-ordered and unreactive MnIVO2. Structure-function relations in Mn-based water-oxidn. catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technol. use.
  28. 28
    Ramírez, A.; Hillebrand, P.; Stellmach, D.; May, M. M.; Bogdanoff, P.; Fiechter, S. Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water. J. Phys. Chem. C 2014, 118, 14073– 14081,  DOI: 10.1021/jp500939d
    Google Scholar
    28
    Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water
    Ramirez, Alejandra; Hillebrand, Philipp; Stellmach, Diana; May, Matthias M.; Bogdanoff, Peter; Fiechter, Sebastian
    Journal of Physical Chemistry C (2014), 118 (26), 14073-14081CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Different manganese oxide phases were prepd. as thin films to elucidate their structure-function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnOx films anodically deposited on F:SnO2/glass and annealed at different temps. (to improve film adherence and crystallinity) were tested in neutral and alk. electrolytes. Differential electrochem. mass spectroscopy showed that the anodic current correlated well with the onset of the expected oxygen evolution, where in 1 M KOH, the anodic current of cryst. α-Mn2O3 films was detd. to onset at an overpotential (η) of 170 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE. Amorphous MnOx films heated at 573 K (MnOx-573 K) were found to improve their adherence to F:SnO2/glass substrate after heat treatment with a slight crystn. detected by Raman spectroscopy. The onset of water oxidn. of MnOx-573 K films was identified at η = 230 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE (1 M KOH). The least active of the investigated manganese oxides was Mn3O4 with an onset at η = 290 mVRHE (at J = 0.1 mA/cm2) and current densities of ca. 10 mA/cm2 at η = 570 mVRHE (1 M KOH). In neutral soln. (1 M KPi), a similar tendency was obsd. with the lowest overpotential found for α-Mn2O3 followed by MnOx-573 K and Mn3O4. XPS revealed that after electrochem. treatment, the surfaces of the manganese oxide electrodes exhibited oxidn. of Mn II and Mn III toward Mn IV under oxygen evolving conditions. In the case of α-Mn2O3 and MnOx-573 K, the manganese oxidn. was found to be reversible in KPi when switching the potential above and below the oxygen evolution reaction (OER) threshold potential. Furthermore, SEM (SEM) images displayed the presence of an amorphous phase on top of all manganese oxide films here tested after oxygen evolution. The results indicate that structural changes played an important role in the catalytic activity of the manganese oxides, in addn. to oxidn. states, a large variety of Mn-O bond lengths and a high concn. of oxygen point defects. Thus, compared to Mn3O4, cryst. α-Mn2O3 and MnOx-573 K are the most efficient catalyst for water oxidn. in the manganese-oxygen system.
  29. 29
    Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. In Situ X-Ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525– 8534,  DOI: 10.1021/ja3104632
    Google Scholar
    29
    In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction
    Gorlin, Yelena; Lassalle-Kaiser, Benedikt; Benck, Jesse D.; Gul, Sheraz; Webb, Samuel M.; Yachandra, Vittal K.; Yano, Junko; Jaramillo, Thomas F.
    Journal of the American Chemical Society (2013), 135 (23), 8525-8534CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In situ x-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochem. systems, with the ability to elucidate the chem. nature of electrocatalysts under reaction conditions. The authors perform in situ XAS measurements on a bifunctional Mn oxide (MnOx) catalyst with high electrochem. activity for the O redn. reaction (ORR) and the O evolution reaction (OER). Using x-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), exposure to an ORR-relevant potential of 0.7 V vs. RHE produces a disordered MnII3,III,IIIO4 phase with negligible contributions from other phases. After the potential is increased to a highly anodic value of 1.8 V vs. RHE, relevant to the OER, the authors observe an oxidn. of ∼80% of the catalytic thin film to form a mixed MnIII,IV oxide, while the remaining 20% of the film consists of a less oxidized phase, likely corresponding to unchanged MnII3,III,IIIO4. XAS and electrochem. characterization of two thin film catalysts with different MnOx thicknesses reveals no significant influence of thickness on the measured oxidn. states, at either ORR or OER potentials, but demonstrates that the OER activity scales with film thickness. Probably the films have porous structure, which does not restrict electrocatalysis to the top geometric layer of the film. As the portion of the catalyst film that is most likely to be oxidized at the high potentials necessary for the OER is that which is closest to the electrolyte interface, the authors hypothesize that the MnIII,IV oxide, rather than MnII3,III,IIIO4, is the phase pertinent to the obsd. OER activity.
  30. 30
    Huynh, M.; Bediako, D. K.; Nocera, D. G. A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid. J. Am. Chem. Soc. 2014, 136, 6002– 6010,  DOI: 10.1021/ja413147e
    Google Scholar
    30
    A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid
    Huynh, Michael; Bediako, D. Kwabena; Nocera, Daniel G.
    Journal of the American Chemical Society (2014), 136 (16), 6002-6010CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    First-row metals were a target for the development of O evolution reaction (OER) catalysts because they comprise noncrit. elements. The authors now report a comprehensive electrochem. characterization of Mn oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissoln. during turnover. To study this process in detail, the O evolution mechanism of MnOx was studied electrokinetically over a pH range spanning acidic, neutral, and alk. conditions. In the alk. pH regime, a ∼ 60 mV/decade Tafel slope and inverse 1st-order dependence on proton concn. were obsd., whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concn. The results reflect two competing mechanisms: a 1-electron 1-proton PCET pathway that is dominant under alk. conditions and a Mn3+ disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of H2O oxidn. as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.
  31. 31
    Huynh, M.; Shi, C.; Billinge, S. J. L.; Nocera, D. G. Nature of Activated Manganese Oxide for Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 14887– 14904,  DOI: 10.1021/jacs.5b06382
    Google Scholar
    31
    Nature of Activated Manganese Oxide for Oxygen Evolution
    Huynh, Michael; Shi, Chenyang; Billinge, Simon J. L.; Nocera, Daniel G.
    Journal of the American Chemical Society (2015), 137 (47), 14887-14904CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Electrodeposited manganese oxide films (MnOx) are promising stable oxygen evolution catalysts. They are able to catalyze the oxygen evolution reaction in acidic solns. but with only modest activity when prepd. by const. anodic potential deposition. We now show that the performance of these catalysts is improved when they are "activated" by potential cycling protocols, as measured by Tafel anal. (where lower slope is better): upon activation the Tafel slope decreases from ∼120 to ∼70 mV/decade in neutral conditions and from ∼650 to ∼90 mV/decade in acidic solns. Electrochem., spectroscopic, and structural methods were employed to study the activation process and support a mechanism where the original birnessite-like MnOx (δ-MnO2) undergoes a phase change, induced by comproportionation with cathodically generated Mn(OH)2, to a hausmannite-like intermediate (α-Mn3O4). Subsequent anodic conditioning from voltage cycling or water oxidn. produces a disordered birnessite-like phase, which is highly active for oxygen evolution. At pH 2.5, the c.d. of activated MnOx (at an overpotential of 600 mV) is 2 orders of magnitude higher than that of the original MnOx and begins to approach that of Ru and Ir oxides in acid.
  32. 32
    Morgan Chan, Z.; Kitchaev, D. A.; Nelson Weker, J.; Schnedermann, C.; Lim, K.; Ceder, G.; Tumas, W.; Toney, M. F.; Nocera, D. G. Electrochemical Trapping of Metastable Mn3+ Ions for Activation of MnO2 Oxygen Evolution Catalysts. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, E5261– E5268,  DOI: 10.1073/pnas.1722235115
    Google Scholar
    There is no corresponding record for this reference.
  33. 33
    Chinnadurai, D.; Nallal, M.; Kim, H.-J.; Li, O. L.; Park, K. H.; Prabakar, K. Mn3+ Active Surface Site Enriched Manganese Phosphate Nano-Polyhedrons for Enhanced Bifunctional Oxygen Electrocatalyst. ChemCatChem 2020, 12, 2348– 2355,  DOI: 10.1002/cctc.202000164
    Google Scholar
    33
    Mn3+ Active Surface Site Enriched Manganese Phosphate Nano-polyhedrons for Enhanced Bifunctional Oxygen Electrocatalyst
    Chinnadurai, Deviprasath; Nallal, Muthuchamy; Kim, Hee-Je; Li, Oi Lun; Park, Kang Hyun; Prabakar, Kandasamy
    ChemCatChem (2020), 12 (8), 2348-2355CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Manganese-based electrocatalyst has a great attention for the oxygen evolution reaction (OER) and oxygen redn. reaction (ORR) applications, since the discovery of active center in nature photosynthesis system. The Mn oxidn. state optimization and structural defect engineering are essential to get a highly active Mn-based catalytic materials. MnP showed a good water oxidn. activity with a lower overpotential of 286 mV to reach the c.d. of 10 mA/cm2 and a Tafel slope of 76 mV/dec. Also, the electron transfer no. calcd. from both the rotating disk electrode and rotating ring-disk electrode techniques is a quasi-4 electron transfer process with an onset and half-wave potential of 0.998 V and 0.936 V vs. RHE resp. MnP achieved a higher limiting kinetic current of 5.7 mA/cm2 and a very low H2O2 yield of 1.6%. Chronoamperometry and cyclic voltammetry studies confirmed the long-term stability and durability of the prepd. catalyst. The variance metrics ΔE [Ej10-Ej-3] is used to est. the overall activity from the p.d. between OER overpotential at 10 mA/cm2 and ORR kinetic current at 3 mA/cm2. MnP shows very low ΔE (0.58 V) which demonstrate an efficient bifunctional activity in ORR and OER reactions. This work might shed new light on the development of MnP based bifunctional oxygen electrocatalyst.
  34. 34
    Shao, C.; Yin, K.; Liao, F.; Zhu, W.; Shi, H.; Shao, M. Rod-Shaped α-MnO2 Electrocatalysts with High Mn3+ Content for Oxygen Reduction Reaction and Zn-Air Battery. J. Alloys Compd. 2021, 860, 158427,  DOI: 10.1016/j.jallcom.2020.158427
    Google Scholar
    34
    Rod-shaped α-MnO2 electrocatalysts with high Mn3+ content for oxygen reduction reaction and Zn-air battery
    Shao, Chenrui; Yin, Kui; Liao, Fan; Zhu, Wenxiang; Shi, Huixian; Shao, Mingwang
    Journal of Alloys and Compounds (2021), 860 (), 158427CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    Manganese dioxide (MnO2) is a potential efficient candidate for the catalysis of oxygen redn. reactions (ORR) in alk. electrolyte. In this work, potassium permanganate (KMnO4) was reduced by silicon nanowires (SiNWs) and hydrofluoric acid (HF) to prep. α-MnO2 with a rod-like structure. The α-MnO2 prepd. at 120°C (α-MnO2-120) demonstrated excellent activity for ORR and had an oxygen redn. pathway of four-electron. In 0.1 M KOH electrolyte, it showed a large half-wave potential of 0.85 V vs. RHE and a high initial potential of 1.01 V vs. RHE. The diffusion-limiting current of α-MnO2-120 catalyst (7.0 mA cm-2) is also quite stable. The excellent ORR activity of α-MnO2-120 is attributed to the high Mn3+ content and sufficient oxygen vacancies. In addn., the zinc-air battery using the α-MnO2-120 catalyst displays an open circuit potential of 1.27 V, a max. power d. of 240 mW cm-2 at a c.d. of 0.33 Acm-2 and superior stability.
  35. 35
    Melder, J.; Mebs, S.; Heizmann, P. A.; Lang, R.; Dau, H.; Kurz, P. Carbon Fibre Paper Coated by a Layered Manganese Oxide: A Nano-Structured Electrocatalyst for Water-Oxidation with High Activity over a Very Wide pH Range. J. Mater. Chem. A 2019, 7, 25333– 25346,  DOI: 10.1039/c9ta08804k
    Google Scholar
    35
    Carbon fibre paper coated by a layered manganese oxide: a nano-structured electrocatalyst for water-oxidation with high activity over a very wide pH range
    Melder, Jens; Mebs, Stefan; Heizmann, Philipp A.; Lang, Rebekka; Dau, Holger; Kurz, Philipp
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (44), 25333-25346CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Manganese oxide coated carbon fiber paper electrodes (MnOx/CFP), prepd. via an easily scalable redox-deposition route, have been reported as promising materials for electrochem. water oxidn. catalysis (WOC). In the study presented here, the catalyst mass-dependence of the activity of such nanostructured WOC electrodes and their activity and stability in phosphate buffer electrolytes over nearly the entire pH range (pH 1-14) was explored. For all pH values, current densities of >1 mA cm-2 at overpotentials η of 350-500 mV were obsd., which are very good values for electrodes contg. only earth-abundant elements (Mn, O and C in this case). The very decent performance obsd. for the mildly acidic pH regime (pH 2-6) was esp. noteworthy. For the dependence of η on the pH, an ideal Nernstian behavior was detected for the pH window pH 1-10. Above pH 10, a pronounced deviation from the expected trend was found, as most electro-kinetic parameters indicated even higher activities for the strongly alk. regime. Concerning anode stabilities, current-densities of 2 mA cm-2 could be maintained at all studied pHs over at least 24 h of continuous operation. Pre- and post-operando spectroscopic analyses (e.g. vibrational and X-ray absorption spectroscopy, SEM) revealed only minor changes of the catalyst structure, compn., morphol. or the av. Mn oxidn. state induced by the electrocatalytic operation, which confirms the good stabilities found in the electrochem. measurements. Thus, we report on an earth-abundant, easily-prepd. type of WOC electrode, which exhibits promising activities and stabilities for applications in alk., neutral and even acidic electrolytes.
  36. 36
    Melder, J.; Bogdanoff, P.; Zaharieva, I.; Fiechter, S.; Dau, H.; Kurz, P. Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions. Z. Phys. Chem. 2020, 234, 925– 978,  DOI: 10.1515/zpch-2019-1491
    Google Scholar
    36
    Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions
    Melder, Jens; Bogdanoff, Peter; Zaharieva, Ivelina; Fiechter, Sebastian; Dau, Holger; Kurz, Philipp
    Zeitschrift fuer Physikalische Chemie (Muenchen, Germany) (2020), 234 (5), 925-978CODEN: ZPCFAX; ISSN:0942-9352. (Oldenbourg Wissenschaftsverlag GmbH)
    A review. The efficient catalysis of the four-electron oxidn. of water to mol. oxygen is a central challenge for the development of devices for the prodn. of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biol. catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides ("MnOx") for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and compn. of the "MnOx" is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are esp. good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been obsd. that the electrocatalytic performance is also largely dependent on the electrolyte's compn. and pH and that a no. of equil. accompany the catalytic process, resulting in "adaptive changes" of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidn. electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the compn. of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates obsd. for iridium, ruthenium or nickel-iron oxide anodes in acidic or alk. solns., resp. So at the end of the article, two fundamental questions will be addressed: (1) are there tech. applications where MnOx materials could actually be the first choice as OER electrocatalysts and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: "Why did nature choose manganese to make oxygen".
  37. 37
    Deibert, B. J.; Zhang, J.; Smith, P. F.; Chapman, K. W.; Rangan, S.; Banerjee, D.; Tan, K.; Wang, H.; Pasquale, N.; Chen, F.; Lee, K.-B.; Dismukes, G. C.; Chabal, Y. J.; Li, J. Surface and Structural Investigation of a MnOx Birnessite-Type Water Oxidation Catalyst Formed under Photocatalytic Conditions. Chem.─Eur. J. 2015, 21, 14218– 14228,  DOI: 10.1002/chem.201501930
    Google Scholar
    37
    Surface and Structural Investigation of a MnOx Birnessite-Type Water Oxidation Catalyst Formed under Photocatalytic Conditions
    Deibert, Benjamin J.; Zhang, Jingming; Smith, Paul F.; Chapman, Karena W.; Rangan, Sylvie; Banerjee, Debasis; Tan, Kui; Wang, Hao; Pasquale, Nicholas; Chen, Feng; Lee, Ki-Bum; Dismukes, G. Charles; Chabal, Yves J.; Li, Jing
    Chemistry - A European Journal (2015), 21 (40), 14218-14228CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Catalytically active MnOx species have been reported to form in situ from various Mn-complexes during electrocatalytic and soln.-based water oxidn. when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnOx catalysts form without supports in situ under photocatalytic conditions. Our most active 4MnOx catalyst (∼0.84 mmol O2 mol Mn-1 s-1) forms from a Mn4O4 bearing a metal-org. framework. 4MnOx is characterized by pair distribution function anal. (PDF), Raman spectroscopy, and HR-TEM as a disordered, layered Mn-oxide with high surface area (216 m2g-1) and small regions of crystallinity and layer flexibility. In contrast, the SMnOx formed from Mn2+ salt gives an amorphous species of lower surface area (80 m2g-1) and lower activity (∼0.15 mmol O2 mol Mn-1 s-1). We compare these catalysts to cryst. hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p3/2 core levels detects enriched Mn3+ and Mn2+ content on the surfaces, which indicates possible disproportionation/comproportionation surface equil.
  38. 38
    Sadtler, B.; Burgos, S. P.; Batara, N. A.; Beardslee, J. A.; Atwater, H. A.; Lewis, N. S. Phototropic Growth Control of Nanoscale Pattern Formation in Photoelectrodeposited Se–Te Films. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 19707– 19712,  DOI: 10.1073/pnas.1315539110
    Google Scholar
    38
    Phototropic growth control of nanoscale pattern formation in photoelectrodeposited Se-Te films
    Sadtler, Bryce; Burgos, Stanley P.; Batara, Nicolas A.; Beardslee, Joseph A.; Atwater, Harry A.; Lewis, Nathan S.
    Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (49), 19707-19712,S19707/1-S19707/2CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Photoresponsive materials that adapt their morphologies, growth directions, and growth rates dynamically in response to the local incident electromagnetic field would provide a remarkable route to the synthesis of complex 3D mesostructures via feedback between illumination and the structure that develops under optical excitation. We report the spontaneous development of ordered, nanoscale lamellar patterns in electrodeposited Se-Te alloy films grown under noncoherent, uniform illumination on unpatterned substrates in an isotropic electrolyte soln. These inorg. nanostructures exhibited phototropic growth in which lamellar stripes grew toward the incident light source, adopted an orientation parallel to the light polarization direction with a period controlled by the illumination wavelength, and showed an increased growth rate with increasing light intensity. Furthermore, the patterns responded dynamically to changes during growth in the polarization, wavelength, and angle of the incident light, enabling the template-free and pattern-free synthesis, on a variety of substrates, of woodpile, spiral, branched, or zigzag structures, along with dynamically directed growth toward a noncoherent, uniform intensity light source. Full-wave electromagnetic simulations in combination with Monte Carlo growth simulations were used to model light-matter interactions in the Se-Te films and produced a model for the morphol. evolution of the lamellar structures under phototropic growth conditions. The expts. and simulations are consistent with a phototropic growth mechanism in which the optical near-field intensity profile selects and reinforces the dominant morphol. mode in the emergent nanoscale patterns.
  39. 39
    Carim, A. I.; Batara, N. A.; Premkumar, A.; Atwater, H. A.; Lewis, N. S. Self-Optimizing Photoelectrochemical Growth of Nanopatterned Se–Te Films in Response to the Spectral Distribution of Incident Illumination. Nano Lett. 2015, 15, 7071– 7076,  DOI: 10.1021/acs.nanolett.5b03137
    Google Scholar
    39
    Self-Optimizing Photoelectrochemical Growth of Nanopatterned Se-Te Films in Response to the Spectral Distribution of Incident Illumination
    Carim, Azhar I.; Batara, Nicolas A.; Premkumar, Anjali; Atwater, Harry A.; Lewis, Nathan S.
    Nano Letters (2015), 15 (10), 7071-7076CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Photoelectrochem. growth of Se-Te films spontaneously produces highly ordered, nanoscale lamellar morphologies with periodicities that can be tuned by varying the illumination wavelength during deposition. This phenomenon was characterized further herein by detg. the morphologies of photoelectrodeposited Se-Te films in response to tailored spectral illumination profiles. Se-Te films grown under illumination from 4 different sources, having similar av. wavelengths but having spectral bandwidths that spanned several orders of magnitude, all nevertheless produced similar structures which had a single, common periodicity as quant. identified via Fourier anal. Film deposition using simultaneous illumination from 2 narrowband sources, which differed in av. wavelength by several hundred nanometers, resulted in a structure with only a single periodicity intermediate between the periods obsd. when either source alone was used. This single periodicity could be varied by manipulating the relative intensity of the 2 sources. An iterative model that combined full-wave electromagnetic effects with Monte Carlo growth simulations, and that considered only the fundamental light-material interactions during deposition, was in accord with the morphologies obsd. exptl. Simulations of light absorption and concn. in idealized lamellar arrays, in conjunction with all of the available data, addnl. indicated that a self-optimization of the periodicity of the nanoscale pattern, resulting in the maximization of the anisotropy of interfacial light absorption in the 3-dimensional structure, is consistent with the obsd. growth process of such films.
  40. 40
    Lowe, J. M.; Yan, Q.; Benamara, M.; Coridan, R. H. Direct Photolithographic Patterning of Cuprous Oxide Thin Films Via Photoelectrodeposition. J. Mater. Chem. A 2017, 5, 21765– 21772,  DOI: 10.1039/c7ta05321e
    Google Scholar
    40
    Direct photolithographic patterning of cuprous oxide thin films via photoelectrodeposition
    Lowe, James M.; Yan, Qigeng; Benamara, Mourad; Coridan, Robert H.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (41), 21765-21772CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    External fields can be used to regulate the morphol. and chem. properties of electrochem. synthesized materials. One example is phototropism, where the coaction of light absorption and carrier generation can induce controllable variations in the material as it grows under illumination. Here we describe the photoelectrodeposition of photocathodic cuprous oxide (Cu2O). Illuminating the growing film with photon energies in excess of the band gap of Cu2O results in nanoscale morphol. changes in the structure of thin films and intrinsically dopes the material during growth. The result is a 'black' Cu2O film that is chem. distinct but crystallog. identical to an ordinary film grown in the dark. The flat band potential of the film is controlled by the growth illumination intensity and the photodoping is reversible under thermal oxidn. We explore the nature of the intrinsic dopant, the emergence of nanocryst. Cu metal inclusions in the Cu2O matrix, and the potential of for this effect to be used in the fabrication of new semiconductor heterostructures. This chem. distinction between the native and black Cu2O can be exploited to pattern films with a photomask as an example of direct photolithog.
  41. 41
    Tan, C.; Qin, C.; Sadtler, B. Light-Directed Growth of Metal and Semiconductor Nanostructures. J. Mater. Chem. C 2017, 5, 5628– 5642,  DOI: 10.1039/c7tc00379j
    Google Scholar
    41
    Light-directed growth of metal and semiconductor nanostructures
    Tan, Che; Qin, Chu; Sadtler, Bryce
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (23), 5628-5642CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    A review. External parameters such as temp., pressure, and concn. are typically used to control chem. transformations in mols. and materials. Many classes of inorg. materials are also responsive to external stimuli, such as elec., magnetic, and electromagnetic fields. This Review article will explore examples of how light can guide the growth of metal and semiconductor nanostructures. Plasmon excitation in metals and electronic excitation in semiconductors mediate interfacial charge-transfer reactions to alter the growth rates of these materials at the nanoscale. Light-driven growth of inorg. materials provides a route to achieve morphol. control over colloidal particles and nanostructured films for potential applications in solar energy conversion, photonic computing, and holog.
  42. 42
    Carim, A. I.; Hamann, K. R.; Batara, N. A.; Thompson, J. R.; Atwater, H. A.; Lewis, N. S. Template-Free Synthesis of Periodic Three-Dimensional PbSe Nanostructures via Photoelectrodeposition. J. Am. Chem. Soc. 2018, 140, 6536– 6539,  DOI: 10.1021/jacs.8b02931
    Google Scholar
    42
    Template-Free Synthesis of Periodic Three-Dimensional PbSe Nanostructures via Photoelectrodeposition
    Carim, Azhar I.; Hamann, Kathryn R.; Batara, Nicolas A.; Thompson, Jonathan R.; Atwater, Harry A.; Lewis, Nathan S.
    Journal of the American Chemical Society (2018), 140 (21), 6536-6539CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Highly periodic, geometrically directed, anisotropic Se-Pb films have been synthesized at room temp. from an isotropic aq. soln. without the use of phys. templates by photoelectrodeposition using a series of discrete input illumination polarizations and wavelengths from an unstructured, uncorrelated, incoherent light source. Dark growth did not generate deposits with substantial long-range order, but growth using unpolarized illumination resulted in an ordered, nanoscale, mesh-type morphol. Linearly polarized illumination generated Se-Pb deposits that displayed an ordered, highly anisotropic lamellar pattern wherein the long axes of the lamellae were aligned parallel to the light polarization vector. The pitch of the lamellar features was proportional to the input light wavelength, as confirmed by Fourier anal. Full-wave electromagnetic and Monte Carlo growth simulations that incorporated only the fundamental light-matter interactions during growth successfully reproduced the exptl. obsd. morphologies and quant. matched the pattern periodicities. Electrochem. postprocessing of the as-deposited Se-Pb structures resulted in the generation of stoichiometric, cryst. PbSe while preserving the nanopatterned morphol., thus broadening the genus of materials that can be prepd. with controlled three-dimensional morphologies through maskless photoelectrodeposition.
  43. 43
    Meier, M. C.; Cheng, W.-H.; Atwater, H. A.; Lewis, N. S.; Carim, A. I. Inorganic Phototropism in Electrodeposition of Se–Te. J. Am. Chem. Soc. 2019, 141, 18658– 18661,  DOI: 10.1021/jacs.9b10579
    Google Scholar
    43
    Inorganic Phototropism in Electrodeposition of Se-Te
    Meier, Madeline C.; Cheng, Wen-Hui; Atwater, Harry A.; Lewis, Nathan S.; Carim, Azhar I.
    Journal of the American Chemical Society (2019), 141 (47), 18658-18661CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Photoelectrochem. deposition of Se-Te on isolated Au islands using an unstructured, incoherent beam of light produces growth of Se-Te alloy toward the direction of the incident light beam. Full-wave electromagnetic simulations of light absorption indicated that the induced spatial growth anisotropy was a function of asym. absorption in the evolving deposit. Inorg. phototropic growth is analogous to biol. systems such as palm trees that exhibit phototropic growth wherein phys. extension of the plant guides the crown toward the time-averaged position of the sun, to maximize solar harvesting.
  44. 44
    Qin, C.; Campbell, B. M.; Shen, M.; Zhao, T.; Sadtler, B. Light-Driven, Facet-Selective Transformation of Cuprous Oxide Microcrystals to Hollow Copper Nanoshells. Chem. Mater. 2019, 31, 8000– 8011,  DOI: 10.1021/acs.chemmater.9b02240
    Google Scholar
    44
    Light-driven, facet-selective transformation of cuprous oxide microcrystals to hollow copper nanoshells
    Qin, Chu; Campbell, Brandon M.; Shen, Meikun; Zhao, Tong; Sadtler, Bryce
    Chemistry of Materials (2019), 31 (19), 8000-8011CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Photoexcitation can be used to control the compn. and nanoscale morphol. of inorg. materials. Here we report the photoinduced transformation of faceted cuprous oxide (Cu2O) microcrystals to hollow particles consisting of an inner region of cuprous oxide and an outer shell of copper metal. When cuprous oxide microcrystals with mixed {100} and {111} facets are held at a neg. bias (-1.0 V vs Ag/AgCl) in a soln. of sodium hydroxide (NaOH), light mediates the growth of copper metal selectively on the {100} facets while the crystal interior is etched at {111} facets. Conformal Cu layers grow to connect at vertices of the cuboctahedral microcrystals and form a hollow shell. This process is only obsd. in the presence of illumination. Without an applied bias the {100} facets are preferentially etched under illumination in the same NaOH soln. We propose this light-driven, facet-selective transformation arises from the potential-dependent structure and energetics of the semiconductor/electrolyte interface, which lead to facet-selective extn. of photogenerated electrons from the {100} facets when the applied bias is more neg. than the flat-band potential of the Cu2O microcrystals. Growth of the Cu shell protects the {100} facets while the {111} facets are chem. etched in the presence of oxygen and hydroxide.
  45. 45
    Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. Photocurrent Generation from Semiconducting Manganese Oxide Nanosheets in Response to Visible Light. J. Phys. Chem. B 2005, 109, 9651– 9655,  DOI: 10.1021/jp0500485
    Google Scholar
    45
    Photocurrent Generation from Semiconducting Manganese Oxide Nanosheets in Response to Visible Light
    Sakai, Nobuyuki; Ebina, Yasuo; Takada, Kazunori; Sasaki, Takayoshi
    Journal of Physical Chemistry B (2005), 109 (19), 9651-9655CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Unilamellar nanosheet crystallites of manganese oxide generated the anodic photocurrent under visible light irradn. (λ < 500 nm), while the nanosheets themselves were stable as revealed by in-plane XRD and UV-visible absorption spectra. The band gap energy was estd. to be 2.23 eV on the basis of the photocurrent action spectrum. The mol. thickness of ∼0.5 nm may facilitate the charge sepn. of excited electrons and holes, which is generally very difficult for strongly localized d-d transitions. The monolayer film of MnO2 nanosheets exhibited the incident photon-to-electron conversion efficiency of 0.16% in response to the monochromatic light irradn. (λ = 400 nm), which is comparable to those for sensitization of monolayer dyes adsorbed on a flat single-crystal surface. The efficiency declined with increasing the layer no. of MnO2 nanosheets, although the optical absorption was enhanced. The recombination of the excited electron-hole pairs may become dominant when the carriers need to migrate a longer distance than 1 layer through multilayered nanosheets.
  46. 46
    Pinaud, B. A.; Chen, Z.; Abram, D. N.; Jaramillo, T. F. Thin Films of Sodium Birnessite-Type MnO2: Optical Properties, Electronic Band Structure, and Solar Photoelectrochemistry. J. Phys. Chem. C 2011, 115, 11830– 11838,  DOI: 10.1021/jp200015p
    Google Scholar
    46
    Thin Films of Sodium Birnessite-Type MnO2: Optical Properties, Electronic Band Structure, and Solar Photoelectrochemistry
    Pinaud, Blaise A.; Chen, Zhebo; Abram, David N.; Jaramillo, Thomas F.
    Journal of Physical Chemistry C (2011), 115 (23), 11830-11838CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Inexpensive and earth-abundant, Mn oxides (MnxOy) have attracted considerable attention for catalysis, but fewer efforts have focused on their semiconducting properties. Cryst. Na birnessite-type MnO2 thin films were studied for their surface and bulk chem. in the context of H prodn. by photoelectrochem. H2O splitting. Thin films were synthesized by electrodeposition onto F-doped Sn oxide (FTO) substrates, imaged by SEM, and characterized by XRD as well as XPS and UV-visible spectroscopy. Three different electrochem. methods (illuminated open circuit potential, potential of photocurrent onset, and Mott-Schottky plots) were used to probe the flatband potential required to construct a band diagram for the material. Photostability, cond., and band structure are discussed as potential causes of the low external quantum efficiency (<1%) for the birnessite-type MnO2 photoanode. The position of the conduction band well below the H evolution potential likely mitigates this material's potential use in a single absorber configuration, but its chem., optical, and electronic characteristics as shown in this work may be well-suited for a photoanode in a tandem device.
  47. 47
    Hsu, Y.-K.; Chen, Y.-C.; Lin, Y.-G.; Chen, L.-C.; Chen, K.-H. Birnessite-Type Manganese Oxides Nanosheets with Hole Acceptor Assisted Photoelectrochemical Activity in Response to Visible Light. J. Mater. Chem. 2012, 22, 2733– 2739,  DOI: 10.1039/c1jm14355g
    Google Scholar
    47
    Birnessite-type manganese oxides nanosheets with hole acceptor assisted photoelectrochemical activity in response to visible light
    Hsu, Yu-Kuei; Chen, Ying-Chu; Lin, Yan-Gu; Chen, Li-Chyong; Chen, Kuei-Hsien
    Journal of Materials Chemistry (2012), 22 (6), 2733-2739CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    A cost-effective and simple electroplating technique has been developed to prep. layered manganese oxide (MnO2) arrays as a promising material for solar hydrogen prodn. and wastewater treatment by photoelectrochem. process. The microstructure of these MnO2 nanosheets can be referenced to Birnessite-type, as characterized by Raman spectra and transmission electron microscopy. The bandgap energy of the as-grown nanosheets detd. from UV-visible spectroscopy is ∼2.1 eV. Mott-Schottky plots show the flat band potential of the MnO2 nanosheets to be -0.01 V and a donor concn. of 4.68 × 1020 cm-3. Remarkable photocurrent in response to visible light is obsd. in the presence of hole acceptors, such as sodium formate and methanol, which efficiently suppress the recombination loss of electron-hole pairs from localized d-d transitions within manganese ion. Meanwhile, the transient photocurrent-time responses and the effect of different hole acceptors on photoelectrochem. activity are studied with an increase of resp. hole acceptor concn., and the results reveal the crit. role in the process of absorption and decompn. of the hole acceptor. Significantly, the MnO2 nanosheets exhibit an incident photon-to-electron conversion efficiency of 7% in response to the monochromatic wavelength of 400 nm, which is comparable to that from hematite (α-Fe2O3). These results demonstrate the nanoporous MnO2 nanosheets have great potential in solar hydrogen applications and org. pollutant cleaning.
  48. 48
    Sunda, W. G.; Huntsman, S. A. Photoreduction of Manganese Oxides in Seawater. Mar. Chem. 1994, 46, 133– 152,  DOI: 10.1016/0304-4203(94)90051-5
    Google Scholar
    48
    Photoreduction of manganese oxides in seawater
    Sunda, William G.; Huntsman, Susan A.
    Marine Chemistry (1994), 46 (1-2), 133-52CODEN: MRCHBD; ISSN:0304-4203.
    Expts. were conducted on the photoreactive dissoln. of 54Mn-labeled synthetic oxides, prepd. from MnO42- oxidn. of 54Mn(II), and natural labeled oxides formed in seawater from microbial oxidn. of 54Mn(II). Sunlight increased the dissoln. rate of synthetic oxides in seawater, an effect that increased with the duration of light exposure. The photodissoln. of these oxides was found to result primarily from Mn redn. by H2O2, produced in seawater from the photoredn. of O by dissolved org. matter. This conclusion was based on the previously obsd. marked stimulation of photodissoln. by added humic compds., the obsd. reductive dissoln. of the oxides by added H2O2 and on the almost complete reversal of photodissoln. by enzymic (catalase) removal of H2O2. Sunlight had an even larger stimulatory effect on the reductive dissoln. of 54Mn-labeled natural oxides. It increased specific dissoln. rates to values of 6-13% h-1, 6-70 times higher than rates in the dark. In contrast to synthetic oxides, rates for natural oxides did not increase measurably with the duration of light exposure, were not appreciably altered by humic acid addn. or by photolytic removal of org. matter, and were not substantially reduced by catalase addn. Furthermore, rates for reductive dissoln. of natural oxides by H2O2 were only about 1/6 of those for synthetic oxides. These results indicate that the photoreductive dissoln. of natural oxides in seawater is not primarily related to the photoprodn. of H2O2, although such prodn. appears to account for a small portion (ca. 10-20%) of the overall effect. Instead, both the chromophore and the reductant(s) involved in the reaction appear to reside with the bacterial/Mn oxide aggregates themselves. Although several possibilities can be postulated, the exact mechanism of the photochem. reaction remains obscure.
  49. 49
    Sherman, D. M. Electronic Structures of Iron(III) and Manganese(IV) (Hydr)Oxide Minerals: Thermodynamics of Photochemical Reductive Dissolution in Aquatic Environments. Geochim. Cosmochim. Acta 2005, 69, 3249– 3255,  DOI: 10.1016/j.gca.2005.01.023
    Google Scholar
    49
    Electronic structures of iron(III) and manganese(IV) (hydr)oxide minerals: Thermodynamics of photochemical reductive dissolution in aquatic environments
    Sherman, David M.
    Geochimica et Cosmochimica Acta (2005), 69 (13), 3249-3255CODEN: GCACAK; ISSN:0016-7037. (Elsevier Inc.)
    The reactivity of Fe-Mn (hydr)oxide minerals to sunlight-induced photochem. dissoln. is detd. by the electronic structure of the mineral-water interface. Oxygen K-edge absorption and emission spectra were used to det. the electronic structures of iron(III) (hydr)oxides (hematite, goethite, lepidocrocite, akaganeite and schwertmannite) and manganese(IV) oxides (pyrolusite, birnessite, cryptomelane). The band gaps in the iron(III) (hydr)oxide minerals are near 2.0-2.5 eV; the band gaps in the manganese (IV) oxide phases are 1.0-1.8 eV. Using published values for the electrochem. flat-band potential for hematite together with exptl. pHpzc values for the (hydr)oxides, it is possible to predict the electrochem. potentials of the conduction and valence bands in aq. solns. as a function of pH. The band potentials enable semiquant. predictions of the susceptibilities of these minerals to photochem. dissoln. in aq. solns. At pH 2 (e.g., acid-mine waters), photoredn. of iron(III) (hydr)oxides could yield millimolal concns. of aq. Fe2+ (assuming surface detachment of Fe2+ is not rate limiting). In seawater (pH 8.3), however, the direct photo-redn. of colloidal iron(III) (hydr)oxides to give nanomolal concns. of dissolved, uncomplexed, Fe2+ is not thermodynamically feasible. This supports the hypothesis that the apparent photodissoln. of iron(III) (hydr)oxides in marine systems results from Fe3+ redn. by photochem. produced superoxide. In contrast, the direct photoredn. of manganese oxides should be energetically feasible at pH 2 and 8.3.
  50. 50
    Kwon, K. D.; Refson, K.; Sposito, G. On the Role of Mn(IV) Vacancies in the Photoreductive Dissolution of Hexagonal Birnessite. Geochim. Cosmochim. Acta 2009, 73, 4142– 4150,  DOI: 10.1016/j.gca.2009.04.031
    Google Scholar
    50
    On the role of Mn(IV) vacancies in the photoreductive dissolution of hexagonal birnessite
    Kwon, Kideok D.; Refson, Keith; Sposito, Garrison
    Geochimica et Cosmochimica Acta (2009), 73 (14), 4142-4150CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
    Photoreductive dissoln. of layer type Mn(IV) oxides (birnessite) under sunlight illumination to form sol. Mn(II) has been obsd. in both field and lab. settings, leading to a consensus that this process is a key driver of the biogeochem. cycling of Mn in the euphotic zones of marine and freshwater ecosystems. However, the underlying mechanisms for the process remain unknown, although they have been linked to the semiconducting characteristics of hexagonal birnessite, the ubiquitous Mn(IV) oxide produced mainly by bacterial oxidn. of sol. Mn(II). One of the universal properties of this biogenic mineral is the presence of Mn(IV) vacancies, long-identified as strong adsorption sites for metal cations. The possible role of Mn vacancies in photoreductive dissoln. is investigated theor. using quantum mech. calcns. based on spin-polarized d. functional theory (DFT). DFT study demonstrates unequivocally that Mn vacancies significantly reduce the band-gap energy for hexagonal birnessite relative to a hypothetical vacancy-free MnO2 and thus would increase the concn. of photo-induced electrons available for Mn(IV) redn. upon illumination of the mineral by sunlight. Calcns. of the charge distribution in the presence of vacancies, although not fully conclusive, show a clear sepn. of photo-induced electrons and holes, implying a slow recombination of these charge-carriers that facilitates the two-electron redn. of Mn(IV) to Mn(II).
  51. 51
    Marafatto, F. F.; Strader, M. L.; Gonzalez-Holguera, J.; Schwartzberg, A.; Gilbert, B.; Peña, J. Rate and Mechanism of the Photoreduction of Birnessite (MnO2) Nanosheets. Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 4600– 4605,  DOI: 10.1073/pnas.1421018112
    Google Scholar
    51
    Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets
    Marafatto, Francesco Femi; Strader, Matthew L.; Gonzalez-Holguera, Julia; Schwartzberg, Adam; Gilbert, Benjamin; Pena, Jasquelin
    Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (15), 4600-4605CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The photoreductive dissoln. of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidn. of org. matter and fate of trace elements assocd. with Mn oxides, but the intrinsic rate and mechanism of mineral dissoln. in the absence of org. electron donors is unknown. The photoredn. of δ-MnO2 nanosheets is investigated at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradn. and quantified the yield and timescales of Mn(III) prodn. The study of transient intermediate states using time-resolved optical and x-ray absorption spectroscopy showed key roles for chem. distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. It is proposed that irreversible Mn redn. is coupled to hole-scavenging by surface water mols. or hydroxyl groups, with assocd. radical formation. This work demonstrates the importance of direct MnO2 photoredn. in environmental processes and provides a framework to test new hypotheses regarding the role of org. mols. and metal species in photochem. reactions with Mn oxide phases. The timescales for the prodn. and evolution of Mn(III) species and a catalytic role for interlayer Ca2+ identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidn.
  52. 52
    McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977– 16987,  DOI: 10.1021/ja407115p
    Google Scholar
    52
    Benchmarking heterogeneous electrocatalysts for the oygen evolution reaction
    McCrory, Charles C. L.; Jung, Suho; Peters, Jonas C.; Jaramillo, Thomas F.
    Journal of the American Chemical Society (2013), 135 (45), 16977-16987CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Objective evaluation of the activity of electrocatalysts for water oxidn. is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for detg. electrochem. active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a c.d. of 10 mA cm-2 per geometric area, approx. the c.d. expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can det. electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alk. solns.: CoOx, CoPi, CoFeOx, NiOx, NiCeOx, NiCoOx, NiCuOx, NiFeOx, and NiLaOx. The oxygen-evolving activity of an electrodeposited IrOx catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alk. soln., every non-noble metal system achieved 10 mA cm-2 current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrOx was unstable under oxidative conditions in acidic solns.
  53. 53
    Huynh, M.; Bediako, D. K.; Liu, Y.; Nocera, D. G. Nucleation and Growth Mechanisms of an Electrodeposited Manganese Oxide Oxygen Evolution Catalyst. J. Phys. Chem. C 2014, 118, 17142– 17152,  DOI: 10.1021/jp501768n
    Google Scholar
    53
    Nucleation and Growth Mechanisms of an Electrodeposited Manganese Oxide Oxygen Evolution Catalyst
    Huynh, Michael; Bediako, D. Kwabena; Liu, Yi; Nocera, Daniel G.
    Journal of Physical Chemistry C (2014), 118 (30), 17142-17152CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The authors study the mechanisms of nucleation and steady-state growth of a Mn oxide catalyst (MnOx) electrodeposited from Mn2+ solns. in a weakly basic electrolyte. Early catalyst growth was probed through chronoamperometry transients, which were fit to reveal a progressive nucleation mechanism for initial catalyst formation. Time-dependent at. force microscopy snapshots of the electrode surface reveal a rapid increase in nucleus size together with a sluggish rise in coverage, which is also characteristic of progressive nucleation. Electrochem. kinetic studies of the catalyst growth yield a Tafel slope of ∼2.3 × RT/2F and a rate law consisting of a 2nd-order and inverse 4th-order dependence on [Mn2+] and proton activity, resp. These results are consistent with a deposition mechanism involving rate-limiting disproportionation of aq. Mn3+, resolving a longstanding ambiguity surrounding the deposition of Mn oxides under nonacidic conditions.
  54. 54
    Nesbitt, H. W.; Banerjee, D. Interpretation of XPS Mn(2p) Spectra of Mn Oxyhydroxides and Constraints on the Mechanism of MnO2 Precipitation. Am. Mineral. 1998, 83, 305– 315,  DOI: 10.2138/am-1998-3-414
    Google Scholar
    54
    Interpretation of XPS Mn(2p) spectra of Mn oxyhydroxides and constraints on the mechanism of MnO2 precipitation
    Nesbitt, H. W.; Banderjee, D.
    American Mineralogist (1998), 83 (3-4), 305-315CODEN: AMMIAY; ISSN:0003-004X. (Mineralogical Society of America)
    Calcd. Mn(2p3/2) X-ray photoelectron spectra (XPS) of Mn2+, Mn3+, and Mn4+ free ions are strikingly similar to Mn(2p3/2) spectra of Mn2+-, Mn3+-, and Mn4+-oxides and oxyhydroxides, indicating that these ions adopt high spin states in MnO, manganite, and birnessite. The Mn(2p) peak structures reveal the presence of only Mn3+ in manganite but Mn2+, Mn3+, and Mn4+ are present in the near-surface of synthetic birnessite at about 5, 25, and 70%, resp. Pptn. of birnessite by reaction of Mn2+(aq) with an oxidant includes two electron transfer steps: (1) oxidn. of Mn2+(aq) to produce Mn3+-oxyhydroxide, an intermediate reaction product that forms on the surface of synthetic birnessite and (2) subsequent oxidn. of Mn3+-oxyhydroxide surface species to produce synthetic birnessite. Some surface Mn3+, however, remains unoxidized and is incorporated into birnessite. As for this synthesis (KMnO4 used as oxidant), oxidn. may not proceed to completion in natural settings (as O2 is the oxidant) leading to Mn3+ incorporation into Mn-oxides. The hypothesis explains the abundance of non-stoichiometric MnO2 phases in sedimentary environments. The MnO2 pptn. scheme proposed by Stumm and Morgan (1981) includes the surface species Mn2+·MnO2. This and other studies indicate that the reactive intermediate is a Mn3+-bearing surface species. The formation rate of birnessite is probably controlled by one of these redox reactions. The proposed rate expression of Davies and Morgan (1989), however, needs no modification provided surface area is a reasonable measure of the surfaces d. of the reactive intermediate.
  55. 55
    Gupta, R. P.; Sen, S. K. Calculation of Multiplet Structure of Core p-Vacancy Levels. Phys. Rev. B: Solid State 1974, 10, 71– 77,  DOI: 10.1103/physrevb.10.71
    Google Scholar
    55
    Calculation of multiplet structure of core p-vacancy levels
    Gupta, R. P.; Sen, S. K.
    Physical Review B: Solid State (1974), 10 (1), 71-7CODEN: PLRBAQ; ISSN:0556-2805.
    The spin-orbit and crystal-field effects were included. The cases studied are Mn2+ in MnF2 and the free ion Fe3+. For 3p vacancies these effects appear to be unimportant from the point of view of photoelectron spectroscopy owing to the limited resoln. of the technique. For 2p vacancies, the spin-orbit interaction is comparable to the electrostatic interaction between the electrons and both the interactions must be considered simultaneously. The conclusion is that the x-ray photoelectron spectra of inner-core electrons are more suitable for chem. anal. than those ofouter electrons which, in addn., contain large correlation and many-body effects.
  56. 56
    Gupta, R. P.; Sen, S. K. Calculation of Multiplet Structure of Core p-Vacancy Levels. II. Phys. Rev. B: Solid State 1975, 12, 15– 19,  DOI: 10.1103/physrevb.12.15
    Google Scholar
    56
    Calculation of multiplet structure of core p-vacancy levels. II
    Gupta, R. P.; Sen, S. K.
    Physical Review B: Solid State (1975), 12 (1), 15-19CODEN: PLRBAQ; ISSN:0556-2805.
    The multiplet structure of core 2p-vacancy levels for 36 ions belonging to 3d transition metals were calcd. and their plots presented. Only the ground-state configuration 2p53dn (n = 1,...,9) is considered for each ion. The spin-orbit interaction was incorporated exactly, but the crystal-field effect is ignored. While there is general agreement with the available exptl. works, higher-energy resolution in x-ray-photoelectron measurements is necessary for detailed comparison of the spectra presented here. On the theor. side inclusion of crystal field appears to be important. One should go beyond single-configuration approxn. to have the obsd. satellite structure in 2p-x-ray-photoelectron spectra appear in the calcns.
  57. 57
    Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717– 2730,  DOI: 10.1016/j.apsusc.2010.10.051
    Google Scholar
    57
    Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni
    Biesinger, Mark C.; Payne, Brad P.; Grosvenor, Andrew P.; Lau, Leo W. M.; Gerson, Andrea R.; Smart, Roger St. C.
    Applied Surface Science (2011), 257 (7), 2717-2730CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Chem. state x-ray photoelectron spectroscopic anal. of 1st row transition metals and their oxides and hydroxides is challenging due to the complexity of their 2p spectra resulting from peak asymmetries, complex multiplet splitting, shake-up and plasmon loss structure, and uncertain, overlapping binding energies. The previous paper in which the authors examd. Sc, Ti, V, Cu and Zn species, showed that all the values of the spectral fitting parameters for each specific species, i.e. binding energy (eV), full wide at half max. (FWHM) value (eV) for each pass energy, spin-orbit splitting values and asym. peak shape fitting parameters, are not all normally provided in the literature and data bases, and are necessary for reproducible, quant. chem. state anal. A more consistent, practical and effective approach to curve fitting was developed based on a combination of (1) std. spectra from quality ref. samples, (2) a survey of appropriate literature databases and/or a compilation of literature refs. and (3) specific literature refs. where fitting procedures are available. This paper extends this approach to the chem. states of Cr, Mn, Fe, Co and Ni metals, and various oxides and hydroxides where intense, complex multiplet splitting in many of the chem. states of these elements poses unique difficulties for chem. state anal. The curve fitting procedures proposed use the same criteria as proposed previously but with the addnl. complexity of fitting of multiplet split spectra which was done based on spectra of numerous ref. materials and theor. XPS modeling of these transition metal species. Binding energies, FWHM values, asym. peak shape fitting parameters, multiplet peak sepn. and peak area percentages are presented. The procedures developed can be used to remove uncertainties in the anal. of surface states in nanoparticles, corrosion, catalysis and surface-engineered materials.
  58. 58
    Julien, C.; Massot, M.; Baddour-Hadjean, R.; Franger, S.; Bach, S.; Pereira-Ramos, J. P. Raman Spectra of Birnessite Manganese Dioxides. Solid State Ionics 2003, 159, 345– 356,  DOI: 10.1016/s0167-2738(03)00035-3
    Google Scholar
    58
    Raman spectra of birnessite manganese dioxides
    Julien, C.; Massot, M.; Baddour-Hadjean, R.; Franger, S.; Bach, S.; Pereira-Ramos, J. P.
    Solid State Ionics (2003), 159 (3,4), 345-356CODEN: SSIOD3; ISSN:0167-2738. (Elsevier Science B.V.)
    Structural features of layered Mn dioxides of the birnessite family were studied using Raman scattering spectroscopy. This local probe is capable of analyzing directly the near-neighbor environment of O coordination around Mn and Li cations. Four types of sol-gel birnessite (SGB) are considered: Li birnessite (Li-Bir), Na birnessite (Na-Bir), sol-gel birnessite (SG-Bir), and sol-gel Co-doped birnessite (SGCo-Bir). Thus, in a 1st approach, the authors consider the overall spectral features of birnessites such as the superposition of the spectra of local structures, while the lattice modes are discussed in the spectroscopic symmetry. Results show the specific spectroscopic fingerprints of SG-Bir single phases, the site occupancy of Co ions in the substituted SGCo-Bir compd., and vibrations due to Li ions with their O neighbors in Li-Bir, Li0.32MnO2·0.6H2O. A correlation between the interlayer d-spacing and the stretching mode frequencies of birnessite oxides was established.
  59. 59
    Chen, D.; Ding, D.; Li, X.; Waller, G. H.; Xiong, X.; El-Sayed, M. A.; Liu, M. Probing the Charge Storage Mechanism of a Pseudocapacitive MnO2 Electrode Using in Operando Raman Spectroscopy. Chem. Mater. 2015, 27, 6608– 6619,  DOI: 10.1021/acs.chemmater.5b03118
    Google Scholar
    59
    Probing the Charge Storage Mechanism of a Pseudocapacitive MnO2 Electrode Using in Operando Raman Spectroscopy
    Chen, Dongchang; Ding, Dong; Li, Xiaxi; Waller, Gordon Henry; Xiong, Xunhui; El-Sayed, Mostafa A.; Liu, Meilin
    Chemistry of Materials (2015), 27 (19), 6608-6619CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    While Mn oxide (MnO2) was extensively studied as an electrode material for pseudo-capacitors, a clear understanding of its charge storage mechanism is still lacking. Here the authors report the findings in probing the structural changes of a thin-film model MnO2 electrode during cycling using in operando Raman spectroscopy. The spectral features (e.g., band position, intensity, and width) are correlated quant. with the size (Li+, Na+, and K+) of cations in different electrolytes and with the degree of discharge to gain better understanding of the cation-incorporation mechanism into the interlayers of pseudo-capacitive MnO2. Also, theor. calcns. of phonon energy assocd. with the models of interlayer cation-incorporated MnO2 agree with the exptl. observations of cation-size effect on the positions of Raman bands. Also, the cation-size effects on spectral features at different potentials of MnO2 electrode are correlated quant. with the amt. of charge stored in the MnO2 electrode. The understanding of the structural changes assocd. with charge storage gained from Raman spectroscopy provides valuable insights into the cation-size effects on the electrochem. performances of the MnO2 electrode.
  60. 60
    Scheitenberger, P.; Euchner, H.; Lindén, M. The Hidden Impact of Structural Water – How Interlayer Water Largely Controls the Raman Spectroscopic Response of Birnessite-Type Manganese Oxide. J. Mater. Chem. A 2021, 9, 18466– 18476,  DOI: 10.1039/d1ta05357d
    Google Scholar
    60
    The hidden impact of structural water - how interlayer water largely controls the Raman spectroscopic response of birnessite-type manganese oxide
    Scheitenberger, Phillip; Euchner, Holger; Linden, Mika
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (34), 18466-18476CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Birnessite-type manganese oxides, consisting of stacked MnOx sheets, sepd. by charge-balancing metal ions and structural water are potential candidates for electrochem. applications. Due to their structural complexity, Raman spectroscopy is one of the most widely used techniques for studying this class of materials. However, the interpretation of the Raman spectra is still debated. In fact, the obsd. Raman bands are often intuitively assigned to either Mn-O or M-O vibrations, where M corresponds to charge balancing metal ions such as Na, Ca, and K-ions which are present in natural forms of birnessite. Here, we report a combined exptl. and computational study that, opposite to the commonly accepted assignments, strongly suggests that many of the characteristic Raman bands can be attributed to vibrations related to the interlayer water. Our computational findings are compared with detailed ex situ and in situ Raman spectroscopy/X-ray diffraction, and cyclovoltammetry results for K-birnessite, allowing for a full explanation of potential-dependent changes in the Raman spectra. Furthermore, the excellent correlation between the intensity of a band pos. assigned to water vibrations and the d-spacing, give support for the important influence of interlayer water on the Raman spectra. This study points to the crucial role of arrangement and content of structural water and deepens the current understanding of hydrated birnessites.
  61. 61
    Wu, T.-H.; Hesp, D.; Dhanak, V.; Collins, C.; Braga, F.; Hardwick, L. J.; Hu, C.-C. Charge Storage Mechanism of Activated Manganese Oxide Composites for Pseudocapacitors. J. Mater. Chem. A 2015, 3, 12786– 12795,  DOI: 10.1039/c5ta03334a
    Google Scholar
    61
    Charge storage mechanism of activated manganese oxide composites for pseudocapacitors
    Wu, Tzu-Ho; Hesp, David; Dhanak, Vin; Collins, Christopher; Braga, Filipe; Hardwick, Laurence J.; Hu, Chi-Chang
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (24), 12786-12795CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Manganese oxides can undergo an electrochem. activation step that leads to greater capacitances, of which the structural change and mechanism remains poorly understood. Herein a wide-ranging study is presented on a manganese oxide synthesized by annealing manganese(II) acetate precursor to 300°, which includes in operando monitoring of the structural evolution during the activation process via in situ Raman microscopy. Based on powder X-ray diffraction, XPS, transmission electron and ex-situ Raman microscopy, the as prepd. manganese oxide was characterized as hausmannite-Mn3O4 with a minor portion of MnO2. The activation process of converting as-prepd. hausmannite-Mn3O4 into amorphous MnO2 (with localized birnessite structure) by electrochem. cycling in 0.5 M Na2SO4 was examd. After activation, the activated MnOx exhibited capacitive performance of 174 F g-1 at a mass loading of 0.71 mg cm-2. The charge storage mechanism is proposed as the redox reaction between Mn(III) and Mn(IV) at outer surface active sites, since the disordered birnessite-MnO2 does not provide an ordered layer structure for cations and/or protons to intercalate.
  62. 62
    Yang, L.; Cheng, S.; Ji, X.; Jiang, Y.; Zhou, J.; Liu, M. Investigations into the Origin of Pseudocapacitive Behavior of Mn3O4 Electrodes Using in Operando Raman Spectroscopy. J. Mater. Chem. A 2015, 3, 7338– 7344,  DOI: 10.1039/c5ta00223k
    Google Scholar
    62
    Investigations into the origin of pseudocapacitive behavior of Mn3O4 electrodes using in operando Raman spectroscopy
    Yang, Lufeng; Cheng, Shuang; Ji, Xu; Jiang, Yu; Zhou, Jun; Liu, Meilin
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (14), 7338-7344CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    A detailed understanding of the phase changes of manganese oxides in a pseudocapacitor during cycling is vital to reveal the mechanism by which they store energy and to achieve a rational design of improved electrode materials. Here we report the results of our probing the phase changes of a Mn3O4 electrode material in a pseudocapacitor during a charging/discharging process using in operando Raman spectroscopy. Along with enhancing performance, the spinel Mn3O4 was transformed to a layered birnessite-type MnO2 upon potential cycling and two types of processes were shown to contribute to the energy storage: intercalation/deintercalation of Na+ and oxidn./redn. of Mn2+. After charging and discharging for ∼10,000 cycles, the specific capacitance of manganese oxide increased almost 3-fold; and the max. specific capacitance approached 230 F/g at a cycling rate of 0.5 A/g.
  63. 63
    Boumaiza, H.; Renard, A.; Rakotomalala Robinson, M.; Kervern, G.; Vidal, L.; Ruby, C.; Bergaoui, L.; Coustel, R. A Multi-Technique Approach for Studying Na Triclinic and Hexagonal Birnessites. J. Solid State Chem. 2019, 272, 234– 243,  DOI: 10.1016/j.jssc.2019.02.017
    Google Scholar
    63
    A multi-technique approach for studying Na triclinic and hexagonal birnessites
    Boumaiza, Hella; Renard, Aurelien; Rakotomalala Robinson, Mbolantenaina; Kervern, Gwendal; Vidal, Loic; Ruby, Christian; Bergaoui, Latifa; Coustel, Romain
    Journal of Solid State Chemistry (2019), 272 (), 234-243CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)
    Birnessite is a ubiquitous natural Mn oxide. This layered material is involved in a large variety of ion-exchange and redox reaction in soils and sediments. Synthetic Na-triclinic birnessite with a general formula [MnIII0.35 MnIV0.65 O2][Na+0.35 0.7H2O] was converted into hexagonal birnessite by acidic treatment at pH = 3.5. Both solids were fully characterized by TEM, SEM, PXRD, FTIR, Raman scattering, 23Na as well as the first 1H MAS NMR birnessite spectra were reported. The surfaces of both forms were fully analyzed by XPS: a particular attention was paid to the detn. of the oxidn. state of Mn derived from the fitting of XPS Mn 3p or Mn 2p3/2 features. An excess of reduced Mn at the surface in comparison to the bulk materials was evidenced.
  64. 64
    Scheitenberger, P.; Brimaud, S.; Lindén, M. XRD/Raman Spectroscopy Studies of the Mechanism of (De)Intercalation of Na+ from/into Highly Crystalline Birnessite. Mater. Adv. 2021, 2, 3940– 3953,  DOI: 10.1039/d1ma00161b
    Google Scholar
    64
    XRD/Raman spectroscopy studies of the mechanism of (de)intercalation of Na+ from/into highly crystalline birnessite
    Scheitenberger, Philipp; Brimaud, Sylvain; Linden, Mika
    Materials Advances (2021), 2 (12), 3940-3953CODEN: MAADC9; ISSN:2633-5409. (Royal Society of Chemistry)
    Due to its low-cost and environmental friendliness, birnessite-type manganese oxide has attracted wide interest for use as a cathode material in electrochem. energy storage applications. The mechanisms of energy storage and release have been studied in some detail during the last decade, but despite some agreement, some aspects of the storage and release mechanisms are still under debate. The main reason for this, we argue, is the varying interpretations of Raman spectroscopy data in the literature. Therefore, we undertook a detailed correlative Raman spectroscopy/XRD study in combination with cyclic voltammetry. Raman spectroscopy allowed for straightforward differentiation between symmetry changes during the (de)intercalation of Na-ions. More specifically, through the use of highly cryst. birnessite samples it is suggested that Raman spectra are sensitive to the lattice parameters β and d001, which allowed us to derive unprecedented details of the changes in the birnessite structure that occur upon Na+ (de)intercalation. Furthermore, it is shown that the reversible hexagonal/monoclinic symmetry transition during the course of a charge/discharge cycle is a prerequisite for effective charge storage. Based on the results, a detailed mechanism describing the (de)intercalation of Na+ from/into birnessite is presented.
  65. 65
    Nam, K. W.; Kim, S.; Yang, E.; Jung, Y.; Levi, E.; Aurbach, D.; Choi, J. W. Critical Role of Crystal Water for a Layered Cathode Material in Sodium Ion Batteries. Chem. Mater. 2015, 27, 3721– 3725,  DOI: 10.1021/acs.chemmater.5b00869
    Google Scholar
    65
    Critical Role of Crystal Water for a Layered Cathode Material in Sodium Ion Batteries
    Nam, Kwan Woo; Kim, Sangryun; Yang, Eunjeong; Jung, Yousung; Levi, Elena; Aurbach, Doron; Choi, Jang Wook
    Chemistry of Materials (2015), 27 (10), 3721-3725CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Layered transition metal oxides are considered promising cathodes for sodium ion batteries (SIBs) due to their superior specific capacities. However, they usually suffer from insufficient cycling and rate performance mainly from the structural instability during repeated cycles. These longstanding challenges are overcome by engaging crystal water in the interlayer space of sodium manganese oxide under the Birnessite framework. The crystal water enhances Na ion diffusion both in the crystal host and at the interface, suppresses fatal Mn2+ dissoln., and improves long-term structural stability, leading to excellent performance in rate capability and cycle life. The current study suggests that many hydrated materials can be good candidates for electrode materials of emerging rechargeable batteries that need to deal with the large size or multivalent charge of their carrier ions.
  66. 66
    Lu, K.; Hu, Z.; Xiang, Z.; Ma, J.; Song, B.; Zhang, J.; Ma, H. Cation Intercalation in Manganese Oxide Nanosheets: Effects on Lithium and Sodium Storage. Angew. Chem., Int. Ed. 2016, 55, 10448– 10452,  DOI: 10.1002/anie.201605102
    Google Scholar
    66
    Cation Intercalation in Manganese Oxide Nanosheets: Effects on Lithium and Sodium Storage
    Lu, Ke; Hu, Ziyu; Xiang, Zhonghua; Ma, Jizhen; Song, Bin; Zhang, Jintao; Ma, Houyi
    Angewandte Chemie, International Edition (2016), 55 (35), 10448-10452CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The rapid development of advanced energy-storage devices requires significant improvements of the electrode performance and a detailed understanding of the fundamental energy-storage processes. In this work, the self-assembly of two-dimensional manganese oxide nanosheets with various metal cations is introduced as a general and effective method for the incorporation of different guest cations and the formation of sandwich structures with tunable interlayer distances, leading to the formation of 3D MxMnO2 (M=Li, Na, K, Co, and Mg) cathodes. For sodium and lithium storage, these electrode materials exhibited different capacities and cycling stabilities. The efficiency of the storage process is influenced not only by the interlayer spacing but also by the interaction between the host cations and shutter ions, confirming the crucial role of the cations. These results provide promising ideas for the rational design of advanced electrodes for Li and Na storage.
  67. 67
    Shan, X.; Guo, F.; Charles, D. S.; Lebens-Higgins, Z.; Abdel Razek, S.; Wu, J.; Xu, W.; Yang, W.; Page, K. L.; Neuefeind, J. C.; Feygenson, M.; Piper, L. F. J.; Teng, X. Structural Water and Disordered Structure Promote Aqueous Sodium-Ion Energy Storage in Sodium-Birnessite. Nat. Commun. 2019, 10, 4975,  DOI: 10.1038/s41467-019-12939-3
    Google Scholar
    67
    Structural water and disordered structure promote aqueous sodium-ion energy storage in sodium-birnessite
    Shan Xiaoqiang; Guo Fenghua; Charles Daniel S; Teng Xiaowei; Lebens-Higgins Zachary; Abdel Razek Sara; Piper Louis F J; Wu Jinpeng; Yang Wanli; Xu Wenqian; Page Katharine L; Neuefeind Joerg C; Feygenson Mikhail; Feygenson Mikhail
    Nature communications (2019), 10 (1), 4975 ISSN:.
    Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; however, its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity. Herein we report a sodium rich disordered birnessite (Na0.27MnO2) for aqueous sodium-ion electrochemical storage with a much-enhanced capacity and cycling life (83 mAh g(-1) after 5000 cycles in full-cell). Neutron total scattering and in situ X-ray diffraction measurements show that both structural water and the Na-rich disordered structure contribute to the improved electrochemical performance of current cathode material. Particularly, the co-deintercalation of the hydrated water and sodium-ion during the high potential charging process results in the shrinkage of interlayer distance and thus stabilizes the layered structure. Our results provide a genuine insight into how structural disordering and structural water improve sodium-ion storage in a layered electrode and open up an exciting direction for improving aqueous batteries.
  68. 68
    Chigane, M.; Ishikawa, M. XRD and XPS Characterization of Electrochromic Nickel Oxide Thin Films Prepared by Electrolysis–Chemical Deposition. J. Chem. Soc., Faraday Trans. 1998, 94, 3665– 3670,  DOI: 10.1039/a806459h
    Google Scholar
    68
    XRD and XPS characterization of electrochromic nickel oxide thin films prepared by electrolysis-chemical deposition
    Chigane, Masaya; Ishikawa, Masami
    Journal of the Chemical Society, Faraday Transactions (1998), 94 (24), 3665-3670CODEN: JCFTEV; ISSN:0956-5000. (Royal Society of Chemistry)
    Electrochromic (EC) behaviors of nickel oxide (NiOx) thin films, grown by chem. process in a nickel ammine complex soln. [Ni(NH3)x2+] after electrolysis of this soln. (EL-C method), were studied using various alk. electrolyte solns.; potassium borate buffer (pH 12), KOH (pH 12.9), (CH3)4NOH and Et4NOH. The fully oxidized films in all adopted electrolytes commonly showed broadened UV-visible spectra in the visible and near IR region. However, voltammetry studies assocd. with photoabsorption of the films revealed an apparent difference in the EC dynamics among the electrolytes caused by the variation of cation sizes and electrolyte soln. pH. Characterization of as-prepd., oxidized and reduced films in the electrolytes by x-ray diffraction (XRD) and XPS were performed to explain the mechanism of the films based on the conversion between layered α-Ni(OH)2 and γ2-2NiO2·NiOOH. A model according to the extn./insertion of cations or water mols. from/into the interlayer is proposed.
  69. 69
    Chigane, M.; Ishikawa, M. Manganese Oxide Thin Film Preparation by Potentiostatic Electrolyses and Electrochromism. J. Electrochem. Soc. 2000, 147, 2246,  DOI: 10.1149/1.1393515
    Google Scholar
    69
    Manganese oxide thin film preparation by potentiostatic electrolysis and electrochromism
    Chigane, Masaya; Ishikawa, Masami
    Journal of the Electrochemical Society (2000), 147 (6), 2246-2251CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    Manganese oxide (MnOx) thin films were deposited on transparent conducting tin oxide glass substrates by potentiostatic anodic electrolysis of alk. soln. of a manganese ammine complex at 298 K. The effects of varying deposition potentials on the microstructure and the electrochromic (EC) properties of the films were studied. Characterization of films by x-ray diffraction revealed that two distinct potential regions (lower and >0.3 V vs. Ag/AgCl) were available for the film deposition; the crystal structure of the film deposited at lower and higher regions were γ-Mn2O3 and/or Mn3O4 and Mn7O13·5H2O, resp. XPS analyses of the films featuring exchange splitting effect on Mn 3s spectra indicated that the valence of manganese in the films prepd. at lower and higher potential regions are mixts. of divalence-trivalence and of trivalence-tetravalence, resp. The XPS anal. also revealed that terminal chem. bonding species of the films are a mixt. of hydroxide (Mn-O-H) and (Mn-O-Mn). The mechanism of the EC process, by which the color change between brown and light yellow occurs, could be explained in terms of the transformation between these two oxygen groups in Mn-O-H and Mn-O-Mn, accompanied by the change in valence of Mn. The EC durability of the films in switching performance was also assessed.

Cited By

Click to copy section linkSection link copied!
Citation Statements
Explore this article's citation statements on scite.ai
powered by  Scite

This article is cited by 4 publications.

  1. Yang Liu, Shiqing Zhang, Shaokai Ma, Xinyu Sun, Ying Wang, Fang Liu, Ying Li, Yuanhui Ma, Xuewen Xu, Yanming Xue, Chengchun Tang, Jun Zhang. Electronic Structure Modification of MnO2 Nanosheet Arrays with Enhanced Water Oxidation Activity and Stability by Nitrogen Plasma. ACS Applied Materials & Interfaces 2024, 16 (28) , 36498-36508. https://doi.org/10.1021/acsami.4c07973
  2. Masaharu Nakayama, Wataru Yoshida. Electrodeposited Manganese Dioxides and Their Composites as Electrocatalysts for Energy Conversion Reactions. ChemSusChem 2025, 18 (5) https://doi.org/10.1002/cssc.202401907
  3. Chu Qin, Shijun Tian, Jialong Wu, Junliang Mou, Lan Feng, Zhongqing Jiang. Plasma‐Assisted Surface Engineering to Stabilize Mn 3+ in Electrodeposited Manganese Oxide Films for Water Oxidation. ChemCatChem 2024, 16 (22) https://doi.org/10.1002/cctc.202401033
  4. Elisa Morales, Lauren Formanski, Shaner Sarah, Stone Kari. Characterizing Biogenic MnOx Produced by Pseudomonas putida MnB1 and Its Catalytic Activity towards Water Oxidation. Life 2024, 14 (2) , 171. https://doi.org/10.3390/life14020171
Download PDF
Go to ACS Nanoscience Au
Get e-Alerts
Get e-Alerts

ACS Nanoscience Au

Cite this: ACS Nanosci. Au 2023, 3, 4, 310–322
Click to copy citationCitation copied!
https://doi.org/10.1021/acsnanoscienceau.3c00002
Published April 23, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY-NC-ND 4.0 .

Article Views

1738

Altmetric

-

Citations

4
Learn about these metrics

Article Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.

Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.

The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated.

  • Abstract

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 1

    Figure 1. (a,b) CV scans (using IUPAC convention) of MnOx films measured in 0.1 M KOH and 0.9 M KNO3. (a) CV scans of the as-synthesized films grown under illumination (red trace) and in the dark (blue trace). (b) CV scans of the same illuminated film (orange trace) and dark film (light-blue trace) after 1 h of constant-current electrolysis at a current density of 10 mA/cm2. The arrows indicate the onset potentials for the OER. (c) Applied potential vs time for an illuminated MnOx film (red trace) and a dark film (blue trace) during constant-current electrolysis at a current density of 10 mA/cm2 to test the stability of the films during the OER.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 2

    Figure 2. SEM images of (a) as-synthesized MnOx film grown under illumination with a 405 nm LED, (b) MnOx film synthesized under illumination after 30 min of electrolysis at 3 mA/cm2, (c) as-synthesized MnOx film grown in the dark, and (d) MnOx film synthesized in the dark after 30 min of electrolysis at 3 mA/cm2. The scale bar of 1 μm applies to all images.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 3

    Figure 3. Electrochemical characterization of electrodeposited MnOx films. (a) Tafel plots of MnOx films synthesized under illumination before (red triangles) and after (orange circles) electrolysis at 5 mA/cm2 for 40 min and films synthesized in the dark before (blue triangles) and after (light-blue squares) electrolysis. The symbols represent the actual data, and the lines connecting the symbols represent linear fits of the data used to calculate Tafel slopes. The errors were determined by linear fitting, and the R2 values of the linear fits were greater than 0.995. (b) Mott–Schottky plots of MnOx films synthesized under illumination before (red triangles) and after (orange circles) electrolysis at 5 mA/cm2 for 1 h, and films synthesized in the dark before (blue triangles) and after (light-blue squares) electrolysis. The lines represent linear fits of the raw data used to calculate the Mott–Schottky slopes. (c) Nyquist plots of MnOx films synthesized under illumination before (red) and after (orange) electrolysis at 3 mA/cm2 for 1.5 h, and films synthesized in the dark before (blue) and after (light blue) electrolysis. The inset shows the equivalent circuit model used to fit the Nyquist plots. R1 represents the uncompensated resistance from the solution, FTO substrate, and MnOx film, C2 represents the double-layer capacitance, R2 represents the polarization resistance, and W2 represents a Warburg impedance.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 4

    Figure 4. XPS of electrodeposited MnOx films in the binding energy region of Mn 2p3/2 electrons. (a,b) MnOx films synthesized under illumination before (a) before and (b) after electrolysis at 5 mA/cm2 for 1 h (c,d) MnOx films synthesized in the dark (c) before and (d) after electrolysis. For each spectrum, the shaded regions show the deconvolution of the peak into contributions from Mn4+ (purple), Mn3+ (green), and Mn2+ (light red). The gray circles show the raw data, and the black line shows the sum of fitting the peak to the different oxidation states of Mn. The dashed line shows the residual of the fit. The peak maxima for each peak in the deconvolution are provided in Table S3 of the Supporting Information.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 5

    Figure 5. (a) Raman spectra of electrodeposited MnOx films. Red trace: as-synthesized MnOx film grown under illumination; orange trace: illuminated film after electrolysis at 5 mA/cm2 for 1 h; blue trace: as-synthesized MnOx film grown in the dark; light-blue trace: dark film after electrolysis at 5 mA/cm2 for 1 h. (b) Crystal structure of potassium-intercalated birnessite hydrate, K0.27(Mn0.98O2)(H2O)0.51. Mn atoms are pink, oxygen atoms are gray, and potassium atoms are purple. The oxygen atoms within the layer represent intercalated H2O (hydrogen atoms not shown). The structure was made using collection code no 55407 from the Inorganic Crystal Structure Database.

    High Resolution Image
    Download MS PowerPoint Slide

  • References

    This article references 69 other publications.

    1. 1
      Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729– 15735,  DOI: 10.1073/pnas.0603395103
      1
      Powering the planet: chemical challenges in solar energy utilization
      Lewis, Nathan S.; Nocera, Daniel G.
      Proceedings of the National Academy of Sciences of the United States of America (2006), 103 (43), 15729-15735CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A review. Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, ≥2-fold by mid century relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atm. demands that holding atm. CO2 levels to even twice their pre-anthropogenic values by mid century will require invention, development, and deployment of schemes for C-neutral energy prodn. on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 h to the earth than all of the energy consumed by humans in an entire year. In view of the intermittence of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An esp. attractive approach is to store solar-converted energy as chem. bonds, i.e., in a photosynthetic process at a year-round av. efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy as chem. bonds, producing O from H2O and a reduced fuel such as H, methane, MeOH, or other hydrocarbon species.
    2. 2
      Tahir, M.; Pan, L.; Idrees, F.; Zhang, X.; Wang, L.; Zou, J.-J.; Wang, Z. L. Electrocatalytic Oxygen Evolution Reaction for Energy Conversion and Storage: A Comprehensive Review. Nano Energy 2017, 37, 136– 157,  DOI: 10.1016/j.nanoen.2017.05.022
      2
      Electrocatalytic oxygen evolution reaction for energy conversion and storage: A comprehensive review
      Tahir, Muhammad; Pan, Lun; Idrees, Faryal; Zhang, Xiangwen; Wang, Li; Zou, Ji-Jun; Wang, Zhong Lin
      Nano Energy (2017), 37 (), 136-157CODEN: NEANCA; ISSN:2211-2855. (Elsevier Ltd.)
      Water oxidn. or oxygen evolution reaction (OER) electrocatalysis got much attention in the last few years because of its prime role in water splitting, rechargeable metal-air batteries and fuel cells. Therefore, the development of efficient, abundant and economical catalysts for water oxidn. reaction is one of the main subjects of present study in renewable energies. This review article summarizes the very recent efforts in the field of OER electrocatalysis along with the faced challenges. The solns. to these challenges also outline with appropriate examples of scientific literatures. Significantly, the present review will provide the stds. to evaluate the activity and stability for heterogeneous OER catalysts. It will clearly summarize the future directions and applications, esp. the combination of sustainable energy utilization (like triboelec. nanogenerator) with water splitting. The providing study will help to explore and develop better catalysts and units for practical applications and will offer basic understanding of OER process along with the std. parameters to evaluate the performance.
    3. 3
      Vesborg, P. C. K.; Jaramillo, T. F. Addressing the Terawatt Challenge: Scalability in the Supply of Chemical Elements for Renewable Energy. RSC Adv. 2012, 2, 7933– 7947,  DOI: 10.1039/c2ra20839c
      3
      Addressing the terawatt challenge: scalability in the supply of chemical elements for renewable energy
      Vesborg, Peter C. K.; Jaramillo, Thomas F.
      RSC Advances (2012), 2 (21), 7933-7947CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The energy infrastructure for fossil fuels is well-established, accounting for approx. 87% of the 16 TW of power consumed globally. For renewable and sustainable energy conversion technologies to play a relevant role at the terrestrial scale, they must be able to scale to the TW level of deployment. This would place a significant demand on the current and future supply of raw materials (chem. elements) used by those technologies. Oftentimes, the av. crustal abundance of a chem. element is cited as a measure of its scalability, however another important metric for scalability is the existence (of lack thereof) of mineable ores with a high concn. of the targeted element. This paper aims to provide an overview of the availability of all elements. This is accomplished via a compilation of data for global primary prodn. rates for each element, as a measure of availability at the present time. This work also addresses the potential future availability based on current and possible future primary sources.
    4. 4
      Gaultois, M. W.; Sparks, T. D.; Borg, C. K. H.; Seshadri, R.; Bonificio, W. D.; Clarke, D. R. Data-Driven Review of Thermoelectric Materials: Performance and Resource Considerations. Chem. Mater. 2013, 25, 2911– 2920,  DOI: 10.1021/cm400893e
      4
      Data-Driven Review of Thermoelectric Materials: Performance and Resource Considerations
      Gaultois, Michael W.; Sparks, Taylor D.; Borg, Christopher K. H.; Seshadri, Ram; Bonificio, William D.; Clarke, David R.
      Chemistry of Materials (2013), 25 (15), 2911-2920CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      A review. In this review, the authors describe the creation of a large database of thermoelec. materials prepd. by abstracting information from over 100 publications. The database has over 18,000 data points from multiple classes of compds., whose relevant properties were measured at several temps. Appropriate visualization of the data immediately allows certain insights to be gained with regard to the property space of plausible thermoelec. materials. Of particular note is that any candidate material needs to display an elec. resistivity value that is ∼1 mΩ cm at 300 K, i.e., samples should be significantly more conductive than the Mott min. metallic cond. The Herfindahl-Hirschman index, a commonly accepted measure of market concn., was calcd. from geol. data (known elemental reserves) and geopolitical data (elemental prodn.) for much of the periodic table. The visualization strategy employed here allows rapid sorting of thermoelec. compns. with respect to important issues of elemental scarcity and supply risk.
    5. 5
      Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F. Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production Via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6, 1983– 2002,  DOI: 10.1039/c3ee40831k
      5
      Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry
      Pinaud, Blaise A.; Benck, Jesse D.; Seitz, Linsey C.; Forman, Arnold J.; Chen, Zhebo; Deutsch, Todd G.; James, Brian D.; Baum, Kevin N.; Baum, George N.; Ardo, Shane; Wang, Heli; Miller, Eric; Jaramillo, Thomas F.
      Energy & Environmental Science (2013), 6 (7), 1983-2002CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Photoelectrochem. water splitting is a promising route for the renewable prodn. of hydrogen fuel. This work presents the results of a tech. and economic feasibility anal. conducted for four hypothetical, centralized, large-scale hydrogen prodn. plants based on this technol. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calc. the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day prodn. scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from $1.60-$10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O2 and H2 in a single bed system and long mol. transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity anal. revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the prodn. costs are consistent with the Department of Energy's targeted threshold cost of $2.00-$4.00 per kg H2 for dispensed hydrogen, demonstrating that photoelectrochem. water splitting could be a viable route for hydrogen prodn. in the future if material performance targets can be met.
    6. 6
      Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution. J. Am. Chem. Soc. 2012, 134, 17253– 17261,  DOI: 10.1021/ja307507a
      6
      Solution-Cast Metal Oxide Thin Film Electrocatalysts for Oxygen Evolution
      Trotochaud, Lena; Ranney, James K.; Williams, Kerisha N.; Boettcher, Shannon W.
      Journal of the American Chemical Society (2012), 134 (41), 17253-17261CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Water oxidn. is a crit. step in water splitting to make hydrogen fuel. The authors report the soln. synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) electrocatalytic properties of ∼2-3 nm thick films of NiOx, CoOx, NiyCo1-yOx, Ni0.9Fe0.1Ox, IrOx, MnOx, and FeOx. The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochem. properties of the oxides. Ni0.9Fe0.1Ox is the most active water oxidn. catalyst in basic media, passing 10 mA/cm2 at an overpotential of 336 mV with a Tafel slope of 30 mV/dec with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrOx control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni0.9Fe0.1OOH oxyhydroxide species with nearly every Ni atom electrochem. active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, NiyCo1-yOx thin films showed decreasing activity relative to the pure NiOx films with increasing Co content. This finding is explained by the suppressed in situ formation of the active layered oxyhydroxide with increasing Co. The high OER activity and simple synthesis make these Ni-based catalyst thin films useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting or in high-surface-area electrodes for water electrolysis.
    7. 7
      Sun, K.; Park, N.; Sun, Z.; Zhou, J.; Wang, J.; Pang, X.; Shen, S.; Noh, S. Y.; Jing, Y.; Jin, S.; Yu, P. K. L.; Wang, D. Nickel Oxide Functionalized Silicon for Efficient Photo-Oxidation of Water. Energy Environ. Sci. 2012, 5, 7872– 7877,  DOI: 10.1039/c2ee21708b
      7
      Nickel oxide functionalized silicon for efficient photo-oxidation of water
      Sun, Ke; Park, Namseok; Sun, Zhelin; Zhou, Jigang; Wang, Jian; Pang, Xiaolu; Shen, Shaohua; Noh, Sun Young; Jing, Yi; Jin, Sungho; Yu, Paul K. L.; Wang, Deli
      Energy & Environmental Science (2012), 5 (7), 7872-7877CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      We report a nickel oxide (NiOx) thin film, from a cost-effective sol-gel process, coated n-type silicon (n-Si) as a photoanode for efficient photo-oxidn. of water under neutral pH condition. The NiOx thin film has three functions: (a) serves as a protection layer to improve the chem. stability of the Si photoelectrode, (b) acts as an oxygen evolution catalyst, and (c) provides junction photovoltage to further reduce overpotential. The oxygen evolution onset potential is reduced to below the thermodn. water oxidn. level and oxygen evolution was obsd. at low overpotentials. Our results demonstrate the fabrication of robust photoelectrodes from low-cost NiOx and Si, which enable a practical solar water oxidn. with high efficiency.
    8. 8
      Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 342, 836– 840,  DOI: 10.1126/science.1241327
      8
      High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation
      Kenney, Michael J.; Gong, Ming; Li, Yanguang; Wu, Justin Z.; Feng, Ju; Lanza, Mario; Dai, Hongjie
      Science (Washington, DC, United States) (2013), 342 (6160), 836-840CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Si's sensitivity to corrosion has hindered its use in photoanode applications. Deposition of a ∼2-nm Ni film on n-type Si (n-Si) with its native oxide affords a high-performance metal-insulator-semiconductor photoanode for photoelectrochem. (PEC) H2O oxidn. in both aq. KOH (KOH, pH = 14) and aq. borate buffer (pH = 9.5) solns. The Ni film acted as a surface protection layer against corrosion and as a nonprecious metal electrocatalyst for O evolution. In 1 M aq. KOH, the Ni/n-Si photoanodes exhibited high PEC activity with a low onset potential (∼1.07 V vs. reversible H electrode), high photocurrent d., and durability. The electrode showed no sign of decay after ∼80 h of continuous PEC H2O oxidn. in a mixed Li borate-K borate electrolyte. The high photovoltage was attributed to a high built-in potential in a metal-insulator-semiconductor-like device with an ultrathin, incomplete screening Ni/NiOx layer from the electrolyte.
    9. 9
      Zou, S.; Burke, M. S.; Kast, M. G.; Fan, J.; Danilovic, N.; Boettcher, S. W. Fe (Oxy)Hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution. Chem. Mater. 2015, 27, 8011– 8020,  DOI: 10.1021/acs.chemmater.5b03404
      9
      Fe (Oxy)hydroxide Oxygen Evolution Reaction Electrocatalysis: Intrinsic Activity and the Roles of Electrical Conductivity, Substrate, and Dissolution
      Zou, Shihui; Burke, Michaela S.; Kast, Matthew G.; Fan, Jie; Danilovic, Nemanja; Boettcher, Shannon W.
      Chemistry of Materials (2015), 27 (23), 8011-8020CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Fe cations dramatically enhance oxygen evolution reaction (OER) activity when incorporated substitutionally into Ni or Co (oxy)hydroxides, serving as possible OER active sites. Pure Fe (oxy)hydroxides, however, are typically thought to be poor OER catalysts and are not well-understood. Here, we report a systematic investigation of Fe (oxy)hydroxide OER catalysis in alk. media. At low overpotentials of ∼350 mV, the catalyst dissoln. rate is low, the activity is dramatically enhanced by an AuOx/Au substrate, and the geometric OER c.d. is largely independent of mass loading. At higher overpotentials of ∼450 mV, the dissoln. rate is high, the activity is largely independent of substrate choice, and the geometric c.d. depends linearly on loading. These observations, along with previously reported in situ cond. measurements, suggest a new model for OER catalysis on Fe (oxy)hydroxide. At low overpotentials, only the first monolayer of the electrolyte-permeable Fe (oxy)hydroxide, which is in direct contact with the conductive support, is OER-active due to elec. cond. limitations. On Au substrates, Fe cations interact with AuOx after redox cycling, leading to enhanced intrinsic activity over FeOOH on Pt substrates. At higher overpotentials, the cond. of Fe (oxy)hydroxide increases, leading to a larger fraction of the electrolyte-permeable catalyst film participating in catalysis. Comparing the apparent activity of the putative Fe active sites in/on different hosts/surfaces supports a possible connection between OER activity and local structure.
    10. 10
      Wang, H.; Lee, H.-W.; Deng, Y.; Lu, Z.; Hsu, P.-C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional Non-Noble Metal Oxide Nanoparticle Electrocatalysts through Lithium-Induced Conversion for Overall Water Splitting. Nat. Commun. 2015, 6, 7261,  DOI: 10.1038/ncomms8261
      10
      Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting
      Wang, Haotian; Lee, Hyun-Wook; Deng, Yong; Lu, Zhiyi; Hsu, Po-Chun; Liu, Yayuan; Lin, Dingchang; Cui, Yi
      Nature Communications (2015), 6 (), 7261CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Developing earth-abundant, active and stable electrocatalysts which operate in the same electrolyte for water splitting, including oxygen evolution reaction and hydrogen evolution reaction, is important for many renewable energy conversion processes. Here we demonstrate the improvement of catalytic activity when transition metal oxide (iron, cobalt, nickel oxides and their mixed oxides) nanoparticles (∼20 nm) are electrochem. transformed into ultra-small diam. (2-5 nm) nanoparticles through lithium-induced conversion reactions. Different from most traditional chem. syntheses, this method maintains excellent elec. interconnection among nanoparticles and results in large surface areas and many catalytically active sites. We demonstrate that lithium-induced ultra-small NiFeOx nanoparticles are active bifunctional catalysts exhibiting high activity and stability for overall water splitting in base. We achieve 10 mA cm-2 water-splitting current at only 1.51 V for over 200 h without degrdn. in a two-electrode configuration and 1 M KOH, better than the combination of iridium and platinum as benchmark catalysts.
    11. 11
      Hill, J. C.; Landers, A. T.; Switzer, J. A. An Electrodeposited Inhomogeneous Metal–Insulator–Semiconductor Junction for Efficient Photoelectrochemical Water Oxidation. Nat. Mater. 2015, 14, 1150– 1155,  DOI: 10.1038/nmat4408
      11
      An electrodeposited inhomogeneous metal-insulator-semiconductor junction for efficient photoelectrochemical water oxidation
      Hill, James C.; Landers, Alan T.; Switzer, Jay A.
      Nature Materials (2015), 14 (11), 1150-1155CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      The photoelectrochem. splitting of water into hydrogen and oxygen requires a semiconductor to absorb light and generate electron-hole pairs, and a catalyst to enhance the kinetics of electron transfer between the semiconductor and soln. A crucial question is how this catalyst affects the band bending in the semiconductor, and, therefore, the photovoltage of the cell. A simple and inexpensive electrodeposition method is introduced to produce an efficient n-Si/SiOx/Co/CoOOH photoanode for the photoelectrochem. oxidn. of water to oxygen. The photoanode functions as a solid-state, metal-insulator-semiconductor photovoltaic cell with spatially non-uniform barrier heights in series with a low overpotential water-splitting electrochem. cell. The barrier height is a function of the Co coverage; it increases from 0.74 eV for a thick, continuous film to 0.91 eV for a thin, inhomogeneous film that has not reached coalescence. The larger barrier height leads to a 360 mV photovoltage enhancement relative to a solid-state Schottky barrier.
    12. 12
      Jiao, F.; Frei, H. Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts. Energy Environ. Sci. 2010, 3, 1018– 1027,  DOI: 10.1039/c002074e
      12
      Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts
      Jiao, Feng; Frei, Heinz
      Energy & Environmental Science (2010), 3 (8), 1018-1027CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      A review. Recent development of new methods of prepg. cobalt oxide and manganese oxide clusters has led to oxygen evolving catalysts that operate under mild conditions and modest overpotentials at rates approaching practical utility. Synthesis of nanostructured Co3O4 and Mn oxide clusters in mesoporous silica scaffolds affords catalysts with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites. Stacking of the nanoclusters of these earth abundant, durable oxide catalysts in the scaffold results in turnover frequencies per projected area that are sufficient for keeping up with the photon flux at high solar intensity. Opportunities for expanding the metal oxide/silica interface approach to heterogeneous water oxidn. catalysis to a more general approach for multi-electron catalyst designs based on core/shell constructs are discussed. The results are reviewed in the context of all-inorg. materials for catalytic water oxidn. reported recently from other labs., in particular electrodeposits generated from Co phosphate solns., a mol. water oxidn. catalyst based on a polyoxotungstate featuring a Co oxide core, and Mn oxide materials with incorporated Ca ions.
    13. 13
      Hocking, R. K.; Brimblecombe, R.; Chang, L.-Y.; Singh, A.; Cheah, M. H.; Glover, C.; Casey, W. H.; Spiccia, L. Water-Oxidation Catalysis by Manganese in a Geochemical-Like Cycle. Nat. Chem. 2011, 3, 461– 466,  DOI: 10.1038/nchem.1049
      13
      Water-oxidation catalysis by manganese in a geochemical-like cycle
      Hocking, Rosalie K.; Brimblecombe, Robin; Chang, Lan-Yun; Singh, Archana; Cheah, Mun Hon; Glover, Chris; Casey, William H.; Spiccia, Leone
      Nature Chemistry (2011), 3 (6), 461-466CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Water oxidn. in all oxygenic photosynthetic organisms is catalyzed by the Mn4CaO4 cluster of Photosystem II. This cluster has inspired the development of synthetic Mn catalysts for solar energy prodn. A photoelectrochem. device, made by impregnating a synthetic tetranuclear-Mn cluster into a Nafion matrix, was shown to achieve efficient H2O oxidn. catalysis. The authors reported in situ x-ray absorption spectroscopy and TEM studies that demonstrate that this cluster dissocs. into Mn(II) compds. in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the obsd. photochem. H2O-oxidn. catalysis. The original Mn cluster serves only as a precursor to the catalytically active material. The behavior of Mn in Nafion parallels its broader biogeochem., which is also dominated by cycles of oxidn. into solid Mn(III/IV) oxides followed by photoredn. to Mn2+.
    14. 14
      Iyer, A.; Del-Pilar, J.; King’ondu, C. K.; Kissel, E.; Garces, H. F.; Huang, H.; El-Sawy, A. M.; Dutta, P. K.; Suib, S. L. Water Oxidation Catalysis Using Amorphous Manganese Oxides, Octahedral Molecular Sieves (OMS-2), and Octahedral Layered (OL-1) Manganese Oxide Structures. J. Phys. Chem. C 2012, 116, 6474– 6483,  DOI: 10.1021/jp2120737
      14
      Water Oxidation Catalysis using Amorphous Manganese Oxides, Octahedral Molecular Sieves (OMS-2), and Octahedral Layered (OL-1) Manganese Oxide Structures
      Iyer, Aparna; Del-Pilar, Joselyn; King'ondu, Cecil K.; Kissel, Edward; Garces, Hector F.; Huang, Hui; El-Sawy, Abdelhamid M.; Dutta, Prabir K.; Suib., Steven L.
      Journal of Physical Chemistry C (2012), 116 (10), 6474-6483CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Water oxidn. is the bottleneck in artificial photosynthetic systems that aim to split water into hydrogen and oxygen. However, water oxidn. occurs readily in plants, catalyzed by the Mn4O4Ca manganese cluster. In addn. to this, manganese minerals are ubiquitous in nature displaying layered and tunnel structures. In this study, mixed valent porous amorphous manganese oxides (AMO), along with cryptomelane type tunnel manganese oxides (OMS-2) and layered birnessite (OL-1) have been used as water oxidn. catalysts. Significantly higher turnovers were obtained with AMO (290 mmol O2/mol Mn) compared to tunnel structure OMS-2 (110 mmol O2/mol Mn) and layered structure OL-1 (27 mmol O2/mol Mn) in water oxidn. tests with Ce4+. Oxygen evolution was also confirmed under photochem. conditions using Ru(bpy)32+ as a photosensitizer and persulfate as a sacrificial agent. The differences in catalytic activity among these catalysts have been probed using X-ray diffraction, transmission electron microscopy, Raman and Fourier transform IR (FTIR) spectroscopy, av. oxidn. state, and compositional analyses. Comparison of AMO against prominent manganese catalysts described in literature shows AMO provided the highest turnover nos. AMO catalyst was also reusable after regeneration. O-18 labeling studies proved that water was the source of dioxygen and IR proved the structural stability of AMO after reaction. AMO is related to hexagonal birnessites such as layered biogenic manganese oxides or H+-birnessite that have cation vacancies in the MnO2 sheets rather than completely filled Mn3+/Mn4+ sheets, and this is influential in catalytic activity.
    15. 15
      Wiechen, M.; Zaharieva, I.; Dau, H.; Kurz, P. Layered Manganese Oxides for Water-Oxidation: Alkaline Earth Cations Influence Catalytic Activity in a Photosystem II-Like Fashion. Chem. Sci. 2012, 3, 2330– 2339,  DOI: 10.1039/c2sc20226c
      15
      Layered manganese oxides for water-oxidation: alkaline earth cations influence catalytic activity in a photosystem II-like fashion
      Wiechen, Mathias; Zaharieva, Ivelina; Dau, Holger; Kurz, Philipp
      Chemical Science (2012), 3 (7), 2330-2339CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      In reaction sequences for light driven water-splitting into H2 and O2, water-oxidn. is a crucial reaction step. In vivo, the process is catalyzed within a photoenzyme called photosystem II (PSII) by a μ-oxido CaMn4 cluster, the oxygen-evolving complex (OEC). The OEC is known to be virtually inactive if Ca2+ is removed from its structure. Activity can be restored not only by the addn. of Ca2+ but also Sr2+ ions. We have recently introduced layered calcium manganese oxides of the birnessite mineral family as functional synthetic model compds. for the OEC. Here, we present the syntheses of layered manganese oxides where we varied the interlayer cations, prepg. a series of K-, Ca-, Sr- and Mg-contg. birnessites. Structural motifs within these materials were detd. using X-ray absorption spectroscopy (XAS) showing that all materials have similar at. structures despite their different elemental compns. Water-oxidn. expts. were carried out to elucidate structure-reactivity relations. These expts. demonstrated that the oxides-like the OEC-require the presence of calcium in their structures to reach max. catalytic activity. As another similarity to the OEC, Sr2+ is the "second best choice" for the secondary cation. The results thus support mechanistic proposals which involve an important catalytic role for Ca2+ in biol. water-oxidn. Addnl., they offer valuable hints for the development of synthetic, manganese-based water-oxidn. catalysts for artificial photosynthesis.
    16. 16
      Boppana, V. B. R.; Yusuf, S.; Hutchings, G. S.; Jiao, F. Nanostructured Alkaline-Cation-Containing δ-MnO2 for Photocatalytic Water Oxidation. Adv. Funct. Mater. 2013, 23, 878– 884,  DOI: 10.1002/adfm.201202141
      16
      Nanostructured Alkaline-Cation-Containing δ-MnO2 for Photocatalytic Water Oxidation
      Boppana, Venkata Bharat Ram; Yusuf, Seif; Hutchings, Gregory S.; Jiao, Feng
      Advanced Functional Materials (2013), 23 (7), 878-884CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Oxygen evolution from water is one of the key reactions for solar fuel prodn. Here, two nanostructured K-contg. δ-MnO2 are synthesized: K-δ-MnO2 nanosheets and K-δ-MnO2 nanoparticles, both of which exhibit high catalytic activity in visible-light-driven water oxidn. The role of alk. cations in oxygen evolution is first explored by replacing the K+ ions in the δ-MnO2 structure with H+ ions through proton ion exchange. H-δ-MnO2 catalysts with a similar morphol. and crystal structure exhibit activities per surface site approx. one order of magnitude lower than that of K-δ-MnO2, although both nanostructured H-δ-MnO2 catalysts have much larger Brunauer-Emmett-Teller (BET) surface areas. Such a low turnover frequency (TOF) per surface Mn atom might be due to the fact that the Ru2+(bpy)3 sensitizer is too large to access the addnl. surface area created during proton exchange. Also, a prepd. Na-contg. δ-MnO2 material with an identical crystal structure exhibits a TOF similar to that of the K-contg. δ-MnO2, suggesting that the alk. cations are not directly involved in catalytic water oxidn., but instead stabilize the layered structure of the δ-MnO2.
    17. 17
      Robinson, D. M.; Go, Y. B.; Mui, M.; Gardner, G.; Zhang, Z.; Mastrogiovanni, D.; Garfunkel, E.; Li, J.; Greenblatt, M.; Dismukes, G. C. Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis. J. Am. Chem. Soc. 2013, 135, 3494– 3501,  DOI: 10.1021/ja310286h
      17
      Photochemical Water Oxidation by Crystalline Polymorphs of Manganese Oxides: Structural Requirements for Catalysis
      Robinson, David M.; Go, Yong Bok; Mui, Michelle; Gardner, Graeme; Zhang, Zhijuan; Mastrogiovanni, Daniel; Garfunkel, Eric; Li, Jing; Greenblatt, Martha; Dismukes, G. Charles
      Journal of the American Chemical Society (2013), 135 (9), 3494-3501CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Manganese oxides occur naturally as minerals in at least 30 different crystal structures, providing a rigorous test system to explore the significance of at. positions on the catalytic efficiency of water oxidn. In this study, the authors chose to systematically compare eight synthetic oxide structures contg. Mn(III) and Mn(IV) only, with particular emphasis on the five known structural polymorphs of MnO2. The authors have adapted literature synthesis methods to obtain pure polymorphs and validated their homogeneity and crystallinity by powder x-ray diffraction and both transmission and scanning electron microscopies. Measurement of water oxidn. rate by oxygen evolution in aq. soln. was conducted with dispersed nanoparticulate manganese oxides and a std. ruthenium dye photooxidant system. No Ru was absorbed on the catalyst surface as obsd. by XPS and EDX. The post reaction at. structure was completely preserved with no amorphization, as obsd. by HRTEM. Catalytic activities, normalized to surface area (BET), decrease in the series Mn2O3 > Mn3O4 » λ-MnO2, where the latter is derived from spinel LiMn2O4 following partial Li+ removal. No catalytic activity is obsd. from LiMn2O4 and four of the MnO2 polymorphs, in contrast to some literature reports with polydispersed manganese oxides and electro-deposited films. Catalytic activity within the eight examd. Mn oxides was found exclusively for (distorted) cubic phases, Mn2O3 (bixbyite), Mn3O4 (hausmannite), and λ-MnO2 (spinel), all contg. Mn(III) possessing longer Mn-O bonds between edge-sharing MnO6 octahedra. Electronically degenerate Mn(III) has antibonding electronic configuration eg1 which imparts lattice distortions due to the Jahn-Teller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidn. at the surface.
    18. 18
      Pokhrel, R.; Goetz, M. K.; Shaner, S. E.; Wu, X.; Stahl, S. S. The “Best Catalyst” for Water Oxidation Depends on the Oxidation Method Employed: A Case Study of Manganese Oxides. J. Am. Chem. Soc. 2015, 137, 8384– 8387,  DOI: 10.1021/jacs.5b05093
      18
      The "Best Catalyst" for Water Oxidation Depends on the Oxidation Method Employed: A Case Study of Manganese Oxides
      Pokhrel, Ravi; Goetz, McKenna K.; Shaner, Sarah E.; Wu, Xiaoxia; Stahl, Shannon S.
      Journal of the American Chemical Society (2015), 137 (26), 8384-8387CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Manganese oxides are a highly promising class of water-oxidn. catalysts (WOCs), but the optimal MnOx formulation or polymorph is not clear from previous reports in the literature. A complication not limited to MnOx-based WOCs is that such catalysts are routinely evaluated by different methods, ranging from the use of a chem. oxidant such as Ce4+, photoactive mediators such as [Ru(bpy)3]2+, or electrochem. techniques. Here, we report a systematic study of nine cryst. MnOx materials as WOCs and show that the identity of the "best" catalyst changes, depending on the oxidn. method used to probe the catalytic activity.
    19. 19
      Frey, C. E.; Kurz, P. Water Oxidation Catalysis by Synthetic Manganese Oxides with Different Structural Motifs: A Comparative Study. Chem.─Eur. J. 2015, 21, 14958– 14968,  DOI: 10.1002/chem.201501367
      19
      Water Oxidation Catalysis by Synthetic Manganese Oxides with Different Structural Motifs: A Comparative Study
      Frey, Carolin E.; Kurz, Philipp
      Chemistry - A European Journal (2015), 21 (42), 14958-14968CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Manganese oxides are considered to be very promising materials for water oxidn. catalysis (WOC), but the structural parameters influencing their catalytic activity have so far not been clearly identified. For this study, a dozen manganese oxides (MnOx) with various solid-state structures were synthesized and carefully characterised by various phys. and chem. methods. WOC by the different MnOx was then investigated with Ce4+ as chem. oxidant. Oxides with layered structures (birnessites) and those contg. large tunnels (todorokites) clearly gave the best results with reaction rates exceeding 1250 mmolO2 molMn-1 h-1 or about 50 μmolO2 m-2 h-1. In comparison, catalytic rates per mol of Mn of oxides characterized by well-defined 3D networks were rather low (e.g., ca. 90 mmolO2 molMn-1 h-1 for bixbyite, Mn2O3), but impressive if normalized per unit surface area (>100 μmolO2 m-2 h-1 for marokite, CaMn2O4). Thus, two groups of MnOx emerge from this screening as hot candidates for manganese-based WOC materials: 1) amorphous oxides with tunnelled structures and the well-established layered oxides; 2) cryst. MnIII oxides. However, synthetic methods to increase surface areas must be developed for the latter to obtain good catalysis rates per mol of Mn or per unit catalyst mass.
    20. 20
      Smith, P. F.; Deibert, B. J.; Kaushik, S.; Gardner, G.; Hwang, S.; Wang, H.; Al-Sharab, J. F.; Garfunkel, E.; Fabris, L.; Li, J.; Dismukes, G. C. Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (γ-MnOOH). ACS Catal. 2016, 6, 2089– 2099,  DOI: 10.1021/acscatal.6b00099
      20
      Coordination Geometry and Oxidation State Requirements of Corner-Sharing MnO6 Octahedra for Water Oxidation Catalysis: An Investigation of Manganite (γ-MnOOH)
      Smith, Paul F.; Deibert, Benjamin J.; Kaushik, Shivam; Gardner, Graeme; Hwang, Shinjae; Wang, Hao; Al-Sharab, Jafar F.; Garfunkel, Eric; Fabris, Laura; Li, Jing; Dismukes, G. Charles
      ACS Catalysis (2016), 6 (3), 2089-2099CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Surface-directed corner-sharing MnO6 octahedra within numerous manganese oxide compds. contg. Mn3+ or Mn4+ oxidn. states show strikingly different catalytic activities for water oxidn., paradoxically poorest for Mn4+ oxides, regardless of oxidn. assay (photochem. and electrochem.). This is demonstrated herein by comparing cryst. oxides consisting of Mn3+ (manganite, γ-MnOOH; bixbyite, Mn2O3), Mn4+ (pyrolusite, β-MnO2) and multiple monophasic mixed-valence manganese oxides. Like all Mn4+ oxides, pure β-MnO2 has no detectable catalytic activity, while γ-MnOOH (tetragonally distorted Mn3+O6, D4h symmetry) is significantly more active and Mn2O3 (trigonal antiprismatic Mn3+O6, D3d symmetry) is the most active. γ-MnOOH deactivates during catalytic turnover simultaneous with the disappearance of crystallog. defined corner-sharing Mn3+O6 and the appearance of Mn4+. In a comparison of 2D-layered cryst. birnessites (δ-MnO2), the monovalent Mn4+ form is catalytically inert, while the hexagonal polymorph, contg. few out-of-layer corner-sharing Mn3+O6, has ∼10-fold higher catalytic activity than the triclinic polymorph, contg. in-plane edge-sharing Mn3+O6. These electronic and structural correlations point toward the more flexible (corner-shared) Mn3+O6 sites, over more rigid (edge-shared) sites as substantially more active catalytic centers. Electrochem. measurements show and ligand field theory predicts that, among corner-shared Mn3+O6 sites, those possessing D3d ligand field symmetry have stronger covalent Mn-O bonding to the six equiv. oxygen ligands, which we ascribe as responsible for more efficient and faster electrolytic water oxidn. In contrast, D4h Mn3+O6 sites have weaker Mn-O bonding to the two axial oxygen ligands, have sepd. electrochem. oxidn. waves for Mn and O, and are catalytically less efficient and exhibit slower catalytic turnover. By controlling the ligand field geometry and strength to oxygen ligands, we have identified the key variables for tuning water oxidn. activity by manganese oxides. We apply these findings to propose a mechanism for water oxidn. by the CaMn4O5 catalytic site of natural photosynthesis.
    21. 21
      Thenuwara, A. C.; Cerkez, E. B.; Shumlas, S. L.; Attanayake, N. H.; McKendry, I. G.; Frazer, L.; Borguet, E.; Kang, Q.; Remsing, R. C.; Klein, M. L.; Zdilla, M. J.; Strongin, D. R. Nickel Confined in the Interlayer Region of Birnessite: An Active Electrocatalyst for Water Oxidation. Angew. Chem., Int. Ed. 2016, 55, 10381– 10385,  DOI: 10.1002/anie.201601935
      21
      Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation
      Thenuwara, Akila C.; Cerkez, Elizabeth B.; Shumlas, Samantha L.; Attanayake, Nuwan H.; McKendry, Ian G.; Frazer, Laszlo; Borguet, Eric; Kang, Qing; Remsing, Richard C.; Klein, Michael L.; Zdilla, Michael J.; Strongin, Daniel R.
      Angewandte Chemie, International Edition (2016), 55 (35), 10381-10385CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni2+ ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 at. %)-intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA/cm2. This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst β-Ni(OH)2 (η≈550 mV). Mol. dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chem. in the spatially confined interlayer region.
    22. 22
      Kang, Q.; Vernisse, L.; Remsing, R. C.; Thenuwara, A. C.; Shumlas, S. L.; McKendry, I. G.; Klein, M. L.; Borguet, E.; Zdilla, M. J.; Strongin, D. R. Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2017, 139, 1863– 1870,  DOI: 10.1021/jacs.6b09184
      22
      Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction
      Kang, Qing; Vernisse, Loranne; Remsing, Richard C.; Thenuwara, Akila C.; Shumlas, Samantha L.; McKendry, Ian G.; Klein, Michael L.; Borguet, Eric; Zdilla, Michael J.; Strongin, Daniel R.
      Journal of the American Chemical Society (2017), 139 (5), 1863-1870CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The dependence is investigated of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to det. the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers pos. correlates with the size of the alkali cation: NS/Cs+/NS > NS/Rb+/NS > NS/K+/NS > NS/Na+/NS > NS/Li+/NS. The thermodn. origins of these bilayer heights were investigated using mol. dynamics simulations. The overpotential (η) for the OER correlates with the interlayer spacing; NS/Cs+/NS has the lowest η (0.45 V), while NS/Li+/NS exhibits the highest η (0.68 V) for OER at a c.d. of 1 mA/cm2. Kinetic parameters (η and Tafel slope) assocd. with NS/Cs+/NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies.
    23. 23
      Kölbach, M.; Fiechter, S.; van de Krol, R.; Bogdanoff, P. Evaluation of Electrodeposited α-Mn2O3 as a Catalyst for the Oxygen Evolution Reaction. Catal. Today 2017, 290, 2– 9,  DOI: 10.1016/j.cattod.2017.03.030
      There is no corresponding record for this reference.
    24. 24
      Li, A.; Ooka, H.; Bonnet, N.; Hayashi, T.; Sun, Y.; Jiang, Q.; Li, C.; Han, H.; Nakamura, R. Stable Potential Windows for Long-Term Electrocatalysis by Manganese Oxides under Acidic Conditions. Angew. Chem., Int. Ed. 2019, 58, 5054– 5058,  DOI: 10.1002/anie.201813361
      24
      Stable Potential Windows for Long-Term Electrocatalysis by Manganese Oxides Under Acidic Conditions
      Li, Ailong; Ooka, Hideshi; Bonnet, Nadege; Hayashi, Toru; Sun, Yimeng; Jiang, Qike; Li, Can; Han, Hongxian; Nakamura, Ryuhei
      Angewandte Chemie, International Edition (2019), 58 (15), 5054-5058CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Efficient, earth-abundant, and acid-stable catalysts for the oxygen evolution reaction (OER) are missing pieces for the prodn. of hydrogen via water electrolysis. Here, we report how the limitations on the stability of 3d-metal materials can be overcome by the spectroscopic identification of stable potential windows in which the OER can be catalyzed efficiently while simultaneously suppressing deactivation pathways. We demonstrate the benefits of this approach using gamma manganese oxide (γ-MnO2), which shows no signs of deactivation even after 8000 h of electrolysis at a pH of 2. This stability is vastly superior to existing acid-stable 3d-metal OER catalysts, but cannot be realized if there is a deviation as small as 50-mV from the stable potential window. A stable voltage efficiency of over 70 % in a polymer-electrolyte membrane (PEM) electrolyzer further verifies the availability of this approach and showcases how materials previously perceived to be unstable may have potential application for water electrolysis in an acidic environment.
    25. 25
      Takashima, T.; Hashimoto, K.; Nakamura, R. Mechanisms of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO2 Electrocatalysts. J. Am. Chem. Soc. 2012, 134, 1519– 1527,  DOI: 10.1021/ja206511w
      25
      Mechanisms of pH-Dependent Activity for Water Oxidation to Molecular Oxygen by MnO2 Electrocatalysts
      Takashima, Toshihiro; Hashimoto, Kazuhito; Nakamura, Ryuhei
      Journal of the American Chemical Society (2012), 134 (3), 1519-1527CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Mn oxides function as efficient electrocatalysts for H2O oxidn. to O2 in strongly alk. conditions, but are inefficient at neutral pH. To provide new insight into the mechanism underlying the pH-dependent activity of the electrooxidn. reaction, the authors performed UV-visible spectroelectrochem. detection of the intermediate species for H2O oxidn. by a Mn oxide electrode. Layered Mn oxide nanoparticles, δ-MnO2 (K0.17[Mn4+0.90Mn3+0.07.box.0.03]O2·0.53H2O) deposited on F-doped Sn oxide electrodes catalyze H2O oxidn. at pH from 4 to 13. At this pH range, a sharp rise in absorption at 510 nm was obsd. with a concomitant increase of anodic current for O2 evolution. Using pyrophosphate as a probe mol., the 510 nm absorption was attributable to Mn3+ on the surface of δ-MnO2. The onset potential of the H2O oxidn. current was const. at ∼1.5 V vs. SHE from pH 4 to pH 8, but sharply shifted to neg. at pH > 8. Strikingly, this behavior was well reproduced by the pH dependence of the onset of 510 nm absorption, indicating that Mn3+ acts as the precursor of H2O oxidn. Mn3+ is unstable at pH < 9 due to the disproportionation reaction giving Mn2+ and Mn4+, whereas it is effectively stabilized by the comproportionation of Mn2+ and Mn4+ in alk. conditions. Thus, the low activity of Mn oxides for H2O oxidn. under neutral conditions is most likely due to the inherent instability of Mn3+, whose accumulation at the surface of catalysts requires the electrochem. oxidn. of Mn2+ at a potential of ∼1.4 V. This new model suggests that the control of the disproportionation and comproportionation efficiencies of Mn3+ is essential for the development of Mn catalysts that afford H2O oxidn. with a small overpotential at neutral pH.
    26. 26
      Takashima, T.; Hashimoto, K.; Nakamura, R. Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation under Neutral Conditions. J. Am. Chem. Soc. 2012, 134, 18153– 18156,  DOI: 10.1021/ja306499n
      26
      Inhibition of Charge Disproportionation of MnO2 Electrocatalysts for Efficient Water Oxidation under Neutral Conditions
      Takashima, Toshihiro; Hashimoto, Kazuhito; Nakamura, Ryuhei
      Journal of the American Chemical Society (2012), 134 (44), 18153-18156CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The development of Mn-oxide electrocatalysts for the oxidn. of H2O to O2 was the subject of intensive researches not only for their importance as components of artificial photosynthetic systems, but also as O2-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidn. reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochem. techniques, the stabilization of surface-assocd. intermediate Mn3+ species relative to charge disproportionation is an effective strategy to lower the overpotential for H2O oxidn. by MnO2. The formation of N-Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO2 electrodes effectively stabilized the Mn3+ species, resulting in an ∼500-mV neg. shift of the onset potential for the O2 evolution reaction at neutral pH.
    27. 27
      Zaharieva, I.; Chernev, P.; Risch, M.; Klingan, K.; Kohlhoff, M.; Fischer, A.; Dau, H. Electrosynthesis, Functional, and Structural Characterization of a Water-Oxidizing Manganese Oxide. Energy Environ. Sci. 2012, 5, 7081– 7089,  DOI: 10.1039/c2ee21191b
      27
      Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide
      Zaharieva, Ivelina; Chernev, Petko; Risch, Marcel; Klingan, Katharina; Kohlhoff, Mike; Fischer, Anna; Dau, Holger
      Energy & Environmental Science (2012), 5 (5), 7081-7089CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      In the sustainable prodn. of non-fossil fuels, water oxidn. is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochem. water oxidn. at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at const. anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biol. paragon, the water-splitting Mn4Ca complex of photosynthesis, with respect to mean Mn oxidn. state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidn., we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di-μ-oxo bridging between Mn ions is obsd. However in the MnCat, the voltage-cycling protocol resulted in formation of MnIII sites and prevented formation of well-ordered and unreactive MnIVO2. Structure-function relations in Mn-based water-oxidn. catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technol. use.
    28. 28
      Ramírez, A.; Hillebrand, P.; Stellmach, D.; May, M. M.; Bogdanoff, P.; Fiechter, S. Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water. J. Phys. Chem. C 2014, 118, 14073– 14081,  DOI: 10.1021/jp500939d
      28
      Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water
      Ramirez, Alejandra; Hillebrand, Philipp; Stellmach, Diana; May, Matthias M.; Bogdanoff, Peter; Fiechter, Sebastian
      Journal of Physical Chemistry C (2014), 118 (26), 14073-14081CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Different manganese oxide phases were prepd. as thin films to elucidate their structure-function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnOx films anodically deposited on F:SnO2/glass and annealed at different temps. (to improve film adherence and crystallinity) were tested in neutral and alk. electrolytes. Differential electrochem. mass spectroscopy showed that the anodic current correlated well with the onset of the expected oxygen evolution, where in 1 M KOH, the anodic current of cryst. α-Mn2O3 films was detd. to onset at an overpotential (η) of 170 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE. Amorphous MnOx films heated at 573 K (MnOx-573 K) were found to improve their adherence to F:SnO2/glass substrate after heat treatment with a slight crystn. detected by Raman spectroscopy. The onset of water oxidn. of MnOx-573 K films was identified at η = 230 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE (1 M KOH). The least active of the investigated manganese oxides was Mn3O4 with an onset at η = 290 mVRHE (at J = 0.1 mA/cm2) and current densities of ca. 10 mA/cm2 at η = 570 mVRHE (1 M KOH). In neutral soln. (1 M KPi), a similar tendency was obsd. with the lowest overpotential found for α-Mn2O3 followed by MnOx-573 K and Mn3O4. XPS revealed that after electrochem. treatment, the surfaces of the manganese oxide electrodes exhibited oxidn. of Mn II and Mn III toward Mn IV under oxygen evolving conditions. In the case of α-Mn2O3 and MnOx-573 K, the manganese oxidn. was found to be reversible in KPi when switching the potential above and below the oxygen evolution reaction (OER) threshold potential. Furthermore, SEM (SEM) images displayed the presence of an amorphous phase on top of all manganese oxide films here tested after oxygen evolution. The results indicate that structural changes played an important role in the catalytic activity of the manganese oxides, in addn. to oxidn. states, a large variety of Mn-O bond lengths and a high concn. of oxygen point defects. Thus, compared to Mn3O4, cryst. α-Mn2O3 and MnOx-573 K are the most efficient catalyst for water oxidn. in the manganese-oxygen system.
    29. 29
      Gorlin, Y.; Lassalle-Kaiser, B.; Benck, J. D.; Gul, S.; Webb, S. M.; Yachandra, V. K.; Yano, J.; Jaramillo, T. F. In Situ X-Ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 8525– 8534,  DOI: 10.1021/ja3104632
      29
      In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction
      Gorlin, Yelena; Lassalle-Kaiser, Benedikt; Benck, Jesse D.; Gul, Sheraz; Webb, Samuel M.; Yachandra, Vittal K.; Yano, Junko; Jaramillo, Thomas F.
      Journal of the American Chemical Society (2013), 135 (23), 8525-8534CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In situ x-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochem. systems, with the ability to elucidate the chem. nature of electrocatalysts under reaction conditions. The authors perform in situ XAS measurements on a bifunctional Mn oxide (MnOx) catalyst with high electrochem. activity for the O redn. reaction (ORR) and the O evolution reaction (OER). Using x-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), exposure to an ORR-relevant potential of 0.7 V vs. RHE produces a disordered MnII3,III,IIIO4 phase with negligible contributions from other phases. After the potential is increased to a highly anodic value of 1.8 V vs. RHE, relevant to the OER, the authors observe an oxidn. of ∼80% of the catalytic thin film to form a mixed MnIII,IV oxide, while the remaining 20% of the film consists of a less oxidized phase, likely corresponding to unchanged MnII3,III,IIIO4. XAS and electrochem. characterization of two thin film catalysts with different MnOx thicknesses reveals no significant influence of thickness on the measured oxidn. states, at either ORR or OER potentials, but demonstrates that the OER activity scales with film thickness. Probably the films have porous structure, which does not restrict electrocatalysis to the top geometric layer of the film. As the portion of the catalyst film that is most likely to be oxidized at the high potentials necessary for the OER is that which is closest to the electrolyte interface, the authors hypothesize that the MnIII,IV oxide, rather than MnII3,III,IIIO4, is the phase pertinent to the obsd. OER activity.
    30. 30
      Huynh, M.; Bediako, D. K.; Nocera, D. G. A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid. J. Am. Chem. Soc. 2014, 136, 6002– 6010,  DOI: 10.1021/ja413147e
      30
      A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid
      Huynh, Michael; Bediako, D. Kwabena; Nocera, Daniel G.
      Journal of the American Chemical Society (2014), 136 (16), 6002-6010CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      First-row metals were a target for the development of O evolution reaction (OER) catalysts because they comprise noncrit. elements. The authors now report a comprehensive electrochem. characterization of Mn oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissoln. during turnover. To study this process in detail, the O evolution mechanism of MnOx was studied electrokinetically over a pH range spanning acidic, neutral, and alk. conditions. In the alk. pH regime, a ∼ 60 mV/decade Tafel slope and inverse 1st-order dependence on proton concn. were obsd., whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concn. The results reflect two competing mechanisms: a 1-electron 1-proton PCET pathway that is dominant under alk. conditions and a Mn3+ disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of H2O oxidn. as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.
    31. 31
      Huynh, M.; Shi, C.; Billinge, S. J. L.; Nocera, D. G. Nature of Activated Manganese Oxide for Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 14887– 14904,  DOI: 10.1021/jacs.5b06382
      31
      Nature of Activated Manganese Oxide for Oxygen Evolution
      Huynh, Michael; Shi, Chenyang; Billinge, Simon J. L.; Nocera, Daniel G.
      Journal of the American Chemical Society (2015), 137 (47), 14887-14904CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Electrodeposited manganese oxide films (MnOx) are promising stable oxygen evolution catalysts. They are able to catalyze the oxygen evolution reaction in acidic solns. but with only modest activity when prepd. by const. anodic potential deposition. We now show that the performance of these catalysts is improved when they are "activated" by potential cycling protocols, as measured by Tafel anal. (where lower slope is better): upon activation the Tafel slope decreases from ∼120 to ∼70 mV/decade in neutral conditions and from ∼650 to ∼90 mV/decade in acidic solns. Electrochem., spectroscopic, and structural methods were employed to study the activation process and support a mechanism where the original birnessite-like MnOx (δ-MnO2) undergoes a phase change, induced by comproportionation with cathodically generated Mn(OH)2, to a hausmannite-like intermediate (α-Mn3O4). Subsequent anodic conditioning from voltage cycling or water oxidn. produces a disordered birnessite-like phase, which is highly active for oxygen evolution. At pH 2.5, the c.d. of activated MnOx (at an overpotential of 600 mV) is 2 orders of magnitude higher than that of the original MnOx and begins to approach that of Ru and Ir oxides in acid.
    32. 32
      Morgan Chan, Z.; Kitchaev, D. A.; Nelson Weker, J.; Schnedermann, C.; Lim, K.; Ceder, G.; Tumas, W.; Toney, M. F.; Nocera, D. G. Electrochemical Trapping of Metastable Mn3+ Ions for Activation of MnO2 Oxygen Evolution Catalysts. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, E5261– E5268,  DOI: 10.1073/pnas.1722235115
      There is no corresponding record for this reference.
    33. 33
      Chinnadurai, D.; Nallal, M.; Kim, H.-J.; Li, O. L.; Park, K. H.; Prabakar, K. Mn3+ Active Surface Site Enriched Manganese Phosphate Nano-Polyhedrons for Enhanced Bifunctional Oxygen Electrocatalyst. ChemCatChem 2020, 12, 2348– 2355,  DOI: 10.1002/cctc.202000164
      33
      Mn3+ Active Surface Site Enriched Manganese Phosphate Nano-polyhedrons for Enhanced Bifunctional Oxygen Electrocatalyst
      Chinnadurai, Deviprasath; Nallal, Muthuchamy; Kim, Hee-Je; Li, Oi Lun; Park, Kang Hyun; Prabakar, Kandasamy
      ChemCatChem (2020), 12 (8), 2348-2355CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Manganese-based electrocatalyst has a great attention for the oxygen evolution reaction (OER) and oxygen redn. reaction (ORR) applications, since the discovery of active center in nature photosynthesis system. The Mn oxidn. state optimization and structural defect engineering are essential to get a highly active Mn-based catalytic materials. MnP showed a good water oxidn. activity with a lower overpotential of 286 mV to reach the c.d. of 10 mA/cm2 and a Tafel slope of 76 mV/dec. Also, the electron transfer no. calcd. from both the rotating disk electrode and rotating ring-disk electrode techniques is a quasi-4 electron transfer process with an onset and half-wave potential of 0.998 V and 0.936 V vs. RHE resp. MnP achieved a higher limiting kinetic current of 5.7 mA/cm2 and a very low H2O2 yield of 1.6%. Chronoamperometry and cyclic voltammetry studies confirmed the long-term stability and durability of the prepd. catalyst. The variance metrics ΔE [Ej10-Ej-3] is used to est. the overall activity from the p.d. between OER overpotential at 10 mA/cm2 and ORR kinetic current at 3 mA/cm2. MnP shows very low ΔE (0.58 V) which demonstrate an efficient bifunctional activity in ORR and OER reactions. This work might shed new light on the development of MnP based bifunctional oxygen electrocatalyst.
    34. 34
      Shao, C.; Yin, K.; Liao, F.; Zhu, W.; Shi, H.; Shao, M. Rod-Shaped α-MnO2 Electrocatalysts with High Mn3+ Content for Oxygen Reduction Reaction and Zn-Air Battery. J. Alloys Compd. 2021, 860, 158427,  DOI: 10.1016/j.jallcom.2020.158427
      34
      Rod-shaped α-MnO2 electrocatalysts with high Mn3+ content for oxygen reduction reaction and Zn-air battery
      Shao, Chenrui; Yin, Kui; Liao, Fan; Zhu, Wenxiang; Shi, Huixian; Shao, Mingwang
      Journal of Alloys and Compounds (2021), 860 (), 158427CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      Manganese dioxide (MnO2) is a potential efficient candidate for the catalysis of oxygen redn. reactions (ORR) in alk. electrolyte. In this work, potassium permanganate (KMnO4) was reduced by silicon nanowires (SiNWs) and hydrofluoric acid (HF) to prep. α-MnO2 with a rod-like structure. The α-MnO2 prepd. at 120°C (α-MnO2-120) demonstrated excellent activity for ORR and had an oxygen redn. pathway of four-electron. In 0.1 M KOH electrolyte, it showed a large half-wave potential of 0.85 V vs. RHE and a high initial potential of 1.01 V vs. RHE. The diffusion-limiting current of α-MnO2-120 catalyst (7.0 mA cm-2) is also quite stable. The excellent ORR activity of α-MnO2-120 is attributed to the high Mn3+ content and sufficient oxygen vacancies. In addn., the zinc-air battery using the α-MnO2-120 catalyst displays an open circuit potential of 1.27 V, a max. power d. of 240 mW cm-2 at a c.d. of 0.33 Acm-2 and superior stability.
    35. 35
      Melder, J.; Mebs, S.; Heizmann, P. A.; Lang, R.; Dau, H.; Kurz, P. Carbon Fibre Paper Coated by a Layered Manganese Oxide: A Nano-Structured Electrocatalyst for Water-Oxidation with High Activity over a Very Wide pH Range. J. Mater. Chem. A 2019, 7, 25333– 25346,  DOI: 10.1039/c9ta08804k
      35
      Carbon fibre paper coated by a layered manganese oxide: a nano-structured electrocatalyst for water-oxidation with high activity over a very wide pH range
      Melder, Jens; Mebs, Stefan; Heizmann, Philipp A.; Lang, Rebekka; Dau, Holger; Kurz, Philipp
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (44), 25333-25346CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Manganese oxide coated carbon fiber paper electrodes (MnOx/CFP), prepd. via an easily scalable redox-deposition route, have been reported as promising materials for electrochem. water oxidn. catalysis (WOC). In the study presented here, the catalyst mass-dependence of the activity of such nanostructured WOC electrodes and their activity and stability in phosphate buffer electrolytes over nearly the entire pH range (pH 1-14) was explored. For all pH values, current densities of >1 mA cm-2 at overpotentials η of 350-500 mV were obsd., which are very good values for electrodes contg. only earth-abundant elements (Mn, O and C in this case). The very decent performance obsd. for the mildly acidic pH regime (pH 2-6) was esp. noteworthy. For the dependence of η on the pH, an ideal Nernstian behavior was detected for the pH window pH 1-10. Above pH 10, a pronounced deviation from the expected trend was found, as most electro-kinetic parameters indicated even higher activities for the strongly alk. regime. Concerning anode stabilities, current-densities of 2 mA cm-2 could be maintained at all studied pHs over at least 24 h of continuous operation. Pre- and post-operando spectroscopic analyses (e.g. vibrational and X-ray absorption spectroscopy, SEM) revealed only minor changes of the catalyst structure, compn., morphol. or the av. Mn oxidn. state induced by the electrocatalytic operation, which confirms the good stabilities found in the electrochem. measurements. Thus, we report on an earth-abundant, easily-prepd. type of WOC electrode, which exhibits promising activities and stabilities for applications in alk., neutral and even acidic electrolytes.
    36. 36
      Melder, J.; Bogdanoff, P.; Zaharieva, I.; Fiechter, S.; Dau, H.; Kurz, P. Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions. Z. Phys. Chem. 2020, 234, 925– 978,  DOI: 10.1515/zpch-2019-1491
      36
      Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions
      Melder, Jens; Bogdanoff, Peter; Zaharieva, Ivelina; Fiechter, Sebastian; Dau, Holger; Kurz, Philipp
      Zeitschrift fuer Physikalische Chemie (Muenchen, Germany) (2020), 234 (5), 925-978CODEN: ZPCFAX; ISSN:0942-9352. (Oldenbourg Wissenschaftsverlag GmbH)
      A review. The efficient catalysis of the four-electron oxidn. of water to mol. oxygen is a central challenge for the development of devices for the prodn. of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biol. catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides ("MnOx") for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and compn. of the "MnOx" is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are esp. good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been obsd. that the electrocatalytic performance is also largely dependent on the electrolyte's compn. and pH and that a no. of equil. accompany the catalytic process, resulting in "adaptive changes" of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidn. electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the compn. of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates obsd. for iridium, ruthenium or nickel-iron oxide anodes in acidic or alk. solns., resp. So at the end of the article, two fundamental questions will be addressed: (1) are there tech. applications where MnOx materials could actually be the first choice as OER electrocatalysts and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: "Why did nature choose manganese to make oxygen".
    37. 37
      Deibert, B. J.; Zhang, J.; Smith, P. F.; Chapman, K. W.; Rangan, S.; Banerjee, D.; Tan, K.; Wang, H.; Pasquale, N.; Chen, F.; Lee, K.-B.; Dismukes, G. C.; Chabal, Y. J.; Li, J. Surface and Structural Investigation of a MnOx Birnessite-Type Water Oxidation Catalyst Formed under Photocatalytic Conditions. Chem.─Eur. J. 2015, 21, 14218– 14228,  DOI: 10.1002/chem.201501930
      37
      Surface and Structural Investigation of a MnOx Birnessite-Type Water Oxidation Catalyst Formed under Photocatalytic Conditions
      Deibert, Benjamin J.; Zhang, Jingming; Smith, Paul F.; Chapman, Karena W.; Rangan, Sylvie; Banerjee, Debasis; Tan, Kui; Wang, Hao; Pasquale, Nicholas; Chen, Feng; Lee, Ki-Bum; Dismukes, G. Charles; Chabal, Yves J.; Li, Jing
      Chemistry - A European Journal (2015), 21 (40), 14218-14228CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Catalytically active MnOx species have been reported to form in situ from various Mn-complexes during electrocatalytic and soln.-based water oxidn. when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnOx catalysts form without supports in situ under photocatalytic conditions. Our most active 4MnOx catalyst (∼0.84 mmol O2 mol Mn-1 s-1) forms from a Mn4O4 bearing a metal-org. framework. 4MnOx is characterized by pair distribution function anal. (PDF), Raman spectroscopy, and HR-TEM as a disordered, layered Mn-oxide with high surface area (216 m2g-1) and small regions of crystallinity and layer flexibility. In contrast, the SMnOx formed from Mn2+ salt gives an amorphous species of lower surface area (80 m2g-1) and lower activity (∼0.15 mmol O2 mol Mn-1 s-1). We compare these catalysts to cryst. hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p3/2 core levels detects enriched Mn3+ and Mn2+ content on the surfaces, which indicates possible disproportionation/comproportionation surface equil.
    38. 38
      Sadtler, B.; Burgos, S. P.; Batara, N. A.; Beardslee, J. A.; Atwater, H. A.; Lewis, N. S. Phototropic Growth Control of Nanoscale Pattern Formation in Photoelectrodeposited Se–Te Films. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 19707– 19712,  DOI: 10.1073/pnas.1315539110
      38
      Phototropic growth control of nanoscale pattern formation in photoelectrodeposited Se-Te films
      Sadtler, Bryce; Burgos, Stanley P.; Batara, Nicolas A.; Beardslee, Joseph A.; Atwater, Harry A.; Lewis, Nathan S.
      Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (49), 19707-19712,S19707/1-S19707/2CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Photoresponsive materials that adapt their morphologies, growth directions, and growth rates dynamically in response to the local incident electromagnetic field would provide a remarkable route to the synthesis of complex 3D mesostructures via feedback between illumination and the structure that develops under optical excitation. We report the spontaneous development of ordered, nanoscale lamellar patterns in electrodeposited Se-Te alloy films grown under noncoherent, uniform illumination on unpatterned substrates in an isotropic electrolyte soln. These inorg. nanostructures exhibited phototropic growth in which lamellar stripes grew toward the incident light source, adopted an orientation parallel to the light polarization direction with a period controlled by the illumination wavelength, and showed an increased growth rate with increasing light intensity. Furthermore, the patterns responded dynamically to changes during growth in the polarization, wavelength, and angle of the incident light, enabling the template-free and pattern-free synthesis, on a variety of substrates, of woodpile, spiral, branched, or zigzag structures, along with dynamically directed growth toward a noncoherent, uniform intensity light source. Full-wave electromagnetic simulations in combination with Monte Carlo growth simulations were used to model light-matter interactions in the Se-Te films and produced a model for the morphol. evolution of the lamellar structures under phototropic growth conditions. The expts. and simulations are consistent with a phototropic growth mechanism in which the optical near-field intensity profile selects and reinforces the dominant morphol. mode in the emergent nanoscale patterns.
    39. 39
      Carim, A. I.; Batara, N. A.; Premkumar, A.; Atwater, H. A.; Lewis, N. S. Self-Optimizing Photoelectrochemical Growth of Nanopatterned Se–Te Films in Response to the Spectral Distribution of Incident Illumination. Nano Lett. 2015, 15, 7071– 7076,  DOI: 10.1021/acs.nanolett.5b03137
      39
      Self-Optimizing Photoelectrochemical Growth of Nanopatterned Se-Te Films in Response to the Spectral Distribution of Incident Illumination
      Carim, Azhar I.; Batara, Nicolas A.; Premkumar, Anjali; Atwater, Harry A.; Lewis, Nathan S.
      Nano Letters (2015), 15 (10), 7071-7076CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Photoelectrochem. growth of Se-Te films spontaneously produces highly ordered, nanoscale lamellar morphologies with periodicities that can be tuned by varying the illumination wavelength during deposition. This phenomenon was characterized further herein by detg. the morphologies of photoelectrodeposited Se-Te films in response to tailored spectral illumination profiles. Se-Te films grown under illumination from 4 different sources, having similar av. wavelengths but having spectral bandwidths that spanned several orders of magnitude, all nevertheless produced similar structures which had a single, common periodicity as quant. identified via Fourier anal. Film deposition using simultaneous illumination from 2 narrowband sources, which differed in av. wavelength by several hundred nanometers, resulted in a structure with only a single periodicity intermediate between the periods obsd. when either source alone was used. This single periodicity could be varied by manipulating the relative intensity of the 2 sources. An iterative model that combined full-wave electromagnetic effects with Monte Carlo growth simulations, and that considered only the fundamental light-material interactions during deposition, was in accord with the morphologies obsd. exptl. Simulations of light absorption and concn. in idealized lamellar arrays, in conjunction with all of the available data, addnl. indicated that a self-optimization of the periodicity of the nanoscale pattern, resulting in the maximization of the anisotropy of interfacial light absorption in the 3-dimensional structure, is consistent with the obsd. growth process of such films.
    40. 40
      Lowe, J. M.; Yan, Q.; Benamara, M.; Coridan, R. H. Direct Photolithographic Patterning of Cuprous Oxide Thin Films Via Photoelectrodeposition. J. Mater. Chem. A 2017, 5, 21765– 21772,  DOI: 10.1039/c7ta05321e
      40
      Direct photolithographic patterning of cuprous oxide thin films via photoelectrodeposition
      Lowe, James M.; Yan, Qigeng; Benamara, Mourad; Coridan, Robert H.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2017), 5 (41), 21765-21772CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      External fields can be used to regulate the morphol. and chem. properties of electrochem. synthesized materials. One example is phototropism, where the coaction of light absorption and carrier generation can induce controllable variations in the material as it grows under illumination. Here we describe the photoelectrodeposition of photocathodic cuprous oxide (Cu2O). Illuminating the growing film with photon energies in excess of the band gap of Cu2O results in nanoscale morphol. changes in the structure of thin films and intrinsically dopes the material during growth. The result is a 'black' Cu2O film that is chem. distinct but crystallog. identical to an ordinary film grown in the dark. The flat band potential of the film is controlled by the growth illumination intensity and the photodoping is reversible under thermal oxidn. We explore the nature of the intrinsic dopant, the emergence of nanocryst. Cu metal inclusions in the Cu2O matrix, and the potential of for this effect to be used in the fabrication of new semiconductor heterostructures. This chem. distinction between the native and black Cu2O can be exploited to pattern films with a photomask as an example of direct photolithog.
    41. 41
      Tan, C.; Qin, C.; Sadtler, B. Light-Directed Growth of Metal and Semiconductor Nanostructures. J. Mater. Chem. C 2017, 5, 5628– 5642,  DOI: 10.1039/c7tc00379j
      41
      Light-directed growth of metal and semiconductor nanostructures
      Tan, Che; Qin, Chu; Sadtler, Bryce
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (23), 5628-5642CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      A review. External parameters such as temp., pressure, and concn. are typically used to control chem. transformations in mols. and materials. Many classes of inorg. materials are also responsive to external stimuli, such as elec., magnetic, and electromagnetic fields. This Review article will explore examples of how light can guide the growth of metal and semiconductor nanostructures. Plasmon excitation in metals and electronic excitation in semiconductors mediate interfacial charge-transfer reactions to alter the growth rates of these materials at the nanoscale. Light-driven growth of inorg. materials provides a route to achieve morphol. control over colloidal particles and nanostructured films for potential applications in solar energy conversion, photonic computing, and holog.
    42. 42
      Carim, A. I.; Hamann, K. R.; Batara, N. A.; Thompson, J. R.; Atwater, H. A.; Lewis, N. S. Template-Free Synthesis of Periodic Three-Dimensional PbSe Nanostructures via Photoelectrodeposition. J. Am. Chem. Soc. 2018, 140, 6536– 6539,  DOI: 10.1021/jacs.8b02931
      42
      Template-Free Synthesis of Periodic Three-Dimensional PbSe Nanostructures via Photoelectrodeposition
      Carim, Azhar I.; Hamann, Kathryn R.; Batara, Nicolas A.; Thompson, Jonathan R.; Atwater, Harry A.; Lewis, Nathan S.
      Journal of the American Chemical Society (2018), 140 (21), 6536-6539CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Highly periodic, geometrically directed, anisotropic Se-Pb films have been synthesized at room temp. from an isotropic aq. soln. without the use of phys. templates by photoelectrodeposition using a series of discrete input illumination polarizations and wavelengths from an unstructured, uncorrelated, incoherent light source. Dark growth did not generate deposits with substantial long-range order, but growth using unpolarized illumination resulted in an ordered, nanoscale, mesh-type morphol. Linearly polarized illumination generated Se-Pb deposits that displayed an ordered, highly anisotropic lamellar pattern wherein the long axes of the lamellae were aligned parallel to the light polarization vector. The pitch of the lamellar features was proportional to the input light wavelength, as confirmed by Fourier anal. Full-wave electromagnetic and Monte Carlo growth simulations that incorporated only the fundamental light-matter interactions during growth successfully reproduced the exptl. obsd. morphologies and quant. matched the pattern periodicities. Electrochem. postprocessing of the as-deposited Se-Pb structures resulted in the generation of stoichiometric, cryst. PbSe while preserving the nanopatterned morphol., thus broadening the genus of materials that can be prepd. with controlled three-dimensional morphologies through maskless photoelectrodeposition.
    43. 43
      Meier, M. C.; Cheng, W.-H.; Atwater, H. A.; Lewis, N. S.; Carim, A. I. Inorganic Phototropism in Electrodeposition of Se–Te. J. Am. Chem. Soc. 2019, 141, 18658– 18661,  DOI: 10.1021/jacs.9b10579
      43
      Inorganic Phototropism in Electrodeposition of Se-Te
      Meier, Madeline C.; Cheng, Wen-Hui; Atwater, Harry A.; Lewis, Nathan S.; Carim, Azhar I.
      Journal of the American Chemical Society (2019), 141 (47), 18658-18661CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Photoelectrochem. deposition of Se-Te on isolated Au islands using an unstructured, incoherent beam of light produces growth of Se-Te alloy toward the direction of the incident light beam. Full-wave electromagnetic simulations of light absorption indicated that the induced spatial growth anisotropy was a function of asym. absorption in the evolving deposit. Inorg. phototropic growth is analogous to biol. systems such as palm trees that exhibit phototropic growth wherein phys. extension of the plant guides the crown toward the time-averaged position of the sun, to maximize solar harvesting.
    44. 44
      Qin, C.; Campbell, B. M.; Shen, M.; Zhao, T.; Sadtler, B. Light-Driven, Facet-Selective Transformation of Cuprous Oxide Microcrystals to Hollow Copper Nanoshells. Chem. Mater. 2019, 31, 8000– 8011,  DOI: 10.1021/acs.chemmater.9b02240
      44
      Light-driven, facet-selective transformation of cuprous oxide microcrystals to hollow copper nanoshells
      Qin, Chu; Campbell, Brandon M.; Shen, Meikun; Zhao, Tong; Sadtler, Bryce
      Chemistry of Materials (2019), 31 (19), 8000-8011CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Photoexcitation can be used to control the compn. and nanoscale morphol. of inorg. materials. Here we report the photoinduced transformation of faceted cuprous oxide (Cu2O) microcrystals to hollow particles consisting of an inner region of cuprous oxide and an outer shell of copper metal. When cuprous oxide microcrystals with mixed {100} and {111} facets are held at a neg. bias (-1.0 V vs Ag/AgCl) in a soln. of sodium hydroxide (NaOH), light mediates the growth of copper metal selectively on the {100} facets while the crystal interior is etched at {111} facets. Conformal Cu layers grow to connect at vertices of the cuboctahedral microcrystals and form a hollow shell. This process is only obsd. in the presence of illumination. Without an applied bias the {100} facets are preferentially etched under illumination in the same NaOH soln. We propose this light-driven, facet-selective transformation arises from the potential-dependent structure and energetics of the semiconductor/electrolyte interface, which lead to facet-selective extn. of photogenerated electrons from the {100} facets when the applied bias is more neg. than the flat-band potential of the Cu2O microcrystals. Growth of the Cu shell protects the {100} facets while the {111} facets are chem. etched in the presence of oxygen and hydroxide.
    45. 45
      Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. Photocurrent Generation from Semiconducting Manganese Oxide Nanosheets in Response to Visible Light. J. Phys. Chem. B 2005, 109, 9651– 9655,  DOI: 10.1021/jp0500485
      45
      Photocurrent Generation from Semiconducting Manganese Oxide Nanosheets in Response to Visible Light
      Sakai, Nobuyuki; Ebina, Yasuo; Takada, Kazunori; Sasaki, Takayoshi
      Journal of Physical Chemistry B (2005), 109 (19), 9651-9655CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Unilamellar nanosheet crystallites of manganese oxide generated the anodic photocurrent under visible light irradn. (λ < 500 nm), while the nanosheets themselves were stable as revealed by in-plane XRD and UV-visible absorption spectra. The band gap energy was estd. to be 2.23 eV on the basis of the photocurrent action spectrum. The mol. thickness of ∼0.5 nm may facilitate the charge sepn. of excited electrons and holes, which is generally very difficult for strongly localized d-d transitions. The monolayer film of MnO2 nanosheets exhibited the incident photon-to-electron conversion efficiency of 0.16% in response to the monochromatic light irradn. (λ = 400 nm), which is comparable to those for sensitization of monolayer dyes adsorbed on a flat single-crystal surface. The efficiency declined with increasing the layer no. of MnO2 nanosheets, although the optical absorption was enhanced. The recombination of the excited electron-hole pairs may become dominant when the carriers need to migrate a longer distance than 1 layer through multilayered nanosheets.
    46. 46
      Pinaud, B. A.; Chen, Z.; Abram, D. N.; Jaramillo, T. F. Thin Films of Sodium Birnessite-Type MnO2: Optical Properties, Electronic Band Structure, and Solar Photoelectrochemistry. J. Phys. Chem. C 2011, 115, 11830– 11838,  DOI: 10.1021/jp200015p
      46
      Thin Films of Sodium Birnessite-Type MnO2: Optical Properties, Electronic Band Structure, and Solar Photoelectrochemistry
      Pinaud, Blaise A.; Chen, Zhebo; Abram, David N.; Jaramillo, Thomas F.
      Journal of Physical Chemistry C (2011), 115 (23), 11830-11838CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Inexpensive and earth-abundant, Mn oxides (MnxOy) have attracted considerable attention for catalysis, but fewer efforts have focused on their semiconducting properties. Cryst. Na birnessite-type MnO2 thin films were studied for their surface and bulk chem. in the context of H prodn. by photoelectrochem. H2O splitting. Thin films were synthesized by electrodeposition onto F-doped Sn oxide (FTO) substrates, imaged by SEM, and characterized by XRD as well as XPS and UV-visible spectroscopy. Three different electrochem. methods (illuminated open circuit potential, potential of photocurrent onset, and Mott-Schottky plots) were used to probe the flatband potential required to construct a band diagram for the material. Photostability, cond., and band structure are discussed as potential causes of the low external quantum efficiency (<1%) for the birnessite-type MnO2 photoanode. The position of the conduction band well below the H evolution potential likely mitigates this material's potential use in a single absorber configuration, but its chem., optical, and electronic characteristics as shown in this work may be well-suited for a photoanode in a tandem device.
    47. 47
      Hsu, Y.-K.; Chen, Y.-C.; Lin, Y.-G.; Chen, L.-C.; Chen, K.-H. Birnessite-Type Manganese Oxides Nanosheets with Hole Acceptor Assisted Photoelectrochemical Activity in Response to Visible Light. J. Mater. Chem. 2012, 22, 2733– 2739,  DOI: 10.1039/c1jm14355g
      47
      Birnessite-type manganese oxides nanosheets with hole acceptor assisted photoelectrochemical activity in response to visible light
      Hsu, Yu-Kuei; Chen, Ying-Chu; Lin, Yan-Gu; Chen, Li-Chyong; Chen, Kuei-Hsien
      Journal of Materials Chemistry (2012), 22 (6), 2733-2739CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      A cost-effective and simple electroplating technique has been developed to prep. layered manganese oxide (MnO2) arrays as a promising material for solar hydrogen prodn. and wastewater treatment by photoelectrochem. process. The microstructure of these MnO2 nanosheets can be referenced to Birnessite-type, as characterized by Raman spectra and transmission electron microscopy. The bandgap energy of the as-grown nanosheets detd. from UV-visible spectroscopy is ∼2.1 eV. Mott-Schottky plots show the flat band potential of the MnO2 nanosheets to be -0.01 V and a donor concn. of 4.68 × 1020 cm-3. Remarkable photocurrent in response to visible light is obsd. in the presence of hole acceptors, such as sodium formate and methanol, which efficiently suppress the recombination loss of electron-hole pairs from localized d-d transitions within manganese ion. Meanwhile, the transient photocurrent-time responses and the effect of different hole acceptors on photoelectrochem. activity are studied with an increase of resp. hole acceptor concn., and the results reveal the crit. role in the process of absorption and decompn. of the hole acceptor. Significantly, the MnO2 nanosheets exhibit an incident photon-to-electron conversion efficiency of 7% in response to the monochromatic wavelength of 400 nm, which is comparable to that from hematite (α-Fe2O3). These results demonstrate the nanoporous MnO2 nanosheets have great potential in solar hydrogen applications and org. pollutant cleaning.
    48. 48
      Sunda, W. G.; Huntsman, S. A. Photoreduction of Manganese Oxides in Seawater. Mar. Chem. 1994, 46, 133– 152,  DOI: 10.1016/0304-4203(94)90051-5
      48
      Photoreduction of manganese oxides in seawater
      Sunda, William G.; Huntsman, Susan A.
      Marine Chemistry (1994), 46 (1-2), 133-52CODEN: MRCHBD; ISSN:0304-4203.
      Expts. were conducted on the photoreactive dissoln. of 54Mn-labeled synthetic oxides, prepd. from MnO42- oxidn. of 54Mn(II), and natural labeled oxides formed in seawater from microbial oxidn. of 54Mn(II). Sunlight increased the dissoln. rate of synthetic oxides in seawater, an effect that increased with the duration of light exposure. The photodissoln. of these oxides was found to result primarily from Mn redn. by H2O2, produced in seawater from the photoredn. of O by dissolved org. matter. This conclusion was based on the previously obsd. marked stimulation of photodissoln. by added humic compds., the obsd. reductive dissoln. of the oxides by added H2O2 and on the almost complete reversal of photodissoln. by enzymic (catalase) removal of H2O2. Sunlight had an even larger stimulatory effect on the reductive dissoln. of 54Mn-labeled natural oxides. It increased specific dissoln. rates to values of 6-13% h-1, 6-70 times higher than rates in the dark. In contrast to synthetic oxides, rates for natural oxides did not increase measurably with the duration of light exposure, were not appreciably altered by humic acid addn. or by photolytic removal of org. matter, and were not substantially reduced by catalase addn. Furthermore, rates for reductive dissoln. of natural oxides by H2O2 were only about 1/6 of those for synthetic oxides. These results indicate that the photoreductive dissoln. of natural oxides in seawater is not primarily related to the photoprodn. of H2O2, although such prodn. appears to account for a small portion (ca. 10-20%) of the overall effect. Instead, both the chromophore and the reductant(s) involved in the reaction appear to reside with the bacterial/Mn oxide aggregates themselves. Although several possibilities can be postulated, the exact mechanism of the photochem. reaction remains obscure.
    49. 49
      Sherman, D. M. Electronic Structures of Iron(III) and Manganese(IV) (Hydr)Oxide Minerals: Thermodynamics of Photochemical Reductive Dissolution in Aquatic Environments. Geochim. Cosmochim. Acta 2005, 69, 3249– 3255,  DOI: 10.1016/j.gca.2005.01.023
      49
      Electronic structures of iron(III) and manganese(IV) (hydr)oxide minerals: Thermodynamics of photochemical reductive dissolution in aquatic environments
      Sherman, David M.
      Geochimica et Cosmochimica Acta (2005), 69 (13), 3249-3255CODEN: GCACAK; ISSN:0016-7037. (Elsevier Inc.)
      The reactivity of Fe-Mn (hydr)oxide minerals to sunlight-induced photochem. dissoln. is detd. by the electronic structure of the mineral-water interface. Oxygen K-edge absorption and emission spectra were used to det. the electronic structures of iron(III) (hydr)oxides (hematite, goethite, lepidocrocite, akaganeite and schwertmannite) and manganese(IV) oxides (pyrolusite, birnessite, cryptomelane). The band gaps in the iron(III) (hydr)oxide minerals are near 2.0-2.5 eV; the band gaps in the manganese (IV) oxide phases are 1.0-1.8 eV. Using published values for the electrochem. flat-band potential for hematite together with exptl. pHpzc values for the (hydr)oxides, it is possible to predict the electrochem. potentials of the conduction and valence bands in aq. solns. as a function of pH. The band potentials enable semiquant. predictions of the susceptibilities of these minerals to photochem. dissoln. in aq. solns. At pH 2 (e.g., acid-mine waters), photoredn. of iron(III) (hydr)oxides could yield millimolal concns. of aq. Fe2+ (assuming surface detachment of Fe2+ is not rate limiting). In seawater (pH 8.3), however, the direct photo-redn. of colloidal iron(III) (hydr)oxides to give nanomolal concns. of dissolved, uncomplexed, Fe2+ is not thermodynamically feasible. This supports the hypothesis that the apparent photodissoln. of iron(III) (hydr)oxides in marine systems results from Fe3+ redn. by photochem. produced superoxide. In contrast, the direct photoredn. of manganese oxides should be energetically feasible at pH 2 and 8.3.
    50. 50
      Kwon, K. D.; Refson, K.; Sposito, G. On the Role of Mn(IV) Vacancies in the Photoreductive Dissolution of Hexagonal Birnessite. Geochim. Cosmochim. Acta 2009, 73, 4142– 4150,  DOI: 10.1016/j.gca.2009.04.031
      50
      On the role of Mn(IV) vacancies in the photoreductive dissolution of hexagonal birnessite
      Kwon, Kideok D.; Refson, Keith; Sposito, Garrison
      Geochimica et Cosmochimica Acta (2009), 73 (14), 4142-4150CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
      Photoreductive dissoln. of layer type Mn(IV) oxides (birnessite) under sunlight illumination to form sol. Mn(II) has been obsd. in both field and lab. settings, leading to a consensus that this process is a key driver of the biogeochem. cycling of Mn in the euphotic zones of marine and freshwater ecosystems. However, the underlying mechanisms for the process remain unknown, although they have been linked to the semiconducting characteristics of hexagonal birnessite, the ubiquitous Mn(IV) oxide produced mainly by bacterial oxidn. of sol. Mn(II). One of the universal properties of this biogenic mineral is the presence of Mn(IV) vacancies, long-identified as strong adsorption sites for metal cations. The possible role of Mn vacancies in photoreductive dissoln. is investigated theor. using quantum mech. calcns. based on spin-polarized d. functional theory (DFT). DFT study demonstrates unequivocally that Mn vacancies significantly reduce the band-gap energy for hexagonal birnessite relative to a hypothetical vacancy-free MnO2 and thus would increase the concn. of photo-induced electrons available for Mn(IV) redn. upon illumination of the mineral by sunlight. Calcns. of the charge distribution in the presence of vacancies, although not fully conclusive, show a clear sepn. of photo-induced electrons and holes, implying a slow recombination of these charge-carriers that facilitates the two-electron redn. of Mn(IV) to Mn(II).
    51. 51
      Marafatto, F. F.; Strader, M. L.; Gonzalez-Holguera, J.; Schwartzberg, A.; Gilbert, B.; Peña, J. Rate and Mechanism of the Photoreduction of Birnessite (MnO2) Nanosheets. Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 4600– 4605,  DOI: 10.1073/pnas.1421018112
      51
      Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets
      Marafatto, Francesco Femi; Strader, Matthew L.; Gonzalez-Holguera, Julia; Schwartzberg, Adam; Gilbert, Benjamin; Pena, Jasquelin
      Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (15), 4600-4605CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The photoreductive dissoln. of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidn. of org. matter and fate of trace elements assocd. with Mn oxides, but the intrinsic rate and mechanism of mineral dissoln. in the absence of org. electron donors is unknown. The photoredn. of δ-MnO2 nanosheets is investigated at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradn. and quantified the yield and timescales of Mn(III) prodn. The study of transient intermediate states using time-resolved optical and x-ray absorption spectroscopy showed key roles for chem. distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. It is proposed that irreversible Mn redn. is coupled to hole-scavenging by surface water mols. or hydroxyl groups, with assocd. radical formation. This work demonstrates the importance of direct MnO2 photoredn. in environmental processes and provides a framework to test new hypotheses regarding the role of org. mols. and metal species in photochem. reactions with Mn oxide phases. The timescales for the prodn. and evolution of Mn(III) species and a catalytic role for interlayer Ca2+ identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidn.
    52. 52
      McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977– 16987,  DOI: 10.1021/ja407115p
      52
      Benchmarking heterogeneous electrocatalysts for the oygen evolution reaction
      McCrory, Charles C. L.; Jung, Suho; Peters, Jonas C.; Jaramillo, Thomas F.
      Journal of the American Chemical Society (2013), 135 (45), 16977-16987CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Objective evaluation of the activity of electrocatalysts for water oxidn. is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for detg. electrochem. active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a c.d. of 10 mA cm-2 per geometric area, approx. the c.d. expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can det. electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alk. solns.: CoOx, CoPi, CoFeOx, NiOx, NiCeOx, NiCoOx, NiCuOx, NiFeOx, and NiLaOx. The oxygen-evolving activity of an electrodeposited IrOx catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alk. soln., every non-noble metal system achieved 10 mA cm-2 current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrOx was unstable under oxidative conditions in acidic solns.
    53. 53
      Huynh, M.; Bediako, D. K.; Liu, Y.; Nocera, D. G. Nucleation and Growth Mechanisms of an Electrodeposited Manganese Oxide Oxygen Evolution Catalyst. J. Phys. Chem. C 2014, 118, 17142– 17152,  DOI: 10.1021/jp501768n
      53
      Nucleation and Growth Mechanisms of an Electrodeposited Manganese Oxide Oxygen Evolution Catalyst
      Huynh, Michael; Bediako, D. Kwabena; Liu, Yi; Nocera, Daniel G.
      Journal of Physical Chemistry C (2014), 118 (30), 17142-17152CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors study the mechanisms of nucleation and steady-state growth of a Mn oxide catalyst (MnOx) electrodeposited from Mn2+ solns. in a weakly basic electrolyte. Early catalyst growth was probed through chronoamperometry transients, which were fit to reveal a progressive nucleation mechanism for initial catalyst formation. Time-dependent at. force microscopy snapshots of the electrode surface reveal a rapid increase in nucleus size together with a sluggish rise in coverage, which is also characteristic of progressive nucleation. Electrochem. kinetic studies of the catalyst growth yield a Tafel slope of ∼2.3 × RT/2F and a rate law consisting of a 2nd-order and inverse 4th-order dependence on [Mn2+] and proton activity, resp. These results are consistent with a deposition mechanism involving rate-limiting disproportionation of aq. Mn3+, resolving a longstanding ambiguity surrounding the deposition of Mn oxides under nonacidic conditions.
    54. 54
      Nesbitt, H. W.; Banerjee, D. Interpretation of XPS Mn(2p) Spectra of Mn Oxyhydroxides and Constraints on the Mechanism of MnO2 Precipitation. Am. Mineral. 1998, 83, 305– 315,  DOI: 10.2138/am-1998-3-414
      54
      Interpretation of XPS Mn(2p) spectra of Mn oxyhydroxides and constraints on the mechanism of MnO2 precipitation
      Nesbitt, H. W.; Banderjee, D.
      American Mineralogist (1998), 83 (3-4), 305-315CODEN: AMMIAY; ISSN:0003-004X. (Mineralogical Society of America)
      Calcd. Mn(2p3/2) X-ray photoelectron spectra (XPS) of Mn2+, Mn3+, and Mn4+ free ions are strikingly similar to Mn(2p3/2) spectra of Mn2+-, Mn3+-, and Mn4+-oxides and oxyhydroxides, indicating that these ions adopt high spin states in MnO, manganite, and birnessite. The Mn(2p) peak structures reveal the presence of only Mn3+ in manganite but Mn2+, Mn3+, and Mn4+ are present in the near-surface of synthetic birnessite at about 5, 25, and 70%, resp. Pptn. of birnessite by reaction of Mn2+(aq) with an oxidant includes two electron transfer steps: (1) oxidn. of Mn2+(aq) to produce Mn3+-oxyhydroxide, an intermediate reaction product that forms on the surface of synthetic birnessite and (2) subsequent oxidn. of Mn3+-oxyhydroxide surface species to produce synthetic birnessite. Some surface Mn3+, however, remains unoxidized and is incorporated into birnessite. As for this synthesis (KMnO4 used as oxidant), oxidn. may not proceed to completion in natural settings (as O2 is the oxidant) leading to Mn3+ incorporation into Mn-oxides. The hypothesis explains the abundance of non-stoichiometric MnO2 phases in sedimentary environments. The MnO2 pptn. scheme proposed by Stumm and Morgan (1981) includes the surface species Mn2+·MnO2. This and other studies indicate that the reactive intermediate is a Mn3+-bearing surface species. The formation rate of birnessite is probably controlled by one of these redox reactions. The proposed rate expression of Davies and Morgan (1989), however, needs no modification provided surface area is a reasonable measure of the surfaces d. of the reactive intermediate.
    55. 55
      Gupta, R. P.; Sen, S. K. Calculation of Multiplet Structure of Core p-Vacancy Levels. Phys. Rev. B: Solid State 1974, 10, 71– 77,  DOI: 10.1103/physrevb.10.71
      55
      Calculation of multiplet structure of core p-vacancy levels
      Gupta, R. P.; Sen, S. K.
      Physical Review B: Solid State (1974), 10 (1), 71-7CODEN: PLRBAQ; ISSN:0556-2805.
      The spin-orbit and crystal-field effects were included. The cases studied are Mn2+ in MnF2 and the free ion Fe3+. For 3p vacancies these effects appear to be unimportant from the point of view of photoelectron spectroscopy owing to the limited resoln. of the technique. For 2p vacancies, the spin-orbit interaction is comparable to the electrostatic interaction between the electrons and both the interactions must be considered simultaneously. The conclusion is that the x-ray photoelectron spectra of inner-core electrons are more suitable for chem. anal. than those ofouter electrons which, in addn., contain large correlation and many-body effects.
    56. 56
      Gupta, R. P.; Sen, S. K. Calculation of Multiplet Structure of Core p-Vacancy Levels. II. Phys. Rev. B: Solid State 1975, 12, 15– 19,  DOI: 10.1103/physrevb.12.15
      56
      Calculation of multiplet structure of core p-vacancy levels. II
      Gupta, R. P.; Sen, S. K.
      Physical Review B: Solid State (1975), 12 (1), 15-19CODEN: PLRBAQ; ISSN:0556-2805.
      The multiplet structure of core 2p-vacancy levels for 36 ions belonging to 3d transition metals were calcd. and their plots presented. Only the ground-state configuration 2p53dn (n = 1,...,9) is considered for each ion. The spin-orbit interaction was incorporated exactly, but the crystal-field effect is ignored. While there is general agreement with the available exptl. works, higher-energy resolution in x-ray-photoelectron measurements is necessary for detailed comparison of the spectra presented here. On the theor. side inclusion of crystal field appears to be important. One should go beyond single-configuration approxn. to have the obsd. satellite structure in 2p-x-ray-photoelectron spectra appear in the calcns.
    57. 57
      Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717– 2730,  DOI: 10.1016/j.apsusc.2010.10.051
      57
      Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni
      Biesinger, Mark C.; Payne, Brad P.; Grosvenor, Andrew P.; Lau, Leo W. M.; Gerson, Andrea R.; Smart, Roger St. C.
      Applied Surface Science (2011), 257 (7), 2717-2730CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Chem. state x-ray photoelectron spectroscopic anal. of 1st row transition metals and their oxides and hydroxides is challenging due to the complexity of their 2p spectra resulting from peak asymmetries, complex multiplet splitting, shake-up and plasmon loss structure, and uncertain, overlapping binding energies. The previous paper in which the authors examd. Sc, Ti, V, Cu and Zn species, showed that all the values of the spectral fitting parameters for each specific species, i.e. binding energy (eV), full wide at half max. (FWHM) value (eV) for each pass energy, spin-orbit splitting values and asym. peak shape fitting parameters, are not all normally provided in the literature and data bases, and are necessary for reproducible, quant. chem. state anal. A more consistent, practical and effective approach to curve fitting was developed based on a combination of (1) std. spectra from quality ref. samples, (2) a survey of appropriate literature databases and/or a compilation of literature refs. and (3) specific literature refs. where fitting procedures are available. This paper extends this approach to the chem. states of Cr, Mn, Fe, Co and Ni metals, and various oxides and hydroxides where intense, complex multiplet splitting in many of the chem. states of these elements poses unique difficulties for chem. state anal. The curve fitting procedures proposed use the same criteria as proposed previously but with the addnl. complexity of fitting of multiplet split spectra which was done based on spectra of numerous ref. materials and theor. XPS modeling of these transition metal species. Binding energies, FWHM values, asym. peak shape fitting parameters, multiplet peak sepn. and peak area percentages are presented. The procedures developed can be used to remove uncertainties in the anal. of surface states in nanoparticles, corrosion, catalysis and surface-engineered materials.
    58. 58
      Julien, C.; Massot, M.; Baddour-Hadjean, R.; Franger, S.; Bach, S.; Pereira-Ramos, J. P. Raman Spectra of Birnessite Manganese Dioxides. Solid State Ionics 2003, 159, 345– 356,  DOI: 10.1016/s0167-2738(03)00035-3
      58
      Raman spectra of birnessite manganese dioxides
      Julien, C.; Massot, M.; Baddour-Hadjean, R.; Franger, S.; Bach, S.; Pereira-Ramos, J. P.
      Solid State Ionics (2003), 159 (3,4), 345-356CODEN: SSIOD3; ISSN:0167-2738. (Elsevier Science B.V.)
      Structural features of layered Mn dioxides of the birnessite family were studied using Raman scattering spectroscopy. This local probe is capable of analyzing directly the near-neighbor environment of O coordination around Mn and Li cations. Four types of sol-gel birnessite (SGB) are considered: Li birnessite (Li-Bir), Na birnessite (Na-Bir), sol-gel birnessite (SG-Bir), and sol-gel Co-doped birnessite (SGCo-Bir). Thus, in a 1st approach, the authors consider the overall spectral features of birnessites such as the superposition of the spectra of local structures, while the lattice modes are discussed in the spectroscopic symmetry. Results show the specific spectroscopic fingerprints of SG-Bir single phases, the site occupancy of Co ions in the substituted SGCo-Bir compd., and vibrations due to Li ions with their O neighbors in Li-Bir, Li0.32MnO2·0.6H2O. A correlation between the interlayer d-spacing and the stretching mode frequencies of birnessite oxides was established.
    59. 59
      Chen, D.; Ding, D.; Li, X.; Waller, G. H.; Xiong, X.; El-Sayed, M. A.; Liu, M. Probing the Charge Storage Mechanism of a Pseudocapacitive MnO2 Electrode Using in Operando Raman Spectroscopy. Chem. Mater. 2015, 27, 6608– 6619,  DOI: 10.1021/acs.chemmater.5b03118
      59
      Probing the Charge Storage Mechanism of a Pseudocapacitive MnO2 Electrode Using in Operando Raman Spectroscopy
      Chen, Dongchang; Ding, Dong; Li, Xiaxi; Waller, Gordon Henry; Xiong, Xunhui; El-Sayed, Mostafa A.; Liu, Meilin
      Chemistry of Materials (2015), 27 (19), 6608-6619CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      While Mn oxide (MnO2) was extensively studied as an electrode material for pseudo-capacitors, a clear understanding of its charge storage mechanism is still lacking. Here the authors report the findings in probing the structural changes of a thin-film model MnO2 electrode during cycling using in operando Raman spectroscopy. The spectral features (e.g., band position, intensity, and width) are correlated quant. with the size (Li+, Na+, and K+) of cations in different electrolytes and with the degree of discharge to gain better understanding of the cation-incorporation mechanism into the interlayers of pseudo-capacitive MnO2. Also, theor. calcns. of phonon energy assocd. with the models of interlayer cation-incorporated MnO2 agree with the exptl. observations of cation-size effect on the positions of Raman bands. Also, the cation-size effects on spectral features at different potentials of MnO2 electrode are correlated quant. with the amt. of charge stored in the MnO2 electrode. The understanding of the structural changes assocd. with charge storage gained from Raman spectroscopy provides valuable insights into the cation-size effects on the electrochem. performances of the MnO2 electrode.
    60. 60
      Scheitenberger, P.; Euchner, H.; Lindén, M. The Hidden Impact of Structural Water – How Interlayer Water Largely Controls the Raman Spectroscopic Response of Birnessite-Type Manganese Oxide. J. Mater. Chem. A 2021, 9, 18466– 18476,  DOI: 10.1039/d1ta05357d
      60
      The hidden impact of structural water - how interlayer water largely controls the Raman spectroscopic response of birnessite-type manganese oxide
      Scheitenberger, Phillip; Euchner, Holger; Linden, Mika
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2021), 9 (34), 18466-18476CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Birnessite-type manganese oxides, consisting of stacked MnOx sheets, sepd. by charge-balancing metal ions and structural water are potential candidates for electrochem. applications. Due to their structural complexity, Raman spectroscopy is one of the most widely used techniques for studying this class of materials. However, the interpretation of the Raman spectra is still debated. In fact, the obsd. Raman bands are often intuitively assigned to either Mn-O or M-O vibrations, where M corresponds to charge balancing metal ions such as Na, Ca, and K-ions which are present in natural forms of birnessite. Here, we report a combined exptl. and computational study that, opposite to the commonly accepted assignments, strongly suggests that many of the characteristic Raman bands can be attributed to vibrations related to the interlayer water. Our computational findings are compared with detailed ex situ and in situ Raman spectroscopy/X-ray diffraction, and cyclovoltammetry results for K-birnessite, allowing for a full explanation of potential-dependent changes in the Raman spectra. Furthermore, the excellent correlation between the intensity of a band pos. assigned to water vibrations and the d-spacing, give support for the important influence of interlayer water on the Raman spectra. This study points to the crucial role of arrangement and content of structural water and deepens the current understanding of hydrated birnessites.
    61. 61
      Wu, T.-H.; Hesp, D.; Dhanak, V.; Collins, C.; Braga, F.; Hardwick, L. J.; Hu, C.-C. Charge Storage Mechanism of Activated Manganese Oxide Composites for Pseudocapacitors. J. Mater. Chem. A 2015, 3, 12786– 12795,  DOI: 10.1039/c5ta03334a
      61
      Charge storage mechanism of activated manganese oxide composites for pseudocapacitors
      Wu, Tzu-Ho; Hesp, David; Dhanak, Vin; Collins, Christopher; Braga, Filipe; Hardwick, Laurence J.; Hu, Chi-Chang
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (24), 12786-12795CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Manganese oxides can undergo an electrochem. activation step that leads to greater capacitances, of which the structural change and mechanism remains poorly understood. Herein a wide-ranging study is presented on a manganese oxide synthesized by annealing manganese(II) acetate precursor to 300°, which includes in operando monitoring of the structural evolution during the activation process via in situ Raman microscopy. Based on powder X-ray diffraction, XPS, transmission electron and ex-situ Raman microscopy, the as prepd. manganese oxide was characterized as hausmannite-Mn3O4 with a minor portion of MnO2. The activation process of converting as-prepd. hausmannite-Mn3O4 into amorphous MnO2 (with localized birnessite structure) by electrochem. cycling in 0.5 M Na2SO4 was examd. After activation, the activated MnOx exhibited capacitive performance of 174 F g-1 at a mass loading of 0.71 mg cm-2. The charge storage mechanism is proposed as the redox reaction between Mn(III) and Mn(IV) at outer surface active sites, since the disordered birnessite-MnO2 does not provide an ordered layer structure for cations and/or protons to intercalate.
    62. 62
      Yang, L.; Cheng, S.; Ji, X.; Jiang, Y.; Zhou, J.; Liu, M. Investigations into the Origin of Pseudocapacitive Behavior of Mn3O4 Electrodes Using in Operando Raman Spectroscopy. J. Mater. Chem. A 2015, 3, 7338– 7344,  DOI: 10.1039/c5ta00223k
      62
      Investigations into the origin of pseudocapacitive behavior of Mn3O4 electrodes using in operando Raman spectroscopy
      Yang, Lufeng; Cheng, Shuang; Ji, Xu; Jiang, Yu; Zhou, Jun; Liu, Meilin
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (14), 7338-7344CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A detailed understanding of the phase changes of manganese oxides in a pseudocapacitor during cycling is vital to reveal the mechanism by which they store energy and to achieve a rational design of improved electrode materials. Here we report the results of our probing the phase changes of a Mn3O4 electrode material in a pseudocapacitor during a charging/discharging process using in operando Raman spectroscopy. Along with enhancing performance, the spinel Mn3O4 was transformed to a layered birnessite-type MnO2 upon potential cycling and two types of processes were shown to contribute to the energy storage: intercalation/deintercalation of Na+ and oxidn./redn. of Mn2+. After charging and discharging for ∼10,000 cycles, the specific capacitance of manganese oxide increased almost 3-fold; and the max. specific capacitance approached 230 F/g at a cycling rate of 0.5 A/g.
    63. 63
      Boumaiza, H.; Renard, A.; Rakotomalala Robinson, M.; Kervern, G.; Vidal, L.; Ruby, C.; Bergaoui, L.; Coustel, R. A Multi-Technique Approach for Studying Na Triclinic and Hexagonal Birnessites. J. Solid State Chem. 2019, 272, 234– 243,  DOI: 10.1016/j.jssc.2019.02.017
      63
      A multi-technique approach for studying Na triclinic and hexagonal birnessites
      Boumaiza, Hella; Renard, Aurelien; Rakotomalala Robinson, Mbolantenaina; Kervern, Gwendal; Vidal, Loic; Ruby, Christian; Bergaoui, Latifa; Coustel, Romain
      Journal of Solid State Chemistry (2019), 272 (), 234-243CODEN: JSSCBI; ISSN:0022-4596. (Elsevier B.V.)
      Birnessite is a ubiquitous natural Mn oxide. This layered material is involved in a large variety of ion-exchange and redox reaction in soils and sediments. Synthetic Na-triclinic birnessite with a general formula [MnIII0.35 MnIV0.65 O2][Na+0.35 0.7H2O] was converted into hexagonal birnessite by acidic treatment at pH = 3.5. Both solids were fully characterized by TEM, SEM, PXRD, FTIR, Raman scattering, 23Na as well as the first 1H MAS NMR birnessite spectra were reported. The surfaces of both forms were fully analyzed by XPS: a particular attention was paid to the detn. of the oxidn. state of Mn derived from the fitting of XPS Mn 3p or Mn 2p3/2 features. An excess of reduced Mn at the surface in comparison to the bulk materials was evidenced.
    64. 64
      Scheitenberger, P.; Brimaud, S.; Lindén, M. XRD/Raman Spectroscopy Studies of the Mechanism of (De)Intercalation of Na+ from/into Highly Crystalline Birnessite. Mater. Adv. 2021, 2, 3940– 3953,  DOI: 10.1039/d1ma00161b
      64
      XRD/Raman spectroscopy studies of the mechanism of (de)intercalation of Na+ from/into highly crystalline birnessite
      Scheitenberger, Philipp; Brimaud, Sylvain; Linden, Mika
      Materials Advances (2021), 2 (12), 3940-3953CODEN: MAADC9; ISSN:2633-5409. (Royal Society of Chemistry)
      Due to its low-cost and environmental friendliness, birnessite-type manganese oxide has attracted wide interest for use as a cathode material in electrochem. energy storage applications. The mechanisms of energy storage and release have been studied in some detail during the last decade, but despite some agreement, some aspects of the storage and release mechanisms are still under debate. The main reason for this, we argue, is the varying interpretations of Raman spectroscopy data in the literature. Therefore, we undertook a detailed correlative Raman spectroscopy/XRD study in combination with cyclic voltammetry. Raman spectroscopy allowed for straightforward differentiation between symmetry changes during the (de)intercalation of Na-ions. More specifically, through the use of highly cryst. birnessite samples it is suggested that Raman spectra are sensitive to the lattice parameters β and d001, which allowed us to derive unprecedented details of the changes in the birnessite structure that occur upon Na+ (de)intercalation. Furthermore, it is shown that the reversible hexagonal/monoclinic symmetry transition during the course of a charge/discharge cycle is a prerequisite for effective charge storage. Based on the results, a detailed mechanism describing the (de)intercalation of Na+ from/into birnessite is presented.
    65. 65
      Nam, K. W.; Kim, S.; Yang, E.; Jung, Y.; Levi, E.; Aurbach, D.; Choi, J. W. Critical Role of Crystal Water for a Layered Cathode Material in Sodium Ion Batteries. Chem. Mater. 2015, 27, 3721– 3725,  DOI: 10.1021/acs.chemmater.5b00869
      65
      Critical Role of Crystal Water for a Layered Cathode Material in Sodium Ion Batteries
      Nam, Kwan Woo; Kim, Sangryun; Yang, Eunjeong; Jung, Yousung; Levi, Elena; Aurbach, Doron; Choi, Jang Wook
      Chemistry of Materials (2015), 27 (10), 3721-3725CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Layered transition metal oxides are considered promising cathodes for sodium ion batteries (SIBs) due to their superior specific capacities. However, they usually suffer from insufficient cycling and rate performance mainly from the structural instability during repeated cycles. These longstanding challenges are overcome by engaging crystal water in the interlayer space of sodium manganese oxide under the Birnessite framework. The crystal water enhances Na ion diffusion both in the crystal host and at the interface, suppresses fatal Mn2+ dissoln., and improves long-term structural stability, leading to excellent performance in rate capability and cycle life. The current study suggests that many hydrated materials can be good candidates for electrode materials of emerging rechargeable batteries that need to deal with the large size or multivalent charge of their carrier ions.
    66. 66
      Lu, K.; Hu, Z.; Xiang, Z.; Ma, J.; Song, B.; Zhang, J.; Ma, H. Cation Intercalation in Manganese Oxide Nanosheets: Effects on Lithium and Sodium Storage. Angew. Chem., Int. Ed. 2016, 55, 10448– 10452,  DOI: 10.1002/anie.201605102
      66
      Cation Intercalation in Manganese Oxide Nanosheets: Effects on Lithium and Sodium Storage
      Lu, Ke; Hu, Ziyu; Xiang, Zhonghua; Ma, Jizhen; Song, Bin; Zhang, Jintao; Ma, Houyi
      Angewandte Chemie, International Edition (2016), 55 (35), 10448-10452CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The rapid development of advanced energy-storage devices requires significant improvements of the electrode performance and a detailed understanding of the fundamental energy-storage processes. In this work, the self-assembly of two-dimensional manganese oxide nanosheets with various metal cations is introduced as a general and effective method for the incorporation of different guest cations and the formation of sandwich structures with tunable interlayer distances, leading to the formation of 3D MxMnO2 (M=Li, Na, K, Co, and Mg) cathodes. For sodium and lithium storage, these electrode materials exhibited different capacities and cycling stabilities. The efficiency of the storage process is influenced not only by the interlayer spacing but also by the interaction between the host cations and shutter ions, confirming the crucial role of the cations. These results provide promising ideas for the rational design of advanced electrodes for Li and Na storage.
    67. 67
      Shan, X.; Guo, F.; Charles, D. S.; Lebens-Higgins, Z.; Abdel Razek, S.; Wu, J.; Xu, W.; Yang, W.; Page, K. L.; Neuefeind, J. C.; Feygenson, M.; Piper, L. F. J.; Teng, X. Structural Water and Disordered Structure Promote Aqueous Sodium-Ion Energy Storage in Sodium-Birnessite. Nat. Commun. 2019, 10, 4975,  DOI: 10.1038/s41467-019-12939-3
      67
      Structural water and disordered structure promote aqueous sodium-ion energy storage in sodium-birnessite
      Shan Xiaoqiang; Guo Fenghua; Charles Daniel S; Teng Xiaowei; Lebens-Higgins Zachary; Abdel Razek Sara; Piper Louis F J; Wu Jinpeng; Yang Wanli; Xu Wenqian; Page Katharine L; Neuefeind Joerg C; Feygenson Mikhail; Feygenson Mikhail
      Nature communications (2019), 10 (1), 4975 ISSN:.
      Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; however, its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity. Herein we report a sodium rich disordered birnessite (Na0.27MnO2) for aqueous sodium-ion electrochemical storage with a much-enhanced capacity and cycling life (83 mAh g(-1) after 5000 cycles in full-cell). Neutron total scattering and in situ X-ray diffraction measurements show that both structural water and the Na-rich disordered structure contribute to the improved electrochemical performance of current cathode material. Particularly, the co-deintercalation of the hydrated water and sodium-ion during the high potential charging process results in the shrinkage of interlayer distance and thus stabilizes the layered structure. Our results provide a genuine insight into how structural disordering and structural water improve sodium-ion storage in a layered electrode and open up an exciting direction for improving aqueous batteries.
    68. 68
      Chigane, M.; Ishikawa, M. XRD and XPS Characterization of Electrochromic Nickel Oxide Thin Films Prepared by Electrolysis–Chemical Deposition. J. Chem. Soc., Faraday Trans. 1998, 94, 3665– 3670,  DOI: 10.1039/a806459h
      68
      XRD and XPS characterization of electrochromic nickel oxide thin films prepared by electrolysis-chemical deposition
      Chigane, Masaya; Ishikawa, Masami
      Journal of the Chemical Society, Faraday Transactions (1998), 94 (24), 3665-3670CODEN: JCFTEV; ISSN:0956-5000. (Royal Society of Chemistry)
      Electrochromic (EC) behaviors of nickel oxide (NiOx) thin films, grown by chem. process in a nickel ammine complex soln. [Ni(NH3)x2+] after electrolysis of this soln. (EL-C method), were studied using various alk. electrolyte solns.; potassium borate buffer (pH 12), KOH (pH 12.9), (CH3)4NOH and Et4NOH. The fully oxidized films in all adopted electrolytes commonly showed broadened UV-visible spectra in the visible and near IR region. However, voltammetry studies assocd. with photoabsorption of the films revealed an apparent difference in the EC dynamics among the electrolytes caused by the variation of cation sizes and electrolyte soln. pH. Characterization of as-prepd., oxidized and reduced films in the electrolytes by x-ray diffraction (XRD) and XPS were performed to explain the mechanism of the films based on the conversion between layered α-Ni(OH)2 and γ2-2NiO2·NiOOH. A model according to the extn./insertion of cations or water mols. from/into the interlayer is proposed.
    69. 69
      Chigane, M.; Ishikawa, M. Manganese Oxide Thin Film Preparation by Potentiostatic Electrolyses and Electrochromism. J. Electrochem. Soc. 2000, 147, 2246,  DOI: 10.1149/1.1393515
      69
      Manganese oxide thin film preparation by potentiostatic electrolysis and electrochromism
      Chigane, Masaya; Ishikawa, Masami
      Journal of the Electrochemical Society (2000), 147 (6), 2246-2251CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      Manganese oxide (MnOx) thin films were deposited on transparent conducting tin oxide glass substrates by potentiostatic anodic electrolysis of alk. soln. of a manganese ammine complex at 298 K. The effects of varying deposition potentials on the microstructure and the electrochromic (EC) properties of the films were studied. Characterization of films by x-ray diffraction revealed that two distinct potential regions (lower and >0.3 V vs. Ag/AgCl) were available for the film deposition; the crystal structure of the film deposited at lower and higher regions were γ-Mn2O3 and/or Mn3O4 and Mn7O13·5H2O, resp. XPS analyses of the films featuring exchange splitting effect on Mn 3s spectra indicated that the valence of manganese in the films prepd. at lower and higher potential regions are mixts. of divalence-trivalence and of trivalence-tetravalence, resp. The XPS anal. also revealed that terminal chem. bonding species of the films are a mixt. of hydroxide (Mn-O-H) and (Mn-O-Mn). The mechanism of the EC process, by which the color change between brown and light yellow occurs, could be explained in terms of the transformation between these two oxygen groups in Mn-O-H and Mn-O-Mn, accompanied by the change in valence of Mn. The EC durability of the films in switching performance was also assessed.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.3c00002.

    • Procedures used to synthesize manganese oxide powders and obtain their diffraction patterns, variations of the square-wave potential used to electrodeposit MnOx films, procedure for obtaining the double-layer capacitances and ECSAs, and characterization of electrodeposited MnOx films and reference powders by XANES; summary of electrochemical characterization of MnOx films and the parameters used to fit the X-ray photoelectron spectra of different MnOx films and reference powders; additional characterization by linear sweep voltammetry and constant-current electrolysis to measure the stability of MnOx films, CV used to measure the double-layer capacitances of the MnOx films, Mott–Schottky plots of MnOx films at different frequencies, X-ray absorption near edge spectroscopy and linear fitting of XANES data, Raman spectrum of a MnO2 film grown at constant potential, and XPS of electrodeposited MnOx films in the region for K 2p electrons (PDF)


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.