Enhancement Effect of Noble Metals on Manganese Oxide for the

Oct 7, 2015 - Developing improved catalysts for the oxygen evolution reaction (OER) is key to the advancement of a number of renewable energy technolo...
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Enhancement Effect of Noble Metals on Manganese Oxide for the Oxygen Evolution Reaction Linsey C. Seitz,† Thomas J. P. Hersbach,† Dennis Nordlund,‡ and Thomas F. Jaramillo*,† †

Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305, United States SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, Menlo Park, California, 94025, United States

‡

S Supporting Information *

ABSTRACT: Developing improved catalysts for the oxygen evolution reaction (OER) is key to the advancement of a number of renewable energy technologies, including solar fuels production and metal air batteries. In this study, we employ electrochemical methods and synchrotron techniques to systematically investigate interactions between metal oxides and noble metals that lead to enhanced OER catalysis for water oxidation. In particular, we synthesize porous MnOx films together with nanoparticles of Au, Pd, Pt, or Ag and observe significant improvement in activity for the combined catalysts. Soft X-ray absorption spectroscopy (XAS) shows that increased activity correlates with increased Mn oxidation states to 4+ under OER conditions compared to bare MnOx, which exhibits minimal OER current and remains in a 3+ oxidation state. Thickness studies of bare MnOx films and of MnOx films deposited on Au nanoparticles reveal trends suggesting that the enhancement in activity arises from interfacial sites between Au and MnOx.

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The work presented here further explores the interaction between MnOx and noble metals for the OER by systematically characterizing the enhancement of MnOx thin films deposited on top of a low loading of noble metal nanoparticles. Electrochemical activity is systematically measured for several sets of samples and correlated with electronic structure data from soft X-ray absorption spectroscopy, probing the effect of the various noble metals on the Mn oxidation state. Furthermore, a study of MnOx films of varying thickness on Au nanoparticles provides evidence to suggest that the enhanced active site is the interface between the two metals. Experimental methods for sample preparation, electrochemical testing, as well as investigation of catalyst morphology and electronic structure is included in the Supporting Information. Cyclic voltammetry was used to assess the activity of all catalysts, which were deposited on flat glassy carbon (GC) disks and studied in an alkaline electrolyte with a carbon rod counter electrode. Figure 1a shows the geometric area normalized activity of the bare noble metals used in this study (Au, Pd, Pt, and Ag), each of which is deposited with an identical, low loading. The bare noble metals exhibit rather low activity, with the exception of Pd, which nearly reaches 1 mA/ cm2geo at 1.65 V versus RHE. We note that large oxidation peaks are visible from the Ag catalyst, but the OER does not onset (defined for our purposes as OER current density reaching 0.2 mA/cm2geo) until 1.85 V versus RHE. Figure 1b shows the geometric area normalized activity of the MnOx catalysts in this study, prepared by electrodeposition. These

he oxygen evolution reaction (OER) is the complementary half reaction to a number of processes that are central to renewable energy research involving the electrocatalytic production of chemicals and fuels. The large overpotential required to drive the OER is a significant limitation in producing efficient devices for water splitting, a promising method for the clean production of hydrogen if powered by a renewable energy source such as wind or solar.1−5 Significant research efforts over the past few decades have focused on fundamental understanding of OER catalysts as well as developing more active catalysts.6−15 Additionally, the use of nonprecious metals to develop highly active catalysts is beneficial for making these processes scalable and economical.16−19 Manganese oxide (MnOx) has been widely studied as a catalyst for the OER with a range of reported activities.20−31 Recently, it was found that an electrochemical MnOx catalyst is significantly more active when interacting with gold.32−34 Additionally, studies have shown similar findings for cobalt oxide, (intentionally or unintentionally) iron-doped nickel oxyhydroxide, and cerium oxide on noble metals including Au, Pt, and Pd.35−38 These studies suggest charge transfer from the transition metal oxide to the more electronegative noble metal, which stabilizes a more active oxidation state or phase of the transition-metal oxide. Other potential mechanisms for improving OER activity by stabilization of reaction intermediates with Au or Ru in conjunction with first row transitionmetal oxide catalysts have been suggested in the literature as well.39,40 This mechanism would decrease the voltage requirement of the most energetically uphill step by stabilizing the *OOH intermediate, thereby decreasing the overall overpotential to drive the reaction. © XXXX American Chemical Society

Received: September 2, 2015 Accepted: October 1, 2015

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used to compare the electronic structure of the catalysts both with and without noble metals, as prepared and under simulated OER conditions. XAS is a powerful tool that can be used to probe electron transitions from the 2p to 3d orbitals of first row transition metals providing surface-sensitive, elemental-specific information about the valence structure.41 The strong dipole transition between the 2p core−electron and the 3d final states results in sharp and strong transitions whose fine spectral features are dominated by multiplet effects.41 Figure 2a shows the Mn L-edge XAS scans for manganese

Figure 1. Cyclic voltammograms for identical loadings of bare noble metal nanoparticles (a), as well as identical amounts of bare MnOx and MnOx deposited on top of noble metal nanoparticles (b) in 0.1 M KOH. The combined activity of MnOx on noble metal nanoparticles is increased significantly.

include a thin film of bare MnOx as well as a similar thin film of MnOx deposited on top of low loadings of noble metal nanoparticles similar to those described in Figure 1a. The bare MnOx sample exhibits minimal activity in the potential range tested. However, for the composite catalysts consisting of both MnOx and the noble metal nanoparticles, significant enhancements in activity are observed. The enhancement effect is greatest for MnOx combined with Au, followed in order by Pd, Pt, and Ag. Further quantification of the average activity and standard deviations for three sets of the samples described above are shown in Figure S2a in the Supporting Information. The high activity of the composite catalysts cannot be accounted for by simply adding the OER current densities of the individual noble metal nanoparticles and of the bare MnOx (Figure S2b). These results motivate the investigations described below, exploring the origins of enhanced catalyst activity. Increased film conductivity due to the presence of the noble metals was initially considered as a plausible origin of the enhanced electrocatalytic activity; however, conductivity alone cannot describe the observations in Figure 1b. For one, the MnOx films shown in Figure 1b are very thin (approximately 18 nm) and would likely not suffer from substantial conductivity limitations. Second, the four noble metal nanoparticles investigated are all highly conductive, yet the enhancements in activity are shown to differ substantially among them. We hypothesize that the enhancement effect arises primarily from physical−chemical interactions at the interface between the oxide and metal nanoparticles. Evidence of such interactions can be observed in the cyclic voltammetry, where substantial changes to the redox behavior of Ag can be viewed in comparing bare Ag (Figure 1a) to the case of MnOx on Ag (Figure 1b). As the redox features of MnOx are not clearly visible on these samples, we employed synchrotron-based spectroscopies to understand such effects on MnOx. To understand in greater detail the effect of the noble metals on the MnOx, soft X-ray absorption spectroscopy (XAS) was

Figure 2. XAS for standard powders (a), MnOx catalysts as prepared (b), and under “simulated” OER conditions at 1.65 V versus RHE (c). Bare MnOx undergoes no significant change at these potentials, but MnOx deposited on noble metals oxidizes to a 4+ state. The inset in (c) presents an enlarged view of the LIII edge, showing that the MnOx is most oxidized at these conditions when deposited on Au, followed by Pd, Pt, and then Ag.

reference powders at different oxidation states, including the spin−orbit split LIII (Mn 2p3/2) and LII (Mn 2p1/2) regions. Generally, the spectral weight shifts to higher energies when manganese is more oxidized, but the scans also reflect changes in the local bonding environment with a rich multiplet structure, consistent with earlier studies.41,42 As prepared, the MnOx phases of the catalysts with and without noble metals are indistinguishable and best match a mixture of the Mn3O4 and Mn2O3 reference powder spectra, as can be seen in Figure 2b. However, after they have been electrochemically oxidized under OER conditions (1.65 V versus RHE; see the methods for details), we observe significant changes in the spectra. Figure 2c shows that after exposure to OER conditions, the bare MnOx catalyst is only slightly oxidized from its as-prepared state, resembling a 4179

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The Journal of Physical Chemistry Letters mixture of the Mn3O4 and Mn2O3 reference powder spectra, and thus primarily exists in a mixed 2+/3+ oxidation state. In contrast, the MnOx films deposited on the noble metals have all oxidized significantly after exposure to OER conditions and display spectra similar to the LIII edge in the MnO2 spectra corresponding to a 4+ oxidation state. If we zoom in on the LIII edge (as shown in the inset of Figure 2c), we see that there is a distinct trend in the oxidation state, which can be qualitatively (and quantitatively, which is not pursued here) evaluated by observing the absence (or presence) of intensity in the “valley” of the two distinct transitions observed for Mn 4+; a deeper valley with respect to the peaks indicates greater 4+ character. We observe that the degree of Mn 4+ character depends on the support material and decreases in the order of Au, Pd, Pt, and Ag, which matches the trend in electrochemical activity (as characterized by the measured OER current at 1.65 V versus RHE; see Figure 1). This suggests that the more oxidized the MnOx, the more active the catalyst. Probing further into this relationship between electronic structure and catalytic activity, we observe that the bare MnOx catalyst can be oxidized to a 4+ state if it is swept to and held at more oxidizing potentials. Figure S3 in the Supporting Information shows a comparison of three XAS spectra for MnOx: as prepared, at 1.65 V versus RHE, and at 1.85 V versus RHE. The LIII edge of the bare MnOx catalyst at 1.85 V versus RHE exhibits the same valley shape characteristic of MnO2, but the feature is shallower than that for MnOx on noble metals at 1.65 V versus RHE, indicating that it still has slightly less 4+ character, despite having been tested at a more oxidizing potential. Additionally, the bare MnOx catalyst does not onset for the OER until 2.3 V versus RHE. These results indicate that all of our MnOx catalysts are in a 4+ state when driving the OER and suggests that the presence of noble metals helps MnOx to reach this state at a lower potential, thereby allowing for an earlier OER activity onset. Understanding the correlation between the MnOx oxidation state and catalytic activity is key knowledge needed to comprehend the origins of its catalytic activity. Several studies of MnOx catalysts for the OER have reported mixed phases or even more reduced phases (Mn 2 O 3 ) for their active catalyst.23−25,34 However, many highly active MnOx catalysts have large 4+ contribution, resulting in mixed phases with oxidation states between 3+ and 4+, and in some cases, active MnOx catalysts with fully 4+ states have been reported.26−31 Additionally, DFT calculations suggest that MnO2 is the expected stable phase at relevant OER potentials and that it should also be the more active phase compared to Mn2O3.9,22 Many of these reports also indicate that the preparation method, including the deposition technique, thermal treatment, and so forth, has a strong effect on the oxidation state of the resulting material as prepared and also dictates how prone the material is to change when exposed to OER conditions. To further investigate the origins of the enhanced activity of the Au/MnOx catalyst system, we conducted a thickness study, preparing a set of samples of MnOx films with thicknesses ranging from approximately 5 to 180 nm on bare GC and on Au nanoparticles on GC. SEM images for a subset of these samples are shown in Figure 3, both with Au nanoparticles (top two rows) and without Au nanoparticles (bottom two rows). Additionally, each sample is imaged in the same spot using a secondary electron detector (top image of each set, Figure 3a,e,i,c,g,k) and a concentric backscatter detector (bottom image of each set, Figure 3b,f,j,d,h,l), which is sensitive to

Figure 3. SEM images of two sets of catalysts, MnOx with and without Au nanoparticles, at three different controlled amounts of deposited MnOx (three different estimated thicknesses). Each sample is imaged in the same spot using a secondary electron detector (EDT) and a concentric backscatter detector (CBS), which emphasizes elements with higher atomic number.

atomic number. The backscatter detector images highlight the Au nanoparticles when they are present beneath thinner films of MnOx. The thicker films both on Au nanoparticle-decorated and bare GC clearly show the characteristic structure seen in many other electrodeposited MnOx films.26,43−45 However, we observe that for thinner films, the MnOx deposits quite differently on the two substrates; MnOx nucleates mostly around Au nanoparticles when they are present, whereas on the flat GC substrate without Au, the MnOx films are more evenly distributed. Due to the fact that MnOx nucleates around the Au nanoparticles, through this synthesis procedure, we can control the amount of Au/MnOx interface area as a function of MnOx deposited. Catalytic activity was measured for four sets of bare MnOx and Au/MnOx samples, and the resulting activity trend is shown in Figure 4. When plotting the geometric area normalized current density of these catalysts as a function of the amount and/or thickness of the MnOx films (Figure 4a), we see that the activity of the bare MnOx is fairly constant. Although the films’ porosity would lead us to expect increased surface area (and therefore increased active sites) with additional material deposited, our data suggest that there may exist limitations from, for example, mass transport and/or film conductivity that prevent additional current collection through the thicker films. In contrast, the geometric area normalized current density of MnOx on Au nanoparticles was 3−4 times higher than that of bare MnOx at the lowest MnOx loading tested. As more MnOx is deposited on the Au nanoparticles, thereby increasing the interface area between MnOx and Au, the activity increases up until a maximum activity is reached at approximately 24 nm of MnOx (16 nmol); a 16-fold increase in current density is observed relative to the bare MnOx film. Depositing additional MnOx from this point results in decreased activity approaching that of bare MnOx for films of approximately 100 nm and thicker. We hypothesize that the 4180

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noble metals helps oxidize the MnOx at less positive potentials, thereby allowing an earlier onset of current for the OER. Lastly, our study of activity trends for MnOx of varying film thicknesses deposited on Au nanoparticles suggests that the interfacial sites between Au and MnOx experience the greatest enhancement effect and are likely the active sites contributing most to the higher activity. The geometric area normalized activity of this combined catalyst system increases with MnOx loading as the interfacial area increases, up to a certain point after which increasing MnOx film thickness further results in a decrease in activity until it matches that of a bare MnOx film of similar thickness, likely due to decreased access to the interfacial Au/MnOx sites at the base of the film.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01928. Details on experimental methods, table of unit cell parameters for relevant MnOx phases, calibration curves relating the amount of MnOx deposited and estimated film thicknesses as well as the procedure to complete these calculations, bar chart showing the average current density for multiple sets of bare MnOx, bare noble metal, and MnOx on noble metal samples, XAS spectra for bare the MnOx sample as prepared and at 1.65 and 1.85 V versus RHE, and capacitance-normalized catalytic activity trend for thickness study of MnOx on Au (PDF)

Figure 4. Electrochemical activity trends for MnOx films of varying thickness deposited on bare GC as well as on Au nanoparticledecorated GC. Activities are presented on a geometric current density basis (a) and a mass activity basis (b) at 1.63 V versus RHE.

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decrease in activity observed with increasing film thickness could result from reduced accessibility of highly active interfacial sites between Au and MnOx located at the base of the film. When the Au nanoparticles are essentially fully covered by the thick MnOx film, the catalyst activity returns to nearly match a bare MnOx film, as expected. Improved conductivity from the presence of Au may also play a role, but the substantial activity enhancement of the thinner MnOx films, where conductivity is less of an issue, suggests that improved conductivity is not the main source of activity enhancement for these catalysts. Upon normalizing the activity to the amount of MnOx in the form of mass activity (A/g), Figure 4b, for each case (with or without Au), the activity trend is such that the thinnest films of MnOx have the highest mass activity. This trend might be expected because for thicker films, access is reduced to subsurface MnOx. For MnOx film thicknesses up to 50 nm, the combined Au/MnOx catalysts show 2−5 times greater mass activity than the bare MnOx catalysts. The activity was also normalized to the capacitance of each catalyst as a first-order approximation of the catalyst surface area (shown in Figure S4 in the Supporting Information). This results in a similar trend to the mass activity where for each case (with or without Au), the thinnest films of MnOx are the most active. We have explored the relationship between OER catalytic activity and oxidation state of electrodeposited porous films of MnOx, either alone or in the presence of small amounts of noble metal nanoparticles. We found that the presence of Au enhanced the activity of MnOx the most, followed by Pd, Pt, and Ag, respectively. Higher activity was found to be correlated with increased oxidation of the MnOx to a 4+ state after exposure to OER conditions, consistent with other examples of MnOx in the literature. We hypothesize that the presence of

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 650 498 6879. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Center on Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Grant Number DE-SC0001060. Part of this work was performed at the Stanford Nano Shared Facilities. The authors acknowledge Guangchao Li at the Stanford Environmental Measurements Facility for technical assistance with ICP-OES measurements.

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REFERENCES

(1) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305 (5686), 972−974. (2) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (43), 15729−15735. (3) Gray, H. B. Powering the planet with solar fuel. Nat. Chem. 2009, 1 (1), 7−7. (4) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446−6473. (5) Seitz, L. C.; Chen, Z.; Forman, A. J.; Pinaud, B. A.; Benck, J. D.; Jaramillo, T. F. Modeling Practical Performance Limits of Photoelectrochemical Water Splitting Based on the Current State of Materials Research. ChemSusChem 2014, 7 (5), 1372−1385. (6) Trasatti, S. Electrocatalysis in the anodic evolution of oxygen and chlorine. Electrochim. Acta 1984, 29 (11), 1503−1512.

4181

DOI: 10.1021/acs.jpclett.5b01928 J. Phys. Chem. Lett. 2015, 6, 4178−4183

Letter

The Journal of Physical Chemistry Letters (7) Matsumoto, Y.; Sato, E. Electrocatalytic properties of transition metal oxides for oxygen evolution reaction. Mater. Chem. Phys. 1986, 14 (5), 397−426. (8) Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. Electrolysis of water on oxide surfaces. J. Electroanal. Chem. 2007, 607 (1−2), 83−89. (9) Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martinez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Norskov, J. K.; Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3 (7), 1159−1165. (10) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3 (3), 399−404. (11) 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 (41), 17253−17261. (12) Trotochaud, L.; Boettcher, S. W. Precise oxygen evolution catalysts: Status and opportunities. Scr. Mater. 2014, 74 (0), 25−32. (13) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel−Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136 (18), 6744−6753. (14) Stoerzinger, K. A.; Qiao, L.; Biegalski, M. D.; Shao-Horn, Y. Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2. J. Phys. Chem. Lett. 2014, 5 (10), 1636−1641. (15) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137 (13), 4347− 4357. (16) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334 (6061), 1383−1385. (17) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y.-L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution. Nat. Commun. 2013, 4, 2439. (18) 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 (7), 1983−2002. (19) Gattrell, M.; Gupta, N.; Co, A. Electrochemical reduction of CO2 to hydrocarbons to store renewable electrical energy and upgrade biogas. Energy Convers. Manage. 2007, 48 (4), 1255−1265. (20) Brimblecombe, R.; Koo, A.; Dismukes, G. C.; Swiegers, G. F.; Spiccia, L. Solar Driven Water Oxidation by a Bioinspired Manganese Molecular Catalyst. J. Am. Chem. Soc. 2010, 132 (9), 2892−2894. (21) Najafpour, M. M.; Govindjee. Govindjee. Oxygen evolving complex in Photosystem II: Better than excellent. Dalton Trans. 2011, 40 (36), 9076−9084. (22) Su, H.-Y.; Gorlin, Y.; Man, I. C.; Calle-Vallejo, F.; Norskov, J. K.; Jaramillo, T. F.; Rossmeisl, J. Identifying active surface phases for metal oxide electrocatalysts: a study of manganese oxide bi-functional catalysts for oxygen reduction and water oxidation catalysis. Phys. Chem. Chem. Phys. 2012, 14 (40), 14010−14022. (23) Gorlin, Y.; Jaramillo, T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132 (39), 13612−13614. (24) Mette, K.; Bergmann, A.; Tessonnier, J.-P.; Hävecker, M.; Yao, L.; Ressler, T.; Schlögl, R.; Strasser, P.; Behrens, M. Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting. ChemCatChem 2012, 4 (6), 851−862. (25) Pickrahn, K. L.; Park, S. W.; Gorlin, Y.; Lee, H.-B.-R.; Jaramillo, T. F.; Bent, S. F. Active MnOx Electrocatalysts Prepared by Atomic

Layer Deposition for Oxygen Evolution and Oxygen Reduction Reactions. Adv. Energy Mater. 2012, 2 (10), 1269−1277. (26) 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 (5), 7081−7089. (27) Grimaud, A.; Carlton, C. E.; Risch, M.; Hong, W. T.; May, K. J.; Shao-Horn, Y. Oxygen Evolution Activity and Stability of Ba6Mn5O16, Sr4Mn2CoO9, and Sr6Co5O15: The Influence of Transition Metal Coordination. J. Phys. Chem. C 2013, 117 (49), 25926−25932. (28) Indra, A.; Menezes, P. W.; Zaharieva, I.; Baktash, E.; Pfrommer, J.; Schwarze, M.; Dau, H.; Driess, M. Active Mixed-Valent MnOx Water Oxidation Catalysts through Partial Oxidation (Corrosion) of Nanostructured MnO Particles. Angew. Chem., Int. Ed. 2013, 52 (50), 13206−13210. (29) Bergmann, A.; Zaharieva, I.; Dau, H.; Strasser, P. Electrochemical water splitting by layered and 3D cross-linked manganese oxides: correlating structural motifs and catalytic activity. Energy Environ. Sci. 2013, 6 (9), 2745−2755. (30) Shevchenko, D.; Anderlund, M. F.; Styring, S.; Dau, H.; Zaharieva, I.; Thapper, A. Water oxidation by manganese oxides formed from tetranuclear precursor complexes: the influence of phosphate on structure and activity. Phys. Chem. Chem. Phys. 2014, 16 (24), 11965−11975. (31) Pickrahn, K. L.; Gorlin, Y.; Seitz, L. C.; Garg, A.; Nordlund, D.; Jaramillo, T. F.; Bent, S. F. Applications of ALD MnO to electrochemical water splitting. Phys. Chem. Chem. Phys. 2015, 17 (21), 14003−14011. (32) El-Deab, M. S.; Awad, M. I.; Mohammad, A. M.; Ohsaka, T. Enhanced water electrolysis: Electrocatalytic generation of oxygen gas at manganese oxide nanorods modified electrodes. Electrochem. Commun. 2007, 9 (8), 2082−2087. (33) Mohammad, A. M.; Awad, M. I.; El-Deab, M. S.; Okajima, T.; Ohsaka, T. Electrocatalysis by nanoparticles: Optimization of the loading level and operating pH for the oxygen evolution at crystallographically oriented manganese oxide nanorods modified electrodes. Electrochim. Acta 2008, 53 (13), 4351−4358. (34) Gorlin, Y.; Chung, C.-J.; Benck, J. D.; Nordlund, D.; Seitz, L.; Weng, T.-C.; Sokaras, D.; Clemens, B. M.; Jaramillo, T. F. Understanding Interactions between Manganese Oxide and Gold That Lead to Enhanced Activity for Electrocatalytic Water Oxidation. J. Am. Chem. Soc. 2014, 136, 4920. (35) Yeo, B. S.; Bell, A. T. Enhanced Activity of Gold-Supported Cobalt Oxide for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2011, 133 (14), 5587−5593. (36) Yeo, B. S.; Bell, A. T. In Situ Raman Study of Nickel Oxide and Gold-Supported Nickel Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Phys. Chem. C 2012, 116 (15), 8394−8400. (37) Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11 (6), 550−557. (38) Acerbi, N.; Tsang, S. C.; Golunski, S.; Collier, P. A practical demonstration of electronic promotion in the reduction of ceria coated PGM catalysts. Chem. Commun. 2008, 13, 1578−1580. (39) Frydendal, R.; Busch, M.; Halck, N. B.; Paoli, E. A.; Krtil, P.; Chorkendorff, I.; Rossmeisl, J. Enhancing Activity for the Oxygen Evolution Reaction: The Beneficial Interaction of Gold with Manganese and Cobalt Oxides. ChemCatChem 2015, 7 (1), 149−154. (40) Halck, N. B.; Petrykin, V.; Krtil, P.; Rossmeisl, J. Beyond the volcano limitations in electrocatalysis - oxygen evolution reaction. Phys. Chem. Chem. Phys. 2014, 16 (27), 13682−13688. (41) Cramer, S. P.; DeGroot, F. M. F.; Ma, Y.; Chen, C. T.; Sette, F.; Kipke, C. A.; Eichhorn, D. M.; Chan, M. K.; Armstrong, W. H. Ligand field strengths and oxidation states from manganese L-edge spectroscopy. J. Am. Chem. Soc. 1991, 113 (21), 7937−7940. 4182

DOI: 10.1021/acs.jpclett.5b01928 J. Phys. Chem. Lett. 2015, 6, 4178−4183

Letter

The Journal of Physical Chemistry Letters (42) Morales, F.; de Groot, F. M. F.; Glatzel, P.; Kleimenov, E.; Bluhm, H.; Hävecker, M.; Knop-Gericke, A.; Weckhuysen, B. M. In Situ X-ray Absorption of Co/Mn/TiO2 Catalysts for Fischer−Tropsch Synthesis. J. Phys. Chem. B 2004, 108 (41), 16201−16207. (43) Nakayama, M.; Kanaya, T.; Lee, J.-W.; Popov, B. N. Electrochemical synthesis of birnessite-type layered manganese oxides for rechargeable lithium batteries. J. Power Sources 2008, 179 (1), 361− 366. (44) Larabi-Gruet, N.; Peulon, S.; Lacroix, A.; Chaussé, A. Studies of electrodeposition from Mn(II) species of thin layers of birnessite onto transparent semiconductor. Electrochim. Acta 2008, 53 (24), 7281− 7287. (45) 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 (23), 11830−11838.

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