One- or Two-Electron Water Oxidation, Hydroxyl Radical, or H2O2EvolutionSamira Siahrostami,† Guo-Ling Li,‡,§ Venkatasubramanian Viswanathan,∥ and Jens K. Nørskov*,†,‡

†SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, 443 Via Ortega,Stanford, California 94305, United States‡SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park,California 94025, United States§School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, China∥Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, UnitedStates

*S Supporting Information

ABSTRACT: Electrochemical or photoelectrochemcial oxidation of water to form hydrogenperoxide (H2O2) or hydroxyl radicals (

•OH) offers a very attractive route to waterdisinfection, and the first process could be the basis for a clean way to produce hydrogenperoxide. A major obstacle in the development of effective catalysts for these reactions is thatthe electrocatalyst must suppress the thermodynamically favored four-electron pathwayleading to O2 evolution. We develop a thermochemical picture of the catalyst properties thatdetermine selectivity toward the one, two, and four electron processes leading to •OH, H2O2,and O2.

Water can be oxidized electrochemically or photo-electrochemically to form OH radicals, H2O2 or O2.The number of electrons transferred characterizes the threehalf-cell reactions leading to these products:The one-electron process:1

→ • + + ° =+ − EH O OH(aq) (H e ) 2.73 V2 (1)

The two-electron process:2

→ + + ° =+ − E2H O H O 2(H e ) 1.76 V2 2 2 (2)

The four-electron process:2

→ + + ° =+ − EH O O 4(H e ) 1.23 V2 2 (3)

.where the standard reduction potentials are reported inreference to the reversible hydrogen electrode. In the presentLetter, we present a thermodynamic analysis of the free energyof intermediates adsorbed on different catalyst surfaces for thethree reactions. This leads us to a set of criteria characterizingcatalysts with a tendency for selectivity toward the differentproducts. The approach rationalizes a number of observationsin the literature, and provides a set of catalyst selection rules.The background for the present study is the desire to find

electrocatalysts for direct production of hydrogen peroxide(H2O2) in a process that is cleaner and more sustainable thanthe presently used anthraquinone process.3 Hydrogen peroxideis an important chemical with a wide range of applications in

industry including paper and textile manufacturing. It is also avery clean oxidant in water treatment.3 Direct synthesis ofH2O2 from its elements hydrogen (H2) and oxygen (O2) hasbeen studied extensively in order to reduce the danger ofexplosion and simultaneously find selective and activecatalysts.4−9 An electrochemical route based on solar or windenergies that could be implemented at the point of use wouldbe very desirable for applications such as water cleaning.Electrochemical reduction of molecular oxygen to hydrogenperoxide has shown considerable promise, the main challengesbeing to find a cheap, efficient, and selective catalyst.10−16 Theoxidation of water to hydrogen peroxide (eq 2), is the simplestpossible process. It requires only water as the reactant anddirectly produces H2O2 in addition to H2.

17−19 Unfortunately,it has proven difficult to find good catalysts for this reaction.There are experimental reports on the electrochemicaloxidation of water to hydrogen peroxide over manganeseoxide.17,18 Hydroxyl radicals, hydrogen peroxide, and super-oxide anions have been detected at the surface of titaniumdioxide under UV irradiation.20−24 Small amounts of hydroxylradicals have also been detected over bismuth vanadate(BiVO4) exposed to visible light.

25−28 Most recently, Fuku et

Received: December 12, 2016Accepted: February 23, 2017Published: February 23, 2017

Letter

pubs.acs.org/JPCL

© 2017 American Chemical Society 1157 DOI: 10.1021/acs.jpclett.6b02924J. Phys. Chem. Lett. 2017, 8, 1157−1160

pubs.acs.org/JPCLhttp://dx.doi.org/10.1021/acs.jpclett.6b02924

al. have reported a successful example of a photoelectrodesystem made of tungsten trioxides (WO3) and bismuthvanadate capable of producing and accumulating hydrogenperoxide effectively.29 Tin oxide (SnO2) has been reported forelectrochemical wastewater treatment30,31 and has beenproposed as a possible electrocatalyst for H2O2 generation.

19

In the following, we use potential-dependent free energydiagrams for the three reactions (1−3) to analyze the selectivitytrends. Figure 1 shows examples of the free energy diagrams for

three representative catalysts, TiO2, WO3, and IrO2 and withthe three different major products included: the OH radical,H2O2, and O2, respectively. The free energy diagrams havebeen constructed for the adsorption free energies of relevantintermediates of the one- (eq 1), two- (eq 2) and four-electron(eq 3) water oxidation reactions, i.e., OH*, O*, and OOH*.We use the computational hydrogen electrode (CHE) model,which exploits that the chemical potential of a proton−electronpair is equal to that of gas-phase H2, at U = 0.0 V versus thereversible hydrogen electrode. The effect of the electrodepotential on the free energy of the intermediates is taken intoaccount by shifting the electron energy by −eU, where e and Uare the elementary charge and the electrode potential.32 Foreach catalyst, the adsorption free energy of the different keyintermediates has been taken from reported density functionaltheory calculations using the RPBE functional to describeexchange and correlation effects including corrections for zeropoint energies and entropy contributions based on theharmonic approximation.16,19,33,34 The energies of the finalstates, solvated •OH (details in Supporting Information),solvated H2O2, and gas phase O2 are taken from experi-ment.35−37 The thermodynamic analysis can only be taken asqualitative, since it does not include activation barriers, but ishas proven useful in rationalizing trends for a number ofelectrochemical reactions involving oxygen.33,38,39

Consider first the free energy diagram for TiO2, Figure 1a.Here the largest free energy step in the process is the firstoxidation step to form adsorbed OH (OH*). In fact this step isso close in energy to the solvated hydroxyl that once *OH isformed it is equally favorable to desorb and form solvatedhydroxyl, •OH(aq) or recombine at the surface and form H2O2.This analysis suggests that TiO2 should have a propensity toform OH radicals, and that the potential needed to apply tomake that possible is quite large, U•OH ∼ 2.4 V. This is in goodagreement with observations for UV illuminated titania.20−23

Several reports suggest that OH radicals are formed, and theenergy of the holes in TiO2 during UV radiation corresponds toroughly a potential of 2.5 V.20−23,40 We note that the freeenergy of O* is considerably higher than that of H2O2,suggesting that hydrogen peroxide is a likely additionaloxidation product. This is in agreement with some experimentalreports on detecting a small amount of H2O2 under UVillumination of TiO2.

20−23

The analysis above can be generalized to suggest that acriterion for forming hydroxyl radicals is that the free energy ofOH* on the catalyst surface is higher (more endergonic relativeto water) than the free energy of •OH(aq). eU•OH ∼ 2.4 eV.This sets an upper limit for the free energy of OH* at thesurface for H2O2 evolution. In other words, the minimumrequirement to avoid the one-electron oxidation reaction (eq 1)and proceed through the two- electron oxidation reaction (eq2) is ΔGOH ≲ 2.4 eV.Figure 1b shows the free energy diagram for IrO2(110),

which is known both experimentally41−43 and theoretically33,38

to be an excellent catalyst for O2 evolution. IrO2 stronglyadsorbs OH*33 and the free energy of OH* is far below thefree energy for OH radical formation. In addition, the freeenergy of O* is far below the one for H2O2 (ΔGH2O2 = 3.52eV), suggesting that the IrO2 surface has a strong driving forcefor the complete four-electron oxidation.Overall, based on the thermodynamic analysis, we identify

two requirements for catalysts in terms of OH* and O* binding

Figure 1. Free energy diagram for water oxidation reaction on (a)TiO2(110), (b) IrO2(110), and (c) WO3(100) surfaces plotted at zeropotential. The OH*, O*, and OOH* binding energies of TiO2(110)and IrO2(110) are adapted from ref 33. The relevant binding energiesfor WO3(100) are adapted from ref 16.

The Journal of Physical Chemistry Letters Letter

DOI: 10.1021/acs.jpclett.6b02924J. Phys. Chem. Lett. 2017, 8, 1157−1160

1158

http://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6b02924/suppl_file/jz6b02924_si_001.pdfhttp://dx.doi.org/10.1021/acs.jpclett.6b02924

energies that would lead the product selectivity toward H2O2evolution, including ΔGO ≳ 3.5 eV, and ΔGOH ≲ 2.4 eV.Figure 1c shows the free energy diagram for WO3 as an

example of a material that satisfies these criteria.The two criteria developed above are included in the two-

dimensional selectivity diagram shown in Figure 2. The

selectivity diagram shows that there is a large region in(ΔGO, ΔGOH) space, where O2 evolution (highlighted in blue)is expected to be dominating (the overpotential may be high,but that is another matter), another window associated withweak O adsorption energies (highlighted in green) that has adriving force toward H2O2 evolution, and a region for veryweakly interacting catalysts where OH radical formation(highlighted in red) is expected to be dominant. The mapalso shows why it is hard to find catalysts for selective H2O2synthesis. The O and OH adsorption energies tend to becorrelated as indicated by the dashed scaling line, and onlycrosses a corner of the region of H2O2 selectivity.A number of different reported oxides have been included in

Figure 2. Most of the data are adapted from previousreports.16,19,33,34,44 In addition, we calculated the BiVO4 inorder to be able to make comparisons with all current theexperimental reports. Details of calculations are in theSupporting Information.The selectivity diagram rationalizes the experimental

observations for OH radical, H2O2, and O2 evolution. Itshows that WO3, BiVO4, MnO2, and SnO2, which are known toproduce H2O2, are well into the green region, whereas TiO2,which is known to form both OH radical and H2O2, is close tothe border of weak OH adsorption (red region). Lastly,catalysts such as IrO2, RhO2, and PtO2, which are known toevolve O2 as the major product, are thermodynamically locatedin the selectivity region for O2 evolution. Given the ability tosystematize known catalyst trends, we suggest that theselectivity diagram can also be used as a tool to identify leads

for new H2O2 production catalysts. Promising leads need to befurther investigated to make sure that there are not kineticbarriers preventing their use. We also note that we have notaddressed the question of production rates.In summary we developed a cohesive understanding of water

oxidation incorporating the formation of hydroxyl radicals, aone-electron water oxidation reaction. This analysis provides asolid theoretical background for rationalizing product selectivityin a number of different experimental reports.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpclett.6b02924.

Computational details, description of the BiVO4structure and adsorption energies, and estimated freeenergy of OH radical (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: norskov@stanford.edu.ORCIDSamira Siahrostami: 0000-0002-1192-4634Guo-Ling Li: 0000-0002-1703-3699Venkatasubramanian Viswanathan: 0000-0003-1060-5495NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe gratefully acknowledge support from the U.S. Departmentof Energy, Office of Sciences, Office of Basic Energy Sciences,to the SUNCAT Center for Interface Science and Catalysis. S.Sacknowledges support from the Global Climate Energy Project(GCEP) at Stanford University (Fund No.52454). Part of thecalculations was financially supported by Henan University ofScience and Technology (No. 2013ZCX018) and NationalNatural Science Foundation of China (Nos. U1404212 and11404098).

■ REFERENCES(1) Armstrong, D. A.; Huie, R. E.; Lymar, S.; Koppenol, W. H.;Mereńyi, G.; Neta, P.; Stanbury, D. M.; Steenken, S.; Wardman, P.Standard Electrode Potentials Involving Radicals in Aqueous Solution:Inorganic Radicals. BioInorg. React. Mech. 2013, 9, 59−61.(2) Bard, A. J.; Parsons, R.; Hordan, J. Standard Potentials in AqueousSolution, 1st ed.; Bard, A. J., Parsons, R., Hordan, J., Eds.; M. Dekker:New York, 1985.(3) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G.Hydrogen Peroxide Synthesis: An Outlook beyond the AnthraquinoneProcess. Angew. Chem., Int. Ed. 2006, 45, 6962−6984.(4) Samanta, C. Direct Synthesis of Hydrogen Peroxide fromHydrogen and Oxygen: An Overview of Recent Developments in theProcess. Appl. Catal., A 2008, 350, 133−149.(5) Edwards, J. K.; Freakley, S. J.; Lewis, R. J.; Pritchard, J. C.;Hutchings, G. J. Advances in the Direct Synthesis of HydrogenPeroxide from Hydrogen and Oxygen. Catal. Today 2015, 248, 3−9.(6) Rankin, R. B.; Greeley, J. Trends in Selective Hydrogen PeroxideProduction on Transition Metal Surfaces from First Principles. ACSCatal. 2012, 2, 2664−2672.(7) Edwards, J. K.; Solsona, B. E.; Landon, P.; Carley, A. F.; Herzing,A.; Kiely, C. J.; Hutchings, G. J. Direct Synthesis of HydrogenPeroxide from H2 and O2 Using TiO2-Supported Au−Pd Catalysts. J.Catal. 2005, 236, 69−79.

Figure 2. Phase diagram in terms of the binding energies of O* versusOH*. The black solid line displays the scaling line between O* andOH* on different oxides adapted from refs 16, 19, 33, and 34. Thecorresponding adsorption energies for IrO2, RhO2, and PtO2 areadapted form ref 33. For MnO2, the energies are from ref 44 withselected Hubbard U value of 2 as recommended by Wang et al.45

BiVO4 adsorption energies are calculated in this work. Blue, green, andred highlighted colors indicate regions in which O2, H2O2, or OHradical are expected to be the major product, respectively, in terms ofpurely thermodynamic constraints.

The Journal of Physical Chemistry Letters Letter

DOI: 10.1021/acs.jpclett.6b02924J. Phys. Chem. Lett. 2017, 8, 1157−1160

1159

http://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6b02924/suppl_file/jz6b02924_si_001.pdfhttp://pubs.acs.orghttp://pubs.acs.org/doi/abs/10.1021/acs.jpclett.6b02924http://pubs.acs.org/doi/suppl/10.1021/acs.jpclett.6b02924/suppl_file/jz6b02924_si_001.pdfmailto:norskov@stanford.eduhttp://orcid.org/0000-0002-1192-4634http://orcid.org/0000-0002-1703-3699http://orcid.org/0000-0003-1060-5495http://dx.doi.org/10.1021/acs.jpclett.6b02924

(8) Freakley, S. J.; He, Q.; Harrhy, J. H.; Lu, L.; Crole, D. A.; Morgan,D. J.; Ntainjua, E. N.; Edwards, J. K.; Carley, A. F.; Borisevich, A. Y.;et al. Palladium-Tin Catalysts for the Direct Synthesis of H2O2 withHigh Selectivity. Science 2016, 351, 965−968.(9) Edwards, J. K.; Solsona, B.; N, E. N.; Carley, A. F.; Herzing, A. A.;Kiely, C. J.; Hutchings, G. J. Switching off Hydrogen PeroxideHydrogenation in the Direct Synthesis Process. Science 2009, 323,1037−1041.(10) Siahrostami, S.; Verdaguer-Casadevall, A.; Karamad, M.; Deiana,D.; Malacrida, P.; Wickman, B.; Escudero-Escribano, M.; Paoli, E. a;Frydendal, R.; Hansen, T. W.; et al. Enabling Direct H2O2 Productionthrough Rational Electrocatalyst Design. Nat. Mater. 2013, 12, 1137−1143.(11) Verdaguer-Casadevall, A.; Deiana, D.; Karamad, M.;Siahrostami, S.; Malacrida, P.; Hansen, T. W.; Rossmeisl, J.;Chorkendorff, I.; Stephens, I. E. L. Trends in the ElectrochemicalSynthesis of H2O2: Enhancing Activity and Selectivity by Electro-catalytic Site Engineering. Nano Lett. 2014, 14, 1603−1608.(12) Fellinger, T.-P.; Hasche,́ F.; Strasser, P.; Antonietti, M.Mesoporous Nitrogen-Doped Carbon for the Electrocatalytic Syn-thesis of Hydrogen Peroxide. J. Am. Chem. Soc. 2012, 134, 4072−4075.(13) Choi, C. H.; Kwon, H. C.; Yook, S.; Shin, H.; Kim, H.; Choi, M.Hydrogen Peroxide Synthesis via Enhanced Two-Electron OxygenReduction Pathway on Carbon-Coated Pt Surface. J. Phys. Chem. C2014, 118, 30063−30070.(14) Park, J.; Nabae, Y.; Hayakawa, T.; Kakimoto, M. HighlySelective Two-Electron Oxygen Reduction Catalyzed by MesoporousNitrogen-Doped Carbon. ACS Catal. 2014, 4, 3749.(15) Choi, C. H.; Kim, M.; Kwon, H. C.; Cho, S. J.; Yun, S.; Kim, H.-T.; Mayrhofer, K. J. J.; Kim, H.; Choi, M. Tuning Selectivity ofElectrochemical Reactions by Atomically Dispersed Platinum Catalyst.Nat. Commun. 2016, 7, 10922.(16) Siahrostami, S.; Björketun, M. E.; Strasser, P.; Greeley, J.;Rossmeisl, J. Tandem Cathode for Proton Exchange Membrane FuelCells. Phys. Chem. Chem. Phys. 2013, 15, 9326−9334.(17) Izgorodin, A.; Izgorodina, E.; MacFarlane, D. R. LowOverpotential Water Oxidation to Hydrogen Peroxide on a MnOxCatalyst. Energy Environ. Sci. 2012, 5, 9496.(18) McDonnell-Worth, C.; MacFarlane, D. R. Ion Effects in WaterOxidation to Hydrogen Peroxide. RSC Adv. 2014, 4, 30551.(19) Viswanathan, V.; Hansen, H. A.; Nørskov, J. K. SelectiveElectrochemical Generation of Hydrogen Peroxide from WaterOxidation. J. Phys. Chem. Lett. 2015, 6, 4224−4228.(20) GOTO, H. Quantitative Analysis of Superoxide Ion andHydrogen Peroxide Produced from Molecular Oxygen on Photo-irradiated TiO2 Particles. J. Catal. 2004, 225, 223−229.(21) Hirakawa, T.; Yawata, K.; Nosaka, Y. Photocatalytic Reactivityfor O2− and OH Radical Formation in Anatase and Rutile TiO2Suspension as the Effect of H2O2 Addition. Appl. Catal., A 2007, 325,105−111.(22) Cai, R.; Kubota, Y.; Fujishima, A. Effect of Copper Ions on theFormation of Hydrogen Peroxide from Photocatalytic TitaniumDioxide Particles. J. Catal. 2003, 219, 214−218.(23) Zhang, J.; Nosaka, Y. Quantitative Detection of OH Radicals forInvestigating the Reaction Mechanism of Various Visible-Light TiO 2Photocatalysts in Aqueous Suspension. J. Phys. Chem. C 2013, 117,1383−1391.(24) Sańchez-Quiles, D.; Tovar-Sańchez, A. Sunscreens as a Sourceof Hydrogen Peroxide Production in Coastal Waters. Environ. Sci.Technol. 2014, 48, 9037−9042.(25) Park, H. S.; Leonard, K. C.; Bard, A. J. Surface InterrogationScanning Electrochemical Microscopy (SI-SECM) of Photoelectro-chemistry at a W/Mo-BiVO 4 Semiconductor Electrode: Quantifica-tion of Hydroxyl Radicals during Water Oxidation. J. Phys. Chem. C2013, 117, 12093−12102.(26) Saison, T.; Chemin, N.; Chaneác, C.; Durupthy, O.; Mariey, L.;Mauge,́ F.; Brezova,́ V.; Jolivet, J.-P. New Insights Into BiVO 4Properties as Visible Light Photocatalyst. J. Phys. Chem. C 2015, 119,12967−12977.

(27) Rettie, A. J. E.; Lee, H. C.; Marshall, L. G.; Lin, J.-F.; Capan, C.;Lindemuth, J.; McCloy, J. S.; Zhou, J.; Bard, A. J.; Mullins, C. B.Combined Charge Carrier Transport and PhotoelectrochemicalCharacterization of BiVO4 Single Crystals: Intrinsic Behavior of aComplex Metal Oxide. J. Am. Chem. Soc. 2013, 135, 11389−11396.(28) Nishikawa, M.; Hiura, S.; Mitani, Y.; Nosaka, Y. EnhancedPhotocatalytic Activity of BiVO4 by Co-Grafting of Metal Ions andCombining with CuBi2O4. J. Photochem. Photobiol., A 2013, 262, 52−56.(29) Fuku, K.; Sayama, K. Efficient Oxidative Hydrogen PeroxideProduction and Accumulation in Photoelectrochemical Water SplittingUsing a Tungsten Trioxide/bismuth Vanadate Photoanode. Chem.Commun. (Cambridge, U. K.) 2016, 52, 5406−5409.(30) Stucki, S.; Kotz, R.; Carcer, B.; Suter, W. Electrochemical WasteWater Treatment Using High Overvoltage Anodes Part II: AnodePerformance and Applications. J. Appl. Electrochem. 1991, 21, 99−104.(31) Trasatti, S. Electrochemistry and Environment: The Role ofElectrocatalysis. Int. J. Hydrogen Energy 1995, 20, 835−844.(32) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.;Kitchin, J. R.; Bligaard, T.; Jońsson, H. Origin of the Overpotential forOxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108,17886−17892.(33) Man, I. C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H. A.; Martínez,J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.;Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis onOxide Surfaces. ChemCatChem 2011, 3, 1159−1165.(34) Montoya, J. H.; Garcia-Mota, M.; Nørskov, J. K.; Vojvodic, A.Theoretical Evaluation of the Surface Electrochemistry of Perovskiteswith Promising Photon Absorption Properties for Solar WaterSplitting. Phys. Chem. Chem. Phys. 2015, 17, 2634−2640.(35) Hamad, S.; Lago, S.; Mejías, J. A. A Computational Study of theHydration of the OH Radical. J. Phys. Chem. A 2002, 106, 9104−9113.(36) Pabis, A.; Szala-Bilnik, J.; Swiatla-Wojcik, D. MolecularDynamics Study of the Hydration of the Hydroxyl Radical at BodyTemperature. Phys. Chem. Chem. Phys. 2011, 13, 9458−9468.(37) Cabral do Couto, P.; Guedes, R. C.; Costa Cabral, B. J.;Martinho Simões, J. A. The Hydration of the OH Radical:Microsolvation Modeling and Statistical Mechanics Simulation. J.Chem. Phys. 2003, 119, 7344.(38) Rossmeisl, J.; Qu, Z.-W.; Zhu, H.; Kroes, G.-J.; Nørskov, J. K.Electrolysis of Water on Oxide Surfaces. J. Electroanal. Chem. 2007,607, 83−89.(39) Viswanathan, V.; Hansen, H. A.; Rossmeisl, J.; Nørskov, J. K.Universality in Oxygen Reduction Electrocatalysis on Metal Surfaces.ACS Catal. 2012, 2, 1654−1660.(40) Li, L.; Salvador, P. A.; Rohrer, G. S. Photocatalysts with InternalElectric Fields. Nanoscale 2014, 6, 24−42.(41) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y.Synthesis and Activities of Rutile IrO 2 and RuO 2 Nanoparticles forOxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett.2012, 399−404.(42) Hu, J.-M.; Zhang, J.-Q.; Cao, C.-N. Oxygen Evolution Reactionon IrO2-Based DSA® Type Electrodes: Kinetics Analysis of TafelLines and EIS. Int. J. Hydrogen Energy 2004, 29, 791−797.(43) Cruz, J. C.; Baglio, V.; Siracusano, S.; Ornelas, R.; Ortiz-Frade,L.; Arriaga, L. G.; Antonucci, V.; Arico,̀ A. S. Nanosized IrO2Electrocatalysts for Oxygen Evolution Reaction in an SPE Electrolyzer.J. Nanopart. Res. 2011, 13, 1639−1646.(44) Xu, Z.; Rossmeisl, J.; Kitchin, J. R. A Linear Response DFT+ UStudy of Trends in the Oxygen Evolution Activity of Transition MetalRutile Dioxides. J. Phys. Chem. C 2015, 119, 4827−4833.(45) Wang, L.; Maxisch, T.; Ceder, G. Oxidation Energies ofTransition Metal Oxides within the GGA + U Framework. Phys. Rev.B: Condens. Matter Mater. Phys. 2006, 73, 195107.

The Journal of Physical Chemistry Letters Letter

DOI: 10.1021/acs.jpclett.6b02924J. Phys. Chem. Lett. 2017, 8, 1157−1160

1160

http://dx.doi.org/10.1021/acs.jpclett.6b02924