Electrochemical Synthesis of H2O2 by Two-Electron Water Oxidation Reaction

Abstract

Electrochemical synthesis of H2O2 represents a growing interest in the distributed production of valuable chemicals with renewable electricity, as evidenced by several recently published reviews on this topic. However, a review focusing exclusively on the electrochemical two-electron water oxidation reaction (2e-WOR) to produce H2O2 is still needed with tutorial styles to provide both theoretical and experimental fundamentals on this topic. Moreover, the related protocols and standardized metrics, which are crucially important for more consistent and reliable fundamental studies, are not well developed yet from the current reports. To fill in the gaps, our work discusses the current understanding and status of catalyst development for 2e-WOR from both theoretical and experimental perspectives. This review also outlines the questions to be answered for 2e-WOR and future research directions, including how to evolve H2O2 more efficiently and application development for accumulated H2O2. Hydrogen peroxide (H2O2) is a high-value green chemical oxidant widely used for industrial bleaching, chemical synthesis, and disinfection. Industrially, H2O2 is produced through the energy-intensive anthraquinone process and distributed to the point of use. There is a growing interest in electrochemically producing H2O2 onsite to mitigate transportation cost and safety concerns and leveraging renewable electricity. Most research has been dedicated to the two-electron oxygen reduction to produce H2O2. For the past decade, growing attention has been paid to the two-electron water oxidation reaction (2e-WOR) to produce H2O2. This review focuses on the research progress on 2e-WOR, including basic principles, catalyst development, and H2O2 detection. Computational approaches to study candidate materials for 2e-WOR are detailed, and various experimental reports on catalysts are summarized. Ulterior electrochemical factors that impact H2O2 production are discussed, along with device-level design. Finally, a holistic perspective on water oxidation reaction is offered, and open questions for future work are presented. Hydrogen peroxide (H2O2) is a high-value green chemical oxidant widely used for industrial bleaching, chemical synthesis, and disinfection. Industrially, H2O2 is produced through the energy-intensive anthraquinone process and distributed to the point of use. There is a growing interest in electrochemically producing H2O2 onsite to mitigate transportation cost and safety concerns and leveraging renewable electricity. Most research has been dedicated to the two-electron oxygen reduction to produce H2O2. For the past decade, growing attention has been paid to the two-electron water oxidation reaction (2e-WOR) to produce H2O2. This review focuses on the research progress on 2e-WOR, including basic principles, catalyst development, and H2O2 detection. Computational approaches to study candidate materials for 2e-WOR are detailed, and various experimental reports on catalysts are summarized. Ulterior electrochemical factors that impact H2O2 production are discussed, along with device-level design. Finally, a holistic perspective on water oxidation reaction is offered, and open questions for future work are presented. Hydrogen peroxide (H2O2) is a high-value green chemical oxidant widely used for chemical and medical applications both in industry and routine life. It is used for paper and fiber bleaching,1Cai J.Y. Evans D.J. Smith S.M. Bleaching of natural fibers with TAED and NOBS activated peroxide systems.AATCC Rev. 2001; 1: 31-34Google Scholar synthesis of other chemical species,2Sheldon R. Metal-Catalyzed Oxidations of Organic Compounds: Mechanistic Principles and Synthetic Methodology Including Biochemical Processes. Elsevier, 2012Google Scholar disinfection, and water treatment.3Wilmotte, R., Lebeau, B., Irurzun, J.-P., and Marechal, F. (2003). Disinfecting composition based on H2O2, acids and metal ions. US Patent USOO6660289B1, filed April 25, 2000, and granted December 9, 2003.Google Scholar Industrially, H2O2 is produced through the reduction/oxidation of anthraquinone.4Campos-Martin J.M. Blanco-Brieva G. Fierro J.L. Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process.Angew. Chem. Int. Ed. Engl. 2006; 45: 6962-6984Crossref PubMed Scopus (1335) Google Scholar Although this approach is efficient for large-scale production of H2O2, it requires large plants and infrastructure, and the H2O2 produced is crude, requiring costly extraction of solvents and reactants. Additionally, H2O2 is relatively unstable; hence, long-distance transportation presents substantial safety concerns. An alternative synthesis approach was devised to produce H2O2 from hydrogen (H2) oxidation without organic reactants.5Samanta C. Choudhary V.R. Direct formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in aqueous acidic reaction medium over halide-containing Pd/SiO2 catalytic system.Cat. Commun. 2007; 8: 2222-2228Crossref Scopus (41) Google Scholar This approach has been well studied in laboratory environments and reaches some promising efficiencies with the proper catalysts.6Solsona B.E. Edwards J.K. Landon P. Carley A.F. Herzing A. Kiely C.J. Hutchings G.J. Direct synthesis of hydrogen peroxide from H2 and O2 using Al 2 O3 supported Au−Pd catalysts.Chem. Mater. 2006; 18: 2689-2695Crossref Scopus (173) Google Scholar However, this route necessitates harsh conditions, such as high temperatures and pressures, as well as the use of flammable H2, both of which hinder its adoption. Electrochemical routes provide an alternative means to produce H2O2 onsite. Though the use of electricity limits this route to small-scale development, this lends itself well to portable and distributed devices, which circumvent transportation safety concerns. Figure 1 shows three possible electrochemical devices for H2O2 production. Figure 1A shows a fuel cell that converts O2 to H2O2, instead of H2O, at the cathode via the two-electron O2 reduction reaction (2e-ORR).7Xia C. Xia Y. Zhu P. Fan L. Wang H. Direct electrosynthesis of pure aqueous H2O2 solutions up to 20% by weight using a solid electrolyte.Science. 2019; 366: 226-231Crossref PubMed Scopus (212) Google Scholar Figure 1B shows an electrolyzer that oxidizes H2O to H2O2, instead of O2, at the anode via the two-electron water oxidation reaction (2e-WOR). Figure 1C shows a light-driven fuel cell for two-sided production of H2O2 at the cathode via 2e-ORR and the anode via 2e-WOR. Regardless of the device configuration, two reactions, i.e., 2e-ORR and 2e-WOR, are responsible for the electrochemical production of H2O2. The commonly studied electrochemical pathway to generate H2O2 is through the 2e-ORR.8Sánchez-Sánchez C.M. Bard A.J. Hydrogen peroxide production in the oxygen reduction reaction at different electrocatalysts as quantified by scanning electrochemical microscopy.Anal. Chem. 2009; 81: 8094-8100Crossref PubMed Scopus (194) Google Scholar 2e-ORR is an undesirable and competing reaction with the commonly studied four-electron ORR to produce water (H2O) in fuel cells.9Nørskov J.K. Rossmeisl J. Logadottir A. Lindqvist L. Kitchin J.R. Bligaard T. Jónsson H. Origin of the overpotential for oxygen reduction at a fuel-cell cathode.J. Phys. Chem. B. 2004; 108: 17886-17892Crossref Scopus (5506) Google Scholar,10Qu L. Liu Y. Baek J.B. Dai L. Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells.ACS Nano. 2010; 4: 1321-1326Crossref PubMed Scopus (3380) Google Scholar So far, several highly efficient electrocatalysts, such as alloys of Pt and Pd-Hg11Siahrostami 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 production through rational electrocatalyst design.Nat. Mater. 2013; 12: 1137-1143Crossref PubMed Scopus (584) Google Scholar and, most recently, carbon-based materials12Chen S. Chen Z. Siahrostami S. Kim T.R. Nordlund D. Sokaras D. Nowak S. To J.W.F. Higgins D. Sinclair R. et al.Defective carbon-based materials for the electrochemical synthesis of hydrogen peroxide.ACS Sustainable Chem. Eng. 2018; 6: 311-317Crossref Scopus (155) Google Scholar,13Jiang K. Back S. Akey A.J. Xia C. Hu Y. Liang W. Schaak D. Stavitski E. Nørskov J.K. Siahrostami S. Wang H. Highly selective oxygen reduction to hydrogen peroxide on transition metal single atom coordination.Nat. Commun. 2019; 10: 3997Crossref PubMed Scopus (238) Google Scholar have been developed for 2e-ORR,14Qiang Z. Chang J.H. Huang C.P. Electrochemical generation of hydrogen peroxide from dissolved oxygen in acidic solutions.Water Res. 2002; 36: 85-94Crossref PubMed Scopus (388) Google Scholar,15Lu Z. Chen G. Siahrostami S. Chen Z. Liu K. Xie J. Liao L. Wu T. Lin D. Liu Y. et al.High-efficiency oxygen reduction to hydrogen peroxide catalysed by oxidized carbon materials.Nat. Cat. 2018; 1: 156-162Crossref Scopus (571) Google Scholar and a few excellent reviews have been compiled for 2e-ORR.16Jiang Y. Ni P. Chen C. Lu Y. Yang P. Kong B. Fisher A. Wang X. Selective Electrochemical H2 O2 production through two-electron oxygen electrochemistry.Adv. Energy Mater. 2018; 8: 1801909Crossref Scopus (247) Google Scholar,17Fukuzumi S. Lee Y.M. Nam W. Solar-driven production of hydrogen peroxide from water and dioxygen.Chemistry. 2018; 24: 5016-5031Crossref PubMed Scopus (61) Google Scholar The 2e-WOR has also attracted increasing attention in recent years to produce H2O2. The 2e-WOR, in comparison with 2e-ORR, does not rely on the gas-phase reactant and provides a new approach for electrochemical H2O2 production. Figure 2 summarizes representative research on 2e-WOR chronologically. The top of the timeline focuses on experimental research and the bottom on theoretical research.18Wang L. Cao S. Guo K. Wu Z. Ma Z. Piao L. Simultaneous hydrogen and peroxide production by photocatalytic water splitting.Chin. J. Cat. 2019; 40: 470-475Crossref Scopus (0) Google Scholar Initially, photocatalysis studies suggested the formation of H2O2 using wide-band-gap metal oxides such as TiO2.19Shiraishi F. Nakasako T. Hua Z. Formation of hydrogen peroxide in photocatalytic reactions.J. Phys. Chem. 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ZnO as an active and selective catalyst for electrochemical water oxidation to hydrogen peroxide.ACS Catal. 2019; 9: 4593-4599Crossref Scopus (79) Google Scholar In Theoretical Aspects for the Rational Design of 2e-WOR Catalysts, we first discuss the theoretical aspects of the rational design of 2e-WOR electrocatalysts, including different possible mechanisms, computational details for constructing the free energy diagram, and understanding trends of activity among different catalyst materials. We also discuss how catalyst stability can be estimated using computational material design. Lastly, we discuss the effects of solvation and density functional theory (DFT) functional choice on the energetics of reaction intermediates. Together, these theoretical frameworks provide important insights and guidance on the selection of 2e-WOR catalysts for experimental efforts.28Shi X. Siahrostami S. Li G.L. Zhang Y. Chakthranont P. Studt F. Jaramillo T.F. Zheng X. Nørskov J.K. Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide.Nat. Commun. 2017; 8: 701Crossref PubMed Scopus (175) Google Scholar,29Siahrostami S. Li G.L. Viswanathan V. Nørskov J.K. One-or two-electron water oxidation, hydroxyl radical, or H2O2 evolution.J. Phys. Chem. Lett. 2017; 8: 1157-1160Crossref PubMed Scopus (124) Google Scholar In Experimental Methods for Characterizing 2e-WOR Catalysts we discuss experimental methods for characterizing 2e-WOR electrocatalysts, detail H2O2-quantification methods, and finally express our opinions on standardized reporting of electrocatalyst performance metrics. In Experimental Results of 2e-WOR Catalysts with Benchmarking Metrics, we summarize the most recent performance benchmarks of previously studied electrocatalysts. We further discuss the other factors impacting 2e-WOR in Other Important Factors Affecting 2e-WOR to Produce H2O2 and provide future perspectives for the field in Summary and Outlook. There are three possible pathways for electrochemical water oxidation reactions (WORs).28Shi X. Siahrostami S. Li G.L. Zhang Y. Chakthranont P. Studt F. Jaramillo T.F. Zheng X. Nørskov J.K. Understanding activity trends in electrochemical water oxidation to form hydrogen peroxide.Nat. Commun. 2017; 8: 701Crossref PubMed Scopus (175) Google Scholar,29Siahrostami S. Li G.L. Viswanathan V. Nørskov J.K. One-or two-electron water oxidation, hydroxyl radical, or H2O2 evolution.J. Phys. Chem. Lett. 2017; 8: 1157-1160Crossref PubMed Scopus (124) Google Scholar The reaction steps for the three WORs are schematically illustrated for a catalyst surface in Figure 3 and described in Equations 1, 2, and 3. The most studied WOR is the four-electron process (4e-WOR, Equation 1) used in electrolysis to produce O2, which is commonly referred to as the oxygen evolution reaction (OER). Water oxidation can also proceed via a two-electron pathway to produce H2O2 (Equation 2) and a one-electron pathway to produce OH radicals (OH•, Equation 3). Notably, all three WORs start with the same first step, i.e., forming adsorbed OH (OH∗). Both H2O2 and OH• are of practical value due to their strong oxidizing abilities, and both can be used for sanitization and water disinfection. OH• generally has a short lifetime, so H2O2 is of more practical interest. Thermodynamically, 4e-WOR is the most favorable reaction, as it has the lowest equilibrium potential (E°=1.23VRHE). Producing H2O2 from water (E°=1.76VRHE) requires a 530 mV higher potential than producing O2, making selective production of H2O2 intrinsically more challenging. The 4e-WOR:2H2O→O2+4(H++e−)(E°=1.23VRHE)(Equation 1) ∗+H2O→OH∗+(H++e−)(Equation 1A) OH∗→O∗+(H++e−)(Equation 1B) O∗+H2O→OOH∗+(H++e−)(Equation 1C) OOH∗→∗+O2+(H++e−)(Equation 1D) The 2e-WOR:2H2O→H2O2+2(H++e−)(E°=1.76VRHE)(Equation 2) ∗+H2O→OH∗+(H++e−)(2A, the same as 1A) OH∗+H2O→H2O2+(H++e−)+∗(Equation 2B) The one-electron WOR (1e-WOR):H2O→OH⋅(aq)+(H++e−)(E°=2.38VRHE)38Bard A. Standard Potentials in Aqueous Solution. Routledge, 2017Crossref Google Scholar(Equation 3) ∗+H2O→OH∗+(H++e−)(Equation 3A, the same as 1A) OH∗→OH⋅(aq)+∗(Equation 3B) To evaluate the energetics of WOR pathways, one needs to use DFT calculations to calculate the binding free energies of reaction intermediates, such as O∗, OH∗, and OOH∗ (ΔGO∗,ΔGOH∗,andΔGOOH∗). The free energy of H2 in the gas phase and H2O in the liquid phase are used as the reference states. This is to avoid the explicit use of O2 electronic energy due to the typical DFT error in describing triplet O2 molecules.39Pellegrini F. Montangero S. Santoro G.E. Fazio R. Adiabatic quenches through an extended quantum critical region.Phys. Rev. B. 2008; 77: 140404Crossref Scopus (55) Google Scholar Instead, standard reduction potential values are used, i.e., ΔGOH⋅=2.38eV,ΔGH2O2=2×1.76eV,and ΔGO2=4×1.23eV for the 1e−, 2e−, and 4e−WOR, respectively (Figure 4). To incorporate the effect of the electrode potential on the free energy, the computational hydrogen electrode (CHE)9Nørskov J.K. Rossmeisl J. Logadottir A. Lindqvist L. Kitchin J.R. Bligaard T. Jónsson H. Origin of the overpotential for oxygen reduction at a fuel-cell cathode.J. Phys. Chem. B. 2004; 108: 17886-17892Crossref Scopus (5506) Google Scholar is used. The CHE sets the chemical potential of a proton-electron pair equivalent to that of gas-phase H2 at potential U = 0 V, and when the potential U is applied, the chemical potential of an electron is shifted by −eU, where e and U are the elementary charge and electrode potential, respectively. Figure 4A illustrates the use of free energy diagrams to capture the activity and selectivity of a hypothetical catalyst for the three WOR pathways. The first oxidation step, which is the same for the three WOR pathways, leads to adsorbed OH∗ species on the catalyst surface. If the OH∗-binding free energy on the catalyst surface is weaker than the formation energy of aqueous hydroxyl radical, i.e., ΔGOH∗>2.38eV, it will be energetically favorable to release OH into the solution as OH•(1e-WOR). In contrast, if ΔGOH∗<2.38eV, the adsorbed OH∗ could be further oxidized to either H2O2 or adsorbed O∗ formation, depending on the ΔGO∗. Since the formation energy of H2O2, i.e., ΔGH2O2, in the solution is constant at 3.52 eV, catalyst materials with a weaker O∗-binding free energy than ΔGH2O2, i.e., ΔGO∗>3.52eV, favor the formation of H2O2 (2e-WOR), while those stronger than 3.52eV prefer the O2 evolution pathway (4e-WOR). This thermodynamic analysis suggests that ΔGOH∗ is the activity-determining factor, and ΔGO∗ determines the selectivity between the three WOR products.40Krishnamurthy D. Sumaria V. Viswanathan V. Quantifying robustness of DFT predicted pathways and activity determining elementary steps for electrochemical reactions.J. Chem. Phys. 2019; 150041717Crossref PubMed Scopus (12) Google Scholar This analysis allows us to construct a selectivity map (Figure 4B) and rationalize product selectivity on many reported experimental catalysts for WORs.29Siahrostami S. Li G.L. Viswanathan V. Nørskov J.K. One-or two-electron water oxidation, hydroxyl radical, or H2O2 evolution.J. Phys. Chem. Lett. 2017; 8: 1157-1160Crossref PubMed Scopus (124) Google Scholar Next, we determine the overpotentials (ηH2O2) and the limiting potentials (UL) for 2e-WOR using scaling relations. DFT calculations of heterogeneous catalysis often observe linear scaling relations between binding energies of reaction intermediates that interact with surfaces through the same atomic species (e.g., N∗ with NH∗ and NH2∗, C∗ with CH∗, CH2∗, and CH3∗).41Abild-Pedersen F. Greeley J. Studt F. Rossmeisl J. Munter T.R. Moses P.G. Skúlason E. Bligaard T. Nørskov J.K. Scaling properties of adsorption energies for hydrogen-containing molecules on transition-metal surfaces.Phys. Rev. Lett. 2007; 99016105Crossref PubMed Scopus (936) Google Scholar These scaling relations have facilitated reducing the dimension of reaction networks, making it possible to predict catalytic activities for a large number of catalyst materials from simple binding-energy calculations rather than constructing full free energy diagrams. For various oxide systems, scaling relations have been established between reaction intermediates; ΔGO∗ = 2ΔGOH∗ + 0.28 for metal-oxide surfaces42Man I.C. Su H.Y. Calle-Vallejo F. 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For 4e-WOR, UL has been expressed as UL,O2=max(ΔGO∗−ΔGOH∗,3.2eV−[ΔGO∗−ΔGOH∗]) based on the scaling relation between ΔGOOH∗ and ΔGOH∗.42Man 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 on oxide surfaces.ChemCatChem. 2011; 3: 1159-1165Crossref Scopus (2214) Google Scholar This expression can be further simplified using the scaling relation between ΔGO∗ and ΔGOH∗ as follows: UL,O2=max( ΔGOH∗+0.28eV,2.98eV− ΔGOH∗). For 1e-WOR and 2e-WOR, since OH∗ is the only reaction intermediate, the catalytic properties for both are solely determined by ΔGOH∗, as depicted by the solid black lines and the boundary between the green and red regions in Figure 4C. For 2e-WOR, the overpotentials (ηH2O2) and the limiting potentials (UL) are determined based on ΔGOH∗ (Equation 4A and Equation 4B) as follows:UL,H2O2=[max(ΔGOH∗,1.76eV−ΔGOH∗)]/e(Equation 4A) ηH2O2=UL,H2O2−1.76eV(Equation 4B) Figure 4C shows the volcano plots of UL,H2O2 and UL,O2 as a function of ΔGOH∗. When UL,H2O2< UL,O2, 4e-WOR is thermodynamically favored over 2e-WOR. The middle section of Figure 4C is the preferred range for 2e-WOR, where the OH∗ binds to the catalyst surface neither too strongly nor too weakly (1.6 to 2.4 eV). The optimum ΔGOH∗ is close to 1.76 eV. Using this approach, we have calculated the UL,H2O2 for various oxide materials as shown in Figure 4D. These DFT predictions were validated by experimental results as discussed in the section "Activity and Selectivity of Various Catalysts toward 2e-WOR". The reaction conditions of 2e-WOR expose catalysts to high oxidation potentials over 1.76 V versus reversible hydrogen electrode (RHE), so the electrochemical stability of catalysts is a critical factor for H2O2 production. The Pourbaix diagram can be generated by using Pymatgen44Ong S.P. Richards W.D. Jain A. Hautier G. Kocher M. Cholia S. Gunter D. Chevrier V.L. Persson K.A. Ceder G. Python materials genomics (pymatgen): a robust, open-source python library for materials analysis.Comp. Mater. Sci. 2013; 68: 314-319Crossref Scopus (1360) Google Scholar and Materials Project45Jain A. Ong S.P. Hautier G. Chen W. Richards W.D. Dacek S. Cholia S. Gunter D. Skinner D. Ceder G. Persson K.A. Commentary: the materials project: a materials genome approach to accelerating materials innovation.Apl. Mater. 2013; 1011002Crossref Scopus (3005) Google Scholar and is a great thermodynamic tool to predict the electrochemical stability of catalysts. For example, Figure 5 shows a hybrid computational and experimental Bi-V Pourbaix diagram, which shows that BiVO4 is only stable46Toma F.M. Cooper J.K. Kunzelmann V. McDowell M.T. Yu J. Larson D.M. Borys N.J. Abelyan C. Beeman J.W. Yu K.M. et al.Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes.Nat. Commun. 2016; 7: 12012Crossref PubMed Scopus (160) Google Scholar in the low potential range of 0.3 ∼ 1.1 VRHE in the pH range (∼8.3) of bicarbonate-based electrolytes often used in 2e-WOR studies, while vanadium oxides (VO4−) dissolve at higher potentials. Indeed, BiVO4 shows poor electrochemical stability as reported by Toma et al. (Figure 5)46Toma F.M. Cooper J.K. Kunzelmann V. McDowell M.T. Yu J. Larson D.M. Borys N.J. Abelyan C. Beeman J.W. Yu K.M. et al.Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes.Nat. Commun. 2016; 7: 12012Crossref PubMed Scopus (160) Google Scholar even though it has good catalytic activity for 2e-WOR. We note that the Pourbaix diagram only accounts for the thermodynamic stability, not kinetics. Nevertheless, the Pourbaix diagram, together with the descriptors discussed in Computational Details, should be used to identify both active and stable electrocatalysts for 2e-WOR for further experimental studies. The accuracy of DFT calculations for catalytic activities is critically affected by the choice of the exchange-correlation functional. Due to the high oxidation potential required for 2e-WOR (>1.76 VRHE), metal oxides may be the only materials that are somewhat stable catalysts for this reaction, which has led to their widespread use. However, the electronic and thermodynamic properties of bulk metal oxides are known to be overestimated by the standard exchange-correlation functionals, such as generalized gradient approximation (GGA) or local density approximation (LDA).47Franchini C. Podloucky R. Paier J. Marsman M. Kresse G. Ground-state properties of multivalent manganese oxides: density functional and hybrid density functional calculations.Phys. Rev. B. 2007; 75: 195128Crossref Scopus (249) Google Scholar The overestimation is due to a self-interaction error of localized d- and f-electrons. To solve this problem, the Hubbard U correction48Dudarev S.L. Botton G.A. Savrasov S.Y. Humphreys C.J. Sutton A.P. Electron-energy-loss spectra and the structural stability of nickel oxide: an LSDA+ U study.Phys. Rev. B. 1998; 57: 1505-1509Crossref Scopus (8168) Google Scholar is generally used to treat the strong onsite Coulomb interaction in oxides. However, it is not trivial to determine the Hubbard U values, which often need to be benchmarked against experimental data. Cococcioni and Gironcoli suggested that the Hubbard U values can be calculated via a linear-response method.49Cococcioni M. De Gironcoli S. Linear response approach to the calculation of the effective interaction parameters in the LDA+ U method.Phys. Rev. B. 2005; 71035105Crossref Scopus (2156) Google Scholar Hu et al. applied this method50Xu Z. Rossmeisl J. Kitchin J.R. A linear response DFT+ U study of trends in the oxygen evolution activity of transition metal rutile dioxides.J. Phys. Chem. C. 2015; 119: 4827-4833Crossref Scopus (66) Google Scholar on various rutile metal oxides for 4e-WOR and found that applying U correction shifts the adsorption energies endothermically but does not significantly affect the general trend established without U correction, such as scaling relations of binding energies. Overall, applying U correction improves the agreement between the calculated limiting potential and the experimental onset potential. Though the GGA functionals are not accurate for calculating the band gap and bulk properties of metal oxides, they have predicted chemisorption51Wellendorff J. Silbaugh T.L. Garcia-Pintos D. Nørskov J.K. Bligaard T. Studt F. Campbell C.T. A benchmark database for adsorption bond energies to transition metal surfaces and comparison to selected DFT functionals.Surf. Sci. 2015; 640: 36-44Crossref Scopus (268) Google Scholar properties with great accuracy. The DFT calculated values for adsorption energies are typically found to be within ±0.2 eV of experimental results, depending on the specific GGA functional used.51Wellendorff J. Silbaugh T.L. Garcia-Pintos D. Nørskov J.K. Bligaard T. Studt F. Campbell C.T. A benchmark database for adsorption bond energies to transition metal surfaces and comparison to selected DFT functionals.Surf. Sci. 2015; 640: 36-44Crossref Scopus (268) Google Scholar These results explain why it is still useful to use GGA functionals for predicting binding energies. Another example is a report by Chuasiripattana et al., which shows CO binding energy on the ZnO (0001) surface using the PBE functional is in agreement with experiments.52Chuasiripattana K. Warschkow O. Delley B. Stampfl C. Reaction intermediates of methanol synthesis and the water–gas-shift reaction on the ZnO (0001) surface.Surf. Sci. 2010; 604: 1742-1751Crossref Scopus (30) Google Scholar Also, a microkinetic study using BEEF-vdW qualitatively predicted experimentally observed methanol synthesis rates,53Medford A.J. Sehested J. Rossmeisl J. Chorkendorff I. Studt F. Nørskov J.K. Moses P.G. Thermochemistry and micro-kinetic analysis of methanol synthesis on ZnO (0 0 0 1).J. 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