Role of Hydroxyl Species in Hydrogen Oxidation Reaction: A DFT Study

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Role of Hydroxyl Species in Hydrogen Oxidation Reaction: A DFT Study Zhiping Feng, Li Li, Xingqun Zheng, Jing Li, Na Yang, Wei Ding, and Zidong Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04731 • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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The Journal of Physical Chemistry

Role of Hydroxyl Species in Hydrogen Oxidation Reaction: a DFT Study Zhiping Feng, Li Li*, Xingqun Zheng, Jing Li, Na Yang, Wei Ding*, Zidong Wei* The State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing Key Laboratory of Chemical Process for Clean Energy and Resource Utilization, Chongqing Key Laboratory of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Chongqing University, Shazhengjie 174, Chongqing 400044, P. R. ABSTRACT: Slow kinetics of hydrogen oxidation reaction (HOR) in alkaline electrolyte impedes the development of alkaline fuel cell systems. In this work, density functional theory (DFT) calculations were used to study the HOR mechanism on several metals (Pt(110), Ir(110), Pd(110), Ni(110) and PtRu(110), particularly by additionally considering the adsorption of hydroxyl species (OH*) on these metals. We found that the formation of OH* can transfer the potential determining step (PDS) of HOR mechanism from H* oxidation to H2O* desorption under remarkably different effects of OH* on H* and H2O*. The comprehensive rG-U relational diagrams for HOR/HER show that, apart from the widely accepted activity descriptor – H* adsorption free energy ( GH ), OH* adsorption free energy ( GOH ) and H2O* adsorption free energy ( GH2O ) also should be involved in predicting HOR catalytic activity of metal catalysts in alkaline electrolyte. When OH* formation free energy change ( rGOH = GOH , at equilibrium GH , at equilibrium potential), GH as potential) is more positive than H* oxidation free energy change ( rGH H2O = GH2O the sole descriptor indicates the HOR activity of catalysts due to hardly forming of OH* and relatively weak H2O* adsorption at a relatively low overpotential, which is the case happened on Pt(110) and Pd(110). When rGOH and rGH H2O has little difference as that occurred on Ir(110), both OH* formation and H* oxidation affects the HOR, then OH and the enhanced GH2O by OH* should be involved in evaluating the HOR activity. On Ni(110), the much lower value of rGOH than rGH H2O causes the surface mostly blocked by OH*, which suppresses the HOR. The combination of GOH , GH and GH2O gives a more precise and comprehensive description of HOR mechanism for metallic catalysts at different electrode potentials.

in the current prevailing theory was proposed to be the “unique and sole” predictor for the performance of HOR/HER electrocatalysts, it cannot afford a deep understanding of the intrinsic HOR/HER activity of catalysts. For example, the enhanced HOR/HER activity of PtRu was attributed to the Ruinduced weakening of the HBE on Pt as reflected by the more negative HUPD stripping peak position.22 While Ma’s team indicated that the negative HUPD might arise from the surface Ru rather than Pt sites, because Hupd peak of the PtRu alloy largely overlapped that of Ru/C23. Then the superior HOR activity of PtRu should not be governed by the Ru-induced change of the HBE. Additionally, it is still puzzling why HBE on catalysts increases with pH, and what strengthens HBE in alkaline electrolytes. Markovic et al. found that the HOR/HER are more sensitive to the catalysts’ surface structure in alkaline media than in acids. They attributed the different HOR/HER activity on Pt low-index face to the structure sensitive adsorption of Hupd and OH species and the effect of these species on the formation of Hopd.24-26 It means that the interaction between the H and OH should be involved in HOR/HER mechanism. Considering the rate-determining step of HOR in alkaline2729, Volmer reaction (H + OH H2O + e ), the role of OH in HOR cannot be omitted. Markovic’s team claimed that the adsorbed OH (OH*) rather than OH- is the key species for the alkaline HOR/HER.27 They proposed that the most active

1. INTRODUCTION Upon environmental protection requirements, the sustainable and clean resource, such as H2, with its high energy density and the nature of environmental friendliness, has attracted lots of scientific attention.1-3 Electrocatalytic hydrogen production by water splitting (hydrogen evolution reaction, HER), and hydrogen utilization by oxidization back to water (hydrogen oxidation reaction, HOR), are two key reactions in the energy cycle.4-6 To date, a large number of experimental and theoretical methods have been applied to design and develop HER catalysts,7-10 however, there is much less fundamental work directing towards the HOR catalysts, especially those used in basic conditions.11-12 Simultaneously, the kinetics of both HOR and HER are two orders of magnitude slower in alkaline than in acid electrolytes,13-15 and the mechanism in alkaline media is still under discussion. A mass of experiments and calculations have shown that the activity of HOR/HER catalysts is linked to the hydrogen binding energy (HBE), corresponding to the volcanic relationship and Sabatier’s principle.16-19 Yan’s team reported the sites on Ir/C catalysts with low HBE are the most active sites for HOR/HER, and the population of the sites with the smallest HBE correlates with the HOR/HER activity.20 The pH-dependent HBE mechanism also proved the linear correlation between HOR/HER and HBE.20-21 Although HBE 1

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catalysts provide the optimal interaction energies of H2/H* and OH* with the metal surface. Recently, Ma’s team also verified the promoting role of OH* on PtRu alloy by giving experimentally electrochemical and in situ spectroscopic data for HOR kinetics in alkaline media.23 Then, the HOR rate should co-depend upon the HBE and the binding of OH* to the surface in alkaline media.30 However, such a level of accounting for the HOR activity in alkaline solution is still far from clear. How does OH* influence the HOR activity of metal catalysts in alkaline? Especially, OH* can not only promote oxidation of H*, but also block H adsorption in relatively positive HOR potential region. And is there any other species affecting the HOR mechanism? Herein, a calculation model was established based on Pt(110), which represents the high active and stable noble metal catalyst for HOR in alkaline solutions26, 31-33, to quest the HOR mechanism. On the basis of density functional theory (DFT) calculations, we analyzed the Gibbs free energy of each elementary step, ascertained the potential determining step (PDS), and subsequently confirmed the HOR thermodynamic mechanism with and without the effect of OH*. We found that the formation of OH* can change the HOR mechanism by significantly strengthening the H2O adsorption on the surface. Apart from the H* adsorption free energy ( GH ) as one widely accepted HOR activity descriptor, OH* adsorption free energy ( OH ) and H2O* adsorption free energy ( GH2O ) also should be involved in predicting HOR catalytic activity of metal at a relatively high overpotential. The DFT calculations on a range of other metal catalysts, including Pd(110), Ir(110), Ni(110) and PtRu(110), were further performed to prove the generality of GH , GH2O , and OH as the descriptors.

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Tafel-Volmer pathway would be the reaction mechanism with the Volmer step as the PDS.21, 38-40 So here the thermodynamic pathways of HOR for calculation are outlined as follows: 1

H (g) + 2 2

H

(1) H + OH

H2O + e

H2O

(2)

H2O(l) +

(3)

The sign * stands for an active site on the catalyst surface; (l) and (g) are the symbol of liquid and gas phases, respectively; H* and H2O* are adsorbed intermediates of HOR/HER.

The Gibbs free energy change ( rG) for the elementary reaction is calculated according to equations (4) - (6): (4) rGH = H rGH

H2O

rGH2O

=

=

GH2O

GH2O + Gl

GH g

Gl

g

+ GH +

Ge

(5) (6)

The species Gibbs free energy is determined by 41. Specifically, the G = + H is calculated SH ; the GH2O is according to GH = EH + ZPEH SH2O , where calculated as GH2O = EH2O + ZPEH2O * E the H adsorption energy ( H ) and the H2O* adsorption energy ( EH2O ) are calculated by taking the separated H2 and single H2O as a reference state, respectively19, 42: EH = EH 1

E

E EH2O = EH2O E EH2O. The ZPEH 2 H2 and and ZPEH2O are the zero point energy difference for H* and H2O*; SH and SH2O are the entropy change between the adsorbed state and the gas phase, and T represents the temperature (298.15 K). Further, SH originates from SH

2. METHODS

1

S 2 H2

at T = 298.15 K. The Ge is represented by Ge = eU, in which U is the potential measured versus normal hydrogen electrode (NHE) at standard conditions and e is the elementary charge. The pH correction is determined by GH + = kBTln 10 × pH (kB is the Boltzmann constant; pH = 14). The Gl g is equal to RTln(p p " ), which represents the difference between GH2O(l) and GH2O(g) at 298.15 K and 3.5 kPa. More detailed calculation results are shown in Table S1 and the partial of Table S2. According to the water dissociation reaction of H2O + OH + H + + e in acid solution43-44, the Gibbs free energy change of OH* formation ( rGOH ) in alkaline solution, OH + OH + e , is calculated45-46 as rGOH = OH + GH + , where OH* adsorption free energy is e OH = EOH + ZPEOH , and OH* adsorption energy is calculated 1 E (EH2O 2EH2) (Noteworthy, the by EOH = EOH E value of EOH in Table 1 follows EOH = EOH EOH , where the EOH is the energy of sole OH). When r GOH = 0, OH* formation potential (UOH ) is obtained. Table S3 shows the oxidation potential of metals (Uoxidation). And for transition state calculation, activation energy (Ea) is represented by Ea = ETS EIS, ETS and EIS are the energies of the transition state and initial state, respectively.

In present work, all periodic DFT calculations were performed by employing the generalized gradient approximation (GGA) and the exchange-correlation energy of interacting electrons determined by the Perdew-BurkeErnzerhof (PBE) functional, which were implemented in the Vienna Ab Initio Simulation Package (VASP).34-35 The ionelectron interaction was described with the projector augmented wave (PAW) method.36 A basis set of plane waves was up to an energy cutoff of 520 eV. The Monkhorst-pack method with the centered k-point grid (3 × 3 ×1) and (5 × 5× 1) was used for structure optimization and surface calculations respectively. All of the calculations were continued until the force has converged to less than 0.01 eV Å-1, and energies have converged within 10-5 eV. The solvent effect was calculated using implicit solvation model with considering dipolar correction . The calculated models consists of the periodic (3 × 3) supercells (exceptionally, (4 × 2) supercell for PtRu(110) because of its special crystal texture) with five layers of metal atoms, and three bottom layers were fixed while two top layers were fully relaxed, such as Pt(110) shown in Figure 1. The vacuum height is 15 Å along the Z-axis to avoid artificial interactions from its periodic images. All activation barriers were calculated using the Nudged Elastic Band (NEB) method.37 The overall reaction scheme of HOR in the alkaline H2 environment is typically considered as H2(g) +2OH O(l) + 2e , which involves Tafel-Volmer pathway or Heyrovsky-Volmer pathway. Most researchers found that the 2


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The Journal of Physical Chemistry OH* adsorbe on PtRu(110), the H2O* desorption then becomes the PDS, revealing the role of GOH and GH2O in HOR.

GH2O ) as PDS Pt(110) and the rGH2O ( rGH2O = increases to about 0.402 eV. The negative rGOH and the positive rGH2O indicate that Pt(110) surface is mainly occupied by OH* and H2O*, which would block the adsorption and oxidation of H*, and then hinder the subsequent HOR. Thus, the adsorption of OH* ( OH ) and the enhanced adsorption of H2O* ( GH2O ) by OH* should be involved in describing HOR activity when the potential beyond UOH . Noticeably, GOH , which not only represents oxophilicity of catalysts, but also reveals the existence of competitive adsorption in HOR, determines the dominant activity descriptors of HOR at different electrode potentials. Once the reaction potential goes beyond Uoxidation (yellow transparent region), the oxidation surface would largely impede the whole HOR reaction. For HER, OH* hardly appears on Pt(110) surface due to the negative potential region (-1.200 V ~ -0.828 V, blue transparent region). Then, the combination of H* step is the PDS ( rGPDS = 0.365 eV), which is governed by Pt-H* binding, that is, GH is the sole activity descriptor for HER.

Figure 6 and Figure S11 show the rG-U relationship and the values of rGOH , rGH H2O and rGH2O when U equals 0 or U (U=UOH + 0.828 V) for all investigated metal catalysts. It verifies that combined consideration of OH , GH and GH2O can describe the HOR activity of metal catalysts more comprehensively and reasonably. And it also provides a practicable strategy to design and improve the catalytic activity for HOR. The UOH on investigated metals (Figure S12) follows the order: Ni(110) W PtRu(110)-Ru site < Ir(110) < Pd(110) < Pt(110), namely that Ni(110), Ru sites of PtRu(110) and Ir(110) can easily form OH* at lower overpotentials, and Pt(110) and Pd(110) exhibit strong antioxidation ability. In Figure 6a and 6b, similar to that on Pt(110), the rGOH on Pd(110) is larger than the rGH H2O , thus GH can predict the HOR activity of these precious metals reasonably at relatively large reaction potential region 6U lower than 0.711 V) due to the hardly forming of OH* and weakly H2O* adsorption. While on Ir(110) shown in Figure 6c and 6d, the rGOH is relative lower than the rGH H2O , indicating the possible coexistence of OH* formation reaction and H* oxidation reaction on Ir sites. The formation of OH* simultaneously has positive and negative effects on HOR. On the one hand, it can benefit the H* oxidation. While on the other hand, it might occupy the active sites. When U is higher than 0.188 V (green transparent region), the formed OH* on surface changes the PDS from H* oxidation to H2O* desorption. As a result, too positive value of rGH2O causes the decreasing of the HOR activity, and the negative value of * rGOH results in the blocking of Ir sites by OH . Then GOH and GH2O inevitably play an important part in affecting activity for HOR on Ir(110). Our calculation results are consistent with experiments. Durst’s team compared the HOR mechanism on Pt/C, Ir/C and Pd/C in alkaline solution53. They found that the HOR currents on Ir/C electrode decrease obviously at overpotential higher than ~0.3 V due to the early formation of OH* species. Thus, for Ir(110), depressing the formation of OH* (increasing GOH ) and accelerating the desorption of H2O* (pushing GH2O close to zero) can improve the HOR activity and stability in a wide reaction potential region. On Ni(110), in Figure 6e and 6f, rGOH is much lower than near the equilibrium potential (or U?2 V), rGH H2O indicating that OH* formation is much easier than oxidation of H*. Instead of HOR, the dominant OH* formation impedes the adsorption and oxidation of H* on Ni(110), leading to low HOR activity. The too negative UOH and Uoxidation further confirm the oxophilic behavior of Ni(110) in alkaline solution. When U equals 0.026 V or rGOH equals zero, the value of rGH2O becomes too positive, denoting hardly desorption of H2O* and the poor HOR catalytic activity of Ni(110). Then the key for improving HOR catalytic activity of transition metal, such as Ni, is to restrain the formation of OH* or decrease GH close to zero. It not only provides a clean surface for H* adsorption and oxidation, but also makes possible tuning HOR in a relatively wide potential region. Yan’s team fabricated a series of Ni-based catalysts, such as Ni/CNTs and CoNiMo catalysts17, 54 for HOR. Though the initial HOR activities of the Ni-based catalysts were improved obviously by decreasing

Therefore, OH* plays an important part in determining the HOR mechanism. Combining OH , GH and GH2O might give a comprehensive description about HOR mechanism of catalysts at different electrode potentials.

3.3 Thermodynamic HOR Mechanism on other Metal Catalysts To make above calculation results more convincing, the study was further extended to other metals including Pd(110), Ir(110), Ni(110) and alloy PtRu(110). The EH and EH2O with and without OH* coadsorption in vacuum were investigated thoroughly, and the optimized and most stable adsorption geometries of intermediates on these metal surfaces were displayed in Figure S5-S8. The EH and EH2O on all investigated metal surfaces further confirm the weakening effect of OH* on H* and the enhancing effect of OH* on H2O* (Figure 5), and the enhancing effect is more notable than the weakening effect. Compared with Pt(110), all other metal surfaces have lower EH . Co-adsorption energies of H* with OH* on Ni(110) and PtRu(110) show an inconsistent change which is resulted from the relatively strong OH* binding energy and different adsorption sites (Figure S9). (a)

(b)

Figure 5. Adsorption energies of (a) H* and (b) H2O* on different metals surfaces with and without OH*.

Similar as that of Pt(110), the Gibbs free energy diagrams of all investigated metals in Figure S10 show that the dissociation of H2 is downward and exothermic on clean metal surfaces, which means that the PDS of HOR is H* oxidation at equilibrium potential. That is, GH indicates the HOR activity on clean metal surfaces. When OH* forms on surface, the H2O* desorption step on Pd(110), Ir(110) and Ni(110) becomes the PDS as that on Pt(110). While on PtRu(110), due to Ru atom as OH* adsorption site and Pt atom as H* adsorption site, the H* oxidation remains the PDS. When two 5



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Comprehensive analysis of rGOH , rGH H2O and r GH2O on all investigated catalysts at different overpotentials reveals that the change of activity descriptors for HOR depends on the potential and the oxophilicity of catalysts, in which the oxophilicity of catalysts can be indicated by OH . At low overpotentials, GH can be the sole HOR activity descriptor for metal catalysts before the formation of OH*. With the increasing of overpotentials, the role of OH , GH2O in HOR should be considered, especially when OH* forms on surface. For Pt(110) and Pd(110), GH is the uniquely proper activity descriptor of HOR at a large wide potential region, because rGOH is more positive than r GH H2O . For Ir(110) the relative low value of rGH H2O * relative to rGOH shows the possible coexistence of OH formation reaction and HOR, then GOH and the enhanced GH2O by OH* should be combined with GH to indicate the HOR activity. For Ni(110), the much low value of rGOH compared to rGOH indicates that OH* formation is the dominate reaction compared with HOR, then GOH represents the poor activity of Ni(110) though the GH on Ni(110) is close to zero.

the HBE, these high activities cannot be reamined as the applied overpotential exceeds 0.1 V17, 54. The narrow catalytic potential region of Ni-based catalysts is consistent with our calculation, that is, the formation of OH* suppresses the HOR. Therefore, inhibiting OH* formation (increasing GOH ) is one of the most important challenges to develop non-precious metal electrocatalysts for HOR. In Figure 6g and 6h, Ru shows a similar oxophilic behavior as Ni. Ru sites on PtRu(110) not only modulate the GH by changing the electronic structure of Pt, but also act as OH* formation site to benefit the H* oxidation. Though the value of * rGOH is lower than that of rGH H2O , OH would not * occupy the active Pt sites of H oxidation, because OH* forms on Ru sites. Correspondingly, the large potential interval between UOH on Ru and Uoxidation on Pt provides a wide potential region for HOR. As shown in Figure 6h, rGH H2O and rGH2O on PtRu(110) are all close to zero whatever at low or high overpotential, indicating the most optimum HOR catalytic activity. It means that constructing specific adsorption sites for H* and OH* respectively is more beneficial to modulate the rGOH and rGH2O of catalysts.

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(10) Gao, G. P.; O'Mullane, A. P.; Du, A. J. 2d Mxenes: A New Family of Promising Catalysts for the Hydrogen Evolution Reaction. ACS Catal. 2017, 7, 494-500 (11) Sun, Y.; Dai, Y.; Liu, Y.; Chen, S. A Rotating Disk Electrode Study of the Particle Size Effects of Pt for the Hydrogen Oxidation Reaction. Phys. Chem. Chem. Phys. 2012, 14, 22782285. (12) Liao, J.; Ding, W.; Tao, S.; Nie, Y.; Li, W.; Wu, G.; Chen, S.; Li, L.; Wei, Z. Carbon Supported IrM (M = Fe, Ni, Co) Alloy Nanoparticles for the Catalysis of Hydrogen Oxidation in Acidic and Alkaline Medium. Chin. J. Catal. 2016, 37, 1142-1148. (13) Sheng, W. C.; Gasteiger, H. A.; Shao-Horn, Y. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes. J. Electro. Soc. 2010, 157, 1529-1536. (14) Pan, J.; Chen, C.; Li, Y.; Wang, L.; Tan, L.; Li, G.; Tang, X.; Xiao, L.; Lu, J.; Zhuang, L. Constructing Ionic Highway in Alkaline Polymer Electrolytes. Energy Environ. Sci. 2014, 7, 354360. (15) Ram Subbaraman, D. T.; Dusan S.; Kee-Chul C.; Masanobu U.; Arvydas P. P.; Vojislav S.; N. M. Markovic. Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li4-Ni(OH)5-Pt Interfaces. Sci. 2011, 43, 1256-1260. (16) Laursen, A. B.; Varela, A. S.; Dionigi, F.; Fanchiu, H.; Miller, C.; Trinhammer, O. L.; Rossmeisl, J.; Dahl, S. Electrochemical Hydrogen Evolution: Sabatier’s Principle and the Volcano Plot. J. Chemi. Edu. 2012, 89, 1595-1599. (17) Sheng, W. C.; Bivens, A. P.; Myint, M.; Zhuang, Z. B.; Forest, R. V.; Fang, Q. R.; Chen, J. G.; Yan, Y. S.. Non-precious Metal Electrocatalysts with High Activity for Hydrogen Oxidation Reaction in Alkaline Electrolytes. Energy Environ. Sci. 2014, 7, 1719-1724. (18) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen Evolution by a Metalfree Electrocatalyst. Nat Commun. 2014, 5, 3783-3791. (19) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current for Hydrogen Evolution. Cheminform 2005, 36, 1215412154. (20) Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. S. Correlating Hydrogen Oxidation and Evolution Activity on Platinum at Different pH with Measured Hydrogen Binding Energy. Nat. Commun 2015, 6, 5848-5854. (21) Zheng, J.; Sheng, W. C.; Zhuang, Z. B.; Xu, B. J.; Yan, Y. S. Universal Dependence of Hydrogen Oxidation and Evolution Reaction Activity of Platinum-group Metals on pH and Hydrogen Binding Energy. Sci. Adv. 2016, 2, 1501602-1501610. (22) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Pt–Ru Catalyzed Hydrogen Oxidation in Alkaline Media: Oxophilic Effect or Electronic Effect? Energy Environ. Sci. 2015, 8, 177-181. (23) Li, J.; Ghoshal, S.; Bates, M. K.; Miller, T. E.; Davies, V.; Stavitski, E.; Attenkofer, K.; Mukerjee, S.; Ma, Z. F.; Jia, Q.Y. Experimental Proof of the Bifunctional Mechanism for the Hydrogen Oxidation in Alkaline Media. Angew Chem. Int. Ed. Engl. 2017, 56, 15594-15598. (24) Markovic, N. M.; Sarraf, S. T.; Gasteiger, H. A.; Ross, P. N. Hydrogen Electrochemistry on Platinum Low-index SingleCrystal Surfaces in Alkaline Solution. J. Chem. Soc. Faraday T. 1996, 92, 3719-3725. (25) Schmidt, T. J.; Ross, P. N.; Markovic, N. M. Temperature Dependent Surface Electrochemistry on Pt Single Crystals in Alkaline Electrolytes: The Hydrogen Evolution/Oxidation Reaction. J. Electro. Chem. 2002, 52, 252-260. (26) Schmidt, T. J.; Ross, P. N.; Markovic, N. M. Temperature Dependent Surface Electrochemistry on Pt Single Crystals in Alkaline Electrolytes Part 2: The Hydrogen Evolution/Oxidation Reaction. J. Electro. Chem. 2002, 524, 252-260. (27) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der Vliet, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Improving the Hydrogen Oxidation Reaction Rate by Promotion of Hydroxyl Adsorption. Nat. Chem. 2013, 5, 300-306.

On Ir(110) and PtRu(110), the combination of GOH , GH and GH2O determines the activity of HOR because of the coexistence of the OH* formation and H* oxidation. On Ni(110), too easily forming of OH* hampers the HOR, then GOH denotes the HOR activity. Therefore, according to the effect of OH* on H* and H2O* adsorption, modulating GH and GH2O by changing GOH should be an effective way to rationally design HOR catalysts, especially non-precious metal catalysts, with high activity and stability in a wide reaction potential region.

ASSOCIATED CONTENT Supporting Information The bonding length and –ICOHP values of Pt-O(H2O*) on Pt(110) with and without OH*; The figure of transition state; The geometric structures and adsorption energies of H* and H2O* on other metals with and without OH*; OH* adsorption energy on different metals; Gibbs free energy diagrams.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Li); [email protected] (Ding); [email protected] (Wei), Tel: +86 2365678945

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No.: 21576032, 21822803, and 21761162015). In addition, the work was carried at Lvliang Cloud Computing Center of China and the calculations were performed on TianHe-2.

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Role of Hydroxyl Species in Hydrogen Oxidation Reaction: A DFT Study

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