Theoretical Insight into the Mechanism of Photoelectrochemical

Aug 9, 2017 - Oxygen evolution reaction (OER) is the limiting step in a photoelectrochemical (PEC) water splitting process. In this paper, the effect ...
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Theoretical Insight into the Mechanism of Photoelectrochemical Oxygen Evolution Reaction on BiVO4 Anode with Oxygen Vacancy Jun Hu,†,‡ Xin Zhao,† Wei Chen,† Haibin Su,*,† and Zhong Chen*,† †

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore School of Chemical Engineering, Northwest University, Xi’an, P. R. China 710069

‡

S Supporting Information *

ABSTRACT: Oxygen evolution reaction (OER) is the limiting step in a photoelectrochemical (PEC) water splitting process. In this paper, the effect of oxygen vacancies (Ovac) on BiVO4 photoanode for PEC water splitting is studied using firstprinciples calculations. The results indicate that the holes transfer at the electrode/electrolyte interface play a defining role in determining the surface catalytic activities, and thus functional characteristics of BiVO4 photoanode. There are two main reasons behind the enhancement of OER on the surface of the photoanode. First, the V site becomes the active site for PEC water splitting and the number of the active sites are greatly increased by inducing oxygen vacancies. Second, the adsorption energies of H2Oads, OHads, and Oads are higher in the presence of Ovac, which implies enhanced hole transfer from the photoanode surface to the electrolyte. The change of Gibbs free energy indicates a high possibility of spontaneous charge transfer to the electrolyte, facilitating OER on surfaces with Ovac. These results provide important insights into the roles of Ovac on BiVO4 surface for photocatalytic reactions.

1. INTRODUCTION Photoelectrochemical (PEC) water-splitting is an attractive technique for the production of hydrogen and oxygen from water without causing pollution to the environment. Monoclinic clinobisvanite bismuth scheelite (ms-BiVO4) has attracted a lot of attention recently for on-demand oxygen production with advantageous properties such as suitable conduction band edge position, the ability to withstand oxidizing conditions, and material abundance.1,2 Up to now, great progress has been made to improve the photocurrent density of ms-BiVO4, through ways such as morphology control,3−7 n-type doping,8−11 heterojunction formation,12,13 cocatalyst loading, and surface states generation.14−16 However, from a practical point of view, there is still a long way to go before the ms-BiVO4 can be used for longterm large scale water-splitting due to its poor electron mobility, short hole diffusion length, slow water oxidation kinetics, and significant electron−hole recombination.17 Recently, in situ experimental results show that the oxygen vacancies (Ovac) on a crystal facet could yield impressive performance due to enhanced electron transport in most of metal oxides such as TiO2, WO3 and BiVO4.18−26 For example, Kong and Zhao et al. reported that surface Ovac in TiO2 can more effectively improve PEC efficiency than bulk Ovac.27,28 In this regard, it was found that the PEC performance of BiVO4 photoanodes was greatly enhanced by H2 treatment to form Ovac.29,30 Kim et al. reported solar water splitting that exceeds 2% efficiency by nitrogen-treated BiVO4 photoanodes by nitrogen doping.31 Apart from the experimental work, research has also © 2017 American Chemical Society

been devoted to studying the effect of electronic structure of bulk BiVO4 and specific crystal facets using quantum chemical simulation.1,32 Knowledge on the electronic properties of materials indeed helps to understand the mechanism of PEC catalysis. For example, Kim et al. studied the effect of Ovac on the band structure of BiVO4, and found Ovac will increase the density and mobility of holes. This would then make electron−hole pairs more easily separated.31 Ovac, which can introduce mobile electrons to improve the electron transport in BiVO4, plays an important role in determining the conductivities.12 Wang et al. and Yin et al. calculated the formation energies of Ovac in BiVO4 and found Ovac as shallow donors with low formation energy allows better charge separation.29,33 Despite the recent advances in experimental and theoretical analyses on the fundamental understanding of the electronic properties of BiVO4 with Ovac, the detailed information on the effect of Ovac in the process of adsorption, bond-breaking and bond making on BiVO4, is still missing from the literature. It is generally accepted that the oxygen evolution reaction (OER) is the bottleneck of the overall water splitting process, because OER involves four-electron-transfer and is energetically an uphill reaction.34 Therefore, understanding the OER heterogeneous reactions requires not only electronic properties of the semiconductor but also detailed knowledge of how reaction Received: June 15, 2017 Revised: August 9, 2017 Published: August 9, 2017 18702

DOI: 10.1021/acs.jpcc.7b05884 J. Phys. Chem. C 2017, 121, 18702−18709

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The Journal of Physical Chemistry C intermediates interact with the surface, and how the bondbreaking and bond making processes occur at the active sites.35−37 Further studies, especially the ones on the charge transfer across the electrode/electrolyte interface are critical for us to understand the reasons for the observed slow O2 evolution rate and the significant electron−hole recombination. In this paper, a systematic theoretical analysis on the roles by Ovac is presented using the (010) facet of BiVO4 compound. We show that Ovac increases not only the number of active sites on BiVO4 anode, but also the adsorption energies of reaction intermediates, which leads to enhanced holes transfer from electrolyte to the electrode surface. This study also confirms that Ovac on the surface affects the Gibbs free energy in the elementary steps of the catalysis process.

(ZPE) corrections were not considered during the above calculations.44 On the basis of previous experiments and simulations, a plausible pathway of water splitting reactions on BiVO4 photoanode has been reported as a series of reactions, reactions R1−R7, where the subscript of “ads” represents the adsorbed species.35,45

2. COMPUTATIONAL DETAILS The CASTEP module of the Materials Studio software (Accelrys Inc.) was employed for the quantum chemistry calculations. During the calculations, the (010) facet of BiVO4 was chosen, since it is one of the most stable and highest photocatalytic activity facets under the practical conditions, and thus has been widely used to explore the structural and catalytic activities of BiVO4.38−40 The BiVO4 (010) facet was obtained from the optimized ms-BiVO4 bulk (space group 15) with a vacuum region of 15 Å.35 A supercell with size of 10.4 × 14.6 × 25.6 Å3 was built with 12 layers, and the bottom six layers were constrained. During the calculations, self-consistent periodic density functional theory (DFT) was adopted to explore the electronic structure and catalytic activities on the BiVO4 (010) facet. Perdew−Burke−Ernzerhof (PBE) approximation was selected as the generalized gradient approximation (GGA) method to calculate the exchange-correlation energy. The Broyden−Fletcher−Goldfarb−Shanno (BFGS) scheme was selected as the minimization algorithm. And DFT-D correction was used for dispersion corrections. Hubbard U-corrections to the d electrons of V (LDA+U, effective U(V) = 2.5 eV) and spinpolarized were performed during the calculations. The energy cutoff is 380 eV and the SCF tolerance is 1.0 × 10−6 eV/atom. The optimization is completed when the energy, maximum force, maximum stress and maximum displacement are smaller than 5.0 × 10−6 eV/atom, 0.01 eV/Å, 0.02 GPa, and 5.0 × 10−4 Å, respectively. The Γ point only k-point samplings were used with 2 × 2 supercells; the results with different k-points and supercells were compared as shown in Figure S1. Fermi level is simply defined as the valence band maximum (VBM) in the CASTEP code.41,42 The calculated crystallographic parameters and XRD data are listed in Table S1 and Figure S2, and are compared with experimental results. The vacancy formation energy (Evf) can be estimated by

Oads + H2Oads + h+ + e− ↔ Oads + H + + e− + OHads

Evf = Eds + EO2 − Eps

(R1)

H 2Oads + h+ + e− ↔ H+ + e− + OHads

(R2)

OHads + h+ + e− ↔ H+ + e− + Oads

(R3)

Oads + H 2O ↔ Oads + H 2Oads

(R4)

(R5) +

−

+

−

Oads + OHads + h + e ↔ H + e + O2ads

(R6)

O2ads ↔ O2

(R7)

The Gibbs free energy for reaction can quantitatively describe the ability of adsorption and desorption. Take reaction R2 for example, ΔGT,P(R2) can be expressed as ΔGT , P (R2) = GT , P (surface@OH) + GT , P(H+ + e−) − GT , P (surface@H 2O) − GT , P(h+ + e−) (3)

Because ΔGT,P of a solid is not significantly changed in the wide ranges of temperature and pressures,46,47 the Gibbs free energies of defect-free BiVO4 (010) facet and contaminated surfaces can be predicted by using DFT according to DFT GT , P (surface@OH) = G0,0(surface@OH) = Esurface@OH

(4) DFT GT , P (surface@H 2O) = G0,0(surface@H 2O) = Esurface@H 2O

(5)

In contrast to a solid phase, the Gibbs free energies of a gas phase is greatly influenced by pressure and temperature.48 Taking O2 for example, the Gibbs free energies of O2 can be estimated by using eq 6:49 GT , P (O2 ) = EODFT + μO (T , P0) + kT ln(PO2 /P0) 2 2

EDFT O2 ,

EDFT surface@OH

(6)

EDFT surface@H2O

where and are total energies of O2, BiVO4 (010) facet with adsorbed OHabs, and H2Oabs by DFT calculation, respectively. μO2(T,P0) are the chemical potentials of the gas O2 molecules at temperature T. kT ln(PO2/P0) is a pressure-dependent value from the Maxwell relation, where k is the Boltzmann constant, and PO2/P0 is equal to 0.21 for an atmospheric environment. On the basis of the definition of standard hydrogen electrode (SHE), we obtained eq 7 as below:

(1)

where Eds is the total energy of defective surface, Eps is the total energy of perfect surface and EO2 is the energy of an O2 molecule in the gas phase.43 The adsorption energy (Eads) between a BiVO4 (010) facet and adsorbed particles was computed as Eads = Emolecule + surface − Emolecule − Esurface

H 2O ↔ H 2Oads

ΔG0,0(H) = 0.5G295.15K,1bar(H 2) − G0,0(H+ + e−) = 0 (7)

(2)

G0,0(H+ + e−) is the free energy needed for the separation of a pair of electron and hole. In OER, this potential is the sum of equilibrium potential and OER over potential. Therefore, the OER onset potential in PEC experiments are estimated by Eg

where Emolecule+surface is the total energy of the BiVO4 (010) facet and adsorbent, Emolecule is the energy of adsorption molecules, and Esurface is the energy of BiVO4 (010) facet. Zero-point energy 18703

DOI: 10.1021/acs.jpcc.7b05884 J. Phys. Chem. C 2017, 121, 18702−18709

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The Journal of Physical Chemistry C (the bandgap of BiVO4) and Ev (the applied electrical energy at the onset potential for the water oxidation reaction): G(h+ + e−) = e|U | = e|E 0 + UOER| = Eg + Ev

band, which lies from around 0.96 eV above Fermi energy to about 1.29 eV below Fermi energy. Near the Fermi level, there are resonance peaks between O 2p and V 3d in the V by removing O adatom as shown in Figure 1). The Fermi level is pinned above these localized states, indicating that impurity levels are occupied. This suggests that the Ovac defects in the BiVO4 (010) facet will introduce deep level states, not act as shallow donors. The negative ionization energy of Ovac centers will change into 2+ charge state by spontaneous ionization and the two electrons present at Ovac site will be ionized and localized at the V atoms. This is very helpful for donated electrons to form small electron polarons via self-trapping at the V site, resulting in occupied electronic states deep within the BiVO4 bandgap. This result is also supported by Copper and Kim’s results.27,31 Vacancies induced small polarons were also reported and discussed in SrTiO3.56 More information about band DOS can be found in Figure S3. Figure 2d shows work function for BiVO4 (010) facets with Ovac are greatly reduced. It means holes can be easily transferred from the solid to the surface. 3.2. Adsorption Proprieties. The adsorption on the BiVO4 surface plays an important role in the PEC water oxidation reactions. Figure S4 shows the highest occupied molecular orbital (HOMO) region changed from bulk to the surface when Ovac were generated on the surface. Since the energy of the HOMO orbital is directly related to the ionization potential and characterizes the susceptibility of the molecule to attacks by electrophiles, introduction of surface Ovac will greatly increase the electron donor ability of the surface and easy adsorption of O adatom in H2O, which has electron acceptor ability on O−H antibonding orbitals 4a1 and 2b2. Therefore, Ovac on the surface may influence the adsorption properties and the adsorption of H2Oads, OHads, Oads on different BiVO4 surfaces with or without Ovac were first investigated. Top site (above V or Bi adatom) and bridge site (between V or Bi adatom) were considered during adsorption, and the most stable adsorption site was characterized according to the minimum energy of the system. Figure 3 shows the adsorption structures, energies and electron density difference of H2Oads, OHads, and Oads involved in a water splitting process on BiVO4 (010) facet with Ovac. Figure 3 manifests detailed information about the adsorption of several key species on BiVO4 (010) facet with Ovac. Compared to the cases without Ovac, the most stable adsorption position has significantly changed, i.e. H2O is likely to be adsorbed on Bi top site on perfect BiVO4 (010) facet with the adsorption energy of −0.63 eV, while it is more easily adsorbed on the bridge site of Bi−V on BiVO4 (010) facet with Ovac with the adsorption energy of −0.74 eV. So the active sites are totally different for BiVO4 (010) facet with Ovac or without Ovac. This can be explained as follows. A V atom on the perfect surface can not be further oxidized because its highest valence is +5, while the Bi on the same surface can be further oxidized to a higher valence state (from +3 to +5). So the Bi is the only active site on the perfect surface, which is also supported by the results obtained by Oshikiri.57 For BiVO4 (010) facet with Ovac, Bi and V have an unshared pair of electrons due to the loss of O, so Bi and V can accept holes (losing one unshared electron). This means Ovac has changed the V atoms to be active sites. Figure S5 also indicates adsorbents can be more easily adsorbed on the position of Ovac on BiVO4 (010) surface because the surface states on the band gap are gradually decreased until complete disappearance along with the adsorption of H2O, OH, and O. Furthermore, the adsorption energies on the facet with Ovac are more negative than that of facet without Ovac, indicating Ovac can also increase the

(8)

On the basis of previous experimental results, the bandgap of bulk BiVO4 is around 2.4 eV and the onset potential for water splitting using BiVO4 photoelectrode is −0.3 V (vs NHE). Therefore, G(h+ + e−) in this system should be 2.1 eV.35 The change of the Gibbs free energies for water splitting reactions on the BiVO4 (010) facets can be obtained based on Table S2.

3. RESULTS 3.1. Geometric and Electronic Structure. The optimized unit cell of bulk monoclinic BiVO4, BiVO4 (010) facet, and Ovac with (010) facet are showed in Figure 1.

Figure 1. (a) Optimized unit cell of bulk ms-BiVO4, where the cleavage plane (010) and surface vectors named u⟨001⟩, v⟨100⟩ are also indicated. (b) Optimized BiVO4 (010) facet and BiVO4 (010) facet with Ovac. From the top view, surface atoms are shown in colored spheres and other atoms are only indicated by a stick connection point. There are 16 Bi (purple), 16 V (gray), and 64 O (red) atoms on BiVO4 (010) facet, amounting to a total of 96 atoms.

Figure 1 shows the optimized structure during the calculation. It shows loss of oxygen will make V atom closer to adjacent oxygen atom while there are no obvious changes for the positon of the Bi atom. The calculated band structure and density of states (DOS) of bulk BiVO4, BiVO4 (010), and BiVO4 (010) facets with Ovac (Figure 2) are plotted and compared along high symmetry directions in the brillouin zone. As illustrated in Figure 2a, we found that bulk ms-BiVO4 is an indirect band gap semiconductor and the predicted band gap is nearly 2.147 eV, which are consistent with previous calculations.50−54 The calculated band gap is smaller than the experimental values of 2.4−2.5 eV, due to underestimating the band gap by using DFT.55 It can be obviously observed that there are Bi 6s and O 2p resonance peaks at the valence band maximum (VBM), while there are V 3d and O 2p resonance peaks at the conduction band minimum (CBM). This shows electrons are mainly excited from bond of Bi 6s and O 2p orbitals to the bond of V 3d and O 2p orbitals, leaving behind photogenerated holes on VBM. Figure 2b shows that the BiVO4 (010) facet is a direct band gap semiconductor located in the midway between F(0,0.5,0) and Q(0,0.5,0.5) and the predicted band gap is nearly 2.131 eV. VBM and CBM are nearly the same as that of bulk msBiVO4. However, the band structure is greatly changed when there are Ovac moieties on the BiVO4 (010) facet. As shown in Figure 2c, Fermi energy level moves into the middle of forbidden 18704

DOI: 10.1021/acs.jpcc.7b05884 J. Phys. Chem. C 2017, 121, 18702−18709

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Figure 2. Calculated band structure of BiVO4 and the corresponding partial density of states (PDOS): (a) band structure and PDOS of bulk BiVO4, (b) band structure and PDOS of BiVO4 (010) facet, and (c) band structure and PDOS of BiVO4 (010) facet with Ovac. (d) Electrostatic potentials profile along the Z axis of the BiVO4 (010) facets with and without Ovac.

Figure 3. Adsorption structures, energies and electron density difference of H2Oads, OHads and Oads involved in a water splitting process on BiVO4 (010) facet with Ovac. The surface atoms are shown in colored spheres, and other atoms are shown as a stick-connection point in order to clearly observe the surface. The “-”, “--” and “@” signs stand for bond, close contact, and adsorption state on the facet, respectively. In the electron density difference map, a loss of electrons is indicated in blue, while electron enrichment is indicated in red. White color indicates regions with very little change in electron density.

activity of the surface active sites. The results are consistent with

The different charge density gives an important insight into the bonding properties and the electron redistribution due to the adsorption. As shown in Figure 3, only top of the Bi loses electrons on defect-free surface. This indicates the potential

the frontier molecular orbital and work functions analysis, as shown in Figures S4 and S6. 18705

DOI: 10.1021/acs.jpcc.7b05884 J. Phys. Chem. C 2017, 121, 18702−18709

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Figure 4. Free-energy profiles of OER on BiVO4 (010) facets and BiVO4 (010) facets with Ovac with G(h++e−) = 0 and 2.1 eV. The unit of Gibbs free energy is eV.

Figure 5. Schematic picture of the PEC water splitting process on BiVO4 (010) facet with or without Ovac. Adsorption energy along surface is illustrated.

presence of Ovac. On the basis of the above analysis, it can be concluded that the introduction of Ovac will not only increase the number of active sites but also enhance the bonding between the adsorption species and the surface, which makes charge transfer easier from the surface to the electrolyte. 3.3. Gibbs Free Energy during Reactions. Gibbs free energy indicates the direction of a chemical reaction based on thermodynamics. The calculated Gibbs free energy during OER process on the BiVO4 (010) facets is shown in Figure 4. It can be seen that it is impossible to generate oxygen without light due to the high positive value of ΔG. While it becomes possible under visible-light irradiation and applied potential bias as ΔG becomes negative. When comparing the two processes, we found the reaction processes are different with or without Ovac although the initial and final ΔG are same. For defect-free BiVO4 (010) surface, the H2Oads is easily adsorbed on top of Bi, and the adsorption energy is −0.63 eV. The adsorbed H2O moiety will lose one of the proton over the Bi position with the help of the photogenerated hole (ΔG = 0.01 eV). After that, the OHads will

adsorbed sites are the Bi adatoms through O for various species. The feasible reason is that atomic O, having many electrons, prefers the acidic Bi adatom. When H2Oads are adsorbed on top of Bi, the bond length of Bi−O becomes 2.541 Å, which is longer than that in the bulk BiVO4 (2.417 Å). This indicates that charge transfer on the surface is more difficult than in the bulk and recombination will happen more easily for defect-free surface. OHads are still adsorbed on the Bi adatom with slight move to the hollow site with bond length of Bi−O becoming 2.176 Å because unsaturated species prefer the bridge or hollow site.44 The short length makes the hole transfer from Bi to O easier. For O adatom, it is likely to be adsorbed on the bridge of Bi−V with a lower adsorption energy. On the Ovac surface, Bi and V have the ability to absorb H2Oads, OHads, and Oads on the bridge of Bi−V. In general, all of the selected species on BiVO4 (010) facet with Ovac have more negative adsorption energies and shorter bond length than those on perfect BiVO4 (010) facet. It means that charge transfer from surface to solution is made easier on BiVO4 (010) facet with 18706

DOI: 10.1021/acs.jpcc.7b05884 J. Phys. Chem. C 2017, 121, 18702−18709

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The Journal of Physical Chemistry C

5. CONCLUSIONS The effects of Ovac on BiVO4 photoanode surface for PEC water splitting are studied using DFT calculations. The results indicate there are at least two reasons for the enhancement of OER on the surface with Ovac. The first is that the V site becomes the active sites for PEC water splitting by introducing Ovac. The other is the adsorption energies of H2Oads, OHads and Oads are higher on the facet with Ovac than that without Ovac, which will enhance holes transfer from the anode surface to the electrolyte. These results provide the insight into the roles of Ovac on BiVO4 (010) facet and are important for the design of efficient faceted semiconductor photocatalysts for water splitting.

lose another H adatom and generate O adatom. The resulting O adatom is extremely electrophilic and immediately obtains an electron by bonding to an adjacent V. Then another H2O will be adsorbed on top of Bi spontaneously. The loss of the two protons on this H2O is thermodynamically stable because the ΔG is negative for the two reactions. The last step is O−O separation from (010) surface with the help of potential bias. For the BiVO4 (010) surface with Ovac, the H2Oads are easily adsorbed on the bridge of Bi−V and the adsorption generates V− O bond. The bond length and bond energy are 2.304 Å and −0.74 eV, respectively. The more negative adsorption energy will induce the hole transfer from V to H2O and remove one of the proton in H2O. The ΔG goes down to −2.62 eV after the first proton is lost. After that, the adsorbed OH moiety loses another proton and O adatom will generate a bond with Bi. Then, another H2O is adsorbed on the hollow site. In this case, the most negative ΔG (nearly −4.76 eV) is obtained, indicating this structure is more stable. The photogenerated hole then transfers from Bi to O and one of the proton in H2O is removed. The negative ΔG indicates this reaction is thermodynamically favorable. At last, the ΔG goes to −2.99 eV when the O−O is desorbed from the surface. In the whole process, the Gibbs free energy change on Ovac surface is more negative than that on defect-free surface, which indicates a higher possibility of spontaneous chemical reaction on Ovac surface under appropriate external driving forces.36 Furthermore, this free-energy profiles also provides a means to understand some of the key points to consider involving elemental water splitting reactions. In general, the binding of Oads and OHads on BiVO4 (010) facets is too weak to transfer the holes from the interface to the electrolyte. Therefore, the reactions are limited by the adsorption of Oads and OHads, whereas strong-binding of Oads and OHads on BiVO4 (010) facets with Ovac are limited by desorption process (bondbreaking processes).58

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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.jpcc.7b05884. Simulation verification, more information about the density of states, highest occupied molecular orbital, and work function (Tables S1and S2 and Figures S1−S6) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.). *E-mail: [email protected] (Z.C.). ORCID

Haibin Su: 0000-0001-9760-6567 Zhong Chen: 0000-0001-7518-1414 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Education of Singapore (RG15/16), the National Natural Science Foundation of China (No. 21676216), the China Postdoctoral Science Foundation (No. 2014M550507; 2015T81046), and the Scientific Research Plan Projects of the Shaanxi Education Department (14JK1751) is greatly acknowledged.

4. DISCUSSION On the basis of above analysis, the mechanism of the effect of Ovac on the PEC water splitting process is proposed, as shown in Figure 5. The adsorption of several key species on BiVO4 (010) facet with Ovac plays a defining role in enhancing the PEC performance of BiVO4 photoanodes. There are at least two main reasons for the improved performance of the defect BiVO4 (010) surface. The first is that Ovac greatly increases the number of active sites on the facet. Only the Bi site is the active site for PEC water splitting process for BiVO4 (010) facet without Ovac, while Bi and V sites, especially the V sites, are active for BiVO4 (010) facet with Ovac. As a result, the number of active sites are greatly increased by introducing Ovac. Another important reason is that the Ovac improves the hole transfer from surface to adsorbents due to close adsorption of reaction species on the surface. This is consistent with the experimental results where Ovac decreases the onset potential for about 100 mV.27 As shown in Figure 5, the high adsorption energy on V site will induce a decrease in the adsorption length. The orbits of Bi 6s and V 3d overlap with the O 2p in key adsorption species, leading to enhanced electrons transfer from solution to anode surface. Thus, the four electron reduction of O2 on the BiVO4 (010) facet could occur more efficiently in the presence of Ovac. The change of Gibbs free energy also verify that there is a high possibility of spontaneous OER on the Ovac surface.

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REFERENCES

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DOI: 10.1021/acs.jpcc.7b05884 J. Phys. Chem. C 2017, 121, 18702−18709

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DOI: 10.1021/acs.jpcc.7b05884 J. Phys. Chem. C 2017, 121, 18702−18709