Computational Investigation of the Oxygen Evolution Reaction

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C: Energy Conversion and Storage; Energy and Charge Transport

Computational Investigation of the Oxygen Evolution Reaction Catalyzed by Nickel (Oxy)Hydroxide Complexes Soran Jahangiri, and Nicholas J. Mosey J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06614 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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The Journal of Physical Chemistry

Computational Investigation of the Oxygen Evolution Reaction Catalyzed by Nickel (Oxy)hydroxide Complexes

Soran Jahangiri and Nicholas J. Mosey* Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, ON, Canada, K7L 3N6 *e-mail: [email protected]

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Abstract The catalytic behaviour of nickel oxy(hydroxide) complexes containing one to four nickel atoms towards oxygen evolution reaction was investigated with density functional theory (DFT). The accuracy of the methodology was verified by comparison with the results of complete active space self-consistent field and coupled cluster calculations performed on a model nickel hydroxide system. The DFT methodology was found to be a suitable choice for investigating the oxygen evolution reaction catalyzed by nickel oxy(hydroxide) complexes. The results of the DFT calculations demonstrate the importance of the oxidation state of nickel in determining the catalytic efficiency of the clusters. A cubic cluster containing four Ni(III) atoms was found to have the best catalytic efficiency, with hydrogen abstraction free energies and an overpotential comparable to those of the bulk nickel oxyhydroxide. This cluster represents the smallest model of the bulk material. The presence of iron in a model cluster was found to increase the catalytic efficiency, which is in agreement with experimental findings on the bulk catalysts. The results presented here are expected to help efforts to design more efficient nickel oxy(hydroxide) cluster catalysts for water oxidation.


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1. Introduction The discovery of clean and renewable sources of energy is essential to circumventing challenges such as global warming and environmental pollution that are associated partly with the consumption of fossil fuels. Solar energy is one the most promising candidates for providing clean and sustainable energy; although, its low energy density and absence during night demand the development of clean and efficient methods for energy storage and transportation.1 Water splitting is an efficient process for providing hydrogen as a clean and sustainable energy carrier. This endothermic reaction can be carried out with energy provided by visible light or electrical energy generated from renewable sources using homogenous or heterogeneous catalysts.2–5 The water splitting reaction involves the transfer of four electrons and is composed of the hydrogen evolution and oxygen evolution reactions (HER and OER, respectively) that occur according to Scheme 1.4,6 Scheme 1. Hydrogen evolution and oxygen evolution reactions at different pH conditions. 4H2O + 4e−  2H2 + 4OH−

(HER, pH ≥ 7)

(1),

4H+ + 4e−  2H2

(HER, pH < 7)

(2),

4OH−  O2 + 2H2O + 4e−

(OER, pH > 7)

(3),

2H2O  O2 + 4H+ + 4e−

(OER, pH ≤ 7)

(4).

In the overall reaction, two water molecules are converted to one oxygen and two hydrogen molecules (2H2O  O2 + 2H2). The efficiency of the water splitting reaction depends on the kinetics of the OER, which is considered to consist of four sequential steps involving the transfer of four electrons and four protons.7 The intermediate species that appear during these steps can be stabilized by forming strong bonds with catalysts containing transition metals. The catalytic behaviour of a broad range of materials for increasing the efficiency of the OER has been investigated.6–8 These catalysts usually contain expensive noble metals such as Ru and Ir. Unfortunately, the cost of these metals renders these catalysts too expensive for practical use 3 ACS Paragon Plus Environment


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and instead the large-scale production of OER catalysts requires the use of abundant metals, such as first-row transition metals like Mn, Co and Ni. Nickel-containing materials are promising candidates for OER catalysis.9 Nickel hydroxide, which is the active component of nickel-based batteries, has been known as an OER catalyst for a long time.10,11 Homogenous nickel-based catalysts containing nickel complexes have also been investigated as homogenous OER catalysts that can oxidize water at low overpotential and neutral pH.12 Detailed inspection of the active intermediates and the catalytic mechanism has provided guidelines for improving the catalytic activity of such complexes.12 The photo-catalytic activities of nickel complexes containing nickel oxide cores have been investigated by Han et al.13 The catalysts were found to have high activities towards the visible light-driven water oxidation reaction.13 The structures of the nickel oxide cores of such complexes are presented in Figure 1. These nickel oxide cores have similar structures to the manganese-containing cluster in photosystem

II

protein

complex

that

catalyses

the

photosynthetic oxygen evolution in plants.14,15 The cubic structure (Figure 1b) is also similar to cobalt-containing complexes that have been used as homogenous OER catalysts.16 Such nickel complexes might also represent a small site of the bulk β-NiOOH heterogeneous catalyst as presented in Figure 1c. A number of clusters containing cubane nickel oxide cores have been synthesised experimentally but their catalytic activities towards OER remains to be investigated.17–22 Understanding the mechanism and energetics of the OER is essential to develop more efficient catalysts. Li and Selloni have investigated the mechanism of OER on the surface of pure and Fe-doped β-NiOOH and γ-NiOOH using density functional theory (DFT) calculations.23 Based on their results, the Fe-doped β-NiOOH was identified as a robust catalyst for water oxidation with an activity higher than that of noble metal oxide catalysts such as RuO2.23 Fidelsky and Toroker investigated the role of the Fe dopant on the improved efficiency of NiOOH catalysts towards water oxidation using DFT calculations.24 They found that the ability of Fe to have several oxidation states, as well as its ability to change the oxidation state easily, is the main factor in improving the electrocatalytic efficiency of NiOOH for water oxidation.24 Tkalych et al. also performed DFT calculations on the mechanism of OER on Fe-doped β-NiOOH and 4 ACS Paragon Plus Environment

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concluded that several competing mechanism might occur under the OER electrochemical conditions.25 The results of DFT calculations have indicated that doping the NiOOH surface with small amounts of Co might also increase the efficiency of the catalyst.26 The mechanism of the OER catalyzed by (oxy)hydroxides of other transition metal elements such as Mn and Co have been also investigated due to the importance of such materials in nature and industry. In particular, the mechanisms of the OER catalyzed by model Mn and Co dimer and tetramer structures have been comprehensively investigated.27–33 However, unlike the Mn and Co containing catalysts, the OER mechanistic investigations of the nickel-based materials have been limited to the heterogeneous catalysts. This limitation has prevented the development of a general description of the manner in which the OER is catalyzed by nickel-containing materials. Furthermore, a comprehensive evaluation of the DFT approaches that are typically used in the investigations of bulk systems is lacking because the application of more accurate reference quantum chemistry methods is computationally limited to smaller systems. In this work, the OER catalyzed by nickel (oxy)hydroxide clusters is investigated computationally using density functional theory. The accuracy of the DFT models used in the calculations is validated through comparison with the results of ab initio calculations for a model system representing the nickel hydroxide catalyst. The complexes investigated here contain 1–4 nickel atoms and could represent the active sites of homogenous and heterogeneous nickel (oxy)hydroxide OER catalysts. The model systems and computational details are described in Section 2, the results are presented and discussed in Section 3, and conclusions are provided in Section 4. 2. Models and methods The mechanism of the OER catalyzed by nickel (oxy)hydroxide clusters is described in Section 2.1. The model systems used to investigate the OER mechanism are described in Section 2.2. Details of the methods implemented and the calculations performed to investigate this mechanism are described in Section 2.3.



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2.1. Mechanism of the OER The mechanism of the OER catalyzed by transition-metal hydroxides involves the transfer of four electrons. In neutral and acidic media, two water molecules are oxidized and four protons and one oxygen molecule are released during the course of the reaction. Under basic conditions, hydroxide anions are oxidized and two water molecules are produced along with the oxygen molecule. The mechanism of the OER in pH ≤ 7 is considered to involve the steps presented in Scheme 2, where M represents an active site of the catalyst.34 Scheme 2. Mechanism of oxygen evolution reaction in pH ≤ 7. HO—M—OH2  HO—M—OH• + e− + H+ HO—M—OH•  HO—M—O + e− + H+ HO—M—O + H2O  H2O—M—OOH H2O—M—OOH  HO—M—OOH• + e− + H+ HO—M—OOH•  HO—M—OO + e− + H+ HO—M—OO  HO—M + O2 HO—M + H2O  HO—M—OH2

(5), (6), (7), (8), (9), (10), (11).

The first two steps (Eqs. 5,6) in this mechanism correspond to the removal of two protons and two electrons from a water molecule to form an O atom directly bonded to the catalyst. In the third step (Eq. 7), a water molecule is added to the catalyst to form an OOH group and transform another OH group to water. The fourth and fifth steps (Eqs. 8,9) are also proton removal steps from the OOH and water groups such that an oxygen molecule is formed on the catalyst. Addition of the second water molecule releases the oxygen molecule and reproduces the original structure of the catalyst (Eqs. 10,11). For pH > 7 conditions, the OER mechanism could be represented by the reactions in Scheme 3:34–36


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Scheme 3. Mechanism of oxygen evolution reaction in pH > 7. HO—M—OH2 + OH–  HO—M—OH• + H2O + e− HO—M—OH• + OH–  HO—M—O + H2O + e− HO—M—O + H2O  H2O—M—OOH H2O—M—OOH + OH–  HO—M—OOH• + H2O + e− HO—M—OOH• + OH–  HO—M—OO + H2O + e− HO—M—OO  HO—M + O2 HO—M + H2O  HO—M—OH2

(12), (13), (14), (15), (16), (17), (18).

This mechanism is analogous to that presented for the neutral and acidic pHs but under the basic conditions, hydroxide groups are involved in capturing protons from the species adsorbed on the catalyst. In this manner, the protons are simply transferred on the hydroxide anions to form water molecules in contrast to the formation of hydronium cations under neutral and acidic conditions. Comparison of the reaction steps suggested for the OER in acidic, neutral, and basic media (Eqs. 5–18) reveals that the key intermediates are similar in both cases. As a result, performing DFT calculations on one set of the intermediate structures allows the calculation of the reaction free energies for the OER steps in all pH conditions. In Figure 2, the process of proton transfer in different pHs is presented schematically. 2.2. Model systems The nickel (oxy)hydroxide clusters investigated here are presented in Figure 3. The smallest model systems containing only one nickel atom are Ni1(OH)2 and Ni1(OH)4 (Figures 3a and 3b). The Ni1(OH)2 model was also used in the validation of the DFT method with computationally expensive higher level ab initio methods. These two systems represent the smallest possible active site of a nickel (oxy)hydroxide catalyst. Furthermore, increasing the oxidation state of the Ni atom from +2 to +4, by only changing the number of hydroxide groups, also helps rationalize the effect of the oxidation state on the mechanism and energetic of the OER catalysed by nickel (oxy)hydroxide in the absence of bulk effects. The cluster containing two nickel atoms has the chemical formula Ni2(OH)8 (Figure 3c). The structure of this system is analogous to those of Mn33 and Co30 dimer complexes that have been suggested as catalysts for water oxidation. 7 ACS Paragon Plus Environment


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Besides the potential of the Ni2(OH)8 complex to be used as the active core of homogeneous catalysts for water oxidation, this model is the smallest unit that allows investigating the effect of doping the catalyst with other transition metals such as Fe. The clusters containing three and four nickel atoms (Figures 3d and 3e, respectively) are particularly important because they are the core active sites of materials that have been suggested as light-driven water oxidation catalysts.13 These clusters have structures that are similar to the active site of the water oxidation catalyst in Photosystem II and a complex containing cobalt.

14–16

The clusters

containing three and four nickel atoms can be also considered as the smallest units representing the bulk NiOOH material which is an efficient catalyst for water oxidation (see Figure 1). The model systems presented here provide a framework for investigating the systematic change in the size of the clusters and the oxidation state of the Ni atoms on the catalytic behaviour of nickel (oxy)hydroxide catalyst. Such systematic investigations provide informative insight into the mechanism of OER on the smallest active sites of the catalysts and help rationalizing the change in the mechanism of OER by extending the minimal active sites of the catalysts towards the extended surface. Investigating a variety of clusters with a rational order in their size and oxidation state will also help finding trends for designing more efficient active cores of homogenous catalysts containing nickel clusters. 2.3. Computational details The DFT calculations were performed with the exchange-correlation functional of Perdew, Burke and Ernzerhof developed into a hybrid functional by Adamo (PBE0).37,38 The accuracy of the DFT calculations was validated using the coupled cluster method with single, double and non-iterative triple excitations [CCSD(T)]39,40 and complete active space self-consistent field (CASSCF)41–48 calculations performed on the DFT geometries obtained on the Ni1(OH)2 model system. The effects of dynamical electron correlation were included in the CASSCF calculations by performing MP2 corrections to the CASSCF energy.49 This model will be referred to as CASMP2. The CASSCF and CASMP2 calculations were performed by including 8 electrons in 10 active orbitals. Dunning's aug-cc-pVDZ correlation-consistent basis set augmented with diffuse functions was used in all calculations.50 The cluster structures were optimized with the Berny 8 ACS Paragon Plus Environment

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algorithm51 using a force threshold of 1 x 10−5 au. The reaction free energies were calculated based on the rigid rotor-harmonic oscillator approximation using the DFT harmonic vibrational frequencies. All calculations were performed with the Gaussian09 program.51 3. Results and discussion The results of the validation of the DFT method is presented in Section 3.1 and the results of DFT calculations examining the OER catalyzed by clusters containing 1–4 nickel atoms are presented in Sections 3.2–3.5, respectively. The overpotentials were calculated as the difference between the highest free energy obtained during the OER with respect to the total free energy of water dissociation per one electron/proton transfer. The highest free energies and the overpotentials obtained for the systems investigated here as well as the enthalpic and entropic contributions of the free energies are summarized in Table 1. 3.1. Method validation DFT has been used routinely to investigate the catalytic behaviour of transition metal hydroxides towards water oxidation.23–30,32,33 One of the challenges in describing the electronic structure of such catalysts is the presence of multiple nearly degenerate excited states with energies very close to that of the ground state.31,52,53 In particular, previous CASSCF calculations performed on a Mn dimer complex revealed the presence of multiple almost degenerate excited states.31 The presence of such states requires the use of multiconfiguration wavefunction methods to properly describe the OER mechanism. In this work, the accuracy of the DFT method used was assessed by performing two sets of ab initio calculations. First, we examined the necessity of performing multiconfiguration calculations by performing CASSCF and CASMP2 calculations on different spin states of a sample catalyst and comparing their relative energies with those of DFT. In the second set of calculations, the accuracy of DFT in describing the reaction energies was verified by performing CCSD(T) calculations on the minimum-energy structures of the OER reaction steps. The computational expenses associated with the CASSCF and CCSD(T) calculations prevent the use of these methods for all the clusters investigated in this work. Accordingly, the Ni1(OH)2.2H2O 9 ACS Paragon Plus Environment


model system (Figure 3a) was used to evaluate the accuracy of the DFT calculations. This model can properly describe the mechanism of OER on the smallest possible active site of a nickel hydroxide system. The DFT, CASSCF and CASMP2 calculations were performed on the electronic low-spin state and three electronic excited states of the model systems in the OER reaction described in Eqs. 5–18 and illustrated in Figure 4. The relative energies are presented in Table 2. For all structures, it can be inferred that the DFT calculations correctly predict the lowest-energy electronic structures. It is noted that for the relative energies of the Ni1(OH)2O.2H2O intermediate, there is a disagreement between the CASSCF and CASMP2 while the DFT relative energies are consistent with the CASMP2 results (cf. Table 2). Furthermore, the ab initio relative energies of the highest energy structures are at least 0.1 eV larger than those of the minimumenergy structures in all cases. These observations indicate that single-reference DFT calculations can properly describe the energetics of the reaction steps of the OER mechanism. It is noted that the active space in the ab initio calculations was composed of the four highest occupied molecular orbitals and six unoccupied molecular orbitals that are localized on the OH and H2O groups. These orbitals were chosen based on the involvement of these groups in the OER steps. The shape of the highest occupied and lowest unoccupied natural orbitals of Ni1(OH)2 used to construct the active space are presented in Figure 5. The DFT and CCSD(T) energies of the reaction steps of OER for the model system illustrated in Figure 4 are presented and compared in Figure 6a. The DFT values are in close agreement with the CCSD(T) values, with an average deviation of 0.1 eV. Despite the small differences in the magnitude of the predicted energies, the DFT method reproduces the CCSD(T) trend correctly. The agreement between the DFT and CCSD(T) results is comparable to those reported for a Co-containing cluster in a previous investigation.29 It is noted that the reaction energies predicted by both methods are pH independent. The reaction energies reported for the neutral and acidic media were calculated relative to the H2  2H• dissociation reaction and the reaction energies of the basic conditions were obtained with respect to the water dissociation reaction H2O  ½ H2 + OH•. These references were chosen to eliminate the calculation of proton and electron free energies.30,32,33 In the following, the discussions will be 10 ACS Paragon Plus Environment

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focused on the OER mechanism at neutral pH due to its simplicity and consistency with the literature.23,26,30,32,33 The free energy of the water splitting reaction (∆𝐺) can be related to the difference between the OER and HER half cell potentials (𝐸0𝑐𝑒𝑙𝑙) as:54 (19),

∆𝐺 = ―𝑛𝐹𝐸0𝑐𝑒𝑙𝑙

where n and F represent the number of electrons and the Faraday constant, respectively. The experimental 𝐸0𝑐𝑒𝑙𝑙 is −1.23 V and the corresponding ∆𝐺 is 4.92 eV.2 The free energy of the water splitting reaction obtained from the DFT calculations is 4.59 eV. This free energy corresponds to a hypothetical 𝐸0𝑐𝑒𝑙𝑙 of −1.15 V which is very close to the experimental value of −1.23 V. The ∆𝐺 obtained here (4.59 eV) is also in close agreement with the value obtained from other DFT calculations (4.56 eV) with the BP86 exchange-correlation functional. 32 3.2. Ni1(OH)2,4.2H2O The free energies of the four deprotonation steps of the OER catalyzed by Ni1(OH)2 obtained from the DFT calculations are plotted in Figure 7. The highest free energy is associated with the second deprotonation step. In this step, one proton is removed from an OH group, which eventually bonds to another OH group to form OOH (see Figure 4). The resulting structure in this step has a minimum energy when the spin multiplicity is 3, in which case the unpaired electrons are localized mainly on the nickel atom. The calculated free energy for this step is 1.98 eV, which corresponds to an overpotential of 0.83 V. The enthalpic contribution (ΔH) to the free energy has a positive value of 2.24 eV reflecting the endothermic nature of the proton abstraction reaction. The entropic contribution to the free energy (−TΔS) has a small negative value of −0.26 eV demonstrating that removing a proton from the complex results in a relatively less structured system and the reaction is entropically favourable. The mechanism of the OER catalyzed by Ni1(OH)4 is illustrated in Figure 8 and the free energies obtained for the four deprotonation steps of the Ni1(OH)4.2H2O system are presented in Figure 7. The maximum free energy value (2.12 eV) is associated with the first deprotonation 11 ACS Paragon Plus Environment


step. This value is larger than that obtained for the Ni1(OH)2.2H2O system very likely due to the change in the oxidation state of nickel from Ni(II) to Ni(IV), as well as the possible structural destabilization associated with the presence of two extra OH groups in Ni1(OH)4.2H2O. The change in the oxidation state prevents the charge transfer from the nickel atoms to the hydroxide groups and destabilizes the structure formed after the first deprotonation step. This observation indicates that the lower oxidation state of nickel in nickel hydroxide clusters containing only one nickel atom leads to a better catalytic efficiency for OER compared to the higher oxidation state. The maximum deprotonation free energy obtained for the Ni1(OH)4.2H2O system is also dominated by the enthalpic contribution (cf. Table 1). The free energies of the four deprotonation steps in the Ni1(OH)2,4.2H2O systems obtained from the low-spin structures are presented in Figure 9. The maximum free energies obtained for the low-spin structures are larger than those obtained for the minimum-energy structures by 0.4 eV and 0.8 eV for the Ni1(OH)2.2H2O and Ni1(OH)4.2H2O systems, respectively. The increase in the spin multiplicities presumably allows for the redistribution of the electrons such that some of the intermediates appeared during the OER become more stable. Inspection of the natural orbitals of the Ni1(OH)2.2H2O system with spin multiplicity of one, presented in Figure 5, demonstrates that the highest-energy doubly-occupied orbital is localized on an OH group. Increasing the spin multiplicity to two, results in two degenerate singly-occupied orbitals that are located on different OH groups as presented in Figure 10. The partial atomic charges on the nickel and oxygen atoms for the Ni1(OH)2.2H2O system with spin multiplicities of one and three, obtained from natural population analysis, are presented in Table 3. The complex with higher spin multiplicity has higher amounts of electron transferred from the nickel atom to the hydroxide and water oxygen atoms. The changes in the electron distribution associated with the change in the spin multiplicity lead to the formation of more stable intermediates during the OER, which generally decrease the magnitude of the maximum reaction free energies. The DFT and CCSD(T) energies of the OER steps for the Ni1(OH)4.2H2O model system illustrated in Figure 8 are presented and compared in Figure 6b. The DFT values are in close agreement with the CCSD(T) values analogous to the results obtained for the Ni1(OH)2.2H2O system (cf. Figure


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6). It is noted that the OER reaction steps for these two systems involve intermediates with various oxidation states of Ni ranging from +2 to +6. 3.3. Ni2(OH)8.2H2O The free energies obtained for the deprotonation steps of the Ni2(OH)8.2H2O system and the structures obtained during the OER are presented in Figure 11 and Figure 12, respectively. The highest free energy in the OER catalyzed by Ni2(OH)8.2H2O is 2.65 eV which corresponds to the first deprotonation step. Considering the Ni2(OH)8 system as a dimer of the Ni1(OH)4 complex, the maximum free energies of these two systems are expected to be analogous. However, formation of the structure containing a Ni atom coordinated by six hydroxide groups, after the first deprotonation step (Figure 12), might be the reason for the higher free energy required to abstract proton from the Ni2(OH)8.2H2O system. The enthalpic contribution also dominates the maximum deprotonation free energy obtained for this system and the entropic contribution is analogous to the values obtained for the clusters containing one nickel atom (cf. Table 1). The effect of Fe on the catalytic efficiency of nickel hydroxide was investigated by replacing one of the Ni atoms in Ni2(OH)8.2H2O with Fe. This system was chosen because there is only one possible way for arranging the Ni and Fe atoms. The free energies of the deprotonation steps obtained for this system are plotted in Figure 11. The maximum deprotonation free energy is 2.54 eV. Interestingly, the presence of Fe lowers the maximum free energy by 0.11 eV compared to the Ni2(OH)8.2H2O system. This change in free energy is in agreement with the experimental observations of the effect of Fe on increasing the efficiency of nickel hydroxide catalysts for OER.55 The presence of Fe lowers the free energy of the first deprotonation step while increasing the free energy of the second step which means that the first deprotonated structure is stabilized in the presence of Fe. Inspection of the atomic partial charges on this structure indicated that the Fe atom has a larger positive partial charge compared to the corresponding Ni atoms in Ni2(OH)8.2H2O. The partial atomic charges obtained from natural population analysis for Fe and Ni atoms are 1.0 and 0.9 au, respectively. The ability of Fe in adopting a larger partial charge, which leads to a greater degree of charge



transfer from the metal atom to the OH groups, might explain the stabilization of the first deprotonated structure. 3.4. Ni3O4(OH)4.5H2O The Ni3O4(OH)4.5H2O system has a structure that is similar to the NiOOH active site (cf. Figures 1 and 3d); although, the nickel atoms have a +4 oxidation state in the complex while the oxidation state of nickel is +3 in the bulk material. The free energies obtained for the deprotonation steps of the Ni3O4(OH)4.5H2O system are presented in Figure 13. The highest free energy step in the OER catalyzed by Ni3O4(OH)4 system is 2.34 eV, which belongs to the first deprotonation step, and is smaller than that obtained for the Ni2(OH)8.2H2O system by 0.31 eV. The larger size of the Ni3O4(OH)4 cluster, which allows for the formation of a larger number of hydrogen bonds to stabilize of the hydroxide groups, is very likely the reason for lowering the maximum deprotonation free energy compared to the system containing two Ni(IV) atoms (vide infra). The maximum deprotonation free energy in the Ni3O4(OH)4.5H2O system is also dominated by the enthalpic effects (cf. Table 1). The structures obtained during the OER catalyzed by Ni3O4(OH)4 are presented in Figure 14. The water molecules in the original structure are hydrogen bonded to the OH groups of the neighboring nickel atoms. Removing one proton from any of these water molecules leads to the formation of a hydrogen bond between two OH groups. The length of such hydrogen bonds increases from 1.49 Å, for the water–OH interaction, to 1.96 Å, for the OH–OH bond. This increase in the hydrogen bond length decreases the strength of the hydrogen bond and makes the structure less stable. Removing a proton from the water molecule coordinated to a nickel atom also leads to structural changes in the Ni3O4 framework due to the redistribution of the atomic charges and spin densities. This structural change along with the increased hydrogen bond length between the OH groups destabilize the resulting structure after the first deprotonation of Ni3O4(OH)4.5H2O. Removing the second proton forms an OOH group which bridges between two nickel atoms and stabilizes the Ni3O4 core again.


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3.5. Ni4O4(OH)4.4H2O The Ni4O4(OH)4.4H2O system can be considered as the smallest representative of the bulk NiOOH materials. In fact the cluster could be represented as [NiOOH.H2O]4 in which the oxidation state of nickel is +3, analogous to the bulk catalyst. The structures obtained during the OER catalyzed by Ni4O4(OH)4 are presented in Figure 15 and the free energies obtained for the deprotonation steps of the OER catalyzed by the Ni4O4(OH)4 complex are presented in Figure 13. Analogous free energies obtained for the bulk material from periodic DFT calculations are also presented for comparison.26 As the results in Figure 13 show, there is a very good agreement between the predicted free energies of the cluster and the bulk systems indicating that the Ni4O4(OH)4.4H2O complex can represent the NiOOH catalyst properly. The maximum free energy step in the complex corresponds to the second deprotonation step, in agreement with the bulk system, which results in an OOH group bridged between two nickel atoms. The free energy of this step is 1.66 eV which is very close to the value of 1.51 eV obtained for the bulk material.26 This values corresponds to overpotentials of 0.52 eV and 0.28 eV for the cluster and bulk,26 respectively. It is noted that the difference between the free energies may originate from the differences between the DFT exchange-correlation functionals used for the cluster and bulk calculations. The highest free energy values are also comparable to what obtained for Co (1.62 eV)30 and Mn (1.7–1.8 eV)32 cubane complexes investigated previously. The Ni4O4(OH)4 system is the most efficient catalyst among the series of clusters investigated here. This efficiency, which is indicated by the small deprotonation free energies compared to the other clusters, might be related to the stability of the structures that are formed during the OER reaction. The nickel oxide core (Ni4O4) in the Ni4O4(OH)4 complex , which can be considered as a small unit cell of bulk nickel oxide, provides a very stable framework for the formation of the Ni4O4(OH)4.4H2O complex. The average Ni–Ni distance in the Ni4O4 core during the OER process changes by only 0.03 Å (2.73 Å to 2.76 Å) and the cubic structure of the core is fully preserved (Figure 15). The average spin density on the Ni atoms changes by only 0.2 au. (0.89 to 1.08). These small variations in the structure and electronic spin distribution of the Ni4O4 core might be related to the stability of the complex during the proton removal in the 15 ACS Paragon Plus Environment


OER and explain the small magnitude of the free energies obtained for the deprotonation steps. It is noted that the enthalpic contributions are dominant in the total values of the free energies analogous to the other systems investigated here (cf. Table 1). 4. Conclusions The results of electronic structure calculations of the mechanism of OER catalyzed by model nickel (oxy)hydroxide complexes are presented in this work. The CASSCF and CASMP2 calculations performed on a model nickel hydroxide complex revealed non-degenerate energy states that differ from the lowest-energy spin state by at least 0.1 eV. The DFT method employed here also predicts the lowest-energy spin state with a very good accuracy compared to the ab initio results. Assuming that the DFT model predicts accurate energetics for the different spin-states of the larger complexes investigated here, it can be concluded that the electronic structure of the nickel (oxy)hydroxide complexes can be properly described by singlereference wave-function DFT methods. Comparison of the DFT results with those of CCSD(T) calculations also revealed a fair agreement between the OER predicted energies. According to the validations performed here, it can be concluded that DFT properly describe the mechanism of OER in nickel (oxy)hydroxide systems. Our results indicate that structures with different spin multiplicities should be optimized in order to obtain the lowest-energy structure although multi-configuration calculations are not necessary in most cases. Inspection of the free energies obtained from DFT for the OER catalyzed by nickel (oxy)hydroxide complexes reveals that the oxidation state of the nickel atoms plays a key role in determining the efficiency of the catalyst. Those complexes in which nickel atoms have an oxidation state of +2 and +3 demonstrated higher efficiency compared to systems containing Ni(IV) atoms. This conclusion is valid for both nickel hydroxide and nickel oxy-hydroxide complexes. This observation could be explained based on the electronic charge transferred from the nickel atoms to the oxygen atoms of the OH and water. The presence of a large number of electron withdrawing OH groups, coordinated to the nickel atoms, hinders the formation of extra OH groups from the water molecules during the OER process. In the catalysts containing Ni(IV), the first oxidation step involves the formation of Ni(V) while the +5 oxidation 16 ACS Paragon Plus Environment

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state is not favorable for Ni atoms. The DFT calculations also revealed that replacing a nickel atom in the dimer complex with an iron atom, which might better adopt higher oxidation states, results a smaller overpotential for the OER which is in agreement with the experimental observations. The presence of the Fe atom causes charge redistribution between Ni and Fe atoms which leads to a slight decrease in the Ni positive charge. The deprotonation free energies obtained for all systems investigated in this work are dominated by the enthalpic contributions. The entropic contributions favour the proton abstraction reactions and contribute to about 10 % of the total free energies. The Ni4O4(OH)4 nickel oxyhydroxide complex has an OER overpotential comparable to the bulk NiOOH value. This complex can be considered as the smallest unit that represents the bulk NiOOH catalyst. The results presented here show that clusters containing cubic nickel oxyhydroxide cores can be considered as potential candidates in the design of homogenous water oxidation catalysts. In such catalysts, complexes with lower oxidation state of nickel are very likely more efficient towards OER. Replacing one nickel atom of the Ni4 complex with Fe or Co atoms might further improve the efficiency of such catalysts. Acknowledgements This research was conducted as part of the Engineered Nickel Catalysts for Electrochemical Clean Energy project administered from Queen's University and supported by Grant No. RGPNM 477963-2015 under the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Frontiers Program. Computing resources were provided by Compute Canada. The authors are grateful to Prof. Gregory Jerkiewicz for useful comments.



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(36) L. Doyle, R.; G. Lyons, M. E. An Electrochemical Impedance Study of the Oxygen Evolution Reaction at Hydrous Iron Oxide in Base. Phys. Chem. Chem. Phys. 2013, 15, 5224– 5237. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (38) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158–6170. (39) Purvis, G. D.; Bartlett, R. J. A Full Coupled‐cluster Singles and Doubles Model: The Inclusion of Disconnected Triples. J. Chem. Phys. 1982, 76, 1910–1918. (40) Pople, J. A.; Head‐Gordon, M.; Raghavachari, K. Quadratic Configuration Interaction. A General Technique for Determining Electron Correlation Energies. J. Chem. Phys. 1987, 87, 5968–5975. (41) Hegarty, D.; Robb, M. A. Application of Unitary Group Methods to Configuration Interaction Calculations. Mol. Phys. 1979, 38, 1795–1812. (42) Eade, R. H. A.; Robb, M. A. Direct Minimization in Mc Scf Theory. the Quasi-Newton Method. Chem. Phys. Lett. 1981, 83, 362–368. (43) Schlegel, H. B.; Robb, M. A. MC SCF Gradient Optimization of the H2CO→H2 + CO Transition Structure. Chem. Phys. Lett. 1982, 93, 43–46. (44) Bernardi, F.; Bottoni, A.; McDouall, J. J. W.; Robb, M. A.; Schlegel, H. B. MCSCF Gradient Calculation of Transition Structures in Organic Reactions. Faraday Symp. Chem. Soc. 1984, 19, 137–147. (45) Frisch, M.; Ragazos, I. N.; Robb, M. A.; Bernhard Schlegel, H. An Evaluation of Three Direct MC-SCF Procedures. Chem. Phys. Lett. 1992, 189, 524–528. (46) Yamamoto, N.; Vreven, T.; Robb, M. A.; Frisch, M. J.; Bernhard Schlegel, H. A Direct Derivative MC-SCF Procedure. Chem. Phys. Lett. 1996, 250, 373–378. (47) Siegbahn, P. E. M. A New Direct CI Method for Large CI Expansions in a Small Orbital Space. Chem. Phys. Lett. 1984, 109, 417–423. (48) Klene, M.; Robb, M. A.; Frisch, M. J.; Celani, P. Parallel Implementation of the CIVector Evaluation in Full CI/CAS-SCF. J. Chem. Phys. 2000, 113, 5653–5665. (49) McDouall, J. J. W.; Peasley, K.; Robb, M. A. A Simple MC SCF Perturbation Theory: Orthogonal Valence Bond Møller-Plesset 2 (OVB MP2). Chem. Phys. Lett. 1988, 148, 183–189. (50) Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007–1023. (51) Gaussian 16, Revision A.03, Frisch, M. J. et al. Gaussian, Inc., Wallingford CT, 2016. (52) Reiher, M.; Salomon, O.; Hess, B. A. Reparameterization of Hybrid Functionals Based on Energy Differences of States of Different Multiplicity. Theor. Chem. Acc. 2001, 107, 48–55. (53) Sears, J. S.; Sherrill, C. D. The Electronic Structure of Oxo-Mn(Salen): Single-Reference and Multireference Approaches. J. Chem. Phys. 2006, 124, 144314. (54) Light, Water, Hydrogen: The Solar Generation of Hydrogen by Water Photoelectrolysis; Grimes, C., Varghese, O., Ranjan, S., Eds.; Springer US, 2008. (55) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M.-J.; Sokaras, D.; Weng, T.-C.; Alonso-Mori, R.; et al. Identification of Highly Active Fe Sites in (Ni,Fe)OOH for Electrocatalytic Water Splitting. J. Am. Chem. Soc. 2015, 137, 1305–1313.


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Table 1. The enthalpy (ΔH) and entropy (−TΔS) components of the highest OER free energies (ΔG), and overpotentials (η) of the model catalysts obtained from DFT Complex

a Includes

ΔH (eV) a

−TΔS (eV)

ΔG (eV)

η (V)

Ni1(OH)2.2H2O

2.24

−0.26

1.98

0.83

Ni1(OH)4.2H2O

2.33

−0.21

2.12

0.97

Ni2(OH)8.2H2O

2.87

−0.22

2.65

1.50

NiFe(OH)8.2H2O

2.81

−0.27

2.54

1.40

Ni3O4(OH)4.5H2O

2.57

−0.23

2.34

1.20

Ni4O4(OH)4.4H2O

1.85

−0.19

1.66

0.52

the zero-point energy.



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Table 2. The relative energies (in eV) of the compounds appear during the OER catalyzed by Ni1(OH)2 obtained from DFT, CASSCF and CASMP2a PBE0 CASSCF CASMP2 LS ES1 ES2 ES3 LS ES1 ES2 ES3 LS ES1 ES2 ES3 Ni1(OH)2.2H2O 0.5 0.0 4.2 10.1 1.0 0.0 4.7 7.9 2.8 0.0 7.2 11.9 Ni1(OH)3.1H2O 0.0 0.2 5.4 11.2 0.0 0.1 3.9 7.5 0.0 0.4 5.2 10.0 Ni1(OH)2O.1H2O 0.3 0.0 0.5 5.4 1.0 0.0 0.4 5.7 3.6 0.0 2.2 8.8 Ni1(OH)2O.2H2O 0.2 0.0 0.4 5.8 0.0 0.8 1.6 4.0 12.4 0.0 2.6 7.3 Ni1(OH)1O2.2H2O 0.1 0.0 2.4 6.5 1.8 0.0 1.1 3.5 0.2 0.0 3.7 7.7 Ni1(OH)2O2.1H2O 1.7 0.3 0.0 3.9 4.1 0.1 0.0 5.5 2.3 3.0 0.0 4.7 Ni1(OH)2.1H2O 1.1 0.0 3.9 10.2 1.2 0.0 4.2 7.2 1.3 0.0 6.0 10.9 a The low-spin and first, second, and third electronic excited states are represented by LS, ES1, ES2, and ES3, respectively. The lowest-energy is considered as 0.0 eV. b The structures are presented in Figure 4. Complexb


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Table 3. Partial atomic charges on the nickel and oxygen atoms of Ni1(OH)2.2H2O system obtained from natural population analysis performed with DFTa atom LS ES1 Ni 0.83 1.13 O -0.90 -0.95 O -1.06 -1.15 O -0.90 -0.95 O -1.06 -1.15 a The low-spin and first electronic excited state are represented by LS and ES1, respectively.



(a)

(c)

(b)

Figure 1. The structure of nickel oxide clusters containing (a) three and (b) four nickel atoms that represent the active cores of homogenous13 nickel-based catalysts and (c) bulk NiOOH. The parallelogram in (c) represents the unit cell and the circle indicates the similarity between a βNiOOH site and nickel oxide clusters containing (a) three and (b) four nickel atoms. The analogy is consistent with that reported for Co containing materials.16


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H

H

OH2

H

O

O

M

M

(a)

OH− H

+ H3O+

H

H

O

O

M

M

(b)

+ H2O

Figure 2. Schematic representation of the first step in the mechanism of the OER catalyzed by M in (a) pH ≤ 7 and (b) pH > 7 conditions.



(a)

(b)

(c)

Page 26 of 39

(d)

(e)

Figure 3. The structures of the model systems used here for investigating the OER mechanism catalyzed by nickel (oxy)hydroxide clusters. (a) Ni1(OH)2.2H2O, (b) Ni1(OH)4.2H2O, (c) Ni2(OH)8.2H2O, (d) Ni3O4(OH)4.5H2O, (e) Ni4O4(OH)4.4H2O.


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+

+

−H − e− 1

−H − e− 2

+ H2O 3

+

+

−H − e− 5

−H − e− 4

+ H2O

− O2

7

6

Figure 4. The mechanism of the OER catalyzed by Ni1(OH)2. The structures were obtained from DFT calculations. The yellow circles indicate the proton removal sites and the blue oval represents the catalyst active site indicates by M in Eqs. 5-18.



30 (1.9983, −16.2)

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31 (1.9952, −15.6)

32 (1.9498, −13.9)

33 (1.9493, −13.9)

34 (0.0000, 5.5)

35 (0.0459, 7.5)

36 (0.0000, 9.1)

37 (0.0000, 9.4)

38 (0.0000, 11.6)

39 (0.0003, 11.9)

40 (0.0003, 11.9)

41 (0.0213, 12.7)

42 (0.0213, 12.7)

43 (0.0000, 12.8)

44 (0.0184, 14.1)

Figure 5. The 4 highest-energy occupied and 11 lowest-energy unoccupied natural orbitals of Ni1(OH)2.2H2O. The orbitals used as the active space of the CASSCF and CASMP2 calculations are indicated by bold orbital numbers. The orbital occupation numbers and energies (in eV) are presented in the parenthesis.


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(a)

(b)

Figure 6. The reaction energies of the OER catalyzed by a) Ni1(OH)2 and b) Ni1(OH)4 obtained from DFT and CCSD(T) calculations. The reaction steps are defined in Figure 4 and Figure 8 for Ni1(OH)2 and Ni1(OH)4, respectively.



Figure 7. The deprotonation free energies of the OER obtained from DFT calculations for Ni1(OH)2.2H2O and Ni1(OH)4.2H2O. The reaction steps are defined in Figures 4 and 8.


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+

+

−H − e− 1

−H − e− 2

+ H2O 3

+

+

−H − e− 5

−H − e− 4

+ H2O

− O2

7

6

Figure 8. The mechanism of the OER catalyzed by Ni1(OH)4. The structures were obtained from DFT calculations. The yellow circles indicate the proton removal sites.



Figure 9. The deprotonation free energies of OER obtained from DFT calculations for Ni1(OH)2.2H2O and Ni1(OH)4.2H2O using the low-spin structures. The reaction steps are defined in Figures 4 and 8.


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30 (1.9961, −18.3)

31 (1.9785, −17.7)

32 (1.9785, −17.7) 33 (0.9880, −12.4) 34 (0.9880, −12.4)

Figure 10. The 5 highest-energy occupied natural orbitals of Ni1(OH)2.2H2O with spin multiplicity of 3. The orbital numbers are presented for comparison with the natural orbitals of the same system with a spin multiplicity if 1, presented in Figure 5. The orbital occupation numbers and energies (in eV) are presented in the parenthesis.



Figure 11. The deprotonation free energies of the OER obtained from DFT calculations for Ni2(OH)8.2H2O and NiFe(OH)8.2H2O. The reaction steps are defined in Figure 12.


Page 34 of 39

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1

3 + H2O

2 +

−H − e−

+

−H − e−

+

−H − e−

+ H2O

− O2

7

6

4

+

−H − e− 5

Figure 12. The mechanism of the OER catalyzed by Ni2(OH)8. The structures were obtained from DFT calculations. The yellow circles indicate the proton removal sites.



Figure 13. The deprotonation free energies of the OER obtained from DFT calculations for Ni3O4(OH)4.5H2O, Ni4O4(OH)4.4H2O and bulk β-NiOOH.26 The reaction steps are defined in Figures 14 and 15.


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1

3 + H2O

2 +

−H − e−

+

−H − e−

+

−H − e−

+ H2O

− O2

7

6

4

+

−H − e− 5

Figure 14. The mechanism of the OER catalyzed by Ni3O4(OH)4. The structures were obtained from DFT calculations. The yellow circles indicate the proton removal sites.



1

3 + H2O

2 +

−H − e−

Page 38 of 39

+

−H − e−

+

−H 4 − e−

+ H2O

− O2

7

6

+

−H − e− 5

Figure 15. The mechanism of the OER catalyzed by Ni4O4(OH)4. The structures were obtained from DFT calculations. The yellow circles indicate the proton removal sites.


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TOC Graphic


Computational Investigation of the Oxygen Evolution Reaction

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