Article pubs.acs.org/JPCC
Enhanced Water Oxidation Catalysis of Nickel Oxyhydroxide through the Addition of Vacancies Vicky Fidelsky† and Maytal Caspary Toroker*,‡ †
The Nancy and Stephen Grand Technion Energy Program and ‡Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel S Supporting Information *
ABSTRACT: Enhancing catalytic efficiency for the oxygen evolution reaction (OER) is of extreme importance for the future of sustainable energy. One of the best-performing catalysts reported to date is nickel oxyhydroxide (NiOOH). However, during operating conditions, NiOOH has varying hydrogen content. To understand how hydrogen vacancies affect catalytic efficiency, we use density functional theory + U calculations that model oxygen evolution reaction catalysis for NiOOH with hydrogen vacancies. Our calculations reveal that these defects destabilize the surface by altering local oxidation states and therefore reduce the overpotential. Our results agree with a very recent experiment that shows enhanced OER activity of NiOOH upon deprotonation.
1. INTRODUCTION Developing efficient catalytic materials is vital for obtaining clean energy, particularly through the water-splitting reaction.1,2 Heterogeneous water splitting is based on an electrochemical cell, where water oxidizes and is reduced to produce oxygen gas and hydrogen fuel, respectively.3 The water oxidation reaction (OER) at the anode is especially challenging because there are very few materials that are able to perform the catalysis efficiently.4−7 One of the best known OER catalysts that has attracted significant interest in recent years is nickel oxide (NiO).8−19 Under an aqueous environment, NiO exists in several phases, including α-Ni(OH) 2 , β-Ni(OH) 2 , γ-NiOOH, and βNiOOH.20 The transformation from α-Ni(OH)2 to βNiOOH occurs during the application of bias that induces the reduction of Ni(II) to Ni(III).21 After further electrochemical charging, β-NiOOH can transform into γ-NiOOH, which is characterized by water molecules penetrating between the material layers. All of these phases include layers that are held by van der Waals interactions between O and H atoms.22 However, one of the unresolved fundamental questions regarding this catalyst is the location and amount of H atoms.16,23,24 The quantity of H atoms changes during the application of voltage, and as a result, vacancies of H atoms may be present.25,26 In addition, the concentration of H atoms can be controlled through fabrication.27,28 Furthermore, because the amount of H vacancies can also be altered through pH, a recent experiment using in situ surface enhanced Raman spectroscopy (SERS) showed that high pH induces surface deprotonation of NiOOH and enhances the OER activity.29 © 2016 American Chemical Society
Enhancement of OER activity as a result of introducing vacancies has been studied theoretically for oxides such as TiO2 and Fe2O3.30−33 The absence of atoms at the surface creates dangling bonds associated with surface charging that changes the binding energy of adsorbates and hence the required overpotential. By means of density functional theory + U (DFT +U) calculations, the water oxidation catalysis was modeled for Fe2O3 in the presence of oxygen and iron vacancies.32,33 Iron vacancies do not change the overpotential but add excess holes to the crystal and thereby increase the free energies required for early intermediate deprotonation reactions.32,33 Oxygens at the surface were shown to dramatically increase the overpotential, but these could be passivated at different operating conditions.34 In contrast, oxygen vacancies one layer away from the surface were found to reduce the overpotential by 0.2−0.3 eV,32,35 in agreement with experiment.36 In this paper, we use first-principles calculations to model the OER reaction on β-NiOOH in the presence of vacancies. We focus on the β-NiOOH phase because reducing Ni(OH)2 to NiOOH is required for OER enhancement, because the β phase may be one of the chemically active phases of NiOOH,37,38 and because the γ phase has a more complex structure. We consider both H and OH vacancies in order to obtain fundamental understanding of positive and negative charging at the surface, respectively. We find that vacancy defects destabilize the formal charge of Ni at the surface and reduce the required overpotential for the OER reaction. Received: August 5, 2016 Revised: October 1, 2016 Published: October 19, 2016 25405
DOI: 10.1021/acs.jpcc.6b07931 J. Phys. Chem. C 2016, 120, 25405−25410
Article
The Journal of Physical Chemistry C
Figure 1. Unit cells showing H or OH vacancy locations in yellow for intermediate A.
2. METHODS AND CALCULATION DETAILS Spin-polarized DFT calculations were performed with the VASP program.39,40 The DFT+U formalism of Duradev et al. was used with the Perdew−Burke−Ernzerhof (PBE) functional and an on-site Coulomb repulsion41 with an effective U−J term of 5.5 eV for Ni.42 Previous literature reported deriving this U− J value from linear response theory and using this value for NiOOH.15,16,22,23,43 Projected-augmented wave (PAW) potentials replaced the core electrons of Ni 1s2s2p3s3p and O 1s.44,45 The bulk unit cell structure of β-NiOOH15,46−49 was cleaved at the (01̅5) facet because this high-index facet was interpreted to be chemically active and because we wanted to compare to previous literature that studied this facet.15 The slab sizes chosen were the same as those in ref 15 for comparison; the vacuum length was taken to be 15 Å for all slabs.43 We assume that a single monolayer of water is sufficient for converging the free-energy calculations because previous literature18 showed that even a water-uncovered model of NiOOH gave similar overpotential values and other properties, including the effect of Fe-doping in reducing the overpotential. Dipole corrections were not introduced because in our previous work14 we tested the effect of adding adsorbates symmetrically on both sides of the slabs and found that this symmetric model gives similar free energies relative to the asymmetrical model used in ref 15. The energy cutoff and k-point grid were converged to within
