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Dual Mechanisms: Hydrogen Transfer During Water Oxidation Catalysis of Pure and Fe-doped Nickel Oxyhydroxide Yuval Elbaz, and Maytal Caspary Toroker J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04142 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017
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The Journal of Physical Chemistry
Dual Mechanisms: Hydrogen Transfer during Water Oxidation Catalysis of Pure and Fe-doped Nickel Oxyhydroxide Yuval Elbaz and Maytal Caspary Toroker* Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel
Abstract Metal oxides are often used as catalysts for the oxygen evolution reaction which is of significant importance for water splitting as an alternative energy source. One of the best performing catalysts reported to date for water oxidation under alkaline conditions is nickel oxyhydroxide (NiOOH) doped with iron. However, NiOOH has hydrogen atoms whose positions are not known and may transfer through the material. In order to understand how hydrogen transfer affects catalytic efficiency, we use Density Functional Theory+U (DFT+U) calculations that model oxygen evolution reaction catalysis for pure and Fe-doped NiOOH. Our calculations reveal that hydrogen transfer is possible in the Fe-doped case, but is less probable in the pure case. The duality of proton and charge transfer at the surface of reactive materials provides further evidence to the effectiveness of doping for improving catalysis. *Corresponding author: E-mail:
[email protected] , Tel.: +972 4 8294298.
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1. Introduction Discovering new materials for catalysis is at the front of scientific developments. Especially in the field of energy conversion, one of the goals is to enhance the efficiency of catalysts that split water to hydrogen fuel.1 In order to design better catalysts for water splitting, it is important to understand what controls catalysis, and in particular, to reveal what makes efficient catalysts successful.2-3 One of the best known water oxidation catalysts at alkaline conditions is Fedoped NiOOH. Recent experiments identified that NiOOH is able to catalyze water oxidation well when it is doped with Fe.4 Furthermore, several experiments show superior catalytic performance for mixed Fe-Ni oxides.5-6 Several studies followed in attempt to understand how Fe improves catalysis.711 A joint experimental and theoretical study found that Fe acts as the active site for catalysis.12 Our own recent theoretical study13 revealed that Fe is a successful dopant for NiOOH since Fe is able to have several oxidation states, which are important for catalysis. Interest in the excellent efficiency of NiOOH provoked theoretical characterization of the NiOOH crystal structure. Although NiOOH is one of the best catalysts for water oxidation, the crystal structure has not been fully resolved, since the positions of the hydrogen atoms are not easily detectable.14 In one study, the crystal structure was suggested to have hydrogen atoms in staggered positions.15 In a second study, the bulk structure had hydrogen atoms at several positions, and some structures with different hydrogen positions had the same energy.16 These studies suggest that the position of hydrogen atoms may not be fixed during catalysis. In this work, in order to understand how hydrogen transfer beneath the surface has an effect on water oxidation catalysis on top of the surface, we perform Density Functional Theory +U (DFT+U) calculations on pure and Fe-doped 𝛽-NiOOH. This study focuses on the 𝛽 phase since this phase is known to be one of the active phases of NiOOH and since this phase is conceptually simpler as it does not have intercalating water molecules. In order to test the duality of the hydrogen transfer and catalytic process, we considered the water oxidation catalysis mechanism and altered the position of a hydrogen atom closest to the active site.
2. Methods and Calculation Details DFT calculations were performed with the VASP program.17-18 The DFT+U formalism of Duradev et al. was used with the Perdew-Burke-Ernzerhof (PBE) functional19 and a U value of 5.5 eV for Ni and 3.3 eV for Fe,20-25 as done previously for pure and Fe-doped NiOOH.21-25 Projected-augmented wave (PAW) potentials
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included contribution of the core electrons of Ni 1s2s2p3s3p, Fe 1s2s2p3s3p , and O 1s.26-27 The bulk unit cell structure of NiOOH22, 28-31 was cleaved at the (01̅5) facet since we wanted to compare to previous literature that studied catalysis on this facet.22 The same slab sizes were chosen as in ref. 22 for comparison: the vacuum length was taken to be 15Å for all slabs.24 The energy cutoff of 600 eV and a Gamma-centered k-point mesh of 2x2x1 grid were converged to within
