Mechanism and activity of water oxidation on selected surfaces of pure and Fe-Doped NiO_x

Mixed nickel-iron oxides have recently emerged as promising electrocatalysts for water oxidation because of their low cost and high activity, but the composition and structure of the catalysts active phase under working conditions are not yet fully established. We present here density functional theory calculations with on-site Coulomb repulsion of the energetics of the oxygen evolution reaction (OER) on selected surfaces of pure and mixed Ni-Fe oxides that are possible candidates for the catalysts active phase. The investigated surfaces are pure β-NiOOH(011̄5) and γ-NiOOH(101), Fe-doped β-NiOOH(011̄5) and γ-NiOOH(101), NiFe₂O ₄(001), and Fe₃O₄(001). We find that Fe-doped β-NiOOH(011̄5) has by far the lowest overpotential (η = 0.26 V), followed by NiFe₂O₄(001) (η = 0.42 V). Our results indicate that Fe-doped β-NiOOH and, to a lesser extent, NiFe ₂O₄ could be the phases responsible for the enhanced OER activity of NiO_x when it is doped with Fe.