Noninnocent Influence of Host β‑NiOOH Redox Activity on Transition-Metal Dopants’ Efficacy as Active Sites in Electrocatalytic Water Oxidation

Catalysts for electrochemical water oxidation or the oxygen evolution reaction (OER) are dominated by transition-metal (TM) complexes (homogeneous) and oxides (heterogeneous). For the latter class of catalysts, iron-doped nickel oxyhydroxide (Ni_1–xFe_xOOH) has shown promise for replacing precious-metal-based catalysts such as RuO₂ and IrO₂ for alkaline water electrolysis. Recently, we used first-principles quantum mechanics to illuminate the precise roles of Fe and Ni in this compound with respect to the mechanism of oxygen evolution. From periodic density functional theory using a screened-exchange hybrid exchange-correlation functional, we identified that the combined abilities of Fe to form an optimally metastable terminal oxo as Fe­(IV)O and Ni­(III)’s relatively facile reduction to Ni­(II) yield a very low overpotential OER. In pursuit of other dopant(s) that can form other such optimal terminal oxos, here we assess the oxygen evolution mechanism and energetics when alternatively doping the β-NiOOH­(1211) surface, a catalytically active facet, with other first-row TMs, namely, V, Cr, Mn, and Co, and some second-row TMs, namely, Mo, Ru, and Rh. These dopants were chosen because their cations are known to form terminal oxos of varying strengths, and some are known to form oxides that are effective OER catalysts. Rather than finding an alternative dopant better than Fe, this study further confirms the unique chemistry of Fe that brings optimal OER activity. Co is found to be a competitive alternative to Fe, with only 0.07 V higher predicted thermodynamic overpotential. More importantly, this study also reveals the strong influence of Ni­(III) on the oxidation state of the dopants, in that most dopants investigated could oxidize spontaneously to their respective high oxidation states, while lattice Ni­(III) ion(s) reduce(s) concomitantly to Ni­(II). Of note, V, Cr, Mo, Ru, and Rh could easily form +5, +6, +6, +7, and +5 TM-oxo groups, respectively. The enhanced stability of these high-oxidation-state cations due to the Ni­(III)/Ni­(II) redox render these dopants inactive toward reductive elimination of O₂, except for V, and thus we predict that they will be ineffective OER catalysts.