A Density Functional + U Assessment of Oxygen Evolution Reaction Mechanisms on β-NiOOH

NiO_x has long been studied both as a battery cathode material and electrocatalyst for the oxygen evolution reaction (OER). Numerous investigations have demonstrated that Fe-doped nickel oxyhydroxide (NiOOH) is one of the most active OER catalysts in alkaline media. Despite extensive research, however, many unanswered questions pertaining to the OER mechanism on this material remain. Here, using density functional theory + U calculations, we compare several surfaces of β-NiOOH studied for the OER and determine that unlike some earlier models selected, the (001) surface is the most stable surface under electrochemical conditions. We then examine several magnetic states of this material and predict that, unlike bulk β-NiOOH, (001)-β-NiOOH manifests a slight preference to be ferromagnetic. We then use the resulting structural model to compare in detail four commonly proposed OER mechanisms. In addition to excluding a proposed mechanism involving hydrogen peroxide formation, we identify multiple binuclear mechanisms with slightly lower overpotentials than the commonly studied associative mechanism. All exhibit overpotentials that coincide well with measured values. However, the similarity in calculated overpotentials highlights the fact that several mechanisms are likely to be operative under electrochemical conditions on β-NiOOH. This finding suggests that much of the complexity of studying the OER on NiOOH is due to multiple competing mechanisms occurring under given conditions, which should be accounted for in subsequent analyses.