Mechanism of Nickel-Iron Water Oxidation Electrocatalysts

Abstract

Hotly debated these days is whether nickel or iron is the active site in nickel-iron water oxidation electrocatalysts. We have previously argued that iron is a likely candidate for highly active materials because it can reach high-oxidation (high-ox) states at potentials relevant to water splitting. Here, we further assert that nickel is likely not an active site for water oxidation electrocatalysis in these materials. Our 3-fold argument is supported by electrochemical measurements on rigorously planar electrodes produced by pulsed laser ablation in liquids: (1) nickel cannot achieve high-ox states in aqueous environments at relevant potentials; (2) large steady-state concentrations of metal sites preclude them from being active, thereby indicating that even more oxidizing moieties are critically important; and (3) unlike nickel sites, high-ox iron sites documented experimentally are neither rare nor unreasonably reactive.