Mechanism of Nickel–Iron Water Oxidation Electrocatalysts

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.