Electromodified NiFe Alloys as Electrocatalysts for Water Oxidation: Mechanistic Implications of Time‐Resolved UV/Vis Tracking of Oxidation State Changes

Facile electromodification of metallic NiFe alloys leads to a series of NiFe oxyhydroxide surface films with excellent electrocatalytic performance in alkaline water oxidation. During cyclic voltammetry and after sudden potential jumps between noncatalytic and catalytic potentials, Ni oxidation/reduction was tracked with millisecond time resolution by a UV/Vis reflectance signal. Optimal catalysis at intermediate Ni/Fe ratios is explained by two opposing trends for increasing Fe content: a) pronounced slowdown of the Ni2+/Ni3+ oxidation step and b) increased reactivity of the most oxidized catalyst state detectable at catalytic potentials. This state may involve an equilibrium between Ni4+ ions and Ni2+ ions with neighboring ligand holes, possibly in the form of bound peroxides. Redox in slow motion: Facile electromodification of metallic NiFe alloys leads to a series of NiFe oxyhydroxide surface films with excellent electrocatalytic performance in alkaline water oxidation. Increasing Fe content slows down Ni2+/Ni3+ oxidation state changes and enhances oxygen evolution reactivity involving an equilibrium between Ni4+ and Ni2+ states with ligand holes.