Redox-mediated origin of Fe–Ni synergy in oxygen evolution catalysis

Abstract

Fe–Ni layered hydroxides are among the most active oxygen evolution reaction (OER) catalysts, yet the mechanistic origin of their long-recognized synergy remains under debate. Here, we report a solution-phase strategy that decouples structural Fe incorporation from redox mediation by introducing a soluble Fe³⁺–triethanolamine (Fe³⁺(TEA)) complex into alkaline electrolytes. Electrochemical measurements reveal the emergence of highly oxidizing Fe redox states accessible under OER-relevant potentials, enabling redox-mediated activation of Ni sites without lattice doping. Operando Ni K-edge X-ray absorption spectroscopy demonstrates that Fe-mediated oxidation promotes the transformation of β-NiOOH into catalytically active γ-NiOOH at reduced overpotentials. Coulometric titration further quantifies an average transfer of ∼1.6 electrons per Ni atom in the presence of Fe³⁺(TEA), establishing a quantitative link between redox mediation and Ni oxidation. These results identify solution-phase redox mediation as the governing origin of Fe–Ni synergy in OER catalysis and provide a general framework for engineering multimetallic electrocatalysts through electrolyte-controlled redox interactions.