A Hypothesis on the Function of High‐Valent Fe in NiFe (Hydr)oxide in the Oxygen‐Evolution Reaction

Abstract This study investigated the dynamic changes in NiFe (hydr)oxide and identified the role of high‐valent Fe in the oxygen‐evolution reaction (OER) within alkaline media via in situ techniques. Several high‐valent Fe ions were found to remain considerably stable in the absence of potential in NiFe (hydr)oxide, even 96 hours after the OER. For Ni 2+ hydroxide treated with 57 Fe ions, where Fe sites are introduced onto the surface of Ni 2+ hydroxide, no Fe 4+ species were detected at the rate‐determining step (RDS). The findings of this study suggested that the oxidation of bulk Fe ions, similar to Ni ions, to high valent forms, is charge accumulation without a direct role in OER; these results offered a novel perspective on manipulating Fe states to optimize OER efficacy. The prevailing hypothesis suggested that trace amounts of high‐valent Fe ions, notably those on the surface, directly participate in OER.