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 Ni2+ hydroxide treated with 57Fe ions, where Fe sites are introduced onto the surface of Ni2+ hydroxide, no Fe4+ 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.
Keywords: NiFe (hydr)oxide * Oxygen evolution reaction * High-valent Fe4+ * In-situ techniques * Electrocatalysts * Alkaline conditions * Mechanism * Active sites*.
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