Stabilizing High-Valence Ni/Fe Through Li Vacancy Engineering in Li(1–x)NiFeO2/NiFeOOH Heterostructures for Enhanced Oxygen Evolution Reaction

Ni/Fe (oxy)hydroxides have been extensively studied as highly effective electrocatalysts for oxygen evolution reactions (OERs) in alkaline media. The ability of stable, higher-valence Ni/Fe ions (Ni³⁺/Fe³⁺) to enhance the OER activity has been well documented. In this work, we propose a cost-effective strategy for fabricating efficient OER catalysts through the electrochemical in situ delithiation of layered LiNi_1–xFe_xO₂ (LNFO) in an alkaline solution. This process leads to the formation of a NiFeOOH phase with highly oxidative Ni³⁺/Fe³⁺ species at the catalyst surface. The ingenious heterostructure, resulting from the lithium vacancies generated on the LNFO surface, stabilizes the high-valence Ni and Fe species, significantly enhancing the intrinsic OER activity. The as-prepared NiFeOOH/LNFO catalyst shows good OER performance, achieving a current density of 10 mA cm^–2 at an overpotential (η) of 250 mV. In situ Raman and quasi-in situ XPS analyses reveal that the continuous electrochemical delithiation process resulted in the presence of highly oxidative Ni^3+δ/Fe^3+δ species and more amorphous defective structures on the surface of NiFeOOH/LNFO during OER. The high activity (U = 1.72 V at 1 A cm^–2) and durability (continuous 45 h at 500 mA cm^–2) of a membrane electrode assembly (MEA) cell also highlight its potential for practical large-scale applications.