Rational Design and Operando Characterization of Hierarchical α‐ <scp>F</scp> e <scp>OOH</scp> ‐ <scp>F</scp> e <scp>P</scp> / <scp>N</scp> i ₃ <scp>S</scp> ₂ Catalysts for High‐Rate Alkaline Water Electrolysis

Advancing alkaline water electrolysis for renewable energy technologies requires oxygen evolution reaction electrocatalysts that combine high activity, long‐term durability, and mechanistic clarity. Herein, we report a hierarchically engineered α‐FeOOH–FeP/Ni 3 S 2 electrocatalyst supported on 3D Ni foam, synthesized via a stepwise hydrothermal sulfidation, gas‐phase phosphidation, and chemical impregnation strategy. This integrated multi‐phase architecture exhibits strong interfacial coupling, enabling accelerated charge transfer and favorable oxygen evolution reaction kinetics under alkaline conditions. In situ/operando Raman, UV–vis, and electrochemical impedance spectroscopy uncover dynamic surface reconstruction under operating conditions, with reversible Fe 3+ /Fe 4+ redox cycling within the α‐FeOOH overlayer, pinpointing transient Fe 4+ –O species as key catalytic intermediates. The optimized catalyst attains low overpotentials of 223 and 251 mV at 10 and 100 mA cm −2 and sustains industrial‐level operation (&gt;500 mA cm −2 ) with outstanding durability in 1.0 m KOH. When deployed in a symmetric anion exchange membrane water electrolyzer, it delivers a cell voltage of only 1.47 V at 10 mA cm −2 , outperforming benchmark noble‐metal‐based systems. Mechanistic studies including kinetic isotope effect and pH‐dependent analysis support a proton‐coupled electron transfer mechanism, with O–H bond cleavage as the rate‐determining step. These findings elucidate key structure–function relationships and establish a modular design strategy for advanced alkaline oxygen evolution reaction electrocatalysts.