Rational Design and Operando Characterization of Hierarchical α-FeOOH-FeP/Ni₃S₂ Catalysts for High-Rate Alkaline Water Electrolysis

Abstract

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/Ni₃S₂ 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 Fe³⁺/Fe⁴⁺ redox cycling within the α-FeOOH overlayer, pinpointing transient Fe⁴⁺–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 (>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.

Keywords

electrocatalysis / oxygen evolution reaction / heterostructured catalysts / oxyhydroxides / transition metal phosphides

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2025 The Author(s). Energy & Environmental Materials published by John Wiley & Sons Australia, Ltd on behalf of Zhengzhou University.