Tailoring copper content in high-entropy oxides for enhanced water oxidation catalysis.

High-entropy oxides (HEOs) have recently gained considerable attention as next-generation electrocatalysts owing to their exceptional compositional flexibility, structural stability, and synergistic multimetal effects. In this study, a series of Cu-modified high-entropy oxide catalysts (HEO-Cu) with different Cu loadings (20%, 30%, 40%, 50%, and 60%) supported on nickel foam (NF) were synthesized via a facile co-precipitation method and comprehensively characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS) analyses. The structural results confirmed the formation of a homogeneous single-phase oxide with uniform elemental distribution. Electrochemical measurements revealed that the incorporation of Cu significantly improved the oxygen evolution reaction (OER) performance. The linear sweep voltammetry (LSV) curves showed a remarkable decrease in overpotential from 570 mV for bare NF to 470, 376, 371, 262, and 252 mV at 10 mA/cm for 20%, 30%, 40%, 50%, and 60% Cu loadings, respectively. The HEO-Cu 40%/NF electrode exhibited the lowest overpotential (252 mV) and a Tafel slope of 64.5 mV/dec, indicating the most favorable OER kinetics among all compositions. Electrochemical impedance spectroscopy (EIS) confirmed a significant reduction in charge transfer resistance (Rct) from 212 Ω (NF) to 82 Ω (HEO-Cu 40%/NF), while chronoamperometric stability tests demonstrated that the optimized electrode maintained over 95% of its initial current density after 36,000 s of continuous operation.