Dynamic Cu valence state cycling regulates urea–OH− adsorption competition: electrocatalytic mechanism and application of CuxS/FeS/NF

Abstract

The urea oxidation reaction (UOR), with its low theoretical potential, can replace the oxygen evolution reaction (OER) at the anode in water electrolysis, thereby reducing the thermodynamic barrier of electrochemical water splitting. However, in practical UOR electrocatalysts, the adsorption competition between urea and OH⁻ constitutes a kinetic bottleneck. Conventional catalysts lack the mechanism of dynamic regulation, which ultimately leads to the occurrence of competitive adsorption. In this study, we fabricated a Cu_xS/FeS/NF heterojunction electrocatalyst via a hydrothermal method, achieving highly efficient catalysis for UOR-assisted overall water splitting. The heterojunction featured an open and disordered hierarchical porous structure, significantly increasing the specific surface area (with a double-layer capacitance of 8.33 mF cm⁻²). More importantly, the reversible Cu⁺/Cu²⁺ valence state cycling on the heterojunction surface dynamically modulated urea adsorption sites, addressing the issue of OH⁻ occupying active centers and accelerating reaction kinetics (with a Tafel slope as low as 26.28 mV dec⁻¹). In situ Raman tests confirmed that Cu²⁺ formed strong coordinations with –NH₂ via empty orbitals, while Cu⁺ weakened the adsorption of OH⁻ through high electron density; the two species synergistically accelerated the reaction kinetics. Electrochemical tests revealed that in the UOR‖HER system, Cu_xS/FeS/NF required a potential of only 1.35 V vs. RHE to achieve a current density of 10 mA cm⁻², while for the HER, it required merely 78 mV to reach the same current density. When applied to UOR-assisted overall water splitting in a 1 M KOH + 0.33 M urea electrolyte, the system required only 1.440 V vs. RHE cell voltage to deliver a current density of 50 mA cm⁻² and maintained stable operation for 60 hours, with a urea degradation rate of 66.4% after 20 hours. This work, through the dynamic regulation mechanism of copper valence state cycling, provides a novel approach for designing UOR catalysts with both high activity and stability, which holds significant application potential in low-energy hydrogen production and wastewater treatment.