Nickel sulfides for urea oxidation: mechanistic understanding, design strategies and advanced characterization

The urea oxidation reaction (UOR) is a promising alternative to the oxygen evolution reaction for hydrogen production and wastewater treatment, but its practical application is limited by sluggish kinetics. Nickel sulfides have attracted significant attention because of their high activity, tunable electronic structures, and low cost. This review summarizes recent advances in catalyst design through heterostructure, doping, defect, and structural engineering strategies, which enhance catalytic performance by optimizing electronic structures, intermediate adsorption, and active-site exposure. UOR mechanisms under different pH conditions are discussed, together with insights gained from in situ characterization techniques. Finally, key challenges, including unclear reaction pathways, limited selectivity, stability concerns, and scale-up issues, are highlighted, and future directions for the rational development of efficient UOR electrocatalysts are proposed.