Enhanced Urea Electrooxidation Reaction by Synergistic Nickel and Cobalt Mixed Oxides/Hydroxides Catalyst

Hydrogen emerges as a promising and sustainable energy resource; however, its production faces limitations due to the sluggish kinetics of the oxygen evolution reaction (OER, 1.23V) during the water electrolysis. As a potentially superior alternative to the OER, the urea oxidation reaction (UOR) presents a lower theoretical potential when integrated into the electrolytic cell to replace OER. Extensive research has focused on nickel-based catalysts for their high activity in facilitating the UOR and cost-effectiveness compared to noble metals based electrocatalysts. However, the UOR reaction mechanism and UOR-active phases of Ni-based electrocatalysts still remain ongoing debates.<br/><br/>In this study, we investigated a series of Ni/Co electrocatalysts for UOR, including mixed-phase hydroxides and oxides and single-phase oxides. We found that the synergistic effect between Ni and Co enhanced UOR activity, showing decreased onset potential, fast reaction kinetics, and good selectivity against OER, outperforming Ni and Co monometallic compounds (e.g., NiO, NiOOH, Co₃O₄) and benchmark commercial Pd catalysts. We also found that Ni²⁺ and Co³⁺ species at low anodic potential showed plausible UOR activity and selectivity. While Ni³⁺ formed at high anodic potential showed much enhanced UOR activity, it also accounted for unwanted OER, evidenced by staircase voltammetry electrochemical measurements, synchrotron X-ray absorption and X-ray diffraction, and XPS analysis. This work highlights the importance of valence details and crystalline phase to obtain highly active and selective UOR electrocatalysts and provides a leap forward in understanding the design of high-performance and selective UOR electrocatalysts.