Insight into the High Activity of Nickel Phosphide Precatalysts for Urea Oxidation

The quest for advanced Ni-based (pre)­catalysts for the urea oxidation reaction (UOR) has been significantly impeded by a lack of understanding regarding the catalytic structures and mechanisms after the surface reconstruction, particularly for metalloid compounds like nickel phosphide. This study systematically investigates the UOR performance of Ni­(OH)₂, nickel phosphate (Ni–P_i), and Ni₂P, shedding light on the role of electronic structure and surface morphology in dictating catalytic activity. Through the electrochemical experiments and in situ spectroscopic techniques, we demonstrate that the superior activity of Ni₂P originates from its unique electronic conductivity and the presence of residual phosphate ions, which facilitate the formation of highly active, coordinatively unsaturated sites following the surface reconstruction as well as the faster electron transport. A novel descriptor based on the reversibility of the Ni³⁺/Ni²⁺ redox couple is proposed to underscore the importance of NiOOH formation and regeneration kinetics in the UOR process. The findings reveal that the rapid UOR dynamics on Ni₂P results in minimal accumulation of intermediates, indicative of its high catalytic efficiency. This research not only elucidates the catalytic mechanisms of metal-nonmetal compounds in UOR but also offers a strategic framework for the design of efficient electrocatalysts for sustainable energy applications.