Electron Delocalized Ni Active Sites in Spinel Catalysts Enable Efficient Urea Oxidation

The urea oxidation reaction (UOR) has attracted much attention as an efficient alternative reaction to oxygen evolution reaction (OER) due to its low required overpotential. Despite significant progress in efficient nickel-based catalysts, the fundamental issues regarding product selectivity control and dissociation mechanism during the UOR process have not been clarified. Here, we report that tuning the electron delocalization strength of Ni sites significantly affects the OH^- binding sites, altering urea molecule dissociation patterns in alkaline systems. Using spinel NiCo₂O₄ as a model catalyst, the charge delocalization strength of nickel active centers is influenced by the degree of phosphorus-substituted anions. Specifically, complete phosphorus-substituted spinel enhances N₂ selectivity up to 26.8 % with a peak current density of 300 mA ⋅ cm^-2, while partial phosphorus-substituted spinel achieves a Faraday efficiency of 78.1 % for liquid products (NO_x ^-). Experiments and theory reveal that strong charge delocalization at Ni sites favors remote-site attack on urea molecules by hydroxide ions, whereas weak delocalization prefers nearby-site attack, enhancing UOR efficiency and suppressing OER in both pathways.