Nitrogen vacancies enriched Ce-doped Ni₃N hierarchical nanosheets triggering highly-efficient urea oxidation reaction in urea-assisted energy-saving electrolysis

Urea oxidation reaction (UOR), which has favorable thermodynamic energy barriers compared with oxygen evolution reaction (OER), can provide more cost-effective electrons for the renewable energy systems, but is trapped by its sluggish UOR kinetics and intricate reaction intermediates formation/desorption process. Herein, we report a novel and effective electrocatalyst consisting of carbon cloth supported nitrogen vacancies-enriched Ce-doped Ni₃N hierarchical nanosheets (Ce-Ni₃N@CC) to optimize the flat-footed UOR kinetics, especially the stiff rate-determine CO₂ desorption step of UOR. Upon the introduction of valance state variable Ce, the resultant nitrogen vacancies enriched Ce-Ni₃N@CC exhibits an enhanced UOR performance where the operation voltage requires only 1.31 V to deliver the current density of 10 mA cm⁻², which is superior to that of Ni₃N@CC catalyst (1.36 V) and other counterparts. Density functional theory (DFT) results demonstrate that the incorporation of Ce in Ni₃N lowers the formation energy of nitrogen vacancies, resulting in rich nitrogen vacancies in Ce-Ni₃N@CC. Moreover, the nitrogen vacancies together with Ce doping optimize the local charge distribution around Ni sites, and balance the adsorption energy of CO₂ in the rate-determining step (RDS), as well as affect the initial adsorption structure of urea, leading to the superior UOR catalytic performance of Ce-Ni₃N@CC. When integrating the Ce-Ni₃N catalyst in UOR//HER and UOR//CO₂R flow electrolyzer, both of them perform well with low operation voltage and robust long-term stability, proofing that the thermodynamically favorable UOR can act as a suitable substitute anodic reaction compared with that of OER. Our findings here not only provide a novel UOR catalyst but also offer a promising design strategy for the future development of energy-related devices.