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

Abstract

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.