Revealing the Reaction Pathways and Interfacial Regulation Mechanisms of Urea Electro-Oxidation on Nickel-Based Catalysts

Abstract

With an equilibrium potential of 0.37 V versus RHE, much lower than that of the oxygen evolution reaction, the electrocatalytic urea oxidation reaction has been widely regarded as a promising anodic half reaction for reducing the energy consumption of hydrogen production while treating nitrogen containing wastewater. Its practical application, however, is still limited by sluggish kinetics arising from the complex six electron, six proton transfer process, even in high performance nickel based catalysts. In addition, the limited thermodynamic selectivity at nickel catalytic interfaces often prevents the complete conversion of urea into environmentally benign N2. Instead, competing pathways and over oxidation of nitrogen containing intermediates can lead to the formation of soluble byproducts such as NO2- and NO3-. The release of these species not only weakens the remediation effect, but may also cause secondary pollution, including eutrophication.This review summarizes recent progress in nickel based UOR electrocatalysts, with emphasis on reaction pathways, in situ identification of active sites, and interface engineering across multiple scales. Particular attention is given to dynamic surface reconstruction under operando conditions and to the competition among the conventional Ni3+ mediated indirect mechanism, direct oxidation on high valence Ni4+ species, and the lattice oxygen mechanism. We also discuss how the two step pathway involving ammonia intermediates and dual site synergistic mechanisms may help relieve the linear scaling constraints associated with intermediate adsorption. On this basis, we outline a framework for catalyst regulation that includes atomic scale electronic structure tuning, mesoscopic control of internal electric fields in heterojunctions, and macroscopic microenvironment engineering, including ionic regulation and superaerophobic interfaces. Finally, by combining spatiotemporally resolved in situ characterization with standardized quantitative protocols, we highlight key challenges in capturing transient intermediates, resisting interference in real wastewater, and maintaining dynamic stability at ampere level current densities