Recent development of nickel-based catalysts and in situ characterization techniques for mechanism understanding of the urea oxidation reaction

Along with the emerging modern urea-based energy conversion technologies, electrocatalytic urea oxidation reaction (UOR) plays an important role in urea-rich wastewater treatment and hydrogen production. However, the sluggish kinetics of UOR which involves 6-electron transfer process has impeded its widespread applications. Hence, the development of efficient electro-catalysts is vital for the application of UOR and considerable progress has recently been reported. Among the various UOR catalysts, Ni-based catalysts have been widely studied due to their high UOR activity, abundant resources and long-term sustainability. Their high activity seems to be derived from the surface transformation behavior it undergoes during UOR. The surface transformation behavior results in some intermediates which are the real active sites in the reaction. Further investigation of the surface transformation behavior of Ni-based catalysts is critical and prerequisite for unraveling the underlying UOR mechanisms. Some advanced in situ characterization techniques should be combined during investigation. In this way, some effective methods for synthesis and modification of Ni-based catalysts can be proposed to boost UOR. In this review, we make a compressive summary of the widely accepted UOR mechanisms and the application of several key in situ characterization techniques in revealing surface transformation behavior and reaction mechanism of UOR. Recent advances of Ni-based electro-catalysts were discussed, especially in the methods of improving their UOR performance. We also propose some challenges and prospects for the future development of advanced characterization technologies and corresponding Ni-based catalysts in UOR.