Insights into the Understanding of the Nickel-Based Pre-Catalyst Effect on Urea Oxidation Reaction Activity

Nickel-based catalysts are regarded as the most excellent urea oxidation reaction (UOR) catalysts in alkaline media. Whatever kind of nickel-based catalysts is utilized to catalyze UOR, it is widely believed that the in situ-formed Ni³⁺ moieties are the true active sites and the as-utilized nickel-based catalysts just serve as pre-catalysts. Digging the pre-catalyst effect on the activity of Ni³⁺ moieties helps to better design nickel-based catalysts. Herein, five different anions of OH^-, CO₃^2-, SiO₃^2-, MoO₄^2-, and WO₄^2- were used to bond with Ni²⁺ to fabricate the pre-catalysts β-Ni(OH)₂, Ni-CO₃, Ni-SiO₃, Ni-MoO₄, and Ni-WO₄. It is found that the true active sites of the five as-fabricated catalysts are the same in situ-formed Ni³⁺ moieties and the five as-fabricated catalysts demonstrate different UOR activity. Although the as-synthesized five catalysts just serve as the pre-catalysts, they determine the quantity of active sites and activity per active site, thus determining the catalytic activity of the catalysts. Among the five catalysts, the amorphous nickel tungstate exhibits the most superior activity per active site and can catalyze UOR to reach 158.10 mA·cm^-2 at 1.6 V, exceeding the majority of catalysts. This work makes for a deeper understanding of the pre-catalyst effect on UOR activity and helps to better design nickel-based UOR catalysts.