Directing the Selectivity and Activity of Urea Oxidation Reaction Using Spinel Catalysts

To date, the synergic contribution of trimetallic Ni_xCu_(1−x)Co₂O₄ catalyst towards UOR activity and selectivity has never been reported. We have carried out the synthesis of three catalysts: NCC-1 (Ni_0.25Cu_0.75Co₂O₄), NCC-2 (Ni_0.50Cu_0.50Co₂O₄), and NCC-3 (Ni_0.25Cu_0.75Co₂O₄) by thermal decomposition technique. Physical characterizations, including ICP-OES, XRF, XRD, UV–vis spectroscopy, HR-SEM, and HR-TEM, were conducted. Among the catalysts, the NCC-3 catalyst exhibits long crystalline order, the lowest optical bandgap, and a continuous morphology, all of which are ideal for improved UOR activity. Additionally, the NCC-3, NCC-2, and NCC-1 catalysts produce a stable current for about 10 h, 7 h, and 5 h, respectively, before dropping to lower or almost zero current. The lowest optical bandgap and highest ECSA are other significant features contributing to the enhanced UOR performance of the NCC-3 catalyst. The integral product analysis reveals that the Ni_xCu_(1−x)Co₂O₄ catalysts direct the UOR through the overoxidation route with nitrate as the major product. The concentrations of nitrate, quantified by mathematical and spectroscopic methods, are compared. From the phase transformation tests, although no new crystalline phases evolved, and the spinel structure was preserved; however, nominal leaching of Cu ions into the electrolyte during prolonged electrode suspension in a highly alkaline solution was observed.