Anodic Process of Nano-Ni Hydroxides for the Urea Oxidation Reaction and Its Electrochemical Removal with a Lower Energy Input

Ni-hydroxide catalysts coated on nickel foam electrodes were prepared to explore their intrinsic nature to favor the urea electro-oxidation reaction in an alkaline solution. Urea electrolysis takes place at oxidative potentials of around 0.5–0.6 V, reaching its maximum current density at ∼0.575 V, while oxygen release occurs above 0.6 V, indicating that electrochemical urea removal can be achieved under low-energy-input conditions. Scanning electron microscopy and cyclic voltammetry were used to investigate the electrocatalytic performance of the Ni catalyst on an Au-layered glassy carbon electrode for urea oxidation. In situ surface-enhanced Raman spectroscopy (SERS) was used to measure the valence of Ni catalysts, indicating urea-dependent changes in the intensity of the NiOOH peak and thus revealing that the NiIII is an active catalytic species. It evidenced that Raman bands corresponding to the speciation of Ni hydroxides were formed as oxidative mediates. It can be concluded that the electrolysis of urea is mediated by the NiOOH/Ni(OH)2 redox pair accompanied by regeneration of the catalyst.