Controlled synthesis of ACo₂O₄ (A = Fe, Cu, Zn, Ni) as an environmentally friendly electrocatalyst for urea...

Water electrolysis is relatively an environmentally friendly hydrogen production technology, but due to the slow transfer of four electrons in the anodic oxidation reaction, it needs a theoretical voltage of up to 1.23 V. Therefore, in this experiment, a series of transition metal oxides, ACo₂O₄ (A = Fe, Cu, Zn, Ni), was synthesized on Ni foam current collectors by a hydrothermal and calcination method, and the material was applied in urea electrolysis to produce hydrogen. What is noteworthy is that the CuCo₂O₄ electrode has a unique flower-like nanoneedle structure, and has a larger electrochemical active area, more reactive active sites, and a faster charge transfer rate. In 1.0 M KOH and 0.5 M urea solution, CuCo₂O₄ provides a potential of only 1.268 V at a current density of 10 mA cm^-2 during the urea oxidation reaction (UOR), while in 1.0 M KOH solution, with the same current density, the oxygen evolution reaction (OER) is required to provide a potential of 1.53 V, indicating that the UOR can effectively replace the OER. Density functional theory calculations show that the CuCo₂O₄ material exhibits Gibbs free energy of the hydrogen closest to zero, thus promoting the electrochemistry performance of the electrode. In a cell composed of CuCo₂O₄//CuCo₂O₄, the current density of 10 mA cm^-2 can be achieved by providing a potential of only 1.509 V. This work offers a novel scheme for reducing energy consumption of the OER and improving catalytic performance of the UOR.