Role of Zr in promoted activity of urea oxidation reaction and oxygen evolution reaction for NiFe layered double hydroxide

Electrolysis of water and splitting of urea to produce hydrogen are identified as one of the most attractive approach to reduce environmental pollution. However, this approach is both promising and challenging. In the paper, the NiFeZr LDH material was in situ grown on Ni foam through a one-step hydrothermal process. The electrochemistry activity of the NiFeZr LDH electrode was investigated by two probe reactions, one for the oxidation of urea and the other for the oxidation of water. It is worth mentioning that whether it is the oxidation reaction of urea (potential of 1.42 V @ 10 mA cm-2) or water (overpotential of 350 mV @ 10 mA cm-2), the doping of Zr improves the catalytic activity of the material. What is worth mentioning that the oxidation of urea does significantly decrease energy consumption compared to the oxidation of water. However, experiments show that the catalytic active sites are different in the two reactions. We speculate that iron is the active site for the oxidation of water, while zirconium is the active site for the oxidation of urea. Experiments demonstrate that the promoted performance is assigned to the exposure of the centre site, the rapid charge transfer rate and the improved electron configuration. Density functional theory analysis indicates that this NiFeZr LDH electrode presents optimal adsorption energy for urea, which improves the catalytic activity of the material.