Insights into the synergistic effect between nickel and molybdenum for catalyzing urea electrooxidation

The efficiency of many clean technologies, such as urea fuel cells, hydrogen production from urea-containing water splitting, and sewage treatment, and so forth, is restricted due to the sluggish kinetics of the urea oxygen reaction (UOR). Mo-doped Ni-based catalysts are excellent for UOR because of the synergistic effect between Ni and Mo. However, such applications have indeed been hampered by a lack of mechanistic comprehension. Here, we fabricate Mo-Ni(OH)₂ nanosheets for efficient UOR electrocatalysis with activity at least twice higher than Ni(OH)₂ nanosheets. Various in situ characterizations including Raman, differential electrochemical mass spectrometry and Fourier transform infrared spectroscopy reveals that the doping of Mo lowers the onset potential required for Ni²⁺/Ni³⁺ conversion and promotes the coupling of the N–N bond inside the molecule and the fracture of the double bond between C and O in the urea molecule. In addition, an entirely flexible direct urea fuel cell with the Mo-Ni(OH)₂ electrocatalyst delivers a maximum output power density of 0.78 mW cm⁻² and complete flexibility for the first time. Our research significantly promotes the mechanistic understanding of UOR electrocatalysis and its applications.