Built-in electrophilic/nucleophilic domain of nitrogen-doped carbon nanofiber-confined NiP/NiN nanoparticles for efficient urea-containing water-splitting reactions.

Transferring urea-containing waste water to clean hydrogen energy has received increasing attention, while challenges are still faced in the sluggish catalytic kinetics of urea oxidation. Herein, a novel hybrid catalyst of NiP/NiN embedded in nitrogen-doped carbon nanofiber (NiP/NiN/NCNF) is developed for energy-relevant urea-containing water-splitting reactions. The built-in electrophilic/nucleophilic domain resulting from the electron transfer from NiP to NiN accelerates the formation of high-valent active Ni species and promotes favourable urea molecule adsorption. A spectral study and theoretical analysis reveal that the negatively shifted Ni d-band centre in NiP/NiN/NCNF weakens the adsorption of intermediate CO and facilitates its desorption, thereby improving the urea oxidation reaction kinetics. The overall reaction process is also optimized by minimizing the energy barrier of the rate-determining step. Following the stability test, the surface reconstruction of the pre-catalyst is discussed, where an amorphous layer of NiOOH as the real active phase is formed on the surface/interface of NiP/NiN for urea oxidation. Benefiting from these characteristics, a high current density of 151.11 mA cm at 1.54 V RHE is obtained for urea oxidation catalysed by NiP/NiN/NCNF, exceeding that of most of the similar catalysts. A low cell voltage of 1.39 V is required to reach 10 mA cm for urea electrolysis, which is about 200 mV less than that of the general water electrolysis. The current work will be helpful for the development of advanced catalysts and their application in the urea-containing waste water transfer to clean hydrogen energy.