Hierarchically cactus-like nickel sulfide-lanthanum carbonate hydroxide composite for urea-assisted water splitting

Hydrogen production from water electrolysis is mainly affected by the high overpotential and slow reaction kinetics of the anodic oxygen evolution reaction (OER). Replacing slow oxygen evolution reaction (OER) with thermodynamically favorable urea oxidation reaction (UOR) is one of the feasible strategies for achieving energy-saving hydrogen production. In this work, a cactus-like nickel sulfide-lanthanum carbonate hydroxide (NiSx-LaCH) composite was synthesized on the surface of Ni foam by a hydrothermal method and sulfurization process for the first time. This cactus-like structure can significantly improve the specific surface area of the catalyst and the electrolyte accessibility. The coupling of NiSx and LaCH brought about the electron structure redistribution and enhanced the stability and conductivity. The generated synergistic effect of NiSx-LaCH/NF improved the adsorption of urea/water molecules and the intrinsic catalytic activity. It exhibited outstanding electrocatalytic performance for UOR and hydrogen evolution reaction, which can drive the current density of 100 mA cm-2 at low potentials of 1.36 V and -0.26 V, respectively. Assembling NiSx-LaCH/NF into urea electrolysis system can reduce the cell voltage by 0.22 V compared to water electrolysis. This indicates that using UOR instead of OER could achieve energy-saving hydrogen production and has promising prospects in treating urea-rich wastewater.