Heterojunction-Induced Rapid Transformation of Ni 3+ /Ni 2+ Sites which Mediates Urea Oxidation for Energy-Efficient Hydrogen Production.

Water electrolysis is an environmentally-friendly strategy for hydrogen production but suffers from significant energy consumption. Substituting urea oxidation reaction (UOR) with lower theoretical voltage for water oxidation reaction adopting nickel-based electrocatalysts engenders reduced energy consumption for hydrogen production. The main obstacle remains strong interaction between accumulated Ni 3+ and *COO in the conventional Ni 3+ -catalyzing pathway. Herein, a novel Ni 3+ /Ni 2+ mediated pathway for UOR via constructing a heterojunction of nickel metaphosphate and nickel telluride (Ni 2 P 4 O 12 /NiTe), which efficiently lowers the energy barrier of UOR and avoids the accumulation of Ni 3+ and excessive adsorption of *COO on the electrocatalysts, is developed. As a result, Ni 2 P 4 O 12 /NiTe demonstrates an exceptionally low potential of 1.313 V to achieve a current density of 10 mA cm -2 toward efficient urea oxidation reaction while simultaneously showcases an overpotential of merely 24 mV at 10 mA cm -2 for hydrogen evolution reaction. Constructing urea electrolysis electrolyzer using Ni 2 P 4 O 12 /NiTe at both sides attains 100 mA cm -2 at a low cell voltage of 1.475 V along with excellent stability over 500 h accompanied with nearly 100% Faradic efficiency.