In-situ reconstitution of Ni(III)-based active sites from vanadium doped nickel phosphide/metaphosphate for super-stable urea-assisted water electrolysis at large current densities

Abstract

Efficient bifunctional electrocatalysts towards oxygen evolution reaction (OER) and urea electrooxidation reaction (UOR) are urgently needed for hydrogen production from urea-containing wastewater electrolysis. The main challenge lies in the sluggish UOR kinetics and the stability of catalyst under practical high current density. Here, a vanadium doped heterostructure of Ni(PO₃)₂/Ni₂P with shaggy nanosheet morphology was successfully synthesized. The doping of V atoms promotes the formation of Ni(PO₃)₂/Ni₂P heterojunction in phosphating process. It is demonstrated that V-doped Ni(PO₃)₂/Ni₂P accelerates the generation of real active site V@NiOOH in OER and UOR processes, which can also be stabilized by the PO₃⁻ ions. The in-situ formed V@NiOOH increases the adsorption energy of urea molecule, and reduces the adsorption energy of key intermediates *COO, thus facilitating the release of CO₂ product from the catalyst surface. The energy barrier of *HNCON to *NCON is also reduced dramatically, promoting the kinetics of UOR. In addition, the shaggy nanosheets morphology provides large number of catalytic sites and transport channels, which are conducive to mass transfer under high current density. As a result, the V-Ni(PO₃)₂/Ni₂P electrode based anion-exchange membrane (AEM) electrolyzer needs only 1.61 V to drive the total urea electrolysis at an industrial grade current density of 550 mA cm⁻² with an outstanding durability of 700 h. This work paves the way for designing practical efficient and stable electrocatalyst for urea contained wastewater electrolysis to produce hydrogen.