Revealing the Reactant Mediation Role of Low-Valence Mo for Accelerated Urea-Assisted Water Splitting

Construction of bifunctional catalysts with balanced adsorption toward multiple reactants and intermediates is crucial for efficient urea-assisted water splitting. Nickel Sulfide (NiS) is considered as a promising choice for both urea oxidation reaction (UOR) and hydrogen evolution reaction (HER), whereas the highly occupied d orbital of Ni sites results in too weak reactant adsorption to achieve satisfactory bifunctionality. Herein, considering the adsorption-energy scaling limitations, another Mo4+ site with empty d orbital is introduced into NiS crumpled nanoflowers as a reactant mediator for accelerated urea electrolysis. Based on the in situ characterizations and theoretical calculations, a dual-center catalytic mechanism is proposed that the Mo4+ sites govern the reactant adsorption, subsequently cooperate with nearby Ni sites to promote the reactant dissociation, intermediate formation, and product desorption. The crumpled flower-like nanostructure also provides abundant active sites and rapid mass transfer. Consequently, the Mo-NiS exhibits excellent UOR/HER bifunctionality in an anion-exchange membrane (AEM) flow electrolyzer. Compared with the pure-water electrolysis system, the urea-assisted water electrolyzer saves 15% energy to produce hydrogen. Besides, this electrolyzer obtains an ampere-level current density of 1 A cm(-2) at the cell voltage of 2.0 V, about 2.56 times higher than that of the assembly of RuO2||Pt/C, and robust durability.

Automated summary

The construction of bifunctional catalysts with balanced adsorption towards multiple reactants and intermediates is crucial for efficient urea-assisted water splitting. In this study, a dual-center catalytic mechanism is proposed for urea electrolysis, where NiS with introduced Mo4+ sites exhibits excellent UOR/HER bifunctionality. The crumpled flower-like nanostructure provides abundant active sites and rapid mass transfer, leading to energy savings and high current density in the urea-assisted water electrolyzer.