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

Abstract 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 Mo 4+ 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 Mo 4+ 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 RuO 2 ||Pt/C, and robust durability.