Cooling Crystallization at Solid Interfaces en Route to a Janus Nanocatalyst

Hydrogen production via water electrolysis is limited by the sluggish kinetics of the oxygen evolution reaction (OER), however replacing OER with the urea oxidation reaction (UOR) provides a promising, energy‐efficient alternative. The development of Janus electrocatalysts offers new opportunities to address the intrinsic complexity problem of the UOR, which involves multiple electron transfers. Here, a cooling crystallization strategy is reported, guided by carbophilicity differences, to synthesize multi‐metallic RuNiW/W 2 C Janus nanoparticles as efficient catalysts for catalyzing the UOR. In the synthesis, W 2 C forms during the initial heating stage and serves as a substrate that promotes the exsolution and subsequent growth of the RuNiW phase upon cooling. This Janus electrocatalyst demonstrates outstanding bifunctional performance, requiring only 1.40 V versus RHE for the UOR and an overpotential of 133 mV for the hydrogen evolution reaction to achieve a current density of 100 mA cm −2 . Moreover, it enables urea‐assisted electrolyzer to operate continuously for over 200 h. Both experiments and calculations confirm that the Janus structure effectively modulates the valence state of Ni component, optimizing urea adsorption and reducing the energy barrier of the rate‐determining step. This work provides a new avenue for designing highly efficient Janus electrocatalysts for sustainable hydrogen production.