Abstract

Abstract Constructing built‐in electric field (BIEF) in heterojunction catalyst is an effective way to optimize adsorption/desorption of reaction intermediates, while its precise tailor to achieve efficient bifunctional electrocatalysis remains great challenge. Herein, the hollow Mo/MoS Vn nanoreactors with tunable BIEFs are elaborately prepared to simultaneously promote hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) for sustainable hydrogen production. The BIEF induced by sulfur vacancies can be modulated from 0.79 to 0.57 to 0.42 mV nm −1 , and exhibits a parabola‐shaped relationship with HER and UOR activities, the Mo/MoS V1 nanoreactor with moderate BIEF presents the best bifunctional activity. Theoretical calculations reveal that the moderate BIEF can evidently facilitate the hydrogen adsorption/desorption in the HER and the breakage of N─H bond in the UOR. The electrolyzer assembled with Mo/MoS V1 delivers a cell voltage of 1.49 V at 100 mA cm −2 , which is 437 mV lower than that of traditional water electrolysis, and also presents excellent durability at 200 mA cm −2 for 200 h. Life cycle assessment indicates the HER||UOR system possesses notable superiority across various environment impact and energy consumption. This work can provide theoretical and experimental direction on the rational design of advanced materials for energy‐saving and eco‐friendly hydrogen production.