Spatial Electron Density Modulation by Engineering Ordered Built‐in Electric Fields for Efficient and Stable Water Splitting

Abstract Rational modulating spatial electron density through interfacial charge engineering presents a promising approach to enhance water electrolysis. Herein, a porous Co/Co 6 Mo 6 C 2 heterostructure is reported, featuring an ordered built‐in electric field (BIEF) directed from internal Co core to external Co 6 Mo 6 C 2 shell, engineered by the work function difference. This BIEF generates spatially separated electron‐deficient and electron‐rich regions, synergistically facilitating oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The porous structure ensures a large surface area and abundant active sites. X‐ray absorption spectroscopy, potential‐dependent X‐ray photoelectron spectroscopy, and in situ Raman spectroscopy reveal that the high‐valent Mo promotes Co conversion to CoOOH, accelerating OER kinetics and enabling a low overpotential of 170 mV at 10 mA cm −2 in 1 m KOH. A slight increase in Mo content yields an optimized HER catalyst (Co/Co 6 Mo 6 C 2 ‐H) with an overpotential of 39 mV at 10 mA cm −2 . Theoretical calculations combined with experiments corroborate that the ordered BIEF optimizes the adsorption of OER and HER intermediates through electronic redistribution and d‐band center regulation. The corresponding anion exchange membrane water electrolyzer (AEMWE) device demonstrates a voltage of 1.77 V at 500 mA cm −2 with 500 h durability in 1 m KOH, showing potential for industrial‐scale green hydrogen production.