Crystalline‐Amorphous Interface‐Triggered Electron Redistribution on Copper(II) Sulfide@Metal (Ni, Co, and Fe) Oxyhydroxides for Ultra‐Efficient Overall Water/Seawater Splitting

Abstract Rearranging the electronic orbitals of metal sites through interface engineering is the breakthrough for achieving high efficiencies in hydrogen/oxygen evolution reactions (HER/OER) on bimetallic catalysts. Here, via a multistep liquid‐phase synthesis strategy, the crystalline‐amorphous (c‐a) interface is built by coating amorphous oxyhydroxide layer on the surface of crystallized copper(II) sulfide (CuS@MOOH, M = iron (Fe), cobalt (Co) and nickel (Ni)) with an internal cavity. For HER, c‐a interface facilitates the electron filling of the 3d orbitals of Cu, thereby enhancing the coordination between Cu sites (Cu²⁺/Cu⁺) and *H and reducing the energy barrier for *H adsorption. For OER, c‐a interface triggers electronic rearrangement in the 3d orbitals of M sites, prompting electron transition from the t2g orbitals to the eg orbitals to achieve a half‐filled state, optimizing the oxygen‐intermediates adsorption on M sites (M³⁺/M⁴⁺). Among CuS@MOOH, the as‐marked CuS@CoOOH‐6 exhibits the best activities with ultra‐low overpotentials of 62 mV (HER) and 136 mV (OER). Only 1.52 V is sufficient to power the electrolyzer with CuS@CoOOH‐6‐based cathode/anode, maintaining a ultra‐stable efficiency (96.9 %) over 72 h. Notably, CuS@CoOOH‐6 also exhibits impressive activity/durability for natural seawater electrolysis. This study enhances understanding of the properties and electronic structure of the c‐a interface for water splitting.