Space charge transfer is crucial for an efficient electrocatalytic process, especially for narrow-band-gap metal sulfides/selenides. Herein, we designed and synthesized a core–shell structure which is an ultrathin MoSe2 nanosheet coated CuS hollow nanoboxes (CuS@MoSe2) to form an open p–n junction structure. The space charge effect in the p–n junction region will greatly improve electron mass transfer and conduction, and also have abundant active interfaces. It was used as a bifunctional electrocatalyst for water oxidation at a wide pH range. It exhibits a low overpotential of 49 mV for the HER and 236 mV for the OER at a current density of 10 mA·cm –2 in acidic pH, 72 mV for the HER and 219 mV at 10 mA·cm –2 for the OER in alkaline pH, and 62 mV for the HER and 230 mV at 10 mA·cm –2 for the OER under neutral conditions. The experimental results and density functional theory calculations testify that the p–n junction in CuS@MoSe2 designed and synthesized has a strong space charge region with a synergistic effect. The built-in field can boost the electron transport during the electrocatalytic process and can stabilize the charged active center of the p–n junction. This will be beneficial to improve the electrocatalytic performance. This work provides the understanding of semiconductor heterojunction applications and regulating the electronic structure of active sites.

Rational design and construction of bifunctional electrocatalysts with excellent activity and durability is imperative for water splitting. The regulation of the local electronic structure of the active metal sites in polymetallic composites provides a basic idea. Herein, the ultrathin bimetallic molybdate nanosheets are evenly deposited on the CuOx hollow nanotubes on the copper foam substrate to form a hierarchical heterostructure, which can be used as an efficient bifunctional electrocatalyst for overall water splitting. Experimental results prove that the introduction of Ni to form a double metal salt molybdate and CuOx heterostructure design can be optimized by Co cation as best electronic structure of electric catalytic activity center, and can effectively promote the generation of CoOOH active phase at the same time, so that can skillfully optimize the binding strength between the Co site and the oxygen‐containing intermediate, and enhancing catalytic activity. When it is used in the overall water splitting of a double‐electrode alkaline electrolytic cell, the cell voltage of CoNiMoO4‐21/CuOx/CF is 1.532 V at 50 mA cm−2, far exceeding most of the reported conventional bifunctional electrocatalyst. This work has contributed to understanding the central active sites of polymetallic composites and has provided a useful value for the design of efficient electrocatalysts.

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