Developing high-efficiency electrocatalysts for hydrogen evolution reaction (HER) has been considered as a crucial way to reduce energy loss in watersplitting. Herein, heterogeneous NiSe 2 /MoSe 2 (H-NiSe 2 /MoSe 2 ) nanocomposite constructed by metallic NiSe 2 nanocrystallites embedded in few-layer MoSe 2 nanosheets has been in situ synthesized by a simple hydrothermal reaction. As an HER electrocatalyst, it delivers superior performance with an ultra-small onset overpotential of 103 mV, a small overpotential of 147 mV at 10 mA cm −2 , a low Tafel slope of 43.5 mV dec −1 , and long-term stability up to 18 h in 0.5 M H 2 SO 4 . The superior performance for HER can be ascribed to the unique embedded nanostructure, which can not only improve the overall conductivity of the nanocomposite but also greatly increase the exposed active sites. Furthermore, the synergistic effect of NiSe 2 and MoSe 2 hybrid facilitates the rapid electrons transfer from the electrode to the exposed MoSe 2 edges to take part in the HER reaction, and thus promotes the HER performance. This study provides a simple route for fabricating bi-metallic selenides-based nanocomposite as an efficient and stable electrocatalyst for HER.The excessive consumption of natural resources has brought about severe ecological environment and energy crises. [1][2][3] Hydrogen is considered a renewable energy source to replace natural fossil fuels due to its clean combustion products and high energy density. [4][5][6] Up to now, hydrogen evolution from electrochemical water splitting has been regarded as an eco-friendly and promising strategy to fabricate the hydrogen energy resource. 7-10 However, the high overpotential on the cathode electrode of water splitting severely increases the

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