Unveiling the Origin of the High Catalytic Activity of Ultrathin 1T/2H MoSe2 Nanosheets for the Hydrogen Evolution Reaction: A Combined Experimental and Theoretical Study

2 D transition metal dichalcogenide materials with layered nanostructures and specific phases usually exhibit excellent catalytic activities for the hydrogen evolution reaction (HER). A facile solvothermal process was used to prepare ultrathin noble‐metal‐free 2 D biphasic MoSe2 nanosheets composed of a metastable metallic 1T phase and a semiconducting 2H phase. High metallic 1T phase content and few‐layer‐thick MoSe2 nanosheets were obtained by tuning the amount of NaBH4 used in the reaction. The optimal integration of a metallic 1T phase and an environmentally stable 2H phase in MoSe2 electrocatalysts provides abundant active sites and good conductivity beneficial for the HER. The combination of experimental results and DFT calculations implies that the electrocatalytic activity for the HER on the MoSe2 surface goes through a collaborative Heyrovsky and Volmer reaction process. The theoretical studies suggest that the presence of 1T‐MoSe2 could reduce the band energy relative to 2H‐MoSe2 and, consequently, accelerate the sluggish HER kinetics of 2H‐MoSe2. This work provides valuable and novel insights into the understanding of the structure–activity relationships in 2 D transition metal dichalcogenide electrocatalysts. A formidable combination! Ultrathin 2 D MoSe2 nanosheets with a high metallic 1T phase content are fabricated. The resulting nanostructured MoSe2 electrocatalysts exhibit superior activity for the hydrogen evolution reaction (HER) with remarkable stability. The mechanism responsible for the enhanced HER performance is revealed by a powerful combination of DFT calculations and experimental evidence.