Engineered MoSe2‐Based Heterostructures for Efficient Electrochemical Hydrogen Evolution Reaction

2D transition metal‐dichalcogenides are emerging as efficient and cost‐effective electrocatalysts for the hydrogen evolution reaction (HER). However, only the edge sites of their trigonal prismatic phase show HER‐electrocatalytic properties, while the basal plane, which is absent of defective/unsaturated sites, is inactive. Herein, the authors tackle the key challenge of increasing the number of electrocatalytic sites by designing and engineering heterostructures composed of single‐/few‐layer MoSe2 flakes and carbon nanomaterials (graphene or single‐wall carbon nanotubes) produced by solution processing. The electrochemical coupling between the materials that comprise the heterostructure effectively enhances the HER‐electrocatalytic activity of the native MoSe2 flakes. The optimization of the mass loading of MoSe2 flakes and their electrode assembly via monolithic heterostructure stacking provides a cathodic current density of 10 mA cm−2 at overpotential of 100 mV, a Tafel slope of 63 mV dec−1, and an exchange current density (j0) of 0.203 µA cm−2. In addition, thermal and chemical treatments are exploited to texturize the basal planes of the MoSe2 flakes (through Se‐vacancies creation) and to achieve in situ semiconducting‐to‐metallic phase conversion, respectively, thus they activate new HER‐electrocatalytic sites. The as‐engineered electrodes show a 4.8‐fold enhancement of j0 and a decrease in the Tafel slope to 54 mV dec−1. Thermo‐induced texturization, chemically induced material phase conversion, and the monolithic stacked assembly of solution‐processed MoSe2‐based heterostructures provide advanced tools that are required to efficiently produce electrochemical hydrogen.