  Two-dimensional transition metal-dichalcogenides are emerging as efficient
and cost-effective electrocatalysts for 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. Here, we tackle the key challenge
that is 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 (SWNTs))
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 provided a cathodic current density of 10mAcm-2 at
overpotential of 100mV, a Tafel slope of 63mVdec-1 and an exchange current
density (j0) of 0.203 Acm-2. In addition, electrode thermal annealing in a
hydrogen environment and chemical bathing in n-butyllithium 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 54mVdec-1.
