The global energy landscape requires the development of sustainable energy production sources capable of replacing finite fossil fuels and reducing environmental problems. Renewable energies are currently in use; however, their intermittency makes it necessary to convert them into energy vectors suitable for long-term storage. The production of hydrogen from water using sustainable processes, such as photocatalysis or electrocatalysis powered by an external source of renewable electricity, is believed to be a promising solution. Unfortunately, most active catalysts capable of carrying out these transformations are based on expensive precious metals. Therefore, the design of highly active, stable, and precious metal-free photocatalysts and electrocatalysts is of utmost importance for the energy transition.

In this regard, molybdenum sulfide-based materials have emerged as potential precious metal-free candidates towards hydrogen evolution reaction (HER). Bulk molybdenum disulfide (MoS₂) shows low activity in HER because its activity originates mainly at the edge sites, while the basal planes are catalytically inert. Generally, most strategies used to improve the activity of MoS₂-derived catalysts have focused on the nanostructuring of these materials to maximize the exposure of active edge sites. Nevertheless, the most convenient approach to achieve this goal should involve the activation of basal planes by defect engineering.

Recently, we have established an innovative bottom-up synthetic strategy that uses Mo₃S_4-7 molecular cluster complexes as precursors to engineer defective molybdenum sulfide nanomaterials (called {Mo₃S₄₋₇}_n) with a greater number of defects in both the edge positions and the basal planes and, therefore, with higher activity. Remarkably, the unique structural configuration of these subunits has made it possible to obtain advanced heterogeneous catalysts for hydrogenation and dehydrogenation reactions in fine chemical synthesis.^[1,2,3] In this communication, it will be shown how, thanks to the extended molecular nature of the {Mo₃S₄₋₇}_n nanomaterials and their processability in the form of heterojunctions with conductive carbon supports and inorganic semiconductors, highly active HER electrocatalysts and photocatalysts have been obtained, respectively.^[4]  Furthermore, the ability to adjust the composition of the molecular cluster precursor allows the derived materials to be precisely tuned and, therefore, the nature of the HER active sites to be deciphered.