Sunlight-driven water splitting is one of the most attractive methods for solar energy conversion and storage. To achieve efficient water splitting, active catalysts for the hydrogen evolution reaction (HER) are required. Although platinum and a few other precious metals are highly active HER catalysts, their high cost and low abundance are severe hurdles for large-scale applications. The limitations of precious HER catalysts prompts the search for and development of non-precious catalysts. 

Molybdenum-based materials (molybdenum sulfides, molybdenum borides) and molecules have drawn much attention recently. In particular, molybdenum sulfides have been shown to exhibit high HER activity. There is experimental and computational evidence that the active sites in these catalysts are located at the Mo edge, where there are unsaturated sulfur atoms. These sulfur atoms can adsorb hydrogen atoms and mediate hydrogen evolution^[1].

Figure 1. Crystal structures of molybdenum borides^[2].

Heron Vrubel and Xile Hu reported that molybdenum boride (MoB) and carbide (Mo2C) are active HER catalysts^[1]. Although Mo2C has been shown to be a good support for Pt in HER, its own HER activity has not been studied in detail. The results of this research article are interesting in several aspects:
1) The catalysts are non-precious and commercially available. They are also active under both acidic and basic conditions, which are rare properties for HER catalysts.
2) As the structures of molybdenum boride and carbide are different from that of MoS2, the activity of these catalysts is not easily explained by the presence of unsaturated edge sites. The results presented here might invoke new theoretical and experimental investigations of molybdenum-containing HER catalysts.
3) The work might inspire the development of molecular catalysts containing unusual boride and carbide ligands.

References

[1] Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in both Acidic and Basic Solutions.
DOI:10.1002/anie.201207111

[2] The thermodynamic and mechanical properties of Earth-abundant metal ternary boride Mo2(Fe,Mn)B2 solid solutions for impact- and wear-resistant alloys. Mater. Adv., 2023, 4, 3822.