The Holy Grail in Platinum-Free Electrocatalytic Hydrogen Evolution: Molybdenum-Based Catalysts and Recent Advances

Electrochemical water splitting towards hydrogen production is of significance for our sustainable development in the future. Although platinum-group catalysts possess the highest activity for hydrogen evolution reaction (HER), their scarcity renders potential applications commercially inviable and thus many efforts are constantly devoted into this arena. Over the last decade, various molybdenum-based catalysts, covering, for example, sulfides, carbides, phosphides, borides, nitrides and alloys, have been found to be very active, and are rapidly becoming popular. In this Review, we comprehensively summarize this topic of emerging Mo-based catalysts for HER by stating and discussing their catalytic mechanisms, synthesis methods, optimization tactics, and promise to outbalance platinum-based systems. We use typical examples from recent advances of various Mo-based catalysts to elaborate their merits and shortcomings for HER, as well as how to further optimize their activities, varying from surface area maximization and active site exposure to electron/atom/molecule/phase-level manipulation, and to external field-assisted activity enhancement. We also propose various challenges and perspectives for these emerging Mo-based catalysts and discuss how these challenges might be addressed.