The mechanism of hydrogen adsorption on transition metal dichalcogenides as hydrogen evolution reaction catalyst

Two-dimensional transition metal dichalcogenides (TMDs) have been widely considered as potential hydrogen evolution reaction (HER) catalysts because of their low cost and good electrochemical stability in acid conditions. The mechanism of hydrogen adsorption on TMDs plays an important role in optimizing HER activity. In this research, a series of TMDs (MX₂, M = Co, Cr, Fe, Mn, Mo, Nb, Ni, Re, Sc, Tc, Ti, V, W, Zr, and X = S, Se, Te) in 2H- and 1T-phases were investigated using density functional theory to determine the relationship between hydrogen adsorption free energy (ΔG_H) and electronic structure using a simple descriptor. The results showed a positive linear relation between ΔG_H and the work required of the H electron to fill the unoccupied electronic states of the TMDs. Based on such linear relationships, the various defects (B-, C-, N-, O-, F-, P-, Se-doping and S-vacancy) were used to activate the inert basal planes of the 2H-phase molybdenum disulfide, which can introduce impurity states in the lower energy level to effectively accommodate the H electron. Furthermore, HER activity can be further optimized with the increasing concentration of the defects. These findings provide a practicable map of the HER performances, as well as indicating an appropriate direction for optimizing HER activity.