Origin of hydrogen adsorption free energy variation in electrochemical hydrogen evolution in transition-metal dichalcogenides

Two-dimensional transition-metal (TM)-doped transition metal dichalcogenides (TMDs) are among the most promising materials for water-splitting catalysts. Among various methods applied to promote the hydrogen evolution reaction (HER) in TMDs, doping with TM heteroatoms has attracted attention because this strategy allows optimization of hydrogen adsorption and H₂ generation reactions. Herein, we conduct in-depth, systematic analyses of the trends in the change in the hydrogen adsorption free energy (ΔG_H∗)—the most well-known descriptor for evaluating HER performance—for doped TMDs. The total of 150 different doped TMDs are used to conduct an atomic-level analysis of the origin of ΔG_H∗ changes upon TM heteroatom doping. Moreover, we suggest two key factors that govern hydrogen adsorption over doped TMDs: 1) changes in the charge of chalcogen atoms, where hydrogen atoms are adsorbed onto early-TM-doped structures and 2) structural deformation energies accompanying the introduced dopants of the late-TM-doped structures. Furthermore, we propose a new perspective on how vacancies in TM-doped TMDs can result in the enhanced ΔG_H∗ for the HER. We suggest that introducing electrostatic and structural controls in early- and late-TM-doped systems, respectively, are effective strategies for achieving thermoneutral ΔG_H∗ in TMDs and promote the design of new TMD catalysts with superior water-splitting capabilities.