In-situ coalesced vacancies on MoSe2 mimicking noble metal: Unprecedented Tafel reaction in hydrogen evolution

Highlights: • Vacancy MoSe₂ was synthesized by hydrogen reactivity control during CVD process without any post-treatment. • The in-situ synthesized vacancy-MoSe₂ exhibited outstanding HER performance with exceptionally low Tafel slope. • Our results present a new perspective for developing high performance TMDs-based electrocatalysts. -- Abstract: Transition metal dichalcogenides (TMDs) have shown promising potential as electrocatalyst materials for the hydrogen evolution reaction (HER). However, the low catalytic activity in the basal planes of TMDs results in only limited HER activity, and several strategies to overcome this bottleneck have been proposed, involving various post-synthesis treatments such as introducing chalcogen vacancies or applying mechanical strain. Herein, we demonstrate in-situ modulation of chalcogen vacancy sites during the chemical vapor deposition synthesis of molybdenum diselenides (MoSe₂) for application in the HER. We demonstrate for the first time that the Tafel reaction can be activated via in-situ vacancy-engineered MoSe₂, resulting in improved onset potential and an exceptionally low Tafel slope, which exhibits one of the lowest values reported for TMDs to date in our knowledge. Density functional theory calculations revealed that vacancy coalescence in the MoSe₂ lattice reduced the hydrogen adsorption free energy and diffusion barrier to activate the Tafel reaction. Our approach could contribute to the development of high-performance TMDs-based electrocatalysts with relatively simple processability to make hydrogen production more viable.