Vacancies-Engineered M2CO2 MXene as an Efficient Hydrogen Evolution Reaction Electrocatalyst.

Vacancy engineering is proposed to effectively modulate the hydrogen evolution reaction (HER) activity of M2CO2 MXene. A single C vacancy slightly weakens the H adsorption, while the introduction of a M vacancy or coupled M+C vacancies can greatly enhance the H binding. For a MXene with intrinsic too-strong H adsorption, double C vacancies are effective in weakening the binding and promoting the activity. The activity tuning is closely correlated to the electronic structures of the defected MXene, where the highest occupied peak position of the surface O electronic states shows an apparent linear trend with ΔGH and can be used to qualitatively predict the activity. The weakened or strengthened H adsorption by a C or M vacancy is attributed to the upshifted or downshifted Fermi level of surface O, respectively. Our results indicate the potential of defect chemistry to tune the catalytic activity of MXene and provide new possibilities to enhance the applications of MXene.