Abstract
Cheap and abundant electrocatalysts for hydrogen evolution reactions (HER) have been widely pursued for their practical application in hydrogen-energy technologies. In this work, I present systematical study of the hydrogen evolution reactions on MXenes (Mo2X and W2X, X = C and N) based on density-functional-theory calculations. I find that their HER performances strongly depend on the composition, hydrogen adsorption configurations, and surface functionalization. I show that W2C monolayer has the best HER activity with near-zero overpotential at high hydrogen density among all of considered pure MXenes, and hydrogenation can efficiently enhance its catalytic performance in a wide range of hydrogen density further, while oxidization makes its activity reduced significantly. I further show that near-zero overpotential for HER on Mo2X monolayers can be achieved by oxygen functionalization. My calculations predict that surface treatment, such as hydrogenation and oxidization, is critical to enhance the catalytic performance of MXenes. I expect that MXenes with HER activity comparable to Pt in a wide range of hydrogen density can be realized by tuning composition and functionalizing, and promotes their applications into hydrogen-energy technologies.
Highlights
Their applications as catalysts in HER is investigated based on the calculations of density-functional theory (DFT)
The overpotentials of pure W2C monolayer at full H-coverage in individual process decreases from 0.15 eV to 0.05 eV, to 0.02 eV as the H-adsorption site changing from hexagonal center (HC) →top of X atom (TX) →top of M atom (TM) (Fig. 3a,c,e)
Both differential and average Gibbs free energies of W2C-TM and W2CH-TM are within a range of −0.2 to 0.1 eV in the whole H-coverage (n = 1~4) (Figs 3e,f and 4e,f), indicating its excellent performance in hydrogen evolution reaction
Summary
Their applications as catalysts in HER is investigated based on the calculations of density-functional theory (DFT). It is predicted that the HER performances of MXenes strongly depend on their composition, surface treatment, hydrogen coverage, and hydrogen adsorption sites. It is found that pure and hydrogenated W2C monolayers are excellent in HER in a wide range of hydrogen density, while oxidization results in the significant reduction of its HER ability. It is shown that oxidized Mo2X monolayer is much better than pure and hydrogenated counterparts in HER.
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