Two-Dimensional, Ordered, Double Transition Metal Carbides (MXenes): A New Family of Promising Catalysts for the Hydrogen Evolution Reaction

Generation of hydrogen by splitting water with the electrocatalytic approach could become a more sustainable way following the discovery of new materials, such as the 2D transition-metal carbides. Developing eco-friendly, low-cost, stable, and highly active nonprecious hydrogen evolution reaction (HER) catalysts is one of key factors for hydrogen energy economy. Two-dimensional metal carbide and nitride (MXenes) materials have shown characteristics of promising HER catalysts. Herein, we explored the conductive and thermal stability and electrocatalyst performance of four 2D ordered double MXenes M2′M″C2, Cr2TiC2, Cr2VC2, Mo2TiC2, and Mo2VC2, and their corresponding oxygen (O*)- or hydroxyl (OH*)-terminated MXenes by using density functional calculations. Results indicated that all the above MXenes are conductive, which are favored to charge transfer during HER. Four MXenes are fully terminated by O* under standard conditions [pH = 0, p(H2) = 1 bar, U = 0 V]. The Gibbs free energy for the adsorption of atomic hydrogen (ΔGH*) on the O*-terminated M2′M″C2 (e.g., Cr2TiC2O2) is close to 0 eV (the ideal value) at suitable H coverage. The formability of oxygen vacancy in the fully O*-terminated M2′M″C2, that is, M2′M″C2O2 was studied, and a linear relationship between the formation energy of oxygen vacancy (Ef) and ΔGH* was obtained. The electronic structure analysis indicates that the more electrons gained by the terminated O* from M2′M″C2, the higher is the occupation of the p orbitals of the terminated O* and thus the weaker is the binding strength between the terminated O* and the adsorbed H. Our results indicated that O*-terminated M2′M″C2 are promising HER electrocatalysts for generating hydrogen by water splitting.