Deep insights into the exfoliation properties of MAX to MXenes and the hydrogen evolution performances of 2D MXenes

The generation of hydrogen by water splitting through the electrocatalytic approach could become a sustainable way to replace traditional fossil energy. The search for alternatives to the precious metals (Pt, Pd, and Ir) for hydrogen production from water splitting is one of the central issues in the area of renewable energy. Two-dimensional metal carbide and nitride (MXenes) materials have shown some characteristics of promising catalysts for the hydrogen evolution reaction (HER). Herein, we performed density functional calculations to systematically explore the exfoliation properties of more than 50 MAX phases and the HER performances of 30 MXenes to search the universal descriptors for the HER performances of MXenes. The exfoliation properties of MAXs were studied and the results showed that for most MAXs, the lower the binding energy of A elements, the higher the exfoliation energy of 2D MXenes from MAXs. A critical value of exfoliation energy of 0.253 eV Å^-2 was obtained, whereby MAXs with an exfoliation energy lower than this value favor exfoliation. For the predication of the HER performances of MXenes, two descriptors, namely N_e and E_f, have been proposed in the literature. The present calculations illustrated that these two parameters could not fully address the influence of hydrogen coverage on the Gibbs free energy of hydrogen adsorption (ΔG_H∗) on MXenes. Further studies indicated that both N_e and E_f relate to the surface-terminated O-p orbital center (ϵ_p), whereby the lower ϵ_p, the larger N_e and E_f, and the stronger interaction between O∗ with MXenes substrates. However, it is not favored for HER, if ϵ_p is either too large or too small. These results show that the surface-terminated O-p orbital center ϵ_p, could be used as a good descriptor for predicting the HER performances of MXenes. The best values of ϵ_p were in the range of -4.1 to -3.3 eV, with the corresponding values of N_e and E_f from 0.93 to 1.15 e and 3.15 to 3.78 eV, respectively, and ΔG_H∗ from -0.3 to 0.3 eV. The result for the energy change in the HER process (Volmer-Heyrovsky and Volmer-Tafel mechanisms) of 10 M₂CO₂, and the minimum energy pathway of Mo₂CO₂ and Zr₂CO₂ confirmed that MXenes with a medium ϵ_p could deliver optimal HER performance.