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 angstrom(-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 (Delta G(H*)) on MXenes. Further studies indicated that both N-e and E-f relate to the surface-terminated O-p orbital center (epsilon(p)), whereby the lower epsilon(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 epsilon(p) is either too large or too small. These results show that the surface-terminated O-p orbital center epsilon(p), could be used as a good descriptor for predicting the HER performances of MXenes. The best values of epsilon(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 Delta G(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 epsilon(p) could deliver optimal HER performance.