Theoretical study of the mechanism of the hydrogen evolution reaction on the V2C MXene: Thermodynamic and kinetic aspects

Both experimentally and theoretically, the MXene family has shown promising hydrogen evolution reaction (HER) capabilities. However, so far, the theoretical approach has been relying on the well-known thermodynamic descriptor, ΔG_H, whereas experimental studies report Tafel plots, containing kinetic rather than thermodynamic information. Aiming to link theory to experiments, the present study explores five different HER pathways over the exemplary V₂C (0001) MXene by density functional theory calculations. While the surface coverage under HER conditions (with either H* or OH* adsorbates) is extracted from a Pourbaix diagram, we determine the energetics of the reaction intermediates and transition states for both surface species as active sites. This enables the construction of free-energy diagrams for the Volmer-Heyrovsky and Volmer-Tafel mechanisms and allows for the simulation of Tafel plots by a rigorous microkinetic framework. While the active-site motif V₂C-OH seems to be less relevant for the HER under typical reaction conditions, we demonstrate that the HER is kinetically facile on the V₂C-H surface. For this surface termination, we report a potential-depending switching of the preferred mechanism from the Volmer-Heyrovsky to the Volmer-Tafel description with increasing overpotential while encountering similarities to the HER over Pt.