The Effect of Constant Potential on the Hydrogen Evolution Reaction Activity of M₂ CO₂ and M₂ NO₂ MXenes

Two-dimensional transition metal carbides and nitrides (MXenes) have been widely explored as electrocatalysts for hydrogen evolution reaction (HER). However, current theoretical understanding of the MXene activity mainly relies on the charge neutral method, which neglects the charge effect from the electrode potential. In this work, using hydrogen adsorption as the testing probe, we compared the HER activity of M₂ CO₂ and M₂ NO₂ MXenes via the constant potential method (CPM) and charge neutral method (CNM) computations. The results indicate that the CNM overestimates the H adsorption strength on most MXenes, and the difference in H adsorption free energy between CNM and CPM enlarges with the increase of potential. The $$ difference is mainly caused by the potential induced charge effects, which affect the chemical reactivity and become more evident at the higher potential. Particularly, the Mo₂ CO₂ is HER more active than Ti₂ CO₂ under CPM computations, which contrasts with the CNM results but shows good agreement with the experiment. We introduced a descriptor φ related to the Fermi-level and geometric properties of MXenes, which is highly correlated with the adsorption strength of H and can be applied as an effective activity descriptor. Our work advances the understanding of the effect of potential on HER and can be extended to other electrochemical reactions in MXene.