The Effect of Constant Potential on the HER Activity of M2CO2 and M2NO2 MXenes.

Abstract

Two-dimensional transition metal carbides and nitrides (MXenes) have been widely explored as electrocatalysts for hydrogen evolution reaction (HER). However, current theoretical understanding of MXene activity mainly relies on charge neutral method, which neglects the charge effect fromelectrode potential. In this work, using hydrogen adsorption as the testing probe, we compared the HER activity of M2CO2 and M2NO2 MXenes via the constant potential method (CPM) and charge neutral method (CNM) computations. The results indicate that the CNM overestimates 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 [[EQUATION]] difference is mainly caused by the potential induced charge effects, which affect chemical reactivity and become more evident at the higher potential. Particularly,Mo2CO2 is HER more active than Ti2CO2 under CPM computations, which contrasts with the CNM results but shows good agreement with experiment. We introduced a descriptor ϕ related to Fermi-level and geometric properties of MXenes, which is highly correlated with adsorption strength of H and can be applied as an effective descriptor. Our work advances understanding of the effect of potential on HER and can be extended to other electrochemical reactions in MXene.