Electrochemical Stability of MXenes in Water Based on Constant Potential AIMD Simulations.

MXene has been recently explored as promising electrocatalytic materials to accelerate the electrocatalytic process for hydrogen evolution, but their dynamic stability under electrochemical conditions remains elusive. Here we performed first-principle ab initio molecular dynamics calculations to reveal the electrochemical stability of Ti2CTx MXene in different aqueous environments. The results revealed the high vulnerability of the pure and vacancy-defected Ti2CO2 MXene towards water attack, leading to surface oxidation of MXene under neutral electrochemical condition that formed adsorbed oxygen species to Ti and dissociated proton in solution. The surface oxidation of Ti2CO2 could be prevented in the acid condition or in the neutral condition under the negative potential. Differently, the fully F- or OH-functionalized Ti2CF2 and Ti2C(OH)2 as well as the mixed functionalized Ti2C(O0.5OH0.5)2 and Ti2CO1.12F0.88 are highly stable under various electrochemical conditions, which can effectively prevent close contact between water and surface Ti atoms via electronic repulsion or steric hindrance. These findings provide atomic level understanding of the aqueous stability of MXene and provide useful strategies to prevent degradation and achieve highly stable MXenes.

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 Δ G C P M - Δ G C N M ${{\rm \Delta }{G}_{CPM}-{\rm \Delta }{G}_{CNM}}$ 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.