Effect of terminations on the hydrogen evolution reaction mechanism on Ti3C2 MXene

Abstract

Two-dimensional (2D) MXene materials are proposed as high-efficiency hydrogen evolution reaction (HER) electrocatalysts. Most computational studies addressed the HER assuming a fully O-termination, even if as-synthesized MXenes feature a mixture of –O, –OH, –F, or even –H surface groups. To better understand the electrocatalytic surface composition and mechanism under HER equilibrium conditions in the Ti₃C₂ MXene model material, we composed Pourbaix diagrams considering ca. 450 topologically different surface terminations, including pristine Ti₃C₂, full –O, –OH, –F, and –H terminations, and binary and ternary situations with different group ratios. Realistic models built from Pourbaix diagrams near HER equilibrium conditions of low pH and U were used to investigate the Volmer–Heyrovsky and Volmer–Tafel mechanisms, with the particularity of considering, or not, the participation of H atoms from –OH or –H termination groups at different reaction stages. Results pointed out that the models close to the HER equilibrium line, O_2/3OH_1/3, F_1/3O_1/3OH_1/3, and F_3/9O_4/9OH_2/9, require an almost negligible overpotential of 0.01 V, while surface charges explain the impact of higher ratios of –O groups on modulating the H bond, and the positive influence of having surface –F groups.