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

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, O2/3OH1/3, F1/3O1/3OH1/3, and F3/9O4/9OH2/9, 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.