Surface termination dependent carbon dioxide reduction reaction on Ti3C2 MXene

Abstract

The use of two-dimensional (2D) MXene materials as highly efficient electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) has gained considerable attention in the last few years. However, current computational studies on the CO₂RR are primarily focused on MXene materials with different types of metals or displaying fully –O or –OH terminated surfaces, which fail to account for the fact that as-synthesized MXenes possess mixtures of –O, –OH, –F, and/or –H surface groups. Here, a comprehensive density functional theory (DFT) study is carried out on the stability and impact of different surface terminations and moiety distributions on CO₂RR performance done on the prototype Ti₃C₂ MXene, analyzing the possible electrocatalytic synthesis of a series of CO₂RR products, from CO to H₂CO, HCOOH, CH₃OH, and CH₄ under favorable low pH and potential, U, reaction conditions, while considering the competitive H₂ evolution reaction (HER). From ca. 450 distinct surface terminations, four F-free models are selected as dominant in Pourbaix surface stability diagrams under low pH and U conditions, namely –OH, –OH_2/3O_1/3, –OH_1/2O_1/2, and –OH_1/3O_2/3, and one F-containing model, –F_1/3OH_1/3O_1/3. Results highlight the participation of surface –OH groups as H-donors, and the benefits of simultaneous hydrogenation from proton reduction and –OH H transfer. In addition, the presence of both –OH and –O groups is beneficial, reducing limiting potential, U_L, costs, as experimentally observed. On the –F_1/3OH_1/3O_1/3 model, the presence of –F is per se non-detrimental, moving the limiting step to an early stage and reducing the U_L. The overall results underscore the competitiveness of MXenes in the CO₂RR with respect to a Cu electrocatalyst reference, and the tunability possibilities to maximize the selectivity towards either the CO₂RR or the HER.