Surface termination dependent carbon dioxide reduction reaction on Ti₃C₂ MXene

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 H-2 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, -OH2/3O1/3, -OH1/2O1/2, and -OH1/3O2/3, and one F-containing model, -F1/3OH1/3O1/3. 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 -F1/3OH1/3O1/3 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.