Modulating CO2 Electroreduction Activity on Mo2C and Promoting C2 Product by Grain Boundary Engineering: Insights from First-Principles Calculations

Recently, two-dimensional transition-metal carbides and/or nitrides (MXenes) have attracted extensive interest owing to their promising applications in electrochemistry, especially in electrocatalysis for the CO₂ reduction reaction (CO₂RR). However, there still exist challenges in developing MXene electrocatalysts with high activity and selectivity. Grain boundaries (GBs) could potentially provide active sites for chemical reactions, and their existence may be helpful for improving various electrocatalytic performances of MXenes. In this work, we constructed nine types of GBs in the Mo₂C monolayer and employed density functional theory (DFT) calculations to systematically investigate their effects on the conversion efficiency of CO₂ and the diversity of CO₂RR products. Our study reveals that the presence of different valence states of Mo atoms at the GBs breaks the symmetry of CO₂ adsorption on Mo₂C, which promotes the activation of CO₂ and diversifies the CO₂RR products. Especially, these GBs exhibited remarkably low limiting potentials for C₁ products, e.g., −0.29 V for CH₄ on 5|7c GB, −0.31 V for CH₃OH on 4|8 GB, and −0.55 V for HCOOH on 4|4a GB. Furthermore, the reduced potential barriers at the GBs, such as 0.26 eV for 5|7b GB and 0.13 eV for 8|8b GB, facilitate the C–C coupling and promote the formation of C₂ products. These findings demonstrate that the introduction of GBs can enhance both the electrocatalytic activity of Mo₂C for the CO₂RR and the diversity of CO₂RR products, therefore paving the way for designing and advancing high-efficiency MXene electrocatalysts through GB engineering.