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.