Theoretical Study of Electrocatalytic CO₂ Reduction Mechanism on Typical MXenes under Realistic Conditions

MXenes are a revolutionary class of two-dimensional materials that have been recently demonstrated to exhibit promising capability of electrocatalytic CO₂ reduction reaction (CO₂RR) in theory and experiment. In electrocatalytic reactions, the active phases, the mechanism, and the performance can be greatly influenced by electrochemical conditions such as applied electrode potential, pH, and electrolyte. Therefore, in this first-principles study, the stable surface structures of three typical MXenes (V₂C, Mo₂C, and Ti₃C₂) with variation of electrocatalytic conditions were determined by the Pourbaix phase diagrams. Additionally, the reaction mechanism for CO₂RR toward C₁ products was investigated based on the thermal dynamically stable phases. The computation revealed that surfaces of all three MXenes are dominated by H* termination throughout the practical CO₂RR electrochemical condition ranges. Meanwhile, the bicarbonate ions, which serve as the major electrolyte in CO₂RR, show thermal dynamic unfavorability to adsorb on the surfaces. Among the three types of MXenes, V₂CH exhibits higher activity in generating CO and HCOOH through the CO₂RR, while Mo₂CH exhibits higher activity in producing HCHO, CH₃OH, and CH₄. This comprehensive study provides crucial insights into the mechanism of electrocatalytic CO₂RR on MXenes under realistic electrochemical conditions.