N-doped Ti₃C₂T_x as an efficient support for single-atom catalysts with high stability and selectivity in CO₂RR

The electrochemical CO₂ reduction reaction (CO₂RR) converts CO₂ into valuable fuels and chemicals, offering an effective strategy to address the carbon cycle imbalance. Single atoms anchored on the surface of MXenes can enhance catalytic activity for CO₂RR. However, the optimal local N/O coordination environment for single atoms on nitrogen-doped Ti₃C₂T_x with mixed -O/-N terminations remains unknown. The role of N/O coordination in governing single-atom stability and CO₂ reduction performance has yet to be established. Herein, we systematically investigate the CO₂RR performance of ten 3d transition metal (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) single-atom catalysts anchored on nitrogen-doped Ti₃C₂T_x (TM@N_doped-Ti₃C₂T_x). We find that the anchoring stability of single atoms increases with the number of coordinating nitrogen atoms, and the three-nitrogen coordination (TM@N₃) exhibits the highest thermodynamic stability. This is attributed to strong covalent interactions and favorable d-p orbital matching between TM and nitrogen atoms. Meanwhile, TM@N₃-Ti₃C₂T_x effectively suppresses the competing HER during CO₂RR. Furthermore, TM@N₃-Ti₃C₂T_x catalysts selectively reduce CO₂ to CO and HCOOH. Among them, Co@N₃-Ti₃C₂T_x exhibits the lowest free energy barrier (0.44 eV) to HCOOH formation while maintaining excellent oxidation resistance. This work establishes the structure-activity relationship of TM@N_doped-Ti₃C₂T_x and provides feasible theoretical guidance for CO₂RR applications.