N-doped Ti3C2Tx 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 Ti3C2Tx 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 Ti3C2Tx (TM@Ndoped-Ti3C2Tx). 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-Ti3C2Tx effectively suppresses the competing HER during CO2RR. Furthermore, TM@N3-Ti3C2Tx catalysts selectively reduce CO2 to CO and HCOOH. Among them, Co@N3-Ti3C2Tx 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@Ndoped-Ti3C2Tx and provides feasible theoretical guidance for CO2RR applications.