Axial Fluorine-Modulated M-N4-C SACs for Electrochemical CO2 Reduction: Mechanistic Insights into Ligand Interaction Strength.

Single-atom catalysts (SACs) based on axial modification have received much attention with respect to the carbon dioxide reduction reaction (CO2RR). Revealing the action mechanism of ligands and, at the same time, constructing the interaction relationship between ligands and the structure and activity of catalysts can provide important guidance for the design of highly efficient electrocatalysts for the CO2RR. In this paper, the mechanism of the reduction of CO2 to methanol (CH3OH) on 19 transition metal-coordinated nitrogen-doped carbon (M-N4-C) and axial F atom modified M-N4-C (M-N4F-C) was studied through density functional theory calculations. Moreover, the influence of axial F atoms on M-N4F-C catalytic activity was further revealed. The electrocatalytic reduction activity of M-N4-C SACs toward CO2 depends strongly on the outermost d-shell electron number and electronegativity of the selected metal. The incorporation of axial atoms alters the coordination structure and charge distribution of the central metal atoms, which not only enhances the stability (especially the electrochemical stability) of the M-N4F-C SACs but also modulates the adsorption strength of the intermediate species, thereby either increasing or decreasing the catalytic activity. The catalytic activity is determined by the intrinsic properties of the ligands and metal atoms. Four SACs (Mn-N4-C, Zn-N4-C, Co-N4F-C, and Ru-N4F-C) that can be utilized for the CO2RR are used in the experiments, exhibiting remarkable catalytic activity and stability. Importantly, the electronegativity of the ligands (ηACL), the number of lone pairs of electrons in the ligand (m), and the electronegativity of the central metal atom (ηM) are proposed as key factors to describe the interaction relationship between the ligand and the metal center. Based on these parameters, a ligand interaction strength (λ) is introduced to quantitatively evaluate this interaction. Furthermore, several ligands with different λ values (CN < F < O < N) were employed for axial modification, demonstrating that λ can effectively elucidate the influence of ligands on the geometric and electronic structures of SACs. By correlating λ with the adsorption energy of critical intermediate *OCHO, V-N4CN-C, Cr-N4CN-C, and Mo-N4CN-C were identified as promising electrocatalysts for the CO2RR. Our study provides useful guidance for understanding the influence of axial ligands on the electrocatalytic CO2RR and for designing highly efficient and stable electrocatalysts.