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

Abstract

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 (η_ACL), the number of lone pairs of electrons in the ligand (m), and the electronegativity of the central metal atom (η_M) 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.