Regulating the Second Coordination Sphere of Cu-Salen–Covalent Organic Frameworks via Substituent Engineering for Electrocatalytic CO2 Reduction to CO

Abstract Single-atom catalysts (SACs) have shown great potential for electrocatalytic CO2 reduction reaction (CO2RR) due to high activity, but the CO2RR selectivity is severely restricted by competing hydrogen evolution reaction (HER). Modifying the electronic structure of single-atom sites to inhibit water adsorption while promoting CO2 adsorption is a promising strategy. Herein, three Cu-Salen–X (X = CF3, H, and OH) covalent organic frameworks with well-defined Cu–N2O2 single-atom sites were synthesized for electrocatalytic CO2RR. The ortho-X substituent serves as a secondary coordination sphere to modulate the electronic state and local microenvironment of Cu centers. The optimized Cu-Salen–CF3 achieves a 95% CO Faradaic efficiency (stable over 50 h) with a partial current density of 11.4 mA cm–2, 4.85- and 10.85-fold higher than those of Cu-Salen–H and Cu-Salen–OH, respectively. Comprehensive investigations and DFT calculations indicate that the electron-withdrawing –CF3 lowers the electron density surrounding Cu and enhances its Lewis acidity, promoting CO2 adsorption and *COOH formation. Concurrently, the d-band center of Cu-Salen–CF3 shifts down to −3.14 eV, facilitating CO desorption and lowering the energy barrier for CO2-to-CO conversion. Additionally, the –CF3 substituent establishes a hydrophobic reaction interface (water contact angle: 120°), which effectively reduces water accumulation near active sites and suppresses the competing HER.