Triazine-Functionalized Covalent Organic Framework Ultrathin Films for Enhancing Local CO₂ Concentrations in Electrochemical CO₂ Reduction

Electrochemical CO2 reduction (CO2R) with transition-metal catalysts often suffers from low selectivity for value-added products. Organic layers have been explored to enhance selectivity by modifying local environments and adjusting the CO2 and CO adsorption, but their role remains unclear due to structural and mechanical instability. Here, we designed a well-defined organic layer model consisting of ∼4 nm-thick covalent organic framework (COF) films with ∼3 nm pores and a cm2-scale area. The COF film, predominantly oriented along the (100) plane, restricted water access, while its triazine moieties enhanced local CO2 concentration through strong CO2 adsorption, improving Faradaic efficiencies with Au, Sn, and Cu. Notably, the suppressed deactivation of Cu was also demonstrated by COF, which exhibited a 2-fold increase in ethylene selectivity. Operando electrochemical spectroscopies and density functional theory (DFT) calculations reveal increased CO coverage and stronger binding of Cu to COF, promoting the formation of OCCOH and HOCCOH intermediates, key to ethylene formation.