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.