Promoting electrochemical CO2 reduction by pyridinium in the secondary coordination sphere of copper-sites anchored on a covalent organic framework

Abstract

The influence of local coordination environments around active sites on catalytic performance for the electrochemical CO₂ reduction reaction (eCO₂RR) has received significant attention. However, the complex interactions among various factors in the catalytic system hinder the establishment of a clear structure–activity relationship, restricting the rational design of efficient catalysts. Herein, single-copper sites coordinated with the bipyridine units of two covalent organic frameworks with different secondary coordination spheres were designed as electrocatalysts for the eCO₂RR. In 1.0 M KHCO₃ solution, the Cu@N⁺-COF catalyst with functionalized cationic pyridinium groups (N⁺) around Cu sites achieved a higher faradaic efficiency for CO₂-to-CO conversion (FE_CO) of 93% with a higher partial current density, superior to the catalyst without pyridinium assistance (Cu@COF) (86%). Notably, a significant difference in catalytic performance was displayed even in the acid electrolyte. The experimental results suggested that the regulation of the secondary coordination sphere by cationic pyridinium groups could not significantly affect the reaction pathway of the Cu sites in Cu@COF and Cu@N⁺-COF. The noteworthy enhancement in catalytic activity and selectivity for Cu@N⁺-COF is attributed to the alternation of the electronic structure of Cu sites caused by the introduction of cationic pyridinium groups into the secondary coordination sphere, which is favorable for stabilizing the *COO⁻ intermediate and inhibiting the competitive hydrogen evolution reaction.