Modulating the density of catalytic sites in multiple-component covalent organic frameworks for electrocatalytic carbon dioxide reduction

It is generally assumed that the more metal atoms in covalent organic frameworks (COFs) contribute to higher activity toward electrocatalytic carbon dioxide reduction (CO₂RR) and hindered us in exploring the correlation between the density of catalytic sites and catalytic performances. Herein, we have constructed quantitative density of catalytic sites in multiple COFs for CO₂RR, in which the contents of phthalocyanine (H₂Pc) and nickel phthalocyanine (NiPc) units were preciously controlled. With a molar ratio of 1/1 for the H₂Pc and NiPc units in COFs, the catalyst achieved the highest selectivity with a carbon monoxide Faradaic efficiency (FE_CO) of 95.37% and activity with a turnover frequency (TOF) of 4713.53 h^-1. In the multiple H₂Pc/NiPc-COFs, the electron-donating features of the H₂Pc units provide electron transport to the NiPc centers and thus improved the binding ability of CO₂ and intermediates on the NiPc units. The theoretical calculation further confirmed that the H₂Pc units donated their electrons to the NiPc units in the frameworks, enhanced the electron density of the Ni sites, and improved the binding ability with Lewis acidic CO₂ molecules, thereby boosting the CO₂RR performance. This study provides us with new insight into the design of highly active catalysts in electrocatalytic systems.