Engineering Conjugation of Covalent Organic Frameworks for Carbon Dioxide Reduction Reaction

Abstract

Covalent organic frameworks (COFs) with tunable electrondonating/withdrawing metal centers and linkers have been utilized in electrocatalysis. However, the roles of conjugated degree of linkers in the catalytic process have not been explored. Here, we develop catalytic COFs with tailored conjugation for formate synthesis via carbon dioxide reduction reaction (CO 2 RR). With extending π lattice sizes from 1.98 to 2.70 nm, the twisted backbones can be controlled, enabling the transmission of π delocalization along the frameworks, the reductive ability and polarity of frameworks, and the electronic positivity of metal sites. Bi-Por-SP-COF with the largest π lattices exhibited high activity and selectivity with a faradaic efficiency (FE) for formate (FE formate ) of 92.8% at −0.9 V and a turnover frequency (TOF) value of 3475.13 h −1 , which were 1.2 and 3.2 times higher than those from the COF with the shortest π lattice, respectively. Moreover, Bi-Por-SP-COF also illustrated a high FE HCOOH of 90.19% at a current density of 138.75 mA cm −2 in the flow cell. The theoretical calculations and in situ experiments revealed that a higher conjugated degree was more conducive to reducing the energy barrier of the reaction intermediate *OCHO and suppressing the competing hydrogen evolution reaction, which contributed to the high activity and selectivity of electrochemical CO 2 RR toward formate. This work gives us remarkable insight into COF and catalytic system design.