Engineering Conjugation of Covalent Organic Frameworks\nfor Carbon Dioxide Reduction Reaction

Covalent\norganic frameworks (COFs) with tunable electron-donating/withdrawing\nmetal centers and linkers have been utilized in electrocatalysis.\nHowever, the roles of conjugated degree of linkers in the catalytic\nprocess have not been explored. Here, we develop catalytic COFs with\ntailored conjugation for formate synthesis via carbon dioxide reduction\nreaction (CO<sub>2</sub>RR). With extending π lattice sizes\nfrom 1.98 to 2.70 nm, the twisted backbones can be controlled, enabling\nthe transmission of π delocalization along the frameworks, the\nreductive ability and polarity of frameworks, and the electronic positivity\nof metal sites. Bi-Por-SP-COF with the largest π lattices exhibited\nhigh activity and selectivity with a faradaic efficiency (FE) for\nformate (FE<sub>formate</sub>) of 92.8% at −0.9 V and a turnover\nfrequency (TOF) value of 3475.13 h<sup>–1</sup>, which were\n1.2 and 3.2 times higher than those from the COF with the shortest\nπ lattice, respectively. Moreover, Bi-Por-SP-COF also illustrated\na high FE<sub>HCOOH</sub> of 90.19% at a current density of 138.75\nmA cm<sup>–2</sup> in the flow cell. The theoretical calculations\nand <i>in situ</i> experiments revealed that a higher conjugated\ndegree was more conducive to reducing the energy barrier of the reaction\nintermediate *OCHO and suppressing the competing hydrogen evolution\nreaction, which contributed to the high activity and selectivity of\nelectrochemical CO<sub>2</sub>RR toward formate. This work gives us\nremarkable insight into COF and catalytic system design.