Single-Point Linkage Engineering in Conjugated Phthalocyanine-Based Covalent Organic Frameworks for Electrochemical CO2 Reduction

The utilization of covalent organic frameworks (COFs) holds great potential for achieving tailorable tuning of catalytic performance through bottom-up modulation of the reticular structure. In this work, we show that a single-point structural alteration in the linkage within a nickel phthalocyanine (NiPc)-based series effectively modulates the catalytic performance of the COFs in electrochemical CO₂ reduction reaction (CO₂RR). A NiPc-based COF series with three members which possess the same NiPc unit but different linkages, including piperazine, dioxin, and dithiine, have been constructed by nucleophilic aromatic substitution reaction between octafluorophthalocyanine nickel and tetrasubstituted benzene linkers with different bridging groups. Among these COFs, the dioxin-linked COF showed the best activity of CO₂RR with a current density of CO (j_CO) = - 27.99 mA cm^-2 at - 1.0 V (versus reversible hydrogen electrode, RHE), while the COF with piperazine linkage demonstrated an excellent selectivity of Faradaic efficiency for CO (FE_CO) up to 90.7% at a pretty low overpotential of 0.39 V. In addition, both a high FE_CO value close to 100% and a reasonable j_CO of - 8.20 mA cm^-2 at the potential of - 0.8 V (versus RHE) were obtained by the piperazine-linked COF, making it one of the most competitive candidates among COF-based materials. Mechanistic studies exhibited that single-point structural alteration could tailor the electron density in Ni sites and alter the interaction between the active sites and the key intermediates adsorbed and desorbed, thereby tuning the electrochemical performance during CO₂RR process.