Facile Solvent-Free Synthesis of Thin Iron Porphyrin COFs on Carbon Cloth Electrodes for CO2 Reduction

We demonstrate the solvent-free synthesis of a covalent organic framework (COF) containing 5,10,15,20-tetra-(4-aminophenyl)porphyrin Fe(III) chloride (FeTAPPCl) and 2,5-dihydroxyterephthalaldehyde (Dha) for the reduction of CO2 to CO with good turnover frequency (>600 h–1 mol–1 of electroactive Fe sites) and reasonable Faradaic efficiency for CO (an average of 80%) at −2.2 V (vs Ag/AgCl) over 3 h in MeCN with 0.5 M trifluoroethanol. Characterization and stability of the COFs were probed by chemical and electrochemical methods. X-ray photoelectron spectroscopy analysis of the COF showed the formation of imine bonds and hemiaminal groups. Cyclic voltammetry of FeDhaTph-COF electrodes revealed three redox couples in MeCN or N,N-dimethylformamide (DMF) systems. The Fe(II/I) and Fe(I/0) redox couples were observed at comparable potentials as FeTAPPCl in solution. Inductively coupled plasma optical emission spectroscopy results suggested that the COF network has retained FeTAPPCl molecules substantially better than simple adsorption in either MeCN or DMF solvent.