Covalent Organic Frameworks Linked by Amine Bonding for Concerted Electrochemical Reduction of CO2

Covalent organic frameworks (COFs) with amine linkage in both three and two dimensions, COF-300-AR and COF-366-M-AR, were synthesized by direct reduction of their corresponding COFs with imine linkage, COF-300 and COF-366-M, respectively. The quantitative reduction was confirmed by Fourier transform infrared and cross-polarization magic angle spinning NMR (both 13C and 15N) spectroscopy. These amine COFs were highly crystalline and exhibited excellent chemical stability in strong acids and bases. The abundant amino groups in the COF-300-AR backbone facilitated the electrochemical reduction of CO2 on silver electrodes in a concerted manner and led to selective generation of CO. Specifically, CO conversion efficiency was raised from 13% to 53% at −0.70 V and from 43% to 80% at −0.85 V (versus a reversible hydrogen electrode) in comparison with that of a bare silver electrode. The porosity of COFs favored molecular diffusion to the electrode surface, and the amine functional groups close to the electrode surface promoted CO2 conversion efficiency by forming carbamate intermediates.