Iron porphyrin with appended guanidyl group for significantly improved electrocatalytic carbon dioxide reduction activity and selectivity in aqueous solutions

Iron porphyrins have high activity and selectivity for electrocatalytic CO2 reduction reaction (CO2RR) in nonaqueous solutions, but they usually display poor or moderate selectivity for CO2RR in aqueous solutions because of the competitive hydrogen evolution reaction. Using water as the electrocatalytic reaction solvent is more favored because not only it is cheap, green and abundant but also it can sufficiently provide protons required for CO2RR. Therefore, developing Fe porphyrins as electrocatalysts for efficient and selective CO2RR in aqueous solutions is of both fundamental and practical significance. Herein, we report the design and synthesis of Fe porphyrin 1 with an appended guanidyl group and its electrocatalytic features for CO2RR in both nonaqueous and aqueous solutions. In acetonitrile, Fe porphyrin 1 and its guanidyl-free analogue, tetrakis(3,4,5-trimethoxyphenyl)porphyrin 2, are both efficient for electrocatalytic CO2-to-CO conversion, but the turnover frequency with 1 (3.9 × 105 s−1) is one order of magnitude larger than that with 2 (1.7 × 104 s−1), showing the critical role of the appended guanidyl group in improving electrocatalytic CO2RR activity. More importantly, in 0.1 mol L−1 KHCO3 aqueous solutions, 1 showed very high selectivity for electrocatalytic CO2-to-CO conversion with a Faradaic efficiency of 96%, while 2 displayed a Faradaic efficiency of 65% for the CO2-to-CO conversion. This work is of significance to show the effect of appended guanidyl group on improving both activity and selectivity of Fe porphyrins for CO2RR electrocatalysis.