The electrochemical reduction of carbon dioxide (CO2) to produce value-added chemicals is of great significance in mitigating environmental and energy concerns. In this work, an iron porphyrin catalyst, FePEG8T, with multiple triazole units tethered to a porphyrin ligand via flexible oxymethylene linkers, is reported for efficient electrocatalytic reduction of CO2 to afford carbon monoxide (CO). The electrocatalyst exhibits an excellent catalytic activity with a current density of –17.5 mA/cm2 and CO Faradaic efficiency of 95% at –2.5 V vs. Fc/Fc+ in acetonitrile using water as the proton source. The maximum turnover frequency (TOFmax) was calculated to be 5.5 x 104 s-1 using foot-of-the-wave analysis (FOWA), which is thirty times higher than the result from our previous zinc complex with the same triazole-porphyrin ligand. Control experiments on iron porphyrin complex without triazole units confirm the contribution of triazole units on high catalytic activity. Long-term electrolysis of 40 hours was also performed and demonstrated high catalyst stability. A normal KIE of 6.92 was obtained with H2O/D2O as the proton source at varying concentration ranges (0.5–5 M), suggesting that protonation of catalyst-substrate intermediate is a rate-limiting step. Further, a Tafel plot was generated for the catalyst FePEG8T for comparison with previously reported iron porphyrin catalysts. This work demonstrates an efficient CO2 reduction catalyst containing an iron metal center and a flexible triazole in the second coordination sphere toward CO formation with high stability, activity, and selectivity.

Homogeneous electrocatalytic CO2 reduction using a porphyrin complex with flexible triazole units in the second coordination sphere

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