Effects of Appended Poly(ethylene glycol) on Electrochemical CO2 Reduction by an Iron Porphyrin Complex

Abstract

Electrochemical carbon dioxide (CO2) reduction is a sustainable approach for transforming atmospheric CO2 into chemical feedstocks and fuels. To overcome the kinetic barriers of electrocatalytic CO2 reduction, catalysts with high selectivity, activity, and stability are needed. Here, we report an iron porphyrin complex, FePEGP, with a poly(ethylene glycol) unit in the second coordination sphere, as a highly selective and active electrocatalyst for the electrochemical reduction of CO2 to carbon monoxide (CO). Controlled-potential electrolysis using FePEGP showed a Faradaic efficiency of 98% and a current density of -7.8 mA/cm2 at -2.2 V versus Fc/Fc+ in acetonitrile using water as the proton source. The maximum turnover frequency was calculated to be 1.4 × 105 s-1 using foot-of-the-wave analysis. Distinct from most other catalysts, the kinetic isotope effect (KIE) study revealed that the protonation step of the Fe-CO2 adduct is not involved in the rate-limiting step. This model shows that the PEG unit as the secondary coordination sphere enhances the catalytic kinetics and thus is an effective design for electrocatalytic CO2 reduction.