Utilization of Electropolymerized Films of Cobalt Porphyrin for the Reduction of Carbon Dioxide in Aqueous Media

Abstract A facile electropolymerization process was utilized to prepare electrodes modified with thin films of cobalt protoporphyrin IX. These thin films exhibited a high Faradaic efficiency (84±2 %) for the reduction of CO 2 to CO in aqueous solutions near neutral pH with 450 mV of overpotential and a turnover frequency at zero overpotential (log(TOF 0 )) of −5.9. The production of CO was stable over several hours at these modest potentials. The use of a 13 CO 2 reactant led exclusively to 13 CO as the product. Polymeric films of the unmetalated porphyrin did not demonstrate catalysis for CO 2 reduction. UV/Vis spectroelectrochemical experiments indicate that the parent Co II complex is reduced to Co I at the electrode surface before interaction with CO 2 . It is proposed that the rate‐determining step in the reduction of CO 2 is the initial reduction of the Co II moiety to Co I , which binds CO 2 and then undergoes a proton‐coupled electron transfer and a loss of water to form CO. Additionally, a new metric for the evaluation of electrocatalysts, the catalytic efficiency, is proposed. The catalytic efficiency is the ratio of the power stored to power consumed for a given electrochemical reaction and can be used to describe both the kinetics and overpotential considerations of a given system.