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

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 CO₂ to CO in aqueous solutions near neutral pH with 450 mV of overpotential and a turnover frequency at zero overpotential (log(TOF₀)) of −5.9. The production of CO was stable over several hours at these modest potentials. The use of a ¹³CO₂ reactant led exclusively to ¹³CO as the product. Polymeric films of the unmetalated porphyrin did not demonstrate catalysis for CO₂ reduction. UV/Vis spectroelectrochemical experiments indicate that the parent Co^II complex is reduced to Co^I at the electrode surface before interaction with CO₂. It is proposed that the rate-determining step in the reduction of CO₂ is the initial reduction of the Co^II moiety to Co^I, which binds CO₂ 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.