Low Overpotential CO<sub>2</sub> Activation by a Graphite-Adsorbed Cobalt Porphyrin
Soumalya Sinha, Rui Zhang, Jeffrey J. Warren
Abstract
The conversion of captured atmospheric carbon dioxide to a useful product is an important chemical challenge. Addressing this challenge requires catalysts that can rapidly turnover with good selectivity and energetic efficiency. Herein, aqueous electrocatalytic CO2-to-CO conversion using drop cast films of cobalt(II) 5-(2-hydroxyphenyl)-10,15,20-triphenyl porphyrin (CoTPOH) are described. Cyclic voltammetry (CV) experiments show redox features that are consistent with formation of a CoIII–CO intermediate at near the CO2/CO thermodynamic potential that then releases CO upon further 1-electron reduction. Analysis of CV and controlled potential electrolysis experiments show that CoTPOH catalyzes the reduction of CO2 to CO with 50% Faradaic efficiency at 0.2 V applied overpotential. With >0.5 V applied overpotential, stable catalytic current density (≥0.2 mA/cm2) is observed during 24 h of electrolysis, with 92% Faradaic efficiency for CO. The behavior of CoTPOH is distinct from the parent Co(II)(5,10,15,20-tetraphenylporphyrin) deposited on basal plane graphite under identical electrochemical conditions. These results demonstrate that CO2 reduction and activation are possible at very little overpotential in aqueous electrolyte, but CO release from Co(III) at low potentials is the limiting factor. The proposed mechanism offers alternative pathways to consider in the design and implementation of CO2 reduction electrocatalysts. The incorporation of proton relays into heterogeneous catalysts remains a challenge, and this study of CoTPOH suggests that they can have a significant impact on catalysis.