CO2 or Carbonates – What is the Active Species in Electrochemical CO2 Reduction over Fe-Porphyrin?

Abstract

CO2 reduction is typically performed at neutral pH. Under these conditions CO2 is in equilibrium with H2CO3, HCO3− and CO32−. However, despite their presence so far most studies solely focus on the contribution of CO2 while carbonate species as alternative reactants are generally neglected. Using density functional theory (DFT) modelling we explore the possible contribution of these carbonate species to the overall CO2 reduction activity for a Fe porphyrin model catalyst. Considering only reaction Gibbs free energies, we find the reduction of carbonic acid (H2CO3), bicarbonate (HCO3−) and CO2 to be equally likely. However, owing to a very high activation barrier for the initial adsorption of CO2 onto the catalyst, bicarbonate and carbonic acid reduction are found to be several orders of magnitude faster. These data are used to model the pH dependence of the reaction rates of the different reactants. These results confirm that carbonic acid and bicarbonate are the most likely reactants independent of the pH and reactor setup.