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

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.