Selectivity of CO2, carbonic acid and bicarbonate electroreduction over Iron-porphyrin catalyst: A DFT study

Carbon dioxide reduction is a promising approach to convert CO2 to value-added products. Carbon dioxide electroreduction is commonly performed at neutral pH where CO2, H2CO3, HCO3-, and CO3-2 are in equilibrium. Despite this, only CO2 is commonly considered as the active species while H2CO3, HCO3-, and CO3-2 (carbonate species) are neglected. Using density functional theory (DFT), we investigate the contribution of carbonate species in the CO2 electroreduction reaction over a Fe-porphyrin (Fe(ppy)) model system. We find that the hydrogen evolution reaction (HER) is blocked by a high activation barrier of 1.54 eV associated with the Fe("I") (ppy)-H formation. This is opposed to the reduction of H2CO3 and HCO3- whose rate-limiting step requires significantly lower activation energies of approximately 0.4 eV. Direct CO2 reduction on the other hand requires to overcome a rate-limiting barrier of 0.95 eV for the CO2 adsorption step. Based on these results we suggest that H2CO3 and HCO3- are the true reactants for the electrochemical "CO2"RR over Fe(ppy).