Modeling the Local Environment within Porous Electrode during Electrochemical Reduction of Bicarbonate

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO₂ supply has been motivated as a means of integrating conversion with upstream CO₂ capture. The way that CO₂ is formed and transported during CO₂-mediated bicarbonate reduction in flow cells is profoundly different from conventional CO₂ saturated and gas-fed systems and a thorough understanding of the process would allow further advancements. Here, we report a comprehensive two-phase mass transport model to estimate the local concentration of species in the porous electrode resultant from homogeneous and electrochemical reactions of (bi)carbonate and CO₂. The model indicates that significant CO₂ is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO₂. However, the in situ formation of CO₂ and subsequent reduction to CO exhibits a plateau at high potentials due to neutralization of the protons by the alkaline reaction products, acting as the limiting step toward higher CO current densities. Nevertheless, the pH in the catalyst layer exhibits a relatively smaller rise, compared to conventional electrochemical CO₂ reduction cells, because of the reaction between protons and CO₃ ^2– and OH^– that is confined to a relatively small volume. A large fraction of the CL exhibits a mildly alkaline environment at high current densities, while an appreciable amount of carbonic acid (0.1–1 mM) and a lower pH exist adjacent to the membrane, which locally favor hydrogen evolution, especially at low electrolyte concentrations. The results presented here provide insights into local cathodic conditions for both bicarbonate cells and direct-CO₂ reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode.