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

• Published in: Industrial & Engineering Chemistry Research
• Main Authors: Kas, Recep, Yang, Kailun, Yewale, Gaurav P., Crow, Allison, Burdyny, Thomas, Smith, Wilson A.
• Language: unknown
• Published: 2023
• Subjects: 30 DIRECT ENERGY CONVERSION; 37 INORGANIC; ORGANIC; PHYSICAL; AND ANALYTICAL CHEMISTRY; Carbonic acid
• Online Access: http://www.osti.gov/servlets/purl/1867378; https://www.osti.gov/biblio/1867378; https://doi.org/10.1021/acs.iecr.2c00352

The electrochemical reduction of bicarbonate to renewable chemicals without external gaseous CO 2 supply has been motivated as a means of integrating conversion with upstream CO 2 capture. The way that CO 2 is formed and transported during CO 2 -mediated bicarbonate reduction in flow cells is profoundly different from conventional CO 2 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 CO 2 . The model indicates that significant CO 2 is generated in the porous electrode during electrochemical reduction, even though the starting bicarbonate solution contains negligible CO 2 . However, the in situ formation of CO 2 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 CO 2 reduction cells, because of the reaction between protons and CO 3 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-CO 2 reduction membrane electrode assembly cells utilizing cation exchange membranes facing the cathode.