CO2 Loss into Solution: An Experimental Investigation of CO2 Electrolysis with a Membrane Electrode Assembly Cell

In pursuit of commercial viability for carbon dioxide (CO2) electrolysis, this study investigates the operational challenges associated with membrane electrode assembly (MEA)-type CO2 electrolyzers, with a focus on CO2 loss into the solution phase through bicarbonate (HCO3-) and carbonate (CO32-) ion formation. Utilizing a silver electrode known for selectively facilitating CO2 to CO conversion, the molar production of CO2, CO, and H-2 is measured across a range of current densities from 0 to 600 mA/cm(2), while maintaining a constant CO2 inlet flow rate of 58 mL/min. The dynamics of CO2 loss are monitored through measurements of pH changes in the electrolyte and carbon elemental balance analysis. Employing the concept of conservation of elemental carbon, a chemical reaction analysis is conducted, identifying the critical role of the hydroxide (OH-) ion. At lower current densities below 125 mA/cm(2), where CO2 reduction predominates, it is observed that CO2 loss is proportional to current density, reaching up to 0.18 mmol/min, and directly correlates with the rate of OH- ion production, indicative of HCO3-/CO32- ion formation. Conversely, at higher current densities above 450 mA/cm(2), where hydrogen evolution is the dominant process, CO2 loss is shown to decouple from the OH- ion production rate with a constant limit condition of 0.12 mmol/min, regardless of the current density. This suggests that electrolyte-induced cathode flooding restricts CO2 access to cathode sites. Additionally, pH change in the electrolyte during the electrolysis further infers differing ion populations in the CO2 reduction and hydrogen evolution regimes, and their movement across the membrane. Continued monitoring of the pH change after the cessation of electricity offers insights into the accumulation of HCO3-/CO32- ion at the cathode, influencing salt formation.