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 (CO₃ ^2-) ion formation. Utilizing a silver electrode known for selectively facilitating CO₂ to CO conversion, the molar production of CO₂, CO, and H₂ is measured across a range of current densities from 0 to 600 mA/cm², 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/cm², 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 ^-/CO₃ ^2- ion formation. Conversely, at higher current densities above 450 mA/cm², 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 ^-/CO₃ ^2- ion at the cathode, influencing salt formation.