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

High Resolution Image Download MS PowerPoint Slide In pursuit of commercial viability for carbon dioxide (CO 2 ) electrolysis, this study investigates the operational challenges associated with membrane electrode assembly (MEA)-type CO 2 electrolyzers, with a focus on CO 2 loss into the solution phase through bicarbonate (HCO 3 – ) and carbonate (CO 3 2– ) ion formation. Utilizing a silver electrode known for selectively facilitating CO 2 to CO conversion, the molar production of CO 2, CO, and H 2 is measured across a range of current densities from 0 to 600 mA/cm 2, while maintaining a constant CO 2 inlet flow rate of 58 mL/min. The dynamics of CO 2 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 CO 2 reduction predominates, it is observed that CO 2 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 HCO 3 – /CO 3 2– ion formation. Conversely, at higher current densities above 450 mA/cm 2, where hydrogen evolution is the dominant process, CO 2 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 CO 2 access to cathode sites. Additionally, pH change in the electrolyte during the electrolysis further infers differing ion populations in the CO 2 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 HCO 3 – /CO 3 2– ion at the cathode, influencing salt formation.