CO<sub>2</sub> Loss\ninto Solution: An Experimental\nInvestigation of CO<sub>2</sub> Electrolysis with a Membrane Electrode\nAssembly Cell

In pursuit of commercial viability for carbon dioxide\n(CO<sub>2</sub>) electrolysis, this study investigates the operational\nchallenges\nassociated with membrane electrode assembly (MEA)-type CO<sub>2</sub> electrolyzers, with a focus on CO<sub>2</sub> loss into the solution\nphase through bicarbonate (HCO<sub>3</sub><sup>–</sup>) and\ncarbonate (CO<sub>3</sub><sup>2–</sup>) ion formation. Utilizing\na silver electrode known for selectively facilitating CO<sub>2</sub> to CO conversion, the molar production of CO<sub>2</sub>, CO, and\nH<sub>2</sub> is measured across a range of current densities from\n0 to 600 mA/cm<sup>2</sup>, while maintaining a constant CO<sub>2</sub> inlet flow rate of 58 mL/min. The dynamics of CO<sub>2</sub> loss\nare monitored through measurements of pH changes in the electrolyte\nand carbon elemental balance analysis. Employing the concept of conservation\nof elemental carbon, a chemical reaction analysis is conducted, identifying\nthe critical role of the hydroxide (OH<sup>–</sup>) ion. At\nlower current densities below 125 mA/cm<sup>2</sup>, where CO<sub>2</sub> reduction predominates, it is observed that CO<sub>2</sub> loss is proportional to current density, reaching up to 0.18 mmol/min,\nand directly correlates with the rate of OH<sup>–</sup> ion\nproduction, indicative of HCO<sub>3</sub><sup>–</sup>/CO<sub>3</sub><sup>2–</sup> ion formation. Conversely, at higher\ncurrent densities above 450 mA/cm<sup>2</sup>, where hydrogen evolution\nis the dominant process, CO<sub>2</sub> loss is shown to decouple\nfrom the OH<sup>–</sup> ion production rate with a constant\nlimit condition of 0.12 mmol/min, regardless of the current density.\nThis suggests that electrolyte-induced cathode flooding restricts\nCO<sub>2</sub> access to cathode sites. Additionally, pH change in\nthe electrolyte during the electrolysis further infers differing ion\npopulations in the CO<sub>2</sub> reduction and hydrogen evolution\nregimes, and their movement across the membrane. Continued monitoring\nof the pH change after the cessation of electricity offers insights\ninto the accumulation of HCO<sub>3</sub><sup>–</sup>/CO<sub>3</sub><sup>2–</sup> ion at the cathode, influencing salt\nformation.