Investigation of CO2 single-pass conversion in a flow electrolyzer

Flow electrolyzers are attracting significant attention because of their unique capability of facilitating carbon dioxide (CO2) electroreduction at high reaction rates. Among all figures of merit, CO2 single-pass conversion is an important factor that can strongly affect the product separation cost of the whole process but often neglected in the literature. In this study, CO2 single-pass conversion was investigated using a flow electrolyzer under various operating conditions to identify the operating constraints on achieving a maximum single-pass CO2 conversion. The maximum amount of CO2 being converted to CO is limited to approximately 43% regardless of CO2 feeding rate, operating current density, and reaction temperature. Further investigation shows that the remaining CO2 feed was mainly consumed by the side reaction of carbonate formation between the CO2 feed and the hydroxide anions generated during the electrolysis. As a result, the gas effluent stream from the cathode chamber contains mainly CO (∼80%), together with 15% H2 and 5% unreacted CO2.