Multi-parameter study of CO2 electrochemical reduction from concentrated bicarbonate feed

Abstract

The electrochemical reduction of CO2 (CO2RR) from a concentrated (bi)carbonate solution represents a novel approach of supplying carbon feedstock to a reactor. While this approach achieves comparable performance to gas-feed systems, it substantially reduces the amount of residual CO2 in the output, simplifying the separation of reduction products from unreacted CO2. In this work, the reduction of CO2 to CO from a 2 M KHCO3 solution will be studied by determining the influence of the membrane’s polarity, anolyte’s pH and temperature on CO2RR in a zero-gap flow cell. These parameters were evaluated by measuring the cell potential, the faradaic efficiency (FECO) and residual CO2 at current densities (J) between 25 and 200 mA cm−2. The results obtained show that bipolar membranes offer a much higher FECO compared to cationic or anionic membranes (∼ 45 % to ∼ 12 % at 200 mA cm−2 at 50 °C) thanks to their ability to dissociate water and produce H+ towards the cathode. However, bipolar membranes exhibit an important drawback in the form of a higher cell potential, and higher residual CO2. The anolyte doesn’t affect FECO significantly when the pH stays within an alkaline value. Also, it was confirmed that higher temperatures improve the FECO (especially at higher current densities) while reducing the cell potential. Finally, it was determined that the residual CO2 in the output gas composition stays within 20 %–55 % (at J ≥ 100 mA cm-2), well below the 60–95 % range obtained in systems with gaseous CO2 feed, demonstrating the benefits of working with a bi(carbonate) feed.