An Experimental- and Simulation-Based Evaluation of the CO₂ Utilization Efficiency of Aqueous-Based Electrochemical CO₂ Reduction Reactors with Ion-Selective Membranes

The CO2 utilization efficiency of three types of electrochemical CO2 reduction (CO2R) reactors by using different ion-selective membranes, including anion exchange membrane (AEM), cation exchange membrane (CEM), and bipolar membrane (BPM), was studied quantitatively via both experimental and simulation methods. The operating current density of the CO2R reactors was chosen to be between 10 and 50 mA cm–2 to be relevant for solar-fuel devices with relatively low photon flux from sunlight. In the AEM-based CO2R reactor with a six-electron per carbon CO2R at the cathode surface, an upper limit of 14.4% for the CO2 utilization efficiency was revealed by modeling and validated by experimental measurements in CO2-saturated aqueous electrolytes without any buffer electrolyte. Improvements in CO2 utilization efficiency were observed when additional buffer electrolyte was added into the aqueous solution, especially in solutions with low bicarbonate concentrations. The effects of the feed rate of the input CO2 stream, the Faradaic efficiency (FE), and the participating electron numbers of the cathode reaction on the CO2 utilization efficiency were also studied in the AEM-based CO2R reactor. The CEM-based CO2R reactor exhibited low CO2 utilization efficiency with recirculation between the catholyte and the anolyte and was unsustainable due to the cation depletion from the anolyte without any recirculation. The BPM-based CO2R reactor operated continuously without a significant increase in the cell voltage and exhibited significantly higher CO2 utilization efficiency, up to 61.4%, as compared to the AEM-based CO2R reactors. Diffusive CO2 loss across the BPM resulted in relatively low CO2 utilization efficiency at low operating current densities. Modeling and simulation also provided target BPM properties for higher CO2 utilization efficiency and efficient cell operation.