Schematic Proposed electrochemical/chemical reactions, ionic species and water transport in a zero-gap AEM CO2 electrolyzer. Humidified CO2, and 10 mM KHCO3 are fed to cathode and anode of a CO2 electrolyzer, respectively. Adapted from 
Charge Carriers in Zero-gap Anion Exchange Membrane CO2 Electrolysis to CO: OH, CO32- or HCO3–?
The electrochemical conversion of CO2 to commercially valuable products, such as carbon monoxide (CO), has been an increasing important research area over the past 20 years. CO in combination with hydrogen is the basis for the Fischer-Tropsch (F-T) process, an important industrial process with the capability of producing a wide variety of fuels and chemicals.
Dioxide Material’s CO2 electrolyzers convert CO2 to CO at high selectivity, stability and efficiency using Sustainion® Membranes . This relies on a zero-gap cell configuration with three key technological developments:
- a cathode gas diffusion electrode (GDE) comprising Ag nanoparticles and an imidazolium-based ionomer as a co-catalyst, reducing cathode overpotential of CO2 conversion to CO.
- an alkaline stable, highly conductive Sustainion® anion exchange membrane (AEM) that conducts anions.
- an anode GDE structure comprising IrO2 nanoparticle catalyst.
Ideally, in zero-gap AEM CO2 electrolyzer, CO2 reacts with H2O on an Ag cathode to generate CO and OH– (Eq. 1); at same time, water is electrolyzed as side reaction to produce H2 and OH– (Eq. 2). OH– ions are transported through the anion exchange membrane and oxidized on an IrO2 anode to evlove O2 (Eq. 3).
The overall reactions of CO2 and water electrolysis are written in Eq. 4 and 5, respectively.
However, CO2 is found in O2 gas stream in CO2 electrolyzers [1-3]. Therefore, it is important to understand the chemistries involved thus optimizing the entire system.
Below is a set of the proposed cathode reactions occuring in the CO2 electrolyzer. CO2 is reduced to CO on an Ag nanoparticle catalyst, forming OH– ions. In the presence of excess CO2, OH– ions are most likely to react with CO2, forming carbonate (Eq. 6) or bicarbonate (Eq. 7) anions. All anions could pass through the anion exchange membrane.
The overall cathode reactions could be as follows:
When carbonate and/or bicarbonate arrives at anode, there are base hydrolysis equilibrium (Eq. 10 and 11). Both continuous supply of CO32- (Eq. 8) and/or HCO3– (Eq. 9) from cathode and consumption of OH– (Eq. 3) in anode push reactions (Eq. 10 and 11) shift to right direction. Therefore, CO2 is released in anode as shown in Eq. 12. The overall anode reactions could be rewritten as Eq. 13 and/or 14.
What are the charge carriers: OH–, CO32-, or HCO3–?
As shown in the schematic below, all three of OH–, CO32-, and HCO3– anions are possible charge carriers in zero-gap AEM CO2 electrolyzers. If OH–, CO32-, or HCO3– ions were charge carriers, CO2/O2 ratio would be 0, 2 and 4, respectively, in anode gas stream (Eq.3, 14, and 13). CO2/O2 ratio were reported to be about 2/1 in anode gas stream [1-3], so it is most likely that CO32- is the main charge carrier in zero-gap AEM CO2 electrolysis to CO. Our papers [1,2,4,5] provide details of the Dioxide Materials’ CO2 electrolyzer. Masel, et al.  provide a detailed review of the performance all of the previous experimental work published on CO2 electrolyzers.
Of course, charge carrier may change when modifying the cell components/configuration and changing the operation conditions. This remains open to further improve CO2 conversion efficiency, paving the way to commercialize CO2 electrolyzers.
- Liu, Z., Yang, H., Kutz, R. and Masel, R. I. CO2 Electrolysis to CO and O2 at High Selectivity, Stability and Efficiency Using Sustainion Membranes. J. Electrochemical Society, 165 (15) J3371-J3377 (2018). https://iopscience.iop.org/article/10.1149/2.0501815jes/pdf
- Kaczur J. J., Yang H, Liu Z, Sajjad S .D. and Masel R. I. Carbon dioxide and water electrolysis using new alkaline stable anion membranes. Front. Chem. 6:263 (2018). https://doi.org/10.3389/fchem.2018.00263.
- Endrodi, B., Kecsenovity, E., Samu, A., Halmagyi, T., Rojas-Carbonell, S., Wang, L., Yan, Y., and Janaky, C. High carbonate ion conductance of a robust PiperION membrane allows industrial current density and conversion in a zero-gap carbon dioxide electrolyzer cell. Energy Environ. Sci., 13, 4098-4105 (2020). https://doi.org/10.1039/D0EE02589E
- Liu, Z., Masel, R. I., Chen, Q., Kutz, R., Yang, H., Lutz, D. R. et al., (2015). “Electrochemical generation of syngas from water and carbon dioxide at industrially important rates.” J. CO2 Util. 15, 50–56. https://doi.org/10.1016/j.jcou.2016.04.011
- Kutz, R. B., Chen Q., Yang, H., Sajjad, S. D., Liu, Z. and Masel, R. I. Sustainion imidazolium-functionalized polymers for carbon dioxide electrolysis. Energy Technology. Energy Technol. 5, 929 – 936 (2017). https://doi.org/10.1002/ente.201600636
- Masel, R. I., Liu, Z., Yang, H., Kaczur, J. J., Carrillo, D., Ren, S., Salvatore, D., and Berlinguette, C. P. An industrial perspective on catalysts for low-temperature CO2 electrolysis. Nature Nanotechnology, Jan., (2021). https://doi.org/10.1038/s41565-020-00823-x