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CO2 transport through the membrane is an undesirable side- process in CO2-splitting applications, as it is a parasitic loss for the system and reduces CO2 utilization efficiency. Pătru et al.95 evaluated cell designs for gas-phase CO2 reduction using CEM, AEM, BPM, and a novel BPM-like configuration. Authors also quantified CO2 production at the anode of the various cell config- urations. In alkaline membrane co-electrolysis of CO2, carbonate and bicarbonate formation at the cathode results in one-half to one CO2 molecule being transferred to the anode side per electron. CO2 is also transported via the hydrogen evolution reaction at the cathode in concert with other side reactions. For the co-electrolyzer, an alkaline membrane was prepared with gold catalyst at the cathode and iridium dioxide and titanium dioxide catalyst at the anode. Similar to Liu et al.,94 the study found that at high current densities, twice the amount of CO2 as O2 was produced at the anode, indicating the formation and transport of carbonate through the membrane. At lower current densities, less CO2 was produced but constituted a larger proportion of the anode outlet gas. Using a BPM with the acidic cation exchange side facing the anode greatly suppressed this CO2-pumping effect. One challenge with a BPM is delamination at the interface due to the formation of water and CO2, and a novel configuration using an alkaline ionomer within and on top of the cathode catalyst layer and an acidic PEM was able to mitigate this problem while still suppressing unwanted CO2 trans- port. On the Effects of CO2 in Proton Exchange Membrane Fuel Cell Systems Although active transport of CO2 (i.e., electrochemical pumping) does not occur in PEM fuel cells, the topic is briefly discussed here. A review of the common perfluorosulfonic acid (PFSA, e.g., Nafion) membranes by Kusoglu et al.97 notes the slight variation in measured CO2 permeability values found in literature, but it is generally the third most permeable molecule after He and H2. CO2 has also been observed to have a pressure-dependent permeability due to its tendency to plasticize fluorocarbon chains. For CO2 in PFSA membranes, both permeability and selectivity increase with in- creasing relative humidity. The effects of CO2 in PEM fuel cells are well studied due to the potential for CO to poison the catalyst.98,99 Even if clean (CO2-free) air is used as the oxidant, CO2 can be formed through oxidation of carbon at the electrode, which is a particularly relevant mode of corrosion during transient start-up or shut-down of the cell.100 A pertinent study by Erbach et al.101 examined the effect of CO2 crossover to the anode side through diffusion when ultra-thin (⩽15 μm) PEMs are used. CO permeation was examined over a 2 range of different membrane materials, thicknesses, temperatures, relative humidities, pressures, and flow rates. A hydrocarbon-based membrane introduced significantly less CO2 crossover compared to PFSA membranes. Higher pressures, higher relative humidities, and higher temperatures were all associated with higher CO2 permeation levels. With a feed concentration of 1% CO2 in Ar, up to 135 ppm CO2 was observed on the permeate side in a single cell configuration using an 8 μm PFSA membrane. Increasing the current density in the full size fuel cell system did not appear to affect CO2 on the anode side, as expected in the absence of electrochemical CO2 pumping. Conclusions The state of the art in electrochemical CO2 separation methods is presented in this work, which provides a basis for comparisons and future work. Significant research advances and material improve- ments during the past several decades have enabled performance enhancements to make electrochemical CO2 separation commer- cially interesting. As shown, CO2 separation is relevant for a wide range of applications, and will only become more critical in the coming years due to the ubiquity of CO2 emissions. Current industrially-implemented methods of CO2 separation have signifi- cant drawbacks—sorbent regeneration requires large heat inputs and/or pressure swings, and cryogenic distillation is very inefficient.47 Electrochemical membrane separation could maintain the desirable aspects of membrane separation, such as high modularity, continuous operation, and relatively low energy inten- sity, while improving aspects such as selectivity and ability to operate with low CO2 concentrations. While early studies show interesting possibilities for electro- chemical CO2 capture, there is still significant research and devel- opment work required to reduce costs and increase performance. Critical to these cost reduction pathways are the catalyst and membrane contributions. Research efforts should move towards platinum-group-metal-free catalysts and higher current densities. If the applicable current densities remain restricted to the 0–10 mA cm−2 range, very large membrane/electrode areas will be required to separate appreciable amounts of CO2, incurring high capital costs. The importance of faradaic efficiency will depend strongly on the price and CO2 intensity of electricity used to drive the process. For example, a relatively low faradaic efficiency process may become attractive if capital costs are low and off-peak renew- able electricity can be utilized. Another topic that needs to be investigated further is system durability across long operation times, and this should be reported on in future studies. The parallel advancements in AEM fuel cell and electrolyzer technology may allow further improvements to the state of the art in electrochemical CO2 separation. Acknowledgments The authors would like to acknowledge Shell’s New Energy Research and Technology (NERT) Program for providing the funding for this work. We would like to acknowledge NERT’s Dense Energy Carriers team (DEC) for their useful inputs and discussions during the course of this work. ORCID https://orcid.org/0000-0003-1535-8032 https://orcid.org/0000-0002-6555-590X https://orcid.org/0000-0002-8338-6994 References Journal of The Electrochemical Society, 2020 167 133504 Alexander P. Muroyama Alexandra Pătru Lorenz Gubler 1. M. Muntean, D. Guizzardi, E. Schaaf, M. Crippa, E. Solazzo, J. Olivier, and E. Vignati, Fossil CO2 Emissions of All World Countries - 2018 Report, Joint Research Centre (European Commission) (2018). 2. S. Pacala and R. Socolow, Science, 305, 968 (2004). 3. D. W. Keith, Science, 325, 1654 (2009). 4. G. T. Rochelle, Science, 325, 1652 (2009). 5. M. Zhao, A. I. Minett, and A. T. Harris, Energy Environ. Sci., 6, 25 (2013). 6. P. Fennell and B. Anthony, Calcium and Chemical Looping Technology for Power Generation and Carbon Dioxide (CO2) Capture (Elsevier, Amsterdam) (2015). 7. A. MacKenzie, D. L. Granatstein, E. J. Anthony, and J. C. Abanades, Energy Fuels, 21, 920 (2007). 8. S. M. Kim, P. M. Abdala, M. Broda, D. Hosseini, C. Copéret, and C. Müller, ACS Catal., 8, 2815 (2018). 9. N. D. Hutson, S. A. Speakman, and E. A. Payzant, Chem. Mater., 16, 4135 (2004). 10. G.-P. Hao, W.-C. Li, D. Qian, and A.-H. Lu, Adv. Mater., 22, 853 (2010). 11. A. Ö. Yazaydın et al., J. Am. Chem. Soc., 131, 18198 (2009). 12. B. Ibeh, C. Gardner, and M. Ternan, Int. J. Hydrogen Energy, 32, 908 (2007). 13. C. L. Gardner and M. Ternan, J. Power Sources, 171, 835 (2007). 14. J. M. Sedlak, J. F. Austin, and A. B. LaConti, Int. J. Hydrogen Energy, 6, 45 (1981). 15. J. Landon and J. R. Kitchin, J. Electrochem. Soc., 157, B1149 (2010). 16. M. Eisaman, D. Schwartz, S. Amic, D. Larner, J. Zesch, F. Torres, and K. Littau, Technical Proceedings of the 2009 Clean Technology Conference and Trade Show, p. 3 (2009). 17. M. D. Eisaman, L. Alvarado, D. Larner, P. Wang, B. Garg, and K. A. Littau, Energy Environ. Sci., 4, 1319 (2011). 18. M. D. Eisaman, L. Alvarado, D. Larner, P. Wang, and K. A. Littau, Energy Environ. Sci., 4, 4031 (2011). 19. H. Nagasawa, A. Yamasaki, A. Iizuka, K. Kumagai, and Y. Yanagisawa, AlChE J., 55, 3286 (2009). 20. S. Datta et al., Ind. Eng. Chem. Res., 52, 15177 (2013). 21. A. Iizuka, K. Hashimoto, H. Nagasawa, K. Kumagai, Y. Yanagisawa, and A. Yamasaki, Sep. Purif. Technol., 101, 49 (2012). 22. M. C. Stern and T. A. Hatton, RSC Adv., 4, 5906 (2014).PDF Image | CO2 Separation and Transport via Electrochemical Methods
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