An Investigation into the Self-Purging Mechanism of Anion Exchange Membrane Fuel Cells Exposed to Carbon Dioxide

Abstract

There is still a gap in the current understanding of how carbon dioxide affects the operation of AEMFCs. Dekel et al. recently reported that only 3% of experimental studies included the effects of ambient CO2 on cell performance3, even though several sources have identified the CO2 problem as a key issue to be addressed in AEMFCs3,5,6. Carbon dioxide from ambient air (or other sources, such as flue gases7,8) can degrade AEMFC performance by reacting with the native hydroxide ions to form bicarbonate and carbonate ions (HCO3 -, CO3 2- respectively). These ions are less mobile than the native hydroxide ions (OH-) and therefore increase ohmic losses in the cell. In addition, the presence of (bi)carbonate species may impact electrochemical processes (and hence activation/thermodynamic losses) in the AEMFC, a phenomenon which is still largely unexplored. Although AEMFC performance is worse when operated in the presence of carbon dioxide, this can be mitigated during normal operation through an effect known as the self-purging mechanism. Several recent reviews and papers have noted that there remains a lack of consensus over the exact mechanism of the self-purging phenomenon1,5,6,9. The two leading theories can be described as a chemical mechanism, and an electrochemical mechanism: Chemical Mechanism: The chemical mechanism proposes that local depletion of OH- near the anode due to the hydrogen oxidation reaction (HOR) shifts the equilibrium of (bi)carbonate-producing chemical reactions to favor OH- production, thereby replenishing OH-, reducing the AEM’s (bi)carbonate content, and releasing CO2 into the anode gas stream via desorption. Electrochemical Mechanism: The electrochemical mechanism proposes that (bi)carbonate ions are direct reactants in the HOR, which would predict their removal from the membrane. This theory also accounts for the decrease in (bi)carbonate concentrations and CO2 emission during operation. In this numerical study we present a transient, spatially-varying AEM model to account for the carbonation process under operating conditions. Within this model we individually simulate the chemical and electrochemical self-purging pathways, and compare the results to those observed experimentally (e.g. conductivity increase with increasing current density, emission of CO2 into the anode gas stream, and increasing anode overpotential with increasing CO2 content). Following the determination of an appropriate self-purging model, we present a parametric investigation into this effect, and make suggestions on how it might be facilitated in order to improve AEMFC performance in the presence of CO2. References Arges, C. G. & Zhang, L. Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency. ACS Appl. Energy Mater. 1, 2991–3012 (2018).Varcoe, J. R. & Slade, R. C. T. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 5, 187–200 (2005).Dekel, D. R. Review of cell performance in anion exchange membrane fuel cells. J. Power Sources 375, 158–169 (2017).Omasta, T. J. et al. Beyond catalysis and membranes: Visualizing and solving the challenge of electrode water accumulation and flooding in AEMFCs. Energy Environ. Sci. 11, 551–558 (2018).Ziv, N., Mustain, W. E. & Dekel, D. R. The Effect of Ambient Carbon Dioxide on Anion-Exchange Membrane Fuel Cells. ChemSusChem 11, 1136–1150 (2018).Gottesfeld, S. et al. Anion exchange membrane fuel cells: Current status and remaining challenges. J. Power Sources 1–15 (2017). doi:10.1016/j.jpowsour.2017.08.010Rigdon, W. A. et al. Carbonate Dynamics and Opportunities With Low Temperature, Anion Exchange Membrane-Based Electrochemical Carbon Dioxide Separators. J. Electrochem. Energy Convers. Storage 14, 020901 (2017).Landon, J. & Kitchin, J. R. Electrochemical Concentration of Carbon Dioxide from an Oxygen/Carbon Dioxide Containing Gas Stream. J. Electrochem. Soc. 157, B1149 (2010).Shiau, H.-S., Zenyuk, I. V. & Weber, A. Z. Elucidating Performance Limitations in Alkaline-Exchange- Membrane Fuel Cells. J. Electrochem. Soc. 164, E3583–E3591 (2017).