Ex-Situ Technique to Measure the Chemical Stability of Anion Exchange Membranes Simulating in-Operando Anion Exchange Membrane Fuel Cell Test Environment

Abstract

One of the major barrier in developing practical anion-exchange membrane fuel cells (AEMFCs) is the limited anion exchange membrane (AEM) chemical stability, since they usually undergo alkaline degradation at fuel cell operating conditions1,2. The existing ex situ stability testing methods in the literature do not allow for a reliable measurement of operando AEM stability (do not mimic the proper environment in an operating AEMFC)3–5. A common ex situ stability test is the long-term submersion of AEM in aqueous KOH solutions, but different groups use different conditions and characterization methods, making it difficult to compare the stability values reported in the literature. This KOH solution soaking method leads to inaccurate results because: (a) Different AEMs absorb liquid KOH electrolyte differently; (b) Carbonation can occur during the various experimental steps if CO2 not rigorously excluded; (c) Excess K+ and OH- co- and counter-ions are present unlike in an operating AEMFC; and mainly, (d) The AEMs are being degraded under maximum hydration conditions, which is not comparable to operando conditions. All of these give a false picture of degradation rates4.This presentation reports a unique ex situ method for determining the stability of AEMs under conditions that mimic the fuel cell operating environment. We apply our technique to determine the alkaline stability of AEMs with different cationic functional groups and polymer backbone configurations (Figure 1). It provides a new ex situ standard to measure AEM stabilities that gives a reliable picture of in situ alkaline stabilities (a meaningful simulation of the high pH, partially hydrated environment found operating AEMFCs).(1) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; Xu, T.; Zhuang, L. Anion-Exchange Membranes in Electrochemical Energy Systems. Energy Environ. Sci. 2014, 7 (10), 3135–3191. https://doi.org/10.1039/c4ee01303d.(2) Dekel, D. R. Review of Cell Performance in Anion Exchange Membrane Fuel Cells. J. Power Sources 2018, 375, 158–169. https://doi.org/10.1016/j.jpowsour.2017.07.117.(3) Willdorf-Cohen, S.; Mondal, A. N.; Dekel, D. R.; Diesendruck, C. E. Chemical Stability of Poly(Phenylene Oxide)-Based Ionomers in an Anion Exchange-Membrane Fuel Cell Environment. J. Mater. Chem. A 2018, 6 (44), 22234–22239. https://doi.org/10.1039/C8TA05785K.(4) Müller, J.; Zhegur, A.; Krewer, U.; Varcoe, J. R.; Dekel, D. R. Practical Ex-Situ Technique to Measure the Chemical Stability of Anion-Exchange Membranes under Conditions Simulating the Fuel Cell Environment. ACS Mater. Lett. 2020, 2 (2), 168–173. https://doi.org/10.1021/acsmaterialslett.9b00418.(5) Diesendruck, C. E.; Dekel, D. R. Water – A Key Parameter in the Stability of Anion Exchange Membrane Fuel Cells. Curr. Opin. Electrochem. 2018, 9, 173–178. https://doi.org/10.1016/j.coelec.2018.03.019. Figure 1