Anion exchange membranes (AEMs) are promising materials for many electrochemical energy conversion and storage systems such as fuel cells, electrolysers, redox flow batteries, reverse electrolysers, just to name a few. We have observed that membrane-processing techniques affect mechanical and chemical properties of an anion exchange membrane, significantly.

Literatures have shown highly conductive AEMs ¹ at fuel cell operating conditions, however, a more study is still needed to find a balance between ionic conductivity and mechanical strength of AEMs for practical applications. We used poly(vinyl benzyl chloride)(PPO-b-PVBC) polymer with IEC of 2.6 mmol/g, synthesis published else where¹, to understand the effect of polymer processing techniques in ionic conductivity and mechanical properties. Elemental analysis of the membrane after melt pressing for 5, 10 and 20 min of solvent cast membranes showed the decrease in chloride in the membrane by ~55, 44 and 44%; respectively from it’s initial 14.5 %w/w of chlorine in the polymer. The reduced chlorine but oxygen in the membrane indicated the presence of crosslinking in hydrophilic group of the polymer. Ionic conductivity, diffusion of ions and water, and dynamic vapor water sorption experiments were performed to understand ionic transport, tensile stretching of membrane were performed at fuel cell operating conditions to understand membrane strength, stiffness and elongation, and small angle x-ray scattering experiments were performed to understand the membrane morphology.

Membranes with higher degree of cross-linking by melt pressed for 40 min, were found to be mechanically robust, but chemically less conductive. The membrane melt pressed for just 10 min, was highly conductive with slightly lower membrane strength. A different degree of cross-linking showed a difference in water uptake (Figure 1). A greater water uptake by membrane had bigger effect of water plasticizing the membrane resulting in poor mechanical performance. A good balance of degree of cross-linking in the membrane to optimize the ionic conductivity, and mechanical properties will be discussed in the presentation.

Figure 1. Vapor water uptake by the membrane at 60°C as a function of humidity. SCMP5, SCMP10, and SCMP20 (solvent cast and then melt pressed for 5, 10 and 20 min; respectively in addition to waiting same amount of time for melting the polymer before the application of pressure)

Acknowledgement

The authors would like to thank the US Army Research Office for the funding under the MURI #W911NF-10-1-0520and Yating Yang and Daniel M. Knauss for providing the polymer for the study.

References

1.              (a) Pandey, T. P.; Maes, A. M.; Sarode, H. N.; Peters, B. D.; Lavina, S.; Vezzu, K.; Yang, Y.; Poynton, S. D.; Varcoe, J. R.; Seifert, S.; Liberatore, M. W.; Di Noto, V.; Herring, A. M., Interplay between water uptake, ion interactions, and conductivity in an e-beam grafted poly(ethylene-co-tetrafluoroethylene) anion exchange membrane. Physical Chemistry Chemical Physics 2015; (b) 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.