Molecular Design of Ether-Free Anion Exchange Membranes: Effects on Ion Conductivity and Alkaline Stability
E4. Materials for Energy Harvesting and ConversionJoel S. Olsson1, Patric Jannasch1
1 Lund University, Department of Chemistry
Introduction/Purpose: The development of durable and high-performance anion exchange membranes (AEMs) that allow production of inexpensive and efficient alkaline fuel cells and electrolyzers is currently attracting attention. These devices will work in tandem with environmentally friendly intermittent energy sources, such as solar and wind. Today, achieving both a high OH− conductivity and a sufficient stability at elevated temperature under alkaline conditions is the main challenge within the field of AEM research.
Methods: In a fundamental study on the alkaline stability of model compounds, alicyclic quaternary ammonium (QA) cations were found to be exceptionally stable [1]. On the basis of these findings we have previously prepared and studied several different AEMs based on poly(arylene piperidinium)s (Scheme a) [2-4]. These membranes combined a high hydroxide conductivity with a good alkaline stability. The predominant degradation mechanism was β-elimination leading to ring-opening of the cyclic cation. This was ascribed to the influence of the rigid backbone on the ring relaxation of the alicylic QA ring, which enabled degradation via β-elimination. The current study was focused on providing further insights on how the attachment of alicyclic QA cations and the configuration (meta or para) of the backbone can improve the membrane properties of poly(arylene alkylene)s.
Results: Poly(terphenyl alkylene)s (Scheme b) were successfully prepared from polymerization of in-house synthesized monomers with terphenyl or 1,1,1-trifluoroacetophenone in superacid catalyzed polyhydroxyalkylations. Subsequent AEMs exhibited high hydroxide conductivities (>100 mS cm-1) coupled to moderate water uptakes at 80 °C. The AEMs also possessed good alkaline stabilities with <10% ionic loss after 30 days in 2 M aq. NaOH at 90 °C.
Conclusions: The results clearly indicated that the position of the cation and the configuration of backbone, of poly(terphenyl alkylene)s, had an effect on both the hydroxide conductivity and the alkaline stability.
Selected references
[1] Marino, M. G., Kreuer, K. D. ChemSusChem 2015, 8, (3), 513-523
[2] Olsson, J. S., Pham, T. H., Jannasch, P., Adv. Funct. Mater. 2018, 28, (2), 1702758
[3] Pham, T. H., Olsson, J. S., Jannasch, P., J. Mater. Chem. A, 2018, 6, 16537-16547
[4] Olsson, J. S., Pham, T. H., Jannasch, P., J. Membr. Sci. 2019, 578, 183-195
