Anion Exchange Membrane Using Fused Expanded Pyridinium As Their Cationic Unit

Abstract

Ion conductivity is expressed as a function of the Faraday constant, ion concentration, and mobility. Thus, an increase in both ion concentration and mobility is crucial for increasing the ion conductivity. Ion mobility reflects ionic motion in the restricted area of ion conduction channels. The general approach to expand ion conduction channels in AEM is to introduce a large number of ionic groups, i.e., to increase ion-exchange capacity (IEC). Because the increase of IEC also increase ion concentration, increase of IEC have been major strategy for increasing ion conductivity. However, this approach causes softening of the AEM because high IEC involves large amount of water uptake (WU).On the other hand, the ion concentration in AEM is also affected by the dissociation constant of the ionic groups. However, conventional ionic groups in AEMs, such as ammonium cations, suffer from a low dissociation constant that leads to low ion conduction in AEM. Previously, we found that a fused expanded pyridinium cation (FEP) having a delocalized positive charge showed high dissociation of OH– owing to the weak interaction between the soft acid (FEP) and the hard base (OH–) which can be explained based on the Pearson Hard Soft Acid Base (HSAB) principle. As the result, AEM with FEP unit exhibited excellent ion conductivity of 123.4 ±13.3 mS cm–1 at 80 °C with very low IEC (0.77 mmol g-1).Recently, we synthesized alkylated fused expanded pyridinium (CxFEP) with alkyl chain lengths (x) of 0, 6, 12, and 18 and introduced into polymers as side chains for AEM applications. FEP is characterized by a large π cation that offers weak electrostatic interactions with anions such as hydroxide (OH–) ions and therefore leads to high OH– conductivity in AEM. CxFEP shows synergetic effect between the π-π interactions of FEP and van der Waals interactions of the alkyl chains and forms a strong assembly, particularly for C6FEP. On the other hand, when CxFEP was introduced into the polymers, C12FEP formed an ordered stacking assembly in the films, whereas the other polymers with shorter (C0 and C6) and longer (C18) alkyl chains exhibited no such ordered structure. For the AEM, all the polymers showed higher ion conductivities than that of commercial ammonium-based AEM, even with their low ion exchange capacities (IEC), probably because of their characteristic weak interactions between the FEP cation and OH–. In particular, the AEM with C12FEP showed the highest ion conductivity of 143.3 ± 27.3 mS cm-1 even with a very low IEC of 0.41 mmol g-1. In addition to the high ion dissociation of FEP, the molecular assembly of FEP provides ionic conduction paths. This study demonstrates the importance of high ion dissociation and formation of a conduction pathway for AEM with high conductivity.