Molecular Assembly of Alkylated Fused Expanded Pyridinium for a Highly Conductive Anion-Exchange Membrane

Alkylated fused expanded pyridinium (C x FEP) with alkyl chain lengths ( x ) of 0, 6, 12, and 18 was newly synthesized and introduced into polymers as side chains for anion-exchange membrane (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. C x FEP shows a 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 C x FEP was introduced into the polymers, C 12 FEP formed an ordered stacking assembly in the films, whereas the other polymers with shorter (C 0 and C 6 ) and longer (C 18 ) alkyl chains exhibited no such ordered structure. For the AEM, all of 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 level of ion dissociation of FEP, the molecular assembly of FEP provides ionic conduction paths. This study demonstrates the importance of high ion dissociation and the formation of a conduction pathway for AEM with high conductivity.