Simultaneous Postfunctionalization and Cross-Linking of Epoxidized Polystyrene-b-polybutadiene-b-polystyrene for Anion Exchange Membrane

Abstract

Anion exchange membrane (AEM) is a key material component of many electrochemical devices, such as fuel cells and electrolyzers. However, its relatively low hydroxide conductivity and insufficient long-term chemical and mechanical stabilities have been major barriers to the wider adoption of AEM-based technologies. To address those issues, various synthetic approaches have been explored with the aim of increasing ion exchange capacity and introducing cross-linkers within the membrane. These approaches, however, typically require multiple synthetic steps to achieve the overall material properties with tailorable functionality. Herein, a facile synthetic method to simultaneously incorporate bromoalkyl side chains and chemical cross-linkers is reported for the preparation of AEMs. Using partially epoxidized polystyrene-b-polybutadiene-b-polystyrene (SBS) as a precursor material, simultaneous polymer functionalization and cross-linking reactions were carried out via UV-initiated cationic epoxy ring-opening polymerization. The extent of functionalization/cross-linking was controlled by tailoring the molar ratio of the epoxy group between epoxidized SBS and 2-(6-bromohexyl)oxirane additive. In contrast to the typical approach to cross-linked AEM, our in situ cross-linked AEMs offer the advantage of molecular design to locate quaternary ammonium cationic groups at the end of flexible alkyl spacers. This AEM material design is likely to result in favorable chain configurations for better ionic aggregation and ion transport facilitated by well-connected ionic conducting channels. The resulting cross-linked SBS AEM exhibits high hydroxide conductivity (>0.1 S/cm at 80 °C) and good mechanical strength and toughness, retaining good alkaline stability (no degradation after immersing in 1 M NaOH solution at 80 °C for 500 h).