Poly(Styrene-b-Isobutylene-b-Styrene) (SIBS)-Based Comb-Shaped Anion Exchange Membranes for Alkaline Fuel Cell with Three-Dimensional Ordered Phase Separation and Enhanced Conductivity

Abstract

Polymers backbones with free of aryl-ether structures are preferred for producing stable anion exchange membranes (AEMs) suitable for alkaline fuel cells. In this study, we utilized the inert all-hydrocarbon polymer poly(styrene-b-isobutylene-b-styrene) (SIBS) as the polymer backbone and integrated tertiary amines with varying carbon chain lengths to synthesize alkaline stable comb-shaped AEMs via halogenation and the Menschutkin reaction. The synthesized QSIBS–OH-C_n membranes demonstrated remarkable film-forming capabilities and mechanical properties, and SAXS analysis revealed the presence of distinct hydrophilic and hydrophobic microphase separation structures, which promote the self-assembly of ion clusters, resulting in the formation of interconnected ion transport pathways within the membrane. Therefore, the QSIBS–OH-C_n membranes demonstrated a significant enhancement in hydroxide conductivity, reaching up to 104 mS cm^–1 at 80 °C, a marked improvement over their poly(phenylene oxide)-based equivalents. Furthermore, the QSIBS–OH-C_n membranes exhibited remarkable alkaline stability, maintaining over 92% of their conductivity after 1800 h at 80 °C in a 1 M NaOH solution, underscoring the significance of the polymer backbone and the com-shaped molecular architecture. Finally, the QSISBS–OH-C_n and QPPO–OH-C_n membranes were utilized in single alkaline fuel cells operating with H₂/O₂ at 60 °C, where the QSIBS–OH–C₁₂ membrane demonstrated a peak power density of 537 mW cm^–2 at a current density of 670 mA cm^–2. Moreover, the QSIBS–OH–C₆ and QSIBS–OH–C₁₂ membranes displayed their stability across the durability tests of fuel cell for over 120 h with 0.3 V constant voltage. Overall, this study emphasizes the significance of the SIBS thermoplastic triblock polymer as a backbone and the integration of comb-shaped molecular architectures in developing robust AEMs, offering a strategic method for optimizing the molecular design of AEMs.