Aryl-ether-free polyphenylene-based anion exchange membranes incorporating N-cyclic quaternary ammoniums for enhanced alkaline fuel cell performance

Selecting appropriate polymer backbones is crucial for the advancement of anion exchange membranes (AEMs) that exhibit both high ionic conductivity and robust chemical stability. In this study, we synthesized rigid and chemically inert aryl-ether-free polyphenylene-based polymer backbones containing alkaline-resistant N-cyclic cations, i.e. dimethyl piperidinium (DMP) or 6-azonia-spiro[5.5]undecane (ASU), as high-performance AEMs. The procedure encompassed several key steps: superacid-catalyzed Friedel-Crafts polycondensation, bromomethylation, azidation, and Cu(I)-mediated azide-alkyne cycloaddition. The synthesized BP-ASU(DMP)-x copolymers demonstrated excellent film-forming capabilities to afford robust AEMs as compared to polyolefin-based counterparts. Owing to the significant incompatibility between the hydrophobic backbones and the hydrophilic cations, the BP-ASU(DMP)-x membranes exhibited a well-defined microphase-separated morphology, resulting in exceptionally high hydroxide conductivity, up to 157.2 mS cm⁻¹ at 80 °C in water. More importantly, BP-ASU-150 membrane exhibited no chemical degradation and a conductivity retention of >97 % when immersed in 1 M NaOH solution at 80 °C for 4000 h. Compared to the poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and polyolefin-based counterparts, the achieved outstanding alkaline stability is related to aryl-ether-free polyphenylene and alkali-stable N-cyclic cations, highlighting the importance of polymer backbones for AEM materials. Finally, all the polyphenylene-based AEMs with incorporated N-cyclic cations were assembled in single H₂/O₂ AEMFCs, with the BP-ASU-150 membrane demonstrating a peak power density (PPD) of 860.4 mW cm⁻² at a current density of 1363 mA cm⁻². We concluded that the BP-ASU(DMP) AEMs demonstrated superior performance across various metrics due to exceptional stability and robustness of polyphenylene polymers, highlighting their potential as backbones for high-performance AEM materials.