Tailoring Ion Transport and Dimensional Stability in Poly(aryl piperidinium) Anion Exchange Membranes via Molecular Branching

In this study, branched poly(aryl piperidinium)-based anion exchange membranes (AEMs) were developed to investigate the effect of branching architecture on ion transport, dimensional stability, and alkaline durability. A linear pristine polymer and branched analogues containing 3 wt% and 5 wt% branching units were synthesized via superacid-catalyzed polycondensation and subsequently quaternized to obtain the corresponding cationic membranes. The chemical structures were confirmed by ¹H NMR and FT-IR spectroscopy, verifying the successful formation of the target polymers. The experimentally determined ion exchange capacities of the pristine, 3 wt%, and 5 wt% membranes were 2.62, 2.89, and 2.99 meq g^-1, respectively.

The incorporation of branching units significantly affected hydration behavior and membrane properties. The hydroxide conductivity increased from 95.8 mS cm^-1 for the pristine membrane to 121.1 and 144.6 mS cm^-1 for the 3 wt% and 5 wt% membranes, respectively, at 90 °C. Furthermore, the 5 wt% membrane exhibited a lower swelling ratio of 15.9% compared with 29.5% for the pristine membrane, indicating improved dimensional stability despite the higher conductivity.

The branched membranes also showed enhanced alkaline durability. After immersion in 3 M KOH at 80 °C for 200 h, the 5 wt% membrane retained 95.6% of its initial conductivity, compared with 90.1% for the pristine membrane. The optimized membrane further demonstrated promising AEMFC performance, achieving a peak power density of 352 mW cm^-2 with stable operation. These results highlight molecular branching as an effective strategy for improving ion conductivity, dimensional stability, and alkaline durability, providing useful design guidelines for high-performance AEMs.