Side Chain Piperidinium Functionalized AEMs with an Ethylene Oxide Spacer for Improving Ion Conductivity and Alkaline Stability

ABSTRACT: In this work, grafting alkaline stable piperidinium cations via ethylene oxide (EO) spacers onto an aryl ether-free poly(oxindole terphenylene) backbone was adopted as a strategy for designing self-aggregating side chain AEMs with optimized alkaline stability. Aryl ether-free poly(oxindole terphenylene) backbones were synthesized via superacid-catalyzed step-growth polycondensation and were subsequently functionalized with either piperidinium containing hydrophilic, dipolar EO or hydrophobic alkyl spacer, aiming to explore the effect of side chain-engineering on conductivity and alkaline stability of the resulting AEMs. The AEM membrane containing dipolar ethylene oxide spacer, despite its lower ion exchange capacity (IEC), exhibited a more pronounced microphase separated morphology as evidenced by TEM, and higher ionic conductivity (reaching up to 30.5 mS cm⁻¹ at 80 °C) compared to the hydrophobic alkyl spacer-containing AEM membrane. This was attributed to its higher water uptake stemming from the EO hydrophilic nature and the formation of interconnected ion-conducting channels due to piperidinium–EO interactions. Additionally, the hydrophilic nature of the ethylene oxide groups endowed the membrane with enhanced alkaline stability, preserving its mechanical integrity and retaining 71.5% of its initial conductivity after 3 weeks of immersion in 2 M KOH at 80 °C. In contrast, the AEM with an alkyl spacer experienced severe degradation under the same conditions. These results suggest that incorporating flexible alkoxy-containing spacers onto an aryl ether-free backbone is a promising and simple route for fabricating mechanically and chemically robust AEMs with sufficient conductivity.