Imidazole-Containing Ether-Free Polyaromatics with Self-Aggregating Piperidinium Side Chains for Alkaline-Stable Anion Exchange Membranes

Abstract Despite being critical components of anion exchange membrane water electrolyzers (AEMWEs), anion exchange membranes (AEMs) still confront substantial bottlenecks in meeting operational requirements due to inadequate alkaline stability and ionic conductivity. In this work, to address these challenges, the integration of self-aggregating cationic side chains onto the backbone was adopted to promote both efficient ion transport and high alkaline stability. Highly alkaline-stable polyaromatic precursors containing isatin and imidazole moieties were prepared via acidic polyhydroxyalkylation, followed by postfunctionalization with either hydrophobic aliphatic chains or flexible hydrophilic ethylene oxide (EO)-containing piperidinium pendants to afford respective AEMs. The synthesized precursors demonstrated excellent film-forming properties due to their high molecular weights along with extraordinary alkaline stability, withstanding 4320 h in 20 wt % KOH solution at 80 °C. Upon cationic functionalization of precursor, AEM bearing dipolar EO spacer (P(OB85–4Im)-OPip) exhibited enhanced electrolyte uptake and an ionic conductivity of 84.7 mS cm–1 at 80 °C, corresponding to a 67% increase compared to its AEM analogue with an alkyl spacer (P(OB85–4Im)-Pip), despite its lower ion exchange capacity (IEC). These enhanced properties are attributed to hydrogen-bonding network formation and dipolar-cation self-assembly, which induced a more pronounced microphase-separated morphology, to facilitate efficient OH– transport, as confirmed by AFM, TEM, and SAXS. Both AEMs successfully scaled up to 20 × 20 cm2, displaying outstanding mechanical stability (tensile strength: 73–74 MPa) and sufficient alkali resistance. After aging in 1 M KOH for 1440 h, both retained their mechanical integrity and more than 65% of their initial conductivity. Finally, the improved ionic conductivity of P(OB85–4Im)-OPip was confirmed by improved performance under alkaline water electrolysis conditions, ascribed to lower area ohmic resistance. Stable operation was achieved for a 250 h test under operational conditions, regarding area ohmic resistance and hydrogen/oxygen separation properties. These results highlight the significant potential of P(OB85–4Im)-OPip membrane for alkaline water electrolysis.