High-performance anion exchange membranes based on poly(oxindole benzofuran dibenzo-18-crown-6)s functionalized with hydroxyl and quaternary ammonium groups for alkaline water electrolysis.

Anion exchange membranes (AEMs) play a vital role in AEM water electrolysis (AEMWE), a promising technology for hydrogen production. However, developing AEMs that simultaneously achieve high OH conductivity, robust alkaline stability and sustained operational performance in AEMWE remains challenging. In the present study, we report on a class of high-performance, readily synthesized AEMs based on poly(oxindole benzofuran dibenzo-18-crown-6)s functionalized with hydroxyl and quaternary ammonium groups. A super-acid-catalyzed Friedel-Crafts reaction was used to copolymerize dibenzofuran (DBF), dibenzo-18-crown-6 (DBC) and isatin to yield a series of poly(oxindole benzofuran dibenzo-18-crown-6)s (P(O-F-C)s). The hydrophilic and π-conjugated DBF units provide the membranes with additional free volume, while the bulky DBC units promote a favorable microphase morphology and impart resistance to hydroxide attack. Quaternization is achieved via a ring-opening reaction with glycidyl trimethyl ammonium chloride (GTA) without requiring a catalyst. The resulting side chains, featuring alkyltrimethylammonium cations with hydroxyl groups in the β-position, introduce hydrogen-bonding networks and enhance OH conductivity. The optimized P(O-F-C)-GTA membrane exhibits a well-developed microphase structure, achieving a Cl conductivity of up to 103 mS cm at 80 °C. Moreover, the presence of DBC groups mitigates the degradation of the hydroxyl-containing side chains, enabling the membrane to retain 81 % of its original conductivity after 600 h of alkaline stability testing in 1 M KOH at 80 °C. In a PGM-free AEMWE, the P(O-F-C)-GTA membrane attains a current density of 3.8 A cm at 2 V and 80 °C. These findings shows the potential of incorporating DBF and DBC moieties in the polymer main chain, along with the GTA-functionalized side chains, offering a promising pathway for advancing AEMs in AEMWE applications.