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-acidcatalyzed 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-Fx-Cy)s). The hydrophilic and it-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 beta-position, introduce hydrogen-bonding networks and enhance OH- conductivity. The optimized P(O-F50%-C50%)-GTA membrane exhibits a well-developed microphase structure, achieving a Cl- conductivity of up to 103 mS cm- 2 at 80 degrees 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 degrees C. In a PGM-free AEMWE, the P(O-F50%-C50%)-GTA membrane attains a current density of 3.8 A cm- 2 at 2 V and 80 degrees 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.