Highly Conductive and Mechanically Robust Isatin-Derived Anion Exchange Membranes for Water Electrolysis

Anion exchange membrane water electrolysis (AEMWE) is a promising approach for hydrogen production, but its efficiency depends on anion exchange membranes (AEMs) that combine high OH– conductivity with excellent alkaline stability and robust mechanical strength. To address these challenges, we synthesized a series of quaternized poly(5,6-difluoroisatin biphenyl) (QPDFIB) and quaternized poly(5,6-difluoroisatin biphenyl-co-dibenzothiophene) (QPDFIBD-x) AEMs by employing highly reactive 5,6-difluoroisatin (DFI) as the ketone monomer in a polyhydroxyalkylation reaction. These polymers exhibited high molecular weights (455.6–594.6 kg mol–1) before quaternization, and their fluorine-containing, ether-free polyaromatic backbones enhanced alkaline resistance, dimensional stability, and mechanical robustness. Incorporation of twisted dibenzothiophene units into the polymer backbone further promoted microphase separation and improved OH– conductivity. The resulting dry membranes displayed tensile strengths of 42.0–56.7 MPa and elongations at break of 10.5–17.7%. Notably, the QPDFIBD-40 membrane achieved the highest OH– conductivity (139.2 mS cm–1 at 80 °C) while maintaining a low swelling ratio (22.3%). It also retained 88.0% of its conductivity after 1200 h in 1 M KOH at 80 °C, indicating excellent alkaline stability. In AEMWE testing, QPDFIBD-40 delivered a current density of 2.6 A cm–2 at 1.86 V and 60 °C, a 36.8% improvement over a commercial PiperION-A80 membrane. These results demonstrate the strong potential of QPDFIBD-40 for practical water electrolysis applications.