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.