Effect of Radical‐Mediated Cross‐Linking on Partially Fluorinated Aromatic Anion Exchange Membranes and their Applications in Alkaline Water Electrolysis Cells

Abstract

To investigate the effect of cross‐linking on partially fluorinated anion exchange membranes tethered with trimethylpropyl side chains (QPAF‐C3), styrene‐based cross‐linker is introduced into the precursor copolymers and then cross‐linked via free radical reaction. The one‐pot cross‐linking and quaternization reactions are successful as confirmed through nuclear magnetic resonance spectra. By solution casting, the resulting polymers provide flexible membranes (xQPAF‐C3‐VB) with 9.1–36.0% degree of cross‐linking. The cross‐linking results in smaller hydrophilic/hydrophobic phase‐separated morphology as confirmed by transmission electron microscopy images. The cross‐linking effect on the membrane properties is observed in the suppressed water uptake and decreased hydroxide ion conductivity. Among the cross‐linked membranes, xQPAF‐C3‐VB membranes with 17.4% degree of cross‐linking and 1.16 meq g −1 of ion exchange capacity exhibit the highest hydroxide ion conductivity (56 mS cm −1 at 30 °C) that is comparable to that of the pristine membrane (54 mS cm −1 ). The cross‐linking contributes to improving the thermomechanical properties with higher glass transition temperature. The cross‐linked xQPAF‐C3‐VB is applied to alkaline water electrolyzer to achieve high efficiency (74%) and reasonable performance (1.67 V at 1.0 A cm −2 ).