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

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).