Fluorinated Aromatic Polyether Ionomers Containing Perfluorocyclobutyl as Cross-Link Groups for Fuel Cell Applications

The cross-linkable copolymers (SHQ<i>x</i>-TFV<i>y</i>s) with varying degrees of sulfonation (DS) from 70 to 95% were prepared from potassium-2,5-dihydroxybenzenesulfonate (SHQ), decafluorobiphenyl (DFBP), and 4-(trifluorovinyloxy)-biphenyl-2,5-diol (TFVOH) as a cross-linkable moiety. To develop a highly stable polymer electrolyte membrane (PEM) for application in polymer electrolyte fuel cells (PEFC)­s, cross-linked membranes were prepared by chemical cross-linking. The cross-linked membranes were synthesized by varying the amount of TFVOH (5–30 mol %) in order to achieve desirable PEM properties. The structures of the cross-linkable monomer and polymers were investigated by ¹H and ¹⁹F NMR and FT-IR spectra. The cross-linked membranes exhibited good glass transition temperature and thermal stability up to 239–271 °C and 290–312 °C, respectively. The crosslinked membranes (DS range 80–95%) exhibited higher proton conductivity (0.098–0.151 S/cm) than Nafion 212 (0.092 S/cm). Moreover, all membranes possessed lower methanol permeability (13–132 × 10^–8 cm²/s) compared with Nafion 212 (163 × 10^–8 cm²/s) under the same measurement conditions. The H₂/O₂ single cell performance tests of the cross-linked membranes and Nafion 212 were performed. The CSHQ90-TFV10 exhibited the higher maximum power density (1.053 W/cm²) than that of Nafion 212 (0.844 W/cm²)