Polymer Electrolyte Membranes Based on Poly(phenylene ether)s with Pendant Perfluoroalkyl Sulfonic Acids

Poly(phenylene ether)s containing pendant perfluoroalkyl sulfonic acids with ion exchange capacity (IEC) of 1.17−1.83 mequiv/g were synthesized by the aromatic nucleophilic substitution reaction of a perfluoromonomer (decafluorobiphenyl or hexafluorobenzene) with 2,5-bis(4′-iodophenyl)hydroquinone, followed by the Ullman coupling reaction with potassium 1,1,2,2-tetrafluoro-2-(1,1,2,2-tetrafluoro-2-iodoethoxy)ethanesulfonate. The membranes prepared by solution casting showed high oxidative stability against hot Fenton’s reagent. Even in the hydrated state, water uptake of the membranes was significantly low (19.3−28.0 wt %) due to the high fluorine content. High proton conductivity (∼1.0 × 10 −1 S/cm) was observed in all membranes under high relative humidity (95% RH). Furthermore, the membrane with IEC = 1.83 mequiv/g still maintained high proton conductivity of 3.3 × 10 −3 S/cm even under low relative humidity (30% RH), which was comparable to that of the Nafion 117 membrane (4.1 × 10 −3 S/cm). This proton conductivity was the highest data among the aromatic polymers containing pendant perfluoroalkyl sulfonic acids. Atomic force microscopy observation supported the formation of the phase-separated structure that the hydrophilic domains were well dispersed and connected to each other.