Perfluorocyclobutyl (PFCB)-based polymer blends for proton exchange membrane fuel cells (PEMFCs)

A phase-separated morphology, as observed in block copolymers, for proton exchange materials (PEM) is expected to result in improved PEM fuel cell performance. Blending hydrophilic and hydrophobic polymers provides an easier route to this phase-separated morphology especially for an otherwise difficult block copolymer synthesis. Blends of hydrophilic and hydrophobic perfluorocyclobutyl (PFCB) polymers were prepared and characterized as PEMFC membranes. The [2p+2p] cyclodimerization of 4,4'-bis(trifluorovinyloxy)biphenyl and 4,4'-sulfonyl-bis(trifluorovinyloxy)biphenyl monomers afforded the hydrophobic homopolymers biphenyl perfluorocyclobutyl (BP-PFCB) and sulfonyl-bridged biphenyl perfluorocyclobutyl (SO2-PFCB), respectively. The hydrophilic homopolymer was prepared by post-sulfonation of the BP-PFCB using chlorosulfonic acid and thionyl chloride, yielding sulfonated biphenyl (sBP-PFCB) polymer. Blends of sBP-PFCB with SO2-PFCB and BP-PFCB were prepared in 1:1 mol ratios, resulting in higher ion exchange capacity (IEC) values and reduced membrane swelling by water when compared to Nafion (R). The pure hydrophilic sBP-PFCB polymer showed a 2.40 mmol/g IEC, while its 1:1 mol ratio blends with BP-PFCB and SO2-PFCB, gave IECs of 1.36 and 1.37 mmol/g, respectively, which are over 1.5x that of Nafion (R)'s. Blending these polymers combined the mechanically and thermally stable PFCB backbone with the highly acidic sulfonated PFCB and displayed nanophase-separated morphologies without the challenge of controlling polydispersities. Proton conductivity of 6 and 8x10-2 S/cm for the sBP-PFCB blend with BP-PFCB and SO2-PFCB respectively, was achieved at 80 degrees C and 100% relative humidity. Testing a 1:1 mol ratio of sBP- and SO2-PFCB blend at similar to 100 degrees C and 50% relative humidity showed a higher proton conductivity of 1.5x10-1 S/cm. These results confirm that the blends of hydrophilic (sulfonated) and hydrophobic PFCB polymers can be promising materials for fuel cell membranes.