Polyphosphazene-Based Proton-Exchange Membranes for Direct Liquid Methanol Fuel Cells

摘要

Proton-exchange membranes for direct methanol fuel cells were prepared from sulfonated poly bis(3-methylphenoxy)phosphazene) and sulfonated poly bis(phenoxy)phosphazene). The methylphenoxy polymer was blended with Kynar Flex and solution cast into thin films, whereas the bisphenoxy material was blended with polybenzimidazole (for acid-base complexation crosslinking) prior to membrane casting. Some of the films containing poly bis(3- methylphenoxy)phosphazene) were UV-crosslinked for added control of swelling and methanol permeability. For most membranes, the proton conductivity was sufficiently high for direct methanol fuel cell (DMFC) applications but the methanol permeability was significantly lower than that in DuPont's Nafion. Membranes composed of sulfonated poly bis(phenoxy)phosphazene) (SPOP) and polybenzimidazole (PBI) worked particularly well in a DMFC (at 60 degrees C 1.0 M methanol, and ambient pressure air). Membrane performance in a DMFC was dependent on the blend composition (ion-exchange capacity of SPOP and wt% of added PBI). For an 82 m thick membrane composed of 1.2 mmol/g IEC SPOP with 3 wt% PBI, the maximum power density was 89 mW/cm2 (versus 96 mW/cm2 with Nafion 117), while the methanol crossover was 2.6 times lower than that with Nafion 117.