Sulfonated polyphosphazene-based membranes for use in direct methanol fuel cells

Novel crosslinked and sulfonated poly[bis(3-methylphenoxy)phosphazene] blended proton exchange membranes were fabricated for use as the solid polymer electrolyte in a direct methanol fuel cell. Three polymers, polybenzimidazole, polyacrylonitrile and polyvinylidene fluoride-co-polyhexafluoropropylene were found to be compatible for blending with sulfonated polyphosphazene. A combination of blending and crosslinking was shown to be an effective method of producing durable, low water swelling films with acceptable proton conductivity.; A novel tracer-diffusion 1H NMR method was developed and used to measure the mutual diffusion of methanol in non-crosslinked and crosslinked membranes composed of sulfonated polyphosphazene. The technique measures the growth of a solute NMR signal in the bulk (external) solution as it diffuses out of a thin film membrane. The transient increase in methanol peak height during analyte (methanol) desorption was fitted to a simple theoretical diffusion model using the methanol diffusion coefficient as an adjustable parameter. This method was found to be fast, reproducible, and accurate to within about ±20%. Diffusion coefficients at 25°C were in the range of 1.0 × 10−8 cm2/s to 4.0 × 10−7 cm2/s for methanol concentrations of 1.0–5.0 M and were significantly smaller than those reported for a Nafion® perfluorosulfonic acid membrane.; Direct liquid methanol fuel cell tests were performed with membrane electrode assemblies (MEAs) fabricated with polyphosphazene-based proton-exchange membranes. MEAs worked best when high ion-exchange capacity (high conductivity) polyphosphazene membrane contacted the electrodes, in which case the fuel cell power output was nearly the same as that with Nafion 117 (for current densities ≤0.15 A/cm2), but the methanol crossover was three times lower than that of Nafion. The electrochemical performance of single-membrane MEAs with low conductivity S-POP/PAN films was poor, although the methanol crossover was low.; A multi-layered membrane MEA was proposed and developed in order to create a fuel cell membrane with low membrane/electrode interfacial resistances and low rates of methanol crossover. With a three-membrane, composite WA (a low methanol permeability S-POP/PAN film sandwiched between two high conductivity membranes), there was a significant decrease in methanol crossover (the flux was about ten times lower than that of Nafion 117) with reasonably good current-voltage behavior.