Ryszard Wycisk

Sulfonated polyphosphazene ion-exchange membranes

Abstract Four phosphazene polymers: poly[(3-methylphenoxy)(phenoxy)phosphazene], poly[(4-methylphenoxy)(phenoxy)phosphazene], poly[(3-ethylphenoxy)(phenoxy)phosphazene] and poly[(4-ethylphenoxy)(phenoxy)phosphazene] were sulfonated in solution with SO3 and cast into membranes from N,N-dimethylacetamide or 1-methyl-2-pyrrolidinone solvents at a temperature of 80°C. Methylphenoxy polymers were resistant to degradation and the sulfonation degree was easily controlled. The ethylphenoxy polymers underwent severe degradation during sulfonation and were unusable as membranes. Depending on the molar ratio of SO3 to the polymer mer, water insoluble membranes from the poly[(methylphenoxy)(phenoxy)phosphazenes] had an ion-exchange capacity ranging from near 0 to 2.3 mmol/g, an ac impedance in 0.1 N NaCl between 48 kohm m and 0.04 ohm m, and swelling in water (SO3H-form) from 0.1 to 0.9 g/g. Poly[(3-methylphenoxy)(phenoxy)phosphazene] was found to be the best starting material, in terms of the ease in controlling the degree of sulfonation and the highest polymer ion-exchange capacity for a water insoluble membrane.

Blended Polyphosphazene/Polyacrylonitrile Membranes for Direct Methanol Fuel Cells

Direct liquid methanol fuel cell tests were performed with membrane electrode assemblies (MEAs) fabricated with polyphosphazene-based proton-exchange membranes. The membranes were prepared from sulfonated poly[bis(3-methylphenoxy)phosphazene] that was blended with polyacrylonitrile and then UV cross-linked using benzophenone as the photoinitiator. MEAs worked best when a 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/cm 2 ), but the methanol crossover was three times lower than that of Nafion. With a three-membrane composite MEA (a methanol-blocking film sandwiched between two high conductivity membranes), there was a significant decrease in crossover (ten times lower than that of Nafion 117) with a modest decrease in current-voltage behavior.

Modified Nafion as the Membrane Material for Direct Methanol Fuel Cells

There have been numerous studies on modifying DuPonts Nafion (a perfluorosulfonic acid polymer) in order to improve the performance of this membrane material in a direct methanol fuel cell. Modifications focused on making Nafion a better methanol barrier, without sacrificing proton conductivity, so that methanol crossover during fuel cell operation is minimized. In this chapter, a brief literature survey of such modifications is presented, along with recent experimental results (membrane properties and fuel cell performance curves) for: (1) thick Nafion films, (2) Nafion blended with Teflon-FEP or Teflon-PFA, and (3) Nafion doped with polybenzimidazole.