Sulfonated and crosslinked polyphosphazene-based proton-exchange membranes

Proton-exchange membranes, for possible use in H-2/O-2 and direct methanol fuel cells have been fabricated from poly[bis(3-methylphenoxy)phosphazene] by first sulfonating the base polymer with SO3 and then solution-casting th in films. The ion-exchange capacity of the membrane was 1.4 mmol/g. Polymer crosslinking was carried out by dissolving benzophenone photoinitiator in the membrane casting solution and then exposing the resulting films after s olvent evaporation to UV light. The crosslinked membranes look particularly promising for possible proton exchange membrane (PEM) fuel cell applicatio ns. A sulfonated and crosslinked polyphosphazene membrane swelled less than Nafion 117 in both water and methanol. Proton conductivities in crosslinke d and non-crosslinked 200 mu m thick water-equilibrated polyphosphazene fil ms at temperatures between 25 degrees C and 65 degrees C were essentially t he same and only 30% lower than those for Nafion 117. Additionally, water a nd methanol diffusivities in the crosslinked polyphosphazene membrane were very low (less than or equal to 1.2 x 10(-7) cm(2)/s). Sulfonated/crosslink ed polyphosphazene films showed no signs of mechanical failure (softening) up to 173 degrees C and a pressure of 800 kPa and did not degrade chemicall y when soaked in a hot hydrogen peroxide/ferrous ion solution. (C) 1999 Els evier Science B.V. All rights reserved.