Processing of sulfonated polysulfone for PEMFC membrane applications:: Part 2.: Polymer in salt form

The possibility of manufacturing sulfonated polysulfone (sPSU) membranes by an extrusion process is explored for low-temperature proton exchange membrane fuel cell applications. Because the acid form of sPSU is not suitable for melt-phase processing, the use of the salt form of the polymer was considered, as sulfonate groups are much less reactive than sulfonic acid groups. This form of sPSU makes it possible to use an extrusion process with the help of a suitable plasticizer, such as poly(ethyleneglycol) (PEG). The residual porosity of the film, induced by removing the soluble PEG with water, was found to improve the proton conductivity of the membrane. The membrane fabrication process has a prominent effect on the final properties of the film. For the same composition, a solvent-casting procedure can produce membranes with proton conductivity twice as high as that attainable with a melt-phase process. However, extruded membranes exhibit a greater longevity than do cast membranes. At the same time, the proton conductivity of extruded membranes is less dependent on the ion-exchange capacity of the polymer.