(92a) Session Keynote - Practical Membranes Based on Heteropoly Acds for Hotter and Drier Automotive Fuel Cells
The proton exchange membrane (PEM) fuel cell is currently being commercialized for automotive applications, but there is a still a need for PEMs that operate at higher and drier operating conditions and for cathode electrode solution that use less platinum for the oxygen reduction reaction. The heteropoly acids (HPAs) are a subset of the polyoxometalates that have very high acidity in the solid state and have interesting interactions with colloidal platinum group metals. We have demonstrated that very high proton conductivity can be achieved in a model system in which we co-polymerized a divinyl substituted lacunary HPA with butyl acrylate and a small amount of hexanedioldiacrylate (for additional cross linking). While this chemistry had been demonstrated before we were the first group to fabricate freestanding films in the acidic proton conducting form. Proton conductivities >0.1 S cm-1 at <90°C and an RH of 50% where demonstrated with this system. Unfortunately, acrylate chemistry is not applicable to practical fuel cells, as the ester linkage in the polymer backbone is not stable to hydrolysis and the methylene units in the polymer are subject to oxidative degradation. We are now developing fuel cell ready hybrid HPA membranes based on perflourinated backbone polymer chemistry. We use dehydrofluorination of a commercial perfluorinated polymer to functionalize the material to allow binding of a lacunary HPA. The emphasis in this project is on film processing to achieve films with nano-scale phase separation and thicknesses of <20 Î¼m. When reproducible film chemistries and processing have been achieved, proton conduction mechanisms are elucidated by pulsed field gradient NMR with EIS and explained based on film morphology elucidated through SAXS and microscopy. The results from these fundamental transport studies help to support simulation studies using Multistate-empirical valance bond theory to explain the exact nature of the proton transport mechanism. One intriguing aspect of the work is that we can use HPA functionalized carbons in the catalysts layer, which should lower the resistance of the membrane electrode interface, while stabilizing the Pt catalysts particles. Recent materials have high enough performance that we are incorporating them into membrane electrode assemblies for single cell fuel cell studies.