Water Uptake in Novel, Water-Stable, Heteropoly Acid Films

Operating a fuel cell at high temperature and low humidity can reduce catalyst and heat and water management cost in addition to improved tolerance to fuel impurities [1]. Traditional perfluorinated sulfonic acid proton exchange membranes do not operate well under these conditions, due to a need for high hydration to facilitate proton transport. Heteropoly acids (HPA)s have demonstrated promise for high temperature, low humidity fuel cell operation, but often are not immobilized, resulting in premature fuel cell performance loss[2]. In order to overcome this, a novel, chemically and thermally robust chemistry has been explored. Phenyl phosphonic acid side chains are grafted onto a 3M polyvinyl fluoride-co-hexafluoropropylene, FC-2178. These phenyl phosphonic acid (PPA) side chains then react with lacunary silicotungstic acid, thus immobilizing the HPAs, via covalent bonds. Different levels of PPA attachment have been explored, giving different ion-exchange capacities.

³¹P and ¹H NMR have been used to determine the side chain attachment and IEC of the PPA functionalized FC-2178. The immobilized tungsten content was measured using PIXE (Proton Induced X-ray Emission) elemental analysis. Knowledge of the IEC and tungsten content elucidates the contributions to IEC from both PPA and HPA. SAXS and AFM have been used to determine the membrane morphology. Samples with a continuous conducting phase have shown proton conductivities up to 20 mS/cm at 110°C and 50% RH.

References

[1] Chandan, A.; Hattenberger, M.; El-kharouf, A.; Du, S.; Dhir, A.; Self, V.; Pollet, B. G.; Ingram, A.; Bujalski, W., High temperature (HT) polymer electrolyte membrane fuel cells (PEMFC) – A review. Journal of Power Sources 2013, 231, 264-278.

[2] Herring, A. M., Inorganic–Polymer Composite Membranes for Proton Exchange Membrane Fuel Cells. Journal of Macromolecular Science, Part C: Polymer Reviews 2006, 46, 245-296.