The Role of Water and/or Morphology in Ion Transport in Novel Ionomers

Andrew M. Herring (Colorado School of Mines)

Proton exchange membrane (PEM) fuel cells are still the most desirable energy conversion device for future zero emission, high efficiency automobiles. However, their unit cost, ease of operation, and reliability must be reduced which includes eliminating the humidifier from the fuel cell system. Currently the commercial proton exchange membrane (PEM) is fabricated from a perfluorosulfonic acid (PFSA) polymer such as Nafion®. Unfortunately PFSA ionomers must be fully hydrated to achieve practical levels of proton conductivity which can only be achieved in vehicles operating at an inlet RH of 80% which still necessitates the use of a humidifier and undesirable complex water management and recovery. To achieve the goal of a PEM that can operate at temperatures from freezing to 120ºC using dry inlet gases it will be necessary to develop new PEMs that are based on new chemistries or dramatically improved morphologies of existing chemistries. Additionally it is necessary to have precious metal catalysts in acid to catalyze fuel cell reactions and so it would be desirable to develop alkaline exchange membrane (AEM) fuel cells which in principle could utilize less expensive metals as the catalysts.

We are developing a wide variety of ionomer materials for energy conversion applications, the properties of which depend strongly on their morphology and water content. Because of this we have made extensive use of SAXS using the synchrotron radiation of beamline 12 C/D or B at the APS. Here we have been able to not only perform environmental measurements at conditions relevant to fuel cell operation, but we have been able to follow the dynamics of morphological changes induced by humidity or temperature changes. In this talk I will show some of this data for Perfluorosulfonic acid materials, their derivatives and composites produced in collaboration with 3M. I will also show data for novel hybrid inorganic polymeric materials based on heteropoly acids or zirconium phosphonate where the SAXS is dominated by the electron dese scatterers. Finally I will discuss novel AEMs both random and structured diblock materials fabricated at CSM and U. Mass Amherst.

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