(588a) New Membrane Morphologies for Pem Fuel Cells

Polymeric membranes play a critical role during the generation of electricity in hydrogen and direct methanol proton-exchange membrane (PEM) fuel cells. The membrane in such devices performs three roles: It physically separates the positive and negative electrodes, prevents mixing of the fuel and oxidant, and provides a conduit for ion (proton) transport between the electrodes. For hydrogen proton-exchange membrane fuel cells, the membrane must exhibit low gas permeability and high proton conductivity. For a direct liquid methanol PEM fuel cell, the ion-exchange membrane must conduct protons and be a good methanol barrier. DuPont's Nafion^® (a perfluorosulfonic acid polymer) is the membrane of choice for hydrogen/air PEM fuel cells that operate at a temperature less than or equal to 80^oC. For a reformate fuel, it is desirable to operate the fuel cell at a temperature greater than 100^oC (to reduce the effects of CO poisoning), but Nafion looses water and the conductivity degrades at such high temperatures unless the water activity in the fuel gas feed is near unity. Nafion has also been used in direct methanol fuel cells, but high methanol crossover leads to a low power output due to cathode depolarization by methanol and unwanted fuel losses. There is considerable research and development around the world to develop new membrane materials for PEM fuel cells with tailored physical and transport properties. Alternatively, researchers are studying modified forms of Nafion (e.g., Nafion blends and composites) in order to selectively improve certain membrane properties. In general, current membrane fuel cell work is focused on: (i) proton conductors that operate at a temperature of 100-120^oC under low humidity conditions (25% relative humidity) for hydrogen/air fuel cells, (ii) polymeric membranes that conduct protons in the dry state at high temperatures (160-200^oC), and (iii) proton conducting ion-exchange membranes with low methanol permeability for operating temperatures in the range of 60-140^oC.

Recent fuel cell membrane work by the present authors has focused on: (i) multi-layer Nafion membranes, (ii) stretched recast Nafion, and (iii) proton conducting membranes based on interconnecting nano-fibers. The over-riding objective of these studies is to control and manipulate the membrane morphology in such a way as to improve membrane properties for fuel cell applications, such as increasing proton conductivity at high temperature and low relative humidity or lowering methanol permeability. In this talk, an overview of the various membrane systems under investigation in the authors' lab will be presented. Membrane fabrication methods and the results of membrane characterization experiments will be described. Physical property data relevant to PEM fuel cell applications will be related to the membrane morphology. Available fuel cell performance data will be also discussed.