Novel Proton Exchange Membrane for High Temperature Fuel Cells
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Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
total pressure at low load levels. While humidification and pressure operation solve the problem for hydrogen operation, they do nothing for cells operating on hydrocarbon fuel. Attempts to substitute even simple hydrocarbons for hydrogen have, to date, failed largely because efficient hydrocarbon electrooxidation will probably always involve operation at temperatures above 150 0 C. All state of the art PEM dry out rapidly when heated above 150C. The result is local cell failure. This reduces the rate of water formation which only makes the situation worse. PEMs are very difficult to rehydrate once dehydrated. This research sought to explore the feasibility of raising the useful operating temperatures of PEMs above 100 0 C through changes in the chemistry of the polymer. The significance of the research lies in the possibility that not only would the system constraints (pressure levels and humidification requirements) be relaxed on hydrogen/oxygen systems, but it is also likely that improvements in electrooxidation of simple hydrocarbons would result. Recently, Yeager et a12 equilibrated Nafion 117 with ortho-H3PO4 in a successful attempt
to demonstrate improvement in the high temperature operating characteristics of the PEM Nafion. This study's findings echoed those of others in that H 3PO 4 acted like a Brbnsted base in conjunction with the sulfonic acid groups and thereby improved the dissociation of the sulfonic acid as demonstrated below: -CF 3 SO 3 H + H3 PO 4
Do H4 PO 4 + + -CF 3 SO3-
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This effect together with what these authors found to be better water retention by the phosphoric acid-equilibrated Nafion probably explains the improved ionic conductivity of these modified membranes at elevated temperatures. The proposed research sought to build on this initial study and offer further improvement to modified perfluoroalkane sulfonic acid membranes through addition of fluorinated phosphonic acid monomers. Perfluorinated phosphonic acid should be a far more powerful Br6nsted base which will accentuate the dissociation of sulfonic acids while maintaining high temperature water retensive properties. The result should be a modified Nafion membrane with excellent high temperature conductivity that can serve as a model for a future polymeric counterpart with similar properties. Such an approach will be necessary since a solid state, polymeric membrane is one sure way to guarantee sustainable improvement to the properties of Nafion. EXPERIMENTAL Perfluorinated phosphonic acids were synthesized by following a literature 3 procedure. The structures of the perfluoroalkylphosphonates and their designations are shown in Table I. The current research focused on compounds 1, 2 and 5. Compounds I and 2 were chosen because they are difunctional and offered the possibility of greatest success. Compound 5 was chosen as an extreme case, since it was the most hydrophobic of this group. Two basic approaches were taken to combine the individual test compou
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