ABSTRACT: We propose to synthesize ion-containing thermally stable polymers, as candidates for high temperature proton exchange membrane/membrane electrode assembly (PEM/MEA) components of fuel cells. The work will comprise several initiatives over a 12-month period. Firstly, sulfonated ion-conducting sites will be introduced during direct polymerization to provide well-defined location and concentration information. The PI and his colleagues have already prepared random copolymers of hydrolytically stable naphthalene-based six-membered ring polyimide copolymers, as well as wholly aromatic ether sulfones and ether phenyl phosphine oxides. Cast films have been demonstrated to have conductivities at ambient temperatures, equal to, or exceeding, Nafion state-of-the-art materials (e.g., 0.1 S/cm). Furthermore, the sulfonated copolymers containing 40 mol% of the disulfonated monomer have demonstrated twice this conductivity in a continuous operation at 120°C for about 4 days (ca. 120 hours), when conducted under 70 psi water pressure.
The possibility of generating conduction mechanisms at elevated temperatures in the absence of water is obviously very challenging. Several approaches have been proposed and these, along with new ideas, will be investigated. Firstly, the utilization of a polymer blend in conjuction with an aromatic sulfonic acid can be utilized as Tsuchida has demonstrated to prepare moderately conducting materials at 150°C . Others have investigated the use of phosphotungstic acid, or PTA in general, which also shows some promise in the 150-170°C temperature range . Savinall and co workers have pioneered a third approach, which has been based upon the doping of polybenzimidazoles with phosphoric acid . These approaches have, in general, shown some promise and are worthy of further investigations using suitably modified new matrix systems and new acidic ion conducting sites. We will particularly determine the upper limits of the well defined sulfonic acid containing copolymers, which has not been reported. Short term stability at more than 200°C has been demonstrated, but there is no certainty that the materials will have appropriate long-term stability at temperatures approaching that value. The PI and his colleagues will characterize this stability using the established sulfonic acid systems developed earlier, which is described in this proposal.
A more novel, but still speculative method will be to utilize our expertise in phosphorous containing polymers. The poly(arylene ether phosphine oxide)s, for example, or phosphine oxide containing polyimides also show strong complexing to a variety of different acids, including various sulfonic acids. It is possible that the interaction may be strong enough to allow for conductivity at elevated temperatures. Even more likely, is the possibility of using the reduced form of the phosphine oxide, typically the phosphine or possibly dihydroxy phosphorus derivative, which can also be obtained. The phosphine shows extremely strong complexations to metals and would likely even complex strong acids, such as triflic acid. Whether the resulting complexes could be induced to show adequate conductivity would be a major objective of the research. The research could be initiated almost immediately because of the availability of two excellent postdoctoral fellows, as well as several students in their 4th or 5th years of their Ph.D. programs, who are "up-to-speed" in these areas.
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