Novel Proton-Conducting Polyelectrolyte Composed of an Aromatic Polyether Containing Main-Chain Pyridine Units for Fuel Cell Applications N. Gourdoupi, A. K. Andreopoulou, V. Deimede, and J. K. Kallitsis* Department of Chemistry, University of Patras, University Campus, GR-26500 Rio-Patras, Greece, and Foundation of Research and Technology Hellas, Institute of Chemical Engineering and High-Temperature Chemical Processes, P.O. Box 1414, GR-26500 Patras, Greece Received August 8, 2003. Revised Manuscript Received October 10, 2003 A new high-molecular-weight, soluble, wholly aromatic polyether bearing polar pyridine and phosphinoxide groups along the main chain is presented. This easily processable polyether presents excellent film-forming properties, high glass-transition temperature (up to 260 °C), and thermal stability up to 500 °C, all together combined with an ability to form ionically conductive materials after doping with phosphoric acid. The polar groups throughout the polymeric chains enable high acid uptake and subsequent high ionic conductivity for the doped membranes in the range of 10 -2 S/cm. Characterization of all polymeric materials prepared was performed using NMR, size exclusion chromatography, thermal and mechanical analysis, and conductivity measurements. The oxidative stability of the materials was studied using hydrogen peroxide, and the treated membranes were further characterized using dynamic mechanical analysis and FT-Raman spectroscopy. The conductivity of the doped membranes was determined as a function of the doping level. The temperature dependence of the conductivity was also studied. Introduction Fuel cells have attracted increasing attention in recent years as a clean, silent, and efficient power source. 1 Polymer electrolyte membrane fuel cells (PEM- FC) operating at about 90 °C are currently the best candidates for automobile applications. 1,2 However, up to now, the low temperature PEMFCs demand hydrogen of high purity and humidification of the feed gases, thus their operation cost increases sufficiently. In these cells high ionic conductivity is obtained at high levels of humidity where proton mobility is combined with a water flow through the membrane (electroosmotic drug effect). On the other hand, when methanol, which is considered an environmentally friendly fuel, is selected in direct methanol fuel cells (DMFCs), permeability through the membrane remains the key disadvantage of current materials in use. 3 By increasing the temper- ature at which FCs operate, advantages such as in- creased catalyst’s activity, decreased susceptibility of the anode’s catalyst to poisoning due to impurities in the fuel stream, easier thermal management than conven- tional PEM fuel cells, etc., can arise. 4-6 Particularly poisoning of the anode’s catalyst, a key parameter if a low-cost H 2 supply is to be used, can be overcome in FCs operating above 150 °C in which more than 1% CO content in the fuel stream does not disturb the operation efficiency. 7 The basic prerequisites for a polymeric material to be used as a membrane for high-temperature PEMFCs are good mechanical, thermal, and chemical stability, high glass-transition temperature, and increased ionic conductivity after doping with a strong acid. Besides polybenzimidazole (PBI), which is a well-established high-temperature polymeric electrolyte, 4,5 there is a significant research effort nowadays toward the devel- opment of some novel polymeric materials which fulfill the above requirements. Keeping these considerations in mind, polyelectrolytes composed of thermally and chemically stable aromatic main chain backbones are proper candidates. Such materials could either be new polymeric structures 8-13 * To whom correspondence should be addressed. Phone: 3061-997- 121. Fax: 3061-997-122. E-mail: J.Kallitsis@upatras.gr. (1) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (2) Dresselhaus, M. S.; Thomas, F. L. Nature 2001, 414, 332. (3) Kreuer, H. D. Chem. Phys. Chem. 2002, 3, 771. (4) Wang, J.-T.; Savinell, R. F.; Wainright, J.-S.; Litt, M.; Yu, H. Electrochim. Acta 1996, 41, 193. (5) Wainright, J.-S.; Wang, J.-T.; Weng, D.; Savinell, R. F.; Litt, M. J. Electrochem. Soc. 1995, 142, L121. (6) Haile, S. M. Mater. Today 2003, 3, 24. (7) Qingfeng, L.; Hjuler, H. A.; Hasiotis, C.; Kallitsis, J. K.; Kontoyannis, C. G.; Bjerrum, N. J. Electrochim. Solid-State Lett. 2002, 5, A125. (8) Poppe, D.; Frey, H.; Kreuer, K. D.; Heinzel, A.; Muelhaupt, R. Macromolecules 2002, 35, 7936. (9) Rulkens, R.; Wegner, G.; Thurm-Albrecht, T. Langmuir 1999, 15, 4022. (10) Vanhee, S.; Rulkens, R.; Lehmann, V.; Rosenauer, C.; Koehler, W.; Wegner, G. Macromolecules 1996, 29, 5136. (11) Brodowski, G.; Horrath, A.; Ballauff, M.; Rehahn, M. Macro- molecules 1996, 29, 6962. (12) Wittemann, M.; Kelch, S.; Blaul, J.; Hicke, P.; Guilleaume, B.; Brodowski, G.; Horrath, A.; Ballauff, M.; Rehahn, M. Macromol. Symp. 1999, 142, 43. (13) Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 7411. 5044 Chem. Mater. 2003, 15, 5044-5050 10.1021/cm0347382 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/25/2003