Proton-Conducting Polymers and Membranes for Fuel Cells-Preparation and Properties
Abstract: The development of new materials for use in fuel cells is of importance for the probable replacement of the combustion engine by this technology in public vehicles in the future. The proton-conducting polymer electrolyte membrane is one of the main components in the fuel cell. The need for improved performance of the proton conducting membrane is large and has led to intensive research worldwide. When new materials are developed, a fundamental understanding of their properties is of importance for the optimization of the material in its application. In the present work two different series of proton-conducting polymer systems were prepared and studied. The first one was a series of amphiphilic copolymers with sulfonic acid groups synthesized from sulfonated and non-sulfonated acrylamide monomers. The copolymers obtained were characterized by solution and aggregation properties using viscometry, light scattering and atomic force microscopy. It was concluded that the polymers behaved as polyelectrolytes in solution. In solutions with copolymers having a low content of sulfonic acid groups the presence of aggregates was detected. The water absorption properties were investigated in humidified air and the absorption was found to increase approximately linearly with the content of sulfonic acid groups. The effect of the amount of water in the samples and the interaction between the water and the polymer were studied in relation to the proton conductivity. High amounts of water and acid groups generally resulted in a high conductivity above 0 °C, however, at temperatures below 0 °C the state of the water was of greater importance. A high degree of interaction between the water and the polymer, and thus the inability of the water to crystallize, was necessary to reach a high conductivity. In order to prepare proton conducting membranes, modification of aromatic polymers such as polysulfone and polyphenylsulfone was performed. These materials are thermally, mechanically and chemically stable materials. Short sulfonated spacers were attached to the polymer main chain by lithiation followed by reaction with an electrophile. Both flexible aliphatic and rigid aromatic spacers were used. The modification of these materials resulted in proton conductivities and thermal stabilities in the range of other sulfonated aromatic materials. The water absorption properties were encouraging and some of the materials had a constant water absorption at temperatures above 120 °C. The materials with the most rigid molecular structure showed a constant water absorption to the highest temperatures. In addition, the interaction between the absorbed water and the polymers was high for these materials. This was shown by the inability of the absorbed water to crystallize when analyzed with DSC. These properties indicated that these materials may be useful in fuel cells operated at elevated temperatures and possibly also in direct methanol fuel cells.
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