Proton conducting inorganic–organic matrices based on sulfonyl- and styrene derivatives functionalized polycondensates via sol–gel processing
Abstract
Proton conducting inorganic–organic hybrid polymer electrolytes were developed in the last decade based on sulfonated, methacryl and epoxy functionalized alkoxysilanes for thin film cells [Electrochim. Acta 45 (2000) 137]. To improve the electrochemical stability of the materials for applications like polymer electrolyte membranes for direct methanol fuel cells, the less stable methacryl and epoxy alkoxysilanes of the former test system [Electrochim. Acta 45 (2000) 137] were replaced by styrene derivative functionalized alkoxysilanes. These alkoxysilanes were synthesized with a new modified Grignard reaction. The sol–gel materials were prepared in a two-step reaction: first, an alkoxysilane containing a sulfonated group and an alkoxysilane containing at least a nitrogen heterocycle, an amine group or a sulfonamide group were separately hydrolyzed and co-condensed each with one half of the amount of a styrene derivative functionalized alkoxysilane. Then, these two co-condensates were mixed. After evaporation of the solvent, the resin was cast in Teflon® moulds or applied on a substrate as a film and finally organically cross-linked via UV and/or thermal curing. The influence of the sample composition on the conductivity and the mechanical properties was studied. Conductivities of 3×10−3 S cm−1 at room temperature were obtained for membranes free of water, whose precursor composition consists of 60% sulfonated alkoxysilane, finally mixed with 2 mol imidazole per mol ---SO3H. If the imidazole is exchanged by water (max. 15 mass% absorption within membrane), the membranes show conductivities up to 8×10−3 S cm−1 at room temperature. A thermal stability of the inorganic–organic matrix of up to 180 °C (<5% weight loss) was measured by thermo-gravimetric (TG) analysis.