Inorganic–organic hybrid protonic polymeric materials for fuel cells based on polycondensed and organically cross-linked sulfonyl- and styrene-functionalized alkoxysilanes

A new class of proton conducting inorganic–organic polymer (ORMOCER®) electrolytes for fuel cells based on polycondensed and organically cross-linked sulfonyl- and styrene-functionalized alkoxysilanes has been developed [1]. Different synthesis processes are used to take account of the different acidities of the starting alkoxydes. System-I is based on the separate hydrolysis and condensation of the acid and basic alkoxysilanes. Sulfonated alkoxysilanes and a styrene derivative functionalized alkoxysilane are hydrolyzed and co-condensed in parallel to the alkoxysilane containing at least a nitrogen heterocycle, an amine group or a sulfonamide group and a styrene derivative functionalized alkoxysilane. The two polycondensates are then mixed; the resulting resins are shaped into thin films and organically cross-linked via UV and/or thermal curing. Improvements of mechanical and electrochemical properties lead to System-II in which the sulfonated functionalized alkoxysilane is first hydrolyzed and condensed, then the two other alkoxysilanes are added for co-condensation. In system-III the three alkoxysilanes used for system-I are hydrolyzed and co-condensed without any partial hydrolysis of one of the components.The three systems present a good thermal stability up to 180 °C. The conductivity of the materials shows an Arrhenius behavior in the temperature range 25–110 °C with activation energies of 0.45 up to 0.78 eV depending on sample composition. A 1.0 × 10−2 S cm−1-conductivity value was measured for system-II/imidazole membranes at 110 °C under an inert gas atmosphere. The conductivity of anhydrous system-III/imidazole membranes goes from 9.1 × 10−3 S cm−1 at 100 °C to 2.0 × 10−2 ± 1.5 × 10−3 S cm−1 at 140 °C.