Direct synthesis of sulfonated mesoporous silica as inorganic fillers of proton-conducting organic-inorganic composite membranes

The mesoporous silica sphere grafted with organic sulfonic acid has been synthesized by a condensation reaction of 1,3-propane sultone and 3-mercaptopropyltrimethoxy silane, followed by hydrothermal reaction with tetramethoxysilane. Proton-conducting organic-inorganic composite membranes were cast together with Nafion® using these functionalized mesoporous silicas as inorganic fillers. The composite membrane showed increased water uptake and higher ionic conductivity relative to recast Nafion membrane under low relative humidity condition. This led to enhanced performance of proton exchange membrane fuel cell employing the composite membrane containing the sulfonated mesoporous silica under the low humidity condition without external humidification of reactant gas. The current density at 0.5 V of the membrane-electrode assembly (MEA) fabricated with composite membrane containing the sulfonated mesoporous silica was ca. 3.3 times higher than that of the MEA fabricated with recast Nafion membrane.