Porous Silica Nanospheres Functionalized with Phosphonic Acid as Intermediate-Temperature Proton Conductors

This paper reports the first study of developing porous silica nanospheres functionalized with phosphonic acid as intermediate-temperature (above 100 °C) proton conductors. These materials were synthesized by the co-condensation of diethylphosphatoethyltriethoxysilane (DPTS) and tetraethoxysilane using surfactant cetyltrimethylammonium bromide as a template, followed by acidification of phosphonate to phosphonic acid. With more DPTS used in the synthesis, the pore structure of the sample changes from the ordered mesoporous MCM-41 structure to the microporous structure, and the particle size simultaneously increases from ca. 80 to ca. 200 nm. The prepared silica nanospheres show promising proton conductivity above 100 °C; for example, the conductivity is as high as 3.0 × 10⁻⁴ to 0.015 S cm⁻¹ at 130 °C when the relative humidity increases from 20 to 100%. The proton conductivity increases with an increase in the content of functionalized phosphonic acid, whereas the conductivity under low humidity conditions is significantly enhanced by the high surface area of the porous structure. Because of their nanosized monodisperse spherical morphology, these novel intermediate-temperature proton conductors are promising in applications as inorganic fillers for preparing composite proton exchange membranes.