Enhancement of Proton Transport by High Densification of Sulfonic Acid Groups in Highly Ordered Mesoporous Silica

Highly ordered sulfonic acid functionalized mesoporous silica films with various pore sizes (2.2–3.9 nm) and acid densities (0.78–2.3 mmol g –1 ) were successfully prepared by the Evaporation Induced Self-Assembly (EISA) process using tetramethoxysilane and 3-mercaptopropyltrimethoxysilane as starting materials. The relationship between the proton diffusivity and the water sorption process in the nanopores was clarified in detail by these mesoporous films. The proton conductivity of mesoporous films increased steeply according to the capillary condensation of water in the uniform mesopores. This steep increase of proton conductivity was controlled by the pore size and the acid density described by the Kelvin equation. Notably, the mesoporous electrolyte with the smallest pore size (2.2 nm) and the highest acid density (2.3 mmol g –1 ) showed the highest proton conductivity at low relative humidity (RH) [e.g., 5.4 × 10 –3 S cm –1 (25 °C) at 20% RH]. Furthermore, the proton diffusivity in the mesopores was enhanced by the high densification of sulfonic acid groups even at very small amounts of adsorbed water [λ(H 2 O/SO 3 H) = 2].