Pore size effect on proton transfer in sol–gel porous silica glasses

Abstract

The proton transfer in nanopores was investigated using the sol–gel-derived porous silica glasses. The glasses with the pore size of 2–25 nm in diameter were prepared using the sol–gel method and the proton conduction through water molecules adsorbed in pores was discussed by the measurement of the conductivity, electrical modulus, and diffusion coefficient. In the porous glasses with pores larger than ∼15 nm, the pores are not filled with the water molecules, resulting into the decreased proton conduction. As the pore size decreases, the pores are filled with the physically bonded water molecules, which act to form the pass way of proton conduction. On the other hand, in the pores smaller than ∼2 nm, much of the adsorbed water is chemically bound and restricts the proton hopping. It was found that the glass having pores ∼4 nm diameter is more appropriate for the fast-proton-conducting glasses.