Microstructure development of mesoporous silica thin films with pore channels aligned perpendicularly to electrode surfaces and application to proton conducting composite electrolyte membranes

We investigated a novel preparation technique of a proton conducting inorganic composite thin membrane consisting of an oxyacid salt proton conductor, CsHSO4, and a mesoporous SiO2 thin film that has highly oriented mesopores. The mesoporous SiO2 thin films were prepared through the removal of the Fe component from Fe–Si–O precursor thin films synthesized by sputter deposition. In this study, we focused on the SEM observation of growth structures of Fe–Si–O precursor thin films and the resultant mesoporous SiO2 thin films. The relationship between the deposition parameters (working gas pressure and target component) and the growth structures of the thin films is discussed. The obtained mesoporous SiO2 thin films showed pore channels aligned perpendicularly to electrode surfaces when relevant deposition parameters were applied. A CsHSO4/SiO2 composite thin membrane was prepared by the impregnation of a CsHSO4 aqueous solution into the mesopores of a SiO2 thin film. The proton conductivity of the CsHSO4/SiO2 composite thin membrane was examined with respect to the difference between proton conduction in the composite thin membrane and that in neat CsHSO4, which suggests that mesopores of a SiO2 thin film play an important role in the enhancement of proton conductivity, especially in the low-temperature region between 60 and 140 °C.