Thermally stable and water-free proton conductor based on Brønsted acidic ionic liquid and silicate network was prepared by sol-gel processing. As a Brønsted acidic ionic liquid, mixture of bis(trifluoromethyl sulfonyl)imide (HTf 2 N) and 1-(1-ethyl-3-imidazolio)butane-4-sulfonate (eim4S) was employed. The sol-gel process was successfully done, since the ionic liquid worked as acidic catalyst for the dehydration and the condensation reactions of tetramethoxysilane. Although the gel containing equimolar eim4S and HTf 2 N showed high ionic conductivity, the thermal stability was poor because of sublimation of the HTf 2 N. Use of a small excess amount of eim4S effectively suppressed the sublimation of HTf 2 N. The obtained glassy ionogel showed high degradation temperature reflecting the thermal stability of both the silicate and the ionic liquid. The ionic conductivity reached to 10 À3 S cm À1 at 150 C without any humidification. Contribution of protons to the ionic conductivity was confirmed by chronoamperometry and impedance spectroscopy under hydrogen atmosphere. 1-4 The polymers generate protons under wet condition and protons migrate in the continuous hydrogen bonding network of water molecules. However, those waterbased proton conductors are not suitable for the cells operated above 100 C or below 0 C. The operation at higher temperature is important to enhance the cell voltage and to suppress the activity of carbon monoxide as a catalyst poison. Alternative proton conductors, durable at wider temperature range, have been accordingly investigated. [5][6][7][8][9] Imidazole and the 2-substituted derivatives are recently examined instead of water because of their relatively small molecules having both proton donating site and proton accepting site. Imidazole-based proton conductor shows good ionic conductivity at the temperature above 100 C, though the thermal and chemical stability are waiting to be improved. [10][11][12][13][14] Some ionic liquids are also expected as new proton conductors. [15][16][17] Ionic liquids generally show high thermal stability beyond 300 C, negligible vapor pressure, low glass transition temperature (T g ), and high ionic conductivity.
18Previously, we proposed a simple method to add acidity to ionic liquids by mixing appropriate zwitterionic salts and super strong acids such as bis(trifluoromethylsulfonyl)imide (HTf 2 N). 19 In such a binary system, imidazolium cationic part in the zwitterion forms low T g domain with Tf 2 N À , while the anionic part is neutralized to be acid form with proton. The mixture of 1-(1-butyl-3-imidazolio)propane-3-sulfonate and HTf 2 N showed ionic conductivity over 10 À3 S cm À1 at 150 C, as well as the decomposition temperature higher than 300 C. Gels including ionic liquid, called ionogels, are currently reported as ion conductor, 20-23 actuator, 24,25 luminescent materials, 17,26 and several related electrochemical devices.
18As the host matrices of the ionogels, we used silicate network which was provided by sol-gel process. 27 The sol-gel reacti...