Proton conductivity of mesoporous silica incorporated with phosphorus under high water vapor pressures up to 150°C

Silica mesoporous structures incorporated with phosphorus were prepared and their proton conductivity was evaluated at temperatures of up to 150 degrees C under saturated or controlled water vapor pressure. The fixing of phosphoric acid through heat treatment after forming the mesostructure had a limited effect on the improvement in proton conductivity only in a relatively low-temperature region. The incorporation of phosphorus during mesoporous silica formation decreased the surface area but was effective in improving proton conductivity. A sample with a low P/Si atomic ratio of 0.07 showed the highest proton conductivity above 10(-2) Scm(-1) at 100 to 120 degrees C under saturated water vapor pressure. A sample with a high P/Si atomic ratio of 0.25 showed relatively high conductivities being maintained even at low relative humidity at 150 degrees C. An adequate phosphorus concentration and a large surface area were found necessary for high and humidity-independent proton conductivity in mesoporous silica.