Comparison of structure and proton conductivity of phosphosilicate gels derived from several kinds of phosphorus-containing compounds

Proton conductive phosphosilicate gels have been prepared from tetraethoxysilane (TEOS) and various kinds of phosphorus-containing compounds such as orthophosphoric acid (H3PO4), triethylphosphate (PO(OEt)3) and 2-(diethoxyphosphoryl)ethyltriethoxysilane (DPTS) by the sol–gel method. Proton conductivities of all the phosphosilicate gels increased with an increase in relative humidity, indicating that continuous paths suitable for fast proton conduction were formed in the gels due to the adsorption of water. The H3PO4-derived gel heat-treated at 150°C showed a high conductivity of 3×10−3 S cm−1 even in a dry atmosphere of 20% relative humidity at room temperature. In the case of heat treatment at 600°C, the conductivity of DPTS-derived phosphosilicate gels was higher than those of the H3PO4-derived and PO(OEt)3-derived ones. The decrease in the amount of isolated phosphoric acid and the formation of crystalline phases in the phosphosilicate gels lowered the proton conductivity.