Proton conductivity at medium temperature range and chemical durability of phosphosilicate gels added with a third component

Abstract

Phosphosilicate (P 2O 5–SiO 2) gels with an addition of Al 2O 3 or B 2O 3 as a third component have been prepared by the sol–gel method aiming at the improvement in chemical durability of the gels. The presence of strongly interacted water with gel matrices was confirmed for all the gels from an endothermic peak with weight loss at around 230 °C in thermal analyses. The addition of Al 2O 3 or B 2O 3 suppressed the formation of Si 5O(PO 4) 6 in phosphosilicate gels during heat treatment and Al 2O 3 remarkably improved the chemical durability of the resultant gels after a heat treatment at 300 °C. Al 2O 3-added gels (P/Si/Al=1:0.9:0.1 molar ratio) showed conductivities of about 10 −4 Scm −1 at 130 °C and 0.7% R.H. after a pre-treatment at 30 °C and 60% R.H. for 3 h, whereas pure phosphosilicate gels (P/Si=1:1) and B 2O 3-added gels (P/Si/B=1:0.9:0.1 molar ratio) showed higher conductivities of about 10 −2 Scm −1 under the same conditions. The lower conductivity of the Al 2O 3-added gel was ascribed to the smaller amounts of H 3PO 4 formed by hydrolysis of Si 5O(PO 4) 6 in the gels during the pre-treatment.