Preparation of Fast Proton Conductors by a Chemicovectorial Method Using Hydration of Calcium Phosphate Glass

Abstract

Novel fast proton conductors could be chemicovectrially prepared by introducing a large amount of water into calcium metaphosphate glass. When fine-sized calcium metaphosphate glass powders were contacted with water, hydrogelation started immediately, resulting in formation of calcium polyphosphates with high viscosity. Calcium metaphosphate glass has a long-chain structure and nonbridging oxygens (NBOs), and calcium ions serve as ionic cross-links between the NBOs of two different chains. The hydration breaks the long-chain phosphate structure and protons break the cross-links, resulting in connection to the NBOs. Subsequently, water molecules are hydrogen-bonded around the protons connected to the NBOs. The hydrogel is the cluster consisting of such hydrated phosphate chains. The hydrogel showed high conductivity of 5 mS/cm even at 30°C. By heat-treatment in saturated vapor at 90°C, monocalcium phosphate monohydrate (MCPM) crystal was precipitated in the hydrogel and its conductivity was much improved to 20 mS/cm at 30°C. The high conductivity was regarded as the fast proton transfer promoted by coexistence of large amounts of acidic POH groups and water molecules.