High proton conductivity and thermal stability at intermediate temperatures of phosphate glasses and their melts containing rare-earth oxides achieved by high thermal stability of proton carriers

Abstract

In this study, the proton conductivities of phosphate glasses containing rare-earth oxides with a composition similar to that of metaphosphate were investigated in the HO1/2-RO-LnO3/2-GeO2-PO5/2 (mol%,R = Mg, Ca, Sr, and Ba, Ln = La, Gd, Dy, and Y) glasses and discussed in terms of the density, mobility, and stability at elevated temperatures of the proton carriers. The proton conductivity at 300 °C increased approximately eight-fold (1 × 10−4 to 8 × 10−4 Scm−1) by increasing the HO1/2 content from 25 to 35 mol%, whereas the proton carrier density only doubled (5 × 1021 to 9 × 1021 cm−3). The 35HO1/2-2RO-5GdO3/2-5GeO2-53PO5/2 glasses exhibited a sustainable proton conductivity of approximately 1 × 10−3 Scm−1 for >300 h sustained at 310 °C. From the observation that deprotonation of the glasses did not occur at temperatures up to 500 °C, the high proton conductivity stable at ∼300 °C is largely due to the extremely high resistance against crystallization and phase separation inducing dehydration of the glass melt. While the Tg decreased from ∼225 °C to ∼170 °C with the increasing HO1/2 content from 25 to 35 mol% as the previously proposed linear regression model predicted, the μH at Tg decreased from 1.5 × 10−8 to 3.0 × 10−9 cm2V−1 s−1 with the increasing HO1/2 content contrary to the prediction.