Thermal stability and proton conductivity of densely proton injected phosphate glasses containing rare-earth elements

Abstract

• Effects of rare-earth element species in proton conducting phosphate glasses were studied. • Tg and glass structures were almost identical irrespective of the rare-earth element species. • Thermal stability was found to be significantly dependent on the rare-earth element species. • Surprisingly, the glass containing dy was stable up to 600 °C. • The glass containing dy exhibited proton conductivity of 4 × 10 −3 Scm −1 at 550 °C. Sodium ions in 25NaO 1/2 –3MgO–7 Ln O 3/2 –3GeO 2 –62PO 5/2 (mol%, Ln = La, Nd, Gd, Dy, Y, Yb) glasses were electrochemically substituted with protons with a substitution rate higher than 90% by using an Alkali-Proton Substitution (APS) technique.. Effects of rare-earth element species on various properties were investigated, such as the glass transition temperature (Tg), structure, thermal stability, and proton conductivity. Whereas Tg and glass network structures were almost mutually identical irrespective of the rare-earth element species, their thermal stability as evaluated from the onset temperature of weight loss during heating was found to be significantly dependent on the rare-earth element species. The glass containing Dy after APS exhibited the highest thermal stability among the glasses: it was stable up to 600 °C, which is ca. 370 °C higher than its Tg, without crystallization. Thermal stability depending on the rare-earth element species was related to the difference in the structure between the glass and the crystalline phase that precipitated from the glass. The proton conductivity was almost independent of the rare-earth element species. Therefore, the glass containing Dy which exhibited the highest thermal stability among the glasses exhibited the highest proton conductivity of 4 × 10 −3 Scm −1 at 550 °C.