Enhancing proton mobility and thermal stability in phosphate glasses with WO3: the mixed glass former effect in proton conducting glasses

Abstract

This study aimed to investigate the impact of WO₃ on the thermal stability of glass, as measured by the glass transition temperature (T_g), as well as the activation energy (E_a) of proton conduction and proton mobility (μ_H). These parameters were analyzed based on variations in the glass network structure and the nature of the P–O and O–H bonds in 35HO_1/2–xWO₃–8NbO_5/2–5LaO_3/2–(52 − x) PO_5/2 (x = 2, 4, 6, and 8) glasses. As previously predicted by a linear regression model, replacing PO_5/2 with WO₃ resulted in an increase in T_g and μ_H at T_g. The observed enhancement rates were +9.1 °C per mol% of WO₃ for T_g and 0.09 per mol% of WO₃ for log(μ_H at T_g [cm² V⁻¹ s⁻¹]), which aligned with the predicted values of +6.5 °C and 0.08, respectively, validating the linear regression model. The increased T_g was attributed to the formation of heteroatomic P–O–W linkages that tightly cross-linked the phosphate chains. The decrease in E_a and increase in μ_H at T_g with increasing WO₃ content were attributed to the reduction of the energy barrier for inter-phosphate chain proton migration owing to the increasing proton migration paths through P–O–W linkages. This μ_H enhancement is distinct from previously reported ones due to the reduction of the energy barrier for proton dissociation from OH groups. This phenomenon can be attributed to the mixed glass former effect in proton conducting glass.