Proton transport properties of proton-conducting phosphate glasses at their glass transition temperatures

The proton transport properties of 32 kinds of proton-conducting phosphate glasses with broad ranges of glass transition temperature, proton conductivity, and the proton carrier concentration were studied. Almost constant proton mobility of around 2 × 10^-8 cm² V^-1 s^-1 at the glass transition temperature, corresponding to a diffusion coefficient of approximately 4 × 10^-10 cm² s^-1, was found for the glasses. The reason why the diffusion coefficient of protons is almost constant in various proton-conducting phosphate glasses was discussed based on the role of the protons as a cross-linker within the phosphate framework via hydrogen bonding. We evaluated the highest proton conductivity of the phosphate glasses and melts based on the almost constant mobility at their glass transition temperatures and obtained a highest expected proton conductivity of 7.5 × 10^-3 S cm^-1 at 300 °C. The potential of proton-conducting phosphate glasses as electrolytes in intermediate temperature fuel cells was also discussed.