Proton-conducting phosphate glass: Recent development as an electrolyte in intermediate temperature fuel cells

Recently, it has been reported that phosphate glasses with high proton conductivity in the intermediate temperature range have been successfully developed using a proton carrier injection technique with a high concentration for electrochemical ion substitution. In this review, we summarize the characteristics of the bonding states between protons and oxygen in phosphates and the concepts of material design for achieving high proton conductivity in phosphate materials. This paper introduces a carrier proton injection technique called the alkali-proton substitution (APS) method, which was developed as a process for increasing the concentration of proton carriers and required a breakthrough in the conventional approach to the development of phosphate-glass-based proton conductors. Additionally, the knowledge obtained regarding the correlations between glass components, glass structure, and proton mobility is summarized. This paper also describes the characteristics of proton-conductive phosphate glasses prepared using the APS method, demonstrating that they have the unique property of being able to carry only protons as charge carriers in any atmosphere. In conclusion, future strategies for improving proton conductivity and the potential for developing new applications are discussed.