Progress in the development of intermediate-temperature electrochemical cells using phosphate glass electrolytes

Intermediate-temperature electrochemical devices operating at approximately 300 °C have attracted increasing attention as a promising platform that bridges the gap between low-temperature polymer electrolyte membrane systems and high-temperature solid oxide devices. This temperature regime offers a balanced combination of high energy conversion efficiency, reduced material constraints, and enhanced system flexibility, enabling expanded application potential in both power generation and electrochemical conversion processes. This article reviews recent progress in the development of phosphate glass electrolytes as proton-conducting materials suitable for intermediate-temperature operation. After summarizing early studies on proton conduction in phosphate glasses, recent advances enabled by the alkali–proton substitution (APS) method are discussed. The APS method allows the introduction of protons into glass frameworks at exceptionally high concentrations, yielding glass electrolytes that exhibit stable and purely protonic conduction in the intermediate-temperature range. The unique proton dissolution and transport mechanisms in phosphate glasses, fundamentally distinct from those in polymer electrolytes and oxide-based proton conductors, are highlighted. Furthermore, key factors governing proton conductivity, including glass network structure, constituent elements, and crosslinking effects, are systematically reviewed, and compositional design guidelines are summarized. Perspectives on electrolyte processing, cell fabrication strategies, and evaluation methodologies toward device integration are also presented. These insights establish phosphate glass electrolytes as a versatile and tunable materials platform for the realization of next-generation intermediate-temperature electrochemical devices.