Enhancing Proton Conductivity and Structural Stability of CsH ₂ PO ₄ Composites with Acid-Treated Zeolite as Solid Acid Electrolyte Materials

Solid acid electrochemical cells operating at intermediate temperatures are promising for efficient hydrogen-energy conversion, but their practical application is limited by the dehydration-driven degradation of CsH 2 PO 4 (CDP), a representative solid acid electrolyte. In this study, a composite electrolyte based on CDP and phosphoric acid-treated ZSM-5 (ZSM-5(A)) is developed to improve proton-conduction behavior and dehydration tolerance under humidified intermediate-temperature conditions. Structural and spectroscopic analyses indicate that phosphoric acid treatment introduces phosphate-derived local chemical environments into the zeolite while preserving the ZSM-5 framework, thereby altering the interfacial environment surrounding the CDP phase. Proton-conductivity measurements show that CDP/ZSM-5(A) exhibits higher conductivity than pristine CDP and CDP/ZSM-5, including enhanced conductivity below the superprotonic transition with an earlier onset of the conductivity increase, approaching 10 −2 S cm −1 at intermediate temperatures. Under humidified operation at 250 °C, pristine CDP and CDP/ZSM-5 show rapid conductivity decay, whereas CDP/ZSM-5(A) maintains substantially more stable proton-conduction behavior over 50 h. Correlated FT-IR, XPS, temperature-resolved XRD, and SEM analyses further indicate that the acid-treated composite undergoes less severe dehydration-associated chemical and structural changes, while preserving a more continuous pellet microstructure. These results support the role of ZSM-5(A) as a phosphoric acid-treated interfacial component in a CDP-dominated proton-conduction network and identify CDP/ZSM-5(A) as a phosphate-modified composite electrolyte for durable intermediate-temperature electrochemical energy systems.