| Summary: | CsH<sub>2</sub>PO<sub>4</sub> is a proton conductor pertaining to the acid salts group and shows a phase transition from monoclinic to cubic phase at 232 ± 2 °C under high-steam atmospheres (>30%). This cubic phase gives rise to the so-called superprotonic conductivity. In this work, the influence of the partial substitution of Cs by Ba and Rb, as well as the partial substitution of P by W, Mo, and S in CsH<sub>2</sub>PO<sub>4</sub> on the phase transition temperature and electrochemical properties is studied. Among the tested materials, the partial substitution by Rb led to the highest conductivity at high temperature. Furthermore, Ba and S-substituted salts exhibited the highest conductivity at low temperatures. CsH<sub>2</sub>PO<sub>4</sub> was used as electrolyte in a fully-assembled fuel cell demonstrating the applicability of the material at high pressures and the possibility to use other materials (Cu and ZnO) instead of Pt as electrode electrocatalyst. Finally, an electrolyzer cell composed of CsH<sub>2</sub>PO<sub>4</sub> as electrolyte, Cu and ZnO as cathode and Pt and Ag as anode was evaluated, obtaining a stable production of H<sub>2</sub> at 250 °C.
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