| Summary: | Using powder X-ray diffraction and AC impedance spectroscopy, we have found that the superprotonic CsH<sub>2</sub>PO<sub>4</sub> (CDP) phase is stable at T = 250 °C when sealed in different volumes (15 mL and 50 mL) of dry air or inert gasses. Under these conditions, CDP’s proton conductivity stays constant at 2.5 × 10<sup>−2</sup> S·cm<sup>−</sup><sup>1</sup> for at least 10 h. On the other hand, removing the gas from the chamber leads to a sharp, two-order-of-magnitude drop in the proton conductivity. Our data show no evidence of a self-generated water vapor atmosphere in the chamber, and the gas pressure at T = 250 °C is several orders of magnitude below the pressures previously used to stabilize CDP’s superprotonic phase. These results demonstrate that hermetically sealing CDP in small gas-filled volumes represents a new method to stabilize the superprotonic phase, which opens new paths for large-scale applications of phosphate-based solid acids as fuel cell electrolytes.
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