sealed in small gas-filled volumes*

We have used x-ray diffraction and ac-impedance spectroscopy to investigate the crystal structure and proton conductivity of CsH₂PO₄ (CDP) sealed in small volumes (15 ml and 50 ml) of air or inert gasses at T=250°C. Our data shows that CDP’s proton conductivity stays constant under these conditions at superprotonic values of ~2.5×10^-2 S·cm^-1 for at least 10 hours. Removing the gas from the chamber leads to a sharp drop in the proton conductivity of two orders of magnitude in ~ 2 hours. The gas pressure is several orders of magnitude below what was previously used to stabilize CDP’s superprotonic phase (~1GPa), and we found no evidence of a self-generated water vapor atmosphere in the chambers. Therefore, hermetically sealing CDP in small gas-filled volumes represents a new method to stabilize its superprotonic phase, which opens new paths for large scale applications of phosphate-based solid acids as intermediate-temperature fuel cell electrolytes.