Thermal behavior and conductivity of rubidium dihydrogen phosphate (RbH₂PO₄) at high temperature

RbH₂PO₄₂ ionic compound has emerged as a viable electrolyte for intermediate-temperature fuel cell, and has been subjected to thermal analysis and impedance spectroscopy to clarify its high-temperature properties. A thermo-analytical peak which was identified at 127°C without any evidence of weight loss is due to structural phase transition rather than thermal decomposition. It has been observed the phase transition temperature Tp, scattering widely over the temperature range of 80 - 120°C, is relatively independent of heating rate. Impedance spectroscopy performed under dry N₂ condition revealed a fast increase in conductivity at °245°C, which is a consequence of dehydration in the heating process. Application of humidified N₂ suppressed the dehydration effectively. Furthermore, it was found that a sharp jump in conductivity by several orders of magnitude appeared at ∼280°C in humidified N₂ atmosphere, indicative of a superprotonic phase transition. The existence of a superprotonic transition under humid condition instead of dry condition, in despite of the same sample, suggests that humidity has an important influence on conductivity for RbH₂PO₄ crystal.