Examination of the superprotonic transition and dehydration behavior of Cs0.75Rb0.25H2PO4 by thermogravimetric and differential thermal analyses

a r t i c l e i n f o Keywords: Cesium rubidium dihydrogen phosphate Cesium dihydrogen phosphate Rubidium dihydrogen phosphate Dehydration Superprotonic transition Thermogravimetric and differential thermal analysis The dehydration and the superprotonic transition behavior of Cs 0.75 Rb 0.25 H 2 PO 4 have been investigated via thermogravimetry and differential thermal analysis. To enable measurements under the high water partial pressures required, a high humidity system (p H 2 O = 0.02–0.8 atm) capable of weight change detection at 0.7% resolution and thermal events with 0.01 °C resolution was assembled. It has been found that the super-protonic phase transition temperature of Cs 0.75 Rb 0.25 H 2 PO 4 is 231 °C, slightly higher than that of CsH 2 PO 4. In contrast, dehydration of Cs 0.75 Rb 0.25 H 2 PO 4 under p H 2 O = 0.35 atm occurs at 265 °C, a somewhat lower temperature than that at which CsH 2 PO 4 undergoes dehydration. Analysis of the temporal evolution of both transformations suggests a mechanism of continuous nucleation of the high temperature phase at a constant nucleation rate combined with three-dimensional growth. Under sufficiently humidified conditions CsH 2 PO 4 (CDP) undergoes a transition at 230 °C (superprotonic transition, T s) to a cubic phase with high proton conductivity [1–6]. It has been demonstrated that above the superprotonic transition and below the thermal dehydration temperature, T d , which depends on the humidification level (~260 °C for p H 2 O = 0.2 atm), CsH 2 PO 4 is a viable electrolyte for intermediate temperature fuel cells [7–9]. In this study we explore the role of chemical substitutions, specifically Rb for Cs, on both the polymor-phic phase transition and dehydration behavior, with the objective of obtaining insight that ultimately enables direct chemical tuning of T s and T d. The compound RbH 2 PO 4 (RDP) displays superprotonic conductivity above 280 °C under high pressure [10]. Compounds in the Cs 1−x Rb x H 2 PO 4 (CRDP) system form solid solutions in which, at room temperature, the P2 1 /m structure of CsH 2 PO 4 occurs in the region of x = 0–0.8 and the P2 1 /c structure of RbH 2 PO 4 in the region of x = 0.9–1 [11,12]. Preliminary investigations reveal superprotonic phase transformations for x = 0–0.7 underp H 2 O ~ 0.4 atm [11], …