Phase behavior and superprotonic conductivity in the Cs1−xRbxH2PO4and Cs1−xKxH2PO4systems

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The solid acid compound CsH2PO4 (CDP) adopts a cubic structure of high conductivity above 228 °C, rendering it attractive as a fuel cell electrolyte for intermediate temperature operation. This superprotonic phase is stable from the phase transition temperature, Ts, to the dehydration temperature, Td, where the latter depends on water vapor pressure (e.g. Td = 290 °C at pH2O = 0.8 atm). In this work we examine the possibility of modifying these temperatures and thereby, amongst other characteristics, fuel cell operating conditions by introduction of Rb and K as substituents for Cs in CDP. The phase behavior of the Cs1−xRbxH2PO4 and Cs1−xKxH2PO4 pseudo-binary systems are determined by in situ X-ray diffraction (XRD) and thermal analysis. It is found that RbH2PO4 (RDP) and CDP are entirely miscible both below and above the transition to the cubic phase. With increasing Rb concentration, Ts increases and Td decreases. In contrast, K has limited solubility in CDP, with a 27 at.% solubility limit in the cubic phase, and both Ts and Td decrease with K content. The eutectoid temperature in the Cs1−xKxH2PO4 system is 208 ± 2 °C and the K solubility decreases sharply below this temperature. In both systems, conductivity decreases monotonically with increasing substituent concentration. Furthermore, even after normalization for cation size, the impact of K is greater than that of Rb, suggesting local disruptions to the proton migration pathway, beyond global changes in unit cell volume. Although this investigation shows unmodified CDP to remain the optimal fuel cell electrolyte material, the study provides a possible framework for elucidating proton transport mechanisms in superprotonic conductors.