Phase behavior and superprotonic conductivity in the Cs1−xRbxH2PO4 and Cs1−xKxH2PO4 systems

Abstract

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, T_s, to the dehydration temperature, T_d, where the latter depends on water vapor pressure (e.g. T_d = 290 °C at p_H2O = 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 Cs_1−xRb_xH₂PO₄ and Cs_1−xK_xH₂PO₄ 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, T_s increases and T_d decreases. In contrast, K has limited solubility in CDP, with a 27 at.% solubility limit in the cubic phase, and both T_s and T_d decrease with K content. The eutectoid temperature in the Cs_1−xK_xH₂PO₄ 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.