High pressure synchrotron x-ray diffraction studies of superprotonic transitions in phosphate based solid acids
Certain phosphate based solid acids, such as CsH2PO4 and RbH2PO4, have been shown to exhibit an abrupt, several-order-of-magnitude increase in their proton conductivity when heated above a temperature threshold. This so called superprotonic behavior allows the above-mentioned materials to function as fuel cell electrolytes at temperatures between 150°C and 300°C, a remarkable application that attracted significant interest especially from the automobile industry. Yet, the microscopic structures and dynamic mechanisms responsible for this behavior are not fully understood. In fact, until very recently, the very nature of the superprotonic behavior has been debate, with some groups attributing the steep proton enhancement to a polymorphic transition and others pointing out to possible chemical modifications. This is mainly due to the fact that heating the title materials under ambient pressure and humidity conditions does indeed lead to their dehydration at temperatures in the immediate vicinity of proton conductivity jump, which, in turn, generate ambiguity on the origin of the superprotonic behavior. The main purpose of the investigations presented in this thesis is to clarify the origin of the above-mentioned heating-induced proton conductivity enhancement. To this end, we have mostly used high-pressure synchrotron x-ray diffraction methods to avoid dehydration and study the structural (and possibly chemical) modifications responsible for the observed proton conductivity behavior. However, for comparison purposes, we carried out similar measurements under ambient-pressure conditions. We investigated CsH2PO4 and RbH2PO4, as well as their counterparts based on smaller size cations, i.e. KH2PO4, NaH2PO 4, and LiH2PO4. Our initial temperature-resolved data collected on polycrystalline CsH 2PO4 demonstrate that even under ambient pressure conditions this solid acid exhibits a transition from its room-temperature monoclinic (P21/m) phase to a cubic (Pm3m) modification. Yet, the cubic phase is not stable under ambient pressure and humidity conditions and dehydrates in minutes even in the absence of further heating. Further measurements on samples subjected to high pressure (P=1GPa) reveal the same monoclinic→cubic polymorphic transition at T∼260°C. In this case, the high temperature cubic phase (Pm3m, a=4.88 Å) is stable, and the transition occurs under the same (P, T) conditions as the 1000-fold jump in CsH2PO 4's proton conductivity. Rietveld analysis confirms that the high-pressure cubic phase has essentially the same crystal structure as its counterpart observed under normal atmosphere. This unambiguously demonstrates that the superprotonic behavior of CsH2PO4 is due to a polymorphic transformation and not to dehydration-driven chemical modifications. For RbH2PO4 we found a transition from its room temperature tetragonal (I-42d) phase to an intermediate temperature monoclinic (P21/m) modification, which, remarkably, is isomorphic to the room the room temperature monoclinic CsH2PO4. This suggests that a monoclinic→cubic polymorphic transition, similar to the one observed in CsH2PO4, is responsible for the Rb-based compound's superprotonic behavior. While further heating under ambient pressure conditions resulted in the sample's chemical decomposition, temperature-resolved data collected under 1 GPa of pressure revealed the existence of a previously unknown high-temperature RbH2PO4 polymorph. Moreover, this new phase has the same cubic symmetry as its Cs-based counterpart, thus confirming our hypothesis that the microscopic aspects that trigger the superprotonic behavior in phosphate solid acids are not cation-dependent, and a general highly-efficient proton conduction mechanism is at work in the high-temperature cubic phases of these compounds. KH2PO4 has been shown not to exhibit a superprotonic behavior although it is isomorphic with RbH2PO4 at room temperature. Our data shows that this isomorphism persists in the intermediate temperature phases, which implies that the cation size plays a key role in determining the existence of a superprotonic behavior. We confirmed this hypothesis through temperature resolved x-ray diffraction measurements on small-cation phosphate solid acids NaH2PO4 and LiH2PO 4. Indeed, these latter materials do not appear to exhibit heating-induced transitions to the highly symmetric phases that enable an efficient proton transport in phosphate solid acids.
Alternative Energy|Solid State Physics|Materials science
Hermosillo, Juan Daniel, "High pressure synchrotron x-ray diffraction studies of superprotonic transitions in phosphate based solid acids" (2012). ETD Collection for University of Texas, El Paso. AAI3512011.