Superprotonic Solid Acid Phase Transitions and Stability

Solid acid proton conductors are viable alternatives to current fuel cell electrolytes. Incorporating solid acid electrolytes, next generation fuel cells would not require humidification of the electrolyte, as in proton exchange membrane fuel cells, and could operate at higher temperatures which would improve catalysis rates. Engineering the properties of these materials for fuel cell electrolyte applications requires an understanding of the structural and chemical parameters that support superprotonic phase transitions over melting or decomposition. In this thesis, the structures of three superprotonic solid acids are presented, and for the first time, a distinction is possible between local versus average structure. An adapted model for configurational entropy based on Pauling’s entropy rules for ice is incorporated to describe the entropy of superprotonic solid acids. Insights from local structural information alleviate discrepancies between this model and experimentally determined entropy values. With clarifications from this work, the calculated configurational entropy of the superprotonic structures of CsD2PO4, RbDSeO4, and CsDSO4, agree well with experimentally determined entropy values. A study of chemical intermediates, CsxRb1-xH2PO4, provides valuable insight into the nature of the cation size effect on superprotonic phase transitions within an isostructural system. For compounds in the series that do exhibit a superprotonic phase, CsH2PO4 – Cs0.3Rb0.7H2PO4, the magnitude of proton conductivity remains neutral to rubidium incorporation. Altering the effective cation size shows a profound impact on transition temperature for compounds with high rubidium content (x < 0.5) while preserving the overall conductivity of the high-temperature and low-temperature phases. X-ray diffraction, thermal analysis, Raman, IR, 133Cs, 87Rb and 1H-NMR spectroscopy all attest to the gradual variation in structural properties across the composition range. The complicated high-temperature properties of Rb3H(SO4)2 have been misinterpreted in earlier literature as a superprotonic phase transition. This work presents a careful analysis of a conglomeration of data from different techniques which definitively conclude that at ambient pressure, Rb3H(SO4)2 degrades via phase separation, Rb3H(SO4)2 → RbHSO4 + Rb2SO4.