Materials Chemistry of Superprotonic Solid Acids

Abstract

Solid acid is a class of materials that shows potential as a fuel cell electrolyte. Understanding the phase and mechanical stability are required for further development of this technology. We addressed both issues in this work. We expanded the use of the crystallographic theory of the phase transformation to three major classes of solid acids. That allowed us to relate material properties hysteresis to fundamental crystallographic and thermodynamic parameters. The understanding of the mechanism of the transformation can guide the effort to create materials with desired hysteresis. Careful investigation of the thermal and phase behavior of CsHSO₄, CsH₂PO₄, Rb₃H(SeO₄)₂ and in Cs1-xRbxH₂PO₄ solid solution series for both low and high temperature phases was performed and crystal symmetry and lattice parameters for Cs0.75Rb0.25H₂PO₄, T=240°C phase were found for the first time. Consistency between predicted and measured properties was shown for all three different classes of solid acids as well as for the isostructural solid solution series. Nanocomposite materials based on cesium hydrogen sulfate and nanometer size silica were characterized. We observed 30-40 nm size surface stabilization of our material at the high temperature phase, otherwise metastable at room temperature. We developed methods to quantitatively study interface phases and its effect on ion mobility. The method allowed us to quantitatively find crystalline and amorphous amounts in the composites. We observed 3-4 order decrease in spin-lattice relaxation values of the metastable phase in the composite. Solid state NMR allowed surface interactions directly and suggest high ion mobility. Strong effect on superprotonic transition temperature in composites was observed. Superprotonic phase was stable in composites at temperatures up to 70°C below phase transition compared with pure phase CsHSO₄. The mechanism and activation energy of the creep plastic deformation in CsHSO4 were found. Based on that, a method to reduce creep by 1-2 orders of magnitude was developed and creep-resistant material was synthesized.