Electrical Characteristics of Grain Boundaries in Oxygen Ion and Proton-Conducting Solid Oxide Electrolytes.

摘要

Polycrystalline solid oxide electrolytes have been applied in a wide variety of scientific fields including: fuel cells, gas sensors, and gas pumps. The key to operating these devices more efficiently lies in the electrical properties of the grain and grain boundary in such electrolytes. This particular study focuses on the electrical properties of mostly grain boundaries with a minor emphasis on grain interiors on a few popular solid oxide electrolytes: doped ZrO2, CeO2, LaGaO3, BaZrO3, and SrZrO3 that conducts either oxide ions or protons. This is because the grain boundaries in these electrolytes often determine the electrical conductivity of the materials. Through the examination of an average width of "electrical" grain boundaries in Y-doped BaZrO 3, it was established that the width increases with decreasing dopant concentration, in accordance with what a space-charge model (i.e., the Mott-Schottky model) predicts. In addition, the current-voltage relationships under applied dc-bias of doped Y-doped CeO2, Sr-doped LaGaO3, Y-doped BaZrO3, and SrZrO3 show an ohmic to superohmic transition as the applied dc-bias exceeds the thermal voltage, consistent with the thermionic emission theory of a current flowing through a Schottky-type potential barrier at a solid-solid interface. These evidences unambiguously verify that the highly resistive nature of grain boundaries of these microcrystalline electrolytes originates from the potential barrier caused by the difference in the chemical potential in the grain and grain boundaries.;Although the grain boundaries of these microcrystalline electrolytes are thought to be highly resistive, a completely different phenomenon occurs at the grain boundaries of doped ZrO2 and CeO2 as the grain size reduces to tens of nanometers—they start to conduct protons in a H2O saturated environment as temperature drops below around 150 °C. Through a dopant concentration dependence of electrical conductivity study on dense fluorite-structured, nanocrystalline Gd-doped CeO2, it was found as hypothesized that the bulk defect chemistry does not play a role in determining such protonic conductivity due to the invariance of the protonic conductivity with different dopant concentrations. Encouragingly, this result implies that the protonic conduction may very well exist in the grain boundaries of other solid electrolytes with similar grain size and not necessarily limited to the fluorite structure.;Finally, the last part of this dissertation focuses on oxygen vacancy dynamics in the grain interior in an extreme case of heavily doped solid oxide electrolyte where high degree of dopant-oxygen vacancies association is expected to occur. In this study, a combined 89Y MAS NMR and impedance spectroscopy was performed on 59 cat% Y-doped ZrO2 over a wide range of temperature (from ambient to 500 °C for NMR and 350 to 700 °C) yielding very similar hopping frequencies and their respective activation energies (∼1.4 eV) between the two spectroscopies. More importantly, an average hopping distance of ∼ 7.5±1.0 Å as determined by NMR allows the computation of the mobilities, which when combined with the conductivity results from impedance spectroscopy, ultimately allows the determination of the concentration of mobile charge carriers, i.e., oxygen vacancies in this system.