The characterization and optimization of metal electrode interfaces with solid electrolytes

The purpose of this research effort was two fold. The first objective was to understand the process and material issues relating to producing mixed ionic and electronic conducting electrodes (MIECE) made from heat-sintered blends of molybdenum and titanium dioxide, and from heat-sintered blends of titanium nitride and titanium dioxide. The second objective was to investigate the fundamental physical characteristics, such as surface contaminants and morphology at the electrode/electrolyte interface, to determine how those characteristics influences the mechanisms and energies which govern the oxidation and reduction reactions at the electrodes. The intent was to learn what were the contributing components to electrical impedances and overpotentials, to measure them directly, or indirectly, and to control them for optimization. Sodium β″-alumina was used as the solid electrolyte ionic conductor with sodium as the mobile species. The major processes and equipment used in this research program were presented, as were the experimental methods and techniques employed to evaluate the results. While the efforts to obtain a MIECE using Mo/TiO₂ were unsuccessful, a MIECE was successfully made from TiN and TiO₂. Test data suggest it behaves as a true MIECE.; The morphology and chemistry of the interface between electrode and the alumina were found to significantly influence the charge transfer impedances and ionic conductance (at the surface) events. The surface of β ″-alumina was mechanically modified using various grit blasting and surface polishing techniques. Once modified, various surface characterization methods were used to understand the morphological and chemical properties of the β″-alumina surface. These methods included scanning electron microscopy, energy depressive X-ray spectroscopy, X-ray photoelectron spectroscopy, and optical interferometry. Sodium ion conductivity in β″-alumina measurements were also made. The primary contributor to impedance was surface impurities on the β″ -alumina. Only after these contaminants were removed was surface roughness found to be influential in charge transfer impedances. These results were confirmed in a series of miniature electrode test cell experiments, and also in a sodium exposure test cell evaluation.