Defect equilibria and chemical expansion in undoped and doped ceria and strontium doped lanthanum cobalt iron oxide oxygen conducting materials and their applications in solid-state electrochemical cells

Ceramic oxygen conducting materials are used in many energy-related applications from fuel reforming to electricity generation. In a typical ceramic oxygen conductor, oxygen ions are transmitted through a dense membrane via defects in the material. Hence, defects control the performance of these materials.;In low oxygen partial pressure, ceria based materials become non-stoichiometric and can form defect complexes that hinder ionic conductivity. In addition, oxygen non-stoichiometry induces strain (chemical expansion) that can result in stresses for composite or constrained applications. In this dissertation the non-stoichiometry and chemical expansion behavior is reported as a function of PO2 for undoped ceria, gadolinium doped ceria (GDC) and strontium doped lanthanum cobalt iron oxide (LSCF). Undoped ceria was measured at 800 °C and GDC and LSCF at 600 - 900 °C in a wide PO2 range. It was found that defect interactions could effectively be modeled in ceria based materials using defect complex formation in a mass action formalism and the results were used to model nonlinear chemical expansion behavior. In LSCF, non-stoichiometry and chemical expansion were modeled using using metallic and semi-conductor models. A semi-conductor model with B-site small polarons best represented the measured behavior. The resulting models and parameters can be used to predict mechanical and electrical behavior of SOFC components.;Surface oxygen non-stoichiometry has also been measured by comparing low surface area to high surface area samples. It was found that surface defect concentration previously ignored in the literature can result in erroneous non-stoichiometry measurements.;In the latter half of this dissertation these materials are applied in SOFC and ceramic oxygen generator (COG) technology. A strategy is presented here for advanced life support systems employs a catalytic layer combined with a COG cell so that CO2 is reduced all the way to solid carbon and oxygen without carbon buildup on the COG cell and subsequent deactivation. In addition, a highly porous GDC anode support is developed that is infiltrated with metal. SOFCs are fabricated that utilize the highly porous support and operate on hydrocarbon fuels that show initial performance > 250 mW/cm 2 at 600 °C on vaporized dodecane.