8 - Oxygen ionic conductor

Abstract

In Solid Oxide Fuel Cells (SOFCs), the oxygen ionic conductor is the most fundamental and important material as the electrolyte, so that many kinds of oxide ionic conductors have been intensively proposed and investigated. Among them, Yttria Stabilized Zirconia (YSZ) electrolytes have been recognized as a well-developed electrolyte for SOFCs to be operated around 1000°C because of its excellent electrical, chemical, thermodynamic, and mechanical stabilities under operational and manufacturing conditions. On the other hand, there has been growing interest in recent years in reducing the operating temperature of SOFCs to utilize metal interconnectors to improve the anti-thermal-shock performance and to lower materials and manufacturing costs. When the operating temperature of SOFCs decreases, YSZ electrolytes are no longer a good candidate because the oxygen ionic conductivity of YSZ decreases rapidly with lowering temperature. Thus, alternative materials that have higher conductivity than YSZ have attracted considerable interest in recent years; some materials have been developed successfully and have already been tested in practical SOFC stacks. In particular, doped-lanthanum gallates exhibit excellent characteristics for intermediate temperature SOFCs. This chapter describes those fast oxygen ionic conductors that have higher oxygen ionic conductivity than YSZ electrolytes. It reviews fundamental features of oxygen ionic conductors and the importance of the ratio of the ionic to the electronic conductivity. It also discusses the current status of oxygen ionic conductors, presents a comparison of typical oxygen ionic conductors from the viewpoint of utilization in energy conversion applications, and describes the physico-chemical properties and related topics of oxygen ionic conductors.