Oxide ion conductors are widely used as electrolytes in solid oxide fuel cells (SOFCs). In this chapter, the current status of oxide ion conductors is overviewed. In addition to oxide ion conductors, proton conductors are also studied as the electrolytes of SOFCs. Therefore, in the first section, a comparison of proton-conducting electrolytes and oxide ion-conducting electrolytes is briefly performed. The conduction mechanism and conduction route of oxide ions in fluorite and perovskite oxides are discussed. After a general explanation of the conventional oxide ion conductors, properties of LaGaO3-based oxide, which is considered as the most promising alternative for Y2O3-ZrO2, are discussed in detail. Another type of new oxide ion conductor is Ba2In2O5, and the effects of dopant on oxide ion conductivity in these new types of oxide ion conductors are also covered. The degradation in electrolyte conductivity is discussed finally.
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