Electrolytes

This chapter discusses fundamental and practical features of fluorite structured electrolytes. The fluorite structure is a face-centered cubic arrangement of cations with anions occupying all the tetrahedral sites, leading to a large number of octahedral interstitial voids. Thus, this structure is a rather open one and rapid ion diffusion might be expected. At high temperatures, zirconia has the fluorite structure, stabilized by addition of divalent or trivalent (that is, aliovalent) cations such as Ca or Y at lower temperatures. Pure ceria also has the fluorite structure. Oxide ion conduction is provided by oxide ion vacancies and interstitial oxide ions. Intrinsic defects are fixed by thermodynamic equilibrium in pure compounds, while extrinsic defects are established by the presence of aliovalent dopants. To maintain electroneutrality, a soluble aliovalent ion in an ionic compound is compensated by an increase in the concentration of an ionic defect. In addition to fluorite structure electrolytes such as stabilized zirconia and ceria, there are many non-fluorite structure oxides which are potentially attractive for solid oxide fuel cells (SOFC) electrolyte application. These include perovskites like lanthanum gallate and to a lesser degree calcium titanate.