Electrolytes

This chapter discusses fundamental and practical aspects of electrolyte materials for solid oxide fuel cells (SOFCs). We focus on two main families of ceramic electrolytes; those with the fluorite crystal structure (ZrO₂- and CeO₂-based) and those with perovskite-related structures (La_{1 - x} Sr _x Ga_{1 - y} Mg _y O_{3 -(x+ y)/2}, CaTiO₃ and Brownmillerite). Fundamental factors influencing the oxide ionic conductivity are discussed, including the crystal structure, along with strategies to improve the conductivity by substitution with aliovalent cations to introduce extrinsic point defects (oxygen vacancies). Protonic conductivity in perovskite-structured ceramics is also discussed. Finally, we consider some of the alternative materials, including apatite-structured silicates and LAMOX. Although the latter two families are interesting, we highlight some of the challenges to be overcome for their implementation in practical SOFC systems.