ABSTRACT
The electrolytes for solid oxide fuel cells (SOFCs) are generally oxygen ion conductors, in which current flow occurs by the movement of oxygen ions through the crystal lattice. This movement is a result of thermally activated hopping of the oxygen ion, moving from one crystal lattice site to its neighbor site. To achieve the movement, the crystal must contain unoccupied sites equivalent to those occupied by the lattice oxygen ions, and the energy involved in the process of migration from one site to the unoccupied equivalent site must be small, certainly less than about 1 eV. This small barrier to migration would seem, at first glance, difficult to attain since the oxygen ions are the largest components of the lattice, with an ionic radius of 0.14 nm. Intuitively, it would be expected that the smaller metal ions would be more likely to have an appreciable mobility in the lattice and, hence, carry the current. However, in certain crystal structures, oxygen defects are predominant, so oxygen ions migrate in the electric field. Examples of these crystal structures with partially occupied oxygen sites are ZrO2-, CeO2-, and Bi2O3-based oxides with fluorite structure, LaGaO3-based perovskites, derivatives of Bi4V2O11 and La2Mo2O9, perovskite-and brownmilleritelike phases (e.g., derived from Ba2In2O5), pyrochlores, and (Gd, Ca)2Ti2O7-d [1].
