Optimizing ionic conductivity in doped ceria

David Andersson et al. report calculations revealing how doped oxides with cubic fluorite structures become effective ionic conductors. Cubic fluorite structures, such as ceria (CeO2), can become effective ionic conductors when doped with cations of lesser valence than the host cations. Doped ceria thus has potential as an electrolyte for environmentally friendly solid oxide fuel cells. Andersson et al. examined how different dopants affect ionic conductivity and found that oxygen-vacancy properties at the atomic level corresponded closely to macroscopic conductivity properties. The interactions between the dopants and the oxygen vacancies could be divided into repulsive elastic and attractive electronic parts, with the elastic part mediated by lattice deformations and the electronic part mediated by redistribution of the electronic structure. In the optimal electrolyte, these parts should balance, minimizing the strain on the crystal lattice. The ideal dopant was calculated to have an effective atomic number between 61 (Pm) and 62 (Sm). Unusual for dopants, combinations of Nd Sm and Pr Gd showed higher ionic conductivity than for each element separately. This finding offers a simple way to narrow the search for superior dopants and combinations of dopants in ceria for potential use as electrolytes. — P.D.