Ceria fluorite electrolytes from ionic to mixed electronic and ionic membranes

Ceria-based electrolytes are one of the most promising alternatives for conventional solid oxide fuel cell (SOFC) yttrium-stabilized zirconia (YSZ). The ceria has the same fluorite structure as zirconia through ion doping to produce oxygen vacancies in the structure thus achieving high O^2- conductivity, which can reach 0.1 S-cm^-1 at 800-^oC, i.e. 200-^oC lower than that of the YSZ. However, chemical instability - e.g. in fuel cell operation, Ce⁴⁺ reduced to Ce³⁺ leading to electronic conductivity and microcracking - hinders applications of ceria for SOFCs. This chapter presents a new perspective from conventional structural doping to surface modifications by the surface doping and finally, non-doping ceria. The undoped ceria is actually most effective for ionic transport resulting in excellent fuel cell performances.