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 O2– 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.