Preparation and properties of rare earth co-doped Ce_0.8Sm_0.2-xY_xO_1.9 electrolyte materials for SOFC

Ceria-rare earth solid solutions are known as solid electrolytes with potential applications in solid oxide fuel cell (SOFC). Trivalent rare earth ions enter into solid solution introducing anion vacancies for charge compensation. These oxygen vacancies are quite mobile at moderate temperatures giving rise to a comparatively high ionic conduction. In this study, rare-earth co-doped Ce_0.8Sm_0.2-xY_xO_1.9 (x = 0.0-0.20) fine powders were prepared by a sol-gel method. The XRD results showed that all powders crystallite calcined at 800 °C were single phase with cubic fluorite structure; the average crystallite size was between 24 and 35 nm, which was in good agreement with the results of TEM and particle size distribution measurements. The SEM results exhibited that electrolyte pellets sintered at 1400 °C were dense, and the relative densities of these pellets were over 99%. Impedance spectra analysis of these electrolytes has been performed at 250-850 °C. The results displayed that Ce_0.8Sm_0.15Y_0.05O_1.9 and Ce_0.8Sm_0.1Y_0.1O_1.9 appeared higher conductivity and lower activation energy than the singly doped ceria with the same dopant concentration (20% trivalent rare earth) at the temperature range of 550-700 °C. The maximum power density and the maximum current density for the single SOFC based on the two samples were higher than singly doped sample with the same vacancy concentration. Therefore, it is concluded that co-doping with an appropriate ratio of samarium and yttrium can further improve the electrical performance of ceria-based electrolytes.