Solid oxide fuel cells (SOFCs) are efficient and environmental friendly devices which directly convert hydrogen and fosil fuels into electric energy. The operation temperature of these devices is mainly determined by the nature of its electrolyte. Conventional SOFCs are composed of yttria based electrolytes requiring operation temperatures around 1000ºC, whereas ceria based electrolytes have allowed reducing the temperature down to the 500-700ºC range. Nevertheless, these materials require sinterization temperatures in the 1300-1600ºC range to achieve complete densification, which produces very large grain sizes and poor mechanical properties. In the last years, several research works have demonstrated that the addition of transition metals oxides is effective in improving the densification and thus, lowering the sinterization temperatures of ceria based electrolytes. Such research works, generally focalize on how the addition of these sintering aids affect the properties (density, sintering temperature, conductivity) of the electrolyte. Nonetheless, it is well established that SOFC performance depends not only on the characteristics and performance of its individual components (i.e. cathode, electrolyte and anode) but also on their interfaces. Hence, a comprehensive study on how the addition of electrolyte sintering aids affects the cell performance is still missing.

In this work, we have studied La_0.4Sr_0.6Co_0.8Fe_0.2O_3-d/Ce_0.8Gd_0.2O_2-d/La_0.4Sr_0.6Co_0.8Fe_0.2O_3-d symmetrical cells containing Ce_0.8Gd_0.2O_2-d electrolytes synthesized by a molten salt route [1] with and without Co oxide doping. La_0.4Sr_0.6Co_0.8Fe_0.2O_3-d nanostructured cathodes with high electrochemical performance [2,3] were deposited by spin coating techniques on both sides of the electrolyte pellets. The assemblies were studied by Electrochemical Impedance Spectroscopy (EIS), X-Ray Diffraction and Transmission and Scanning Electron Microscopy. EIS results indicate that both electrolyte and cathode impedance response are affected by the addition of the sintering aid to the electrolyte, despite using cathodes with similar microstructure according to SEM observations. A possible explanation for such behavior was founded during TEM characterization which reveals the formation of columnar structures at the cathode/electrolyte interface of the cell with cobalt-doped electrolyte. These columnar structures would negatively affect the cell performance hindering the diffusion of oxide ions from the cathode to the electrolyte.

[1] Mendoza-Mendoza E., Padmasree K., Montemayor S. and Fuentes A., J Mater Sci 47 (2012) 6076-6085.

[2] Baqué L, Caneiro A, Moreno MS, Serquis A., Electrochem Commun 10 (2008) 1905-1908.

[3] Soldati A.,  Baqué L.,  Troiani H.,  Cotaro C., Schreiber A., Caneiro A, Serquis A. Int J Hyd Energy 36 ( 2011) 9180-9188.