Caracterización estructural y estudio de las propiedades electroquímicas de óxidos derivados del GdBaCo₂O₅₊δ: nuevos materiales para cátodos de pilas de combustible tipo SOFC

The aim of this work is the synthesis, structural characterisation and study of the physical properties of oxides based on GdBaCo2O5+δ for being used as cathodes in solid oxide fuel cells (SOFCs). We have studied the effect of the substitution of different transition-metal atoms by Co in the GdBaCo2O5+δ compound. In particular, the work has focused in the study of the GdBaCo2-xFexO5+δ, GdBaCo2-xMnxO5+δ, GdBaMn2- xFexO5+δ and Gd0.5Ca0.5BaCo2-xFexO5+δ systems. The oxides have been prepared by ceramic method under different gas atmospheres. The structural characterisation has been carried out by a combination of techniques: xray diffraction (XRD), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM) and in some cases by scanning transmission electron microscopy (STEM). Oxides have also been characterized (metallic composition, average oxidation state of the transition metals and oxygen content) by XEDS, EELS and redox titration. Thermal stability has been evaluated by thermogravimetric analysis in air from room temperature to 900 ºC. Thermal expansion coefficients (TEC) have been determinate from the variation of the lattice parameters of the unit cell with the temperature, obtained by fitting the profile of the XRD patterns at different temperatures. Electrical properties have been studied by DC measurements under different gas atmospheres (air, O2 and N2). Electrochemical behaviour has been investigated by AC impedance spectroscopy on symmetrical cells using CGO as electrolyte..