Synthesis, physical-chemical characterization and electrochemical performance of GdBaCo_2-xNi_xO_5+δ (x = 0-0.8) as cathode materials for IT-SOFC application

GdBaCo_2-xNi_xO_5+δ (x = 0-0.8) cathode materials have been synthesized by a citrate-gel modified chemical route, by which we have achieved a high level of substitution up to x = 0.8. Oxygen stoichiometry at room temperature has been determined by iodometry and the structural evolution as a function of Ni content has been extracted from XRD patterns analysis. All compounds are orthorhombic at room temperature and a DSC analysis reveals that the high-temperature orthorhombic-to-tetragonal phase transition is observed at lower temperature as Ni substitution increases. Three compositions, with x = 0, 0.3 and 0.6, were then chosen for further characterization: high-temperature XRD has been performed confirming the abovementioned structural evolution with temperature and allowing to determine that Ni doping diminishes slightly the thermal expansion coefficient. According to 4-probe measurements, we show that these compounds present a high electronic conductivity, suitable for cathode materials. Finally, electrochemical characterization has been performed by AC impedance spectroscopy with symmetric cells using composite electrode showing an improvement of performance at intermediate substitution levels.