PhD Thesis Defence Presentations - Alexandros Safakas

A comparative assessment of the behavior of a series of eight (8) perovskite oxides with general formula La_0.8Sr_0.2Co_xFe_1-xO₃ (including La_0.8Sr_0.2FeO₃ and La_0.8Sr_0.2CoO₃) was conducted, as catalysts for CO and propane combustion and as oxygen reduction electrocatalysts for application in high temperature solid oxide fuel cells as well as in low temperature alkaline fuel cells. The main conclusion derived from the study of the catalytic behavior of La_0.8Sr_0.2Co_xFe_1-xO₃ powders for CO and propane oxidation, under oxidative and stoichiometric feed conditions, was that the catalytic activity increased with increasing Co/Fe ratio in the perovskite composition, this effect being more pronounced for oxidative conditions. The study of the impedance characteristics of La_0.8Sr_0.2Co_xFe_1-xO₃ electrodes interfaced to CeO₂-Gd₂O₃ electrolyte under oxygen partial pressures PO₂ ranging from 0.7 to 100 kPa and temperatures from 600 to 850 ^οC revealed that these characteristics were determined by up to two main processes whose relative contribution in shaping the open circuit total polarization resistance depended on PO₂, temperature and Co/Fe ratio. Finally, the study of electrocatalysts composed of a mixture of La_0.8Sr_0.2Co_xFe_1-xO₃ and carbon black (performed mainly via the rotating disk technique) as it concerns oxygen reduction in alkaline medium at  room temperature showed that gradual substitution of Fe by Co in the oxide’s composition results in increase of the electrocatalytic activity. Also, an increase in the number of transferred electrons was found, from ca. 2 for La_0.8Sr_0.2FeO₃/C to ca. 4 for La_0.8Sr_0.2CoO₃/C, implying a gradual change in the reaction mechanism. In all above cases, the origin of the observed behavior was associated with changes in the electronic structure of the surface B-sites (Co- and Fe-sites) induced by changing the Co/Fe ratio, as well as with changes in the concentration of oxygen vacancies.