Tesis

One of the most important problems studied in the field of ceramic fuel cells (Solid-Oxide Fuel Cells, SOFCs) is the reduction of their operating temperature. This is desirable to avoid problems of degradation by thermal shock, reactions at the [electrolyte/electrode] interfaces, problems of chemical stability of materials in a reducing or oxidizing atmosphere, etc., which occur in traditional SOFCs, which work at 900-1000 °C. These temperatures are commonly used because most of the best-studied materials only have good electrochemical and/or ion transport properties in that range. However, reducing the operating temperature to the range of 500-800 °C would avoid the above-mentioned problems and it is also very important in order to reduce the costs of these devices. The main problem is the interconnection of cells, since materials with high electronic conductivity that resist high temperatures in both oxidizing and reducing atmospheres are very expensive special ceramics. Instead, at lower temperatures, cheaper materials based on high-temperature resistant steels could be used. To achieve good performance devices at lower temperatures, new materials for electrolyte, cathode and anode that are efficient under these conditions are studied. SOFCs operating at temperatures between 500 and 800 °C are known as "Intermediate-temperature SOFCs" (IT-SOFCs).

In this Master's Thesis, a material of interest for IT-SOFC cathodes was studied: strontium cobaltite (SrCoO_3-δ). We worked on its nanostructuring and studied the possibility of retaining, at room temperature, the high-temperature cubic phase by reducing the crystallite size, without introducing dopants. This phase presents mixed ionic and electronic conductivity, which favors the electrode reaction because it increases the number of active sites. In addition, due to the high concentration of oxygen vacancies, it has a high diffusivity of O^2- ions, making it promising for applications. Two wet-chemical synthesis methods were tested, liquid-mix (also known as Pechini's method) and pore-wetting of polymeric membranes, which resulted in nanopowders and nanotubes of the compound, respectively. These synthesis routes were modified in order to reduce the final synthesis temperature and therefore avoid the growth of nanocrystals. The resulting materials were characterized by X-ray powder diffraction, scanning and transmission electron microscopy, and chemical analysis by energy dispersive spectroscopy

 Keywords: IT-SOFCs, SrCoO_3-d, cathode, mixed conductors, nanomaterials.