SYNTHESIS AND CHARACTERIZATION OF NOVEL CATHODE MATERIALS FOR USE IN SOFCs

Synthesis and characterization of novel cathode materials for use in SOFCs. This master thesis has been carried out at Instituto de Ciencia de Materiales de Madrid and Universidad Complutense de Madrid, Spain; it focuses on the synthesis and characterization of new materials for Solid Oxide Fuel Cell (SOFC) cathodes. SOFCs are high-temperature electrochemical devices for the direct conversion of chemical energy into electrical and thermal energy with low environmental impact and high efficiency. In particular, the chemical compositions of the studied cathodes derive from a theoretical model based on the search for perovskite cathode materials, ABO3, with high electrocatalytic properties, high thermodynamic stability, and cheap and widely available chemical precursors. On top of that, this theoretical model proposed the O-p band center of the oxygen atom as a new design principle for SOFC cathodes as opposed to the more established d band center of the transition metal with which it forms the ionic-covalent bond responsible for the electronic and ionic conduction processes. A sol-gel method, namely the Citrate Method, has been used for the synthesis of SOFC cathodes. Among the six chemical compositions proposed as candidates, only three have been synthesized in a pure form. Moreover, one of the pure phases has been obtained following a modification of the chemical composition to remove the identified secondary phases and simultaneously provide enhanced transport properties. These samples have been then characterized by structural analysis (XRD, NPD, Rietveld Refinement Method), morphological analysis (Scanning Electron Microscopy), thermal analysis (Dilatometry, Thermogravimetry), transport analysis (Four-probe Method, Electrochemical Impedance Spectroscopy), and a Single-cell test (for density power generation). The first sample, synthesized in pure form at 1000 °C, is Ba0.75Sr0.25Fe0.875Ga0.125O3-δ. Instead, the second sample, synthesized in pure form at 1000 °C after changing its chemical composition from the one proposed by the model, is a SrCo0.9Sc0.1O3-δ cobaltite (the one of the model is SrCo0.5Sc0.5O3-δ). Problems associated with thermomechanical stability and transport properties have been identified for both samples. The latter are due to the dominant contribution of oxygen vacancies identified by neutron diffraction. The third sample, synthesized at 1150 °C, is a hexagonal perovskite BaFe0.875Re0.125O3-δ. This sample has been characterized in the same way as the previous candidates and experimental confirmation has been obtained that hexagonal structures, even if containing rhenium, are not suitable for SOFC cathodes. In conclusion, the theoretical model could not be confirmed experimentally. However, in this work three new cathode materials have been synthesized and fully characterized; these new materials can be modified to further improve their properties and can be considered to be a significant step forward in a better understanding of perovskite structures and properties.