SOLID OXIDE FUEL CELL CATHODES: EXPERIMENTS ON MATERIAL STABILITY AND NOVEL TEST SYSTEM DEVELOPMENT
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Cost reduction is driving the development of solid oxide fuel cell (SOFC) technology for operations at lower temperatures (500 °C -700 °C) so as to allow the usage of cheaper balance-of-plant components and enhance the durability of the stack. However, lower temperatures adversely affect the overall performance of the cell and most notably that of the cathode. Development of cathode material exhibiting high performance at lower temperature is one of the goals of SOFC research and development. This thesis work is concerned with two distinct aspects of SOFC cathode development – one concerned with the stability of a recently studied cathode material, La0.5Ba0.5CoO3-δ (LBC), in CO2 containing atmosphere and another concerned with the development of methods for fabrication of reproducible electrodes and rapid electrochemical testing thereof. The study of reaction between LBC and CO2 was carried using a combination of thermogravimetric analysis (TGA), ex-situ X-Ray Diffraction (XRD) of products from TGA experiments and in-situ high-temperature XRD of LBC-CO2 mixtures. The mass change observed during TGA was combined with ex-situ XRD analyses of solid material phases to deduce the overall reactions. In-situ XRD measurements allowed for studying the intermediate reaction products. Isothermal studies at different temperatures in pure CO2 yielded kinetics for the reaction between LBC and CO2. Overall reaction pathway was proposed from these data. In addition, experiments were carried out to determine the thermodynamic carbonate formation temperature at a fixed CO2 partial pressure (pCO2). From the thermodynamic analysis of carbonate formation temperature at three different pCO2, the standard state enthalpy and entropy for the carbonate formation reaction were determined. This work is the first known in-depth study of reaction between LBC and CO2. The second distinct contribution of this thesis is the demonstration of a test system framework for fabricating reproducible miniature electrodes and rapid testing thereof. In particular, inkjet printing method was used to create well defined geometry of porous electrode and micro-contact impedance spectroscopy setup (MICS) was used to study the electrode electrochemical kinetics. The feasibility with electrode fabrication and electrochemical testing methods were demonstrated through the study of multiple silver miniature electrodes printed on single chip made of yttria-stabilized zirconia single crystal wafer.