Analysis of Electrochemical Performance and Degradation of Solid Oxide Fuel Cell Cathodes

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This dissertation presents research on solid oxide fuel cell (SOFC) cathodes. It specifically covers two main topics: the electrochemical performance of porous two-phase composite cathodes, and the degradation mechanisms of porous single-phase mixed conducting cathodes. Current-voltage measurement and electrochemical impedance spectroscopy were used extensively to evaluate the cell performance. The impact of microstructure on electrode resistance was studied by using focused ion beam-scanning electron microscopy (FIB-SEM), utilizing both two-dimensional (2D) and three-dimensional (3D) images. To investigate the mechanisms of electrode degradation during long-term operation, FIB-SEM 3D tomography was used to analyze the microstructural evolution, and a novel selective chemical etching method was developed to examine the surface compositional changes.', '\tDensification of Y0.16Zr0.92O2-δ (YSZ) electrolyte at a reduced temperature, 1250 °C, was achieved by utilizing Fe2O3 as sintering aid, allowing single-step co-firing fabrication of anode- and cathode-supported SOFCs consisting of (La0.8Sr0.2)0.98MnO3-δ (LSM)-YSZ cathodes, YSZ electrolytes, and Ni-YSZ anodes. The resultant LSM-YSZ cathode had higher polarization resistance than a cathode that was fired separately at an optimized temperature of 1175 °C. Analysis of FIB-SEM images showed that co-firing caused more sintering and coarsening than in the optimally-fired LSM-YSZ, reducing triple phase boundary (TPB) density. It was found that reducing the firing time slightly decreased cathode polarization resistance and increased cell power output. To compensate for the over-sintering of LSM-YSZ during co-firing, (Sm0.5Sr0.5)CoO3 (SSC) was infiltrated into LSM-YSZ cathodes in the cathode-supported SOFCs in order to enhance the TPB density. However, coarsening of the nano-scale SSC was observed during the preliminary life tests, leading to significant cell degradation. On the other hand, cathode-supported SOFCs allowed the use of thinner anodes, and hence exhibited high fuel utilization as well as high steam utilization when operating under electrolysis mode.', '\tThe study of degradation mechanisms was conducted on single-phase mixed conducting electrodes, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). Temperature-induced degradation was investigated on both LSCF symmetric-electrode cells and microtubular full cells consisting of LSCF cathodes. It was found by FIB-SEM 3D tomography that particle coarsening only occurred at sufficiently high temperatures,  950 °C. The performance degradation observed at temperatures < 950 °C was attributed to Sr segregation to particle surfaces, which inhibited the oxygen surface exchange process. A selective chemical etching method with inductively couple plasma-optical emission spectrometry (ICP-OES) detection was developed to quantitatively analyze the Sr surface segregation on the porous LSCF electrodes. This method was also found to be applicable to systems other than LSCF, such as quantifying Ba surface segregation in La0.5-XPrxBa0.5CoO3-δ (LPBC) cathodes. Additionally, degradation during reversing current operation was examined on LSCF symmetric-electrode cells. Results showed that the primary source of the degradation was also associated with Sr surface segregation, with a smaller contribution from current-induced coarsening.

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  • 09/30/2019
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