Evaluation of Cathode Materials for Low Temperature (500-700C) Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFC) have gained a great deal of interest, due to their potential for high efficiency power generation and ability to utilize hydrogen fuel, as well as various hydrocarbon-based fuels. A recent trend in SOFC development has been towards lower operating temperatures (500-700°C), which can substantially reduce the cost and complexity of the system. This thesis presents an investigation into state of the art Ba- and La- based cathode materials for use in low temperature (500-700°C) solid oxide fuel cells. Synthesis of A-site deficient [A=0.97] Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) was accomplished by the EDTA-citrate method. This powder was compared against a commercially available La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) powder used as a reference. The phase structure and morphology of the powders was characterized using XRD analysis, SEM imaging, and BET surface area analysis. Thermal expansion and thermogravimetric measurements were performed to characterize the chemical and mechanical properties of the powders up to 800°C. The powders were also found to develop a secondary surface phase after exposure to ambient air at room temperature for as little as 100h. This phase was attributed to surface A-site carbonate formation due to exposure to atmospheric CO2. Symmetric cells were prepared and electrochemically characterized using AC impedance spectroscopy in a range of 500-700°C, and a PO2 of 1.0-0.01 atm. The BSCF was found to have a ~7x lower ASR than LSCF at 600°C (0.4 Ωcm2 vs. 2.95 Ωcm2 at 600°C). The impedance spectra were modeled using an equivalent circuit model, and three distinct processes were identified. These processes were ultimately attributed to a charge transfer process, a surface exchange process, and a gas diffusion process. Using the calculated PO2 and temperature dependencies, a model was proposed to explain the mechanism behind the oxygen reduction reaction on the BSCF cathode. It was found that below 600°C, the primary mechanism is partial reduction of the gaseous oxygen, possibly followed by surface diffusion. Above 600°C, the gaseous oxygen is completely reduced and incorporated into the bulk of the cathode, where it undergoes bulk diffusion to the cathode/electrolyte interface.
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