Ceramic Anode Materials with Nanoscale Electrocatalysts for Solid Oxide Fuel CellsPublic Deposited
The work presented in this dissertation focuses on ceramic anode materials for solid oxide fuel cells (SOFCs). The primary goal was to characterize the anode and relate the electrochemical behavior to the microstructure. The anode that was most extensively studied in this work was a composite of Gd0.10Ce0.90O1.95 (GDC) and La0.80Sr0.20Cr1-xRuxO3 (LSCrRu, x = 0.05 - 0.25). SOFCs with LSCrRu-GDC anodes achieved high power densities, > 500 mW/cm2, and low polarization resistances, 0.16 cm2, at 800ºC. When the cells were tested at constant current, the voltage increased significantly with time. In powders that were reduced in H2 at 800ºC, transmission electron microscopy images revealed the presence of nanometer-scale Ru particles on the surface of lanthanum chromite. Up to 300 h, the Ru particle diameter remained less than 5 nm. The high surface area of Ru, the catalyst phase, was determined to be the main cause for the time-dependent increase in voltage over time. Detailed studies were carried out to determine the effect of anode current collector thickness, Ru content, operating current and temperature on the behavior and overall performance of cells with LSCrRu-GDC anodes. The performance of the cells increased with increasing current collector thickness, Ru content, operating current and temperature. The rate at which the performance improved and reached a maximum or stable voltage also increased. However, the onset of voltage degradation occurred earlier for cells with higher Ru content and those operated at higher temperature. SOFCs with LSCrRu-GDC anodes were also tested with hydrocarbon fuel, fuels containing sulfur and reduction-oxidation cycling. The anode did not suffer significant damage immediately after reduction-oxidation cycling. Cells tested with fuel containing sulfur showed a relatively high degradation rate but the performance was fully recovered by reduction-oxidation cycling. Overall, the LSCrRu-GDC anode material yielded high performance SOFCs and is a good candidate as an alternative anode material. Similar anodes were tested and exhibited time-dependent behavior similar to cells with LSCrRu-GDC. However, the performance was unsatisfactory. Microscopy results showed that nano-catalyst particles precipitated from the host lattice upon reduction, demonstrating that the technique of nanometer-scale catalyst particle precipitation can be adapted to other materials.