Evaluation of cation migration in lanthanum strontium cobalt ferrite solid oxide fuel cell cathodes via in-operando X-ray diffraction
Anode-supported SOFC button cells with LSCF-6428 cathodes were operated at various temperatures under constant current conditions for hundreds of hours in dry or humid (∼3% water) cathode air while continuously collecting 1 hour XRD scans throughout the test duration. Additionally, one cell in dry air was held at OCV, and 12% CO2 was added to the humid air for another cell. Long cumulative XRD count times allowed minor phases in concentrations of less than 0.1 wt% to be identified. In humid air, cell performance increased during the first couple of hundred hours. XRD measured a gradual shift of Fe-rich Fe,Co spinel peaks to lower angles while post-mortem SEM discovered nano-nodules on the LSCF surfaces. When 12% CO2 was added to humid air, performance degradation was observed after the initial conditioning response, unlike in tests without CO2, which were stable after conditioning. XRD also measured a shift in the peaks of the major LSCF phase in the test with CO2 indicative of some degree of gradual decomposition. Cells tested at constant current in dry air exhibited fast initial degradation that decelerated over time, while the cell held at OCV maintained a stable voltage. XRD of the cathodes held at OCV at 750 °C and operated at constant current at 700 °C in dry air measured a slower shift of the Fe-rich spinel peaks to lower angles than was seen in tests in humid air, along with a shift in the peaks of a Co-rich Fe,Co spinel to higher angles. Post-mortem SEM of cells exposed to dry air discovered smaller nano-nodules on the LSCF surfaces than had been present in cells tested in humid air. Cells tested at constant current in dry air at 750 °C and 800 °C did not exhibit any gradual changes in the XRD patterns nor were nano-nodules discovered.