Evaluation of cation migration in lanthanum strontium cobalt ferrite solid oxide fuel cell cathodes via in-operando X-ray diffraction

Abstract

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.