Chromium poisoning of SOFC cathodes: Influence of the LSM-YSZ structure

Abstract

The structure of the cathode strongly influences the degree and rate at which a (La0.8Sr0.2)0.98MnO3 (LSM) - 8% YSZ ceramic cathode assembly will exhibit "chromium poisoning" effects. Under cathodic polarization at 600°C (under flowing, "dry" air), porous films of LSM on dense YSZ in contact with a 430 stainless steel (430SS) contact will lose a significant degree of activity, especially at a high overpotential of -0.5 V vs. Pt/air. Under the same conditions, similar film thicknesses, and using the same materials, structurally graded cathodes composed of 50:50 LSM:YSZ topped with a 100% LSM contact layer exhibit high activity whether a Pt or 430SS contact is employed. Under the same conditions, patterned, fully dense, ca. 80 nm thick LSM films on dense YSZ also show relatively stable activity in the presence of either a Pt or 430SS contact. Electron microprobe analyses of the surfaces of the samples clearly show that Cr2O3 is deposited at and around the perimeter of the cathode film for all samples polarized using a 430SS contact. However, little if any Cr species were found beneath the porous 50:50 LSM:YSZ film, in contrast to the porous LSM film samples, where large concentrations of Cr species were observed at what ostensibly is the buried LSM-YSZ-air triple-phase boundary (TPB). Thus, the stability of the 50:50 LSM:YSZ composite samples seems to be linked to the absence of Cr species at the buried TPB. The same cannot be said for the dense LSM sample, as other factors must be contributing to its resistance to Cr poisoning.