Thermally Controlled Activation and Passivation of Surface Chemistry and Oxygen-Exchange Kinetics on a Perovskite Oxide

The state-of-the-art oxygen electrode materials used in solid oxide cells at 600–800 °C can deteriorate by gradual passivation of their surfaces related to changes the surface chemistry. We have recently observed unexpected recovery oxygen-exchange activity on La0.6Sr0.4FeO3 after short-term exposure high temperatures; a 10 h-long thermal treatment above 800 resulted 40-fold increase coefficient, kchem, from 1 × 10–5 4 10–4 cm/s. present work addresses underlying mechanism this improvement investigating chemical and morphological with respect history. Using repeated sequences treatments 1000 over weeks-long aging around 650 °C, we probed kinetics chemistry concurrently electrical conductivity relaxation X-ray photoelectron spectroscopy (XPS). coefficient decreases following Avrami-type but sharply increases each high-temperature treatment. An increased amount strontium [40% cation fraction; XSr/(XLa + XSr XFe)] is found all thermally treated samples (compared bulk nominal 20%). Hence, passivation/reactivation not due total Sr enrichment as such. By XPS, two different states are (labeled “lattice-bound” “secondary phase”). relative amounts these phases vary time, differently versus °C. “lattice-bound Sr” phase opposite show that it secondary phases, rather than amount, which main culprit for performance degradation. findings further supported observations form SrO, SrCO3, SrSO4 precipitates layers. Decomposition simultaneous dissolution back perovskite lattice rejuvenate lead significant exchange kinetics.