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

Abstract

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