Long-term stability of the IT-SOFC cathode materials La0.6Sr0.4CoO3 − δ and La2NiO4 + δ against combined chromium and silicon poisoning

• Long-term oxygen exchange kinetics of La0.6Sr0.4CoO3 − δ and La2NiO4 + δ at 700 °C
• Segregation of Sr (La0.6Sr0.4CoO3 − δ) and La (La2NiO4 + δ) to the surface
• Strong degradation due to Cr- and Si-poisoning
• Cr-rich crystallites on degraded surfaces confirmed by AFM and XPS
• Small electrically insulating patches of silicon confirmed by SEM-EDXS

Long-term degradation effects of combined Cr- and Si-poisoning for the promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode materials La0.6Sr0.4CoO3 − δ (LSC) and La2NiO4 + δ (LNO) were investigated at 700 °C in dry and humid atmospheres for subsequent periods of 1000 h by using dc-conductivity relaxation measurements. Changes in the chemical composition and morphology of the contaminated sample surfaces were studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy and wavelength dispersive X-ray analysis and by atomic force microscopy (AFM). Upon exposure to dry and humid gas flows containing Cr- and Si-species both materials exhibit a strong decrease of the chemical surface exchange coefficient of oxygen by a factor of 110 and 40 for La0.6Sr0.4CoO3 − δ and La2NiO4 + δ, respectively, which can be attributed to the formation of Cr-containing crystallites on the degraded sample surfaces. AFM-measurements reveal faceted crystallites of lateral extensions of up to 2 μm and heights of up to 500 nm on the surface of La0.6Sr0.4CoO3 − δ. XPS-analyses confirm the presence of high levels of Cr and Sr in these crystals, indicating the decomposition of the perovskite phase through SrCrO4-formation. The significantly smaller crystallites that are found on the surface of La2NiO4 + δ also contain Cr, as well as elevated levels of La. For both compounds, silicon is found to spread in small patches across the entire sample surface, as determined by EDXS elemental mapping analysis.