Long-term degradation effects of combined Cr-and Si-poisoning on the promising IT-SOFC cathode materials La 0.6 Sr 0.4 CoO 3-δ and La 2 NiO 4+δ were investigated at 700°C in dry and humid atmospheres for subsequent periods of 1000 hours using dcconductivity relaxation measurements. Degradation-induced changes in chemical composition and morphology of the contaminated sample surfaces were studied by atomic force microscopy, X-ray photoelectron spectroscopy and scanning electron microscopy with energy and wavelength dispersive X-ray analysis. Upon exposure to humid, Cr-and Si-containing gas flows both materials exhibit a strong decrease of the chemical surface exchange coefficient of oxygen by a factor 110 and 40 for La 0.6 Sr 0.4 CoO 3-δ and La 2 NiO 4+δ , respectively, which can be attributed to the formation of Cr-containing crystallites on the degraded sample surfaces. Post-test analyses confirm large amounts of Cr accompanied by a Sr-enrichment within the first 600 nm of the surface of La 0.6 Sr 0.4 CoO 3-δ , indicating the decomposition of the perovskite phase by SrCrO 4 -formation. For La 2 NiO 4+δ the penetration depth of chromium is significantly less and Cr-traces up to a depth of up to 140 nm were determined by depth profiling. For both compounds silicon was found to spread in small patches across the entire sample surface as determined by elemental mapping analysis.
The influence of simultaneous Cr-and Si-poisoning on the oxygen exchange kinetics of La 2 NiO 4+δ was studied at 800°C in dry and humid O 2 /Ar atmospheres over a period of 4000 hours by means of dc-conductivity relaxation measurements. Chemical and morphological changes of the La 2 NiO 4+δ surface exposed to increasingly harsh ambient conditions were investigated using X-ray photoelectron spectroscopy (XPS) depth profiling, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDXS) and high-resolution scanning transmission electron microscopy with energy dispersive X-ray and electron energy loss spectroscopy (HR-STEM EDXS/EELS). In the presence of Cr/Si-sources and under dry conditions, La 2 NiO 4+δ shows good stability for more than 1000 hours. However, as soon as humidity is introduced into the system a strong decline of the oxygen exchange activity is observed. Post-test analyses reveal newly formed partially blocking Cr-and Si-rich secondary phases on the surface of La 2 NiO 4+δ which impede the surface exchange of oxygen.
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