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The stability of the solid oxide fuel cell ͑SOFC͒ cathode material Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−␦ in CO 2 -containing atmospheres ͑4 ϫ 10 −4 ഛ pCO 2 /bar ഛ 5 ϫ 10 −2 ͒ is investigated by precision thermogravimetry ͑TG͒ and mass spectrometry ͑MS͒ as a function of temperature ͑20 ഛ T/°C ഛ 950͒. The desorption of O 2 and CO 2 from samples with different pretreatments is compared. Oxygen exchange at 300 Ͻ T/°C Ͻ 700 is significantly impaired by CO 2 -containing atmospheres. At 600°C the kinetics of carbonate formation in CO 2 -rich atmosphere is described by a linear-parabolic rate law. A pronounced dependence of the rate constant on the CO 2 content is suggested. Temperature cycles at 0 ഛ pCO 2 /bar ഛ 5 ϫ 10 −2 and pO 2 = 0.2 bar, which show effects due to oxygen exchange of the perovskite and due to CO 2 , are analyzed based on reference experiments in a CO 2 -free atmosphere, and under consideration of the TG-MS results. The decomposition temperature of the carbonate in contact with an atmosphere of pCO 2 = 5 ϫ 10 −2 bar amounts to 807°C. Complementary, the impact of carbonate formation on the oxygen exchange kinetics is investigated by conductivity relaxation measurements. A severe degradation of the surface oxygen exchange coefficient is observed after 3-10 days of exposure to ambient air. Regeneration and activation of the oxygen exchange kinetics is demonstrated after treatment in a CO 2 -free atmosphere at 825°C.The lack of cathodes with excellent efficiency for oxygen reduction at intermediate temperatures ͑IT͒, i.e., 600-750°C, still represents a major obstacle toward lowering the operating temperatures of solid oxide fuel cells ͑SOFCs͒. In this regard, Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−␦ ͑BSCF͒ was recently suggested as a highly promising candidate. 1-3 However, the successful implementation of BSCF will strongly depend on sufficiently low degradation rates under real operating conditions.Initially developed for oxygen permeable membranes, 4-6 BSCF is a mixed ionic-electronic conducting perovskite-type oxide with an exceptionally high oxygen nonstoichiometry. 7-12 A stable cubic perovskite structure was reported from room temperature up to 700-1000°C at 10 −5 Ͻ pO 2 /bar Ͻ 0.2. 10,13,14 However, a recent study indicated that a kinetically slow transformation from the cubic to the hexagonal modification may occur during cooling from high temperatures in air. 15 Under CO 2 -free laboratory conditions, excellent oxygen exchange properties of BSCF in the IT range have been reported. 7,16,17 Nevertheless, the long-term stability and robustness of the material, especially in CO 2 -containing oxidants such as ambient air, is questionable. Investigations performed on oxygen permeable BSCF membranes indicated a degradation of BSCF in atmospheres containing CO 2 or both CO 2 and H 2 O. 4,18 Early studies on the performance of IT-SOFC cells with integrated BSCF cathodes at 450-750°C showed that the presence of relatively small quantities of CO 2 ͑ഛ3 vol %͒ are sufficient to negatively affect the cell performance. 1,...

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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.

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