Stability of the SOFC Cathode Material (Ba,Sr)(Co,Fe)O[sub 3−δ] in CO[sub 2]-Containing Atmospheres

The stability of the solid oxide fuel cell (SOFC) cathode material Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 − δ in C O 2 -containing atmospheres ( 4 × 10 − 4 ⩽ p C O 2 ∕ bar ⩽ 5 × 10 − 2 ) is investigated by precision thermogravimetry (TG) and mass spectrometry (MS) as a function of temperature ( 20 ⩽ T ∕ ° C ⩽ 950 ) . The desorption of O 2 and C O 2 from samples with different pretreatments is compared. Oxygen exchange at 300 < T ∕ ° C < 700 is significantly impaired by C O 2 -containing atmospheres. At 600 ° C the kinetics of carbonate formation in C O 2 -rich atmosphere is described by a linear-parabolic rate law. A pronounced dependence of the rate constant on the C O 2 content is suggested. Temperature cycles at 0 ⩽ p C O 2 ∕ bar ⩽ 5 × 10 − 2 and p O 2 = 0.2 bar, which show effects due to oxygen exchange of the perovskite and due to C O 2 , are analyzed based on reference experiments in a C O 2 -free atmosphere, and under consideration of the TG-MS results. The decomposition temperature of the carbonate in contact with an atmosphere of p C O 2 = 5 × 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 C O 2 -free atmosphere at 825 ° C .