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

Abstract

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,...

Electrode materials: a challenge for the exploitation of protonic solid oxide fuel cells

“…More recently, owing to its high oxygen diffusivity, Ba 1−x Sr x Co 1−y Fe y O 3−δ (BSCF) series has been proposed as a valid cathode material for doped-ceria electrolyte [22]. However, the chemical stability of BSCF in the presence of CO 2 and water vapor was questioned [23], indicating limits for practical applications. New materials showing promising performance, such as double perovskite or bismuth ruthenate pyrochlore, have been proposed as alternative cathodes [24,25].…”

Oxygen tracer diffusion and surface exchange kinetics in Ba0.5Sr0.5Co0.8Fe0.2O3−δ

“…

AbstractHigh density Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) samples with the degree of closed porosity less than 7 % were annealed in 18

…”

“…The first is isotope exchange in an 18 O-enriched atmosphere followed by secondary ion mass spectroscopy (SIMS) depth profiling by sputtering or line scanning [3]. The second is the isotope exchange method with gas phase analysis using a static circulation setup [8].…”

Oxygen permeation flux through 10Sc1YSZ-MnCo 2 O 4 asymmetric membranes prepared by two-step sintering

“…The materials having adequate chemical stability unfortunately typically do not provide sufficient oxygen transport. For example, compounds in the Ba1−xSrxCo1−yFeyO3−δ (BSCF) and La1−xSrxCo1−yFeyO3−δ (LSCF) series have attracted large interest due to their good electronic and ionic conductivity [7][8] [9][10] [11], however, their lack of chemical stability in the presence of CO2 [12][13] [14][15] [16] [17] (especially for the alkaline earth rich and cobalt rich compounds) clearly show the limitations of these compounds for oxy-fuel applications involving direct integration. Indeed, for materials containing La and alkaline earth metals the stability in CO2 and SO2 is low because of the potential formation of La/alkaline carbonates and sulphates.…”