Oxygen Nonstoichiometry and Thermo-Chemical Stability of Perovskite-Type La_0.6Sr_0.4Co_1-yFe_yO_3-δ(y= 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) Materials

Abstract

The oxygen nonstoichiometry of La0.6Sr0.4Co1-yFeyO3-δ (LSCF) mixed-conducting perovskite oxides strongly depends on the iron content: the cobalt-rich compounds (y = 0, 0.2, 0.4) were previously reported to show a sharp decrease in oxygen content with reducing atmosphere, while the oxygen vacancy concentration increased steadily for the iron-rich compounds (y = 0.6, 0.8) with a plateau at intermediate oxygen partial pressures for y = 1. Here we measured the oxygen content of LSCF with y = 0.5 by thermogravimetry and coulometric titration as a function of oxygen partial pressure and temperature. No transitional behavior could be observed for La0.6Sr0.4Co0.5Fe0.5O3-δ, the oxygen nonstoichiometry showed the same trend as for the cobalt-rich compounds. While the cobalt-rich compounds exhibit a metallic band structure, a semiconducting model fits the data best for the iron-rich compounds. Except for y = 1, the thermo-chemical stability of LSCF was little affected by the iron dopant content, all compounds (0 ≤ y ≤ 0.8) decomposed at similar oxygen partial pressures.

Nonlinear Impedance Analysis of La0.4Sr0.6Co0.2Fe0.8O3-δThin Film Oxygen Electrodes

“…Figure 5 compares the measured p O2 dependence of the thin film V SC to that predicted by the p-type, metallic, and empirical thermodynamic models shown in Figure 1. 4 While the empirical model agrees well with the observed capacitance, neither the p-type nor metallic model accurately captures the measured V SC's absolute value and p O2 dependence.…”

“…• C. 4 Comparing the two sample geometries, the specific surface area (SSA) for a 20 nm thick, 200 μm diameter film of LSCF is ∼8 m 2 /g, while we estimate the SSA of samples measured Kuhn et al to be somewhere between 2∼5 m 2 /g (based on comparison/extrapolation of BET measurements made by Bishop et al at higher temperatures). 4,9 These surface areas are similar enough that it is possible that both the thin film and the "bulk" samples exhibit enhanced nonstoichiometry near the surface.…”

“…4,[9][10][11] In the limit of high B-site iron content (y > 0.6), workers have interpreted this δ 0 − p O2 − T relationship using a ptype point-defect model originally developed for LSF (LSCF, with y = 1), 12 where electrons are localized on iron centers of discrete charge and defects are considered dilute.…”

“…[2][3][4] Electrochemical measurements on LSCF-6428, including conductivity relaxation, 3,5 coulometry, 4,6 and impedance techniques 2,7,8 have underscored the importance of bulk oxygen vacancies in governing rates of O 2 reduction and evolution. However, a clear understanding of how these vacancies contribute to the mechanism and rate-limiting step(s) of O 2 exchange remains elusive.…”

“…4,[9][10][11] In the limit of high B-site iron content (y > 0.6), workers have interpreted this δ 0 − p O2 − T relationship using a ptype point-defect model originally developed for LSF (LSCF, with y = 1), 12 where electrons are localized on iron centers of discrete charge and defects are considered dilute. 4,9 For Co-rich compositions (y < 0.4), a metallic model used to describe LSC (LSCF, with y = 0) yields better agreement. 4 While La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ exhibits semiconducting behavior over a wide range of p O2 and temperature, the observed δ 0 − p O2 relationship deviates substantially from both p-type and metallic models under oxidizing conditions ( p O2 > 10 −4 , lower temperature).…”