Stabilization of the cubic perovskite in the system La1-xBa xCo1-yFeyO3-δ (0.7 ≤ x ≤ 0.9) and its electrochemical performance as cathode materials for intermediate-temperature solid oxide fuel cells

Abstract

The effects of the substitution of Co by Fe on the crystal chemistry and electrode reaction of the system La1-xBaxCo 1-yFeyO3-δ (0.7 ≤ x ≤ 0.9), are investigated. The incorporation of Fe stabilizes the cubic perovskite and suppresses the structural transformation of La1-xBa xCoO3-δ from a metastable cubic perovskite to a hexagonal phase below 900 C. The linear expansion decreases with the replacement of Co by Fe. The lowest expansion coefficient value, α ∼ 20.5 × 10-6 K-1, was obtained for the samples with Fe content y = 0.6. AC impedance spectroscopy measurements on symmetrical cells reveal the presence of an intermediate (IF) and a low (LF) frequency contributions at T ≥ 600 C. The LF arc varies with the pO2, at 700 C, according to a power law with exponent n = -1, indicating oxygen diffusion through the porous of the electrode. The minimum Rp value (0.6 cm2 at 600 C) was obtained for La0.3Ba0.7Co0.6Fe 0.4O3-δ. At constant temperature, T = 750 C, only the IF contribution varies at a rate of 3 × 10-4 cm2 h-1, while the LF contribution remains constant. Experimental data suggest the growth of the IF arc is caused by the mismatch in the expansion coefficients of the electrode and electrolyte.