Stabilization of the cubic perovskite La0.8Pr0.2BaCo2-yO6-δ by introducing Co deficiency and its effect on thermal expansion, electrical conductivity and electrochemical properties

Abstract

The effects of B site deficiency on crystal structure, phase relationships, surface and high temperature properties of the cubic perovskite La0.8Pr0.2BaCo2-yO6-δ (0.00 ≤ y ≤ 0.10) have been studied. From X-ray diffraction data the single cubic perovskite phase was obtained for the Co range 0.00 ≤ y ≤ 0.05. X-ray photoelectron spectroscopy (XPS) measurements indicated the presence of Co3+ and Pr3+ and confirmed a lower Co content for the y = 0.05 sample. High temperature X-ray diffraction and dilatometry revealed the formation of secondary phases related to CoOx compounds. The introduction of Co deficiency (y = 0.02 and 0.05) at the B site prevents the formation of these secondary phases while the crystal structure remains cubic. The polarization resistance values for the oxygen reduction reaction in La0.8Pr0.2BaCo2-yO6-δ (0.00 ≤ y ≤ 0.05) electrodes were as low as 0.05 Ω cm2 at 600°C. The impedance spectra as a function of T and pO2 reveal two arcs reproduced with (R, CPE) elements. The variation of the polarization resistance (Rp) with pO2 was analyzed by using the power law. For the low frequency contribution, 0.1 ≤ ν ≤ 3 Hz, the parameter n ∼ -1, corresponds to the oxygen molecule diffusion through the porous of the electrode. For the contribution at higher frequencies, 5 ≤ ν ≤ 500 Hz, the parameter n was in the range -0.17 ≤ n ≤ -0.33, which indicates the main rate-limiting process is the oxygen exchange at the surface of the electrode. The rate of change of Rp with time for the Co-deficient electrodes, 1RpdRpdt∼6×10−4h−1, was found to be slightly slower than that obtained for the stoichiometric sample.