Stabilization of the cubic perovskite La0.8Pr0.2BaCo2-yO6-6 by introducing Co deficiency and its effect on thermal expansion, electrical conductivity and electrochemical properties

The effects of B site deficiency on crystal structure, phase relationships, surface and high temperature properties of the cubic perovskite La0.8Pr0.2BaCo2-yO6-6 (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-yO6-6 (0.00 <= y <= 0.05) electrodes were as low as 0.05 O cm(2) at 600 degrees C. The impedance spectra as a function of T and pO(2) reveal two arcs reproduced with (R, CPE) elements. The variation of the polarization resistance (Rp) with pO(2) was analyzed by using the power law. For the low fre-quency contribution, 0.1 <= v <= 3 Hz, the parameter n similar to-1, corresponds to the oxygen molecule diffusion through the porous of the electrode. For the contribution at higher frequencies, 5 <= v <= 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, 1dRp dt similar to 6 x 10(-4) h(- 1), was Rp found to be slightly slower than that obtained for the stoichiometric sample.