Effects of A/B-ratio in strontium doped lanthanum cobaltite

EFFECTS OF A/B-RATIO IN STRONTIUM DOPED LANTHANUM COBALTITE Martin Søgaard a;b, Peter V. Hendriksen a, Dorthe Lybye a, Mogens Mogensen a and Torben Jacobsen b aRisø National Laboratory, Materials Research Department, Frederiksborgvej 399, DK-4000 Roskilde, Denmark bTechnical University of Denmark, Department of Chemistry, DK-2800 Kgs. Lyngby, Denmark ABSTRACT The electrical conductivity of (La0.85Sr0.15)sCoO3-δ, s = 0.98; s = 1.00; s = 1.02 has been measured. The electrical conductivity of the composition s = 0.98 was significantly higher than those of the other two materials at temperatures below 400°C. The chemical diffusion coefficient, DChem, and the apparent surface exchange coefficient, kEx, were measured using electrical conductivity relaxation. The compositions s = 1.02 and s = 0.98 had within the uncertainty the same surface exchange coefficient. kEx was approximately 3 times larger for the composition s = 0.98. It is speculated that the Co-rich material has a surface layer richer in cobalt, which improves the electrochemical activity. DChem, was the same for the compositions s = 0.98 and s = 1.00, whereas it was significantly lower for the s = 1.02 material by approximately 25%. The activation energy for the vacancy diffusion coefficient, DV, was calculated to 75.2 kJ mol-1 for the s = 1.02 composition. Oxygen permeation measurements were carried out on the s = 1.02 composition using an oxygen-pump. The results were in excellent agreement with fluxes calculated from the measured DChem and kEx values using the Wagner equation combined with surface exchange reactions.