Measurement of oxygen surface exchange kinetics and solid-state diffusive transport in mixed ionic and electronic conducting perovskites using conductivity relaxation

The kinetics of oxygen reduction and incorporation in mixed ionic and electronic conducting (MIEC) perovskite materials was investigated using the electrical conductivity relaxation technique. The transport parameters studied were the chemical surface exchange coefficient, kchem, and the chemical diffusion coefficient, D˜.; A novel technique using conductivity relaxation on porous samples was carried out on La0.6Sr0.4CoO3-delta, La 0.8Sr0.2FeO3-delta and La0.5Sr 0.5CoO3-delta. The experiments were conducted on porous bodies with average particle size much less than the material critical length over a given temperature range. This allows an estimation of k chem at these temperatures that is not influenced by chemical diffusion. The experiments were conducted at temperatures from 250 to 800°C and over the oxygen partial pressure, pO2 , range from 0.02 to 0.21 atm. Conductivity relaxation was also carried out on dense samples of thickness on the order of the material critical length at these temperatures, where the transport kinetics are dictated by both D˜ and kchem.; The results from porous samples revealed that over a certain temperature range the measured kchem increased with decreasing temperature. This is in contrast to the results from dense samples, where kchem is thermally activated and decreases with decreasing temperature. The unusual temperature dependence of kchem from porous samples is indicative of an exothermic process, which is consistent with gas adsorption on a surface. The kchem over the temperature range measured could then be given, at least qualitatively, in terms of the oxygen surface coverage as described by an adsorption isotherm. Conductivity relaxation experiments on porous samples investigating the competitive adsorption of gases using O2-He and O2-CO2 gas mixtures were found to support this hypothesis. However, the extent of the influence of gas diffusion into the porous interstices of the porous samples on the measured relaxation was not clear. Conductivity relaxation on dense samples showed that significant uncertainties exist in both k chem and D˜ obtained by fitting to a single set of conductivity relaxation data. A novel technique in which the kinetics of re-equilibration is predominantly dictated by a single parameter is used to accurately measure both D˜ and kchem in dense samples. The results highlight the importance of studying the kinetic transport parameters, independently of each other.