Oxide diffusion in innovative SOFC cathode materials

Abstract

Oxide diffusion was studied in two innovative SOFC cathode materials, Ba₂Co₉O₁₄ and Ca₃Co₄O_9+δ derivatives. Although oxygen diffusion was confirmed in the promising material Ba₂Co₉O₁₄, it was not possible to derive accurate transport parameters because of an oxidation process at the sample surface which has still to be clarified. In contrast, oxygen diffusion in the well-known Ca₃Co₄O_9+δ thermoelectric material was improved when calcium was partly substituted with strontium, likely due to an increase of the volume of the rock salt layers in which the conduction process takes place. Although the diffusion coefficient remains low, interestingly, fast kinetics towards the oxygen molecule dissociation reaction were shown with surface exchange coefficients higher than those reported for the best cathode materials in the field. They increased with the strontium content; the Sr atoms potentially play a key role in the mechanism of oxygen molecule dissociation at the solid surface.