Theoretical optimisation of a SOFC composite cathode

Theoretical calculations and experimental results have clearly demonstrated that a composite electrode should exhibit low activation polarisation by spreading the electrochemical active area within the volume of the electrode. The present modelling has been performed in order to Live a complete description of such an electrode structure as well as the processes occurring therein. A one-dimension flooded homogeneous model and a microscopic approach were used. The cathode was assumed to be composed of spherical particles of ionic (YSZ) and electronic conductors (M). The porous mixture of spherical grains was described as a face-centred cubic lattice. The microstructural parameters of interest include: the electrode thickness (L), the grain diameter (d(ysz) = d(M) = d(g)), the porosity (6), the specific adsorption surface area (av(ads)), the specific electrochemical surface area (av(tpb)), the pore diameter (d(p)) and the composition (e,). The proposed approach defines three independent parameters: E, dg and E,. For a given electrode composition, the results suggest that the nature of the rate determining step depends on grain size. An optimised porosity value is also determined. In case of a limitation by the charge transfer step, the model predicts that grading both composition and reaction sites is effective in increasing the electrochemical performances whereas grading porosity is not beneficial. (C) 2004 Elsevier Ltd. All rights reserved.