Theoretical optimisation of a SOFC composite cathode

International audience; 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 give 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 (dYSZ = dM = dg), the porosity (ε), the specific adsorption surface area (avads), the specific electrochemical surface area (avtpb), the pore diameter (dp) and the composition (εa). The proposed approach defines three independent parameters: ε, dg and εa. 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.