Finite-element modeling of idealized infiltrated composite solid oxide fuel cell cathodes

The polarization resistance of idealized, branched, composite cathodes was modeled using a two-dimensional finite element calculation. The model structures consisted of micrometer-scale columns and nanoscale branches of ionically conducting materials that were coated with a mixed-conducting material. The structures approximate an ionic-conductor matrix infiltrated first with the same ionic-conductor material and then with a mixed conductor. Increasing the length of the ionically conducting nanobranches, and hence, the surface area of the infiltrated mixed conductor, resulted in a factor of ∼10 polarization resistance decrease compared to mixed-conductor-coated columns without ionically conducting nanobranches. For many solid oxide fuel cell relevant temperatures (500-900°C), cathode geometries, and materials, the cathode resistance was limited by surface oxygen exchange and hence, was inversely proportional to the mixed-conductor surface area. However, for cathode columns or branches with large enough aspect ratios, ionic-conduction losses also limited the polarization resistance. The characteristic length of nanostructured cathodes was found to depend on the cathode surface area ratio in addition to the traditional bulk diffusion constant to surface exchange constant ratio (D/k). Lastly, the effects of materials properties, particularly ionic conductivity and surface resistance, were investigated and discussed for common cathode materials.