Percolation effects in functionally graded SOFC electrodes

A solid oxide fuel cell (SOFC) composite electrode exhibits a superior performance compared to a single phase electrode since the electrochemically active zone is spread into its volume. A functionally graded composite electrode consisting of monosized spherical electrocatalyst and electrolyte particles is sintered numerically by the discrete element method (DEM). The electrochemical performance is evaluated by a resistance network approach using Kirchhoff's current law. In the network discretization each contact between two particles is substituted by a bond resistance defined by contact size and the type of materials in contact. The graded electrode is optimized by varying its composition at the electrolyte/electrode interface and the degree to which the composition decreases linearly towards the current collector/electrode interface. Regarding its electrochemical activity, the graded electrode does not perform significantly better than an optimized uniformly randomly mixed composite electrode but percolation of the graded electrode is improved. In order to demonstrate the importance of percolation effects, a novel better performing electrode is developed which contains electronically conducting particle chains arranged within a random packing of ionically conducting particles.