MONTE-CARLO SIMULATIONS OF CONDUCTIVITY OF COMPOSITE ELECTRODES FOR SOLID OXIDE FUEL-CELLS

Composite electrodes for solid oxide fuel cells were modeled by three- dimensional resistor networks. The networks were generated on a comput er by identifying neighbors in either cubic lattices randomly occupied by electrolyte or electrode particles, or in random packings generate d by sequential deposition of such particles in random order. The resi stance between any two particle sites i and j were taken to be functio ns of the conductivities of the particles residing in site i and j and the periphery of the necks formed between them. An emphasis was put o n parameters believed to be relevant for cermets of Ni and yttria-stab ilized zirconia. The conductivity df the networks were calculated nume rically, and the results of the model are in good agreement with exper imental findings. A sharp transition from low to high conductivity occ urs at approximately 30 volume percent of electrode material for cubic lattices and for the random packings with uniform particle radii. In the bimodal random packings, this percolation threshold increases with increasing electrode-particle radius relative to the electrolyte-part icle radius. This is suggested as a possible explanation for cermet de activation under operation, since upon aggregation of electrode partic les the percolation threshold may increase past the given volume fract ion of electrode material in the composite.