The influence of pore and Ni morphology on the electrical conductivity of porous Ni/YSZ composite anodes for use in solid oxide fuel cell applications

A wide range of porous Ni–YSZ composite microstructures was produced by conventional tape casting and co-sintering using a variety of starting powders including Ni, NiO, graphite and Ni-coated graphite. The graphite additions were added to produce controlled levels of porosity in the final sintered and reduced anode. All materials indicated classical conductivity percolation behaviour with increasing Ni loadings. However, the percolation threshold at which electrical conduction became measurable was lowest for anodes made with Ni-coated graphite and highest for anodes containing large amounts of porosity introduced by large additions of graphite. Sintered and reduced anodes possessed large scale porosity introduced by the graphite additions and a finer scale porosity generated by the incomplete sintering of the Ni/YSZ powder network. A model was developed for predicting the influence of large scale porosity on conductivity and agreed well with the experimental results. The analysis indicates that fine scale porosity will have a more detrimental impact on conductivity compared to a coarse porous structure.