Influence of porous composite microstructure on the processing and properties of solid oxide fuel cell anodes

In this investigation porous Ni/YSZ composite SOFC anode structures were produced by adding separate Ni and graphite (pore former) particles and/or by adding Ni coated graphite powder to a tape casting suspension. The porous volume increased with an increase in graphite content in a similar manner regardless of how the graphite was added (i.e. separately or as the core of a coated powder). The sintering shrinkage of the anodes decreased with an increase in the Ni content. Both sintering behaviour and the CTE of sintered and reduced anodes were similar regardless of how the Ni was added. However for a given Ni content the electrical conductivity was dramatically improved when Ni was added as a coating on a graphite powder. This behaviour was explained on the basis of an “effective” Ni content, VNieff, which includes the graphite portion of the coated particle. In the green tape, VNieff controls the creation of a percolating network of Ni. However, when the graphite burns away it leaves a percolated Ni network at a much lower volume fraction than would be required from a dense Ni particle.