(214a) Design and Synthesis of Optimized Sofc Structures

State-of-the-art Solid Oxide Fuel Cell (SOFC) designs have an Yttria-stabilized Zirconia (YSZ) electrolyte, a Lanthanum-Strontium Manganate cathode and a Ni-YSZ composite anode. SOFC charge transfer reactions are generally assumed to occur at triple-phase-boundaries (TPB's) between electrode, electrolyte and gas phase. Equilibrium space-charge-accumulation lengths suggest the width of the TPB to be in the order of 10 nm. The efficiency of SOFC conversion is most likely affected by:

-Resistive losses in the bulk electrolyte, including constriction effects due to the fact that charge transfer occurs at narrow TPB's only.

-Surface charge transfer at the TPB.

A TPB of only 10 nm thick naturally leads to the suggestion that typical dimensions of cathode and anode material and porosity should also be of the order of 10 nm to obtain the best possible electrode efficiency. In the ideal electrode/electrolyte interface there are many active triple phase boundary contacts, possibly percolating into the porous electrode structure. Further away from the electrolyte interface, both cathode and anode may coarsen to allow for an optimum strength, gas transport and electron conductivity. An electrolyte thickness of 10 nm is achievable by stretching the limits of what is possible with high definition colloidal processing. Optimized structures as described will result in much better performance at lower temperatures but also require much better control of particle morphology. A graded interface between the porous electrode structure and the dense electrolyte structure is likely to result in better adhesion and thermo-chemical stability during lifetime. Design and realization of optimized SOFC structures is a major enterprise that requires a concerted effort of several research groups with special attention for the relation between morphology-properties-synthesis. This paper discusses quantitative descriptions of SOFC transport phenomena that can be used as a starting point to develop optimized cell designs. A finite element description is presented of solid state transport of charge carriers, based on Langmuir lattice statistics, Onsager linear irreversible thermodynamics and the Poisson equation. In addition, unconventional strategies will be presented for realizing the foreseen, ideal graded structures with thin electrolytes. The example is presented of synthesis and consolidation of 10 nm spherical zirconia particles by modified emulsion precipitation. The particles are formed in the confinement of aqueous solution droplets in a normal emulsion, followed by removal of water and subsequent steric stabilization of the particles in the remaining oil medium. The particles, surrounded by their organic polymer stabilizer, are deposited as a thin filtration layer on a macro-porous substrate, followed by removal of the polymer by O₂ plasma oxidation. This treatment results in complete densification of the particle layer at, apparently, near room temperature conditions. The final crystal structure is then formed by crystallization around 600ºC. This temperature is sufficiently low to preserve originally designed 10 nm details.