Development of Tailored Porous Microstructures for Infiltrated Catalyst Electrodes by Aqueous Tape Casting Methods

Recent activities in SOFC research have shown the technique of impregnating fine catalyst structures into porous scaffolds to be an extremely promising route of development. Electrodes made in this way have shown promising performance with improvements in power density, carbon tolerance, redox tolerance and potentially sulphur tolerance. To gain the best cell performance it is likely that a supported electrolyte design will be favoured with the scaffold being fabricated by economically viable and scaleable techniques such as tape casting.  

It is clear that in optimising the advantages offered by the infiltration technique there will be an obvious link between the morphology of the porous scaffold and the infiltrated catalyst which is supported upon it. At present a significant amount of tape casting techniques are based around non-polar solvent systems. The solvents used in these systems, such as methyl ethyl ketone and toluene, can be quite aggressive and so limit the type of polymeric based pore former which can be employed to create porosity. There are also significant health and environmental issues with the use of such systems, such as flammability in exhaust systems at larger production scales and they are also coming under increasingly tight legislation with regards to handling, worker exposure and disposal.  Minimising these make development of an aqueous system an attractive option.

However the application of aqueous systems is not without issue. Attaining stable particle dispersions can be trickier due to the polar nature of the solvent leading to increased dipole interaction between slurry constituents and increased sensitivity to changes in pH. Furthermore the slurry can be more susceptible to local environmental changes such as temperature and humidity resulting in inconsistencies in tape quality.  These factors have led to a slow uptake of aqueous systems in technical ceramic processing. However the potential benefits of aqueous systems for this application, such as a the potential for a far larger range of pore forming materials which may be employed to create specific pore morphologies, secondly the significantly reduced environmental burdens, such as exhaust handling, worker exposure and disposal, make their development for the production of porous scaffolds a valuable goal.

Recent and ongoing activities to develop such systems at University of St Andrews will be described in this paper. A number of pore forming systems are under investigation, acrylic microspheres, rice starch, styrene-acrylic latex and sodium alginate, the latter is of additional interest due to its ability to cross-link allowing gel-casting techniques to be explored.  Tapes have been manufactured using both YSZ and strontium and calcium doped lanthanum titanate (LSCT) as the ceramic phase with the pore formers used both individually and in combination with one another .  The rheological behaviour of the wet tapes was assessed by rotational viscometry along with assessment of casting, drying and green properties. Tapes were fired in air over various times and temperatures with fired microstructures assessed by SEM examination and image analysis using “Image J” analytical software. Areas of discussion will be effects of ceramic particle type and size, the size ratio of pore former to ceramic particle, pore former type and loading and how these affect the interaction with other tape constituents both on the behaviour of the tape during processing and on the final fired morphology. How these relationships may be better understood and what implications they have in designing optimised porous structures in the future will also be discussed.