Page 27 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

8 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

vapour was passed along the outside of the tube, while water vapour passed
down the inside. This deposited a layer of yttria-doped zirconia which first
blocked the pores at the surface of the substrate tube and then subsequently grew
to about 40 pm in thickness to form the electrolyte layer [lo]. The interconnect
strip could also be formed from magnesia-doped lanthanum chromite by the same
principle [ 111. Although tubular SOFCs give good electrochemical performance,
the process is lengthy and expensive when compared with tape casting. Also,
the heavy tubes cannot be heated rapidly and require a 4-6 hour start-up time.
Tape casting was originally used to make thin tape materials for electronic
applications 11121 especially using organic solvents. A slurry of the YSZ powder,
with solvent and dispersing agent, for example methyl ethyl ketone/ethanol
mixture with KD1 (Uniqema), was ball milled for 24 hours to finely grind the
particles and remove agglomerates [13]. Then a polymer and plasticiser mixture
was prepared by milling polyethylene oxide and dibutylphthalate with the
solvent, mixed with the particle dispersion, and followed by further ball milling.
After filtering and vacuum deairing, the slurry was tape cast on a polymer film
and dried for 3 hours before firing at 1300°C.
Water-based tape casting is much more desirable than the organic solvent
system for environmental reasons, and this has been developed by Viking
Chemicals who prepared their own pure zirconia by solvent extraction
techniques [14]. The calcined zirconia powder was bead milled in water with
ammonium polyacrylate solution (Darvan 82 1 A, Vanderbilt) to give a very
stable dispersion. To this suspension, a solution of purified ethyl cellulose was
added, followed by filtering and deairing. This was tape cast onto polymer film,
then dried and fired at 1450°C. Similar dispersions have been screen printed onto
tape cast anode tapes made by a similar casting procedure to give co-fired
supported electrolyte films of reduced thickness which gave enhanced current
capacity [ls]. Such results were originally reported by Minh and Horne [16]
who used the tape calendering method which is similar to tape casting but with a
plastic composition [17]. They also corrugated the plates and made monolithic
designs by sticking corrugated pieces together in a stacked structure. Of course,
the problem with flat plate designs is the thermal shock which prevents rapid
heating or cooling. This was a particular problem for monolithic structures
which cracked very easily when made more than a few centimetres in length.
To prevent the thermal shock problem, smaller diameter tubes have been
produced by extrusion as described in Chapter 8 [18]. Again, these
compositions were prepared by mixing zirconia powder with water and
polymer, for example polyvinyl alcohol. Extrusion through a die gave tubes
which could be as little as 2 mm in diameter and 100-200 pm in wall thickness,
sinterable at 1450°C.

1.6 Electrode Materials and Reactions

Having produced the YSZ electrolyte membrane, it is then necessary to apply
electrodes to the fuel contact surface (anode) and the oxidant side (cathode).