Page 165 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

142 High Temperuture Solid Oxide Fuel Cells: Fundamentals, Design and Applications

solubilities of YSZ and SDC and enhancement effects of water vapour on the
cathode reaction [66].
To minimise/avoid Cr poisoning, several approaches have been tried. These
include coating a dense, electrically conducting oxide, such as cathode-related
perovskites (LSM) or lanthanum chromite-based oxides, on metallic
interconnects, and utilisation of Cr getters. Cr containing species easily react
with other oxides (particularly basic oxides) so that such basic oxides can act
as Cr getters. La203 can be a strong Cr getter, but this material is hard to
handle because of its hygroscopic nature. Lanthanum cobaltite can also be a Cr
getter because the stabilisation energy of LaCr03 is larger than LaCo03.

5.5 Fabrication of Cathodes
In general, cathodes are made by powder processing routes. Cathode material
powders are either made by solid state reaction of constituent oxides or high
surface area powders are precipitated from nitrate and other solutions as a gel
product, which is dried, calcined and comminuted to give crystalline particles in
the 1-10 pm size range.
Fabrication methods for cathodes largely depend on the cell design. For
cathode-supported tubular cells, porous cathode tubes are first extruded and
then sintered at high temperatures [9,69,70]. After sintering, appropriate
porosity and strength develop. State-of-the-art cathode tubes are more than 1.5
m in length. The most important issue when depositing other cell component
layers on the cathode tubes is the adherence of the dense electrolyte and
interconnect. When electrochemical vapour deposition (EVD) process is used to
fabricate a thin and dense YSZ electrolyte as well as the dense LaCr03
interconnect [69], a highly adherent and reliable interface is formed without any
chemical reaction or change in cathode microstructure [69]. A more cost-
effective non-EVD process, a wet slurry/sintering process has also been
developed [9,70] which has been very successful in fabricating reliable cells at a
lower cost than the EVD process.
For planar-type SOFCs, particularly for anode-supported SOFCs, the cathode
layer is usually deposited after preparing the anode/electrolyte assembly. This
makes it possible to use various processes including slurry coating, screen
printing, tape casting, and wet powder spraying for cathode deposition [ 71-74].
After deposition of the cathode slurry, it is dried followed by sintering. In many
cases, the sintering temperature can be lower for anode-supported cells than for
the cathode-supported type, giving higher surface area cathodes. Since the
sintering of lanthanum manganite-based cathode with YSZ at higher than 1473
I. gives rise to a drastic change in microstructure, lower sintering temperatures
are advantageous.
Physical processes have also been used to make cathodes. For example,
vacuum plasma spraying has been tried for fabricating entire anode/electrolyte/
cathode assembly [75]. In this approach, cathode layers are deposited onto a
porous metallic felt substrate.