Page 230 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Cell and Stack Designs 207

Electrophoretic deposition. YSZ particles are deposited from a suspension
onto an electrode of opposite charge upon application of a DC electrical field. The
deposited layer is then fired at elevated temperatures [ 151.
Slip casting. YSZ layers are deposited on a porous substrate by vacuum slurry
coating. After deposition followed by drying, the layer is sintered at high
temperatures [16].
Plasma spraying. Powders injected into a plasma jet are accelerated. melted,
and deposited on the substrate [17].
Other processes investigated for planar SOFC fabrication include electrostatic-
assisted vapour deposition, vapour phase electrolytic deposition, vacuum
evaporation, laser spraying, transfer printing, sedimentation method, and
plasma metal organic chemical vapour deposition.
As discussed in Chapter 7, the interconnect for planar SOFCs is either ceramic
or metallic depending on the cell operating temperature. Ceramic interconnects
are commonly used at 900-1000°C while metallic interconnects at < 800°C.
The most common material for ceramic interconnects is doped lanthanum
chromite (LaCr03). Lanthanum chromite interconnects for planar SOFCs are
often made by conventional ceramic processing methods such as pressing or tape
casting followed by sintering. Flowfields are either embossed into the
interconnect before firing or machined into the sintered interconnect.
Lanthanum chromite is known to be difficult to densify under high oxygen
activity environments; therefore, the material used for making the interconnect
is generally tailored to improve its sinterability under required conditions,
especially under oxidising atmospheres [ 31. The most common metallic materials
are chromium-based alloys and ferritic stainless steels. These materials are
considered for planar SOFC interconnects because their coefficients of thermal
expansion closely match those of cell components. Alloys with chromium oxide
scale formation are often preferred for interconnect applications (as compared to
those with alumina scales) due to the higher conductivity of the chromia scale
formed on the surface of the alloy. The key technological issue with chromium-
containing metallic interconnects relates to migration of chromium species into
the cell, causing cell performance degradation during SOFC operation [18]. The
use of metallic materials permits a variety of conventional forming methods for
manufacture of the interconnect. Flowfields can be formed on metallic
interconnects by machining or stamping.
The majority of planar SOFC stacks require sealing to prevent gas leakage or
cross-leakage. In general, when a planar SOFC is designed, one emphasis is to
minimise sealing and sealing surfaces because the seal requirements are very
stringent. Two types of sealing methods have been used: compressive loads
(with or without gaskets) and high-temperature sealants. Compressive seals
involve use of mechanical loads to compress fuel cell components to form a seal.
This type of sealing has the advantage of requiring no sealants: however,
forming a gas-tight seal and minimising mechanical stress due to compression
of uneven surfaces are the key issues. Gaskets can be used to improve gas
tightness and provide cushion for surface unevenness. High-temperature
sealants include cements, glasses, and glass-ceramics. A sealant selected for