Page 203 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

In an effort to alleviate the Co migration problem, a number of investigators
have studied the (La,Ca)Crl-,03 and (La,Sr)Crl_,O3 systems [12, 17, 26-29].
What has been found is that when powders of these compositions are prepared at
temperatures below 'iOO"C, the powders tend to be multiphase with substantial
quantities of either La, Ca, or Sr chromates present. These chromate phases melt
incongruently in the 1000-1200°C range and the liquid promotes sintering.
Unfortunately. the best sinterable compositions are Cr deficient and the excess A
site components, La, Ca, or Sr tend to segregate to the grain boundaries and
create hydration and cracking problems under both SOFC operating and ambient
conditions. Thus, this method of sintering has not been entirely satisfactory, but
until a better one is discovered, it is the one being most frequently employed in
the planar SOFC configuration. The search is still on to develop a method of
densifying stoichiometric LaCrOs which is both economical and yields stable
interconnects.
A number of methods are used to fabricate the interconnect. The method used
depends on the SOFC design [30]. For the tubular design, fabrication methods
such as electrochemical vapour deposition (EVD), plasma spraying, laser
ablation and slurry coating/sintering have been used, with EVD and plasma
spraying being favoured. Economics is an issue with EVD while porosity and
interfacial cracking are the difficulties with plasma spraying.
In the early 198Os, the monolithic SOFC design made use of tape casting,
lamination and calendaring technology to produce a structure which was then
sintered to produce a completed SOFC stack. On the surface, this process is
attractive since it offers the potential of low cost and high power density. In
practice, it is a very difficult process because it requires the simultaneous
sintering of all cell components. This means that the shrinkages and shrinkage
rates must be matched for all four cell components during sintering. Also, the
interdiffusion between the components under the high-temperature processing
conditions must be minimised. As a result, this design has been abandoned.
A variation of the monolithic design was introduced by Allied Signal [31]
(that first became part of Honeywell and now a part of General Electric Power
Systems). This design co-sinters the electrolyte, cathode and anode, but
fabricates the interconnect separately. This design has eliminated the
fabrication incompatibility problem between the interconnect and other cell
components, but it does have the sealing problems of the planar cell design.
The main advantage of this design is the densification of the interconnect by
itself so that it gives the option of liquid-phase sintering the interconnect
without inducing problems with the other cell components.
The conventional planar cell designs build the gas distribution channels into
the interconnect in a bipolar structure. In this design, good electrical contact
between the cell components must be maintained and the edges sealed gas-tight,
These seals are made by using either glasses or cements which, when heated,
give both gas-tight seals and electrical contact. In addition to interlayer seals,
side seals are required which are both electrically insulating and gas-tight. A
number of different schemes have been tried to provide these two seals, but at the
moment they still remain a major issue with planar SOFCs.