Page 161 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

13 8 High Temperature Solid Oxide Fuel Cells: Fundamentak, Design and Applications

have focused on development of new cathodes appropriate for intermediate
temperature SOFCs and (La,Sr)(Co,Fe)03 cathodes have been optimised for use
with Ce02-based electrolytes [ 11,121, a typical composition being
(Lao.6Sro.4)(Coo.8Fe,,2)03 (LSCF). This cathode has about the same thermal
expansion coefficient as doped ceria.
Ferrite-based cathodes [60] have also been explored for use with YSZ
electrolyte in intermediate temperature SOFCs. However, their high-
temperature phase behaviour and long-term stability remain unclarified and
need further investigation. Use of interlayers between the perovskite cathode
and the electrolyte has also been tried to minimise interfacial reactions. One
typical interlayer consists of a thin layer of ceria based oxide between the
cobaltite-based cathode and YSZ electrolyte because the chemical reactivity and
thermal expansion mismatch between YSZ and cobaltites can be moderated by
inserting ceria between the two materials. Another approach is to use multilayer
cathodes consisting of a manganite at the interface with the electrolyte and a
cobaltite on top of the manganite [68]. In this case, the electrochemical active
sites are located in the manganite part, whereas the cobaltite-rich layer acts as
good electrical conductor.

5.4 Compatibility of Perovskite Cathodes with Interconnects
In addition to compatibility with the electrolyte, compatibility of the cathode
with the interconnect is also important. Both oxide ceramic and metallic
materials are used as interconnects in SOFCs. As expected, these two types
of interconnects present quite different issues in their compatibility with
the cathode.

5.4.1 Compatibility of Cathodes with Oxide Interconnects
The main oxide interconnects for SOFCs are based on lanthanum chromite [5].
In this case, since both the cathode and the interconnect materials are
perovskites, there are no severe chemical reactions between them and
interdiffusion and precipitation of third phases are the main issues. Interdiffusion
can take place in both the A and the B sites of the perovskite lattice. Ideal mixing
in the A and the B sites gives rise to the driving force for interdiffusion. Usually,
the cation diffusivity in perovskite oxides is faster in the A site than in the B site
[61]. Thus, mixing can occur first in the A site and then in the B site. This implies
that when (La,Sr)Mn03 and (La,Ca)Cr03 are contacted, interdiffusion among the
A-site elements will take place to give a constant La or Sr content throughout
the manganite and chromite phases. This is due to the driving force originating
from random mixing. This can be called the ‘entropy effect’. Another driving
force for interdiffusion arises from the difference in stabilisation energy among
the different combinations of A-site and B-site cations. When comparison is made
of the valence stabilities of manganese and chromium ions in a cathode
atmosphere, chromium ion tends to be trivalent, whereas the manganese ion