Page 211 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

18 8 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

Contact materials are used in stack assembly for better electrical contact
between the interconnect and the electrodes and also for compensation of
dimensional tolerances of the parts. Such contact layers have no direct role in
electrochemical reactions, but they can provide a homogeneous contact over the
whole area of the fuel cell and minimise the ohmic losses within the stack. The
maximum assembling temperature depends on the interconnect material used.
For SOPCs with only ceramic components [83, 841, the bond between the cell
and the LaCr03 interconnect is realised by sintering at about 1300°C and a solid,
stiff bond with good electrical contact is obtained requiring no other contact
material. In the case of Cr 5Fe 1Y203 interconnects, sintering can be utilised for
stack assembly providing good contact without any contact material due to the
high melting point (1700°C) of the alloy [38]. However, with ferritic steel
interconnects, the stack assembly temperature cannot be higher than 900-
950°C due to enhanced corrosion and thus contact material is needed for good
electrical contact.
Since there are no electrochemical requirements for the contact materials,
they can be different from the electrode materials and be selected on the basis of
their electrical conductivity and thermal expansion. Lanthanum cobaltites have
high specific conductivities, up to 1700 S/cm [81]. However, the thermal
expansion of these cobaltites has a large mismatch with the other cell
components as mentioned previously. For these ceramic contact materials,
therefore, a compromise between acceptable conductivity and tolerable
mismatch in thermal expansion is generally required.
A chemical interaction between the contact layer and an electrode or the
interconnect should not occur, but cannot be avoided in most cases due to
the reaction of the contact material with the chromia scale formed on the
interconnect. In all cases where alkaline earth-containing chromite contact
materials were used, the formation of chromates was observed [57, 64, 68, 85,
8 61 leading to progressive decomposition of the perovsltite material. The change
in contact resistance (Figure 7.7) is not only due to the scale formation on the
surface of the interconnect but also driven by the reaction between the oxide
scale and the volatile Cr species with the contact material, by the formation of
alkaline earth chromates and the steady depletion of material at the contact
material/interconnect interface due to the volatility of these chromates. The
latter process was demonstrated by Hou et al. [87] by applying different
cathode materials - (i) Pt, (ii) Lao.~Sro.4C003, (iii) Lao.ssSro.lsMn03 +
-
La0.~Sro.~Gao.8~SMg0.1703 onto an un-oxidised Fe-based alloy with
composition similar to X18 CrN 28. They found that the area-specific resistance
of the cobaltite specimen increased at a greater rate than for the other two
material combinations although the cobaltite is more conductive than the
manganite/gallate mixture. For the contact material, it is important to have not
only an initial low contact resistance but also a constant resistance with time (or
even a decreasing resistance as shown in Figure 7.7).
Often the corrosion of the interconnect on the anode side is not an issue
because Ni meshes are used and these make good electrical contact with the
interconnect. However, the Ni wires can also be corrosively attacked during