Page 212 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Interconnects 189

Figure 7.8 Cross-section of damaged Ni wires of a Ni mesh in contact with XlOCrAl18. The stack was
operated for 3700 h at 800°C. The pores along the outerpart of the wires were formed during start-up of the
stackdue topartialoxidationoftheNi meshandsubsequent reductionof theoxidelayer by the fuelgas[89].

long-term stack operation (Figure 7.8), because of the interdiffusion of the
metals across the interface: Fe and Cr diffuse into the Ni wire and Ni diffuses into
the interconnect [57]. Such deterioration of contacts can be minimised by an
additional Ni coating on the interconnect [88]. The interdiffusion cannot be
avoided, but the diffusion zone in this case is not in direct contact with the gas
atmosphere and causes no internal oxidation by the formation of Ni-Cr oxides as
shown in Figure 7.8.

7.5 Summary

The interconnect material is vitally important in connecting cells electrically
and in separating the reactants. The requirements placed upon it are stringent
and range from compatibility with electrodes and chemical stability to corrosion
resistance combined with excellent electronic conductivity. The two types of
materials that have been extensively used are the chromite ceramics and the
chromium-based metallic alloys.
A number of issues can be listed which need to be addressed before a
completely acceptable ceramic interconnect can be developed (Table 7.6).
However, most of them are of secondary importance when compared to the two

Table 7.6 Current issues of ceramic interconnects
Of most importance Of next most importance
1. Fabrication and processing costs 1. Thermal expansion match to YSZ
2. Material costs 2. Chemical compatibility to YSZ and sealing glass or cement
3. Expansion due to loss ofoxygen
4. Mechanical strength and durability in reducing atmosphere
5. Electrical conductivity in reducing atmosphere