Page 197 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

have acceptable properties. These rare earth chromites satisfy most of the
requirements, but have problems in fabrication and have high cost. Metallic
interconnects are easier to fabricate and potentially less costly than oxide
ceramics but their lifetimes under SOFC operating conditions remain to
be demonstrated.
In this chapter the requirements of interconnect materials, the characteristics
that the leading candidate materials possess, and how well these fulfil the
requirements are discussed. The oxide ceramic materials are discussed first
followed by a description of several types of metallic interconnection materials.
Then, the special protective and contact materials applied as coatings on the
interconnects to match them to the electrodes are described.

7.2 Ceramic Interconnects (Lanthanum and Yttrium Chromites)

7.2.7 Electrical Conductivity
The electronic conductivity for an interconnect to perform adequately should be
greater than about 1 S/cm at 1000°C. For either YCr03 or LaCr03 to obtain this
level of conductivity, acceptor doping is required. Tables 7.1 and 7.2 list typical
conductivity values that are obtainable.
Upon exposure to reducing atmospheres, all oxides tend to lose oxygen and
form oxygen vacancies. In the case of p-type oxides like Y and La chromites, the
loss of oxygen results in a decrease in electrical conductivity. Figure 7.1
illustrates the behaviour that these oxides display [7]. Figures 7.2 and 7.3 show

Table 7.1 Electrical conductivity data for substituted LaCrO, (in air)
Dopant Composition Electrical conductivity Activation energy Ref.
(mol%) at 1000°C (S/cm) (ev) (kJ/moU
None 0 1 0.19 (18) 2
Mg 10 3 0.20 (19) 3
Sr 10 14 0.12 (12) 4
Ca 20 35 0.14 (13) 5
Ca, Co 20 Ca, 10 Co 34 0.15 (14) 6

Table 7.2 Electrical conductivity data for Ca-dopedYCr0, (in air)

Ca Electrical conductivity Activation energy Ref.
content at 1000°C (S/cm) (W (kJ/mol)
(mol%)
5 4.5 0.17(16)
10 7.7 0.18 (17)
15 13.0 0.18 (17)
20 15.5 0.18 (17)