Page 196 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Chapter 7

Interconnects

Harlan U. Anderson and Frank Tietz

7.1 Introduction
Two roles of the interconnect in high-temperature solid oxide fuel cells (SOFCs)
are the electrical connection between cells and the gas separation within the cell
stack. The fact that the interconnect must be compatible with all of the cell
components as well as be stable with respect to both oxidising and reducing gases
places very stringent materials requirements on it. These requirements plus the
additional constraints of cost and ease of fabrication tend to limit the possible
choices to only a few materials. These materials come from either perovskite-type
oxide ceramics based on rare earth chromites for operating temperatures in the
900-1 000°C range or metallic alloys for lower temperature cell operation.
The properties which an interconnect must possess are rather extensive and
somewhat dependent upon the particular SOFC configuration [ 11. However,
typical requirements are:

0 High electronic conductivity with low ionic conductivity
0 Chemical stability in both fuel and air
0 Thermal expansion match to other cell components
0 High mechanical strength
0 High thermal conductivity
0 Chemical stability with regard to other cell components

Depending upon the particular SOFC design, additional requirements such as
the ease of fabrication to gas-tight density, the ability to make gas-tight seals
with other cell components, and the material cost also play an important role.
Of the requirements listed above, the first three are crucial and tend to
eliminate most candidate materials. In fact, for operation at temperatures above
8OO0C, the only oxides that fit these criteria are the doped rare earth chromites.
In particular, compositions from the system (La,Sr,Ca)(Cr,Mg)03 are the leading
interconnect materials. However, compositions from the system (Y,Ca)CrQ3 also