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The interconnect of an SOFC stack electrically and physically connects the anode of one fuel cell
to the cathode of the adjacent fuel cell in the stack. The interconnect can be ceramic or metallic
materials. The manufacturing cost of ceramic interconnects is prohibitive for economical use in
SOFC stacks. Therefore, this project investigates metallic alloys for use as interconnects in
SOFC stacks. This project is designed to determine the suitability of metallic alloys with and
without coatings for the interconnect in SOFC stack technology for transportation applications.
The SOFC stacks in these applications will be required to undergo 7000 thermal cycles in 10,000
hours of operation. The interconnect materials used for SOFC stacks must be electronically
conducting, oxidation resistant, impermeable to the diffusion of gases, chemically stable with
fuel cell materials, and the oxide scale formed must not spall from the substrate. Since SOFC
stacks are thermally cycled, the oxidation resistance of the metallic alloys are being tested under
thermal cycling conditions in both air and fuel atmospheres. Robust engineering arrays were
designed to effectively and quickly assess metal alloy candidates and coatings for use as the
SOFC interconnect material.
EXECUTIVE SUMMARY
This report describes the interconnect project work at Delphi for our SOFC stack program. The
interconnect electrically connects the individual fuel cells in a stack so that the fuel cells can be
operated in series. Since the there are two more interconnects in each stack than there are fuel
cells, the economics of the interconnect are of great importance. Further, failure of an
interconnect represents failure of the SOFC stack itself. The work at Delphi has concentrated on
finding an economically feasible, manufacturable interconnect metallic alloy. The robust
engineering studies in this project have been designed to quickly and efficiently assess the
oxidation kinetics, oxide scale adherence and electronic resistance of the oxide scale formed on
12 different alloys and 4 different coatings in air and wet hydrogen under thermal cycling
conditions. The results from this work will determine the best metallic alloy candidate and
interconnect coating configuration for SOFC stacks for both bonded and compressive sealing
strategies.
EXPERIMENTAL
Material choice
The metallic alloys chosen for this study include ferritic stainless steels, austenitic stainless
steels, and Ni-based superalloys. The ferritic stainless steels are 409, 439, 434, Crofer 22, and a
Sumitomo, Inc. alloy. The austenitic stainless steel is 302 SS. The nickel-based superalloys are
Haynes 230, Haynes 242, and Inconel 718. The variety of alloys was chosen because of the
differences in SOFC stack designs. Stack manufacturers with intimately bonded sealing designs
use ferritic stainless steels for the interconnect material because of their better coefficient of
thermal expansion (CTE) match to the fuel cell components. If a close CTE match is not
accomplished throughout the heating and cooling range of the SOFC stack than shear forces can
develop between the fuel cell and the interconnect. Shearing forces can lead to cracking and
leaking of seals and eventual failure of the stack. This is of particular concern for brittle sealing
materials that do not have the strength or ductility to accommodate the shearing forces. Stack
manufacturers with compressive sealing designs can use the superior, oxidation resistance of4
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England, Diane M. SOFC INTERCONNECT DEVELOPMENT, report, June 6, 2003; United States. (https://digital.library.unt.edu/ark:/67531/metadc789025/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.