Several years ago a new material was introduced for use as an interconnect material for solid oxide fuel cell (SOFC) application. This new material is designated UNS S44535 and it has unique properties that make it an ideal choice in application as an interconnect material. The key properties are the coefficient of thermal expansion (CTE), which closely matches the ceramic materials used in the SOFC stack, the good electrical conductivity of the protective oxide scale that is formed during operation of the fuel cell stack and the outstanding oxidation resistance at operating temperatures. Several lessons were learned during the development, scale-up, and production of the UNS S44535 material. The end result is a commercially available material that meets the technical requirements for interconnects in SOFC application.
BACKGROUND
Solid oxide fuel cells (SOFC?s) are designed to produce electricity directly from high temperature flowing gases and use no moving parts. The benefits of SOFC technology include high efficiencies, low emissions, and the ability to integrate with other power generating systems, such as automotive engines or various size gas turbines for power generation. These features of SOFC systems make SOFC systems a very attractive technology for the transportation and power generation industries.
One key issue holding back the advancement of this attractive technology is materials of construction. In particular, there is a need for economical materials for interconnects and seals in SOFC systems. Figure 1 shows a schematic of how a SOFC system operates. The SOFC produces electricity from an electrochemical reaction when oxidizing and reducing gases are introduced on either side of the single fuel cell unit. Multiple units are stacked together to increase power production; this requires the fuel cell units to be physically separated from each other, yet be electrically connected to collect the produced current. A schematic of a repeating fuel cell stack and example of a metallic inter-connect are shown in Figure 2. A successful interconnect material must act as a physical barrier to separate the fuel and oxidant, be a low resistance electrical conduit over the life of the system, and provide mechanical support and stability to the fuel cell stack.
A joint effort between Forschungszentrum Jülich and ThyssenKrupp VDM has recently led to the commercial introduction of a ferritic stainless steel specially designed for use as an interconnect material, which is commercially named Crofer 22APU* (UNS S44535). Table 1 compares properties of UNS S44535 to other materials classes that have been considered and also cites the requirements for a SOFC interconnect. It is clear from Table 1 that UNS S44535 is ideally suited for use an interconnect material in SOFC systems.
MATERIAL PROPERTIES OF UNS S44535
UNS S44535 is specially designed as an interconnect material for SOFC systems. The material is especially balanced to provide the required coefficient of thermal expansion, oxidation resistance, electrical conductivity, low contact resistance, reduced chromia evaporation, and required mechanical properties, as well as be easily fabricated.
The chemistry of UNS S44535 is given in Table 2. UNS S44535 is classified as a ferritic stainless steel. The microstructure of this material consists of equi-axed grains of ferrite (bcc crystal structure) with a small amount of carbides, typical for this class of material.
The mechanical properties of the UNS S44535 at room temperature are given in Table 3. The mechanical properties are typical of other ferritic stainless steels. The material has adequate strength for the service application and also has suff