PNNL

Abstract

The interconnect is one of the key components in a SOFC stack. Stainless steels are typically chosen as interconnect materials for intermediate temperature (650-850oC) SOFCs. However, metallic interconnects can contribute a significant portion of power loss and performance degradation during operation due to the continuous growth of oxide scales and contact resistances. Optimization of interconnect materials and coatings is a necessary, but not sufficient, condition for obtaining stable stack performance. The complete solution to interconnect-related challenges must also take into account the electrode/interconnect interfaces. Thus, it is necessary to develop a complete materials system, which includes not only the interconnect itself, but also stable, high-performance contact materials for electrode/interconnect interfaces to ensure good stack performance for meeting the DOE technical targets and enabling the Fuel Cell Program goals. Requirements for successful development of cathode contact materials include: • High electrical conductivity to minimize the resistance of the contact layer and the contact interfaces; • Chemical compatibility with the interconnect and the perovskite cathode. If reaction occurs, the resulting reaction products should be stable and conductive; • Appropriate thermal expansion behavior and high thermo-chemical and structural stability in the SOFC operating environment; • Appropriate bond-forming ability and sintering activity for increased contact area, decreased contact resistance, as adequate interfacial structural stability and strength. • Low cost. Finding suitable materials for electrical contact layers at the cathode interface can be especially challenging, particularly in intermediate temperature SOFCs where high-temperature oxidation-resistant alloys are used as interconnects. The contact layer must provide a low-resistance contact between a conductive oxide cathode (e.g., LSM, LSF, or LSCF) and the interconnect (probably coated if alloy-based). Therefore, there always exists a ceramic/metal interface, and possible additional ceramic/ceramic interfaces as well, all of which potentially contribute to high contact resistance and power loss. Many conductive oxides, such as those commonly used as cathodes, demonstrate high electrical conductivity and chemical compatibility, but these oxides usually exhibit low sintering activity at typical IT-SOFC stack sealing temperatures (825 – 950ºC). On the other hand, many conductive oxides with a low sintering temperature are highly reactive and demonstrate poor compatibility and stability. Consequently, there is a pressing need for optimized, cost-effective contact materials for cathode-side applications.