Metallic interconnects perovskites

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

Besides the glass seal interfaces, interactions have also been reported at the interfaces of the metallic interconnect with electrical contact layers, which are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, perovskites that are typically used for cathodes and considered as potential contact materials have been reported to react with interconnect alloys. Reaction between manganites- and chromia-forming alloys lead to formation of a manganese-containing spinel interlayer that appears to help minimize the contact ASR [219,220], Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form SrCr04 [219,221],... 

Larring, Y. and Norby, T., Spinel and perovskite functional layers between Plansee metallic interconnect (Cr-5 wt% Fe-1 wt% Y2O3) in ceramic (Lao 85Sro.i5)o.9iMn03 cathode materials for solid oxide fuel cells, J. Electrochem. Soc., 147, 3251-3256 (2000). 

Lanthanum chromite has provided long lifetimes, as long as 69,000 h in Siemens Westinghouse tubular cells, at 900-1000°C. However, metallic interconnects have not yet shown equivalent lifetime performance. Improvements in metallic interconnect compositions and contact layers between cells/interconnects are still issues for materials development. In particular, the metal/ceramic interface in cells should have low corrosion, low contact resistance and low permeability of chromium species. Recent results have shown that optimised steels for SOFC applications are available and alkaline earth-free and cobalt-containing perovskites are the most suitable materials for contact layers however, their long-term performance under fuel cell operation conditions needs to be proven. 

Researchers and product developers are geared towards development of ceramic/perovskite coatings for metallic interconnects for high temperature operation at 1000°C. Improved alloy composition may be looked at as metallic interconnect. 

Broadly, interconnect materials for SOFC fall into two categories conductive ceramic (perovskite) materials for operation at high temperature (900 to 1000 °C) and metallic alloys for lower temperature operation. Though the shape of SOFC interconnects depends heavily on the cell and stack design, the materials choice is almost entirely determined by physical and chemical stability under operating conditions. 

Ttvo roles of the interconnect in high-temperature solid oxide fuel cells (SOFCs) are the electrical connection between cells and the gas separation vvithin 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 900-1000°C range or metallic alloys for lower temperature cell operation. 

Among the two perovskites, namely alkaline earth metal doped LaCr03 and YCvO perovskite oxides, doped LaCr03-based perovskite is widely used as the ceramic interconnect material (Armstrong et al. 

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