Perovskite interconnects

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. 

The perovskite oxides used for SOFC cathodes can react with other fuel cell components especially with yttria-zirconia electrolyte and chromium-containing interconnect materials at high temperatures. However, the relative reactivity of the cathodes at a particular temperature and the formation of different phases in the fuel cell atmosphere... 

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],... 

Kim J-H, Peck D-H, Song R-H, Lee G-Y, Shin D-R, Hyun S-H, Wackerl J, and Hilpert K. Synthesis and sintering properties of (La Ca, 2 lSrx)Cr03 perovskite materials for SOFC interconnect. J. Electroceramics 2006 17 729-733. 

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). 

Clearly, not all these perovskite compositions are useful for oxygen delivery applications. For example, ceramics based on Laj.xAxCrOj (x == Sr, Ba, Ca), Lai, rxCri yMny03.5 and Lai xCaxCri yCOy03 have been proposed for use as interconnection material (separator) in solid oxide fuel cells (SOFC), and therefore should be dense and impermeable in order to prevent burning off of the fuel without generating electricity [137,138]. 

The interconnecting material used in building a stack of fuel cells must have a high electrical conductivity and a coefficient of expansion similar to that of the electrolyte. The perovskite oxide LaCrOs meets these requirements nicely and is widely used although ferritic stainless steels have been used successfully in certain fuel cell geometries. 

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