Interconnection LaCrO

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. 

Si present in the substrate also creates another problem (Fig. 6). As can be seen in the SEM cross section, a continuous Si02 layer forms beneath Cr203 layers when the metallic 446 interconnect coated with 0.2 im porous LaCrOs thin film is annealed at 900°C for 100 hours. Formation of such insulating layer is also detrimental for the electrical performance of the SOFC. Though the cross section analysis of structure of oxide layers did not... 

Figure 6. Backscattered cross section SEM image of446 SS metallic interconnect material coated with 0.2 pm LaCrOs thin film after annealing at 90(f C for 100 hours in air.

Doped LaCrOs generates Cr" " at high oxygen partial pressures and oxygen vacancies in reducing atmospheres. The important function of interconnects is electronic conduction without electrochemical oxygen leak. Thus, the defect chemistry-associated transition and... 

Numerous studies have investigated coating the interconnect with a dense protective oxide layer to reduce surface oxidation and chromium contamination of other key components [124, 125]. Perovskite materials such as LaCrOs typically used for higher temperature ceramic interconnects are ideal as their conductivity and thermal expansion coefficient can be tailored through doping to ensure compatibility and optimise performance. 

The material cost of lanthanum is high so that the thickness of the oxide interconnects should be thin enough to reduce the amount used in cells [31]. On the other hand, the LaCrOs-based intercoimects should be thick enough to prevent the electrochemical oxygen permeation which takes place as the bipolar diffusion of oxide ions and holes inside the interconnect. For such a purpose, Ca-doped LaCrOs is not appropriate because those ceramics are highest in gas permeation among the alkali earth-doped LaCrOs [36, 37]. 

Horita, T. (2009). LaCrOs-based perovskite for SOFC interconnects in T. Ishihara (ed.) Perovskite Oxide for Solid Oxide Fuel Cells. Springer. London. 285-286. 

In the beginning, strong interest arose in utilization of metal interconnects instead of LaCrOs-based oxide interconnects [13]. Because of the severe corrosion of metals at high temperatures, operation temperature needs to be lowered. 

Lanthanum chromite-based perovskite oxides (LaCrOs) have been widely recognized as promising interconnect materials for solid oxide fuel cells (SOFCs). The interconnects must separate fuel and oxidant gases and also have high electronic conductivity at high temperature (773-1273 K). Therefore, interconnects should meet the following requirements ... 

To meet the foregoing requirements, the composition of LaCrOs was modified by doping of lower valence alkaline ions, such as Ca ", Mg +, and Sr, at the La " " or sites. The substitution of La site and Cr site with the other elements can decrease the sintering temperature and increase the electronic conductivity. So far, a number of papers have been published regarding the physical and chemical properties of doped LaCrOs. The present chapter describes the overview of LaCrOs-based ceramics for SOFC interconnects. The following topics are introduced and discussed ... 

The interconnects should posses high electronic conductivity greater than 10 S/ cm at operating temperatures (873-1273 K) under reducing and oxidant atmospheres Po2 = 10 Pa-10 Pa. Doped LaCrOs is a p-type conductor, and the... 

Fig. 15.5 Schematic drawing of oxygen permeation through LaCrOs interconnects...

Fig. 15.6 Oxygen permeation current density through LaCrOs interconnects. Relationship between ionic leak current density and total current density in Lao rCao sCrOs under oxygen potential gradient. The thickness of the LaCrOs plate is assumed to be 3 mm. (Reproduced by permission of The Electrochemical Society [28])...

F. Boroomand, E. Wessel, H. Bausinger, K. Hilpert, Correlation between defect chemistry and expansion during reduction of doped LaCrOs interconnects for SOECs. Solid State Ionics 129, 251-258 (2000)... 

The electronic conductivity for an interconnect to perform adequately should be greater than about 1 S/cm at 1000°C. For either YCrOa or LaCrOs to obtain this level of conductivity, acceptor doping is required. Tables 7.1 and 7.2 list typical conductivity values that are obtainable. 

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