Page 143 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications
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120  High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

           For  intermediate  temperature  SOFCs,  a  composite  cathode  consisting  of
         strontium-doped  lanthanum  manganite  (LSM)  and  YSZ  has  shown  good
         performance [ 101. For use with ceria-based electrolytes [ll], a (La,Sr)(Co,Fe)03
         (LSCF)-based cathode has been developed [ 121.
           In this chapter, the physical  and physicochemical properties of  perovskites
         which are important in understanding electrochemical performance are given in
         detail, emphasising the effect of valence stability on oxygen nonstoichiometry,
         crystal  structure,  electrical  conductivity,  thermal  expansion  and  chemical
         reactivity  with  YSZ  electrolyte.  Then,  practical  aspects  of  the  lanthanum
         manganite  cathodes  are described  with  a  focus  on  how  to  avoid  La2Zr207
         formation.  Cathodes for the intermediate  temperature SOFCs are discussed in
         relation to the conventional LaMn03-based cathodes. Compatibility of  cathode
         materials  with oxide ceramic interconnects and metallic interconnects is also
         examined with emphasis on Cr poisoning. Finally, the fabrication methods for
         cathodes are briefly described.



         5.2 Physical and Physicochemical Properties of  Perovskite
         Cathode Materials


         5.2.1 Lattice Structure, Oxygen Nonstoichiometry,  and Valence Stability
         The lattice structure of  perovskites, AB03, is shown in Figure  5.1. This oxide
         consists  of  three elements, namely  the large cations, A"+, the small  cations,






                                                       A site cation


                                                       B site cation


                                                       Oxygen


                   Figure 5.1  Schematic representation oflattice structure ofperovskite, AB0 3.


         B(6-")+,  and the oxide ions, 02-, where n is the positive charge on the A ions.
         Cations B(6-")+ are surrounded by  6 oxide ions, whereas cations A"+ have 12
         oxide ion coordinates: the sites with 12 coordinates are often called the A sites,
         and the sites with 6 coordinates the B sites. The geometrical fitting of cations and
         anions  to  this  structure is  measured  with  the  Goldschmidt  tolerance factor
         defined as follows:
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