Page 142 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Chapter 5

Cathodes

Harumi Yokokawa and Teruhisa Horita

5.1 Introduction
Cathodes for solid oxide fuel cells (SOFCs) have to possess many properties
including high electrical conductivity, high catalytic activity for oxygen
reduction, and compatibility with other cell components. In the earliest stages of
SOFC development, platinum was used as cathode since other appropriate
materials were not available. However, platinum is expensive and its use in
cost-effective commercial SOFCs for power generation is not practical. Less
expensive perovskites [l] also possess the required properties and have
consequently attracted much interest. In 1969, LaCo03 was tested by Tedmon
et al. [2] and its initial performance in cells was good. However, severe
degradation occurred with increasing time of operation due to reactions with
yttria-stabilised zirconia (YSZ) electrolyte. Investigations on cathodes then
rnoved to lanthanum manganite (LaMn03)-based materials. Although degradation
of lanthanum manganite cathodes was not as severe, some potential reactions
vvith YSZ, particularly at higher cell fabrication temperatures, were recognised [3].
Success of the seal-less tubular cells with electrolyte fabricated using
electrochemical vapour deposition method [4] stimulated investigations of
fabricating SOFC components by a more cost-effective slurry/sintering process
[ 51. To utilise this process successfully, any chemical interactions between
cathode and YSZ electroIyte had to be avoided during cell fabrication without
sacrificing cathode performance. For this purpose, the reactivity of lanthanum
manganites with YSZ was investigated using thermodynamic considerations [6],
and to avoid La2Zr207 formation, A-site (La)-deficient LaMn03 was proposed for
the cathode. Its use inhibited La2Zr207 formation and resulted in better cathode
performance, as confirmed by Dokiya et al. [ 71 on test cells and by Ipponmatsu et
aI. [8] on tubular cells in a 1 kW system. Aizawa et al. [9] also used the A-site
deficient lanthanum manganite cathode to fabricate tubular cells by a wet
slurry/sintering process and achieved about the same cell performance as cells
fabricated by using electrochemical vapour deposition for the YSZ electrolyte [4].