Page 153 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

13 0 High Temperature Solid Oxide FueI CeIIs: Fundamentals, Design and AppIications

5.3 Reactivity of Perovskite Cathodes with Zr02

5.3.7 Thermodynamic Considerations
The perovskite lattice provides a strong stabilisation effect on the transition
metal ions in B sites. For example, the trivalent Co3+ ions are well stabilised in the
perovskite structure, although this valence state is not fully stable in other
crystal structures. This stabilisation is due to the geometrical matching of the
A-site and the B-site ions (see Figure 5.1). When the tolerance factor (Eq. (1))
derived from the ionic sizes is close to unity, large stabilisation energy is achieved
[6,45]. For the rare earth transition metal perovskites, the tolerance factor is less
than unity suggesting that rare earth ions are smaII as A-site cations. Among
rare earth ions, the largest ions, La3+, therefore exhibit the largest stabilisation.
When comparison is made between Ca and Sr, the stabilisation energy of SrO-
based perovskites is generally larger because of better geometrical fitting.
Yttria-stabilised zirconia also exhibits strong stabilisation on forming solid
solutions in the fluorite structure [46]. Pure zirconia has the stable monoclinic
phase in which the zirconium ions are coordinated with 7 oxide ions, whereas
the cubic phase with 8 coordinates becomes stable only at high temperatures. On
doping with Y203, the oxide ion vacancies are formed preferentially around the
zirconium ions, which leads to stabilisation of zirconia in the cubic phase.
Perovsltite cathodes and yttria-stabilised zirconia (YSZ) electrolyte can react
in several ways as discussed below.

5.3.7.7 Reaction of Perovskites with the Zirconia Component in YSZ
The La203 component in perovskite can react with the zirconia component in
YSZ to form lanthanum zirconate, La2Zr207. On reaction, the valence state of the
transition metal ions, M"+, may change as a result of the formation of other
transition metal binary oxides:

LaM03 + ZrO2 = 0.5LazZr207 + MO + 0.2502(g) (9)
LaM03 + Zr02 = 0.5La2Zr207 + 0.5M203 (10)
LaM03 + ZrO2 + 0.2502(g) = 0.5LazZr207 + MO2 (11)

where M is the transition metal and MO, is its binary oxide. During cell
fabrication at high temperatures, reaction (9) becomes important because of its
large entropy change. During cell operation, oxygen potential dependence
becomes important. Since YSZ is an oxide ion conductor, the oxygen potential in
the vicinity of the electrode/electrolyte interface is important in determining
how electrolyte/electrode chemical reactions proceed under cell operation.

5.3.1.2 Reaction of perovskite with the yttria (dopant) component in YSZ
Since the yttria component has a large stabilisation on forming solid solution, it
is rare for it to react with other oxides. One exceptional case is the reaction with
vanadium oxide: