Environmental Engineering Reference
In-Depth Information
Fig. 4.9
Crystal structure
of yttria stabilized zirconia,
YSZ used as the electrolyte
material of SOFC
Y
3+
ions. Introduction of 3 + cations in a 4 + cation site violates the charge neutrality
in the crystal, resulting in the formation of vacancies on the oxide ion site. When
an oxide ion jumps into the vacancy from a neighbouring site leaving a vacancy
behind, a charge 2 + is transported by the vacancy. Oxide ion is thus transported by
this vacancy mechanism.
It is not therefore necessary for a SOFC to use platinum or other precious
metals for the electrode catalyst. This is due to the high operation temperature,
and is regarded as an advantage for SOFC over PEFC or PAFC. The most popu-
lar anode material is nickel-YSZ 'cermet' (a
cer
amics and
met
al composite).
The YSZ particles prevent the nickel particles from agglomerating, and form
three-dimensional paths for oxide ion in the electrode layer. For the cathode,
electronically conductive ceramics are utilized. In the early stage of the devel-
opment of SOFCs, (La, Sr)MnO
3
was regarded as the best candidate due to its
stability and compatibility with the YSZ electrolyte. Recent development of re-
duced temperature SOFCs, requires a cathode with a better performance, and the
use of (La, Sr)(Co, Fe)O
3
has become popular. Since this material reacts to YSZ
to form an insulating layer, a thin interlayer is inserted between the cathode and
the electrolyte.
As with other types of fuel cells, SOFC is also used as a 'stack', piling up mul-
tiple single cells. The shape of the stack however can differ- planar stacks, tubular
stacks, flat tubular stacks, or segment-in-series stacks are possible (Fig.
4.10
). Pla-
nar stacking has an advantage in energy density, while tubular stacks have higher
stability against thermo-mechanical stresses. Segment-in-series type stacks can be
cost-effective since the structure is sustained by a low cost ceramic substrate. The
component connecting single cells is called a separator in a PEFC, but is gener-
ally called an 'interconnect' in a SOFC. Metal or ceramic interconnects are used
depending on the stack design.
The most distinctive feature of the SOFC configuration is its high operating tem-
perature. This causes various advantages and disadvantages for a SOFC relative to
other types of fuel cells as listed below.
• High energy conversion efficiency. High temperature operation is advantageous
for achieving high energy-conversion efficiency, especially when hydrocarbon
