Page 159 - Materials Chemistry, Second Edition

146 2 Solid-State Chemistry

to produce and have some of the highest efficiencies, they require high-purity
oxygen to prevent catalyst poisoning by carbon dioxide. PEMFC designs also suffer
from catalyst poisoning. The presence of small concentrations of CO (>1 ppm)
in the reformate of fuels drastically alters the performance of the anodic catalyst
(e.g., Pt, Pt/Ru).
In this section, we will consider SOFCs, the most relevant to our discussion
of crystal structures. These ceramic-based fuel cells are perhaps best suited for
large-scale stationary power generators that could provide electricity for entire
cities. This is a consequence of its high operating temperature, often greater than

1,000 C. In addition to the primary electricity generated from the electrodes within
the fuel cell, the steam byproduct may be used to turn turbines, generating secondary
electricity. The SOFC can operate on most any hydrocarbon fuel, as well as
hydrogen gas:

ð58Þ H 2 þ O 2 ! H 2O þ 2e ðat anode)

ð59Þ 1=2O 2 þ 2e ! O 2 ðat cathode)
The electrochemical reactions occurring within a SOFC are shown in Eqs. 58
and 59. The anode consists of a porous mixture of a Ni or Co catalyst on yttria-
stabilized zirconia. Such a mixture of metal and ceramic is referred to as a
cermet. The zirconia acts to inhibit grain growth of the catalyst particles of
nickel or cobalt and protects against thermal expansion. The cathode is generally
a Sr-doped LaMnO 3 perovskite. The Sr dopant provides for oxygen transfer to
the cathode–electrolyte interface.
Figure 2.101 shows a variety of stacked cell designs employed by SOFCs. Since
an individual fuel cell produces a low voltage (typically <1 V), a number of cells are
connected in series forming a fuel cell stack. An interconnect comprising a high-density
material is used between the repeating {anode–electrolyte–cathode} units of the stack.
The electrolyte in a SOFC must have the following characteristics:
1. High oxide conductivity
2. Stability in both oxidizing and reducing environments
3. Chemical compatibility with other cell components
4. High density to prevent mixing of fuel and oxidant gases
5. Desirable thermal expansion properties, to prevent cracking of the fuel cell at
high temperatures
One such material that satisfies all of the above requirements is yttrium-stabilized
ZrO 2 . In a SOFC, the electrolyte must allow oxide-ion transport between the anode
and cathode. In its high-temperature cubic form, zirconia is not able to conduct
oxide ions. That is, there are no suitable interstitial sites in the lattice to trap oxide
3þ
ions. However, when a lower valent metal oxide such as Y 2 O 3 (i.e., Y )is
4þ
substituted for ZrO 2 (i.e., Zr ), a vacancy is created in the unit cell of the extended
lattice (Figure 2.96). This site is able to accept an O 2 ion generated at the cathode,
and deliver it to the anode where it is transformed to H 2 O. As illustrated in
Figure 2.102, the oxide ion is transferred among adjacent zirconia unit cells en