Chemistry Reference
In-Depth Information
applications, like portable devices, thus further stimulating research
endeavors targeting lower SOFC operating temperatures.
6,142,143
The most critical component of SOFCs in terms of cell performance is the
electrolyte. To improve the oxide membrane, two major approaches have
been pursued in recent years: first, by decreasing the membrane thickness,
the ohmic loss of the membrane is reduced, since this loss is linearly pro-
portional to the thickness.
138,140,144
By limiting the electrolyte thickness,
even to micro-scale, the operating temperature can be reduced without
losing electric performance. Reducing the thickness, however, decreases the
robustness of the membranes and typically requires more dicult fabri-
cation methods.
The second major approach is the development of new electrolyte ma-
terials with high ionic conductivity to replace conventionally used yttria
stabilized zirconia or doped ceria. These state-of-the-art materials are cubic
fluorite oxides with inadequate ionic conductivity below 700 1C or partial
mixed conduction in a reduced atmosphere or at an elevated tempera-
ture.
145,146
Promising alternative electrolyte materials discovered for SOFC
applications include Ca
12
Al
14
O
33
,
147
Bi
2
O
3
-based materials,
148
perovskite
LaGaO
3
,
149
Ln
10
(SiO
4
)
6
O
3
(Ln
ΒΌ
La, Nd, Sm, Gd and Dy),
150
and La
2
Mo
2
O,
151
to name only a few. The conductivity of several ion conductive materials is
summarized by S. M. Haile.
1
Unfortunately, many of these newly found materials exhibit properties
hindering or even inhibiting successful application in realistic fuel cells. The
refractory and highly reactive LaGaO
3
is dicult to be applied as thin
films.
152-154
Bi
2
O
3
can fail due to its phase structure change.
148
To achieve
satisfactory fuel cell eciency, the ionic conductivity should be above a
critical level of 0.1 S cm
1
.
155
None of these novel materials has yet been able
to fulfill this requirement for temperatures of 600 1C and less.
134
More re-
search is necessary to improve the stability and ionic conductivity of known
materials or to find entirely new materials.
d
n
3
r
4
n
g
|
4
.
5.2.4.3 Nanotechnology in Composite Electrolytes
Nanostructures have emerged as highly promising materials applied as
functional components of advanced energy conversion and storage devices,
such as lithium-ion batteries, supercapacitors, solar cells, low-temperature
fuel cells, and fuel reformers.
6,9,111,156-158
Nanostructured materials have a
very large surface-to-volume ratio and experience a strong effect of grain
boundaries, leading to unique material characteristics that can be exploited
in novel energy technologies. Like low-temperature fuel cells, high-tem-
perature fuel cells including SOFCs can benefit significantly from the use of
nano-scale structures and materials,
111,144,159-162
for example in the form of
nanostructured solid conductors, sometimes referred to as 'nanoionics'.
163
The high cell fabrication temperature of SOFCs can cause diculties
when applying nanostructures due to their instability at high temperatures.
Nano-sized particles,
for example, may form microstructures during
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