Fabrication and characterization of anode-supported tubular SOFCs with zirconia-based electrolyte for reduced temperature operation

Article abstract of DOI:10.1149/1.1768131

An anode-supported tubular solid oxide fuel cell (SOFC) with a thin electrolyte film for reduced temperature operation was fabricated by a wet cofire process. Instead of the traditional extrusion method, the anode substrate was formed by a tape-cast method. This allows design the substrate with some advantageous configurations that can improve the performance of SOFCs. Sc ₂O₃ and Y₂O₃-stabilized zirconia electrolyte films were fabricated on the substrate by a modified slurry dip-coating method and then cofired. In order to use (La_0.6Sr _0.4)CoO₃ cathode, a thin (CeO₂) _0.9(GdO_1.5)_0.1 interlayer was also fabricated by the slurry dip-coating method. The fabricated tubular cell generated electricity successfully between 600 and 850°C. Power density of ca. 170 and 220 mW/cm² was achieved at 600 and 700°C, respectively, however, the ohmic resistance was larger than expected. Because (CeO₂) _0.9(GdO_1.5)_0.1 interlayer was used, ohmic resistance could be high due to the formation of a layer of (Zr,Ce)O₂ solid solution at the zirconia/ceria interface. The gaseous diffusion resistance governed the performance between 700 and 850°C. Better performances of the anode-supported tubular SOFCs can be achieved by further optimizations for the zirconia/ceria interface, control of both porosity and thickness of the anode substrate, as well as improvement of the (La _0.6Sr_0.4)CoO₃ cathode.

Full text of DOI:10.1149/1.1768131

A1230
Journal of The Electrochemical Society, 151 ͑8͒ A1230-A1235 ͑2004͒
0
013-4651/2004/151͑8͒/A1230/6/$7.00 © The Electrochemical Society, Inc.
Fabrication and Characterization of Anode-Supported Tubular
SOFCs with Zirconia-Based Electrolyte for Reduced
Temperature Operation
a,z
a
a
a
a
Tuong Lan Nguyen, Takeo Honda, Tohru Kato, Youko Iimura, Ken Kato,
a
a
b
b
Akira Negishi, Ken Nozaki, Mitsunobu Shiono, Akihisa Kobayashi,
Kan Hosoda, Zifu Cai, and Masayuki Dokiya *
b
b
b,
aNational Institute of Advanced Industrial Science and Technology, Energy Electronics Institute, Fuel Cell
Group, Tsukuba, Ibaraki 305-8568, Japan
b
Graduate School of Environmental and Information Sciences, Yokohama National University, Hodogaya,
Yokohama 240-8501, Japan
An anode-supported tubular solid oxide fuel cell ͑SOFC͒ with a thin electrolyte film for reduced temperature operation was
fabricated by a wet cofire process. Instead of the traditional extrusion method, the anode substrate was formed by a tape-cast
method. This allows design the substrate with some advantageous configurations that can improve the performance of SOFCs.
Sc O and Y O -stabilized zirconia electrolyte films were fabricated on the substrate by a modified slurry dip-coating method and
2
3
2
3
then cofired. In order to use (La0.6Sr0.4)CoO3 cathode, a thin (CeO2)0.9(GdO1.5)0.1 interlayer was also fabricated by the slurry
dip-coating method. The fabricated tubular cell generated electricity successfully between 600 and 850°C. Power density of ca.
2
1
70 and 220 mW/cm was achieved at 600 and 700°C, respectively, however, the ohmic resistance was larger than expected.
Because (CeO2)0.9(GdO1.5)0.1 interlayer was used, ohmic resistance could be high due to the formation of a layer of (Zr,Ce)O2
solid solution at the zirconia/ceria interface. The gaseous diffusion resistance governed the performance between 700 and 850°C.
Better performances of the anode-supported tubular SOFCs can be achieved by further optimizations for the zirconia/ceria
interface, control of both porosity and thickness of the anode substrate, as well as improvement of the (La0.6Sr0.4)CoO cathode.
3
©
2004 The Electrochemical Society. ͓DOI: 10.1149/1.1768131͔ All rights reserved.
Manuscript submitted November 4, 2003; revised manuscript received February 13, 2004. Available electronically July 2, 2004.
In order to make solid oxide fuel cells ͑SOFCs͒ suitable for
whereas the tape-casting method has been widely adopted for fabri-
cating a planar ceramic substrate. In this study we modified the
tape-casting method for preparing the tubular NiO-yttria-stabilized
zirconia ͑YSZ͒ cermet anode substrate, because it is not only ad-
equate to a laboratorial scale but also is capable of mass producing.
There are some available wet methods for fabricating a ceramic thin
film such as dip-coating, screen printing, spraying, tape casting, or
electrophoretic depositing. However, we have selected the dip-
coating method because it is reasonable to expect that the method is
most appropriable for a tubular shape. Furthermore, because both
methods require only simple equipment and also avoid waste of
materials, reduction of fabrication cost is expected.
practical use such as in compact cogeneration systems and transpor-
tation applications, reduced temperature SOFCs have been exten-
sively studied. Operation at around 750°C or less retards materials
degradation and hence prolongs stack lifetime, reducing cost by uti-
lization of metallic materials, simplifying the system. Reduced-
temperature SOFCs can be realized in two parallel ways: employing
materials with high functional properties and/or fabricating SOFCs
with novel thin-film concepts. Among basic SOFC designs, anode-
supported SOFCs with thin-film electrolyte are better suited for op-
eration at reduced temperature. Many groups have investigated the
planar anode substrate SOFC due to its high specific power output
and volumetric power density. However, works on tubular SOFCs
have shown that the tubular designs have an advantage of being
capable of coping with thermal stresses.1 Siemens Westinghouse
multiple tubular cell tests have been successfully conducted for al-
This paper reports the primary results of fabricating reduced tem-
perature, tubular anode-supported SOFCs by a wet cofire process.
Advantages of 10 mol % Sc2O3 ͑10SSZ͒ and 3 mol % Y2O3
͑3YSZ͒-stabilized ZrO electrolyte thin films, (CeO )0.9(GdO1.5)0.1
2
2
2
most 70,000 h. Small-scale tubular SOFCs, as described by Kendall
interlayer, and (La0.6Sr0.4)CoO cathode were combined to promote
3
3
4
and Prica, and Mizusaki et al., could endure rapid heating/cooling
and quickly respond to load variation within 1 min at an operating
temperature of 800°C. Therefore, it can be expected that a tubular
design can be applied to anode substrate SOFCs for cogeneration
and transportation applications.5 In such cases, a benefit on
sealing the stack is expected because the sealing length can be
the electricity generation at reduced temperature. 10SSZ and 3YSZ
were chosen among various stabilized ZrO2 electrolytes because
SSZ electrolytes present the highest oxide ionic conductivity,
whereas 3YSZ has the highest mechanical strength. Such properties
are needed for the thin electrolyte film. In addition, because 10SSZ
is expensive, replacement of 10SSZ by 3YSZ in a Ni cermet allows
not only strengthening the substrate but also cutting down expense
-8
9
7,8
reduced. Previously, Kato et al. estimated the performance at
medium temperature of lightweight Ni porous substrate tubular
SOFC with electrolyte film fabricated by plasma spraying. Because
single tubular cells can be joined by alloy interconnection to build
SOFCs modules and stacks, the weight and cost of SOFC systems
can be reduced drastically. We have investigated the possibility of
fabricating the tubular anode supported SOFC by a wet cofire pro-
cess, because it has the potential for mass production and cost re-
duction for SOFCs. The cofired cell has the basic configuration: Ni
cermet anode-supported tube/electrolyte film/cathode. In an ad-
vanced cell, the substrate thickness can be reduced or it can be
replaced by cheaper ceramic, Ni, or alloy porous tubes. In general,
the extrusion method is used to prepare a tubular ceramic substrate,
of material. Use of (La0.6Sr0.4)CoO is desirable because this mate-
3
rial exhibits high cathodic performance at reduced temperature. In
order to suppress the reaction between (La0.6Sr0.4)CoO and electro-
3
lyte, the fabrication of (CeO2)0.9(GdO1.5)0.1 ͑GDC͒ as an interlayer
on stabilized ZrO films by dip-coating was investigated in detail.
2
Performance characteristics of the cells were evaluated between 600
and 850°C. Based on the evaluated performance characteristics, the
problems of materials and fabrications can be clarified for this type
of SOFC.
Experimental
Preparation of Ni cermet substrate tube.—A mixture of 45 vol %
nickel oxide and 55 vol % 3YSZ ͑Tosoh Co.͒ powders was used to
make an anode substrate. Nickel oxide powder was obtained by
grinding NiCO •2Ni͑OH͒ •nH O ͑High Purity Chemical Co.͒ in an
*
Electrochemical Society Active Member.
E-mail: lan-nguyentuong@aist.go.jp
z
3
2
2
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Journal of The Electrochemical Society, 151 ͑8͒ A1230-A1235 ͑2004͒
alumina mortar and then thermally decomposed at 600°C. A green
A1231
substrate tube was fabricated by tape-casting. An anode slurry tape
with gauze immersed inside was prepared by doctor blading
organic-based slurry of anode powders. The slurry is comprised of
anode powders, pore former ͑poly-methyl-metha-acrylate, Souken
Kagaku Co.͒, and organic ingredients such as binder ͑poly vinyl
butyral͒, dispersant ͑oil fish͒, plasticizer ͑di-n-butyl-phthalate͒,
bubble-killing reagent ͑Triton-X͒, and solvents ͑isopropanol and
1
0
2
toluene͒. The used cotton gauze was 12 ϫ 12 threads per cm
netting ͑type I, Hakujuji Co., Japan͒. Cotton threads were ca. 100-
30 ␮m diam. Slurry tape was wound to form a tube with precise
1
dimensions and then calcined at 900°C in air by a properly con-
trolled temperature program. Because the thermal gravimetric analy-
sis ͑TG/DTA͒ has shown that organic components are burned out
between 150 and 350°C, a slow heating rate of 0.5°C/min was ap-
plied within this range, and a faster heating rate of 2.5°C/min was
used outside this temperature range.
Figure 1. Photograph of green NiO-3YSZ anode substrates, of which the
slurry tapes were ͑a͒ spirally and ͑b͒ cylindrically would around a mandrel.
diam were screen printed at the center on GDC films and then fired
at 950°C for 1 h in air. The ␴ of LSC electrodes was examined by
E
a three-probe method using a frequency analyzer ͑Solatron 1260͒
and a potentiostat ͑Solatron 1286͒ over the frequency range of 1
MHz to 10 mHz using Ϯ10 mV excitation voltage between 600 and
800°C in air. A Pt reference electrode was set on the side of SSZ
pellets. Pt lead wires and Pt mesh current collectors were used. The
Preparation of electrolyte film, GDC interlayer, and
(
%
La0.6Sr0.4)CoO cathode.—Powder of 10 mol % Sc O and 1 mol
3
2
3
CeO -stabilized ZrO ͑10SSZ, Daiichi Kigenso Kogyo Kagaku͒
2 2
and 3YSZ was used to prepare electrolyte films. Particle size of
0SSZ and 3YSZ powders was smaller than ca. 0.5 and 0.1 ␮m,
1
interfacial resistance (R ) of LSC electrodes was deduced from the
E
respectively. Electrolyte powder was dispersed into organic ingredi-
ents to form slurries by using a ball mill. Organic ingredients were
similar with those of the anode slurry. Electrolyte films were pre-
pared by the slurry dip-coating method. In the traditional slurry
dip-coating method, the substrate is coated with electrolyte film by
dipping it several times into a slurry tank and then withdrawing at an
appropriate speed. Here the technique was modified. The green tube
was vertically fixed inside a tank. A green slurry film was deposited
on the outside face of the tube when the slurry was flooded and then
emptied out through a slurry outlet located at the bottom of the tank.
The pressure inside the substrate tube was slightly decreased by
vacuum pump. This breaks away microbubbles of air that could be
trapped between the surface of the tube and the slurry. In this work,
the green electrolyte film was deposited by only one time coating.
The contact time of slurry with the green substrate anode tube was
ca. 5 s. After drying at room temperature in air, the green electrolyte
films were cofired on the substrate at 1300°C in air for 3 h.
12
complex impedance plots. The ␴E was calculated from RE as
1
␴E
ϭ
͓1͔
ARE
where A is area of LSC electrode, and RE is interfacial resistance.
For clarifying the effects of the GDC interlayer, ␴E of LSC on a
GDC disk was also measured.
Performance characteristics of tubular SOFCs.—The perfor-
mance characteristics were evaluated for both cases of 10SSZ and
3YSZ electrolyte cells. The effective cathode area of 10SSZ and
2
3YSZ cells was ca. 10 and 5.6 cm , respectively. Glass O-rings were
used to seal the cell between H and air sites. H was passed at a
2
2
2
rate of 8-15 mL/min/cm and humidified at room temperature.
Current-voltage and ac impedance measurements were made by a
two-probe method between 600 and 850°C by the potentio-
galvanostat ͑Toho Technical Research P/G-stat-2000͒ and a fre-
quency analyzer ͑NF-S-5720C͒ over the frequency range of 1 MHz
to 10 mHz using Ϯ10 mV excitation voltage. Pt lead wires were
used with a four-terminal configuration. Current collectors were Pt
and Ni meshes at the cathode and anode side, respectively.
As a protective layer for stabilized ZrO electrolyte films from
2
unfavorable solid-state reactions with (La0.6Sr0.4)CoO3 cathode, a
GDC film ͑Anan Kasei Co.͒ was also fabricated by the dip-coating
method. The slurry of GDC was prepared in the same way as that of
electrolytes. A green GDC interlayer was either subsequently coated
to the green electrolyte film and then cofired in air at 1300°C, or a
GDC green film was deposited on the dense electrolyte film and
fired at a lower temperature in the secondary step. The latter allows
suppressing unfavorable solid-state reactions between electrolyte
films and GDC.
Results and Discussion
Fabrication of NiO-3YSZ supported tubular SOFC.—In this
study, instead of the traditional extrusion method, tubular anode sub-
strates were manufactured by tape-casting. The slurry tape was
strong because cotton gauze was immersed inside the slurry. This
allows winding the sheet around a polished steel mandrel to form a
tubular shape. Cotton gauze and other organic ingredients were
burned out and removed during the calcinations to obtain a NiO-
Single-phase (La0.6Sr0.4)CoO ͑LSC͒ cathode powder was pre-
3
11
pared by the coprecipitation method. The LSC cathode was depos-
ited by brush painting a LSC paste, which consists of LSC powder,
cellulose acetate, and turpentine oil, on the GDC interlayer followed
by firing at 1000°C for 1 h in air. The heating and cooling rate of
each firing process was 2.5°C/min. The microstructure of samples
was examined by scanning electron microscopy ͑SEM͒.
3YSZ green tubular substrate. The design of the green substrate tube
can be altered by changing the size of the slurry sheet and the
mandrel, and also by changing the way of rolling the sheet around
the mandrel. Figure 1 shows an example of the appearance of tubes
with two open ends. The green anode substrate tubes were annealed
at 900°C in air for 2 h. The tube was shaped by rolling the tape
spirally ͑Fig. 1a͒ or cylindrically ͑Fig. 1b͒. The former technique is
suitable for mass producing long tubes. These green tubes were
fabricated with the thickness of ca. 0.5-1 mm. This is required be-
cause the tubular substrate should have sufficient mechanical
strength for supporting the thin electrolyte film in a finished product.
However, the thickness of the substrate can be reduced by utilizing
a corrugated design.
Measurement of the interfacial conductivity (␴ ) of LSC elec-
E
trode in a 10SSZ pellet/GDC interlayer/LSC half-cell.—In order to
examine the possibility of GDC interlayer for suppressing the unfa-
vorable solid-state reaction between LSC and the electrolyte film as
well as to attain a maximum cathodic activation of LSC, we exam-
ined the influences of firing temperature of the GDC interlayer to the
interfacial conductivity, ␴ , of LSC electrode in 10SSZ pellet/GDC
E
interlayer/LSC half-cells. 10SSZ disks ͑15 mm/diam; 2 mm thick͒
were used as the solid electrolyte. GDC films were prepared by
coating GDC slurry on both faces of 10SSZ disks and then firing
between 1000 and 1300°C in air for 2 h. LSC electrodes of 6 mm
The 10SSZ and 3YSZ electrolyte films were fabricated on the
green anode substrate tube by a slurry dip coat and cofire process.
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Journal of The Electrochemical Society, 151 ͑8͒ A1230-A1235 ͑2004͒
Figure 2. Photograph of cofired NiO-3YSZ anode supported tubular
SOFCs: ͑a, b͒ anode-supported electrolyte film half-cells, ͑c͒ an SOFC with
GDC interlayer, ͑d͒ a half-cell with corrugated substrate, and ͑e͒ a half-cell
combined with a Ni porous substrate tube.
The success of fabricating the film depends on both electrolyte
slurry coating and cofiring processes. An electrolyte green film
should be thin, flawless, and adhere well to the substrate. In order to
avoid cracking of electrolyte film due to possible stress induced by
the cofiring process, shrinkages of the substrate and electrolyte film
were carefully investigated.13 Photographs of cofired samples are
given in Fig. 2. The half-cells with the basic structure as thick NiO-
3
2
YSZ anode-supported tube/thin electrolyte film are shown in Fig.
a and b. A finished product with GDC interlayer and LSC cathode
is given in Fig. 2c. These samples have a good appearance with
straight length and round cross section. The outer diameter and
thickness are 6-10 and 1 mm, respectively. In this work, tubular cells
were fabricated with a longest length of ca. 25 cm, which was lim-
ited by the size of the furnace. Figure 2d shows a cofired half-cell
with a corrugated substrate. The thickness of the anode layer, which
is adjacent to the electrolyte film, was reduced to around 150 ␮m. It
can be expected that the resistance of gaseous diffusion on the anode
side can be improved with the reduction of the thickness of anode
Figure 3. SEM observations of cofired anode-supported tubular SOFCs: ͑a͒
surface of SSZ film, ͑b͒ surface of 3YSZ film, ͑c͒ cross section of 10SSZ
electrolyte cell, ͑d͒ cross section of 3YSZ electrolyte cell, ͑e͒ surface of
GDC interlayer fired at 1200°C, and ͑f͒ cross section of 10SSZ electrolyte
film and GDC interlayer at a high magnification.
1
4
substrate. Figure 2e shows a half-cell that was cofired with a Ni
7,8
porous substrate tube. In order to combine the SOFC with the Ni
substrate tube into one, the NiO-3YSZ green substrate was precisely
prepared with consideration of its shrinkage coefficient. The thick-
ness of the cell and the Ni substrate tube was ca. 0.3 and 0.9 mm,
respectively. Because the cell was cofired in air, the Ni substrate
tube was oxidized to NiO, but it keeps in touch firmly with the
SOFC. This confirms the possibility of developing metallic
substrate-supported thin electrolyte film SOFCs in a cost-effective
way. Details of the fabricating process and properties of this type of
SOFC have been under further investigation and will be reported
elsewhere.
SEM observations of cofired tubular NiO-3YSZ/10SSZ/GDC/
LSC and NiO-3YSZ/3YSZ/GDC/LSC cells are shown in Fig. 3. The
dense electrolyte films were successfully fabricated upon porous
substrate. As shown in Fig. 3a and b, surfaces of 10SSZ and 3YSZ
electrolyte films did not exhibit cracks or pinholes. The grain sizes
of 10SSZ varied between ca. 3 and 7 ␮m ͑Fig. 3a͒, whereas 3YSZ
particles were submicrometer ͑Fig. 3b͒. Grain growth rate of 10SSZ
was relatively higher than that of 3YSZ. For achieving better me-
chanical strength of very thin ceramic film, suppression of grain
grown during the cofire process is important. Hence, there is a need
to further reduce the particle sizes of 10SSZ powder. Cross sections
of the 10SSZ and 3YSZ electrolyte cells are shown in Fig. 3c and d,
respectively. GDC interlayer and LSC cathode were subsequently
deposited on electrolyte films and fired at 1200 and 1000°C, respec-
tively. Dense, thin, and uniform electrolyte films adhered well to the
porous anode substrate and GDC interlayer. Electrolyte films can be
fabricated thinner than 10 ␮m. Because the 3YSZ particles are sub-
micrometer, further reduction in thickness of 3YSZ electrolyte film
is expected by reducing the amount of 3YSZ powder dispersed in
the slurry. Thin electrolyte film with few closed submicrometer
pores was fabricated by sintering at 1300°C without using any sin-
tering aid. This is achieved by optimizing the compositions of slurry
and also the coating technique to form a uniform and high-packed
green film. Figure 3e and f shows the microstructures of ca. 3 ␮m
GDC interlayer, which was deposited upon dense 10SSZ film of
NiO-3YSZ/10SSZ/LSC cell, then fired at 1200°C for 2 h. Cracks
were not found on the GDC interlayer. The GDC particles were
sintered. However, because the half-cell with an electrolyte film pre-
viously cofired at 1300°C did not contract further at 1200°C, only
GDC particles shrunk. Therefore, openings occurred in the sintered
GDC interlayer. This is different in the case of double layers of
electrolyte and GDC cofired on the NiO-3YSZ substrate. In that
case, as shown in Fig. 4, despite that close pores remained inside, a
dense GDC interlayer was also obtained. SEM observations also
show that despite its large thermal expansion coefficient, porous
LSC cathodes with a thickness of ca. 20-30 ␮m were also attached
stably to triplet substrates.
Influences of GDC interlayer on the interfacial conductivities,
␴E , of LSC.—Figure 5 shows the Arrhenius plots of ␴E of LSC
electrode measured between 600 and 800°C in air on 10SSZ/GDC/
LSC half-cells. The firing temperature of LSC electrode was fixed
at 950°C, but the firing temperature of the GDC interlayer was var-
Figure 4. SEM observation of 10SSZ and GDC double layer cofired on a
NiO-3YSZ substrate at 1300°C: ͑a͒ surface of GDC and ͑b͒ cross section of
the cofired 10SSZ/GDC double layers.
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Journal of The Electrochemical Society, 151 ͑8͒ A1230-A1235 ͑2004͒
A1233
Figure 7. Performance curves of a cofired, anode-supported tubular with
SSZ electrolyte film Ni-3YSZ/SSZ/GDC/LSC SOFC at 800°C.
with the increase of firing temperature, GDC particles were further
sintered and became larger. The openings were also enlarged. For
cases of the interlayer fired at 1200 and 1300°C, the size of the
largest opening was ca. 100 and 200 nm, respectively. With the
thickness of a few micrometers, the interlayer with such small open-
ings should be effective to keep electrolyte film from unfavorable
solid-state reactions with LSC. The grain growth of GDC particles
reinforced their connections. Such improvements in microstructure
of the GDC interlayer enhances the oxygen reduction reaction oc-
curring at the triple-phase boundary of LSC-GDC-air. Consequently,
in this study the firing temperature of the GDC layer was kept at
Figure 5. Arrhenius plots of interfacial conductivity, ␴ , measured between
E
6
00 and 800°C in air of LSC electrode on 10SSZ disk/GDC interlayer. The
GDC interlayer was fired at ͑᭞͒ 1000, ͑᭝͒ 1100, ͑᭺͒ 1200, and ͑ᮀ͒ 1300°C.
᭹͒ LSC on a GDC disk.
͑
ied between 1000 and 1300°C. ␴E increased when the firing
temperature of the GDC layer was increased to 1200°C (␴E,1200
Ͼ ␴E,1300 Ͼ ␴E,1100 Ͼ ␴E,1000). ␴E,1300 and ␴E,1200 are higher than
that of LSC deposited on a dense GDC disk. The activation energy
1200°C. However, if the solid-state reaction and interdiffusion phe-
nomena at the interface of zirconia in contact with the GDC, of
which the ionic conductivity of the product is lower by two orders of
(
E ) also showed a reduced tendency with increasing GDC firing
a
magnitude than stabilized ZrO , are taken into consideration, it is
2
temperature
(Ea,1200 ϭ 1.39 eV Ͻ Ea,1300 ϭ 1.47 eV Ͻ Ea,CGO
preferable to fire GDC upon zirconia at as low as possible
ϭ 1.50 eV Ͻ Ea,1100 ϭ 1.61 eV Ͻ Ea,1000 ϭ 1.98 eV). The results
reveal that the GDC interlayer successfully suppressed the unfavor-
15
temperature. Further optimizations for preparing the GDC inter-
layer have been under examination.
able solid-state reaction between LSC and stabilized ZrO electro-
2
lyte films. Preparation of GDC films at a temperature between 1200
and 1300°C gives full scope to the cathodic performance of LSC on
stabilized ZrO2 electrolyte SOFCs. The microstructure of GDC
films fired between 1000 and 1300°C are shown in Fig. 6. Together
Tubular cell performance test.—Preliminary cell performance
tests were carried out on tubular cells with the basic configuration as
shown in Fig. 2c. The cell with 10SSZ electrolyte film showed an
open-circuit voltage ͑OCV͒ of ca. 1.0 V and a power density of 150
2
mW/cm at 800°C ͑Fig. 7͒. Because the OCV was lower than the
theoretical value of 1.1 V, gas leakages were recognized in this cell.
Reasons for this may be a defect in the glass seal, or during the
reduction of NiO in the substrate some microcracks were yielded on
the 10SSZ film. Further investigations are required for improving
the tubular Ni-3YSZ/10SSZ/GDC/LSC SOFC.
Performances of the cell with 3YSZ electrolyte film are shown in
Fig. 8. The cell generated electricity reproducibly with thermal re-
cycles between 600 and 850°C. OCV values were ca. 1.1 V, higher
than that of the SSZ case, and in good agreement with theoretical
values. This indicated that the 3YSZ electrolyte thin film was per-
fectly fabricated. This is expected because 3YSZ is tougher and
1
6
stronger than other electrolyte materials. Furthermore, the utilized
YSZ particles were not only fine but also had a low growth rate.
3
2
Maximum power densities were ca. 170 mW/cm ͑600°C͒, 195
mW/cm ͑650°C͒, 220 mW/cm ͑700°C͒, 220 mW/cm ͑750°C͒,
2
2
2
2
2
2
30 mW/cm ͑800°C͒, and 225 mW/cm ͑850°C͒. The power den-
sity at around 600°C is satisfactory because it is higher than that
of other SOFCs, which were composed of Ni-10SSZ anode sub-
strate, 10SSZ electrolyte film, and different cathode materials such
2
10
2
as 100 mW/cm of Ni-10SSZ/10SSZ/Pt-10SSZ, 60 mW/cm of
1
3
Ni-10SSZ/10SSZ/Ag-Pt-10SSZ, and it is comparable with the
power output of 190 mW/cm achieved for a small disk-type Ni-
3YSZ/10SSZ/GDC/LSC SOFC. However, when increasing tem-
perature,
2
Figure 6. SEM observations of surface of GDC interlayer fired at various
temperatures on 10SSZ disks. The firing temperatures were ͑a͒ 1000, ͑b͒
1
7
1100, ͑c͒ 1200, and ͑d͒ 1300°C.
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Journal of The Electrochemical Society, 151 ͑8͒ A1230-A1235 ͑2004͒
Figure 8. ͑left͒ Cell voltage and ͑right͒ power density as a function of cur-
rent densities for a cofired, anode-supported tubular with 3YSZ electrolyte
film Ni-3YSZ/3YSZ/GDC/LSC SOFC between 600 and 850°C: ͑ϩ͒ 600,
͑᭞͒ 650, ͑छ͒ 700, ͑᭝͒ 750, ͑᭺͒ 800, and ͑ᮀ͒ 850°C.
Figure 10. ͑᭝͒ Ohmic resistance, R , ͑छ͒ polarization resistance of elec-
b
the power densities did not increase much. This means that in this
case performances of the cell were mainly affected by less- or non-
thermal activating processes.
Shown in Fig. 9 are the complex impedance plots measured at
the open-circuit condition. The plots show two arcs. As the first arc
varied but the second arc did not vary much with temperature, we
trodes, R , ͑᭺͒ gaseous diffusion resistance, R , and ͑ᮀ͒ total resistances,
1
2
Rtotal as determined from the impedance spectra. ͑ϫ͒ Ohmic resistance of 15
m thickness of 3YSZ film.
␮
SOFCs.18,19 The resistances are plotted in Fig. 10 as a function of
temperature. Rb is larger than both R1 and R2 . Rb is also larger than
the theoretical value calculated for 15 ␮m thick 3YSZ film. In ad-
dition, temperature dependence of Rb was less than that of 3YSZ
can attribute the Cole-Cole plots to ohmic resistance, R ͑left inter-
b
cept on z axis͒, polarization resistance of electrodes ͑first arc at high
frequency͒, R , and gaseous diffusion resistance, R , ͑second arc at
1
2
low frequency͒. Such behavior of the gaseous diffusion resistance
was also shown in some other studies on anode-supported planar
electrolyte. The apparent activation energy (E ) of 0.3 eV was cal-
a
16
culated for R , while the 3YSZ shows an E of 0.8 eV. The ob-
b
a
servations show that ohmic losses other than electrolyte losses are
significant. Because GDC interlayer was deposited upon the 3YSZ
layer, formation of a very thin layer of ͑Zr,Ce͒O solid solution can
2
be the reason for such behavior of R . This suggests that further
b
control of fabrication of GDC interlayer is required. Because the
current collector was a Pt mesh, limited contact between the cathode
and the Pt mesh may also have contributed to the ohmic resistance.
Regarding the processes occurring on electrodes, because R did not
2
vary much with the change of temperature, below 700°C R1 is
greater than R , but at higher temperatures R is lesser than R2 .
2
1
Together with less dependence of R on temperature, this can ex-
b
plain why more electricity cannot generate when the operating tem-
perature is increased to between 700 and 850°C. The cause of gas-
eous diffusion resistance can be attributed to low porosity of the
thick anode substrate and also to a stagnant gas layer existing out-
1
8
side the porous anode substrate. The low anodic activity of Ni-
YSZ should also be noted. The control of porosity and thickness of
3
the anode substrate are other key issues for the cofire cell. In addi-
tion, further improvement of the LSC cathode is also expected for
improving cell performance.
Conclusions
The present investigation has confirmed the high technological
feasibility of the wet cofired process for fabricating reduced-
temperature operable anode-supported tubular SOFCs. Because the
process is simple and does not require complicated equipment, it is
useful for mass producing cost-effective, tubular-type SOFCs.
It was possible to use Co-based perovskite oxides as cathode
materials with zirconia-based electrolyte in conjunction with a GDC
interlayer, which was prepared by the slurry dip-coating method.
The performance of LSC cathode was enhanced by optimizing the
microstructure of the GDC interlayer. However, because a layer of
Figure 9. Impedance spectra at open-circuit state of the Ni-3YSZ/3YSZ/
GDC/LSC SOFC ͑3% humidified H vs. air͒.
2
Downloaded on 2015-04-04 to IP 132.174.254.155 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 151 ͑8͒ A1230-A1235 ͑2004͒
A1235
the Solid Oxide Fuel Cell Society of Japan, Tokyo, Japan ͑2001͒.
. J. Van Herle, R. Ihringer, N. M. Sammes, G. Tompsett, K. Kendall, K. Yamada, C.
Wen, T. Kawada, M. Ihara, and J. Mizusaki, Solid State Ionics, 132, 333 ͑2000͒.
͑
Ze,Ce͒O2 solid solution with very low conductivity at the
5
ZrO /GDC interface can be formed, which in turn increases the
2
ohmic loss, the fabrication of GDC interlayer should be further op-
timized.
6. R. H. Song, E. Y. Kim, D. R. Shin, and H. Yokogawa, in Solid Oxide Fuel Cells VI,
S. C. Shinghal and M. Dokiya, Editors, PV 99-19, p. 845, The Electrochemical
Society Proceedings Series, Pennington, NJ ͑1999͒.
The cofired anode substrate tubular SOFC with stabilized ZrO2
electrolyte films, GDC interlayer, and LSC cathode showed high
performance at reduced temperature. The performance is able to be
improved by suppressing the influences of gaseous diffusion at the
anode side, and solid-state reaction at the interface of the electrolyte
and interlayer, and also by further improving the cathodic activities.
7
. T. Kato, S. Wang, A. Negishi, S. Nagata, and K. Nozaki, in The Fourth European
Solid Oxide Fuel Cell Forum Proceedings, A. J. McEvoy, Editor, Vol. 2, p. 553,
Lucerne, Switzerland ͑2000͒.
8. T. Kato, S. Wang, N. Iwasita, T. Honda, T. Kaneko, A. Negishi, S. Nagata, and K.
Nozaki, in Solid Oxide Fuel Cells VII, H. Yokogawa and S. C. Shinghal, Editors,
PV 2001-16, p. 1065, The Electrochemical Society Proceedings Series, Pennington,
NJ ͑2001͒.
9
. W. Winkler and J. Kruger, J. Power Sources, 71, 244 ͑1998͒.
The National Institute of Advanced Industrial Science and Technology,
Japan, assisted in meeting the publication costs of this article.
1
0. T. L. Nguyen, K. Kobayashi, Z. Cai, I. Takahashi, K. Yasmoto, M. Dokiya, S.
Wang, and T. Kato, in Solid Oxide Fuel Cells VII, H. Yokogawa and S. C. Shinghal,
Editors, PV 2001-16, p. 1042, The Electrochemical Society Proceedings Series,
Pennington, NJ ͑2001͒.
List of Symbols
1
1. S. Wang, T. Kato, S. Nagata, T. Honda, T. Kaneko, N. Iwasita, and M. Dokiya,
Solid State Ionics, 146, 203 ͑2002͒.
2. M. Shyono, K. Kobayashi, T. L. Nguyen, K. Hosoda, T. Kato, K. Ota, and M.
Dokiya, Solid State Ionics, Submitted.
3. Z. Cai, T. L. Nguyen, S. Wang, and M. Dokiya, Solid State Ionics, 152-153, 583
A
area of (La0.6Sr0.4)CoO3 cathode
RE interfacial resistance
interfacial conductivity
1
1
1
␴
E
Ea activation energy
Rb ohmic resistance of a tubular cell
R1 polarization resistance of electrodes of a tubular cell
R2 gaseous diffusion resistance of a tubular cell
Rtotal total resistance of a tubular cell
͑
2002͒.
4. J. W. Kim and A. V. Virkar, in Solid Oxide Fuel Cells VI, S. C. Shinghal and M.
Dokiya, Editors, PV 99-19, p. 830, The Electrochemical Society Proceedings Se-
ries, Pennington, NJ ͑1999͒.
1
5. A. Tsoga, A. Gupta, A. Naoumidis, and P. Nikolopoulos, Acta Mater., 48, 4709
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