Fabrication and testing of a doped lanthanum gallate electrolyte thin-film solid oxide fuel cell

Article abstract of DOI:10.1149/1.1496485

A supported lanthanum gallate (LSGM) electrolyte thin-film solid oxide fuel cell with Ni-YSZ cermet anode and strontium-doped lanthanum manganite (LSM)-yttria stabilized zirconia (YSZ) composite cathode was, for the first time, fabricated and tested. The cell was prepared by an unconventional approach, in which an LSGM thin film (about 15 μm thick) was first deposited on a porous substrate such as a porous YSZ disk by a wet process and sintered at a high temperature (above 1400°C). NiO was then incorporated into the porous substrate by a carefully controlled impregnation process and fired at a much lower temperature. In this way, the severe reaction between LSGM and NiO at a high temperature, which is required for the full densification of LSGM film, can be avoided. A strontium-doped LaMnO₃ (LSM)-YSZ composite cathode was screen printed on the surface of the LSGM film and then fired at 1250°C. The electrolyte resistances of the SOFC single cells fabricated by this approach are much lower compared to those of thick LSGM film supported cells. A maximum output power density of over 0.85 W/cm² at 800°C with H₂ as fuel and air as oxidant for a fabricated cell was achieved.

Full text of DOI:10.1149/1.1496485

A1132
Journal of The Electrochemical Society, 149 ͑9͒ A1132-A1135 ͑2002͒
0
013-4651/2002/149͑9͒/A1132/4/$7.00 © The Electrochemical Society, Inc.
Fabrication and Testing of a Doped Lanthanum Gallate
Electrolyte Thin-Film Solid Oxide Fuel Cell
z
J. W. Yan, Z. G. Lu, Y. Jiang,* Y. L. Dong, C. Y. Yu, and W. Z. Li
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
A supported lanthanum gallate ͑LSGM͒ electrolyte thin-film solid oxide fuel cell with Ni-YSZ cermet anode and strontium-doped
lanthanum manganite ͑LSM͒-yttria stabilized zirconia ͑YSZ͒ composite cathode was, for the first time, fabricated and tested. The
cell was prepared by an unconventional approach, in which an LSGM thin film ͑about 15 ␮m thick͒ was first deposited on a
porous substrate such as a porous YSZ disk by a wet process and sintered at a high temperature ͑above 1400°C͒. NiO was then
incorporated into the porous substrate by a carefully controlled impregnation process and fired at a much lower temperature. In this
way, the severe reaction between LSGM and NiO at a high temperature, which is required for the full densification of LSGM film,
can be avoided. A strontium-doped LaMnO ͑LSM͒-YSZ composite cathode was screen printed on the surface of the LSGM film
3
and then fired at 1250°C. The electrolyte resistances of the SOFC single cells fabricated by this approach are much lower
2
compared to those of thick LSGM film supported cells. A maximum output power density of over 0.85 W/cm at 800°C with H2
as fuel and air as oxidant for a fabricated cell was achieved.
©
2002 The Electrochemical Society. ͓DOI: 10.1149/1.1496485͔ All rights reserved.
Manuscript submitted November 15, 2001; revised manuscript received March 4, 2002. Available electronically July 16, 2002.
Strontium- and magnesium-doped lanthanum gallate ͑LSGM͒
Experimental
was first reported to be a potential candidate as an electrolyte for
Preparation of porous substrate.—Porous substrates were made
of YSZ. A commercial YSZ powder ͑TZ-8Y͒ was well mixed with a
certain amount of pore former such as carbon powder and then
uniaxially die-pressed into disks of 25 mm diam and 1.5 mm thick.
The disks were prefired at 1000-1100°C to remove the pore former
and form porous substrates. The porosity of the substrate was tai-
lored to at least 60% by the amount of pore former added so that it
could accommodate a large percentage of anode catalyst, which was
subsequently impregnated. The amount of the carbon pore former is
intermediate temperature solid oxide fuel cells ͑SOFCs͒ by Ishihara
1
et al. Following his work, considerable investigation have been car-
ried out regarding its ionic conductivity, stability, and chemical and
thermal compatibility with electrode materials.2-6 Many studies have
7
-9
also been conducted on the fabrication of LSGM-based SOFCs.
In those SOFCs, thick ͑ca. 500 ␮m͒, well-densified LSGM pellets
were generally used as electrolyte. The performances of the LSGM
based cells were indeed much improved compared with thick yttria-
stabilized zirconia ͑YSZ͒ film based SOFCs at the same conditions
because of the high conductivity of the electrolyte and thus, a much
lower ohmic resistance loss. A maximum output power density of a
30-40 wt %. An LSGM film was deposited on the substrate by a
wet process. An LSGM slurry was prepared from
La0.8Sr0.2Ga0.8Mg0.2O3 Ϫ ␦ ͑LSGM͒ ultrafine powder. Ethanol was
used as solvent and polyvinyl butyral ͑PVB͒ as suspension aid. The
slurry was applied quantitatively onto the surface of the substrate,
and then cofired at 1400°C for a couple of hours to form a well-
bonded, dense LSGM film supported on the YSZ porous substrate.
The thickness of the LSGM film was generally controlled at 15-20
2
thick LSGM cell ͑600 ␮m thick͒, around 0.90 W/cm at 800°C, was
reported by Huang et al.,10 which is much higher than those of the
YSZ thick electrolyte based cells operated at 950-1000°C. Although
the power density of an LSGM cell is improved significantly due to
the high ionic conductivity of the LSGM electrolyte, it is still much
lower than that of an SOFC constructed on an anode or a cathode
supported YSZ thin film ͑10 ␮m͒, for which power densities as high
␮m.
2
11-13
as 1.5-1.8 W/cm have been reported.
The problem with a thick
Preparation of anode and cathode.—The anode was prepared by
LSGM cell is that the ohmic loss of the thick electrolyte film, more
than 20 times thicker than that of a YSZ thin film, is still dominant.
Obviously, the reduction of the thickness of LSGM film down to
an impregnation technique frequently used in the preparation of het-
erogeneous catalysts. The impregnation solution was Ni(NO ) wa-
3
2
ter solution prepared from Ni(NO ) •6H O reagent. The porous
3
2
2
10-20 ␮m could possibly lead to a remarkable improvement of
YSZ substrate was carefully impregnated with the solution, dried,
and heated to 700°C to decompose Ni(NO ) into NiO. The impreg-
power density and to a lower operation temperature than 800°C.
However, the preparation of a supported LSGM thin film appears to
be much more difficult. The complexity of the composition of
LSGM makes electrochemical vapor deposition ͑EVD͒ and other
related methods very complicated. The reactive nature of LSGM
with Ni and other transition metals makes it impossible to prepare a
dense LSGM thin film without electronic conductivity on an anode
substrate by the tape-casting technique. Recently, Mathews reported
the preparation of an LSGM thin film on a Pt substrate by electro-
3
2
nation procedure was repeated several times to ensure sufficient in-
take of NiO in the porous substrate. The amount of NiO impreg-
nated should exceed the percolation value so that after reduction, a
continuous and conductive Ni-YSZ cermet anode is formed. The
optimal ratio of NiO in the cermet is about 56 wt %. After the
completion of impregnation, the sample was fired at 1000°C for a
couple of hours to enhance the contact between NiO and the YSZ
substrate. Reduction of NiO to Ni was carried out in situ prior to the
test of a single cell performance. To complete a full single cell, a
La0.8Sr0.2MnO3Ϫ␦ ͑LSM͒-YSZ composite cathode was applied on
the LSGM film surface from an LSM-YSZ paste, and fired in air at
1
4
15
phoretic deposition and by pulsed laser ablation. Unfortunately,
no supported LSGM thin-film-based SOFC was constructed because
of practical difficulties in preparing a realistic complete cell with
these methods. The preparation of LSGM thin-film SOFCs still re-
mains difficult and tempting. In this paper, an unconventional ap-
proach was developed to prepare LSGM thin-film SOFCs. For the
first time, a single cell based on the supported LSGM thin film with
1
250°C for 1 h. The detailed procedure for the preparation of LSM-
16
YSZ cathode can be found elsewhere. The LSM-YSZ cathode was
typically 20 ␮m thick. If necessary, a layer of Ag paste was applied
onto the surface of the cathode to ensure a good current collection.
NiO-YSZ as anode and strontium-doped LaMnO ͑LSM͒-YSZ as
3
cathode was fabricated and tested, showing high performance.
Test of single cells.—The completed cell was sealed on one end
of an alumina tube with Pyrex glass powder. Pt meshes were used as
current collectors and spring-pressed against the cathode and the
anode. A Pt reference electrode was placed on the electrolyte surface
at the cathode side by applying a Pt paste and sintering in air at
*
Electrochemical Society Active Member.
E-mail: yanjw@ms.dicp.ac.cn
z
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Journal of The Electrochemical Society, 149 ͑9͒ A1132-A1135 ͑2002͒
A1133
Figure 1. An SEM micrograph of the cross section of an LSGM thin-film
Figure 2. An SEM micrograph of the top view of the LSGM thin film on a
SOFC with an impregnated Ni anode and an LSM-YSZ cathode.
porous substrate.
9
00°C for 30 min. Resistances of LSGM electrolyte film, i.e., IR
a NiO-YSZ green disk and then cofiring the sample at about
drops of the constructed cells, were measured by the four-point
probe current interruption technique using an EG&G 173
potentiostat/galvanostat, an EG&G 175 universal programmer, and a
high-speed oscilloscope. The performance of the cell was character-
1400°C͒. It can be found that a nearly pure perovskite-phase LSGM
film was formed on the NiO-YSZ substrate prepared by the impreg-
nation route. However, many more impurity peaks appeared on the
XRD pattern of the LSGM film prepared by the traditional proce-
dure, showing that a severe reaction between the LSGM film and
NiO in the anode substrate occurred. So a conclusion can be drawn
that the reaction between LSGM and NiO during the LSGM film
preparation process can be effectively avoided by introducing this
unconventional preparation approach.
ized at 800°C after in situ reduction of the NiO anode in H for
2
several hours. Hydrogen passed over the anode at a flow rate of 150
mL/min, and air flew over the cathode surface at a flow rate of 400
mL/min. Voltages across the anode and cathode as well as voltage
between the anode and the reference were measured at various cur-
rent densities by changing the external load.
The ohmic resistance of the film was measured by a current
interruption technique. The potential transient across the film was
captured by a high-speed oscilloscope after an imposed current den-
Characterization of the chemical compatibility between related
cell materials and the morphology of the LSGM films.—The spent
cells were fractured and then examined by scanning electron micros-
copy ͑SEM͒. The SEM observation was performed on a scanning
electron microscope ͑Hitachi, S-3200N͒. Typical and representative
pictures of both top- and cross-view microstructures were taken by a
digital camera.
2
sity of 2 A/cm was interrupted. The potential transient clearly
showed the linear drop in the initial time interval of less than 5 ␮s,
which was attributed to the potential drop over the ohmic resistance
of the LSGM thin film. The total linear potential drop was about
0.15 V. The specific ohmic resistance across the LSGM film was
X-ray diffraction ͑XRD͒ patterns of the LSGM thin films pre-
pared by this new approach and the traditional route ͑directly depos-
iting LSGM thin film on a NiO-YSZ substrate͒ were collected on a
D-MAX-RB X-ray model diffractometer using Cu K␣ radiation.
The chemical compatibilities of LSGM with YSZ, LSM, and
NiO in the positive-electrolyte-negative ͑PEN͒ assembly constructed
were examined by energy-dispersive X-ray analysis ͑EDX͒. The
EDX patterns were collected by a scanning electron microscope
͑
JEOL JSM-5600LV͒ equipped with an EDX accessory ͑Oxford
University͒.
Results and Discussion
Figure 1 shows an SEM micrograph of the cross section of a
fractured cell after testing. The top layer was the impregnated and
reduced Ni-YSZ cermet anode, showing uniform distribution of Ni
particles in the porous YSZ substrate. The bottom layer was a po-
rous LSM-YSZ composite cathode, the thickness of which was
around 20 ␮m. Shown in the middle, the LSGM film was about 15
␮m thick, well sintered, and dense, although there were a few
sparsely populated pinholes which were not penetrating through the
whole film. The top-view SEM micrograph ͑Fig. 2͒ clearly shows
the grain morphology on the surface of a sintered LSGM film. The
grains were around 1-3 ␮m in size and closely packed. There were
no obvious cracks observed on the film.
Figure 3 shows the XRD pattern of the LSGM thin film prepared
by the unconventional technique compared with that prepared by a
traditional preparation procedure ͑depositing LSGM film directly on
Figure 3. XRD pattern of an LSGM thin film prepared by the unconven-
tional approach compared with that prepared by directly depositing LSGM
film on NiO-YSZ substrate and cofiring it at high temperature ͑about
1400°C͒.
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A1134
Journal of The Electrochemical Society, 149 ͑9͒ A1132-A1135 ͑2002͒
Figure 5. The performance of a typical LSGM thin-film cell with an im-
pregnated Ni anode and an LSM-YSZ cathode at 800°C, H2
Figure 4. EDX analysis results of the YSZ substrate and the supported
LSGM film prepared by the unconventional approach ͑prior to the impreg-
nation of NiO and preparation of LSM-YSZ cathode͒: ͑I͒ LSGM film 15 ␮m
thick, ͑II͒ porous YSZ substrate.
ϭ 200 mL/min, air ϭ 300 mL/min.
loss was no longer a dominant contribution to the total potential loss
as in a thick LSGM electrolyte cell. At a current density of
2
2
.3 A/cm , the ohmic resistance loss was only 0.17 V, comparable
2
2
0
.15 V/2 A/cm ϭ 0.075 ⍀ cm . The conductivity of bulk LSGM
to the overpotential losses of the anode and the cathode. The ca-
thodic overpotential was the largest, and the anodic overpotential at
the maximum current density, although the smallest, reached 0.15 V.
To find out the reason for the relatively low output performance
of the single cell described ͑including the relatively low OCP and
high cathodic overpotential͒, EDX was also applied to analyze the
distribution of elements on the cross section of a PEN assemble
Ϫ1
at 800°C is about 0.08 S cm . In theory, the specific resistance of
a dense LSGM film of 15 ␮m in thickness is 0.0188 ⍀ cm , only a
2
fraction of the measured one. Several reasons might probably ex-
plain the much higher measured specific resistance. The supported
LSGM film was not as well sintered as a bulk LSGM sample, which
is usually obtained by die-pressing green powder and firing at a
higher temperature. The existence of pinholes and the much smaller
grain size might also lower the conductivity. The electrodes on both
sides of the LSGM film might add some resistance to the measured
value when the electrode conductivity is not sufficiently high and
the resistance of the electrode is comparable to that of a thin LSGM
film. Another most possible reason associated with this relatively
high specific resistance of the thin LSGM film was that a reaction
between LSGM and YSZ occurred during the high-temperature fir-
ing process. EDX was applied to confirm this reaction. Figure 4
shows the EDX patterns for all the elements on a cross section near
the interface of the YSZ porous substrate and the LSGM thin film
prior to the impregnation of NiO. No obvious element migration
between the YSZ substrate and the LSGM film can be observed
except for strontium and magnesium. Nearly all strontium and most
magnesium migrated into the YSZ substrate. The loss of strontium
and magnesium in the electrolyte film lowered the electrical conduc-
tivity of the electrolyte LSGM because the doping levels for both A
͑Fig. 7͒. It can be clearly found that after the preparation of LSM
cathode at 1250°C, all elements in LSGM including lanthanum,
strontium, gallium, and magnesium showed some migration into the
YSZ substrate. The migration depths of lanthanum and gallium were
about 15 and 5 ␮m, respectively. However, the migration depth of
magnesium was much higher than those of lanthanum and gallium.
Strontium also showed a migration into the YSZ substrate but the
migration depth could not be determined because of its relatively
low signal intensity. No zirconium and yttrium were observed in the
LSGM thin film. Nickel did not show noticeable migration into the
and B site in LaGaO were significantly decreased.
3
Although the specific resistance of the film is much higher than
the expected one, it is significantly reduced compared with that of a
2
thick LSGM electrolyte ͑0.5 mm, about 0.5 ⍀ cm ͒.
Figure 5 shows the typical cell performance of LSGM thin-film
single cells with impregnated Ni anodes and LSM-YSZ composite
cathodes at 800°C. The open-circuit potential ͑OCP͒, as shown, was
around 0.95 V, somehow lower than that of a thick LSGM film
single cell ͑about 1.1 V͒ on H and air. The maximum power density
2
2
obtained from the thin LSGM cells was around 0.85 W/cm . It is
also worth noting that the slope of the pseudolinear region of I-V
curve, which includes the electrolyte resistance, electrochemical re-
sistance at high polarization, and gas-transport resistance through
the porous electrodes, is significantly reduced, also suggesting a
much smaller electrolyte resistance. Further experiments were con-
ducted with a reference electrode on the cathode side to find out
anodic and cathodic overpotentials and ohmic resistance loss. The
results are shown in Fig. 6. It can be seen that the ohmic resistance
Figure 6. Anodic and cathodic overpotentials and ohmic resistance loss of
the LSGM thin-film cell with an impregnated Ni anode and an LSM
Ϫ YSZ cathode. Same conditions as in Fig. 5
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Journal of The Electrochemical Society, 149 ͑9͒ A1132-A1135 ͑2002͒
A1135
layer between the cathode layer and the LSGM thin film, or by
utilizing better cathode materials such as doped lanthanum cobalt
and samarium cobalt, which are shown to be much more active than
LSM. Optimization in the preparation of LSGM film and the porous
substrate is also necessary to obtain a better quality supported
LSGM thin film, leading to a high OCP and an improved cell
performance.
Conclusion
An SOFC single cell with supported strontium- and magnesium-
doped lanthanum gallate thin film as electrolyte, Ni-YSZ cermet as
anode, and LSM-YSZ composite as cathode was, for the first time,
fabricated by an unconventional approach. The thin LSGM films
prepared on porous YSZ substrate were around 15 ␮m thick and the
2
specific resistance was substantially reduced to 0.075 ⍀ cm . The
Ni anodes, which were prepared by an impregnation method, fired at
a much lower temperature, and reduced in situ, were sufficiently
electrochemically active. The anode overpotential was least among
the overpotentials and ohmic resistance loss while the cathode over-
potential was largest, which may be associated with the migration of
Mn from LSM-YSZ cathode into LSGM thin film. The LSGM thin-
film cells showed a high performance with a power density of about
2
0
.85 W/cm at 800°C. The results also proved that the active SOFC
anodes can be prepared by an impregnation process.
Acknowledgments
This work is supported by the International Joint Research Grant
from New Energy and Industrial Technology Development Organi-
zation ͑NEDO͒, Japan, and by the Research Grant for Outstanding
Scientists from the Natural Science Foundation, China.
Figure 7. EDX analysis results of the YSZ-NiO anode substrate, the sup-
ported LSGM film prepared by the unconventional approach, and the LSM-
YSZ cathode: ͑I͒ LSM-YSZ cathode 20 ␮m thick, ͑II͒ LSGM film of 15 ␮m
thick, and ͑III͒ NiO-YSZ anode substrate.
The Dalian Institute of Chemical Physics assisted in meeting the publi-
cation costs of this article.
LSGM thin film. However, some manganese migrated from LSM-
YSZ cathode into LSGM film and YSZ-NiO substrate. The high
cathodic overpotential of the SOFC single cell can be explained by
the stoichiometric change of manganese in LSM. The loss of Mn in
LSM will lead to a significant decrease of the electrochemical ac-
tivity of LSM. The low OCP of an SOFC single prepared by this
approach may be attributed to the migration of Mn with variable
valance into LSGM thin film. The slight leakage both from sealing
and from the thin LSGM film and the minor short-circuiting caused
by reduced Ni across the film at the edge from impregnation may
also contribute to the lower OCP.
The present work shows the possibility of constructing supported
LSGM thin film SOFCs using an unconventional approach and the
encouraging results on cell performance achieved. Apparently, much
work should be done to improve the thin-film preparation and the
anode preparation, which is the most delicate part of constructing
the cell after an LSGM film is deposited. More attention should be
paid to parameters such as the anode particle size, particle distribu-
tion, morphology, and porosity. Optimization of the impregnation
process to achieve an appropriate Ni particle size, morphology, and
distribution may further improve the anode performance. The cath-
ode performance will be improved by introducing a protective inter-
References
1
2
3
. T. Ishihara, H. Matsuda, and Y. Takita, J. Am. Chem. Soc., 116, 3801 ͑1994͒.
. M. Feng and J. B. Goodenough, J. Am. Ceram. Soc., 79, 1100 ͑1996͒.
. T. Ishihara, J. A. Kilner, M. Honda, N. Sakai, H. Yokokawa, and Y. Takita, Solid
State Ionics, 113Õ115, 593 ͑1998͒.
4. P. Huang and A. Petric, J. Electrochem. Soc., 143, 1644 ͑1996͒.
. J. W. Stevenson, T. R. Armstrong, L. R. Pederson, J. Li, C. A. Lewinsohn, and S.
Baskaran, Solid State Ionics, 113Õ115, 571 ͑1998͒.
. M. S. Khan and M. S. Islam, J. Phys. Chem. B, 102, 3099 ͑1998͒.
. T. Ishihara, S. Takaati, H. Miho, N. Hiroyasu, and Y. Takita, J. Electrochem. Soc.,
147, 1332 ͑2000͒.
. K. Huang, R. Tichy, and J. B. Goodenough, J. Am. Ceram. Soc., 81, 1581 ͑1998͒.
. R. Maric, S. Ohara, T. Fukui, H. Yoshida, M. Nishmura, T. Inagaki, and K. Miura,
J. Electrochem. Soc., 146, 2006 ͑1999͒.
0. K. Huang, J. H. Wan, and J. B. Goodenough, J. Electrochem. Soc., 148, A788
͑2001͒.
1. J. W. Kim, A. V. Virkar, K-Z. Fung, K. Metha, and S. C. Singhal, J. Electrochem.
5
6
7
8
9
1
1
Soc., 146, 69 ͑1999͒.
1
1
2. S. DeSouza, S. J. Visco, and L. C. DeJonghe, Solid State Ionics, 98, 57 ͑1997͒.
3. T. Tsai and S. A. Barnett, Solid State Ionics, 98, 191 ͑1997͒.
14. T. Mathews, N. Rabu, J. R. Seller, and B. C. Muddle, Solid State Ionics, 128, 111
2000͒.
͑
1
5. T. Mathews, P. Manoravi, M. P. Antony, J. R. Seller, and B. C. Muddle, Solid State
Ionics, 135, 397 ͑2000͒.
1
6. S. Wang, Y. Jiang, Y. Zhang, J. Yan, and W. Li, J. Electrochem. Soc., 145, 1932
͑1998͒.
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