A novel method for fabrication of y2O3-stabilized ZrO2 electrolyte films

Article abstract of DOI:10.1111/j.1551-2916.2006.00980.x

Gas-tight Y₂O₃-stabilized ZrO₂ (YSZ) films were prepared on NiO-YSZ and NiO-SDC (Sm_0.2Ce_0.8O _1.9) anode substrates by a novel method. A cell, Ni-YSZ/YSZ(10 μm)/LSM-YSZ, was tested with humidified hydrogen as fuel and ambient air as oxidant. The maximum power densities of 1.64, 1.40, 1.06, and 0.60 W/cm ₂ were obtained at 850°, 800°, 750°, and 700°C, respectively. With methane as fuel, a cell of Ni-SDC/YSZ (12 μm)/LSM-YSZ exhibited the maximum power densities of 1.14, 0.82, 0.49, and 0.28 W/cm ₂ at 850°, 800°, 750°, and 700°C, respectively. The impedance results showed that the performance of the cell was controlled by the electrode polarization rather than the resistance of YSZ electrolyte film.

Full text of DOI:10.1111/j.1551-2916.2006.00980.x

J. Am. Ceram. Soc., 89 [7] 2304–2307 (2006)
DOI: 10.1111/j.1551-2916.2006.00980.x
r 2006 The American Ceramic Society
ournal
J
A Novel Method for Fabrication of Y O -Stabilized ZrO Electrolyte
2
3
2
Films
Yaohui Zhang, Xiqiang Huang, Zhe Lu, Zhiguo Liu, Xiaodong Ge, Jiahuan Xu, Xianshuang Xin,
Xueqing Sha, and Wenhui Su
w,z,z
Center for Condensed Matter Science and Technology, Harbin Institute of Technology, Harbin 150001, China
Key Laboratory of Condensed Matter Science and Technology, Heilongjiang Provincial Universities, Harbin 150001,
China
8
9
10
sputtering, tape casting, spray coating, centrifugal casting,
11
Gas-tight Y O -stabilized ZrO (YSZ) films were prepared on
2
3
2
NiO–YSZ and NiO–SDC (Sm0.2Ce0.8O1.9) anode substrates by
a novel method. A cell, Ni–YSZ/YSZ(10 lm)/LSM–YSZ, was
tested with humidified hydrogen as fuel and ambient air as ox-
and so on. To date, YSZ is still the most promising electrolyte
material for SOFCs commercialization due to its high physical
and chemical stability and high mechanical properties.
12
idant. The maximum power densities of 1.64, 1.40, 1.06, and
In this paper, our aim is to develop a handy and cost-effective
method for thin YSZ film fabrication based on a simple physical
and co-firing process. Some initial experimental results are
reported here.
2
0.60 W/cm were obtained at 8501, 8001, 7501, and 7001C, re-
spectively. With methane as fuel, a cell of Ni–SDC/YSZ (12
lm)/LSM–YSZ exhibited the maximum power densities of
2
1
7
.14, 0.82, 0.49, and 0.28 W/cm at 8501, 8001, 7501, and
001C, respectively. The impedance results showed that the per-
II. Experimental Procedure
formance of the cell was controlled by the electrode polarization
rather than the resistance of YSZ electrolyte film.
(
1) Preparation of Starting Powders
NiO powder was prepared by an ammonia precipitation method
from nickel nitrate solution. Ammonia (analytical reagent,
A.R.) was added to Ni(NO ) ꢀ 6H O (A.R.) solution drop by
I. Introduction
3
2
2
drop to obtain nickel hydroxide deposition. Then, the as-pre-
pared deposition was aged at 701C for 40 min. After washing by
alcohol and drying at 1051C, the resultant deposition was finally
fired at 4001C for 2 h to get nickel oxide powder.
OLID oxide fuel cells (SOFCs), which can convert chemical
energy into electrical energy directly, are attracting more and
more attention because of their high efficiency, low pollution,
and fuel flexibility. From the viewpoint of cost reduction and
long-term stability, many researchers have paid attention to the
fabrication of SOFCs operating at reduced temperatures
(6001B8001C). Reducing the thickness of Y
ZrO (YSZ) electrolyte is an effective approach to lower the
operation temperature of SOFCs, while retaining the high per-
formance. Chen et al. reported a unique thin-film SOFC design
S
1
,2
SDC (Sm0.2Ce0.8O1.9) powder was prepared by a citric-nitrate
process. Stoichiometric amounts of Ce(NO ) ꢀ 6H O (A.R.) and
3
3
2
Sm O
2 3
(A.R.) were dissolved in diluted HNO
3
(A.R.), and citric
2 3
O -stabilized
acid (A.R.) was added as the complexant. The molar ratio of
total cation to citric acid was 1/1.15. The solution was evapo-
rated at 751C to form a transparent gel. After drying at 1001C
for 12 h, the transparent gel changed to a brown dry gel. The dry
gel was subsequently fired at 7501C for 2 h to get SDC powder.
2
3
based on thin-film deposition and microlithographic processes.
In their experiment, 2 mm thick YSZ films were deposited onto 6
mm thick polycrystalline nickel foils by pulsed laser deposition
and the single cell showed a maximum power density of 110
La0.7Sr0.3MnO (LSM) was used as the cathode material in
3
this study. The method for the preparation of LSM powder was
similar to that for SDC powder, as described above. The starting
materials for LSM powder preparation were all analytical rea-
gents in this study. The LSM powder was obtained by firing the
as-prepared dry gel at 10001C for 4 h.
2
4
mW/cm at 5701C. de Souza et al. reported an anode-supported
SOFC with a 10 mm thick YSZ electrolyte film and the Ni–YSZ/
YSZ/LSM cell showed an extremely high power density of 1.9
2
W/cm at 8001C. Recently, the effect of cell parameters (elec-
trolyte thickness, cathode interlayer thickness, anode thickness,
etc.) on polarization in anode-supported SOFCs has been
investigated in detail by Zhao and Virkar. Based on the opt-
imized cell parameters, the corresponding maximum power den-
sity was B1.8 W/cm at 8001C. Dollen and Barnett developed
anode-supported YSZ film SOFCs by screen printing techno-
logy and got a maximum power density of 1.45 W/cm at 8001C.
A variety of film deposition techniques have been explored for
YSZ powder (0.2 mm) bought from Tosoh corporation (TZ-
Y, Tokyo, Japan) was used to prepare thin electrolyte films.
8
5
(
2) Preparation of Anode Substrates
NiO powder and YSZ powder were used to prepare NiO–YSZ
anode substrates. They were weighted and mixed in a weight
ratio of 1:1. To obtain sufficient porosity in the anode subst-
rates, 10 wt% starch was added as pore former. After being
ground in an agate mortar for 1 h, the mixed powder was
pressed into pellets of 13 mm in diameter and 0.5 mm in thick-
ness under a pressure of 260 MPa. Then the pellets were sub-
sequently sintered at 10001C for 2 h to improve their mechanical
property. After sintering, the pellets can be used as the anode
substrates. NiO–SDC anode substrates were also fabricated by
the same method. The NiO–SDC anode contained 50 wt% NiO
powder and 50 wt% SDC powder.
2
6
2
the preparation of dense ceramic films on porous electrode subst-
rates, including chemical vapor deposition, DC magnetron
7
R. Cutler—contributing editor
Manuscript No. 21020. Received September 24, 2005; approved January 16, 2006.
This work is supported by the Ministry of Science and Technology of China under
contract No. 2001AA323090.
(3) Preparation of YSZ Film and Cathode
wAuthor to whom correspondence should be addressed. e-mail: suwenhui@hit.edu.cn
z
z
A terpineol-ethylcellulose mixture was used as organic vehicle to
prepare YSZ paste. The vehicle was prepared by dissolving 6
Department of Condensed Matter Physics, Jilin University, Changchun, China.
International Center for Materials Physics, Academia, Shenyang, China.
2
304

July 2006
Communications of the American Ceramic Society
2305
in an agate mortar and was ground for 1 h to obtain a stable and
homogeneous YSZ paste for compacting YSZ electrolyte film.
The viscosity of as-prepared YSZ paste was 263 Paꢀ S (251C) as
tested by an NDJ-99 rotation viscometer (Chengdu Instrument
Plant, Chengdu, China). The rotation speed of the rotor was 1.5
rpm in the viscosity testing. In the next step, a certain amount of
YSZ paste was dropped on the surface of the fixed anode subst-
rate. Finally, the paste was spread and compacted onto the an-
ode substrate by the revolving rod. The rotating rate of the rod
was 60 rpm. The pressure applied to the revolving rod was 1.3
Revolving rod
YSZ paste
Flat holder
2
N/cm . After coating, the anode/film bilayer was sintered at
14001C for 4 h at a heating rate of 51C/min. By controlling the
Anode substrate
amount of YSZ paste, the thickness deviation for the sintered
YSZ film can be controlled to be 72 mm. The YSZ electrolyte
films were fabricated onto the NiO–YSZ anode substrates and
NiO–SDC anode substrates, respectively, by the same method.
For simplicity, the term ‘‘NiO–YSZ anode-supported cell’’ is
abbreviated to ‘‘NYCELL’’ and the term ‘‘NiO–SDC anode-
supported cell’’ is abbreviated to ‘‘NSCELL’’ hereafter.
2 3 2
Fig. 1. Schematic diagram of Y O -stabilized ZrO (YSZ) film deposi-
tion equipment.
wt% ethylcellulose (A.R., Dongfeng Chemical Reagents Plant,
Wenzhou, China) into 94 wt% terpineol (A.R., Tianjin Kermel
Chemical Reagents Development Centre, Tianjin, China). Fig-
ure 1 shows the schematic diagram of YSZ film deposition
equipment. The equipment was composed of a revolving rod
and a flat holder. The flat holder was used to support and fix the
anode substrate. The revolving rod was used to spread and
compact the YSZ paste onto the anode substrate. YSZ powder
was firstly mixed with the organic vehicle in a weight ratio of 4:1
LSM powder was mixed with YSZ powder in a weight ratio
of 3:2. The mixed powder was then mixed with the terpineol-
ethylcellulose vehicle in an agate mortar and was ground for 1 h
to obtain a cathode printing ink. The viscosity of the ink was 70
Paꢀ S (251C). LSM–YSZ composite cathode was applied onto
the YSZ electrolyte by the screen printing technique. After the
printing, the cathode was sintered at 12001C for 2 h in air. The
2
cathode-effective area was 0.25 cm .
Fig. 2. SEM micrographs of (a) prepared NiO powder, (b) prepared SDC powder, (c) prepared LSM powder, and (d) commercial Tosoh 8YSZ powder.

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Communications of the American Ceramic Society
Vol. 89, No. 7
2 3 2
Fig. 3. SEM micrographs of the Y O -stabilized ZrO (YSZ) films and cells: (a) surface of NiO–YSZ anode-supported YSZ film, (b) surface of
NiO–SDC anode-supported YSZ film, (c) cross-section of NYCELL, and (d) cross-section of NSCELL.
(4) Characterizations of Prepared Powders and Anode-
Supported SOFCs
Scientific Instruments, Ltd., Durham, UK, CuKa radiation,
l 5 0.15418 nm) was used to confirm the crystalline structure
of the prepared powders.
The NYCELL was tested from 6501 to 8501C with humidified
H as fuel and ambient air as oxidant, while the NSCELL was
2
The morphology of prepared powders was examined by a Hit-
achi S-570 (Tokyo, Japan) scanning electron microscope (SEM).
X-ray diffraction (Bede D X-ray diffraction meter, Bede
1
tested from 7001 to 8501C using methane as fuel and ambient air
as oxidant. The cell performance and electrochemical impedance
spectroscopy were measured using SI-1260 impedance/gain-
phase analyzer in combination with SI-1287 electrochemical
interface (Solartron Instruments, Hampshire, UK). A four
probe set up was used to eliminate the resistive loss in the lead
wires. Silver paste was used as the current collector for both
anode and cathode. The microstructures of the cell after testing
were examined by SEM.
1
1
1
0
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
.0
2.0
1.8
1.6
5
6
6
7
50°C
00°C
50°C
00°C
750°C
00°C
850°C
NYCELL
8
1.4
1.2
1.0
0.8
0.6
0
.4
.2
III. Results and Discussion
0
(1) Prepared Powders
0.0
XRD results suggested that all the prepared powders of NiO,
SDC, and LSM exhibited the expected phase structure. Figure 2
shows the SEM micrographs of starting powders for the prep-
aration of anode-supported cells. As can be seen from Figs.
2(a)–(c), the prepared powders of NiO, SDC, and LSM were
foam-like. The commercial Tosoh 8YSZ powder, as shown in
0
.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Current density (A/cm2)
Fig. 4. The I-V-P characteristic of NYCELL with a 10 mm thick Y
stabilized ZrO (YSZ) film. Humidified H was used as fuel and ambient
air as oxidant.
2 3
O -
2
2

July 2006
Communications of the American Ceramic Society
2307
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
1.4
Table I. Characteristics of Cells and their Y O -Stabilized
3
2
7
7
8
8
00 °C
50 °C
00 °C
50 °C
ZrO (YSZ) Electrolyte Films
2
NSCELL
1.2
NYCELLA NYCELLB NYCELLC
1
0
0
0
0
.0
.8
.6
.4
.2
Thickness of YSZ
28
66.1
1.02
8
10
67.1
film (mm)
Activation energy of YSZ
film (kJ/mol)
68.6
Open circuit voltage of
cells at 8001C (V)
Maximum power density at 1.12
1.02
1.35
1.01
1.13
2
8
001C (W/cm )
0.0
0
1
2
3
4
Current density (A/cm2)
2
densities are 1.14, 0.82, 0.49, and 0.28 W/cm at 8501, 8001, 7501,
and 7001C, respectively.
Fig. 5. The I-V-P characteristic of NSCELL with a 12 mm thick Y
2 3
O -
Figure 6 shows two typical impedance spectra under the
open-circuit condition at 8001C for both NYCELL and
NSCELL. The NYCELL was tested using humidified hydro-
gen as fuel, and the NSCELL was operated using methane as
fuel. The total area specific resistance of the NYCELL at 8001C
stabilized ZrO
oxidant.
2
(YSZ) film. Methane was used as fuel and ambient air as
Fig. 2(d), was composed of spherical-shaped particles with an
average particle size of 0.2 mm.
2
2
was 0.35 O ꢀ cm , of which 0.04 O ꢀ cm came from the YSZ
(
2) Microstructure of YSZ Film and Electrode
2
electrolyte film and 0.31 O ꢀ cm from the interfacial polarization
Figures 3(a) and (b) show the surface micrographs of the NiO–
YSZ anode-supported YSZ film and the NiO–SDC anode-sup-
ported YSZ film, respectively. It can be seen that the YSZ films
were composed of irregular grains without cracks or pinholes.
The green YSZ film possessed a relative density of 46% before
sintering. After sintering at 14001C for 4 h, the relative density
increased to 97%. The shrinkage of the NiO–YSZ anode and
the NiO–SDC anode was 21.64% and 21.70%, respectively. The
resistance. So, for the NYCELL, the resistance of YSZ film was
only about 11.4% of the total cell resistance. For the NSCELL,
the resistance of YSZ film was 12.5% of the total cell resistance
when methane was used as fuel. So, the cell performance was
basically limited by the electrode polarization rather than the
YSZ film resistance.
Several NYCELLs were fabricated and tested with humidi-
fied H as fuel and ambient air as oxidant. Table I summarizes
2
the characteristics of these cells including the thickness of YSZ
films, values of the activation energy of YSZ films, OCV of cells,
and maximum power densities.
‘
‘z-direction’’ shrinkage of the YSZ electrolyte film was calcu-
lated by comparing the thickness of the green film with the
sintered film. The results showed that the ‘‘z-direction’’ shrink-
age for the NiO–YSZ anode-supported film was 21.8%, while
this ratio for the NiO–SDC anode-supported film was 22.4%.
Figure 3(c) shows the cross-sectional SEM micrograph of the
NYCELL with a 10 mm thick YSZ film. Figure 3(d) exhibits the
cross-sectional view of the NSCELL with a 12 mm thick YSZ
film. As can be seen from Figs. 3(c) and (d), the YSZ films were
almost fully dense, and both the anode and the cathode exhib-
ited a porous structure. The anode porosity was about 63% as
tested using the standard Archimedes method. The porosity for
cathode was estimated from the SEM photos to be about 45%.
Figure 4 shows the I-V-P characteristic of NYCELL with a
IV. Conclusions
In conclusion, high-quality YSZ electrolyte films were success-
fully fabricated on porous NiO–YSZ and NiO–SDC anode
substrates by a handy film fabrication technique. The cells ex-
hibited excellent performance in the intermediate temperature
range of 6001–8001C. The impedance results showed that the
performance of the cell was controlled by the electrode polari-
zation rather than the resistance of YSZ electrolyte films.
1
1
6
0 mm thick YSZ film. The maximum power densities are 1.64,
.40, 1.06, 0.60, and 0.30 W/cm at 8501, 8001, 7501, 7001, and
501C, respectively. It was seen that the open circuit voltage
2
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(
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3
4
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6
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&