Ni-Sm0.2Ce0.8O1.9 anode-supported YSZ electrolyte film and its application in solid oxide fuel cells

Article abstract of DOI:10.1016/j.jallcom.2006.03.043

Ni-Sm_0.2Ce_0.8O_1.9 anode-supported YSZ electrolyte films were fabricated and a fuel cell based on this film was tested in this paper. The anode/YSZ film interface was investigated by means of SEM-EDX. The SEM results showed that the YSZ film was almost fully dense and crack-free. The analyses of EDX revealed that Ce and Sm cations preferential migrated from the NiO-SDC anode into the YSZ film for less than 3 μm while no significant diffusion occurred for Y and Zr. A cell of Ni-SDC/YSZ(14 μm)/LSM-YSZ was tested from 700 to 850 °C using humidified hydrogen as fuel and ambient air as oxidant. The cell provided maximum power densities of 304, 625, 980 and 1270 mW/cm² at 700, 750, 800 and 850 °C, respectively. Polarization analysis based on the impedance spectra showed that the anode polarization only accounted for 8% of the total electrode polarization while this ratio for cathode was 92%. A negligible anode polarization suggested that the diffusion of Ce and Sm and its negative effect on the performance of the cell were suppressed to an acceptable level in this study.

Full text of DOI:10.1016/j.jallcom.2006.03.043

Journal of Alloys and Compounds 428 (2007) 302–306
Ni–Sm0.2Ce0.8O1.9 anode-supported YSZ electrolyte film and its
application in solid oxide fuel cells
Yaohui Zhanga, Xiqiang Huanga, Zhe Lua, Zhiguo Liua, Xiaodong Gea,
Jiahuan Xua, Xianshuang Xina, Xueqing Shaa, Wenhui Sua,b,c,∗
a Center for Condensed Matter Science and Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
b Department of Condensed Matter Physics, Jilin University, Changchun 130022, People’s Republic of China
c International Center for Materials Physics, Academia, Shenyang 110015, People’s Republic of China
Received 11 January 2006; received in revised form 16 March 2006; accepted 16 March 2006
Available online 5 June 2006
Abstract
Ni–Sm0.2Ce0.8O1.9 anode-supported YSZ electrolyte films were fabricated and a fuel cell based on this film was tested in this paper. The
anode/YSZ film interface was investigated by means of SEM–EDX. The SEM results showed that the YSZ film was almost fully dense and
crack-free. The analyses of EDX revealed that Ce and Sm cations preferential migrated from the NiO–SDC anode into the YSZ film for less
than 3 ␮m while no significant diffusion occurred for Y and Zr. A cell of Ni–SDC/YSZ(14 ␮m)/LSM–YSZ was tested from 700 to 850 ◦C using
humidified hydrogen as fuel and ambient air as oxidant. The cell provided maximum power densities of 304, 625, 980 and 1270 mW/cm2 at 700,
750, 800 and 850 ◦C, respectively. Polarization analysis based on the impedance spectra showed that the anode polarization only accounted for 8%
of the total electrode polarization while this ratio for cathode was 92%. A negligible anode polarization suggested that the diffusion of Ce and Sm
and its negative effect on the performance of the cell were suppressed to an acceptable level in this study.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Ceramics; Fuel cells; Thin films; Electrochemical impedance spectroscopy; Scanning electron microscopy
1. Introduction
as electrode sintering, interfacial diffusion, and thermal mis-
match. Therefore, itisdifficulttoexpectalong-termstability. For
these reasons, great efforts have been made towards the devel-
opment of intermediate temperature SOFCs [5,6]. To reduce
the operation temperature, the following two approaches can
be considered: one way is to replace the traditional YSZ elec-
trolyte with doped-CeO2 or doped-LaGaO3 which has higher
ionic conductivity than YSZ; the other way is to reduce the
thickness of YSZ electrolyte. This paper correlates with the
latter case because of the excellent combination properties of
YSZ.
Ni-YSZ cermet is a conventional anode materials for YSZ
electrolyte-based SOFCs. Compared with the Ni-YSZ cermet
anode, Ni-doped-CeO2 anode has its advantages. The doped-
CeO2 is an ion-and-electron mixed conductor in hydrogen which
can increase the length of three-phase boundary (TPB) within
anode. Doped-CeO2-based anode is more suitable for oxida-
tion of hydrocarbon because of its high activity for hydrocarbon
oxidation and its high ionic conductivity [7]. Among the doped-
CeO2 materials, Samaria-doped ceria (SDC) has the highest
Solid oxide fuel cells (SOFCs), which can convert chemical
energy directly to electrical power with high efficiency and low
pollution, attract more and more attentions in recent decades
[1–4]. To date, YSZ is the most commonly used electrolyte
materials in SOFCs because it possesses unique combination
of properties such as excellent mechanical strength, high chem-
ical and thermal stability in both oxidizing and reducing atmo-
spheres and pure oxygen ionic conductivity over a wide range
of conditions. However, oxygen ionic conductivity of YSZ is
insufficient for SOFCs which operating at intermediate temper-
ature (600–800 ◦C). The conventional SOFCs based on YSZ
electrolyte usually operate at high temperature such as 1000 ◦C
or more to obtain good performance. However, operating SOFCs
at such a high temperature may cause a series of problems such
∗
Corresponding author. Tel.: +86 451 86418570; fax: +86 451 86412828.
E-mail addresses: suwenhui@hit.edu.cn, hitcrazyzyh@126.com (W. Su).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.03.043

Y. Zhang et al. / Journal of Alloys and Compounds 428 (2007) 302–306
303
ionic conductivity [8]. NiO–SDC anode was selected in this
study for fabrication of anode-supported YSZ film SOFCs.
It is reported that doped-CeO2 reacts with stabilized-ZrO2
at elevated temperature forming CeO2–ZrO2-based solid solu-
tions which have a low electrical conductivity [9]. Shiono
et al. [10] introduced a Ce0.9Gd0.1O2−δ (GDC) interlayer
between Scandia-stabilized zirconia (SSZ) electrolyte and
La0.6Sr0.4CoO3−δ (LSC) cathode to avoid the reaction between
them. They found that the reaction between SSZ electrolyte and
GDC interlayer was not observed at 1320 ◦C.
Ni-YSZ anodes were usually used to support the YSZ elec-
trolyte thin films. There are few publications on application of
Ni–SDC anode-supported YSZ films in SOFCs. Xu et al. [11]
fabricated NiO–SDC anode-supported YSZ electrolyte films by
spin-coating and co-firing method. The cell based on this film
showed a maximum power density of 535 mW/cm2 at 750 ◦C.
However, the diffusion at the NiO–SDC anode and YSZ film
interface was not investigated in their reports. In this paper,
YSZ film was prepared on NiO–SDC green anode substrate by
slurry-casting method. The interface between NiO–SDC anode
and YSZ electrolyte was examined by SEM–EDX. The perfor-
mances of the Ni–SDC anode-supported YSZ films SOFCs were
tested and discussed.
Fig. 1. Schematic diagram of YSZ film deposition equipment.
thin electrolyte films in this study. Terpineol-ethylcellulose vehicle was used
to prepare YSZ slurry. YSZ powder was mixed with terpineol-ethylcellulose
vehicle in an agate mortar and ground for about 2 h to get a homogeneous
and stable YSZ paste. The slurry contained 80 wt.% of YSZ and 20 wt.% of
terpineol-ethylcellulose vehicle. Fig. 1 showed a schematic diagram of YSZ
film fabrication equipment. The equipment was composed of a flat holder and
a revolving rod. The flat holder was used to fix and support the anode substrate
for slurry-casting. A certain amount of YSZ slurry was dropped on one side of
anode and compacted by the revolving rod. The casting force helped to obtain
green YSZ films with high density, which led to dense sintered films. After being
dried in air, the anode/YSZ bilayer was fired at 1400 ◦C for 4 h to increase the
density of YSZ film.
2. Experimental
The LSM–YSZ composition cathode was prepared using screen-printing
technique. LSM and YSZ powders were mixed in a weight ratio of 1.5:1, and
then the mixed powders were mixed with terpineol-ethylcellulose vehicle and
groundforabout1 htogetastablecathodeprintingink. Afterthat, theLSM–YSZ
printing ink was printed onto YSZ electrolyte surface by screen-printing tech-
nique and then sintered at 1200 ◦C for 3 h. The cathode size is 0.5 cm2.
2.1. Preparation of starting powders
NiO powder was prepared by ammonia precipitation method. Ammonia
(analytical reagent, A.R.) was added to Ni(NO3)2·6H2O (A.R.) solution drop
by drop to obtain nickel hydroxide deposition. Then, the as-prepared deposition
was aged at 70 ◦C for 40 min. After washing by alcohol and drying at 105 ◦C, the
resultant deposition was finally fired at 400 ◦C for 2 h to get nickel oxide powder.
SDC (Sm0.2Ce0.8O1.9) powder was prepared by a citric-nitrate process.
Stoichiometric amounts of Ce(NO3)3·6H2O (A.R.) and Sm2O3 (A.R.) were
dissolved in diluted HNO3 (A.R.) and citric acid (A.R.) was added as the com-
plexant. The molar ratio of total cation to citric acid was 1/1.15. The solution was
evaporated at 75 ◦C to form a transparent gel. After drying at 100 ◦C for 12 h,
the transparent gel changed to a brown dry gel. The dry gel was subsequently
fired at 750 ◦C for 2 h to get SDC powder.
2.4. Characteristics of YSZ film and anode/YSZ film interface
Morphology and microstructure of the YSZ film were evaluated by scanning
electron microscope (SEM). The density of the film was estimated by the dimen-
sions and the weight of the film. The anode/YSZ film interface was examined by
SEM–EDX. The EDX analysis at interface was carried out in a line-scan mode
with a probe diameter less than 1 ␮m.
La0.7Sr0.3MnO3 (LSM) was used as the cathode material in this study. The
method for preparation of LSM powder was similar to that for SDC powder as
described above. The starting materials for LSM powder preparation were all
analytical reagent in this study. The LSM powder was obtained by firing the
as-prepared dry gel at 1000 ◦C for 4 h.
2.5. Single cell testing
Silver paste was current collectors for both anode and cathode. A four-probe
set-up was adopted to eliminate the resistive loss in the silver wires. A reference
electrode was attached to electrolyte film in the vicinity of the cathode to separate
the cathode polarization from that of anode. Hydrogen saturated with water at
room temperature was used as fuel while oxygen in the ambient air was used
as oxidant. The single cell was tested in the temperature range of 700–850 ◦C.
The cell performance and impedance spectroscopy were measured by SI-1260
impedance/gain-phase analyzer in combination with SI-1287 electrochemical
interface (Solartron Instruments, Hampshire, U.K.).
X-ray diffraction (Bede D1 X-ray diffraction meter) was used to confirm the
crystalline structure of the prepared powders. All the powders of NiO, SDC and
LSM exhibited the expected phase structure without impure phases.
2.2. Preparation of NiO–SDC anode substrates
The as-prepared NiO and SDC powders were mixed in a weight ratio of 1:1.
In addition, 15 wt.% starch was added as pore former to get sufficient porosity.
After mixing and grinding, the anode powder was pressed into pellets of 13 mm
in diameter and 0.5 mm in thickness under a pressure of 260 MPa. The green
pellets were pre-sintered at 1000 ◦C for 3 h to get the NiO–SDC anode substrates.
3. Results and discussion
3.1. Microstructures of supported YSZ film and single cell
2.3. Preparation of YSZ electrolyte film and LSM–YSZ composite
cathode
Fig. 2 showed SEM micrographs of supported YSZ film and
a single cell with a 14-␮m-thick YSZ film. Fig. 2(a) was a sur-
face micrograph of YSZ film before sintering, which showed an
underdeveloped microstructure with small grains. There were
YSZ powder with an average particle size of 0.2 ␮m bought from Tosoh
corporation (TZ-8Y, Tosoh Corporation, Tokyo, Japan) was chosen to make

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Y. Zhang et al. / Journal of Alloys and Compounds 428 (2007) 302–306
Fig. 2. SEM micrographs of (a) surface of YSZ film before sintering, (b) surface of film after sintering, (c) cross-section of anode/film bilayer before sintering, (d)
cross-section of anode/film bilayer after sintering, and (e) cross-section of a single cell with a 14-␮m-thick YSZ film.
some pinholes in the green YSZ film, which came from the
volatilization of terpineol-ethylcellulose vehicle during the film
drying process. Fig. 2(b) showed the YSZ film surface micro-
graph after sintering at 1400 ◦C. All pinholes disappeared and
the YSZ thin film was composed of grains of irregular shape. The
grain size varied from 1 to 5 ␮m without any crack and pinhole.
Fig. 2(c) and (d) presented the cross-sections of anode/YSZ film
bilayer before and after sintering at high temperature, respec-
tively. As shown in Fig. 2(c), there was a clear boundary between
the anode and supported YSZ film. After sintered at high tem-

Y. Zhang et al. / Journal of Alloys and Compounds 428 (2007) 302–306
305
perature, as shown in Fig. 2(d), the boundary disappeared and an
interlocking boundary came into being. The green YSZ film was
verydensewitharelativedensityofabout45%. Aftersinteringat
high temperature, the relative density increased to about 97%. In
this study, the stable YSZ slurry, which contained up to 80 wt.%
of YSZ particles, was used to prepare YSZ film. This paste led to
a green YSZ film with a small amount of organic additive com-
pared with the conventional slurry coating [12]. Furthermore,
the casting force also helped to compress the green YSZ film,
leading a dense sintered film after sintering. It was believed that
the high-quality sintered YSZ film was ascribed to the dense
green YSZ film as shown in Fig. 2(c). Fig. 2(e) showed a single
cell of Ni–SDC/YSZ(14 ␮m)/LSM–YSZ. The electrochemical
testing in this paper was based on this cell. The YSZ film was
fully dense while both the Ni–SDC anode and LSM–YSZ com-
posite cathode were porous.
3.2. SEM–EDX analysis at anode/YSZ film interface
Fig. 3(a) showed the cross-section of the NiO–SDC
anode/YSZ film interface after sintering. The YSZ film was
homogeneous and almost pinhole-free in the cross-section. The
results of EDX analyses for Y, Zr, Sm and Ce at anode/YSZ
interface were presented in Fig. 3(b). It was observed that there
was no significant diffusion occurred for Y and Zr. The Ce and
Sm cations migrated from NiO–SDC anode into the supported
YSZ film for less than 3 ␮m. These diffusions of Ce and Sm
cations resulted in CeO2–ZrO2-based solid solutions in the YSZ
film side. Mitsuyasu et al. [13] have investigated the solid state
reaction at the interface of Y2O3-doped-CeO2 (YDC) and YSZ
by XRD. Their XRD results showed that the solid state reac-
tion between YDC and YSZ accompanied a significant shift of
the zirconia lines to lower angle while no obvious lines shift
observed for ceria. They believed that the reaction between
ceria and zirconia proceeded by preferential migration of the
ceria component into the zirconia side. Tsoga et al. [9] also
found a similar phenomenon between GDC and YSZ interface.
Our experiment results suggested the reaction at the interface
between the NiO–SDC anode and the YSZ film mainly resulted
from the migration of the Ce and Sm cations into the YSZ side.
Fig. 3. (a) SEM micrograph of a cross-section of NiO–SDC anode/YSZ film
interfaceaftersintering;thewhitelineindicatesthepositionoftheEDXanalyses;
(b) EDX analyses of Y, Zr, Sm and Ce in the line-scan mode at the NiO–SDC
anode/YSZ film interface.
3.3. Results of electrochemical testing
The temperature dependence of cell performance in the
temperature range of 700–850 ◦C were shown in Fig. 4. The
observed open circuit voltages (OCVs) of higher than 1.0 V
implied that the gas leakage across YSZ film was insignifi-
cant. The maximum power densities were 304, 625, 980 and
1270 mW/cm2 at 700, 750, 800 and 850 ◦C, respectively. The
present fuel cell performance is better than that of previ-
ous reported Ni–SDC anode-supported YSZ electrolyte films
SOFCs [11].
The impedances of the whole cell and the half cell (cath-
ode side only) tested at 800 ◦C were shown in Fig. 5. For the
whole cell impedance, the intercepts on the real axis at high fre-
quency corresponded to the cell ohmic resistance while the arcs
at intermediate and low frequency were the whole electrodes
Fig. 4. Cell voltage (solid symbols) and power density (open symbols) as func-
tions of current density for a thin film cell with humidified hydrogen as fuel and
ambient air as oxidant from 700 to 850 ◦C.

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Y. Zhang et al. / Journal of Alloys and Compounds 428 (2007) 302–306
because of the good chemical compatibility of Ni for both YSZ
and ceria. They recommended that a ceria–40 vol.% Ni cermet
should be considered when ceria was to be employed at the YSZ
electrolyte and Ni–YSZ anode interface to reduce anode polar-
ization losses. The insignificant reaction between NiO–SDC and
YSZ film in this study seemed to agree with this case.
4. Conclusions
Ni–SDC anode-supported YSZ films were prepared by slurry
casting to study the solid reaction at the interface of the anode
and the supported YSZ film and to evaluate the performances of
the fuel cells. A compacted YSZ green film with relative density
of 45% was obtained after one-time coating. The relative density
of the YSZ film increased to 97% after sintered at 1400 ◦C for
4 h. The SEM–EDX analyses showed that there was not obvious
diffusion occurred for Y and Zr. On the other hand, Ce and Sm
cations migrated from NiO–SDC anode into the supported YSZ
film for less than 3 ␮m. A single cell, Ni–SDC/YSZ/LSM–YSZ,
was tested using humidified hydrogen as fuel and ambient air as
oxidant from 700 to 850 ◦C. The open circuit voltage of above
1.0 V was observed. The cell provided the maximum power den-
sities of 304, 625, 980 and 1270 mW/cm2 at 700, 750, 800 and
850 ◦C, respectively. The cell performance was basically limited
by cathode polarization, instead of anode polarization and elec-
trolyte ohmic resistance. The effect of the interfacial reaction
between NiO–SDC anode and YSZ film on the performance of
the cell was not significant in this study.
Fig. 5. Impedances of the whole cell and half cell (cathode side only) measured
at 800 ◦C under open circuit condition.
Table 1
Fitting results of the whole cell and cathode impedances tested at 800 ◦C under
open circuit condition
The whole cell impedance
(ꢀ cm2)
The cathode impedance
(ꢀ cm2)
R1
R2
R3
R4
0.041
0.027
0.153
0.108
0.288
0.023
0.024
0.129
0.114
0.267
R2 + R3 + R4
Acknowledgement
polarization resistance including both anode and cathode polar-
ization. For the half cell impedance, the intercepts on the real
axis at high frequency also corresponded to the ohmic resistance
while the arcs at intermediate and low frequency were only the
cathode polarization resistance. The inset in Fig. 5 showed an
equivalent circuit which was used to resolve the impedance data.
The fitting results were listed in Table 1. The whole electrode
(including both cathode and anode) polarization resistance is
0.288 ꢀ cm2 while the polarization resistance for cathode pro-
cess is 0.267 ꢀ cm2. It could be readily seen that the cathode
polarization accounted for up to 92% of the total electrode
polarization while the anode polarization was almost negligi-
ble. As shown in Fig. 3, the diffusions of Ce and Sm cations
from NiO–SDC anode into YSZ film resulted in a CeO2–ZrO2-
based solid solution on the YSZ side, which had low electrical
conductivity. This solid solution restricted the anode reaction
and thus resulted in a high anode polarization. From the anal-
yses of impedance, the anode polarization only accounted for
about 8% of the total electrode polarization at 800 ◦C. This result
suggested that the diffusion of Ce and Sm and its effect on per-
formance of the cell were kept at an acceptable level in this study.
Tsoga et al. [9] reported that although pure GDC reacted readily
with YSZ at elevated temperature, suppression of the reaction
between YSZ and GDC could be achieved in the presence of Ni
Financial support from the National Natural Science Founda-
tion of China (grant No.10504005) is gratefully acknowledged.
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