Composite cathode La0.6Sr0.4Co0.2Fe 0.8O3-Sm0.1Ce0.9O1.95-Ag for intermediate-temperature solid oxide fuel cells

Article abstract of DOI:10.1016/j.jallcom.2004.11.056

Sm_0.1Ce_0.9O_1.95 (SDC) and La _0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) were synthesized by a glycine-nitrate process. A composite LSCF-SDC-Ag cathode was prepared and its structure was analyzed by X-ray diffraction. No chemical reactions among LSCF, SDC, and Ag in the composite cathode were found. The composite cathode was applied onto SDC electrolyte and the microstructure of the electrolyte-electrode interface was examined by SEM. The overpotential of the interface was measured. A single SOFC was prepared by using LSCF-SDC-Ag as cathode, SDC as electrolyte, and Ni-SDC as anode and its performance was measured. Compared to a similar SOFC with LSCF-SDC cathode, it is found that adding Ag in the cathode improved the SOFC performance significantly.

Full text of DOI:10.1016/j.jallcom.2004.11.056

Journal of Alloys and Compounds 395 (2005) 322–325
Composite cathode La Sr Co Fe O –Sm Ce O –Ag
0.6 0.4
0.2 0.8 3
0.1
0.9 1.95
for intermediate-temperature solid oxide fuel cells
a
a
a
a
a,b,∗
Jidong Zhang , Yuan Ji , Hongbo Gao , Tianmin He , Jiang Liu
a
Department of Condensed Physics, College of Physics, Jilin University, Chuangchun 130023, China
College of Chemistry Science, South China University of Technology, Guangzhou 510641, China
b
Received 9 October 2004; received in revised form 24 November 2004; accepted 26 November 2004
Available online 7 January 2005
Abstract
3
Sm0.1Ce0.9O1.95 (SDC) and La0.6Sr0.4Co0.2Fe0.8O (LSCF) were synthesized by a glycine–nitrate process. A composite LSCF–SDC–Ag
cathode was prepared and its structure was analyzed by X-ray diffraction. No chemical reactions among LSCF, SDC, and Ag in the composite
cathode were found. The composite cathode was applied onto SDC electrolyte and the microstructure of the electrolyte–electrode interface
was examined by SEM. The overpotential of the interface was measured. A single SOFC was prepared by using LSCF–SDC–Ag as cathode,
SDC as electrolyte, and Ni–SDC as anode and its performance was measured. Compared to a similar SOFC with LSCF–SDC cathode, it is
found that adding Ag in the cathode improved the SOFC performance significantly.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cells; Composite cathode; LSCF; SDC
1
. Introduction
reduced by using the composite cathode, the electronic con-
ductivity of the cathode decreased.
Strontium doped lanthanum manganate (La1−xSrxMnO3,
In this paper, we describe the performance improvement
of LSCF by mixing it with samarium doped ceria (SDC)
and silver Ag. Here the function of SDC is similar to GDC
in the work of Murray et al. [5]. Ag was added based on
the following considerations: (1) Silver is a fine electronic
conductor and it should improve the electronic conduction
LSM) is commonly used as cathode material in traditional
high temperature solid oxide fuel cells [1]. Nowadays, the
operating temperature of SOFCs might hopefully be reduced
◦
to 600–800 C due to improvement of the techniques in mak-
ing a thin electrolyte and the use of some intermediate tem-
perature electrolytes [2,3]. LSM is not suitable as cathode
of SOFCs operating in this temperature range under which
its conductivity is not high enough and its electrochemical
activity is too low. Much work has been done in searching
new cathode materials for SOFCs operating at such interme-
diate temperatures. Lai et al. reported that the conductivity
of the electron-ion mixed conductor La0.6Sr0.4Co0.2Fe0.8O3
◦
of the cathode; (2) the melting point of Ag is 961 C and it
◦
can work at temperatures between 600 and 800 C; (3) as its
melting point is low, it might aid the sintering behavior of the
cathode at lower temperatures.
2. Experiment
(
6
LSCF) was larger than 300 S/cm in the temperature range of
00–800 C [4]. Murray et al. [5] made a composite cathode
◦
SDC and LSCF were synthesized by a glycine–nitrate pro-
LSCF–GDC by mixing LSCF with gadolinium-doped ceria
cess [6]. Weighing Ce(NO ) ·6H O (AP, Shanghai Chemi-
3
3
2
(GDC). While the polarization of the cathode–electrolyte was
2 3
cals) and Sm O (99.9%, Shanghai XinXing Chemicals) ac-
3+
3+
cording to the mole ratio of Ce to Sm as 9:1. Sm2O3 was
dissolved in nitric acid to obtain the corresponding nitrate.
The Ce and Sm nitrates were dissolved in pure water together
∗
Corresponding author. Tel.: +86 20 22236169; fax: +86 20 22236169.
E-mail address: jiangliu@scut.edu.cn (J. Liu).
0
925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.11.056

J. Zhang et al. / Journal of Alloys and Compounds 395 (2005) 322–325
323
with a proper amount of glycine. The solution of the mixture
was heated until it dried and combusted seriously produc-
ing ultra-fine SDC powder. The SDC powder was pre-fired
◦
at 600 C and then pressed into pellets with a diameter of
1
3 mm and thickness of ∼0.5 mm. The pellets were sintered
◦
at 1400 C for 4 h, then, the electrolyte pellets were ready.
A
proper amount of glycine was dissolved in
Ni(NO3)2·6H2O (AP, Shanghai Shanpu Chemical Inc.) solu-
tion and the solution was heated until it combusted producing
fine NiO powder. The NiO powder was mixed with 35 wt.%
SDC powder by grinding them with some ethanol in an agate
mortar. Then some organic binder was added and mixed with
the powder to obtain the anode paste. The NiO–SDC anode
paste was applied on one side of the SDC electrolyte pellet
Fig. 1. The X-ray diffraction patterns of the SDC, LSCF powders and the
sintered composite cathodes.
◦
and sintered at 1200 C in air for 2 h. Similarly, LSCF was
synthesized. The original materials for making LSCF were:
La(NO3)3·6H2O (AP, Beijing Chemical Plant), Sr(NO3)2
action between the two components. As Ag was added and
the composite cathode was sintered at 850 C for 3 h, there
(
Beijing Xinhua Chemical Plant), Co(NO3)3·6H2O (AP, Bei-
◦
jing Yili Chemical Inc.) and Fe(NO3)3·6H2O (AP, Beijing
Chemical Plant). The LSCF powder was mixed with 30 wt.%
SDC by grinding. Some organic binder was added to make
the cathode LSCF–SDC paste. Then, some LSCF–SDC paste
was mixed with Ag paste (DAD-87, Shanghai Institute of
Synthesized Risen) with Ag content of 30 wt.% to make
LSCF–SDC–Ag composite cathode paste. LSCF–SDC paste
was applied on the other side of some of the SDC electrolyte
pellets already with the anode on one side mentioned above,
while LSCF–SDC–Ag paste was applied on the other pel-
lets in the same way. The pellets with LSCF–SDC cathode
was no chemical reaction between other components, either.
This makes it possible for each component in the composite
cathode to function with its own advantages, i.e., the mixed
conduction of LSCF promoting the cathode reactions, SDC
reducing the electrode–electrolyte interface polarization, Ag
enhancing the electronic conductivity of the cathode. A high
performance cathode should be obtained by combining all
these merits.
3.2. SEM
◦
was sintered at 950 C for 3 h and those with LSCF–SDC–Ag
◦
were sintered at 850 C for 3 h.
SEM images were obtained for the electrolyte and the
interfaces of electrode–electrolyte for the tested SOFCs
and they are shown in Fig. 2. Fig. 2(a) shows the SEM
At last, the SOFC units (single cell) were assembled with
Agpasteassealingmaterial [7]. TheSOFCsweretestedusing
H2 as fuel and air as oxidant.
◦
images of the SDC electrolyte sintered at 1400 C with
The performance of the SOFCs was measured by a So-
lartron SI1287 Electrochemical Interface. The measuring
temperature range was 250–750 C. The X-ray spectra of
the specimen were obtained by a Rigaku D/Max-rA X-ray
diffractometer. A SHIMADZU SSX-550 Scanning Electron
Microscope was used to examine the microstructures of the
electrode, the electrolyte and the electrode–electrolyte inter-
face.
a magnification of 20,000. It can be seen that the SDC
electrolyte is relatively dense with some scattered closed
pores. This means that the SDC will not cause any gas
leakage as SOFC electrolytes because of its porosity. The
interface sections of LSCF–SDC/SDC (electrolyte) and
LSCF–SDC–Ag/SDC are shown in Fig. 2(b) and (c), respec-
tively with a magnification of 1000×. There is an obvious line
at the LSCF–SDC–electrolyte interface and the adhesion of
the cathode on the electrolyte is poor as the cathode and the
electrolyte separate at many parts around the interface area.
This will raise the polarization loss at the interface. While, the
adhesion of the cathode with Ag on the electrolyte is much
better with no gap between the cathode and the electrolyte.
Note that the sintering temperature of the cathode with Ag
was lower than that without Ag. This was done considering
the low melting point of Ag as well as the possible improve-
ment of adhesion for the cathode without Ag with higher
sintering temperature. In addition, the microstructure of the
cathode with Ag is more uniform than that without Ag. This
may partly have contributed to the ability of enhancing sin-
tering behavior of Ag, i.e., Ag functioned as a sintering aid
in this case.
◦
3
. Results and discussions
3
.1. XRD analysis
Fig. 1 shows the X-ray spectra of LSCF and SDC powders.
The spectra of the sintered composite cathodes LSCF–GDC
and LSCF–GDC–Ag are also shown in the same figure. It can
be seen that the main phase of the LSCF powder made by the
glycine–nitrate process is the rhombohedral perovskite [8].
The structure of SDC is cubic. After the LSCF–SDC mix-
◦
ture was sintered at 950 C, LSCF and SDC retained their
own structures, respectively and there was no chemical re-

3
24
J. Zhang et al. / Journal of Alloys and Compounds 395 (2005) 322–325
Fig. 3. Relations of overpotential and current density for different cathodes
◦
on the SDC electrolytes measured at 650 and 750 C.
the slowness of the reactions taking place on the surface of
the electrodes, fuel crossover and internal currents through
the electrolyte, Ohmic losses and mass transport or concen-
tration losses result from the change in concentration of the
reactants at the surface of the electrodes as the fuel is used
[9]. The potential of the electrode will deviate from its ideal
value when polarization occurs. Fig. 3 shows the relations of
the overpotential and current density for the composite cath-
odes LSCF–SDC and LSCF–SDC–Ag on SDC electrolytes.
◦
The measuring temperatures were 650 and 750 C. It can be
seen that the overpotential increases with temperature for the
same cathode. At the same temperature, the overpotential of
LSCF–SDC–Ag is much lower than the cathode without Ag.
This is consistent with the SEM images in Fig. 2 showing the
cathode with Ag has better microstructure and adhesion.
3.4. Performance of the SOFCs
Fig. 4 shows the plots of cell voltage and power density
versus current density for the SOFC with LSCF–SDC–Ag
cathode at different temperatures. It can be seen from the
figure that the voltage and the output power density change
with temperature. At the same voltage, the current density,
thus the power density increases with temperature. The max-
◦
imum current density and power density at 750 C are 605.3
2
and 173.8 mW/cm , respectively. While, the open circuit
Fig. 2. SEM images of the SDC electrolyte (a), the cathode–electrolyte in-
terfaces of LSCF–SDC/SDC (b) and LSCF–SDC–Ag/SDC (c).
3
.3. Overpotential
The overpotential is an important factor representing the
electrode performance, and the value of the overpotential af-
fects the performance of the corresponding SOFCs directly.
Overpotential is also called polarization with respect to the
electrode performance of SOFCs. The overpotential or po-
larization is mainly caused by activation losses because of
Fig. 4. The single SOFC performance with LSCF + SDC + Ag composite
cathode at different temperatures.

J. Zhang et al. / Journal of Alloys and Compounds 395 (2005) 322–325
325
4. Conclusions
A composite LSCF–SDC–Ag cathode can be used as the
cathode of intermediate-temperature SOFCs. Silver in this
composite cathode plays an important role in improving the
cathode performance compared to the LSCF–SDC cathode.
Each component in the cathode retains its own phase and
properties after sintering. Ag can enhance the electronic con-
ductivity of cathode as a fine conductor and improve the mi-
crostructure of the cathode as a sintering aid.
Acknowledgements
Fig. 5. The performance comparison of the SOFCs with LSCF–SDC and
LSCF–SDC–Ag cathodes tested at 650 and 750 C.
◦
The authors gratefully acknowledge the financial support
from the Innovation Foundation of Jilin University and the
Young Teacher Fund of Jilin University.
◦
voltage decreases with temperature, from 0.99 V at 450 C
◦
drops to 0.76 V at 750 C. This is because at higher tem-
peratures, some Ce ions in the electrolyte are reduced to
Ce ions in reducing atmosphere causing some electronic
conduction in the electrolyte and results in the OCV drop. A
comparison of the performance of SOFCs with LSCF–SDC
and LSCF–SDC–Ag cathodes at 650 and 750 C (Fig. 5)
shows that the maximum power density of the SOFC with
LSCF–SDC cathode is 124.6 mW/cm . This value is higher
than that in our previous work [10], but lower than the SOFC
4+
3
+
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