J. Mater. Sci. Technol., 2012, 28(9), 828–832.
Cobalt-free Composite Ba0.5 Sr0.5 Fe0.9 Ni0.1 O3−δ –Ce0.8 Sm0.2 O2−δ
as Cathode for Intermediate-Temperature
Solid Oxide Fuel Cell
Xiangfeng Chu1)† , Feng Liu1) , Weichang Zhu2) , Yongping Dong1) , Mingfu Ye1) and Wenqi Sun1)
1) School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243002, China
2) School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China
[Manuscript received September 20, 2011, in revised form January 4, 2012]
New cobalt-free composites consisting of Ba0.5 Sr0.5 Fe0.9 Ni0.1 O3−δ (BSFN) and Ce0.8 Sm0.2 O2−δ (SDC) were
investigated as possible cathode materials for intermediate-temperature solid oxide fuel cell (IT-SOFC). BSFN,
which was synthesized by auto ignition process, was chemically compatible with SDC up to 1100 ◦ C as indicated
by X-ray diffraction analysis. The electrical conductivity of BSFN reached the maximum value of 57 S·cm−1
at 450 ◦ C. The thermal expansion coefficient (TEC) value of BSFN was 30.9×10−6 K−1 , much higher than
that of typical electrolytes. The electrochemical behavior of the composites was analyzed via electrochemical
impedance spectroscopy with symmetrical cells BSFN-SDC/SDC/BSFN-SDC. The area specific interfacial
polarization resistance (ASR) decreased with increasing SDC content of the composite. The area specific
interfacial polarization resistance (ASR) at 700 ◦ C is only 0.49, 0.34 and 0.31 Ω·cm2 when 30, 40, and
50 wt% SDC was cooperated to BSFN, respectively. These results suggest that BSFN-SDC is a possible
candidate for IT-SOFC cathode.
KEY WORDS: Ba0.5 Sr0.5 Fe0.9 Ni0.1 O3−δ ; Cathode; Intermediate-temperature solid oxide fuel cell
(IT-SOFC); Solid oxide fuel cell
1. Introduction
La1−x Srx MnO3−δ perovskite (LSM) is regarded
as one of the most promising cathode materials for
solid oxide fuel cells (SOFC) due to its high thermal and chemical stability[1] to the typical electrolyte.
However, the cathode polarization plays a critical
role in the whole electrode reaction process, especially at low temperatures[2,3] . LSM is limited in the
application of cathode for intermediate-temperature
solid oxide fuel cell (IT-SOFC) because of its high
polarization resistance[4] . In order to improve cathode performance, one effective strategy is to replace
LSM with higher catalytic active materials at lower
temperatures. For example, Wei et al.[5] examined Ba0.5 Sr0.5 Zn0.2 Fe0.8 O3−δ (BSZF) as a cobalt-free
† Corresponding author. Prof., Ph.D.; Tel.: +86 555 2311551;
Fax: +86 555 2311822; E-mail address: xfchu99@ahut.edu.cn
(X.F. Chu).
cathode for IT-SOFC and found that the symmetrical
BSZF cathode on Ce0.8 Sm0.2 O2−δ (SDC) electrolyte
showed the area specific interfacial polarization resistance (ASR) of 0.48 Ω·cm2 at 650 ◦ C. Zhao et al.[6]
prepared Ba0.5 Sr0.5 Fe0.8 Cu0.2 O3−δ (BSFC) via auto
ignition process and found that the ASR of BSFC was
as low as 0.137 Ω·cm2 at 700 ◦ C. Ling et al.[7] reported
that the ASR of a cobalt-free cubic perovskite oxide Sm0.5 Sr0.5 Fe0.8 Cu0.2 O3−δ (SSFCu) was only 0.085
Ω·cm2 at 700 ◦ C. In this work, new cobalt-free composites BSFN-SDC were prepared and their electrochemical performance as the IT-SOFC cathode were
investigated.
2. Experimental
BSFN was synthesized by auto ignition process[8] .
Stoichiometric amount of Ba(NO3 )2 , Sr(NO3 )2 ,
Fe(NO3 )3 ·9H2 O and Ni(NO3 )2 ·6H2 O were dissolved
829
X.F. Chu et al.: J. Mater. Sci. Technol., 2012, 28(9), 828–832.
3. Results and Discussion
3.1 XRD patterns of the BSFN-SDC, BSFN and SDC
Fig. 1 presents XRD patterns of the BSFN powder
after being calcined at 1000 ◦ C for 3 h and the SDC
powder. Sharp lines demonstrate a well-developed
crystallization of BSFN and all the peaks can be
well indexed as a cubic perovskite structure with the
space group of Pm3m (211). Fig. 1 also shows the
XRD spectra of mixed powders of BSFN and SDC
00
Intensity / a.u.
BSFN+SDC 11
BSFN
20
30
40
50
2
60
000
1
SDC
70
600
o
C
o
C
o
C
80
/ deg.
Fig. 1 XRD patterns of the BSFN-SDC, BSFN and SDC
60
-
1
50
Conductivity / S cm
in distilled water to form an aqueous solution, and
then citric acid, as complexation agent, was added
with the mole ratio of citric acid/metal of 1.5:1.
The solution was then heated till self-combustion
occurred. The as-synthesized powders were subsequently calcined at 1000 ◦ C for 3 h to obtain fine
BSFN powders. SDC was synthesized by carbonate
coprecipitation[9] . To assess the phase reaction between BSFN cathode and SDC electrolyte, the chemical compatibility of the BSFN cathode with the SDC
electrolyte was investigated by sintering the mixed
powders of BSFN and SDC in a weight ratio of 1:1
at 1100 ◦ C for 3 h. Phase identification of the calcined powders was performed with X-ray diffraction
(D/Max-gA, Japan).
Electrical conductivity (σ) measurements were
performed with a four-probe d.c. method on H.P.
multimeter (Model 34401) from 400 to 800 ◦ C with
increments of 50 ◦ C in air.
Thermal expansion of the specimen was measured from 30 to 1000 ◦ C using a dilatometer (SHIMADZU50) at a heating rate of 10 ◦ C·min−1 in air.
Symmetrical electrochemical cell with the configuration of BSFN-SDC/SDC/BSFN-SDC applied for
the impedance research was measured by electrochemical impedance spectroscopy (EIS) (IM6e, Zahner,
0.1 Hz–1 MHz) from 550 to 700 ◦ C with increments
of 50 ◦ C. The powder SDC was uniaxially dry-pressed
into pellet (Ø13 mm, 1 mm thick) at 200 MPa, and
the pellet were sintered at 1400 ◦ C for 5 h. Composite
BSFN-SDC in various weight ratios were mixed thoroughly with a 6 wt% ethylcellulose–terpineol binder
to prepare the cathode slurry, respectively, which were
painted on both faces of SDC electrolyte. Then the
cells were fired at 1000 ◦ C for 3 h in air. Afterwards,
a silver paste was screen-printed onto both faces of
the cathode, and then fired at 800 ◦ C for 2 h in air
to form a symmetrical electrochemical cell. Hereafter,
the composite BSFN-SDC in various weight ratios will
be referred to as BSFN70-SDC30, BSFN60-SDC40
and BSFN50-SDC50 for convenience (the number is
weight ratio (%)).
A scanning electron microscopy (SEM, JEOL
JSM-6400) was used to observe the microstructure of
BSFN-SDC composite cathode on the surface of SDC
electrolyte and cross-section of BSFN-SDC/SDC after
EIS testing.
40
30
20
10
0
350 400 450 500 550 600 650 700 750 800 850
Temp. /
o
C
Fig. 2 Temperature dependence of the conductivity for
BSFN sample measured in the range of 400–
800 ◦ C in air
after sintered at 1100 ◦ C for 3 h. All the peaks
can be attributed to either BSFN or SDC, indicating that BSFN is chemically compatible with SDC up
to 1100 ◦ C.
3.2 Temperature dependence of the conductivity for
BSFN sample
Fig. 2 shows the temperature dependence of the
electrical conductivity of the BSFN sample in air. The
total electrical conductivity was measured on a BSFN
rectangular bar sintered at 1250 ◦ C for 5 h in air. It
is clear that the conductivity increases with increasing temperature and reaches the maximum value of
57 S·cm−1 at about 450 ◦ C, then the conductivity begins to decrease when the temperature is high than
450 ◦ C because of the loss of the lattice oxygen at
elevated temperature. The conductivity values of the
BSFN sample in the temperature range of IT-SOFC
(400–600 ◦ C) are 28–57 S·cm−1 . The conductivity of
LSM was 3.5 S·cm−1 at 1000 ◦ C[10] . In fact, several
perovskite electrode materials with low conductivity
have been reported with good electrochemical performance. For example, the maximum conductivities of
Ba0.5 Sr0.5 Co1−y Fey O3−δ (BSCF)[11] were about 20–
830
X.F. Chu et al.: J. Mater. Sci. Technol., 2012, 28(9), 828–832.
match can be minimized with the addition of SDC to
form composite cathode.
0.030
3.4 Electrochemical characterization
0.025
d /
LL
0
0.020
0.015
0.010
0.005
0.000
0
200
400
600
Temp. /
o
800
1000
C
Fig. 3 Thermal expansion of the sample of BSFN at a
heating rate of 10 ◦ C·min−1 in air
40 S·cm−1 , which is very close to that of BSFN.
3.3 Thermal expansion of the BSFN sample
Thermal expansion curve in Fig. 3 shows the average thermal expansion coefficient (TEC) value of
BSFN is 30.9×10−6 K−1 and gradually increases in
the high temperature region. The TEC value of SDC
was 12.8×10−6 K−1[12] , indicating that there should
be significant thermal expansion mismatch between
BSFN cathode and SDC electrolyte. The mismatch of
TEC between the electrolyte and cathode will result
in delamination at the cathode/electrolyte interface,
or cracking of the electrolyte because of the stress developed upon heating and cooling. However, the mis-
The typical impedance spectra of the symmetrical
cell BSFN-SDC/SDC/BSFN-SDC at various temperatures (at 550, 600, 650, 700 ◦ C) in air are shown in
Fig. 4, which is fitted using equivalent circuit program
by Zview. The electrolyte contribution has been subtracted from the overall impedance. The model used
to fit the impedance data was based on the model
shown in Fig. 5. Similar models have been used in
the literature for comparable systems[13–16] . The polarization resistance (Rp ) of a single electrode may
be calculated as Rp =1/2(R1 + R2 ), where R1 and R2
are the total electrode contributions. The ohmic resistances Rs of the electrolyte and Rp from the impedance fitting of samples are presented in Table 1.
As shown in Fig. 6, the ASR decreases with increasing SDC content at the same operating temperature. As expected, the increase of the measuring temperature results in a significant reduction of the ASR.
It is worthy to note that the ASR of the BSFN70SDC30, BSFN60-SDC40 and BSFN50-SDC50 cathode are 0.49, 0.34 and 0.31 Ω·cm2 in air at 700 ◦ C,
respectively. The ASR is typically used in the field
of SOFC to quantify all resistances associated with
the electrode[17] . Although the ASR of BSFN-SDC is
higher than those of BSZF, BSFC and SSFCu, it is
lower than that of LSM-SDC (0.94 Ω·cm2 ) prepared
by screen-printed method at 700 ◦ C[18] . This implies
30
8
(a)
o
550
(b)
C
600
o
C
25
2
cm
15
4
-Z'' /
-Z'' /
cm
2
6
20
10
2
5
0
0
0
5
10
15
Z'
20
25
30
0
/
/
6
8
2
cm
1.0
650
(c)
o
C
(d)
2.5
700
o
C
2
0.8
cm
2
2.0
1.5
-Z'' /
cm
4
Z'
cm
3.0
-Z'' /
2
2
0.6
0.4
1.0
0.2
0.5
0.0
0.0
0.0
0.5
1.0
1.5
Z'
/
2.0
2
cm
2.5
3.0
0.0
0.2
0.4
Z'
0.6
/
0.8
1.0
2
cm
Fig. 4 Impedance spectra of the BSFN-SDC cathode in various weight ratios with SDC serving as the electrolyte
measured at 550–700 ◦ C. ((4) BSFN70-SDC30; (°) BSFN60-SDC40; (¤) BSFN50-SDC50)
X.F. Chu et al.: J. Mater. Sci. Technol., 2012, 28(9), 828–832.
831
Fig. 5 Equivalent circuit of BSFN-SDC in air
Table 1 Fitted equivalent circuit polarization resistances for electrode
Cathode composition
BSFN50-SDC50
BSFN60-SDC40
BSFN70-SDC30
T /◦ C
Rs /Ω·cm2
550
600
650
700
550
600
650
700
550
600
650
700
14.20
7.81
4.74
3.09
12.29
6.79
4.12
2.69
14.09
11.51
6.76
4.21
7.5
7.0
-1
-2
cm )
8.0
ln((
T/ASR) / K
6.5
6.0
5.5
5.0
4.0
3.5
1.00
BSFN50-SDC50
BSFN60-SDC40
BSFN70-SDC30
1.05
resistances (ASR)/Ω·cm2
R2
Rp
7.81
8.11
2.26
2.73
0.80
0.79
0.39
0.31
10.03
10.38
3.12
3.23
0.97
0.92
0.38
0.34
33.23
20.86
4.03
4.26
1.34
1.28
0.57
0.49
that the activity of BSFN-SDC cathode is higher than
that of traditional LSM-SDC cathode.
As shown in Fig. 6, the activation energy (Ea )
of BSFN-SDC cathode decreases with the SDC content. This may result from expanding the reaction
zone beyond three-phase boundaries with increasing
SDC content. However, it is worthy to note that the
electrical conductivity of BSFN-SDC cathode will be
too low with over-high content of SDC in the cathode.
This may result in the difficulty of collecting current
in practice.
8.5
4.5
Area polarization
R1
8.40
3.19
0.79
0.23
10.72
3.34
0.86
0.31
8.49
4.49
1.23
0.41
E =1.60 eV
E =1.66 eV
E =1.80 eV
a
a
a
1.10
1.15
1.20
1.25
T) / K
(1000/
3.5 Microstructure of cathode/electrolyte
-1
Fig. 6 Arrhenius plot of polarization resistance for
BSFN-SDC cathode in various weight ratios
The microstructure of BSFN-SDC/SDC is shown
in Fig. 7. The cathodes are porous, indicating that
Fig. 7 SEM images of BSFN-SDC composite cathode after EIS testing: (a) surface of SDC electrolyte, (b) crosssection of BSFN-SDC/SDC
832
X.F. Chu et al.: J. Mater. Sci. Technol., 2012, 28(9), 828–832.
there is a good absorbility to the O2 . The thickness
of cathodes was determined from SEM micrograph.
The average thickness of BSFN-SDC is about 20 µm.
4. Conclusion
New cobalt-free composites BSFN-SDC as a possible cathode material have been investigated on
their chemical compatibility, electrical conductivity
and polarization resistances in various weight ratios.
BSFN is chemically compatible with SDC at 1100 ◦ C.
The conductivity values of the BSFN sample in the
temperature range of IT-SOFC (400–600 ◦ C) are 28–
57 S·cm−1 . The polarization resistance decreases with
increasing SDC content in cathode material. The polarization resistance of BSFN50-SDC50 is 0.31 Ω·cm2
in air at 700 ◦ C and is much lower than that of traditional LSM-SDC cathode material. Therefore, BSFNSDC is a possible candidate for IT-SOFC cathode material.
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