Journal of New Materials for Electrochemical Systems 7, 145-151 (2004)
© J. New. Mat. Electrochem. Systems
On the Suitability of La0.60Sr0.40Co0.20Fe0.80O3 Cathode for the
Intermediate Temperature Solid Oxide Fuel Cell (ITSOFC)
I . Arul Raja* , A.S. Nesaraja, M. Kumara, F. Tietzb, H.P. Buchkremerb and D. Stoeverb
aFuel
Cells section, Central Electrochemical Research Institute, Karaikudi – 630 006, India
bInstitute for Materials in Energy Systems (IWV 1), FZJ, D-52425, Germany
(Received April 12, 2003; received revised form December 8, 2003)
Abstract: Solid oxide fuel cell can convert fuels rich in H2 into electrical energy directly without pollution by electrochemical reaction with
oxygen. The efficiency of energy conversion and durability of performance mainly depend on the electrocatalytic activity of the cathode and its
thermo-chemical compatibility with the oxide ion conducting solid electrolyte. The global experience gained all these years in the SOFC
development has prompted for a change from the state of the art functional cathode material, La1-xSrxMnO3 (LSM) to a new material which is an
electrocatalyst for the oxygen reduction reaction in the intermediate temperature range (873 – 1073 K). In this work, La0.60Sr0.40Co0.20Fe0.80O3,
(LSCF), a mixed conducting stable perovskite oxide prepared by glycine nitrate combustion route is systematically characterised. Both circular and
rectangular pellets were fabricated by uniaxial compression followed by annealing at different temperatures. The functional properties such as
porosity, percentage thermal shrinkage in volume and percentage densification of the sintered pellets are compiled. It is found that
La0.60Sr0.40Co0.20Fe0.80O3 exhibited high electrical conductivity (350 Scm-1) at ITSOFC operating temperature (1073 K). The thermo-chemical
compatibility of this cathode material with alternate oxide ion conducting solid electrolytes namely, La0.9Sr0.1Ga0.8Mg0.2O3, Ce0.90Gd0.10O2 and
Ce0.80Sm0.20O2 ( LSGM, CDC and SDC respectively ) is also brought out. A brief discussion is made on its suitability for application as
electrocatalytic cathode under ITSOFC operating conditions.
Keywords: cathodes, intermediate temperature solid oxide fuel cells
applications. Waller et al [12] have found that La1-xSrxCo1-yFeyO3
perovskites remain stable with high degree of cation deficiency in
the A-site and that when 0.2<x<0.25, the crystal structure of
LSCF changes from rhombohedral to cubic. Dionissios
Mantzavinos et al [13] have investigated the oxygen stoichiometry
in La1-xSrxCo1-yFeyO3-d perovskites at reduced oxygen partial
pressures and at various temperatures. In line with these global
efforts, in this work, with the objective of demonstrating ITSOFC
real working system, based on the results obtained in a separate
study on several perovskite based cathode materials, we have set a
mandate, to either qualify or disqualify through experimental
measurements the combination between the LSCF cathode and
LSGM, GDC and SDC electrolytes for ITSOFC so that upscalable tape casting experiments to fabricate reproducible large
area thin sections of functional components can be confined only
to the qualified systems.
1. INTRODUCTION
The global experience gained in the solid oxide fuel cell
development had prompted a change from the state of art cathode
material for operating it in the intermediate temperature range
(below1073 K). The cathode material employed currently for the
reduction of oxygen is La1-xSrxMnO3 (LSM) perovskite which
causes the problem of formation of insulating new compounds
across the cathode-electrolyte interface which is detrimental to the
performance of SOFC [1-5]. In recent years there is a growing
interest in exploring alternate perovskite type oxides with mixed
electronic and ionic conduction for the cathode in ITSOFC [610]. Chou et al [11] have found that LSCF material with low Sr
content exhibited acceptable properties for use in electrochemical
*To whom correspondence should be addressed: E-mail:
iarulraj@rediffmail.com; Fax: 0091-44-22542456
145
146
2. EXPERIMENTAL
2.1. Powder preparation
High purity lanthanum oxide, strontium nitrate, cobalt nitrate tetra
hydrate, ferric nitrate nano hydrate and glycine were used. The
stoichiometric composition of mixtures for combustion were
calculated using the total oxidising (O) and reducing (F) valencies
of the components which serve as a numerical coefficient for the
stoichiometric balance. From the stoichiometric calculation, the
LSCF powder was obtained by glycine-nitrate synthesis route
which involved rapid heating of an aqueous concentrated solution
containing respective ions at 823 K. The solution had initially
boiled, undergone rapid volume reduction and foaming followed
by decomposition generating gases. The gases ignite and burn at
temperature 1273 K, yielding voluminous oxide which were
calcined at 1073 K for 3 hours to enrich the most stable mixed
oxide phases.
I.A. Raj et al. / J. New. Mat. Electrochem. Systems 7, 145-151 (2004)
orthorhombic. It is reported earlier that the crystal structure of
La1-xSrxCo0.2Fe0.8O3-d where x ≥ 0.6 changed from orthorhombic
to rhombohedral then to cubic similar to that of LaFeO3-SrFeO3-d
system [7]. The theoretical X-ray density of LSCF was calculated
from the data obtained from XRD data and found to be
5.95 gcm-3. The lattice parameters are given in Table 1.
3.2. Thermal analysis
The TGA/DTA patterns obtained with LSCF is shown in Figure 2.
In the TGA pattern, the LSCF sample showed a gain of weight
(2%) from room temperature to 973K. It is reported that LSCF
samples have undergone reversible reduction and oxidation
respectively on heating and cooling [7]. From the DTA pattern, it
is seen that a broad exothermic peak observed at 628 K due to the
decrease in the weight.
120
2.2. Physico-chemical characterisation
The combustion-derived powders were crushed in an agate mortar
and pelletized by applying uni-axial pressure. These pellets were
subjected to sintering at various temperatures ranging from 1073
K to 1473 K. From the data, the sintering behaviour viz.
percentage densification and percentage shrinkage in volume
were estimated. The thermo-chemical compatibility of LSCF with
alternate oxide ion conducting solid electrolytes was estimated by
mixing LSCF and the chosen electrolyte in the weight ratio of 1:1.
The thoroughly mixed powder was uni-axialy compressed into
circular pellets and then subjected to sintering at 1573 K for 3
hours. The sintered pellets were crushed again and subjected to
XRD for examining the new phases, if any, formed. The annealing
conditions 1573 K, 3 h dwell time and air is chosen arbitrarily for
relative estimation. For the apparent porosity data, measurements
were made on sintered pellets by dioxan retention method in a
device designed for this purpose.
100
80
Intensity(a.u)
The combustion derived LSCF perovskite
powders were
characterised by powder X-ray diffraction (XRD), particle size
analysis, FTIR spectra and TGA/DTA. The XRD patterns were
obtained with JEOL-8030 X- ray diffractometer. Horiba Laser
Particle Size Analyser (LA-910) was used to determine the
particle size distribution. FTIR Spectra was recorded using a
Perkin- Elmer 500 FTIR spectrometer.
60
40
20
0
0
10
20
50
60
70
80
90
Figure 1. XRD pattern obtained of LSCF powder.
Table 1. Crystal
structure
and
La0.60Sr0.40Co0.20Fe0.80O3-d powder.
Properties
For the electrical conductivity measurements rectangular sticks
(40 mm length, 5 mm thickness and 5 mm width ) were made by
hydraulic pressing of the powder and sintered at 1573 K for 5 h in
air. DC four probe measurements were carried out with the help
of a computer controlled equipment fabricated in the laboratory
for this purpose.
Crystal structure
3. RESULTS AND DISCUSSION
Unit cell volume
(A°)3
Theoretical
density (g/cc)
Crystallite Size
(µm)
The XRD pattern obtained from the calcined LSCF powder is
shown in Figure 1. The crystal structure is found to be
40
2θ
2.3. Electrical characterisation
3.1. XRD data
30
Unit cell lattice
parameter (A°)
parameters
obtained
Standard XRD
data for LaFeO3
powder (JCPDS
No. 37-1493)
Orthorhombic
Obtained XRD
data for
La0.60Sr0.40Co0.20
Fe0.80O3-δ powder
Orthorhombic
a=5.5669
a=5.6576
b=7.7847
b=7.8837
c=5.5330
c=5.4939
247.2656
245.0434
6.5200
5.9530
-
10.7840
on
147
On the Suitability of La0.60Sr0.40Co0.20Fe0.80O3 ... / J. New. Mat. Electrochem. Systems 7, 145-151 (2004)
35
30
Volume %
25
20
15
10
5
0
3,
5
4,
1
4,
7
7,
3
8,
5
9,
11 8
,
17 4
,
20 7
,
23 5
,
27 8
,6
32
-5
Figure 2. TGA and DTA curves obtained on LSCF powder.
Particle Size micrometer
3.3. FTIR spectroscopy
Figure 4. Particle Size Distribution data obtained on LSCF powder.
Figure 3 shows the FTIR spectra obtained on LSCF powder. The
broad peak at 600 cm-1 with a split is characteristic of the MO6
octahedra found commonly in perovskite oxide. It is reported
that the peak at 588.43 cm-1 can be assigned to the Co-O
stretching.
120
Figure 4 presents the particle size distribution obtained on LSCF
powder. It is observed that 30 % of particles in bulk remain in the
10 µm range. About 25 % of the particles remain in the 25 µm
range. The particles were found to be spherical and uniform.
Pct of Densification Factor
100
3.4. Particle size measurements
80
60
40
20
0
1000
1100
1200
1300
1400
1500
Tem perature K
Figure 5. The Effect of Temperature on Densification Factor of LSCF
pellets.
Figure 3. FTIR analysis data obtained on LSCF powder.
3.5. Sintering data
The sintering behaviour for LSCF cathode material is investigated
in detail as a function of the sintering temperature. The
densification factor values were calculated from the sintering data
and the apparent porosity values were estimated.
The effect of temperature on densification factor of the LSCF
cathode is shown in Figure 5. The percentage densification factor
increased with increasing temperature. The percentage shrinkage
in volume observed on LSCF pellets due to annealing as a
function of temperature is shown in Figure 6. It is shown that
both the percentage densification factor as well as the percentage
shrinkage factor increase with increase in annealing temperature.
It is drawn that beyond 1350 K, there is a limiting behaviour
observed for LSCF which is indicative of the fact that when this
material is fabricated as thin sections of cathode component for
ITSOFC the limiting temperature of processing must be 1350 K.
3.6. Apparent porosity data
The effect of temperature on the apparent porosity values of the
sintered pellets made from the LSCF cathode material is shown in
148
I.A. Raj et al. / J. New. Mat. Electrochem. Systems 7, 145-151 (2004)
Figure 7. It is noted that the percentage porosity values decrease
with increase in temperature attaining limit at 1350 K. The
maximum attainable porosity ( 44.5 %) is resulted at 1100 K. It is
necessary that the ITSOFC cathode must have an apparent
porosity of minimum 30 % to enable the diffusion of molecular
oxygen into the porous body for taking up adsorption sites on the
LSCF so that electron transfer rate may be enhanced. Apart from
the electrocatalytic activity of the cathode material, the ease of
fabrication of thin layers with optimum porosity is also critical.
60
Pct of Shrinkage Factor
50
3.7. Apparent porosity versus densification factor
The relationship between the percentage densification factor and
apparent porosity values of LSCF cathode is shown in Figure 8. It
is evident that the densification factor decreases with increase in
porosity values. Both the percentage densification and the
percentage apparent porosity values attain the desirable optimum
of 32 % at a sintering temperature of 1175 K. It is a critical point
that has to be considered when a functional component is
fabricated in thin form as section with adequate porosity and high
electrical conductivity (cathode) in conjunction with another
functional component without porosity and high degree of
densification (oxide ion conducting solid electrolyte) for ITSOFC.
It is therefore drawn from these data that when LSCF is chosen to
perform as ITSOFC cathode, the maximum permissible limit of
percentage densification and the acceptable limit of percentage
apparent porosity should be around 32 % for desirable results.
40
50
30
45
40
20
10
0
1000
1100
1200
1300
1400
1500
Temperature K
Figure 6. The Effect of Temperature on Shrinkage Factor of LSCF pellets.
Pct of Densification Factor
35
30
25
20
15
10
50
5
45
0
40
-5
35
0
20
40
60
80
100
120
Pct of Porosity Factor
Pct of Porosity
30
25
Figure 8. The dependence of Densification Factor on porosity of LSCF
pellets.
20
15
3.8. Electrical conductivity data
10
The electrical conductivity data obtained on LSCF rectangular
stick in air are presented in Table 2. as function of temperature.
The Arrhenius plot obtained from these data is shown in Figure 9.
The electrical conductivity of LSCF cathode increases with
temperature until 865 K where it reaches a maximum value of 405
Scm-1 and then decreases with further increase in temperature up
to 1166 K. Below 865 K, the LSCF follows the relationship for
the adiabatic small polaron hopping mechanism, governed by the
following equation as described earlier [7].
5
0
-5
1000
1100
1200
1300
1400
1500
Temperature K
Figure 7. The Effect of Temperature on Porosity Factor of LSCF.pellets.
149
On the Suitability of La0.60Sr0.40Co0.20Fe0.80O3 ... / J. New. Mat. Electrochem. Systems 7, 145-151 (2004)
sT = C exp(-Ea/KBT)
3.9. Chemical compatibility features
LSCF is a P-type conductor and the partial substitution by Sr+2
(acceptor) on the La+3 site increases the electronic conductivity
due to Fe+3 → Fe+4 / Co+3 → Co+4 transitions [14]. From the slope
of the Arrhenius plot, the activation energy (Ea) for small polaron
conduction is calculated as 0.05 eV. The LSCF peak conductivity
ranging from 300-400 Scm-3 was observed between 570-960 K.
This value is significantly higher than that of the LSM cathode
(200 Scm-1 ), which is currently in use and hence LSCF is
declared qualified to be an useful alternate cathode material for
ITSOFC application provided it qualifies the thermo-chemical
compatibility experiments with the alternate oxide ion conducting
solid electrolytes proposed in this work.
3.9.1. Chemical compatibility between LSCF cathode and
LSGM electrolyte
XRD pattern obtained on 1:1 by weight of LSCF and LSGM
powder mixture calcined at 1573 K for 3 h is shown in Figure 10
(a). The comparison of XRD data of the individual powder (LSCF
and LSGM) and mixed powder (LSCF+LSGM) is shown in Table
3. It is reported that La0.60Sr0.40Co0.20Fe0.80O3-d reacts with YSZ
electrolyte at 1273 K forming SrZrO3 perovskite and La2Zr2O7
pyrochlore [15], whereas, no details are available on the thermo-
750
LSCF+LSGM
LSCF+CGO
LSCF+SDC
(c)
550
Intensity ( a.u )
Elevtrical conductivity S/cm
2,6
(b)
2,4
350
* Impurity
(a)
*
2,2
150
0
20
40
60
80
100
2 theta Degree
Figure 10. XRD patterns on the compatibility of LSCF with LSGM, CGO
and SDC.
2
0,5
1,5
2,5
3,5
1/T (1/K)
Figure 9. Arrhenius plot for the conductivity of LSCF pellets in air.
Table 2. Electrical conductivity data obtained on La0.60Sr0.40Co0.20Fe0.80O3-d.
Temperature K
Electrical conductivity (Scm-1)
321
369
469
570
670
768
865
964
1065
1120
1166
118.25
184.23
280.22
339.80
378.39
401.16
405.80
403.12
364.49
315.77
296.87
Table 3. Comparison of XRD data of individual powder with the XRD data
of La0.60Sr0.40Co0.20Fe0.80O3-d + La0.9Sr0.1Ga0.8Mg0.2O3-d powder mixture after
annealing at 1573 K for 3 h.
XRD data for LSCF + LSGM 9182
XRD data for
XRD data for
powder mixture
LSCF
LSGM 9182
Peak
2θ
d
Peak
2θ
d
2θ
d
No.
values
values
assigned for
values
values
values
values
1.
22.600
3.931
LSGM
--
--
2.
27.600
3.229
impurity
--
--
3.
29.800
2.996
LSGM
--
--
30.100 2.9660
4.
32.400
2.761
LSGM/ LSCF
33.000
2.712
32.600 2.7445
5.
35.400
2.534
LSGM
--
--
34.800 2.5760
6.
46.500
1.951
LSGM
--
--
46.700 1.9435
7.
49.500
1.840
LSGM
--
--
49.600 1.8364
8.
57.800
1.594
LSGM/ LSCF
58.600
1.574
58.000 1.5889
9.
67.800
1.381
LSGM/ LSCF
68.900
1.362
68.000 1.3775
22.700 3.9141
--
--
150
I.A. Raj et al. / J. New. Mat. Electrochem. Systems 7, 145-151 (2004)
chemical compatibility of LSCF with LSGM electrolyte. From
Table 3 and Figure 10(a), it is noted that the La0.60Sr0.40Co0.20
Fe0.80O3-d calcined with La0.9Sr0.1Ga0.8Mg0.2O3-d at 1573 K for 3 h
exhibited one new peak at 2θ = 27.6o owing to the formation of
impurity phase. It is therefore drawn that La0.60Sr0.40Co0.20
Fe0.80O3-d is not a suitable candidate cathode for use with
La0.9Sr0.1Ga0.8Mg0.2O3-d electrolyte in ITSOFC.
3.9.2. Chemical compatibility between LSCF cathode and GDC
electrolyte
XRD pattern obtained on 1:1 by weight of LSCF and GDC
powder mixture calcined at 1573 K for 3 h is shown in Figure 10
(b). The comparison of XRD data of the individual powder (LSCF
and GDC) and mixed powder (LSCF+GDC) is shown in Table 5.
As no other impurity peak corresponding to new phase is
observed, which is in line with earlier reports [16,17], it is drawn
that La1-xSrxCo1-yFeyO3-d is a suitable candidate cathode material
for use with GDC electrolyte.
Table 4. Comparison of XRD data of individual powder with the XRD data
of
La0.60Sr0.40Co0.20Fe0.80O3-ä+ Ce0.80Sm0.20O2-ä powder mixture after
annealing at 1573 K for 3 h.
XRD data for LSCF+ SDC8020
XRD data for
powder mixture
LSCF
XRD data for
SDC 8020
Peak
2θ
d
Peak
2θ
d
2θ
d
No.
values
values
assigned for
values
values
values
values
1.
28.200
3.162
SDC
--
--
28.800
3.119
2.
32.600
2.744
LSCF/SDC
33.000
2.712
33.100
2.704
3.
46.900
1.936
LSCF
47.300
1.920
--
--
4.
55.800
1.646
SDC
--
--
56.300
1.633
5.
58.200
1.584
LSCF/ SDC
58.600
1.574
59.000
1.564
Table 5. Comparison of XRD data of individual powder with the XRD data
of
La0.60Sr0.40Co0.20Fe0.80O3-d+ Ce0.90Gd0.10O2-d powder mixture after
annealing at 1573 K for 3 h.
XRD data for LSCF+ CGO9010
XRD data for
XRD data for
powder mixture
LSCF
CGO9010
Peak
2θ
d
Peak
2θ
d
2θ
d
No.
values
values
assigned
values
values
values
values
for
1.
28.300
3.151
CGO
--
--
28.800
3.097
2.
32.600
2.744
LSCF
33.000
2.712
--
--
3.
47.100
1.928
LSCF
47.300
1.920
--
--
4.
56.100
1.638
CGO
--
--
56.600
1.625
5.
58.100
1.586
LSCF
58.600
1.574
--
--
6.
76.400
1.246
CGO
--
--
76.900
1.239
3.9.3 Chemical compatibility between LSCF cathode and SDC
electrolyte
XRD pattern obtained on 1:1 by weight of LSCF and SDC
powder mixture calcined at 1573 K for 3 h is shown in Figure 10
(c). The comparison of XRD data of the individual powder (LSCF
and SDC) and mixed powder (LSCF+SDC) is shown table in
Table 4. From Table 4 and Figure 10 (b), it is noted that the
La0.60Sr0.40Co0.20Fe0.80O3-d calcined with Ce0.80Sm0.20O2-d at 1573
K for 3 h in air exhibited no reaction product. No other impurity
peak corresponding to new phase, if any formed, is observed. It is
therefore drawn that La1-xSrxCo1-yFeyO3-d is a suitable candidate
cathode material for use with SDC electrolyte [16].
4. CONCLUSION
The thermal behaviour of LSCF pellets is brought out from the
steady state sintering experiments to draw useful information on
the inter-dependence of percentage shrinkage in volume and the
percentage densification factor with respect to the maximum
attainable apparent percentage porosity for the first time. From
the DC four probe measurements, LSCF is shown to exhibit
electrical conductivity value (350 Scm-1) when compared with
LSM (200 Scm-1 ) at 1073 K. LSCF is shown to be a suitable
candidate cathode material for CGO and SDC electrolytes and not
for LSGM electrolyte. These results have prompted us, for the
future work, to fabricate thin sections of LSCF by tape casting
onto already fabricated < 1.5 mm thick CGO and SDC electrolyte
support as bilayer, to investigate the influence of co-firing these
bilayer together for wide duration at 1350 K which can throw
more light on the prospect of these combinations and to integrate
the Ni-CGO and Ni-SDC cermet anodes onto the other side of the
cathode-electrolyte bilayer by tape casting to realize practical
PEN structures for ITSOFC.
5. ACKNOWLEDGEMENT
The authors are thankful to Dr.M.Raghavan, Director, CECRI,
Karaikudi. A.S.N. thanks C.S.I.R., New Delhi for Senior Research
Fellowship. Financial assistance by DST, New Delhi under grant
in aid project (06/2001 at CECRI, Karaikudi) is thankfully
acknowledged. Part of this work was carried out in the frame of
the Indo-German bilateral scientific project (No. IND 99/043)
supported by the Council of Scientific and Industrial Research
(CSIR), New Delhi and the German Ministry of Science and
Education, Berlin/Bonn.
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