A high performance intermediate-temperature solid oxide fuel cell using impregnated La0.6Sr0.4CoO3-δ cathode

Article abstract of DOI:10.1016/j.jallcom.2009.08.063

La_0.6Sr_0.4CoO_3-δ (LSC)-impregnated Sm_0.2Ce_0.8O_1.9 (SDC) porous cathode is successfully fabricated on a dense Gd_0.1Ce_0.9O_1.95 (GDC) film by an impregnation m

Full text of DOI:10.1016/j.jallcom.2009.08.063

Journal of Alloys and Compounds 487 (2009) 781–785
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
A high performance intermediate-temperature solid oxide fuel cell using
impregnated La0.6Sr0.4CoO3−ı cathode
Fei Zhaoa,b, Lei Zhanga, Zhiyi Jianga, Changrong Xiaa,∗, Fanglin Chenb
a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering,
University of Science and Technology of China, Hefei, 230026 Anhui, China
b Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2009
Received in revised form 10 August 2009
Accepted 14 August 2009
Available online 22 August 2009
La0.6Sr0.4CoO3−ı (LSC)-impregnated Sm0.2Ce0.8O1.9 (SDC) porous cathode is successfully fabricated on a
dense Gd0.1Ce0.9O1.95 (GDC) film by an impregnation method for intermediate-temperature solid oxide
fuel cells (IT-SOFCs). The effects of LSC loading and firing temperature on the performance of the LSC-
impregnated SDC cathodes are investigated by impedance spectroscopy with symmetrical cells. A single
cell with the LSC-impregnated SDC cathode produces a peak power output of 0.815 W cm−2 at 600 ◦C,
using humidified hydrogen as the fuel and ambient air as the oxidant. The present result shows promising
operational lifetime. Further, the result of thermal cycling stability of the LSC-impregnated cathode shows
that the performance of the single cell under open circuit condition is essentially dominated by the ohmic
Keywords:
Fuel cells
Electrode materials
Nanofabrications
Electrochemical impedance spectroscopy
resistance as well as the contact effect between the current collector and the electrode.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
the SOFCs development. The overall electrochemical performance
of an SOFC will decrease with the reduction in the operating tem-
A solid oxide fuel cell (SOFC) which utilizes solid oxide ceramics
as the electrolytes has shown to be a potentially reliable, durable,
environmentally benign, fuel flexible and high efficient energy con-
version technology. The use of solid electrolyte in SOFC eliminates
the involvement of liquids and the variation of the electrolyte
composition during operation. The high operating temperature
promotes rapid reaction kinetics, eliminates the need of previous
metal catalysts, allows internal reforming of hydrocarbon fuels, and
produces byproduct heat suitable for co-generation [1]. However,
SOFCs are currently not economically competitive to the existing
power generation technologies due to problems associated mainly
with high temperature (>800 ◦C) operation, including performance
degradations of cell components due to high temperature oxi-
dation, corrosion, chemical interdiffusion and reaction, structural
failures due to creep deformation and high thermal stress at the
interfaces of the various SOFCs components, as well as high cost of
the overall SOFCs system. One approach to cost reduction is low-
ering the SOFCs operating temperature to below 700 ◦C, so that
inexpensive metals can be used for interconnects, heat exchangers,
and structural components.
perature due to increased polarization resistance of the electrode
reaction and decreased electrolyte conductivity [2]. The resistive
contribution of the electrolyte can be reduced significantly by
using electrolyte materials with higher ionic conductivity such
as gadolinium-doped ceria (GDC) and thin film electrolytes (i.e.,
5–30 ␮m) on electrode-supported cell structures [3–9]. Therefore,
the shift in emphasis has been driven from the electrolyte to the
electrode where the electrode shows a higher percentage of the
voltage loss (due to higher activation energy) for intermediate-
temperature SOFCs. The cathode has been the center of the focus
in the electrode development largely because oxygen reduction
is the more difficult reaction to activate in SOFCs operated at
reduced temperature [10,11]. Consequently, development of cath-
ode material with high electrocatalytic activity becomes critical for
intermediate-temperature SOFCs. Recently nanoparticles impreg-
nated composite cathode structures have been reported [12–15].
Such cathode structures consist of well-connected cathode frames
and impregnated nanoparticles coated on the surfaces of the cath-
ode frames. The cathode frames are fabricated on top of the
electrolytes and then impregnated with nano-size particles to
improve the electrocatalytic activities of the cathodes. For exam-
ple, Jiang et al. [12] reported a LSM-based electrode modified by
impregnating Gd0.2Ce0.8(NO3)x solution into the LSM structure. At
700 ◦C, the electrode polarization resistance of the impregnated
LSM electrode decreased to 0.72 ꢀ cm2 as compared to 26.4 ꢀ cm2
for pure LSM electrode at similar testing condition. Huang et al. [13]
However, reducing the operation temperature of SOFCs to the
intermediate temperature range of 500–700 ◦C is still a challenge in
∗
Corresponding author. Tel.: +86 551 3607475; fax: +86 551 3601592.
E-mail address: xiacr@ustc.edu.cn (C. Xia).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.08.063

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F. Zhao et al. / Journal of Alloys and Compounds 487 (2009) 781–785
reported that a LSM–YSZ composite cathode prepared by impreg-
nating a porous YSZ cathode frame with nitrate solution of La, Sr,
and Mn to 40 wt% LSM loading exhibited a very low electrode polar-
ization loss of 0.5 ꢀ cm2 at 700 ◦C.
to form well-crystalline oxide powders prior to the subsequent use for fuel cell
fabrication.
Anode supported cells with GDC as electrolytes were prepared using a co-
pressing technique. Anode powders consisted of NiO and 40 wt% SDC were prepared
by mixing and grinding with an agate mortar and pestle. The anode powders were
first pressed as a substrate. GDC powder was then added onto the substrate and co-
pressed at 300 MPa for 3 min to form a bilayer. A SDC ink prepared by ball-milling
SDC powder with an ethyl cellulose binder and a terpineol-based solvent for 24 h
was applied by a screen-printing technique onto the GDC electrolyte to form a cath-
ode substrate layer. The tri-layer was dried at 80 ◦C in an oven for 10 min, followed by
co-sintering at 1400 ◦C for 5 h to form a cell consisted of NiO–SDC anode substrate,
GDC electrolyte dense film, and porous SDC frame. The thicknesses for the porous
SDC layer and dense GDC electrolyte film were ∼60 and ∼30 ␮m, respectively.
La0.6Sr0.4Co(NO3)x nitrate solution prepared by dissolving La(NO3)3·6H2O, Sr(NO3)2
and Co(NO3)2·6H2O in distilled water at a molar ratio of La:Sr:Co = 0.6:0.4:1 was
Since performance and reliability of an SOFC depend critically
on the cathode, reduction in the cathode polarization resistance
and improvement in the cathode stability are expected to signifi-
cantly enhance the SOFC performance [16,17]. The most effective
approach is to fabricate a composite cathode consisting of an
electronically conducting catalyst and an ionically conducting elec-
trolyte oxide. The inclusion of the electrolyte component in the
cathode structure often enhances the ionic conductivity within the
cathode, thereby increasing the electrochemically active region by
providing a “connection” with the electrolyte so that oxygen ions
can transport from the cathode to the electrolyte without barriers,
and helping to provide a thermal expansion match with the elec-
trolyte film. The choice of which electronically conducting oxide
to use in the cathode is often dictated by the need to have a high
electrocatalytic activity toward oxygen reduction and a chemical
compatibility to the other cell components. Lanthanum–strontium
cobaltite (La1−xSrxCoO3−ı, LSC) has been regarded as a potential
cathode material and shows enhanced electrocatalytic activity for
oxygen reduction and increased oxide ionic conductivity to pro-
duce a mixed ionic and electronic conductor from 600 to 800 ◦C
[18]. Furthermore LSC does not react with ceria-based electrolyte at
intermediate temperature range (500–700 ◦C). Consequently, LSC
is a promising cathode material especially for SOFCs using doped
ceria as the electrolytes. Unluckily, the thermal expansion coeffi-
cient (TEC) of LSC (∼20.5 × 10−6 K−1) is much higher than those of
doped ceria electrolytes (∼12 × 10−6 K−1) [1,19]. The mismatch in
the thermal expansion will cause potential stability issues for the
SOFC stacks and systems since the cathodes and the electrolytes
will crack and delaminate due to the thermal stresses causing sub-
stantial increase in the contact resistance.
In order to overcome the barrier of thermal expansion mismatch
between the LSC and the ceria-based electrolyte, a novel cathode
architecture is previously developed in our lab [20]. In this struc-
ture, the electrolyte material is doped ceria, the cathode frame
is SDC (samarium doped ceria) and the cathode catalyst is LSC.
SDC has high oxygen ion conductivity at intermediate operating
temperature while LSC has the highest electrocatalytic activity for
oxygen reduction [10]. Porous SDC layer (cathode frame) is formed
on the dense electrolyte. LSC nanoparticles are attached on the sur-
face of the porous SDC cathode frame, and the LSC nanoparticles are
connected with each other to form electronic conduction paths. LSC
nanoparticles are formed by an impregnation method: mixed La, Sr,
and Co nitrite solution is introduced into the porous cathode frame,
and the nitrites dry and cover the surface of the porous frame. Upon
firing, the nitrates decompose to form individual oxide nanoparti-
cles and then react to produce LSC particles on the surface of the
porous SDC cathode frame. In this work, the effects of LSC firing
temperature and LSC loading on the cathode performance as well
as the life durability and thermal cycling behavior of the SOFCs with
the impregnated LSC cathodes have been studied.
impregnated into the porous SDC cathode frame, followed by firing at 700–1200 ◦
C
for 2 h to form a LSC-impregnated SDC composite cathode. To optimize the perfor-
mance of the impregnated cathodes, symmetric cells were prepared by using SDC
electrolytes as substrates and LSC-impregnated SDC composites as electrodes. The
cathode performances as a function of the LSC loading and firing temperature were
investigated.
Single cells were sealed in alumina tubes with silver paste for cell performance
evaluation. Ag paste and Ag wires were used for current collection in the cells.
Microstructures of the cells were characterized using a JSM-6700F scanning elec-
tron microscope. Impedance spectra were measured on the symmetric cells and
the single cells under open circuit condition, with a frequency range from 0.01 Hz to
100 kHz and a 10 mV ac perturbation, using a ZAHNER IM6e electrochemical station.
The single cell performance based on the I–V curve was measured in the galvanos-
tatic mode. The life durability of the cell was obtained in the potentiostatic mode at
a cell voltage of 0.5 V.
3. Results and discussions
Shown in Fig. 1 is the dependence of interfacial polarization
resistance (Rp) on the LSC loading at temperatures from 500 to
750 ◦C. The resistances were measured with symmetric cells. The
LSC loading is expressed as the mass percentage of LSC in the LSC-
impregnated SDC electrode. The electrode polarization interfacial
resistance was directly measured from the difference of high and
low-frequency intercepts at the real axis in the impedance spec-
trum [1]. Results indicate Rp is influenced by both the loading of
LSC in the LSC-impregnated SDC composite electrode and the oper-
ating temperature of the symmetric cells. Increasing the loading
from 35% to 55% resulted in a continuous decrease in Rp at the
temperature range of 500–600 ◦C. However, when the loading was
further increased up to 65%, Rp increased with the increase of the
LSC loading. For example, Rp at 600 ◦C was 1.037 ꢀ cm2 when the
loading was 35%. It decreased to 0.364 ꢀ cm2 when the loading was
55%, but increased to 0.731 ꢀ cm2 at 65% LSC loading. At increased
operation temperature, the loading corresponding to the lowest
Rp was slightly lower than that at relative low temperature. The
overall electrode performance is not only determined by the inher-
2. Experimental
The cells studied in this work consisted of LSC (La0.6Sr0.4CoO3−ı)-impregnated
SDC (Sm0.2Ce0.8O1.9
) cathodes, GDC (Gd0.1Ce0.9O1.95) electrolytes and Ni–SDC
anodes. Oxide powders for fuel cell components including NiO, SDC and GDC were
prepared using a glycine-nitrate combustion method with nitrate precursors of
Ni(NO3)2, Sm(NO3)3, Gd(NO3)3 and (NH4)2Ce(NO3)6 [21]. Stoichiometric amounts
of the nitrates were dissolved in distilled water to form a precursor solution. Glycine
was then added to the solution with the molar ratio of glycine to total metal
ions of 1:1, 1.9:1 and 2.7:1 for NiO, SDC and GDC, respectively. The solution was
subsequently stirred and heated in a beaker on a hot plate with magnetic stirrer
until it was ignited, spurting out some puffy oxide powders. The as-prepared NiO,
SDC and GDC powders were pre-heated at 600 ◦C to remove carbon residues and
Fig. 1. Effect of LSC loading on the interfacial polarization resistance measured at
temperatures from 500 to 750 ◦C.

F. Zhao et al. / Journal of Alloys and Compounds 487 (2009) 781–785
783
Fig. 2. Impedance spectra measured at 600 ◦C in ambient air for symmetric cells
with impregnated LSC fired at 700, 800, 900 and 1200 ◦C for 2 h, respectively.
ent catalytic property of the electrode material, but also by gas
transport to and away from the reaction zone. At higher operat-
ing temperature, the reaction is highly activated; consequently, fast
reaction rate and a low Rp are expected. However, at relatively high
LSC loading, gas transport might become limited since the elec-
trode porosity decreases. Consequently, further increase in the LSC
loading beyond certain level will increase the electrode interfacial
resistance.
To investigate the effect of the LSC firing temperature on the
electrode performance, symmetric cells with electrodes impreg-
nated with 55 wt% LSC were fabricated and characterized. Shown
in Fig. 2 are the impedance spectra measured at 600 ◦C in ambi-
ent air for symmetric cells with the impregnated LSC electrodes
fired at 700, 800, 900 and 1200 ◦C for 2 h, respectively. Rp that is
the difference between the low-frequency and the high-frequency
intercepts on the real axis for the cell fired at 700 ◦C had the largest
value, which was 0.6 ꢀ cm2. This is probably due to the fact that
pure phase perovskite structured LSC was not completely formed
when the impregnated precursor was fired at 700 ◦C for 2 h. XRD
analysis revealed the presence of Co3O4 as shown in Fig. 3 and sim-
ilar result was also reported by others [22]. The LSC pure perovskite
phase was formed when the firing temperature reached 800 ◦C. The
lowest Rp was achieved with sample fired at 800 ◦C. Rp increased
with further increase in the firing temperature beyond 800 ◦C. The
average size of LSC particles usually increases with the firing tem-
perature. It can be expected that fine electrode microstructure will
enhance electrode performance. Consequently, Rp increased with
the firing temperature when it was above the temperature needed
for the formation of pure LSC perovskite phase.
Fig. 4. Cell performance with a LSC-impregnated SDC cathode. Measurement was
conducted at 650 ◦C under open circuit conditions with humidified H2 (3% H2O) as
the fuel and ambient air as the oxidant. (a) Life durability with a constant output
voltage of 0.5 V; (b) cell voltage and power density as a function of current density.
operated with a constant output voltage of 0.5 V. Shown in Fig. 4 is
the 89 h-run performance of the cell with LSC-impregnated SDC
as the cathode. As shown in Fig. 4(a), when the cell was oper-
ated with a constant output voltage of 0.5 V, the current density
increased in the initial 64 h, probably due to the activating pro-
cess and adaptive microstructure optimization of the impregnated
composite cathode. No obvious decline in the cell performance
was observed within the subsequent operating period. At the
end of the 89 h testing, the cell showed a peak power density of
0.815 W cm−2, which was about 40% higher than the initial value
(Fig. 4(b)). Shown in Fig. 5(a) is the impedance spectra of the cell
measured at 650 ◦C under open circuit condition. The total cell
resistance (Rt), interfacial polarization resistance (Rp) and ohmic
resistance (Ro) determined from the impedance spectra are shown
in Fig. 5(b). Ro, the intercept of high-frequency arc with the real axis,
was almost unchanged with a value of 0.165 ꢀ cm2, whereas, Rp,
the difference between the real axis intercepts of the impedance,
decreased from 0.0754 ꢀ cm2 to 0.0341 ꢀ cm2 in 64 h, and then
was almost unchanged in the subsequent operation. The improve-
ment of the cell performance as shown in Fig. 4 was mainly due
to the reduction of the interfacial polarization resistance, which
was contributed from both the cathode and anode. Compared with
the cathode interfacial contribution, the interfacial resistance from
the anode was generally negligible for the Ni-ceria cermet anode
[23]. The high performance observed in this work was attributed to
the unique cathode structure (well-connected cathode/electrolyte
interface) and impregnated nanosized LSC particles with high catal-
ysis activity which was reported in our previous work [20].
To investigate the life durability of a single cell with the
LSC-impregnated SDC composite cathode at 650 ◦C, the cell was
Another cell with the LSC-impregnated SDC cathode was inves-
tigated in the thermal cycling test. The cell was operated at a
constant output voltage of 0.5 V for 12 h at 600 ◦C prior to each
thermal cycle. Thermal cycling test was conducted at tempera-
Fig. 3. XRD patterns of the SDC cathode frame without impregnation (a), impreg-
nated LSC–SDC cathode fired at 700 ◦C for 2 h (b) and 800 ◦C for 2 h (c). *Co3O4.

784
F. Zhao et al. / Journal of Alloys and Compounds 487 (2009) 781–785
Fig. 5. (a) AC impedance spectra measured at 650 ◦C under open circuit conditions
for the cell as described in Fig. 4. (b) The total cell resistance (Rt), interfacial polar-
ization resistance (Rp), and ohmic resistance (Ro) determined from the impedance
spectra as a function of operation time.
Fig. 7. (a) AC impedance spectra of a cell with LSC-impregnated SDC cathode mea-
sured at 650 ◦C under open circuit conditions at the thermal cycles from 1 to 4 (as
shown in Fig. 6). (b) The total cell resistance (Rt), interfacial polarization resistance
(Rp), and ohmic resistance (Ro) determined from the impedance spectra as a function
of thermal cycle time.
tures between 500 and 650 ◦C with a heating and cooling rate of
5 ◦C/min at open circuit conditions. The cell was held for at least
1 h at each operating temperature prior to current load was applied.
The impedance measurement was collected under open circuit con-
dition. Shown in Fig. 6 is the cell performance as a function of
cell operation time and temperature. As shown in Fig. 6, after the
first thermal cycle treatment, the cell had a peak power output
of 0.851 W cm−2 at 650 ◦C with humidified hydrogen as the fuel
and ambient air as the oxidant. The peak power output decreased
after each thermal cycle, and after the 4th thermal cycle, it was
0.681 W cm−2 at 650 ◦C, which was about 20% lower than that
measured after the first thermal cycle. The decreasing trend was
also observed at lower operating temperature. The performance
reduction resulted from the increase in both ohmic resistance and
interfacial polarization resistance. Shown in Fig. 7(a) are the AC
impedance spectra of the cell with the LSC-impregnated SDC cath-
ode measured at 650 ◦C under open circuit condition after each
thermal cycle. The total cell resistance (Rt), the interfacial resistance
(Rp) and ohmic resistance (Ro) determined from the impedance
spectra are shown in Fig. 7(b). It can be seen that both the ohmic
resistance and interfacial polarization resistance increased with the
number of thermal cycles. However, the ohmic resistance increased
0.071 ꢀ cm2 while the interfacial polarization resistance increased
only 0.015 ꢀ cm2 after a total of four thermal cycles. Therefore, the
reduction of the cell performance during the thermal cycles was
dominated by the change of the ohmic resistance. Furthermore, it
was noticed that the Ag current collecting layer (Ag paste was used
in this test) curled up partially from the surface of the composite
cathode after the thermal cycling test. Since the LSC-impregnated
SDC composite cathode was well bounded to the electrolyte film
as shown in Fig. 8, the increase in cell ohmic resistance is probably
related to the partial delamination at the current collector/cathode
interface during the thermal cycling test due to the TEC mismatch
between the LSC-impregnated SDC cathode and the Ag current col-
Fig. 6. Peak power density of a cell with the LSC-impregnated SDC cathode as a
Fig. 8. SEM micrograph of the cross-sectional view of Ni–SDC anode/GDC/LSC-
function of operation time and temperature.
impregnated SDC cathode tri-layer after testing.

F. Zhao et al. / Journal of Alloys and Compounds 487 (2009) 781–785
785
lecting layer resulting in increased contact resistance [24]. It was
also reported that the polarization losses of the cell increased with
the decrease in the contact area of the current collector [25]. In
our previous study [20,26], the resistance of the LSC-impregnated
SDC cathode kept almost constant during the thermal cycling tests
when Pt current collecting layer was used. Pt current collecting
layer was found to be attached to the LSC-impregnated SDC cath-
ode very well after the thermal cycling tests. And the cell with Pt
as current collecting layer will be tested in the coming work.
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4. Conclusions
The performance of the cell with the LSC-impregnated SDC as
cathode was investigated in this work. Both the loading of LSC
and the firing temperature have significant influence on the per-
formance of the LSC-impregnated SDC composite cathode. With
55 wt% LSC loading and firing at 800 ◦C for 2 h, the impregnated
cathode and single cell showed excellent performance. The cell
showed the peak power output of 0.815 W cm−2 at 650 ◦C after run-
ning with a constant voltage output of 0.5 V for 89 h. After four
thermal cycling tests, the cell performance measured at 650 ◦C
reduced about 20% mainly due to the delamination between the
cathode and Ag current collecting layer.
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Acknowledgements
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This work was supported by the Natural Science Foundation of
China (50672096 and 50730002) and the Ministry of Science and
Technology of China (2007AA05Z151) and the US Department of
Energy (DE-FG36-08GO88116).
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