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Electrochemical performance of a solid oxide fuel cell with anLSCF cathode under different oxygen concentrations
Gianfranco DiGiuseppe*, Li Sun
Mechanical Engineering Department, Kettering University, 1700 University Avenue, Flint, MI 48504, USA
a r t i c l e i n f o
Article history:
Received 16 November 2010
Received in revised form
28 December 2010
Accepted 5 January 2011
Available online 4 March 2011
Keywords:
Solid oxide fuel cells
SOFC
LSCF
ASR
Impedance spectroscopy
a b s t r a c t
A new performance study has been performed on a commercially available anode sup-
ported planar SOFC containing an LSCF cathode. The SOFC cell is tested at different
temperatures and different cathode gas compositions. The temperature and cathode gas
dependence on the electrochemical performance is studied using voltageecurrent density
curves and impedance spectroscopy at different cell voltages. The cell tested shows
excellent performance at all temperatures and is not limited by diffusion losses for the
tested conditions. This new study indicates that the cell impedance spectroscopy is
comprised of at least four semicircles of which two are partially dependent on the cathode
gas conditions. It was found that historical effects play a role in the impedance spectra,
showing some scatter in the ohmic resistance as a function of applied voltage. The cell
ohmic resistance decreases as the temperature increases and as the cathode gas condi-
tions are switched from air to O2eHe mixture. However, the cell ohmic resistance under
pure O2 was found to be higher than the O2eHe mixture. In virtually all IS data, the cell
ohmic resistance showed a maximum value around 0.8 V. The cell ohmic ASR shows that
interfacial resistances are a significant portion of the total ohmic resistance. The total
electrode polarization decreases as the temperature increases and as the cathode gas
conditions are switched from air to O2eHe mixture and to pure O2. Finally, the peak
frequency of the largest semicircle observed at high frequency shows a linear dependence
on the applied voltage in most cases. This behavior is related to the charge transfer that
occurs in the high frequency range and indicates that the electrochemical reactions are
occurring at faster rates as more current flows through the cell.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction
Solid Oxide Fuel Cells (SOFCs) are expected to play amajor role
in many power generation applications. SOFC prototypes are
currently being built for portable, residential, remote, and
power plants applications. SOFCs can be built using different
ceramic materials depending upon the operating tempera-
ture. In general, the trend has been to operate SOFCs between
600 and 800 �C in order to increase the stack reliability and to
use more common components such as metallic intercon-
nects [1,2]. The LSCF cathode is a mixed conductor which has
shown high electrochemical activity as well as long term
stability. In this perovskite (ABO3) material, the A site is
comprised of La and Sr while the B site is comprised of a Fe
and Comixture. The compositions that have been studied and
reported in the literature appear to be La0.8Sr0.2Fe0.8Co0.2O3
* Corresponding author. Tel.: þ1 810 762 9666.E-mail address: gdigiuse@kettering.edu (G. DiGiuseppe).
Avai lab le at www.sc iencedi rect .com
journa l homepage : www.e lsev ie r . com/ loca te /he
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 7
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2011.01.017
Author's personal copy
and La0.6Sr0.4Fe0.8Co0.2O3; however, other compositions have
been tested and reported [3e6]. There is a general agreement
within the literature that the use of reference electrodes
combined with impedance spectroscopy (IS) for thin film
electrolytes SOFCs does not give accurate results, and the
whole cell IS spectra should be analyzed [7,8]. In an attempt to
resolve this concern, researchers have turned to microelec-
trode design in order to minimize the associated errors [9].
Others have looked at the whole cell IS spectra using equiva-
lent circuit analysis [8,10e14]. Though the IS technique is
widely used among scientists and engineers, it still not fully
accepted for SOFC applications and remains subject to inter-
pretation and controversy. In this study, the whole cell IS
approach is chosen because cells can be manufactured using
the same processes used for larger cells and should provide
a better representation of actual cells used in practical
applications.
Nonetheless, impedance spectroscopy is capable of
resolving the ohmic contributions from the total electrodes
polarization very well. Typical impedance data (Z ) is repre-
sented in terms of a complex number as Z ¼ Zre(u) þ jZimg(u)
where a real component (Zre) and an imaginary component
(Zimg) are plotted in Nyquist and Bode plots. The impedance
data is frequency dependent; therefore, cell properties and
processes information can be extracted from plots of different
parameters. In general, the real and imaginarypartsareplotted
against frequency. In many instances an equivalent circuit
model is used to resolve the different electrode processes, i.e.,
activation and diffusion. The high frequency intercept
describes the ohmic resistance (Ro) of the cell. The low
frequency intercept describes the total polarization of the cell.
The difference, subtracting the high frequency intercept from
low frequency intercept, gives the total electrode polarization
(Rp) which include both diffusion and activation contributions.
In other cases, the phase angle f ¼ tan�1ðRimg=RreÞ can be
plotted against the frequency in order to see more clearly the
process dependence on the frequency. In this work, a perfor-
mancemap has been developed at different temperatures and
cathode gas conditions using current-density curves and
impedance spectroscopy. Furthermore, thiswork, unlikewhat
has been reported in the literature, has studied the IS depen-
dence upon the cell voltage at very low applied voltages. In
addition, a phase angle analysiswas performed to identify any
visible effects on the peak frequencies of specific processes.
The behavior of the measured ohmic resistance and total
electrode polarization using IS was studied as a function of
applied voltage and temperature. Finally, the ohmic area
specific resistance (ASR) of the cell was also studied to deter-
mine any additional contributions due to interfacial
resistances.
2. Experimental method
In our laboratory, we have the capability to test several button
cells for extended periods of time to evaluate cell performance
and long term reliability. Typically, commercially available
SOFC cells are tested for approximately a thousand hours and
the voltage is monitored over time. Diagnostic tools, such as
impedance spectroscopy and voltageecurrent density curves,
are used to determine which cell component tends to degrade
over time. These are button cell tests where only the cell
performance is evaluated under very low fuel utilization.
Impurities, poisons, or other disturbances are not used;
however, they are parameters of interest for future endeavors.
The seal used is always the same and does not appear to play
any role in any voltage degradation. The cells are obtained
from a commercial source of SOFC systems. The anode
comprises of the state-of-the-art Ni-YSZ cermet, the electrolyte
material is a thin YSZ, a buffer layer made of doped ceria is
used between the electrolyte to prevent unwanted reactions,
and the cathode is made of lanthanum strontium cobalt
ferrite (LSCF). Current collectors are applied on both sides and
a four probe measurement is made. A typical cross section of
the button cell and the test setup are shown in Fig. 1. The
electrolyte layer is approximately 10 mm thick and the ceria
layer is approximately 4 mm. The experimental setup consists
of an alumina tube where the cell is placed with an appro-
priate seal. On the air side, a porous ceramic cap prevents
cross contamination with the air inside the furnace. A ther-
mocouple is placed close to the cell in order to record an
accurate cell temperature. In general, the furnace tempera-
ture is a few degrees cooler when the cell is under load due to
the cell ohmic heating. The cell setup is first heated in air to
seal the cell, then fuel is introduced and the anode is reduced
in situ. On the anode side, humidified hydrogen at room
temperature (3%) is used at a flow rate of 1.0 SLPM. On the
cathode side air, 21% O2 rest helium, or pure oxygen are used
as the oxidant at a flow rate of 1.0 SLPM. A PARSTAT 2273 from
Princeton Applied Research is used to collect the volta-
geecurrent density curves and the impedance data reported
herein. Voltageecurrent density curves are taken at different
temperature and different oxidant conditions. The tempera-
ture is varied from 650 to 800 �C at 50 �C intervals, and the
voltage is swept at 3 mV/s. The impedance measurements are
done at OCV and at different applied voltages; i.e., from OCV
down to around 0.5 V. The AC amplitude used is 10mVwith 12
datum points per each frequency decade. The frequency
range was primarily between 0.01 Hz and around 100,000 Hz;
however, at OCV frequencies in the range of 0.001 Hz are
reached. All collected IS spectra were analyzed using the
ZSimpWin software obtained from AMETEK Princeton
Applied Research.
3. Theoretical analysis
Voltageecurrent density (Vi) curves can be separated into
three regions according to the dominant polarization losses.
At low current densities, the most dominant losses are due to
activation polarization. In the linear portion of the Vi curve,
the ohmic resistance dominates the losses. At high current
densities, diffusion losses become dominant. Inmathematical
terms, the cell voltage of an SOFC can be described as
Vcell ¼ VN � iRohmic � hact;an � hact;ca � hdiff;an � hdiff;ca (1)
where Vcell is the measured cell voltage and VN is the Nernst
voltage (or theoretical voltage) which can be calculated using
the following equation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 7 5077
Author's personal copy
VN ¼ Eþ RT4F
ln PO2þ RT
2Fln
PH2
PH2O(2)
where the thermodynamic voltage (E ) is defined as
E ¼ �DG=nF, and the other symbols take their usual meaning.
For planar cells the ohmic resistance, Rohmic, can be estimated
from the material conductivities and respective thickness of
each layer as
Rohmic ¼X ti
si(3)
The term hact is called the activation overpotential and
represents the charge transfer losses of both electrodes due to
the electrochemical reactions occurring at the three phase
boundary (TPB). The activation overpotential measures the
change from an equilibrium voltage. In general, the
Fig. 1 e Cross section of typical SOFC cells used in this study and button cell testing setup.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 75078
Author's personal copy
ButlereVolmer equation is used to describe the activation
losses in SOFC as
i ¼ io
�exp
�bnFhact
RT
�� exp
�� ð1� bÞnFhact
RT
��(4)
Equation (4) is applied to each electrode by knowledge of the
appropriate kinetic parameters (exchange current density and
transfer coefficient). The diffusion losses, hdiff, requires
knowledge of the porosity and tortuosity of both electrodes as
well as the partial pressures at the three phase boundary. The
equation below is used to estimate the diffusion losses at the
cathode and the anode
hdiff ¼RTnF
ln
�PO2 ;TPB
PO2 ;bulk
�(5)
The diffusion losses, though present, do not play a major
role in the measured Vi curves because the curve bending at
high current density is not observed in any of our studied
cases. Note that both the activation and diffusion losses are
functions of current density. The cell equation can be quite
cumbersome to work with once all terms are included. In
order to simplify the cell equation, a total cell resistance may
be assumed as
VcellxVN � iRtot (6)
The total resistance now includes the ohmic, activation,
and diffusion resistances which are all functions of current
density. Hence, the ASR can be determined by solving Equa-
tion (6) for the total cell resistance and defining it as
ASRhRtot ¼ VN � Vcell
i(7)
Using the above equation, the ASR can be easily estimated
based on experimental voltageecurrent density curves.
However, Equation (7) includes any fuel leakages since it
contains the Nernst voltage which is related to the fuel partial
pressure.Alternatively,wemaydifferentiateEquation (6)andget
dVcell
di¼ �RtothASR (8)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
)V(egatloV
Current Density (A/cm2)
Cell24_750_97% H2 and 3% H
2O in airCell24_750_97% H
2 and 3% H
2O in air
Cell24_750_97% H2 and 3% H
2O, in 21% O
2 and He
No VJ under 100% O2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
)V(egatloV
Current Density (A/cm2)
Cell24_800_97% H2 and 3% H2O in air
Cell24_800_97% H2 and 3% H2O in 21% O2 and He
Cell24_800_97% H2 and 3% H2O in air
Cell24_800_97% H2 and 3% H2O in 21% O2 and He
Cell24_800_97% H2 and 3% H2O in 100% O2
0.0 0.5 1.0 1.5 2.0 2.50.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
)V(egatloV
Current Density (A/cm2)
Cell24_650_97% H2 and 3% H2O in air
Cell24_650_97% H2 and 3% H2O in 21% O2 and He
Cell24_650_97% H2 and 3% H2O in air
Cell24_650_97% H2 and 3% H2O in 21% O2 and He
Cell24_650_97% H2 and 3% H2O in 100% O2
0.0 0.5 1.0 1.5 2.0 2.5 3.00.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Current Density (A/cm2)
)V(egatloV
Cell24_700_97% H2 and 3% H
2O in air
Cell24_700_97% H2 and 3% H
2O in 21% O
2 and He
Cell24_700_97% H2
H and 3% H2O in air
Cell24_700_97% H2
H and 3% H2O in 21% O
2and He
Cell24_700_97% H2 and 3% H
2O in 100% O
2
a
b d
c
Fig. 2 e Voltageecurrent density curves at different temperatures (a) 650 �C (b) 700 �C (c) 750 �C (d) 800 �C and different
cathode gas conditions.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 7 5079
Author's personal copy
Equation (8) is also an ASR definition but without any fuel
leakages contribution to the total cell resistance. Equations
(7) and (8) estimate the same ASR provided that the fuel
leakages are very small. However, researchers have shown
that Equation (8) is more accurate when compared to IS
measurements [8].
In general, the cell ohmic ASR is dominated by the elec-
trolyte resistance since it has the lowest conductivity when
compared to the other cell components. The YSZ ionic
conductivity can be easily calculated using the following
equation [15]
s ¼ so
Texp
�� Eact
RT
�(10)
The electrolyte contribution to the ASR can be estimated
using Equation (3) and the known thickness of the electrolyte
previously noted as 10 mm.Comparing the electrolyteASRwith
the total cell ohmic ASR can provide information about the
quality of tested cells in terms of ohmic losses and interfacial
resistances. Basically, if the measured total cell ohmic ASR is
significantly larger than the electrolyte ASR, then interfacial
resistances are significant. The ceria interlayer has slightly
higher ionic conductivity than YSZ but still contributes to the
cell ohmic ASR. For instance, the reported ionic conductivity
for ceria electrolyte is about 0.1 S/cm at 800 �C [16]. The elec-
trodes have significantly greater conductivities and their
contribution to the cell ohmic ASR can be neglected [15,17,18].
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
-2.2
-2.4 Z_img Cell24_650_OCV 97% H2 and 3% H2O in air Zimg Cell24_650_1V 97% H2 and 3% H2O in air Zimg Cell24_650_0.9V 97% H2 and 3% H2O in air Zimg Cell24_650_0.8V 97% H2 and 3% H2O in air Zimg Cell24_650_0.7V 97% H2 and 3% H2O in air Zimg Cell24_650_0.65V 97% H2 and 3% H2O in air Zimg Cell24_650_0.6V 97% H2 and 3% H2O in air Zimg Cell24_650_0.55V 97% H2 and 3% H2O in air Zimg Cell24_650_0.5V 97% H2 and 3% H2O in air
mcmho(
gmi_
Z2 )
Z_re (ohm cm 2)
0. 2 0.4 0. 6 0.8 1. 0 1.2 1. 4 1.6 1. 8 2.0 2. 2 2.4 2. 6 2.8 3. 0 3.2
0.01 0.1 1 10 100 100 0 1000 0 10000 0
0
5
10
15
20
25
30
35
40
45
50 Phi Cell24_650_OCV 97% H2 and 3% H2O in air Phi Cell24_650_1V 97% H2 and 3% H2O in air Phi Cell24_650_0.9V 97% H2 and 3% H2O in air Phi Cell24_650_0.8V 97% H2 and 3% H2O in air Phi Cell24_650_0.7V 97% H2 and 3% H2O in air Phi Cell24_650_0.65V 97% H2 and 3% H2O in air Phi Cell24_650_0.6V 97% H2 and 3% H2O in air Phi Cell24_650_0.55V 97% H2 and 3% H2O in air Phi Cell24_650_0.5V 97% H2 and 3% H2O in air
).ged(ihP-
Frequency (Hz)
Fig. 3 e Nyquist and Bode representation of the impedance data in air at 650 �C.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 75080
Author's personal copy
4. Results and discussion
Fig. 2 shows the Vi curves measured at different temperatures
and different cathode gas conditions. The open circuit voltage
is very high indicating good sealing at all temperatures. As
expected, the cell performance increases with temperature
and improves when air is switched to an O2eHe mixture and
to pure O2 for all tested temperatures. The curve bending in
the diffusion region is never observed and the cells are not
limited by diffusion losses for the tested conditions. The initial
bending observed in all curves indicates that activation is the
major contributor to the voltage losses in the SOFC cell fol-
lowed by the ohmic losses. However, ohmic resistance plays
a larger role at 650 �Cwhere a larger linear portion in the curve
is observed. Also, activation losses are less pronounced at
higher temperatures where the electrochemical reactions are
occurring at faster rates.
The performance improvement observed between air and
the O2eHe mixture is mostly due to the higher diffusion
coefficient for oxygen in helium than in nitrogen. The reac-
tion rates should remain the same since the same air oxygen
concentration is used for the helium mixture. When pure
oxygen is used, the improvement in performance is due to
removal of diffusion losses and improved reaction rates in
the cathode given a partial pressure of one for oxygen. The
performance improvement seen as temperature is increased
is due to higher ionic conductivities, higher electrochemical
reaction rates, and higher diffusion coefficients. Diffusion
coefficients increase with temperature because of the
0. 2 0.4 0. 6 0.8 1. 0 1.2 1. 4 1.6 1. 8 2.0 2. 2 2.4 2. 6 2.8 3. 00.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
-2.2
-2.4
mcmho(
gmi_
Z2 )
Z_re (ohm cm 2)
0.01 0.1 1 10 100 100 0 1000 0 10000 0
0
5
10
15
20
25
30
35
40
45
50 Phi Cell24_650_OCV 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_1V 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_0.9V 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_0.8V 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_0.7V 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_0.65V 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_0.6V 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_0.55V 97% H2 and 3% H2O in 21% O2 and He Phi Cell24_650_0.5V 97% H2 and 3% H2O in 21% O2 and He
).ged(ihP-
Frequency (Hz)
Fig. 4 e Nyquist and Bode representation of the impedance data in 21% O2, rest Helium at 650 �C.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 7 5081
Author's personal copy
T1.75 dependence [19]. Nonetheless, the cell performance
is excellent given the high performance cathode used and
the thin electrolyte. For instance under pure oxygen and
at 800 �C, the maximum power density measured reached
2 W/cm2.
Figs. 3, 4 and 5 reports the IS data at 650 �C for air,
O2eHe mixture, and pure O2 respectively. The plots repor-
ted here are typical of all tested conditions, and the other
condition plots are not shown to reduce the size of the
manuscript. A visual inspection of the Nyquist plots indi-
cates that at least three semicircles are present for all gas
conditions at 650 and 700 �C. At higher temperatures, 750
and 800 �C, there seem to be at least four semicircles. The
fourth semicircle is probably present at the lower
temperatures as well; however, it is not clearly visible in
the collected IS data. Note that this visual observation can
only be made near or at OCV.
The small circle observed at low frequency (far right)
significantly decreases when transitioning from air to pure
O2. This is an indication that this semicircle is related or
partially related to a cathodic process and represents
a diffusion process. A portion of this semicircle is probably
related to the anodic diffusion process as well but very
small. The largest semicircle observed at higher frequency
(middle) decreases when transitioning from air to pure O2
as well. This indicates a partial relationship to a cathodic
process related to a charge transfer process but without
a diffusion component.
0. 2 0.4 0. 6 0.8 1. 0 1.2 1. 4 1.6 1. 8 2.0 2. 2 2.40.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
-2.2
-2.4 Z_img Cell24_650_OCV 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_1V 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_0.9V 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_0.8V 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_0.7V 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_0.65V 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_0.6V 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_0.55V 97% H2 and 3% H2O in 100% O2 Zimg Cell24_650_0.5V 97% H2 and 3% H2O in 100% O2
mcmho(
gmi_
Z2 )
Z_re (ohm cm 2)
0.01 0.1 1 10 100 100 0 10000 10000 0 100000 0
0
5
10
15
20
25
30
35
40
45
50 Phi Cell24_650_OCV 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_1V 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_0.9V 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_0.8V 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_0.7V 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_0.65V 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_0.6V 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_0.55V 97% H2 and 3% H2O in 100% O2 Phi Cell24_650_0.5V 97% H2 and 3% H2O in 100% O2
).ged(ihP-
Frequency (Hz)
Fig. 5 e Nyquist and Bode representation of the impedance data in 100% O2 at 650 �C.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 75082
Author's personal copy
As expected, the high frequency intercept (far left) indi-
cates a reasonable constant cell ohmic resistance as a func-
tion of applied voltage in all cases. This is true because the cell
ohmic resistance is not a function of applied voltage or current
drawn. The total electrode resistance decreases as a function
of applied voltage as expected. This is due to decreasing
activation losses as current is drawn from the cell and can be
shown using Equation (4) or a simplified version of it [19]. At
voltages less than 0.9 V, only two semicircles can be visually
observed which makes equivalent circuit analysis quite
challenging since more semicircles are observed at OCV.
Finally, all semicircles become smaller as the applied voltage
decreases, and this behavior is in agreement with reported
cell ASR curves and similar studies [8].
Fig. 6 has been added to better illustrate the dependence on
the gas composition. Here, we can see a decrease in diffusion
resistances as we switch from air to the oxygenehelium
mixture and to pure O2. The same behavior is observed at
different voltages; however, the total resistance decreases as
discussed above.
The cell ohmic resistance and total electrode resistance
obtained from the impedance data are reported in Figs. 7e10.
The ohmic resistance decreases as temperature increases
but some scatter is present at different applied voltages for
each cathode gas condition. The observed scatter gets worse
as temperature increases. This behavior can be attributed to
history effects, and the cell may have not reached a true new
equilibrium. In general, the ohmic resistance under air is the
largest and decreases as the cathode gas is replaced with an
O2eHe mixture and pure O2. However, pure O2 reveals
a higher ohmic resistance than the O2eHe mixture. It is
unclear why this behavior is seen, but it is possible that pure
oxygen changes the properties of the cathode or interfaces
causing some increase in interfacial resistances. Another
trend has emerged from the ohmic resistance data except at
650 �C. A maximum for the ohmic resistance is observed at
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.40.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
Z_im
g (o
hm c
m2)
Z_re (ohm cm2)
Cell24_650_1V 97% H2 and 3% H2O in air Cell24_650_1V 97% H2 and 3% H2O in 21% O2 and He Cell24_650_1V 97% H2 and 3% H2O in 100% O2
10
100
1000
100001
0.1
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1.0
-1.1
-1.2
Z_re (ohm cm2)
Z_im
g (o
hm c
m2)
Cell24_650_0.9V 97% H2 and 3% H2O in air Cell24_650_0.9V 97% H2 and 3% H2O in 21% O2 and He
Cell24_650_0.9V 97% H2 and 3% H2O in 100% O2
0.1
1
10100
1000
10000
Fig. 6 e Bode representation of the impedance data at
different gas condition and voltage at 650 �C. Solid points
indicate frequency decades.
Fig. 7 e Measured ohmic resistance and electrode total
polarization at 650 �C as a function of applied voltage.
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around 0.8 V. Again, it is unclear why this behavior is
observed but probably related to some interfacial phenom-
enon. Finally, the total electrode polarization resistances
measured at the low frequency intercepts decreases as the
temperature increases and as the applied voltage decreases
as expected. Also expected is the decrease in total electrode
polarization resistances when transitioning from air to
O2eHe mixture, and to pure O2. This trend remains the same
at all temperatures except at 800 �C where some history
effects may have played a role (see Fig. 10).
To further understand the cell ohmic behavior, the total
cell ohmic ASR was calculated using an average measured
ohmic resistance for each gas condition. The ln (T/ASR) was
then plotted versus the inverse temperature and is shown in
Fig. 11. The observed linear behavior is typical and is the same
for every cathode gas conditions. In order to understand if
interfacial resistances are a significant portion of the
measured cell ASR, the ohmic ASR for the YSZ and ceria using
literature conductivities is also plotted. The resulting activa-
tion energies for all curves are very close to each other and are
in the order of 0.8 eV. The experimental ohmic ASR for the cell
is larger than the calculated YSZ and ceria ohmic ASR. This
indicates that interfaces play a role in the ohmic losses.
Elementsmigration to the interfaces of SOFCs has beenwidely
reported in the literature [3,8]. Contact resistances have also
been reported to cause increased ohmic resistance due to
imperfect interfaces [20,21]. Some of the higher measured cell
ASR is also due to the observed porosity in the ceria layer but
cannot explain the difference observed. In conclusion, the
interface contributes to the measured cell ASR and may also
explain why some of the scatter in the measured cell ohmic
resistance is observed. It is possible than an imperfect inter-
face may react to different testing conditions such as current
and oxygen partial pressures.
In all cases, air, O2eHemixture, and pure O2, the Bode plots
indicate that the high frequency semicircle peak frequency
has a linear dependence on the applied voltage. In other word,
the peak frequency is shifting to higher values as the applied
voltage decreases for all cathode gas conditions. Fig. 12 plots
the large semicircle peak frequencies observed at high
Fig. 8 e Measured ohmic resistance and total electrode
polarization at 700 �C as a function of applied voltage.
Fig. 9 e Measured ohmic resistance and total electrode
polarization at 750 �C as a function of applied voltage.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 0 7 6e5 0 8 75084
Author's personal copy
frequency as a function of cell applied voltage for the data
collected at 650 �C in air. A clear linear dependence is seen and
the peak frequency decreases as the cell voltage increases.
The same trend is observed at all temperatures and different
cathode gas conditions as can be observed from the Bode
plots. However, a few exceptions are observed at higher
temperatures and lower voltages. To the authors’ knowledge
this behavior has not been reported in the literature. This
behavior is related to the charge transfer that occurs in the
high frequency range and indicates that the electrochemical
reactions are occurring at faster rates as more current flows
through the cell.
Finally, the obtained IS data were fitted with an equiv-
alent circuit comprised of LRo(R1Q1)(R2Q2)(R3Q3)(R4Q4);
however, a good fit was not obtained especially at the lower
applied voltages. Efforts are underway to improve the fitting
by choosing different equivalent circuits. Some initial
results are reported in Table 1 for the data collected at OCV
and at different temperatures and cathode gas conditions.
A clear trend is not readily present and further studies are
required, however a few things can be noted. The cell
ohmic resistance (Ro) decreases with temperature but
shows some scatter as gases are changed. The resistance R2
seems to be the major contribution to the impedance. All
resistances, R1, R2, R3, and R4, decrease with temperature in
general but show scatter as the gas conditions are changed.
At this time, it seems that the software is playing a role in
the scatter where some parameters have to be fixed to
minimize the fitting errors.Fig. 10 e Measured ohmic resistance and total electrode
polarization at 800 �C as a function of applied voltage.
Fig. 11 e Average cell ohmic ASR plotted as a function of
inverse temperature.
Fig. 12 e Typical linear dependence of the peak frequencies
of the high frequency semicircle as a function of cell
voltage.
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5. Conclusion
The electrochemical performance of an SOFC containing an
LSCF cathode was studied at different temperatures and
different cathode gas conditions. The impedance data indicate
that at least four semicircles are present of which two are
partially dependent on cathodic processes. At lower applied
cell voltages, only twosemicircles canbevisually observedand
morescatter isnoticeable. Someof thescatter canbeattributed
to history effects where a true new equilibrium may have not
been reached. The cell ohmic resistance reveals a maximum
around 0.8 V, and interfacial resistances play a role because of
the higher measured area specific resistance. The cell ohmic
resistance is found to be greater in pure oxygen than in oxy-
geneheliummixture possibly due to changing interface prop-
erties. The total electrode polarization decreases as the
cathode gas conditions improve to pure oxygen as expected
from a theoretical point of view. Finally, the large semicircle
peak frequencies observed at high frequency shows a clear
linear dependence on the applied cell voltage showing that the
peak frequency decreases as the cell voltage increases.
Acknowledgment
The authors would like to express their gratitude to Delphi
Corporation for their financial support and cells for the
completion of this project. Special thanks are due to Rick Kerr
for his continued support and guidance.
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Table 1 e Initial simulation results of the IS data collected at OCV at different temperatures and cathode gas conditions.
R0 (U cm2) R1 (U cm2) R2 (U cm2) R3 (U cm2) R4 (U cm2)
Cell24_650_OCV 97% H2 and 3% H2O in air 0.3679 0.0283 2.111 0.4523 0.08511
Cell24_650_OCV 97% H2 and 3% H2O in 21% O2 and He 0.3201 0.6591 1.133 0.4277 0.07458
Cell24_650_OCV 97% H2 and 3% H2O in 100% O2 0.3229 0.0993 0.1106 1.229 0.3361
Cell24_700_OCV 97% H2 and 3% H2O in air 0.2034 1.178 0.1565 0.08728 0.1221
Cell24_700_OCV 97% H2 and 3% H2O in 21% O2 and He 0.1792 0.4715 0.4931 0.2767 0.1235
Cell24_700_OCV 97% H2 and 3% H2O in 100% O2 0.2088 0.01054 0.8601 0.1077 0.06153
Cell24_750_OCV 97% H2 and 3% H2O in air 0.1247 0.00324 0.6244 0.2363 0.1494
Cell24_750_OCV 97% H2 and 3% H2O in 21% O2 and He 0.1247 0.0807 0.3114 0.2785 0.1558
Cell24_750_OCV 97% H2 and 3% H2O in 100% O2 0.1408 0.1156 0.3051 0.1588 0.07368
Cell24_800_OCV 97% H2 and 3% H2O in air 0.1017 0.05927 0.2714 0.2826 0.1598
Cell24_800_OCV 97% H2 and 3% H2O in 21% O2 and He 0.1023 0.08165 0.3298 0.2862 0.1703
Cell24_800_OCV 97% H2 and 3% H2O in 100% O2 0.09464 0.05253 0.2691 0.1731 0.1225
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Nomenclature
DG: change in Gibbs energy, J mol�1
E: thermodynamic voltage, VF: Faraday constant, C eq�1
i: current density, A m�2
i0: exchange current density, A m�2
n: electrons transferred per reactionPi: partial pressures, atmP: pressure, atm
R: universal gas constant, J mol�1 K�1
Ro: ohmic resistance, U m2
Rp: polarization resistance, U m2
Rtot: cell resistance, U m2
T: temperature, Kt: thickness, mVcell: cell voltage, VZ: impedance, U m2
Greek Lettersb: transfer coefficienth: overpotential, Vs: conductivity, S m�1Subscriptsact: activationan: anodeca: cathodediff: diffusioni: component or layerimg: imaginary component of impedanceN: Nernstohmic: ohmic resistancere: real component of impedance
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