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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.

r e f e r e n c e s

[1] Singhal SC. Solid oxide fuel cells for stationary, mobile, andmilitary applications. Solid State Ionics 2002;152e153:405e10.

[2] Dokiya M. SOFC system and technology. Solid State Ionics2002;152e153:383e92.

[3] Simner SP, Anderson MD, Engelhard MH, Stevenson JW.Degradation mechanisms of LaeSreCoeFeeO3 SOFCcathodes. Electrochem Solid-State Lett 2006;9:A478e81.

[4] Mai A, Becker M, Assenmacher W, Tietz F, Hathiramani D,Ivers-Tiffee E, et al. Time-dependent performance of mixed-

conducting SOFC cathodes. Solid State Ionics 2006;177:1965e8.

[5] Kim JY, Sprenkle VL, Canfield NL, Meinhardt KD, Chick LA.Effects of chrome contamination on the performance ofLa0.6Sr0.4Co0.2Fe0.8O3 cathode used in solid oxide fuel cells.J Electrochem Soc 2006;153:A880e6.

[6] Mai A, Haanappel VAC, Uhlenbruck S, Tietz F, Stover D.Ferrite-based perovskites as cathode materials for anode-supported solid oxide fuel cells. Part I. Variation ofcomposition. Solid State Ionics 2005;176:1341e50.

[7] Adler SB. Reference electrode placement in thin solidelectrolytes. J Electrochem Soc 2002;149:E166e72.

[8] Zhou X-D, Pederson LR, Templeton JW, Stevenson JW.Electrochemical performance and stability of the cathodefor solid oxide fuel cells. J Electrochem Soc 2010;157:B220e7.

[9] Dunyushkina LA, Lu Y, Adler SB. Microelectrode array forisolation of electrode polarization on planar solidelectrolytes. J Electrochem Soc 2005;152:A1668e76.

[10] Jensen SH, Hauch A, Hendriksen PV, Mogensen M,Bonanos N, Jacobsen T. A method to separate processcontributions in impedance spectra by variation of testconditions. J Electrochem Soc 2007;154:B1325e30.

[11] Barfod R, Mogensen M, Klemensø T, Hagen A, Liu Y-L,Hendriksen PV. Detailed characterization of anode-supported SOFCs by impedance spectroscopy. J ElectrochemSoc 2007;154:B371e8.

[12] Kungas R, Kivi I, Lust E. Effect of cell geometry on theelectrochemical parameters of solid oxide fuel cell cathodes.J Electrochem Soc 2009;156:B345e52.

[13] Sonn V, Leonide A, Ivers-Tiffee E. Combined deconvolutionandCNLSfitting approachapplied on the impedance responseof technical NiO8YSZ cermet electrodes. J Electrochem Soc2008;155:B675e9.

[14] Leonide A, Ruger B, Weber A, Meulenberg WA, Ivers-Tiffee E.Impedance study of alternative (La, Sr)FeO3�d and (La, Sr) (Co,Fe)O3�d MIEC cathode compositions. J Electrochem Soc 2010;157:B234e9.

[15] Zhu H, Kee RJ, Janardhanan VM, Deutschmann O,Goodwin DG. Modeling elementary heterogeneous chemistryand electrochemistry in solid-oxide fuel cells. J ElectrochemSoc 2005;152:A2427e40.

[16] Kharton VV, Marques FMB, Atkinson A. Transport propertiesof solid oxide electrolyte ceramics: a brief review. Solid StateIonics 2004;174:135e49.

[17] Gazzarri JI, Kesler O. Non-destructive delamination detectionin solid oxide fuel cells. J of Power Sourc 2007;167:430e41.

[18] Swierczek K, Gozu M. Structural and electrical properties ofselected La1-xSrxCo0.2Fe0.8O3 and La0.6Sr0.4Co0.2Fe0.6Ni0.2O3

perovskite type oxide. J Power Sourc 2007;173:695e9.

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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[19] Chan SH, Khor KA, Xia ZT. A complete polarization model ofa solid oxide fuel cell and its sensitivity to the change of cellcomponent thickness. J Power Sourc 2001;93:130e40.

[20] Wanzenberg E, Tietz F, Kek D, Panjan P, Stover D. Influenceof electrode contacts on conductivity measurements of thinYSZ electrolyte films and the impact on solid oxide fuel cells.Solid State Ionics 2003;164:121e9.

[21] Kundracik F, Hartmanova M, Mullerova J, Jergel M, Kostic I,Tucoulou R. Ohmic resistance of thin yttria stabilizedzirconia film and electrodeeelectrolyte contact area. MaterSci Eng B 2001;84:167e75.

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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