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Carbonates formed during BSCF preparation and their effectson performance of SOFCs with BSCF cathode

Zhe Zhao a,b,c, Li Liu a,b,c, Xiaomin Zhang a,b,c, Baofeng Tu a,b, Dingrong Ou a,b,Mojie Cheng a,b,*aDivision of Fuel Cells, Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, PR ChinabDalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR ChinacUniversity of Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:

Received 11 August 2012

Received in revised form

20 September 2012

Accepted 23 September 2012

Available online 24 October 2012

Keywords:

Carbonate

pH

Citrate/metal ions ratio

Ba0.5Sr0.5Co0.8Fe0.2O3�d cathode

* Corresponding author. Division of Fuel CelChina. Tel./fax: þ86 411 84379049.

E-mail address: mjcheng@dicp.ac.cn (M.0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.09.1

a b s t r a c t

Series of Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF) samples have been prepared by modified citrate-

nitrate combustion method from the precursor solutions with different pH values and

citrate/metal ion ratios. The XRD results reveal that BSCF oxide free of impurity phases can

be obtained from a precursor solution with a suitable pH value and a suitable C/M value,

whereas CO2-TPD profiles show that there are minor carbonates species present in all BSCF

samples, but the amount of these carbonates varies with the pH and C/M values of

precursor solutions. The current densityevoltage characteristics indicate that carbonates

in the BSCF samples reduce the cell performance. The electrochemical impedance spectra

(EIS) show that carbonates in BSCF lead to increases in ohmic and polarization resistances.

High performance is achieved on the cell with a cathode using a pure BSCF calcined under

O2 flow at 900 �C.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction a BSCF cathode when CO2 is present in oxygen flow [7e9].

The perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF)

exhibits good electrochemical performance as cathode for

intermediate- and low-temperature solid oxide fuel cells

(SOFCs), mainly due to high ionic and electronic conductivity

as well as excellent catalytic activity [1,2]. Recently, BSCF is

also considered as a promising candidate for the anode of

solid oxide electrolysis cells (SOECs) in which it displays good

performance under anodic polarization condition [3e6].

However, the presence of double alkaline-earth metal ions of

Ba2þ and Sr2þ makes BSCF susceptible to CO2. It has been

found that the adsorption of CO2 and formation of surface

carbonate species on BSCF reduce greatly the performance of

ls, Dalian National Labo

Cheng).2012, Hydrogen Energy P42

Surface carbonates, which are formed when BSCF is exposed

to CO2-containing atmosphere for a long time, degrade

significantly surface oxygen exchange coefficient [10,11].

Therefore, the elimination of carbonates fromBSCF samples is

important for achieving or maintaining high electrochemical

performance.

Perovskite oxides are generally prepared through solid

state reaction of metal oxides or salts and sol-gel combustion

synthesis. The synthesis methods and process parameters

play an important role in the microstructure, composition,

chemical and physical properties of material [12e26]. Sol-gel

combustion process is a simple and popular method, in

which the reactants can be uniformly mixed at a molecular

ratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, PR

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 7 ( 2 0 1 2 ) 1 9 0 3 6e1 9 0 4 4 19037

level by using a large amount of complexing agent [14e24].

The structure, composition and properties of obtained mate-

rials are influenced mainly by the homogeneity and burning

behavior of precursors as well as organic residues [17,21e23].

For instance, the ratio of citrate tometal ion (C/M) in precursor

solution influences the morphology and composition of

synthesized sample. A low C/M ratio leads to inhomogeneous

distribution in particle size while a high C/M ratio results in

the formation of isolated carbonates during combustion,

which increases the formation temperature of pure perovskite

phase [17,22]. Besides, the pH value of precursor solution also

affects the calcination temperature and time to eliminate

carbonate phases [17,24,25]. It takes 5 h to obtain SrFeOw2.85

oxide free of carbonates phase for the sample from

a precursor solution with pH < 5, but the treatment of 100 h is

required in the case of a precursor solution with pH ¼ 8 [24].

Sol-gel combustion synthesis is now adopted for the

preparation of BSCF materials. As a high catalytic active

material for SOFC and SOEC, it is important to make clear

whether there are carbonates residues in the BSCF samples

and what effects of the carbonates are on cell performance. In

this paper, series of Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCFs) are

prepared bymodified citrate-nitrate combustionmethod from

precursor solutions with various pH values and various C/M

ratios. In order to find out carbonate residues, the BSCF

samples are characterized by CO2-TPD, XRD and SEM tech-

niques. Further, the electrochemical performances of the

BSCF samples with surface carbonate residues from prepara-

tion and without carbonates are evaluated on SOFCs.

2. Experimental

2.1. Synthesis of samples

Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF) oxides were prepared by modi-

fied citrate-nitrate combustion method. Metal nitrates

Ba(NO3)2, Sr(NO3)2, Co(NO3)2$6H2O and Fe(NO3)3$9H2O were

used as the starting materials. All metal nitrates at the

nominal ratio for the desired product were added into deion-

ized water under stirring until a transparent and homoge-

neous solution was obtained. Citric acid ammoniumwas then

added into the solution as complexing agent. The first series of

BSCF samples were prepared from the precursor solutions

with a constant C/M ratio of 1.5 and different pH values of 1.5,

2, 2.5, 3, 3.5 and 4. The second series of BSCF samples were

prepared from the precursor solutions with a constant pH

value of 2.5 and different C/M values of 1, 2 and 3. The solution

was evaporated at 90 �C until the transparent sol was formed.

Glycol was added to the sol as dispersant before combustion.

Spontaneous combustion occurred when the sol was heated

to a certain extent, and then primary powders were obtained.

Some primary powders were grinded and subsequently

calcined at 950 �C for 5 h in a muffle furnace under static air.

The others were directly calcined in O2 flow of 50 ml min�1 at

900 �C for 2 h. The sample from the precursor solution with

a pH value of A and a C/M value of B, calcined at temperature

T1 under static air in a muffle furnace, was denoted as AIRT1-

pHA-CMB. The sample, directly calcined at temperature T2

under O2 flow, was denoted as O2T2-pHA-CMB. The sample

AIRT1-pHA-CMB, treated at temperature T3 under O2 flow,was

denoted as O2T3-AIRT1-pHA-CMB.

2.2. Characterization of samples

The XRD patterns of the calcined BSCF samples were

measured on a Rigaku D/max-2500PC X-ray diffractometer at

40 kV and 200 mA using Cu Ka radiation. The range of 2q scan

was from 20 to 80�. The SEM photographs of the samples were

taken on a FEI QUANTA 200F microscope at an accelerating

voltage of 20 kV.

Carbon dioxide temperature programmed desorption (CO2-

TPD) profiles of the BSCF samples were measured on the

flowing reaction system. Sample of 120 mg in 80e120 mesh

was heated from room temperature to 950 oC with a heating

rate of 10 �C min�1 using pure He (99.999%) as carrier gas. The

desorbed CO2 was detected by a mass spectrometer (Pfeiffer

Vacuum, Ominstar GSD 301 O2) with scanning m/z ¼ 44

channel.

2.3. Electrochemical test

Anode-supported single cell with thin GDC electrolyte (30 mm)

was prepared by a dual dry-pressing method. Cathode slurry,

consisting of BSCF powders, organic binders and solvent, was

applied onto the electrolyte film by slurry-coating, and sin-

tered at 950 �C for 2 h. The current density-voltage charac-

teristics were measured in the temperatures 500e600 �C after

the in situ reduction of the NiO-GDC anode at 700 �C.Humidified H2 (3% H2O) and O2 were supplied as fuel and

oxidant at a flow rate of 100 ml min�1, respectively. Electro-

chemical impedance spectra (EIS) were typically measured

under open circuit conditions using a Solartron 1287 poten-

tiostat and a 1260 frequency response analyzer. The frequency

range was from 0.1 Hz to 100 kHz with signal amplitude of

10 mV.

3. Results and discussion

3.1. XRD

Fig. 1 shows the XRD pattern of the primary BSCF powder after

combustion from a precursor solutionwith a pH of 2.5 and a C/

M ratio of 3. Besides the characteristic peaks of perovskite

BSCF, the characteristic peaks of witherite BaCO3 and spinel

CoFe2O4 can be found in the XRD pattern of the primary

powder. Pure BSCF phase can be developed from a further

structural evolution by heating these oxides and carbonates at

high temperatures [17,27,28]. The homogeneity of primary

powders after combustion plays an important role in the

formation of pure perovskite oxides. The inhomogeneity of

precursor makes the perovskite structure evolution difficult

and increases the formation temperature of a pure perovskite

phase [21e23,27,29]. Fig. 2 shows the XRD patterns of the BSCF

samples calcined under static air in a muffle furnace from the

precursor solutions with different pH values and C/M ratios.

Strong diffraction peaks for the typical perovskite BSCF are

shown in the XRD patterns of all samples. But some additional

small peaks can also be seen on some samples, indicating the

20 30 40 50 60 70 802-Theta (o)

Inte

nsity

( a.

u)

BSCFBaCO3CoFe2O4

Fig. 1 e XRD pattern of the primary BSCF powder after

combustion from a precursor solution with pH [ 2.5 and

C/M [ 3.

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 7 ( 2 0 1 2 ) 1 9 0 3 6e1 9 0 4 419038

presence of minor additional phases in BSCF. For the samples

AIR950-pH3.5-CM1.5 and AIR950-pH4-CM1.5, the small addi-

tional peaks are identified as BaCO3 phase. These BaCO3 can

arise from the reaction of alkaline-earth oxides with CO2 from

the combustion of organic residues in primary BSCF powders

in a muffle furnace. For the samples AIR950-pH1.5-CM1.5 and

AIR950-pH2-CM1.5, the small additional peak is identified as

Ba2Fe2O5/Ba2Co2O5 phase. During the synthesis, some white

precipitations are observed in the precursor solutions with

low pH of 1.5 and 2 during heating solution, but this obser-

vation is not found in precursor solutions with higher pH

values. The white precipitations are identified as Ba(NO3)2 by

XRDmeasurements. The occurrence of precipitations could be

due to the low solubility of Ba(NO3)2 on one hand. Another

reason is the weak complexion of citric with Ba2þ ions at low

20 30 40 50 60 70 80

Inte

nsity

( a.

u)

2-Theta o

x

BaCO3c

cc

x

p BSCFx Ba2Co2O5/ Ba2Fe2O5

p pp

p

p

p

p

p

ppH4-C/M1.5

pH3-C/M1.5

pH3.5-C/M1.5

pH2.5-C/M1.5

pH2-C/M1.5

pH1.5-C/M1.5

pH2.5-C/M3

pH2.5-C/M2

pH2.5-C/M1

Fig. 2 e XRD patterns of the BSCF samples from the

precursor solutions with different pH values and C/M ratios

and calcined under static air in a muffle furnace at 950 �Cfor 5 h.

pH of 1.5 and 2. Thereby, the whole precursor solutions

become inhomogeneous due to the formation of precipita-

tions. The further solid reaction between Ba(NO3)2 and other

oxides leads to the formation of Ba2Fe2O5/Ba2Co2O5. As

a result of Ba2Fe2O5/Ba2Co2O5 phase segregation, the produced

BSCF can have non-stoichiometric composition. The defi-

ciency of metal ions such as Ba2þ with large radius can lead to

a lattice expansion due to an increase in average AeO bond

distance.

Fig. 3 shows the XRD patterns of the BSCF samples calcined

under O2 flow from the precursor solutions with different pH

values and C/M ratios. For the samples O2900-pH1.5-CM1.5

and O2900-pH2-CM1.5, the small additional peak for Ba2Fe2O5/

Ba2Co2O5 is still present in the XRD patterns. For the other

samples, all diffraction peaks are well indexed as the perov-

skite structure. The results show that BaCO3 phase in samples

AIR950-pH3.5-CM1.5 and AIR950-pH4-CM1.5 is removed by

heating in O2 flow. The removal of CO2 from the combustion of

carbon residues and the decomposition of carbonates by

flowing O2 during heat treatment are responsible for the

formation of pure BSCF.

3.2. SEM

Fig. 4 shows the SEM micrographs of O2900-pH3-CM1.5,

AIR950-pH3-CM1.5 and AIR950-pH2.5-CM1.5. All samples

show porous structure. The size of primary BSCF particle is

about 1e2 mm. The surfaces of O2900-pH3-CM1.5 particles are

smooth and clean, whereas the surfaces of AIR950-pH3-CM1.5

and AIR950-pH2.5-CM1.5 particles are covered by lots of small

particles sized in 20e150 nm. Similar SEM micrographs have

been observed on the BSCF crystallites treated in a gasmixture

of 1% CO2/O2 at 450 �C for 24 h, and these small particles are

ascribed to the mixed Sr and Ba enriched carbonates [7]. The

mixed Sr and Ba enriched carbonates appear on the BSCF

membrane when CO2 was used as sweep gas [11]. So, it is

reasonable to assign these small particles on BSCF surface to

carbonates.

20 30 40 50 60 70 80

Inte

nsity

( a.

u)

2-Theta o

xx

p BSCFx Ba2Co2O5/ Ba2Fe2O5

p pp

p

p

p

p

p

ppH4-C/M1.5

pH3-C/M1.5

pH3.5-C/M1.5

pH2.5-C/M1.5

pH2-C/M1.5

pH1.5-C/M1.5

pH2.5-C/M3

pH2.5-C/M2

pH2.5-C/M1

Fig. 3 e XRD patterns of the BSCF samples from the

precursor solutions with different pH values and C/M and

directly calcined under O2 flow at 900 �C for 2 h.

Fig. 4 e SEM micrographs of the samples (a) O2900-pH3-CM1.5, (b) AIR950-pH3-CM1.5 and (c) AIR950-pH2.5-CM1.5.

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 7 ( 2 0 1 2 ) 1 9 0 3 6e1 9 0 4 4 19039

3.3. CO2-TPD

If carbonates are in a small quantity, it is very difficult to

detect them just by XRD technique due to the limited

sensitivity. The minor carbonates can be investigated using

CO2-TPD technique with mass spectrometer as detector

[25,30]. Fig. 5 shows the CO2-TPD profiles of the BSCF

calcined in a muffle furnace under static air. The area of CO2

desorption peak indicates the amount of carbonates while

the position and shape of CO2 desorption peak is related to

the decomposition of different carbonates. Three main CO2

desorption peaks are observed on all samples, which

correspond to the three main desorption bands, including

a low-temperature band (LT-CO2) in the temperatures

100e520 �C, an intermediate temperature band (MT-CO2) in

the temperatures 520e640 �C and a high temperature band

(HT-CO2) in the temperatures 640e950 �C. The presence of

different desorption bands indicates that different carbonate

species are present in produced BSCF samples. The area of

CO2 desorption bands varies with pH values and C/M ratios

of precursor solutions. The sample prepared from

a precursor solution with a pH of 2.5 and a C/M ratio of 1.5

contains smallest the amount of carbonates according to the

TPD results.

200 400 600 800

Inte

nsity

(a.u

)

Temperature (oC)

pH1.5-C/M1.5

pH2-C/M1.5

pH2.5-C/M1.5

pH3-C/M1.5

pH3.5-C/M1.5

pH4-C/M1.5

a

b

2

200 400 600 800

Inte

nsity

(a.u

)

Temperature (oC)

pH2.5-C/M3

pH2.5-C/M2

pH2.5-C/M1.5

pH2.5-C/M1

2

Fig. 5 e CO2-TPD profiles of the BSCF samples calcined

under static air in a muffle furnace at 950 �C for 5 h from

the precursor solutions with (a) different pH values and (b)

different C/M ratios.

200 400 600 800

(a)

(b)

(c)

Inte

nsity

(a.u

)

Temperature (oC)

2

Fig. 6 e CO2-TPD profiles of the samples (a) AIR950-pH2.5-

CM2, (b) referenced sample 5 wt% SrCO3/AIR950-pH2.5-

CM2 calcined under static air in a muffle furnace eat 950 �Cfor 5 h and (c) referenced sample 5 wt% BaCO3/ AIR950-

pH2.5-CM2 calcined under static air in a muffle furnace at

950 �C for 5 h.

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 7 ( 2 0 1 2 ) 1 9 0 3 6e1 9 0 4 419040

The thermal decomposition temperature of barium and

strontium carbonates in air is about 808 �C [31]. The HT-CO2

(640e950 �C) band can be from the decomposition of barium

and strontium carbonates. In order to confirm the conclusion,

the sample AIR950-pH2.5-CM2, exhibiting a very small HT-CO2

band, was respectively mixed with 5wt% SrCO3 and 5wt%

BaCO3 as referenced sample. Then the composite materials of

5wt% SrCO3/AIR950-pH2.5-CM2 and 5wt% BaCO3/ AIR950-

pH2.5-CM2 were grinded and calcined at 950 �C for 5 h in

a muffle furnace. The CO2-TPD profiles of referenced samples

are shown in Fig. 6. Referenced sample shows an obvious

increase in HT-CO2 band, suggesting that the HT-CO2 band

arises from the decomposition of barium and strontium

carbonates. The area of HT-CO2 band mainly varies with the

pH of precursor solution, which firstly decreases with pH

value in the low pH range, and then increases when pH value

exceeds 3. Since XRD results have confirmed the presence of

BaCO3 in the samples from the precursor solutions with pH of

3.5 and 4, the samples AIR950-pH3.5-CM1.5 and AIR950-pH4-

CM1.5 show large HT-CO2 bands as compared with other

samples. In the samples AIR950-pH1.5-CM1.5 and AIR950-

pH2-CM1.5, additional phases Ba2Fe2O5/Ba2Co2O5 with many

oxygen vacancies are present, and easily react with released

CO2 during calcination in a muffle furnace and result in the

formation of BaCO3. So the samples AIR950-pH1.5-CM1.5 and

AIR950-pH2-CM1.5 show large HT-CO2 bands. The MT-CO2

(520e640 �C) band can be related to the decomposition of

barium and strontium carbonates highly dispersed on the

crystal surfaces of BSCF. Shrinking Core Model (SCM) is

proposed to describe thermal decomposition of barium and

strontium carbonate, and the decomposition reaction firstly

occurs at the outer layer of the particle, and then moves into

the bulk. Thereby, particle size can influence the decomposi-

tion process. Compared with a sample with larger particle

size, a sample with smaller particle size needs lower

temperature and shorter time to get fully decomposed due to

high efficient heat transfer [32e34]. Decomposition of small

surface SrCO3 crystallites on perovskite oxide at about 600 �Cwas also reported [34,35]. The amount of carbonates corre-

sponding to the MT-CO2 band is relative low for all samples,

suggesting that most of the formed barium and strontium

carbonates can be in the form of isolated carbonates, and only

minor carbonates are dispersed on the crystal surfaces of

BSCF. The LT-CO2 (100e520 �C) band is ascribed to the

decomposition of monodentate carbonates and/or bidentate

carbonates with low thermal stability [36,37]. The area of this

band is almost unchanged with pH and C/M. These chem-

isorbed carbonate species are formed from the sorption of CO2

on the samples after high temperature calcination. In sum,

the presence of minor carbonates on the BSCF samples

depends on pH and C/M.

Fig. 7 shows the CO2-TPD profiles of the BSCF samples

directly calcined under O2 flow. The MT-CO2 and HT-CO2

bands are eliminated by heating in O2 flow. Only the small LT-

Inte

nsity

(a.u

)

Temperature (oC)

800oC 850oC 900oC

200 400 600 800 Isotherm

1

Fig. 8 e CO2-TPD profiles of the BSCF sample AIR950-pH3-

CM1.5 after treatment in O2 flow at 800, 850 and 900 �C for

1 h.

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 7 ( 2 0 1 2 ) 1 9 0 3 6e1 9 0 4 4 19041

CO2 band still remains. The area of LT-CO2 band for the BSCF

sample calcined under O2 flow is smaller than that for the

BSCF sample calcined in static air. The presence of the small

LT-CO2 peak is ascribed to the re-adsorption of trace amount

of CO2 during cooling process.

The decomposition of carbonates depends on temperature.

In order to know the effect of calcination temperature on the

presence of carbonates, the sample AIR950-pH3-CM1.5 was

further treated in O2 flow at 800, 850 and 900 �C for 1 h,

respectively. The corresponding CO2-TPD profiles are shown

in Fig. 8. The area of CO2 desorption bands decreases with

increasing treatment temperature, suggesting that the

amount of carbonates in BSCF gradually decreases. The

presence of HT-CO2 band in O2800-AIR950-pH3-CM1.5 indi-

cates that treatment temperature of 800 �C is not high enough

to completely eliminate carbonates in BSCF. The results also

reveal that carbonates in a BSCF cathode still remain in the

intermediate- and low-operating temperatures even O2 flow is

used as oxidant. The absence of CO2 desorption band in O2900-

AIR950-pH3-CM1.5 shows that carbonates can be eliminated

completely in O2 flow at 900 �C. Afterwards, the sample O2900-

200 400 600 800

Inte

nsity

(a.u

)

Temperature (oC)

pH2.5-C/M2

pH2.5-C/M3

pH2.5-C/M1.5

pH2.5-C/M1

2

200 400 600 800

b

a

Inte

nsity

(a.u

)

Temperature (oC)

pH4-C/M1.5

pH3.5-C/M1.5

pH3-C/M1.5

pH2.5-C/M1.5

pH2-C/M1.5

pH1.5-C/M1.5

2

Fig. 7 e CO2-TPD profiles of the BSCF powders directly

calcined under O2 flow at 900 �C for 2 h from the precursor

solutions with (a) different C/M ratios and (b) different

pH values.

AIR950-pH3-CM1.5 is also added into aqueous solution and

heatedwith stirring. The pH of the solution does not obviously

change, indicating that the sample does not contain alkaline-

earth oxides. Carbonates have decomposed completely and

converted into perovskite structured BSCF during the treat-

ment in O2 flow at 900 �C.

3.4. Electrochemical performance

Fig. 9 shows the voltage and power density versus current

density curves for the cells with AIR950-pH3-CM1.5 and

O2900-AIR950-pH3-CM1.5 cathodes in the temperatures

500e600 �C. The power densities at 0.7 V for the cell with

AIR950-pH3-CM1.5 cathode are 1.08, 0.80 and 0.50 W cm�2 at

600, 550 and 500 �C, respectively, the corresponding values are

1.17, 0.90 and 0.59W cm�2 for the cell with O2900-AIR950-pH3-

CM1.5 cathode, increasing by 8.33, 12.50 and 18.00%, respec-

tively. Fig. 10 illustrates the electrochemical impedance

spectra (EIS) of the cells with AIR950-pH3-CM1.5 and O2900-

AIR950-pH3-CM1.5 cathodes. In the EIS plots, the intercept

with the real axis at high frequencies represents the ohmic

resistance of the cell while the intercept with the real axis at

low frequencies represents the total resistance of the cell.

Compared with the cell with O2900-AIR950-pH3-CM1.5

cathode, the cell with AIR950-pH3-CM1.5 cathode exhibits

larger total resistances and ohmic resistances. The total

resistances for the cell with O2900-AIR950-pH3-CM1.5 cathode

are 0.13, 0.19 and 0.33 U cm2 at 600, 550 and 500 �C, respec-tively, the corresponding values reach to 0.14, 0.23 and

0.42 U cm2 for the cell with AIR950-pH3-CM1.5 cathode,

increasing by 7.69, 20.05 and 27.27%. The ohmic resistances

for the cell with O2900-AIR950-pH3-CM1.5 cathode are 0.12,

0.16 and 0.23 U cm2 at 600, 550 and 500 �C, respectively; thecorresponding values are 0.14, 0.18 and 0.26 U cm2 for the cell

with AIR950-pH3-CM1.5 cathode. The ohmic resistance

usually includes the contributions of electrolyte, electrodes,

the connection wires and interfacial contact of cathode and

electrolyte. Since the same anode and electrolyte are used for

0.1 0.2 0.3 0.4

0.00

0.05

0.10

0.15

0.20 500 oC 550 oC 600 oC

-ImZ

(Ω c

m2 )

Re Z (Ω cm2)

a

b

0.1 0.2 0.3 0.4

0.00

0.05

0.10

0.15

0.20

0.25 500 oC 550 oC 600 oC

500oC550oC

-ImZ

(Ω c

m2 )

Re Z (Ω cm2)

600oC

Fig. 10 e EIS plots of the cells with cathodes of (a) AIR950-

pH3-CM1.5 and (b) O2900-AIR950-pH3-CM1.5 at 500, 550

and 600 �C.

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

Volta

ge (V

)

Current density A cm-2

600oC 550oC 500oC

a

b

0.0

0.4

0.8

1.2

1.6

Power density

W cm

-2

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0 600oC 550oC 500oC

Current density A cm-2

Volta

ge (V

)

0.0

0.4

0.8

1.2

1.6

Power density

W cm

-2

Fig. 9 e Voltage and power density versus current density

curves for the cells with cathodes of (a) AIR950-pH3-CM1.5

and (b) O2900-AIR950-pH3-CM1.5 at 500, 550 and 600 �C.

1 10 100 1000 10000 100000-0.02

0.00

0.02

0.04

0.06

0.08

f (Hz)

ΔZ' (

f)

500 oC 550 oC 600 oC

Fig. 11 e Analysis of difference in impedance spectra (ADIS)

plots of the cellswith AIR950-pH3-CM1.5 and O2900-AIR950-

pH3-CM1.5 cathode in the temperatures 550e800 �C.

DZ0ðfÞ[vZ0ðfÞvlnðfÞ

�����O2900�AIR950�pH3�CM1:5

LvZ0ðfÞvlnðfÞ

�����AIR950�pH3�CM1:5

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 7 ( 2 0 1 2 ) 1 9 0 3 6e1 9 0 4 419042

both cells, and the cathodes are sintered at the same

temperature of 950 �C. The differences in ohmic resistances

are related to the presence of carbonates in the BSCF cathode.

Carbonates with low conductivity on the surfaces of BSCF

particles can lead to the worse interfacial contact between

cathode and electrolyte and the worse contact between the

BSCF particles and block the transfer of electron. On the other

hand, the polarization resistance (RP) is adjusted according to

the equation below because the electronic conduction in GDC

electrolyte is not negligible under operation conditions [38]

RP ¼ RT � Rb

VOC

EN

�1� Rb

RT

�1� VOC

EN

��

where RT is the total resistance of the cell. Rb is the ohmic

resistance of the cell. VOC is the open circuit voltage. EN is the

theoretical Nernst potential across the cell. The estimated

polarization resistances for the cell with O2900-AIR950-pH3-

CM1.5 cathode are 0.01, 0.04 and 0.12 U cm2 at 600, 550 and

500 �C, and the corresponding values are 0.02, 0.07 and

0.19 U cm2 at 600, 550 and 500 �C for the cell with AIR950-pH3-

CM1.5 cathode. Additionally, the impedance spectrum of the

cell especially at low temperature shows a pressed arc

including at least two arcs, suggesting that there are at least

two rate limiting steps involved. Because internal shorting is

present on the cell with GDC electrolyte and influences greatly

cathodic and anodic polarization, and the extent of internal

shorting varies with electrodes and operating conditions,

therefore, it is difficult to propose a fit equivalent circuit that is

applicable to all data at different temperatures to analyze

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 7 ( 2 0 1 2 ) 1 9 0 3 6e1 9 0 4 4 19043

oxygen reduction on BSCF cathode. Herein, the analysis of

difference in impedance spectra (ADIS) plot is used to identify

where the compared EIS spectra of cells with different cath-

odes deviate. A peak in ADIS plots represents a change in

reaction process. Fig. 11 shows ADIS plots of the cells with

AIR950-pH3-CM1.5 and O2900-AIR950-pH3-CM1.5 cathodes.

Only one peak appears in ADIS plot and centers at 26.86, 51.68

and 162.41 Hz at 500, 550 and 600 �C, respectively. The results

show that carbonates in BSCF increase mainly the low

frequency arc, which corresponds to oxygen adsorption and

dissociation process [8,39,40]. Carbonates in the BSCF surface

impede oxygen surface reaction on BSCF cathode, which

accounts for the larger polarization resistances of the cell with

AIR950-pH3-CM1.5 cathode than that of the cell with O2900-

AIR950-pH3-CM1.5 cathode.

4. Conclusions

Series of BSCF samples were synthesized by modified citrate-

nitrate combustion method. The XRD results show that

perovskite oxide free of impurity phases can be obtained

from a precursor solution with a suitable pH value and

a suitable C/M value. However, the CO2-TPD results reveal that

minor carbonates are still present in all samples calcined

under static air in a muffle furnace, and the amount of

carbonates species varies with pH value and C/M value. The

carbonates cover on the surfaces of BSCF particles and lead to

increases in both polarization and ohmic resistances. Higher

performance can be achieved on the cell with a cathode using

a pure perovskite BSCF calcined under O2 flow at 900 �C.

Acknowledgements

The authors gratefully acknowledge financial supports from

the Ministry of Science and Technology of China (No.

2010CB732302 and 2011AA050704), National Natural Science

Foundation of China (No. 21076209, 20876156 and 20803073).

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