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Potentiality of cobalt-free perovskiteBa0.5Sr0.5Fe0.9Mo0.1O3Ld as a single-phase cathodefor intermediate-to-low-temperature solid oxidefuel cells

Yihan Ling a,b, Xiaozhen Zhang c, Zhenbin Wang a, Songlin Wang d,Ling Zhao e,*, Xingqin Liu a, Bin Lin f,*a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering,

University of Science and Technology of China (USTC), Hefei, Anhui 230026, PR Chinab Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japanc Key Laboratory of Jiangxi Universities for Inorganic Membranes, School of Material Science and Engineering,

Jingdezhen Ceramic Institute, Jingdezhen 333001, PR Chinad Department of Mechanical Engineering, Tongling University (TLU), Tongling, Anhui 244061, PR Chinae Department of Materials, China University of Geoscience, Wuhan 430074, PR Chinaf Anhui Key Laboratory of Low Temperature Co-fired Materials, Department of Chemistry, Huainan Normal

University, Huainan, Anhui 232001, PR China

a r t i c l e i n f o

Article history:

Received 10 July 2013

Received in revised form

20 August 2013

Accepted 20 August 2013

Available online 19 September 2013

Keywords:

Single-phase cathode

Intermediate-to-low-temperature

solid oxide fuel cells

Chemically compatible

Polarization resistance

* Corresponding authors.E-mail address: lyhyy@mail.ustc.edu.cn (

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.08.0

a b s t r a c t

Cobalt-free perovskite Ba0.5Sr0.5Fe0.9Mo0.1O3�d (BSFMo) was investigated as a single-phase

cathode for intermediate-to-low-temperature solid oxide fuel cells (IL-SOFCs). The X-ray

diffraction (XRD) Rietveld refinement, electrical conductivity, thermogravimetric (TG)

measurements, the phase reaction were investigated. The doping of high-valence Mo

cations into Fe-site obviously enhanced the electrical conductivity of BSFMo sample with

the maximum value of 174 S cm�1. XRD results showed that BSFMo cathode was chemi-

cally compatible with the BaZr0.1Ce0.7Y0.1Yb0.1O3�d (BZCYYb) electrolyte for temperatures

up to 1000 �C. Laboratory-sized tri-layer cells of NiO-BZCYYb/BZCYYb/BSFMo were oper-

ated from 550 to 700 �C with humidified hydrogen (～3% H2O) as fuel and the static air as

oxidant, respectively. An open-circuit potential of 1.001 V, the maximum power density of

428 mW cm�2, and a low electrode polarization resistance of 0.148 U cm2 were achieved at

700 �C. The experimental results indicated that the single-phase BSFMo is a promising

candidate as cathode material for IL-SOFCs.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction great attention because of their potential long-term stability

Recently, intermediate-to-low-temperature temperature

(400e800 �C) solid-oxide fuel cells (IL-SOFCs) have attracted

B. Lin).2013, Hydrogen Energy P89

and economical competitiveness without significant loss of

the cell efficiency [1e3]. However, the lower operating tem-

perature results in a significant decrease in both the ohmic

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

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 8 ( 2 0 1 3 ) 1 4 3 2 3e1 4 3 2 814324

resistance of the electrolyte materials and the polarization

resistance dependent on the cathode electrical conductivity

and the oxygen-reduction activity [4e6], nevertheless the

polarization resistance increases much rapidly than the

ohmic resistance as the temperature decreases [7]. Therefore,

central to IL-SOFCs is the development of alternative cathode

material which can own high ionic and electronic conductiv-

ities and electroecatalytic activity on oxygen reduction reac-

tion in order to reduce the polarization resistance at the

cathode/electrolyte interface [8e11].

To date, the most promising cathode materials seem to be

the cobalt-containing perovskite oxides with high ionic and

electronic conductivities such as Ba1�xSrxCo1�yFeyO3�d [5,12],

Ln1�xSrxCo1�yFeyO3�d [13e15] and Sm0.5Sr0.5CoO3 [16], which

potentially meet most of the requirements for IL-SOFC

operation at low temperature. These cobalt-based cathodes,

unfortunately, often encountered some problems like high

thermal expansion coefficient, chemical stability, high cost

of cobalt element and easy evaporation and reduction of

cobalt [16,17]. In addition to that, second-phase materials

were introduced forming composite cathodes for improving

the ionic or electronic conductivity [18,19]. Yet the compati-

bility between the multi-phase components and the

complicated preparation process greatly limit their practical

application. Clearly, it is significant to develop efficient

single-phase cathodes with high stability and sufficient cat-

alytic activity for IL-SOFCs which would be a long-term

challenge.

Accordingly, some studies showed that the high-valence

ions doped can enhance the stability of cathode materials

[20e22]. Meanwhile, Mo-doping was also considered as an

efficient factor in improving electroecatalytic activity on ox-

ygen reduction reaction of Ba2CoMo0.5Nb0.5O6�d, LaNi0.75-Mo0.25O3, Sr2Fe1.5Mo0.5O6�d, and Ba0.9Co0.7Fe0.2Mo0.1O3�d

[23e26]. Here we have evaluated the single-phase BSFMo as

promising cathode for proton conducting IL-SOFCs with

BZCYYb electrolyte.

2. Experimental

The BZCYYb powder was synthesized by an ethylenediamine

tetraacetic acid (EDTA)-citrate complexation process, where

citrate and EDTA were employed as parallel complexing

agents. Ba(NO3)2$9H2O, Zr(NO3)4$4H2O, Ce(NO3)3$6H2O, Y2O3,

and Yb2O3 were dissolved at the stoichiometric ratio in

distilled water to form an aqueous solution, and then a proper

amount of citric acid was introduced, the molar ratio of

EDTA: citric acid: total of metal cations was controlled around

1:1.5:1. After converted into viscous gel under heating

and stirring conditions, the solution was ignited to flame

and form the primary powders. The as-synthesized powder

was subsequently calcined at 1000 �C for 3 h to form a pure

perovskite oxide. The BSFMo powder was also synthesized

by an EDTA-citrate complexation process with the raw ma-

terials Ba(NO3)2$9H2O, (NH4)6Mo7O24$9H2O, Sr(NO3)2, and

Fe(NO3)3$9H2O at a proper molar ratio and then calcined at

950 �C for 3 h. XRD (q ¼ 1�/min) was performed to analyze the

phase purity of the BSFMo powders and XRD Rietveld re-

finements were performed using GSAS software.

The mixture of NiO þ BZCYYb þ starch (60%:40%:20% in

weight) was pre-pressed at 200 MPa and formed into an

anode substrate. Then loose BZCYYb powder synthesized

above was uniformly distributed onto the anode substrate,

co-pressed at 250 MPa, and subsequently co-sintered at

1400 �C for 5 h to obtain dense BZCYYb membrane. Fine

BSFMo powder, was mixed thoroughly with a 6 wt %

ethylcellulose-terpineol binder to prepare the cathode slurry,

which was then painted on BZCYYb electrolyte films, and

sintered at 950 �C for 3 h in air to form a tri-layer cell of NiO-

BZCYYb/BZCYYb/BSFMo.

The phase identification of the phase reaction between

electrode and electrolyte were studied with the powder X-ray

diffraction by Cu-Ka radiation (D/Max-gA, Japan). Electrical

conductivity of BSFMo was studied using the standard DC

four-probe technique on H.P. multimeter (Model 34401) from

200 to 800 �C. Weight loss of BSFMo as a function of temper-

ature was obtained from thermogravimetric (TG) measure-

ments up to 850 �C in air. Prior to TG runs, BSFMo powders

were annealed at 950 �C for 10 h to remove any volatile sub-

stance thatmight exist. Agmeshwas attached to cathode side

as current collector for electrical measurements. Single cells

were tested from 550 to 700 �C in a home-developed-cell-

testing system with humidified hydrogen (～3% H2O) as fuel

and the static air as oxidant, respectively. The flow rate of fuel

gas was about 40 ml min�1. The cell voltages and output

current of the cells were measured with digital multi-meters

(GDM-8145). AC impedance spectroscopy (Chi604c, Shanghai

Chenhua) was performed on the cell under open-current

conditions from 550 to 700 �C. A scanning electron micro-

scope (SEM, JEOL JSM-6400) was used to observe the micro-

structure of the cells after testing.

3. Results and discussion

The doping of Mo6þ results in the decrease of the oxygen va-

cancy concentration and the increase of oxygen non-

stoichiometry according to electrostatic neutrality. The defect

reactions may be written as following:

2MoO3 þ 3V��O /

Fe2O32Fe���

Mo þ 3O�O (1)

To understand the influence of Mo on the crystal structure

of BSFMo, Fig. 1 showed the XRD Rietveld refinement of the as-

prepared BSFMo sample, where the strong peaks correspond

to space group Pm-3m (221) and the cubic phase structural

parameters obtained based on this model are with

a¼ 3.943442 nm. The refinement of the BSFMo sample gave c2,

wRp and Rp values of 11.17, 14.92% and10.27% respectively,

indicating a close fit to the experimental data.

Fig. 2(a) showed the electrical conductivities of the ceramic

BSFMo samples at the temperature range from 200 �C to 800 �Cin air. The conductivity increaseswith increasing temperature

up to 400 �C and reaches the maximum value of 174 S cm�1,

then decreases with further increasing temperature, which is

much higher than that of Ba0.5Sr0.5FeO3�d samples [27]. It can

be well interpreted as the valence change of Mo5þ/Mo6þ and

Fe3þ/Fe4þ which could arise with the doping of Mo6þ as shown

in chemical reactions (2) and (3):

20 40 60 80

In

te

ns

ity

(a

rb

.u

nits

)

2θ (deg.)

Obs

Calc

bckgr

diff

Phase

Fig. 1 e XRD Rietveld refinement of BSFMo prepared by an

EDTA-citrate complexation process at 950 �C for 3 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 8 ( 2 0 1 3 ) 1 4 3 2 3e1 4 3 2 8 14325

2Mo�Mo þO�

O/2MoMo þ V��O (2)

2Fe�Fe þOx

O/2FeFe þ V��O (3)

200 400 600 800

0

50

100

150

200

250

BSFMo [in this work]

σ σ (s.cm

-1

)

Temperature (o

C)

(a)

200 400 600 800

0.96

0.98

1.00

Weig

ht (%

)

Temperature (o

C)

BSFMo (b)

Fig. 2 e (a)Temperature dependence of the conductivity for

BSFMo sample measured at 200e800 �C in air. (b) TG

weight loss of BSFMo powder sample as a function of

temperature in air (10 �C/min).

With further increase in temperature (higher than 400 �C),

the lattice oxygen desorption of the BSFMo samples increased

significantly shown in Fig. 2(b). Although the solubility of ox-

ygen vacancies would increase with the lattice oxygen

desorption, accompanied by the disappearance of two times

the electronehole, whichwould also be the electron scattering

centers or capture traps [28], resulting in the electrical con-

ductivities of the BSFMo samples reduced as shown in

chemical reactions (4):

2OxO/2V��

O þ 4e� þO2ðgÞ (4)

To assess the phase reaction between electrode and elec-

trolyte is undesirable for the long-term stability of IL-SOFCs

(Fig. 3). The chemical compatibility of BSFMo cathode with

BZCYYb electrolyte was investigated by mixing thoroughly

BSFMowith BZCYYb in a 1:1 weight ratio, and then sintered at

1000 �C for10 h as well as BSFMo cathode with SDC electrolyte.

There are no new peaks identifiable or shift of XRD peaks in

the patterns indicating that there is no significant reaction

between BSFMo and electrolytes. These results reveal that

BSFMo has a good chemical compatibility with BZCYYb and

SDC electrolytes.

The thermal compatibility between the cathode and elec-

trolyte was examined after cell testing. Fig. 4 shows the

microstructure of BZCYYb electrolyte and the cross-sectional

views of the as-prepared tri-layer cells with BSFMo cathode on

the porous anode support after testing. It can be seen that the

BZCYYb membrane is completely dense and the grains are

quite uniform in the size of 1e2 mm in Fig. 4(a). There are no

obvious pores and cracks on the cross-section of the BZCYYb

membrane. As shown in Fig. 4(b and c), it can be seen that a

distinct interfacial boundary cannot be found between the

BZCYYb electrolyte and the anode substrate and the BZCYYb

membrane and BSFMo cathode are about 25 mm and 20 mm in

thickness, respectively. From Fig. 4(d), it can be observed that

the adhesion of the BSFMo cathode to the BZCYYb electrolyte

seems to be excellent and the cathode layer is porous. The

results demonstrate that a dense and crack-free BZCYYb

electrolyte membrane can be successfully fabricated with a

20 40 60 80

&&&

&

&&

&#

##

#

##

##

*

*

*

**

*

*

*

**

* *

**

**Relative in

ten

sity (a.u

.)

2 theta (degree)

*#

&

: BZCYYb

: SDC

: BSFMo

Fig. 3 e The phase reaction between BSFMo and BZCYYb or

SDC electrolytes sintered at 1000 �C for10 h, respectively.

Fig. 4 e SEM images of (a) the cross-section of BZCYYb electrolyte, (b) cross-section views of the tri-layer cells after testing (c)

the electrolyte layer adhered to the anode (d) the electrolyte layer adhered to the cathode.

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 8 ( 2 0 1 3 ) 1 4 3 2 3e1 4 3 2 814326

heat treatment on porous anode support. Therefore, the

cathode performance would be further enhanced through

optimizing the BSFMo cathode microstructures.

To determine its performance in real fuel-cell conditions,

the electrochemical performance of the as-prepared tri-layer

NiO-BZCYYb/BZCYYb (w25 mm)/BSFMo (w20 mm) cell using

humidified hydrogen (～3%H2O) as the fuel and static ambient

air as the oxidant is experimentally obtained under different

operating temperatures, including IeV and IeP curve. It is well

known that the open-circuit voltage (OCV) of the cell should be

close to its theoretic value of 1.1 V, and is slightly influenced

by operating conditions. As shown in Fig. 5, the maximum

0 500 1000 1500

0.0

0.5

1.0

Po

we

r d

en

sity

(m

W c

m-2

)

Po

te

ntia

l(V

)

Current Density (mA cm-2

)

700oC

650oC

600oC

550oC

0

100

200

300

400

500

Fig. 5 e Performance of the tri-layer cell NiO-BZCYYb/

BZCYYb/BSFMo with hydrogen at different temperatures.

power densities of 428, 336, 221, and 107 mW cm�2 with the

OCV values of 1.001, 1.026, 1.032, and 1.054 V are obtained at

700, 650, 600, and 550 �C for the BSFMo cathode, respectively. It

is worth noting that the high open-circuit voltages indicate

that the BZCYYb electrolyte membrane is sufficiently dense

and a good cell performance can be obtained using the cobalt-

free cathode. The single cells exhibit higher power densities

than those using cobalt-free Sm0.5Sr0.5FeO3�d-BaZr0.1Ce0.7Y0.2O3�d (341 mW cm�2 at 700 �C) cathode [29], even

higher than cobalt-based La0.6Sr0.4Co0.2Fe0.8O3�d e BZCYYb

(260 mW cm�2 at 600 �C) cathode [30].

In order to intensively evaluate the performance of cobalt-

free BSFMo working as cathodes for IL-SOFC, resistances of

the cells under open-circuit conditions surveyed by AC

impedance spectroscopy are shown in Fig. 6. In these spectra,

the intercepts with the real axis at low frequencies represent

the total resistance of the cell and the value of the intercept at

high frequency is the electrolyte resistance, while the differ-

ence of the two values corresponds to the sum of the resis-

tance of the two interfaces: the cathode-electrolyte interface

and the anode-electrolyte interface. The total cell resistance

(Rt), ohmic resistance (Ro), as well as interfacial polarization

resistance (Rp) are then obtained from the impedance spectra

in Fig. 6(a), the results are shown in Fig. 6(b). The ratios of Rp to

Rt increasewith a decrease of the operating temperature, from

35.2% at 700 �C to 80.9% at 550 �C, implying that the cell per-

formance is greatly limited by interfacial polarization resis-

tance at low-temperature conditions, which dominated by

the cathodeeelectrolyte interface. The increase of the mea-

surement temperature resulted in significant reduction of

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0

0.5

1.0

1.5

-Z

''(Ω

.c

m2)

Z'(Ω.cm2)

550o

C

600o

C

650o

C

700o

C

(a)

500 550 600 650 700 750

0

1

2

3

Ro

Rt

Rp

Re

sis

ta

nc

e( Ω

.c

m2

)

Temperature (o

C)

(b)

Fig. 6 e Impedance spectra for (a) the tri-layer cell with

BSFMo cathode at various temperatures under OCV

conditions. (b) The total cell resistances (Rt), interfacial

polarization resistances (Rp), and electrolyte resistances

(Ro) obtained from impedance spectra at different

temperatures of the tri-layer cell with BSFMo cathode.

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 8 ( 2 0 1 3 ) 1 4 3 2 3e1 4 3 2 8 14327

interfacial polarization resistance Rp, typically from 2.02Ucm2

at 550 �C to 0.148 U cm2 at 700 �C, respectively. As compared

to the performance with other cobalt-free cathodes, such

as: SrFe0.9Sb0.1O3�d (0.154 U cm2 at 700 �C) [31] and

1.0 1.1 1.2

-1

0

1

2

3

ln

(1

/R

p)

(Ω

-1c

m-2

)

1000/T (K-1

)

Ea=116 kJmol-1

Fig. 7 e Arrhenius plots of Rp of BSFMo cathode with

BZCYYb as the electrolyte.

BaCe0.5Fe0.5O3�d (0.17 U cm2 at 700 �C) [32], the interfacial

resistance of the cell with BSFM cathode is the same low at or

below 700 �C. The low Rp values indicated that the cobalt-free

BSFMo cathode exhibited high electrochemical activity for

operation at intermediate temperature range in practical fuel

cell system. Fig. 7 shows the Arrhenius plots of the Rp values

for BSFMo on the BZCYYb electrolyte. The activation energy

(Ea) of BSFMo is 116 kJ mol�1, which calculated from the

slope of the fitted line, in order to satisfy the Arrhenius

equations. Comparable to other Ea values reported in the

literature [33], e.g. Ea ¼ 164 kJ mol�1 for La0.8Sr0.2CoO3�d, and

Ea ¼ 202 kJ mol�1 for La0.8Sr0.2Co0.8Fe0.2O3�d, respectively. It is

obvious that BSFMo exhibits promising high activity for oxy-

gen reduction and mobility. The experimental results indi-

cated that the single-phase BSFMo was a promising cathode

candidate for LL-SOFCs.

4. Conclusions

In this work, cobalt-free perovskite BSFMowas investigated as

a single-phase cathode for IL-SOFCs. The XRD Rietveld

refinement of the as-prepared BSFMo sample obtained with

the strong peaks correspond to space group Pm-3m (221) and

the cubic phase structural parameters. The doping of high-

valence Mo cations into Fe-site obviously enhanced the con-

ductivities and the electrical conductivity of BSFMo sample

reached the maximum value of 174 S cm�1at 400 �C. XRD re-

sults showed that BSFMo cathode was chemically compatible

with BZCYYb electrolyte for temperatures up to 1000 �C. Thinproton-conducting BZCYYb electrolyte was prepared over

porous anode substrates composed of NiOeBZCYYb by a one-

step dry-pressing/co-firing process. Laboratory-sized tri-layer

cells of NiO-BZCYYb/BZCYYb/BSFMo were operated from 550

to 700 �C with humidified hydrogen (～3% H2O) as fuel and the

static air as oxidant. An open-circuit potential of 1.001 V,

maximumpower density of 428mWcm�2, and a low electrode

polarization resistance of 0.148 U cm2 were achieved at700 �C.The activation energy of the Rp values for BSFMo on the

BZCYYb electrolyte is 116 kJ mol�1. The experimental results

indicated that the single-phase BSFMo was a promising

candidate as cathode material for IL-SOFCs.

Acknowledgments

The authors wish to thank Japan Society for the Promotion of

Science for the JSPS Postdoctoral Fellowship and Chinese

Natural Science Foundation on contract No.51102107 and

No.21171131 for financial support.

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