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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: [email protected] (
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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