A Comparative Study of Oxygen Reduction Reaction
on Bi- and La-Doped SrFeO
3-
Perovskite Cathodes
Yingjie Niu,
a
Jaka Sunarso,
b
Fengli Liang,
b
Wei Zhou,
b,z
Zhonghua Zhu,
b
and
Zongping Shao
a,z
a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical
Engineering, Nanjing University of Technology, Nanjing 210009, People’s Republic of China
b
School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia
In this work the oxygen reduction reaction on Bi
0.5
Sr
0.5
FeO
3-
BSF and La
0.5
Sr
0.5
FeO
3-
LSF as cathodes for intermediate
temperature solid oxide fuel cells 600–750°C is compared in detail. Partial substitution of Sr in SrFeO
3-
with 50 mole % of Bi
or La results in the distinct structural features which strongly impact its electrochemical characteristics due to the presence of a
lone electron pair in Bi
3+
, which is not available in La
3+
. Cubic structure for BSF favours higher ionic conductivity and enhanced
formation of oxygen vacancies relative to rhombohedral structure for LSF. Large oxygen nonstoichiometry for BSF, nevertheless,
leads to the dominance of Fe
3+
and subsequent low electronic conductivity relative to LSF. It was found that upon cathodic
polarization, the oxygen vacancies are created on LSF which helps reducing its interfacial resistance afterward, which is not the
case for BSF. Overall, BSF demonstrates good electrochemical performance which can be further optimized by enhancing its
electronic conductivity.
© 2010 The Electrochemical Society. DOI: 10.1149/1.3521316 All rights reserved.
Manuscript submitted October 13, 2010; revised manuscript received November 5, 2010. Published December 3, 2010.
Solid oxide fuel cells SOFCs offer additional advantages with
respect to other conventional energy conversion devices in terms of
high efficiency, fuel flexibility e.g., prospect to operate on natural
gas and biogas, environmental compatibility, as well as the possi-
bility to recover exhaust heat.
1-5
The development of robust cathode
materials for SOFCs which operate at intermediate temperatures
500–700°C has attracted much attention because of their potential
to dramatically reduce the cost of the SOFC technology.
6-10
A desir-
able cathode material for the intermediate temperature SOFC should
have high electronic and oxide ion conductivities, low thermal ex-
pansion to be compatible with the electrolyte, as well as high cata-
lytic activity for the oxygen reduction reaction ORR.
11
Mixed
ionic–electronic conducting perovskites e.g., materials with
ABO
3-
structure with the capability to conduct oxygen ions and
electrons simultaneously have become the strong potential candi-
dates as SOFCs’ cathodes in recent years.
12-15
SrCoO
3-
SC-based
perovskites especially have been investigated intensively due to
their high electronic and oxide ion conductivities.
16-20
SC-based per-
ovskites nevertheless exhibit drawbacks mainly associated with their
relatively large thermal expansion coefficients TECs and high re-
activity e.g., readily reacts with yttria-stabilized zirconia YSZ
electrolyte.
21-25
In comparison to SC-based perovskites, SrFeO
3-
SF-based
ones have a lower TEC and excellent chemical compatibility with
YSZ and ceria-based electrolytes.
15,26-30
It has been repeatedly re-
ported that a relatively low cathodic polarization resistance can be
obtained in Ln,SrFeO
3-
-based Ln = lanthanide series, the mag-
nitude of which generally depends on the radius size of Ln
3+
cations
relative to Sr.
31
Nevertheless, not much attention has been given to
doping with nonlanthanide element. In this respect, we have recently
reported that Bi doping into the A-site of SF ABO
3-
represented
by Bi
0.5
Sr
0.5
FeO
3-
BSF compound also results in very low area
specific resistance ASR of 0.06 cm
2
at 750°C, which is much
lower than that of other Co-free perovskite cathodes.
32
Because the
radii of Bi
3+
and La
3+
are very similar, the large discrepancy in the
crystal structure and electrocatalytic performance between them is
derived from the presence of highly polarizable 6s
2
lone electron
pair for the Bi
3+
cation.
33,34
In this work, we report in detail the
electrochemical and ORR performances of BSF and
La
0.5
Sr
0.5
FeO
3-
LSF cathodes including their comparison using
electrochemical impedance spectroscopy EIS and polarization
e.g., current–voltage measurement. This work can be considered as
the extension of our previous communications.
32
Experimental
BSF and LSF powders were synthesized by a combined ethyl-
enediatomictetracetic acid EDTA–citrate complexing process.
Stoichiometric amounts of Bi NO
3
3
·5H
2
O 99.99+%, Sigma-
Aldrich, La NO
3
3
·6H
2
O 99.99+%, Sigma-Aldrich, Sr NO
3
2
99.9+%, Ajax Finechem, Australia, and Fe NO
3
3
·9H
2
O
99.9+%, Sigma-Aldrich were mixed in deionized water and
heated at 80°C. Dissolution of Bi NO
3
3
·5H
2
O was performed by
adding the required amount of HNO
3
67%, Sigma-Aldrich. EDTA
99.4+%, Ajax Finechem, Australia and citric acid 99.5%, Fluka
were then added as the metal ions’ complexing agents. The molar
ratio of total metal ions, EDTA, and citric acid in the solution was
1:1:2. To ensure complete complexation, the solution pH was ad-
justed to 6 by adding NH
3
aqueous solution 28%, Ajax Finechem
Australia. The final powders were obtained from the solution after
water evaporation at 90°C to form a transparent gel, followed by
prefiring of this gel at 250°C and its calcination at 950°C for 5 h in
air.
The crystal structures of the BSF and LSF powders were deter-
mined by X-ray diffraction XRD, Bruker, Karlsruhe, Germany
AXS D8 Advance with filtered Cu-K radiation at 40 kV and
40 mA and a receiving slit of 0.2–0.4 mm. The diffraction patterns
were collected at room temperature by step scanning in the range of
10° 2 90° with the scan rate of 2 min
-1
. Rietveld refine-
ments on the XRD patterns were carried out using DIFFRAC
plus
TO-
PAS 4 software.
35
All the initial parameters for the structures were
taken from Inorganic Crystal Structure Database ICSD#92335
SrFeO
3
for BSF and ICSD#78066 for LSF.
36
During refinements,
general parameters, such as the scale factor, background parameters,
and the zero point of the counter, were optimized. Profile shape
calculations were carried out using the Thompson–Cox–Hastings
function implemented in the program by varying the strain param-
eter. The cell parameter, the isotropic thermal parameters B
eq
, and
oxygen occupancy factor were refined with all the atomic positions
kept constant. For BSF, the initial atomic position of Bi
3+
was set
equal to the position of Sr
2+
with an occupancy factor of 0.5.
The oxygen nonstoichiometry and average valence of Fe in BSF
and LSF were measured by iodometric titration. Approximately
0.1 g of powder was dissolved in a 6.0 mol l
-1
HCl solution under
argon atmosphere to prevent the oxidation of I
-
ions in air before
titration by thiosulfate S
2
O
3
2-
solution. Several drops of starch so-
z
E-mail: shaozp@njut.edu.cn; wei.zhou@uq.edu.au
Journal of The Electrochemical Society, 158 2 B132-B138 2011
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