characterization of lsco|bczy|lsco for potential application in it-sofc
TRANSCRIPT
Characterization of LSCO|BCZY|LSCO for Potential Application in IT-SOFC
Abdullah Abdul Samat1,a, Mohd Azlan Mohd Ishak2,b and Nafisah Osman2,c 1Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
2Faculty of Applied Sciences, Universiti Teknologi MARA, 02600 Arau, Perlis, Malaysia
[email protected], [email protected], c [email protected]
Keywords: LSCO cathode, BCZY electrolyte, EIS, intermediate temperature SOFC
Abstract. A high purity of strontium-doped lanthanum cobaltite with formula of La0.6Sr0.4CoO3-δ
(LSCO) was synthesized via a combined citrate-EDTA route. LSCO slurry was prepared by mixing
LSCO and polyvinyl pyrrolidone (PVP) in ethanol solution. This slurry was manually painted onto
both surfaces of yttrium-doped cerate-zirconate, BaCe0.54Zr0.36Y0.1O2.95 (BCZY) electrolyte to
fabricate a symmetrical cell of LSCO|BCZY|LSCO. The scanning electron microscopy (SEM)
analysis result revealed that the LSCO was well adhered onto the BCZY electrolyte with no
formation of crack or air gap/hole at the LSCO|BCZY interface. Elemental composition of LSCO
cathode and BCZY electrolyte elements such as lanthanum (La), barium (Ba) and cerium (Ce) at the
interface region was confirmed by electron dispersive spectroscopy (EDS) analysis. The
electrochemical performance of the fired cell was analyzed in air by an electrochemical impedance
spectroscopy (EIS) as a function of temperature ranging from 500 – 800°C. It is found that the
fabricated cell exhibits low polarization resistance (Rp) at the operating temperatures and the values
are comparable with those reported in literature. This significant result indicates that LSCO is a
promising candidate to be used as a cathode material for BCZY electrolyte at intermediate
temperatures.
Introduction
Solid oxide fuel cell (SOFC) is considered to be a promising energy conversion device as it
converts chemical energy of fuel gas directly into electrical energy with advantages of high
electrical efficiency and low pollutant emissions. The conventional SOFC operated at high
temperature of up to 1000°C presents some problems related to the cost of materials and
fabrication. Thus, development of intermediate temperature SOFC (IT-SOFC) with proton
conductor operating in the temperature range of 500 – 800°C has attracted much attention in recent
years [1, 2]. Proton-conducting SOFC has some advantages compared with oxygen-conducting
SOFC as it offers low activation energy as well as high energy efficiency. Even though
improvements in proton-conducting SOFC have been achieved, but their performance is still
typically limited by cathode overpotential or cathode polarization resistance [2, 3].
The optimization of the performance of proton-conducting SOFC depends strongly on the
efficiency of cathode materials and cathodic cell structures. The La0.6Sr0.4CoO3-α (LSCO) perovskite
material is an interesting candidate. The potential of this mixed ionic-electronic conductor (MIEC)
as cathode has been proven as it has shown very low polarization resistance, higher electronic
conductivity and good catalytic activity at intermediate temperatures [4, 5]. In addition, the
polarization resistance of the cathode is also dependent on the microstructure and thus on the
preparation process. Synthesis route and its conditions including heat treatment applied are well
known to play an important role on the properties and performance of the final materials. Even
though with the same compositions, the materials would have different characteristics.
Besides the properties of the cathode materials, a good adhesion or good contact between
cathode and electrolyte at the cathode|electrolyte interface also contributes to the significant effect
on the polarization resistance. A strong or good adhesion between cathode and electrolyte at
interfacial layer will reduce the deleterious effect upon the transport of ions as well as charge
Defect and Diffusion Forum Vol. 353 (2014) pp 233-238Online available since 2014/May/21 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/DDF.353.233
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transfer processes and thus reduce the polarization resistance. To improve the adherence of the
porous cathode to the electrolyte, a cathode binder is used in making the cathode slurry to be
painted onto the electrolyte surfaces. The cathode binder helps to join two different materials to
stick together. Polyvinyl pyrrolidone (PVP) was reported as a promising cathode binder in helping
LSCO cathode to be well adhered onto the BaCe0.54Zr0.36Y0.1O2.95 (BCZY) electrolyte without no
formation of crack and air gap at the LSCO|BCZY interface [6].
In this work, the single perovskite phase powder of LSCO synthesized by a combined citrate-
EDTA method was evaluated as a promising cathode for a proton-conducting SOFC based on
cerate-zirconate electrolyte. The electrochemical properties of a symmetrical half-cell of
LSCO|BCZY|LSCO were examined by an AC electrochemical impedance spectroscopy (EIS)
technique as function of temperature. This technique has the advantages of making it possible to
separate different rate processes in the studied frequency domain and is an efficient tool to
determine the main processes involved in the reaction [1]. The results obtained are compared with
the values reported in literature.
Experimental procedure
Synthesis and characterization of LSCO powder. The powder of La0.6Sr0.4CoO3-α (LSCO)
cathode was prepared using a combined citrate-EDTA method. Analytical grade of La(NO3)3.6H2O,
Sr(NO3)2 and Co(NO3)3.6H2O were used as starting materials. A mixture of these starting materials
was mixed with citric acid (CA) in 100 mL deionized water. The solution mixture was stirred and
heated in a water bath at 75°C. Then, ethylenediaminetetra-acetic acid (EDTA) was slowly added
and the pH of the solution was adjusted to be 0.5 after the EDTA was completely dissolved. Finally,
ethylene glycol (EG) was added and the solution was continuously stirred and heated for several
hours. The resulting viscous gel was dried at 150°C for 12 hours and 250°C for 5 hours. The as-
synthesized powder was calcined at 1000°C with a heating/cooling rate of 10°C min-1
for 5 hours to
obtain black powder.
The details of the steps involved in producing the LSCO powder were reported elsewhere [7].
The phase of the calcined LSCO powder was analyzed at room temperature on an X-ray
diffractometer (XRD 6000 Shimadzu) with Ni-filtered and Cu-Kα radiation source (λ = 0.1540558
nm). The XRD was operating at 40 kV and 30 mA using a step scan rate of 0.02° s-1
for the 2θ
range from 20° to 80°.
Fabrication and characterization of LSCO|BCZY|LSCO cell. Dense BaCe0.54Zr0.36Y0.1O2.95
(BCZY) pellet was prepared as previously reported by Osman et al. [8]. About 0.65 g of BCZY
powder was compacted into pellet at 5 tons pressure using a hydraulic press in a mould of 13 mm in
diameter. The green BCZY pellet was sintered at 1400°C for 10 hours. Both surfaces of sintered
BCZY pellet at 1400°C were polished with silicon carbide (SiC) grit paper and the prepared LSCO
cathode slurry was painted onto both sides of the polished surfaces. The symmetrical cell was first
heated up to 500°C and then to 950°C for 2 hours. Next, the symmetrical cell was cross-sectional
mounted in different sample holders using acrylic resin powder and acrylic hardener clear liquid.
The mounted cell was polished using different #SiC grit papers. The polished cell was
ultrasonically cleaned to remove the contamination on the surface of the cell. After that, the cell was
polished with 6 µm and 1 µm diamond polish, accordingly and dried in air for few hours. The
procedure involved in fabricating the symmetrical cell was reported elsewhere [6]. The surface
morphology of the cell was examined by a Zeiss SMT Supra 40VP scanning electron microscope
(SEM) operating at 10 kV. Electron dispersive spectroscopy (EDS) was applied to determine the
elemental composition of cathode, electrolyte and cathode|electrolyte interface.
Measurement of the electrochemical impedance characteristic of the cell was performed at open-
circuit voltage (OCV) in air as a function of temperature ranging from 500 – 800°C at 50°C
intervals by ZIVE SP2 Electrochemical Workstation (ZIVELAB WonATech) connected to a
234 Diffusion in Solids and Liquids IX
BCZY electrolyte LSCO
cathode
LSCO|BCZY interface
personal computer. ZIVE®
Smart Manager™
software was used for data acquisitions and analysis.
The impedance spectra were obtained in the frequency range of 50 KHz to 125 µHz with ten steps
per decade and signal amplitude of 1V.
Results and discussion
The formation of a single perovskite phase for the LSCO powder calcined at 1000°C was verified
by XRD measurement as shown in Figure 1. The powder's XRD pattern of the calcined LSCO
showed high intensity and narrow peaks perovskite phase that matched with the Joint Committee of
Powder Diffraction Standards (JCPDS) file no 48-0121. A SEM micrograph of the cross-sectional
view at the LSCO|BCZY interface is shown in Figure 2. There was no obvious crack or air gap/hole
appearing at the interface region which shows a good adhesion between LSCO cathode and BCZY
electrolyte. The elements present at the interfacial region were lanthanum (La), barium (Ba) and
cerium (Ce). The results were confirmed by EDS analysis as tabulated in Table 1.
Figure 1. XRD pattern of LSCO powder after Figure 2. SEM micrograph at LSCO|BCZY
calcined at 1000°C for 5 hours interface
Table 1. Elemental atomic percentage at LSCO, BCZY and LSCO|BCZY interface areas
Element Elemental atomic percentage at each area (%)
LSCO cathode BCZY electrolyte LSCO|BCZY interface
C 49.79 23.94 44.37
O 31.05 44 36.94
La 4.98 - 3.16
Sr 3.74 - -
Co 10.44 - -
Ba - 17.92 9.52
Ce - 9.08 6.02
Zr - 4.17 -
Y - - -
Figure 3 shows the typical impedance spectra of the sample measured at OCV in air between
temperature ranges of 500 – 800°C. The impedance spectra consist of two arcs indicating that at
least two responses that corresponding to the electrolyte and electrode-electrolyte processes might
happened. The intercept of the real axis at high frequency is associated to the ohmic resistance
(ROhm) or overall electrolyte resistance. The value at the end of semi-circle at low frequency
corresponds to the total resistance (RT) of the cell. Therefore, the difference between the low
Defect and Diffusion Forum Vol. 353 235
frequency and high frequency represents the total interfacial polarization resistance (Rp) which
includes the concentration polarization resistance (mass-transfer or gas-diffusion polarization) and
the effective interfacial polarization resistance associated with the electrochemical reactions at the
electrode|electrolyte interface [3].
Figure 3. Impedance spectra of the prepared cell measured under OCV condition at different
temperatures
As shown in Figure 4, the cell performance is significantly influenced by the Rp. As the
temperatures increased from 500°C to 800°C, the Rp values were significantly decreased, typically
from 4.60 Ω cm2 to 0.13 Ω cm
2 at 500°C and 800°C, respectively. These results are comparable
with the values reported in the literature (Table 2).
The discrepancy in Rp values obtained from this study with those reported in literature might be
due to the use of a different synthesis method in producing the LSCO cathode material and the use
of different electrolyte materials. The different synthesis method gives a different microstructure
and properties which then determine the performance of the produced materials. Furthermore, the
produced materials will show different chemical properties or compatibility with different
electrolyte materials. Besides that, the cell fabrication method will also determine the
electrochemical performance of the fabricated cell. The spraying and screen-printed methods are a
more advanced technique as compared to the manually painted method which is a simple technique.
The first two methods enable the cathode material to well adheres onto electrolyte surfaces and can
control the thickness of cathode material deposited onto the electrolyte surfaces. The data obtained
from this impedance measurement signifies that:
• LSCO cathode is also compatible with BCZY electrolyte instead of GDC electrolyte at
intermediate temperatures range
236 Diffusion in Solids and Liquids IX
• Compliment to the SEM results which shows a good contact between LSCO cathode and
BCZY electrolyte at interface
• Simple fabrication method can also produce a good cell with good electrochemical
performance
Figure 4. Graph of the overall electrolyte resistance (Rohm), total cell resistance (RT) and
polarization resistance (Rp) determined from the impedance spectra of the prepared cell at different
temperatures
Table 2. Rp value of LSCO cathode on various electrolytes at various temperatures
Synthesis
method
Fabrication
method
Electrolyte Rp (ΩΩΩΩ cm2) Ref.
(600°°°°C) (700°°°°C) (750°°°°C)
Citrate-
EDTA
Spraying
method
Ce0.9Gd0.1O1.95 (GDC) - 0.17 0.07 [9]
Flame
spray
Screen
printed Ce0.9Gd0.1O2-δ (GDC) 0.96 0.14 - [10]
Citrate-
EDTA
Manually
painted
BaCe0.54Zr0.36Y0.1O2.95
(BCZY)
0.87 0.29 0.19 [this
work]
Conclusion
In this work, a single perovskite phase of La0.6Sr0.4CoO3-α (LSCO) powder was prepared via a
combined citrate-EDTA method and it was employed as a cathode for a proton-conducting SOFC of
BaCe0.54Zr0.36Y0.1O2.95 (BCZY) electrolyte. In terms of material, the prepared cell exhibits a good
contact between the cathode and electrolyte that gives a better cell performance of
LSCO|BCZY|LSCO. Detailed studies of the separating electrode|electrolyte processes at interfacial
layer are under investigation and the progress will be reported elsewhere.
Defect and Diffusion Forum Vol. 353 237
Acknowledgement
The authors thank to the Ministry of Higher Education, Malaysia for the Research Acculturation
Collaborative Effort (RACE) grant and Research Acculturation Grant Scheme (RAGS) and
Universiti Teknologi MARA (UiTM) for the facilities.
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238 Diffusion in Solids and Liquids IX
Diffusion in Solids and Liquids IX 10.4028/www.scientific.net/DDF.353 Characterization of LSCO|BCZY|LSCO for Potential Application in IT-SOFC 10.4028/www.scientific.net/DDF.353.233