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Metal oxide nanocomposites as anode and cathode for low temperature solid oxide fuel cell
Journal Pre-proof Metal oxide nanocomposites as anode and cathode for low temperature solid oxide fuel cell Kausar Shaheen, Zarbad Shah, Hussain Gulab, Muhammad Bilal Hanif, Hongli Suo PII:
S1293-2558(19)31345-7
DOI:
https://doi.org/10.1016/j.solidstatesciences.2020.106162
Reference:
SSSCIE 106162
To appear in:
Solid State Sciences
Received Date: 18 November 2019 Revised Date:
23 February 2020
Accepted Date: 23 February 2020
Please cite this article as: K. Shaheen, Z. Shah, H. Gulab, M.B. Hanif, H. Suo, Metal oxide nanocomposites as anode and cathode for low temperature solid oxide fuel cell, Solid State Sciences (2020), doi: https://doi.org/10.1016/j.solidstatesciences.2020.106162. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Masson SAS.
Graphical Abstract:
1
Metal Oxide Nanocomposites as Anode and Cathode for Low Temperature Solid Oxide
2
Fuel Cell
3 4
Kausar Shaheena,b,c, Zarbad Shah*d, Hussain Gulabd, Muhammad Bilal Hanife,
5
Hongli Suo*a
6 7
a
8
University of Technology, Beijing-100124, China
9
b
The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing
Department of Physics, University of Peshawar, Peshawar-25120, Khyber Pakhtunkhwa,
10
Pakistan
11
c
12
Khyber Pakhtunkhwa, Pakistan
13
d
14
Pakhtunkhwa, Pakistan
15
e
16
Engineering, Xian Jiaotong University, Xi'an, Shaanxi, China
17
Corresponding author:
18
Zarbad Shah, Assistant Professor, PhD.
19
E-mail: [email protected] and [email protected]
20
Hongli Suo, Professor, PhD.
21
E-mail: [email protected]
22 23 24 25 26 27 28 29 30 31
Department of Physics, Jinnah College for Women, University of Peshawar, Peshawar-25120,
Department of Chemistry, Bacha Khan University Charsadda, Charsadda-24420,
Khyber
State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and
32
Abstract
33
The high operating temperature for Solid Oxide Fuel Cells (SOFCs) is one of the major obstacles
34
in the advancement and marketing of the fuel cell technology. Mixed metal oxides Cu0.5Sr0.5
35
(CS) and La0.2 doped Cu0.4Sr0.4 (LCS) nanocomposites, fabricated through a mild and cost
36
effective pechini method were used as an efficient remedy in this context. The nanocomposites
37
were evaluated for phase purity and structural analysis through X-Ray Diffractometer (XRD) and
38
Scanning Electron Microscope (SEM). Particle size calculated was ~37.21nm (for CS) and
39
62.43nm (for LCS) via XRD, while SEM revealed the size ranged from (43-72)nm. Performance
40
of the symmetrical triple layered cells was tested in the temperature range of 500-600oC with H2
41
fuel. LCS nanocomposites exhibited higher electrical conductivity ~4.70S/cm and greater power
42
density ~782mW/cm2 as compared to CS nanocomposites for which the electrical conductivity
43
and power density were achieved as 4.40S/cm and 725mW/cm2 respectively. The activation
44
energy for CS and LCS were found to be 0.23 and 0.26eV respectively. The reliable and
45
enhanced power densities make the synthesized nanocomposites as potential candidates for low
46
temperature SOFCs as anode and cathode.
47
Key words: Mixed metal oxides; nanocomposites; SOFCs; power density.
48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
63
1. Introduction
64
Nanotechnology has attracted much attention of researchers due to utilization of nanostructured
65
materials for manufacturing of energy devices in order to deal with the energy crisis. Rapid
66
depletion of fossil fuel sources has severely increased this need. One of the serious drawbacks
67
related to conventional fossil fuel energy sources is the climate change due to emission of
68
greenhouse gases. Only the renewable and sustainable alternative energy systems can overcome
69
this problem [1]. Fuel cell is one of those sources, which can provide us with energy on
70
demands, where all the other sources such as sun and wind are totally nature dependent [2]. The
71
devices based on fuel cell can efficiently generate electrical energy at the cost of chemical
72
energy without combustion. In fuel cell devices, electric power is usually generated by oxidation
73
at anode and reduction at cathode, while ions are exchanged between anode and cathode via
74
electrolyte solution. The by-products such as CO2 and H2O are also obtained, depending upon
75
the type of fuel utilized during operation [3]. SOFCs are generally more favored as promising
76
advanced technology due to their effective performance, reliability, and environment friendliness
77
[4, 5].
78
The three important parts of SOFCs are cathode (where air is used as oxidant), anode (utilizing
79
any fuel) and an electrolyte (for exchange of ions between electrodes). The most significant part
80
of SOFCs is anode, which not only catalyze the mobility of the ions through electrolyte but also
81
conduct electrons to external circuit and provides a site of oxidation for the fuel [6]. In general
82
both the electrode materials must have greater stability, better ionic and electronic conductivity,
83
homogeneous fine sized nanostructure, higher porosity, and should be inert towards the
84
interconnector or electrolyte materials with the temperature coefficient comparable to that of
85
electrolyte [7]. Materials with small particle size offer less surface reaction resistance and hence
86
the interface adherence between electrode and electrolyte is improved [8]. Due to various
87
drawbacks in the existing conventional materials, there is always a need to explore some new
88
and suitable materials. A major improvement has been made towards the characteristics of
89
electrode materials and a number of advanced materials have also been prepared. Still there are
90
many issues to be addressed before the materials can be practically utilized. For example, the
91
iron based materials usually has the corrosion problem due to formation of iron oxide layers,
92
reducing the stability and hence the long term usage of such anodes. Cobalt was considered as
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one of the most stable anode material, but the higher cost and carbon deposition hinder its
94
utilization in SOFCs [9]. The most commonly used nickel based anode materials suffer from
95
degradation, blockage of reaction site, mechanical breakage and low tolerance for sulfur based
96
fuels such as H2S [10, 11]. Anode material NiO/YSZ has been successfully used, but its high
97
working temperature such as 800°C, sulphur deposition and squat carbon tolerance limit their
98
inherent use in the operation of SOFCs [12-14].
99
As an alternative, copper based materials are being utilized as cooking resistant anode material
100
[15]. Addition of La2O3 in NiO based anode materials has improved the efficiency and stability
101
of fuel cell by preventing the coarsening of NiO [16]. Similarly, utilization of nanocomposites
102
such as ZnO and CuO is also reported as very effective for the enhanced efficiency of anode
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materials based on Ni [17, 18]. Various tunable properties and controlled synergistic effects can
104
be achieved by utilizing these nanocomposites as an anode materials for SOFCs [19]. Similarly,
105
in the recent past the materials such as Li-NiO, La-Sr-Mg (LSM), La-Sr-Fe (LSF), La-Sr-Co
106
(LSC), Ba-Sr-Co-Fe (BSCF) and La-Sr-Co-Fe (LSCF) were frequently used as cathode in the
107
fabrication of SOFCs [20-24]. However, there is still a need to explore materials well suited,
108
because a very few materials such as ZnO/NiO, NiO, LiNiZn-oxides can be used for this purpose
109
[5-10].
110
Semiconductor metal oxides have also received special attention in various fields of science and
111
technology, such as piezoelectricity, conducting films and catalysis. Co-doping of
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semiconductors with various ions such as Al, Bi, Mn, Co, Cu, and Sr have improved their
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electrical properties. Low operating temperature ~650°C and easy synthesis techniques have
114
made the composites containing semiconductors and metal oxides more attractive [25-27].
115
Previously, ZnO (doped) [14], NiO/ZnO nanocomposites [28], trimetallic oxides Cu-NiO-ZnO
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and LiNi0.8Co0.2O2 have been successfully used in low temperature SOFCs [29, 30]. For the
117
efficient performance of SOFCs at low temperature, the co-doping of metals and semiconductors
118
is one of the best and effective techniques. Grain boundary, grain size, long term mechanical and
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chemical stability, porosity and electro-catalytic efficiency are the major components affecting
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the conductivity of doped metals and making them well suited for electrochemical reactions [31].
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Hence, the field of nano-engineering to develop nanocomposites with enlarged surface area,
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active interface and improved performance for SOFCs are considered more advantageous as
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compared to microscale technology [32, 33]. Hence, it is expected to obtain the improved
124
efficiency and stability of SOFCs through multi-oxide nanocomposite electrode materials.
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In the current work, two nanostructured materials CS and LCS were synthesized via pechini
126
method in order to validate the improved electrochemical performance for SOFCs. XRD and
127
SEM were utilized for phase and microstructural analysis. Electronic conductivity and
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impedance measurements were performed through four probe method and electrochemical
129
impedance spectrometer. Triple layered symmetric fuel cells were tested for open circuit voltage
130
(OCV) and power density through fuel testing instrument (L-43). For this purpose,
131
La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) material was employed as an electrolyte [34-36]. The crystal
132
phase, microstructure and electrical performance evaluated in detail suggested the fabricated
133
nanocomposites as an excellent electrode candidates for SOFCs.
134
2. Experiments
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2.1. Nanocomposites Preparation
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Stoichiometric amount of Cu(NO3)2, Sr(NO3)2, La(NO3)3.6H2O and deionized water were used
137
as the starting materials. The mixture was stirred on a hot plate and magnetic stirrer with 200rpm
138
for 1hr at temperature of 80oC and pH was adjusted while agitating the solution. After pH
139
adjustment, the solution was stirred for 3 more hours, until it became transparent. The solution
140
was then heated and evaporated at 180oC till appearance of a gel. The gel was dried at 110oC in
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oven for 12hrs and then was calcined at 450oC for 1hr. The obtained powder was ball milled in
142
ethanol for 24hrs. Finally, the obtained product was sintered at a temperature of 800oC for 5hrs
143
for phase evolution of the oxides.
144
2.2. Cell Fabrication, Conductivity Setup and Electrochemical Performance
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The powder was dry pressed into pellets with a pressure of 100MPa having thickness ~1-2mm
146
and diameter ~10mm with LSGM electrolyte as a thin layer sandwiched between anode and
147
electrode. The two configurations CS/LSGM/CS and LCS/LSGM/LCS were used as
148
cathode/electrolyte/anode materials as shown in Fig. 1. The pellets were sintered at 600°C for 2
149
hours. After sintering, they were coated with conducting silver paste and were investigated for
150
SOFCs.
151 152 153
154
Fig 1: Schematic diagram for SOFCs.
155 156
Electrical conductivity was measured by utilizing the following mathematical Eq. 1 [37].
157
Eq. 1
158
Where, L, R and A represent the dimensions of the pellet. Similarly, activation energy (Ea) was
159
calculated through Eq. 2, by utilizing the parameters such as conductance (σ), exponential factor
160
(σ0), constant (R) and temperature (T) [38]. exp
161
Eq. 2
162
L-43 testing instrumentation was utilized for electrochemical performance of the synthesized
163
cell. Fuel utilized was in the form of pure H2 at anode with a flow rate of 100ml/min and air with
164
flow rate of 80ml/min was provided as an oxidant at cathode. The current and OCV values were
165
to obtain I-V and I-P curves and to record the power densities.
166
2.3. Characterization
167
Crystalline phase structures for the as synthesized nanocomposites was determined via XRD
168
(D/MAX 2200-PC). SEM (JSM-5910) operating at 15.0kV was used to observe the morphology
169
and microstructure. A standard four probe DC method was followed to calculate electrical
170
conductance at various temperatures, by utilizing a keithley 2400 source meter. Impedance
171
spectra were investigated by utilizing electrochemical spectroscopy (EIS) at a temperature of
172
650oC, with voltage fixed at 10mV. Electrochemical performance of LSGM supported
173
symmetrical cells was investigated by testing machine loaded with variable resistance in a
174
temperature range of 500-600°C, utilizing H2 as a fuel, whereas, the ambient air was used as an
175
oxidant.
176 177
178 179
3. Experimental Results and Discussion
180
3.1. XRD and SEM
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Phase structure of synthesized nanocomposites was carried out by XRD patterns as depicted in
182
Fig. 2(a, b). The developed peaks in XRD pattern were indicating the desired phases of CuO,
183
SrO and La2O3 according to PDF#72-0629, PDF#06-0520 and PDF#84-2041 respectively. Some
184
additional peaks for SrCO3 and Sr(OH)2 were also indicated at 2θ values of 25.2o, 25.8o and
185
46.6o. These extra peaks may be attributed towards the impurities present due to precursors or the
186
hygroscopic reaction of SrO with atmospheric H2O and CO2.
187
SEM micrographs for CS and LCS nanocomposites revealed non homogeneous distribution of
188
porous particles as depicted in Fig. 2 (c, d). Whereas, SEM of cell sintered with the configuration
189
as anode/electrolyte/cathode and after cell testing is shown in Fig. 2 (e-h).
190
Particle size calculated from XRD patterns by utilizing Scherer's formula (Eq. 3) [39] was 37.21
191
and 62.43nm, whereas the size obtained via SEM was ranged from 43-60 and 63-72nm for CS
192
and LCS nanocomposites respectively.
193 194 195
196
197
0.89
Eq. 3
Where s is particle size, λ is wavelength and β is FWHM (full width at half maximum).
198
199 200 201 202
Fig 2: (a, b). XRD patterns and SEM micrographs of (c, d). Sintered nanocomposites, (e, f). Sintered cells and (g, h). After fuel cell testing. 3.2. Conductivity and Activation Energies of CS and LCS Nanocomposites
203
Electrical conductivity for CS and LCS nanocomposites at a temperature ranged from 400-650oC
204
is shown in Fig. 3a. Direct relationship between conductivity and temperature was revealed with
205
maximum conductivity ~4.40 and 4.70S/cm for CS and LCS nanocomposites respectively.
206
Previously, it is reported that the materials with electrical conductivity greater than 1S/cm is
207
suitable for effective performance of fuel cells. Therefore, the much higher values of
208
conductivity for CS and LCS nanocomposites in the present study make them well suited for
209
better efficiency [40, 41]. LCS nanocomposites were reported with greater value of conductivity
210
as compared to CS, which may be attributed towards the doping of La element. Arrhenius plot
211
and linear fit of Arrhenius plot for CS and LCS nanocomposites is shown in Fig. 3(b, c).
212
Semiconducting behaviour was exhibited by both the nanocomposites as is depicted by the
213
corresponding plots. Low activation energies ~0.23 and 0.26eV were calculated from the linear
214
fitting technique for CS and LCS nanocomposites respectively. These low values of activation
215
energies are favorable for effective utilization of these materials as an electrode, as has been
216
reported previously as well [42].
217 218 219
Fig 3: (a). Electrical conductivity and (b, c). Arrhenius plots for CS and LCS nanocomposites. 3.3. Impedance Measurement
220
Impedance spectra for CS and LCS nanocomposites at a temperature of 650°C were obtained in
221
the shapes of two semi circles (i.e. larger and smaller) by plotting the real part of impedance
222
against the imaginary part. The two semi circles were indicating the complex impedance plane
223
format, Z* (=Zʹ - jZʹʹ, j2= -1) attributed towards two distinguishable electro-active regions within
224
the nanocomposites. Larger semi-circle revealed electronic conduction (due to bulk/grain effect),
225
while small semicircle represented ionic behaviour (due to grain boundary effect) [43-48]. An
226
equivalent fitted circuit was also obtained after simulation of the experimental curve as shown in
227
the inset of Fig. 4 (a, b).
228 229 230
Fig 4: Impedance spectra for the synthesized nanocomposites. 3.4. Electrochemical Performance
231
Electrochemical performance including voltage of cell, power density and current density was
232
measured by a cell consisting of three layers, CS/LSGM/CS and LCS/LSGM/LCS at temperature
233
ranged from 500-600°C. Dry H2 oxidant by ambient air was utilized as fuel. Maximum OCV was
234
measured as 1.02, 1.13V and maximum power densities were calculated as 725 and 782mWcm-2
235
at a temperature of 600oC for CS and LCS nanocomposites respectively, as shown in Fig. 5(a, b).
236
Numerical data about the synthesized nanocomposites is summarized in Table I.
Table I: Numerical data for the synthesized nanocomposites.
237
Sample CS LCS
Composition Configuration Cu0.5Sr0.5 CS/LSGM/CS La0.2Cu0.4Sr0.4 LCS/LSGM/LCS
Conductivity 4.40 S/cm 4.70 S/cm
Activation energy 0.23eV 0.26eV
Power density 725mW/cm2 782mW/cm2
238 239
The values for electrochemical performance is much higher than the already reported materials
240
as described in Table II.
241 242
Fig 5: Electrochemical efficiency of symmetric cell (a). CS/LSGM/CS, (b). LCS/LSGM/LCS.
243
Table II: Comparison of electrochemical performance between CS/LSGM/CS and
244
LCS/LSGM/LSC to the reported data.
245 246 247 248 249 250 251 252
(Anode/Electrolyte/Cathode) BCFZ/NKCDC/LSCZ AMNZ/GDC/BSCF NOCO/Y2O3-SDC4/NiOY2O3-DC ATZ/SDC/BSCF LCNZ/BGC/LCNZ ANZ-GDC/GDC/BSCF ATZN-SDC/SDC/BSCF CS/LSGM/CS LCS/LSGM/LCS
Temp Power density 550oC 334mW/cm2 550oC 535mW/cm2 580oC 750mW/cm2 650oC 354mW/cm2 650oC 375mW/cm2 550oC 705mW/cm2 650oC 370mW/cm2 600oC 725mW/cm2 600oC 782mW/cm2
Ref. 49 50 51 52 53 54 55 This work This work
253 254
4. Conclusion
255
In this study, two mixed metal oxide nanocomposites CS and LCS were synthesized through
256
pechini method for low temperature SOFCs. The enhanced crystallinity and porous structure of
257
the nanocomposites favored their improved electrochemical properties. Symmetrical cells with
258
specified configuration such as CS/LSGM/CS and LCS/LSGM/LCS were applied for cell
259
performance at temperature ranged from 500-600oC. Maximum power density ~782mWcm-2 was
260
achieved at 600oC for LCS. Electrical conductivity were measured as 4.40 and 4.70Scm-1 for CS
261
and LCS nanocomposites respectively. Based on all these preliminary results, the synthesized
262
nanocomposites can be effectively used for low temperature SOFCs.
263
Acknowledgment
264
The authors are highly grateful to National Natural Science Foundation of China (51571002),
265
Beijing Natural Science Foundation (2172008), Program of Beijing City and Beijing University
266
of Technology, Evaluation Research for the Performance of Tapes (GH-201809CG005), General
267
Program of Science and Technology, Development Project of Beijing Municipal Education
268
Commission of China (No. KM201810005010), Project of Advanced Discipline (No. PXM2019-
269
014204-500031) and the Department of Chemistry Bacha Khan University Charsadda-24420,
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Charsadda Khyber Pakhtunkhwa Pakistan.
271
Conflict of Interest
272
The Authors confirm that the content of this manuscript has no conflict of interest.
273
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Highlights Mixed metal oxides nanocomposites Symmetrical Solid Oxide Fuel Cells (SOFCs) Low working temperature Enhanced power density
The contribution of the authors is as follow; 1. Conceptualization, Supervision and Data curation = Zarbad Shah 2. Methodology, Writing original draft, Software and Formal analysis = Kausar Shaheen 3. Funding acquisition, Project administration and Resources = Hongli Suo 4. Investigation, Validation and Visualization = Hussain Gulab 6. Writing - review and editing = Shah Faisal and Muhammad Bilal Hanif
S1293-2558(19)31345-7
DOI:
https://doi.org/10.1016/j.solidstatesciences.2020.106162
Reference:
SSSCIE 106162
To appear in:
Solid State Sciences
Received Date: 18 November 2019 Revised Date:
23 February 2020
Accepted Date: 23 February 2020
Please cite this article as: K. Shaheen, Z. Shah, H. Gulab, M.B. Hanif, H. Suo, Metal oxide nanocomposites as anode and cathode for low temperature solid oxide fuel cell, Solid State Sciences (2020), doi: https://doi.org/10.1016/j.solidstatesciences.2020.106162. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Masson SAS.
Graphical Abstract:
1
Metal Oxide Nanocomposites as Anode and Cathode for Low Temperature Solid Oxide
2
Fuel Cell
3 4
Kausar Shaheena,b,c, Zarbad Shah*d, Hussain Gulabd, Muhammad Bilal Hanife,
5
Hongli Suo*a
6 7
a
8
University of Technology, Beijing-100124, China
9
b
The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing
Department of Physics, University of Peshawar, Peshawar-25120, Khyber Pakhtunkhwa,
10
Pakistan
11
c
12
Khyber Pakhtunkhwa, Pakistan
13
d
14
Pakhtunkhwa, Pakistan
15
e
16
Engineering, Xian Jiaotong University, Xi'an, Shaanxi, China
17
Corresponding author:
18
Zarbad Shah, Assistant Professor, PhD.
19
E-mail: [email protected] and [email protected]
20
Hongli Suo, Professor, PhD.
21
E-mail: [email protected]
22 23 24 25 26 27 28 29 30 31
Department of Physics, Jinnah College for Women, University of Peshawar, Peshawar-25120,
Department of Chemistry, Bacha Khan University Charsadda, Charsadda-24420,
Khyber
State Key Laboratory for Mechanical Behaviour of Materials, School of Materials Science and
32
Abstract
33
The high operating temperature for Solid Oxide Fuel Cells (SOFCs) is one of the major obstacles
34
in the advancement and marketing of the fuel cell technology. Mixed metal oxides Cu0.5Sr0.5
35
(CS) and La0.2 doped Cu0.4Sr0.4 (LCS) nanocomposites, fabricated through a mild and cost
36
effective pechini method were used as an efficient remedy in this context. The nanocomposites
37
were evaluated for phase purity and structural analysis through X-Ray Diffractometer (XRD) and
38
Scanning Electron Microscope (SEM). Particle size calculated was ~37.21nm (for CS) and
39
62.43nm (for LCS) via XRD, while SEM revealed the size ranged from (43-72)nm. Performance
40
of the symmetrical triple layered cells was tested in the temperature range of 500-600oC with H2
41
fuel. LCS nanocomposites exhibited higher electrical conductivity ~4.70S/cm and greater power
42
density ~782mW/cm2 as compared to CS nanocomposites for which the electrical conductivity
43
and power density were achieved as 4.40S/cm and 725mW/cm2 respectively. The activation
44
energy for CS and LCS were found to be 0.23 and 0.26eV respectively. The reliable and
45
enhanced power densities make the synthesized nanocomposites as potential candidates for low
46
temperature SOFCs as anode and cathode.
47
Key words: Mixed metal oxides; nanocomposites; SOFCs; power density.
48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
63
1. Introduction
64
Nanotechnology has attracted much attention of researchers due to utilization of nanostructured
65
materials for manufacturing of energy devices in order to deal with the energy crisis. Rapid
66
depletion of fossil fuel sources has severely increased this need. One of the serious drawbacks
67
related to conventional fossil fuel energy sources is the climate change due to emission of
68
greenhouse gases. Only the renewable and sustainable alternative energy systems can overcome
69
this problem [1]. Fuel cell is one of those sources, which can provide us with energy on
70
demands, where all the other sources such as sun and wind are totally nature dependent [2]. The
71
devices based on fuel cell can efficiently generate electrical energy at the cost of chemical
72
energy without combustion. In fuel cell devices, electric power is usually generated by oxidation
73
at anode and reduction at cathode, while ions are exchanged between anode and cathode via
74
electrolyte solution. The by-products such as CO2 and H2O are also obtained, depending upon
75
the type of fuel utilized during operation [3]. SOFCs are generally more favored as promising
76
advanced technology due to their effective performance, reliability, and environment friendliness
77
[4, 5].
78
The three important parts of SOFCs are cathode (where air is used as oxidant), anode (utilizing
79
any fuel) and an electrolyte (for exchange of ions between electrodes). The most significant part
80
of SOFCs is anode, which not only catalyze the mobility of the ions through electrolyte but also
81
conduct electrons to external circuit and provides a site of oxidation for the fuel [6]. In general
82
both the electrode materials must have greater stability, better ionic and electronic conductivity,
83
homogeneous fine sized nanostructure, higher porosity, and should be inert towards the
84
interconnector or electrolyte materials with the temperature coefficient comparable to that of
85
electrolyte [7]. Materials with small particle size offer less surface reaction resistance and hence
86
the interface adherence between electrode and electrolyte is improved [8]. Due to various
87
drawbacks in the existing conventional materials, there is always a need to explore some new
88
and suitable materials. A major improvement has been made towards the characteristics of
89
electrode materials and a number of advanced materials have also been prepared. Still there are
90
many issues to be addressed before the materials can be practically utilized. For example, the
91
iron based materials usually has the corrosion problem due to formation of iron oxide layers,
92
reducing the stability and hence the long term usage of such anodes. Cobalt was considered as
93
one of the most stable anode material, but the higher cost and carbon deposition hinder its
94
utilization in SOFCs [9]. The most commonly used nickel based anode materials suffer from
95
degradation, blockage of reaction site, mechanical breakage and low tolerance for sulfur based
96
fuels such as H2S [10, 11]. Anode material NiO/YSZ has been successfully used, but its high
97
working temperature such as 800°C, sulphur deposition and squat carbon tolerance limit their
98
inherent use in the operation of SOFCs [12-14].
99
As an alternative, copper based materials are being utilized as cooking resistant anode material
100
[15]. Addition of La2O3 in NiO based anode materials has improved the efficiency and stability
101
of fuel cell by preventing the coarsening of NiO [16]. Similarly, utilization of nanocomposites
102
such as ZnO and CuO is also reported as very effective for the enhanced efficiency of anode
103
materials based on Ni [17, 18]. Various tunable properties and controlled synergistic effects can
104
be achieved by utilizing these nanocomposites as an anode materials for SOFCs [19]. Similarly,
105
in the recent past the materials such as Li-NiO, La-Sr-Mg (LSM), La-Sr-Fe (LSF), La-Sr-Co
106
(LSC), Ba-Sr-Co-Fe (BSCF) and La-Sr-Co-Fe (LSCF) were frequently used as cathode in the
107
fabrication of SOFCs [20-24]. However, there is still a need to explore materials well suited,
108
because a very few materials such as ZnO/NiO, NiO, LiNiZn-oxides can be used for this purpose
109
[5-10].
110
Semiconductor metal oxides have also received special attention in various fields of science and
111
technology, such as piezoelectricity, conducting films and catalysis. Co-doping of
112
semiconductors with various ions such as Al, Bi, Mn, Co, Cu, and Sr have improved their
113
electrical properties. Low operating temperature ~650°C and easy synthesis techniques have
114
made the composites containing semiconductors and metal oxides more attractive [25-27].
115
Previously, ZnO (doped) [14], NiO/ZnO nanocomposites [28], trimetallic oxides Cu-NiO-ZnO
116
and LiNi0.8Co0.2O2 have been successfully used in low temperature SOFCs [29, 30]. For the
117
efficient performance of SOFCs at low temperature, the co-doping of metals and semiconductors
118
is one of the best and effective techniques. Grain boundary, grain size, long term mechanical and
119
chemical stability, porosity and electro-catalytic efficiency are the major components affecting
120
the conductivity of doped metals and making them well suited for electrochemical reactions [31].
121
Hence, the field of nano-engineering to develop nanocomposites with enlarged surface area,
122
active interface and improved performance for SOFCs are considered more advantageous as
123
compared to microscale technology [32, 33]. Hence, it is expected to obtain the improved
124
efficiency and stability of SOFCs through multi-oxide nanocomposite electrode materials.
125
In the current work, two nanostructured materials CS and LCS were synthesized via pechini
126
method in order to validate the improved electrochemical performance for SOFCs. XRD and
127
SEM were utilized for phase and microstructural analysis. Electronic conductivity and
128
impedance measurements were performed through four probe method and electrochemical
129
impedance spectrometer. Triple layered symmetric fuel cells were tested for open circuit voltage
130
(OCV) and power density through fuel testing instrument (L-43). For this purpose,
131
La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) material was employed as an electrolyte [34-36]. The crystal
132
phase, microstructure and electrical performance evaluated in detail suggested the fabricated
133
nanocomposites as an excellent electrode candidates for SOFCs.
134
2. Experiments
135
2.1. Nanocomposites Preparation
136
Stoichiometric amount of Cu(NO3)2, Sr(NO3)2, La(NO3)3.6H2O and deionized water were used
137
as the starting materials. The mixture was stirred on a hot plate and magnetic stirrer with 200rpm
138
for 1hr at temperature of 80oC and pH was adjusted while agitating the solution. After pH
139
adjustment, the solution was stirred for 3 more hours, until it became transparent. The solution
140
was then heated and evaporated at 180oC till appearance of a gel. The gel was dried at 110oC in
141
oven for 12hrs and then was calcined at 450oC for 1hr. The obtained powder was ball milled in
142
ethanol for 24hrs. Finally, the obtained product was sintered at a temperature of 800oC for 5hrs
143
for phase evolution of the oxides.
144
2.2. Cell Fabrication, Conductivity Setup and Electrochemical Performance
145
The powder was dry pressed into pellets with a pressure of 100MPa having thickness ~1-2mm
146
and diameter ~10mm with LSGM electrolyte as a thin layer sandwiched between anode and
147
electrode. The two configurations CS/LSGM/CS and LCS/LSGM/LCS were used as
148
cathode/electrolyte/anode materials as shown in Fig. 1. The pellets were sintered at 600°C for 2
149
hours. After sintering, they were coated with conducting silver paste and were investigated for
150
SOFCs.
151 152 153
154
Fig 1: Schematic diagram for SOFCs.
155 156
Electrical conductivity was measured by utilizing the following mathematical Eq. 1 [37].
157
Eq. 1
158
Where, L, R and A represent the dimensions of the pellet. Similarly, activation energy (Ea) was
159
calculated through Eq. 2, by utilizing the parameters such as conductance (σ), exponential factor
160
(σ0), constant (R) and temperature (T) [38]. exp
161
Eq. 2
162
L-43 testing instrumentation was utilized for electrochemical performance of the synthesized
163
cell. Fuel utilized was in the form of pure H2 at anode with a flow rate of 100ml/min and air with
164
flow rate of 80ml/min was provided as an oxidant at cathode. The current and OCV values were
165
to obtain I-V and I-P curves and to record the power densities.
166
2.3. Characterization
167
Crystalline phase structures for the as synthesized nanocomposites was determined via XRD
168
(D/MAX 2200-PC). SEM (JSM-5910) operating at 15.0kV was used to observe the morphology
169
and microstructure. A standard four probe DC method was followed to calculate electrical
170
conductance at various temperatures, by utilizing a keithley 2400 source meter. Impedance
171
spectra were investigated by utilizing electrochemical spectroscopy (EIS) at a temperature of
172
650oC, with voltage fixed at 10mV. Electrochemical performance of LSGM supported
173
symmetrical cells was investigated by testing machine loaded with variable resistance in a
174
temperature range of 500-600°C, utilizing H2 as a fuel, whereas, the ambient air was used as an
175
oxidant.
176 177
178 179
3. Experimental Results and Discussion
180
3.1. XRD and SEM
181
Phase structure of synthesized nanocomposites was carried out by XRD patterns as depicted in
182
Fig. 2(a, b). The developed peaks in XRD pattern were indicating the desired phases of CuO,
183
SrO and La2O3 according to PDF#72-0629, PDF#06-0520 and PDF#84-2041 respectively. Some
184
additional peaks for SrCO3 and Sr(OH)2 were also indicated at 2θ values of 25.2o, 25.8o and
185
46.6o. These extra peaks may be attributed towards the impurities present due to precursors or the
186
hygroscopic reaction of SrO with atmospheric H2O and CO2.
187
SEM micrographs for CS and LCS nanocomposites revealed non homogeneous distribution of
188
porous particles as depicted in Fig. 2 (c, d). Whereas, SEM of cell sintered with the configuration
189
as anode/electrolyte/cathode and after cell testing is shown in Fig. 2 (e-h).
190
Particle size calculated from XRD patterns by utilizing Scherer's formula (Eq. 3) [39] was 37.21
191
and 62.43nm, whereas the size obtained via SEM was ranged from 43-60 and 63-72nm for CS
192
and LCS nanocomposites respectively.
193 194 195
196
197
0.89
Eq. 3
Where s is particle size, λ is wavelength and β is FWHM (full width at half maximum).
198
199 200 201 202
Fig 2: (a, b). XRD patterns and SEM micrographs of (c, d). Sintered nanocomposites, (e, f). Sintered cells and (g, h). After fuel cell testing. 3.2. Conductivity and Activation Energies of CS and LCS Nanocomposites
203
Electrical conductivity for CS and LCS nanocomposites at a temperature ranged from 400-650oC
204
is shown in Fig. 3a. Direct relationship between conductivity and temperature was revealed with
205
maximum conductivity ~4.40 and 4.70S/cm for CS and LCS nanocomposites respectively.
206
Previously, it is reported that the materials with electrical conductivity greater than 1S/cm is
207
suitable for effective performance of fuel cells. Therefore, the much higher values of
208
conductivity for CS and LCS nanocomposites in the present study make them well suited for
209
better efficiency [40, 41]. LCS nanocomposites were reported with greater value of conductivity
210
as compared to CS, which may be attributed towards the doping of La element. Arrhenius plot
211
and linear fit of Arrhenius plot for CS and LCS nanocomposites is shown in Fig. 3(b, c).
212
Semiconducting behaviour was exhibited by both the nanocomposites as is depicted by the
213
corresponding plots. Low activation energies ~0.23 and 0.26eV were calculated from the linear
214
fitting technique for CS and LCS nanocomposites respectively. These low values of activation
215
energies are favorable for effective utilization of these materials as an electrode, as has been
216
reported previously as well [42].
217 218 219
Fig 3: (a). Electrical conductivity and (b, c). Arrhenius plots for CS and LCS nanocomposites. 3.3. Impedance Measurement
220
Impedance spectra for CS and LCS nanocomposites at a temperature of 650°C were obtained in
221
the shapes of two semi circles (i.e. larger and smaller) by plotting the real part of impedance
222
against the imaginary part. The two semi circles were indicating the complex impedance plane
223
format, Z* (=Zʹ - jZʹʹ, j2= -1) attributed towards two distinguishable electro-active regions within
224
the nanocomposites. Larger semi-circle revealed electronic conduction (due to bulk/grain effect),
225
while small semicircle represented ionic behaviour (due to grain boundary effect) [43-48]. An
226
equivalent fitted circuit was also obtained after simulation of the experimental curve as shown in
227
the inset of Fig. 4 (a, b).
228 229 230
Fig 4: Impedance spectra for the synthesized nanocomposites. 3.4. Electrochemical Performance
231
Electrochemical performance including voltage of cell, power density and current density was
232
measured by a cell consisting of three layers, CS/LSGM/CS and LCS/LSGM/LCS at temperature
233
ranged from 500-600°C. Dry H2 oxidant by ambient air was utilized as fuel. Maximum OCV was
234
measured as 1.02, 1.13V and maximum power densities were calculated as 725 and 782mWcm-2
235
at a temperature of 600oC for CS and LCS nanocomposites respectively, as shown in Fig. 5(a, b).
236
Numerical data about the synthesized nanocomposites is summarized in Table I.
Table I: Numerical data for the synthesized nanocomposites.
237
Sample CS LCS
Composition Configuration Cu0.5Sr0.5 CS/LSGM/CS La0.2Cu0.4Sr0.4 LCS/LSGM/LCS
Conductivity 4.40 S/cm 4.70 S/cm
Activation energy 0.23eV 0.26eV
Power density 725mW/cm2 782mW/cm2
238 239
The values for electrochemical performance is much higher than the already reported materials
240
as described in Table II.
241 242
Fig 5: Electrochemical efficiency of symmetric cell (a). CS/LSGM/CS, (b). LCS/LSGM/LCS.
243
Table II: Comparison of electrochemical performance between CS/LSGM/CS and
244
LCS/LSGM/LSC to the reported data.
245 246 247 248 249 250 251 252
(Anode/Electrolyte/Cathode) BCFZ/NKCDC/LSCZ AMNZ/GDC/BSCF NOCO/Y2O3-SDC4/NiOY2O3-DC ATZ/SDC/BSCF LCNZ/BGC/LCNZ ANZ-GDC/GDC/BSCF ATZN-SDC/SDC/BSCF CS/LSGM/CS LCS/LSGM/LCS
Temp Power density 550oC 334mW/cm2 550oC 535mW/cm2 580oC 750mW/cm2 650oC 354mW/cm2 650oC 375mW/cm2 550oC 705mW/cm2 650oC 370mW/cm2 600oC 725mW/cm2 600oC 782mW/cm2
Ref. 49 50 51 52 53 54 55 This work This work
253 254
4. Conclusion
255
In this study, two mixed metal oxide nanocomposites CS and LCS were synthesized through
256
pechini method for low temperature SOFCs. The enhanced crystallinity and porous structure of
257
the nanocomposites favored their improved electrochemical properties. Symmetrical cells with
258
specified configuration such as CS/LSGM/CS and LCS/LSGM/LCS were applied for cell
259
performance at temperature ranged from 500-600oC. Maximum power density ~782mWcm-2 was
260
achieved at 600oC for LCS. Electrical conductivity were measured as 4.40 and 4.70Scm-1 for CS
261
and LCS nanocomposites respectively. Based on all these preliminary results, the synthesized
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nanocomposites can be effectively used for low temperature SOFCs.
263
Acknowledgment
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The authors are highly grateful to National Natural Science Foundation of China (51571002),
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Beijing Natural Science Foundation (2172008), Program of Beijing City and Beijing University
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of Technology, Evaluation Research for the Performance of Tapes (GH-201809CG005), General
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Program of Science and Technology, Development Project of Beijing Municipal Education
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Commission of China (No. KM201810005010), Project of Advanced Discipline (No. PXM2019-
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014204-500031) and the Department of Chemistry Bacha Khan University Charsadda-24420,
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Charsadda Khyber Pakhtunkhwa Pakistan.
271
Conflict of Interest
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The Authors confirm that the content of this manuscript has no conflict of interest.
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Highlights Mixed metal oxides nanocomposites Symmetrical Solid Oxide Fuel Cells (SOFCs) Low working temperature Enhanced power density
The contribution of the authors is as follow; 1. Conceptualization, Supervision and Data curation = Zarbad Shah 2. Methodology, Writing original draft, Software and Formal analysis = Kausar Shaheen 3. Funding acquisition, Project administration and Resources = Hongli Suo 4. Investigation, Validation and Visualization = Hussain Gulab 6. Writing - review and editing = Shah Faisal and Muhammad Bilal Hanif