Journal of Scientific & Industrial Research
Vol. 74, April 2015, pp. 202-208
A Comparative Study on Electrochemical Behaviour of Co3O4 and Co3O4MWCNTs for Supercapacitors
K Chaitra1, V R Thomas1, M S Santosh1, C Srivastava2, N Nagaraju3 and H Kathyayini1*
1
Department of Nanobiosciences, Centre for Emerging Technologies, Jain Global Campus, Jain University, Jakkasandra post,
Kanakapura Taluk, Ramanagara District, Bangalore Rural-562112, Karnataka, India
2
Department of Material Engineering, Indian Institute of Science, Bangalore-560012, Karnataka, India
3
Department of Chemistry, St. Joseph’s College P.G. Centre 46, Langford Road, Shanthinagar, Bangalore-560027, Karnataka India
Received 18 October 2014; revised 29 January 2015; accepted 3 March 2015
Co3O4 and Co3O4/MWCNTs were prepared by hydrothermal process under autogenous pressure in Teflon lined autoclave
and calcined at 2500C. Both samples were characterized by PXRD, FT-IR, SEM-EDS, TEM & FT-Raman to evaluate their
surface and bulk properties. The PXRD pattern of the materials indicated the formation of cubic phase of Co 3O4. FT-IR
results showed the presence of metal oxygen bond in the samples. The SEM and TEM images of the Co 3O4 / MWCNTs
indicated spherical and cubic aggregates of metal oxide particles (10-30 nm) decorated both on the surface and inside the
tubes of carbon nanotubes. The characteristic Ig and Id (graphitic and defects) Raman bands indicated the retention of
tubular structure of MWCNTs even after the deposition of Co 3O4. The calcined Co3O4-MWCNTs composites and Co3O4
exhibited specific capacitance of 284 & 205 F/g at a sweep rate of 2mVs-1 in 6M KOH by cyclic voltammetry.
The psuedocapacitance performances of calcined Co3O4-MWCNTs were found to be better than Co3O4.
Chronopotentiometric studies made for the materials at a current density of 500mA/g indicated 100% columbic efficiency at
2000th cycle for Co3O4 / MWCNTs which is a better electrode material than Co3O4.
Keywords: Multiwalled carbon nanotubes, cyclic voltammetry, chronopotentiometry, columbic efficiency.
Introduction
Supercapacitors are the chief energy storage
devices which are in demand as there is energy crisis
in the present world. As supercapacitors posses’ high
power density and longer cyclability, these devices
find application in electronics, hybrid electric vehicles
etc. 1, 2 Supercapacitors are divided into two classes
based on their energy storage mechanism.
Electrochemical double layer capacitors (EDLCs) are
the first type of supercapacitors using carbon
materials such as activated carbon, graphene, carbon
nanotubes, mesoporous carbon etc. The storage
mechanism here is purely electrostatic and depends on
the surface area3-5.The other is pseudocapacitance that
includes metal oxides and conducting polymers, in
which the capacitance arises due to the redox
reactions and movement of π electrons6,7. Though the
EDLC materials offer very high cycle life they do not
offer high capacitance when compared to the pseudo
capacitor materials and vice versa. Hence, a
——————
Author for correspondence
E-mail: nkathyayini45@gmail.com
composite of both class of materials give an
opportunity to utilize their advantages 8, 9. Among
EDLC materials MWCNTs are interesting as they
posses high accessible surface area, low
electrochemical resistivity (ESI) and when compared
to conventional carbon materials they allow easy
diffusion of ions because of the open mesoporous
network10. E. Frackowiak and K.Metenier have
studied the electrochemical properties of MWCNTs
and have reported a maximum capacitance of
135 F/g 11. Also, in a comparison study done by Seyed
Hamed Aboutalebi and Alfred.T. Chidembo for
Graphene oxide, MWCNTs and their composite, they
found that MWCNTs exhibited a specific capacitance
of 85 F/g only 12. Metal oxides are studied as
electrode materials because of their variable oxidation
states, proficient redox charge transfer and high
pseudocapacitance. Though noble metal oxides such
as RuO2, IrO2 exhibit good pseudocapacitance, due to
their high cost and toxicity, now the focus is on the
oxides of base transition metals such as Co, Ni, Cu,
Fe etc, which are also known to exhibit good
electrochemical properties13. Scientists have realized
KATHYAYINI et al.: A COMPARATIVE STUDY ON ELECTROCHEMICAL BEHAVIOUR OF CO3O4 & MWCNTs
the importance of Co3O4 as supercapacitor material as
it is inexpensive and posse’s high redox activity.
Shenglin Xiong and Changzhou Yuan have obtained a
capacitance value of 92 F/g for mesoporous Co3O4
nanostructures14. Wang et al. have prepared
mesoporous Co3O4 crater-like microspheres with a
specific capacitance of 102F/g15. Hence Co3O4 and
MWCNTs composite is the expected or predicted to
be a better supercapacitor material. For instance, C. T.
Hsieh and J.Y. Lin have reported a capacitance value
of 185F/g (Co3O4-MWCNTs) which was found to be
much higher than the oxidized MWCNTs (83.5 F/g) 16.
Lian Gao and Yan Shan have prepared Co3O4MWCNTs composites by in situ decomposition
of Co(NO3)2 and obtained specific capacitance of
200F/g and only 90 F/g for pristine MWCNTs17.
Mazloumi et al. have studied the electrochemical
properties of aligned carbon nanotubes-Co3O4 strips
and have obtained a specific capacitance value of
123 F/g 18.Hence it is interesting to study the
electrochemical behavior of composite of Co3O4 with
MWCNTs. In the present work, we have synthesized
Co3O4 nanoparticles and it’s composite with
MWCNTs by hydrothermal method followed by
calcination. Characterizations of the materials were
carried out to understand their structural and physical
properties. The electrochemical performance of
these materials in 6M KOH was studied by cyclic
voltammetry, chronopotentiometry and electrochemical
impedance techniques.
Experimental
Materials
All the reagents employed in this research work are
of analytical grade. MWCNTs used in this work were
purchased from Nanocyl Belgium. Cobalt nitrate
hexahydrate, Sodium dodecyl sulphate (SDS),
Ammonia solution, Sulphuric acid (H2SO4), Nitric
acid (HNO3), Hydrogen peroxide (H2O2) were
obtained from Merck Specialties Pvt. Ltd and used as
received.
Method
Synthesis of Co3O4 and Co3O4/MWCNTs
MWCNTs were functionalized using 1:3
HNO3:H2SO4 by sonicating at a frequency of 42 kHz
and 250 watts power for 4 h to develop oxygen
functionalities on its surface19. Further, 0.1g of
functionalized MWCNTs was sonicated in distilled
water for 2h. 0.15M solution of cobalt nitrate
203
hexahydrate was mixed with 10ml of SDS (0.01g)
solution. The resulting solution was added to the
dispersed MWCNTs suspension and further sonicated
for 30 min, followed by slow addition of ammonia
solution (2.5 wt %) with stirring. Subsequently
0.27ml of H2O2 was added and stirred for 5 min. The
above solution was transferred to an autoclave
maintained at 160oC for 5h. The reaction mixture was
cooled, filtered and washed with distilled water; the
resulting precipitate was dried at 100oC for overnight
and was calcined at 2500 C for 3h to get the final
product. For comparison of electrochemical
properties, Co3O4 without MWCNTs was also
prepared by following the same procedure. In this
paper the two materials, Co3O4 and Co3O4/MWCNTs
are denoted as Co-MO and Co-CNTs respectively.
Characterization
The morphology and the surface properties of CoMO and Co-CNTs were studied by the following
techniques. P-XRD was recorded by using Panalytical
Xpert pro X-ray diffractometer with Cu Kα radiation
(λ = 0.154 nm) in 2θ range 5o to 70o at 40 kV and a
scanning rate of 2omin-1, FT-IR spectra was obtained
from Nicolet Model Impact 400D FT-IR spectrometer
with 4 cm–1 resolution using KBr pellet technique,
SEM-EDS for the samples was carried out for
elemental composition by Quanta 200 FEI instrument.
Transmission electron microscopy (TEM) images
were recorded using a Tecnai 10 philips microscope.
Raman spectra were recorded using Bruker
instrument.
Fabrication of Electrode and Electrochemical Measurements
The working electrode was fabricated by mixing
85 wt% of the material with 10 wt% ketjen black EC
600JD in an agate mortar. To this mixture, a binder,
5 wt% polyvinylidene difluoride (PVDF) dissolved in
solvent 1-methyl-2- pyrrolidinone (NMP) was mixed
well and the slurry prepared was coated on a
pretreated battery grade Ni foil of area 1cm2 and
0.2mm thickness. Further the working electrode was
vacuum dried at 70oC for 8h. In this study the active
mass of sample Co-MO and Co-CNTs coated on Ni
foil was 1.3mg and 0.4mg respectively.
Electrochemical studies such as cyclic voltammetry,
chonopotentiometry and electrochemical impedance
spectroscopy (EIS) were performed using CHI608E
electrochemical workstation. The materials coated on
Ni foil was used as working electrode, Pt wire as
counter electrode, Ag/AgCl as reference electrode and
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J SCI IND RES VOL 74 APRIL 2015
deoxygenated aqueous 6M KOH as the electrolyte.
The stability of the working electrode materials were
checked from galvanostatic charge-discharge cycles
upto 2000cycles. AC impedance studies were made
by imposing a sinusoidal alternating frequency from
0.01 Hz to 105 Hz, alternating current (ac) amplitude
of 5mV at open circuit potential.
Results and Discussion
Characterization of the materials
P-XRD is an important tool for the structural
characterization of composite materials. The x-ray
diffraction patterns for Co-MO and Co-CNTs are
given in Figure 1. The diffraction peaks at 2Ө values
18.9o, 31.4o, 36.8o, 38.7o, 44.9o, 55.9o, 59.6o, 65.4o
obtained in Co-MO corresponds to the different hkl
planes (111) (220) (311) (222) (400) (422) (511)
(440) of cubic phase of Co3O4 respectively (JCPDS
Card No. 43-1003) 20.The main P-XRD diffraction
peaks at 31.2o, 36.8o, 65.2o corresponding to (220)
(311) (440) planes are the clear indication of cubic
spinel structure of Co3O4 as described by Shin et al.21.
The XRD pattern of functionalized MWCNTs
contains prominent diffraction peaks at 2Ө value
25.8 o, 43 o and 53.5 o which corresponds to (002)
(100) and (004) hkl planes of graphite22,23.Co-CNTs
exhibited prominent diffraction peaks for Co3O4 and
diffraction peaks of MWCNTs were not noticeable
due to Co3O4 particles which are deposited on
MWCNTs. Further from TEM and SEM analysis of
Co-CNTs reveals the formation of Co3O4 on
MWCNTs with the particle size ranging from
10-30nm. In FT-IR spectrum of Co-CNTs, a broad
peak at 3426cm-1 is due to the stretching vibrations of
OH– groups of H2O molecules24. The two distinct
bands in Co-CNTs present at 673 cm-1 and 579 cm-1 is
due to the stretching vibrations of metal-oxygen bond.
The peak at 673 cm-1 is attributed to the M–O
vibrations in which M is Co2+ and is tetrahedral
coordinated, while the peak at 579 cm-1 is due to the
stretching vibration mode of M–O in which M is Co3+
and is octahedral coordinated indicating the formation
of Co3O4 oxide 25. (FT-IR spectrum given as
supplementary data) SEM-EDS analysis of Co-CNTs
was carried out to determine its surface morphology
and elemental composition. The SEM image of
Co-CNTs shows more or less uniform spherical
Co3O4 nanoparticles deposited on the micro tubular
structured MWCNTs. The elemental composition of
the material indicates the presence of Co, O and C in
Co-CNTs. Further insight on the morphology of the
Co-CNTs is obtained from the TEM analysis.
It clearly shows the presence of metal oxide
nanoparticles on the surface and inside the walls of
MWCNTs. It is also observed that the metal oxide
nanoparticles are in the form of spherical as well
as cubic aggregates with a diameter ranging from
10-30 nm. (SEM, TEM images and EDS profile given
as supplementary data) Raman spectra of
functionalized CNTs and Co-CNTs were studied.
The three characteristic bands obtained for these
materials at 1285cm-1, 1600 cm-1, 2565cm-1 are
attributed to D- band, G- band and G’ band
respectively. The G band in MWCNTs represent the
in plane vibration of C-C bonds and the D band
represents hydridized vibrational mode of graphene
edges indicating the disorder of carbon atoms in
MWCNTs 26. The disorder density on the MWCNTs
sidewall is measured from the ratio of ID/IG 27. ID/IG
ratio was found to be 1.0 and 1.34 in functionalized
MWCNTs and Co-CNTs respectively. The values of
ID/IG indicated that there is greater extent of disruption
of the graphitic planes of MWCNTs in Co-CNTs. The
raman spectra of Co-CNTs and Co-MO was also
compared. The raman bands for Co-O bond in
Co-MO at 479, 620 and 695 cm-1 28 was observed with
more intensity while the same appeared in Co-CNTs
with slightly lower intensity. Thus these
characterization analyses clearly indicate the presence
of Co3O4 in MWCNTs.
Electrochemical studies
Cyclic Voltammetry
Fig.1—PXRD pattern of Co-MO and Co-CNTs
Cyclic voltammetry (CV) measurements of Co-MO
and Co-CNTs were studied at different sweep rates in
KATHYAYINI et al.: A COMPARATIVE STUDY ON ELECTROCHEMICAL BEHAVIOUR OF CO3O4 & MWCNTs
205
the fixed potential range of 0 - 0.45V (Vs Ag/AgCl
electrode) in 6M KOH solution. Initially the
electrodes were stabilized at a fixed potential range
for 25 CV cycles at a scan rate of 5mV s-1. The CV
patterns obtained for the Co-MO and Co-CNTs
exhibited mainly redox peaks indicating faradic
behavior. The multiple anodic (oxidation) and
cathodic (reduction) peaks in the CV patterns as
shown in Figure 2a is attributed to different phases of
cobalt oxide having different oxidation states. These
peaks are due to the Co (II) Co  (III) and Co (III)
Co (IV) redox transitions 29,30. The following are two
plausible reactions.
Co3O4+ OH + H2O  3CoOOH + e3CoOOH + e-  Co3O4 + OH + H2O
The shape of CV curve becomes more asymmetric
with increase in scan rate indicating the kinetic
irreversibility of the electrode material. The graph of
anodic current against square root of scan rate of the
materials showed linear behavior indicating a
diffusion controlled process at the electrode surface
Figure 2b & 2c 31,32. The specific capacitance
(Cs in F/g) values of both samples at different scan
rates (ν, Vs-1) were calculated graphically by
integrating the area under CV patterns using the
equation as described by Ranga Rao et al 33. Co-MO
and Co-CNTs exhibited specific capacitance of
205 and 284 F/g respectively at 2mVs-1. The higher
specific capacitance of Co-CNTs is presumed due to
the presence of MWCNTs, which allows the easy
movement of OH- ions through the electrode material.
It was observed that the specific capacitance values
decreases from lower scan rate to higher, as OH- ions
can only access the outer surface of the electrode
where as at low scan rates the ions can percolate
efficiently through the surface of electrode, in
particularly the MWCNTs which can lead to higher
specific capacitance as observed in Table 1. The
increased specific capacitance of Co-CNTs at lower
scan rate can be due to the presence of MWCNTs
which allows easy percolation of OH- ions and
provide larger surface for the redox reaction to occur.
Chronopotentiometry
The stability and cycle life of the electrode material
is an important parameter for their application in
supercapacitors which was studied by charge
discharge cycles. The charge discharge cycles of these
materials at different current densities 0.2, 0.5, 1, 2.5,
5 and 10 A/g at potential window of 0 – 0.36V in 6M
Fig.2—(a) Cyclic volammetric curve of Co-CNTs at various
sweep rate, (b) and (c) Square root of scan rate Vs anodic peak
current of Co-MO and Co-CNTs respectively.
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J SCI IND RES VOL 74 APRIL 2015
KOH solution was studied. The materials exhibited a
nonlinear charge discharge curve which is primarily
due to cobalt oxide reaction occurring at the electrode
surface indicating psuedocapacitance behavior. The
discharge curves obtained for Co-CNTs are shown in
Figure 3. The specific capacitance values from
chronopotentiometry are determined using the
equation as reported earlier 34. The calculated specific
capacitance values of Co-MO and Co-CNTs at
different current densities are given in Table 2. It was
observed that at higher current densities also CoCNTs exhibit higher specific capacitance. The
specific capacitance decreased with increase in the
applied current density which is attributed to large IR
drop, polarization and insufficient utility of electro
active material at very high current densities 35. The
stability of electrode material was tested for 2000
charge discharge cycles and was found that Co-CNTs
was stable for 2000 complete charge discharge cycles.
But, Co-MO was stable for only 1000 cycles. The
columbic efficiency (η) is calculated from each
charge discharge cycles using the following equation.
Table 1—Specific capacitance of Co-MO & Co-CNTs
at various scan rates
Sl. No. Scan rate (mV/s) Specific capacitance in F/g from CV
measurements
Co-MO
Co-CNTs
1
2
205
284
2
5
197
243
3
10
195
227
4
20
189
206
5
40
182
195
6
50
178
191
7
80
171
185
8
100
163
181
Fig.3—Charge discharge curves obtained for Co-CNTs at
different applied current density
η = tD / tC x100
Where tD and tC represents discharge and charge time
respectively. It was seen that in Co-MO columbic
efficiency was 86% initially which decreased to 80%
after 1000 cycles, while Co-CNTs exhibited 100%
columbic efficiency even at 2000th cycle indicating
the improved efficiency of Co3O4 due to the presence
of MWCNTs in the Co-CNTs. This may be
attributable to the backbone structure of MWCNTs
for the metal oxide and helps in maintaining its
structural stability. Hence Co-CNTs was found to be
an excellent material in a life cycle test, when
compared to Co-Mo.
Electrochemical Impedance spectroscopy
AC impedance studies were carried out to study the
frequency dependent specific capacitance behavior.
The impedance study of the two compounds was
carried out at their open circuit potentials in the
frequency range of 0.01Hz to 10e+5 Hz. The specific
capacitance of the materials was calculated from the
experimental data36.The obtained specific capacitance
of Co-MO & Co-CNTs was 160F/g and 240 F/g
respectively Figure 4. The Co-CNTs showed better
Table 2—Specific capacitance of Co-MO & Co-CNTs at various
current densities
Sl. No
Current density
(A/g)
1.
2.
3.
4.
5.
0.2
0.5
1
2.5
5
Specific capacitance in F/g from CP
measurements
Co-MO
Co-CNTs
86
129
88
114
83
104
81
93
72
86
Fig.4—Plot of Specific capacitance obtained from impedance data
for Co-MO and Co-CNTs
KATHYAYINI et al.: A COMPARATIVE STUDY ON ELECTROCHEMICAL BEHAVIOUR OF CO3O4 & MWCNTs
specific capacitance performance by all the above
experiments which could be due to improved mobility
of ions at lower frequency through the tubes of
the MWCNTs. Due to presence of MWCNTs in
Co-CNTs which provide high surface area for redox
reactions to occur at the electrode surface and they
offer low resistance, indicating Co-CNTs a better
material for supercapacitor applications.
Conclusions
In this work, a comparative electrochemical
performance of Co-MO and Co-CNTs prepared by
simple
hydrothermal
method
was
studied.
Characterization studies and results indicated
formation of cubic or spherical Co3O4 with particle
size ranging from 10-30 nm on MWCNTs. The
electrochemical studies showed Co-CNTs exhibiting
100% columbic efficiency even at 2000th cycle and
also showed high specific capacitance in comparison
to Co-MO. This improved performance of Co-CNTs
is owing to the presence of MWCNTs. Hence there is
a significant role of MWCNTs in Co-CNTs in
enhancing the columbic efficiency.
Acknowledgement
The authors thank NRB- Naval Research Board for
the financial support given for this research work.
Project Number: NRB-290/MAT/12-13. Authors also
thank Dr. Krishna Venkatesh, CEO, Centre for
Emerging Technologies and Dr. Chenraj Roychand,
President Jain University trust for their constant
support in encouraging this research work.
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