Facile synthesis of Co3O4 with different morphology and their application in supercapacitors

Article abstract of DOI:10.1039/c4ra17075j

In this article, direct growth of Co₃O₄ with different morphologies on nickel foam is successfully achieved via a simple hydrothermal method by changing the volume ratio between ethanol and water. The morphology and structure of the as-prepared samples are examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The electrochemical performance of the Co₃O₄ electrodes is investigated as pseudocapacitor material by cyclic voltammetry and galvanostatic charge/discharge test in 3 mol L^-1 KOH solution. Results show that the solvent composition plays an important role not only in the morphology but also in the capacitance. Co₃O₄ with a honeycomb structure obtained from the volume ratio of C₂H₅OH/H₂O = 1 exhibits the highest capacitive performance, 2509.4 F g^-1 at 1 A g^-1 and 1754 F g^-1 at 10 A g^-1, which is much larger than that prepared in the pure water and pure ethanol solvent. The electrode also has a satisfactory cycling performance with capacity retention of 74% after 1000 cycles at 10 A g^-1. The enhanced electrochemical performance is ascribed to the honeycomb nanostructure allowing facile electrolyte flow which speeds up electrochemical reaction kinetics. These findings may open up the opportunity for optimizing the hydrothermal synthesis conditions to control the morphology and performance of the products.

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Facile synthesis of Co3O4 with different
morphology and their application in
supercapacitors
Cite this: DOI: 10.1039/x0xx00000x
Tian Ouyang, Kui Cheng*, Shuying Kong, Ke Ye, Yinyi Gao, Dingfu Zhang,
Guiling Wang, Dianxue Cao *
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
In this article, Co3O4 with different morphologies direct growth on nickel foam is successfully
synthesized via a simple hydrothermal method by changing the volume ratio between ethanol and
water. The morphology and structure of the asꢀprepared samples are examined by Scanning
electron microscopy, Transmission electron microscopy, Xꢀray diffractometer and Fourier
transform infrared spectroscopy. The electrochemical performances of the Co3O4 electrodes are
investigated as pseudocapacitor material by cyclic voltammetry and galvanostatic
charge/discharge test in 3 mol Lꢀ1 KOH solution. Results show that the solvent composition plays
an important role not only in the morphology but also in the capacitance. Co3O4 with a
honeycomb structure obtained from the volume ratio of C2H5OH/H2O=1 exhibits the highest
capacitive performance, 2509.4 F gꢀ1 at 1 A gꢀ1 and 1754 F gꢀ1 at 10 A gꢀ1, which is much larger
than that prepared in the pure water and pure ethanol solvent. The electrode also has a
satisfactory cycling performance with capacity retention of 74 % after 1000 cycles at 10 A gꢀ1.
The enhanced electrochemical performances are ascribed to the honeycomb nanostructure
allowing facile electrolyte to flow which fastens electrochemical reaction kinetics. These findings
may open up the opportunity on optimizing the hydrothermal synthesis conditions to control the
morphology and performance of the products.
www.rsc.org/
based pseudocapacitors mainly depends on properties such as
Introduction
surface area, morphology, and specific orientation of different
facets. Therefore, the control over the morphology of
nanomaterials with wellꢀdefined shape and uniform size
remains an important goal of modern synthetic chemistry,
especially nanomaterials direct growth on conductive substrate.
In comparison to the conventional Co3O4/carbon/PTFE
electrode18ꢀ21, oneꢀdimensional (1D) Co3O4 materials grow on
currentꢀcollecting substrates could enhance the power density
(or rate capability) and cycling stability since this structure not
only have the ability to increase the contact area between the
electrolyte and active material but also provide accessible
channels that facilitate mass and charge transport22ꢀ24. In order
to solve this issue, great efforts have been devoted to creating
nanostructured materials with large surface area to enhance the
kinetics of ion and electron transport and also shorten the
diffusion path of ions and electrons25.
Electrochemical capacitors, also known as supercapacitors or
ultracapacitors, have drawn an increasing attention as the most
promising candidates for nextꢀgeneration energy storage
devices due to their high power densities, fast chargeꢀdischarge
processes, long cycling life, and safe operation1, 2. According to
the different energy storage mechanisms, electrochemical
supercapacitors are divided into two types. One is electrical
doubleꢀlayer capacitors (EDLCs) based on carbonaceous
materials which store/deliver energy by nonꢀfaradaic surface
interactions, leading to higher power density and long cycling
life but low specific capacitance3, 4. The other one is
pseudocapacitors based on conducting polymers and transition
metal oxides/hydroxide, where charge is stored/delivered via a
fast and reversible surface faradic redox reaction5, 6. Despite the
specific capacitance of pseudocapacitors is several times larger
than EDLCs, pseudocapacitive materials still present
a
There are several methods used to prepare cobalt oxides
with the controlled morphology, however, most reports focus
on the use of surfactant or template, which greatly increased the
complicated process to remove these foreign substance. As a
facile and effective synthesis method, hydrothermal is widely
used for the preparation of Co3O4 nanomaterials and various
structures have been obtained by changing the temperature,
reaction time or reagents8ꢀ11. However, a very few reports have
systematically investigated the effects of solvents composition
on the morphology and electrochemical performance of Co3O4.
Therefore, in this work, Co3O4 with different morphology has
been successfully synthesized through adding different volume
ratio ethanol. The obtained Co3O4 electrode exhibited a unique
compromise between the power performance and reversibility
because of the fast faradic reaction leading to a high demand
for the electron transport and fast ion transport7. Therefore, a lot
of research works have been done in the past few years to
improve the performance of the electrochemical capacitor
electrode.
Among these pseudocapacitive materials, Co3O4 is
considered to be one of the most attractive materials owing to
its lower toxicity, high theoretical specific capacitance (3560 F
gꢀ1), good capability retention and efficient redox charge
transfer8ꢀ13. Thus, various morphology of Co3O4 has been
reported, such as nanoparticles14, nanosheets15, nanowires16,
nanotubes17 and nanoflowers18 etc. It has been found that Co3O4
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3D hierarchical structure, which delivered large specific channels. As it shown in Figure 1a and b, EꢀCo3O4 sample
capacitances and good electrochemical stability at high rates. exhibits the sheetꢀlike structure. The nanosheets interconnected
This work demonstrate that solvents seem to play a crucial role with each other, creating loose porous nanoꢀstructures with
in shaping the morphology of Co3O4 crystals, which could be abundant open space and electroꢀactive surface sites, however,
used to effectively control their structural organization and after calcined, the nanosheets have melted and bonded together,
further improve the capacitive performance.
Experimental
Synthetic Methods
The formation process of Ni foam supported Co3O4 electrodes
were based on a hydrothermal methods followed by an
annealing process. The experimental detail was as follows: The
Ni foam was pretreated by degreasing with acetone, etching
with 6.0 mol Lꢀ1 HCl for 15 min, rinsing with water, prior to
use. In a typical procedure, 4 mmol Co(NO)3 and 10 mmol
CO(NH2)2 were added to 24 mL C2H5OH/H2O mixed solvent in
a volume ratio of 1:0, 1:1 and 0:1 with vigorous stirring. After
stirring for 15 min, the mixture was then transferred into a 30
mL Teflonꢀlined stainless steel autoclave. Afterwards, a piece
of Ni foam was put into the autoclave and immersed in the
solution. The autoclave was heated gradually to 90 °C and
maintained at this temperature for 10 h to allow the growth of
Co3O4. After that, the autoclave was cooled down to room
temperature naturally, then, the Ni foam was removed from the
growth solution and rinsed with distilled water several times
before drying at 60 °C in an oven. Finally, the asꢀprepared
samples were converted to Co3O4 via thermal decomposition at
300 °C for 3 h in air.
Materials Characterization
The electrode morphology was examined by a scanning
electron Microscope (SEM, JEOL JSMꢀ6480) and
a
transmission electron microscope (TEM, FEI Teccai G2SꢀTwin,
Philips). The structure was analyzed using an Xꢀray
diffractometer (Rigaku TTR III) with Cu Kα radiation.
Chemical bonding information on metalꢀoxygen and precursor
was investigated with Fourier transform infrared spectroscopy
(FTIR, Equinos55, Bruker) using the potassium bromide pellet
technique.
Figure 1 SEM images of EꢀCo3O4 before (a) and after (b)
calcined, EWꢀCo3O4 before (c) and after (d) calcined, and Wꢀ
Co3O4 before (e) and after (f) calcined. Insert is the highꢀ
magnified SEM images of the asꢀprepared samples after
calcined.
and cracks appeared on the surface. According to Figure 1c and
d, the morphology of EWꢀCo3O4 sample remained no obvious
changes before and after calcination, which shows
honeycombꢀlike structure consists of dense nanowires and
nanosheets attached to each other. For the sample prepared in
pure deionized water (WꢀCo3O4), only nanowire structure was
grown on the Ni foam (Figure 1e and f).
The TEM images of the asꢀprepared Co3O4 samples before
and after calcined are demonstrated in Figure 2, which provide
an insight of individual structures. The TEM images of Eꢀ
Co3O4 (Figure 2a) and WꢀCo3O4 (Figure 2c) samples confirm
its assembly as a sheetꢀlike structure and only nanowireꢀlike,
respectively. While for the sample EWꢀCo3O4 (Figure 1c), it
can be clearly observed that the sample is composed of
nanowire and nanosheet. However, all the three Co3O4 samples
have an obvious change in the morphology that previously solid
structure acquired after hydrothermal were converted to a
porous structure composed of interconnected nanoparticles with
about 20 nm in diameter after calcination. Those porous
structures could enhance the accessible channels with the
electroꢀactive species, further facilitating mass and charge
transport.
Electrochemical measurements
The electrochemical measurements were carried out in a threeꢀ
electrode electrochemical cell containing 3 mol Lꢀ1 KOH
aqueous solution as the electrolyte, the prepared electrode as
the working electrode, a saturated calomel electrode (SCE) as
reference electrode and a Pt foil as counterꢀelectrode. The
cyclic voltammetry (CV) studies were performed on an
electrochemical workstation (VMP3, BioLogic, France).
Galvanostatic chargeꢀdischarge test was conducted using
computer controlled cycling equipment (LAND, Wuhan China).
All solutions were made with analytical grade chemical
reagents and MilliꢀQ water (18 MΩ cm).
a
Results and discussion
By using different solvents composition, three samples with
different morphologies were obtained. In order to distinguish
with each other, the samples prepared using C2H5OH/H2O
volume ratios of 1:0, 1:1 and 0:1 were denoted as EꢀCo3O4,
EWꢀCo3O4 and WꢀCo3O4, respectively. Their morphologies
were detailed characterized by SEM and TEM. Figure 1 shows
the SEM images of asꢀprepared Co3O4 samples before and after
calcined. It can be seen that all the three samples exhibit
relatively uniform, wellꢀdispersed morphology and have its
own characteristic, completely different with each other. It can
be clearly seen from Figure S1 that Ni foam substrate was
completely covered by Co3O4. Ni foam as the substrate for the
direct growth of Co3O4 has many advantages, since that a 3D
network structure with micro open cages and zigzag flow
Figure 3 shows the XRD patterns of asꢀprepared Co3O4
samples before (a) and after (b) annealing, respectively, and the
standard date of spinelꢀphased Co3O4 crystal (JCPDS NO. 43ꢀ
2 | J. Name., 2012, 00, 1-3
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Thus, the precursor of EWꢀCo3O4 was
Co(OOCCH3)2ꢁ4H2O and Co(CO3)0.5(OH)0.11H2O. After heat
treatment, the peaks appeared at 2 =19.0°, 31.3°, 36.8°, 44.8°,
a mixture of
θ
49.1°, 55.7°, 59.4°, 65.2° and 77.3° can be successfully indexed
to (111), (220), (311), (222), (400), (331), (511), (440) and (533)
plane reflections of the spinel Co3O4 phase (JCPDS NO. 43ꢀ
1003), implying that the crystalꢀline Co3O4 has been formed
after annealing treatment (Figure 3b).
Figure 2 TEM images of EꢀCo3O4 before (a) and after (b)
calcined, EWꢀCo3O4 before (c) and after (d) calcined, and Wꢀ
Co3O4 before (e) and after (f) calcined.
1003) is also shown in Figure 3b for comparison. To avoid the
effect of Ni foam, all the samples were examined by the
powders scratched from nickel foam. As shown, all the samples
before the annealing are different and weak crystal. The peak of
WꢀCo3O4 could be assigned to the Co(CO3)0.5(OH)0.11H2O
(JCPDS NO. 48ꢀ0083), which is agree with the literature
reported and the major reactions occurred is as followed26, 27
:
Figure 3 XRD patterns of asꢀprepared EꢀCo3O4, EWꢀCo3O4
and WꢀCo3O4 before (a) and after (b) annealing at 300 °C for 3
h in air, respectively.
CO(NH ) + 4H O → H CO + 2NH ⋅ H O
(1)
(2)
(3)
2 2
2
2
3
3
2
NH3 ⋅ H2O → NH+4 +OHꢀ
H2CO3 → 2H+ +CO32ꢀ
Figure 4a shows the FTIR spectra of the precursor of the three
samples. As can been seen, the 1630 and 1384 cmꢀ1 bands
appeared in both EꢀCo3O4 and EWꢀCo3O4 are the characteristic
stretching vibration of the –COO– stretching vibration.
However, as the CH3CH2OH ratio decreased, the relative
strength of –C=O is decreased, revealing that the amount of –
COOH is decreased in the precursor of EWꢀCo3O4. The peaks
at 1630 and 1384 cmꢀ1 are due to the mode of the –COO–
stretching vibration28. The EWꢀCo3O4 and WꢀCo3O4 precursor
film contain stretching vibrations of νOCO22(91,43909 cmꢀ1) and
(x+y)Co2+ +yCO32− + xOHꢀ + zH2O
(4)
→ xCo(OH)2 ⋅ yCoCO3 ⋅ zH2O
However, when the ethanol was used as solvent, for the sample
EꢀCo3O4, the peak appeared at 12.8° could be assigned to
Co(OOCCH3)2ꢁ4H2O (JCPDS NO. 29ꢀ0465), indicating that
ethanol began to be oxidized during the process of
hydrothermal, further reacted with Co2+. The possible reaction
is as followed:
νCO3ꢀ2(1384 cmꢀ1, 830 cmꢀ1and 746 cmꢀ1)
as well as
vibration bands of νCo–O (517 cmꢀ1) and νCo–OH (960 cmꢀ1)31.
After annealing treatment, the bands of the precursor film
disappeared and two very strong peaks centered at 661 and 565
cmꢀ1characteristic of spinel Co3O4 are noticed, which are
consistent with the XRD result. Combined the result of XRD,
SEM, TEM and FTIR, it can be found that the solvent
CH3CH2OH+O2 → CH3COOH+H2O
(5)
2CH3COOH+Co2+ → (CH3COO)2Co+2H+
(6)
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influences not only the morphology but also the composition of capacitance of nickel foam can be ignored which the peak
precursor.
currents reached less than 2 mA cmꢀ2. Clearly seen that the CV
In order to demonstrate the excellent electrochemical area of EWꢀCo3O4 sample is the largest among the three
properties of the asꢀprepared Co3O4 electrode, we scientifically samples, declares the most ideal capacitive behavior.
evaluated its supercapacitance behavior through CV and
galvanostatic chargeꢀdischarge in 3.0 mol Lꢀ1 KOH aqueous
electrolyte using the conventional threeꢀelectrode configuration
Figure 5 Cyclic voltammograms of EꢀCo3O4 (a), EWꢀCo3O4 (b)
and WꢀCo3O4 (c) in 3.0 mol Lꢀ1 KOH solution at different scan
rates, respectively. (d) The Cyclic voltammograms comparision
of EꢀCo3O4, EWꢀCo3O4 WꢀCo3O4 and Ni foam after the
solvothermal process in 3.0 mol Lꢀ1 KOH at a 10 mV s−1 scan
rate.
Figure 6 shows the discharge curves of EꢀCo3O4 (a), EWꢀCo3O4
(b) and WꢀCo3O4 (c) in 3.0 mol Lꢀ1 KOH solution at different
current densities. The specific capacitances were calculated
according to the following equation:
C
=(i × ꢀt) / (ꢀV × m)
(7)
m
where Cm (F gꢀ1) is the specific capacitance,
discharge current, ꢂ (s) is the discharge time, ꢂ
i
V
(A) is the
(V) is the
t
Figure 4 FTꢀIR spectra of asꢀprepared EꢀCo3O4, EWꢀCo3O4
and WꢀCo3O4 before (a) and after (b) annealing, respectively.
with a Pt plate counter electrode and a SCE reference electrode.
Figure 5 shows the typical CVs of EꢀCo3O4 (a), EWꢀCo3O4 (b)
and WꢀCo3O4 (c) in 3.0 mol Lꢀ1 KOH solution with various
sweep rates, respectively. The CV curves of the three Co3O4
with different morphology show two oxidation/reduction peaks
in the potential range of ꢀ0.5 to 0.65 V. The peaks can be
respectively attributed to the transition of the redox couple
Co(II)/Co(III) and Co(III)/Co(IV)32, revealing their pseudoꢀ
capacitive characteristics. With the increase of the scan rate, the
redox current density increased and the anodic and cathodic
peaks shifted toward higher and lower potentials, respectively,
indicating the quasiꢀreversible feature of the redox couples.
Figure 5d shows the comparison CVs of the asꢀprepared Co3O4
samples with the nickel foam after the solvothermal process at a
scan rate of 10 mV sꢀ1 in 3.0 mol Lꢀ1 KOH solution. It can been
seen from the insert of Figure 5d that Ni foam only showed a
pair of redox peaks which is corresponding to the
interconversion between Ni(II) and Ni(III)33. However,
compared with the asꢀprepared Co3O4 samples, the specific
Figure 6 The discharge curves of EꢀCo3O4 (a), EWꢀCo3O4 (b)
and WꢀCo3O4 (c) in 3.0 mol Lꢀ1 KOH solution at different
current densities. (d) Plots of specific capacitance per gram and
per area versus current density for the asꢀprepared Co3O4
samples.
voltage window for discharge, and
m (g) is the total mass of
active material in the electrode. The specific capacitance per
gram and per area versus current density for the asꢀprepared
Co3O4 samples is shown in Figure 6d. The mass loadings of
EꢀCo3O4, EWꢀCo3O4 and WꢀCo3O4 (in mg of electrochemically
active material per cm2 of nickel foam) are 3.8, 2.1 and 3 mg
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4RA17075J
cmꢀ2, respectively. As shown in the result, EWꢀCo3O4 sample which is caused by EWꢀCo3O4 sample have a large specific
displays an extremely performance for supercapacitors during surface area, further will benefit for enhancing the kinetics of
the three samples and the specific capacitance values were ion and electron transport.
measured to be 2509.4, 2004.2, 1754.9, 1221.6, 342.8 F g−1
Cycling stability or cycling life is an important
(5.26, 4.21, 3.69, 2.57 and 0.72 F cmꢀ2) at the current density of requirement for the asꢀprepared electrode materials in the
1, 5, 10, 20, and 50 A gꢀ1, respectively. It was also found that practical supercapacitor applications. The cyclic performance of
the capacitance decreased with an increase in discharge current the EWꢀCo3O4 sample examined by galvanostatic chargeꢀ
density, which was caused by the electrode resistance and the discharge tests at a current density of 10 A gꢀ1 for 1000 cycles
relatively low utilization of the active material under higher is depicted in Figure 7. The columbic efficiency is also shown
discharge current densities.
in Figure 7, which was calculated based on the following
In order to better evaluation the electrochemical properties equation:
of the asꢀprepared Co3O4 materials, we prepared the electrode
η
= td / tc ×100
(8)
using the conventional Co3O4/carbon/PTFE technology.
Typically, 85 wt.% Co3O4 powder (scratched from nickel foam),
10 wt.% acetylene black as a conducting agent and 5 wt.%
polytetrafluorethylene (PTFE) as a binder were homogeneously
mixed and pressed into a nickel foam (1 cm*1 cm) current
collector under a pressure of 8 MPa. The mass loading of each
electrode is about 6 mg cmꢀ2. Their supercapacitance behaviors
were tested through CV and galvanostatic chargeꢀdischarge
methods in 3.0 mol Lꢀ1 KOH aqueous electrolyte using the
conventional threeꢀelectrode. The results are shown in Figure
S2, S3 and S4. Cleary, the electrochemical performance of the
three samples is far below that of direct growth on the Ni foam,
which may cause by these nanostructures, can improve the
contact between electroactive materials and current collectors
which facilitates better kinetics. However, among these three
samples, EWꢀCo3O4 sample shows the highest performance,
In this equation, tc and td represent the time of charge and
discharge, respectively. It is apparent that the specific
capacitance decreased obviously in the first 200 with cycles. In
order to explain the initial capacitance decay, EIS analysis is
performed and the result is displayed in the insert of Figure 7.
All the impedance spectra were composed of one semicircle
component at highꢀfrequency followed by a linear component
at the lowꢀfrequency. Obviously, the EWꢀCo3O4 electrode
before cycles exhibits lower transfer resistance and closer
straight line to 90◦, demonstrating the lower diffusion resistance
of ions and improved electrolyte ions diffusion34. After 1000
cycles, the specific capacitance remains 1225.7 F gꢀ1 with
capacitance retention of 74%. The columbic efficiency remains
above 99% within 1000 cycles.
Table 1. Comparison of the specific capacity at different current densities for recently reported Co3O4 materials
Current Density A g-1
1
2
5
10
290
271
20
272
258
Cycle
Method
Electrode
Reference
31
Co3O4 nanowire array
ꢀ
323 a
ꢀ
Not given
Hydrothermal
Hydrothermal
Grown on Ni foam
Grown on Ni foam
35
18
36
15
37
19
38
21
39
hollow Co3O4 nanowire
ꢀ
295
ꢀ
82% returned after 7500
cycles at 10 A gꢀ1
Co3O4/C flowerꢀlike
nanostructures
290.9
ꢀ
255.3
364
351
524
394.4
ꢀ
ꢀ
330
ꢀ
ꢀ
ꢀ
ꢀ
Not given
Not given
Solvothermal
Powder + acetylene
black+ PTFE
Co3O4 /rGO/CNTs Hybrid
porous Co3O4 nanoflake array
Co3O4 ꢀCNFs
ꢀ
Hydrothermal Grown on rGO/CNTs
Hybrid Paper
369
552
ꢀ
264
ꢀ
242
97.4% returned after 4000
cycles at 2 A gꢀ1
Hydrothermal
Electrospinning
annealing
Grown on Ni foam
ꢀ
ꢀ
99% returned after 2000
cycles at 4 A gꢀ1
Powder + acetylene
black+ PVDF
Co3O4 hollow microspheres
brushꢀlike Co3O4 nanowires
Co3O4 nanofibers
386
ꢀ
360
ꢀ
319
92% returned after 500
cycles at 2 A gꢀ1
Powder + acetylene
black+ PVDF
1525
340
ꢀ
ꢀ
94% returned after 5000
cycles at 1 A gꢀ1
Solvothermal
electrospinning
Grown on carbon
fiber paper
ꢀ
315
770
2004
296
484
1754
ꢀ
ꢀ
94% returned after 1000
cycles at 1 A gꢀ1
Powder + acetylene
black+ PVDF
Reduced Co3O4 NWs
EWꢀ Co3O4
978
ꢀ
90% returned after 2000
cycles at 2 A gꢀ1
solution
reduction
Grown on Ni foam
Grown on Ni foam
2509
1221
74% returned after 1000
cycles at 10 A gꢀ1
Solvothermal
This work
a specific capacitance per gram
As presented in Table 1, we have summarized the rate EWꢀCo3O4 electrode. Clearly, this is a rather high value
performance of recently reported Co3O4 based materials with our compared to literature results (Table 1)15, 18, 19, 21, 31, 35ꢀ39. The
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enhanced performance of the EWꢀCo3O4 electrode can be
ascribed to the follow reasons: (1) The electroꢀactive materials
were directly grown onto nickel foam without any conducting
additive and binder, therefore, the utilization of the active
material is high. (2) The EWꢀCo3O4 sample provided a large
specific surface area with many holes, benefitting from
enhancing the kinetics of ion and electron transport and also
shortening the diffusion path of ions and electrons.
Engineering, Harbin Engineering University, Harbin, 150001,
P.R.China. Eꢀmail: chengkui@hrbeu.edu.cn, caodianxue@hrbeu.edu.cn.
†
Electronic Supplementary Information (ESI) available: Experimental
details and characterizations. See DOI: 10.1039/b000000x/
1. J. R. Miller, R. Outlaw and B. Holloway, Science, 2010, 329, 1637ꢀ
1639.
2. T. Brezesinski, J. Wang, S. H. Tolbert and B. Dunn, Nature
Materials, 2010,
9, 146ꢀ151.
3. L. Wei and G. Yushin, Nano Energy, 2012,
1
, 552ꢀ565.
Conclusions
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In summary, Co3O4 nanostructure with different morphologies
has been easily obtained through a simple hydrothermal method
with changing the solvents composition. The electrochemical
performance of the asꢀprepared Co3O4 samples has been done
without adding any binder or carbon additive. The specific
capacitance of EWꢀCo3O4 electrode prepared in C2H5OH/H2O
volume ratios 1/1 is 2509.4 Fꢁgꢀ1 at 1 A gꢀ1 and 1754 Fꢁgꢀ1 at 10
A gꢀ1, which is much larger than that on the Co3O4 nanowire
arrays prepared in the pure water and pure ethanol solvent, as
well as a rather high value compared to literature results. The
enhanced performance is possibly due to its unique structure
that could greatly raise the utilization efficiency of active
materials and guarantee a superior property for electrolyte
diffusion. The asꢀprepared honeycombꢀlike Co3O4 electrode has
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We gratefully acknowledge the financial support of this
research by National Nature Science Foundation of China
(21403044), the Heilongjiang Postdoctoral Fund (LBHꢀ
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