Co0.56 Ni0.44 oxide nanoflake materials and activated carbon for asymmetric supercapacitor

Article abstract of DOI:10.1149/1.3497298

In this work, Co_0.56 Ni_0.44 oxide nanoflakes with a maximum specific capacitance of 1227 F/g (136 mAh/g) are successfully synthesized by a facile chemical coprecipitation method. To enhance energy density, an asymmetric supercapacitor with high energy and power density has been constructed with Co_0.56 Ni_0.44 oxide nanoflakes as the positive electrode and activated carbon as the negative electrode. The performance of the asymmetric supercapacitor was characterized by cyclic voltammetry and chronopotentiometry technology. By using the nanoflakes Co _0.56 Ni_0.44 oxides electrode, the asymmetric supercapacitor exhibits high energy density and stable power characteristics; the maximum specific capacitance of 97 F/g and specific energy of 34.5 W h/kg are demonstrated for a cell voltage between 0 and 1.6 V.

Full text of DOI:10.1149/1.3497298

Journal of The Electrochemical Society, 157 ͑12͒ A1341-A1346 ͑2010͒
A1341
0
013-4651/2010/157͑12͒/A1341/6/$28.00 © The Electrochemical Society
Co Ni
Oxide Nanoflake Materials and Activated Carbon
0
.56 0.44
for Asymmetric Supercapacitor
a
a,b,z
a
b
b
Jun-Wei Lang, Ling-Bin Kong,
Min Liu, Yong-Chun Luo, and Long Kang
a
b
State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials and School of Materials Science
and Engineering, Lanzhou University of Technology, Lanzhou 730050, People’s Republic of China
In this work, Co0.56Ni0.44 oxide nanoflakes with a maximum specific capacitance of 1227 F/g ͑136 mAh/g͒ are successfully
synthesized by a facile chemical coprecipitation method. To enhance energy density, an asymmetric supercapacitor with high
energy and power density has been constructed with Co0.56Ni0.44 oxide nanoflakes as the positive electrode and activated carbon
as the negative electrode. The performance of the asymmetric supercapacitor was characterized by cyclic voltammetry and
chronopotentiometry technology. By using the nanoflakes Co0.56Ni0.44 oxides electrode, the asymmetric supercapacitor exhibits
high energy density and stable power characteristics; the maximum specific capacitance of 97 F/g and specific energy of
3
©
4.5 W h/kg are demonstrated for a cell voltage between 0 and 1.6 V.
2010 The Electrochemical Society. ͓DOI: 10.1149/1.3497298͔ All rights reserved.
Manuscript submitted May 24, 2010; revised manuscript received August 12, 2010. Published October 16, 2010.
Supercapacitors provide a higher energy density than dielectric
capacitors and a higher power density than batteries. They are par-
ticularly adapted for applications which require energy pulses during
nied by its energy storage performance in basic electrolytes and
environment compatibility, have made it one of the most promising
2
9
electrode materials for pseudocapacitors. However, issues related
to its electrochemical characteristics, such as conductivity and the
1-3
short periods of time. However, for the electrochemical double-
layer capacitors ͑EDLCs͒ and most pseudocapcitors, the specific
energy of electrochemical capacitors is still much less than that of
3
0
utilization of the active material, sill remain unresolved. In order
to improve the nickel oxide performance, great efforts have been
made. In fact, cobalt compounds are also studied as cheaper candi-
dates with good capacitive characteristics due to its many important
technological applications. Moreover, cobalt compounds are poten-
tially good additions for improving the properties of the nickel elec-
4-6
advance batteries. Therefore, in order to satisfy the industrial de-
mand, the development of new materials and new concepts for su-
percapacitors should be proposed.
In its charged state, a supercapacitor is equivalent to two elec-
3
1,32
trode in the nickel-metal hydride ͑Ni/MH͒ battery.
Adding co-
trodes of capacitances C and C in series. As the capacity of the
1
2
balt compounds can significantly enhance nickel oxide utilization as
cobalt can reduce the resistance of the nickel electrode and raise the
two electrodes is different, even in a symmetric capacitor, the ca-
pacity of the total system C is determined by the electrode with the
smaller capacity value. Moreover, the later electrode operates in a
larger potential window than the other one, what consequently re-
duces the voltage range of the device. This drawback can be circum-
vented by balancing the respective masses of the electrodes or by
3
3-35
oxygen overpotential.
In this work, nickel–cobalt oxide nanoflakes were successfully
synthesized by a facile chemical coprecipitation method followed by
a simple calcinations process. The studies show that the composites
nickel–cobalt oxides nanoflakes with less crystallization were com-
posed of NiO and Co O . The maximum specific capacitance of
7
,8
using different materials working in their optimal potential range.
The asymmetric design is an attractive approach to increasing the
energy density of supercapcitors as it can lead to an almost doubling
of device capacitance.
3
4
1
227 F/g ͑136 mAh/g͒ is the highest report of nickel–cobalt oxides
for electrochemical capacitors, which shows better rate capability
and great potential as the electrode materials for electrochemical
capacitors. To enhance energy density, an asymmetric-type
pseudocapacitor/electric double-layer capacitor is considered, where
Co0.56Ni0.44 oxide nanoflakes and activated carbon act as the posi-
tive and negative electrodes, respectively. Electrochemical measure-
ments were carried out in 2 M aqueous KOH in a half-cell setup
configuration at room temperature using CV, chronopotentiometry,
and impedance spectroscopy. Values for the maximum specific ca-
pacitance and specific energy of 97 F/g and 34.5 W h/kg, respec-
tively, are demonstrated for a cell voltage between 0 and 1.6 V. By
using the nanoflakes Co0.56Ni0.44 oxide electrode, the asymmetric
supercapacitor exhibits high energy density and excellent power
characteristics.
For the asymmetric supercapacitor, a new concept of high energy
density electrochemical capacitor, one electrode stores charge
through a reversible, nonfaradaic reaction of ion adsorption/
desorption on the surface of the an activated carbon ͑AC͒, and the
other electrode is to utilize a reversible redox faradaic reaction in a
transition metal oxide. It is possible to reach the high working volt-
age and high energy density by choosing a proper electrode material,
contributed to a significant increase in the overall energy density of
9-11
the supercapacitor devices.
Supercapacitors make use of three main classes of materials:
1
2-15
16-19
carbon,
metal oxide,
and electronically conducting
Among these electrode materials, AC, as the represen-
tative electrode material of the EDLCs, has rapid current–voltage
I–V͒ response and rectangle cyclic voltammetry ͑CV͒ shape,
2
0-22
polymer.
͑
2
3
whereas its specific capacitance is relatively low. Electroactive
materials with several oxidation states or structures, such as transi-
tion metal oxides and conducting polymers, can provide a large
reversible capacity, good cycling performance, and high recharging
Experimental
Preparation of the electrode material.— All of the chemicals
were of analytical grade and were used without further purification.
Co0.56Ni0.44 oxides were synthesized by a facile chemical coprecipi-
tation method followed by a simple calcinations process. First, a
mixed aqueous solution of 1 M NiCl ·6H O and 1 M CoCl ·6H O
2
4-28
rates for pseudocapacitors.
Conducting-polymer capacitors have
been reported to display high power densities, but their specific
capacitances are much lower than those of metal oxide capacitors.
Hence, it would be of great interest to develop a metal oxides elec-
trode material with the large capacity and high rate capability for
supercapacitors.
2
2
2
2
was made, the molar ratios of the CoCl ·6H O:NiCl ·6H O was 1:3.
2
2
2
2
The pH value of the mixed solution was slowly adjusted to 9 by
adding slowly 5 wt % NH ·H O at room temperature. The resulting
3
2
The natural abundance and low cost of nickel oxide, accompa-
suspension was kept stirring at this temperature for 3 h. Then the
solid was filtered, washed with distilled water several times, and
dried in air at 250°C for 6 h.
z
E-mail: konglb@lut.cn
Commercial AC ͑The ShaoWu XinSen Carbon Company, China,
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A1342
Journal of The Electrochemical Society, 157 ͑12͒ A1341-A1346 ͑2010͒
2
with a specific surface area of 2000 m /g and a diameter of
5
–10 ␮m͒ was used as the negative electrode material without fur-
ther treatment.
Structure characterization.— The obtained products were char-
acterized by field emission scanning electron microscope ͑SEM͒
͑
͑
JEOL, JSM-6701F, Japan͒, X-ray diffraction ͑XRD͒ measurements
Rigaku, D/Max-2400, Japan͒, and nitrogen adsorption and desorp-
NiO
tion experiments ͑Micromeritics, ASAP 2020M, USA͒. The surface
area was calculated using the Brunauer–Emmett–Teller ͑BET͒ equa-
tion. Pore-size distributions were calculated by the Barrett–Joyner–
Halenda ͑BJH͒ method using the desorption branch of the isotherm.
The elemental composition in the as-prepared samples was deter-
mined using the energy dispersive spectrum ͑EDS͒ coupled to the
SEM instrument.
Co
Ni
.56 0.44
Oxide
0
Co O
3
4
1
0
20
30
40
50
60
70
80
Preparation of the electrode.— The working electrodes were
prepared according to the method reported in Ref. 1. Nickel–cobalt
oxide powder ͑80 wt %͒ was mixed with 7.5 wt % of acetylene
black ͑ Ͼ 99.9%͒ and 7.5 wt % of conducting graphite in an agate
mortar, until a homogeneous black powder was obtained. To this
mixture, 5 wt % of poly͑tetrafluoroethylene͒ was added with a few
drops of ethanol. After briefly allowing the solvent to evaporate, the
resulting paste was pressed at 10 MPa to a nickel gauze with a
nickel wire for an electric connection. The electrode assembly was
dried for 16 h at 80°C in air. The AC electrodes were prepared by
the same method as the negative electrode described above.
2
ؑ Angle (degrees)
Figure 1. The XRD patterns of the nickel oxide, Co0.56Ni0.44 oxide, and
cobalt oxide.
2
E = CU /2 ͑W h/kg͒
͓4͔
max
where C is the system capacitance for a cell and Umax is the maxi-
mum cell voltage.
Results and Discussion
Electrochemical test of the single electrode and asymmetric su-
percapacitor.— The electrochemical measurements of single
nickel–cobalt oxide and AC electrodes were carried out using an
electrochemical working station ͑CHI660C, Shanghai, China͒ in a
three-electrode cell at room temperature. A platinum gauze electrode
and a saturated calomel electrode ͑SCE͒ served as the counter elec-
trode and the reference electrode, respectively. Each electrode con-
tained about 8 mg of electroactive material and had a geometric
Characterization of Co0.56Ni0.44 oxide materials.— The atomic
percentages ͑atom %͒ of the elements Co and Ni in nickel–cobalt
oxides were obtained by means of EDS spectroscopy. The result
shows that the Co0.56Ni0.44 oxide material is obtained when the mo-
lar ratio of the CoCl ·6H O:NiCl ·6H O in mixed solution was 1:3.
2
2
2
2
The XRD patterns of the nickel oxide, Co0.56Ni0.44 oxide, and
cobalt oxide are shown in Fig. 1. All the diffraction peaks of pure
nickel oxide sample, including not only the peak positions but also
their relative intensities, are in good accordance with the cubic crys-
talline structure of NiO ͑Powder Diffraction File ͑PDF͒, no. 22-
1189͒. All diffraction peaks of the cobalt oxides sample are in good
accordance with the diffraction data of the spinel structure Co O
2
surface area of about 1 cm .
The nickel–cobalt oxide cathode and AC anode were pressed
together and separated by a porous nonwoven cloth separator. The
mass ratio of active materials ͑anode/cathode͒ was 22:8 mg. Each
2
electrode had a geometric surface area of about 1 cm . The electro-
3
4
chemical measurements of the asymmetric supercapacitor were also
carried out using the electrochemical working station in a two-
electrode cell at room temperature.
The specific capacitance of the asymmetric capacitor can be
evaluated from the charge/discharge test together with the following
equation
͑PDF, no. 42-1467͒. It is obvious that the Co0.56Ni0.44 oxide is com-
posed of NiO and Co O . It is difficult to differentiate between the
3
4
two phases, since they have similar structures and their diffraction
peaks are very close. The peaks of the Co0.56Ni0.44 oxide are broad,
which indicates the low degree of crystallization of the products; the
crystallite size of the Co0.56Ni0.44 oxide was 44.6 Å which was cal-
culated from the Scherrer formula.
SEM measurement was performed to the Co0.56Ni0.44 oxide. As
seen in Fig. 2, the Co0.56Ni0.44 oxide has a spherical morphology
with 2–3 ␮m diameter, which is built up of many interleaving thin
nanoflakes. It can be observed that the Co0.56Ni0.44 oxide possesses
many macropores that are hundreds of nanometers in size on the
surface.
C = I/͓͑dE/dt͒ ϫ m͔ Ϸ I/͓͑⌬E/⌬t͒ ϫ m͔ ͑F/g͒
͓1͔
where C is the specific capacitance, I is the constant discharging
current, ⌬t is the time period for the potential change ⌬E, and m is
the mass of the corresponding electrode materials measured.
The charge on the electrode can be evaluated from the charge/
discharge test together with the following equation
Surface area and pore-size distribution analysis of the
Co0.56Ni0.44 oxide materials were conducted using N2 adsorption
and desorption experiments. As shown in Fig. 3, the profile of the
hysteresis loop indicates an adsorption–desorption characteristic of
the porous materials. The BET specific surface area of the
CЈ = I⌬t/m ͑C/g͒
͓2͔
where I is the constant discharging current, ⌬t is the time period for
the potential change, and m is the mass of the corresponding elec-
trode materials measured.
The real power density Preal is determined from the constant
2
Co0.56Ni0.44 oxide was 110 m /g. The inset is the pore diameter
3
6
current charge/discharge cycles as follows
distributions of the Co0.56Ni
oxide sample. As shown in this fig-
0.44
ure, the Co0.56Ni0.44 oxides possess a narrow mesoporous distribu-
tion at around 2–7 nm. The BJH desorption average pore diameter
for the Co0.56Ni0.44 oxide was 5.9 nm.
Preal = ⌬EI/m ͑W/kg͒
͓3͔
where ⌬E = ͑Emax + Emin͒/2 with Emax being the potential at the
end of the charge and Emin being the potential at the end of the
discharge, Emin is the applied current ͑amperes͒, and m is the weight
of active material in the electrode ͑kilograms͒.
The electrochemical characterizations of the Co0.56Ni0.44 oxide
material and AC electrodes.— To evaluate the electrochemical
properties of the prepared Co0.56Ni0.44 oxides material and the com-
The specific energy Ereal is defined as
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Journal of The Electrochemical Society, 157 ͑12͒ A1341-A1346 ͑2010͒
A1343
0
0
0
0
0
0
.10
.08
.06
.04
.02
.00
b
a
-
-
-
-
-
0.02
0.04
0.06
0.08
0.10
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Potential (V vs. SCE)
Figure 4. Cyclic voltammogram of ͑a͒ AC and ͑b͒ Co0.56Ni0.44 oxide elec-
trodes at a scan rate of 5 mV/s in 2 M KOH solution.
Figure 2. SEM images of the Co0.56Ni0.44 oxide material.
dow of 0–0.4 V at a current density of 625 mA/g. The shape of the
discharge curves does not show the characteristic of a pure double-
layer capacitor, but mainly pseudocapacitance, which agrees with
mercial AC, we directly use these two materials to fabricate elec-
trodes for supercapacitors. CV and chronopotentiometry measure-
ments have been used to evaluate the electrochemical properties and
quantify the specific capacitance of the Co0.56Ni0.44 oxides and AC
electrodes. CV curves for AC and Co0.56Ni0.44 oxides electrodes in
a
0.4
2
M KOH aqueous solution are shown in Fig. 4. The AC electrode
was performed within a potential window of −1.0 to 0 V ͑versus
SCE͒, and Co0.56Ni0.44 oxides was employed within a potential win-
dow of 0–0.5 V ͑versus SCE͒ at a scan rate of 5 mV/s. For the AC
electrode, a nearly rectangular CV curve is obtained, indicating a
typical e EDLC behavior because no peaks of oxidation and reduc-
tion are observed. The CV shapes of Co0.56Ni0.44 oxides reveal that
the capacitance characteristic is very distinguished from that of the
electric double-layer capacitance. Obviously, electrochemical polar-
ization of electrodes does not appear from CV curves at the two
edges of potential window, which indicates that electrochemical ca-
pacitors with wider potential window can be chosen.
0
0
.3
.2
0.1
0
.0
Figure 5a shows the typical galvanostatic charge/discharge
0
200 400 600 800 1000 1200 1400 1600 1800
curves of the Co0.56Ni0.44 oxides electrode within a potential win-
Time (s)
2
2
1
1
50
00
50
00
b
0
.0
1
1
1
0
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
.0
Desorption
-0.2
-
-
-
-
0.4
0.6
0.8
1.0
0
20
40
60
80
100
120
Pore Diameter (nm)
Adsorption
5
0
0
0
100
200
300
400
500
600
Time (s)
0
.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure
(
P/Po
)
Figure 5. Charge/discharge curves of the ͑a͒ Co0.56Ni0.44 oxide electrode in
the potential window 0 to 0.4 V and ͑b͒ AC electrode in the potential win-
dow of −1 to 0 V at a current density of 625 mA/g in 2 M KOH solution.
Each electrode contained about 8 mg of electroactive material and had a
Figure 3. The N adsorption–desorption isotherm of Co0.56Ni0.44 oxide ma-
terials; the inset is the BJH pore-size distributions of Co0.56Ni0.44 oxide ma-
terials.
2
2
geometric surface area of about 1 cm .
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A1344
Journal of The Electrochemical Society, 157 ͑12͒ A1341-A1346 ͑2010͒
0
.05
.04
.03
.02
.01
.00
100
a
9
8
7
6
5
0
0
0
0
0
0
0
0
0
0
a
1
0 mV/s
5 mV/s
2
.5 mV/s
40
b
3
2
1
0
0
0
0
-
-
-
0.01
0.02
0.03
-
0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0
200 400 600 800 1000 1200 1400 1600 1800
Cell voltage (V)
Current density (mA/g)
1
1
1
1
1
0
0
0
0
0
.8
.6
.4
.2
.0
.8
.6
.4
.2
.0
Figure 7. Specific capacitance as a function of discharge currents for ͑a͒
asymmetric supercapacitor and ͑b͒ AC-based EDLC capacitor.
6
66.7 mA/g 333.3 mA/g
b
1
66.7 mA/g
trode and the AC electrode is 176:490, and the loads of the cathode
Co0.56Ni0.44 oxide electrode and the AC anode electrode materials
are of 8 and 22 mg, respectively.
The electrochemical characterizations of the asymmetric capaci-
tors.— It is very important to polarize each electrode at the same
potential before cycling the device; otherwise, there is a risk of
damaging the cell during the first cycles. The typical CV of the
asymmetric capacitor at voltage scan rates of 2.5, 5, and 10 mV/s is
depicted in Fig. 6a. The hybrid device was cycled between 0 and
1.6 V with a good reversibility in this cell voltage window. It should
be noted that the CV curves can be approximately considered as
rectangle shapes when the scan rate is 2.5 mV/s; when the scan rate
increases to 5 mV/s, the curve begins to deform. Also, the current
increases with scan rate, indicating rapid I–V response.
1000 mA/g
1
333.3 mA/g
666.7 mA/g
1
0
500
1000
1500
2000
Time (s)
Figure 6. Electrochemical properties of the asymmetric supercapacitor in
M KOH solution within a cell voltage range from 0.0 to 1.6 V: ͑a͒ cyclic
voltammograms at different scan rates and ͑b͒ charge/discharge behavior at
different current densities. The mass loads of the cathode Co0.56Ni0.44 oxide
electrode material are 8 and 22 mg of the AC anode electrode material.
2
Galvanostatic constant current charge/discharge measurements at
different current densities were applied to evaluate the electrochemi-
cal properties and to quantify the specific capacitance of the asym-
metric supercapacitor in 2 M KOH electrolyte. Figure 6b shows the
typical galvanostatic charge–discharge curves of the hybrid capaci-
tor between 0 and 1.6 V at different current densities. As shown in
Fig. 6b, during the charging and discharging steps, though an almost
linear variation in the cell voltage is observed in the curves, perfect
linear curves are not obtained compared with EDLC. This is due to
a typical pseudocapacitance behavior resulting from the electro-
chemical adsorption/absorption or redox reactions at interfaces be-
the result of the CV curves. The slope of charge/discharge curves
indicates the potential dependent nature of the faradaic reaction. The
specific capacitance value of the Co0.56Ni0.44 oxide is calculated to
be 1227 F/g ͑136 mAh/g͒, according to Eq. 1. Since the size range
of the hydrated ions in the electrolyte is typically 6–7.6 Å, the pore
size at the range of 8–50 Å is the effective one required to increase
3
6
tween electrodes and electrolyte. On account that the real galvano-
static discharge window of the Co0.56Ni0.44 oxide electrode is about
0.5 V, the result of the present work shows that it is possible to
reach the high working voltage by choosing a proper electrode ma-
terial. All these profit from using the active carbon as the negative
electrode materials with large surface and proper pore distribution,
which ensure that the Co0.56Ni0.44 oxides precede with the faradaic
reaction in the larger applied potential range. Furthermore, as the
discharge current increases, the large voltage drop is produced and
finally the capacity decreases. This phenomenon may be explained
by referring to OH− ions diffusion processes during the charging/
discharging for the electrode. When the electrode at high sweep
3
7
either the pseudocapacitance or electric double-layer capacitance.
Furthermore, the unique microstructure of the Co0.56Ni0.44 oxides
can accommodate the electroactive species in the solid bulk elec-
trode material. Therefore, the obtained high specific capacitance of
227 F/g from the synthesized Co0.56Ni0.44 oxides materials is
mainly attributed to the effective distributions of the pore size and
the high specific surface area.
1
The linear charge/discharge curves are observed in Fig. 5b of the
AC electrode within a potential window of −1 to 0 V at a current
density of 625 mA/g. The linear shape of the curve is attributed to
the linear correlation of the absorbed charge on the interface with
the applied potential. That means the specific capacitance is inde-
pendent of the applied potential nature, distinctly accompanied with
a nonfaradaic process on the interface. The charge on the
Co0.56Ni0.44 oxide electrode and the usual AC electrode in their own
potential window is 490 and 176 C/g, respectively, according to Eq.
−
rates corresponds to a high current density, massive OH ions are
required to intercalate swiftly at the interface of the electrode/
−
electrolyte. However, relatively low concentration of OH ions
could not meet this demand and the processes would be controlled
3
8
by the ion diffusion.
In comparison with the asymmetric capacitor, we fabricate a two-
electrode AC symmetric capacitor and it is cycled galvanostatically
between 0 and 1.0 V. Each electrode contained about 22 mg of AC
2
. Therefore, the optimal mass ratio of the Co0 Ni
oxide elec-
0.44
.56
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Journal of The Electrochemical Society, 157 ͑12͒ A1341-A1346 ͑2010͒
A1345
1
1
1
400
200
000
a
8
7
0
0
1.8
1
1
1
1
.6
.4
.2
.0
60
50
8
6
4
2
00
00
00
00
0
4
3
2
1
0
0
0
0
0
0.8
b
0
0
.6
.4
0.2
.0
0
0
1000
2000
3000
4000
5000
6000
Time (s)
0
5
10
15
20
25
30
35
40
0
200
400
600
800
1000
Energy density
(
Wh/kg)
Cycle number
Figure 8. Ragone plot relating power density to achievable energy density of
͑
a͒ asymmetric supercapacitor and ͑b͒ AC-based EDLC capacitor.
Figure 9. Cycle life of the asymmetric supercapacitor at the current density
of 333.3 mA/g. The inset is charge/discharge curves of the asymmetric su-
percapacitor.
2
material and had a geometric surface area of about 1 cm . Figure 7
shows the specific capacitance of the double electrode cell as a
function of the discharge current density for the Co0.56Ni0.44 oxide
electrode-based asymmetric supercapacitor and the AC-based
EDLC. The specific capacitances ͑calculated based on the total
weight of the active materials in the supercapacitor͒ of the hybrid
capacitor at 166.7, 333.3, 666.7, 1000, 1333.3, and 1666.7 mA/g
were 97, 87.4, 77.5, 72.3, 68.6, and 63.2 F/g, respectively. The spe-
cific capacitances of the EDLC of the total weight of the active
material for both electrodes at 113.6, 227.3, 454.5, 681.8, 909.1, and
a simple calcinations process. The studies show that the as-prepared
Co0.56Ni0.44 oxide was composed of NiO and Co O with a less
crystallization, nanoflake structure. The specific capacitance value of
3
4
1
227 F/g ͑136 mAh/g͒ is obtained for the Co0.56Ni0.44 oxide
sample. Additionally, an asymmetric supercapacitor based on
Co0.56Ni0.44 oxide nanoflakes as the positive electrode and AC as the
negative electrode with high operating voltage and high energy den-
sity was fabricated in the 2 M KOH electrolyte. The specific capaci-
tance and specific energy of the cell reach 97 F/g and 34.5 W h/kg
within the cell voltage range from 0 to 1.6 V, respectively. The hy-
brid supercapacitor also demonstrated a good cycling performance
with an attenuation of capacitance of 17% over 1000 cycle numbers.
1
136.4 mA/g were 30.5, 29.3, 27.8, 26.8, 26.3, and 25.6 F/g, re-
spectively. The results show that the specific capacitance of the
asymmetric supercapacitor is much higher than that of the EDLC.
Even though under the large current density of 1666.7 mA/g, nearly
65.2% of the initial amount can be reached for the hybrid capacitor,
Acknowledgments
so the large specific energy and well rate capability of the asymmet-
ric capacitor makes it attractive, particularly for a practical applica-
tion.
This work was supported by the National Natural Science Foun-
dation of China ͑grant no. 50602020͒ and the National Basic Re-
search Program of China ͑grant no. 2007CB216408͒.
High power and energy densities are always objects for superca-
pacitor to pursue. Therefore, we also evaluate the relation between
power and energy densities of the asymmetric supercapacitor and
the EDLC. The values of power and energy densities are calculated
from Eq. 3 and 4, respectively, and are depicted in Fig. 8. The data
clearly demonstrated that the asymmetric supercapacitor based on
AC and Co0.56Ni0.44 oxide has a good specific energy and power
density. For example, the specific energy was 34.5 W h/kg at a
power density of 133.3 W/kg and still keeps 22.5 W h/kg at a
power density of 1333.3 W/kg. Clearly both the energy density and
power density greatly increased compared with the EDLC type. This
can be explained by the energy and power densities, which critically
depend on the real working voltage. The specific energy increases
by more than eight times compared with that of a symmetric AC-
based EDLC capacitor using an aqueous KOH electrolyte.
The cycling stability of the asymmetric supercapacitor was per-
formed by charge–discharge at a current density of 333.3 mA/g
within a voltage range of 0–1.6 V in the 2 M KOH electrolyte. As
shown in Fig. 9, the capacitance of the hybrid supercapacitor de-
creases with the growth of the cycle number. After a continuous
Lanzhou University of Technology assisted in meeting the publication
costs of this article.
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