The nickel oxide/CNT composites with high capacitance for supercapacitor

Article abstract of DOI:10.1149/1.3425624

A simple hydrothermal synthesis method is adopted to prepare nickel oxide/carbon nanotube (NiO/CNT) composites. X-ray diffraction, differential scanning calorimetry/thermogravimetric analysis, transmission electron microscopy, scanning electron microscopy, and N₂ -adsorption/ desorption techniques are employed for morphology and structure characterizations. The different morphologies of NiO are obtained, which change from a two-dimensional flake to a zero-dimensional mesoporous sphere, dispersing on the surface of CNTs by changing the sodium dodecyl sulfate's fraction in the reacting system. The electrochemical performance of NiO/CNT composites is largely affected by the morphology and distribution of the NiO phase. The zero-dimensional mesoporous sphere NiO shows the largest specific capacitance of 1329 F g^-1 as well as a good cycle life during 1000 cycles in a 1 M KOH electrolyte at a very high current density of 84 A g^-1 by chronopotentiometry measurement.

Full text of DOI:10.1149/1.3425624

A818
Journal of The Electrochemical Society, 157 ͑7͒ A818-A823 ͑2010͒
0013-4651/2010/157͑7͒/A818/6/$28.00 © The Electrochemical Society
The Nickel Oxide/CNT Composites with High Capacitance
for Supercapacitor
Pei Lin,a Qiujie She,a Binling Hong,a Xiaojing Liu,a Yining Shi,a Zhan Shi,b
Mingsen Zheng,a,z and Quanfeng Donga,z
aChemistry Department, College of Chemistry and Chemical Engineering, State Key Laboratory of
Physical Chemistry of Solid Surfaces, and bDepartment of Materials Science and Engineering, College of
Materials, Xiamen University, 361005 Xiamen, China
A simple hydrothermal synthesis method is adopted to prepare nickel oxide/carbon nanotube ͑NiO/CNT͒ composites. X-ray
diffraction, differential scanning calorimetry/thermogravimetric analysis, transmission electron microscopy, scanning electron
microscopy, and N2-adsorption/desorption techniques are employed for morphology and structure characterizations. The different
morphologies of NiO are obtained, which change from a two-dimensional flake to a zero-dimensional mesoporous sphere,
dispersing on the surface of CNTs by changing the sodium dodecyl sulfate’s fraction in the reacting system. The electrochemical
performance of NiO/CNT composites is largely affected by the morphology and distribution of the NiO phase. The zero-
dimensional mesoporous sphere NiO shows the largest specific capacitance of 1329 F g−1 as well as a good cycle life during 1000
cycles in a 1 M KOH electrolyte at a very high current density of 84 A g−1 by chronopotentiometry measurement.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3425624͔ All rights reserved.
Manuscript submitted March 3, 2010; revised manuscript received April 6, 2010. Published May 21, 2010.
Due to the fossil fuel energy depletion and global warming is-
improve the high resistance and low electrochemical utilization. To
the best of our knowledge, NiO/CNT composites, which are pre-
pared hydrothermally, have not been investigated so far. In this pa-
per, a simple hydrothermal synthesis method is adopted to prepare
NiO/CNT composites for supercapacitors. We obtain different mor-
phologies of NiO, which change from a two-dimensional flake to a
zero-dimensional mesoporous sphere, dispersing on the surface of
CNTs by changing the content of the sodium dodecyl sulfate ͑SDS͒
in the reaction system.
sues, we need to explore and exploit the new energy such as solar
radiation, wind, and waves,1 which are variable in time and diffuse
in space. These energy sources require more advanced energy stor-
age and management devices. Supercapacitors offering transient but
extremely high powers are probably the most important next-
generation energy storage device.2
To boost the specific capacitance of supercapacitors, the size and
morphology of the electrode materials need to be designed to pro-
vide a large amount of superficial electroactive species to participate
in faradaic redox reactions. In addition, suitable mesopore sizes of
the porous electrode materials are critical to ease the mass transfer
of electrolytes via the pores for fast redox reactions and double-layer
charging/discharging.3-6
Experimental
Synthesis of NiO/CNT composites.— The NiO/CNT composites
were prepared by a simple hydrothermal precipitation followed by
thermal annealing. The CNTs used were multiwalled CNTs pur-
chased from Shenzhen Nanotechnologies Co. ͑China͒. First, 1 g
CNTs, 1 g NiCl2·6H2O, and 40 g urea were dispersed in 50 mL
deionized water by magnetic stirring for 3 h to get a transparent
solution. Then, surfactant SDS was added into the solution, and a
vigorous stirring was carried on until the solution was homoge-
neous. The mixture was put into an autoclave and kept stirring at
80°C for 4 h. Finally, the precipitant was filtered, washed, and dried
in vacuum at 60°C for 12 h. The resulting NiO/CNT composites
were calcined in air at 300°C for 6 h. We used three different SDS
contents of 0, 10, and 20 g, and the final products were marked as
NiO/CNT-0, NiO/CNT-10, and NiO/CNT-20, respectively.
As to the electrode material, electroactive materials possessing
multiple oxidation states that enable rich redox reactions for
pseudocapacitance generation are desirable for supercapacitors.
Transition-metal oxides are a class of materials that have drawn
extensive and intensive research attention in recent years. Among
them, RuO2 is the most prominent one with a specific capacitance as
7
high as 1580 F g−1
,
probably the highest ever reported, but its
high cost limits its commercial use. Nickel oxide is considered a
potential electrode material for supercapacitors in alkaline electro-
lytes because of its high specific capacity, low cost, easy prepara-
tion, and environmental compatibility. The specific capacitance of
nickel oxide electrode materials ranges from 150 to 750 F g−1
͑from a single electrode͒ depending on the method of synthesis,8-12
which is still far from the theoretical value of 2584 F g−1 within
0.5 V, indicating the low electrochemical utilization of nickel oxide
materials. Furthermore, nickel oxide has a serious shortcoming
caused by high resistance for practical application to supercapaci-
tors. Therefore, it is crucial to enhance the conductivity and electro-
chemical utilization of the nickel oxide material to improve the en-
ergy density and power density. Nam et al.13 reported NiO/carbon
nanotube ͑CNT͒ films prepared by electrochemical precipitation,
and a high specific capacity of 1000 F g−1 was obtained, but the
prepared method is very complex. Gao et al.14 used chemical deposit
methods followed by thermal annealing to prepare NiO/CNT com-
posites, and a specific capacity of 523 F g−1 was acquired. How-
ever, the large rate capability of the NiO/CNT composites is not
satisfactory.
Measurement of structure and morphology.— A differential scan-
ning calorimetry ͑DSC͒/thermogravimetric analysis ͑TGA͒ ͑SDT-
Q600͒ in air was carried out in the composite first. Then, to inves-
tigate the microstructure of the NiO/CNT composites, X-ray
diffraction ͑XRD͒ ͑Philips X’Pert Pro Super X-ray diffractometer,
Cu K␣ radiation, an angular range of 10–90° with a step width of
0.0167°͒, scanning electron microscopy ͑SEM͒ ͑Leo-1530 SEM sys-
tem͒, and transmission electron microscopy ͑TEM͒ ͑Tecnai F30,
300kv FEG͒ were used to characterize the crystal structure, the sur-
face morphology, and the nanostructure of the composites. The com-
posites’ Brunauer, Emmett, and Teller ͑BET͒ specific surface areas,
Barrett–Joyner–Halender ͑BJH͒ pore volume, and BJH pore-size
distribution were obtained from the N2-adsorption/desorption iso-
therms recorded at 77 K ͑Tristar3000, Micromeritics͒. The BET spe-
cific surface areas were calculated using the BET equation. Pore-
size distributions were calculated by the BJH method using the
desorption branch of the isotherm.
Based on the above consideration, CNTs were considered for
controlling the nanometer scale and the microstructure of NiO to
Measurement of electrochemical properties.— All electrochemi-
cal tests were achieved by a three-electrode system equipped with a
working electrode ͑sample electrode͒, an active carbon counter elec-
z E-mail: mszheng@xmu.edu.cn; qfdong@xmu.edu.cn
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Journal of The Electrochemical Society, 157 ͑7͒ A818-A823 ͑2010͒
A819
Figure 1. DSC–TGA curve of synthesized Ni͑OH͒ in air.
2
Figure 3. XRD patterns of Ni͑OH͒2, CNTs, and Ni͑OH͒2/CNT composites,
which are prepared with 0, 10, and 20 g SDS, respectively. ͑A͒ Ni͑OH͒2, ͑B͒
Ni͑OH͒2/CNT-0, ͑C͒ Ni͑OH͒2/CNT-10, ͑D͒ Ni͑OH͒2/CNT-20, and ͑E͒
CNTs.
trode, and a mercury oxidation electrode ͑Hg/HgO͒. Working elec-
trodes were prepared by mixing the active material with 20 wt %
carbon ͑super-p͒ and 5 wt % poly͑tetrafluoroethylene͒. Then, this
mixture was dispersed in a small amount of isopropyl alcohol to
obtain a homogeneous mixture, which was pressed on a treated
nickel grid ͑1.2 ϫ 107 Pa͒. Electrochemical performance was in-
vestigated by cyclic voltammetry ͑CV͒ using a Perkin-Elmer 273A
potentiostat. An Arbin battery tester BT2000 was adopted for the
charge and discharge test of the active materials. All measurements
were carried out in 1 M KOH electrolyte solution.
according to the weight loss at different temperature ranges. At tem-
peratures of 400 and 800°C, the weight percentages are 88 and
21.2%, which means the mass fractions of the NiO/CNTs and NiO,
respectively. Therefore, the loading amount of NiO is 24.09%
͑21.2%/88%͒, which is very close to the theory loading amount
͑calculated based on 1 g NiCl2·6H2O͒, 23.91%.
Structure and morphology.— The power XRD patterns of the
samples prepared in our experiment are shown in Fig. 3 and 4. For
comparison, the pure CNTs are also listed. As shown in Fig. 3, the
characteristic graphitic ͑002͒ peak of the CNTs at 26° was clearly
Results and Discussion
DSC/TGA
.— For the synthesized Ni͑OH͒2, as shown in Fig. 1,
the apparent weight loss from room temperature to 200°C corre-
sponds to the loss of chemically adsorbed H2O. The very sharp peak
is present between 250 and 350°C on the differential TGA curve
observed.16 The XRD pattern of the Ni͑OH͒ and Ni͑OH͒2/CNT
2
composites prepared without SDS corresponded to the well-known
layered ␣-Ni͑OH͒ structure with a characteristic ͑001͒ peak at
corresponding to a weight loss via the conversion of Ni͑OH͒ into
2
2
10.2°. Moreover, the intensity of the ͑001͒ reflection decreased with
NiO and H2O. When the temperature is above 350°C, the weight
decreases slowly because of the removal of a small amount of
hydroxyl-group residue during the development of the NiO phase.15
For the Ni͑OH͒2/CNT composites ͑Fig. 2͒, an inconspicuous weight
loss appears between 250 and 350°C, which also corresponds to a
Ni͑OH͒ dispersing on the surface of CNTs. However, the charac-
2
teristic ͑001͒ peak of the Ni͑OH͒2/CNT composites synthesized
with SDS disappears, and their XRD pattern represents amorphous
Ni͑OH͒2/CNT composites. In Fig. 4, the characteristic peaks for
rocksalt NiO at 2␪ = 36.9, 42.8, and 62.4° ͓corresponding to ͑111͒,
͑200͒, and ͑220͒ reflections, respectively͔ as well as the CNTs at 26°
weight loss via the conversion of Ni͑OH͒ into NiO and H2O. A
2
very sharp peak is present between 500 and 650°C on the DSC
curve, which corresponds to an oxidation of CNTs. According to
TGA, we chose 300°C as the calcining temperature in this study.
The percentage of NiO in NiO/CNT composites can be estimated
Figure 4. XRD patterns of NiO, CNTs, and NiO/CNT composites, which are
prepared with 0, 10, and 20 g SDS, respectively. ͑A͒ NiO, ͑B͒ NiO/CNT-0,
͑C͒ NiO/CNT-10, ͑D͒ NiO/CNT-20, and ͑E͒ CNTs.
Figure 2. DSC–TGA curve of synthesized Ni͑OH͒2/CNT composites in air.

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Journal of The Electrochemical Society, 157 ͑7͒ A818-A823 ͑2010͒
Figure 5. SEM images of NiO and NiO/
CNT composites, which are prepared with
0, 10, and 20 g SDS, respectively. ͑a͒ NiO,
͑b͒ NiO/CNT-0, ͑c͒ NiO/CNT-10, and ͑d͒
NiO/CNT-20.
were observed in the XRD patterns of the NiO/CNT composites
heated at 300°C. This confirmed that nickel oxide with a rocksalt
structure had formed on the NiO/CNT composites after heating the
Ni͑OH͒2/CNT composites to 300°C. The characteristic peaks of
rocksalt NiO seriously broaden. It means that we obtain the finer
size and amorphous state of electrode materials to provide a large
amount of superficial electroactive species to participate in faradaic
redox reactions.
the composites are constructed by nanofilaments and have many
voids that ensure an easier electrolyte ion transport and more super-
ficial electroactive species.
Pore distributions.— Then, the mesoporous characteristics of the
product were investigated with the N2-adsorption/desorption iso-
therms. Important structural parameters were derived from the iso-
therms and tabulated in Table I. From Table I, we found that the
BET specific surface areas are gradually decreased, but the BJH
pore sizes are gradually increased after the addition of CNTs and
SDS. It must be considered that the content of NiO in samples
NiO/CNT-0, NiO/CNT-10, and NiO/CNT-20 is only 24 wt % and
the BET surface of the CNTs is only about 20–50 m2 g−1. So, the
addition of CNTs and SDS actually increases the BET surface area,
which means that a better nanostructure is obtained. The typical
pore-size distribution of the samples is shown in Fig. 7, which ex-
hibits the major BJH pore diameter centered at 4.4 nm.
To further investigate the different NiO/CNT composites, SEM
was adopted. Figure 5a shows the SEM images of pure NiO powder,
and Fig. 5b-d displays the SEM images of NiO/CNT powder. With-
out the addition of CNTs or SDS, a lot of sheets clearly congregate
into large secondary particles, and the sizes of NiO are of the mi-
crometer order of magnitude, as shown in Fig. 5a. After the addition
of CNTs, the SEM image of NiO/CNT-0 shows uniform sheet-
shaped NiO dispersing on the surface of CNTs, which demonstrates
that CNTs restrain the agglomeration of sheet-shaped NiO, and the
sizes of NiO are of the nanometer order of magnitude. Furthermore,
with the addition of SDS, the zero-dimensional NiO sphere dispers-
ing on the surface of CNTs and the very fine particles of NiO nano-
particles were obtained ͑Fig. 5c and d͒. With the increasing addi-
tional mass of SDS, the shape of nano-NiO changes from nanodisk
to nanofilament and results in finer zero-dimensional particles. We
mainly ascribe the phenomena to attractive interactions between
dodecyl sulfate anions and the precursor of bivalent nickel ions with
opposite charges. TEM was adopted to observe the nanostructure of
the samples Ni͑OH͒2/CNT-20 and NiO/CNT-20, which also shows
In summary, the SEM results exhibit the morphology of
Ni͑OH͒2, changing from a two-dimensional disk to a zero-
dimensional sphere with an increasing additional amount of SDS,
and these results are also demonstrated by the characteristic ͑001͒
peak of the layered ␣-Ni͑OH͒ structure that disappears.
2
N2-adsorption/desorption results also demonstrate that the addition
of CNTs brings a nanostructure of NiO with a larger size pore di-
ameter.
Electrochemical performance.— The electrochemical properties
of the products were compared by performing CV in a 1 M KOH
solution. Figure 8 shows the cyclic voltammograms of CNTs, NiO,
and NiO/CNT-20 composite electrodes at a scan rate of 10 mV s−1
in a 1 M KOH electrolyte. The cyclic voltammograms of NiO show
a roughly rectangular mirror image with respect to the zero-current
line, a representation of a capacitive behavior.17,18 The cyclic volta-
mmograms of NiO/CNT-20 shows the diagnostic peaks of anodic/
cathodic redox reactions at 0.54 and 0.34 V ͑vs the Hg/HgO elec-
trode͒, which are responsible for the Ni͑OH͒2/NiOOH redox
reaction.8,13,19,20
that the nanostructure of the as-prepared Ni͑OH͒ is almost the
2
same as that of the NiO after being calcined at 300°C, as shown in
Fig. 6. It seems that the calcined temperature is low enough to keep
the original nanostructure of the nickel oxide. The Ni͑OH͒2 and NiO
nanoparticles seem to be formed with nanofilaments, which form a
loose porous sphere structure adsorbed on the surface of CNTs.
Here, the SDS plays a key role in giving a template for the growth
of the nano-nickel oxide. In addition, the CNTs give a very good
original nanostructure for the nucleation, which strongly affects the
nanostructure of the nickel oxide. The zero-dimensional spheres in

Journal of The Electrochemical Society, 157 ͑7͒ A818-A823 ͑2010͒
A821
Figure 7. Pore-size distribution plots of sample NiO/CNT-20.
zero-dimensional porous sphere has a looser pore structure and a
larger specific surface area, which means that the access of electro-
lyte ions to reaction sites is easier and that there are more formation
sites of nickel hydroxide. So, compared with NiO, NiO/CNT-0,
NiO/CNT-10, and NiO/CNT-20, sample NiO/CNT-20 shows the
largest pore size in a series of composites, indicating the highest
capacitance. Such enhanced capacity proves our deduction from the
above section. Adding SDS to NiO/CNT composites can improve
the dispersion of Ni͑OH͒ nucleation on the surface of CNTs and
2
can change the crystalline structure, size, and morphology of
Ni͑OH͒ so as to change the conductivity and electrochemical uti-
2
lization of NiO.
To further evaluate the electrochemical capacity of the synthe-
sized NiO/CNT composites, we used these composites to fabricate
electrodes for supercapacitors and characterized them with chrono-
potentiometric measurements.
Our research on CNTs shows that its capacity is about 7.9 F g−1
in 1 M KOH; hence, we can deduce that the capacitive behaviors of
these composites are mainly attributed to the pseudocapacity of
NiO. So, the specific capacity of the NiO/CNT composites ͑C͒ and
NiO in the composites ͑CNiO͒ is calculated as Eq. 1
Figure 6. The TEM images of Ni͑OH͒2/CNT-20 and NiO/CNT-20 compos-
ites prepared with 20 g SDS. d1: Ni͑OH͒2/CNT-20; d2: NiO/CNT-20.
It is believed that NiO, in contact with an alkaline solution, tends
to change into nickel hydroxide at the surface, forming a surface
layer onto NiO probably within several angstroms of the nickel
oxide.8,13,19,20 To rationalize the above results, possibly the morphol-
ogy and size effect of NiO may affect the generation of nickel hy-
droxide. With the increasing additional mass of SDS, the shape of
nano-NiO changes from a two-dimensional disk to a zero-
dimensional sphere. Compared with the two-dimensional flake, the
Table I. Structural parameters of the samples.
BET specific
surface area
BJH pore
volume
BJH pore
diameter
͑nm͒
Sample
͑m2 g−1
͒
͑cm3 g−1
͒
NiO
176.3506
128.9194
111.9441
111.3016
0.1193
0.1833
0.1765
0.1246
4.3661
7.3492
7.3776
7.7310
NiO/CNT-0
NiO/CNT-10
NiO/CNT-20
Figure 8. Cyclic voltammograms of CNTs, NiO, and NiO/CNT-20 compos-
ite electrodes at a scan rate of 10 mV/s in 1 M KOH electrolyte.

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Journal of The Electrochemical Society, 157 ͑7͒ A818-A823 ͑2010͒
Figure 9. Cyclic life of the different conditions of composites at a constant
current density of 84 A g−1. The pure NiO powder measured at 20 A g−1
.
C
I ϫ ⌬t
CNiO
=
=
͓1͔
m ϫ ␪ ⌬V ϫ m ϫ ␪
where I is the discharge current, m is the mass of the composite, ⌬V
is the potential drop during discharging, ⌬t is the total discharge
time, and C is the specific capacity. ␪ is the weight percent of NiO
in NiO/CNTs.
The cycle life of all samples is monitored by a chronopotentiom-
etry measurement at 84 A g−1 in 1 M KOH, as shown in Fig. 9.
Meanwhile, based on Eq. 1, the highest measured capacity of each
electrode is listed in Table II. The SEM micrographs of the initial
and cycled samples of NiO/CNT-20 composite electrodes are shown
in Fig. 10a and b. Evidently, no readily distinguishable difference in
morphology can be observed before and after cycling. The results
indicate that the composites have excellent stability in terms of mor-
phology structural integrity ͑devoid of any pulverization͒ even after
1000 cycles. Compared with pure NiO and NiO/CNT electrodes, the
two-dimensional sheet NiO dispersing on the surface of CNTs
shows a significant improvement in specific capacity. We think that
the size of sheet-shaped NiO becomes smaller and has more forma-
tion sites of nickel hydroxide than pure NiO. The zero-dimensional
porous sphere NiO dispersing on the surface of CNTs showed the
highest specific ͑1329 F g−1͒ capacity at 84 A g−1, which may be
because the zero-dimensional mesoporous sphere has large specific
surface areas and easier electrolyte ion transport. All the electrodes
reveal a stable specific capacity during consecutive cycle tests.
Figure 10. The SEM images of NiO/CNT-20 composite electrodes. ͑a͒ Be-
fore and ͑b͒ after cycling for 1000 cycles.
chemical performance of NiO/CNT composites is largely affected
by the morphology, size, and crystalline structure of NiO. The NiO/
CNT composite electrode shows a maximum specific capacitance of
1329 F g−1 as well as a good cycle life during 1000 cycle in a 1 M
KOH electrolyte at a very high current density, 84 A g−1
.
Acknowledgments
Conclusion
The authors gratefully acknowledge the financial support by the
Key Project of NSFC ͑200933005 and 20903077͒, the National 973
Program ͑2009CB220102͒, the Key Project Founded by Fujian
Province ͑2008H0087͒, and the project of NSFC ͑50702047͒.
A simple hydrothermal synthesis method is adopted to prepare
nickel oxide/CNT ͑NiO/CNT͒ composites. We obtain different mor-
phologies of NiO, which change from a two-dimensional disk to a
zero-dimensional mesoporous sphere, dispersing on the surface of
CNTs by altering reaction experimental conditions. The electro-
Xiamen University assisted in meeting the publication costs of this ar-
ticle.
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Table II. The specific capacity of NiO in the composite was cal-
culated from charge/discharge curves according to Eq. 1.
Weight percent
of NiO in
NiO/CNT composite
Capacity of the NiO
Sample
͑wt %͒
in composite at 84 A/g ͑F g−1
͒
NiO
100.00
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23.91
260.89 ͑20 A g−1
707.37
͒
NiO/CNT-0
NiO/CNT-10
NiO/CNT-20
1145.40
1329.02

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