design and synthesis of nico2o4–reduced graphene oxide composites for high performance...
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Cite this: J. Mater. Chem., 2011, 21, 10504
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Design and synthesis of NiCo2O4–reduced graphene oxide composites for highperformance supercapacitors†
Huan-Wen Wang, Zhong-Ai Hu,* Yan-Qin Chang, Yan-Li Chen, Hong-Ying Wu, Zi-Yu Zhangand Yu-Ying Yang
Received 20th February 2011, Accepted 27th April 2011
DOI: 10.1039/c1jm10758e
In the present work, we used charge-bearing nanosheets as building blocks to construct a binary
composite composed of NiCo2O4 and reduced graphene oxide (RGO). Co–Ni hydroxides intercalated
by p-aminobenzoate (PABA) ion and graphite oxide (GO) were exfoliated into positively charged
hydroxide nanosheets and negatively charged graphene oxide nanosheets in water, respectively, and
then these oppositely charged nanosheets were assembled to form heterostructured nanohybrids
through electrostatic interactions. The subsequent thermal treatment led to the transformation of the
hydroxide nanosheets into spinel NiCo2O4 and also to the reduction of graphene oxide. The as-
obtained NiCo2O4–RGO composite exhibits an initial specific capacitance of 835 F g�1 at a specific
current of 1 A g�1 and 615 F g�1 at 20 A g�1. More interestingly, the specific capacitance of the
composite increases with cycling numbers, reaches 1050 F g�1 at 450 cycles and remains at 908 F g�1
(higher than the initial value) after 4000 cycles. The high specific capacitance, remarkable rate
capability and excellent cycling ability of the composites mean that they show promise for application
in supercapacitors. Comparison with the capacitive behavior of pure NiCo2O4 and NiCo2O4
mechanically mixed with RGO displays the importance of the self-assembly of the nanosheets in
making a wide range of graphene-based composite materials for applications in electrochemical energy
storage.
1. Introduction
During the past decade, nanosheets, as a new class of two
dimensional (2D) nanoscale materials, have attracted intense
research activity because 2D quantum confinement may result in
intriguing properties that differ from those of bulk lamellar
systems.1,2 One of the most effective methods for synthesizing 2D
nanosheets is exfoliation, the separation of layered inorganic
solids into individual layers through the intercalation of bulky
organic molecules.3–5 Since exfoliated nanosheets possess
a considerable amount of surface charge, they can be used as
building blocks for heterostructured nanohybrids and hierar-
chically assembled multilayered films.6 Especially, sophisticated
functionalities or nanodevices may be designed through the
Key Laboratory of Eco-Environment-Related Polymer Materials ofMinistry of Education, Key Laboratory of Polymer Materials of GansuProvince, College of Chemistry and Chemical Engineering, NorthwestNormal University, Lanzhou, 730070, PR China. E-mail: [email protected]; Fax: +86 931 8859764; Tel: +86 931 7973255
† Electronic supplementary information (ESI) available: Raman spectraof RGO and the NiCo2O4–RGO composite, XRD pattern, TEM imageof the residual sediment obtained from the HCl leaching of theNiCo2O4–RGO composite, and electrochemical behaviors ofNiCo2O4–RGO mixture: See DOI: 10.1039/c1jm10758e/
10504 | J. Mater. Chem., 2011, 21, 10504–10511
selection of nanosheets and the combination with heterogeneous
species.
As a carbon nanosheet, graphene has sparked unprecedented
research fever due to its remarkable electronic properties.7,8
Recent progress in large-scale synthesis of graphene sheets from
GO9,10 has promoted extensive exploration. Graphene-based
materials have shown promising application in nanoelectronics
and energy storage recently. Because of their high specific surface
area and excellent conductivity, graphene sheets are expected to
be excellent electrode materials for supercapacitors.11–13
However, graphene materials exhibit unsatisfactory capacitance
performance (generally tens–264 F g�1) due to unavoidable
aggregation or restacking of graphene nanosheets.14–16 There-
fore, keeping the graphene sheets separated within the electrode
materials by loading redox-active materials will improve the
electrochemical performance of graphene-based supercapacitors.
The preparation of graphene oxide dispersed composite mate-
rials and the subsequent in situ reduction is considered as an
effective method to fabricate graphene hybrid materials with
molecule-level dispersion and maximum interfacial interaction
between the nanofiller and the matrix.17
In recent years, exfoliation of layered double hydroxides
(LDHs) into single layers has provided a new type of nanosheet
with ultimate two-dimensional anisotropy and positive charge.
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These nanosheets could be used as synthons in the preparation of
hybrid materials. In particular, the direct assembly of positively
and negatively charged nanosheets will provide a new and
rational way to design new materials with a precisely controlled
nanostructure. For example, graphene oxide–intercalated
a-hydroxides were synthesized in butanol by metathesis.18 In
addition, exfoliated oxide nanosheets such as Ti0.91O2 and
Ca2Nb3O10 and LDH nanosheets of Mg2/3Al1/3(OH)2 were
restacked into inorganic sandwich layered materials in form-
amide.19 However, the LDHs with high layer charge density and
integrated hydrogen bonding are often exfoliated in organic
solvents such as butanol and formamide by rendering the inter-
layer region organophilic.20,21 It is well known that most organic
solvents are expensive and toxic. The excessive consumption of
them usually causes a series of environmental problems. There-
fore, it would be ideal to achieve exfoliation of LDHs in water as
this would lead to a cheaper and greener route to large scale
production of composites that are potential electrode materials.
In the present work, the heteroassembly of oppositely charged
nanosheets at the molecular layer was used to design and
synthesize RGO-based composites. Positively charged hydroxide
nanosheets were produced by exfoliation of PABA intercalated
Co–Ni hydroxides with a Co/Ni molar ratio of 2 : 1 in water,
while negatively charged graphene oxide nanosheets were
directly obtained from the exfoliation of GO in water. The
precursors with heterostructure were formed through self-
assembly driven by electrostatic interactions. Subsequent
thermal treatments resulted in the formation of the NiCo2O4–
RGO composites. In the composites, highly-dispersed electro-
active particles on the RGO sheets possess multiple oxidation
states/structures, enabling rich redox reactions for pseudocapa-
citance generation. In addition, the loading of fine NiCo2O4
particles is able to reduce serious agglomeration and restacking
of the RGO sheets, and consequently provides a higher available
electrochemically active surface area for increasing energy
storage. As a consequence, the NiCo2O4–RGO composite shows
a very high specific capacitance of 1050 F g�1, with a remarkable
rate capability and excellent cycling ability, thus showing great
potential as an electrode material for high performance
supercapacitors.
2. Experiment
2.1 Synthesis of starting materials
GO was synthesized from graphite powder by a modified
Hummers method.22,23 PABA intercalated Co–Ni hydroxides
were synthesized on the basis of a previous report with modifi-
cation.24 Typically, 5 mmol of Co(NO3)2$6H2O and 2.5 mmol of
Ni(NO3)2$6H2O were dissolved in 100 ml of deionized water.
Afterwards, 75 ml of a 0.5MNH3 solution containing 6.75 mmol
of p-aminobenzoic acid was added dropwise into the above
solution under vigorous stirring. The solid product formed was
immediately centrifuged, and then dried at 60 �C for 12 h under
vacuum.
Colloidal dispersion of negatively charged graphene oxide
nanosheets was obtained by dispersing 50 mg of GO in 100 ml of
water by sonication for about half an hour. The PABA–Co–Ni
hydroxide (200 mg) was washed with deionized water, and as the
This journal is ª The Royal Society of Chemistry 2011
pH of the supernatant was�7 the solid tended to form an opaque
colloid. Finally, the suspensions containing positively charged
hydroxide nanosheets were obtained.
2.2 Self-assembly of two oppositely charged nanosheets
The colloidal dispersion of graphene oxide nanosheets was
slowly added to the suspensions containing hydroxide nano-
sheets under continuous stirring and then sonicated for 60 min.
The resulting dispersion was then stirred at 60 �C for 12 h. The
black precipitate obtained at the end of 12 h was repeatedly
washed with ethanol to remove the surfactants and the solid was
dried at 60 �C for 12 h under vacuum.
2.3 Synthesis of the NiCo2O4–RGO composite
The above black precipitate was heated in an air atmosphere at
200 �C for 6 h, resulting in the formation of the NiCo2O4–RGO
composite. The content of RGO was determined by weighing the
residue that was obtained after processing the NiCo2O4–RGO
composite at 800 �C for 8 h in air atmosphere. The weight percent
of RGO in the composite was 16%.
For comparison, NiCo2O4 and RGO were synthesized by the
same procedure as the composites in the absence of GO and the
Co–Ni hydroxide, respectively. The as-prepared NiCo2O4 and
RGO were added into a given volume of deionized water with
a weight ratio of 84 : 16 (NiCo2O4 : RGO) and then sonicated to
form their mechanical mixture. The mixture sample was dried at
60 �C for 12 h under vacuum.
2.4 Characterization
The products were characterized by field emission scanning
electron microscopy (FESEM; JEOL, JSM-6701F, Japan), and
transmission electron microscope (TEM; JEOL, JEM-2010,
Japan). The structure of the samples was examined by X-ray
diffraction (XRD; D/Max-2400) with Cu-Ka radiation (l ¼1.5418 �A) operating at 40 kV, 60 mA. The components of the
materials were measured by a Nicolet Nexus 670 FT–IR instru-
ment. Raman spectra were measured and collected using
a 1064 nm laser with RFS 100/S under ambient conditions, with
a laser spot size of about 1 mm.
2.5 Electrochemical measurement
The working electrodes were prepared by pressing mixtures of
the as-prepared samples, acetylene black and polytetrafluoro-
ethylene (PTFE) binder (weight ratio of 75 : 20 : 5) onto a nickel
foam current collector. Each working electrode contained about
2 mg of electroactive material and had a geometric surface area
of about 1 cm2. A typical three-electrode experimental cell
equipped with a working electrode, a platinum foil counter
electrode, and an Hg/HgO reference electrode was used for
measuring the electrochemical properties of the working elec-
trode. All the electrochemical measurements were carried out in 6
M KOH aqueous solution as the electrolyte. Cyclic voltammetry
(CV) and galvanostatic charge–discharge were carried out on
a CHI660B electrochemical working station. The specific
capacitance was calculated by C ¼ I � t/(V � m), where I is the
discharging current, t is the discharging time, V is the potential
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drop during discharge, and m is the mass of active material in
a single electrode.
3. Results and discussion
3.1 Synthesis of the NiCo2O4–RGO composite
Graphene-based hybrid materials are quite intriguing from the
perspectives of both fundamental science and technology
because the hybrid can enable versatile and tailor-made prop-
erties with performances far beyond those of the individual
materials.25 Currently, most of the graphene-based composites
for energy storage applications are prepared by reduction of GO
followed by the load of pseudocapacitive nanomaterials.
Consequently, they cannot fully reveal the excellent electric and
surface properties of graphene sheets because of the agglomer-
ation of the graphene sheets during the reaction processes. Here,
we present a new route, as shown in Scheme 1, through which
graphene-based composites with high capacitive performance are
designed and synthesized by combining the self-assembly from
2D nanosheets and thermal treatment. Graphene oxide nano-
sheets are highly negatively charged when dispersed in water,
apparently as a result of ionization of the carboxylic acid and
phenolic hydroxyl groups that are known to exist on the GO
sheets.26 In the water/PABA intercalated Co–Ni hydroxides
systems, the development of partial positive charge on the amine
end of the intercalated anion causes repulsion between the layers,
leading to exfoliation and colloidal suspension of positively
charged nanosheets.24 When two components with opposite
charges were carefully introduced together, the mutual electro-
static interactions drove them to self-assemble into hetero-
structured nanohybrids (assemblies). During a subsequent
thermal treatment (at 200 �C), Co–Ni hydroxide nanosheets in
the assemblies were transformed into NiCo2O4, while graphene
oxide was simultaneously reduced by divalent cobalt. The
resultant NiCo2O4 can serve as spacers to prevent aggregation of
individual graphene sheets, and form a well-dispersed NiCo2O4–
RGO composite.
3.2 Material characterization
The presence of hydroxide nanosheets in the colloidal suspen-
sions was confirmed by TEM. The low magnification image
(Fig. 1a) shows several folded hydroxide nanosheets while indi-
vidual hydroxide nanosheets are seen in the high magnification
Scheme 1 Preparation process of NiCo2O4–RGO composite.
10506 | J. Mater. Chem., 2011, 21, 10504–10511
image (Fig. 1b). The electron transparent nature, low contrast
and large area observed in the TEM images reveal hydroxide
nanosheets with thicknesses in the order of nanometres and
lateral dimensions in the range of micrometres.
The crystal phase and structure information of the samples
were obtained by XRD measurements. Fig. 1c shows XRD
patterns of GO, PABA–Co–Ni hydroxide and assemblies. GO
has a dominant peak centered at 2q¼ 10.4�, corresponding to the
basal spacing of 8.3 �A. The basal spacing of PABA–Co–Ni
hydroxide is calculated to be 15.7 �A from the basal reflections.
This value indicates that the PABA ions are successfully incor-
porated in the interlayer space during the synthesis. After self-
assembly of the exfoliated hydroxide nanosheets and graphene
oxide nanosheets, the basal spacing is reduced to 8.1 �A. The
reduced basal spacing of the assemblies compared to that of the
PABA–Co–Ni hydroxides indicates that the surfactant ions are
no longer present in the interlayer region. Possibly, negatively
charged graphene oxide nanosheets are introduced into the
interlayer region of the hydroxides compensating for the positive
charge. Besides, it should be noted that the basal spacing (8.1 �A)
of the assemblies correspond to the sum of the layer thicknesses
of the hydroxide (4.6 �A) and graphene oxide (3.4 �A). The result
indirectly demonstrates that hydroxide nanosheets and graphene
oxide nanosheets are alternately reassembled. Furthermore, to
our surprise, weak diffraction peaks at 2q values of 18.9�, 31.2�,36.7�, 44.7�, 59.1�, 65.0� on the XRD spectrum of assemblies are
observed (Fig. 1c), indicating that the formation of the assem-
blies is also accompanied by a minor amount of NiCo2O4 crys-
talline phase (marked by ;).
From Fig. 1d, we find that all the diffraction peaks of the
NiCo2O4–RGO composite are perfectly indexed to the cubic
spinel NiCo2O4 (JCPDS, No. 20–0781) after calcination at
200 �C.27,28 The broad XRD patterns show that the size of the
NiCo2O4 crystals is in the nanometre range. Moreover, no
conventional stacking peak of graphene sheets is detected, thus
suggesting that the graphene sheets are homogeneously anchored
by NiCo2O4, resulting in the low degree of graphitization. Pure
NiCo2O4 and RGO are also synthesized by the same procedure
for comparison. The unit cell dimension of the spinel structure,
a0, is determined from the observed d-spacing for the (3 1 1) plane
by using the formula for a cubic lattice: a0 ¼ d(h2 + k2 + l2)1/2,
where h, k, and l are the Miller indices. The calculated values for
the NiCo2O4–RGO composite and pure NiCo2O4 are calculated
as 8.1065 and 8.1147 �A, respectively, which are very close to the
value of 8.1100 �A given in the JCDS 20–0781 file for NiCo2O4.
Additionally, to verify if graphene oxide is converted to RGO in
the composite, the NiCo2O4–RGO composite is subjected to acid
treatment. The XRD pattern of the residual sediment is shown in
Fig. S1†. A broad peak with a basal spacing of 0.37 nm at 2q ¼23.7� is observed, and it is attributed to the characteristic peak of
RGO. This result suggests that GO nanosheets in the assemblies
are mostly reduced during the thermal process.
To further support the XRD study, the FT-IR spectra of the
samples are presented in Fig. 2a. The peaks at 3400 cm�1 and
1624 cm�1 for GO are attributed to O–H stretching and bending
vibration, respectively. The other oxygen-containing functional
groups are revealed by the bands at 1725, 1065, 1245 and 1375
cm�1, which correspond to C]O in COOH, C–O, C–OH and C–
O–C (epoxy), respectively.29 The presence of PABA ion in the
This journal is ª The Royal Society of Chemistry 2011
Fig. 1 (a) low and (b) high magnification TEM images of Co–Ni hydroxide nanosheets. (c) XRD patterns of GO, PABA–Co–Ni hydroxide and
assemblies. (d) XRD patterns of the NiCo2O4–RGO composite, RGO and NiCo2O4.
Fig. 2 (a) The FTIR spectra of GO, PABA–Co–Ni hydroxide and
assemblies. (b) Cross-sectional SEM image of the assemblies. For cross-
sectional SEM observation, the sample was filtered on a porous alumina
membrane (Anopore) filter with pores of 0.2 mm, followed by cutting the
membrane to view the profile.
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interlayer space of the PABA–Co–Ni hydroxide is confirmed by
the bands at 1607 (N–H bending vibration), 1512 (C]C inplane
vibrations of the aromatic ring), 1383 (carboxylate anion
stretching) and 790 cm�1 (aromatic ring C–H bending).30 The
FT-IR spectrum of the assemblies (Fig. 2a) has none of these
bands due to the absence of PABA. This also suggests the
presence of negatively charged graphene oxide sheets in the
interlayer space, which is in agreement with the XRD analyses.
The cross-sectional SEM image shows clearly the ordered
sandwich structures (Fig. 2b). The lamellar fringe is not
completely parallel but slightly twisted in some areas, which may
be due to the high flexibility of the nanosheets. Meanwhile, the
assemblies are loosely and not compactly packed. This unique
structure directly demonstrates the self-assembly between posi-
tively charged hydroxide nanosheets and negatively charged
graphene oxide nanosheets. The Raman spectra of RGO and
NiCo2O4–RGO display similar G and D bands appearing at
1588 and 1348 cm�1 respectively, indicating that the structure of
graphene has stayed in the composite (Fig. S2†).
To further characterize the structure of the NiCo2O4–RGO
composite, FESEM and TEM studies were carried out (Fig. 3).
Fig. 3a shows the FESEM images of the as-synthesized RGO. It
is evident that RGO are almost transparent. Graphene layers
interact with each other to form an open network. The FESEM
image of the NiCo2O4–RGO composite is shown in Fig. 3b. In
comparison with the FESEM image of RGO, the surface of the
composite is much rougher than that of RGO, which is attrib-
uted to the growth of the NiCo2O4 nanoparticles on RGO. The
TEM image of RGO (Fig. 3c) shows that RGO have some
wrinkles and folds, which result from the fact that the 2D
membrane structure becomes thermodynamically stable via
J. Mater. Chem., 2011, 21, 10504–10511 | 10507
Fig. 3 FESEM and TEM images of the as-prepared RGO (a, c) and NiCo2O4–RGO (b, d).
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bending.31 From Fig. 3d, it can be seen that the nanoscale
NiCo2O4 particles are uniformly dispersed on the surfaces of
graphene. In addition, a TEM image of the residual sediment
obtained from the HCl leaching of the NiCo2O4–RGO
composite is shown in Fig. S3†, displaying a typical graphene
structure.
3.3 Electrochemical measurements
The potential of using the NiCo2O4–RGO composite as an
electrode material for supercapacitors was tested by cyclic vol-
tammetry (CV) and a galvanostatic charge–discharge technique.
NiCo2O4 is generally regarded as a mixed valence oxide that
adopts a spinel structure in which the nickel ion occupies the
octahedral sites and the cobalt ion is distributed over both the
octahedral and tetrahedral sites. The solid-state redox couples
Co2+/Co3+ and Ni2+/Ni3+ are present in the structure, which
provide two kinds of active centers for generating pseudocapa-
citance. Fig. 4a shows CV curves of the NiCo2O4–RGO
composite at various scan rates. Two pairs of redox current peaks
correspond to the reversible reactions of Co2+/Co3+ and Ni2+/Ni3+
transitions (Fig. 4a). It should be pointed out that such a feature
was not reported previously for NiCo2O4 synthesized by the
epoxide-driven sol–gel28 or the coprecipitation decomposition
method,32 where only the Co2+/Co3+ transitions are observed.
This may be due to strong chemical interactions between the
nanoscale NiCo2O4 particles and the residual oxygen-contain-
ing functional groups on the RGO, or van der Waals interac-
tions between NiCo2O4 and the RGO. This intimate binding
affords facile electron transport between individual NiCo2O4
and the RGO, which is a key to the presence of redox peaks of
Ni2+/Ni3+ transitions. Fig. 4b shows galvanostatic discharge
curves of NiCo2O4–RGO composite at various specific
10508 | J. Mater. Chem., 2011, 21, 10504–10511
currents. The NiCo2O4–RGO composite exhibits a specific
capacitance as high as 835 F g �1 at a specific current of 1 A
g�1. (Fig. 4b, 4c). The specific capacitance is still as high as 615
F g�1 even at a high specific current of 20 A g�1 (Fig. 4b, 4c).
Importantly, the columbic efficiency is nearly 100% for each
cycle of charge and discharge (Fig. 4d). The excellent cycle
stability of the NiCo2O4–RGO composite is demonstrated in
Fig. 4e. Interestingly, the specific capacitance of the sample
increases with cycling numbers, instead of decreasing as in most
cycle stability tests. At a cycle number of 450, the specific
capacitance reaches a very high value of 1050 F g�1 (Fig. 4e),
indicating that the additional 450 cycles are needed to fully
activate the sample.33 After a 4000-cycle test, the specific
capacitance is still 908 F g�1, which is higher than its initial
value. These results reveal that the high specific capacitance,
remarkable rate capability and excellent cycle stability are
achieved in the NiCo2O4–RGO composite material for elec-
trochemical supercapacitors.
We also performed electrochemical measurements for pure
NiCo2O4 prepared under similar conditions. As shown in Fig. 5a,
only the redox peaks of Co2+/Co3+ transitions are observed for
pure NiCo2O4, which is different from the feature of CV for the
NiCo2O4–RGO composite (Fig. 4a). This shows that RGO have
a significant influence on electrochemical activity of NiCo2O4 in
the composite. Pure NiCo2O4 exhibits an inferior rate capability
as compared with the NiCo2O4–RGO composite. The specific
capacitance is 662 F g�1 at a specific current of 1 A g�1, whereas
the specific capacitance is only 349 F g�1 at a high specific current
of 16 A g�1, 53% of that at 1 A g�1 (Fig. 5b, 5c). In particular, in
strong contrast to the NiCo2O4–RGO composite, pure NiCo2O4
shows very poor cycle stability and the decay in specific capaci-
tance based on the initial value after a 300-cycle test reaches 52%
(Fig. 5d).
This journal is ª The Royal Society of Chemistry 2011
Fig. 4 Electrochemical characteristics of the NiCo2O4–RGO composite: (a) CV curves at various scan rates. (b) Galvanostatic discharge curves at
various specific current. (c) Specific capacitance at different specific currents. (d) Galvanostatic charge and discharge curves at a specific current of 20 A
g�1. (e) Evolution of the specific capacitance versus the number at 2 A g�1.
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For further comparison, NiCo2O4 was physically mixed with
RGO in the same NiCo2O4–RGO mass ratio as the synthesized
NiCo2O4–RGO composite material. As shown in Fig. S4†, the
electrochemical performance of the physical mixture is inferior to
that of the NiCo2O4–RGO composite, albeit the cycle stability is
obviously improved in comparison with pure NiCo2O4. At
a specific current of 1 A g�1, the specific capacitance was 376 F
g�1 (Fig. S4b†), a factor of 2.2 lower than that of the NiCo2O4–
RGO composite at the same specific current. The specific
capacitance further decreased to 247 F g�1 at a specific current of
14 A g�1 (Fig. S4b, 4c).
3.4 Discussion
Recent work has shown applications of oxides/hydroxides13,34–38
and conducting polymers39,40 coupled with graphene sheets in
supercapacitors. However, to our best knowledge, little work
involves the spinel NiCo2O4 anchored on RGO sheets for
This journal is ª The Royal Society of Chemistry 2011
electrochemical energy storage. Usually, 2D nanosheets show
significantly enhanced host capabilities as redox active electrode
materials, due to their extremely high electrochemically active
surface area and the full utilization of the surface active sites for
redox reaction.41 To further exploit the unique properties of 2D
hydroxide nanosheets and graphene materials, we designed and
synthesized the NiCo2O4–RGO composite via the self-assembly
of oppositely charged nanosheets followed by thermal treatment.
The composites exhibited excellent electrochemical capacitance
and high cycling stability, making them potentially useful for
high performance supercapacitor materials. According to a gal-
vanostatic charge–discharge test in a three electrode system,
a specific capacitance of as high as 835 F g�1 was achieved at the
initial cycle for the NiCo2O4–RGO composite. Moreover, the
specific capacitance reached 1050 F g�1 after the materials were
fully activated through electrochemical reactions during charge–
discharge. This value is even higher than that of bare RuO2
(648 F g�1) and other reported values (500–788 F g�1) of RuO2.42
J. Mater. Chem., 2011, 21, 10504–10511 | 10509
Fig. 5 Electrochemical behavior of the as-prepared NiCo2O4: (a) CV curves at various scan rates. (b) Galvanostatic discharge curves at various specific
current. (c) Specific capacitance at different specific currents. (d) The evolution of the specific capacitance versus the number of cycles at 2 A g�1.
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This composite material also outperforms the spinel NiCo2O4
thin film (580 F g�1).27 Taking into account the low cost and
environmentaly friendly nature of NiCo2O4 and graphene
materials, our obtained composite is a promising candidate for
use as an electrode material for high performance
supercapacitors.
Several factors endow the NiCo2O4–RGO composite with
superior electrochemical performance in supercapacitors. First,
the controlled assembly of 2D hydroxide nanosheets and gra-
phene oxide nanosheets and the subsequent in situ reduction are
beneficial to the formation of graphene hybrid materials with
molecule-level dispersion and high interfacial interaction
between the graphene sheets and the NiCo2O4 matrix. Second,
the intimate combination of binary oxide nanocrystals with
electronically conducting graphene provides electron super-
highways,43,44 allowing for rapid and efficient charge transport
and leading to an increase in the overall electronic conductivity.
Third, grafting these NiCo2O4 particles onto the mechanically
stable RGO can effectively prevent the agglomeration of elec-
troactive material, appropriately decrease restacking of graphene
sheet ensembles, and consequently provide a higher available
electrochemically active surface area for exploiting the full
advantages of NiCo2O4 pseudocapacitance and graphene-based
double-layer capacitance.
In our experiment, pure NiCo2O4 shows inferior rate capa-
bility and poor cycle stability. This further confirmed that the
conductivity and dispersive state of graphene sheets are impor-
tant for enhancing the pseudocapacitance of NiCo2O4. We also
10510 | J. Mater. Chem., 2011, 21, 10504–10511
observed that the mechanical mixture of NiCo2O4 and RGO
showed significantly lower specific capacitances than the
NiCo2O4–RGO composite at all specific currents. This result
highlights again the value of our experimental method. Taken
together, we conclude that the method we have developed is
highly effective for the construction of advanced graphene based
composites directly from charge-bearing 2D nanosheets.
4. Conclusions
In summary, NiCo2O4–RGO as electrode materials for super-
capacitors were prepared by combining the self-assembly of
positively and negatively charged nanosheets and subsequent
thermal treatment. The composite displayed high specific
capacitances, remarkable rate capability and excellent cycle
stability, which was significantly superior to pure NiCo2O4 and
the mechanical mixture of NiCo2O4 and RGO. Superior
conductivity and the dispersive action of graphene sheets have
a significant influence on the electrochemical activity of
NiCo2O4. The present work provides a deeper understanding of
the concept of the self-assembly of 2D nanosheets as a straight-
forward processing protocol for the build-up of graphene based
composites with advanced electrochemical properties.
Acknowledgements
The authors gratefully acknowledge the financial support offered
by the National Natural Science Foundation of China
This journal is ª The Royal Society of Chemistry 2011
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(20963009), Gansu Science and Technology Committee
(0803RJA005) and the postgraduate advisor program of
Provincial Education Department of Gansu.
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