J. Cent. South Univ. (2014) 21: 2596−2603
DOI: 10.1007/s11771-014-2218-7
Excellent supercapacitive performance of a reduced graphene oxide/Ni(OH)2
composite synthesized by a facile hydrothermal route
ZHENG Cui-hong(郑翠红), LIU Xin(刘欣), CHEN Zhi-dao(陈志道),
WU Zhen-fei(伍振飞), FANG Dao-lai(方道来)
Anhui Key Laboratory of Metal Materials and Processing (School of Materials Science and Engineering,
Anhui University of Technology), Ma’anshan 243002, China
© Central South University Press and Springer-Verlag Berlin Heidelberg 2014
Abstract: A reduced graphene oxide/Ni(OH)2 composite with excellent supercapacitive performance was synthesized by a facile
hydrothermal route without organic solvents or templates used. XRD and SEM results reveal that the nickel hydroxide, which
crystallizes into hexagonal -Ni(OH)2 nanoflakes with a diameter less than 200 nm and a thickness of about 10 nm, is well combined
with the reduced graphene oxide sheets. Electrochemical performance of the synthesized composite as an electrode material was
investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge measurements. Its
specific capacitance is determined to be 1672 F/g at a scan rate of 2 mV/s, and 696 F/g at a high scan rate of 50 mV/s. After 2000
cycles at a current density of 10 A/g, the composite exhibits a specific capacitance of 969 F/g, retaining about 86% of its initial
capacitance. The composite delivers a high energy density of 83.6 W·h/kg at a power density of 1.0 kW/kg. The excellent
supercapacitive performance along with the easy synthesis method allows the synthesized composite to be promising for
supercapacitor applications.
Key words: supercapacitors; reduced graphene oxide; nickel hydroxide; hydrothermal method; electrochemical performance
1 Introduction
Electrochemical
capacitors,
often
called
supercapacitors, have the unique characteristic of larger
power density and longer cycle life than secondary
batteries, and higher energy density than conventional
capacitors [1]. Nowadays, supercapacitors have been
used as energy-storage devices to complement or replace
batteries in many fields, such as uninterruptible power
supplies, consumer electronics and industrial power and
energy management [2]. However, energy density of
supercapacitors is still much lower than that of batteries.
Recently, for further increasing energy density of
supercapacitors, various pseudocapacitive materials such
as oxides [3], polymers [4] and hydroxides [5], whose
charge-storage mechanism is based on faradic redox
reactions, have been extensively explored as electrode
materials. Among the pseudocapacitive materials,
hexagonal-layered Ni(OH)2 is believed to be one of the
most promising electrode materials for supercapacitors,
due to its large theoretical specific capacitance (SC),
well-defined redox behavior and low cost [6]. However,
compared with carbonaceous materials for electrical-
double-layer capacitors, these pseudocapacitve materials
exhibit not only lower electrochemical reversibility, but
also much smaller electrical conductivity, which can not
meet fast electron transport during high-rate charge/
discharge process. As a result, the increased energy
density of the pseudocapacitors usually compromises
their rate capability and reversibility.
Newly found graphene is a fascinating twodimensional carbonaceous material with the nature of
light weight, large surface area, high electrical
conductivity, high flexibility, and good chemical
tolerance [7], which is recognized as an ideal substrate
for growth of nanomaterials for energy storage [8]. Very
recently, a lot of works have been reported concerning
synthesis of graphene/Ni(OH)2 composites with
improved supercapacitive performance [9−12]. However,
these synthesis routes usually adopt a considerable
quantity of organic solvents such as ethylene glycol [9],
N,N-dimethylformamide [10] and N-methylpyrrolidone
[11], or surfactant templates such as benzenesulfonate
[12], which inevitably lead to the difficulty in the
removal of organic solvents or surfactant templates, a
rise in cost, and potential environmental pollution,
consequently, infeasibility of the large-scale production.
Foundation item: Project(KJ2012A045) supported by the Natural Science Foundation of Education Commission of Anhui Province, China
Received date: 2013−04−27; Accepted date: 2013−09−02
Corresponding author: FANG Dao-lai, Associate Professor, PhD; Tel: +86−555−2311570; Fax: +86−555−2311570; E-mail: fangdl@ahut.edu.cn
J. Cent. South Univ. (2014) 21: 2596−2603
To the best of our knowledge, easy and environmentally
friendly synthesis routes to high-performance graphene/
Ni(OH)2 composites have been scarcely reported until
now.
In this work, a reduced graphene oxide (RGO)/
-Ni(OH)2 composite with excellent supercapacitive
performance was synthesized by a facile hydrothermal
route without organic solvents or surfactant templates
used. Phase compositions and morphology of the
obtained composite were investigated. Also its
electrochemical performance was evaluated and
compared with that of the pure -Ni(OH)2 synthesized
under the same conditions.
2 Experimental
2.1 Synthesis of electrode materials
All the reagents, purchased from Sinopharm
Chemical Reagent Co. Ltd., were of analytical grade, and
used as received. Graphene oxide was prepared by a
modified Hummers’ method. In a typical procedure, 5 g
of natural flake graphite powder was added into a
mixture of 125 mL of 98 % sulfuric acid and 3.5 g of
sodium nitrate, and the formed suspension was strongly
stirred for 15 min in a 500 mL reaction beaker immersed
in a water-glycol bath controlled at about 0 C. Then,
15 g of potassium permanganate was added slowly into
the suspension, which was then stirred at about 0 C for
another 15 min. Subsequently, the obtained suspension
continued to react at an elevated temperature of 35 C for
40 min, and 200 mL of de-ionized water was slowly
added to the suspension, during which the suspension
was rapidly stirred to control its temperature not beyond
90 C. Afterwards, the obtained suspension was kept at
about 90 C for 30 min. After this, the above suspension
was further diluted with de-ionized water and
ultrasonically treated for 40 min, and 40 mL of 30%
hydrogen peroxide was added to the ultrasonically
treated suspension to reduce residual permanganate to
soluble manganese ions, followed by filtrating and
washing the suspension to obtain graphene oxide. Finally,
100 mL of 50% hydrazine hydrate was added into the
graphene oxide suspension, and the reaction system was
stirred and refluxed in a silicon oil bath at 100 C for 5 h,
resulting in the RGO.
The RGO/Ni(OH)2 composite was synthesized by a
hydrothermal route. For obtaining a RGO/Ni(OH)2
composite with a mass ratio of RGO: Ni(OH)2 of 1:10,
1.2547 g of Ni(NO3)26H2O was dissolved in 50 mL
de-ionized water, and 0.0400 g of the obtained RGO was
ultrasonically dispersed in the prepared Ni(NO3)2
solution to form a homogeneous suspension. Then let the
obtained suspension stand for 24 h, followed by adding
2597
40 mL of 0.22 mol/L NaOH solution into it under
vigorous agitation. After further stirring for 0.5 h, the
reacting suspension was transferred to an autoclave,
which was subsequently kept in an oven at 180 C for
10 h. Finally, the solid product formed was separated
from the reaction system, and washed and dried in air at
60 C, resulting in the RGO/Ni(OH)2 composite. For
comparison, pure Ni(OH)2 was also synthesized under
the same conditions.
2.2 Characterization of structure and electrochemical
performance
A Philips X’pert Pro X-ray diffractometer with Cu
K radiation (=1.5406 Å) was used to analyze phase
compositions of the synthesized samples. Diffraction
data were collected in the 2 range from 10 to 75,
using the step-scan mode with a scanning speed of 0.02
step size and 1 s per step. Their morphology was
observed by using a field emission scanning electron
microscope (FESEM) called Nano SEM 430.
The RGO/Ni(OH)2 composite and the pure Ni(OH)2
were used as the electroactive materials. The electrodes
fabricated for electrochemical measurements were
composed of the electroactive material, acetylene black
(AB) and polytetrafluoroethylene (PTFE), whose mass
ratio was 80:15:5. The electroactive material and
acetylene black were fully mixed and ground, then a
proper amount of PTFE binder was added into the
ground mixture to achieve a homogeneous slurry,
followed by painting the prepared slurry onto a nickel
foam current collector with an area of 1 cm1 cm.
Finally, the painted current collector was dried for 10 h at
60 C, and pressed under a pressure of 10 MPa to form a
reliable electrode. Each electrode contained about 2.0 mg
of the electroactive material.
All the electrochemical measurements concerned
were carried out in 6 mol/L KOH aqueous electrolyte.
Cyclic voltammetry and electrochemical impedance
spectroscopy (EIS) were measured on a CHI604C
electrochemical workstation (Shanghai Chen hua
Instruments Co. Ltd., China) in a three-electrode cell
set-up, using an electrode fabricated above as the
working electrode, a platinum foil as the counter
electrode, and a saturated calomel electrode (SCE) as the
reference electrode. Cyclic voltammograms (CVs) at
various scan rates of 2−50 mV/s were recorded between
0 and 0.6 V vs SCE. EIS of the electrode was measured
applying an a.c. amplitude of 5 mV in a frequency range
of 0.01−105 Hz. The galvanostatic charge/discharge
performances at various current densities were
determined by a battery test system of Land CT2001A
(Wuhan Land Electronics Co. Ltd., China) in the
potential range of 0−0.45 V vs SCE.
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3 Results and discussion
3.1 Phase compositions of RGO, RGO/Ni(OH)2 and
pure Ni(OH)2
The XRD patterns of the RGO, the RGO/Ni(OH)2
composite and the pure Ni(OH)2 are shown in Fig. 1(a).
Clearly, the diffraction peaks of the RGO are weak and
highly dispersive, suggesting the disordered structure
formed in the RGO due to the oxidation and ultrasonic
exfoliation. The apparent asymmetry of the (002) peak
discloses that interlayer spacing and the size in c-axis
direction of the RGO are not uniform. Lorentzian fit for
(002) reflection of the RGO is shown in Fig. 1(b), and
the characteristic parameters obtained are given in Table
1. The fitted (002) profile in Fig. 1(b) yields two 2
values of 24.95 and 26.14, which are corresponding to
the interlayer spacing of 3.57 Å and 3.41 Å, respectively.
From the Bragg angle  and the full width at half
maximum (FWHM) intensity of the fitted (002) peaks, Lc,
Fig. 1 XRD patterns of RGO, RGO/Ni(OH)2 composite and pure
Ni(OH)2 (a) and Lorentzian fit for (002) reflection of RGO (b)
Table 1 Characteristic parameters obtained from fitted (002)
peak of RGO
2 value/
FWHM/()
Interlayer
spacing/Å
Lc, size in c-axis
direction/Å
Number of
layers
24.95/3.12
3.57
26(3)
7
26.14/1.19
3.41
69(13)
20
the size in c-axis direction, calculated by Scherrer’s
equation, is about 26 Å and 69 Å, corresponding to the
average thickness of 7 and 20 stacked RGO layers,
respectively. The interlayer spacing of the RGO,
especially the one of 3.57 Å, is remarkably larger than
the d-spacing of the well ordered graphite (3.36 Å),
implying incomplete reduction of the graphene oxide
(GO) to pristine graphene, and a certain amount of
oxygen-containing functional groups remaining on the
RGO sheets [13]. As shown in Fig. 1(a), the XRD pattern
of the RGO/Ni(OH)2 composite is in good agreement
with that of the hexagonal -Ni(OH)2 (JCPDS file
No.14-0117 ), confirming the formation of the pure
hexagonal - Ni(OH)2 under hydrothermal conditions.
Apparently, the XRD pattern of the -Ni(OH)2 in Fig. 1(a)
exhibits the selective broadening of the non-(hk0)
reflections, which is due to the presence of a large
amount of stacking faults in the crystal lattice, and of the
interstratification [14]. The selective broadening of the
non-(hk0) reflections signifies high electrochemical
reactivity and large capacity of the -Ni(OH)2 in the
composite [15]. Based on the fitted (001) diffraction
peak, the size of the -Ni(OH)2 in c-axis direction can be
calculated to be about 10 nm. The characteristic peaks of
the RGO can hardly been observed on the XRD pattern
of the composite, probably due to highly disordered
structure and low relative content (10%, mass fraction)
of the RGO in the composite. Also, XRD pattern in Fig.
1(a) confirms that the pure Ni(OH)2 synthesized under
the same conditions is composed of hexagonal
-Ni(OH)2.
3.2 Morphology of RGO, RGO/Ni(OH)2 and pure
Ni(OH)2
FESEM images of the RGO, the RGO/Ni(OH)2 and
the pure Ni(OH)2 are shown in Fig. 2. As shown in
Fig. 2(a), the RGO sheets are rippled and entangled with
each other, resembling crumpled silk veil waves. It was
reported that corrugating and scrolling are intrinsic to
graphene sheets, because thermodynamic stability of the
2D membrane is resulted from microscopic crumpling
via bending or buckling [16]. Apparently, a
morphological difference is observed for the RGO sheets,
part of which look less wrinkled, and the other part of
which seem more wrinkled. The less wrinkled sheets
might correspond to the RGO with the larger size Lc in
c-axis direction, and the more wrinkled sheets might
correspond to those with the smaller size Lc. The FESEM
result of the RGO is in accordance with the XRD result
in Fig. 1.
As shown by the low-magnification FESEM image
in Fig. 2(b), the prepared RGO/Ni(OH)2 composite
nearly retains the corrugating and scrolling feature of the
RGO sheets, and all the ultra-fine Ni(OH) 2 particles
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Fig. 2 FESEM images of RGO (a), RGO/Ni(OH)2 composite under low (a) and high (b) magnifications and pure Ni(OH)2 (d)
adhere to the crumpled RGO sheets. Almost no large
Ni(OH)2 agglomerates isolated are observed in the whole
FESEM field of view. The high-magnification image in
Fig. 2(c) discloses that the ultra-fine Ni(OH)2 particles
are practically in the shape of thin flake, whose diameter
is less than 200 nm. Importantly, these Ni(OH)2 flakes
are well combined with the RGO sheets, and stack
loosely on or between the surfaces of the RGO sheets,
forming a porous composite. Usually, there exist a
certain number of oxygen-containing functional groups
decorating on the RGO sheets [17], which is also
supported by the XRD result in Fig. 1. When the asprepared RGO is soaked in nickel nitrate solution for
24 h, a great number of nickel ions can be adsorbed onto
the surfaces of the RGO sheets due to the electrostatic
force between the nickel ions and negatively charged
residual oxygen-containing functional groups. When
adding NaOH precipitating agent, the oxygen-containing
functional groups, which have anchored nickel ions, act
as the energetically favorable sites for nucleation of the
Ni(OH)2 flakes. Consequently, the Ni(OH)2 flakes grow
on the RGO sheets, avoiding the formation of large
Ni(OH)2 agglomerates in the composite. Furthermore,
intercalation of the Ni(OH)2 species into the RGO sheets
may effectively restrain their restacking, which allows
the RGO sheets to keep a highly electroactive surface
area. Due to the favorable morphology formed, the
RGO/Ni(OH)2 composite is expected to exhibit high
electrochemical performance. The FESEM image in
Fig. 2(d) shows that the pure Ni(OH)2 is also
flake-shaped with a diameter of about 200 nm.
3.3 Electrochemical performance of RGO/Ni(OH)2
and pure Ni(OH)2 electrodes
CVs of the RGO/Ni(OH)2 composite and pure
Ni(OH)2 electrodes at various scan rates of 2−50 mV/s
are shown in Figs. 3(a) and (b), respectively. All the CVs
present a pair of intense redox peaks, which correspond
to the conversion between different oxidation states of Ni
ions according to the following equation [18]:
Ni(OH)2 + OH↔NiOOH+H2O+e
(1)
The CV of the RGO/Ni(OH)2 electrode at a scan
rate of 2 mV/s exhibits a cathodic peak at 0.22 V, and an
anodic peak at 0.36 V, while that of the pure Ni(OH)2
electrode shows a cathodic peak at 0.17 V, and an anodic
peak at 0.34 V. The potential difference (Eac) between
the anodic and cathodic peaks characterizes the
reversibility of the electrochemical redox reaction: the
higher the reversibility, the smaller the Eac. Clearly, the
Eac (0.14 V) for the RGO/Ni(OH)2 electrode is smaller
than that for the pure Ni(OH)2 electrode (0.17 V),
demonstrating higher reversibility of the RGO/Ni(OH)2
electrode. As seen in Figs. 3(a) and (b), with increasing
scan rate, the anodic and cathodic peaks shift to the
positive and negative directions, respectively, due to the
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where Cs is the SC (F·g1), m is the mass (g) of the
electroactive material, v is the potential scan rate (V·s1),
and I is the even current response (A) defined by
1
I
idV (Vc and Va represent the lowest and
2(Va  Vc ) 
highest potentials (V), respectively). Scan rate
dependence of the SC values for the two electrodes is
shown in Fig. 3(c). The RGO/Ni(OH)2 electrode shows a
SC of 1672 F/g at a scan rate of 2 mV/s, and it still
delivers a considerable SC of 696 F/g at an increasing
scan rate of 50 mV/s, retaining 42% of its SC at 2 mV/s.
The pure Ni(OH)2 electrode exhibits a SC of 1535 F/g at
a scan rate of 2 mV/s, and of 421 F/g at a scan rate of
50 mV/s, retaining 27% of its SC at 2 mV/s. Apparently,
the RGO/Ni(OH)2 electrode shows larger SC and higher
rate capability.
For further investigation of the actual
electrochemical diffusion process, EIS spectra of the
RGO/Ni(OH)2 and pure Ni(OH)2 electrodes are
measured in the frequency range from 0.01 to 105 Hz
with an a.c. excitation signal of 5mV, as shown in Fig. 4.
The EIS spectra are analyzed by using the CNLS fitting
method based on the equivalent circuit given in the inset
of Fig. 4. The Rs, Cdl Rct, Zw and Cl values obtained are
given in Table 2. Rs is the internal resistance, composed
of the ionic resistance of electrolyte, the intrinsic
resistance of the active material, and the contact
resistance at the active material/current collector
interface; Cdl is the double-layer capacitance on the grain
surface; Rct is the interfacial charge-transfer resistance
during the faradic reactions, which is often the main
limiting factor for rate capability of an electrode
Fig. 3 CVs for RGO/Ni(OH)2 composite (a), pure Ni(OH)2 (b)
electrodes at various scan rates of 2−50 mV/s, and scan rate
dependence of SC values of electrodes (c)
limitation of the ion diffusion rate to meet electronic
neutralization during the redox reaction. However, the
shape of the CVs for the RGO/Ni(OH)2 electrode
changes less remarkably with increasing scan rate from 2
to 50 mV/s, compared with that of the CVs for the pure
Ni(OH)2 electrode. This suggests higher rate capability
of the RGO/Ni(OH)2 electrode. Specific capacitance (SC)
values of the two types of electrode materials at various
scan rates are calculated from the CVs according to the
following formula:
Cs 
I
mv
(2)
Fig. 4 EIS within frequency range of 0.01−105 Hz of
RGO/Ni(OH)2 and pure Ni(OH)2 electrodes
Table 2 Calculated values of Rs, Cdl Rct, Zw and Cl from CNLS
fitting of EIS based on proposed equivalent circuit in Fig. 4
Cdl/F
Rct/
Zw/
Cl/F
RGO/Ni(OH)2 0.927
0.00129
0.758
1.182
2.274
Pure Ni(OH)2 1.267
0.00107
2.417
1.345
1.796
Sample
Rs/
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material; Cl is the limit capacitance; Zw is the Warburg
resistance, corresponding to the 45 slope of the curve,
resulting from the frequency dependence of ion
diffusion/transport in the electrolyte. In comparison with
the pure Ni(OH)2 electrode, the RGO/Ni(OH)2 electrode
shows obviously smaller Rs and Rct, implying that charge
transfer and ion diffusion are easier during the
electrochemical reactions in the RGO/Ni(OH)2 electrode.
The EIS results are in accordance with the CV results in
Fig. 3.
Galvanostatic charge/discharge profiles and SC
variation with current density for the RGO/Ni(OH)2 and
pure Ni(OH)2 electrode are shown in Fig. 5. As seen
from Figs. 5(a) and (b), all the galvanostatic charge/
discharge profiles at various current densities present a
similar feature: the potential changes linearly with time
in the lower potential range, and nonlinearly with time in
the higher potential range. For example, the charging
profile at a current density of 1 A/g in Fig. 5(a) displays a
linear potential increase in the potential range of
0−0.25 V and a nonlinear one in the potential range of
0.25−0.45 V, and the discharging counterpart exhibits a
nonlinear potential decrease in the potential range of
0.45−0.20 V and a linear one in the potential range below
0.20 V. The nonlinear potential variation signifies the
pseudocapacitance behavior, which results from the
electrochemical adsorption/ desorption or redox reaction
at the interface at certain potentials, while the linear
potential variation with time indicates the double-layer
capacitance behavior, which is caused by the charge
separation taking place at an electrode/electrolyte
interface [18]. As seen from galvanostatic charge/
discharge profiles in Figs. 5(a) and (b), the nonlinear
potential variation dominates the charge/discharge
process, suggesting that the charge- storage capacity of
the RGO/Ni(OH)2 composite and pure Ni(OH)2
electrodes mainly originates from the faradic redox
reaction of the Ni(OH)2 species in the composite.
SC values of the RGO/Ni(OH)2 and pure Ni(OH)2
electrodes are calculated according to the following
equation:
Cs 
It
mV
(3)
where Cs is the SC (F/g), I is the constant current (A), t is
the discharge time (s), V is the total potential deviation
(V) (i.e. 0.45V in our case), and m is the mass of the
active material in the electrode. Variation of SC value
with charge/discharge current density for the
RGO/Ni(OH)2 and pure Ni(OH)2 electrodes is given in
Fig. 5(c). The RGO/Ni(OH)2 electrode exhibits a SC
value as large as 1702 F/g at a current density of 1 A/g,
and when increasing current density to 40 A/g it still
Fig. 5 Galvanostatic charge/discharge profiles of
RGO/Ni(OH)2 (a) and pure Ni(OH)2 electrodes (b) at various
current densities of 1−40 A/g, and variation of SC value with
discharging current density for electrodes (c)
delivers a considerable SC of 873 F/g, showing a
capacitance retention of about 51%. The pure Ni(OH)2
electrode possesses a SC of 1563 F/g at 1 A/g, and of
593 F/g at 40 A/g, showing a capacitance retention of
38%. Obviously, the RGO/Ni(OH)2 composite exhibits
larger SC and higher rate capability than the pure
Ni(OH)2. Using a non-aqueous approach in the medium
of ethylene glycol, LEE et al [9] synthesized a
RGO/Ni(OH)2 composite, whose SC was determined to
be 1215 F/g at a scan rate of 5 mV/s, and 521 F/g at 50
mV/s. WANG et al [10] obtained a Ni(OH)2/graphene
composite using a two-step method in a 10:1 N,N-
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2602
dimethylformamide (DMF)/H2O mixed solvent, and its
SC was 1335 F/g at a discharge current density of
2.8 A/g, and 935 F/g at 45.7 A/g. SUN and LU [12]
achieved RGO/Ni(OH)2 composites by a solid-state
reaction route using benzenesulfonate as the surfactant
template, and the obtained composite with the optimum
performance showed a SC of 820 F/g at a discharge
current density of 4 A/g, and 420 F/g at 11.2 A/g.
Apparently, the SC and rate capability of the
RGO/Ni(OH)2 composite we prepared are superior or
equivalent to those of the composites reported in the
above works [10−12]. This may be attributed to the more
favorable compositions and morphology of the
RGO/Ni(OH)2 composite, in which the loosely stacked
Ni(OH)2 thin flakes stick closely to the RGO sheets, as
revealed by the FESEM images in Figs. 2(b) and (c). The
close combination of the Ni(OH)2 flakes with the RGO
sheets allows the prepared composite to have a high
electrical conductivity, while the thin-flake shape of the
Ni(OH)2 species and the formed porosity yield a large
electroactive surface area. Consequently, during the
high-rate
charge/discharge
process,
rapid
ion
intercalation/deintercalation and fast electron transfer
become possible, and a high percentage of electroactive
material is available for the electrochemical reactions.
Furthermore, the RGO/Ni(OH)2 composite we prepared
is derived from the hydrothermal route without organic
solvents or surfactant templates used, which is very
simple and low-cost, thereby avoiding potential
environmental pollutions and the difficulty in the
separation of the prepared composites from the reaction
systems.
To evaluate their long-term cycling stability, the
RGO/Ni(OH)2 and pure Ni(OH)2 electrodes were
subjected to 2000 cycles at a current density of 10 A/g,
as shown in Fig. 6(a). For the RGO/Ni(OH)2 electrode,
during the first 68 cycles, the SC increases from 1133 to
1199 F/g, rising by about 6%. The SC rise is probably
due to the activation process, which increases the number
of the available active sites, and allows the trapped ions
to gradually diffuse out [17]. During the next 600 cycles,
the SC decreases rapidly from 1199 to 1054 F/g. During
the following cycles, the SC decays slowly, and tends to
be stable. After 2000 cycles, the RGO/Ni(OH)2 electrode
still preserves a SC of 969 F/g, about 86% of its initial
SC. During 2000 cycles, the SC of the pure Ni(OH)2
electrode decreases almost continuously. After 2000
cycles, it only retains 72% of its initial capacitance.
Evidently, the RGO/Ni(OH)2 electrode shows higher
cycling stability, which may be attributed to its favorable
morphology. The RGO sheets may act as a flexible
cushion that releases the mechanical strains generated in
the RGO/Ni(OH)2 electrode due to the volume change of
the -Ni(OH)2 nanoflakes during charge/discharge
cycling [19].
Fig. 6 Long-term cycling stability measured at current density
of 10 A/g (a) and Ragone plot of energy density dependence
of power density for RGO/Ni(OH)2 and pure Ni(OH)2
electrodes (b)
Ragone plot, which describes the relation between
energy density and power density, is an efficient way to
evaluate the capacitive performance of supercapacitor
electrode materials. The Ragone plot for the
RGO/Ni(OH)2 and pure Ni(OH)2 electrodes is derived
from their CVs at various scan rates, as given in Fig. 6(b).
The power density and energy density are estimated
based on the following formulae:
E
1
Cs (V ) 2
2
(4)
E
(5)
t
where E is the average energy density (W·h/kg), Cs is the
SC (F/g) based on the mass of the RGO/Ni(OH)2
composite, V is the potential window of discharge (V),
P is the average power density (kW/kg), and t is the
discharge time (s). The RGO/Ni(OH)2 electrode exhibits
an ultra-high energy density of 83.6 W·h/kg at a high
power density of 1.0 kW/kg. Importantly, the energy
density decreases slowly with increasing power density,
and the RGO/Ni(OH)2 electrode still delivers a
considerably high energy density of 34.8 W·h/kg at a
P
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power density as high as 10.4 kW/kg. The pure Ni(OH)2
electrode possesses a considerably high energy density of
76.7 W·h/kg at a power density of 0.92 kW/kg, however,
its energy density decays more rapidly with increasing
power density, decreasing to 21.0 W·h/kg when
increasing power density to 6.3 kW/kg. In terms of the
energy density and the power density, the capacitive
performance of the RGO/Ni(OH)2 composite we
synthesized is superior to that of the graphene/Ni(OH)2
composites synthesized in organic solvents [9−12], and
comparable to that of the Ni(OH)2/graphene composite
derived from a chemical precipitation approach [20].
The electroactive material of the RGO/Ni(OH)2
composite we prepared exhibits large SC, high rate
capability, good cycling stability and high energy density.
The excellent supercapacitive performance could be
attributed to the synergistic effect between the Ni(OH)2
nanoflakes and the RGO sheets in the prepared
composite [20]. The close combination of the loosely
stacked Ni(OH)2 nanoflakes with the RGO sheets not
only makes a large active surface available for
electrochemical reactions, but also facilitates fast
electron transfer and rapid ion intercalation/
deintercalation during high-rate charging/discharging
process.
4 Conclusions
1) A RGO/-Ni(OH)2 composite is synthesized by a
simple and low-cost hydrothermal route without organic
solvents or surfactant templates involved. In the
composite, the Ni(OH)2 nanoflakes formed are closely
combined with the RGO sheets.
2) The composite exhibits excellent supercapacitive
performance. Its SC is 1672 F/g at a scan rate of 2 mV/s,
and 696 F/g at 50 mV/s. After 2000 cycles at a current
density of 10 A/g, it still retains 86% of its initial SC. Its
maximum energy density is 83.6 W·h/kg at a power
density of 1.0 kW/kg.
3) The composite shows much superior
electrochemical performance, compared with the pure
Ni(OH)2 synthesized under the same conditions. This
might be attributed to the synergistic effect between the
Ni(OH)2 nanoflakes and the RGO sheets.
2603
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[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
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(Edited by FANG Jing-hua)