Electrochimica Acta 129 (2014) 334–342
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Enhanced Symmetric Supercapacitive Performance of Co(OH)2
Nanorods Decorated Conducting Porous Graphene Foam Electrodes
U.M. Patil a,1 , Su Chan Lee a,1 , J.S. Sohn a , S.B. Kulkarni a , K.V. Gurav b , J.H. Kim b ,
Jae Hun Kim c , Seok Lee c , Seong Chan Jun a,∗
a
Nano-Electro Mechanical Device Laboratory, School of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea
Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea
c
Sensor System Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, South Korea
b
a r t i c l e
i n f o
Article history:
Received 28 November 2013
Received in revised form 24 January 2014
Accepted 13 February 2014
Available online 26 February 2014
Keywords:
Symmetric supercapacitor
Co(OH)2 nanorods
Graphene foam
Specific capacitance
Energy and power density.
a b s t r a c t
Hierarchical entangled Co(OH)2 nanorods (NRs) are anchored on graphene foam (GF) electrodes by using
a facile chemical bath deposition (CBD) method. The porous, conducting, and higher specific area access
offered by the interconnected 3D graphene framework along with unique Co(OH)2 NRs morphology of
electrode displays ultrahigh specific capacitance, energy and power density. The Co(OH)2 /GF electrode
reveals maximum specific capacitance about 1139 F g−1 at 10 A g−1 charge-discharge current density
in 1 M KOH aqueous solution. Moreover, Co(OH)2 /GF electrode in the symmetric supercapacitor device,
reveals a high energy (13.9 Wh kg−1 ) with power (18 kW kg−1 ) density. These results promote the potential applicability of Co(OH)2 /GF electrode in the supercapacitor field with effective boosting in charge
transfer and storage mechanism.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
The ever-increasing energy and power demands in the applications such as cordless electric tools, hybrid electric vehicles,
day/night storage, and industrial energy management [1–4]. Therefore, over the past few years, intense efforts have been made to
develop high-energy and power electrochemical capacitors (ECs)
due to their faster charge and discharge processes (seconds) compared to batteries. However, ECs are unable to achieve high energy
density (normally <10 Wh kg−1 ) as compare to batteries [5]. To
address this problem, the innovation of new materials is essential
to offer enhanced energy and power densities of energy storage
devices [6,7]. On the other hand, fabricating symmetric/asymmetric
devices is an effective approach to increase the supercapacitors the
energy and power density [8–10].
Generally, despite of poor specific energy density, carbonaceous materials are used in the fabrication of symmetric and
asymmetric supercapacitors devices due to exceptional cyclic stability and high interfacial capacitance [11–13]. The obstacle of low
energy density can be overcome by employing pseudo-capacitive
∗ Corresponding author. Tel.: +82 2 2123 5817; fax: +82 2 312 2159.
E-mail address: [email protected] (S.C. Jun).
1
These authors contributed equally.
http://dx.doi.org/10.1016/j.electacta.2014.02.063
0013-4686/© 2014 Elsevier Ltd. All rights reserved.
materials such as metal oxides/hydroxides and conducting polymers in supercapacitor devices [13]. To date, various materials such
as transition metal oxides, metal hydroxides, and electronically
conducting polymer materials have been extensively investigated
for possible applications in supercapacitors such as NiO, CoOx,
MnO2 , Mn3 O4 , CuO, Ni(OH)2 and Co(OH)2 [14–20]. The Co(OH)2
is one of the most promising candidates for applications in high
energy storage devices, especially in supercapacitors due to its high
theoretical specific capacitance, low cost and well-defined electrochemical redox activity [21,22].
Up to now, different reports have been made by researchers on
the performance evaluation of symmetric and asymmetric supercapacitors [8–10]. Recently, Jagadale et al. reported symmetric
supercapacitive performance of electrodeposited Co(OH)2 on stainless steel substrate with maximum energy density about 3.96 Wh
kg−1 [9]. However, asymmetric type of supercapacitor formed with
Co(OH)2 and graphene has been proposed to be a power-oriented
supercapacitive device by Hu et al. with lower value of specific
energy 1.7 Wh kg−1 [13]. From past few years, to improve surface area and electrical conductivity, intensive studies have been
focused on combining conductive carbon materials (e.g., Go, rGo,
CNT) with Co(OH)2 [22–24]. However, these materials have limitations in both energy and power density mainly due to their
irregular pore sizes and relatively low electrical conductivity [17].
The Co(OH)2 -cabon composite based electrodes are commonly
U.M. Patil et al. / Electrochimica Acta 129 (2014) 334–342
binder-enriched electrodes made by the traditional slurry-coating
technique for electrochemical evaluation. The typical addition of a
polymeric binder will not only hamper the charge transport rate,
but also increase the total mass of the electrode.
Recently, light weight graphene foam (GF) (density ∼1020 mg cm−3 ), a structure consisting of an ultrathin graphene
skeleton with high electrical conductivity is used as an alternate
to conventional current collectors such as nickel foam or carbon
paper [25–27]. Advanced multifunctional structures of graphene is
a strong contender as an electrode material due to many fascinating properties such as their enormous electron mobility, extremely
high thermal conductivity, and extraordinary elasticity and stiffness [26,27]. X. Dong et al. reported specific capacitance about 1100
F g−1 for cobalt oxide on graphene foam by hydrothermal method
[27].
Till date there is no report on the symmetric supercapacitor performance based on Co(OH)2 /GF electrodes. In the present work,
direct synthesis of Co(OH)2 nanorods (NRs) on graphene foam was
successfully accomplished by using a facile chemical bath deposition (CBD) method. Furthermore, the supercapcitive performance
of conventional symmetric devices fabricated by Co(OH)2 /SS and
Co(OH)2 /GF electrodes were investigated and compared systematically.
2. Experimental Methods
2.1. Preparation of Co(OH)2 nanorods on 3D graphene foam
The fabrication of self-supported Co(OH)2 on 3D graphene foam
consists of two stage procedure; preparation of the GF and fastening
of Co(OH)2 NRs on it. The graphene were grown on Ni foam by
chemical vapour deposition (CVD) method, followed by etching of
Ni foam through etchant, the details of process is described in ref.
[26].
The Co(OH)2 NRs were prepared by mixing Co(NO3 )2 .6H2 O
(Sigma-Aldrich USA) as Co source and urea as an oxidant (SigmaAldrich USA) in 50 ml of DI water. The aqueous bath was prepared
from 0.1 M Co(NO3 )2 .6H2 O and 0.1 M urea (CO(NH2 )2 ). The 3D
skeleton of graphene foam was dipped in a prepared bath with the
support of glass micro slides. Then, the prepared bath was heated at
90 ◦ C for 4 hours. After cooling to room temperature, the graphene
foams with Co(OH)2 deposits were washed with deionized (DI)
water and dried at room temperature. Further, the prepared pink
colour Co(OH)2 /GF was used as an electrode in a supercapacitor cell.
For comparison, Co(OH)2 NRs from the same bath were deposited
on a commercial stainless steel (SS) substrate and used in the fabrication of a conventional supercapacitor device.
335
2.2. Characterization
The electrode materials were structurally characterized by
XRD, XPS and FESEM measurements. The X-ray diffraction
(XRD) was carried out on a Rigaku Ultima diffractometer using
Cu-K␣radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a thermo scientific ESCALAB 250
(Thermo Fisher Scientific, UK). The morphology of the composite was examined by field-emission scanning electron microscopy
(FESEM, JSM-7001F, JEOL).
2.3. Electrochemical Measurements
2.3.1. Half-test cell testing of Co(OH)2 /SS and Co(OH)2 /GF
electrodes
The individual electrochemical performances of Co(OH)2 /GF
and Co(OH)2 /SS electrodes were measured by half-test cell testing. A conventional half-test cell contains three-electrode system
comprises with Co(OH)2 /SS or Co(OH)2 /GF as working electrode,
Ag/AgCl as reference electrode and platinum (Pt) as counter electrode in a 1 M KOH aqueous electrolyte.
2.3.2. Symmetric Co(OH)2 /1 M KOH/Co(OH)2 test cell fabrication
The symmetric supercapacitor test cells were fabricated with
a two-electrode configuration. The symmetric cells were made
of both identical Co(OH)2 /SS and Co(OH)2 /GF electrodes. Fig. 1
shows the schematic of the supercapacitive test cells which consists of two Co(OH)2 electrodes separated by a thin polypropylene
separator in 1 M KOH aqueous electrolyte solution. Actual photographs of Co(OH)2 , pink in color, coated GF substrates (having
an area of 2 × 4 cm2 ) shown in Fig. 1. Cyclic Voltammetry (CV),
galvanostatic charge/discharge tests and EIS measurements were
performed using ZIVE SP2 LAB analytical equipment (South Korea).
3. Results and Discussion
3.1. Fabrication of Co(OH)2 /SS and Co(OH)2 /GF electrodes
For the fabrication of GF, graphene networks were grown by
methane using CVD on Ni foam. The graphene foam maintains a 3D
porous structure with a smooth and thin graphene skeleton after
the exclusion of Ni (figure S1 (a, b, c)) (see ESI). The thickness of the
graphene foam is ∼1.4 mm and the width of an individual graphene
sheet is around ∼25 ␮m, as shown in figure S1 (d) (see ESI). The
recorded Raman spectra of 3D graphene at different places on the
foam exhibited two distinct peaks at ∼1,559 cm−1 (G-band) and
∼2,699 cm−1 (2D-band), as shown in figure S2 (see ESI) [28,29]. The
integral ratio of the 2D and G band indicates few layered domains
Fig. 1. The actual photograph of Co(OH)2 /GF electrode and schematic of symmetric device based on Co(OH)2 /GF electrodes.
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growth [30]. The Co(OH)2 has been deposited on a SS and GF substrate by the slow hydrolysis of cobalt nitrate.
In the formation Co(OH)2 , as the temperature of solution bath
increased to 90 ◦ C, decomposition of urea (reactions 1) took place,
producing CO2 and NH3 gradually [31].
NH2 CONH2 + H2 O−→2NH3 + CO2
Fig. 2. The XRD patterns of grapheme foam (GF) and Co(OH)2 /GF electrode.
contained as-grown GF. The TEM image (shown in figure S3) at
lower magnification reveals that, a graphene sheet comprises with
some wrinkles, which may form due to the grain boundaries of Ni.
Moreover, the edge of Graphene foam is found to be ∼2-3 nm in
thickness at higher magnification. This finding suggests that the
prepared graphene foam consists of multilayered domains, which
is in good agreement with the Raman analysis. The Raman, SEM and
TEM studies confirm the formation of strongly interconnected high
quality 3D graphene foam. Such interconnected 3D graphene foam
further employed as a substrate for Co(OH)2 as a supercapacitor
electrode.
Synthesis of Co(OH)2 on two different kinds of substrates, stainless steel (SS) and graphene foam, was achieved by using the CBD
method. The CBD method is based on the formation of a solid phase
from solution which involves two steps: nucleation and particle
(1)
After decomposition of urea as per above reaction, the solution
became alkaline, such an alkaline condition is more favorable to
generate more hydroxyl and epoxy group on a graphene surface.
Increased hydroxyl and epoxy groups are constructive to create
more nucleation sites.
Simultaneously, Co2+ from Co(NO3 )2. 6H2 O get reacts with NH3
which forms amine complex. Such amine complexed metal ions
easily get adsorbed on the heterogeneous surface of the substrate
by electrostatic or Van-der Wall force. The high specific surface area
of GF makes it easy to adsorb a large number of amine complexed
Co2+ .
Co2+ + NH3 → Co(NH3 )2+
4
(2)
As deposition time prolongs solution become alkalescent. At this
alkalescent bath condition, the Co(NH3 )4 2+ is unstable and the following reaction occur to form metal hydroxides on the graphene
surface in the form of NRs.
−
Co[(NH)3 ]2+
4 + 2OH → Co(OH)2 + 4NH3
(3)
In the film formation NH3 , released from urea, introduced as a
complexing agent in the bath and exerts itself to control the release
velocity of Co2+ ions for deposition of Co(OH)2 NRs. According to
the above reaction mechanism, the growth of pink colored Co(OH)2
fastened on GF surface (shown in Fig. 1).
Fig. 3. Full XPS spectrum of (a) Co(OH)2 /GF and elemental spectra of (b) C 1s, (c) Co 2p, (d) O1s.
U.M. Patil et al. / Electrochimica Acta 129 (2014) 334–342
3.2. Structural analysis
Fig. 2 shows the X-ray diffraction (XRD) profiles of graphene
foam and Co(OH)2 /GF electrodes. Two significant diffraction peaks
(2␪) at 26.5◦ and 54.6◦ are attributed to the (002) and (004) reflections of the crystalline peak of hexagonal graphite carbon marked
as ‘x’ (JCPDS: 75-1621). The XRD pattern of Co(OH)2 /GF reveals
that, characteristic peaks at ∼17.02◦ , 23◦ , 33.5◦ , 34◦ , 34.4◦ , 39.1◦ ,
46.8◦ and 58.9◦ correspond to the (020), (220), (300), (221), (301),
(231), (304) and (412) diffraction planes, respectively and it can be
indexed as cobalt hydroxide, which are consistent with the results
337
in the literature or the standard card (JCPDS Card No. 38-0547)
[21]. It is well known that cobalt hydroxide can be crystallized
into a hexagonal layered structure with two polymorphs: ␣ and
␤. The Co(OH)2 is isostructural with hydrotalcite-like compounds
and consist of stacked Co(OH)2-x layers intercalated with various
anions (e.g., nitrate, carbonate, hydrate etc.) in the interlayer space
to restore charge neutrality and can be termed as cobalt hydroxide
nitrate hydrate (Co(OH)2-x (NO3 )x .nH2 O). Depending on the intercalated anions, ␣ phase has a larger interlayer spacing (7.0 Å) than
the brucite-like ␤ phase (4.6 Å) which can result higher electrochemical activity [23]. However, in this study, interlayer spacing is
Fig. 4. (a, b) The SEM micrographs of graphene foam at different magnifications, (c, d) SEM micrographs of Co(OH)2 on graphene foam at different magnifications, (e, f) SEM
images of Co(OH)2 nanorods on graphene surface at higher magnifications. (g, h) TEM images of Co(OH)2 NRs on graphene foam at different magnifications.
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found to be 5.5 Å indicating probably formation of mixed phase (␣
and ␤) of Co(OH)2 on GF surface.
3.3. X-ray photoelectron spectroscopy (XPS) studies
X-ray photoelectron spectroscopy (XPS) was used to measure
the binding energy, from which we estimated the various chemical states of the bonded elements. A typical XPS spectrum of
Co(OH)2 /GF sample presented in Fig. 3 (a). Fig. 3 (b) represents the
spectra of C (1s) for Co(OH)2 /GF electrode. The curve yields three
components at sp2 -C-C (284.7 eV), C-NR2 (amine, 285.7 eV), C-H
(epoxyl, 286.01 eV) and O = C-OH- (carboxyl, 289.2 eV) [32]. Fig. 3
(c) shows XPS spectra for Co (2p) of Co(OH)2 /GF electrode. The
graph shows that, peaks of Co (2p3/2 ) and Co (2p1/2 ) are centred
at 780.4 and 796.0 eV (with splitting of 5.4 eV) respectively, were
attributed to the presence of Co2+ chemical state as an indication
for formation of Co(OH)2 [33]. In addition, the presence of shakeup satellite peaks of the Co (2p3/2 ) and Co (2p1/2 ) around 786.5
and 802.9 eV, respectively, confirmed the formation of Co3+ on the
surface [34]. To have detailed insight and the presence of hydroxide, O (1s) core level of Co(OH)2 was examined (Fig. 3 (d)). It has
been observed that the O (1s) core level region is composed of a
broad peak at 532.0 eV, which is associated with bound hydroxide
groups (OH− ) [35]. Hence, there is possibility of formation of cobalt
hydroxide than cobalt oxide. The XPS result suggests that formation
of cobalt hydroxide rather than cobalt oxide on GF surface.
3.4. Surface Morphology
The FE-SEM micrographs of GF and Co(OH)2 thin film deposited
on the GF at different magnifications are shown in Fig. 4. Fig. 4 (a)
shows that, the GF replicates and gets the 3D structure of the nickel
foam template, and all the graphene sheets in the foam are interconnected with each other without breaks. The 3D porous graphene
foam reveals a smooth and thin graphene skeleton with width
of individual graphene sheets around ∼25 ␮m (Fig. 4 (b)). In the
SEM image of Co(OH)2 /GF electrode shown in Fig. 4 (c), it is found
that graphene skeleton is entirely and uniformly covered by the
Fig. 5. Electrochemical studies of Co(OH)2 /GF and Co(OH)2 /SS electrodes. Cyclic voltammograms of (a) Co(OH)2 /GF and (b) Co(OH)2 /SS electrodes within optimized potential
range of -0.2 to 0.4 V in aqueous 1 M KOH at different scan rates. Galvanostatic charge-discharge (GCD) plots of (c) Co(OH)2 /GF and (d) Co(OH)2 /SS electrodes within potential
window of 0 to 0.4 V at constant charging current from 10 to 30 A g−1 . (e) Graph of charge-discharge current dependent specific capacitance for Co(OH)2 /GF and Co(OH)2 /SS
electrodes. (f) Typical Ragone plot of energy density and power density for Co(OH)2 /GF and Co(OH)2 /SS electrodes.
U.M. Patil et al. / Electrochimica Acta 129 (2014) 334–342
Co(OH)2 . The Fig. 4 (d) shows Co(OH)2 nanflowers are well grown
on 20-25 ␮m wide graphene skeleton with some overgrowth at the
edges. Fig. 4 (e) reveals that, all Co(OH)2 nanoflowers composed of
entangled NRs are distributed on the surface of graphene. The high
magnified SEM image of nanoflowers shown in Fig. 4 (f) reveals that
individual Co(OH)2 NRs are ∼160-180 nm in diameter with needle
like tip. Similarly, the TEM images shows the formation of entangled Co(OH)2 on the graphene foam (Fig. 4 (g)). The high resolution
TEM image (Fig. 4 (h)) evidently reveals needle like tip (5-20 nm) of
Co(OH)2 nanorods (∼160 nm). The SEM and TEM analysis confirms
that formation of entangled NRs like structure of Co(OH)2 over 3D
graphene foam surface. Such nanorods-like morphology leads to be
a high specific surface area, which provides the structural foundation for the high specific capacitance. The 3D structure of GF and
Co(OH)2 NRs together provide a large accessible surface area; play
vibrant role in enhancement supercapacitive performance [36].
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3.5. Supercapacitive performance of Co(OH)2 /SS and Co(OH)2 /GF
electrodes
The individual electrochemical performances of Co(OH)2 /SS and
Co(OH)2 /GF electrodes were tested by forming half-test cell. Cyclic
voltammetry (CV) is an appropriate tool to illustrate the capacitive behaviour and quantify the specific capacitance of an electrode
material. Fig. 5 (a) and (b) shows the CV curves at different scan
rates ranging from 25 to 125 mV s−1 for Co(OH)2 /GF and Co(OH)2 /SS
in a aqueous 1 M KOH solution, respectively. It is noticeable that
all of the CV curves exhibit two intense redox peaks arising from
the reversible faradaic reaction in the alkaline electrolyte. The
shapes of the CV curves are not generally close to the ideal rectangular shape of the EDLC. This clearly indicates pseudocapacitive
capacitance based on a redox mechanism. The anodic peak (+ve current density) observed at ∼0.16 V (vs. Ag/AgCl) corresponds to an
Fig. 6. Cyclic voltammograms of symmetric devices based on (a) Co(OH)2 /SS and (b) Co(OH)2 /GF electrodes within optimized potential range of 0 to 1.2 V in aqueous 1 M KOH
at different scan rates. Galvanostatic charge-discharge (GCD) plots of symmetric devices based on (c) Co(OH)2 /SS and (d) Co(OH)2 /GF electrodes within potential window of
0 to 1.2 V at a constant charging current from 5 to 15 A g−1 . (e) Graph of charge-discharge current dependent specific capacitance for symmetric devices based on Co(OH)2 /GF
and Co(OH)2 /SS electrodes. (f) Typical Ragone plot of energy density and power density of symmetric devices based on Co(OH)2 /GF and Co(OH)2 /SS electrodes.
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oxidation reaction Co(OH)2 to CoOOH, while the cathodic peak (-ve
current density) which occurred around ∼0.08 V (vs. Ag/AgCl) indicates the reverse process, i.e., a reduction reaction. The intercalation
and de-intercalation reaction of ions involved in a basic electrolyte
is the characteristic source of pseudocapacitance as follows [20].
Co(OH)2 + OH − ↔ CoOOH + H2 O + e−
(4)
The Co(OH)2 /GF electrode contributed higher current under
the curve and maximum area with prominent redox peaks compared to the Co(OH)2 /SS electrode. This result demonstrates the
maximum pseudocapacitive charge storage due to more porous,
conducting, and higher specific area access offered by the interconnected graphene framework. The current under curve increasing
with scan rate; concludes that the voltammetric current is directly
proportional to the scan rates of CV and this is the indication of the
supercapacitive behavior of electrode. Fig. 5 (c and d) shows the
galvanostatic charge–discharge of the Co(OH)2 /GF and Co(OH)2 /SS
at different constant current densities from 10 to 30 A g−1 . The
shape of the charge–discharge curves do not show the characteristic of a pure double-layer capacitor (linear charge-discharge), but
mainly pseudocapacitance (non-linear charge-discharge), which
corresponds with the result of the CV test. Fig. 5 (e) shows the graph
of specific capacitance with different charge-discharge current
density. The graph reveals that, the specific capacitance decreases
with increase in charge-discharge current density. The decreasing
specific capacitance is consequence of less active material access,
which reduces the effective utilization of material at a higher
scan rate. Maximum specific capacitances 1139 and 616 F g−1 are
observed for Co(OH)2 /GF and Co(OH)2 /SS electrodes, respectively,
at low charge-discharge current density 10 A g−1 . The improved
synergistic interaction of graphene and Co(OH)2 ensures the superior electrode compared to Co(OH)2 on stainless steel. The specific
capacitance of Co(OH)2 /GF is much higher than that of Co(OH)2 /SS
electrode. The calculated energy and power density from chargedischarge curves is shown in the typical Ragone plot (Fig. 5 (f)).
The maximum energy density about 49 and 21 Wh kg−1 at power
density 12 kW kg−1 observed for Co(OH)2 /GF and Co(OH)2 /SS electrodes respectively. The energy density of Co(OH)2 /GF electrode is
two times higher than the Co(OH)2 /SS electrode.
Fig. 6 (a and b) shows the CVs of the symmetric Co(OH)2 /SS
and Co(OH)2 /GF supercapacitor devices measured in the voltage
window of 0 to 1.2 V at different scan rates of 25, 50, 75, and
100 mV s−1 . The Fig. 6 (a) shows an asymmetric rectangular shape
of the CV curves with unclear oxidation and reduction peaks. However, Fig. 6 (b) shows two distinct redox peaks at 0.9 and 0.7 V
for the Co(OH)2 /GF device. The current under the CV curves of the
Co(OH)2 /GF device is higher than Co(OH)2 /SS device. Fig. 6 (c) and
(d) show galvanostatic charge-discharge (GCD) plots of Co(OH)2 /SS
and Co(OH)2 /GF, respectively, in the symmetric mode with a cell
voltage of 1.2 V at different constant currents of 5 to 15 A g−1 .
Easily one can observe, a higher IR drop for Co(OH)2 /SS device as
compared to Co(OH)2 /GF, signifies the low internal resistance of
Co(OH)2 /GF electrodes device. The current density dependant specific capacitance plots of symmetric Co(OH)2 /SS and Co(OH)2 /GF
devices implies maximum specific capacitance values of 40 F g−1
and 69 F g−1 , respectively, (shown in Fig. 6 (e)). Ganesh et al.
reported a specific capacitance value of 22 F g−1 for a porous Ni
|KOH| porous Ni symmetric supercapacitor cell [37]. Previously,
Jagadale et al. reported 44 F g−1 specific capacitance for a symmetric supercapacitor formed from Co(OH)2 on stainless steel substrate
[9]. The enhanced specific capacitance of 69 F g−1 for Co(OH)2 /GF
symmetric device is probably due to the enhanced surface area
and easy charge transfer offered by the 3D graphene. The deviation from perfect linearity of the charge-discharge curve results
from the active involvement of pseudocapacitance in the total specific capacitance. The typical Ragone plot for symmetric devices of
Fig. 7. (a) Nyquist plots of symmetric devices based on Co(OH)2 /GF and Co(OH)2 /SS
electrodes. (b) Comparative Ragone plot of energy and power density with previously reported work. Inset shows schematic of the charge transport mechanism of
Co(OH)2 /GF and Co(OH)2 /SS electrodes.
Co(OH)2 /SS and Co(OH)2 /GF electrodes for different charging current densities is shown in Fig. 6 (f). The symmetric device based
on Co(OH)2 /GF electrodes demonstrated maximum energy and
power density about 13.9 Wh kg−1 and 18 kW kg−1 , respectively.
The energy density of Co(OH)2 /GF device is higher than Co(OH)2 /SS
device (8 Wh kg−1 ). The Galvonostatic charge-discharge (GCD)
cyclic stability is studied for Co(OH)2 /GF based symmetric device
over 1000 cycles at 10 A g−1 (shown in Fig. S4). The specific capacitance of device is marginally decreased by 15% up to 200 cycles
however; thereafter, specific capacitance dropped only by 11% for
next 800 cycles. Hence, the stability of device retained to ∼74% after
1000 cycles.
Fig. 7 (a) present the complex plane electrochemical impedance
spectra (EIS) of the symmetric Co(OH)2 /SS and Co(OH)2 /GF devices.
The EIS measurements were carried out by measuring the open
circuit potential (OCP) over the frequency range of 10 MHz to
0.01 Hz. A sharp increase of the imaginary part of the EIS at
lower frequencies is due to the capacitive behavior of the cell,
where a semi-circular loop at higher frequencies is due to chargetransfer resistance (Rct) [38]. For an ideal double-layer capacitor,
the impedance plot should be a vertical line parallel to the imaginary axis, which is generally observed for carbon-based materials
such as activated carbon, graphite, CNTs, and graphene. The “Rct”
can be calculated from the radius of the initial curvature at higher
frequencies [39]. The “Rct” for Co(OH)2 /GF (0.8 ) is lower than that
of the Co(OH)2 /SS (7.5 ) device. As shown in Fig. 7 (a), the solution
resistance (Rs) of the Co(OH)2 /GF electrode (0.5 ) is much smaller
than that of the Co(OH)2 /SS electrode (8.2 ). The high “Rct” and
U.M. Patil et al. / Electrochimica Acta 129 (2014) 334–342
“Rs” of the Co(OH)2 /SS device restrict to achieve maximum specific
capacitance, energy density, and power density.
A rough comparison of energy and power shown in Fig. 7 (b)
indicates that this value is also significantly higher than those of
other Co(OH)2 based ECs with aqueous electrolyte solutions. The
energy density obtained in the present work surpasses previous
literature values reported for aqueous electrolyte based Co(OH)2
symmetric [9] and asymmetric supercapacitors [11], with the
exception of a power density of devices. As demonstrated here,
graphene foam is uniquely advantageous to serve as a 3D support of large capacity to uniformly anchored Co(OH)2 NRs. The
Co(OH)2 NRs over GF electrode offer many advantages over the typically used conventional Co(OH)2 /SS electrodes in supercapacitors.
Such conventional Co(OH)2 /SS architectures offer self-dimensional
restrictions in charge transportation during charge-discharge process (schematic mechanism shown in the inset of Fig. 7 (b)).
However, herein we presented effective utilization of 3D graphene
foam and Co(OH)2 NRs as supercapacitor electrode by fabricating a
novel architecture. In such a structure, 3D GF serves as the current
collector with high surface area to improve the electron conductivity and utilization of Co(OH)2 nanostructure. Also, the hierarchical
1D nanostructures of Co(OH)2 on 3D GF possess a large surface area
with porous structure, which allows them to interact effectively
with the electrolyte ions. As demonstrated here, GF is uniquely
advantageous to serve as a 3D support of large capacity and high
energy density to uniformly anchored Co(OH)2 NRs.
4. Conclusions
In summary, a chemical bath deposition method was successfully employed for the deposition of Co(OH)2 NRs onto GF. The
Co(OH)2 NRs on GF is more proficient to provide maximum specific
capacitance up to 1139 F g−1 . The Co(OH)2 NRs on different kinds of
substrates were assembled as a symmetric Co(OH)2 /KOH/Co(OH)2
supercapacitor device. As compared to conventional Co(OH)2 /SS
electrodes, the improved supercapacitive performance of symmetric device emanates from the synergistic cooperation between
graphene and Co(OH)2 NRs, leading to a high specific capacitance up
to 69 F g−1 along with increased specific power and energy densities
of 18 kW kg−1 and 13.9 Wh kg−1 , respectively. Thus, the enhanced
supercapacitive performance of Co(OH)2 /GF electrodes in symmetric device, elaborate better suitability in energy device applications
than that of conventional one.
Acknowledgement
This work was partially supported by the Pioneer Research Center Program (2010-0019313), and the Priority Research Centers
Program (2009-0093823) through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future
Planning.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.
2014.02.063.
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