Preparation of metallic cobalt-graphene composites with enhanced electrochemical activity

Article abstract of DOI:10.1016/j.electacta.2012.05.022

Metallic cobalt-graphene composites were synthesized through coprecipitation synthetic method followed by hydrothermal treatment at 180°C. The composite material as prepared was characterized using X-ray diffraction (XRD), SEM, TEM, Energy dispersive spectrometer (EDS) analysis and Thermogravimetric (TG) analysis. The results indicate that metallic cobalt particles possess a good dispersion on the surface of graphene. The electrochemical performance of metallic cobalt-graphene composites was characterized by Cyclic voltammetry (CV) and galvanostatic charge-discharge tests. The specific capacitance of the composite material approaches 1340 F g^-1 at current density of 1.5 A g^-1 with a capacitance retention rate of 82% at current density of 3.5 A g^-1. The results suggest that the combination of metallic cobalt and graphene leads to synergistic performance, which inherits the overall system with enhanced electrochemical performance.

Full text of DOI:10.1016/j.electacta.2012.05.022

Electrochimica Acta 76 (2012) 282–287
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Preparation of metallic cobalt–graphene composites with enhanced
electrochemical activity
Lin Wanga, Dianlong Wanga,∗, Xinchun Hua,b, Junsheng Zhua, Xiaoshi Lianga
a School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, China
b Tian Neng Power International Limited, Wuhu 241000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Metallic cobalt–graphene composites were synthesized through coprecipitation synthetic method fol-
lowed by hydrothermal treatment at 180 ◦C. The composite material as prepared was characterized using
X-ray diffraction (XRD), SEM, TEM, Energy dispersive spectrometer (EDS) analysis and Thermogravimetric
(TG) analysis. The results indicate that metallic cobalt particles possess a good dispersion on the surface
of graphene. The electrochemical performance of metallic cobalt–graphene composites was character-
ized by Cyclic voltammetry (CV) and galvanostatic charge–discharge tests. The specific capacitance of
the composite material approaches 1340 F g−1 at current density of 1.5 A g−1 with a capacitance reten-
tion rate of 82% at current density of 3.5 A g−1. The results suggest that the combination of metallic
cobalt and graphene leads to synergistic performance, which inherits the overall system with enhanced
electrochemical performance.
Received 17 December 2011
Received in revised form 4 May 2012
Accepted 6 May 2012
Available online 14 May 2012
Keywords:
Metallic cobalt
Cobalt hydroxide
Graphene
Hydrothermal reaction
Supercapacitor
© 2012 Elsevier Ltd. All rights reserved.
1. Introduction
monolayer graphene/NiO nanosheets, which indicated that the
specific capacitance of the composite was 528 F g−1 and much
Graphene as a rising star has triggered an exciting new area
in the field of carbon science, due to its extraordinary electrical
[1], chemical stability [2], high surface-to-volume [3] and capac-
itive performance [4–6]. Moreover, the interleaved network of
graphene produces pathways for electron transport, and can obvi-
ously improve the electrical conductivity of the electrode. The large
surface of graphene provides the platform for the decoration of
nanoparticles to avoid their agglomeration, hence more utilization
of nanoparticles during the electrochemical reaction. Because of
these unique properties of graphene, the combination of graphene
with a second phase material can further enhance the electro-
chemical properties of the obtained composites. Therefore, many
materials, including various metal [7], metal oxide nanoparticles
[8–11], carbon materials [12], and polymers [13,14], have been used
as the second phase to incorporate with graphene. Recently, Lee
et al. [15] reported that the specific capacitance of MnO2–graphene
composite electrode was 399.82 F g−1 at 10 mV s−1, which demon-
strated that graphene layers improved the electro-conductivity of
MnO2 and the interaction between the electrolyte and the elec-
trode. Zhao et al. [16] developed an effective method to prepare
larger than that of pure NiO. Results from Chen et al. [17] shown
that the specific capacitance of Co(OH)2–graphene nanocomposite
reached a value as high as 972.5 F g−1, which leaded to a significant
improvement in relation to each individual counterpart (137.6 and
726.1 F g−1 for graphene and Co(OH)2, respectively).
Metallic cobalt has been regarded as a magnetic material [18,19]
and several methods have been available for the production of this
magnetic material, including physical methods [20], electrochemi-
cal methods [21] and chemical methods [22,23]. Herein, to expand
the extent of graphene-based composites as electrode materials,
we attempted to develop a facile approach to prepare metallic
cobalt–graphene composites via coprecipitation synthetic method
combined with hydrothermal treatment. The results show that
metallic cobalt particles possess a good dispersion on the surface
of graphene, which prevents the agglomeration of metallic cobalt
particles. The high specific capacitance of the composite material
could be expected for the synergistic electrochemical result of the
combination of metallic cobalt and graphene in alkaline electrolyte.
2. Experimental
2.1. Preparation of thermal reduced graphene
∗
Corresponding author at: School of Chemical Engineering and Technology, P.O.
Thermal reduced graphene (TRG) was prepared from natural
graphite powder according to modified Hummers method [24].
6.5 g of graphite powder was placed in 150 mL of cold (0 ◦C)
Box 411, Harbin Institute of Technology, Harbin 150001, China.
Tel.: +86 451 86413751; fax: +86 451 86413721.
E-mail address: dlwang@hit.edu.cn (D. Wang).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.05.022

L. Wang et al. / Electrochimica Acta 76 (2012) 282–287
283
concentrated H2SO4, and then 19.5 g of KMnO4 was added grad-
ually with stirring and cooling, and the temperature of the solution
was not allowed to go up to 20 ◦C. The mixture was stirred at
35 3 ◦C for 2 h, and 460 mL of distilled water was added slowly
to an increase in temperature to 98 ◦C. The temperature was held
at 60 ◦C for 1 h. Finally, 1.4 L of distilled water and 100 mL of 30%
H2O2 solution were added after the reaction. The mixture was held
at room temperature for 24 h, and then filtered and washed with 5%
HCl aqueous solution until sulfate could not be detected with BaCl2.
The graphite oxide obtained was dried at 50 ◦C for 72 h. Thermal
reduced graphene was obtained after graphite oxide being put into
a muffle oven preheated to 1073 K for 60 s.
MCGC
10
15
20
25
30
35
40
MCGC
BMC
Co(OH)2-graphene
Co(OH)2
TRG
2.2. Preparation of metallic cobalt–graphene composites
10
20
30
40
50
60
70
80
90
Metallic cobalt–graphene composites (MCGC) were synthesized
through a two-step synthetic method. The typical route was as
follows: firstly, 997 mg of cobalt acetate was dissolved in 160 mL
of distilled water, and then 27 mg of TRG was dispersed in the
above solution with ultrasonication for 1 h. Subsequently, 100 mL
of 0.1 mol L−1 NaOH aqueous solution was added into the above
slurry with vigorous stirring. Co(OH)2–graphene composites (CGC)
were collected by filtration, washed with distilled water, and then
dried in air at 60 ◦C.
2 theta / degree
Fig. 1. XRD patterns of the samples as prepared.
tests were measured between −1.05 and −0.5 V (vs. Hg/HgO) and
investigated by a Neware battery testing workstation.
3. Results and discussion
Then, MCGC were obtained by hydrothermal reaction of CGC
with hydrazine hydrate at 180 ◦C. 70 mg of CGC and 10 mL of
80 wt% hydrazine hydrate were added to 40 mL of distilled water
in a Teflon-lined autoclave (85 mL). After stirring of 1 min with a
stainless-steel muddler, the autoclave was sealed, and heated at
180 ◦C for 24 h. The system was then allowed to cool to room tem-
perature. The resulting product was collected by filtration, washed
with distilled water, and then dried at 60 ◦C. For comparison, cobalt
hydroxidebare (Co(OH)2) and bare metallic cobalt(BMC) were syn-
thesized via a similar procedure in the absence of graphene.
The XRD patterns of TRG, Co(OH)2 and BMC extracted from com-
posites, as well as CGC and MCGC are shown in Fig. 1. The diffract
peak of TRG at around 21.5◦ corresponding to 0 0 2 crystal plane
of carbon is weak and broad, which indicates that graphene has a
highly amorphous nature through thermal treatment. The results
of XRD patterns show that diffract peaks of pristine Co(OH)2 as
prepared are in good agreement with the standard profile of hexag-
onal ␤-Co(OH)2 (JCPDS 30-0443). Moreover, the crystal state of CGC
coincides well with that of Co(OH)2. The reflection peak of graphene
disappears in the XRD pattern of CGC, which shows that graphene
covered by Co(OH)2 is in a state of increasingly disordered stacking.
The XRD patterns confirm that BMC consists of hexagonal phase
(JCPDS 05-0727) and cubic phase (JCPDS 15-0806). Compared to
BMC, a diffraction hump of MCGC appears in the range of 20–27◦,
which originates for the diffract peak of graphene. This result sug-
gests that the disorderedly stacked structure of graphene tends to
an ordered structure through hydrothermal treatment.
The microstructures of TRG, Co(OH)2, BMC, CGC and MCGC are
characterized by FESEM in Fig. 2, respectively. The wrinkle and
transparent graphene sheets can be seen in Fig. 2(a). Fig. 2(b) shows
that the compound Co(OH)2 exists as flat hexagonal platelets in
around 300 nm. The FESEM image in Fig. 2(c) reveals that metal-
lic cobalt particles are aggregated together with the particle size
of around 400 nm. Fig. 2(d) indicates that Co(OH)2 within CGC is
encapsulated in graphene sheets, and CGC possess a layer by layer
assembled structure. The FESEM image of MCGC shows that metal-
lic cobalt particles are anchored on the surface of graphene sheets
in Fig. 2(e), which prevents metallic cobalt particles from agglom-
eration. To confirm the elemental composition of MCGC, EDS is
employed in Fig. 2(f). The EDS image displays that MCGC consist
of cobalt element and carbon element with little of oxygen ele-
ment which is derived from functional groups on the surface of
graphene. The microstructure of MCGC is also observed by TEM in
Fig. 3. The TEM image in Fig. 3(a) reveals that metallic cobalt par-
ticles possess a good dispersion on the surface of graphene and
a maximum particle diameter of 300 nm. Moreover, Fig. 3(b) dis-
plays that metallic cobalt particles within MCGC are wrapped by
graphene. As will be discussed later, this unique microstructure
is beneficial for good accessibility and fast diffusion of the redox
phase, which results in the enhanced electrochemical capacitance
of the composite material as prepared.
2.3. Materials characterization
Powder X-ray diffraction (XRD) analysis was performed on
Rigaku D/MAX-RC X-ray diffractometer in order to identify the
crystalline phases of materials. Morphologies of products as
prepared were observed on a field emission scanning electron
microscopy(FESEM, Hitachi, S-4800, 15 kV) and a transmission
electron microscopy (TEM, Hitachi, H-7650, 100 kV). The element
compositions of electrode materials as prepared were determined
by an energy dispersive X-ray spectroscopy (EDS, Hitachi, S-4800).
Thermogravimetric (TG) analysis was performed using Perkin
Elmer Pyris 1 TGA thermo balance. In the TG experiment, the sam-
ples were heated in air from 30 to 700 ◦C at 10 ◦C min−1
.
2.4. Electrochemical measurements
Cyclic
voltammetry
(CV)
curves
and
galvanostatic
charge–discharge tests of electrodes as prepared were inves-
tigated under a conventional three-electrode cell with 6 mol L−1
KOH aqueous solution as electrolyte at room temperature. The
working electrode was fabricated by mixing powders as prepared
with 20 wt% acetylene black and 10 wt% polytetrafluorene ethylene
(PTFE) binder. A small amount of distilled water was added to
the mixture to produce a homogeneous paste. The mixture was
pressed onto nickel foam current collectors (1.0 cm × 1.0 cm) to
make electrodes. A platinum foil and an Hg/HgO electrode served
as counter electrode and reference electrode, respectively. CV
curves were collected between −1.3 and −0.2 V (vs. Hg/HgO) and
conducted on a CHI 604B electrochemical workstation (Shanghai
CH Instrument Company, China). Galvanostatic charge–discharge

284
L. Wang et al. / Electrochimica Acta 76 (2012) 282–287
Fig. 2. FESEM micrographs of the samples as prepared: (a) FESEM image of TRG; (b) FESEM image of Co(OH)2; (c) FESEM image of BMC; (d) FESEM image of CGC; (e) FESEM
image of MCGC; (f) EDS image of MCGC.
In order to quantify the mass ratio of metallic cobalt to graphene
within MCGC, TG curves of TRG, BMC and MCGC are shown in Fig. 4.
Normally, some vacancies and topological defects can be simulta-
neously produced on graphene because of the release of carbon
dioxide during thermal treatment. These defect sites lead to the
thermal decomposition of graphene at elevated temperature. From
this figure, only one step for mass loss of TRG begins at 483 ◦C with
the mass loss of 91.6% [25]. The initial oxidation temperature of
Fig. 3. TEM micrographs of MCGC with (a) low and (b) high magnification.

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285
surface and edge of graphene via electrostatic interaction. There-
fore, Co2− ions can be selectively bonded with functional groups
through mutual electrostatic attraction. Then, Co2− ions, which are
absorbed on the surface of graphene, react with added NaOH to pro-
duce Co(OH)2 platelets on the surface of graphene. These platelets
are encapsulated in graphene sheets, and increase disorderedly
stacked structure of graphene. Through the reduction of CGC in
presences of hydrazine hydrate, Co(OH)2 platelets are transformed
into metallic cobalt particles which can be firmly attached to the
surface of graphene. Moreover, this synthetic methodology is easy
and straightforward, therefore can be readily extended as a general
procedure to preparation of other metal–graphene nanocompos-
ites.
180
160
140
120
100
80
TRG
BMC
MCGC
60
40
20
0
To explore the potential applications of the composite material
as prepared, the samples were fabricated into supercapaci-
tor electrodes and characterized with CV and galvanostatic
charge–discharge measurements. Interestingly, the combination
of metallic cobalt and graphene enhances electrochemical perfor-
mance of the whole system. CV curves of TRG, BMC and MCGC
at scan rate of 10 mV s−1 are shown in Fig. 6(a). The loop shape
curve of TRG is close to rectangle, which indicates its ideal capac-
itive performance. A couple of peaks in the CV curve of BMC is
observed, which is related to the following redox reaction [27]:
Co(OH)2 + 2e ↔ Co + 2OH−. The oxidation and reduction peak of
metallic cobalt appear at the potential of around −0.696 and
−1.072 V, respectively. However, the reduction peak of metallic
cobalt is weak in comparison with the oxidation peak, due to slow
reduction reaction rate of Co(OH)2 caused by its diffusion pro-
cess [28]. The CV curve of MCGC shows that the current density
of the oxidation peak is obviously enhanced when the oxidation
peak migrates to high potential [29]. However, the reduction peak
of MCGC is inconspicuous due to that this peak shifts to low
potential and is covered by the current of hydrogen evolution reac-
tion. Meanwhile, the reduction current density of the composite
material is obviously elevated. The results of CV clearly illustrate
that the well-organized interleaved structure consisting of metal-
lic cobalt and graphene facilitates Faraday reaction of metallic
cobalt.
0
100 200 300 400 500 600 700
Temperature / oC
Fig. 4. TG curves of TRG, BMC and MCGC.
metallic cobalt is 343 ◦C and its residue mass is 136.0% of the origi-
nal amount. This result indicates that metallic cobalt is completely
oxidized to Co3O4 while it is exposure to air at this temperature
range [26], due to that the molecular weight of Co3O4 is 36% higher
than that of metallic cobalt. Accordingly, the mass ratio of metallic
cobalt to graphene within MCGC can be determined through the TG
curve of the composite material. The mass gain behavior of MCGC
undergoes a broader temperature range and the mass gain of the
composite material is 25.3%, which is assigned to the decomposi-
tion of graphene and the generation of Co3O4. The mass ratio of
metallic cobalt to graphene within MCGC is estimated to be around
8.6:1.
The overall synthetic strategy toward the composite material
consisting of metallic cobalt and graphene is outlined in Fig. 5. It is
well known that graphene has its basal plane decorated with epoxy
and hydroxyl groups, while carboxyl groups are located at the edge.
These functional groups, which act as anchor sites, have a high
capability of enabling dissolved metal ions to be absorbed on the
Fig. 5. Synthetic strategy illustration of the composite material consisting of metallic cobalt and graphene.

286
L. Wang et al. / Electrochimica Acta 76 (2012) 282–287
Fig. 6. (a) CV curves of TRG, BMC and MCGC between −1.3 V and −0.2 V (vs. Hg/HgO) in 6 mol L−1 KOH aqueous electrolyte; (b) CV curves of MCGC at different scan rates
(10 mV s−1, 20 mV s−1, 30 mV, 40 mV s−1 and 50 mV s−1); (c) galvanostatic charge–discharge curves of TRG, BMC and MCGC at current density of 1.5 A g−1; (d) rate performance
of TRG, BMC and MCGC at different current densities(1.5 A g−1, 2.0 A g−1, 2.5 A g−1, 3.0 A g−1 and 3.5 A g−1).
To further explore the electrochemical behavior of MCGC, CV
curves of MCGC at different CV scan rates are shown in Fig. 6(b).
CV curves of MCGC show that the oxidation current density of
MCGC increases with the enhancement of CV scan rate until CV
scan rate is increased to 20 mV s−1. However, the oxidation peak
current density retains around 25 A g−1 as well as the profile of the
oxidation peak becomes slightly broad, when CV scan rate proceeds
to increase. The results from CV curves of MCGC in Fig. 6(b) suggest
that the electrochemical reaction between Co and Co(OH)2 may
be controlled by the dissolution–precipitation process of Co(OH)2
rather than the diffusion process of dissolved Co(OH)2 in alkaline
solution [28], when CV scan rate is increased adequately.
The comparison of charge–discharge curves of TRG, BMC and
MCGC at current density of 1.5 A g−1 indicates enhanced electro-
chemical performance of MCGC in Fig. 6(c). The results obtained
from Fig. 6(c) show that the charge–discharge curve of TRG is highly
liner and symmetrical, which is a typical characteristic of an ideal
capacitive material. MCGC have a longer charge–discharge plateau
than BMC, which indicates that MCGC possess a higher capacitance
than BMC. In this case, the synergistic effect of the combination
of metallic cobalt and graphene on electrochemical performance
of the composite material is excellently demonstrated. Graphene
is anticipated to serve two roles during the electrochemical reac-
tion. On one hand, graphene prevents the agglomeration of metallic
cobalt particles, which can provide unobstructed pathways for
ionic transport during the electrochemical reaction. On the other
hand, the presence of graphene can contribute to the double-
layer which increases the facility of OH− transportation during the
charge–discharge process.
Fig. 6(d) shows specific capacitances of TRG, BMC and MCGC at
different current densities. Specific capacitances of the samples are
measured according to the following equation [30]:
I × ꢀt
Cs =
(1)
ꢀV × m
where Cs is the specific capacitance (F g−1), I is the constant dis-
charge current (A), ꢀt is the discharge time (s), and ꢀV is the
potential change during discharge process (V), m is the mass of
the active materials in electrode (g). Specific capacitances of TRG,
BMC and MCGC are 214 F g−1, 460 F g−1 and 1340 F g−1 at current
density of 1.5 A g−1, respectively. Capacitance retention rates of
TRG, BMC and MCGC are 89%, 68% and 82% at current density of
3.5 A g−1, respectively. The results obtained suggest that the three-
dimensional network of graphene sheets plays an important role in
improving the rate capability of metallic cobalt, due to that metal-
lic cobalt particles are anchored homogeneously on conductive
graphene which can promote both the electrochemical utilization
of metallic cobalt and the electrical conductivity of the electrode.
As mentioned above, one main reason for the synergistic effect
of the combination of metallic cobalt and graphene on electro-
chemical performance of the composite material is due to that
metallic cobalt particles possess a good dispersion on the surface of
graphene, which improves the interaction between the electrolyte
and the electrode. And the flexible and electrically conductive
graphene layers, in which metallic cobalt particles are embedded,
can enhance the electrical conductivity of the composite material
to reduce the electrochemical polarization of the electrode during
the electrochemical reaction. In additional, double-layer capacitive

L. Wang et al. / Electrochimica Acta 76 (2012) 282–287
287
performance of graphene increases the facility of OH− transporta-
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In summary, MCGC are prepared through a two-step synthetic
methodology. Through the reduction of CGC possessing a layer
by layer assembled structure, metallic cobalt particles are firmly
attached to the surface of graphene. The specific capacitance of
MCGC is 1340 F g−1 at current density of 1.5 A g−1 in comparison
with the specific capacitance of 460 F g−1 without graphene. Capac-
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at current density of 3.5 A g−1, respectively. As a result, the addition
of graphene enhances the storage capacitance and rate capability
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We gratefully acknowledge the testing support offered by
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