1) Mn3O4 nanoparticles were anchored onto graphene nanosheets through a simple ultrasound-assisted synthesis at room temperature to create a nanocomposite for use as a supercapacitor electrode. 2) Characterization showed the Mn3O4 nanoparticles were uniformly 4-8 nm in size and anchored on the graphene nanosheets. 3) Electrochemical testing found the nanocomposite exhibited a high specific capacitance of 312 F/g, approximately three times that of pristine Mn3O4, and maintained 76% of its capacitance after 1000 charge/discharge cycles.

1. Ultrasound assisted synthesis of Mn3O4 nanoparticles anchored
graphene nanosheets for supercapacitor applications
Balasubramaniam Gnana Sundara Raj a
, Rajasekharan Nair Radhika Ramprasad a
,
Abdullah M. Asiri b
, Jerry J Wu c
, Sambandam Anandan a,
*
a
Nanomaterials and Solar Energy Conversion Lab Department of Chemistry, National Institute of Technology, Trichy 620 015, India
b
The Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21413, P.O. Box 80203, Saudi Arabia
c
Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan
A R T I C L E I N F O
Article history:
Received 10 October 2014
Received in revised form 10 December 2014
Accepted 12 January 2015
Available online 13 January 2015
Keywords:
Ultrasound
Mn3O4 nanoparticles
Graphene nanosheets
Nanocomposite
Supercapacitors
A B S T R A C T
Mn3O4 nanoparticles anchored graphene nanosheets (MG) have been successfully synthesized by a
simple ultrasound assisted synthesis at room temperature without the use of any templates or
surfactants for supercapacitor applications. Upon ultrasound assisted synthesis, the formation of Mn3O4
nanoparticles and the graphene oxide reduction occurs simultaneously. The crystalline structure of thus
prepared MG nanocomposite have been characterized by the powder X-ray diffraction (XRD) analysis.
Thermo Gravimetric Analysis (TGA) is used to determine the mass content of graphene (17 wt%) in the MG
nanocomposite. Transmission electron microscopy (TEM) and Atomic force microscopy (AFM) studies
shows that the Mn3O4 nanoparticles (4–8 nm) were uniformly anchored on the surface of graphene
nanosheets. The electrochemical properties of the MG nanocomposite were investigated by employing
cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy
(EIS). The capacitive properties of MG nanocomposite studied in the presence of 1 M Na2SO4 exhibited
high specific capacitance of 312 F gÀ1
which was approximately three times greater than that of pristine
Mn3O4 (113 F gÀ1
) at the same current density of 0.5 mA cmÀ2
in the potential range from -0.1 to +0.9 V.
About 76% of the initial capacitance was retained even after 1000 cycles establishes the fact that MG
nanocomposite exhibited good electrochemical stability and capacitance retention capability also.
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Metal oxide nano particles due to their versatile structural
properties leading to a wide variety of nano architectures finds
innumerable applications in catalysis [1], energy storage devices
[2], fuel cells [3], degradation and absorption of dyes [4] and even
removal of heavy metal ions from real water samples [5]. Due to
the depleting energy sources all around the world, the energy
storage applications of metal oxides have been vastly investigated
by the scientific community. Even though several electrochemical
energy storage and consumption devices were developed in the
past decades such as lithium ion batteries, supercapacitors and fuel
cells etc, supercapacitors separate among them with their amazing
properties like enormous charge–discharge in seconds and ability
to store huge power density in an astounding low cost of
manufacture have various potential applications in hybrid
electrical vehicles, new generation electronic devices and power
back up systems [6,7]. Classifications based on their ability of
charge - storage mechanisms the supercapacitors have been
categorized into two types namely –electrical double layer
capacitors (EDLCs) and pseudo capacitors [8]. Carbon materials
with their astonishing speed of absorption/desorption of electro-
lyte ions on their surface serves the purpose of electrode in
electrical double layer capacitors [9,10], while faradic electron
transfer process enables the storage of the charge and their release
on pseudo capacitors by the means of conducting polymers and
metal oxides [11–15]. Conducting polymers had been extensively
studied for the fabrication of supercapacitors but the major
disadvantage of it is the poor cycling performance due to
continuous charge–discharge process [16]. To overcome these
difficulties, different metal oxides have been investigated for the
successful replacement of conducting polymers as the alternative
electrode material for supercapacitors. This will increase the
specific capacitance by the rate of charge- discharge process with a
better cycling performance and in addition enhance the energy
density.
* Corresponding author. Tel.: +91 431 2503639; fax: +91 431 2500133.
E-mail addresses: sanand@nitt.edu, sanand99@yahoo.com (S. Anandan).
http://dx.doi.org/10.1016/j.electacta.2015.01.052
0013-4686/ã 2015 Elsevier Ltd. All rights reserved.
Electrochimica Acta 156 (2015) 127–137
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journal homepage: www.elsevier.com/locate/electacta

2. Transition metal oxides have multiple oxidation states and their
simultaneous existence for considerably long potential range
without any phase transitions, the versatility to accommodate
protons in their lattice positions during reduction process makes it
is a potential candidate for electrode materials in supercapacitors
[17]. Ruthenium oxide have been widely synthesized and vastly
exploited because of its outstanding specific capacitance, electrical
conductivity and good chemical stability. In spite of its high specific
capacitance (720 F gÀ1
), it has limited practical applications due to
high cost of manufacturing and toxic effects [18]. Mean while the
current research works focus on utilizing the capacitance
properties of comparatively low cost transition metal oxides like
NiO [19], Co3O4 [20], MnO2 [21] and Mn3O4 [22]. Manganese
oxides, which were believed to be a successful replacement for
RuO2, because of its low cost synthesis and comparatively high
specific capacitance generation due to its capability to exist in
seven different crystalline forms having Mn in different oxidation
states such as Mn2+
, Mn3+
and Mn4+
[23,24]. In the case of
manganese oxides, Mn3O4 have a normal spinel structure in room
temperature in which Mn3+
occupying the octahedral sites and
Mn2+
occupying the tetrahedral sites. Even though Mn3O4 was also
found to be environmentally benign but its intrinsic insulating
properties ($10À7
to 10À8
S/cm) [25] limited practical applications
of it. To enhance the electronic conductivity and in order to rise the
capacitance of the Mn3O4, a promising approach is to make
composite electrodes with Mn3O4 nanoparticles anchored with
highly conductive porous matrix materials.
2D material graphene, exhibits an extremely high surface area
(2630 m2
gÀ1
) [26], huge Young’s modulus (1100 GPa) [27], high
tensile strength, ballistic electrical conductivity (200000 cm2
vÀ1
sÀ1
) and thermal stability ($4840–5300 WmÀ1
KÀ1
) [28]. These
interesting properties make graphene an excellent candidate to
host active nanomaterials for many applications such as energy
storage devices [29] and sensors [30]. Several transition metal
oxide/hydroxide - graphene composites were then reported such
as RuO2 [31], NiO [32], Co3O4 [33], and manganese oxide [34,35]
have been anchored on graphene as electrode materials for
supercapacitors by using chemical reactions or physical mixing,
which exhibited remarkable higher capacitances in comparison
with pure graphene or individual metal oxides. However, the
electrochemical capacitive properties of Mn3O4–graphene com-
posites have received relatively little attention. Recently, various
attempts have been made to prepare Mn3O4–graphene nano-
composite as an electrode material for using energy storage
applications by following different synthetic approaches [36–41].
Wang et al. reported a two-step solution-phase method for
growing Mn3O4 nanoparticles on graphene surface to form Mn3O4/
graphene nanocomposite for Li-ion battery applications [36].
Zhang et al. reported a solvothermal method for the preparation of
Mn3O4–graphene nanocomposite with a specific capacitance of
147 F gÀ1
in 1 M Na2SO4 [37]. Wang et al. reported that Mn3O4
nanoparticles deposited on the both sides of the graphene
nanosheets by a hydrothermal method exhibits a specific
capacitance of 236 F gÀ1
in 2 M KOH solution [38]. Fan et al.
reported that the Mn3O4 nanoparticles anchored on graphene
nanosheets were synthesized using a facile in situ one-pot
hydrothermal method generates a specific capacitance of 171 F gÀ1
in 1 M Na2SO4 aqueous electrolyte [39]. Subramani et al. reported
that chemical decomposition of the manganese hexacyanoferrate
complex directly on the graphene surface yields Mn3O4–reduced
graphene oxide nanocomposites with a high specific capacitance of
131 F gÀ1
in 1 M Na2SO4 electrolyte at a current density of 0.5 A gÀ1
[40]. Zhu et al. [41] reported that Mn3O4/graphene composite is
obtained by using KMnO4 as both the oxidant and Mn source in a
Fig. 1. Schematic representation of the formation mechanism of Mn3O4 nanoparticles anchored graphene nanosheets (MG) by sonochemical synthesis.
128 B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137

3. modified Hummers’ method. It exhibits the specific capacitance of
271.5 F gÀ1
in 6 M KOH at the current density of 0.1 A gÀ1
. Even
though as mentioned above, most of the works related Mn3O4-
graphene explains that the method of preparation, size, morphol-
ogy of the Mn3O4, and the nature of the electrolyte used to examine
the supercapacitance performance. To achieve a better electrode
performance, Mn3O4 nanoparticles may uniformly anchored on
the graphene and further the particle size, morphology and
porosity of the materials mainly depend on the method of
preparation.
Hence in this manuscript, a facile one-step sonochemical
approach was followed to prepare Mn3O4 nanoparticles anchored
graphene nanosheets (MG) at room temperature without the use
of any templates or surfactants for supercapacitor applications.
Under Ultrasound irradiation, the conversion of graphene oxide to
graphene and manganese acetate to Mn3O4 takes place simulta-
neously. The results showed uniform distribution of Mn3O4
nanoparticles all around the surface of graphene nanosheets,
which may responsible for enhancing the capacitive properties of
the metal oxide by dramatic increase in the conductivity.
Electrochemical studies revealed a steep increase in the capaci-
tance of Mn3O4–graphene composite while compared with that of
pristine Mn3O4.
2. Experimental
2.1. Materials
Graphite powder (99%) purchased from Sigma-Aldrich, India.
Sodium nitrate (NaNO3, 99 wt%), Potassium permanganate
(KMnO4, 99.5 wt%), Hydrogen peroxide (H2O2, 30 wt%), Hydrazine
hydrate (H6N2O, 99 wt%), Sulphuric acid (H2SO4, 98 wt%), Manga-
nese acetate [Mn (CH3COO)2, 95wt%], Dimethylformamide were
purchased from Merck Co.Ltd., Sodium hydroxide pellets (NaOH,
98 wt%) and Ethanol (CH2CH2OH, 99 wt%) were purchased from
SRL Pvt. Ltd India. Vulcan XC-72, poly-vinylidene fluoride (PVdF)
was purchased from Fluka. N-methyl-2-pyrrolidone (NMP) was
purchased from Merck. All purchased chemicals and reagents were
of analytical grade and used as received without any further
purification.
2.2. Synthesis of Mn3O4 nanoparticles anchored graphene nanosheets
(MG)
Natural graphite powder obtained from Sigma-Aldrich had
been used for the synthesis of Graphene Oxide (GO) by modified
Hummer’s method as previously reported [42]. Then, 70 mg of GO
synthesized by Hummer’s method was dispersed on a solution
(100 mL) containing DMF and water in a ratio of 10:1. To the brown
colored solution, 5 mL of 0.05 M Manganese acetate was then
added drop wise under sonication. After 6 h of sonication, solution
color changes from brown to black indicating the formation of
Mn3O4 nanoparticles anchored graphene nanosheets (MG). The
resulted MG composite was washed with double distilled water for
several times and centrifuged. Finally, the product was dried under
vacuum oven at 60
C overnight to obtain pure MG composite. By
following the same procedure for preparation of pristine Mn3O4
without graphene was synthesized for comparative studies.
2.3. Characterization of materials
In order to confirm the crystal structure and phase purity of the
product, powder X-ray diffraction patterns were recorded on a
Philips XPertPro X-ray diffractometer with Cu Ka (l= 0.15418 nm)
radiation. FT-IR spectra were obtained with a Thermo Scientific
Nicolet iS5 FT-IR spectrometer using a KBr pellet. TGA spectra were
recorded in the TGA 4000-PerkinElmer in the range of 0–600
C at a
heating rate of 10
C/min in air atmosphere. The morphology and
structural properties of the MG composite were studied using a
JEOL 7401 F (Scanning electron microscope) and JEOL JEM 2010
(High resolution transmission electron microscope) model. Atomic
force microscope (AFM) images were studied under non-contact
mode with a XE-100 scanning probe microscope, Park systems,
South Korea.
Fig. 2. a) The XRD patterns of Graphene Oxide (GO), Graphene, (b) pristine Mn3O4, MG composite samples.
Fig. 3. FT-IR spectra of Graphene Oxide (GO), MG composite samples.
B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137 129

4. 2.4. Electrochemical measurements
A supercapacitor device was fabricated using a high-purity
stainless steel (SS) plate as a current collector. The plate was
polished with successive grades of emery paper, cleaned with soap
solution, washed with DD water, rinsed with acetone, dried and
weighed. The working electrode is made up of MG composite
(75 wt.%) as an active material, Vulcan XC-72 carbon (20 wt.%) as a
conductive agent, PVdF (5 wt.%) as a binder were ground in a
mortar, and a few drops of NMP was added to form slurry. It was
coated onto the pretreated SS plate (coating electrode area is
1.0 cm2
) and dried at 100
C in vacuum for 12 h. Cyclic voltammetry
(CV), galvanostatic charge–discharge and electrochemical imped-
ance spectroscopy (EIS) techniques were carried at room
temperature by using a potentiostat/galvanostat (AUTOLAB
302 N module) testing system.
The supercapacitance studies were carried out in a standard
three-electrode system containing the MG composite coated
stainless steel plate as a working electrode, Pt foil as a counter
electrode and Ag/AgCl as a reference electrode. The electrolyte
used was aqueous solution of 1 M Na2SO4. The performance of
supercapacitor studies were evaluated by cyclic voltammetry (CV)
and galvanostatic charge–discharge techniques within the poten-
tial range of - 0.1 to +0.9 V at different scan rates (5, 10, 20, 40,
80 and 160 mV sÀ1
) and different current densities (0.5–15 mA
cmÀ2
) respectively. Electrochemical impedance spectroscopy
measurements were performed under open circuit voltage in an
alternating current frequency range of 0.1–1,00,000 Hz with an
excitation signal of 10 mV. As a comparison, the electrochemical
performance of pristine Mn3O4 was also investigated under the
same conditions.
3. Results and discussion
The formation of Mn3O4 nanoparticles anchored on graphene
surface layer as shown in Fig.1 and is explained on the basis of XRD,
FTIR and TEM results. Firstly, the manganese acetate solution was
slowly added into GO dispersion to form a stable aqueous
suspension. As it is well-known that the GO nanosheets consists
of plentiful carboxyl and epoxide functionalities, they are
hydrophilic and intrinsically negatively charged [43]. In aqueous
solution, manganese ions are positively charged. So in the solution,
Mn2+
ions get coordinated to GO nanosheets by electrostatic
interaction via several number of oxygenated groups like epoxy,
hydroxyl and carboxylic acid [44]. It is reported that in alkaline
atmosphere, GO gets reduced to graphene [45] and also Mn+2
ion
oxidized to Mn+3
ion which leads to the formation of Mn3O4.
However, DMF: H2O mixture is used here which generates a good
source of hydroxyl ions by hydrolysis mechanism [46] (see Eq.(1))
and in addition ultrasound also produces hydroxyl radicals thus
maintains the basic medium. As produced hydroxyl ions contribute
the conversion of GO to graphene and Mn2+
ion are oxidized to Mn3
+
ion which leads to the formation Mn3O4 simultaneously.
The XRD patterns of the graphene oxide (GO), graphene,
pristine Mn3O4 and Mn3O4-graphene composite (MG) are shown
in Fig. 2a b. In the case of GO, the diffraction peak at 10.5
denotes
the interlayer spacing between graphene oxide nanosheets of
0.85 nm. GO reduced by DMF and water solution will give two very
broad peaks at 26
and 41
which corresponds to that of graphene
nanosheets having random arrangement of material resembling
graphite. The positions and relative intensities of the peaks
associated with the MG composite and pristine Mn3O4 can be
indexed perfectly and matches well with the reported tetragonal
structure of Mn3O4 nanoparticles (JCPDS no. 024-0734). There is no
any other impurity peaks were observed in the XRD pattern,
indicating the high phase purity of the prepared samples. In
addition to that, the obtained diffraction peaks are relatively broad
and of low intensity, suggesting that the prepared samples are
comparatively small in particle size. The diffraction peaks of
graphene cannot be detected in the MG composite indicates that
the graphene nanosheets are stacked in a disordered manner with
a low degree of graphitization because the Mn3O4 nanoparticles
loaded on the graphene nanosheets prevented graphene nano-
sheets from restacking [47].
The FT-IR spectra of the prepared GO, MG composite are shown
in Fig. 3. Various functional groups are present on GO such as
hydroxyl, epoxy, carboxyl, alkoxy and carbonyl groups. The strong
peak observed at around 3432 cmÀ1
corresponds to the presence
hydroxyl groups in GO nanosheets. The C–H stretching peak at
2923 cmÀ1
, a peak around at 1742 cmÀ1
assigned to carbonyl/
carboxyl C¼O, 1627 cmÀ1
assigned to aromatic C¼C, C–O in C–OH/
C–O–C functional moieties can be seen at 1078 cmÀ1
, respectively
Fig. 4. The TG analysis spectrum of pristine Mn3O4, MG composite samples in air.
(1)
130 B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137

5. [40]. The MG composite showed two new peaks at 625 cmÀ1
and
527 cmÀ1
representing to the vibration of the Mn-O stretching
modes of tetrahedral and octahedral sites, respectively. Moreover,
the intensity of all of these absorption peaks related to the oxygen
containing functional groups decreased significantly in the
spectrum of MG composite, indicating the complete reduction
of GO and restored aromatic nature in the composite [38]. In
addition, a new absorption peak that appears around at 1594 cmÀ1
represents to the C¼C stretching vibration of aromatic skeleton of
the graphene nanosheets, and its presence further confirms the
reduction of GO.
The mass percentage of Mn3O4 and graphene in MG composite
was determined by thermo gravimetric analysis (TGA). The TGA
curves of MG composite compared with pristine Mn3O4 are shown
in Fig. 4. The weight loss occurring at above 100
C is reasonably
attributed to the evaporation of physically and chemically
adsorbed water in the case of pristine Mn3O4 and the same for
MG composite also. The weight loss starting from 150–400
C
indicates the decomposition of graphene into CO2 and H2O. While
reaching 500
C, the entire graphene content has been removed
from MG composite. Generally, graphene and graphene oxide are
completely burned to CO2 in air below 500
C [49]. Pristine Mn3O4
does not undergo any degradation after 500
C, indicating that
Mn3O4 is very stable within the temperature region of TGA. The
weight loss of graphene in MG composite is calculated as 17%,
which suggests that the mass percentage of Mn3O4 is 83%.
The morphology, particle size and crystallinity of the MG
composite have been investigated using FESEM and TEM images.
Fig. 5a b shows a low and high magnification of FESEM images. It
can be seen that Mn3O4 nanoparticles are anchored and densely
dispersed on the surface of graphene nanosheets. Fig. 6 shows TEM
HRTEM images of MG composite. For MG composite, after
investigation of several graphene nanosheets in TEM and all of
them were found to be of size 0.2–0.5 mm and one such sheet
shown in Fig. 6a is of 0.5 mm. The morphology of the Mn3O4
nanoparticles is spherical in shape and also closely anchored on the
surfaces of graphene nanosheets in Fig. 6b. It should be noted that
the Mn3O4 nanoparticles are tightly bound with graphene nano-
sheets even after long time sonication, which not only facilitates
the aggregation of Mn3O4 nanoparticles but also minimizes the
restacking of graphene nanosheets. This result is expected to lead
easily the diffusion and movement of electrolyte ions into the
interior of the electrodes [47]. Random measurement of hundred
particles on graphene nanosheets indicates that the size of most of
the nanoparticles falls in the range of 4–8 nm, as shown in the inset
of Fig. 6b. In spite of that, the particle size of the Mn3O4 was
determined around 7 nm. HRTEM image (Fig. 6c) was focused on
lattice planes of Mn3O4 nanoparticles. The (211) crystal planes of
Mn3O4 lattice fringes are clearly seen and matches well with a d–
spacing of 0.25 nm. The SAED pattern (Fig. 6d) shows crystalline
nature of Mn3O4 nanoparticles and the four diffraction rings, which
are assigned to the (101), (112), (2 0 0) and (211) planes of Mn3O4
nanoparticles are consistent with the XRD results (Fig. 2). The EDX
spectrum and elemental mapping (Fig. 6e) also shows the presence
of Mn, O and C as elements in the MG composite. The morphology
and nano-scale structure of MG composite was further examined
using atomic force microscopy (AFM) in noncontact mode at
different scales shown in Fig. 7. Several numbers of graphene
nanosheets are stacked with Mn3O4 nanoparticles in the image.
Mn3O4 nanoparticles are identified as dots, anchored on the
surface of graphene nanosheet. The particle size can be determined
by the height differences of the desired particles. The values of the
particle size are evaluated to be 7 nm respectively, consistent with
the results obtained from TEM analysis ($4–8 nm).
The electrochemical properties of the pristine Mn3O4, MG
composite as electrode materials for supercapacitors were
investigated using cyclic voltammetry (CV) and galvanostatic
charge–discharge in a three-electrode system with aqueous 1 M
Na2SO4 as electrolyte from -0.1 to +0.9 V (vs. Ag/AgCl). Fig. 8a
compared the CV measurements of pristine Mn3O4, graphene and
MG composite at a scan rate of 10 mVsÀ1
on 1 M Na2SO4, it can be
clearly understood that both the curves are rectangular in shape
without any redox peak currents exhibiting the ideal capacitive
behavior of the prepared electrodes. As, the integrated area under
the CV curve is proportional to the specific capacitance, it is evident
from the CV curves that MG composite electrode has a greater
specific capacitance in compared with pristine Mn3O4, graphene.
The capacitance of the MG composite was enhanced owing to the
synergistic effect between the conductive graphene, pseudocapa-
citive Mn3O4 which can provide free diffusion pathways for the fast
ion transport and facile ion accessibility to storage sites [49–51].
Fig. 8b shows the CV curves of MG composite electrode under
different scan rates in 1 M Na2SO4 and all the curves are
rectangular in shape and exhibit mirror image characteristics
which indicate that the Faraday redox reactions are electrochemi-
cally reversible as well as an ideal electrochemical capacitive
behavior [52]. It can be seen that the current under curve increases
with an increase in the scan rate which indicates that the
voltammetric current is always directly proportional to the scan
rate [53] and the CVs of the MG composite retain a similar shape
even at high scan rate, indicating an excellent capacitance behavior
and the fast diffusion of electrolyte ions into the MG composite
electrode [45].
The galvanostatic charge-discharge studies were carried out to
acquire more information on the capacitive performance of the
pristine Mn3O4 and MG electrode. Fig. 9a shows charge-discharge
Fig. 5. FESEM images of MG composite (a and b) at different magnifications, respectively.
B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137 131

6. curves of pristine Mn3O4, graphene and MG composite electrode at
current density of 0.5 mA cmÀ2
in 1 M Na2SO4 as electrolyte
solution in the potential range between -0.1 to 0.9 V (vs. Ag/AgCl). A
linear variation of the potential with time during charging and
discharging processes can be seen with the electrodes, which is
another criterion for capacitive behavior of the material. But in the
MG composite, the charge curve is almost symmetric with its
corresponding discharge counterpart, with a small internal
resistance (IR) drop, indicating the pseudocapacitive contribution,
along with the double layer contribution [54]. The increase in the
charging and discharging time indicates the higher capacitance of
the MG composite. The discharge specific capacitance (SC) of
electrode material at different current densities can be calculated
using the following formula
SC = I t/m DE (2)
where, I is the charge–discharge current in amps, t is the discharge
time in seconds, m is the mass of the active material present on the
electrode in grams and DE is the operating potential window in
volts of charge or discharge. The specific capacitance calculated
Fig. 6. TEM images of MG composite (a, b). The inset in (b) is a statistical histogram of the sizes of Mn3O4 nanoparticles anchored on graphene surface. HRTEM image of MG
composite (c), corresponding SAED pattern (d) and EDX (e) spectrum.
132 B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137

7. from the discharge curves (Fig. 9a) is 312 F gÀ1
for the MG
composite, which is much higher than that for the pristine Mn3O4
(113 F gÀ1
) and graphene (153 F gÀ1
) at a current density of
0.5 mA cmÀ2
in 1 M Na2SO4 electrolyte, respectively. Approximate-
ly, the specific capacitance of the MG composite is three times
larger than that of pristine Mn3O4. The significantly enhanced
capacitance of the MG composite is probably attributed to the
improved electrical conductivity of the doped graphene and a high
utilization speed of electro active Mn3O4 nanoparticles [47]. Fig. 9b
shows the galvanostatic charge discharge curves of MG composite
electrode at a current density of 0.5 mA cmÀ2
. Table 1 summarizes
electrochemical performance reported in literature for Mn3O4/
Graphene composites as a supercapacitor electrode material.
As the good cycling stability are most important criteria for
supercapacitors. The cycling stability of the MG composite was
examined by charge–discharge cycling at a current density of
0.5 mA cmÀ2
in 1 M Na2SO4 as electrolyte (Fig. 9c). The MG
composite retained 76% of the initial specific capacitance even
after 1000 cycles, indicating the excellent electrochemical stability
and hence capacitance retention capability. Fig. 9d. shows the
galvanostatic charging-discharging curves of the MG composite at
different current densities (0.5–15 mA cmÀ2
) and their specific
capacitances were shown in Fig. 9e. The specific capacitance was
312, 307, 297, 274, 255, 236, 221, 220 and 219 F/g at 0.5, 1, 3, 5, 7, 9,
11, 13 and 15 mA cmÀ2
current density, respectively. This phenom-
enon can be explained by the inconsistent insertion-deinsertion
behavior of ions from the electrolyte to electrode materials [55]. In
this cycling test, charge/discharge time increased at low current
density, as the ions from the electrolyte can be fully occupied the
active sites at electrode/electrolyte interface. Similarly, charge/
discharge time decreased at high current density, as the ions
occupy the limited number of active sites at electrode/electrolyte
interface. It reveals that the specific capacitance is inversely
proportional to the current density [56]. When the current rate is
increased from 0.5 to 15 mA cmÀ2
(312 to 219 F gÀ1
), 70% of the
capacitance was retained. This implies that the MG composite has
an excellent rate capability at high current density, which is a
crucial key factor for evaluating the power applications of
supercapacitors.
The electrochemical impedance spectroscopy (EIS) measure-
ments were evaluated to study the mechanistic characteristics of
pristine Mn3O4 and MG composite electrode over a frequency
Fig. 7. AFM images of MG composite at different scales.
Fig. 8. CV curve for (a) pristine Mn3O4, graphene and MG composite at 10 mV sÀ1
scan rate, (b) CV curve for MG composite at different scan rates 5 mV sÀ1
, 10 mV sÀ1
,
20 mV sÀ1
, 40 mV sÀ1
, 80 mV sÀ1
, and 160 mV sÀ1
in the potential range of -0.1 to 0.9 V vs. Ag/AgCl in aqueous solution of 1 M Na2SO4 as electrolyte (a-f).
B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137 133

8. range of 0.1 to 100000 Hz. Fig. 10(a) shows the Nyquist plots of the
EIS for pristine Mn3O4 and MG composite. According to the
analysis, the MG composite exhibited a semicircle over the high
frequency region, followed by a vertical part in the low frequency
range. The diameter of the semicircle in the plot corresponds to the
charge transfer resistance (Rct), which is mainly generated at the
electrode/electrolyte interface [39,56]. Obviously, the smaller
semicircle in MG composite means charge transfer resistance
decreased extremely when compared to pristine Mn3O4 electrode
can be attributed to the presence of conductive graphene, revealing
that the graphene facilitates the conductivity. Low charge transfer
resistance implies that the electrolyte ions can easily diffuse into
the pores of the electrode material and access the surface of active
electrode material. At low frequencies, the vertical shape is an
indication of pure capacitive behaviour and low diffusion
resistance of ions in the structure of MG composite electrode.
Fig. 9. a) Charge–discharge cycles of pristine Mn3O4, graphene and MG composite in aqueous solution of 1 M Na2SO4 at a current density of 0.5 mA cmÀ2
between -0.1 to
+0.9 V vs. Ag/AgCl. Area of the electrode: 1.0 cm2
(b) Charge–discharge cycles of MG composite at a current density of 0.5 mA cmÀ2
(c) Cycling behavior of MG composite (d)
Galvanostatic charge–discharge curves at different current densities 0.5 mA cmÀ2
, 1 mA cmÀ2
, 3 mA cmÀ2
, 5 mA cmÀ2
(e) Specific capacitance variation at different current
densities.
134 B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137

9. Table 1
Summary of electrochemical performance reported in literature for Mn3O4/Graphene composites as a supercapacitor electrode material.
Preparation method Nature of Mn3O4/Graphene
Composite
Current
collector
Mass of electro active
materials
(mg/cm2
)
Electrolyte Measurement
protocol $
Maximum
capacitance
(F/g)*
Ref
(year)
ultrasonication processing and
heat treatment
powder platinum foil – 1 M Na2SO4
2 M KOH
CV (v = 5 mV/
s)
175
256
[24]
(2010)
solvothermal powder stainless
steel foil
– 1 M Na2SO4 Cp (i = 0.1 A/g) 147 [37]
(2012)
hydrothermal powder nickel grid – 2 M KOH Cp (i = 1 A/g)
Cp (i = 8 A/g)
236.7
133
[38]
(2012)
hydrothermal powder nickel foam 3 1 M Na2SO4 Cp (i = 0.1 A/g) 171 [39]
(2013)
hydrothermal self-assembly hydrogel carbon
flake
2 1 M Na2SO4 Cp (i = 1 A/g) 148 [43]
(2013)
hydrothermal nitrogen-doped graphene–
Mn3O4 nanohybrids
nickel foam 7.8 0.5 M Na2SO4 with
5 mM NaHCO3
Cp (i = 1 A/g) 206 [48]
(2013)
chemical decomposition powder stainless
steel mesh
3–5 1 M Na2SO4 Cp (i = 0.5 A/
g)
131 [40]
(2014)
ultrasonication powder stainless
steel plate
1.2 1 M Na2SO4 Cp
(i = 0.5 m A/
cm2
)
Cp (i = 15 m A/
cm2
)
312
219
this
work
$
CV = cyclic voltammetry, Cp = chronopotentiometry, v = scan rate, and i = current density.
*
Maximum specific capacitance reported.
Fig. 10. a) Electrochemical impedance spectra of pristine Mn3O4 and MG omposite. The inset shows the enlarged EIS at the low frequency region. b) Electrochemical
impedance spectra of for MG composite before and after 1000 cycles (c) FE-SEM micrographs of MG composite after 1000 cycles.
B.G.S. Raj et al. / Electrochimica Acta 156 (2015) 127–137 135

10. Fig. 10(b) shows Nyquist plots of the EIS for MG composite before
and after 1000 cycles, the measured charge transfer resistance (Rct)
was increased from 3 to 33 Vcm2
, this further leads to the decrease
in specific capacitance after long term cyclic stability test. Such
observed significant changes in the MG composite after the long
cyclic stability tests may be due to morphology changes takes place
at the electrode which is analyzed to FE-SEM. The FE-SEM image
(Fig. 10(c)) revealed the dissolution, aggregation, and the volume
change occurred in the electrode material after 1000 cycles
which may in turn decrease the specific capacitance of the
electrodes.
The electrochemical parameters, such as energy and power
density, play an important role in the capacitive behavior of the
electrochemical cells. The energy and power density can be
evaluated using Eqs. (3) and (4),[57]
E = (I*t*V)/(7.2*M) Wh/kg (3)
P = 3.6 E/t W/kg (4)
where E and P is the energy and power density respectively. I
is the applied current (mA), t is the discharge time of the
active material (s), V is the potential window (V) and M is the mass
of the active material (mg). The calculated energy density and
power density of the composite electrodes were evaluated
43 Wh kgÀ1
and 207 W kgÀ1
at a discharge current density of
0.5 mA cm2
.
The high capacitance, good cycle performance and excellent
rate capability delivered in the case of MG composite electrode
can be illuminated by the following factors. First, the graphene
nanosheets provide as a conductive matrix to promote fast
Faradaic charging and discharging property of the Mn3O4
nanoparticles, favoring the long-term electrochemical stability
and the excellent rate performance. Second, well-dispersed
Mn3O4 nanoparticles anchored on the graphene nanosheets
can significantly improve the electrochemical utilization of
Mn3O4 and hence minimize the transport length of Na+
during
the charge–discharge process. Third, the graphene prevents
aggregation of the Mn3O4 nanoparticles, preserving the high
interfacial area between the Mn3O4 nanoparticles and the
electrolyte. As an overall results clearly demonstrate that the
MG composite have considerably improved electrochemical
performance compared with pristine Mn3O4. The MG composite
is expected to be a promising electrode material having great
potential application in manufacture of electrochemical super-
capacitors.
4. Conclusions
In summary, Mn3O4 nanoparticles anchored graphene nano-
sheets (MG) has been synthesized by a simple ultrasound assisted
synthesis at room temperature without the use of any templates
or surfactants. The phase composition, morphology, and structure
of the as-prepared products were studied in detail. The
electrochemical performance of MG composite delivers a high
specific capacitance of 312 F gÀ1
which was approximately three
times greater than that of pristine Mn3O4 (113 FgÀ1
) at 0.5 mA
cmÀ2
current density in the potential range from -0.1 to +0.9 V. In
addition to that a good rate capability of 219 F gÀ1
at 15 mA cmÀ2
and long cycling life with 76% capacitance retention after
1000 cycles was exhibited by MG composite. Since MG composite
has these attractive and enhanced properties like high
capacitance, good rate capability and capacitance retention we
believe that it is promising candidate for supercapacitor
applications.
Acknowledgements
The research work was financially supported by Council of
Scientific and Industrial Research (CSIR), New Delhi (CSIR
Reference No.02 (0021)/11/EMR-II).
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