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Original Article

g-MnS nanoparticles anchored reduced graphene oxide: Electrode

materials for high performance supercapacitors

S. Ranganatha

*

, N. Munichandraiah

Department of Inorganic& Physical Chemistry, Indian Institute of Science, C V Raman Avenue, Bengaluru 560012, India

a r t i c l e i n f o

Article history:
Received 1 June 2018
Received in revised form
27 June 2018

Accepted 2 July 2018
Available online 6 July 2018
Keywords:

Supercapacitors
Reduced graphene oxide

g-MnS
Composite
rGO

a b s t r a c t

g-MnS/reduced graphene oxide composites (g-MnS/rGO) were successfully synthesized by a simple one
pot solvothermal route. Their structure, morphology and electrochemical properties were studied with

respect to applications as a supercapacitor electrode material. The specific capacity ofg-MnS/rGO is
1009 C/g at 1 A/g and retains 82% of initial capacity over 2000 cycles at 2 A/g whereas pristineg-MnS
delivers only 480 C/g at 1 A/g with a capacity retention of 64%. Thus,g-MnS/rGO proves to be a promising
electrode material, which exhibits high the specific capacity and stable long cycle life.

© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />

1. Introduction

In the current scenario, more and more efforts are focussed on
suitable and environmentally friendly energy converting and
en-ergy storage materials. Electrochemical supercapacitors are one
such class of materials, which offer high energy density, fast charge/
discharge rate, long cycle life, etc. They are mechanically classified
as electrochemical double layer capacitors and pseudocapacitors

[1_e5]. Some of the recent literature reports on transition metal
oxides and sulphides have described their significance as potential
candidates for supercapacitor electrode materials[6e10].
Manga-nese sulphide being a wide gap semiconductor, has found potential
applications in short wavelength opto electronics, luminescents
and magnetic semiconductors technologicalfields. These materials
are used for semiconductor spin-based electronics or spintronics
due to their magnetic and magneto-optical properties which arise
from spin-exchange interactions between the dopant ions and the
semiconductor charge carriers[1,3,4]. Recent reports clarify that
metal sulphides, in particular MnS, are promising materials for
supercapacitors. MnS is known to exhibit strong redox peaks in
the cyclic voltammogram which is attributed to the non-linear

dependence of charge storage vs. potential advocating it's
fara-daic or battery type behaviour[11]. MnS is known to crystallize in
three different polymorphic forms, namely,

a

-MnS with rock salt
structure,

b

-MnS with zinc-blends structure and

g

-MnS with
wurzite structure.

g

-MnS stands superior in electrochemical
per-formances due to its laminar nanostructure facilitating easy
pene-tration of electrolytes and intercalation of ions affecting the
capacitive behaviour positively [12e17]. MnS is less focussed for
supercapacitive applications due to its poor cycling ability and low
electronic conductivity[18,19].

Carbon based materials like activated carbon, carbon nanotubes,
graphenes etc. are very potential electrode candidates for
super-capacitors and batteries which offer high power density and long
cycle life. Unfortunately the charge storage mechanism limits its
energy density. Recently, scientists targeting bridging the gap of this
power density and energy density by combining the contributions
of both pseudocapacitive materials like metal sulphides/oxides with
conducting materials. Popularly, the conducting polymer
polyani-line is being used to wrap pseudocapacitive materials, thus to
enhance the performance. In recent past, studies dealing with
anchoring the metal sulphides/oxide nanoparticles to graphene
sheets are gaining importance because of their high conductivity
and very high specific surface area[20]. Using graphene as a matrix
for MnS will be a good idea to facilitate large electrode/electrolyte
interfaces for charge/discharge reactions and to enhance the
con-ductivity[21e26].

* Corresponding author.

E-mail address:(S. Ranganatha).
Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j s a m d

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In this work, the MnS anchored reduced graphene oxide (rGO)
composite has been successfully designed by the facile solvothermal
method. Its supercapacitive performance has also been evaluated
showing that

g

-MnS/rGO composite possesses better capacities
compared to the pristine

g

-MnS.

2. Experimental

In brief, rGO was synthesized using the oxidation of graphite by
KMnO4and H2O2and NaNO2, and by the subsequent hydrothermal

reduction with an ammonia solution [26].

g

-MnS/rGO was
pre-pared using a solvothermal procedure based on rGO dispersed
glycerol, MnCl2 4H2O and thioacetamide at 190
C for 5 h[21].

Samples were characterized by various techniques, such as X-ray
diffraction (XRD), Tunneling electron microscopy (TEM), Raman
spectroscopy and X-ray photoelectron spectroscopy (XPS).
2.1. Preparation of rGO

To synthesize the graphene oxide, 10 g of graphite powders and
5 g NaNO3were mixed and added to 220 mL conc. H2SO4which

was kept in an ice bath. 30 g of KMnO4was slowly added with

constant stirring. After 30 min, the mixture was further stirred at
35
C for 3 h. 460 mL of water and 80 mL of H2O2were then added

slowly to the solution. After cooling, the mixture wasfiltered and
washed with 10% HCl and deionized water until the sulfate ions

Fig. 1. (a) XRD patterns of GO& rGO, (b)g-MnS,g-MnS/rGO, (c) TEM image and SAED pattern ofg-MnS, (d) TEM image ofg-MnS/rGO, rGO sheets as inset, (e) HRTEM ofg-MnS, (f)
TEM and SAED pattern ofg-MnS/rGO.

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were removed. As-prepared GO was reduced by the solvothermal
method using NH4OH. Nearly 50 mg GO was dispersed in 60 mL

ethanol and sonicated for 3 h. 10 mL of NH4OH was added and

reduced hydrothermally at 180
C for 10 h[27].
2.2. Preparation of

g

-MnS/rGO

To prepare the

g

-MnS/rGO, 60 mg of rGO was dispersed in 60 mL
of glycerol by sonication for 1 h. 0.01 mmol of MnCl2$4H2O and

0.01 mmol Thioacetamide were dissolved in 10 mL distilled water
individually. Both of these solutions were added to 60 mL of
glyc-erol, well mixed and stirred then transferred into a Teflon lined
autoclave of 100 mL capacity. The autoclave was sealed and
main-tained at 190
C for 5 h. Precipitates were washed and dried. The

same procedure was followed to synthesize the

g

-MnS, except the
addition of rGO.

2.3. Characterization

Powder X-ray diffraction (XRD) patterns were recorded using a
PANylatical diffractometer with Cu K

a

(Wavelength ¼ 1.5438 Å)

incident radiation as the source. The surface area and the pore size
distribution of the samples were measured using the micromeritics
surface area analyzer of the model ASAP 2020. The X-ray
photo-electron spectra (XPS) were collected on an AXIS ULTRA X-ray
photoelectron spectrometer. Microscopy images of the samples
were recorded using the FEI Tecnai T-20 e 200 kV transmission
electron microscope (TEM) and FEI Co. equipped with an EDAX
system at an accelerating voltage of 10 kV. The Raman spectra were
measured by a Horiba Jobin Yvon LabRam HR spectrometer having
an 0.2 mW power laser of 514.5 nm wavelength illustrating the
sample surface.

2.4. Preparation of electrodes and electrochemical experiments
For the fabrication of the electrodes, the active material
(70 wt.%), conductive carbon (Ketjen black, 15 wt.%) and
poly-vinylidinefluoride (15 wt.%) were mixed in a mortar. A few drops of
N-methyl pyrrolidone were added to form a slurry. This slurry was
coated on a carbon paper with a geometrical area of 1 cm2and then
dried at 100
C under reduced pressure. The coating and drying
steps were repeated to get the mass of the active material

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0.8e1 mg/cm2_{. The electrodes were} _{finally dried for 12 h. An}

electrochemical cell was assembled using the material coated
car-bon paper, Pt and a saturated calomel electrode (SCE) as the
working, counter and reference electrodes, respectively, in a glass
container.

All potential values are reported against SCE reference. The
cy-clic voltammetry (CV) and the galvanostatic charge/discharge
cycling were measured by the Biologic SA multichannel
potentio-stat/galvanostat of the model VMP3, in a 6M KOH solution. The
electrochemical impedance spectroscopic measurements (EIS)
were done using the Electrochemical Analyzer model CHI608C in
the range 0.01 Hze100 kHz with an alternating voltage
perturba-tion of 5 mV. The galvanostatic charge/discharge cycling tests were
performed and the discharge speci_{fic capacity (C) was calculated}
using the relation C¼ It/m, where I is the current, t the discharge
time,

D

E the potential window and m the mass of the active
ma-terial on the working electrode.

3. Results and discussion

XRD patterns for graphite oxide and reduced graphene oxide
are shown in Fig. 1(a). A peak at 10.6
in GO indicates the
oxidation of graphite. This characteristic peak vanishes as rGO
forms, indicating the periodic layered structure of rGO sheets. The
peak emerged at 24
advocates the formation of graphene and its
amorphous structure.Fig. 1(b) refers to the XRD pattern of the

g

-MnS (JCPDS file # 40-1289) with the characteristic diffraction
peaks at 26
, 28
, 37.8
, 46
, 50.2
, 54.5
, 61.5
, 70
and 78.5

corresponding to (100), (002), (102), (110), (103), (112), (202),

(203) and (105), respectively [21,23]. Fig. 1(c) shows the TEM
image and the SAED pattern of the

g

-MnS indicating its well
dispersed nanoparticles and polycrystalline nature with distinct
diffraction rings.Fig. 1(d) and (f) show the TEM images of

g

-MnS/
rGO wherein the rGO sheets are shown with the arrow marks
demonstrating the anchoring of

g

-MnS on to the rGO sheets. Also,
an image of the individual rGO sheets is provided as an inset in

Fig. 1(d). The SAED pattern of

g

-MnS/rGO with no distinct
diffraction rings suggests the amorphous nature of this material.
The HRTEM image of the composite shown in Fig. 1(e) clearly
pronounces the characteristic lattice fringes with a lattice plane
space of 0.32 nm, which corresponds to the (002) plane of

g

-MnS
in agreement with the XRD results.

Fig. 2(a) shows a broad XPS survey spectrum of

g

-MnS
indi-cating the presence of the n, S, C and O elements. The binding
en-ergies at about 642.1 and 655.1 eV (Fig. 2(b)) can be assigned to Mn
2p3/2and Mn 2p1/2, respectively. The peaks at 162 and 164.5 eV

(Fig. 2(c)) are attributed to the binding energies of S 2p3/2and S

2p1/2, respectively. These values are matched with corresponding

literature values and confirmed that Mn2ỵand S2are present in
the sample. The peak at around 169 eV, suggests that a part of S2
on the MnS surface in the as-synthesized material has been
oxidized [21,22]. In Raman spectra of GO, rGO and

g

-MnS/rGO
(Fig. 2(d)), D and G bands of the graphene are exhibited by the
curves at 1349 and 1586 cm1, the representing poorly crystallized

graphite and crystal graphite's stretching mode, respectively. ID/IG

ratio convey the quality of graphene and it gets improved from 0.98
to 1.36 for GO to rGO and it is 1.38 for that of

g

-MnS/rGO which is
ascribed to the new and smaller sp2 domains formed during the
reduction of GO. The composite shows a characteristic peak at
645 cm1confirming the presence of

g

-MnS[21e24].

The specific surface area was calculated using the
Bru-nauereEmmetteTeller (BET) method from the adsorption branch
of isotherms in p/p0 range of 0.1e0.2 (Fig. 3). The inset of 3(a)
depicts the isotherms of the as-prepared

g

-MnS. The adsorption
and the desorption branches (Fig. 3(a)) exhibit a loop at the high
relative pressure indicating a porous nature of the compound. In
the case of

g

-MnS/rGO, there were 28 cm3/g of N2 adsorbed at

p/p0¼ 0.99 and the sample possesses a specific surface area of
6.8 m2_{/g whereas for}

_g

_{-MnS the corresponding values are 2.5 cm}3_/g

and 1.2 m2/g, respectively. According toFig. 3(b), the BJH curves of
the composite depict a pore size distribution with a prominent
maximum at around 20 nm.

Fig. 4(a) and (b) depict the CV diagrams of the

g

-MnS and

g

-MnS/rGO electrodes, respectively. Broad voltammograms with
current peaks corresponding to redox reactions are observed at all
scan rates suggesting a faradaic nature of

g

-MnS. This behavior is
contrary to the electric double layer capacitor behaviour, where
rectangular CV is the signature. The experimental result showing
the incremental current responses with the increasing scan rates

signify the diffusion limited redox behavior. Furthermore, the redox
peaks move with the increasing scan rates can be attributed as
being responsible for the limitation of the diffusion rate to satisfy
the electronic neutralization[28,29]. The variable oxidation states
of Mn (Mn2ỵ/Mn3ỵ/Mn4ỵ) in MnS contribute to the faradaic

Fig. 3. (a) Adsorptione desorption isotherms from the BET experiment (The inset: isotherms of MnS enlarged), (b) pore size distribution for theg-MnS andg-MnS/rGO.
S. Ranganatha, N. Munichandraiah / Journal of Science: Advanced Materials and Devices 3 (2018) 359e365

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capacity. The CV diagrams reflect a good reversibility of the
corre-sponding electrode processes; and also the large integrated area
assures a consequential remarkable capacity. The following redox
reactions can be proposed[21e26].

MnSỵ OH_{4MnSOH ỵ e}

MnSOHỵ OH_{4MnSO ỵ H}
2Oỵ e

The charge-discharge voltage proles registered at different
specic currents are shown inFig. 4(ced). The symmetric
charac-teristics of the charge_{edischarge curves suggests a satisfactory}
reversibility w.r.t. faradaic reactions. The pristine

g

-MnS shows a
specific capacity of 480 C/g at 1 A/g and 47 C/g at 15 A/g, whereas,

g

-MnS/rGO offers a high capacity 1009 C/g at 1 A/g and 90 C/g at
high specific current 15 A/g (Fig. 4(e)). Also, to test the cyclic
sta-bility of the materials, 2000 cycles were run at 2 A/g (Fig. 4(f)). The
composite retains 82% of the initial capacity whereas the pristine

g

-MnS retains only 64%. In Nyquist plots (Fig. 4(g)) the intersection
of the semi-circle at high frequencies on the real axis reflects the

solution resistance RS, whereas, the diameter of the semi-circle is

equated to the charge-transfer resistance Rctof the interface

elec-trode/electrolyte. The Rctvalues for

g

-MnS and

g

-MnS/rGO are 2

U

and 0.3

U

, respectively, suggesting a lower intrinsic resistance and a
better capacitive behavior of

g

-MnS/rGO. The straight line or a
spike seen in the low frequency region in the case of

g

-MnS/rGO
represents the resistance to the diffusion of the electrolyte ions to
the electrode interior. The angle of the straight line with respect to
the horizontal axis closely to 90
suggests the fast electrolyte
diffusion and adsorption to the electrode surface attesting the ideal
capacitor characteristics[28e30].

A quick review on the previous reports on MnS as a
super-capacitor material manifests the superiority and novelty of the
pre-sent work. Quan et al., fabricated the

a

-MnS/rGO solvothermally and
studied its electrochemical properties. It exhibits 513 C/g at 1 A/g of
specific current[31]. Recently, MnS/rGO was fabricated by Xu et al.
also using the solvothermal method. The group could obtain a high
capacity up to 540 C/g at 1 A/g [32].

a

-MnS nanosheets were
generated adopting the hydrothermal route by Li et al. The maximum
capacity of that synthesized material was 137.6 C/g at 0.5 A/g[33].
In a recent study by Hou et al., aimed to synthesize a

g

-MnS/CNT
hybrid by the two steps hydrothermal method. They obtained a
capacity up to 353 C/g at 1 A/g from the

g

-MnS anchored CNT
hybrid material[30]. In an attempt to synthesise MnS nanocrystals,
Tang et al., obtained MnS nanospheres with appreciable
electro-chemical properties including a specific capacity of 490 C/g[34]. In

comparison with these literature reports, our

g

-MnS/rGO hybrid
material, wherein

g

-MnS is anchored on the surface of the highly
conducting rGO, exhibits superior electrochemical capacitive
char-acteristics making it a reliable candidate for high performance
supercapacitors. This superiority can be attributed to the few
ad-vantages of the composite. Anchoring is possible due to the direct
covalent bonding and Van der Waal's attraction at oxygen, containing
the functional groups on rGO.

g

-MnS anchored on rGO sheets can
interact well and favor the electron transportation. The diffusion
paths for the electrolyte ions are significantly shortened due to the
particle anchoring to rGO sheets by spacing effect, thereby increasing
the effective surface contact to the electrolyte. Lastly, the outstanding
electron transportation from the particles to the underlying rGO
sheets speeds up the faradaic reactions, even at the high specific
currents. So, this unique structural feature of the composite material
provides a condition wherein the electrolyte utilizes both

g

-MnS and
rGO to the maximum.

4. Conclusion

We successfully synthesized the

g

-MnS anchored rGO
com-posite for an electrochemical supercapacitor via the solvothermal
method. The material showed an appreciably high capacity of
1009 C/g at 1 A/g and a 82% capacity retention over 2000 cycles at
2 A/g. This material with high effective contact surface and
elec-tronic conductivity facilitates effective faradaic reactions at
the interface of

g

-MnS and the electrolyte. As a consequence,

g

-MnS/rGO exhibits a superior specific capacity and an
excep-tional cycling stability.

Acknowledgments

S R acknowledges the financial support from the University
Grant Commission (UGC), Government of India, under Dr. D.S.
Kothari postdoctoral fellowship program [Ref. No. F.4-2/2006(BSR)/
CH/14-15/0133].

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