gama mns nanoparticles anchored reduced graphene oxide electrode materials for high performance supercapacitors

Original Articleg-MnS nanoparticles anchored reduced graphene oxide: Electrode materials for high performance supercapacitors S.. Their structure, morphology and electrochemical properti

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 (http://creativecommons.org/licenses/by/4.0/)

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

[1e5] 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: kamath.ranganath@gmail.com (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

https://doi.org/10.1016/j.jsamd.2018.07.001

2468-2179/© 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

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 pristineg-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)

g

S Ranganatha, N Munichandraiah / Journal of Science: Advanced Materials and Devices 3 (2018) 359e365 360

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 ofg-MnS/rGO

To prepare theg-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 theg-MnS, except the

addition of rGO

2.3 Characterization

Powder X-ray diffraction (XRD) patterns were recorded using a

PANylatical diffractometer with Cu Ka(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

Fig 2 (a) XPS spectrum ofg-MnS, (b) High resolution spectrum of Mn, (c) High resolution spectrum of S, (d) Raman spectra of GO, rGO andg-MnS/rGO.

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 specific capacity (C) was calculated

using the relation C¼ It/m, where I is the current, t the discharge

time,DE 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 theg

-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 ofg-MnS/

rGO wherein the rGO sheets are shown with the arrow marks

demonstrating the anchoring ofg-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 ofg-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 Mn2þand 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 ofg-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 ofg-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-preparedg-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 ofg-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 forg-MnS the corresponding values are 2.5 cm3/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 ofg-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 (Mn2þ/Mn3þ/Mn4þ) in MnS contribute to the faradaic

e desorption isotherms from the BET experiment (The inset: isotherms of MnS enlarged), (b) pore size distribution for theg g

S Ranganatha, N Munichandraiah / Journal of Science: Advanced Materials and Devices 3 (2018) 359e365 362

Fig 4 (aeb) CV diagrams ofg-MnS andg-MnS/rGO, (ced) Galvanostatic charge/discharge profiles ofg-MnS andg-MnS/rGO, (e) Variation of the specific capacity with the specific current, (f) Stability of the electrodes upon cycling, (g) Nyquist plots for samples.

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]

MnSþ OH4MnSOH þ e

MnSOHþ OH4MnSO þ H2Oþ e

The charge-discharge voltage profiles registered at different

specific currents are shown inFig 4(ced) The symmetric

charac-teristics of the chargeedischarge curves suggests a satisfactory

reversibility w.r.t faradaic reactions The pristineg-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 forg-MnS andg-MnS/rGO are 2U

and 0.3U, 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 ofg-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 thea-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 ag-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, whereing-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 bothg-MnS and

rGO to the maximum

4 Conclusion

We successfully synthesized theg-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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