Ultrasonics Sonochemistry 21 (2014) 1933–1938

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Ultrasonics Sonochemistry

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Sonochemically synthesized MnO2 nanoparticles as electrode materialfor supercapacitors

1350-4177/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ultsonch.2013.11.018

⇑ Corresponding author. Tel.: +91 431 2503639; fax: +91 431 2500133.E-mail addresses: sanand@nitt.edu, sanand99@yahoo.com (S. Anandan).

Balasubramaniam Gnana Sundara Raj a, Abdullah M. Asiri b, Abdullah H. Qusti 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, Indiab Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21413, P.O. Box 80203, Saudi Arabiac 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 21 October 2013Received in revised form 26 November 2013Accepted 28 November 2013Available online 8 December 2013

Keywords:Sonochemical synthesisAmorphous materialsX-ray diffractionElectrochemical propertiesEnergy storageSupercapacitor

a b s t r a c t

In this study, manganese oxide (MnO2) nanoparticles were synthesized by sonochemical reduction ofKMnO4 using polyethylene glycol (PEG) as a reducing agent as well as structure directing agent underroom temperature in short duration of time and characterized by powder X-ray diffraction (XRD), Fouriertransform infrared spectroscopy (FT-IR), Scanning electron microscope (SEM), Transmission electronmicroscopy (TEM) and Brunauer–Emmett–Teller (BET) analysis. A supercapacitor device constructedusing the ultrasonically-synthesized MnO2 nanoparticles showed maximum specific capacitance (SC)of 282 Fg�1 in the presence of 1 M Ca(NO3)2 as an electrolyte at a current density of 0.5 mA cm�2 inthe potential range from 0.0 to 1.0 V and about 78% of specific capacitance was retained even after1000 cycles indicating its high electrochemical stability.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Electrochemical capacitors (ECs) or supercapacitors have at-tracted increasing attention in recent years due to their higherpower density and longer cycle life than batteries and higher en-ergy density than conventional capacitors [1]. Accordingly, theyhave been utilized in a wide range of applications such as con-sumer electronics, memory back-up systems and hybrid electricvehicles [2,3]. Electrochemical capacitors bridge the gap betweenbatteries and conventional capacitors. Depending on the chargestorage mechanism, electrochemical capacitors are basically classi-fied into two types as electrical double layer capacitor (EDLC) andpseudo capacitors. In the EDLC, capacitance arises as a result ofcharge separation at the electrode–electrolyte interface, whereasthe charge transfer from reversible faradaic reactions takes placeat the electrode surface of pseudo capacitance.

Many researchers have been focusing on the development ofelectrode active materials with improved electrochemical proper-ties [4–6]. Generally, high surface area carbons [7], conductingpolymers [8] and transition metal oxides are used as electrode ac-tive materials for supercapacitors [9–12]. Various transition-metaloxides have been shown to be excellent electrode active materials

due to their chemical stability, variable valence etc. [13]. Manga-nese oxide (MnO2) acts as an attractive electrode active materialfor supercapacitors because of its structure flexibility, long cyclelife, environmental compatibility and low cost [14–16]. The perfor-mance of MnO2 depends on different synthetic methods owing toits crystal structure, particle size and morphology. MnO2 usuallyhas low intrinsically electronic conductivity and clustered mor-phology [17]. MnO2 nanoparticles were synthesized by differentmethods including co-precipation [18,19], micro emulsion [20],sol–gel [21], sonochemical [22], hydrothermal [23] and electro-chemical methods [14,24]. Sonochemical method is a useful tech-nique for synthesising of nanostructured metal oxide materials atroom temperature, ambient pressure and short reaction times.The benefits of sonochemistry, in creating nanostructures materi-als arise principally from acoustic cavitation; the formation,growth, and implosive collapse of bubbles in a liquid. Bubble col-lapse stimulated by cavitation produces intense local heating andhigh pressures [25]. The storage capacity of MnO2 electrolytes withunivalent cations has been accounted by one electron transfer pro-cesses. However, studies with electrolytes containing polyvalentcations are scarce. The specific capacitance of MnO2 in electrolytescontaining bivalent cations is greater than that of univalent cation[26,27]. Recently, a high specific capacitance of 283 Fg�1 was re-ported for MnO2 in bivalent cation (Ca2+) containing electrolytes,whereas 188 Fg�1 was reported in monovalent cation (Na+) con-taining electrolytes [28].

10 20 30 40 50 60 70 80

Inte

nsi

ty /

a.u

.

2 θ / degree

(001)

(111)

(005)

Fig. 1. XRD spectrum of sonochemically prepared MnO2 nanoparticles.

4000 3500 3000 2500 2000 1500 1000 500

% T

ran

smit

tan

ce

Wavenumber (cm-1)

υ(Mn-O)

υ(O-H) str

υ(O-H) bend

Fig. 2. FT-IR spectrum of sonochemically prepared MnO2 nanoparticles.

1934 B. Gnana Sundara Raj et al. / Ultrasonics Sonochemistry 21 (2014) 1933–1938

In this investigation, we followed a simple route to prepareMnO2 nanoparticles using a sonochemical method by reduction ofKMnO4 using PEG as a reducing agent as well as structuredirecting agent under room temperature in short reaction time.The supercapacitive behavior of the product was evaluated in aque-ous electrolytes containing bivalent cations, which showed that theultrasonically prepared MnO2 nanoparticles exhibited remarkablecapacitive behavior in bivalent cation containing electrolytes.

2. Experimental section

2.1. Materials

Reagent-grade KMnO4, Ca(NO3)2 (MERCK) and Poly EthyleneGlycol (PEG; mw: 55,000) (Aldrich), Vulcan XC-72, poly-vinylidenefluoride (PVdF) and N-methyl-2-pyrrolidone (NMP) were used. Allsolutions for the experiment were prepared with doubly distilled(DD) water.

2.2. Material preparation

MnO2 was prepared by reduction of KMnO4 with PEG aided bysonication. A horn type 20 kHz Sonics sonifier (100 W/cm2) with atip diameter of 13 mm was used. Typically, 0.5 g of KMnO4 was dis-solved in 60 ml of double distilled (DD) water and then 5.5 g of PEGwas added with continuous stirring for 5 min in a 100 mlsonochemical glass vessel and then ultrasonicated for 20 min. After20 min, brown color precipitate was formed. Then the precipitatewas filtered, washed in DD water repeatedly for several times fol-lowed by ethanol washing three times and dried in an air oven at70 �C for 8 h.

2.3. Characterization

In order to confirm the crystal structure and phase purity of theproduct, powder X-ray diffraction patterns were recorded on aPhilips XPertPro X-ray diffractometer with Cu Ka (k = 0.15418 nm)radiation. FT-IR spectra were obtained using a BRUKER OptikGmbH MODEL TENSOR 27 FT-IR spectrometer with a detector RTDLaTGS using a KBr pellet. The surface property measurementsNitrogen adsorption–desorption was carried out by using microm-eritics surface area analyzer model ASAP 2020. The morphologyand structural properties of the as-prepared manganese oxideswere studied using a JEOL 7401F and JEOL JEM 2010 model.

2.4. Electrode fabrication and electrochemical characterization

For electrochemical characterization methods, electrodes wereprepared on high-purity stainless steel plate as a current collector.The plate was polished with successive grades of emery paper,cleaned with soap solution, washed with DD water, rinsed withacetone, dried and weighed. As prepared MnO2 (75 wt.%) as an ac-tive 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 dropsof NMP was added to form slurry. It was coated onto the pretreatedSS plate (coating electrode area is 1.0 cm2) and dried at 100 �C un-der vacuum for 12 h. Electrochemical studies were carried outusing a potentiostat/galvanostat (AUTOLAB 302 N module) in athree-electrode system with the MnO2 coated plate as the workingelectrode, Pt foil as the counter electrode and Ag/AgCl as the refer-ence electrode. The discharge specific capacitance (SC) of MnO2

was calculated using the formula

SC ¼ It=mDE ð1Þwhere I is the charge–discharge current in amps, t is the dischargetime in seconds, m is the mass of the active material present on the

electrode in grams and DE is the operating potential window involts of charge or discharge.

3. Results and discussion

As mentioned in the experimental details, sonochemical prepa-ration processes is followed for the synthesis of MnO2 nanoparti-cles based only on the redox reactions between potassiumpermanganate and polyethylene glycol (PEG) without any otheradditives such as templates or surfactants, under mild conditions(room temperature) in short duration of time (20 min) [29,30].PEG is used as a structure directing agent used for the preparationof porous nanomaterials [31]. Using PEG, the nanostructured mate-rials such as nanoparticles, nanowires, nanorods, etc. are formeddue to the presence of both hydrophilic and hydrophobic groupswhich can form micelles in aqueous solutions [32]. During theultrasonic irradiation, the organic radicals (glycols or aldehydes)that are generated during pyrolysis of PEG due to the extreme tem-perature conditions generated within the cavitation bubbles leadto the reduction of KMnO4 to MnO2 [33,34]. Fig. 1. shows theXRD pattern of the as prepared MnO2 show broad peaks positionsat 2h values 12.6�, 37.2� and 66� matches well with the d-MnO2

Fig. 3. SEM (a), TEM (b), HRTEM (c), SAED (d) and EDX (e) images of sonochemically prepared MnO2 nanoparticles.

0

100

200

300

400

Qu

anti

ty A

dso

rbed

(cm

3 /g S

TP

)

Relative Pressure (p/p°)

(a)

0.0 0.2 0.4 0.6 0.8 1.0 0 10 20 30 40 50 60 70 80 90 100

0.000

0.002

0.004

0.006

0.008

0.010

0.012

Po

re a

rea

m2 /g

.nm

Pore diameter / nm

(b)

Fig. 4. (a) N2 adsorption–desorption isotherms and (b) BJH pore-size distribution curve of sonochemically prepared MnO2 nanoparticles.

B. Gnana Sundara Raj et al. / Ultrasonics Sonochemistry 21 (2014) 1933–1938 1935

JCPDS value (JCPDS No. 80-1098). All the above reflections can beindexed to the corresponding crystal planes ((hkl) (001), (111)

and (005)) [35,36]. The peaks broadening suggest that the sampleis poorly crystalline in nature.

0.0 0.2 0.4 0.6 0.8 1.0-0.008

-0.004

0.000

0.004

0.008

0.012

0.016

Cu

rren

t d

ensi

ty /

A c

m-2

Potential / V vs. Ag/AgCl

abc

d

e

f

0.0 0.2 0.4 0.6 0.8 1.0-0.008

-0.004

0.000

0.004

0.008

0.012

0.016

Cu

rren

t d

ensi

ty /

A c

m-2

Potential / V vs. Ag/AgCl

Fig. 5. CV for sonochemically prepared MnO2 nanoparticles at different scan rates5 mV s�1,10 mV s�1, 20 mV s�1, 40 mV s�1, 80 mV s�1, 160 mV s�1 in the potentialrange of 0.0–1.0 V vs. Ag/AgCl in aqueous solution of 1 M Ca(NO3)2 electrolyte (a–f).Inset shows the fitted rectangular shape in the cyclic voltammetric behavior.

1936 B. Gnana Sundara Raj et al. / Ultrasonics Sonochemistry 21 (2014) 1933–1938

The FT-IR spectrum for the as-prepared MnO2 nanoparticles isshown in Fig. 2. The absorption band at 525 cm�1 can be assignedto Mn–O bending vibrations, arise from MnO6 octahedron vibra-

0.0

0.2

0.4

0.6

0.8

1.0

Po

ten

tial

/ V

vs.

Ag

/Ag

Cl

Time / s

(a)

0 1000 2000 3000 4000 5000 6000

0 200 40100

150

200

250

300

Sp

ecif

ic C

apac

itan

ce /

Fg

-1

Cy

(c)

Fig. 6. (a) Charge–discharge cycles of sonochemically prepared MnO2 nanoparticles in aq1.0 V vs. Ag/AgCl. Area of the electrode: 1.0 cm2, (b) charge–discharge curves of sonochem(c) cycling behavior of sonochemically prepared MnO2 nanoparticles.

tion mode. The observed broad band around 3400 cm�1 is attrib-uted due to hydroxyl stretching vibrations and the weak bandaround 1630 cm�1 corresponds to the bending vibrations of theOH group, which are related to adsorbed crystalline water mole-cules [37].

The morphology of the as prepared MnO2 nanoparticles isspherical as suggested by SEM and TEM micrographs (Fig. 3a andb). The size range of the particles is in the range from 10 to20 nm. From high resolution transmission electron microscopy(HRTEM) image (Fig. 3c), lattice fringes are clearly seen and matchwell with the MnO2 planes. The amorphous pattern is viewed inthe selected area electron diffraction (SAED) (Fig. 3d). The chemicalcomposition of the as prepared MnO2 was determined by Energy-dispersive X-ray spectroscopy (EDX) analysis (Fig. 3e) indicates thepresence of pure manganese and oxygen as elements in thenanoparticles.

The nitrogen adsorption/desorption isotherm and pore-size dis-tribution were measured for the prepared MnO2 nanoparticles andit is shown in Fig. 4. The observed Brunauer–Emmett–Teller (BET)surface area of the sample is 60 m2 g�1 and it shows type IV hyster-esis loops for a typical mesoporous structure as defined by theIUPAC [38]. The Barrett–Joyner–Halenda (BJH) analysis shows anarrow pore-size distribution of about 10–14 nm. The BET surfacearea is not unique influence parameter for the capacitance, but an

0 100 200 300 400 500 600

0.0

0.2

0.4

0.6

0.8

1.0

1st

cycle

500th

cycle

1000th

cycle

Po

ten

tial

/ V

vs.

Ag

/Ag

Cl

Time/s

(b)

0 600 800 1000

cle numbers

ueous solution of 1 M Ca(NO3)2 at a current density of 0.5 mA cm�2 between 0.0 andically prepared MnO2 nanoparticles recorded on 1st, 500th and 1000th cycles, and

B. Gnana Sundara Raj et al. / Ultrasonics Sonochemistry 21 (2014) 1933–1938 1937

increase tendency of the specific capacitance is connected with therelative large BET surface area [39,40].

For electrochemical measurements, cyclic voltammetry andcharge–discharge cycling experiments were performed in aqueouselectrolytes. There are two mechanisms proposed for charge stor-age in MnO2 nanoparticles [41,42]. The first step is based on theintercalation/deintercalation of protons (H+) or alkali metal cations(M+) such as Li+ in the bulk of the material upon reduction/oxida-tion of Mnn+ into MnO2.

MnO2 þMþ þ e�¢ MnOOMðMþ ¼ Liþ;Naþ;KþÞ ð2Þ

The second step is based on the surface process, which involvesthe adsorption/desorption of electrolyte cations (M+) on MnO2.

ðMnO2Þsurface þMþ þ e�¢ ðMnO�2 MþÞsurface ð3Þ

Electrochemical studies of MnO2 nanoparticles were reportedusing several electrolytes and as it is known that MnO2 nanoparti-cles exhibit higher SC in Ca (NO3)2 electrolyte than in the conven-tional Na2SO4 electrolyte because it contains bivalent cationswhich can reduce two Mn4+ to Mn3+ ions [26,27,43], so the electro-chemical capacitance properties of MnO2 was studied in 1 M Ca(NO3)2 electrolyte. Fig. 5 shows cyclic voltammetric data of as pre-pared MnO2 nanoparticles electrode at various scan rates from 5 to160 mV s�1 in the potential range between 0.0 and 1.0 V vs.Ag/AgCl in aqueous 1 M Ca(NO3)2 solution. The rectangular shapeof voltammogram (see inset Fig. 5.) without any redox currentpeaks indicates the ideal capacitive behavior. The size of the vol-tammogram increases with an increase in the sweep rate whichindicates that the voltammetric currents are directly proportionalto the scan rate [44]. At the low-scan rate, the ions from the elec-trolyte can occupy all the available sites in the active electrodematerial, because the ions have enough time to diffuse into allthe sites which leads to the higher capacitance. On the other hand,at high-scan rate, the ions from electrolyte confront the difficultyto access all the available sites in the active electrode materialdue to their partial rate of movement in the electrolyte [45].

As prepared MnO2 nanoparticles were subjected togalvanostatic charge–discharge cycling studies. Charge–dischargecurves recorded at a current density (c.d) of 0.5 mA cm�2 in thepotential range from 0.0 to 1.0 V in 1 M Ca(NO3)2 as electrolytesolution are shown in Fig 6a. There is a linear variation of potentialwith time during charging and discharging processes, which isanother criterion for capacitive behavior of as prepared MnO2

electrode material. The initial value of discharge capacitance inCa (NO3)2 was 282 Fg�1. Ragupathy et al. report that the capacitivebehavior of amorphous manganese oxides with short rangecrystallinity is predominantly due to the surface adsorption–desorption of cations rather than to insertion/deinsertion [46],which yield lower SC value for MnO2 nanoparticle while comparedto its theoretical value. However, the higher SC in the bivalentcation system may possibly be assigned to the bivalent electrolytecation reducing two Mn4+ to Mn3+, which means a doubling of thenumber of electrons involved compared to that with a univalentcation-containing electrolyte system [28,34].

The observed specific capacitance values are comparativelyhigh when compared with the literature values especially,169 Fg�1 at a current density of 250 mA g�1 in 1 M Na2SO4

reported by Yu et al. [47]. Jiang et al. reported that the specificcapacitance of MnO2 electrode is increased from 77 to 176 Fg�1

in the presence of added surfactant P123, since the particle sizeof MnO2 was decreased [37]. Fig. 6b presents charge–dischargecurves of as prepared MnO2 recorded on 1st, 500th and 1000thcycles. The stability of the as prepared MnO2 was studied bycharge–discharge cycling at a current density of 0.5 mA cm�2

(Fig. 6c). The discharge capacitance falls from an initial value of

282–262 Fg�1 after 100 cycles. The SC of MnO2 after 500 cycleswas 247 Fg�1, which is 87% of the initial capacitance. At the endof 1000 cycles, 78% of the initial specific capacitance (220 Fg�1) isretained indicates its high electrochemical stability.

4. Conclusions

In this work, a simple sonochemical route is followed for thesynthesis of MnO2 nanoparticles using PEG as a reducing agentand studied its impact on structure, morphology and electrochem-ical performance. XRD results indicate that the prepared MnO2

nanoparticle is poorly crystalline in nature. The morphology studyshows that the particles are spherical in shape and the size rangesfrom 10 to 20 nm. The electrochemical performance of MnO2

delivers a maximum SC of 282 Fg�1 at a current density of0.5 mA cm�2 in 1 M Ca(NO3)2 electrolyte. The higher SC of MnO2

was assigned to the narrow pore size distribution with small poresand using an electrolyte containing bivalent cations. The preparedMnO2 nanoparticle exhibits high electrochemical stability suggest-ing that it is a suitable electrode material for electrochemicalsupercapacitors.

Acknowledgment

The research work was financially supported by Council of Sci-entific and Industrial Research (CSIR), New Delhi (CSIR ReferenceNo. 02 (0021)/11/EMR-II).

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