Journal of Electroanalytical Chemistry 719 (2014) 98–105

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Journal of Electroanalytical Chemistry

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Supercapacitive properties of nanostructured MnO2/exfoliated graphitesynthesized by ultrasonic vibration

http://dx.doi.org/10.1016/j.jelechem.2014.02.0111572-6657/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +98 21 44580751; fax: +98 21 44580781.E-mail addresses: hrnaderi@ut.ac.ir (H.R. Naderi), mortaheb@ccerci.ac.ir

(H.R. Mortaheb), zolfaghari@ccerci.ac.ir (A. Zolfaghari).

Hamid Reza Naderi, Hamid Reza Mortaheb ⇑, Alireza ZolfaghariChemistry and Chemical Engineering Research Center of Iran, Tehran, Iran

a r t i c l e i n f o

Article history:Received 1 September 2013Received in revised form 4 February 2014Accepted 7 February 2014Available online 19 February 2014

Keywords:SupercapacitorNanostructured MnO2

Exfoliated graphiteComposite electrodeUltrasonic synthesis

a b s t r a c t

Nanostructured manganese oxides/exfoliated graphite (MnO2/EG) are prepared using ultrasonic vibrationin an aqueous solution. The structures of the MnO2/EG synthesized with different contents of EG areexamined by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis(TGA) and Brunauer–Emmett–Teller (BET) method. The electrochemical performances of composite elec-trodes are investigated using cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemicalimpedance spectroscopy (EIS). Electrochemical data of the composite electrodes tested in the 0.5 M Na2-

SO4 solution shows supercapacitive behavior in the voltage window of 0–1.0 V versus Ag/AgCl. At themass ratio of MnO2/EG of equal to 60:40, the specific capacitance value of the sample calculated fromthe cyclic voltammetry curves at the scan rate of 2 mV s�1 is 358 F g�1 whereas this value for the purec-MnO2 is 289 F g�1. The specific capacitance of the MnO2/EG at the scan rate 30 mV s�1 is 240 F g�1,which is decreased to 209 F g�1 after 1000 cycles. The capacitance retention of the composite is 87%.The MnO2/EG composite electrode represents an energy density of 51 W h kg�1 at a power density of500 W kg�1, which is much higher than that of pure c-MnO2.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Supercapacitors as energy storage devices have advantages incomparison to the batteries such as higher power densities, fastcharging/discharging rates, and excellent cycling stabilities. Sup-ercapacitors are used particularly for the applications involvingin high power electronics, electrical utilities, transportation, medi-cal electronics and military defense systems [1,2]. The charge stor-age mechanisms on the basis of applied electrode materials areclassified to: (a) electrical double-layer capacitors (EDLCs), whichemploy carbon or other similar materials as electrode materials,and (b) pseudocapacitors, in which electroactive materials suchas transition metal oxides (e.g. RuO2, MnO2, etc.) or conductingpolymers (polyaniline, polypyrrole, polythiophene) are employedas electrodes [3,4]. RuO2�xH2O shows great performance as an elec-trode material for pesudocapacitors. However, the high cost of hy-drous ruthenium oxide hinders its wide application [5]. MnO2 as asupercapacitor electrode has attracted many attentions due to itsrelatively low cost, and being environmental friendly as neutralaqueous electrolytes [6,7]. However, the low electrical conductivity

(10�5–10�6 S cm�1) and low electrochemical kinetic of MnO2 limitits charge storage and thus its commercial usage [8,9].

The mechanism of charge storage in the MnO2-based pseudoc-apacitors can be explained by two proposed mechanisms. The firstmechanism is based on the intercalation of protons (H+) or alkalimetal cations (M+) such as Na+ in the bulk of the material viareduction reaction followed by deintercalation through oxidationreaction as described by the following reaction [10,11]:

MnO2 þMþ þ e� ����! ����charge

dischargeMnOOM ð1Þ

The second mechanism involves the surface adsorption/desorp-tion of alkali metal cations (M+) on MnO2 [6]:

ðMnO2Þsurface þMþ þ e� ����! ����charge

dischargeðMnOOMÞsurface ð2Þ

The main difference between the two mechanisms is the differ-ence in the corresponded reacting sites. The majority of ‘‘bulk’’ Mnsites are involved in the intercalation/deintercalation mechanismwhile the surface Mn sites have priority in the reaction based onthe adsorption/desorption mechanism [12]. Toupin et al. usingX-ray photoelectron spectroscopy confirmed that the amorphousor poorly crystalline manganese dioxide shows the adsorption/desorption mechanism. On the other hand, Kuo et al. by in situ

H.R. Naderi et al. / Journal of Electroanalytical Chemistry 719 (2014) 98–105 99

synchrotron X-ray diffraction analysis demonstrated that the inter-calation/deinteracalation process occurs mostly on the crystallineMnO2 compounds.

The electrical conductivity of MnO2 could be improved by incor-porating various types of carbonaceous compounds such as graph-ite [13], graphene oxide [14], graphene [15,16], carbon nanotube[17,18], mesoporous carbon [19], and carbon black [20,21] by var-ious methods.

Exfoliated graphite can increase the conductivity of metaloxide-based supercapacitors. The MnO2/EG are synthesized by var-ious methods such as hydrothermal route [13], sol–gel route [22],and microwave methods [23]. The specific capacitances of thiscomposite electrode were 276.3 F g�1 at the current density of2 mA cm�2, 398 F g�1 at the scan rate of 2 mV s�1, and 97.5 F g�1

at the scan rate of 1 mV s�1, respectively.In the present study, a simple method for preparation of nano-

structured MnO2/EG as the supercapacitor electrodes is introducedusing ultrasonic vibration. Applying ultrasonic waves is a simplemethod for the synthesis of nanostructures [23]. The process,known as cavitation, is based on incorporating bubbles in the li-quid, where they grow up and finally collapse. The bursting of bub-bles results in very high local temperature and pressure leading toreactions in the solution [24]. Nanostructured MnO2 and MnO2/EGare characterized by XRD, SEM, TGA and BET. The pseudocapacitivebehaviors of the MnO2 and MnO2/EG composite electrodes areinvestigated by CV, EIS and galvanostatic charge/discharge mea-surements in a 0.5 M Na2SO4 electrolyte.

2. Experimental

2.1. Synthesis

Predetermined amounts of graphite (30, 40 and 50 wt.%) wereadded to aqueous solutions of 0.25 M MnSO4 (Merck, researchgrade) and 0.5 M KBrO3 (Merck, research grade) in a total volumeof 100 mL. The mixture was irradiated with a high intensity ultra-sound radiation with 480 W cm�2 input power and working fre-quency of 24 kHz. A titanium horn transducer system (UP 200 H,Dr. Hielscher GmbH) with a 2 mm microtip was employed. Thetitanium horn was inserted to a depth of 2 cm into the solution.During the reaction, the mixture temperature was within 51–55 �C.

2.2. Structural characterization

The structures of nanostructured MnO2 and MnO2/EG werecharacterized using a powder X-ray diffractometer (XRD, Bruker,D8-advance, 2h = 4–70�, 40 kV/30 mA with monochromatized CuKa radiation, k = 1.541874 Å). TGA for the samples were recordedin the temperature range of 20–900 �C in an air atmosphere at aheating rate of 10 �C per min using a TG 209 F1 (Netzsch) tester.The morphologies of the samples were examined by a scanningelectron microscope (SEM, philips, XL30). The pore size distribu-tion and surface area of the samples were determined by nitrogenadsorption at 77 K with an automated adsorption apparatus (BEL-SORP-miniII). The pore size distribution was determined by theBarret–Joyner–Halenda (BJH) method from the desorption iso-therm. The surface area was derived from the BET equation.

2.3. Electrochemical measurement

The working electrodes (composite electrodes) were fabricatedby mixing the prepared MnO2/EG, acetylene black (Alfa Aesar,>99.9%, S.A. 80 m2 g�1) and polytetrafluoroethylene (PTFE) (Merck)in a mass ratio of 80:15:5 and then dispersing the mixture in eth-anol. The resulting suspension was coated onto stainless steel grid

substrates (0.75 cm2) using a spatula following by drying them atroom temperature for 1 h and then pressing at 15 MPa for30 min. The weight of resulted composite on the electrode was3 mg (active material on the electrode was 2.8 mg).

All the electrochemical measurements were performed using athree-electrode setup where the coated stainless steel grid wastreated as the working electrode, a platinum foil electrode wasused as the counter, and an Ag/AgCl electrode (Metrohm AG9101 Herisau, 3 M KCl, 0.207 V versus SHE at 25 �C) was used asthe reference.

All electrochemical characteristics were evaluated by cyclic vol-tammetry (CV) and chronopotentiometry (charge/discharge), andelectrochemical impedance spectroscopy (EIS) measurements onan Autolab, Eco Chemie. B.V. potentiostat/galvanostat. The CVswere performed in a voltage window of 0–1.0 V at different scanrates in a range of 2–200 mV s�1. The chronopotentiometry wascarried out in a voltage window of 0–1.0 V under different currentdensities from 1 to 50 A g�1. The EISs were investigated in the fre-quency range of 0.01–105 Hz at an applied potential of 0.5 V. Allmeasurements were taken using 0.5 M Na2SO4 as electrolyte withpH of 3.3 at room temperature.

3. Results and discussion

3.1. Material characterization

Fig. 1a shows the XRD patterns of nanostructured MnO2 andMnO2/EG with various ratios. The diffraction peaks in Fig. 1a at2h = 21.63�, 37.19�, 38.71�, 41.9�, 42.42�, 55.56�, 66.67� and68.20� can be readily indexed as the pure orthorhombic c-MnO2

crystal planes of the (120), (230), (300), (002), (160), (242),(421) and (003), respectively (JCPDS card No. 14-644) [25]. Noother impurity peak (such as a- or b-MnO2, Mn(OH)2 and Mn2O3)is detected. The crystalline structure of MnO2 is composed of oneMn atom surrounded by six O atoms to form an octahedron. TheMnO6 octahedral subunits share vertices and edges to form crystal-line tunnel structures by continuously linking to the neighboringsubunits. The structure of c-MnO2 is widely accepted as a randomintergrowth of 1 � 1 tunnels of pyrolusite and 1 � 2 tunnels oframsdellite [26,27]. According to Fig. 1a, the XRD pattern of thegraphite shows a peak at 2h = 26.51� and 53.48� corresponding tothe (002) and (004) planes. Graphite peak (002) is very sharp.The y-axis in Fig. 1a is truncated since the intense peaks of graphitestructure diminish the MnO2 peaks. The figure shows that bydecreasing the graphite content in the MnO2/EG, the intensitiesof graphite peaks (can be tracked by peak of (004) plane) aredeclined.

The TGA profiles of the c-MnO2 and MnO2/40% EG powders areshown in Fig. 1b for a heating rate of 10 �C min�1 from room tem-perature to 900 �C in an air atmosphere. The total weight loss ofpure c-MnO2 powders across the whole temperature range is24 wt.%. The initially high weight loss of 15 wt.% below 150 �Ccan be mostly attributed to the dehydration of the surface physi-cally adsorbed water. As the temperature increases from 150 �Cto around 500 �C, the small weight loss is attributed to the dehy-dration of the bounded water inside the tunnels [28]. The weightloss between 500 and 550 �C is probably due to the reduction ofmanganese from being tetravalent to trivalent and to the releaseof generated oxygen from the transformation in the manganeseoxide chemical structure [29]. The small weight loss in the range850–900 �C is related to the loss of oxygen and phase transitionfrom Mn2O3 to Mn3O4 [30]. The TGA thermogram of MnO2/EGpowder represents a total weight loss of 49 wt.% across the wholetemperature range. The initially weight loss of 9 wt.% from theambient to 350 �C can be related to the dehydration of crystalline

Fig. 1. Typical XRD patterns of as-prepared nanostructured MnO2 and MnO2/EG (a), TGA curves of pure MnO2 and MnO2/40% EG composite (b), nitrogen adsorption/desorption isotherms of pure MnO2 (c), and MnO2/40% EG (d).

100 H.R. Naderi et al. / Journal of Electroanalytical Chemistry 719 (2014) 98–105

or the physically adsorbed water. The abrupt weight loss of 40 wt.%in the temperature range of 400–600 �C is related to the combus-tion of EG from the sample.

The specific surface area greatly affects the characteristics of ananomaterial such as its specific capacitance. The specific surfacearea and pore size distribution of the MnO2 and MnO2/40% EG wereobtained from an analysis of the desorption branch of N2 gas iso-therms using the density function theory. Since MnO2/40% EGshows the best supercapacitive performance compared to theother samples, this sample were selected to investigate the struc-tural and morphological properties. Fig. 1c and d shows the nitro-gen adsorption–desorption isotherms of MnO2 and MnO2/40% EG,respectively. Considering the small amount of nitrogen adsorptionat low relative pressure, it can be concluded that the contributionof micropores in the total pore volume is low for the MnO2 andMnO2/40% EG. In terms of shape, the isotherm of the MnO2 andMnO2/40% EG can be classified as type IV according to the IUPCclassification, which is characteristic of mesoporous materials[31]. The nitrogen adsorption–desorption isotherm (Fig. 1c and d)show a hysteresis loop in the relative pressure (p/p0) range of0.45–0.95, which is explained in the mesoporous structure. It alsoshows a sharp increase in adsorption near the relative saturationpressure and a sharp decrease in desorption at certain relativepressures, which is a typical adsorption–desorption characteristicof a uniform pore systems with parallel wall slit-like pore struc-ture. According to the BET analysis, a total specific surface area of95 m2 g�1 is obtained for the pure MnO2 (Fig. 1c), which is muchless than that of the MnO2/40% EG (115 m2 g�1, see Fig. 1d). The

calculation indicates that narrow distribution of pores for MnO2/40% EG is around 15.2 nm.

The surface morphologies of graphite, MnO2 and MnO2/40% EGare shown in Fig. 2. The EG in Fig. 2a shows a typical exfoliatestructure. The SEM micrograph clearly shows that the synthesizedMnO2 has an apparent spherical-like morphology with diameter ofaround 50–80 nm (Fig. 2b), which are aggregated in larger agglom-erations. The size of particles in the SEM image of MnO2/EG inFig. 2c is estimated in the range of 30–60 nm (Fig. 2c). In addition,Fig. 2c shows that the surface of EG is covered uniformly by MnO2

particles with almost the same size. Furthermore, the agglomera-tion of MnO2 particles in the MnO2/EG is much lower than thatof pure MnO2 shown in Fig. 2b.

3.2. Electrochemical properties

Supercapacitive performance of the MnO2/EG composite elec-trodes is examined by electrochemical experiments such as CV,CP and EIS. The electrochemical experiments were performed ina 0.5 M Na2SO4 solution with pH of 3.3, in which the electrodeshave the best performance [25]. Fig. 3a and b shows the CV curvesof the pure MnO2 and MnO2/40% EG composite electrodes at thescan rates of 5, 10, 20, 30, 40, and 50 mV s�1 with potential win-dows ranging from 0 to 1.0 V versus Ag/AgCl. The shapes of thesecurves are quasi-rectangular indicating the ideal electrical dou-ble-layer capacitance behavior and the fast charging/dischargingprocess characteristic. For the pure MnO2 (Fig. 3a), the CV curvesat different scan rates have quasi-rectangular shapes. However,

Fig. 2. SEM micrographs of graphite (a), MnO2 (b), and MnO2/40% EG (c).

H.R. Naderi et al. / Journal of Electroanalytical Chemistry 719 (2014) 98–105 101

the distance between leveled anodic and cathodic currents at thesame scan rates is much smaller than that of the MnO2/40% EGcomposite electrodes indicating its smaller specific capacitance.Fig. 3c presents the CV curves of the EG, MnO2 and MnO2/40% EGcomposite electrodes at a sweep potential window of 0–1.0 Vand a scan rate of 20 mV s�1. All the CV curves show approximatemirror images with respect to the zero-current line and a rapidcurrent response on voltage reversal at each end potential (thequasi-rectangular shapes and symmetric I–V responses) indicatingideal pseudocapacitive behavior of the materials.

The specific capacitance (SC) of the electrodes can be calculatedfrom CVs according to the following equations [32].

SC ¼ 12DV m v0

Z V f

Vi

jij dV ð3Þ

where i is the response current density (A). DV is the potentialrange (mV), m0 is the scan rate (mV s�1), m is the amount of activematerial (g), and Vi and Vf are the initial and final voltages (mV),respectively. The SC values of the EG, pure MnO2 and the MnO2/40% EG composite electrodes according to Eq. (1) are 16 F g�1,289 F g�1 and 358 F g�1 at the scan rate of 2 mV s�1, respectively.The SC for the MnO2/EG composite electrode is 24% higher thanthat of pure MnO2. The enhanced in SC can be attributed to thesynergic effects from each pristine component. It can be also

attributed to the smaller particle size, the higher specific surfacearea, and the electrochemical stability, which contribute to in-crease the number of available active sites and thus increase theamount of energy that can be stored in the composite [21].

Fig. 3d shows the CV curves of the MnO2/EG composite elec-trodes with various mass ratios. The rectangular area of the CVloop in the MnO2/40% EG composite electrodes is significantly lar-ger than that of the other composite electrodes. The calculated SCsof MnO2 electrode and MnO2/EG composite electrodes at differentpotential scan rates are summarized in Fig. 3e. The SC for pureMnO2 and composite electrodes decreases with the increase of po-tential scan rates. However, the capacitance of MnO2/40% EG com-posite electrode is much higher than that of the pure MnO2 and theother composite electrodes. It can be seen from Fig. 3e that the SCsfor MnO2/40% EG composite electrodes decrease from 358 to180 F g�1 by increasing the potential scan rate from 2 to200 mV s�1. At slower scan rates, the electrolyte ion (Na+ or H+)can be inserted into almost all available pores and all spaces ofthe MnO2, which lead to a high effective redox reaction and a highcapacitance. However, when the scan rate is increased, the effec-tive interaction between the electrolyte ion and the pores is greatlyreduced. The effective area for the redox reaction has been limitedonly to the outer surface of the composite electrode, leading to areduction in the capacitance.

The stability and reversibility of an electrode material areimportant issues for its usage as an electrochemical supercapaci-tor. The cyclic performance of pure MnO2 and MnO2/40% EG com-posite electrodes were measured by CV tests at the potential scanrate of 30 mV s�1. As shown in Fig. 3f, 84% of MnO2’s supercapaci-tance is retained after 1000 cycles. The retained SC of the MnO2/40% EG composite electrode under the same condition is 87%. Thisindicates that the electrochemical stability of the pure MnO2 isslightly lower than that of the composite electrode. It also confirmsthat the incorporation of appropriate amount of EG can signifi-cantly enhance the SC of the composite electrode.

In order to gain further understanding about the supercapacitiveperformance, the galvanostatic charge/discharge of MnO2/EG com-posite electrodes are measured at the potential range of 0–1.0 Vand current density of 2.0 A g�1 (Fig. 4a). The electrodes have stableelectrochemical properties in the 0.5 M Na2SO4 electrolyte wherethe capacitor voltage is varied linearly with the time during bothcharging and discharging. Almost no voltage drop was observed dur-ing the discharge process, giving high values of charge/dischargeefficiency. Fig. 4b shows the charge/discharge of the MnO2/40% EGcomposite electrode. The cutoff voltages for charging/dischargingwere 0.0 and 1.0 V at various current densities. A good linear varia-tion of potential versus time was observed for all the curves, which isanother typical characteristic of an ideal capacitor. The small iR dropindicates a conductive characteristic of a composite material. This isrelated to the formation of a highly conductive network by addingEG, which facilitates the interface contact between the electrolyteand the MnO2 of the composite electrode. Therefore, the internalresistance within the composite electrode itself is reduced and goodelectronically conducting pathways are provided for protonic andelectronic transportation during the rapid charge/discharge process[13]. The shape of the charge/discharge curves shows agreementwith the result of the CV curves. The SC from charge/dischargecurves can be calculated using the following relation:

SC ¼ I � td

m� Vð4Þ

where I is the charge/discharge current (A), t is the dischargetime (s), m is the active mass (g) and V is the potential drop duringdischarge (V). The SCs of the MnO2/40% EG composite electrodes is280 F g�1 in current density of 2.0 A g�1 representing good agree-ment with the SC values obtained from CV (scan rate of 10 mV s�1).

Fig. 3. CVs of the pure MnO2 and MnO2/40% EG composite electrodes at various scan rates (5, 10, 20, 30, 40, 50 mV s�1) (a and b), CVs of the EG, pure MnO2 and MnO2/EGcomposite electrodes at scan rate of 20 mV s�1 (c), CVs of the MnO2/40% EG composite electrodes with various EG contents at scan rate of 20 mV s�1 (d), SC of the pure MnO2

and MnO2/EG composite electrodes with various EG contents at different scan rates (e), and long-term stability of the pure MnO2 and MnO2/40% EG composite electrodesupon CVs at a scan rate of 30 mV s�1 (f).

102 H.R. Naderi et al. / Journal of Electroanalytical Chemistry 719 (2014) 98–105

The Ragone plots of Fig. 4c summarize the performance of thecapacitor tested under various charge/discharge currents between0 and 1.0 V. The specific power density (P) and energy density (E)of the MnO2/EG composite electrodes are calculated as follows[33]:

P ¼ V � Im

ð5Þ

E ¼ V � I � td

mð6Þ

where active mass (m) in the composite electrode for the calcula-tion of power and energy densities is 2.8 mg. The MnO2/40% EGcomposite electrode shows significant enhancement in both powerand energy densities with the maximum energy density of51 W h kg�1 at the power density of 500 W kg�1, which is higher

Fig. 4. Chronopotentiograms of MnO2/EG composite electrodes with different EGcontents between potential range of 0–1.0 V at charge/discharge current density of2.0 A g�1 (a), Chronopotentiograms of MnO2/40% EG composite electrode atdifferent discharge current densities (2, 4, 8, 16 and 24 A g�1) (b), and Ragoneplots of MnO2/EG composite electrodes with different EG contents (c).

H.R. Naderi et al. / Journal of Electroanalytical Chemistry 719 (2014) 98–105 103

than that of the other composite electrodes such as graphene/CNT/MnO2 (<20 W h kg�1) [34], MnO2/graphitic petals (<20 W h kg�1)[35], MnO2/grapheme (<20 W h kg�1) [36]. More importantly, theenergy density shows very good stability by increasing the powerdensity. The energy density reaches to 29 W h kg�1 even at a highpower density of 15,000 W kg�1, which is much higher than mostof the current commercial supercapacitors.

The electrochemical impedance spectroscopy (EIS) analysis isan important technique for examining the fundamental behavior

of electrode materials for supercapacitors [1]. The Nyquist imped-ance plots for MnO2/EG composite electrodes are shown in Fig. 5a.All plots are composed of a very small arc in the high-frequency re-gion, which is related to the charge transfer process at electrode/electrolyte interface, and of a line in the low-frequency region,which is associated to the diffusion process in the composite elec-trodes. At very high frequencies, the intercept of the real part (Z0),Rs, is a combination of the electrolyte’s ionic resistance, the intrin-sic resistance of the active material, and the contact resistance be-tween the active material and the current collector. The obtainedvalues of Rs are close together for different electrodes.

The EIS curves obtained from the composite electrodes wereanalyzed by complex nonlinear least square (CNLS) fitting method[1] on the basis of the equivalent circuit given in Fig. 5b (inset). Thecircuit consists of five elements; Rs, Rct, Cdl, ZW, and CF. Rs is theinternal resistance includes the bulk electrolyte solution resis-tance, the intrinsic resistance of active material and the ionic resis-tance of the electrolyte at current collector/electrode interface[37]. Rct is the charge-transfer resistance. Cdl and CF in the circuitrepresent, respectively, the electrical double layer capacitance atthe interface of electrode/electrolyte and the pseudocapacitance,which is accounted for the Faradic reaction. ZW is the Warburgresistance resulted from the frequency dependence of ion diffu-sion/transport in the electrolyte to the electrode surface [38]. Ta-ble 1 represents the values of Rs, Cdl, Rct, Zw and CF calculatedfrom the CNLS fitting of the experimental impedance spectra.Fig. 5b compares the experimental curve and fitted curve calcu-lated from the CNLS fitting of the experimental impedance spectra.These curves are in good agreement and the mean error of modu-lus is smaller than 3%. The major difference is observed in thesemicircle at the high-frequency range, which corresponds to theRct caused by the Faradaic reactions and Cdl on the grain’s surface.As expected, Rct is decreased by increasing EG in the compositeelectrodes due to conductive characteristics of the graphite. Fur-thermore, Cdl is decreased by increasing EG in the composite elec-trodes because of lower surface area of EG than that of thesynthesized MnO2 that reduces the available surface area per unitmass, accordingly.

As seen in the table, the MnO2/40% EG composite electrodeshows a better electrochemical performance than the other com-posite electrodes. The highest value of CF is corresponded to theMnO2/40% EG composite electrode. Regarding the EIS data, whenthe amount of EG in the MnO2/EG reaches to 40%, a balance be-tween the specific capacity and the conductivity in the compositeelectrode is attained. Further increase of EG to more than 40%causes a reduction in Faradic capacity and an increment in the dou-ble layer capacity. However, the increment of double layer capacityis much smaller than the reduction of Faradic capacity. This data isconsistent with CVs and chronopotentiograms data [39].

4. Conclusion

Nanostructured MnO2/EG with various contents of EG weresynthesized using an ultrasonic vibration. The XRD patterns con-firmed the orthorhombic structure of c-MnO2 and the layer struc-ture of graphite in the synthesized MnO2/EG. The optimumsupercapacitive performance of the composite electrodes were ob-tained for the sample with 40 wt.% EG, for which the SC of the com-posite electrode in a 0.5 M Na2SO4 electrolyte was approximately358 F g�1 at the scan rate of 2 mV s�1. The exfoliated graphitecould promote the conductivity of the composite electrode. Asthe composite electrodes are based on a combination of doublelayer capacitance and faradaic pseudocapactance. The SC can bemarkedly increased by incorporating graphite in the active mareri-als. Long-term cycling experiments performed on the MnO2/EG

Fig. 5. Nyquist plots of the impedance spectra experimentally measured on the MnO2/EG composite electrodes at an applied potential of 0.5 V (a), and experimental curveand circuit curve calculated from the CNLS fitting of the experimental impedance spectra and the equivalent model circuit (inset) (b).

Table 1Calculated values of Rs, Cdl, Rct, ZW and CF by CNLS fitting of the experimentalimpedance spectra based on the proposed equivalent circuit.

Fitting parameters MnO2/30% EG MnO2/40% EG MnO2/50% EG

Rs (mOhm) 152 162 216Cdl (mF) 0.628 0.467 0.477Rct (mOhm) 0.798 0.690 0.566ZW 0.660 0.86 0.87CF (mF) 453 580 402

104 H.R. Naderi et al. / Journal of Electroanalytical Chemistry 719 (2014) 98–105

composite electrodes showed an excellent behavior, with a totalloss of 13% compared to the initial capacitance after 1000 cycles.A good linear variation of potential versus time was observed forall the charge/discharge curves, which is another typical character-istic of an ideal capacitor. The MnO2/EG composite electrodesexhibited high energy densities at high-level power densities,which were attributed to the combination of high pseudocapaci-tance of MnO2 with high electronic conductivity of the EG sheets.

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