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Synthetic Metals 175 (2013) 62– 67

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

journa l h om epa ge: www.elsev ier .com/ locate /synmet

novel multilayered architecture of graphene oxide nanosheets for highupercapacitive performance electrode material

. Zabihinpoura,∗, H.R. Ghenaatianb

Department of Physics, Payame noor University, P.O. Box 19395-3697, Tehran, IranFaculty of Basic Science, Jahrom University, Jahrom, Iran

a r t i c l e i n f o

rticle history:eceived 11 November 2012eceived in revised form 6 April 2013ccepted 17 April 2013vailable online 28 May 2013

eywords:upercapacitorpecific capacitance

a b s t r a c t

Multilayer graphene oxide (MGO) nanosheets have been prepared by a simple chemical method. TheMGO has been employed as a novel and low cost electrode material in construction of a high energyaqueous symmetric supercapacitor MGO/MGO. Scanning electron micrographs reveal the formation ofnanosheet structure with thickness ranging from 30 to 50 nm The Fourier transform infrared spectroscopy(FTIR) results confirm the existence of oxygenated functional groups in the MGO nanosheets. Cyclicvoltammetry (CV), galvanostatic charge–discharge (CD) and electrochemical impedance spectroscopy(EIS) methods are carried out to characterize electrochemical performance of the symmetric superca-pacitor MGO/MGO. The maximum cell applied potential 1.2 V is obtained associates with 60 F g−1 of

ultilayered graphene oxide nanosheetraphite oxide

total electrode materials and maintains about 85% of the initial capacitance after 10,000 cycles. Further-more, the symmetric supercapacitor MGO/MGO shows high specific energy, specific power and maximumpower values of 12 Wh kg−1, 300 W kg−1 and 20,751 W kg−1 based on the total mass of the active elec-trode materials, respectively, at a current density of 5 mA cm−2 in 1 M H2SO4. The performance of theproposed supercapacitor has been compared with a symmetric supercapacitor based on graphite oxide(GO).

. Introduction

Supercapacitors are high-power density energy storage sys-ems that in recent years have drawn much attention to theirecause of their ability in storage and redeliverance of electri-al energy in a short time [1]. They can complement or replaceatteries in electrical energy storage and harvesting applications,hen high power delivery or uptake is needed. A notable improve-ent in performance has been achieved through recent advances

n understanding charge storage mechanisms and the developmentf advanced nanostructured materials [2–5]. Performances of theseystems are determined mainly by their electrode materials [6]. Theain drawback of the supercapacitors is their low specific energy.

he specific energy (Wh kg−1) of a supercapacitor is given by Eq.1) [7]:

pecific energy (SE) = 12

C�V2 = I�Vt

2M × 3600(1)

here C is the specific capacitance (F g−1), �V is the working poten-ial window (V), I is the discharge current (A), t is discharge time (s)nd M is the amount of active materials (kg) in the supercapacitor

∗ Corresponding author. Tel.: +98 9171912873.E-mail address: m zabihin@pnu.ac.ir (M. Zabihinpour).

379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.synthmet.2013.04.023

© 2013 Elsevier B.V. All rights reserved.

(including positive and negative electrode weight). Based on Eq. (1),two main strategies are envisioned to improve the specific energy ofsupercapacitors: (i) using of nanomaterials with high surface areaand (ii) increasing working potential window (�V). The stabilityand the nature of the electrolyte and the electroactive material ofa supercapacitor are important parameters for determination of itsoperating working potential [8].

Based on the charge-storage mechanisms, supercapacitorscan be classified into two categories: (i) the electric double layercapacitors (EDLCs), mainly by using conducting porous materialssuch as the pure carbon [9], in which the capacitance arises fromthe charge separation at an electrode/electrolyte interface. (ii) Themultielectron-transfer Faradic reaction at or within 2-dimensionalsurfaces of the electroactive materials [2,10] such as transitionmetal oxides [11], electrically conducting polymers (ECPs) [12] andheteroatom-containing carbon materials [13]. Supercapacitors canstore energy by using a single or combination of the charge storagemechanisms. Low specific capacitance of the pure carbon materialsis a big drawback [14]. To overcome this problem, heteroatom-containing carbon materials which have various functional groupssuch as oxygen, nitrogen, boron and dispersed metal on their

surfaces, have been considered in the supercapacitor field [9,15].Recently, carbon materials with some oxygen containing func-tional groups (such as COOH, CO, and others) show promisingpseudocapacitance characteristics [16]. Their abilities arise from

M. Zabihinpour, H.R. Ghenaatian / Synthetic Metals 175 (2013) 62– 67 63

a) MG

cg

srabrssm

2

2

oopweeci01Ae

eMwstfpcgf

Fig. 1. Scanning electron micrographs of (

ontributing of electrochemically active and acidic functionalroups in the formation of EDLCs and Faradic reactions [1,6,9].

In this study, the MGO nanosheets have been prepared by aimple chemical method and used as a new high performance mate-ial in construction of a symmetric supercapacitor MGO/MGO inqueous electrolyte solution. This symmetric supercapacitor showsoth pseudo-capacitive behavior using EDLs formation and Faradiceactions as energy storage mechanisms. The system demon-trates a higher capacitance and specific energy than symmetricupercapacitor GO/GO and can operate up to 1.2 V in aqueousedia.

. Experimental

.1. Reagents and instrumentation

Graphite powders (raw samples) and analytical grade reagentsf KOH, HCl, Na2SO4, KClO3, and H2SO4 were prepared from Merkr Fluka as supplied without further purification. All solutions wererepared by using doubly distilled water. The electrolyte solutionsere degassed by using pure Argon during all experiments. All

lectrochemical experiments were carried out by an Autolab Gen-ral Purpose System PGSTAT 30 (Eco-chemie, The Netherlands). Aonventional three electrode cell was used in order to character-ze the MGO and the GO using CV in potential windows −0.8 to.9 V at scan rate 50 mV s−1. A MGO or GO plate (0.5 mm thick and

cm × 1 cm area) were used as the working electrode, a Pt wire andg/AgCl (KCl, saturated) were utilized as the counter and referencelectrodes, respectively.

Symmetric supercapacitors were built in a home-made twolectrode cell: (i) MGO/MGO and (ii) GO/GO with two identicalGO and GO electrodes, respectively. These were constructedith 5 mg cm−2 for each electrode using cellulose acetate as

eparator in 1 M H2SO4 aqueous solution. Electrochemical inves-igations of the symmetric supercapacitors were performed asollows: the CV and the CD experiments were done in cell applied

otential 0.0–1.2 V at scan rates 50–200 mV s−1 and constanturrent densities 5–40 mA cm−2, respectively. The EIS investi-ations were performed at cell applied potential 1.2 V and inrequency range 10−2 to 105 Hz. Cycle life tests of the symmetric

O nanosheets, (b) GO dense agglomerates.

supercapacitors MGO/MGO and GO/GO were done at a constantcharge–discharge current density of 5 mA cm−2 for continuous10,000 cycles.

Morphological studies of prepared samples were carried outby using SEM (Philips XL 30). A Bomem (Quebec, Canada) MB102FTIR spectrometer equipped with a DTGS mid-range detector,CsI optics and a global source were employed to carry out the IRmeasurements.

2.2. Preparation of MGO

The MGO was prepared from natural graphite based on the fol-lowing procedures: the raw graphite powder was added graduallyin the fuming nitric acid with ultrasonic radiation using an ultra-sonic bath. Then mixture’s temperature was fixed at 70 ◦C and thisproduct was oxidized using potassium chlorate for 12 h. In the nextstep, the remaining liquid and solid particles were centrifuged andsolid product (GO) was repeatedly washed with 5% HCl solutionand distilled water and dried in an oven at 50 ◦C for 24 h. Finally, inorder to prepare the MGO, the GO powder was dispersed in 3 M KOHsolution using an ultrasonic irradiation and dispersed solid particleswere centrifuged in order to recover the MGO solid product. Subse-quently, it was washed using doubly distilled water repeatedly anddried in an oven at 50 ◦C for 24 h. Then, the powders were plated bymixing 80 wt% of MGO or GO powder with 15 wt% acetylene blackpowder and 5 wt% of poly(tetraflouroethylene) (PTFE).

3. Results and discussion

3.1. SEM and FTIR

Morphology and dimension of the as-prepared MGO and GOwere investigated by scanning electron microscopy (SEM) and theresults are shown in Fig. 1a, a′, b and b′, respectively. As shownin Fig. 1a and a′, the structure of the MGO is consisted of some

nanosheets with thickness between 30 and 50 nm The resultsobviously demonstrate that the dense agglomerated GO can bereadily exfoliated as individual MGO nanosheets in the presenceof ultrasonic irradiation and KOH solution [17].

64 M. Zabihinpour, H.R. Ghenaatian / Synthetic Metals 175 (2013) 62– 67

tcbhtclWsp

Fp1tr

3

mnciaFGoFm0bmcsdptui

pFoavst

Fig. 2. FTIR of the MGO nanosheets.

Previous studies [18] show that the surface charge (zeta poten-ial) of as-prepared graphene oxide sheets is highly negativelyharged when dispersed in water. The proposed mechanism cane explained by ionization of the carboxylic acid and phenolicydroxyl groups using KOH in aqueous medium. So, exfoliation ofhe GO is due to electrostatic repulsion forces between negativelyharged GO layers. Moreover, formation of the stable MGO col-oids is attributed to electrostatic repulsion forces against van der

aals interactions between the graphene layers [18]. These resultshow the critical role of KOH addition and ultrasonic irradiation inreparation of the MGO nanosheets.

The FTIR spectra of the MGO are shown in Fig. 2. Similar toTIR spectra of the GO [19], the MGO (Fig. 2) depicts a strong –OHeak at about 3400 cm−1 and other C O functionalities, COOH at720 cm−1 and COC/C OH in the range 1384–1051 cm−1. The spec-rum also shows a C C peak at 1619 cm−1 corresponding to theemaining sp2 character.

.2. Cyclic voltammetry studies

Cyclic voltammetry is one of the most suitable electrochemicalethods for estimating the difference between the Faradic and the

on-Faradic phenomena in electrochemical systems. Fig. 3 showsyclic voltammograms of the raw graphite, the GO and the MGOn 1 M H2SO4 and the MGO in 1 M Na2SO4 electrolyte solutionst scan rate of 50 mV s−1 in three electrode cell configurations. Asig. 3 shows, the larger current density response of the MGO and theO electrodes than raw graphite electrode is due to participationxygen-containing functional groups in formation of the EDL andaradic reactions. Predominantly, the MGO and the GO’ s voltam-ograms show a couple of oxidation and reduction peak between

.1 and 0.6 V (versus Ag/AgCl) which are results of the transitionetween quinone/hydroquinone states and are typical for carbonaterials with oxygen-containing functionalities [20]. The higher

urrent density response of the MGO nanosheets (Fig. 3) is due tohortening the diffusion and migration length of the electrolyte ionsuring the fast charge/discharge process and increase the partici-ation of the MGO electrochemical reactions. These results confirmhat the MGO nanosheets could be reversibly charged/dischargedsing Faradic and non-Faradic phenomena better than the GO and

n the stable form.The CV studies were also conducted across a wide working

otential window of 0.0–1.2 V to evaluate both the Faradic and non-aradic charge storage mechanisms and to determine a suitableperating working potential window of the symmetric supercap-

citors MGO/MGO and GO/GO. Fig. 4a and b shows the cyclicoltammograms of both symmetric supercapacitors at differentcan rates 50–200 mV s−1 in 1 M H2SO4 solution. It is worth men-ioning that during the charge process of these systems (anodic

Fig. 3. Cyclic voltammograms of the raw graphite, the GO and the MGO in 1 M H2SO4

and the MGO in 1 M Na2SO4 electrolyte solutions at scan rate of 50 mV s−1 in threeelectrode cell configuration.

branches), the potential of the electrodes are split and in thedischarge process (anodic branches) electrode potentials will beequaled finally.

The rectangle shape of the CV curves of the MGO/MGO cell(Fig. 4a) at higher scan rates confirms that this system has a goodsupercapacitive performance. Whereas, the parallelogram shape ofCV profiles (Fig. 4b) of the GO/GO system is due to its higher internalresistance than the MGO/MGO cell. Moreover, larger normalizedcurrents of the MGO/MGO than the GO/GO at the same voltage scanrates are due to shortening the diffusion and migration length of theelectrolyte ions during the fast charge/discharge processes [21].

The specific capacitance of a supercapacitor could be calculatedaccording to Eq. (2):

Specific capacitance (F g−1) =∫

IdV

sM�V(2)

where I is the response current density (A), V is the voltage (V),s is the potential scan rate (V s−1), �V is cell applied potentialrange (V) and M is the amount of active materials (g) in the super-capacitor (includes positive and negative electrodes) [10,22]. Thespecific capacitances of the symmetric supercapacitors MGO/MGOand GO/GO at different voltage scan rate were calculated using Eq.(2) based on the cyclic voltammograms (Fig. 4a and b) and areshown in Fig. 4c. As it is shown, the specific capacitance of theMGO/MGO cell at high scan rate 200 mV s−1 is 49.6 F g−1 whichis about twice of the specific capacitance of the GO/GO system(25 F g−1) at some scan rate. While, at lower scan rate 50 mV s−1

the specific capacitance of the MGO/MGO cell is 61 F g−1, and about1.37 times more than the specific capacitance of the GO/GO cell(44.3 F g−1). These results show facile diffusion of the electrolyteions at higher scan rates in the bulk of the MGO nanosheets in viewof the fact that the porosity of the MGO nanosheets are higher thanthe GO dense agglomerates. So, the electrolyte ions are more easilyinserted and de-inserted in the MGO nanosheets.

3.3. Galvanostatic charge–discharge and performance evaluation

Galvanostatic charge/discharge experiments were consid-ered to evaluate the electrochemical performance of theMGO/MGO and the GO/GO cells. Fig. 5a and b shows thegalvanostatic charge–discharge profiles of the symmetric super-

capacitors MGO/MGO and GO/GO at different current densities(5–40 mA cm−2). As shown (Fig. 5a and b), the potential varia-tions of both systems are almost sloping with some steps, whichagrees with the CV results (Fig. 4a and b). This is attributed to the

M. Zabihinpour, H.R. Ghenaatian / Syn

Fig. 4. Cyclic voltammetry responses of the symmetric supercapacitors MGO/MGO(e

pGsnn

b

S

a) and GO/GO (b), at different scan rates 50–200 mV s−1 in 1 M H2SO4 solution andffect scan rate on specific capacitance of both symmetric cells (c).

otential-dependent nature of Faradic redox reactions of MGO andO. It means that upon exchange of the electrons from a neutralegment of positive and negative electrodes, some local positive oregative charge deformations occur and according to the chargeeutrality, the ions flow to the electrodes.

The calculated specific capacitance of both symmetric cells

ased on Eq. (3) versus current density has been shown in Fig. 5c.

pecific capacitance (F g−1) = It

�VM(3)

thetic Metals 175 (2013) 62– 67 65

where I denotes the applied constant discharge current (A), tis discharge time (s) and �V is discharge potential window(V) [23].

As it is shown (Fig. 5c), the specific capacitance of the MGO/MGOcell at high current density 40 mA cm−2 is 43.6 F g−1 which is about1.89 times more than the specific capacitance of the GO/GOsystem (23 F g−1). While, at lower current density 5 mA cm−2 thespecific capacitance of the MGO/MGO cell is 60 F g−1 and about1.42 times more than the specific capacitance of the GO/GO cell(42.3 F g−1). The results reveal that the MGO nanosheets canexhibit higher electrode/electrolyte interface areas and providingmore electroactive regions than the GO.

Another useful parameter used to evaluate supercapacitor per-formance is equivalent series resistance (ESR), which refers to theresistance that adds to the impedance due to imperfections withinthe electrolyte and capacitor’s material [3]. It shows the powerproperties of the system during the discharge process. The ESR anal-yses of the MGO/MGO and the GO/GO cells were calculated usingthe charge–discharge profiles of Fig. 5a and b. The output potentialwindow dropped sharply at the beginning of each discharge exper-iment, proportional to the ESR of the systems. The ESR (�) couldbe calculated according to Eq. (4):

ESR = Echarge − Edischarge

2I(4)

where I, Echarge and Edischarge are the applied constant discharge cur-rent (A), the output potential window of the cell at the end of chargeand at the beginning of discharge after the Ohmic drop (V), respec-tively [3]. These average values of 1.5 and 8 (�) for the MGO/MGOand the GO/GO systems, respectively, were obtained at constantcharge–discharge current density of 5 mA cm−2. These low internalresistance value of the MGO/MGO cell confirms that this system hashigher power characteristic than the GO/GO cell.

The good performance of the symmetric supercapacitorMGO/MGO was highlighted using the Ragone plots derived fromthe results of the galvanostatic discharge curves (Fig. 5a and b) usingEqs. (1) and (5) and are shown in Fig. 5d.

Specific power (SP) = I�V

2M(W kg−1) (5)

As a result, it is concluded that the MGO/MGO system exhibitspromising performance compared to the GO/GO system. For exam-ple, at a constant current density of 5 mA cm−2, the high specificenergy of 12 Wh kg−1 was obtained at the corresponding specificpower of 300 W kg−1 for the MGO/MGO cell, whereas the specificenergy of 8.45 Wh kg−1 was obtained for the GO/GO system. So, thespecific energy of the symmetric supercapacitor MGO/MGO wassignificantly enhanced upon raising the specific capacitance of theMGO/MGO cell. These results clearly show that application of theMGO nanosheets as electroactive electrode materials can be oneof the best ways for fabrication of high energy supercapacitors. Theoutstanding performance of the MGO nanosheets material is due todecrement of ion mobility limitation during the charge–dischargeprocesses and facile formation of the EDLCs and Faradic reaction[16].

The symmetric supercapacitors MGO/MGO and GO/GOwere subjected to a prolonged cycle-life test at a constantcharge–discharge current density of 5 mA cm−2 for continuous10,000 cycles and these cycling performances are shown inFig. 6.

As it is seen, the symmetric supercapacitor MGO/MGO repre-sents high specific capacitance of 60 F g−1 at initial cycles, whichwas decreased slowly to 51 F g−1 after 10,000 cycles (15% decrease).

Whereas, the GO/GO alternative revealed a low specific capacitanceof 42.3 F g−1 which was decreased sharply to less than 15 F g−1 after10,000 cycles (65% decrease). This high attenuation in the capac-itance of the GO/GO system can be reasonably attributed to the

66 M. Zabihinpour, H.R. Ghenaatian / Synthetic Metals 175 (2013) 62– 67

F /MGOH tric ces

acbMfni

FM5

ig. 5. Galvanostatic charge–discharge curves of symmetric supercapacitors MGO2SO4 solution and effect of current density on specific capacitance of both symme

ymmetric systems (d).

gglomeration of the GO. Conversely, the modest decrease of theapacitance of the MGO/MGO cell suggests good cycle stability cane reasonably attributed to the surface negative charges of theGO nanosheets which prevent aggregation and reducing its sur-

ace area. So, from the results, it can be expected that the MGO

anosheets feature good performance as supercapacitor materials

n the aqueous solutions.

ig. 6. Dependence of the specific capacitance of the symmetric supercapacitorsGO/MGO and GO/GO with cycle number at charge–discharge current density of

mA cm−2.

(a) and GO/GO (b) at different constant current densities 5–40 mA cm−2 in 1 Mlls (c) and Ragon plots relating power density to achievable energy density of both

3.4. EIS analysis

The EIS analysis is a powerful and nondestructive method whichcan be used to separate different processes and evaluate the kinet-ics of the under investigation systems. This method has often beenused to study the power sources and electrochemical systems,especially supercapacitors.

Here, the impedance spectra of the symmetric supercapacitorsMGO/MGO and the GO/GO were performed in 1.0 M H2SO4 elec-trolyte at frequency range 10−2 to 105 Hz and shown in Fig. 7 in cellapplied potential 1.2 V. As it is seen, the almost vertically straightlines and semicircle in low and high frequencies of both systemsdefinitely demonstrate the pseudocapacitive behaviors. From theintersecting point at high frequency with the real axis, the inter-nal resistances of the supercapacitor MGO/MGO is less than theGO/GO system. To better evaluate the performance of these twosystems, the experiment data were fitted to the equivalent circuitmodel shown in Fig. 7 and were represented in Table 1. Due tothe porous surface of the MGO and the GO electrodes, constantphase elements CPEdl and CPEF are used to express Cdl and CF in theequivalent circuit. The impedance of the constant-phase elementis defined as ZCPE = [Q(jω)n]−1 with −1 ≤ n ≤ 1, where Q, ω and nare the frequency-independent constant representing capacitance,angular frequency and a correction factor between 0 and 1, respec-tively. The mean error of the modulus is less than 1% implying that

the parameter values obtained from EIS fitting via such proposedcircuit are reliable. The analysis of Z (ω) is based on Eq. (6):

Z(ω) = Rs + ZI(ω) + ZF(W) (6)

M. Zabihinpour, H.R. Ghenaatian / Synthetic Metals 175 (2013) 62– 67 67

Table 1Electrical parameters for the supercapacitors MGO/MGO and GO/GO evaluated from EIS test at applied voltage 1.2 (V) in 1.0 M H2SO4 solution.

Cell Rs (�) Rct1 (�) Rct2 (�) CPEdl CPEF

Cdl (F cm−2) n1 CF (F cm−2) n2

MGO/MGO 1.79 0.65 370

GO/GO 6.09 1.04 740

FMT

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4

tasc

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ig. 7. Equivalent circuit model and EIS plots of the symmetric supercapacitorsGO/MGO and GO/GO at cell applied potential 1.2 (V) in 1.0 M H2SO4 solution.

he solid lines represent the fitting results according to this equivalent circuit.

1ZI(ω)

= 1Rct1 + ZW

+ jωCdl (7)

1ZF(ω)

= 1Rct2

+ jωCF (8)

Every element has physical meaning where ω, Rs, ZI, ZF, Rct1,ct2, Cdl, ZW and CF are the angular frequency, solution resistance,

mpedance of modified film/electrolyte interfaces, bulk faradicmpedance, ionic charge-transfer resistance of the redox transitionsf the polymer films, double layer capacitance, Warburg diffu-ion impedance and bulk faradic pseudocapacitance, respectively24–26]. As it is seen in Table 1, the double layer capacitance andulk faradic pseudocapacitance of the MGO/MGO is much higherhan the GO/GO system which is consistence with the results ofhe CV and CD experiments. Also, the Rct1 in both cases is lowerapproximately 500 times) than the Rct2, which suggests a higherate of energy storage by the electrically double layer formation asompared to the Faradic reactions.

. Conclusion

Nanostructured MGO has been prepared by chemical oxida-

ion followed by ultrasonic irradiation in the presence of KOHnd is used for constructing of a new high energy symmetricupercapacitor. This system shows very promising energy storageharacteristic up to 1.2 V. Based on the obtained results, the specific

[

[[

0.02 0.68 0.57 0.950.016 0.62 .38 0.89

capacitance, specific energy and specific power of 60 F g−1,12 Wh kg−1 and 300 W kg−1 were calculated, respectively. Thisstudy highlights and opens a new rich world of possibilities todevelop carbon materials with large facile synthesis and low costfor supercapacitor applications.

Acknowledgement

I would like to thank the Payam e Nour University ResearchCouncil is gratefully acknowledged for their financial support.

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