Symmetric supercapacitor based on partially exfoliated and reducedgraphite oxide in neutral aqueous electrolyte

S. Shivakumara n, Brij Kishore, Tirupathi Rao Penki, N. MunichandraiahDepartment of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o

Article history:Received 21 July 2014Received in revised form22 August 2014Accepted 24 August 2014by E.V. SampathkumaranAvailable online 6 September 2014

Keywords:A. Exfoliated graphenesB. Thermal exfoliationC. High voltage supercapacitorsD. Symmetric capacitors

a b s t r a c t

The partially exfoliated and reduced graphite oxide (PE-RGO) is prepared by low temperature thermalexfoliation of graphite oxide under air atmosphere. A symmetric carbon/carbon supercapacitor is studiedin a Na2SO4 aqueous electrolyte. The discharge capacitance is 92 F g�1, when symmetric cell is cycledbetween the potential ranges from 0 to 1.6 V. This system demonstrates a stable charge/discharge cyclebehavior up to 3000 cycles when the cell is operated at a voltage window of 1.6 V. The utilization ratio ofpotential window is 90% for this system is attributed to the more negative value of electrodes potentialwhen the cell voltage is set to 0 V. The low-temperature exfoliation approach is convenient for massproduction of graphenes at low cost and it can be used as electrode material for energy storageapplications.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Electrochemical capacitors (ECs), also called supercapacitors orultracapacitors, have attracted growing interests because of theirhigh power density, excellent reversibility and long cycle lifeespecially for electric vehicles [1]. Compared with the batteries,the energy density of supercapacitors is often limited to less than10 Wh/kg. Efforts have been made to improve the energy densityof the active materials by either improving its capacitance or byincreasing cell voltage according to the Eq. (1).

E¼ 0:5CV2 ð1Þ

where C is the capacitance and V is the voltage, the energy densityis proportional to the specific capacitance and the square ofvoltage [1–3]. Currently extensive research has been focused onoperating voltage of supercapacitors. Ideally, voltage is limited bythe stability of the potential window of electrolytes. Organicelectrolytes are preferred for application due to a high voltage(�2.7 V) than aqueous electrolytes (�1 V), while aqueous elec-trolytes has the advantages of being cheaper, environment friendlyand easy to construct than organic ones [4,5]. Thus, aqueouselectrolytes seem to be more promising one from an industrialpoint of view in the long term. Generally, H2SO4 and KOH are themost commonly studied aqueous electrolytes for supercapacitors.However, only limited energy densities could be obtained with the

acid or basic aqueous electrolytes due to their low maximumvoltages (usually below 1 V) and the low over potential forhydrogen evolution. It has been observed that the di-hydrogenover-potential is higher in neutral aqueous electrolytes than inH2SO4 or KOH. For instance, a stable potential window of 2.0 V hasbeen reported for activated carbon three-electrode cell in 0.5 MNa2SO4 [5]. Consequently, an operational potential window of1.6 V has been reported for carbon/carbon symmetric supercapa-citors in 0.5 M Na2SO4 neutral aqueous solution. An even highervoltage of 2.4 V has been claimed for seaweed carbon in 0.5 MNa2SO4 [6]. Such a high voltage has been attributed to theimportant dehydration energy of ions and to the comparableconcentrations of protons and hydroxyl groups at neutral pHwhich do not favor hydrogen or oxygen evolution. Moreover, thecell voltage can also be increased by introducing heteroatoms tothe surface of the carbonaceous materials [6,7]. Thus recently highvoltage symmetric capacitors using heteroatoms-rich carbons inaqueous electrolyte have attracted much attention.

Many approaches to prepare heteroatoms-rich functionalizedcarbons, especially oxygen rich carbons have been reported [8–10].However, these carbon materials have limited energy storagecapacity and rate capability that restricts their applications. Alter-natively, graphene-based nanostructures are of particular interestbecause of their exceptional electrical, mechanical properties andelectrochemical performance [11]. In this contest, thermallyexfoliated-reduced graphenes with desired properties have beenstudied electrochemically performance [12–15]. These grapheneshave been prepared by intercalating the oxygen functionalities into the graphite layer and quickly removed by rapid heat treatment

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Solid State Communications

http://dx.doi.org/10.1016/j.ssc.2014.08.0140038-1098/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ91 80 22932240.E-mail address: elessk@gmail.com (S. Shivakumara).

Solid State Communications 199 (2014) 26–32

under different atmospheric conditions. Among these approaches,partially exfoliated and reduced graphite oxide (PE-RGO) at lowtemperature (�200 1C) under air atmosphere is the cost effectiveand efficient approach for the mass production of graphenes [12].The resultant graphenes contains some of the oxygen functional-ities left behind on the surface and these functionalities areexpected to enhance the electrochemical performance of thegraphenes.

In the present study, a cost effective method to synthesizepartially exfoliated-reduced graphite oxide by low temperatureexfoliation of GO under air atmosphere is reported. The three- andtwo-electrode cells have been constructed to study the electro-chemical performance of these electrodes and those results arediscussed. The PE-RGO electrodes exhibit a wider potential win-dow in neutral Na2SO4 electrolyte than in H2SO4 or even in KOHelectrolytes. The PE-RGO electrodes have been used to constructsymmetric supercapacitor system in an environmental friendlyand less corrosive aqueous neutral electrolyte. The constructedsymmetric device can operate reversibly up to 1.6 V and theutilization ratio of potential window of about 90%.

2. Experimental section

2.1. Preparation of partially exfoliated-reduced graphiteoxide (PE-RGO)

GO was prepared by a modified Hummers method [16]. In atypical synthesis, 92 mL of sulfuric acid (98 wt%) was taken in a500 mL conical flask, which was kept in an ice bath. 2 g of graphitepowders and 2 g of NaNO3 were added to the above solution. 6 g ofpotassium permanganate was slowly added to the mixture andcontinuosly stirred for 2 h at room temperature. To the abovemixture 92 mL of deionized water was slowly added. A suitableamount of hydrogen peroxide was added it. Finally the mixturewas filtered and washed with 5% HCl and deionized water until nosulfate ions in the filtrate was detected, the solid was dried in avacuum oven for 24 h. The partial exfoliation of GO was carried outby keeping small quantity of GO in a pre-heated furnace (200 1C)under air atmosphere. After 10 min, the volume of the GO sampleexpanded for several times and few-layered partially exfoliatedreduced graphenes were obtained.

2.2. Characterization electrochemical measurements

Powder X-ray diffraction (XRD) patterns of the samples wererecorded using Bruker D8 advance X-ray diffractometer withmonochromatized Cu Kα (λ¼1.5418 Å) incident radiation as thesource. The X-ray photoelectron spectra (XPS) were collected onAXIS ULTRA X-ray photoelectron spectrometer. The Raman spectrawere measured by a Horiba Jobin Yvon LabRam HR spectrometerhaving a laser of 514.5 nm and power 0.2 mW on the samplesurface. Surface area and pore size distribution of the sampleswere measured using micromeritics surface area analyzer modelASAP 2020. The microstructures of the materials were examinedby using scanning electron microscopy (SEM) model Sirion andtransmission electron microscopy (TEM) model JEOL JEM 2100F.

For fabrication of electrodes, PE-RGO (85 wt%), conductivecarbon (acetylene black, 10 wt%) and polyvinylidene fluoride(5 wt%) were mixed and ground in a mortar pestle. Few drops ofN-methyl pyrrolidone were added to form syrup. The syrup wascoated on the pre-treated stainless steel (SS) current collector anddried at 100 1C under reduced pressure for 12 h. The loading massof each electrode was about 0.8 mg. Electrochemical experimentswere carried out in both three- and two- electrode systems. Forthree-electrodes system, PE-RGO coated SS, Pt foils and saturated

calomel electrode (SCE) as the working, counter and referenceelectrodes, respectively were used in a glass container containingelectrolyte. The electrolytic solutions used are 6 M KOH, 1 M H2SO4

and 1 M Na2SO4. The symmetric electrode system fabricated withPE-RGO was studied using a home made Teflon Swagelok typesystem with absorptive glass mat separator and SS currentcollector in a 1 M Na2SO4 aqueous electrolyte. The cyclic voltam-metry (CV) and galvanostatic charge/discharge were measured bya Biologic SA multichannel potentiostat/galvanostat model VMP3.

3. Results and discussion

3.1. Structure and morphology

Fig. 1. shows the XRD patterns of pristine graphite, GO and PE-RGO. The diffraction peaks observed at 26.61 and 10.61 in Fig. 1.corresponds to pristine graphite and GO respectively. The evolu-tion of a new peak at 10.61 indicates the graphite has beenoxidized. After the heat-treatment at 200 1C under ambient atmo-sphere, the characteristic peak observed for GO at 10.61 vanishes,indicating that the periodic layered structure of GO sheets hasbeen exfoliated randomly. A very weak diffraction peak (shown ininset) emerged at 241 in PE-RGO sample, indicate the exfoliation ofGO and amorphous structures of as-prepared graphene.

In addition to the low temperature exfoliation, GO sheets couldalso be reduced during the thermal treatment process. The Fig. 2areperesents the survey XPS spectra of GO and PE-RGO. Fig. 2(b) and(c) shows the C 1s spectra of GO and PE-RGO, respectively. The XPSresults further confirmed the formation of oxygen functionalities inPE-RGO; with oxygen content deduced from XPS data decreasingfrom 26.06 to 11.84 at% after exfoliation, suggesting partial reduc-tion during the exfoliation process. The oxygen functionalitiespresent in both GO and PE-RGO are presented in Table 1. The C1s XPS spectrum of PE-RGO (Fig. 2c) clearly indicates the existenceof carbonyl (O–CQO, 289.0 eV) and alkoxy (C–O, 286.7 eV) [14].

The thermal reduction was further confirmed by Raman spec-tra. As shown in Fig. 3, the G band of PE-RGO shifted from1570 cm�1 to 1588 cm�1, which is close to the value of pristinegraphite (1570 cm�1), indicating the successful reduction of GO[17–19]. The G band of PE-RGO is close to that of natural graphite.Due to disordered vibrations of carbon bond and defects, theintensity of D band increased dramatically and G band is broaden.Compared to natural graphite, the ratio of intensities of D and Gband (ID/IG) was dramatically increased. It is mainly attributed tooxygen-containing defects caused by fiercely oxidation, as well asvacancies and topological defects caused by the release of carbondioxide etc. during decomposition [20].

Fig. 1. XRD patterns of graphite, GO and PE-RGO (inset is the pattern of PE-RGO).

S. Shivakumara et al. / Solid State Communications 199 (2014) 26–32 27

The nitrogen adsorption–desorption isotherms and pore sizedistribution are plotted in Fig. 4a. The specific surface area wascaluculated using the Brunauer–Emmett–Teller (BET) method fromadsorption branch of isotherm in p/p1 range of 0.1–0.2. Theisotherms of PE-RGO sample resemble type IV isotherm withhysteresis in the p/p1 range of 0.5–0.99. The adsorption–desorptionloop indicates the presence of meso pores in the samples. Barreett–Joyner–Halenda (BJH) curves from desorption branch of isothermsare presented in Fig. 4b. The BET surface area is 395 m2 g�1 and thesample exhibit a pore size distribution of 3.1 nm.

Fig. 5 represents SEM of GO (Fig. 5a) and PE-RGO (Fig. 5b)shows that these layers are exfoliated to a very high extent. Themorphology (Fig. 5b) is exhibiting a multi-layer structure quite

similar to that of graphene prepared at high temperature exfolia-tion of 1050 1C or low temperature exfoliation under high vacuum[12,17,21]. The graphene layers stacked together with nanoscalepores that facilitate the electrolyte ions to percolate between thelayers, and hence the electrochemical performance [12]. Thepressure generated by decomposition of oxygen functional groupsmight not be high enough to overcome the Van der Waals forcesthat resulted in the stacked graphene layers [11,12]. Fig. 5(c)and (d) presents transmission electron microscopy (TEM) imagesof PE-RGO. Under low magnification TEM the PE-RGO look likewrinkled transparent flakes as shown in Fig. 5c. It is seen from thehigh magnification TEM image (Fig. 5d) that PE-RGO resembles atransparent ultra thin film with a few thin ripples within the

Table 1XPS surface characterization of GO and PE-RGO.

Sample C at% O at% N at%

GO 72.32 26.06 1.62PE-RGO 87.99 11.84 0.18

1000 1200 1400 1600 1800 200

Raman shift (cm-1)

Inte

nsity

(a.u

.)

Graphite

GO

PE-RGO

Fig. 3. Raman spectra of Pristine graphite, GO and PE-RGO.

Fig. 4. Typical N2 adsorption–desorption isotherm and BJH pore-size distributioncurve of PE-RGO.

Fig. 2. XPS general spectrum (a), C 1s curves of GO (b), and PE-RGO (c).

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plane. The ripples are intrinsic properties of thin graphene sheets,due to extra thermodynamic stability of the 2D membranes arisingfrom microscopic crumpling [22].

3.2. Electrochemical characterization

3.2.1. Three-electrode cell characterization using different aqueouselectrolytes

Fig. 6a shows the cyclic voltammograms of PE-RGO electrode inthree-electrode cells with aqueous electrolytes 1 M H2SO4, 6 MKOH and 1 M Na2SO4, respectively. In H2SO4 media, the cathodicand anodic peaks are observed at around 0.39 and 0.46 V vs SCE,respectively (Fig. 6a, right side). These peaks are generally attrib-uted to the reversible oxygenated functionalities such as quinone/hydroquinone transformation [23]. In KOH (Fig. 6a, left side) thecathodic/anodic shoulders related to the redox of oxygenatedfunctionalities corresponds to �0.3 V and �0.5 V vs SCE, respec-tively. In alkaline electrolytic solution, the redox reactions invol-ving surface quinone groups are less [6].

The stability potential window is about twice in Na2SO4 than inacidic and basic electrolyte. Actually, in neutral electrolyte, thepotential window is high reaching up to 2 V due to the high overpotential for di-hydrogen evolution, owing to the storage ofnascent hydrogen on graphene sheets below the thermodynamicpotential for water decomposition [24]. Fig. 6b shows the cyclicvoltammogram of PE-RGO electrode with a three-electrode cell in1 M Na2SO4 electrolyte. The two vertical lines correspond to thepotentials of water reduction and oxidation, respectively [6,7]. It isobvious that the potential range can extend to more negative value(�1.0 V) than the water reduction potential (�0.38 V), this overpotential is related with an increase in pH locally due to theformation of nascent hydrogen on carbon surface [25]. The chemi-sorbed hydrogen can provide additional capacitance through the

Fig. 5. SEM photographs of (a) GO, (b) PE-RGO, TEM photographs of (c) PE-RGO, (d) HRTEM of PE-RGO.

Fig. 6. (a) Cyclic voltammograms of (10 mV s�1) showing the potential stabilitywindow of PE-RGO in 6 M KOH, 1 M H2SO4 and 1 M Na2SO4 and, (b) Cyclicvoltammograms of PE-RGO in 1 M Na2SO4.

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electro-oxidation of hydrogen during the anodic scan and anodichump appears at 0.1 V vs. SCE. On the other hand, a slight increaseof current takes place at 0.8 V during the anodic scan, correspond-ing to the electrochemical oxidation of graphenes. According tothe previous studies, a higher content of oxygenated surfacefunctionalities on graphene will protect it from irreversible oxida-tion during electrochemical process [4–7].

3.2.2. Two-electrode cell characterizationCyclic voltammograms and the charge/discharge profiles of

symmetric carbon/carbon electrode cell based PE-RGO in 1 MNa2SO4 solution are presented in Fig. 7. The cyclic voltammograms(Fig. 7a) operating at different cell voltage exhibit a rectangularshape at voltage upto 1.6 V, which is a characteristic of a purecapacitive behavior. The anodic current increases proportionally asthe cathodic current with the cell voltage extending from 1.0 to1.6 V, which suggests a good reversibility of the charge storageprocess, as well as the increase of capacitance, which has beenconfirmed by galvanostatic charge/discharge tests. However, whenthe cell voltage is increased to 1.8 V, a sharp increase in currenttook place during the anodic scan, and its counterpart is notsymmetric during the negative scan, these phenomena are corre-sponding to the irreversible oxidation of carbon at positiveelectrode or hydrogen evolution at negative electrode. When thevoltage is higher than 1.8 V, the graphene sheets are no longerstable and will be electrochemically oxidized during electroche-mical measurements [26]. Galvanostatic charge/discharge cycles(Fig. 7b) recorded for all voltage windows show symmetriccharacteristics, confirming a pure capacitive behavior. The

capacitance increases when the maximum voltage increases from1.0 to 1.6 V, which are presented in Table 2. These results suggestthat the symmetric carbon/carbon system should be able tooperate up to 1.6 V with high specific capacitance.

Fig. 8a shows the CVs of the symmetric carbon/carbon super-capacitor from 0 to 1.6 V at different scan rates from 5 to100 mV s�1. The cell maintains the rectangular shape of the CVseven at a high scan rate of 100 mV s�1, indicating ideal capacitivebehavior and desirable fast charging–discharging property forpower devices. The galvanostatic charge–discharge curves from0 to 1.6 V at different current densities, as presented in Fig. 8b,shows the linear voltage–time relation, characteristic of an elec-trical double layer capacitance. The average discharge capacitance(Cs) of both electrodes were calculated according to the galvano-static charge/discharge tests by Eq. (2) [5].

Cs¼ 4CUNIT ¼ 4nIΔt=MV ð2Þ

where CUNIT is total capacitance of the cell, I is the dischargecurrent, Δt is the discharge time, M is total mass of the twoelectrodes, V is the operating cell voltage. The SC of symmetriccarbon/carbon electrodes evaluated from the discharge curves are92, 83, 73, 78 and 64 F g�1 at the current densities of 0.1, 0.2, 0.3,0.4 and 0.5 A g�1 respectively. This value is high in comparisonwith the values reported in the literature [5–7] for symmetriccarbon/carbon supercapacitor electrodes. Thus, specific capaci-tance of about 92 F g�1 measured in the present study for PE-RGO is quite attractive. A high specific capacitance is retained(56 F g�1) at high current density (5 A g�1) as shown in Fig. 8c,indicating excellent high power capability.

The energy densities (E) of the two-electrode cell operating atdifferent cell voltage were calculated according to the Eq. (3):

E¼ 0:5CUNITV2 ð3Þ

Table 2 reports the specific capacitances and energy densities ofthe cell when the operating cell voltage increases from 1.0 to 1.8 V.The specific capacitances increased marginally from 88 to92 F g�1, while the energy density increased dramatically from3.1 to 10.4 Wh kg�1 with the increase of operating cell voltage.

Fig. 8d presents the capacitance retention versus the galvano-static cycles up to 1.6 V for PE-RGO based symmetric capacitor in1 M Na2SO4 at a current density of 0.5 A g�1. The capacitancekeeps almost constant till 3000 cycles after a slight decreaseduring the first 1000 cycles at 1.6 V. After 3000 cycles, thesymmetric capacitor retains a high specific capacitance of about83% of its initial capacitance.

Fig. 9. shows the evolution of the potential measured at cellvoltage is set to 0 V (E0V) and of the extreme electrode potentialsreached by positive and negative electrodes, when the cell ischarged up to different operating voltage. It is found that when thecell voltage is charged up to 1.6 V, the operating potentialwindows for positive [�0.33; 0.66] and negative [�0.34; �0.94]electrodes are within the stable window [�1.0; 0.8] V vs. SCE,which is shown by two horizontal lines in Fig. 9. Even though,when the cell voltage is charged up to 1.8 V, the operatingpotentials windows for positive [�0.33; 0.8] and negative[�0.33; �1.0] electrodes are still within the stable potentialwindow.

Fig. 7. (a) cyclic voltammograms (2 mV s�1) and (b) galvanostatic (0.1 A g�1)charge/discharge recorded for an symmetric supercapacitor in 1 M Na2SO4.

Table 2Specific capacitances and energy densities of PE-RGO electrodes based symmetricsupercapacitor working at different cell voltages.

Cell voltage (V) 1.0 1.2 1.4 1.6 1.8Specific capacitance (F g�1) 88 89 90 92 92Energy density (Wh kg�1) 3.1 4.5 6.1 8.2 10.4

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The maximum utilization of the potential window (90%) isachieved in present study as compared with the previous studies[5,6]. For instance, the larger stable potential window of 2.0 V foractivated carbon [5] and 2.4 V for seaweed carbon [6] wasachieved in aqueous Na2SO4 solution. However, the cell voltagesare limited to 1.6 V irreversible carbon oxidation in positiveelectrodes, and the entire stability of the potential window towardmore negative potential values underused. The potential windowutilization ratio was about 80% for activated carbon and 67% forseaweed carbon. The measured electrochemical parameters ofhigh voltage supercapacitors based on different carbon materialsare tabulated in Table 3. Based on these results we found that PE-RGO based supercapacitor shows a more negative value (�0.33 V)than activated carbon based supercapacitors (0.09 V) and seaweedcarbon based supercapacitors (0.39 V), which is important toeliminate the oxidation of positive carbon and take more use ofthe potential window to more negative potential values.

4. Conclusions

The PE-RGO electrodes have been tested for supercapacitors inH2SO4, KOH and Na2SO4 aqueous electrolytes. A high value of 2.0 Vstable potential window was observed for PE-RGO electrodes inNa2SO4 electrolyte. A high voltage value of 1.6 V was achieved with asymmetric carbon/carbon supercapacitor by using PE-RGO inNa2SO4 aqueous electrolyte. Based on the results, the graphene-based supercapacitors can operate with the voltage of 1.6 and even

1.8 V in the neutral electrolyte with good charge–discharge cyclabil-ity. The value of E0V strongly influences the utilization of completestable potential window. The method of preparation is convenientfor mass production of graphenes. The electrochemical performanceshows that a neutral aqueous electrolyte was very attractive fordeveloping a new generation of environment friendly carbon/carbonsymmetric supercapacitors able to compete with the organic elec-trolyte based systems.

Fig. 8. (a) Cyclic voltammograms of the symmetric supercapacitor between 0 to 1.6 V at different scan rates from 5 to 100 mV s�1. (b) The galvanostatic charge/dischargecurves of the symmetric supercapacitor at different current densities. (c) Specific capacitance as a function of different discharge current densities of symmetric capacitorelectrode, and (d) The cyclic performance of the symmetric supercapacitor in the voltage window between 0 to 1.6 V at a current density of 0.5 A g�1.

Fig. 9. Extreme potential reached by the positive (Eþ) and the negative electrode(E�) electrode at different maximum voltage in 1 M Na2SO4. The Horizontal linesrepresent the extreme potential values determined in three-electrode cell.

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Acknowledgments

SS and TRP acknowledge the financial support from the Uni-versity Grant Commission (UGC), Government of India, underDr. D.S. Kothari post doctoral fellowship programme [Ref. no. F.4-2/2006(BSR)/13-626/2012(BSR)] and senior research fellowship,respectively.

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Table 3Comparison of measured electrochemical parameters of symmetric supercapacitors based on different carbon electrode materials.

Electrode material Electrolyte Capacitance (F g�1) Cell voltage (V) Energy density (Wh kg�1) Utilization of potential window (%) Ref.

Activated carbon 0.5 M Na2SO4 135 1.6 10.0 67 5Seaweed carbon 0.5 M Na2SO4 125 1.6 10.7 80 6Ball-milled graphite 0.5 M Na2SO4 81 1.6 7.2 89 7PE-RGO 1 M Na2SO4 92 1.6 8.2 90 present work

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