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ECS Electrochemistry Letters, 4 (8) A87-A89 (2015) A87

Symmetric Supercapacitor Based on Reduced Graphene Oxide inNon-Aqueous ElectrolyteS. Shivakumara,z Brij Kishore,∗ Tirupathi Rao Penki,∗ and N. Munichandraiah∗∗

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

Reduced graphene oxide (RGO) is prepared by thermal exfoliation of graphite oxide in air. Symmetric RGO/RGO supercapacitorsare constructed in a non-aqueous electrolyte and characterized. The values of energy density are 44 Wh kg−1 and 15 Wh kg−1,respectively at 0.15 and 8.0 kW kg−1. The symmetric supercapacitor exhibits stable charge/discharge cycling tested up to 3000cycles. The low-temperature thermal exfoliation approach is convenient for mass production of RGO at low cost and it can be usedas electrode material for energy storage applications.© The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in anymedium, provided the original work is properly cited. [DOI: 10.1149/2.0031508eel] All rights reserved.

Manuscript submitted April 22, 2015; revised manuscript received May 20, 2015. Published June 2, 2015.

Lithium ion batteries (LIBs) and supercapacitors (SCs) are promis-ing energy storage devices for portable electronics, digital commu-nications, hybrid electric vehicles, electric vehicles and renewableenergy systems.1–4 Electrical double layer capacitors (EDLCs) are theimportant electrochemical energy storage devices with long cyclingstability in aqueous electrolytes. The specific capacitance measuredin aqueous electrolytes is generally higher than in organic electrolyte.However, organic electrolytes are more attractive as they can with-stand a higher operation voltage (up to 3 V)5 than aqueous electrolyte(1.6 V for symmetric6 and 2.0 V for asymmetric7 supercapacitors).Compared with the batteries, the energy density of supercapacitors isoften limited to less than 10 Wh kg−1. Efforts have been made to im-prove the energy density (E) of a supercapacitor by either improvingits capacitance (C) or by increasing cell voltage (V) according to theequation,

E = 0.5CV 2.

The energy is proportional to capacitance and square of voltage.2–4

Ideally, voltage is limited by the stable potential window of the elec-trolyte. Since the potential window of organic electrolytes is largerthan that of aqueous electrolytes, higher voltages can be achieved andthus higher energy can be realized.

Various carbon based EDLCs have been studied in non-aqueouselectrolytes.8–12 However, these carbon materials have limited energystorage capacity and rate capability that restricts their applications. Al-ternatively, graphene-based materials are of particular interest becauseof their exceptional electrical, mechanical properties and electrochem-ical performance.13,14 Thermally exfoliated RGO with desired prop-erties have been studied for electrochemical performance.14–20 Thesegraphenes have been prepared by intercalating the oxygen function-alities into the graphite layer and quickly removing them by rapidheat-treatment under different atmospheric conditions. Among theseapproaches, the reduction of graphite oxide at low temperature in airis the cost effective and efficient approach for the mass production ofreduced graphene oxide (RGO).15 The resultant RGO contains someof the oxygen functionalities left behind on the surface and these func-tionalities are expected to enhance the electrochemical performance ofthe RGO. There are no reports on symmetric supercapacitor behaviorof thermally prepared RGO in non aqueous electrolytes, to the best ofauthors’ knowledge. A high specific capacitance with increased volt-age obtained in the present study suggests the suitability of thermallyprepared RGO for supercapacitor application.

Experimental

Graphene oxide (GO) was prepared by a modified Hummer’smethod.6,15 In a typical synthesis, 92 mL of sulfuric acid (98 wt%)

∗Electrochemical Society Student Member.∗∗Electrochemical Society Active Member.

zE-mail address: elessk@gmail.com

was taken in a 500 mL conical flask, which was kept in an ice bath.2 g of graphite powders and 2 g of NaNO3 were added to the abovesolution. 6 g of potassium permanganate was slowly added to themixture and stirred for 2 h at room temperature. To the above mixture92 mL of deionized water was slowly added followed by the additionof suitable amount of hydrogen peroxide. Finally, the mixture wasfiltered and washed with 5% HCl and deionized water until no sulfateions in the filtrate was detected, the solid was dried in a vacuum ovenfor 24 h. The thermal exfoliation of GO was carried out by keepingsmall quantity of GO in a pre-heated furnace (200◦C) under air at-mosphere. After 10 min, the volume of the GO sample expanded forseveral times and few-layered partially exfoliated reduced grapheneswere obtained.

The Powder X-ray diffraction (XRD) pattern were recorded usingBruker D8 advance X-ray diffractometer and Raman spectra weremeasured by a Horiba Jobin Yvon LabRam HR spectrometer. Sur-face area and pore size distribution of the samples were measuredusing micromeritics surface area analyzer model ASAP 2020. Themicrostructures of the materials were examined by using scanningelectron microscopy (SEM) model Sirion and transmission electronmicroscopy (TEM) model JEOL JEM 2100F.

For the fabrication of electrodes, the active material (RGO,85 wt%), conductive material (Acetylene black, 10 wt%) andpolyvinylidene fluoride (PVDF, 5 wt%) were mixed and ground ina mortar. Few drops of n-methyl pyrolidinone (NMP) were added toform slurry. The slurry was coated on the pre-treated stainless steeldisk (16 mm diameter) and dried at 110◦C under reduced pressurefor 12 h. The loading mass of each electrode was about 1.5–2.0 mgcm−2. Electrochemical experiments were carried out in two-electrodecoin cell system. The symmetric RGO/RGO cell was constructed bysandwiching the two electrodes separated by Celgard porous propy-lene membrane (2400). A commercial electrolyte of 1 M LiPF6 inethylene carbonate, diethyl carbonate and dimethyl carbonate (2:1:2v/v) was used as the electrolyte. Coin cells CR2032 (Hohsen Cor-poration, Japan) were assembled in argon filled glove box MBraunmodel UNILAB. The cyclic voltammetry (CV) and galvanostaticcharge/discharge cycling were measured by a Biologic SA multi-channel potentiostat/galvanostat model VMP3.

Results and Discussion

Fig. 1a. shows the XRD patterns of GO and RGO. The diffractionpeak at 10.6◦ corresponds to GO. After the heat-treatment at 200◦Cin air, the characteristic peak observed for RGO at 24.4◦ indicates theformation of RGO.20 The Raman intensity of D band increased whilethat of G band decreased and the ID/IG ratio is found to be 1.1. It ismainly attributed to the formation of vacancies and topological defectscaused by the release of CO and CO2 during decomposition.21 Thespecific surface area of RGO is found to be 395 m2 g−1 with a poresize distribution at 3.1 nm. The SEM micrograph of RGO (Fig. 1b)

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A88 ECS Electrochemistry Letters, 4 (8) A87-A89 (2015)

10 20 30 40 50 60 702-Theeta (degree)

Inte

nsi

ty (

a.u

.)

GO

RGO

(a)

(c)

(b)

Figure 1. (a) XRD patterns of GO and RGO, (b) SEM, and (c) TEM pho-tographs of RGO.

shows that there is a large exfoliation of graphene layers exhibitinga multi-layer structure. Under low magnification TEM the RGO looklike wrinkled transparent flakes (Fig. 1c) and at high magnification(Fig. 1c, inset) it resembles a transparent ultra thin film with a fewthin ripples within the plane.

Figure 2a presents the CV curves of a symmetric supercapacitoroperating at cell voltage between 0 and 3 V, and cycled at differentscan rates in the range 10–100 mV s−1. The CVs exhibit a rectangularshape with a good reversibility, which is a characteristic of a purecapacitive behavior. As per energy storage mechanism, both anions(PF6

−) and cations (Li+) are adsorbed on the graphene sheets duringcharging process, whereas the process is reversed during discharge.5

It is obvious to notice that increasing sweep rate leads to a decreasein the net charge under the curve. At the high scan rates the surfaceof the active material (RGO) is only involved in the electrochemicalreaction rather than the bulk, which results in a decrease in specificcapacitance.22

Galvanostatic charge/discharge experiments at various currentdensities between 0 and 3.0 V were carried out for symmetric su-percapacitor (Fig. 2b). The specific capacitance of 140, 132, 127, 121and 116 F g−1 are observed at a current density of 0.1, 0.2, 0.3, 0.4 and0.5 A g−1, respectively. The obtained values are highly comparable tothe previous reports.5,16,17 Recently, Mhamane et al., reported the ca-pacitance of 97 F g−1 at a current density of 0.1 A g−1.5 Also, Ruoff’sand co-workers reported the maximum capacitance of 112 F g−1 inan ionic liquid electrolyte (1 M tetraethylammonium tetrafluoroboratein acetonitrile or propylene carbonate).16,17 The specific capacitanceobtained in the present study is significantly greater than the valuesreported in the literature for RGO prepared by the other routes. Thespecific capacitance values at different current density are presentedin Fig. 2c. The maximum capacitance of 84 F g−1 was observed at acurrent density of 10 A g−1 indicating excellent high power capability.

Figure 2d, shows the plot of specific energy vs specific powerdensities. RGO based symmetric supercapacitor system is capableof delivering maximum energy density of 44 Wh kg−1. At a powerdensity of 8 kW kg−1, the energy density obtained is 15 Wh kg−1. Theobserved values in the present study are comparatively higher thanthat of carbon based systems.5,16,17

The cycling stability of symmetric RGO/RGO cell was subjectedto galvanostatic charge-discharge cycling at a specific current densityof 0.2 A g−1 (Fig. 3). The capacitance retention of about 86% wasobtained after 3000 cycles.

(a) (b)

(c) (d)

Figure 2. (a) Cyclic voltammograms of thesymmetric supercapacitor between 0 to 3 V atdifferent scan rates from 10 to 100 mV s−1.(b) The galvanostatic charge/discharge curvesat different current densities. (c) Specific ca-pacitance as a function of different dischargecurrent densities, and (d) The energy densityand power density calculated from the data ob-tained from the charge-discharge experiments.

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ECS Electrochemistry Letters, 4 (8) A87-A89 (2015) A89

500 1000 1500 2000 2500 3000Cycle number

0

20

40

60

80

100

120

Cap

acit

ance

ret

enti

on (

%)

0 5000 10000 15000 20000Time (sec)

0

0.5

1

1.5

2

2.5

3

Vo

lta

ge (

V)

Figure 3. The cyclic performance of the symmetric supercapacitor in thevoltage window between 0 to 3 V at a current density of 0.2 A g−1, and firstfew charge/discharge cycles (inset).

Conclusions

The RGO sheets were prepared in bulk by a low-temperature ther-mal exfoliation of GO. A symmetric supercapacitor with cell voltageup to 3 V was studied in a non-aqueous electrolyte. The system de-livered the maximum energy and power densities of 44 Wh kg−1 and8 kW kg-1, respectively.

Acknowledgments

S. S. and T. R. P. acknowledge the financial support from theUniversity Grant Commission (UGC), Government of India, un-der Dr. D. S. Kothari post doctoral fellowship programme [Ref.No. F.4-2/2006(BSR)/13-626/2012(BSR)] and senior research fellow-ship, respectively.

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