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Nano Energy (2014) 8, 231–237

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High performance supercapacitor for efficientenergy storage under extremeenvironmental temperatures

Ranjith Vellacheri, Ahmed Al-Haddad, Huaping Zhao,Wenxin Wang, Chengliang Wang, Yong Lein

Ilmenau University of Technology, Institute of Physics & IMN Macronanos (ZIK), Prof. Schmidt-Str. 26,98693 Ilmenau, Germany

Received 27 April 2014; received in revised form 10 June 2014; accepted 10 June 2014Available online 20 June 2014

KEYWORDSEnergy storage;Supercapacitor;Capacitance;Graphene;Aqueous electrolyte

0.1016/j.nanoen.2lsevier Ltd. All rig

thor. Tel.: +49 36ong.lei@tu-ilmean

AbstractOperating temperature greatly influences the performance of supercapacitors. In order toachieve a very consistent performance at various working temperatures, we develop a low-costand high performance supercapacitor utilizing graphene-based electrodes and Li2SO4-basedaqueous electrolyte. The fabricated supercapacitor shows excellent charge storage character-istics from �20 1C to 45 1C. Specific capacitance of the electrode obtained from charge–discharge measurements, which are carried on two electrode cells, is 91 F/g at roomtemperature and there is no significant change observed when tested at various environmentsin a large temperature range (e.g. 74 F/g and 99 F/g at �20 1C and 45 1C, respectively). Thissupercapacitor also exhibits an outstanding capacitance retention and cycleability during itsoperation at different temperatures. Impedance measurements reveal the exceptionally lowequivalent series resistance (ESR) and charge transfer resistance of the supercapacitorelectrodes. Study of the relationship between charge storage and temperature by employingArrhenius-type equation shows very low activation energy for the charge storage process. Theremarkable electrochemical performances obtained here elucidate the great potential of thegraphene-based supercapacitor for the efficient energy storage at various environmentaltemperatures.& 2014 Elsevier Ltd. All rights reserved.

014.06.015hts reserved.

77 693748; fax: +49 3677 693746.u.de (Y. Lei).

R. Vellacheri et al.232

Introduction

Supercapacitor, also known as ultracapacitor or electroche-mical capacitor, has gained immense attention in recentyears as an effective energy storage device for high powerdemanding applications. Supercapacitors are very helpful tomeet the high power requirements in electric vehicles,hybrid electric vehicles, industrial forks, electromechanicaldevices, wind mills etc. [1,2]. Based on the charge storagemechanisms, supercapacitor can be classified into twotypes: electrochemical double layer capacitor (EDLC) andpseudocapacitor [1,3]. Pseudocapacitive materials (metaloxides such as RuO2 and MnO2, and conducting polymerssuch as polyaniline, polypyrrole and PEDOT) usually storemore charges than those of EDLC materials (carbon materi-als comprising activated carbon, carbon nanotube, gra-phene etc.) [4–11]. However, the use of metal oxides andconducting polymers for supercapacitor devices are par-tially limited due to their low chemical stability in certainelectrolytes, while carbon-based EDLC electrodes are stablein most of the electrolytes. It makes carbon as a versatilematerial for supercapacitor electrode fabrication. Amongall the carbon materials, graphene is the most promisingelectrode material for energy storage applications becauseof its high surface area, cost effectiveness, high electricalconductivity etc. [12–14].

Performance of a supercapacitor is primarily depends onits electrodes, electrolytes and operating conditions. Manyresearchers have extensively studied the effect of structureand composition of electrodes, and characteristics of elec-trolytes on performance of supercapacitor. But so far onlyfew studies have been conducted regarding the perfor-mance of supercapacitor electrodes at various operatingconditions and also about the development of supercapaci-tors for the application in different surroundings. Differentregions on the earth experience distinct climate conditions,for example many European countries go through frigidwinter periods (temperature generally falls below 0 1Cduring winter) while many countries in Asia and Africa facethe ablaze of the summer (temperature may goes above40 1C). Because of this, the supercapacitors reported inmany literatures that are focusing on room temperature(RT) performances may not be suitable for applicationsunder above-mentioned extreme environmental tempera-tures. The performance of many supercapacitors assembledin normal aqueous and non-aqueous electrolytes may dete-riorate at low temperature due to the freezing of theaqueous electrolyte and the increased viscosity of thenon-aqueous electrolytes which results in reduced powerdensity [15–18]. For example, even for organic electrolytebased commercial supercapacitors, their cell resistanceunder low temperatures can be increased 10,000 timescompared to that of RT [17]. There are some recent resultsfor designing suitable electrodes and/or electrolytes forsupercapacitors using in extreme temperature conditions,including MnO2/C with aqueous electrolyte [15], activatedgraphene with ionic liquid electrolyte [18], and MnO2

nanorods in p-phenylenediamine/KOH electrolyte [16].However, these systems have disadvantages especially atlow temperature such as low specific capacitance, poor ratecapability, high equivalent series resistance (ESR) andcharge transfer resistance. Generally, there are a few

technical challenges for realizing high-performance super-capacitors for extreme temperature applications, mainlyincluding: (i) simultaneous achievement of very high specificcapacitance, rate capability, energy density, power densityand cycle life over a wide range of operating temperatures;(ii) demonstration of performance using standard two-electrode cell at different operating temperatures andcurrent densities through charge–discharge measurements;(iii) attaining the cost-effectiveness by using cheap compo-nents (for example, use of low-priced electrolytes insteadof expensive ionic liquid and organic electrolytes, avoidingthe use of costly Pt-disk which is needed in certain cases toreduce the contact resistance etc.) [18–20].

In this paper, using an innovative combination ofgraphene-based electrode and Li2SO4-based aqueous elec-trolyte, we developed a supercapacitor that has excellentperformance over a wide range of temperatures from�20 1C to 45 1C. The two-electrode supercapacitor cellshows high specific capacitance, good rate capability, highpower density and long cycle life, which originate from thecombination of graphene-based electrode and Li2SO4-basedelectrolyte. Approximately 80% of specific capacitance wasretained at �20 1C comparing to that of RT, showingexcellent charge storage capability of the supercapacitorat low temperature. The values obtained from this two-electrode cell shall be more reliable than those of theconventional three-electrode cells. Besides the above-mentioned advantages, the combination of inexpensivegraphene and Li2SO4-based electrolyte allows the fabrica-tion of affordable supercapacitor for energy storage underextreme environmental temperatures.

Experimental

Graphite and PTFE (60%) were procured from AldrichChemicals. Hydrogen peroxide (30%), sulfuric acid (98%),sodium nitrate, Li2SO4, ethanol, hydrazine monohydrate(98%) were purchased from Alfa Aesar. All the chemicalswere used as received without any further purification.Carbon nanofibers for the use in electrode fabrication werepurchased from Pyrograf Products Inc., and purified by H2O2

[7]. Poly (tetrafluoroethylene) (PTFE) filter papers (poresize, 0.45 mm; Sartorius) were used for filtration.

Preparation of graphene

Graphene was obtained by chemical reduction of grapheneoxide (GO) by hydrazine monohydrate. GO was prepared bymodified Hummers method. For GO synthesis, concentratedH2SO4 (23 mL) was added to a mix of graphite (1.0 g) andNaNO3 (0.5 g), and then mixture was cooled to 0 1C. KMnO4

(3.0 g) was added very slowly as small amounts whilekeeping the reaction temperature below 20 1C. The reactionwas heated to 35 1C for 30 min with stirring, subsequently DIwater (46 mL) was added slowly to generate an exotherm to98 1C. External heating was added to keep up the reactiontemperature at 98 1C for 15 min, then the heating wasremoved and the reaction was cooled using a water bath.Afterwards, DI water (140 mL) and 30% H2O2 (1 mL) wereadded and created a new exotherm. After cooling, the

233High performance supercapacitor

mixture was purified (sifting, filtration, multiple washings,centrifugations and decanting, vacuum drying) [12,21,22].

Graphene was prepared by the chemical reduction of GOusing hydrazine monohydrate. Hydrazine monohydrate(2 ml, 98%) was added into the dispersion of GO in water(1 mg/ml) and the solution was refluxed for 24 h. After thereflux process, solution was filtered and graphene waswashed several times with water and methanol.

Preparation of electrodes and electrolytes

Symmetric two electrode cells were fabricated to analyzethe charge-storage capabilities of the graphene-based elec-trodes in Li2SO4/ethylene glycol/water electrolyte. Electro-des were fabricated by coating mixture of graphene (75%),Carbon nanofiber (15%) and PTFE (10%) on a stainless steelsubstrate (3.1 cm2). The addition of the carbon nanofiberhelps to reduce the internal resistance of the cell. The massof the each electrode was 2 mg. The electrolyte composi-tion was tailored for obtaining wide temperature range forthe smooth operation of supercapacitor and electrolyte forthe present study was prepared by mixing 70% (by mass) of1.4 M aq. Li2SO4 with 30% (by mass) ethylene glycol [15].Before the analysis, all the assembled cells were equili-brated at desired temperature for more than 5 h.

Characterizations

Scanning electron microscopy (SEM) images were collectedon a Hitachi S-4800 operated at 5 kV. Transmission electronmicroscopy (TEM) images were recorded on a JEOL JEM-2100F operated at 200 kV; the samples were dispersed inethanol and dropped on a carbon coated copper grid. X-raydiffraction (XRD) patterns of the sample were analyzed bySiemens D5000 using Cu K-alpha radiation. Cyclic voltam-metry, galvanostatic charge/discharge, and electrochemicalimpedance spectroscopy were carried out on test cells withgraphene electrodes and Li2SO4/Ethylene glycol/Waterelectrolyte using a Biologic electrochemical work station.Cyclic voltammograms were scanned at different scan rates(from 5 to 200 mV/s) from 0 to 1 V. Charge–dischargemeasurements were carried out at different current den-sities from 1 to 10 mA/cm2.

The specific capacitance (F/g) of the electrode wascalculated from each galvanostatic charge–discharge curve

Figure 1 (a) TEM and (b) SEM images of graphene nanosheets

according to

C¼ 4It=Vm ð1Þwhere I is the discharging current, m is the mass of theelectrodes, and V is the potential window and t is thedischarge time.

Energy density was calculated using the equation,

E ¼ CV2=8 ð2Þwhere C is the capacitance value measured from dischargecurve of the charge–discharge profile and V is the potentialwindow (here it is 1 V). Cycleability of the supercapacitorwas analyzed by carrying out charge–discharge measure-ments over 5000 cycles.

Electrochemical impedance analysis was performed inthe frequency range from 0.01 Hz to 1 MHz with an AC signalof 10 mV amplitude. Power density was calculated by usingthe equation,

P¼ V2=4ESRm ð3Þwhere ESR is equivalence series resistance obtained fromimpedance spectrum.

Results and discussion

Figure 1 shows the TEM and SEM images of the grapheneprepared by reducing graphene oxide using hydrazinemonohydrate. Both TEM and SEM images confirm the thinsheet like morphology of graphene. The TEM image alsoexposes the transparent nature of the graphene sheets. XRDmeasurements are carried out to identify the formation ofgraphene. XRD pattern of the GO (Figure S1 in the Supple-mentary information) shows a sharp peak centered at2θ=111. But after the reduction, that peak was completelyvanished and a new broad peak appeared between 2θ=201and 2θ=301 which indicates the conversion of GO intographene after the reduction.

To investigate the charge-storage properties of thegraphene-based electrodes at different temperatures, sym-metric two-electrode cells are fabricated. The fabricatedtwo-electrode cells are analyzed by cyclic voltammetry andgalavanostatic charge–discharge. Figure 2 shows the cyclicvoltammograms obtained at a scan rate of 20 mV/s over awide range of temperatures at RT, 45 1C, 0 1C and �20 1C.The obtained cyclic voltammograms (from 0 to 1 V) havenearly rectangular shape. And similar shapes are obtained

prepared by the chemical reduction of GO using hydrazine.

Figure 2 Cyclic voltammograms of graphene-based superca-pacitor measured over a wide range of temperatures at RT,45 1C, 0 1C, and �20 1C (at a scan rate of 20 mV/s). Therectangular shape of the cyclic voltammograms specifies theideal charge storage behavior at different temperatures.

Figure 3 (a) Charge–discharge profiles of the graphene-basedsupercapacitor at different temperatures (measured at a dis-charge current of 1 mA/cm2) (b) Variation of capacitance atdifferent temperatures and current densities.

R. Vellacheri et al.234

over a wide range of scan rates from 5 to 200 mV/s (FigureS2 in the Supporting information). These rectangular shapesof the cyclic voltammograms point toward the ideal capa-citive behavior due to the good transmission of charge in theelectrodes and also the effective double layer chargestorage mechanism. The ideal characteristics of cell specifythat the graphene-based electrode can be easily accessedby ions in the electrolyte even at extreme temperatureconditions. Along with this, low electrical resistance of theelectrodes and high ionic conductivity of the electrolytealso make significant contribution for obtaining excellentcharge storage characteristics. The graphene electrodesalso maintained good capacitive currents at all measuredtemperatures. The capacitive current slightly increasedwhen the temperature increased to 45 1C, which is a normalresult of higher ionic conductivity of electrolyte at elevatedtemperatures. The ions in the electrolyte can reach moreareas of the electrode surface compared to the case at RT.The capacitive current slightly decreased at low tempera-ture, which might be resulted from the reduced diffusion ofions to the electrode. In all cases the capacitive currentincreases with the scan rates (Figure S2 in the Supportinginformation).

Figure 3a shows the galvanostatic charge–discharge pro-files of the supercapacitor at a current density of 1 mA/cm2.Charge–discharge profiles are nearly symmetrical with linearslop even at low temperatures. Specific capacitances of thegraphene-based electrodes are calculated from charge–discharge profiles. At RT, the specific capacitance is 91 F/gand it increases to 99 F/g at 45 1C. The specific capacitanceof the electrode shows a slight drop when the temperatureis decreased. Specific capacitances are 80 and 74 F/g at0 and �20 1C, respectively. That is, approximately 80% ofspecific capacitance is retained at �20 1C comparing to thecase of RT. And this high retention of specific capacitance ismore promising than the performance of many previouslyreported supercapacitors [15,16]. Figure 3b shows the goodrate capability of the supercapacitor at different currentdensities (the corresponding charge–discharge profiles areshown in Figure S3 at the Supporting information), which

indicates good charge storage properties of the supercapa-citor. High specific capacitance and very small IR drop (dueto low internal resistance) obtained at the beginning ofdischarge curves indicate that the graphene-based elec-trode can simultaneously maintain both the excellent iondiffusion path for the easy motion of the ions in theelectrolyte during charge–discharge process and high elec-trical conductivity to obtain good performance under var-ious operating temperature. The relative low viscosity ofLi2SO4-based electrolyte is also helpful in obtaining goodperformance at different temperatures.

The energy densities of supercapacitors at differentoperating temperatures are calculated using the specificcapacitance obtained from charge–discharge profiles. Energydensities of supercapacitor are 3.44 Wh/kg, 3.16 Wh/kg,2.78 Wh/kg and 2.57 Wh/kg at 45 1C, RT, 0 1C and �20 1C,respectively. The energy densities at low temperatureobtained here are more promising than the values obtainedfor aqueous electrolyte based supercapacitors in previousliteratures (Supporting information Table S1).

The better knowledge of the correlation between chargestorage and operating temperature is very important for thepractical applications. Generally, the charge storage pro-cess in a graphene-based supercapacitor occurs through theformation and disruption of the electrochemical double

Figure 4 The Arrhenius plot of ln C vs. 1/T at differentcurrent densities.

Figure 5 Cycle life of the supercapacitor at different tem-peratures measured at a discharge current of 4 mA/cm2.

Figure 6 Nyquist plots obtained for graphene-based super-capacitor at different temperatures. Inset figure shows thesemicircle in the high frequency range which indicates thecharge transfer resistance.

235High performance supercapacitor

layer at the electrode–electrolyte interface during chargeand discharge process, respectively. During the charging,the ions in the electrolyte move towards electrode surfaceand form the electrochemical double layer. As seen inFigure 3b, the decrease of the temperature and theincrease of the current density decline the specific capaci-tance. So the motion of ions towards electrode surface isvery critical in the charge storage process. In such cases,Arrhenius-type equation might be useful to get moreinformation about the mechanisms of the charge storageprocess. Hence, we could use Arrhenius-type equationC=C0 exp

�Ea/RT or ln C= ln C0�Ea/RT, where C is the chargestored during the process, C0 is the pre-exponential factor,Ea is activation energy, T is absolute temperature and R isuniversal gas constant. The slope (=�Ea/R) of the plot ofln C vs. 1/T (Figure 4) for each current density can be usedto find the value of Ea. The resulting activation energyvalues are varying from 0.72 to 1.88 kcal/mol at the currentdensities from 1 to 5 mA/cm2. In comparison with the Eaobtained for graphene-based supercapacitor with ionicliquid electrolyte (13–26 kcal/mol for RT to 80 1C), thevalue obtained here is more promising [23]. Low Ea indi-cates that the electrochemical double layer formation–dissipation process occurring on graphene electrodes inour present system can be achieved very easily.

Long cycle life is also a very important parameter when itcomes to practical applications. In order to obtain thedetails regarding the cyclic stability of the supercapacitorat different temperatures, 5000 cycles of charge–dischargemeasurements are carried at a current density of 4 mA/cm2.From the plot of specific capacitance vs. cycle number asshown in Figure 5, it is clear that the supercapacitors canmaintain excellent high cyclic stability at wide range ofextreme operating temperatures. The long cycle life of thesupercapacitor can be attributed to the high electrochemi-cal stability of the graphene at various operating conditions.It also indicates the good compatibility of graphene-basedelectrodes with Li2SO4-based electrolyte.

The electrochemical impedance spectroscopy also has beendone to perceive the electrochemical behavior of thegraphene-based supercapacitors at various temperatures.Figure 6 shows the Nyquist plot obtained from electrochemical

impedance spectroscopy at different temperatures. The dou-ble layer charging, charge transfer resistance and diffusioncontrolled kinetics are divided by the complex plane plotsconcerning to different frequency regions. The charge transferprocess at the electrode–electrolyte interface is explained bythe distinct region represented by a semicircle at higherfrequencies, whilst the straight line inclined at an angle of451 to the real axis specifies the diffusion controlled electrodekinetics at low frequency region. The variation of super-capacitor performances at different temperatures can beobtained by comparing diameter of the semicircle, equivalentseries resistance and position of the straight line in the lowfrequency region. The charge transfer resistances obtainedfrom impedance plots are 0.95 Ω, 1.24 Ω, 1.62 Ω and 2.06 Ωat RT, 45 1C, 0 1C and �20 1C, respectively. The vertical shapeobtained at the low frequency region indicates the idealcapacitive behavior which is a characteristic of diffusion ofions to the electrode. The more vertical the shape, the moreintensely the supercapacitor performs as an ideal capacitor. Inthe present case, the variation of temperature not effectingvery significantly on the diffusion of ions and is evident fromposition of vertical lines at different temperature. The veryconsistent specific capacitance of the electrode at different

R. Vellacheri et al.236

operating temperatures obtained from charge–discharge mea-surements also confirms this. The ESR values are, veryimportant for determining power density of a supercapacitor,about 0.97 Ω, 0.63 Ω, 1.44 Ω and 3.13 Ω at RT, 45 1C, 0 1C and�20 1C, respectively. The ESR increased only 3.2 times whentemperature lowered to �20 1C from RT. The extremely lowESR will result high power densities during the practicalapplications. Power densities of supercapacitor are64.43 kW/kg, 99.20 kW/kg, 43.40 kW/kg and 19.97 kW/kg atRT, 45 1C, 0 1C and �20 1C, respectively.

Conclusions

Inexpensive and highly reliable energy storage at wide rangeof temperatures is achieved by a graphene-based super-capacitor. Cyclic voltammetry and galvanostatic charge–discharge measurements show the ideal charge storagebehavior of the supercapacitor. Specific capacitance of theelectrode is 91 F/g at RT and it also shows very goodsteadiness in its specific capacitance at different operatingtemperatures. Charge–discharge measurements also con-firmed the good capacitance retention and cycle life ofthe supercapacitor at various conditions. Low ESR andcharge transfer resistance obtained from impedance mea-surements support the ability of the supercapacitor toattain high power densities even at very low temperatures.Our results will pave a new way for the development andapplication of supercapacitors for energy storage undervarying environmental conditions.

Acknowledgments

We are grateful to European Research Council (ThreeDsur-face: 240144), BMBF (ZIK-3DNanoDevice: 03Z1MN11), BMBF(Meta-ZIK-BioLithoMorphie: 03Z1M511), and Volkswagen-Stiftung (Herstellung funktionaler Oberflächen: I/83 984)for the financial support to this work.

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2014.06.015.

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Ranjith Vellacheri is pursuing Ph.D. in theInstitute of Physics at Ilmenau University ofTechnology under the supervision of Prof.Yong Lei, Germany. His research interestincludes the preparation of 3D nanostruc-tures using alumina templates, synthesis ofvarious carbon materials and their applica-tions to the development of high perfor-mance energy storage and conversiondevices.

Ahmed Al-Haddad received his BachelorDegree in physics (2000) and Master'sDegree in Solid State Physics (2003) fromAl-Mustansiriyah University, Iraq. From 2006to 2010, he worked as a lecturer in Al-Mustansiriyah University, Iraq. He is now aPh.D. student in Ilmenau University of Tech-nology supervised by Prof. Yong Lei. Hiscurrent research is concentrating on thedevelopment of highly ordered nanostruc-

tures using anodic alumina membranes and their applications insolar cell and optoelectronic devices.

Huaping Zhao received his Ph.D. in MaterialScience from Shandong University in 2007.Then he worked as postdoc in Institute ofChemistry Chinese Academy of Sciences andUniversity of Muenster from 2007 to 2011.From 2012, he joined Ilmenau University ofTechnology as postdoc until now. His currentresearch is focused on the preparation ofthree-dimensional nanostructures and theirapplications in energy-related devices.

237High performance supercapacitor

Wenxin Wang received master degree in2011 from Harbin Engineering University,and then joined Prof. Dr. Yong Lei's groupas a Ph.D. candidate in Muenster University,Germany. From 2012 to now, he is proceed-ing Ph.D. program in Ilmenau University ofTechnology, Germany. His topic is aboutfabrication and application of multi-functional nanostructure.

Chengliang Wang was born in 1983. Hereceived his Bachelor degree in Chemistryfrom Nanjing University in 2005 and Ph.D.degree from the Institute of Chemistry, theChinese Academy of Sciences (CAS) in 2010.His research work includes the design andsynthesis of novel organic semiconductors,organic semiconductor nanostructures andtheir applications in optoelectronics as wellas energy storage and energy harvesting.

Yong Lei received his Bachelor degree inSun Yat-Sen University in 1991 and Ph.D.degree from Chinese Academy of Sciencesin 2001. After two years of postdoc researchin Singapore-MIT Alliance, he worked as anAlexander von Humboldt Fellow at Karls-ruhe Research Center in 2003–2006. Heworked in University of Muenster as a groupleader from 2006 and was promoted to a W1Professor in 2009. He joined Ilmenau Uni-

versity of Technology in 2011 as a W2 Professor. The currentresearch topics of his group are mainly focusing on template-realized functional nanostructures, 3D nano-structuring and surfacenano-patterning, energy-related and optoelectronic applications.Prof. Lei received a few prestigious funding including EuropeanResearch Council and BMBF (Federal Ministry of Education andResearch of Germany) ZIK projects.