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Available online at www.sciencedirect.com
http://dx.doi.org/0272-8842/& 20
nCorrespondinE-mail addre
[email protected]@nPoiSim.Khiew@
1 (2015) 715–724
Ceramics International 4 www.elsevier.com/locate/ceramintOne-step green synthesis of graphene/ZnO nanocomposites forelectrochemical capacitors
Ejikeme Raphael Ezeigwea,n, Michelle T.T. Tana, Poi Sim Khiewa, Chiu Wee Siongb
aDivision of Materials, Mechanics and Structures, Center of Nanotechnology and Advanced Materials, Faculty of Engineering,University of Nottingham, Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia
bLow Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received 13 July 2014; received in revised form 28 August 2014; accepted 29 August 2014Available online 22 September 2014
Abstract
A facile, green and efficient approach consisted of a novel liquid phase exfoliation and solvothermal synthesis method has been adopted toprepare graphene/ZnO nanocomposites as electrode material for electrochemical capacitors. Highly pristine graphene was produced from mildsonication treatment of raw highly pyrolytic graphite in a solution at a proper ratio of ethanol and water. The X-ray diffraction (XRD) analysisrevealed the formation of pure ZnO structure from the zinc nitrate hexahydrate precursor during solvothermal synthesis. The surfacecharacteristics and elemental composition of the nanocomposites have been studied by means of field emission scanning electron microscopy(FESEM), energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM). The electrochemical properties of the graphene/ZnO nanocomposites were examined by cyclic voltammetry (CV), galvanostatic charge–discharge tests and electrochemical impedancespectroscopy (EIS). The graphene/ZnO nanocomposites displayed an improved capacitive performance of 236 F/g at a scan rate of 10 mV/s,excellent cyclic performance, and an average energy density of 11.80 W h/kg. The improved electrochemical performance of the nanocompositescan be ascribed to the high electrical conductivity of the synthesized pristine graphene and the good electroactive property of ZnO.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Green approach; Pristine graphene; Solvothermal synthesis; Graphene/ZnO nanocomposites; Electrochemical capacitor
1. Introduction
There are concerns over the limited availability of fossilfuels and its association with global warming and environ-mental pollution. This growing interest has generated the needfor the advancement of renewable energy sources along withnew technologies related to effective, facile, and environmentalfriendly method for energy conversion and storage [1–3].Energy conversion and storage systems may embrace thetechnology and systems of an external thermal interface orthat of an external electrical interface [4] and they are
10.1016/j.ceramint.2014.08.12814 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
g author. Tel.: þ60 16 6707692.sses: [email protected],gham.edu.my (E.R. Ezeigwe),ottingham.edu.my (M.T.T. Tan),nottingham.edu.my (P.S. Khiew).
categorised into groups by the amount of energy and poweravailable for the load [5] which includes batteries, fuel cells,capacitors and supercapacitors [1,5].Supercapacitors also known as electrochemical capacitors
have been a subject to many applications, research and devel-opment due to its high power density, environmental friendliness,long shelf life, long life cycle [3,4] and it bridges the energy gapbetween capacitors (high power output) and fuel cells/batteries(high energy storage) [1,2]. Much research and developmenthave been carried out to improve the overall performance ofsupercapacitors via the use of carbonaceous compound, metaloxides, conducting polymers and the associated hybrids/ com-posites as promising electrode materials [1–3,6,7].Graphene is highly anticipated to be an excellent electrode
material due to its notable characteristics such as high surface areato volume ratio, good electrical conductivity, good flexibility,fast electron mobility and good thermal and electrochemical
E.R. Ezeigwe et al. / Ceramics International 41 (2015) 715–724716
properties [8–11]. The introduction of graphene and metal oxidecomposites has demonstrated a better energy density, powerdensity and capacitance for supercapacitors [3,8,12]. The syner-getic effect between the metal oxide and graphene clearlyindicates that metal oxides play the roles of spaces to expandthe surface area of graphene and improves capacitance as a resultof the redox reaction while graphene reduces the resistance byproviding a conductive pathway for the easy diffusion of ions andmovement of electrons [8]. Among the many metal oxidesavailable, zinc oxide (ZnO) has attracted much attention as apromising electrode material for supercapacitors due to its lowcost, abundant availability, environmental friendly nature andelectrochemical activity [13]. ZnO is a battery active materialwhich attains an energy density of 650 A/g. It has a large excitonbinding energy (60 meV) and a wide band-gap (3.37 eV) [14–16].In addition, it has strong emission of light at room temperatureand it is transparent in nature [15].
In view of the outstanding individual properties of grapheneand ZnO as electrode materials, the combination of graphenewith ZnO nanoparticles as electrodes may result in improvedsupercapacitor performance. Several methods employed inpreparing graphene–ZnO nanocomposites for supercapacitorshave been reported. Lu et al. [17] prepared graphene–ZnOnanocomposites via a microwave-assisted system, where thecomposite compound exhibited an improved electrochemicalcapacitance of 146 F/g at a scan rate of 2 mV/s. Li et al. [18]reported a hydrothermal technique to grow ZnO nanorods thatwere distributed homogeneously on the graphene nanosheets.The nanocomposite materials were found to possess a specificcapacitance of 156 F/g at a scan rate of 5 mV/s. Zhang et al. [19]synthesised graphene–ZnO composites films for supercapacitorsapplication by depositing ZnO on graphene by ultrasonic spraypyrolysis (USP). They reported that graphene/ZnO compositeshad demonstrated a better reversible capacitive process thangraphene material alone. Wang et al. [3] reported a successfulsynthesis of graphene nanosheets/ZnO composites prepared by anin situ crystallization method which displayed an improvedcapacitance of 62 F/g at a current density of 5 mA/cm2. However,most of these studies had utilised graphene nanomaterialssynthesised via the modified Hummers method, followed bychemical reduction producing the reduced graphene oxide (rGO).By using this conventional method, the graphene produced areless pristine due to the used of harsh acids in the synthesisprocesses, resulting in many oxygenated groups formed on thegraphene surface [20,21]. The presence of these oxygenatedgroups results in significant compositional and structural defectswhich affects its electrical properties. Though the reductionprocess is aimed at removing the oxygen moieties, literatureshave indicated that in general, this chemical reduction processesare incomplete and that the oxygenated groups cannot becompletely removed resulting in less superior electrical propertiesor greater crystal defects when compared to the pristine graphene[20–24].
Consequently, the development of a facile green approachfor synthesising high-quality graphene is essential and crucialfor practical sustainability. Therefore in this work; a novelgreen synthesis method is used to produce the graphene
material. This method involves the liquid phase exfoliationof graphite through the sonication of raw highly pyrolyticgraphite flakes in an optimum ratio of ethanol and de-ionizedwater, as reported in our previous work [25]. Such methoddoes not require strong oxidizing agents (potassium perman-ganate, sodium nitrate and concentrated sulphuric acid) andreducing agents (hydrazine) thereby making the process simpleand fast. The use of a common reagent (ethanol) as theexfoliation medium which has a low boiling point of 78.37 1Cfor the synthesis of graphene makes the synthesis processrelatively safe and preserves the pristine structure of graphenematerial.Our composite synthesis method incorporates such simple
and green method of synthesising graphene–ZnO nanocompo-site material using facile solvent exfoliation and low tempera-ture solvothermal processing approaches. It is our aim to fullyharvest the desired high electrical conductivity of graphene andthe good electrocatalytic property of ZnO in the nanocompo-sites material. In this work, the surface characteristics, struc-tural and electrochemical properties of the graphene/ZnOnanocomposites as an electrode material for supercapacitorswere evaluated thoroughly.
2. Experimental
2.1. Materials
Commercially available raw highly pyrolytic graphite (HOPG)flakes (99% carbon purity) were purchased from Bay Carbon,zinc nitrate hexahydrate Zn(NO3)2 � 6H2O (reagent grade 98%)were bought from Sigma Aldrich. Activated carbon (pCH-0020) was purchased from RHE resources, ethanol was obtainedfrom Merck and sodium hydroxide (NaOH) was purchased fromR&M Chemicals. All chemicals were used as received withoutfurther purification. Deionized water (DI) from the Milliporesystem was used throughout the experiment.
2.2. Synthesis of graphene
The graphene nanomaterial was prepared via a facile greensolvothermal synthesis of graphene and ZnO as illustrated inFig. 1. Graphite was used as the starting reagent for synthesisof graphene via liquid phase exfoliation method [25]. Typi-cally graphite (50 mg) material was dispersed in 100 mlsolution of ethanol and deionized water (2:3 ratio) andsonication treatment was carried out in a conventional ultra-sonic bath with working frequency of 50/60 Hz at roomtemperature for 3 h to form a darkish black suspension. Thesolution was centrifuged under 1000 rpm for 30 min to removeaggregates and the remaining residue was dried at a tempera-ture of 80 1C for 10 h.
2.3. Preparation of graphene–ZnO composites
Graphene (0.2 g) nanomaterial was then re-dispersed in50 ml solution of ethanol and deionized water (2:3) undersonication for 30 min. Zinc nitrate hexahydrate was added to
Fig. 1. The schematic diagram of graphene–ZnO composites prepared via a facile green solvothermal synthesis. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)
Table 1Different mass ratios of graphene to zinc precursor used to form graphene–ZnO nanocomposites.
Name ofsample
Mass ofgraphene (g)
Mass of zinc nitratehexahydrate (g)
Weight ratio ofgraphene: zinc precursor
GZn1 0.2 0.2 1:1GZn2 0.2 0.4 1:2GZn3 0.2 1.6 1:8GZn4 0.2 3.2 1:16
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the graphene solution and stirred for 15 min to produce auniform dispersion. Diluted sodium hydroxide solution wasthen added to the mixture till a pH value of 12 is obtained andwas stirred for 30 min. The mixture was transferred to a 50 mlTeflon stainless steel autoclave and put in an oven for 10 h at90 1C. The solid precipitation was isolated from the solutionby centrifugation, washed repeatedly with excess water andethanol, respectively and dried overnight at 70 1C in an oven.
The addition of NaOH solution in the mixture generates anintermediate hydroxide (Zn(OH)2) colloids upon reaction withZn(NO3)2 � 6H2O as stated in Eq. (1), part of the Zn(OH)2colloids dissolves into Zn2þ and OH� during the hydrother-mal process according to Eq. (2). ZnO nuclei are formed whenthe concentration of Zn2þ and OH� reaches the supersatura-tion degree of ZnO according to Eq. (3). The possible reactionsprocess can be expressed in the equations as follows:
Zn(NO3)2 � 6H2Oþ2NaOH¼Zn(OH)2 (gel)þ2NaNO3þ6H2O (1)
Zn(OH) (gel)þ2H2O¼Zn2þ2OH�
þ2H2O¼Zn(OH)42�þ2Hþ (2)
Zn(OH)42¼ZnOþH2Oþ2OH� (3)
Table 1 shows detailed information of the different ratios ofgraphene to zinc nitrate hexahydrate which were used toprepare the graphene–ZnO nanocomposites.
2.4. Characterization
The crystalline structure of the materials were analysed byan X-ray diffractometer (XRD) (Philip XRD) operated at33 mA and 45 kV with Cu Kα radiation (λ¼1.54056 Å) inthe range of 101 to 701 with step size 0.031. The surfacecharacteristics and elemental composition of the compositeswere analysed using a scanning electron microscope (SEM)
(FEI Quanta-400 FESEM). The morphology of the grapheneand the composites were evaluated by a transmission electronmicroscope (TEM) (JEOL JEM-2100F) at an operating voltageof 200 kV. The Raman spectra on the powdered samples ofgraphene and graphene–ZnO composites were recorded on aRenishaw inVia Raman microscope at room temperature. Thesystem is equipped with a CCD detector and a holographicnotch filter, using a 514 nm diode laser excitation source withpower below 0.5 mW.
2.5. Preparation of electrodes and electrochemicalcharacterization
To evaluate the electrochemical properties of the compo-sites, the electrodes were prepared by mixing the electroactivematerials (graphene–ZnO composites), activated carbon andpolyvinylidene fluoride (PVDF) in a mass ratio of 70:20:10and dispersed in N-methyl-2-pyrrolidinone (NMP). The resul-tant mixture was coated on an aluminium foil and vacuumeddried at 70 1C for 7 h. The electrochemical capacitor cell wasassembled by sandwiching two symmetric electrodes separatedby a thin filter paper in 1 M KOH electrolyte. The electrodeswere connected in a two-electrode configuration for thegalvanostatic charge/discharge analysis to an Arbin Instrument
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(BT-2000) using the MITS Pro 4.0512.12 software. The cyclicvoltammetry and electrochemical impedance spectroscopy(EIS) were also studied in a two-electrode configuration usinga VersaSTAT 3 (VE-400) electrochemical working stationdriven by the V3 studio version 1.0286 software.
3. Results and discussion
The XRD patterns of the prepared graphene–ZnO compo-sites, graphite and zinc oxide are shown in Fig. 2. It can beseen that the ZnO XRD patterns of the nanocomposites withdifferent mass ratios are similar to that of pure zinc oxide,indicating that no other impurity peaks were detected. It can bereadily assigned to pure ZnO with hexagonal structure (ICSDNo. 98-002-7781) with lattice constants a¼b¼3.2540 Å,c¼5.2150 Å. The peaks at 2θ values of 32.0, 34.6, 36.5,47.7, 56.8, 63.0, 66.6 and 68.11 can be indexed to (1 0 2 ),
Fig. 2. XRD patterns of graphite, zinc oxid
Fig. 3. Raman spectra of graphene and
(1 0 0), (0 1 2), (0 1 �2), (0 1 4), (2 �2 1), (2 0 1) and(2 �2 3) crystal planes, respectively. This clearly indicatedthat ZnO nanoparticles were formed from the precursor. Inaddition, with the increase in mass loading of ZnO, theincrease in intensities of the respective diffraction peaks ofthe composites were also observed. The two diffraction peaksof graphite at 2θ values of 26.8 and 54.91 are attributed to the(0 0 2) and (1 �2 0) reflections of graphite (ICSD No. 98-005-2916), and these diffraction maximums were clearly observedfor the graphene–ZnO nanocomposites. These results indicatethat the crystalline structure of the graphene material was intactand its pristine nature was preserved.The Raman spectra of graphene and graphene/ZnO nano-
composites respectively are shown in Fig. 3. Three distinctpeaks are associated with the graphene Raman spectra; namelythe G band at �1580 cm�1 which corresponds to the in-planevibration of sp2 carbon atoms, the 2D band at �2719 cm�1
e and graphene–ZnO composites.
graphene–ZnO composites.
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and the D band at �1351 cm�1 ascribed to the first-order zoneboundary phonons, which is absent from defect-free graphene [26].However, the formation of new edges which can be seen as defectsfrom the exfoliated sheets due to the exfoliation process ofgraphene from graphite leads to the existences of a D peak[25,27,28]. After the hydrothermal treatment, four peaks indicatedas ‘X’ in the graphene/ZnO spectrum belongs to ZnO. The peak at�432 cm�1 agrees with the finger signal of the characteristic E2mode of ZnO wurtzite structure [29], while the peaks at �327cm�1 and �568 cm�1 are well indexed to the transversal opticalmodes with A1 symmetry and the longitudinal optical modes. Thebroad peak at �1127 cm�1 corresponds to 2A1 (LO), 2E1(LO)and 2LO mode of ZnO [26,29]. The D-band, G-band and 2D-bandwere located at �1349 cm�1, �1574 cm�1 and �2717 cm�1;the shift of the bands are likely due to the doping effects of ZnO[30]. The Raman spectrum after the hydrothermal treatmentconfirms that the structure of graphene was not destroyed andthe graphene/ZnO composites were formed, which is in goodagreement with the XRD results (Fig. 2).
Fig. 4a–d shows the FESEM images of graphene–ZnOcomposites synthesised from different precursor mass ratios.The ZnO nanoparticles formed on the graphene sheets werefound to be of spherical to globular structures with diameter
Fig. 4. FESEM images of (a) GZn1, (b) GZn2, (c) GZn3, (d) GZn4, TEM images of (e) g
around 1500 nm for GZn1 and GZn2 (Fig. 4a and b) and 200 nmfor GZn3 and GZn4 (Fig. 4c and d), respectively. The resultsillustrated that with an increase in the loading mass of Znprecursor, more ZnO nanoparticles will be formed on the surfaceand edges of the graphene sheets. It was also observed that inGZn1 and GZn2, the ZnO nanoparticles were not uniformlyspread on the graphene surface. On the other hand, heavy clusters(agglomerated form) of ZnO nanoparticles were found on thegraphene surface of GZn4, and these aggregates tend to formnon-conducting layers that can affect its charge storage property[31]. Fig. 4c shows that a uniform and homogenous spread ofZnO nanoparticles were anchored well on the edges and surfaceof the graphene sheet. The evidence of the formation of ZnOnanoparticles deposited on the surface of graphene is supplemen-ted by images provided by TEM analysis (Fig. 4e and f). TheTEM results display the two dimensional structure of grapheneand a well spread graphene sheet decorated with ZnO. Fig. 4fillustrates that the ZnO nanoparticles were quasi-spherical inshape and are well deposited on the inside interlayer of thegraphene sheet. No free ZnO nanoparticles were observed outsidethe graphene sheet.The chemical composition of the composites were analysed
by an energy dispersive spectroscopy (EDS). The peaks of C,
raphene (f) GZn3 (g) the EDS spectrum and its corresponding SEM image of GZn3.
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O, Zn in the EDS spectrum depict the presence of carbon,oxygen and zinc compounds only, therefore confirming thepurity of the composites material (Fig. 4g). The peak corre-sponding to Si in the EDS spectrum is due to the siliconsubstrate used for the characterization.
The cyclic voltammograms (CV) tests were carried outwithin a potential range of �0.1 to 0.5 V at different scan ratesvarying from 5 to 100 mV/s using 1 M KOH electrolyte.Fig. 5a compares the CV curves of graphene, GZn1, GZn2,GZn3 and GZn4 at a scan rate of 10 mV/s. It can be seen thatall the CV curves exhibited a nearly rectangular shape alongthe current–potential axis indicating good capacitive perfor-mance. In comparison to the graphene electrode, the graphene–ZnO composites electrode showed higher integrated area onthe current–potential axis, disclosing better charge storageperformance. This clearly indicates that the addition of ZnOincreases the total capacitance of the electrode. The specificcapacitance (Cs) can be calculated from the CV curves byEq. (4) below [32]:
Csð Þ ¼ 2Is� m
ð4Þ
where I, s and m are the charge current, scan rate and mass ofthe single electrode respectively. Based on Eq. (4), thecalculated Cs values for these electrochemical capacitors were:236 F/g (GZn3)4230 F/g (GZn2)4188 (GZn4)4185(GZn1)4100 F/g (graphene) at a scan rate of 10 mV/s. Fromthe calculated results, it is noticed that the increase in massloading of ZnO nanoparticles was not proportional to theimprovement in Cs of the electrochemical capacitors. SampleGZn4 demonstrated lower specific capacitance as compared toGZn3 and GZn2, which can be attributed to excess agglomerations
Fig. 5. Cyclic voltammograms (CV) curves of (a) GZn1, GZn2, GZn3, GZn4 and gr5–100 mV/s (c) variation of specific capacitance of GZn3 at different scan rates.
of ZnO nanoparticles on the graphene surface (Fig. 4d). Theparticle aggregation tends to form non-conducting layersthat can increase its intrinsic resistivity and deteriorate thecharge storage capacity. In addition, these non-conductinglayers on the graphene surface reduces the amount ofsurface area accessible to the electrolyte ions thereby limit-ing its capacitive performance [31]. The CV analyses onGZn3 at various scan rates were performed in order toobtain further information on its capacitive performance(Fig. 5b). The CV curves for the composite materialexhibited a nearly rectangular shape within a potentialwindow of �0.1 V to 0.5 V signifying good capacitiveperformance. Generally, it was noticed that an increase scanrate produced an increase in integrated area on the current–potential axis, indicating good storage rate ability, which issimilar to previous work reported [18,32,33]. Nevertheless,as clearly indicated in Fig. 5c, a decrease in specificcapacitance with increase in scan rate is observed instead.At a scan rate of 5 mV/s, a specific capacitance of 246 F/gwas achieved while at higher scan rate of 100 mV/s therecorded specific capacitance was significantly reduced to112 F/g. The results clearly implies that lower scan ratesgenerally is more favourable, since it allows adequate timefor ion adsorption and diffusion within the intrinsic porestructures of the active material, leading to a higher chargestorage capacity [33].The long cycling life is an important requirement for
practical applications of supercapacitors [18]. A cycling lifetest of graphene–ZnO composite electrode (GZn3) was carriedout by repeating the CV measurements between �0.1 to 0.5 Vat a scan rate of 50 mV/s for 200 cycles. As shown in Fig. 6a,the respective CV curves retained their quasi-rectangular shapes
aphene at a scan rate of 10 mV/s (b) GZn3 at different scan rates in the range of
Fig. 6. Cyclic voltammograms (CV) curves of (a) GZn3 at a scan rate of 50 mV/s for 200 cycles (b) cyclic performance of GZn3 electrode along the 200 cycles.
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throughout the whole cycling test. Furthermore, it was observedthat more than 90% of its initial specific capacitance was retainedafter 200 cycles, indicating good electrochemical stability duringcycling and clearly implies its potential practical usage in variousapplications (Fig. 6b).
The galvanostatic charge–discharge tests were performed byapplying a series of charging and discharge currents onto theelectrochemical capacitors and measuring the voltage response.The voltage response of graphene, GZn1, GZn2, GZn3 andGZn4 electrodes were studied at a constant current density of10 mA/g. The energy loss due to internal or equivalent seriesresistance is shown by the IR drop at the turning point of thecharge and discharge curves, as seen in Fig. 7a [34]. Fromthese results, GZn3 sample has the least internal resistance,thereby having the highest capacitance when evaluated withrespect to their individual mass. These findings supports theresults of the calculated specific capacitances obtained fromthe CV curves earlier (Fig. 5). The IR drop observed in thecharge–discharge curve may be due to the internal resistanceof the electrodes. Fig. 7b shows the representative galvano-static charge–discharge measurements of the GZn3 electrode atdifferent current densities of 1, 2, 3, 5 and 10 mA/g under anapplied potential between 0 and 1 V. The charge–dischargecurves of the GZn3 electrode exhibits near linear andsymmetric triangles, indicating good capacitive performanceand excellent reversibility [35,36]. The IR drops on all thecurves are negligible, implying low resistance and goodcontact between the collectors and the electrode material.The discharge curves were not straight lines suggesting thatthe capacitance includes the double-layer and redox capaci-tance [36]. The specific capacitance of graphene, GZn1, GZn2,GZn3 and GZn4 at different current densities were comparedas shown in Fig. 7c. It can be observed that with increase ofcurrent densities from 1 to 10 mA/g, there is a correspondingdecrease in specific capacitance of the electrode materials. Thiscould be explained by the discrepant insertion–desertionbehaviour of ions from electrolyte to electrode material. Therending ions (OH�) fail to fully occupy the sites at theelectrolyte/electrode interface if compared to lower currentdensity because of ions’ limited migration velocity and fixedroute in the interface thereby leading to an uncompletedinsertion reaction [37]. GZn3 electrode exhibited a drop of
20.8% from its initial capacitance value as the current densityincreased from 1 to 10 mA/g compared to GZn2, GZn4, GZn1and graphene which had a drop of 22.7%, 27%, 29% and 60%capacitance values respectively. In order to further evaluate thecycling stability of GZn3 electrode, galvanostatic charge–discharge analysis was performed at a constant current densityof 10 mA/g between 0 to 1 V for consecutive 1000 cycles. Asdisplayed in Fig. 7d, the specific capacitance of GZn3electrode retains 83% of its initial capacitance value, reflectinggood electrochemical stability and reversibility upon repetitivecharge–discharge process [35].Electrochemical impedance spectroscopy (EIS) measurements
are very essential in assessing the resistive characteristics of theelectrode. It shows the response of components performance inthe frequency domain [34]. EIS were carried out in a frequencyrange from 100 kHz to 0.01 Hz in order to evaluate the frequencyresponse of both graphene and graphene/ZnO nanocomposites.The Nyquist plots for the electrodes are depicted in Fig. 8. The EISdata was analysed using Nyquist plots and each data point is at adifferent frequency. The Nyquist plots consists of two frequencyregions, a high frequency region denoted by a semicircle whichrepresents the transfer of charges occurring at the electrode/electrolyte interface and the low frequency region signified by astraight line representing the diffusion of ions in the electrolyte.The equivalent series resistance (ESR) can be obtained from the x-intercept of the Nyquist plot and the charge transfer resistance Rctcan be directly measured as the diameter of the semicircle arc onthe real axis [3,34,38]. The ESR and charge transfer resistance Rctobtained from the Nyquist plot (Fig. 8) are depicted in Table 2.The ESR and Rct values of the electrochemical capacitors
decreases in the order of graphene4GZn14GZn24GZn44GZn3 which is indirectly proportional to results of thespecific capacitance, specifically, the higher the Rct value, thelower the specific capacitance of the electrochemical capacitor[39].Graphene having the highest ESR and Rct values affirmsthat the addition of ZnO to graphene results in an improvedcharge transfer performance. From Table 2, GZn3 electro-chemical capacitor has the smallest ESR and this could be dueto the fact that the graphene/ZnO hybrid facilitates the highestaccessibility of ions into the surface of graphene sheets madepossible by the uniform dispersion of ZnO particles on thegraphene surface, as evidenced in the SEM analysis (Fig. 4c).
Fig. 7. Charge–discharge curves of (a) graphene, GZn1, GZn2, GZn3 and GZn4 (b) for GZn3 at a current density of 10 mA (b) charge–discharge curves of GZn3 atdifferent current densities (1, 2, 3, 5 and 10 mA/g) (c) Plots of specific capacitance of graphene, GZn1, GZn2, GZn3 and GZn4 as a function of current densities (d)Cycling test for GZn3 at the current density of 10 mA/g up to 1000 cycles.
Fig. 8. Nyquist plots for (a) GZn1, GZn2, GZn3, GZn4 (b) graphene.
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The average energy density (E) and power density (P) of theelectrochemical capacitors were calculated using the followingequations [18],
E¼ 1000 Cs ∇ V2
2� 3600ð5Þ
P¼ E � 3600Δt
ð6Þ
where E is the energy density of the electrode (W h/kg), P isthe power density (kW/kg), Cs is the specific capacitance of theelectrochemical capacitor (F/g), Δt in seconds is the time for asweep segment and ΔV is the voltage range for one sweepsegment. The calculated average energy and power densitiesare tabulated in Table 2.On the basis of the results provided by Table 2, the
graphene/ZnO nanocomposites electrodes showed a higherspecific capacitance, improved charge transfer performance
Table 2Electrochemical performance of graphene and graphene/ZnO composite electrodes.
Samples ESR (Ω) Rct (Ω) Specific capacitance (F/g) Energy density (W h/kg) Power density (kW/kg)
Graphene 4.34 2.31 100 5.00 18GZn1 1.49 0.77 185 9.24 33.26GZn2 1.04 0.61 230 11.52 41.47GZn3 0.94 0.59 236 11.80 42.48GZn4 1.14 0.91 188 9.40 33.84
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and a significantly enhanced energy and power densities whencompared to the graphene electrode. The resulting specificcapacitance of the graphene/ZnO nanocomposites is signifi-cantly higher than previous work reported on graphene/ZnObased supercapacitor [3,17,18,33]. The improved electroche-mical performance of graphene/ZnO electrodes can be attrib-uted to the following aspects: (i) the pristine nature of thesynthesized graphene which improves the electrical conduc-tivity of the hybrid structure. (ii) The electroactive property ofZnO anchored to the graphene sheets provides a threedimensional conductive system which provides more activesites for the formation of electric double layer. This distinctivesystem also aids the easy diffusion of ions into the innerchannels hereby reducing diffusion resistance, similar to thosereported in previous studies [31]. (iii) The pseudo-capacitancefrom the ZnO combined with the double-layer capacitancefrom graphene provides a higher specific capacitance [19,40].
4. Conclusions
In summary, graphene/ZnO nanocomposites as an electrodefor electrochemical capacitors was successfully synthesizedfrom highly pyrolytic graphite via a green, facile, effective andscalable solvothermal technique. The structural analysis andelectrochemical properties of graphene/ZnO nanoparticles havebeen studied comprehensively. In comparison with the pristinegraphene electrode, the graphene/ZnO nanocomposite electro-des demonstrated an improved electrochemical performance.The graphene/ZnO nanocomposite electrode material with aweight ratio of 1:8 (graphene: ZnO) displayed the highestspecific capacitance of 236 F/g at a scan rate of 10 mV/s withenergy and power densities of 11.80 W h/kg and 42.48 kW/kg,respectively. This simple, facile and green synthesis methodcan be extended for other graphene/metal oxide nanocompo-sites based high performance electrochemical capacitor.
Acknowledgements
Financial support from the Ministry of Science, Technologyand Innovation research grant (E-Science code: 04-02-12-SF0198), Early Career Research and Knowledge TransferScheme Award (ECKRT: A2RHM), HIR-Chancellory UM.C/625/1/HIR/079) and HIR-MOHE (UM.C/625/HIR/MOHE/SC/06) and The University of Nottingham are gratefullyacknowledged. The authors also express profound gratitudeto Mr Anandan Shanmugam for access to the supercapacitorunit and its analytical equipment. The assistance of Ms Low
Sze Shin and Ms Joanna Su Yuin Chia are gratefully acknowl-edged for the preparation of the SEM samples.
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