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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 33 (2015) 89–93

http://d1369-80

n CorrE-m

journal homepage: www.elsevier.com/locate/mssp

Short Communication

Electrochemical investigation of graphene/nanoporouscarbon black for supercapacitors

Mahdi Robat Sarpoushi a,n, Mahdi Nasibi a,b, Masoud Moshrefifar c,Mohammad Mazloum-Ardakani d, Zaki Ahmad e,f, Hamid Reza Riazi g

a Technical Inspection Engineering Department, Petroleum University of Technology, Abadan, Iranb Health, Safety and Environment (HSE) Engineering Office, NIOPDC, Yazd Region, Yazd, 89167-84395, Iranc Materials and Mining Engineering Department, Yazd University, Yazd, Irand Department of Chemistry, Faculty of Science, Yazd University, Yazd 89195-741, Irane COMSATS University, Lahore, Pakistanf KFUPM, Dhahran, Saudi Arabiag Materials Science and Engineering Department, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e i n f o

Keywords:SupercapacitorNanoporous carbonGrapheneEnergyCapacitance

x.doi.org/10.1016/j.mssp.2015.01.03701/& 2015 Elsevier Ltd. All rights reserved.

esponding author. Tel: þ98 9155725460.ail address: Mehdi.sarpoushi@gmail.com (M

a b s t r a c t

In this paper, mixing effect of nanoporous carbon black (NCB) and graphene nanosheets(GNS) on surface morphology and electrochemical performance of prepared electrodes wereinvestigated. 80:10:10 (NCB:GNS:PTFE) prepared electrodes show a maximum specific capa-citance as high as 10.22 F g�1 in 3 M NaCl electrolyte. Addition of nanoporous carbon blackincreases outer to total charge stored (qn

O/qn

T) on the electrode from 0.024 to 0.037 whichconfirms the higher current response and higher voltage reversal at the end potentials.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon base materials due to their different allotropes(graphite, graphene, nanotubes, etc.), various microtextures(more or less ordered) owing to the degree of graphitization,and ability for exist under different forms (from powders tofibers, foams, fabrics and composites) represent a very attrac-tive material for electrochemical applications, especially forthe storage of energy. Apart from it, carbon materials areenvironment friendly [1]. Graphene consists of a 2D layers ofsp2 hybridized carbon atoms bonded together and the shapethat is resulted from a “honeycomb” lattice, notable for itshigh regularity. Graphene exhibits superior electrical conduc-tivity, high surface areas of over 2600 m2/g and a broadelectrochemical window [2].

Among different energy storage systems, supercapacitorsare recognized as highly attractive energy storage devices [3].High cycle life, high life time, high energy efficiency and high

. Robat Sarpoushi).

self-discharge rate are some of the characteristics of super-capacitors [4,5]. Depending on the charge-storage mechanisms,supercapacitors can be classified in three types: Electrochemicaldouble layer capacitors (EDLC), faradic pseudo-capacitors andhybrid capacitors [6,7]. EDLCs store the electric charge directlyacross the DL of the electrode [6]. Since no chemical action isinvolved, the effect is easily reversible with minimal degrada-tion in deep discharge or overcharge and the typical life cycle ishundreds of thousands of cycles [8]. In this paper, the doublelayer capacitance and pseudo-capacitance characteristics ofdifferent GNS/NCB electrodes were investigated. The aim ofthis work is to fabricate GNS/NCB electrodes using mechanicalpressing as a fast and easy method and characterizes the effectof sheet-like and nanoporous structures on charge storageability of the prepared electrodes.

2. Experimental

2.1. Materials

Graphene nanosheets (60 nm Flakes, multi-layered) withthe specific surface area of 15 m2/g and purity of 98.5% were

M. Robat Sarpoushi et al. / Materials Science in Semiconductor Processing 33 (2015) 89–9390

purchased from graphene supermarket and polytetrafluor-oethylene (o2 μm) from Aldrich company. Nanoporous(o10 nm in diameter) carbon black (NCB) micro-sized parti-cles (o2 μm) were purchased from Degussa, Germany. Allother chemicals used in this study were purchased fromMerck. In order to prepare the electrodes, the mixture cont-aining different wt% NCB, GNS and 10 wt% polytetrafluor-oethylene (PTFE) were mixed well in paste form in ethanol byultrasonic wave for about 30min. After drying the paste andpowdering, the prepared complex was pressed onto a 316 Lstainless steel plate (50 MPa) which served as a currentcollector (surface area was 1.4 cm2). Typical mass load of theelectrode material was 45 mg. The used electrolyte was3 M NaCl.

2.2. Characterization

The electrochemical behavior of prepared electrode wascharacterized using CV and EIS tests. Electrochemical mea-surements were performed using PGSTAT 302 N ModelAutolab (Netherlands). CV tests were performed within therange of �0.45 and þ0.3 V (vs. SCE), using scan rates of 10,20, 50, 100 and 200 mV s�1. EIS measurements were alsocarried out in frequency range of 100 kHz–0.01 Hz at open

Fig. 1. Scanning electron microscopy image obtained from

Fig. 2. XRD pattern obtain

circuit potential with an AC amplitude of 10 mV. The specificcapacitance can be estimated from the voltammetric chargesurrounded by the CV curve according to the followingformula [9,10]:

C ¼ qaþ qc��

��

2mΔVð1Þ

where qa, qc are the sum of anodic and cathodic voltam-metric charges on positive and negative sweeps, respectively,m is the mass of active material (regardless of mass of PTFE)and ΔV is the potential window of CV. The morphologies andstructures of the samples were observed by a scanningelectron microscope (TESCAN, USA).

3. Results and discussion

Specific surface area and conductivity are two importantparameters to prepare highly efficient electrodes for super-capacitors. Carbonaceous materials possess both high specificsurface area and high conductivity. But, only some parts ofthe surface are always accessible by electrolyte ions to beadsorbed, because the porosity influences the ions diffusion.3D porosity structure of electrode material will lead to highspecific surface area due to high ratio of volume to mass, andhigh surface area of electrode material which is achieved via

90:10:10 (a), 60:30:10 (b) and 0:90:10 (c) electrodes.

ed from graphene.

M. Robat Sarpoushi et al. / Materials Science in Semiconductor Processing 33 (2015) 89–93 91

porous structure of the primary used particles. As expected,SEM images obtained from the surface of 90:00:10 (GNS:NCB:PTFE) electrodes show only a flat and smooth surface onthe electrode (Fig. 1 (a)). Mixing the nanosheets with CBmacroparticles will have a significant effect on the morphol-ogy and nature of the prepared electrodes. Addition of NCBparticles into the electrode material increases distances bet-ween GNSs more and arrange them in different directions(Fig. 1 (b)). These changes will change the specific surfacearea, electrical resistance, diffusion characteristics and sub-sequently capacitance and reversibility of the preparedelectrode. Also the structure of NCB is shown in Fig. 1(c).To evaluate the distance between graphene nanosheets XRDtest carried out (Fig. 2). This distance was about 3.36 Å whichis a characteristic of multi-layered graphene.

One of the principal factors determining the powerspectrum of porous electrode is the resistivity of the electro-lyte, Re, in the pores and the extent of its physical contactwith maximum accessible area of the pores on the matrix. Inaddition minimum contact resistance, Rc, between particlesof electrode is desirable. This is a function of the used ma-terial for electrode fabrication. Diminution of contact resis-tance can be achieved by compression of the matrix, but thisusually diminishes the volume fraction of electrolyte, result-ing in a relative increase of Re. An optimized combination

Fig. 3. CV curves at 10 mV s�1 (a) and Nyquist diagrams (b) of differentelectrodes in 3 M NaCl electrolyte.

must be achieved [11]. In this research mechanical pressingwith pressure of 5�107 Pa was selected.

In Fig. 3(a), all CVs do not exhibit the parallelogram andrectangle-like shaped profile, which is a major character-istic of Faradaic process in an EDLC. As the carbon contentof electrode increases the capacitance versus potentialrelation will deviate from the classical square waveform,expected for a pure capacitor but maximum current andtotal capacitance are increased. This may be due to themore frequency dependence of NCB than GNS and thelarger pores of NCB than GNS.

Sheet-like structure of graphene containing electrodes ma-kes it easy for ions to adsorb on and desorb from the surfaceof the prepared electrodes. But, this sheet-like morphologydecreases the specific surface area of the electrodes and subs-equently the active sites on the surface of electrode. One ofthe precious parameters for graphene is low potential-dep-endence (as shown in Fig. 3(a)).

It may be concluded that addition of nanoporous to sheet-like materials simultaneously during electrode fabrication willhave two reverse effects on electrochemical properties of theprepared electrodes: increasing potential-dependence andincreasing specific surface area. From viewpoint of capacitanceand reversibility, 60:30:10 (CB:GNS:PTFE) electrode showslower current dependence among all proposed electrodes.

As shown in Fig. 3 (b), it could be obviously seen that allobtained impedance spectra almost have similar form, com-posed of one semicircle at high frequency end followed by anearly vertical line at lower frequencies. The radius of semic-ircle at the high frequency region reflects the impedance onelectrode/electrolyte interface [12]. Almost always, the practicalEDLC devices suffer from the high charge leakage (defined as aself-discharge) which results from potential-dependent chargetransfer reactions. These deviations from the ideal capacitivebehavior of EDLC are attributed mainly to ionic chemical/physical adsorption and diffusional impedance, incompletepolarization of the porous electrode, and Faradaic chargetransfer resistance caused by the voltage differential acrossthe electrode/electrolyte interface [13–15]. The equivalentcircuit of the NCB/GNS electrodes is given in Fig. 4. Proposed

Fig. 4. Equivalent circuit of NCB/GNS electrode in 3 M NaCl electrolyteat OCP.

Table 1Numerical values of the proposed equivalent circuit of prepared GNS/CBelectrodes.

Type of electrode Rs(Ω) Cdl (F/g) Rct(Ω) CF (F/g) Rl(Ω)

20:70:10 8.475 1.262 1.351 5.644 221.840:50:10 9.039 0.933 0.973 7.066 199.360:30:10 9.379 0.248 0.979 9.555 13780:10:10 9.342 0.092 1.556 10.533 156.8

Fig. 6. Capacitance vs. frequency curves obtained from different electro-des in 3 M NaCl electrolyte.

M. Robat Sarpoushi et al. / Materials Science in Semiconductor Processing 33 (2015) 89–9392

equivalent circuit contains the bulk solution resistance element,Rs, the double layer capacitance, Cdl, the charge transferresistance, Rct, the pseudocapacitance, Cp, and leakage resis-tance, Rl. Table 1 shows numerical values of the proposedequivalent circuit of prepared GNS/CB electrodes. The maindifferences of the electrodes are between Cdl and Cp values.Increasing carbon content will result in a sharp increasing in Cpand a sharp decreasing in Cdl. This was confirmed by thedeviation of CV curves from rectangular sharp.

In practice, some frequency dependence is commonlyobserved, i.e., the phase angle for the double layer capacitancemay not have the ideal value of 901 at all frequencies. Inpractical electrochemical capacitor devices, frequency depen-dence of the overall capacitance is generally observed and isdue, in addition, to the “porous electrode” effect, to couplingwith other equivalent series resistance (ESR) components [16].The efficiency of an electrochemical capacitor is related to theloss factor, dc [17]:

dc ¼ tan ð90�φÞ ð2Þ

where φ is the phase angle between real and imaginarycomponents of the impedance. Bode plots of the four electro-des is shown in Fig. 5. Considering Fig. 5, the qualitativebehavior of the 20:70:10 electrode is the best and theminimum loss factor of 0.378 and dissipated power of 0.362(φ¼68.96) is achieved at a frequency of 20 mHz (Table 2).Relaxation times are 13.90, 26.88, 37.31 and 51.81 s for20:70:10, 40:50:10, 60:30:10 and 80:10:10 electrodes,respectively.

The Bode plots shown in Fig. 6 describe the capacitance asa functions of the frequency. Specifically, the resistance at thehigh frequency range shows mainly the value of Relec, whilethe Faradaic leakage or charge transfer resistance from bulkelectrolytes (Rbulk) appears at the medium frequency rangebetween 10 Hz and 1 kHz. The low frequency range between1 Hz and 20mHz includes Rint caused by interfacial processes,and represents the summation of Relec, Rbulk and Rint. On theother hand, Fig. 6 demonstrates the frequency dependence of

Fig. 5. Bode plot obtained from different electrodes in 3 M NaCl electro-lyte at OCP.

Table 2Loss factor for prepared GNS/CB electrodes.

Type of electrode 20:70:10 40:50:10 60:30:10 80:10:10

Loss factor 0.378 0.395 0.515 0.490

the capacitance. The capacitance reaches its maximum at thelow frequency because the ions have sufficient time to reachto the pores. The value of capacitance at the lowest frequencyhence represents the overall capacitance [18].

As the scan rate increases (Fig. 7) the CV curves woulddeviate from the classical square waveform, expected for purecapacitance, due to a marked decrease in the accessible surfa-ce area at such a high scan rate and the resistance down thenano-sized pores [19], therefore, capacitance and accordinglythe energy delivered decreases dramatically. The efficiency isanother important parameter affecting the capacitance inthese high sweep rates [20]. It is clear that increasing thecarbon content will result in more current dependence ofpotential (compare Fig. 7(a) and Fig. 7(b)).

In order to gain quantitative information on utilization ofthe prepared electrodes, voltammograms were analyzed as afunction of scan rate, according to the procedure reported by

Fig. 7. CV curves obtained from 80:10:10 (a) and 20:70:10 (b) electrodesusing different scan rates in 3 M NaCl electrolyte.

Fig. 8. (a) Extrapolation of q to υ¼0 from the q�1 vs. υ0.5 plot given thetotal charge and (b) extrapolation of q to υ¼1 from the q vs. υ�0.5 plotgiven the outer charge for NCB/GNS electrodes.

M. Robat Sarpoushi et al. / Materials Science in Semiconductor Processing 33 (2015) 89–93 93

Ardizzone et al. [21]. Scan rate dependence of the capaci-tance can be related to the less accessible surface area(pores, cracks, etc.) which becomes excluded as the reactionrate is enhanced. In charge and discharge cycles, totalcharge can be written as the sum of an inner charge fromthe less accessible reaction sites and an outer charge fromthe more accessible reaction sites, i.e., qn

T ¼ qnI þqn

O, whereqnT, q

nI and qn

O are the total charge, charges related to theinner and the outer surface, respectively. Extrapolation of qn

to υ¼1 (υ�1/2¼0) from qn vs. υ�1/2 plot (Fig. 8(a)) givesthe outer charge qn

O, which is the charge on the mostaccessible active surface. In addition, extrapolation of qn toυ¼0 (υ1/2¼0) from the 1/qn vs. υ1/2 plot (Fig. 8(b)) gives thetotal charge qn

T, which is the charge related to the entireactive surface of the electrode. These electrodes show a lowratio of the outer charge to total charge (qn

O/qnT) of 0.024 and

0.037 in 20:70:10 and 80:10:10 electrodes, respectively,which confirms the low current response of the electrodesat high scan rates.

4. Conclusions

Addition of NCB particles into the electrode material incre-ases distances between GNSs more and arrange them indifferent directions which result in more accessible sites, thelarger specific surface area and subsequentlymore capacitance.Considering CV curves, 20:70:10, 40:50:10, 60:30:10 and80:10:10 electrodes showed a maximum capacitance of ashigh as 5.47, 6.89, 9.40 and 10.22 F g�1 respectively, in 3MNaCl electrolyte which demonstrate the effect of NCB in incre-asing of capacitance. Although increasing the carbon contentincreased capacitance significantly but this method resulting inmore current dependence of potential. The 20:70:10 electrodeshowed a higher charge separation capability at electrolyte/electrode interface and a lower ratio of outer to total charge(qn

O/qnT) of 0.024 which confirms the lower current response at

each end potential.

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