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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 24 (2014) 260–264

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Short Communication

Effect of zirconium oxide nanoparticles on surfacemorphology and energy storage of electrochemical capacitors

Mahdi Nasibi a,b,n, Mahdi R. Sarpoushi a, Rouhallah Hesan c,Mohammad Ali Golozar d, Masoud Moshrefifar c

a Technical Inspection Engineering Department, Petroleum University of Technology, Abadan, Iranb Health and Safety Engineering (HSE) Office, NIOPDC, Yazd Region, Yazd, IrancMaterials and Mining Engineering Department, Yazd University, Yazd, Irand Materials Science and Engineering Department, Isfahan University of Technology, Isfahan, Iran

a r t i c l e i n f o

Keywords:SupercapacitorsNanomaterialsMorphologyZrO2

x.doi.org/10.1016/j.mssp.2014.03.03701/Crown Copyright & 2014 Published by E

esponding author. Tel.: þ98 9113708480; faail address: mahdi.nasibi@gmail.com (M. Na

a b s t r a c t

In this study, the effect of mixing zirconium oxide nanoparticles and carbon black particleson surface morphology and electrochemical performance of prepared electrodes wereinvestigated. Scanning electron microscopy was used to characterize microstructure andnature of nanocomposites. Charge stored (q) on different nanoparticle containingelectrodes was calculated and the effect of surface morphology on charge storage wasdiscussed. It is concluded that charge stored on the electrode shows an n-like change byincreasing nanoparticle content of electrodes. Addition of nanoparticle increases qnO/qnT(from 0.05 to 0.18) which confirms the higher current response and higher voltagereversal at the end potentials.

Crown Copyright & 2014 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Electrochemical capacitors (ECs), with a combination ofhigh power density and high energy density, can be usedas a complementary energy-storage device along with aprimary power source, such as a battery or a fuel cell, forpower enhancement in short pulse applications [1]. Highcycle life, high energy efficiency and high self-dischargerate are some of the supercapacitors characteristics [2,3].Today, many laboratories are actively engaged in develop-ment of well-known type of supercapacitors, viz., electro-chemical double-layer, pseudo and hybrid supercapacitors,and most research has been focused on development ofdifferent electrode materials [4,5]. For practical applica-tions, an EC must fulfill the following technical require-ments: high specific capacitance, long cycle life and

lsevier Ltd. All rights reserv

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high charge/discharge rate. Today, using nanoparticles isof interest in order to improve these parameters. So,nanoparticles distribution quality on the electrode surfaceis of most important parameters [6]. In our previous work,we investigated the effect of different mixing processes ofelectrode material on dispersion quality of nanoparticleswhich change their electrochemical performance. In thiswork, we investigate the effect of nanoparticle contents ofthe electrode material on microstructure and nature ofprepared electrodes using scanning electron microscopy,and potentiodynamic polarization techniques. At theend, quantitative measurements were reported for furtherinvestigations.

2. Experimental

2.1. Materials

High purity (499%) nano-sized zirconium oxide (ZrO2)particles (o100 nm), nickel foil (99.99% with 0.125 mm

ed.

M. Nasibi et al. / Materials Science in Semiconductor Processing 24 (2014) 260–264 261

thickness) and polytetrafluoroethylene (o2 μm) werepurchased from Aldrich, USA. All other chemicals used inthis study were purchased from Merck, Germany. Carbonblack particles (o2 μm) were purchased from Degussa,Germany. In order to prepare electrodes, the mixturecontaining different wt% ZrO2 and carbon black (CB) and10 wt% polytetrafluoroethylene (PTFE) was well mixed inethanol to form a paste and then was pressed onto thenickel foil (25 MPa), which served as a current collector(surface was 0.785 cm2). The typical mass weight of

Fig. 1. (a–d) Schematic illustration of the surface changes by addition ofnanoparticles into the electrode material and, (e) SEM image obtainedfrom 30:60:10 electrode. (For interpretation of the references to color inthis figure, the reader is referred to the web version of this article.)

electrode material was 30 mg. The used electrolyte was2 M KCl.

2.2. Characterization

The electrochemical measurements were performedusing an Autolab (Netherlands) potentiostat ModelPGSTAT 302N. CV tests were conducted at various scanrates (s) with recording of potential response currents, I,which is related by C¼ I/s where C is the capacitance of theelectrode interface. The specific capacitance C (Fg�1) of theactive material was determined by integrating either theoxidative or reductive parts of the cyclic voltammogramcurve to obtain voltammetric charge Q (C). This charge wasdivided by mass of active material m (g) in the electrodeand width of the potential window of the cyclic voltam-mogram ΔE (V), i.e., C¼Q/(ΔEm) [7]. The morphology andnature of the prepared electrodes were studied usingscanning electron microscopy (TESCAN, USA).

3. Results and discussion

Nanoparticles distribution quality on the electrodesurface is one of the key parameters which controls theelectrical performance of nanoparticle containing electro-des for supercapacitors. Using macroparticles like usedcarbon black particles which store electrical energythrough the double layer mechanism, will make macro-pores and macrogrooves with deep and hollow shapes onsurface of the electrode. Using the nanoparticles will makenanopores with shallow shapes. Therefore, mixing thenanoparticles with macroparticles will have a significanteffect on the morphology and nature of the preparedelectrodes. One of the thinkable morphology changesby mixing of the nanoparticles with macroparticles isschematically illustrated in Fig. 1. In the absence of thenanoparticles, macrogrooves produced between CB parti-cles and these grooves are exposed to the electrolyte forcharge storage (Fig. 1(a)). As the nanoparticle content of

Fig. 2. Nyquist diagrams of different ZrO2-content electrodes in 2 M KClelectrolyte.

M. Nasibi et al. / Materials Science in Semiconductor Processing 24 (2014) 260–264262

the electrodes increases the macropores are filled withnanoparticles and the depth of the macrogrooves aredecreased (Fig. 1(b) and (c)). Then, all grooves are filledand the CB particles on the surface of the electrode arecovered with a thin layer of ZrO2 nanoparticles and finally,macrogrooves are replaced with the nanoporous structureprepared by the nanoparticles (Fig. 1(d)). Therefore, activesurface used for charge storage on the surface of theelectrode is replaced and this increases the specific surfacearea of the prepared electrodes. SEM images obtained from30:60:10 (CB:ZrO2:PTFE) electrodes (Fig. 1(e)) confirm thepresence of macrogrooves made between the CB particles(were shown by red lines) which nearly filled with ZrO2

nanoparticles. In these electrodes, CB particles can providea conductive channel due to their excellent conductivity.Unlike the CB particles, metal oxides like ZrO2 are lowconductive materials but, have a pseudo capacitive char-acteristic which can take place in redox reactions and

Fig. 3. Capacitance vs. potential curves obtained from (a) 90:00:10, (b) 50:40:10,

improve the energy storage capability of the electrodes.This low conductivity decreases the charge stored on theelectrodes, especially at high sweep rates. Therefore, as thenanoparticle content of the electrode increases, it isproposed that the total charge stored on the electrodesurface increases, at first, due to increasing the specificsurface area and changing the charge storage mechanismfrom double layer to pseudo, and then decreases due toincreasing the electrical resistance of the electrode.

The principle reaction involved in the charging anddischarging processes of zirconium dioxide in an aqueouselectrolyte can be described by reaction (1)

ZrIVO2þλþ þe�2ZrIIIOOλ ð1Þwhere λ denotes Kþ , Hþ .

Additionally, two mechanisms can be proposed forsupercapacitive charge storage in ZrO2. The first mechan-ism is based on the intercalation/extraction of protons or

(c) 30:60:10, (d) 20:70:10 and (e) 10:80:10 electrodes in 2 M KCl electrolyte.

Fig. 4. (a) Extrapolation of q to v¼0 from the q�1 vs. v0.5 plot given thetotal charge and (b) extrapolation of q to v¼1 from the q vs. v�0.5 plotgiven the outer charge for CB electrodes. (c) Total and outer charge storedon the different nanoparticle containing electrodes.

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alkali cations into the oxide particles (denoted as reaction(2)), whereas the second mechanism involves the surfaceadsorption/desorption of proton or alkali cations probably(denoted as reaction (3)):

ZrO2þMþ þe� ¼ ZrOOM ð2Þand

ðZrO2ÞsurfaceþMþ þe� ¼ ðZrOOMÞsurface ð3Þwhere Mþ denotes Kþ or H3Oþ .

In order to gain a quantitative information on the effect ofsurface changes on charge storage mechanism of the ZrO2/CBelectrodes by increasing the nanoparticle content of theelectrodes, the voltammograms obtained from differentnanoparticle containing electrodes were analyzed as a func-tion of scan rate, according to the procedure reported byArdizzone et al. [8]. Then, the total charge and charge relatedto the most accessible surface area were calculated.

Four prominent characteristics can be reported by chan-ging the nanoparticle content of the electrodes fromobtained CV curves (Fig. 3): changing the shape of the CVcurves, changing the specific capacitance, deviation of the CVcurves from the classical square waveform expected for apure capacitor by increasing the sweep rate and, increasingthe voltage reversal at end potentials. As the nanoparticlecontent of the electrodes increases electrical resistance of theelectrode increases (Fig. 2) and, up to 60%, the energy storedon the electrode increases at first and then decreases due tothe electrical resistance and specific surface area changes ofthe electrodes. Additionally, scan rate dependence of thecapacitance can be related to the less accessible surface area(pores, cracks, etc) which become excluded as the ratereaction is enhanced [9,10]. Ion diffusion ability of theelectrolyte into the surface of the electrode will have asignificant effect on the voltage reversal of the electrodes.Improvement of the voltage reversal of the high nanoparticlecontaining electrodes may be related to surface morphologychanges which change the active energy storage sites fromdown the deep macropores to shallow nanopores. Calculat-ing the total charge stored on electrodes and the chargestored on less and more accessible surface area of theelectrodes are efficient indicators which indicate the chargedistribution on the electrode. In charge and discharge cycles,the total charge can be written as a sum of an inner chargefrom the less accessible reaction sites and an outer chargefrom the more accessible reaction sites, i.e., q*T¼q*Iþq*O,where q*T, q*I and q*O are the total charge and charges relatedto the inner and the outer surfaces, respectively [11]. Theextrapolation of q* to v¼0 from 1/q* vs. v1/2 plots obtainedfrom different nanoparticle containing electrodes (Fig. 4(a))give the total charge qT which is the charge related to theentire active surface of the electrode. In addition, extrapola-tion of q* to v¼1 (v�1/2¼0) from the q* vs. v�1/2 plots (Fig.4(b)) give the outer charge q*O, which is the charge due toredox process on the most accessible active surface [8,11].Total and outer charges vs. nanoparticle content of theelectrode obtained from different nanoparticle electrodeswere plotted in Fig. 4(c). Total and outer charge plots confirmthat the addition of nanoparticles increases and thendecreases the charge stored on the electrode surface, asexpected and explained above. Although, electrodes which

contain higher nanoparticle contents show higher ratio of theouter charge to total charge (q*O/q*T) (increases from 0.05 to0.18) which confirms the higher current response on voltagereversal of high nanoparticle containing electrodes. Finally, itis concluded that the 40:50:10 (CB:ZrO2:PTFE) electrodesshow better charge storage capability (�57C g�1 cm�2)(Fig. 4(c)). It may be due to the synergistic effect of the doublelayer characteristic of the CB particles and the pseudocharacteristic of the ZrO2 nanoparticles. Changing the surfacemorphology from macrogrooves to nanoporous structuremakes the 10:80:10 electrode to show the high outer chargeto total charge (q*O/q*T) ratio of as-high-as 0.18 which confirmsits high current response on voltage reversal.

4. Conclusions

In this study, effect of nanoparticle content of the ZrO2/carbon black nanocomposite electrodes on the microstruc-ture, nature and the electrochemical performance of theprepared electrode were investigated. SEM images confirm

M. Nasibi et al. / Materials Science in Semiconductor Processing 24 (2014) 260–264264

the surface changes from hallow macrogrooves to shallownanoporous structure by increasing the nanoparticle contentof the electrodes. Charge stored on the electrode surfaceincreases (�57C g�1 cm�2 obtained from 40:50:10 elec-trode) at first, due to the synergistic effect of the carbonblack and ZrO2 nanoparticles, and then decreases by increas-ing the nanoparticle content due to the electrical resistanceof the electrode. Finally, it is concluded that the currentresponse of the electrodes (q*O/q*T ratio of 0.18 obtained from10:80:10 electrode) increases by addition of the nanoparti-cles due to the surface changes from hollowmacrogrooves tothe nanoporous structure.

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