Journal of Power Sources 195 (2010) 74807483

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Journal of Power Sources

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Short communication

Brushe gpolyan

Qiang Liua Department o , Cleveb Department o

a r t i c l

Article history:Received 26 FeReceived in reAccepted 1 JunAvailable onlin

Keywords:SupercapacitoUltracapacitorFlexible superNanomaterialNanocompositPainted capaci

tructiet ofnanoolymeets we and

1. Introduction

Energy storage for portable electronic devices, which arebecoming ifar the largencing innoelectronicsenergy storElectrochembilities are pto be integrbe made e[3,4]. Also, tinexpensive

Recentlybasic unit fogrow lmssubstrates ulosedissolvinltrated iembeddingnanocompopacitor was

CorresponE-mail add

papers back-to-back. Flexible supercapacitors were also fabricatedusing a spraying technique [6]. CNT dissolved inwaterwas sprayedon plastic substrates to form exible electrodes. The supercapaci-

0378-7753/$ doi:10.1016/j.ncreasingly important to the present society, forms byest mobile energy storage market today and is experi-vation for future applications such as exible/printedand display [1,2]. To operate exible devices, exibleage needs to be integrated with the active devices.ical supercapacitorswithhigh energy andpower capa-otentially capable of poweringexible devices. In orderated with exible electronics, supercapacitors shouldxible with sheet-like structures, which are lightweighthe fabricationmethod of the supercapacitors should beand high-throughput., a paper-like nanocomposite material was used as ther assemblingexible capacitors [5]. Therst stepwas toof vertically aligned carbon nanotubes (CNT) on siliconsing thermalchemical vapor-deposition. Then, cellu-ed inan ionic liquid thatwasalsousedaselectrolytewasnto the CNT to form a lm of cellulose and electrolyte,the CNT. After solidication on dry ice, the resultingsite paper was peeled from the substrate. A superca-made by bonding two pieces of the nanocomposite

ding author. Tel.: +1 216 875 9783; fax: +1 216 687 5405.ress: s.yau@csuohio.edu (S.-T. Yau).

tor was formed by sandwiching a exible gel/solid electrolyte withtwo exible electrodes.

This paper reports a simple approach for constructing exiblesheets of supercapacitor. The construction is based on paintinga sheet of exible plastic electrolyte with a composite materialmade of a conducting polymer and carbon nanotubes. To fulll therequirements of exible electronics applications, solid electrolytesare preferred over liquid electrolytes, which evaporate and there-fore require enclosures. Carbon nanotubes were used to reducethe internal resistance of the conducting polymer. The fabricationapproach is inexpensive and potentially high-throughput. Stacksof capacitor sheets were used to light up a system of light-emittingdiodes.

2. Experimental

The two-step construction of the capacitor sheet is schemat-ically described in Fig. 1(a). Polyvinyl alcohol (PVA) powder(SigmaAldrich) was mixed with water with a ratio of 1:9 byweight. The PVA mixture was then mixed with phosphoric acid(H3PO4, 85%) with a ratio of 1:1 by volume. Films of PVA wereformed by rst casting the nalmixture on a glass surface and thenpeeling the cast off from the surface after water had evaporated.The free-standing lms had a thickness of about 0.3mm. A volume

see front matter 2010 Elsevier B.V. All rights reserved.jpowsour.2010.06.002d-on exible supercapacitor sheets usiniline and carbon nanotubesa, Munir H. Nayfehb, Siu-Tung Yaua,

f Electrical and Computer Engineering, Cleveland State University, 2121 Euclid Avenuef Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

e i n f o

bruary 2010vised form 31 May 2010e 2010e 8 June 2010

r

capacitor

e materialtor sheet

a b s t r a c t

A simple, two-step method for consstruction is based on painting a sheof a conducting polymer and carbonpseudocapacitance produced by the pnanotubes. Stacks of the capacitor shThe method suggests an inexpensivsupercapacitors./ locate / jpowsour

a nanocomposite of

land, OH 44115, USA

ng exible sheets of supercapacitors is described. The con-exible plastic electrolyte with a composite material madetubes. The total capacitance of the supercapacitor consists ofer and electrical double-layer capacitance produced by carbonere used to light up a system of three light-emitting diodes.potentially high-throughput approach for making exible

2010 Elsevier B.V. All rights reserved.

Q. Liu et al. / Journal of Power Sources 195 (2010) 74807483 7481

Fig. 1. (a) Schsheet. Therstmaterial is thesheet.

of 0.1ml ofand 0.003gtions) wereused as puPANICNT adene the sat 40C forwThe two-dimElectrical coper sheets.

Cyclic vdischargingusing a comtrochemicaa commerciand a platitry measurChargingdrent densitmeasuremeH2SO4 aquDirect Q3,buffer solutning electrinstrument

3. Results

Themoried using SEa brillar swhich has bmorphologCyclic voltashows the cematic illustration of the two-step construction of the supercapacitorstep is the formation of the free-standing electrolyte sheet. The activen painted on both sides of the electrolyte using a brush. (b) A capacitor

polyaniline (PANI) dispersed in xylene (SigmaAldrich)of single-walled carbon nanotubes (CNT) (Carbon Solu-mixed in N-dimethylformamide. Both materials wererchased. A painting brush was used to transfer thequeousmixture on both sides of a PVA lm to form andhape of a capacitor. Painted sheets were put in an ovenater to be evaporated. Fig. 1(b) shows a capacitor sheet.ensional size of the capacitorwas about 5mm5mm.

ntacts weremade by either usingmetallic clips or cop-

oltammetry of PANI and galvanostatic chargingmeasurements of capacitor sheets were performedmercial potentiostat (CH Instrument 660C). The elec-l cell used was a conventional three-electrode cell withalAg/AgCl (3MKCl) electrodeas the referenceelectrodenum wire as the counter electrode. For voltamme-ements, the cell potential was scanned at 50mV/s.ischarging measurements were made at different cur-ies, with cutoff voltages of 0 and 0.8V. Voltammetrynts were made with electrodes immersed in 0.5Meous electrolyte. Deionized water ( =18.2M cm,Millipore) was used to prepare solutions. Phosphateion (PBS, 100mM) was prepared for general use. Scan-on microscopy was performed using an Amray 1820.

and discussion

phology of the PANICNT compositematerial was stud-M as shown in Fig. 2(a). The composite material showstructure with a cross-sectional size of about 250nm,een observed previously [7]. The absence of CNT in they indicates that CNT was embedded in the PANI layer.mmetry of the purchased PANIwas performed. Fig. 2(b)yclic voltammograms (CVs) of PANI and the PANICNT

Fig. 2. (a) SEMcomposite in 1

compositem(HOPG) eleimmersed ipeaks, namrespectiveldine salt th[8]. A2/C2coupled pothat the puand therefocapacitancethe composposite. Thepositive 100that the totdue to the player capacelectrode.

Initiallytested. Tracof the PANICNT composite. (b) CVs of PANI and the PANICNTM H2SO4. (c) CVs of PANI and the PANICNT composite at pH 4.

aterial deposited onhighly orientedpyrolytic graphitectrodes. Both CVs were obtained with the electrodesn 1M H2SO4. CV a of PANI shows three pairs of redoxely, A1/C1, A2/C2 and A3/C3. A1/C1 and A3/C3 indicatey the transition between leucoemraldine and emeral-e transition between emeraldine salt and pernigranilinehas been attributed to either the presence of ortho-lymers [9]. The appearance of the redox peaks indicatesrchased PANI was functional as conducting polymerre the redox processes of PANI gave rise to pseudo-of the electrode [10]. The evidence that CNT was inite is indicated by CV b taken with the PANICNT com-three pairs of redox peaks still appear in CV b with amV shift in potential. Comparing the twoCVs indicates

al current level the PANICNT composite was enhancedresence of CNT, which contributes an electrical double-itance [11] component to the total capacitance of the

capacitor sheetsmade of PANI onlywere fabricated ande a of Fig. 3(a) is the chargingdischarging characteristic

7482 Q. Liu et al. / Journal of Power Sources 195 (2010) 74807483

Fig. 3. (a) TraPANI electrodcapacitor sheesheet.

of the PANIseries resistof the capacwas introduwas obtaineESR is almosthe composing to CS = I/mass of thethe dischartrace. For aevaluated t

The Ragobetween thpacitor, is ssheet has aimum specthose obserlow energyce a and trace b are the charging/discharging characteristics of thee and the composite electrode, respectively. (b) Ragone plot of thet. (c) Charging/discharging cycling stability of a composite capacitor

capacitor sheet. The trace indicates a large equivalentance (ESR),which inprinciple reduces thepoweroutputitor sheet [12]. To reduce the observed large ESR, CNTced into PANI to form the composite. Trace b of Fig. 3(a)d with the composite capacitor sheet and it shows thatt absent in the composite. The specic capacitanceCS ofite capacitor sheet was evaluated using Trace b accord-(mdV/dt), where I is the discharging current,m is theelectrode material and dV/dt is the rate of change in

ging potential obtained from the chargingdischargingdischarging current density of 0.16mA/cm2, CS was

o be 16F/g.ne plot of the capacitor sheet,which shows the relatione energy storage and power capabilities of a superca-hown in Fig. 3(b). The plot shows that the capacitormaximum specic energy of 0.5Whkg1 and a max-ic power of 0.3 kWkg1, which are much lower thanved previously with exible supercapacitors [5,6]. Theand power capabilities are attributed to the fact that

Fig. 4. (a) Disc(b) Three LEDs

a mild acidtrolyte. Fig.composite (CVs: disapptwo materiH2SO4 (seepower capaaccordinglyFig. 3(c). Thof a capacitduring thethe way toitance withand shrinkidegradation

The poteby using thA problemtion voltagerequiremenin series wacharging chthree (curvare proportsheets was

4. Conclus

In summfor constrution is basea compositnanotubes.pseudocapalayer capacelectrolytecapabilitiesHowever, wharging characteristics of three difference stacks of capacitor sheets.powered by a stack of three capacitor sheets.

, H3PO4, was used in the preparation of the solid elec-2(c) shows the CVs of PANI (CV a) and the PANICNTCV b)measured at pH4. Twoeffects are indicated by theearing redox peaks and diminished current levels of theals compared to those obtained in strong acid such asFig. 2(b)). Thus, the capacitance, the energy storage andbilities of the PVA-based capacitor sheet were affected. The cycling stability of the capacitor sheet is shown ine specic capacitance of 16 F/g obtained at 0.16mA/cm2

or sheet shows a fast decrease by 50% of the initial valuerst 200 cycles. After that, CS shows a mild decrease allthe 650th cycle. The fast decrease in the specic capac-the cycle number is likely to be caused by the swellingng of conducting polymers, which are known to cause

during cycling [12].ntial application of the capacitor sheet is demonstratede capacitor sheet to power light-emitting diodes (LED).encountered in the demonstration was the low opera-(1V) provided by the sheet. To satisfy the operatingt of LEDs, a stack of several capacitor sheets connecteds used to increase the voltage. Fig. 4(a) shows the dis-aracteristics of stacksof one (curvea), two (curveb), ande c) capacitor sheets. The initial voltages of the stacksionally higher, reaching 4V. A stack of three capacitorused to drive three red LEDs as shown in Fig. 4(b).

ion

ary, we have demonstrated a simple, two-stepmethodcting exible sheets of supercapacitors. The construc-d on painting a sheet of exible plastic electrolyte withe material made of a conducting polymer and carbonThe total capacitance of the supercapacitor consists ofcitance produced by the polymer and electrical double-itance produced by carbon nanotubes. The use of solidhas resulted in considerably small energy and poweras reected in the Ragone plot of the capacitor sheet.e have used stacks of such capacitor sheets to light up

Q. Liu et al. / Journal of Power Sources 195 (2010) 74807483 7483

a system of three light-emitting diodes in order to demonstratethe potential applications of the capacitor sheet. Our future workwill focus on improving the energy and power capabilities and thestability of the capacitor sheet.

Acknowledgement

The support from Cleveland State University is acknowledged.

References

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Brushed-on flexible supercapacitor sheets using a nanocomposite of polyaniline and carbon nanotubesIntroductionExperimentalResults and discussionConclusionAcknowledgementReferences