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Applied Surface Science 313 (2014) 704–710

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

facile method to prepare a high performance solid-state flexibleaper-based supercapacitor

en-Yu Shieha, Sheng-Hui Zhanga, Cheng-Hung Wua, Hsin Her Yub,∗

Institute of Electro-Optical and Materials Science, National Formosa University, 64 Wenhua Road, Huwei, Yunlin 63208, TaiwanDepartment of Biotechnology, National Formosa University, 64 Wenhua Road, Huwei, Yunlin 63208, Taiwan

r t i c l e i n f o

rticle history:eceived 1 April 2014eceived in revised form 9 June 2014ccepted 9 June 2014vailable online 17 June 2014

eywords:nergy storage

a b s t r a c t

We propose a low cost and simple method to prepare a paper-based supercapacitor in this study.Multi-walled carbon nanotubes (MWCNTs) were dispersed with a pectin solution under an ultrasonichomogenizer. Carbon nanotube suspension was prepared using a centrifuge to eliminate impurities. Thedispersed MWCNTs suspension was dropped and dried onto the shallow surface of commercial copypaper. A paper-based conductive paper was formed as the electrodes. The electrical conductivity anddispersed morphology of the paper-based conductive paper were examined by four probes, atomic forcemicroscope (AFM), scanning electron microscope (SEM) and transmission electron microscope (TEM).

arbon nanotubesonductive paperolid-state electrolyteaper-based supercapacitor

The solid-state electrolyte was prepared by casting a solution of phosphoric acid and polyvinyl alco-hol onto a glass plate. The paper-based supercapacitor was constructed with one solid-state electrolyteinserted between two electrodes, which were assembled into a sandwich structure by hot press. Thespecific capacitance and cycle-life stability of the paper-based supercapacitor was investigated by cyclicvoltammetry analysis.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Portable electronic devices (e.g., mobile phones, notebooks,ameras) are reaching a point where increased functionality isimited by existing technologies for energy management. Futureevelopments are moving toward thin, light, cheap and more flex-

ble solutions, with wearable electronics as one typical application1]. Charge storage devices include batteries and supercapacitors,heir common feature being that the energy-providing processesake place at the phase boundary of the electrode/electrolyte inter-ace. Different features are the energy storage and conversion

echanisms. In batteries, electrical energy is generated by theonversion of chemical energy via redox reactions at the anodend cathode. In supercapacitors (or called electrochemical capaci-ors, ES), energy may not be delivered via redox reactions, but byhe movement of electrolyte ions between the electrode interfacend the electrolyte interface. The energy is accumulated with theure physical charge at the electrode interface [2]. Therefore, the

eaction time of batteries is longer, as supercapacitors can chargend discharge quickly. The advantages of supercapacitors includ-ng economizing on energy, a long lifetime, safety, environmental

∗ Corresponding author.E-mail address: hhyu@nfu.edu.tw (H.H. Yu).

ttp://dx.doi.org/10.1016/j.apsusc.2014.06.059169-4332/© 2014 Elsevier B.V. All rights reserved.

protection, a wide range of operating temperatures, etc. [3]. Withthin-based supercapacitors, Pushparaj et al. [4] used carbon nano-tubes (CNTs) as electrode materials, which could replace chargecollectors because of their high conductivity. Furthermore, CNTshave features, such as excellent mechanical performance, no chem-ical reaction with the electrolyte, a highly specific surface area,thermal conductivity, etc. [5,6]. Therefore, in this study, CNTs wereused as the electrode material. Lufrano et al. [7] discovered that thelarger surface area of carbon exhibited the higher specific capaci-tance by the supercapacitor.

Ki et al. [8] fabricated transparent conductive CNT film on glassby the solvent evaporation method. This method of solution-baseddeposition has several advantages, including a low temperature(

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re dispersed in the solvent with surfactant or organic solvent ashe dispersant. However, dispersants can harm the environment;herefore, in our laboratory, we used apple pectin as the disper-ant, since pectin is an ingredient of the polysaccharide from theell walls of plants. It not only creates a thermodynamically suitableurface in water, but also provides steric or electrostatic repulsionmong the dispersed CNTs, thus preventing aggregation [23].

In addition, in choosing the substrate, the major choices arelass, metal film, silicon and plastic. The biggest problem is thedhesion between the CNTs and the substrate. Yu et al. [24] usedow-cost porous material such as cellulose, textile and sponge toeplace the traditional substrate (glass, metal film, silicon and plas-ic) as the supporting substrate. In this study, we used copy papers the substrate. Paper is formed with a combination of long fibersnd short fibers. The surface of the paper absorbed the CNT suspen-ion when CNT suspension was dropped onto the paper surface,ith CNTs absorbed between the opening pores of the paper fibers,ith more suspension progressively evaporated by drying, so thatNTs were deposited on the substrate surface. The conformal coat-

ng and strong binding between CNTs and paper are attributed toarge capillary effect, maximized contact areas, and strong van der

aals forces. Paper fibers can provide more contact areas, resultingn more energy density at the interface of electrode and electrolytes.

To have a flexible and thin supercapacitor, we chose integratedolid electrolyte. Solid electrolytes offer dual functionality, as theyombine the separator and the electrolyte into a single layer. Thisvoids potential leakage, since the electrolyte is bound within theolymer matrix, while the use of a liquid electrolyte in a superca-acitor requires both robust encapsulation to prevent leakage and aeparator to avoid electrical contact between the electrodes. In thistudy, polyvinyl alcohol (PVA) and phosphoric acid (H3PO4) wereixed to form a solid-state electrolyte. The polymer gel electrolytesere used as both separator and electrolyte, simultaneously. PVA

s a water-soluble polymer, and its excellent mechanical propertiesre between those of plastic and rubber [25].

The aim of this study was to use solvent evaporation to dropNTs onto the paper surface in order to form conductive paper ashe electrode. Therefore, a paper-based supercapacitor was con-tructed by the electrode and solid electrolyte. In addition to aiscussion of the characteristics of this supercapacitor, this studylso presents the TEM analysis of the degree of CNT dispersion byifferent dispersants (water and pectin). We compared the depo-ition of CNT suspension dropped on different substrates, such asaper and glass, and measured the variation of sheet resistance andhe number of bending tests by four point probe in order to deter-

ine the adhesion of CNTs and paper in conductive paper. Finally,he capacitance behavior and cycle stability of the paper-basedupercapacitor was investigated by cyclic voltammetry.

. Materials and methods

.1. Preparation of paper-based conductive paper

In this experiment, CNT suspension was prepared by usingpple pectin (SIGMA) to disperse multi-walled carbon nanotubesMWCNTs, VGCFTM-X, Showa Denko K.K. Japan). The 0.5 wt% pectinolution was heated to 60 ◦C and stirred for about 1 h, and then 0.5 gWCNTs were dispersed in it by ultrasonication (Vibra-Cell Ultra

onics Processor, VCX 750, SONICS). To ensure uniform distribu-ion of the MWCNTs, the suspension was centrifuged at 4000 rpmor 15 min (tabletop centrifuge, LEGEND MACH 1.6R, SORVALL) to

emove the MWCNT entanglements and dispersant. The centrifuga-ion was repeated several times, and a uniform MWCNT suspensionas gained. The suspension was dropped onto copier paper (PAP-

RONE A4/70gsm, April Fine Paper) and dried under a vacuum oven

Fig. 1. (a) A schematic illustration for preparing the conductive paper, (b) structureof paper-based supercapacitor.

at 60 ◦C for 6 h. A schematic illustration for preparing the conduc-tive paper is shown in Fig. 1(a).

2.2. Preparation of solid-state electrolyte

The gel electrolyte was prepared by mixing 10 wt% poly(vinylalcohol) (PVA, MW: 25000, Polysciences, Inc.) powder with de-ionized water. 1.2 g H3PO4 (purity 99%, Sigma) was added to thegel electrolyte and stirred thoroughly. Upon evaporation of excesswater, the electrolyte solidified.

2.3. Assembly of paper-based supercapacitor

The paper-based supercapacitor was constructed with onesolid-state electrolyte inserted between two electrodes, whichwere assembled into a sandwich structure by hot press. A schematicillustration of the paper-based supercapacitor is shown in Fig. 1(b).

706 J.-Y. Shieh et al. / Applied Surface Science 313 (2014) 704–710

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ig. 2. TEM images of sonicated MWCNTs dispersed by (a) DI water, and (b) apple peposited on paper surface with (c) water-dispersant, and (d) apple pectin-dispersa

.4. Electrochemical characterization

The degree of CNT dispersion was investigated by transmissionlectron microscopy (TEM, JEM-2010, JEOL). The surface morphol-gy of the conductive paper was investigated by field emissioncanning electron microscopy (FESEM, JSM-6700F, JEOL) and byn atomic force microscope (AFM, P47H, NT-MDT). The variationf sheet resistance was measured by the number of bending testsmicro-computer tensile tester, JIA-802, Justice) and a four pointrobe (2400, Kiethly). The capacitance behavior and cycle stabil-

ty of the paper-based supercapacitor was investigated by cyclicoltammetry (611C, CHI). Electrochemical measurements werearried out using the three-electrode cell consisting of paper-basedlectrode as a working electrode, Pt wire and Ag/AgCl electrodes ashe counter and the reference electrodes, respectively.

In the three-electrode cell, the specific capacitance of the con-uctive paper is equal to the cell capacitance divided by the weightf MWCNTs in the working electrode by Eq. (1). The specific capac-tance of paper-based supercapacitor also can be calculated by Eq.1), but m is equal to two times weight of MWCNTs in one electroden the two-electrode cell.

=∫

idvm · s · �V (1)

here i is the discharge current, m is the weight of active materialMWCNTs), s is the scan rates, �V represents the operating voltageindow.

. Results and discussion

The microstructural changes of the MWCNTs both before andfter the pectin was added with sonication were observed by TEM.ue to the hydrophobic properties of MWCNTs, agglomeration was

ormed by van der Waals attraction and Brownian motion between

The insets in (a), (b) are their corresponding photographs. SEM images of MWCNTs

the MWCNTs, as shown in Fig. 2(a). The inset in Fig. 2(a) shows that,after sonication, the suspension was segregated after 2 h placed atroom temperature. On the other hand, the pectin in suspensionwas used as the dispersant to disperse the MWCNTs by sonication.The pectin was adsorbed on the MWCNT surface via cavitation. Theuniformity of the suspension was achieved by the separation of theMWCNTs by the decreasing of van der Waals attraction, as shownin Fig. 2(b). The inset in Fig. 2(b) shows the dispersion effect of thepectin on the MWCNTs, which was to maintain the stability of thedispersion at room temperature for two months.

The conductive paper with water-dispersant, as shown inFig. 2(c). MWCNTs agglomerated as bundles, the diameter of whichwas 28–67 nm. The conductive paper with pectin-dispersant, asshown in Fig. 2(d). MWCNTs were uniformly dispersed by pectin insolution, resulting in smaller diameter bundles of about 14–28 nm,which was similar to the MWCNTs provided by the vendor(15–20 nm). The results indicated that MWCNTs reached the bestdegree of dispersion. When the suspension was dropped onto thepaper, the solvent was progressively evaporated against the papersurface; however, the MWCNTs did not agglomerate. Therefore,pectin enhanced the stability of the suspension and the uniformityof deposition on the paper surface.

In order to understand the adhesion between substrate and sus-pension, the suspension with pectin was dropped onto paper, andthe cross-section was observed by SEM. The suspension only infil-trated the surface of the paper fiber by capillarity, as shown in Fig. 3.Because colloidal liquid was formed by the pectin-added suspen-sion, the time of infiltration was longer. As the solution evaporated,some of the MWCNTs were deposited on the shallow paper surface;therefore, the remaining MWCNTs were not able to completely

infiltrate the paper.

The surface roughness of the conductive papers prepared bydifferent dispersants was analyzed by AFM. Fig. 4(a) shows AFMimage of the conductive paper with water-dispersant (labeled as

J.-Y. Shieh et al. / Applied Surface Science 313 (2014) 704–710 707

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Fig. 4. AFM image of (a) water dispersed conductive paper, and (b) pectin dispersedconductive paper. The x-coordinate and the left side of the y-coordinate are the scan-

Fig. 3. SEM image of the cross-section of MWCNTs deposited on paper.

ater-cp), in which the x-coordinate and the left side of the y-oordinate indicate the scanning area, and the right side of the-coordinate represents the height of the scanning area. Surfacekewness (Ssk) was −0.265. The negative quantity indicated thathe water-cp surface was composed of a high peak and many deeprooves. The agglomerated bundles formed a larger agglomera-ion because the solvent evaporated, resulting in an undulatingurface. Average roughness (Ra) was 566 nm. The AFM image ofhe conductive paper with pectin-dispersant (labeled as pectin-cp)as shown in Fig. 4(b), in which the surface skewness (Ssk) was

0.386. The positive number indicated that the surface of pectinispersed conductive paper was composed of many hills. Whenniformly dispersed suspension was dropped onto the paper andfter the solvent had evaporated, the separation of MWCNTs wasreserved by the pectin, resulting in a uniform surface, with anverage roughness (Ra) of 313 nm. Jain et al. [26] used a spray torepare a CNT electrode, and the results showed that the electrodead a nanoporous morphology formed by an overlapping accu-ulation. Besides, Winter et al. [2] proposed that one property of

upercapacitors was that the energy conversion was at the inter-ace between the nanoporous electrode and the electrolyte. Whenhe electrolyte was full of the nanoporous structure of electrode,

ore energy conversion was produced.The variation in the sheet resistance of the conductive paper was

nvestigated using different dispersants through 100, 300 and 500epetitions of the bending test as measured by a four point probe,s shown in Fig. 5. Before the bending tests, the sheet of water-cpas 186 �, which increased to 200 � after 500 repetitions of the

ending test. The sheet resistance of the pectin-cp was 7 � beforehe bending test, and 7–8 � after the bending test. When wateras used as the dispersant, the MWCNTs agglomerated, causing

n uneven distribution. On the contrary, when pectin was used ashe dispersant, it was absorbed on the MWCNTs, which uniformlyispersed on the paper. That is, pectin-cp had excellent electric con-uctivity. According to the results of the bending test for two kindsf conductive paper, as shown in Fig. 5, MWCNTs on the paper sur-ace did not peel off. If the situation of peeling was produced, theheet resistance of the conductive paper increased significantly. Itlso showed the effective anchoring role of MWCNTs infiltrating theaper fiber. The variation in the resistance of the pectin-cp as mea-ured by multimeter before and after being folded was 47.9 � and

5.3 �, respectively. Both results were close to each other, revealinghe excellent flexibility and stable conductivity of pectin-cp.

Fig. 6 shows the CV of the supercapacitor with different elec-rolytes, as electrochemically analyzed for PVA/H3PO4 solid-state

ning area; the right side of the y-coordinate represents the height of the scanningarea.

electrolyte and 1 M H3PO4 liquid-state electrolyte, respectively. Thearea of the CV curve was the capacitance of the supercapacitor. Theideal CV curve is rectangular in shape without internal resistance(IR). However, the real CV curve showed a graph departure becauseof IR, but it remained a complete parallelogram, showing excel-lent capacitance behavior and energy storage stability. The voltagerange of CV was from −0.67 V to 0.67 V, with a 50 mV/s scan rate.The electrochemical behavior of a capacitor with the solid-stateelectrolyte or the liquid-state electrolyte in two-electrode system.The specific capacitance values for the cells based on both elec-trolytes calculated from the cyclic voltammetry. By Eq. (1), thespecific capacitance is 47 F/g with solid-state electrolyte, and is53 F/g with liquid-state electrolyte. During the experiment, it was

observed that the supercapacitor with the liquid-state electrolytewas faster in the initial charge period, as shown in Fig. 6(a) (fromA to B). Later, the current showed a downward trend (from B to C),

708 J.-Y. Shieh et al. / Applied Surface Science 313 (2014) 704–710

(a)

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ig. 5. Variation of sheet resistance and the number of bending tests by a four pointrobe measurement: (a) water dispersed conductive paper, and (b) pectin dispersedonductive paper.

nd it reached a peak after a period of time (from C to D). A sim-lar phenomenon was observed in the discharge process, that is,he supercapacitor was faster in the discharge period, followed byhe downward and upward trend of current, as shown in Fig. 6(a)D→E→F). Kimizuka et al. [27] used a supercapacitor which wasrepared by SWCNT film to discover the relationship between theV curve and the electrical properties of CNTs. CNTs were convertedrom conductors into semiconductors when starting the chargeeriod, and the current decreased after a while. At the time, CNTsonverted from semiconductors into conductors. The conversionf electrical properties increased the rate of charge and discharge,esulting in the pseudo-butterfly shape of the CV curve, as shownn Fig. 6(a). However, the supercapacitor with the solid-state elec-rolyte did not produce this phenomenon. Thus, the rate of chargend discharge was a little lower, as shown in Fig. 6(b). Kaemp-en et al. [12] discovered that there was no significant differenceetween PVA/H3PO4 solid-state electrolyte and 1 M H3PO4 liquid-tate electrolyte, but that the process of assembling a liquid-statelectrolyte was more complex. In order to prevent leakage ando avoid electrical contact between the electrodes, the thickness,exibility and even the limitation of the temperature range muste taken into consideration. Because of the excellent mechanicalroperties of polymer film and the ability to control the thicknessf electrolyte during the preparation process, the operating tem-

erature range of the supercapacitor with solid-state electrolyteould be wider, which conformed the possibility of a thin energytorage device.

(b)

(a)

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A

B C

E

D

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ig. 6. Cyclic voltammetry measurement of the paper-based supercapacitor in (a)olid-state electrolyte, and (b) liquid electrolyte, respectively.

Fig. 7. Cyclic voltammograms for the supercapacitor with solid-state electrolyte ata scanning rate of (a) 20, (b) 40, (c) 60, (d) 80 and (e) 100 mV/s.

In order to investigate the effects of solid-state electrolyte onthe capacitance properties and cycle stability of the supercapacitor,we set the CV with various scan rates in the two-electrode cell. Asshown in Fig. 7, at a scan rate between 20 mV/s and 100 mV/s, the CVvoltage ranged from −0.67 V to 0.67 V. The CV curve of conductivepaper recorded within a potential window of −0.67 V to ∼0.67 Vdisplays a nearly rectangular shape with no obvious gaseous evo-lution. When the scan rate was 20 mV/s, the slower scan rate, thediffusion of ions from the electrolyte could gain access to almostall available pores of the electrodes, and the penetration of elec-trolyte ions into the pores was a lot deeper, leading to a high specificcapacitance [28]. However, when the scan rate was increased, theefficient infiltration of ions into the porous electrode was progres-sively lower and, as a result, there was a reduction in capacitance.This result was consistent with the results obtained for other super-capacitors.

Finally, the paper-based supercapacitor was constructed withone solid-state electrolyte inserted between two electrodes, whichwere assembled into a sandwich structure by hot press. Fig. 8presents the charge/discharge curves of the supercapacitor withsolid-state electrolyte at various current density of 3 A/g. From thewell-retained triangular-shaped curve, an apparent feature is thatthe electrode mainly exhibits a double-layer capacitor behaviorresulted from the electrostatic attraction. Cyclic durability is one ofthe most significant electrochemical performance of supercapaci-tor. Fig. 9 shows the cycling stability of the supercapacitor based

on solid-state electrolyte was measured by charge/discharge at acurrent density of 3 A/g. The device still retains about 96% of theinitial capacitance after 2000 cycles. In addition, to demonstrate

0.0

0.5

1.0

1.5

2.0

0 100 20 0 30 0 40 0 50 0 60 0

Vol

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(V)

Tim e (s)

Fig. 8. Galvanostatic charge/discharge curves of supercapacitor with solid-stateelectrolyte at a current density of 3 A/g.

J.-Y. Shieh et al. / Applied Surface Science 313 (2014) 704–710 709

0

20

40

60

80

100

0 500 100 0 150 0 200 0Cap

acita

nce

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ntio

n (%

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Cycle number

Fig. 9. Variations of the electrode specific capacitances of supercapacitor with solid-state electrolyte as a function of cycle number measured at the charge/discharge currentdensity of 3 A/g.

Table 1Specific capacitance of various supercapacitors.

Item Electrode material Electrolyte Voltage range (V) Current load or Scan rate Specific capacitance (F/g) Ref

1 MWCNTs PVA/H3PO4 −0.67 to 0.67 50 mV s−1 472 MWCNTs 6 N KOH 0.0–1.0 0.78 mA cm−2 21 [29]3 SWCNTs PVA/H3PO4 0.0–1.0 30 mA mg−1 36 [12]4 SWCNTs 1 M LiClO4 0.0–3.0 0.75A g−1 35 [30]5 SWCNTs 1 M H2SO4 0.0–1.0 20 mV s−1 44 [31]6 CNTs 1 M H SO 0.0–1.0 20 mV s−1 33 [31]

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7 SWCNTs 6 M KOH2 M H2SO4 0.0–0.8

ote: Item 1 = our research work.

he potential use of these flexible solid-state paper-based super-apacitors, we used as-fabricated devices which could provide aorking voltage of 1.983 V to drive red light-emitting-diodes (LED,

.8 V). This device and could light a red LED well for about 5 minfter charging, later the light was dark gradually and distinguishedt last.

Table 1 shows the comparison of the paper-based supercapac-tor and the supercapacitors in other research. Items 1–6 wereompared, and the results showed that the specific capacitance ofhe paper-based supercapacitor was obviously better than that ofther supercapacitors. We concluded that the paper-based super-apacitor had specific advantages. First, with apple pectin chosens the dispersant, there was uniform dispersion of the MWCNTs.ompared to other dispersants, such as surfactants and organicolvents, the pectin was both safe and environmentally friendly.econd, with the selection of a solid-state electrolyte, leakage wasrevented and electrical contact between the electrodes avoided,hich ensured excellent mechanical properties. Third, paper was

hosen as the substrate material. With a plastic-based electrode,he conductive layer usually adheres to the plastic with the aid of

binder; while with a glass-based electrode, the flexibility of thelectrode is obviously limited. In this study, with paper used as theubstrate, the suspension infiltrated the surface of the paper fibery capillarity, resulting in the good adhesion of the conductive layernd the paper. Because of the good flexibility of paper, the resis-ance was retained even when the paper was folded. Because ofhe use of plant fiber in the paper, it was environmentally friendlynd recyclable.

. Conclusions

In this study, MWCNT suspension was dropped onto paper,nd the conductive paper formed after drying was used as thelectrodes. The paper-based supercapacitor was constructed byne solid-state electrolyte inserted between two electrodes, which

ere assembled into a sandwich structure by hot press. The results

f the experiment showed that apple pectin was a good dispersantor MWCNTs, which reached the optimal dispersion by sonication.

hen the suspension was dropped onto paper, it only infiltrated

2–150 mV s−1 30–140 [32]

the surface of the paper fiber by capillarity, resulting in the goodadhesion of the conductive layer and the paper. After many repe-titions of the bending test, the sheet resistance of the conductivepaper was quite stable. This showed the effective anchoring roleof the MWCNTs infiltrating the paper fiber, while the conductivepaper revealed excellent flexibility and stable conductivity. Theresults of the analysis of the paper-based supercapacitor by CVconfirmed its excellent capacitance behavior and good cycle per-formance, and that the specific capacitance of the paper-basedsupercapacitor was obviously better than that of other supercapa-citors. The paper-based supercapacitor has the proven advantagesof excellent flexibility, fast charge and discharge and safety. In thefuture, paper-based supercapacitors have potential applications inportable electronic devices, the green energy industry and thinenergy storage devices.

Acknowledgement

The authors gratefully acknowledge the funding of the researchby the National Science Council of Taiwan (NSC102-2221-E-150-052).

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A facile method to prepare a high performance solid-state flexible paper-based supercapacitor1 Introduction2 Materials and methods2.1 Preparation of paper-based conductive paper2.2 Preparation of solid-state electrolyte2.3 Assembly of paper-based supercapacitor2.4 Electrochemical characterization

3 Results and discussion4 ConclusionsAcknowledgementReferences