Direct printing and reduction of graphite oxide for flexible supercapacitorsHanyung Jung, Chang Ve Cheah, Namjo Jeong, and Junghoon Lee Citation: Applied Physics Letters 105, 053902 (2014); doi: 10.1063/1.4890840 View online: http://dx.doi.org/10.1063/1.4890840 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Engineering of high performance supercapacitor electrode based on Fe-Ni/Fe2O3-NiO core/shell hybridnanostructures J. Appl. Phys. 117, 105101 (2015); 10.1063/1.4913218 Flexible solid-state symmetric supercapacitors based on MnO2 nanofilms with high rate capability and longcyclability AIP Advances 3, 082129 (2013); 10.1063/1.4820353 Graphene metal oxide composite supercapacitor electrodes J. Vac. Sci. Technol. B 30, 03D118 (2012); 10.1116/1.4712537 Flexible solid-state paper based carbon nanotube supercapacitor Appl. Phys. Lett. 100, 104103 (2012); 10.1063/1.3691948 Scalable nanoimprint patterning of thin graphitic oxide sheets and in situ reduction J. Vac. Sci. Technol. B 29, 011023 (2011); 10.1116/1.3533936

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Direct printing and reduction of graphite oxide for flexible supercapacitors

Hanyung Jung,1 Chang Ve Cheah,2 Namjo Jeong,3 and Junghoon Lee1,2,4,a)

1Department of Nano Science and Technology, Graduate School of Convergence Science and Technology,Seoul National University, Seoul, South Korea2Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul, South Korea3Energy Materials and Convergence Research Department, Korea Institute of Energy Research, Daejeon,South Korea4Division of WCU Multiscale Mechanical Design, School of Mechanical and Aerospace Engineering,Seoul National University, Seoul, South Korea

(Received 27 March 2014; accepted 19 June 2014; published online 5 August 2014)

We report direct printing and photo-thermal reduction of graphite oxide (GO) to obtain a highly po-

rous pattern of interdigitated electrodes, leading to a supercapacitor on a flexible substrate. Key pa-

rameters optimized include the amount of GO delivered, the suitable photo-thermal energy level

for effective flash reduction, and the substrate properties for appropriate adhesion after reduction.

Tests with supercapacitors based on the printed-reduced GO showed performance comparable with

commercial supercapacitors: the energy densities were 1.06 and 0.87 mWh/cm3 in ionic and or-

ganic electrolytes, respectively. The versatility in the architecture and choice of substrate makes

this material promising for smart power applications. VC 2014 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4890840]

Supercapacitors are becoming increasingly popular

because of their potential as alternatives to current batteries.1

One of the key hallmarks of the supercapacitor is a power

density or rapid charging/discharging capability that is sev-

eral orders of magnitude higher than that of conventional bat-

teries.2 Rather than relying on a chemical redox reaction, the

supercapacitor directly drives electrical charges in and out of

electrical double layers, resulting in an extended number of

charge/discharge cycles (>1 00 000) far beyond the capability

of current batteries.3 Nevertheless, the energy density of

supercapacitors falls short of that of batteries by a few orders

of magnitude.4 Thus, there is now intensive research effort in

developing supercapacitors with reasonable energy density

by maximizing the porosity of the electrode materials.5

Another area of research for supercapacitors is their use

in advanced electronics applications that require handling of

power signals on flexible platforms. For example, energy har-

vesting for sensors and electronics on flexible substrates

involves the use of large-capacity charge storage directly

integrated with, and controlled by electronics in a smart man-

ner.6 Previous approaches in this field relied on the use of dis-

crete power components that not only increase the size and

complexity of the whole system but also bring challenges in

seamless integration of charge storage with electronics.

Therefore, it is important to realize the supercapacitor on a

flexible substrate in a compatible manner.7

Recently, interdigitated electrodes configuration via pat-

terning has been introduced as an alternative to the conven-

tional stacking approach because of benefits such as flexible

fabrication, scalability, and process compatibility. Due pri-

marily to its hydrophilic property, graphite oxide (GO) can

be completely dispersed in deionized water and used as an

“ink” for patterning. Then the printed pattern, after drying,

can be reduced to highly porous reduced GO (rGO). The pat-

terning and reduction of GO by photo-thermal exposure with

a mask has been demonstrated for the potential use in super-

capacitors.8 Onion-like carbon particles have been electro-

phoretically deposited on a pattern of interdigitated gold

electrodes for supercapacitor applications.9 This approach

required an additional step with precisely controlled deposi-

tion conditions. Recently, a supercapacitor has been devel-

oped based on the direct patterning and reduction of a GO

film by a commercial DVD laser system after drop cast-

ing.10,11 However, direct printing of nanoporous material has

only been demonstrated with carbon nanotubes.12

Here, we report an approach to use GO printing to

achieve an interdigitated pattern for a flexible planar super-

capacitor, including substrate material selection, multiple

dispensing for high resolution GO electrodes, and optimiza-

tion of photo-thermal energy. The most straightforward

approach is inkjet-like printing of GO and subsequent reduc-

tion to obtain the patterned supercapacitor configuration. Yet

limited by the difficulties in delivering sufficient porous ma-

terial and maintaining resolution, such printing method is

rarely tried. Inkjet printing, for example, can only deliver a

very small droplet that contains porous material far below

the needed amount to construct enough surface area for

charge storage. We used direct printing of GO into a specific

pattern with a liquid dispenser, which was originally

designed as a DNA array, and subsequent reduction of the

GO to make the pattern highly porous. This method allows

for freedom in the choice and the amount of material that

can be delivered. For instance, the amount of GO deposited

on the surface can be determined by controlling the droplet

volume in the liquid dispenser. Multiple dispensing is needed

because the stacking of GO deposited during the printing is

critical for high-resolution patterning. The GO pattern after

drying is reduced to rGO using an optimized level of photo-

thermal energy with a commercial camera flash device.

a)Author to whom correspondence should be addressed. Electronic mail:

jleenano@snu.ac.kr

0003-6951/2014/105(5)/053902/5/$30.00 VC 2014 AIP Publishing LLC105, 053902-1

APPLIED PHYSICS LETTERS 105, 053902 (2014)

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There are a variety of GO reduction methods: thermal

reduction at high temperature, chemical reduction using a

reducing agent like hydrazine,13 and photo-thermal reduc-

tion. Being able to trigger deoxygenating reaction of GO by

local heating, the photo-thermal reduction is desirable when

the heating of the whole substrate, e.g., thin plastic film,

needs to be avoided.14 Photo-thermal reduction can result

from various intensive light sources, such as UV light,15

excimer laser,16 and camera flash. We used a xenon flash, an

easily accessible camera accessory that can rapidly produce

the photo-thermal effect for in-depth reduction. Testing with

the supercapacitor fabricated through the above process

showed comparable energy density without using additional

components such as current collectors, separator, and chemi-

cal binders. This approach provides a solution to obtaining

printable charge storage devices that can be used in the area

of smart devices with energy harvesting and storage compo-

nents integrated on a flexible substrate.17

Substrate material selection is a vital process leading to

effective GO reduction and good adhesion of rGO. Several

candidates, such as transparent glass, transparent polyethyl-

ene terephthalate (PET) film, copy paper, polydimethylsilox-

ane (PDMS), and CYTOP (CTL-809 A, Asahi Glass, Japan),

were chosen as shown in Fig. 1. The reduced spots show a

clear change of color from brown to black. Some spots

remained unchanged in color when the local light energy

delivered was below the threshold to cause reduction. On all

of the substrates except transparent glass, almost every GO

spot was photo-thermally reduced via flash irradiation.

The contact angle of the GO droplet on the material sur-

face is an essential feature that has an effect on the adhesion

of rGO on the substrate after photo-thermal reduction. A GO

droplet forms a wide range of contact angles on different

surfaces depending on the characteristic hydrophobicity of

the material. We observed that GO deposited on a substrate

with a larger contact angle was more easily reduced com-

pared to those with smaller contact angles. While the total

amount of GO in a droplet was the same, a higher contact

angle resulted in a smaller area of deposition where more

GO was focused after drying. For example, the droplet area

on the glass substrate was about four times larger than that

on the CYTOP substrate. GO reduction hardly took place on

the glass slide where the GO droplet spread to the largest

extent, resulting in a thin, transparent layer of GO after dry-

ing. On the other hand, GO droplet with a high contact angle

led to the loss of some spots after reduction. GO goes

through a volume expansion during the reduction into the

rGO. The rGO spot was under stress against substrate due to

such volume expansion, and prone to delamination when the

interfacial energy was high, as reflected in the high contact

angle. Many spots with GO droplets that formed a high con-

tact angle, e.g., 110� on the CYTOP substrate delaminated

and detached from the surface after flash reduction.

However, we obtained stable and effective reduction without

delamination when the contact angle was around 80�, as

shown for the PET film.

Multiple dispensing is critical to obtain an adequate pat-

tern. The minimum dispensing volume from our dispenser

(sciFLEXARRAYER DW, Scienion, Germany) was 300 pL,

and multiple droplets of such volume were dispensed on the

PET surface. Pattern was maintained when the drying period

was provided between each dispensing cycle. Time for dry-

ing a single spot after dispensing takes a few seconds

because of quick evaporation of small volume liquid.

Obviously, the interval for serial dispensing takes much lon-

ger than such drying time, which ensured that the dispensing

occurred on a dry surface every time. This result is in con-

trast with dispensing the whole volume at once. Two cases

FIG. 1. Camera flash reduction characteristic on different substrates: 10 nl

GO droplets with 5 g/l concentration were dispensed on different materials.

FIG. 2. (a) Multiple dispensing of ten

300 pl droplets and (b) single dispens-

ing of 3 nl droplet of GO solution for

producing straight lines on the PET

film. The coalescence of the GO solu-

tion failed to produce a straight line in

the single dispensing printing method.

Although total droplets volume and

substrate material were the same con-

dition except printing method, a con-

nected line was only produced by the

multiple dispensing printing method.

(c) 0.79 J/cm2 flash reduction. (d)

1.50 J/cm2 flash reduction. GO was

reduced evenly over the whole surface

area. (e) 6.36 J/cm2 flash reduction.

Although high photo-thermal energy

reduced most of GO surface, it dam-

aged the surface.

053902-2 Jung et al. Appl. Phys. Lett. 105, 053902 (2014)

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compares, i.e., multiple dispensing of 10 times and single

dispensing with the same target volume in Figs. 2(a) and

2(b). While the line pattern with multiple dispensing shows

that the spacing of 80 lm can be readily achieved, whereas

the line pattern failed with the single dispensing of the whole

volume. The merging of large droplets resulted in disconnec-

tion and loss of the space between lines.

Photo-thermal energy optimization is crucial to maintain

the structure of the printed electrode. At low photo-thermal

energy (0.79 J/cm2), GO was not reduced as shown in Fig.

2(c). As photo-thermal energy increased gradually, GO could

be easily reduced (Fig. 2(d)). However, at high photo-

thermal energy (6.36 J/cm2), the GO surface was torn apart

and broken into pieces which would result in disconnection

of the electrode as shown in Fig. 2(e). Therefore, it is neces-

sary to determine the threshold energy that triggers GO

reduction without inducing any damages in the reduced

domain.

Photo-thermal reduction (1.50 J/cm2, Style RX 1200,

Elinchrom, Switzerland) produced rGO with less oxygen-

containing functional groups than GO, which was verified

through Raman spectroscopy and X-ray photoelectron spec-

troscopy (XPS) measurements. Sharpness of the D peak

(1350 cm�1) and G peak (1580 cm�1) in Raman spectrum

increased and the remarkable increase of 2D peak

(2690 cm�1) was observed, indicating better graphitization

of rGO than GO (Fig. 3(a)). Increase of the intensity of the D

peak is commonly observed after photo-thermal reduction.

This explains that oxygen-containing functional groups in

GO were removed, resulting in the restoration of sp2 hybri-

dized C-C bonds. The total amount of oxygen was detected

by XPS measurements in Figs. 3(b) and 3(c). C–O

(286.5 eV, hydroxyl and epoxy) and C¼O (287.8 eV) bond-

ing for GO were 19.8 at. % and 4.16 at. %, while for rGO

they both significantly decreased to 6.09 at. % and 3.86

at. %. The intensity of sp2 carbon also increased two times.

This indicates that oxygen-containing functional groups in

GO were comparably removed after the reduction process.18

The patterning process of interdigitated rGO electrodes

and the packing of the supercapacitor on a flexible substrate

are illustrated in Fig. 4. A liquid dispenser was programmed

to eject GO droplets at every designated spot on a substrate

to produce an interdigitated pattern. The thickness of the GO

pattern could be precisely controlled by direct printing

because the structure of the GO pattern was stacked up by

multiple dispensing. A pair of 4-interdigitated patterns was

produced by direct printing of GO on a thin PET film with

dimensions of 1.750 mm� 1.890 mm including the spacing

between interdigitated fingers. After applying an electrolyte,

the top layer of PET film was placed and then sealed with

KaptoneVR

tape.

Three types of electrolytes were used to test the per-

formance of the printed-reduced GO (PRGO)-based superca-

pacitor: 1-ethyl-3-methylimidazolium tetrafluoroborate

(EMIMBF4, Sigma Aldrich), tetraethylammonium tetrafluor-

oborate (TEABF4, Alfa Aesar) in acetonitrile (CH3CN, Dae

Jung, Korea), and sulfuric acid (H2SO4). Ionic liquid electro-

lyte, EMIMBF4 was used without any other treatments;

while 1.0 M organic electrolyte was prepared by dissolving

TEABF4 in acetonitrile. Concentrated H2SO4 was diluted to

1.0 M of concentration in deionized water to be used as aque-

ous electrolyte. As previously reported,10 the PRGO-based

supercapacitor with the ionic liquid electrolyte showed the

best performance in terms of energy and power densities

among the three electrolytes. The energy density with the

ionic liquid and the organic electrolyte was approximately

10 times higher than that with the aqueous electrolyte

because the ionic liquid electrolyte (3.5 V) and the organic

electrolyte (2.75 V) have a relatively larger potential window

than the aqueous electrolyte (1 V).

The results of cyclic voltammetry (CV) and galvano-

static charging/discharging (CC) tests for the PRGO-based

supercapacitor with organic electrolyte are shown in Figs.

FIG. 3. (a) Raman spectroscopy of GO and rGO. The ID/IG intensity areas of

GO and rGO were 1.63 and 1.28, respectively. XPS spectrum of (b) GO and

(c) rGO.

053902-3 Jung et al. Appl. Phys. Lett. 105, 053902 (2014)

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5(a) and 5(b). An electrochemical workstation (IVIUMSAT,

Ivium technologies BV Co., The Netherlands) was used for

the test. The current was measured at each potential applied

through the symmetric electrodes in the CV test and the

potential was measured with time in the CC test. Based on

the CC test results, we calculated the specific capacitance,

energy density, and power density of the PRGO-based super-

capacitor. The total charge stored in one cycle was calculated

using

Csp ¼It

DEV; (1)

where I, t, E, and V are the electric current, discharging time,

potential, and volume, respectively. Specific capacitance in a

specific area was calculated by dividing the total volume

with the size of a potential window: DE¼ 1 V.The energy density (Ed) is the energy density per unit

volume and is given by

Ed ¼1

2CspDE2: (2)

The power density (Pd) was obtained by dividing the energy

density by the discharging time of the device

Pd ¼Ed

Dt: (3)

The maximum achievable values of specific capaci-

tance, energy density, and power density for the three

FIG. 4. Patterning process of GO solu-

tion (a) GO solution (5 g/L) for liquid

dispenser ink. (b) Direct printing 300

pl droplet of GO solution and interdigi-

tated GO patterning.19 (c) After xenon

camera flash irradiation, where the

brownish patterned-GO was reduced to

black rGO and grew up in an arbitrary

direction, observed in cross-sectional

scanning electron microscopy image.

(d) Electrolyte injection. (e)

Interdigitated PRGO was packaged by

symmetric PET films for an rGO

supercapacitor and carbon paste for

electrode connection. The active mate-

rial thickness and area of a supercapa-

citor were 10 lm and 0.033 cm2,

respectively.

FIG. 5. (a) CV graph. Potential win-

dow of 0–2.75 V and current flow in

organic electrolyte at 50, 100, and

200 mV/s scan rates. (b) CC curves at

2, 4, and 8 lA. (c) Ragone plot, energy

density, and power density of

PRGObased supercapacitor. Li thin-

film battery. Reprinted by permission

from Macmillan Publishers Ltd:

Nature Nanotechnology (D. Pech, M.

Brunet, H. Durou, P. Huang, V.

Mochalin, Y. Gogotsi, P.-L. Taberna,

and P. Simon, Nat. Nanotechnol. 5, 651

(2010)), copyright (2010).9 Commercial

AC-EC and Al electrolytic capacitor.

Reprinted with permission from M. F.

El-Kady, V. Strong, S. Dubin, and

R. B. Kaner, Science 335, 1326 (2012).

Copyright 2012 AAAS.10 (d) Bending

test from 0 to 180 with an organic elec-

trolyte at 50 mV/s scan rate.

053902-4 Jung et al. Appl. Phys. Lett. 105, 053902 (2014)

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07:02:09

different electrolytes by volume and mass at given condi-

tions are shown in Table I. More detailed performance speci-

fications compared to other approaches appear on the

Ragone plot in Fig. 5(c).10

In addition to the high performance and benefits dis-

cussed above, our supercapacitor can be realized on various

substrates. Therefore, PRGO could be coated on many sub-

strates such as thin films and device surfaces. In the case of

the supercapacitor on a flexible substrate, it showed sus-

tained operation even though the substrate was highly

deformed. Fig. 5(d) shows the CV characteristics when the

PRGO-based supercapacitor was made to bend at different

angles. It shows that the operation property hardly varied at

extreme bending, as illustrated by the CV graph and corre-

sponding photo in Fig. 5(d).

We have demonstrated an innovative approach to fabri-

cate an interdigitated supercapacitor through patterning and

photo-thermal reduction of GO. Direct printing technology

was utilized to perform the patterning of GO which enables

the realization of the supercapacitor on various substrates.

The amount of GO, substrate properties, dispensing proce-

dures, and photo-thermal energy threshold were optimized to

obtaining desirable printed electrodes. Despite the absence

of current collector, separator, or chemical binders, the

PRGO-based supercapacitor outperformed commercial

supercapacitors and electrolytic capacitors in terms of energy

density. Furthermore, the fact that our supercapacitor did not

suffer degradation in performance even under extreme bend-

ing is promising for its application in flexible energy storage

devices, in addition to various smart power applications.

This work was supported by the Center for Integrated

Smart Sensors funded by the Ministry of Science, ICT &

Future Planning as Global Frontier Project (CISS-

2012M3A6A6054193) and a grant to Bio-Mimetic Robot

Research Center funded by Defense Acquisition Program

Administration (UD130070ID). Fabrication and experiments

were performed at the Interuniversity Semiconductor

Research Center (ISRC) in Seoul National University.

1P. Simon and Y. Gogotsi, Nat. Mater. 7, 845 (2008).2M. D. Stoller, S. Park, Y. Zhu, J. An, and R. S. Ruoff, Nano Lett. 8, 3498

(2008).3T. Y. Kim, H. W. Lee, M. Stoller, D. R. Dreyer, C. W. Bielawski, R. S.

Ruoff, and K. S. Suh, ACS Nano 5, 436 (2011).4C. Liu, Z. Yu, D. Neff, A. Zhamu, and B. Z. Jang, Nano Lett. 10, 4863

(2010).5B. G. Choi, M. Yang, W. H. Hong, J. W. Choi, and Y. S. Huh, ACS Nano

6, 4020 (2012).6Line technology, LT3501 Monolithic Dual Tracking 3A Step Down

Switching Regulator.7S. Hu, R. Rajamani, and X. Yu, Appl. Phys. Lett. 100(10), 104103 (2012).8L. J. Cote, R. Cruz-Silva, and J. Huang, J. Am. Chem. Soc. 131, 11027

(2009).9D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L.

Taberna, and P. Simon, Nat. Nanotechnol. 5, 651 (2010).10M. F. El-Kady, V. Strong, S. Dubin, and R. B. Kaner, Science 335, 1326

(2012).11M. F. El-Kady and R. B. Kaner, Nat. Commun. 4, 1475 (2013).12P. Chen, H. Chen, J. Qiu, and C. Zhou, Nano Res. 3(8), 594 (2010).13I. K. Moon, J. Lee, R. S. Ruoff, and H. Lee, Nat. Commun. 1, 1 (2010).14Y. Zhu, S. Murali, M. D. Stoller, A. Velamakanni, R. D. Piner, and R. S.

Ruoff, Carbon 48, 2118 (2010).15G. Williams, B. Seger, and P. V. Kamat, ACS Nano 2, 1487 (2008).16D. A. Sokolov, C. M. Rouleau, D. B. Geohegan, and T. M. Orlando,

Carbon 53, 81 (2013).17C. Abbey and G. Joos, IEEE Trans. Ind. Appl. 43(3), 769 (2007).18M. Wang, J. Oh, T. Ghosh, S. Hong, G. Nam, T. Hwang, and J. D. Nam,

RSC ADV. 4(7), 3284 (2014).19See supplementary material at http://dx.doi.org/10.1063/1.4890840 for

interdigitated electrodes patterning via a liquid dispenser.

TABLE I. Specific capacitance, energy density, and power density of the three electrolytes.

Electrolyte

Specific capacitance Energy density Power density

Volume (F/cm3) Mass (F/g) Volume (mWh/cm3) Mass (Wh/kg) Volume (W/cm3) Mass (kW/kg)

Ionic 0.63 39 1.06 63.0 0.408 25.5

Organic 0.82 51 0.87 54.2 0.259 16.2

Aqueous 0.70 43 0.09 6.0 0.089 5.6

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