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Nano Energy 26 (2016) 746–754

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Highly transparent and flexible supercapacitorsusing graphene-graphene quantum dots chelate

Keunsik Lee a,b, Hanleem Lee a,c, Yonghun Shin d, Yeoheung Yoon a, Doyoung Kim a,c,Hyoyoung Lee a,b,c,n

a Centre for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), 2066 Seoburo, Jangan-gu, Suwon 16419, Republic of Koreab Department of Chemistry, Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon 16419, Republic of Koreac Department of Energy Science, Sungkyunkwan University, 2066 Seoburo, Jangan-gu, Suwon 16419, Republic of Koread Department of Electrical and Computer Engineering, Inter-university Semiconductor Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu,Seoul 08826, Republic of Korea

a r t i c l e i n f o

Article history:Received 24 February 2016Received in revised form16 June 2016Accepted 17 June 2016Available online 18 June 2016

Keywords:GrapheneGraphene quantum dotsSupercapacitorMicrosupercapacitorTransparent supercapacitorFlexible supercapacitor

x.doi.org/10.1016/j.nanoen.2016.06.03055/& 2016 Elsevier Ltd. All rights reserved.

esponding author at: Centre for Integrated Naof Basic Science (IBS), Sungkyunkwan Un

16419, Republic of Korea.ail addresses: [email protected], hyoyoung@

a b s t r a c t

Nowadays, transparent and flexible energy storage devices are attracting a great deal of research interestdue to their great potential as integrated power sources. In order to take full advantage of transparentand flexible devices, however, their power sources also need to be transparent and flexible. In the presentwork we fabricated new transparent and flexible micro-supercapacitors using chelated graphene andgraphene quantum dots (GQDs) by a simple electrophoretic deposition (EPD) method. Through a chelateformation between graphene and GQDs with metal ions, the GQD materials were strongly adhered on aninterdigitated pattern of graphene (ipG-GQDs) and its resulting porous ipG-GQDs film was used as theactive material in the micro-supercapacitors. Amazingly, these supercapacitor devices showed hightransparency (92.97% at 550 nm), high energy storage (9.09 μF cm�2), short relaxation time (8.55 ms),stable cycle retention (around 100% for 10,000 cycles), and high stability even under severe bendingangle 45° with 10,000 cycles.

& 2016 Elsevier Ltd. All rights reserved.

1. Introduction

In the area of miniaturized and portable electronics, there hasbeen rapid technological advancement and a great deal of demandfor small, thin, lightweight, flexible, and even transparent elec-tronics, and this has stimulated the development of microscalepower sources with increasing power density [1–4]. Reducing thesize of energy storage devices down to the micrometer scale hasbecome an important issue. Development of demands for flexibleand transparent electronics has also sharply increased, however,achieving flexible and transparent energy storage devices withhigh power performance remains a great challenge because thesedevices' performance depends strongly on the electrical and me-chanical properties of their electrode materials.

Electric double layer capacitors (EDLCs), also named super-capacitors (SCs) store energy by using accumulating oppositelycharged electrolyte ions to form a double layer on

nostructure Physics (CINAP),iversity, Seoburo, Jangan-gu,

hotmail.com (H. Lee).

electrochemically stable electrodes with high specific surface area(SSA) [5]. Additionally, because the charge storage takes placethrough physical adsorption/desorption of charge carriers, we canhandle currents leading to a specific capacitance [6]. Therefore, SCsshow ideal energy storage from renewable resources, especially foron-chip operation with extremely high cycle endurance. The ap-plication of SCs in micropower systems (micro-supercapacitors,MSCs) has become a hot research topic [7–9]. As an device archi-tecture, small-sized interdigitated electrode designs consisting oftwo coplanar electrodes have gained more interest in comparisonwith conventional 2D stacked thin film electrodes due to theirstrong advantages with regard to kinetic performance [10,11].

Transparent and flexible electronics are essential components foroptoelectronics systems. Recently, transparent and flexible energyconversion and storage devices have attracted increasing attentiondue to their great potential as integrated power sources for rollupdisplays, field effect transistors, and building and automobile win-dows [12–15]. Transparent SCs are necessary for devices requiringintegrated power, and adding flexibility can ensure stable perfor-mance even under unpredictable mechanical deformations such asbending or twisting. To take full advantage of transparent and flexibledevices, their power sources should also be transparent and flexible.

K. Lee et al. / Nano Energy 26 (2016) 746–754 747

Carbon nanomaterials, and especially graphene, are more suitable fortransparent and flexible SCs, and are thus more promising to fulfill therequirements of transparent and flexible electronic devices [16].

Graphene, a 2D sp2-hybridized carbon sheet of single-atomthickness, has received attention because of its unique structureand properties [17–19]. Especially, its high theoretical SSA(2630 m2/g) and its high electrical conductivity combined withoptical transparency make it very promising as an electrode and/or active material for energy storage systems [3,20–22]. Graphenequantum dots (GQDs) are single or few-layer graphene particleswith nanometer-scale size, and have unique properties thatemerge from their graphene material combined the small scale ofquantum dots (QDs). Surprisingly, because of their desirablestrengths of high electron mobility and dispersibility in varioussolvents, the potential application field of GQDs is extensive, in-cluding light-emitting diodes, and even supercapacitors [7,23,24].Considering the advantages of GQDs, it is reasonable to expect it tobe an ideal electrode material for fabrication of high-performancesupercapacitors [25]. It is expected that due to the tiny size ofGQDs [26], as-deposited GQDs on a graphene electrode couldimprove the SSA of electrode materials and simultaneously createadditional micropores that would significantly contribute to theoverall electrochemical capacitance, because the SSA and the mi-cropores are the most important features of supercapacitors.Therefore, it is strongly expected that transparent GQDs nano-materials on the graphene electrode will be applicable as excellentelectrode materials for transparent and flexible energy storagedevices. Despite of their superior characteristics for the use ofactive materials in the field of supercapacitor, there are very fewreports on the combined bulk graphene and GQDs materials and/or films [26]. However, the use of a combined monolayer grapheneand GQDs to keep high transparency and high flexibility is stillunique concept since the technique of combining the monolayergraphene and GQDs is very difficult due to the difficulty of tightand stable bonding these materials together.

Herein, we report a new concept to fabricate highly transparentand flexible MSCs via a monolayer graphene and GQDs chelateformation by using a novel and simple electrophoretic depositionmethod of combining graphene and GQDs to yield a G-GQDselectrode. An interdigitated graphene pattern was designed andused as both a current collector and an active material on atransparent and flexible polyethylene terephthalate (PET) sub-strate. And then GQDs were deposited on the patterned grapheneto give ipG-GQDs film. For tight bonding of GQDs onto the pat-terned graphene, the GQDs were electrophoretically depositedonto the graphene with the help of chelating metal cations. It isexpected that the resulting chelated and porous G-GQDs film havea rough surface, high transparency, and high conductivity, allow-ing for the fast diffusion of electrolyte ions. The supercapacitordevice using the ipG-GQDs film is expected to allow for opticallytransparent and mechanically flexible energy storage devices thatcould be integrated into unique applications such as rollup dis-plays, wearable devices, and organic solar cell platforms. By usingtheir use of polymer gel electrolyte, the newly developed ipG-GQDs micro-supercapacitors (ipG-GQDs-MSC) show a high specificcapacitance and high transparency.

2. Experimental section

2.1. Synthesis of GQDs

Graphene oxide (GO) was prepared from natural graphite byusing a modified Hummer's method [19] including the use ofsulfuric acid, potassium permanganate, and sodium nitrate. Intypical synthesis, GO (800 mg) was dispersed in DMF (80 mL) and

then the suspension was transferred to a poly(tetra-fluoroethylene)-lined autoclave (100 mL). The suspension washeated to 200 °C for 12 h and then allowed to cool to room tem-perature. Then, the mixture was filtered through a 0.2 mm micro-porous membrane and the resulting brown dispersion of GQDs inDMF was collected. Dry GQDs powder was obtained by rotaryevaporation of the DMF at 80 °C under vacuum.

2.2. Preparation of interdigitated patterned graphene (ipG)

Monolayer graphene on Cu foil, prepared by CVD method, wassupplied by the Graphene Centre of Sungkyunkwan University.This CVD graphene was transferred onto PET substrates by PMMA-mediated methods. The graphene transferred onto the PET wasthen shielded by a shadow mask with an interdigitated patternand then this mask was used to deposit Al (50 nm) as an etchingmask. The exposed graphene was etched by using O2 plasmatreatment (100 W for 1 min). The Al mask was etched, yieldinginterdigitated patterned graphene (ipG) on the PET substrate.

2.3. Electrophoretic deposition of GQDs

GQDs were electrodeposited onto the ipG electrodes by im-mersing them in DMF solution (50 mL) containing GQDs (5.0 mg)and Mg(NO3)2 �6H2O (6 mg) and applying a constant DC voltage of80 V for 60 min. The two connected ipG electrodes with a con-ductive Cu tape and a Pt plate were set as the cathode and theanode, respectively. After the GQDs were deposited, the resultingGQDs on ipG electrodes (ipG-GQDs) was washed with deionizedwater and dried in air.

2.4. Preparation of polymer gel electrolyte

To prepare a polymer gel electrolyte, polyvinyl alcohol (PVA)powder was mixed with water (1 g PVA per 10 g H2O). This mix-ture was heated at 90 °C under vigorous stirring until it turnedclear, and then allowed to cool. Then, 0.8 g of concentrated H3PO4

solution was added to the solution and mixed thorough stirring.This yielded the PVA/H3PO4 polymer gel electrolyte.

2.5. Fabrication of ipG-GQDs microsupercapacitors (ipG-GQDs-MSCs)

The fabricated ipG-GQDs were directly used as MSCs after ap-plying an overcoat of gel-type electrolyte (either PVA/H3PO4) bypouring. Briefly, Cu tape was applied along the edges to improveelectrical contact, and the interdigitated area was defined byKapton tape (PI tape). The uncovered parts of the ipG-GQDs werecoated by slowly pouring the gel-type electrolyte (0.1 mL electro-lyte per 1 cm2). The resulting ipG-GQDs-MSC was allowed to restunder ambient conditions for 5 h to ensure that the electrolytecompletely wetted the electrodes and to allow for evaporation ofany excess water.

2.6. Material characterization

Structural characterizations were performed using a JEOL JSM-7404F field emission scanning electron microscope (SEM) operatedat 15 kV and using a JEOL JEM-2100F transmission electron mi-croscope (TEM). X-ray photoelectron spectroscopy (XPS) mea-surements were performed on a Thermo VG Microtech ESCA 2000equipped with a monochromatic Al-Kα X-ray source operated at100 W. Raman spectra were recorded on a WITEC alpha300equipped with a 532 nm excitation source.

K. Lee et al. / Nano Energy 26 (2016) 746–754748

2.7. Electrochemical characterization

Cyclic voltammograms, galvanometric charge–discharge curves,and electrochemical impedance spectra over the frequency range of0.01 Hz to 100 kHz with the AC amplitude of 10 mV were acquiredusing a CHI660c electrochemical workstation. Specific capacitance (Cs)was calculated from galvanometric charge–discharge curves by fol-lowing formula.

= ( Δ Δ )I A V tC 4 / /s

here, I is the constant current, A is the total area of both electrodes,and ΔV/Δt is the slope of a straight line fitted to the discharge curve.The multiplier of 4 in the equation is included to adjust the specificcapacitance of the cell and the combinedmass of the two electrodes tothe capacitance and mass of a single electrode.

The evolution of the changes in the real (C′) and imaginary (C″)parts of the capacitance versus frequency were plotted using thefollowing equations.

′( ω) = − ″( ω) ω ( ω)ZC / Z2

Fig. 1. Schematic illustration for fabrication of an ipG-GQDs-MSC. (a) Monolayer graphento prepare interdigitated graphene electrodes, ipG. (c) GQDs are deposited onto the ipG band a DC voltage is applied, yielding ipG-GQDs. (d) Illustrations of a fabricated ipG-GQlustration of the mechanism of the G-GQDs EPD process.

″( ω) = ′( ω) ω ( ω)ZC / Z2

here, ω is the pulsation, C′(ω) is the real part of the capacitance, C″(ω) is the imaginary part of the capacitance, Z(ω) is the electro-chemical impedance, and Z′(ω) and Z″(ω) are the real and ima-ginary parts of the impedance, respectively, defined as Z′2þ Z″2¼ |Z (ω)|2.

The energy density and power density of the electrodes werecalculated using the following equations.E ¼ Cs (ΔV)2/8

( ) ( )= −P V V R A/4max drop ESR2

here, Cs is the specific capacitance of the electrode, ΔV is the po-tential range, E is the energy density, P is the power density, A isthe area of the electrode materials (both electrodes), and the RESR

is estimated by using the voltage drop at the beginning of thedischarge (Vdrop) at a certain constant current Icons, by using theformula RESR¼Vdrop/(2Icons).

e prepared by CVD is transferred onto a PET substrate. (b) The graphene is patternedy using an EPD method: ipG is immersed in a GQDs dispersion including Mg(NO3)2Ds-MSC. (e) Optical image of an ipG-GQDs-MSC under bending. (f) Schematic il-


3. Results and discussion

3.1. Fabrication and characterizations of ipG-GQDs-MSC

The ipG-GQDs-MSC fabrication process is illustrated in Fig. 1and described as follows. First, monolayer graphene prepared bychemical vapor deposition (CVD) method is transferred onto a PETsubstrate. Then, an Al layer 50 nm thick was deposited over thetransferred graphene as a shadow mask to describe an 18-fingerinterdigitated pattern (9 positive and 9 negative microelectrodeseach 300 mm�12.4 mm in size; 300 mm gaps between micro-electrodes; total electrode dimensions of 0.67 cm2). The exposedgraphene was etched by O2 plasma, and then the Al mask wasremoved by etching in 0.1 M KOH, yielding interdigitated gra-phene (ipG) on the PET substrate. To prove for monolayer ipG, wecharacterized in Fig. S1. Finally, the GQDs were deposited by usingan electrophoretic deposition (EPD) method (Fig. S2) [7]. For themechanism process of EPD, Fig. 1(f) shows a schematic illustration.The Mg(NO3)2 was selected as an electrolyte due to a high ionicmobility and a low dissociation constant [27]. In colloidal sus-pension of the GQDs with Mg(NO3)2 in DMF, the carboxyl group(COOH) of GQDs and Mg(NO3)2 can easily ionized as COO� andMg2þ ions due to a high dielectric medium offered by DMF, re-spectively. Then, the negatively charged carboxyl group (COO�) of

Fig. 2. Characterizations of as-prepared GQDs. (a) HR-TEM image of as-prepared GQ(b) Raman spectra of GQDs. (c) XPS C1s peaks of GQDs. (d) PL spectra of GQDs under excsolution, showing blue fluorescence under excitation by 365 nm UV light.

GQDs was easily complexed with the positively charged Mg2þ ionsto form chelates through a chemical bonding to give Mg2þ-GQD[28]. When the graphene electrode was negatively applied, theresulting Mg2þ-GQD complexes moved toward the cathode sur-face under the constant electric field [25], leading to the deposi-tion of GQDs by electrostatic force. The as-fabricated ipG-GQDs-MSCs displayed superior electrochemical properties including highcapacitance, high rate capability, and stability during cycling, evenunder bending. The encouraging results achieved by using thecombined G-GQDs material suggest its great potential for enablingtransparent and flexible high-performance supercapacitors.

As-prepared GQDs were characterized to confirm their prop-erties. A high-resolution transmission electron microscope (HR-TEM) image showed uniform GQDs with size less than 20 nm. Theclear atomic lattice structure of GQDs exhibited a lattice spacing of0.25 nm that crystal state of graphite [29–31], showing manycrystalline domains highlighted white circle (Fig. 2(a) and inset).Raman spectroscopy confirmed the synthesis and structuralcharacterization of the GQDs (Fig. 2(b)). The D band at 1334 cm�1

arose from the A1g symmetry mode caused by breathing vibrationsof six-membered sp2 carbon rings; this mode becomes active inthe presence of disorder [32]. The G band at 1584 cm�1 was re-lated to the E2g vibrational mode of the aromatic domain in the 2Dhexagonal lattice structure [33]. The broad bands centered at 2687

Ds with showing nano-sized crystalline domains highlighted with white circle.itation at various wavelengths from 320 to 440 nm. Inset: photographs of the GQDs


and 2891 cm�1 are named as 2D and D þ G bands, respectively.The broadening and arising bands are caused by a splitting ofelectronic band structure of GQDs and a phonon scattering atboundaries with defects in GQDs [32,34]. The GQDs were alsocharacterized by X-ray photoelectron spectroscopy (XPS) to con-firm their structure (Fig. 2(c)). Primary C1s peaks were observed at284.6 eV, assigned as sp2 carbon peaks (C–C/CQC). The otherpeaks at the binding energies of 286.4 and 288.8 eV were assignedas the sp3 carbons of carbonyl (CQO) with hydroxyl carbon peak(C–OH), and carboxyl peak (C(O)O), respectively [35]. To analyzethe optical properties of GQDs, photoluminescence (PL) spectralanalysis was performed using GQDs suspended in DMF (Fig. 2(d)).Upon excitation at 380 nm, the GQDs emitted a strong fluores-cence centered at 450 nm. The emission peaks in the PL spectrashifted as the excitation wavelength was varied from 320 to440 nm, indicating that the PL emission depends on the excitationwavelength. The blue emission was attributed to a zig-zag effectwith a carbine-like triplet ground state of s1π1 [29,36]. The insetoptical image shows the blue luminescence under 365 nm UV lightillumination.

Fig. 3(a) shows a scanning electron microscopy (SEM) image oftransferred graphene on PET substrate, verifying clean and flatsurface. The GQDs deposited by using EPD on graphene for Fig. 3(b) show different morphology comparing with graphene. Inset ofFig. 3(b) also indicates G-GQDs surface at high magnification,which means rough surface by EPD process of GQDs. In addition,energy dispersive X-ray spectroscopy (EDS) was conducted toprovide element maps of C, O, and Mg (Fig. 3(c)–(f)), which con-firmed that the Mg(NO3)2/DMF contained in the GQD suspensionacted as a glue between the GQDs and the graphene surface, al-lowing successful deposition of GQDs onto the cathode.

Fig. 3. Characterizations of transferred graphene and GQDs deposited on graphene sheetmagnification, inset showing at high magnification. (c–f) EDS element mapping of ipG-

3.2. Electrochemical characterizations of G-GQDs supercapacitor

The energy storage effect of the G-GQDs material was character-ized by electrochemical measurements. To prepare an electrochemicalcell from the G-GQDs material, current collector (Cu tape) was appliedalong the edges to improve the electrical contact. The uncovered partsof the G-GQDs were coated by pouring PVA/ H3PO4, a gel-type elec-trolyte. Fig. S3 shows an optical image of a sandwich-type transparentG-GQDs supercapacitor cell, including an illustration of its cell struc-ture. Cyclic voltammetry (CV) was first used to determine the capa-citive performance of the G-GQDs (Fig. 4(a)). The resulting CV curveswere nearly rectangular under a variety of scan rates (0.1–1.0 V s�1),indicating ideal supercapacitor performance during the charge–dis-charge process. A linear relation was observed for the dischargingprocess (Fig. 4(b)), with rectangular CV shapes observed even at thehigh scan rate of 100 V s�1 (Fig. S4), indicating that the dischargeprocess was very fast over the entire electrode surface due to fastdiffusion of the electrolyte and fast charge transport [7,37–39]. Becausethe rough surfaces and high conductivity of the G-GQDs material al-lows fast diffusion of electrolyte ions from the deposited GQDs elec-trode (Fig. 3(b)), the CV curves could maintain their fully rectangularshape regardless of the scan rate. Fully symmetric galvanostaticcharge–discharge curves of G-GQDs were observed under the appli-cation of the current densities of 0.1, 0.2, 0.5, and 1.0 μA cm�2 (Fig. 4(c)), verifying that the only current source was non-faradaic current,attributable to the electric double layer capacitor process alone. Thespecific capacitance was 9.09 μF cm�2 as calculated from the dis-charge curve under the current density of 0.1 μA cm�2. Moreover, noIR drop was observed at the beginning of the discharge curves, sug-gesting very low equivalent series resistance (ESR) in the G-GQDsmaterial.

To further confirm the supercapacitor behavior, electrochemicalimpedance spectroscopy (EIS) measurements of the G-GQDs

s by EPD method. SEM image of (a) transferred graphene and (b) ipG-GQDs at sameGQDs.

Fig. 4. Electrochemical performance of a sandwich-type G-GQDs supercapacitor cell including a gel-type electrolyte (PVA/H3PO4). (a) Cyclic voltammetry (CV) curvescollected under various scan rates. (b) The dependence of the capacitive current (extracted from the CV curves at 0.4 V for discharge) versus applied scan rate.(c) Galvanostatic charge–discharge curves collected under various current densities. (d) Bode plot of G-GQDs cell. (e) Nyquist plot in frequency range from 1 MHz to 10 mHzwith a 10 mV AC amplitude. (f) Evolution of the real (C′, black circle line) and imaginary (C″, red circle line) parts of the capacitance of the G-GQDs cell.


supercapacitor cell was performed in the frequency range from100 MHz to 10 MHz using the AC amplitude of 10 mV. A Bode plot ofthe G-GQDs supercapacitor cell showed its excellent frequency re-sponse and short relaxation time (Fig. 4(d)). The phase angle wasclose to �90° at low frequencies, verifying the capacitive behavior ofthe cell [22,40]. The ESR obtained from the real-axis intercept of aNyquist plot was only 368 Ω, attributed to the ionic conductivitybetween the electrode materials and the electrolyte (Fig. 4(e))[22,41]. This plot featured a vertical curve that indicated the nearlycapacitive performance of the cell. To consider the supercapacitor asa whole by using the impedance data, Fig. 4(f) presents the real partof the capacitance (C′; open circle, black line) and the imaginary partof the capacitance change (C″; open square, red line) versus fre-quency for G-GQDs. When the frequency decreased, C′ increasedsignificantly and tended to be less frequency-dependent, which isrelated to the electrode structure and the interface between theelectrode and the electrolyte [4,42]. However, the C″ curve showed apeak at frequency f0, corresponding to the dielectric relaxation timeconstant of τ0¼1/f0¼8.55 ms. This represents the minimum timeneeded to discharge all the energy from the cell with efficiencygreater than 50% [43].

3.3. Electrochemical characterizations of ipG-GQDs-MSC

To improve the performance of the ipG-GQDs-MSC, the un-covered parts of the ipG-GQDs were coated by pouring the gel-typeelectrolyte (PVA/H3PO4). Fig. 5(a) shows an optical image of an ipG-GQDs-MSC and Fig. 5(b) illustrates the ipG-GQDs-MSC structureduring tests under bending. In Fig. 5(c), the CV curves of the ipG-GQDs-MSC were fully rectangular at various scan rates (0.1–1.0 V s�1), indicating ideal supercapacitor performance during thecharge–discharge process. Rectangular CV curves were observed evenat the high scan rate of 100 V s�1 (Fig. S5), indicating fast diffusion ofthe electrolyte and fast charge transport [37–39]. Fig. 5(d) shows

galvanostatic charge–discharge curves for the ipG-GQDs-MSC; curvescollected under various current densities were observed to havesymmetric triangular shapes. Only non-faradaic current was ob-served, attributable to the electric double layer capacitor processalone. The specific capacitance was 7.02 μF cm�2 as calculated fromthe discharge curve under the current density of 0.02 μA cm�2. EISmeasurements were also performed in the frequency range from100 kHz to 10 MHz using the AC amplitude of 10 mV (Fig. S6). Therelaxation time (τ0) of the ipG-GQDs-MSC was 8.55 ms, indicatingfast ion diffusion [44]. Although the capacitance of ipG-GQDs-MSCwas slightly smaller value than non-patterned device of G-GQDsdevice due to pattering process, the ideal supercapacitor behaviorswere remained including rectangular shape CV curves, symmetrictriangular shapes of galvanostatic charge–discharge (GCD) curves,and short relaxation time from EIS.

3.4. Investigation of transparent and flexible properties of ipG-GQDs-MSC

The ipG-GQDs-MSC was highly transparent, with the opticaltransmittance of 92.97% at 550 nm (Fig. 6(a)). The wavy transmittanceis caused from the interference fringes produced by the thin filmcoated flatways on substrates in the UV–vis light path [45,46]. Inset ofFig. 6(a) shows digital photograph of ipG-GQDs-MSC, exhibiting agood transparency. Furthermore, the effect of bending upon the ipG-GQDs-MSC supercapacitor performance was investigated (Fig. 6(b)).Insets in Fig. 6(b) show optical images of three bending positions,denoted as no bending, bending 1, and bending 2. To verify bendingdegree, we described in Fig. S7. The bending angles of no bending,bending 1, and bending 2 were calculated as 180°, 75°, and 45°, re-spectively. As the bending angles decrease from 180° to 45°, the ipG-GQDs-MSC was suffered from more severe bending condition. Al-though considerable bending was applied, the ipG-GQDs-MSC re-tained fully rectangular CV shapes with stable capacitive behavior.

Fig. 5. Electrochemical performance of an ipG-GQDs-MSC including a gel-type electrolyte (PVA/H3PO4). (a) Optical image of ipG-GQDs-MSC. (b) Illustration for fabricatedstructure of ipG-GQDs-MSC during bending test. (c) Cyclic voltammetry (CV) curves collected under various scan rates. (d) Galvanostatic charge–discharge curves collectedunder various current densities.


Furthermore the ipG-GQDs-MSC was performed repeating bending upto 10,000 cycles as shown in Fig. 6(c). The capacitance is retained as87.9%, which means stable supercapacitor performance as well asmaintenance strong bond between graphene and GQDs even harshmechanical attack condition. The use of graphene derivatives as theelectrode materials of the proposed ipG-GQDs-MSC allowed the de-vice to remain stable even under severe bending. In addition, the ipG-GQDs-MSC was tested under the scan rate of 10 V s�1 for 10,000 cy-cles with around 100% of the capacitance retention (Fig. 6(d)), in-dicating very stable supercapacitor even numerous charge-dischargerepeat. Furthermore, the aging behavior of ipG-GQDs-MSC was testedat constant elevated cell voltage of 1.5 V (floating voltage) [47]. Al-though the floating voltage was higher as 1.5 V than cell voltage(0.8 V) up to 10 h, the capacitance retention is still maintained asshown in Fig. S8, indicating stable supercapacitor performance evenunder severe electrochemical condition [43].

To demonstrate the overall performance of ipG-GQDs-MSC, aRagone plot is shown in Fig. 6(e). The maximum energy densityand power density of ipG-GQDs-MSC were estimated as727 nWh cm�2 and 83.4 μW cm�2, respectively. As a result, thesuperior performances of the ipG-GQDs-MSC including roughsurface, low resistance, fast relaxation time can syntheticallyprovide high power performance as well as high energy storageeffects, which were higher than previously reported performancesof transparent and flexible MSCs (Table S1). The ipG-GQDs-MSCwas compared with various transparent supercapacitors bytransparency versus capacitance (Fig. 6(f)). The energy density orareal capacitance are much more reliable performance metrics forsupercapacitor devices compared with gravimetric capacitance.Especially, it is more pronounced in case of a thin film electrode on

a chip is negligible [48]. To evaluate a performance for the large-area device in details, we fabricated the large size cell in whichelectrode is 1 mm�25 mm. Although the area is increased 6 timeslarger than micro-sized one based on active area, the CV curves ofFig. S9a showed rectangular shape even up to 100 V s�1 of scanrate. Surprisingly, there is no big difference in CV curves betweenthe large-area cell and the micro-sized ipG-GQDs-MSC shown inFig. S9b, maintaining a good capacitive performance even forlarge-area device. Our results show the highest transparency andhigh capacitance as much as other transparent graphene super-capacitors [49–51]. We strongly expect that the proposed our ipG-GQDs-MSC, which has high transparency, flexibility, and me-chanical stability, will be applicable for flexible, bendable, foldable,and even stretchable devices.

4. Conclusions

We developed a highly transparent and flexible micro-supercapacitor based on an interdigitated graphene pattern de-posited with GQDs by using a simple electrophoretic depositionmethod. We found that through a chelate formation between gra-phene and GQDs with metal ions, the GQD materials were stronglyadhered on the interdigitated pattern of graphene (ipG-GQDs) and itsresulting porous ipG-GQDs film was used as the active anode nano-materials for supercapacitor with high energy storage. The grapheneand GQDs were characterized by PL, Raman, XPS, TEM, SEM, and EDS.The ipG-GQD electrode materials were covered with gel electrolyte(H3PO4/PVA) and the resulting ipG-GQDs-MSC showed high energystorage performance with highly stable cycling, even under severe

Fig. 6. Transparent and flexible performance of an ipG-GQDs-MSC including a gel-type electrolyte (PVA/H3PO4). (a) UV spectra of the ipG-GQDs-MSC, showing the highoptical transmittances of 92.97% at 550 nm (bare from air), inset: optical image of ipG-GQDs-MSC. (b) Cyclic voltammetry (CV) curves collected under various bending states(Insets: optical images of each bending position, bending angles of no bending, bending 1, and bending 2 were 180°, 75°, and 45°, respectively). (c) Bending test from 180° to45° for 10,000 cycles. (d) The cyclic test at scan rate of 10 V s�1 for 10,000 cycles. (e) The Ragone plot showed energy density and power density (f) The ipG-GQDs-MSC wascompared with various transparent energy storage devices by transparency versus areal capacitance (Graphene network: ref. [49], Wrinkled graphene: ref. [50], Graphenefilm: ref. [51], This work: ipG-GQDs-MSC).


bending motion. After fabrication of the ipG-GQDs-MSCs, theirtransparency was about 92.97% at 550 nm, which is the highest valueuntil now. Their specific capacitance was 9.09 μF cm�2 and theyretained around 100% of their capacitance after 10,000 charge–dis-charge cycles. And the ipG-GQDs-MSCs have high stability even un-der severe bending angle 45° with 10,000 cycles. Also, their relaxa-tion time was very short, 8.55 ms, allowing high power performanceand fast diffusion of electrolyte ions through a rough surface and highconductivity of GQDs. Based on these superior properties, thetransparent and flexible ipG-GQDs-MSC has strong potential to beutilized as integrated power sources in industrial energy storageapplications, including uses in rollup displays, wearable devices, andbuilding and automobile windows.

Acknowledgment

This work was supported by IBS-R011-D1.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.nanoen.2016.06.030.

References

[1] B.Y. Ahn, E.B. Duoss, M.J. Motala, X.Y. Guo, S.I. Park, Y.J. Xiong, J. Yoon, R.G. Nuzzo, J.A. Rogers, J.A. Lewis, Science 323 (2009) 1590.

[2] J.J. Boland, Nat. Mater. 9 (2010) 790–792.[3] M.F. El-Kady, V. Strong, S. Dubin, R.B. Kaner, Science 335 (2012) 1326.

[4] W.P. Si, C.L. Yan, Y. Chen, S. Oswald, L.Y. Han, O.G. Schmidt, Energy Environ. Sci.6 (2013) 3218.

[5] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845.[6] B.W. Ricketts, C. Ton-That, J. Power Sources 89 (2000) 64.[7] W.W. Liu, Y.Q. Feng, X.B. Yan, J.T. Chen, Q.J. Xue, Adv. Funct. Mater. 23 (2013)

4111.[8] X. Li, B.Q. Wei, Nano Energy 2 (2013) 159.[9] D. Pech, M. Brunet, H. Durou, P.H. Huang, V. Mochalin, Y. Gogotsi, P.L. Taberna,

P. Simon, Nat. Nanotechnol. 5 (2010) 651.[10] J. Chmiola, C. Largeot, P.L. Taberna, P. Simon, Y. Gogotsi, Science 328 (2010)

480.[11] M. Beidaghi, C.L. Wang, Electrochim. Acta 56 (2011) 9508.[12] C.C. Chen, L.T. Dou, R. Zhu, C.H. Chung, T.B. Song, Y.B. Zheng, S. Hawks, G. Li, P.

S. Weiss, Y. Yang, ACS Nano 6 (2012) 7185.[13] H.Y. Jung, M.B. Karimi, M.G. Hahm, P.M. Ajayan, Y.J. Jung, Sci. Rep. 2 (2012) 773,

http://dx.doi.org/10.1038/srep00773.[14] X.S. Li, Y.W. Zhu, W.W. Cai, M. Borysiak, B.Y. Han, D. Chen, R.D. Piner,

L. Colombo, R.S. Ruoff, Nano Lett. 9 (2009) 4359.[15] Z.Q. Niu, W.Y. Zhou, J. Chen, G.X. Feng, H. Li, Y.S. Hu, W.J. Ma, H.B. Dong, J.Z. Li,

S.S. Xie, Small 9 (2013) 518.[16] T. Chen, H.S. Peng, M. Durstock, L.M. Dai, Sci. Rep. 4 (2014) 3612, http://dx.doi.

org/10.1038/srep03612.[17] K.S. Novoselov, V.I. Fal’ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, Nature

490 (2012) 192.[18] X. Huang, X.Y. Qi, F. Boey, H. Zhang, Chem. Soc. Rev. 41 (2012) 666.[19] I.K. Moon, J. Lee, R.S. Ruoff, H. Lee, Nat. Commun. 1 (2010) 73, http://dx.doi.

org/10.1038/ncomms1067.[20] M.D. Stoller, S.J. Park, Y.W. Zhu, J.H. An, R.S. Ruoff, Nano Lett. 8 (2008) 3498.[21] M. Beidaghi, C.L. Wang, Adv. Funct. Mater. 22 (2012) 4501.[22] M.F. El-Kady, R.B. Kaner, Nat. Commun. 4 (2013) 1475, http://dx.doi.org/

10.1038/ncomms2446.[23] S.J. Zhu, J.H. Zhang, C.Y. Qiao, S.J. Tang, Y.F. Li, W.J. Yuan, B. Li, L. Tian, F. Liu,

R. Hu, H.N. Gao, H.T. Wei, H. Zhang, H.C. Sun, B. Yang, Chem. Commun. 47(2011) 6858.

[24] R. Sekiya, Y. Uemura, H. Murakami, T. Haino, Angew. Chem. Int. Ed. 53 (2014)5619.

[25] W.W. Liu, X.B. Yan, J.T. Chen, Y.Q. Feng, Q.J. Xue, Nanoscale 5 (2013) 6053.[26] Q. Chen, Y. Hu, C.G. Hu, H.H. Cheng, Z.P. Zhang, H.B. Shao, L.T. Qu, Phys. Chem.

Chem. Phys. 16 (2014) 19307.[27] S.D. Sharma, M. Bhalwar, R. Gupta, J. Chromatogr. Sci. 39 (2001) 463.[28] M. Das, C. Dhand, G. Sumana, A.K. Srivastava, R. Nagarajan, B.D. Malhotra, J.


Mater. Chem. 22 (2012) 2727.[29] Y. Shin, J. Lee, J. Yang, J. Park, K. Lee, S. Kim, Y. Park, H. Lee, Small 10 (2014) 866.[30] Y. Shin, J. Park, D. Hyun, J. Yang, J.H. Lee, J.H. Kim, H. Lee, Nanoscale 7 (2015)

5633.[31] J. Peng, W. Gao, B.K. Gupta, Z. Liu, R. Romero-Aburto, L.H. Ge, L. Song, L.

B. Alemany, X.B. Zhan, G.H. Gao, S.A. Vithayathil, B.A. Kaipparettu, A.A. Marti,T. Hayashi, J.J. Zhu, P.M. Ajayan, Nano Lett. 12 (2012) 844.

[32] S. Niyogi, E. Bekyarova, M.E. Itkis, H. Zhang, K. Shepperd, J. Hicks, M. Sprinkle,C. Berger, C.N. Lau, W.A. Deheer, E.H. Conrad, R.C. Haddon, Nano Lett. 10(2010) 4061.

[33] D.Y. Pan, J.C. Zhang, Z. Li, M.H. Wu, Adv. Mater. 22 (2010) 734.[34] F.C. Liu, Y.Y. Sun, Y.P. Zheng, N.J. Tang, M. Li, W. Zhong, Y.W. Du, RSC Adv. 5

(2015) 103428.[35] K. Lee, D. Kim, Y. Yoon, J. Yang, H.G. Yun, I.K. You, H. Lee, RSC Adv. 5 (2015)

60914.[36] S.J. Zhu, J.H. Zhang, X. Liu, B. Li, X.F. Wang, S.J. Tang, Q.N. Meng, Y.F. Li, C. Shi,

R. Hu, B. Yang, RSC Adv. 2 (2012) 2717.[37] Y. Yoon, K. Lee, C. Baik, H. Yoo, M. Min, Y. Park, S.M. Lee, H. Lee, Adv. Mater. 25

(2013) 4437.[38] J. Lin, C.G. Zhang, Z. Yan, Y. Zhu, Z.W. Peng, R.H. Hauge, D. Natelson, J.M. Tour,

Nano Lett. 13 (2013) 72.[39] M. Beidaghi, Y. Gogotsi, Energy Environ. Sci. 7 (2014) 867.[40] T. Kim, G. Jung, S. Yoo, K.S. Suh, R.S. Ruoff, ACS Nano 7 (2013) 6899.[41] Y. Yoon, K. Lee, S. Kwon, S. Seo, H. Yoo, S. Kim, Y. Shin, Y. Park, D. Kim, J.Y. Choi,

H. Lee, ACS Nano 8 (2014) 4580.[42] P.L. Taberna, P. Simon, J.F. Fauvarque, J. Electrochem. Soc. 150 (2003) A292.[43] P.W. Ruch, D. Cericola, A. Foelske, R. Kotz, A. Wokaun, Electrochim. Acta 55

(2010) 2352.[44] K.X. Sheng, Y.Q. Sun, C. Li, W.J. Yuan, G.Q. Shi, Sci. Rep. 2 (2012) 247, http://dx.

doi.org/10.1038/srep00247.[45] Y.J. Kim, S.J. Yu, Y.G. Jeong, RSC Adv. 6 (2016) 30106.[46] X.Y. Liu, Y.Q. Gao, G.W. Yang, Nanoscale 8 (2016) 4227.[47] P. Ratajczak, K. Jurewicz, F. Beguin, J. Appl. Electrochem. 44 (2014) 475.[48] Y. Gogotsi, P. Simon, Science 334 (2011) 917.[49] X.L. Fan, T. Chen, L.M. Dai, RSC Adv. 4 (2014) 36996.[50] T. Chen, Y.H. Xue, A.K. Roy, L.M. Dai, ACS Nano 8 (2014) 1039.[51] Y. Gao, Y.S. Zhou, W. Xiong, L.J. Jiang, M. Mahjouri-samani, P. Thirugnanam,

X. Huang, M.M. Wang, L. Jiang, Y.F. Lu, APL Mater. 1 (2013) 012101, http://dx.doi.org/10.1063/1.4808242.

Keunsik Lee is currently a Ph.D. candidate under Prof.Hyoyoung Lee at department of Chemistry and Centrefor Integrated Nanostructure Physics (CINAP), Instituteof Basic Science (IBS) in Sungkyunkwan University(SKKU). His research interests are modification, synth-esis of materials and application in energy storage de-vices for enhancing performances.

Hanleem Lee is currently a Ph.D. candidate under Prof.Hyoyoung Lee at department of Energy Science andCentre for Integrated Nanostructure Physics (CINAP),Institute of Basic Science (IBS) in Sungkyunkwan Uni-versity (SKKU). Her research interests are flexible/stretchable/transparent conductive film and exfoliationof TMD materials.

Dr. Yonghun Shin received his Ph.D. degree in de-partment of Energy Science under supervision of Prof.Hyoyoung Lee in Sungkyunkwan University (SKKU). Heis now working at department of Electrical and Com-puter Engineering, Inter-university Semiconductor Re-search Centre in Seoul National University. He is in-terested in synthesis and characterization of perovskitequantum dots for QLED application.

Dr. Yeoheung Yoon received his Ph.D. degree in de-partment of Energy science under supervision of Prof.Hyoyoung Lee in Sungkyunkwan University (SKKU). Heis now working as postdoctoral researcher at Centre forIntegrated Nanostructure Physics (CINAP), Institute ofBasic Science (IBS) in Sungkyunkwan University(SKKU). He is interested in synthesis and application ofMXene materials for energy storage devices.

Doyoung Kim is currently a Ph.D. candidate under Prof.Hyoyoung Lee at department of Energy Science andCentre for Integrated Nanostructure Physics (CINAP),Institute of Basic Science (IBS) in Sungkyunkwan Uni-versity (SKKU). His research interests are synthesis andcharacterization of carbon based materials for applica-tion of batteries.

Hyoyoung Lee is a professor of department of chem-istry and energy science in Sungkyunkwan University(SKKU), Suwon, Korea. He received his Ph.D. degree inOrganic chemistry from University of Mississippi, MS,USA in 1997. His research interests include synthesisand exfoliation of TMD materials, reduced TiO2, OTFTmaterials and graphene based materials for applicationon energy conversion/storage devices, FET devices.

nano energy - swb.skku.edu · nano energy 26 (2016) 746–754. carbon nanomaterials, and especia...

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