Stacked Bilayer Graphene and Redox-Active Interlayer for

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Stacked Bilayer Graphene and Redox-Active Interlayer for Transparent and Flexible High-Performance Supercapacitors Kyungmin Jo, Sangbong Lee, Sang-Min Kim, Jung Bin In, SeungMo Lee, Jae-Hyun Kim, Hak-Joo Lee, and Kwang-Seop Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504801r • Publication Date (Web): 24 Apr 2015 Downloaded from http://pubs.acs.org on April 29, 2015

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Chemistry of Materials

Stacked Bilayer Graphene and Redox-Active Interlayer for Transparent and Flexible HighPerformance Supercapacitors Kyungmin Jo,† Sangbong Lee,† Sang-Min Kim,†,‡ Jung Bin In,§ Seung-Mo Lee,† Jae-Hyun Kim,† Hak-Joo Lee,† and Kwang-Seop Kim*,† †

Nano-Convergence Mechanical Systems Research Division, Korea Institute of Machinery

& Materials (KIMM), Daejeon 305-343, Republic of Korea ‡

Graduate School of Energy Environment Water and Sustainability (EEWS), Korea

Advanced Institute of Science & Technology (KAIST), Daejeon 305-701, Republic of Korea §

School of Mechanical Engineering, Chung-Ang University, Seoul 156-756, Republic of

Korea

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ABSTRACT Fabrication of a transparent and flexible supercapacitor requires electrode materials that are optically transparent and mechanically flexible. Although chemical vapor deposition (CVD)grown graphene is a promising electrode material, its use in supercapacitor applications is greatly limited by the low area-specific capacitance of the fabricated supercapacitor. Here, we demonstrate transparent and flexible high-performance supercapacitor using stacked bilayer graphene and an ultrathin redox-active interlayer. By inserting the redox-active layer between stacked bilayer graphene, we achieved an almost 20-fold enhancement of the area-specific capacitance (from 5.6 µF/cm2 to 101 µF/cm2) with a thickness of electrode material for each electrode less than 2 nm. In addition, the fabricated supercapacitor exhibited excellent transparency of 75% (including the substrate’s transparency) and flexibility (bending radius down to 5 mm) by virtue of the outstanding transparency and flexibility of the stacked bilayer graphene and redox-active interlayer.


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1. INTRODUCTION Recently, there has been growing interest in incorporating aesthetically pleasing properties such as transparency and flexibility in addition to the intrinsic performance of electronic devices for future electronics.1–11 In this regard, extensive research has been performed on the development of transparent and flexible electrodes2,3, as well as electronic devices such as transistors.4,5 light-emitting diodes,6,7 displays,8,9 and solar cells.10,11 However, the actualization of such devices is limited by less (or non-) transparent and flexible energystorage devices, which are a key component to power the device. Therefore, development of a highly transparent and flexible energy-storage device is important for practical realization of transparent and flexible electronic devices. Supercapacitors represent one type of energy-storage device that can deliver high power density, excellent cyclic stability, and fast charge/discharge rates. To fabricate a supercapacitor, carbon-based materials have been extensively used as electrode materials due to their high specific surface area, good electrical conductivity, low cost, and electrochemical stability.12 Among them, graphene is a promising candidate especially for transparent and flexible supercapacitors because of its high electrical conductivity,13 mechanical flexibility,14 and optical transparency.15 Despite their outstanding flexibility, the majority of graphenebased supercapacitors are not (or marginally) transparent, as graphene-based electrode material is too thick (from a few to a few hundred micrometers) to be transparent. Recent studies have shown that transparent graphene-based supercapacitors can be fabricated by reducing the thickness of electrode material to less than 100 nm with reduced-graphene oxide (r-GO)16 or even to a few nanometers using few-layer chemical vapor deposition (CVD)grown graphene.17,18 Although these ultrathin graphene film-based supercapacitors were optically transparent (with transmittance values less than 73%) with thicknesses of the electrode material ranging from a few to a few tens of nanometers, their area-specific 3/23 ACS Paragon Plus Environment


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capacitances (~5 µF/cm2) were almost three orders of magnitude lower, due to a tradeoff between transparency and thickness (or amount of loading) of electrode materials, compared to that of non-transparent graphene-based supercapacitor (~4.0 mF/cm2) with a fewmicrometer thick electrode material.19. Therefore, further enhancement of the energy density of ultrathin graphene film-based supercapacitors is required to make them practically appealing, while maintaining their unique transparent and flexible nature. Enhancing the energy density of the supercapacitor typically involves the addition of redoxactive materials to redox-inactive electrode materials that exhibit only electrical double-layer capacitance.20,21 In this regard, various redox-active materials such as conducting polymers,22,23 metal hydroxides,24,25 and metal oxides26,27 have been combined with graphene materials to enhance the capacitance of graphene-based supercapacitors. However, it remains challenging to construct a graphene-based supercapacitor incorporated with redox-active materials that are both highly transparent and flexible with an enhanced electrochemical performance, while maintaining the ultrathin thickness of graphene-based films. In this study, we fabricated graphene electrodes by stacking monolayer CVD graphene with the insertion of a redox-active interlayer composed of redox-active molecules, and assembled a supercapacitor using fabricated graphene electrodes. Particularly, we chose p-aminophenol (p-AP) as a redox-active molecule due to the following reasons. First, p-AP is one of the most-well known electro-active organic molecules,28,29 which is frequently employed to generate electrochemical signal in enzyme-based electrochemical biosensors.30,31 Second, pAP, as an aromatic molecule, is likely to be adsorbed onto the surface of CVD-grown graphene predominantly through π-π interaction, as CVD-grown graphene is almost free of oxygen-containing active chemical functional groups (such as carboxy and epoxy groups) unlike reduced graphene obtained from reduction of graphene oxides.32 We demonstrated that the redox-active layer inserted between stacked CVD graphene sheets with a thickness of less 4/23 ACS Paragon Plus Environment

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than 1 nm enables nearly 20-fold enhancement of the area-specific capacitance of the assembled supercapacitor without appreciably altering its transparency. In addition, the fabricated supercapacitor showed good electrochemical cyclic stability and exhibited reliable electrochemical performance even at bending state. We found that the redox-active interlayer is likely formed through the polymerization of redox-active molecules and remains stable between stacked bilayer graphene, which is responsible for the improved capacitance of the ultrathin graphene-based supercapacitor.

2. EXPERIMENTAL SECTION Synthesis of Monolayer Graphene. Monolayer graphene was synthesized on 25-µm-thick Copper (Cu) foils (Alfa Aesar) using the CVD method. The Cu foil was loaded into a quartz tube and then heated to 1000°C and annealed (45 min) at a low pressure (~400 mTorr) with Ar (100 sccm)/H2(50 sccm) flow. Subsequently, graphene growth was performed with a mixture gas flow of CH4:H2:Ar (4 sccm:50 sccm:100 sccm) for 35 min. The sample was then rapidly cooled to room temperature. Preparation of the Graphene Electrode. Graphene electrodes with monolayer graphene (1L Gr), bilayer graphene (2L Gr), and bilayer graphene incorporating a redox-active interlayer (2L Gr (R)) were prepared by poly(methyl methacrylate) (PMMA, Microchem)based transfer of graphene and etching of Cu foils. To fabricate the 1L Gr electrode, PMMA was spin-coated (3000 rpm, 30 s) onto graphene grown on Cu foil (graphene/Cu) and baked (10 min at 120°C). The resulting PMMA/graphene/Cu foil was then floated onto 0.1 M ammonium persulfate ((NH4)2S2O8, Sigma-Aldrich) solution to etch away Cu. After Cu etching, PMMA/graphene film was floated onto deionized (DI) water to rinse off any impurities (30 min) and transferred to target substrates. For the 2L Gr electrode, PMMA/graphene film was transferred to the graphene/Cu foil to obtain PMMA/bilayer 5/23 ACS Paragon Plus Environment


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graphene/Cu foil. After Cu etching and rinsing with DI water, we obtained PMMA/bilayer graphene and transferred it to target substrates. For the 2L Gr (R) electrode, we immersed graphene/Cu foil into 1 mM p-AP (Sigma-Aldrich) solution for 2 h and thoroughly rinsed with DI water to generate the redox-active layer/graphene/Cu foil. We then transferred PMMA/graphene to redox-active layer/graphene/Cu foil to fabricate PMMA/graphene/redoxactive interlayer/graphene/Cu foil. With subsequent Cu etching and DI rinsing, we obtained PMMA/graphene/redox-active interlayer/graphene and transferred it to target substrates. Characterization of the Graphene. UV-vis spectra were measured using a spectrophotometer (Mecasys, OPTIZEN 2120UV) to determine the transmittance and absorbance of the sample. For optical imaging, an optical microscope (Nikon, Eclipse LV100D) was used. The Raman measurement was performed using confocal Raman microscope (Renishaw, inVia) using a 514-nm laser beam as an excitation source. The beam size was 2 µm and a 50× objective lens was used. X-ray photoelectron spectroscopy (XPS) was performed using a photoelectron spectrometer (Kratos, AXIS Nova) equipped with a monochromatic Al X-ray source (1486.6 eV). For sheet resistance mapping of graphene samples, a four-point probe resistivity meter (Bega technologies, DHY-ARS-200) was used. Topographic images and their height profiles were obtained using an atomic force microscope (AFM; Park Systems, XE-100) with a non-contact cantilever. Fabrication of the Supercapacitor. The all-solid supercapacitor was fabricated by employing a graphene electrode and poly vinyl alcohol (PVA)-H2SO4 gel electrolyte. The gel electrolyte was coated over a predetermined area (2 cm x 2 cm) of graphene electrode, leaving the uncoated parts for electrical connection, and dried under ambient conditions for several hours. The supercapacitor was then assembled by stacking electrolyte-coated graphene electrodes together with electrolyte-coated parts facing each other. The gel electrolyte was prepared by mixing 0.8 g of concentrated H2SO4 (Sigma-Aldrich) with 10 mL 6/23 ACS Paragon Plus Environment

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of DI water and then adding 1 g of PVA (Sigma-Aldrich, Mw = 89,000 to 98,000). The gel electrolyte solution was then heated at 85°C with vigorous stirring to obtain clear gel electrolyte solution. Evaluation of the Electrochemical Performance of the Supercapacitor. The electrochemical performance of the supercapacitor was investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (CD) experiments using an electrochemical workstation (CH Instruments, CHI 660E). For electrochemical measurements with the threeelectrode setup, we used a graphene electrode as a working electrode and Ag/AgCl (3M NaCl) and Pt wire as a reference and counter electrode, respectively. Calculations. The area-specific capacitance (Carea) values were calculated from the CV curves according to the following equation (1): Carea =

ଵ

‫׬‬ ∆௏×஺

ூሺ௏ሻௗ௏ ఔ

(1)

Where ΔV is the voltage window (V), A is the total area of the electrode (cm2), and I(V) is the voltammetric current (A), and ν is the scan rate (Vs-1). The area-specific capacitance values were also calculated from the CD curves according to the following equation (2): Carea =

ଶ×ூ ሺௗ௏/ௗ௧ሻ×஺

(2)

Where I is the applied charge/discharge current (A), dV/dt is the slope of the discharge curve (V/s), and A is the total area of the electrode (cm2). Note that we use area-specific capacitance (F/cm2) for the evaluation of the electrochemical performance of supercapacitor, and we avoid the use of gravimetric capacitance (F/g) and volumetric capacitance (F/cm3) due to the following facts: (1) gravimetric capacitance rather



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overestimates the actual capacitance of transparent supercapacitor, as the mass of loaded electrode material is typically very low (less than few µg); (2) it is impossible to measure the exact mass of CVD-graphene; (3) the volumetric capacitance of our device can be exaggerated by the atomically thin CVD-graphene used as an electrode material, although volumetric capacitance of our device is estimated to be 505 F/cm3 by considering 2-nm-thick electrode material.

3. RESULTS AND DISCUSSION Figure 1 schematically illustrates a procedure for preparation of graphene electrodes, in which 1L Gr, 2L Gr, and 2L Gr (R) are transferred to target substrates. The procedure consists of PMMA-based transfer of graphene, in which removable PMMA polymer is coated onto the graphene grown on Cu foil and used as a flexible supporting layer to minimize undesired mechanical damage during the transfer process, and etching of Cu foil.33,34 Using this transfer and etching process, we prepared 1L Gr electrode (as seen in Figure 1) and confirmed that the graphene used in this work is a high-quality monolayer and that the applied transfer method induces negligible mechanical damage by analyzing optical images, Raman spectra, and map of sheet resistance of the graphene transferred to a 300-nm-thick SiO2/Si wafer (Figure S1). We prepared a 2L Gr electrode by transferring PMMA-coated graphene to a graphene-grown Cu foil (rather than by repeating transfer of PMMA-coated graphene to the target substrate and subsequent removal of PMMA) to construct a stacked bilayer graphene free of polymeric residues between the stacked graphene layers.35 To prepare a 2L Gr (R) electrode, we immersed graphene-grown Cu foil in a solution containing p-AP for 2 h and sufficiently rinsed it with DI water, followed by transfer of PMMA-coated graphene to the p-AP-coated graphene-grown Cu foil. The successful preparation of 2L Gr and 2L Gr (R) electrodes was verified by optical imaging of 2L Gr and 2L Gr (R) transferred to the SiO2/Si wafer substrate 8/23 ACS Paragon Plus Environment

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(Figure S2a). Negligible amounts of tears and cracks in the optical images of both samples implied that the process for preparation of 2L Gr and 2L Gr (R), which involves repeated transfer and etching steps, is reliable for the fabrication of graphene electrode. In addition, we determined an electrode area for both samples by considering that transferred graphene is almost free of voids. Although 2L Gr and 2L Gr (R) samples were optically indistinguishable, the presence of a redox-active interlayer, which is the major difference between 2L Gr and 2L Gr (R), was confirmed based on XPS measurements (Figure S2b). Particularly, we compared core N 1s spectra for both graphene samples transferred to the SiO2/Si wafer, as the main component of p-AP contains nitrogen in its chemical structure, as seen in Figure 1, while the synthesized CVD graphene does not. We observed an intense N 1s peak (~400 eV) for 2L Gr (R) and the noise-level signal without a distinct peak for 2L Gr, indicating that 2L Gr (R) has a nitrogen-containing interlayer between stacked graphene sheets. In addition to the spectroscopic study, the redox-active layer was identified by means of electrochemical measurements (Figure S2c). CV curves recorded using the 2L Gr and 2L Gr (R) electrodes as working electrodes in 1 M H2SO4 electrolyte solution with a three-electrode setup demonstrated that the redox-active interlayer causes significant redox current to flow with multiple oxidation/reduction peaks (red line in Figure S2). Considering that the electrochemical reaction of the p-AP proceeds exclusively by proton-coupled electron transfer and is thus sensitive to the concentration of protons (H+),36,37 we further investigated the H+ concentration-dependent electrochemical properties of the redox-active interlayer using electrolyte solutions containing different H+ concentrations. When the CV measurement was conducted in 1 M Na2SO4 electrolyte solution (blue line in Figure S2c), where the concentration of H+ was considerably lower compared to that in H2SO4 electrolyte solution, the multiple oxidation/reduction peaks previously observed in the CV curve obtained in H2SO4 electrolyte solution were absent. However, these peaks appeared again 9/23 ACS Paragon Plus Environment


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when the H2SO4, which acts as an H+ source, was added to the Na2SO4 electrolyte solution (green line in Figure S2c). These results demonstrate that the redox-active interlayer is sensitive to the concentration of H+, indicating that the H+-sensitive p-AP species are incorporated in the redox-active interlayer. Besides, it also demonstrate that the redox chemistry of redox-active interlayer is largely dependent on the concentration of H+ similar to the case of most conducting polymers, although exact mechanism requires further investigations. Moreover, it is important to note that the interlayer structure of redox-active layer adopted here exhibits better stability than the redox-active layer without upper graphene layer (Figure S3), and that the thickness of the redox-active layer is estimated to be 0.4 nm based on AFM measurement (Figure S4). Figure 2a shows a schematic depiction of the preparation of transparent and flexible supercapacitors with a simple symmetric parallel-plate geometry using ultrathin graphenebased films transferred to a polyethylene terephthalate (PET) substrate as supercapacitor electrodes and PVA-H2SO4 gel as electrolyte. Notably, the PVA-H2SO4 gel electrolyte functions not only as an electrolyte, but also as a separator, and ensures the transparency and flexibility of the supercapacitor. The electrochemical performance of the fabricated supercapacitor was investigated by performing CV and galvanostatic CD experiments. Figure 2b presents CV curves obtained for the supercapacitor prepared using 1L Gr electrodes (1L Gr-SC), 2L Gr electrodes (2L Gr-SC), and 2L Gr (R) electrodes (2L Gr (R)-SC) at a scan rate of 100 mV/s. In comparison with CV curves of 1L Gr-SC and 2L Gr-SC, which exhibited almost ideal double-layer capacitive behavior with a rectangular CV shape, the CV curve of 2L Gr (R)-SC showed enhanced electrochemical performance with characteristic oxidation and reduction peaks associated with the redox reaction of the redox-active interlayer. Note that the enhanced electrochemical performance was primarily due to the presence of redoxactive layer, as the redox-active layer itself is not conductive and the difference in the 10/23 ACS Paragon Plus Environment

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electrical conductivity between 2L Gr and 2L Gr (R) is negligible. The enhancement of electrochemical performance was also verified by CD curves obtained at a current density of 1.8 µA/cm2 (Figure 2c). CD curves for 2L Gr (R)-SC showed deviation from the typical triangular shape and suggested contribution of the faradaic redox reactions to the charge storage process, whereas 2L Gr-SC showed CD curves of triangular shape corresponding to its non-faradaic capacitive behavior. In addition, the discharge time obtained for 2L Gr (R)SC was about 18-fold higher than that for 2L Gr-SC, although the enhancement was reduced at higher current densities. To accurately assess the enhancement of electrochemical performance, we calculated area-specific capacitance from CV and CD curves obtained for 2L Gr-SC and 2L Gr(R)-SC (Figure 2d, 2e, S5, and S6). The area-specific capacitance values for 2L Gr (R)-SC were 101.0 µF/cm2 (from the CV curve measured at a scan rate of 10 mV/s) and 99.4 µF/cm2 (from the CD curve obtained at the current density of 1.8 µA/cm2), whereas the values for 2L Gr-SC were 5.6 µF/cm2 and 5.5 µF/cm2, respectively. Although calculated area-specific capacitance values were slightly different for the type of electrochemical measurement, the values for 2L Gr-SC were comparable to those reported previously for supercapacitors based on CVD-graphene electrodes with four or a few graphene layers.17,18 In addition, we observed an almost 20-fold (at lower charge/discharge rates) or 10-fold (at higher charge/discharge rates) improvement in the area-specific capacitance values when the redox-active interlayer was incorporated between stacked bilayer graphene. Figure 3a shows the supercapacitors assembled using bare PET substrates and PET substrates on which 1L Gr, 2L Gr, and 2L Gr (R) were transferred as supercapacitor electrodes. The fabricated supercapacitors positioned above the logo of the Korea Institute of Machinery & Materials (KIMM) exhibited good transparency as well as large-area uniformity of the graphene electrodes. To quantify the transparency of the assembled supercapacitor, we assessed its optical transmittance (Figure S7). At a wavelength of 550 nm, the supercapacitor 11/23 ACS Paragon Plus Environment


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prepared using bare PET substrate without graphene sheets as the supercapacitor electrode possessed 88.7% transmittance, resulting from the inherent transmittance of the gel electrolyte and PET substrate. Supercapacitors prepared using PET substrates coated with 1 L Gr, 2L Gr, and 2L Gr (R) films as supercapacitor electrodes showed transmittance values of 83.7%, 78.7%, and 75.0%, respectively. We attributed the excellent transparency of the supercapacitor demonstrated in this study to the high transparency of graphene, which has a theoretical transmittance of 97.7%, together with the negligible reduction in transmittance by the redox-active interlayer. Figure 3b shows a comparison of the transparency and the electrochemical performance of the fabricated supercapacitor obtained in this work with data reported previously.17,18 Compared to graphene films consisting of four or a few layers of graphene with the area-specific capacitance of 4–5 µF/cm2 and transparency of 48–73%, 2L Gr yielded a fabricated supercapacitor of comparable area-specific capacitance with significantly improved transparency (~79%) due to the decrease in the number of graphene layers, and 2L Gr (R) showed enhancement of both electrochemical performance (~101 µF/cm2) and transparency (~75%) due to the incorporated redox-active interlayer. Particularly, the electrochemical performance of the transparent supercapacitor fabricated in this work was superior compared to that in previous studies in terms of the volumetric power and energy densities (Figure S8). It is important to note that the transparency of the fabricated supercapacitor can be improved by using substrates with better transparency, because the transmittance value of PET substrate used in this study (~89%) is lower than that of PDMS substrate (~95%) which is frequently used as a substrate for the fabrication of the transparent and flexible supercapacitor. To evaluate its flexibility, we obtained CV curves of the supercapacitor under different bending conditions (Figure 3c). The CV curves remained stable at a bending radius ranging from 25 mm to 5 mm, indicating that the supercapacitor can be bent without loss of 12/23 ACS Paragon Plus Environment

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performance. We also investigated the electrochemical cyclic stability of the bent supercapacitor with a bending radius of 12.5 mm at a scan rate of 500 mV/s; 86.5% of the initial capacitance was maintained after 1000 cycles (Figure 3d). To understand plausible mechanism for the process of formation of the redox-active layer on the surface of the graphene following immersion in p-AP solution, we investigated the mechanism responsible for generation of the redox-active layer. In particular, we examined the oxidation behavior of p-AP in the absence and presence of a reducing agent, as p-AP is readily oxidized which leads to the formation of poly-aminophenol species (poly-AP) via pAP polymerization under ambient conditions38 (note that we controlled the extent of polymerization of p-AP by using 2 h immersion time throughout the study, although longer immersion time could lead to further polymerization). In addition, the oxidation of p-AP can be prevented by adding a reducing agent (Figure 4a).30,31 The color of p-AP solution changed from transparent to brown due to p-AP oxidation during formation of the redox-active layer by immersion of graphene-grown Cu foil in p-AP solution for 2 h. However, the p-AP remained transparent when hydrazine (HZ) was added as a reducing agent (Figure 4b). This behavior was further confirmed based on UV-vis measurements (Figure 4c). The absorbance increased after aging of 1 mM p-AP solution for 2 h (2 h aged), while the absorbance of freshly prepared 1 mM p-AP solution, 10 mM HZ solution, and the 2 h-aged mixture of 1 mM p-AP and 10 mM HZ remained unchanged. UV-vis data indicated that the oxidation of pAP leads to the generation of poly-AP39 and that the oxidation can be suppressed by the reducing agent. Importantly, poly-AP generation was also confirmed by the presence of multiple oxidation and reduction peaks from the CV curve (red line in Figure S2c), because polymerized redox-active molecules generally represent CV curves with multiple oxidation and reduction peaks.40 To explore the effect of p-AP oxidation on the formation of the redoxactive layer and the electrochemical performance of the resulting supercapacitor, we 13/23 ACS Paragon Plus Environment


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fabricated a supercapacitor using stacked bilayer graphene and redox-active interlayers that were prepared using two methods: (i) p-AP solution without adding HZ (2L Gr (AP)-SC), and (ii) a mixture of p-AP and HZ (2L Gr (AP+HZ)-SC). As seen in Figure 4d, the electrochemical signal for 2L Gr (AP+HZ)-SC was markedly smaller compared to those for 2L Gr (AP)-SC and 2L Gr-SC. Particularly, the capacitance value for 2L Gr (AP+HZ)-SC (9.4 µF/cm2) was slightly higher than that for 2L Gr-SC (5.2 µF/cm2) which does not contain a redox-active layer. Although adsorbed p-AP molecules might exist between stacked bilayer graphene for 2L Gr (AP+HZ)-SC, the enhancement of the capacitance due to p-AP was much less (2-fold) compared to that due to poly-AP (20-fold). The UV-vis and electrochemical data suggested that the redox-active interlayer was likely formed via (1) polymerization of the pAP and (2) subsequent adsorption of poly-AP to the surface of graphene via π-π interaction during immersion in the p-AP solution. In addition, the results suggested that formation of the redox-active interlayer is impeded by inhibition of p-AP oxidation by a reducing agent. Despite the fact that the redox-active layer is generated via spontaneous oxidation of p-AP, we found that the redox-active interlayer between stacked bilayer graphene remains stable after prolonged cyclic testing (for 1000 cycles) and several days of storage under ambient conditions (Figure 3d and S9).

4. CONCLUSIONS In conclusion, we fabricated a highly transparent and flexible supercapacitor based on an ultrathin graphene film composed of stacked CVD graphene bilayers and a redox-active interlayer, with an overall thickness less than 2 nm. The resulting supercapacitor was highly transparent, with transparency of 75% (using PET as a transparent and flexible substrate) at a wavelength of 550 nm, and demonstrated good flexibility without deterioration of performance with a bending radius down to 5 mm. In addition, we observed an almost 2014/23 ACS Paragon Plus Environment

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fold improvement of the area-specific capacitance of the supercapacitor following insertion of the redox-active layer between stacked graphene sheets, with only a minor decrease in transparency. Further investigation revealed that the redox-active interlayer was generated by the spontaneous polymerization of the redox-active molecule, and that the redox-active interlayer was stable once formed and inserted between stacked bilayer graphene. Our results could be used to fabricate transparent and flexible energy storage devices, which are essential for the realization of transparent and flexible future electronic devices.

■ ASSOCIATED CONTENT Supporting Information Optical, Raman spectroscopic, and electrical characterization of the transferred monolayer graphene (Figure S1). Optical, XPS, and electrochemical characterization of the transferred bilayer graphene with and without a redox-active interlayer (Figure S2). Thickness of graphene-based films transferred to the SiO2/Si substrate (Figure S4). Electrochemical performance of the fabricated supercapacitors (Figure S5). CV curves of the fabricated supercapacitors (Figure S6). Transmittances of the fabricated supercapacitors (Figure S7). Storage stability of the supercapacitor electrode (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes 15/23 ACS Paragon Plus Environment


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The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We would like to acknowledge the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of Republic Korea (Grant CAP-13-2-ETRI), and the internal research program of Korea Institute of Machinery and Materials (SC1020 and NK192C).

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FIGURES

Figure 1. Schematic of the preparation of graphene electrodes with 1L Gr, 2L Gr, and 2L Gr (R).



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Figure 2. Fabrication and evaluation of the electrochemical performance of the supercapacitor. (a) Schematic of fabrication of the supercapacitor. (b) CV curves of 1L GrSC, 2L Gr-SC, and 2L Gr (R)-SC at a scan rate of 100 mV/s. (c) Galvanostatic CD curves of 2L Gr-SC and 2L Gr (R)-SC at a current density of 1.8 µA/cm2. (d) The area-specific capacitances of 1L Gr-SC, 2L Gr-SC, and 2L Gr (R)-SC calculated from the CV curves at the indicated scan rates. (e) The area-specific capacitances of 2L Gr-SC, and 2L Gr (R)-SC calculated from the CD curves at the indicated current densities.


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Figure 3. Transparency and bending stability of the supercapacitor. (a) Digital photographs of the supercapacitor fabricated using PET substrates with and without transferred graphene films as supercapacitor electrodes. (b) Comparison of the area-specific capacitance of an ultrathin-graphene-film-based transparent and flexible supercapacitor with respect to the transmittance of the fabricated supercapacitor. Supercapacitors were fabricated based on planar (open circle) and wrinkled (closed circle) few-layer graphene, buckled 4-layer graphene without tensile strain (open square) and with 40% tensile strain (closed square), and stacked 2-layer graphene without (open star) and with (closed star) a redox-active interlayer. (c) CV curves of the 2L Gr (R)-SC obtained at a scan rate of 100 mV/s before and after bending with the indicated bending radii. (d) Normalized area-specific capacitance of 2L (R)SC as a function of cycle number in a bending state with a bending radius of 12.5 mm.



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Figure 4. Plausible mechanism for generation of the redox-active interlayer. (a) Reaction pathways for the oxidation of a p-AP and its inhibition in the presence of a reducing agent HZ. (b) Digital photographs of the p-AP solution (i) following its preparation and (ii) after 2 h, and (iii) a mixture of p-AP and HZ after 2 h. (c) Absorbance of 1 mM p-AP solution without HZ (fresh and after 2 h) and with 10 mM HZ after 2 h, and of 10 mM HZ. (d) CV curves of 2L Gr-SC, 2L Gr (AP)-SC, and 2L Gr (AP+HZ)-SC. CV curves were obtained at a scan rate of 50 mV/s.


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TABLE OF CONTENTS


Stacked Bilayer Graphene and Redox-Active Interlayer for

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