Dye Wastewater Cleanup by Graphene Composite Paper for

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Dye Wastewater Cleanup by Graphene Composite Paper for Tailorable Supercapacitors Dandan Yu, Hua Wang, Jie Yang, Zhiqiang Niu, Huiting Lu, Yun Yang, Liwei Cheng, and Lin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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ACS Applied Materials & Interfaces

Dye Wastewater Cleanup by Graphene Composite Paper for Tailorable Supercapacitors Dandan Yu†, Hua Wang*,†, Jie Yang†, Zhiqiang Niu‡, Huiting Lu*,†, Yun Yang†, Liwei Cheng†, Lin Guo*,† †

School of Chemistry, Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of

Ministry of Education, Beihang University, Beijing 100191, China ‡

Key Laboratory of Advanced Energy Materials Chemistry of Ministry of Education, Nankai University,

Tianjin 300071, China

ABSTRACT: Currently, energy crisis and environmental pollution are two critical challenges confronted by human beings. The development of smart strategies to address the above-mentioned issues simultaneously is significant. As the main accomplices for water pollution, several kinds of organic dyes with intrinsic redox functional groups such as phenothiazines derivatives, anthraquinone and indigoid dyes are potential candidates to replace the conventional pseudocapacitive materials. In this work, three typical organic dyes can be efficiently removed by a facile adsorption procedure using reduced graphene oxide-coated cellulose fiber (rGO@CF) paper. Flexible supercapacitors based on dye/rGO@CF electrodes exhibit excellent electrochemical performances, which are superior than or comparable to those of conventional pseudocapacitive materials based devices, presenting a new type of

1 Environment ACS Paragon Plus


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promising electrode materials. Moreover, benefiting from the high flexibility and considerable mechanical strength of the graphene composite paper, the operating potential and capacitance of the devices can be easily adjusted by tailoring the hybrid electrodes into different specific shapes followed by rational integrating. The smart design of these dye/rGO@CF paper-based electrodes shows that energy storage and environmental remediation can be achieved simultaneously.

KEYWORDS: flexibility, supercapacitor, pseudocapacitance, dye wastewater, graphene paper

INTRODUCTION With the booming development of portable and wearable electronics, the exploration of highlyefficient flexible energy-storage devices is drawing great attention.1-10 Generally, electrode materials dominate the electrochemical performance and fabrication cost of these flexible electronic devices.11-16 In order to improve the energy and power densities of flexible electrodes, conventional pseudocapacitive materials including transition metal oxides and hydroxides as well as conducting polymers, are often incorporated into capacitor-type carbonaceous materials.17-27 However, limited electrochemical performance, tedious fabrication procedure and rising manufacturing cost of these hybrid electrodes hamper their large-scale applications. For example, due to the poor electrical conductivity, unsustainable exploitation and rising price, the widespread use of transition compounds-based electrodes will be constrained in the near furture.17-20,22 Additionally, besides the complicated and time-consuming polymerization process, conducting polymers usually suffer from poor electrochemical stability and mass transport limitations within thick polymer layers.14,23-27 Thus, the development of novel flexible electrodes based on abundant, cheap and intrinsically redox-active materials for high-performance energy-storage devices is significant and challenging.


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Organic dyes, one of the most common pollutions in wastewater, are usually toxic and carcinogenic, which bring a large threat to the survival of marine organisms and serious risks to human health.28-31 Many technologies including adsorption, coagulation, microbiological treatments, photocatalysis and electrochemical oxidation/reduction have been utilized for the removal of dye pollutants, and most of the dyes are difficult to eliminate owing to their complex structures.32-35 Thanks to the existence of reversible redox pairs, many types of widely-used organic dyes such as phenothiazine,36-38 anthraquinone39-42 and indigoid42-45 have the potential ability for charge storage, and they may be utilized as active materials in energy-storage devices.46-49 For example, with phenothiazine functional group, methylene blue (MB) can store charges reversibly via a two-electron redox reaction.37 As a typical anthraquinone dye, alizarin red S (ARS) can be a candidate of pseudocapacitive materials, since a reversible transformation is conducted between anthraquinone and hydroquinone groups.39,42 Indigo carmine (IC), known as a water-soluble indigoide dye, can perform a reversible charge transfer redox reaction corresponding to conjugated carbonyl groups.44,45 Based on the above studies, low-cost and abundant organic dyes are promising pseudocapacitive materials to replace conventional transition metal oxides and hydroxides as well as conducting polymers. Herein, we present a new type of flexible electrodes based on dye-modified rGO@CF composited paper, in which MB, ARS and IC were loaded on rGO@CF paper through the fast and facile adsorption removal of organic dyes from their aqueous solution (Scheme 1a). Due to the good conductivity of rGO as well as the high flexiblity and bendability of graphene composite paper, flexible supercapacitors can be prepared by symmetrically assembling the hybrid electrodes with H2SO4-PVA gel electrolyte (Scheme 1a). The pseudocapacitive behavior of dye molecules enhance the specific capacitance, energy and power densities of graphene composite paper-based supercapacitors. Moreover, the operating potential and capacitance of the devices can be controllably adjusted by tailoring the hybrid electrodes



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into different shapes and rational integrating. Considering the diversity of molecular structures, other low-cost organic dyes containing redox functional groups can be used as substitutions of conventional pseudocapacitive materials. Our work provides new insights for the treatment and resource utilization of dye wastewater. RESULTS AND DISCUSSION With delocalized π electrons within sp2 carbon domains and surface oxygen-containing groups including hydroxyl and carboxyl, rGO can effectively interact with dye molecules via π-π stacking, hydrogen bonding, and electrostatic attraction (Scheme 1b).50 The dye/rGO hybrid materials are expected to achieve charge-storage properties due to the synergetic effects of pseudocapacitive organic dyes and electrical double-layer capacitive rGO, and should be an ideal electrode for flexible supercapacitors. First of all, we characterized the as-prepared rGO@CF paper. Being different from the original white CF paper, dark grey rGO@CF paper (Figure 1a) is caused by the appearance of rGO. Obviously, the size and shape of the as-fabricated rGO@CF paper depend on the CF substrate. Due to its suitable flexibility and mechanical strength, rGO@CF paper (Figure 1b) can be easily tailored and manipulated to construct various models (e.g., paper windmill). SEM images (Figure 1c and d) clearly show the surface and pores of CF are coated by the layered rGO sheets, while some wrinkles formed on rGO can improve its active sites. The composition of CF, GO@CF, and rGO@CF paper is probed by Raman and FTIR spectra. Raman spectra (Figure 1e) reveal that there are no characteristic peaks of pure CF paper and the intensity ratio ID/IG of rGO@CF is remarkable increased compared with that of GO@CF, demonstrating that GO was chemically reduced to rGO.50,51 Meanwhile, in constrast to GO@CF, the characteristic band of rGO@CF at 1726 cm-1 attributed to the stretching vibration of C=O weakens, further confirming the successful transformation of GO to rGO (Figure S1a).52-54 In addition, strain-


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stress curves are also measured to describe the toughness of graphene composite paper (Figure 1f). The tensile strength of rGO@CF is 4.94 MPa, which is higher than that of pure CF (0.76 MPa) and GO@CF (1.40 MPa) paper, and no rGO sheets are detached from rGO@CF paper when stretching deformation happens, suggesting the good mechanical stability of [email protected] Then, CV measurement is conducted to investigate the redox behavior of three rationally chosen organic dyes (Figure 2a). MB has two reversible peaks located at 0.39 and 0.34 V, and anodic peak at 0.15 V together with cathodic peak at 0.11 V can be observed in the CV curve of IC, while the peaks at 0.04 and -0.11 V are assigned to ARS. As described in Figure 2b, the reversible redox peaks are attributed to phenothiazine, anthraquinone and conjugated carbonyl groups in MB, ARS, and IC molecules, respectively.37,42,43 Furthermore, it can be found the color of dye solution with the initial concentration of 5 mg L-1 decolorizes within 24 h in the presence of rGO@CF paper including six-ply tissues with a length of 3.6 cm and a width of 3.4 cm (Figure 2c). In order to demonstrate that dye molecules have been adsorbed by rGO@CF, the as-fabricated dye/rGO@CF samples are detected by FTIR spectra (Figure 2d and S1b). In the spectrum of MB/rGO@CF, the stretching vibrations of C-N at 1335 cm-1 and C=C group in aromatic ring at 1600 cm-1 as well as the bending vibration of N-H at 885 cm-1, which are belonging to MB, can be observed.55 The characterization of ARS contained in ARS/rGO@CF paper is revealed by the asymmetric stretching vibration of C=O group at 1664 and 1637 cm-1, sulfonic group stretching vibration at 1238 cm-1 as well as S-O stretching mode at 812 cm-1.56 In the case of IC/rGO@CF, the bands at 1640 and 1613 cm-1 assigned to stretching vibration of C=O group and the peak at 818 cm-1 corresponding to that of S-O originate from IC.57 In addition, the experimental conditions were optimized to achieve high specific capacitance of dye/rGO@CF electrodes by adjusting the initial concentration of dye solution and the adsorption times.



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The plots in Figure S2 show that integral area of CV curves increases as the concentration enhances, but it is almost unvaried when the initial concentration of dye solution is high enough to a certain value. Besides, no obvious capacitance variation is presented as the times of adsorption increase. In the following fabrication procedures of the hybrid materials, the initial concentration of MB, ARS, and IC is kept at 200, 200, and 600 mg L-1, respectively, meanwhile, the adsorption experiments are only carried out for one time. Furthermore, as a proof-of-concept, the electrochemical properties of dye/rGO@CF electrodes are evaluated. The CV curves of dye/rGO@CF electrodes display one couple of redox peaks which should be attributed to the functional groups of dye molecules, suggesting that single dye has been combined with rGO@CF paper (Figure 3a). In addition, the peak current increases as the scan rate varies from 2 to 20 mV s-1 (Figure S3a-c) and the shapes of CV curves also remain (Figure S3a-d). Moreover, as revealed by DPV (Figure 3b), the mid-point potentials are consistent with anodic peak potentials in their corresponding CV curves, which further confirm the contribution of redox functional groups to the charge-storage capacity of dye/rGO@CF. The EIS reveals that the width of the semicircle impedance loop of rGO@CF is smaller than that of dye/rGO@CF electrodes due to the incorporation of insulating organic dyes (Figure S3e).58,59 Then, symmetric supercapacitors based on graphene composite paper electrodes were fabricated and studied. After dye immobilization, the capacitance of as-obtained dye/rGO@CF based supercapacitors is drastically boosted due to the redox functional groups of dye molecules, while MB/rGO@CF electrode has much higher capacitance than others (Figure 3c). The capacitances of these supercapacitors were further evaluated by galvanostatic charge/discharge test (Figure 3d). The linear charge/discharge profiles indicate ideal capacitive behaviors and the calculated specific capacitance of MB/rGO@CF, ARS/rGO@CF, IC/rGO@CF, and rGO@CF at the current density of 0.25 A g-1 are 179.6, 153.8, 161.5,


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and 131.3 F g-1, respectively, which is consistent with the conclusion derived from CV test. When the scan rate varies from 2 to 100 mV s-1, 53.5%, 55.9%, 58.7%, and 41.7% of original capacitance can be retained for supercapacitors based on MB/rGO@CF, ARS/rGO@CF, IC/rGO@CF, and rGO@CF electrodes, respectively, indicating their good rate performance (Figure 3e). In addition, the capacitances of dye/rGO@CF supercapacitors are almost unchanged under different bending states (Figure 3f), which demonstrate the superior flexibility of our devices. Furthermore, dye/rGO@CF supercapacitors exhibit excellent electrochemical stability with ~90% capacitive retention of their initial capacitances after 4000 charge/discharge cycles (Figure S4). In pratical situation, wastewater usually contains multicomponent organic dyes. Thus, rGO@CF paper was applied to adsorb the mixed dye solution including MB, ARS, and IC to get MB-ARS-IC/rGO@CF electrode. Compared with that of pure CF paper (Figure S5a), the SEM image of MB-ARS-IC/rGO@CF (Figure S5) is brighter, which is attributed to the good conductivity of rGO. Moreover, the elemental mapping images demonstrate that dye molecules are homogeneously distributed onto the surface of rGO@paper. In a three-electrode cell, three couples of reversible redox peaks corresponding to the functional groups of dye molecules presents in the CV curves (Figure 4a), which demonstrates that MB, ARS, and IC have been successfully incorporated into rGO@CF paper together. In addition, the peak current and seperation increase with the san rate, suggesting the quasi reversible electron transfer of organic dyes. Then, supercapacitors based on MB-ARS-IC/rGO@CF electrode were fabricated and characterized. The shapes of CV (Figure S6a) and charge/discharge curves (Figure S6b) can be almost maintained when the operating potential extends to 1.2 V, which demonstrate the good capacitive performance of MB-ARS-IC/rGO@CF supercapacitors. To keep the stable capacitance delivery and high coulombic effciency of supercapacitors, the potential window of the following experiments is determined to be 0-1 V. Moreover, no evident shape change of CV curves (Figure 4b) can be observed



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under different bending states, demonstrating the high flexiblity of MB-ARS-IC/rGO@CF supercapacitors. When the current density increases from 0.05 to 2.5 A g-1, the specific capacitances vary from 178 to 113 F g-1 for MB-ARS-IC/rGO@CF supercapacitors and 154 to 105 F g-1 for rGO@CF supercapacitors (Figure 4c). Once the current is set as its initial value, the capacitances can be restored to 172 and 151 F g-1, respectively, suggesting their superior rate performance and considerable stability. Meanwhile, the long-time cyclability was also investigated, and it revealed that the specific capacitance of MB-ARSIC/rGO@CF supercapacitors still remained at 160 F g-1 (∼90.9%) after 5000 charge-discharge cycles (Figure 4d). Besides, the time constant τ0 (τ0 = 1/f0) that represents the minimum time needed to discharge all the energy from the device with an efficiency of >50% is calculated to be 6.25 s based on the plot of the phase angle versus the frequency (Figure 4e), in which f0 is the characteristic frequency at the phase angle of -45°.26,62 EIS measurement reveals MB-ARS-IC/rGO@CF supercapacitors have an equivalent series resistance (ESR) of 22.4 Ω and the relatively vertical curve in the low frequency region indicates the device has an ideal capacitive performance, indicating the porous structure and high conductivity of graphene composite paper are beneficial to the effective ion diffusion and electron transport throughout the electrodes.60-62 Considering the practical demands for energy and power densities, flexible MB-ARS-IC/rGO@CF supercapacitors were integrated in series or parallel configurations using a traditional method. When the number of cells connected in series varies from one to four, the working voltage can be extended from 1 to 4 V with no capacitance decay as revealed by applying CV and galvanostatic charge-discharge tests (Figure S6c and d). In addition, the multi-fold capacitances can been vertified when multiple supercapacitors are connected in parallel (Figure S6e and f). In order to further evaluate the practical applicability of MBARS-IC/rGO@CF supercapacitors, the Ragone plot (Figure 4f) is constructed on the basis of the curve


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of specific capacitance versus current density. The maximum energy density of our devices is calculated to be 3.89 Wh kg-1 at the power density of 1250 W kg-1 and it is even higher than or comparable to that of some symmetric supercapacitors based on conducting polymers and transition metal compounds.16,6370

The electric double-layer capacitance and good conductivity of rGO as well as the reversible

functional groups of dye molecules synergistically contribute to the excellent electrochemical performances of these paper-based supercapacitors. Due to the high flexibility, proper mechanical strength, and controllable area of graphene composite paper, sophisticated energy-storage integrated devices can be fabricated by simply tailoring and assembling MB-ARS-IC/rGO@CF. For example, as depicted in Figure 5a and b, three supercapacitors connected in parallel can be achieved by stacking two E shape electrodes (Figure S7a) with H2SO4-PVA gel electrolyte. The specific capacitance of this device is triple that of single supercapacitors, which can be calculated from the CV (Figure 5d) and charge/discharge (Figure 5e) curves. When the left-hand supercapacitors was cut down (Figure 5c), two thirds of the initial capacitance could be remained. In addition, if the hybrid electrodes are tailored into T and L shapes electrodes (Figure S7b), we construct another device, in which mutiple cells are connected in two parallel included two series (Figure 5f and g). The working potential can be extended to 2 V (Figure 5i and j) and the capacitance can be proportionally controlled when two cells connected in parallel were seperated (Figure 5h). It can be inferred that the paper-based electrodes can be tailored into different shapes and sizes to meet various energy-storage requirements. CONCLUSION In summary, a new type of flexible electrodes based on dye-modified rGO@CF paper has been successfully prepared by a facial adsorption and drying strategy. Three kinds of widely-used organic dyes including phenothiazine, anthraquinone and indigoide dyes can be efficiently removed from their



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aqueous solution, realizing the wastewater treatment and resource utilization. The flexible supercapacitors based on these dye/rGO@CF paper electrodes exhibit excellent electrochemical performances, and the energy and power densities are higher than or comparable to those of some symmetric supercapacitors based on conducting polymers and transition metal compounds. The porous structure and high conductivity of graphene composite paper facilitate the effective ion diffusion and electron transport throughout the binder-free and conductor-free paper-based electrodes, and the synergetic effects of pseudocapacitive organic dyes and electrical double-layer capacitive rGO are favorable to achieve high sepecific capacitance. Moreove, being different from traditional series or parallel connections, advanced integrated devices can be fabricated by stacking the hybrid electrodes with specific shapes due to the good tailorability, high flexibility and considerable mechanical strength of rGO@CF paper. With intrinsic redox functional groups, other phenothiazine derivatives, anthraquinone and indigoide dyes should be promising substitutions of conventional pseudocapacitive materials. It can be expected that the specific capacitance of the devices can be further enhanced by increasing the adsoption content of organic dyes in electrode materials, implying their significantly potential application in other energy-storage devices such as lithium-ion batteries. Our work achieves the function that kills two birds with one stone towards the treatment of organic dye wastewater and development of highly efficient electrochemical energy-storage materials, and may be expanded to the resource-utilization of other versatile effluent containing the redox moieties. MATERIALS AND METHODS Synthesis of rGO@CF Paper. Graphene-coated CF paper was prepared on the basis of previous literature.14 The aqueous GO dispersion (flake diameter < 500 nm) was purchased from XianFeng Nano. A piece of CF paper with three-ply tissues was flated on a beaker of GO suspension with an initial concentration of 2.0 mg mL-1. GO would be adsorbed by CF paper until saturated. To evaporate the


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residual water, GO@CF paper was left in the fume hood after being seperated from the GO dispersion. Repeated adsorption and drying were conducted until GO was completed loaded on the surface CF paper. GO@CF paper (12.5 × 5 cm2) was put into a Teflon-lined autoclave containing 20 mL GO dispersion (3.5 mg mL-1) and 1 mL hydrazine hydrate (80%). Then it was heated at 100 ℃ for 6 h to reduce GO. After cooling down to room temperature naturally, dark grey rGO@CF paper was obtained after being washed with deionized water and ethanol, respectively. The mass loading of rGO sheets on the CF paper was measured to be 0.48 mg cm-2. Preparation of Dye-Modified rGO@CF Paper. Organic dyes such as MB, ARS, and IC could be loaded on the as-fabricated rGO@CF paper by a facile adsorption. Batch adsorption experiments were performed under the same condition. Briefly, rGO@CF paper (2.5 × 1 cm2) was immersed in 20 mL dye solution. The composite paper was seperated from the solution an hour later. After being heated at 60 ℃ for 12 h, dye/rGO@CF paper was obtained. Besides, MB, ARS, and IC were dissloved into water all together. In this mixture, the concentration of MB, ARS, and IC was 200, 200, and 600 mg L-1, respectively. Similar experimental procedures were conducted to fabricate MB-ARS-IC/rGO@CF paper. Fabrication of All-Solid-State Supercapacitors. Firstly, H2SO4-PVA gel electrolyte was prepared to fabricate supercapacitors based on dye-modified rGO@CF paper. 9 g H2SO4 was added into 90 mL deionized water and then 15 g PVA (Mw = 67 000, Aladdin) was mixed with this solution. The mixture was kept at 85 ℃ under continuous stirring until PVA was completely dissloved. Then, the surface of dye-modified rGO@CF paper (1.5 × 0.35 cm2) was coated with H2SO4-PVA gel electrolyte. Carbon wires were served as current collectors between two-ply tissues, while CF paper was used as separators to avoid short circuit. These units were assembled together on the glass slide. Finally, the excess water contained in the supercapacitor was vaporized naturally to obtain flexible supercapacitors, which could be peeled off the glass.



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Characterizations. The morphology and the microstructure of rGO@CF paper were captured by using a field-emission gun scanning electron microscope (FEI Quanta 250 FEG). Fourier transform infrared (FTIR) spectra were collected by using Nicolet iN10MX spectrophotometer in the ATR mode. Raman spectra were obtained at an excitation wavelength of 514 nm. The optical images were captured by Canon 600D. The tensile mechanical properties were measured using a Shimadzu AGS-X Tester with a dynamic mechanical analyzer (DMA). Static tensile tests were evaluated at a load speed of 0.5 mm min-1. Electrochemical Measurements. Cyclic voltammetry (CV), galvanostatic charge-discharge, differential pulse stripping voltammetry (DPV), and electrochemical impedance spectroscopy (EIS) measurements were carried out at a CHI 660D electrochemical workstation. In a three-electrode system, CV curves of paper-based electrodes were obtained in H2SO4-PVA gel electrolyte with carbon cloth as the counter electrode and Ag/AgCl as the reference electrode. The potential windows for MB, ARS, and IC were -0.1-0.7, -0.5-0.3, and -0.3-0.5 V, respectively. In the DPV test, the parameters were set as follow: potential increment 4 mV, amplitude 50 mV, pulse width 0.05 s, and pulse period 20 ms. EIS measurement was performed at an open circuit voltage in the frequency range from 0.01 Hz to 100 MHz at a amplitude of 5 mV. The electrochemical cycling performances of supercapacitors were detected in LAND testing system at the current density of 1A g-1. The specifc capacitance (C) of an electrode in flexible supercapacitors, energy density (E) and power density (P) of supercapacitors can be calculated on the basis of the following equations (1)-(3)

C=

4 × I × ∆V M × ∆t

E=

C × ∆V 2 8

(1)

(2)


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P=

E ∆t

(3)

in which I represents the discharge current, M is the mass loading of active materials based on two electrodes, ∆V and ∆t correspond to the time and the potential window of discharge curves, respectively. ASSOCIATED CONTENT Supporting Information FTIR spectra of CF, GO@CF, rGO@CF, MB, ARS and IC, SEM images of CF and MB-ARSIC/rGO@CF paper, CV curves and EIS of dye/rGO@CF (dye = MB, ARS, and IC) measured in a threeelectrode system, cycling performance of supercapacitors based on rGO@CF and dye/rGO@CF (dye = MB, ARS, and IC) electrodes, CV and charge/discharge curves of MB-ARS-IC/rGO@CF supercapacitors, and the schematic fabrication process of tailored supercapacitors. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Tel&Fax: +86-10-82338162. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT



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The authors acknowledge the financial support of the National Natural Science Foundation of China (51502009, 51532001), National Key Basic Research Program of China (2014CB31802). REFERENCES (1)

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Scheme 1. (a) Fabrication of dye-modified rGO@CF electrodes and construction of flexible graphene composite paper-based supercapacitors. (b) Schematic illustration of the interactions between MB and rGO in MB/rGO@CF paper as well as the mechanisms of hydrogen ion storage and electron transport.



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Figure 1. Optical images of (a) a flat rGO@CF paper and (b) paper windmill produced by tailoring rGO@CF. (c,d) SEM images of rGO@CF paper. (e) Raman spectra and (f) stress-strain curves of CF, GO@CF, and rGO@CF paper.


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Figure 2. (a) CV curves of MB, ARS, and IC aqueous solution at the scan rate of 20 mV s-1 measured in a three-electrode cell using 1 M H2SO4 as electrolyte. (b) Redox mechanisms of MB, ARS, and IC in the acid electrolyte. (c) Photographs of MB, ARS, and IC aqueous solution absorbed by rGO@CF paper. (d) FTIR spectra of dye/rGO@CF and rGO@CF.



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Figure 3. (a) CV curves at the scan rate of 2 mV s-1 and (b) DPV curves of graphene composite paperbased electrodes measured in a three-electrode system. (c) CV curves of graphene composite paperbased supercapacitors at the scan rate of 2 mV s-1. (d) Charge/discharge curves of supercapacitors at the current density of 0.25 A g-1. (e) The specific capacitance of electrodes in supercapacitors against san rate. (f) The specific capacitance of electrodes in flexible supercapacitors under different bending states at the scan rate of 20 mV s-1.


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Figure 4. Electrochemical performances of MB-ARS-IC/rGO@CF supercapacitors: (a) CV curves of the hybrid electrodes at different scan rates in a three-electrode cell. (b) CV curves of supercapacitors under different bending states at the scan rate of 20 mV s-1. (c) Cycling stability of supercapacitors upon progressively varying the current density. (d) Specific capacitance of MB-ARS-IC/rGO@CF electrodes in supercapacitors against the number of charge/discharge cycles at the current density of 1.0 A g-1. The inserted plot is the charge/discharge curves at different current densities. (e) The plot of impedance phase angle versus frequency in the EIS of supercapacitors. The inset is the Nyquist plot. (f) Ragone plots of MB-ARS-IC/rGO@CF supercapacitors as compared to that of some symmetric supercapacitors.



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Figure 5. Tailorable and integrated devices based on MB-ARS-IC/rGO@CF electrodes: (a) Simulated structures of three supercapacitors connected in series by stacking two E shape electrodes. (b,c) Photographs of the tandem devices before and after one-third cut. (d) CV and (e) charge/discharge curves of the integrated devices before and after being tailored. (f) Configurations of two parallel cells contained two tandem supercapacitors by assembling T and L shapes electrodes. (g,h) Optical images of the series-parallel device before and after being symmetric tailored. (i) CV and (j) charge/discharge curves of the series-parallel devices before and after being tailored. The insets in Figure 5d and 5i represent the corresponding circuits. The scan rate is 10 mV s-1, and the current density is 1 A g-1.


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