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graphene nanocomposites for supercapacitors of high volumetric energy density
Accepted Manuscript Compact, flexible conducting polymer/graphene nanocomposites for high volumetric supercapacitors Mahmoud Moussa, Maher F. El-Kady, Safwat Abdel-Azeim, Richard B. Kaner, Peter Majewski, Jun Ma PII:
S0266-3538(17)32365-5
DOI:
10.1016/j.compscitech.2018.02.033
Reference:
CSTE 7109
To appear in:
Composites Science and Technology
Received Date: 17 October 2017 Revised Date:
26 January 2018
Accepted Date: 20 February 2018
Please cite this article as: Moussa M, El-Kady MF, Abdel-Azeim S, Kaner RB, Majewski P, Ma J, Compact, flexible conducting polymer/graphene nanocomposites for high volumetric supercapacitors, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.02.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Compact, Flexible Conducting Polymer/Graphene Nanocomposites for High Volumetric Supercapacitors
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Mahmoud Moussa1, 2, 3, Maher F. El-Kady4, 5, Safwat Abdel-Azeim6, Richard B. Kaner4, , Peter Majewski1, Jun Ma1,2*
Future Industries Institute and 2School of Engineering, University of South Australia, Mawson Lakes, SA5095, Australia,
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Department of Chemistry, Faculty of Science, Beni-Suef University, Beni-Suef 62111, Egypt,
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Department of Chemistry & Biochemistry and California NanoSystems Institute, University of California, Los Angeles
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(UCLA), Los Angeles, USA,
Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt,
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Centre for Petroleum and Minerals, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Saudi Arabia
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ABSTRACT
Graphene is extensively utilized in energy storage devices because of its high surface area and electron conductivity as well as ease of electrode fabrication. But graphene sheets often stack themselves in polymeric matrices leading to poor capacitive performance. This
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problem was addressed in this study by developing and inserting respectively two types of nano-sized conducting polymers into graphene interlayer spacing. The resulting hydrogel
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composite electrodes demonstrated efficient electron transfer for fast and reversible Faradaic reactions at the interface. Theoretical modelling by the density functional theory
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suggested that the reduction involve 2H+ transfer steps from polyaniline to graphene oxide: the first step would be an epoxy-ring opening process after activation of the C–O bond, and the second step would be C–O rupture leading to a de-epoxidation process. This binder-free electrode demonstrated high cycling performance and ultrahigh volumetric capacitance 612 F cm−3, being 10 times higher than the activated carbon used in the current industry. The
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study represents a step forward towards the fabrication of flexible, high-energy density supercapacitors.
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Keywords: graphene, conducting polymers, volumetric capacitance 1. Introduction
Batteries and supercapacitors have received great attention due to their increasing
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importance to our daily life. Batteries store and deliver charge through Faradaic processes
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[1-5], providing high energy density, but they are limited by low power density and cycle life [6]. On the other hand, supercapacitors store and deliver charge in electrochemical double layers (EDLs), and they feature high power density, rabid charge-discharge, long cycling stability, no maintenance and environmental friendliness [7-9]. Nevertheless, it remains a formidable challenge to improve the volumetric capacitance rather than
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gravimetric capacitance, because current portable electronics and electric vehicles need to deliver maximum energy in a very limited space [10-12]. Carbon-based materials have been the best option to-date for energy storage due to their highly porous nature and high
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conductivity. However, these materials suffer poor volumetric capacitance because of their
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low packing density [13-17]. Moreover, binders and conductive additives are needed, which reduce the volumetric capacitance, since these additives have little or no contribution to the overall capacitance. Graphene has been widely examined as an important class of active electrode
materials for ultracapacitors owing to its unique flexible and 2D structure. Recently, many forms of graphene have been developed such as fibres, membranes and foams [4, 14, 1822], but most of these deliver only a low volumetric capacitance. A compressed, three-
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dimensional, porous graphene oxide framework showed a gravimetric capacitance of 310 F g−1 and a volumetric capacitance of 220.1 F cm−3 in aqueous electrolyte. The hierarchical
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porous structure of holey graphene provided large ion-accessible surface area and efficient electron and ion transport pathways [23]. Graphene compact film provided a volumetric capacitance of 255.5 F cm−3 with two-electrode configuration; the liquid-mediated process
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avoided the restacking of graphene layers to obtain low ion transport resistance [19]. Highly dense graphene (with a density of 1.58 g cm-3) was developed by evaporation-
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induced drying of a graphene hydrogel, which showed volumetric capacitance 376 F cm−3 for neat graphene materials [10]. MXenes have shown promise for electrochemical capacitors with volumetric capacitance of 900 F cm−3 [24], but their fabrication processes require multi-steps and more efforts than those for conducting polymer/graphene composites [24, 25]. From our perspectives, composite materials would be the best solution
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to improve the overall performance due to synergistic effects. Recently Chen et al. introduced a novel direct electrode deposition of graphenepolyaniline using a filtration method [26], and this one-cell supercapacitor provided a
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volumetric capacitance of 199 F cm-3 at a current density of 5 A g-1 with a film density of 2
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g cm-3. Wu et al. used a similar method to prepare a composite film of chemically converted graphene and polyaniline (PANi) nanofibers [27], where the nanofibers were uniformly sandwiched between graphene sheets, producing a high-quality flexible film. The electrode of 0.76 g cm-3 in density showed a volumetric capacitance of 160 F cm-3. These capacitance values are not ideal due to low out-of-plane electrical conductivity caused by the weak interlayer van der Waals interactions [28]. Therefore, it is indispensable to develop new
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graphene electrodes possessing a 3D structure with high density to improve the volumetric performance.
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Here we demonstrate a cost-effective, novel method for the development of conducting polymer/graphene composite electrodes, where two types of nano-sized conducting polymers play multi-roles such as reducing agents, spacers and pseudo-
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capacitive materials. Graphene oxide is reduced by conducting polymers such as PANi in an emeraldine salt form, and this process is supported by a density functional theory. The
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PANi/graphene composite film delivers a volumetric capacitance of 369.5 F cm−3 at 1 A g-1 in 1.0 M H2SO4, which increases to 612 F cm−3 after further reduction by hydroiodic acid. 2. Experimental
2.1 Synthesis of conducting polymer/graphene composite films Polyaniline nanotubes (PANi-NT), polypyrrole (PPy) and graphene oxide (GO)
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were prepared as described in the supporting information. Conducting polymer/graphene composites were synthesised as follows: all the composites were designed to contain 10
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wt% polymers; 60 mg GO and 7 mg polymer were ultrasonically dispersed for one hour in the deionized water, respectively. Then, the two batches of suspension were mixed and
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sonicated for one more hour. Next, the mixture was kept at 95 °C for 30 h without any disturbance to form a cylindrically shaped composite, which was then smashed by vigorous shaking in 60 ml deionized water. Next, the suspension was filtered through a PTFE membrane to produce a freestanding composite film, followed by drying under vacuum for 4 h at room temperature. For further reduction, the PANi-NT/graphene composite was
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immersed in HI solution at 70 °C for 7 h. Note that the PPy/graphene composite film has no sufficient mechanical stability for further reduction with HI.
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2.2 Supercapacitor fabrication To fabricate our supercapacitors, each composite film was cut into small (1 cm2) rectangular pieces, followed by dipping in 1.0 M H2SO4 overnight. The two composite
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films were attached to platinum foils as the current collectors, between which a piece of filter paper was sandwiched as a separator. All electrochemical tests including cyclic
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voltammetry (CV), galvanostatic charge/discharge (CD) and electrochemical impedance spectroscopy (EIS) were carried out using a two-electrode cell configuration by a CHI 660E electrochemical workstation. 2.3 Quantum calculations
Density functional theory (DFT) calculations were carried out to study the
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mechanism of graphene oxide reduction by PANi, focusing on the de-epoxidation of graphene oxide by the emeraldine oxidation state of PANi. The proposed model consists of
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a large graphene sheet (123 atoms) with an epoxy functional group and one PANi molecule consisting of four aniline monomers. The accuracy of DFT has been verified by
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benchmarking with the coupled cluster CCSD(T) method [29]. The geometric optimizations were performed using M05-2X /6-31G** [30-32]. Frequency calculations were carried out on all stationary points of the reaction profile to verify the maxima (characterized with only one negative frequency corresponding to the reaction coordinate) and the minima (characterized with positive frequencies). The vibrational corrections to the electronic energies were estimated at the same level of the geometric optimization (M05-
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2X/6-31G**). Single point energy calculations were performed to explore the solvent effects on the reaction profile using the Polarizable Continuum Model (PCM) [33] as
3. Results and discussion
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3.1 Conducting polymer/graphene composites
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implemented in the Gaussian 09 code [34].
We propose the following criteria for design of flexible supercapacitor electrode
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materials to obtain high volumetric capacitance. First, such materials should be selected to create large charge storage capacity and thus high energy density. Second, it is essential to control the electrode packing density for ideal volumetric performance. Third, the materials should have interconnected porous structure with robust mechanical durability. Figure 1 illustrates these design criteria. Figure 2 contains a schematic of the methodology for the
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fabrication of conducting polymer/graphene composites. The methodology suits all composites containing any conducting polymers, although we use polyaniline (PANi) and
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polypyrrole (PPy) as two examples in this study. Mixing the colloidal GO solution with either PANi nanotubes (PANi-NT) or a PPy solution created an electrostatic interaction
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between GO surface and PANi-NT or PPy. Heating the mixture at 95 °C for 12 h converted the brown colour of GO into black, which indicated reduction; PANi-NTs or PPy thus had four roles: (i) reducing GO, (ii) physically crosslinking the composites, (iii) working as a spacer and (iv) contributing to pseudo-capacitance [35, 36]. Cylinder-like samples were produced through continuous heat treatment for 30 h, and then these were dispersed in deionized water by vigorous shaking to produce colloidal suspensions that can be filtered
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and easily peeled off, resulting in compact, flexible and freestanding composite films (Figure 3a and 3d). SEM images of these films are shown in Figure 3c, 3f, S1c and S1f,
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where the graphene composite films exhibit an ordered format in their layered structure with obvious porosity caused by filtration. The micrographs also show that PANi-NT and PPy are firmly embedded between graphene layers, which is a desired morphology for the
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composite because it is well known for sufficient interspace and high accessibility to electrolyte ions [37].
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The structural integrity of graphene oxide and its composites was investigated using Raman scattering (Figure 4a). Graphene displays a D-band (1348 cm−1) and a G-band (1590 cm−1), confirming the conversion of sp2 to sp3-hybridized carbon and the vibration of an sp2-hybridized carbon, respectively. The ID/IG intensity ratio provides an important tool to gauge the degree of disorder and information on sp2 domains. After GO reduction by the
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conducting polymers, the ID/IG ratio increases from 0.88 for GO to 1.06 and 1.01 respectively for PANi-NT/graphene and PPy/graphene. Figure 4b and S2 provide the XRD
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patterns of GO, PANi-NT, PPy, PANi-NT/graphene and PPy/graphene. The sharp, intense diffraction located at 2θ = 10.6° to the interplanar spacing of 0.83 nm for GO sheets – a
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substantial increase of the interlayer spacing from 0.33 nm for graphite to 0.83 nm for GO indicating significant oxidation [38]. PANi shows its characteristic diffraction peaks (at 2θ values of 20.1° and 25.3°), which are attributed to the crystallinity and coherence length of polymer chain orientation [39]. The amorphous nature of neat PPy is confirmed by a broad diffraction at 2?? = 26°. The reducing ability of conducting polymers is clearly evident in the XRD pattern of PANi-NT/graphene and PPy/graphene, where the characteristic
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diffraction of GO has disappeared, and broad ones appear at 2θ = 24.7° and 24.91° for PANi-NT/graphene and PPy/graphene, respectively.
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The elemental composition of GO and its composites was identified using highresolution X-ray photoelectron spectroscopy (XPS; Figure 3c and S2c). The GO shows a C/O ratio of 2.23, indicating a significant degree of oxidation. After reduction with the 10
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wt% of conducting polymers, PANi-NT/graphene and PPy/graphene respectively exhibit C/O ratios of 7.58 and 7.57. This should enhance the π–π interactions between the graphene
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and the conducting polymer backbone, thus improving the electron transfer and the electrochemical characteristics [40]. The C1s spectrum of GO (Figure 4d) contains three main peaks: the sp2 aromatic rings (C–C; 285.05 eV) and the oxygen containing groups (C– O 287.1 eV and C=O 288.5 eV). Figure 4e presents the C1s of the PANi-NT/graphene composite, confirming the presence of C=C (284.8 eV), C–N– (285.39 eV) and C–O (286.9
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eV) bonding [41]. The weak intensity of the C–O bond located at 286.9 eV is due to residual oxygen containing functional groups left in the graphene composite after reduction. For the N1s spectrum of the PANi-NT/graphene composite (Figure 4f), the curve exhibits
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three components: =NH– (399.5 eV), –NH– (401.0 eV) and –NH+– (402.1 eV) [42],
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indicating successful incorporation of PANi-NTs into the composite film. The analysis agrees with the preceding SEM observation. 3.2 Quantum Chemical Calculations We by DFT calculations investigated the mechanism involved in the effective
removal of epoxide groups from GO by PANi which acted as a reducing agent (deepoxidation) (Figure 5a). During the reduction, the first proton transfer facilitates C–O
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bond activation and opens the epoxy ring (Figure 5b), producing an intermediate (INT1); this step needs 7.19 kcal mol-1 in vacuum, and it is overall an exothermic step (-9.45 kcal
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mol-1). In solution, the epoxy-ring opening has an even higher activation barrier of 11.54 kcal mol-1; thus the reaction is less exothermic (-4.66 kcal mol-1) because of the need to stabilize the reactant (R) induced by interaction with the solvent; the interaction is
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responsible for the energetic changes in the reaction profile. INT1 would form after the C– O activation rearranges itself to receive a second proton transfer from PANi, creating the
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final product – reduced graphene oxide (P). This step involves higher activation barriers of 26.45 kcal mol-1 and 28.04 kcal mol-1 in vacuum and solution, respectively. The final release of water is highly favorable with free energies of -22.17 (in vacuum) and -27.20 kcal mol-1 (in solution). Our calculations estimate that the de-hydroxylation should be a rate-determining step for de-epoxidation. The profile for GO reduction indicates that both
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de-epoxidation and de-hydroxylation through PANi are feasible at room temperature. 3.3 Electrochemical performance of the composite
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Our composite design represents an advance in the development of electrode materials for supercapacitors through (i) the incorporation of conducting polymers that not
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only contributes pseudocapacitance but prevents graphene sheets from re-stacking, (ii) graphene sheets in the matrices that can markedly improve the electrical conductivity and also the potential cycling performance of the polymers, (iii) the inter-connected porous structure that enhances the electrochemical interfacial area (Figure 3 and S1) and (iv) no binder needed. Figure 6a illustrates the cyclic voltammetry (CV) curves of a PANiNT/graphene composite film in 1.0 M H2SO4. The curves are nearly rectangular with a redox peak, indicating pseudo-capacitance contributed by the presence of PANi-NTs and
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residual oxygen groups on the reduced graphene sheets. The CV curves of the PANiNT/graphene electrodes at various scan rates, demonstrating their high capacitance even
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when operated over a wide range of scan rates. The charge/discharge (CD) curves of the PANi-NT/graphene composite films are presented in Figure 6b; the slight deviation from linearity should be caused by pseudo-capacitance from the PANi-NTs. The discharge time
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is longer due to a combination of EDLC from the graphene sheets and Faradiac capacitance from the PANi-NTs. Figure 6c illustrates the gravimetric (Cwt) and volumetric (Cvol)
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capacitances, which were calculated at a film density of 1.19 g cm-3 from the charge/ discharge curves according to the following equations:
∁wt = ∆
∆
∁vol =
4 (1) ∆ ( ) ∆
, m and V are the applied constant current (A), the discharge curve slope, and
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where I,
4 ∆ ( ) ∆
the total mass (g) and volume (cm3) of the two electrodes, respectively. The PANiNT/graphene composite electrode shows a Cwt 323.11 F g-1 at a current density of 1 A g-1,
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which is higher than those tested under similar conditions, such as 214.48 F g-1 for the
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PANi-PAMPA/graphene composite electrode and 310 F g-1 for a holey graphene framework [19, 23]. Using the active material volume, the composite Cvol was calculated as 369.5 F cm-3 at 1 A g−1, which is higher than the reported values measured under similar conditions. The dense graphene was reported to have a Cvol of 376 F cm-3 when measured at 0.1 A g−1, but such a current density may not meet practical requirements [18, 19, 43, 44]. The electrochemical performance of PPy/graphene composite film is illustrated in Figure S3. The high electrochemical performance of these two groups of composites means that
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our composite design should suit all of conducting polymers for the development of flexible supercapacitors, because the unique composites morphology can provide large
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accessible graphene surfaces, reduce the dynamic resistance of electrolyte ions and facilitate the electron transfer due to the strong π–π electron and hydrogen bond interaction (Figure 3c).
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The ion transfer properties of the electrodes based on these two composites were studied using electrochemical impedance spectroscopy (EIS). Figure 6d displays the
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Nyquist plots of these composite electrodes in 1.0 M H2SO4; both represent a straight line in the low-frequency region and a semicircle (insert in Figure 6d) in the high-frequency region. The PANi-NT/graphene composite electrode demonstrates a more vertical graph, which is explained by a nearly ideal capacitance behaviour. Its equivalent series resistance (ESR) is estimated to be 0.42 Ω, which is lower than that of a PPy/graphene composite
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(0.62 Ω); thus it carries low resistance due to the geometry of PANi-NTs, providing more interface for enhancement of the charge transfer. The dependence of the phase angle on the
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frequency is shown in Figure S3d. The PANi-NT/graphene phase angle is close to -90° at low frequencies, showing an ideal capacitive behaviour. The time constant τ0 (the inverse
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of the characteristic frequency (ƒ0) at a phase angle of –45°) was found to be 0.2 s for the PANi-NT/graphene composite and 1.08 s for PPy/graphene composite; this rapid frequency response of the PANi-NT/graphene composite demonstrates a significantly enhanced ion transport rate due to the large, accessible surface area (Figure 3c). 3.4 Volumetric and gravimetric capacitance enhancement for PANi-NT/graphene composite
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Hydriodic acid (HI) was selected, because it was able to (i) assist a further reduction of GO sheets while maintaining the film flexibility and (ii) act as a source of redox active
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electrolyte to enhance the device performance [45, 46]. The redox active electrolyte comprises iodine anions and neutral iodine (I−/I2). While the composite electrode stores the charge through (i) fast yet reversible redox reactions that are intrinsic to PANi and (ii) ion
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adsorption on graphene surface, the I-/I2 species may increase the capacitance further by contributing to the redox reactions (Figure 7a). These processes may work synergistically
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to produce ultrahigh volumetric capacitance as seen from the CV graphs in Figure 7b. A large capacitance response is found in comparison with the unreduced one. The gravimetric charge/discharge (CD) curves of the reduced PANi-NT/graphene electrodes are shown in Figure 7d, demonstrating a competitive electrochemical performance over a wide range of current densities. Figure 7e contains gravimetric (Cwt) and volumetric (Cvol) capacitances of
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the reduced composite film in 1.0 M H2SO4 at different current densities; it is apparent that the Cwt of 513.5 F g-1 and the Cvol of 612 F cm-3 for the reduced electrodes are much higher than those for the unreduced composite (Cwt = 323.11 F g-1 and Cvol = 369.5 F cm-3) at 1 A
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g-1. We can say with strong confidence that the value of Cvol obtained for our PANi-
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NT/graphene composite-HI electrodes is the highest one reported to date amongst all the flexible carbon-based supercapacitor electrodes in H2SO4 at 1 A g-1. Figure 8a exhibits a Ragone plot with the energy (Evol) and the power (Pvol) densities for the hydrogel films and other carbon based materials [47-52] based on these equations:
Evol =
0.125 ∁vol (∆ )2 3.6
Pvol =
vol # 3600
∆
(2)
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For the unreduced PANi-NT/graphene composite electrodes, the maximum energy and power density values are respectively 8.21 Wh l-1 and 2896.9 W l-1. The reduction was
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found to lift the energy density up to 13.6 Wh l-1. The supercapacitor based on PANi-NT/graphene with HI was investigated for flexibility during charging and discharging. In Figure 8b, the charge/discharge curves
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under different bending conditions (0, 90 and 180°) all overlap well, meaning that this device exhibits nearly ideal electrochemical performance during deformation owing to the
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high mechanical strength of the composite film. Therefore, the electrode materials we have developed are suitable to many flexible and portable devices applications. A tandem cell with supercapacitor units connected in series is a common method to produce high voltage for practical applications. We made a 3-unit tandem cell with a potential window up to 2.4 V (Figure 8c) by assembling three-supercapacitor units in series based on a PANi-
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NT/graphene with HI in H2SO4. When it was used to power a red light emitting diode (LED), the high capability in each unit can be confirmed from the unchanged discharge
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time. Figure S4b shows snapshots of the 3-unit tandem cell after charging for 30 seconds at 2.4 V; this can deliver sufficient energy to power the LED for over 5 minutes. The cycling
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stability of the composite electrodes was tested at a current density of 20 A g−1 in 1.0 M H2SO4 for 5000 cycles (Figure 8d), providing a capacity retention of 97.6%. The charge/discharge curves for the first and last three cycles are nearly the same (see the inset to Figure 8d), indicating high reversibility and long-term stability due to the 3D hydrogel structure which improves the mechanical strength and stabilizes the composite during the charge-discharge process.
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4. Conclusions In summary, we have developed a facile one-step synthesis approach to fabricate
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3D flexible freestanding electrodes for energy storage devices. This approach should suit all of conducting polymers. The conducting polymer serves not only as an excellent reducing agent for graphene oxide, but as a spacer that prevents graphene sheets from
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restacking in the composite film; this provides a three-dimensional structure where graphene sheets are exfoliated, offering more interfaces for reversible pseudo-capacitance
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reactions. Theoretical modelling indicates that the graphene oxide reduction involves 2H+ transfer from PANi; the first proton transfer causes the epoxy-ring opening and the second transfer assists the C–O rupture which is a rate-determining step for the de-epoxidation process. The PANi-NT/graphene supercapacitor device delivered a gravimetric capacitance of 323.1 F g-1 and a volumetric capacitance 369.5 F cm-3 at 1 A g −1 in 1.0 M H2SO4. After
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further reduction of the electrode materials by HI, the capacitances were increased to 513.5 F g-1 and 612 F cm-3. This new fabrication process for graphene/conducting polymer
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composites could provide an important contribution to future energy storage devices. Acknowledgment
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The authors are thankful for financial support by the Australian Research Council (Grant LP140100605). The first author acknowledges the scientific and technical assistance of Dr Nobuyuki Kawashima for XRD and Dr Andrew Michelmore for XPS at University of South Australia. The authors thank Benjamin Wade, Animesh Basak and Angus Netting for technical support at Adelaide Microscopy. References [1] Zhou G, Li F, Cheng H-M. Progress in flexible lithium batteries and future prospects. Energy & Environmental Science. 2014;7(4):1307-1338.
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[19] Yang X, Cheng C, Wang Y, Qiu L, Li D. Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage. Science. 2013;341(6145):534537. [20] Chen C, Yang Q-H, Yang Y, Lv W, Wen Y, Hou P-X, et al. Self-Assembled FreeStanding Graphite Oxide Membrane. Advanced Materials. 2009;21(29):3007-3011. [21] Lv W, Li Z, Zhou G, Shao J-J, Kong D, Zheng X, et al. Tailoring Microstructure of Graphene-Based Membrane by Controlled Removal of Trapped Water Inspired by the Phase Diagram. Advanced Functional Materials. 2014;24(22):3456-3463. [22] Hu H, Zhao Z, Wan W, Gogotsi Y, Qiu J. Ultralight and Highly Compressible Graphene Aerogels. Advanced Materials. 2013;25(15):2219-2223. [23] Xu Y, Lin Z, Zhong X, Huang X, Weiss NO, Huang Y, et al. Holey graphene frameworks for highly efficient capacitive energy storage. Nat Commun. 2014;5. [24] Ghidiu M, Lukatskaya MR, Zhao M-Q, Gogotsi Y, Barsoum MW. Conductive twodimensional titanium carbide ‘clay’ with high volumetric capacitance. Nature. 2014;516:78. [25] Alhabeb M, Maleski K, Anasori B, Lelyukh P, Clark L, Sin S, et al. Guidelines for Synthesis and Processing of Two-Dimensional Titanium Carbide (Ti3C2Tx MXene). Chemistry of Materials. 2017;29(18):7633-7644. [26] Chen W, Xia C, Alshareef HN. Graphene based integrated tandem supercapacitors fabricated directly on separators. Nano Energy. 2015;15(0):1-8. [27] Wu Q, Xu Y, Yao Z, Liu A, Shi G. Supercapacitors Based on Flexible Graphene/Polyaniline Nanofiber Composite Films. ACS Nano. 2010;4(4):1963-1970. [28] Xue Y, Ding Y, Niu J, Xia Z, Roy A, Chen H, et al. Rationally designed graphenenanotube 3D architectures with a seamless nodal junction for efficient energy conversion and storage. Science Advances. 2015;1(8). [29] Bartlett RJ. Many-Body Perturbation Theory and Coupled Cluster Theory for Electron Correlation in Molecules. Annual Review of Physical Chemistry. 1981;32(1):359-401. [30] Zhao Y, Schultz NE, Truhlar DG. Design of Density Functionals by Combining the Method of Constraint Satisfaction with Parametrization for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions. Journal of Chemical Theory and Computation. 2006;2(2):364-382. [31] Hehre WJ, Ditchfield R, Pople JA. Self—Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian—Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. The Journal of Chemical Physics. 1972;56(5):2257-2261. [32] Hariharan PC, Pople JA. Accuracy of AH n equilibrium geometries by single determinant molecular orbital theory. Molecular Physics. 1974;27(1):209-214. [33] Barone V, Cossi M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. The Journal of Physical Chemistry A. 1998;102(11):1995-2001. [34] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al. Gaussian 09. Wallingford, CT, USA: Gaussian, Inc.; 2009. [35] Rana U, Malik S. Graphene oxide/polyaniline nanostructures: transformation of 2D sheet to 1D nanotube and in situ reduction. Chemical Communications. 2012;48(88):10862-10864.
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Figures captions Figure 1. Conceptual representation of a flexible supercapacitor whose electrodes consist of a high density graphene and polyaniline (PANi) nanotube composite. Figure 2. Synthesis of PANi-NT/graphene and polypyrrole (PPy)/graphene composites. Figure 3. SEM images of the cross-section of PANi-NT/graphene (b and c) and PPy/graphene films (e and f). Figure 4. Spectroscopic characterization of GO, PANi-NT, PPy and their composites: (a) Raman, (b) XRD, (c) XPS spectra, (d) XPS high-resolution C1s spectra of GO, (e) C1s of the PANi-NT/graphene composite, and (f) N1s spectra of the PANi-NT/graphene composite Figure 5. (a) Graphene oxide and PANi molecular models used in DFT calculations. (b) The profile of the graphene oxide reduction using PANi in its emeraldine salt form. Free energies (kcal mol-1) calculated using M05-2X/6-31** levels of theory are reported in vacuum (black) and in water solution (blue). For the sake of clarity, the stationary points are shown using small model (orange), and some key distances are also reported in Å. Figure 6. Electrochemical performance of PANi-NT/graphene and PPy/graphene symmetric supercapacitors in 1.0 M H2SO4 electrolyte: (a) cyclic voltammograms (CVs) at 10 mV s-1, (b) charge/discharge curves (CDs) collected at 1 A g-1, (c) volumetric and gravimetric capacitances calculated under different current densities, and (d) Nyquist plots of PANi-NT/graphene hydrogel and PPy/graphene. The inset is the amplified Nyquist plots in the high frequency region. Figure 7. Electrochemical performance of PANi-NT/graphene symmetric supercapacitor in 1.0 M H2SO4 electrolyte with hydriodic acid (HI): (a) schematic illustration showing the charge storage mechanism, (b) cyclic voltammograms (CVs), (c) CVs at different scan rates, (d) charge/discharge curves (CDs) at different current densities, and (e) volumetric and gravimetric capacitances. Figure 8. (a) Ragone plot showing the volumetric power vs. volumetric energy for the PANi-NT/graphene and the PANi-NT/graphene with HI, (b) the charge/discharge curves (CVs) of the PANi-NT/graphene composite with HI at different bending angles, (c) CD curves of the composite with HI for a single cell and tandem cells, and (d) capacity retention versus cycle number for the composite with HI, where the inset shows CD curves for the first and last three cycles.
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S0266-3538(17)32365-5
DOI:
10.1016/j.compscitech.2018.02.033
Reference:
CSTE 7109
To appear in:
Composites Science and Technology
Received Date: 17 October 2017 Revised Date:
26 January 2018
Accepted Date: 20 February 2018
Please cite this article as: Moussa M, El-Kady MF, Abdel-Azeim S, Kaner RB, Majewski P, Ma J, Compact, flexible conducting polymer/graphene nanocomposites for high volumetric supercapacitors, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.02.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Compact, Flexible Conducting Polymer/Graphene Nanocomposites for High Volumetric Supercapacitors
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Mahmoud Moussa1, 2, 3, Maher F. El-Kady4, 5, Safwat Abdel-Azeim6, Richard B. Kaner4, , Peter Majewski1, Jun Ma1,2*
Future Industries Institute and 2School of Engineering, University of South Australia, Mawson Lakes, SA5095, Australia,
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Department of Chemistry, Faculty of Science, Beni-Suef University, Beni-Suef 62111, Egypt,
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Department of Chemistry & Biochemistry and California NanoSystems Institute, University of California, Los Angeles
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(UCLA), Los Angeles, USA,
Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt,
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Centre for Petroleum and Minerals, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Saudi Arabia
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ABSTRACT
Graphene is extensively utilized in energy storage devices because of its high surface area and electron conductivity as well as ease of electrode fabrication. But graphene sheets often stack themselves in polymeric matrices leading to poor capacitive performance. This
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problem was addressed in this study by developing and inserting respectively two types of nano-sized conducting polymers into graphene interlayer spacing. The resulting hydrogel
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composite electrodes demonstrated efficient electron transfer for fast and reversible Faradaic reactions at the interface. Theoretical modelling by the density functional theory
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suggested that the reduction involve 2H+ transfer steps from polyaniline to graphene oxide: the first step would be an epoxy-ring opening process after activation of the C–O bond, and the second step would be C–O rupture leading to a de-epoxidation process. This binder-free electrode demonstrated high cycling performance and ultrahigh volumetric capacitance 612 F cm−3, being 10 times higher than the activated carbon used in the current industry. The
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study represents a step forward towards the fabrication of flexible, high-energy density supercapacitors.
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Keywords: graphene, conducting polymers, volumetric capacitance 1. Introduction
Batteries and supercapacitors have received great attention due to their increasing
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importance to our daily life. Batteries store and deliver charge through Faradaic processes
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[1-5], providing high energy density, but they are limited by low power density and cycle life [6]. On the other hand, supercapacitors store and deliver charge in electrochemical double layers (EDLs), and they feature high power density, rabid charge-discharge, long cycling stability, no maintenance and environmental friendliness [7-9]. Nevertheless, it remains a formidable challenge to improve the volumetric capacitance rather than
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gravimetric capacitance, because current portable electronics and electric vehicles need to deliver maximum energy in a very limited space [10-12]. Carbon-based materials have been the best option to-date for energy storage due to their highly porous nature and high
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conductivity. However, these materials suffer poor volumetric capacitance because of their
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low packing density [13-17]. Moreover, binders and conductive additives are needed, which reduce the volumetric capacitance, since these additives have little or no contribution to the overall capacitance. Graphene has been widely examined as an important class of active electrode
materials for ultracapacitors owing to its unique flexible and 2D structure. Recently, many forms of graphene have been developed such as fibres, membranes and foams [4, 14, 1822], but most of these deliver only a low volumetric capacitance. A compressed, three-
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dimensional, porous graphene oxide framework showed a gravimetric capacitance of 310 F g−1 and a volumetric capacitance of 220.1 F cm−3 in aqueous electrolyte. The hierarchical
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porous structure of holey graphene provided large ion-accessible surface area and efficient electron and ion transport pathways [23]. Graphene compact film provided a volumetric capacitance of 255.5 F cm−3 with two-electrode configuration; the liquid-mediated process
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avoided the restacking of graphene layers to obtain low ion transport resistance [19]. Highly dense graphene (with a density of 1.58 g cm-3) was developed by evaporation-
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induced drying of a graphene hydrogel, which showed volumetric capacitance 376 F cm−3 for neat graphene materials [10]. MXenes have shown promise for electrochemical capacitors with volumetric capacitance of 900 F cm−3 [24], but their fabrication processes require multi-steps and more efforts than those for conducting polymer/graphene composites [24, 25]. From our perspectives, composite materials would be the best solution
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to improve the overall performance due to synergistic effects. Recently Chen et al. introduced a novel direct electrode deposition of graphenepolyaniline using a filtration method [26], and this one-cell supercapacitor provided a
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volumetric capacitance of 199 F cm-3 at a current density of 5 A g-1 with a film density of 2
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g cm-3. Wu et al. used a similar method to prepare a composite film of chemically converted graphene and polyaniline (PANi) nanofibers [27], where the nanofibers were uniformly sandwiched between graphene sheets, producing a high-quality flexible film. The electrode of 0.76 g cm-3 in density showed a volumetric capacitance of 160 F cm-3. These capacitance values are not ideal due to low out-of-plane electrical conductivity caused by the weak interlayer van der Waals interactions [28]. Therefore, it is indispensable to develop new
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graphene electrodes possessing a 3D structure with high density to improve the volumetric performance.
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Here we demonstrate a cost-effective, novel method for the development of conducting polymer/graphene composite electrodes, where two types of nano-sized conducting polymers play multi-roles such as reducing agents, spacers and pseudo-
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capacitive materials. Graphene oxide is reduced by conducting polymers such as PANi in an emeraldine salt form, and this process is supported by a density functional theory. The
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PANi/graphene composite film delivers a volumetric capacitance of 369.5 F cm−3 at 1 A g-1 in 1.0 M H2SO4, which increases to 612 F cm−3 after further reduction by hydroiodic acid. 2. Experimental
2.1 Synthesis of conducting polymer/graphene composite films Polyaniline nanotubes (PANi-NT), polypyrrole (PPy) and graphene oxide (GO)
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were prepared as described in the supporting information. Conducting polymer/graphene composites were synthesised as follows: all the composites were designed to contain 10
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wt% polymers; 60 mg GO and 7 mg polymer were ultrasonically dispersed for one hour in the deionized water, respectively. Then, the two batches of suspension were mixed and
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sonicated for one more hour. Next, the mixture was kept at 95 °C for 30 h without any disturbance to form a cylindrically shaped composite, which was then smashed by vigorous shaking in 60 ml deionized water. Next, the suspension was filtered through a PTFE membrane to produce a freestanding composite film, followed by drying under vacuum for 4 h at room temperature. For further reduction, the PANi-NT/graphene composite was
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immersed in HI solution at 70 °C for 7 h. Note that the PPy/graphene composite film has no sufficient mechanical stability for further reduction with HI.
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2.2 Supercapacitor fabrication To fabricate our supercapacitors, each composite film was cut into small (1 cm2) rectangular pieces, followed by dipping in 1.0 M H2SO4 overnight. The two composite
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films were attached to platinum foils as the current collectors, between which a piece of filter paper was sandwiched as a separator. All electrochemical tests including cyclic
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voltammetry (CV), galvanostatic charge/discharge (CD) and electrochemical impedance spectroscopy (EIS) were carried out using a two-electrode cell configuration by a CHI 660E electrochemical workstation. 2.3 Quantum calculations
Density functional theory (DFT) calculations were carried out to study the
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mechanism of graphene oxide reduction by PANi, focusing on the de-epoxidation of graphene oxide by the emeraldine oxidation state of PANi. The proposed model consists of
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a large graphene sheet (123 atoms) with an epoxy functional group and one PANi molecule consisting of four aniline monomers. The accuracy of DFT has been verified by
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benchmarking with the coupled cluster CCSD(T) method [29]. The geometric optimizations were performed using M05-2X /6-31G** [30-32]. Frequency calculations were carried out on all stationary points of the reaction profile to verify the maxima (characterized with only one negative frequency corresponding to the reaction coordinate) and the minima (characterized with positive frequencies). The vibrational corrections to the electronic energies were estimated at the same level of the geometric optimization (M05-
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2X/6-31G**). Single point energy calculations were performed to explore the solvent effects on the reaction profile using the Polarizable Continuum Model (PCM) [33] as
3. Results and discussion
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3.1 Conducting polymer/graphene composites
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implemented in the Gaussian 09 code [34].
We propose the following criteria for design of flexible supercapacitor electrode
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materials to obtain high volumetric capacitance. First, such materials should be selected to create large charge storage capacity and thus high energy density. Second, it is essential to control the electrode packing density for ideal volumetric performance. Third, the materials should have interconnected porous structure with robust mechanical durability. Figure 1 illustrates these design criteria. Figure 2 contains a schematic of the methodology for the
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fabrication of conducting polymer/graphene composites. The methodology suits all composites containing any conducting polymers, although we use polyaniline (PANi) and
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polypyrrole (PPy) as two examples in this study. Mixing the colloidal GO solution with either PANi nanotubes (PANi-NT) or a PPy solution created an electrostatic interaction
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between GO surface and PANi-NT or PPy. Heating the mixture at 95 °C for 12 h converted the brown colour of GO into black, which indicated reduction; PANi-NTs or PPy thus had four roles: (i) reducing GO, (ii) physically crosslinking the composites, (iii) working as a spacer and (iv) contributing to pseudo-capacitance [35, 36]. Cylinder-like samples were produced through continuous heat treatment for 30 h, and then these were dispersed in deionized water by vigorous shaking to produce colloidal suspensions that can be filtered
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and easily peeled off, resulting in compact, flexible and freestanding composite films (Figure 3a and 3d). SEM images of these films are shown in Figure 3c, 3f, S1c and S1f,
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where the graphene composite films exhibit an ordered format in their layered structure with obvious porosity caused by filtration. The micrographs also show that PANi-NT and PPy are firmly embedded between graphene layers, which is a desired morphology for the
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composite because it is well known for sufficient interspace and high accessibility to electrolyte ions [37].
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The structural integrity of graphene oxide and its composites was investigated using Raman scattering (Figure 4a). Graphene displays a D-band (1348 cm−1) and a G-band (1590 cm−1), confirming the conversion of sp2 to sp3-hybridized carbon and the vibration of an sp2-hybridized carbon, respectively. The ID/IG intensity ratio provides an important tool to gauge the degree of disorder and information on sp2 domains. After GO reduction by the
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conducting polymers, the ID/IG ratio increases from 0.88 for GO to 1.06 and 1.01 respectively for PANi-NT/graphene and PPy/graphene. Figure 4b and S2 provide the XRD
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patterns of GO, PANi-NT, PPy, PANi-NT/graphene and PPy/graphene. The sharp, intense diffraction located at 2θ = 10.6° to the interplanar spacing of 0.83 nm for GO sheets – a
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substantial increase of the interlayer spacing from 0.33 nm for graphite to 0.83 nm for GO indicating significant oxidation [38]. PANi shows its characteristic diffraction peaks (at 2θ values of 20.1° and 25.3°), which are attributed to the crystallinity and coherence length of polymer chain orientation [39]. The amorphous nature of neat PPy is confirmed by a broad diffraction at 2?? = 26°. The reducing ability of conducting polymers is clearly evident in the XRD pattern of PANi-NT/graphene and PPy/graphene, where the characteristic
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diffraction of GO has disappeared, and broad ones appear at 2θ = 24.7° and 24.91° for PANi-NT/graphene and PPy/graphene, respectively.
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The elemental composition of GO and its composites was identified using highresolution X-ray photoelectron spectroscopy (XPS; Figure 3c and S2c). The GO shows a C/O ratio of 2.23, indicating a significant degree of oxidation. After reduction with the 10
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wt% of conducting polymers, PANi-NT/graphene and PPy/graphene respectively exhibit C/O ratios of 7.58 and 7.57. This should enhance the π–π interactions between the graphene
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and the conducting polymer backbone, thus improving the electron transfer and the electrochemical characteristics [40]. The C1s spectrum of GO (Figure 4d) contains three main peaks: the sp2 aromatic rings (C–C; 285.05 eV) and the oxygen containing groups (C– O 287.1 eV and C=O 288.5 eV). Figure 4e presents the C1s of the PANi-NT/graphene composite, confirming the presence of C=C (284.8 eV), C–N– (285.39 eV) and C–O (286.9
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eV) bonding [41]. The weak intensity of the C–O bond located at 286.9 eV is due to residual oxygen containing functional groups left in the graphene composite after reduction. For the N1s spectrum of the PANi-NT/graphene composite (Figure 4f), the curve exhibits
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three components: =NH– (399.5 eV), –NH– (401.0 eV) and –NH+– (402.1 eV) [42],
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indicating successful incorporation of PANi-NTs into the composite film. The analysis agrees with the preceding SEM observation. 3.2 Quantum Chemical Calculations We by DFT calculations investigated the mechanism involved in the effective
removal of epoxide groups from GO by PANi which acted as a reducing agent (deepoxidation) (Figure 5a). During the reduction, the first proton transfer facilitates C–O
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bond activation and opens the epoxy ring (Figure 5b), producing an intermediate (INT1); this step needs 7.19 kcal mol-1 in vacuum, and it is overall an exothermic step (-9.45 kcal
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mol-1). In solution, the epoxy-ring opening has an even higher activation barrier of 11.54 kcal mol-1; thus the reaction is less exothermic (-4.66 kcal mol-1) because of the need to stabilize the reactant (R) induced by interaction with the solvent; the interaction is
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responsible for the energetic changes in the reaction profile. INT1 would form after the C– O activation rearranges itself to receive a second proton transfer from PANi, creating the
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final product – reduced graphene oxide (P). This step involves higher activation barriers of 26.45 kcal mol-1 and 28.04 kcal mol-1 in vacuum and solution, respectively. The final release of water is highly favorable with free energies of -22.17 (in vacuum) and -27.20 kcal mol-1 (in solution). Our calculations estimate that the de-hydroxylation should be a rate-determining step for de-epoxidation. The profile for GO reduction indicates that both
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de-epoxidation and de-hydroxylation through PANi are feasible at room temperature. 3.3 Electrochemical performance of the composite
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Our composite design represents an advance in the development of electrode materials for supercapacitors through (i) the incorporation of conducting polymers that not
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only contributes pseudocapacitance but prevents graphene sheets from re-stacking, (ii) graphene sheets in the matrices that can markedly improve the electrical conductivity and also the potential cycling performance of the polymers, (iii) the inter-connected porous structure that enhances the electrochemical interfacial area (Figure 3 and S1) and (iv) no binder needed. Figure 6a illustrates the cyclic voltammetry (CV) curves of a PANiNT/graphene composite film in 1.0 M H2SO4. The curves are nearly rectangular with a redox peak, indicating pseudo-capacitance contributed by the presence of PANi-NTs and
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residual oxygen groups on the reduced graphene sheets. The CV curves of the PANiNT/graphene electrodes at various scan rates, demonstrating their high capacitance even
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when operated over a wide range of scan rates. The charge/discharge (CD) curves of the PANi-NT/graphene composite films are presented in Figure 6b; the slight deviation from linearity should be caused by pseudo-capacitance from the PANi-NTs. The discharge time
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is longer due to a combination of EDLC from the graphene sheets and Faradiac capacitance from the PANi-NTs. Figure 6c illustrates the gravimetric (Cwt) and volumetric (Cvol)
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capacitances, which were calculated at a film density of 1.19 g cm-3 from the charge/ discharge curves according to the following equations:
∁wt = ∆
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, m and V are the applied constant current (A), the discharge curve slope, and
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where I,
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the total mass (g) and volume (cm3) of the two electrodes, respectively. The PANiNT/graphene composite electrode shows a Cwt 323.11 F g-1 at a current density of 1 A g-1,
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which is higher than those tested under similar conditions, such as 214.48 F g-1 for the
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PANi-PAMPA/graphene composite electrode and 310 F g-1 for a holey graphene framework [19, 23]. Using the active material volume, the composite Cvol was calculated as 369.5 F cm-3 at 1 A g−1, which is higher than the reported values measured under similar conditions. The dense graphene was reported to have a Cvol of 376 F cm-3 when measured at 0.1 A g−1, but such a current density may not meet practical requirements [18, 19, 43, 44]. The electrochemical performance of PPy/graphene composite film is illustrated in Figure S3. The high electrochemical performance of these two groups of composites means that
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our composite design should suit all of conducting polymers for the development of flexible supercapacitors, because the unique composites morphology can provide large
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accessible graphene surfaces, reduce the dynamic resistance of electrolyte ions and facilitate the electron transfer due to the strong π–π electron and hydrogen bond interaction (Figure 3c).
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The ion transfer properties of the electrodes based on these two composites were studied using electrochemical impedance spectroscopy (EIS). Figure 6d displays the
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Nyquist plots of these composite electrodes in 1.0 M H2SO4; both represent a straight line in the low-frequency region and a semicircle (insert in Figure 6d) in the high-frequency region. The PANi-NT/graphene composite electrode demonstrates a more vertical graph, which is explained by a nearly ideal capacitance behaviour. Its equivalent series resistance (ESR) is estimated to be 0.42 Ω, which is lower than that of a PPy/graphene composite
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(0.62 Ω); thus it carries low resistance due to the geometry of PANi-NTs, providing more interface for enhancement of the charge transfer. The dependence of the phase angle on the
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frequency is shown in Figure S3d. The PANi-NT/graphene phase angle is close to -90° at low frequencies, showing an ideal capacitive behaviour. The time constant τ0 (the inverse
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of the characteristic frequency (ƒ0) at a phase angle of –45°) was found to be 0.2 s for the PANi-NT/graphene composite and 1.08 s for PPy/graphene composite; this rapid frequency response of the PANi-NT/graphene composite demonstrates a significantly enhanced ion transport rate due to the large, accessible surface area (Figure 3c). 3.4 Volumetric and gravimetric capacitance enhancement for PANi-NT/graphene composite
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Hydriodic acid (HI) was selected, because it was able to (i) assist a further reduction of GO sheets while maintaining the film flexibility and (ii) act as a source of redox active
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electrolyte to enhance the device performance [45, 46]. The redox active electrolyte comprises iodine anions and neutral iodine (I−/I2). While the composite electrode stores the charge through (i) fast yet reversible redox reactions that are intrinsic to PANi and (ii) ion
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adsorption on graphene surface, the I-/I2 species may increase the capacitance further by contributing to the redox reactions (Figure 7a). These processes may work synergistically
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to produce ultrahigh volumetric capacitance as seen from the CV graphs in Figure 7b. A large capacitance response is found in comparison with the unreduced one. The gravimetric charge/discharge (CD) curves of the reduced PANi-NT/graphene electrodes are shown in Figure 7d, demonstrating a competitive electrochemical performance over a wide range of current densities. Figure 7e contains gravimetric (Cwt) and volumetric (Cvol) capacitances of
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the reduced composite film in 1.0 M H2SO4 at different current densities; it is apparent that the Cwt of 513.5 F g-1 and the Cvol of 612 F cm-3 for the reduced electrodes are much higher than those for the unreduced composite (Cwt = 323.11 F g-1 and Cvol = 369.5 F cm-3) at 1 A
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g-1. We can say with strong confidence that the value of Cvol obtained for our PANi-
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NT/graphene composite-HI electrodes is the highest one reported to date amongst all the flexible carbon-based supercapacitor electrodes in H2SO4 at 1 A g-1. Figure 8a exhibits a Ragone plot with the energy (Evol) and the power (Pvol) densities for the hydrogel films and other carbon based materials [47-52] based on these equations:
Evol =
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Pvol =
vol # 3600
∆
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For the unreduced PANi-NT/graphene composite electrodes, the maximum energy and power density values are respectively 8.21 Wh l-1 and 2896.9 W l-1. The reduction was
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found to lift the energy density up to 13.6 Wh l-1. The supercapacitor based on PANi-NT/graphene with HI was investigated for flexibility during charging and discharging. In Figure 8b, the charge/discharge curves
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under different bending conditions (0, 90 and 180°) all overlap well, meaning that this device exhibits nearly ideal electrochemical performance during deformation owing to the
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high mechanical strength of the composite film. Therefore, the electrode materials we have developed are suitable to many flexible and portable devices applications. A tandem cell with supercapacitor units connected in series is a common method to produce high voltage for practical applications. We made a 3-unit tandem cell with a potential window up to 2.4 V (Figure 8c) by assembling three-supercapacitor units in series based on a PANi-
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NT/graphene with HI in H2SO4. When it was used to power a red light emitting diode (LED), the high capability in each unit can be confirmed from the unchanged discharge
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time. Figure S4b shows snapshots of the 3-unit tandem cell after charging for 30 seconds at 2.4 V; this can deliver sufficient energy to power the LED for over 5 minutes. The cycling
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stability of the composite electrodes was tested at a current density of 20 A g−1 in 1.0 M H2SO4 for 5000 cycles (Figure 8d), providing a capacity retention of 97.6%. The charge/discharge curves for the first and last three cycles are nearly the same (see the inset to Figure 8d), indicating high reversibility and long-term stability due to the 3D hydrogel structure which improves the mechanical strength and stabilizes the composite during the charge-discharge process.
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4. Conclusions In summary, we have developed a facile one-step synthesis approach to fabricate
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3D flexible freestanding electrodes for energy storage devices. This approach should suit all of conducting polymers. The conducting polymer serves not only as an excellent reducing agent for graphene oxide, but as a spacer that prevents graphene sheets from
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restacking in the composite film; this provides a three-dimensional structure where graphene sheets are exfoliated, offering more interfaces for reversible pseudo-capacitance
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reactions. Theoretical modelling indicates that the graphene oxide reduction involves 2H+ transfer from PANi; the first proton transfer causes the epoxy-ring opening and the second transfer assists the C–O rupture which is a rate-determining step for the de-epoxidation process. The PANi-NT/graphene supercapacitor device delivered a gravimetric capacitance of 323.1 F g-1 and a volumetric capacitance 369.5 F cm-3 at 1 A g −1 in 1.0 M H2SO4. After
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further reduction of the electrode materials by HI, the capacitances were increased to 513.5 F g-1 and 612 F cm-3. This new fabrication process for graphene/conducting polymer
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composites could provide an important contribution to future energy storage devices. Acknowledgment
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The authors are thankful for financial support by the Australian Research Council (Grant LP140100605). The first author acknowledges the scientific and technical assistance of Dr Nobuyuki Kawashima for XRD and Dr Andrew Michelmore for XPS at University of South Australia. The authors thank Benjamin Wade, Animesh Basak and Angus Netting for technical support at Adelaide Microscopy. References [1] Zhou G, Li F, Cheng H-M. Progress in flexible lithium batteries and future prospects. Energy & Environmental Science. 2014;7(4):1307-1338.
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Figures captions Figure 1. Conceptual representation of a flexible supercapacitor whose electrodes consist of a high density graphene and polyaniline (PANi) nanotube composite. Figure 2. Synthesis of PANi-NT/graphene and polypyrrole (PPy)/graphene composites. Figure 3. SEM images of the cross-section of PANi-NT/graphene (b and c) and PPy/graphene films (e and f). Figure 4. Spectroscopic characterization of GO, PANi-NT, PPy and their composites: (a) Raman, (b) XRD, (c) XPS spectra, (d) XPS high-resolution C1s spectra of GO, (e) C1s of the PANi-NT/graphene composite, and (f) N1s spectra of the PANi-NT/graphene composite Figure 5. (a) Graphene oxide and PANi molecular models used in DFT calculations. (b) The profile of the graphene oxide reduction using PANi in its emeraldine salt form. Free energies (kcal mol-1) calculated using M05-2X/6-31** levels of theory are reported in vacuum (black) and in water solution (blue). For the sake of clarity, the stationary points are shown using small model (orange), and some key distances are also reported in Å. Figure 6. Electrochemical performance of PANi-NT/graphene and PPy/graphene symmetric supercapacitors in 1.0 M H2SO4 electrolyte: (a) cyclic voltammograms (CVs) at 10 mV s-1, (b) charge/discharge curves (CDs) collected at 1 A g-1, (c) volumetric and gravimetric capacitances calculated under different current densities, and (d) Nyquist plots of PANi-NT/graphene hydrogel and PPy/graphene. The inset is the amplified Nyquist plots in the high frequency region. Figure 7. Electrochemical performance of PANi-NT/graphene symmetric supercapacitor in 1.0 M H2SO4 electrolyte with hydriodic acid (HI): (a) schematic illustration showing the charge storage mechanism, (b) cyclic voltammograms (CVs), (c) CVs at different scan rates, (d) charge/discharge curves (CDs) at different current densities, and (e) volumetric and gravimetric capacitances. Figure 8. (a) Ragone plot showing the volumetric power vs. volumetric energy for the PANi-NT/graphene and the PANi-NT/graphene with HI, (b) the charge/discharge curves (CVs) of the PANi-NT/graphene composite with HI at different bending angles, (c) CD curves of the composite with HI for a single cell and tandem cells, and (d) capacity retention versus cycle number for the composite with HI, where the inset shows CD curves for the first and last three cycles.
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