In-situ reduced graphene oxide nanosheets–polypyrrole nanotubes nanocomposites for supercapacitor applications

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In-situ reduced graphene oxide nanosheets–polypyrrole nanotubes nanocomposites for supercapacitor applications Madhabi Devi, A. Kumar* Material Research Laboratory, Department of Physics, Tezpur University, Napaam, Tezpur, 784028, Assam, India

A R T I C L E I N F O

Article history: Received 26 July 2016 Received in revised form 5 November 2016 Accepted 10 November 2016 Available online xxx Keywords: Supercapacitor Polypyrrole nanotube Reduced graphene oxide Nanocomposite Specific capacitance

A B S T R A C T

Nanocomposites of reduced graphene oxide (RGO) nanosheets and polypyrrole nanotubes (PPyNTs) have been synthesized by in-situ reduction of graphene oxide in presence of PPyNTs as spacers. The morphology and structure of the samples were investigated by High resolution transmission electron microscope, Scanning electron microscope, X-ray diffraction, Fourier transform infrared and MicroRaman spectroscopy. Electrical and electrochemical properties of the samples were characterized by current-voltage characteristics, cyclic voltammetry and electrochemical impedance spectroscopy studies. The nanocomposites exhibit high specific capacitance and good electrochemical stability as electrode material for supercapacitors. The specific capacitance gradually increases to 281 F g
1 with the increase in feeding of RGO and the capacitive retention is about 77.28% of the initial capacitance after 1000 cycles. A maximum energy density of 39 W h kg
1 and power density of 3105 W kg
1 are achieved by utilizing the surface area of RGO nanosheets with the help of PPyNTs as spacers. These results demonstrate the synergistic effect of 1D PPyNTs and 2D RGO nanosheets for their possible applications as high performance energy storage devices. ã 2016 Elsevier B.V. All rights reserved.

1. Introduction The increasing world-wide demand for a novel, eco-friendly and high performance energy storage devices has driven intense research efforts towards the development of energy storage and delivery systems. Supercapacitors have generated great interest as one of the attractive alternative for portable electronics and automotive applications due to their high power density and long durability than secondary batteries [1]. However, the energy storage density of supercapacitors is lower than that of batteries [2]. In order to develop an advanced supercapacitor device, a high performance electrode material with improved energy density while maintaining the high power density and cycling stability is essential [3]. Conducting polymers (CPs) such as polypyrrole (PPy), polyaniline (PAni) and polyethylenedioxythiophene (PEDOT) with good electrical conductivity, redox reversibility and large pseudocapacitance have aroused wide interest as electrode materials for supercapacitor applications [4]. The rising interest of CP based materials is stimulated by the demand of high power and energy density as well as low fabrication cost. CPs can store charges not only in electrical double layer (EDL) but also through rapid faradic

* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Kumar).

charge transfer (pseudocapacitance) and hence exhibit higher capacitance than that of carbon electrodes [5]. To improve the performances and extend the functions of the devices, CPs are usually nanostructured [6]. The large surface area created by nanoscale structures enhances the efficiency of an electrode material by increasing the electrolyte transport rate and therefore improves the electrode performance. Among the CPs, PPy is considered as a highly promising electrode material for energy storage applications owing to its low cost, easy synthesis, high energy storage capacity, environmental stability and relatively high conductivity [7–9]. However, its practical application is limited by the low power density and poor cycling stability during charging/discharging owing to the easily damaged structure of the material during the redox process [10,11]. In order to resolve this issue, nanocomposites of CPs with carbon nanotubes, graphene or reduced graphene oxide (RGO), graphene oxide (GO), inorganic materials have been fabricated and improved performance of these nanocomposites compared to pristine CPs have been observed [12–15]. Graphene, a two-dimensional monolayer of sp2-bonded carbon atoms, has attracted much attention in energy storage application because of its large specific surface area along with exceptional thermal, mechanical and electrical properties [16]. As an essential characteristic of an electrode material, the very large

http://dx.doi.org/10.1016/j.synthmet.2016.11.004 0379-6779/ã 2016 Elsevier B.V. All rights reserved.

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electrochemically active surface area of graphene is the most notable feature, which determines the capacitance [17]. But large scale production of high quality graphene still remains a challenge using current technologies that include micromechanical cleavage, epitaxial growth on silicon carbide or metal surface and chemical vapor deposition [18]. Therefore, RGO derived from the chemical reduction of GO is used as a desirable alternative to pristine graphene because of its easy processing of chemical exfoliation and nearly graphene like properties [19]. However, restacking of RGO especially after chemical reduction reduces the intrinsic capacitance of an individual graphene sheet and significantly hampers their performance as electrodes. One of the possible prevention of aggregation of RGO is the use of CP nanostructures as spacers. Insitu chemical reduction of GO in appropriate polymer solutions can not only restore the conductivity of RGO but also prevent their restacking because of the presence of CP in their solutions during the reduction [20]. Furthermore, the synergistic effect between CP nanostructures and RGO may further boost the specific capacitance of the nanocomposites by simultaneously combining two kinds of charge storage mechanisms; (i) Pure electrostatic attraction between ions and the charged electrode surface (i.e. EDL capacitance) and, (ii) Faradic pseudocapacitance arising from PPy redox reactions. Moreover, PPy can serve as a spacer to further enhance the surface area of RGO, while RGO nanosheets can serve as a stable and underlying conductive network, whereby good electrical conductivity and improved cycling stability can be achieved [21]. Recently, several groups have reported nanocomposites of PPy with GO, graphene or RGO wherein most of the reports are based on the bulk form of PPy [22–28]; and the use of PPy nanostructures are limited to a few systems [29–32]. Apart from that, the nanotube structured PPy is not much investigated and the detailed study of the role of in-situ reduced GO on the structural and electrochemical properties of the tubular structure is still lacking in the literature. Compared to other CP nanostructures, one-dimensional nanotubes provide facile ion transportation, large aspect ratio and internal hollow structure thereby making accessible both its interior and exterior surface to the electrolyte ions. In addition to that, the insitu reduction of GO in presence of polypyrrole nanotubes (PPyNTs) will prevent the irreversible restacking of RGO and will help in achieving a homogeneous dispersion of RGO nanosheets within the polymer matrix. It is therefore interesting to explore the effect of RGO content on the structural and electrochemical properties of PPyNTs. In the present paper, we report nanocomposites of RGO and PPyNTs via an in-situ reduction of GO by hydrazine hydrate. The weight % of GO with respect to PPyNTs was tuned to obtain the nanocomposite with the best performance. The morphology, chemical composition and electrochemical performance of the nanocomposites have been investigated and compared with that of pristine PPyNTs. The effect of RGO on the electrochemical stability of the nanocomposites is especially emphasized and the possible application of the best performance nanocomposite as electrode material in supercapacitors has been presented and discussed. 2. Experimental 2.1. Materials Monomer (pyrrole, 98%, Sigma Aldrich) was vacuum dried prior to use. Graphite powder was obtained from Loba Chemie, sulphuric acid (98%) from Rankem, potassium permanganate, hydrogen peroxide (30%) and ethanol from Merck, hydrazine hydrate (80%) from SD fine chemicals limited (SDFCL), ferric chloride and methyl

orange from Sisco Research Laboratory (SRL). All the chemicals, except pyrrole, were used as-received without further purification. 2.2. Synthesis of PPyNTs PPyNTs were synthesized via reactive self-degrade methyl orange (MO)-FeCl3 template method. In a typical procedure, 0.243 gm (1.5 mmol) of FeCl3 was dissolved in 30 mL of 5 mM MO deionized water (0.15 mmol) that results in the formation of MOFeCl3 template. This MO-FeCl3 complex acts as a template in the formation of PPyNTs which degrades automatically during polymerization. Subsequently, 105 mL (1.5 mmol) of pyrrole was added to the reaction solution. The solution appeared black after a little while indicating the polymerization of pyrrole and was allowed to stir at room temperature for 24 h to complete the polymerization. The precipitate was washed with deionized water and ethanol several times until the filtrate became colorless and was separated by centrifugation. PPyNTs, thus formed, were dried in a desiccator overnight. 2.3. Synthesis of RGO-PPyNTs nanocomposites Graphite oxide was synthesized by oxidation of graphite powder according to modified Hummers method [33,34]. The RGO-PPyNTs nanocomposites were chemically synthesized by insitu reduction of GO in presence of PPyNTs using hydrazine hydrate as reducing agent as illustrated in Fig. 1. For this purpose, a required amount of uniform GO suspension (2 mg/mL) in double distilled (DD) water was first prepared through ultrasonication for 1 h. Then, a definite amount of preweighted PPyNTs were added and dispersed in the GO suspension by ultrasonication for 3 h. The resulting solution was treated with 4 mL/mg of hydrazine hydrate with respect to GO and stirred at 95  C for 1 h. Finally, the obtained nanocomposite was filtered and washed with water and ethanol thoroughly followed by drying at 60  C for 12 h. Four different compositions were prepared by varying the concentration of GO as 5, 10, 20 and 40 wt.% with respect to PPyNTs. For comparison, pure RGO was also prepared with similar procedure without using PPyNTs. 2.4. General characterization The crystalline structures of the materials were determined by a powder X-ray diffraction system (Rigaku miniflex X-ray diffractometer) equipped with Cu Ka radiation (l = 0.15406 nm). Fourier transform infrared (FTIR) spectra of the samples were carried out in the form of KBr pellets in transmission mode using a Nicolet Impact 410 spectrometer. The morphology of the samples was investigated by a scanning electron microscope (SEM) JEOL model JSM 6390 LV at 15 kV accelerating voltage and high resolution transmission electron microscope (HRTEM) model JEOL JEM 2100 at an accelerating voltage of 200 kV. Micro-Raman spectra were recorded in the range of 500–3000 cm
1 by a Renishaw in-via spectrometer (Renishaw, Wotton-under-Edge, UK) using 514.5 nm argon ion laser as an excitation source. The current-voltage (I–V) characteristics were studied in the voltage range of
5 V–5 V using a Keithley source metre model 2400-C. For this purpose, the samples were compressed in the form of pellets of 5 mm diameter and 1 mm thickness. Electrical conductivity measurements of the pellets were performed with a standard four probe resistivity measurement set-up using the following equation [35]. Resistivity (r, ohm-cm) = pt (V/I)/ln 2 = 4.53  t  (resistance) Conductivity (s , S/cm) = 1/r

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Fig. 1. Illustration of the process for preparation of RGO-PPyNTs nanocomposites.

where t is the thickness of the sample, I is the source current through the outer two probes and V is the measured voltage across the inner two probes.

of the sample were sandwiched between the electrodes. An ac sinusoidal signal of peak voltage 3 mV with varying frequencies was applied to measure the desired parameters.

2.5. Electrochemical characterization

3. Results and discussion

The working electrodes were fabricated as described elsewhere [36,37]. Briefly, the as-prepared sample, carbon black, and Nmethyl-2-pyrrolidone (NMP) were mixed in a mass ratio of 85:10:5 and dispersed in ethanol by ultrasonication for 1 h. The resulting mixture was then dropped onto a 2 mm diameter platinum working electrode and dried at 40  C for 12 h. The mass loading of the sample on the platinum electrode was 1 mg. Cyclic voltammetry (CV) measurements were carried out using a AEW2 Sycopel potentiostat (Sycopel Scientific, UK) and galvanostatic chargedischarge experiments were conducted on an electrochemical workstation Biologic SP-150 in 1 M KCl aqueous electrolyte solution using a three electrode system, in which a platinum counter electrode and Ag/AgCl reference electrode were used. The electrochemical impedance spectroscopy (EIS) was recorded using a Hioki 3532-50 LCR HiTester in the frequency range of 42 Hz to 5 MHz at room temperature. A two stainless-steel electrode system was employed for impedance measurements, wherein the pellets

3.1. Electron microscopy studies The morphology of RGO, PPyNTs and nanocomposites were investigated by HRTEM as shown in Fig. 2. HRTEM micrograph of RGO displays a flat paper-like morphology with wrinkles on the surface indicating that the sheets are very thin. On the other hand, Fig. 2(b) demonstrates hollow tubular structure of PPy with an average outer diameter around 132 nm confirming the formation of PPyNTs. From the HRTEM micrograph of the 10 wt.% nanocomposite, it is observed that the PPyNTs are well organized on the large surface area of the RGO nanosheets. As depicted in Fig. 3, the SEM images of the nanocomposites exhibit a coating of PPyNTs onto the large RGO nanosheets. At 5 wt.% of RGO, PPyNTs are seen to aggregate due to its higher ratio, but on increasing the wt.% of RGO, a homogeneous distribution of PPyNTs is observed from the SEM micrographs. It is interesting to note that the nanotubes have been wrapped by the

Fig. 2. HRTEM micrographs of (a) RGO nanosheets, (b) PPyNTs and (c) 10 wt.% RGO-PPyNTs nanocomposite.

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Fig. 3. SEM images of RGO-PPyNTs nanocomposites with (a) 5 wt.%, (b) 10 wt.%, (c) 20 wt.% and (d) 40 wt.% of RGO.

RGO nanosheets which indicate the incorporation of the nanotubes into the interlayer galleries of RGO or between the RGO nanosheets. This is possible owing to the strong van der Waal’s force of attraction between the highly aromatic RGO basal plane and the p conjugated polymer matrix which assembles these two nanostructures on top of another resulting in a sandwiched structure. This sandwich structure is helpful in preventing the reaggregation of the RGO nanosheets.

which is attributed to the p-p interaction of partial PPy chains [40]. Similarly, the RGO-PPyNTs nanocomposites also exhibit a broad diffraction peak in the 2u range of 20 –35 [Fig. 4(ii)]. This broad peak can be ascribed to the overlapping of the reflection peak of PPyNTs around 20 –30 with that of the RGO at 24.6 . The peak of GO at 2u = 9.93 is absent in the nanocomposites which confirms the successful in-situ reduction of GO in presence of PPyNTs.

3.2. X-ray diffraction analysis

3.3. FTIR spectroscopy

X-ray diffraction (XRD) study is performed to investigate the crystal structure of RGO, PPyNTs and nanocomposites. XRD patterns of graphite, GO, RGO and PPyNTs are presented in Fig. 4(i). Graphite exhibits an intense diffraction (002) peak at 2u = 26.6 corresponding to an interlayer spacing of 0.33 nm [38]. On chemical oxidation of graphite, the (002) diffraction peak shifts to lower angle at 2u = 9.93 indicating a larger interlayer spacing of 0.88 nm. This increase in interlayer spacing is due to the incorporation of oxygen functional groups like carboxyl, epoxide, hydroxyl and water molecules into the interlayer galleries of GO thus supporting the formation of graphite oxide [39]. In the XRD pattern of RGO, the (002) peak at 9.93 vanishes and a new broad peak at 2u value of 24. 6 is observed that indicates the loss of long range stacking order and exfoliation of graphite oxide. This gives an interlayer spacing of 0.36 nm which is much smaller than 0.88 nm for GO and is closer to graphite thereby confirming the efficient removal of oxygen functional groups from GO. Pristine PPyNTs show a broad amorphous peak centered around 2u = 20 –30 ,

FTIR spectra of RGO, PPyNTs and all the nanocomposites are shown in Fig. 5. The FTIR spectrum of PPyNTs exhibits characteristic absorption bands at 1544 and 1460 cm
1 that are assigned to C¼C stretching and C

N stretching vibration, respectively [41]. The vibrations observed at 1037 cm
1 and 1312 cm
1 are attributed to C

H inplane deformation vibrations [42]. The strong bands near 1178 and 903 cm
1 indicate the doped states of PPyNTs [43]. The spectrum of RGO shows an absorption band at 1628 cm
1 for C¼C stretching, indicating the restoration of the graphene network on reduction [44]. The weak absorption peak at 1023 cm
1 represents C

O stretching vibrations suggesting the presence of residual oxygen groups after reduction [45]. The disappearance of distinct characteristic absorption bands of RGO in the FTIR spectra of the nanocomposites is attributed to the overlapping of 3438, 1628 and 1023 cm
1 bands of RGO with the 3416, 1544 and 1037 cm
1 bands of PPyNTs, respectively. The signals related to CH and CH2 stretching appears at 2853 and 2924 cm
1 for both PPyNTs and

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Fig. 4. X-ray diffraction patterns of i(a) graphite, i(b) GO, i(c) RGO, i(d) PPyNTs and RGO-PPyNTs nanocomposites with ii(a) 5 wt.%, ii(b) 10 wt.%, ii(c) 20 wt.% and ii(d) 40 wt.% of RGO.

The significant structural changes of the nanocomposites have been investigated by Micro-Raman spectroscopy and are displayed in Fig. 6.

The Raman spectrum of RGO exhibits two prominent bands: D and G bands at 1355 cm
1 and 1595 cm
1, respectively. The D band known as the disorder or the defect band represents a breathing mode from sp2 carbon rings caused by the first-order zone boundary phonons and requires a defect for its activation [47]. The G band arises due to the in-plane optical mode vibration of two neighbouring carbon atoms on a sp2-hybridized RGO layer [48]. The appearance of the D band in the RGO spectrum indicates the presence of structural defects and partially disordered structures formed during the chemical synthesis of RGO [49]. Furthermore, a 2D band at 2700 cm
1 is observed along with the DG combination band at 2935 cm
1 in the Raman spectra of RGO, which corresponds to the second order of zone-boundary phonons [50]. This broad 2D band is a signature of a multilayered structure of RGO [51]. The Raman spectra of PPyNTs include the peaks at 930 cm
1, 982 cm
1, 1265 cm
1, 1382 cm
1 and 1588 cm
1. The characteristic bands at 1588 and 1382 cm
1 are attributed to the stretching of the p conjugated structure and the ring stretching mode of PPyNTs respectively [52]. The band located at 1265 cm
1 is

Fig. 5. FTIR spectra of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.%, (e) 40 wt.% of RGO and (f) RGO.

Fig. 6. Micro-Raman spectra of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.%, (e) 40 wt.% of RGO and (f) RGO.

RGO as well as in the nanocomposites. The broad absorption peaks centering at around 3438 and 3416 cm
1 for RGO and PPyNTs corresponds to the O

H and N

H stretching vibrations both of which overlaps in the FTIR spectra of the nanocomposites. Compared to pure PPyNTs, it is observed that the peaks in the nanocomposites corresponding to C¼C, C

N and C

H vibrations shift a little to the higher wave number side. It is worth noting that all the characteristic peaks experiencing such shift are related to carbon groups which may be ascribed to the change in chemical environment during the in-situ reduction of GO in the nanocomposites involving PPy [35]. The shifting reveals interactions such as p-p stacking between PPy backbone and RGO or hydrogen bonding for the residual oxygen functional groups on RGO [46]. 3.4. Micro-Raman spectroscopy

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assigned to the C

H in-plane stretching vibration. The small peaks at 930 and 982 cm
1 are associated with ring deformation of dication and radical cation, respectively, which justify the doped state of PPy [53]. Simultaneously displaying the peaks at 922, 982, 1382 and 1588 cm
1 in the Raman spectrum of the nanocomposites reveals the coexistence of RGO and PPyNTs. In addition, the broadening of the D and G bands for the RGO-PPyNTs nanocomposites indicates the interaction between the RGO nanosheets and PPyNTs.

Table 1 Fitting parameters, low field conductance (G0), voltage scale factor (V0) and h to Kaiser equation for PPyNTs and RGO-PPyNTs nanocomposites with different RGO concentrations. Sample

G0 (S)

V 0 (V)

h

PPyNTs 5 wt.% RGO 10 wt.% RGO 20 wt.% RGO 40 wt.% RGO

1.23  10
4 2.62  10
4 4.3  10
4 6.3  10
4 9.4  10
4

1.55 1.87 2.45 2.72 3.12

0.65 0.72 0.80 0.82 0.90

3.5. I–V characteristics and electrical conductivity studies The I–V characteristics of the samples have been carried out using a two probe unit. The measurement is performed on pelletshaped samples pressed from powders within a voltage range of
5 to 5 V. The I–V characteristics of PPyNTs, RGO and RGO-PPyNTs nanocomposites have been presented in Fig. 7(i). From the plot, it is observed that the I–V characteristics of pristine PPyNTs and the nanocomposites are non-linear but symmetric in nature. The non-linear I–V characteristic of PPyNTs is a result of non-homogeneous structure of PPyNTs, having ordered chain folding (metallic regions) in their structures that are separated by disordered dielectric barriers [54]. The nanocomposites also exhibit non-linear I–V characteristics, which is attributed to their inhomogeneous structure due to dispersion of highly conductive RGO nanosheets within relatively less conducting PPyNTs matrix. It is observed from the plot that the non-linearity increases with the applied voltage signifying space charge limited conduction (SCLC) [55]. During SCLC, the density of injected carriers from the electrodes exceeds the density of intrinsic charge carriers of the sample resulting in the increase of current with the applied voltage. In order to understand the non-linear behavior of I–V characteristics in a quantitative manner, we have used the model proposed by Kaiser et al. [56]:
 G0 exp VV0 I
 i h ð1Þ G¼ ¼ V 1 þ h exp V =
1 V0

where G0 is temperature dependent low field conductance i.e., the value of G at a specific temperature for V tends to zero. The parameter h is equal to the ratio of low field conductance to the  saturated conductance at high field i.e. Go G < 1. V 0 is the voltage

scale factor, which provides an estimate of the exponential increase in conductance at high field. We note that all these I–V characteristics are essentially symmetric upon reversal of the voltage direction, so only the positive sides of I–V characteristics are fitted according to the above equation and the resulting graphs are presented in Fig. 7(ii). The parameters derived from the fitted curves are listed in Table 1. It is observed that the fitting parameters namely, low field conductance ðG0 Þ, voltage scale factor ðV 0 Þ and h exhibit higher values on increasing the concentration of RGO in the RGO-PPyNTs nanocomposites. The increase in V 0 with increasing RGO concentration indicates a lessening of non-linearity in the I–V characteristics. Also, an increase in the value of h from 0.65 (PPyNTs) to 0.90 (40 wt.% of RGO) is noticed which corresponds to the increase in conductance at low field. Electrical conductivity of PPyNTs, RGO and RGO-PPyNTs nanocomposites has been measured using a four probe set-up. The measured value of conductivity of pure PPyNTs is 0.34 S cm
1 and that of pure RGO is 9.21 S cm
1. The conductivity of RGO-PPyNTs nanocomposites with 5, 10, 20 and 40 wt.% loading of RGO is enhanced to 1.31, 1.94, 2.76 and 4.47 S cm
1, respectively as compared to pure PPyNTs. The increase in conductivity of the nanocomposites with increase in RGO concentration can be attributed to the formation of connected network of RGO within less conducting PPyNTs matrix [57]. RGO nanosheets act as “conduction link” within PPyNTs providing a path of small contact resistance for intertube charge transfer [58]. Moreover, the high aspect ratio of RGO nanosheets with one-dimensional structure of PPyNTs facilitates the electron transfer process within the nanocomposites.

h

Fig. 7. (i) I–V characteristics and (ii) Kaiser equation fitted I–V curves of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.% and (e) 40 wt.% of RGO.

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3.6. Cyclic voltammetry and galvanostatic charge-discharge measurements CV measurements have been conducted in order to investigate the electrochemical properties of PPyNTs and RGO-PPyNTs nanocomposites. The measurements have been carried out within the potential range of
0.2 to 0.8 V at scan rates of 10, 25, 50, 75 and 100 mV s
1 in 1 M KCl aqueous electrolyte solution and are shown in Fig. 8.

7

The specific capacitance is calculated from the CV curves by using the following equation: R IdV ð2Þ C sp ¼ 2  m  n  DV where Csp is the specific capacitance (F g
1), I is the current (A), V is the potential (V) vs. Ag/AgCl, n is the voltage scan rate (V s
1), m is the mass of the electrode material (g), and DV is the sweep potential range or the working potential window, respectively. The

Fig. 8. CV curves at different voltage scan rate of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.% and (e) 40 wt.% of RGO.

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Fig. 9. Variation of specific capacitance with voltage scanning rate of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.% and (e) 40 wt.% of RGO.

variation of specific capacitance with increasing voltage scan rate for PPyNTs and RGO-PPyNTs nanocomposites are displayed in Fig. 9. It is observed that the charge storage capacity of the nanocomposites is greatly influenced by the amount of RGO loading and increases with the increase in RGO concentration. Nanocomposite with 40 wt.% of RGO exhibits the highest charge storage capacity with largest current response (Fig. 8(e)), which is comparatively much higher than that of PPyNTs (Fig. 8(a)). PPyNTs exhibits smallest specific capacitance with non-rectangular geometry of the CV curve due to the larger equivalent series resistance as to be discussed in impedance spectroscopy results. Moreover, the redox groups within PPy are also responsible for the irregular shape of the CV curve [59]. It is also observed that PPyNTs failed to maintain the capacitive response with increasing voltage scanning rate from 10 to 100 mV s
1. We ascribe this to the discrepant insertion-deinsertion behaviors of alkali ion (K+) from the electrolyte to PPyNTs. At a low voltage scan rate, the diffusion of ions from the electrolyte can gain access to almost all available surface area of the electrode leading to a complete insertion reaction, and therefore a higher Csp. However, with increase in scan rate, the effective interaction between the ions and the PPyNTs electrode is greatly reduced, and as a result there is a reduction in capacitance [60]. In contrast to PPyNTs, the measured voltammetry curves of the nanocomposites have the most rectangular geometry indicating excellent charge propagation in the electrodes and have larger specific capacitance. Moreover, the nearly ideal rectangular shaped cyclic voltammogram observed for the nanocomposites even at increasing voltage scanning rate clearly demonstrates improved ionic and electronic transport within the RGO incorporated PPyNTs configuration. The improved electrochemical performance of the nanocomposites is also confirmed from galvanostatic charge-discharge measurements performed between 0 and 0.8 V at a current density of 0.5 A g
1. As shown in Fig. 10, the charge-discharge curves are almost symmetric with a slight curvature, which arises due to the double layer contribution of RGO along with the pseudocapacitive contribution of PPyNTs. As the RGO concentration is increased in the nanocomposites, the discharge curves are observed to be more symmetric and linear indicating nearly capacitive behavior. The discharge time for the nanocomposites is substantially

Fig. 10. Galvanostatic charge-discharge curves of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.% and (e) 40 wt.% of RGO at a current density of 0.5 A g
1.

prolonged compared to pristine PPyNTs exhibiting the highest duration for 40 wt.% of RGO, which demonstrates a higher capacitance. Also, a significant decrease in internal resistance (IR drop) has been observed after incorporation of RGO in PPyNTs indicating an improved capacitive behavior of the nanocomposites. The specific capacitance is evaluated from the discharge curves according to the equation, C sp ¼ ðI  Dtd Þ=ðm  DV Þ

ð3Þ

and the coulombic efficiency h is calculated using the following equation,

h¼

Dt d  100 Dt c

ð4Þ

where, I is the constant discharge current, m is the mass of the electrode material, DV is the potential drop during discharge, and Dtd and Dtc are the times for discharging and charging, respectively. The specific capacitance of PPyNTs and RGO-PPyNTs nanocomposites with 5, 10, 20 and 40 wt.% of RGO are calculated to be 108.8, 141.79, 174.93, 220.83 and 288.57 F g
1, respectively, which are consistent with the CV results. The coulombic efficiency of PPyNTs and RGO-PPyNTs nanocomposite electrodes with increasing concentration of RGO is determined to be 89, 92, 93, 94 and 96%, respectively. The higher coulombic efficiency of the nanocomposites compared to pristine PPyNTs is due to the increasing linear shape of the charge and discharge curves with enhanced RGO loading. The extended discharge duration of the nanocomposites may be attributed to the combined double-layer and pseudocapacitive contribution to the total capacitance, while the short discharge duration in PPyNTs indicates the ideal pseudocapacitive nature of the fabricated electrode [61]. Thus a synergistic effect is observed in the nanocomposite system. This enhanced electrochemical behavior of the nanocomposites is attributed to the nanoscale structure of RGO and PPyNTs which ensures the following advantages to its functions: (i) the highly conductive RGO can provide a easy pathway for the transportation of ions and electrons, which in turn reduces the internal resistance of the nanocomposites, (ii) PPyNTs can act as a spacer that prevents the aggregation of RGO nanosheets resulting in an enhanced ion exchange rate, and (iii) the high surface area of RGO nanosheets provides enough contact area between PPyNTs and RGO nanosheets and also with the electrolyte. Although quite a few studies have been performed on RGO-PPy nanocomposites with different

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PPy nanostructures, not much work have been reported on RGOPPyNTs nanocomposites. Different groups have investigated RGOPPy nanocomposites for supercapacitor application and reported a maximum specific capacitance of 267 F g
1, 248 F g
1 and 68 F g
1, respectively [29–31]. The obtained specific capacitance of the RGOPPyNTs nanocomposites is much larger than that of a previously reported PPy-GO nanocomposite (92.2 F g
1) [23], a multilayered nanoarchitecture of graphene nanosheets and PPy nanowires (165 F g
1) [32], PPy coated graphene film (254 F g
1), singlewalled carbon nanotubes and PPy nanocomposite (265 F g
1) and a free-standing PPy-graphene nanocomposite (277 F g
1) [24,12,25]. Energy density and power density are critical factors for evaluating the practical application of the as-prepared nanocomposites as supercapacitor electrodes. The energy density and power density values are calculated from the CV curves by using the equations: 1 2 Energy densityðEÞ ¼ C sp ðDV Þ 2

ð5Þ

where Csp is the specific capacitance (F g
1) and DV is the working potential window (V). Power densityðPÞ ¼

E t

ð6Þ

where E is energy density and t is the time in second for a complete CV cycle. The as calculated energy density and power density of different samples are tabulated in Table 2. From the table, it is clearly seen that the energy density decreases with increasing scan rate, similar to the specific capacitance value. The 40 wt.% RGO-PPyNTs nanocomposite shows the highest energy density value of 39 W h kg
1 at a 10 mV s
1 scan rate. Excellent power density values of 696.2, 1292.3, 1933.9, 2567.3 and 3105.8 W kg
1 has been obtained for the nanocomposite at voltage scanning rates of 10, 25, 50, 75 and 100 mV s
1, respectively. These results are consistent with those reported in the literature. An energy density of 37.9 W h kg
1 with a power density of 141.1 W kg
1 has been reported for a graphene/ polyaniline material synthesized by in-situ polymerization [62]. An energy density and power density of around 11.3 W h kg
1 and 106.7 W kg
1 have been reported for a polyaniline film coated on ultra-thin graphene nanosheets [63]. A graphene-polypyrrole composite film has been found to exhibit a maximum energy density of 32.9 W h kg
1 and a power density of 1184 W kg
1 [64]. The improved energy density and power density values are attributed to the extended conductive network structure of RGO and electron rich PPy matrix. Thus, it reduces the internal resistance and increases the electrolyte ion transport rate during electrochemical performance. Hence compared with PPyNTs, the

Table 2 Energy density and power density of PPyNTs and RGO-PPyNTs nanocomposites at different voltage scan rates. Sample

Energy density (W h kg 10 mV s
1

PPyNTs

15.7/ 280.3 5 wt.% RGO 19.7/ 352.5 10 wt.% RGO 24/428.2 20 wt.% RGO 30.4/ 542.8 40 wt.% RGO 39/696.2


1

)/Power density (W kg

RGO-PPyNTs nanocomposites prove to be a promising electrode material for supercapacitors. The electrochemical stability of PPyNTs and its nanocomposites is investigated by repeating the CV measurement at a scan rate of 100 mV s
1 in 1 M KCl aqueous solution for 1000 cycles. The variation of specific capacitance with increasing number of CV cycles for PPyNTs and the nanocomposites is presented in Fig. 11(i). It is observed that about 59.56% (26.96 F g
1) of the initial specific capacitance has been retained by PPyNTs when the number of cycles was increased to 1000 cycles. It should be noted that the specific capacitance of the nanocomposites after successive cycles increases with increasing RGO concentration and the nanocomposite with 40 wt.% of RGO retains about 77.28% (105.6 F g
1) of the initial capacitance after 1000 cycles. The CV curves of 40 wt.% nanocomposite for 1st, 50th, 100th, 200th, 500th and 1000th cycles are shown in Fig. 11(ii). From the figure, it is seen that the difference between the 1st cycle and the 1000th cycle of 40 wt. % nanocomposite is negligible, suggesting good long-term cyclic stability of the electrode. The discrepancy in electrochemical stability between PPyNTs and 40 wt.% RGO-PPyNTs may be attributed to the combined double-layer and pseudocapacitive contributions. It is well-known that the double-layer process only involves a physical rearrangement of charges, while pseudocapacitive is related to the faradic charge transfer reactions and therefore the double-layer capacitors have a higher electrochemical stability but lower Csp as compared to those for pseudocapacitors [65]. Accordingly, the 5, 10, 20 wt.% RGO-PPyNTs nanocomposites exhibit enhanced electrochemical stability retaining 64.39%, 67.19% and 71.98% of the initial capacitance after 1000 cycles. Capacitive retention rate of around 70% have been reported for a graphene/polyaniline nanofiber composites after 1000 cycles [66] and for a three-dimensional graphene/polyaniline hybrid prepared by electropolymerization of polyaniline on macroporous graphene foam after 600 cycles [67]. While, the electrolyte ionic accessibility inside the PPy electrode is highly favorable from the hollow structured network, it appears that the presence of multilayered RGO nanosheets coated with one dimensional nanotubes facilitates enhanced electronic transport with high capacitive response even with increasing number of cycles. Generally for polymer electrodes, the polymer volume increases when they are doped with ions in electrolyte (charging) and decreases when these ions diffuse back to electrolyte (discharging). Hence, most polymer electrodes cannot tolerate the mechanical stress created by the volume change and thus break into small pieces during the cycling process [68]. In the present study, it appears that the high surface area of the flat nanosheets of RGO can easily adapt to the volume change that the PPy electrode undergoes during the charge transfer reactions, that allows the swelling or shrinkage to be avoided in the nanocomposite electrode and hence a more stable capacitance with good electrochemical stability could be obtained with the nanocomposites. 3.7. EIS investigation


1

)

25 mV s
1

50 mV s
1

75 mV s
1

100 mV s
1

9.9/437.1

8.4/719.1

6.9/859.5

6.2/1027.6

12.9/571

10.3/880.3

9/1144.1

8.1/1440

16.6/733.6

13.5/1161.6

11.4/1434.9

21.1/931.9

16.9/ 1449.3 22.6/ 1933.9

29.3/ 1292.3

9

10.2/ 1682.1 15.1/1888.4 13.3/ 2189.4 20.6/ 18.9/ 2567.3 3105.8

The enhanced electronic transport of RGO-PPyNTs nanocomposites with the incorporation of RGO nanosheets is also evident from the electrochemical impedance analysis. The equivalent circuit of different elements from the impedance spectra is presented in the inset of Fig. 12(i). The equivalent circuit consists of equivalent series resistance (Rs), the charge transfer resistance (Rct), the constant-phase element (CPE) and the Warburg impedance (Zw). Rs includes the ionic resistance of the electrolyte, the intrinsic resistance of the active material and the contact resistance at the interface active material [69]. Rct is the charge transfer resistance at the electrode/

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Fig. 11. (i) Variation of specific capacitance with cycle number measured at a scan rate of 100 mV s
1 of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt. %, (d) 20 wt.% and (e) 40 wt.% of RGO; (ii) CV curves with increasing cycle number of 40 wt.% RGO-PPyNTs nanocomposite.

Fig. 12. (i) Nyquist plots of (a) PPyNTs and RGO-PPyNTs nanocomposites with (b) 5 wt.%, (c) 10 wt.%, (d) 20 wt.% and (e) 40 wt.% of RGO (Inset shows equivalent circuit) and (ii) Magnified view of the Nyquist plot in Z' range of 0–60 V.

electrolyte interface. In addition, a frequency dispersion is often revealed in the impedance results for a solid electrode/electrolyte interface. This frequency dispersion is generally attributed to a capacitance dispersion expressed in terms of a CPE. The impedance of the CPE is defined as [70]: 1  Z CPE ¼  Q ðjvÞn

ð7Þ

where Q is a constant that arises from the combination of properties related to the surface and the electroactive species, j is an imaginary number, v is the angular frequency and the exponent n denotes the correction factor pertaining to the roughness of the electrode. These components form a mixed-controlled Randle’s circuit, which is usually used to model capacitance in a process with a combination of kinetic and diffusion processes determining the rate. At high frequencies, the capacitor impedance becomes much smaller and the system resistance is dominated by the resistors and polarization impedances. At low frequencies, an ideal polarizable capacitance would give a straight line with large angles with respect to Z0 or almost parallel to Z00 [71].

The EIS data are analyzed using Nyquist plots as shown in Fig. 12(i) and (ii) (magnified view). Nyquist plots display the frequency response of the electrode/electrolyte system and are a plot of the imaginary component (Z00 ) of the impedance against the real component (Z0 ). Each data in the plot is related to a different frequency with the lower left portion of the curve corresponding to higher frequencies. The values of Rs and Rct of PPyNTs and the nanocomposites obtained from the Nyquist plot are depicted in Table 3.

Table 3 Values of equivalent series resistance (Rs), charge transfer resistance (Rct) and n of PPyNTs and RGO-PPyNTs nanocomposites. Sample

Rs (V)

Rct (V)

n

PPyNTs 5 wt.% RGO 10 wt.% RGO 20 wt.% RGO 40 wt.% RGO

12.78 12.07 10.74 8.82 6.94

35.69 24.62 16.63 10.15 6.26

0.55 0.58 0.63 0.69 0.71

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All the Nyquist plots exhibit a semicircle in the high-frequency region and a straight line in the low-frequency region, where the high frequency semicircle intercepts the real axis at Rs and (Rs + Rct) respectively. The high frequency semicircle is associated with charge transfer resistance caused by the Faradic reactions and the double-layer capacitance on the electrode surface, while the straight line is determined by ion diffusion [72]. From the small intercepts, it is evident that Rs is very low for all the nanocomposites compared to PPyNTs. The difference in Rs value between PPyNTs and 40 wt.% RGO-PPyNTs being about 5.84 V, it appears that high conductivity of the nanocomposite due to the presence of RGO is contributing to the low Rs of the electrode. This clearly demonstrates the reduced interfacial contact resistance between the RGO nanosheets and PPyNTs inside the electrode material. Rs data also determines the rate that the supercapacitor can be charged/discharged [73]. The values of Rct are also found to be low in the nanocomposites. The lowest Rct is 6.26 V for 40 wt.% RGO-PPyNTs nanocomposite, which is much smaller than that of PPyNTs. This meant that the contribution of polarization impedance reduced, while mass capacitance remained the dominant component of the system impedance. The decreased semicircle in the complex impedance plane has been explained by high ionic conductivity at the electrode/electrolyte interface [74]. It also points to the strength of the underlying van der Waal’s force of interaction between PPyNTs and large aromatic RGO basal plane for facile interlayer charge transport within the nanocomposite matrix. Furthermore, the slope of the 45 portion of the curve in the Nyquist plot is a signature of the Warburg impedance [75]. The vertical shape at lower frequencies indicates a pure capacitive behavior, representative of the ion diffusion/transport in the electrolyte to the electrode surface. The more vertical the curve, the more closely the supercapacitor behaves as an ideal capacitor [76]. Among all the electrodes, the slope of the straight line for 40 wt.% of RGO incorporated PPyNTs electrode is largest, closest to the imaginary impedance axis (Z00 ). This reduced Warburg resistance for the nanocomposite indicates short ion diffusion path lengths and decreased obstruction of ion movement within the electrode [66]. Thus it can be attributed that the network of the RGO structure helps in facilitating efficient access of electrolyte ions to the nanocomposite electrode surface thus shortening the ion diffusion path. On the other hand, imperfectness of the supercapacitive materials can be expressed by CPE. The CPE has been considered to represent a circuit parameter with limiting behavior as a capacitor for n = 1, a resistor for n = 0, and a Warburg impedance for n = 0.5 [77]. The value of n is obtained from the linear fit of Bode plot of log |Z| versus log f which is also presented in Table 3, where |Z| is the absolute value of impedance and f is the frequency. It is observed that the value of n is 0.55 for PPyNTs, signifying the Warburg behavior. However in the case of nanocomposites, n increases with increase in RGO loading attaining a maximum value of 0.71 for 40 wt.% nanocomposite indicating moderate capacitive nature of the sample. Hence it can be concluded that the nanocomposite with 40 wt.% of RGO exhibited the highest ionic conductivity or lowest internal resistance, including polarization impedance, very good charge transfer and is a promising electrode material for supercapacitor applications. 4. Conclusion In this paper, RGO-PPyNTs nanocomposites for supercapacitor electrode applications are synthesized by in-situ reduction of GO in the presence of PPyNTs. HRTEM micrographs display a paper-like morphology for RGO and an average outer diameter of 132 nm is measured for PPyNTs. A uniform distribution of RGO and PPyNTs is observed from the SEM images with increasing concentration of RGO. XRD patterns confirm the reduction of GO to RGO within the

11

polymer matrix. In FTIR spectra, the shifting of bands associated with the carbon groups indicates the interaction between PPyNTs and RGO. The significant structural changes in the nanocomposites after incorporation of in-situ reduced GO are also revealed from the Micro-Raman spectra. Increase in conductivity of the nanocomposites is observed from I–V curves. The obtained nanocomposites show enhanced capacitive behavior with a highest specific capacitance of 281 F g
1 along with a maximum energy density and power density values of 39 W h kg
1 and 3105 W kg
1, respectively. Also, a good electrochemical stability is observed for the nanocomposites with a highest capacitive retention of 77.28% after 1000 cycles which can be ascribed to the unique properties of both RGO and PPyNTs. EIS studies depict an increased electronic charge transport and short diffusion path length for ions in the nanocomposites after incorporation of RGO. Acknowledgement The authors gratefully acknowledge the help extended for HRTEM measurements by the Electron Microscopy Division, SAIF, Shillong. References [1] H. Gómez, M.K. Ramb, F. Alvia, P. Villalba, E. Stefanakos, A. Kumar, J. Power Sources 196 (2011) 4102–4108. [2] A. Burke, J. Power Sources 91 (2000) 37–50. [3] J. Yan, T. Wei, Z. Fan, W. Qian, M. Zhang, X. Shen, F. Wei, J. Power Sources 195 (2010) 3041–3045. [4] F. Alvi, M.K. Ram, P.A. Basnayaka, E. Stefanakos, Y. Goswami, A. Kumar, Electrochim. Acta 56 (2011) 9406–9412. [5] Z. Gao, W. Yang, J. Wang, H. Yan, Y. Yao, J. Ma, B. Wang, M. Zhang, L. Liu, Electrochim. Acta 91 (2013) 185–194. [6] C. Li, H. Bai, G. Shi, Chem. Soc. Rev. 38 (2009) 2397–2409. [7] C. Zhu, J. Zhai, D. Wen, S. Dong, J. Mater. Chem. 22 (2012) 6300–6306. [8] F. Liu, S. Wang, G. Han, R. Liu, Y. Chang, Y. Xiao, J. Appl. Polym. Sci. 131 (2014) 41023. [9] S. Sahoo, G.C. Nayak, C.K. Das, Macromol. Symp. 315 (2012) 177–187. [10] D.W. Wang, F. Li, J. Zhao, W. Ren, Z.G. Chen, J. Tan, Z.S. Wu, I. Gentle, G.Q. Lu, H. M. Cheng, ACS Nano 3 (2009) 1745–1752. [11] Y. Huang, J. Liang, Y. Chen, Small 8 (2012) 1805–1834. [12] K.H. An, K.K. Jeon, J.K. Heo, S.C. Lim, D.J. Bae, Y.H. Lee, J. Electrochem. Soc. 149 (2002) A1058–A1062. [13] H. Cao, X. Zhou, Y. Zhang, L. Chen, Z. Liu, J. Power Sources 243 (2013) 715–720. [14] J. Xu, K. Wang, S.Z. Zu, B.H. Han, Z. Wei, ACS Nano 4 (2010) 5019–5026. [15] L. Li, K. Xia, L. Li, S. Shang, Q. Guo, G. Yan, J. Nanopart. Res. 14 (2012) 908. [16] Z. Gao, W. Yang, J. Wang, B. Wang, Z. Li, Q. Liu, M. Zhang, L. Liu, Energy Fuels 27 (2013) 568–575. [17] A.G. Pandolfo, A.F. Hollenkamp, J. Power Sources 157 (2006) 11–27. [18] L. Mao, K. Zhang, H.S.O. Chan, J. Wu, J. Mater. Chem. 22 (2012) 80–85. [19] J. Zhang, Y. Yu, L. Liu, Y. Wu, Nanoscale 5 (2013) 3052–3057. [20] J. Du, H.M. Cheng, Macromol. Chem. Phys. 213 (2012) 1060–1077. [21] N.A. Kumar, J.B. Baek, Chem. Commun. 50 (2014) 6298–6308. [22] J. Wang, Y. Xu, J. Zhu, P. Ren, J. Power Sources 208 (2012) 138143. [23] C. Bora, S.K. Dolui, Polymer 53 (2012) 923–932. [24] Y.S. Lim, Y.P. Tan, H.N. Lim, N.M. Huang, W.T. Tan, J. Polym. Res. 20 (2013) 156. [25] H.P. Oliveira, S.A. Sydlik, T.M. Swager, J. Phys. Chem. C 117 (2013) 10270–10276. [26] P.A. Basnayaka, M.K. Ram, L. Stefanakos, A. Kumar, Graphene 2 (2013) 81–87. [27] F.H. Hsu, T.M. Wu, Synth. Met. 162 (2012) 682–687. [28] Y. Yang, C. Wang, B. Yue, S. Gambhir, C.O. Too, G.G. Wallace, Adv. Energy Mater. 2 (2012) 266–272. [29] S. Bose, N.H. Kim, T. Kuila, K.t. Lau, J.H. Lee, Nanotechnology 22 (2011) 295202. [30] J. Zhang, X.S. Zhao, J. Phys. Chem. C 116 (2012) 5420–5426. [31] X. Feng, Z. Yan, R. Li, X. Liu, W. Hou, Polym. Bull. 70 (2013) 2291–2304. [32] S. Biswas, L.T. Drzal, Chem. Mater. 22 (2010) 5667–5671. [33] S. Park, J. An, J.R. Potts, A. Velamakanni, S. Murali, R.S. Ruoff, Carbon 49 (2011) 3019–3023. [34] M. Wojtoniszak, X. Chen, R.J. Kalenczuk, A. Wajda, J. Łapczuk, M. Kurzewski, M. Drozdzik, P.K. Chu, E. Borowiak-Palen, Colloids Surf. B 89 (2012) 79–85. [35] S. Bose, T. Kuila, M.E. Uddin, N.H. Kim, A.K.T. Lau, J.H. Lee, Polymer 51 (2010) 5921–5928. [36] S. Maiti, B.B. Khatua, RSC Adv. 3 (2013) 12874–12885. [37] Y. Liu, H. Wang, J. Zhou, L. Bian, E. Zhu, J. Hai, J. Tang, W. Tang, Electrochim. Acta 112 (2013) 44–52. [38] H. Hu, G. Chen, Polym. Compos. 31 (2010) 1770–1775. [39] M. Acik, C. Mattevi, C. Gong, G. Lee, K. Cho, M. Chhowalla, Y.J. Chabal, ACS Nano 4 (2010) 5861–5868. [40] J. Upadhyay, A. Kumar, Mater. Sci. Eng. B 178 (2013) 982–989. [41] J. Jang, J.H. Oh, Adv. Funct. Mater. 15 (2005) 494–502.

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G Model SYNMET 15493 No. of Pages 12

12

M. Devi, A. Kumar / Synthetic Metals xxx (2016) xxx–xxx

[42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58]

J. Jang, H. Yoon, Chem. Commun. (2003) 720–721. J. Upadhyay, A. Kumar, Mater. Sci. Eng. C 33 (2013) 4900–4904. A. Abulizi, G.H. Yang, J.J. Zhu, Ultrason. Sonochem. 21 (2014) 129–135. M. Sawangphruk, P. Srimuk, P. Chiochan, T. Sangsri, P. Siwayaprahm, Carbon 50 (2012) 5156–5161. D. Zhang, X. Zhang, Y. Chen, P. Yu, C. Wang, Y. Ma, J. Power Sources 196 (2011) 5990–5996. J. Zeng, H.J. Yao, S.X. Zhang, P.F. Zhai, J.L. Duan, Y.M. Sun, G.P. Li, J. Liu, Nucl. Instrum. Methods Phys. Res. B 330 (2014) 18–23. V. Chandra, K.S. Kim, Chem. Commun. 47 (2011) 3942–3944. S. Some, P. Bhunia, E. Hwang, K. Lee, Y. Yoon, S. Seo, H. Lee, Chem. Eur. J. 18 (2012) 7665–7670. G. Compagnini, F. Giannazzo, S. Sonde, V. Raineri, E. Rimini, Carbon 47 (2009) 3201–3207. Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Adv. Mater. 22 (2010) 3906–3924. J. Zhang, Y. Yu, L. Liu, Y. Wu, Nanoscale 5 (2013) 3052–3057. V. Varade, G.V. Honnavar, P. Anjaneyulu, K.P. Ramesh, R. Menon, J. Phys. D Appl. Phys. 46 (2013) 365306–365313. L. Yun-Ze, Y. Zhi-Hua, L. Meng-Meng, G. Chang-Zhi, D. Jean-Luc, J. Ai-Zi, W. Mei-Xiang, Chin. Phys. B 18 (2009) 2514–2522. an, M. Sag lam, A. Türüt, J. Phys.: Condens. Matter 18 (2006) 2665– S. Aydog 2676. Z.H. Yin, Y.Z. Long, C.Z. Gu, M.X. Wan, J.L. Duvail, Nanoscale Res. Lett. 4 (2009) 63–69. S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nat. Lett. 442 (2006) 282–286. J. Upadhyay, A. Kumar, Compos. Sci. Technol. 97 (2014) 55–62.

[59] S. Sahoo, G. Karthikeyan, G.C. Nayak, C.K. Das, Macromol. Res. 20 (2012) 415– 421. [60] W. Zhang, C. Ma, J. Fang, J. Cheng, X. Zhang, S. Dong, L. Zhang, RSC Adv. 3 (2013) 2483–2490. [61] S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, ACS Nano 4 (2010) 2822–2830. [62] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, Nanoscale 2 (2010) 2164–2170. [63] Q. Wang, J. Yan, Z. Fan, T. Wei, M. Zhang, X. Jing, J. Power Sources 247 (2014) 197–203. [64] A. Davies, P. Audette, B. Farrow, F. Hassan, Z. Chen, J.-Y. Choi, A. Yu, J. Phys. Chem. C 115 (2011) 17612–17620. [65] P. Si, S. Ding, X.W. Lou, D.H. Kim, RSC Adv. 1 (2011) 1271–1278. [66] K. Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Chem. Mater. 22 (2010) 1392–1401. [67] X. Dong, J. Wang, J. Wang, M.B. Chan-Park, X. Li, L. Wang, W. Huang, P. Chen, Mater. Chem. Phys. 134 (2012) 576–580. [68] T. Liu, L. Finn, M. Yu, H. Wang, T. Zhai, X. Lu, Y. Tong, Y. Li, Nano Lett. 14 (2014) 2522–2527. [69] J. Gamby, P.L. Taberna, P. Simon, J.F. Fauvarque, M. Chesneau, J. Power Sources 101 (2001) 109–116. [70] J.B. Jorcin, M.E. Orazem, N. Pébère, B. Tribollet, Electrochim. Acta 51 (2006) 1473–1479. [71] L. Wang, Y. Ye, X. Lu, Z. Wen, Z. Li, H. Hou, Y. Song, Sci. Rep. 3 (2013) 3568. [72] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Adv. Funct. Mater. 21 (2011) 2366–2375. [73] C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Nano Lett. 10 (2010) 4863–4868. [74] X. Li, J. Rong, B. Wei, ACS Nano 4 (2010) 6039–6049. [75] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, ACS Nano 4 (2010) 1963–1970. [76] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. 8 (2008) 3498–3502. [77] P. Zoltowski, J. Electroanal. Chem. 443 (1998) 149–154.

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