Electrochemical reduced graphene oxide-polyaniline as effective nanocomposite film for high-performance supercapacitor applications

Electrochemical reduced graphene oxide-polyaniline as effective nanocomposite film for high-performance supercapacitor applications

Accepted Manuscript Title: Electrochemical reduced graphene oxide-polyaniline as effective nanocomposite film for high-performance supercapacitor appl...

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Accepted Manuscript Title: Electrochemical reduced graphene oxide-polyaniline as effective nanocomposite film for high-performance supercapacitor applications Authors: Mehdi Shabani-Nooshabadi, Fatemeh Zahedi PII: DOI: Reference:

S0013-4686(17)31149-0 http://dx.doi.org/doi:10.1016/j.electacta.2017.05.152 EA 29587

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

20-4-2017 20-5-2017 23-5-2017

Please cite this article as: Mehdi Shabani-Nooshabadi, Fatemeh Zahedi, Electrochemical reduced graphene oxide-polyaniline as effective nanocomposite film for high-performance supercapacitor applications, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.05.152 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.

Electrochemical reduced graphene oxide-polyaniline as effective nanocomposite film for high-performance supercapacitor applications

Mehdi Shabani-Nooshabadi*, Fatemeh Zahedi Department of Analytical Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-51167, Kashan, Iran Corresponding author* Tel.: +98 3155912357; fax: +98 315555293 E-mail address: m.shabani @kashanu.ac.ir (M.Shabani-Nooshabadi).

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The increasing societal demands for portable electronics vehicles prompted researchers to develop electrochemical energy storage systems such as supercapacitors and lithium-ion batteries. Supercapacitors has been renowned as one of the most next generation energy storage devices due to its long cycle life high power density and. Therefore, in this study, we reported the synthesis electrochemical reduced graphene oxide (ERGO)-polyaniline (PANI) nanocomposites as supercapacitors.

HIGHLIGHTS 

PANI nanospherical are grown on ERGO layer by CV electrodeposition.



The application of ERGO-PANI nanocomposites for supercapacitors studies systematically.



The as-synthesized nanocomposites of ERGO-PANI show good electrochemical property and cycling stability.



Performance of ERGO-PANI nanocompsites were improved by synergistic effects between two components.

Abstract In this paper, high performance supercapacitor based on electrochemical reduced graphene oxide (ERGO)-polyaniline (PANI) nanocomposite film was prepared in facile two steps. ERGO sheets synthesized on surface of glassy carbon electrode (GCE) by an effective and environment-friendly electrochemical reduction of graphene oxide (GO), followed PANI formed on ERGO surface via cyclic voltammetry electrodeposition at potential window -0.20 to 0.80 V at a scan rate of 50 mV s-1. Electrochemical results indicate that ERGO-PANI nanocomposites exhibit a good capacitance performance with maximum specific capacitance of 1084 Fg-1, and high specific energy density of 53.12Wh kg-1 with specific power density of 500.61W kg-1 at a current density of 3.22 mA cm-2 in 1M H2SO4 aqueous solution. Improved electrochemical performance of the nanocomposites arises from synergistic combination of electrically conductive ERGO and highly capacitive behavior of PANI. Therefore, the ERGO-PANI nanocomposite electrodes are promising candidate for application in energy storage devices. Moreover, the products have been successfully characterized by X-ray diffraction (XRD), Fourier transformation spectroscopy (FT-IR), scanning electron microscope (SEM), energy dispersive spectrometry (EDS) and atomic force microscopy (AFM). Keywords: Supercapacitor; Energy storage; Polyaniline; Electrochemical reduced graphene oxide; Electrodeposition.

1. Introduction With rapid technological development, increasing depletion of fossil energy and the global environmental concerns, there is an urgent require to research on renewable and sustainable energy storage sources. Supercapacitors, also known as electrochemical capacitors or ultracapacitors, are considered as one of the significant innovations in electrical energy storage systems which require high power density in short time periods such as pulse lasers, portable electronic devices, aerospace, memory back-up systems, emergency power systems, and hybrid electrical vehicles (HEV) and etc. [1-8]. Generally, supercapacitors have longer cycle life (> 100000 cycles) and higher power density than lithium secondary batteries and higher energy density than conventional dielectric capacitors [9, 10]. Nevertheless, in compared with batteries, supercapacitors suffer poorer specific capacity (or energy density). Overall, supercapacitors are considered as intermediate systems between conventional dielectric capacitors and batteries in energy world. Regarding energy storage mechanism, supercapacitors can be classified into two types [11].The first one is electrochemical double layer capacitors (EDLC) where energy stored via opposite charges separation at the electrode/electrolyte interface [12]. Carbon nanomaterials like activated carbons (ACs) [13], ordered mesoporous carbons (OMCs) [14], carbon nanotubes [15], graphene nanosheets (GNS) [16] and graphdiyne [17], are used as electrode materials for this type of supercapacitors. The other type of supercapacitor, pseudocapacitors are, where storage of energy occurs by reversible and fast faradaic reactions at surface or near of the active materials. Peseudocapacitors typically can deliver much greater specific capacitance and energy density than EDLC. Because the energy storage (fast redox reactions) in pseudocapacitors occur both near and surface of electrode, whereas the energy storage of

EDLC confine to the surface regions of carbon materials only. Pseudocapacitior electrode materials including transition metal oxides and hydroxides such as, CuO [18], MnO2 [19], Ni)OH)2 [20], La2O3[21], Co3O4 [22] and conducting polymers such as polyaniline (PANI) [23], polypyrrol (PPy) [24], poly-3,4-ethlyenedioxythiophene (PEDOT) [25], and polythiophene (PTH) [26] and their derivatives. Among various pseudocapacitive materials, PANI is recognized as one of the most promising conducting polymer which is extensively studies for supercapacitors in last decades due to its controllable electric conductivity, simple synthesis, high energy density, environmental friendliness and quick oxidation-reduction reactions [27-30]. Essentially, different oxidation forms of PANI (Leucoemeraldine, Emeraldine and Pernigraniline) contribute to its high theoretical capacitance. However, relatively poor mechanical stability of PANI caused by large volumetric swelling and shrinking during charge/discharge process, thus, leading limitation of PANI as individual electrode materials for electrochemical energy storage applications. To reinforce the stability thus improve the electrochemical performance of PANI, inorganic and organic materials have been combined with PANI to make PANI-based nanocomposites [31, 32]. Graphene nanosheet is a two dimensional (2D) monolayer of sp2-hybridized carbon atoms in a hexagonal crystal lattice with distinctive property such as: high surface-to-volume ratio, large specific surface area (2630 m2 g-1), exceptional mechanical strength, very high mobility of charge carriers (200000 cm2 V s-1), excellent chemical stability and suitable thermal and electrical conductivities [33-35]. Different methods of reduction GO have been developed for the synthesis of graphene, such as thermal, chemical, solvothermal and hydrothermal, photocatalytic and electrochemical reduction. Chemical reduction of GO needs excessive reducing agent that leading environmental pollutions and even could contaminate the resulting reduced graphene oxide. Additionally, deoxygenation of GO cannot be completely accomplished via only a reluctant action [36].

More recently, electrochemical methods are one promising innovative and green strategy for graphene synthesis that provide several advantages as compared with chemical methods such as: easy to control, relatively simple, fast, economic, operates at ambient temperature and pressure and no toxic solvent is used [37, 38]. In chemical method, the resulting graphene incline to agglomerate upon reduction in liquid phase. Whilst, electrochemical route produce graphene directly onto the working electrode substrate which could be used for various applications without further steps. Furthermore, thickness of produced graphene film in the method can be easily tuned by adjusting the electrode potential. Diverse electrochemical approaches containing potentiodynamic (CV), potentiostatic, linear sweep voltammetry (LSV) and differential pulse voltammetry (DPV) can be used for GO reduction in a threeelectrode electrochemical scheme. Among these techniques, CV method have been attracted significant interest, since it provides controllability in layer thickness on electrode surface, information about potentials of redox, and reaction reversibility [39]. Until now, few reports have been investigated by researchers on the supercapacitance performance of ERGO-based nancomposites. Chen Tsai et al. synthesized ERGO/PPy film onto polished gold electrode by electrochemical-co deposition. Compared with pristine PPy (108 F g-1), specific capacitance ERGO/PPy composite was 352 F g-1 at a charge current density of 1 A g-1. Also, a high specific capacitance 424 F g-1 for this composite with value of 6mg ml-1 GO obtained [40]. Shahrokhian et al. studied fabrication three dimensional ERGONiO composite by electrochemical method on nickel foam which show specific capacitance 1715.5 F g-1 at current density of 2 A g-1 with retention life 78.8% after 2000 charge/discharge cycles [41]. The fundamental objective of the present study was to evaluate supercapacitive properties of ERGO-PANI by cyclic voltammetry, charge-discharge and electrical impedance spectroscopy measurements. Furthermore, structural and morphological properties were studied using XRD, FT-IR, SEM-EDS and AFM. The synthesized ERGO-

PANI nanocomposite in this work revealed higher specific capacitance than those reported in the literature. 2. Experimental 2.1 Materials and methods Graphene oxide with purity 99% was purchased from Us Research Nanomaterials company (USA). Other chemical reagents with analytical grade obtained from Merck company (Germany) and used without any further purification, excluding aniline monomers that was freshly distilled to eliminate oxidation impurities. Resulting colorless liquid retained in darkness at low temperature (5 ºC) before use. Deionized water was employed for the preparation of solution during all experiments. First, GCE (glassy carbon electrode with 2 mm diameter) was carefully polished with 0.05 µm alumina powders on polishing cloth, then sonicated sequentially in ethanol and deionized water before casting action. GO was dispersed in deionized water by probe sonication (30 W) for 15 min to form a uniform brown colloidal dispersion. Later, 2 µl of GO dispersion (3 mg ml-1) was drop casted on surface of the polished GC in such a way that its surface area was fully covered, then dried slowly at room temperature. A potentiostat–galvanostat (Sama 500, Iran) was used for the electrodeposition, in standard three-electrode system, including an Ag/AgCl (3.0 M KCl) as the reference electrode, Pt rod as the counter electrode and GCE as the working electrode. Electrochemical reduction of GO film was accomplished at -0.40 to -1.5 V (vs. Ag/AgCl) for 20 cycles in 0.1 M phosphate buffer solution (PBS, pH: 3.0) with purged nitrogen for 5 minutes. After wards, ERGO coating was rinsed several times with water and dried at ambient temperature. The ERGO-PANI was prepared through electrodeposition process in an electrolyte solution containing 0.1 M aniline monomer and 0.5 M sulfuric acid. Electropolymerization of aniline was performed via cyclic voltammetry at a potential range of -0.2 to 0.8 V onto the ERGO

surface at scan rate of 50 mV s-1 for different cycles from 5 to 90 cycles. Finally, product washed with deionized water to eliminate any absorbed soluble monomers. For comparison, pure PANI (with cycles number of 50) and ERGO were also prepared under similar conditions. Mass of PANI films can be approximated from total faradic charges consumed in electrodeposition process through faraday’s law of electrolysis by following equation (1) [42]:

m=

(1)

Where m is the mass of PANI (g), Q is total faradic charges and F, Mm, Md and ᵞ are Faraday constant (96485 C mol-1), molar mass of aniline monomer (93.13 g mol-1), molar mass of hydrogen sulfate dopant (97.0 g mol-1) and doping level (0.50 for PANI) [43, 44], respectively. 2.2. Electrochemical characterization All the electrochemical tests were carried out in a three electrode cell by using μAUTOLAB potentiostat/galvanostat model μIII AUTO 71174 equipped with NOVA 1.6 software. GCE, Pt rod and Ag/AgCl (3.0 M KCl) were used as the working, counter and reference electrodes. Cyclic voltammetry (CV) carried out in the potential window of -0.200.60V at various scan rates of 15-200 mV s-1. Galvanostatic charge/discharge (GCD) examinations were conducted at the potential window between 0 to 0.6V and at the different current densities from 3.22 mA cm-2 to 25.81 mA cm-2. The electrochemical impedance spectroscopy (EIS) tests were measured in the frequency range from 10 mHz to 100 kHz to at a cell potential of 0.2 V vs. Ag/AgCl by applying an AC voltage of 10 mV amplitude. All electrolytes used above were1 M H2SO4 aqueous solution. All measurements were achieved at room temperature. 2.3. Structural characterization

The crystalline structure of the prepared nanocomposites investigated by X-ray diffractometer (X’pert Pro MPD, Philips, Netherlands) with operation 40kV and 30mA using Cu Kα radiation source (0.154 nm) in range 2ϴ=10-80°. FT-IR spectrophotometer (Magna IR550, Nicolet, USA) utilized to characterize surface functionality of samples. Spectrum was collected at the scanning wavenumbers (4000-400 cm-1) by KBr pellets. The structures morphologies and elemental analysis of the prepared nanocomposites were characterized by scanning electron microscope (Prox, Phenom, Netherlands) coupled with an energy dispersive X-ray spectrometry (EDS) at an accelerating voltage of 10 kV. Topography of film surfaces prepared on GCE were characterized by AFM (NT-MDT Solver P47, Zelenograd, RUS) with a 278 kHz resonance frequency and scanning area 5 μm×5 μm in tapping mode using silicon tip.

3. Results and discussion 3.1. Synthesis mechanism Fig. S1(a) (see Supporting Information) shows the typically cyclic voltammograms obtained during electrochemical reduction of GO from potential range -0.40 to -1.50 at scan rate of 50 mV s-1 . Cathodic current peak around -1.15 V ascribed to reduction of surface oxygen species on GO. It should be note that this reduction peak decreased slowly with successive potential scans due to irreversible of reduction process. The selection of suitable reduction potential plays essential role for complete reduction of GO. It is noteworthy that, the high cathodic potential of -1.5 V can overwhelmed energy barriers for reduction of some oxygen functionalities groups including epoxide (C-O-C) bonding, hydroxyl (C-O-H) on GO plane [45]. In the other words, these functional groups are very hard to be reduced, thus the use of negative potential of (-1.5 V) increases electrochemical reduction rate of GO to ERGO, so the oxygen-containing functional groups can be efficiently removed.

Fig. S1(b) (Supporting Information) shows CV curves recorded for electrochemical synthesis of PANI on ERGO in potential window -0.20 to 0.80 V for 50 cycles. In first cycle, anodic irreversible peak around 0.80V corresponds to oxidation of aniline monomers and beginning of polymerization process. In following cycles, prominent oxidation and reduction peaks PANI were evidently observed. First anodic peak around 0.10V is attributed exchange fully reduced form (Leucoemerladine) into half oxidized form (Emeraldine) and it shifts slightly in positive direction in each sequential cycle. Second anodic peak at 0.50V related to formation hydrolysis products of PANI [46]. Continuous growth of PANI film can be approved by increased peaks current with increasing number of potential cycles. Superficially, creation of deep green coating was confirmed formation PANI on ERGO surface. 3.2. Structure and morphology characterizations The crystal structures of samples were characterized initially by XRD as shown in Fig. 1(a). XRD pattern of GO revealed a high intense sharp at 2θ=11.15 º, corresponding to interplanar spacing of 0.79 nm using Bragg’s equation:

d=

(2)

Where λ is wavelength of X-ray beam, θ is Bragg angle and n is 1. In the case of the ERGO, broad diffraction peak at 2θ=24.70º is ascribed to structural change of GO during electrochemical reduction reaction. Additionally, inter planar spacing between ERGO sheets (0.40 nm) smaller than GO sheets, which could be attributed to removal oxygen containing functional groups on GO sheets after reduction process. For ERGO-PANI, peaks centered at 2θ=19.49º and 25.26º which corresponding emerldine salt of PANI. It is worth noting that peak ERGO might be overlapped with peaks of PANI [47]. Fig. 1(b) displays FT-IR spectra of GO, ERGO and ERGO-PANI noanococ. FT-IR spectrum of GO presented C─O (alkoxy), C─O─C (epoxy) and C=O (carbonyle) stretching

vibrations at 1052.30, 1222.42 and 1704.66 cm-1. Broad band around 3395.93 cm-1 attributed to stretching vibration of ─OH group in GO nanosheets [48]. Again in FT-IR of ERGO, intensity most of absorption bands of oxygen functionalities decreased obviously, indicating that GO could be reduced by CV electrochemical reaction, indicating that electrochemical reduction as an effective approach provided production of ERGO. ERGO-PANI revealed typical bands of PANI including 825.48 cm-1 (C─H out plane bending vibration in 1,4 disubstituted aromatic ring), 1299 cm-1 (C─N stretching of secondary aromatic amines), 1236.48 cm-1 (C=N stretching vibration) and 3300 cm-1 (N─H stretching vibration of aromatic amines). Also, bands at 1471.30 and 1565.11 cm-1 are assigned to C=C stretching vibrations benzenoid and quinoid rings [49]. Presence of these characteristic bands suggested PANI formed successfully on ERGO. Trivial Shift in absorption bands of ERGO/PANI compared with ERGO might be associated interaction between PANI and ERGO. The agglomerated morphology of GO was presented in Fig. 2(a). SEM image of ERGO (Fig. 2 (b)) displayed a non-uniform coating and particles with average diameter about 40 nm however nanoparticles agglomeration on GCE surface observed. SEM images of as-syntheses ERGO-PANI (50) nanocomposite was shown in Fig. 3(a,b). The PANI particle formed homogenously onto skeletons ERGO. Apparently, most nanostructures were spherical form. It is believed that nanometer dimension of electrodes due to their unique chemical, electrical and mechanical properties critical influence on improvement of supercapacitors performance [50]. Nanostructure with high surface area provide shorter transportation path for both ions electron and compared to bulk materials. Fig. 2(c, d) shows EDS spectrum of GO and ERGO, which obviously confirmed existing of C and O elements. Existence of Au peaks in spectrums due to gold thin coating which covered on samples for SEM analysis. It can be observed that the C/O atomic ratio of GO as compared to ERGO increased remarkably from 1.61 to 7.3, demonstrating that most of oxygen-containing functional groups in ERGO sheets

were eliminated. According to spectrum of Fig. 3(c) to existence of new peak at 0.30 keV in ERGO-PANI spectrum assigned to N element due to formation PANI on ERGO surface. AFM images of ERGO and ERGO-PANI(50) nanocomposite in Fig. 4(a-d) show that nanocomposite has greater root mean square of roughness (Rrms: 38.21nm) compared to ERGO electrode (Rrms:18.95nm). Greater Rrms, might enhance interfacial contact between electroactive sites of PANI and electrolyte ions that for supercapacitor applications are necessary. 3.3. Capacitance performance To investigate of influence mass loading or coating density in samples on capacitive performance, nanocomposites in coating density range of 0.28 to 3.03 mg cm-2 of PANI were electrosynthesized by 5 to 90 CV cycles at scan rate 50 mV s-1, and specific capacitances were calculated from CV tests methodically, which is revealed in Fig. 5. It is observable that coating of density increases with growing CV cycles. Further increasing cycles from 50 to 90, resulting to the specific capacitance decline. At high coating density (3.03 mg cm-2), PANI chains cannot be completely utilized during charge/discharge process due to long diffusion pathway ions of electrolyte to PANI, thus leading to a decline specific capacitance of nanocomposite. Based on electrodeposition cycles of 15, 25 and 50 PANI onto ERGO layer referred to as ERGO-PANI(15), ERGO-PANI(25) and ERGO-PANI(50). Therefore, three samples were investigated to detailed measurements of electrochemical. Fig. 6(a) presents the CV curves of the ERGO-PANI(15), ERGO-PANI(25), ERGOPANI(50), ERGO, pure PANI and GCE in the potential range of -0.20 to 0.60 V at a scan rate of 15 mV s-1. The enlarged CV plots for PANI, ERGO and GCE was presented in inset Fig. 6(a). The specific and area capacitances of the samples were estimated from CV curves according to following equations [51, 52]:

Cs =

Ca =





(3)

(4)

Where Cs and Ca are the specific capacitance (F g-1) and area capacitances (F cm-2), ∫ is integrated area under CV curve loop, m, A, υ and ∆V are mass of electroactive material (g), the surface area of electroactive material (cm2), scan rate (V s-1) and the potential window Vc to Va (V), respectively. According to the formula 3, the electrochemical capacitance is directly proportional to the area under the CV curve. The area surrounded by CV curves for the ERGO-PANI(50) is obviously larger than those other electrodes at the same scan rate. It is clear that the CV curve for graphene electrode is quasi rectangular, revealing the typical electrochemical double-layer capacitive behavior however presence of redox peaks on each CV plot for the ERGO-PANI or pure PANI indicates the existence of the faradic processes at electrode/electrolyte surface. Importantly, existence very small current of GCE show that its contribution to total specific capacitance can be ignored as compared with ERGO-PANI nanocomposites or even pure PANI and ERGO. All nanocomposites have two pairs of redox peaks: the first couple of peaks (symbolized as A1/C1) are attributed to the redox transition of PANI between a semiconducting state (Leucoemeraldine) and a conducting state (Emeraldine): the second couple of peaks (symbolized as A2/C2) are due to the transformation between the p-benzoquinone/hydroquinone couple due to cross-linking reaction among chains of polyamine It should be noted that graphene layer as support material of PANI with suitable electrical conductive and high surface area facilitates accessibility electrolyte ions to most of PANI chins, leading improvement capacitance. Fig. 6(b-d) show the CV curves of nanocomposites measured at scan rates of 15, 25, 50, 100 and 200 mV s-1, respectively. The redox current density increases evidently with increasing

scan rate, indicating good rate ability. All the CV curves of ERGO-PANI maintain two pairs of redox peaks even at a scan rate of 200 mV s-1. But it could be seen the oxidation peaks and the reduction peaks shifted to positively and negatively directions with the increase of scan rate, which resulted from resistance of the electrodes. The plots of the first anodic peak current versus the square root of the scan rate (insets of Fig. 6(b-d)) illustrations a good linear correlation, which is indicative redox reactions under diffusion-controlled process. The scan rate dependent specific and areal capacitances of three nanocomposites are revealed in Fig. S2 (a-c) (see Supporting Information). For example, the specific and areal capacitance values for ERGO-PANI (15), ERGO-PANI(25) and ERGOPANI(50) samples are 800.00 F g-1 ( 0.62 F cm-2), 862.21 F g-1 (1.0 F cm-2) and 898.55 F g-1 (2.0 F cm-2) at scan rate of 15 mV s-1. These results noticeably indicate that the specific and areal capacitances values are decreased, when the scan rate is increased. Since active sites of PANI involving in the faradic reactions inaccessible at high rate scans. With increasing electrodepositon cycles of PANI from 15 to 50, specific capacitance is usually enhance due to the higher number of redox electroactive species PANI chains in nanocomposite. The GCD plots of different capacitors at current density of 3.22 mA cm-2 were displayed in Fig. 7(a). The nonlinear and nearly symmetric charge-discharge curves of nanocomposites suggest a pseudocapacitance behavior of electrode materials. Notably, the IR drop is not apparent at low current density indicating low internal resistance and effective use of a capacitance current. The special capacitance (Cs, F g-1) was calculated from the discharge stage according to the by equation 5 [53]:

Cs =

(5)

where I (A), Δt (s), m and ΔV (V) are the discharge current, discharge time, mass of active materials and potential window (excluding IR drop at beginning of discharge), respectively. Evidently, the charge/discharge times are different at a constant current density for the electrodes, signifying various specific capacitances. On the basis of the charge/discharge curves, it is observed that ERGO-PANI (50) has largest specific capacitances with 1084 F g-1, while that specific capacitances of ERGO-PANI(25), ERGO-PANI(15), at same current density was 1027, 989.189 F g-1, respectively. The improved specific capacitance of nanocomposites was probably ascribed to strong synergistic effect between ERGO sheets and PANI. ERGO layer as active substrates with high electrical conductivity accelerated effective and quick ions diffusion lengths and electron transference into PANI matrix, while PANI with outstanding electrochemical activity and reversibility have chief contribution to the whole nanocomposite capacitance. In addition, to understand the rate capability of nanocomposites the GCD measurements were achieved and shown in Fig. 7(b- d) at different current densities from 3.22 to 25.81mA cm-2. Specific capacitances against current are plotted in Fig. 8(a). The specific capacitance for all the electrodes reduced gradually with the increasing current density. At high current of 25.81 mA cm-2, intercalation/deintercalation or (doping/dedoping reactions) of electrolyte ions cannot occur at deeper redox active sites of electrode material, and therefore only the outer surface or shallow pores can be employed for the charge storage [52]. As the current density increase from 3.22 to 25.81mA cm-2, retention of specific capacitances for ERGO-PANI(50) (95% from 1084 to 1033.90 F g-1 ) was somewhat higher than ERGO-PAN (25) (95% from 1027.0 to 981.8 F g-1) and ERGO-PAN(15) (91% from 989.2 to 902.7 F g-1 ), demonstrating a great rate capability. Ragone plots have been frequently used to evaluate supercapacitors performance as energy storage devices. Specific energy density (S.E) and specific power density (S.P) were

also calculated from the galvanostatic charge /discharge curves at diverse current densities by following equations [54]

S.E =

(6)

S.P =

(7)

Where S.E (Wh kg-1), S.P (W kg-1), C (F g-1), ΔV (V) and ∆t (s) are specific energy density, specific power density, specific capacitance, scanned potential window and discharge time, respectively. For comparison, the Ragone plots of nanocomposites and conventional energy storage systems were presented in Fig. 8(b). Moreover, S.E and S.P values of nanocomposites at different current densities were listed in Table. 1. The maximum specific energy density about 53.12 Wh kg-1 (with specific power density 500.61 W Kg-1) at current density of 3.22 mA cm-2, and the maximum specific power density 6703.2 W kg-1 (at specific energy density37.24 Wh kg-1) at current density of 25.81 mA cm-2 were obtained. It is interesting to note that mass loading of electrode material has an important effect on some parameters of supercapacitors such as S.E and S.P. On the other hand, thick PANI coating with large amounts of electroactive material can lead high energy density but thin PANI coating can delivered great density power. Besides, it is well shown that as prepared nanocomposites as supercapacitor can fill gap between conventional capacitors and lithium batteries. Electrochemical stability of electrode is one of the most critical requirements for practical applications of the supercapacitors. The cycling stability and coulombic efficiency of ERGO-PANI(50) electrode and pure PANI were investigated by repeating CV test at a scan rate of 50mV/s during 1000 cycles of charge/discharge, as displayed in Fig. 8 (c, d). Insets Fig. 8 show shape of CV curves of samples at first and the 1000th cycles. Coulombic efficiency (η) of supercapictors can be determined based on the following equation

η = ×100

(8)

Where td and tc are the times of charge and discharge (s). After 1000 cycles, capacitance of ERGO-PANI(50) and pure PANI retain about 86% and 76% of their initial values. ERGO layer with exceptional mechanical property, reduces cycling degradation of polymer matrix from seriously swelling and shrinkage of PANI chains during fast electrochemical doping/dedoping reactions, and thus leads to improve stability of the nanocomposite. Remarkably, the ERGO-PANI(50) and pure PANI electrodes still retain good redox reactions reversibility with coulombic efficiency of 89% and 84% even after 1000 cycles. EIS tests were also measured to analyze internal resistance of electrode materials and their capacitance properties at a bulk and interface of electrode-electrolyte. The Nyquist plots of nanocomposites and pure PANI were revealed in Fig. S3(a) (see Supporting Information). All the curves were quite similar. At very high frequencies, equivalent series resistance (ESR) or RS obtained from the intercept point on real exist of Nyquist plot, indicating intrinsic resistance of electrode, bulk electrolyte resistance. A semicircle behavior in high frequencies, corresponds to Faradic reactions in electrode- electrolyte interface , which is described as charge transfer resistance (Rct) [55]. At low frequencies, the 45º portion curve reveals Warburg element due to ion diffusion or transport from electrolyte to electrode surface. The relatively low Rct for all samples were evidently observed, suggesting fast electron transference, which facilitates ions doping and dedoping in electrodes. The Bodephase curves in Fig. S3 (b)(Supporting Information) illustrations that an approach to ideal capacitor at low frequency can be known by reaching to the -90º. Phase angle values of ERGO-PANI(15), ERGO-PANI(25), ERGO –PAN(50) and pure PANI at 10 mHz frequency were -80.66º, -81.57º, -82.19º and -72.46º, demonstrate that presence of ERGO in nanocomposites improves the capacitive actions of the pure PANI. Fig. S3(c) (Supporting Information) displays that ERGO-PANI (50) has the smallest impedance value among the samples analyzed. In order to better understand the fundamental and important properties of

supercapacitors complex capacitance analysis employed using impedance spectra [56]. The complex capacitance dependent on frequency of supercapacitor can be expressed by equation 9, 10 and 11.

C ( ) = Cʹ Cʹ ( ) = Cʺ( ) =

)- iC"( )

(9) (10)

|

| ʹ

|

(11) |

Where Cʹ ( ) and C" ( ) are real and imaginary capacitance, C(

,

and |Z( )| are the

angular frequency and absolute impedance. Cʹ ( ) characterizes the real capacitance of the electrode at very low frequency, while C″ correspond to the loss capacitance by kinds of irreversible processes in electrodes. The changes of real and imaginary capacitance versus frequency for all supercapacitors were revealed in Fig. 9(a- c). Small time constants are very essential factor in design electrochemical supercapacitors, which ensure rapid charge and discharge characteristics, so that deliver higher power density. Time constant or relaxation time defined as τ0 = 1/fm , the time which the supercapacitor changes from resistive to capacitive performance [57]. These values were calculated using maximum imaginary parts of capacitance (C") at frequency fm. τ0 for the ERGO-PANI (15), ERGO-PANI (25) and ERGO-PANI (50) were about 0.52, 1.93 and 5.18 s, demonstrating great power density of all nanocomposites, which is accordance with galvanostatic charge-discharge and CV results. The real part of the capacitance Cʹ (ω) decrease with enhance the frequency and shows that as-prepared supercapacitors behave similar a pure resistance at high frequency. The real capacitance of ERGO-PANI(50) electrode at frequency of 10 mHz revealed highest value (26.3mF), compared with ERGO-PANI(25)(7.1 mF), ERGO-PANI(15)(4.45 mF) and pure PANI(1.32 mF). Considering all electrochemical tests, ERGO-PANI(50) exhibits best supercapacitor performance among three nanocomposites. 4. Conclusions

In summary, we have reported two consecutive electrochemical steps to preparation of ERGO-PANI nanocomposites on GCE as substrate. XRD and FTIR measurements of the ERGO signify that some oxygen functional groups of GO removed by electrochemical reduction. Furthermore, SEM and EDS results indicate homogenous distribution of PANI in total support layer of ERGO. Maximum specific capacitance of nanocomposite obtained 1084 F g-1 at current density of 3.22 mA cm-2 and exceptional rate capability of 97 % retentions (when current density was increased from 3.22 mA cm-2 to 25.81 mA cm-2). In addition, nanocomposite illustrates remarkable cycling stability with capacitance retention of 86% at scan rate of 50 mV s-1 during 1000 charge/discharge cycles. Acknowledgements The authors would like to express their deep gratitude to University of Kashan for supporting this work by Grant No. 682209-1.

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Figure captions Fig. 1. XRD (a) and FT-IR (b) spectra GO, ERGO and ERGO-PANI with 50 electrodeposition cycles. Fig. 2. SEM images of (a) GO and (b) ERGO. EDS spectrums of (c) GO and (d) ERGO. Fig. 3. SEM images of prepared ERGO-PANI (50) at different magnifications (a,b), EDS spectrum of ERGO-PANI (50) nanocomposite. Fig. 4. AFM images of ERGO (a) 2D and (b) 3D, AFM images of ERGO-PANI nanocomposite (c) 2D and (d) 3D. Fig. 5. Comparison of specific capacitance and mass density PANI in nanocomposite during different deposition cycles (5-90 cycles). Fig. 6. Cyclic voltammograms (CV) of (a) ERGO-PANI(15), ERGO-PANI(25), ERGOPANI(50), PANI, ERGO samples and GCE at 15 mV s-1 scan rate (inset show magnified of CV curves for pure PANI, ERGO and GCE), CV curves of (b) ERGO-PANI(15), (c) ERGOPANI(25) and (d) ERGO-PANI(50) at different scan rates (insets are plots of current versus square root rate scan). Fig. 7. Galvanostatic charge-dicharge curves (a) ERGO-PANI(15), ERGO-PANI(25), ERGO-PANI(50), PANI, ERGO at constant current density of 3.22 mA cm-2, Galvanostatic charge-discharge curves of (b) ERGO-PANI(15), (c)ERGO-PANI(25) and (d) ERGOPANI(50) at diverse current densities. Fig. 8. (a) plots of specific capacitance as a function of current density of ERGO-PANI(15), ERGO-PANI(25) and ERGO-PANI(50), (b) Ragone plots of ERGO-PANI(15), ERGOPANI(25) and ERGO-PANI(50) electrodes compared with different energy storage devices, Specific capacitance retention and Coulombic efficiency as cycle number for (c) ERGOPANI (50) and (d) pure PANI. Insets show CV curves of ERGO-PANI (50) and pure PANI at 50 mV s-1 scan rate for 1st and 1000th cycles. Fig. 9. Real and imaginary parts of capacitance plotted versus frequency (a) ERGOPANI(15), (b) ERGO-PANI(25) ,(c) ERGO-PANI(50) and (d) pure PANI.

Table caption: Table. 1. Specific energy density and specific power density of nanocomposites at different current densities.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Table. 1