High performance electrode material prepared through in-situ polymerization of aniline in the presence of zinc acetate and graphene nanoplatelets for supercapacitor application

Journal of Electroanalytical Chemistry 739 (2015) 10–19

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High performance electrode material prepared through in-situ polymerization of aniline in the presence of zinc acetate and graphene nanoplatelets for supercapacitor application Amit Kumar Das, Sandip Maiti, B.B. Khatua ⇑ Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India

a r t i c l e

i n f o

Article history: Received 8 August 2014 Received in revised form 30 October 2014 Accepted 10 December 2014 Available online 18 December 2014 Keywords: Supercapacitor Polyaniline Zinc acetate dihydrate GNP Electrical conductivity

a b s t r a c t This study discusses a simple and feasible method that involves in-situ polymerization of aniline in the presence of zinc acetate dihydrate [Zn(CH3COO)22H2O] and graphene nanoplatelets (GNP) for the preparation of high performance electrode material (PZG composite) for supercapacitor application. In the presence of metal salt [Zn(CH3COO)22H2O], the capacitance value is changed and increased enormously compared to the capacitance value of PANI/GNP (PG) composite. Thus, the proposed method gives specific capacitance value for the PZG composite is 688 F/g at a 10 mV/s scan rate which is very high compared to the specific capacitance value (340 F/g) of PG composite at the same scan rate. In the PZG composite, zinc acetate and GNP are successfully coated by PANI, which provides more active sites for nucleation and electron transfer path. In addition, the inter- and intra-molecular interactions among them facilitate the electron transfer path which plays an important role to enhance the capacitance value of the composite. Moreover, the prepared composite is electrically conducting in nature and shows electrical conductivity in the order of 4.67  102 S cm1. In addition, PZG composite shows semiconducting behavior. Field Emission Scanning Electron Microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) have been studied for the morphological analysis of the PZG composite. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Supercapacitor has become a new generation energy storage device in the current science and technologies. It has been used as a bridge between low power devices (low temperature fuel cells and batteries) and low specific energy devices (conventional capacitors). In addition, it has been considered most promising material for various energy storage devices due to its long cycle life (>100,000 cycles), high rate capability, low maintenance cost, rapid charging–discharging rates, environmental friendliness and light weight [1–4]. Two types of capacitors are generally classified depending on their charge storage mechanism, such as, (a) electrical double-layer capacitors (EDLCs) composed of porous carbonaceous materials of high surface area as electrodes, where achievement of electrostatic charge storage of the electrical energy occurs due to the separation of charge at the interface between the surface of a conductor electrode and an electrolytic solution

⇑ Corresponding author. Tel.: +91 3222 283982. E-mail address: [email protected] (B.B. Khatua). http://dx.doi.org/10.1016/j.jelechem.2014.12.018 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.

and this whole process is non-Faradaic in nature and (b) pseudocapacitors, often called redox electrochemical capacitors, where storage of the electrical energy occurs due to some fast, reversible redox reactions of the electrode materials generally composed of conducting polymers and transition metal oxides, with the electrolyte and this process is Faradaic in nature [5]. Since pseudocapacitors deal with fast and reversible redox reactions, comparatively higher capacitance value will be obtained for pseudocapacitors than EDLCs. To improve the capacitance value, different electro-active transition metal oxides such as RuO2, MnO2, TiO2, Fe3O4, MoOx, CoOx, NiOx [6–13] have been used for the preparation of supercapacitor materials. Besides these, various conducting polymers like polypyrrole (PPY), polyaniline (PANI), and polythiophene (PTH) have been extensively used along with metal oxides or metal salts to enhance the ability of the energy storage of the materials. Among all of them PANI has been much more attractive and promising basic material due to its low cost, high electrical conductivity [14], high capacitive characteristic, environmental stability, ease of synthesis, simple acid doping-base dedoping chemistry [15–17]. However, electrodes comprised of conducting polymers, such as PANI suffer from some serious

A.K. Das et al. / Journal of Electroanalytical Chemistry 739 (2015) 10–19

problems such as low electronic conductivity, degradation through mechanical shrinkage and swelling during charging–discharging and hence, high resistance during cycling [18–21]. So for improving electrochemical energy storage property of PANI, concept of using composite electrode had evolved, where PANI is combined with a particular amount of porous, highly conducting carbonaceous materials of high surface area [19,22]. Capacitance can further be improved by doping conducting polymers with transition metal ions like Zn2+, Cu2+ and Fe3+, as these serve as catalysts in the redox process increasing the energy density. Many research groups have prepared electrode materials for supercapacitors based on different transition metal ions, PANI along with highly surface reactive conducting nanofillers. For instance, Maiti and Khatua [23] have prepared CoCl2 doped PANI/ GNP composite (PGC) as a supercapacitor electrode material through in-situ polymerization and the reported specific capacitance value was 634 F/g at a 10 mV/s scan rate. Li et al. [24] have synthesized Zn2+ and H+ co-doped PANI by simple chemical polymerization method and the obtained capacitance value of the composites was 369 F/g. Wang et al. [25] have synthesized a flexible graphene/PANI hybrid material as an electrode material by in-situ polymerization–reduction/dedoping–redoping process. They have achieved high specific capacitance value (1126 F/g) with 84% retention of life after 1000 cycles. Baek et al. [18] have studied the specific capacitance value of PANI/reduced graphene oxide (rGO) composite. The obtained capacitance value was 250 F/g with high electrical conductivity of 8.66 S cm1. The composite has been synthesized through in-situ polymerization. Cong et al. [26] have achieved high specific capacitance value of 763 F/g for PANI/graphene composites. They have prepared flexible composite paper using in-situ electro-polymerization technique. Xia et al. [27] have prepared PANI/rGO/molybdenum oxide (MoO) ternary composites as electrode material and reported specific capacitance value was 553 F/g in 1 M H2SO4 at a scan rate of 1 mV/s. Sahoo et al. [28] have prepared PANI/surface modified graphene composites as supercapacitor materials. They have observed that specific capacitance value of the composite decreased from 242 F/g to 193 F/g for modification of graphene. Giri et al. [29] have developed an electrically conducting (70.8 S cm1) PANI/ graphene/zirconium oxide (ZrO) nanocomposite with high specific capacitance value of 1360 F/g at 1 mV/s. The present study discusses about the capacitance performance of the PANI/Zn(CH3COO)2/GNP (PZG) composite. The PZG composite has been synthesized through in-situ polymerization of aniline in the presence of zinc acetate and GNP at low temperature. This is the one of the simplest and commercially successful methods. In addition, the composite has delivered high specific capacitance value which is 688 F/g at 10 mV/s scan rate which is higher than the specific capacitance value (340 F/g) of the PANI/GNP (PG) composite at the same scan rate. Thus, the presence of zinc acetate plays a crucial role to enhance the specific capacitance value of the PZG composite. In the PZG composite, a well ordered nanostructure is developed, which reduces the ionic diffusion path and facilitates ionic motion to the inner part. So, it can be considered as one of the most promising electrode materials for supercapacitor application. Moreover, the PZG composite shows high cycle stability and 83% of specific capacitance has retained after 500 cycles. In presence of conducting PANI and nanofiller like GNP, the PZG composite becomes highly electrically conducting and observed conductivity value is 4.67  102 S cm1, indicating that it can also be used in various fields of application. We also studied the semi-conducting property of the composite. Thus, proposed simple cost effective method, high capacitance value, better cycle stability and high conductivity value of the PZG composite will play a key role for developing next generation high performance supercapacitor.

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2. Experimental 2.1. Materials details Zinc acetate dihydrate [Zn(CH3COO)22H2O], ammonium persulfate (APS) [(NH4)2S2O8] and cetyl trimethylammonium bromide (CTAB) were purchased from Loba Chemie Pvt. Ltd. India. Aniline and potassium chloride were purchased from Merck, Germany. Multi-layered graphene nanoplatelets (GNPs, carbon purity:>99.5%) having thickness and diameter of 8–10 nm and 5–25 lm respectively, were purchased from J.K. Impex, Mumbai, India. These GNPs have appreciable electrical conductivity of 107 S m1. All the chemicals were of analytical grade and used as received without any further chemical modification. 2.2. Preparation of the PZG composite The PANI/Zn(CH3COO)2/GNP composite (PZG) was synthesized through simple in-situ polymerization of aniline in the presence of Zn(CH3COO)22H2O and GNP. At the beginning, the calculated amount of surfactant (CTAB) was dissolved in 150 ml of 1.5 (M) HCl solution and then required amount of GNP was added to this acidic solution followed by strong ultra-sonication for 30 min. The presence of CTAB plays a crucial role for homogeneous dispersion of multi-layered GNPs in the acidic solution. The acidic suspension of GNP was stirred vigorously in an ice-bath using magnetic stirrer for another 40 min. Then, aqueous solution of Zn(CH3COO)22H2O was prepared using de-ionized (DI) water and drop-wise added to the acidic solution at the same ice-cool condition under constant stirring for another 35 min. Finally, 1 ml of aniline was poured into the acidic reaction mixture followed by the controlled addition of acidic APS solution (prepared by dissolving of 2 gm APS in 150 ml 1.5 (M) HCl solution). The observation showed that the solution immediately changed to bluish color first and then deep green after sometime. The reaction mixture was stirred vigorously and kept for 6 h at this ice-cool condition. After 6 h of reaction, a deep greenish color solution was observed and filtered the reaction mixture. The residue was collected and washed with DI water followed by ethanol for several times. Finally, washed residue was air dried and then, kept in an air-oven at 60 °C for 24 h. The PANI/GNP (PG) composite was prepared following the same steps written above except the addition of zinc acetate salt during the polymerization of aniline. A schematic representation for the preparation of the PZG composite is shown in Fig. 1. 3. Characterizations 3.1. Fourier Transform Infrared (FTIR) Spectroscopic Analysis FTIR studies of pure PANI, pristine GNP and PZG composite were done using a NEXUS 870 FTIR (Thermo Nicolet) to evaluate the bonding and structure of the PZG composite. The samples under investigation were prepared by mixing those with potassium bromide (KBr) in the ratio of 1:10 (w/w) and then were pelletized. 3.2. Wide Angle X-ray Diffraction (WAXD) Analysis To investigate the crystal structures of pristine GNP, pure PANI and PZG composite, X-ray diffraction analysis was performed using a wide angle X-ray diffractometer (WAXD, X’pertPRO, PANalytical, Netherlands) with nickel-filtered Cu Ka line (k = 0.15404 nm) at an accelerating voltage of 40 kV and 30 mA current density and a scanning rate of 0.5°/min. The sample-to-detector distance was 400 mm. All the materials were scanned in the range of 2h = 10– 70°.

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Fig. 1. Schematic diagram representing the preparation of PZG composite.

3.3. Raman Spectroscopic Analysis

3.6. Electrochemical characterization

Raman spectra of pristine GNP, pure PANI and PZG composite were recorded using Horiba Jobin–Yvon LabRAM Raman microscope (J–Y HORIBA, model: T64000) with an excitation wavelength of 632.8 nm Ar ion laser to confirm the presence of PANI in the PZG composite.

Electrochemical characterization of as-synthesized PZG composite was carried out with a BioLogic (SP-150) electrochemical analyzer employing a classical three-electrode method for all measurements: a glassy carbon electrode of 3 mm diameter coated with 0.18 mg PZG composite was directly used as working electrode; the Ag/AgCl (KCl saturated; +197 mV) and Pt wire served as reference and counter electrodes, respectively. The Cyclic Voltammetry measurements were done in 1 (M) KCl aqueous electrolyte solution at a potential window from 0.4 to +0.8 V and at different scan rates from 10 mV/s to 100 mV/s. The specific capacitances (Csp) of PZG composite at different scan rates were calculated on the basis of the following equation [27,29]:

3.4. Field Emission Scanning Electron Microscopy (FESEM) Study The surface morphologies of pure PANI, pristine GNP and the PZG composite were studied through Field Emission Scanning Electron Microscopy (FE-SEM, Carl Zeiss-SUPRA™ 40), with an accelerating voltage of 5 kV. The samples for the test were prepared through dispersion of the composite in dichloro methane solvent and putting a drop of that on Al-foil. After solvent evaporation, the Al-foil was pasted onto the carbon tape and coated with a thin layer (approx 5 nm) of gold to avoid the accumulation of electrical charging during the scanning process. These gold coated composite samples were scanned in the vacuum order of 104–106 mm Hg.

R v2

Specific capacitance;C sp ¼

iðv Þdv v1 ðv 2  v 1Þv m

ð1Þ

where i (A) is the response current, v (V/s) is the scan rate, v1 and v2 are the potential limits in volt, and m (g) is the mass of active electrode material. The numerator of Eq(1) actually denotes the area under the CV-curve. The Electrochemical Impedance Spectroscopic (EIS) measurements were performed by sweeping frequencies from 1 MHz to 100 mHz. 3.7. I–V measurement

3.5. High-resolution transmission electron microscopy (HRTEM) study The TEM analysis of the PZG composite was investigated by high resolution transmission electron microscopy (HRTEM: JEM2100, JEOL, Tokyo, Japan), operated at an accelerating voltage of 200 kV. For HRTEM analysis, dispersion of a small amount of the PZG composite was firstly done in dichloro methane solvent by sonication and then on the copper (Cu) grid the dispersed PZG composite was dropped and after evaporation of the solvent, the Cu grid was taken for HRTEM analysis. The energy-dispersive X-ray (EDX) analysis was performed with the PZG composite sample to validate the presence of different elements in the composite.

Current–Voltage relationship of pellet sample of PZG composite was investigated with a Keithley 2400 source meter (lab view 18.1 protocol) within the potential range of 3 V to +3 V at room temperature. Two contacts were taken out of the probe station where the sample was kept. Using the Keithley source meter, positive voltage was set on the top contact of the material. 3.8. Electrical conductivity The DC electrical conductivity of the PZG composite was determined by applying four probe method and following the equations [30]:

A.K. Das et al. / Journal of Electroanalytical Chemistry 739 (2015) 10–19

Resistivityðq; ohm cmÞ ¼ pt=In2ðV=IÞ

ð2Þ

Conductivityðr; S cm1 Þ ¼ 1=q

ð3Þ

where I is the applied current, V is the measured voltage and t represents the samples’ thickness. The conductivity data shown in this paper is an average of the several test results for each specimen. A computer controlled precision impedance analyzer (Agilent 4294A) was used to determine the frequency dependent AC conductivity of the disc type sample of PZG composite with 0.3 cm thickness and 1.88  101 cm2 area employing an alternating electric field of 1.000 volt amplitude across the sample cell in the frequency range of 40 Hz–10 MHz. For all the electrical measurements, parallel plate configuration with Pt probe sample holder was used. The parameters like dielectric permittivity (e0 ) and dielectric loss tangent (tan d) are dependent on frequency. The AC conductivity (rac) was estimated from the dielectric data using the relation [23,31]:

rac  xeo e0 tan d

ð4Þ

where x = 2pf (f = frequency), and eo is permittivity of the vacuum. The dielectric permittivity (e0 ) was estimated from the following equation [23]:

e0  C p =C o

ð5Þ

where Cp is the observed capacitance of the sample in parallel configuration and Co is the capacitance of the cell, value of which was calculated using the area (A) and thickness (d) of the sample as the relation shown below [23,31]:

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The weak absorbance peaks in the range of 3500–3000 cm1 and 3000–2800 cm1 represent the NAH and CAH bond stretching, respectively. FTIR spectra of pure PANI show additional peaks around 1290 cm1 and 1236 cm1, which were assigned as characteristic peaks for CAN stretching mode in quinoid–benzenoid– quinoid unit and the same in benzenoid unit, respectively and also at 1105 cm1 and 877 cm1, for CAH in-plane bending mode in quinoid and benzenoid rings, respectively [33,34]. Again in the FTIR spectrum of pure GNP (Fig. 2b), appearance of a strong band near 3431 cm1 indicates the vibrational stretching of OAH bond, as because hydrophilic hydroxyl group remains attached to the edge of the basal plane of the GNP [35]. The peak around 2346 cm1 indicates aromatic sp2 CAH stretching vibration [23]. GNP also showed a strong peak at 1638 cm1 indicative of vibrations of the graphitic double bonds [36]. The FTIR spectrum for PZG composite (Fig. 2c) contains the peaks characteristic for both pure PANI and GNP though somewhat shifted in positions and with enhanced intensities, which is because of the structural change in the polymer backbone caused by the strong molecular interactions among PANI, metal salt and GNP in the composite. Hence FTIR analysis unambiguously confirms the presence of PANI and GNP in the PZG composite.

4.2. Wide Angle X-ray Diffraction (WAXD) Analysis

To elucidate the structure and bonding of PZG composite, FTIR analyses for pure PANI, pure GNP and the composite were done and the spectra, represented in Fig. 2, were recorded in the range of 4000–400 cm1 averaging 32 scans at the resolution of 4 cm1. The peaks in the FTIR spectra of pure PANI (Fig. 2a) at 1560 cm1 and 1481 cm1 are associated with the C@C stretching vibration in quinoid ring and the same in benzenoid ring, respectively [32].

To monitor the nanostructure of pristine GNP, pure PANI and PZG composite, X-ray diffraction technique was employed and the evolved results are represented in Fig. 3. GNP (Fig. 3a) produced a high intense sharp peak at 2h = 26.5° and other characteristic peaks of low intensity at 42.3°, 44.3°, 54.3°, which can be assigned to the (0 0 2), (1 0 0), (1 0 1) and (0 0 4) planes of hexagonal graphite, respectively [37]. From Fig. 3b, three characteristic peaks can be observed from PANI at 2h = 15.1°, 20.8° and 25.4° which can be indexed to (0 1 1), (0 2 0) and (2 0 0) crystalline planes of emeraldine salt form of PANI, respectively as reported elsewhere [38]. The X-ray diffractogram for the composite (Fig. 3c) contains all the characteristic peaks for GNP and PANI with a little bit change in the peak positions, which is indicative of the change in crystal structures due to the presence of Zn(CH3COO)22H2O. Also, additional peaks observed in the X-ray diffractogram for the composite confirms the presence of Zn(CH3COO)22H2O. The low intense peaks for the composite compared to those obtained for PANI and GNP could be as a result of coating of PANI over the GNP.

Fig. 2. FTIR spectra of (a) pure PANI, (b) pristine GNP, and (c) PZG composite.

Fig. 3. XRD patterns of (a) pristine GNP, (b) pure PANI, and (c) PZG composite.

C o  ðeo  AÞ=d

ð6Þ

4. Results and discussion 4.1. FTIR Spectra Analysis

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4.3. Raman Spectroscopic Analysis To assign the microstructures of different carbonaceous materials such as GNP, PANI and the composite, Raman spectroscopic analyses were carried out and Fig. 4 represents the recorded data. Two clear peaks at 1326 cm1 and 1579 cm1 contained by Raman spectrum of pristine GNP (Fig. 4a) indicate the D-band and G-band, respectively [39]. The peaks displayed by the Raman spectrum of pure PANI (Fig. 4b) at 1161, 1218, 1331, 1468, 1590 cm1 are indicative of in-plane bending of CAH in benzenoid ring, CAN stretching of single bonds, semiquinone radical structure (CAN+ stretching vibration), C@C stretching of quinoid ring and C@C stretching of benzenoid ring, respectively [40–42]. The PZG composite revealed two broad peaks containing a range of peaks around at 1347 and 1574 cm1 which indicate the presence of PANI in the composite and possible interactions of the same with the constituents in the composite. 4.4. Morphology study The morphological characteristics of the pure PANI, GNP and PZG composite were shown in Fig. 5. The FESEM image of pure PANI revealed fiber-like structure as observed in Fig. 5a. Fig. 5b shows the plate-like (flake) morphology for the pure GNP. The FESEM images of PZG composite were shown in Fig. 5c and d at different magnifications. Fig. 5c represents the FESEM image of PZG composite at very lower magnification. From this image, it is clearly seen that GNPs are coated by PANI homogeneously, which helps to improve the ionic motion to the inner part increasing the capacitance value of the composite. Again, the interconnected fiber-like/thread-like morphology was observed for PZG composite in the image, captured at higher magnification (Fig. 5d). This kind of morphology helps to improve the capacitance value of the composite since this provides more active sites for electron transfer path in the PZG composite and plays a crucial role to improve the interactions among the PANI, GNP and metal salt. Moreover, the presence of Zn2+ ions in the composite increases the current density and the specific capacitance through generation of less resistive pathway for the electron transport between the active materials and current collectors. Therefore, this PZG composite will be very much useful as supercapacitor electrode material. HRTEM images of PZG composite were recorded at two different magnifications represented by Fig. 6a and b. These images also

Fig. 4. Raman spectra of (a) pristine GNP, (b) pure PANI, and (c) PZG composite.

supported the formation of fiber-like/thread-like morphology in PZG composite, which plays a major role to enhance the capacitance value of the composite as well as electrical conductivity of the same. The circularly marked black regions in Fig. 6b denote the presence of Zn2+ ions which can act as linkages connecting neighboring PANI chains via coordination with the N-atoms present in those polymer chains [43]. This kind of linked-morphology was also supported by FESEM micrographs (Fig. 5d). Furthermore, energy-dispersive X-ray (EDX) analysis was done to confirm the presence of different elements in the PZG composite and the EDX spectrum, represented in Fig. 7, confirms the presence of Zn in the composite. 4.5. Electrochemical characterizations 4.5.1. Cyclic Voltammetry (CV) Analysis The CV measurements were studied using three-electrode configuration in 1 (M) KCl solution used as electrolyte and the data were recorded in Fig. 8, where Fig. 8a represents the CV analysis curves of the PG and PZG composite at same scan rate of 10 mV/s and Fig. 8b shows the same of PZG composite at various scan rates of 10, 30, 50, 80 and 100 mV/s. As can be seen from Fig. 8a, the CV plot of PG composite is rectangular in nature, which is indicative of ideal capacitive behavior and the specific capacitance (Csp), calculated on the basis of Eq. (1) came to be 340 F/g. Again, the deviation in ideal capacitor behavior curve meaning non-rectangular curve was observed for PZG composite (Fig. 8a). This indicated the reduced interactions among electrode and electrolyte and some redox charge transfer processes were taking place, which are reflected in the pseudo-capacitive characteristic of PZG composite. The Faradic charge storage mechanism was observed in the case of PZG composite. The negative and positive current regions in the CV curve were developed due to the cathodic reduction and anodic oxidation processes, respectively. The estimated Csp value of the PZG composite was 688 F/g at the scan rate of 10 mV/s, which is much higher compared to the same for PG composite (340 F/g) at the same scan rate. Due to this high specific capacitance value, PZG composite can be used as promising capacitor material in different fields of applications. Inter and intra-molecular interactions among the PANI, existing metal ion and GNP are the key reason behind the high specific capacitance value of the composite. This type of interactions is facilitated as PANI is anchored on the GNP sheets of high surface area and Zn2+ ion can form a charge transfer complex via coordination to the lone pair of electrons present on the imine and amine N-atoms of the PANI chains. In addition, the p–p interaction in the PZG composite also helped to enhance the capacitance value. Thus, the presence of zinc acetate salt plays a crucial role to enhance the capacitance value of the PZG composite. The Csp values of PZG composite at different scan rates were estimated and the variation in Csp values with scan rate was shown in Fig. 9. As observed, the specific capacitance value strongly depends on the scan rate and decreased gradually with increasing the scan rate. This is because of limited diffusion rates of the electrolyte ions due to the lack in utilization of the active material because of polarizations of concentration as the scan rate is increased. So, the low diffusion rates of electrolyte ions lowers the extent of electrochemical reactions of the electrode material at higher scan rates which helped to reduce the specific capacitance value of the composite [44–47]. 4.5.2. Constant Current Charging–Discharging (CCD) Analysis To determine the electrochemical behavior of the PZG composite as electrode material, CCD analysis of the composite was studied at constant current density of 1.85 A/g and within the potential window of 0.3 to 0.7 V, as shown in Fig. 10a. A non-linear nature of the CCD curve is indicative of pseudocapacitive behavior of the

A.K. Das et al. / Journal of Electroanalytical Chemistry 739 (2015) 10–19

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Fig. 5. FESEM images of (a) pure PANI, (b) Pure GNP, and PZG composite at (c) lower and (d) higher magnification.

Fig. 6. HRTEM micrographs of PZG composite at (a) low and (b) high magnification.

composite. However, the CCD plot for the composite also shows a potential drop (0.05 V) due to internal resistance of the electrode material (i.e., the PZG composite). From the galvanostatic charge– discharge curve, the specific capacitance value of the composite was calculated for CCD analysis with the help of following relation [22,23,29]:

Specific capacitance;C sp ¼

ðI  DtÞ ðm  DVÞ

ð7Þ

where Csp (F/g) stands for the specific capacitance, I (A) represents the discharge current, DV (V) signifies the potential range, t (s) is the discharge time and m (g) is the mass of the active material, i.e., PZG composite. Thus, calculating on the basis of Eq. (7), the highest capacitance value of 670 F/g was achieved for PZG composites at 1.85 A/g of current density. To know the effect of high current on the electrode material, the charging–discharging test was also carried out at higher current densities (Fig. 10b) and it was observed that the specific capacitance value of the PZG composite follows a decreasing trend when a higher current density is

applied. For example, the specific capacitances of PZG composite at 3.3, 4.5, 8 A/g current densities came to be 657, 648, 600 F/g, respectively (Fig. 10c). Application of high current leads to less availability of the inner redox active sites in the electrode and limited diffusion of cations, which, in turn, reduce the specific capacitance values. The test for cyclic stability of the PZG composite was done for 500 cycles in 1 (M) KCL electrolyte at a current density of 2 A/g for the evaluation of durability. Fig. 10d shows the variation of specific capacitance of PZG composite with cycle number, where almost 83% of specific capacitance is retained after 500 cycles for the composite. The unique microstructure and superior charge transfer complex formation through p–p interaction in the PZG composite increasing the active sites for the pseudo-capacitance, are believed to be the reasons behind its cyclic stability [48,23]. 4.5.3. Electrochemical Impedance Spectroscopic (EIS) Analysis Fig. 11a shows the EIS analysis of the PZG composite in the frequency range of 1 MHz–100 mHz to study the transportation of

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Fig. 9. Specific capacitances of PZG composite at different scan rates.

Fig. 7. EDX spectra of the PZG composite.

electrons and ions in electrode material. The imaginary component (Zimg) and the real component (Zreal) in the Nyquist plot (Fig. 11a) indicate the capacitive and ohmic behavior of the composite, respectively [49]. From Fig. 11a, a suitable equivalent fitting circuit has been designed for this PZG composite, as shown in Fig. 11b. In this circuit (Fig. 11b), Rs represents the solution resistance including resistance of the substrate and resistance at the electrode/electrolyte interface as well as at the bulk, Rct stands for the charge transfer resistance, W signifies the Warburg diffusion element or impedance, and CPE denotes the constant phase element. In Fig. 11a, the value of Rs of the electrode has been considered as the intersection point of the semicircle at higher frequency region and the value of Rct is considered as the diameter of that semicircle arc. The intermediate frequency region corresponds to the Warburg impedance (W) [50–52]. From Fig. 11a, a semicircle is observed at high frequency and a straight line is seen at the low frequency region. The intra and inter-molecular interaction between the conducting PANI, metal ion (Zn+2) and GNP in the PZG composite greatly affects the electrolyte accessibility to the electrode surface by the shortening the ion diffusion path. The capacitive behavior of the PZG composite with low diffusion coefficient value was supported by the vertical line which was observed at low frequency region in Fig. 11a. The capacitive behavior of the composites changed with the changing of the ‘n’ value obtained from the CPE. If the material shows ‘n’ value equal to zero,

then material will behave like an insulator. However, if ‘n’ value is one, then, material will behave like an ideal supercapacitor which is impossible in nature. The present study (PZG composite) shows ‘n’ value in between 0.5 and 1.0 (0.76), suggesting the prepared PZG composite would behave like a moderate capacitor. 4.6. Current (I)–voltage (V) relationship The current (I)–voltage (V) relation of the PZG composite is shown in Fig. 12. From this plot, it is clearly seen that a non-linear curve was observed for PZG composite in the potential range of 3 to +3 V. Hence, this non-linear curve indicates the semi conducting nature of the PZG composite and increased its application area in various electronic fields [53]. 4.7. Electrical properties 4.7.1. DC electrical conductivity The PZG composite is electrically conducting in nature and electrical conductivity (rDC) of 4.67  102 S cm1 was achieved for this composite. The high value of electrical conductivity was achieved due to the presence of conducting GNP and PANI in the composite. The high capacitance value of the PZG composite also concluded the conducting nature of the composite. In general, the tunneling of charge carriers are the responsible for the development of electrical conductivity in the composite [54]. The electron tunneling happened between the neighboring nanofillers in

Fig. 8. CV plots of (a) PZG and PG composite at the scan rate of 10 mV/s and (b) PZG composite at different scan rates.

A.K. Das et al. / Journal of Electroanalytical Chemistry 739 (2015) 10–19

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Fig. 10. (a) CCD plot of PZG composite at a constant current density of 1.85 A/g, (b) CCD plots of PZG composite at different current densities, (c) specific capacitances of PZG composite at different current densities, and (d) cyclic stability of PZG composite at current density of 2 A/g.

Fig. 11. (a) Nyquist plot of PZG composite and (b) the equivalent circuit fitting the Nyquist plot of the composite.

the PZG composite. In addition, the high energy barrier of insulating or less conductive polymer in the composites restricted the movement of electrons between the two electrodes. However, this energy barrier gap can be reduced by applying of voltage which plays a driving force for the movement of electrons between the electrodes by tunneling conduction mechanism. Thus, the electrical conductivity of the composite increased and behaved like a conducting material.

frequency region and the dipoles/induced dipoles get sufficient time for the orientation of themselves with the direction of applied electric field. Thus, the value of rAC actually indicates the rDC for a conductive system at low frequency region. However, this interfacial polarization effect becomes negligible at high frequency region and the dipoles do not get sufficient time to orient themselves in the direction of applied electric field. Thus, the value of rAC was increased with increasing the frequency.

4.7.2. AC electrical conductivity The variation of AC electrical conductivity (rAC) of the PZG composite was shown in Fig. 13. As observed, the value of rAC was increased with increasing the frequency (f) which is general characteristics of the conducting materials. The polarization effect plays a crucial role for changing the AC electrical conductivity of the composite. The interfacial polarization effect is more significant at low

4.7.3. Dielectric permittivity Fig. 14 represents the variation of dielectric permittivity of PZG composite in the frequency range of 40 Hz–10 MHz. From the figure, a quick decrease in the dielectric permittivity value of the composite with increasing frequency is observed at lower frequency zone (within 104 Hz) and the rate of decrease becomes slow at higher frequency (above 104 Hz), which is the characteristic behavior of a

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of applied electric field producing high dielectric permittivity owing to appreciable polarization. With increase in frequency, short relaxation time affects the polarization of the dielectric material leading to a sharp decrease in the dielectric permittivity [57]. So, above a certain frequency (104 Hz), the rate of decrease in dielectric permittivity with increasing frequency becomes moderately slow due to insignificant polarization (Fig. 14). 5. Conclusion

Fig. 12. Current–voltage relationship of PZG composites.

In the present study, a high performance electrode material (PZG composite) has been prepared for supercapacitor application by a cost effective and simple method involving in-situ polymerization of aniline in presence of zinc acetate [Zn(CH3COO)22H2O] and GNP. The obtained PZG composite shows high capacitance value of 688 F/g at 10 mV/s scan rate, which is higher than the specific capacitance value of PG composite (340 F/g) at same scan rate (10 mV/s). The existence of conducting nanofiller like GNP and zinc acetate in the PZG composite greatly improved the capacitance value of the composites. In addition, the PZG composite is electrically conducting in nature and shows high electrical conductivity (4.67  102 S cm1). Moreover, PZG composite is semi-conducting in nature and can be used in various device applications. Thus, the composite can be used as promising electrode materials for next generation supercapacitor applications. The capacitive behavior, conducting properties and morphological study of the PZG composite were studied thoroughly. Conflict of interest There is no conflict of interest. References

Fig. 13. Relation of AC conductivity of PZG composite with frequency.

Fig. 14. Relation of dielectric permittivity of PZG composite with frequency.

dielectric material [55]. Now, in a dielectric material, the dielectric permittivity directly varies with its polarizability, i.e., separation of the electric charges while exposed to an electric field [56]. At lower frequencies, the molecules of the dielectric material get extended relaxation time to get oriented themselves in the direction

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