Electrochemical characterization of in situ polypyrrole ...

Synthetic Metals 161 (2011) 1713–1719

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Electrochemical characterization of in situ polypyrrole coated graphene nanocomposites Sumanta Sahoo ∗ , G. Karthikeyan, Ganesh Ch. Nayak, Chapal Kumar Das Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India

a r t i c l e

i n f o

Article history: Received 28 February 2011 Received in revised form 1 June 2011 Accepted 6 June 2011 Available online 30 June 2011 Keywords: Graphene Polypyrrole Cyclic voltammetry Specific capacitance

a b s t r a c t Graphene/polypyrrole nanocomposites were prepared by in situ oxidative polymerization method by varying the weight percentage of graphene. FTIR study confirmed the formation of polypyrrole in presence of graphene. Field Emission Scanning Electron Microscopy (FESEM) and High Resolution Transmission Electron Microscopy (HRTEM) were used to characterize the morphology of the nanocomposites which showed a uniform coating of graphene with the polypyrrole. From the cyclic voltammetry (CV) measurement it was found that the capacitances of the nanocomposites were increased up to a certain percentage of graphene and after which it showed a downward trend. The maximum capacitance value and energy density, among the composites studied, were found to be 409 F/g and 227.2 Wh/Kg at 10 mV/s scan rate. However, maximum power density achieved was 4617 W/kg at a scan rate of 200 mV/s. © 2011 Elsevier B.V. All rights reserved.

1. Introduction A great deal of researches has been devoted towards the development of various types of energy storage devices, for the last two decades. Lithium ion battery and other secondary batteries are the results of these researches [1]. But for today’s energy demands these are not sufficient, as a result of which the present research has been diverted towards development of supercapacitors. There are two types of supercapacitor, which are under development, namely electronic double layer capacitor (EDLC) and redox capacitor [2].Among these two, EDLC have longer cycle life than batteries and also possess higher energy density with respect to conventional capacitor. Further they have higher power capability, wide thermal operating range, and low maintenance cost [3]. The electrode for the fabrication of supercapacitors is mainly consists of carbon, metal oxide and conducting polymer [4]. Many carbon containing material like activated carbon, masoporous carbon, carbon nanotubes (CNTs), etc. are used as the carbon source for electrode material. Conducting polymers such as polypyrrole (PPy), polyaniline, polythiophene are also used as another part of the electrode material. Composites based on conducting polymer and CNTs have been studied as supercapacitor electrode and a good thermal stability have been achieved [5–7]. But the major drawbacks of these EDLCs are their low energy density and low specific capacitance, due

∗ Corresponding author. Tel.: +91 9733661435; fax: +91 3222 255303. E-mail address: [email protected] (S. Sahoo). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.011

to the presence of pristine CNTs (generally less than 100F/g) [8]. Presently graphene based supercapacitor electrodes are studied hugely by the researchers around the world. Graphene is a transparent single layer of carbon atoms, arranged in a “honeycomb” fashion. It has attracted rapidly growing research interest since it was discovered in 2004 by Geim [9–11], due to its unique electrical, thermal and mechanical properties. Graphene and chemically modified graphene sheets possess high conductivity, high surface area, and good mechanical properties [12], comparable with or even better than CNTs. Further it possesses higher capacitance (10–135 F/g) value than CNTs [13]. Among the conducting polymers, PPy was widely investigated as a part of electrode material due to its good chemical and thermal stability, easy synthesis, high specific capacitance and high electrical conductivity [14–17]. Presently many researches are going on to develop graphene based supercapacitor. Biswas et al. studied the electrochemical properties of PPy/graphene nanocomposites and reported a specific capacitance of 165 F/g [18]. But the effect of graphene percentage on the electrochemical properties of graphene/PPy has not been studied by any research group. In this paper we focused on graphene/PPy composites, prepared by in situ oxidative polymerization method using ammonium persulfate as oxidant. The objective of this work is to study the effect of graphene percentage on the electrochemical properties of the graphene/PPy composites. We have prepared four composites at different weight percentage of graphene. Further electrochemical characterizations of these composites were done using the cyclic voltammetry.

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Table 1 Composition of the composites. Sample codes

Graphene (wt%)

Pyrrole (wt%)

GP1 GP2 GP3 GP4

12.5 25 37.5 50

87.5 75 62.5 50

2. Materials Graphene was obtained from Sinocarbon Materials Technology Co. Ltd. China. Ammonium persulphate (APS) and Cetyltrimethylammonium bromide (CTAB) were supplied by Loba Chemie Pvt. Ltd. Mumbai (India). Pyrrole was obtained from E. Merck Ltd. (India). All the chemicals were used as received, without any further purification. 2.1. Composite preparation Graphene/PPy composites were prepared by in situ polymerization method by a process which has already in the literature [19]. The composition of the composites are given in Table 1. Briefly, in 300 ml distilled water, graphene and CTAB were sonicated for 30 min to get well dispersed solution. Then pyrrole was added to the above solution and the whole solution was sonicated for further 10 min. During the sonication 100 ml distilled water containing APS was added. Then sonication was continued for further 30 min. After sonication the reaction vessel was kept in the refrigerator at 1–5 ◦ C for 24 h. Resulting black colored precipitate was filtered and washed with distilled water and ethanol several times. Resulting precipitate was dried at 70 ◦ C for 12 h to get the composites. 2.2. Characterization techniques 2.2.1. Fourier Transform Infrared Spectroscopy (FTIR) FTIR of nanocomposites was done using a NEXUS 870 FTIR (Thermo Nicolet) to investigate the structure of graphene based composites. For the IR spectrum a small amount of material was mixed with KBr in adequate level to make a disk and the disk was analyzed for getting the spectrum. 2.2.2. Field Emission Scanning Electron Microscopy (FESEM) The surface morphologies of the nanocomposites were analyzed by using Carl Zeiss-SUPRATM 40 FESEM, with an accelerating voltage of 5 kV. A small amount of the sample was adhered to the sample holder using the carbon tape and then analyzed by FESEM. 2.2.3. High Resolution Transmission Electron Microscopy (HR-TEM) The nanocomposites were analyzed by high resolution transmission electron microscopy (HR-TEM, JEOL 2100) to check the uniformity of the coating of PPy on the graphene. A small amount of the sample was put in the acetone and sonicated for 15 min in an ultrasonic bath. Then a drop of this dispersed solution was put on the copper grid for HRTEM analysis. 2.3. Electrochemical characterization Electrochemical experiments such as cyclic voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS) were carried out by using three electrode system where platinum and saturated calomel electrode (SCE) were used as counter and reference electrodes respectively. Cyclic voltammetry (Gamry Instrument, 750 mA and 2 V) measurements were performed in 1 M KCl solu-

Fig. 1. FTIR spectrum of pure PPy, graphene, GP1, GP2, GP3 and GP4.

tion at different scan rates from 10 to 200 mV/s. The capacitances were calculated by the following equation [20] Csp =

(I+ − I− ) v×m

(1)

where I+ and I− are maximum currents in positive and negative voltage scan respectively, v is the scan rate and m is the mass of the composite electrode materials. Impedance measurements were carried out for all the composites in 1 M aqueous KCl solution (electrolyte) by EIS. The samples, in the pellet shape of 1 cm diameter, were prepared by pressing the composite materials at 10 MPa for 1 min. The electrodes were used for electrochemical characterization without any polymer binder. 3. Results and discussion 3.1. FTIR study The FTIR plots of the composites are depicted in Fig. 1.The peaks at 1562, 1469 and 3410, for pure PPy, are corresponds to the C–C, C–N and N–H stretching vibration in the pyrrole ring. Whereas, the peaks at 2855 and 2962 cm−1 are associated with the symmetric and asymmetric vibrations of CH2 [21]. The characteristic peaks of polypyrrole at 1469 cm−1 and 1562 cm−1 are also observed for the composites, indicating towards the formation of PPy in presence of graphene. It is important to note that the intense peak of PPy at 1562 cm−1 becomes broaden in GP3, which indicates the better interaction between the aromatic ring of pyrrole and Graphene, due to a uniform coating of PPy upon graphene (discussed in the FESEM section). Further the broad peak at 3410 cm−1 is due to the overlap between N–H stretching of PPy and the O–H stretching of water (from moisture). 3.2. Field Emission Scanning Electron Microscopy (FESEM) The capacitance of the electrode material depends on the specific surface area and its mesoporosity. The synergistic effect of high conductivity and surface are of graphene and the mesoporous structure of PPy can enhance the capacitance of PPy coated graphene nanocomposites as compared to both pure PPy and graphene. Uniform and thin coating of graphene by PPy is a basic criteria to achieve better capacitance properties. To analyze the coating status of the graphene by PPy, FESEM images were taken at higher magnification and the represented in Fig. 2a–e. FESEM image of pure PPy shows polymer domains of diameter 300 nm


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Fig. 2. FESEM images of (a) pure PPy, (b) GP1, (c) GP2, (d) GP3 and (e) GP4.

(approximately) (Fig. 2a). A thick coating of PPy onto the graphene is observed for the GP1 nanocomposite (Fig. 2b), along with pure PPy domains without graphene. This may be due to the presence of higher amount of pyrrole during the in situ polymerization reaction which promotes the formation of PPy on the graphene surface resulting in a thick coating. However, due to the greater availability of pyrrole monomer in the reaction mixture, polymerization also occurred outside the graphene surface which gives rise to clusters of pure PPy domains (as shown in Fig. 2a). When the graphene percentage was increased, the coating thickness of the PPy on the graphene surface was decreased but some graphene surfaces were remain uncoated (shown in Fig. 2c). The reason for this observation was not fully understood. However, for the GP3 nanocomposite a uniform coating of PPy is observed without any uncoated graphene sheets (Fig. 2d). However, Fig. 2e shows that further increase in the graphene percentage produced uncoated graphene sheets due to the lesser availability of pyrrole monomer at each graphene sheet during polymerization reaction.

tion (SAED) image is shown in the inset. A regular pattern of six membered carbon atoms is seen in the SAED image of graphene. However, the pattern is missing in the SAED image of the nanocomposite (Fig. 3b). This is due to the coating of graphene sheets with the PPy. Since PPy is an amorphous material, no SAED pattern is observed for the PPy coated graphene sheets. This supports the FESEM analysis results where a uniform coating of PPy was observed for the GP3 nanocomposites. 3.4. Electrochemical characterization Cyclic voltammograms of pure PPy and their composites were shown in Fig. 4. Their corresponding specific capacitances were summarized in Table 2. The nanocomposites were analyzed within

Table 2 Specific capacitance of PPy and composites at different scan rate. Sample

3.3. High Resolution Transmission Electron Microscopy (HRTEM) The coating of the graphene sheets by the PPy, in the GP3 nanocomposite, was further analyzed by HRTEM and the images were presented in Fig. 3a and b. Fig. 3a shows the HRTEM image of pure graphene and respective selected area electron diffrac-

PPy GP1 GP2 GP3 GP4

Sp. capacitance (F/g) at scan rate 10 mV/s

20 mV/s

50 mV/s

100 mV/s

200 mV/s

192 186.5 219 409 366.4

145.2 142.92 205.75 346.8 335

85.62 83.85 92.7 297 296.46

51 54.68 82.75 145.1 165.93

46.6 33.5 31 92.4 90.93

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S. Sahoo et al. / Synthetic Metals 161 (2011) 1713–1719 Table 3 Energy density of PPy and composites at different scan rate. Sample

PPy GP1 GP2 GP3 GP4

Energy density (Wh/kg) at scan rate 10 mV/s

20 mV/s

50 mV/s

100 mV/s

200 mV/s

106.6 103.6 121.6 227.2 203.5

80.6 79.4 114.3 192.6 186.1

47.5 46.6 51.5 165 164.6

28.3 30.37 46 80.72 92.2

25.9 18.6 17.2 51.3 50.5

The use of supercapacitor also depends on its energy density and power density. Major drawback of supercapacitors is their low energy density. In our work, all the composites along with PPy show better energy density as well as power density. Energy densities of composites were calculated by the equation: 1 (CV 2 ) 2

Energy density (E) =

(2)

where, C = specific capacitance in F/g, and V = operating voltage, where as power densities were calculated by the equation: Power density (P) =

E t

(3)

where, t = time in second for a complete cycle. Energy densities of all the composites at various scan rates are summarized in Table 3. The highest energy density of 227.2 Wh/Kg is obtained for GP3 at 10 mV/s. This value is higher than the value reported by Wang et al. [23]. Power density at different scan rate for all the composite are shown in the Table 4. The highest power density of 4617 W/Kg is obtained for GP3 at 200 mV/s. 3.5. Impedance study

Fig. 3. HR-TEM images of (a) graphene and (b) GP3. Insets are the respective SAED images.

the applied voltage range of −1.0 V to 1.0 V. The mass of the active materials were around 10 mg. Among the nanocomposites, GP3 was found to be having maximum capacitance of 409 F/g. This capacitance value is higher than MWCNTs/PPy nanocomposites reported by Zhou et al. [22]. GP1 is having capacitance of 186.5 F/g which was lower than the specific capacitance of pure PPy (192 F/g). This decrease in the specific capacitance can be ascribed to the specific surface area accessible by the electrolyte. As shown in Fig. 2b, clusters of PPy domains are formed in the GP1 nanocomposite, which can restrict the access of electrolyte to the surface of individual PPy domains and therefore decreasing the effective surface area of the electrode and hence the capacitance. The specific capacitance was found to be increasing with increase in graphene wt % (GP2 and GP3) and then it decreases, which was evident from lower specific capacitance of GP4. As shown in FESEM image, in case of GP2 and GP4, some graphene sheets were remained uncoated which affects the synergetic contribution of graphene and PPy towards their specific capacitance. However, uniform coating in GP3 composite, revealed from FESEM and HRTEM studies, enhanced the specific capacitance due to the synergetic effect of graphene and PPy [23]. For all the composites, it was found that with increase in the scan rate, the specific capacitance values were decreased, which is shown in Fig. 5. This can be attributed to the greater charge mobilization per unit time.

Electrochemical impedance spectra of pure PPy and their composites were analyzed between the frequencies of 0.1 Hz and 1 MHz. Nyquist plot (imaginary component (Z ) vs. real component (Z )) explains the frequency dependence of electrode/electrolyte system was shown in Fig. 6a–d. The intercept of the real component gives the equivalent series resistance (ESR) of the electrode material. ESR determines the rate of charging/discharging of the electrode material. Semicircle observed in the higher frequency region attributes to the interfacial charge transfer resistance. Larger the diameter of semicircle indicates the higher interfacial charge transfer resistance and it indicates the poor conductivity of the electrodes. ESR of 1.12 , 1.32 , 1.99 , 1.46 and 3.55 were obtained for PPy, GP1, GP2, GP3 and GP4, respectively. Larger semicircle of PPy and GP4 showed the poor conductivity of the electrode materials. GP2 and GP3 showed ideal capacitor behavior. Larger Warburg resistances were observed for GP1 and GP2 which was expected due to the difficulty of ion movement [5]. Nyquist plots are usually interpreted by fitting the experimental impedance spectra to an equivalent electrical circuit. The suitable equivalent

Table 4 Power density of PPy and the composites at different scan rate. Sample

PPy GP1 GP2 GP3 GP4

Power density (W/kg) at different scan rate 10 mV/s

20 mV/s

50 mV/s

100 mV/s

200 mV/s

479.7 466.2 547.2 1022.4 915.75

725.4 714.6 1028.7 1733.4 1675

1068.75 1048.5 1158.75 3712.5 3703.5

1273.5 1366.65 2070 3800 4149

2331 1674 1548 4617 4545


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Fig. 4. CV curves at different scan rate of (a) PPy, (b) GP1, (c) GP2, (d) GP3, (e) GP4.

electrical circuit simulated to the experimental data is shown in Fig. 7. Fitting data of polypyrrole composites to the above mentioned circuit is shown in Table 5. In real-world systems, the capacitors are not ideal. These imperfect capacitors are represented as constant phase element (CPE), which are attributed by depressed semicircle in Nyquist plots. CPE may arise from (i) distribution of relaxation times as a result of inhomogenities at the electrode/electrolyte interface; (ii) porosity; (iii) the nature of the electrode; (iv) dynamic disorder associated with diffusion [24]. The steep rise in curve in lower frequency region attributes the

Table 5 Fitting data for equivalent electrical circuit elements of polypyrrole based composites. Sample Rs () Rct () W (S − s0.5 ) × 10−2 Cdl (F) CPE (S − sn ) × 10−3 n GP1 GP2 GP3 GP4

1.327 1.772 1.5 3.535

102.6 64.77 15.08 28.13

0.63 1.062 0.048 0.12

0.99 0.26 8.5 3.04

0.316 0.126 2.08 0.61

0.72 0.55 0.60 0.27

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Fig. 7. Equivalent electrical circuit used in EIS fitting data.

4. TGA analysis

Fig. 5. Specific capacitance vs. scan rate curves of the nanocomposites and pure PPy.

material to behave more like a capacitor. The slope of 45◦ of the curve indicates the Warburg resistance of the electrode material. It results from the frequency dependence of the ion transport in the electrolyte.

The effect of graphene percentage on thermal stability of the composites was analyzed by TGA, in nitrogen atmosphere, and the plots are depicted in Fig. 8. As can be seen from Fig. 8, pure PPy follows a two-step weight loss process, where the first weight loss corresponds to the evaporation of water or volatile impurities [25]. The TGA results of the composites are summarized in Table 6. Pure PPy shows around 10% weight loss at 100 ◦ C which decreased to 7.5 and 4.8% for GP1 and GP3, respectively. Similar weight loss trend was observed for the composites, with increase in temperature. This suggests the thermal stability of the GP3 composite is higher than pure PPy and GP1. This enhancement in the thermal stability of the composites can be attributed to the restriction imposed by the graphene nano sheets on the mobility and thermal vibration PPy chains at the graphene–PPy interface

Fig. 6. Nyquist plot of (a) GP1, (b) GP2, (c) GP3, (d) GP4.


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References

Fig. 8. TGA plot of PPy based nanocomposites.

Table 6 Thermal stability data of different composites extracted from TGA plots. Sample code

PPy GP1 GP3

Weight loss at temperature (%) 100 ◦ C

200 ◦ C

300 ◦ C

400 ◦ C

10.4 7.5 4.8

15.2 11.3 6.5

27.5 20.1 10.2

35.9 27.8 14.1

[26]. This restricted thermal vibration delayed the degradation of the PPy chains. As discussed in the FESEM section uniform coating was achieved for the GP3 composite, which increased the graphene–PPy interaction area and hence improved the thermal stability. 5. Conclusion Graphene/PPy composites were prepared by in situ oxidative polymerization method. The morphology and chemical structure of the composites and pure PPy are characterized by different characterization techniques such as FTIR, FESEM, HR-TEM. From the morphological study it is clear that PPy and Graphene forms a uniform thin coating at nanometer scale in GP3. From the electrochemical studies it has been found that the electrochemical performances of composites are remarkably enhanced compared with the PPy. Among the composites highest capacitance value (at 10 mv/s scan rate), as well as energy density (at 10 mv/s scan rate) and power density (at 200 mv/s scan rate) values are achieved for GP3.

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