Investigation of the capacitive performance of tobacco solution reduced graphene oxide

Materials Chemistry and Physics 151 (2015) 72e80

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Investigation of the capacitive performance of tobacco solution reduced graphene oxide Milan Jana a, b, Sanjit Saha a, Pranab Samanta a, Naresh Chandra Murmu a, Joong Hee Lee c, *, Tapas Kuila a, * a Surface Engineering & Tribology Division, Council of Scientific and Industrial Research e Central Mechanical Engineering Research Institute, Durgapur 713209, India b Academy of Scientific and Innovative Research (AcSIR), Anusandhan Bhawan, 2 Rafi Marg, New Delhi 110001, India c Advanced Materials Research Institute for BIN Fusion Technology (BK Plus Global, Program), Department of BIN Fusion Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Reduced graphene has been prepared by bio-reduction of graphene oxide.
Few layers of graphene has been synthesised as observed by Raman spectra.
Two electrode based supercapacitors are fabricated.
Highest specific capacitance is found to be 206 F g1.
Retention in specific capacitance is 112% after 1000 chargeedischarge cycles.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 July 2014 Received in revised form 29 September 2014 Accepted 14 November 2014 Available online 24 November 2014

A facile and green approach for the reduction of graphene oxide (GO) using tobacco leaves solution was reported. The benefits of this approach were the use of green and cheap reducing agent as compared to the commercially available toxic and hazardous chemicals. Moreover, the purification of reduced GO (rGO) sheets can be avoided by using naturally occurring reducing agents. The obtained rGO sheets were characterised by Ultra violet visible, Fourier transform infrared, Raman and X-ray photo electron spectroscopy analysis. The morphologies were recorded by transmission electron and field emission scanning electron microscopy analysis and these showed the formation of a few layer rGO sheets. The electrical conductivity of rGO was found to be ~410 S m1 at room temperature. Electrochemical performances were characterised by cyclic voltammetry, chargeedischarge and electrochemical impedance spectroscopy analysis. A two electrode symmetric supercapacitor device was designed using nickel foam as current collector. The specific capacitance of the two-electrode device reached to 206 F g1 at a current density of 0.16 A g1. The retention in specific capacitance was found to be ~112% after 1000 charge edischarge cycles. © 2014 Elsevier B.V. All rights reserved.

Keywords: Nanostructures Electrochemical techniques X-ray photo electron spectroscopy (XPS) Electrical conductivity

1. Introduction * Corresponding authors. E-mail addresses: [email protected] (J.H. Lee), [email protected] (T. Kuila). http://dx.doi.org/10.1016/j.matchemphys.2014.11.037 0254-0584/© 2014 Elsevier B.V. All rights reserved.

Supercapacitors, also known as ultracapacitors or electrical double layer capacitors (EDLC) or more scientifically (generic)

M. Jana et al. / Materials Chemistry and Physics 151 (2015) 72e80

electrochemical capacitors, have drawn significant attention to the research community, due to its rapid chargingedischarging rate, long cycle life, high power density and wide operating temperature range [1]. Electrochemical capacitors can serve individually or by coupling with batteries to provide greater pulse of peak power on uphill gradients during acceleration [2]. Supercapacitors provide several orders of magnitude higher energy than that of other electrical double layer capacitors [3]. It can act either by EDLC mechanism or battery like pseudocapacitive mechanism or by both. In case of EDLC, there is a reversible adsorptionedesorption of charges at the electrodeeelectrolyte interface forming a high surface area double layer capacitance. Electrolyte ions are reversibly adsorbed onto the surface of electrode materials to store the charge. Therefore, the increase of specific surface area, fast transport of electrolyte ions through the consistent pores and good wettability of the electrode materials are the key factors in EDLC mechanism. On the other hand, redox capacitors provide capacitance by oxidationereduction reaction and generate pseudocapacitance with metal oxides or conducting polymer as supercapacitor electrode [4,5]. Pseudocapacitive reactions afford greater specific capacitance than EDLC, but, the faradic reactions result in the change of phase, lowering their life-time, energy density and power density. Different types of materials like metal oxides, conducting polymers and nano-structured carbon materials have been widely investigated as supercapacitor electrode for energy storage applications [6,7]. Among all these materials studied so far, graphene has been regarded as the ideal supercapacitor electrode material providing very high EDLC due to its high specific surface area and good electrochemical cyclic stability [8]. Graphene, the two dimensional monolayer allotrope of carbon, consists of sp2 hybridised carbon atoms arranged in a honeycomb crystal moiety has shown remarkable potential for different applications in the areas of flexible electronics, energy storage, biosensors, polymer composites, lubricants, nanofluids, drug delivery, field effect transistors, hydrogen storage, etc. [9e17]. In order to harness these potential aspects, scaled up production of graphene is required. So it's a challenge to the material scientists to develop a clean and cost effective method to obtain graphene in pilot-scale. Several methods have been attempted to prepare graphene in standard lab-scale [13,15]. Single layered graphene sheets can be obtained from highly oriented pyrolytic graphite by repeatedly peeling with a sticky tape [9]. Large area monolayer graphene has been synthesised by chemical vapour deposition (CVD) method [13,15]. However, pilot-scale synthesis of graphene by CVD technique is still challenging. Intercalation of different organic functional groups in the graphite moiety followed by exfoliation to the monolayer graphene sheets is a well known method to obtain high quality graphene. Among these techniques, chemical oxidation of graphite to multilayer graphite oxide stack, conversion of these graphite oxide to monolayer graphene oxide (GO) by ultrasonic exfoliation followed by reduction to graphene has been accepted by most of the laboratories [18]. Several types of reducing agents and techniques have been introduced to achieve reduced GO (rGO) as reported in the literature [22]. Thermal reduction of GO (600e1100  C) takes place within 30e45 s and has been regarded as suitable technique for the mass production of rGO [19]. However, annealing of GO at elevated temperature causes ruptures in the rGO sheets and forms amorphous carbonaceous material. Among all these procedures reported so far, chemical reduction of GO is the most accepted for large scale production [20e23]. The commonly used reducing chemicals include hydrazine monohydrate, sodium borohydride, hydroxylamine etc. are toxic and environmentally hazardous [20,21]. Recent literature shows that there are many promising substituent of hazardous and toxic reducing agents, such as alkaline solutions (sodium

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hydroxide, potassium hydroxide), alcohols (ethyl alcohol, methyl alcohol, isopropyl alcohol), various metals (zinc, aluminium and iron), reducing sugars (glucose, sucrose and fructose), phenols (tea ploy solution, gallic acid, dopamine, tanin), and others (microbes like baker's yeast and escherica coli, vitamic c, protein brovine serumabumin, sodium citrate, drained water from moong bean), etc. [22,23]. However, there are various challenges with these green reducing agents due to their low reducing ability, ease of availability, high cost and consumption of food stuffs in material research. Therefore, a green and effective technique without consuming food stuffs is still a matter of research. Herein, we described a clean and cost effective bio-reduction of GO using tobacco leaves solution (TS). Tobacco leaves contain various kinds of amino acids (like glutamine, asparagine), volatile acids (like formic, malic, citric and oxalic acid) reducing sugar as dextrose [24,25]. Previous reports confirm that amino acids, formic acid, malic acid, citric and oxalic acid and dextrose can successfully reduce GO in aqueous medium [26e30]. Therefore, it is expected that TS can be used as an efficient reducing agent to obtain rGO. The electrochemical performances of rGO have been evaluated through cyclic voltammetry (CV), chargeedischarge cyclic stability and electrochemical impedance spectroscopy (EIS) analysis. 2. Experimental 2.1. Materials Natural graphite flakes and polyvinylidene fluoride (PVDF) were obtained from SigmaeAldrich. Potassium permanganate, sulphuric acid, hydrogen peroxide and N,N-dimethyl formamide (DMF) were purchased from Merck, Mumbai, India. The reducing agent tobacco leaves was purchased from a local market in Durgapur, West Bengal, India. Conducting carbon black (EC-600JD, purity: >95%) was purchased from Akzo Nobel Amides Co., Ltd, South Korea. Nickel foam was purchased from Shanghai Winfay New Material Co., Ltd., China. 2.2. Reduction of GO using tobacco leaves soaked solution GO was prepared from natural flake graphite by modified Hummers method as reported earlier [31]. For the reduction of GO, about 15 g of tobacco leaves was washed thoroughly with DI water. The clean tobacco leaves were then added to ~300 ml of DI water and socked for overnight. The tobacco leaves were then separated by filtration and the filtrate (TS) was collected as a green reducing agent. About 100 mg of GO was dispersed in 100 ml of DI water by 1 h ultra-sonication. The GO dispersion was centrifuged for ~5 min at 4000 rpm to remove un-exfoliated graphite oxide. The clear GO dispersion was taken in a 500 ml round bottom flask and 100 ml of TS was added to it followed by stirring for ~24 h at room temperature. The black residue was collected by filtration followed by washing thoroughly with DI water. The collected product was dried in vacuum at ~60  C for ~72 h and designated as TBG1. In order to investigate the effect of temperature, reduction of GO was carried out at 100  C for ~24 h and the resulting product was designated as TBG2. 2.3. Characterisation Ultra violetevisible (UVevis) spectroscopy measurements of GO, TBG1 and TBG2 were performed by using Hitachi Q-3010 spectrophotometer. Fourier transform infrared (FT-IR) spectroscopy was recorded using a Nicolet 6700 spectrometer (Thermo scientific, USA). X-ray photo electron spectroscopy (XPS) was recorded with AxiseNova, Kratos Analytical Ltd., Manchester, UK. Transmission

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electron microscopy (TEM) was recorded using JEOL JEM-2200 FS. For sample preparation, rGO was dispersed in water (~0.1 mg ml1) by 20 min ultrasonication followed by drop casting onto the fresh lacey carbon copper grid. Field emission scanning electron microscopy (FE-SEM) images were recorded with Sigma HD, Carl Zeiss, Germany. Raman spectra of pure GO, TBG1 and TBG2 were measured by a Nanofinder 30 (Tokyo Instruments Co., Osaka, Japan) with Laser wavelength of 514 nm and 100 mm spot size. The electrical conductivity of GO, TBG1 and TBG2 was measured using KEITHLEY Delta System consisting of AC&DC current source, model: 6221 and Nanovoltmeter, model: 2182A. The powdered samples were compressed into a pellet (1 cm diameter), the conductivity of the rGO pellets were analysed by a 4 probe method using the formula.

1


Conductivity ¼ 4:53 

I V

d

where I ¼ current in ampere, V ¼ voltage in volt and d is the measured thickness (metre) of the pellet. The electrochemical analysis including CV, EIS and chargeedischarge cyclic stability were performed using a three electrode configuration in 2 (N) H2SO4 electrolyte (aqueous) with PARSTAT 4000 (Princeton Applied Research, USA) electrochemical workstation. Glassy carbon electrode was used as working electrode, AgCl/Ag as reference electrode and platinum wire as counter electrode. The electrochemical performances (CV, EIS and chargeedischarge) were also evaluated in a symmetric two electrode configuration using 1(M) Li2SO4 as electrolyte. The electrode material for the two electrode configuration was prepared by mixing 80 wt% of rGO, 15 wt% of carbon black and 5 wt% PVDF in DMF solution. The slurry was then drop casted on two pieces of coin shaped (1.0 cm diameter) nickel foam and dried under vacuum for overnight at 65  C. 3. Results and discussions 3.1. UVevis spectra analysis UVevis spectra of GO, TBG1 and TBG2 (dispersed in DI water) are shown in Fig. 1. GO reflected two absorption peaks, the pep* transition peak of C]C at ~230 nm and another shoulder at

Fig. 1. UVevis spectra of GO, TBG1, and TBG2.

~300 nm due to the nep* transition of C]O bonds [23]. The peak corresponding to nep* of C]O bonds was absent in the rGO (TBG1 and TBG2). In addition, the peak at ~230 nm was red (in the web version) shifted to ~265 nm in the UVevis spectra of TBG1 and TBG2. Interestingly, it was seen that the peak intensity of TBG2 at ~265 nm was significantly higher than that of TBG1. In contrast, a broad hump was recorded in case of TBG1. It may be due to the improvement in restoration of p-electronic conjugated structure at 100  C as compared to RT. 3.2. FT-IR spectra analysis FT-IR spectra of GO, TBG1 and TBG2 are shown in Fig. 2. The broad band at 3427 cm1 can be ascribed to the stretching vibration of eOH functional groups present on the surface of GO [31]. The peaks at 1725 and 1631 cm1 were assigned to the eCOOH functional groups and skeletal vibration of un-oxidised graphitic carbon, respectively [32]. The peak at 1394 cm1 can be ascribed to the eOH deformation peak and bending vibration of intercalated water molecules. Stretching vibration peaks of CeO (alkoxy) and CeOeC (epoxy) appeared at 1061 and 1231 cm1, respectively [31]. The FTIR spectra of TBG1 and TBG2 were sharply different from that of GO. The peak corresponding to eCOOH functional groups at 1725 cm1 was absent in the TBG1 and TBG2. The intensities of the peaks related to the alkoxy and epoxy functionalities at 1061 and 1231 cm1 decreased significantly in the reduced GO (TBG1 and TBG2), confirming the elimination of oxygen functionalities during tobacco reduction [29]. The FT-IR data also revealed that the intensities of the oxygen functional groups of TBG2 were significantly lowered than that of TBG1 further suggesting superior reduction at 100  C as compared to RT. 3.3. XPS analysis XPS is a practical technique to analyse the structure and chemical composition of graphite like materials to assess the sp3 and sp2 hybridised carbon atoms. The C1s peaks of TBG1 and TBG2 are shown in Fig. 3(a,b). The C1s spectrum of GO was reported in our previous publication and each of the C1s peaks were explained in detail [31]. Original GO showed peaks at 286.8, 287.3 and 289.2 eV due to the CeO (epoxy and aloxy), C]O and OeC]O functional groups, respectively, along with the main peak at

Fig. 2. FT-IR spectra of GO, TBG1 and TBG2. The inset shows the stretching vibration peak for epoxy functional groups at 1231 cm1.

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Fig. 3. C1s XPS of (a) TBG1 and (b) TBG2.

284.8 eV, responsible for the carbon atoms (C]C/CeC) in aromatic rings of graphitic backbone [33,34]. It was seen that the peak intensities of the oxygen functional groups were decreased significantly in the TBG1 and TBG2, illustrating the removal of oxygen functional groups. Interestingly, the intensities of the C1s peaks of the oxygen functional groups in TBG1 were sufficiently greater than that of TBG2. It may be attributed to the higher reducing efficiency of TS at 100  C as compared to that at RT. It was also noted that the peak positions were changed in the TBG1 and TBG2 as compared to pure GO. In case of TBG1, the peaks corresponding to C]C/CeC (aromatic), CeO (epoxy and alkoxy), and C]O appeared at 283.7, 285.2, 287.4 eV, respectively. In contrast, these peaks were visible at 284.05, 285.8, 287.1 eV, respectively in the TBG2. XPS elemental analysis showed that the C/O atomic ratios in GO, TBG1 and TBG2 were 2.3, 2.8 and 5.1, respectively. All these observations, confirmed that TS can reduce GO to rGO and the reducing efficiency was much better at 100  C as compared to RT. Although the C/O atomic ratios of TBG2 was lower than other rGOs obtained by using different types of reducing agents, suggesting mild reducing effect of TS and presence of residual oxygen functionalities on rGO surface [21,35e38]. 3.4. Raman spectra analysis Raman spectroscopy of GO, TBG1 and TBG2 are shown in Fig. 4. Raman spectra of graphite and graphene are analysed by two bands, the G band (E2g mode of sp2 carbon atoms) at 1575 cm1, and 2D (G0 ) band at ~2700 cm1. The 2D band is associated with the two point phonon double resonance process of the band structure of graphene layers. Another sharp peak (D) appeared at ~1345 cm1 for the chemically derived graphene, is a measure of disarray and arises due to the k-point phonons breathing mode of A1g symmetry. In the case of GO, the G band was shifted to 1590 cm1 and the D band appeared below to 1334 cm1 due to the extensive oxidation of graphite. The D band of TBG1 and TBG2 was up-shifted to 1344 cm1. In contrast, the G band of TBG1 and TBG2 appeared at 1580 and 1565 cm1, respectively. The 2D band of TBG1 and TBG2 appeared at ~2685 cm1. Interestingly, the intensity of the 2D band was significantly higher in TBG2 than that of TBG1 further confirming better reducing ability of TS at 100  C. However, the intensity of this band was relatively lower than that of the single layer of graphene as reported by Ferrari et al. [39]. The comparative study of the 2D band confirmed the formation of a few layer rGO in the case of TBG2. The degree of disorder in the TBG1 and TBG2 can be derived by the intensity ratio of the D to the G band (ID/IG). The ID/IG

Fig. 4. Raman spectra of GO, TBG1 and TBG2.

ratios for GO, TBG1 and TBG2 were found to be 1.12, 0.97 and 0.60, respectively. The significant decrease in ID/IG ratios in TBG2 indicated that most of the oxygen functional groups were removed during reduction at 100  C. The low ID/IG ratio also suggested better restoration of electronic conjugated structure and low defect concentration in TBG2 as compared to TBG1. The intensity ratio of the 2D and G band (I2D/IG) was also related to the recovery of C]C bonds in the graphitic structure [40]. The I2D/IG ratios of GO, TBG1 and TBG2 was calculated to be 0.23, 0.24 and 0.60 respectively. The

Table 1 Comparative study of the electrical conductivity and C/O atomic ratios (obtained from XPS elemental analysis) of rGO obtained by using different reducing agents. Reducing agents

C/O atomic ratios

Electrical conductivity (S m1)

References

Hydrazine monohydrate L-cysteine Dextran Methanol Ethanol Green tea solution Baker's yeast Sodium borohydride TS

10.3 e e 4.0 6.0 e 5.9 15.1 5.1

200 0.124 1.1 3.2  105 1.8  104 53 43 500 410

20 43 30 43 43 44 46 45 Present work

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increase in peak intensity ratio in TBG2, clearly revealed the better restoration of C]C bonds in TBG2 than in TBG1. 3.5. Electrical conductivity Comparison of electrical conductivity of GO derivatives is one of the best techniques to measure the extent of the reduction. The presence of sp3 bonded carbons within the sp2 networks of graphene decreases its electrical conductivity and makes GO nonconductive [41,42]. Therefore, the measurement of electrical conductivity is a useful technique to examine the extent of restoration of p-electronic conjugation after GO reduction. The electrical conductivity of GO, TBG1 and TBG2 were measured at RT and the recorded values were 9.3  102, 5.5 and 410 S m1, respectively. Table 1 shows the electrical conductivity of various types of rGO obtained by using different types of reducing agents [20,30,43e46]. The significant increase in electrical conductivity in TBG2 is in good agreement with the observations of UVeVis, FT-IR, Raman, and XPS elemental analysis further confirming successful reduction of GO using TS. 3.6. FE-SEM and TEM image analysis The morphology of TBG2 was checked by the FE-SEM, TEM and high resolution TEM (HR-TEM) image analysis. FE-SEM images of TBG2 with different scale bars are shown in Fig. 5(a,b). The graphene sheets were associated, crumpled and aggregated to each other forming a continuous conducting network [3]. The formation of aggregated structure of the TBG2 can be ascribed to the pep stacking of the individual rGO sheets during the reduction of GO. A large graphene sheets was located on top of the copper grid (Fig. 6(a)) in TEM image of TBG2. The stability of TBG2 under high energy electron beam was corroborated by the appearance of transparent sheets of TBG2. However, it was seen that the transparency of the TBG2 sheets was relatively lower than that of the single-layer graphene sheets reported earlier [23]. It may be attributed to the stacking (or overlapping) of rGO sheets during reduction using TS at 100  C. In order to further investigate the morphology of TBG2, HR-TEM was carried out to identify the number of layers at different locations as shown in Fig. 6(b). The edges of the graphene sheets tend to fold back to access the cross sectional views. The number of lattice fringes visible on the edges of the folded regions represents the number of layers in a graphene sheet, suggesting the formation of a few-layer graphene sheets. Presence of oxidative debris in the surface of GO masks the graphene like lattice [23,47,48]. The HR-TEM image of TBG2 showed a little bit of disordered or amorphous nature. The residual oxidative

debris present on the surface of TBG2 as evidenced by XPS elemental analysis may mask the crystalline nature of graphene [23,47,48]. The morphological features obtained from FE-SEM and TEM image analysis showed greater degree of accessibility of electrolyte not only in the outer region but also in the inner surfaces of the materials [3]. It revealed that both sides of the TBG2 sheets were exposed to the electrolyte suggesting the improvement in capacitance performance. 3.7. Electrochemical properties Reduced GO has been recognised as the promising supercapacitor electrode materials due to its high electrical conductivity and electrochemical cyclic stability [41,49e54]. In order to evaluate the electrochemical performances, CV was carried out within the chosen voltage range of 0.6 to 0.8 V (vs. AgCl/Ag) in a three electrode system using 1 M H2SO4 as electrolyte. Fig. 7(a,b) shows the CV curves of TBG1 and TBG2 at various scan rates of 10e200 mV s1. The CV curves were near rectangular in shape. However, the presence of redox humps in the CV curves was clearly visible due to the faradic reactions of the remaining oxygen functionalities of TBG1 and TBG2. Interestingly, the intensities of the redox peaks decreased in the TBG2 as compared to that of the TBG1. It is found that there was no distortion in the CV curves even at high scan rate at 200 mV s1, indicating higher capacitive performances of TBG1 and TBG2. It may be attributed to the controlled diffusion of ions in solution and establishment of ion adsorptionedesorption equilibrium at the electrodeeelectrolyte interfaces [50]. In addition, it can also be concluded that the diffusion of ions in solution took place at the same rate to that of the electronic transitions occurred at the active sites of the electrode materials [62]. The CV curves clearly suggested that the capacitive behaviour of TBG1 and TBG2 was dominated by EDLC rather than the pseudocapacitance. In order to investigate the supercapacitor performance of an electrode material, galvanostatic chargeedischarge is more reliable. The galvanostatic chargeedischarge curves of TBG1 and TBG2 at various current densities are shown in Fig. 7(c,d). Specific capacitance was calculated by using the formula C ¼ (I  Dt/ m  DV), where C, I, m, Dt and DV are specific capacitance, applied current, mass of active materials, discharging time and potential window of the chargeedischarge curves, respectively. The specific capacitances of TBG1 and TBG2 were calculated as 169 and 138 F g1, respectively at a current density of 1.6 A g1. However, the specific capacitance of TBG2 was higher than that of TBG1 at high current densities as shown in Fig. 8(a). The high specific capacitance of TBG1 at lower current density may be attributed to the high content of residual oxygen functionalities as evidenced by the

Fig. 5. (a) Low magnification FE-SEM and (b) high magnification FE-SEM of TBG2.

M. Jana et al. / Materials Chemistry and Physics 151 (2015) 72e80

77

Fig. 6. (a) TEM and (b) HR-TEM images of TBG2.

Fig. 7. CV curves of (a) TBG1 and (b) TBG2 at various scan rates. Chargeedischarge cycles of (c) TBG1 and (d) TBG2 at different current densities.

XPS, FT-IR and Raman spectroscopy analysis [57]. These residual oxygen functionalities took part in battery type redox reactions, which were absent in higher current densities due to the fast ion transfer between electrodeeelectrolyte interfaces [51,52]. The coulombic efficiency of TBG1 and TBG2 was calculated from the chargeedischarge curves. It has been found that the coulombic efficiency of TBG1 and TBG2 were decreased with increasing

current density. However, the rate of decrease in coulombic efficiency of TBG1 was greater than that of TBG2 as presented in Fig. 8(a). EIS was carried out in the frequency range of 10,000 to 1 Hz at 10 mV amplitude to evaluate the capacitive performances of TBG1 and TBG2. The EIS of TBG1 and TBG2 are shown in Fig. 8(b). An ideal supercapacitor electrode can be identified by the appearance of a

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Fig. 8. (a) Variation of specific capacitance and coulombic efficiency with current density of TBG1 and TBG2, (b) EIS plot of TBG1 and TBG2.

negligible resistor capacitor (RC) or semicircle loop in the high frequency region [53]. The vertical line in the low frequency region is associated with the diffusion resistance (in the interior of the electrode) and ion diffusion of the electrolyte into the electrode surface [54]. The almost vertical slope at the low frequency region of TBG1 and TBG2 indicated the capacitive behaviour of these materials [53]. It is seen that the solution resistance (Rs) of TBG2 (4.20 U) was smaller than that of TBG1 (5.63 U). It may be attributed to the presence of different extent of remaining oxygen functionalities in the TBG1 and TBG2. Inspection of CV curves, specific capacitance, coulombic efficiency and EIS spectra suggested that

TBG2 can perform better as supercapacitor electrode materials as compared to that of the TBG1. The capacitive performance of the electrode materials is strongly dependant on the cell configuration and it has been found that the specific capacitance values obtained from three electrode configurations are always higher than that of the two electrode systems [55,56]. Therefore, two electrode symmetric configurations were designed to check the capacitive performance of TBG2 in device level. Two coin shaped nickel foam (1 cm diametre) were used as current collector as well as for materials support. 1 M Li2SO4 aqueous solution was used as electrolyte. The CV curves of the

Fig. 9. (a) CV curves of TBG2, (b) variation of specific capacitance with scan rate, (c) chargeedischarge cycles, (d) variation of specific capacitance and coulombic efficiency with current density in two electrode system.

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fabricated device were studied at different scan rates of 10e200 m V s1 in the voltage range of 0e1 V. The CV curves were rectangular in shape as shown in Fig. 9(a). The specific capacitance of the TBG2 electrode can be calculated from the CV loops using the formula.

  specific capacitance C ¼ 4

Z1

 Im dV nV

0

where Im is current density response, V is the potential window and n is the scan rate in mV s1. It is seen that the specific capacitance of TBG2 decreased (240e40 Fg1) with increasing the scan rate (10e200 mV s1) as shown in Fig. 9(b). The decrease in specific capacitance with increasing scan rate may be attributed to the diffusion limits of the electrolyte ions [57]. It is anticipated that at higher scan rates the inner active material surfaces of the electrode could not properly participate in faradic transitions, rather the outer active surfaces were taking part in the charge storage mechanism. The gradual decrease in specific capacitance with scan rates suggested that some parts of the electrode were unreachable at high chargingedischarging rate and the capacitance at low scan rate was the actual capacitance of the electrode [58,59]. The galvanostatic chargeedischarge curves at various current densities are shown in Fig. 9(c). The specific capacitance was calculated using the formula C ¼ (4I  Dt)/(m  DV), where the symbols bear the same meaning as in the case of three-electrode configurations. The specific capacitance was calculated to be 206 F g1 at a current density of 0.16 A g1 and found to decrease gradually with increasing current density as shown in Fig. 9(d). Variation of coulombic

79

efficiency with current density is also presented in Fig. 9(d). It was seen that the specific capacitance values calculated from chargeedischarge and CV curves were almost comparable to each other. The electrochemical cyclic stability of the designed device was also examined up to 1000 chargeedischarge cycles (at a constant current density of 1 A g1) and the cyclic stability results are shown in Fig. 10(a). The specific capacitance values were found to increase after 1000 chargeedischarge cycles. XPS elemental analysis showed that various oxygen containing functional groups were present on the surface of TBG2 and the C/O atomic ratio was found to be ~5.1 (TBG2). The atomic ratio of TBG2 was low as compared to the other rGOs (reported in the literature) obtained by using different types of reducing agents [21,35e38]. It is expected that the residual non conducting oxygen functional groups of TBG2 were reduced during the galvanostatic chargeedischarge cycles resulting the enhancement of specific capacitance [60e65]. Also the activation of electrode materials may lead to high specific capacitance due to better interaction between electrolyte and electro active materials [63e65]. Indeed, the original specific capacitance of TBG2 was recorded as 132 F g1 at a current density of 1 A g1. The value was increased to only 148 F g1 after 1000 chargeedischarge cycles. It should also be noted that there was a four fold change in specific capacitance with the variation of discharging time as evidenced from the equation, C ¼ (4I  Dt)/(m  DV). Where, the symbols bear same meaning as stated earlier. The EIS of the two-electrode system are shown in Fig. 10(b). The Nyquist plot showed that the Rs obtained from the two and three electrode configurations were almost comparable. However, the nature of the curve differed significantly. The Nyquist plot intersected the real axis (X axis) with an angle of 45 , indicating the

Fig. 10. (a) Cycling performance of TBG2 up to 1000 cycles, (b) EIS of TBG2, (c) Ragone plot in two electrode system.

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porous nature of the electrode when saturated with the electrolyte [57]. Based on the specific capacitance values obtained from the two electrode configuration, the energy density (E) and power density (P) of the system were also calculated, using the formula E ¼ 1/8CV2, and P ¼ E/Dt. The energy density vs. power density plot is shown in Fig. 10(c). The highest energy density was found to be 7.15 Wh Kg1 at a power density of 79.7 W kg1. 4. Conclusions Reduction of GO using TS was described in detail. Various amino acids (asparagine, glutamine), reducing sugar (dextrose), and carboxylic acid (like formic acid) present in TS were responsible for the reduction of GO. FT-IR spectra and XPS analysis clearly revealed the removal of oxygen containing functional groups from the surfaces of GO. The restoration of p-electronic conjugation after reduction of GO and formation of a fewer defects in rGO (ID/IG ratio 0.6) were illustrated by the UVevis and Raman spectra analysis. The increase in electrical conductivity in TBG2 also confirmed the reduction of GO using TS. The as prepared rGO (TBG1 and TBG2) could be used as supercapacitor electrode material. The specific capacitance of TBG2 was found to be 206 F g1 at current density of 0.16 A g1 as illustrated by galvanostatic chargeedischarge using 1(M) Li2SO4 solution in a two electrode configuration. The designed supercapacitor device was highly stable and showed ~112% retention in specific capacitance after 1000 chargeedischarge cycles. The present method is cost effective and eco-friendly in comparison to the available reduction methods using hazardous and toxic chemicals. Acknowledgements Authors are thankful to Dr. P. Pal Roy, Director of CSIR-CMERI. Authors are also thankful to Department of Science and Technology, New Delhi, India (IFA12-CH-47) for the financial supports vide DST-INSPIRE Faculty Scheme e INSPIRE Programme and Council of Scientific and Industrial Research, New Delhi, India for funding MEGA Institutional project (ESC0112/RP-II). References [1] Y.W. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W.W. Cai, P.J. Ferreira, Science 332 (2011) 1537. [2] C.Z. Yuan, X.G. Zhang, B. Gao, J. Li, Co(OH)2, Mater. Chem. Phys. 101 (2007) 148. [3] Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, J. Phys. Chem. C 113 (2009) 13103. [4] Z.B. Lei, L. Lu, X.S. Zhao, Energy Environ. Sci. 5 (2012) 6391. [5] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. 8 (2008) 3498. [6] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, Nanoscale 5 (2013) 72. [7] G. Wang, L. Zhang, J. Zhang, Chem. Soc. Rev. 41 (2012) 797. [8] Y. Chen, X. Zhang, D. Zhang, P. Yu, Y. Ma, Carbon 49 (2011) 573. [9] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [10] H. Lui, L. Liu, K.F. Mak, G.W. Flynn, T.F. Heinz, Nature 462 (2009) 339. [11] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nanotechnol. 3 (2008) 270. [12] K.P. Loh, Q. Bao, G. Eda, M. Chhowalla, Nat. Chem. 2 (2010) 1015. [13] J. Hou, Y. Shao, M.W. Ellis, R.B. Moored, B. Yie, supercapacitors and lithium ion batteries, Phys. Chem. Chem. Phys. 13 (2011) 15384. [14] S.M. Hafiza, R. Ritikosa, T.J. Whitchera, N.M. Razibb, D.C.S. Bienb, N. Chanlek, H. Nakajima, T. Saisopa, P. Songsiriritthigul, N.M. Huang, S.A. Rahman, Sens. Actuat. B Chem. 193 (2014) 692. [15] T. Kuila, S. Bose, P. Khanra, A.K. Mishra, N.H. Kim, J.H. Lee, Biosens. Bioelectron. 26 (2011) 4637. [16] T. Kuila, S. Bose, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Polym. Test. 31 (2012) 31. [17] K. Spyrou, D. Gournis, P. Rudolf, ECS J. Solid Sci. Tech. 2 (2013) M3160. [18] T. Kuila, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Nanoscale 5 (2013) 52.

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