Carbon Nanotubes-Graphene-Solidlike Ionic Liquid Layer-Based

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Carbon Nanotubes-Graphene-Solidlike Ionic Liquid Layer-Based Hybrid Electrode Material for High Performance Supercapacitor P Tamailarasan and S Ramaprabhu* Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai −600036, India

ABSTRACT: Carbon nanomaterials are promissing electrode materials for supercapacitor applications due to their unique properties. Electrolyte accessibility is a big challenge in ionic liquid electrolyte-based supercapacitors with carbon nanomaterials as electrodes. In this study, an ultrahigh performance supercapacitor electrode, based on solidlike ionic liquid layer coated carbon nanotubes-hydrogen exfoliated graphene nanocomposite is demonstrated with hydrophobic ionic liquid as electrolyte. The presence of solidlike layers of ionic liquid is confirmed by structural and morphological analysis. The nanocomposite shows extremely high energy density (171 Wh/kg) and high specific capacitance (201 F/g) at a large specific current of 2 A/g, in terms of the mass of active electrode material, along with wide operating voltage (3.5 V). The Ragone fit shows that the time constant, maximum stored energy, and maximum available power are 0.575 s, 170.66 Wh/kg, and 148.43 kW/kg, respectively. The improvement in performance of the nanocomposite is mainly attributed to the presence of solidlike layers of ionic liquid on the surface of carbon nanomaterials, which effectively increases the electrolyte accessibility and number of shortest, directional ion transport paths. Here, carbon nanotubes play a role as a smart conductive spacer.

1. INTRODUCTION The capacitors, with superior energy as well as power densities, are called supercapacitors (also known as electrochemical capacitor or ultracapacitor). This capacitance arises due to either electrostatic interaction (electric double layer capacitor, EDLC) or Faradaic charge transfer (pseudocapacitance) at the electrode−electrolyte interface.1−3 In EDLC, energy is stored through polarization followed by adsorption of ionic charges on the surface of the electrode and hence surface area of the electrode plays a major role. Because of high surface area, porous structure, chemical inertness, and good electrical conductivity, carbon nanomaterials are promising electrode materials for supercapacitors (SCs).4,5 In 2004, Novoselov et al. have extracted graphene, the mother of all carbon materials, from graphite by mechanical exfoliation.6 Thenceforth, the scientific community has given more attention to graphene as an electrode material for supercapacitor applications due to its unique properties over other carbon nanomaterials.7 Increasing capacitor voltage is a way to enhance the power and energy densities significantly. Organic electrolytes, instead of aqueous electrolyte, provide a wide potential window. However, the thermal stability and toxicity of organic © XXXX American Chemical Society

electrolytes limits their use as electrolytes in supercapacitors. Ionic liquids, the room temperature molten salts with large organic cation (and anion), are promising electrolytes due to their low vapor pressure, wide liquid range, high ionic conductivity, good electrochemical as well as thermal stability along with its wide potential window (generally, in the range of 3 to 7 V.).8 Hence, ionic liquids can play a role as an ultimate, environmentally friendly alternate for organic electrolytes. Supercapacitors based on ionic liquid electrolyte have been reported using various carbon materials-based electrodes like activated carbon,9 carbon nanotubes,10 and graphene.13 However, the presence of water molecules may decrease the potential window of ionic liquids significantly.11 Hence, it requires moisture free atmosphere, which makes a glow box a mandatory requisite and complicates the practical use of ionic liquid as an electrolyte. Hydrophobic ionic liquids can sort this problem out effectively. Received: December 22, 2011 Revised: June 1, 2012

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MWNTs/HEG/[BMIM] [TFSI]) ternary electrode materials for supercapacitors.

A study on graphene-based supercapacitors, carried out by Vivekchand et al., shows specific capacitance of 117 and 95 F/g with aqueous and ionic liquid electrolytes respectively at a scan rate of 100 mV/s.12 The loss (19%) observed in capacitance in the case of ionic liquid electrolyte may be attributed to lack of wetting ability of highly viscous ionic liquids and electrolyte accessibility of graphene-based electrode with large electrolyte ions. There are few attempts to use a mixture of ionic liquid and acetonitrile, as electrolyte, to reduce viscosity and increase wetting ability.13,14 Kim et al. achieved well wetted graphene electrode by coating poly [ionic liquid] on the surface of graphene. They could obtain a specific capacitance of 159 F/g with specific current of 2 A/g.15 Here, the cations of ionic liquids are immobilized by polymerization. Introducing ionic liquid monomers, instead of poly [ionic liquid], can make both anion and cation mobile inside the electrode material. This can enhance ion transport into the electrode by effectively increasing the number of ion diffusion paths. The alkyl tails of ionic liquid cation interact with graphene surfaces and make a uniform solidlike ordering of ionic liquid cations at the solid− liquid interface, as reported by Bovio et al., which ensures better wetting ability.16 The morphology of graphene plays a major role in ionic liquid-based supercapacitor electrode system because the electrolyte has large ions. In this case, wrinkled graphene can have much better electrolyte accessibility than planar graphene as the wrinkles present on the surface of graphene effectively increase the amount of mesopores. However, the presence of a layer of ionic liquid on the surface of graphene increases the surface tension and makes graphene prone to agglomerate, which results in the reduction in capacitance. There has been a significant interest in introducing 1D carbon nanotubes with 2D graphene sheets to prevent face-to-face agglomeration where MWNTs play an additional role as a bridge for electron transfer between graphene sheets.17 A recent study shows that the presence of single walled carbon nanotubes along with graphene attains a specific capacitance of 161 F/g at 3.1 A/g specific current where [EMIM][TFSI] was used as electrolyte.18 The supercapacitor based on graphene oxide/MWNTs also has been demonstrated by Aboutalebi et al. with aqueous electrolyte, which shows the specific capacitance of 251 F/g at the scan rate of 5 mV/s.19 Inspiringly, Lu et al. have reported a specific capacitance of 188 F/g with activated carbon/carbon nanotubes/ionic liquid (AC/CNT/IL) ternary composite electrode with IL electrolyte where the high surface area of AC and high electrical conductivity of CNT have been utilized to attain enhancement in capacitance.20 But the electrical conductivity of AC limits the specific capacitance. Graphene can be a unique alternate to AC in this ternary nanocomposite-based supercapacitor. In the present study, we have evaluated the performance of functionalized multiwalled carbon nanotube/hydrogen exfoliated graphene/1-butyl-3-methylimidazolium bis (trifluoromethyl sulfonyl)imide (f-MWNTs/HEG/[BMIM] [TFSI]) ternary nanocomposite electrode-based supercapacitor, where the same ionic liquid was used as electrolyte, using a more practical twoelectrode supercapacitor assembly. The f-MWNTs were utilized as a spacer as well as a bridge for electron transfer between graphene sheets. The hydrophobic nature of [BMIM] [TFSI] ensures a wide potential window (3.5 V) even at ambient conditions, which results in high energy and power densities. To the best of our knowledge, this is the first study on (f-

2. EXPERIMENTAL SECTION 2.1. Synthesis of Materials. 2.1.1. Carbon Nanotubes. Multiwalled carbon nanotubes (MWNTs) were synthesized by catalytic chemical vapor deposition (CCVD) method, where hydrogen decrepitated MmNi3 alloy was used as catalyst material. Acetylene was pyrolyzed at 700 °C in argon atmosphere in a tubular furnace, which results in the growth of MWNTs on the surface of the catalyst. We have employed air oxidation and acid treatment to remove amorphous carbon and catalytic impurities from the as grown MWNTs, respectively. These purified MWNTs were further functionalized by refluxing in concentrated HNO3 for 2 h to make them hydrophilic in nature and labeled as f-MWNTs.21 2.1.2. Graphene. Graphene was prepared from graphitic oxide in a hydrogen atmosphere. Briefly, graphitic oxide was prepared by oxidation of pure graphite using a water-free mixture of concentrated sulphuric acid, sodium nitrate, and potassium permanganate (Hummers’ method).22 This graphitic oxide was further thermally exfoliated at 200 °C under hydrogen atmosphere in a tubular furnace. The graphene, synthesized by hydrogen exfoliation technique (labeled as HEG), has a wrinkled structure due to the rapid removal of oxygen containing functional groups.23 2.1.3. Ionic Liquid. 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([BMIM][TFSI]) have been synthesized from 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) by metathesis reaction with sodium tetrafluoroborate and lithium bis(trifluoromethylsulfonyl)imide, respectively.24 As-synthesized ionic liquids were kept under dynamic vacuum at 60 °C for 6 h to remove the residual solvent species present and stored in a desiccator. 2.1.4. Preparation of Nanocomposite Electrode. The supercapacitor electrodes have been fabricated as follows: The active electrode materials (f-MWNTs and/or HEG) and ionic liquid (1:1 w/w) were ultrasonicated in isopropanol medium with 5% nafion (1 mL per gram of active material) solution as a binder. We maintained a 1:1 ratio of f-MWNTs and HEG, whereas they present together in active electrode material. The dispersion was coated (∼1 mg of active material/ electrode) on carbon paper (2 × 2 cm2, SGL Germany) using a simple brush coating technique. The supercapacitor setup contains nanocomposite-coated carbon paper as electrodes, polypropylene membrane as separator, the corresponding ionic liquids as electrolyte, and the stainless steel sheets as current collectors. A separator soaked with ionic liquid was sandwiched between two electrodes. This assembly was further packed between Perspex sheets along with the current collectors. 2.2. Characterization Techniques. The nanocomposites were characterized by microscopy and spectroscopy techniques. X’Pert Pro PANalytical powder X-ray diffractometer has been employed to analyze the structure of the nanocomposite. The morphological analysis was done by field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) using FEI Quanta and Technai G-20, respectively. The Raman spectra were obtained with a WITec alpha 300 Confocal Raman system equipped with a Nd:YAG laser (532 nm) as the excitation source. A Fourier transform infrared (FTIR) study was also carried out to examine the vibrational characteristics of B

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The (002) diffraction peak of graphene shifts toward lower 2θ value and observed at 19.9°. Ionic liquids may be adsorbed on the surface of graphene in dispersion phase. While dry, the nanocomposite graphene tries to restack where ionic liquid may stay in between graphene layers and increase the interlayer distance. Here, the alkyl chain of cation interacts parallel with the surface of graphene and the imidazolium rings are slightly tilted to the surface.27 Because the surface is wrinkled, the tilt may be arbitrary which results in the distribution of inter layer distance cantered at 19.9°. 3.2. Morphological Study. The morphological studies of the f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite were carried out by electron microscopy techniques. SEM image (part a of Figure 2) reveals the well established porous structure, due to the intrusion of f-MWNTs into the graphene layers, which results in increment in mesoporosity of nanocomposite. In addition, the wrinkled nature of graphene layers, which enables ionic liquid to accommodate and make more diffusion path for electrolyte, is also picturized clearly. Parts b and c of Figure 2 are the high-resolution TEM images of the nanocomposite. The arrow marks in part b of Figure 2 point out the presence ionic liquid in the wrinkles of the graphene and on the surface of f-MWNTs. The inset in part b of Figure 2 is TEM image of pure graphene, which clearly shows uniform transparency in planar (wrinkleless) region, whereas the ionic liquid containing graphene shows variation in transparency even at planar region as shown in part b of Figure 2. This is evidence of the presence of ionic liquid uniformly in the whole electrode material. An intercalated f-MWNT is highlighted by the arrow mark in part c of Figure 2. The presence of ionic liquid was confirmed by Energy dispersive Xray (EDX) analysis. The elemental analysis (part d of Figure 2) contains peaks corresponding to fluorine and sulfur, which belong to bis(trifluoromethyl sulfonyl)imide anion of ionic liquid. In addition, along with carbon peaks, the oxygen peak corresponding to the functional groups present in f-MWNTs due to functionalization and the presence of finite amount of residual functional groups in graphene also appears. 3.3. Surface Area and Porosity Analysis. The nitrogen adsorption−desorption isotherm has been employed to study specific surface area (SSA) and porosity of the f-MWNTs/HEG nanocomposite. Brunauer−Emmett−Teller (BET) method and Barrett-Joyner-Halenda (BJH) method are employed to find SSA and porosity, respectively.28,29 Figure 3 shows the BET isotherm of f-MWNTs/HEG nanocomposite at standard temperature and pressure (STP), which reveals the SSA of 166.47 m2 /g. The hysteresis between adsorption and desorption isotherm along with sharp fall in adsorbed amount, at higher relative pressures, can be assigned to the mesoporous nature of nanocomposite. The inset of Figure 3 shows desorption dV/dlog(r) pore volume as a function of pore radius, calculated using BJH method. It clearly exhibits the distribution of average pore size (diameter) with maxima at 3.54 nm corresponding to an average pore volume of 4.429 cm3. Because the average pore size of the nanocomposite is higher than the dimension of the ionic liquid ions (∼0.7 nm), it enables ions to accommodate inside the pores and thus results in better electrolyte accessibility and enhanced specific capacitance. 3.4. Raman Spectrogram Analysis. Figure 4 shows the Raman spectrum of nanocomposites of interest. Raman spectrum of f-MWNTs/HEG (part a of Figure 4) shows the D-band (1335.3 cm−1) and G-band (1567.4 cm−1) with an ID/

nanocomposite using a PerkinElmer FTIR spectrometer, whereas the surface area and porosity of nanocomposite was analyzed by ASAP 2020 V3.00 H surface area analyzer. Electrochemical behavior was studied using CH instrument (CHI608C).

3. RESULTS AND DISCUSSION 3.1. Powder X-ray Diffractogram Analysis. The diffractogram of f-MWNTs/HEG (part a of Figure 1) reveals

Figure 1. X-ray diffraction pattern for (a) f-MWNTs/HEG and (b) fMWNTs/HEG/[BMIM] [TFSI] ternary nanocomposites.

the peak at 26.3° corresponding to the (002) plane of fMWNTs. The XRD pattern of f-MWNTs/HEG/[BMIM] [TFSI] ternary nanocomposite (part b of Figure 1) reveals three peaks corresponding to the interplanar distances of 0.7, 0.45, 0.34 nm centered at 12.6°, 19.9°, 26.3°, respectively. The ternary nanocomposite shows no significant shift in diffraction pattern from (002) plane of f-MWNTs, which may reveal that the presence of IL did not disturb the interplanar distance of concentric tubular structure of MWNTs. Liu et al. have observed solidlike layers of 1-butyl-3methylimidazolium hexafluorophosphate adjacent to the surface of atomically flat mica substrate using atomic force microscopy.25 The periodicity of layers was observed to be 0.7−0.8 nm, which is comparable to the length of an imidazolium cation. A similar study has been carried out by Bovio et al., with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide on a series of atomically flat surfaces, which confirms the existence of ordered structures of ionic liquids using atomic force microscopy (AFM).16 However, there is no evidence of ordered structures could be found on highly oriented pyrolitic graphite (HOPG) substrates using AFM. Carmichael et al. has employed X-ray reflectivity to describe the phase change in the interface of imidazolium-based ionic liquid and silicon substrate. This study presents the X-ray diffraction peak at 10.7° for the 1-butyl-3-methylimidazolium cation-based ionic liquid.26 In the present study, we observed a peak at 12.2° corresponding to the 0.7 nm d-spacing. This observation is in good agreement with the literature and confirms the existence of solidlike ordering of [BMIM] [TFSI] on the surface of carbon nanomaterials (graphene and fMWNTs). The peak at 12.2° shows the full width half maxima of 3.6°, which is corresponding to the crystallite size of 2.3 nm. Hence, it can be concluded that, as an average, three solidlike layers of ionic liquid exist on the surface of carbon nanomaterials. C

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Figure 2. (a) SEM, (b,c) TEM, and (d) EDAX analysis of f-MWNTs/HEG/[BMIM] [TFSI] ternary nanocomposite.

stretching. The vibrational modes of [TFSI] anion comes from the major contribution of CF3 symmetric stretching (1260 cm−1) along with other Raman active vibrational modes.32 We observed a small shift in G, D, and 2D bands of Raman

Figure 3. Nitrogen adsorption−desorption isotherm (BET) of fMWNTs/HEG nanocomposite. Inset: Pore volume vs pore radius using the BJH method.

IG value of 0.793. G-band corresponds to the tangential modes (E2g) of vibrations and the D-band corresponds to the presence of defect in MWNTs.30,31 Part b of Figure 4, Raman spectra of [BMIM] [TFSI], shows the characteristic vibrational modes in CH stretching region (2800−3000 cm−1), where 2958 cm−1 is assigned to the CH stretching vibrations of the butyl chain of the [BMIM] cation. The strong Raman band observed at 1431 cm−1 is assigned to imidazolium ring in-plane antisymmetric

Figure 4. Raman spectrum of (a) f-MWNTs/HEG, (b) [BMIM] [TFSI], and (c) f-MWNT/HEG/[BMIM][TFSI] nanocomposites. D

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spectrum of f-MWNTs/HEG in the presence of ionic liquid (part c of Figure 4), numerically, 9.9, 8.9, and 21.1 cm−1, respectively. Because resolution of the Raman spectrometer is 3 cm−1, these shifts are significant. These shifts may be attributed to the presence of solidlike ordering of ionic liquid cations at the solid−liquid interface, which provides a net compression on the surface of carbon nanostructures.16 Schadler et al. have reported that the shift in the Raman 2D peak position with compressive strain is about 7 wave numbers/percentage applied strain.33 Hence, it can be inferred from the shift in 2D band (21.1 cm−1) that the compression, applied by the solidlike layering of [BMIM][TFSI] ionic liquid thin films, is around 3% of strain of graphene. There was no shift observed in Raman modes corresponding to [BMIM] [TFSI] in the spectrum of nanocomposite. 3.5. Fourier Transform Infrared Spectrogram Analysis. The vibrational characteristics of nanocomposites of interest have been determined by Fourier Transform Infrared (FTIR) spectrum analysis. FTIR spectra of f-MWNTs/HEG, [BMIM] [TFSI] and f-MWNTs/HEG/[BMIM][TFSI] nanocomposites are shown in Figure 5. The band corresponding to stretching

both f-MWNTs/HEG and [BMIM] [TFSI] only. Here, the alkyl chain of the cation physically interacts parallel with the surface of graphene and the imidazolium rings are slightly tilted to the surface.27 3.6. Electrochemical Analysis. The electrochemical analysis has been carried out at ambient conditions using a more practical two-electrode supercapacitor setup and performance is reported in terms of mass of active electrode materials (f-MWNTs/HEG). The comparison of cyclic voltammograms (Figure 6) of the materials of interest is shown at 10 mV/s scan

Figure 6. Cyclic voltammograms of supercapacitor based on (a) fMWNT/HEG/[BMIM][BF4], (b) f-MWNT/HEG/[BMIM][TFSI], and (c) f-MWNT/HEG nanocomposites.

rate with a two-electrode supercapacitor setup. Initially, 100 cycles were scanned at 100 mV/s scan rate to stabilize the performance of capacitance for each studied material. Specific capacitance (Cs) was measured using the equation-

Cs = 2 ×

Figure 5. FTIR spectra of (a) f-MWNT/HEG, (b) [BMIM] [TFSI], and (c) f-MWNT/HEG/[BMIM][TFSI] nanocomposite.

I m×

dV dt

(1)

where, I is the average current, dV/dt is the potential sweep rate, and m is the mass of active electrode material at each electrode. A factor of 2 is incorporated due to the series capacitance formed in two-electrode system.1 Part a of Figure 6 shows the cyclic voltammogram of the fMWNTs/HEG/[BMIM] [BF4] ternary nanocomposite, which shows the potential window less than 2 V. Though the electrochemical window of dry [BMIM][BF4] was 4.10 V, the presence of moisture reduces it to 1.95 V. Schroder et al. demonstrated a considerable reduction in both the anodic and cathodic limits of ionic liquids upon the presence of water.11 Hence, the hydrophilicity of ionic liquid plays a major role in potential window of electrolyte. We chose [BMIM] [TFSI], a hydrophobic ionic liquid, as electrolyte. Part b of Figure 6 shows the cyclic voltammogram of fMWNTs/HEG/[BMIM] [TFSI] ternary nanocomposite, where [BMIM] [TFSI] is used as electrolyte. It is clear that [BMIM] [TFSI] has wide potential window (3.5 V) even at ambient conditions due to its hydrophobic nature. The curve shows rectangular behavior which ensures the better supercapacitive behavior with specific capacitance of 254 F/g in terms of mass of active electrode material. This performance is 36.22% higher than that of supercapacitor with f-MWNTs/ HEG electrode where [BMIM] [TFSI] was used as electrolyte. Further, the cyclic voltammetric curve (part c of Figure 6) of fMWNTs/HEG electrode shows slow transient responses at both ends, that is a slow charge storage and delivery kinetics.

vibrations of hydroxyl group occurs at 3440 cm−1, whereas the antisymmetric and symmetric stretching vibrations of =CH2 occur at 2925 and 2853 cm−1 respectively for f-MWNTs/HEG. This can be attributed to the presence of functional groups in fMWNTs and the residual oxygen containing functional groups in HEG. The vibrational spectrum of dry [BMIM] [TFSI] are shown in part b of Figure 5, which confirms the structure. In brief, the presence of −CH2− (2944, 2872 cm−1), C-aromatic carbon (3085, 3148, 1572 cm−1), C−CH3 (2969, 1460 cm−1), and the side chain −CH2−CH2−CH2− (741 cm−1) groups are identified by the FTIR spectrum.34 The strong peak appearing at 1353 cm−1 belongs to the sulfonamide group of the ionic liquid anion. There is no significant peak observed corresponding to −OH stretching for dry [BMIM] [TFSI], which is a consequence of the hydrophobic nature of [BMIM] [TFSI]. This can be attributed to the low self-diffusion coefficient (on the order of 10−11m2/s) of water + [BMIM] [TFSI] system.35 However, a slight deviation from the baseline occurs around 3400 cm−1, which may be attributed to the presence of moisture on the surface of ionic liquid. The FTIR spectrum of f-MWNTs/HEG/[BMIM][TFSI] shows no significant new vibrational signal other than that for fMWNTs/HEG and [BMIM] [TFSI], which indicates that there is no significant chemical interaction of [BMIM] [TFSI] with f-MWNTs/HEG and shows the peaks corresponding to E

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method using (1) where dV/dt was obtained from the slop of the straight line fit of discharge curves from Vmax to Vmax/2, which is a widely accepted method to calculate capacitance.37 Here, the intercept of linear fit with potential axis gives the potential drop (Vdrop) due to RESR. The performance of the supercapacitor with charge−discharge cycles is reported with different current at the Coulombic efficiency greater than 90%. Because the supercapacitors, based on neat carbon nanomaterials (carbon nanotubes and/or graphene) have been reported in literature,12,38,39 it is worth to do a comparative study on carbon nanomaterials/ionic liquid nanocomposites by the same method. In this regards, we have studied the discharge behavior of HEG/[BMIM] [TFSI], f-MWNTs/[BMIM] [TFSI], and f-MWNTs/HEG along with f-MWNTs/HEG/ [BMIM] [TFSI] nanocomposites at specific current of 2 A/g (Figure 9). We observed a better charge transport behavior in

This may be attributed to the slow diffusion of large sized ionic liquid ions into the porous electrode. The diffusion is limited due to discontinuities occurs in the paths of ions. However, in the case of f-MWNTs/HEG/IL, the presence of solidlike layers of ionic liquid in the electrode material effectively increases the continuity of ion diffusion path. The enhancement in performance may be attributed to three major reasons: first, the presence of solidlike layers of ionic liquid in electrode material, which enhances the electrolyte accessibility and kinetics of ion transport by effectively increasing the number of ion diffusion paths inside the fMWNT/HEG electrode and also decreases the interfacial resistance between the electrolyte and the active electrode materials, thereby hence improving the performance. Second, the large amount of wrinkles present in the surface of HEG, which enhances the amount of mesopores that can be accessible to the large ionic liquid molecules. Third, presence of f-MWNTs prevents the face-to-face agglomeration as a smart spacer, which further enhances the amount of mesopores and accessible surface area of the nanocomposite. In addition, fMWNTs play a role as a bridge for electron transfer between graphene layers.36 Figure 7 reveals the effect of scan rate on specific capacitance, which shows the decrease in capacitance with scan rate. This is

Figure 9. Comparison of discharge behavior of (a) f-MWNTs/HEG/ [BMIM] [TFSI], (b) HEG/[BMIM] [TFSI], (c) f-MWNTs/[BMIM] [TFSI] and (d) f-MWNTs/HEG nanocomposites at specific current of 2 A/g. Inset: Bar chart comparison of performance of the nanocomposites.

the case of f-MWNTs/HEG and f-MWNTs/HEG/[BMIM] [TFSI] nanocomposites as the discharge curves are nearly linear. In the case of HEG/[BMIM] [TFSI] and f-MWNTs/ [BMIM] [TFSI], the discharge curves are more curved, which can be assigned to the slow charge delivery of the device. The pore size distribution and occupancy of pores by electrolyte are the primary factors in determining equivalent series resistance, thus the charge transport behavior.1 Presence of f-MWNTs results in maxima of pore distribution in mesoporous region, thus the nanocomposite can accommodate large sized electrolyte ions more conveniently. But the relatively low capacitance of f-MWNTs/HEG than that of f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite can be attributed to the hurdles in accessing inner pores of electrode material. The straight line fit from Vmax to Vmax/2 shows that HEG/[BMIM] [TFSI] and fMWNTs/[BMIM] [TFSI] gives relatively low capacitance which can be attributed to the face-to-face agglomeration of HEG in the presence of ionic liquid and less surface area of fMWNTs, respectively. Figure 10 shows the specific capacitance as a function of specific current. The f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite shows specific capacitance of 201 F/g at specific current of 2 A/g. The increase in current results in the decrease in capacitance due to the rapid motion of massive ionic liquid ions, which results in poor accommodation of ions inside the pores of active electrode material. At high currents, the ions may not get sufficient time to diffuse into the pores of active

Figure 7. Specific capacitance of f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite as a function of scan rate.

the direct consequence of mobility of massive electrolyte ions and the viscosity of electrolyte. Figure 8 shows the discharge behavior of f-MWNTs/HEG/ [BMIM] [TFSI] nanocomposite with different specific current. Specific capacitance (Cs) was measured by charge−discharge

Figure 8. Discharge behavior of f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite with different specific currents. F

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Because the voltage decay is faster during a discharge at high power, the Ragone plot drops faster with the power. The maximum available power (Pmax) and the maximum stored energy (Wmax) in the supercapacitor have been extracted by fitting the Ragone equation to the experimental data. The Ragone equation is represented as, WL =

Wmax ⎛ ⎜⎜1 + 2 ⎝

1−

PL ⎞ ⎟⎟ Pmax ⎠

(4)

where WL is available energy to the external load, PL is the available power to the load, Wmax is the maximum stored energy, and Pmax is maximum available power.40 The fitting parameters reveal the values of Pmax and Wmax and are found to be 148.43 kW/kg and 170.66 Wh/kg, respectively. The Pmax and Wmax can be related by

Figure 10. (a) Specific capacitance of the f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite as a function of specific current. Inset: Cyclic stability of the nanocomposite at specific current of 10 A/g.

τo =

electrode material, as a consequence of large size of ions. This may result in a simple polarization followed by the accumulation of charges at the surface of the electrode rather than accommodate inside the pores. The inset in Figure 10 shows the cyclic stability of the nanocomposite with charge− discharge cycles, which clearly shows that the nanocomposite retains more than 98% of its initial capacitance even after 1000 cycles with the specific current of 10 A/g. The Ragone plot (Figure 11) of the f-MWNTs/HEG/ [BMIM] [TFSI] nanocomposite electrode-based supercapaci-

Wmax 2Pmax

(5)

where, τo (=RESRC) is the time constant, that is the time necessary to discharge the capacitor to 36.8% of its initial voltage in a short.40 The time constant has been evaluated using (5) and found to be 0.575 s. The lower time constant indicated the fast charge−discharge functioning of the device. Here. RESR (=Vdrop/I), the equivalent series resistance, is composed of an ionic and an electronic part. The ionic contribution comes from the mobility of the ions in the electrolyte and electrode− electrolyte interface resistance, whereas the electronic contribution comes from the ohmic resistance in the current collector and electrode. The inset in Figure 11 shows the power dissipated in the equivalent series resistance (PESR = I2RESR) of the supercapacitor with respect to specific current. Figure 12 shows Nyquist plots of different nanocomposite electrode material-based supercapacitor, whereas the inset

Figure 11. Ragone plot of f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite-based supercapacitor. Inset: Power dissipated in equivalent series resistance RESR (PESR).

tor relates the energy density and power density. The energy density (E) and power density (P) have been calculated using (2) and (3). 1 E = CcellV 2 (2) 2 P=

E Δt

Figure 12. Nyquist plot of (a) f-MWNTs/HEG/[BMIM] [TFSI], (b) f-MWNTs/[BMIM] [TFSI], (c) HEG/[BMIM] [TFSI], and (d) fMWNTs/HEG. Inset: R ESR of corresponding nanocomposite (amplitude, 5 mV; range, 1 mHz to 0.1 MHz).

(3)

shows the equivalent series resistance of corresponding nanocomposite. In the lower frequency region, f-MWNTs/ HEG/[BMIM] [TFSI] nanocomposite supercapacitor shows better capacitive behavior, whereas the frequency response of fMWNTs/HEG nanocomposite shows a shift toward Warburg impedance. This may be assigned to the relatively low electrolyte accessibility of f-MWNTs/HEG nanocomposite. In the high frequency region, Nyquist plot shows the intercept with real axis at 2.8. 4.3, 5.2, and 5.6, which reveals the equivalent series resistances (RESR, in ohms) for f-MWNTs/

where Ccell is the specific capacitance of the total cell, V is the cell potential, and Δt is the discharge time for the potential drop V.1 Figure 11 shows the energy and power densities in extremely high scale as a consequence of wide potential window of ionic liquid electrolyte. The specific capacitance, energy density, and specific power density of the nanocomposite at 2 A/g specific current are 201 F/g, 170.9 Wh/kg, and 5.71 kW/kg, respectively, in terms of mass of active electrode material. G

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HEG/[BMIM] [TFSI], f-MWNTs/[BMIM] [TFSI], HEG/ [BMIM] [TFSI], and f-MWNTs/HEG, respectively. It is clear that the f-MWNTs/HEG/[BMIM] [TFSI] nanocomposite has much lesser RESR than f-MWNTs/HEG, which is a consequence of better electrolyte accessibility resulted by the presence of solidlike ionic liquid layers. It is important to notice that HEG/[BMIM] [TFSI] has more RESR than f-MWNTs/ [BMIM] [TFSI] nanocomposite. This may be due to the planar morphology of HEG that is prone to aggregate in the presence of ionic liquid, which results in the reduction in accessible surface area and limited interaction with electrolyte, whereas the randomly entangled tubular morphology of f-MWNTs provides the better interaction with large sized ionic liquid molecules.

4. CONCLUSIONS The ultra high performance supercapacitor based on fMWNTs/HEG/[BMIM] [TFSI] ternary nanocomposite electrode has been demonstrated, which shows extremely high energy density and power density. The performance enhancement has been mainly attributed to the presence of solidlike layers of ionic liquid in the electrode material and the wrinkled nature of graphene. In addition, the presence of solidlike layering of ionic liquid facilitates fast ion transport by the providing shortest directional path for ion diffusion. The hydrophobic ionic liquid ([BMIM] [TFSI]) shows a wide potential window (3.5 V) even at ambient conditions, which ensures stability of the device. The specific capacitance and energy density of the nanocomposite at 2 A/g specific current are 201 F/g and 171 Wh/kg, respectively. The maximum stored energy and maximum available power were extracted from Regone plot and found to be 148.43 kW/kg and 170.66 Wh/ kg, respectively. The low time constant (0.575 s) ensures the fast charge−discharge ability of the device. Nyquist plot of fMWNTs/HEG/[BMIM] [TFSI] shows that RESR is 2.8 Ω. This supercapacitor device retains 98% of its initial capacitance after 1000 cycles, which ensures good cyclic stability. Hence, the f-MWNTs/HEG/[BMIM] [TFSI] ternary nanocomposite can be a promising electrode material in large scale production of high performance supercapacitors.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Indian Institute of Technology Madras (IITM), Chennai for the financial supports and SAIFIITM for FTIR analysis.

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REFERENCES

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