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‘Bucky gel’ of multiwalled carbon nanotubes as electrodes for high performance, flexible electric double layer capacitors

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 465704 (http://iopscience.iop.org/0957-4484/24/46/465704) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 24 (2013) 465704 (10pp)

doi:10.1088/0957-4484/24/46/465704

‘Bucky gel’ of multiwalled carbon nanotubes as electrodes for high performance, flexible electric double layer capacitors Manoj K Singh, Yogesh Kumar and S A Hashmi Department of Physics and Astrophysics, University of Delhi, Delhi-110007, India E-mail: [email protected]

Received 11 July 2013, in final form 28 August 2013 Published 24 October 2013 Online at stacks.iop.org/Nano/24/465704 Abstract We report the preparation of a gelled form of multiwalled carbon nanotubes (MWCNTs) with an ionic liquid 1-butyl-1-methyl pyrrolidinium bis(trifluoromethane sulfonyl)imide (BMPTFSI)), referred to as ‘bucky gel’, to be used as binderless electrodes in electrical double layer capacitors (EDLCs). The characteristics of gelled MWCNTs are compared with pristine MWCNTs using transmission electron microscopy, x-ray diffraction and Raman studies. A gel polymer electrolyte film consisting of a blend of poly(vinylidene fluoride-cohexafluoropropylene) and BMPTFSI, exhibiting a room temperature ionic conductivity of 1.5 × 10−3 S cm−1 , shows its suitability as an electrolyte/separator in flexible EDLCs. The performance of EDLCs, assembled with bucky gel electrodes, using impedance spectroscopy, cyclic voltammetry and charge–discharge analyses, are compared with those fabricated with pristine MWCNT-electrodes. An improvement in specific capacitance (from 19.6 to 51.3 F g−1 ) is noted when pristine MWCNTs are replaced by gelled MWCNT-binderless electrodes. Although the rate performance of the EDLCs with gelled MWCNT-electrodes is reduced, the pulse power of the device is sufficiently high (∼10.5 kW kg−1 ). The gelled electrodes offer improvements in energy and power densities from 2.8 to 8.0 Wh kg−1 and 2.0 to 4.7 kW kg−1 , respectively. Studies indicate that the gel formation of MWCNTs with ionic liquid is an excellent route to obtain high-performance EDLCs. (Some figures may appear in colour only in the online journal)

1. Introduction

and exploited at the commercial scale. AC-based EDLCs deliver a high specific capacitance (and hence a high specific energy); however, they encounter various drawbacks, including poor cyclability [5, 11]. Only 10–20% of the maximum theoretical capacitance can be achieved in practice, due to the dominant presence of micropores which are inaccessible to the solvated ions of electrolytes and wetting limitations of the electrolyte on the electrode surface [12]. In recent years, single- or multiwalled carbon nanotubes (SWCNTs or MWCNTs) have attracted great attention as potential supercapacitor electrodes [13–15]. CNTs are well

Supercapacitors are unique charge storage devices, which are complementary to power sources such as rechargeable batteries and fuel cells [1–3]. Among the different types of supercapacitor, symmetrical electric double layer capacitors (EDLCs) are the most widely reported devices, employing various forms of carbon, such as activated carbons [4–8], carbon xerogels [9], and carbon nanofibers [10], as electrode materials. Activated carbons (ACs) in the forms of powder, fabric, fiber, etc are predominantly used as EDLC-electrodes 0957-4484/13/465704+10$33.00

1

c 2013 IOP Publishing Ltd Printed in the UK & the USA

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M K Singh et al

known to have a cylindrical nano-sized structure consisting of rolled graphene sheets built from sp2 carbon atoms, with ordered and uniform mesopores (2–50 nm size) [16, 17]. The high electron conductance due to the large π-electron surface and their mesoporous character make CNTs an attractive electrode material for EDLCs. Low mass density, high mechanical strength, moderate surface area, chemical and thermal stability, and high ion diffusion are some additional important characteristics of CNTs which indicate their suitability as EDLC-electrodes [18]. CNTs are not only used in EDLCs but also as electrodes in many other electrochemical devices, including rechargeable batteries, fuel cells and sensors [19–22]. Although the overall performance of SWCNTs is more attractive, MWCNTs are cost effective, favoring their practical application in the fabrication of EDLCs. Apart from the many advantages described above, poor wettability and dispersibility in different liquid media are the major drawbacks of CNTs, limiting their processability for many applications [23, 24]. For example, as graphene sheets are hydrophobic in nature and have attractive van der Waals forces between them, CNTs (the rolled form of graphene layers) have a tendency to get entangled heavily in aqueous media, forming bundles/agglomerates, and thus showing poor dispersibility [25]. Many reports exist in the literature in which the CNTs are physically, chemically and electrochemically modified by functionalizing them with suitable functional groups to improve their dispersibility for easy processing [26, 27]. Ionic liquids (ILs), which are basically molten salts composed of bulky organic cations and small inorganic anions, have been used in different ways to modify various electrodes for different applications, including sensors and biosensors [28–30]. Ionic liquids are reported as being used to modify the electrodes as droplets/films deposited on them, composite electrodes containing ILs as one of the components, carbon paste electrodes with ILs as a binder, electrodes modified with attached ILs, and electrodes prepared from the gelation of carbon nanotubes by ILs [30]. Recently, Fukushima et al [31] demonstrated the gel formation of SWCNTs with imidazolium-ion-based room temperature ILs (RTILs) referred as a ‘bucky gel of ionic liquids’. According to their report, the bundles of SWCNTs, which are heavily entangled three-dimensional networks, are exfoliated during the process of gelation with the RTILs. There exist various reports on the bucky gel formation of SWCNTs, primarily with imidazolium-based RTILs [32–34]. Several gelled forms of MWCNTs and mesoporous carbons have also been reported with imidazolium RTILs for application as electrodes for bioanalytical applications [30]. Kachoosangi et al [35] recently investigated a paste electrode of MWCNTs with an ionic liquid N-butyl N-methyl pyrrolidinium bis(trifluoromethyl sulfonyl)imide for electrochemical applications, although the gel formation of MWCNTs was not indicated in that report. Supercapacitors/EDLCs based on a gelled form of CNTs are hardly mentioned in the literature, except for a few reports. For example, an EDLC with a ‘bucky gel’ of SWCNTs has recently been reported by Katakabe et al [32], showing improvements in terms of high specific

capacitance and lower electrode resistance as compared to pristine SWCNTs. Supercapacitors of the latest generation employ polymerbased electrolytes as separators to substitute the liquid electrolytes [17, 36] in order to overcome various associated problems (leakage, etc) and to develop portable and flexible devices with a high specific energy/power. Gel polymer electrolytes (GPEs), composed of liquid electrolytes (e.g. lithium salts in organic solvents or ionic liquids) entrapped in a host polymer, are primarily used in such flexible supercapacitors. Ionic liquids immobilized in poly(vinylidene fluoride-co-hexafluoropropylene) (PVdFHFP) are one of the widely reported classes of GPEs, as these are high ion-conducting free-standing flexible films, employed in supercapacitors [5]. Binary combinations of polymers/ILs are not useful electrolytes in battery applications due to the absence of target ions (e.g. Li+ , Na+ ion, etc); however, they have sufficient amounts of charge for capacitive storage at the interfaces. In this paper, we report the preparation and characterization of MWCNT-electrodes gelled with an IL 1-butyl1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide, (BMPTFSI) for application in EDLCs. Comparative performance studies are presented on flexible, quasi-solid-state EDLCs fabricated using the binderless electrodes of a bucky gel of MWCNTs and pristine MWCNTs (with PVdF-HFP as binder). A polymeric gel film composed of BMPTFSI entrapped in PVdF-HFP has been used as a separator/electrolyte to fabricate flexible EDLCs. The gelled MWCNT-electrodes were characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD) and Raman studies. The performance characteristics of EDLCs were evaluated using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and constant current charge–discharge tests over numerous cycles.

2. Experimental details 2.1. Preparation of gel polymer electrolyte and EDLC-electrodes The copolymer PVdF-HFP (average MW ∼400 000) and ionic liquid BMPTFSI were obtained from Sigma-Aldrich and dried in vacuum at ∼80 ◦ C before use. A solution-cast method was adopted to prepare the gel polymer electrolyte film. The polymer PVdF-HFP was dissolved in acetone and the ionic liquid BMPTFSI was mixed with the PVdF-HFP/acetone solution. The mixture was stirred magnetically for ∼12–14 h, thereafter it was poured in glass Petri dishes, and the acetone was allowed to evaporate slowly. The ratio of the ionic liquid to polymer was kept at 80:20 weight/weight. Finally, a semi-transparent free-standing flexible GPE (thickness ∼200–250 µm) was obtained. The gel film was stored in a dry chamber to avoid moisture adsorption. A powder of MWCNTs was procured from SigmaAldrich, having a specification of OD × ID × L = 10–15 nm × 2–6 nm × 0.1–10 µm, with an average wall thickness of 5–15 graphene layers. The MWCNT-powder was 2

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Table 1. Porosity parameters of the MWCNTs (pristine) and the gelled MWCNT-1 and MWCNT-20 electrode materials. Electrode code

Sample composition

BET surface area (m2 g−1 )

Total pore volume (cm3 g−1 )

MWCNT MWCNT-1 MWCNT-20

MWCNT (pristine) MWCNT:BMPTFSI (99:1 w/w) MWCNT:BMPTFSI (80:20 w/w)

261 93 113

0.70 0.35 0.43

Dielectric/Impedance Analyser (C-50 Alpha A, Novocontrol, Germany) over the frequency range from 0.1 Hz to 100 kHz for the temperature range from 15 to 90 ◦ C, varied at a heating rate of 1 ◦ C min−1 in a dynamic N2 atmosphere. The ESW of the film was evaluated by linear sweep voltammetry using an electrochemical analyzer (608C, CH Instruments, USA) at a scan rate of 5 mV s−1 . Brunauer–Emmett–Teller (BET) surface area and porosity measurements of MWCNTs (before and after gelation) were evaluated using a Surface Area and Pore Size Analyzer (Gemini-V, Micromeritics, USA). X-ray diffraction (XRD) patterns of pristine and gelled MWCNTs were recorded with a high-resolution x-ray diffractometer (D8-Discover, Bruker, USA) using Cu Kα radiation in the Bragg angle (2θ ) range from 10◦ to 80◦ . A Renishaw Invia Raman spectrometer, equipped with a charge-coupled device (CCD) detector, was employed for Raman measurements. An Ar-ion laser with a 514.5 nm wavelength and a spot diameter of 1–2 µm was used to excite the samples. TEM images of the MWCNTs were obtained using a transmission electron microscope (Tecnai G2T30, U-TWIN) in which a LiB6 filament was used as the electron source and the working voltage was kept in the range 50–100 kV. The samples for TEM were prepared on carbon-coated copper grids. The performance characteristics of the EDLCs were evaluated using EIS, cyclic voltammetry (CV) and galvanostatic charge–discharge tests. The EIS studies were performed in the frequency range from 10 mHz to 100 kHz using the electrochemical analyzer mentioned above. The CV responses were also recorded with the same electrochemical analyzer. The charge–discharge test was performed at constant current using a charge–discharge unit (BT-2000, Arbin Instruments, USA).

Figure 1. Schematic representation of the fabrication of an EDLC assembly with a gel polymer electrolyte.

vacuum dried at ∼100 ◦ C overnight before its use as capacitor electrodes. The gel of MWCNTs was prepared by grinding a mixture of MWCNT-powder and ionic liquid BMPTFSI, taking the ratio of 99:1 and 80:20 w/w. After a slow and thorough grinding for about half an hour with a mortar and agate, a gelled form of the MWCNT/IL mixture was obtained. A small amount of N-methyl-2-pyrrolidone (NMP) was added and ground again to obtain the slurries. These slurries were coated on flexible high-density graphite sheets (25 µm thick, Nickunj Eximp Entp., India). These electrodes were vacuum dried overnight at ∼100 ◦ C before using them as EDLC-electrodes. Electrodes of pristine MWCNTs were also prepared for comparison by adding ∼20 wt% of a binder PVdF-HFP following the above-mentioned process. Different electrode compositions are coded for rest of the text and the coded names are given in table 1. 2.2. Fabrication of EDLC cells In order to construct a symmetrical configuration of EDLCs, the dilute slurry of gel polymer electrolyte was coated on the electrodes and dried in vacuum. Two such systems were stacked over each other (as schematically shown in figure 1) to obtain fully symmetrical EDLC cells. Cell configurations with gel polymer electrolyte (ILGPE) and three types of electrodes are given as follows:

3. Results and discussion 3.1. Characterization of electrode materials

Cell#1: MWCNT | ILGPE | MWCNT Cell#2: MWCNT-1 | ILGPE | MWCNT-1 Cell#3: MWCNT-20 | ILGPE | MWCNT-20.

Figure 2 shows typical comparative N2 -adsorption– desorption isotherms of the MWCNTs (pristine) and gelled MWCNTs (MWCNT-20) recorded at 77 K. Both the materials show almost similar patterns of isotherms for the entire relative pressure (P/P0 ) range from 0.005 to 0.95, which are categorized as type-IV isotherms according to the IUPAC system [15]. A sudden small initial jump was observed in the isotherm of the MWCNT (pristine)-powder (figure 2(a)), indicating a small proportion of microporosity in the material. This is due to the presence of residual carbon as an impurity in the MWCNT-powder. This initial jump in the isotherm is significantly reduced (figure 2(b)), indicating a substantial

2.3. Instrumentation The electrochemical performance of the GPE film was tested by measuring the ionic conductivity and electrochemical stability window (ESW). The ionic conductivity was evaluated by electrochemical impedance spectroscopy (EIS). The impedance spectra were recorded using a Broadband 3

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M K Singh et al

their swelling/exfoliation) become mesoporous with almost no microporosity. Figure 3 shows the TEM images and XRD patterns of MWCNTs (pristine) and gelled MWCNTs (MWCNT1 and MWCNT-20). On grinding the MWCNTs with a small amount (∼1 wt%) of BMPTFSI, the heavily entangled MWCNT bundles get converted into finer bundles (figure 3(B)). Possible interactions between π -electrons of carbon and π -cations of ionic liquid are responsible for such changes and cross-linking of these finer bundles [33]. The IL is entrapped in these cross-linked networks to form the gel [32]. The addition of more IL (∼20 wt%) during the grinding process of MWCNTs leads to swelling (i.e. an increase in diameter of tubes) of the bundles followed by their exfoliation (i.e. peeling of tube walls). Some of the tubes even appear to be broken, as indicated in the TEM picture (figure 3(C)). It may be noted that all these modifications in the texture of MWCNTs are physical changes [31, 37], as no significant crystallographic changes are observed due to the gelation of MWCNTs with IL, as indicated from the comparative XRD results (figure 3(D)). Comparative Raman spectral studies have been performed on pristine MWCNTs and their gelled form with 1 and 20 wt% of BMPTFSI (figure 4(A)). The intense peaks of the ionic liquid at 2256, 2951 and 2983 cm−1 , which are observed in the spectral range of 500–3200 cm−1 , disappear or become almost negligible when MWCNTs are ground with IL (figures 4(A)–(C)). Similar behavior has been reported when styrene–butadiene rubber (SBR) is thoroughly mixed with MWCNTs and, as a result, all the Raman bands of

Figure 2. N2 -adsorption–desorption isotherms of (a) pristine MWCNTs, and (b) gelled MWCNT-20 electrode material measured at 77 K.

reduction in microporosity in the gelled MWCNTs. This is a possible reason for the observed lowering in the BET specific surface area and total pore volume of gelled MWCNTs as compared to the pristine material (table 1). Furthermore, the gradual increase in the isotherms with respect to the relative pressure and the presence of hysteresis loops during N2 -desorption indicate that the pristine or gelled MWCNT materials possess predominantly mesoporous interiors. The studies indicate that the gelled MWCNTs (after

Figure 3. TEM micrographs of (A) pristine MWCNTs and gelled MWCNTs: (B) MWCNT-1 and (C) MWCNT-20. (D) shows XRD patterns of (a) pristine MWCNTs, (b) MWCNT-1 and (c) MWCNT-20. 4

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Figure 4. (A) Comparative Raman spectra of (a) pristine MWCNT, (b) MWCNT-1, (c) MWCNT-20, and (d) pure ionic liquid BMPTFSI. (B) and (C) expanded representation of the D- and G-bands, respectively. Table 2. Raman bands and their assignments in the pristine MWCNTs. Raman bands

Peak positions (cm−1 )

Assignments/remarks

References

D

1320–1350

[38, 39]

G

1570–1585

D0

1602–1625

G0 or 2D

2640–2700

Attributed to sp2 hybridized disorder and defects in curved graphene sheet, sensitive to the nanotube alignments, provide information about the amount of impurities and degree of disorder In-plane vibrations of the graphitic wall, not very sensitive to the orientation of nanotubes, provide information about the amount of impurities and degree of disorder Tetrahedral sp3 band (also indicative of sp2 bonded carbon representing surface defect modes), from the intravalley, defect-induced, double-resonance process Originate from defect-free sp2 carbons, not sensitive to the nanotube alignments

SBR disappear [38]. The expanded representations of Raman spectra in the spectral ranges from 1250 to 1450 cm−1 and from 1500 to 1750 cm−1 are respectively shown in figures 4(B) and (C). The primary signatures of the carbon nanotube structures are the four important Raman bands, namely D, G, D0 and G0 (or 2D). Their peak positions along with their assignments are given in table 2. It may be noted that on the gelation of MWCNTs with ∼1 wt% of BMPTFSI, no significant changes in the D and G bands, which are sensitive to the tube alignment and impurities, have been observed. This further confirms the physical changes obtained in the MWCNTs, observed in the form of finer bundles. Significant changes in the D and G bands are observed in terms of their shifts towards lower wavenumbers by 5–6 cm−1 when ∼20 wt% ionic liquid is ground with MWCNT for gelation. These shifts in the Raman bands towards lower energy are direct evidence of weakening in the C–C bonds due to interactions with the IL. The weakening of the C–C bonds leads to an elongation in the bond lengths, resulting in swelling in the nanotubes. This leads to the exfoliation of CNTs, observed at different portions of TEM images (figure 3(C)). It may be noted that such changes at the molecular level are not observable in x-ray diffraction in terms of peak positions or full width half maxima (FWHM),

[38]

[39, 40]

[38, 41]

suggesting that the addition of IL does not influence the structures of MWCNTs significantly [39]. 3.2. Electrochemical properties of GPE The ionic conductivity of the GPE PVdF-HFP/BMPTFSI is observed to be ∼1.5 × 10−3 S cm−1 at 25 ◦ C, which is close to that of pure BMPTFSI (i.e. ∼2.4 × 10−3 S cm−1 ) [42]. Figure 5(a) shows the temperaturedependent ionic conductivity (i.e. log σ versus 1/T plot) of the GPE film. A curved nature of the log σ versus 1/T plot has been found, which can be described by the non-Arrhenius Vogel–Tammann–Fulcher (VTF) relationship:   −B σ = AT −1/2 exp (1) T − To where the parameter B is a pseudo-activation energy which is associated with the critical free volume for the ion transport, A is the pre-exponential factor and To is the equilibrium glass transition temperature. The ln(σ T 1/2 ) versus 1/(T − To ) plot shows a linear variation, as shown in figure 5(a) (inset). These parameters B, A and To have been evaluated using nonlinear least squares (NLS) fitting of the data and found to be 0.056 eV, 6.24 S cm−1 K−1/2 and 180 K, respectively. 5

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Table 3. Electrical parameters of EDLC cells from the impedance analysis. EDLC cells

Rb ( cm2 )

Rct ( cm2 )

Csp Ra ( cm2 ) (F g−1 )

Knee frequency (Hz)

Cell#1 Cell#2 Cell#3

3.1 4.2 4.2

0.24 0.14 1.13

341 176 56

117 117 31

a

19.6 24.9 51.3

Overall resistance of EDLC cell at noted 10 mHz.

accompanied by high-frequency semicircular features owing to the bulk property of GPE and the electrode–electrolyte interface, as shown in the expanded representation of the impedance responses (figure 6(B)). The values of bulk resistance (Rb ) and interfacial charge-transfer resistance (Rct ) of different capacitor cells have been evaluated from the intercepts at the real axis of the EIS responses. The value of capacitance (C) of each EDLC cell was evaluated by the expression: C = 1/(2π f |Z 00 |), where Z 00 is the imaginary part of the impedance (Z) of the cell, estimated at a frequency (f ) of 10 mHz. The specific capacitance Csp was evaluated using the expression: Csp = 2 C m−1 , where m is the mass of the single electrode. The values of Csp along with the resistive parameters Rb and Rct are listed in table 3. The bulk resistance values (Rb ) of the electrolyte are found to be almost the same, whereas the Rct and overall resistance (R) values are substantially different according to the electrodes employed in EDLCs. The Rct value reduces slightly when MWCNT-gel (MWCNT-1) is employed as the electrodes. This indicates that the finer bundles of MWCNTs (gelled with 1 wt% of IL) show easy charge transfer with GPE. On the other hand, the charge-transfer processes get hindered when a sufficiently swollen gel of MWCNTs (MWCNT-20) is used as the electrodes. It may be noted that the overall resistance (R) of the cell reduces substantially (table 3) when the binderless gelled MWCNTs form interfaces with GPE. This is due to the flexible nature of gelled electrodes and GPE, and hence the formation of proper interfacial contacts. The absence of the binder in the gelled electrodes is another factor responsible for reducing the resistance of the cell. Furthermore, a remarkable improvement has been observed in the specific capacitance (Csp ) (table 3) when gelled MWCNT-electrodes are employed to fabricate EDLCs. The enhancement in Csp and the reduction in overall resistance R ensure improvements in the energy and power densities of the device with gelled MWCNT-electrodes. In order to compare the rate capability and pulse power performance of the EDLCs, fabricated from the MWCNTs (pristine) and gelled MWCNTs (MWCNT-1 and MWCNT-20), parameters such as the knee frequency, characteristic response frequency (fo ) and response time (τo ) have been evaluated from the impedance responses. The knee frequency is the frequency below which the impedance response is observed to be predominantly capacitive (i.e. Z 00 rises steeply with respect to Z 0 ) [5]. The response time (τo ), according to Miller’s approach, is the reciprocal of the characteristic response frequency fo at which the Z 0 and Z 00 versus log f (Bode plots) cross each other [43]. A Bode

Figure 5. (a) Ionic conductivity (σ ) as a function of temperature for the GPE BMPTFSI/PVdF-HFP; the corresponding ln (σ T 1/2 ) versus 1/(T − To ) plot is depicted in the inset. (b) LSV response recorded at room temperature (25 ◦ C) at a scan rate of 5 mV s−1 .

The electrochemical stability window, ESW (i.e. working voltage range) for the GPE has been evaluated using linear sweep voltammetry (LSV) tested on the cell: SS| polymeric electrolyte| Ag, using stainless steel (SS) as a working electrode and Ag as both the counter and reference electrodes. It can be seen from figure 5(b) that the GPE film is electrochemically stable up to ∼3.7 V versus Ag, which is a sufficient range for electrochemical applications, particularly for EDLCs. 3.3. Performance of EDLCs As mentioned earlier, the comparative performance of EDLCs, fabricated from MWCNTs (pristine) and gelled MWCNTs, have been characterized using EIS, CV and charge–discharge tests. Figure 6(A) shows the EIS responses of different EDLC cells, indicating that both the MWCNTelectrode materials (pristine or gelled) show capacitive behavior when interfaced with the GPE, as reflected by the steeply rising behavior of the impedance responses in the low-frequency range. This capacitive impedance response is 6

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Figure 6. (A) EIS plots of different EDLC cells recorded at room temperature in the frequency range from 100 kHz to 10 mHz. (B) Expanded representations of the impedance plots in the high-frequency region. (C) Bode plot of a typical Cell#3. (D) Variation of the complex imaginary capacitance (C00 ) as a function of frequency.

influence the rate capability of the cell, as indicated from the approximately one-fourth reduction in the knee frequency (table 4). Further, the characteristic response frequency fo for MWCNTs (pristine)-based EDLCs, which significantly reduces (the response time τo increases accordingly) when gelled MWCNTs are used as electrodes (table 4). The available specific energy (Eo ) of each cell has been evaluated from the expression: Eo = 12 Co V 2 /M, where Co is the capacitance at fo and V is the rated voltage of the EDLCs (2 V in the present case). The pulse power density (Po ) of each cell has been evaluated from: Po = Eo /τo and listed in table 4. It may be noted that the pulse power density of the cells decreases when pristine MWCNT-electrodes are replaced by the gelled MWCNT-electrodes. However, this reduction in Po is not observed with the same ratio as that of the increase in τo . This is because of the gelled MWCNT-electrodes, which are of higher available specific energy (table 4). The cyclic voltammetry has been performed in a two-electrode configuration of each cell and presented in figure 7. The comparative CV patterns of the EDLC cells (Cell#1, Cell#2 and Cell#3), recorded at a scan rate of 100 mV s−1 (figure 7(a)), show higher currents for the capacitors with gelled electrodes as compared to those with pristine MWCNT-electrodes (with binder). The highest current and hence the largest specific capacitance value (49.8 F g−1 at 10 mV s−1 ) has been obtained for Cell#3 with binderless gelled MWCNT-electrodes (MWCNT-20). This observation is in agreement with the result obtained from impedance analysis (discussed earlier), which shows

Table 4. Characteristic parameters of the EDLC cells at fφ = −45◦ . Response EDLC frequency cells fo (Hz)

Available Response specific energy at time τo (s) fo Eo (Wh kg−1 )

Pulse power Po (kW kg−1 )

Cell#1 5.8 Cell#2 2.6 Cell#3 0.9

0.17 0.38 1.12

17.7 13.8 10.5

0.84 1.46 3.28

plot for the typical cell (Cell#3) is shown in figure 6(C). These parameters have been evaluated for each EDLC and are listed in table 4. The response time of each cell has also been estimated following the approach described by Taberna et al [44]. Accordingly, the imaginary capacitance value (C00 ) is plotted against log f (figure 6(D)) and the peak shape for each cell is obtained at the characteristic response frequency, the reciprocal of which is the response time (τo ). The response times evaluated from this approach (indicated in figure 6(D)) are almost in conformity with previously mentioned values following Miller’s approach as given in table 4. It may be noted that the knee frequency is not affected significantly when the gelled MWCNT-electrodes (MWCNT1) are applied instead of pristine MWCNT-electrodes (with binder). This indicates that the transformation of MWCNTs to their finer bundles, while the gelation process with a small amount (1 wt%) of IL does not affect the rate capability of the capacitor cell. On the other hand, substantially swollen MWCNTs with a high amount of IL (MWCNT-20) 7

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M K Singh et al

Figure 7. (a) Comparative cyclic voltammograms of EDLC cells at 100 mV s−1 , and CV responses of (b) Cell#1, (c) Cell#2 and (d) Cell#3 at different scan rates. Scan rates (in mV s−1 ) are marked on each voltammogram.

resistance (table 3), an increase in response time and a decrease in pulse power density (table 4). These indicate that swollen/exfoliated MWCNT-electrodes gelled with IL have the ability to store larger amounts of charge at the capacitive interfaces (hence offer higher specific energy); however, the charge-transfer/ion switching rate is reduced, leading to lowering in the pulse power performance. Figure 8(a) shows the charge–discharge characteristics of all the EDLC cells (Cell#1, Cell#2 and Cell#3) recorded at a constant current of 1 mA cm−2 . All the cells were charged from 0 to 2.0 V at room temperature. The discharge characteristics of the cells are observed to show a capacitive nature as characterized by almost linear discharge patterns. The discharge specific capacitance (Cd ) values have been evaluated from the discharge characteristic regions using the expression:

the proper compatibility and the formation of excellent capacitive interfaces of the gelled MWCNT-electrodes with ionic liquid-based GPE. The scan rate dependent voltammograms for the EDLC cells Cell#1 (with pristine MWCNT-electrodes) and Cell#2 (with MWCNT-1 electrodes) show the capacitive rectangular shaped patterns up to the scan rate of 1500 mV s−1 (figures 7(b) and (c)). A slight deviation from the capacitive shape has been obtained for Cell#3 (with MWCNT-20 electrodes) beyond 1000 mV s−1 (figure 7(d)). The rectangular CV responses up to high scan rates (1000–1500 mV s−1 ) indicate the high rate performance of the cells. This was also indicated from the impedance analysis (discussed earlier) in terms of knee frequencies and characteristic response times/pulse power density values (table 4). Weak but distinct and reversible humps are observed in the voltammograms of Cell#2 in the voltage range 7–10 mV (figure 7(c)), which are related to possible defects created in π –π interactions in the MWCNT-1 electrodes during the gelation process of MWCNTs with a small amount of IL. This shows the weak pseudo-capacitive nature of the fine bundled MWCNT-1 electrodes in addition to the double layer capacitance. Such pseudo-capacitive features are not visible in MWCNT (pristine) and MWCNT-20 gelled electrodes (figures 7(b) and (d)). The slight deviation from capacitive rectangular shape beyond 1000 mV s−1 for Cell#3 indicates a slightly lower rate capability. The rate capability reduction of Cell#3, employing swollen/exfoliated MWCNT-electrodes (MWCNT-20), was indicated earlier in impedance analysis in terms of a lowering in the knee frequency and charge-transfer

2i1t (2) m1V where i is the constant discharge current, 1t is the discharge time interval for a voltage range of 1V, excluding the IR drop, and m is the mass of single electrode material without a current collector. The values of internal resistance (or ESR), evaluated by the initial sudden drop in the discharge profile, and the discharge specific capacitance, evaluated using equation (2), are listed in table 5. A more than twofold decrease in ESR and an almost threefold increase in discharge capacitance (Cd ) have been observed when gelled MWCNT-20 electrodes are employed to fabricate EDLCs with respect to the values with MWCNT Cd =

8

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Table 5. Various parameters evaluated from charge–discharge curves at a current 1.0 mA cm−2 at room temperature. EDLC cells

ESR ( cm2 )

Energy density Cd (F g−1 ) (Wh kg−1 )

Power density (kW kg−1 )

Cell#1 Cell#2 Cell#3

252 155 107

20.0 28.4 57.5

2.0 3.2 4.7

2.8 3.9 8.0

(MWCNT-20). The studies indicate the attractive approach of employing bucky gel of MWCNTs with ionic liquid as electrodes to fabricate flexible, solid-state EDLCs with improved energy and power densities. The charge–discharge reversibility, i.e. cyclic performance is one of the important aspects associated with supercapacitors/EDLCs. The cyclic performance of each EDLC cell has been evaluated by galvanostatic charge–discharge tests at a constant current of 1 mA cm−2 for 10 000 cycles, as shown in figure 8(b). The performance for initial charge–discharge cycles up to 1000 cycles is shown in the inset of figure 8(b). After initial fading, each capacitor cell shows almost stable specific capacitance values up to 104 cycles. It may be noted that even after the fading, the bucky gel-based EDLC (Cell#3) shows substantially higher capacitance values with respect to the device with pristine MWCNT-electrodes. The present results on the performance characteristics of EDLCs may be compared with the conventional EDLCs commonly configured with activated carbon electrodes and the following important points may be considered. Activated carbons are, in general, predominantly microporous, so possessing a large surface area and hence showing higher capacitance values (few hundreds of F g−1 ) [4–8]. However, such capacitors show lower rate capability and exhibit stable performance for a limited number (4000–5000) of charge–discharge cycles due to the possible permanent blockage of some micropores by electrolyte ions [4–8, 11, 17]. On the other hand, CNTs are mesoporous materials, so possessing a smaller surface area and hence offer lower values of specific capacitance (∼30–60 F g−1 ) [5, 17, 18]. However, the CNT-based capacitors show excellent rate capability and cyclic performance for large (105 –106 ) numbers of cycles [5]. This is due to easy switching of electrolyte ions through the mesoporous structure of the CNT-electrodes. Further improvement in the capacitive performance has been observed in the present studies when the binderless ‘bucky gel’ of MWCNTs was employed in place of pristine MWCNT-electrodes with a binder to form EDLCs.

Figure 8. (a) Galvanostatic charge–discharge curves of the EDLC cells at 1.0 mA cm−2 , and (b) variation of Cd of the cells, evaluated at constant current of 1.0 mA cm−2 , as a function of charge–discharge cycles.

(pristine)-containing binder (table 5). A decrease in ESR by more than 2 times and an ∼1.5 times enhancement in the specific capacitance have also been recently reported on an EDLC fabricated with a pure ionic liquid as electrolyte and gelled electrodes of SWCNTs with respect to the device with SWCNT-electrodes without gelation [32]. The decrease in ESR and the increase in specific capacitance are directly associated with the improvement in energy and power densities, respectively. The energy density (E) and power density (P) have been estimated using the expressions: 1 Cd V 2 (3) 8m V2 P= (4) 8mESR where V is the rated voltage for charging the capacitor cells. The values of energy and power densities of all the cells, evaluated respectively from equations (3) and (4), are listed in table 5. A substantial (∼2.5 times) enhancement in the energy and power densities has been observed when pristine MWCNT-electrodes are replaced by gelled MWCNTs E=

4. Conclusions Modified MWCNTs in the form of ‘bucky gel’ with pyrrolidinium-based IL have been employed as binderless electrodes to fabricate flexible high-performance EDLCs. A GPE film comprising a blend of PVdF-HFP and IL BMPTFSI with high ionic conductivity (∼1.5 × 10−3 at room temperature) has been incorporated as separator to provide a flexible nature to the EDLCs. The porosity and specific surface area of the MWCNTs have been significantly reduced 9

Nanotechnology 24 (2013) 465704

M K Singh et al

due to their gelation; however, morphological changes such as exfoliation/swelling of the carbon chains due to the interaction with IL enhance their compatibility with the GPE and provide improved capacitive performance. Performance characteristics of the MWCNT-gel-based capacitors have been evaluated from EIS, CV and charge–discharge analyses and compared with the EDLCs fabricated with pristine MWCNT-electrodes prepared with binder. As evidenced from EIS studies, an approximately 2.5 times enhancement in specific capacitance (from 19.6 to 51.3 F g−1 ) has been observed when pristine MWCNT-electrodes are substituted with gelled MWCNTs. On the other hand, the rate capability of the gelled MWCNT-based EDLC (measured in terms of the knee frequency and response time) is reduced; however, the corresponding pulse power value does not reduce in the same proportion due the enhanced available energy density of the device. As indicated from the scan rate dependence of CV responses, the rate performance of the EDLC with gelled MWCNTs is slightly lower than that of the device with pristine MWCNTs; however, it shows a rate capability better than many activated carbon-based EDLCs and redox supercapacitors. With respect to pristine MWCNTs, the gelled MWCNT-electrodes show substantial improvements in the energy and power densities (from 2.8 to 8.0 Wh kg−1 and from 2.0 to 4.7 kW kg−1 , respectively), evaluated from the charge–discharge studies. These studies indicate that the bucky gel formation of MWCNTs to be used as EDLC-electrodes is an excellent approach to obtain high specific capacitance and high energy/power densities with a substantially high rate capability.

[11] Niu C, Sichel E K, Hoch R, Moy D and Tennent H 1997 Appl. Phys. Lett. 70 1480–2 [12] Sun Y, Wu Q and Shi G 2011 Phys. Chem. Chem. Phys. 13 17249–54 [13] Frackowiak E 2007 Phys. Chem. Chem. Phys. 9 1774–85 [14] Ruch P W, Hardwick L J, Hahn M, Foelske A, Kotz R and Wokaun A 2009 Carbon 47 38–52 [15] Honda Y, Ono T, Takeshige M, Morihara N, Shiozaki H, Kitamura T, Yoshikawa K, Morita M, Yamagata M and Ishikawa M 2009 Electrochem. Solid-State Lett. 12 A45–9 [16] Ajayan P M 1999 Chem. Rev. 99 1787–99 [17] Lu W, Hartman R, Qu L and Dai L 2011 J. Phys. Chem. Lett. 2 655–60 [18] Iijima S 1991 Nature 354 56–8 [19] Baibarac M, Lira-Cantu M, Oro-Sole J, Casan-Pastor N and Gomez-Romero P 2006 Small 2 1075–82 [20] Claye A S, Fischer J E, Huffman C B, Rinzler A G and Smalley R E 2000 J. Electrochem. Soc. 147 2845–52 [21] Kaempgen M, Lebert M, Nicoloso N and Roth S 2008 Appl. Phys. Lett. 92 94101–3 [22] Suehiro J, Zhou G and Hara M 2003 J. Phys. D: Appl. Phys. 36 L109–14 [23] Chen D T N, Chen K, Hough L A, Islam M F and Yodh A G 2010 Micromolecule 43 2048–53 [24] Hu L, Pasta M, Mantia F L, Cui L, Jeong S, Deshazer H D, Choi J W, Han S M and Cui Y 2010 Nano Lett. 10 708–14 [25] Tasis D, Tagmatarchis N, Bianco A and Prato M 2006 Chem. Rev. 106 1105–36 [26] Ma P-C, Siddiqui N A, Marom G and Kim J-K 2010 Composites A 41 1345–67 [27] Datsyuk V, Kalyva M, Papagelis K, Parthenios J, Tasis D, Siokou A, Kallitsis I and Galiotis C 2008 Carbon 46 833–40 [28] Zhao Y D, Bi Y H, Zhang W D and Luo Q M 2005 Talanta 65 489–94 [29] Zhang Y and Zheng J 2008 Electrochim. Acta 54 749–54 [30] Opallo M and Lesniewski A 2011 J. Electroanal. Chem. 656 2–16 [31] Fukushima T, Kosaka A, Ishimura Y, Yamamoto T, Takigawa T, Ishii N and Aida T 2003 Science 27 2072–4 [32] Katakabe T, Kaneko T, Watanabe M, Hukushima T and Aida T 2005 J. Electrochem. Soc. 152 A1913–6 [33] Lee J and Aida T 2011 Chem. Commun. 47 6757–62 [34] Mukai K, Asaka K, Kiyohara K, Takeuchi I, Sugino T, Terasawa N, Hata K, Fukushima T and Aida T 2008 Electrochim. Acta 53 5555–62 [35] Kachoosangi R T, Wildgoose G G and Compton R G 2007 Electroanalysis 19 1483–9 [36] Pan H, Li J and Feng Y P 2010 Nanoscale Res. Lett. 5 654–68 [37] Aida T and Fukushima T 2007 Phil. Trans. R. Soc. A 365 1530–52 [38] Yan X, Kitahama Y, Sato H, Suzuki T, Han X, Itoh T, Bokobza L and Ozaki Y 2012 Chem. Phys. Lett. 523 87–91 [39] Maurin G, Henn F, Simon B, Colomer J-F and Nagy J B 2001 Nano Lett. 1 75–9 [40] Wang H, Maiyalagan T and Wang X 2012 ACS Catal. 2 781–94 [41] Costa S, Borowiak-Palen E, Kruszy˜nska M, Bachmatiuk A and Kalenczuk R J 2008 Mater. Sci.-Poland 2 433–41 [42] Choi J-A, Shim E-G, Scrosati B and Bull D-W K 2010 Korean Chem. Soc. 31 3190–4 [43] Miller J R 1998 8th Int. Seminar on Double Layer Capacitor and Similar Energy Storage Devices (Deerfield Beach, FL) [44] Taberna P L, Simon P and Fauvarque J F 2003 J. Electrochem. Soc. 150 A292–300

Acknowledgments Authors acknowledge financial support received from the DST, New Delhi (Sanction No. DST/TSG/PT/2009/93) and University of Delhi (11-17 Research Fund). One of us (MKS) is grateful to the University Grants Commission, New Delhi for providing a Junior Research Fellowship.

References [1] Conway B E 1999 Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (New York: Kluwer Academic/Plenum) [2] Hashmi S A 2004 Natl Acad. Sci. Lett. 27 27–46 [3] Simon P and Gogotsi Y 2008 Nature Mater. 7 845–54 [4] Qu D and Shi H 1998 J. Power Sources 74 99–107 [5] Kumar Y, Pandey G P and Hashmi S A 2012 J. Phys. Chem. C 116 26118–27 [6] Hwang S R and Teng H J 2002 J. Electrochem. Soc. 149 A591–6 [7] Hung K, Masarapu C, Ko T and Wei B 2009 J. Power Sources 193 944–9 [8] Hashmi S A, Latham R J, Linford R G and Schlindwein W S 1997 J. Chem. Soc. Faraday Trans. 93 4177–82 [9] Fang B, Wei Y-Z, Maruyama K and Kumagai M 2005 J. Appl. Electrochem. 35 229–33 [10] Tao X Y, Zhang X B, Zhang L, Cheng J P, Liu F, Luo J H, Luo Z Q and Geise H J 2006 Carbon 44 1425–8

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