Structural and Electrochemical Characterization of

Article pubs.acs.org/JPCC

Structural and Electrochemical Characterization of Ethylenediaminated Single-Walled Carbon Nanotubes Prepared from Fluorinated SWCNTs Kohei Bushimata,†,# Shin-ichi Ogino,†,# Kazutaka Hirano,‡ Tatsuhiro Yabune,‡ Kenta Sato,§ Takashi Itoh,∥ Kenichi Motomiya,† Koji Yokoyama,† Daiki Mabuchi,† Hikaru Nishizaka,† Go Yamamoto,⊥ Toshiyuki Hashida,⊥ Kazuyuki Tohji,† and Yoshinori Sato*,† †

Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki, Aoba-ku, Sendai 980-8579, Japan Stella Chemifa Corporation, 1-41, Rinkai-cho, Izumiotsu, Osaka 595-0075, Japan § Netzsch Japan K. K., 3-9-13, Moriya-cho, Kanagawa-ku, Yokohoma, Kanagawa 221-0022, Japan ∥ Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan ⊥ Fracture and Reliability Research Institute, Tohoku University, Aoba 6-6-11, Aramaki, Aoba-ku, Sendai 980-8579, Japan ‡

S Supporting Information *

ABSTRACT: We prepared ethylenediaminated single-walled carbon nanotubes (SWCNTs) from fluorinated SWCNTs by substituting fluorine groups with ethylenediamine groups. The ethylenediaminated SWCNTs were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman scattering spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer−Emmett−Teller surface area measurement by nitrogen adsorption, contact angle measurement, zeta potential analysis, and thermogravimetry. In addition, the properties of 30 wt % sulfuric acid aqueous electrolyte-based electric double-layer supercapacitors (EDLSCs) with free-standing ethylenediaminated SWCNT electrodes were investigated. The degree of ethylenediamine functionalization was 0.603 mmol/g and 1.46 μmol/m2, and the specific surface area was ∼413.3 m2/g. From HRTEM observation, isolated nanotubes disentangled from the bundled SWCNTs were present in many observed areas, and the structures retained a nanotube skeleton. The properties of the EDLSCs with the ethylenediaminated SWCNT electrodes included an average specific capacitance of 94 F/g at a low scan rate of 10 mV/s and an energy density of 2.6 Wh/kg at a power density of 0.24 kW/kg. The EDLSCs exhibited an average specific capacitance of 67 F/g at a high scan rate of 1000 mV/s and an energy density of 1.3 Wh/kg at a power density of 24 kW/kg, values that were superior to those of carboxylated SWCNT electrodes.

1. INTRODUCTION Electric double-layer supercapacitors (EDLSCs) with carbonbased electrodes exhibit significant features that can be utilized to yield a lightweight, high-energy-density, high-power-density, and maintenance-free device with a long life.1−3 Activated carbons are normally used as carbon electrodes because of their high specific surface area. However, the energy density of EDLSCs with activated carbon electrodes at a high scan rate is low because activated carbons have a large internal resistance. Recently, single-walled carbon nanotubes (SWCNTs), which possess a high specific surface area and high conductivity, have received much attention as a new electrode material for EDLSCs.4,5 The specific surface area of SWCNTs depends on the nanotube diameter and the bundled diameter, which is approximately 1100−1200 m2/g for individual SWCNTs6,7 and approximately 300−500 m2/g for bundled SWCNTs.8−11 In addition, SWCNTs possess ballistic conduction, and the π electrons of SWCNTs are capable of hopping between © 2014 American Chemical Society

SWCNTs. To investigate these features, EDLSCs using binder-free SWCNT films10 or vertically aligned SWCNTs12,13 as electrodes were studied. These SWCNT electrodes have a low loss of electron transfer, and electrolyte ions smoothly adsorb onto and desorb from the surface of the SWCNTs at a high scan rate since the surface of binder-free SWCNTs is open to the electrolyte. Thus, the power density of SWCNT electrode-based EDLSCs is larger than that of activated carbon electrode-based EDLSCs at a high scan rate. Consideration of the practical and production aspects of EDLSCs highlights the fact that aqueous electrolytes are superior in terms of cost, safety, and reliability in comparison with organic electrolytes, which are weak in regard to moisture. Additionally, ions are easy to move in aqueous electrolytes Received: March 29, 2014 Revised: June 3, 2014 Published: June 12, 2014 14948

dx.doi.org/10.1021/jp503124n | J. Phys. Chem. C 2014, 118, 14948−14956

The Journal of Physical Chemistry C

Article

reactants under a nitrogen atmosphere at 80 °C for 5 h (Scheme 1).18 The resultant suspension was filtered using a

since the viscosity of aqueous electrolytes is low. This is the reason the power density of aqueous electrolyte-based EDLSCs is large. Actually, a 30−40 wt % sulfuric acid (H2SO4) aqueous solution has been used because of its nonvolatility and high conductivity. The charge breakdown voltage of a single cell of aqueous electrolyte-based EDLSCs is about 1.2 V because of the use of water, a value that is lower than that of organic electrolyte-based EDLSCs (2.5 V). This indicates that the energy stored with aqueous electrolyte-based EDLSCs is lower than that stored with organic electrolyte-based EDLSCs at a constant electric charge. To achieve the high energy density of sulfuric acid−based EDLSCs, it is necessary to form an extensive electric double layer onto the electrode surface. In an effort to achieve this outcome, many electrolyte ions must adsorb onto electrodes with improved wettability by inducing polarizable functional groups while maintaining the high specific surface area of the electrodes. The properties of EDLSCs with negatively charged carboxylated SWCNT (COOH-SWCNT) electrodes in an aqueous sulfuric acid solution have been investigated.14,15 The main ions in a 30−40 wt % aqueous H2SO4 solution are hydronium (H3O+) and hydrogen sulfate ions (HSO4−). To utilize HSO4− as an electric double layer, it is preferable to use an SWCNT electrode that is modified with a positively charged functional group in a sulfuric acid aqueous solution rather than negatively charged COOHSWCNT electrodes. The properties of EDLSCs with positively aminated SWCNT electrodes have never been reported. In this study, we prepared and characterized crystalline ethylenediaminated SWCNTs from fluorinated SWCNTs by substituting fluorine groups with ethylenediamine groups, and we investigated the properties of a 30 wt % sulfuric acid aqueous electrolyte-based EDLSC with free-standing ethylenediaminated SWCNT electrodes.

Scheme 1

PTFE membrane filter with an average pore diameter of 3.0 μm, and the filtered cake was washed with ethanol to remove the residual ethylenediamine on the nanotube surface. Finally, samples were dried in ambient atmosphere at 60 °C for 24 h. The product, ethylenediaminated SWCNTs, is hereafter referred to “EDA-SWCNTs.” 2.3. Supercapacitor Assembly. The procedure of the electrode preparation was as follows: SWCNTs (10 mg) and 30 wt % H2SO4 aqueous solution (5 drops) were mixed to form a slurry. The slurry was fully injected in a 3.5 mm-diameter hole in the center of a butylene gasket (465 μm thick and 20 mm in diameter) on a stainless steel collector, making a single electrode. The weight of each material (SWCNTs, collector, 30 wt % H2SO4 aqueous solution, and butylene gasket) was measured at each step. The average weight of SWCNTs was approximately 1.0 mg for each single electrode. A single-cell EDLSC device was fabricated with two SWCNT electrodes separated by a mixed polymer film of polyethylene and polyoxyethylene (100 μm thick and 13 mm in diameter). To minimize surface resistance between the collector and the SWCNTs, the two electrodes were fixed by pressing at 1372 N. 2.4. Structural Characterization. The sample morphologies were determined by scanning electron microscopy (SEM) (S-4100, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM) (HF-2000, Hitachi, Japan), both equipped with a field emission gun. The SEM and HRTEM were operated at 5 and 200 kV, respectively. Raman scattering spectroscopy (Jobin-Yvon T64000, Horiba Co. Ltd., Japan) studies were used to analyze the vibrational modes of the graphitic materials. These measurements were carried out at room temperature using a semiconducting laser (473.0 nm). Xray diffraction (XRD) measurements were recorded on an Xray diffractometer (Multi Flex, Rigaku Co. Ltd., Japan) equipped with Cu Kα radiation. XPS was performed using an AXIS-ULTRA instrument (Shimadzu Ltd., Japan) with a monochromatized Al Kα line. Thermogravimetry with differential thermal analysis (TG-DTA) (STA 449 F1 Jupiter, NETZSCH-Geratebau GmbH, Germany) and mass spectroscopy (QMS 403 C Aëolos, NETZSCH-Geratebau GmbH, Germany) were used to detect the gas products resulting from the thermal decomposition of the samples in helium (flow rate: 40 sccm) at room temperature and up to 1000 °C (10 °C/ min). The reason helium was used as a carrier gas instead of argon is to prevent the mass fragmentation patterns of samples from overlapping on those of argon, Ar2+ (m/z 20),36Ar (m/z 36), and 38Ar (m/z 38). The specific Brunauer−Emmett− Teller (BET) surface area was measured on a NOVA 1200 porosimeter (Quantachrome Instruments) by nitrogen molecule adsorption at −196 °C. The surface charges of the samples were characterized by measuring the zeta potential (ZEECOM

2. EXPERIMENTAL SECTION 2.1. Preparation of Fluorinated SWCNTs. We synthesized as-grown SWCNTs by the arc discharge method using a mixture of Fe/Ni particles as a metal catalyst.16 The as-grown SWCNTs were air oxidized and treated with 6.0 mol/L hydrochloric acid to remove amorphous carbon and catalytic metal particles, respectively. The resultant suspension was filtered using a polytetrafluoroethylene (PTFE) membrane filter with an average pore diameter of 3.0 μm, and the filtered cake was washed with purified water. The resulting sample was dried in vacuum at 200 °C for 24 h. The purified-SWCNTs were then annealed under a high vacuum (2.0 × 10−5 Pa) at 1200 °C for 3 h16 and are subsequently referred to “highly crystalline SWCNTs” (hc-SWCNTs). The residue metal catalyst in the hc-SWCNTs after annealing was approximately 1.55 wt %. We conducted direct fluorination of hc-SWCNTs to an approximate CF0.5 stoichiometry. The hc-SWCNTs were fluorinated at 250 °C using a mixture of F2 (20%) and N2 (80%) for 2 h (flow rate of 25 mL/min). Subsequently, thermal annealing was carried out at 250 °C for 4 h in a PTFE cell under nitrogen flow (20 mL/min). The products were then characterized by X-ray photoelectron spectroscopy (XPS),17 and the C:F stoichiometries were determined to be CF0.41 (see Figure S1). The fluorinated SWCNTs are hereafter referred to as “F-SWCNTs”. 2.2. Preparation of Ethylenediaminated SWCNTs. The F-SWCNTs (40 mg) were transferred to a two-necked roundbottom flask with ethylenediamine (20 mL) in the presence of pyridine (10 drops) as a catalyst and refluxed by stirring the 14949



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ZC2000, Microtec, Japan). Samples were dispersed in a given solution at different pH values prepared by hydrochloric acid or aqueous sodium hydroxide, and the dispersions were measured at room temperature. The water contact angles of the samples were measured using a contact angle meter (face CA-DS, Kyowa Interface Science Co., Ltd., Japan). For the contact angle measurements, films filtrated by vacuum filtration were prepared with a size of approximately 16 mm in diameter and 20 μm thick. A 30 wt % H2SO4 aqueous solution was used as a dropping solvent. 2.5. Electrochemical Characterization. The properties of the EDLSCs were evaluated with cyclic voltammetry (CV) using a potentiostat (Model 263A, Princeton Applied Research, USA). CV measurements were conducted in the voltage range of 0.0−0.9 V under various scan rates ranging from 10 to 10000 mV/s. The best estimates of the electrochemical properties were determined from the mean values of three independent experiments. The capacitance, energy, and power of the EDLSCs were calculated from the CV curve. The specific capacitances were estimated according to the following equation:

∫ (I dt )/ΔV }/m

Cp = C /m = (Q /ΔV )/m = {

where Cp is the specific capacitance; C, the total capacitance; m, the average weight of a single SWCNT electrode; Q, the total charge (integral area of CV curve when the current is greater than zero); I, the current; dt, the measurement interval; and ΔV, the applied voltage. The energy E and power W were calculated by the following equation: W = I × V,

E=

Figure 1. XPS spectra of the (a) C1s and (b) N1s region for the EDASWCNTs. The dotted lines are the deconvolved peaks.

region of the EDA-SWCNTs. The obtained N1s peak was deconvolved into four peaks centered at 398.3, 399.3, 400.1, and 401.7 eV. The peak of the XPS spectrum of the N1s region of stand-alone ethylenediamine is 400.2 eV, which corresponds to nitrogen atoms in a free primary amine (Figure S2). The binding energy of −NH3+ is reported to be in the 401.8 eV region.19 Thus, the third and fourth peaks at 400.1 and 401.7 eV are associated with nitrogen atoms in free primary amines and −NH3+, respectively. The binding energy of 398.3 and 399.3 eV is considered to be a binding (Ctube−NH−) between a carbon atom of the nanotube frame (Ctube) and an amine. Since there are some binding sites of amines to the nanotube frame, which depend on binding sites of fluorine to the nanotube frame, the binding energy of Ctube−NH− is expected to be different. The XPS spectrum of the hc-SWCNTs shows that they bear several oxygen-containing functional groups such as hydroxyl, lactone, and carboxylic groups (Figure S3). The TG-mass curve of the hc-SWCNTs indicates that a mass number equivalent to CO2 can be found at around 320 and 650 °C, along with H2O (m/z 18) as shown in Figure S4; this means that the decomposition of the oxygen-containing functional groups on the hc-SWCNTs occurs at around 350 and 650 °C.20−22 In FSWCNTs, the weight loss drastically decreased at around 500 °C (Figure S5). The mass number m/z 19 behavior in the range 100−300 °C represents stand-alone fluorine groups desorpted from the F-SWCNTs, which supports the interpretation that the fluorine groups are modified on the outermost surface of the bundled nanotubes. The m/z 19 peak detected around 350−600 °C is related to dissociated mass fragments of CF4, CF3, and CHF3, in which a proton (H+) is added to CF3 (see the mass fragmentation patterns of carbonfluoride compounds in Figure S6). The mass number m/z 19

∫ W dt

Here, we define 0 A of the discharge current as the initial discharge point and 0.6 V of the discharge potential as the final discharge point when a discharging potential is scanned from 0.9 to 0.0 V. We chose the discharge potential 0.6 V as the final discharge point in order to compare the properties of EDLSCs at potential areas without peaks attributed to the redox reactions due to the functional groups on the SWCNTs for all scan rates of each sample. The energy was estimated by integrating the power over time. The power was calculated by the current at 0.6 V. The specific energy density and power density were evaluated by dividing each value by the average weight of a single SWCNT electrode.

3. RESULTS AND DISCUSSION 3.1. Characterization of EDA-SWCNTs. Figure 1a shows the XPS spectrum of the C1s region of the EDA-SWCNTs. The peak obtained in a high-resolution scan was deconvolved into eight peaks. The first peak (284.4 eV) is associated with carbon atoms in sp2-hybridized graphitic structures. The third peak (285.2 eV) can be assigned to sp3-hybridized carbon atoms. The last peak (291.2 eV) corresponds to the characteristic vibration mode of carbon atoms in aromatic structures. The second peak (284.7 eV) is associated with carbon atoms in carbon−hydrogen bonds of ethylenediamine. The fourth peak (286.2 eV) is related to carbon atoms in hydroxyl groups or amines (−NH2). The peaks (287.8, 288.4, and 290.2 eV, respectively) are related to carbon atoms in carbonyl/ether, carbon fluoride, and carboxylic/ester functional groups, respectively. Figure 1b shows the XPS spectrum of the N1s 14950



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behavior over 600 °C represents stand-alone fluorine groups desorbed from the interstitial sites of the bundled nanotubes. Peaks for the mass numbers, m/z 50, 51, and 69 were detected in the range from 350 to 600 °C, and each peak was centered at around 550 °C. These correspond to CF2, CHF2, and CF3, respectively, and are interpreted as dissociated fragments of CF4 or CHF3 (Figure S6). Judging from the intensity ratio of their mass fragments, most of the modified fluorine groups desorb as carbon-fluoride compounds, such as CF3 and CF4. Given these results, the desorption of stand-alone fluorine groups below 300 °C, the desorption of carbon fluorides in the temperature range from 300 to 600 °C, and the observation of several C−F binding energies in the XPS C1s spectrum of the FSWCNTs (Figure S1b), fluorine groups are likely modified at several different addition sites on the nanotube skelton.23 Figure 2 shows the TG-mass curve of the EDA-SWCNTs. Since a fragment ion of ethyleneamine was detected at 320 °C (m/z 30; Figure S7), the ethylenediamines modified to SWCNTs were found to desorb at 320 °C. The fragment ions m/z 50 (CF2) of CF4 and CF3 and the fragment ions m/z 51 (CHF2) of CHF3 were detected in the temperature range 120−320 °C, showing that fluorine functional groups remain in the EDA-SWCNTs. In addition, mass numbers equivalent to H2O (m/z 18) and CO2 (m/z 44) were detected at approximately 320 °C. In this temperature range, the decomposition of the oxygen-containing functional groups functionalized to EDA-SWCNTs is found to occur. These results suggest that the EDA-SWCNTs prepared in this study bore hydroxyl, carboxyl, fluorine, and ethylenediamine groups, which indicates that the substitution reaction from fluorine to ethylenediamine groups was not complete. Since the desorption temperature of ethylenediamine functionalized on EDA-SWCNTs is the same as that of hydroxyl and carboxyl groups on the modified EDA-SWCNTs, the functional degree of ethylenediamine on SWCNTs cannot be calculated by thermogravimetric analysis.24,25 Here, we evaluated the functional degree of ethylenediamine groups from the chemical composition of the EDA-SWCNTs as measured by XPS. Table 1 shows the chemical composition of hcSWCNTs, F-SWCNTs, and EDA-SWCNTs. The elemental ratio in the EDA-SWCNTs is C94.3O0.8N4.1F0.8. Since an ethylenediamine group has two carbon atoms and two nitrogen atoms, the composition ratio including ethylenediamine groups is C 9 0 . 2 O 0 . 8 (C 2 N 2 ) 2 . 0 5 F 0 . 8 , also written as CO0.009(C2N2)0.023F0.009. The ethylenediamine group content is approximately 2.6 times that of fluorine groups. Calculating the CC distance as 1.42 nm, the area of a beneze ring within graphene is 5.24 × 10−20 m2, in which two entire carbon atoms exist. On the basis of the average specific surface areas of the EDA-SWCNTs (413.3 m2/g, see Table 2), the number of carbon atoms associated with this surface area per unit mass is 1.58 × 1022 atoms. Supposing that the chemical composition results of the EDA-SWCNTs derive only from the outermost surfaces of the bundled nanotubes, ethylenediamine groups are associated with 3.63 × 1020 atoms in these carbon nanotubes. Therefore, the functional degree of ethylenediamine groups per unit mass is estimated to be 0.603 mmol/g, and the average density, which is the number of ethylenediamine groups per unit surface area, is about 1.46 μmol/m2. In addition, the ratio of ethylenediamine groups and carbon atoms associated with the surface area of the EDA-SWCNTs is 1:44. Thus, one ethylenediamine group is functionalized per 44 carbon atoms on a nanotube frame. In contrast, the remaining amount of

Figure 2. TG curve of EDA-SWCNTs (top) measured in a helium atmosphere. Ion current data plots for ions with m/z = 18, 19, 30, 44, 50, 51, and 69, characterizing volatile products of the thermal degradation of EDA-SWCNTs.

Table 1. Chemical Composition of hc-, F-, and EDASWCNTs Measured by XPS Chemical composition measured by XPS samples

C (atom %)

O (atom %)

N (atom %)

F (atom %)

hc-SWCNTs F-SWCNTs EDA-SWCNTs

98.4 70.0 94.3

1.6 1.0 0.8

0.0 0.0 4.1

0.0 29.0 0.8

fluorine groups within the EDA-SWCNTs (per unit mass) is 0.236 mmol/g, and the average density is about 0.571 μmol/ 14951



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Table 2. Specific Surface Area, Contact Angle, Zeta Potential, and Raman Scattering Intensity Ratio (ID/IG; R Value) of D-Band to G-Band of hc-, F-, and EDA-SWCNTs

a

materials

hc-SWCNTs

F-SWCNTs

EDA-SWCNTs

Specific surface area (m2/g) Contact angle (deg) Zeta potential (mV)a ID/IG (R value)

323.6 ± 47.4 91.0 ± 3.8 0.1 ± 3.6 0.011 ± 0.002

221.8 ± 37.6 99.8 ± 3.2 −0.5 ± 4.0 0.932 ± 0.030

413.3 ± 48.2 NDb 23.6 ± 3.9 0.278 ± 0.056

These values are at a pH of 3.0. bUnmeasurable level because of high wettability.

m2. The production yield of EDA-SWCNTs is approximately 5.6% (see the definition of “production yield” in the Supporting Information). The surface charge of the EDA-SWCNTs was positive (see Figure 3 and Table 2; 23.6 ± 3.9 mV at pH 3.0), indicating that

Figure 3. pH-dependent zeta potential of hc-SWCNTs (black), FSWCNTs (green), and EDA-SWCNTs (red).

the free primary amine groups were positively charged in the acid solution. Since droplets of the 30 wt % H2SO4 aqueous solution quickly soaked into the EDA-SWCNT film, we could not estimate the contact angle of the EDA-SWCNT film (Table 2). Thus, the wettability of the EDA-SWCNTs for a 30 wt % H2SO4 aqueous solution is extremely good. These results indicate that we succeeded in replacing fluorine groups with ethylenediamine groups. Figure 4a shows the Raman scattering spectra of each sample. A peak at 180 cm−1 appears in the radial breathing mode (RBM) of the Raman spectrum for the hc-SWCNTs. The average diameter of the hc-SWCNTs used in this study was 1.5 nm,16 and the Raman intensity ratio (ID/IG; known as the “R value”) of the D-band (1350 cm−1) to the G-band (1590 cm−1) of the hc-SWCNTs is 0.011 (Table 2), which indicates that the crystallinity of the nanotubes is high. As compared with the hcSWCNTs, the peak intensity of the RBM of the EDA-SWCNTs is low and their R value increases. In contrast, the peak intensity of the RBM of the F-SWCNTs was not detected and their R value (0.917 ± 0.052) is the largest of all the samples (Table 2). Figure 4b shows the low-angle XRD profile of each sample. Although a (10) XRD peak associated with intertubule spacing (d10; 2θ = 6.05°) within bundles of nanotubes was detected for the hc-SWCNTs,26,27 there were no peaks detected at the d10 position in the other samples. As shown in Figure 5a, the hc-SWCNTs possessed a highly crystalline bundled structure, with a bundled diameter of 10− 30 nm. The F-SWCNTs were observed to be isolated tubule structures, although they mainly exhibited a bundled structure (Figure 5b). It has been reported that fluorinated SWCNTs are compatible with polar solvents such as water, amines, and

Figure 4. (a) Raman scattering spectra and (b) XRD profiles of hcSWCNTs (black), F-SWCNTs (green), and EDA-SWCNTs (red).

alcohols.28,29 When the F-SWCNTs for TEM observation were sonicated in ethanol, the bundled structures seemed to disentangle, as shown in Figure 5b. Meanwhile, since the FSWCNTs were dispersed in ethylenediamine and exhibited either an isolated or thinly bundled morphology, the synthesized EDA-SWCNTs are considered to exist as both isolated and bundled structures (Figure 5c). Additionally, the EDA-SWCNTs retained the nanotube skeleton. It has been reported that the increase of D-band intensity occurs not only by the increase of topological defects to CNTs30 but also by the increase in the number of sp3hybridized covalent bonds that the modified CNTs possess.31−34 In fluorinated SWCNTs with a CF0.41 of C/F stoichiometry, fluorine groups are functionalized all over the SWCNTs and bond with carbon atoms of the nanotube frame (Ctube) by an sp3-hybridized covalent bond. Concerning the disappearance of peaks in the range of RBM, Ctube atoms with sp3-hybridized covalent bonds are considered to suppress the RBM vibration of SWCNTs.31,33,34 Since the substitution reaction from fluorine into ethylenediamine groups, as previously mentioned, does not occur completely, there is a case in which the fluorine groups are defluorinated alone without substitution with ethylenediamine. Actually, we heated the F-SWCNTs under a nitrogen atmosphere at 80 °C for 5 h in order to investigate a behavior that fluorine groups are 14952



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became lower and the R value (0.858 ± 0.036) decreased compared to the F-SWCNTs (see Figure S8), indicating that the fluorine groups can be defluorinated alone. In these cases, sp3-hybridized carbon atoms bonded with fluorine are considered to reform into the sp2-hybridized carbon atoms of the nanotube frame. The decrease of D-band intensity and the appearance of an RBM peak support this speculation. Meanwhile, Margrave et al. reported that the thermal defluorination of fluorinated SWNTs occurred in an argon atmosphere and that CF4 started evolving from the fluorinated SWCNTs at 560 °C, introducing defects on the fluorinated SWCNTs.35 Although our reaction system is a solution process at low temperature (80 °C), there might a possibility that defects by defluorination alone occur. The reasons the d10 peak in the XRD profiles of the F- and EDA-SWCNTs disappears are considered to be due to (i) the increase of individual SWCNTs as compared to the total number of SWCNTs and (ii) the lack of periodic bundled structures by functionalization or defects induced to the nanotube frames. 3.2. Properties of EDLSCs with EDA-SWCNT Electrodes. Activated carbon (Aldrich, #24227−6) and COOHSWCNTs were used as reference samples and subjected to electrochemical measurement. The activated carbon used in this study has the average specific surface area of 716 ± 25 m2/ g. The COOH-SWCNTs were synthesized by refluxing the hcSWCNTs in a 6.8 mol/L aqueous nitric acid solution at 150 °C for 4 h because electrodes of the COOH-SWCNTs refluxed for 4 h showed better EDLSC performance in comparison with prepared COOH-SWCNT electrodes refluxed for 1 or 8 h (Figure S9). The basic properties, such as specific surface area, functional groups (see “Functional groups of COOHSWCNTs” and Figures S10 and S11) in the Supporting Information), contact angle, zeta potential, and R value, of the COOH-SWCNTs are shown in Table S1.

Figure 5. HRTEM images of (a) hc-SWCNTs, (b) F-SWCNTs, and (c) EDA-SWCNTs.

defluorinated alone without ethylenediamine. As a result, the Raman peak intensity of the D-band of the heated F-SWCNTs

Figure 6. Cyclic voltammograms of EDLSCs with activated carbon, hc-, EDA-, and COOH-SWCNT electrodes at scan rates of (a) 50, (b) 100, (c) 1000, and (d) 2000 mV/s. 14953



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Figure 7. (a) Specific capacitance of each capacitor as a function of scan rate. (b) Variation of specific capacitance ratio of each EDLSC normalized by each specific capacitance at a scan rate of 10 mV/s when the scan rate increases. (c) Correlation between energy density and power density for each EDLSC. This correlation displayed the representatives for each EDLSC. Activated carbon (brown), hc-SWCNT (black), EDA-SWCNT (red), and COOH-SWCNT (blue) electrodes.

discharging at a high scan rate. The adsorption−desorption rate of ions is therefore considered to be incapable of following the potential rates within a high scan rate region in the EDLSCs with COOH-SWCNT electrodes. Since the HRTEM lattice image of COOH-SWCNTs is not clear (Figure S12), the surfaces of the COOH-SWCNTs are considered to be broken by surface damage due to the nitric acid treatment,30 which will result in poor mobility of the π electrons. EDLSCs with EDASWCNT electrodes exhibit an energy density of 2.2 Wh/kg and a power density of 5.2 kW/kg (and have an average specific capacitance of 83 F/g at a scan rate of 200 mV/s) as shown in Figure 7c, showing the high-performance capacitance properties of high energy and high power density as the energy− power density curve of EDLSCs with hc-SWCNT electrodes shifted to the right on the plot. This is evidence that EDASWCNTs maintain nanotube skeletons, and the attraction of many electrolyte ions around amino functional groups in conjunction with EDLSC properties yields good capacitor performance. The improvement in the properties of the EDLSCs using EDA-SWCNT electrodes is considered to be the result of two factors. A first factor is the wettability of the EDA-SWCNTs. A free primary amine group, which is an amino-terminal of the EDA functionalized on an SWCNT, is a protonated −NH3+ ion in the H2SO4 electrolyte. The protonated −NH3+ ion strongly interacts with HSO4−, and thus the wettability of the EDASWCNTs is dramatically improved in comparison with both the hc- and COOH-SWCNTs. A second factor is that the nanotube skeleton is retained, despite the functionalization of EDA. The CV curves of EDLSCs with EDA-SWCNT electrodes are close to the ideal rectangular shape at a high scan rate, which shows that the capacitors are the smallest ESR.

The cyclic voltammograms of EDLSCs with hc- and EDASWCNT electrodes at scan rates of 50, 100, 1000, and 2000 mV/s are shown in Figure 6. Figure 7a shows the specific capacitance of each capacitor as a function of scan rate. (Table S2 displays the specific capacitance of each electrode.) Although the CV current density of EDLSCs with EDASWCNT electrodes differs from that of EDLSCs with hcSWCNT electrodes, both CV curves are close to an ideal rectangular shape at a high scan rate, indicating that the capacitors correspond to the smallest equivalent series resistance (ESR). The specific capacitance for EDLSCs with EDA-SWCNT electrodes is 96 F/g at a scan rate of 10 mV/s, which shows the largest specific capacitance of all the capacitors except for EDLSCs with an activated carbon electrode at scan rates of 10 and 20 mV/s. In contrast, the CV curve of EDLSCs with COOH-SWCNT electrodes is not rectangular in shape, indicating a large ESR. In addition, the CV curves of EDLSCs with COOH-SWCNT electrodes exhibits peaks at approximately 0.1 V (charging side) and 0.0 V (discharging side) at low scan rates, which correspond to the redox reaction (faradaic reaction) protonation/deprotonation of carboxyl, hydroxyl, and phenol groups.36 Figure 7b shows the variation of specific capacitance ratio of each EDLSC with increasing scan rate normalized by each specific capacitance at a scan rate of 10 mV/s. The normalized specific capacitance ratios decrease with increasing scan rate in the following order: hc-SWCNTs ≈ EDA-SWCNTs > COOH-SWCNTs > activated carbon. Although the specific capacitance of EDLSCs with EDASWCNT electrodes differs from that of EDLSCs with hcSWCNT electrodes due to differences in the number of absorbing ions, this result reveals that both electrodes with the high crystalline nanotube network (Figure 5a,c) are capable of 14954



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Author Contributions

In addition, the power density of EDLSCs with EDA-SWCNT electrodes is superior to that of EDLSCs with COOHSWCNTs, which possess surface defects (Figure S12). These results indicate that EDA-SWCNTs have large mobility of π electrons. As indicated by these two factors, EDLSCs with EDA-SWCNT electrodes possess a high energy density and high power density. The crystalline nanotube skeleton of EDA-SWCNTs is characteristics of SWCNTs prepared from F-SWCNTs. If the functionalization degree of EDA increases, the wettability will improve even more. On the other hand, sp3 hybrid orbital covalent bonds will increase on the nanotube frame, and the π electron mobility on the surface of the nanotubes is expected to decrease. Therefore, it is thought that an optimum functionalization degree of amination exists in order for the ESR of EDLSC cells to decrease as it has been reported that EDLSCs with excellent capacitor characteristics have a suitable functionalization degree in relation to the carboxyl group.14,15

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These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.S. would like to thank Prof. H. Sakurai for useful discussion. This work was supported by Grants-in-Aid for Young Research (A) 23686092 from the Japan Society for the Promotion of Science (JSPS).

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4. CONCLUSIONS We synthesized EDA-SWCNTs from F-SWCNTs with an element ratio of CF0.41, and we investigated the properties of EDLSCs with EDA-SWCNTs as electrodes. The EDASWCNTs exhibited a highly crystalline isolated or thin bundled structure with good wettability. The EDLSCs with the EDASWCNT electrodes possessed an average specific capacitance of 94 F/g at a low scan rate of 10 mV/s and an energy density of 2.6 Wh/kg at a power density of 0.24 kW/kg. At a high scan rate of 1000 mV/s, the capacitors exhibited an average specific capacitance of 67 F/g and an energy density of 1.3 Wh/kg at a power density of 24 kW/kg. Therefore, free-standing EDASWCNT electrodes for EDLSCs are effective as electrodes and improve the properties of EDLSCs in comparison to binderfree COOH-SWCNT electrodes. These results were brought about by the crystalline isolated or thin bundled structure and the high wettability that ethylenediaminated SWCNTs prepared from F-SWCNTs possess. In the feature, we will reveal an optimal EDA concentration to achieve better EDLSC performance.

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ASSOCIATED CONTENT

S Supporting Information *

Definition of “production yield” of EDA-SWCNTs, XPS spectra of the wide range and C1s region of F-SWCNTs, XPS spectrum of the N1s region of stand-alone ethylenediamine, XPS spectrum of the C1s region of hc-SWCNTs, TG-mass curves of hc-SWCNTs and F-SWCNTs, mass fragmentation patterns of carbon fluoride and amine compounds, Raman scattering spectrum of F-SWCNTs heated under N2 atmosphere at 80 °C for 5 h, preparation of carboxylated SWCNTs, properties of EDLSCs with COOH-SWCNT electrodes, XPS spectrum of the C1s region of the COOH-SWCNTs, TG-mass curve of COOH-SWCNTs, HRTEM image and characteristics and of COOH-SWCNTs refluxed in 6.8 mol/L HNO3 aqueous solution for 4 h, and specific capacitance of each electrode versus scan rates. This material is available free of charge via the Internet at http://pubs.acs.org.

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