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Electrochemical capacitors using nitrogen-doped vertically aligned multi-walled carbon nanotube electrodes prepared by defluorination
Accepted Manuscript Electrochemical capacitors using nitrogen-doped vertically aligned multi-walled carbon nanotube electrodes prepared by defluorination Rei Nonomura, Takashi Itoh, Yoshinori Sato, Koji Yokoyama, Masashi Yamamoto, Tetsuo Nishida, Kenichi Motomiya, Kazuyuki Tohji, Yoshinori Sato PII:
S0008-6223(18)30201-X
DOI:
10.1016/j.carbon.2018.02.071
Reference:
CARBON 12907
To appear in:
Carbon
Received Date: 25 September 2017 Revised Date:
2 February 2018
Accepted Date: 14 February 2018
Please cite this article as: R. Nonomura, T. Itoh, Y. Sato, K. Yokoyama, M. Yamamoto, T. Nishida, K. Motomiya, K. Tohji, Y. Sato, Electrochemical capacitors using nitrogen-doped vertically aligned multi-walled carbon nanotube electrodes prepared by defluorination, Carbon (2018), doi: 10.1016/ j.carbon.2018.02.071. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Electrochemical capacitors using nitrogen-doped vertically aligned multi-walled carbon nanotube
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electrodes prepared by defluorination
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Rei Nonomura,a Takashi Itoh,b Yoshinori Sato,c Koji Yokoyama,a Masashi Yamamoto,c Tetsuo Nishida,c Kenichi Motomiya,a Kazuyuki Tohji,a Yoshinori Sato*a,d a
Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki, Aoba-
ku, Sendai 980-8579, Japan
Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University, Aoba 6-3,
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b
Aramaki, Aoba-ku, Sendai 980-8579, Japan c
Stella Chemifa Corporation, 1-41, Rinkai-cho, Izumiotsu, Osaka 595-0075, Japan Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research, Shinshu
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d
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University, Asahi 3-1-1, Matsumoto 390-8621, Japan
*
Corresponding author. Tel: +81-22-795-3215. E-mail: [email protected] (Y. S.)
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ABSTRACT:
Nitrogen-doped vertically aligned multi-walled carbon nanotubes (N-VAMWCNTs)
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were prepared by reacting fluorinated VAMWCNTs with ammonia gas at temperatures of 300– 600 °C. The N-VAMWCNTs were characterized using scanning electron microscopy, highresolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman
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scattering spectroscopy. In addition, the electrochemical properties of capacitors with NVAMWCNT electrodes were evaluated by cyclic voltammetry and AC impedance spectroscopy
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using a two-electrode coin-type cell in an electrolyte of propylene carbonate containing triethylmethylammonium tetrafluoroborate. All the samples were prepared without destroying the alignment structure of the nanotubes. The ratios between the concentration of fluorine, carbon, and nitrogen (F/C and N/F) and the R value (degree of crystallinity) of the samples
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indicate that the N-VAMWCNTs prepared at 500 °C (N500-VAMWCNTs) had the highest level of nitrogen doping and the best crystallinity among the samples. Nitrogen atoms were doped at a concentration of 5.26 at% into the nanotube frames, thus enriching the N500-VAMWCNTs with
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pyridinic nitrogen species. The average specific capacitance of the N500-VAMWCNT electrodes was 12.0 F/g at a scan rate of 100 mV/s, which is approximately 1.8 times the value obtained for
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the as-grown VAMWCNT electrodes (6.5 F/g).
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1. Introduction Activated carbon is usually used as polarized electrodes in electric double-layer capacitors
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(EDLCs). Since it has an intricate micropore structure and high contact resistance between the particles, the inefficiency of ion diffusion through its structure is cause for concern. Meanwhile, the high specific surface area and high conductivity of carbon nanotubes (CNTs) make them
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fascinating materials for polarized electrodes in EDLCs [1,2]. EDLCs that employ vertically aligned CNTs (VACNTs; the nanotubes were vertically grown on a substrate) allow swift ion
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diffusion owing to the oriented structure of the CNTs. In addition, the internal resistance of an EDLC can be reduced because one end of all the nanotubes is in contact with a collector electrode [3-10]. The specific capacitance of CNTs that were modified by chemical functionalization is hitherto known to be superior to that of non-surface-modified CNTs because of the increase in ion adsorption resulting from the wettability that is improved by solvents in the
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electrolyte [11,12]. Electrochemical devices with surface-modified CNT electrodes are not pure EDLCs, but electrochemical capacitors, because redox reactions occur at the functional groups
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during charge and discharge.
There have been few reports on electrochemical capacitors with surface-modified VACNT
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electrodes because there are limited ways to alter the surface of VACNTs. Lu et al. investigated electrochemical capacitors by utilizing vertically aligned multi-walled CNTs (VAMWCNTs) with negatively charged oxygen-containing functional groups as electrodes and ionic liquids as the electrolyte. They reported that the electrochemical capacitors showed high capacitance owing to the combined contribution from the electric double-layer capacitance and redox pseudocapacitance [13]. Meanwhile, pyrrolic- and graphitic-nitrogen species that introduced to a graphene skeleton have positive charges, which are known to improve the capacitance [14], and
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the properties of nitrogen-doped VACNTs for electrochemical capacitors have been reported [15-17]. These nitrogen-doped VACNTs were prepared by the direct synthesis method of chemical vapor deposition, using a carbon- and nitrogen-containing reaction gas as the source,
degree was low, which affected the electrochemical properties.
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and the produced nitrogen-doped VACNTs had a bamboo-like structure and the graphitization
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Recently, we successfully synthesized nitrogen-doped single-walled carbon nanotubes
(SWCNTs) through a defluorination-assisted nanotube-substitution reaction using ammonia gas
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[18,19]. Their levels of nitrogen doping (1.38–3.04 at%) were fairly high, and the CNTs were enriched with pyridinic- and pyrrolic-nitrogen species. In this new post-synthesis doping method, when the fluorinated SWCNTs are heated at a low temperature (300–600 °C), the fluorine groups are detached along with carbon atoms from the nanotube skeleton to form CxFy such as CF4. The remaining dangling-bonded carbon atoms in the nanotube skeleton react with the
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ammonia molecules, and nitrogen atoms are introduced to the nanotube frame. Since the fluorine functional groups modify the entire CNT surface, the whole surface is doped with nitrogen atoms. In addition, this method can selectively introduce pyridinic- and pyrrolic-nitrogen species to the
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nanotube frame while maintaining the vertically aligned nanotube structure. Here we report the
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fabrication and performance of electrochemical capacitors with nitrogen-doped VAMWCNT electrodes prepared by defluorination.
2. Experimental 2.1. Synthesis of Nitrogen-Doped VAMWCNTs via Defluorination
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The substrate used for the synthesis of VAMWCNTs was a p-type silicon wafer bearing a thermal oxide (SiO2). An iron thin film (thickness: 3 nm) was deposited on the substrate using electron-beam evaporation. The wafer was scored on the back and snapped into rectangles
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measuring 7 × 8 mm. The cut pieces were placed on a quartz plate (0.5 × 25 × 120 mm) that was then slid into the reactor from the outlet end to the center position. After loading and closure of the reactor, it was evacuated and a flow of He (99.9999%) was supplied into the chamber. A He
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flow rate of 496 sccm was used as the temperature was ramped for 20 min to the reaction
temperature of 678 °C and then held for 10 min. Following this procedure, VAMWCNTs were
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synthesized at 678 °C under a He atmosphere containing 3 mol% acetylene (99.9999%), supplied at a total flow rate of 512 sccm for 15 min. Next, the synthesized VAMWCNTs were placed on a Ni boat that was slid into the reactor from the outlet end to the center position. After the asgrown VAMWCNTs were annealed in vacuum at 250 °C for 2 h to remove absorbed water
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molecules, they were fluorinated at 250 °C for 30 min using a gas mixture of 20% F2 and 80% N2. Subsequently, they were subjected to thermal annealing at 250 °C for 1 h in a nitrogen flow. The resulting fluorinated VAMWCNTs are hereafter referred to as F-VAMWCNTs. Next, the F-
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VAMWCNTs were placed in a quartz glass boat, and the boat was set at the center of a quartz glass tube (inner diameter: 21.4 mm, length: 700 mm), which was then placed at the center of a
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tubular electric furnace. The F-VAMWCNTs were annealed in vacuum at 110 °C for 1 h to remove absorbed water molecules. They were then made to react at a given temperature (300– 600 °C) for 30 min in a flow of a mixture of 1.0% NH3 and 99.0% N2 gas. After the completion of the reaction, the sample was left to cool to room temperature in a N2 gas flow before it was removed from the furnace. Hereafter, the resulting samples that reacted at x °C are referred to as Nx-VAMWCNTs.
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2.2. Structural Characterization
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The morphology of the samples was determined with a scanning electron microscope (SEM; S-4100, Hitachi, Japan) and a high-resolution transmission electron microscope (HRTEM; HF2000, Hitachi, Japan). The SEM and HRTEM were operated at 5 and 200 kV, respectively. X-
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ray photoelectron spectroscopy (XPS) was performed using a K-Alpha+ system (ThermoFisher Scientific Inc., USA) with a monochromatic Al Kα X-ray source to analyze the elemental
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composition and chemical bonding state of the samples. A Raman scattering spectrometer (Jobin Yvon T64000, Horiba Co. Ltd., Japan) was used to analyze the vibrational modes of the graphitic materials. These measurements were performed in the backscattering mode at room temperature using a diode-pumped solid-state laser (Cobolt Blues™, Cobolt, Sweden) with an
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excitation wavelength of 473.0 nm (output power: 9.2 mW).
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2.3. Electrochemical Characterization
For the testing of the capacitors, a two-electrode coin-type cell of aluminum (HS Flat Cell,
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Hosen Co., Japan) was used. The reference electrode and the auxiliary electrode were connected together as one electrode, coupling with the working electrode to form the two-electrode system. The test cell consisted of a pair of Nx-VAMWCNT electrodes, with propylene carbonate (PC) containing 1.96 M triethylmethylammonium tetrafluoroborate (TEMABF4) as the electrolyte [4]. A Nx-VAMWCNT/Fe/SiO2/Si sample, the size of which was approximately 7 × 8 mm, was transferred to an Al sheet (thickness: 50 µm), which served as the current collector, without
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applying an electrically conductive glue. The separator used for the cell was made of cellulose (TF4050, Nippon Kodoshi Co., Japan). The capacitor assembly and electrochemical experiments were carried out in a purge-type glove box (DBO-1KP-SRS, Miwa, Japan). Cyclic voltammetry
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(CV) was performed at scan rates that were varied from 100 to 10000 mV/s in a cell voltage range between −2.5 and +2.5 V using a potentio-galvanostat (Model 263A, Princeton Applied Research, USA). The capacitance of the capacitors was calculated from the CV curve. The
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specific capacitances were estimated according to the following equation [12,20]:
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Cp = C/m = (Q/∆V)/m = {∫(I·dt)/∆V}/m,
where Cp is the specific capacitance measured in parallel; C is the total capacitance; m is the weight of the Nx-VAMWCNT electrode; Q is the total charge (area under the CV curve when the current is greater than zero); I is the current; dt is the measurement interval; and ∆V is the applied
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voltage.
The impedance measurements were carried out at a DC bias of 0 V, with a sinusoidal signal whose amplitude was 10 mV, using a frequency response analyzer (1260A, Solartron Analytical,
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USA) to drive the potentiostat (1287A, Solartron Analytical, USA). The frequencies were in the
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range of 0.1 Hz to 20 kHz.
With relation to the as-grown VAMWCNT and F-VAMWCNT, a test cell with a pair of VAMWCNT electrodes was also prepared via the same procedure, and their electrochemical characteristics were measured.
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3. Results and Discussion 3.1. Characterization of Nitrogen-Doped VAMWCNTs
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Fig. 1 shows SEM images of all the samples. The length of the as-grown VAMWCNTs
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varied from 220 to 280 µm. All the resulting samples were prepared without destroying the
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Fig. 1. SEM photographs of the (A) as-grown VAMWCNTs, (B) F-VAMWCNTs, (C) N300VAMWCNTs, (D) N400-VAMWCNTs, (E) N500-VAMWCNTs, and (F) N600-VAMWCNTs. Each inset is a magnified image of the area at the side of each sample.
alignment structure of the nanotubes. Table 1 shows the chemical composition of each sample, which was estimated from the corresponding XPS wide spectrum (Fig. S1). The concentration of
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fluorine atoms in the Nx-VAMWCNTs dramatically decreased when the reaction temperature was above 400 °C. Nitrogen atoms were detected in all of the Nx-VAMWCNT samples, and the maximum concentration of nitrogen atoms was 6.12 at% in the N400-VAMWCNTs. The N1s XPS
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peak of each Nx-VAMWCNT sample could be deconvolved into seven peaks (Fig. 2). The peaks labeled N1, N2, N3, N4, and N5 are attributed to the pyridinic-type nitrogen bond (398.5 ± 0.1 eV) [16,19,21], primary amine bond (399.3 ± 0.1 eV) [12,18,19,22], pyrrolic-type nitrogen bond
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(400.0 ± 0.2 eV) [16,19,21], graphitic-type nitrogen bond with the center nitrogen atom (401.2 ±
Table 1. Chemical composition of the as-grown VAMWCNTs, F-VAMWCNTs, and NxVAMWCNTs and the concentration of various nitrogen species in the Nx-VAMWCNTs as
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estimated from the results of the XPS analysis.
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Fig. 2. N1s XPS spectra of the (A) N300-VAMWCNTs, (B) N400-VAMWCNTs, (C) N500-
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VAMWCNTs, and (D) N600-VAMWCNTs.
0.1 eV) [16,19,21,23], and graphitic-type nitrogen bond with the valley nitrogen atom (402.4 ± 0.1 eV) [19,23]. The XPS peaks with higher binding energies labeled N6 (403.9 ± 0.2 eV) and
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N7 (405.3 ± 0.2 eV) are assigned to nitrogen oxide groups such as the pyridine-N-oxide bond and nitro groups [19,23-25]. All Nx-VAMWCNTs with the exception of N600-VAMWCNTs were
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enriched with pyridinic nitrogen species. In the N600-VAMWCNTs, the nitrogen concentration decreased to 2.02 at% and the pyrrolic nitrogen content was high among the nitrogen species. HRTEM revealed that the as-grown VAMWCNTs comprised MWCNTs with an average inner diameter of 5.0–6.5 nm and outer diameter of 10–13 nm, which indicate that they consisted of 9–12 layers. The TEM image of each sample is shown in Fig. 3. In the as-grown VAMWCNTs, all of the observed layers were stacked together with regular interlayer spacing.
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In contrast, a few outside layers of the F-VAMWCNTs expanded. It is considered that fluorine atoms penetrated the interlayer spaces, where the fluorine atoms bonded with the carbon atoms of the nanotube frame, and the interlayer spacing increased [26]. Similar increase in the
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interlayer spacing was observed in the N300-VAMWCNTs, whose fluorine concentration was equal to that of the F-VAMWCNTs. The outside of the nanotubes prepared at a reaction
temperature above 400 °C was observed to have no expanded interlayer spacing, but wavy and
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turbostratic layers where the nitrogen-based dopants resided. The Raman scattering intensity ratio of the D-band (1350 cm-1) to the G-band (1580 cm-1), ID/IG, also known as the R value, of
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the F-VAMWCNTs was the strongest of all the samples owing to the presence of C−F sp3hybridized carbon atoms. The R value of Nx-VAMWCNTs decreased with increasing reaction temperature at which defluorination occurred [18] because the fluorine groups were gradually
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detached from the nanotube skeleton with increasing reaction temperature (see Fig. S2 and
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Fig. 3. HRTEM images of the (A,a) as-grown VAMWCNTs, (B,b) F-VAMWCNTs, (C,c) N300VAMWCNTs, (D,d) N400-VAMWCNTs, (E,e) N500-VAMWCNTs, and (F,f) N600-VAMWCNTs. Each image on the right shows a magnified view of the area outlined by the square in the HRTEM image on the left.
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Table S1). All the Nx-VAMWCNTs showed high R values (>0.1) when compared with that of the as-grown VAMWCNTs, which originated from not only the nitrogen-based dopants, but also the remaining fluorine groups and the vacancy formed by defluorination. As we reported in our
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previous works [18,19], the doping mechanism of nitrogen atoms proceeded as follows: when the F-VAMWCNTs were heated at temperatures of 300–600 °C, the fluorine groups
functionalized on the outside nanotubes were detached along with carbon atoms from the
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nanotube skeleton and formed carbon fluorides [12,27,28], leaving topological defects. Pyridinic nitrogen and pyrrolic nitrogen are believed to be introduced only to the edges of large vacancy-
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type defects.
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Fig. 4. F/C and N/F concentration ratios and the R value of Nx-VAMWCNTs as functions of the reaction temperature.
Fig. 4 shows the ratio between the concentration of various components (F/C and N/F) and the R value (degree of crystallinity), which is deeply related to the conductivity of carbon materials, as functions of the reaction temperature. The N/F ratio had the maximum value of 3.29
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at 500 °C, indicating that the N500-VAMWCNTs contained many nitrogen-based dopants and a small amount of fluorine groups. Since the F/C ratio was almost zero at reaction temperatures above 400 °C, the samples synthesized at these temperatures possessed very few fluorine groups.
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Considering that the R value of Nx-VAMWCNTs decreased with increasing reaction temperature, N500-VAMWCNTs had the best crystallinity and the highest level of nitrogen doping among all of the samples. The concentrations of the pyridinic-type (N1), primary amine-type (N2),
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pyrrolic-type (N3), graphitic-type (N4, N5), and pyridine-N-oxide-type (N6, N7) nitrogen
species were 2.17, 0.16, 1.60, 1.11, and 0.22 at.%, respectively, and their percentages with
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respect to the total nitrogen species were 41.3, 3.0, 30.4, 21.1, and 4.2%, respectively. In the remaining experiments, we measured the electrochemical capacitance using N500-VAMWCNT
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electrodes.
3.2. Electrochemical Capacitance of N500-VAMWCNTs The CV curves of the electrochemical capacitors with as-grown VAMWCNT electrodes, Nx-
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VAMWCNT electrodes, and F-VAMWCNT electrodes at scan rates of 100, 500, 1000, 2000,
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5000, and 10000 mV/s are shown in Fig. 5. Note that the CV curve of the electrochemical capacitors with F-VAMWCNT electrodes at a scan rate of 10000 mV/s is not shown, since it was not measured due to overload of the measurement device. First, let us consider the CV curves of each sample at low scan rates (100, 500, and 1000 mV/s). The CV curves of the as-grown VAMWCNTs exhibit a butterfly shape [13,29-31], indicating that the current density increased with increasing cell polarization without Ohmic distortion and suggesting that the internal series resistance was low. The electrochemical capacitor with N500-VAMWCNT electrodes exhibited
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higher current densities than those of the capacitor with as-grown VAMWCNT electrodes, indicating that the nitrogen-based dopants affected the increase in specific capacitance. The shapes of the CV curves are considered to include the Faradaic current by redox reaction as well
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as the deterioration of the current’s response by resistance. The current densities of the capacitor with F-VAMWCNT electrodes were found to be lower than that of all other electrodes. This is
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due to the decreasing number of π-electrons in the nanotube owing to the introduction of
Fig. 5. Cyclic voltammograms of capacitors with as-grown VAMWCNT electrodes, N500VAMWCNT electrodes, and F-VAMWCNT electrodes at scan rates of (A) 100, (B) 500, (C) 1000, (D) 2000, (E) 5000, and (F) 10000 mV/s.
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fluorine groups with sp3-hybridized covalent bonds into the nanotube skeleton [26]. The CV curves of the capacitor with as-grown VAMWCNT electrodes obtained at high scan rates (2000, 5000, and 10000 mV/s) exhibit the butterfly shape similar to that of the CV curves obtained at
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low scan rates, indicating the excellent output derived from the low internal series resistance. The CV curves of the capacitor with N500-VAMWCNT electrodes become elongated at the upper right corner owing to the increase in internal series resistance. The CV curves of the capacitor
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with F-VAMWCNT electrodes showed further lower current densities than those at low scan rates. Fig. 6 shows the specific capacitance of each capacitor as a function of the scan rate. The
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specific capacitance of the capacitor with the F-VAMWCNT electrode was the lowest of the three electrodes at all scan rates, while that at scan rates of more than 1000 mV/s was almost zero, indicating that VAMWCNTs modified with a number of fluorine groups are not suitable as
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electrodes for electrochemical capacitors.
Fig. 6. Specific capacitance of each capacitor as a function of the scan rate.
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At a scan rate of 100 mV/s, the specific capacitance of the capacitor with N500-VAMWCNT electrodes was 12.0 F/g, which is approximately 1.8 times that of the capacitor with as-grown VAMWCNT electrodes (6.5 F/g), and the specific capacitance decreased to 1.8 F/g at a scan rate
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of 10000 mV/s. The specific capacitance of the capacitor with the N500-VAMWCNT electrode was larger than that of the capacitor with the as-grown VAMWCNT electrode at scan rates of less than 2000 mV/s. The decrease ratio in the specific capacitance of the capacitor with N500-
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VAMWCNT electrodes is larger than that with as-grown VAMWCNT electrodes, showing that N500-VAMWCNT electrodes have weaker power characteristics than the as-grown VAMWCNT
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electrodes; this is mainly attributed to the defective structure resulting from defluorination. The electrochemical capacitive behavior of the capacitors with as-grown VAMWCNT electrodes, N500-VAMWCNT electrodes, and F-VAMWCNT electrodes was also confirmed by AC impedance analysis. Fig. 7A depicts the Nyquist plot of samples and the magnified plot of
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the high-frequency region (Fig. 7B). Generally, a high-frequency region represents the sum of the electrolyte resistance, internal resistance of the carbon materials, and contact resistance between the carbon electrode and current collector [32]. Consider the case when substance
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transport in solution occurs simultaneously with charge transfer at the interface between the
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electrodes and electrolyte ions (capacitive semicircle in the high-frequency region). The capacitive semicircle is modeled by a parallel combination of an interfacial charge transfer resistance and a double-layer capacitance. The substance transport manifests as Warburg impedance inclined at 45° to the straight line (impedance of the diffused substance) in the lowfrequency region. The impedance spectra of the capacitors with as-grown VAMWCNT electrodes and the capacitors with N500-VAMWCNT electrodes showed a depressed capacitive semicircle in the high-frequency region with a negligible 45° Warburg region and steep low-
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frequency line. The capacitive semicircle in the impedance spectrum of the capacitor with N500VAMWCNT electrodes is larger than that of the capacitor with as-grown VAMWCNT electrodes. This is one reason why N500-VAMWCNTs have high internal resistance owing to
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vacancy defects at which nitrogen atoms do not dope in defluorination. Another reason is that conductive glue was not used to fix the VAMWCNTs to the Al current collector electrode, due to which the interfacial resistance between the VAMWCNTs and collector may be large.
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Furthermore, the interfacial charge transfer resistance is considered to be high in the capacitor with N500-VAMWCNT electrodes. The semicircles in the Nyquist plots of N-doped porous
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carbon are associated with the interfacial charge transfer resistance related to the Faradaic pseudocapacitive interaction between the ions and nitrogen species in nonaqueous electrolyte [33], although the mechanism of pseudocapacitive interaction between the ions and nitrogen species is not clear. The semicircle in the Nyquist plot of the capacitor with N500-VAMWCNT
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electrodes is larger than that of the capacitor with as-grown VAMWCNT electrodes. Because the two samples have almost the same alignment of nanotubes and use the same electrolyte, this difference in the semicircle size of the Nyquist plot can be ascribed to the difference in the
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pseudotransfer resistance (interfacial charge transfer resistance) [34], suggesting that N500VAMWCNTs have redox sites which do not exist in as-grown VAMWCNTs. These redox sites
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are considered to be the doping nitrogen species and the defective structure produced by defluorination. The Warburg impedance of the capacitor with N500-VAMWCNT electrodes is larger than that of the capacitor with as-grown VAMWCNT electrodes, suggesting that electrolyte ions diffuse to the vacancy defects of the nanotube skeleton. The knee frequency of the capacitor with as-grown VAMWCNT electrodes was 317 Hz, which is larger than that of the capacitor with N500-VAMWCNT electrodes (80 Hz), showing the ability of the capacitor with as-
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grown VAMWCNT electrodes to respond to fast charging and discharging events. Meanwhile, the Nyquist plot of the capacitor with the F-VAMWCNT electrodes had no capacitive semicircle and displayed an expanded Warburg region (indicating slow ion transport) and a less steep low-
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frequency line, suggesting that ion diffusion resistance to the electrodes is large. Electrolyte ions are considered difficult to penetrate F-VAMWCNT electrodes, since the distance between the nanotubes is narrow owing to modification with fluorine groups. Therefore, impedance analysis
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electrodes for electrochemical capacitors.
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also proves that F-VAMWCNTs modified with a number of fluorine groups are not suitable as
It is hitherto reported that the specific capacitance of electrochemical capacitors using nitrogen-doped carbon electrodes is improved in aqueous and non-aqueous electrolytes. One reason for this improvement is the enhanced wettability between the nanotube surface and electrolyte by doping nitrogen species to the nanotube skeleton [35-39]. Another reason is the
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existence of charged nitrogen species with pseudocapacitance. Some studies attribute the pseudocapacitance in N-doped carbons to not only negatively charged groups located at the edges of carbon skeletons such as pyridine [40-42] but also positively charged groups like
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pyridine [33,43], pyrrole [14, 33], graphitic N [14], and pyri-N-oxides [43]. However, it is not
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clear which nitrogen species are critical sites for surface charge transfer, because nitrogen-doped carbon materials with single-species nitrogen atoms cannot be synthesized. In the N500VAMWCNTs in this study, the percentages of the pyridinic-type (N1), primary amine-type (N2), pyrrolic-type (N3), graphitic-type (N4, N5), and pyridine-N-oxide-type (N6, N7) nitrogen species with respect to the total nitrogen species were 41.3, 3.0, 30.4, 21.1, and 4.2%, respectively. Moreover, vacancy defects produced by defluorination, without nitrogen species, exist in the nanotubes. Although these nitrogen species and defects may be redox reaction sites at
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which charge transfer occurs, the redox reaction sites could not be identified in this study. In order to synthesize N-doped VAMWCNTs with higher-performance electrochemical capacitor properties, it is necessary to investigate the influence of doping nitrogen species and their
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quantities on the electrochemical properties in detail. Current methods for doping nitrogen
species to a nanotube skeleton are classified into “direct-doping” and “post-doping” methods. It is difficult to control nitrogen species in the former method. On the other hand, our
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defluorination-assisted nanotube-substitution reaction, which is a type of the latter method,
allows us to control the doping of nitrogen species to CNTs. In future, we will synthesize defect-
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free N-doped VAMWCNTs with single-species nitrogen atoms and analyze their electrochemical
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properties to understand the relationship between the amount and type of doping nitrogen species.
Fig. 7. (A) Nyquist plots of each capacitor. (B) A magnified view of the high-frequency region.
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4. Conclusion We synthesized N-VAMWCNTs by reacting F-VAMWCNTs with ammonia gas at temperatures of 300–600 °C. The ratio of concentration of the components (F/C and N/F) and the
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R value (degree of crystallinity) of the resulting samples showed that the N-VAMWCNTs
prepared at 500 °C (N500-VAMWCNTs) had the highest level of nitrogen doping and the best crystallinity among the samples. In the N500-VAMWCNTs, nitrogen atoms were doped into the
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nanotube frames at a concentration of 5.26 at%, showing that the N500-VAMWCNTs were enriched with pyridinic nitrogen species. The capacitor with N500-VAMWCNT electrodes
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exhibited a specific capacitance of 12.0 F/g, which was superior to that of the capacitor with asgrown VAMWCNTs electrodes (6.5 F/g).
ACKNOWLEDGMENT
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Y. S. was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant
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S0008-6223(18)30201-X
DOI:
10.1016/j.carbon.2018.02.071
Reference:
CARBON 12907
To appear in:
Carbon
Received Date: 25 September 2017 Revised Date:
2 February 2018
Accepted Date: 14 February 2018
Please cite this article as: R. Nonomura, T. Itoh, Y. Sato, K. Yokoyama, M. Yamamoto, T. Nishida, K. Motomiya, K. Tohji, Y. Sato, Electrochemical capacitors using nitrogen-doped vertically aligned multi-walled carbon nanotube electrodes prepared by defluorination, Carbon (2018), doi: 10.1016/ j.carbon.2018.02.071. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Electrochemical capacitors using nitrogen-doped vertically aligned multi-walled carbon nanotube
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electrodes prepared by defluorination
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Rei Nonomura,a Takashi Itoh,b Yoshinori Sato,c Koji Yokoyama,a Masashi Yamamoto,c Tetsuo Nishida,c Kenichi Motomiya,a Kazuyuki Tohji,a Yoshinori Sato*a,d a
Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki, Aoba-
ku, Sendai 980-8579, Japan
Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University, Aoba 6-3,
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b
Aramaki, Aoba-ku, Sendai 980-8579, Japan c
Stella Chemifa Corporation, 1-41, Rinkai-cho, Izumiotsu, Osaka 595-0075, Japan Institute for Biomedical Sciences, Interdisciplinary Cluster for Cutting Edge Research, Shinshu
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d
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University, Asahi 3-1-1, Matsumoto 390-8621, Japan
*
Corresponding author. Tel: +81-22-795-3215. E-mail: [email protected] (Y. S.)
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ABSTRACT:
Nitrogen-doped vertically aligned multi-walled carbon nanotubes (N-VAMWCNTs)
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were prepared by reacting fluorinated VAMWCNTs with ammonia gas at temperatures of 300– 600 °C. The N-VAMWCNTs were characterized using scanning electron microscopy, highresolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman
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scattering spectroscopy. In addition, the electrochemical properties of capacitors with NVAMWCNT electrodes were evaluated by cyclic voltammetry and AC impedance spectroscopy
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using a two-electrode coin-type cell in an electrolyte of propylene carbonate containing triethylmethylammonium tetrafluoroborate. All the samples were prepared without destroying the alignment structure of the nanotubes. The ratios between the concentration of fluorine, carbon, and nitrogen (F/C and N/F) and the R value (degree of crystallinity) of the samples
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indicate that the N-VAMWCNTs prepared at 500 °C (N500-VAMWCNTs) had the highest level of nitrogen doping and the best crystallinity among the samples. Nitrogen atoms were doped at a concentration of 5.26 at% into the nanotube frames, thus enriching the N500-VAMWCNTs with
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pyridinic nitrogen species. The average specific capacitance of the N500-VAMWCNT electrodes was 12.0 F/g at a scan rate of 100 mV/s, which is approximately 1.8 times the value obtained for
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the as-grown VAMWCNT electrodes (6.5 F/g).
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1. Introduction Activated carbon is usually used as polarized electrodes in electric double-layer capacitors
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(EDLCs). Since it has an intricate micropore structure and high contact resistance between the particles, the inefficiency of ion diffusion through its structure is cause for concern. Meanwhile, the high specific surface area and high conductivity of carbon nanotubes (CNTs) make them
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fascinating materials for polarized electrodes in EDLCs [1,2]. EDLCs that employ vertically aligned CNTs (VACNTs; the nanotubes were vertically grown on a substrate) allow swift ion
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diffusion owing to the oriented structure of the CNTs. In addition, the internal resistance of an EDLC can be reduced because one end of all the nanotubes is in contact with a collector electrode [3-10]. The specific capacitance of CNTs that were modified by chemical functionalization is hitherto known to be superior to that of non-surface-modified CNTs because of the increase in ion adsorption resulting from the wettability that is improved by solvents in the
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electrolyte [11,12]. Electrochemical devices with surface-modified CNT electrodes are not pure EDLCs, but electrochemical capacitors, because redox reactions occur at the functional groups
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during charge and discharge.
There have been few reports on electrochemical capacitors with surface-modified VACNT
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electrodes because there are limited ways to alter the surface of VACNTs. Lu et al. investigated electrochemical capacitors by utilizing vertically aligned multi-walled CNTs (VAMWCNTs) with negatively charged oxygen-containing functional groups as electrodes and ionic liquids as the electrolyte. They reported that the electrochemical capacitors showed high capacitance owing to the combined contribution from the electric double-layer capacitance and redox pseudocapacitance [13]. Meanwhile, pyrrolic- and graphitic-nitrogen species that introduced to a graphene skeleton have positive charges, which are known to improve the capacitance [14], and
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the properties of nitrogen-doped VACNTs for electrochemical capacitors have been reported [15-17]. These nitrogen-doped VACNTs were prepared by the direct synthesis method of chemical vapor deposition, using a carbon- and nitrogen-containing reaction gas as the source,
degree was low, which affected the electrochemical properties.
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and the produced nitrogen-doped VACNTs had a bamboo-like structure and the graphitization
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Recently, we successfully synthesized nitrogen-doped single-walled carbon nanotubes
(SWCNTs) through a defluorination-assisted nanotube-substitution reaction using ammonia gas
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[18,19]. Their levels of nitrogen doping (1.38–3.04 at%) were fairly high, and the CNTs were enriched with pyridinic- and pyrrolic-nitrogen species. In this new post-synthesis doping method, when the fluorinated SWCNTs are heated at a low temperature (300–600 °C), the fluorine groups are detached along with carbon atoms from the nanotube skeleton to form CxFy such as CF4. The remaining dangling-bonded carbon atoms in the nanotube skeleton react with the
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ammonia molecules, and nitrogen atoms are introduced to the nanotube frame. Since the fluorine functional groups modify the entire CNT surface, the whole surface is doped with nitrogen atoms. In addition, this method can selectively introduce pyridinic- and pyrrolic-nitrogen species to the
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nanotube frame while maintaining the vertically aligned nanotube structure. Here we report the
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fabrication and performance of electrochemical capacitors with nitrogen-doped VAMWCNT electrodes prepared by defluorination.
2. Experimental 2.1. Synthesis of Nitrogen-Doped VAMWCNTs via Defluorination
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The substrate used for the synthesis of VAMWCNTs was a p-type silicon wafer bearing a thermal oxide (SiO2). An iron thin film (thickness: 3 nm) was deposited on the substrate using electron-beam evaporation. The wafer was scored on the back and snapped into rectangles
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measuring 7 × 8 mm. The cut pieces were placed on a quartz plate (0.5 × 25 × 120 mm) that was then slid into the reactor from the outlet end to the center position. After loading and closure of the reactor, it was evacuated and a flow of He (99.9999%) was supplied into the chamber. A He
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flow rate of 496 sccm was used as the temperature was ramped for 20 min to the reaction
temperature of 678 °C and then held for 10 min. Following this procedure, VAMWCNTs were
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synthesized at 678 °C under a He atmosphere containing 3 mol% acetylene (99.9999%), supplied at a total flow rate of 512 sccm for 15 min. Next, the synthesized VAMWCNTs were placed on a Ni boat that was slid into the reactor from the outlet end to the center position. After the asgrown VAMWCNTs were annealed in vacuum at 250 °C for 2 h to remove absorbed water
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molecules, they were fluorinated at 250 °C for 30 min using a gas mixture of 20% F2 and 80% N2. Subsequently, they were subjected to thermal annealing at 250 °C for 1 h in a nitrogen flow. The resulting fluorinated VAMWCNTs are hereafter referred to as F-VAMWCNTs. Next, the F-
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VAMWCNTs were placed in a quartz glass boat, and the boat was set at the center of a quartz glass tube (inner diameter: 21.4 mm, length: 700 mm), which was then placed at the center of a
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tubular electric furnace. The F-VAMWCNTs were annealed in vacuum at 110 °C for 1 h to remove absorbed water molecules. They were then made to react at a given temperature (300– 600 °C) for 30 min in a flow of a mixture of 1.0% NH3 and 99.0% N2 gas. After the completion of the reaction, the sample was left to cool to room temperature in a N2 gas flow before it was removed from the furnace. Hereafter, the resulting samples that reacted at x °C are referred to as Nx-VAMWCNTs.
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2.2. Structural Characterization
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The morphology of the samples was determined with a scanning electron microscope (SEM; S-4100, Hitachi, Japan) and a high-resolution transmission electron microscope (HRTEM; HF2000, Hitachi, Japan). The SEM and HRTEM were operated at 5 and 200 kV, respectively. X-
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ray photoelectron spectroscopy (XPS) was performed using a K-Alpha+ system (ThermoFisher Scientific Inc., USA) with a monochromatic Al Kα X-ray source to analyze the elemental
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composition and chemical bonding state of the samples. A Raman scattering spectrometer (Jobin Yvon T64000, Horiba Co. Ltd., Japan) was used to analyze the vibrational modes of the graphitic materials. These measurements were performed in the backscattering mode at room temperature using a diode-pumped solid-state laser (Cobolt Blues™, Cobolt, Sweden) with an
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excitation wavelength of 473.0 nm (output power: 9.2 mW).
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2.3. Electrochemical Characterization
For the testing of the capacitors, a two-electrode coin-type cell of aluminum (HS Flat Cell,
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Hosen Co., Japan) was used. The reference electrode and the auxiliary electrode were connected together as one electrode, coupling with the working electrode to form the two-electrode system. The test cell consisted of a pair of Nx-VAMWCNT electrodes, with propylene carbonate (PC) containing 1.96 M triethylmethylammonium tetrafluoroborate (TEMABF4) as the electrolyte [4]. A Nx-VAMWCNT/Fe/SiO2/Si sample, the size of which was approximately 7 × 8 mm, was transferred to an Al sheet (thickness: 50 µm), which served as the current collector, without
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applying an electrically conductive glue. The separator used for the cell was made of cellulose (TF4050, Nippon Kodoshi Co., Japan). The capacitor assembly and electrochemical experiments were carried out in a purge-type glove box (DBO-1KP-SRS, Miwa, Japan). Cyclic voltammetry
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(CV) was performed at scan rates that were varied from 100 to 10000 mV/s in a cell voltage range between −2.5 and +2.5 V using a potentio-galvanostat (Model 263A, Princeton Applied Research, USA). The capacitance of the capacitors was calculated from the CV curve. The
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specific capacitances were estimated according to the following equation [12,20]:
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Cp = C/m = (Q/∆V)/m = {∫(I·dt)/∆V}/m,
where Cp is the specific capacitance measured in parallel; C is the total capacitance; m is the weight of the Nx-VAMWCNT electrode; Q is the total charge (area under the CV curve when the current is greater than zero); I is the current; dt is the measurement interval; and ∆V is the applied
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voltage.
The impedance measurements were carried out at a DC bias of 0 V, with a sinusoidal signal whose amplitude was 10 mV, using a frequency response analyzer (1260A, Solartron Analytical,
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USA) to drive the potentiostat (1287A, Solartron Analytical, USA). The frequencies were in the
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range of 0.1 Hz to 20 kHz.
With relation to the as-grown VAMWCNT and F-VAMWCNT, a test cell with a pair of VAMWCNT electrodes was also prepared via the same procedure, and their electrochemical characteristics were measured.
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3. Results and Discussion 3.1. Characterization of Nitrogen-Doped VAMWCNTs
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Fig. 1 shows SEM images of all the samples. The length of the as-grown VAMWCNTs
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varied from 220 to 280 µm. All the resulting samples were prepared without destroying the
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Fig. 1. SEM photographs of the (A) as-grown VAMWCNTs, (B) F-VAMWCNTs, (C) N300VAMWCNTs, (D) N400-VAMWCNTs, (E) N500-VAMWCNTs, and (F) N600-VAMWCNTs. Each inset is a magnified image of the area at the side of each sample.
alignment structure of the nanotubes. Table 1 shows the chemical composition of each sample, which was estimated from the corresponding XPS wide spectrum (Fig. S1). The concentration of
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fluorine atoms in the Nx-VAMWCNTs dramatically decreased when the reaction temperature was above 400 °C. Nitrogen atoms were detected in all of the Nx-VAMWCNT samples, and the maximum concentration of nitrogen atoms was 6.12 at% in the N400-VAMWCNTs. The N1s XPS
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peak of each Nx-VAMWCNT sample could be deconvolved into seven peaks (Fig. 2). The peaks labeled N1, N2, N3, N4, and N5 are attributed to the pyridinic-type nitrogen bond (398.5 ± 0.1 eV) [16,19,21], primary amine bond (399.3 ± 0.1 eV) [12,18,19,22], pyrrolic-type nitrogen bond
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(400.0 ± 0.2 eV) [16,19,21], graphitic-type nitrogen bond with the center nitrogen atom (401.2 ±
Table 1. Chemical composition of the as-grown VAMWCNTs, F-VAMWCNTs, and NxVAMWCNTs and the concentration of various nitrogen species in the Nx-VAMWCNTs as
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estimated from the results of the XPS analysis.
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Fig. 2. N1s XPS spectra of the (A) N300-VAMWCNTs, (B) N400-VAMWCNTs, (C) N500-
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VAMWCNTs, and (D) N600-VAMWCNTs.
0.1 eV) [16,19,21,23], and graphitic-type nitrogen bond with the valley nitrogen atom (402.4 ± 0.1 eV) [19,23]. The XPS peaks with higher binding energies labeled N6 (403.9 ± 0.2 eV) and
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N7 (405.3 ± 0.2 eV) are assigned to nitrogen oxide groups such as the pyridine-N-oxide bond and nitro groups [19,23-25]. All Nx-VAMWCNTs with the exception of N600-VAMWCNTs were
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enriched with pyridinic nitrogen species. In the N600-VAMWCNTs, the nitrogen concentration decreased to 2.02 at% and the pyrrolic nitrogen content was high among the nitrogen species. HRTEM revealed that the as-grown VAMWCNTs comprised MWCNTs with an average inner diameter of 5.0–6.5 nm and outer diameter of 10–13 nm, which indicate that they consisted of 9–12 layers. The TEM image of each sample is shown in Fig. 3. In the as-grown VAMWCNTs, all of the observed layers were stacked together with regular interlayer spacing.
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In contrast, a few outside layers of the F-VAMWCNTs expanded. It is considered that fluorine atoms penetrated the interlayer spaces, where the fluorine atoms bonded with the carbon atoms of the nanotube frame, and the interlayer spacing increased [26]. Similar increase in the
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interlayer spacing was observed in the N300-VAMWCNTs, whose fluorine concentration was equal to that of the F-VAMWCNTs. The outside of the nanotubes prepared at a reaction
temperature above 400 °C was observed to have no expanded interlayer spacing, but wavy and
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turbostratic layers where the nitrogen-based dopants resided. The Raman scattering intensity ratio of the D-band (1350 cm-1) to the G-band (1580 cm-1), ID/IG, also known as the R value, of
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the F-VAMWCNTs was the strongest of all the samples owing to the presence of C−F sp3hybridized carbon atoms. The R value of Nx-VAMWCNTs decreased with increasing reaction temperature at which defluorination occurred [18] because the fluorine groups were gradually
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detached from the nanotube skeleton with increasing reaction temperature (see Fig. S2 and
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Fig. 3. HRTEM images of the (A,a) as-grown VAMWCNTs, (B,b) F-VAMWCNTs, (C,c) N300VAMWCNTs, (D,d) N400-VAMWCNTs, (E,e) N500-VAMWCNTs, and (F,f) N600-VAMWCNTs. Each image on the right shows a magnified view of the area outlined by the square in the HRTEM image on the left.
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Table S1). All the Nx-VAMWCNTs showed high R values (>0.1) when compared with that of the as-grown VAMWCNTs, which originated from not only the nitrogen-based dopants, but also the remaining fluorine groups and the vacancy formed by defluorination. As we reported in our
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previous works [18,19], the doping mechanism of nitrogen atoms proceeded as follows: when the F-VAMWCNTs were heated at temperatures of 300–600 °C, the fluorine groups
functionalized on the outside nanotubes were detached along with carbon atoms from the
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nanotube skeleton and formed carbon fluorides [12,27,28], leaving topological defects. Pyridinic nitrogen and pyrrolic nitrogen are believed to be introduced only to the edges of large vacancy-
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type defects.
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Fig. 4. F/C and N/F concentration ratios and the R value of Nx-VAMWCNTs as functions of the reaction temperature.
Fig. 4 shows the ratio between the concentration of various components (F/C and N/F) and the R value (degree of crystallinity), which is deeply related to the conductivity of carbon materials, as functions of the reaction temperature. The N/F ratio had the maximum value of 3.29
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at 500 °C, indicating that the N500-VAMWCNTs contained many nitrogen-based dopants and a small amount of fluorine groups. Since the F/C ratio was almost zero at reaction temperatures above 400 °C, the samples synthesized at these temperatures possessed very few fluorine groups.
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Considering that the R value of Nx-VAMWCNTs decreased with increasing reaction temperature, N500-VAMWCNTs had the best crystallinity and the highest level of nitrogen doping among all of the samples. The concentrations of the pyridinic-type (N1), primary amine-type (N2),
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pyrrolic-type (N3), graphitic-type (N4, N5), and pyridine-N-oxide-type (N6, N7) nitrogen
species were 2.17, 0.16, 1.60, 1.11, and 0.22 at.%, respectively, and their percentages with
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respect to the total nitrogen species were 41.3, 3.0, 30.4, 21.1, and 4.2%, respectively. In the remaining experiments, we measured the electrochemical capacitance using N500-VAMWCNT
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electrodes.
3.2. Electrochemical Capacitance of N500-VAMWCNTs The CV curves of the electrochemical capacitors with as-grown VAMWCNT electrodes, Nx-
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VAMWCNT electrodes, and F-VAMWCNT electrodes at scan rates of 100, 500, 1000, 2000,
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5000, and 10000 mV/s are shown in Fig. 5. Note that the CV curve of the electrochemical capacitors with F-VAMWCNT electrodes at a scan rate of 10000 mV/s is not shown, since it was not measured due to overload of the measurement device. First, let us consider the CV curves of each sample at low scan rates (100, 500, and 1000 mV/s). The CV curves of the as-grown VAMWCNTs exhibit a butterfly shape [13,29-31], indicating that the current density increased with increasing cell polarization without Ohmic distortion and suggesting that the internal series resistance was low. The electrochemical capacitor with N500-VAMWCNT electrodes exhibited
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higher current densities than those of the capacitor with as-grown VAMWCNT electrodes, indicating that the nitrogen-based dopants affected the increase in specific capacitance. The shapes of the CV curves are considered to include the Faradaic current by redox reaction as well
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as the deterioration of the current’s response by resistance. The current densities of the capacitor with F-VAMWCNT electrodes were found to be lower than that of all other electrodes. This is
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due to the decreasing number of π-electrons in the nanotube owing to the introduction of
Fig. 5. Cyclic voltammograms of capacitors with as-grown VAMWCNT electrodes, N500VAMWCNT electrodes, and F-VAMWCNT electrodes at scan rates of (A) 100, (B) 500, (C) 1000, (D) 2000, (E) 5000, and (F) 10000 mV/s.
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fluorine groups with sp3-hybridized covalent bonds into the nanotube skeleton [26]. The CV curves of the capacitor with as-grown VAMWCNT electrodes obtained at high scan rates (2000, 5000, and 10000 mV/s) exhibit the butterfly shape similar to that of the CV curves obtained at
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low scan rates, indicating the excellent output derived from the low internal series resistance. The CV curves of the capacitor with N500-VAMWCNT electrodes become elongated at the upper right corner owing to the increase in internal series resistance. The CV curves of the capacitor
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with F-VAMWCNT electrodes showed further lower current densities than those at low scan rates. Fig. 6 shows the specific capacitance of each capacitor as a function of the scan rate. The
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specific capacitance of the capacitor with the F-VAMWCNT electrode was the lowest of the three electrodes at all scan rates, while that at scan rates of more than 1000 mV/s was almost zero, indicating that VAMWCNTs modified with a number of fluorine groups are not suitable as
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electrodes for electrochemical capacitors.
Fig. 6. Specific capacitance of each capacitor as a function of the scan rate.
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At a scan rate of 100 mV/s, the specific capacitance of the capacitor with N500-VAMWCNT electrodes was 12.0 F/g, which is approximately 1.8 times that of the capacitor with as-grown VAMWCNT electrodes (6.5 F/g), and the specific capacitance decreased to 1.8 F/g at a scan rate
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of 10000 mV/s. The specific capacitance of the capacitor with the N500-VAMWCNT electrode was larger than that of the capacitor with the as-grown VAMWCNT electrode at scan rates of less than 2000 mV/s. The decrease ratio in the specific capacitance of the capacitor with N500-
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VAMWCNT electrodes is larger than that with as-grown VAMWCNT electrodes, showing that N500-VAMWCNT electrodes have weaker power characteristics than the as-grown VAMWCNT
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electrodes; this is mainly attributed to the defective structure resulting from defluorination. The electrochemical capacitive behavior of the capacitors with as-grown VAMWCNT electrodes, N500-VAMWCNT electrodes, and F-VAMWCNT electrodes was also confirmed by AC impedance analysis. Fig. 7A depicts the Nyquist plot of samples and the magnified plot of
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the high-frequency region (Fig. 7B). Generally, a high-frequency region represents the sum of the electrolyte resistance, internal resistance of the carbon materials, and contact resistance between the carbon electrode and current collector [32]. Consider the case when substance
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transport in solution occurs simultaneously with charge transfer at the interface between the
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electrodes and electrolyte ions (capacitive semicircle in the high-frequency region). The capacitive semicircle is modeled by a parallel combination of an interfacial charge transfer resistance and a double-layer capacitance. The substance transport manifests as Warburg impedance inclined at 45° to the straight line (impedance of the diffused substance) in the lowfrequency region. The impedance spectra of the capacitors with as-grown VAMWCNT electrodes and the capacitors with N500-VAMWCNT electrodes showed a depressed capacitive semicircle in the high-frequency region with a negligible 45° Warburg region and steep low-
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frequency line. The capacitive semicircle in the impedance spectrum of the capacitor with N500VAMWCNT electrodes is larger than that of the capacitor with as-grown VAMWCNT electrodes. This is one reason why N500-VAMWCNTs have high internal resistance owing to
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vacancy defects at which nitrogen atoms do not dope in defluorination. Another reason is that conductive glue was not used to fix the VAMWCNTs to the Al current collector electrode, due to which the interfacial resistance between the VAMWCNTs and collector may be large.
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Furthermore, the interfacial charge transfer resistance is considered to be high in the capacitor with N500-VAMWCNT electrodes. The semicircles in the Nyquist plots of N-doped porous
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carbon are associated with the interfacial charge transfer resistance related to the Faradaic pseudocapacitive interaction between the ions and nitrogen species in nonaqueous electrolyte [33], although the mechanism of pseudocapacitive interaction between the ions and nitrogen species is not clear. The semicircle in the Nyquist plot of the capacitor with N500-VAMWCNT
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electrodes is larger than that of the capacitor with as-grown VAMWCNT electrodes. Because the two samples have almost the same alignment of nanotubes and use the same electrolyte, this difference in the semicircle size of the Nyquist plot can be ascribed to the difference in the
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pseudotransfer resistance (interfacial charge transfer resistance) [34], suggesting that N500VAMWCNTs have redox sites which do not exist in as-grown VAMWCNTs. These redox sites
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are considered to be the doping nitrogen species and the defective structure produced by defluorination. The Warburg impedance of the capacitor with N500-VAMWCNT electrodes is larger than that of the capacitor with as-grown VAMWCNT electrodes, suggesting that electrolyte ions diffuse to the vacancy defects of the nanotube skeleton. The knee frequency of the capacitor with as-grown VAMWCNT electrodes was 317 Hz, which is larger than that of the capacitor with N500-VAMWCNT electrodes (80 Hz), showing the ability of the capacitor with as-
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grown VAMWCNT electrodes to respond to fast charging and discharging events. Meanwhile, the Nyquist plot of the capacitor with the F-VAMWCNT electrodes had no capacitive semicircle and displayed an expanded Warburg region (indicating slow ion transport) and a less steep low-
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frequency line, suggesting that ion diffusion resistance to the electrodes is large. Electrolyte ions are considered difficult to penetrate F-VAMWCNT electrodes, since the distance between the nanotubes is narrow owing to modification with fluorine groups. Therefore, impedance analysis
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electrodes for electrochemical capacitors.
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also proves that F-VAMWCNTs modified with a number of fluorine groups are not suitable as
It is hitherto reported that the specific capacitance of electrochemical capacitors using nitrogen-doped carbon electrodes is improved in aqueous and non-aqueous electrolytes. One reason for this improvement is the enhanced wettability between the nanotube surface and electrolyte by doping nitrogen species to the nanotube skeleton [35-39]. Another reason is the
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existence of charged nitrogen species with pseudocapacitance. Some studies attribute the pseudocapacitance in N-doped carbons to not only negatively charged groups located at the edges of carbon skeletons such as pyridine [40-42] but also positively charged groups like
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pyridine [33,43], pyrrole [14, 33], graphitic N [14], and pyri-N-oxides [43]. However, it is not
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clear which nitrogen species are critical sites for surface charge transfer, because nitrogen-doped carbon materials with single-species nitrogen atoms cannot be synthesized. In the N500VAMWCNTs in this study, the percentages of the pyridinic-type (N1), primary amine-type (N2), pyrrolic-type (N3), graphitic-type (N4, N5), and pyridine-N-oxide-type (N6, N7) nitrogen species with respect to the total nitrogen species were 41.3, 3.0, 30.4, 21.1, and 4.2%, respectively. Moreover, vacancy defects produced by defluorination, without nitrogen species, exist in the nanotubes. Although these nitrogen species and defects may be redox reaction sites at
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which charge transfer occurs, the redox reaction sites could not be identified in this study. In order to synthesize N-doped VAMWCNTs with higher-performance electrochemical capacitor properties, it is necessary to investigate the influence of doping nitrogen species and their
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quantities on the electrochemical properties in detail. Current methods for doping nitrogen
species to a nanotube skeleton are classified into “direct-doping” and “post-doping” methods. It is difficult to control nitrogen species in the former method. On the other hand, our
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defluorination-assisted nanotube-substitution reaction, which is a type of the latter method,
allows us to control the doping of nitrogen species to CNTs. In future, we will synthesize defect-
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free N-doped VAMWCNTs with single-species nitrogen atoms and analyze their electrochemical
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properties to understand the relationship between the amount and type of doping nitrogen species.
Fig. 7. (A) Nyquist plots of each capacitor. (B) A magnified view of the high-frequency region.
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4. Conclusion We synthesized N-VAMWCNTs by reacting F-VAMWCNTs with ammonia gas at temperatures of 300–600 °C. The ratio of concentration of the components (F/C and N/F) and the
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R value (degree of crystallinity) of the resulting samples showed that the N-VAMWCNTs
prepared at 500 °C (N500-VAMWCNTs) had the highest level of nitrogen doping and the best crystallinity among the samples. In the N500-VAMWCNTs, nitrogen atoms were doped into the
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nanotube frames at a concentration of 5.26 at%, showing that the N500-VAMWCNTs were enriched with pyridinic nitrogen species. The capacitor with N500-VAMWCNT electrodes
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exhibited a specific capacitance of 12.0 F/g, which was superior to that of the capacitor with asgrown VAMWCNTs electrodes (6.5 F/g).
ACKNOWLEDGMENT
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Y. S. was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant
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