SWNTs-in-Binary-Polymer Nanofiber Structures and Their Use as

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SWNTs-in-Binary-Polymer Nanofiber Structures and Their Use as Carbon Precursors for Electrochemical Application Yukyung Kim, Thanh-Hai Le, Saerona Kim, Geunsu Park, Kap Seung Yang, and Hyeonseok Yoon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12145 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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The Journal of Physical Chemistry

SWNTs-in-Binary-Polymer Nanofiber Structures and Their Use as Carbon Precursors for Electrochemical Application Yukyung Kim,2,† Thanh-Hai Le,2,† Saerona Kim,2 Geunsu Park 2 Kap Seung Yang2,* and Hyeonseok Yoon1,2,*

1

School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea. 2

Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea.

†

These authors contributed equally to this work.

Corresponding Authors: *E-mail: [email protected], [email protected]

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ABSTRACT

Hierarchical structuring of materials in the nanometer regime provides opportunities to achieve extraordinary characteristics of the resulting products. Here, we report unique one-dimensional hierarchical nanostructures consisting of single-walled carbon nanotubes (SWNTs), polyvinyl alcohol (PVA), and polyacrylonitrile (PAN). First, SWNTs-in-binary-polymer nanofiber (SbPNF) structures were obtained through the incorporation of PVA-wrapped SWNTs into PAN, followed by the electrospinning of the SWNT/PVA/PAN solution. Importantly, the SbPNFs exhibited an aligned SWNTs-in-nanofiber structure and enhanced ordering of the polymer chains. The SbPNFs were successfully converted to carbonized products (SbCNFs) with enhanced crystallinity and tunable electrochemical properties. Compared to the control samples (no SWNT), the charge transfer resistances and surface area of the SbCNFs were two orders of magnitude lower and 11−20% higher, respectively, which resulted in better electrochemical properties. The major factors determining the properties of the SbCNFs included the SWNT content and PVA/PAN microphase behavior. Furthermore, the removal of the PVA phase from the SbPNFs provided another opportunity to control the textural properties of the carbonized products. It was found that meso- and macropores were more developed in the carbonized products (SCNFs). The specific capacitance of the SCNFs increased to a maximum of 577 F g−1, which was 3.7 times higher than that of the SbCNFs. The SCNF with the best properties was successfully applied to electrochemical capacitors as the electrode material. It is believed that further optimization of the hierarchical nanostructures will impart attractive properties for various applications.


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INTRODUCTION A variety of nanostructured materials with fascinating properties have been developed for realizing new, and challenging target applications.1-7 Versatile nanostructures consisting of polymers, inorganic semiconductors, and their hybrids have been discovered for electronics, optics, medicine, and energy applications.8-14 In particular, various nanostructured carbon allotropes have been found, such as fullerenes, nanotubes, and graphene, and considerable efforts continue to be applied to research focused on expanding the range of applications of nanocarbon species.15-21 There are several critical issues to be overcome in the purification, separation, manipulation, and processing of nanocarbons.18, 22-23 As a typical nanocarbon, single-walled carbon nanotubes (SWNTs) have diameters of the order of one nanometer and lengths ranging from tens of nanometers to a few micrometers.24-26 The electrical/electronic and optical properties of SWNTs are determined by their diameter and chirality.27-29 Although SWNTs have well-defined morphologies, it remains difficult to produce monodisperse SWNTs with uniform diameter and chirality.30-33 The high aspect ratio of the SWNTs also makes them difficult to manipulate for specific applications.34 For example, the use of SWNTs has presented difficulties in the controlled deposition/assembly on a substrate or dispersion in a continuous phase medium. As a result, there is a great demand for developing efficient technological strategies to circumvent these problems. It is important to structurally design and fabricate novel nanostructured materials, which may facilitate the attainment of fascinating functions for advanced applications.35-36 Advances in nanotechnology have provided opportunities to fabricate hierarchical nanostructures for achieving further innovative material properties.37-39 Most hierarchical nanostructures contain more than two component phases, in which the different phases are micro-arranged with two or more levels in a hierarchical structure.40-42 Well-designed hierarchical nanostructures can possess all the advantages that individual components may have, which results in extraordinary physical/chemical properties.43-45 However, it remains a great challenge to construct hierarchical nanostructures over a long length scale in a controlled manner.46-47 Like other materials, in the nanometer regime, the major properties of ACS Paragon Plus Environment

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carbon materials highly depend on their type of orbital hybridization, defects, and crystallinity.2, 48-49 Therefore, the precise structural control of carbon materials at the nanoscale can lead to unprecedented and beneficial properties that are substantially different from those of their bulk counterparts. In this work, we report an efficient strategy for fabricating a new hierarchically nanostructured material, namely “SWNTs-in-binary-polymer nanofiber” (SbPNF) structures. The SbPNF was structurally designed to amplify the unique characteristics of the individual components through hierarchical combination. Specifically, SWNTs were homogeneously dispersed in a water-soluble polymer, polyvinyl alcohol (PVA) matrix. The SWNT/PVA dispersion was mixed with a typical carbon precursor polymer, polyacrylonitrile (PAN), and the resulting dispersion solution was electrospun to produce SbPNFs. A thermal treatment of the SbPNFs under appropriate conditions then yielded “SWNTs-incarbon nanofiber” (SbCNF) structures. The SbPNFs had a hierarchical structure of SWNTs aligned in polymer nanofibers, providing high surface area with enhanced crystallinity. The use of SbPNFs as carbon precursors provided a good opportunity to control the major properties of the resulting SbCNFs.50 The textural properties of the SbCNFs, as well as the electrical/electrochemical properties, were dependent on the concentration of the incorporated SWNT and the PVA-to-PAN ratio. Additionally, the removal of one polymer phase from the SbPNFs led to a significant change in the textural properties of their carbonized products. This synthetic strategy can be extended to various combinations of core nanotubes with nanofiber matrix, which can result in unexpected and interesting characteristics.


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METHODS Materials. PVA (Mw = 9,000–10,000, 80% hydrolyzed), PAN (Mw = 150,000), SWNT ((6,5) chirality ≥ 40%) and dimethyl sulfoxide (DMSO) were purchased from Sigma Aldrich. Fabrication of nanofibers. First, 1 g of PVA was dissolved in 10 mL of DMSO, and SWNT powder was then added to the PVA/DMSO solution. The SWNT/PVA/DMSO solution was subjected to ultrasonication for 2 h, which yielded a black dispersion. Next, 0.8 g of PAN was dissolved in 5 mL of DMSO, following which the solution was mixed with 2 mL of the as-obtained SWNT/PVA/DMSO solution. The concentrations of SWNT in the final products were controlled to be 0.02, 0.04, and 0.06 wt%. The mixture solution was further stirred for 4 h at 60 °C and then for 12 h at 25 °C. The resultant solution was transferred into a syringe and delivered to the needle tip (0.838 mm inner diameter) of the syringe at a feeding rate of 1.0 mL h−1. A 20-kV voltage was applied to the solution via the stainless steel needle of the syringe. The ejecting nanofibers were collected on a cylindrical mandrel (9 cm diameter) revolving at 1,000 rpm, with a separation distance between the needle tip and mandrel of 18 cm. The electrospun SWNT/PVA/PAN ternary nanoﬁbers were calcined at 280 °C for 1 h under an air atmosphere, followed by carbonization at 800 °C under a nitrogen atmosphere. The removal of PVA from the SbPNFs was effected by immersing them in excess water for 1 h. Electrochemical measurements. All measurements were performed using a Metrohm Auto B.V. PGSTAT101 potentiostat/galvanostat. A polyvinylidine fluoride binder (14 wt%, containing 7 wt% Denka Black) was mixed with the electrode materials, and the resulting mixture was coated on stainless steel. Cyclic voltammetry (CV) and galvanostatic charge/discharge experiments were carried out with a three-electrode cell (100 mL) containing 1 M sulfuric acid solution (60 mL) as an electrolyte, and using a Pt counter electrode and a Ag/AgCl reference electrode. Electrochemical impedance spectroscopy (EIS) Nyquist plots were recorded using the same three-electrode system in the frequency range of 100 mHz to 0.1 MHz. Stainless-steel symmetric capacitor cells were assembled with the two electrode materials, cellulose (Whatman, pore size 0.45 µm) separator, and sulfuric acid electrolyte (1 M).


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Characterization. The morphological characteristics were analyzed using scanning electron microscopy (SEM, JEOL JSM-7500F) and transmission electron microscopy (TEM, TECNAI F20 ST). Raman spectroscopy analysis was performed with a 532-nm excitation source using a Nanobase XperRam 200 spectrometer. Optical micrographs were taken using a LEICA DM750P optical microscope. Polarizing optical microscopy (POM) imaging was performed with crossed polarizers and a 530-nm full-wavelength retardation plate. Nitrogen sorption experiments were carried out with a MicrotracBEL BELSORP-max at 77 K.


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RESULTS and DISCUSSION

Figure 1. a) Scheme describing the preparation of SbPNFs, where a dispersion of PVA-wrapped SWNTs in PAN/DMSO solution was electrospun to produce the nanofibers. b) Both dispersions of (left) PVA-wrapped SWNTs and (right) SWNT/PVA/PAN ensembles in DMSO solution exhibited good colloidal stability. c) Evolution of color in the electrospun products depending on the SWNT content, from 0.00 to 0.06 wt%.


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Figure 2. Optical micrographs of SWNT/PVA/PAN DMSO solutions with different SWNT contents: a) 0.00, b) 0.02, c) 0.04, and d) 0.06 wt%. Images were tinted green for clarification of phases; the scale bar is 50 µm.

Figure 3. Representative optical microscopic images of (a,b) SbPNFs (0.04 wt% SWNT) and (c,d) PVA/PAN nanofibers under cross-polarized light (a,c) before and (b,d) after carbonization. The scale bar is 20 µm.


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SWNTs were incorporated into polymeric nanofibers by electrospinning dispersions of SWNT/PVA/PAN in DMSO with different SWNT contents (Figure 1). The SbPNFs prepared with SWNT concentrations of 0.02, 0.04, and 0.06 wt% were labeled as SbPNF-1, SbPNF-2, and SbPNF-3, respectively. First, SWNTs were wrapped by PVA in DMSO with the aid of ultrasonication, where the concentration of SWNTs was controlled. The SWNT/PVA dispersion solution underwent a centrifugation process to obtain only colloidally stable SWNT/PVA ensembles, during which undesirable impurities were removed. PAN was employed as a host polymer. The SWNT/PVA dispersion was mixed with the PAN in DMSO solution, and the concentrations of the individual components were optimized to achieve an appropriate viscosity for electrospinning. SWNT/PVA was miscible with the PAN in DMSO (Figure 1b). The formation of hydrogen bonding between the hydroxyl group of PVA and the cyano group of PAN would contribute to the miscibility of PVA and PAN. However, microphase separation was observed in the SWNT/PVA/PAN blend, as displayed in Figure 2. Spherical SWNT/PVA microdomains of diameter 20 to 60 µm were dispersed in the PAN continuous phase, depending on the SWNT content. Interestingly, the size of the microdroplets in the blend decreased as the SWNT content increased from 0.02 to 0.04 wt% and then showed a slight increase at 0.06 wt% SWNT content. The PVA-to-PAN molar ratio was kept constant for all the samples. However, increases in the PVA-wrapped SWNT content were accompanied by decreases in the portion of free PVA that could interact with the continuous phase, PAN. Thus, it is considered that the change in the SWNT/PVA-to-free PVA ratio led to the variation in the size of the microdroplets. Both PVA and PAN serve as carbon precursors, but the properties of the resulting carbons are significantly different. The SWNT/PVA/PAN blends were subjected to heat treatment under an inert atmosphere for carbonization. The final product SbCNFs prepared with SWNT concentrations of 0.02, 0.04, and 0.06 wt% were labeled as SbCNF-1, SbCNF-2, and SbCNF-3, respectively. As seen in Figure 1c, the addition of SWNTs to the polymeric nanofibers changed the color of the resulting products to dark gray. The color of the products changed from white to dark gray with increasing SWNT content. In addition, there was ACS Paragon Plus Environment

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clearly no inhomogeneity in color, indicating that the SWNTs were well dispersed in the nanofibers at the molecular level. The dispersion of SWNTs in the nanofibers was further examined by POM. As seen in Figures 3a and 3b, unique and distinct optical textures were observed for the SWNT/PVA/PAN nanofibers before and after carbonization. Considering the one-dimensional structural characteristics of SWNTs, the strong birefringence in the POM image of the SWNT/PVA/PAN nanofibers indicates the presence of nematic long-range ordering along the fiber axis.51-53 It should be noted that the SWNT/PVA/PAN nanofibers were not subjected to any processing treatment to increase their orientation and crystallinity, such as drawing or annealing. Interestingly, birefringent domains were visible even after the carbonization, although there were topological defects between the domains. Importantly, the SWNT/PVA/PAN solution used for electrospinning exhibited featureless and lowcontrast images under cross-polarized light (Figure S1), indicating that the solution was entirely amorphous. In the electrospinning process, the discharged SWNT/binary-polymer solution jet was subject to a whipping process, wherein SWNTs and polymer chains were highly stretched and aligned with solvent evaporation. In other words, it is considered that the SWNT/PVA-in-PAN dispersions underwent an isotropic-to-nematic phase transition during the electrospinning process. Furthermore, in the absence of SWNTs, relatively little birefringence was observed from the nanofibers, as shown in Figures 3c and 3d. As a result, the POM images demonstrated the macroscopic orientation of SWNTs in the nanofiber, and furthermore, may support the enhanced crystallinity of the surrounding matrices.


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Figure 4. SEM images of SbPNFs with different SWNT contents (a–d) before and (e–h) after carbonization: (a,e) SbPNF-1, (b,f) SbPNF-2, (c,g) SbPNF-3, and (d,h) control; the scale bar is 1 µm.

G

a)

G' D

RBM

b) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c)

500

1000

1500

2000

2500

3000

Raman shift (cm−1)

Figure 5. Representative Raman spectra of (a,b) SbPNFs (0.04 wt% SWNT) (a) before and (b) after carbonization and (c) carbonized sample of the control containing no SWNTs.

Figures 4a to 4d present SEM images of the as-electrospun SWNT/polymeric products. It was found that the nanofibers ranging diameters of 250‒600 nm were well formed, without any undesirable byproducts. The SWNT/binary-polymer nanofibers were successfully converted to carbonaceous nanofibers through a heat treatment process under an inert atmosphere, as presented in Figures 4e to 4h. Only the diameter of the nanofibers decreased by approximately 10‒20% owing to the formation of more compact structures accompanied by dehydrogenation and aromatization in the carbonization


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process.54-55 The precursor SWNT/binary-polymer nanofibers and the carbonized nanofibers were characterized using Raman spectroscopy (Figure 5). The SbPNFs exhibited a radial breathing mode (RBM) around 300 cm−1, which corresponded to the characteristic RBM of the employed SWNT.56-57 The disorder-induced mode (D), the tangential first-order mode (G: G+/G−), and the high-frequency second-order mode (Gʹ) were also observed in common at ~1350, ~1500‒1600, and ~2650 cm−1. The G+/G− peaks are a unique feature of SWNTs. Specifically, the G+ mode originates from the vibration of carbon atoms along the nanotube axis, whereas the G− mode stems from the vibration of carbon atoms along the nanotube circumference.58-59 The intensity of the D peak depends on the number of defects in SWNTs and thus the ratio of the D-to-G+ peaks provides a quantitative measure of the level of defects in SWNTs.57-58 The SbPNFs had the average D-to-G+ ratio of as low as 0.08. After the carbonization, it was difficult to find the SWNT characteristics in the Raman spectra, indicating that the SWNTs were integrated into the surrounding carbon matrix. The SbCNFs showed the D-to-G+ ratio of approximately 0.88, which was lower than that (more than 1) of the control prepared without SWNT. It was clear that the SWNTs contributed to the enhanced crystallinity of the resulting carbon matrix, as exhibited in Figure 3b.


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a) 2

−1

I (A g )

1 0 -1

SbCNF-1 SbCNF-2 SbCNF-3 Control

-2 -0.2

0.0

0.2

0.4

0.6

0.8

E (V vs. Ag/AgCl)

E (V vs. Ag/AgCl)

b)

0.8


0.6 0.4 0.2 0.0 0

500

1000

1500

2000

2500

8

10

Time (s)

c) −1

Capacitance (F g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

150

100

50

0 0

2

4

6

−1

Current density (A g )

Figure 6. Electrochemical properties of SbCNFs with different SWNT contents and a control (no SWNT). (a) CV curves recorded at a scan rate of 25 mV s−1, (b) galvanostatic charge/discharge curves recorded at a current density of 0.1 A g−1, and (c) plot of SbCNF-2 specific discharge capacitance versus current density. 1 M sulfuric acid was used as an electrolyte.


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−Z" (kOhm)

a)

1.5

SbCNF-1 SbCNF-2 SbCNF-3

1.0

CPE2

Rct Rs

0.5

0.0 0.0

CPE1 0.5

1.0

1.5

2.0

Z' (kOhm)

b)

−Z" (kOhm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

Control

10

Rct Rs 5

CPE1 0 0

5

10

15

20

Z' (kOhm)

Figure 7. EIS Nyquist plots of (a) SbCNFs with different SWNT contents and (b) a control (no SWNT) recorded in the 100 mHz to 0.1 MHz frequency range. Insets: equivalent circuit models, where RS, Rct, and CPE denote solution resistance, charge transfer resistance, and constant phase element, respectively.


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Table 1. Equivalent circuit component values calculated through the fitting of the impedance plot in Figure 7.[a,b] Sample

Rs

Rct

(Ω)

(Ω)

CPE1

n1

CPE2

n2

Control

1.1

284 × 102

2.7 × 10−5

0.90

-

-

SbCNF-1

2.0

294

9.9 × 10−6

0.83

6.3 × 10−4

0.69

SbCNF-2

2.0

209

9.5 × 10−6

0.86

1.4 × 10−3

0.66

SbCNF-3

2.2

245

9.8 × 10−6

0.85

7.9 × 10−4

0.66

[a]

CPE is defined by ZCPE = Q−1 (jw)−n, where Q and n are frequency-independent constants, and w is the angular frequency. [b] The exponent n is a correction factor related to the roughness of the electrode, ranging from 0 to 1. For example, the CPE element is an ideal capacitor at n = 1, whereas it is a resistor or Warburg element at n = 0.0 or 0.5, respectively.

The heat treatment of the polymer precursor nanofibers in an inert atmosphere resulted in SbCNFs with electrochemical activities. CV was used to examine the electrochemical properties of the SbCNFs (Figure 6a). The shape of all the CV curves was distorted rectangular, in which remarkable redox peaks were not found. The CV curves deviate from an ideal rectangular shape in the presence of the equivalent series resistance.60 The integrated areas of the CV curves were ordered as follows: control < SbCNF-1 < SbCNF-3 < SbCNF-2. It was expected that the incorporation of SWNTs into the nanofibers would contribute to the electrical/electrochemical properties of the SbCNFs. All the SbCNFs showed larger CV areas compared to the control (no SWNTs). However, the largest integrated area of the CV curves was observed for SbCNF-2, which was prepared with a SWNT content of 0.04 wt%, not SbCNF-3, indicating that the electrochemical activity of the SbCNFs was not necessarily proportional to the amount of SWNTs incorporated in the nanofiber. Figure 6b shows representative galvanostatic charge/discharge curves of SbCNFs measured under the same conditions as were used for the CV analysis. The specific discharge capacitances were 50±12, 157±20, and 54±17 F g−1 for SbCNF-1, ACS Paragon Plus Environment

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SbCNF-2, and SbCNF-3, respectively; this trend was commensurate with that observed in the CV data. Figure 6c shows the dependence of the specific discharge capacitance of SbCNF-2 with the highest value of current density. An EIS analysis was performed for the SbCNFs to clarify the effect of the SWNT content on their electrical/electrochemical properties. Figure 7 displays EIS Nyquist plots of the SbCNFs and the control (no SWNT) with equivalent circuit models. The Nyquist plots of the SbCNFs were characterized by a semicircular shape at high frequencies and a straight line at low frequencies (Figure 7a). The fitting of the Nyquist plot to an equivalent circuit model resulted in a modified Randles circuit, in which the double-layer capacitance (Cdl) and Warburg element were replaced by constant phase elements (CPEs). However, the control containing no SWNTs showed only a single semicircle in the same frequency range, which resulted in a simpler equivalent circuit model with a solution resistance (Rs), charge transfer resistance (Rct), and CPE (Figure 7b). The values of the equivalent circuit components calculated by fitting are summarized in Table 1. First of all, the high Rct value of the control indicated that the incorporation of SWNTs in the nanofiber significantly decreased Rct. The Rct values of the SbCNFs decreased in the order of SbCNF-1 < SbCNF-3 < SbCNF-2. CPE2 was two or three orders of magnitude higher than CPE1, implying that the capacitance was predominantly determined by CPE2. SbCNF-2 had the lowest Rct with the highest CPE2, which accounted for the largest CV curve area and the highest discharge capacitance, as observed in Figure 6. The incorporated SWNTs may always have a positive effect on the electrical and crystalline properties of the SbCNFs. However, although both PVA and PAN can be converted to carbonized products, as mentioned before, the textural and chemical properties of the resulting products are significantly different. It should be re-emphasized that the size of the phase-separated microdroplets was not simply proportional to the SWNT content in Figure 2. As a result, it is evident that there is an optimal condition of the SWNT-to-PAN and free PVA-to-PAN ratios to produce SbCNFs with the best electrical/electrochemical properties.


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a) 160 −1

120

3

Vadsorbed (cm g )

140

100 80 60


40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

3

−1

−1

dVp/dDp (cm g nm )

b)

1.4 SbCNF-1 SbCNF-2 SbCNF-3 Control

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Dp (nm)

c)

0.2 SbCNF-1 SbCNF-2 SbCNF-3 Control

3

−1

−1

dVp/dlogDp(cm g nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

0.0 0

20 40 60 80 100 120 140 160 180

Dp (nm)

Figure 8. Textural properties of the SbCNFs: (a) Nitrogen adsorption/desorption isotherms (the filled and unfilled symbols indicate adsorption and desorption branches, respectively), (b) micropore size distribution plots resulted from the MP method, and (c) BJH plots showing the mesopore size distribution.


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Table 2. Major textural parameters of the SbCNFs made from the SbPNFs. Sarea[a]

Vmicro[b]

Vmeso[c]

Vmicro/Vtotal

Vmeso/Vtotal

(m2 g−1)

(10−2 cm3 g−1)

(10−2 cm3 g−1)

(10−1)

(10−1)

SbCNF-1

416.5

17.9

1.9

9.0

1.0

SbCNF-2

447.0

22.8

6.4

7.8

2.2

SbCNF-3

429.1

19.0

2.2

9.0

1.0

Control

373.6

19.5

1.2

9.4

0.6

Sample

[a]

Specific surface area calculated by the BET method. Micropore volume calculated by MP method. [c] Mesopore (2–50 nm) volume calculated by the BJH method. [b]

The textural properties of the SbCNFs were examined using nitrogen sorption experiments at 77 K. As seen in Figure 8a, the adsorption–desorption isotherms all show a steep increase in the adsorbed volume up to a relative pressure of approximately 0.05, and then leveled off. This type of isotherm can be classified to Type I, indicating that the SbCNFs had significant microporosity.50, 61 SbCNF-2 was exceptional, exhibiting a slight increase in the adsorbed amount at relative pressures of more than 0.05, with a hysteresis in the relative pressure range 0.4–0.7. Such a sorption behavior is ascribed to the presence of mesopores in SbCNF-2. The micropore analysis (MP) and Barrett–Joyner–Halenda (BJH) plots showing the size distributions of micropores and mesopores, respectively, indicated that SbCNF-2 had higher adsorption volumes in micro/mesopores with diameters of 1.4–7.2 nm, compared to other SbCNFs (Figures 8b and 8c). Table 2 summarizes the major textural parameters of the SbCNFs derived from the isotherm analysis. Compared to the control, all the SbCNFs showed enlarged specific surface areas. SbCNF-2 had the largest Brunauer–Emmett–Teller (BET) surface area, which was likely due to the largest mesopore volume. The mesopore volume of SbCNF-2 was calculated to be approximately three times larger than those of SbCNF-1 and SbCNF-3. As a result, it is believed that the higher porosity of SbCNF-2 contributed to its superior electrochemical properties.


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Figure 9. SEM images of SPNFs with different SWNT contents (a–d) before and (e–h) after carbonization: (a,e) SPNF-1, (b,f) SPNF-2, (c,g) SPNF-3, and (d,h) control; the scale bar is 1 µm.


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Figure 10. Representative TEM images of (a) SbCNFs and (b) SCNFs (SWNT 0.04 wt%). The scale bar is 100 nm.


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a) 180

3

−1

Vadsorbed (cm g )

160 140 120 100 80 60

SCNF-1 SCNF-2 SCNF-3 Control

40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

3

−1

−1

dVp/dDp (cm g nm )

b)

1.4 SCNF-1 SCNF-2 SCNF-3 Control

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Dp (nm)

c) SCNF-1 SCNF-2 SCNF-3 Control

−1

dVp/dlogDp(cm g nm )

−1

0.2

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

0.0 0

20 40 60 80 100 120 140 160 180

Dp (nm)

Figure 11. Textural properties of the SCNFs made from SPNFs: (a) Nitrogen adsorption/desorption isotherms (the filled and unfilled symbols indicate adsorption and desorption branches, respectively), (b) micropore size distribution plots resulted from MP method, and (c) BJH plots showing the mesopore size distribution.


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Table 3. Major textural parameters of the SCNFs made from the SPNFs. Sarea[a]

Vmicro[b]

Vmeso[c]

Vmacro[d]

Vtotal

(m2 g−1)

(10−2 cm3 g−1)

(10−2 cm3 g−1)

(10−2 cm3 g−1)

(10−2 cm3 g−1)

SCNF-1

346.0

22.7

6.4

1.1

30.2

SCNF-2

327.0

19.5

13.8

1.7

35.0

SCNF-3

265.0

17.4

11.6

1.8

30.8

Control

263.9

17.9

4.2

1.2

23.3

Sample

[a]

Specific surface area calculated by the BET method. Micropore volume calculated by the MP method. [c] Mesopore (2–50 nm) volume calculated the BJH method. [d] Macropore (>50 nm) volume calculated by the BJH method. [b]

The integration of two polymers into the nanofibers provided a unique opportunity to control the textural properties of the SbCNFs. The textural properties of the SbCNFs were further developed by removing PVA from the SbPNFs. The PVA phase was easily removed from the SbPNFs by immersing them into an aqueous solution. The resultant products containing no PVA were labelled as SPNFs and their carbonized products were labeled as SCNFs, using the same labeling scheme to indicate the SWNT content as was used earlier. Figure 9 displays SEM images of the SPNFs and SCNFs. It was difficult to find any remarkable change in the morphology of the nanofibers before and after the removal of the PVA. Figure 10 displays representative TEM images of SbCNFs and SCNFs for comparison. Whereas the SbCNFs showed a solid nanofiber morphology, the SCNFs clearly revealed many meso- and macropores throughout the nanofiber body. In other words, the PVA acted as a sacrificial porogen in the nanofibers. The SCNFs were also characterized using nitrogen sorption experiments. As shown in Figure 11a, the isotherms of the SCNFs were completely different from those of the SbCNFs. All SCNFs showed hysteresis in the adsorption/desorption behavior, indicating the generation of meso- and macropores in the nanofibers. In particular, SCNF-2 showed a large hysteresis loop in the relative pressure range of 0.1 to 0.9. Figures 11b and 11c provide information on the pore size distribution of the SCNFs. Apparently, the volume of the pores with diameters of less than 40 nm appeared to increase ACS Paragon Plus Environment

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compared to the SbCNFs. The calculated major textural parameters are summarized in Table 3. Compared to the SbCNFs, the pore volume of the SCNFs increased by 17–46%, and their surface area became decreased by 17–38%. The micropore volumes of the SCNFs remained comparable to those of the SbCNFs. However, it was found that the mesopore volumes of the SCNFs increased dramatically compared to the SbCNFs.

3

a)

2

−1

I (A g )

1 0 -1 SCNF-1 SCNF-2 SCNF-3 Control

-2 -3 -4 -0.2

0.0

0.2

0.4

0.6

0.8

E (V vs. Ag/AgCl)

E (V vs. Ag/AgCl)

b)

0.8

SCNF-1 SCNF-2 SCNF-3 Control

0.6 0.4 0.2 0.0 0

1000 2000 3000 4000 5000 6000

Time (s)

c) −1

Capacitance (F g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

600

400

200

0 0

2

4

6

8

10

−1

Current density (A g )


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Figure 12. Electrochemical properties of SCNFs with different SWNT contents and a control (no SWNT). (a) CV curves recorded at a scan rate of 25 mV s−1, (b) galvanostatic charge/discharge curves recorded at a current density of 0.1 A g−1, and (c) plot of SCNF-2 specific discharge capacitance versus current density. 1 M sulfuric acid was used as an electrolyte.

The electrochemical properties of the SCNFs were also examined by CV analysis and galvanostatic charge/discharge measurements (Figure 12). The shape of the CV curves was similar to that of the CV curves of the SbCNFs (Figure 12a). However, the integrated CV area of the SCNFs increased by 49−249% compared to the SbCNFs. Remarkably, the specific capacitance was calculated to increase by 98−267% (Figure 12b). The discharge capacitance of SCNF-2, with the highest pore volume, reached a maximum of 577 F g−1 at a current density of 0.1 A g−1 (Figure 12c). As a result, it is believed that the removal of the PVA phase from the SbCNFs led to increased meso- and macroporosity in the SCNFs, which contributed to their improved electrochemical activities.


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CONCLUSIONS The incorporation of PVA-wrapped SWNTs into PAN, followed by the electrospinning of the SWNT/PVA/PAN solution resulted in interesting and significant characteristics, such as an aligned SWNTs-in-nanofiber structure and enhanced ordering of the polymer chains. It should be noted that the nanofibers, SbPNFs, reported in this study were not subjected to any special processing treatment. Drawing during or after the spinning and post-synthetic annealing would result in increased crystallinity and orientation. The SbPNFs were successfully converted to SbCNFs with tunable electrochemical properties. Compared to the control (no SWNT), the charge transfer resistances of the SbCNFs were two orders of magnitude lower. Interestingly, SbCNF-2, prepared with a SWNT content of 0.04 wt%, showed the best electrochemical activity owing to its low charge transfer resistance and high pore volume. Along with the SWNT content, the microphase behavior between PVA and PAN, which are qualitatively different carbon precursors, determines the major properties of the SbCNFs. The removal of the PVA phase from the SbPNFs provided the opportunity to further control the textural properties of their carbonized products, which produced SCNFs with enhanced meso-/macropore volumes. The specific discharge capacitance of the SCNFs was found show a maximum value of 577 F g−1. Finally, SCNF-2, which showed the best properties, was successfully used in a model application as the electrode material for an electrochemical capacitor.


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ASSOCIATED CONTENT Supporting Information. POM images of SWNT/PVA/PAN solution.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (the Ministry of Science and ICT) (NRF-2015R1A2A2A01007166).


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SWNTs-in-Binary-Polymer Nanofiber Structures and Their Use as

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