KR101766143B1 - Preparing method of aligned activative carbon nanofibers using electrospinning - Google Patents

Abstract

The present invention relates to a method for preparing aligned activated carbon nanofibers using an electrospinning method, and the preparation method presents a novel method. The preparation method can prepare the aligned activated carbon nanofibers having improved electrical characteristics by adjusting the degree of alignment according to the driving speed of a rotating current collector, and especially, when the rotating current collector is driven in one direction, providing an excellent path for increasing the ion transport speed. When the activated carbon nanofibers prepared by the preparation method are used as an electrode for a super capacitor, excellent electrochemical characteristics can be exhibited at the time of rapid charging and discharging.

Description

[0001] The present invention relates to an aligned activated carbon nanofibers using electrospinning,

The present invention relates to a method for preparing an aligned activated carbon nanofiber by electrospinning.

Recent research on high-performance electrical energy storage systems (EES) has focused on eco-friendly technologies that can be applied to portable electric devices and large electric devices. The efficient storage and release of electrical energy can enable continuous utilization of currents generated from intermittent energy sources. Especially among electric energy storage devices, supercapacitors have received considerable attention as one of the most promising modern energy storage systems to provide electrical energy to various portable devices due to its unique electrochemical characteristics.

Supercapacitors can be classified as electrical double layer capacitors (EDLCs) and pseudo-capacitors depending on the reaction mechanism for generating electrical charge and discharge.

The electric double layer capacitor stores and emits electrical energy through a non-faradic reaction of the electric double layer between the electrolyte and the electrode surface. Since the electric double layer capacitor does not include the redox FERRADE reaction, the reaction mechanism is very fast and stable, has excellent long-term cycling stability and high speed characteristics, and low non-storage capacity.

Another type of supercapacitor, a pseudo-capacitor, has an electrochemical reaction mechanism, in which a pseudo-capacitor acquires and emits electrical energy through a redox ferrete reaction between the electrode surface and the electrolyte.

In general, the pseudo-capacitor has a high energy density, but a high power density can not be obtained. To utilize the advantages of the electric double layer capacitors and the pseudo capacitors, hybrid electrodes are generally used for industrial applications. Metal oxides and conductive polymers, which are composite electrodes, store electric energy through a redox ferrode reaction mechanism, Hybrid supercapacitors need to have a structure that supports high electrical conductivity and transfer of electrons or ions so that the high electrical energy generated by the redox FERRARDI reaction between the transition metal and the electrolyte can be efficiently transferred .

One-dimensional carbon materials such as carbon nanotubes (CNTs), carbon nano-coils, and carbon nanofibers between various carbon materials for charge transfer have high aspect ratio and excellent electron mobility, Can be accelerated.

Moreover, the carbon nanofibers are being studied as a famous material for charge transfer because of their high aspect ratio, electrical conductivity, and good productivity. One technique for fabricating the carbon nanofibers is electrospinning, and electrospinning has been used for a long time due to a simple process.

Recently, various researches on the alignment of nanofibers have received considerable attention due to their excellent structural characteristics. In particular, composite materials, reinforcing agents, electrochemical sensors, etc., Fibers are used.

Recently, studies on carbon nanofibers aligned in industry, academia, and research fields have been actively conducted, but they are not used as energy storage materials. Particularly, research on the change of surface area, pore volume, electrical conductivity, and electrochemical characteristics .

Therefore, it is necessary to develop an aligned carbon nanofiber electrode having improved surface characteristics and electrical conductivity by using electrospinning for application to an electrode for supercapacitor, which is one of energy storage systems, and to apply it to a high-performance electric energy storage device It is a situation.

Korea Patent Publication No. 2013-0073481

It is an object of the present invention to provide a method of manufacturing an activated carbon having an ordered electrospinning process capable of increasing the specific surface area and the pore volume by aligning the rotating current collector in one direction at a high speed and providing an excellent path for increasing the ion transport rate, And a method for manufacturing nanofibers.

In order to accomplish the above object, the present invention provides a method of manufacturing an electrospinning solution, comprising the steps of: (1) preparing an electrospinning solution; Connecting the prepared electrospinning solution to a syringe pump, and then operating a rotary collector at a speed of 250 to 2000 rpm to prepare aligned polyacrylonitrile nanofibers (second step); Drying and stabilizing the prepared polyacrylonitrile nanofibers (step 3); Carbonizing the stabilized polyacrylonitrile nanofibers to produce aligned carbon nanofibers (Step 4); And activating the carbon nanofibers to produce activated carbon nanofibers (step 5). The present invention also provides a method for producing activated carbon fibers using the aligned carbon nanofibers.

The present invention also provides aligned activated carbon nanofibers, which are produced according to the above-mentioned production method.

The manufacturing method according to the present invention can adjust the degree of alignment according to the operation speed of the rotating current collector, and in particular, when the rotating current collector is operated at a high speed, the activated carbon nanofiber having improved surface properties and electrical characteristics can be produced. The activated carbon nanofibers prepared by the production method can be aligned in one direction to provide an excellent path for increasing the ion transport speed and can exhibit high electrochemical characteristics at the time of fast charging / discharging when used for a supercapacitor electrode.

1 is a schematic view showing the production of CNFs by electrospinning.
2 shows SEM images of CNFs produced at various rpm of 250 rpm, 500 rpm, 1000 rpm, and 2000 rpm of a rotating current collector. CNFs 250 (a), CNFs 500 (b), CNFs 1000 ), And CNFs 2000 (d).
3 is a view showing an alignment chart of CNFs 250 (a), CNFs 500 (b), CNFs 1000 (c), and CNFs 2000 (d).
4 is a graph showing N ₂ adsorption / desorption analysis (a), surface area (b), average diameter (c), and electric conductivity (d) of CNFs as the speed of the rotating current collector is increased.
FIG. 5 shows the relationship between the CV curve (a) of the CNFs, the constant current charge-discharge curve (b), the remaining storage capacity (c) as the scan speed increases from 10 mV / s to 50 mV / ), And a summary (d) of velocity characteristics.
Figure 6 shows the CV curves of CNFs 250 electrode (a), CNFs 500 electrode (b), CNFs 1000 electrode (c), and CNFs 2000 electrode (d) at various scan rates from 10 mV / s to 50 mV / FIG.
7 shows the Nyquist plot (a) and cyclic stability (b) of CNFs 250 electrode (a), CNFs 500 electrode (b), CNFs 1000 electrode (c), and CNFs 2000 electrode Fig.
8 is an SEM image and an alignment diagram of ACNFs 250 (a, c) and ACNFs 2000 (b, d).
Figure 9 shows the BET isotherm (a), the CV curve b at a scan rate of 10 mV / s, the constant current charge-discharge curve c at a 1 A / g current density, (ACNFs 250 (e), and ACNFs 2000 (f)) as a function of scan speed from mV / s to 50 mV / s.
10 is a graph showing a Nyquist plot (a) and circulation stability (b) of ACNFs 250 and ACNFs 2000 electrodes for 1000 times.

Hereinafter, the present invention will be described in more detail.

The present inventors have found that when the rotating current collector is operated at a high speed in manufacturing activated carbon nanofibers by electrospinning, the activated carbon nanofibers are aligned in one direction, thereby providing an excellent path for increasing the ion transport speed, It has been found that when fibers are used as an electrode for a supercapacitor, they can exhibit excellent electrochemical characteristics at the time of fast charging and discharging, thereby completing the present invention.

The present invention relates to a method of preparing an electrospinning solution, Connecting the prepared electrospinning solution to a syringe pump, and then operating a rotary collector at a speed of 250 to 2000 rpm to prepare aligned polyacrylonitrile nanofibers (second step); Drying and stabilizing the prepared polyacrylonitrile nanofibers (step 3); Carbonizing the stabilized polyacrylonitrile nanofibers to produce aligned carbon nanofibers (Step 4); And activating the carbon nanofibers to produce activated carbon nanofibers (step 5). The present invention also provides a method of manufacturing aligned activated carbon nanofibers using electrospinning.

The electrospinning solution may be prepared by mixing polyacrylonitrile with N, N-dimethylformamide in a weight ratio of 1: 7 to 10, but is not limited thereto.

The rotating current collector may be operated at a speed of 2000 rpm, but is not limited thereto.

In the third step, the fabricated polyacrylonitrile nanofiber may be dried and stabilized by heating to 250 to 350 ° C, but is not limited thereto.

In the fourth step, the polyacrylonitrile nanofiber stabilized in a nitrogen atmosphere may be carbonized by heating to 800 to 1000 ° C, but is not limited thereto.

In the fifth step, the carbon nanofibers produced in a CO ₂ atmosphere may be activated at a temperature of 800 to 1000 ° C, but are not limited thereto.

The present invention also provides aligned activated carbon nanofibers, which are produced according to the above-mentioned production method.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the present invention is not limited by these examples.

Example 1 Synthesis of Carbon Nanofibers (CNFs 250) and Activated Carbon Nanofibers (ACNFs 250)

1. Preparation of materials

Polyacrylonitrile (PAN, Mw = 150,000 g / mol) was purchased from Sigma-Aldrich and N, N-dimethylformamide (MF, assay 99.0% min) and sulfuric acid ₂ SO ₄ , 95%) were purchased from OCI (Korea).

2. Synthesis of carbon nanofibers (CNFs 250)

To prepare the CNFs, a 12 wt% PAN (2.4 g) / DMF (20 g) solution was first prepared and the prepared solution was spun using an electrospinning apparatus (Nano NC, South Korea). Referring to FIG. 1, the PAN / DMF solution for electrospinning was connected to a syringe pump and pushed at a rate of 1 mL / h.

The distance between the tip of the needle and the rotary collector substrate was fixed at 18 cm, the applied voltage was 18 kV, the diameter of the rotating current collector was 25 cm, and the width for capturing nanofibers was fixed at 1.7 cm Respectively. Then, the electrospun PAN nanofibers were fabricated by operating the rotating current collector by fixing the speed of the rotating current collector at 250 rpm.

The aligned electrospun PAN nanofibers were dried in a vacuum oven at 60 DEG C for 24 hours and heated to 300 DEG C in a tube furnace (Korea Furnace Development Co., Ltd., Korea) at a heating rate of 1.5 DEG C / min And stabilized by keeping the temperature at 300 캜 for 1 hour in the atmosphere.

The stabilized electrospun PAN nanofibers were then carbonized by heating from 300 ° C to 900 ° C at a heating rate of 5 ° C / min in a tube furnace. After maintaining the temperature at 900 ° C for 1 hour, N ₂ gas was blown, To synthesize carbon nanofibers. The carbon nanofibers synthesized by the above method were named 'CNFs 250'.

3. Synthesis of activated carbon nanofibers (ACNFs 250)

Activated carbon nanofibers (ACNFs) were prepared by exchanging N ₂ gas with CO ₂ gas after maintaining the synthesized CNFs 250 at 900 ° C. for 1 hour. Activated carbon nanofibers synthesized by the above method were called 'ACNFs 250 ".

Example 2 Synthesis of Carbon Nanofibers (CNFs 500) and Activated Carbon Nanofibers (ACNFs 500)

The carbon nanofibers and activated carbon nanofibers synthesized by the above method were referred to as 'CNFs 500' and 'ACNFs 500', respectively, except that the speed of the rotating current collector was fixed at 500 rpm. Respectively.

Example 3 Synthesis of Carbon Nanofibers and Activated Carbon Nanofibers

Except that the speed of the rotating current collector was fixed at 1000 rpm. The carbon nanofibers and the activated carbon nanofibers synthesized by the above method were respectively referred to as 'CNFs 1000' and 'ACNFs 1000' Respectively.

Example 4 Synthesis of Carbon Nanofibers and Activated Carbon Nanofibers

Except that the speed of the rotating current collector was fixed at 2000 rpm. The carbon nanofibers synthesized by the above method and the activated carbon nanofibers were respectively referred to as 'CNFs 2000' and 'ACNFs 2000' Respectively.

<Experimental Example 1> Morphological analysis

The morphology of CNFs prepared according to the preceding Examples 1 to 4 In order to analyze the characteristics, Scanning Electron Microscope (SEM) analysis was performed at 15 kV using SEM (Hitachi, S-4300) and all samples were sputter coated with platinum before analysis.

The specific surface area and pore size distribution of all the samples were measured using an instrument (ASAP 2020, Micromeritics) according to the Brunauere-Emmette-Teller (hereinafter 'BET') equation. The total pore volume is the amount absorbed at P / P ₀ of 0.99.

FIG. 2 shows a representative morphological SEM image of carbon nanofibers prepared by electrospinning, stabilizing and carbonizing processes. FIGS. 2 (a) -2 (d) , 500 rpm, 1000 rpm, and 2000 rpm) of the carbon nanofibers.

As shown in FIG. 2, when the speed of the rotating current collector increases, it can be seen that the carbon nanofibers are gradually aligned in one direction. Further, referring to FIG. 2 (d), it can be seen that the carbon nanofibers were aligned in one direction when the speed of the rotating current collector was 2000 rpm. When the applied electrostatic force overcomes the surface tension of the polymer solution, it is possible to produce an electrically charged polymer jet, which, after drying the polymer jet, forms a charged fiber having a micro or nano-sized diameter I could. These residual charges were sufficient to exert a repelling force on the deposited fibers.

The speed of the rotating current collector was increased to reduce the time that the charged fibers could be randomly arranged on the charged surface of the rotating current collector. As a result, when the speed of the rotating current collector reached 2000 rpm, the fibers were arranged more aligned.

&Lt; Experimental Example 2 >

FIG. 3 shows alignment of carbon nanofibers synthesized using various speeds (250, 500, 1000, and 2000 rpm) of a rotating current collector. FIG. 3 (a) shows an alignment chart of the CNFs 250 synthesized according to the first embodiment, and CNFs 250 shows a wide range of alignment. On the other hand, when the speed of the rotating current collector is increased, it can be seen that the shape of the graph to be represented is a convergent shape. These results are similar to the morphological shape shown in FIG. 2. As in FIG. 2, the degree of alignment of CNFs reached a maximum value (93.8%) when reaching 2000 rpm.

Experimental Example 3 Surface Characterization, Microstructure, and Electrical Conductivity of Synthesized CNFs

The microstructure of all the samples was analyzed by Fourier Transform Raman (FT-Raman), Bruker RFS-100 / S analysis and X-ray diffraction (XRD), Phillips X'Pert -PRO MRD) analysis.

An X-ray tube (wavelength: 0.154 nm) operated with CuKa radiation at 40 kV, 30 mA was used and the XRD pattern at a 2θ angle of 2 ° to 60 ° at a scan rate of 5 ° / . Electrical conductivity of each sample was measured using a resistivity meter (Loresta-GP, Mitsubishi Chemical Co., Japan).

The degree of alignment for CNFs was calculated using the following equation (1).

[Equation 1]

The N and _θ N _T means the number of fibers each containing a specific angle, and the total number of carbon nanofibers.

Referring to the SEM image of FIG. 2, the corresponding statistical analysis for calculating the degree of alignment of the carbon nanofibers was performed using an angle between the edge of the image and the longitudinal axis of the fiber. The statistical analysis was also performed with 500 carbon nanofibers.

4 (a) shows N ₂ adsorption isotherms of CNFs performed at various speeds (250 rpm, 500 rpm, 1000 rpm, and 2000 rpm) of a rotating current collector. ), It can be seen that while the median value is kept relatively constant, it varies considerably at low and high pressure.

Sample
Specific surface area
(m ² / g) Total pore volume
(cm &lt; ³ &gt; / g) Pore volume (cm ³ / g) Micro-porous Meso CNFs 250 533 0.26 0.19 0.08 CNFs 500 542 0.26 0.19 0.08 CNFs 1000 573 0.35 0.21 0.14 CNFs 2000 635 0.50 0.24 0.26

Table 1 shows the specific surface area, total pore volume, micro or macro pore volume of CNFs manufactured using various rotational speeds. Referring to FIG. 4 (b), CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 were 533 m ² / g, 542 m ² / g, 573 m ² / g, and 635 m ² / g, respectively. The total pore volume was 0.26 cm ³ / 0.26 cm ³ / g, 0.35 cm ³ / g, and 0.50 cm ³ / g.

Referring to FIG. 4 (c), the average diameter of CNFs decreased from 522 nm to 441 nm. The average diameter was the mean value calculated from the Scion image program using 500 SEM images. These results show that the PAN nanofibers are related to the change of the rotating current velocity. Generally, in the spinning process for fabric production, the speed of the rotating current collector is increased to increase polymer crystals and decrease fiber average diameter. By increasing the speed of the rotating current collector, the fibers receive a strong tensile strength from the surface of the rotating current collector. As a result, the fibers have a smaller average diameter due to the higher tensile strength generated by the higher rotating current velocities.

In order to align the nanofibers, the speed of the spinning current was increased from 250 rpm to 2000 rpm, and the diameter of the nanofibers was reduced due to the tensile strength between the nanofibers and the surface area of the rotating current collector, similar to the spinning process.

4 (d) shows the electrical conductivities of CNFs using a rotating current collector at various speeds (250 rpm, 500 rpm, 1000 rpm, and 2000 rpm), and the electrical conductivities of CNFs were measured in the vertical and horizontal directions. The vertical direction is the same as the aligned direction of the CNFs, and the horizontal direction is the opposite direction. 4 (d), the electric conductivity of the CNFs in the vertical direction increased from 342 S / cm to 626 S / cm when the speed of the rotating current collector increased from 250 rpm to 2000 rpm.

These results are related to the alignment of one-dimensional structures. Carbon materials having a one-dimensional structure such as carbon nanotubes, carbon nanowires, and carbon nanofibers have high aspect ratios, and these results lead to high electrical conductivity. Furthermore, aligning the materials in one direction provides an excellent path for increasing the ion transport rate and reduces electrical resistance.

Referring to FIG. 4 (d), the electrical conductivity of the CNFs with respect to the horizontal direction is slightly increased relative to the vertical direction, and the horizontal direction provides an excellent path for moving the electrons. It is related to repulsion during the electrospinning process.

Due to the fibers containing the same charge, they can be attracted to each other on the rotating current collector surface, thereby increasing the space between the fibers. These generated spaces can limit the movement of electrons and ions. On the other hand, by increasing the speed of the rotating current collector, the repulsive force is minimized since it is possible to reduce the time that the repulsive force can occur between the fibers. Therefore, it can be seen that the aligned CNFs have a higher electrical conductivity than the CNFs oriented arbitrarily because the space between the fibers is relatively narrow.

<Experimental Example 4> Electrochemical analysis

All prepared samples were directly used as electrodes for electrochemical analysis and the electrochemical properties of the samples were performed in a 3-electrode system potentiostat (Bio-Logic Science Instruments, France) Was composed of 1 MH ₂ SO ₄ , a platinum plate and Ag / AgCl as a counter electrode and a reference electrode, respectively.

1. Cyclic voltammetry (CV) analysis

The circulation of the electrodes in potential windows between -0.2 V and 0.8 V with various scan rates (10 mV / s, 20 mV / s, 30 mV / s, 40 mV / s, and 50 mV / s) Cyclic voltammetry (CV) was performed.

5 (a) shows the CV curves of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 using a 10 mV / s scan rate and a 1 MH ₂ SO ₄ electrolyte solution. , And all CV curves for CNFs did not show any redox curve.

The respective non-storage capacities were calculated using the following equation (2).

&Quot; (2) &quot;

Wherein C ,? V, q , and m are respectively the ratio of the total capacitance (F / g), the maximum potential range (V) allowed by the electrochemical window, the amount of generated charge (C) .

The calculated storage capacities of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 were 122 F / g, 125 F / g, 134 F / g, and 165 F / g, respectively. With increasing alignment of CNFs, the reserve capacity increased and the non-storage capacity of CNFs 2000 was improved by 35.5% compared to the non-storage capacity of CNFs 250 at a scan rate of 10 mV / s. The above results are related to the enhancement of the surface properties caused by the tensile strength caused by the increase of the rotating current collector speed.

Referring to Table 1, the surface area and the pore volume of the CNFs were improved by the increase of the speed of the rotating current collector. As a result, the micropore volume was increased, and CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 Were 0.19 cm ³ / g, 0.19 cm ³ / g, 0.21 cm ³ / g, and 0.24 cm ³ / g, respectively.

In a variety of carbon material electrodes, such as EDLCs, the micropore volume serves as an important factor for the non-accumulation capacity, and micropores (below 2 nm) can provide a rich adsorption space for ions, The surface area was increased compared to CNFs 250, and the non-storage capacity of CNFs 2000 was much higher than that of CNFs 250. The enhanced surface properties through the electrospinning alignment process could improve the non-storage capacity of CNFs.

2. Constant current charge-discharge analysis

The constant current charging / discharging curve was shown by measuring the non-accumulating capacity of the electrode at a current density of 1 A / g in a potential window between -0.2 V and 0.8 V.

5 (b) shows the constant current charge-discharge curves of CNFs 250, CNFs 500, CNFs 1000, and CNF 2000 at a current density of 1 A / g. After 100 cycles, all constant current charge-discharge curves were obtained.

The charge-discharge curves for all the samples were nearly linear and showed a triangular shape between -0.2 V and 0.8 V. This suggests that all electrodes of CNFs exhibit the ideal bilayer charge capacity activity.

The non-accumulation capacity of all the samples was calculated from the charge-discharge curve in Fig. 5 (b) using the following equation (3).

&Quot; (3) &quot;

I ,? T ,? V, and m mean discharge current (A), discharge time (s), potential range (V), and sample weight (m).

The calculated non-storage capacities of the CNFs 250, CNFs 500, CNFs 1000, and CNF 2000 were 98.7 F / g, 99.5 F / g, 108 F / g, and 135 F / g, respectively. CV curve, and the non-accumulation capacity obtained from the constant current charge-discharge curve of CNFs was improved by increasing the degree of alignment of CNFs.

3. Speed characteristics

The rate capability was calculated using the following equation (4).

&Quot; (4) &quot;

The C ₁₀ and C ₅₀ represent the non-accumulating capacities at scan rates of 10 mV / s and 50 mV / s, respectively. Figure 5 (c) shows the non-storage capacities of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 for various scan rates from 10 mV / s to 50 mV / s in a 1M H ₂ SO ₄ electrolyte solution.

6 shows the CV curves of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 according to the scan speed change from 10 mV / s to 50 mV / s. Referring to FIG. 6 (d) It can be seen that the CNFs 2000 curve has a larger area than the curve. CNFs 2000 has more active area compared to other CNFs and is an ideal EDLC. Referring to Table 1 above, the improved surface properties affect the formation of CNFs 2000's ideal shape.

Referring to FIG. 5D, the remaining values of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 after 100 times of use according to a change in scan speed are 52.4%, 54.2%, 57.8%, and 67.3%.

As a result, the speed characteristic of the CNFs electrode was improved by 28.4% due to the increase of the degree of alignment. In addition, CNFs 2000 exhibited the highest non-storage capacity at low scan rates, as well as a non-storage capacity value of 111 F / g at the fastest scan rate of 50 mV / s. The CNFs 2000 electrode enabled rapid ion diffusion and showed better electrochemical properties than the other electrodes. These results can be explained by the surface properties and electrical conductivity of CNFs 2000.

Table 1 shows the specific surface area increased when the speed of the rotating current collector was increased from 250 rpm to 2000 rpm. In addition, the mesopore volume (2-50 nm) of CNFs increased from 0.08 cm ³ / g to 0.26 cm ³ / g as the degree of alignment increased.

The increased mesopore volume is related to the tensile strength generated by the electrospinning process. The mesopore pore size ratio increased from 29.6% to 52.1% due to the alignment of CNFs. The mesopore channels enable electrolyte ion diffusion into the electrode material. The ordered one-dimensional structure affects the improvement of ion accessibility under a high current density meter. Therefore, the rate characteristic increased from 52.4% to 67.3% due to the increase of the mesopore volume for the CNFs induced by the increase in alignment.

In addition, electrical conductivity is a very important factor for improving the speed characteristics of EDLCs. High electrical conductivity assists in the rapid charge transfer generated by the charge-discharge process between the electrode surface and the electrolyte. Referring to FIG. 4 (d), the electrical conductivity increases with increasing alignment of CNFs. As a result, the increased surface properties are related to the pore size distribution and the improved electrical conductivity, and the aligned one-dimensional structure affects the enhanced speed characteristics of the CNFs.

4. Electrochemical Impedance Spectroscopy (EIS) Analysis

Electrochemical impedance spectroscopy (EIS) was performed with an open-circuit potential of 10 mV perturbation amplitude and a frequency range of 100 kHz to 10 mHz.

7 (a) shows Nyquist plots of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000. All Nyquist plots showed linearity in the low frequency range, but in the high frequency range And did not show a semicircle shape. These results indicate that there is no influence of induction current and it is reproducible behavior. It can be seen that the type of Nyquist plot is a typical EDLCs behavior.

There is a clear distinction between the four lines, such as the internal resistance (Rs), the resistance of the electrolyte, and the resistivity, including the intercept of the actual axis at high frequencies, the contact resistance interface between the active material and the current collector.

The Rs decreased from 7.43 Ω to 7.09 Ω as the speed of the rotating current increased from 250 rpm to 2000 rpm. This is related to the increased surface properties and electrical conductivity of CNFs 2000. The warband impedance in the intermediate frequency region was reduced by the alignment of the CNFs and the change in the bug impedance contributed to the smaller and bug impedance of the CNFs pores compared to other CNFs electrodes, It shows easy electrolyte diffusion.

This is related to the enhanced porosity and electrical conductivity of the ordered structure of CNFs 2000. In particular, increased mesopore and electrical conductivity affect the ionic fluidity in the mid-range.

Referring to FIG. 7 (a), it can be seen that the slope of the CNFs 2000 electrode in the intermediate frequency region is almost perpendicular to that of the CNFs electrode. This can contribute to improved surface area, micropore volume, and ordered structure. The enhanced surface area of CNFs 2000, and the micropore volume, mean that they have multiple active areas, and the aligned structure provides an excellent path for ions to move into the pores of the CNFs 2000 electrode.

<Experimental Example 5> Circulation stability analysis

Referring to FIG. 7 (b), cyclic stability tests of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 were performed at 1 A / g current density in 1 M H ₂ SO ₄ electrolyte for 1000 cycles.

During 1000 cycles, the non-accumulation capacity decreased generally over time, and after 1000 cycles the non-storage capacities of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 were 18.1%, 17.2%, 15.4%, and 10.8% Respectively. Since CNFs are EDLCs without redox ferreday reactions, the above results suggest that there is almost no structural change in one of the electrodes caused by the non-Ferdey reaction within the voltage range between -0.2 V and 0.8 V.

As a result, through the analysis of long circulation stability, it was found that all the CNFs were excellent as an electrode material for a supercapacitor, and in particular, CNFs 2000 is a material providing better circulation stability.

&Lt; Experimental Example 6 >

The effect of alignment by electrospinning was analyzed in a commonly used activation process to improve the surface area of the carbon material. To improve the surface area, the activation process was performed in a 900 ° C tube furnace under a CO ₂ atmosphere for 1 hour. 8 shows the morphology and alignment of the activated carbon nanofibers (ACNFs 250 and ACNFs 2000) fabricated by electrospinning using the speeds of the various rotating current collectors (250 and 2000 rpm).

Similar to FIG. 3, when the speed of the rotating current collector was increased from 250 rpm to 2000 rpm, the degree of alignment of ACNFs shown in FIG. 8 increased from 14.8% to 91.1%. When the speed of 2000 rpm was reached, ACNFs were successfully aligned in one direction.

Sample
Specific surface area
(m ² / g)
Total pore volume
(cm &lt; ³ &gt; / g) Pore volume
(cm &lt; ³ &gt; / g) Electrical conductivity
(S / cm) Average diameter
(nm) Micro
pore Meso
pore Vertical direction Horizontal direction ACNFs 250 808 0.51 0.19 0.08 0.29 0.10 364 ACNFs 2000 1097 0.62 0.24 0.26 1.42 0.89 246

The surface properties of ACNFs were analyzed by BET analysis. Table 2 shows the surface area, the pore volume, the electric conductivity, and the average diameter of ACNFs. The obtained specific surface areas of ACNFs 250 and ANCFs 2000 calculated using the BET method were 815 m ² / g and 1097 m ² / g, respectively.

This tendency is identical to that observed in CNFs. When the speed of the rotating current collector reached 2000 rpm, the surface properties were enhanced by the tensile strength between the nanofibers and the collector surface.

Due to the CO ₂ activation process, the surface area of ACNFs is greater than the surface area of CNFs. Table 2 above shows the average diameter and electrical conductivity of ACNFs 250 and ACNFs 2000.

The ACNFs average diameter decreased from 364 nm to 246 nm due to the tensile strength generated as the speed of the rotating current increased. This mean diameter value is smaller than the value of CNFs. This means that the CO ₂ activation process reduces the ACNFs diameter.

The electrical conductivity of ACNFs increased as the spinning speed increased from 250 rpm to 2000 rpm. Aligned one-dimensional structures of ACNFs 2000, like aligned CNFs 2000, can provide an excellent path for migration of ions and electrons. However, the electrical conductivity of ACNFs was lower than that of CNFs. These results suggest that CO ₂ Is connected to the physical activation process used.

The physical activation process involves deformation of the carbon substrate due to controlled gasification, which is generally carried out at high temperatures (700-1100 ° C) under CO ₂ conditions, and during controlled gas burning, Through the removal of volatile pyrolysis products, the CO ₂ atmosphere increases the surface area and pore volume of the carbon material. The high degree of activation plays a crucial role in reducing carbon strength, low material density, and electrical conductivity.

9 (b) shows the CV curves of ACNFs 250 and ACNFs 2000 using a 10 mV / s scan rate and 1 MH ₂ SO ₄ electrolyte, and the CV curves of ACNFs 250 and ACNFs 2000 do not show redox peaks . Referring to FIG. 9 (b), the CV curve of ACNFs 2000 is larger than that of ACNFs 250, and the non-accumulating capacities of ACNFs 250 and ACNFs 2000 calculated from CV curves are 203 F / g and 254 F / g, respectively.

FIG. 9 (c) shows the constant current charge-discharge curves of ACNFs 250 and ACNFs 2000 at a current density of 1 A / g in a 1.0 MH ₂ SO ₄ solution. Referring to FIG. 9 (c) And a triangular shape between -0.2 V and 0.8 V indicating an ideal bilayer capacitive capacity activity like CNFs and the non-storage capacities of ACNFs 250 and ACNFs 2000 obtained from constant current charge-discharge curves are respectively 204 and 230 F / g, respectively.

The non-capacitances obtained from the CV curves and the constant current charge-discharge curves show that the ACNFs 2000 electrode has better electrical properties than the ACNFs 250 electrode. According to the alignment of the ACNFs, pore volume (from ^{0.19 cm 3 / g 0.24 cm 3} / g) is associated with. As a result, improved surface properties and electrical conductivity affect the increase in the storage capacity of ACNFs.

FIG. 9 (d) shows the non-storage capacities of ACNFs 250 and ACNFs 2000 according to the scan speed from 10 mV / s to 50 mV / s. The non-storage capacities of ACNFs 250 and ACNFs 2000 were 39.6% and 33.6% Respectively. The ACNFs 2000 electrode has a higher rate characteristic than the ACNFs 250 electrode, which is related to the electrical conductivity and mesopore volume enhanced by the alignment process.

10 (a) shows a Nyquist plot of ACNFs 250 and ACNFs 2000. Referring to FIG. 10 (a), at high frequencies, the semicircle generally contributes to the charge-discharge reaction process, Means a charge transfer resistance (Rct). The Rct values of ACNFs 250 and ACNFs 2000 were 1.18? And 0.67?, Respectively.

Also, as the degree of alignment increased, Rs decreased from 7.96 Ω to 7.43 Ω. In addition, in the low frequency region, the slope of the Nyquist line was close to 90 degrees as the speed of the rotating current increased from 250 rpm to 2000 rpm in the intermediate frequency region. Thus, the ordered structure of ANCFs is easier to transport than randomly oriented ANCFs, and exhibits higher non-accumulation capacities, thus enhancing surface properties and electrical conductivity.

Referring to FIG. 10 (b), cyclic stability tests of ACNFs 250 and ACNFs 2000 were performed at 1 A / g current density in a 1 M H ₂ SO ₄ electrolyte for 1000 cycles. When the constant current charge-discharge test was performed for 1000 cycles, the non-accumulating capacity was continuously decreased and decreased by 8.04% and 3.51% for ACNFs 250 and ACNFs 2000, respectively. Therefore, from the long circulation stability test for ACNFs 250 and ACNFs 2000, it can be seen that it is an excellent electrode material for supercapacitor application. In addition, ACNFs 2000, which contains a one-dimensional aligned structure, exhibited inherent electrical properties.

Aligned CNFs were fabricated by electrospinning using a high speed rotating current collector. When the speed was increased, the degree of alignment of CNFs increased. When the speed of the rotating current collector reached 2000 rpm, the fibers were aligned. The increased tensile strength between the surface and the fibers of the rotating current collector for the electrospinning process was increased by increasing the speed of the rotating current collector to cause a decrease in the average diameter of the CNFs. Alignment of CNFs increased the surface area from 533 m ² / g to 635 m ² / g. Moreover, the electrical conductivity of CNFs 2000 has been increased by its inherent one-dimensional orientation structure.

The electrochemical properties of CNFs 250, CNFs 500, CNFs 1000, and CNFs 2000 were measured by CV, constant current charge-discharge curve, and EIS analysis. Due to the alignment of the CNFs, the non-accumulation capacity obtained from the CV curve increased to 35.4%.

The CV characteristics were analyzed at various scan rates from 10 mV / s to 50 mV / s. The speed characteristics of CNFs 2000 were 14.9% higher than those of CNFs 250.

The Nyquist plot showed improved electrochemical properties of CNFs due to Rs reduction in the low frequency region, bug resistance in the intermediate frequency region, and a leading slope in the high frequency region.

This result implies enhanced electrical conductivity and surface properties of CNFs 2000 because the one-dimensional ordered structure affects the improvement of the inherent electrochemical properties. Moreover, CNFs 2000 exhibited excellent cyclic stability, and the capacity after 1000 cycles was reduced by 10.8%.

In order to improve the surface area, an activation process by CO ₂ treatment was performed at 900 ° C for 1 hour. Similar to the tendency of CNFs, the degree of alignment was highest (91.1%) when the speed of the rotating current collector reached 2000 rpm, and the surface area and electrical conductivity were higher than those of arbitrarily oriented ACNFs.

The non - storage capacity and velocity characteristics of ACNFs 2000 calculated from CV curves were 25.1% and 9.8% higher than ACNFs 250, respectively. As a result, electrochemical properties of CNFs and ACNFs were improved by improved surface properties and electrical conductivity through alignment by electrospinning.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It is to be understood that various modifications and changes may be made without departing from the scope of the appended claims.

Claims (7)

Preparing an electrospinning solution (first step);
Connecting the prepared electrospinning solution to a syringe pump, and then operating a rotary collector at a speed of 2000 rpm to produce aligned polyacrylonitrile nanofibers (second step);
Drying and stabilizing the prepared polyacrylonitrile nanofibers (step 3);
Carbonizing the stabilized polyacrylonitrile nanofibers to produce aligned carbon nanofibers (Step 4); And
And activating the carbon nanofibers to produce active carbon nanofibers (Step 5). The method of manufacturing an activated carbon nanofibers according to any one of claims 1 to 5, The electrospinning solution according to claim 1, wherein the electrospinning solution is prepared by mixing the polyacrylonitrile with N, N-dimethylformamide at a weight ratio of 1: 7 to 10, Lt; RTI ID = 0.0 &gt; of activated carbon nanofibers. delete [2] The method of claim 1, wherein the polyacrylonitrile nanofibers are dried and stabilized by heating to 250 to 350 [deg.] C. The method according to claim 1, wherein the fourth step is carbonization of the polyacrylonitrile nanofibers stabilized in a nitrogen atmosphere by heating to 800 to 1000 ° C. [6] The method according to claim 1, wherein the step 5) activates the carbon nanofibers produced under a CO ₂ atmosphere at a temperature of 800 to 1000 ° C. The aligned activated carbon nanofibers produced according to the process of any one of claims 1, 2 and 4 to 6.

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