Method for preparing porous carbon nanofibers containing a metal oxide, porous carbon nanofibers prepared using the method, and carbon nanofiber products including same

Abstract

The present invention relates to carbon nanofibers, and more particularly, to a method capable of preparing metal oxide-containing porous carbon nanofibers having a high specific surface area by changing the composition of a spinning solution, which is used in a process of preparing carbon nanofiber by electrospinning, and to metal oxide-containing porous carbon nanofibers prepared by the method, and carbon nanofiber products comprising the same.

Claims

1. A method for preparing porous carbon fibers, the method comprising: preparing a carbon nanofiber precursor solution containing a carbon nanofiber precursor and a metal alkoxide [M(OR).sub.n]; electrospinning the precursor solution to obtain electrospun fibers, wherein the precursor solution is prepared to contain the carbon nanofiber precursor and the metal alkoxide at a ratio of 70-99: 30-1 wt %; stabilizing the electrospun fibers to obtain stabilized fibers; and carbonizing or physically activating the stabilized fibers to obtain porous carbon fibers.

2. The method of claim 1, wherein the stabilized fibers are physically activated to obtain the porous carbon fibers.

3. The method of claim 1, wherein the metal alkoxide is Si-alkoxide, Ti-alkoxide, Al-alkoxide, Zn-alkoxide, or a combination thereof.

4. A method for preparing porous carbon fibers, the method comprising: preparing a carbon nanofiber precursor solution containing a carbon nanofiber precursor and a metal alkoxide [M(OR).sub.n]; electrospinning the precursor solution to obtain electrospun fibers; stabilizing the electrospun fibers to obtain stabilized fibers, wherein the stabilization is carried out by placing the electrospun fibers in an air circulating furnace, supplying compressed air to the furnace at a flow rate of 5-20 mL/min, and maintaining the fibers at 200300 C. at a heating rate of 1 C./min for 30 minutes or more; and carbonizing or physically activating the stabilized fibers to obtain porous carbon fibers.

5. A method for preparing porous carbon fibers, the method comprising: preparing a carbon nanofiber precursor solution containing a carbon nanofiber precursor and a metal alkoxide [M(OR).sub.n]; electrospinning the precursor solution to obtain electrospun fibers; stabilizing the electrospun fibers to obtain stabilized fibers, wherein the stabilized fibers are carbonized to obtain the porous carbon fibers, and the carbonizing is carried out by heating the fibers to 7001500 C. at a rate of 5 C./min in an inert or vacuum atmosphere, and then maintaining the fibers at that temperature for 30 minutes or more.

6. The method of claim 2, wherein the physical activation is carried out by heating the fibers to 700850 C. at a rate of 5 C./min, and then maintaining the fibers in an atmosphere of 150-250 mL/min of inert gas and 5-15 vol % of steam for 30 minutes or more.

7. The method of claim 1, wherein one or more of a diameter and a surface porosity of the carbon nanofibers is controlled by controlling one or more of the concentration of the metal alkoxide, the carbonization temperature and time, and the physical activation temperature and time.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic flow diagram showing a method of preparing porous carbon nanofibers according to the present invention and the application of the porous carbon nanofibers to a high-capacitance supercapacitor.

(2) FIG. 2 shows a mechanism in which the sol-gel reaction of a metal alkoxide occurs.

(3) FIG. 3 is a graphic diagram showing the results of thermal analysis of electrospun fibers obtained in Example 1.

(4) FIG. 4 is a graphic diagram showing the results of differential scanning calorimetry (DSC) analysis carried out by heating electrospun fibers and stabilized fibers, obtained in Example 1 of the present invention, at a heating rate of 10 C./min in a nitrogen atmosphere.

(5) FIG. 5 is a high-magnification scanning electron microscope (SEM) photograph of carbon nanofibers 1 obtained at a carbonization of 800 C. in Example 2 of the present invention.

(6) FIG. 6 is a scanning electron microscope (SEM) photograph of porous carbon nanofibers 2 obtained at a carbonization of 900 C. in Example 3 of the present invention.

(7) FIG. 7 is a scanning electron microscope (SEM) photograph of porous carbon nanofibers 3 obtained at a carbonization of 1000 C. in Example 4 of the present invention.

(8) FIG. 8 is a graph showing the results of energy dispersive X-ray (EDX) analysis of porous carbon nanofibers 3 carbonized at 1000 C. in Example 4 of the present invention.

(9) FIG. 9 is a high-magnification scanning microscope (SEM) photograph of porous carbon nanofibers 1 obtained at a carbonization temperature of 800 C. in Example 2 of the present invention.

(10) FIG. 10 is a high-magnification scanning microscope (SEM) photograph of porous carbon nanofibers 2 obtained at a carbonization temperature of 900 C. in Example 3 of the present invention.

(11) FIG. 11 is a high-magnification scanning microscope (SEM) photograph of porous carbon nanofibers 3 obtained at a carbonization temperature of 1000 C. in Example 4 of the present invention.

(12) FIG. 12 shows cyclic voltammogram (CV) graphs of capacitors comprising porous carbon nanofibers 4 to 6, obtained in Examples 5 to 7 of the present invention, as a function of carbonization temperature in an aqueous electrolyte solution of 6M KOH.

(13) FIG. 13 is a graphic diagram showing the specific capacitances of capacitors comprising porous carbon fibers 4 to 6, obtained in Examples 5 to 7 of the present invention, as a function of carbonization temperature in the voltage range of 0 to 1.

(14) FIG. 14 is a graphic diagram showing the results of separating the peak of a carbon element (C) in porous carbon nanofibers 4, obtained in Example 5 of the present invention, by X-ray photoelectron spectrometry (XPS).

(15) FIG. 15 is a graphic diagram showing the results of separating the peak of an oxygen element (O) in porous carbon nanofibers 4, obtained in Example 5 of the present invention, by X-ray photoelectron spectrometry (XPS).

(16) FIG. 16 is a graphic diagram showing the results of separating the peak of a nitrogen element (N) in porous carbon nanofibers 4, obtained in Example 5 of the present invention, by X-ray photoelectron spectrometry (XPS).

(17) FIG. 17 is a set of Ragon graphs showing the results of measuring the energy and power densities of capacitors comprising porous carbon nanofibers 4 to 6, obtained in Examples 5 to 7 of the present invention, as a function of the carbonization temperature of the porous carbon nanofibers.

(18) FIG. 18 is a graphic diagram showing the specific capacitances of capacitors comprising porous carbon fibers 7 and 8, obtained in Examples 8 and 9 of the present invention, in the voltage range of 0 to 2.7 as a function of the weight ratio of TEOS in the porous carbon nanofibers.

(19) FIG. 19 is a set of Ragon graphs showing the results of measuring the energy and power densities of capacitors comprising porous carbon nanofibers 7 and 8, obtained in Examples 8 and 9 of the present invention, as a function of the weight ratio of TEOS in the porous carbon nanofibers.

DETAILED DESCRIPTION

(20) Although the usage of the general terms which are currently widely used was selected within the realm of the possible in the present invention, terms optionally selected by the applicant will be used in specific cases, wherein their meaning will be defined and described in detail in the corresponding detailed description of the present invention. Accordingly, the terms used in the present invention should be understood by their true meaning.

(21) Hereinafter, the technical configuration of the present invention will be described in detail with reference to an embodiment of the present invention.

(22) However, the present invention is not limited to the embodiment described herein and may also be embodied in other forms. Throughout the specification, like reference numerals used to describe the present invention indicate like elements.

(23) In the method of preparing carbon nanofibers by electrospinning according to the present invention, a metal alkoxide is added to a carbon fiber precursor solution at a specific weight ratio, so that porous carbon nanofibers can be prepared using the sol-gel reaction of the metal alkoxide. The porous carbon nanofibers thus prepared have excellent properties, including high porosity, high specific surface area, high energy density and high energy storage capacity. Due to such excellent properties, various products comprising the porous carbon nanofibers of the present invention, including supercapacitors, fuel cells, secondary battery electrodes, electromagnetic wave-shielding materials, highly conductive materials, and adsorbents, also have excellent quality.

(24) Thus, the inventive method for preparing porous carbon nanofibers comprises the steps of: preparing a carbon nanofiber precursor solution containing a metal alkoxide [M(OR).sub.n]; electrospinning the precursor solution to obtain sound precursor fibers; stabilizing the electrospun fibers to obtain stabilized fibers; and then either carbonizing the stabilized fibers to obtain porous carbon nanofibers, or physically activating the stabilized fibers to obtain porous carbon nanofibers.

(25) Herein, the carbon nanofiber precursor that is used in the present invention may be any known material, but, for example, may be a polyacrylonitrile (PAN) for fiber use. The PAN (molecular weight=about 160,000) may be a PAN homopolymer or a PAN copolymer containing 5-15 wt % of a comonomer. Herein, the comonomer may be itaconic acid or methylacrylate (MA).

(26) The metal alkoxide [(M(OR).sub.n] that is used in the present invention may be any known material, but, for example, may be any one of metal alkoxides in which M is Si, Ti, Al or Zr, that is, Si-alkoxide, Ti-alkoxide, Al-alkoxide and Zn-alkoxide.

(27) The metal alkoxide which is contained in the carbon fiber precursor solution may be used at a ratio of 1-30 wt % relative to the carbon fiber precursor. Thus, the carbon fiber precursor solution is prepared to contain the carbon nanofiber precursor and the metal alkoxide at a ratio of 70-99:301 wt %.

(28) As a solvent in which the carbon nanofiber precursor and the metal alkoxide are dissolved, any solvent may be used without particular limitation, so long as it can dissolve both the carbon fiber precursor and the metal alkoxide. For example, any one of dimethylformamide (DMF), DMSO and THF is used.

(29) The concentration of the carbon fiber precursor in the carbon fiber precursor solution may be 5-30%. If the concentration of the carbon fiber precursor is out of this range, the precursor will be difficult to spin and fibers will not be easily formed.

(30) In the method of the present invention, the stabilizing step is carried out by placing the fibers in a hot air circulating furnace, supplying compressed air to the furnace at a flow rate of 5-20 mL/min, and maintaining the fibers at 200300 C. at a heating rate of 1 C./min for 30 minutes or more.

(31) Then, when the stabilized fibers are carbonized, the carbonization may be carried out by heating the fibers to 7001500 C. at a rate of 5 C./min in an inert or vacuum atmosphere and then maintaining the fibers at that temperature for 30 minutes or more. As shown in FIG. 2, when a metal oxide is produced from the metal alkoxide by a sol-gel reaction, water and alcohol are produced as byproducts and are partially removed from the metal oxide during the high-temperature carbonization step, and thus carbon nanofibers having surface pores are prepared.

(32) The inventive porous carbon nanofibers thus obtained have a diameter of 100-300 nm and a specific surface area of 700-1300 m.sup.2/g and include fine pores having a size of 1-3 nm. As described above, the specific surface of the porous carbon nanofibers and the size of the fine pores can be controlled depending on the content of the metal alkoxide and the heat-treatment temperature.

(33) Meanwhile, when the stabilized fibers are activated, the activation may be carried out by heating the fibers to 700850 C. at a rate of 5 C./min and then maintaining the fibers at that temperature in an atmosphere of 150-250 mL/min of inert gas and 5-15 vol % of steam for 30 minutes, and, for example, in an atmosphere of 200 mL/min of inert gas and 10 vol % of steam for 60 minutes or more.

(34) Specifically, steam which is used in the physical activation of the fibers promotes the hydrolysis and condensation of the metal alkoxide, and water and alcohol which are removed during the production of metal oxide accelerate the sol-gel reaction of the metal alkoxide according to Le Chatelier's principle, thereby producing an increased number of fine pores and enlarged pores in the surface.

(35) The inventive porous activated carbon nanofibers obtained by physical activation as described above have a diameter of 100-250 nm and a specific surface area of 1300-1700 m.sup.2/g and include fine pores and mesopores, which have a size 2 nm or more. When such carbon nanofiber structures are applied to electrodes for capacitors, they can match the size of organic solvent electrolyte ions, making it possible to fabricate supercapacitors having high energy density.

(36) According to the present invention, one or more of the diameter and surface porosity (including specific surface area and the size of fine pores and mesopores) of the carbon nanofibers can be controlled by controlling the concentration of the metal alkoxide in the carbon nanofiber precursor solution, the carbonization temperature and time and the activation temperature and time.

Example 1

(37) A polyacrylonitrile (PAN) homopolymer as a carbon nanofiber precursor and tetraethyl orthosilicate (Si(OEt).sub.4, TEOS) (Si-alkoxide) as a metal alkoxide [(M(OR).sub.n] were prepared. The prepared PAN and TEOS were dissolved in the solvent DMF, thereby preparing a carbon nanofiber precursor solution. Herein, the carbon nanofiber precursor solution was prepared to have a carbon nanofiber precursor concentration of 10% and to contain PAN and Si-alkoxide at a ratio of 80:20 wt %.

(38) Then, the prepared carbon nanofiber precursor solution was electrospun to prepare nonwoven fabric webs consisting of nanofibers, thereby obtaining electrospun fibers.

(39) In the electrospinning process, a voltage of 25 kV was applied to each of the nozzle and collector of the electrospinning apparatus, and the distance between the spinneret and the collector was set at about 20 cm and could be changed if necessary.

(40) The electrospun fibers resulting from the electrospinning process were stabilized by placing the fibers in a hot air circulating furnace, supplying compressed air to the furnace at a flow rate of 20 mL/min, heating the fibers at a rate of 1 C./min to 280 C. and maintaining the fibers at that temperature for 50 minutes, thereby obtaining stabilized insoluble fibers.

Example 2

(41) The stabilized fibers were carbonized by heating the fibers to 800 C. at a rate of 5 C./min in an inert atmosphere and then maintaining the fibers at that temperature for 50 minutes, thereby preparing porous carbon nanofibers 1.

Example 3

(42) The procedure of Examples 1 and 2 was repeated, except that the carbonization temperature was increased to 900 C., thereby preparing porous carbon nanofibers 2.

Example 4

(43) The procedure of Examples 1 and 2 was repeated, except that the carbonization temperature was increased to 1000 C., thereby preparing porous carbon nanofibers 3.

Example 5

(44) The procedure of Example 2 was repeated, except that the carbon nanofiber precursor solution was prepared to contain PAN and Si-alkoxide at a ratio of 70:30 wt %, thereby preparing porous carbon nanofibers 4.

Example 6

(45) The procedure of Example 3 was repeated, except that the carbon nanofiber precursor solution was prepared to contain PAN and Si-alkoxide at a ratio of 70:30 wt %, thereby preparing porous carbon nanofibers 5.

Example 7

(46) The procedure of Example 4 was repeated, except that the carbon nanofiber precursor solution was prepared to contain PAN and Si-alkoxide at a ratio of 70:30 wt %, thereby preparing porous carbon nanofibers 6.

Example 8

(47) The stabilized fibers obtained in Example 1 were heated to 800 C. at a rate of 5 C./min and then maintained at that temperature for 60 minutes in an atmosphere of 200 mL/min of inert gas and 10 vol % of steam, thereby preparing porous carbon nanofibers 7.

Example 9

(48) The procedure of Example 8 was repeated, except that the carbon nanofiber precursor solution was prepared to contain PAN and Si-alkoxide at a ratio of 70:30 wt %, thereby preparing porous carbon nanofibers 8.

Example 10

(49) Each of the porous carbon fibers 4 to 6 prepared in Examples 5 to 7 were cut to a suitable size and placed on Ni foam collectors to form positive and negative electrodes, and a Celgard (polypropylene) separator was inserted between the positive and negative electrodes, after which a 6M KOH aqueous electrolyte solution was impregnated into the resulting structure, thereby fabricating supercapacitors 1 to 3.

Example 11

(50) Each of the porous carbon fibers 7 and 8 prepared in Examples 8 and 9 were cut to a suitable size and placed on Ni foam collectors to form positive and negative electrodes, and a Celgard (polypropylene) separator was inserted between the positive and negative electrodes, after which a 1.5M organic solvent electrolyte solution of tetraethylammonium tetrafluoroborate in acetonitrile was impregnated into the resulting structure, thereby fabricating supercapacitors 4 and 5.

COMPARATIVE EXAMPLE

(51) Comparative carbon nanofibers were prepared in the same manner as in Example 3, except that only PAN was used as the solute without using metal alkoxide.

Test Example 1

(52) The electrospun fibers obtained in Example 1 were thermally analyzed at a heating rate of 10 C./min in an atmosphere of N.sub.2. The analysis results are shown in FIG. 3.

(53) As can be seen in the thermogravimetric analysis (TGA) graph in FIG. 3, a major change in weight appeared at 280330 C. At 330 C. or higher, a slow decrease in weight appeared, and at 1000 C., a residual amount of 28% appeared. In addition, in the differential scanning calorimetry (DSC) in FIG. 3, a very high exothermic peak appeared at 280330 C., as in the TGA graph, suggesting that thermal decomposition of the precursor fibers occurred in this temperature range.

Test Example 2

(54) The electrospun fibers and stabilized fibers obtained in Example 1 were analyzed by differential scanning calorimetry (DSC) at a heating rate of 10 C./min in a nitrogen atmosphere. The results are shown graphically in FIG. 4.

(55) As can be seen in FIG. 4, the electrospun fibers showed a very strong exothermic peak at around 287 C., and this was believed to be because of the influence of the cyclization of the nitrile group during the stabilization process. In the DSC graph of the stabilized fibers, an exothermic peak appeared over the broad range of 320350 C., because the cyclization or crosslinking of the nitrile group remaining in the precursor fibers continuously took place. In addition, unlike the electrospun fibers, the stabilized fibers showed a strong endothermic peak at around 100 C. due to the evaporation of water or alcohol, because the stabilized fibers contained increased amounts of functional groups such as CO, CO and COH.

Test Example 3

(56) Porous carbon fibers 1 to 3 obtained in Examples 2 to 4 were observed with a scanning electron microscope (SEM), and the SEM photographs are shown in FIGS. 5 to 7. As can be seen in FIGS. 5 to 7, carbon nanofibers could be easily produced by the method of the present invention without producing particles or beads, and the diameter of the carbon nanofibers decreased as the carbonization temperature increased.

(57) Particularly, carbon nanofibers, including porous carbon nanofibers 1 to 6, were prepared at varying carbonization temperatures and the average diameters thereof were measured. As a result, as shown in Table 1 below, the average diameter decreased as the carbonization temperature increased.

(58) TABLE-US-00001 TABLE 1 Carbonization temperature ( C.) Average diameter (nm) 800 200-300 900 170-260 1000 150-175

Test Example 4

(59) Porous carbon nanofibers 3 obtained in Examples 4 were analyzed by energy dispersive X-ray (EXD) analysis, and the analysis results are shown in FIG. 8.

(60) As can be seen in FIG. 8, C, O and Si elements could be observed in the carbon nanofibers prepared using tetraethyl orthosilicate (Si(OEt).sub.4, TEOS) as the metal alkoxide.

Test Example 5

(61) The surfaces of porous carbon nanofibers 1 to 3 obtained in Examples 2 to 4 were observed with a high-magnification scanning electron microscope (SEM), and the SEM photographs are shown in FIGS. 9 to 11.

(62) As can be seen in FIGS. 9 to 11, the size of fine pores in the surface increased as the carbonization temperature increased.

Test Example 6

(63) Porous carbon nanofibers 1 to 3, obtained in Examples 2 to 4, and the comparative carbon nanofibers obtained in the Comparative Example were measured for BET specific surface areas, pore volumes and average pore sizes, and the measurement results are shown in Table 2 below.

(64) As can be seen in Table 2, as the carbonization temperature increased, the BET specific surface area, pore volume and average pore size of the porous carbon nanofibers prepared using the metal alkoxide increased, and these were higher than those of the comparative carbon nanofibers prepared using PAN only. Thus, it can be seen that that electric double-layer capacitors (EDLCs) made of the porous carbon nanofibers prepared using the metal alkoxide as described in the present invention have high specific capacitance and a stable cycle life, because the surface of the porous carbon nanofibers has developed fine pores.

(65) TABLE-US-00002 TABLE 2 BET Pore Average surface volume pore Carbonization temperature area (m.sup.2/g) (cm.sup.3/g) size () Porous carbon nanofibers 1 732.20 0.287 15.7 carbonized at 800 C. Porous carbon nanofibers 2 950.78 0.376 16.2 carbonized at 900 C. Porous carbon nanofibers 3 1300.25 0.782 18.0 carbonized at 1000 C. Comparative carbon nanofibers 336.75 0.136 15.2 carbonized at 1000 C.

Test Example 7

(66) The BET specific surface area, pore volume and average pore size of porous carbon nanofibers 1 to 3, obtained in Examples 2 to 4, and porous carbon nanofibers 7 and 8 obtained in Examples 7 and 8 were measured, and the measurement results are shown in Table 3 below.

(67) TABLE-US-00003 TABLE 3 T-plot Total Mesopore Micropore Average surface volume volume volume pore area (cm.sup.3/g) fraction (%) fraction (%) size () Porous 986.3 0.374 20 80 15.73 carbon nanofibers 1 Porous 1045.6 0.404 22 78 16.25 carbon nanofibers 2 Porous 1200.2 0.536 28 72 18.08 carbon nanofibers 3 Porous 1386.9 0.571 35 65 19.65 carbon nanofibers 7 Porous 1483.3 0.604 40 60 20.18 carbon nanofibers 8

(68) As can be seen in Table 3, the BET specific surface area, pore volume and average pore size of porous carbon fibers 7 and 8 prepared using the physical activation process increased compared to those of porous carbon nanofibers 1 to 3 prepared using the carbonization process. This suggests that steam in the activation process promotes the hydrolysis and condensation of TEOS to accelerate the sol-gel reaction of the TEOS, thereby increasing the mesopore volume fraction. In addition, it can be seen that, as the weight ratio of TEOS increases, the amount of precursor that can be subjected to the sol-gel reaction increases, and thus the specific surface area increases.

Test Example 8

(69) The cyclic voltammograms of supercapacitor electrodes 1 to 3 comprising porous carbon nanofibers 4 to 6 prepared in Examples 5 to 7 were measured in a 6M KOH aqueous solution as an electrolyte, and the measurement results are shown in FIG. 12. Herein, the measurement was carried out at a voltage of 0-1.0 V and a scanning speed of 25 mV/s. As can be seen in FIG. 12, as the carbon temperature decreased (1000.fwdarw.800 C.), the specific surface area increased, but the capacitance decreased. In addition, the cyclic voltammograms did not show a rectangular shape, which is shown in typical EDLCs, but showed a quasi-rectangular shape at the end portion corresponding to 1V.

Test Example 9

(70) The specific capacitances of supercapacitor electrodes 1 to 3 comprising porous carbon nanofibers 4 to 6 prepared in Example 10 were measured as a function of carbonization temperature, and the results are shown graphically in FIG. 13. The specific capacitances of the carbon nanofibers as a function of carbonization temperature in FIG. 13 showed a tendency similar to those in the results of measurement of CVs shown in FIG. 12, and the carbon nanofibers showed a high specific capacitance of 161.39 F/g at a low carbonization temperature of 800 C. This is because the carbon nanofibers carbonized at a low temperature of 800 C. contained large amounts of heteroatoms such as silicon (Si), oxygen (O) and nitrogen (N), which allowed the electrode to exhibit high specific capacitance, high energy density and high power density because of electric double-layer capacitance and pseudo-capacitance.

Test Example 10

(71) Porous carbon nanofibers 1 to 6 obtained in Examples 2 to 7 were quantitatively analyzed by X-ray photoelectron spectroscopy (XPS), and the analysis results are shown in Table 4 below.

(72) TABLE-US-00004 TABLE 4 Carbonization temperature C 1s % O 1s % N 1s % Si 2p % Porous carbon nanofibers 83.4 9.90 6.00 0.70 1,4 carbonized at 800 C. Porous carbon nanofibers 85.2 9.48 4.58 0.74 2,5 carbonized at 900 C. Porous carbon nanofibers 89.9 6.01 3.51 0.55 3,6 carbonized at 1000 C.

(73) As can be seen in Table 4 above, as the carbonization temperature increased, the ratio of the carbon (C) atom increased, but the ratio of heteroatoms such as silicon (Si), oxygen (O) and nitrogen (N) decreased.

Test Example 11

(74) The peaks of carbon (C), oxygen (O) and nitrogen (N) elements in porous carbon nanofibers 4 obtained in Example 4 were analyzed by X-ray photoelectron spectroscopy (XPS). As a result, it can be seen in FIGS. 14 to 16 that carbon, oxygen and nitrogen were separated into 4, 3 and 2 binding energy peaks, respectively.

(75) In addition, for porous carbon fibers 1 to 6 obtained in Examples 2 to 7, the peaks of carbon (C), oxygen (O) and nitrogen (N) elements were analyzed by X-ray photoelectron spectroscopy, and the percentages of functional groups in these peaks, that is, the XPS of C1s, the XPS of O1s and the XPS of N1s, are shown in Tables 5 to 7, respectively. These results show that as the carbonization temperature decreases, the percentage of crystalline graphite structures in the carbon nanofibers decreases, and thus the crystallinity of the fibers decreases, whereas the percentages of functional groups in the carbon nanofiber surface increase.

(76) TABLE-US-00005 TABLE 5 CC CC COH, COOH, Carbonization temperature graphite amorphous COR COOR Porous carbon nanofibers 6.25 53.56 19.67 9.32 1,4 carbonized at 800 C. Porous carbon nanofibers 9.32 52.6 20.14 4.93 2,5 carbonized at 900 C. Porous carbon nanofibers 11.87 49.93 18.42 4.46 3,6 carbonized at 1000 C.

(77) TABLE-US-00006 TABLE 6 Amides, CO anhydrides, and Ether oxygen carbonyl oxygen atoms atoms in atoms in hydroxyls esters and Carbonization temperature in ester or ethers anhydrides Porous carbon nanofibers 2.76 5.38 2.70 1,4 carbonized at 800 C. Porous carbon nanofibers 2.20 3.39 2.12 2,5 carbonized at 900 C. Porous carbon nanofibers 2.38 2.39 1.65 3,6 carbonized at 1000 C.

(78) TABLE-US-00007 TABLE 7 N-6 N-Q Carbonization temperature (pyridinic nitrogen) (quaternary nitrogen) Porous carbon nanofibers 3.92 2.9 1,4 carbonized at 800 C. Porous carbon nanofibers 3.67 1.84 2,5 carbonized at 900 C. Porous carbon nanofibers 2.79 0.64 3,6 carbonized at 1000 C.

Test Example 12

(79) The energy and power densities of supercapacitor electrodes 1 to 3 of Example 10 comprising porous carbon nanofibers obtained in Examples 5 to 7 were measured as a function of carbonization temperature in an aqueous electrolyte solution of 6M KOH, and the results are shown in FIG. 17 as Ragon graphs. As can be seen in FIG. 17, the supercapacitors showed a high energy density of 22 Wh/kg and a high power density of 100 kW/kg.

Test Example 13

(80) The specific capacitances of supercapacitors 4 and 5 of Example 11 comprising porous carbon nanofibers 7 and 8 obtained in Examples 8 and 9 were measured as a function of the weight ratio of TEOS of the porous carbon nanofibers in the voltage range of 0-2.7 V, and the measurement results are shown in FIG. 18.

(81) As can be seen in FIG. 18, the capacitors showed specific capacitances of 103.95 and 120.93 F/g, suggesting that these are supercapacitors.

Test Example 14

(82) The energy and power densities of supercapacitors 4 and 5 of Example 11 comprising porous carbon nanofibers 7 and 8 obtained in Examples 8 and 9 were measured as a function of the weight ratio of TEOS, and the measurement results are shown in FIG. 19 as Ragon graphs. As can be seen in FIG. 19, the supercapacitors showed an energy density of 90-35 Wh/kg and a power density of 2-100 kW/kg in a 1.5 M organic solvent electrolyte solution of tetraethylammonium tetrafluoroborate in acetonitrile.

(83) From the above test results, it can be seen that the porous carbon nanofibers prepared by carbonization in the preparation method of the present invention contain the metal alkoxide introduced into the carbon crystal, and have a diameter of 100-300 nm and a specific surface area of 700-1500 m.sup.2/g and include fine pores having a size of 1-3 nm. Thus, it is concluded that these porous carbon nanofibers can increase the dielectric constant between an electrolyte and the electrode surface or induce the Faraday reaction, thus improving energy density.

(84) In addition, the carbon nanofibers prepared at low carbonization temperature show fast selective adsorption and desorption even when the potential changes slightly. Thus, such carbon nanofibers allow the cost of the carbonization process to be reduced and can be provided with high conductivity, a high specific surface area and fine pores having various sizes, which are controlled depending on the carbonization temperature.

(85) Furthermore, the porous carbon nanofibers prepared through physical activation in the preparation method of the present invention have a diameter of 100-250 nm and a specific surface area of 1300-1700 m.sup.2/g and include fine pores and mesopores, which have a size of 2 nm or greater. Thus, it can be seen that steam accelerates the sol-gel reaction in the physical activation process to increase the specific surface area and the formation of fine pores and mesopores, which contain the metal oxide. Specifically, steam which is used in the physical activation process promotes the hydrolysis and condensation of the metal alkoxide to produce a metal oxide, and water and alcohol which are removed when the metal oxide is produced accelerate the sol-gel reaction according to Le Chatelier's principle to increase the production of fine pores in the surface and to produce mesopores.

(86) As a result, it can be seen that the porous carbon nanofibers prepared using the carbonization process or physical activation process of the present invention all have excellent electrochemical properties, capacitance, power density and energy density, and particularly, the porous carbon nanofibers prepared using the physical activation process can provide supercapacitors having high energy density, compared to capacitors comprising the porous carbon nanofibers prepared using the carbonization process.

(87) Thus, the porous carbon nanofiber nanofibers prepared by the method of the present invention have excellent properties as demonstrated in the above test examples, and can be used in supercapacitors. In addition, although specifically not described, the porous carbon nanofibers of the present invention can be used in various industrial fields, including adsorbent materials.

(88) Moreover, because the porous carbon nanofibers of the present invention have pores exposed to the outside, contaminants can get close to the carbon nanofibers, suggesting that the carbon nanofibers can be used as filter materials. Also, the porous carbon nanofibers are highly useful as materials for electric double-layer supercapacitors and secondary batteries, as well as electromagnetic wave-shielding materials and highly conductive materials.

(89) One or more embodiments of the present invention has the following excellent effects.

(90) First, according to the present invention, highly porous carbon nanofibers can be prepared by adding a metal alkoxide to a carbon fiber precursor solution and carrying out a sol-gel reaction of the metal alkoxide without a chemical activation process. The carbon nanofibers prepared by the present invention have a large specific surface area and improved electrical conductivity.

(91) Also, according to the present invention, carbon nanofibers having ultrafine and highly porous fiber webs are prepared through carbonization or physical activation, and thus activated/carbon nanofibers and products comprising the same can be prepared in a cost and time-effective manner.

(92) In addition, the properties of the carbon nanofibers are easily controlled as desired by controlling one or more of the concentration of the metal alkoxide, the heat-treatment temperature and time and the activation process to control the specific surface area and pore size distribution of the metal alkoxide-containing carbon nanofibers.

(93) Furthermore, according to the present invention, a metal alkoxide is introduced into the carbon crystals of the carbon nanofibers so that the dielectric constant between an electrolyte and the electrode surface can be increased to increase energy density, and the carbon nanofibers can show fast adsorption and desorption even when energy changes slightly, and thus have excellent storage properties and provide high-performance capacitors.

(94) Moreover, according to the present invention, a high-capacitance capacitor having excellent electrochemical properties, charge/discharge characteristics, and energy and power densities can be provided by applying carbon nanofibers having porous fiber webs, prepared by a carbonization process, to an aqueous electrolyte.

(95) In addition, according to the present invention, a supercapacitor can be provided by applying porous carbon nanofibers, prepared through an activation process using a steam-containing gas such that a metal oxide remains, to an organic solvent electrolyte, the capacitor having significantly improved electrochemical characteristics, capacitance, power density and energy density and being capable of being applied to power storage devices.

(96) Although the exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.