KR20040019537A - Novel structural fibrous carbon - Google Patents

Abstract

PURPOSE: Provided is a fibrous carbon with novel structure, which has specific surface area 2 to 6 times greater than that of conventional carbon nanofiber and exhibits double-layer capacitance. CONSTITUTION: The fibrous carbon comprises regularly arranged webbed carbon hexagon faces at intervals of 3.345 to 3.8 Å and has a diameter of 3.5 to 500 nm, a length 10 times or more greater than the diameter, and specific surface area of 200 to 1000 m¬2/g, wherein a unit structure of the fibrous carbon includes at least two hexagon faces and each unit structure is distanced from adjacent unit structure by more than 10 nm and wherein the unit structure preferably includes 30 to 120 carbon hexagon faces.

Description

New structured fibrous carbon {NOVEL STRUCTURAL FIBROUS CARBON}

The present invention relates to a novel structured fibrous carbon, and more particularly, carbon hexagonal meshes formed by spherically bonding two-dimensional carbon atoms of sp ² atomic structure are regularly arranged at an interplanar distance of 3.345 to 3.8 Å. Exhibits a fibrous shape with a diameter of 3.5 to 500 nm and a length of at least 10 times the diameter, and a unit structure composed of at least two or more carbon hexagonal network planes is separated by at least 10 nm with respect to an adjacent unit structure. It is to provide a fibrous carbon, characterized in that the specific surface area is 200 to 1,000 m 2 / g.

Recently, nano-sized carbon materials represented by fullerene, carbon nanotubes, carbon nanofibers, etc. have attracted much attention. Among them, carbon nanofibers are allotropic materials in which carbon hexagonal meshes, in which carbon atoms of sp ² atomic structure are firmly bonded, are continuously arranged with a diameter of 2 to 100 nm and a length of 5 to 100 μm in a predetermined direction with respect to the fiber axis. to be.

Such carbon nanofibers can be prepared by generally decomposing a gaseous compound containing carbon at a high temperature and growing it in a fiber form on a metal catalyst. ) Carbon nanofibers are obtained.

Carbon nanofibers can be classified into four types of structures from a microscopic point of view. A herringbone structure in which carbon hexagonal net faces are arranged at an angle with the fiber axis, and a flat plate in which carbon hexagonal net faces are arranged perpendicular to the fiber axis. There is a platelet structure, a spiral structure in which the carbon hexagonal meshes are spirally wound about the fiber axis, and a ribbon-like arrangement in which the carbon hexagonal network surfaces are arranged perpendicular to the fiber axis. These microstructures are determined by various factors such as catalyst type, composition and preparation conditions, synthesis temperature, reaction gas type and amount pressure.

Each carbon hexagonal network constituting the carbon nanofibers is arranged at a distance of approximately 3.345 to 3.8 과 from the adjacent carbon hexagonal network. The total specific surface area of the carbon nanofibers based on such a structure is generally 20 to 200 m 2 / g. Is in range. Therefore, carbon nanofibers are used in various fields using high conductivity, specific surface area, strength, and the like, and their application fields are also gradually being widened. One such example is an electric double layer capacitor.

An electric double layer capacitor is a device for storing electric charge by charging ions on an electrolyte and electrons on an electrode in an electric double layer formed at an interface between an electrode and an electrolyte. Since the electric double layer capacitor has a faster charge / discharge rate than the secondary battery, it is advantageous in high current density discharge conditions and has the advantage of high cycle characteristics.

An electric double layer capacitor is composed of an equivalent circuit in which double-layer capacitance and equivalent series resistance (ESR) are connected in series. In this case, the double layer capacitance is proportional to the surface area of the electrode, and ESR is the sum of the resistance of the electrode, the resistance of the electrolyte solution, and the resistance of the electrolyte in the electrode pores. The amount of charge that can be stored in the double layer capacitor decreases as the charge and discharge rate increases, which is determined by the size of the ESR. That is, the larger the ESR, the lower the charge storage capacity, and this phenomenon becomes worse as the charge / discharge rate increases.

Therefore, the electrode material of the electric double layer capacitor must satisfy the following conditions: 1) the large surface area of the double layer capacitance is large due to the large specific surface area, 2) the resistance of the electrode must be small due to the high electrical conductivity of the electrode material, and 3) the electrode The resistance of the electrolyte in the pores should be small.

Until now, powdered activated carbon, activated carbon fiber, and the like have been used as electrode materials for electric double layer capacitors. However, these materials have a problem of having low capacitance expressed compared to specific surface area, low density, high resistance, etc. when the high specific surface area is expressed, resulting in low electrode density, and high IR resistance during capacitance expression.

On the other hand, carbon nanofibers having excellent electrical conductivity, strength and the like are considered as alternative materials such as activated carbon and activated carbon fibers. However, since the conventional carbon nanofibers have a small specific surface area of 20 to 200 m 2 / g as described above, the double layer capacitance is small to 2 to 8 F / g in the case of the organic bipolar test. Therefore, there is a higher demand for a carbon material having a high specific surface area and conductivity, and thus high capacitance.

In addition, not only an electric double layer capacitor, but also various fields that require a high specific surface area and a conductive carbon material exist. Representative examples thereof include an electrode for a secondary battery, a catalyst carrier, a hydrogen storage medium, an electromagnetic wave shielding agent, an antistatic conductive coating agent, an adsorbent, a filter, and the like.

Therefore, an object of the present invention is to solve the problems of the prior art and the technical problems that have been requested from the past.

That is, an object of the present invention is to provide a carbon material having a novel structure that can be used in various applications requiring high conductivity and high specific surface area. In particular, it is to provide a novel structure of fibrous carbon having a high specific surface area and a high double layer capacitance which can be used as an electrode material for an electric double layer capacitor.

Another object of the present invention is to provide products using such carbon materials.

After repeated studies and numerous experiments, the present inventors have been able to produce fibrous carbon in which the unit structures of carbon hexagonal mesh planes are separated by a predetermined distance, and the present invention has been confirmed by confirming their characteristics. This is a new structure of carbon material that has not been reported at all.

1 is a graph showing changes in activity and specific surface area according to reaction temperature of carbon nanofibers (CNF-1) according to Example 1;

2 is a graph showing a change in specific surface area according to reaction time of carbon nanofibers (CNF-1) according to Example 1;

Figure 3 is a graph showing the change in XRD stack structure parameters with the reaction time of carbon nanofibers (CNF-1) according to Example 1;

4A and 4B are scanning electron microscope (SEM) photographs before and after activation of carbon nanofibers (CNF-3) according to Example 3;

5A and 5B are transmission electron microscope (TEM) photographs (× 150,000, × 600,000) before activation of carbon nanofibers (CNF-1) according to Example 1;

6A and 6B are transmission electron microscope (TEM) photographs (× 150,000, × 600,000) after activation of carbon nanofibers (CNF-1) according to Example 1;

7 is a graph showing the change of the double layer capacitance according to the specific surface area of the fibrous carbon and the pitch-based activated carbon fiber, coke and the like according to an embodiment of the present invention.

The fibrous carbon of the novel structure according to the present invention is characterized in that the unit structure of the carbon hexagonal network faces is at least 10 nm wide and has a specific surface area of 200 to 1000 m 2 / g.

The "carbon hexagonal network" is a carbon structure sheet (graphene sheet) in which carbon atoms of sp ² atomic structure are firmly bonded in two dimensions. It has a diameter and a length of 10 times or more, and forms a fibrous shape as a whole.

The term "unit structure" of the carbon hexagonal network means a structure consisting of at least two carbon hexagonal network surfaces, in which these carbon hexagonal network surfaces are regularly arranged at intervals of 3.345 to 3.8 mm3 from each other. Preferably, the number of carbon hexagonal network faces constituting the unit structure is 30 to 120.

As the unit structures are spread to each other at least 10 nm or more, the specific surface area of the fibrous carbon increases, and the specific surface area varies depending on the number and distance of the carbon hexagonal networks forming the unit structure. The stretched distance of the unit structure is not particularly limited, but is preferably 10 to 40 nm. If the gap distance of the unit structure is 10 nm or less, it may not have a specific surface area within the above set range. On the contrary, if the gap distance is too large, the structural stability is poor, and it is difficult to exert the shape or physical properties required for the carbon material. do. In some cases, there may be a carbon material which has a high specific surface area but does not satisfy the above conditions, i.e., the unit structure of the carbon hexagonal nets is extended by a certain distance, but this is because the arrangement of the carbon hexagonal nets is partially collapsed. And / or as a result of a high specific surface area due to rearrangement, ultimately one of the properties required for fibrous carbon cannot maintain high strength.

The carbon hexagonal network surfaces may be a structure arranged at right angles to the fiber axis, or may be a structure arranged at an angle with respect to the fiber axis. This variety can be determined according to the properties of the materials used when producing the fibrous carbon of the present invention or may vary depending on the reaction conditions and the like.

The specific surface area of the fibrous carbon of the present invention 200 ~ 1000 m 2 / g is a value of about 2 to 6 times larger than the specific surface area of conventional carbon nanofibers. Therefore, when the fibrous carbon according to the present invention is used as an electrode material of, for example, an electric double layer capacitor, a high double layer capacitance is exhibited due to the high specific surface area.

The method for producing fibrous carbon of the novel structure according to the present invention is as follows.

That is, the fibrous carbon of the novel structure according to the present invention is a process of treating a mixture of carbon nanofibers and potassium hydroxide (KOH) in a weight ratio of 1: 1 to 1: 6 at 400 to 1,000 ° C. under an inert atmosphere for 0.5 to 5 hours. It is prepared by a method comprising a.

The carbon nanofibers as raw materials are not particularly limited as long as they are carbon nanofibers having a diameter of 3.5 to 500 nm, which are generally manufactured by various known methods. As described above, a herringbone structure and a platelet are used. Carbon nanofibers having a microscopic structure such as a structure, a spiral structure, and the like can all be used. However, the carbon nanofibers are not limited thereto, and may include all of the nanofibers, composites, aggregates, mixtures, and the like, of microscopic structures that are not yet disclosed.

Although the reactor before the potassium hydroxide spreads the unit structure is not clearly known, when the mixing weight ratio with carbon nanofibers is less than 1: 1, the phenomenon of unit structure is hardly observed, on the contrary, when the mixing weight ratio is 1: 6 or more. It is not economical because it uses a large amount of potassium hydroxide. More preferred mixing weight ratio is 1: 3 to 1: 5. In some cases, other compounds or mixtures may be added to the mixture to promote an active reaction or increase the reaction rate, and the addition thereof is included in the scope of the present invention so long as the composition and effects of the present invention are not impaired. Should be interpreted as

The mixture is treated in an inert atmosphere, for example, an atmosphere such as Ar, N ₂ or the like may be used. Since the reaction is in a liquid phase, the pressure does not particularly affect, and is preferably performed at normal pressure.

On the other hand, if the reaction temperature is 400 ° C. or less, it is difficult to achieve a desired effect, and if it is 1,000 ° C. or more, an apparatus for forming a high temperature is not economical. More preferable reaction temperature is 650-900 degreeC.

When the reaction time is 0.5 hours or less, it is not preferable because it is difficult to achieve the desired effect, and when it is 5 hours or more, it is not economical. More preferred reaction time is 0.8 to 2 hours.

These factors for the reaction of spreading the unit structure of carbon hexagonal networks (hereinafter, also referred to as 'activation reaction') have a mutual correlation. For example, when a large amount of potassium hydroxide is used, the reaction time can be shortened, or when the reaction temperature is high, the reaction time can be shortened relatively. Thus, optimum conditions for imparting a higher specific surface area while maintaining the shape and physical properties of the fibrous carbon while the bulging distance of the unit structure is at least 10 nm or more can be achieved through an appropriate combination of these reaction factors.

In some cases, acid treatment and / or distilled water washing may be further performed to completely remove potassium hydroxide remaining after the activation reaction.

However, the present invention is not limited to the above method, and may be any method for producing fibrous carbon having a unit surface structure of carbon hexagonal network surfaces of at least 10 nm or more, and thus having a specific surface area of 200 to 1,000 m 2 / g. However, these methods should be construed as being included in the scope of the present invention.

The present invention also relates to a product of each application field using the fibrous carbon, and examples thereof include, as described above, an electrode for an electric double layer capacitor, an electrode for a secondary battery, a catalyst carrier, a hydrogen storage medium, an electromagnetic shielding agent, and an antistatic agent. Conductive coating agents, adsorbents, filters, and the like. Especially, when the fibrous carbon which concerns on this invention is used as an electrode material for electric double layer capacitors, it shows very high double layer capacitance compared with the conventional thing. Since the manufacturing method of these products using a carbon material is already known, the description is abbreviate | omitted in this specification.

Hereinafter, the content of the present invention will be described in more detail with reference to Examples according to the present invention, but the scope of the present invention is not limited thereto.

EXAMPLE

One. Compounds Used in Experiments

Metal catalysts are required for the production of carbon nanofibers used in the experiments, and materials for preparing such catalysts are copper nitrate (Cu (NO ₃ ) ₃ · H ₂ O) and nickel nitrate (Ni (NO ₃ ) ₃ · H ₂ O), ammonium bicarbonate (NH ₄ HCO ₃ ) and potassium hydroxide were those purchased from Wako Inc. (Japan). Hydrogen gas (> 99.9999%), ethylene gas (99.9%) and helium gas (> 99.9999%) were purchased from Asahi Sanso Inc. (Japan) and used without purification.

2. Preparation of Carbon Nanofibers Used in Experiments

Ammonium bicarbonate, as described in the Best and Russell paper (Best RJ, Russell WW. Nickel, copper and some of their alloys as catalysts for ethylene hydrogenation. J. Am. Chem. Soc. 1954; 76: 838-842). Copper and nickel carbonate were precipitated from the nitrate solution using to prepare a copper / nickel alloy catalyst for use in the production of carbon nanofibers.

For the production of carbon nanofibers, a modified flow reactor heated in a Lindberg horizontal tubular furnace was used. The MKS flow controller was used to precisely identify and control the flow of gas into the reactor, allowing for constant delivery of the composition. 30-50 mg of powdered catalyst specimens were placed in an aluminum boat located in the center of the reactor in the reactor, reduced to a suitable temperature for 2 hours in a 10% H ₂ / He mixture, and the reaction system was purged with helium gas for 30 minutes. . Then, the reaction gas C ₂ H ₄ / H ₂ mixture was flowed over the catalyst for a predetermined time. After the reaction system was cooled to room temperature, the amount of carbon deposited for a predetermined time was measured.

3. Preparation of Fibrous Carbon of New Structure

Using a stainless steel flow reactor heated by a conventional upright tube furnace, a mixture of carbon nanofibers and KOH in a weight ratio of 1: 4 (1 g carbon nanofibers / 4 g KOH) was placed in a stainless steel pot. , And heated to a set temperature for a predetermined time in an argon atmosphere. Setting of time and temperature was performed in various changes as demonstrated later.

4. Analysis method

Data on crystallinity were collected using an X-ray rotation analyzer (Rigaku, CuKα target), and the parameter on crystallinity (d ₀₀₂ ) was calculated according to the Gakushin (JSPS) method.

After degassing at 150 ° C. for 8 hours on the sample, the BET surface area was measured by adsorption and desorption of nitrogen using a surface area analyzer (Sorptomatic 1990 ^™ , FISONS Instruments). In addition, the scanning electron microscope (JSM-6320F ^TM, JEOL) and high resolution transmission electron microscope using (JEM-100CX ^TM, JEOL) were ensure nanostructure and morphology of the carbon nanofiber, and Raman spectroscopy (NRS-2000B ^TM, JASCO) was used to measure the change in carbonization. 514.5 nm Ar ion laser light was used.

5. Measurement of Double Layer Capacitance

Capacitance was measured in a coin-type stainless steel cell. The test cell consisted of an electrolyte in which a pair of electrodes and tetraethylammonium tetrafluoroborate (Et ₄ NBF ₄ ) were dissolved in 1 mol of propylene carbonate solvent.

6. Effect of reaction temperature and time on specific surface area

Three kinds of carbon nanofibers as shown in Table 1 were synthesized, and KOH activity was performed. The carbon nanofibers (CNF-1) of Example 1 were reacted for 1 hour at intervals of 100 ° C. in a temperature range of 500 to 1,000 ° C. As a result, as can be seen in Figure 1, it was confirmed that there is an active effect at 600 ℃ or more. After 1 hour of reaction at 900 ° C, the specific surface area was greatly improved from 107 m 2 / g to 429 m 2 / g. As shown in FIG. 2, the reaction time was optimal when about 3 hours, and the specific surface area was found to decrease slightly over 4 hours.

Example d ₀₀₂ (nm) Average diameter (nm) Specific surface area (㎡ / g) Before activation After activation 1 (CNF-1) 0.3412 150 105 511 2 (CNF-2) 0.3410 110 110 461 3 (CNF-3) 0.3506 40 229 1765

7. Effect of KOH Activation on Carbon Nanofibers

Table 1 shows the results of reacting three types of carbon nanofibers at 900 ° C. for 3 hours, and it can be seen that the specific surface area of the carbon nanofibers increased by approximately five times under the above conditions, which is a new structure of carbon. It means that the material has been created.

8. Structure of surface change

As a result of calculating the structural parameters from the XRD on the generated fibrous carbon, it was confirmed that the crystallinity of the carbon nanofibers decreased as KOH activation proceeded (see FIG. 3). Raman results (not shown) also showed a similar trend, meaning that carbon nanofibers lost graphite properties, such extreme examples being shown in Figures 4a (before activation) and 4b (after activation). TEM images before activation (FIGS. 5A and 5B) and TEM images after activation (FIGS. 6A and 6B) for 3 hours at 900 ° C., while the herringbone microstructure was maintained after KOH activation, the carbon hexagonal meshes were spaced as regular units This opens up, showing that the graphite properties are reduced as a result. In particular, in Figures 6a and 6b it can be seen that the unit structure of the 30 to 120 hexagonal network surface is open, this structure is a completely new structure has not been reported earlier.

9. Double layer capacitance of fibrous carbon

Experimental results for the double layer capacitance is shown in Figure 7, the capacitance of the carbon nanofibers having a specific surface area of ~ 100 ㎡ / g was ~ 6 F / g, after activation of KOH, the capacitance of the activated carbon nanofibers As the specific surface area increased to ~ 500 m 2 / g, it increased to ~ 21 F / g. Compared with electrode materials such as activated carbon fibers or mesophase pitch-based carbon fibers, it can be seen that the fibrous carbon according to the present invention exhibits high capacitance even with a relatively small specific surface area.

Those skilled in the art to which the present invention pertains may make various applications and modifications within the scope of the present invention based on the above contents.

The fibrous carbon according to the present invention is a carbon material having a completely new structure, which has not been reported so far, and has a specific surface area of about 2 to 6 times wider than that of conventional carbon nanofibers, thereby showing a high double layer capacitance. Therefore, it can be used in various applications requiring a high specific surface area and conductive carbon material, and can be particularly useful as an electrode material for electric double layer capacitors.

Claims (6)

Carbon hexagonal meshes formed by spherically bonding two-dimensional carbon atoms of sp ² atoms are formed into a fibrous structure with a diameter of 3.5 to 500 ㎚ and a length of more than 10 times the diameter by regular arrangement with an interplanar distance of 3.345 to 3.8 Å. A fibrous carbon exhibiting a shape, wherein a unit structure composed of at least two carbon hexagonal network surfaces is at least 10 nm wider than an adjacent unit structure, whereby the specific surface area is 200 to 1,000 m 2 / g. The fibrous carbon according to claim 1, wherein the number of carbon hexagonal mesh planes forming the unit structure is 30 to 120. The fibrous carbon according to claim 1, wherein the gap of the unit structure is 10 to 40 nm. The fibrous carbon according to claim 1, wherein the carbon hexagonal net faces are arranged at right angles to the fiber axis, or are arranged at an angle with respect to the fiber axis. The fibrous carbon according to claim 1, wherein the microstructure of the fibrous carbon is a herringbone structure, a platelet structure, a spiral structure, or a ribbon-like structure. An electric double layer capacitor using the fibrous carbon according to any one of claims 1 to 6 as an electrode material.