JP4147294B2 - Method for producing activated carbon for electric double layer capacitor - Google Patents

Description

[0001]
【Technical field】
The present invention relates to a method for producing activated carbon for an electric double layer capacitor, and more specifically, a capacitor having a high capacitance by adjusting the volatile matter and mesophase amount of a coal-based pitch which is a raw material, and then performing an activation treatment. The present invention relates to a production method capable of obtaining activated carbon.
[0002]
[Prior art]
In recent years, electric double layer capacitors applying the principle of electric double layers have attracted attention as energy storage devices.
[0003]
For example, positive and negative charges are arranged and distributed through a very short distance at an interface where two different phases contact, such as a solid electrode and an electrolyte solution.
When the electrode is positively charged, anions are arranged on the solution side, and a layer generated by this charge arrangement is an electric double layer.
A capacity that appears at the electrode interface as the electric double layer is generated is called an electric double layer capacity, and an electric double layer capacitor stores energy using this principle.
[0004]
The electric double layer capacitor is charged / discharged by ion adsorption / desorption, which is a physical mechanism, without a chemical reaction. Therefore, the deterioration of characteristics in repeated use is extremely small, and the operating temperature range is wide. In addition, it has many features such as excellent high-speed charge / discharge characteristics, and is widely used as a backup power source for small electronic devices such as computers.
In the future, it is also expected as a combined power source for electric vehicles.
[0005]
An important characteristic of the electric double layer capacitor is that the capacitance is high, and this capacitance is expressed by the following equation.
Capacitance C = ∫ε / (4πδ) · dS
Where ε is the dielectric constant of the electrolyte, δ is the distance from the electrode interface to the ion center, and S is the surface area of the electrode interface.
[0006]
Therefore, in view of the polarizable electrode used in the capacitor, the capacitance of the capacitor increases when an electrode having a large surface area is used.
For this reason, activated carbon powder and activated carbon fiber having a large specific surface area and conductivity are mainly used as materials for electric double layer capacitors.
[0007]
Currently used activated carbon powder is produced by activating coal, coconut shell, phenol resin, etc. in an oxidizing gas (water vapor, carbon dioxide, air, etc.) atmosphere, all of which have a specific surface area. Although it is a large material, since it is optically isotropic, it has insufficient conductivity.
That is, when the conductivity is low, the internal resistance of the capacitor increases, resulting in a decrease in positive capacitance.
[0008]
Further, according to Randin, J and Yeagea, E (J. Electroan. Chem., 36, 257 (1972)), when the activated carbon surface is formed by the edge surface of the graphite layer surface, the capacitance is greatly improved.
However, since the conventional activated carbon is an optically isotropic material, the arrangement of the edge surface and the basal surface of the graphite layer surface cannot be controlled, and there is a drawback that improvement in this surface cannot be performed. It was.
[0009]
Because of these problems, a method for producing activated carbon having a large specific surface area and high conductivity is desired.
[0010]
[Problems of the Invention]
In view of the above situation, the present inventor provides a method capable of producing activated carbon for electric double layer capacitors having a large specific surface area, sufficient conductivity, and high capacitance.
[0011]
[Means for solving problems]
In order to solve the above problems, the present inventor has intensively studied. As a result, it is preferable to use a mesophase pitch which is an optically anisotropic material. The present invention was completed by focusing on the fact that an activated carbon for a capacitor having excellent characteristics can be obtained by adjusting the amount of volatile matter and mesophase by heat treatment after adjusting to a specific amount.
[0012]
That is, the present inventor proposes that a coal-based pitch having a metaphase amount of 3 to 20% is heat-treated to adjust the volatile content to 5% or less and the metaphase amount to 80% or more.
A method for producing activated carbon for an electric double layer capacitor, characterized by pulverizing and activating treatment at 600 to 1000 ° C.
[0013]
The present invention is described in detail below.
[0014]
First, coal-based pitch is used as a starting material.
A feature of the present invention is that the amount of metaphase in the coal-based pitch is adjusted to 3 to 20%.
If it is less than 3%, the generation of pores is suppressed, and activated carbon having a large specific surface area cannot be obtained. If it exceeds 20%, the amount of mesophase pitch after heat treatment to be described later cannot be increased to 80% or more. Is also not preferred.
[0015]
The amount of metaphase is adjusted by adding metaphase separated from the pitch.
Although the metaphase separation method is not limited, it can be easily separated by centrifugation from the tar that is the raw material of the pitch, or by extraction with a pitch solvent (for example, petroleum-based absorbent oil, quinoline, etc.). .
[0016]
This adjustment of the metaphase amount is important in order to reduce the crystallinity of the mesophase pitch, to obtain activated carbon having a large specific surface area and high capacitance.
[0017]
That is, Metafe produced by heat treatment of pitch - figure homogeneous and excellent crystallinity, and molecular orientation in one direction at Netsusho sense. As a result, when the obtained mesophase is pulverized and activated in an oxidizing gas atmosphere, the oxidation proceeds planarly, pores are hardly generated, and activated carbon having a large specific surface area cannot be obtained.
[0018]
In order to solve this problem, the crystallinity of the mesophase pitch is lowered by adjusting the metaphase amount of the pitch before the heat treatment.
Even with this treatment, activation is less likely to proceed than optically isotropic materials, but this eventually promotes the generation of mesopores (2-50 nm) with the optimum pore size for the activated carbon for capacitors, resulting in high electrostatic capacity. Capacitance capacitors can be obtained.
[0019]
After adjusting the metaphase amount as described above, this pitch is heat-treated at 450 to 600 ° C. for 5 to 60 hours in a nitrogen or inert gas atmosphere.
By this heat treatment, the amount of mesophase produced for the obtained heat-treated product is adjusted to be 80% or more and the volatile content is 5% or less.
[0020]
When the amount of mesophase is less than 80%, the capacitance of the final product decreases. When the amount of volatile components exceeds 5%, fusion of mesophase pitch fine powder occurs in the activation of the post-process, and the characteristics of the final product. Is not preferable because of the lowering of.
[0021]
Next, the heat-treated product is pulverized into a fine powder.
The particle size is not particularly limited, but it is usually appropriate that the average particle size is 50 μm or less.
[0022]
After pulverization, activation is performed at 600 to 1000 ° C. in an oxidizing gas atmosphere to obtain activated carbon for capacitors according to the present invention.
As the oxidizing gas, water vapor, carbon dioxide, air or the like can be used, but water vapor activation is preferable in view of control of activation reaction and cost.
Although activation using an alkali metal compound such as potassium hydroxide or potassium chloride is possible, it is not preferable because much labor is required for separation and removal of the alkali metal after activation.
[0023]
Activated carbon obtained in the present invention, the specific surface area is 200m ² / g~2000m ² / g, and preferably has no difference between the nitrogen gas adsorption (BET) adsorption and desorption isotherm obtained by methods.
When the specific surface area is less than 200 m ² / g, the capacitance of the final product is lowered, and when it exceeds 2000 m ² / g, the density is lowered and the strength is lowered.
Further, if there is a difference between the adsorption and desorption isotherms of the nitrogen gas adsorption (BET) method, the amount of irreversible adsorption of the electrolyte increases, and as a result, the capacitance of the final product decreases, which is not preferable.
[0024]
The activated carbon for electric double layer capacitors of the present invention can be obtained by the manufacturing method as described above.
[0025]
When the activated carbon obtained as described above is used for a polarizable electrode of an electric double layer capacitor, usable binders are polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR). Etc. can be used.
As the electrolytic solution, aqueous sulfuric acid and hydrochloric acid, non-aqueous ethylene carbonate, propylene carbonate, γ-butyllactone, acetonitrile, dimethylformamide and the like can be used.
In addition, tetraalkylammonium salts having excellent stability such as (CH ₃ ) ₄ NBF ₄ , (C ₂ H ₅ ) ₄ NBF ₄ , (C ₃ H ₇ ) ₄ NBF ₄ , (CH ₃ ) ₄ NPF ₆ are used as the electrolyte. Etc. can be used.
[0026]
【The invention's effect】
The activated carbon obtained by the method of the present invention has a large specific surface area and sufficient conductivity,
When used for an electric double layer capacitor, a capacitor having a high electrostatic capacity can be obtained.
[0027]
Examples and Comparative Examples
[Example 1]
Commercially available coal pitch with a metaphase content of 5.3% in a nitrogen gas atmosphere,
Heat treatment was performed at 500 ° C. for 24 hours to obtain a mesophase pitch.
The heat-treated pitch had a volatile content of 4.5% and a mesophase amount of 95% as measured with a polarizing microscope.
After the pitch was pulverized to an average particle size of 25 μm, activated carbon was obtained by activation at 950 ° C. for 2 hours in a steam atmosphere using a small rotary kiln.
The obtained activated carbon had a specific surface area of 450 m ² / g, and there was no difference between the adsorption and desorption isotherms obtained by the nitrogen gas adsorption method.
[0028]
To 100 parts by weight of the activated carbon, 10 parts by weight of a conductive additive and 10 parts by weight of polytetrafluoroethylene are added, mixed at room temperature, then rolled and pressed onto an aluminum mesh,
Further, an electrode sheet was prepared by punching to a diameter of 10 mm.
[0029]
Next, in a glove box in an argon gas atmosphere, a porous polypropylene sheet was sandwiched between a pair of electrode sheets and fixed to assemble the cell.
To this was added propylene carbonate (concentration 1M) in which tetraethylammonium tetrafluoroborate was dissolved as an electrolytic solution, and the capacitance was measured at room temperature. As a result, it was 15.6 F / g.
[0030]
[Example 2]
Ten times the amount of coal-based absorbent oil was added to the coal-based pitch of Example 1, heated to 100 ° C. to dissolve the pitch, and then filtered to separate and extract the metaphase.
This metaphase was added so that the amount of metaphase of the coal-based pitch was 10%. Thereafter, heat treatment, pulverization and activation were performed in the same manner as in Example 1 to obtain activated carbon.
The mesophase pitch had a volatile content of 3.8% and the mesophase amount measured by an optical microscope was 98%.
The activated carbon surface area after activation was 650 m ² / g, and there was no difference between the absorption and desorption isotherms obtained by the nitrogen gas adsorption method.
Next, an electrode was produced using this activated carbon in the same manner as in Example 1, and the capacitance was measured. As a result, it was 18.3 F / g.
[0031]
[Comparative Example 1]
Except that the amount of metaphase of the coal-based pitch in Example 1 was set to 30%, heat treatment, pulverization, and activation were all performed in the same manner as Example 1 to obtain activated carbon.
The mesophase pitch had a volatile content of 3.8% and the mesophase amount measured with an optical microscope was 30%.
The specific surface area of the activated carbon after activation was 780 m ² / g, and there was no difference between the absorption and desorption isotherms obtained by the nitrogen gas adsorption method.
Next, an electrode was produced using this activated carbon in the same manner as in Example 1, and the capacitance was measured. As a result, it was 9.5 F / g.
[0032]
[Comparative Example 2]
Using the coal-based pitch in Example 1, heat treatment, pulverization, and activation were carried out in the same manner as in Example 1 except that the heat treatment was performed at 450 ° C. for 24 hours to obtain activated carbon.
The mesophase pitch had a volatile content of 6.5% and the mesophase amount measured by an optical microscope was 55%.
However, the activated carbon has particles adhering to each other and is not in a state where it can be used for a capacitor.
[0033]
[Comparative Example 3]
Using the coal-based pitch in Example 1, heat treatment, pulverization, and activation were performed in the same manner as in Example 1 except that the activation condition was set at 800 ° C. for 3 hours to obtain activated carbon.
The obtained activated carbon had a specific surface area of 110 m ² / g, and a difference was observed between the adsorption and desorption isotherms obtained by the nitrogen gas adsorption method.
Next, using this activated carbon, an electrode was produced in the same manner as in Example 1, and the capacitance was measured. The result was 6.2 F / g.

Claims (3)

After the coal-based pitch having a metaphase amount of 5.3 to 10% is heat-treated at 450 to 600 ° C. in a nitrogen or inert gas atmosphere, the volatile content is adjusted to 5% or less and the mesophase amount is set to 80% or more. A method for producing an activated carbon for an electric double layer capacitor, comprising pulverizing and subjecting the finely pulverized product to activation treatment at 600 to 1000 ° C. in a water vapor atmosphere. In claim 1, the specific surface area of the resulting activated carbon characterized in that there is no difference between 200m in ² / ^{g~2000m 2} / g, and a nitrogen gas adsorption (BET) adsorption and desorption isotherm obtained by method Manufacturing method of activated carbon for electric double layer capacitor. The electric double layer capacitor using the activated carbon obtained by the manufacturing method of Claim 1 or Claim 2.