JP2001302225A - Method for producing porous carbon material, porous carbon material and electric double layer capacitor produced by using the material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a porous carbon material having high specific surface area in high yield at a low cost by an oxidizing gas activation method which can be easily incorporated in a process. SOLUTION: This porous carbon material is produced by heating a soft carbon material in the presence of oxygen at a temperature lower than the activation temperature and activating the obtained pretreated product with an oxidizing gas. The pretreatment is preferably carried out at 200-500 deg.C. A porous carbon material having a specific surface area of >=1,000 m2/g and usable as an electrode material for an electric double layer capacitor having high electrical capacitance can be produced by the process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a porous carbon material by an oxidizing gas activation method, a porous carbon material having a large specific surface area and a low cost obtained by the method, and an electric double layer containing the same. Related to capacitors.

[0002]

2. Description of the Related Art Generally, at an interface where two different phases are in contact with each other, positive and negative charges are arranged and distributed opposite to each other at an extremely short distance. The charge distribution layer is called an electric double layer. When a DC voltage is applied to a pair of polarizable electrodes immersed in the electrolyte, anions in the electrolyte are electrostatically applied to the positively polarized electrode, and cations in the electrolyte are applied to the negatively polarized electrode. To form a space charge layer called an electric double layer at the interface between the electrode and the electrolyte. The electric double layer capacitor is an element configured to use electric energy of the electric charge stored in the electric double layer.

[0003] Such electric double layer capacitors have excellent instantaneous charge / discharge characteristics and a characteristic that the performance is less likely to decrease due to repetition. Therefore, such electric double layer capacitors are used as backup power supplies for electronic devices such as microcomputers equipped with IC memories. It is being widely used in An electric double layer capacitor having excellent instantaneous charge / discharge characteristics is also useful as a power supply for electric vehicles.

In an electric double layer capacitor composed of a pair of polarizable electrodes and an electrolyte as described above, the capacitance C stored in the electric double layer formed at the interface between the polarizable electrode and the electrolyte is expressed by the following equation (1). ). C = ∫ [ε / (4πδ)] dS (1) (where ε: the dielectric constant of the electrolytic solution, δ: the distance from the electrode surface to the ion center, and S: the surface area of the electrode interface)

Therefore, when a polarizable electrode material having a large specific surface area is used, an electric double layer capacitor having a high capacitance can be obtained. Activated carbons (porous carbon materials) are conventionally known as polarizable electrode materials having a large specific surface area. Activated carbon is obtained by activating (porosity-forming) raw materials such as organic wastes such as resins, pulp production residues, coal, coal coke, wood, and coconut shells. As an activation method, a method using an oxidizing gas such as steam, air, oxygen, CO ₂ , or the like, or a method of activating a chemical using an alkali chemical is generally used. A large specific surface area can be easily obtained by activating with an alkali chemical. However, the industrial mainstream is an oxidizing gas activating method in view of the problem of equipment corrosion, chemical cost, and need for cleaning.

[0006] The oxidizing gas activation method can be carried out in a simple process without using any special chemicals, has an advantage in cost, and also in terms of performance, the porous carbon material obtained by the method is polarized. There is an advantage that a change with time when used as an electrode material is small.

However, when the mesocarbon-based activation raw material is activated with an oxidizing gas without using a chemical such as an alkali, it is difficult to obtain an essentially large specific surface area, and an increase in the specific surface area leads to a decrease in yield. It is also limited in terms of productivity. In particular, when mesocarbon microbeads (mesophase microspheres) are used among the mesocarbon-based activation raw materials, a porous carbon material having a uniform particle size can be obtained, and a polarizable electrode material having a small electric resistance and a high packing density can be obtained. However, the mesophase microspheres are harder to activate compared to fibrous ones. Mesophase microspheres have a smooth surface and a specific surface area of 1000
It is difficult to activate up to m ² / g. For example, as shown in JP-A-1-230414, the specific surface area is about 640 m ² / g in the ordinary steam activation of mesophase spherules. For this reason, it is desired to develop a method for producing a porous carbon material having a large specific surface area, a high packing density, and a large capacitance by using an oxidizing gas activation method without using a chemical, with high productivity. Was.

[0008]

SUMMARY OF THE INVENTION In view of the above prior art, the present invention provides a method for producing a porous carbon material having a large specific surface area and a high packing density by an oxidizing gas activation method with high productivity. It is an object of the present invention to provide a porous carbon material having a large capacitance and an electric double layer capacitor including the same.

[0009]

Means for Solving the Problems The present inventor examined the activation of the oxidizing gas of the soft carbon-based carbon material, and prior to the activation treatment, added a heating treatment in the presence of oxygen at a temperature lower than the activation temperature. As a result, the inventors have found that a porous carbon material having a large specific surface area can be obtained with a high yield, and have completed the present invention.

That is, the present invention relates to a method for producing a porous carbon material in which a soft carbon-based carbon material is subjected to a pretreatment of heating at a temperature lower than an activation temperature in the presence of oxygen and then activated with an oxidizing gas. is there. As the soft carbon-based carbon material, mesophase small spheres and / or bulk mesophase can be used. The above pre-processing is 20
It is preferable to carry out at a temperature of 0 to 500 ° C.

In the present invention, the porous carbon material is obtained by the above-described method for producing a porous carbon material, and has a specific surface area of 1000 m ² / g.
The porous carbon material described above is also provided. Also provided is an electric double layer capacitor using this porous carbon material as a polarizable electrode material.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below more specifically. In the present invention, a porous carbon material is manufactured by performing a pretreatment of heating a soft carbon-based carbon material in the presence of oxygen at a temperature lower than the activation temperature and then activating with an oxidizing gas. As the soft carbon-based carbon material, mesophase small spheres and / or bulk mesophase can be used. Known mesophase spheres and bulk mesophases can be widely used and are not particularly limited, but those having a particle size in a range of 5 to 100 μm are preferable, and those having a particle size of 5 to 40 μm are more preferable.

As a raw material for producing mesophase small spheres and bulk mesophase, pitches such as coal-based tar or pitch, petroleum heavy oil or pitch are generally used. If these pitches are subjected to a heat treatment at 350 to 500 ° C. for 0.5 hours to 10 hours, preferably in a nitrogen stream, first, mesophase small spheres are formed in the pitch as the heating proceeds, and then the mesophase small spheres are formed. The spheres aggregate to form a bulk mesophase. Therefore, the generation of the mesophase microspheres and / or the bulk mesophase may be performed by appropriately selecting the heat treatment conditions as desired. From the heated pitch, mesophase microspheres and / or bulk mesophase can be obtained by a solvent separation operation using, for example, quinoline.

The details of the mesophase microspheres and bulk mesophase used as the starting material in the present invention are described in, for example, JP-A-1-230414 and JP-A-8-85794.
And Japanese Patent No. 2950781. These descriptions can also be cited in the present specification. In the present invention, these commercially available products may be used. Either the mesophase sphere or the bulk mesophase may be used, or a mixture thereof may be used. Such a soft carbon-based carbon material is a graphite precursor, which is preferably graphitized in the activation process and easily formed into a graphite structure, so that a fine structure having a small internal resistance can be formed after activation. That is, it is preferable to use a soft carbon-based carbon material because a porous carbon material having a lower electric resistance can be obtained as compared with a case where a hard carbon-based material is used as a raw material.

In the present invention, prior to the activation of the soft carbon-based carbon material as described above, a pre-heating treatment is performed at a temperature lower than the activation temperature in the presence of oxygen. Specifically, in an atmosphere in which an oxygen-containing gas such as oxygen, air, or oxygen-enriched air is present, the temperature is raised to a pretreatment temperature of 200 ° C. or higher and lower than the activation temperature. Other gases such as chlorine gas may coexist as long as the effects of the present invention are not impaired. If the pretreatment temperature is high, the yield in the pretreatment step is reduced. In particular, when the temperature exceeds 500 ° C., the yield is rapidly lowered to an unsuitable level although it varies depending on the oxygen concentration and the like. It is desirable to be about 500 ° C. In addition, 90 degrees by raising the temperature to 400 ° C.
When the temperature was raised to 500 ° C. under the condition that a yield of about% could be secured, the yield was reduced to 5% or less.

The heating rate and the holding time at the pretreatment temperature are not particularly limited, but it is desirable to perform the heating at a heating rate of about 1 to 20 ° C./min. After the temperature is raised, cooling may be started without holding the heating.
It is desirable to hold for 0 hours. Preprocessing is 0.03 ~
The pressure can be increased by about 0.2 MPa, and the pretreatment time can be reduced by the pressure.

The soft carbon-based carbon material pretreated as described above is then subjected to an activation treatment. The activation can be performed according to a normal activation method using an oxidizing gas, and is usually performed by heating under an atmosphere of an oxygen-containing gas such as water vapor, carbon monoxide, or the above air. The heating (activation) temperature varies depending on the atmosphere, but when activated by steam (such as a steam-saturated inert gas atmosphere), it is usually about 600 to 1000 ° C, preferably 700 to 950 ° C, and about 10 minutes to 6 hours. It is preferable to hold. When activated by an oxygen-containing gas such as air, it is preferable that the temperature is maintained at a temperature of about 400 to 700 ° C., preferably about 500 to 600 ° C. for several minutes. At the time of heating (activation), the temperature may be raised at a fixed rate and then maintained at a desired temperature, or may be maintained several times in the middle until reaching the final temperature. Among them, steam activation is preferable. The activation can be performed under pressure using a pressure vessel.

According to the present invention as described above, a porous carbon material having a large specific surface area can be obtained. Specifically, in the present invention, it is possible to obtain a porous carbon material having a specific surface area of 1000 m ² / g or more, which has been difficult by the conventional oxidizing gas activation without using an alkali chemical.

Further, the porous carbon material obtained by the present invention has a low electric resistance and a high packing density, so that the capacitance per unit volume is high. A specific method for measuring the packing density will be described later in Examples. In the present invention, a yield of 10% or more can be obtained even with a specific surface area of 1000 m ² / g or more. The yield is the ratio of the obtained porous carbon material (mass) to the soft carbon-based carbon material (mass) used as the raw material.

As described above, the porous carbon material obtained in the present invention is suitable as a polarizable electrode material for an electric double layer capacitor. In the present invention, the porous carbon material is used as a polarizable electrode material. A capacitor is also provided.

The porous carbon material substantially corresponds to the shape of a soft carbon-based carbon material such as a raw material mesophase small sphere or bulk mesophase, and has an average particle size of 5 to 100 μm.
The polarizable electrode can be manufactured according to a general method using these materials. Generally, if necessary, a binder and a conductive agent are appropriately added to the porous carbon material,
The porous carbon material layer is formed by molding into a circular or rectangular disk or sheet.

As the binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (P
VDF) can be used. The binder can be used usually in an amount of about 0.1 to 20% by mass based on the porous carbon material. When the amount of the above-mentioned binder is too large, the internal resistance of the battery tends to increase, and when too small, the adhesion between the porous carbon material particles and the current collector tends to be insufficient. As the conductive agent, carbon black or the like is usually used as needed. When used, usually 3 to 20% by mass based on the porous carbon material
Of the order of magnitude.

The polarizable electrode has a structure in which a conductive current collector layer is provided on one surface of the porous carbon material layer. The conductive current collector layer is formed of a porous carbon material, a binder, and a mixture of a conductive agent. When the porous carbon material layer is formed, the current collector may be electrically connected to one surface of the porous carbon material layer formed in advance by a method such as compression molding.

In order to simultaneously form a thin porous carbon material layer having a thickness of about 10 to 200 μm and a current collector made of metal or the like, it is preferable to use the above binder. Specifically, when polyvinylidene fluoride is used as the binder, this is
A method of dissolving in a solvent such as methyl-2-pyrrolidone, adding a porous carbon material and, if necessary, a conductive agent to form a paste, uniformly applying the paste on a current collector, and drying the paste is preferred.
After drying, it is also possible to increase the packing density of the porous carbon material layer by pressing at room temperature or by heating.

In the case of producing a disk or a thick sheet-like molded article of a porous carbon material, polytetrafluoroethylene or the like is preferably used as a binder, and the porous carbon material, the binder and, if necessary, the conductive agent are mixed at room temperature. Alternatively, a method of kneading under heating and compression molding at room temperature or under heating is preferable.

The current collector may be electrically connected to the porous carbon material layer by spraying a metal such as aluminum to form a current collector, or by pressing a current collector made of a metal foil or a metal net of aluminum or the like. And the like.

The unit cell of the electric double layer capacitor generally uses a pair of polarizable electrodes obtained as described above, and opposes each other via a liquid-permeable separator made of a nonwoven fabric or another porous material as necessary. It is formed by dipping in an electrolytic solution. Note that the pair of polarizable electrodes may be the same or different. In using the electric double layer capacitor, the unit cells are used alone or a plurality of unit cells are connected in series and / or in parallel.

As the electrolytic solution, either a non-aqueous solvent system or an aqueous system can be used. The non-aqueous solvent-based electrolyte is a solution in which an electrolyte is dissolved in an organic solvent.Examples of the organic solvent include ethylene carbonate, propylene carbonate, γ-butyl lactone, dimethyl sulfoxide, dimethyl formamide, acetonitrile, tetrahydrofuran, dimethoxyethane, and the like. Can be used. Mixtures of two or more of these can also be used.

As the electrolyte, (C ₂ H ₅ ) ₄ PB
F ₄ , (C ₃ H ₇ ) ₄ PBF ₄ , (C ₂ H ₅ ) ₄ NBF
₄ , (C ₃ H ₇ ) ₄ NBF ₄ , (C ₂ H ₅ ) ₄ PP
F ₆ , (C ₂ H ₅ ) ₄ PCF ₃ SO ₃ , LiBF ₄ , L
iClO ₄ , LiCF ₃ SO _3, or the like can be used. The electrolyte of the aqueous electrolyte is NaCl, NaO
H, HCl, H ₂ SO _{4 and the} like can be used.

[0030]

EXAMPLES Next, the present invention will be described specifically with reference to examples, but the present invention is not limited to these examples. (Examples 1 to 3) In a heat-resistant container, mesophase microspheres (trade name: KMFC manufactured by Kawasaki Steel Co., Ltd.) having an average particle size of 17 μm were placed at a predetermined temperature shown in Table 1 under an air flow of 10 L / min. Heating (pretreatment) for hours. Then, 900 L / min under a 33% H ₂ O saturated nitrogen stream.
Heat treatment (steam activation) at 3 ° C. for 3 hours. Table 1 shows the pore structure (specific surface area and pore diameter) and packing density of the obtained porous carbon material. Further, Table 1 shows the capacitance of the electric double layer capacitor manufactured using the porous carbon material.

<Pore Structure> The specific surface area and the pore diameter were determined by a nitrogen adsorption BET method. The specific surface area and pore size are mi
Calculated by the BET method based on the adsorption isotherm by adsorption and desorption of N _{2 at} 77 K using ASAP2400 manufactured by crometrics. The average pore diameter is 4 × (pore volume) / (specific surface area).

<Density> 80 mg of a carbon material was mixed with 1 mg of polytetrafluoroethylene (PTFE) and press-formed into a disk having a diameter of 13 mm (pressure 14.7 G).
Pa), and the packing density (g / cm ³ ) was calculated from the diameter, thickness, and mass of the disk.

<Preparation of Electrode> To 80 g of the porous carbon material, 10 g of carbon black and 10 g of PVDF were added at a ratio (mass ratio) of 10 g, and the mixture was dry-mixed. Pressurization (9.8 GP
a / cm ² ) to form a polarizable electrode material. This was dried at 160 ° C. for 6 hours under reduced pressure (133.3 Pa).

<Preparation of Electric Double Layer Capacitor> In a glove box through which high-purity argon is flowed at a dew point of -60 ° C., porous polypropylene (pore diameter) is interposed between the pair of polarizable electrode materials prepared above. 0.20 μm)
Was inserted into a two-electrode cell manufactured by Hosensha, and filled with an electrolyte to prepare a cell. The electrolyte used was a solution of tetraethylammonium tetrafluoroborate ((C ₂ H ₅ ) ₄ NBF ₄ ) dissolved in propylene carbonate at a concentration of 1M.

<Measurement of Capacitance> The charge / discharge measurement was performed using a Hokuto Denko charge / discharge tester (HJR-110mSM6), a constant current charge of 0.5 mA / cm ² was performed, and the potential was 2.4 V. After that, the operation was shifted to constant voltage charging, and charging was performed for 2 hours. A constant current discharge of 0.5 mA / cm ² was performed to make the final voltage 0 V. This was performed for 10 cycles. The capacitance was calculated as follows. From the discharge curve of the third cycle (discharge voltage-discharge time), the total discharge energy (W ·
s) The seek, seeking capacitance using a capacitance (F) = 2 × total discharge energy (W · s) / (a discharge start voltage (V)) ² relationship, minute the capacitance The capacitance per unit mass was divided by the mass of the carbon material of the polar electrode material (positive electrode + negative electrode, unit: g). To the capacitance per unit mass,
The value multiplied by the packing density (g / cm ³ ) of the polarizable electrode material was defined as the capacitance per unit volume (F / cm ³ ).

(Comparative Example 1) Steam activation was performed in the same manner as in Example 1 except that the pretreatment was not performed. Table 1 shows the results
Shown in

[0037]

[Table 1]

[0038]

According to the method for producing a porous carbon material of the present invention,
By the oxidizing gas activation method, which is low cost and easy to process, a porous carbon material having a large specific surface area can be manufactured with high yield. Further, the porous carbon material of the present invention produced as described above has a high capacitance and a high packing density. Such a porous carbon material can be used for various applications in which such various properties are required, but is particularly suitably used as an electrode material for an electric double layer capacitor requiring high density energy.

Claims (5)

[Claims] 1. A method for producing a porous carbon material, wherein a soft carbon-based carbon material is subjected to a pretreatment of heating at a temperature lower than an activation temperature in the presence of oxygen and then activated with an oxidizing gas. 2. The method for producing a porous carbon material according to claim 1, wherein the soft carbon-based carbon material is mesophase small spheres and / or bulk mesophase. 3. The method for producing a porous carbon material according to claim 1, wherein the pretreatment is performed at a temperature of 200 to 500 ° C. 4. A method for producing a porous carbon material according to claim 1, which has a specific surface area of 1000 m
² / g or more of a porous carbon material. 5. An electric double layer capacitor using the porous carbon material according to claim 4 as a polarizable electrode material.