JP2001302226A - 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 provide a method for the production of a porous carbon material having high electrical capacitance and high energy density and suitable as an electrode material for an electric double layer capacitor by an industrially applicable alkali activation method. SOLUTION: This porous carbon material is produced by activating a soft carbon material with an alkali containing potassium hydroxide and sodium hydroxide at a weight ratio of 99/1 to 30/70. The activation treatment is carried out preferably at a temperature lower than the boiling point of metallic potassium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high specific surface area, high packing density when used as a polarizable electrode material for an electric double layer capacitor, and an extremely large capacitance per volume as well as per mass. A method for producing a porous carbon material capable of industrially producing the obtained porous carbon material by alkali activation, and a porous material having a high energy density obtained thereby and suitable as a high-capacity small-sized carbon material for an electric double layer capacitor The present invention relates to a carbon material and an electric double layer capacitor including the same.

[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) If a large polarizable electrode material is used, an electric double layer capacitor having a high capacitance per unit mass (F / g) can be obtained.

[0005] With the recent tendency to reduce the size and weight of electronic devices and electric vehicles, it is desired to reduce the size and weight of electric double-layer capacitors mounted thereon. ing. For this reason, not only is the capacitance per unit mass (F / g) improved, but also the capacitance per unit volume (F / cm ³ ) is required to be improved. The capacitance per unit volume (F / cm ³ ) is obtained by multiplying the capacitance per unit mass (F / g) by the packing density, and specifically, is as extremely high as 20 F / cm ³ or more. Value has been a recent goal.

Conventionally, the above-mentioned polarizable electrode materials include organic wastes such as resins, pulp production residues, coal,
Activated carbons whose specific surface area has been increased by activating raw materials such as coal coke, wood, and coconut shells are well known.In recent years, in order to obtain a porous carbon material with a high packing density, the particle size has become uniform. Use of mesocarbon microbeads (mesophase microspheres) as an activator has been studied.

As an activation method, according to the conventional oxidizing gas activation method using an oxidizing gas such as steam, air, oxygen or CO ₂ which is a method for producing activated carbon, no special chemical is used. Although there is a cost advantage without the problem of equipment corrosion, it is difficult to obtain an essentially large specific surface area by this method.

[0008] The mesophase microspheres are made of KOH, Na
It is known that, when activated by using an alkali chemical such as OH, CsOH, or LiOH, the specific surface area can be easily increased and the yield is high as compared with the oxidizing gas activation method (Japanese Patent Application Laid-Open No. 1-230414). No., Patent No. 26346
No. 58, JP-A-10-199767, etc.). In the alkali activation, it is generally known that the specific surface area can be increased by increasing the amount of alkali, and among the alkalis, KOH can obtain a large specific surface area. For example, 3 times (mass ratio) NaOH to mesophase microspheres
When using, the specific surface area is at most 1500 m ² / g
, But if KOH is used in the same ratio, 2000 m
A specific surface area of ² / g or more can be easily obtained. In addition, a high yield of 60% or more can be obtained by KOH activation.

On the other hand, in the alkali activation method, in order to obtain a high surface area, it is necessary to use more expensive KOH than the raw material in an amount of three times or more. Further, a preferable activation temperature (see JP-A-1-230414) is 800 to 800 KOH.
The temperature is as high as about 1000 ° C., which is higher than the 600 ° C. of NaOH. Therefore, in order to industrially implement the alkali activation method using KOH, there are problems of chemical cost and equipment corrosion.

Further, when the mesocarbon small spheres having a smooth surface are alkali-activated, the packing density decreases as the specific surface area increases. For example, the specific surface area is increased by activating KOH to increase the capacitance per unit mass (F / g). Although it can be increased, the capacitance per unit volume (F / cm
³ ) There is a limit. With NaOH activation, a higher capacitance per unit mass or per unit volume than KOH cannot be obtained.

[0011]

DISCLOSURE OF THE INVENTION The present invention provides a porous carbon material having a high capacitance per unit mass and per unit volume, and which can be carried out on an industrial level. The method for producing the material and the high packing density obtained by this method,
It is an object of the present invention to provide a porous carbon material suitable as an electrode material for a miniaturized electric double layer capacitor, and an electric double layer capacitor using the porous carbon material.

[0012]

Means for Solving the Problems The present inventor studied the alkali activation of a soft carbon-based carbon material, and found that a mixture of KOH and NaOH was used.
In the use of the same amount as the mixture, KOH or NaOH
It has been found that a porous carbon material having a higher capacitance, particularly a higher capacitance per unit volume (F / cm ³ ), can be obtained as compared with the case where each is used alone. This is KOH
It is presumed that the use of the mixture of NaOH and NaOH formed a large number of pores suitable for use as an electrode material for electric double layer capacitors at 2 nm (20 °) or more effective for desorbing the electrolyte. Like this, KOH and N
The effect of dramatically increasing the capacitance due to the mixture with aOH could not be expected from the effect when each was used alone.

When a KOH-NaOH mixed agent is used,
It has also been found that the above effect can be obtained at a temperature lower than the KOH activation usually performed at 850 ° C., and particularly, that the desired effect can be obtained even when the temperature is lower than the boiling point of metal potassium (762 ° C.). . In addition, it was also found that when carried out at a temperature lower than the boiling point of metallic potassium, evaporation of metallic potassium was hardly observed, and the problem of corrosion was reduced.
In addition, the inventors have found that if the activation temperature is lowered, the heating time can be shortened, the energy can be reduced, and the invention can be carried out with good industrial productivity, and the present invention has been completed.

That is, the present invention relates to a porous carbon for alkali-activating a soft carbon-based carbon material using an alkali containing potassium hydroxide and sodium hydroxide in a quantitative ratio of 99/1 to 30/70 (mass ratio). It is a method of manufacturing a material.
In the above, if the usage ratio of KOH is reduced, the chemical cost can also be reduced. As the soft carbon-based carbon material, mesophase small spheres and / or bulk mesophase can be used. In the present invention, the activation treatment is preferably performed at a temperature lower than the boiling point of metal potassium. Specifically, the activation treatment can be performed at about 600 to 750 ° C.

According to the method for producing a porous carbon material of the present invention as described above, a porous carbon material having a packing density of 0.5 g / cm ³ or more can be obtained. Further, in the present invention, when used as a polarizable electrode material for an electric double layer capacitor, a porous carbon material having a high capacitance of 30 F / g or more per unit mass can be obtained. In the present invention, the capacitance per unit volume obtained by the above manufacturing method is 20%.
A porous carbon material having F / cm ³ or more is also provided. The present invention also provides an electric double layer capacitor using the above-described porous carbon material as a polarizable electrode material.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below more specifically. In the present invention, a soft carbon-based carbon material is activated using a mixed alkali of potassium hydroxide and sodium hydroxide to produce a porous carbon material. As the soft carbon-based carbon material, mesophase spherules and / or
Alternatively, a bulk mesophase can be used. Known mesophase spherules 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 5 to 40 μm
m is more preferred.

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, and graphitization proceeds to some extent even in the activation process, so that it is easy to take a graphite structure, so that it is possible to form a microstructure having a small internal resistance after activation, which is preferable. .

In the present invention, the above soft carbon-based carbon material is alkali-activated by using a mixture of potassium hydroxide and sodium hydroxide. Potassium hydroxide and sodium hydroxide have a KOH / NaOH (mass ratio) of 9
It is used in a quantitative ratio of 9/1 to 30/70, preferably 95
/ 5 to 40/60 ratio. KOH within this range
If the amount ratio is reduced, the chemical cost can be reduced. Other alkalis may be added as long as the object of the present invention is not impaired.

The amount of the mixed alkali used in the present invention varies depending on the finally desired specific surface area and the like.
When a specific surface area of about 00 to 2000 m ² / g is to be obtained, the raw material (soft carbon-based carbon material)
It is used in an amount of 1.5 to 2.5 times (mass ratio).

The activation treatment of the soft carbon-based carbon material is as follows:
Usually, the slurry is formed by forming a slurry of a soft carbon-based carbon material, an alkali containing a KOH / NaOH mixture, and if necessary, a solvent and the like, drying the obtained slurry to form a dry mixture, and then heating. The alkali activation treatment is preferably performed in an inert atmosphere such as a nitrogen gas or an argon gas, whereby a further decarburizing effect can be obtained.

The activation can be carried out at a temperature of 300 ° C. to 1500 ° C., preferably at a temperature not higher than the boiling point of metallic potassium, and more preferably at about 550 to 750 ° C. At the time of activation, various heating devices such as a rotary kiln, a fluidized bed, and a moving bed can be used. Activation heating time is usually about 10 minutes to 24 hours. This heating (activation) may be carried out at a predetermined rate and then maintained at a desired temperature, or may be carried out intermediately several times before reaching the final temperature. After the activation treatment, usually, the porous carbon material is obtained by neutralizing with a hydrochloric acid solution or the like and then washing with ion-exchanged water or the like.

Production of the porous carbon material of the present invention as described above
According to the manufacturing method, when the average pore diameter (BET method) is 2 nm or more,
Has many pores of a size effective for electrolyte desorption,
Porous carbon material with high capacitance per unit mass
You can get the fee. Therefore, the porosity obtained by the present invention
High-quality carbon materials, such as those obtained by KOH activation
It is not necessary to have a large specific surface area.
Area (BET method) is 500-2000m^Two/ G about that
Good, 1000-2000m ^Two/ G about that
I just need.

Further, if the porous carbon material obtained by the present invention is used, a high packing density can be obtained, and usually 0.5 g / cm
³ or more, preferably 0.6 g / cm ³ or more is possible. Therefore, the capacitance per unit volume is also increased, and a polarizable electrode having a high energy density can be obtained. A specific method for measuring the packing density will be described later in Examples. According to the present invention, a porous carbon material having the above desired characteristics can be obtained with a yield of 60% 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 by the present invention exhibits a high capacitance capability. Therefore, it is suitable as a polarizable electrode material for electric double layer capacitors,
The present invention also provides an electric double layer capacitor using the porous carbon material as a polarizable electrode material.

The performance when specifically evaluated with a polarizable electrode is higher per unit mass with respect to the specific surface area than the conventional alkali activation method, and a capacitance of 30 F / g or more can be obtained. It is also possible to obtain a capacitance of 20 F / cm ³ or more per unit volume. For example, if the same raw material is activated to a specific surface area of about 2500 m ² / g by conventional KOH activation, a high capacitance of 25 F / g or more can be obtained per unit mass, but the packing density is low and 20 F / g per unit volume.
It is a general evaluation of electric double layer capacitors that it is difficult to obtain a capacitance of / cm ³ or more.

The above-mentioned porous carbon material almost corresponds to the shape of a soft carbon-based carbon material such as raw material mesophase small spheres and 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 made 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 can 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.

[0036]

EXAMPLES Next, the present invention will be described specifically with reference to examples, but the present invention is not limited to these examples.
The physical properties of the carbon materials obtained in Examples and Comparative Examples were measured as follows. (1) Filling density 1 mg of polytetrafluoroethylene (PTFE) was mixed with 80 mg of carbon material, and press-formed (pressure 14.7 GPa) into a disk having a diameter of 13 mm, and filled from the diameter, thickness and mass of the disk. The density (g / cm ³ ) was calculated.

(2) Pore Structure The specific surface area and pore diameter were determined by the nitrogen adsorption BET method. The specific surface area and pore size are micrometrics ASAP2
Using BET 400, it was calculated by the BET method based on the adsorption isotherm due to N ₂ adsorption and desorption at 77K. The average pore diameter is 4 × (pore volume) / (BET specific surface area) MP value.

(Example) To 150 g of mesophase microspheres (trade name: KMFC manufactured by Kawasaki Steel) having an average particle size of 17 μm was added K
225 g of OH and 225 g of NaOH were added and uniformly mixed to form a slurry, which was dried at 80 ° C. for 3 hours and at 160 ° C. for 24 hours. Then, heat treatment (activation treatment) was performed for 3 hours at each temperature shown in Table 1 under an argon stream. After the activation, the sample was neutralized with hydrochloric acid and washed with ion-exchanged water until the washing solution became neutral. The yield was determined as a mass ratio (%) to the raw material of the obtained porous carbon material. Table 1 shows the packing density and pore structure of the obtained porous carbon material.

(Comparative Example) Either KOH or NaOH was used in combination with the total amount of KOH and NaOH (45
0g) in the same manner as in Example except that the same amount as in Table 1 was used.
Activated at the temperature indicated. Table 1 shows the results.

Using each of the porous carbon materials obtained in Examples and Comparative Examples, an electric double layer capacitor was produced as follows, and the performance was evaluated. The results are shown in Table 1, FIG. 2 and FIG.
Shown in <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.
After dry mixing, the aluminum mesh is used as a current collector and pressed at room temperature into a disc having a diameter of 13 mm (9.8 GPa / cm ² ).
It was formed into a polarizable electrode material. This was reduced under reduced pressure (133.
3Pa) and dried at 160 ° C. for 6 hours.

<Preparation of Electric Double Layer Capacitor> In a glove box through which high-purity argon is passed 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), charging at a constant current of 0.5 mA / cm ² , and a potential of 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 ).

[0043]

[Table 1]

[0044]

According to the method for producing a porous carbon material of the present invention as described above, a porous carbon material having a high capacitance per unit mass can be obtained. Further, since the porous carbon material has a high packing density, a high capacitance per unit volume can be obtained. In addition, the above-mentioned effects can be obtained even at a temperature lower than the conventional KOH activation temperature, and the yield is high, so that the problems of corrosion and production cost due to alkali activation can be greatly improved. 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.

[Brief description of the drawings]

FIG. 1 is a diagram showing the capacitance per unit mass with respect to the specific surface area of the porous carbon materials obtained in Examples and Comparative Examples.

FIG. 2 is a diagram showing the capacitance per unit volume with respect to the specific surface area of the porous carbon materials obtained in Examples and Comparative Examples.

Claims (5)

[Claims] (1) a method of mixing potassium hydroxide and sodium hydroxide;
A method for producing a porous carbon material in which a soft carbon-based carbon material is alkali-activated by using an alkali containing a mixing ratio of 9/1 to 30/70 (mass ratio). 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 activation treatment is performed at a temperature lower than the boiling point of metallic potassium. 4. A method for producing a porous carbon material according to claim 1, which has a capacitance per unit volume of 20 F when used as a polarizable electrode material for an electric double layer capacitor. / Cm ³ or more. 5. An electric double layer capacitor using the porous carbon material according to claim 4 as a polarizable electrode material.