JP2004047613A - Activated carbon and electrode for electric double-layer capacitor employing active carbon - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide activated carbon which shows a big electrostatic capacity and a low internal resistance per volume, in an electrode material for an electric double-layer capacitor. <P>SOLUTION: The activated carbon, obtained by activating a pitch series carbonaceous material employing an alkaline metal hydroxide, is specified so as to have the total amount of surface functional group of 0.80-1.50meq per gram of activated carbon and a specific surface area of 300-1,500m<SP>2</SP>/g. The alkaline metal hydroxide is preferable to be sodium hydroxide or potassium hydroxide. The active carbon is suitable for the material of an electrode for electric double-layer capacitor. <P>COPYRIGHT: (C)2004,JPO

Description

＊　圧力上昇
【００４７】
表１から、実施例１〜３で作製した活性炭は容積当たりの静電容量が比較例１及び２のものに比べて高いことがわかる。また、実施例１〜３で作製した電気二重層キャパシタは、比較例１及び２のものに比べて内部抵抗が小さいことがわかる。
【００４８】
【発明の効果】
本発明により、容積当たりの静電容量が大きく、内部抵抗が小さい活性炭を得ることができる。かかる活性炭は、成形して電気二重層キャパシタ用の電極として好適に使用される。
【図面の簡単な説明】
【図１】電気二重層キャパシタの一例を示す概略断面図である。
【符号の説明】
１，２　集電部材、３，４　分極性電極、５　セパレータ、６　ガスケット、７　ケース[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to activated carbon obtained by alkaline activation of a pitch-based carbonaceous material, and an electrode for an electric double layer capacitor using the same.
[0002]
[Prior art]
In recent years, electric double layer capacitors using activated carbon as polarizable electrodes have been used as relatively small memory backup power supplies, auxiliary power supplies, and the like due to their excellent capacitance. Recently, in addition to a relatively small and small-capacity product such as a conventional memory backup power supply, a large-capacity product such as an auxiliary power supply such as a motor has been developed.
[0003]
The capacitance of the electric double layer capacitor mainly depends on the surface area of the polarizable electrode on which the electric double layer is formed, the electric double layer capacitance per unit area, the resistance of the electrode, and the like. In practical terms, increasing the density of the electrodes themselves is also an important issue in order to increase the capacitance per unit volume and reduce the volume of the electric double layer capacitor.
[0004]
The following activated carbons (1) to (4) are known as activated carbon for a polarizable electrode of such an electric double layer capacitor.
(1) Activated carbon obtained by activating a carbonaceous material such as resin material, coconut shell, pitch, and coal under acidic conditions such as steam and gas (Large-capacity capacitor technology and materials, published by CMC Corporation (1998) )reference);
(2) Activated carbon obtained by activating a carbonaceous material such as a resin material, coconut shell, pitch and coal with a chemical having a strong oxidizing power (for example, KOH or the like) (WO91 / 12203, JP-A-10-203) 1997767);
(3) The total amount of surface functional groups obtained by activating a carbonaceous material such as resin material, coconut shell, pitch and coal with a chemical having strong oxidizing power (for example, KOH) for a long time is 0.20. And (4) activated carbon having a surface acidic functional group content of about 1.5 meq / g (surface Vol. 34, No. 2 (1996)).
[0005]
[Problems to be solved by the invention]
However, as described above, despite the demand for high capacitance and low resistance for the electrode for the electric double layer capacitor, the above-mentioned activated carbon (1) is used for the electric double layer capacitor. When it is used as an electrode material for a battery, a sufficient capacitance cannot be obtained, and in order to obtain a required capacitance, there is a problem that an electric double layer capacitor becomes a large device. . In the case of the activated carbon of (2), if the activated carbon is activated at a high temperature by using an activator having a strong oxidizing power such as potassium hydroxide, a certain high capacity can be expected. At present is not reached.
[0006]
In the case of the activated carbon (3), although the capacitance is improved, the activation time during the production is too long, and it cannot be said that the activated carbon can be produced industrially advantageously. In the case of activated carbon (4), many functional groups are introduced on the surface. However, since the specific surface area is large and the specific gravity is small, it is difficult to increase the electrode density. Has the drawback that the capacitance of the device becomes small.
[0007]
An object of the present invention is to provide an activated carbon which exhibits a large capacitance per volume and a low internal resistance in an electrode material of an electric double layer capacitor and can be industrially advantageously produced.
[0008]
[Means for Solving the Problems]
The present inventors have achieved, in order to achieve the above object, the total amount of surface functional groups of activated carbon obtained by alkali-activating a pitch-based carbonaceous material known to have a high carbon content, and the specific surface area is Have been found to be closely related to the capacitance per volume, and have completed the present invention.
[0009]
That is, the present invention is an activated carbon obtained by activating a pitch-based carbonaceous material using an alkali metal hydroxide, wherein the total amount of surface functional groups is 0.80 to 1.50 meq per 1 g of activated carbon, and a specific surface area provide the activated carbon, which is a 300m ² / g~1500m ² / g.
[0010]
Further, the present invention provides an electrode material for an electric double layer capacitor obtained by molding the above-mentioned activated carbon.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0012]
The activated carbon of the present invention is obtained by activating a pitch-based carbonaceous material obtained by a conventional method from a known optically anisotropic pitch obtained from petroleum or coal using an alkali metal hydroxide. In this case, the total amount of the surface functional groups and the specific surface area are each limited to a specific range.
[0013]
First, when the total amount of the surface functional groups of the activated carbon (for example, at least one of a carboxyl group, a lactone group, a hydroxyl group, and a quinone group) is too small, the electrostatic attraction is insufficient and the capacitance is reduced. In this case, gas is generated due to decomposition of the functional group, and the internal resistance increases. Therefore, the total amount of the surface functional groups (for example, at least one of carboxyl group, lactone group, hydroxyl group, and quinone group) of the activated carbon of the present invention is 0.80 to 1.50 meq, preferably 0.90 to 1 per gram of activated carbon. .30 meq.
[0014]
The amount of the surface functional groups of the activated carbon can be determined by a generally known method (for example, see Surface Vol. 34, No. 2 (1996); Catal. 16, 179 (1966)). Specifically, 2 g of each activated carbon sample was placed in a 100 ml Erlenmeyer flask, and an N / 10 alkaline reagent ((a) sodium hydrogen carbonate, (b) sodium carbonate, (c) caustic soda, (d) sodium ethoxide) Was added, and the mixture was shaken for 24 hours, filtered off, and the unreacted alkaline reagent was titrated with N / 10 hydrochloric acid. The carboxyl groups were all alkaline reagents (a) to (d), and the lactone groups were alkaline reagents. (B) to (d), the hydroxyl group reacts with the alkali reagents (c) to (d), and the quinone group reacts with the alkali reagent (d). It can be quantified.
[0015]
Also, when the specific surface area of the activated carbon is too small, the number of adsorption sites of the electrolyte is not sufficient, so that the capacitance is small. Conversely, when the specific surface area is too large, the capacitance per weight tends to increase. Per unit capacitance is reduced. Therefore, the specific surface area of the activated carbon of the present ^{invention, 300m 2 / g~1500m 2 / g} , preferably from 400m ² / g~1100m ² / g.
[0016]
The specific surface area of the activated carbon can be measured by the BET adsorption method (see the new edition "Activated Carbon-Fundamentals and Applications-1997", pp. 23-40).
[0017]
As the pitch-based carbonaceous material which is a raw material for producing the activated carbon of the present invention, a pitch-based graphite-based carbonaceous material is preferably exemplified, and among them, a mesophase pitch-based carbon fiber is preferably exemplified. In particular, as the mesophase pitch-based carbon fiber, a pitch-based carbon fiber containing 50% by volume or more, preferably 80% by volume or more of an optically anisotropic phase is preferable from the viewpoint of excellent conductivity.
[0018]
The shape of the pitch-based carbonaceous material is not limited, and various shapes such as granules, fine powders, fibers, and sheets can be used.
[0019]
Examples of the alkali metal hydroxide which is another production raw material of the activated carbon of the present invention include, for example, sodium hydroxide, potassium hydroxide, lithium hydroxide, and cesium hydroxide. To obtain it, it is preferable to use sodium hydroxide or potassium hydroxide. These are usually used alone, but may be used in combination of two or more.
[0020]
The amount of the alkali metal hydroxide to be used for the pitch-based carbonaceous material of the present invention, if the latter is too small, the activation operation becomes difficult to be performed uniformly and sufficiently, and the properties of the desired activated carbon may vary, Conversely, if too large, not only is not economical, there is a risk that activation may proceed too much, the capacitance per weight tends to increase, but not only the capacitance per volume decreases, The introduction amount of the acidic functional group becomes too large, and when it is used as an electrode for an electric double layer capacitor, the life and stability may be significantly impaired. Therefore, the amount of the alkali metal hydroxide used is preferably at least 1 part by weight based on 1 part by weight of the pitch-based carbonaceous material, and 1.2 to 4 parts by weight in consideration of the stability of activation. Considering the amount to be introduced, it is 1.3 to 2.5 parts by weight.
[0021]
The activated carbon of the present invention is obtained by activating a pitch-based carbonaceous material using an alkali metal hydroxide. Specifically, the activated carbon is obtained by mixing a pitch-based carbonaceous material with an alkali metal hydroxide. Then, after activation by heating to 500 ° C. or more in an inert gas, it is obtained by washing with warm water, acid washing, and drying.
[0022]
Here, the activation temperature is a factor that can control the total amount of the surface functional groups of the activated carbon.If the activation temperature is too high, the number of functional groups introduced into the activated carbon is reduced, and when used as an electric double layer capacitor electrode, The capacitance tends to decrease, and the surface area of the activated carbon increases, but the danger increases because the metal potassium generated in the activation operation evaporates. Also, if the activation temperature is too low, a fine structure that is gasified by the activation action and should be removed outside the system is not removed, and may exhibit relatively high electric resistance, making it difficult to use as an electrode material. Sometimes. Therefore, the activation temperature is preferably set to 500 ° C. to 780 ° C., more preferably set to 600 ° C. to 750 ° C., and particularly preferably set to 650 ° C. to 750 ° C. in consideration of the amount of the functional groups.
[0023]
Further, when producing the activated carbon of the present invention, after the activation temperature reaches the intended maximum temperature, it is possible to quickly cool or maintain the maximum temperature, but the holding time is too short. When the retention time is too long, the amount of the functional group may be too small, the capacitance may be reduced, and the development of the graphite structure may be reduced. And the capacitance tends to decrease. Therefore, the maximum activation temperature holding time is 0.5 to 5 hours, preferably 0.5 to 3 hours.
[0024]
In the activation, the temperature rise and the cooling after completion of the activation need to be performed, for example, in a stream of an inert gas such as nitrogen or argon in order to suppress the combustion of the activated carbon raw material. When flowing an inert gas, the moving speed of the inert gas gas in the furnace is usually 0.1 cm / min or more, although it depends on the reaction method. preferable. The embodiment for performing the activation may be a fixed bed in which the mixture is fixed, a moving bed accompanied by movement such as rotation and stirring, or a batch type or a continuous type.
[0025]
After the activation is completed, the alkali metal hydroxide as an activator is removed by washing with hot water.Washing with hot water is also an important factor that can control the total amount of surface functional groups of the activated carbon. The functional group introduced into the site can be decomposed and removed. That is, the portion having increased water solubility by alkali hydrolysis can be removed by this operation.
[0026]
If the temperature of the hot water used for this hot water cleaning is too high, not only special equipment is required, but also the structure generated by activation may be destroyed, and if the temperature of the hot water is too low, Since hydrolysis hardly proceeds and the water solubility does not increase, the efficiency of removing the functional group may decrease. Therefore, the temperature of the hot water is preferably from 50 ° C to 120 ° C, more preferably from 60 ° C to 100 ° C, and particularly preferably from 65 ° C to 90 ° C, from the viewpoint of economy that no special device is required.
[0027]
The time required for washing with hot water is usually in the range of 30 minutes to 6 hours from the viewpoint of the efficiency of hydrolysis.
[0028]
After the above washing with warm water, washing with hydrochloric acid and drying are carried out in a conventional manner to obtain the activated carbon of the present invention.
[0029]
As a specific method for controlling the total amount of surface functional groups of the activated carbon of the present invention, it is possible to employ conditions that appropriately combine factors that can control the total amount of surface functional groups, such as an activation temperature and a temperature of hot water washing. For example, by performing activation at 700 to 750 ° C and performing hot water washing at 80 to 90 ° C, the total amount of surface functional groups can be 0.90 to 1.50 meq per gram of activated carbon.
[0030]
Further, as a specific method of controlling the specific surface area of the activated carbon of the present invention, it is possible to employ a condition in which the amount of the alkali metal hydroxide used and the activation temperature are combined. For example, by setting the amount of the alkali metal hydroxide used to 1.7 to 2.3 parts by weight with respect to 1 part by weight of the pitch-based carbonaceous material and setting the activation temperature to 700 to 750 ° C., the ratio of the activated carbon is reduced. the surface area can be in the range of 300m ² / g~1500m ² / g.
[0031]
The activated carbon of the present invention described above is useful as a raw material of a polarizable electrode for an electric double layer capacitor, and a method generally known to manufacture such a polarizable electrode can be applied. . For example, a commercially available material known as a binder such as polyvinylidene fluoride or polytetrafluoroethylene or a conductive material such as carbon black may be added to a few percent if necessary, and the mixture may be kneaded. Then, it is formed into a sheet by pressure forming or rolling into a sheet and punched into a required shape to form an electrode. At that time, if necessary, an organic compound such as alcohol or N-methylpyrrolidone, a solvent such as water, a dispersant, or various additives may be used. It is also possible to apply heat. An unnecessarily high temperature affects not only the deterioration of the binder component used but also the physical properties, for example, the specific surface area, of the activated carbon component, so that it is needless to say that the temperature condition must be considered.
[0032]
At the time of molding, a conductive substance such as conductive carbon may be added to reduce the resistance of the electrode. This is effective in lowering the internal resistance of the polarizable electrode and reducing the electrode volume. In addition to the sheet electrodes as described above, a kneaded material may be applied to a current collector to form an applied electrode.
[0033]
The polarizable electrode described above is useful as an electrode of an electric double layer capacitor as shown in FIG. 1 (schematic sectional view). Each component constituting the capacitor of FIG. 1 can have the same configuration as a known electric double layer capacitor except that the polarizable electrode according to the present invention is used. , 3 and 4 are polarizable electrodes made of the activated carbon of the present invention, 5 is a separator made of polypropylene non-woven fabric and the like, 6 is a gasket made of polypropylene, polyethylene, polyamide, polyamide imide, polybutylene, etc. And 7 indicate a case made of a material such as stainless steel.
[0034]
In order to function as an electric double layer capacitor, a known electrolyte such as tetraethylammonium tetrafluoroborate or tetramethylammonium tetrafluoroborate is placed in the case 7 with a carbonate such as ethylene carbonate, dimethyl carbonate, diethyl carbonate or propylene carbonate. , Acetonitrile and other nitriles, α-methyl-γ-butyrolactone and other lactones, dimethylsulfoxide and other sulphoxides, and dimethylformamide and other amides and the like must be filled with an electrolytic solution.
[0035]
【Example】
Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited thereto. The capacitance and the internal resistance of the electric double layer capacitor were measured by the following methods.
[0036]
Capacitance A low current charge is performed at 3 mA / cm ² per unit surface area of the electrode until the ultimate voltage reaches 2.5 V, and a supplementary charge is performed at 2.5 V for 30 minutes under a low voltage. / Cm ² . The capacitance is determined from the discharge gradient from 1.2 V to 1.0 V at that time.
Internal resistance After charging is completed, the circuit is opened for one second, the reduced voltage ([Delta] E) is measured, and the divided voltage is divided by the current value immediately before the circuit is opened to obtain the internal resistance.
[0037]
Example 1
A 2-inch nickel reactor equipped with a thermometer and a stirrer was charged with 10 g of mesophase carbon fiber (MCF, manufactured by Petka) and 20 g of 85% potassium hydroxide, and the reaction system was replaced with nitrogen. (Flow rate: 1 cm per minute) The temperature was raised to 700 ° C at a rate of 200 ° C / hour. After reaching 700 ° C., stirring was continued for 1 hour, and then cooled to room temperature over 2 hours. Nitrogen was passed through a distilled water bubbler for 1 hour, potassium hydroxide was removed with 90 ° C. ion-exchanged water, neutralized with 10% hydrochloric acid, washed with distilled water to remove salts, and dried. Thus, 6.8 g of activated carbon was obtained.
[0038]
The obtained activated carbon was pulverized to an average particle size of 5 to 20 μm to obtain powdered activated carbon, and a mixture comprising 80% by weight of the powdered activated carbon, 10% by weight of conductive carbon, and 10% by weight of polytetrafluoroethylene was prepared and kneaded. Next, the mixture was formed into a sheet having a thickness of 300 μm by roll rolling, and punched into a circular shape having a diameter of 2 cm using a punch. Further, the electrode was dried at 150 ° C. for 4 hours under a reduced pressure of 26.7 Pa to produce a sheet-like electrode.
[0039]
As shown in FIG. 1, the prepared electrode was placed in a stainless steel case in a glow box having a dew point of −80 ° C. or less, a current collector, a sheet-shaped polarizable electrode, a polypropylene nonwoven fabric separator, a sheet-shaped polarizable electrode, and After laminating the current collecting member, the polarizable electrode was impregnated with a propylene carbonate solution containing 1 mol of tetraethylammonium tetrafluoroborate, and was caulked and sealed on the stainless steel upper lid using a polypropylene insulating gasket. A capacitor was manufactured.
[0040]
The obtained double electric layer capacitor was subjected to a constant current up to 2.5 V and 10 charge / discharge cycle tests at room temperature using an electric double layer capacitor evaluation device manufactured by Hioki Electric Co., Ltd. g) and internal resistance (Ω) were measured. The average value of the capacitance obtained from the discharge curve by a conventional method was 22 F / cc.
[0041]
Further, regarding the activated carbon produced in each of the examples and comparative examples, the functional group amount (meq / g), the specific surface area (m ² / g) (measurement apparatus: BELSORP18 manufactured by Nippon Bell Co., Ltd.), and the capacitance (F / cc) was measured. Table 1 shows the obtained results.
[0042]
Example 2
In Example 1, an electrode and a double electric layer capacitor were produced by repeating the same operation as in Example 1 except that the activation temperature was changed to 650 ° C., and evaluated in the same manner as in Example 1. Table 1 shows the obtained results.
[0043]
Example 3
In Example 1, an electrode and a double electric layer capacitor were produced by repeating the same operation as in Example 1 except that the activation temperature was changed to 750 ° C., and it was evaluated in the same manner as in Example 1. Table 1 shows the obtained results.
[0044]
Comparative Example 1
In Example 1, an electrode and a double electric layer capacitor were produced by repeating the same operation as in Example 1 except that the activation temperature was changed to 800 ° C., and evaluated in the same manner as in Example 1. Table 1 shows the obtained results.
[0045]
Comparative Example 2
In Example 1, an electrode and a double electric layer capacitor were produced by repeating the same operation as in Example 1 except that the cleaning temperature was set to 40 ° C., and the same as in Example 1 was evaluated. Table 1 shows the obtained results.
[0046]
[Table 1]

* Pressure rise [0047]
Table 1 shows that the activated carbon produced in Examples 1 to 3 has a higher capacitance per volume than those of Comparative Examples 1 and 2. Further, it can be seen that the electric double layer capacitors manufactured in Examples 1 to 3 have lower internal resistance than those of Comparative Examples 1 and 2.
[0048]
【The invention's effect】
According to the present invention, activated carbon having a large capacitance per volume and a small internal resistance can be obtained. Such activated carbon is suitably molded and used as an electrode for an electric double layer capacitor.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing an example of an electric double layer capacitor.
[Explanation of symbols]
1,2 current collecting member, 3,4 polarity electrode, 5 separator, 6 gasket, 7 case

Claims (3)

An activated carbon obtained by activating a pitch-based carbonaceous material using an alkali metal hydroxide, wherein the total amount of surface functional groups is 0.80 to 1.50 meq / g of activated carbon, and the specific surface area is 300 m ^2. / G to 1500 m ² / g. The activated carbon according to claim 1, wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide. An electrode for an electric double layer capacitor obtained by molding the activated carbon according to claim 1.

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