JP2008013394A - Activated carbon and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of an activated carbon having a high meso-hole occupancy rate. <P>SOLUTION: The manufacturing method comprises using a Mg salt of an organic acid having 6 or more carbons in the structural formula, for example, a Mg salt of citric acid as a raw material, heating the Mg salt of citric acid at 300°C or higher under an inert atmosphere, followed by acid cleaning. When heating the Mg salt of an organic acid at 300°C or higher under an inert atmosphere, fine magnesium oxide (MgO) is formed through oxidation of Mg in the Mg salt of the organic acid, and a carbon film is formed around the MgO, resulting in covering the MgO with C. Thereafter, the MgO is eluted by washing it with a solution for example such as sulphuric acid, hydrochloric acid or the like that can dissolve the raw material, whereby only the carbon film remains and the inside of the carbon film is converted into pores. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing activated carbon using activated carbon and organic acid Mg as raw materials characterized by the thickness of the carbon film constituting the outer shell of the pores.

Since the electric double layer capacitor has a large Farad-class capacitance and is excellent in charge / discharge cycle characteristics, it is used as a backup power source for various devices including automobiles.
The electric double layer capacitor is configured such that a pair of polarizable electrodes are provided opposite to each other with a separator interposed therebetween, and these polarizable electrodes are impregnated with an electrolytic solution so that each acts as an anode and a cathode. The polarizable electrode of this electric double layer capacitor is obtained by combining activated carbon with a binder resin such as PTFE (polytetrafluoroethylene) to form a thin sheet.
One of the methods for producing activated carbon constituting this polarizable electrode is an air activation method. Since the air activation method is a relatively simple treatment method in which the carbonized material is exposed to air while maintaining a predetermined temperature, the production cost of activated carbon is low, and this air activation method can be adopted industrially. It is advantageous.

  By the way, the pores of the activated carbon are divided into macropores exceeding 50 nm, mesopores of 2 to 50 nm, and micropores of less than 2 nm depending on the diameter. Of these, activated carbon used for polarizable electrodes of electric double layer capacitors has many mesopores in the case of organic electrolytes that are often used as backup power sources for automobiles, etc., depending on the electrolyte. This is advantageous for the permeation of the electrolyte and the movement of ions, and it is said that good rate characteristics can be obtained.

  However, when producing activated carbon by a general air activation method in which the carbonized raw material is exposed to air while being kept at a predetermined temperature, first, micropores are created in the carbonized raw material, and the micropores grow into mesopores. Take the process of going. In that case, micropores remained and it was difficult to increase the proportion of mesopores.

Further, as a method for producing activated carbon having a moderate ratio of micropores to mesopores by an air activation method, there is one described in Patent Document 1. This is because the air activation treatment is divided into a primary performed at a high temperature and a secondary performed at a low temperature, mesopores are formed on the surface of the carbonized material by the primary air activation treatment, and the carbonized product is formed by the next secondary activation treatment. The micropores are grown from the mesopores on the surface of the raw material to the inside.
JP-A-2005-286170

As described above, the air activation method is industrially advantageous, but there is a problem that it is difficult to produce activated carbon having a high mesopore occupation ratio. For this reason, development of the method of manufacturing industrially advantageous activated carbon with a high mesopore occupation rate has been calculated | required.
In addition, activated carbon constituting the polarizable electrode of an electric double layer capacitor is required to have a large pore volume per unit weight, in particular, for weight reduction.
Accordingly, a first object of the present invention is to provide activated carbon having a large pore volume per unit weight, and a second object is to provide a method capable of producing activated carbon having a high mesopore occupation ratio. There is to offer.

The activated carbon of the present invention is characterized in that the carbon film constituting the outer shell of the pores has a thickness of 20 nm or less (claim 1). The thickness is preferably 1 to 10 nm.
As described above, the pores of the activated carbon are divided into macropores, mesopores, and micropores depending on the diameter. The outer shell of these pores is constituted by a carbon film. The thinner this carbon membrane, the lighter it becomes and the pore volume per unit weight becomes larger.

  A product manufactured using such activated carbon having a large pore volume per unit weight is lightweight. For this reason, by using the activated carbon of the present invention as the activated carbon of the polarizable electrode of the electric double layer capacitor (Claim 2), a lightweight electric double layer capacitor can be manufactured, for example, as a backup power source mounted in an automobile It can cope with the request for weight reduction when using.

When activated carbon is used as a raw material for producing a polarizable electrode of an electric double layer capacitor, it is preferably activated carbon having many mesopores, particularly when the capacitor uses an organic electrolyte. Activated carbon having many mesopores can be obtained by the production method of claim 3.
The method for producing activated carbon according to claim 3 uses an organic acid, in particular, an organic acid Mg having a structural formula of C of 6 or more as a raw material, and heats the organic acid Mg to 300 ° C. or higher in an inert atmosphere. It is characterized by acid cleaning. Examples of the organic acid Mg having a structural formula where C is 6 or more include Mg citrate and Mg gluconate.

  When the organic acid Mg is heated to 300 ° C. or higher in an inert atmosphere, the Mg of the organic acid Mg is oxidized to form fine magnesium oxide (MgO). As shown in FIG. A carbon film is formed around MgO to produce a product. By using an organic acid Mg as a raw material having a structural formula of C or more, a normal carbon film can be formed without causing C deficiency. In addition, the average particle diameter of the MgO crystallite size by the said organic acid Mg is 2-60 nm. Thereafter, the product is acid washed, that is, washed with a solution of, for example, sulfuric acid or hydrochloric acid capable of dissolving MgO to dissolve MgO. Thereby, as shown in FIG. 3B, only the carbon film remains, and the inside of the carbon film becomes pores. Thus, activated carbon is produced.

The pores of the activated carbon thus produced have a size centered on the mesopores and have a large proportion of mesopores. Further, since the treatment is performed using the organic acid Mg as a raw material, the carbon film constituting the outer shell of the pores is a thin film having a thickness of 20 nm or less. A method of forming pores by forming a carbon film around a certain substance and then eluting the substance inside the carbon film as in the method of the present invention is called a template method.
In addition, the surface area per mass unit of a target object can be made high by using only the said organic acid Mg as a raw material, without using an activation adjuvant. Further, the cleaning solution after the cleaning can be concentrated to precipitate the organic acid Mg and recycled again to the starting material.

  The heating temperature in the inert atmosphere can be 800 ° C. or higher. The heating temperature affects the crystallization of the carbon film. When heated to 800 ° C. or higher, crystallization proceeds and graphitizes, and the electrical resistance decreases. Further, it is advantageous for promoting the formation of the carbon film itself and for making the distribution of pores uniform. When used as a raw material for producing a polarizable electrode of an electric double layer capacitor, the carbon film is preferably graphitized at a heating temperature of 800 ° C. or higher in order to lower the electric resistance.

  In the method for producing activated carbon of the present invention, heating to 300 ° C. or higher in an inert atmosphere is preferably performed at a temperature rising rate of 5 ° C. or lower per minute. With this rate of temperature increase, the generation of MgO and the generation (crystallization) of the carbon film around the MgO proceed more favorably.

  After heating to a temperature of 300 ° C. or higher and holding at the heating temperature for a certain period of time promotes the formation of the carbon film, it helps to make the carbon film to an appropriate thickness. That is, the carbon film thickness can be controlled by controlling the holding time. However, this holding time is preferably 1 hour or less.

  Thus, the rate of temperature rise when heating to the target temperature and the holding time at the target heating temperature both affect the growth of the carbon film. Since the growth of the carbon film is affected by the amount of applied heat energy, the rate of temperature rise and the holding time may be determined in consideration of the balance between the target thickness of the carbon film and the heating temperature.

[Example 1]
A chemical reagent (powder form) manufactured by Wako Pure Chemical Industries, Ltd., and a trade name “trimagnesium dicitrate nonahydrate” was used as a raw material Mg citrate. After putting 100 g of this raw material Mg citrate into a heating furnace and replacing the inside of the heating furnace with an inert gas such as argon, helium or nitrogen, and heating to 900 ° C. at a rate of 5 ° C. per minute. did. By this heating, MgO is generated in the raw material, and C is coated around the MgO to form a carbon film, thereby generating a product. After heating the heating furnace to 900 ° C., the heating furnace is immediately cooled to room temperature by air cooling (natural cooling for about 2 hours), the cooled product is taken out from the heating furnace, and MgO inside the carbon film is eluted. For this purpose, for example, the acid was washed with 1 mol of aqueous sulfuric acid. After this acid cleaning, the product was washed with water and dried to obtain activated carbon (Example Product 1).

[Example 2]
The same raw material as in Example 1 was used, and heat treatment was performed under the same conditions as in Example 1 except that the material was held at a heating temperature of 900 ° C. for 30 minutes. Then, the temperature of the heating furnace was lowered, and the cooled product was washed with acid, washed with water and dried in the same manner as in Example 1 to obtain activated carbon (Example Product 2).

[Example 3]
The same raw material as in Example 1 was used, and heat treatment was performed under the same conditions as in Example 1 except that the material was held at a heating temperature of 900 ° C. for 1 hour. Then, the temperature of the heating furnace was lowered, and the cooled product was washed with acid, washed with water and dried in the same manner as in Example 1 to obtain activated carbon (Example Product 3).

[Example 4]
The same raw material as in Example 1 was used, and the heat treatment was performed under the same conditions as in Example 1 except that the heating temperature was maintained at 700 ° C. for 30 minutes. Then, the temperature of the heating furnace was lowered, and the cooled product was washed with acid, washed with water and dried in the same manner as in Example 1 to obtain activated carbon (Example Product 4).

  The inventor measured the pore size distribution of the above-mentioned Example product 1. The pore distribution was measured by the BJH (Barrett-Joyner-Halenda) method and the DFT (Density-Functional-Theory) method. The pore distribution measurement results are shown in FIGS. In FIGS. 1 and 2, the vertical axis represents the pore volume and the horizontal axis represents the pore diameter. From FIG. 1 and FIG. 2, it is understood that the activated carbon according to the present invention has fine pores with a small micropore occupancy and concentrated in mesopores, particularly pore diameters of 2 to 10 nm centered on 5 nm. The

Moreover, in order to compare with said Example goods 1-4, this inventor obtained the comparative example goods as follows.
[Comparative Example 1]
The same raw material as in Example 1 was used, and the heat treatment was performed under the same conditions as in Example 1 except that the heating temperature was maintained at 250 ° C. for 30 minutes. Then, the temperature of the heating furnace was lowered, and the cooled product was washed with acid, washed with water and dried in the same manner as in Example 1. However, as shown in Table 1, pores were not formed, and activated carbon was obtained. There wasn't. (Comparative product 1).

[Comparative Example 2]
In Comparative Example 2, activated carbon is produced by the casting method as in the present invention. First, 50 g of PVA (polyvinyl alcohol) was added as an activation aid to 50 g of the same Mg citrate as in Example 1, and both were mixed evenly. Using this mixture as a raw material, this raw material was put in a heating furnace, and the inside of the heating furnace was made an inert atmosphere in the same manner as in Example 1. Then, the temperature was raised to 900 ° C. at a temperature rising rate of 5 ° C. per minute. For 30 minutes. Thereafter, the temperature of the heating furnace was lowered to room temperature, the product was taken out of the heating furnace, and acid-washed with 1 mol of sulfuric acid aqueous solution to elute MgO inside the carbon film. After this acid cleaning, the product was washed with water and dried to obtain activated carbon (Comparative Example Product 2).

[Comparative Example 3]
As a carbonized material, for example, 100 g of a carbonized spherical phenol resin was prepared, this raw material was placed in a heating furnace, heated to 400 ° C. while being exposed to air, and held at this temperature for 40 hours. In the raw material, pores are formed by the air activation treatment in the heating furnace. Thereafter, the temperature of the heating furnace was lowered to room temperature, and activated carbon of Comparative Example Product 3 was obtained.

  This inventor performed the measurement of the carbon film thickness, and the specific surface area, the pore volume, the electrostatic capacity, and the test of the rate characteristic about the said Example goods 1-4 and the comparative example goods 1-3. The results are shown in Table 1 below. The specific surface area was determined by the BET (Brunauer-Emmett-Teller) method. The rate characteristic is a value obtained by dividing the capacitance at 200 mA discharge by the capacitance at 20 mA discharge as a percentage.

  As can be seen from Table 1, the Examples 1 to 4 have a larger pore volume than the Comparative Examples 1 to 3, and the carbon membranes constituting the outer shell of the pores when measured by electron microscope observation In Examples 1 to 4, the thickness of the sample was 20 nm or less, and in Comparative Example 1 the pores were not formed. In Comparative Example 3, the thickness exceeded 20 nm (average shortest pore distance / 2). Is in the micron order. In addition, the example products 1 to 4 have a larger capacitance than the comparative example products 1 to 3, and in particular, the example products 1 to 3 show high rate characteristics that are close to 90%.

  4 is a schematic diagram of the activated carbon of Example Product 3, and FIG. 5 is a schematic diagram of the activated carbon of Comparative Example Product 3. The activated carbon of Comparative Example Product 3 has many micropores as pores and a long distance between pores. In contrast, in the activated carbon of Example Product 3, the pores are centered on the mesopores, and it can be seen that the thickness of the carbon film is relatively thin compared to the size of the pores.

Graph showing the distribution ratio of pore size Graph showing the distribution ratio of size for micropores Conceptual diagram of carbon film coated around MgO Schematic diagram of activated carbon of Example product 3 Schematic diagram of activated carbon of Comparative Example Product 3

Claims (7)

In activated carbon with fine pores,
Activated carbon in which the carbon film constituting the outer shell of the pore has a thickness of 20 nm or less.   The activated carbon according to claim 1, which is used as a raw material for producing a polarizable electrode of an electric double layer capacitor.   A method for producing activated carbon, characterized in that an organic acid Mg having a structural formula of C of 6 or more is used as a raw material, the organic acid Mg is heated to 300 ° C. or higher in an inert atmosphere, and then cooled and acid washed. .   The method for producing activated carbon according to claim 3, wherein the heating temperature in the inert atmosphere is 800 ° C. or higher.   The method for producing activated carbon according to claim 3 or 4, wherein the heating to 300 ° C or higher in the inert atmosphere is performed at a heating rate of 5 ° C or lower per minute.   The method for producing activated carbon according to any one of claims 3 to 5, wherein after heating to 300 ° C or higher, the holding time of the heating temperature is 1 hour or less.   The method for producing activated carbon according to any one of claims 3 to 6, which is a method for producing activated carbon used as a raw material for producing a polarizable electrode of an electric double layer capacitor.

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