JP5004501B2 - Activated carbon and electric double layer capacitor using the same - Google Patents

Description

  The present invention relates to activated carbon, and more particularly to activated carbon for obtaining a durable electric double layer capacitor and an electric double layer capacitor using the activated carbon.

Activated carbon is a porous structure activated by carbon materials such as carbonized coconut palm, petroleum coke, and coal coke. Porous activated carbon having a large surface area is frequently used for electrode materials such as adsorbents, catalyst carriers, electric double layer capacitors, and lithium secondary batteries. In particular, in an electric double layer capacitor used in a hybrid car or the like, in order to increase energy density, that is, electrostatic capacity, activated carbon having a high degree of crystallinity in which micropores are effectively formed is required as the electrode material. ing.
In the industrial production of activated carbon in which fine pores that can be used for the electrode material of such an electric double layer capacitor are effectively formed, a carbon material such as petroleum coke and an alkali metal compound such as potassium hydroxide are contained in an inert gas atmosphere. For example, an activation method is generally used in which, for example, heating is performed in a range of 600 to 1200 ° C., and an alkali metal is allowed to enter between graphite crystal layers to cause a reaction. In such activation, an alkali metal penetrates into a layered structure in which layered condensed polycyclic carbon compounds are laminated, and micropores are formed. Alkali metal acts in a gaseous state in the activation process, and most of it is removed from the activated carbon together with the inert gas. However, a part of the alkali metal remains in the activated carbon, and activated carbon is manufactured through a cleaning process for removing the part.

In addition, the electrochemical stability is impaired by functional groups such as chemically unstable carboxyl groups (—COOH) and hydroxyl groups (—OH) generated by activation, and the activated carbon is used as a polarizable electrode of an electric double layer capacitor. When applied, it can be considered that the electrolytic solution is decomposed by a chemical reaction or charging of the polarizable electrode is inhibited, and the electrostatic capacity is reduced or the internal resistance is increased.
Therefore, it is necessary to limit the functional group content in the activated carbon. In Patent Document 1 (Patent No. 3012240), a carbon material having a total acidic group amount based on a carboxyl group or a hydroxyl group of 0.45 μeq / m ² or more, It is suitable for capacitors and electrochromic devices. However, when a capacitor is manufactured using such a carbon material, an improvement in capacitance is recognized, but Patent Document 1 does not mention durability. The activated carbon used for the electrode material of the electric double layer capacitor is strongly required to have good durability such as cycle life and storage stability in addition to its large capacitance.
Japanese Patent No. 3012240

  The present invention has been made in view of such a situation, and an object thereof is to provide an activated carbon excellent in durability and an electric double layer capacitor excellent in durability using the activated carbon.

As a result of intensive studies on the above problems, the present inventors have completed the present invention.
That is, the present invention per unit weight obtained by dry-treating activated carbon obtained by activation treatment of graphitizable carbon with an alkali metal hydroxide under an inert gas atmosphere having an oxygen concentration of 2000 ppm by volume or less. Relates to activated carbon having a total acid group content of 0.2 to 1.2 mmol / g.

The present invention also relates to the activated carbon described above, wherein the graphitizable carbon is petroleum coke.
The present invention also relates to the activated carbon described above, wherein the BET specific surface area is 500 m ² / g or more.

In addition, the present invention is characterized in that the electrostatic capacity after application of 2.7 V and 200 hours at a temperature of 60 ° C. exhibits a value of 90% or more with respect to the electrostatic capacity before application. The above-mentioned activated carbon.
The present invention also relates to an electric double layer capacitor using the activated carbon described above.

  By applying the activated carbon of the present invention to an electric double layer capacitor, an electric double layer capacitor having excellent cycle characteristics and excellent durability, a large capacitance, and excellent voltage resistance can be obtained. .

Hereinafter, the present invention will be described in detail.
General activated carbon is made of synthetic polymer carbon such as phenol resin or plant-derived carbon such as coconut shell as a carbon source and is called non-graphitizable carbon called non-graphitizable carbon. The crystal structure is composed of a turbulent layer structure in which graphite layers are turbulently arranged.
On the other hand, the crystal structure of activated carbon using graphitizable carbon as a carbon source is called a crystallite in which small graphite layers are stacked in parallel. It is characterized by a mixture of crystal structures. This is due to the fact that the raw carbon material already contains a portion showing a crystal structure and an undeveloped portion. Therefore, the produced activated carbon also contains a crystal structure and an undeveloped crystal structure. .

Examples of such graphitizable carbon include carbonized petroleum coke and coal pitch coke, and mesophase pitch and mesophase carbon fiber spun from it, which are infusible and carbonized. In the invention, carbonized petroleum coke is particularly preferably used.
The method for carbonizing petroleum coke is not particularly limited, and examples thereof include a method of coking at a temperature of 400 to 600 ° C. for several hours. Petroleum coke becomes a carbide containing microcrystalline carbon similar to graphite by laminating condensed polycyclic aromatics produced by a pyrolysis reaction in the coking process.

The carbide (carbon raw material) is then activated by heat treatment in a nitrogen gas or inert gas atmosphere together with the metal hydroxide.
Specific examples of the metal hydroxide include alkali metal hydroxides such as potassium hydroxide, sodium hydroxide and lithium hydroxide, and alkaline earth metal hydroxides. In addition, two or more kinds can be used in combination. Of these, potassium hydroxide is particularly preferable because it can form fine pores efficiently.

  The carbon raw material / metal hydroxide is preferably used in a ratio of carbon raw material / metal hydroxide (mass ratio) of 0.1 to 2, more preferably 0.2 to 1. By setting the carbon raw material / metal hydroxide (mass ratio) to 2 or less, micropores can be sufficiently formed in the carbon raw material, and activated carbon having a sufficient surface area can be obtained. Further, by making the mass ratio of the carbon material / metal hydroxide 0.1 or more, efficient activation can be performed without lowering the bulk density.

In the activation of the carbon raw material, examples of the activation temperature include 500 ° C. or more and 1200 ° C. or less. Preferably it is 600 degreeC or more and 1000 degrees C or less, More preferably, it is 800 degrees C or less. When the activation temperature is in the above range, activated carbon having sufficient fine pores can be obtained efficiently. The activation treatment time can be appropriately selected in relation to conditions such as temperature, and can be, for example, 3 to 6 hours.
Examples of the inert gas at the time of activation include inert gas and nitrogen gas. For example, the inert gas is preferably introduced as a supply amount that can maintain the oxygen concentration of the activation atmosphere at 100 ppm by volume or less.

In the present invention, the activated carbon obtained by the activation treatment is further subjected to a dry treatment.
Prior to the dry treatment, it is preferable to remove the metal hydroxide and the like by appropriately performing treatments such as alkali washing, acid washing, water washing, drying and pulverization on the activated product. For example, it is desirable to perform cleaning so that the pH of the cleaning wastewater is about 7 to 8 and to remove alkali metal as much as possible.

  It is important that the dry treatment is performed in an inert gas atmosphere having an oxygen concentration of 2000 ppm by volume or less, preferably 1000 ppm by volume or less. If the oxygen concentration in the inert gas is more than 2000 ppm by volume, the total amount of acidic groups in the activated carbon is not sufficiently reduced, and the durability of the electric double layer capacitor using this activated carbon is insufficient. On the other hand, the lower limit of the oxygen concentration is not particularly limited, but is preferably 10 ppm by volume or more. When the oxygen concentration is less than 10 ppm by volume, the total amount of acidic groups contained in the activated carbon decreases, and when used as an electrode for an electric double layer capacitor, the affinity with the electrolyte is insufficient and the capacitance is unsatisfactory. It becomes.

The dry processing temperature is preferably in the range of 200 to 800 ° C, more preferably in the range of 300 to 700 ° C. The dry processing time is not particularly limited, and is usually 0.5 to 2 hours.
The BET specific surface area of the activated carbon thus obtained is preferably 500 m ² / g or more, and more preferably 700 m ² / g or more.

  The total amount of acidic groups contained in the activated carbon thus treated, that is, the total amount of hydroxyl groups and carboxyl groups is 0.2 to 1.2 mmol / g, preferably 0.3, as the total amount of acidic groups per unit weight. Activated carbon in the range of -1.0 mmol / g can be obtained. When the total amount of acidic groups contained in the activated carbon exceeds 1.2 mmol / g, gas is generated by an electrochemical reaction when the capacitor is operated, and a reaction with the electrolytic solution occurs, which adversely affects durability. On the other hand, when the total amount of acidic groups is less than 0.2 mmol / g, the affinity with the electrolytic solution is reduced and the capacitance is lowered.

  The activated carbon of the present invention is excellent in durability, and the capacitance after application of 2.7 V and 200 hours at a temperature of 60 ° C. shows a value of 90% or more with respect to the capacitance before application. .

Next, the electric double layer capacitor of the present invention will be described.
The electric double layer capacitor of the present invention comprises an electrode containing activated carbon (carbon material for electrode) prepared as described above.
The electrode may be configured by adding, for example, an electrode carbon material and a binder, more preferably a conductive agent, and may be an electrode integrated with a current collector.

  As the binder used here, known materials can be used, for example, polyolefins such as polyethylene and polypropylene, fluorine such as polytetrafluoroethylene, polyvinylidene fluoride, and a fluoroolefin / vinyl ether copolymer crosslinked polymer. And polymerized cellulose, celluloses such as carboxymethylcellulose, vinyl polymers such as polyvinylpyrrolidone and polyvinyl alcohol, and polyacrylic acid. Although content of the binder in an electrode is not specifically limited, Usually, it suitably selects in the range of about 0.1-30 mass% with respect to the total amount of the carbon material for electrodes, and a binder.

  As the conductive agent, powders of carbon black, powdered graphite, titanium oxide, ruthenium oxide and the like are used. Although the compounding quantity of the electrically conductive agent in an electrode is suitably selected according to a compounding purpose, it is 1-50 mass% normally with respect to the total amount of the carbon material for electrodes, a binder, and a electrically conductive agent, Preferably it is 2 It is appropriately selected within a range of about 30% by mass.

  In addition, as a method for mixing the electrode carbon material, the binder, and the conductive agent, a known method is appropriately applied. For example, a solvent having a property of dissolving the binder is added to the above components to form a slurry. A method of uniformly coating the current collector on the current collector, or a method of kneading the above components without adding a solvent and then press molding at room temperature or under heating is employed.

  As the current collector, a known material and shape can be used. For example, a metal such as aluminum, titanium, tantalum, or nickel, or an alloy such as stainless steel can be used.

  The unit cell of the electric double layer capacitor of the present invention generally uses a pair of the above electrodes as a positive electrode and a negative electrode, is opposed to each other through a separator (polypropylene fiber nonwoven fabric, glass fiber nonwoven fabric, synthetic cellulose paper, etc.), and is immersed in an electrolytic solution. Formed by.

  As the electrolytic solution, a known aqueous electrolytic solution or organic electrolytic solution can be used, but it is more preferable to use an organic electrolytic solution. As such an organic electrolyte, those used as a solvent for an electrochemical electrolyte can be used. For example, propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, sulfolane, sulfolane derivatives, 3 -Methylsulfolane, 1,2-dimethoxyethane, acetonitrile, glutaronitrile, valeronitrile, dimethylformamide, dimethylsulfoxide, tetrahydrofuran, dimethoxyethane, methyl formate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, etc. . In addition, you may mix and use these electrolyte solutions.

Further, the supporting electrolyte in the organic electrolytic solution is not particularly limited, but various salts such as salts, acids, alkalis and the like that are usually used in the field of electrochemistry or the field of batteries can be used. For example, alkali metal salts Inorganic ion salts such as alkaline earth metal salts, quaternary ammonium salts, cyclic quaternary ammonium salts, quaternary phosphonium salts, and the like. Specifically, (C ₂ H ₅ ) ₄ NBF ₄ , (C ₂ H ₅ ) ₃ (CH ₃ ) NBF ₄ , (C ₂ H ₅ ) ₄ PBF ₄ , (C ₂ H ₅ ) ₃ (CH ₃ ) PBF _{4 and the} like are preferable. The concentration of these salts in the electrolytic solution is appropriately selected within the range of usually about 0.1 to 5 mol / l, preferably about 0.5 to 3 mol / l.

  The specific configuration of the electric double layer capacitor is not particularly limited. For example, the electric double layer capacitor is accommodated in a metal case through a separator between a pair of thin sheet-like or disk-like electrodes (positive electrode and negative electrode) having a thickness of 10 to 500 μm. A coin type, a wound type in which a pair of electrodes are wound through a separator, and a stacked type in which a large number of electrode groups are stacked through a separator.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[Example 1]
<Manufacture of activated carbon>
Petroleum raw coke was used as the graphitizable carbon raw material, and carbonized at 500 ° C. for 3 hours. 1 part by weight of the carbide particles (particle size 2 mm or less) and 2.5 parts by weight of potassium hydroxide (KOH) are mixed, put into a nickel reaction vessel, heated at 750 ° C. for 1 hour under a nitrogen stream, and activated. Went.
After the activation treatment, the reaction mixture inside the reaction vessel was cooled to 300 ° C., and carbon dioxide was passed instead of nitrogen to deactivate the metal potassium. Thereafter, the reaction mixture was washed with hydrochloric acid, and washing with water was repeated until the washing solution became neutral. The reaction mixture was dried by heating at 150 ° C. The obtained activated carbon (activated carbon A) was further subjected to dry heating at 700 ° C. for 1.5 hours under a nitrogen stream containing 100 ppm by volume of oxygen to obtain activated carbon B.

<Measurement of total acid group content>
0.1N NaOH (50 ml) was added to activated carbon B (1 g) after the dry treatment, and shaken for 24 hours. This was filtered, 10 ml of the filtrate was collected, placed in 50 ml of water, and titrated with 0.1N HCl. It was 0.55 mmol / g when the total amount of acidic groups of the activated carbon B was determined by titration.

<Production and evaluation of electric double layer capacitor>
Activated carbon B (0.8 g) obtained above, ketjen black (0.1 g), and PTFE (0.1 g) were mixed in a mortar. This mixture was sandwiched between two 0.1 mm thick triacetate films and rolled by passing 20 times between nip rolls having a width of 160 mm, an upper and lower roll interval of 0.7 mm, and a pressure of 23.0 MPa. Two circles having a diameter of 16 mm were punched out from the rolled sheet to obtain carbon electrodes. The carbon electrode was dried in a vacuum dryer for 2 hours.
A cellulose separator having a thickness of 50 μm is sandwiched between two electrodes impregnated with an electrolytic solution (one mol of (C ₂ H ₅ ) ₃ (CH ₃ ) NBF ₄ dissolved in 1 liter of propylene carbonate) It enclosed in the coin cell made from SUS316 of diameter 20mm. In this case, a current collector carbon coating applied to the surface of a 20 μm thick aluminum foil was used as a current collector, and the paint side was sandwiched between the carbon electrode and the cell so as to face the carbon electrode.
The electric double layer capacitor cell thus produced was left in a high temperature bath at 60 ° C. for 200 hours while applying a voltage of 2.7V. The capacitance of the cell after being left was 93% of the capacitance of the cell before being left.

[Comparative Example 1]
An electric double layer capacitor was produced in the same manner as in Example 1 except that activated carbon A was used instead of activated carbon B in Example 1.
The electric double layer capacitor cell thus produced was left in a high temperature bath at 60 ° C. for 200 hours while applying a voltage of 2.7V. The capacitance of the cell after being left was 75% of the capacitance of the cell before being left.

Claims (4)

  The activated carbon obtained by activating the graphitizable carbon with an alkali metal hydroxide is further heated at 200 to 800 ° C. for 0.5 to 2 hours in an inert gas atmosphere having an oxygen concentration of 10 to 2000 ppm by volume. A method for producing activated carbon, comprising dry-treating to produce activated carbon having a total amount of acidic groups per unit weight of 0.3 to 1.2 mmol / g.   The method for producing activated carbon according to claim 1, wherein the graphitizable carbon is petroleum coke. The method for producing activated carbon according to claim 1, wherein the BET specific surface area is 500 m ² / g or more. The capacitance after applying 2.7 V and 200 hours at a temperature of 60 ° C. shows a value of 90% or more with respect to the capacitance before application. Manufacturing method of activated carbon.