JP2006012938A - Carbon material for electric double-layer capacitor electrode and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbon material for electric double-layer capacitors for improving energy density by increasing capacitance density, and to provide a method for manufacturing the carbon material. <P>SOLUTION: The carbon material for electric double-layer capacitor electrodes is obtained by heat-treating coke at a temperature of 550 - 1,000°C in atmosphere containing organic gas or ammonium gas before activation treatment. In the manufacturing method of the carbon material, the coke is heat-treated at a temperature of 550 - 1,000°C in atmosphere containing organic gas or ammonium gas before activation treatment. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a carbon material for an electric double layer capacitor electrode having a high capacitance density and a method for producing the same.

  In recent years, with the growing awareness of resource saving and environmental issues, the development of power storage systems is progressing rapidly. Examples of the electricity storage device include various secondary batteries. Among them, the electric double layer capacitor has excellent features such as rapid charging / discharging, high output density, no chemical reaction, and long life with little deterioration due to charging / discharging. Development of applications such as power supplies for electric power, memory backup power supplies for electronic information devices, nighttime power storage, solar system power storage, emergency power supplies, and auxiliary power supplies is expected.

  The electric double layer capacitor uses an electric double layer generated at the interface between the polarizable electrode and the electrolyte, and it is the polarizable electrode that determines basic performance such as energy density. This polarizable electrode must be electrically and chemically stable. Moreover, in order to obtain many electric double layers and to obtain a high energy density, it is necessary that there are many voids of appropriate pores for holding the electrolyte. For this reason, in general, polarizable electrodes are often mainly composed of activated carbon having a high specific surface area. Examples of the activated carbon include coconut shell, phenol resin, coke and the like.

  In particular, in recent years, many methods have been reported in which microcrystalline carbon having a layered crystal structure similar to graphite made of raw materials such as coke is activated with an alkali metal hydroxide, and the resulting carbon material is used as the main material of a polarizable electrode. And carbonization and activation treatment methods related to this are disclosed (for example, see Patent Documents 1 to 8).

  Alkali hydroxide activated charcoal of microcrystalline carbon (hereinafter referred to as graphitizable carbon raw material carbon) having a layered crystal structure similar to graphite derived from raw materials such as coke is the above-mentioned coconut shell raw material and phenol resin raw material carbon. Although the specific surface area is often smaller than activated carbon made of a material, it is known that a higher capacitance density can be obtained when used as a polarizable electrode of an electric double layer capacitor.

  Unlike the activated carbon obtained from the carbon material of the coconut shell raw material and the phenol resin raw material, the alkali metal hydroxide activated charcoal of the graphitizable carbon raw material coal does not necessarily exhibit a high capacitance density, unlike the activated carbon obtained from the carbon material of the phenolic resin raw material. Even if the specific surface area is small, if there is a voltage history in which the electrolyte can be sufficiently intercalated between the graphite layers, a high capacitance density is exhibited. However, the electric double layer capacitor using the alkali metal hydroxide activated carbon has a drawback in that the capacitance density is greatly reduced during repeated use.

Alkali metal hydroxide activated charcoal can suppress a decrease in capacitance density by heat treatment, and the method has already been disclosed (for example, see Patent Documents 6 and 9). Patent Document 8 describes a method of heat-treating activated charcoal at 200 to 850 ° C. in a reducing gas atmosphere in the presence of a transition metal catalyst, but this method suppresses a decrease in capacitance density. Although it is possible, there is a problem that the capacitance density is not improved.
JP-A-5-258996 JP-A-10-1997767 Japanese Patent Laid-Open No. 11-135380 JP-A-11-222732 Japanese Patent Laid-Open No. 2002-15958 Japanese Patent Laid-Open No. 2002-25867 JP 2003-206121 A Republished No. 00-11688 JP 2002-362912 A

  The present invention provides a carbon material for an electric double layer capacitor electrode that overcomes the above problems and increases the energy density by increasing the capacitance density.

  As a result of intensive studies, the present inventors have found that a carbon material for an electric double layer capacitor electrode giving a high energy density can be obtained by heat-treating coke under specific conditions before the activation treatment. The invention has been completed.

That is, the present invention is as follows.
(1) A carbon material for an electric double layer capacitor electrode obtained by subjecting coke to heat treatment in an atmosphere containing organic gas or ammonia gas at a temperature of 550 to 1000 ° C. and then activation treatment.
(2) A method for producing a carbon material for an electric double layer capacitor electrode, wherein coke is heat-treated at a temperature of 550 to 1000 ° C. in an atmosphere containing an organic gas or ammonia gas, and then activated.
(3) The method for producing a carbon material for an electric double layer capacitor electrode according to the above (2), wherein the organic gas is a lower alcohol gas or an organic acid ester gas of a lower alcohol.
(4) The method for producing a carbon material for an electric double layer capacitor electrode according to the above (2), wherein the organic gas is methanol, ethanol, propanol, methyl formate, ethyl formate, or methyl acetate.
(5) The method for producing a carbon material for an electric double layer capacitor electrode according to the above (2), wherein the organic gas is brought into contact with a metal containing nickel, iron or cobalt.
(6) The method for producing a carbon material for an electric double layer capacitor electrode according to the above (2), wherein a metal containing nickel, iron, or cobalt is used for a heat treatment vessel or furnace material.

  The carbon material for an electric double layer capacitor electrode of the present invention can provide a high capacitance density and improve the energy density. Moreover, the production of the carbon material for an electric double layer capacitor electrode of the present invention is simple, and the carbon material can be produced at a low cost. Therefore, its industrial significance is extremely great.

  As the raw material coke, petroleum coke and coal coke are used. The coke used in the present invention is produced from heavy petroleum oil or heavy coal oil, and examples include needle coke, semi-coke, pitch coke, foundry coke, blast furnace coke, and gasification coke. . These can be used as they are, but they may be used after heat treatment at a temperature of 550 to 950 ° C. for 0.5 to 10 hours.

  In addition, the coke used for activation in the present invention may be a coke obtained by heat treatment of petroleum pitch, coal pitch, and synthetic pitch as starting materials. In this case, it is obtained from a step of removing volatile components, a calcining step of developing a microcrystalline structure by heat-treating it at a higher temperature, and a continuous step thereof. These steps are generally performed in an inert gas atmosphere. The step of removing volatile components is performed at 550 ° C. or lower, but the temperature and time are not particularly limited. The calcination step is performed at 550 to 950 ° C. for 0.5 to 10 hours, preferably at 600 to 850 ° C. for 1 to 5 hours. Moreover, these two processes can also be performed continuously. The shape of the raw material before these steps is not particularly limited.

  As described above, the coke used in the present invention includes petroleum-based coke, coal-based coke, or synthetic-type coke, and is not particularly limited. Since it is excellent in chemical purity and quality stability as compared with coal-based coke, it is preferably used. As the synthetic pitch, those obtained by polymerizing condensed polycyclic hydrocarbons or substances containing them in the presence of hydrogen fluoride and boron trifluoride are preferably used. Such synthetic pitches include naphthalene, monomethylnaphthalene, dimethylnaphthalene, anthracene, phenanthrene, acenaphthene, pyrene, and the like, as shown in Japanese Patent Nos. It is obtained by polymerizing a condensed polycyclic hydrocarbon having a skeleton of the above, a mixture thereof or a substance containing these.

  Heat treatment in an atmosphere containing an organic gas or ammonia gas is performed at a temperature of 550 to 1000 ° C. for 0.5 to 10 hours. Preferably, it is performed at a temperature of 600 to 850 ° C. for 1 to 5 hours. When the heat treatment temperature is lower than 550 ° C., the microcrystalline structure on which the alkali metal acts is insufficiently developed in the activation with the alkali metal hydroxide in the next step. On the other hand, when the heat treatment temperature exceeds 1000 ° C., graphitization progresses due to the development of the microcrystalline structure, and on the contrary, the alkali metal becomes difficult to act and activation becomes difficult.

The organic gas is not particularly limited, and examples thereof include lower alcohols and organic acid esters of lower alcohols. Examples of the lower alcohol include methanol, ethanol, propanol and the like. Examples of organic acid esters of lower alcohols include methyl formate, ethyl formate, and methyl acetate. These organic gases may be combined, or may be diluted with an inert gas such as argon or nitrogen. The concentration of the gas is 0.1 to 100 vol%, preferably 20 to 100 vol%. The flow rate (GHSV) of the organic gas is 100 to 100,000 hr ⁻¹ , and preferably 500 to 5000 hr ⁻¹ .

The ammonia gas may be diluted with an inert gas such as argon or nitrogen. The concentration of the gas is 0.1 to 100 vol%, preferably 20 to 100 vol%. Ammonia gas flow rate (GHSV) is 100～100000Hr ^-1, preferably from 500~5000hr ^-1.

  Moreover, it can carry out by making organic gas contact with the metal containing nickel, iron, or cobalt with coke in heat processing. This is because the catalytic action of the metal component causes the organic gas to come into contact with these metals to promote decomposition, the highly reactive cracked gas reacts with the coke, or the carbon component generated during the decomposition process is the coke surface. This is due to the fact that it has grown rapidly. In practice, it is desirable that the furnace material that comes into contact with the coke during heat treatment, or the container in which the coke is contained, contain these metals.

  The heat-treated coke thus obtained (hereinafter sometimes referred to as calcined charcoal) is given performance as a carbon material for an electric double layer capacitor electrode in the next activation treatment step. The calcined charcoal can be activated using an alkali metal hydroxide. The activation with the alkali metal hydroxide is one in which the alkali metal erodes the microcrystalline structure in the calcined coal or acts between layers of the microcrystalline structure. The activated charcoal obtained in this way forms voids with appropriate pores for holding the electrolyte, or forms voids easily by intercalation of the electrolyte during charging and discharging, and the electric double layer capacitor electrode carbon Appropriate performance is imparted to the material.

  The activation treatment with the alkali metal hydroxide is performed by mixing the alkali metal hydroxide with calcined charcoal and heating it in an inert gas. Here, potassium hydroxide is used as the alkali metal hydroxide, but other alkali metal hydroxides or mixtures thereof may be used. Moreover, although the shape of the calcined charcoal before activation treatment is preferably fine powder, the particle size is not particularly limited.

  In the case of potassium hydroxide, the mixing ratio of the alkali metal hydroxide and calcined charcoal is 1 to 4 parts by weight, preferably 2 parts by weight with respect to 1 part by weight of calcined charcoal. The activation treatment is performed at a temperature of 550 to 900 ° C. for 0.5 to 10 hours, and preferably at a temperature of 650 to 750 ° C. for 1 to 3 hours.

  The obtained activated charcoal is washed to remove the alkali metal hydroxide component. The washing method is not particularly limited, but in general, water washing, steam washing, dilute hydrochloric acid washing, or a combination of these washings can be used. The cleaning must be performed as much as possible until the alkali metal hydroxide component and the acid used for the cleaning are not eluted. If these components remain, it is said that the long-term performance of the capacitor is adversely affected. The obtained activated charcoal is heat-dried, but is preferably dried in an inert gas or vacuum-dried in order to suppress oxidation during heating.

  The obtained activated charcoal is greatly affected by the raw material, the calcining temperature, the activation temperature, and the treatment time thereof, and the specific surface area shows a wide range of about several tens to 1000 m <2> / g depending on the conditions. In general, when the calcination temperature and the activation temperature are high, or when the treatment time is long, the interlayer distance of the graphite-like microcrystals due to the tightening tends to decrease, and the specific surface area tends to decrease. Moreover, about 0.01-5 wt% potassium remains after the said washing | cleaning. It is said that the higher the heat treatment temperature and the activation temperature, the higher the residual potassium concentration. When this residual potassium concentration is high, the long-term performance of the capacitor is adversely affected. Here, although it does not limit about the specific surface area and potassium concentration of activated carbon, Preferably, a specific surface area is 300-800 m <2> / g, and potassium concentration is 0.05 wt% or less.

  A polarizable electrode is produced using the activated charcoal thus obtained, but the method is not particularly limited. For example, a method in which activated charcoal powder, a conductive agent such as carbon black, and a binder such as Teflon (registered trademark) are mixed and molded, and activated carbon and a conductive agent are molded with resin, pitch, etc. and then fired to obtain a high density component. A method of manufacturing a polar electrode can be employed. Further, in order to increase the capacitance per volume, the packing density can be increased by a pressure press or the like.

The electrolyte can be used by dissolving in a non-aqueous solvent (hereinafter, this solution is referred to as an electrolytic solution).
Electrolyte solution is not particularly limited, but electrolytes such as tetraalkylammonium, tetraalkylphosphonium, imidazolium quaternary ammonium borofluoride, phosphorous fluoride, trifluoromethanesulfonylimide and the like are non-aqueous electrolyte propylene. It can be used by dissolving in carbonate, acetonitrile, sulfolane and the like.

Hereinafter, the present invention will be described in detail with reference to examples. In addition, this invention is not limited to the following Example.
The manufacturing method of the polarizable electrode and the measuring method of capacitance density in the examples were performed by the following methods.
(I) Method for producing polarizable electrode After kneading a mixture consisting of 100 parts by weight of activated charcoal powder, 10 parts by weight of carbon black and 10 parts by weight of polytetrafluoroethylene, a pressure sheet was formed. The obtained sheet was punched into a disk shape to obtain a polarizable electrode (diameter 16 mm, thickness 0.55 mm), and vacuum dried at 220 ° C. for 12 hours to obtain an electrode.
(II) Method for Measuring Capacitance Density The electrodes were placed opposite each other via a polyethylene separator and housed in a stainless steel case. Then, the electrolytic solution was impregnated and contained under reduced pressure to obtain an electric double layer capacitor cell. As the electrolytic solution, a 1.8 mol / l triethylmethylammonium tetrafluoroborate propylene carbonate solution was used.
The obtained electric double layer capacitor cell was charged at 25 ° C. with a constant current of 5 mA up to 2.7 V and charged for 100 minutes, then discharged to 0 V with a constant current of 5 mA and charged and discharged for 10 cycles, Next, charge and discharge were repeated for 30 cycles with an applied voltage of 3.5V. The capacitance density (F / cc) was determined from the energy during discharge.
That is, the capacitance density (F / cc) is obtained by adding the capacitance C (F) to the capacitance C (F) obtained by the equation of capacitance C (F) = 2 × U × 3600 / (V1 × V1). Was calculated by dividing. Here, U (Wh) is a value obtained by integrating the product of the discharge voltage (V) and the discharge current (A) from the start of discharge to the end of discharge, and V1 is the charge voltage (V). It is.

Example 1
Synthetic mesophase pitch obtained by catalytic polymerization of naphthalene in the presence of hydrogen fluoride and boron trifluoride (softening point by elevated flow tester method: 235 ° C., H / C atomic ratio: 0.65, optical Anisotropy content: 100%) was kept at 550 ° C. for 2 hours under a nitrogen stream to be coked. After cooling to room temperature, it was pulverized to a mean particle size of 30 μm or less by a ball mill. Furthermore, this was heat-treated in a nickel boat under a mixed gas atmosphere of ammonia / nitrogen (50 vol% each) (GHSV: 2000 hr ⁻¹ ) at 750 ° C. for 4 hours. Then, it cooled to room temperature and obtained calcined charcoal 50g. 20 g of potassium hydroxide (special grade reagent) was uniformly mixed in 10 g of this calcined charcoal in a ceramic container, and activated at 700 ° C. for 2 hours under a nitrogen stream. After cooling to 100 ° C., the activated material was sufficiently wetted by flowing steam, and then cooled to room temperature and taken out. This activated product was repeatedly subjected to ultrasonic water washing (10 minutes) and suction filtration with 100 parts by weight of water. This was dried at 100 ° C. for 2 hours and further vacuum dried at 220 ° C. for 5 hours to obtain a carbon material for a polarizable electrode.

Example 2
A carbon material for a polarizable electrode was obtained in the same manner as in Example 1 except that the atmosphere gas in the heat treatment step was changed to a methanol / nitrogen (50 vol% each) mixed gas.

Example 3
A carbon material for a polarizable electrode was obtained in the same manner as in Example 1 except that the atmosphere gas in the heat treatment step was changed to a mixed gas of methyl formate / nitrogen (50 vol% each).

Example 4
The raw material was petroleum-based needle coke (manufactured by Koa Oil Co., Ltd., H / C atomic ratio: 0.38). A carbon material for a polar electrode was obtained.

Example 5
A carbon material for a polarizable electrode was obtained in the same manner as in Example 2 except that the container for the heat treatment step was changed from a nickel container to a ceramic container.

Comparative Example 1
A carbon material for a polarizable electrode was obtained in the same manner as in Example 1 except that the atmosphere gas in the heat treatment step was changed to nitrogen gas.

Comparative Example 2
The raw material was petroleum needle coke (manufactured by Koa Oil Co., Ltd., H / C atomic ratio: 0.38), which was held at 550 ° C. for 2 hours under a nitrogen stream, and then the atmospheric gas was changed to nitrogen gas. The same operation as 1 was performed to obtain a carbon material for a polarizable electrode.

The results are shown in Table 1.
(Examples 1-3)
The potassium hydroxide activated charcoal of calcined charcoal obtained by heat treating synthetic mesophase pitch-derived coke in an ammonia gas atmosphere or an organic gas atmosphere of methanol gas or methyl formate gas is the nitrogen gas atmosphere of Comparative Example 1. Compared with activated charcoal when heat-treated in the medium, all showed high capacitance density.
Example 4
The calcined charcoal potassium hydroxide activated charcoal obtained by heat treating petroleum-based needle coke in an ammonia gas atmosphere has a higher electrostatic capacity than the activated charcoal when heat treated in the nitrogen gas atmosphere of Comparative Example 2. The capacity density is shown.
(Example 5)
The alkali metal hydroxide activated carbon of calcined charcoal obtained by performing heat treatment in a methanol gas atmosphere in a ceramic container is compared with the activated charcoal obtained through heat treatment in a nitrogen gas atmosphere of Comparative Example 1. Although it showed a high capacitance density, it showed a low value as compared with Example 2 under the same conditions carried out in a nickel container.

Claims (6)

  A carbon material for an electric double layer capacitor electrode obtained by subjecting coke to heat treatment at a temperature of 550 to 1000 ° C. in an atmosphere containing organic gas or ammonia gas, and then activation treatment.   A method for producing a carbon material for an electric double layer capacitor electrode, wherein coke is heat-treated at a temperature of 550 to 1000 ° C. in an atmosphere containing an organic gas or ammonia gas and then activated.   The method for producing a carbon material for an electric double layer capacitor electrode according to claim 2, wherein the organic gas is a lower alcohol gas or an organic acid ester gas of a lower alcohol.   The method for producing a carbon material for an electric double layer capacitor electrode according to claim 2, wherein the organic gas is methanol, ethanol, propanol, methyl formate, ethyl formate, or methyl acetate.   The method for producing a carbon material for an electric double layer capacitor electrode according to claim 2, wherein the organic gas is brought into contact with a metal containing nickel, iron or cobalt.   The manufacturing method of the carbon material for electric double layer capacitor electrodes of Claim 2 using the metal containing nickel, iron, or cobalt for the container or furnace material for heat processing.