JP2006012939A - 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 capacitor electrodes with a small amount of decrease in capacitance retention, and also to provide a method for manufacturing the carbon material. <P>SOLUTION: The carbon material for electric double-layer capacitor electrodes is obtained by activating coke in atmosphere containing organic gas (low-class alcohol gas or low-class alcohol organic acid ester gas). In the manufacturing method of the carbon material, the coke is activated in atmosphere containing organic gas (low-class alcohol gas or low-class alcohol organic acid ester gas). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a carbon material for an electric double layer capacitor electrode and a method for producing the same, in which a decrease in capacitance density is small during long-term use.

  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 memory backup power supply for electronic information devices, nighttime power storage, solar system power storage, emergency power supply, auxiliary power supply is expected in the future.

  The electric double layer capacitor uses an electric double layer generated at the interface between the polarizable electrode and the electrolyte, and one of the basic properties such as energy density is determined by the polarizable electrode. This polarizable electrode is electrically and chemically stable, and has many pores in the appropriate pores that hold the electrolyte in order to generate many electric double layers and to obtain a high energy density. Was needed. 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, pitch, and the like. In particular, many methods have recently been reported in which microcrystalline carbon having a layered crystal structure similar to graphite made of raw materials such as pitch is activated with an alkali metal hydroxide and the resulting carbon material is used as the main material of a polarizable electrode. The carbonization and activation treatment methods are disclosed (see Patent Documents 1 to 8).

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

  However, the electric double layer capacitor using alkali metal hydroxide activated carbon obtained from graphitizable carbon raw material coal has a capacitance density at the time of repeated use as compared with the case using palm shell raw material and phenol resin raw material. It was a drawback that the decrease was large. As one of the causes of the decrease, alkali metal hydroxide activated charcoal has many hetero-functional functional groups such as COOH, CHO, OH, etc., and these functional groups and the electrolyte react chemically, and the gas and reaction generated at this time It is presumed that the product causes various problems such as blocking the voids of the carbon material, and as a result, the capacitance density during repeated use is lowered (decrease in capacitance retention).

The hetero-element functional group of the alkali metal hydroxide activated carbon can be removed by removing it in the form of CO2, H2O, CO, etc. by heat treatment, and the method has already been disclosed (Patent Document). 9 and 10). Patent Document 9 describes a method of desorbing a heteroelement-containing functional group by heat treatment in the presence of a transition metal catalyst in the presence of a transition metal catalyst in a hydrogen gas or ammonia gas stream. However, this method requires a complicated process of separating the catalyst after the heat treatment, and a simple method for producing a carbon material for an electric double layer capacitor that improves the capacitance retention has been desired.
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 Japanese Patent Laid-Open No. 2002-25867 JP 2002-362912 A

  A carbon material for an electric double layer capacitor electrode that overcomes the above-described problems and causes little reduction in capacitance retention is provided by a simple method at a low cost.

  As a result of intensive studies, the present inventors have found that a carbon material giving a high capacitance retention can be obtained by activating the coke under specific conditions, and the present invention has been completed. It was.

That is, the present invention is as follows.
(1) A carbon material for an electric double layer capacitor electrode obtained by activating coke in an atmosphere containing an organic gas.
(2) A method for producing a carbon material for an electric double layer capacitor electrode, wherein coke is activated in an atmosphere containing an organic gas.
(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 a lower alcohol organic acid ester gas.
(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 the organic gas is brought into contact with an activation treatment vessel or furnace material containing nickel, iron or cobalt.
(7) The coke obtained by heat-treating a pitch synthesized by polymerizing a condensed polycyclic hydrocarbon or a substance containing the same in the presence of hydrogen fluoride and boron trifluoride (2 The manufacturing method of the carbon material for electric double layer capacitor electrodes of description.

  The carbon material for electric double layer capacitor electrodes of the present invention provides a high capacitance retention. In addition, the method for producing the carbon material is simple and the carbon material can be produced at a low cost, so that its industrial significance is extremely great.

  Examples of the coke used in the present invention include petroleum coke and coal coke. 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.

  The coke used in the present invention may be one obtained by coking by using a petroleum-based pitch, a coal-based pitch, or a synthetic pitch as a starting material and heat-treating them. 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 coke, coal coke, or synthetic coke, and is not particularly limited, but synthetic coke obtained by heat treatment of synthetic pitch is petroleum coke. It is preferably used because it is superior in chemical purity and quality stability compared to coal-based coke.

  Further, as a synthetic pitch used as a raw material for synthetic coke, a pitch obtained by polymerizing a condensed polycyclic hydrocarbon or a substance containing the same in the presence of hydrogen fluoride and boron trifluoride is preferably used. It is done. 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.

  The above coke is given performance as an electric double layer capacitor electrode carbon material in the next activation step. The coke can be activated with an alkali metal hydroxide. In this activation treatment with an alkali metal hydroxide, the alkali metal erodes the microcrystalline structure in the coke 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.

  Here, potassium hydroxide is used as the alkali metal hydroxide, but other alkali metal hydroxides and mixtures thereof may be used and are not particularly limited. The shape of the coke before the activation treatment is preferably fine powder, but the particle size is not particularly limited. In the case of potassium hydroxide, the mixing ratio of the alkali metal hydroxide and coke is 1 to 4 parts by weight, preferably 2 parts by weight with respect to 1 part by weight of coke. 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 material in contact with the alkali metal hydroxide is preferably nickel which is corrosion resistant to the alkali metal hydroxide.

The atmosphere gas in the activation process is an organic gas. Examples of the organic gas include, but are not limited to, methanol, ethanol, propanol, methyl formate, ethyl formate, and methyl acetate. These gases 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 .

  In the activation treatment in an organic gas atmosphere, the organic gas is preferably brought into contact with a metal containing nickel, iron, or cobalt together with coke at the activation temperature. This is because the organic gas is brought into contact with these metal components, and the carbon component generated by the decomposition is effectively acting. In practice, it is desirable that the furnace material that comes into contact with the coke during activation or the container containing the coke contain these metals.

  The obtained activated charcoal is washed to remove the alkali metal hydroxide component. The cleaning method is not particularly limited, but in general, it can be performed by water cleaning, steam cleaning, dilute hydrochloric acid cleaning, or a combination of these cleanings. 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 in vacuum 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, and the specific surface area shows a wide range of 1 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. Further, the concentration of the remaining potassium after the washing is higher as the calcining temperature and the activation temperature are higher. A high concentration is said to adversely affect the long-term performance of the capacitor. Here, the specific surface area of the activated carbon is not particularly limited, but the specific surface area is preferably 50 to 800 m 2 / g.

  A polarizable electrode is produced using the activated charcoal thus obtained, but the method is not particularly limited. For example, a method of blending activated charcoal powder with a conductive agent such as carbon black and a binder such as Teflon (registered trademark) and molding, activated carbon and conductive agent are molded with resin, pitch, etc., and then fired to obtain high-density polarizability A method of manufacturing an 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 is not particularly limited, but electrolytes such as tetraalkylammonium, tetraalkylphosphonium, imidazolium quaternary ammonium borofluoride, phosphorous fluoride, trifluoromethanesulfonylimide, etc., which are non-aqueous electrolytes 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. Thereafter, the electrolytic solution was impregnated under reduced pressure to obtain a sealed electric double layer capacitor cell. As the electrolyte, a 1.8 mol / l triethylmethylammonium fluoroborate propylene carbonate solution was used.
This electric double layer capacitor cell was placed in a constant temperature bath at 60 ° C., charged to a voltage of 3.5 V at a constant current of 5 mA and charged for 5 hours, and then discharged to 0 V at a constant current of 5 mA. This cycle is repeated 20 cycles, and the electrostatic capacity density per electrode volume (F / cc) in the first cycle and the 20th cycle and the electrostatic capacity density in the first cycle are taken as 100%. Retention rate (%) was determined. The capacitance density per electrode volume is the capacitance C (F) obtained by the formula of capacitance C (F) = 2 × U × 3600 / (V1 × V1). It 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
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, optically anisotropic Volatile content was kept at 550 ° C. for 2 hours under a nitrogen stream to remove volatile components and coke. After cooling to room temperature, it was pulverized to a mean particle size of 30 μm or less by a ball mill. Furthermore, heat treatment was performed by holding at 750 ° C. for 4 hours under a nitrogen stream. After cooling to room temperature, 50 g of coke powder to be used was obtained. To 10 g of the coke powder, 20 g of potassium hydroxide (special grade reagent) is uniformly mixed in a nickel container, and mixed at 700 ° C. in a methanol / nitrogen (50 vol% each) mixed gas atmosphere (GHSV: 2000 hr ⁻¹ ). The activation treatment was carried out by holding for a time. After cooling to 100 ° C., steam was flowed to sufficiently wet the activated material, 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 activation process was changed to a mixed gas of methyl formate / nitrogen (50 vol% each).

Example 3
The raw material was petroleum-based 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 thereafter the same operation as in Example 1 was performed. A carbon material for a polar electrode was obtained.

Example 4
A carbon material for a polarizable electrode was obtained in the same manner as in Example 1 except that the container in the activation process was changed from nickel to ceramic.

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 activation process was changed to nitrogen gas.

Comparative Example 2
The raw material is petroleum-based needle coke (manufactured by Koa Oil Co., Ltd., H / C atomic ratio: 0.38), and after holding this at 550 ° C. for 2 hours under a nitrogen stream, the atmosphere gas in the activation process is changed to nitrogen gas, Thereafter, the same operation as in Example 1 was performed to obtain a carbon material for a polarizable electrode.

The results are shown in Table 1.
Example 1
The activated charcoal obtained by treating coke derived from synthetic mesophase pitch in a methanol / nitrogen mixed gas atmosphere had a capacitance retention of 100.7% and no deterioration was observed. On the other hand, the activated charcoal obtained by processing in the nitrogen atmosphere of Comparative Example 1 was found to be deteriorated with a capacitance retention of 42.5%.
(Example 2)
The activated charcoal obtained by treating coke derived from synthetic mesophase pitch in a methyl formate / nitrogen mixed gas atmosphere had a capacitance retention of 100.0% and no deterioration was observed. On the other hand, the activated charcoal obtained by processing in the nitrogen atmosphere of Comparative Example 1 was found to be deteriorated with a capacitance retention of 42.5%.
Example 3
The activated carbon obtained by treating petroleum needle coke in a methanol / nitrogen mixed gas atmosphere had a capacitance retention of 100.0% and no deterioration was observed. On the other hand, the activated carbon obtained by processing in the nitrogen atmosphere of Comparative Example 2 was found to be deteriorated with a capacitance retention of 39.7% (Example 4).
The activated charcoal obtained by processing coke derived from synthetic mesophase pitch in a ceramic container in a methanol / nitrogen mixed gas atmosphere had a capacitance retention of 82.0%. Although it improved compared with the comparative example 1, it fell compared with Example 1 which put into the nickel container and performed the activation process.

Claims (7)

  A carbon material for an electric double layer capacitor electrode obtained by activating coke in an atmosphere containing an organic gas.   A method for producing a carbon material for an electric double layer capacitor electrode, wherein coke is activated in an atmosphere containing an organic gas.   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 a lower alcohol organic acid ester gas.   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 activation processing.   3. The electricity according to claim 2, wherein the coke is obtained by heat-treating a pitch synthesized by polymerizing a condensed polycyclic hydrocarbon or a substance containing the same in the presence of hydrogen fluoride and boron trifluoride. Manufacturing method of carbon material for double layer capacitor electrode.