JP2006024747A - Carbon material for electric double-layer capacitor electrode, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive carbon material for electric double-layer capacitor electrode in which capacitance sustention rate does not lower significantly, using a simple method. <P>SOLUTION: The carbon material for electric double-layer capacitor electrode is produced, by activating coke using an alkaline metal hydroxide and then heat treating it at a temperature of 900-1,100°C under an inert gas or reduced gas atmosphere. <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, 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 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 (for example, see Patent Documents 1 to 8).

  Activated charcoal with alkali metal hydroxide of microcrystalline carbon (hereinafter referred to as graphitizable carbon raw material carbon) having a layered crystal structure similar to graphite obtained from raw materials such as pitch is the above-mentioned coconut shell raw material, phenol resin raw material. It is known that the specific surface area is often smaller than that of the carbon steam activated carbon obtained from the above, but when used as a polarizable electrode of an electric double layer capacitor, a higher capacitance density can be obtained.

  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-functional functional group of the alkali metal hydroxide activated charcoal can be removed and removed by heat treatment in the form of CO ₂ , H ₂ O, CO, etc., and the method has already been disclosed. (For example, see Patent Documents 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 inventors of the present invention can obtain a carbon material that provides an electric double layer capacitor with little decrease in capacitance retention by heat-treating alkali metal hydroxide activated carbon under specific conditions. And found the present invention. That is, the present invention is as follows.
(1) A carbon material for an electric double layer capacitor electrode obtained by activating coke with an alkali metal hydroxide and then heat-treating the coke at a temperature of 900 to 1100 ° C. in an inert gas or reducing gas atmosphere.
(2) The carbon material for an electric double layer capacitor electrode according to the above (1), wherein the oxygen content in the carbon material after the heat treatment is 0.05 wt% or less.
(3) The carbon material for an electric double layer capacitor electrode according to the above (1) or (2), wherein the alkali metal content in the carbon material after the heat treatment is 0.05 wt% or less.
(4) A method for producing a carbon material for an electric double layer capacitor electrode, wherein coke is activated with an alkali metal hydroxide and then heat-treated at 900 to 1100 ° C. in an atmosphere of an inert gas or a reducing gas.
(5) The method for producing a carbon material for an electric double layer capacitor electrode according to the above (4), wherein the inert gas or reducing gas is nitrogen gas, hydrogen gas or ammonia gas.
(6) The method for producing a carbon material for an electric double layer capacitor electrode according to the above (4) or (5), wherein the oxygen content in the carbon material after the heat treatment is 0.05 wt% or less.
(7) The method for producing a carbon material for an electric double layer capacitor electrode according to any one of the above (4) to (6), wherein the alkali metal content in the carbon material after the heat treatment is 0.05 wt% or less.
(8) The above (4), 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. )-(7) The manufacturing method of the carbon material for electric double layer capacitor electrodes in any one of.

  The carbon material for an electric double layer capacitor electrode of the present invention provides a high capacitance retention. Further, the method for producing the carbon material is simple and the carbon material can be produced at low cost.

  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 activated using 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.

  Examples of the alkali metal hydroxide include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Here, potassium hydroxide is used, but a mixture thereof may be used. 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 activated charcoal thus obtained 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 specific surface area of the activated charcoal obtained is in a wide range of several 10 to 1000 m ² / g, and is greatly affected by the raw material, calcining temperature, activation temperature, and time thereof. In general, the higher the calcination temperature and the activation temperature, and the longer the time, the smaller the specific surface area. The specific surface area of the activated charcoal is preferably 300 to 800 m ² / g.

When the atmosphere gas during the heat treatment performed after the activation treatment is an inert gas such as nitrogen gas or argon gas, the treatment is performed at 850 to 1100 ° C. for 0.5 to 10 hours, preferably 850 to 950 ° C., 0.5 to 2 hours. The concentration of the gas is 0.1 to 100 vol%, preferably 20 to 100 vol%. The gas flow rate (GHSV) is 100~100000hr ^-1, preferably 500~5000hr ^-1.

When the atmospheric gas during the heat treatment is a reducing gas such as hydrogen gas or ammonia gas, the temperature is 900 to 1100 ° C. and 0.5 to 10 hours, preferably 900 to 1000 ° C. and 0.5 to 2 hours. It is. The concentration of the gas is 0.1 to 100 vol%, preferably 20 to 100 vol%. The gas flow rate (GHSV) is 100~100000hr ^-1, preferably 500~5000hr ^-1.

  The degree of removal of the hetero-containing functional group can be confirmed by measuring the oxygen concentration by elemental analysis. The sample for this measurement must be vacuum-dried at 220 ° C. for 5 hours or more immediately before being subjected to the analyzer, and the measurement operation must be performed while eliminating the influence of adsorbed moisture as much as possible. The measurement method for elemental analysis is not particularly limited as long as it is a method for directly quantifying oxygen. The oxygen content in the carbon material after the heat treatment is preferably 0.05 wt% or less.

  Moreover, after performing the said activation process and wash | cleaning, 0.01-5 wt% potassium remains. The higher the calcining temperature and the activation temperature, the higher the residual amount of potassium. When the residual amount of potassium is high, the capacitor performance is deteriorated. The potassium content in the carbon material after the heat treatment is preferably 0.05 wt% or less. The measurement of the amount of residual potassium is not particularly limited as long as it is a method in which carbon is completely decomposed by a method in which potassium does not lose and potassium is directly measured. As the decomposition method, dry ashing or wet decomposition with an oxidizing agent is preferable. As a measuring method, ICP analysis and atomic absorption analysis are desirable.

  The production of the polarizable electrode is not particularly limited, but for example, a method of blending and molding a carbon material powder and a conductive agent such as carbon black and a binder such as Teflon (registered trademark), the carbon material and the conductive agent may be resin or A known method such as a method of producing a high-density polarizable electrode after forming with a pitch or the like 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 production method of the polarizable electrode and the measurement method of the capacitance retention in the examples were performed by the following methods.
(I) Method for Producing Polarizable Electrode A mixture of 100 parts by weight of the obtained carbon material powder, 10 parts by weight of carbon black, and 10 parts by weight of polytetrafluoroethylene was kneaded and then formed into a pressure sheet. 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 Retention Rate The electrodes were opposed to each other via a polyethylene separator and housed in a stainless steel case. Then, the electrolytic solution was impregnated under reduced pressure to obtain a sealed electric double layer capacitor cell. As the electrolytic solution, a 1.8 mol / L triethylmethylammonium tetrafluoroborate propylene carbonate solution was used.
The initial charge at 25 ° C. was applied at a constant current of 5 mA to a voltage of 3.2 V, charged for 100 minutes, and discharged to 0 V at a constant current of 5 mA. Thereafter, 10 cycles of charge and discharge were similarly performed at an applied voltage of 3.0 V. Thereafter, the temperature was raised to 70 ° C., and charging and discharging were repeated for 280 hours by charging for 300 minutes, and the capacitance density (F / cc) per electrode volume in the first and 280th cycles was determined, and the static charge in the first cycle was The capacitance retention rate (%) at the 280th cycle was determined by setting the capacitance density to 100%. 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
Synthetic mesophase pitch obtained by catalytic polymerization of naphthalene in the presence of hydrogen fluoride and boron trifluoride (softening point by elevated flow tester: 235 ° C., H / C atomic ratio: 0.65, optically different (Isotropic content: 100%) was held at 550 ° C. for 2 hours under a nitrogen stream to remove volatile components, cooled to room temperature, and then ground to an average particle size of 30 μm or less by a ball mill. Furthermore, this was kept at 750 ° C. for 4 hours under a nitrogen stream and cooled to room temperature to obtain coke. With respect to 1 part by weight of the coke, 2 parts by weight of potassium hydroxide (special grade reagent) was uniformly mixed in a nickel container and kept at 700 ° C. for 2 hours under a nitrogen stream for activation treatment. 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 activated charcoal.
The activated charcoal was heat-treated in a nitrogen gas atmosphere (GHSV: 2000 hr ⁻¹ ): 1) 700 ° C., 2 hours 2) 800 ° C., 2 hours 3) 850 ° C., 2 hours 4) 900 ° C., 2 hours 5) 950 2 hours, 6) 1000 ° C., 2 hours, 7) 1050 ° C., 2 hours, 8) 1100 ° C., 2 hours.

Example 2
A carbon material for a polarizable electrode was obtained in the same manner as in Example 1 except that the atmosphere gas when heat-treating the activated charcoal was changed to a mixed gas of hydrogen / nitrogen (50 vol% each).

Example 3
A carbon material for a polarizable electrode was obtained in the same manner as in Example 1 except that the atmosphere gas when heat-treating the activated charcoal was changed to a mixed gas of ammonia / nitrogen (50 vol% each).

Example 4
The raw material was petroleum needle coke (manufactured by Koa Oil Co., Ltd., H / C atomic ratio: 0.38), which was kept at 550 ° C. for 2 hours under a nitrogen stream and cooled to room temperature. Crushed. Furthermore, this was hold | maintained at 750 degreeC under nitrogen stream for 4 hours. Thereafter, the same operation as in Example 1 was performed to obtain a carbon material for a polarizable electrode.

Example 5
A carbon material for a polarizable electrode was obtained in the same manner as in Example 4 except that the atmosphere gas when heat-treating the activated charcoal was changed to a mixed gas of hydrogen / nitrogen (50 vol% each).

Comparative Example 1
The activated charcoal obtained in the same manner as in Example 1 was used as a polarizable electrode without heat treatment.

Comparative Example 2
The activated charcoal obtained in the same manner as in Example 4 was used as a polarizable electrode without heat treatment.

Comparative Example 3
A carbon material for a polarizable electrode was obtained in the same manner as in Example 1 except that the step of obtaining coke was changed to hold at 1050 ° C. for 4 hours, and the heat treatment of the activated charcoal was only 900 ° C. and 2 hours. .

The carbon materials for polarizable electrodes obtained in the examples and comparative examples were evaluated for the performance of electric double layer capacitors in accordance with (I) a method for producing polarizable electrodes and (II) a method for measuring capacitance retention. . The results are shown in Table 1.
In Example 1, in the heat treatment under a nitrogen stream using activated charcoal derived from the synthetic mesophase pitch, the capacitance retention was improved at 800 ° C. or higher as compared with the case of heat untreated in Comparative Example 1. In addition, in the case of heat treatment in Comparative Example 1, the oxygen content in the carbon material is 2.30%, whereas in Example 1, it is 850 ° C. or more and 0.05 wt% or less (below the detection lower limit value). Met.
In Example 2, in the heat treatment under a hydrogen / nitrogen stream using activated charcoal derived from the synthetic mesophase pitch, the capacitance retention was improved at 700 ° C. or higher as compared with the case of heat untreated in Comparative Example 1. . In contrast, the heat-untreated oxygen concentration in Comparative Example 1 was 2.30%, whereas in Example 2, it was 700 ° C. or higher and 0.05 wt% or lower (lower than the detection lower limit value).
In Example 3, in the heat treatment under an ammonia / nitrogen stream using activated charcoal derived from the synthetic mesophase pitch, the capacitance retention was improved at 900 ° C. or higher as compared with the case of heat untreated in Comparative Example 1. . In contrast, the oxygen concentration in the case of heat treatment in Comparative Example 1 was 2.30%, whereas in Example 3, it was 700 ° C. or higher and 0.05 wt% or lower (lower than the detection lower limit value).
In Example 4, in the heat treatment under a nitrogen stream using activated carbon derived from petroleum needle coke, the capacitance retention was improved at 800 ° C. or higher as compared to the case of heat untreated in Comparative Example 2. Moreover, in the case of the heat-unprocessed of Comparative Example 2, the oxygen content in the carbon material is 2.71%, whereas in Example 4, it is 900 ° C. or higher and 0.05 wt% or lower (lower than the detection lower limit). Met.
In Example 5, in heat treatment under a hydrogen / nitrogen stream using activated carbon derived from petroleum-based needle coke, the capacitance retention was improved at 700 ° C. or higher as compared with the case of heat untreated in Comparative Example 2. . Further, in the case of the heat-untreated sample of Comparative Example 2, the oxygen content in the carbon material was 2.71%, whereas it was not lower than 900 ° C. and not higher than 0.05 wt% (below the lower limit of detection).
In Comparative Example 3, heat treatment for 2 hours at 900 ° C. under a nitrogen stream using activated charcoal derived from synthetic mesophase pitch (held at 1050 ° C. for 4 hours in the process of obtaining coke) has a potassium content of 1 in the carbon material. .62%. On the other hand, the potassium content of Example 1, which was carried out in the same manner as in Comparative Example 3, except that the coke was obtained at 750 ° C. for 4 hours, was 0.03%. Further, the capacitance retention rate was 86.0% in Example 1, whereas it was reduced to 71.2% in Comparative Example 3.

Claims (8)

  A carbon material for an electric double layer capacitor electrode obtained by subjecting coke to an activation treatment using an alkali metal hydroxide and then a heat treatment at a temperature of 900 to 1100 ° C. in an atmosphere of an inert gas or a reducing gas.   The carbon material for an electric double layer capacitor electrode according to claim 1, wherein the oxygen content in the carbon material after the heat treatment is 0.05 wt% or less.   The carbon material for an electric double layer capacitor electrode according to claim 1 or 2, wherein the alkali metal content in the carbon material after the heat treatment is 0.05 wt% or less.   A method for producing a carbon material for an electric double layer capacitor electrode, wherein coke is activated using an alkali metal hydroxide and then heat treated at a temperature of 900 to 1100 ° C. in an atmosphere of an inert gas or a reducing gas.   The method for producing a carbon material for an electric double layer capacitor electrode according to claim 4, wherein the inert gas or the reducing gas is nitrogen gas, argon gas, hydrogen gas or ammonia gas.   6. The method for producing a carbon material for an electric double layer capacitor electrode according to claim 4, wherein the oxygen content in the carbon material after the heat treatment is 0.05 wt% or less.   The method for producing a carbon material for an electric double layer capacitor electrode according to any one of claims 4 to 6, wherein the alkali metal content in the carbon material after the heat treatment is 0.05 wt% or less.   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. The manufacturing method of the carbon material for electric double layer capacitor electrodes in any one.