JP4101681B2 - Method for producing activated carbon fiber for capacitor electrode and electric double layer capacitor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing an activated carbon fiber for a capacitor electrode and an electric double layer capacitor.
[0002]
[Prior art]
In recent years, with the rapid spread of new electronic devices such as mobile phones and notebook computers, secondary batteries used as power sources are also required to have higher performance and smaller size. Further, in addition to power saving, the secondary battery is expected to be applied to a large capacity field such as an auxiliary power source of an electric vehicle. Among these, lithium ion secondary batteries have been put into practical use as secondary batteries that satisfy such demands.
[0003]
However, the lithium ion secondary battery is not fully satisfied in terms of operating temperature range, charge / discharge cycle (life), electric capacity, charge / discharge speed, and cost.
[0004]
For this reason, electric double layer capacitors have been developed. In particular, in the field of power sources for electric vehicles, it is essential to develop a high-performance electric double layer capacitor since it has a long life of 10 years or more and energy regeneration efficiency during deceleration is essential.
[0005]
The electric double layer capacitor goes back to Helmholtz's theory in 1879 and is based on the phenomenon that positive and negative charges generated at the interface when two different layers come into contact with each other are arranged at a short distance. The generated positive and negative charge distribution is called an electric double layer. The electric double layer capacitor is a large-capacity capacitor using an electric double layer formed at the interface between the polarizable electrode and the electrolytic solution, and has the following characteristics as compared with a conventional secondary battery.
[0006]
1) Since the internal resistance is lower than that of a conventional secondary battery, large current discharge is possible.
[0007]
2) Since there is no chemical reaction during charging / discharging, rapid charging / discharging with a large current is possible and energy regeneration efficiency is remarkably high.
[0008]
3) Since there is almost no deterioration of an electrode, it has a long life of several tens to several hundred times as compared with a conventional secondary battery.
[0009]
4) Shows stable charge / discharge behavior over a wide temperature range.
[0010]
5) There is no failure even when short-circuited, and there are no restrictions during charging and discharging.
[0011]
6) Environmentally friendly because it does not contain harmful heavy metals such as cadmium and lead.
[0012]
As the electrode material of the electric double layer capacitor described above, activated carbon and activated carbon fiber having a large specific surface area are considered to be optimal. This is because the electric capacity C accumulated when the electric double layer capacitor is charged is proportional to the specific surface area of the electrode.
[0013]
Conventionally, the activated carbon is manufactured by activating a non-graphitizable carbon material such as coconut shell, coal, phenol resin, that is, a raw material of hard carbon with water vapor or carbon dioxide. However, the electric double layer capacitor using the activated carbon as an electrode material cannot obtain sufficient performance.
[0014]
For this reason, Patent Document 1 discloses a method for producing high-performance activated carbon by so-called alkali activation in which a carbon material is activated in the presence of an alkali metal compound. A feature of this alkali activation is that only a non-graphitizable carbon material can be activated by conventional gas activation, whereas an easily graphitizable carbon material such as mesophase pitch can be used as a raw material as well. For this reason, attempts have been made to alkali activate carbon materials obtained by carbonizing mesophase pitch. For example, Patent Document 2 discloses a method for producing activated carbon fibers having a high specific surface area of 2000 m ² / g or more by spinning pitches containing 50% or more of mesophase components to obtain pitch fibers and activating them with alkali. Has been.
[0015]
Furthermore, in recent years, attempts have been made to use activated carbon fibers obtained by directly subjecting an easily graphitizable carbon material to an alkali activation treatment as an electrode material for an electric double layer capacitor. For example, in Patent Document 3, activated carbon fibers having a specific surface area of 3000 m ² / g or more are directly activated by alkali activation of mesophase pitch-based carbon fibers, the activated carbon fibers are decalcified with water or acids, and then the shape of the fibers is obtained. An activated carbon fiber for an electric double layer capacitor, which is pulverized and molded to such an extent that does not remain, is disclosed. When the activated carbon fiber obtained by such alkali activation treatment is used as an electrode material for an electric double layer capacitor, the activated carbon fiber produced by the conventional gas activation method has a specific surface area smaller than that of the activated carbon fiber. High charge / discharge capacity can be obtained. For this reason, it has the advantage which can improve the charge / discharge capacity per unit volume.
[0016]
[Patent Document 1]
Japanese Patent Laid-Open No. 1-133785
[Patent Document 2]
Japanese Patent Laid-Open No. 5-247731
[Patent Document 3]
JP-A-5-258996
[Problems to be solved by the invention]
However, in the conventional method for producing activated carbon fibers by the alkali activation method, the alkali activation compound is subjected to a state change in which the alkali metal compound as the activation material is once melted and solidified in the alkali activation treatment step. An operation such as sticking to the inner wall and scraping off the sticking activation treatment product from the reaction container becomes necessary, and handling becomes very difficult. As a result, in order to produce activated carbon fibers on an industrial scale by the alkali activation method, the process passability and post-treatment difficulty of the alkali activation treatment product has become a major obstacle, which has hindered mass production.
[0020]
The present invention divides the alkali activation treatment into the previous step and the present step, and improves the handleability of the solid processed material in which the carbon fiber is coated with the alkali metal compound in the previous step, whereby the alkali activation in the subsequent step is performed. An object of the present invention is to provide a method for producing activated carbon fibers for capacitor electrodes capable of carrying out the treatment by an extremely simple operation, and an electric double layer capacitor having a capacitor electrode having activated carbon fibers obtained by this method. It is.
[0021]
[Means for Solving the Problems]
Method for producing activated carbon fibers capacitor electrode according to the present invention, while forcibly stirring in an inert atmosphere in a reaction vessel equipped with a rotary shaft having a stirring blade and a carbon fiber and an alkali metal compound, 300 A step of producing a massive processed product having an average particle size of 1 to 30 mm mainly by melting, solidifying and crushing at a temperature of ˜550 ° C .;
And a step of heat-treating the treated product at 600 to 800 ° C. to subject the carbon fibers in the treated product to an alkali activation treatment.
[0022]
The electric double layer capacitor according to the present invention includes a capacitor electrode including activated carbon fibers for a capacitor electrode manufactured by the above method.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0024]
(1st process; pre-process of alkali activation treatment)
While forcibly stirring the carbon fiber and the alkali metal compound in an inert gas atmosphere, the carbon fiber and the alkali metal compound are processed at a temperature of 300 to 550 ° C. to be melted and solidified to mainly produce a lump processed product.
[0025]
As the carbon fiber, for example, those produced by 1) raw material pitch through 2) spinning step, 3) infusibilization step, 4) carbonization step, and 5) pulverization step are used.
[0026]
1) Raw material pitch Examples of the raw material pitch include petroleum-based or coal-based mesophase pitch. The mesophase content of this pitch is desirably 50% or more, preferably 80% or more, and more preferably 100%.
[0027]
2) Spinning Step In this spinning step, a general method such as a melt spinning method, a centrifugal spinning method, or a melt blow spinning method can be employed. Among these, the melt blow spinning method is preferable from the viewpoint of cost.
[0028]
3) Infusibilization step This infusibilization step is performed at 250 to 350 ° C, preferably 270 to 320 ° C in an oxidizing atmosphere (preferably an air atmosphere) such as air, nitrogen dioxide or iodine. Performed at temperature.
[0029]
4) Carbonization step This carbonization step is performed, for example, at a temperature of 500 to 800 ° C, preferably 600 to 750 ° C, more preferably 650 to 700 ° C in a nitrogen atmosphere for the carbon material after the infusibilization treatment. If the carbonization operation is performed at less than 500 ° C., oxygen, hydrogen, etc. may remain in the carbonized carbon material and the subsequent activation reaction may not proceed uniformly. On the other hand, if the carbonization operation is performed at a temperature exceeding 800 ° C., the carbonization reaction proceeds excessively, and the subsequent activation reaction may not easily proceed.
[0030]
5) Pulverization step This pulverization step is performed using a pulverizer such as a jet mill, a ball mill, a disk mill, and a high-speed rotary mill. In particular, it is preferable to use a jet mill or a high-speed rotating mill in order to hold the carbon fiber in a cylindrical shape.
[0031]
As the alkali metal compound, for example, potassium, sodium, lithium hydroxide or oxide can be used. In particular, from the viewpoint of increasing reaction efficiency, it is preferable to use potassium hydroxide or sodium hydroxide as the alkali metal compound.
[0032]
The compounding amount of the alkali metal compound is 150 to 300 parts by weight, preferably 160 to 200 parts by weight, with respect to 100 parts by weight of the carbon fiber. When the blending amount of the alkali metal compound is less than 150 parts by weight, it becomes difficult to sufficiently activate the carbon fiber. On the other hand, when the compounding amount of the alkali metal compound exceeds 300 parts by weight, the alkali activation reaction may proceed excessively.
[0033]
The melting and solidifying treatment is a pretreatment for alkali activation, and the surface of the carbon fiber is uniformly coated while dehydrating the alkali metal compound, and is mainly processed in a lump form, that is, the alkali activation treatment following this step. This process is performed in order to produce a processed product that has good transportability and is easy to handle. Here, the mainly massive form means that the majority is a massive substance and a part of the granular substance is allowed.
[0034]
From the viewpoint of improving the transportability, the processed product mainly in the form of a lump preferably has an average diameter of 1 to 30 mm, more preferably 5 to 20 mm.
[0035]
In the melting and solidifying treatment, an inert gas atmosphere is used to prevent oxidation of the carbon fiber. Examples of the inert gas include a rare gas such as nitrogen gas or argon gas.
[0036]
When the temperature during the melting and solidifying treatment is less than 300 ° C., the alkali metal compound is not sufficiently dehydrated and it becomes difficult to obtain a solidified processed product. On the other hand, when the temperature during the melting and solidifying treatment exceeds 550 ° C., an undesirable alkali activation reaction may proceed. More preferably, the temperature during the melting and solidifying treatment is 350 to 500 ° C.
[0037]
As the forced stirring treatment, any means may be adopted as long as the carbon fiber and the alkali metal compound can be stirred in a period until they are melted and solidified at the temperature. For example, the forced stirring process employs a method in which the carbon fiber and the alkali metal compound are placed in a reaction vessel having a rotating shaft having a stirring blade, and the stirring blade of the rotating shaft is rotated while being heated at the temperature. be able to. The blades attached to the rotating shaft are arbitrary such as two blades, three blades, and four blades. Moreover, you may attach a blade | wing not only to one place of a rotating shaft but two or three along the axial direction. The reaction vessel, the rotating shaft, and the blade are made of a metal having excellent corrosion resistance, for example, a nickel-based alloy such as alloy or hastelloy.
[0038]
(Second step; this step of alkali activation treatment)
The mainly lump treated product obtained in the first step is heat-treated at 600 to 800 ° C., and the carbon fibers in the treated product are subjected to alkali activation treatment to make the carbon fibers porous, that is, increase the specific surface area.
[0039]
When the heat treatment temperature is less than 600 ° C., it becomes difficult to sufficiently activate the carbon fibers in the treated product. On the other hand, if the heat treatment temperature exceeds 800 ° C., secondary reactions other than activation may proceed to impair the carbon fiber porosity. The heat treatment temperature is more preferably 650 to 750 ° C.
[0040]
In the alkali activation treatment, the processed product is mainly in the form of a lump and has good transportability. Therefore, an activation processing container for storing the processed product and a belt-type continuous furnace in which the activation processing container continuously conveys, Or a rotary kiln-type continuous furnace to which the processed material is supplied.
[0041]
After the second step described above, the activated carbon fiber for capacitor electrode is obtained by eluting and removing the alkali metal mainly from the lump by using an acid such as pure water and hydrochloric acid according to a conventional method, followed by drying. Manufacturing.
[0042]
Next, the electric double layer capacitor according to the present invention will be described.
[0043]
This electric double layer capacitor has a structure in which a pair of capacitor electrodes (anode and cathode) containing activated carbon fibers manufactured by the above-described method are disposed with a separator interposed therebetween, and the capacitor electrode and the separator are impregnated with an electrolyte. Have
[0044]
The capacitor electrode is, for example, kneaded with a conductive agent such as activated carbon fiber and graphite powder and a binder such as polytetrafluoroethylene, and the kneaded product is rolled and formed into a sheet, or the kneaded product is made of aluminum. Either one or both sides of a current collector such as a foil are rolled and formed into a sheet.
[0045]
As the separator, a non-woven fabric of polyolefin such as polyethylene, a porous polyolefin sheet having fine holes opened, or the like can be used.
[0046]
The electrolytic solution is roughly classified into an aqueous electrolytic solution and an organic electrolytic solution, and a capacitor using the latter organic electrolytic solution can greatly increase the energy density.
[0047]
The aqueous electrolyte solution is an aqueous sulfuric acid solution.
[0048]
The organic electrolyte includes an organic solvent such as ethylene carbonate, propylene carbonate, γ-butyrolactone, dimethyl sulfoxide, acetonitrile, tetrahydrofuran and dimethoxyethane, a quaternary ammonium salt, LiClO ₄ , NaBF ₄ , and LiPF ₄ . It has a composition in which an electrolyte is dissolved.
[0049]
Specific examples of such an electric double layer capacitor include a coin-type capacitor and a cylindrical capacitor.
[0050]
As described above, according to the present invention, it is possible to provide a method for producing activated carbon fibers for capacitor electrodes, which can carry out the alkali activation treatment with a very simple operation.
[0051]
That is, in the conventional method for producing activated carbon fibers by the alkali activation method, the alkali metal compound as the activator is once melted in the step of the alkali activation treatment, and is solidified after generating gas accompanied by dehydration reaction and generating bubbles. Due to the change, the alkali activated product adheres to the inner wall of the reaction vessel. Since such an activation treatment product is hard and firmly fixed to the reaction vessel, the activation treatment item fixed to the reaction vessel should be used in order to shift to a subsequent process such as washing for removing the alkali metal. It will be necessary to scrape. As a result, not only does it take a long time to take out the activation treatment product, but the production of a large amount of the activation treatment product is limited. Further, in this scraping operation, the wall portion of the reaction vessel is damaged and its life is shortened, or the operation of taking out the activation treatment product obtained by the next activation treatment is further complicated due to the occurrence of scratches. Therefore, in the conventional method, it has been substantially difficult to produce activated carbon fibers by an alkali activation method on an industrial scale.
[0052]
For this reason, the present invention separates the alkali activation treatment into the previous step and the present step, and forcibly stirs the carbon fiber and the alkali metal compound, for example, put in the reaction vessel in the previous step in an inert atmosphere. However, it processes at predetermined | prescribed temperature (temperature of 300-550 degreeC). At this time, as described above, the alkali metal compound is once melted, undergoes gas generation accompanied by dehydration reaction, undergoes a state change that solidifies after foaming, but solidifies after foaming by forcibly stirring. Since the action of crushing in the process works, it is possible to produce a processed material mainly in the form of a lump without sticking to the inner wall of the reaction vessel. Such mainly lump-like processed products have good transportability and are easy to handle, so that the alkali activation treatment can be easily performed from the reaction vessel without performing a complicated scraping operation from the reaction vessel as in the past. It can transfer to this process, ie, the process which heat-processes the said processed material at 600-800 degreeC, and carries out the alkali activation process of the carbon fiber in this processed material. As a result, the time spent for the alkali activation treatment can be remarkably shortened by improving the permeability of the treated product and reducing the maintenance of the reaction vessel, so that the activated carbon fiber can be produced on an industrial scale. In addition, since it is possible to produce a bulky processed material that has good transportability and is easy to handle in the previous step, the continuous processing apparatus including the activation processing container and the belt-type continuous furnace described above for the alkali activation processing, or the rotary kiln type It becomes possible to carry out using a continuous furnace, and the activated carbon fiber can be produced in a further mass production.
[0053]
Moreover, since the scraping operation of the processed material from the reaction vessel is not required in the previous step of the alkali activation treatment, the reaction vessel can be prevented from being damaged and its service life can be extended.
[0054]
Further, the dehydration reaction can be promoted by forcibly stirring in the previous step of the alkali activation treatment, and even if the amount of the alkali metal compound to be blended is reduced, the surface of the carbon fiber is uniformly coated with the alkali metal compound. Can be achieved. As a result, it is possible to shorten the alkali activation treatment time and reduce the raw material cost.
[0055]
Furthermore, by forcibly stirring in the preceding step of the alkali activation treatment, gas escape associated with the dehydration reaction can be promoted to suppress the bubbling phenomenon, so that the reaction vessel can be enlarged in consideration of volume expansion. Can be avoided.
[0056]
The electric double layer capacitor according to the present invention includes a capacitor electrode including activated carbon fiber manufactured by the above-described method, and thus has an electric double layer capacitor having a capacitor electrode including activated carbon fiber in charge and discharge. You can get no high capacity.
[0057]
【Example】
Hereinafter, preferred embodiments of the present invention will be described.
[0058]
Example 1
First, a petroleum mesophase pitch was spun by a melt blow method, then infusible at 300 ° C. in an air atmosphere, carbonized at 650 ° C. in a nitrogen atmosphere, and pulverized by a jet mill to obtain powdered carbon fibers.
[0059]
Subsequently, the powdered carbon fiber and potassium hydroxide were charged into a reaction vessel equipped with a rotating shaft having two stirring blades at a weight ratio of 1: 2.5. The reaction vessel, the rotating shaft and the blade were all made of a nickel-based alloy. Subsequently, while introducing nitrogen gas into the reaction vessel, the mixture was heated from room temperature to 400 ° C. at a rate of temperature increase of 5 ° C./min, and at the same time, the rotating shaft was rotated at a rate of 100 rpm, Potassium hydroxide was forcibly stirred, and after reaching 400 ° C., the mixture was held for 1 hour while continuing stirring to obtain a mass-treated product having an average diameter of 15 mm.
[0060]
Subsequently, the granular processed material in the reaction vessel is conveyed and stored in a nickel tray (activation processing vessel), and this tray is transferred to a belt type continuous furnace having a heating zone heated to 750 ° C. The alkali activation process was implemented by conveying a heating zone over 2 hours. The nickel tray after conveyance was cooled to room temperature in a nitrogen atmosphere, then washed with pure water and hydrochloric acid until no metallic potassium was eluted, and dried to produce activated carbon fibers activated with potassium.
[0061]
The obtained activated carbon fiber was confirmed to have a specific surface area of 1150 m ² / g by the BET nitrogen adsorption method.
[0062]
After kneading 85 parts by weight of the activated carbon fiber, 10 parts by weight of graphite as a conductive agent, and 5 parts by weight of polytetrafluoroethylene (PTFE) as a binder, this kneaded product is used as an aluminum foil as a current collector. Capacitor electrodes were prepared by rolling and sheeting on one side. Two electrodes obtained are prepared, and these electrodes (anode, cathode) are arranged so that the kneaded material sheets face each other, and a polypropylene non-woven fabric as a separator is interposed between the anode and cathode, and an electric double layer A capacitor (test cell) was assembled. The anode, cathode and polypropylene nonwoven fabric were impregnated with an electrolytic solution (composition of tetramethylammonium tetrafluoroborate {(C ₂ H ₅ ) ₄ NF ₄ } in propylene carbonate dissolved at 1.8 mol / L).
[0063]
As a result of conducting a charge / discharge test on the obtained test cell, a capacitor capacity of 35 F / cc could be taken out.
[0064]
(Example 2)
Activated carbon fibers activated with potassium were produced in the same manner as in Example 1 except that the potassium activation treatment was performed using a rotary kiln type continuous furnace. This activated carbon fiber was confirmed to have a specific surface area of 1210 m ² / g by the BET nitrogen adsorption method.
[0065]
A test cell similar to that of Example 1 was assembled using the obtained activated carbon fiber. As a result of conducting a charge / discharge test on the test cell, a capacitor capacity of 34 F / cc could be taken out.
[0066]
(Comparative Example 1)
After adding 200 parts by weight of potassium hydroxide to 100 parts by weight of powdered carbon fiber obtained by the same method as in Example 1 and mixing them well, this mixture was stored in a nickel tray (activation processing container). Subsequently, this tray was placed in a heating furnace, heated from room temperature to 400 ° C. at a heating rate of 5 ° C./min, held at the same temperature for 2 hours, and further heated at a heating rate of 5 ° C./min at 750 ° C. The alkali activation treatment was carried out by heating to the temperature of and holding at that temperature for 2 hours. The nickel tray was taken out of the heating furnace and cooled to room temperature in a nitrogen atmosphere, and the inside of the tray was observed. As a result, the activation treatment product was fixed to the entire tray. For this reason, it was necessary to scrape the activation treatment product from the tray for a long time. Thereafter, the activated product thus scraped off was washed with pure water and hydrochloric acid until metal potassium was not eluted, and dried to produce activated carbon fibers activated with potassium.
[0067]
A test cell similar to that of Example 1 was assembled using the obtained activated carbon fiber. As a result of conducting a charge / discharge test on the test cell, a capacitor capacity of 32 F / cc could be taken out.
[0068]
【The invention's effect】
As described above in detail, according to the present invention, the alkali activation treatment is divided into a pre-process and a main process. In the pre-process, the carbon fiber is coated with an alkali metal compound, and the transportability is good and easy to handle. By providing this, it is possible to carry out the alkali activation treatment in this subsequent step by an extremely simple operation, and provide a method capable of producing activated carbon fibers for capacitor electrodes on an industrial scale in a mass production manner. it can.
[0069]
Further, according to the present invention, it is equivalent to or equivalent to an electric double layer capacitor comprising a capacitor electrode comprising activated carbon fibers obtained by the above method and comprising a capacitor electrode comprising activated carbon fibers produced by a conventional method in charge and discharge. An electric double layer capacitor capable of taking out such a high capacity can be provided.

Claims (5)

While forcibly stirring in an inert atmosphere in a reaction vessel equipped with a rotary shaft having a stirring blade and a carbon fiber and an alkali metal compound, melting was at a temperature of 300 to 550 ° C., solidification, crushing A step of mainly producing a lump processed product having an average particle diameter of 1 to 30 mm ,
And a step of heat-treating the treated product at 600 to 800 ° C. to subject the carbon fiber in the treated product to an alkali activation treatment.   The method for producing activated carbon fibers for capacitor electrodes according to claim 1, wherein the alkali activation treatment is performed using a continuous treatment apparatus.   The activated carbon fiber for a capacitor electrode according to claim 2, wherein the continuous processing apparatus includes an activation processing container for storing the processed material, and a belt-type continuous furnace that the activation processing container continuously conveys. Manufacturing method.   The said continuous processing apparatus is a rotary kiln type | mold continuous furnace with which the said processed material is supplied, The manufacturing method of the activated carbon fiber for capacitor electrodes of Claim 2 characterized by the above-mentioned.   An electric double layer capacitor comprising a capacitor electrode comprising activated carbon fibers for a capacitor electrode manufactured by the method according to claim 1.