JP2004087988A - Carbon electrode for electric double-layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbon electrode for an electric double-layer capacitor of high capacity. <P>SOLUTION: In an electric double-layer capacitor, expanded carbon fiber is used as the carbon electrode and sulfuric acid is used as the electrolyte. The expanded carbon fiber is obtained by performing electrolytic treatment for raw material carbon fiber such as pitch-based carbon fiber, by forming a graphite intercalation compound, such as nitric acid-graphite intercalation compound, and by decomposing thereafter the graphite intercalation compound by heat treatment. <P>COPYRIGHT: (C)2004,JPO

Description

【００３０】
従来の活性炭電極を用いた場合の１００Ｆ／ｇ程度のキャパシスタンスに比べ、表１に示されるように、本発明の炭素電極のキャパシタンスは大きく、特に濃硫酸を電解質として用いた場合に著しく大きなキャパシタンスが得られることがわかる。
【００３１】
【発明の効果】
本発明の電気二重層キャパシタ用炭素電極を用いることによって、従来の活性炭電極を利用する電気二重層キャパシタよりも高容量の電気二重層キャパシタを製造することができる。さらに、電解質を硫酸とすることによって非常に高い電気二重層キャパシタを製造することができる。
【図面の簡単な説明】
【図１】原料炭素繊維、硝酸中電解後の黒鉛層間化合物、及び、熱処理後の膨張化炭素繊維のＸ線回折図である。
【図２】原料炭素繊維（ａ）、硝酸中電解後の黒鉛層間化合物（ｂ）、及び、熱処理後の膨張化炭素繊維（ｃ〜ｅ）の走査型電子顕微鏡写真である。
【図３】実施例に用いた作用極の説明図である。
【図４】実施例に用いた３極式電気化学セルの説明図である。
【図５】実施例１のサイクリックボルタモグラムを示す図である。
【図６】実施例１の定電流充放電曲線を示す図である。
【図７】実施例２のサイクリックボルタモグラムを示す図である。
【図８】実施例２の定電流充放電曲線を示す図である。
【符号の説明】
１　　炭素材料
２　　白金網
３　　ガラス繊維濾紙
４，５　　ポリテトラフルオロエチレン板
６　　作用極
７　　電解液
８　　窒素ガス
９　　対極
１０　　参照極[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electric double layer capacitor and power storage, and more particularly, to a carbon electrode for an electric double layer capacitor.
[0002]
[Prior art]
An electric double layer capacitor is a capacitor that uses an electric double layer generated at an interface between a solid and a liquid, can be charged and discharged with a large current, and is expected to be used for electric vehicles and for power storage. The structure of the electric double layer capacitor is typically composed of a polarizable electrode composed of a positive electrode and a negative electrode opposed to each other with a separator interposed therebetween, and an electrolytic solution. Conventionally, a carbon material having a high specific surface area has been used as a polarizable electrode of an electric double layer capacitor, and activated carbon powder and activated carbon fiber are generally used for the purpose (for example, D and Qu (D. Qu)). and H. Shi), Journal of Power Sources, vol. 74, (1998) 99, etc.).
[0003]
Conventionally, it has been considered that increasing the specific surface area of the carbon material increases the capacitance of the electric double layer capacitor electrode. As a method for producing a carbon material having a high specific surface area, that is, activated carbon, techniques such as steam activation, CO ₂ activation, and chemical activation have been established. In recent years, a method for producing a high specific surface area carbon material utilizing a dehalogenation reaction of a halogen-containing carbon precursor has also been reported. However, there is a limit in increasing the specific surface area of the carbon material. It has also been clarified that the capacitance of an electric double layer capacitor electrode is not completely proportional to the specific surface area, and optimization of the pore structure, that is, the pore structure (average pore diameter, The search for pore distribution, pore volume) is continuing.
For example, JP-A-2002-128514 discloses that in a pore distribution of a carbonaceous material obtained from a nitrogen adsorption isotherm, a carbonaceous material having a pore radius of 10 ° or less and a pore volume of 70% or less is used as an electric double layer. It has been proposed to use for polarizable electrodes of capacitors.
[0004]
[Problems to be solved by the invention]
However, in recent years, further improvement in performance has been demanded for electric double layer capacitors, and development of higher capacity electric double layer capacitor polarizable electrodes has been demanded. An object of the present invention is to provide a higher capacity carbon electrode for an electric double layer capacitor.
[0005]
[Means for Solving the Problems]
Means for Solving the Problems As a result of diligent studies, the present inventors have found that expanded carbon fibers exhibit typical electric double layer behavior as an electrode. The present invention has been made based on this finding.
[0006]
That is, the present invention
(1) a carbon electrode for an electric double layer capacitor, characterized by using expanded carbon fibers;
(2) The carbon electrode for an electric double layer capacitor according to (1), wherein the carbon electrode is used for an electric double layer capacitor in which the electrolytic solution is sulfuric acid.
(3) The carbon electrode for an electric double layer capacitor according to (1) or (2), wherein the expanded carbon fiber is obtained by decomposing the raw material carbon fiber as the graphite intercalation compound. Things.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, the expanded carbon fiber refers to a fiber in which the carbon fiber lamination layers are expanded so that the pore volume determined by the mercury porosimetry becomes 3 cm ³ / g or more. The carbon fibers have a specific surface area of 30 m ² / g or more as determined by the nitrogen gas adsorption method. Preferably, the carbon fiber has a pore volume of 5 cm ³ / g or more and a specific surface area of 100 m ² / g or more. More specifically, such expanded carbon fibers use carbon fibers as a starting material to synthesize an intercalation compound in which sulfuric acid molecules and the like are inserted between the carbon lamination layers, and rapidly heat this to form the carbon lamination layers. Can be manufactured as expanded fiber. The manufacturing method is described in, for example, JP-A-2001-207376 and JP-A-2001-53301.
The measurement by the mercury intrusion method can be performed at room temperature with a commercially available mercury porosimeter (PASCAL240, manufactured by AMC).
In the measurement by the nitrogen gas adsorption method, the nitrogen adsorption isotherm at 77K is measured using a commercially available constant volume gas adsorption device (BELSORP28SA, manufactured by Bell Japan Co., Ltd.), and the specific surface area is calculated by the BET method. That can be done.
[0008]
Examples of the raw carbon fibers of the expanded carbon fibers that can be used in the present invention include pitch-based carbon fibers, polyacrylamide-based carbon fibers, rayon-based carbon fibers, and the like. Among these, pitch-based carbon fibers are preferred because the graphite structure is likely to develop during firing.
As a raw material of the pitch-based carbon fiber, a petroleum-based pitch, a coal-based pitch, and a synthetic system can be preferably used. Specific examples of petroleum pitch include decant oil pitch and ethylene tar pitch, and specific examples of coal pitch include coal tar pitch and coal liquefied pitch. Specific examples of the synthetic pitch include various pitches such as a naphthalene pitch.
The pitch can be spun by a usual melt spinning method. The obtained pitch fiber is kept in an oxidizing gas atmosphere at a temperature of usually 100 to 360 ° C, preferably 130 to 320 ° C for usually 10 minutes to 10 hours, preferably 1 to 6 hours to perform insolubilization treatment. As the oxidizing gas, oxygen, air, ozone or a mixture thereof with carbon dioxide, chlorine or the like is usually used. The insolubilized fiber is subjected to graphitization at a temperature of 2000 ° C. or higher, preferably 2600 ° C. or higher, more preferably 2900 to 3000 ° C. in an atmosphere of an inert gas such as nitrogen or argon to obtain pitch-based carbon fiber. be able to.
[0009]
In the present invention, the formation of the graphite intercalation compound is preferably performed by electrolytically treating the raw carbon fiber. By the electrolytic treatment, a graphite intercalation compound in which atoms, ions, molecules, and the like are taken in between the graphite plane layers can be formed. As the electrolytic solution used in the electrolytic treatment of the raw carbon fiber, an acidic solution can be usually used, and the acidic solution is not particularly limited as long as it causes electrolysis. Acids include organic acids, inorganic acids, and mixtures thereof.Inorganic acids include sulfuric acid, concentrated sulfuric acid, nitric acid, concentrated nitric acid, phosphoric acid, and the like.Organic acids include formic acid and acetic acid. And dilute sulfuric acid (concentration: 9 mol / dm ³ or less) are preferred, and concentrated nitric acid is particularly preferred. Acid concentration in this case is preferably from 5 to 20 mol / dm ^3, more preferably from 6~20mol / dm ^3. When concentrated nitric acid is used, a nitric acid-graphite intercalation compound is synthesized.
[0010]
As the electrodes and devices used for the above electrolytic treatment, those conventionally used for electrolytic oxidation can be appropriately used. For example, a raw carbon fiber is fixed at the tip of both platinum electrodes, immersed in a concentrated nitric acid electrolytic solution, and subjected to constant current electrolysis to synthesize a nitric acid-graphite intercalation compound.
In addition, the current value and time during electrolysis are selected according to the amount of the raw carbon fiber used for electrolysis, the type of acid, and the selection of the concentration. These conditions can be determined by a person skilled in the art by simple preliminary experiments. For example, when concentrated nitric acid (13 mol / dm ³ ) is used as an electrolytic solution using 0.135 g of pitch-based carbon fiber heat-treated at 3000 ° C., the potential reaches saturation with an increase in the amount of electricity at 0.1 to 2.0 A. , That is, preferably for 10 to 120 minutes, more preferably for 0.5 to 1.0 A for about 20 to 60 minutes.
The formation of the graphite intercalation compound by the electrolytic treatment means that the interlayer distance measured using an X-ray diffractometer is larger than the carbon fiber value before the electrolytic treatment, which is 0.3 to 0.4 nm, which is 0.65 to 0.65 nm. It can be confirmed by being at 0.9 nm.
[0011]
The graphite intercalation compound is then preferably decomposed by heat treatment, the interlayer is expanded, and the fiber is expanded. The decomposition by heat treatment is preferably rapid decomposition by rapid heat treatment so as to instantaneously expand the interlayer, and is preferably performed at 300 to 1200 ° C. for 5 to 60 seconds, and performed at 800 to 1000 ° C. for 5 to 10 seconds. Is more preferable. The heat treatment can be performed using a normal electric furnace or the like.
[0012]
The formation of the expanded carbon fiber can be confirmed by the naked eye, since the fiber is greatly swollen by the expansion treatment. X-ray diffraction of the expanded carbon fiber shows that the carbon laminated structure has been recovered, and further, it can be confirmed by scanning electron micrographs that fibrillation has occurred and that the expanded carbon fiber has been broken into fine structures.
The bulk density of the expanded carbon fiber used in the present invention is preferably 0.001 to 0.01 g / cm ³ .
[0013]
In the present invention, expanded carbon fibers are used as a material for a carbon electrode for an electric double layer capacitor. The method for preparing the carbon electrode for an electric double layer capacitor from the expanded carbon fiber is not particularly limited, and by using the expanded carbon fiber instead of the activated carbon, a conventionally known method for manufacturing an electrode can be appropriately performed. Can be used. For example, an electrode can be formed by sandwiching expanded carbon fibers together with a platinum mesh and glass fiber filter paper on a polytetrafluoroethylene plate. It is also possible to add a binder such as polyethylene, polytetrafluoroethylene, or polyvinylidene fluoride to the expanded carbon fiber, form a pressure roll, and form a sheet or plate to form an electrode.
[0014]
The electric double layer capacitor having the carbon electrode of the present invention can be used with an aqueous electrolyte or a non-aqueous electrolyte, but it is preferable to use an aqueous electrolyte. Examples of the aqueous electrolyte include an aqueous sodium chloride solution, an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, hydrochloric acid, and sulfuric acid. Of these, sulfuric acid is preferred, and concentrated sulfuric acid is particularly preferred, since a remarkably large capacitor capacity can be obtained. Sulfuric acid concentration in this case is preferably 1~18mol / dm ^3, more preferably from 5~18mol / dm ^3.
[0015]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0016]
Example 1
Expanded carbon fiber sample 0.135 g (length about 50 mm) of pitch-based carbon fiber heat-treated at 3,000 ° C. was subjected to constant-current electrolysis at 0.5 A for 40 minutes using 13 mol / dm ³ of nitric acid as an electrode solution. A nitric acid-graphite intercalation compound was synthesized.
This nitric acid-graphite intercalation compound is inserted into an electric furnace maintained at 1000 ° C., and is heat-treated and rapidly decomposed by holding for 5 seconds. Expanded carbon fibers having a measured pore volume of 10.0 cm ³ / g and a specific surface area of 292 m ² / g determined by a nitrogen gas adsorption method using a constant volume gas adsorption apparatus (manufactured by Nippon Bell Co., Ltd., BERSORP28SA). Obtained. This was designated as sample a.
[0017]
FIG. 1 is an X-ray diffraction diagram of the raw carbon fiber used in this example, the graphite intercalation compound after electrolysis in nitric acid, and the expanded carbon fiber after heat treatment. Si shown in the figure is a diffraction line by silicon used as an internal standard. According to FIG. 1, after the electrolysis in nitric acid, the graphite intercalation compound is formed, the interlayer distance becomes 0.78 nm, and the carbon laminated structure is restored in the expanded carbon fiber after the heat treatment. .
[0018]
FIG. 2 is a scanning electron micrograph showing the shapes of the raw carbon fiber (a), the graphite intercalation compound (b) after electrolysis in nitric acid, and the expanded carbon fiber (ce) after heat treatment. It is clearly shown that, unlike the raw material carbon fiber and the graphite intercalation compound after electrolysis, the sample subjected to the expansion treatment by the rapid heat treatment is cut into fine fibers reflecting the fibril structure of the carbon fiber.
[0019]
Electrode sample a 0.003 g (length about 25 mm) was weighed, and a platinum mesh (80 mesh, immersion part 13 × 30 × 0.08 mm) and a glass fiber filter paper having a pore diameter of 1 μm (Whatman, GF / B) Grade, 13 × 30 mm) and a jig made of polytetrafluoroethylene to form a working electrode as shown in FIG.
[0020]
Measurement Cell As shown in FIG. 4, a container made of heat-resistant glass (Pyrex) having a capacity of 80 ml was filled with about 60 ml of 1 mol / dm ³ sulfuric acid as an electrolytic solution, and the working electrode and the counter electrode (platinum plate 10 × 30 × 0.05 mm) were filled. ) And a reference electrode (Hg / Hg ₂ SO ₄ ) were immersed to form a measurement cell of a three-electrode electrochemical cell. Nitrogen gas was always bubbled through the measurement cell to remove dissolved oxygen.
[0021]
Electrochemical measurement An electrochemical measurement was performed using the above-described measurement cell. Using a potentiostat / galvanostat (HA-501G, manufactured by Hokuto Denko) and an arbitrary function generator (HB-105, manufactured by Hokuto Denko), the self potential was measured for about 10 hours, and then the cyclic voltammetry (CV measurement) was performed. In addition, constant current charge / discharge measurement (GC measurement) was performed using a constant current charge / discharge device (HJ-201B manufactured by Hokuto Denko).
FIG. 5 shows the result of the CV measurement (scanning speed 1 mV / s). No redox couple was observed on the cyclic voltammogram obtained as a result of the measurement, indicating typical electric double layer behavior. Based on the equation of I = C · dE / dt, a capacitance of 160 F / g at 0 mV and 115 F / g at 200 mV was calculated for the Hg / Hg ₂ SO ₄ reference electrode.
In addition, a constant current charge / discharge measurement was performed under the condition of a charge / discharge current of 0.2 mA. FIG. 6 shows a constant current charge / discharge curve obtained as a result of the measurement. The discharge capacity (0.5 to 0 V) in the sixth cycle, calculated from the temporal change rate of the capacitor voltage, was 117 F / g, assuming that the curve at the time of discharging was a straight line, which was in good agreement with the CV measurement result. I have.
[0022]
Example 2
CV measurement and GC measurement were performed in exactly the same manner as in Example 1 except that the electrolytic solution of Example 1 was changed to concentrated sulfuric acid (18 mol / dm ³ ). The cyclic voltammogram obtained by the CV measurement is shown in FIG. FIG. 8 shows a constant current charge / discharge curve obtained by GC measurement. CV measurement (scanning speed 1 mV / s) showed a capacitance of 433 F / g at 0 mV and 460 F / g at 200 mV. The capacity determined from the constant current charge / discharge measurement (0.2 mA) was 555 F / g.
[0023]
Example 3
The capacitance was measured by the CV measurement at 200 mV and the GC measurement at 0.5 to 0 V in exactly the same manner as in Example 1 except that the amount of the sample a used for the electrode was changed to 0.0064 g.
[0024]
Example 4
The capacitance was measured in exactly the same manner as in Example 3, except that the electrolytic solution of Example 3 was replaced with concentrated sulfuric acid (18 mol / dm ³ ).
[0025]
Example 5
0.05 g of pitch-based carbon fiber that had been heat-treated at 3,000 ° C. was subjected to constant current electrolysis at 0.5 A for 40 minutes using 13 mol / dm ³ of nitric acid as an electrode solution to synthesize a nitric acid-graphite intercalation compound.
This nitric acid-graphite intercalation compound is inserted into an electric furnace maintained at 1000 ° C., and is heat-treated and rapidly decomposed by holding it for 30 seconds. The mercury intrusion method using a mercury porosimeter (PASCAL240, manufactured by Amco Corporation) Expanded carbon fibers having a measured pore volume of 9.91 cm ³ / g and a specific surface area of 220 m ² / g determined by a nitrogen gas adsorption method using a constant-volume gas adsorption apparatus (manufactured by Nippon Bell Co., Ltd., BERSORP28SA). Obtained. This was designated as sample b.
The capacitance was measured by CV measurement in the same manner as in Example 1, except that 0.0036 g of the sample b was used in place of the sample a.
[0026]
Example 6
The CV measurement was performed in exactly the same manner as in Example 5, except that the electrolytic solution of Example 5 was replaced with concentrated sulfuric acid (18 mol / dm ³ ). In addition, GC measurement was performed in the same manner as in Example 1, and the capacitance was measured.
[0027]
Example 7
The CV measurement was performed in exactly the same manner as in Example 5, except that the amount of the sample b used for the electrode was changed to 0.0076 g. In addition, GC measurement was performed in the same manner as in Example 1, and the capacitance was measured.
[0028]
Example 8
The capacitance was measured in exactly the same manner as in Example 7, except that the electrolytic solution of Example 7 was replaced with concentrated sulfuric acid (18 mol / dm ³ ).
Table 1 shows the measurement results of Examples 1 to 8 described above.
[0029]
[Table 1]

[0030]
As shown in Table 1, the capacitance of the carbon electrode of the present invention is larger than the capacitance of about 100 F / g when the conventional activated carbon electrode is used, and particularly when the concentrated sulfuric acid is used as the electrolyte, the capacitance is remarkably large. Is obtained.
[0031]
【The invention's effect】
By using the carbon electrode for an electric double layer capacitor of the present invention, an electric double layer capacitor having a higher capacity than an electric double layer capacitor using a conventional activated carbon electrode can be manufactured. Further, an extremely high electric double layer capacitor can be manufactured by using sulfuric acid as the electrolyte.
[Brief description of the drawings]
FIG. 1 is an X-ray diffraction diagram of a raw carbon fiber, a graphite intercalation compound after electrolysis in nitric acid, and an expanded carbon fiber after heat treatment.
FIG. 2 is a scanning electron micrograph of a raw carbon fiber (a), a graphite intercalation compound (b) after electrolysis in nitric acid, and an expanded carbon fiber (ce) after heat treatment.
FIG. 3 is an explanatory diagram of a working electrode used in an example.
FIG. 4 is an explanatory view of a three-electrode electrochemical cell used in an example.
FIG. 5 is a diagram showing a cyclic voltammogram of Example 1.
FIG. 6 is a diagram showing a constant current charge / discharge curve of Example 1.
FIG. 7 is a diagram showing a cyclic voltammogram of Example 2.
FIG. 8 is a diagram showing a constant current charge / discharge curve of Example 2.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Carbon material 2 Platinum net 3 Glass fiber filter paper 4,5 Polytetrafluoroethylene plate 6 Working electrode 7 Electrolyte solution 8 Nitrogen gas 9 Counter electrode 10 Reference electrode

Claims (3)

A carbon electrode for an electric double layer capacitor, characterized by using expanded carbon fibers. 2. The carbon electrode for an electric double layer capacitor according to claim 1, wherein the carbon electrolyte is sulfuric acid. 3. The carbon electrode for an electric double layer capacitor according to claim 1, wherein the expanded carbon fiber is obtained by decomposing the raw material carbon fiber as the graphite intercalation compound.

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