JP2003082533A - Carbon fiber of vapor phase and use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an electric double layer capacitor having improved charge- discharge behavior in a high current density. SOLUTION: This carbon fiber of vapor phase method has the volume of micropore having <=2 nm pore diameter in a range of 0.01-0.4 ml/g and the isothermal line of nitrogen adsorption and desorption exhibiting II or III type. This method for carbon fiber of vapor phase method comprises activating carbon fiber of vapor phase method having >=0.33 nm lattice spacing (d002 ) of crystal measured by X-ray diffraction method in the presence of an alkali metal compound at <=1,000 deg.C temperature.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a vapor grown carbon fiber treated with an alkali metal compound, a polarizable electrode containing the carbon fiber as an electrode material, and an electric double layer capacitor using the polarizable electrode.

[0002]

2. Description of the Related Art Development of high-performance energy devices such as solar cells, fuel cells, batteries and capacitors is indispensable for supporting information and communication technology, environment and energy technology in the 21st century.

Electric double layer capacitors are capable of 1) rapid charging / discharging, 2) resistant to overcharging / discharging, 3) long life due to no chemical reaction, 4) usable in a wide temperature range, and 5) heavy metals. It has characteristics not found in batteries, such as being environmentally friendly because it does not contain it, and has been used for memory backup of electronic devices. Furthermore, in recent years, large-capacity development has rapidly progressed, and application development for high-performance energy devices has progressed. Utilization for power storage systems combined with solar cells and fuel cells, hybrid car engine assist (auxiliary power), etc. Are also being considered.

The polarizable electrode used as an engine assist for a hybrid car not only has a high electric capacity,
It is important to maintain high electric capacity even at high current density.

The following techniques are known in order to improve the charge / discharge characteristics of conventional electric double layer capacitors at high current densities. An electric double layer capacitor is manufactured using a polarizable electrode to which graphitized carbon whiskers are added (JP-A-7-307250). By using this method, the carbon whiskers and the activated carbon particles are electrically connected to reduce the internal resistance, but the charge / discharge characteristics at high current density are not sufficient.

An electric double layer capacitor comprising mesophase carbon fibers or activated carbon fibers obtained by alkali-activating an optically isotropic pitch as a main component of a polarizable electrode is disclosed in JP-A-11-
It is disclosed in JP-A-233383, JP-A-11-222732, and JP-A-11-293527. However, by these methods, a polarizable electrode having a high electric capacity per electrode weight (F / g) can be obtained, but the charge / discharge characteristics at high current density are not sufficient, and the activated carbon fiber used is bulky and Electric capacity per volume (F /
However, there is a problem in that

[0007]

The problem to be solved by the present invention is to obtain an electric double layer capacitor having improved charge and discharge characteristics at high current density.

[0008]

Means for Solving the Problems As a result of diligent studies on the above problems, the present inventor has made an electrode material of a vapor grown carbon fiber in which the distribution of micropores (pores having a diameter of about 2 nm) is controlled within a certain range. It was found that the above problem can be solved by using a polarizable electrode added as a conductive auxiliary agent.

The vapor grown carbon fiber of the present invention, which is added as a conduction aid, can be produced by heat treating so-called vapor grown carbon fiber with an alkali metal compound. This is because the vapor-grown carbon fiber has a more developed crystal structure than the pitch-based fiber and is hard to be activated by the alkali metal compound at room temperature. Usually, when a carbon material is subjected to activation treatment, a crystal structure is not developed, that is, an unstructured portion is selectively decomposed and consumed, and fine pores in the carbon structure are released to develop micropores. The micropores developed in this way function effectively in charging and discharging at low current densities, but at high current densities such as 40 mA / cm ² or more, the diffusion of electrolyte ions cannot catch up and the double layer capacity is increased. Will not contribute to. Therefore, it becomes a factor that causes a large decrease in the electric capacity at a high current density.

However, since the vapor grown carbon fiber has very few undeveloped crystal structures, compared with carbon fiber having low crystallinity such as pitch fiber, even if activated with an alkali metal compound. Less likely to be consumed and micropores less likely to develop. Further, it becomes possible to manufacture a vapor grown carbon fiber having many pores effective for charging and discharging at a high current density.

A polarizable electrode containing the above-mentioned carbon fiber treated with an alkali metal compound can be prepared to provide an electric double layer capacitor having the polarizable electrode. In the polarizable electrode, it is preferable to use the vapor grown carbon fiber as a conductive additive. This is because if the polarizable electrode is contained in a large amount, the bulky carbon fiber will reduce the electrode density and the electric capacity per electrode volume (F / ml) will decrease.

That is, the present invention provides the following vapor grown carbon fiber. 1) The volume of micropores having a pore diameter of 2 nm or less is
Gas phase carbon fiber having a nitrogen adsorption / desorption isotherm of type II or type III in the range of 0.01 to 0.4 ml / g, 2) BET specific surface area of 30 to 1000 m ² / g. The vapor grown carbon fiber according to 1) above, 3) the hollow carbon structure inside, the outer diameter of 2 to 500 nm, and the aspect ratio of 10 to 15000, wherein the gas is above 1) or 2) Phase method carbon fiber, 4) Crystal lattice spacing (d ₀₀₂ ) measured by X-ray diffraction method
Is 0.36 nm or less, the vapor grown carbon fiber according to any one of 1) to 3) above, 5) D (1360 cm ^-1 ) in Raman spectrum
The gas phase carbon fiber according to any one of 1) to 4) above, wherein the ratio of the G (1580 cm ^-1 ) peak height to the peak height is 1.4 or more, 6) measured by X-ray diffraction method Crystal lattice spacing (d ₀₀₂ )
Of 0.33 nm or more is activated at a temperature of 1000 ° C. or lower in the presence of an alkali metal compound, 7) A method for producing a vapor grown carbon fiber, 7) wherein the alkali metal compound is sodium, 8. The method for producing vapor grown carbon fiber according to 6) above, which is at least one selected from the group consisting of hydroxides, carbonates, sulfides or sulfates of potassium or calcium, 8) Polarizable electrode containing vapor grown carbon fiber according to any one of 1) to 5), 9) Vapor grown carbon obtained by the method for producing vapor grown carbon fiber according to 6) or 7) above. An electric double layer capacitor comprising a polarizable electrode containing fibers, and 10) the polarizable electrode according to 8) or 9) above.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.

The carbon fibers used as a raw material are those produced by a vapor phase method. The carbon fiber preferably has a hollow structure inside, an outer diameter of 2 to 500 nm, an aspect ratio of 10 to 15000, and a length of 100 μm or less. d ₀₀₂ is preferably 0.33 to 0.35 nm, more preferably 0.336 to 0.346 nm, in order to obtain a sufficient dispersed state and an appropriate activated state.

The vapor grown carbon fiber, even as produced,
Any heat-treated product can be used, but the heat-treatment temperature is preferably 1500 ° C. or lower. This is because when the vapor grown carbon fiber is heat-treated at 1500 ° C. or higher, crystallization proceeds too much and the heat-treatment effect of the alkali metal compound cannot be sufficiently obtained.

The vapor grown carbon fiber is treated with an alkali metal compound. In this case, it is not particularly limited as long as it is a compound containing an alkali metal, but among them, hydroxides, carbonates, sulfides and sulfates of potassium, sodium, calcium and cesium are preferable. For example, potassium hydroxide, sodium hydroxide, calcium hydroxide, cesium hydroxide,
Potassium carbonate, sodium carbonate, calcium carbonate, cesium carbonate, potassium sulfide, sodium sulfide, potassium thiocyanate, potassium sulfate and sodium sulfate can be used. You may use these 1 type or in mixture of 2 or more types.

For example, when potassium hydroxide is used, the mass ratio of potassium hydroxide to the raw material carbon fiber is 0.
It is about 5 to 10, more preferably about 2 to 6. If the addition amount is less than 0.5 times, the carbon fiber-like surface treatment becomes non-uniform and the desired effect cannot be obtained. If the addition amount is 10 times or more, the amount of surface functional groups of the fibrous carbon increases, which may increase the leakage current in the polarizable electrode.

The heat treatment temperature is preferably 250 to 1000 ° C, more preferably 500 to 900 ° C.
C., more preferably 600 to 800.degree. C. If the heat treatment temperature is 400 ° C. or lower, the progress of activation is insufficient, and if the heat treatment temperature is 1000 ° C. or higher, carbon fibers having desired physical properties cannot be obtained.

The carbon fiber thus obtained is II
Type or type III nitrogen adsorption isotherm and micropores (pores of 2 nm (20 Å) or less) volume of 0.01 to 0.
It becomes 40 ml / g.

The adsorption isotherm here is generally BDDT.
It is classified into 5 types from type I to type V called "classification of"("Science of adsorption", pages 32-33, Maruzen Co., Ltd.).
Issue). Carbon fibers having a small micropore ratio such that the adsorption isotherm becomes type II or type III are very effective as a conductive aid. Then, the micropores (2n
The volume of pores of m or less) is preferably 0.60 ml / g or less, and more preferably 0.01 to 0.40 m.
1 / g. This value can be controlled by the amount of alkali metal compound added.

It is conventionally known that the measurement of Raman spectrum is one of the analysis methods for carbon materials. Generally Raman spectrum of the carbon material appears G peak and 1360 cm ^-1 near the D peak near 1580 cm ^-1. The vapor grown carbon fiber of the present invention is obtained as the peak height of G peak and D peak as the height from the baseline to the peak point in the peak curve obtained by Raman spectrum measurement. This G / D peak height ratio hardly changes depending on measurement conditions and the like. This G / D peak height ratio is
Those of 1.4 or more, preferably 1.4 or more and 6.7 or less, and more preferably 1.6 or more and 6.6 or less are good.
This is because the vapor grown carbon fiber of the present invention does not significantly reduce the degree of graphitization due to the activation reaction, and thus functions sufficiently as a conductive additive.

The amount of fibrous carbon added as a conductive aid is preferably 0.02 to 50% by mass, more preferably 0.05 to 30% by mass. If the content is 0.02% by mass or less, the effect of increasing the number of contact points with the activated carbon particles is small, and thus a sufficient effect cannot be obtained. When it is 50% by mass or more, the content of activated carbon in the polarizable electrode is reduced and the electric capacity is reduced.

Further, BET of the main raw material activated carbon for the polarizable electrode
The specific surface area is preferably 500 m ² / g or more.
When it is 500 m ² / g or less, the ion adsorption area is small, so that the polarizable electrode cannot sufficiently perform its function and the electric capacity becomes small. The activated carbon having a particle size of 1 μm to 100 μm is preferably used. 1
This is because the use of activated carbon having a particle size of 100 μm or less or 100 μm or more makes it difficult to form an electrode sheet.

The raw material of the activated carbon may be any of synthetic resins such as phenol resin, natural materials such as coconut shell, petroleum coke and coal coke.

A polarizable electrode for an electric double layer capacitor can be manufactured by adding the carbon fiber obtained in the present invention. The carbon fiber, activated carbon, polyethylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or other binder dissolved in a solvent is mechanically kneaded with a planetary mixer or the like to form a slurry, and a paste Turn into. The obtained paste is used as a foil for aluminum, carbon-coated aluminum, stainless steel, titanium, or the like, or a plate-shaped metal current collector (conductive substrate) (thickness 1
0 μm to 0.5 mm is preferred) to a predetermined thickness, the solvent is evaporated at room temperature or by heating, and thereafter, if necessary, pressure treatment is performed with a roll press or the like to obtain an electrode sheet.

The electrolyte used may be a known one, and as the aqueous one, a sulfuric acid aqueous solution, a sodium sulfate aqueous solution, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, an ammonium hydroxide aqueous solution, a potassium chloride aqueous solution, a carbonic acid is used. Examples thereof include aqueous potassium solutions.

As the non-aqueous type, R ¹ R ² R ³
A cation represented by R ⁴ N ⁺ or R ¹ R ² R ³ R ⁴ P ⁺ (R ¹ , R ² , R ³ , and R ⁴ each independently have 1 to 1 carbon atoms.
0 is an alkyl group or an allyl group. ) And, BF ₄ ^-,
Diethyl ether, dibutyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl, using a quaternary ammonium salt or quaternary phosphonium salt consisting of anions such as PF ₆ ⁻ and ClO ₄ ⁻ as an electrolyte. Ethers such as ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol dimethyl ether, ethylene glycol phenyl ether; formamide, N-methylformamide, N, N-
Dimethylformamide, N-ethylformamide, N,
N-diethylformamide, N-methylacetamide,
Amides such as N, N-dimethylacetamide, N-ethylacetamide, N, N-diethylacetamide, N, N-dimethylpropionamide, hexamethylphosphorylamide; sulfur-containing compounds such as dimethylsulfoxide and sulfolane; methylethylketone, methylisobutylketone Such as dialkyl ketones; cyclic ethers such as ethylene oxide, propylene oxide, tetrahydrofuran, 2-methoxytetrahydrofuran, 1,2-dimethoxyethane, and 1,3-dioxolane; carbonates such as ethylene carbonate and propylene carbonate; γ-butyrolactone; N-methyl Pyrrolidone; a solution of an organic solvent such as acetonitrile or nitromethane is preferable. Furthermore, preferably, a carbonate-based non-aqueous solvent such as ethylene carbonate or propylene carbonate can be used. Two or more kinds of electrolytes or solvents may be used.

The separator to be interposed between the electrodes as required may be a porous separator which allows the permeation of ions, such as a microporous polyethylene film, a microporous polypropylene film, a polyethylene non-woven fabric, a polypropylene non-woven fabric and a glass fiber mixed paper non-woven fabric. A glass mat filter or the like can be preferably used.

The prepared electrode is inserted into a container with a separator cut into a predetermined size and shape interposed between both electrodes, and then an electrolytic solution is injected, and a sealing plate and a gasket are used to caulk and seal the electrode. It can be a polar cell.

[0030]

The present invention will be described in more detail below by showing typical examples. Note that these are merely examples for description, and the present invention is not limited to these.

(Example 1) Vapor grown carbon fiber (average fiber diameter: about 500 nm, length: about 20000 n) produced by a conventional method
m) 10 g of potassium hydroxide 50 g and distilled water 100
What was added and stirred well was put in the crucible. After raising the temperature to 750 ° C at 3 ° C / min in a nitrogen stream, 750 ° C
It was held for 30 minutes and cooled in a nitrogen stream. The treated fibrous carbon is washed with 1N hydrochloric acid and then with distilled water,
Residual alkali and metal impurities were removed.

(Method for measuring nitrogen adsorption-desorption isotherm and method for calculating micropore volume and BET specific surface area) NOVA1200, manufactured by Quantachrome Co., was used, and P / P0 = 0.01-
The adsorption-desorption isotherm of nitrogen was measured at the liquid nitrogen temperature in the range of 0.99. The data obtained are shown in FIG.
The specific surface area was calculated by the BET method using the nitrogen adsorption amount at P / P0 = 0.01 to 0.3. Micro hole (2
The volume of pores of 0 Å or less) was calculated from the nitrogen adsorption amount at P / P0 = 0.19.

(Measurement of Capacitance) 10 parts by mass of PTFE (polytetrafluoroethylene) and 10 parts by mass of carbon black were added to 80 parts by mass of activated carbon having an average particle diameter of 30 μm, kneaded in an agate mortar and thickened with a rolling roller. 0.5m
It was rolled into a sheet of m. This sheet was punched into a disk having a diameter of 13 mm, vacuum dried at 200 ° C. for 12 hours, and used as a polarizable electrode. The above electrode was used in a glove box in which high-purity argon was circulated to assemble and evaluate a cell as shown in FIG. In FIG. 1, 1 is an aluminum upper lid, 2 is a fluororubber ring, 3 is a current collector made of aluminum, 4 is an insulator made of Teflon (registered trademark), 5 is an aluminum container, and 6 is an aluminum plate. A spring, 7 is a polarizable electrode, and 8 is a glass fiber separator having a thickness of 1 mm. PC (propylene carbonate) is used as a solvent for the electrolyte,
(C ₂ H ₅ ) ₄ NBF ₄ as an electrolyte Toyama Pharmaceutical Co., Ltd.
The product name LIPASTE-P / EAFIN manufactured by K.K.

HJ-101S manufactured by Hokuto Denko was used for charge / discharge measurement.
Charge and discharge was performed at 0 to 2.5 V using M6. Charge and discharge current density was evaluated by ^{1.6mA / cm 2, 48mA / cm} 2, using a discharge curve, was calculated capacitance per poles activated carbon weight of the electric double layer capacitor (F / g).
The results are shown in Table 1.

(Example 2) Example 2 was carried out in the same manner as in Example 1 except that the vapor grown carbon fiber produced by a conventional method was heat-treated at 1000 ° C in an argon stream as a raw material. The results are shown in Table 1.

(Example 3) The amount of potassium hydroxide mixed was 50
Same as Example 2 except that g was used. The results are shown in Table 1.
It was shown to.

Example 4 Example 4 was repeated except that 30 g of calcium carbonate was mixed. The results are shown in Table 1.

(Comparative Example 1) The same procedure as in Example 1 was carried out except that pitch-based carbon fiber was used as a raw material. The results are shown in Table 1.

Comparative Example 2 Graphitized vapor grown carbon fiber (registered trademark: VGCF, Showa Denko) was used as a conductive additive. The results are shown in Table 1.

Comparative Example 3 Carbon black (trade name: Denka Black, manufactured by Denki Kagaku Kogyo) was used as a conductive additive. The results are shown in Table 1.

[0041]

[Table 1]

[0042]

EFFECTS OF THE INVENTION By using the carbon fiber of the present invention, a polarizable electrode and an electric double layer capacitor having excellent charge / discharge characteristics at high current density can be obtained.

[0043]

[Brief description of drawings]

FIG. 1 is a sectional view of a cell for evaluating the performance of an electric double layer capacitor.

2 is a nitrogen adsorption / desorption isotherm of the carbon fiber of the present invention obtained in Example 1. FIG.

[Explanation of symbols]

1 upper pig 2 O-ring 3 Current collector 4 insulator 5 containers 6 leaf spring 7-minute polar electrode 8 separator

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoichi Nanba             Showa Denko Co., Ltd. 6850 Omachi, Omachi City, Nagano Prefecture             Company Omachi Production & Technology Division (72) Inventor Tsutomu Masuko             Showa Denko Co., Ltd. 6850 Omachi, Omachi City, Nagano Prefecture             Company Omachi Production & Technology Division F-term (reference) 4L037 AT02 CS03 CS04 FA05 PA01                       UA04

Claims (10)

[Claims] 1. A gas phase method in which the volume of micropores having a pore diameter of 2 nm or less is in the range of 0.01 to 0.4 ml / g and the nitrogen adsorption / desorption isotherm shows type II or type III. Carbon fiber. 2. A BET specific surface area of 30 to 1000 m ^2.
/ G, The vapor grown carbon fiber according to claim 1. 3. A hollow structure having an outer diameter of 2 to 500
nm, aspect ratio 10-15000, The vapor grown carbon fiber according to claim 1 or 2, which is characterized in that. 4. The vapor grown carbon fiber according to claim 1, which has a crystal lattice spacing (d ₀₀₂ ) of 0.36 nm or less measured by an X-ray diffraction method. 5. D (1360) in Raman spectrum
cm ^-1) vapor-grown carbon fiber as described in any one of claims 1 to 4 ratio of G (1580 cm ^-1) peak height to the peak height is 1.4 or more. 6. A vapor phase carbon fiber having a crystal lattice spacing (d ₀₀₂ ) of 0.33 nm or more measured by an X-ray diffraction method is activated at a temperature of 1000 ° C. or less in the presence of an alkali metal compound. A method for producing a vapor grown carbon fiber. 7. The method according to claim 6, wherein the alkali metal compound is at least one selected from the group consisting of sodium, potassium or calcium hydroxide, carbonate, sulfide and sulfate. Method for producing vapor grown carbon fiber. 8. A polarizable electrode comprising the vapor grown carbon fiber according to any one of claims 1 to 5. 9. A polarizable electrode containing a vapor grown carbon fiber obtained by the method for producing a vapor grown carbon fiber according to claim 6. 10. An electric double layer capacitor, comprising the polarizable electrode according to claim 8 or 9.