Activated carbon, electrode material using it, and electric double layer capacitor

The present invention relates to activated carbon and polar electrode materials useful as electric double layer capacitors (also referred to as electric double layer capacitors), and methods for producing the same. More specifically, an electrode for an electric double-layer capacitor using an activated carbon that can be suitably used as an electrode material for a capacitor having a high electric (electrostatic) capacity and high durability and a polarized electrode material containing preferably a vapor phase method carbon fiber (minutes). Polar electrode), an electric double layer capacitor having that electrode.

Electric double layer capacitors have characteristics that batteries do not have, such as rapid charge / discharge resistance, resistance to overcharge / discharge, long life because they do not involve chemical reactions, use in a wide temperature range, and environmental friendliness because they do not contain heavy metals. It has and has been used as a memory backup power supply and so on. Furthermore, in recent years, the development of large capacity has progressed rapidly, the development of applications for high-performance energy devices has been promoted, and the use for power storage systems combined with solar cells and fuel cells, engine assist of hybrid cars, etc. has been considered. There is.

The electric double layer capacitor has a structure in which a pair of positive electrode and negative electrode polarizing electrodes (also simply called electrodes) made of activated carbon or the like are opposed to each other in a solution containing electrolyte ions via a separator. .. When a DC voltage is applied to the electrodes, anions in the solution are attracted to the electrodes polarized to the positive (+) side, and cations in the solution are attracted to the electrodes polarized to the negative (-) side, which attracts the electrodes and the solution. The electric double layer formed at the interface with is used as electric energy.

While conventional electric double layer capacitors have excellent power density, they have the problem of inferior energy density, and further development of larger capacities is required for use in energy device applications. In order to increase the capacity of the electric double layer capacitor, it is indispensable to develop an electrode material that forms many electric double layers between solutions. Therefore, the use of activated carbon with a large specific surface area has been studied in order to form more electric double layers, but such activated carbon is superior to the electric capacity per mass (F / g), but the electrode density is reduced. There was a problem that the electric capacity per volume (F / ml) did not increase so much in order to cause the above.

Further, it has been proposed to produce activated carbon having graphite-like microcrystals having an interlayer distance (average interplanar spacing) of 0.365 nm to 0.385 nm and use it as a raw material for a polarizing electrode (see Patent Document 1). An electric double layer capacitor using the activated carbon as a raw material for a polarizing electrode can be said to be an excellent raw material in that it has a large capacitance (F / ml) per volume. However, this example was also problematic and unsatisfactory. That is, since this activated carbon expands when a voltage is applied, a dimension limiting structure is required to suppress the expansion of the electrodes as described in the patent publication, and there is a big problem in the capacitor assembly operation. In addition, since the electric capacity is not developed unless a voltage of about 4 V is applied in advance, there is a risk of decomposing the electrolytic solution.

In recent years, an activated carbon that has been activated after mixing a carbon source that forms graphitized carbon by heating and a carbon source that forms graphitized carbon by heating has been proposed (see Patent Document 2). Activated carbon using a carbon source that forms graphitized carbon has a high volume (F / ml) per volume due to its high bulk density, but the electrode expands significantly when a voltage is applied, while graphitizable carbon is used. Activated carbon using the carbon source to be formed complements the advantages and disadvantages of low volume per volume (F / ml) but low expansion of graphite when a voltage is applied. However, simply mixing the two as in this example creates a trade-off between capacity and electrode expansion, and the capacity required for power storage systems combined with solar cells and fuel cells, engine assist for hybrid cars, etc. It was difficult to reduce the electrode expansion. Further, for the purpose of improving the conductivity of the electric double layer capacitor electrode and the strength of the electrode sheet, a mixture of activated carbon of a carbide powder of polyvinylidene chloride resin (hereinafter referred to as PVDC) and vapor phase carbon fiber is used. It is proposed to use. (Refer to Patent Document 3) However, since the capacitance (F / ml) per volume is small in these examples, the needs for high capacitance required for capacitors in recent years could not be satisfied. Furthermore, for the purpose of increasing the capacitance of activated carbon for electric double layer capacitors and reducing internal resistance, it has been proposed to remove alkali metal compounds by washing with water after alkali activation, and then further activate with steam ( See Patent Document 4). However, as in this example, it is not possible to control the pore distribution of 4 Å or less, which affects electrode expansion, simply by performing further steam activation after alkali activation, so the effect of reducing electrode expansion is small, and the electrical capacity is reduced. It is not preferable.

<patcit num="1"><text>Japanese Unexamined Patent Publication No. 11-317333</text></patcit><patcit num="2"><text>Japanese Unexamined Patent Publication No. 2002-83748</text></patcit><patcit num="3"><text>Japanese Unexamined Patent Publication No. 9-171946</text></patcit><patcit num="4"><text>Japanese Unexamined Patent Publication No. 2000-40645</text></patcit>

<p> An object of the present invention is to provide a polarized electrode for an electric double layer capacitor having a large capacitance (F / ml) per volume and reduced expansion of the electrode when a voltage is applied. </p>

<p> The present invention has been made as a result of diligent research to achieve the above object, and has the following configuration. (1) The pore volume of 4 angstroms or less obtained by the nitrogen adsorption method is 0.01 ml / g or less, and the pore volume of 10 to 13 angstroms is 0.07 ml / g or more. Activated carbon having a disordered layer structure at least in part in the image. (2) The activated carbon according to (1) above, wherein the pore volume of 10 to 13 angstroms is 0.07 ml / g or more and 0.5 ml / g or less. (3) The activated carbon according to (1) above, wherein the pore volume of 10 to 13 angstroms is 0.085 ml / g or more and 0.45 ml / g or less. (4) The BET specific surface area determined by the nitrogen adsorption method is 800 to 1600 m.²The activated carbon according to any one of (1) to (3) above, which is characterized by being in the range of / g.</p><p>(5) The electrode material containing the activated carbon and the vapor phase carbon fiber according to any one of (1) to (4) above. (6) The electrode material according to (5) above, wherein the ratio of the average length of the vapor phase carbon fibers to the average particle size of the activated carbon is in the range of 1: 0.5 to 1: 5. (7) Vapor phase method The length of carbon fibers is 1 to 1000 μm, and the BET specific surface area determined by the nitrogen adsorption method is 10 to 500 m.²The electrode material according to (5) or (6) above, which is in the range of / g and has an average particle size of activated carbon of 2 to 50 μm. (8) The above-mentioned (5) to (7), wherein the vapor phase carbon fiber has a hollow structure inside, has an outer diameter of 2 to 500 nm, and has an aspect ratio of 10 to 15000. Electrode material. (9) Vapor phase carbon fiber has a micropore volume of 0.01 to 0.4 ml / g and a BET specific surface area of 10 to 500 m.²The electrode material according to any one of (5) to (8) above, which is characterized by being / g. (10) The electrode material according to any one of (5) to (9) above, wherein the vapor phase carbon fiber has a branched structure. </p><p>(11) Electrode material according to (5) to (10) above, wherein the amount of carbon fiber added is 0.02% by mass to 50% by mass with respect to activated carbon. (12) Activated carbon is activated by mixing and heating a coal-based pitch with an alkali metal compound at a temperature of 800 ° C or less in the presence of an inert gas and water vapor. At least one selected from the group consisting of those fired at C, those that have been graphitized at 2200 ° C or higher, and those that have been activated at a temperature of 800 ° C or lower (5) to (11) above. The electrode material according to any one of. (13) The electrode material according to any one of (5) to (12) above, wherein the expansion coefficient of the electrode material when a 2.5 V voltage is applied is 25% or less, and the electrode density of the electrode material is 0.7 g / ml or more. (14) The electrode material according to any one of (5) to (13) above, wherein the electric capacity at the time of 2.5V charge / discharge is 37 F / g or more and 30 F / ml or more. (15) After mixing the activated carbon, the vapor phase method carbon fiber, the conductive particles, and the binder resin according to any one of (1) to (4) above, the mixed powder is mixed with an organic solvent having a boiling point of 200 ° C. or less. A method for producing an electrode material, which comprises adding, kneading, rolling, and then drying at 100 to 200 ° C. (16) The method for producing an electrode material according to (15) above, wherein the conductive particles are at least one selected from the group consisting of carbon black, natural graphite, artificial graphite, titanium oxide, and ruthenium oxide. (17) The binder resin is polytetrafluoroethylene, polyvinylidene fluoride, fluoroolefin / vinyl ether copolymer, carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl alcohol, polyacrylic acid or a salt thereof (15) or (16). ). The method for producing an electrode material.</p><p>(18) The electrode material obtained by the production method according to any one of (15) to (17) above. (19) A laminate of an electrode sheet containing the electrode material according to any one of (5) to (14) above and a current collector. (20) A laminate of an electrode sheet containing the electrode material according to (18) above and a current collector. (21) The laminate according to (19) or (20) above, wherein the current collector is selected from the group consisting of aluminum, copper, titanium, tantalum, nickel or alloys thereof or stainless steel. (22) The electrode made of the laminate according to any one of (19) to (21) above. (23) An electric double layer capacitor using the electrode of (22) above. (24) The above (23), wherein an electrolyte salt containing at least one selected from the group consisting of a quaternary ammonium salt, a quaternary imidazolium salt, a quaternary pyridinium salt, and a quaternary phosphonium salt is used. ). The electric double layer capacitor.</p>

<p> By using the activated carbon of the present invention, it is possible to obtain an activated carbon having a high electric capacity (F / ml), a low electrode expansion rate, and good durability without applying an excessive voltage. Further, by mixing the activated carbon with the vapor phase carbon fiber, it is possible to manufacture an electrode and an electric double layer capacitor having better characteristics.</p>

Hereinafter, the present invention will be described in detail. (Activated carbon) The electrical characteristics of activated carbon are greatly affected by the structural properties such as the specific surface area, pore distribution, and crystal structure of activated carbon. The structural characteristics of such activated carbon are determined by the structure of the raw material, carbonization conditions, and activation conditions. Therefore, in order to obtain activated carbon useful as an electrode material, it is necessary to optimize the structure of the raw material, carbonization conditions, and activation conditions. In order to achieve the above, it has been found that it is preferable to select a coal-based pitch as a raw material. Compared to petroleum-based carbon raw materials, activated carbon made from coal-based pitch is a compound because it has fewer side chains, a higher proportion of aromatic compounds, and a mixture of polycyclic aromatic compounds with various molecular structures. This is because it is considered that various complex microcrystal structures and the like are formed and excellent electrical characteristics are exhibited. The coal-based pitch to be selected is not particularly limited, but a softening point of 100 ° C or less, more preferably 60 ° C to 90 ° C is used.

Then, this coal-based pitch is carbonized in two stages at temperatures of 400 ° C or more and less than 600 ° C and 600 ° C or more and 800 ° C or less. When the carboniferous pitch is heated between 400 and 800 ° C, a thermal decomposition reaction occurs, gas and light fractions are desorbed, and the residue undergoes polycondensation and finally solidifies. In the first step of this carbonization step, the microbonding state between carbon atoms is almost determined, and the structure of carbon crystals determined in this step determines the basis of the structure of activated carbon, which is the final product. .. If the heating temperature in the first stage is less than 400 ° C, the pyrolysis reaction is insufficient and carbonization does not proceed. When heated to 600 ° C or higher in the first stage, the structure of the carbon crystallite becomes a random structure because it solidifies without going through a sufficient molten state at the stage where the pitch is thermally decomposed, and the electric capacity of the activated carbon after activation decreases. It is not preferable. In this first stage carbonization step, the heating rate is 3 to 10 ° C / hr, more preferably 4 to 6 ° C / hr, and the holding time at the maximum temperature is 5 to 20 hr, more preferably 8 to. It is done by setting it to 12 hours.

Next, a second stage of carbonization is performed at a temperature of 600 to 800 ° C. Also in this second stage carbonization step, the heating rate is 3 to 10 ° C / hr, more preferably 4 to 6 ° C / hr, and the holding time at the maximum temperature is 5 to 20 hr, more preferably 8 to. It is done by setting it to 12 hours. If the heating temperature of the second stage is less than 600 ° C, it is the same as the heating temperature of the first stage and the heating effect of the second stage does not appear. It is not preferable because it is done. By this carbonization treatment, the true density d of the carbonized product has 1.50 g / ml d 1.70 g / ml. This is effective in uniformly activating the carbonized product with alkali to obtain activated carbon having a high electric capacity. The activated carbon thus obtained has a structure having at least a part of a disordered layer structure (non-oriented fiber) having no crystalline carbon portion (graphite-like microcrystalline structure portion) in a bright field image in a transmission electron microscope. Therefore, it was preferably composed of only a disordered layer structure or mainly a disordered layer structure. When a graphite-like microcrystalline structure portion is formed, many gaps between layers are formed by prying open the carbon layers with metallic potassium generated by reducing KOH during the alkali activation reaction. Therefore, when a capacitor voltage is applied, the gap between the carbon layers of 3.35 to 4.0 angstroms (Å) is intercalated with tetraethylammonium cations (solvate ionic radius 3.7 Å) as electrolyte ions, for example, to spread the layers. Because it is adsorbed in the pores, the volume of the activated carbon particles is expanded, and as a result, the expansion of the entire electrode is increased. On the other hand, in activated carbon in which a microcrystalline structure portion similar to graphite is not formed, the number of pores formed by carbon consumption by water or carbon dioxide gas during the alkali activation reaction increases, and the gap between carbon layers due to metallic potassium increases. As a result, the expansion of the electrode is also reduced. Therefore, it is presumed that due to such an incomplete crystal structure, high electric capacity can be exhibited without going through the step of inserting ions between graphite layers by applying an excessive voltage.

It is also effective to carry out the above carbonization step in the vapor of an alkali metal. Alkali metals act catalytically in the carbonization process. That is, the cross-linking between aromatics in the pitch is promoted, and the carbonization reaction proceeds.

Next, the charcoal material is pulverized into grains of about 1 to 5 mm or fine powder having an average particle size of 1 to 50 μm, mixed with an alkali metal compound and heated to form pores in the charcoal material to obtain activated carbon. The alkali metal compound used in the alkali activation reaction is not particularly limited, but potassium, sodium, and calcium hydroxides, carbonates, sulfides, and sulfates are preferable. For example, sodium hydroxide, potassium hydroxide, cesium hydroxide, potassium carbonate, sodium carbonate, potassium sulfide, sodium sulfide, potassium thiocyanate, potassium sulfate, sodium sulfate and the like can be used. Potassium hydroxide and sodium hydroxide are preferable, and potassium hydroxide is more preferable. One of these types or a mixture of two or more types may be used. The activation temperature is preferably 800 ° C. or lower, preferably 600 ° C to 800 ° C, and more preferably 700 ° C to 760 ° C. The alkali metal compound is mixed in an amount of 1.5 to 7 times, more preferably 2.5 to 5 times the amount of the carbon material. Activation process is N₂, Ar gas or other inert gas atmosphere, and in the presence of water vapor, carbon dioxide, etc. After the activation treatment, wash with water, acid, etc. For acid cleaning, mineral acids such as sulfuric acid, phosphoric acid, hydrochloric acid and nitric acid, and organic acids such as formic acid, acetic acid and citric acid can be used. Hydrochloric acid and citric acid are preferable from the viewpoint of cleaning efficiency and residue. The acid concentration is 0.01 to 20 and preferably 0.1 to 10. As a cleaning method, stirring may be performed after adding an acid, but the cleaning efficiency is improved by boiling or heating at 50 to 90 ° C. It is also effective to use an ultrasonic cleaner. The cleaning time is 0.5 to 24 hours, but is usually 1 to 5 hours. In the case of acid cleaning, the container used for cleaning is preferably glass lining, Teflon (registered trademark) or the like.

After performing these acid washings, wash thoroughly with water. As a cleaning method, it is effective to boil or heat at 50 to 90 ° C and stir. The washing time is 0.5 to 24 hours, but usually it is carried out in 1 to 5 hours, and the number of washings is 1 to 10 times, but usually it is carried out in 1 to 3 times. The container used for cleaning is preferably a glass lining, Teflon (registered trademark) or the like. The activated carbon washed after activation is heated and vacuum dried as necessary, and in the case of 1 to 5 mm particles, it is pulverized to an average particle size of 2 to 50 μm. The crushing method may be a normal crushing method such as a jet mill, a vibration mill, or a valverizer. The activated carbon fine powder thus obtained is subjected to magnetic sorting and sieving treatment in order to remove metallic foreign substances and the like. The magnetic separation and sieving treatment may be carried out by either a wet slurry or a dry powder. For the magnetic separator, use 5000G ~ 12000G Nb, ferrite magnet, etc. The sieving treatment is carried out with an opening of 20 to 100 μm, preferably 45 to 60 μm. The activated carbon thus obtained has the characteristics that it exhibits a high electric capacity from the first cycle without applying an excessive voltage, and has a high retention rate of the electric capacity. The ratio of the D peak height to the G peak height (height from the baseline to the peak point in the actual measurement curve) of the Raman spectrum was 0.8 to 1.2. Here, the intensity ratio of the D peak to the G peak of the Raman spectrum is used as an index indicating the degree of graphitization of the carbon material, but when this intensity ratio is shown as the peak height ratio, the degree of graphitization is high. The value becomes smaller. In the bright field image of a transmission electron microscope, the value was 0.8 to 1.2 in the case of the activated carbon having no crystalline carbon portion (graphite-like microcrystal structure portion). Further, it is considered that the amount of functional groups on the carbon surface is reduced and the deterioration of the electric capacity is suppressed by undergoing a sufficient carbonization step. When the tap density of the activated carbon was measured with a tap density meter (manufactured by Kuramochi Kagaku Kikai Seisakusho), it was 0.35 to 0.70 g / ml when the number of taps was 50, and the powder resistance was 0.4 Ωcm or less at 1.0 MPa. Met.

The activated carbon of the present invention has a pore volume of 4 Å or less, which is obtained by determining the integrated pore distribution of micro + mesopores by density functional theory (DFT method) using the nitrogen adsorption method, and has a pore volume of 0.01 ml / g or less, and is 10 to 10 to The pore volume of 13 Å is 0.07 ml / g or more. The upper limit can be up to about 0.5 ml / g. The desired range of pore volume of 10 to 30 Å is 0.085 ml / g or more and 0.45 ml / g or less, and more preferably 0.08 ml / g or more and 0.4 ml / g or less. The DFT method is suitable for evaluating a smaller pore volume than the conventional method for evaluating the distribution of mesopores (20 Å or more) by the BJH (Barrett, Joiner and Halenda) method. When the pore volume of 4 Å or less exceeds 0.01 ml / g, electrolyte ions (solvate ionic radius 3.7 Å) intercalate and expand the pores, resulting in large electrode expansion. A pore volume of 10 to 13 Å contributes significantly to the capacitor capacity, and if it is less than 0.07 ml / g, the capacitor capacity becomes insufficient. In the range of the pore volume of 10 to 13 Å, the BET specific surface area may be reduced to make the volume smaller, and the BET specific surface area may be made larger to make the volume larger. Large pores of 20 Å or more contribute to the electric capacity per mass, but cause a decrease in electrode density. Therefore, if the pores increase, the electric capacity per volume decreases. BET surface area is preferably 800-1600g / m²Is. Supplementing the pores, 0.7 nm (7 Å) or less is called ultra-micro pores, and it is possible to measure up to about 0.35 nm (3.5 Å) ("Zeolite" Vol.19, No.4 (2002)). .. The above DFT (Density Function Theory) method is a method for analyzing the pore structure based on characteristic values such as specific surface area, pore volume, and pore distribution determined by the gas adsorption method. Micropores (20 Å or less) to mesopores (20 Å or less) It is possible to analyze with one theory over a wide range from 20 to 50 Å).

(Vaic phase carbon fiber) Further, by adding the vapor phase carbon fiber to the activated carbon thus obtained, the characteristics can be further improved. The vapor phase carbon fiber in this case can be used, for example, one produced by spraying benzene and metal catalyst particles in a hydrogen stream at about 1000 ° C. It has a hollow structure inside and has an outer diameter of 2 to 2. It is characterized by having an aspect ratio of 10 to 15000 at 500 nm. The fiber length is preferably 1 to 1000 μm. Further, the fiber preferably has a branch. By mixing this vapor phase carbon fiber with the activated carbon, the contact resistance between the particles is reduced, the electrode strength is improved, and the durability as a polar electrode is improved. The length of the vapor phase carbon fiber has a ratio of the activated carbon particles to the average particle size in the range of 1: 0.5 to 1: 5, preferably 1: 0.8 to 1: 3. If the length ratio of the vapor phase carbon fibers is shorter than 0.5, the particles cannot be bridged and the conductivity becomes insufficient, and if the length ratio exceeds 5, they cannot enter the gaps between the activated carbon particles and the polarized electrode. The strength decreases.

As the vapor phase method carbon fiber, a carbon fiber as it is produced, which is calcined at 1000 to 1500 ° C., or a carbon fiber which is further graphitized at 2000 ° C. or higher can be used. In addition, the gas phase method carbon fiber is gas activated (steam, CO).₂Etc.) and / or chemical-activated (zinc chloride, potassium hydroxide, sodium hydroxide, etc.) can be used, but in this case, micropores (pores of 20 Å or less) volume 0.01 to 0.4 ml. / g, BET specific surface area 10 ~ 500m²It is better to use the one whose surface structure is controlled so as to be / g. This is because when carbon fibers having many micropores are mixed, the ion diffusion resistance inside the electrode increases. As with activated carbon, the activation temperature is preferably 800 ° C or lower, preferably 600 ° C to 800 ° C, and more preferably 700 ° C to 760 ° C. The mixed amount of the vapor phase carbon fibers in this case is preferably 0.02% by mass to 50% by mass, more preferably 0.5 to 10% by mass with respect to the activated carbon. If it is less than 0.02% by mass, the effect of increasing the contact point with the activated carbon particles is small, so that a sufficient effect cannot be obtained. If it exceeds 50% by mass, the activated carbon content in the polarizable electrode decreases and the electric capacity decreases. By mixing this vapor phase carbon fiber with the activated carbon, the contact resistance between the particles is reduced, the conductivity and the electrode strength are improved, and the electrode expansion rate with voltage applied is reduced. By adding vapor-phase carbon fiber, in addition to improving heat dissipation by taking advantage of the high conductivity and high thermal conductivity of vapor-phase carbon fiber, electrode expansion cushioning material due to the mixture of fibrous material in agglomerated activated carbon particles. It is effective in suppressing an increase in the coefficient of expansion of the electrode with voltage applied because the role of the carbon fiber is enhanced.

(Method for Producing Polarizing Electrode) A polarizing electrode and an electric double layer capacitor can be manufactured from the activated carbon of the present invention or the activated carbon and the vapor phase carbon fiber. That is, the polarized electrode is a method of adding a conductive agent and a binder to activated carbon and kneading and rolling, a method of adding a conductive agent, a binder resin and a solvent to the activated carbon to form a slurry, and applying the activated carbon to the conductive material. It is produced by a method such as mixing uncarbinized resin with and sintering it. For example, carbon black and vapor-phase carbon fiber are added as conductive agents to activated carbon powder having an average particle size of about 10 to 50 μm, and a binder resin such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride is added and blended. Dry mixing. Next, an organic solvent having a boiling point of 200 ° C. or lower is added to the mixed powder to swell it, knead it, form it into a sheet having a thickness of about 0.1 to 0.5 mm, and vacuum dry it at a temperature of about 100 to 200 ° C.

As the binder resin, for example, polytetrafluoroethylene, polyvinylidene fluoride, fluoroolefin / vinyl ether copolymer, carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl alcohol, polyacrylic acid and the like can be used. The content of the binder resin in the electrode is preferably about 0.5 to 20% by mass in the total amount of the activated carbon and the binder resin. If the amount of the binder resin is less than 0.5% by mass, the strength of the electrode is insufficient, and if it exceeds 20% by mass, the electric resistance increases and the capacity decreases, which is not preferable. From the viewpoint of the strength and volume balance of the electrode, it is more preferable that the blending amount of the binder resin is 0.5 to 10% by mass. The fluoroolefin / vinyl ether copolymer may be a crosslinked polymer, and examples of the crosslinked agent include amines, polyamines, polyisocyanates, bisphenols, and peroxides.

As the conductive particles (conductive agent), powders of carbon black, natural graphite, artificial graphite, titanium oxide, ruthenium oxide and the like are used. Of these, it is preferable to use Ketjen black or acetylene black, which is one of carbon blacks, because even a small amount has a large effect of improving conductivity. The blending amount of the conductive particles such as carbon black in the electrode is preferably 5% by mass or more, particularly 10% by mass or more, in the total amount with the activated carbon powder so as to sufficiently improve the conductivity. If the amount of the conductive particles is too large, the ratio of activated carbon is reduced and the capacitance of the electrode is reduced. Therefore, the amount of the conductive particles in the electrode is 40% by mass or less, especially 30% by mass or less. It is preferable to do so.

The organic solvent used when producing the polarization electrode is preferably one that can dissolve the binder resin. For example, hydrocarbons such as toluene, xylene and benzene, ketones such as acetone, methyl ethyl ketone and butyl methyl ketone, methanol and ethanol. , Alcohols such as butanol, and esters such as ethyl acetate and butyl acetate are preferable as long as they are organic solvents having a boiling point of 200 ° C. or less, and toluene, acetone, ethanol and the like are more preferable. It is not preferable to use an organic solvent having a boiling point exceeding 200 ° C because the organic solvent remains in the sheet when it is dried at 100 to 200 ° C after forming the polar electrode sheet.

This sheet is used as a punched electrode material in a predetermined shape. The expansion coefficient of the electrode material when a 2.5 V voltage is applied is preferably 25% or less, more preferably 20% or less, and the density of the electrode material is preferably 0.7 g / ml or more. A metal plate, which is a current collector, is laminated on this electrode material to form an electrode, and two sheets are stacked with the metal plate on the outside via a separator and immersed in an electrolytic solution to form an electric double layer capacitor. The capacitor can have an electric capacity of 37 F / g or more and 30 F / ml or more when charged and discharged at 2.5 V. The current collector is not particularly limited as long as it is an electrochemically and chemically corrosion-resistant conductor, and for example, aluminum, copper, titanium, tantalum, nickel, alloys thereof, stainless steel, or the like is used. Of these, aluminum and stainless steel are preferable. The shape of the current collector may be foil-like, net-like, fibrous, or may have micropores penetrating the surface. The surface of the current collector may be coated with a compound other than metal as long as the conductivity is not lost, and for example, carbon-coated aluminum or the like is used. As the electrolytic solution of the electric double layer capacitor, either a known non-aqueous electrolyte or an aqueous electrolyte can be used and is not limited, but the porous metal current collector and the alkali-activated activated carbon of the present invention are used in the non-aqueous electrolyte. Is more suitable.

Examples of the non-aqueous electrolyte used in the present invention include known organic electrolytes, polymer solid electrolytes and polymer gel electrolytes, and ionic liquids. Examples of the organic solvent used in the organic electrolytic solution include carbonates such as ethylene carbonate, propylene carbonate, diethyl carbonate, butylene carbonate, dimethyl carbonate and vinylene carbonate; lactones such as γ-butyrolactone; nitriles such as acetonitrile and benzonitrile. Ethers such as 1,2-dimethoxyethane, diethylene glycol dimethyl ether, ethylene oxide, propylene oxide, tetrahydrofuran, 2-methoxy tetrahydrofuran, 1,2-dimethoxyethane, 1,3-dioxolane; sulfur-containing compounds such as dimethylsulfoxide and sulfolane; formamide , N-methylformamide, N, N-dimethylformamide, N-ethylformamide, N, N-diethylformamide, N-methylacetamide, N, N-dimethylacetamide, N-ethylacetamide, N, N-diethylacetamide, N , N-dimethylpropionamide, amide such as hexamethylphosphorylamide; dialkylketone such as methylethylketone and methylisobutylketone; N-methylpyrrolidone; organic solvent such as acetonitrile and nitromethane. Preferred examples thereof include ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, vinylene carbonate carbonates, lactones such as γ-butyrolactone, dimethyl sulfoxide, acetonitrile, tetrahydrofuran and the like. These solvents can be used alone or in admixture of two or more.

Polymers used in polymer solid electrolytes and polymer gel electrolytes include polyethylene oxide derivatives and polymers containing the derivatives, polypropylene oxide derivatives and polymers containing the derivatives, phosphate ester polymers, polycarbonate derivatives and the derivatives. Examples thereof include polymers containing. As these solutes (electrolyte salts), quaternary ammonium salts, quaternary imidazolium salts, quaternary pyridinium salts, quaternary phosphonium salts and the like are used alone or as a mixture of two or more kinds. Among these solutes, ionic liquids are liquid, such as 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium trifluorosulfonate, even if they are not dissolved in a solvent. Can be mentioned.

Known separators such as glass filters, non-woven fabrics, porous papermaking, and polyolefin-based microporous films can be used, but non-aqueous electrolyte-based separators are polyethylene or polypropylene with a thickness of 5 to 50 μm and a porosity of 40% or more. It is preferable that the microporous film produced can be made thin, and even when it is made thin, short circuits do not occur, the strength is good, and the compatibility with the electrolytic solution is good. The electric double layer capacitor is a coin type capacitor housed in a metal case together with an electrolytic solution via a separator between a pair of sheet-shaped electrodes, a winding type in which a pair of positive electrodes and a negative electrode are wound via a separator, and a separator. Any configuration such as a laminated type in which a large number of sheet-shaped electrodes are stacked can be taken. A multilayer type or winding type capacitor is preferable.

Hereinafter, the present invention will be specifically described with reference to Examples. The measurement method of each characteristic in this embodiment is as follows. (BET specific surface area) Using NOVA1200 manufactured by Quantachrome, it was calculated using the BET method and DFT method from the adsorption isotherm of nitrogen at the liquid nitrogen temperature. The amount of nitrogen adsorbed was measured at a relative pressure (P / P0) of 0.01 to 1.0. (Measurement by transmission electron microscope) The sample was dispersed in ethanol, mounted on a microgrid, and cut with an ultramicrotome. Analysis was performed with a transmission electron microscope (manufactured by Philips, CM120) at an acceleration voltage of 120 KV, and a bright-field image at a magnification of 200,000 was obtained. (Measurement of true density) The true density was measured using a continuous automatic powder and granular material true density measuring device "Autotoludencer MAT-7000" manufactured by Seishin Enterprise Co., Ltd. using n-butanol as a solvent. (Measurement of Raman spectrum) Using an Ar laser 514.5 nm as the excitation light and a CCD (Charge Coupled Device) as the detector, the Raman spectrum of the carbon material as a raw material for activated carbon was measured with a slit of 500 μm and an exposure of 60 seconds. (Measurement of pore volume) Calculated using the DFT method from the adsorption isotherm of nitrogen at the liquid nitrogen temperature using AUTOSORB 1 manufactured by Quantachrome. The relative pressure (P / P0) is 10 for the amount of nitrogen adsorbed.^-8Measured at ~ 1.0.

(Preparation of electrodes) 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 size of 10 μm, kneaded and rolled into a sheet having a thickness of 0.5 mm. This sheet was punched into a disk having a diameter of 20 mm, vacuum dried at 200 ° C. for a whole day and night, and used as a polar electrode.

(Assembly of Electric Double Layer Capacitor) The above electrode was used for evaluation by assembling an evaluation cell as shown in FIG. 1 in a glove box in which high-purity argon is circulated. In Fig. 1, 1 is an aluminum top lid, 2 is a fluororubber O-ring, 3 is a current collector made of aluminum, 4 is an insulating material made of Teflon (registered trademark), 5 is an aluminum container, and 6 is made of aluminum. Leaf spring, 7 is a polarizing electrode, and 8 is a 1 mm thick separator made of glass fiber. The trade name LIPASTE-P / EAFIN (1 mol / liter) manufactured by Tomiyama Pure Chemical Industries, Ltd., which uses PC (propylene carbonate) as a solvent and (C2H5) 4NBF4 as an electrolyte, was used as the electrolyte. When measuring the change in the coefficient of expansion of the electrode during charging and discharging, the evaluation cell as shown in FIG. 2 is used, and the displacement in the electrode thickness direction is measured using the indicator. The electrode presser coil spring 9 in FIG. 2 can be used that requires a load of about 0.1 to 1.0 kgf to compress 1 cm, but a load of 0.3 kgf is required for this measurement. I used what I did. The measurement temperature was room temperature (20 to 30 ° C). Here, for example, the electrode expansion coefficient (%) when a 2.5 V voltage is applied is obtained by (electrode thickness at 2.5 V charge-electrode thickness before charging) / (electrode thickness before charging) × 100.

The charge / discharge measurement uses the charge / discharge test device HJ-101SM6 manufactured by Hokuto Denko Co., Ltd., and charges / discharges at 0 to 2.5V or 0 to 3.0V at 5mA, and the discharge curve obtained by the second constant current discharge. From this, the capacitance per mass (F / g) and the capacitance per volume (F / ml) of the bipolar activated coal of the electric double layer capacitor were calculated. Durability was evaluated by the capacity retention rate of the electric capacity (electric capacity after the cycle test / electric capacity after the second charge / discharge) after 200 charge / discharge cycle tests.

(Example 1) The coal pitch at a softening point of 86 ° C was raised at 50 ° C / hour, held at 500 ° C for 10 hours for the first stage of carbonization, and this was raised under the above conditions to 670 °. The second stage carbonization was carried out by holding at C for 1 hour. When the obtained charcoal material was jet mill crushed to an average particle size of 10 μm, the true density of the charcoal material was 1.55 g / ml. The obtained charcoal material was mixed with 2.5 times the mass ratio of KOH and filled in a crucible. This was heated to 750 ° C at 3 ° C / hr in a nitrogen atmosphere containing 80% by volume of water vapor, and then held at 750 ° C for 60 minutes for activation. The activated carbon material was washed by boiling with 1N hydrochloric acid and then washed with boiling water to remove residual KOH and metal impurities. This was vacuum dried at 200 ° C. to obtain activated carbon having an average particle size of 10 μm.

The specific surface area of this activated carbon is 1300m²It was / g, and no crystalline carbon moiety was observed in the bright field image under a transmission electron microscope (TEM). The pore volume of 4 Å or less by the DFT method was 0.006 ml / g, and the pore volume of 10 to 13 Å was 0.025 ml / g. 1% by mass of vapor-phase carbon fiber (length 5 μm), 9% by mass of carbon black and 10% by mass of PTFE (polytetrafluoroethylene) are dry-mixed with the activated carbon, and then an organic solvent is added to the mixed powder. After adding and swelling, the mixture was kneaded, rolled, and vacuum dried at 200 ° C. to prepare an electrode material. The electric capacities at the time of 2.5V charge / discharge were 40.3F / g and 31.4F / ml, and the electrode density was 0.78g / ml. The capacity retention rate after 200 cycles of charging and discharging was 98.2%. The coefficient of expansion of the electrodes when the voltage was applied was 13%.

(Example 2) Activated carbon was produced in the same manner as in Example 1, and 5% by mass of vapor phase carbon fibers (length 20 μm) were mixed to prepare a polar electrode material. The electric capacities at the time of 2.5V charge / discharge were 39.5F / g and 30.4F / ml, and the electrode density was 0.77g / ml. The capacity retention rate after 200 cycles of charging and discharging was 98.7%. The coefficient of expansion of the electrode when the voltage was applied was 8%.

(Example 3) The activated carbon obtained by the method of Example 1 was alkali-activated by the vapor phase method carbon fiber (micropore volume: 0.3 ml, BET specific surface area 490 m).²/ g, length 8 μm) 3% by mass was mixed to prepare a polar electrode material. The electric capacity during 2.5V charge / discharge was 40.1 F / g and 30.9 F / ml, and the electrode density was 0.77 g / ml, and the capacity retention rate after 200 cycles of charge / discharge was 99.0%. The coefficient of expansion of the electrode when the voltage was applied was 10%.

(Example 4) The coal pitch at a softening point of 86 ° C was carbonized in the first stage at 500 ° C and in the second stage at 650 ° C. The obtained charcoal material was mixed with 2.5 times the mass ratio of KOH and filled in a crucible. The true density of the obtained carbonaceous material was 1.52 g / ml. This was heated to 750 ° C at 3 ° C / hr in the presence of water vapor, and then held at 750 ° C for 60 minutes for activation. The activated carbon material was washed by boiling with 1N hydrochloric acid and then washed with boiling water to remove residual KOH and metal impurities. This was vacuum dried at 200 ° C. and then pulverized to obtain activated carbon having an average particle size of 10 μm.

The specific surface area of this activated carbon is 1416m²It was / g, and no crystalline carbon moiety was observed in the bright field image under a transmission electron microscope (TEM). The pore volume of 4 Å or less by the DFT method was 0.003 ml / g, and the pore volume of 10 to 13 Å was 0.26 ml / g. After dry mixing 9% by mass of carbon black and 10% by mass of PTFE (polytetrafluoroethylene) with the activated carbon, an organic solvent is added to the mixed powder to swell it, kneading it, rolling it, and then 200 ° C. The electrode material was prepared by vacuum drying with C. The electric capacities at the time of 2.5V charge / discharge were 39.2F / g and 30.3F / ml, and the electrode density was 0.77g / ml. The capacity retention rate after 200 cycles of charging and discharging was 98.5%. The coefficient of expansion of the electrode when the voltage was applied was 8%.

(Example 5) The coal pitch at a softening point of 86 ° C was carbonized in the first stage at 500 ° C and in the second stage at 650 ° C. The obtained charcoal material was mixed with 2.5 times the mass ratio of KOH and filled in a crucible. The carbonaceous material was jet milled to an average particle size of 4 μm. The true density was 1.53 g / ml. This was heated to 750 ° C at 3 ° C / hr, steam was introduced, and the temperature was maintained at 750 ° C for 60 minutes for activation. The activated carbon material was washed by boiling with 1N hydrochloric acid and then washed with boiling water to remove residual KOH and metal impurities. This was vacuum dried at 200 ° C. to obtain activated carbon having an average particle size of 4 μm.

The specific surface area of this activated carbon is 1338m²It was / g, and no crystalline carbon moiety was observed in the bright field image under a transmission electron microscope (TEM). The pore volume of 4 Å or less by the DFT method was 0.003 ml / g, and the pore volume of 10 to 13 Å was 0.25 ml / g. After dry mixing 9% by mass of carbon black and 10% by mass of PTFE (polytetrafluoroethylene) with the activated carbon, an organic solvent is added to the mixed powder to swell it, kneading it, rolling it, and then 200 ° C. The electrode material was prepared by vacuum drying with C. The electric capacities at the time of 2.5V charge / discharge were 40.5F / g and 30.4F / ml, and the electrode density was 0.75g / ml. The capacity retention rate after 200 cycles of charging and discharging was 98.5%. The coefficient of expansion of the electrodes when the voltage was applied was 11%.

(Comparative Example 1) The coal pitch at a softening point of 86 ° C was carbonized in the first stage at 500 ° C and in the second stage at 800 ° C. The obtained charcoal material was mixed with 2.5 times the mass ratio of KOH and filled in a crucible. The true density was 1.60 g / ml. This was heated to 750 ° C at 3 ° C / hr and then held at 750 ° C for 60 minutes for activation. The activated carbon material was boiled in 1N hydrochloric acid and then washed with boiling water to remove residual KOH and metal impurities. This was vacuum dried at 200 ° C. and then pulverized to obtain activated carbon having an average particle size of 10 μm.

The specific surface area of this activated carbon is 800m²It was / g, and a crystalline carbon portion was observed in a bright field image under a transmission electron microscope (TEM). The pore volume of 4 Å or less by the DFT method was 0.07 ml / g, and the pore volume of 10 to 13 Å was 0.082 ml / g. The activated carbon was used as a polar electrode material. The electric capacities at the time of 2.5V charge / discharge were 38.4F / g and 32.3F / ml, and the electrode density was 0.84g / ml. The capacity retention rate after 200 cycles of charging and discharging was 96.7%. The coefficient of expansion of the electrode when the voltage was applied was 33%.

(Comparative Example 2) 5% by mass of vapor phase carbon fiber (length 5 μm) was mixed with the activated carbon of Comparative Example 1 to prepare a polar electrode material. The electric capacities at the time of 2.5V charge / discharge were 37.8F / g and 30.2F / ml, and the electrode density was 0.80g / ml. The capacity retention rate after 200 cycles of charging and discharging was 98.2%. The coefficient of expansion of the electrodes when the voltage was applied was 26%.

(Comparative Example 3) After washing, the activated carbon of Example 1 is filled in a crucible, the temperature is raised to 600 ° C at 3 ° C / hr, steam is introduced, and the crucible is held at 600 ° C for 60 minutes to activate the steam. did. The activated carbon material was washed with 1N hydrochloric acid and then with distilled water to remove residual KOH and metal impurities. This was vacuum dried at 200 ° C. and then pulverized to obtain activated carbon having an average particle size of 10 μm. Table 1 shows the results from Example 1 to Comparative Example 3 above.

<tables num="1"><img file="JP2005136397A_D0001.tif" /></tables>

The electric double layer capacitor using the electrode material containing activated carbon of the present invention can be used for applications to high-performance energy devices, for example, a power storage system combined with a solar cell or a fuel cell, an engine assist of a hybrid car, and the like.

<figref num="1">It is sectional drawing of the cell for evaluation of an electric double layer capacitor.</figref><figref num="2">It is sectional drawing of the electrode expansion measuring apparatus.</figref><figref num="3">It is a pore size distribution map of the activated carbon of Example 1.</figref><figref num="4">It is a pore size distribution map of the activated carbon of Comparative Example 1.</figref>

Code description

1 Top lid 2 O-ring 3 Current collector 4 Insulator 5 Container 6 Leaf spring 7 Electrode 8 Separator 9 Coil spring