AU2007203187A1

AU2007203187A1 - Manufacturing method of activated carbon and electric double-layer capacitor

Info

Publication number: AU2007203187A1
Application number: AU2007203187A
Authority: AU
Inventors: Tatsuaki Yamaguchi
Original assignee: MASATADA SAKAMOTO; SANGYO GIJUTSU KENKYUSHO KK; TOHRU YAMANAKA
Current assignee: Sakamoto Masatada; SANGYO GIJUTSU KENKYUSHO KK
Priority date: 2006-08-10
Filing date: 2007-07-09
Publication date: 2008-02-28
Also published as: BRPI0703505A2; JP2008037733A

Description

Cl (TRANSLATION) SJAPAN PATENT OFFICE This is to certify that the annexed is a true copy of the following application as filed with this Office.

00 ps Date of Application :August 10, 2006 CI Application Number Patent Application No. 2006-218316 The country code and number of your priority application, to be used for filing abroad under the Paris Convention, is JP2006-218316 Applicant(s) Kabushiki Kaisha Sangyo Gijutsu Kenkyusho; and Tatsuaki Yamaguchi April 17, 2007 Commissioner, Japan Patent Office: Makoto Nakajima (Seal) Application Certification No. ACP2007-3026578 p..

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oo 1, 1 Regulation 3.2

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicants: Actual Inventor: Address for Service: Invention title: Kabushiki Kaisha Sangyo Gijutsu Kenkyusho, Tatsuaki Yamaguchi, Masatada Sakamoto, and Tohru Yamanaka Tatsuaki Yamaguchi MADDERNS, 1st Floor, 64 Hindmarsh Square, Adelaide, South Australia, Australia MANUFACTURING METHOD OF ACTIVATED CARBON AND ELECTRIC DOUBLE-LAYER

CAPACITOR

The following statement is a full description of this invention, including the best method of performing it known to us.

(PatAU132) MANUFACTURING METHOD OF ACTIVATED CARBON AND ELECTRIC DOUBLE- LAYER CAPACITOR c, Field of the Invention [0001] The present invention relates to a manufacturing 0_ method of activated carbon large in specific surface area.

SThe activated carbon is the best to use for a major ingredient of polarizable electrodes of an electric doublelayer capacitor or other similar applications. The present invention also relates to a manufacturing method of an electric double-layer capacitor including a pair of polarizable electrodes, a separator disposed between the polarizable electrodes, a pair of collecting electrodes connected to the respective polarizable electrodes, and electrolyte being in contact with the polarizable electrodes.

Background of the Invention [0002] An electric double-layer capacitor is known in which activated carbon is used as a major ingredient of polarizable electrodes. For example, Japanese Patent No.

3330235 discloses an electric double-layer capacitor including a pair of polarizable electrodes opposed to each other through a separator, and a pair of collecting electrodes attached to the outer surfaces of the respective polarizable electrodes on the sides opposite to the separator.

Electrolyte is interposed between the polarizable electrodes laeach containing activated carbon as a major ingredient.

C Electric charges are accumulated in a pair of electric double-layers formed at the respective interfaces between the electrolyte and the polarizable electrodes. The above Patent further describes that activated carbon obtained from phenol 00 resin is superior in performance to activated carbon obtained C by carbonizing/activating cellulose such as pulp, for the purpose of activated carbon to be used for a major ingredient of the polarizable electrodes of the electric double-layer capacitor. In the above Patent, as activated carbon to be used for a major ingredient of the polarizable electrodes of the electric double-layer capacitor, a porous carbon material is used that is obtained by crushing a paper-base phenol resin laminate into a powder, carbonizing the powder, and then activating the powder with alkali metal hydroxide.

[0003] On the other hand, sugar cane bagasse, that is, cane trash, can not fully be consumed as heat sources for sugar factories and so on, and the material for electric power generation for sugar factories and so on. A considerable amount of bagasse is disposed. An amount of sugar cane bagasse beyond the use for the heat sources and the material for electric power generation is also used for papers, soil conditioners, and adsorbents. For the use as an adsorbent, the sugar cane bagasse is carbonized and activated with gas or a chemical. The BET surface area of the' adsorbent is about 1,000 m 2 The sugar cane bagasse is made up of a hard fibrous rind part having a honeycomb O structure caused by capillary tubes, and a pith part constituted by parenchyma cells at the center of the rind part. No particular detailed examination has been done for activated carbon obtained from the respective rind and pith 00 parts obtained by separating the sugar cane bagasse into the rind part and the pith part.

[0004] In the electric double-layer capacitor disclosed in the above Patent, a paper-base phenol resin laminate is used as the raw material for activated carbon to be used as a major ingredient of the polarizable electrodes. In the electric double-layer capacitor, therefore, no sufficient capacitance can be obtained. In addition, the raw material cost is high.

Summary of the Invention [0005] An object of the present invention is to provide activated carbon, that is, porous carbon, suitable for use in an electric double-layer capacitor in which a large capacitance is obtained and the raw material cost is low, and other similar applications.

[0006] According to a first aspect of the present invention, a manufacturing method of activated carbon is characterized by comprising a step of obtaining carbide by carbonizing a raw material obtained from bagasse of sugar cane made in, for example, Okinawa; and a step of obtaining activated carbon by activating said carbide with alkali. In O this case, the raw material is preferably a rind part of the sugar cane bagasse obtained by separating the sugar cane bagasse into a pith part and the rind part. The BET surface area of the activated carbon obtained by the alkali 0_ activation is preferably within a range of 3,000 to 3,500 m 2 and more preferably within a range of 3,250 to 3,400 m 2 /g.

[0007] According to the first aspect of the present invention, the carbonized temperature in the carbonization is preferably within a range of 673 k to 1,073 k, and more preferably within a range of 773 K to 973 K. Alkali to be used in the alkali activation is preferably alkali metal hydroxide such as sodium hydroxide and/or potassium hydroxide.

The weight ratio of alkali to the carbide in the alkali activation is preferably within a range of 4 to 9, and more preferably within a range of 5 to 8. The activated temperature in the alkali activation is preferably within a range of 823 k to 1,323 K, and more preferably within a range of 923 K to 1,223 K. The activated temperature in the alkali activation is higher than the carbonized temperature in the carbonization preferably by a range of 100 K to 300 K, and more preferably by a range of 150 k to 250 k. The activated carbon is preferably for an electric double-layer capacitor.

[0008] According to a second aspect of the present invention, a manufacturing method of an electric double-layer -4capacitor comprising a pair of polarizable electrodes; a separator disposed between the polarizable electrodes; a pair of collecting electrodes connected to the respective polarizable electrodes; and electrolyte being in contact with the polarizable electrodes, is characterized in that 00 activated carbon is used for a major ingredient of at least one of the polarizable electrodes, and the activated carbon is prepared through a step of obtaining carbide by carbonizing a raw material obtained from bagasse of sugar cane made in, for example, Okinawa; and a step of obtaining activated carbon by activating the carbide with alkali.

[0009] According to the second aspect of the present invention, the raw material is preferably a rind part of the sugar cane bagasse obtained by separating the sugar cane bagasse into a pith part and the rind part. The electric double-layer capacitance of the activated carbon is preferably within a range of 170 to 195 F/g, and more preferably within a range of 175 to 190 F/g. Alkali to be used in the alkali activation is preferably alkali metal hydroxide such as sodium hydroxide and/or potassium hydroxide.

[0010] In either of the first and second aspects of the present invention, one, two, or three of the raw material, the carbide, and the activated carbon can be cut, crushed, and/or ground through a suitable cutting process, a suitable crushing process, and/or a suitable grinding process, any of which may be known. However, when cutting, crushing, and/or grinding are not required in particular, the cutting process, the crushing process, and/or the grinding process may be omitted.

[0011] According to the present invention, activated carbon large in specific surface area and high in capacitance can be obtained with a low raw material cost through a Srelatively simple manufacturing process.

[0012] According to claims 2 to 6 and 9 to 14, activated carbon higher in capacitance can be obtained.

[0013] According to claim 16, an electric double-layer capacitor large in specific surface area and high in electric double-layer capacitance can be obtained with a low raw material cost through a relatively simple manufacturing process.

[0014] According to claim 17, an electric double-layer capacitor higher in electric double-layer capacitance can be obtained.

[0015] According to claims 18 and 19, an electric double-layer capacitor can be obtained that is very high in practicality because of a sufficiently high electric doublelayer capacitance.

Brief Description of the Drawings [0016] FIG. 1 is a partially cut-away schematic perspective view of the whole of an electric double-layer capacitor according to an embodiment of the present -6invention; FIG. 2 is an exploded perspective view of the electric double-layer capacitor of FIG. 1 though a case is omitted; FIG. 3A is a SEM image of a pith part of sugar cane bagasse to be used for manufacture of a polarizable electrode 0_ of the electric double-layer capacitor of FIG. 1;

C.

SFIG. 3B is a SEM image of a rind part of sugar cane bagasse to be used for manufacture of a polarizable electrode of the electric double-layer capacitor of FIG. 1; FIG. 4 is a graph showing relations between the carbonized temperature, the carbon yield, and the elemental ratio, on pith and rind parts of sugar cane bagasse to be used for manufacture of a polarizable electrode of the electric double-layer capacitor of FIG. 1; FIG. 5A is a SEM image of the carbonized pith part of sugar cane bagasse shown in FIG. 3A; FIG. 5B is a SEM image of the carbonized rind part of sugar cane bagasse shown in FIG. 3B; FIG. 6 is a graph showing relations between the alkali ratio, the BET surface area, and the yield, on activated carbons obtained from the pith part of sugar cane bagasse shown in FIG. 5A, the rind part of sugar cane bagasse shown in FIG. 5B, and the pith rind part; FIG. 7A is a SEM image of activated carbon having a BET surface area of 901 m 2 obtained from the pith part of sugar cane bagasse shown in FIG. FIG. 7B is a SEM image of activated carbon having a BET O surface area of 1,695 m 2 obtained from the pith part of sugar cane bagasse shown in FIG. FIG. 7C is a SEM image of activated carbon having a BET surface area of 2,113 m 2 obtained from the pith part of 0_ sugar cane bagasse shown in FIG. SFIG. 7D is a SEM image of activated carbon having a BET surface area of 3,120 m 2 obtained from the pith part of sugar cane bagasse shown in FIG. FIG. 8A is a SEM image of activated carbon having a BET surface area of 889 m 2 obtained from the rind part of sugar cane bagasse shown in FIG. FIG. 8B is a SEM image of activated carbon having a BET surface area of 1,682 m 2 obtained from the rind part of sugar cane bagasse shown in FIG. FIG. 8C is a SEM image of activated carbon having a BET surface area of 2,030 m 2 obtained from the rind part of sugar cane bagasse shown in FIG. FIG. 8D is a SEM image of activated carbon having a BET surface area of 3,328 m 2 obtained from the rind part of sugar cane bagasse shown in FIG. FIG. 9 is a graph showing N 2 adsorption isotherms at 77 K, on activated carbons obtained from the pith rind part of sugar cane bagasse, the pith part of sugar cane bagasse, and the rind part of sugar cane bagasse; and FIG. 10 is a graph showing relations between the BET -8surface area and the capacitance, on activated carbons O obtained from the pith rind part of sugar cane bagasse, the pith part of sugar cane bagasse, and the rind part of sugar cane bagasse.

0_ Detailed Description of the Preferred Embodiments [0017] Hereinafter, an embodiment of the present invention in which the present invention is applied to an electric double-layer capacitor will be described with reference to FIGS. 1 to 10 with dividing into sections of "1.

General Construction of Whole of Electric Double-layer Capacitor", Manufacturing Method of Polarizable Electrode", and Characteristic Test for Porous Carbon" [0018] 1. General Construction of Whole of Electric Double-layer Capacitor As shown in FIGS. 1 and 2, an electric double-layer capacitor 1 according to an embodiment of the present invention includes a pair of upper and lower polarizable electrodes 3 and 4 opposed to each other through a separator 2, and a pair of upper and lower collecting electrodes 5 and 6 attached to the outer surfaces of the respective polarizable electrodes 3 and 4 on the sides opposite to the separator 2. The layered structures 2 to 6 are housed in a substantially rectangular parallelepiped case 7 made of a proper synthetic resin or the like. The case 7 is filled up with proper electrolyte. Thus, the electrolyte is interposed -9between the polarizable electrodes 3 and 4. Electric charges O are accumulated on each of the upper and lower electric double-layers formed at the upper and lower interfaces between the electrolyte and the upper and lower polarizable electrodes 3 and 4.

0_ [0019] As shown in FIGS. 1 and 2, the separator 2 may be formed into a substantially rectangular thin sheet somewhat smaller than the inner periphery of the case 7. The separator 2 may be made of a paper such as a filter paper, or a nonwoven fabric of cellulose, glass fibers, or the like.

Each of the upper and lower polarizable electrodes 3 and 4 may be formed into a substantially rectangular thick sheet somewhat smaller than the separator 2. Each of the polarizable electrodes 3 and 4 may be made of porous carbon such as activated carbon. Details of each polarizable electrode will be described in the next section "2.

Manufacturing Method of Polarizable Electrode". Each of the upper and lower collecting electrodes 5 and 6 may be formed into a thin sheet. Each collecting elettrode 5 or 6 may be made up of a collecting electrode main body 5a or 6a and a substantially rectangular upper or lower thin terminal 5b or 6b. The collecting electrode main body 5a or 6a may be formed into a substantially rectangular thin sheet substantially equal to or somewhat smaller than the inner periphery of the case 7. The terminal 5b or 6b protrudes from the corresponding collecting electrode main body 5a or 6a beyond the case 7. Each of the collecting electrodes O and 6 may be made of a conductive material, for example, metal such as aluminum or titanium.

[0020] 2. Manufacturing Method of Polarizable Electrode Each of the upper and lower polarizable electrodes 3 0_ and 4 shown in FIGS. 1 and 2 may be manufactured as follows.

Activated carbon as porous carbon, which is obtained as will be described later in detail, is mixed with a conductive agent such as carbon black (made by Sigma-Aldrich Corporation), and a binder such as polytetrafluoroethylene (PTFE: made by Sigma-Aldrich Corporation) at a weight ratio of, for example, 8:1:1. The mixture is put in a mold and then pressed to form the shape shown in FIGS. 1 and 2.

[0021] For manufacturing activated carbon, bagasse of sugar cane made in Okinawa was used as the raw material. The sugar cane bagasse is made up of two parts, that is, a hard rind part and a pith part constituted by parenchyma cells.

The weight ratio between the rind and pith parts is about 18:7. In this embodiment, three kinds of raw materials were used, which were the pith part and the rind part obtained by separating the sugar cane bagasse into the respective parts, and the unseparated bagasse in which the pith and rind parts remain to be united with each other, which is referred to as "pith rind part" in this specification.

[0022] Each of the above three kinds of raw materials was dried for about 24 hours. The material was then put in a -11platinum crucible in a quartz glass tube. With flowing, for example, helium at a flow rate of, for example, about :Z ml/min, the temperature in the crucible was raised to, for example, about 973 K at a temperature rising rate of, for example, about 5 K/min. The material was kept at the OO temperature for about one hour to be carbonized. Each of Cthree kinds of carbides thus obtained as samples was treated Swith alkali, that is, activated with alkali, as follows. In this case, for example, sodium hydroxide (made by Wako Pure Chemical Industries, Ltd.: special grade chemical) was used as an activator. For the alkali treatment, each carbide was put in an alkali aqueous solution of, for example, about ml, prepared such that the weight ratio of the sodium hydroxide was 0.1 to 8.0 to 1.0 g of the carbide. The solution was agitated for, for example, one hour. After the agitation, each of the three kinds of samples was put in a platinum crucible in a quartz glass tube. The temperature in the crucible was raised to, for example, about 1073 K at a temperature rising rate of, for example, about 5 k/min. Each sample was kept at the temperature for, for example, about one hour. Three kinds of samples thus activated with alkali, that is, three kinds of activated carbons, were washed with pure water and dilute hydrochloric acid till pH after washing becomes about 7.

[0023] Three kinds of activated carbons prepared as described above were thermally dried in a vacuum. Each -12activated carbon was then mixed with, for example, carbon O black (made by Sigma-Aldrich Corporation), and, for example, PTFE (made by Sigma-Aldrich Corporation) at a weight ratio of, for example, 8:1:1, as described above. Each activated carbon was then put in a mold and pressed to form the shape 0_ shown in FIGS. 1 and 2. Three kinds of polarizable Selectrodes were thus prepared. In this specification, in the manufacturing methods of activated carbons, and therefore, polarizable electrodes or activated carbon electrodes, as described above, the manufacturing method of activated carbon, and therefore a polarizable electrode or an activated carbon, from the pith part of sugar cane bagasse is referred to as specific example 1; the manufacturing method of activated carbon, and therefore a polarizable electrode or an activated carbon, from the rind part of sugar cane bagasse is referred to as specific example 2; and the manufacturing method of activated carbon, and therefore a polarizable electrode or an activated carbon, from the pith rind part of sugar cane bagasse is referred to as specific example 3.

[0024] 3. Characteristic Test for Porous Carbon Three kinds of porous carbons, that is, activated carbons, prepared by the specific examples 1 to 3 as described in the above Manufacturing Method of Polarizable Electrode", were subjected to characteristic test.

For this purpose, 0.05 g of each of three kinds of mixtures as described above was put in a 13 mm-diameter mold and then -13pressed to be formed into a button shape. A collecting O electrode made of, for example, a titanium mesh, was attached Zby pressing to each of the button-shape compacts. Three kinds of testing activated carbon electrodes were thus prepared. These activated carbon electrodes were compared 00 with each other in characteristics.

[0025] "Characteristic Test for Porous Carbon" as will be described below in detail was carried out in the manner as described in the following to (d) The porosity of porous carbon, that is, activated carbon, was measured with a nitrogen gas absorption type Micromeritics Accelerated Surface Area and Porosimetry system (available by Shimadzu Corporation: ASAP2020) by the constant volume method at 77 k, and at this time, the micropore volume was analyzed and calculated by the MP method, and the mesopore volume was analyzed and calculated by the BJH method; The surface of activated carbon was observed with a scanning electron microscope (SEM); In the elemental analysis, the respective contents of carbon, hydrogen, and nitrogen were measured with a C/H/N simultaneous determination system (CHN corder MT-6 available by Yanaco), and the value obtained by subtracting the contents of three kinds of elements from the whole was considered the content of oxygen; and The electric double-layer capacitance was measured -14in a triode type cell, in which one of the above-described O three kinds of activated carbons, that is, the specific Zexamples 1 to 3, was used for an operation electrode; an activated carbon electrode (one of the specific examples 1 to 3) having six times the weight of the operation electrode was 0_ used for an opposite electrode; and a saturated calomel electrode was used for a reference electrode. 500 ml of 1 mol sulfuric acid aqueous solution was used for electrolyte.

The electric double-layer capacitance C was calculated by the following expression from the gradients of the profiles of the potential of the electrode and the time in the state wherein the positive and negative processes were set to a constant current, for example, 100 mA/g.

C I x AT/(V 2

V

1 I x AT/Av I: constant current

V

1 charging or discharging start potential

V

2 charging or discharging end potential AT: time required for charging or discharging [0026] First will be described characteristics of sugar cane bagasse, that is, the pith rind part of the sugar cane bagasse, and each part of the sugar cane bagasse, that is, the pith and rind parts of the sugar cane bagasse, which were used as the raw materials for the above-described three kinds of activated carbons. The following Table 1 shows results of the elemental analysis of the sugar cane bagasse. In this case, the rind and pith parts were substantially equal to 00 Co each other in the content of each of carbon, hydrogen, and oxygen. The ash content of the pith part was higher, as about 5.8 wt%, than that of the rind part.

[Table 1] Molar ratio Elemental analysis [wt%] Ash 0 O C H N H/C O/C (rest) [wt%] Pith rind 0.6 47.6 5.7 0.2 46.5 1.44 0.73 (7:18) Pith 5.8 48.5 5.9 0.5 45.2 1.45 0.70 Rind 1.9 48.3 5.8 0.2 45.6 1.45 0.71 d.a.f: dry ash free d.b: dry base [0027] FIG. 3A shows a SEM image of the pith part of sugar cane bagasse. FIG. 3B shows a SEM image of the rind part of sugar cane bagasse. From FIG. 3B, it is found that the rind part has a large number of about 10 pm-diameter pores caused by capillary tubes. Contrastingly, from FIG. 3A, it is found that the pith part does not have a large number of pores as of the rind part, and the pith part is in a powdered state.

[0028] Next will be described changes in a physical -16property of sugar cane bagasse and each part of the sugar O cane bagasse when they are carbonized. Two kinds of raw materials, that is, the pith and rind parts of sugar cane bagasse, were carbonized in a temperature range from about 573 K to about 973 K. FIG. 4 shows relations of the 0 carbonized temperature for each of the pith and rind parts of Ssugar cane bagasse to the carbon yield, the H/C ratio, and the O/C ratio. From FIG. 4, it is found that, in either case of the above two kinds of raw materials, the yield sharply drops when the carbonized temperature exceeds about 600 K, and the yield becomes substantially constant when the carbonized temperature exceeds about 873 K. As for the elemental ratios, it is found from FIG. 4 that the H/C ratio decreases as the carbonized temperature increases, and the O/C ratio becomes substantially constant when the carbonized temperature exceeds about 873 K. Further, in either case of the above two kinds of raw materials, the carbon content became more than 90% when the carbonized temperature exceeds about 973 K. It is thinkable that the above phenomenon is because dehydrogenating and deoxidation have progressed as water and carbon dioxide by heat decomposition of the pith part and the rind part. The BET surface area of carbide obtained at this time was about 20 m 2 /g when the carbonized temperature is 973 K. In the present invention, therefore, from the viewpoint of practical utility, in general, the carbonized temperature is preferably not less than 673 K and -17not more than 1,073 K, and more preferably not less than 773 K and not more than 973 K.

[0029] From the above characteristic found in connection with FIG. 4, it is thinkable that the carbonized temperature largely influences changes in structure of the pith and rind 00 parts of sugar cane bagasse. FIG. 5A shows a SEM image of Sthe structure of carbide obtained by carbonizing the pith part of sugar cane bagasse at about 973 K (specific example FIG. 5B shows a SEM image of the structure of carbide obtained by carbonizing the rind part of sugar cane bagasse at about 973 K (specific example From FIG. 5A, it is found that the carbide obtained from the pith part keeps substantially the same structure as its raw material shown in FIG. 3A. From FIG. 5B, it is found that the carbide obtained from the rind part substantially keeps the same structure as its raw material shown in FIG. 3B though some portions in which capillary structure has been broken due to deoxidation by thermal decomposition are observed in comparison with the raw material shown in FIG. 3B, that is, the rind part before carbonization.

[0030] Next will be described characteristics of the above-described three kinds of activated carbons prepared by alkali activation from the above-described three kinds of carbides obtained by carbonizing sugar cane bagasse and each part of the sugar cane bagasse. FIG. 6 shows relations of the weight ratio of alkali, in this case, sodium hydroxide, -18to carbide, to the BET surface area and the yield, for each O of the above-described three kinds of activated carbons, that is, FIG. 6 shows influence of the alkali ratio on the yield and the BET surface area. From FIG. 6, it is found that, in either case of the three kinds of activated carbons obtained 0_ from the pith rind part, the pith part, and the rind part, the BET surface area of activated carbon increases as the alkali ratio in the activated carbon increases. It is also found that activated carbon having its BET surface area of more than 3,000 m 2 /g is obtained when the alkali ratio is From the viewpoint of practical utility, in general, the activated temperature is preferably not less than 100 k and not more than 300 k higher than the carbonized temperature, and more preferably, not less than 150 k and not more than 250 K higher than the carbonized temperature. Therefore, from the viewpoint of practical utility, in general, the activated temperature is preferably not less than 823 k and not more than 1,323 k, and more preferably, not less than 923 k and not more than 1,223 k.

[0031] FIGS. 7A to 7D show SEM images of activated carbons obtained from the pith part of sugar cane bagasse (specific example FIG. 7A shows a case wherein the alkali ratio is 1.7 and the BET surface area is 901 m 2 /g.

FIG. 7B shows a case wherein the alkali ratio is 2.9 and the BET surface area is 1,695 m 2 FIG. 7C shows a case wherein the alkali ratio is 3.7 and the BET surface area is 2,113 -19m 2 FIG. 7D shows a case wherein the alkali ratio is O and the BET surface area is 3,120 m 2 From FIGS. 7A to 7D, it is found that, in the case of activated carbon obtained from the pith part, the size of particles constituting the activated carbon decreases as the BET surface area increases 0_ due to alkali activation. FIGS. 8A to 8D show SEM images of Sactivated carbons obtained from the rind part of sugar cane bagasse (specific example FIG. 8A shows a case wherein the alkali ratio is 1.0 and the BET surface area is 889 m 2 /g.

FIG. 8B shows a case wherein the alkali ratio is 2.0 and the BET surface area is 1,682 m 2 FIG. 8C shows a case wherein the alkali ratio is 3.0 and the BET surface area is 2,030 m 2 FIG. 8D shows a case wherein the alkali ratio is and the BET surface area is 3,328 m 2 From FIGS. 8A to 8D, it is found that, in the case of activated carbon obtained from the rind part, a small-size capillary tube structure remains though the capillary tube structure is somewhat broken as the BET surface area increases due to alkali activation. It is thus found that, in the case of activated carbon obtained from the rind part, the capillary tube structure is kept even when the BET surface area is increased.

In the present invention, therefore, from the viewpoint of practical utility, in general, the weight ratio of alkali to carbide is preferably not less than 4 and not more than 9, and more preferably, not less than 5 and not more than 8. In the present invention, from the viewpoint of practical utility, in general, the BET surface area of activated carbon is preferably not less than 3,000 m 2 /g and not more than 3,500 m 2 and more preferably, not less than 3,250 m 2 /g and not more than 3,400 m 2 /g.

[0032] FIG. 9 shows N 2 adsorption isotherms at 77 K for 00 each of the above-described three kinds of activated carbons.

SThe N 2 adsorption isotherms of activated carbon having its SBET surface area of more than 3,000 m 2 obtained from each of the above-described three kinds of raw materials, were Type II of the adsorption/desorption isotherm classification by International Union of Pure and Applied Chemistry (IUPAC).

In FIG. 9, P represents the adsorption equilibrium pressure and Po represents the saturated vapor pressure. The following Table 2 shows pore structure parameters calculated by analyzing the N 2 adsorption isotherms shown in FIG. 9.

From the Table 2, it is found that mesopore growth is promoted in either case of the above-described three kinds of activated carbons.

[Table 2] Alkali S Raw Total S Total Vp Vp [cm 3 /g] ratio [m 2 /g] material [m 2 [cm 3 /g] meso micro meso Pith rind 6.2 3343 2606 2.07 0.17 1.85 (7:18) -21- S: BET surface area Vp: pore volume [0033] Next will be described the electric double-layer 00 characteristic of bagasse activated carbon, that is, C-i activated carbon obtained from sugar cane bagasse, which is Smesoporous and high in specific surface area. FIG. 10 shows a relation between the BET surface area and the electric double-layer capacitance on each of the above-described three kinds of activated carbon electrodes. From FIG. 10, it is found that the electric double-layer capacitance Cdl increases as the BET surface area increases. In particular, it is found that the activated carbon electrode obtained from the rind part (specific example 2) is higher in the electric double-layer capacitance at each value of the BET surface area than the activated carbon electrode obtained from the pith part (specific example 1) and the activated carbon electrode obtained from the pith rind part (specific example It is also found that the maximum value of the electric double-layer capacitance of the activated carbon electrode obtained from the rind part is about 183 F/g at a BET surface area of about 3,328 m 2 which is about 10 to F/g higher than the maximum value of the electric doublelayer capacitance of the activated carbon electrode obtained from the rind part or the activated carbon electrode obtained -22 from the pith rind part. In the present invention, O therefore, from the viewpoint of practical utility, in general, the electric double-layer capacitance is preferably not less than 170 F/g and not more than 195 F/g, and more preferably, not less than 175 F/g and not more than 190 F/g.

00 [0034] Contrastingly, in the pore structure parameters Sshown in the above Table 2, no remarkable characteristic feature can be observed in accordance with the kind of raw material, that is, each part of sugar cane bagasse. In addition, although the chlorine exchange capacity (CEC) of activated carbon in which the highest electric double-layer capacitance was obtained, was measured from the viewpoint that the acid functionality of activated carbon is caused by an increase in electric double-layer capacitance, no particular characteristic feature could be obtained because the CEC value of the activated carbon in which the highest electric double-layer capacitance was obtained, was 0.11 to 0.13 mmol/g. From this, as shown by the SEM image of the activated carbon obtained from the rind part, about 10 pm 20 pores caused by capillary tubes exist in the activated carbon obtained from the rind part. It is found that the pores are caused by an increase in the electric double-layer capacitance of the activated carbon obtained from the rind part. It is thus found that activated carbon obtained from, 2 in particular, the rind part of sugar cane bagasse is the most suitable for a major ingredient of the polarizable -23electrode of the electric double-layer capacitor.

O [0035] An embodiment of the present invention has been described above. However, the present invention is never limited to the embodiment. Various changes and modifications can be made within the scope of the invention described in 00 the claims.

[0036] For example, in the above embodiment, the case 7 is substantially rectangular parallelepiped, and the separator 2, a pair of polarizable electrodes 3 and 4, and the collecting electrode main bodies 5a and 6a of a pair of collecting electrodes 5 and 6, are substantially rectangular parallelepiped. In a modification, however, the case 7 may be substantially cylindrical, and the separator 2, a pair of polarizable electrodes 3 and 4, and the collecting electrode main bodies 5a and 5b of a pair of collecting electrodes and 6, may be substantially cylindrical. In another modification, the separator 2 and a pair of polarizable electrodes 3 and 4 may be formed into long shapes. The separator 2 and the polarizable electrodes 3 and 4 are put in layers. They are rolled into a cylinder. The cylinder is put in a substantially cylindrical case 7.

[0037] In the above embodiment, the extraction electrodes of a pair of collecting electrode main bodies and 6a are formed as the terminals 5b and 6b integral with the respective collecting electrode main bodies 5a and 6a.

In a modification, however, the extraction electrodes may be -24 formed into separate bodies from the respective collecting electrode main bodies 5a and 6a.

00 0D

Claims

1. A manufacturing method of activated carbon, Zcharacterized by comprising: n a step of obtaining carbide by carbonizing a raw material obtained from sugar cane bagasse; and 00 a step of obtaining activated carbon by activating said carbide with alkali.

2. The method according to claim i, characterized in that said raw material is a rind part of said sugar cane bagasse obtained by separating said sugar cane bagasse into a pith part and said rind part.

3. The method according to claim 1 or 2, characterized in that the BET surface area of said activated carbon obtained by the alkali activation is within a range of 3,000 to 3,500 m 2 /g.

4. The method according to claim 1 or 2, characterized in that the BET surface area of said activated carbon obtained by the alkali activation is within a range of 3,250 to 3,400 m 2 /g.

5. The method according to any of claims 1 to 4, characterized in that the carbonized temperature in the carbonization is within a range of 673 k to 1,073 k.

6. The method according to any of claims 1 to 4, characterized in that the carbonized temperature in the carbonization is within a range of 773 k to 973 k.

7. The method according to any of claims 1 to 6, -26- characterized in that alkali to be used in the alkali O activation is alkali metal hydroxide.

8. The method according to any of claims 1 to 6, characterized in that alkali to be used in the alkali activation is sodium hydroxide and/or potassium hydroxide. 00

9. The method according to any of claims 1 to 8, Scharacterized in that the weight ratio of alkali to said carbide in the alkali activation is within a range of 4 to 9. The method according to any of claims 1 to 8, characterized in that the weight ratio of alkali to said carbide in the alkali activation is within a range of 5 to 8.

11. The method according to any of claims 1 to characterized in that the activated temperature in the alkali activation is within a range of 823 k to 1,323 K.

12. The method according to any of claims 1 to characterized in that the activated temperature in the alkali activation is within a range of 923 k to 1,223 K.

13. The method according to any of claims 1 to 12, characterized in that the activated temperature in the alkali activation is higher than the carbonized temperature in the carbonization by a range of 100 K to 300 K.

14. The method according to any of claims 1 to 12, characterized in that the activated temperature in the alkali activation is higher than the carbonized temperature in the carbonization by a range of 150 K to 250 K. The method according to any of claims 1 to 14, -27- characterized in that said activated carbon is for an electric double-layer capacitor.

16. A manufacturing method of an electric double-layer capacitor comprising a pair of polarizable electrodes (3, a separator disposed between said polarizable 00 electrodes a pair of collecting electrodes 6) connected to said polarizable electrodes 4), Srespectively; and electrolyte being in contact with said polarizable electrodes characterized in that activated carbon is used for a major ingredient of at least one of said polarizable electrodes and said activated carbon is prepared through a step of obtaining carbide by carbonizing a raw material obtained from sugar cane bagasse; and a step of obtaining activated carbon by activating said carbide with alkali.

17. The method according to claim 16, characterized in that said raw material is a rind part of said sugar cane bagasse obtained by separating said sugar cane bagasse into a pith part and said rind part.

18. The method according to claim 16 or 17, characterized in that the electric double-layer capacitance of said activated carbon is within a range of 170 to 195 F/g.

19. The method according to claim 16 or 17, characterized in that the electric double-layer capacitance of said activated carbon is within a range of 175 to 190 F/g. The method according to any of claims 16 to 19, -28- characterized in that alkali to be used in the alkali activation is alkali metal hydroxide.

21. The method according to any of claims 16 to 19, characterized in that alkali to be used in the alkali activation is sodium hydroxide and/or potassium hydroxide. 00 c mD 0D -29-