JP2005343706A - Method of manufacturing porous carbon material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing porous carbon material simply with good precision, for preventing the deterioration with time of a porous carbon material for electric double layer capacitor, which has a high specific surface area and is obtained by alkali-treating mesocarbon microbeads or the like. <P>SOLUTION: The porous carbon material is obtained by treating a suspension containing a porous carbon material by electrodialysis. And specially, the porous carbon material having such superior characteristics that the total content of alkali metals is 100 mass ppm or lower, the total content of transition metals is 100 mass ppm or lower, the pH of an extract obtained by extraction processing with an ion exchange water is 5-8, and the conductance is 10 μS/cm or lower, is obtained by rapidly reducing the remained alkali metals and transition metals by the electrodialysis treatment. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a porous carbon material used as an active material for an electrode for an electric double layer capacitor, for example.

The porous carbon material is generally called activated carbon. As a method for producing this activated carbon, a gas activation method in which an oxidizing gas such as water vapor, air, oxygen, CO _{2 is} allowed to act on a carbonaceous material such as coal, coal coke, coconut shell, wood powder, resin, or In general, pores are formed by a chemical activation method in which chemicals such as zinc chloride and potassium hydroxide are allowed to act.

  Conventionally, activated carbon has been used as an adsorbent for environmental conservation and improvement. Recently, activated carbon has been studied for new uses such as a pressure swing adsorption carrier, a chromatographic separation carrier, and a catalyst carrier.

  Furthermore, attempts have been made to use activated carbon in the electronic field, and among these, electric double layer capacitors are attracting attention. An electric double layer capacitor is one of energy storage devices, and is composed of a pair of polarizable electrodes including a porous carbon material, a separator, an electrolyte solution, and the like.

  The charge / discharge mechanism of a secondary battery, which is a general energy storage device, involves an electrochemical reaction, but the charge / discharge mechanism of an electric double layer capacitor does not involve an electrochemical reaction. This is done by simple adsorption / desorption of positive and negative ions.

  Therefore, the electric double layer capacitor has an excellent feature not found in the secondary battery. That is, it has excellent instantaneous charge / discharge characteristics, stable charge / discharge characteristics over a wide temperature range, and low performance deterioration due to repeated charge / discharge.

  Since the capacitance of the electric double layer capacitor is proportional to the surface area of the active material, a porous material having a large specific surface area is used as the active material, and an electric double layer capacitor having a large capacitance is used. Obtaining has been studied.

  As a porous material having a large specific surface area, a porous carbon material, that is, activated carbon has been mainly used because it exhibits high conductivity, is electrochemically relatively stable, and is easily available.

  By the way, an electric double layer capacitor used for various electronic devices / electronic parts, various electric vehicles and the like is desired to have no performance deterioration due to repeated charge and discharge, that is, to have excellent durability. It is said that the main cause of performance deterioration due to repeated charge and discharge is due to metals remaining in the porous carbon material.

  However, for example, in a porous carbon material in which pores are formed by chemical activation, there is a possibility that alkali metals used as chemicals and transition metals mixed from the apparatus / container may remain in the porous carbon material.

These metals remaining in the porous carbon material catalyze decomposition of the electrolytic solution during charge / discharge and cause gas generation. Due to this gas generation, (1) increase in internal resistance and decrease in capacitance due to peeling of the active material from the current collector (2) decrease in capacitance due to decrease in effective specific surface area due to gas adsorption to the pore openings ( 3) A decrease in electrostatic capacity and an increase in leakage current due to a partial drop of the active material are caused, and the durability of the electric double layer capacitor is significantly decreased.

  As the existence form of metals remaining in the porous carbon material, a physical adsorption state, a chemical adsorption state, and the like are conceivable. Therefore, as a technique for removing these, generally, ion-exchanged water or the like is used as a washing solvent, and this is combined with acids to reduce or remove metals in a physical adsorption state or a chemical adsorption state. I came.

  For example, there is a method of eluting and removing heavy metals and other impurities contained in activated carbon by mixing the activated carbon powder with purified water and boiling, followed by filtering the required number of times (for example, (See Patent Document 1).

  In this technology, desorption is promoted mainly by boiling heavy metals and other impurities that are in a physically adsorbed state. Therefore, if the amount of residual metals is reduced to some extent, the concentration difference that is the driving force for desorption is reduced. It is necessary to repeat the washing operation many times.

Further, it is difficult to remove heavy metals and other impurities mainly in a chemically adsorbed state by this technique. For this reason, in order to reduce and remove these effectively, acids may be added. However, also in this case, it is necessary to repeat the washing operation many times in order to remove the added acids, which requires a lot of labor and time.
Japanese Patent Laid-Open No. 7-142295

  The present invention is a porous material having excellent characteristics used as an active material of an electrode for an electric double layer capacitor, that is, the total content of alkali metals is 100 mass ppm or less, and the total content of transition metals Provides a method for easily and accurately producing a porous carbon material having an extract of 100 mass ppm or less, an extract obtained by extraction with ion-exchanged water having a pH of 5 to 8, and an electrical conductivity of 10 μS / cm or less. For the purpose.

  The present invention is a method for producing a porous carbon material, characterized by electrodialyzing a suspension containing the porous carbon material.

  In the present invention, the suspension containing the porous carbon material is electrodialyzed, the total content of alkali metals is 100 mass ppm or less, the total content of transition metals is 100 mass ppm or less, ions A method for producing a porous carbon material, comprising producing a porous carbon material in which the pH of the extract obtained by the extraction treatment with exchange water is 5 to 8, and the conductivity of the extract is 10 μS / cm or less It is.

  Electrodialysis treatment is a treatment based on the principle of separation based on differences in electrostatic properties, shape, and size of ions and molecules. Alkali metals remaining in the porous carbon material contained in the suspension In addition, transition metals can be easily reduced.

  By applying this treatment, it is possible to obtain a porous carbon material that has the above-described characteristics and hardly deteriorates with time.

  When the porous carbon material is a material obtained by chemically activating a carbon material with an alkali metal compound such as lithium hydroxide, sodium hydroxide, or potassium hydroxide, the method of the present invention uses these alkali metals and transition metals. Can be easily reduced with high accuracy.

  Further, it is preferable that the carbon material is a mesophase microsphere and / or a bulk mesophase.

  According to the present invention, a suspension containing a porous carbon material is electrodialyzed to reduce alkali metals and transition metals remaining in the porous carbon material, and has excellent durability for an electric double layer capacitor. The active material of the electrode can be easily manufactured. That is, the total content of alkali metals is 100 mass ppm or less, the total content of transition metals is 100 mass ppm or less, and the pH of the extract obtained by the extraction treatment with ion-exchanged water is 5 to 8, And the porous carbon material whose electrical conductivity of this extract is 10 microsiemens / cm or less can be obtained easily.

  Embodiments of the present invention will be described below with reference to the drawings.

  As a result of intensive studies on a method for reducing alkali metals, transition metals, and the like, which are impurities remaining in the porous carbon material, the present inventor includes porous carbon materials in which alkali metals and transition metals remain. It has been found that the suspension can be reduced by electrodialysis treatment in a unit operation and by a very short treatment.

  That is, the present invention is a method for producing a porous carbon material by electrodialysis treatment for reducing alkali metals and transition metals remaining in the porous carbon material.

  In the present invention, the total content of alkali metals is 100 mass ppm or less, and the total content of transition metals is 100 mass ppm or less. This is because sufficient durability cannot be obtained when used as an active material for an electrode for an electric double layer capacitor. In addition, the pH of the extract obtained by the extraction treatment with ion-exchanged water was set to 5 to 8 and the conductivity was set to 10 μS / cm or less, as the active material of the electrode for the electric double layer capacitor when deviating from these ranges. This is because the durability becomes insufficient when used.

  In the present invention, ion-exchanged water is purified water having a conductivity of 0.1 μS / cm or less, and is obtained by treating with ion-exchange resin.

  The extraction process defined in the present invention is specifically performed by the following method.

  That is, 1 g of a porous carbon material sample (as a dry weight) is accurately weighed and transferred to a 100 ml Erlenmeyer flask. To this, 50 ml of accurately measured ion-exchanged water is poured to suspend the sample, and the mixture is stirred for 16 hours at 150 rpm at room temperature with a shaker (manufactured by Taitec Corp., double shaker, NR-30 type). The suspension is filtered with a filter paper (Advantech Toyo, 5C). The conductivity of the extract (filtrate) obtained by this extraction operation was measured with a conductivity meter (manufactured by Horiba, ES-14), and the pH was measured by a pH meter (manufactured by Horiba, F-14) at room temperature ( 25 ° C.). Of course, as long as each device is calibrated correctly, it is possible to use a general conductivity meter and pH meter regardless of these unique devices.

  According to this extraction method, alkali metals and transition metals remaining in the porous carbon material are eluted by ion-exchanged water, and are retained in the porous carbon material by measuring conductivity and pH. It is possible to indirectly know the amount of alkali metals and transition metals to be used.

The porous carbon material of the present invention is preferably obtained by chemically activating a carbon material with an alkali metal compound such as lithium hydroxide, sodium hydroxide, or potassium hydroxide. The amount of these alkali metal compounds used varies depending on the specific surface area of the desired porous material, but when trying to obtain a specific surface area of 500 m ² / g or more, the mass ratio is 0.5 to Five times the amount, preferably 1-3 times the amount may be used.

  As the carbon material used in the present invention, a soft carbon material is preferable. Among the soft carbon materials, mesophase microspheres and / or bulk mesophase are more preferable. As the mesophase microspheres and / or the bulk mesophase, known ones can be widely used, and those obtained by pulverizing them may be used. The particle size is not particularly limited, but those having an average particle size in the range of 2 to 100 μm are preferable, and those having an average particle size of 2 to 40 μm are more preferable.

  As raw materials for mesophase spherules and / or bulk mesophase, generally, pitches such as coal-based tar or pitch, petroleum-based heavy oil, or pitch are used.

  These pitches are preferably heated in a nitrogen atmosphere at 350 to 500 ° C. for 0.5 to 10 hours, so that mesophase spherules are first formed in the pitch and then mesophase globules are aggregated. Generate. Therefore, the generation of the mesophase microspheres and / or the bulk mesophase may be appropriately selected according to the heat treatment conditions as desired.

  The mesophase microspheres and / or the bulk mesophase can be obtained from the pitch containing the mesophase microspheres and the bulk mesophase after the heating by a solvent separation operation using, for example, quinoline.

  In the present invention, these commercially available products may be used. Either mesophase microspheres or bulk mesophase may be used, or a mixture thereof may be used.

  Such a soft carbon material is a graphite precursor, and graphitization progresses in the activation process, and it is easy to form a graphite layer structure. Therefore, a fine structure with less internal resistance can be formed after activation. .

  The atmosphere at the time of activation is preferably an inert gas atmosphere such as nitrogen or argon because generation of transition metal oxides is suppressed during the activation process. In the present invention, the activation treatment using the alkali metal hydroxide of the carbon material is not particularly limited with respect to the order of addition of these materials. For example, an aqueous solution of an alkali metal hydroxide and a carbon material can be mixed to easily obtain a suspension.

  Next, the obtained suspension is heat-treated to obtain a dry mixture or a low-moisture-containing mixture, and then an activation treatment is performed by heating to 300 to 1500 ° C., preferably 500 to 900 ° C. in an inert gas atmosphere. In this heating, various heating devices such as a rotary kiln, a fluidized bed, a moving bed, and a fixed bed can be used. The activation time is usually about 10 minutes to 24 hours. This heating may be performed at a desired temperature after the temperature is raised at a certain rate, or may be held several times in the middle until the final temperature is reached.

  After the activation treatment, an electrodialysis treatment which is an important treatment of the present invention is performed. Prior to the electrodialysis treatment, it is preferable to wash away an alkali metal compound present in a considerable amount in the porous carbon material after the activation treatment with ion-exchanged water or acids.

  In electrodialysis treatment, anion exchange membranes that allow only anions to pass through and cation exchange membranes that allow only cations to pass through are alternately arranged to give a potential gradient in a direction perpendicular to the membrane, and electrostatic characteristics, shape, and size. The components are separated by the difference between the two. When a suspension (electrolyte solution) containing a porous material to be treated is flowed in parallel to the ion exchange membrane, the solution exiting the desalting chamber is desalted by electrophoresis based on the potential gradient of the electrolyte and selective membrane permeation, The solution leaving the salt concentration chamber is concentrated.

  In the electrodialysis treatment, the porous carbon material immediately after being made porous may be suspended in ion-exchanged water or the like to form a suspension, and this suspension may be subjected to electrodialysis treatment as it is. What was washed several times with ion exchange water or acids may be electrodialyzed. However, in the former case, the concentration of the concentrate eluted as a result of electrodialysis becomes very high and the operation becomes complicated, so electrodialysis is performed several times in advance with a washing solvent such as ion-exchanged water or acids. The latter is preferred.

  In addition, in order to remove metals present in the pores, a nonionic interface that has little electrical influence on hydrophilic organic solvents such as methanol and electrodialysis so that the washing solvent completely penetrates into the pores. It is preferable to use ion-exchanged water or the like to which an activator is added as a washing solvent. Furthermore, it is effective for the same reason to use ultrasonic treatment and vacuum deaeration treatment in combination.

  As a treatment condition of the electrodialysis treatment, a constant voltage application or a constant current application is performed using a DC power source. Although it varies depending on the size of the electrodialyzer and the amount of the liquid to be treated, it is preferable to apply a constant voltage in the range of 1 to 10 V. In the case of constant current application, it is preferably performed in the range of 5 mA to 10 A. By setting the voltage and current conditions in the above ranges, alkali metal soot and transition metals can be effectively reduced.

  The higher the temperature at which electrodialysis is performed, the better because the viscosity of the suspension decreases, but the treatment at around room temperature is preferred from the viewpoint of the heat resistance of the ion exchange membrane.

  Although the speed at which the liquid to be treated is passed through the desalting chamber varies depending on the size of the electrodialysis apparatus, it usually prevents sedimentation of the porous carbon material in the suspension when it is performed in the range of 10 to 1000 ml / min. This is preferable.

  According to the method for producing a porous carbon material of the present invention as described above, alkali metals and transition metals remaining in the porous carbon material are obtained by electrodialyzing the suspension containing the porous carbon material. The total content of alkali metals is 100 mass ppm or less, the total content of transition metals is 100 mass ppm or less, and the pH of the extract obtained by the extraction treatment with ion-exchanged water is 5 to 8 And the porous carbon material which has the outstanding characteristic whose electrical conductivity of this extract is 10 microsiemens / cm or less can be obtained easily.

  Examples of the present invention will be described. However, the present invention is not limited to only the examples described below.

(Example 1)
Mesophase microspheres (JFE Chemical, trade name KMFC, average particle size 18 μm) 1 kg and 2 kg of potassium hydroxide (made by Wako Pure Chemicals, special grade) dissolved in 2 kg of ion-exchanged water are mixed in a nickel container. And heat-treated at 120 ° C. for 24 hours. Thereafter, the entire vessel was charged into an electric furnace, heated to 800 ° C. at a heating rate of 5 ° C./min in a nitrogen atmosphere, heated for 2 hours, cooled, and the activation treatment was completed.

  The activated product was washed twice with 10 liters of ion exchange water to obtain a porous carbon material. As a result of ICP analysis, this contained 8000 ppm by mass of potassium and 450 ppm by mass of nickel. 200 g of this porous carbon material was suspended in 2 liters of ion exchange water. The pH of this suspension was 10.7, and the conductivity was 300 μS / cm.

  This suspension was injected into an electrodialysis tank, and electrodialysis was performed at a current of 5 A and a flow rate of 200 ml / min.

  The electrodialysis solution was sampled every 30 minutes and the pH and conductivity were measured. The results are shown in FIGS. FIG. 1 and FIG. 2 are graphs showing the progress with the treatment time taken on the horizontal axis and the vertical axis taken pH and conductivity, respectively. Lines 1 and 2 indicate the pH and conductivity of the examples subjected to the electrodialysis treatment. As is apparent from FIGS. 1 and 2, the example showed pH 5.8 and conductivity 1.5 μS / cm after 6 hours.

  Further, after 6 hours of electrodialysis treatment, the porous carbon material was collected and dried. As a result of analyzing potassium and nickel remaining in the porous carbon material, they were 70 mass ppm and 10 mass ppm, respectively. The porous carbon material was extracted with ion exchange water as described above. The pH of the extract obtained by this extraction treatment was 6.2, and the conductivity was 2.3 μS / cm.

(Comparative Example 1)
After the activation treatment as in Example 1, 200 g of the porous carbon material that had been washed twice was suspended in 5 liters of ion-exchanged water, and the suspension was stirred for 50 minutes and filtered for 10 minutes. The obtained porous carbon material cake after stirring and filtration was suspended again in 5 liters of ion-exchanged water, stirred for 50 minutes, and suspended for 10 minutes. The filtrate was sampled for each stirring and filtration operation, and the pH and conductivity were measured. The results are shown by lines 3 and 4 in FIGS. 1 and 2, respectively. The extract obtained after 6 hours of the above operation and after 6 hours showed pH 9.43 and conductivity 19.7 μS / cm, respectively. The porous carbon material after the sixth operation was collected and dried. When potassium and nickel remaining in the porous carbon material were analyzed, they were 2050 ppm by mass and 250 ppm by mass, respectively. Further, the pH of the extract obtained by the extraction treatment with ion-exchanged water of the porous carbon material was 9.2, and the conductivity was 21.6 μS / cm.

  As described above, in the conventional method, it takes a long time to manufacture a porous carbon material having characteristics suitable as an active material for an electrode for an electric double layer capacitor.

  On the other hand, the method according to the present invention can produce a porous carbon material having characteristics suitable as an active material for an electrode for an electric double layer capacitor.

It is a graph which shows the relationship between the processing time of Example 1 by electrodialysis processing, and the comparative example 1 by stirring filtration processing, and pH. It is a graph which shows the relationship between the process time of Example 1 by an electrodialysis process, and the comparative example 1 by a stirring filtration process, and electrical conductivity.

Explanation of symbols

  1-4 lines

Claims (4)

  A method for producing a porous carbon material, comprising subjecting a suspension containing the porous carbon material to electrodialysis.   The suspension containing the porous carbon material is electrodialyzed, and the total content of alkali metals is 100 mass ppm or less, the total content of transition metals is 100 mass ppm or less, and the extraction treatment is performed with ion-exchanged water. A method for producing a porous carbon material, comprising producing a porous carbon material having a pH of 5 to 8 of the obtained extract and a conductivity of the extract of 10 μS / cm or less.   3. The method for producing a porous carbon material according to claim 1, wherein the porous carbon material is obtained by chemically activating a carbon material with an alkali metal compound.   The method for producing a porous carbon material according to claim 3, wherein the carbon material is a mesophase microsphere and / or a bulk mesophase.

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