JP2015056502A - Active carbon for hybrid capacitor and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide active carbon with a high performance contributing to construction of a hybrid capacitor excellent in low temperature characteristics where capacity drop at low temperature is hard to occur, and to provide a method for manufacturing the active carbon.SOLUTION: Active carbon for a hybrid capacitor is active carbon used as electrode material for a hybrid capacitor. In the active carbon, a basic functional group amount on the basis of neutralization titration of the active carbon is regulated so as to be 0.3 mmol/g or less.

Description

  The present invention relates to activated carbon and a method for producing the same. Specifically, the present invention relates to activated carbon used for an electrode material of a hybrid capacitor and a method for manufacturing the same.

  In recent years, in order to improve the energy density of an electric double layer capacitor, in addition to the electric double layer, a hybrid capacitor (asymmetric type electric double layer capacitor) that stores energy by a redox reaction at the positive electrode or the negative electrode has been proposed. For example, Patent Document 1 discloses a lithium ion capacitor that uses activated carbon as a positive electrode and a carbon material capable of reversibly occluding and releasing lithium ions as a negative electrode.

JP 2011-165998 A

  By the way, since the hybrid capacitor can be rapidly charged and discharged, the use of the hybrid capacitor as an electric power source at the time of starting the engine and an assist at the time of acceleration in a vehicle such as an automobile has been studied. Such a vehicle must be able to operate at a low temperature such as in a cold region or in winter, and the requirement as an electric storage device to be provided in this case is excellent electrical characteristics at a low temperature. However, even though a conventional general hybrid capacitor exhibits a relatively high capacitance at room temperature (for example, 25 ° C.), the capacitance may be significantly reduced at a low temperature (for example, −30 ° C.). It was. This invention is made | formed in view of this point, The main objective is to provide the high performance activated carbon which contributes to construction | assembly of the hybrid capacitor which a capacity | capacitance fall at low temperature does not occur easily.

  The present inventor has thought that in a hybrid capacitor using activated carbon as a positive electrode or a negative electrode, the factor that the capacitance decreases at a low temperature (for example, −30 ° C.) is a basic functional group present on the surface of the activated carbon. The present inventors have found that by reducing the amount of such a basic functional group, it is possible to effectively suppress a decrease in capacity at a low temperature, thereby completing the present invention.

  That is, the activated carbon for a hybrid capacitor provided by the present invention is activated carbon used as an electrode material of a hybrid capacitor, and the basic functional group amount based on neutralization titration of the activated carbon is 0.3 mmol / g or less. It is stipulated in.

  This makes it difficult for the capacitance at a low temperature to decrease as compared with the conventional activated carbon in which the amount of the basic functional group exceeds 0.3 mmol / g. That is, since the amount of the basic functional group that inhibits the desorption of the charge body adsorbed on the activated carbon (lithium ion in the case of a lithium ion capacitor) is suppressed to 0.3 mmol or less per 1 g of the activated carbon, the charge adsorbed on the activated carbon. It is difficult to cause a situation where body desorption is hindered. Therefore, it is possible to suppress a decrease in capacitance at a low temperature that may occur when the desorption of the charge bodies adsorbed on the activated carbon is inhibited.

  In a preferred embodiment of the activated carbon disclosed herein, the carbon dioxide adsorption amount at a temperature of −196 ° C. is 0.2 ml / g or more (for example, 0.2 ml / g to 0.4 ml / g, preferably 0.25 ml / g or more). ). Within such a range of carbon dioxide adsorption, a high capacity at room temperature can be realized.

  In a preferred embodiment of the activated carbon disclosed herein, the basic functional group has at least a nitrogen-containing functional group (for example, an amino group). If a basic functional group having an electron donating property such as an amino group is present on the surface of the activated carbon, the desorption of the charge body adsorbed on the activated carbon tends to be hindered, resulting in inconveniences such as a decrease in capacitance. According to the invention, since the basic functional group amount of the activated carbon is suppressed to 0.3 mmol / g or less, such inconvenience hardly occurs.

  The present invention also provides a hybrid capacitor comprising any activated carbon disclosed herein. That is, the hybrid capacitor includes a positive electrode, a negative electrode, and an electrolyte, and the positive electrode includes activated carbon as a positive electrode material. And it is prescribed | regulated so that the basic functional group amount based on the neutralization titration of the said activated carbon may be 0.3 mmol / g or less. According to such a configuration, it is possible to provide a high-performance hybrid capacitor in which a decrease in capacity at a low temperature is suppressed. In this case, the activated carbon may be defined such that the carbon dioxide adsorption amount at a temperature of −196 ° C. is 0.2 ml / g or more. In this case, it is possible to provide an optimum hybrid capacitor that has a high capacity at room temperature and maintains a capacity even at a low temperature. The activated carbon may have at least a nitrogen-containing functional group as the basic functional group.

  As another aspect, the present invention provides a method for producing activated carbon used as an electrode material of a hybrid capacitor. This manufacturing method includes an activated carbon precursor preparation step of preparing an activated carbon precursor. Moreover, the activation process which activates the activated carbon precursor prepared at the said activated carbon precursor preparation process and obtains activated carbon is included. Furthermore, the acid washing process which wash | cleans the activated carbon obtained at the said activation process with an acid is included. Moreover, the water washing process which wash | cleans the activated carbon wash | cleaned by the said acid washing process with an aqueous medium is included. Furthermore, it includes a heat treatment step of heat treating the activated carbon washed in the water washing step. And the said water-washing process is performed so that the amount of basic functional groups based on the neutralization titration of the said activated carbon after the said heat processing process may be 0.3 mmol / g or less.

According to the knowledge of the present inventor, when the activated carbon is washed with an acid (for example, nitric acid) in the acid washing step, an acid-derived functional group (for example, -NO, -NO ₂ ) remains on the surface of the activated carbon. When these residual functional groups are exposed to a reducing atmosphere in the heat treatment step, they are reduced to basic functional groups (for example, —NH ₂ ), which may increase the amount of basic functional groups. On the other hand, in the production method of the present invention, the activated carbon is washed with an acid and then washed with water to remove residual functional groups derived from the acid. And the said heat processing is performed with respect to the activated carbon from which the residual functional group derived from an acid was removed. This makes it possible to obtain activated carbon with fewer basic functional groups present on the surface of the activated carbon (the amount of basic functional groups is suppressed to 0.3 mmol / g or less) as compared with the prior art. Such activated carbon can be a high-performance activated carbon that contributes to the construction of a capacitor in which a decrease in capacity at a low temperature is suppressed.

In a preferred embodiment of the production method disclosed herein, the water washing step is performed according to JIS of the activated carbon after the water washing step.
This is done until the pH based on K 1474 is at least above 4.0. By washing with water until the pH exceeds 4.0, the acid-derived functional group is effectively removed. Therefore, it is possible to reliably obtain activated carbon having a smaller amount of basic functional groups than conventional ones.

  In a preferred embodiment of the production method disclosed herein, the activation step is performed at 600 ° C. after adding 3 equivalents (3 parts by mass) or more of an alkali metal compound to 1 equivalent (1 part by mass) of the activated carbon precursor. Including heat treatment in a temperature range of ˜1000 ° C. According to this method, activated carbon having a relatively large specific surface area can be obtained.

  In a preferable aspect of the production method disclosed herein, the acid cleaning step includes a treatment of immersing the activated carbon in an inorganic acid (for example, nitric acid) for 5 hours or more. According to this method, the alkali component remaining at the time of activation can be removed.

  In a preferable aspect of the production method disclosed herein, the heat treatment step includes heat-treating the activated carbon in a temperature range of 600 ° C. to 1000 ° C. in a reducing gas atmosphere. According to this method, activated carbon with few impurities can be obtained.

  In a preferred embodiment of the production method disclosed herein, after the activated carbon precursor preparation step and before the activation step, the activated carbon precursor is pulverized so as to have an average particle size of 80 μm to 120 μm. According to this method, activated carbon can be obtained which has an increased activation effect and has a small diameter and a narrow particle size distribution.

  In a preferred embodiment of the production method disclosed herein, after the water washing step and before the heat treatment step, the activated carbon is pulverized so as to have an average particle size of 1 μm to 5 μm. According to this method, it is possible to obtain high-performance activated carbon that contributes to the construction of a capacitor having a large capacitance at room temperature.

It is a figure which shows the manufacture flow of activated carbon which concerns on one Embodiment of this invention. CO ₂ adsorption amount and is a graph showing the relationship between the 25 ° C. capacitance. It is a graph which shows the relationship between basic functional group amount and a -20 degreeC electrostatic capacitance maintenance factor.

  Several preferred embodiments of the present invention will be described below with reference to the drawings. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be implemented based on the contents disclosed in the present specification and technical knowledge in the field. Hereinafter, a case where the present invention is mainly applied to activated carbon used for a positive electrode material for a lithium ion capacitor will be described as an example, but the present invention is not intended to be limited thereto.

  The activated carbon provided by the present invention is activated carbon used as a positive electrode material for a lithium ion capacitor. The amount of the basic functional group based on the neutralization titration of the activated carbon is suitably about 0.3 mmol / g or less, preferably 0.25 mmol / g or less, particularly preferably 0.2 mmol / g or less.

Here, the neutralization titration is performed as follows. That is, after dispersing a predetermined amount of activated carbon in hydrochloric acid and separating the activated carbon from hydrochloric acid by filtration, the filtrate is neutralized and titrated with sodium hydroxide to determine the titration amount up to the neutralization point near pH 8. The amount α (mmol / g) of basic functional groups present on the surface of a predetermined amount of activated carbon can be determined. Specifically, α can be obtained using the following equation (1).
α (mmol / g) = (C × D−A × B) ÷ E (1)
A: Sodium hydroxide titration to the neutralization point (mL)
B: Molar concentration of sodium hydroxide (mol / L)
C: Amount of hydrochloric acid (ml)
D: Molar concentration of hydrochloric acid (mol / L)
E: Mass of activated carbon (g)

  The neutralization titration is preferably carried out with the time for which the activated carbon is dispersed in hydrochloric acid being approximately the same. As a result, the amount of the basic functional group can be grasped more accurately. In a preferred embodiment, the dispersion time is set to a substantially constant time (for example, 15 minutes) set between about 10 minutes to 20 minutes. After a predetermined time (dispersion time) has elapsed since the activated carbon is dispersed in hydrochloric acid, the activated carbon is separated from the hydrochloric acid by filtration. Then, it is good to add sodium hydroxide to a filtrate and perform neutralization titration.

  The activated carbon disclosed here is defined such that the basic functional group amount based on neutralization titration of activated carbon obtained by the above formula (1) is 0.3 mmol / g or less. This makes it difficult for the capacitance at a low temperature to decrease as compared with the conventional activated carbon in which the amount of the basic functional group exceeds 0.3 mmol / g. That is, according to the knowledge of the present inventor, activated carbon has many functional groups on the surface, but if a basic functional group having an electron donating property such as an amino group is present, the activated carbon is adsorbed on the activated carbon. The lithium ion desorption is hindered, which can be a factor in reducing the capacitance. In particular, at the time of charging and discharging at low temperature, the movement rate of lithium ions is slow, and the interaction with the basic functional group is strengthened, so that the capacity is likely to decrease. In contrast, according to the present configuration, the amount of basic functional groups that inhibit desorption of lithium ions is suppressed to 0.3 mmol or less per gram of activated carbon, so that it is adsorbed on activated carbon by interaction with basic functional groups. It is difficult to cause a situation in which the desorption of lithium ions is hindered. That is, lithium ions adsorbed on the activated carbon can be smoothly desorbed even at low temperatures. Therefore, it is possible to suppress a decrease in capacitance that may occur when lithium ion desorption is inhibited at low temperatures.

  For example, the activated carbon provided by the present invention preferably has a basic functional group amount of 0.3 mmol / g or less, more preferably 0.25 mmol / g or less, and 0.2 mmol / g or less. More preferably it is. On the other hand, activated carbon having a basic functional group amount of less than 0.1 mmol / g is not very advantageous because it takes a long time to produce (manufacture) and also slows the effect of increasing the capacitance at low temperatures. From the viewpoint of productivity, the basic functional group amount of the activated carbon is preferably 0.1 mmol / g or more. For example, activated carbon having a basic functional group amount of 0.1 mmol / g or more and 0.3 mmol / g or less (especially 0.15 mmol / g or more and 0.25 mmol / g or less) can improve the electrostatic capacity improvement effect and productivity at low temperature. It is appropriate from the viewpoint of achieving both.

The activated carbon disclosed herein is preferably regulated so that the carbon dioxide adsorption amount is 0.2 ml / g or more. Within such a range of carbon dioxide adsorption, pores of small size (typically 1 nm or less) that contribute to the capacitance increase, so that high capacity at room temperature can be realized. Here, the CO ₂ adsorption amount is measured at −196 ° C. after drying a predetermined amount of activated carbon dried at 115 ° C. for 1 hour or more in a vacuum state at 200 ° C. for 3 hours. For example, as the activated carbon disclosed herein, the carbon dioxide adsorption amount is preferably 0.2 ml / g or more, more preferably 0.25 ml / g or more, and 0.3 ml / g or more. Is particularly preferred. On the other hand, activated carbon having a carbon dioxide adsorption amount of more than 0.5 ml / g is difficult to produce (manufacture), and also increases the capacity at room temperature at low temperatures, so there is not much merit. From the viewpoint of easy production, the carbon dioxide adsorption amount of the activated carbon is preferably 0.5 ml / g or less (for example, 0.2 ml / g or more and 0.5 ml / g or less).

  Preferred examples of the activated carbon disclosed herein include those having a basic functional group amount of 0.3 mmol / g or less and a carbon dioxide adsorption amount of 0.2 ml / g or more at −196 ° C. The group amount is 0.25 mmol / g or less, the carbon dioxide adsorption amount at −196 ° C. is 0.25 ml / g or more, the basic functional group amount is 0.2 mmol / g or less, and − A carbon dioxide adsorption amount at 196 ° C. of 0.3 ml / g or more is included. By having both the basic functional group amount and the carbon dioxide adsorption amount within such a predetermined range, it is possible to achieve a high capacity at room temperature while maintaining a relatively high capacity even at a low temperature. Activated carbon can be obtained.

  Next, the manufacturing method of the activated carbon for lithium ion capacitors which concerns on this embodiment is demonstrated, referring FIG.

  As shown in FIG. 1, the method according to the present embodiment is a method for producing activated carbon used as a positive electrode material for a lithium ion capacitor: an activated carbon precursor preparation step (S10) for preparing an activated carbon precursor, and activated carbon. Obtained in the primary pulverization step (S11) in which the precursor is pulverized so that the average particle size is 80 μm to 120 μm, the activation step (S12) in which the activated carbon precursor is activated to obtain activated carbon, and the activation step. An acid washing step (S13) for washing the activated carbon with an acid, a water washing step (S14) for washing the activated carbon washed in the acid washing step with an aqueous medium, and pulverizing the activated carbon so that the average particle diameter is 1 μm to 5 μm. A secondary pulverization step (S15) for performing the treatment and a heat treatment step (S16) for heat treating the activated carbon are included. And it is characterized by performing a water washing process (S14) so that the amount of basic functional groups based on neutralization titration of activated carbon after a heat treatment process (S16) may be 0.3 mmol / g or less. Hereinafter, each process will be described.

  In the activated carbon precursor preparation step (S10), an activated carbon precursor is prepared. The activated carbon precursor is obtained by carbonizing a predetermined raw material for producing activated carbon, and can be produced, for example, by firing a carbonaceous material such as mineral, plant or resin. Examples of mineral carbonaceous materials include coal (lignite, lignite, bituminous coal, anthracite, etc.), cokes, infusible pitch, oil carbon, and the like. Examples of plant-based carbonaceous materials include charcoal, coconut shells, sawdust, wood chips, and grass charcoal. An example of the resin-based carbonaceous material is a phenol resin. Carbonization (firing) of the carbonaceous material can be performed by a conventionally known method. Or you may purchase and use the activated carbon precursor (commercially available product) marketed.

  In the primary pulverization step (S11), the activated carbon precursor is pulverized so that the average particle diameter is 80 μm to 120 μm. The pulverization of the activated carbon precursor may be performed by a known pulverizer such as a hammer mark crusher, a cone crusher, a disk crusher, a rotary crusher, a ball mill, a centrifugal roll mill, a ring roll mill, or a centrifugal ball mill. The average particle diameter (D50 diameter) of the activated carbon precursor is suitably about 80 μm to 120 μm, preferably 90 μm to 110 μm, in order to effectively proceed with the activation step (S12) described later. Such a particle size is also suitable from the viewpoint of obtaining activated carbon having a particle size of several μm or less and a narrow particle size distribution in the secondary pulverization step (S15) described later. In this specification, the average particle diameter refers to the median diameter (d50), and can be easily measured by a particle size distribution measuring apparatus based on various commercially available laser diffraction / scattering methods.

  In the activation step (S12), the pulverized activated carbon precursor is activated to obtain activated carbon. In this embodiment, the activation treatment is performed by adding a powdery alkali metal compound to the activated carbon precursor and heat-treating it. As the alkali metal compound, potassium hydroxide, lithium hydroxide, sodium hydroxide, potassium carbonate, potassium chloride and the like are preferably used. You may use these 1 type or in mixture of 2 or more types. Of these, the use of potassium hydroxide is preferred. The addition amount of the alkali metal compound is suitably 3 equivalents or more (for example, 3 equivalents to 10 equivalents), preferably 3.2 equivalents or more, particularly preferably 3 to 1 equivalent of the activated carbon precursor. .5 equivalents or more. For example, it is appropriate to add 3 parts by mass or more (for example, 3 parts by mass to 10 parts by mass) of an alkali metal compound with respect to 1 part by mass of the activated carbon precursor, preferably 3.2 parts by mass or more, and particularly preferably. 3.5 parts by mass or more. If the addition amount of the alkali metal compound is too small, a sufficient activation effect may not be obtained. On the other hand, if the addition amount is too large, an unreacted alkali metal compound may remain excessively, which is not preferable.

The heat treatment performed after the alkali metal compound is added to the activated carbon precursor may be determined, for example, in a range of 600 ° C. to 1000 ° C. in an inert gas (for example, nitrogen gas) atmosphere. If the heat treatment temperature is too low, activation does not proceed sufficiently, and activated carbon having appropriate pores may not be obtained. On the other hand, if the heat treatment temperature is too high, the alkali metal compound melts, and activation may be insufficient. Accordingly, the heat treatment temperature in the activation step is generally appropriately 600 ° C to 1000 ° C, preferably 650 ° C to 900 ° C, and particularly preferably 700 ° C to 800 ° C. The heat treatment time (holding time at the highest temperature reached) may be a time until the activation sufficiently proceeds, and is usually 5 hours to 30 hours, preferably 8 hours to 20 hours, particularly preferably. 10 hours to 15 hours.
Preferably, the temperature is raised from room temperature to a temperature range of 600 ° C. to 1000 ° C. (eg, 700 ° C.) over about 3 hours to 10 hours (preferably 5 hours to 10 hours) (eg, 1 ° C./min to 3 ° C. / min). And it is good to heat-process for about 5 hours-30 hours in the said highest ultimate temperature range. By heat-treating the activated carbon precursor according to such a heating schedule, activated carbon having a relatively large specific surface area can be obtained.

The activated carbon obtained by the heat treatment according to the schedule as described above is preferably cooled and then filtered and washed with pure water until the pH becomes about 11 to 12, thereby remaining the alkaline component (typically unreacted) at the time of activation. Alkali metal compounds and produced alkali impurities) can be removed. The pH of the activated carbon here is JIS
It shall be grasped by measurement based on K1474.

In the acid washing step (S13), the activated carbon obtained in the activation step is washed with an acid. After the activation step, the activated carbon is acid-washed to reliably remove alkali components (typically unreacted alkali metal compounds and generated alkali impurities) remaining during activation. The acid cleaning may be performed, for example, by immersing activated carbon in an inorganic acid. As the inorganic acid used for washing, any acid that dissolves the residual alkali component can be used without particular limitation. As a preferred example, it is preferable to use a nitrogen acid such as nitrous acid (HNO ₂ ) or nitric acid (HNO ₃ ). You may wash | clean with sulfuric acid or hydrochloric acid after wash | cleaning with the nitrogen acid containing. The concentration of the acid used for washing is not particularly limited as long as it is a concentration capable of dissolving the residual alkali component. For example, when nitric acid is used, the concentration may be about 0.05 mol / L to 0.5 mol / L. Appropriate, preferably 0.1 mol / L to 0.3 mol / L. The acid washing time may be a time until the residual alkali component is sufficiently dissolved, and is usually 5 hours or longer (for example, 5 hours to 30 hours), preferably 10 hours to 20 hours, Preferably, it is 10 hours to 15 hours.

In the water washing step (S14), the activated carbon washed in the acid washing step is washed with water. The washing with water may be performed, for example, by filtration washing in which an aqueous medium is passed through activated carbon on the filter. As the aqueous medium, water or a mixed medium mainly composed of water is preferably used. As a medium component other than water constituting such a mixed medium, one or more organic mediums (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. For example, it is preferable to use an aqueous medium in which 80% by mass or more (more preferably 90% by mass or more, more preferably 95% by mass or more) of the aqueous medium is water. A particularly preferable example is an aqueous medium substantially consisting of water.
Here, the water washing step is one important factor from the viewpoint of reducing the amount of the basic functional group of the activated carbon. Preferably, water washing (in this case, filtration washing) is repeated until the pH of the activated carbon after the water washing step is at least 4.0 (preferably 4.4 or more, more preferably 4.6 or more) based on JIS K 1474. Thus, (for example, in the case of nitric _acid, -NO 2, -NO _3, etc.) functional group derived from an acid remaining in the activated carbon can be removed. Since the functional group derived from the acid is reduced to a basic functional group in a heat treatment step to be described later, if the washing with water is insufficient, the amount of the basic functional group of the activated carbon may increase. According to this, since washing with water is repeated until the pH exceeds 4.0, the basic functional group amount of the activated carbon can be adjusted to a lower value as compared with the conventional case.

  In the secondary pulverization step (S15), the activated carbon washed by the water washing step as described above is preferably dried (for example, after drying at 100 ° C. to 120 ° C. for 10 hours to 24 hours), and the average particle size is 5 μm. It grind | pulverizes so that it may become below (for example, 1 micrometer-5 micrometers), preferably 3 micrometers or less (for example, 1 micrometer-3 micrometers). The activated carbon may be pulverized by a known pulverizer such as a vibration mill, a centrifugal roll mill, a ring roll mill, a centrifugal ball mill, a hammark crusher, a cone crusher, a disc crusher, a rotary crusher, or a ball mill. By setting the average particle diameter of the activated carbon to 5 μm or less, it is possible to obtain activated carbon (high-performance activated carbon that contributes to the construction of a capacitor having a large capacitance at room temperature) with a specific surface area increased by reducing the diameter.

In the heat treatment step (S16), the secondary pulverized activated carbon is heat-treated in a reducing gas atmosphere to reduce functional groups remaining on the surface of the activated carbon (for example, —COOH, —CHO, —OH, etc.). Remove. As the reducing gas, methane (CH ₄ ) gas, carbon dioxide (CO ₂ ) gas, or the like can be used in addition to hydrogen gas or a mixed gas containing hydrogen gas or ammonia gas. The reducing gas may be supplied after being diluted with an inert gas such as nitrogen (N ₂ ) or argon (Ar). Preferably, it is preferable to form a mixed gas atmosphere of hydrogen _{(H 2)} and nitrogen _{(N 2).} Although the degree of dilution is not particularly limited, it is usually sufficient if the reducing gas concentration is 1% by volume or more, for example, 1% to 10% by volume (preferably 1% to 5% by volume). Is preferred.

  As the heat treatment temperature, it is preferable to determine the highest temperature within the range of 600 ° C to 1000 ° C. If the heat treatment degree is too low, removal of residual functional groups may be insufficient, while if the heat treatment degree is too high, energy may be uneconomical. Accordingly, the temperature in the heat treatment step is generally appropriately 600 ° C to 1000 ° C, preferably 650 ° C to 900 ° C, and particularly preferably 700 ° C to 800 ° C. The heat treatment time (holding time at the highest temperature reached) may be a time until the remaining functional groups are sufficiently removed, and is usually 1 hour to 15 hours, preferably 3 hours to 10 hours. Particularly preferred is 5 to 8 hours. Preferably, the temperature is raised from room temperature to a temperature range of 600 ° C. to 1000 ° C. over about 3 hours to 10 hours (preferably 5 hours to 10 hours). And it is good to heat-process in the said highest ultimate temperature range for about 1 hour-15 hours. Residual functional groups can be efficiently removed by heat-treating (sintering) the activated carbon according to such a heating schedule.

The activated carbon thus obtained has a smaller amount of basic functional groups remaining on the surface than in the prior art. Typically, the amount of basic functional groups based on the neutralization titration described above is 0.3 mmol / g or less (preferably 0.25 mmol / g or less). Show. The obtained activated carbon has a large specific surface area and has appropriate pores as compared with the conventional activated carbon. Typically, carbon dioxide (CO ₂ ) adsorption amount at −196 ° C. is 0.2 ml / g or more (preferably 0.25 ml / g or more), and this type of activated carbon exhibits extremely high CO ₂ adsorption amount. . From this, the activated carbon disclosed here can be suitably used as a positive electrode material of a hybrid capacitor (typically a lithium ion capacitor).
Then, a lithium ion capacitor can be constructed by adopting the same materials and processes as those in the prior art except that the activated carbon disclosed here is used.

  For example, the activated carbon disclosed herein can be mixed with carbon black such as acetylene black and ketjen black and other (graphite and the like) powdered carbon materials as the conductive material. In addition to activated carbon and conductive material, a binder (binder) such as styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC) may be added. it can. By dispersing these in an appropriate dispersion medium and kneading, a paste-like (including slurry or ink form; the same applies hereinafter) positive electrode forming composition can be prepared. An appropriate amount of this paste is preferably applied onto a current collector made of aluminum or an alloy containing aluminum as a main component, and further dried, whereby a positive electrode for a lithium ion capacitor can be produced.

On the other hand, the negative electrode for a lithium ion capacitor serving as a counter electrode can be produced by the same technique as in the prior art. For example, the negative electrode material may be any material that can occlude and release lithium ions. Typical examples include powdery carbon materials made of graphite, amorphous carbon, and the like. Alternatively, a material such as polyacetylene or Sn may be used. In particular, graphite particles can be a negative electrode material more suitable for rapid charge / discharge (for example, high power discharge) because of its small particle size and large surface area per unit volume.
As in the case of the positive electrode, a paste-like negative electrode forming composition can be prepared by dispersing and kneading such a powdery material together with an appropriate binder (binder) in an appropriate dispersion medium. A negative electrode for a lithium ion capacitor can be produced by applying an appropriate amount of this paste onto a current collector preferably made of copper, nickel, or an alloy thereof and further drying.
In the lithium ion capacitor using the activated carbon of the present invention as the positive electrode material, the same separator as the conventional one can be used. For example, a porous sheet made of a polyolefin resin (polyethylene, polypropylene, or a laminate thereof) can be used. Alternatively, a cellulose-based nonwoven fabric may be used.

As the electrolyte, the same electrolyte as a non-aqueous electrolyte (typically, an electrolytic solution) conventionally used for lithium ion capacitors can be used without particular limitation. Typically, the composition includes a supporting salt in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include one or two selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. More than seeds can be used. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ). ₃ , 1 type, or 2 or more types of lithium compounds (lithium salt) selected from LiI etc. can be used.
Moreover, as long as the activated carbon disclosed here is employed as the positive electrode material, the shape (outer shape and size) of the lithium ion capacitor to be constructed is not particularly limited. The outer case may be a thin sheet type constituted by a laminate film or the like, and the outer case may be a cylindrical or rectangular parallelepiped capacitor, or may be a small coin shape.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the specific examples.

<Manufacture of activated carbon>
<Sample 1>
First, coal as an activated carbon precursor was pulverized to an average particle size of 100 μm using a pulverizer (Atomizer AIIW-5 manufactured by Dalton Co., Ltd.) to obtain a primary pulverized product. To 1 equivalent of this primary pulverized product, 3.2 equivalent of powdered potassium hydroxide was added and fired in an elevating-type firing furnace (manufactured by Mino Ceramics Co., Ltd.) for activation treatment (activation step). The firing temperature (maximum temperature reached) was 700 ° C., the firing time (calcination time at the maximum temperature reached) was 10 hours, and the temperature was increased from room temperature at 2 ° C./min. The obtained activated carbon was filtered and washed with pure water until the pH reached about 11 to 12, and further washed with 0.1N nitric acid for 12 hours to remove residual alkali components adhering to the activated carbon (acid washing). Process). Next, filtration and washing were repeated with pure water, and washing was performed until the pH based on JIS K 1474 became 4.4 (water washing step). The obtained washed product was dried at 110 ° C. for 24 hours, and then charged into a pulverizer (continuous pulverizer manufactured by Eurus Techno Co., Ltd.), and pulverized to an average particle size of 3 μm to obtain a secondary pulverized product. This secondary pulverized product was fired in an elevating-type firing furnace (manufactured by Mino Ceramics Co., Ltd.) in a 3% by volume hydrogen atmosphere. The firing temperature (maximum temperature reached) was 700 ° C., the firing time (calcination time at the maximum temperature reached) was 5 hours, and the temperature was raised from room temperature at 2 ° C./min. In this way, activated carbon according to Sample 1 was obtained.

<Sample 2>
Filtration washing was repeated until the pH reached 4.6 in the water washing step. The structure was the same as Sample 1 except that the water was washed until the pH reached 4.6 in the water washing step.

<Sample 3>
Filtration washing was repeated until the pH reached 3.5 in the water washing step. The configuration was the same as Sample 1 except that filtration and washing were repeated until the pH reached 3.5 in the water washing step.

<Sample 4>
Filtration washing was repeated until the pH reached 3.7 in the water washing step. The structure was the same as Sample 1 except that the filtration and washing were repeated until the pH reached 3.7 in the water washing step.

<Sample 5>
Filtration washing was repeated until the pH reached 4.0 in the water washing step. The structure was the same as Sample 1 except that the filtration and washing were repeated until the pH reached 4.0 in the water washing step.

<Sample 6>
The amount of potassium hydroxide added was changed to 2.1 equivalents in the activation step, and filtration and washing were repeated until the pH reached 3.5 in the water washing step. It was set as the same structure as the sample 1 except having changed the addition amount of potassium hydroxide to 2.1 equivalent in the activation process, and repeating filtration washing until pH became 3.5 in the water washing process.

<Sample 7>
Commercially available alkaline activated carbon (manufactured by Kansai Thermochemical Co., Ltd .: MSP-20) was used as the activated carbon of Sample 7 as it was.

<Measurement of basic functional group amount>
The amount of basic functional groups was measured for the activated carbon of each sample obtained above. Specifically, 2 g of activated carbon of each sample dried at 115 ° C. for 1 hour or more was weighed, mixed with 20 ml of 0.05 mol / L hydrochloric acid, stirred for 15 minutes, and then filtered. Then, 1 w / v% phenolphthalein reagent was added dropwise to 5 ml of the filtrate and titrated with a 0.05 mol / L sodium hydroxide aqueous solution until the color of the indicator disappeared, and the amount of basic functional group was calculated from the amount added. The results are shown in Table 1. As shown in Table 1, it was confirmed that the basic functional group amount tends to decrease as the pH of the activated carbon after the water washing step increases. Moreover, it was confirmed that the commercially available alkaline activated carbon has a basic functional group amount greatly exceeding 0.3 mmol / g.

<Measurement of carbon dioxide (CO ₂ ) adsorption amount>
The CO ₂ adsorption amount was measured for the activated carbon of each of the obtained samples. Specifically, 0.015 g of activated carbon of each sample dried at 115 ° C. for 1 hour or more was weighed, and dried in a vacuum state at 200 ° C. for 3 hours using Quadrasorb SI (manufactured by Cantachrome Instruments Japan GK). After that, the CO ₂ adsorption amount was measured at −196 ° C. The results are shown in Table 1.

<Construction of lithium ion capacitor>
A lithium ion capacitor for evaluation was constructed using the activated carbon of each sample obtained above. Here, an evaluation capacitor was produced as follows. First, after the obtained activated carbon was dried at 110 ° C. for 2 hours, the activated carbon, SBR as a binder, and carbon black as a conductive material were weighed so as to have a mass ratio of 90: 5: 5. And it mixed in water and the paste-form composition for positive electrodes was prepared. The paste-like positive electrode composition is applied to both surfaces of an aluminum foil (positive electrode current collector, thickness 30 μm) using a doctor blade, and dried at 110 ° C. to provide a positive electrode material on one surface of the positive electrode current collector. The obtained positive electrode sheet was obtained.

The positive electrode sheet was punched into a 2 cm ² coin shape to produce a positive electrode. A triode cell was constructed using this positive electrode (working electrode), metallic lithium as a negative electrode (counter electrode), metallic lithium as a reference electrode, a separator, and a nonaqueous electrolyte. Specifically, a negative electrode (counter electrode) was disposed on both surfaces of the positive electrode via separators, and a three-electrode cell having a configuration in which metallic lithium was used as a reference electrode was produced.
A cellulose-based nonwoven fabric having a thickness of 30 μm was used for the separator. As a non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1: 1 was made to contain LiPF ₆ as a supporting salt at a concentration of about 1.5 mol / liter. Things were used.

<Measurement of capacitance (25 ° C., −20 ° C.)>
Each of the evaluation triode cells according to each sample obtained as described above was temperature-adjusted for 1 hour or more in a thermostatic bath at 25 ° C., and then charged to 3.8 V with a constant current of ² mA / cm ^2. The battery was charged by a constant voltage method until the total charging time was 1 hour. After 10 minutes of rest, the battery was discharged to 2.2 V with a constant current of ² mA / cm ² . The capacitance per unit volume was determined based on the discharge time during which the cell voltage (capacitor voltage) was 3.8V-2.2V during the discharge. Moreover, the set temperature of the thermostat was changed to −20 ° C., and the capacitance was measured in the same procedure. And the -20 degreeC capacitance maintenance factor was computed by (Capacitance in -20 degreeC / Capacitance in 25 degreeC) x100. The charging / discharging device used was HJ-1001 SD8 manufactured by Hokuto Denko Corporation. The results are shown in Table 1, FIG. 2 and FIG. FIG. 2 is a graph showing the relationship between the carbon dioxide adsorption amount of each sample and the 25 ° C. capacitance, and FIG. 3 shows the relationship between the basic functional group amount of each sample and the −20 ° C. capacitance retention rate. It is a graph.

As shown in Table 1 and FIG. 2, the 25 ° C. capacitance tended to increase as the CO ₂ adsorption amount of the activated carbon increased. In the case of the test cell used here, an extremely high 25 ° C. capacitance of 22 F / ml could be achieved by setting the CO ₂ adsorption amount of the activated carbon to 0.2 ml / g or more. From the viewpoint of improving the capacitance at room temperature, the CO ₂ adsorption amount of the activated carbon is preferably about 0.2 ml / g or more, and particularly preferably 0.25 ml / g or more. Moreover, as shown in Table 1 and FIG. 3, the −20 ° C. capacity retention rate tended to increase as the basic functional group amount of the activated carbon decreased. In the case of the test cell used here, by setting the basic functional group amount of activated carbon to 0.3 mmol / g or less, it was possible to achieve an extremely high -20 ° C. capacity retention rate of 70% or more. From the viewpoint of maintaining the capacitance at a low temperature, the basic functional group amount of the activated carbon is preferably about 0.3 mmol / g or less, and particularly preferably 0.25 mmol / g or less.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment and Example are only illustrations and what changed and changed the above-mentioned specific example is contained in the invention disclosed here. For example, the type of the hybrid capacitor is not limited to the above-described lithium ion capacitor, and may be capacitors having various contents with different electrode materials and electrolytes. Also, the size of the capacitor and other configurations can be appropriately changed depending on the application (typically for in-vehicle use).

  The hybrid capacitor according to the present invention can suppress a decrease in capacitance at a low temperature while realizing an increase in capacitance at room temperature. Due to such characteristics, the hybrid capacitor according to the present invention can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile, an electric power source when starting an engine, or an auxiliary power source that assists during acceleration.

S10 Activated carbon precursor preparation step S11 Primary grinding step S12 Activation step S13 Acid washing step S14 Water washing step S15 Secondary grinding step S16 Heat treatment step

Claims (13)

  Activated carbon for hybrid capacitors, which is an activated carbon used as an electrode material for a hybrid capacitor, wherein the amount of basic functional groups based on neutralization titration of the activated carbon is specified to be 0.3 mmol / g or less.   2. The activated carbon for a hybrid capacitor according to claim 1, wherein the carbon dioxide adsorption amount at a temperature of −196 ° C. is defined to be 0.2 ml / g or more.   The activated carbon for hybrid capacitors according to claim 1 or 2, which has at least a nitrogen-containing functional group as the basic functional group. A hybrid capacitor comprising a positive electrode, a negative electrode and an electrolyte,
The positive electrode contains activated carbon as a positive electrode material,
A hybrid capacitor that is defined such that the basic functional group amount based on neutralization titration of the activated carbon is 0.3 mmol / g or less.   5. The hybrid capacitor according to claim 4, wherein the activated carbon is defined such that the carbon dioxide adsorption amount at a temperature of −196 ° C. is 0.2 ml / g or more.   The hybrid capacitor according to claim 4, wherein the activated carbon has at least a nitrogen-containing functional group as the basic functional group. A method for producing activated carbon used as an electrode material of a hybrid capacitor,
An activated carbon precursor preparation step for preparing an activated carbon precursor;
An activation step of activating the activated carbon precursor prepared in the activated carbon precursor preparation step to obtain activated carbon;
An acid washing step of washing the activated carbon obtained in the activation step with an acid;
A water washing step of washing the activated carbon washed in the acid washing step with an aqueous medium;
A heat treatment step of heat treating the activated carbon washed in the water washing step,
Here, the activated carbon manufacturing method which performs the said water washing process so that the basic functional group amount based on the neutralization titration of the said activated carbon after the said heat processing process may be 0.3 mmol / g or less.   The activated water production method according to claim 7, wherein the water washing step is performed until a pH based on JIS K 1474 of the activated carbon after the water washing step exceeds at least 4.0.   The activated carbon production method according to claim 7 or 8, wherein the activation step includes adding 3 equivalents or more of an alkali metal compound to 1 equivalent of the activated carbon precursor and performing a heat treatment in a temperature range of 600 ° C to 1000 ° C. .   The activated acid production method according to any one of claims 7 to 9, wherein the acid cleaning step includes a treatment of immersing the activated carbon in an inorganic acid for 5 hours or more.   The said heat treatment process is an activated carbon manufacturing method as described in any one of Claims 7-10 including heat-treating the said activated carbon in the temperature range of 600 to 1000 degreeC under reducing gas atmosphere.   The activated carbon according to any one of claims 7 to 11, wherein after the activated carbon precursor preparation step and before the activation step, the activated carbon precursor is pulverized so as to have an average particle size of 80 µm to 120 µm. Production method. The activated carbon production method according to any one of claims 7 to 12, wherein a treatment for pulverizing the activated carbon so as to have an average particle diameter of 1 µm to 5 µm is performed after the water washing step and before the heat treatment step.

Images