JP2020088119A - Manufacturing method of carbon material for electrical double layer capacitor - Google Patents

Abstract

A method for producing a carbon material for an electric double layer capacitor is provided which is simpler than the conventional nitrogen doping method and has improved high voltage charging resistance. The production method comprises mixing the porous carbon and the nitrogen-containing heterocyclic compound having an amino group in a mass ratio of 8:1 to 1:4 to obtain a nitrogen-containing heterocyclic compound having the amino group. Obtaining a mixture with a nitrogen heterocyclic compound, and heat-treating the mixture under an inert gas atmosphere to introduce nitrogen atoms into the porous surface. [Selection drawing] Fig. 1

Description

TECHNICAL FIELD The present invention relates to a method for producing a carbon material used for an electrode of an electric double layer capacitor.

An electric double layer capacitor (EDLC) that can be charged and used repeatedly is a capacitor that stores electric charges in the adsorption layer of ions formed in the pores of a porous carbon electrode such as activated carbon, that is, the electric double layer. is there. Since this electric double layer capacitor has a long life and a high output, it has been widely used as a backup power source for computer memories, and recently, it has been receiving a great deal of attention as an electric power storage system mounted on railway vehicles and an auxiliary power source for hybrid vehicles. ing.

Electric double layer capacitors are generally (1) faster charge/discharge than secondary batteries, (2) high reversibility of charge/discharge cycle, (3) long cycle life, (4) electrode Also, because it does not use heavy metals in the electrolyte, it is environmentally friendly. These characteristics are derived from the fact that the electric double layer capacitor does not use heavy metals, operates by physical adsorption/desorption of ions, and does not involve electron transfer reaction of chemical species.

The energy (E) stored in the electric double layer capacitor is proportional to the product of the square of the charging voltage (V) and the electric double layer capacity (C) (E=CV ² /2). It is effective to improve the capacity and charging voltage.

However, current electric double layer capacitors have a problem that the energy density is lower than that of secondary batteries and the like, and there is also a problem that reliability in a charge/discharge cycle in a harsh environment is low. Therefore, in order to develop the above new applications, it is necessary to improve the energy density and reliability of the electric double layer capacitor, and it is necessary to increase the capacity of the electrode material and capacity stability under harsh environments. ing. Double-layer capacity such as weight specific capacity, volume specific capacity, and area specific capacity depends on the nanostructure such as the pore structure, crystal structure, and chemical composition of the activated carbon electrode, so it is necessary to design an electrode material suitable for capacitors. It was

As a study for solving the above-mentioned problems, the present inventors performed heat treatment in a nitrogen monoxide (NO)-containing gas atmosphere prepared by diluting porous carbon such as activated carbon to several thousand ppm with an inert gas such as helium. , A method for easily producing a porous carbon material into which nitrogen is introduced by a dry method (hereinafter, this method is referred to as NO method) has been developed (see, for example, Patent Documents 1 and 2).

In this NO method, by heat treatment in the atmosphere described above, nitrogen is present as a nitrogen-containing functional group on the surface of porous carbon in an N/C atomic ratio of about 1 to 2% according to the reaction scheme shown in the following formula (1). Introduced in proportion.

C + NO → C(N) + CO ……(1)
When the obtained nitrogen-introduced porous carbon material was evaluated as an electrode main material of an electric double layer capacitor, the capacity did not decrease even when charged/discharged at a charging voltage of 3 V or higher. The inventors have found that the rate is improved from 80% to 90%, and succeeded in suppressing the deterioration of the electric double layer capacitor due to high voltage charging.

However, when the nitrogen-introduced porous carbon material by the NO method shown in Patent Documents 1 and 2 above is put to practical use, nitric oxide is diluted to several thousands ppm, but nitric oxide is used. When it is released into the atmosphere in large quantities, it turns into corrosive nitric acid gas, which is a safety issue.

For this reason, the inventors of the present invention pass an inert gas through a filling cylinder filled with one or more reagents selected from the group consisting of ammonium carbamate, ammonium hydrogen carbonate, and ammonium carbonate to remove porous carbon. Proposed a method for producing a carbon material for an electric double layer capacitor, characterized by performing a heat treatment in an inert gas atmosphere containing at least ammonia and carbon dioxide generated by the decomposition reaction of the above reagent (see, for example, Patent Document 3). ..). In the invention disclosed in Patent Document 3, as compared with the conventional manufacturing method using nitric oxide, which has a problem in safety, ammonium carbamate or the like, which is relatively safe, is used as a reagent and is generated by the decomposition reaction of this reagent. Since the porous carbon is heat-treated in an inert gas atmosphere containing a small amount of ammonia and carbon dioxide to produce nitrogen-introduced porous carbon, it is a highly safe production method (hereinafter referred to as “CA method”). ).

JP 2008-141116 A (claim 2, paragraphs [0018] to [0020] in the specification) JP, 2010-135647, A (claim 5, paragraphs [0025]-[0029] of the specification). Japanese Patent No. 5817286 (claim 1, paragraphs [0020] to [0028] in the specification)

However, in the invention disclosed in Patent Document 3, nitrogen doping is performed by heat-treating (800 to 1000° C.) porous carbon in the presence of ammonia, carbon dioxide, etc. generated by thermally decomposing an ammonium salt. In this method, a rotary electric furnace for uniformly heat-treating the porous carbon and the decomposition gas and an electric furnace for thermally decomposing the ammonium salt were separately required. Therefore, there has been a demand for a method for producing a carbon material for an electric double layer capacitor, which is simpler than the conventional nitrogen doping method CA and has improved high voltage charging resistance.

An object of the present invention is to provide a method for producing a carbon material for an electric double layer capacitor, which is simpler than the conventional nitrogen doping method CA and has improved high voltage charging resistance.

A first aspect of the present invention is to mix the porous carbon and the amino group by mixing the porous carbon and the nitrogen-containing heterocyclic compound having an amino group in a mass ratio of 8:1 to 1:4. A step of obtaining a mixture with a nitrogen-containing heterocyclic compound having, and a step of heat-treating the mixture under an inert gas atmosphere to introduce a nitrogen atom into the surface of the porous, a carbon material for electric double layer capacitor It is in the manufacturing method.

A second aspect of the present invention relates to the method for producing a carbon material for an electric double layer capacitor based on the first aspect, wherein the heat treatment is performed by holding at 600 to 1000° C. for 30 minutes to 2 hours in a horizontal tubular furnace. is there.

A third aspect of the present invention is the method for producing a carbon material for an electric double layer capacitor based on the first or second aspect, wherein the nitrogen atom content is 1 to 7 atom %.

A fourth aspect of the present invention is the method for producing a carbon material for an electric double layer capacitor according to any one of the first to third aspects, wherein the nitrogen contained in the carbon material is pyridine type nitrogen.

In the production method according to the first aspect of the present invention, a porous carbon and a nitrogen-containing heterocyclic compound having an amino group are mixed to obtain a nitrogen-containing heterocyclic compound having the porous carbon and the amino group. And a step of heat-treating the mixture under an inert gas atmosphere to introduce a nitrogen atom into the porous surface, thereby providing a rotary structure having a complicated structure required in the conventional CA method. An electric furnace is not required, and porous carbon suitable for an electric double layer capacitor having improved high voltage charging resistance can be obtained by a simple method.

In the method of the second aspect of the present invention, since the heat treatment is performed by holding in a horizontal tubular furnace, a method for producing a carbon material for an electric double layer capacitor, which is simpler than the conventional CA method, can be obtained. ..

In the method of the third aspect of the present invention, since the content of the nitrogen atom is 1 to 7 atom %, a method for producing a carbon material for an electric double layer capacitor, which has further improved high voltage charging resistance, can be obtained. ..

In the method according to the fourth aspect of the present invention, since the nitrogen contained in the carbon material is pyridine type nitrogen, decomposition of the electrolytic solution due to electrochemically active sites such as oxygen-containing surface functional groups of porous carbon is suppressed. Therefore, the life of the electric double layer capacitor can be extended.

It is a figure showing typically the manufacturing method of the carbon material for electric double layer capacitors of the embodiment of the present invention. It is a figure which shows the structure of the bipolar cell for electric double layer capacitor evaluation used in the Example, the comparative example, and the reference example. It is a figure which shows the nitrogen desorption adsorption isotherm of the carbon material of an Example, a comparative example, and a reference example. It is a figure which shows the Nls spectrum of the carbon material of an Example, a comparative example, and a reference example. It is a figure which shows the binding mode of various nitrogen functional groups, and the binding energy of Nls. It is a figure which shows the charging/discharging curve before and behind the endurance test of the electric double layer capacitor of an Example, a comparative example, and a reference example. It is a figure which shows the relationship between the specific surface area and N/C regarding nitrogen doping using melamine.

First, an outline of a mode for carrying out the present invention will be described with reference to the drawings.

As shown in FIG. 1, the carbon material for an electric double layer capacitor of the present invention is obtained by mixing porous carbon for an electric double layer capacitor with a nitrogen-containing heterocyclic compound having an amino group, and mixing the mixture under a nitrogen atmosphere. It can be obtained by raising the temperature at a constant rate of heating and maintaining that temperature for a certain period of time.

Next, a mode for carrying out the present invention will be described in detail.

(A) Mixing Porous Carbon Powder for Electric Double Layer Capacitor and Nitrogen-Containing Heterocyclic Compound Having Amino Group The porous carbon for electric double layer capacitor to be treated by the present production method is not limited to powdered activated carbon. Instead, it may be fibrous, cloth-like, or block-like porous carbon. For example, fine activated carbon (YP50F) powder manufactured by Kuraray Co., Ltd. is used. The porous carbon to be treated in the present production method is not limited to activated carbon activated by the activation treatment, and porous carbon that has not been activated can also be applied. Coconut shell-based activated carbon, wood-based activated carbon, pitch-based activated carbon, coal-based activated carbon, carbon gel, carbon nanofiber, mesoporous carbon and the like can also be used. When producing a nitrogen-doped carbon material by containing nitrogen in the porous carbon, the pores tend to be blocked due to the generation of the nitrogen-containing surface functional group, and the specific surface area tends to be small, so that the porous carbon is at least 1000 m ² / g or more, preferably preferred to use a material having a specific surface area of 1200~2500m ² / g.

As a doping source for doping the porous carbon with nitrogen, a nitrogen-containing heterocyclic compound having an amino group, for example, melamine is used. Besides, acetoguanamine, adenine, 1,4-phenylenediamine and the like can also be used.

First, the porous carbon powder and the melamine powder are mixed using a mortar. When the porous carbon is fibrous, cloth-like or block-like activated carbon, the melamine powder is sprinkled and mixed on the porous carbon. The mass ratio of the porous carbon to the melamine powder is preferably 8:1 to 1:4, more preferably 4:1 to 1:2. When the ratio of porous carbon exceeds the upper limit, the ratio of nitrogen in the porous carbon becomes low, and the obtained double layer capacitor does not have sufficient high voltage charge resistance, and when the ratio is less than the lower limit, the specific surface area of the porous carbon This is because there is a problem that the obtained double layer capacitor does not have sufficient capacitance. Further, the mass ratio is preferably 4:1 to 1:2 because the nitrogen doping tends to become non-uniform when the porous carbon ratio exceeds 4, and the melamine ratio exceeds 2 The reason is that the ratio of nitrogen in the porous carbon does not increase so much.

(B) Heat treatment of a mixture of porous carbon and melamine Next, the mixture of porous carbon and melamine is placed on an alumina boat and placed in a heat treatment furnace. A horizontal tubular electric furnace is used as the heat treatment furnace. Next, the heat treatment furnace is heated in an inert gas atmosphere to raise the temperature from room temperature to 600 to 1000° C., preferably 700 to 900° C. at a heating rate of 1 to 10° C./min. In a gas atmosphere, heat treatment is performed by holding the temperature at a raised temperature for 30 minutes to 2 hours. After the heat treatment, the electric furnace is gradually cooled to room temperature. By carrying out the heat treatment under the above conditions, it is possible to dope nitrogen on the surface of the pores of the porous carbon powder to easily produce a carbon material containing 1 to 7 atomic% of nitrogen (hereinafter referred to as “melamine method”). ")). Here, it is considered that nitrogen doping proceeds by the thermal decomposition product of melamine reacting with the carbon surface. A gas such as nitrogen, argon, or helium is used as the inert gas. The temperature to be raised is regulated within the above range, there is a problem that most of the pores are closed when the amount is less than the lower limit value, and there is a problem that the necessary amount of nitrogen is not doped on the carbon surface when the upper limit value is exceeded. Because. Further, the heat treatment at a treatment time of less than the lower limit does not contain the necessary amount of nitrogen on the carbon surface, and the effect of the present invention is not exhibited. This is because the yield is very small.

The carbon material for an electric double layer capacitor obtained by the production method of the present invention is a state in which nitrogen is introduced on the carbon surface and contains nitrogen in an atomic ratio of 1 to 7 atomic %, and a nitrogen-containing surface functional group similar to pyridine is produced. Exists in. By containing pyridine type nitrogen on the surface of the porous carbon, the decomposition of the electrolytic solution by the oxygen-containing surface functional groups of the porous carbon is suppressed, so that the life of the electric double layer capacitor can be extended. By using a carbon material having an atomic ratio of nitrogen as described above, an electric double layer capacitor having an improved double layer capacitance per unit area, like a carbon material obtained by a conventional CA method, is manufactured. You can Further, the capacitor using the carbon material for electric double layer capacitors obtained by the manufacturing method of the present invention can realize higher high voltage charge resistance than the capacitor using the carbon material obtained by the conventional CA method.
If the nitrogen content is less than the lower limit value, the effect of suppressing electrolysis is reduced, and the high voltage charging resistance of the electric double layer capacitor deteriorates. Further, if the content of nitrogen is more than the upper limit value, the capacity of the electric double layer capacitor is reduced due to the reduction of the specific surface area, and the ionic conductivity in the pores is reduced due to the pore clogging, and the internal resistance is reduced. There is a problem of increase.

In order to form an electrode for an electric double layer capacitor, it is preferable to add a conductive auxiliary agent and a binder in a predetermined ratio to the carbon material obtained by the manufacturing method of the present invention, knead the mixture, and then mold it into an arbitrary shape. Is. Carbon black is mentioned as a conductive auxiliary agent. Examples of the binder include PTFE (polytetrafluoroethylene), PVdF (polyvinylidene fluoride), polyacrylic acid, and SBR (styrene-butadiene rubber)-based binder. Further, as the current collector, the separator, etc., it is possible to apply existing materials that have been conventionally known.

Next, examples of the present invention will be described in detail together with comparative examples.

<Example 1>
First, coconut shell-based activated carbon (steam activated product) (YP50F manufactured by Kuraray Co., Ltd.) was prepared as porous carbon. Coconut shell-based activated carbon is a typical microporous activated carbon as an electrode main material for electric double layer capacitors, similar to phenol resin-based activated carbon. The coconut shell-based activated carbon was also used as a reference example. 0.3 g of this coconut shell-based activated carbon powder and 0.3 g of melamine powder (manufactured by Tokyo Kasei Kogyo Co., Ltd.) were uniformly mixed while pulverizing both powders using a mortar (melamine/YP (mass ratio)=1). ). Next, this mixture was placed on an alumina boat having a size of 30×15×118 mm, heated to 800° C. at a heating rate of 5° C./min in a nitrogen atmosphere in a horizontal tubular furnace, and kept for 1 hour, Nitrogen-doped activated carbon was obtained.

<Comparative Example 1>
First, the same coconut shell-based activated carbon as in Example 1 was prepared as the porous carbon. Next, 2 g of ammonium carbamate was filled in a filling cylinder, and 1.0 g of coconut shell-based activated carbon was put in a rotary kiln electric furnace which is a heat treatment furnace. Then, the filling cylinder was kept at 40° C., and the carrier gas was passed through the filling cylinder at a flow rate of 800 ml/min. Next, a carrier gas containing ammonia and carbon dioxide generated in the filling cylinder was supplied into the heat treatment furnace, and the atmosphere in the furnace was changed to a carrier gas atmosphere containing ammonia and carbon dioxide, and heat treatment was performed at a temperature of 850° C. for 4 hours. , Nitrogen-doped activated carbon was obtained.

<Reference example 1>
The same coconut shell activated carbon as in Example 1 was prepared, and this was used as the porous carbon of Reference Example 1.

<Comparative test 1 and evaluation>
The physical properties of the activated carbon (carbon material) obtained in Example 1, Comparative Example 1 and Reference Example 1 were measured.

BET Specific Surface Area, Mesopore Volume, Micropore Volume, and Average Micropore Width The activated carbon obtained in Example 1, Comparative Example 1 and Reference Example 1 was subjected to nitrogen adsorption/desorption measurement at 77K. From the obtained adsorption isotherm, BET specific surface area (S _BET ), mesopore volume (V _meso ) using the Dollimore-Heal (DH) method, micropore volume (V _micro ) using the Dubinin-Radushkevich (DR) method And the average micropore width (w _micro ) were determined. In addition, the micropores are less than 2 nm, and the mesopores are pores in the range of 2 to 50 nm. The nitrogen content ratio (N/C atomic ratio) in the activated carbon was obtained by elemental analysis for combustion.

FIG. 3 shows nitrogen desorption adsorption isotherms of Example 1, Comparative Example 1 and Reference Example 1. The horizontal axis of FIG. 3 represents the relative pressure (the ratio of a certain pressure P to the saturated vapor pressure P _o ), and the vertical axis represents the amount of gas (nitrogen) adsorbed on the surface of the solid molecule. In addition, Table 1 shows the BET specific surface area, mesopore volume, micropore volume and average micropore width, yield, and nitrogen content, which are pore structure parameters. From FIG. 3, like Example 1 and Reference Example 1, Example 1 showed an I-type isotherm with developed micropores. Further, it can be seen from Table 1 that the specific surface area and the pore volume are slightly reduced by nitrogen doping with melamine. Since the N/C atomic ratios of Example 1 and Comparative Example 1 are 0.029 and 0.027, respectively, which are almost the same, the nitrogen doping by the melamine method is comparable to the CA method from the viewpoint of simple nitrogen content. Can be said.

FIG. 4 shows X-ray photoelectron spectroscopy (XPS) spectra in the Nls peak range of the nitrogen-doped activated carbons of Example 1 and Comparative Example 1. Further, FIG. 5 shows the binding modes of various nitrogen functional groups and the binding energy of Nls. Further, in Table 2, the nitrogen/carbon atomic ratio (N/C _XPS ) obtained from XPS analysis and the abundance ratio of various nitrogen functional groups obtained by peak separation are summarized.

As is clear from FIG. 4, in both the Nls spectra of Example 1 and Comparative Example 1, peaks were confirmed near 398.5 eV and 401.5 eV, and the spectrum shapes were also very similar. Further, as shown in FIG. 4, when the spectrum is separated into waveforms, they are assigned to pyridine type nitrogen, pyrrole/pyridone type nitrogen, quaternary nitrogen, and oxidized type nitrogen in the order of lower energy. The pyridine-type nitrogen had the largest contribution in all the samples, which indicates that the doped nitrogen exists mainly in the pyridine-type bonded state. In addition, it can be seen from Table 2 that N/C _XPS of Example 1 shows a clearly larger value than that of the N/C _{combustion method} , and Comparative Example 1 has almost the same values. From the past research results, it is known that N/C _XPS is almost the same as N/C _{combustion method in} nitrogen doping by CA method. From these results, considering that XPS is a surface analysis, in the CA method, the activated carbon surface and the inside are uniformly nitrogen-doped, whereas in the melamine method, the activated carbon surface is more doped with nitrogen. It was suggested.

<Comparative test 2 and evaluation>
(Preparation of electrode for electric double layer capacitor)
The activated carbon powders obtained in Example 1, Comparative Example 1 and Reference Example 1 were dried, kneaded with a conductive auxiliary agent and a binder, molded into a disk shape with a pressing machine to form an activated carbon electrode, and then a current collector. A conductive adhesive coating was applied to the body, and the activated carbon electrode was overlaid and adhered, whereby the activated carbon electrode and the current collector were integrated to produce an electrode for an electric double layer capacitor.

Specifically, first, the activated carbon powders of Example 1, Comparative Example 1, and Reference Example 1 were dried at 200° C. for 2 hours with a thermal vacuum dryer. Next, the activated carbon powder after drying, acetylene black (Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent, and polytetrafluoroethylene (PTFE) as a binder (Mitsui DuPont) PTFE6.J) manufactured by Fluorochemicals Co., Ltd. was kneaded at a mass ratio of 85:10:5 and then molded into a disk shape by a press machine. Next, a conductive adhesive paint for EDLC (Hitasol GA.715, manufactured by Hitachi Chemical Co., Ltd.) was applied to an etched aluminum foil (manufactured by Nihon Denki Kogyo Co., Ltd., thickness: 20 μm) as a current collector for EDLC. The disk-shaped activated carbons of Example 1, Comparative Example 1 and Reference Example 1 were attached to each other to produce electrodes.

(Fabrication of bipolar cell for electric double layer capacitor)
In order to perform the capacitance measurement and the durability test of the electric double layer capacitor, two activated carbon electrodes obtained in Example 1, Comparative Example 1 and Reference Example 1 were used as a positive electrode and a negative electrode. Aluminum bipolar cells each having the structure shown were produced. In this bipolar cell, a positive electrode side aluminum electrode body 23, a separator 24, a fluororesin guide 25 and a negative electrode side electrode 26 are stacked in this order on a positive electrode side aluminum body 21 having electric wiring, and an electrolytic solution is impregnated between both electrodes. After that, an electrode holder 31 provided with a spring 29 and a negative electrode side aluminum body 32 having electric wiring are placed on the negative electrode side current collector 27 that has been overlapped, and a positive electrode side aluminum body 21 and a negative electrode side aluminum body 32 are provided. It was sandwiched between. A propylene carbonate solution (manufactured by Toyo Gosei Co., Ltd.) containing 1.0 M concentration of triethylmethylammonium tetrafluoroborate ((C2H5)3CH3NBF4) as an electrolyte salt was used as the electrolytic solution of the electric double layer capacitor. This electrolytic solution is generally used as an organic electrolytic solution for electric double layer capacitors.

Further, the impregnation of the activated carbon electrode portion with the electrolytic solution is performed by drying the activated carbon electrode at 200° C. for 2 hours with a thermal vacuum dryer, and then transferring the activated carbon electrode into an argon glove box together with the cellulose-based separator and leaving the activated carbon electrode immersed in the electrolytic solution. It was carried out by holding for 30 minutes.

(Evaluation of initial capacity and capacity retention rate)
The charge/discharge curves before and after the durability test of the electric double layer capacitors using the activated carbons of Example 1, Comparative Example 1 and Reference Example 1 as the electrode main material, and the initial capacity and the capacity retention rate after the durability test were determined.

(An endurance test)
The capacitance measurement for evaluating the durability of the electric double layer capacitor was performed at 40° C. by the constant current method (current density: 80 mA/g; measurement voltage range: 0 to 2.5 V). First, charge and discharge were performed for 5 cycles, and the capacity at the 5th cycle was used as the initial capacity. Next, after measuring the capacity, a durability test was performed by applying a voltage of 3.4 V to the cell at 70° C. for 100 hours. Then, after the durability test, the temperature was returned to 40° C. again, and the capacity was determined by the constant current method (current density: 80 mA/g: measurement voltage range: 0 to 2.5 V). After that, charging and discharging were performed for 5 cycles, and the capacity at the 5th cycle was defined as the final capacity. The capacity ratio before and after the durability test (ratio between the final capacity and the initial capacity) was taken as the capacity retention rate. FIG. 6 shows charge/discharge curves before and after the endurance test of the electric double layer capacitors using the activated carbons of Example 1, Comparative Example 1 and Reference Example 1. Table 3 shows the initial capacity and the capacity retention rate after the durability test. The capacity is a specific capacity (weight specific capacity) obtained by normalizing the capacity obtained from the charge/discharge curve with the total mass of the positive and negative electrodes. Further, the internal resistance of the capacitor was calculated by the following formula using the value of the voltage drop for 0.1 second from the start of discharge.
R=ΔV/(2I) (R: cell resistance (Ω), ΔV: voltage drop for 0.1 second, I: current)

Comparing the results of FIG. 6, all the samples showed a linear charge/discharge curve peculiar to the capacitor before the durability test. However, after the durability test, in Example 1, Comparative Example 1, and Reference Example 1, the charge/discharge curves after the durability test changed, and the time required for discharging decreased. This means that the capacity was reduced by the durability test. In Reference Example 1, the change in the charge/discharge curve after the durability test was large.

On the other hand, from Table 3, Example 1 showed almost the same initial capacity as Comparative Example 1 and Reference Example 1. Further, in the electric double layer capacitor using the non-nitrogen-doped activated carbon of Reference Example 1, although the capacity retention rate after the durability test is only 48%, the electric double layer capacitor using the activated carbon into which nitrogen was introduced by the melamine method of Example 1 was used. The double layer capacitor had a capacity retention rate of 75%, which was higher than 72% of Comparative Example 1 in which nitrogen was introduced by the CA method. From this, it became clear that the melamine method can prepare a nitrogen-doped activated carbon having high-voltage charge resistance as in the CA method.

<Comparative test 3 and evaluation>
(Dependence of doping nitrogen amount on melamine amount and charging amount)

<Example 2>
Nitrogen-doped activated carbon was prepared in the same manner as in Example 1 except that the amount of melamine powder was 0.0375 g (melamine/YP (mass ratio)=0.125) and an aluminum boat having a size of 20×13×80 mm was used. Obtained.

<Example 3>
Nitrogen doping was performed in the same manner as in Example 1 except that the amount of melamine powder was 0.075 g (melamine/YP (mass ratio)=0.25) and an aluminum boat of 20×13×80 mm was used. Activated carbon was obtained.

<Example 4>
Nitrogen doping was performed in the same manner as in Example 1 except that the amount of melamine powder was 0.15 g (melamine/YP (mass ratio)=0.5) and an aluminum boat having a size of 20×13×80 mm was used. Activated carbon was obtained.

<Example 5>
Nitrogen-doped activated carbon was obtained in the same manner as in Example 1 except that an aluminum boat having a size of 20×13×80 mm was used.

<Example 6>
Nitrogen-doped activated carbon was obtained in the same manner as in Example 1 except that the amount of melamine powder was set to 0.6 g (melamine/YP (mass ratio)=2) and an aluminum boat having a size of 20×13×80 mm was used. ..

<Example 7>
Nitrogen-doped activated carbon was obtained in the same manner as in Example 1 except that the amount of melamine powder was 1.2 g (melamine/YP (mass ratio)=4) and an aluminum boat having a size of 20×13×80 mm was used. ..

<Example 8>
A nitrogen-doped activated carbon was obtained in the same manner as in Example 1 except that the amount of activated carbon powder was 2 g and the amount of melamine was 2 g (melamine/YP (mass ratio)=1).

<Example 9>
A nitrogen-doped activated carbon was obtained in the same manner as in Example 1 except that the amount of activated carbon powder was 3 g and the amount of melamine was 3 g (melamine/YP (mass ratio)=1).

Table 4 summarizes the mass ratio of melamine to activated carbon of nitrogen-doped activated carbon, the charged amount of activated carbon, the nitrogen content by the combustion method, and the yield. In Examples 1 to 7, it is considered that the nitrogen doping amount increases as the mass ratio of melamine increases, and that the maximum doping amount is about 5 atom% (N/C=0.05). On the other hand, as shown in Examples 8 and 9, when the mass ratio of melamine and activated carbon was fixed to 1 and the amount of activated carbon was increased to 2 g and 3 g, the nitrogen content was 5.2 atomic% (N/C= 0.052), 5.3 atomic% (N/C=0.053), which was higher than in the case of Examples 1 to 6 in which the amount of activated carbon was 0.3 g. From this, it can be said that in the melamine method, the nitrogen doping amount can be increased as the charging amount increases.

Table 5 summarizes the pore structure data of the samples shown in Table 4. The larger the mass ratio of melamine/activated carbon or the larger the amount of activated carbon charged, the more remarkable the decrease in specific surface area and pore volume. FIG. 7 shows the relationship between the specific surface area and the nitrogen content ratio (N/C) regarding nitrogen doping by the melamine method. The horizontal axis represents the nitrogen content ratio (N/C atomic ratio) in the activated carbon, and the vertical axis represents the BET specific surface area. As shown in FIG. 7, in the melamine method, it is considered that there is a trade-off relationship between the nitrogen doping amount and the maintenance of the pore structure. Considering these results, in order to obtain an electric double layer capacitor that can realize high voltage charging resistance and does not cause a capacity decrease, it is considered that the mass ratio of activated carbon and melamine to be mixed should be 1:1. Be done.

<Example 10>
A nitrogen-doped activated carbon was obtained in the same manner as in Example 1 except that the amount of activated carbon powder was 1 g and the amount of melamine was 1 g (melamine/YP (mass ratio)=1).

<Comparative example 2>
Nitrogen-doped activated carbon was obtained by the CA method in the same manner as in Comparative Example 1 except that ammonium carbonate was used as the nitrogen source and the heat treatment time was 2 hours.

The physical properties of the carbon materials obtained in Example 10 and Comparative Example 2 were measured. The results are shown in Table 6 below.

<Comparative test 4 and evaluation>
An endurance test was conducted in the same manner as in Comparative Test 2 except that the holding voltage of the electric double layer capacitor using the activated carbon of Example 10, Comparative Example 1 and Reference Example 1 as the electrode main material was set to 3.2 V, and the initial capacity was changed. And the capacity retention rate after the durability test was determined. The results are shown in Table 7.

As is clear from Table 7, Example 10 showed almost the same initial capacity as Comparative Example 2 and Reference Example 1. Further, in the electric double layer capacitor using the non-nitrogen-doped activated carbon of Reference Example 1, the capacity retention rate after the durability test was only 74%, but the electricity using the activated carbon into which nitrogen was introduced by the melamine method of Example 9 was used. In the double layer capacitor, the capacity retention rate was 92%, which was higher than the value of 91% in Comparative Example 1 in which nitrogen was introduced by the CA method. From this, it was confirmed that the melamine method can significantly improve the high voltage charge resistance as in the CA method.

<Comparative test 5 and evaluation>
(Heat treatment temperature dependence with melamine)

<Example 11>
Nitrogen-doped activated carbon was obtained in the same manner as in Example 10 except that the treatment temperature was 600°C.

<Example 12>
Nitrogen-doped activated carbon was obtained in the same manner as in Example 10 except that the treatment temperature was 1000°C.

<Comparative example 3>
A nitrogen-doped activated carbon was obtained in the same manner as in Example 10 except that the treatment temperature was 400°C.

Table 8 summarizes the treatment temperature of nitrogen-doped activated carbon, the nitrogen content by the combustion method, the yield, and the BET specific surface area. In Examples 10 to 12 in which the treatment temperature was 600 to 1000°C, the nitrogen doping amount and the yield decreased as the treatment temperature increased, but the specific surface area slightly increased. On the other hand, in Comparative Example 3 in which the treatment temperature was 400° C., the nitrogen doping amount was about 30 atomic% (N/C=0.3), which was extremely high, but the specific surface area was only 40 m ² /g. It is presumed that a large amount of the thermal decomposition product of melamine was deposited on the surface of the activated carbon and blocked the pores. Considering that a specific surface area of 1000 m ² /g or more is required to show sufficient double layer capacity as a porous carbon electrode for capacitors, it can be said that a heat treatment temperature of 600° C. or more is suitable for the melamine method. ..

In addition to Example 10, a durability test was performed in the same manner as Comparative Test 4 except that the activated carbons of Example 11, Example 12, and Comparative Example 3 were used as the electrode main material, and the initial capacity and the capacity after the durability test were performed. The maintenance rate was calculated. The results are shown in Table 9.

As is clear from Table 9, Examples 11 and 12 showed almost the same initial capacity and capacity retention rate as Example 10. In Comparative Example 3 in which the treatment temperature was 400° C., not only the initial capacity was considerably small, but also the capacity retention rate was very low. From this, it became clear that an electric double layer capacitor excellent in high voltage charge resistance could be produced if the heat treatment temperature in the melamine method was in the range of 600° C. or higher and 1000° C. or lower.

<Comparative test 6 and evaluation>
(About the effects of nitrogen sources other than melamine)

<Example 13>
A nitrogen-doped activated carbon was obtained in the same manner as in Example 10 except that 1 g of acetoguanamine was used as a nitrogen source (acetoguanamine/YP (mass ratio)=1).

<Example 14>
Nitrogen-doped activated carbon was obtained in the same manner as in Example 10 except that 2 g of adenine was used as the nitrogen source (adenine/YP (mass ratio)=2).

Table 10 shows the nitrogen source of nitrogen-doped activated carbon, the charged amount of activated carbon, the mass ratio of the nitrogen source and activated carbon, the nitrogen content by the combustion method, the yield, the BET specific surface area, the mesopore volume, the micropore volume and the average micropore width. Summarized. It can be said that even when the nitrogen source is acetoguanamine or adenine, it is possible to prepare a nitrogen-doped activated carbon having a nitrogen content and a pore structure that are almost the same as when melamine is used.

In addition to Example 10, a durability test was performed in the same manner as Comparative Test 4 except that the activated carbons of Examples 13, 14 and Comparative Example 3 were used as the electrode main material, and the initial capacity and the capacity after the durability test were performed. The maintenance rate was calculated. The results are shown in Table 11.

As is clear from Table 11, Examples 13 and 14 showed almost the same initial capacity as that of Example 10, and the capacity retention rate was also substantially the same. From this, by using nitrogen-containing heterocyclic compounds having amino groups such as acetoguanamine and adenine as well as melamine, the porous carbon electrode was nitrogen-doped to produce an electric double-layer capacitor excellent in high voltage charge resistance. It was confirmed that it was possible.

Since the carbon material manufactured by the method of the present invention can significantly improve the high voltage charging resistance of the electric double layer capacitor, it is used as an electric power storage system mounted on a railway vehicle or an auxiliary power source for a hybrid vehicle.

Claims (4)

By mixing the porous carbon and the nitrogen-containing heterocyclic compound having an amino group in a mass ratio of 8:1 to 1:4, the porous carbon and the nitrogen-containing heterocyclic compound having an amino group can be prepared. A method for producing a carbon material for an electric double layer capacitor, comprising: a step of obtaining a mixture; and a step of heat-treating the mixture under an inert gas atmosphere to introduce nitrogen atoms into the porous surface. The method for producing a carbon material for an electric double layer capacitor according to claim 1, wherein the heat treatment is performed by holding at 600 to 1000° C. for 30 minutes to 2 hours in a horizontal tubular furnace. The method for producing a carbon material for an electric double layer capacitor according to claim 1, wherein the content of the nitrogen atom is 1 to 7 atom %. The method for producing a carbon material for an electric double layer capacitor according to claim 1, wherein the nitrogen contained in the carbon material is pyridine type nitrogen.

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