Method for producing carbon material for electric double layer capacitors

Abstract

To provide a manufacturing method of carbon material for electrical double layer capacitor, capable of being executed more easily and more significantly improving high voltage charge tolerance than a conventional nitrogen dope method.SOLUTION: The manufacturing method includes a process of acquiring mixture of porous carbon and nitrogen-containing heterocyclic compound with an amino group by blending the porous carbon and the nitrogen-containing heterocyclic compound with an amino group at a mass ratio of 8:1 to 1:4 and a process of heat-treating the mixture under an inert gas atmosphere and introducing nitrogen atoms to a porous surface.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a carbon material used in electrodes of an electric double layer capacitor. [Background technology]

[0002] Electric double layer capacitors (EDLCs), which can be recharged and reused, are capacitors that store electric charge in an ion adsorption layer, i.e., an electric double layer, formed in the pores of a porous carbon electrode such as activated carbon. Because of their long life and high output, electric double layer capacitors are widely used as backup power sources for computer memory, and recently have been attracting increasing attention as auxiliary power sources for power storage systems installed in railway vehicles and hybrid vehicles.

[0003] Electric double layer capacitors generally have the following characteristics compared to secondary batteries: (1) they can be charged and discharged at high speeds, (2) they have high reversibility in the charge and discharge cycle, (3) they have a long cycle life, and (4) they are environmentally friendly because they do not use heavy metals in the electrodes or electrolytes. These characteristics come from the fact that electric double layer capacitors do not use heavy metals and operate by the physical absorption and desorption of ions, without involving electron transfer reactions of chemical species.

[0004] The energy (E) stored in an electric double layer capacitor is proportional to the product of the square of the charging voltage (V) and the electric double layer capacitance (C), so E=CV 2 / 2) To improve energy density, it is effective to increase capacity and charging voltage.

[0005] However, current electric double layer capacitors have problems such as low energy density compared to secondary batteries and low reliability in charge-discharge cycles under harsh environments. Therefore, in order to develop the above-mentioned new applications, it is necessary to improve the energy density and reliability of electric double layer capacitors, and there is a demand for electrode materials with high capacity and capacity stability under harsh environments. Because double layer capacitance, such as weight-specific capacitance, volume-specific capacitance, and area-specific capacitance, depends on the nanostructure of the activated carbon electrode, such as its pore structure, crystalline structure, and chemical composition, it was necessary to design an electrode material suitable for capacitors.

[0006] As part of research aimed at solving the above problems, the present inventors have developed a simple dry method for producing nitrogen-introduced porous carbon materials by heat-treating porous carbon such as activated carbon in an atmosphere of nitric oxide (NO)-containing gas diluted to several thousand ppm with an inert gas such as helium (hereinafter, this method will be referred to as the NO method) (see, for example, Patent Documents 1 and 2).

[0007] In this NO method, nitrogen is introduced as nitrogen-containing functional groups on the surface of porous carbon at an N / C atomic ratio of approximately 1 to 2% by heat treatment under the above atmosphere according to the reaction scheme shown in formula (1) below.

[0008] C + NO → C(N) + CO ……(1) The resulting nitrogen-introduced porous carbon material was evaluated as the main electrode material for electric double-layer capacitors, and it was found that the capacity did not decrease even when charged and discharged at a charging voltage of 3 V or more; specifically, the capacity retention rate improved from 80% to 90%, successfully suppressing the degradation of electric double-layer capacitors during high-voltage charging.

[0009] However, when the nitrogen-introduced porous carbon materials using the NO method shown in Patent Documents 1 and 2 are put into practical use, even though the nitric oxide used is diluted to several thousand ppm, if released into the atmosphere in large quantities, it will turn into corrosive nitric acid gas, and there is a safety problem.

[0010] For this reason, the present inventors have proposed a method for producing a carbon material for an electric double layer capacitor, which comprises passing an inert gas through a packed tube filled with one or more reagents selected from the group consisting of ammonium carbamate, ammonium hydrogen carbonate, and ammonium carbonate, and heat-treating porous carbon in an inert gas atmosphere containing at least ammonia and carbon dioxide generated by the decomposition reaction of the reagents (see, for example, Patent Document 3). The invention disclosed in Patent Document 3 is a safer production method than conventional production methods using nitric oxide, which have safety issues, because it uses a relatively safe reagent such as ammonium carbamate, and produces nitrogen-infused porous carbon by heat-treating porous carbon in an inert gas atmosphere containing trace amounts of ammonia and carbon dioxide generated by the decomposition reaction of this reagent (hereinafter referred to as the "CA method"). [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2008-141116 A (Claim 2, paragraphs

[0018] to

[0020] of the specification) [Patent Document 2] JP 2010-135647 A (Claim 5, paragraphs

[0025] to

[0029] of the specification) [Patent Document 3] Patent No. 5817286 (Claim 1, specification paragraphs

[0020] to

[0028] ) Summary of the Invention [Problem to be solved by the invention]

[0012] However, in the invention disclosed in Patent Document 3, nitrogen doping is performed by heat treating porous carbon (at 800 to 1000°C) in the presence of ammonia, carbon dioxide, etc., generated by thermal decomposition of ammonium salt. This method requires a separate rotary electric furnace for uniformly heat treating the porous carbon and decomposition gas, and an electric furnace for thermally decomposing the ammonium salt. Therefore, there has been a demand for a method for producing carbon materials for electric double layer capacitors that is simpler than the conventional nitrogen doping method, the CA method, and has improved high-voltage charging resistance.

[0013] 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 CA method, which is a conventional nitrogen doping method, and has improved high-voltage charging resistance. [Means for solving the problem]

[0014] A first aspect of the present invention resides in a method for producing a carbon material for an electric double layer capacitor, comprising the steps of: mixing porous carbon with a nitrogen-containing heterocyclic compound having an amino group in a mass ratio of 8:1 to 1:4 to obtain a mixture of the porous carbon and the nitrogen-containing heterocyclic compound having an amino group; and heat-treating the mixture in an inert gas atmosphere to introduce nitrogen atoms into the surface of the porous carbon.

[0015] A second aspect of the present invention resides in a method for producing a carbon material for an electric double layer capacitor based on the first aspect, wherein the heat treatment is carried out by holding the material in a horizontal tubular furnace at 600 to 1000°C for 30 minutes to 2 hours.

[0016] A third aspect of the present invention resides in a method for producing a carbon material for an electric double layer capacitor based on the first or second aspect, wherein the content of nitrogen atoms is 1 to 7 atomic %.

[0017] A fourth aspect of the present invention resides in a method for producing a carbon material for an electric double layer capacitor based on any one of the first to third aspects, wherein the nitrogen contained in the carbon material is pyridine-type nitrogen. [Effects of the Invention]

[0018] The production method according to the first aspect of the present invention comprises the steps of: mixing porous carbon with a nitrogen-containing heterocyclic compound having an amino group to obtain a mixture of the porous carbon and the nitrogen-containing heterocyclic compound having an amino group; and heat-treating the mixture in an inert gas atmosphere to introduce nitrogen atoms into the surface of the porous carbon. This eliminates the need for a rotary electric furnace with a complicated structure that is required in the conventional CA method, and provides a simple method to obtain porous carbon for electrodes that is suitable for electric double layer capacitors and has improved high-voltage charging resistance.

[0019] In the method according to the second aspect of the present invention, the heat treatment is carried out by holding the material in a horizontal tubular furnace, and therefore a simpler method for producing a carbon material for an electric double layer capacitor can be obtained compared to the conventional CA method.

[0020] In the method according to the third aspect of the present invention, the content of nitrogen atoms is 1 to 7 atomic %, so that a method for producing a carbon material for an electric double layer capacitor having further improved resistance to high voltage charging can be obtained.

[0021] In the method of the fourth aspect of the present invention, the nitrogen contained in the carbon material is pyridine-type nitrogen, which suppresses decomposition of the electrolyte due to electrochemically active sites such as oxygen-containing surface functional groups of the porous carbon, thereby enabling the life of the electric double layer capacitor to be extended. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a diagram schematically illustrating a method for producing a carbon material for an electric double layer capacitor according to an embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing the structure of a bipolar cell for evaluating an electric double layer capacitor used in Examples, Comparative Examples, and Reference Examples. [Figure 3] FIG. 1 is a graph showing nitrogen adsorption / desorption isotherms of the carbon materials of Examples, Comparative Examples, and Reference Examples. [Figure 4] FIG. 1 is a diagram showing Nls spectra of carbon materials of Examples, Comparative Examples, and Reference Examples. [Figure 5]FIG. 1 shows the binding modes of various nitrogen functional groups and the binding energies of Nls. [Figure 6] FIG. 1 is a diagram showing charge / discharge curves of the electric double layer capacitors of Examples, Comparative Examples, and Reference Examples before and after a durability test. [Figure 7] FIG. 1 is a graph showing the relationship between specific surface area and N / C for nitrogen doping using melamine. DETAILED DESCRIPTION OF THE INVENTION

[0023] First, an outline of an embodiment of the present invention will be described with reference to the drawings.

[0024] As shown in FIG. 1, the carbon material for electric double layer capacitors of the present invention can be obtained by mixing porous carbon for electric double layer capacitors with a nitrogen-containing heterocyclic compound having an amino group, heating the mixture at a constant heating rate in a nitrogen atmosphere, and maintaining the mixture at that temperature for a certain period of time.

[0025] Next, an embodiment of the present invention will be described in detail.

[0026] (a) Mixing porous carbon powder for electric double layer capacitors with nitrogen-containing heterocyclic compounds having amino groups The porous carbon for electric double layer capacitors treated by this manufacturing method is not limited to powdered activated carbon, but may also 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 by this manufacturing method is not limited to activated carbon activated by activation treatment, and unactivated porous carbon can also be used. Coconut shell-based activated carbon, wood-based activated carbon, pitch-based activated carbon, coal-based activated carbon, carbon gel, carbon nanofiber, mesoporous carbon, etc. can also be used. When producing a nitrogen-doped carbon material by incorporating nitrogen into porous carbon, the pores tend to be blocked with the generation of nitrogen-containing surface functional groups, resulting in a small specific surface area. Therefore, the porous carbon should be at least 1000 m 2 / g or more, preferably 1200 to 2500m 2It is preferable to use a material with a specific surface area of ​​1 / g.

[0027] As a doping source for doping the porous carbon with nitrogen, a nitrogen-containing heterocyclic compound having an amino group, such as melamine, can be used. Other compounds that can be used include acetoguanamine, adenine, and 1,4-phenylenediamine.

[0028] First, porous carbon powder and melamine powder are mixed in a mortar. When the porous carbon is fibrous, cloth-like, or block-shaped activated carbon, the melamine powder is mixed by scattering it on the porous carbon. The mass ratio of porous carbon to melamine powder is preferably 8:1 to 1:4, and more preferably 4:1 to 1:2. If the porous carbon ratio exceeds the upper limit, the nitrogen ratio in the porous carbon decreases, resulting in a double-layer capacitor with insufficient high-voltage charging resistance. If the porous carbon ratio is below the lower limit, the specific surface area of ​​the porous carbon decreases, resulting in a double-layer capacitor with insufficient capacity. Furthermore, a mass ratio of 4:1 to 1:2 is preferred because a porous carbon ratio exceeding 4 tends to result in non-uniform nitrogen doping, and a melamine ratio exceeding 2 does not significantly increase the nitrogen ratio in the porous carbon.

[0029] (b) Heat treatment of the 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. The heat treatment furnace is then heated in an inert gas atmosphere from room temperature to 600-1000°C, preferably 700-900°C, at a temperature increase rate of 1-10°C / min. The mixture is then maintained at the elevated temperature for 30 minutes to 2 hours under the inert gas atmosphere. After the heat treatment, the electric furnace is slowly cooled to room temperature. By performing the heat treatment under the above conditions, nitrogen is doped into the pore surfaces of the porous carbon powder, allowing for the convenient production of a carbon material containing 1-7 atomic percent nitrogen (hereinafter referred to as the "melamine method"). Here, nitrogen doping is believed to occur through the reaction of the thermal decomposition product of melamine with the carbon surface. The inert gas used is nitrogen, argon, helium, or other gases. The reason why the temperature range is specified is that if the temperature is below the lower limit, most of the pores will be blocked, and if the temperature is above the upper limit, the carbon surface will not be doped with the required amount of nitrogen. Also, if the heat treatment is performed for a time less than the lower limit, the required amount of nitrogen will not be incorporated into the carbon surface, and the effects of the present invention will not be achieved, and if the heat treatment is performed for a time exceeding the upper limit, the yield of the produced carbon material will be very low.

[0030] The carbon material for electric double layer capacitors obtained by the production method of the present invention has nitrogen introduced onto the carbon surface, contains 1 to 7 atomic % of nitrogen, and exists in a state in which nitrogen-containing surface functional groups similar to pyridine are formed. The inclusion of pyridine-type nitrogen on the surface of the porous carbon suppresses decomposition of the electrolyte due to the oxygen-containing surface functional groups of the porous carbon, thereby extending the life of the electric double layer capacitor. By using a carbon material with the above-described nitrogen atomic ratio, it is possible to produce an electric double layer capacitor with improved double layer capacitance per unit area, similar to carbon materials obtained by conventional CA methods. Furthermore, capacitors using the carbon material for electric double layer capacitors obtained by the production method of the present invention can achieve higher high-voltage charging resistance than capacitors using carbon materials obtained by conventional CA methods. If the nitrogen content is less than the lower limit, the effect of suppressing electrolysis is reduced, resulting in a problem of a decrease in the high-voltage charging resistance of the electric double layer capacitor.If the nitrogen content is more than the upper limit, problems arise such as a decrease in the specific surface area, which causes a decrease in the capacity of the electric double layer capacitor, and a decrease in the ionic conductivity in the pores due to pore blockage, which causes an increase in internal resistance.

[0031] To form electrodes for electric double layer capacitors, it is preferable to add a conductive additive 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 any desired shape. Examples of conductive additives include carbon black. Examples of binders include PTFE (polytetrafluoroethylene), PVdF (polyvinylidene fluoride), polyacrylic acid, and SBR (styrene-butadiene rubber). Conventional materials can also be used for current collectors, separators, and other materials. [Example]

[0032] Next, examples of the present invention will be described in detail together with comparative examples.

[0033] Example 1 First, coconut shell-based activated carbon (steam-activated product) (YP50F, manufactured by Kuraray Co., Ltd.) was prepared as porous carbon. Like phenolic resin-based activated carbon, coconut shell-based activated carbon is a typical microporous activated carbon used as the main electrode material for electric double-layer capacitors. This coconut shell-based activated carbon was also used as a reference sample. 0.3 g of this coconut shell-based activated carbon powder and 0.3 g of melamine powder (manufactured by Tokyo Chemical Industry Co., Ltd.) were mixed uniformly while grinding the two powders in a mortar (melamine / YP (mass ratio) = 1). Next, this mixture was placed in an alumina boat measuring 30 × 15 × 118 mm and heated to 800 °C at a rate of 5 °C / min in a horizontal tubular furnace under a nitrogen atmosphere, and maintained at this temperature for 1 hour to obtain nitrogen-doped activated carbon.

[0034] <Comparative Example 1> First, coconut shell-based activated carbon similar to that used in Example 1 was prepared as porous carbon. Next, 2 g of ammonium carbamate was packed into a packing tube, and 1.0 g of coconut shell-based activated carbon was placed in a rotary kiln electric furnace, which served as a heat treatment furnace. Next, the packing tube was kept at 40°C, and a carrier gas was passed through the packing tube at a flow rate of 800 ml / min. Next, a carrier gas containing ammonia and carbon dioxide generated in the packing tube was supplied into the heat treatment furnace, and the furnace atmosphere was changed to a carrier gas atmosphere containing ammonia and carbon dioxide. Heat treatment was then carried out at a temperature of 850°C for 4 hours to obtain nitrogen-doped activated carbon.

[0035] <Reference example 1> The same coconut shell activated carbon as in Example 1 was prepared and used as the porous carbon of Reference Example 1.

[0036] <Comparative Test 1 and Evaluation> The properties of the activated carbons (carbon materials) obtained in Example 1, Comparative Example 1, and Reference Example 1 were measured.

[0037] BET specific surface area, mesopore volume, micropore volume and average micropore width Nitrogen adsorption and desorption measurements were carried out at 77 K for each of the activated carbons obtained in Example 1, Comparative Example 1, and Reference Example 1. From the obtained adsorption isotherms, the BET specific surface area (S BET ), and mesopore volume (V meso ), and micropore volume (V) using the Dubinin-Radushkevich (DR) method. micro ) and average micropore width (w micro ) was determined. Micropores are pores less than 2 nm, and mesopores are pores in the range of 2 to 50 nm. The nitrogen content (N / C atomic ratio) in the activated carbon was determined by elemental analysis using combustion.

[0038] 3 shows nitrogen desorption isotherms for Example 1, Comparative Example 1, and Reference Example 1. The horizontal axis of FIG. 3 represents the relative pressure (a certain pressure P and the saturated vapor pressure P oThe graph shows the ratio of the specific surface area to the carbon dioxide (CO₂) adsorption amount (ratio to the carbon dioxide (CO₂)), and the vertical axis shows the amount of gas (nitrogen) adsorbed on the solid molecular surface. Table 1 also shows the pore structure parameters, namely, the BET specific surface area, mesopore volume, micropore volume and average micropore width, yield, and nitrogen content. As shown in FIG. 3, Example 1, like Comparative Example 1 and Reference Example 1, exhibited a type I isotherm indicating developed micropores. Table 1 also shows that the specific surface area and pore volume were slightly reduced by nitrogen doping with melamine. The N / C atomic ratios of Example 1 and Comparative Example 1 were almost the same, at 0.029 and 0.027, respectively. Therefore, from the perspective of simple nitrogen content, nitrogen doping by the melamine method can be said to be comparable to the CA method.

[0039] [Table 1]

[0040] Figure 4 shows the X-ray photoelectron spectroscopy (XPS) spectra in the Nls peak range of the nitrogen-doped activated carbons of Example 1 and Comparative Example 1. Figure 5 shows the bond modes of various nitrogen functional groups and the Nls bond energies. Table 2 also shows the nitrogen / carbon atom ratio (N / C) obtained from the XPS analysis. XPS ) and the abundance ratios of various nitrogen functional groups obtained by peak separation are summarized.

[0041] As is clear from Figure 4, the Nls spectra of Example 1 and Comparative Example 1 both confirmed peaks around 398.5 eV and 401.5 eV, and the spectral shapes were very similar. Furthermore, as shown in Figure 4, when the spectra were subjected to waveform separation, the peaks were attributed to pyridine-type nitrogen, pyrrole / pyridone-type nitrogen, quaternary nitrogen, and oxidized nitrogen, respectively, in ascending order of energy. In both samples, the largest contributor was pyridine-type nitrogen, which indicated that the doped nitrogen was mainly present in a pyridine-type bond state. Furthermore, Table 2 shows that the N / C of Example 1 XPS N / C 燃焼法 The values ​​are clearly larger than those of Comparative Example 1, and it can be seen that both values ​​are similar. XPS N / C燃焼法 Considering that XPS is a surface analysis, these results suggest that the surface and interior of the activated carbon are uniformly doped with nitrogen in the CA method, whereas the melamine method dopes more nitrogen on the surface of the activated carbon.

[0042] [Table 2]

[0043] <Comparative Test 2 and Evaluation> (Fabrication of electrodes for electric double layer capacitors) The activated carbon powders obtained in Example 1, Comparative Example 1, and Reference Example 1 were dried, kneaded with a conductive additive and a binder, and molded into a disk shape using a press to form an activated carbon electrode. A conductive adhesive coating was then applied to a current collector, and the activated carbon electrode was then superimposed and adhered to the current collector, thereby integrating the activated carbon electrode and the current collector to produce an electrode for an electric double layer capacitor.

[0044] Specifically, the activated carbon powders of Example 1, Comparative Example 1, and Reference Example 1 were first dried at 200°C for 2 hours in a thermal vacuum dryer. Next, the dried activated carbon powder was mixed with acetylene black (Denka Black®, manufactured by Denki Kagaku Kogyo Kabushiki Kaisha) as a conductive additive and polytetrafluoroethylene (PTFE) (PTFE6·J, manufactured by DuPont-Mitsui Fluorochemicals Co., Ltd.) as a binder in a mass ratio of 85:10:5, and then molded into a disk shape using a press. Next, an etched aluminum foil (20 μm thick, manufactured by Nippon Capacitor Industrial Co., Ltd.) serving as a current collector for EDLC was coated with a conductive adhesive paint for EDLC (Hitasol GA·715, manufactured by Hitachi Chemical Co., Ltd.), and the disk-shaped activated carbon of Example 1, Comparative Example 1, and Reference Example 1 was attached to this to prepare electrodes.

[0045] (Fabrication of bipolar cells for electric double layer capacitors) To measure the capacity and perform durability tests on the electric double layer capacitors, aluminum bipolar cells having the structure shown in FIG. 2 were fabricated using two sheets of the activated carbon electrodes obtained in Example 1, Comparative Example 1, and Reference Example 1 as the positive and negative electrodes. These bipolar cells were fabricated by stacking a positive electrode 23, separator 24, fluororesin guide 25, and negative electrode 26 in this order on a positive aluminum body 21 with electrical wiring. The space between the two electrodes was impregnated with an electrolyte. An electrode presser 31 equipped with a spring 29 and a negative aluminum body 32 with electrical wiring were then placed on the stacked negative current collector 27, and the resulting structure was sandwiched between the positive aluminum body 21 and the negative aluminum body 32. The electrolyte used for the electric double layer capacitor was a propylene carbonate solution (manufactured by Toyo Gosei Co., Ltd.) containing 1.0 M triethylmethylammonium tetrafluoroborate ((C2H5)3CH3NBF4) as an electrolyte salt. This electrolyte is generally used as an organic electrolyte for electric double layer capacitors.

[0046] The activated carbon electrode was impregnated with the electrolyte by drying the electrode at 200°C for 2 hours in a thermal vacuum dryer, then transferring it together with the cellulose separator into an argon glove box and leaving the electrode immersed in the electrolyte for 30 minutes.

[0047] (Initial capacity and capacity retention rate evaluation) 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 main electrode material, as well as the initial capacity and capacity retention rate after the durability test were determined.

[0048] (Durability test) The capacity measurement for evaluating the durability of the electric double layer capacitor was performed by a constant current method (current density: 80 mA / g; measurement voltage range: 0 to 2.5 V) at 40°C. First, five charge / discharge cycles were performed, and the capacity at the fifth cycle was defined as the initial capacity. Next, after the capacity measurement, a durability test was performed by applying a voltage of 3.4 V to the cell at 70°C for 100 hours. Subsequently, after the durability test, the temperature was returned to 40°C, and the capacity was determined by a constant current method (current density: 80 mA / g; measurement voltage range: 0 to 2.5 V). Thereafter, five charge / discharge cycles were performed, and the capacity at the fifth cycle was defined as the final capacity. The ratio of the capacities before and after the durability test (the ratio of the final capacity to the initial capacity) was defined as the capacity retention rate. Figure 6 shows the charge / discharge curves before and after the durability test for the electric double layer capacitors using the activated carbons of Example 1, Comparative Example 1, and Reference Example 1. Table 3 shows the initial capacities and the capacity retention rates after the durability test. The capacity is the specific capacity (weight-specific capacity) obtained by normalizing the capacity obtained from the charge / discharge curve by the total mass of the positive and negative electrodes. The internal resistance of the capacitor was calculated using the voltage drop value for 0.1 seconds from the start of discharge using the following formula. R = ΔV / (2I) (R: cell resistance (Ω), ΔV: voltage drop over 0.1 seconds, I: current)

[0049] [Table 3]

[0050] Comparing the results in Figure 6, all samples showed linear charge-discharge curves typical of capacitors before the durability test. However, after the durability test, the charge-discharge curves changed in Example 1, Comparative Example 1, and Reference Example 1, and the time required for discharge decreased. This means that the capacity decreased due to the durability test. Note that in Reference Example 1, the change in the charge-discharge curve after the durability test was significant.

[0051] On the other hand, as shown in Table 3, Example 1 exhibited an initial capacity almost the same as that of Comparative Example 1 and Reference Example 1. Furthermore, the electric double layer capacitor using the nitrogen-undoped activated carbon of Reference Example 1 exhibited a capacity retention rate of only 48% after the durability test, whereas the electric double layer capacitor using the activated carbon into which nitrogen was introduced by the melamine method of Example 1 exhibited a capacity retention rate of 75%, which was higher than the 72% of Comparative Example 1 in which nitrogen was introduced by the CA method. This demonstrates that the melamine method can also be used to prepare nitrogen-doped activated carbon with high-voltage charging resistance, just like the CA method.

[0052] <Comparative Test 3 and Evaluation> (Dependence of doped nitrogen content on melamine content and charge amount)

[0053] <Example 2> Nitrogen-doped activated carbon was obtained 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 measuring 20 × 13 × 80 mm was used.

[0054] Example 3 Nitrogen-doped activated carbon was obtained 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 measuring 20 × 13 × 80 mm was used.

[0055] Example 4 Nitrogen-doped activated carbon was obtained 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 measuring 20 × 13 × 80 mm was used.

[0056] <Example 5> Nitrogen-doped activated carbon was obtained in the same manner as in Example 1, except that an aluminum boat measuring 20×13×80 mm was used.

[0057] Example 6 Nitrogen-doped activated carbon was obtained in the same manner as in Example 1, except that the amount of melamine powder was 0.6 g (melamine / YP (mass ratio) = 2) and an aluminum boat measuring 20 × 13 × 80 mm was used.

[0058] 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 measuring 20 × 13 × 80 mm was used.

[0059] Example 8 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).

[0060] Example 9 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).

[0061] Table 4 summarizes the mass ratio of melamine to activated carbon, the amount of activated carbon charged, the nitrogen content by the combustion method, and the yield of nitrogen-doped activated carbon. In Examples 1 to 7, as the mass ratio of melamine increased, the amount of nitrogen doped increased, and it is believed that doping up to approximately 5 atomic % (N / C = 0.05) was possible. On the other hand, as shown in Examples 8 and 9, when the mass ratio of melamine to activated carbon was fixed at 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) and 5.3 atomic % (N / C = 0.053), respectively, which was higher than in Examples 1 to 6, where 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 by increasing the charged amount.

[0062] [Table 4]

[0063] Table 5 summarizes the pore structure data for the samples shown in Table 4. The larger the melamine / activated carbon mass ratio or the amount of activated carbon added, the more pronounced the decrease in specific surface area and pore volume. Figure 7 shows the relationship between specific surface area and nitrogen content (N / C) for nitrogen doping using the melamine method. The horizontal axis represents the nitrogen content (N / C atomic ratio) in the activated carbon, and the vertical axis represents the BET specific surface area. As shown in Figure 7, with the melamine method, there appears to be a trade-off between the amount of nitrogen doping and maintaining the pore structure. Considering these results, a 1:1 mass ratio of activated carbon to melamine is considered optimal for obtaining an electric double-layer capacitor that can withstand high-voltage charging without causing capacity loss.

[0064] [Table 5]

[0065] Example 10 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).

[0066] <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 set to 2 hours.

[0067] The physical properties of the carbon materials obtained in Example 10 and Comparative Example 2 were measured, and the results are shown in Table 6 below.

[0068] [Table 6]

[0069] <Comparative Test 4 and Evaluation> A durability test was conducted in the same manner as in Comparative Test 2, except that the electric double layer capacitors using the activated carbons of Example 10, Comparative Example 1, and Reference Example 1 as the main electrode material were subjected to a holding voltage of 3.2 V, and the initial capacity and the capacity retention rate after the durability test were determined. The results are shown in Table 7.

[0070] As is clear from Table 7, Example 10 exhibited an initial capacity almost the same as that of Comparative Example 2 and Reference Example 1. Furthermore, the electric double layer capacitor using the nitrogen-undoped activated carbon of Reference Example 1 had a capacity retention rate of only 74% after the durability test, whereas the electric double layer capacitor using the activated carbon into which nitrogen was introduced by the melamine method of Example 9 had a capacity retention rate of 92%, which was higher than the 91% of Comparative Example 1 in which nitrogen was introduced by the CA method. This confirmed that the melamine method, like the CA method, could also significantly improve high-voltage charging resistance.

[0071] [Table 7]

[0072] <Comparative Test 5 and Evaluation> (Heat treatment temperature dependency with melamine)

[0073] Example 11 Nitrogen-doped activated carbon was obtained in the same manner as in Example 10, except that the treatment temperature was 600°C.

[0074] Example 12 Nitrogen-doped activated carbon was obtained in the same manner as in Example 10, except that the treatment temperature was 1000°C.

[0075] <Comparative Example 3> Nitrogen-doped activated carbon was obtained in the same manner as in Example 10, except that the treatment temperature was 400°C.

[0076] Table 8 summarizes the treatment temperature, nitrogen content by combustion method, yield, and BET specific surface area of ​​nitrogen-doped activated carbon. In Examples 10 to 12, where the treatment temperature was 600 to 1000°C, the nitrogen doping amount and yield decreased as the treatment temperature increased, but the specific surface area increased slightly. In contrast, in Comparative Example 3, where the treatment temperature was 400°C, the nitrogen doping amount was very high at about 30 atomic % (N / C = 0.3), but the specific surface area was only 40 m 2 / g. It is presumed that a large amount of melamine pyrolysis products precipitated on the surface of the activated carbon, blocking the pores. In order to exhibit sufficient double layer capacitance as a porous carbon electrode for a capacitor, the specific surface area must be 1000 m 2 / g or more, it can be said that the heat treatment temperature in the melamine method should be 600°C or more.

[0077] [Table 8]

[0078] Durability tests were conducted in the same manner as in Comparative Test 4, except that the activated carbons of Examples 11, 12, and Comparative Example 3, in addition to Example 10, were used as the main electrode material, and the initial capacity and the capacity retention rate after the durability test were determined. The results are shown in Table 9.

[0079] [Table 9]

[0080] As is clear from Table 9, Examples 11 and 12 exhibited initial capacities and capacity retention rates that were almost the same as those of Example 10. Comparative Example 3, in which the treatment temperature was 400°C, not only exhibited a significantly small initial capacity but also an extremely low capacity retention rate. This demonstrates that an electric double layer capacitor with excellent high-voltage charging resistance can be produced if the heat treatment temperature using the melamine method is in the range of 600°C to 1000°C.

[0081] <Comparative Test 6 and Evaluation> (Effects of nitrogen sources other than melamine)

[0082] Example 13 Nitrogen-doped activated carbon was obtained in the same manner as in Example 10, except that 1 g of acetoguanamine was used as the nitrogen source (acetoguanamine / YP (mass ratio) = 1).

[0083] 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).

[0084] The nitrogen source, amount of activated carbon, mass ratio of nitrogen source to activated carbon, nitrogen content by combustion method, yield, BET specific surface area, mesopore volume, micropore volume, and average micropore width of the nitrogen-doped activated carbon are summarized in Table 10. It can be said that when the nitrogen source is acetoguanamine or adenine, nitrogen-doped activated carbon with a nitrogen content and pore structure almost equivalent to that when melamine is used can be prepared.

[0085] [Table 10]

[0086] Durability tests were conducted in the same manner as in Comparative Test 4, except that the activated carbons of Examples 13, 14, and Comparative Example 3 were used as the main electrode material in addition to Example 10, and the initial capacity and the capacity retention rate after the durability test were determined. The results are shown in Table 11.

[0087] [Table 11]

[0088] As is clear from Table 11, Examples 13 and 14 exhibited initial capacities and capacity retention rates that were almost the same as those of Example 10. This confirmed that electric double layer capacitors with excellent high-voltage charging resistance can be produced by doping a porous carbon electrode with nitrogen using nitrogen-containing heterocyclic compounds having an amino group, such as melamine, acetoguanamine, and adenine. [Industrial Applicability]

[0089] The carbon material produced by the method of the present invention can significantly improve the high voltage charging resistance of electric double layer capacitors, and is therefore used as an auxiliary power source for power storage systems mounted on railway vehicles and hybrid vehicles.

Claims

1. The method includes a step of mixing porous carbon and a nitrogen-containing heterocyclic compound having an amino group in a mass ratio of 8:1 to 1:4 to obtain a mixture of the porous carbon and the nitrogen-containing heterocyclic compound having an amino group, and a step of heat-treating the mixture in an inert gas atmosphere to introduce nitrogen atoms into the surface of the porous carbon, The BET specific surface area of ​​the porous carbon is 1200 to 1580 m ２ / g, The content of the nitrogen atoms is 1 to 7 atomic %, and the content ratio of the nitrogen atoms (N / C 燃焼法 ) content ratio (N / C ＸＰＳ ) ratio ((N / C ＸＰＳ ) / (N / C 燃焼法 )) is 1.5 or more, and a capacity retention rate of 75% or more is achieved by applying a voltage of 3.4 V per cell for 100 hours.

2. 2. The method for producing a carbon material for an electric double layer capacitor according to claim 1, wherein the heat treatment is carried out by holding the material in a horizontal tubular furnace at 600 to 1000° C. for 30 minutes to 2 hours.

3. 3. 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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