JP2021143103A - Method for producing reduced graphene oxide - Google Patents

Abstract

To provide a method for producing a reduced graphene oxide that has a low content of manganese and a large specific surface area and is suitable as an electrode of an electric double-layer capacitor.SOLUTION: A water dispersion of graphene oxide is prepared, which has a specific surface area of 600 m2/g or more, as calculated by methylene blue adsorption. To the graphene oxide, 10-50 wt.% of ascorbic acid is added, and then hydrazine is added for heating and reduction.

Description

The present invention relates to a method for producing reduced graphene oxide, and more particularly to a reduced graphene oxide that can be suitably used as an electrode material for an electric double layer capacitor and a method for producing the same.

Graphene is a nanocarbon material from which each layer of the graphite layered structure is peeled off, and is excellent in electrical conductivity, thermal conductivity, gas impermeableness, transparency, high specific surface seating, etc., which are not found in conventional carbon materials. It has the above characteristics, and its application to each device is being actively studied.

There are roughly two ways to take out graphene. One of them is a method of peeling graphene from graphite by mechanical energy, for example, a method using a crusher such as a ball mill, or ultrasonic dispersion of graphite in a solution in the presence of a dispersant to obtain graphene. This is a method of exfoliating and dispersing. With these methods, there is a problem that the peeling cannot be performed uniformly, a problem that impurities are mixed in from the media of the ball mill, or a problem that an excess dispersant adheres. Another method is to prepare reduced graphene oxide having characteristics close to those of graphene by oxidizing graphite to form graphene oxide and then reducing it (Patent Document 1).

As a method for preparing graphene oxide from graphite, a method is known in which graphene oxide is prepared by reacting graphite with a strong oxidizing agent in an acid solvent, and then separated and purified. Concentrated sulfuric acid is used as the acid. , The Hummers method using potassium permanganate as a strong oxidizing agent is known (Non-Patent Document 1).

Further, regarding the Hummers method, improvements such as added raw materials and reaction conditions have been added, and it has been reported as an improved Hummers method (Non-Patent Document 2).

The graphene oxide obtained by the Hummers method or the improved Hummers method can be reduced by various methods such as reduction by heat and reduction by light. Among them, hydrazine or ascorbic acid was used as the reducing agent. Many chemical reductions have been reported (Non-Patent Document 3, Non-Patent Document 4, Non-Patent Document 5).

Conventionally, activated carbon has been used as an electrode of an electric double layer capacitor. In an electric double layer capacitor, an electric double layer is formed on the surface of activated carbon, and when a voltage is applied, positive and negative electrolytes are adsorbed at each pole to store electricity. Therefore, the larger the adsorption surface, the larger the capacity, and it can be said that the larger the specific surface area of the activated carbon, the better the electrode material.

When voltage is applied to both electrodes in the electricity storage state, if there are impurities that electrochemically cause redox inside, the surface of the electrolyte, electrolyte, and activated carbon undergoes a chemical reaction through the redox reaction of the impurities. , There is a concern that the performance of the electrolytic double layer capacitor will deteriorate. In particular, a metal having a variable valence is not particularly preferable as an impurity, and an electrode material having high purity is preferable.

Japanese Unexamined Patent Publication No. 2015-160766

William S. Hummers, et al, Journal of American Chemical Society, 1958, 80, 1339 N. I. Kovtyukhova, et al, Chem. Mater. 11, 771 (1999) Sasha Stankovich, et al, Carbon 45 (2007) 1558-1565 Sungjin Park, et al, Carbon 49 (2011) 3019-3023 Sina Abdolhosseinzadeh, et al, Scientific Reports, 2015, 5, 10160 Jiali Zhang, et al, Chem. Commun., 2010, 46, 1112-1114

In the Hummers method or the improved Hummers method, graphene oxide is prepared by oxidizing graphite with potassium permanganate, and then purified by repeating the operation of dilution with water and sedimentation with a centrifuge many times. The manganese component, which is an impurity, is removed each time the dilution with water and the precipitation are repeated, but there is a problem that approximately 1 to 3 wt% of manganese remains.

^{In Non-Patent Document 3, reduced graphene oxide having a high specific surface area (466 m 2} / g) can be obtained by reducing the dispersion of graphene oxide prepared by the above-mentioned Hummers method or the improved Hummers method with hydrazine. However, there is no mention of the residual amount of manganese, which is an impurity metal. In Non-Patent Document 4, ^{reduced graphene having a high specific surface area (487 m 2} / g) is obtained by reduction with hydrazine, but similarly, impurity metal is not mentioned.

Non-Patent Document 5 reports that the manganese content of reduced graphene oxide is reduced by adding ascorbic acid at the time before purification of the prepared graphene oxide and reducing it as it is. However, since there is no mention of the specific surface area, it is unclear whether or not it is suitable as an electrode for an electric double layer capacitor. Non-Patent Document 6 also uses ascorbic acid as a reducing agent to obtain reduced graphene oxide, but does not mention the specific surface area.

When the present inventors prepared a reduced graphene oxide using ascorbic acid as a reducing agent with respect to the graphene oxide prepared by the Hummers method or the improved Hummers method, the manganese content was 346 ppm, but the specific surface area was It was 279.8 m ² / g, and there was room for improvement in using it as an electrode for an electric double layer capacitor.

As described above, a method for preparing reduced graphene having a low content of manganese as an impurity and a high specific surface area has not yet been reported.

In view of the above circumstances, the present inventors have obtained the following findings as a result of sincere examination of a method for preparing reduced graphene oxide suitable for an electrode of an electric double layer capacitor using an organic electrolytic solution.

That is, in order to obtain reduced graphene oxide having a high specific surface area and a low content of manganese, which is an impurity, graphene oxide having a high specific surface area is prepared as a raw material, and the reducing power is high even under acidic conditions. It is necessary to reduce the manganese remaining with a certain ascorbic acid with hydrazine while maintaining the state of being reduced to an easily water-soluble divalent state, and wash with water.

The present invention has been completed based on the above findings, and the gist thereof is to prepare an aqueous dispersion of graphene oxide ^{having a specific surface area of 600 m 2 / g or more calculated by the methylene blue adsorption method, and to the graphene oxide.} A method for producing reduced graphene oxide, which comprises adding 10 to 50 wt% of ascorbic acid and then adding hydrazine to reduce heat.

According to the present invention, it is possible to obtain a reduced graphene oxide having a large specific surface area and reduced manganese impurities, which is suitable for an electrode of an electric double layer capacitor using an organic electrolytic solution.

First, a dispersion of graphene oxide is prepared. As the preparation method, in addition to the method described in Patent Document 1, the Hummers method (Non-Patent Document 1) or the improved Hummers method (Non-Patent Document 2) can be adopted.

That is, graphite is oxidized by dispersing the raw material graphite in concentrated sulfuric acid and then adding potassium permanganate little by little while cooling. Then, pure water is added to stop the oxidation reaction, and then hydrogen peroxide solution is added until no bubbles are generated. Next, graphene oxide is purified using a centrifuge. Purification is performed by repeating washing with pure water and sedimentation with a centrifuge.

The oxidation reaction changes depending on conditions such as the temperature and time of the reaction and the amount of potassium permanganate added. As the oxidation progresses, the oxygen functional groups in the graphene oxide increase, and the individual graphene oxide sheets are easily peeled off. As the exfoliation progresses, the specific surface area of graphene oxide increases. By using graphene oxide having a large specific surface area and exfoliation, it is possible to obtain reduced graphene oxide having a large specific surface area after reduction. It is difficult to obtain reduced graphene oxide having a large specific surface area from graphene oxide having a small specific surface area and not being exfoliated.

As described above, the value of the specific surface area of graphene oxide is important, but in the present invention, the value of the specific surface area of graphene oxide is measured by the methylene blue adsorption method. The specific surface area of graphene oxide by the methylene blue adsorption method can be calculated with reference to "Pedo Montes-Navajas, et. Al, Langmuir, 2013, 29, 13443-13448".

That is, after adding graphene oxide to an aqueous solution of methylene blue having a known concentration, methylene blue is adsorbed on graphene oxide by stirring for a certain period of time. After that, the amount of methylene blue adsorbed is measured using an ultraviolet / visible spectrophotometer, and the amount of methylene blue adsorbed on 1 g of graphene oxide is calculated. ^{Next, the specific surface area (m 2} / g) of graphene oxide can be calculated by calculating the ^{surface area as 2.54 m 2} with respect to 1 mg of adsorbed methylene blue.

In order to obtain reduced graphene oxide, which is the object of the present invention, it is important that the specific surface area of graphene oxide measured by the above-mentioned measuring method is 600 m ² / g or more, preferably 700 m ² / g or more. be. The upper limit of the specific surface area is not particularly limited, but is usually 850 m ² / g.

Next, it is prepared into an aqueous dispersion of purified graphene oxide. The concentration is usually 0.1 wt% to 1 wt%, preferably 0.1 wt% to 0.5 wt%, and more preferably 0.2 wt% to 0.4 wt%. If the concentration is less than 0.1 wt%, the productivity is poor and it is not preferable. If the concentration is higher than 1 wt%, the specific surface area tends to decrease, which is not preferable.

Next, the prepared aqueous dispersion of graphene oxide is subjected to a dispersion treatment of graphene oxide. An ultrasonic cleaner, an ultrasonic disperser, a homomixer, a homodisper, or the like can be used for the dispersion treatment.

Next, ascorbic acid is added to the aqueous dispersion of graphene oxide to dissolve it. Isoascorbic acid, which is a stereoisomer of ascorbic acid, may be used. The amount of ascorbic acid and / or ascorbic acid added is 10 wt% to 50 wt%, preferably 10 wt% to 30 wt% with respect to graphene oxide. If the amount added is more than 50 wt%, the specific surface area of reduced graphene oxide becomes small, which is not preferable. If the amount added is less than 10 wt%, the amount of manganese remaining increases, which is not preferable.

Next, hydrazine is added and reduced by heating. Usually, hydrazine / monohydrate is used, and the amount used is usually selected to be an appropriate amount so as to maximize the specific surface area of the reduced graphene produced, but it is usually 0.5 per 1 g of graphene oxide. ~ 1.5 mL, preferably 1-1.5 mL. The heating temperature is usually 80 to 100 ° C. The reduced graphene oxide produced after heating under reflux is subjected to normal filtration, washing with pure water, and then dried.

The reduced graphene oxide obtained by the production method according to the present invention has a manganese content of 500 ppm or less. When the manganese content exceeds 500 ppm, when it is used as an electrode of an electric double layer capacitor, a decrease in capacitance and an increase in internal impedance are observed when used for a long period of time, which is not preferable.

The reduced graphene oxide obtained by the production method according to the present invention has a BET specific surface area of 400 m ² / g or more. When the BET specific surface area is ^{less than 400 m 2} / g, sufficient capacitance cannot be obtained when used as an electrode of an electric double layer capacitor, which is not preferable.

The reduced graphene oxide obtained by the production method according to the present invention has a large specific surface area and a reduced manganese content, and is therefore suitable for an electrode of an electric double layer capacitor using an organic electrolytic solution. In addition, it can be expected to be applied to applications such as conductive auxiliary materials.

Hereinafter, the present invention will be described in more detail with reference to specific examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded.

The manganese content of the powder of reduced graphene oxide obtained by drying was measured by an inductively coupled plasma emission spectrophotometer "ICAP6500DuO (manufactured by Thermo Fisher Scientific Co., Ltd.). The specific surface area was measured by the specific surface area measuring device". It was measured by the BET method using Multisoap 16 (manufactured by Kantachrome).

Example 1:
(Preparation of graphene oxide)
While stirring with a mechanical stirrer, 3 g of graphite (SP-1 manufactured by Bay Carbon) was added to 90 mL of concentrated sulfuric acid, and the mixture was ice-cooled. Then, 6 g of potassium permanganate was added, and the mixture was stirred at 35 ° C. for 2 hours. It was ice-cooled again, 90 mL of water was added, 30% hydrogen peroxide solution was added until no bubbles appeared, and the mixture was stirred at room temperature for 30 minutes. Centrifugation was repeated using pure water to remove excess sulfuric acid and manganese from the product to prepare graphene oxide.

(Calculation of specific surface area by methylene blue adsorption method)
A part of the obtained graphene oxide was taken out and freeze-dried. 2.5 mg of graphene oxide after freeze-drying was put into 50 mL of a pre-prepared methylene blue aqueous solution (100 μmol / L), and 30 were dispersed in an ultrasonic cleaner. Then, the dispersion was filtered through a 0.2 μm membrane filter, and the obtained filtrate was used for measurement with an ultraviolet-visible spectrophotometer.

The amount of methylene blue adsorbed by graphene oxide per gram was calculated from the rate of decrease in the absorption peak intensity of methylene blue at 610 nm. ^{Next, the specific surface area of graphene oxide was calculated with the surface area of 2.54 m 2} per 1 mg of methylene blue adsorbed on 1 g of graphene oxide, and the specific surface area was calculated to be 763.8 m ² / g.

A 0.25 wt% aqueous dispersion solution (1 kg) was prepared using the above graphene oxide. Next, 10 wt% of ascorbic acid was added to graphene oxide, and then dispersion treatment was performed for 30 minutes using an ultrasonic disperser. The dispersion solution was transferred to a 2 L separable flask, 2.5 mL of hydrazine / monohydrate was added, and the mixture was heated under reflux at 90 ° C. for 2 hours with stirring using a mechanical stirrer. The obtained reduced graphene oxide was suction-filtered and washed with water, and dried by freeze-drying to obtain reduced graphene oxide. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 322 ppm. The BET specific surface area was 423.3 m ² / g.

Example 2:
In Example 1, the oxidation conditions were changed to prepare a 0.3 wt% aqueous dispersion solution (1 kg) of graphene oxide having ^{a specific surface area of 753.3 m 2 / g by the methylene blue adsorption method, and then the aqueous dispersion was prepared.} Ascorbic acid was added to graphene oxide in an amount of 30 wt%, and 3 mL of hydrazine / monohydrate was added to obtain reduced graphene oxide in the same manner as in Example 1. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 356 ppm. The BET specific surface area was 433.5 m ² / g.

Example 3:
In Example 1, the oxidation conditions were changed to prepare a 0.35 wt% aqueous dispersion solution (1 kg) of graphene oxide ^{having a specific surface area of 780.2 m 2 / g by the methylene blue adsorption method, and then the ascorbic acid was oxidized.} Reduced graphene oxide was obtained by the same operation as in Example 1 except that 20 wt% was added to graphene and 3.5 mL of hydrazine / monohydrate was added. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 249 ppm. The BET specific surface area was 401.2 m ² / g.

Example 4:
In Example 1, the oxidation conditions were changed to prepare a 0.1 wt% aqueous dispersion solution (1 kg) of graphene oxide ^{having a specific surface area of 780.2 m 2 / g by the methylene blue adsorption method, and then ascorbic acid was oxidized.} Reduced graphene oxide was obtained by the same operation as in Example 1 except that 10 wt% was added to graphene and 1 mL of hydrazine / monohydrate was added. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 250 ppm. The BET specific surface area was 432.2 m ² / g.

Comparative Example 1:
A 0.3 wt% aqueous dispersion solution (1 kg) was prepared using graphene oxide (specific surface area 763.8 m ^{2 / g) prepared in Example 1.} Next, 500 wt% of ascorbic acid was added to graphene oxide, and then dispersion treatment was performed for 30 minutes using an ultrasonic disperser. The dispersion solution was transferred to a 2 L separable flask, and the mixture was heated under reflux at 90 ° C. for 2 hours with stirring using a mechanical stirrer without adding hydrazine / monohydrate. The obtained reduced graphene oxide was suction-filtered and washed with water, and dried by freeze-drying to obtain reduced graphene oxide. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 346 ppm. The BET specific surface area was 279.8 m ² / g.

Comparative Example 2:
An aqueous dispersion solution (1 kg) of 0.3 wt% of graphene oxide having a specific surface area of 763.8 m ² / g by the methylene blue adsorption method was prepared, and 3 mL of hydrazine / monohydrate was added without adding ascorbic acid. Reduced graphene oxide was obtained by the same operation as in Comparative Example 1 except for the above. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 9823 ppm. The BET specific surface area was 499.8 m ² / g.

Comparative Example 3:
An aqueous dispersion solution (1 kg) of 0.5 wt% of graphene oxide having a specific surface area of 350.2 m ² / g by the methylene blue adsorption method was prepared, 10 wt% of ascorbic acid was added to graphene oxide, and hydrazine / monohydrate was added. Reduced graphene oxide was obtained by the same operation as in Comparative Example 1 except that 5 mL of the product was added. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 430 ppm. The BET specific surface area was 276.5 m ² / g.

Comparative Example 4:
An aqueous dispersion solution (1 kg) of 0.3 wt% of graphene oxide having a specific surface area of 780.2 m ² / g by the methylene blue adsorption method was prepared, and 3 mL of hydrazine / monohydrate was added without ascorbic acid. Obtained reduced graphene oxide by the same operation as in Comparative Example 1. The amount of manganese contained in the obtained reduced graphene oxide was measured and found to be 4423 ppm. The BET specific surface area was 478.5 m ² / g.

Table 1 below shows the results of the above-mentioned Examples and Comparative Examples.

Claims (2)

Prepare an aqueous dispersion of graphene oxide having a specific surface area of 600 m ² / g or more calculated by the methylene blue adsorption method, add 10 to 50 wt% ascorbic acid to graphene oxide, and then add hydrazine to reduce heat. A method for producing reduced graphene oxide. The method for producing reduced graphene according to claim 1, wherein the obtained reduced graphene oxide has a manganese content of 500 ppm or less and a BET specific surface area of 400 m ^{2 / g or more.}