Porous reduced graphene oxide and method for producing the same

Abstract

A porous reduced graphene oxide that is a porous structure that includes reduced graphene oxide and has mesopores having a pore diameter of at least 10 nm but less than 50 nm and macropores that have a pore diameter of at least 50 nm but less than 160 nm. In a C1s spectrum measured using XPS, the proportion of bonds attributed to C = O is 3% to 10%; in an O1s spectrum measured using XPS, the proportion of bonds attributed to C = O is 25% to 55%; and the macropore pore volume is greater than mesopore pore volume.

Description

[Technical Field]

[0001] The present invention relates to porous reduced graphene oxide and a method for producing the same. [Background technology]

[0002] In recent years, concerns about fossil fuel shortages have led to a rapid increase in demand for alternative energy sources and a need for the development of sustainable, high-power energy sources. In this context, supercapacitors are attracting attention as batteries with long cycle life due to their high power density and ability to store energy quickly and efficiently. In the future, they are expected to be used as large-scale capacitors for applications such as electric vehicle power supplies.

[0003] However, the precious metal-free carbon electrodes currently under consideration have a low energy density and cannot store enough electricity to be used in long-range electric vehicles. Therefore, the development of technologies for creating precious metal-free electrode materials with high electrode capacity is becoming essential.

[0004] Reduced graphene oxide is attracting attention as a precious metal-free electrode material (see Non-Patent Documents 1 and 2). Reduced graphene oxide is a material obtained by reducing graphene oxide, which is obtained by the oxidation of graphite. In reduced graphene oxide, some of the oxygen-containing functional groups that were present in graphene oxide often remain unreduced, so reduced graphene oxide can be positioned as a material that is intermediate between graphene and graphene oxide.

[0005] In recent years, in order to obtain a larger electrode capacity, methods for producing porous materials of reduced graphene oxide, known as "porous reduced graphene oxide," have been investigated (see Non-Patent Document 3). [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Advanced Materials Research, 2014, 1004-1005, 1013-1016 [Non-Patent Document 2] Nature Communications 1, Article number: 73, doi:10.1038 / ncomms1067 [Non-Patent Document 3] Carbon, 2016, 100, 7-15 [Overview of the project] [Problems that the invention aims to solve]

[0007] The method described in Non-Patent Document 3 has several drawbacks, including the large number of steps involved and the long time required for manufacturing (e.g., 20 hours or more), as well as the need for processing under special conditions such as an H2 / Ar atmosphere and at high temperatures (e.g., 300°C).

[0008] Furthermore, while porous reduced graphene oxide obtained by the method described in Non-Patent Document 3 has a large specific volume, there is a need to develop a method that can further increase this specific volume.

[0009] Therefore, one of the objectives of the present invention is to enable the production of porous reduced graphene oxide by a simpler method. Another objective of the present invention is to provide porous reduced graphene oxide with a larger specific volume and a method for producing the same. [Means for solving the problem]

[0010] The present inventors conducted diligent research to solve the above problems and discovered that by irradiating a composite containing graphene oxide and metal oxide particles with microwaves in the presence of alcohols and acids, the reduction of graphene oxide and the removal of metal oxide particles can be smoothly carried out, and porous reduced graphene oxide can be efficiently obtained, thus completing the present invention.

[0011] In other words, one aspect of the present invention relates to a method for producing porous reduced graphene oxide, comprising the steps of: a) preparing a composite containing graphene oxide and metal oxide particles; and b) reducing the graphene oxide in the composite and removing the metal oxide particles from the composite, wherein step b is either step b1 of irradiating the composite with microwaves in the presence of alcohols and an acid, or step b2 of irradiating the composite with microwaves in the presence of alcohols to obtain a reduced composite, and then irradiating the reduced composite with microwaves in the presence of an acid.

[0012] According to the above manufacturing method, reduction of graphene oxide and formation of pores can be achieved in a short number of steps and in a short time, and since it does not require special atmospheres or high-temperature treatment, porous reduced graphene oxide can be manufactured more easily compared to conventional methods.

[0013] Furthermore, the above manufacturing method tends to produce a higher specific volume of porous reduced graphene oxide compared to conventional methods. In other words, the above manufacturing method tends to produce porous reduced graphene oxide with superior specific volume. The reason for this is not clear, but one possible reason is that because graphene oxide and metal oxide particles are compounded before the reduction of graphene oxide, aggregation of graphene caused by the reduction of graphene oxide is less likely to occur. In addition, in conventional methods that use strong bases such as sodium hydroxide, oxygen-containing functional groups are excessively reduced by the base, whereas in the above manufacturing method, acid is used to remove metal oxide particles and alcohols with a mild reducing action are used as reducing agents, making it easier to control the reduction of oxygen-containing functional groups. As a result, it is thought that porous reduced graphene oxide with an appropriate amount of oxygen-containing functional groups remaining is more easily obtained.

[0014] The above step b is preferably step b1, in which the composite is irradiated with microwaves in the presence of alcohols and acids. This method allows for the production of porous reduced graphene oxide in a single pot.

[0015] The above alcohols may contain alkylene glycol. In this case, the reduction of graphene oxide tends to proceed gently, and the specific capacity of the obtained porous reduced graphene oxide tends to be larger.

[0016] The above acid may contain at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and acetic acid. In this case, while graphene oxide is reduced by microwave heat and the reducing action of alcohols, the removal of metal oxide particles from the surface and interlayer of the composite tends to proceed, and the specific capacity of the obtained porous reduced graphene oxide tends to be larger.

[0017] The content of the metal oxide particles in the above composite may be 200 to 500 parts by mass with respect to 100 parts by mass of graphene oxide. In this case, the aggregation of graphene is suppressed, and pores with an appropriate amount and shape are likely to be formed, so that the specific capacity of the obtained porous reduced graphene oxide tends to be larger.

[0018] Here, the aggregation of graphene refers to the aggregation occurring between graphene layers. Graphene is likely to aggregate between graphene layers due to the influence of π-π electrons. This aggregation not only greatly reduces the specific surface area but also prevents other substances such as electrolyte ions from entering the graphene layer and limits the amount of active sites. Therefore, non-porous reduced graphene oxide tends to have a smaller specific capacity than porous reduced graphene oxide.

[0019] Another aspect of the present invention is a porous structure including reduced graphene oxide, having mesopores with a pore diameter of 10 nm or more and less than 50 nm and macropores with a pore diameter of 50 nm or more and less than 160 nm, in the C1s spectrum measured by XPS, the ratio of the bond attributed to C=O is 3 to 10%, and in the O1s spectrum measured by XPS, the ratio of the bond attributed to C=O is 25 to 55%, and the pore volume of the macropores is larger than the pore volume of the mesopores, relating to porous reduced graphene oxide.

[0020] Because the porous reduced graphene oxide described above has a larger specific capacitance, it is suitably used as an electrode material for supercapacitors. In other words, using the porous reduced graphene oxide described above, an electrode material for supercapacitors with a large electrode capacitance can be obtained.

[0021] The pore volume of the above mesopores is 0.02-0.4 cm³. 3 The density may be / g, and the pore volume of the above macropores is 0.4 to 4.0 cm³. 3 It can be expressed as / g. In this case, a larger specific volume is more likely to be obtained.

[0022] In the porous reduced graphene oxide described above, the pore volume of micropores with a pore diameter of less than 10 nm is 0.02 cm³. 3 It may be less than or equal to / g. In this case, a larger specific volume is more likely to be obtained. Note that the lower limit of the pore volume of the micropores is 0. That is, the porous reduced graphene oxide described above does not need to have micropores. [Effects of the Invention]

[0023] According to the present invention, porous reduced graphene oxide can be produced by a simpler method. Furthermore, according to the present invention, porous reduced graphene oxide having a larger specific volume and a method for producing the same can be provided. [Brief explanation of the drawing]

[0024] [Figure 1] These are FE-SEM images of zinc oxide particles, reduced graphene oxide, and porous reduced graphene oxide. [Figure 2] This graph shows the pore size distribution curves for Example 1 and Comparative Example 1. [Figure 3] These are the XPS(C1s) spectra of Example 1 and Comparative Example 1. [Figure 4] These are the XPS(O1s) spectra of Example 1 and Comparative Example 1. [Modes for carrying out the invention]

[0025] In this specification, numerical ranges indicated using "~" represent a range that includes the numbers before and after "~" as the minimum and maximum values, respectively. When multiple numerical ranges are described in stages in this specification, the upper or lower limit of one stage of the numerical range may be replaced with the upper or lower limit of another stage of the numerical range. Alternatively, the upper or lower limit of a numerical range may be replaced with the values ​​shown in the examples. Furthermore, the upper and lower limits described individually can be combined in any way. In addition, unless otherwise specified, the materials exemplified below may be used individually or in combination of two or more types.

[0026] Preferred embodiments of the present invention will be described below. However, the present invention is not limited in any way to the embodiments described below.

[0027] <Porous Reduced Graphene Oxide> One embodiment of porous reduced graphene oxide is a porous structure containing reduced graphene oxide. Porous reduced graphene oxide is, for example, in powder form.

[0028] Porous reduced graphene oxide may consist solely of reduced graphene oxide, but may also contain other components (e.g., components that are inevitably mixed in). For example, it may contain metals or oxides that constitute metal oxide particles. These may be residual metals or oxides that constitute metal oxide particles used in the manufacturing method described later, without being removed. The amount of the above metals and oxides in the porous reduced graphene oxide (e.g., residual amount) is preferably 3% by mass or less, more preferably 1% by mass or less, and even more preferably 0% by mass, based on the total mass of the porous reduced graphene oxide.

[0029] The reduced graphene oxide that constitutes porous reduced graphene oxide is a partially reduced form of graphene oxide. Therefore, porous reduced graphene oxide has oxygen-containing functional groups (more specifically, reduced graphene oxide has oxygen-containing functional groups). Examples of oxygen-containing functional groups include epoxy groups (-COC-), hydroxyl groups (-OH), carbonyl groups (-CO-), and carboxyl groups (-COOH). From the viewpoint of obtaining a larger specific volume, it is preferable that porous reduced graphene oxide has carbonyl groups as oxygen-containing functional groups. The presence of carbonyl groups in porous reduced graphene oxide can be confirmed by analysis using XPS (X-ray photoelectron spectroscopy).

[0030] In the C1s spectrum of porous reduced graphene oxide measured by XPS, the proportion of bonds attributed to C=O may be 3-10%, 4-9%, or 6-8% from the viewpoint of obtaining a larger specific volume. Similarly, in the O1s spectrum of porous reduced graphene oxide measured by XPS, the proportion of bonds attributed to C=O may be 25-55%, 30-50%, or 35-45%. Here, the C1s spectrum refers to the region of the XPS spectrum corresponding to the energy peak position of the 1s orbital of C, and the O1s spectrum refers to the region of the XPS spectrum corresponding to the energy peak position of the 1s orbital of O. Furthermore, the proportion of bonds attributed to C=O in the C1s spectrum refers to the proportion of bonds attributed to C=O in the total number of bonds attributed from the peaks observed in the C1s spectrum (e.g., C=C / CC, CO, C=O, and OC=O). Furthermore, the proportion of bonds assigned to C=O in the O1s spectrum refers to the proportion of bonds assigned to C=O within the total number of bonds assigned to peaks observed in the O1s spectrum (e.g., C=O, CO, and COH).

[0031] The energy peak position of the 1s orbital of C and the energy peak of the 1s orbital of O are attributed to any bond containing C or O based on their peak positions. The ratio of the bond attributed to C=O in the above C 1s spectrum is a value obtained from the peak area ratio of the C 1s spectrum, and can also be referred to as the atomic concentration (unit: atm%) of the carbon atom constituting the C=O bond. Similarly, the ratio of the bond attributed to C=O in the above O 1s spectrum is a value obtained from the peak area ratio of the O 1s spectrum, and can also be referred to as the atomic concentration (unit: atm%) of the oxygen atom constituting the C=O bond. The above XPS can be measured by the method described in the examples.

[0032] The porous reduced graphene oxide preferably has mesopores (pore diameter: 10 nm or more and less than 50 nm) and macropores (pore diameter: 50 nm or more and less than 160 nm). From the viewpoint of obtaining a larger specific capacity, the pore volume of the macropores is preferably larger than the pore volume of the mesopores.

[0033] The pore volume of the mesopores may be 0.02 to 0.4 cm 3 / g, may be 0.05 to 0.3 cm 3 / g or 0.1 to 0.25 cm 3 / g. The pore volume of the mesopores can be measured by the method described in the examples.

[0034] The pore volume of the macropores may be 0.4 to 4.0 cm 3 / g, may be 0.7 to 3.5 cm 3 / g or 1.0 to 3.0 cm 3 / g. The pore volume of the macropores can be measured by the method described in the examples.

[0035] The porous reduced graphene oxide may have pores with a pore diameter of less than 10 nm (micropores). The pore volume of the micropores may be smaller than the pore volume of the mesopores. For example, 0.02 cm 3It may be less than / g. If porous reduced graphene oxide has micropores, the pore volume of the micropores is 0.001 to 0.02 cm³. 3 It may be / g. Also, porous reduced graphene oxide may have pores with a diameter of 160 nm or more.

[0036] The porous reduced graphene oxide described above can be suitably used as a precious metal-free conductive material, and in particular, it can be suitably used as an electrode material for supercapacitors.

[0037] The porous reduced graphene oxide described above can be obtained by the method for producing porous reduced graphene oxide described later.

[0038] <Method for producing porous reduced graphene oxide> One embodiment of a method for producing porous reduced graphene oxide comprises step a of preparing a composite containing graphene oxide and metal oxide particles, and step b of reducing the graphene oxide in the composite and removing the metal oxide particles from the composite.

[0039] (Step a) In step a, a composite is prepared. The composite contains at least graphene oxide (GO) and metal oxide (ZnO) particles.

[0040] Graphene oxide can be obtained, for example, by the oxidation of graphite by the Hummers method described in Non-Patent Document 1. The Hummers method generally includes a first step of pre-treating graphite by reacting it with a compound selected from ammonium persulfate, phosphorus pentoxide, and sulfuric acid, and a second step of oxidizing the pre-treated graphite with sulfuric acid and a strong oxidizing agent (e.g., potassium permanganate). Typically, the pre-treated graphite is washed with water, dried, and then subjected to the second step. The graphite (graphene oxide) oxidized in the second step may be washed with hydrogen peroxide, hydrochloric acid, and water, etc. This method yields powdered graphene oxide.

[0041] Graphene oxide has oxygen-containing functional groups produced by oxidizing graphite. These oxygen-containing functional groups may include, for example, at least one functional group selected from the group consisting of epoxy groups (-COC-), hydroxyl groups (-OH), carbonyl groups (-CO-), and carboxyl groups (-COOH).

[0042] The graphene oxide content in the composite is preferably 17% by mass or more, more preferably 18% by mass or more, even more preferably 19% by mass or more, and particularly preferably 20% by mass or more, based on the total mass of the composite. When the graphene oxide content is 17% by mass or more, it is easier to obtain porous reduced graphene oxide with a larger specific volume. The graphene oxide content in the composite is preferably 34% by mass or less, more preferably 32% by mass or less, even more preferably 29% by mass or less, and particularly preferably 25% by mass or less, based on the total mass of the composite. When the graphene oxide content is 34% by mass or less, it is easier to suppress the aggregation of graphene, and it is easier to obtain porous reduced graphene oxide with a larger specific volume. From these viewpoints, the graphene oxide content is preferably 17 to 34% by mass, based on the total mass of the composite.

[0043] Metal oxide particles are particles whose main component is a metal oxide. In the composite, the metal oxide particles are compounded with graphene oxide in a dispersed state within graphene oxide. For example, the metal oxide particles may be linked to graphene oxide by non-covalent interactions (e.g., electrostatic interactions between positive and negative charges). The metal oxide is not particularly limited as long as it can be compounded with graphene oxide by non-covalent interactions. The metal oxide is preferably zinc oxide (ZnO), magnesium oxide (MgO), calcium oxide (CaO), lead oxide (PbO), or mercury oxide (HgO), and more preferably zinc oxide.

[0044] The metal oxide particles may contain components other than metal oxides (for example, components that are inevitably mixed in). The content of metal oxide in the metal oxide particles is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more. The metal oxide particles may consist only of metal oxides. In this embodiment, the content of zinc oxide in the metal oxide may be within the above range, the content of magnesium oxide may be within the above range, the content of calcium oxide may be within the above range, the content of lead oxide may be within the above range, and the content of mercury oxide may be within the above range.

[0045] The shape of the metal oxide particles may be spherical, or it may be non-spherical, such as flake-shaped or rod-shaped.

[0046] As described above, porous reduced graphene oxide tends to exhibit a large specific capacity when it has mesopores with a pore diameter of 10 nm or more and less than 50 nm, and macropores with a pore diameter of 50 nm or more and less than 160 nm, and it tends to exhibit an even larger specific capacity when the pore volume of the macropores is larger than the pore volume of the mesopores. Therefore, from the viewpoint of making it easier to obtain porous reduced graphene oxide having the above-mentioned mesopores and macropores, the average particle diameter of the metal oxide particles is preferably 2 to 500 nm. From the viewpoint of making it easier to obtain a larger specific capacity, the average particle diameter of the metal oxide particles may be 10 nm or more, 50 nm or more, 100 nm or more, 200 nm or more, or 250 nm or more, and may be 500 nm or less, 400 nm or less, 300 nm or less, or 250 nm or less. When the average particle diameter of the metal oxide particles is 50 nm or more, the pore volume of the macropores tends to be larger than the pore volume of the mesopores. The average particle diameter refers to the average diameter of metal oxide particles if they are spherical, and to the average length of the longest part (major axis) of the particles if they are non-spherical. The average particle diameter can be determined by measuring the diameter or major axis of 100 particles using a scanning electron microscope (SEM) and calculating the average value.

[0047] The content of metal oxide particles in the composite is preferably 200 parts by mass or more, more preferably 250 parts by mass or more, and even more preferably 300 parts by mass or more, per 100 parts by mass of graphene oxide. In this case, aggregation of graphene is less likely to occur, and porous reduced graphene oxide with a large specific volume is more easily obtained. The content of metal oxide particles in the composite is preferably 500 parts by mass or less, more preferably 450 parts by mass or less, and even more preferably 400 parts by mass or less, per 100 parts by mass of graphene oxide. In this case, pores of an appropriate amount and shape are more easily formed, and the specific volume of the resulting porous reduced graphene oxide tends to be larger. From these viewpoints, the content of metal oxide particles in the composite is preferably 200 to 500 parts by mass per 100 parts by mass of graphene oxide.

[0048] The composite is, for example, in powder form. The composite can be obtained, for example, by dispersing metal oxide particles in a solution containing graphene oxide, sonicating it, and then filtering and drying it. Examples of the solvent in the solution (dispersion medium for graphene oxide) include water and ethanol. The amount of solvent may be 30,000 to 200,000 parts by mass per 100 parts by mass of graphene oxide.

[0049] Ultrasonic treatment can be carried out using an ultrasonic generator, such as the ultrasonic cleaner (M2800HJ) manufactured by Yamato Scientific Co., Ltd. The ultrasonic treatment may be carried out at temperatures below 0°C (for example, in an ice bath). The duration of the ultrasonic treatment may be 0.5 to 2 hours. In the above method, stirring may be performed after the dispersion of metal oxide particles and after the ultrasonic treatment. The stirring after the dispersion of metal oxide particles may be carried out for 12 to 36 hours. The stirring after the ultrasonic treatment may be carried out for 0.5 to 2 hours. Drying may be carried out at 35 to 80°C for 6 to 24 hours.

[0050] (Step b) In step b, porous reduced graphene oxide is obtained by reducing graphene oxide and removing metal oxide particles from the composite.

[0051] In step b, the reduction of graphene oxide and the removal of metal oxide particles may be carried out simultaneously, or the reduction of graphene oxide may be performed first, followed by the removal of metal oxide particles. That is, step b may be step b1, in which the composite is irradiated with microwaves in the presence of alcohols and an acid, or step b2, in which the composite is irradiated with microwaves in the presence of alcohols to obtain a reduced composite, and then the reduced composite is irradiated with microwaves in the presence of an acid. From the viewpoint of being able to carry out the reduction of graphene oxide in the composite and the removal of metal oxide particles from the composite in a single pot, and producing porous reduced graphene oxide more easily, it is preferable that step b is step b1. In addition, step b1 tends to yield porous reduced graphene oxide with a larger specific volume.

[0052] Alcohols, having reducing properties, function as reducing agents in step b (steps b1 and b2). Examples of alcohols include propanol, butanol, and alkylene glycols such as ethylene glycol, methylene glycol, propylene glycol, and trimethylene glycol. Among the alcohols, alkylene glycols are preferably used because they tend to slow down the reduction of graphene oxide and tend to increase the specific volume of the resulting porous reduced graphene oxide. More preferably, at least one selected from the group consisting of ethylene glycol, methylene glycol, propylene glycol, and trimethylene glycol is used.

[0053] The acid may be an organic acid or an inorganic acid. From the viewpoint of easily removing metal oxides, at least one selected from the group consisting of hydrochloric acid (HCl), sulfuric acid (H2SO4), and acetic acid (CH3COOH) is preferably used as the acid. The acid is usually used in the form of an aqueous solution mixed with water. That is, in step b, an aqueous solution containing the acid (for example, dilute hydrochloric acid, dilute sulfuric acid, dilute acetic acid, etc.) may be used.

[0054] The following describes processes b1 and b2 separately.

[0055] [Process b1] In step b1, graphene oxide is reduced by the reducing action of alcohols and microwaves to form reduced graphene oxide, and metal oxide particles are removed from the composite by the action of an acid. Step b1 may be, for example, a step in which the composite is placed in a solution (e.g., an aqueous solution) containing alcohols and an acid to obtain a composite-containing solution, and then microwaves are irradiated onto the obtained composite-containing solution.

[0056] The amount of alcohol used may be 1,000 to 3,000 parts by mass per 100 parts by mass of the composite. When using a solution containing alcohol and acid, the concentration of alcohol in the solution may be 40 to 90% by mass based on the total mass of the solution.

[0057] The amount of acid used may be 70 to 600 parts by mass per 100 parts by mass of the composite. When using a solution containing alcohols and acid, the concentration of the acid in the solution may be 10 to 30% by mass based on the total mass of the solution.

[0058] When using solutions containing alcohols and acids, the amount of the complex added may be 1 to 10% by mass, based on the total mass of the complex-containing solution.

[0059] Microwave irradiation can be performed using a microwave generator, such as a microwave oven (Discover SP manufactured by CEM Corporation).

[0060] The microwave output power varies depending on the microwave irradiation time, the amount of material being processed (e.g., composite-containing solution), and the size of the device, but can be, for example, 50 to 300 W. The microwave irradiation time can be, for example, 2 to 30 minutes. The microwave frequency can be, for example, 915 MHz to 2455 MHz.

[0061] Microwave irradiation may be carried out at room temperature (e.g., 25°C) or while heating. The ambient temperature during microwave irradiation (e.g., the set temperature inside the microwave generator) may be 25-200°C, 100-200°C, or 150-180°C.

[0062] After microwave irradiation, the solution containing the obtained porous reduced graphene oxide may be filtered, and then the porous reduced graphene oxide may be washed (e.g., washed with water) and dried. The drying temperature may be 40 to 100°C, and the drying time may be 8 to 24 hours.

[0063] [Step b2] Step b2 includes step b2-1, which involves irradiating the complex with microwaves in the presence of alcohols to obtain a reduced complex, and step b2-2, which involves irradiating the reduced complex with microwaves in the presence of an acid.

[0064] In step b2-1, graphene oxide is reduced by the reducing action of alcohols and microwaves to form reduced graphene oxide. That is, the reduced composite obtained in step b2-1 is a composite containing reduced graphene oxide and metal oxide particles. Step b2-1 may be, for example, a step of adding the composite to a solution containing alcohols (e.g., an aqueous solution) to obtain a first composite-containing solution, and then irradiating the obtained first composite-containing solution with microwaves. Some or all of the metal oxide particles may be reduced in step b2-1. That is, the reduced composite may contain zinc.

[0065] The amount of alcohol used in step b2-1 (based on 100 parts by mass of the composite) may be the same as the amount of alcohol used in step b1 (based on 100 parts by mass of the composite). When a solution containing alcohol is used, the concentration of alcohol in the solution may be 40% by mass or more, or 100% by mass, based on the total mass of the solution.

[0066] In step b2-2, metal oxides are removed from the reduced complex by the action of an acid to form porous reduced graphene oxide. Step b2-2 may be a step in which the reduced complex is added to an acid-containing solution (e.g., an aqueous solution) to obtain a second complex-containing solution, and then the obtained second complex-containing solution is irradiated with microwaves.

[0067] The amount of acid used in step b2-2 (based on 100 parts by mass of the reduced complex) may be the same as the amount of acid used in step b1 (based on 100 parts by mass of the complex). When using a solution containing acid, the concentration of the acid in the solution may be 10 to 30% by mass, based on the total mass of the solution.

[0068] The microwave irradiation in steps b2-1 and b2-2 can be carried out using the same equipment and under the same conditions as in step b-1.

[0069] After microwave irradiation in steps b2-1 and b2-2, the solution containing the product (reduced complex or porous reduced graphene oxide) may be filtered in the same manner as in step b-1, followed by washing and drying of the product.

[0070] In step b above, some or all of the oxygen-containing functional groups of graphene oxide are reduced, but in this embodiment, it is preferable to reduce some of the oxygen-containing functional groups (i.e., leave some of the oxygen-containing functional groups unreduced). The degree of reduction of graphene oxide can be adjusted by the type of alcohol, the microwave irradiation conditions, etc.

[0071] According to the method of this embodiment described above, it is possible to produce porous reduced graphene oxide in a simpler manner compared to conventional methods. Furthermore, porous reduced graphene oxide produced by the method of this embodiment tends to have a larger specific volume compared to porous reduced graphene oxide produced by conventional methods.

[0072] The porous reduced graphene oxide produced by the method of this embodiment will be described below.

[0073] The porous reduced graphene oxide produced by the method of this embodiment may consist solely of reduced graphene oxide, or it may contain components other than reduced graphene oxide (for example, components that are inevitably mixed in). For example, some of the metal or its oxide that constitutes the metal oxide particles may remain in the porous reduced graphene oxide without being removed. The amount of the above-mentioned metal and its oxide remaining in the porous reduced graphene oxide is preferably 3% by mass or less, more preferably 1% by mass or less, and even more preferably 0% by mass, based on the total mass of the porous reduced graphene oxide.

[0074] The porous reduced graphene oxide produced by the method of this embodiment preferably has an oxygen-containing functional group. The porous reduced graphene oxide preferably has a carbonyl group (-CO-) as the oxygen-containing functional group. In the C1s spectrum of porous reduced graphene oxide measured by XPS (X-ray photoelectron spectroscopy), the proportion of bonds attributed to C=O (the proportion of each bond attributed to C=C / CC, CO, C=O, and OC=O) may be 3-10%. Furthermore, in the O1s spectrum of porous reduced graphene oxide measured by XPS (X-ray photoelectron spectroscopy), the proportion of bonds attributed to C=O (the proportion of each bond attributed to C=O, CO, and C-OH) may be 25-55%. XPS can be measured by the method described in the examples.

[0075] The porous reduced graphene oxide produced by the method of this embodiment preferably has mesopores (pore diameter: 10 nm or more to less than 50 nm) and macropores (pore diameter: 50 nm or more to less than 160 nm). The pore volume of the mesopores is 0.02 to 0.4 cm³. 3 The value may be / g. The pore volume of the mesopore can be measured by the method described in the examples.

[0076] The porous reduced graphene oxide produced by the method of this embodiment is usually in powder form. If the powder has aggregated into lumps, it may be pulverized into a fine powder before use.

[0077] The porous reduced graphene oxide produced by the method of this embodiment can be suitably used as a precious metal-free conductive material, and in particular, can be suitably used as an electrode material for supercapacitors. [Examples]

[0078] The present invention will be described in more detail below using examples and comparative examples. However, the present invention is not limited to the following examples.

[0079] <Example 1: Synthesis of porous reduced graphene oxide> (Synthesis of graphene oxide) Graphene oxide (GO) was synthesized from graphite powder (average particle size: 45 μm) by a modified Hummers process. Specifically, first, 60 ml of sulfuric acid (H2SO4, 95% by mass) was added to a 500 ml beaker and heated to 80°C. 3.0 g of ammonium persulfate ((NH4)2S2O8, 98% by mass) was added while stirring and dissolved to obtain a solution. Then, 3.0 g of phosphorus pentoxide (P2O5, 98% by mass) was gradually added to the above solution to obtain a mixture. During this time, the temperature was confirmed to be maintained at 80°C. Next, 3.0 g of graphite was added to the obtained mixture and reacted at 80°C with stirring for 4.5 hours to obtain a graphite mixture. After the reaction, this was kept below 10°C in an ice bath. Then, 450 ml of DI water (ion-exchanged water) was slowly added dropwise to the graphite mixture, taking care to avoid a sudden rise in temperature. The ice bath was then removed, and the mixture was left to stand overnight at room temperature. The resulting mixture was filtered, and the filtrate was washed with 2000 ml of DI water. The washed filtrate was then dried overnight at approximately 45°C. This yielded treated graphite.

[0080] After placing 3.0 g of the obtained treated graphite into a beaker, 150 ml of sulfuric acid was added to the beaker for further oxidation, and the mixture was cooled to near 0°C in an ice bath. To the cooled solution, 18.0 g of potassium permanganate (KMnO4, 99.3% purity) was gradually added, ensuring the temperature did not exceed 35°C, to obtain a mixture. The mixture was stirred for 15 minutes, then the ice bath was removed, and the mixture was heated to 35°C and stirred for 2 hours to allow the reaction to proceed. Next, the stirred mixture was cooled again to 0°C in an ice bath, and 255 ml of DI water was slowly added, taking care to avoid a sudden rise in temperature. Then, the ice bath was removed, and the mixture was stirred for 2 hours while maintaining the solution temperature below 35°C. To the solution after stirring, 750 ml of DI water was added while stirring, and then 10 ml of hydrogen peroxide (H2O2, 30% by mass) was added and stirred for 2 hours, after which it was allowed to stand overnight. Subsequently, the supernatant was filtered to obtain the filter paper precipitate and the precipitate at the bottom of the beaker (beaker precipitate). The obtained filter paper precipitate and beaker precipitate were placed in the same beaker, 250 ml of hydrochloric acid (10% by mass) was added, and after stirring for 2 hours, the mixture was filtered. After repeating the same procedure twice, the filter paper precipitate and beaker precipitate were dissolved in 250 ml of DI water to prepare an aqueous solution containing graphene oxide (GO aqueous solution).

[0081] (Fabrication of GO / ZnO composites) 0.19 g of zinc oxide (ZnO) particles (average particle size: 250 nm, ZnO content: 99% by mass, shape: rod-shaped) were dispersed in 30 ml of a 1.6 mg / ml GO aqueous solution. The resulting mixture was then stirred overnight and sonicated for 1 hour while cooling in an ice bath. After obtaining a nearly homogeneous solution, it was stirred for another hour. The stirred solution was filtered, and the filtrate was dried overnight at 40°C to obtain a powdered GO / ZnO composite (GO:ZnO particle (mass ratio) = 1:4).

[0082] (Preparation of porous reduced graphene oxide) 0.238 g of the obtained powdered GO / ZnO composite was placed in a container containing 3 ml of ethylene glycol and 3 ml of hydrochloric acid (HCl, 35-37% by mass, specific gravity: 1.18 g / ml) to obtain a composite-containing solution. This solution was then irradiated with microwaves. Microwave irradiation was performed continuously for 6 minutes using a microwave oven (CEM Corporation, product name: Discover SP) at a frequency of 2455 MHz, output of 100 W, and ambient temperature of 180 °C. This yielded a solution containing porous reduced graphene oxide (prGO). The obtained solution was filtered, and the filtrate was washed with DI water to remove residual impurities. The washed filtrate was then dried overnight at 40 °C to obtain powdered porous reduced graphene oxide (prGO).

[0083] <Example 2> Powdered porous reduced graphene oxide (prGO) was obtained in the same manner as in Example 1, except that the GO aqueous solution and ZnO particles were used so that the GO:ZnO particle (mass ratio) in the powdered GO / ZnO composite was 1:3.

[0084] <Example 3> A powdered GO / ZnO composite was prepared in the same manner as in Example 1. Next, 0.238 g of the powdered GO / ZnO composite was placed in a container to which 3 ml of ethylene glycol was added to obtain a first composite-containing solution. The obtained first composite-containing solution was then irradiated with microwaves. Microwave irradiation was performed continuously for 6 minutes using a microwave oven (CEM Corporation, product name: Discover SP) at a frequency of 2455 MHz, an output of 100 W, and an ambient temperature of 180 °C. This obtained a solution containing an rGO / ZnO composite (a composite containing reduced graphene oxide (rGO) and ZnO particles). The obtained solution was filtered, and the filtrate was washed with DI water to remove residual impurities. After washing, the filtrate was dried overnight at 40 °C to obtain a powdered rGO / ZnO composite.

[0085] 0.22 g of the obtained powdered rGO / ZnO composite was placed in a container to which 3 ml of hydrochloric acid (HCl, 35-37% by mass, specific gravity: 1.18 g / ml) was added to obtain a second composite-containing solution. This second composite-containing solution was then irradiated with microwaves. Microwave irradiation was performed using a Discover SP at a frequency of 2455 MHz, an output of 100 W, and an ambient temperature of 180 °C for 6 minutes continuously. This yielded a solution containing porous reduced graphene oxide (prGO). The obtained solution was filtered, and the filtrate was washed with DI water to remove residual impurities. After washing, the filtrate was dried overnight at 40 °C to obtain powdered porous reduced graphene oxide (prGO).

[0086] <Comparative Example 1> In the same manner as in Example 1, an aqueous solution containing graphene oxide (GO aqueous solution) was obtained. The obtained GO aqueous solution was filtered, and the filtrate was dried overnight at 40°C to obtain powdered graphene oxide (GO powder). The obtained GO powder was placed in a container to which 3 ml of ethylene glycol and 3 ml of hydrochloric acid (HCl, 35-37% by mass, specific gravity: 1.18 g / ml) were added to obtain a GO-containing solution. Except for using the GO-containing solution instead of the composite solution, the GO-containing solution was irradiated with microwaves, filtered, washed, and dried in the same manner as in Example 1 to obtain powdered reduced graphene oxide (rGO).

[0087] <Analysis and Evaluation> (Morphological observation) The ZnO particles used in the above examples, the porous reduced graphene oxide (prGO) obtained in Example 1, and the reduced graphene oxide (rGO) obtained in Comparative Example 1 were observed using a field emission scanning electron microscope (FE-SEM, product name: JSM-7001F) manufactured by JEOL Ltd. The observed FE-SEM images are shown in Figure 1. Figure 1(a) shows the ZnO particles, Figure 1(b) shows rGO, and Figures 1(c) and (d) show prGO. Figure 1(d) is a magnified image of Figure 1(c).

[0088] As shown in Figure 1, the ZnO particles used were rod-shaped cubes, and it was confirmed that in prGO, the pores (mesopores and macropores) corresponding to the ZnO particles were formed to be unevenly distributed between the layers of the prGO sheet having a wrinkled surface. Although not shown in the figures, morphological observations using FE-SEM were also performed on the prGO of Examples 2 and 3, and it was confirmed that they had a similar morphology to the prGO of Example 1.

[0089] (Measurement of pore size distribution) The Brunauer-Emmett-Teller (BET) pore size distribution of porous reduced graphene oxide (prGO) obtained in Example 1 and reduced graphene oxide (rGO) obtained in Comparative Example 1 was determined from N2 adsorption at 77K (-321℃) using a pore size distribution analyzer (product name: BELSORP-miniX) manufactured by Microtrac-Bel Corporation. Figure 2 is a graph showing the pore size distribution curves of prGO from Example 1 and rGO from Comparative Example 1, determined based on the measured N2 adsorption / desorption isotherms. In Figure 2, the horizontal axis represents the pore size, and the vertical axis represents the difference in pore volume (dV) divided by the difference value d (logD) of the pore size treated as a logarithm.

[0090] Figure 2 shows that the pores of prGO are mainly distributed in the 10-160 nm range, and the pore distribution is mainly concentrated in macropores (50 nm to less than 160 nm) corresponding to the size of ZnO particles. Furthermore, from the magnified view shown in the upper left of the graph in Figure 2 (magnified view of the region with a pore diameter of 10 nm or less), it was confirmed that both prGO and rGO have a small number of micropores, but prGO has more micropores than rGO. Specifically, the pore volume of the micropores of the porous reduced graphene oxide (prGO) obtained in Example 1 was 0.007 cm³. 3 The value is / g, and the pore volume of the mesopore is 0.20 cm³. 3 The value is / g, and the pore volume of the macropores is 1.8 cm³. 3 The value was / g. On the other hand, the pore volume of the micropores of the reduced graphene oxide (rGO) obtained in Comparative Example 1 was 0.002 cm³. 3 The value is / g, and the pore volume of the mesopore is 0.006 cm³. 3The value is / g, and the pore volume of the macropore is 0.02 cm³. 3 It was / g.

[0091] (XPS analysis) The porous reduced graphene oxide (prGO) obtained in Example 1 and the reduced graphene oxide (rGO) obtained in Comparative Example 1 were analyzed by X-ray photoelectron spectroscopy (XPS) using an ESCA3400 spectrometer manufactured by Shimadzu Corporation. MgKα (hv=1.2536 keV) was used as the X-ray source. Figure 3 shows the region corresponding to the energy peak position of the 1s orbital of C in the XPS spectra of Example 1 and Comparative Example 1, and Figure 4 shows the region corresponding to the energy peak position of the 1s orbital of O in the XPS spectra of Example 1 and Comparative Example 1. The four peaks in the XPS spectra of Figure 3 are attributed to C=C / CC (284.8 eV), CO (286.1 eV), C=O (287.6 eV), and O-C=O (288.2 eV), respectively. The three peaks in the XPS spectrum shown in Figure 4 are attributed to C=O (530.5 eV), CO (532.1 eV), and C-OH (533.5 eV), respectively.

[0092] Figure 3 shows that in the C1s spectrum of prGO, the proportion of bonds assigned to C=O was 6.47%, which is higher than that of rGO (4.26%). This result was further confirmed by Figure 4. Specifically, in the O1s spectrum of prGO, the proportion of bonds assigned to C=O was 40.7%, which is significantly higher than that of rGO (23.82%).

[0093] (Measurement of specific capacity) The specific volume of porous reduced graphene oxide (prGO) obtained in Examples 1-3 and the reduced graphene oxide (rGO) obtained in Comparative Example 1 was measured by the following method. Specifically, it was measured by the following method.

[0094] First, the obtained prGO or rGO was ground into a fine powder to prepare the measurement sample. After immersion in a 0.1% Nafion solution ("nafion" is a registered trademark), the resulting mixture was sonicated for several minutes to disperse the measurement sample almost uniformly in the solution. Next, the obtained solution (concentration of the measurement sample: 2 mg / ml) was deposited onto a glassy carbon electrode (GCE) by dropping it using a micropipette to prepare the working electrode.

[0095] In the three-electrode measurements, cyclic voltammetry (CV) and constant current charge-discharge (GCD) measurements were performed using the fabricated working electrode. Platinum wire electrodes and saturated calomel (Hg / HgCl2) electrodes were used as the counter electrodes and reference electrodes, respectively. The measurements were performed at room temperature using a BioLogic SP300-SK-S electrochemical workstation in a 1M H2SO4 aqueous solution electrolyte. The potential window was set to 0-1V, the scan rate to 5-200mV / s, and the current density to 1-10A / g.

[0096] The specific capacities of prGO and rGO were measured from the obtained cyclic voltammetry curves according to equation (1) below.

number

[0097] From the obtained constant current charge-discharge curves, the specific capacities of prGO and rGO were measured according to equation (2) below.

number

[0098] [Table 1]

[0099] As shown in Table 1, the specific capacities of prGO in Examples 1 to 3 were found to be much larger than those of rGO in Comparative Example 1, and also larger than those of prGO produced by the method described in Non-Patent Document 3 (specific capacity of 337.2 F / g at a scan rate of 5 mV / s).

Claims

1. Step a involves preparing a composite containing graphene oxide and metal oxide particles, The process includes step b, which involves reducing the graphene oxide in the composite and removing the metal oxide particles from the composite. The aforementioned step b is, Step b1 is to irradiate the composite with microwaves in the presence of alcohols and acids, or A method for producing porous reduced graphene oxide, comprising step b2 of irradiating the complex with microwaves in the presence of alcohols to obtain a reduced complex, and then irradiating the reduced complex with microwaves in the presence of an acid.

2. The method for producing porous reduced graphene oxide according to claim 1, wherein step b is step b1 of irradiating the composite with microwaves in the presence of alcohols and an acid.

3. A method for producing porous reduced graphene oxide according to claim 1 or 2, wherein the alcohols include alkylene glycol.

4. A method for producing porous reduced graphene oxide according to any one of claims 1 to 3, wherein the acid comprises at least one selected from the group consisting of hydrochloric acid, sulfuric acid, and acetic acid.

5. A method for producing porous reduced graphene oxide according to any one of claims 1 to 4, wherein the content of metal oxide particles in the composite is 200 to 500 parts by mass per 100 parts by mass of graphene oxide.

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