WO2024180364A1

WO2024180364A1 - Method and system for producing reduced graphene oxide aerogel

Info

Publication number: WO2024180364A1
Application number: PCT/IB2023/051792
Authority: WO
Inventors: Montree SAWANGPHRUK; Worapol TEJANGKURA; Thitiphum SANGSANIT
Original assignee: Vidyasirimedhi Institute Of Science And Technology
Priority date: 2023-02-27
Filing date: 2023-02-27
Publication date: 2024-09-06

Abstract

The present invention relates to a method for producing reduced graphene oxide aerogel. The method comprises the steps of preparing a graphene oxide aerogel from a graphene oxide and reducing the said graphene oxide aerogel to obtain a reduced graphene oxide aerogel, wherein the reduction of graphene oxide aerogel is performed using a microwave activation with a power ranging from 50-300 W for a period of time ranging from 30-600 seconds at a temperature ranging from 50-300°C with a pressure ranging from 50-100 psi. The invention further relates to a system for producing said reduced graphene oxide aerogel. The method and the system according to the present invention can produce graphene materials, e.g., graphene oxide, graphene oxide aerogel, and reduced graphene oxide aerogel, while being non-toxic to the environment, highly safe, high in percentage yield, and effective, and are thus suitable for use on an industrial scale.

Description

METHOD AND SYSTEM FOR PRODUCING REDUCED GRAPHENE

OXIDE AEROGEE

TECHNICAL FIELD

Chemistry and materials science related to a method and a system for producing reduced graphene oxide aerogel.

BACKGROUND OF THE INVENTION

Graphene materials, such as graphite oxide, graphene oxide (GO), and reduced graphene oxide (rGO), are highly popular materials due to their light weight, strength, and high flexibility, which make them a good heat and electrical conductor. Graphene materials find their use in various fields, for example, electric circuit, electronic devices, solar circuit, detectors, material for enhancing strength and flexibility of other materials, and semipermeable cell for a chemical process.

Graphene oxide can also be used as a starting material for producing graphene oxide aerogel (GO_ae) by starting from making a dispersion of graphene oxide in a liquid and removing a solvent to make the graphene oxide dispersion become dry to form a graphene oxide aerogel material having a 3D network structure, particularly light weight, high porosity, low density, high specific surface area, high mechanical strength, and high flexibility. Graphene oxide aerogel is commonly used as electrodes for energy storage devices, such as battery, supercapacitor, absorbent material, composite material, materials used in the space technology, and sensor. Furthermore, graphene oxide aerogel can also be reduced under a chemical or thermal treatment to produce reduced graphene oxide aerogel (rGO_ae) with attributes that are superior to those of graphene oxide aerogel.

Furthermore, graphene, graphene oxide, graphene oxide aerogel, and reduced graphene oxide aerogel materials can be applied to other materials to form a composite material with a variety of properties as desired which is suitable for using in , for example, electrodes of energy storage devices such as battery and supercapacitor, which is an alternative use which currently receives great attention.

Currently, the methods commonly used for preparing graphene materials and related materials are chemical vapor deposition and chemical exfoliation. The chemical vapor deposition is an arrangement of carbon atoms on a metal sheet. The method starts from releasing methane gas at room temperature into a furnace internally filled with hydrogen gas. When the methane gas is exposed to the metal sheet at a temperature of over 1,000 °C, it is decomposed, leaving only the carbon atoms attached to the metal sheet with its structure arranged as graphene. The graphene produced can be isolated by removing the metal sheet with an etching process to obtain graphene with the size and the number of stacking carbon layers that can be controlled.

Examples of the prior art related to the preparation of graphene and reduced graphene oxide using the chemical vapor deposition method are as follows.

Thai petty patent no. 12891 discloses a method for preparing graphene polydimethylsiloxane and carbon nanotube composite material by depositing graphene on a nickel foam using the chemical vapor deposition method and coating with polydimethylsiloxane mixed with carbon nanotube for using in the study of electrochemical properties.

US 2016/0060120 Al discloses a method for preparing reduced graphene oxide by reducing graphene oxide coated on a starting metal sheet at a temperature of 200-l,500°C using the chemical vapor deposition process to form a carbon layer on the starting metal sheet before performing an oxidation reaction to obtain graphene oxide.

On the other hand, the chemical exfoliation method is performed by using a chemical to intervene the structure of graphite to cause graphene to be exfoliated as a sheet. One of the popular methods is the modified Hummers’ method which starts from subjecting graphite to the oxidation reaction by adding an oxidizing agent to form a functional group between the graphite layers to separate the layers from one another and stirring using the ultrasonic wave to allow graphite to be exfoliated more easily. The products obtained are graphite oxide and graphene oxide.

Examples of the prior art related to the chemical exfoliation method are as follows.

Thai petty patent no. 13041 discloses a preparation of material for absorbing and releasing fragrance which uses reduced graphene oxide as a fragrance absorbent. Graphene oxide is prepared using Hummers’ method by using 3 g graphite starting material, 1.5 g sodium nitrate, and 9g potassium permanganate, which make up a weight ratio of 1 to 0.5 to 3, respectively.

US 2013/0190449 Al discloses a preparation of graphene oxide using Hummers’ method by using 5 g graphite starting material, 4.5 g potassium nitrate, and 22.5 g potassium permanganate, which make up a weight ratio of 1 to 0.9 to 4.9, respectively. Although the production of graphene using said chemical exfoliation method is highly popular since it allows the product’ s attributes to be controlled to make it suitable for application, does not form toxic gases, produces high percentage yield, and has lower production cost than other methods, a drawback remains in that the modified Hummers’ method can cause a strong, highly deleterious chemical reaction which can cause ignition or explosion, which is a result of the oxidizing agent addition step and the water dilution step. It is thus cautioned to appropriately control the temperature of the process. For that reason, this method remains complicated for a mass production or an industrial-scale production. Additionally, the product obtained from the modified Hummers’ method, such as graphene oxide, has functional groups attached thereto, such as hydroxyl, epoxide, carbonyl, and carboxyl groups, which provide the product with hydrophilic property. These functional groups lower the electrical conductivity property. The obtained graphene oxide therefore needs to be subjected to the reduction under a chemical or thermal treatment to remove said functional groups or to lower the number of functional groups to obtain the reduced graphene oxide with high specific surface area, increased electrical conductivity, and improved mechanical and electrochemical properties. These properties depend on the method used and the reduction efficiency.

Furthermore, there is an attempt to develop a system, or a reactor used to produce the graphene materials which is safe and convenient to use and even more suitable for use on an industrial scale. Examples of the prior art are as follows.

Thai patent publication no. 1901005282 A discloses a pilot machine for producing reduced graphene oxide with the main components being a two -layer reaction tank having a system for controlling the temperature and the pressure and a mixing system connected to the stirring impellers, a water-rinsing tank and a centrifugal precipitation system, a sonication tank, a chemical tank equipped with a chemical pump, a water storage tank and a wastewater tank, a chemical pump and a water pump, a cooling system, a controlling chamber for controlling the operation through a processing unit, a system for alerting when the machine is operating having a base structure fixed to a number of wheels in order to be able to move around, and a system for controlling the operation.

Despite various approaches, methods, and systems for producing graphene oxide, graphene oxide aerogel, reduced graphene oxide, and reduced graphene oxide aerogel mentioned above, a system for producing said materials that offers safety, large-scale preparation, high percentage yield, and suitability for industrial applications has yet to be developed. SUMMARY OF THE INVENTION

The first object of the present invention is to develop a method for producing reduced graphene oxide aerogel using a microwave-based technology to reduce a graphene oxide aerogel which is advantageous in terms of safety as this developed method does not use chemicals or high temperature in the preparation and offers shorter preparation time, for example, not longer than 10 minutes per 1 sample. Furthermore, the invention can also control the key variables of the operation to be in a suitable range, for example, activation power, activation time, activation temperature, and activation pressure, to obtain the reduced graphene oxide aerogel with the desired attributes. The obtained material can also be applied to other types of material to form a composite material, for example, to provide the material with different properties and to make them suitable to be used as, for example, electrodes of energy storage devices such as battery and supercapacitor.

Another object of the present invention is to develop a system for producing graphene materials, e.g., graphene oxide, graphene oxide aerogel, and reduced graphene oxide aerogel that is non-toxic to the environment, highly safe, high in percentage yield, effective, and potent for use on an industrial scale.

To achieve the aforementioned objects, an aspect of the present invention discloses a method for producing reduced graphene oxide aerogel comprising the steps of preparing a graphene oxide aerogel from a graphene oxide and reducing the said graphene oxide aerogel to obtain a reduced graphene oxide aerogel, wherein the reduction of graphene oxide aerogel is performed using a microwave activation with a power ranging from 50-300 W for a period of time ranging from 30-600 seconds at a temperature ranging from 50-300°C with a pressure ranging from 50-100 psi, preferably with a pulse number ranging from 1-20 cycles.

Another aspect of the invention discloses a system for producing reduced graphene oxide aerogel comprising:

- a graphene oxide reactor system for preparing a graphene oxide,

- a lyophilizer which receives and dries the graphene oxide obtained from the graphene oxide reactor system, and

- a microwave activator which receives the graphene oxide aerogel from the lyophilizer to reduce the graphene oxide aerogel to a reduced graphene oxide aerogel.

According to a preferred embodiment of the present invention, the graphene oxide reactor system comprises a first reactor for mixing a mixture used in the preparation of graphene oxide, a second reactor connected to the first reactor to filter-wash and adjust pH of the mixture obtained from the first reactor, a storage tank connected to the second reactor to receive the mixture subjected to the filter-wash and the pH adjustment from the second reactor to further adjust the pH, and a centrifuge connected to the storage tank for centrifuging the mixture obtained from the storage tank to adjust the pH as a last step. Preferably, the first reactor comprises an internal tank, an external tank enclosing the internal tank, at least one of stirring impeller assembly provided inside the internal tank, a temperature control means provided between the internal tank and the external tank to control a temperature of the internal tank, and a condenser connected to the internal tank for condensing chemical vapor generated during the reaction to prevent its release into an environment.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram showing different components of the system for producing reduced graphene oxide aerogel according to the present invention.

Fig. 2 shows an exemplary embodiment of the graphene oxide reactor system according to the present invention.

Fig. 3 is images obtained from the scanning electron microscope (SEM) showing the surfaces of the graphene oxide aerogel and reduced graphene oxide aerogel samples which were subjected to the microwave activation at different pulse number, wherein:

Fig. 3(a) is an image of the surface of the graphene oxide aerogel sample at a 10,000x magnification;

Fig. 3(b) is an image of the surface of the graphene oxide aerogel sample at a 25,000x magnification;

Fig. 3(c) is an image of the surface of the reduced graphene oxide aerogel sample which was subjected to 1 cycle of the microwave activation at a 10,000x magnification;

Fig. 3(d) is an image of the surface of the reduced graphene oxide aerogel sample which was subjected to 1 cycle of the microwave activation at a 100,000x magnification;

Fig. 3(e) is an image of the surface of the reduced graphene oxide aerogel sample which was subjected to 5 cycles of the microwave activation at a 10,000x magnification;

Fig. 3(f) is an image of the surface of the reduced graphene oxide aerogel sample which was subjected to 5 cycles of the microwave activation at a 100,000x magnification; Fig. 3(g) is an image of the surface of the reduced graphene oxide aerogel sample which was subjected to 10 cycles of the microwave activation at a 10,000x magnification; and

Fig. 3(h) is an image of the surface of the reduced graphene oxide aerogel sample which was subjected to 10 cycles of the microwave activation at a 100,000x magnification.

Fig. 4 is graphs showing the identification results of the graphene oxide aerogel and reduced graphene oxide aerogel samples prepared according to the present invention which were analyzed using different techniques, wherein:

Fig. 4(a) is a graph obtained from the surface area and porosity measuring instrument (Gas adsorption, BET);

Fig. 4(b) is a graph obtained from the X-ray diffractometer (XRD);

Fig. 4(c) is a graph obtained from the Fourier transform infrared spectroscopy (FTIR); and

Fig. 4(d) is a graph obtained from the Raman spectroscopy.

Fig. 5 is graphs showing the identification results of the reduced graphene oxide aerogel samples which were subjected to the microwave activation with the pulse number of 1, 5, and 10 cycles, wherein:

Fig. 5(a) is a graph obtained from the surface area and porosity measuring instrument; and

Fig. 5(b) is a graph showing the thermogravimetric analysis.

Fig. 6 is graphs obtained from the X-ray photoelectron spectroscopy (XPS) of the graphene oxide aerogel and reduced graphene oxide aerogel samples which were subjected to the microwave activation with the pulse number of 1, 5, and 10 cycles, wherein:

Fig. 6(a) is a graph of the graphene oxide aerogel sample;

Fig. 6(b) is a graph of the reduced graphene oxide aerogel sample which was subjected to 1 cycle of the microwave activation;

Fig. 6(c) is a graph of the reduced graphene oxide aerogel sample which was subjected to 5 cycles of the microwave activation; and

Fig. 6(d) is a graph of the reduced graphene oxide aerogel sample which was subjected to 10 cycles of the microwave activation. Fig. 7 is graphs and images showing the identification results of the reduced graphene oxide-silver aerogel composite material sample prepared according to the present invention which were analyzed using different techniques, wherein:

Fig. 7(a) is an image of the surface obtained from the scanning electron microscope at a 50,000x magnification;

Fig. 7(b) is an image of the surface obtained from the transmission electron microscope (TEM) at a 30,000x magnification;

Fig. 7(c) is a graph obtained from the surface area and porosity measuring instrument;

Fig. 7(d) is a graph obtained from the X-ray diffractometer;

Fig. 7(e) is a graph showing the thermogravimetric analysis; and

Fig. 7(f) is a graph obtained from the Raman spectroscopy.

Fig. 8 is images obtained from the scanning electron microscope which show the surface of the graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with different amounts of the graphene oxide aerogel, wherein:

Fig. 8(a) is an image of the surface of the graphene oxide-nanosilicon aerogel composite material sample with the amount of graphene oxide aerogel being 0.5% by weight, based on the total mixture, at a 50,000x magnification;

Fig. 8(b) is an image of the surface of the graphene oxide-nanosilicon aerogel composite material sample with the amount of graphene oxide aerogel being 1% by weight, based on the total mixture, at a 50,000x magnification;

Fig. 8(c) is an image of the surface of the graphene oxide-nanosilicon aerogel composite material sample with the amount of graphene oxide aerogel being 2% by weight, based on the total mixture, at a 50,000x magnification; and

Fig. 8(d) is an image of the surface of the graphene oxide-nanosilicon aerogel composite material sample with the amount of graphene oxide aerogel being 4% by weight, based on the total mixture, at a 50,000x magnification.

Fig. 9 is a graph obtained from the X-ray diffractometer which shows the crystal structure of the graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with different amounts of the graphene oxide aerogel and those of a comparative example. Fig. 10 is images obtained from the scanning electron microscope showing the surface of the reduced graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with different amounts of the reduced graphene oxide aerogel, wherein:

Fig. 10(a) is an image of the surface of the reduced graphene oxide-nanosilicon aerogel composite material sample with the amount of reduced graphene oxide aerogel being 0.5% by weight, based on the total mixture, at a 50,000x magnification;

Fig. 10(b) is an image of the surface of the reduced graphene oxide-nanosilicon aerogel composite material sample with the amount of reduced graphene oxide aerogel being 1% by weight, based on the total mixture, at a 50,000x magnification;

Fig. 10(c) is an image of the surface of the reduced graphene oxide-nanosilicon aerogel composite material sample with the amount of reduced graphene oxide aerogel being 2% by weight, based on the total mixture, at a 50,000x magnification; and

Fig. 10(d) is an image of the surface of the reduced graphene oxide-nanosilicon aerogel composite material sample with the amount of reduced graphene oxide aerogel being 4% by weight, based on the total mixture, at a 50,000x magnification.

Fig. 11 is a graph obtained from the X-ray diffractometer which shows the crystal structure of the reduced graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with different amounts of the reduced graphene oxide aerogel and those of a comparative example.

Fig. 12 is images obtained from the scanning electron microscope which show the surface of the reduced graphene oxide-activated carbon aerogel composite material sample prepared according to the present invention with different amounts of the activated carbon, wherein:

Fig. 12(a) is an image of the surface of the reduced graphene oxide-activated carbon aerogel composite material sample with the amount of activated carbon being 10% by volume, based on the total mixture, at a 5,000x magnification;

Fig. 12(b) is an image of the surface of the reduced graphene oxide-activated carbon aerogel composite material sample with the amount of activated carbon being 10% by volume, based on the total mixture, at a 50,000x magnification;

Fig. 12(c) is an image of the surface of the reduced graphene oxide-activated carbon aerogel composite material sample with the amount of activated carbon being 25% by volume, based on the total mixture, at a 5,000x magnification; Fig. 12(d) is an image of the surface of the reduced graphene oxide-activated carbon aerogel composite material sample with the amount of activated carbon being 25% by volume, based on the total mixture, at a 50,000x magnification;

Fig. 12(e) is an image of the surface of the reduced graphene oxide-activated carbon aerogel composite material sample with the amount of activated carbon being 50% by volume, based on the total mixture, at a 5,000x magnification; and

Fig. 12(f) is an image of the surface of the reduced graphene oxide-activated carbon aerogel composite material sample with the amount of activated carbon being 50% by volume, based on the total mixture, at a 50,000x magnification.

Fig. 13 is images obtained from the scanning electron microscope which show the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample prepared according to the present invention with different amounts of the multi-walled carbon nanotube and those of a comparative example, wherein:

Fig. 13(a) is an image of the surface of the comparative example at a 10,000x magnification;

Fig. 13(b) is an image of the surface of the comparative example at a 100,000x magnification;

Fig. 13(c) is an image of the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 10% by volume, based on the total mixture, at a 10,000x magnification;

Fig. 13(d) is an image of the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 10% by volume, based on the total mixture, at a 100,000x magnification;

Fig. 13(e) is an image of the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 25% by volume, based on the total mixture, at a 10,000x magnification;

Fig. 13(f) is an image of the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 25% by volume, based on the total mixture, at a 100,000x magnification;

Fig. 13(g) is an image of the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 50% by volume, based on the total mixture, at a 10,000x magnification; and

Fig. 13(h) is an image of the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 50% by volume, based on the total mixture, at a 100,000x magnification. Fig. 14 is images obtained from the scanning electron microscope which show the surface of the reduced graphene oxide-multi-walled carbon nanotube aerogel composite material sample prepared according to the present invention with different amounts of the multi-walled carbon nanotube, wherein:

Fig. 14(a) is an image of the surface of the reduced graphene oxide-multi- walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 10% by volume, based on the total mixture, at a 10,000x magnification;

Fig. 14(b) is an image of the surface of the reduced graphene oxide-multi- walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 10% by volume, based on the total mixture, at a 100,000x magnification;

Fig. 14(c) is an image of the surface of the reduced graphene oxide-multi- walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 25% by volume, based on the total mixture, at a 10,000x magnification;

Fig. 14(d) is an image of the surface of the reduced graphene oxide-multi- walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 25% by volume, based on the total mixture, at a 100,000x magnification;

Fig. 14(e) is an image of the surface of the reduced graphene oxide-multi- walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 50% by volume, based on the total mixture, at a 10,000x magnification; and

Fig. 14(f) is an image of the surface of the reduced graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amount of multi-walled carbon nanotube being 50% by volume, based on the total mixture, at a 100,000x magnification.

Fig. 15 is images of the surface of the reduced graphene oxide-2D manganese oxide nanosheet aerogel composite material sample prepared according to the present invention and those of a comparative example, wherein:

Fig. 15(a) is an image of the surface of the comparative example at a 50,000x magnification, and

Fig. 15(b) is an image of the surface of the reduced graphene oxide-2D manganese oxide nanosheet aerogel composite material at 5,000x magnification and 10,000x magnification.

Fig. 16 is a graph obtained from the X-ray diffractometer which shows the crystal structure of the graphene oxide-2D manganese oxide nanosheet aerogel composite material sample prepared according to the present invention and those of a comparative example.

Fig. 17 is a graph obtained from the Raman spectroscopy of the graphene oxide-2D manganese oxide nanosheet aerogel composite material sample prepared according to the present invention.

Fig. 18 is graphs showing the charge-discharge efficiency of the button cell symmetric supercapacitor comprising the graphene oxide-2D manganese oxide nanosheet aerogel composite material sample prepared according to the present invention and those of a comparative example, wherein:

Fig. 18(a) shows a cyclic voltammogram graph at a scan rate of 10 mV/s;

Fig. 18(b) is a graph showing the specific capacitance at different scan rates;

Fig. 18(c) is a graph showing the charge-discharge characteristics using the galvanostatic technique with a density of 1 A/g;

Fig. 18(d) is a graph showing the specific capacitance at different current densities; and

Fig. 18(e) is a graph showing the supercapacitor stability with the current density of 3 A/g.

Fig. 19 is graphs showing the charge-discharge efficiency of the anode comprising the reduced graphene oxide-silver aerogel composite material sample, wherein:

Fig. 19(a) is a graph showing the charge-discharge efficiency at 1 cycle with the current of 0.1- 5 A/g; and Fig. 19(b) is a graph showing the stability after testing the charge-discharge performance for 150 cycles with the current of 1 A/g.

DETAILED DESCRIPTION

Any aspects shown herein shall encompass the application to other aspects of the present invention as well, unless specified otherwise.

Any tools, devices, methods, materials, or chemicals mentioned herein, unless specified otherwise, mean the tools, devices, methods, materials, or chemicals generally used or practiced by a person skilled in the art, unless explicitly specified as special or exclusive tools, devices, methods, or chemicals for the present invention.

The terms “comprise(s),” “consist(s) of,” “have/has,” “contain(s),” and “include(s)” are open-end verbs. For example, any method which “comprises,” “consists of,” “has,” “contains” or “includes” one component or multiple components or one step or multiple steps is not limited to only one component or one step or multiple steps or multiple components as specified, but also encompasses components or steps that are not specified.

All components and/or methods disclosed and claimed in the present invention are intended to cover the aspects of the invention obtained from an action, a practice, a modification or a change of any factors which does not require any experiment that is substantially different from the present invention and gives properties and utility and provides the same effect as the aspects according to the present invention according to the judgement of a person of ordinary skill in the art, although not specifically stated in the claims. Therefore, substitutions or analogues of the aspects according to the present invention, including any slight modifications or changes that are clearly apparent to a person of ordinary skill in the art, are considered to be within the spirit, the scope, and the concept according to the present invention as well.

Technical and scientific terms used herein have meanings as understood by a person of ordinary skill in the art, unless specified otherwise.

In the first aspect, the present invention discloses a method for producing reduced graphene oxide aerogel comprising the steps of:

(a) preparing a graphene oxide aerogel from a graphene oxide, and

(b) reducing the graphene oxide aerogel obtained from step (a) to obtain a reduced graphene oxide aerogel, wherein the reduction of graphene oxide aerogel is performed using a microwave activation with a power ranging from 50-300 W for a period of time ranging from 30-600 seconds at a temperature ranging from 50-300°C with a pressure ranging from 50-100 psi. Preferably, the reduction of graphene oxide aerogel is performed using the microwave activation with a pulse number ranging from 1-20 cycles, more preferably 1-10 cycles.

The microwave activation can be performed using different devices or tools that are commonly used or known to a skilled person. For example, it may be performed using a microwave activator.

In a preferred embodiment, the preparation of graphene oxide aerogel in step (a) comprises the steps of: preparing a mixture of graphene oxide used in the preparation of graphene oxide aerogel, and drying the mixture obtained from the preparation of the mixture of graphene oxide used in the preparation of graphene oxide aerogel.

In an exemplary embodiment, the preparation of the mixture of graphene oxide used in the preparation of graphene oxide aerogel comprises the steps of:

(i) preparing a mixture of graphite, sodium nitrate, and sulfuric acid,

(ii) adding potassium permanganate to the mixture obtained from step (i),

(iii) adding deionized water to the mixture obtained from step (ii),

(iv) adding hydrogen peroxide to the mixture obtained from step (iii),

(v) adding hydrochloric acid to the mixture obtained from step (iv), and

(vi) filtering the mixture obtained from step (v).

Preferably, the aforementioned steps of preparing the graphene oxide mixture are performed as follows: step (i) is performed by stirring at a speed ranging from 100-300 rpm at a room temperature for 2-4 hours and controlling the reaction temperature to be in a range of 0-10°C; step (ii) is performed by stirring at a speed ranging from 50-200 rpm for 72-120 hours at a temperature not exceeding 20°C; step (iii) is performed by stirring at a speed ranging from 50-200 rpm for 1-2 hours at a temperature not exceeding 20°C; step (iv) is performed by stirring at a speed ranging from 50-200 rpm for 24 hours at a room temperature; and step (v) is performed by stirring at a speed ranging from 50-200 rpm for 24 hours at a room temperature.

In a preferred embodiment, the preparation of the graphene oxide mixture further comprises a step of adjusting the pH of the mixture obtained from step (vi) to be in a range of 4-6.

In another preferred embodiment, drying the mixture obtained from the preparation of the mixture of graphene oxide used in the preparation of graphene oxide aerogel is performed by freezing the obtained mixture in liquid nitrogen and drying the mixture using a lyophilizer for 120-168 hours. In another exemplary embodiment, the obtained mixture may be filtered using a vacuum filter and dried at a temperature ranging from 60-80°C for 120-240 hours.

The second aspect of the present invention relates to the system for producing reduced graphene oxide aerogel. A preferred exemplary embodiment of the components in the system includes an arrangement of different components as shown in Figs. 1 and 2.

In an embodiment, the system for producing reduced graphene oxide aerogel according to the present invention comprises: a graphene oxide reactor system (1) for preparing a graphene oxide, a lyophilizer (2) which receives and dries the graphene oxide obtained from the graphene oxide reactor system to obtain a graphene oxide aerogel, and a microwave activator (3) which receives the graphene oxide aerogel from the lyophilizer (2) to reduce the graphene oxide aerogel to a reduced graphene oxide aerogel.

Preferably, the graphene oxide reactor system (1) comprises a first reactor (1.1) for mixing a mixture used in the preparation of graphene oxide, a second reactor (1.2) connected to the first reactor (1.1) to filter-wash and adjust pH of the mixture obtained from the first reactor (1.1), a storage tank (1.3) connected to the second reactor (1.2) to receive the mixture subjected to the filter-wash and the pH adjustment from the second reactor (1.2) to further adjust the pH, and a centrifuge (1.4) connected to the storage tank (1.3) for centrifuging the mixture obtained from the storage tank (1.3) to adjust the pH as a last step. The first reactor (1.1) comprises an internal tank (1.1.1), an external tank (1.1.2) enclosing the internal tank (1.1.1), at least one of stirring impeller assembly (1.1.3) provided inside the internal tank (1.1.1), a temperature control means (1.1.4) provided between the internal tank (1.1.1) and the external tank (1.1.2) to control a temperature of the internal tank (1.1.1), and a condenser (1.1.5) connected to the internal tank (1.1.1) for condensing chemical vapor generated during the reaction to prevent its release into an environment.

An example of the temperature control means (1.1.4) is a coolant which is operated by an external cooler (1.5). According to a preferred embodiment, the temperature control means (1.1.4) is provided between the internal tank (1.1.1) and the external tank (1.1.2) to control a temperature during the reaction in the internal tank (1.1.1), thus providing the operator with a safe process of preparing the mixture.

An example of the condenser (1.1.5) is a coil condenser glass tube.

In a preferred embodiment, the first reactor (1.1) may further comprise a safety glass (1.1.6) provided such that it covers the front, lateral, and back sides, an internal temperature sensor (1.1.7) provided inside the internal tank (1.1.1) for detecting a reaction temperature in the internal tank (1.1.1), and a rupture disk (1.1.8) provided at an opening at the bottom of the internal tank (1.1.1) to relieve excessive pressure during the reaction.

An example of a preferred material for the condenser (1.1.5) is a coil condenser glass tube. An example of a preferred material for the safety glass (1.1.6) of the first reactor (1.1) is borosilicate glass as this type of glass can withstand the pressure in a range of -1 to 0.4 bar or more and can withstand the reaction taking place at a temperature ranging from -60 to 200°C. This can prevent an accident from occurring in the reaction, thereby offering more safety to the operator.

An example of a preferred material for making the rupture disk (1.1.8) is a Teflon-coated graphite material as it can withstand chemicals and prevent an explosion from occurring due to excessive pressure generated during the reaction inside the reactor.

Alternatively, the first reactor (1.1) can also be provided with a temperature display monitor which is connected to the internal temperature sensor (1.1.7) to control the temperature to be within a safe range and suitable for the reaction.

A preferred example of the internal temperature sensor (1.1.7) is a temperature sensor encapsulated with a chemical-resistant material. Moreover, such temperature sensor may be equipped with a 4-pin signal cable or a Lemo connector together with a temperature controller having a pressure gauge of -1 to 1.5 bar. The dimension and the length of such temperature sensor can be adjusted as appropriate.

According to the present invention, the at least one of stirring impeller assembly (1.1.3) of the first reactor (1.1) comprises a first stirring impeller (1.1.3.1) provided close to an upper side of the internal tank (1.1.1) and a second stirring impeller (1.1.3.2) provided below the first stirring impeller ( 1.1.3.1 ) .

According to an exemplary embodiment, said first stirring impeller (1.1.3.1) comprises 4 vanes, each vane being tilted 60 degrees from a central axis. The second stirring impeller (1.1.3.2) comprises 4 vanes, each vane being tilted 0 degrees from the central axis.

An exemplary preferred type of the stirring impeller assembly (1.1.3) is a turbine impeller, which is a replaceable, turbine-like, threaded impeller. Furthermore, preferred examples of the material used for making the stirring impeller assembly (1.1.3) are polytetrafluoroethylene (PTFE), Teflon, and stainless steel, such as grade 304 stainless steel coated with perfluoroalkoxy alkane (PFA), since they have good resistance to highly acidic and strong alkaline conditions.

Preferred examples of an arrangement of the first stirring impeller (1.1.3.1) and the second stirring impeller (1.1.3.2) of the stirring impeller assembly (1.1.3) are as follows.

The first stirring impeller (1.1.3.1) may be located on an upper side or at the centre of the internal tank (1.1.1), wherein each vane may have a rectangular shape. The second stirring impeller (1.1.3.2) may be located below the first stirring impeller (1.1.3.1) wherein each vane may have a crescent shape. The stirring impeller assembly (1.1.3) to be installed in the internal tank (1.1.1) should have a special design to be able to effectively stir a substance with high viscosity or concentration.

In a preferred embodiment, the present invention may provide an explosion-proof stirring motor by externally connecting it to the stirring impeller assembly (1.1.3). Preferably, the stirring motor may have a power of not lower than 0.18 kW to control the stirring speed, which can be from 1 to 500 rpm.

In a preferred embodiment, a chemical pump may be provided externally by connecting it to the first reactor (1.1) to draw the chemical in and out through a conveying tube. A material used to make such conveying tube may be Teflon since it has good resistance to the chemicals used, such as concentrated sulfuric acid, hydrogen peroxide, or hydrochloric acid.

According to an exemplary embodiment of the present invention, the second reactor (1.2) comprises a tank (1.2.1), at least one of stirring impeller assembly (1.2.2) provided inside the tank (1.2.1), a filter sheet (1.2.3) provided on a lower side of the tank (1.2.1) for filtering a substance and precipitate. Preferably, the stirring impeller assembly (1.2.2) of said second reactor (1.2) comprises a first stirring impeller (1.2.2.1) provided close to an upper side of the tank (1.2.1), and a second stirring impeller (1.2.2.2) provided below the first stirring impeller (1.2.2.1).

The second reactor (1.2) may further comprise a safety glass (1.2.4) provided such that it covers the front, lateral, and back sides, an internal temperature sensor (1.2.5) provided inside the tank (1.2.1) for detecting a reaction temperature in the tank (1.2.1), and a rupture disk (1.2.6) provided at an opening at the bottom of the tank (1.2.1) to relieve excessive pressure during the reaction.

The glass type and the function of the safety glass (1.2.4) are the same as those of the safety glass (1.1.6) of the first reactor (1.1) described above. The material and the function of the rupture disk (1.2.6) are the same as those of the rupture disk (1.1.8) of the first reactor (1.1) described above.

Like the stirring impeller of the first reactor (1.1), the first stirring impeller (1.2.2.1) of the second reactor (1.2) preferably comprises 4 vanes, each vane being tilted 60 degrees from a central axis. The second stirring impeller (1.2.2.2) comprises 4 vanes, each vane being tilted 0 degrees from the central axis. The arrangement, the appearance, and the function of the stirring impeller assembly (1.2.2) of the second reactor (1.2) are the same as those of the stirring impeller assembly (1.1.3) of the first reactor (1.1).

Moreover, the present invention may provide a stirring motor connected to the stirring impeller assembly (1.2.2) of the second reactor (1.2) like the stirring impeller assembly (1.1.3) of the first reactor (1.1) and may provide a chemical pump for drawing a substance in and out and a chemical pump for filtering a substance.

An example of a preferred material for making the filter sheet (1.2.3) is Teflon. Such filter sheet serves to filter the substance which is a product and precipitate of sulfur and manganate compound.

An example of a preferred material for making the first reactor (1.1), the second reactor (1.2), and the storage tank (1.3) is glass, preferably borosilicate glass. The dimension and the shape of the first reactor (1.1), the second reactor (1.2), and the storage tank (1.3) can be adjusted to make them suitable for application and the amount of graphene materials required to be produced.

The system according to the present invention is especially preferred for the preparation of graphene materials such as graphite oxide, graphene oxide, and reduced graphene oxide which must be prepared using a chemical reaction under an extreme condition, for example, Hummers’ method. Particularly, the system developed can be used to produce graphene materials in a more convenient and safer manner since the design of the devices focuses on the resistance to heat, acids and bases. There are also devices which improve safety of the operator such as the safety glass and the condenser for the condensation of chemical vapor to prevent the chemical vapor from escaping to the environment.

The invention will now be described in more detail with reference to examples and experiments as described below. However, the examples and the experiment results described are merely intended to illustrate the invention and are not intended to particularize the aspects of the invention in any way.

Example

Preparation of graphene oxide aerogel

The preparation of graphene oxide and graphene oxide aerogel was performed using a 30L glass reactor by starting from mixing graphite in an amount ranging from 35-70 g with sodium nitrate in an amount ranging from 52.5-105 g in the glass reactor. Then, sulfuric acid having a concentration of 98% was added in an amount ranging from 3.5-7.0 L and stirred at a speed of 150 rpm at room temperature for 2 hours. The cooler was then turned on to control the reaction temperature to be in a range of 0 to 10°C. Next, potassium permanganate was slowly added in an amount ranging from 280-560 g. In order to prevent an explosion in the reaction caused by the addition of such substance, the reaction temperature was maintained below 20°C. After the addition was completed, the stirring was continued at a speed of 80 rpm for 72 hours. Deionized water was then added slowly in an amount ranging from 3.5-7.0 L. In order to prevent a strong reaction between water and acid, the reaction temperature was maintained below 20°C. The stirring was continued for one more hour. Hydrogen peroxide solution was then added in an amount ranging from 1.1 -2.2 L and stirred at room temperature for 24 hours. Then, hydrochloric acid with 5% concentration was added in an amount ranging from 5-10 L and stirred for 24 hours. Subsequently, the chemical pump was used to draw the whole substance obtained and transfer it to another 30L reactor for filter-washing to adjust the pH. Then, the chemical pump was turned on to draw out the compound of sulfate and manganate to be reused in other processes. After the filtration was completed, approximately 20 L of deionized water was added and stirred at a speed of 60 rpm for 30 minutes. The filtration was repeated two times to adjust the pH. The chemical pump was used to draw out a brown suspension, which was then stored in a 30L storage tank. The pH of the substance was adjusted using deionized water to obtain a pH of 4. Then, the pH of the substance was adjusted to 5.5 using the centrifuge at a speed of 10,000 rpm to obtain the substance in a slurry form. The substance obtained can be prepared in two modes to obtain the product. The first mode is performed by filtering the obtained substance through a filter paper using the vacuum filter and drying the substance at a temperature of 60°C for 7 days. The substance was then ground to obtain a fine, brownish black powder of graphene oxide. The second mode is performed by freezing the substance in liquid nitrogen before drying it using the lyophilizer for 120 hours to obtain a product which is graphene oxide aerogel.

Preparation of reduced graphene oxide aerogel

The preparation of reduced graphene oxide aerogel was performed using the microwave activator. Graphene oxide aerogel obtained from the above process was transferred to a 35 ml glass container and put in the microwave activator. The attributes from the reduction process can be controlled by controlling the following variants of the microwave activation:

1) a power in a range of 50-300 W;

2) a duration being varied from 30-600 seconds;

3) a safe temperature being set in a range of 50-300°C;

4) a safe pressure within the container containing the substance being controlled to be in a range of 50-100 psi; and

5) a pulse number being in a range of 1-20 cycles. The pulse number used in the substance activation being controlled to reach the determined power before lowering the pulse number to a resting state and reactivating the substance. The attributes of the product obtained depend on the pulse number.

The product obtained after the microwave activation is the reduced graphene oxide aerogel.

Test results

The graphene oxide aerogel and reduced graphene oxide aerogel obtained using the method and the system according to the present invention were tested for different characteristics and for identification. The results are shown in Figs. 3-6.

Fig. 3 are images obtained from the scanning electron microscope showing the surfaces of the graphene oxide aerogel sample and the reduced graphene oxide aerogel sample prepared according to the present invention.

In Figs. 3(a) and 3(b), the graphene oxide aerogel with the surface structure of overlapping small crystal sheets is observed. Furthermore, after the graphene oxide aerogel was microwave-activated with the pulse number of 1, 5, and 10 cycles and analysed using the scanning electron microscope, the characteristics of the surface as shown in Figs. 3(c), 3(d), 3(e), 3(f), 3(g) and 3(h) show that the reduced graphene oxide aerogel has the surface structure of sponge-like pores and that the increased pulse number in the microwave activation significantly affected the size and the number of pores on the surface of the obtained reduced graphene oxide aerogel.

Fig. 4 shows the identification results by comparing the graphene oxide aerogel sample to the reduced graphene oxide aerogel sample using different techniques.

Fig. 4(a) shows a graph of the analysis of the surface area and the porosity. It was found that the specific surface area of graphene oxide aerogel was approximately 48 m²/g. After being subjected to the reduction reaction under the microwave activation until the reduced graphene oxide aerogel was obtained, the specific surface area was increased to approximately 750 m²/g.

Fig. 4(b) shows the X-ray diffraction graph. It was found that the graphene oxide aerogel appeared as one apparent narrow peak at a diffraction angle of 10.67 degrees which indicates a crystalline, ordered structure of the graphene oxide aerogel, whereas the reduced graphene oxide aerogel appeared as a broad peak at a diffraction angle ranging from 25 to 35 degrees which indicates an amorphous, less ordered structure as different functional groups on the structure were reduced by the microwave activation.

Fig. 4(c) shows the Fourier transform infrared spectroscopy analysis graph. It was found that the graphene oxide aerogel appears as a peak at a wave number ranging from 3,200 to 3,600 which is a characteristic of the O-H stretching, which comes from different functional groups such as hydroxyl (-OH) or carboxylic (-COOH) group on the structure of the graphene oxide aerogel. On the contrary, the reduced graphene oxide aerogel does not appear as a peak in such range as the functional groups on the structure were subjected to the reduction reaction with the microwave activation.

Fig. 4(d) shows the Raman spectroscopy analysis graph which shows the relationship between the ordered structure of the graphite (G band) and the orderless structures (D band) of both the graphene oxide aerogel and the reduced graphene oxide aerogel.

Fig. 5 shows the identification results of the reduced graphene oxide aerogel sample obtained from the microwave activation with the pulse number of 1, 5, and 10 cycles by using different techniques. Fig. 5(a) shows a graph of the analysis of the attributes and the amount of the surface area and Fig. 5(b) shows the thermogravimetric analysis.

The amount of the surface area, the pore size, and the thermal stability are shown in Table 1.

Table 1 shows the amount of the surface area, the pore size, and the thermal stability of the reduced graphene oxide aerogel sample obtained from the microwave activation at different pulse number.

Table 1

According to the results, it can be seen that the increased pulse number of the microwave activation provides the obtained reduced graphene oxide aerogel with a greater amount of specific surface area and a greater number of pores.

Fig. 6 are graphs obtained from the X-ray photoelectron spectroscopy (XPS) of the graphene oxide aerogel sample and the reduced graphene oxide aerogel sample which were subjected to the microwave activation with the pulse number of 1, 5, and 10 cycles. Fig. 6(a) is a graph of the graphene oxide aerogel sample. Fig. 6(b) is a graph of the reduced graphene oxide aerogel sample which was subjected to one cycle of the microwave activation. Fig. 6(c) is a graph of the reduced graphene oxide aerogel sample which was subjected to 5 cycles of the microwave activation. Fig. 6(d) is a graph of the reduced graphene oxide aerogel sample which was subjected to 10 cycles of the microwave activation.

The ratio of carbon to oxygen present in the structure in Table 2.

Table 2 shows the ratio of carbon to oxygen present in the structures of the graphene oxide aerogel sample and the reduced graphene oxide aerogel sample which were subjected to the microwave activation at different pulse number. Table 2

According to the test results, it was found that the ratio of carbon to oxygen of the graphene oxide aerogel was 57.15 to 42.85. On the other hand, the reduced graphene oxide aerogel which was subjected to the microwave activation with increased pulse number shows an increase in the amount of carbon in the structure and a decrease in the amount of oxygen, respectively.

According to the present invention, the graphene materials obtained will be applied to other types of materials, for example, to form a composite material to increase the variety of properties, especially electrodes of energy storage devices, such as battery and supercapacitor, for example.

Preparation of reduced graphene oxide-silver aerogel composite material

The preparation of the reduced graphene oxide-silver aerogel composite material with different powers and durations of the microwave activation starts from mixing 250 mg of graphene oxide aerogel with 50 ml of ethylene glycol, stirring and increasing the reaction temperature to 80°C. Then, 425 mg of silver nitrate was added and stirred, and the temperature was controlled to be at 80°C for 30 minutes. The obtained substance was washed with deionized water until the pH reached 5.5. The substance obtained was separated using the centrifuge. The obtained mixture was poured into a beaker and frozen in liquid nitrogen for 30 minutes. The mixtures were then put into the lyophilizer for 120 hours to obtain the graphene oxide- silver aerogel composite material. Then, the obtained material was put into a 35 ml microwave activator with a power ranging from 50 to 300 W for a period of time from 30 seconds to 600 seconds at a safe temperature ranging from 50 to 300°C at a safe pressure ranging from 50 to 100 psi to obtain the reduced graphene oxide-silver aerogel composite material as a final product. Preparation of graphene oxide-nanosilicon aerogel composite material

The preparation of the graphene oxide-nanosilicon aerogel composite material with different amounts of the graphene oxide aerogel starts from fine-grinding the nanosilicon using a ball mill at a speed of 20 Hz for 60 minutes and mixing with the graphene oxide aerogel at a ratio of graphene oxide aerogel in a range of 0.5-4.0% by weight using the ball mill with a speed ranging from 1-20 Hz for 30-120 minutes to obtain the graphene oxide-nanosilicon aerogel composite material as a product.

Preparation of reduced graphene oxide-nanosilicon aerogel composite material

The preparation of the reduced graphene oxide-nanosilicon aerogel composite material with the reduced graphene oxide aerogel in a range of 0.5 to 4.0% by weight starts from loading the graphene oxide-nanosilicon composite material obtained from the above process into a glass tube for placing in a 35 ml microwave activator with a power ranging from 50 to 300 W for a period of from 30 seconds to 600 seconds at a safe temperature ranging from 50 to 300°C at a safe pressure ranging from 50 to 100 psi to obtain the reduced graphene oxide-nanosilicon aerogel composite material as a final product.

Preparation of reduced graphene oxide-activated carbon aerogel composite material

The preparation of the reduced graphene oxide-activated carbon aerogel composite material with different percent by volume of the activated carbon starts from preparing the reduced graphene oxide aerogel and the activated carbon in an amount ranging from 10-50% by volume and mixing them in a mixer using a mechano-fusion technique at a speed of 500-5,000 rpm for 10-60 minutes to obtain reduced graphene oxide-activated carbon aerogel composite material as a product.

Preparation of graphene oxide-multi-walled carbon nanotube aerogel composite material The preparation of the graphene oxide-multi-walled carbon nanotube aerogel composite material with different percent by volume of the multi-walled carbon nanotube starts from preparing the graphene oxide and the multi-walled carbon nanotube in an amount ranging from 10-50% by volume and mixing them directly in deionized water for 72 hours and freezing in liquid nitrogen before drying with the lyophilizer for 120 hours to obtain the graphene oxide- multi-walled carbon nanotube aerogel composite material as a product. Preparation of reduced graphene oxide-multi-walled carbon nanotube aerogel composite material

The preparation of the reduced graphene oxide-multi-walled carbon nanotube aerogel composite material with the amount of the multi-walled carbon nanotube ranging from 10-50% by volume starts from loading the graphene oxide-multi-walled carbon nanotube aerogel composite material obtained from the above process into a glass tube for placing in a 35 ml microwave activator with a power ranging from 50-300 W for a period of from 30 seconds to 600 seconds at a safe temperature ranging from 50-300°C at a safe pressure ranging from 50-100 psi with the pulse number of from 1-20 cycles to obtain the reduced graphene oxide- multi-walled carbon nanotube aerogel composite material as a product.

Preparation of graphene oxide-2D manganese oxide nanosheet aerogel composite material

The preparation of graphene oxide-2D manganese oxide nanosheet aerogel composite material starts from preparing graphene oxide and 2D manganese oxide nanosheet at a volume ratio of 1 to 1 and mixing until homogeneous in deionized water for 72 hours and freezing in liquid nitrogen before drying with the lyophilizer for 120 hours to obtain the graphene oxide-2D manganese oxide nanosheet aerogel composite material as a product.

Preparation of reduced graphene oxide-2D manganese oxide nanosheet aerogel composite material

The preparation of the reduced graphene oxide-2D manganese oxide nanosheet aerogel composite material starts from loading the graphene oxide-2D manganese oxide nanosheet aerogel composite material obtained from the above process into a glass tube for placing in a 35ml microwave activator with the power ranging from 50-300 W for a period of from 30 seconds to 600 seconds at a safe temperature ranging from 50-300°C at a safe pressure ranging from 50 to 100 psi with the pulse number of from 1 to 20 cycles to obtain the reduced graphene oxide-2D manganese oxide nanosheet aerogel composite material as a final product.

The composite materials obtained above were tested for their properties and for identification. The results are shown in Figs. 7-17.

Fig. 7 shows the identification results of the reduced graphene oxide-silver aerogel composite material sample prepared according to the present invention using different techniques as follows. Fig. 7(a) is an image obtained from the scanning electron microscope (SEM). Fig. 7(b) is an image obtained from the transmission electron microscope (TEM). Fig. 7(c) is a graph obtained from the surface area and porosity measuring instrument. Fig. 7(d) is a graph obtained from the X-ray diffractometer. Fig. 7(e) is a graph showing the thermogravimetric analysis. Fig. 7(f) is a graph obtained from the Raman spectroscopy.

Fig. 8 is images obtained from the scanning electron microscope showing the surface of the graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with different amounts of the graphene oxide aerogel. Figs. 8(a), 8(b), 8(c), and 8(d) show the surface of the graphene oxide-nanosilicon aerogel composite material sample with the amounts of graphene oxide aerogel being 0.5, 1, 2, and 4% by weight, respectively, based on the total mixture.

Fig. 9 is a graph obtained from the X-ray diffractometer showing the crystal structure of the graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with the amounts of graphene oxide aerogel being 0.5, 1, 2, and 4% by weight, based on the total mixture, and the crystal structure of a comparative example.

Fig. 10 is images obtained from the scanning electron microscope showing the surface of the reduced graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with different amounts of the reduced graphene oxide aerogel. Figs. 10(a), 10(b), 10(c), and 10(d) show the surface of the reduced graphene oxide-nanosilicon aerogel composite material sample with the amounts of the reduced graphene oxide aerogel being 0.5, 1, 2, and 4% by weight, respectively, based on the total mixture.

Fig. 11 is a graph obtained from the X-ray diffractometer which shows the crystal structure of the reduced graphene oxide-nanosilicon aerogel composite material sample prepared according to the present invention with the amounts of the reduced graphene oxide aerogel being 0.5, 1, 2, and 4% by weight, respectively, based on the total mixture, and the crystal structure of a comparative example.

Fig. 12 is images obtained from the scanning electron microscope showing the surface of the reduced graphene oxide-activated carbon aerogel composite material sample with the amounts of the activated carbon being 10, 25, and 50% by volume, respectively, based on the total mixture. According to Figs. 12 (a), 12(b), 12(c), 12(d), 12(e), and 12(f), it can be seen that the reduced graphene oxide-activated carbon aerogel composite material is characterized by several overlapping sheets. Fig. 13 is images obtained from the scanning electron microscope showing the surface of the graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amounts of the multi-walled carbon nanotube being 10, 25, and 50% by volume, respectively, based on the total mixture, and the surface of a comparative example.

According to Figs. 13 (a) and Fig. 13 (b), it can be seen that the surface of the multiwalled carbon nanotube is rod-like or tube-like. Upon mixing the multi-walled carbon nanotube aerogel material with the graphene oxide aerogel material as shown in Figs. 13 (c), 13(d), 13(e), 13(f), 13(g), and 13(h), it was found that the surface of the multi-walled carbon nanotube aerogel material is in a sheet of the graphene oxide aerogel.

Fig. 14 is images obtained from the scanning electron microscope showing the surface of the reduced graphene oxide-multi-walled carbon nanotube aerogel composite material sample with the amounts of the multi-walled carbon nanotube being 10, 25, and 50% by volume, respectively, based on the total mixture. According to Figs. 14 (a), 14(b), 14(c), 14(d), 14(e), and 14(f), it was found that the surface of the multi-walled carbon nanotube aerogel material is in a thin, sponge-like sheet of the reduced graphene oxide aerogel.

Fig. 15 is images of the surface of the reduced graphene oxide-2D manganese oxide nanosheet aerogel composite material sample prepared according to the present invention and the surface of a comparative example. Fig. 15(a) of the comparative example shows the surface which is a sheet of the 2D manganese oxide nanosheet. According to Fig. 15(b), it can be seen that the surface of the reduced graphene oxide-2D manganese oxide nanosheet aerogel composite material is a sheet which is a unique characteristic of the surface of both materials combined.

Fig. 16 is a graph obtained from the X-ray diffractometer showing the crystal structures of the 2D manganese oxide nanosheet material and the graphene oxide-2D manganese oxide nanosheet aerogel composite material sample. The graph indicates that both materials have the same X-ray diffraction angle, the peaks obtained thus appear in the same position.

Fig. 17 is a graph obtained from the Raman spectroscopy of the graphene oxide-2D manganese oxide nanosheet aerogel composite material sample prepared according to the present invention. Example of the application of the graphene oxide-2D manganese oxide nanosheet aerogel composite material as electrodes in a supercapacitor

The graphene oxide-2D manganese oxide nanosheet aerogel composite material obtained above was used to prepare electrodes in a 2032 button cell symmetric supercapacitor by starting from weighing the graphene oxide-2D manganese oxide nanosheet aerogel composite material, carbon black, and polyvinylidene fluoride and pouring them into a container at a ratio of 8 to 1 to 1, respectively. Then, a solution of NMP (normal methyl pyrrolidone) was added with the calculated solid concentration per solvent volume of 40% and stirred until homogeneous. Said mixture was then coated on nickel foam electrodes having a diameter of 20 mm and dried at a temperature of 80°C. The coating was performed until the substance in an amount ranging from 2-3 mg was obtained. Subsequently, the electrodes which were coated and completely dried were compressed into a 2032 button cell symmetric supercapacitor using IM concentrated sodium sulfate as an electrolyte.

The electrodes obtained were assembled into a 2032 button cell symmetric supercapacitor, with the cathode and the anode made of the same material. Then, the electrodes were tested for their electrochemical efficiency by comparing them to the electrodes made of graphene oxide aerogel only using an electrochemical instrument to analyze the charge and discharge characteristics. Next, the electrodes were tested using a battery tester to study the stability of the supercapacitor. The results are shown in Fig. 18.

Fig. 18(a) shows a cyclic voltammetry graph at a scan rate of 10 mV/s. It can be seen that the area under the cyclic voltammetry graph of the supercapacitor comprising the electrodes made of the graphene oxide-2D manganese oxide nanosheet aerogel composite material is larger than that of the comparative example, which is the electrodes made of the graphene oxide aerogel material only, which indicates greater charging capacity.

Fig. 18(b) is a graph showing a specific capacitance at different scan rates. It can be seen that the supercapacitor comprising the electrodes made of the graphene oxide-2D manganese oxide nanosheet aerogel composite material shows from 10-100 mV/s higher charging capacity in every scan rate at the scan rate of 10 mV/s. The electrodes made of the graphene oxide-2D manganese oxide nanosheet aerogel composite material have the specific capacitance of 98 F/g, whereas the electrodes made of the graphene oxide aerogel material only have the specific capacitance of 36 F/g. Fig. 18(c) is a graph showing a charge-discharge using the galvanostatic technique with a density of 1 A/g. It is shown that the supercapacitor comprising the electrodes made of the graphene oxide-2D manganese oxide nanosheet aerogel composite material has a higher chargedischarge rate.

Fig. 18(d) is a graph showing a specific capacitance with different current densities. It is shown that the supercapacitor comprising the electrodes made of the graphene oxide-2D manganese oxide nanosheet aerogel composite material has greater specific capacitance than the supercapacitor comprising the electrodes made of the graphene oxide aerogel material only in every current density.

Fig. 18(e) is a graph showing the stability of the supercapacitor with a current density of 3 A/g. It is shown that the supercapacitor comprising the electrodes made of the graphene oxide- 2D manganese oxide nanosheet aerogel composite material has 65% stability retention, whereas the supercapacitor comprising the electrodes made of the graphene oxide aerogel material only has 73% stability retention after 5,000 cycles of use.

Example of the application of the reduced graphene oxide-silver aerogel composite material as electrodes of a lithium battery

The reduced graphene oxide-silver aerogel composite material obtained above was used to prepare electrodes of a 2032 button cell lithium battery by starting from weighing the reduced graphene- silver aerogel composite material, carbon black, and polytetrafluoroethylene at a ratio of 8 to 1 to 1, respectively, and pouring them into a mortar for grinding substance. All of the substances were stirred in the mortar for grinding substance until homogeneous, extruded into a sheet and spliced to copper foam electrodes until the substances in an amount ranging from 2-3 mg per electrode were obtained. The copper foam spliced with said composite material was extruded and spliced to a lithium strip for 3 hours using 120 pL lithium hexafluorophosphate electrolyte for compressing into a 2032 button cell lithium battery.

The 2032 button cell lithium battery obtained above was tested for its charge-discharge efficiency. The results are shown in Fig. 19.

Fig. 19(a) is a graph showing a charge-discharge performance of the anode comprising the reduced graphene oxide-silver aerogel composite material after being spliced to the lithium strip for 3 hours, which provides a capacity of 967-144 mAh/g at a current of 0.1-5 A/g.

Fig. 19(b) is a graph showing the stability of the anode comprising the reduced graphene oxide-silver aerogel composite material after the addition of lithium. It was found that the capacity was decreased from 478 mAh/g to 311 mAh/g, a decrease of approximately 35% after being tested at a current of 1 A/g for 150 cycles.

BEST MODE OF THE INVENTION

Best mode of the invention is as described in the detailed description of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method for producing reduced graphene oxide aerogel comprising the steps of:

(a) preparing a graphene oxide aerogel from a graphene oxide, and

(b) reducing the graphene oxide aerogel obtained from step (a) to obtain a reduced graphene oxide aerogel, wherein the reduction of graphene oxide aerogel is performed using a microwave activation with a power ranging from 50-300 W for a period of time ranging from 30-600 seconds at a temperature ranging from 50-300°C with a pressure ranging from 50-100 psi.

2. The method according to claim 1, wherein the reduction of graphene oxide aerogel is performed using the microwave activation with a pulse number ranging from 1-20 cycles.

3. The method according to claim 2, wherein the reduction of graphene oxide aerogel is performed using the microwave activation with a pulse number ranging from 1-10 cycles.

4. The method according to claim 1, wherein the preparation of graphene oxide aerogel in step (a) comprises the steps of: preparing a mixture of graphene oxide used in the preparation of graphene oxide aerogel, and drying the mixture obtained from the preparation of the mixture of graphene oxide used in the preparation of graphene oxide aerogel.

5. The method according to claim 4, wherein the preparation of the mixture of graphene oxide used in the preparation of graphene oxide aerogel comprises the steps of:

(i) preparing a mixture of graphite, sodium nitrate, and sulfuric acid,

(ii) adding potassium permanganate to the mixture obtained from step (i),

(iii) adding deionized water to the mixture obtained from step (ii),

(iv) adding hydrogen peroxide to the mixture obtained from step (iii),

(v) adding hydrochloric acid to the mixture obtained from step (iv), and

(vi) filtering the mixture obtained from step (v).

6. The method according to claim 5, wherein step (i) is performed by stirring at a speed ranging from 100-300 rpm at a room temperature for 2-4 hours and controlling the reaction temperature to be in a range of 0-10°C.

7. The method according to claim 5, wherein step (ii) is performed by stirring at a speed ranging from 50-200 rpm for 72-120 hours at a temperature not exceeding 20°C.

8. The method according to claim 5, wherein step (iii) is performed by stirring at a speed ranging from 50-200 rpm for 1-2 hours at a temperature not exceeding 20°C.

9. The method according to claim 5, wherein step (iv) is performed by stirring at a speed ranging from 50-200 rpm for 24 hours at a room temperature.

10. The method according to claim 5, wherein step (v) is performed by stirring at a speed ranging from 50-200 rpm for 24 hours at a room temperature.

11. The method according to claim 5 further comprising a step of adjusting pH of the mixture obtained from step (vi) to be in a range of 4-6.

12. The method according to claim 4, wherein drying the mixture obtained from the preparation of the mixture of graphene oxide used in the preparation of graphene oxide aerogel is performed by freezing the obtained mixture in liquid nitrogen and drying the mixture using a lyophilizer for 120-168 hours.

13. The method according to claim 4, wherein drying the mixture obtained from the preparation of the mixture of graphene oxide used in the preparation of graphene oxide aerogel is performed by filtering the obtained mixture using a vacuum filter and drying the mixture at a temperature ranging from 60-80°C for 120-240 hours.

14. A system for producing reduced graphene oxide aerogel comprising a graphene oxide reactor system (1) for preparing a graphene oxide, a lyophilizer (2) which receives and dries the graphene oxide obtained from the graphene oxide reactor system (1) to obtain a graphene oxide aerogel, and a microwave activator (3) which receives the graphene oxide aerogel from the lyophilizer (2) to reduce the graphene oxide aerogel to a reduced graphene oxide aerogel, wherein the graphene oxide reactor system (1) comprises a first reactor

(1.1) for mixing a mixture used in the preparation of graphene oxide, a second reactor (1.2) connected to the first reactor (1.1) to filter-wash and adjust pH of the mixture obtained from the first reactor (1.1), a storage tank (1.3) connected to the second reactor (1.2) to receive the mixture subjected to the filter-wash and the pH adjustment from the second reactor (1.2) to further adjust the pH, and a centrifuge (1.4) connected to the storage tank (1.3) for centrifuging the mixture obtained from the storage tank (1.3) to adjust the pH as a last step, characterized in that, the first reactor (1.1) comprises an internal tank (1.1.1), an external tank (1.1.2) enclosing the internal tank (1.1.1), at least one of stirring impeller assembly (1.1.3) provided inside the internal tank (1.1.1), a temperature control means (1.1.4) provided between the internal tank (1.1.1) and the external tank (1.1.2) to control a temperature of the internal tank (1.1.1), and a condenser (1.1.5) connected to the internal tank (1.1.1) for condensing chemical vapor generated during the reaction to prevent its release into an environment.

15. The system according to claim 14, wherein the temperature control means (1.1.4) is a coolant which is operated by an external cooler (1.5)

16. The system according to claim 14, wherein the condenser (1.1.5) is a coil condenser glass tube.

17. The system according to claim 14, wherein the first reactor (1.1) further comprises a safety glass (1.1.6) provided such that it covers the front, lateral, and back sides, an internal temperature sensor (1.1.7) provided inside the internal tank

(1.1.1) for detecting a reaction temperature in the internal tank (1.1.1), and a rupture disk (1.1.8) provided at an opening at the bottom of the internal tank

(1.1.1) to relieve excessive pressure during the reaction.

18. The system according to claim 14, wherein the at least one of stirring impeller assembly (1.1.3) of the first reactor (1.1) comprises a first stirring impeller

(1.1.3.1) provided close to an upper side of the internal tank (1.1.1), and a second stirring impeller (1.1.3.2) provided below the first stirring impeller (1.1.3.1).

19. The system according to claim 14, wherein the second reactor (1.2) comprises a tank (1.2.1), at least one of stirring impeller assembly (1.2.2) provided inside the tank (1.2.1), a filter sheet (1.2.3) provided on a lower side of the tank (1.2.1) for filtering a substance and precipitate.

20. The system according to claim 19, wherein the at least one of stirring impeller assembly (1.2.2) of the second reactor (1.2) comprises a first stirring impeller (1.2.2.1) provided close to an upper side of the tank (1.2.1), and a second stirring impeller (1.2.2.2) provided below the first stirring impeller (1.2.2.1).

21. The system according to claim 14, wherein the second reactor (1.2) further comprises a safety glass (1.2.4) provided such that it covers the front, lateral, and back sides, an internal temperature sensor (1.2.5) provided inside the tank (1.2.1) for detecting a reaction temperature in the tank (1.2.1), and a rupture disk (1.2.6) provided at an opening at the bottom of the tank (1.2.1) to relieve excessive pressure during the reaction.