WO2023017870A1 - Reduced graphene oxide and method for preparing same - Google Patents

Abstract

The present invention efficiently provides reduced graphene oxide which, by reducing graphene oxide in an environment lacking hydrogen by applying a specific pressure and temperature in the presence of a catalyst, to have a specific average crystalline size and oxygen content, has low resistance, high electrical conductivity, and excellent quality. In addition, in a method according to an embodiment of the present invention, reduction is carried out in an environment lacking hydrogen and under low-temperature conditions, and carbon loss during the reduction process can be minimized.

Description

Reduced graphene oxide and its preparation method

The present invention relates to reduced graphene oxide having an average crystal size and oxygen (O) content within a specific range, and a method for preparing the same.

Graphene oxide (GO) is a material with a two-dimensional planar structure that can be widely used in future applications, and is in the spotlight for its excellent physical properties and mass production.

Accordingly, various reduction methods for obtaining high-quality graphene sheets from such graphene oxide have been studied.

Methods for reducing graphene oxide include a deoxidation method by chemical treatment and a recrystallization method by thermal reduction.

First, in the case of the deoxidation method by chemical treatment, a chemical reducing agent such as a hydrazine compound and sodium borohydride is used to remove the oxygen group of graphene oxide, which may cause structural destruction by chemical treatment, and the reduction There is a problem that toxic chemicals can be generated during the process.

On the other hand, the recrystallization method by thermal reduction is more practical in terms of electrical conductivity than the deoxidation method by chemical treatment, but heat treatment at a high temperature of 1000 ° C. or more generates carbon-containing gases such as CO ₂ and CO as by-products, Since it is a method of decomposing oxygen-containing groups, a large amount of carbon is lost during the reduction process, which causes structural defects to deteriorate the properties of graphene.

Therefore, there is an urgent need for research on a method for reducing graphene oxide that does not have these problems.

[Prior art literature]

[Patent Literature]

(Patent Document 1) Republic of Korea Patent Registration No. 101470927

An object of the present invention is a high-quality reduced graphene oxide capable of minimizing structural defects and vacancies and providing excellent electrical properties by having an average crystal size and oxygen (O) content within a specific range is to provide

Another object of the present invention is to provide a method for producing reduced graphene oxide capable of minimizing carbon loss in a hydrogen-sufficient environment by applying a specific pressure and temperature in the presence of a catalyst.

The present invention is a reduced graphene oxide having an oxygen (O) content of 6 atomic% to 18 atomic% based on the total content of carbon (C), hydrogen (H) and oxygen (O), Provides a reduced graphene oxide having an average crystalline size of (average crystalline size).

In addition, the present invention includes (1) mixing graphene oxide and an iron-based catalyst; And (2) reducing graphene oxide by treating the mixture at a pressure of 3,000 kgf / cm 2 to 4,500 kgf / cm 2 and a temperature of 150 ° C to 450 ° C; to provide.

According to an embodiment of the present invention, reduced graphene oxide having a specific average crystal size and oxygen content having low resistance and high electrical conductivity by reducing graphene oxide in the presence of a catalyst in a specific pressure, temperature, and hydrogen-sufficient environment can provide efficiently.

In addition, in the method according to an embodiment of the present invention, reduction is performed in a hydrogen-sufficient environment and under low-temperature conditions, and carbon loss during the reduction process can be minimized.

Hereinafter, the present invention will be described in more detail through the accompanying drawings.

1 shows (i) thermally and chemically reduced graphene oxide, (ii) reduced graphene oxide using a catalyst in the presence of hydrogen, and (iii) reduced graphene oxide using a catalyst in a hydrogen-deficient environment (Example It is a schematic diagram showing the manufacturing process of Example 1).

2 to 4 are graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, CVD-grown graphene (CVD-graphene) of Comparative Example 2 and Examples, respectively. Raman spectrum of reduced graphene oxide (CA-rGO) of 1 (FIG. 2), X-ray photoelectron spectroscopy (FIG. 3) and Fourier transform infrared spectroscopy (FT-IR) (FIG. 4) show the measurement results.

Figure 5 (a) shows the reduction mechanism of the reduced graphene oxide according to an embodiment of the present invention, (b) shows the binding energy of O atoms and C atoms in the graphene sheet and FeCuO ₂ .

Figure 6 (a) is an optical image of the reduced graphene oxide (CA-rGO) of Example 1, (b) to (f) are each of the reduced graphene oxide (CA-rGO) of Example 1 It shows the Raman map.

7 (a) to (d) show Raman maps of graphene oxide (GO) of Preparation Example 1, and (e) to (h) are thermally reduced graphene oxide (TrGO) of Comparative Example 1. It shows the Raman map of

8 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and bay carbon (BC). Shows the scanning electron microscope (SEM) measurement results of.

9 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and CVD-rGO of Comparative Example 2. It is a graph showing the crystal size (y-axis) according to the number of layers (I _2D /I _G ) (x-axis) and the ratio (I _D /I _G ) from Raman spectroscopic measurements of grown graphene (CVD-graphene).

10 shows gases (CO, CO ₂ , O ₂ , H generated from graphene oxide (GO) of Preparation Example 1 and reduced graphene oxide (CA-rGO) of Example 1 during heat treatment at 0 ° C. to 500 ° C. ₂ O) is a graph showing the GC-Mass analysis results, and FIG. 11 is a graph comparing the total amount of the generated gas.

FIG. 12 is a pie graph showing the gas analysis results generated from the reduced graphene oxide (CA-rGO) of Example 1 for the results of FIGS. 10 and 11.

13(a) shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1 and Comparative Example 2 is a graph comparing the electrical properties of CVD-grown graphene (CVD-graphene), and (b) is an LED display connected using paper coated with reduced graphene oxide (CA-rGO) of Example 1 14 and 15 are graphene oxide (CA-rGO) reduced in Example 1, graphene oxide (GO) in Preparation Example 1, and thermally reduced graphene in Comparative Example 1. Linear sweeping current-voltage curves for oxygen evolution reactions and hydrogen evolution reactions of oxide (TrGO), CVD-grown graphene of Comparative Example 2, and bay carbon (BC) are shown.

16 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and CVD-rGO of Comparative Example 2. It is a graph showing the resistance of grown graphene (CVD-graphene) and bay carbon (BC).

17 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and bay carbon (BC). shows the Tafel slope (a) and overpotential ((b) and (c)) of the operating potential for Pt of the carbon material catalyst of (d) and (f) the Tafel slope of the operating potential for RHE (d ) and overpotentials ((e) and (f)).

18 is an X-ray photoelectron spectroscopy of reduced graphene oxide (CA-rGO) of Example 1 and reduced graphene oxide of Comparative Examples 3 to 5. It shows the ratio of sp2, sp3, COC and OC=0 by XPS) measurement.

19 is a graph showing the correlation between internal temperature and external temperature according to pressure during reduction treatment.

Figure 20 (a) is a graph showing the ratio of I _D / I _G according to pressure and temperature change, (b) is 4,000 kgf / cm ² pressure and sp2 by XPS measurement according to time change at a temperature of 250 ℃ , sp3, COC, and the ratio of OC=0.

Hereinafter, the present invention will be described in detail.

In the following description of embodiments, it is described that one component is formed on or under another component, which means that one component is directly above or under another component, or indirectly through another component. Including everything formed by That is, when an element such as a layer, region or substrate is referred to as being “on” another element, it is to be understood that it may be directly on the other element or intervening elements may exist. There will be.

In addition, the criteria for the top/bottom of each component will be described based on the drawings. The size of each component in the drawing may be exaggerated for description, and may differ from the actually applied size.

"Including" a component in this specification means that other components may be further included, not excluding other components, unless otherwise stated.

In addition, it should be understood that all numerical ranges representing physical property values, dimensions, etc. of components described in this specification are modified by the term "about" in all cases unless otherwise specified.

In this specification, singular expressions should be interpreted as meaning including the singular or plural interpreted in context unless otherwise specified.

The reduced graphene oxide according to an embodiment of the present invention has an oxygen (O) content of 6 atomic% to 18 atomic% based on the total content of carbon (C), hydrogen (H) and oxygen (O). As a graphene oxide, it has an average crystalline size of 120 nm to 250 nm.

Since the reduced graphene oxide has a crystal size and oxygen (O) content within a specific range, structural defects and vacancies can be minimized, and stable and ordered structures can be obtained, resulting in low resistance and improved electrical conductivity can be achieved.

Hereinafter, the reduced graphene oxide according to an embodiment of the present invention will be described in detail.

Reduced Graphene Oxide

Reduced graphene oxide according to an embodiment of the present invention may have an average crystalline size of 120 nm to 250 nm. Specifically, the reduced graphene oxide may have an average crystal size of 130 nm to 230 nm, 140 nm to 220 nm, 150 nm to 220 nm, or 150 nm to 200 nm.

When the average crystal size of the reduced graphene oxide satisfies the above range, resistance may be lowered and electrical conductivity may be greatly improved. If the average crystal size of the reduced graphene oxide is less than 120 nm, resistance may increase and electrical conductivity may decrease, and if it exceeds 250 nm, the electrochemically active area is reduced, resulting in degradation of performance there may be

On the other hand, the reduced graphene oxide according to an embodiment of the present invention has an oxygen (O) content of 6 atomic% to 18 atomic% based on the total content of carbon (C), hydrogen (H) and oxygen (O). . Specifically, the reduced graphene oxide has an oxygen (O) content of 8 atomic % to 18 atomic %, 10 atomic % to 17 atomic %, 12 atomic % to 17 atomic %, or 12 atomic % to 16 atomic %. can

When the oxygen (O) content of the reduced graphene oxide satisfies the above range, the minimum amount of oxygen required for high-quality reduced graphene oxide can be realized and a completely reconstituted carbon structure can be formed. Electrical properties can be further improved. If the oxygen (O) content of the reduced graphene oxide is less than 6 atomic %, there may be a problem of performance degradation due to a decrease in electrochemical reactivity, and if it exceeds 18 atomic %, structural defects And it may be difficult to form an aligned structure due to the occurrence of vacancies, and there may be problems in that resistance increases and electrical conductivity decreases.

The carbon/oxygen (C/O) atomic ratio of the reduced graphene oxide may be 3 to 5, 3.5 to 5, 3.8 to 4.8, or 3.8 to 4.5. By having a carbon/oxygen (C/O) atomic ratio within the above range, structural defects and vacancies may be minimized to have a well-ordered structure.

In addition, the reduced graphene oxide may have a carbon (C) content of 45 atomic % to 65 atomic %, 45 atomic % to 60 atomic %, 48 atomic % to 55 atomic %, or 48 atomic % to 53 atomic %. there is.

In addition, the reduced graphene oxide may have a hydrogen (H) content of 30 atomic % to 50 atomic %, 32 atomic % to 48 atomic %, 32 atomic % to 42 atomic %, or 32 atomic % to 40 atomic %. there is.

In particular, the reduced graphene oxide according to an embodiment of the present invention may have a significantly low content of oxygen on the graphene oxide base surface.

Specifically, the reduced graphene oxide includes an epoxy group, a hydroxyl group, and a carboxyl group. Specifically, the reduced graphene oxide may include an epoxy group as an oxygen group on a basal surface and a carboxyl group and a hydroxy group as an oxygen group on an edge portion. there is.

Since oxygen groups on the basal surface may be due to structural deformation and defects, it may have a decisive effect on the electrical properties of the reduced graphene oxide. In addition, since the oxygen group of the edge part can serve as a functional group, it can affect the electrocatalytic activity.

Compared to the graphene oxide before reduction, the reduced graphene oxide according to an embodiment of the present invention may have a significantly lower content of epoxy groups present on the basal surface and a higher content of carboxyl and hydroxy groups present on the edge portion.

Specifically, in the reduced graphene oxide, the content of the epoxy group may be 0.2 to 0.6 atomic %, 0.2 to 0.5 atomic %, or 0.2 to 0.48 atomic %.

Meanwhile, the content of the hydroxy group may be 1.0 to 1.8 atomic %, 1.0 to 1.7 atomic %, 1.0 to 1.6 atomic %, or 1.0 to 1.5 atomic %.

The content of the carboxyl group may be 10 to 20 atomic %, 10 to 18 atomic %, 10 to 16 atomic %, or 11 to 15 atomic %.

According to an embodiment of the present invention, electrical conductivity can be greatly improved by minimizing structural deformation and defects by significantly lowering the content of the epoxy group included in the basal surface of the reduced graphene oxide.

In addition, when the hydroxy group and carboxyl group contents included in the edge portion of the reduced graphene oxide are satisfied within the above range, since the reduced graphene oxide (CA-rGO) has an abundant functional group at the edge portion, in this case, When the reduced graphene oxide is used as an oxygen evolution reaction (OER) catalyst and a hydrogen evolution reaction (HER) catalyst, it exhibits high activity and can provide OER and HER with excellent performance.

Specifically, the reduced graphene oxide is 0.2 to 0.6 atomic% of the epoxy group; 1.0 to 1.8 atomic % of hydroxy groups; 10 to 20 atomic % carboxyl groups; and the remaining amount of carbon (C).

Since the reduced graphene oxide according to an embodiment of the present invention simultaneously satisfies the contents of the epoxy group, hydroxyl group, and carboxyl group within the above range, electrical characteristics and activity of the electrocatalyst can be further improved.

Meanwhile, the reduced graphene oxide may have a carbon sp3/sp2 ratio of 0.1 to 0.9, 0.1 to 0.7, 0.1 to 0.5, or 0.1 to 0.3. The sp3/sp2 ratio is an index indicating the degree of graphitization, and when the sp3/sp2 satisfies the above range, the sp2 ratio is relatively increased, so that the graphite structure can be perfectly reconstructed, thereby improving electrical properties. there is.

Meanwhile, the reduced graphene oxide may satisfy the condition of Equation 1 below when measuring Raman spectroscopy.

[Equation 1]

0.05 ≤ I _D /I _G ≤ 0.45

In Equation 1 above,

I _D represents the intensity of a peak existing in the range of 1350 to 1380 cm ^-1 in the Raman spectrum,

I _G represents the intensity of a peak present in the range of 1550 to 1650 cm ^-1 in the Raman spectrum.

Specifically, the reduced graphene oxide may have a D band around 1361 cm ^-1 and a G band around 1590 cm ^-1 in a Raman spectrum. At this time, the precise positions of the D band and G band may be moved by the effect of the defect concentration, structure, and vacancies of the reduced graphene oxide.

Specifically, the reduced graphene oxide may have I _D /I _G of Formula 1 of 0.05 to 0.3, 0.05 to 0.25, 0.05 to 0.18, 0.05 to 0.15, or 0.05 to 0.1. As the I _D is lower within the above range, the crystallinity of the reduced graphene oxide may increase.

When I _D /I _G of Equation 1 satisfies the above range, the reduced graphene oxide has a low defect density and is structurally well aligned.

In addition, when the reduced graphene oxide is measured by Raman spectroscopy, I _2D /I _G may be 0.44 to 1.084. The I _2D /I _G may indicate the number of layers of reduced graphene oxide. For example, the reduced graphene oxide may be stacked in two to several tens of layers by being redeposited by an enhanced π-π interaction under a pressure condition according to an embodiment of the present invention.

^The defect density of the reduced graphene oxide according to an embodiment of the present invention is 2X10 ¹⁰ cm ^-2 to 7X10 10 cm ^-2 , 2X10 ¹⁰ cm ^-2 to 5X10 ¹⁰ cm ^-2 , or 2X10 ¹⁰ cm ^-2 to 4.5X10 ¹⁰ cm ^-2 .

Since the reduced graphene oxide according to an embodiment of the present invention has a defect density within the above range, structural defects and vacancies are minimized and structurally well aligned, thereby further improving electrical properties.

Electrical resistance of the reduced graphene oxide may be 40 μΩm to 60 μΩm, 40 μΩm to 58 μΩm, or 40 μΩm to 55 μΩm. When the electrical resistance of the reduced graphene oxide satisfies the above range, electrical conductivity may be further improved to realize excellent electrical characteristics.

Manufacturing method of reduced graphene oxide

A method for producing reduced graphene oxide according to an embodiment of the present invention includes (1) mixing graphene oxide and an iron-based catalyst; and (2) reducing graphene oxide by treating the mixture at a pressure of 3,000 kgf/cm 2 to 4,500 kgf/cm 2 and a temperature of 150° C. to 450° C.

The method for producing reduced graphene oxide according to an embodiment of the present invention characteristically employs a Fischer-Tropsch reaction in a hydrogen-sufficient environment using an iron-based catalyst.

The Fischer-Tropsch reaction is generally a reaction that produces long-chain hydrocarbons from CO or CO ₂ under abundant hydrogen gas conditions.

In this regard, FIG. 1 shows (i) thermally and chemically reduced graphene oxide, (ii) catalytically reduced graphene oxide in the presence of hydrogen, and (iii) catalytically reduced graphene oxide in a hydrogen-deficient environment. It is a schematic diagram showing the manufacturing process of pin oxide (Example 1), and with reference to this, specific features according to the manufacturing method of the present invention will be described in detail.

Referring to FIG. 1, as shown in (i), in the case of thermally and chemically reduced graphene oxide, carbon loss occurs by generating a carbon-containing gas during reduction, and due to high-temperature heat treatment, The decomposition of oxygen-containing functional groups into carbon-containing gases, such as CO ₂ and CO, can leave structural defects and vacancies in the graphene sheet.

As shown in (ii), in the case of reduced graphene oxide using a catalyst in the presence of hydrogen, since CHx (hydrogenated graphene sheet) can be produced by the combination of carbon and hydrogen on the catalyst surface, the final product, CHx can also be generated from reduced graphene oxide.

On the other hand, as shown in (iii), in the case of reduced graphene oxide using a catalyst in a hydrogen-sufficient environment according to an embodiment of the present invention, oxygen of graphene is adsorbed on the surface of the catalyst and then separated from carbon. This can be done to minimize carbon loss.

Specifically, referring to (a) of FIG. 5, a method for producing reduced graphene oxide using a Fischer-Tropsch reaction according to an embodiment of the present invention is (i) by mixing graphene oxide and an iron-based catalyst, Deoxidation of graphene oxide can be achieved by adsorbing oxygen of graphene on the catalyst surface and then (ii) dissociating oxygen from carbon. Then, (iii) graphitization by treating the carbon that has lost oxygen molecules by the deoxidation under a hydrogen-sufficient environment, that is, a pressure of 3,000 kgf / cm 2 to 4,500 kgf / cm 2 and a temperature of 150 ° C to 450 ° C can be done Then (iv) under the above conditions, oxygen adsorbed on the catalyst surface is discharged in the form of a gas, so that desorption of oxygen and restoration of the catalyst surface can be achieved.

That is, since the method according to the embodiment of the present invention can adsorb oxygen of graphene on the surface of the iron-based catalyst and then separate it from carbon, carbon loss can be minimized during deoxidation and graphitization processes.

In addition, by applying a pressure in the above range to the mixture of the graphene oxide and the iron-based catalyst, it is possible to forcibly block the entry and exit of hydrogen, and as a result, through direct bonding of sufficient carbon and deoxidized carbon atoms in an environment where hydrogen is insufficient. It can generate inert surface carbon species. This can be represented by Reaction Scheme 1 below.

[Scheme 1]

Here, k _c represents the reaction rate constant and C _i represents the inert surface carbon species.

That is, the graphene oxide reduction treatment according to the embodiment of the present invention applies a pressure in the specific range to create a semi-closed system in which gas exchange between the internal and external systems is difficult, thereby forcibly blocking the entry and exit of hydrogen from the outside. It may be a system that

Referring to (b) of FIG. 5, the binding energy between the iron-based catalyst (FeCuO ₂ ) and the oxygen (O) atom is about 3.57 eV, which is higher than the binding energy between the graphene sheet and the oxygen (O) atom (about 2.15 eV). Since the energy is high, oxygen can be easily adsorbed (3.57 eV) on the surface of the iron-based catalyst (FeCuO ₂ ), so deoxidation can be achieved, and the binding energy between the graphene sheet and carbon (C) atoms is about 15.3 eV. , Since it is higher than the binding energy (7.03 eV) between an iron-based catalyst (FeCuO ₂ ) and a carbon (C) atom, formation of carbides that cause carbon loss can be suppressed, and graphitization can be easily performed.

Since the reduced graphene oxide prepared in this way can minimize carbon loss and structural defects and generation of vacancies, it is possible to efficiently provide reduced graphene oxide of excellent quality with low resistance and high electrical conductivity.

Hereinafter, each process will be described in detail step by step:

(One) Mixing graphene oxide and iron-based catalyst

In the method for producing reduced graphene oxide according to an embodiment of the present invention, step (1) is a step of mixing graphene oxide and an iron-based catalyst.

The iron-based catalyst may include iron oxide (Fe ₂ O ₃ ), an iron-based compound having a delafossite structure, or a combination thereof. Specifically, the iron-based catalyst may include an iron-based compound having a delafossite structure.

The iron-based compound having the delafossite structure may be FeCuO ₂ .

(2) reducing graphene oxide by treating the mixture at a pressure of 3,000 kgf/cm 2 to 4,500 kgf/cm 2 and a temperature of 150° C. to 450° C.

In the method for producing reduced graphene oxide according to an embodiment of the present invention, the step (2) is to treat the mixture at a pressure of 3,000 kgf / cm 2 to 4,500 kgf / cm 2 and a temperature of 150 ° C to 450 ° C This is the step of reducing graphene oxide.

According to an embodiment of the present invention, by applying a pressure in the above range, it is possible to forcibly block the entry and exit of hydrogen, which results in direct bonding of sufficient carbon and deoxidized carbon atoms under a low temperature of 450 ° C. or less and an environment lacking hydrogen. this can be done

Specifically, the pressure may be 3,000 kgf/cm 2 to 4,500 kgf/cm 2 , 3,000 kgf/cm 2 to 4,200 kgf/cm 2 , 3,000 kgf/cm 2 to 4,000 kgf/cm 2 , or 3,500 kgf/cm 2 to 4,000 kgf/cm 2 . When the reduction is performed under a pressure in the above range, gas exchange between the inside of the reactor and the external system is difficult, and an environment in which hydrogen is insufficient may be provided. Due to this, since carbon loss and occurrence of structural defects can be suppressed, reduced graphene oxide of excellent quality having low resistance and high electrical conductivity can be efficiently provided.

If the pressure is less than 3,000 kgf/cm 2 , graphitization through recrystallization after deoxidation may not occur because the temperature inside the reaction is less than the temperature at which the graphitization reaction occurs (eg, 280 ° C. or higher), and high resistance and Graphene oxide films with low electrical conductivity can be produced. On the other hand, when the pressure exceeds 4,500 kgf/cm 2 , structural deformation of the catalyst and graphene oxide may occur.

During the reduction treatment, the temperature may be 150 °C to 450 °C, 200 °C to 400 °C, 200 °C to 350 °C, or 220 °C to 350 °C. The temperature may be based on the internal temperature of the reactor. If the temperature is less than 150 ° C, reduction does not occur, so it may not be easy to prepare reduced graphene oxide having a structure and effect desired in the present invention, and if the temperature exceeds 450 ° C, the catalyst used and graphene oxide Structural transformation of pinoxide may occur.

Meanwhile, the mixture may be in powder form. If the mixture is in liquid form, liquid leakage may occur due to pressure and it may be difficult to block the supply of hydrogen from the external environment. In the case of powder form, it may be easier to stably implement an environment lacking in hydrogen. .

Specifically, when the mixture of graphene oxide and iron-based catalyst in step (1) is a mixture in liquid form, the liquid mixture is oven-dried at a temperature range of about 50 ° C to 150 ° C, specifically 80 ° C to 120 ° C A mixture in powder form can be obtained.

Meanwhile, In the reduction treatment of graphene oxide according to an embodiment of the present invention, reduction may be performed after covering the mixture with metal foil to prevent contamination and create a hydrogen-sufficient environment before treatment at the pressure and temperature.

Specifically, the mixture may be encapsulated by disposing the mixture, for example in powder form, on a metal foil to completely cover the metal foil. Thereafter, by applying a pressure in the specific range to create a semi-closed system in which gas exchange between the inside of the reactor and the external system is difficult, the entry and exit of hydrogen can be forcibly blocked.

As long as the effect desired in the present invention is not impaired, various materials may be used for the metal foil, and for example, one or more selected from the group consisting of copper, nickel, and aluminum may be included.

On the other hand, the method for producing reduced graphene oxide according to an embodiment of the present invention may further include adding an acid solution to the reduced graphene oxide obtained in step (2) to remove the iron-based catalyst therefrom. .

The acid solution can remove the iron-based catalyst and can be used without limitation as long as the effect of the present invention is not impaired. Specifically, the acid solution may include at least one selected from the group consisting of hydrochloric acid, nitric acid, and sulfuric acid.

In addition, after adding an acid solution to the reduced graphene oxide, the catalyst may be removed therefrom by maintaining it for about 5 hours to 30 hours.

In the method for producing reduced graphene oxide according to an embodiment of the present invention, reduced graphene without carbon loss is obtained by applying a specific pressure and temperature to graphene oxide in the presence of an iron-based catalyst to perform a Fischer-Tropsch reaction in a hydrogen-sufficient environment. Oxides can be produced efficiently.

In addition, the reduced graphene oxide prepared by the above method has a large average crystal size and a low oxygen content by minimizing the loss of carbon in the reduction process, unlike the reduced graphene oxide prepared by the conventional reduction method. , thereby minimizing structural defects and improving electrical properties.

Furthermore, when the reduced graphene oxide is used as an OER (Oxygen Evolution Reaction) catalyst and a Hydrogen Evolution Reaction (HER) catalyst, it exhibits high activity, thereby providing OER and HER with excellent performance. there is. Therefore, the reduced graphene oxide according to an embodiment of the present invention can be usefully used as a catalyst in various fields.

The present invention will be described in more detail by the following examples. The following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

Preparation Example 1: Preparation of graphene oxide (GO)

After adding and mixing 0.5 g of graphite powder (Bay Carbon) and 0.5 g of NaOH ₃ in a two-necked flask, 23 ml of H ₂ SO ₄ was added and stirred. Thereafter, 3 g of KMnO ₄ was slowly added to the flask, and the mixture was stirred at 35 °C for 2 hours. Thereafter, after adding 40 ml of an aqueous solution to the flask, the mixture was stored at 95° C. for 1 hour. Then, 100 ml of deionized water (DI) was added to the mixture and stirred at room temperature for 30 minutes. The graphene oxide solution thus prepared was mixed with 200 ml of deionized water and filtered through a glass fiber membrane. The filtrate was then washed with 1 wt% HCl and sufficient deionized water, and the pH of the solution was adjusted by centrifugation at 4000 RPM for 10 to 15 minutes until the solution turned into a nearly neutral supernatant.

Preparation Example 2: Catalyst Synthesis (Delafossite, FeCuO ₂ )

FeCl ₂ (II) 50 mM, Cu(NO ₃ ) ₂ 50 mM, and NaOH 4 g were added to 50 ml deionized water and mixed. Then, 0.6 ml of propionaldehyde (reducing agent) was added to the mixed solution. The solution was heat treated at 180 °C (18 °C/min) for 2 hours using a microwave oven. After the reaction, the product was washed with ethanol to remove contaminants, and FeCuO ₂ , an iron-based catalyst having a delafossite structure, was obtained.

<Preparation of reduced graphene oxide>

Example 1: Preparation of reduced graphene oxide (CA-rGO) using the Fischer-Tropsch reaction method

300 mg/L of the graphene oxide solution obtained in Preparation Example 1 was mixed with a catalyst (delafossite, FeCuO ₂ ) using ultrasonic waves. The mixture was oven dried at 80 °C overnight to obtain a powder form. The powder was completely covered with copper foil (1 cm ² ) to prevent contamination of the surroundings. Then, the sample was treated at a pressure of 4,000 kgf/cm ² and a temperature of 250 °C using a heating compressor to reduce graphene oxide. Thereafter, a hydrochloric acid (1 M) solution was added to the product obtained by the reduction treatment and treated for 24 hours to obtain reduced graphene oxide (CA-rGO) from which the iron-based catalyst was removed.

Comparative Example 1: Preparation of thermally reduced graphene oxide (TrGO)

One surface of a SiO ₂ wafer was coated with 300 mg/L of the graphene oxide solution obtained in Preparation Example 1 (3,000 RPM for 60 seconds). After placing it in a quartz tube and putting it in a chamber, the chamber was evacuated to 3 mTorr under an argon (A, 100 sccm) atmosphere, and the temperature was raised to 1,000 ° C. and heat treatment was performed for 30 minutes. Thereafter, the temperature was cooled to room temperature to obtain graphene oxide (TrGO, blue) by thermal reduction.

Comparative Example 2: Preparation of CVD-grown graphene

A high-purity copper foil (99.999%) 1 cm ² was placed on one side of the SiO ₂ wafer. After placing it in a quartz tube and putting it into a chamber, the chamber was evacuated to 170 mTorr under an argon/hydrogen (Ar/H ₂ ,1000/50 sccm) atmosphere, and the temperature was raised to 1,000 °C (20 °C/min). Then, methane gas (CH _4, 24 sccm) was additionally injected into the chamber, maintained for 30 minutes, and graphene was synthesized through cooling (200 °C/min). The synthesized graphene was synthesized using a polymethyl methacrylate (PMMA) solution (molecular weight 996 kD, 4% by weight, ethyl lactate, 2000 rpm for 1 minute) to graphene on a SiO ₂ wafer (wafer) on a copper foil. ) was transcribed. Finally, the coated PMMA was completely removed using acetone. Thus, CVD-grown graphene (brown) was obtained.

Comparative Example 3: Preparation of PGO

Reduced graphene oxide (PGO) was obtained in the same manner as in Example 1, except that the temperature was changed to room temperature in Example 1.

Comparative Example 4: Preparation of HGO

Reduced graphene oxide (HGO) was prepared in the same manner as in Example 1, except that the pressure was changed to 1 atm in Example 1.

Comparative Example 5: Preparation of PHGO

The reduced graphene oxide (PHGO) was prepared in the same manner as in Example 1, except that the reduction treatment was performed using the graphene oxide solution obtained in Preparation Example 1 without using a catalyst in Example 1. manufactured.

tester

The test apparatus used in the above examples is as follows.

(One) Raman spectroscopic measurements

The structure was analyzed using WItec, alpha300R (532 nm, 2 mW, 50x lens).

(2) Average grain size and defect density

The defect density (n _D ) and average crystallite size (L _D ) of the reduced graphene oxide can be determined by applying the results obtained from the Raman spectrum to the Cancado equation in Equation 2 below:

[Equation 2]

In Equation 2, E _L is the laser energy used (here, 2.33 eV),

I _D and I _G are as defined above.

(3) X-ray photoelectron spectroscopy (X-ray photoelectron spectroscopy, XPS)

X-ray photoelectron spectroscopy (XPS) spectra were measured using a Thermo Fisher Scientific EXCALAB 250XI with a monochromatic Al-Kα X-ray source at 1.0X10 ^-9 Torr base pressure.

Meanwhile, the internal temperature (y-axis) shown in FIG. 19 was determined based on Equation 3 below:

[Equation 3]

where T ₁ and P ₁ are the initial (external) temperature and pressure;

T ₂ and P ₂ are the final (internal) temperature and pressure;

n is a multi-row index (1 to 1.4).

To apply the above equation, the experimental system was considered as a closed system. When the pressure was 4,000 kgf/cm ² at 250 °C (external temperature), the internal temperature increased to about 450 °C.

(4) Fourier transform infrared spectroscopy, FTIR)

Fourier transform infrared spectroscopy (FTIR) spectra were measured using a Cary 670 FTIR spectrometer (Agilent).

(5) Scanning Electron Microscope (SEM)

Scanning electron microscopy measurements were made at 15 kV using a Nano-SEM 230.

(6) GC-Mass (Gas Chromatography-Mass Spectrometry) analysis

TGA/GC-mass (TA Instruments/Hiden Analytical LLC, TGA Q500 Auto/QGA) measurements were performed by heating the sample in Ar gas at a maximum of 500 °C for 100 min (5 °C/min).

(7) Evaluation of electrical characteristics

Two electrodes (5 μm) coated with GO (1.6 μm), TrGO (2.1 μm), CVD-graphene (monolayer), and CA-rGO films (2.7 μm) on 40 nm Au deposited on a SiO ₂ wafer. ) was measured for current-voltage (IV) characteristics using a device (MS TECH, MODEL 4000A, keithley 2636B SMU software).

(8) Density functional theory (DFT) calculation

Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP).

The reciprocal-correlation function used the general gradient approximation (CGA) of Perdew-Burke-Ernzerhof (PBE). The electron plane wave interception energy was set to 400 eV, and a 4×4×1 k-point mesh was used. The vacuum layer between neighboring images was set larger than 15 Å, and the force of all atoms was less than 0.01 eV/Å.

The results and other physical properties measured in the above experimental example are described in detail below with reference to Tables 1 to 3 and FIGS. 1 to 20:

Test Example 1: Structural characteristics of reduced graphene oxide (CA-rGO)

2 to 4 are graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, CVD-grown graphene (CVD-graphene) of Comparative Example 2 and Examples, respectively. Raman spectrum of reduced graphene oxide (CA-rGO) of 1 (FIG. 2), X-ray photoelectron spectroscopy (FIG. 3) and Fourier transform infrared spectroscopy (FT-IR) (FIG. 4) show the measurement results.

Specifically, as shown in FIG. 2, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and CVD-grown graphene (CVD-graphene of Comparative Example 2) ) and the reduced graphene oxide (CA-rGO) of Example 1 both had a D band near 1361 cm ^-1 and a G band near 1590 cm ^-1 in the Raman spectrum. The reduced graphene oxide (CA-rGO) of Example 1 and the CVD-grown graphene (CVD-graphene) of Comparative Example 2 were sharper than the thermally reduced graphene oxide (TrGO) of Comparative Example 1. It exhibited highly ordered, graphene-like Raman features with highly symmetric G and 2D bands and almost no D band.

On the other hand, as can be seen in Figure 3, the binding energy of CC (sp ² ) corresponds to 284.5 ± 0.2 eV, from which chemical shifts of +0.5, +1.5, +2.5 and +4.0 eV are typically sp ³ hybridization, Corresponds to C-OH, C = O, and COOH functional groups, respectively. In the reduced graphene oxide (CA-rGO) of Example 1, it was confirmed that the XPS of the C 1s core level appeared at 284.5 ± 0.3 and 285.0 ± 0.3 eV. did It has been identified as sp ² and sp ³ hybridized forms of carbon. The sp ³ /sp ² ratio of the reduced graphene oxide (CA-rGO) of Example 1 was 0.147, whereas the sp ³ /sp ² ratio of the graphene oxide (GO) of Preparation Example 1 was 2.759. That is, the low value of sp ³ /sp ² of the reduced graphene oxide (CA-rGO) of Example 1 indicates that the sp ² bond is reconstituted, which has a graphite structure and improves electrical properties.

As can be seen in Figure 4, as a result of Fourier transform infrared spectroscopy (FT-IR) measurement, the band observed at 1720 cm ^-1 corresponding to the carboxyl group of the reduced graphene oxide (CA-rGO) of Example 1 is Preparation Example 1 was higher in intensity than the band of graphene oxide (GO).

Figure 6 (a) is an optical image of the reduced graphene oxide (CA-rGO) of Example 1, (b) to (f) are each of the reduced graphene oxide (CA-rGO) of Example 1 It shows the Raman map.

Specifically, Figure 6 is a Raman map of the reduced graphene oxide (CA-rGO) of Example 1 in a 150 x 150 μm ² area of a SiO ₂ wafer, as shown in (b) of FIG. 6, the D band In this case, a bright region showing a strong D band was observed around the edge and wrinkled sheet of the reduced graphene oxide. On the other hand, as can be seen in (c) and (d) of FIG. 6, the G and 2D bands were evenly distributed throughout the reduced graphene oxide sheet, and similar results to the optical images of FIG. 6 (a) were obtained. showed up

Meanwhile, (e) and (f) of FIG. 6 are images showing I _D /I _G and I _2D /I _G ratios, respectively. As can be seen in (e) of FIG. 6, the background portion of the reduced graphene oxide has a high ratio of I _D /I _G (bright part), but a uniformly low ratio (dark part) in the sheet of reduced graphene oxide. and the intensities of the D and 2D bands were lower than those of the G band (<1). In addition, when I _D /I _G is as low as about 0.07 ± 0.1, it indicates that the defect density is low and the structure is well aligned.

On the other hand, (a) to (d) of FIG. 7 show Raman maps of graphene oxide (GO) of Preparation Example 1, and (e) to (h) are thermally reduced graphene oxide of Comparative Example 1 ( TrGO) shows the Raman map. It can be seen that (a) to (d) of FIG. 7 represent Raman maps clearly distinguished from the reduced graphene oxide (CA-rGO) of Example 1 of FIG. 6 .

8 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and bay carbon (BC). Shows the scanning electron microscope (SEM) measurement results of.

As shown in Figure 8, the roughness of the surface of the reduced graphene oxide (CA-rGO) of Example 1 is the graphene oxide (GO) of Preparation Example 1, the thermally reduced graphene oxide (TrGO of Comparative Example 1) ), and significantly rougher than the surface roughness of the bay carbon (BC). An increase in surface roughness causes an increase in surface area and improves electrochemical reactions.

9 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and CVD-rGO of Comparative Example 2. It is a graph showing the crystal size (y-axis) according to the number of layers (I _2D /I _G ) (x-axis) and the ratio (I _D /I _G ) from Raman spectroscopic measurements of grown graphene (CVD-graphene).

As can be seen in (f) of FIG. 9 and FIG. 6, I _2D /I _G of the reduced graphene oxide (CA-rGO) of Example 1 ranges from 0.44 to 1.084, and the pressure according to the embodiment of the present invention It can be seen that CA-rGO is stacked in two to several tens of layers due to the re-stacking between the sheets due to the π-π interaction under the condition.

In addition, the crystal size of the reduced graphene oxide (CA-rGO) of Example 1 ranges from 60 nm to 300 nm with an average crystal size ranging from about 120 nm to 250 nm, and the average value of I _2D /I _G is about It was 0.62 ± 0.2.

On the other hand, in Table 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, CVD-grown graphene of Comparative Example 2 (CVD-graphene) and Example 1 The average crystal size and defect density of reduced graphene oxide (CA-rGO) were shown.

As can be seen in Table 1, the reduced graphene oxide (CA-rGO) of Example 1 has a very large average compared to the reduced graphene oxide of Comparative Examples 1 and 2, and the graphene oxide (GO) of Preparation Example 1. It was confirmed to have a crystal size and a very low defect density.

Specifically, the average crystal size (L _D ) of the reduced graphene oxide (CA-rGO) of Example 1 is 174.0 nm (measured from 9 samples), the reduced graphene oxide of Comparative Examples 1 and 2, and Compared to the graphene oxide of Preparation Example 1, it increased more than 2 to 3 times.

On the other hand, the average crystal size (L _D ) of the thermally reduced graphene oxide (TrGO) of Comparative Example 1 was the smallest at 42.13 nm. That is, due to the removal and random rearrangement of functional groups during the deoxidation process, the average crystal size of the thermally reduced graphene oxide (TrGO) of Comparative Example 1 is greater than that of the graphene oxide (GO) (61.77 nm) of Preparation Example 1. It can be seen that it is much smaller.

In addition, the average crystal size (L _D ) of the reduced graphene oxide (CA-rGO) of Example 1 is about 1.66 times greater than the average crystal size of the CVD-grown graphene of Comparative Example 2 increased. In the reduced graphene oxide (CA-rGO) of Example 1, only oxygen was separated from carbon, and reorganization occurred on the surface of the iron-based catalyst, so that the diamond structure (sp ³ bond) was converted into a graphite structure (sp ² bond). because it has been

On the other hand, the defect density of the reduced graphene oxide (CA-rGO) of Example 1 is 4.462x10 ¹⁰ , which is significantly reduced compared to the reduced graphene oxide of Comparative Examples 1 and 2 and the graphene oxide of Preparation Example 1. confirmed.

In particular, the defect density of the reduced graphene oxide (CA-rGO) of Example 1 was reduced by 0.35 times more than that of the CVD-grown graphene of Comparative Example 2.

On the other hand, Table 2 shows the elemental content of graphene oxide (GO) of Preparation Example 1 and reduced graphene oxide (CA-rGO) of Example 1.

As can be seen in Table 2, the oxygen (O) content of the reduced graphene oxide (CA-rGO) of Example 1 is significantly reduced compared to the oxygen (O) content of the graphene oxide (GO) of Preparation Example 1, and C It can be seen that the /O atomic ratio is greatly improved.

Specifically, the oxygen (O) content of the reduced graphene oxide (CA-rGO) of Example 1 is 12.15 atomic%, and the graphene oxide (GO) of Preparation Example 1 having an oxygen (O) content of 25.96 atomic% decreased by more than 50% compared to

In addition, the C / O atomic ratio of the reduced graphene oxide (CA-rGO) of Example 1 is 4.16, more than three times as compared to the graphene oxide (GO) of Preparation Example 1 having a C / O atomic ratio of 1.30 improvement was confirmed.

In addition, Table 3 shows the element content of graphene oxide (GO) of Preparation Example 1 and reduced graphene oxide (CA-rGO) of Example 1.

As can be seen in Table 3, the reduced graphene oxide (CA-rGO) of Example 1 has an increased content of hydroxyl and carboxyl groups compared to graphene oxide (GO) of Preparation Example 1, whereas the content of epoxy groups is significantly reduced can know that

Specifically, in the reduced graphene oxide (CA-rGO) of Example 1, the content of hydroxy groups increased from 0.94 atomic % to 1.14 atomic % compared to the graphene oxide (GO) of Preparation Example 1, and the content of carboxyl groups increased. While it increased from 9.51 atomic % to 13.64 atomic %, the content of epoxy group rapidly decreased from 48.84 atomic % to 0.48 atomic %.

Test Example 2: Analysis of gas generated from reduced graphene oxide (CA-rGO)

10 (a) shows gases (CO, CO 2 , CO ₂ , O ₂ , H ₂ O) is a graph showing the results of GC-Mass analysis, and FIG. 11 is a graph comparing the total amount of the generated gas.

FIG. 12 is a pie graph showing the gas analysis results generated from the reduced graphene oxide (CA-rGO) of Example 1 for the results of FIGS. 10 and 11.

As can be seen in Figure 10, graphene oxide (GO) of Preparation Example 1 and reduced graphene oxide (CA-rGO) of Example 1 are CO (m / z ratio = 28), CO ₂ (m / z ratio = 28) in an argon atmosphere /z ratio = 44), O ₂ (m/z ratio = 16), and H ₂ O (m/z ratio = 18). In addition, the average temperature with the maximum peak intensity of each generated gas of the reduced graphene oxide (CA-rGO) of Example 1 was lower than that of the graphene oxide (GO) of Preparation Example 1, which means that the catalyst accelerates the deoxidation reaction. showed that it did.

In addition, as can be seen in FIGS. 11 and 12, the reduced graphene oxide (CA-rGO) of Example 1 contains much less carbon-containing gas (CO, CO ₂ ) than the graphene oxide (GO) of Preparation Example 1 and Oxygen containing gases (O ₂ and H ₂ O) were released. Specifically, it was confirmed that the reduced graphene oxide (CA-rGO) of Example 1 emitted 24.35% of the exhaust gas amount (based on 100%) of the graphene oxide (GO) of Preparation Example 1. This is due to the adsorption of oxygen (with a binding energy of 3.57 eV) on the catalyst surface at low temperatures.

Specifically, the reduced graphene oxide (CA-rGO) of Example 1 released 75.65% less gas than the graphene oxide (GO) of Preparation Example 1. In addition, 92.8% of the amount of carbon-containing gas generated from the graphene oxide (GO) of Preparation Example 1 was reduced, indicating that the reduced graphene oxide (CA-rGO) of Example 1 was reduced during the reduction process to the graphene of Preparation Example 1. This means that 7.2% gas was released compared to oxide (GO). In each sample, the ratio of carbon-containing gases such as CO and CO ₂ based on the total gas content was 51% for the graphene oxide (GO) of Preparation Example 1 and the reduced graphene oxide (CA- rGO) was confirmed to decrease to 21%.

Test Example 3: Evaluation of Electrical Characteristics and Catalytic Activity of Reduced Graphene Oxide (CA-rGO)

13(a) shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1 and Comparative Example 2 is a graph comparing the electrical properties of CVD-grown graphene, (b) is an LED display connected using paper coated with reduced graphene oxide (CA-rGO) of Example 1 14 (a) and (b), and 15 (a) and (b) show the reduced graphene oxide (CA-rGO) of Example 1, Preparation Example 1 Oxygen evolution reaction and hydrogen of graphene oxide (GO), thermally reduced graphene oxide (TrGO) of Comparative Example 1, CVD-grown graphene (CVD-graphene) and bay carbon (BC) of Comparative Example 2 A linear sweeping current-voltage curve for the resulting response is shown.

16 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and CVD-rGO of Comparative Example 2. It is a graph showing the resistance of grown graphene (CVD-graphene) and bay carbon (BC).

As can be seen in (a) and 16 of FIG. 13, the reduced graphene oxide (CA-rGO) of Example 1 is the graphene oxide (GO) of Preparation Example 1 and the thermally reduced graphene of Comparative Example 1. The electrical properties were superior to oxide (TrGO) and CVD-grown graphene of Comparative Example 2 (CVD-graphene).

In particular, the reduced graphene oxide (CA-rGO) of Example 1 is graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1 and CVD-growth of Comparative Example 2 Its resistance was significantly lower than that of CVD-graphene, and it showed a resistance value similar to that of graphite.

In particular, reduced graphene oxide (CA-rGO) of Example 1 (about 55 μΩm) has significantly improved electrical conductivity compared to graphene oxide (GO) of Preparation Example 1 (about 3.7 MΩm).

In addition, as can be seen in (b) of FIG. 13, when the paper tissue coated with the reduced graphene oxide (CA-rGO) sheet of Example 1 is connected to the coin cell (6V), the LED bulb is turned on. Confirmed.

On the other hand, the reduced graphene oxide (CA-rGO) of Example 1 is the graphene oxide (GO) of Preparation Example 1, the thermally reduced graphene oxide (TrGO) of Comparative Example 1, and the bay carbon (BC) phase. After preparing the Ni(OH) ₂ active material, electrical conductivity and electrochemical performance were confirmed.

As can be seen in (a) of FIG. 14, the reduced graphene oxide (CA-rGO) of Example 1 is the graphene oxide (GO) of Preparation Example 1 and the thermally reduced graphene oxide (TrGO of Comparative Example 1). ), and the current densities for electrodes using bay carbon (BC) all suddenly increased from 1.45 V to 1.8 V potential, indicating an obvious OER catalytic activity of Ni(OH) ₂ . means The overpotential required to reach a current density of 10 mA/cm ² using the Ni(OH) ₂ @CA-rGO electrode was 1.58 V versus the reference electrode (RHE). This was lower than Ni(OH) ₂ @GO (1.70 V) and Ni(OH) ₂ @TrGO (1.61 V) and similar to Ni(OH) ₂ @bay carbon (BC, 1.57 V), and exhibited excellent electrical conductivity. have

In addition, as shown in (b) of FIG. 14, a carbon current collector for hydrogen evolution reaction (HER) compared to a reversible hydrogen electrode (RHE) at a current density of -10 mA/cm ² The operating potentials of were -0.43 V for bay carbon (BC), -0.34 V for GO, -0.32 V for TrGO, and -0.30 V for CA-rGO. Therefore, it can be seen that the Ni(OH) ₂ @CA-rGO electrode using excellent electrical coupling between the active material and the carbon current collector exhibits excellent HER performance. On the other hand, the Ni(OH) ₂ @BC electrode using bay carbon (BC) had poor HER performance, which could be attributed to the lack of functional groups including -OH and -COOH.

Meanwhile, in order to investigate the effect of functional groups on the electrocatalytic activity of carbon materials, the OER and HER performances of various carbon materials as active materials were investigated in an alkaline medium, and the results are shown in (a) and (b) of FIG. was

As can be seen in (a) of FIG. 15, the HER performance of the reduced graphene oxide (CA-rGO) of Example 1 was compared to that of bay carbon (BC), graphene oxide (GO) of Preparation Example 1, and Comparative Example 1. It can be confirmed that it exhibits significantly higher activity than the HER performance of thermally reduced graphene oxide (TrGO).

As can be seen in (b) of FIG. 15, in the case of OER performance, the potential required to reach a current density of 10 mA/cm ² using the reduced graphene oxide (CA-rGO) catalyst of Example 1 is RHE was 1.65 V for The potential is carbon such as bay carbon (BC) (1.72 V), graphene oxide (GO) (1.71 V) of Preparation Example 1, and thermally reduced graphene oxide (TrGO) (1.69 V) of Comparative Example 1. It decreased compared to the case of using the material catalyst.

17 shows reduced graphene oxide (CA-rGO) of Example 1, graphene oxide (GO) of Preparation Example 1, thermally reduced graphene oxide (TrGO) of Comparative Example 1, and bay carbon (BC). shows the Tafel slope (a) and overpotential ((b) and (c)) of the operating potential for Pt of the carbon material catalyst of (d) and (f) the Tafel slope of the operating potential for RHE (d ) and overpotentials ((e) and (f)).

As can be seen in (a) and (f) of Figure 17, the Tafel slope of the reduced graphene oxide (CA-rGO) of Example 1 is the graphene oxide (GO) of Preparation Example 1, thermally of Comparative Example 1 It was significantly lower than the Tafel slopes of reduced graphene oxide (TrGO) and bay carbon (BC), which may indicate that the reduced graphene oxide (CA-rGO) catalyst of Example 1 has faster OER catalyst kinetics. .

In addition, as can be seen in (b) and (c) of FIG. 17, the operating potential at a current density of -10 mA/cm ² is -0.3 V for bay carbon (BC), graphene oxide of Preparation Example 1 It was -0.29 V for (GO), -0.27 V for thermally reduced graphene oxide (TrGO) of Comparative Example 1, and -0.23 V for reduced graphene oxide (CA-rGO) of Example 1.

In conclusion, as can be seen in (a) to (c) of FIG. 17, the reduced graphene oxide (CA-rGO) of Example 1 has excellent kinetic characteristics with the lowest Tafel slope at 125.8 mV dec ^-1 , and It showed the highest hydrogen generation ability with high current density at -0.3 V overpotential.

This result is because the reduced graphene oxide (CA-rGO) of Example 1 has abundant -COOH groups and -OH at the edge, which can improve the electrocatalytic activity. In addition, this deformation of the edge portion of the carbon material can reduce the Gibbs free energy (ΔG _H ) of hydrogen adsorption and rapidly promote the 4-electron transfer kinetics for OER. In particular, the reduced graphene oxide (CA-rGO) of Example 1 improved electrical conductivity by having a defect-free base surface and a -COOH functionalized edge portion, and exhibited electrical conductivity suitable for OER and HER performance.

Test Example 4: Physical property change of reduced graphene oxide (CA-rGO) according to pressure and temperature

18 is an X-ray photoelectron spectroscopy of reduced graphene oxide (CA-rGO) of Example 1 and reduced graphene oxide of Comparative Examples 3 to 5. It shows the ratio of sp2, sp3, COC and OC=0 by XPS) measurement.

As can be seen in FIG. 18, the reduced graphene oxide (CA-rGO) of Example 1 significantly increased the ratio of sp2 and significantly decreased the ratio of sp3 compared to the reduced graphene oxide of Comparative Examples 3 to 5. . In addition, the ratio of O-C=O increased, and the ratio of -O-C-O decreased.

On the other hand, in the case of graphene oxide (PGO) of Comparative Example 1 subjected to reduction treatment at room temperature, the ratio of sp2 rather decreased and the ratio of sp3 increased more than that of graphene oxide (GO).

In addition, in the case of reduced graphene oxides (HGO and PHGO) of Comparative Examples 4 and 5 in which the pressure was changed to 1 atm and no catalyst was used, the ratio of sp2 was increased compared to graphene oxide (GO), but in Example It was significantly lower than that of reduced graphene oxide (CA-rGO) of 1.

19 is a graph showing the correlation between internal temperature (y-axis) and external temperature (x-axis) according to pressure during reduction treatment.

As can be seen in FIG. 19, based on the internal temperature (y-axis) of the graph, when a pressure of 1,000 kgf/cm ² or more is applied, reduction is observed at a temperature higher than 200° C. (external temperature 180° C. or higher). It was confirmed that the graphitization process occurred at an internal temperature of 280 °C or higher (external temperature of 250 °C or higher). In particular, at pressures of 3,000 kgf/cm ² and 4,000 kgf/cm ^{2 ,} reduction was performed at 150° C. (external temperature).

Figure 20 (a) is a graph showing the ratio of I _D / I _G according to pressure and temperature change, (b) is 4,000 kgf / cm ² pressure and sp2 by XPS measurement according to time change at a temperature of 250 ℃ , sp3, COC, and the ratio of OC=0. As shown in (a) of FIG. 20, it was assumed that graphene oxide was reduced and rearranged at a pressure of 4,000 kgf/cm ² and a temperature of 250 °C.

As can be seen in (b) of FIG. 20, the oxygen content in the basal surface of the reduced graphene oxide (CA-rGO) of Example 1 was less than 0.48 atomic%, and is close to the value of graphite. In addition, as the reaction time increases from 0 to 24 hours, the proportion of sp ² increases from 24.4 atomic % to 77.8 atomic % and sp ³ decreases from 22.86 atomic % to 6.9 atomic %, while the oxygen content of the basal plane (epoxy group) ) decreased from 43.8 atomic % to 0.48 atomic %, and the oxygen content (carboxyl group or hydroxyl group) at the edge portion increased from 8.89 atomic % to 14.79 atomic %. After 24 hours, the total oxygen content decreased from 52.73 atomic % to 15.27 atomic %.

Therefore, in the reduced graphene oxide according to an embodiment of the present invention, the content of epoxy groups present on the basal surface is significantly reduced, and the content of carboxyl and hydroxy groups present on the edge portion is increased compared to graphene oxide before reduction, sp It was confirmed that the ratio of ² significantly increased.

Claims (13)

1. A reduced graphene oxide having an oxygen (O) content of 6 atomic% to 18 atomic% based on the total content of carbon (C), hydrogen (H) and oxygen (O),

   A reduced graphene oxide having an average crystalline size of 120 nm to 250 nm.
2. According to claim 1,

   The reduced graphene oxide having a carbon / oxygen (C / O) atomic ratio of 3 to 5, the reduced graphene oxide.
3. According to claim 1,

   The reduced graphene oxide

   0.2 to 0.6 atomic % of epoxy groups;

   1.0 to 1.8 atomic % of hydroxy groups;

   10 to 20 atomic % carboxyl groups; and

   The remaining amount of carbon (C)

   Containing, the reduced graphene oxide.
4. According to claim 1,

   The reduced graphene oxide having an sp3 / sp2 ratio of carbon of 0.1 to 0.9.
5. According to claim 1,

   The reduced graphene oxide having a defect density of 2X10 ¹⁰ cm ^-2 to 7X10 ¹⁰ cm ^-2 and an electrical resistance of 40 μΩm to 60 μΩm, the reduced graphene oxide.
6. According to claim 1,

   Reduced graphene oxide that satisfies the condition of Equation 1 below when the reduced graphene oxide is measured by Raman spectroscopy:

   [Equation 1]

   0.05 ≤ I _D /I _G ≤ 0.45

   In Equation 1 above,

   I _D represents the intensity of a peak existing in the range of 1350 to 1380 cm ^-1 in the Raman spectrum,

   I _G represents the intensity of a peak present in the range of 1550 to 1650 cm ^-1 in the Raman spectrum.
7. (1) mixing graphene oxide and an iron-based catalyst; and

   (2) reducing graphene oxide by treating the mixture at a pressure of 3,000 kgf/cm 2 to 4,500 kgf/cm 2 and a temperature of 150° C. to 450° C.;

   Method for producing a reduced graphene oxide comprising a.
8. According to claim 7,

   The iron-based catalyst comprises iron oxide (Fe ₂ O ₃ ), an iron-based compound having a delafossite structure, or a combination thereof, a method for producing reduced graphene oxide.
9. According to claim 8,

   The iron-based compound having the delafossite structure is FeCuO ₂ Method for producing a reduced graphene oxide.
10. According to claim 7,

    A method for producing reduced graphene oxide in which the mixture of step (2) is covered with metal foil and then treated.
11. According to claim 10,

    Method for producing a reduced graphene oxide, wherein the metal foil contains at least one selected from the group consisting of copper, nickel and aluminum.
12. According to claim 7,

    Method for producing reduced graphene oxide, wherein the mixture of step (2) is in powder form.
13. According to claim 7,

    The method for producing reduced graphene oxide, wherein the method further comprises the step of removing the iron-based catalyst by adding an acid solution to the reduced graphene oxide obtained in step (2).

Images