Graphene / activated carbon composite material as well as preparation method and application thereof

Abstract

The invention provides a graphene / activated carbon composite material and a preparation method and application thereof, and belongs to the technical field of nano materials, the graphene / activated carbon composite material comprises a carbon matrix and a two-dimensional sheet material covering the carbon matrix; the carbon matrix is an activated carbon material and is a carbon material prepared by performing high-temperature pyrolysis on magnesium citrate in a high-temperature self-propagating reaction process; the two-dimensional sheet material is a graphene material and is a few-layer carbon material which is prepared by converting CO2 through a high-temperature self-propagating reaction and is formed by arranging carbon atoms according to a honeycomb-shaped two-dimensional lattice. According to the invention, graphene is used as a conductive and structural dual skeleton; and a hierarchical pore structure is constructed by utilizing gas production and template effect in the reaction process, so that the electrochemical performance of the active carbon positive electrode is improved. And a proper amount of graphene can provide more active sites and maintain the structural stability at the same time, so that the specific capacity and the cycle life are remarkably improved and prolonged, and the material has a wide application prospect and is particularly suitable for preparing a high-performance lithium ion capacitor positive electrode material.

Description

Technical Field

[0001] This invention relates to the field of nanomaterials technology, and in particular to a graphene / activated carbon composite material, its preparation method, and its application. Background Technology

[0002] With the development of portable electronics, transportation electrification, and renewable energy grid integration, energy storage devices urgently need to possess both high energy density and high power density. While lithium-ion batteries boast high energy densities of 150-300 Wh / kg, their power density (<1000 W / kg) and cycle stability are insufficient due to limited intercalation / deintercalation reaction kinetics. Supercapacitors, on the other hand, offer power densities of >5000 W / kg and cycle lives of >100,000 cycles, but are limited by energy densities of <10 Wh / kg. Lithium-ion capacitors offer a trade-off and improvement in energy and power by coupling the positive electrode, which involves rapid surface processes, with the negative electrode, which involves intercalation reactions. However, traditional activated carbon positive electrodes rely on an electrical double-layer for energy storage, which suffers from poor matching of specific surface area and pore structure with electrolyte ions, high bulk / interface resistance, and insufficient electron-ion co-channels, limiting rate and capacity output.

[0003] Graphene possesses a two-dimensional conjugated structure, excellent electrical conductivity, and a conductivity as high as 2630 μm. 2 With a theoretical specific surface area of ​​approximately 550 F / g and a theoretical specific capacitance of approximately 550 F / g, graphene-based composites can construct a continuous conductive network within the electrode and enhance wetting and pseudocapacitance. However, they are prone to agglomeration and stacking, resulting in loss of effective surface area and hindering ion diffusion. Therefore, a graphene / activated carbon composite structure is designed. By "anchoring" small-sized, wrinkled graphene sheets onto the activated carbon framework, agglomeration can be suppressed, maintaining a high effective specific surface area. Simultaneously, interconnected electron channels and hierarchical pores (micro / meso / macropore coupled inter-sheet channels) can be formed, shortening the ion diffusion path and reducing polarization. Furthermore, appropriate amounts of oxygen-containing functional groups can enhance wetting and introduce surface pseudocapacitance, thereby achieving a synergistic improvement in high capacity, high rate capability, and high coulombic efficiency over a wide voltage window. However, traditional methods for preparing graphene-based composites (such as chemical vapor deposition and electrochemical exfoliation) typically require toxic or expensive precursors, and the preparation process is complex, limiting the large-scale application of this technology. Therefore, developing a green, simple, and low-cost method for preparing graphene-based composites has become a key research focus in this field. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a graphene / activated carbon composite material, its preparation method, and its application. The graphene / activated carbon composite material of this invention has higher electrical conductivity and a stable structure.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] A graphene / activated carbon composite material includes a carbon matrix and a two-dimensional sheet material covering the carbon matrix;

[0007] The carbon matrix is ​​an activated carbon material;

[0008] The two-dimensional sheet material is graphene.

[0009] Preferably, the carbon matrix is ​​an activated carbon material, which is a carbon material prepared by high-temperature pyrolysis of magnesium citrate during a high-temperature self-propagating reaction.

[0010] Preferably, the two-dimensional sheet material is graphene, which is a few-layer carbon material prepared by converting CO2 through a high-temperature self-propagating reaction. It consists of carbon atoms arranged in a honeycomb-like two-dimensional lattice.

[0011] This invention also provides a method for preparing the graphene / activated carbon composite material described in the above technical solution, comprising the following steps:

[0012] Magnesium powder and magnesium citrate were manually premixed and then placed in an air jet mill. The powder in the mixture was fully dispersed under the action of air jet shear and turbulence by the action of high-speed air jet to obtain a uniform mixed powder A.

[0013] A homogeneous mixture of powder A is placed in a sealed 20 L reaction vessel, and pure CO2 gas is introduced. After being electrically heated by a nickel-chromium wire, a high-temperature self-propagating reaction is initiated. After a few seconds, the mixture is allowed to cool naturally for 10-20 minutes to obtain the precursor powder product B.

[0014] The precursor powder product B was acid-washed, filtered, and freeze-dried to obtain the graphene / activated carbon composite material.

[0015] Preferably, the mass ratio of the magnesium powder to magnesium citrate is 1:1 to 1:5.

[0016] Preferably, the pickling agent is a 2 mol / L dilute sulfuric acid solution.

[0017] Preferably, the initial temperature of the high-temperature self-propagating reaction is 25 °C, the duration is 3-5 s, and the high-temperature self-propagating reaction is carried out in a CO2 atmosphere.

[0018] The present invention also provides the application of the graphene / activated carbon composite material described in the above technical solution or the graphene / activated carbon composite material prepared by the preparation method described in the above technical solution in the positive electrode of lithium-ion capacitor.

[0019] The present invention provides a graphene / activated carbon composite material, comprising a carbon matrix and a two-dimensional sheet material covering the carbon matrix; wherein the carbon matrix is ​​an activated carbon material and the two-dimensional sheet material is a graphene material.

[0020] This invention designs a graphene / activated carbon composite structure by "anchoring" small-sized, wrinkled graphene sheets onto an activated carbon framework. This not only suppresses agglomeration and maintains a high effective specific surface area, but also forms interconnected electron channels and hierarchical pores, shortening ion diffusion paths and reducing polarization. Furthermore, by introducing appropriate amounts of oxygen-containing functional groups to enhance wetting and introduce surface pseudocapacitance, a synergistic improvement in high capacity, high rate capability, and high coulombic efficiency is achieved over a wide voltage window. The graphene / activated carbon composite material of this invention has broad application prospects, and is particularly suitable for preparing high-performance lithium-ion capacitor cathode materials.

[0021] This invention also provides a method for preparing the graphene / activated carbon composite material described in the above technical solution, comprising the following steps: Magnesium powder and magnesium citrate are manually premixed and then placed in an air jet mill. The powder in the mixture is fully dispersed under the shearing and turbulent action of the high-speed airflow to obtain a uniform mixed powder A. The uniform mixed powder A is placed in a sealed 20 L reaction vessel, and pure CO2 gas is introduced. After being electrically heated by a nickel-chromium wire, a high-temperature self-propagating reaction is initiated. After several seconds, the mixture is naturally cooled for 10-20 minutes to obtain the precursor powder product B. The precursor powder product B is then acid-washed, filtered, and freeze-dried to obtain the graphene / activated carbon composite material.

[0022] The present invention also discloses a positive electrode material for a lithium-ion capacitor, comprising the above-mentioned graphene / activated carbon composite material.

[0023] The present invention also discloses an energy storage device using the above-mentioned positive electrode material.

[0024] Beneficial effects:

[0025] This invention utilizes a high-temperature self-propagating reaction to reduce CO2 from magnesium powder, generating free carbon atoms and releasing a large amount of heat. This leads to the pyrolysis and reduction of magnesium citrate, producing porous activated carbon and magnesium oxide particles. Simultaneously, the free carbon atoms adsorb onto the surface of the magnesium oxide particles, gradually forming graphene sheets that interpenetrate and cover the gaps and surface of the activated carbon, ultimately forming a graphene / activated carbon composite material. By "anchoring" small-sized, wrinkled graphene sheets onto the activated carbon framework, aggregation is suppressed, maintaining a high effective specific surface area. Furthermore, interconnected electron channels and hierarchical pores are formed, shortening ion diffusion paths and reducing polarization. Appropriate amounts of oxygen-containing functional groups enhance wetting and introduce surface pseudocapacitance, thereby achieving a synergistic improvement in high capacity, high rate capability, and high coulombic efficiency within a wide voltage window. Moreover, the preparation method provided by this invention is simple, directly scalable, and universally applicable, and the prepared graphene / activated carbon composite material exhibits good structural stability. Attached Figure Description

[0026] Figure 1X-ray diffraction pattern of the graphene / activated carbon composite material prepared in Example 1;

[0027] Figure 2 The Raman spectrum of the graphene / activated carbon composite material prepared in Example 1;

[0028] Figure 3 The low-temperature N2 adsorption-desorption isotherm of the graphene / activated carbon composite material prepared in Example 1;

[0029] Figure 4 XPS elemental analysis diagram of the graphene / activated carbon composite material prepared in Example 1;

[0030] Figure 5 Scanning electron microscope image of the graphene / activated carbon composite material prepared in Example 1;

[0031] Figure 6 Transmission electron microscope image of the graphene / activated carbon composite material prepared in Example 1;

[0032] Figure 7 The elemental energy distribution diagram of the graphene / activated carbon composite material prepared in Example 1;

[0033] Figure 8 The graph shows the lithium storage rate performance of the graphene / activated carbon composite material prepared in Example 1. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. However, the following embodiments are only for explaining the present invention, and the scope of protection of the present invention should include all the contents of the claims. Moreover, through the description of the following embodiments, those skilled in the art can fully implement all the contents of the claims of the present invention.

[0035] The present invention provides a graphene / activated carbon composite material (MCG), comprising a carbon matrix and a two-dimensional sheet material covering the carbon matrix;

[0036] The carbon matrix is ​​an activated carbon material;

[0037] The two-dimensional sheet material is graphene.

[0038] Unless otherwise specified, all raw materials used in this invention are preferably commercially available products.

[0039] The graphene / activated carbon composite material provided by this invention includes a carbon matrix and a two-dimensional sheet-like material covering the carbon matrix, wherein the carbon matrix is ​​activated carbon material and the two-dimensional sheet-like material is graphene material. In this invention, the preparation method of the graphene / activated carbon composite material preferably includes the following steps:

[0040] Magnesium powder and magnesium citrate were manually premixed and then placed in an air jet mill. The powder in the mixture was fully dispersed under the action of air jet shear and turbulence by the action of high-speed air jet to obtain a uniform mixed powder A.

[0041] In this invention, the mass ratio of magnesium powder to magnesium citrate is preferably 1:1 to 5, and more preferably 1:5.

[0042] A homogeneous mixture of powder A is placed in a sealed 20 L reaction vessel, and pure CO2 gas is introduced. After being electrically heated by a nickel-chromium wire, a high-temperature self-propagating reaction is initiated. After a few seconds, the mixture is allowed to cool naturally for 10-20 minutes to obtain the precursor powder product B.

[0043] In this invention, the high-temperature self-propagating reaction is preferably carried out under a pure CO2 atmosphere.

[0044] In this invention, the natural cooling time is 10 to 20 minutes, more preferably 20 minutes.

[0045] The precursor powder product B was acid-washed, filtered, and spray-dried to obtain the graphene / activated carbon composite material.

[0046] In this invention, the pickling aqueous solution is dilute sulfuric acid, and the concentration of the pickling aqueous solution is preferably 2 mol / L.

[0047] In this invention, the initial temperature of the high-temperature self-propagating reaction is preferably 25 °C;

[0048] In this invention, the washing reagent is preferably water. This invention does not specify the amount or number of times the washing reagent is used, and washing is sufficient until the pH value of the washing solution is greater than 7.

[0049] In this invention, the drying method is preferably freeze-drying.

[0050] The present invention also provides the application of the graphene / activated carbon composite material described in the above technical solution or the graphene / activated carbon composite material prepared by the preparation method described in the above technical solution in the positive electrode of lithium-ion capacitor.

[0051] The present invention does not impose specific limitations on the application of the graphene / activated carbon composite material; any application method known to those skilled in the art can be used.

[0052] The following detailed description of the graphene / activated carbon composite material, its preparation method, and its application provided by the present invention, with reference to specific embodiments, should not be construed as limiting the scope of protection of the present invention.

[0053] Example 1

[0054] (1) Weigh 1 g of magnesium powder and 5 g of magnesium citrate into a 100 mL beaker and stir with a glass rod to complete the first premix. After sealing the beaker with sealing film, rotate and shake the beaker to complete the second premix. Then, transfer the sample to an air jet mill, where the powder in the mixture is fully dispersed under the action of high-speed airflow shear and turbulence to obtain a uniform mixed powder A.

[0055] (2) The uniformly dispersed mixed powder A was loaded into a graphite boat, and a nichrome wire was embedded in it for subsequent heating by current. The graphite boat was transferred to a 20 L sealed reaction vessel. The vacuum was evacuated to -0.1 MPa, and then pure CO2 gas was introduced to 0.2 MPa. This cycle was repeated three times. Then, 0.4 MPa of CO2 gas was introduced into the sealed reaction vessel to participate in the reaction. A 5 A current was introduced into the nichrome wire through an external program. After 3 s, the pressure monitoring of the vessel showed a significant increase. After about 3-5 s of reaction time, the pressure stopped rising, and the current switch was immediately turned off. The pressure rise was about 1 MPa. After natural cooling for 20 min, the precursor powder product B was removed.

[0056] (3) Transfer the precursor powder B to a beaker, add an appropriate amount of deionized water, and place it on a magnetic stirrer at 1000 rpm for uniform stirring to form a slurry. Use 1.5 L of 2 mol / L dilute sulfuric acid as an acid pickling agent and slowly add it to the slurry. Then, maintain a stirring speed of 1000 rpm for 1 h, and let it stand for 12 h, then pour off the supernatant. Separate the filtrate and filter cake by negative pressure filtration, and maintain the pH value of the filter cake above 7. Finally, freeze-dry the filter cake to obtain the graphene / activated carbon composite material (MCG-5).

[0057] Example 2

[0058] MCG-1 was prepared according to the method of Example 1. The difference from Example 1 is that the amount of magnesium citrate added in step (1) was adjusted to 1 g, and the mass ratio of magnesium powder to magnesium citrate was about 1:1.

[0059] Example 3

[0060] MCG-2 was prepared according to the method of Example 1. The difference from Example 1 is that the amount of magnesium citrate added in step (1) was adjusted to 2 g, and the mass ratio of magnesium powder to magnesium citrate was about 1:2.

[0061] Example 4

[0062] MCG-3 was prepared according to the method of Example 1. The difference from Example 1 is that the amount of magnesium citrate added in step (1) was adjusted to 3 g, and the mass ratio of magnesium powder to magnesium citrate was about 1:3.

[0063] The X-ray diffraction pattern of the graphene / activated carbon composite material prepared in this embodiment is shown below. Figure 1 As shown. In this embodiment, obvious MgO characteristic peaks appeared in MCG-1 and MCG-2 of the synthesized samples, indicating the presence of MgO impurities. However, as the proportion of magnesium citrate increased, no obvious MgO characteristic peaks appeared in MCG-3 and MCG-5. This is because the main inorganic byproducts in this system come from two parts: (1) MgO generated by the magnesium thermal reduction of CO2; (2) MgO in the process of magnesium citrate pyrolysis / carbonization. The magnesium thermal reaction is exothermic, while the magnesium citrate decomposition is endothermic and produces gas. The coupling of the two causes carbon free radicals to preferentially nucleate on the surface / internal MgO of magnesium citrate, forming an "open" graphene-MgO contact interface. At the same time, the gas generated by decomposition constructs interconnected mesopores and inhibits the formation of dense carbon films, improving the accessibility and dissolution efficiency of the acid solution. As a result, MgO is no longer covered by a dense carbon layer and is difficult to dissolve, making acid washing more thorough. Furthermore, a diffraction peak corresponding to the (100) crystal plane of the graphene sheet appeared at approximately 42°, and the diffraction peak of the (100) crystal plane of the graphene sheet gradually weakened with the increase of the proportion of magnesium citrate in the raw material, which indicates a decrease in the graphitization degree of the activated carbon / graphene composite material. This change was also accompanied by a large amount of sp in the product. 3 The formation of hybrid structures corresponds to edge defects and pore structures in the sample.

[0064] The Raman spectrum of the graphene / activated carbon composite material prepared in this embodiment is shown below. Figure 2 As shown, the Raman spectra of MCG-1, MCG-2, MCG-3, and MCG-5 all exhibit D and G peaks at shifts of 1332 and 1585, respectively. The D peak represents carbon atom hybridization (i.e., sp...). 3 Type disordered carbon) and G peak sp 3 Represents graphitized carbon. MCG-1, MCG-2, MCG-3, and MCG-5 contain I... D / I G The strength ratios were 0.86, 0.9, 0.97 and 1.11, respectively, indicating that the higher the proportion of magnesium citrate in the raw materials, the higher the degree of disorder in the composite material, which is consistent with the XRD test results.

[0065] The nitrogen adsorption-desorption isotherm curve of the graphene / activated carbon composite material prepared in this embodiment is as follows: Figure 3 As shown, based on BET theory calculations, the specific surface areas of MCG-1, MCG-2, MCG-3, and MCG-5 are 448.67, 488.00, 1030.92, and 1173.49 m², respectively. 2 / g. Furthermore, according to IUPAC classification, the sample exhibits a typical Type IV isotherm, with a large H4 type hysteresis loop appearing near 0.5-1.0 P / P0, indicating the presence of abundant mesopores (2-50 nm) in the MCG. Further analysis of the pore size distribution of the sample using the DFT method revealed that the mesopore size in the MCG is concentrated between 2 and 20 nm, and the enriched average pore size of the composite material decreases with increasing magnesium citrate content.

[0066] The high-resolution X-ray photoelectron spectroscopy of the graphene / activated carbon composite material prepared in this embodiment is as follows: Figure 4 As shown, from Figure 4 It can be seen that, in addition to C and O elements, Auger peaks of Mg were also detected in the materials of MCG-1 and MCG-2, indicating the presence of Mg element impurities, which is consistent with the XRD test results. Figures 3-5 The high-resolution XPS spectrum of C 1s for MCG-5 shows peaks at 284.7 eV and 284.3 eV binding energies corresponding to the main sp1 values, respectively. 3 CC and sp 2 CC, along with -CO- at 286.18 eV, -CO= at 288.38 eV, and OC=O at 290.98 eV.

[0067] The scanning electron microscope image of the graphene / activated carbon composite material prepared in this embodiment is shown below. Figure 5 As shown, the MCG material prepared by high-temperature self-propagating reaction exhibits an irregularly shaped, large-particle microstructure. The surface of the material has a distinct porous structure with pore sizes ranging from nanometers to micrometers. Notably, as the proportion of magnesium citrate in the raw materials increases, the overall particle size gradually decreases, and the pore structure transitions from larger macropores to mesopores and micropores. This change may be closely related to the regulatory role of magnesium citrate on the precursor structure during the reaction. From the perspective of growth mechanism, magnesium citrate, as a precursor, not only provides a carbon source in the self-propagating high-temperature reaction but also acts as a template and expanding agent.

[0068] The transmission electron microscope image of the graphene / activated carbon composite material prepared in this embodiment is shown below. Figure 6As shown, the surface of the small-sized graphene sheets in MCG-5 is accompanied by abundant wrinkles. The presence of these wrinkles is not only a result of the spontaneous folding and curling of the graphene sheets at the nanoscale, but may also be closely related to the dynamic reaction environment caused by gas release during the preparation process. This wrinkled structure increases the specific surface area of ​​the material and provides more active sites, which is beneficial for improving the electrochemical performance of the material in energy storage devices. Clear and bright diffraction spots can be observed in the selected area electron diffraction (SAED) mode. These diffraction spots confirm the crystal structure of the graphene nanosheets in the MCG material, indicating that the graphene sheets have a highly ordered lattice arrangement, further verifying the effectiveness of the self-propagating high-temperature reaction.

[0069] The elemental energy distribution diagram of the graphene / activated carbon composite material prepared in this embodiment is as follows: Figure 7 As shown, from Figure 7 It can be seen that carbon and oxygen elements are uniformly distributed on the material surface, indicating the possible presence of oxygen-containing functional groups. These oxygen-containing functional groups may originate from the pyrolysis products of magnesium citrate or surface oxidation reactions. The introduction of oxygen-containing functional groups can not only improve the hydrophilicity of the material, but may also significantly enhance its energy storage performance through pseudocapacitive effects.

[0070] The electrochemical testing method involves assembling lithium half-cells to test the material performance. Graphene / activated carbon composite materials with different mass ratios are mixed with conductive carbon black and PVDF and stirred in N-methylpyrrolidone at a mass ratio of 7:2:1. The mixture is then coated onto aluminum foil and vacuum dried to obtain electrode sheets. Figure 8 For the rate performance of graphene / activated carbon composite materials, from Figure 8 It can be seen that MCG-5 exhibits specific capacities of 77.76, 72.88, 68.38, 60.95, 55.7, and 49.02 mAh / g at current densities of 0.05, 0.1, 0.2, 0.5, 1, and 2 A / g, respectively, and still maintains 73.34 mAh / g when the current density returns to 0.05 A / g. These results demonstrate that MCG-5 possesses excellent kinetic performance and high specific capacity for lithium-ion capacitor cathode applications.

[0071] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A graphene / activated carbon composite material, characterized in that, It includes a carbon matrix and a two-dimensional sheet material covering the carbon matrix; The carbon matrix is ​​an activated carbon material; The two-dimensional sheet material is graphene.

2. The graphene / activated carbon composite material according to claim 1, characterized in that, The carbon matrix is ​​an activated carbon material, which is prepared by high-temperature pyrolysis of magnesium citrate during a high-temperature self-propagating reaction.

3. The graphene / activated carbon composite material according to claim 1, characterized in that, The two-dimensional sheet material is graphene, a few-layer carbon material prepared by converting CO2 through a high-temperature self-propagating reaction. It consists of carbon atoms arranged in a honeycomb-like two-dimensional lattice.

4. The method for preparing the graphene / activated carbon composite material according to any one of claims 1 to 3, characterized in that, Includes the following steps: Magnesium powder and magnesium citrate were manually premixed and then placed in an air jet mill. The powder in the mixture was fully dispersed under the action of air jet shear and turbulence by the action of high-speed air jet to obtain a uniform mixed powder A. The homogeneous mixed powder A is placed in a sealed 20 L reaction vessel and pure CO2 gas is introduced. After being heated by conductive heating with a nickel-chromium wire, a high-temperature self-propagating reaction is initiated. After a few seconds, it is naturally cooled for 10-20 minutes to obtain the precursor powder product B. The precursor powder product B was acid-washed, filtered, and dried to obtain the graphene / activated carbon composite material.

5. The preparation method according to claim 4, characterized in that, The mass ratio of magnesium powder to magnesium citrate powder is 1:1 to 1:

5.

6. The preparation method according to claim 4, characterized in that, The initial temperature of the high-temperature self-propagating reaction is 25°C, and the duration is 3-5 seconds. The high-temperature self-propagating reaction is carried out under CO2 atmosphere conditions.

7. The preparation method according to claim 4, characterized in that, The acid washing was performed using a 2 mol / L dilute sulfuric acid solution.

8. The application of the graphene / activated carbon composite material according to any one of claims 1 to 3 or the graphene / activated carbon composite material prepared by the preparation method according to any one of claims 4 to 6 in the positive electrode of a lithium-ion capacitor.

9. A positive electrode material for a lithium-ion capacitor, characterized in that, It includes the graphene / activated carbon composite material according to any one of claims 1 to 3.

10. An energy storage device, characterized in that, The cathode material described in claim 9 is used.

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