Electrochemical preparation method of universal covalent functionalized graphene

Abstract

The invention belongs to the technical field of graphene material preparation, and particularly relates to a universal electrochemical preparation method of covalent functionalized graphene (CFG), which comprises the following steps: (1) in an oil seal state of light sealing oil, performing electrochemical intercalation treatment on a graphite electrode by using concentrated sulfuric acid; (2) after intercalation is completed, light sealing oil is injected into the electrolytic bath while concentrated sulfuric acid is discharged; (3) in the electrolytic bath, constructing a micro-liquid membrane reaction zone by using density and intermiscibility differences of three working liquids, namely light sealing oil, aqueous electrolyte and heavy sealing oil, and enabling the micro-liquid membrane reaction zone to be in contact with the intercalated graphite electrode to generate electrochemical oxidation to generate CFG; (4) controlling the micro-liquid membrane reaction area to directionally move relative to the intercalated graphite electrode, so that the whole intercalated graphite electrode is gradually and completely converted into CFG; and (5) putting the CFG structure after the reaction into a solvent, carrying out mechanical stripping to prepare a dispersion liquid of CFG, and drying to obtain CFG powder.

Description

Technical Field

[0001] The present invention belongs to the technical field of graphene material preparation, and specifically is a universal covalently functionalized graphene (CFG) electrochemical preparation method. By using this technology, the efficient preparation of CFG materials with different functional characteristics can be achieved by regulating the chemical composition of the electrolyte and the parameters of the electrochemical reaction process. Technical Background

[0002] Graphene is composed of sp 2 The two-dimensional crystalline material composed of hybrid carbon atoms with a thickness at the atomic scale has a unique two-dimensional single atomic layer conjugated structure and various excellent properties, such as high carrier mobility, high specific surface area, high light transmittance, high thermal conductivity, high mechanical strength, etc., which makes it have broad application prospects in energy storage, microelectronic devices, biomedicine, gas adsorption and separation, etc. However, the hexagonal benzene ring structure of graphene also determines its high chemical stability and surface inertness. At the same time, the strong π-π interaction between the sheets makes it easy to restack, so the dispersion of graphene in water and common organic solvents is extremely poor, which is the main reason for the difficulties in graphene in macroscopic applications. Therefore, graphene usually needs to use chemical grafting functional groups or physical adsorption of small molecules to improve its affinity with solvents or other materials. This new material with obvious differences in properties from intrinsic graphene is called functionalized graphene, and the functionalized graphene connected to the graphene carbon lattice by covalent bonds is called CFG. Graphene oxide (GO) is the most common and currently the most widely used CFG. Its structure is a large number of oxygen-containing functional groups such as hydroxyl, epoxy, and carboxyl grafted onto the graphene carbon plane. The presence of these functional groups makes GO hydrophilic, easy to disperse and assemble, and is the main unit of graphene macroscopic applications (such as thermal conductive films, transparent conductive films, water treatment materials, and biomaterials). The presence of oxygen-containing functional groups makes GO highly reactive, and its application field can be expanded through further chemical reactions to graft active substances. Fluorinated graphene is to partially or completely fluorinate the carbon atoms in graphene to make sp 2 Hybridized carbon atoms are converted to sp 3Hybrid morphology, but maintains a planar two-dimensional structure, so it is also called "two-dimensional polytetrafluoroethylene"; fluorinated graphene has physical and chemical properties that are different from graphene, such as low surface energy, strong hydrophobicity, high chemical stability, and wide band gap, and has important application value in the fields of protective coatings, wear resistance and drag reduction, and energy storage devices. The covalent functionalization of nitrogen-containing groups (amino, amide, azide, etc.) and the substitution and doping effect of nitrogen atoms on lattice carbon atoms are also another important material form for constructing CFG; the high chemical activity of nitrogen-containing groups makes it extremely useful in composite materials, and the defects brought by doped atoms can be used as active sites for various reactions, and have broad application prospects in the fields of electrocatalysis, electrochemical energy storage, and gas sensing. Therefore, CFG is the main material form for graphene to achieve diversified functional applications and has important application value.

[0003] At present, the preparation of CFG is mainly based on a small amount of laboratory research. Among them, the preparation of CFG with covalent functionalization of functional groups as the main form usually uses GO as the raw material to achieve the replacement of functional group types through chemical reactions. However, due to the instability of GO's own chemical properties, CFG generated by the replacement reaction usually has problems such as low degree of functionalization, poor controllability of functional group types, low reaction yield, and high impurity content; in addition, some functionalization reagents, such as fluorine gas or XeF2, have harsh reaction conditions and high reagent toxicity. The preparation of doped CFG represented by nitrogen-doped graphene usually requires high temperature or newly generated carbon atom structure (for example: chemical vapor deposition) to obtain, so the preparation is more difficult, resulting in the slow development of CFG preparation and application technology.

[0004] The preparation of CFG by electrochemical method is an important direction of development in this field in recent years. The high activity and high selectivity of electrochemical reaction are considered to be an effective way to solve the above problems. It also has the advantages of high controllability, high environmental friendliness and high safety. The academic paper "One-step electrochemical strategy for in-situ synthesis of S, N-codoped graphene as metal-free catalyst for oxygen reduction reaction" (Carbon 2018, 134: 316-325.) reported a method for preparing S, N co-doped CFG by cyclic voltammetry electrolysis of melamine and (NH4)2SO4 aqueous solution, which achieved the functionalization of two elements at one time. Its ORR performance is even better than that of Pt / C system, but its low stripping rate caused by too small specific surface area also limits its further application. The academic paper "Electrochemically Scalable Production of Fluorine-Modified Graphene for Flexible and High-Energy Ionogel-Based Microsupercapacitors" (Journal of the American Chemical Society 2018, 140 (26): 8198-8205.) reported the technology of preparing fluorinated graphene by electrolysis using NaBF4 as an electrolyte, but its doping rate was only 3 at.% (atomic percentage). In terms of GO preparation, a large number of experimental studies have attempted to prepare GO using electrochemical processes, but there are always problems of low oxidation and insufficient oxidation, and it is impossible to effectively obtain a high oxidation GO product equivalent to the chemical oxidation method. The inventor's previous technical invention "A method for continuous preparation of graphene oxide microsheets" proposed a patented technology of "two-step electrochemical intercalation-oxidation" (publication number: CN107215867A), which can achieve a high degree of oxidation, but there is still a problem of high content of unoxidized graphite impurities in the reaction product, that is, the uniformity and sufficiency of the electrochemical oxidation treatment are insufficient; and the presence of impurities will have a significant adverse effect on the application of functionalized graphene such as graphene oxide, and therefore it is urgently needed to be improved. Summary of the invention

[0005] In order to solve the technical difficulties existing in the technology of preparing CFG by electrochemical method, the purpose of the present invention is to provide a universal electrochemical preparation method of covalently functionalized graphene, which has made major technical improvements on the basis of the technology of preparing graphene oxide by two-step electrochemical reaction. Through the technical scheme of oil sealing and constructing micro-liquid membrane reaction zone, the control of "water", a key influencing factor in the process of preparing CFG by electrochemical reaction is achieved, and the problem of insufficient oxidation uniformity and sufficiency in the technology of preparing graphene oxide by electrochemical oxidation is effectively solved.

[0006] The technical solution of the present invention is:

[0007] A universal electrochemical preparation method for covalently functionalized graphene uses four working liquids with different properties, including: working liquid 1 light sealing oil, working liquid 2 aqueous electrolyte, working liquid 3 heavy sealing oil, working liquid 4 concentrated sulfuric acid. The specific process is as follows:

[0008] (1) In the state of light sealing oil oil sealing, the graphite electrode is electrochemically intercalated with concentrated sulfuric acid to achieve a first-order intercalation state;

[0009] (2) After the intercalation is completed, light sealing oil is injected into the electrolytic cell while the concentrated sulfuric acid is discharged, and it is ensured that the intercalated graphite is always immersed in the light sealing oil and does not come into contact with the ambient air;

[0010] (3) In the electrolytic cell, a micro-liquid membrane reaction zone is constructed by utilizing the density and solubility differences of three working liquids, namely, light sealing oil, aqueous electrolyte, and heavy sealing oil; when the first-order intercalated graphite electrode contacts the micro-liquid membrane under power-on conditions, an electrochemical reaction will occur in the contact area, converting the carbon lattice in the graphite electrode in this area into a covalently functionalized graphene (CFG) structure; by controlling the relative movement between the micro-liquid membrane reaction zone and the intercalated graphite electrode, the contact reaction zone is gradually moved from one end of the intercalated graphite electrode to the other end, thereby uniformly converting the entire intercalated graphite electrode into a CFG structure; in this process, a sufficient amount of light or heavy sealing oil must be retained, and by increasing the amount of light sealing oil or heavy sealing oil, it is ensured that the intercalated graphite electrode is always immersed in oil and does not contact with air; at the same time, the aqueous electrolyte is supplemented to avoid changes in the thickness of the liquid membrane due to the consumption of the aqueous electrolyte reaction;

[0011] (4) The CFG structure after the reaction is placed in a solvent for mechanical exfoliation to prepare a CFG dispersion, and the CFG powder is obtained after drying.

[0012] In the universal electrochemical preparation method of covalently functionalized graphene, the working liquid 1 is a light sealing oil, including but not limited to liquid paraffin, mineral oil or petroleum ether, which has the common characteristics of being poorly soluble or insoluble in water and having a density of less than 1g / cm 3; Working liquid 2 aqueous electrolyte is an aqueous solution of one or more combinations of various organic or inorganic electrolytes, which generates active free radicals related to the electrolyte components through electrode reactions during the electrochemical reaction; Working liquid 3 heavy sealing oil includes but is not limited to a combination of one or more solvents selected from carbon disulfide, carbon tetrachloride, and carbon trichloride, and its common characteristics are that it is poorly soluble or insoluble in water and has a density greater than 1.1 g / cm 3 ; Working liquid 4 concentrated sulfuric acid refers to a sulfuric acid-water mixture, a sulfuric acid-organic solvent mixture, pure sulfuric acid or fuming sulfuric acid with a sulfuric acid mass fraction of more than 70%.

[0013] The universal electrochemical preparation method of covalently functionalized graphene, in step (1), the graphite electrode includes but is not limited to flexible graphite paper, electrode graphite rod, graphite plate, graphite powder press, pyrolytic graphite or artificial graphite; the oil seal refers to a layer of light sealing oil layer covering the surface of the electrolytic cell after concentrated sulfuric acid is added to the electrolytic cell, and the thickness of the sealing oil is not less than the thickness of a single molecule oil film.

[0014] The universal electrochemical preparation method of covalently functionalized graphene, in step (1), the electrochemical intercalation treatment refers to a process in which a graphite electrode is used as an anode and an inert electrode is used as a cathode, and the graphite electrode is immersed in concentrated sulfuric acid and energized to perform an electrochemical reaction, so that sulfuric acid molecules and ions are inserted into the graphite electrode; the electrochemical intercalation reaction is carried out in a constant voltage mode, the external power supply voltage required for the intercalation is 0.1V to 5V, the time required for the electrochemical intercalation is related to the mass of the graphite electrode, and the time required for completing the intercalation of a graphite electrode with a unit mass of 1g is 1s to 1h.

[0015] In the universal electrochemical preparation method of covalently functionalized graphene, in step (1), the first-order intercalation state is an intercalation state of graphite intercalation compounds, which means that a layer of sulfuric acid molecules / ions is uniformly inserted between any two graphite atomic layers in terms of microstructure, which is the limit state of intercalation of the graphite crystal structure, and macroscopically makes the graphite electrode appear blue.

[0016] In the universal electrochemical preparation method of covalently functionalized graphene, in step (3), the micro-liquid membrane refers to an aqueous electrolyte liquid membrane sandwiched between a light sealing oil and a heavy sealing oil, and the liquid membrane thickness is 1 μm to 50 cm.

[0017] The universal electrochemical preparation method of covalently functionalized graphene, in step (3), the power-on condition refers to the intercalated graphite electrode as one electrode and the inert electrode as the first electrode, which are respectively connected to the positive electrode or the negative electrode of the external power supply and are simultaneously inserted into the electrolyte micro-liquid membrane to form a conductive loop, so that an electrochemical functionalization reaction occurs in the area where the intercalated graphite electrode contacts the micro-liquid membrane, and the electrochemical functionalization reaction is carried out under constant voltage or constant current mode; in constant voltage mode, the external power supply voltage range required for the electrochemical reaction is 0.1V to 500V; in constant current mode, the current density range required for the electrochemical reaction is 1mA / m 2 ~10A / mm 2 The time required for the electrochemical functionalization reaction is related to the volume of the intercalated graphite electrode immersed in the micro-liquid membrane area, with a unit volume of 1 mm 3 The time required for the intercalated graphite electrode to complete the electrochemical reaction is 0.1s to 10min.

[0018] In the universal electrochemical preparation method of covalently functionalized graphene, in step (3), the relative movement between the micro-liquid membrane reaction zone and the intercalated graphite electrode is controlled by one of the following methods: (1) the relative position of the micro-liquid membrane in the electrolytic cell is kept unchanged, and the intercalated graphite electrode sheet moves upward or downward to achieve relative movement; (2) the relative position of the intercalated graphite electrode sheet in the electrolytic cell is kept unchanged, and the liquid level of the lower heavy sealing oil is controlled to make the micro-liquid membrane move upward or downward to achieve relative movement; (3) the intercalated graphite electrode sheet and the micro-liquid membrane perform synchronous or asynchronous reverse movement through operations similar to the above process to achieve relative movement; the speed range of the relative movement is 0.1 mm / min to 10 cm / s.

[0019] In the universal electrochemical preparation method of covalently functionalized graphene, in step (3), the aqueous electrolyte is added to the micro-liquid membrane area by one of the following methods: (1) the aqueous electrolyte is directly added to the uppermost light sealing oil at a certain speed by dripping, and the dripped electrolyte will spontaneously sink and flow into the micro-liquid membrane layer to replenish the consumption of the micro-liquid membrane layer; (2) the aqueous electrolyte is injected from the lower part of the micro-liquid membrane into the area occupied by the heavy sealing oil through a pipeline at a certain speed, and the injected electrolyte will spontaneously rise and flow into the micro-liquid membrane layer to replenish the consumption of the micro-liquid membrane layer; (3) the aqueous electrolyte is directly added into the micro-liquid membrane area through a pipeline connecting the micro-liquid membrane area; the speed of adding the aqueous electrolyte is related to the reaction consumption speed of the micro-liquid membrane, and the speed control range includes but is not limited to 1μL / h~1L / s.

[0020] The universal electrochemical preparation method of covalently functionalized graphene, in step (4), the solvent includes but is not limited to water, ethanol, acetone, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, a mixture of one or more of fluorine solvents and surfactant solutions of different solvents; the mechanical exfoliation method includes but is not limited to ultrasonic crushing, ball milling, high shear crushing, high pressure homogenization, sand milling, mechanical grinding and other technologies, one or more methods of step-by-step treatment; the drying method includes but is not limited to freeze drying, vacuum drying, microwave drying, spray drying and one or more methods of step-by-step treatment.

[0021] The principle of the present invention is:

[0022] The inventors have discovered that the essence of using electrochemical reactions to achieve covalent functionalization of graphite materials is a direct chemical reaction between the active free radicals generated by the reactants in the electrolyte during the electrode reaction and the highly active charged carbon planes, so that the free radicals adsorbed on the surface of the graphite electrode are converted into functional groups that are bound to the carbon planes through covalent bonds.

[0023] Electrochemical reactions have the characteristics of supporting reactions through energy input from external power sources and high electrode potentials, which enable chemical reactions that are difficult to carry out under normal conditions to be realized on the electrode surface. Most molecules or ions of organic or inorganic electrolytes can generate free radicals of related structures under the stimulation of external power sources. Their multi-electron or electron-deficient states make it easier for them to react with substances in the opposite electrochemical state to generate new compounds. However, the reaction activity time of free radicals is usually short (nanoseconds to microseconds), so the corresponding substances that react with them are also required to have high reactivity. Graphite is a highly chemically inert substance, extremely stable at room temperature, and difficult to react chemically. The reactivity of graphite can be improved by intercalating sulfuric acid to a first-order intercalation state. When sulfuric acid intercalated graphite is used as an electrolytic electrode, its reactivity can be further improved. Therefore, a sufficient and stable first-order intercalation state is a necessary condition for realizing efficient electrochemical functionalization of graphite electrodes.

[0024] However, the first-order sulfuric acid intercalation state of graphite is extremely sensitive to water in the environment. When the first-order intercalated graphite contacts moist air (relative humidity>50%) or is immersed in an aqueous electrolyte, rapid deintercalation occurs, increasing the intercalation order to more than 2 orders, which will lead to a sharp decrease in the reactivity of the graphite electrode. Under the microscopic state, the graphite carbon plane in the deintercalation state will not be effectively grafted with functional group structures through electrochemical reactions, which is the main reason why conventional electrochemical preparation technology is difficult to effectively prepare CFG.

[0025] The main feature of the technical solution proposed by the present invention is that it can effectively reduce the adverse effects of water in the environment (air environment and electrolyte environment) on the electrochemical functionalization reaction of graphite, so that the chemical reaction between the active free radicals and the activated carbon plane is effectively realized, ensuring the efficient and controllable preparation of CFG. Under the condition of effectively eliminating the adverse effects of water, the technical solution proposed by the present invention is universal for the preparation of different types of CFG.

[0026] The technical solution of the present invention mainly eliminates the influence of water in the environment through the following key settings, specifically including: (1) isolating the contact between the first-order intercalation graphite electrode and the water in the air through the upper layer of sealing oil, thereby ensuring that it is in a stable first-order intercalation state before participating in the electrochemical reaction; (2) effectively controlling the contact area between the aqueous electrolyte and the intercalation graphite electrode through the construction of a micro-liquid membrane, thereby slowing down the speed of water absorption and deintercalation of the intercalation graphite electrode, making the expansion speed of the electrochemical functionalization reaction area greater than the expansion speed of the water absorption and deintercalation area, thereby achieving uniform and sufficient electrochemical functionalization; (3) using the lower layer of sealing oil to exclude moisture from the reacted graphite electrode area, thereby inhibiting the diffusion of moisture from the reacted area to the reaction area and the unreacted area, and further reducing the adverse effect of water on the electrochemical functionalization reaction.

[0027] The advantages and beneficial effects of the present invention are as follows:

[0028] 1. The technology of the present invention can realize efficient, sufficient and uniform preparation of graphene oxide.

[0029] 2. The functionalized graphene prepared by the technology of the present invention has high selectivity, high product purity and few by-products.

[0030] 3. The present invention has excellent versatility and ease of use. Different types of functionalized graphene can be prepared by selecting electrolytes in aqueous electrolytes and regulating corresponding electrochemical reaction parameters (voltage, current density, relative movement speed of micro-liquid membrane and intercalated graphite electrode).

[0031] 4. The present invention has extremely low acid and water consumption and has significant environmental advantages compared to chemical functionalization treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The micro-liquid membrane layered structure based on four typical working liquids in Example 1 (left) and the electrochemical oxidation reaction state of the graphite electrode intercalated by micro-liquid membrane electrolysis (right). Working liquid 1: paraffin oil; working liquid 2: 30wt.% sulfuric acid aqueous solution; working liquid 3: CCl4; working liquid 4: industrial sulfuric acid with a concentration of 98wt.%.

[0033] Figure 2. Schematic diagram of the process of preparing graphite intercalation compounds (GIC) by electrolytic intercalation of flexible graphite paper strips in the first step of preparing CFG using the two-step micro-liquid membrane electrolysis preparation technology in Example 1.

[0034] Figure 3 . Schematic diagram of the second step of preparing CFG using the two-step micro-liquid membrane electrolysis preparation technology in Example 1: realizing the up and down movement process of the micro-liquid membrane reaction zone by regulating the relative amounts of working liquid 1 and working liquid 3 during the electrolytic oxidation process.

[0035] Attached Figure 1-3 The numbers in the figure refer to the names of the substances: ① Working liquid 1: upper sealing oil (paraffin oil), ② Working liquid 2: aqueous electrolyte (30wt.% sulfuric acid aqueous solution, electrolyte liquid membrane reaction zone), ③ Working liquid 3: lower sealing oil (CCl4), ④ Working liquid 4: concentrated sulfuric acid, ⑤ cathode electrode rod, ⑥ GIC electrode (before reaction), ⑦ Reaction product: CFG strip (after reaction), ⑧ electrolytic oxidation reaction zone, ⑨ graphite electrode conductive fixture, ⑩ external power supply, ⑾ lower sealing oil injection port, ⑿ upper sealing oil overflow port, ⒀ electrolytic anode. DETAILED DESCRIPTION

[0036] During the specific implementation process, four working liquids with different properties are required, including: light sealing oil (working liquid 1), aqueous electrolyte (working liquid 2), heavy sealing oil (working liquid 3), and concentrated sulfuric acid (working liquid 4).

[0037] Working fluid 1 Light sealing oil includes but is not limited to liquid paraffin, mineral oil, petroleum ether, etc., which is characterized by a density of less than 1g / cm 3 , a non-polar solvent that is immiscible with water and has a dielectric constant of less than 5. It can maintain its chemical stability for a long time after contact with concentrated sulfuric acid or dilute sulfuric acid.

[0038] The types of working liquid 2 aqueous electrolyte include but are not limited to aqueous solutions of one or more combinations of various organic or inorganic electrolytes, and its function is to generate active free radicals related to electrolyte components through electrode reactions during the electrochemical reaction.

[0039] The types of working liquid 3 heavy sealing oil include but are not limited to a combination of one or more solvents of carbon disulfide, carbon tetrachloride, and carbon trichloride, and their common feature is that the density is greater than 1.1g / cm 3 , a non-polar solvent that is immiscible with water and has a dielectric constant of less than 10. It can maintain its chemical stability for a long time after contact with concentrated or dilute sulfuric acid.

[0040] Working liquid 4 concentrated sulfuric acid refers to a sulfuric acid-water mixture with a sulfuric acid mass fraction of more than 70%, a sulfuric acid-organic solvent mixture, pure sulfuric acid, and a sulfuric acid / sulfur trioxide mixture (fuming sulfuric acid).

[0041] The present invention provides a universal electrochemical preparation method of covalently functionalized graphene, comprising the following steps:

[0042] 1. In the state of light sealing oil oil sealing, the graphite electrode is electrochemically intercalated with concentrated sulfuric acid to achieve the state of first-order intercalation.

[0043] Graphite electrodes include but are not limited to flexible graphite paper, electrode graphite rods, graphite plates, graphite powder presses, pyrolytic graphite, artificial graphite, etc. Their common characteristics are that they have a layered graphite crystal structure in their microstructure and a volume conductivity of not less than 0.1S / cm.

[0044] Oil seal refers to a layer of light sealing oil layer covering the surface of the electrolytic cell after adding concentrated sulfuric acid to the electrolytic cell. The fluidity and hydrophobicity of the light sealing oil layer are used to prevent the concentrated sulfuric acid in the electrolytic cell and the graphite electrode from contacting and absorbing moisture in the air. The thickness of the sealing oil is not less than the thickness of a single molecule oil film (≥0.5nm).

[0045] Electrochemical intercalation treatment refers to the process of using an external power source to immerse a graphite electrode as an anode and an inert electrode as a cathode in concentrated sulfuric acid and pass electricity to perform an electrochemical reaction, so that sulfuric acid molecules and ions are inserted into the interior of the graphite electrode.

[0046] The electrochemical intercalation reaction is carried out under constant voltage mode. The external power supply voltage required for intercalation is 0.1V to 5V, and the preferred voltage range is 0.8V to 2V. The time required for electrochemical intercalation is related to the mass of the graphite electrode. The time required to complete the intercalation of a unit mass (1g) of graphite electrode is 1s to 1h.

[0047] The first-order intercalation state is an intercalation state of graphite intercalation compounds. Microscopically, it means that a layer of sulfuric acid molecules / ions is uniformly inserted between any two layers of graphite atoms. It is the limit state of intercalation in the graphite crystal structure. Macroscopically, it can make the graphite electrode appear blue.

[0048] 2. After the intercalation is completed, inject light sealing oil into the electrolytic cell while discharging concentrated sulfuric acid, and ensure that the intercalated graphite is always immersed in the light sealing oil and does not come into contact with the ambient air.

[0049] 3. In the electrolytic cell, a micro-liquid membrane reaction zone is constructed by utilizing the density and solubility differences of three working liquids, namely, light sealing oil, aqueous electrolyte, and heavy sealing oil. When the first-order intercalated graphite electrode contacts the micro-liquid membrane under power-on conditions, an electrochemical reaction will occur in the contact area, converting the carbon lattice in the graphite electrode in this area into a CFG structure. By controlling the relative movement between the micro-liquid membrane reaction zone and the intercalated graphite electrode, the contact reaction zone is gradually moved from one end of the intercalated graphite electrode to the other end, thereby uniformly converting the entire intercalated graphite electrode into a CFG structure. In this process, a sufficient amount of light or heavy sealing oil must be retained, and the amount of light sealing oil or heavy sealing oil must be increased to ensure that the intercalated graphite electrode is always immersed in the oil and does not contact the air. At the same time, the aqueous electrolyte is replenished at a certain speed to avoid changes in the thickness of the liquid membrane due to the consumption of the aqueous electrolyte reaction.

[0050] The micro-liquid membrane refers to an aqueous electrolyte liquid membrane sandwiched between a light sealing oil and a heavy sealing oil, and the liquid membrane thickness is 1 μm to 50 cm, and the preferred liquid membrane thickness is 500 μm to 5 cm.

[0051] The power-on condition refers to the intercalated graphite electrode as one electrode and the inert electrode as the second electrode, which are respectively connected to the positive or negative electrode of an external power source and inserted into the electrolyte micro-liquid membrane at the same time to form a conductive circuit, so that an electrochemical functionalization reaction occurs in the area where the intercalated graphite electrode contacts the micro-liquid membrane.

[0052] The electrochemical functionalization reaction can be carried out in constant voltage or constant current mode. In constant voltage mode, the external power supply voltage range required for the electrochemical reaction is 0.1V to 500V, and the preferred voltage range is 3V to 20V; in constant current mode, the current density range required for the electrochemical reaction is 1mA / m 2 ~10A / mm 2 , the preferred current density range is: 1A / m 2 ~20A / m 2 The time required for the electrochemical functionalization reaction is related to the volume of the intercalated graphite electrode immersed in the micro-liquid membrane area, per unit volume (1mm 3 )The time required for the intercalated graphite electrode to complete the electrochemical reaction is 0.1s to 10min.

[0053] The relative movement between the micro-liquid membrane reaction zone and the intercalated graphite electrode can be controlled by one of the following methods or not limited to: (1) Keeping the relative position of the micro-liquid membrane in the electrolytic cell unchanged, the intercalated graphite electrode sheet moves upward or downward to achieve relative movement; (2) Keeping the relative position of the intercalated graphite electrode sheet in the electrolytic cell unchanged, the micro-liquid membrane moves upward or downward by controlling the liquid level of the lower heavy sealing oil to achieve relative movement; (3) The intercalated graphite electrode sheet and the micro-liquid membrane perform synchronous or asynchronous reverse movement through operations similar to the above process to achieve relative movement. The speed range of relative movement is 0.1mm / min to 10cm / s.

[0054] The replenishment of aqueous electrolyte to the micro-liquid membrane area may be carried out by one of the following methods or not limited to: (1) directly adding the aqueous electrolyte to the top layer of light sealing oil at a certain speed by dripping, the dripped electrolyte will spontaneously sink and flow into the micro-liquid membrane layer, replenishing the consumption of the micro-liquid membrane layer; (2) injecting the aqueous electrolyte from the bottom of the micro-liquid membrane into the area occupied by the heavy sealing oil through a pipeline at a certain speed, the injected electrolyte will spontaneously rise and flow into the micro-liquid membrane layer, replenishing the consumption of the micro-liquid membrane layer; (3) directly replenishing the aqueous electrolyte into the micro-liquid membrane area through a pipeline connecting the micro-liquid membrane area.

[0055] The speed of replenishing the aqueous electrolyte is related to the reaction consumption rate of the micro-liquid membrane, and the speed control range includes but is not limited to 1 μL / h to 1 L / s.

[0056] 4. The CFG flake structure after the reaction is placed in a solvent for mechanical exfoliation to prepare a CFG dispersion, and the CFG powder can be obtained after drying.

[0057] The solvent includes, but is not limited to, a mixture of one or more of water, ethanol, acetone, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, fluorine solvents, etc., and surfactant solutions of different solvents, etc. The common feature is that CFG can form a dispersion with a higher concentration in the solvent and has good stability.

[0058] The mechanical stripping method includes, but is not limited to, one or more methods of ultrasonic pulverization, ball milling, high shear pulverization, high pressure homogenization, sand milling, mechanical grinding and other processing techniques in a step-by-step manner.

[0059] The drying treatment method includes, but is not limited to, freeze drying, vacuum drying, microwave drying, spray drying, etc., one or more methods are performed in steps.

[0060] The present invention is further described in detail below through examples and drawings.

[0061] Example 1

[0062] The product objective of this embodiment is to prepare graphene oxide (GO) slurry and powder with a high degree of oxidation.

[0063] As attached Figure 1 As shown in the left figure, four working liquids are used in this embodiment: paraffin oil (working liquid 1), 30wt.% sulfuric acid aqueous solution (working liquid 2), CCl4 (working liquid 3) and 98wt.% industrial sulfuric acid (working liquid 4). The four working liquids form four layers of liquid films distributed in parallel up and down due to differences in density and compatibility.

[0064] The electrochemical reaction process of preparing GO is divided into two steps, namely the first step: electrolytic intercalation process (attached Figure 2 ) and the second step: electrolytic oxidation process (attached Figure 3 ).

[0065] In the first step of electrolytic intercalation, Figure 2 As shown in state 1, concentrated sulfuric acid ④ is injected into the electrolytic cell to occupy most of the height of the electrolytic cell, and then a layer of paraffin oil ① with a thickness of 3 mm is added on the upper surface of the concentrated sulfuric acid to isolate the concentrated sulfuric acid from the air. A flexible graphite paper strip that has been fully dried is used as an electrolytic anode ⒀ and an inert cathode electrode rod ⑤ (platinum electrode rod) are inserted into the electrolytic cell filled with the electrolyte; in order to ensure sufficient electrical contact, the flexible graphite paper strip needs to be clamped at its top with a graphite electrode conductive clamp ⑨, and then connected to the positive electrode of the external power supply ⑩; the upper end of the cathode electrode rod ⑤ is connected to the negative electrode of the external power supply ⑩. Adjust the external power supply voltage to a constant 1.8V, then turn on the external power supply and perform sulfuric acid intercalation treatment on the electrolytic anode ⒀ (flexible graphite paper strip) for 10min to 30min to achieve the first-order intercalation state (state 2); at this time, the part of the flexible graphite paper strip immersed in concentrated sulfuric acid has been converted into the first-order intercalated GIC electrode ⑥ (anode intercalated graphite paper strip), which will act as an anode to participate in the second-step electrolytic oxidation reaction.

[0066] Since concentrated sulfuric acid is no longer required in the second step of the electrolytic oxidation reaction, it is necessary to discharge the concentrated sulfuric acid from the electrolytic cell after the electrolytic intercalation is completed. In order to ensure the stability of the intercalation state of the GIC electrode, it is necessary to add paraffin oil to the electrolytic cell while discharging the concentrated sulfuric acid to ensure that the GIC electrode is immersed in the paraffin oil without contacting the air during the entire process. Usually, the above effect can be achieved by ensuring that the discharge speed of the concentrated sulfuric acid is the same as the injection speed of the paraffin oil. The state after the concentrated sulfuric acid is completely discharged is as shown in the attached figure. Figure 2 As shown in state 3. At this point, the first step of the electrolytic intercalation process is completed.

[0067] In the second step of electrolytic oxidation, Figure 3As shown, before the electrolytic oxidation reaction begins (time point: t0), there are three kinds of working liquids in the electrolytic cell, namely: paraffin oil ① that completely immerses the GIC electrode ⑥, a 5mm thick dilute sulfuric acid electrolyte layer (30wt.% sulfuric acid aqueous solution ②, liquid level height is h0) that exists below the GIC electrode ⑥ but does not contact the GIC electrode ⑥, and CCl4③ below the dilute sulfuric acid electrolyte layer. In order to construct the above-mentioned multi-layer liquid film structure and realize the control of the height of the dilute sulfuric acid electrolyte layer, it is necessary to set the liquid inlet and outlet of the electrolytic cell used for the reaction, as shown in FIG. Figure 3 As shown, a lower sealing oil injection port ⑾ is added at the bottom of the electrolytic cell to inject the working liquid from the bottom; at the same time, an upper sealing oil overflow port ⑿ is added at the top side of the electrolytic cell at a height equivalent to the liquid level to discharge the excess paraffin oil caused by the injection of CCl4. With this electrolytic cell, the height of the dilute sulfuric acid electrolyte layer can be increased by injecting CCl4, and the rising speed can be controlled by the injection speed of CCl4, while keeping the GIC electrode always immersed in paraffin oil without contact with air.

[0068] During the preparation operation, first turn on the external power supply and adjust the output voltage to 3.5V. At this time, since there is no electrolyte connection between the GIC electrode and the inert electrode (cathode electrode rod ⑤), the electrolysis circuit is not conductive; when CCl4 is injected ③ to raise the liquid level of the dilute sulfuric acid electrolyte layer to contact with the bottom of the GIC electrode ⑥, the electrolysis circuit is conductive, and the part of the electrolyte film in contact with the GIC electrode ⑥ begins to produce an electrochemical reaction, oxidizing this part of the GIC electrode into graphite oxide to form CFG graphite oxide strips ⑦; and as the electrolyte film gradually rises, the area where the GIC electrode has been in contact with the electrolyte film will gradually be completely oxidized into graphite oxide, and the intermediate state is as shown in the attached Figure 1 The right picture and attached Figure 3 At time points t1 and t2, the electrolyte film rises at a rate of 0.1 mm / s. During the reaction, due to the consumption of dilute sulfuric acid electrolyte, 30 wt% dilute sulfuric acid is added to the top layer of light sealing oil at a rate of 0.01 mL / min. The added electrolyte will spontaneously sink and flow into the micro-liquid film layer to replenish the consumption of the micro-liquid film layer.

[0069] When the electrolyte film rises to the height of the upper sealing oil overflow port ⑿, the preparation operation is completed. Figure 3The time point is shown as t3. At this time, the GIC electrode ⑥ is completely converted into the graphite oxide state, but the appearance still maintains a complete strip shape. When the length of the raw material flexible graphite paper strip is 15 cm, the time required to complete the electrolytic oxidation reaction in the electrolytic oxidation reaction zone ⑧ is 25 minutes. After that, the paraffin oil and most of the dilute sulfuric acid electrolyte are completely discharged through the upper sealing oil overflow port ⑿ above the electrolytic cell, and all CCl4 and the remaining dilute sulfuric acid electrolyte are discharged through the lower sealing oil injection port ⑾ below. After the working liquid in the electrolytic cell is emptied, the graphite oxide strip can be taken out.

[0070] The taken-out CFG oxidized graphite strip ⑦ is placed in pure water for high-speed shear stripping, acid removal and other conventional operations to obtain GO slurry. The GO slurry is freeze-dried to obtain GO powder. The obtained GO product has the characteristics of high oxidation degree (carbon-oxygen atomic ratio <1.5) and high monolayer rate (monolayer rate>98%). The yield of GO product relative to the original graphite raw material can reach 181%, and the utilization rate of graphite raw material is>95%.

[0071] Example 2

[0072] The product goal of this embodiment is to prepare nitrogen-doped graphene (NG) slurry and powder with high nitrogen content.

[0073] This embodiment adopts the same preparation device and process flow as that of embodiment 1. The main technical differences are the selection of working liquid, the setting of electrochemical reaction parameters and the regulation of micro-liquid membrane movement speed. The specific description is as follows:

[0074] Four working liquids are used in this embodiment: petroleum ether (working liquid 1), 3 mol / L CO(NH2)2+1 mol / L(NH4)2SO4 aqueous solution (working liquid 2, as electrolyte), CCl3 (working liquid 3) and industrial sulfuric acid with a concentration of 98 wt.% (working liquid 4).

[0075] In this embodiment, flexible graphite paper is used as the graphite raw material, the voltage used in the first step of electrolytic intercalation is 1.8V, and the time used to complete the electrolytic intercalation is 15min to 30min.

[0076] In the second step of electrolytic oxidation in this embodiment, the thickness of the micro-liquid membrane used is 8 mm, the voltage used is 3-5 V, the micro-liquid membrane rising rate is 0.07 mm / s, the electrolyte replenishment rate is 0.01 mL / min, and the time required to complete the second step of electrolytic oxidation reaction is 45 min.

[0077] After the second step of electrolytic oxidation is completed, the obtained nitrogen-doped graphite strip is taken out from the electrolytic cell, and ultrasonic stripping is performed using a 40wt% ethanol aqueous solution to obtain a dispersion containing sulfuric acid impurities, and then the sulfuric acid in the dispersion is removed by electrodialysis to prepare a dispersion of nitrogen-doped graphene, and NG powder material can be obtained after vacuum drying. The obtained NG product has the characteristics of high nitrogen content (>10wt.%) and high monolayer rate (monolayer rate>95%), and the yield of NG product relative to the original graphite raw material can reach 120%, and the utilization rate of graphite raw material is>90%.

[0078] Example 3

[0079] The product goal of this embodiment is to prepare sulfur-doped graphene (SG) slurry and powder with high sulfur content.

[0080] This embodiment adopts the same preparation device and process flow as that of embodiment 1. The main technical differences are the selection of working liquid, the setting of electrochemical reaction parameters and the regulation of micro-liquid membrane movement speed. The specific description is as follows:

[0081] Four working liquids are used in this embodiment: petroleum ether (working liquid 1), saturated Na2S and 1 mol / L NaHCO3 aqueous solution (working liquid 2, as electrolyte), CS2 (working liquid 3) and industrial sulfuric acid with a concentration of 98 wt.% (working liquid 4).

[0082] In this embodiment, flexible graphite paper is used as the graphite raw material, the voltage used in the first step of electrolytic intercalation is 1.8V, and the time used to complete the electrolytic intercalation is 15min to 30min.

[0083] In this embodiment, the thickness of the micro-liquid membrane used in the second step of electrolytic oxidation is 8 mm, the voltage used is 3-5 V, the micro-liquid membrane rising rate is 0.07 mm / s, the electrolyte replenishment rate is 0.01 mL / min, and the time required to complete the second step of electrolytic oxidation reaction is 45 min.

[0084] After the second step of electrolytic oxidation is completed, the obtained sulfur-doped graphite strip is taken out from the electrolytic cell, and ultrasonic stripping is performed using a 40wt% ethanol aqueous solution to obtain a dispersion containing sulfuric acid impurities, and then the sulfuric acid in the dispersion is removed by electrodialysis to prepare a dispersion of sulfur-doped graphene, and SG powder material can be obtained after vacuum drying. The obtained SG product has the characteristics of high sulfur content (>3wt.%) and high monolayer rate (monolayer rate>95%), and the yield of SG product relative to the original graphite raw material can reach 100%, and the utilization rate of graphite raw materials is>90%.

[0085] Example 4

[0086] The product goal of this embodiment is to prepare fluorinated graphene (FG) slurry and powder with a high degree of fluorination.

[0087] This embodiment adopts the same preparation device and process flow as that of embodiment 1. The main technical differences are the selection of working liquid, the setting of electrochemical reaction parameters and the regulation of micro-liquid membrane movement speed. The specific description is as follows:

[0088] Four working liquids are used in this embodiment: petroleum ether (working liquid 1), 50 wt.% hydrofluoric acid solution (working liquid 2, used as electrolyte), CCl4 (working liquid 3) and analytical grade trifluoroacetic acid (working liquid 4).

[0089] In this embodiment, flexible graphite paper is used as the graphite raw material, the voltage used in the first step of electrolytic intercalation is 2.0 V, and the time used to complete the electrolytic intercalation is 15 minutes to 30 minutes.

[0090] In this embodiment, the thickness of the micro-liquid membrane used in the second step of electrolytic oxidation is 8 mm, the voltage used is 4 V, the micro-liquid membrane rising rate is 0.05 mm / s, the electrolyte replenishment rate is 0.01 mL / min, and the time required to complete the second step of electrolytic oxidation reaction is 1 h.

[0091] After the second step of electrolytic oxidation is completed, the obtained fluorinated graphene strip is taken out from the electrolytic cell, and ultrasonic stripping is performed using an ethanol aqueous solution with a concentration of 40wt% to obtain a dispersion containing trifluoroacetic acid and hydrofluoric acid impurities, and then the impurities are cleaned by multiple centrifugation, and the product is dispersed in NMP to prepare a dispersion of fluorinated graphene, and FG powder material can be obtained after vacuum drying. The obtained FG product has the characteristics of high fluorine content (>25wt.%) and high monolayer rate (monolayer rate>85%), and the yield of the FG product relative to the original graphite raw material can reach 100%, and the utilization rate of the graphite raw material is>80%.

[0092] The implementation results show that the present invention, based on the two-step electrochemical reaction technology for preparing graphene oxide, achieves efficient controlled preparation of various CFGs including graphene oxide by regulating the composition of the electrolyte involved in the reaction and the electrical parameters of the electrochemical reaction, thereby obtaining a universal CFG electrochemical preparation technology. This technical invention is an important advancement in the field of graphene preparation technology and has great application value.

Claims

1. A universal electrochemical preparation method of covalently functionalized graphene, characterized in that: Four working liquids with different properties are used, including: working liquid 1 light sealing oil, working liquid 2 aqueous electrolyte, working liquid 3 heavy sealing oil, working liquid 4 concentrated sulfuric acid. The specific process is as follows: (1) In the state of light sealing oil oil sealing, the graphite electrode is electrochemically intercalated with concentrated sulfuric acid to achieve a first-order intercalation state; (2) After the intercalation is completed, light sealing oil is injected into the electrolytic cell while the concentrated sulfuric acid is discharged, and it is ensured that the intercalated graphite is always immersed in the light sealing oil and does not come into contact with the ambient air; (3) In the electrolytic cell, a micro-liquid membrane reaction zone is constructed by utilizing the density and solubility differences of three working liquids, namely, light sealing oil, aqueous electrolyte, and heavy sealing oil; when the first-order intercalated graphite electrode contacts the micro-liquid membrane under power-on conditions, an electrochemical reaction will occur in the contact area, converting the carbon lattice in the graphite electrode in this area into a covalently functionalized graphene (CFG) structure; by controlling the relative movement between the micro-liquid membrane reaction zone and the intercalated graphite electrode, the contact reaction zone is gradually moved from one end of the intercalated graphite electrode to the other end, thereby uniformly converting the entire intercalated graphite electrode into a CFG structure; in this process, a sufficient amount of light or heavy sealing oil must be retained, and by increasing the amount of light sealing oil or heavy sealing oil, it is ensured that the intercalated graphite electrode is always immersed in oil and does not contact with air; at the same time, the aqueous electrolyte is supplemented to avoid changes in the thickness of the liquid membrane due to the consumption of the aqueous electrolyte reaction; (4) The CFG structure after the reaction is placed in a solvent for mechanical exfoliation to prepare a CFG dispersion, and the CFG powder is obtained after drying.

2. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: Working fluid 1 Light sealing oil includes but is not limited to liquid paraffin, mineral oil or petroleum ether, which has the common characteristics of being poorly soluble or insoluble in water and having a density less than 1g / cm 3 ; Working liquid 2 aqueous electrolyte is an aqueous solution of one or more combinations of various organic or inorganic electrolytes, which generates active free radicals related to the electrolyte components through electrode reactions during the electrochemical reaction; Working liquid 3 heavy sealing oil includes but is not limited to a combination of one or more solvents selected from carbon disulfide, carbon tetrachloride, and carbon trichloride, and its common characteristics are that it is poorly soluble or insoluble in water and has a density greater than 1.1 g / cm 3 ; Working liquid 4 concentrated sulfuric acid refers to a sulfuric acid-water mixture, a sulfuric acid-organic solvent mixture, pure sulfuric acid or fuming sulfuric acid with a sulfuric acid mass fraction of more than 70%.

3. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (1), the graphite electrode includes but is not limited to flexible graphite paper, electrode graphite rod, graphite plate, graphite powder press, pyrolytic graphite or artificial graphite; the oil seal refers to a layer of light sealing oil layer covering the surface of the electrolytic cell after concentrated sulfuric acid is added to the electrolytic cell, and the thickness of the sealing oil is not less than the thickness of a monomolecular oil film.

4. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (1), the electrochemical intercalation treatment refers to a process in which a graphite electrode is used as an anode and an inert electrode is used as a cathode, and the graphite electrode is immersed in concentrated sulfuric acid and energized to perform an electrochemical reaction, so that sulfuric acid molecules and ions are inserted into the graphite electrode. The electrochemical intercalation reaction is carried out in a constant voltage mode, and the external power supply voltage required for the intercalation is 0.1V to 5V. The time required for the electrochemical intercalation is related to the mass of the graphite electrode. The time required for completing the intercalation of a graphite electrode with a unit mass of 1g is 1s to 1h.

5. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (1), the first-order intercalation state is an intercalation state of the graphite intercalation compound. Microscopically, it means that a layer of sulfuric acid molecules / ions is uniformly inserted between any two graphite atomic layers. It is the limit state of intercalation of the graphite crystal structure, and macroscopically, it makes the graphite electrode appear blue.

6. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (3), the micro-liquid membrane refers to an aqueous electrolyte liquid membrane sandwiched between light sealing oil and heavy sealing oil, and the liquid membrane thickness is 1 μm to 50 cm.

7. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (3), the power-on condition refers to the intercalated graphite electrode as one electrode and the inert electrode as the first electrode, which are respectively connected to the positive electrode or the negative electrode of the external power supply and inserted into the electrolyte micro-liquid membrane at the same time to form a conductive loop, so that an electrochemical functionalization reaction occurs in the area where the intercalated graphite electrode contacts the micro-liquid membrane, and the electrochemical functionalization reaction is carried out under constant voltage or constant current mode; in constant voltage mode, the external power supply voltage range required for the electrochemical reaction is 0.1V to 500V; in constant current mode, the current density range required for the electrochemical reaction is 1mA / m 2 ~10A / mm 2 The time required for the electrochemical functionalization reaction is related to the volume of the intercalated graphite electrode immersed in the micro-liquid membrane area, with a unit volume of 1 mm 3 The time required for the intercalated graphite electrode to complete the electrochemical reaction is 0.1s to 10min.

8. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (3), the relative movement between the micro-liquid membrane reaction zone and the intercalated graphite electrode is controlled by one of the following methods: (1) the relative position of the micro-liquid membrane in the electrolytic cell is kept unchanged, and the relative movement is achieved by the upward or downward movement of the intercalated graphite electrode sheet; (2) the relative position of the intercalated graphite electrode sheet in the electrolytic cell is kept unchanged, and the liquid level of the lower layer of heavy sealing oil is controlled to make the micro-liquid membrane move upward or downward to achieve relative movement; (3) the intercalated graphite electrode sheet and the micro-liquid membrane perform synchronous or asynchronous reverse movement through operations similar to the above process to achieve relative movement; the speed range of the relative movement is 0.1 mm / min to 10 cm / s.

9. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (3), the aqueous electrolyte is replenished into the micro-liquid membrane area by one of the following methods: (1) the aqueous electrolyte is directly added to the uppermost light sealing oil at a certain speed by dripping, and the dripped electrolyte will spontaneously sink and flow into the micro-liquid membrane layer to replenish the consumption of the micro-liquid membrane layer; (2) the aqueous electrolyte is injected from the lower part of the micro-liquid membrane into the area occupied by the heavy sealing oil through the pipeline at a certain speed, and the injected electrolyte will spontaneously rise and flow into the micro-liquid membrane layer to replenish the consumption of the micro-liquid membrane layer; (3) the aqueous electrolyte is directly replenished into the micro-liquid membrane area through the pipeline connecting the micro-liquid membrane area; the speed of replenishing the aqueous electrolyte is related to the reaction consumption rate of the micro-liquid membrane, and the speed control range includes but is not limited to 1μL / h~1L / s.

10. The universal electrochemical preparation method of covalently functionalized graphene according to claim 1, characterized in that: In step (4), the solvent includes but is not limited to water, ethanol, acetone, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, a mixture of one or more of fluorine solvents and surfactant solutions of different solvents; the mechanical exfoliation method includes but is not limited to ultrasonic crushing, ball milling, high shear crushing, high pressure homogenization, sand milling, mechanical grinding and other technologies, one or more methods of step-by-step treatment; the drying treatment method includes but is not limited to freeze drying, vacuum drying, microwave drying, spray drying and one or more methods of step-by-step treatment.

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