A general method for the electrochemical preparation of covalently functionalized graphene

Abstract

The application belongs to the technical field of graphene material preparation, and particularly relates to a general covalent functionalized graphene (CFG) electrochemical preparation method, which comprises the following steps: (1) under the condition of a light sealing oil seal, graphite electrodes are subjected to electrochemical intercalation treatment by using concentrated sulfuric acid; (2) after the intercalation is completed, light sealing oil is injected into an electrolytic cell while the concentrated sulfuric acid is discharged; (3) in the electrolytic cell, a micro-liquid film reaction zone is constructed by using the density and phase solubility difference of the light sealing oil, the aqueous electrolyte and the heavy sealing oil, and the micro-liquid film reaction zone is contacted with the intercalated graphite electrode to make the intercalated graphite electrode electrochemically oxidized to generate CFG; (4) by controlling the relative intercalated graphite electrode of the micro-liquid film reaction zone to move directionally, the whole intercalated graphite electrode is gradually and completely converted into CFG; (5) the CFG structure after the reaction is placed into a solvent to be mechanically exfoliated to prepare a CFG dispersion liquid, and the CFG powder is obtained after drying treatment.

Description

Technical Field

[0001] This invention belongs to the field of graphene material preparation technology, specifically a universal electrochemical preparation method for covalently functionalized graphene (CFG). Using this technology, CFG materials with different functionalization properties can be efficiently prepared by controlling the chemical composition of the electrolyte and the parameters of the electrochemical reaction process. Technical Background

[0002] Graphene is composed of sp 2 Two-dimensional crystalline materials composed of hybrid carbon atoms, with a thickness at the atomic scale, possess a unique two-dimensional monolayer conjugated structure and various excellent properties, such as high carrier mobility, high specific surface area, high light transmittance, high thermal conductivity, and high mechanical strength. This makes them promising for applications in energy storage, microelectronic devices, biomedicine, and gas adsorption and separation. However, graphene's hexagonal benzene ring structure also determines its high chemical stability and surface inertness. Furthermore, the strong π-π interactions between its layers make it prone to recombination, resulting in extremely poor dispersibility in water and common organic solvents. This is the main reason for the difficulties in macroscopic applications of graphene. Therefore, graphene typically requires chemical grafting of functional groups or physical adsorption of small molecules to improve its affinity with solvents or other materials. These new materials, exhibiting significant property differences from intrinsic graphene, are called functionalized graphene. Functionalized graphene, where functionalized groups are covalently bonded to the graphene carbon lattice, is called CFG. Graphene oxide (GO) is the most common and widely used type of CFG. Its structure consists of numerous oxygen-containing functional groups, such as hydroxyl, epoxy, and carboxyl groups, grafted onto the carbon planes of graphene. The presence of these functional groups gives GO hydrophilicity, making it easy to disperse and assemble, and it is currently the main building block for macroscopic applications of graphene (such as thermally conductive films, transparent conductive films, water treatment materials, and biomaterials). The presence of oxygen-containing functional groups also gives GO strong reactivity, allowing for further chemical reactions to graft active substances and expand its application areas. Fluorinated graphene is produced by partially or completely fluorinating the carbon atoms in graphene, making the sp... 2 Hybridized carbon atoms are converted to sp 3Fluorinated graphene, while exhibiting a hybrid morphology, retains a planar two-dimensional structure and is therefore also known as "two-dimensional polytetrafluoroethylene" (PTFE). Fluorinated graphene possesses physicochemical properties distinct from traditional graphene, including low surface energy, strong hydrophobicity, high chemical stability, and a wide bandgap, making it valuable for applications in protective coatings, wear resistance and drag reduction, and energy storage devices. Covalent functionalization of nitrogen-containing groups (amino, amide, azide, etc.) and the substitution doping of lattice carbon atoms by nitrogen atoms also constitute another important class of CFG materials. The high chemical reactivity of nitrogen-containing groups makes them highly effective in composite materials, while the defects introduced by doped atoms can serve as active sites for various reactions, showing broad application prospects in electrocatalysis, electrochemical energy storage, and gas sensing. Therefore, CFG is a primary material form for achieving diverse functional applications of graphene and holds significant application value.

[0003] Currently, the preparation of CFG is mainly carried out in small-scale laboratory research. CFG preparation primarily involving covalent functionalization typically uses GO as a raw material to achieve functional group substitution through chemical reactions. However, due to the inherent chemical instability of GO, CFG generated by substitution reactions often suffers from low functionalization levels, poor controllability of functional group types, low reaction yields, and high impurity content. Furthermore, some functionalizing reagents, such as fluorine or XeF2, present problems such as demanding reaction conditions and high reagent toxicity. The preparation of doped CFG, represented by nitrogen-doped graphene, usually requires high temperatures or conditions involving the formation of new carbon atom structures (e.g., chemical vapor deposition), making preparation even more difficult and resulting in relatively slow development of both CFG preparation and application technologies.

[0004] The preparation of CFG using electrochemical methods has become an important direction in this field in recent years. The high activity and selectivity of electrochemical reactions are considered an effective way to solve the above problems, while also having 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.) reports a method for preparing S,N co-doped CFG by electrolyzing an aqueous solution of melamine and (NH4)2SO4 using cyclic voltammetry. This method achieves the functionalization of both elements in one step, and its ORR performance is even better than that of the Pt / C system. However, the low stripping rate and the resulting small specific surface area limit 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.) reports a technique for preparing fluorinated graphene using NaBF4 as an electrolyte, but its doping rate is only 3 at.% (atomic percentage). Regarding GO preparation, numerous experimental studies have attempted to prepare GO using electrochemical processes, but the problems of low oxidation degree and insufficient oxidation persist, making it impossible to effectively obtain GO products with a high degree of oxidation comparable to chemical oxidation methods. The inventor's previous invention, "A Method for Continuous Preparation of Graphene Oxide Microsheets," proposed a patented "two-step electrochemical intercalation-oxidation" technology (publication number: CN107215867A), which can achieve a high degree of oxidation. However, it still suffers from the problem of a high content of unoxidized graphite impurities in the reaction products, i.e., insufficient uniformity and sufficiency of the electrochemical oxidation process. The presence of impurities will have a significant adverse impact on the application of functional graphene such as graphene oxide, and therefore urgently needs improvement. Summary of the Invention

[0005] To address the technical challenges in the electrochemical preparation of CFG, this invention aims to provide a universal electrochemical method for preparing covalently functionalized graphene. Based on the two-step electrochemical reaction for graphene oxide preparation, significant technical improvements have been made. By employing oil sealing and the construction of a micro-liquid membrane reaction zone, the key influencing factor "water" in the electrochemical CFG preparation process is controlled, effectively solving the problems of insufficient oxidation uniformity and sufficiency in the electrochemical oxidation preparation of graphene oxide.

[0006] The technical solution of this invention is:

[0007] A general electrochemical method for preparing covalently functionalized graphene employs four working liquids with different properties: working liquid 1 is a light sealing oil, working liquid 2 is an aqueous electrolyte, working liquid 3 is a heavy sealing oil, and working liquid 4 is concentrated sulfuric acid. The specific process is as follows:

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

[0009] (2) After the intercalation is completed, while draining the concentrated sulfuric acid, inject light sealing oil into the electrolytic cell and ensure 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 film reaction zone is constructed by utilizing the density and compatibility differences of three working liquids: light sealing oil, aqueous electrolyte, and heavy sealing oil. When the first-order intercalated graphite electrode comes into contact with the micro-liquid film under energized conditions, an electrochemical reaction will occur in the contact area, transforming 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 film reaction zone and the intercalated graphite electrode, the contact reaction zone gradually moves from one end of the intercalated graphite electrode to the other end, thereby uniformly transforming the entire intercalated graphite electrode into a CFG structure. During this process, a sufficient amount of light or heavy sealing oil needs to be retained. By increasing or decreasing the amount of light or heavy sealing oil, it is ensured that the intercalated graphite electrode is always immersed in the oil and does not come into contact with air. At the same time, aqueous electrolyte is replenished to avoid changes in the liquid film thickness due to the consumption of aqueous electrolyte in the reaction.

[0011] (4) The CFG structure after reaction is placed in a solvent for mechanical exfoliation to prepare a CFG dispersion, which is then dried to obtain CFG powder.

[0012] The aforementioned general-purpose electrochemical preparation method for covalently functionalized graphene uses a working liquid 1, a light sealing oil, including but not limited to liquid paraffin, mineral oil, or petroleum ether, which is characterized by being sparingly soluble or insoluble in water and having a density of less than 1 g / cm³. 3Working fluid 2 is an aqueous electrolyte, which is an aqueous solution of one or more organic or inorganic electrolytes. During the electrochemical reaction, it generates active free radicals related to the electrolyte components through electrode reactions. Working fluid 3 is a heavy sealing oil, including but not limited to one or more solvents such as carbon disulfide, carbon tetrachloride, and carbon trichloride. Its common characteristics are that it is sparingly soluble or insoluble in water and has a density greater than 1.1 g / cm³. 3 Working liquid 4 Concentrated sulfuric acid refers to sulfuric acid water mixtures, sulfuric acid organic solvent mixtures, pure sulfuric acid, or fuming sulfuric acid with a sulfuric acid mass fraction of 70% or more.

[0013] The general-purpose covalent functionalized graphene electrochemical preparation method, in step (1), the graphite electrode includes, but is not limited to, flexible graphite paper, electrode graphite rod, graphite plate, graphite powder compressible, pyrolytic graphite or artificial graphite; the oil seal refers to the addition of concentrated sulfuric acid to the electrolytic cell and then covering its surface with a light sealing oil layer, the thickness of which is not less than the thickness of a monomolecular oil film.

[0014] The general-purpose electrochemical preparation method for covalently functionalized graphene, in step (1), electrochemical intercalation refers to the process of using an external power source to immerse a graphite electrode as the anode and an inert electrode as the cathode in concentrated sulfuric acid to carry out an electrochemical reaction, so that sulfuric acid molecules and ions are inserted into the interior of the graphite electrode; the electrochemical intercalation reaction is carried out in constant voltage mode, the external power supply voltage required for intercalation is 0.1V to 5V, and the time required for electrochemical intercalation is related to the mass of the graphite electrode, the time for a unit mass of 1g of graphite electrode to complete intercalation is 1s to 1h.

[0015] In the general covalent functional graphene electrochemical preparation method, in step (1), the first-order intercalation state is an intercalation state of graphite intercalation compound. Microscopically, it refers to the uniform insertion of a layer of sulfuric acid molecules / ions between any two layers of graphite atoms. It is the limit state of graphite crystal structure intercalation and macroscopically makes the graphite electrode appear blue.

[0016] In the general covalent functionalized graphene electrochemical preparation method, in step (3), the micro liquid film refers to the aqueous electrolyte liquid film sandwiched between light sealing oil and heavy sealing oil, and its liquid film thickness is 1μm~50cm.

[0017] In the general electrochemical preparation method of covalently functionalized graphene, step (3) refers to the following: the energizing condition is that the intercalated graphite electrode is used as one electrode and the inert electrode is used as the other electrode. Both are connected to the positive or negative electrode of an external power supply, and simultaneously inserted into the electrolyte microfilm to form a conductive circuit. This allows an electrochemical functionalization reaction to occur in the area where the intercalated graphite electrode contacts the microfilm. The electrochemical functionalization reaction is carried out in constant voltage or constant current mode. In constant voltage mode, the external power supply voltage required for the electrochemical reaction is in the range of 0.1V to 500V. In constant current mode, the current density required for the electrochemical reaction is in the range of 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 microliquid membrane region, 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 general-purpose electrochemical preparation method of covalently functionalized graphene, in step (3), the relative movement between the microfluidic membrane reaction zone and the intercalated graphite electrode is controlled by one of the following methods: (1) keeping the relative position of the microfluidic membrane in the electrolytic cell unchanged, and achieving relative movement by moving the intercalated graphite electrode sheet upward or downward; (2) keeping the relative position of the intercalated graphite electrode sheet in the electrolytic cell unchanged, and achieving relative movement by controlling the liquid level of the lower layer of heavy sealing oil; (3) the intercalated graphite electrode sheet and the microfluidic membrane move synchronously or asynchronously in opposite directions through operations similar to the above process; the speed range of the relative movement is 0.1 mm / min to 10 cm / s.

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

[0020] In the general-purpose electrochemical preparation method of covalently functionalized graphene, in step (4), the solvent includes, but is not limited to, one or more of water, ethanol, acetone, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, and fluorinated solvents, as well as surfactant solutions of different solvents; the mechanical exfoliation method includes, but is not limited to, one or more of ultrasonic pulverization, ball milling, high-shear pulverization, high-pressure homogenization, sand milling, and mechanical grinding, etc., for step-by-step processing; the drying method includes, but is not limited to, one or more of freeze drying, vacuum drying, microwave drying, and spray drying, for step-by-step processing.

[0021] The principle of this invention is:

[0022] The inventors discovered that the essence of using electrochemical reactions to achieve covalent functionalization of graphite materials is that the active free radicals generated by the reactants in the electrolyte during the electrode reaction directly undergo a chemical reaction with the highly active charged carbon plane, transforming the free radicals adsorbed on the graphite electrode surface into functional groups that are covalently bonded to the carbon plane.

[0023] Electrochemical reactions, characterized by energy input from an external power source and high electrode potentials, enable chemical reactions that are difficult to occur under conventional conditions to take place on the electrode surface. Most organic or inorganic electrolyte molecules or ions can generate free radicals with relevant structures under external power stimulation. Their electron-rich or electron-deficient states make them more likely to react with substances in opposite electrochemical states, forming new compounds. However, the reaction time of free radicals is typically short (nanosecond to microsecond scale), thus requiring the corresponding substances to also possess high reactivity. Graphite is a highly chemically inert material, extremely stable at room temperature, and difficult to react chemically. However, intercalation with sulfuric acid to achieve a first-order intercalation state in graphite can improve its reactivity. Using sulfuric acid-intercalated graphite as an electrolytic electrode can further enhance its reactivity. Therefore, a sufficient and stable first-order intercalation state is a necessary condition for achieving highly 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 first-order intercalated graphite comes into contact with humid air (relative humidity > 50%) or is immersed in an aqueous electrolyte, rapid deintercalation occurs, increasing its intercalation order to above level 2. This leads to a sharp decrease in the reactivity of the graphite electrode. At the microscopic level, the deintercalated carbon plane of graphite cannot be effectively grafted with functional groups through electrochemical reactions, which is the main reason why conventional electrochemical preparation techniques are difficult to effectively prepare CFG.

[0025] The main feature of the technical solution proposed in this invention is that it can effectively reduce the adverse effects of water in the environment (air and electrolyte environment) on the electrochemical functionalization reaction of graphite, thereby enabling the effective chemical reaction between active free radicals and the active carbon plane, ensuring the efficient and controllable preparation of CFG. By effectively eliminating the adverse effects of water, the technical solution proposed in this invention is universally applicable to 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 first-order intercalated graphite electrode from water in the air by the upper sealing oil, thereby ensuring that it is in a stable first-order intercalated state before participating in the electrochemical reaction; (2) effectively controlling the contact area between the aqueous electrolyte and the intercalated graphite electrode by constructing a micro-liquid film, thereby slowing down the rate of water absorption and deintercalation of the intercalated graphite electrode, making the expansion rate of the electrochemical functionalization reaction area greater than the expansion rate of the water absorption and deintercalation area, thereby achieving uniform and sufficient electrochemical functionalization; (3) using the lower sealing oil to remove the water in the already reacted graphite electrode area, thereby inhibiting the diffusion of water from the already reacted area to the reaction area and the unreacted area, further reducing the adverse effects of water on the electrochemical functionalization reaction.

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

[0028] 1. The technology of this invention can achieve efficient, complete and uniform preparation of graphene oxide.

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

[0030] 3. This invention has excellent versatility and ease of use. Different types of functionalized graphene can be prepared by selecting the electrolyte in the aqueous electrolyte and controlling the corresponding electrochemical reaction parameters (voltage, current density, relative motion speed between the microfilm and the intercalated graphite electrode).

[0031] 4. This invention has extremely low acid and water consumption, and has significant environmental advantages compared to chemical functionalization treatment. Attached Figure Description

[0032] Figure 1 The image shows a microfluidic membrane layered structure (left) constructed based on four typical working liquids in Example 1, and an electrochemical oxidation reaction state of an intercalated graphite electrode electrolyzed using the microfluidic membrane (right). Working liquid 1: paraffin oil; working liquid 2: 30 wt.% sulfuric acid aqueous solution; working liquid 3: CCl4; working liquid 4: 98 wt.% industrial sulfuric acid.

[0033] Figure 2Schematic diagram of the first step in the preparation of CFG using the two-step microliquid membrane electrolysis technique in Example 1: preparation of graphite intercalation compound (GIC) by electrolytic intercalation of flexible graphite paper strips.

[0034] Figure 3 The second step in the preparation of CFG using the two-step microfluidic membrane electrolysis technique in Example 1 is a schematic diagram of the process of adjusting the relative amounts of working liquid 1 and working liquid 3 to achieve the up-and-down movement of the microfluidic membrane reaction zone during the electrolytic oxidation process.

[0035] Appendix Figure 1-3 The numbers in the table refer to the following substances: ① Working liquid 1: Upper sealing oil (paraffin oil), ② Working liquid 2: Aqueous electrolyte (30wt.% sulfuric acid aqueous solution, electrolyte film 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 clamp, ⑩ External power supply, ⑪ Lower sealing oil inlet, ⑫ Upper sealing oil overflow outlet, ⑬ Electrolytic anode. Detailed Implementation

[0036] In the specific implementation process, four different types of working fluids are required, including: light sealing oil (working fluid 1), aqueous electrolyte (working fluid 2), heavy sealing oil (working fluid 3), and concentrated sulfuric acid (working fluid 4).

[0037] Working fluid 1: Light sealing oils include, but are not limited to, liquid paraffin, mineral oil, petroleum ether, etc., characterized by a density of less than 1 g / cm³. 3 It is a nonpolar solvent that is immiscible with water and has a dielectric constant of less than 5. It can maintain its long-term chemical stability after contact with concentrated or dilute sulfuric acid.

[0038] The working liquid 2 contains aqueous electrolytes, including but not limited to aqueous solutions of one or more combinations of various organic or inorganic electrolytes. Its function is to generate active free radicals related to electrolyte components through electrode reactions during electrochemical reactions.

[0039] The working fluid, a heavy-duty sealing oil, includes, but is not limited to, a combination of one or more solvents such as carbon disulfide, carbon tetrachloride, and carbon trichloride, all of which share the characteristic of having a density greater than 1.1 g / cm³. 3 It is a nonpolar solvent that is immiscible with water and has a dielectric constant of less than 10. It can maintain its long-term chemical stability after contact with concentrated or dilute sulfuric acid.

[0040] Working fluid 4, concentrated sulfuric acid, refers to sulfuric acid water mixtures with a sulfuric acid mass fraction of 70% or higher, sulfuric acid organic solvent mixtures, pure sulfuric acid, and sulfuric acid / sulfur trioxide mixtures (fuming sulfuric acid).

[0041] This invention proposes a universal electrochemical preparation method for covalently functionalized graphene, comprising the following steps:

[0042] 1. Under the condition of light sealing oil, the graphite electrode is subjected to electrochemical intercalation treatment with concentrated sulfuric acid to achieve the first-order intercalation state.

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

[0044] An oil seal refers to a layer of light sealing oil applied to the surface of an electrolytic cell after concentrated sulfuric acid is added. The fluidity and hydrophobicity of this light sealing oil layer prevent the concentrated sulfuric acid and graphite electrodes from contacting each other and absorb moisture from the air. The thickness of the sealing oil is not less than the thickness of a monomolecular oil film (≥0.5 nm).

[0045] Electrochemical intercalation refers to the process of using an external power source to immerse a graphite electrode as the anode and an inert electrode as the cathode in concentrated sulfuric acid to conduct an electrochemical reaction, thereby allowing sulfuric acid molecules and ions to intercalate into the interior of the graphite electrode.

[0046] The electrochemical intercalation reaction is carried out in constant voltage mode. The external power supply voltage required for intercalation is 0.1V to 5V, with the preferred voltage range being 0.8V to 2V. The time required for electrochemical intercalation is related to the mass of the graphite electrode. The time for intercalation to be completed for 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 refers to the uniform insertion of a layer of sulfuric acid molecules / ions between any two layers of graphite atoms. It is the limiting state of intercalation in graphite crystal structure and macroscopically, it can make graphite electrodes appear blue.

[0048] 2. After intercalation is completed, while draining concentrated sulfuric acid, inject light sealing oil into the electrolytic cell, 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 film reaction zone is constructed by utilizing the density and compatibility differences of three working liquids: light sealing oil, aqueous electrolyte, and heavy sealing oil. When the first-order intercalated graphite electrode comes into contact with the micro-liquid film under energized conditions, an electrochemical reaction occurs in the contact area, transforming the carbon lattice in the graphite electrode in this area into a CFG structure. By controlling the relative movement between the micro-liquid film reaction zone and the intercalated graphite electrode, the contact reaction zone gradually moves from one end of the intercalated graphite electrode to the other end, thereby uniformly transforming the entire intercalated graphite electrode into a CFG structure. During this process, a sufficient amount of light or heavy sealing oil must be maintained. The amount of light or heavy sealing oil needs to be increased or decreased to ensure that the intercalated graphite electrode is always immersed in oil and does not come into contact with air. At the same time, aqueous electrolyte is replenished at a certain rate to avoid changes in the liquid film thickness due to the consumption of aqueous electrolyte in the reaction.

[0050] A microfilm is a liquid film containing an aqueous electrolyte sandwiched between light and heavy sealing oils. Its thickness is 1 μm to 50 cm, with a preferred thickness of 500 μm to 5 cm.

[0051] The energizing condition refers to using an intercalated graphite electrode as one electrode and an inert electrode as the other, both of which are connected to the positive or negative terminal of an external power source and simultaneously inserted into the electrolyte microfilm to form a conductive circuit, thereby causing an electrochemical functionalization reaction in the area where the intercalated graphite electrode contacts the microfilm.

[0052] Electrochemical functionalization reactions can be carried out in constant voltage or constant current modes. In constant voltage mode, the external power supply voltage required for the electrochemical reaction ranges from 0.1V to 500V, with a preferred range of 3V to 20V. In constant current mode, the required current density for the electrochemical reaction ranges from 1mA / m. 2 ~10A / mm 2 The preferred current density range is 1 A / 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 microliquid membrane region, per unit volume (1 mm²). 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 microfluidic membrane reaction zone and the intercalated graphite electrode can be controlled by, or is not limited to, one of the following methods: (1) keeping the relative position of the microfluidic membrane in the electrolytic cell constant, and achieving relative movement by moving the intercalated graphite electrode sheet upward or downward; (2) keeping the relative position of the intercalated graphite electrode sheet in the electrolytic cell constant, and achieving relative movement by controlling the liquid level of the lower layer of heavy sealing oil; (3) achieving relative movement by having the intercalated graphite electrode sheet and the microfluidic membrane move synchronously or asynchronously in opposite directions through operations similar to the above process. The speed range of the relative movement is 0.1 mm / min to 10 cm / s.

[0054] The following methods may be used to replenish the microfilm region with aqueous electrolyte: (1) Add aqueous electrolyte directly to the top layer of light sealing oil at a certain speed by dripping. The dripped electrolyte will sink spontaneously and flow into the microfilm layer to replenish the consumption of the microfilm layer; (2) Inject aqueous electrolyte from the bottom of the microfilm into the area occupied by heavy sealing oil at a certain speed through a pipeline. The injected electrolyte will rise spontaneously and flow into the microfilm layer to replenish the consumption of the microfilm layer; (3) Directly replenish aqueous electrolyte into the microfilm region through a pipeline that connects to the microfilm region.

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

[0056] 4. The CFG sheet-like structure after reaction can be mechanically exfoliated in a solvent to prepare a CFG dispersion, which can then be dried to obtain CFG powder.

[0057] Solvents include, but are not limited to, one or more of water, ethanol, acetone, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, fluorinated solvents, and surfactant solutions in different solvents. Their common characteristic is that CFG can form a high concentration dispersion in the solvent and has good stability.

[0058] Mechanical stripping methods include, but are not limited to, one or more of the following processing techniques: ultrasonic crushing, ball milling, high-shear crushing, high-pressure homogenization, sand milling, and mechanical grinding, which involve stepwise processing.

[0059] The drying process includes, but is not limited to, one or more of the following methods: freeze drying, vacuum drying, microwave drying, spray drying, etc., in a step-by-step manner.

[0060] The present invention will now be further described in detail with reference to embodiments and accompanying drawings.

[0061] Example 1

[0062] The 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), 30 wt.% sulfuric acid aqueous solution (working liquid 2), CCl4 (working liquid 3) and 98 wt.% industrial sulfuric acid (working liquid 4). The four working liquids form four parallel liquid films due to differences in density and compatibility.

[0064] The electrochemical reaction process for preparing GO consists of two steps: the first step is the electrolytic intercalation process (see attached diagram). Figure 2 Step 2: Electrolytic oxidation process (attached) Figure 3 ).

[0065] In the first step of electrolytic intercalation, as shown in the attached... Figure 2 As shown in state 1, concentrated sulfuric acid ④ is injected into the electrolytic cell, occupying most of the cell's height. Then, a 3mm thick layer of paraffin oil ① is added to the upper surface of the concentrated sulfuric acid to isolate it from air. A fully dried flexible graphite paper strip is inserted into the electrolytic cell filled with electrolyte, serving as the anode ⒀ and an inert cathode electrode rod ⑤ (platinum electrode rod). To ensure sufficient electrical contact, the flexible graphite paper strip's top is held by a graphite electrode conductive clamp ⑨ and then connected to the positive terminal of an external power supply ⑩. The upper end of the cathode electrode rod ⑤ is connected to the negative terminal of the external power supply ⑩. Adjust the external power supply voltage to a constant 1.8V, then turn on the external power supply to perform sulfuric acid intercalation treatment on the electrolytic anode ⒀ (flexible graphite paper strip). The intercalation treatment time is 10min to 30min, so that it reaches 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 transformed into the first-order intercalated GIC electrode ⑥ (anode intercalated graphite paper strip), which will participate in the second step of electrolytic oxidation reaction as the anode.

[0066] Since concentrated sulfuric acid is no longer needed in the second-step electrolytic oxidation reaction, it needs to be drained from the electrolytic cell after electrolytic intercalation. To ensure the stability of the GIC electrode intercalation state, paraffin oil needs to be added to the electrolytic cell simultaneously with the draining of concentrated sulfuric acid. This ensures that the GIC electrode remains submerged in paraffin oil throughout the process and does not come into contact with air. Generally, ensuring that the rate of concentrated sulfuric acid draining is the same as the rate of paraffin oil injection achieves this effect. The state after complete draining of concentrated sulfuric acid is shown in the attached figure. Figure 2 As shown in state 3. At this point, the first step of the electrolytic intercalation process is complete.

[0067] In the second step of electrolytic oxidation, as shown in the attached... Figure 3As shown, before the start of the electrolytic oxidation reaction (time point: t0), three working liquids exist in the electrolytic cell: paraffin oil ① completely immersing the GIC electrode ⑥; a 5mm thick layer of dilute sulfuric acid electrolyte (30wt.% sulfuric acid aqueous solution ②, liquid level height h0) below but not in contact with the GIC electrode ⑥; and CCl4 ③ below the dilute sulfuric acid electrolyte layer. To construct the above multilayer liquid film structure and control the height of the dilute sulfuric acid electrolyte layer, the electrolytic cell used for the reaction needs to be equipped with inlet and outlet ports, such as... Figure 3 As shown, a lower sealing oil inlet (11) is added to the bottom of the electrolytic cell for injecting the working fluid from the bottom; simultaneously, an upper sealing oil overflow outlet (12) is added to the top side of the electrolytic cell at a height equivalent to the liquid level to drain excess paraffin oil resulting from the injection of CCl4. Using this electrolytic cell, the height of the dilute sulfuric acid electrolyte layer can be increased by injecting CCl4, and the rate of increase can be controlled by the injection rate of CCl4, while ensuring that the GIC electrode remains immersed in paraffin oil and does not come into contact with air.

[0068] In the preparation process, an external power supply is first connected, and the output voltage is adjusted to 3.5V. At this time, since there is no electrolyte connection between the GIC electrode and the inert electrode (cathode rod ⑤), the electrolytic circuit is not conductive. However, when CCl4 ③ is injected to raise the liquid level of the dilute sulfuric acid electrolyte layer to contact the bottom of the GIC electrode ⑥, the electrolytic circuit is activated. An electrochemical reaction begins to occur in the part of the electrolyte film that contacts the GIC electrode ⑥, oxidizing this part of the GIC electrode into graphite oxide, forming CFG graphite oxide strips ⑦. 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. The intermediate state is shown in the attached figure. Figure 1 Right image and appendix 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 dropwise 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 microfilm layer, replenishing the consumed microfilm layer.

[0069] The preparation operation is complete when the electrolyte film rises to the height of the upper sealing oil overflow port (12). Its state is shown in the attached figure. Figure 3Mid-time point: t3. At this point, the GIC electrode ⑥ is completely converted to graphite oxide, but still retains its complete strip shape. When the length of the raw material flexible graphite paper strip is 15cm, the time required to complete the electrolytic oxidation reaction in the electrolytic oxidation reaction zone ⑧ is 25min. Afterward, 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 inlet ⑾ below. After emptying the working liquid in the electrolytic cell, the graphite oxide strip can be removed.

[0070] After the extracted CFG graphite oxide strips ⑦ are placed in pure water for high-speed shearing and peeling, acid removal and other conventional operations, GO slurry can be obtained. After freeze drying, GO powder can be obtained. The obtained GO product has the characteristics of high oxidation degree (carbon-oxygen atomic ratio <1.5) and high monolayer rate (monolayer rate >98%). Moreover, the yield of GO product relative to the original graphite raw material can reach 181%, and the graphite raw material utilization rate is >95%.

[0071] Example 2

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

[0073] This embodiment uses the same preparation apparatus and process as Example 1. The main technical differences lie in three aspects: the selection of the working liquid, the setting of electrochemical reaction parameters, and the control of the microfilm movement speed. The specific descriptions are as follows:

[0074] In this embodiment, four working liquids are used: 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 for the first step of electrolytic intercalation is 1.8V, and the time required to complete the electrolytic intercalation is 15min to 30min.

[0076] In this embodiment, the thickness of the microfluidic membrane used in the second step of electrolytic oxidation is 8 mm, the voltage used is 3-5 V, the microfluidic membrane rise 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, the obtained nitrogen-doped graphite strips are removed from the electrolytic cell and ultrasonically exfoliated using a 40wt% ethanol aqueous solution to obtain a dispersion containing sulfuric acid impurities. The sulfuric acid in the dispersion is then removed by electrodialysis to prepare a nitrogen-doped graphene dispersion. After vacuum drying, NG powder material is obtained. The obtained NG product has a high nitrogen content (>10wt.%) and a high monolayer ratio (>95%), and the yield of the NG product relative to the original graphite raw material can reach 120%, with a graphite raw material utilization rate >90%.

[0078] Example 3

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

[0080] This embodiment uses the same preparation apparatus and process as Example 1. The main technical differences lie in three aspects: the selection of the working liquid, the setting of electrochemical reaction parameters, and the control of the microfilm movement speed. The specific descriptions are as follows:

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

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

[0083] In this embodiment, the thickness of the microfluidic membrane used in the second step of electrolytic oxidation is 8 mm, the voltage used is 3-5 V, the microfluidic membrane rise 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, the obtained sulfur-doped graphite strips are removed from the electrolytic cell and ultrasonically exfoliated using a 40wt% ethanol aqueous solution to obtain a dispersion containing sulfuric acid impurities. The sulfuric acid in the dispersion is then removed by electrodialysis to prepare a sulfur-doped graphene dispersion. After vacuum drying, SG powder material is obtained. The obtained SG product has high sulfur content (>3wt.%), high monolayer ratio (>95%), and the yield of the SG product relative to the original graphite raw material can reach 100%, with a graphite raw material utilization rate >90%.

[0085] Example 4

[0086] The objective of this embodiment is to prepare highly fluorinated graphene (FG) slurry and powder.

[0087] This embodiment uses the same preparation apparatus and process as Example 1. The main technical differences lie in three aspects: the selection of the working liquid, the setting of electrochemical reaction parameters, and the control of the microfilm movement speed. The specific descriptions are as follows:

[0088] In this embodiment, four working liquids are used: petroleum ether (working liquid 1), 50 wt.% hydrofluoric acid solution (working liquid 2, 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 for the first step of electrolytic intercalation is 2.0V, and the time required to complete the electrolytic intercalation is 15min to 30min.

[0090] In this embodiment, the thickness of the microfluidic membrane used in the second step of electrolytic oxidation is 8 mm, the voltage used is 4 V, the microfluidic membrane rise 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 hour.

[0091] After the second step of electrolytic oxidation, the obtained fluorinated graphene strips are removed from the electrolytic cell and ultrasonically exfoliated using a 40wt% ethanol aqueous solution to obtain a dispersion containing trifluoroacetic acid and hydrofluoric acid impurities. The impurities are then washed away by repeated centrifugation, and the product is dispersed in NMP to prepare a fluorinated graphene dispersion. After vacuum drying, FG powder material is obtained. The obtained FG product has high fluorine content (>25wt.%), high monolayer ratio (>85%), and the yield of FG product relative to the original graphite raw material can reach 100%, with a graphite raw material utilization rate >80%.

[0092] The results demonstrate that, based on the two-step electrochemical reaction for graphene oxide preparation, this invention achieves highly efficient and controlled preparation of various CFGs, including graphene oxide, by adjusting the composition of the electrolyte and the electrochemical reaction parameters. This results in a universal CFG electrochemical preparation technology. This invention represents a significant advancement in the field of graphene preparation technology and has substantial application value.

Claims

1. A universal electrochemical preparation method for covalently functionalized graphene, characterized in that, Four different working fluids with varying properties are used: working fluid 1, light sealing oil; working fluid 2, aqueous electrolyte; working fluid 3, heavy sealing oil; and working fluid 4, concentrated sulfuric acid. The specific process is as follows: (1) Under the condition of light sealing oil, the graphite electrode is electrochemically intercalated with concentrated sulfuric acid to achieve the first-order intercalation state. An oil seal refers to a layer of light sealing oil that is applied to the surface of an electrolytic cell after concentrated sulfuric acid has been added to it. The first-order intercalation state is an intercalation state of graphite intercalation compounds. Microscopically, it refers to the uniform insertion of a layer of sulfuric acid molecules / ions between any two layers of graphite atoms. It is the limiting state of intercalation in graphite crystal structure and macroscopically makes the graphite electrode appear blue. (2) After the intercalation is completed, while discharging concentrated sulfuric acid, inject light sealing oil into the electrolytic cell and ensure 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, the density and compatibility differences of the three working liquids—light sealing oil, aqueous electrolyte, and heavy sealing oil—are used to construct a micro-liquid film reaction zone. When the first-order intercalated graphite electrode comes into contact with the micro-liquid film under energized conditions, an electrochemical reaction occurs in the contact area, transforming the carbon lattice in the graphite electrode in this area into a covalently functionalized graphene structure. By controlling the relative movement between the micro-liquid film reaction zone and the intercalated graphite electrode, the contact reaction zone gradually moves from one end of the intercalated graphite electrode to the other end, thereby uniformly transforming the entire intercalated graphite electrode into a covalently functionalized graphene structure. During this process, a sufficient amount of light or heavy sealing oil needs to be retained. By increasing or decreasing the amount of light or heavy sealing oil, it is ensured that the intercalated graphite electrode is always immersed in the oil and does not come into contact with air. At the same time, aqueous electrolyte is added to avoid changes in the liquid film thickness due to the consumption of aqueous electrolyte. Among them, micro liquid film refers to the aqueous electrolyte liquid film sandwiched between light sealing oil and heavy sealing oil; (4) The covalently functionalized graphene structure after the reaction is placed in a solvent for mechanical exfoliation to prepare a dispersion of covalently functionalized graphene, which is then dried to obtain covalently functionalized graphene powder. Working fluid 1, light sealing oil, includes liquid paraffin, mineral oil, or petroleum ether, which are characterized by being sparingly soluble or insoluble in water and having a density of less than 1 g / cm³. 3 Working fluid 2 is an aqueous electrolyte solution composed of one or more organic or inorganic electrolytes. During the electrochemical reaction, it generates active free radicals related to the electrolyte components through electrode reactions. Working fluid 3 is a heavy sealing oil containing one or more of carbon disulfide, carbon tetrachloride, and carbon trichloride. Its common characteristics are that it is sparingly soluble or insoluble in water and has a density greater than 1.1 g / cm³. 3 Working liquid 4 Concentrated sulfuric acid refers to sulfuric acid water mixtures, sulfuric acid organic solvent mixtures, pure sulfuric acid, or fuming sulfuric acid with a sulfuric acid mass fraction of 70% or more.

2. The electrochemical preparation method for covalently functionalized graphene according to claim 1, characterized in that, In step (1), the graphite electrode includes flexible graphite paper, electrode graphite rod, graphite plate, graphite powder compressible, pyrolytic graphite or artificial graphite.

3. The electrochemical preparation method for covalently functionalized graphene according to claim 1, characterized in that, In step (1), electrochemical intercalation refers to the process of using an external power source to immerse a graphite electrode as the anode and an inert electrode as the cathode in concentrated sulfuric acid to carry out an electrochemical reaction, so that sulfuric acid molecules and ions are inserted into the interior of the graphite electrode. The electrochemical intercalation reaction is carried out in constant voltage mode. The external power supply voltage required for intercalation is 0.1V~5V. The time required for electrochemical intercalation is related to the mass of the graphite electrode. The time for intercalation of a unit mass of 1g of graphite electrode is 1s~1h.

4. The electrochemical preparation method for covalently functionalized graphene according to claim 1, characterized in that, In step (3), the thickness of the microfilm is 1 μm to 50 cm.

5. The electrochemical preparation method for covalently functionalized graphene according to claim 1, characterized in that, In step (3), the energizing condition refers to using an intercalated graphite electrode as one electrode and an inert electrode as the other, both connected to the positive or negative terminal of an external power supply, and simultaneously inserted into the electrolyte microfilm to form a conductive circuit. This allows an electrochemical functionalization reaction to occur in the area where the intercalated graphite electrode contacts the microfilm. The electrochemical functionalization reaction is carried out in constant voltage or constant current mode. In constant voltage mode, the external power supply voltage required for the electrochemical reaction ranges from 0.1V to 500V; in constant current mode, the current density required for the electrochemical reaction ranges from 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 microliquid membrane region, 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.

6. The electrochemical preparation method for 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) keeping the relative position of the micro-liquid membrane in the electrolytic cell unchanged, and achieving relative movement by moving the intercalated graphite electrode sheet upward or downward; (2) keeping the relative position of the intercalated graphite electrode sheet in the electrolytic cell unchanged, and achieving relative movement by controlling the liquid level of the lower layer of heavy sealing oil; (3) the intercalated graphite electrode sheet and the micro-liquid membrane move synchronously or asynchronously in opposite directions to achieve relative movement; the speed range of the relative movement is 0.1 mm / min to 10 cm / s.

7. The electrochemical preparation method for covalently functionalized graphene according to claim 1, characterized in that, In step (3), the aqueous electrolyte is replenished to the micro-liquid membrane area using one of the following methods: (1) The aqueous electrolyte is added directly to the uppermost light sealing oil at a certain speed by dripping. 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 at a certain speed through the pipeline. 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 replenishment speed of the aqueous electrolyte is related to the reaction consumption speed of the micro-liquid membrane, and the speed control range is 1μL / h~1L / s.

8. The electrochemical preparation method for covalently functionalized graphene according to claim 1, characterized in that, In step (4), the solvent includes one or more of the following: water, ethanol, acetone, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, tetrahydrofuran, and fluorinated solvents; the mechanical stripping method includes one or more of the following: ultrasonic pulverization, ball milling, high-shear pulverization, high-pressure homogenization, sand milling, and mechanical grinding; and the drying method includes one or more of the following: freeze drying, vacuum drying, microwave drying, and spray drying.

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