Graphene preparation method by formic acid electrochemical intercalation and ultrasonic exfoliation

Abstract

A method for preparing graphene by formic acid electrochemical intercalation and ultrasonic exfoliation belongs to the field of graphene preparation technology. The method includes the following steps: using graphite material as the working electrode, immersing it in a high-concentration formic acid aqueous solution as the electrolyte and intercalating agent, applying a constant voltage to carry out an electrochemical intercalation reaction, obtaining an expanded graphite intercalation precursor; washing and drying the precursor, dispersing it in a liquid medium, and performing ultrasonic exfoliation treatment to obtain a graphene dispersion; finally, obtaining the graphene material through solid-liquid separation and drying. This invention uses formic acid as the intercalation medium, avoiding the use of traditional strong acids and oxidants, significantly improving the cleanliness of the process. The combination of mild electrochemical intercalation conditions and subsequent ultrasonic treatment achieves efficient exfoliation of graphene and high fidelity of the lattice structure. The prepared graphene material has low defect density and excellent structural integrity and intrinsic properties. This invention has a simple process flow, mild operating conditions, and is environmentally friendly, suitable for the large-scale preparation of high-quality graphene.

Description

Technical Field

[0001] This invention belongs to the field of graphene preparation technology, specifically relating to a method for preparing graphene by formic acid electrochemical intercalation and ultrasonic exfoliation. This method achieves controllable intercalation of graphite through electrochemical means, combined with ultrasonic exfoliation technology, to achieve efficient preparation of high-quality, few-layer graphene. Background Technology

[0002] Graphene is a type of carbon atom arranged in sp... 2 Two-dimensional honeycomb crystalline materials formed by hybridization have shown broad application prospects in high-end electronic devices, high-efficiency energy storage (such as lithium-ion batteries and supercapacitors), sensitive sensing, biomedicine, and high-performance composite materials due to their ultra-high carrier mobility, excellent electrical and thermal conductivity, and extremely high specific surface area. However, the strong van der Waals forces between graphene sheets make them prone to recombination and aggregation. Therefore, how to achieve efficient, large-scale preparation and stable dispersion of high-quality, low-layer graphene has always been a core technological bottleneck restricting its industrial application.

[0003] Currently, the main methods for large-scale preparation of graphene include the following:

[0004] (1) Mechanical exfoliation method: It can obtain monolayer graphene with complete crystal structure, but the efficiency is extremely low and it is difficult to achieve controllable large-scale production.

[0005] (2) Chemical vapor deposition (CVD): It can prepare large-area, high-quality graphene films, but the process is complex, the equipment is expensive, the cost is high, and it is difficult to obtain powder materials directly.

[0006] (3) Oxidation-reduction method: Represented by the Hummers process and its modifications, graphite is oxidized to graphene oxide using strong acids and strong oxidants (such as concentrated sulfuric acid and potassium permanganate), and then chemically reduced to obtain graphene. Although mass production is possible, this process severely damages the spline properties of graphene. 2 The crystal structure introduces a large number of oxygen-containing functional groups and structural defects (the intensity ratio of the D peak to the G peak in its Raman spectrum is I). D / I G Typically greater than 1.0), leading to significant degradation of its intrinsic properties such as electrical and thermal conductivity. Furthermore, this method uses large quantities of highly corrosive and oxidizing chemicals, generating toxic and harmful waste liquids, resulting in significant environmental pollution problems.

[0007] (4) Liquid-phase ultrasonic exfoliation: Graphite is dispersed in a solvent and the graphite layers are exfoliated using the cavitation force of ultrasound. This method is simple, but the exfoliation efficiency is low, and the product has many layers and is not uniform. To obtain a stable dispersion, organic solvents or surfactants are often required, which limits the purity and application of the product.

[0008] (5) Electrochemical exfoliation: As an emerging preparation technology, this method involves applying voltage in an electrolyte to drive electrolyte components (such as ions or molecules) to embed into the graphite interlayers, accompanied by gas evolution, thereby expanding and exfoliating the graphite sheets. Compared with the redox method, this process does not require strong oxidants, the conditions are relatively mild and controllable, and the resulting graphene has fewer defects, making it a more promising green preparation route.

[0009] Despite the numerous advantages of electrochemical exfoliation, current mainstream technologies still have significant shortcomings. Their electrolytes often use aqueous solutions containing inorganic salts (such as ammonium sulfate and sodium nitrate) or strong acids (such as sulfuric acid and nitric acid). Under high voltage, water molecules readily undergo vigorous electrolysis, generating large amounts of hydrogen and oxygen bubbles. While the rapid generation of these bubbles facilitates physical exfoliation, it also easily leads to excessive oxidation and mechanical damage to the graphene sheets, introducing additional defects. Furthermore, these electrolyte systems introduce heterogeneous anions such as sulfate and nitrate, or metal cations, which may not only remain in the product and affect purity, but also generate acidic or metal-containing waste liquids, creating a burden for subsequent treatment and environmental risks.

[0010] To overcome the aforementioned problems, the industry has attempted to combine electrochemical intercalation as a pretreatment step with liquid-phase ultrasonic exfoliation to synergistically improve the exfoliation efficiency and product quality of graphene. For example, Chinese invention patent CN108502874A discloses a method for "preparing graphene dispersion by electrochemical-assisted ultrasonication," which uses inorganic acid or salt solutions to electrochemically expand graphite, followed by ultrasonic exfoliation in an organic solvent. This method has demonstrated the effectiveness of the combined "electrochemical pretreatment + ultrasonication" strategy, achieving some progress in yield and layer number control. However, its core technology still relies on inorganic strong acid / salt electrolytes, failing to fundamentally solve the oxidation defect problem caused by side reactions of aqueous electrolytes, and subsequent processing still requires the use of organic solvents. Therefore, there is still considerable room for improvement in the environmental friendliness of the process and the intrinsic quality of the product.

[0011] In recent years, researchers have begun to explore milder and cleaner intercalation systems. Formic acid, as a weak organic acid, functions as both a solvent and an intercalating agent. Under electrochemical conditions, it can decompose to produce small molecule gases such as CO2 and H2, expanding graphite without introducing heteroatoms such as sulfur and nitrogen, making the production process clean and environmentally friendly. Existing research has confirmed the feasibility of using formic acid for electrochemical intercalation of graphite. However, if electrochemical intercalation is used alone, the degree of exfoliation is limited, making it difficult to directly obtain a high proportion of monolayer or few-layer graphene, and exfoliation efficiency becomes a new constraint.

[0012] In view of this, this invention proposes a method for preparing graphene by combining electrochemical intercalation and ultrasonic exfoliation. First, formic acid, a green medium, is used to achieve mild and controllable electrochemical intercalation, maximizing the preservation of graphite lattice integrity. Then, optimized ultrasonic treatment efficiently cleaves the pre-expanded graphite into few-layer graphene. This organic combination of "green electrochemical intercalation" and "precision ultrasonic exfoliation" holds promise for achieving truly green, efficient, and controllable preparation of high-performance, low-defect graphene, which is of great significance for promoting the upgrading and industrial application of graphene preparation technology. Summary of the Invention

[0013] The purpose of this invention is to overcome the problems of serious environmental pollution, numerous product defects, and poor dispersibility in existing graphene preparation technologies, and to provide an environmentally friendly, mild, efficient and controllable graphene preparation method, thereby obtaining high-quality, low-defect, and easily dispersible graphene materials.

[0014] To achieve the above objectives, this invention provides a method for preparing graphene by formic acid electrochemical intercalation and ultrasonic exfoliation, comprising the following steps:

[0015] (1) In the electrochemical system, the graphite material to be stripped is used as the working electrode and immersed in formic acid electrolyte;

[0016] (2) Apply a constant voltage to the working electrode to carry out an electrochemical intercalation reaction, so that the electrolyte components are inserted into the graphite interlayer to obtain an expanded graphite intercalation precursor.

[0017] (3) The graphite intercalation precursor obtained in step (2) is washed, dried, dispersed in a liquid medium, and subjected to ultrasonic treatment to obtain a graphene dispersion.

[0018] (4) The graphene dispersion obtained in step (3) is subjected to solid-liquid separation and drying to obtain graphene material.

[0019] Furthermore, the electrochemical system described in step (1) adopts a three-electrode system, with graphite material as the working electrode, platinum sheet as the counter electrode, and mercury / mercurous sulfate electrode as the reference electrode.

[0020] Further, the graphite material mentioned in step (1) is selected from one of natural graphite, expanded graphite, highly oriented pyrolytic graphite or graphite fiber.

[0021] Further, the formic acid electrolyte in step (1) is an aqueous formic acid solution with a mass fraction of 70%–98%.

[0022] Furthermore, before step (2), a step of pre-activating the working electrode by cyclic voltammetry is included, wherein the potential scan range of the pre-activation is 0–1.5 V and the number of scan cycles is 3–5.

[0023] Furthermore, the constant voltage in step (2) is in the range of 0.5–3 V, and the electrochemical intercalation time is 3–5 h.

[0024] Furthermore, the solvent used for washing in step (3) is water, ethanol, or a mixture thereof; the drying temperature is 30–80°C. ℃.

[0025] Furthermore, the liquid medium mentioned in step (3) is selected from water, ethanol, and isopropanol.

[0026] Furthermore, the ultrasonic frequency of the ultrasonic treatment in step (3) is 20–100 kHz, the treatment time is 5–10 h, and the system temperature is maintained at 20–40 °C during the ultrasonic treatment process by cooling control.

[0027] Furthermore, the solid-liquid separation in step (4) is performed by filtration or centrifugation.

[0028] Furthermore, the drying described in step (4) is vacuum drying or freeze drying.

[0029] The graphene material prepared by the above method is characterized by: the intensity ratio of the D peak to the G peak in its Raman spectrum is I. D / I G < 0.1 (e.g., up to 0.05), exhibiting a low defect density graphene structure; it is a few-layer graphene with 5–15 layers; it can be dispersed in water or ethanol at 25 °C to form a stable dispersion with a concentration of not less than 0.1 mg / mL, which shows no significant sedimentation after standing for 24 hours.

[0030] Compared with existing technologies, the method for preparing graphene by combining electrochemical intercalation and ultrasonic exfoliation provided by this invention has the following significant advantages:

[0031] (1) Green and environmentally friendly: The entire preparation process uses only formic acid aqueous solution as electrolyte and intercalating agent, without introducing traditional strong acids (such as sulfuric acid and nitric acid) or metal salts, avoiding the use of toxic reagents and heavy metal ions. The by-products are only carbon dioxide and hydrogen, without the need for complicated waste liquid treatment, and the process is green and environmentally friendly.

[0032] (2) Mild and controllable: The electrochemical intercalation process is carried out under constant voltage conditions at low voltage. Compared with the traditional rapid stripping at high current density, the reaction rate can be better controlled, avoiding excessive oxidation. The entire process is operated at room temperature and pressure, which has high safety and low energy consumption;

[0033] (3) High product quality: The graphene sheets obtained by this invention have a complete layer structure and extremely low defect density. Raman spectroscopy analysis shows that its I... D / I GThe value < 0.1 indicates that very few defects were introduced into the graphene lattice, preserving a crystal structure and properties close to those of intrinsic graphene. Furthermore, the obtained graphene is mostly 5–15 layers of few-layer graphene, exhibiting excellent electrical conductivity and mechanical properties.

[0034] (4) Good product dispersibility: The graphene material prepared by this invention can form a long-term stable dispersion in water or ethanol without the addition of surfactants, and is suitable for the preparation of uniform films or composite materials by processes such as spraying and filtration.

[0035] (5) Simple and scalable process: The process of this invention is simple, and the main operation steps include electrolysis and ultrasound. The raw materials (graphite and formic acid) are inexpensive and readily available, the equipment is conventional, the parameters are clear and easy to control, and it has good repeatability and prospects for large-scale production. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments are briefly described below. Obviously, the drawings are merely illustrative of some embodiments of the present invention, and those skilled in the art can obtain other related drawings based on these drawings without any creative effort.

[0037] Figure 1 The images show the cyclic voltammetry (CV) plot (a) of samples 1–1.5 V pre-activation, and the X-ray diffraction (XRD) patterns (b) and Raman spectra (c) of samples 1–3.

[0038] Figure 2 The images are scanning electron microscope (SEM) images of the cross section (a, b) and plane (c, d) of sample 3 obtained in Example 1.

[0039] Figure 3 Optical photographs of samples 1–3 in Example 2 after ultrasonic treatment.

[0040] Figure 4 The image shows the Raman spectrum of sample 4 obtained in Example 2.

[0041] Figure 5 The images shown are (a) a high-resolution transmission electron microscope (HR-TEM) image and (b) a selected area electron diffraction (SAED) pattern of sample 4 prepared in Example 2. Detailed Implementation

[0042] To further illustrate the present invention, the method for preparing graphene materials provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments. However, these embodiments should not be construed as limiting the scope of protection of the present invention. Any product that is the same as or similar to the present invention obtained under the guidance of the present invention or by combining the features of the present invention with other prior art should fall within the scope of protection of the present invention.

[0043] The specific operating methods and testing techniques used in this invention are all conventional methods in the art. Unless otherwise specified, they can be performed according to the conventional experimental operations or conditions described in the literature in this field. The reagents involved in this invention are all commercially available standard products and can be purchased and used directly.

[0044] Example 1

[0045] A three-electrode system was constructed at room temperature. A 4 mm × 4 mm × 0.1 mm block of highly oriented pyrolytic graphite (HOPG) was used as the working electrode, a mercury / mercurous sulfate electrode as the reference electrode, and a platinum sheet as the counter electrode. The electrolyte was a 98% (w / w) high-concentration formic acid aqueous solution. Before the electrochemical intercalation reaction, the working electrode was pretreated by cyclic voltammetry (CV) scanning to activate the HOPG electrode surface. The potential scan range was 0–1.5 V (relative to the reference electrode), the scan rate was 0.1 mV / s, and the number of cycles was 3–5. The CV curves are shown below. Figure 1 As shown in Figure a. After pre-activation, constant voltages of 0.55 V, 0.90 V, and 1.50 V were applied for electrochemical intercalation reactions, with a reaction time of 5 h for each. The obtained graphite intercalation precursors are designated as Sample 1, Sample 2, and Sample 3, respectively. The samples were characterized by X-ray diffraction (XRD) and Raman spectroscopy, and the results are shown in Figure a. Figure 1 b and Figure 1 c.

[0046] Example 2

[0047] The graphite intercalation precursor prepared in Example 1 was washed several times with anhydrous ethanol and then vacuum dried at 50 °C. Subsequently, the dried precursor was dispersed in 10 mL of anhydrous ethanol and ultrasonically treated for 10 h using an ultrasonic cleaner (frequency: 45 kHz). After ultrasonic treatment, the resulting suspension was vacuum filtered and further washed with ethanol, and finally vacuum dried to obtain the final graphene product. The graphene product obtained after ultrasonic exfoliation of Sample 3 was designated as Sample 4. Raman spectroscopy analysis was performed on Sample 4. Figure 4 The results showed that the intensity ratio of its D peak to G peak, ID / IG = 0.05, indicating that very few defects were introduced into the graphene structure, preserving a crystal structure and properties close to intrinsic graphene. Further high-resolution transmission electron microscopy (HR-TEM) observations... Figure 5 a) Sample 4 is shown to be approximately 10 layers of few-layer graphene; Selected area electron diffraction (SAED) pattern ( Figure 5 (b) This shows that it has a highly ordered crystal structure, further verifying the high quality characteristics of the product.

[0048] The product prepared from sample 4 had a concentration of 0.1 mg / mL and was a stable dispersion with no obvious sedimentation after standing for 24 hours.

Claims

1. A method for preparing graphene, characterized in that, Includes the following steps: (1) Immerse the graphite material as the working electrode in the formic acid electrolyte; (2) An intercalation reaction is carried out by applying a constant voltage to the working electrode under electrochemical conditions to obtain a graphite intercalation precursor; (3) The precursor is washed, dried and dispersed in a liquid medium and subjected to ultrasonic exfoliation to obtain a graphene dispersion. (4) The graphene dispersion is subjected to solid-liquid separation and drying to obtain graphene material.

2. The method for preparing graphene according to claim 1, characterized in that, The formic acid electrolyte is an aqueous formic acid solution with a mass fraction of 70–98%.

3. The method for preparing graphene according to claim 1, characterized in that, The electrochemical system is a three-electrode system, the working electrode is made of graphite, the counter electrode is a platinum sheet, and the reference electrode is a mercury / mercurous sulfate electrode.

4. The method for preparing graphene according to claim 1, characterized in that, The graphite material is selected from one of the following: natural graphite, expanded graphite, highly oriented pyrolytic graphite, or graphite fiber.

5. The method for preparing graphene according to claim 1, characterized in that, The constant voltage ranges from 0.5 to 3V, and the intercalation reaction time is 3 to 5 hours.

6. The method for preparing graphene according to claim 1, characterized in that, It also includes a step of pre-activating the working electrode by cyclic voltammetry, wherein the pre-activation potential scan range is 0–1.5 V and the number of scan cycles is 3–5.

7. The method for preparing graphene according to claim 1, characterized in that, The solvent used for washing is water, ethanol, or a mixture thereof, and the drying temperature is 30–80 °C.

8. The method for preparing graphene according to claim 1, characterized in that, The ultrasonic frequency of the ultrasonic treatment is 20–100 kHz, the treatment time is 5–10 h, and the system temperature is controlled at 20–40 ℃ during the treatment by cooling.

9. The method for preparing graphene according to claim 1, characterized in that, The liquid medium is one of water, ethanol, or isopropanol.

10. A graphene material prepared by the method according to any one of claims 1–9, characterized in that, The intensity ratio of the D peak to the G peak in its Raman spectrum is I D / I G < 0.1, and it can form a stable dispersion with a concentration of not less than 0.1 mg / mL in water or ethanol at 25 ℃, which shows no obvious sedimentation after standing for 24 hours.

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