Two-dimensional carbon plate as well as preparation method and application thereof

Abstract

The invention relates to the technical field of preparation of two-dimensional carbon plate electrode materials, and discloses a two-dimensional carbon plate and a preparation method and application thereof. The two-dimensional carbon plate contains a lamellar structure and further contains doping elements Fe, B and N. On the basis of the total amount of the two-dimensional carbon plate, the content of the doping element Fe is 0.8 wt%-3.5 wt%, the content of the doping element B is 5 wt%-8.8 wt%, and the content of the doping element N is 2.8 wt%-3.5 wt%. The conductivity of the material and the affinity of the material to a solution are enhanced through the Fe, B and N anion-cation co-doped carbon nanosheets; and the pseudocapacitance performance of the derived carbon material is also improved by doping multiple anions and cations. The two-dimensional nanostructure ensures that active sites derived from doping defects are highly exposed on the surface of the material, so that the active sites are effectively utilized.

Description

Technical Field

[0001] The present invention relates to the technical field of two-dimensional carbon sheet electrode material preparation, and in particular to a two-dimensional carbon sheet and a preparation method and application thereof. Background Art

[0002] Capacitive desalination is a desalination technology with electrochemical capacitive behavior (ECB) as its core mechanism. Among them, electrochemical capacitive behavior is a capacitive mechanism derived from the adsorption of charged ions in the electrolyte on the electrode surface by an external electric field. This mechanism has the advantages of fast adsorption rate, high efficiency and high reversibility. Therefore, its derived practical capacitive deionization (CDI) technology also has these advantages. In CDI, the electrode is the main factor affecting the performance of both. The electrode is mainly composed of active materials, binders, conductive agents and electrode matrices. Among them, the active material is the main material for ion adsorption and is the main factor in the capacitive performance of the electrode. The other parts do not play a role in capacitive adsorption. Therefore, increasing the proportion of active materials in the electrode by reducing the use of additives such as binders is one of the effective strategies to improve the overall capacitive performance of the electrode.

[0003] Two-dimensional carbon materials have always been typical capacitor materials with many advantages. Among them, most of the two-dimensional carbon materials prepared with biomass as precursors are solid biomass. For example, Zhao et al. (J. Mater. Chem. A, 2017, 5, 15243-15252) used kapok fibers as precursors, decomposed the carbonized kapok fibers into two-dimensional fragments, and prepared two-dimensional carbon nanosheet materials with high specific surface area and porosity after KOH activation. Chen et al. (Mater. Lett., 2018, 12, 187-190) based on the white melaleuca bark precursor, after carbonization, the nano-paper-like white melaleuca bark can be directly converted into two-dimensional carbon nanosheets. Most of the existing reports involve the use of strong alkaline and highly corrosive salts. These operations not only cause great harm to equipment and the environment, but also have the disadvantages of high risk and high energy consumption. Of course, some activators with lower corrosiveness have also been developed, but they also have some disadvantages. While the existing two-dimensional carbon materials improve their capacitance performance, they will reduce their conductivity.

[0004] It is of great significance to study a two-dimensional carbon material with high-efficiency capacitance and conductivity properties. Summary of the invention

[0005] The purpose of the present invention is to overcome the problem in the prior art that two-dimensional carbon materials cannot have both capacitance and conductivity, and to provide a two-dimensional carbon sheet and a preparation method and application thereof.

[0006] In order to achieve the above-mentioned purpose, the first aspect of the present invention provides a two-dimensional carbon sheet, wherein the two-dimensional carbon sheet contains a layered structure and also contains doping elements Fe, B and N. wherein, based on the total amount of the two-dimensional carbon sheet, the content of the doping element Fe is 0.8-3.5wt%, the content of the doping element B is 5-8.8wt%, and the content of the doping element N is 2.8-3.5wt%.

[0007] A second aspect of the present invention provides a method for preparing a two-dimensional carbon sheet, comprising the following steps:

[0008] (1) mixing water, biomass, a boron source and an iron source to obtain a hydrogel;

[0009] (2) placing the hydrogel at a low temperature below -5°C for standing and low temperature drying to obtain an aerogel;

[0010] (3) Carbonizing the aerogel to obtain the two-dimensional carbon sheet.

[0011] The third aspect of the present invention provides a preparation method for obtaining a two-dimensional carbon sheet.

[0012] A fourth aspect of the present invention provides an application of the two-dimensional carbon sheet in the preparation of a capacitor device.

[0013] Through the above technical solution, the present invention provides a two-dimensional carbon sheet and a preparation method and application thereof, which have at least the following beneficial effects:

[0014] (1) The present invention enhances the conductivity and affinity of the material to the solution by co-doping the carbon nanosheets with Fe, B, and N anions and cations; multiple anion and cation doping also improves the pseudocapacitive performance of the derived carbon material, and has good specific capacitance and rate performance. The two-dimensional nanostructure will ensure that the active sites derived from the doping defects are highly exposed on the surface of the material, so that they can be effectively utilized.

[0015] (2) The present invention uses a boron source to assist in the preparation of two-dimensional carbon sheets, which revolutionizes the disadvantages of traditional templates. No corrosive agents or high-temperature melting operations are required, and the template can be removed at room temperature, further reducing the operational difficulty and technical cost of the process. The aqueous solution in the hydrogel phase is converted into ice at low temperatures, and the ice film plate formed has a volume difference that will effectively shape the material. The use of a boron source enhances the controllability of the ice film plate over the thickness of the two-dimensional nanosheet; the ice template reduces the amount of boron source used and alleviates the damage to the equipment caused by the traditional boron source method that produces a large amount of gas during preparation.

[0016] (3) The two-dimensional carbon sheet obtained in the present invention can form a binder-free self-supporting electrode on the titanium mesh, which increases the proportion of active materials in the electrode and also reduces the preparation operation of the electrode slurry, effectively reducing the complexity of the electrode preparation process, which is of great significance for practical applications.

[0017] (4) The present invention does not require toxic reagents or high-risk operations such as high pressure during the preparation process, and has low requirements for the environment and equipment. At the same time, compared with the traditional boron source preparation method, the use of auxiliary ice templates effectively reduces the amount of boron source used, and the ice crystals grown in the carbon precursor initially construct the basic pores and lamellar structure, effectively reducing the resistance of the boron source during high-temperature expansion. Therefore, while ensuring the benefits, the amount of boron source used is effectively reduced, and the reagent cost is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 1.

[0019] Figure 2 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 2.

[0020] Figure 3 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 3.

[0021] Figure 4 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 4.

[0022] Figure 5 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 5.

[0023] Figure 6 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 6.

[0024] Figure 7 XRD patterns of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0025] Figure 8 Raman images of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0026] Fig. 9 This is the XPS graph of the two-dimensional carbon sheet synthesized in Example 1.

[0027] Fig.10 This is the N spectrum of the two-dimensional carbon sheet synthesized in Example 1.

[0028] Fig.11 This is the B spectrum of the two-dimensional carbon sheet synthesized in Example 1.

[0029] Fig.12 This is the Fe spectrum of the two-dimensional carbon sheet synthesized in Example 1.

[0030] Fig.13 Three-electrode CV curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0031] Fig.14 CV graphs of the two-dimensional carbon sheet synthesized in Example 1 at different current rates.

[0032] Fig.15 Column chart showing the pseudocapacitance ratio of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0033] Fig.16 The three-electrode GCD curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0034] Fig.17 The three-electrode GCD curves of the two-dimensional carbon sheet synthesized in Example 1 under multiple current densities.

[0035] Fig.18 Three-electrode impedance curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0036] Fig.19 This is the CV curve of the symmetrical capacitor of the two-dimensional carbon sheet synthesized in Example 1.

[0037] Fig. 20 This is the GCD curve of the symmetrical capacitor of the two-dimensional carbon sheet synthesized in Example 1.

[0038] Fig.21 The ion adsorption-time curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0039] Fig. 22 The solution conductivity-time curve of the two-dimensional carbon sheet synthesized in Example 1 in salt water of different concentrations.

[0040] Fig.23 This is the electrode cycle desalination-regeneration curve of the two-dimensional carbon sheet synthesized in Example 1.

[0041] Fig.24 This is a physical picture of the two-dimensional carbon sheet synthesized in Example 1.

[0042] Fig.25 This is a scanning electron microscope image of the carbon material synthesized in Comparative Example 1. DETAILED DESCRIPTION

[0043] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0044] A first aspect of the present invention provides a two-dimensional carbon sheet, wherein the two-dimensional carbon sheet contains a layered structure and also contains doping elements Fe, B and N. Based on the total amount of the two-dimensional carbon sheet, the content of the doping element Fe is 0.8-3.5wt%, the content of the doping element B is 5-8.8wt%, and the content of the doping element N is 2.8-3.5wt%.

[0045] In the present invention, the content of the doping element can be measured by X-ray photoelectron spectroscopy (XPS) method.

[0046] In the present invention, the two-dimensional carbon sheet co-doped with Fe, B, and N anions and cations enhances the conductivity and affinity of the material to the solution; multiple anion and cation doping also improves the pseudocapacitive performance of the derived carbon material, with good specific capacitance and rate performance, and the two-dimensional nanostructure ensures that the active sites derived from the doping defects are highly exposed on the surface of the material, so that they can be effectively utilized. The two-dimensional carbon sheet has a strong capacitive desalination capacity, and after continuous operation, it still maintains a high adsorption capacity.

[0047] In some specific embodiments of the present invention, preferably, the content of the doping element Fe is 1.5-2.5wt%, the content of the doping element B is 7-8wt%, and the content of the doping element N is 3-3.5wt%. When the content of the doping element is within the above range, the two-dimensional carbon sheet has good specific capacitance and rate performance.

[0048] In some specific embodiments of the present invention, more preferably, the content of the doping element Fe is 1.8-2.2wt%, the content of the doping element B is 7.2-7.8wt%, and the content of the doping element N is 3.1-3.4wt%. When the content of the doping element is within the above range, the two-dimensional carbon sheet has good specific capacitance and rate performance.

[0049] In some specific embodiments of the present invention, the doping element Fe contained in the two-dimensional carbon sheet (at least partially) forms an Fe-C bond with the carbon element in the two-dimensional carbon sheet.

[0050] In some specific embodiments of the present invention, the doping element B (at least partially) contained in the two-dimensional carbon sheet forms a BN bond with the doping element N.

[0051] In some specific embodiments of the present invention, the thickness of the two-dimensional carbon sheet is 50-100 nm, which can be measured by scanning electron microscopy.

[0052] In some specific embodiments of the present invention, the specific capacitance of the two-dimensional carbon sheet is 295-320 F g -1 , the rate of the two-dimensional carbon sheet is 73%-88%. "Specific capacitance" refers to the amount of electricity that can be released by a unit mass of active material (i.e., carbon material), which can be measured by a constant current charge and discharge method. "Rate" refers to the discharge performance under different currents, mainly referring to the specific capacitance under a large current compared to the specific capacitance retention rate under a small current, which can be measured by a constant current charge and discharge method under different current densities. The present invention enhances the specific capacitance and rate performance of the two-dimensional carbon sheet by using Fe, B, and N anion-cation co-doped carbon nanosheets.

[0053] A second aspect of the present invention provides a method for preparing a two-dimensional carbon sheet, comprising the following steps:

[0054] (1) mixing water, biomass, a boron source and an iron source to obtain a hydrogel;

[0055] (2) placing the hydrogel at a low temperature below -5°C for standing and low temperature drying to obtain an aerogel;

[0056] (3) Carbonizing the aerogel to obtain the two-dimensional carbon sheet.

[0057] In the present invention, the boron source can be selected from various common substances capable of providing B, such as boric acid, borates, etc. In some specific embodiments of the present invention, the boron source is selected from one or more of boric acid, borates and boron oxide.

[0058] In the present invention, the iron source can be selected from various common substances that can provide Fe, such as iron salts. In some specific embodiments of the present invention, the iron source is selected from one or more of Fe2(SO4)3, FeCl3 and FeSO4.

[0059] In the present invention, the biomass can be any substance that can form a three-dimensional network structure gel in the presence of water. In some specific embodiments of the present invention, the biomass is provided by one or more of jelly seeds, sea stone flowers, and stone flower seeds. When jelly seeds and the like are used as the source of biomass, they are generally placed in water and scrubbed until the viscosity of the liquid no longer changes.

[0060] In some specific embodiments of the present invention, the mass ratio of the biomass to water is 1:6-10. Controlling the water ratio within the above preferred range can more effectively ensure the construction of the pores, further increase the effective specific surface area and further improve the structural stability of the material, thereby further improving the performance of the obtained two-dimensional carbon sheet.

[0061] In the present invention, in order to avoid the influence of impurities, deionized water is used.

[0062] In some specific embodiments of the present invention, the mass ratio of the boron source to the biomass is 1: 2-4. Controlling the boron source content within the above range is beneficial to further expand the carbon sheet, effectively regulating the thickness of the carbon sheet, and improving the electrochemical cycle stability of the carbon material.

[0063] In some specific embodiments of the present invention, the mass ratio of the iron source to the biomass is 1: 200-300. Controlling the iron source ratio within the above range is conducive to further uniform doping of Fe ions in the derived carbon, improving the uniformity of material performance, and will not form iron alone, which is beneficial to the single property of the carbon material.

[0064] In some specific embodiments of the present invention, in step (2), the temperature of the low temperature standing is -15°C to -25°C, and the time of the low temperature standing is 10-15 hours. Controlling the temperature of the low temperature standing within the above range is conducive to the further growth of ice crystals, controlling the growth rate, increasing the effective specific surface area, and is conducive to improving the adsorption of ions by the material.

[0065] In some specific embodiments of the present invention, the preparation method further comprises: low-temperature drying after the low-temperature standing, the temperature of the low-temperature drying is -15°C to -25°C, and the time of the low-temperature drying is 20-28 hours.

[0066] In some specific embodiments of the present invention, in step (3), the carbonization temperature is 600-800° C., and the carbonization time is 1-3 hours. The carbonization time refers to the time maintained at the carbonization temperature.

[0067] In some specific embodiments of the present invention, in step (3), the heating rate of carbonization is 1-2°C min -1 , i.e., at 1-2°C min -1 The temperature is raised to the carbonization temperature. Controlling the heating rate within the above range is beneficial to further control the shrinkage of the carbon sheet, improve the toughness of the carbon sheet, and the expander in the carbon nanosheet can be fully expanded, and the pore size distribution is more uniform.

[0068] In some specific embodiments of the present invention, the method for preparing the two-dimensional carbon sheet comprises the following steps:

[0069] (1) mixing water, biomass, a boron source and an iron source to obtain a hydrogel;

[0070] (2) placing the hydrogel at a low temperature below -5°C for standing and low temperature drying to obtain an aerogel;

[0071] (3) carbonizing the aerogel to obtain the two-dimensional carbon sheet;

[0072] Among them, preferably, the mass ratio of the biomass to water is 1:7-9; the mass ratio of the boron source to biomass is 1:2.5-3.5; the mass ratio of the iron source to biomass is 1:220-280; the low-temperature standing temperature is -18°C to -22°C, and the low-temperature standing time is 10-15h; the carbonization temperature is 650-750°C, and the carbonization time is 1.5-2.5h.

[0073] The third aspect of the present invention provides a preparation method for obtaining a two-dimensional carbon sheet.

[0074] A fourth aspect of the present invention provides an application of the two-dimensional carbon sheet in the preparation of a capacitor device.

[0075] The present invention will be described in detail below through examples.

[0076] In the following examples and comparative examples, if no specific conditions are specified, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used, if no manufacturer is specified, are all conventional products that can be obtained through commercial channels.

[0077] Example 1

[0078] 1) Weigh 5 g of jelly seeds, soak them in 40 g of deionized water and continuously scrub them until the viscosity of the scrubbing liquid no longer changes.

[0079] 2) Mix the scrubbing liquid with 15g of boric acid and 0.02g of Fe2(SO4)3 and pour into a culture dish equipped with a titanium mesh. After it is completely solidified, transfer it to a constant temperature box at -20℃ and keep it for 12 hours.

[0080] 3) After the completely frozen samples were dried at low temperature (-20°C, 24h), they were heated in N2 atmosphere at 1.5°C min -1 The temperature was raised to 700°C at a rate of 100 °C and kept at this temperature for 2 h. The final product was obtained after cooling.

[0081] Figure 1 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this embodiment. Figure 1 As shown in the figure, the material structure is obviously flaky and has a high aspect ratio. It can also be seen from the figure that the flaky structure is thin and there is no serious breakage or cracks in the structure, which indicates that it has good structural stability.

[0082] Example 2

[0083] 1) Weigh 5 g of jelly seeds, soak them in 25 g of deionized water and continuously scrub them until the viscosity of the scrubbing liquid no longer changes.

[0084] 2) Mix the scrubbing liquid with 15g of boric acid and 0.05g of Fe2(SO4)3 and pour into a culture dish equipped with a titanium mesh. After it is completely solidified, transfer it to a constant temperature box at -20℃ and keep it for 12 hours.

[0085] 3) After the completely frozen samples were dried at low temperature (-20°C, 24h), they were heated in N2 atmosphere at 1.5°C min -1 The temperature was raised to 700°C at a rate of 100 °C and kept at this temperature for 2 h. The final product was obtained after cooling.

[0086] Figure 2 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this embodiment. Figure 2 As shown in the figure, due to the low water content in the ratio of water to jelly seeds, the material structure is obviously flaky, but the thickness of the lamella is relatively thick. At the same time, obvious Fe aggregates appear in the lamella, which has an adverse effect on the structural stability and ion adsorption capacity of the material. The thicker lamellae show a dense structure, which proves that it lacks an effective microporous structure and reduces the effective specific surface area.

[0087] Example 3

[0088] 1) Weigh 5 g of jelly seeds, soak them in 55 g of deionized water and continuously scrub them until the viscosity of the scrubbing liquid no longer changes.

[0089] 2) Mix the scrubbing liquid with 15g of boric acid and 0.02g of Fe2(SO4)3 and pour into a culture dish equipped with a titanium mesh. After it is completely solidified, transfer it to a constant temperature box at -20℃ and keep it for 12 hours.

[0090] 3) After the completely frozen samples were dried at low temperature (-20°C, 24h), they were heated in N2 atmosphere at 1.5°C min -1 The temperature was raised to 700°C at a rate of 100 °C and kept at this temperature for 2 h. The final product was obtained after cooling.

[0091] Figure 3 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this embodiment. Figure 3 As shown in the figure, due to the excessive water content in the ratio of water to jelly seeds, the ice crystals grew excessively, destroying the integrity of the nanosheets. Although the material structure showed obvious flakes, the thickness of the flakes was too thin and there was obvious breakage and powdering. Too thin a flake structure cannot guarantee its stability during the ion adsorption process. At the same time, the broken nanosheets are very easy to agglomerate, which is not conducive to maintaining stable capacitance for a long time.

[0092] Example 4

[0093] 1) Weigh 5 g of jelly seeds, soak them in 40 g of deionized water and continuously scrub them until the viscosity of the scrubbing liquid no longer changes.

[0094] 2) Mix the scrubbing liquid with 25g of boric acid and 0.01g of Fe2(SO4)3 and pour into a culture dish equipped with a titanium mesh. After it is completely solidified, transfer it to a constant temperature box at -20℃ and keep it for 12 hours.

[0095] 3) After the completely frozen samples were dried at low temperature (-20°C, 24h), they were heated in N2 atmosphere at 1.5°C min -1 The temperature was raised to 700°C at a rate of 100 °C and kept at this temperature for 2 h. The final product was obtained after cooling.

[0096] Figure 4 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this embodiment. Figure 3 As shown in the figure, the boric acid content is too high and the carbonization process expands excessively. Although the material structure shows obvious flakes, the thickness of the flakes is too thin and there is obvious stacking phenomenon. The stacked nanosheets reduce the effective specific surface area and show a large amount of stacked ineffective area during the ion adsorption process, which is not conducive to the effective adsorption of ions.

[0097] Example 5

[0098] 1) Weigh 5 g of jelly seeds, soak them in 40 g of deionized water and continuously scrub them until the viscosity of the scrubbing liquid no longer changes.

[0099] 2) Mix the scrubbing liquid with 9g of boric acid and 0.02g of Fe2(SO4)3 and pour into a culture dish equipped with a titanium mesh. After it is completely solidified, transfer it to a constant temperature box at -20℃ and keep it for 12 hours.

[0100] 3) After the completely frozen samples were dried at low temperature (-20°C, 24h), they were heated in N2 atmosphere at 1.5°C min -1 The temperature was raised to 700°C at a rate of 100 °C and kept at this temperature for 2 h. The final product was obtained after cooling.

[0101] Figure 5 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this embodiment. Figure 5 As shown in the figure, the boric acid content is too low, and the expansion assistance is insufficient during the carbonization process. Therefore, the nanosheet thickness is relatively large, but no obvious Fe agglomeration occurs. This shows that the lack of boric acid expansion will directly affect the sheet thickness, but has little effect on ion doping. Of course, too thick a sheet is not conducive to ion adsorption.

[0102] Example 6

[0103] 1) Weigh 5 g of jelly seeds, soak them in 40 g of deionized water and continuously scrub them until the viscosity of the scrubbing liquid no longer changes.

[0104] 2) Mix the scrubbing liquid with 15g of boric acid and 0.02g of Fe2(SO4)3 and pour into a culture dish equipped with a titanium mesh. After it is completely solidified, transfer it to a constant temperature box at -30℃ and keep it for 12 hours.

[0105] 3) After the completely frozen samples were dried at low temperature (-20°C, 24h), they were heated in N2 atmosphere at 1.5°C min -1 The temperature was raised to 700°C at a rate of 100 °C and kept at this temperature for 2 h. The final product was obtained after cooling.

[0106] Figure 6 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 6. Figure 6 As shown, since the ice crystal growth temperature is too low, it is not conducive to the formation of uniform pores, so the material exhibits uneven distribution of holes, and most of them are large pores.

[0107] Comparative Example 1

[0108] 1) Weigh 5 g of jelly seeds, soak them in 40 g of deionized water and continuously scrub them until the viscosity of the scrubbing liquid no longer changes.

[0109] 2) Mix the scrubbing liquid with 15 g of boric oxide and 0.02 g of CoSO4 and pour into a culture dish equipped with a titanium mesh. After it is completely solidified, transfer it to a constant temperature box at -20°C and keep it for 12 hours.

[0110] 3) After the completely frozen samples were dried at low temperature (-20°C, 24h), they were heated in N2 atmosphere at 1.5°C min -1 The temperature was raised to 700°C at a rate of 100 °C and kept at this temperature for 2 h. The final product was obtained after cooling.

[0111] Fig.25 is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this comparative example. Fig.25 As shown in the figure, the material structure is a porous block structure with uneven pore distribution. It can also be seen from the figure that the block structure is relatively messy, and the pore structure on its structure is mostly shallow pore structure, which indicates that its actual effective specific surface area is not high.

[0112] Characterization results

[0113] Figure 7 The XRD patterns of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4 are shown in FIG. Figure 7As shown: The XRD curves of the four samples all show two characteristic peaks near 26° and 43°, corresponding to the (002) and (101) crystal planes of amorphous carbon, respectively. Compared with the pure carbon material (24°), the (002) peak has a certain offset due to the abundant heteroatom defects in its carbon lattice. The XRD of Example 2 at 43° (101) crystal plane is weaker than that of the other three, indicating that the excessive doping and agglomeration of Fe caused by too little water content reduces the graphitization of the material.

[0114] Figure 8 The Raman images of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4. Figure 8 Visible: In the Raman spectrum, the D band (representing disordered graphite) and the G band (from crystal defects) can be observed, and their intensity ratio (I D / I G ) proved that heteroatom doping can lead to slight destruction of the carbon lattice, thereby reducing the degree of graphitization of carbon materials.

[0115] Fig. 9 This is the XPS image of the two-dimensional carbon sheet synthesized in Example 1. Fig. 9 It can be seen that although biomass with complex components is used as raw material, the obtained product is relatively pure in elemental composition. It mainly contains carbon as the main body, a large amount of doping elements N, B and Fe elements, and O element in the surface functional group.

[0116] Fig.10 is the N spectrum of the two-dimensional carbon sheet synthesized in Example 1. Fig.10 It can be seen that there are three different N valence bonds, namely pyridinic-N, graphitic-N and pyrrolic-N, by peak separation of the N spectrum. The electron-rich N replaces some carbon atoms and brings electrons into the delocalized π system of the carbon material, which is beneficial to improve the conductivity. In addition, during the ion adsorption process, the N atoms will also serve as active sites for ion adsorption, providing additional pseudocapacitance.

[0117] Fig.11 This is the B spectrum of the two-dimensional carbon sheet synthesized in Example 1. Fig.11 It can be seen that the four peaks in the B1s spectrum can be indexed as -BC3, -BN, BC2O and -BCO2 respectively. Electron-poor boron doping causes the Fermi level to shift toward the conduction band, which will adjust the surface charge distribution through π electron redistribution. At the same time, the unbalanced surface electrostatic potential generated by the incorporation of boron atoms enhances the surface polarity. This polarization of the carbon surface will improve the wettability of electrolyte polar molecules and enable faster transmission of electrolyte ions.

[0118] Fig.12 The Fe spectrum of the two-dimensional carbon sheet synthesized in Example 1. Fig.12It can be seen that in the Fe 2p spectrum, the two strong peaks at 711.5eV and 724.6eV belong to Fe 2p 3 / 2 and Fe 2p 1 / 2 , which can be further fitted to Fe 2+ (711.0eV and 724.2eV) and Fe 3+ (713.5eV and 726.9eV), which also includes the valence bond relationship derived from Fe-C.

[0119] Fig.14 CV graphs of the two-dimensional carbon sheet synthesized in Example 1 at different current rates. Fig.14 It can be seen that the CV curve of the sample prepared in Example 1 at different current rates does not show obvious distortion, which indicates that it has good rate performance.

[0120] Fig.15 The pseudocapacitance ratio bar graph of the two-dimensional carbon sheets synthesized in Examples 1, 2, and 3. Fig.15 It can be seen that when the scan rate is increased from 5mV s -1 Rise to 100mV s -1 As the scanning rate increases, the capacitance contribution of the surface control gradually increases, which means that the double-layer capacitance plays a more dominant role as the scanning rate increases. At the same scanning rate, the capacitance contribution of the surface control process changes in the order of Example 4 < Example 3 < Example 2 < Example 1. Due to the increase in the degree of graphitization, the percentage of double-layer capacitance of Example 1 is higher than that of the other three. This is mainly because it has the best specific surface area and lamellar structure, which allows it to exert a more effective double-layer mechanism.

[0121] Application Examples

[0122] 1. Use the prepared two-dimensional carbon sheets to prepare semi-capacitive devices.

[0123] 1) The active material, namely the two-dimensional carbon sheet, conductive graphite and binder polytetrafluoroethylene (PTFE), was mixed with an appropriate amount of ethanol in a mass ratio of 85:10:5 and then coated on nickel foam (1 cm×1 cm) to obtain a working electrode.

[0124] 2) Dry the nickel foam working electrode in an oven at 60°C to a constant weight, and then press the electrode into a sheet at a pressure of 10 MPa.

[0125] 3) Prepare the working electrode, reference electrode, counter electrode, salt bridge and electrolyte, and assemble the semi-capacitive system.

[0126] 2. Use the prepared two-dimensional carbon sheets to prepare symmetrical capacitor devices.

[0127] 1) The active material, namely the two-dimensional carbon sheet, conductive graphite and binder polytetrafluoroethylene (PTFE), was mixed with an appropriate amount of ethanol in a mass ratio of 85:10:5 and then coated on nickel foam (1 cm×1 cm) to obtain a working electrode.

[0128] 2) Dry the nickel foam working electrode in an oven at 60°C to a constant weight, and then press the electrode into a sheet at a pressure of 10 MPa.

[0129] 3) Assemble symmetrical capacitors with electrode sheets, diaphragms and electrode liquids of the same mass of active material.

[0130] 3. Use the prepared two-dimensional carbon sheet to prepare a capacitive deionization device.

[0131] 1) The symmetrical capacitive deionization device is assembled from two 4cm×4cm titanium mesh electrodes loaded with active materials.

[0132] 2) The complete system includes a recovery container, a peristaltic pump, a CDI device, a DC power supply and a conductivity meter for detecting the salt solution. The specific operating parameters are: the feed salt solution concentration is 200 mg L -1 , 500mg L -1 and 1000mg L -1 ; Flow rate: 32mLmin -1 ;

[0133] 3) The total volume of the salt solution was 80 mL; the applied potential differences were 1.0 V, 1.2 V, 1.4 V and 1.6 V; and the solution temperature was 25 °C.

[0134] Application Characterization

[0135] Fig.13 The three-electrode CV curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4. Fig.13 It can be seen that the shapes of the CV curves of the four are close to rectangles, proving that the ion adsorption mechanism of the four is dominated by double-layer capacitance. At the same time, Example 1 shows the largest CV area. This proves that Example 1 has the best capacitance adsorption capacity. This is due to its appropriate sheet thickness and stable structure.

[0136] Fig.16 The three-electrode GCD curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4. Fig.16 It can be seen that the GCD curves of the four are close to the standard symmetrical isosceles triangle, and the coulombic efficiency of the four is close to 100%, which shows that their capacitance behavior is highly reversible. And Example 1 has the highest specific capacitance, which once again proves the rationality of the design scheme of Example 1.

[0137] Fig.17The three-electrode GCD curves of the two-dimensional carbon sheet under multiple current densities in Example 1. Fig.17 It can be seen that the GCD curves of Example 1 at different current densities show similar shapes, which indicates that it has good rate performance. -1 , 1.0A g -1 , 2.0A g -1 , 5.0A g -1 and 10A g -1 At a current density of , the specific capacitance of Example 1 is 306.05 F g -1 , 297.6F g -1 , 273.2F g -1 , 251.5F g -1 and 243F g -1 At a current density magnified twenty times, its specific capacitance can still be maintained at about 79.3%, which indicates its good rate performance.

[0138] Fig.18 The three-electrode impedance curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, and 3. Fig.18 It can be seen that Example 1 shows the lowest impedance data, which is because its large aspect ratio is conducive to the rapid transmission of electrons, and the thinner layer thickness also reduces the transmission path of electrons inside the layer.

[0139] Fig.19 CV curve of the symmetrical capacitor of the two-dimensional carbon sheet synthesized in Example 1. Fig.19 It can be seen that the CV curve of the symmetrical capacitor shows a standard rectangle, which indicates that the double-layer capacitance mechanism occupies the main position in the capacitor. At the same time, the CV curves at different rates show similar shapes, which also means that it has good rate performance.

[0140] Fig. 20 The GCD curve of the symmetrical capacitor of the two-dimensional carbon sheet synthesized in Example 1. Fig. 20 It can be seen that the symmetrical capacitor GCD assembled by the synthetic material of Example 1 is a standard symmetrical isosceles triangle. Its coulomb efficiency close to 100% indicates that its energy storage mechanism is highly reversible.

[0141] Fig.21 The ion adsorption-time curves of the CDI devices assembled from the two-dimensional carbon sheets synthesized in Examples 1, 2, and 3. Fig.21 It can be seen that when a potential (1.2V) is applied, the concentration of the NaCl solution under the corresponding electrodes of the four samples will drop rapidly. Among them, Example 1 shows the best ion adsorption capacity, which is much greater than the other three. This shows that the ion adsorption capacity of the material is enhanced by optimizing the sheet thickness and doping with heteroatoms.

[0142] Fig. 22 The solution conductivity-time curve of the CDI device assembled with the two-dimensional carbon sheet synthesized in Example 1 under different concentrations of salt water. Fig. 22 It can be seen that as the concentration of salt water increases, the amount of salt adsorbed by the electrode continues to increase. This phenomenon can be attributed to the following two reasons: 1) In high-concentration salt solutions, carbon nanosheets and their pores can more easily capture salt ions; 2) High-concentration salt solutions have higher conductivity, which is conducive to the rapid transmission of ions. At the same time, this also shows that the material has good adsorption capacity in different salt solutions.

[0143] Fig.23 The cyclic desalination-regeneration curve of the CDI device assembled with the two-dimensional carbon sheet synthesized in Example 1. Fig.23 It can be seen that after 1000 min of continuous operation, it still maintains more than 97.3% of the adsorption capacity, showing excellent cycle stability.

[0144] Fig.24 This is a physical picture of the two-dimensional carbon sheet synthesized in Example 1. Fig.24 It can be seen that the material has a stable and fluffy modular structure.

[0145] The performance of the two-dimensional carbon sheets obtained in the examples and comparative examples was tested. The thickness of the sheets was measured by transmission electron microscopy. The specific capacitance was measured by 0.1A g -1 The constant current charge and discharge method was used to measure the rate. -1 and 1A g -1 ) under constant current charge and discharge method; the element proportions were measured by XPS method, and the results are shown in Table 1.

[0146] Table 1

[0147]

[0148] From the results in Table 1, it can be seen that the two-dimensional nanocarbon sheet prepared by Example 1 of the present invention has an optimal sheet thickness, that is, it has a high specific surface area and maintains high structural stability. Correspondingly, its specific capacitance, rate performance and other aspects are significantly better.

[0149] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A two-dimensional carbon sheet, characterized in that: The two-dimensional carbon sheet has a layered structure and also contains doping elements Fe, B and N. Based on the total amount of the two-dimensional carbon sheet, the content of the doping element Fe is 0.8-3.5wt%, the content of the doping element B is 5-8.8wt%, and the content of the doping element N is 2.8-3.5wt%.

2. The two-dimensional carbon sheet according to claim 1, wherein: The content of the doping element Fe is 1.5-2.5wt%, the content of the doping element B is 7-8wt%, and the content of the doping element N is 3-3.5wt%; Preferably, the doping element Fe contained in the two-dimensional carbon sheet forms an Fe-C bond with the carbon element in the two-dimensional carbon sheet; Preferably, the doping element B contained in the two-dimensional carbon sheet forms a BN bond with the doping element N.

3. The two-dimensional carbon sheet according to claim 1 or 2, wherein: The two-dimensional carbon sheet has a sheet thickness of 50-100 nm.

4. The two-dimensional carbon sheet according to any one of claims 1 to 3, wherein: The specific capacitance of the two-dimensional carbon sheet is 295-320 F g -1 , the magnification of the two-dimensional carbon sheet is 78-88%.

5. A method for preparing a two-dimensional carbon sheet, characterized in that: The steps include: (1) mixing water, biomass, a boron source and an iron source to obtain a hydrogel; (2) placing the hydrogel at a low temperature below -5°C for standing and low temperature drying to obtain an aerogel; (3) Carbonizing the aerogel to obtain the two-dimensional carbon sheet.

6. The preparation method according to claim 5, wherein: The boron source is selected from one or more of boric acid, borate and boron oxide; Preferably, the iron source is selected from one or more of Fe2(SO4)3, FeCl3 and FeSO4; Preferably, the biomass is selected from one or more of agar-agar seeds, sea agar, and agar-agar seeds.

7. The preparation method according to claim 5 or 6, wherein: The mass ratio of the biomass to water is 1:6-10; Preferably, the mass ratio of the boron source to the biomass is 1:2-4; Preferably, the mass ratio of the iron source to the biomass is 1:200-300.

8. The preparation method according to any one of claims 5 to 7, wherein: In step (2), the low temperature standing temperature is -15°C to -25°C, and the low temperature standing time is 10-15h; Preferably, the low-temperature drying temperature is -15°C to -25°C, and the low-temperature drying time is 20-28h; Preferably, in step (3), the carbonization temperature is 600-800° C. and the carbonization time is 1-3 h; Preferably, the heating rate of carbonization is 1-2°C min -1 .

9. A two-dimensional carbon sheet obtained by the preparation method according to any one of claims 5 to 8.

10. Use of the two-dimensional carbon sheet according to any one of claims 1 to 4 and 9 in preparing a capacitor device.

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