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 which contains a lamellar structure, a doping element N and an oxygen element, the atom number of the doping element N accounts for 4.5-8.8% of the total atom number, and the atom number of the oxygen element accounts for 12% or above of the total atom number. The sheet layer surface of the two-dimensional carbon sheet has abundant and uniform high-oxygen-content functional groups, shows high pseudocapacitance performance, and can be used as an adsorption site of salt ions. The thickness of the carbon plate has a highly adaptive regulation range, so that the desalting adaptability to different water bodies is enhanced.

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] Electrochemical capacitance behavior is a capacitance 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. Based on this mechanism, a variety of applications have been derived, such as supercapacitors and capacitive deionization. For devices derived from electrochemical capacitance behavior, the capacitance performance of the electrode material is the main factor affecting the devices derived from electrochemical capacitance behavior. As a typical high capacitance material, two-dimensional carbon sheet materials have received widespread application and attention.

[0003] Two-dimensional carbon sheets are often used as electrode materials due to their large lateral size, large specific surface area and thin thickness. Biomass, which is abundant, non-toxic and cheap, is a new type of two-dimensional carbon material precursor with great application prospects. At present, most of the research is carried out on solid biomass. For example, Wu et al. (RSC Adv., 2016, 6, 29996-30003) used water hyacinth (WHs) as a carbon precursor, and prepared two-dimensional macroporous sheets with large specific surface area and large void volume through acid treatment, carbonization and KOH alkaline activation. Chen et al. (J. Electroanal. Chem., 2016, 5, 18-26) used wheat platycodon containing a multi-layered structure of cellulose, hemicellulose and lignin to synthesize highly graphitized graphene materials through hydrothermal and graphitization. At present, most of the research is related to strong corrosive activators such as KOH. 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, there are also some green activators. In recent years, people have begun to use boric acid as an activator. For example, Zhang et al. (J. Hazard. Mater., 2022, 2, 127333) used boric acid as an activator to activate hickory sawdust to make biochar have a higher specific surface area. Boric acid is a low-corrosive, high-efficiency, and recyclable activator, but it produces a large amount of gas during activation, causing the volume of the material to expand, which puts forward new requirements for the capacity and carrying capacity of the device. At the same time, biomass-derived carbon materials often have rich surface functional groups and heteroatom defects. However, this type of structure is often difficult to regulate and unify, so its capacitance performance undergoes some uncontrollable changes and reductions. In short, designing a reasonable and safe strategy for biomass conversion to two-dimensional carbon sheets is an important research direction, and it is necessary to take into account the regulation of non-carbon elements on the surface of two-dimensional carbon sheets. Summary of the invention

[0004] The purpose of the present invention is to overcome the problems of the prior art in that the surface functional groups of two-dimensional carbon materials are not uniform, the thickness of the carbon sheets is difficult to control, and the material uniformity and electrical properties are poor, and to provide a two-dimensional carbon sheet and its preparation method and application.

[0005] 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, a doping element N and an oxygen element, wherein the percentage of the number of atoms of the doping element N to the total number of atoms is 4.5-8.8%, and the percentage of the number of atoms of the oxygen element to the total number of atoms is more than 12%.

[0006] A second aspect of the present invention provides a method for preparing a two-dimensional carbon sheet, comprising the following steps:

[0007] (1) in the presence of water, allowing the nitrogen-containing polysaccharide raw material to undergo a cross-linking reaction to obtain a hydrogel;

[0008] (2) placing the hydrogel at a temperature below 0°C for a low-temperature static treatment, and then low-temperature drying to obtain an aerogel;

[0009] (3) carbonizing the aerogel to obtain a carbon sheet material;

[0010] (4) Under an oxygen atmosphere, the carbon sheet material is subjected to high-temperature oxidation 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 the following beneficial effects:

[0014] (1) The surface of the two-dimensional carbon sheet of the present invention has abundant and uniform high oxygen-containing functional groups, exhibiting high pseudocapacitive performance and can serve as adsorption sites for salt ions. The thickness of the carbon sheet has a highly adaptable control range, enhancing the desalination adaptability to different water bodies.

[0015] (2) The present invention regulates the thickness of the sheet and the interface functional groups by oxygen induction, thereby improving the problem of uncontrollable and non-uniform functional groups on the surface of traditional bio-derived materials and effectively regulating the capacitance stability of bio-derived materials. The controllable regulation of the sheet thickness makes the material have better application applicability and prospects. Under low temperature, the water component in the hydrogel is converted into ice with a larger volume at an appropriate rate, and the biomass is squeezed to achieve the purpose of shaping. The basic structure of the sheet is constructed with the assistance of the ice template, and then the thickness of the sheet is controlled by slow oxidation. The high corrosiveness and high risk of conventional carbon sheet activation methods are effectively avoided.

[0016] (3) 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. The low-cost and reusable ice template used in this work constructs the morphological matrix, and then the sheet thickness is slowly oxidized and adjusted through a low-risk oxidation treatment. Compared with the highly corrosive and high-risk traditional strategies, this process has low equipment requirements and reduces equipment costs. Low risk also reduces operating costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 1.

[0018] Figure 2 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 2.

[0019] Figure 3 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 3.

[0020] Figure 4 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 4.

[0021] Figure 5 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 5.

[0022] Figure 6 This is a transmission electron microscope image of the two-dimensional carbon sheet synthesized in Example 1.

[0023] Figure 7 This is a transmission electron microscope image of the two-dimensional carbon sheet synthesized in Example 2.

[0024] Figure 8 This is a transmission electron microscope image of the two-dimensional carbon sheet synthesized in Example 3.

[0025] Fig. 9 XRD patterns of the two-dimensional carbon sheets synthesized in Examples 1, 2 and 3.

[0026] Fig.10 Raman images of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0027] Fig.11 Infrared spectra of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3, 4 and 5.

[0028] Fig.12 This is a three-electrode CV curve of the two-dimensional carbon sheet synthesized in Example 1.

[0029] Fig.13 This is a three-electrode GCD curve of the two-dimensional carbon sheet synthesized in Example 1.

[0030] Fig.14This is the three-electrode impedance curve of the two-dimensional carbon sheet synthesized in Example 1.

[0031] Fig.15 Three-electrode CV curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0032] Fig.16 This is the full XPS spectrum of the two-dimensional carbon sheet synthesized in Example 1.

[0033] Fig.17 This is the XPS analysis C fine spectrum of the two-dimensional carbon sheet synthesized in Example 1.

[0034] Fig.18 This is the N fine spectrum of the XPS analysis of the two-dimensional carbon sheet synthesized in Example 1.

[0035] Fig.19 The three-electrode GCD curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4.

[0036] Fig. 20 This is the CV curve of the symmetrical capacitor of the two-dimensional carbon sheet synthesized in Example 1.

[0037] Fig.21 This is the GCD curve of the symmetrical capacitor of the two-dimensional carbon sheet synthesized in Example 1.

[0038] Fig. 22 The solution conductivity-time curves of the two-dimensional carbon sheets synthesized in Examples 1, 2 and 3 in saline solutions of different concentrations.

[0039] Fig.23 The material desalination capacity-time curves of the two-dimensional carbon sheets synthesized in Examples 1, 2 and 3 under different brine concentrations.

[0040] Fig.24 This is the electrode cycle desalination-regeneration curve of the two-dimensional carbon sheet synthesized in Example 1.

[0041] Fig.25 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Comparative Example 1. DETAILED DESCRIPTION

[0042] 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.

[0043] The first aspect of the present invention provides a two-dimensional carbon sheet, wherein the two-dimensional carbon sheet contains a layered structure, a doping element N and an oxygen element, wherein the percentage of the doping element N atoms to the total atomic number is 4.5-8.8%, and the percentage of the oxygen element atoms to the total atomic number is more than 12%.

[0044] In the present invention, the sheet thickness and interface functional groups are regulated by oxygen induction, which improves the problem of uncontrollable and non-uniform surface functional groups of traditional bio-derived materials, effectively adjusts the capacitance stability of bio-derived materials, and makes the two-dimensional carbon sheet have better specific capacitance and desalination ability. The controllable regulation of sheet thickness makes the material have better application applicability and prospects. Under low temperature, the water component in the hydrogel is converted into ice with a larger volume at an appropriate rate, and the biomass is squeezed to achieve the purpose of shaping. The basic structure of the sheet is constructed with the assistance of the ice template, and then the thickness of the sheet is controlled by slow oxidation. The high corrosiveness and high risk of conventional carbon sheet activation methods are effectively avoided.

[0045] In some specific embodiments of the present invention, the percentage of the doping element N atoms in the total atoms is 5-8%, and the percentage of the oxygen atoms in the total atoms is more than 12%, preferably, the percentage of the oxygen atoms in the total atoms is 12-14%. It can be measured by X-ray photoelectron spectroscopy (XPS) method.

[0046] In some specific embodiments of the present invention, more preferably, the percentage of doping element N atoms in the total number of atoms is 6.5-7.5%, and the percentage of oxygen atoms in the total number of atoms is 12-14%, so that the two-dimensional carbon sheet has better specific capacitance and desalination ability.

[0047] In some specific embodiments of the present invention, the thickness of the two-dimensional carbon sheet is 20-40 nm, which can be measured by scanning electron microscopy.

[0048] In some specific embodiments of the present invention, the specific capacitance of the two-dimensional carbon sheet is 295-320 F g -1 , the desalination capacity at 1.2 V was 22–32 mg g -1 Specific capacitance refers to the amount of electricity that can be released by a unit mass of active material (i.e., carbon material); it can be measured by constant current charge and discharge method. "Desalination capacity" refers to the amount of salt (NaCl) that can be adsorbed by a unit mass of active material, which can be measured by applying a constant voltage method.

[0049] A second aspect of the present invention provides a method for preparing a two-dimensional carbon sheet, comprising the following steps:

[0050] (1) in the presence of water, allowing the nitrogen-containing polysaccharide raw material to undergo a cross-linking reaction to obtain a hydrogel;

[0051] (2) placing the hydrogel at a temperature below 0°C for a low-temperature static treatment, and then low-temperature drying to obtain an aerogel;

[0052] (3) carbonizing the aerogel to obtain a carbon sheet material;

[0053] (4) Under an oxygen atmosphere, the carbon sheet material is subjected to high-temperature oxidation to obtain the two-dimensional carbon sheet.

[0054] In some specific embodiments of the present invention, the mass ratio of the nitrogen-containing polysaccharide raw material to water is 1:9-13. 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.

[0055] In the present invention, the nitrogen-containing polysaccharide raw material 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 nitrogen-containing polysaccharide raw material is selected from one or more of ice powder seeds, sea stone flowers, and stone flower seeds. When using jelly seeds and the like as the nitrogen-containing polysaccharide raw material, it is generally placed in water and rubbed until the liquid viscosity no longer changes.

[0056] In some specific embodiments of the present invention, in step (2), the temperature of the low-temperature static treatment is -15°C to -25°C, and the time of the low-temperature static treatment is 10-15 hours. Controlling the temperature of the low-temperature static treatment within the above range is conducive to the further growth of ice crystals, controlling the growth rate, increasing the effective specific surface area, and improving the adsorption of ions by the material.

[0057] In some specific embodiments of the present invention, the preparation method further comprises: low-temperature drying after the low-temperature standing treatment, 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.

[0058] 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 h.

[0059] In some specific embodiments of the present invention, the carbonization heating rate 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 uniform.

[0060] In some specific embodiments of the present invention, in step (4), the temperature of the high temperature oxidation is 250-350°C, and the time of the high temperature oxidation is 30-60 minutes. Controlling the temperature of the high temperature oxidation within the above range is conducive to further uniformly allowing oxygen to enter the microstructure of the carbon material, so that oxygen induction occurs inside the material, achieving complete modification of the material, and being able to well control the thickness of the sheet. Controlling the high temperature oxidation within the above range is conducive to further improving the toughness of the material, increasing the yield of the material, being conducive to the stable occurrence and uniformity of the oxidation reaction, being able to well control the thickness of the sheet, and being conducive to efficient production and application.

[0061] In some specific embodiments of the present invention, the heating rate of the high temperature oxidation is 1-2°C min -1 , i.e., at 1-2°C min -1 The temperature is raised to the high-temperature oxidation temperature; controlling the heating rate within the above range is beneficial to further improve the oxidation reaction stability of the material and improve the mechanical strength of the material.

[0062] In some specific embodiments of the present invention, the oxygen atmosphere is a mixture of oxygen and an inert gas, wherein the inert gas is selected from N2 and / or Ar.

[0063] In some specific embodiments of the present invention, the mixing volume ratio of oxygen to inert gas is 1:17-20. Controlling the mixing ratio of oxygen to inert gas within the above range is beneficial to further ensure the integrity of the sheet structure and ensure the stable and uniform oxidation reaction, thereby further improving the performance of the obtained two-dimensional carbon sheet.

[0064] According to the most preferred embodiment of the present invention, the method comprises:

[0065] (1) in the presence of water, allowing the nitrogen-containing polysaccharide raw material to undergo a cross-linking reaction to obtain a hydrogel;

[0066] (2) placing the hydrogel at a temperature below 0°C for a low-temperature static treatment, and then low-temperature drying to obtain an aerogel;

[0067] (3) carbonizing the aerogel to obtain a carbon sheet material;

[0068] (4) Under an oxygen atmosphere, the carbon sheet material is subjected to high temperature oxidation to obtain the two-dimensional carbon sheet;

[0069] Among them, the mass ratio of the nitrogen-containing polysaccharide raw material to water is 1:9-13; the temperature of the low-temperature standing treatment is -18°C to -22°C, and the time of the low-temperature standing treatment is 10-15h; the mixed volume ratio of oxygen to inert gas is 1:18-20.

[0070] The third aspect of the present invention provides a preparation method for obtaining a two-dimensional carbon sheet.

[0071] A fourth aspect of the present invention provides an application of the two-dimensional carbon sheet in the preparation of a capacitor device.

[0072] The present invention will be described in detail below through examples.

[0073] 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.

[0074] Example 1

[0075] 1) Weigh 5 g of ice powder seeds, soak them in 55 g of deionized water and continuously scrub them until the viscosity of the scrubbing solution no longer changes. After standing, the solution itself undergoes a cross-linking reaction to obtain a hydrogel.

[0076] 2) The hydrogel was transferred to a low-temperature environment of -20°C for 12 hours, and the ice crystals in the hydrogel grew rapidly to squeeze the biomass and initially build abundant pores. The hydrogel was then completely dried at -20°C for 24 hours to obtain a fluffy aerogel.

[0077] 3) Place the aerogel in an inert gas environment at 1.5 °C min -1 The temperature was raised to 700°C at a rate of 2 h and a thicker carbon sheet material was obtained after the constant temperature time was 2 h.

[0078] 4) Place the corundum plate with a thin layer of carbon flake material dispersed in a high temperature environment and introduce a mixture of oxygen and argon with a volume ratio of 1:19 at 1.5℃min -1 The temperature is raised to 300°C and kept at this temperature for 40 minutes. Finally, the high capacitance two-dimensional nanosheet material is obtained after the temperature is lowered.

[0079] Figure 1 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this embodiment. Figure 1 It can be seen that the material exhibits a two-dimensional nanosheet structure with a large aspect ratio and a relatively thin thickness of about 20nm. In addition, obvious curling can be observed but no breakage occurs, which indicates that the thickness and material toughness are well coordinated. This will show good stability during the capacitive ion adsorption process.

[0080] Example 2

[0081] 1) Weigh 5 g of ice powder seeds, soak them in 40 g of deionized water and continuously scrub them until the viscosity of the scrubbing solution no longer changes. After standing, the solution itself undergoes a cross-linking reaction to obtain a hydrogel.

[0082] 2) The hydrogel was transferred to a low-temperature environment of -25°C for 12 hours, and the ice crystals in the hydrogel grew rapidly to squeeze the biomass and initially build abundant pores. Then, the hydrogel was completely low-temperature dried at -20°C for 24 hours to obtain fluffy aerogel.

[0083] 3) Place the aerogel in an inert gas environment at 2°C min -1 The temperature was raised to 700°C at a rate of 2 h and a thicker carbon sheet material was obtained after the constant temperature time was 2 h.

[0084] 4) Place the corundum plate with dispersed carbon flakes in a high temperature environment and introduce a mixture of oxygen and argon with a volume ratio of 1:19 at 1.5℃min -1 The temperature is raised to 300°C and kept at this temperature for 40 minutes. Finally, the high capacitance two-dimensional nanosheet material is obtained after the temperature is lowered.

[0085] Figure 2 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in this embodiment. Figure 2 It can be seen that due to the low water content in the ratio of water to ice powder seeds, the biomass agglomerates more tightly. This makes the derived carbon material thicker and exhibits a low specific surface area. At the same time, due to the fast heating rate and high temperature, the material has obvious cracks or even breaks. This further makes the material have low capacitance stability.

[0086] Example 3

[0087] 1) Weigh 5 g of ice powder seeds, soak them in 55 g of deionized water and continuously scrub them until the viscosity of the scrubbing solution no longer changes. After standing, the solution itself undergoes a cross-linking reaction to obtain a hydrogel.

[0088] 2) The hydrogel was transferred to a low-temperature environment of -20°C for 12 hours, and the ice crystals in the hydrogel grew rapidly to squeeze the biomass and initially build abundant pores. The hydrogel was then completely dried at -20°C for 24 hours to obtain a fluffy aerogel.

[0089] 3) Place the aerogel in an inert gas environment at 1.5 °C min -1 The temperature was raised to 700°C at a rate of 2 h and a thicker carbon sheet material was obtained after the constant temperature time was 2 h.

[0090] 4) Place the corundum plate with dispersed carbon flakes in a high temperature environment and introduce a mixture of oxygen and argon with a volume ratio of 1:15 at 1.5℃min -1 The temperature is raised to 300°C and kept at this temperature for 40 minutes. Finally, the high capacitance two-dimensional nanosheet material is obtained after the temperature is lowered.

[0091] Figure 3 FIG. 3 is a scanning electron microscope image of the two-dimensional carbon sheet of this embodiment. Figure 3It can be seen that due to the high proportion of oxygen in the oxygen-induced carbon oxidation process, the oxidation reaction is relatively fast and violent. Therefore, the carbon nanosheets are obviously broken, and due to the uneven oxidation, the thickness of the carbon nanosheets is obviously uneven. This will lead to a significant reduction in the uniformity of material properties.

[0092] Example 4

[0093] 1) Weigh 5 g of ice powder seeds, soak them in 70 g of deionized water and continuously scrub them until the viscosity of the scrubbing solution no longer changes. After standing, the solution itself undergoes a cross-linking reaction to obtain a hydrogel.

[0094] 2) The hydrogel was transferred to a low-temperature environment of -10°C for 12 hours, and the ice crystals in the hydrogel grew rapidly to squeeze the biomass and initially build abundant pores. Then, the hydrogel was completely low-temperature dried at -20°C for 24 hours to obtain fluffy aerogel.

[0095] 3) Place the aerogel in an inert gas environment at 1.5 °C min -1 The temperature was raised to 700°C at a rate of 2 h and a thicker carbon sheet material was obtained after the constant temperature time was 2 h.

[0096] 4) Place the corundum plate with dispersed carbon flakes in a high temperature environment and introduce a mixture of oxygen and argon with a volume ratio of 1:19 at 1.5℃min -1 The temperature is raised to 300°C and kept at this temperature for 40 minutes. Finally, the high capacitance two-dimensional nanosheet material is obtained after the temperature is lowered.

[0097] Figure 4 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 4. Figure 4 It can be seen that due to the excessive water content in the hydrogel and the high ice crystal growth temperature, the growing ice crystals are uneven, and the derived carbon nanosheets also become of different thicknesses, which directly affects the uniformity of the material's capacitance performance.

[0098] Example 5

[0099] 1) Weigh 5 g of ice powder seeds, soak them in 55 g of deionized water and continuously scrub them until the viscosity of the scrubbing solution no longer changes. After standing, the solution itself undergoes a cross-linking reaction to obtain a hydrogel.

[0100] 2) The hydrogel was transferred to a low-temperature environment of -25°C for 12 hours, and the ice crystals in the hydrogel grew rapidly to squeeze the biomass and initially build abundant pores. Then, the hydrogel was completely low-temperature dried at -20°C for 24 hours to obtain fluffy aerogel.

[0101] 3) Place the aerogel in an inert gas environment at 1.5 °C min -1The temperature was raised to 700°C at a rate of 2 h and a thicker carbon sheet material was obtained after the constant temperature time was 2 h.

[0102] 4) Place the corundum plate with dispersed carbon flakes in a high temperature environment and introduce a mixture of oxygen and argon with a volume ratio of 1:19 at 2.5℃min -1 The temperature is raised to 350°C and kept at this temperature for 70 minutes. Finally, the high capacitance two-dimensional nanosheet material is obtained after the temperature is lowered.

[0103] Figure 5 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Example 5. Figure 5 It can be seen that due to the excessively fast oxygen-induced temperature rise and the long constant temperature time, the oxidation reaction occurs excessively. Although the material exhibits a uniform flake shape, its mechanical strength is significantly reduced, and it breaks and fractures. This is not conducive to the capacitor application of the material.

[0104] Comparative Example 1

[0105] 1) Weigh 5 g of ice powder seeds, soak them in 55 g of deionized water and continuously scrub them until the viscosity of the scrubbing solution no longer changes. After standing, the solution itself undergoes a cross-linking reaction to obtain a hydrogel.

[0106] 2) The hydrogel was transferred to a low-temperature environment of -20°C for 12 hours, and the ice crystals in the hydrogel grew rapidly to squeeze the biomass and initially build abundant pores. The hydrogel was then completely dried at -20°C for 24 hours to obtain a fluffy aerogel.

[0107] 3) Place the aerogel in an inert gas environment at 1.5 °C min -1 The temperature was raised to 700°C at a rate of 2 h and a thicker carbon sheet material was obtained after the constant temperature time was 2 h.

[0108] Fig.25 This is a scanning electron microscope image of the two-dimensional carbon sheet synthesized in Comparative Example 1. Fig.25 It can be seen that the material exhibits a two-dimensional nanosheet structure with a large aspect ratio, but the sheets are relatively thick. This is due to the lack of oxidation to control the sheet thickness.

[0109] Characterization results

[0110] Figure 6 This is a transmission electron microscope image of the two-dimensional carbon sheet synthesized in Example 1. Figure 6 It can be seen that the material exhibits a distinct flake structure and its thickness is uniform. It is worth noting that there is no breakage or fragmentation at the bending and folding of the carbon sheet, which indicates that after the thickness of the sheet is adjusted, it has good mechanical toughness and strength.

[0111] Figure 7 This is a transmission electron microscope image of the two-dimensional carbon sheet synthesized in Example 2. Figure 7It can be seen that due to the small number of ice crystals, the basic morphology of the material is poorly constructed, and it exhibits a thicker lamellar structure, which will lead to a decrease in the specific surface area of ​​the material and fail to provide sufficient capacitive adsorption area.

[0112] Figure 8 This is a transmission electron microscope image of the two-dimensional carbon sheet synthesized in Example 3. Figure 8 It can be seen that due to excessive oxygen induction, the material is broken and powdered, and shows agglomeration in TEM. This shows that the material cannot maintain a uniform two-dimensional sheet structure, and a large amount of invalid volume appears, which reduces the volume capacitance of the material.

[0113] Fig. 9 The XRD patterns of the two-dimensional carbon sheets synthesized in Examples 1, 2 and 3 are shown in FIG. Fig. 9 See: The XRD curves of the three samples all show the (002) and (101) crystal planes of amorphous carbon. The (002) peak is offset to about 27° due to the abundant heteroatom defects in the carbon lattice. The XRD crystal plane of Example 2 is weaker than the other two, which is due to its weaker degree of graphitization.

[0114] Fig.10 The Raman images of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4. Fig.10 It can be seen that the D band (representing disordered graphite) and the G band (from crystal defects) can be observed in the Raman spectrum. Therefore, it can be observed that excessive oxygen induction reduces the degree of graphitization of carbon sheets.

[0115] Fig.11 Infrared images of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3, 4 and 5. Fig.11 It can be seen that although biomass with complex components is used as raw material, its surface functional groups show uniformity through oxygen induction treatment. The different degrees of oxygen induction only change the characteristic peak intensity, which corresponds to the number of functional groups. This shows that the oxygen induction in this work is highly efficient in regulating non-carbon elements.

[0116] Fig.16 This is the full XPS spectrum of the two-dimensional carbon sheet synthesized in Example 1. Fig.16 It can be seen that the carbon sheet synthesized in Example 1 mainly contains three kinds of heteroatoms: C, N and O, and the N response peak is significantly lower than that of general biomass-derived carbon materials. This shows that the oxygen induction process has a good regulatory effect on the heteroatoms in the material, reducing the uncontrollability of the disordered functional groups.

[0117] Fig.17 This is the XPS analysis C spectrum of the two-dimensional carbon sheet synthesized in Example 1. Fig.17It can be seen that there are four main carbon bonds in the C spectrum, namely CC (77.98%), CN (9.36%), CO (7.74%) and C=O (4.92%). This shows that oxygen-induced regulation of functional groups does not completely eliminate the existence of functional groups, but regulates them to oxygen-containing functional groups and reduces other heteroatom functional groups.

[0118] Fig.18 This is the XPS analysis N fine spectrum of the two-dimensional carbon sheet synthesized in Example 1. Fig.18 It can be seen that there are three different N valence bonds in the N spectrum, namely pyridine-N, graphite-N and pyrrole-N. The N element replaces a small number of carbon atoms, bringing unmatched electrons into the delocalized π system of the carbon material, which is beneficial to improve the conductivity. In addition, the defective areas derived from N atoms will also serve as active sites, providing additional pseudocapacitance.

[0119] Application Examples

[0120] 1. Use the prepared ultra-thin two-dimensional nanosheet material to prepare semi-capacitive devices.

[0121] 1) The active material, namely two-dimensional nanosheets, 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.

[0122] 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.

[0123] 3) Prepare the working electrode, reference electrode, counter electrode, salt bridge and electrolyte, and assemble the semi-capacitive system.

[0124] 2. Use the prepared ultra-thin two-dimensional nanosheet material to prepare symmetrical capacitor devices.

[0125] 1) The active material, namely two-dimensional nanosheets, 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.

[0126] 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.

[0127] 3) Assemble symmetrical capacitors with electrode sheets, diaphragms and electrode liquids of the same mass of active material.

[0128] 3. Use the prepared ultra-thin two-dimensional nanosheet material to prepare capacitive deionization devices.

[0129] 1) The symmetrical capacitive deionization device is assembled by applying the above-prepared capacitive slurry on an electrode plate consisting of a 4 cm×4 cm titanium plate.

[0130] 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 , 500mgL -1 and 1000mg L -1 ; Flow rate: 32 mL min -1 ;

[0131] 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.

[0132] Application Characterization

[0133] Fig.12 The three-electrode CV curve of the two-dimensional carbon sheet synthesized in Example 1 is shown in FIG. Fig.12 It can be seen that in the three-electrode structure, the material exhibits a non-rectangular CV shape, which indicates that there is a certain pseudocapacitive reaction mechanism in the material. -1 There is no obvious deformation in the CV curve within the large current rate range, which indicates that it has good rate performance.

[0134] Fig.13 The three-electrode GCD curve of the two-dimensional carbon sheet synthesized in Example 1. Fig.13 It can be seen that the material exhibits a coulombic efficiency close to 100%, which shows that its ion adsorption mechanism is highly reversible. And from the GCD curve, it can be seen that it has a high ion adsorption capacity. This will provide sufficient potential for its application in capacitive devices.

[0135] Fig.14 The three-electrode impedance curve of the two-dimensional carbon sheet synthesized in Example 1. Fig.14 It can be seen from the impedance diagram that the material has a low impedance, which is conducive to the rapid transmission of electrons and ions. This also explains the reason for the excellent rate performance of the material.

[0136] Fig.15 The three-electrode CV curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4. Fig.15 It can be seen that the material prepared in Example 1 has the largest CV area, which indicates that it has the highest capacitance adsorption capacity. This is because it has the best sheet thickness and specific surface area.

[0137] Fig.19 The three-electrode GCD curves of the two-dimensional carbon sheets synthesized in Examples 1, 2, 3 and 4. Fig.19It can be seen that Example 1 exhibits the largest specific capacitance, which indicates that Example 1 has the best ion adsorption capacity. And all four have high coulombic efficiency, which indicates that the ion adsorption mechanism of the material itself has good reversibility.

[0138] Fig. 20 CV 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 exhibits a symmetrical CV curve, and no obvious polarization phenomenon occurs after the current rate is exaggerated. This shows that the material has excellent potential for symmetrical capacitor applications.

[0139] Fig.21 The GCD curve of the symmetrical capacitor of the two-dimensional carbon sheet synthesized in Example 1. Fig.21 It can be seen that the GCD curves are all close to the standard symmetrical isosceles triangle, and the Coulomb efficiency of the four is close to 100%. This shows that its capacitance behavior is highly reversible. At the same time, it can also be found that it has good energy density and power density.

[0140] Fig. 22 The solution conductivity-time curves of the CDI devices assembled with the two-dimensional carbon sheets synthesized in Examples 1, 2 and 3 under different saline concentrations. Fig. 22 It can be seen that the conductivity change in the circulating salt solution is recorded to evaluate the CDI performance of each material. When the external voltage is initially turned on, the solution conductivity decreases rapidly, and finally gradually stabilizes after the cut-off voltage reaches 1.2V. It can also be observed that the carbon sheet synthesized in Example 1 has the largest decrease in salt water conductivity, which indicates that it has the best ion adsorption capacity.

[0141] Fig.23 The material desalination capacity-time curve of the CDI device assembled with the two-dimensional carbon sheets synthesized in Examples 1, 2 and 3 under different salt concentrations. Fig.23 It can be seen that as time goes by, the desalination capacity of the material gradually increases until it reaches the upper limit. Fig. 22 Consistently, the two-dimensional nanosheets synthesized in Example 1 have the largest salt adsorption amount.

[0142] Fig.24 The cyclic desalination-regeneration curve of the CDI device assembled with the two-dimensional carbon sheet synthesized in Example 1. Fig.24 It can be seen that after 1000 minutes of continuous operation, it still maintains more than 96.5% of the adsorption capacity, showing excellent cyclic stability. This excellent stability will provide possibilities for the practical application of the material.

[0143] 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 -1The desalination capacity was measured by the constant current charge and discharge method; the desalination capacity was measured by the 1.2V constant voltage method; the element proportion was measured by the XPS method, and the results are shown in Table 1.

[0144] Table 1

[0145]

[0146] From the results in Table 1, it can be seen that the two-dimensional carbon sheet prepared by Example 1 of the present invention has a sheet structure with appropriate thickness and can maintain stable structural toughness. In addition, the supercapacitor and capacitor desalination assembled therefrom have significantly better effects in terms of ion adsorption capacity and specific capacitance performance.

[0147] 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 contains a sheet structure, doping element N and oxygen element, wherein the percentage of doping element N atoms to the total atomic number is 4.5-8.8%, and the percentage of oxygen atoms to the total atomic number is more than 12%.

2. The two-dimensional carbon sheet according to claim 1, wherein: The percentage of the doping element N atoms to the total atoms is 5-8%, and the percentage of the oxygen atoms to the total atoms is more than 12%; And / or, the two-dimensional carbon sheet has a sheet thickness of 20-40 nm.

3. The two-dimensional carbon sheet according to claim 1 or 2, wherein: The specific capacitance of the two-dimensional carbon sheet is 295-320 F g -1 , the desalination capacity at 1.2 V was 22–32 mg g -1 .

4. A method for preparing a two-dimensional carbon sheet, characterized in that: The steps include: (1) in the presence of water, allowing the nitrogen-containing polysaccharide raw material to undergo a cross-linking reaction to obtain a hydrogel; (2) placing the hydrogel at a temperature below 0°C for a low-temperature static treatment, and then low-temperature drying to obtain an aerogel; (3) carbonizing the aerogel to obtain a carbon sheet material; (4) Under an oxygen atmosphere, the carbon sheet material is subjected to high-temperature oxidation to obtain the two-dimensional carbon sheet.

5. The preparation method according to claim 4, wherein The mass ratio of the nitrogen-containing polysaccharide raw material to water is 1:9-13; And / or, the nitrogen-containing polysaccharide raw material is selected from one or more of ice powder seeds, sea stone flowers, and stone flower seeds.

6. The preparation method according to claim 4 or 5, wherein: In step (2), the temperature of the low-temperature static treatment is -15°C to -25°C, and the time of the low-temperature static treatment is 10-15h; Preferably, the low-temperature drying temperature is -15°C to -25°C, and the low-temperature drying time is 20-28 hours.

7. The preparation method according to any one of claims 4 to 6, wherein: In step (3), the carbonization temperature is 600-800° C. and the carbonization time is 1-3 h; Preferably, the heating rate of the carbonization is 1-2°C min -1 .

8. The preparation method according to any one of claims 4 to 7, wherein: In step (4), the temperature of the high temperature oxidation is 250-350° C., and the time of the high temperature oxidation is 30-60 min; Preferably, the heating rate of the high temperature oxidation is 1-2°C min -1 ; Preferably, the oxygen atmosphere is provided by a mixture of oxygen and an inert gas, wherein the inert gas is selected from N2 and / or Ar; Preferably, the mixing volume ratio of oxygen to inert gas is 1:17-20.

9. A two-dimensional carbon sheet obtained by the preparation method according to any one of claims 4 to 8.

10. Use of the two-dimensional carbon sheet according to any one of claims 1 to 3 and 9 in preparing a capacitor device.

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