Nitrogen-doped MXenes aerogel as well as preparation method and application thereof

Abstract

The invention relates to nitrogen-doped MXenes aerogel as well as a preparation method and application thereof, and belongs to the technical field of materials. The preparation method of the nitrogen-doped MXenes aerogel provided by the invention comprises the following steps: S1, etching MAX by adopting an aqueous solution chemical etching method to obtain MXenes; s2, uniformly mixing the turbid liquid of the MXenes with urea, and then carrying out hydrothermal reaction, so as to obtain nitrogen-doped MXenes hydrogel; then, the MXenes aerogel is subjected to freeze drying, and the nitrogen-doped MXenes aerogel is obtained. The mass ratio of the MXenes to the urea in the step S2 is (0.15 to 0.2): (4 to 7); the temperature of the hydrothermal reaction is 120-180 DEG C, and the time is 12-18 hours. The nitrogen-doped MXenes aerogel prepared by the preparation method disclosed by the invention has good electrochemical performance and excellent rate stability.

Description

Technical Field

[0001] The present invention relates to the field of material technology, and in particular to a nitrogen-doped MXenes aerogel and a preparation method and application thereof. Background Art

[0002] MXenes are a class of two-dimensional transition metal carbides or nitrides that possess unique layered structures, abundant surface functional groups, high electrical conductivity, high toughness, hydrophilicity, high specific surface area, stability, and adjustable surface chemistry. They are widely used in electrocatalysis, photocatalysis, electrochemical energy storage, electromagnetic shielding, and other fields. However, traditional MXenes are prone to stacking due to van der Waals forces and hydrogen bonding interactions between nanosheets, resulting in low specific surface area and ion transport efficiency. Currently, this problem is mainly addressed through three-dimensional porous structure design and nitrogen doping. However, existing preparation methods have problems such as complex processes, low doping efficiency, and insufficient structural stability of the resulting MXenes, resulting in poor electrochemical performance and rate stability of MXenes. Summary of the Invention

[0003] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a nitrogen-doped MXenes aerogel and its preparation method and application.

[0004] To achieve the above object, the technical solution adopted by the present invention is:

[0005] In a first aspect, the present invention provides a method for preparing nitrogen-doped MXenes aerogel, comprising the following steps:

[0006] S1. Etching MAX to obtain MXenes using an aqueous chemical etching method (e.g., direct hydrofluoric acid etching, in-situ hydrofluoric acid etching, alkaline solution etching, acid-base alternating etching, etc.);

[0007] S2. The MXenes suspension is uniformly mixed with urea and subjected to a hydrothermal reaction to obtain a nitrogen-doped MXenes hydrogel; the hydrogel is then freeze-dried to obtain a nitrogen-doped MXenes aerogel;

[0008] The mass ratio of MXenes to urea in S2 is (0.15-0.2):(4-7) (for example, the mass ratio of MXenes to urea can be any one of 0.2:4, 0.19:4.5, 0.18:5, 0.17:5.5, 0.16:6, 0.15:7 or any two thereof); the temperature of the hydrothermal reaction is 120-180°C (for example, 120°C, 125°C, 130°C). , 135℃, 140℃, 145℃, 150℃, 160℃, 165℃, 170℃, 175℃, 180℃, any one or any two of the range values), the time is 12-18h (for example, it can be 12h, 12.5h, 13h, 13.5h, 14h, 14.5h, 15h, 15.5h, 16h, 16.5h, 17h, 17.5h, 18h, any one or any two of the range values).

[0009] If MXenes materials are modified only by nitrogen doping, the problem of re-stacking of MXenes nanosheets cannot be solved, and the modified MXenes nanosheets still cannot fully contact the electrolyte; if only a three-dimensional porous structure is constructed, the specific surface area will increase relatively, but the active sites on the surface of the nanosheets will not increase, and the electrochemical performance cannot be significantly improved.

[0010] The preparation method of the present invention uses urea as a nitrogen source and performs nitrogen doping and gelation treatment on MXenes materials through a one-step hydrothermal method, which not only provides a large number of electrochemical reaction active sites for MXenes nanosheets, but also can utilize the NH4 generated by the decomposition of urea. + , interacting with the electronegative MXenes nanosheets on the surface, causing the MXenes nanosheets to bend and cross-link into a three-dimensional porous structure, forming a self-supporting three-dimensional porous nitrogen-doped MXenes hydrogel; at the same time, combined with freeze-drying to remove water in the three-dimensional porous structure, a nitrogen-doped MXenes aerogel with a high specific surface area is obtained, which can make it easier for the electrolyte to contact the MXenes nanosheets, thereby greatly improving the electrochemical performance and rate stability of the MXenes material.

[0011] The inventors discovered that the mass ratio of MXenes to urea in the MXene suspension plays a key role in the performance of nitrogen-doped MXene aerogels. When urea dissolves in the MXene suspension, it tends to adsorb on the MXene surface, isolating water molecules and dissolved oxygen, and acting as an antioxidant. If the urea dosage is too low, the antioxidant effect is less pronounced, and the MXene material undergoes local oxidation during the hydrothermal process, resulting in a lack of significant electrochemical performance enhancement, leading to poor electrochemical performance of the nitrogen-doped MXene aerogel. Furthermore, urea absorbs heat during dissolution. Excessive urea addition can hinder dissolution, leading to incomplete dissolution. Furthermore, steric hindrance can cause coagulation in the MXene suspension, leading to accumulation of MXene nanosheets.

[0012] In addition, if the temperature of the hydrothermal reaction is too low, the reaction cannot be achieved or the reaction is incomplete, resulting in a decrease in the nitrogen doping amount and difficulty in effectively improving the electrochemical performance; if the temperature of the hydrothermal reaction is too high, it will lead to the rapid decomposition of urea, and then cause the oxidation of MXenes.

[0013] As a preferred embodiment of the method for preparing nitrogen-doped MXenes aerogels of the present invention, the MAX in step S1 includes at least one of Ti3AlC2, Ti2AlC, V4AlC3, and V2AlC.

[0014] As a preferred embodiment of the method for preparing nitrogen-doped MXenes aerogels according to the present invention, the concentration of the suspension in step S2 is 6 to 8 g / L, for example, it can be any one of 6 g / L, 6.2 g / L, 6.4 g / L, 6.6 g / L, 6.8 g / L, 7 g / L, 7.2 g / L, 7.4 g / L, 7.6 g / L, 7.8 g / L, and 8 g / L, or any two of the range values.

[0015] As a preferred embodiment of the method for preparing nitrogen-doped MXenes aerogel according to the present invention, step S1 specifically involves etching the MAX with a fluorine-containing acidic etching solution.

[0016] As a preferred embodiment of the method for preparing nitrogen-doped MXenes aerogels according to the present invention, the fluorine-containing acidic etching solution includes hydrochloric acid and lithium fluoride, and the volume mass ratio of hydrochloric acid to lithium fluoride mL / g is (20-30):1, for example, any one of 20:1, 21:1, 22:1, 23:1, 24:1, 26:1, 27:1, 28:1, 29:1, and 30:1, or any two thereof.

[0017] As a preferred embodiment of the method for preparing nitrogen-doped MXenes aerogels of the present invention, the mass concentration of the hydrochloric acid is 35% to 40%. For example, the mass concentration of the hydrochloric acid can be any one of 35%, 35.5%, 36%, 36.5%, 37%, 37.5%, 38%, 38.5%, 39%, 39.5%, and 40%, or any two of these.

[0018] As a preferred embodiment of the preparation method of the nitrogen-doped MXenes aerogel of the present invention, the etching is specifically to immerse MAX in a fluorine-containing acidic etching solution until no bubbles are generated, and then stir it at 40-50°C (for example, it can be any one of 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C, 50°C or any two of the range values) for 45-50h (for example, it can be any one of 45h, 45.5h, 46h, 46.5h, 47h, 47.5h, 48h, 48.5h, 49h, 49.5h, 50h or any two of the range values).

[0019] As a preferred embodiment of the preparation method of the nitrogen-doped MXenes aerogel of the present invention, the freeze-drying temperature is -75 to -50°C (for example, it can be any one of -75°C, -70°C, -65°C, -60°C, -55°C, and -50°C, or any two of the range values), and the time is 12 to 24 hours (for example, it can be any one of 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, and 24 hours, or any two of the range values).

[0020] In a second aspect, the present invention provides a nitrogen-doped MXenes aerogel prepared by the above preparation method.

[0021] In a third aspect, the present invention provides an application of the above-mentioned nitrogen-doped MXenes aerogel in the preparation of hydrogen ion supercapacitors.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The preparation method of the present invention uses urea as a nitrogen source and performs nitrogen doping and gelation treatment on MXenes materials through a one-step hydrothermal method, which not only provides a large number of electrochemical reaction active sites for MXenes nanosheets, but also can utilize the NH4 generated by the decomposition of urea. +, interacting with the electronegative MXenes nanosheets on the surface, causing the MXenes nanosheets to bend and cross-link into a three-dimensional porous structure, forming a self-supporting three-dimensional porous nitrogen-doped MXenes hydrogel; at the same time, combined with freeze-drying to remove water in the three-dimensional porous structure, a nitrogen-doped MXenes aerogel with a high specific surface area is obtained, which can make it easier for the electrolyte to contact the MXenes nanosheets, thereby greatly improving the electrochemical performance and rate stability of the MXenes material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a process flow chart of the preparation method in Example 1;

[0025] Figure 2 This is an optical photograph of the MXenes suspension in Example 1;

[0026] Figure 3 The figures are the physical pictures of the products obtained in Comparative Example 1 and Example 1;

[0027] Figure 4 This is a physical picture of the product obtained in Comparative Example 2;

[0028] Figure 5 The scanning electron microscope images of the products obtained in Comparative Example 1, Comparative Example 2 and Example 1 are shown;

[0029] Figure 6 The nitrogen adsorption-desorption curves of the products obtained in Comparative Example 1, Comparative Example 2 and Example 1 are shown;

[0030] Figure 7 The XPS spectra of the products obtained in Comparative Example 1, Comparative Example 2 and Example 1 are shown;

[0031] Figure 8 The cyclic voltammetry curves of the products obtained in Comparative Example 1, Comparative Example 2 and Example 1 at a scan rate of 1 mV / s are shown. DETAILED DESCRIPTION

[0032] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0033] Unless otherwise specified, other materials and reagents used in the examples can be obtained from commercial sources.

[0034] Example 1

[0035] A method for preparing nitrogen-doped MXenes aerogels comprises the following steps (e.g. Figure 1 shown):

[0036] S1. Place 2 g of the precursor material (Ti3AlC2) into an etching solution (composed of 50 mL of 36% hydrochloric acid and 2 g of lithium fluoride) and etch until no bubbles are generated in the mixture; then, stir at 150 rpm at 45 ° C for 48 h, filter the solid, and then wash with deionized water; the washed product is ultrasonically dispersed in 200 mL of deionized water for 1 h, and then centrifuged at 2000 rpm to obtain a MXenes suspension with a concentration of 7 g / L (such as Figure 2 As shown, an obvious Tyndall effect is exhibited, proving that the prepared MXenes suspension has good dispersion).

[0037] S2. Take 25 mL of the MXenes suspension in S1 and add urea thereto (the mass ratio of MXenes to urea in the MXenes suspension is 0.175:5), stir until the urea is completely dissolved, and then transfer to a reactor for hydrothermal reaction at 180°C for 18 hours to obtain nitrogen-doped MXenes hydrogel;

[0038] S3. The nitrogen-doped MXenes hydrogel in S2 was repeatedly soaked and cleaned with deionized water, and then freeze-dried at -55°C for 24 hours to obtain the nitrogen-doped MXenes aerogel.

[0039] Example 2

[0040] A method for preparing nitrogen-doped MXenes aerogel is different from Example 1 only in that the precursor material in step S1 is Ti2AlC.

[0041] Example 3

[0042] A method for preparing nitrogen-doped MXenes aerogel, which differs from Example 1 only in that the temperature of the hydrothermal reaction in step S2 is 150° C. and the time is 12 h.

[0043] Example 4

[0044] A method for preparing nitrogen-doped MXenes aerogel is different from Example 1 only in that the mass ratio of MXenes to urea in the MXenes suspension in step S2 is 0.15:7.

[0045] Example 5

[0046] A method for preparing nitrogen-doped MXenes aerogel is different from Example 1 only in that the mass ratio of MXenes to urea in the MXenes suspension in step S2 is 0.2:4.

[0047] Comparative Example 1

[0048] A method for preparing a MXenes film comprises the following steps:

[0049] 2 g of the precursor material (Ti3AlC2) was placed in an etching solution (consisting of 50 mL of 36% hydrochloric acid and 2 g of lithium fluoride) and etched until no bubbles were generated in the mixture; then, the mixture was stirred at 45°C at a speed of 150 rpm for 48 hours, the solid was filtered, and then washed with deionized water; the washed product was placed in 200 mL of deionized water for ultrasonic dispersion for 1 hour, and then centrifuged at 2000 rpm to obtain a MXenes suspension; 25 mL of the MXenes suspension was vacuum filtered and dried at room temperature to form a MXenes film.

[0050] Comparative Example 2

[0051] A method for preparing nitrogen-doped MXenes aerogel is different from Example 1 only in that the mass ratio of MXenes to urea in the MXenes suspension in step S2 is 0.175:2.

[0052] Comparative Example 3

[0053] A method for preparing nitrogen-doped MXenes aerogel, which differs from Example 1 only in that urea in step S2 is replaced by pyridine.

[0054] Performance Testing

[0055] The nitrogen-doped MXenes aerogels in Examples 1 to 5 and Comparative Examples 2 to 3 and the MXenes film in Comparative Example 1 were processed into test samples. The specific processing method was as follows: 20 mg of nitrogen-doped MXenes aerogel or MXenes film was taken and cold-pressed into tablets using a tablet press (the tableting load was 5 MPa and the loading time was 5 min), and then trimmed into 1×1 cm 2 The square electrode specimens were tested.

[0056] The electrochemical performance was tested by a CHI660 electrochemical workstation. The electrolyte was 3M sulfuric acid solution. The test system was a three-electrode system. The working electrode was a square electrode, the counter electrode was a Pt electrode, and the reference electrode was Ag / AgCl. After the test, the mass specific capacitance of the electrode was calculated, and the measured curve was integrated. The integrated area was the mass specific capacitance of the electrode material at the current scan rate. The results are shown in Table 1.

[0057] Table 1

[0058]

[0059] According to the data in Table 1 and Figure 8 (Example 1 is N-Ti3C2T x-Aerogel, Comparative Example 1 is Ti3C2T x -film, Comparative Example 2 is N-Ti3C2T x -2) It can be seen that the mass specific capacitance of the nitrogen-doped MXenes aerogels in Examples 1 to 5 at a scanning speed of 1 mV / s all reaches more than 495 F / g, and as the scanning rate increases, the charge capacity can still be maintained at more than 60%, indicating that the nitrogen-doped MXenes aerogels prepared by the present invention have good electrochemical properties and excellent rate stability.

[0060] At the same time, according to Figure 3 It can be found that the MXenes film in Comparative Example 1 ( Figure 3 a) in Example 1 shows a typical thin film structure, with obvious re-stacking of nanosheets, which is not conducive to the infiltration and diffusion of the electrolyte; while the nitrogen-doped MXenes aerogel in Example 1 ( Figure 3 b) in the figure shows a self-supporting three-dimensional porous structure on a macro scale, and has a certain strength and low density. It can support a 50g weight and stand on the flowers and leaves, showing good mechanical processing properties.

[0061] In addition, according to Figure 4 It can be seen that although the nitrogen-doped MXenes aerogel in Comparative Example 2 also has a three-dimensional porous structure, some white substances can be observed on the surface. This is TiO2 generated by the oxidation of MXenes during the hydrothermal process. The generation of these oxides is not conducive to the occurrence of electrochemical reactions and will reduce the electrochemical performance of nitrogen-doped MXenes aerogel.

[0062] according to Figure 5 It can be seen that the MXenes film in Comparative Example 1 ( Figure 5 a) in the comparative example still shows obvious heavy stacking characteristics at the microscopic scale, while the nitrogen-doped MXenes aerogel in comparative example 2 ( Figure 5 b) and the nitrogen-doped MXenes aerogel in Example 1 ( Figure 5 Figure c) shows an obvious three-dimensional porous structure, which is conducive to the increase of specific surface area, exposing more electrochemical reaction active sites and providing more channels for the diffusion of electrolyte.

[0063] according to Figure 6 It can be seen that the nitrogen-doped MXenes aerogel (N-Ti3C2T x -Aerogel) has a specific surface area of ​​up to 200.8m 2 / g, compared with the MXenes film in Comparative Example 1 (Ti3C2T x -film) specific surface area (8.7m 2 / g) increased by about 25 times, while the nitrogen-doped MXenes aerogel in Comparative Example 2 (N-Ti3C2T x -2) has a specific surface area of ​​95.6 m 2 / g, although it is also improved compared with Comparative Example 1, it is still far from reaching the specific surface area of ​​Example 1, indicating that the amount of urea added during the hydrothermal reaction plays an important role in the specific surface area of ​​nitrogen-doped MXenes aerogel.

[0064] according to Figure 7 It can be seen that the MXenes film in Comparative Example 1 (Ti3C2T x -film) was not subjected to nitrogen doping treatment, so no nitrogen element was detected; while the nitrogen-doped MXenes aerogel in Example 1 (N-Ti3C2T x -Aerogel) and Comparative Example 2 nitrogen-doped MXenes aerogel (N-Ti3C2T x -2) After urea hydrothermal treatment, an obvious nitrogen peak can be detected from the XPS spectrum. It has been calculated that the nitrogen content of the nitrogen-doped MXenes aerogel in Example 1 is 5%. The presence of these nitrogen elements can greatly increase the number of electrochemically active sites of the MXenes material and improve the electrochemical performance; while the nitrogen-doped MXenes aerogel in Comparative Example 2, due to the small amount of urea used, can only detect a nitrogen content of about 1.7%, which cannot provide more electrochemical reaction active sites, so the improvement of electrochemical performance is limited.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing nitrogen-doped MXenes aerogel, characterized in that: The following steps are involved: S1, using aqueous chemical etching to etch MAX to obtain MXenes; S2. The MXenes suspension is uniformly mixed with urea and subjected to a hydrothermal reaction to obtain a nitrogen-doped MXenes hydrogel; the hydrogel is then freeze-dried to obtain a nitrogen-doped MXenes aerogel; The mass ratio of MXenes to urea in S2 is (0.15-0.2):(4-7); the temperature of the hydrothermal reaction is 120-180° C. and the time is 12-18 h.

2. The preparation method according to claim 1, wherein The MAX in step S1 includes at least one of Ti3AlC2, Ti2AlC, V4AlC3, and V2AlC.

3. The preparation method according to claim 1, wherein The concentration of the suspension in step S2 is 6-8 g / L.

4. The preparation method according to claim 1, wherein Step S1 specifically involves etching the MAX using a fluorine-containing acidic etching solution.

5. The preparation method according to claim 4, wherein The fluorine-containing acidic etching solution includes hydrochloric acid and lithium fluoride, and the volume mass ratio of the hydrochloric acid to the lithium fluoride is (20-30):1 in mL / g.

6. The preparation method according to claim 4, wherein The mass concentration of the hydrochloric acid is 35% to 40%.

7. The preparation method according to claim 4, wherein The etching is specifically performed by immersing the MAX in a fluorine-containing acidic etching solution until no bubbles are generated, and then stirring at 40-50° C. for 45-50 hours.

8. The preparation method according to claim 1, wherein The freeze-drying temperature is -75 to -50°C and the time is 12 to 24 hours.

9. Nitrogen-doped MXenes aerogel prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the nitrogen-doped MXenes aerogel according to claim 9 in the preparation of hydrogen ion supercapacitors.

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