MHM electrode material and preparation method and application thereof

Abstract

The invention provides an MHM electrode material and a preparation method and application thereof, and belongs to the technical field of electrochemical desalination. The MHM electrode material is Ti3C2Tx-MXene with a three-dimensional hollow microspherical structure, polystyrene microspheres are treated by a polydiene dimethyl ammonium chloride solution and then stirred to react with the Ti3C2Tx-MXene, and the MHM electrode material is obtained through freeze drying and pyrolysis. According to the invention, Ti3C2Tx-MXene is constructed into a three-dimensional spherical structure, so that the stacking problem of traditional MXene sheet layers is avoided, the advantages of good conductivity, large surface area, excellent mechanical property and the like are fully exerted, and the structural stability, the electrochemical performance and the desalting capacity are remarkably improved; the MHM electrode shows excellent adsorption capacity, high ion transmission rate and good cycling stability in the CDI device, and a new thought is provided for CDI material design.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrochemical desalination, and particularly relates to a MHM electrode material and a preparation method and application thereof. BACKGROUND

[0002] As an efficient and low-energy electrochemical desalination method, CDI (Capacitive Deionization) technology has become one of the important technologies for solving water resource shortage and environmental pollution problems since Blair, Murphy and others proposed the concept of "electrochemical desalination" in the late 1960s. Its core principle is to realize water purification through ion adsorption and desorption of electrode materials under the action of an electric field. With the advantages of simple operation, low energy consumption and environmental friendliness, CDI technology has shown broad application prospects in seawater desalination, wastewater treatment, resource recovery and other fields. The performance improvement of CDI technology is highly dependent on the innovation and system optimization of electrode materials. In early studies, electrode materials mainly relied on traditional materials such as activated carbon, oxides and polymers. Although these materials have certain adsorption capacity, they are limited by low specific surface area and limited active sites, making it difficult to meet the actual demand for adsorption capacity and desalination efficiency. Since the 1990s, the emergence of new carbon materials such as graphene, carbon nanotubes and MXene has provided key support for the breakthrough of CDI technology. These materials have higher specific surface area, excellent electrical conductivity and rich surface functional groups, significantly improving ion adsorption rate and capacity, and becoming a current research hotspot.

[0003] To further optimize the performance of CDI electrode materials, researchers have explored from multiple dimensions: at the material design level, by adjusting the microstructure (such as increasing the specific surface area and constructing a porous framework) to expose more active sites and improve ion adsorption and transmission efficiency; at the surface modification level, by adjusting the distribution of valence bonds and functional groups to increase the surface charge density and achieve selective adsorption of specific ions; at the operating condition level, by optimizing voltage (such as low voltage <1.4V to avoid water electrolysis side reactions), solution concentration, flow rate and other parameters to adapt to different application scenarios. However, there are still many key problems to be solved in existing technologies, which restrict the industrial application of CDI technology:

[0004] First, although Ti3C2T x Although the new carbon materials represented by MXene have high electrical conductivity, their two-dimensional layered structure is prone to stacking due to van der Waals forces and hydrogen bonding, resulting in low specific surface area utilization and blocked ion transmission path, limiting the further improvement of adsorption capacity. At the same time, the two-dimensional structure is prone to interlayer collapse in repeated ion adsorption and desorption cycles, which lacks stability and causes rapid degradation of cycle performance, making it difficult to meet long-term operation requirements.

[0005] Secondly, the balance between ion selectivity and desalination efficiency is still a difficulty. The existing materials have limited selective adsorption capacity for specific ions, and the pore structure and surface functional group regulation are insufficient, resulting in slow ion migration rate in low concentration salt solution (the main scene of actual water treatment), and limited adsorption capacity due to the electrostatic repulsion between ions in high concentration solution, which cannot maintain high desalination performance in a wide concentration range.

[0006] Thirdly, the electrochemical performance of the material is not coordinated. Materials with strong redox activity (such as some metal oxides) can improve the adsorption performance through the pseudo-capacitance mechanism, but their low conductivity will hinder the charge transfer; while high-conductivity materials (such as pure MXene) can ensure fast electron conduction, but lack sufficient redox active sites, making it difficult to balance high capacitance characteristics and fast reaction kinetics.

[0007] In addition, the existing materials have poor adaptability to operating conditions. The adsorption capacity is insufficient at low voltage, and high voltage can easily cause side reactions such as water electrolysis, resulting in energy loss and electrode damage; at the same time, it is sensitive to salt concentration fluctuations, and it is difficult to maintain stable performance in complex water quality environments, limiting the expansion of its actual application scenarios. Therefore, developing CDI electrode materials with high adsorption capacity, excellent cycle stability, good ion selectivity and wide operating condition adaptability has become the core demand to further develop CDI technology.

[0008] Therefore, the present application provides a kind of MHM electrode material and its preparation method and application. SUMMARY

[0009] Because Ti3C2T x -MXene is limited to the interlayer ion adsorption dominated mechanism, there are problems such as low adsorption capacity and insufficient cycle stability, and materials with strong redox reaction kinetics are limited by low conductivity to limit CDI performance, the present application provides a kind of MHM electrode material and its preparation method and application, through the "three-dimensional porous framework + dual mechanism synergy" design strategy, by polystyrene microsphere template method constructs Ti3C2T x -MXene hollow microspheres, and Ti3C2T x -MXene hollow microspheres (MHM).

[0010] To achieve the above purpose, the present application provides the following technical solutions:

[0011] One of the technical solutions of the present application is:

[0012] A preparation method of the above-mentioned MHM electrode material, comprising the following steps:

[0013] (1) preparing polystyrene (PS) microspheres by emulsion polymerization method, with polyvinylpyrrolidone (PVP) as emulsifier, styrene as monomer, and 2,2'-azobis isobutyronitrile (AIBN) as initiator;

[0014] (2) etching Ti3C2T2 powder by hydrofluoric acid (HF), and then intercalating and exfoliating Ti3C2T x -MXene by dimethyl sulfoxide;

[0015] (3) stirring the PS microspheres treated by polydimethyl diallyl ammonium chloride (PDDA) solution with Ti3C2T x -MXene, and then freeze-drying and pyrolyzing to obtain Ti3C2T x -MXene hollow microspheres, which are the MHM electrode material.

[0016] Further, in step (1), the reaction temperature of the emulsion polymerization method is 65°C, and the time is 16h.

[0017] Further, in step (1), the amount ratio of polyvinylpyrrolidone, styrene and 2,2'-azobis isobutyronitrile in the emulsion polymerization method is 5g:55mL:1g.

[0018] Further, in step (3), the mass ratio of the PS microspheres to Ti3C2T x -MXene is 10:1.

[0019] Further, in step (3), the amount ratio of the PS microspheres to the polydimethyl diallyl ammonium chloride solution is 0.1mg:1mL, and the concentration of the polydimethyl diallyl ammonium chloride solution is 5mg / mL.

[0020] Further, in step (3), the PS microspheres are treated by the polydimethyl diallyl ammonium chloride solution for 30min.

[0021] Further, in step (3), the stirring reaction time is 2h, the pyrolysis temperature is 500°C, the heating rate is 5°C / min, and the holding time is 2h.

[0022] Further, in step (3), the pyrolysis process is carried out in a nitrogen atmosphere.

[0023] Further, in step (2), the Ti3C2T xThe MXene preparation process is divided into two steps of etching and exfoliation, the etching of MXene refers to removing the A group elements or other atomic layers by chemical etching of the MXene precursor material (such as titanium aluminum carbide), so as to obtain the MXene layer, and the typical etching method includes acid treatment, fluoride treatment and the like, and the exfoliation refers to separating the sheet layer in the multi-layer MXene to obtain a single-layer or few-layer MXene material, and the exfoliation is usually realized by mechanical exfoliation, ion intercalation exfoliation and the like, these methods can effectively exfoliate the MXene layer from the multi-layer structure to obtain the required MXene material, and the few-layer MXene has excellent mechanical properties, reduces the stacking between the MXene layers, increases the specific surface area and active site density, optimizes the electronic transmission path, improves the electronic transmission performance and conductivity, and improves the performance in electrochemistry, catalysis and adsorption and the like.

[0024] The second technical scheme of the application:

[0025] A MHM electrode material prepared according to the above preparation method has a three-dimensional hollow microspherical structure.

[0026] Technical principle: the application re-designs Ti3C2T x MXene structure into a three-dimensional hollow microspherical structure, avoids the stacking problem existing in the sheet structure, improves the stability of the material in water, enhances the cycle stability, further increases the specific surface area, maximally exposes the active sites on the surface of the MXene, improves the ion transmission rate, enhances the performance of the material in the electrochemical test, such as the significant improvement of the specific capacitance, reduces the internal resistance of the material, and improves the adsorption performance of the material as a CDI electrode, and the design of the three-dimensional structure will be beneficial to the further doping modification of the material, selectively controls the pore structure and surface functional groups, realizes the higher adsorption performance, and improves the selectivity of the composite material to specific ions, and improves the deionization efficiency of the CDI device.

[0027] The third technical scheme of the application:

[0028] An asymmetric capacitive deionization (CDI) device, the above-mentioned MHM electrode material is used as a negative electrode, and activated carbon (AC) is used as a positive electrode.

[0029] The fourth technical scheme of the application:

[0030] A salt water desalination method, the above-mentioned asymmetric capacitive deionization device is used to treat a NaCl solution with a salt concentration of 300-1500 mg / L under a voltage of 0.8-1.4 V.

[0031] Compared with the prior art, the application has the following advantages and technical effects:

[0032] (1) The application constructs two-dimensional sheet material Ti3C2T x -MXene into a three-dimensional hollow microsphere structure MHM, so that it has a regular hollow microsphere structure and a larger specific surface area, and the carbon skeleton left after pyrolysis of the PS microspheres ensures the stability of the spherical structure and avoids the stacking of sheet layers affecting the material performance. The mesoporous structure has a large surface area and pore volume, which can provide more adsorption sites, and is also beneficial to the diffusion and adsorption of ions on the electrode surface. The application constructs Ti3C2T x -MXene into a three-dimensional spherical structure, which avoids the stacking problem of traditional MXene sheet layers, fully utilizes the good conductivity, large surface area and excellent mechanical properties of MXene, and significantly improves the structural stability, electrochemical performance and desalination capacity of the MHM electrode. The excellent adsorption capacity, fast ion transport rate and good cycle stability of the MHM electrode in the CDI device provide a new idea for the design of CDI materials.

[0033] (2) The MHM prepared by the application has excellent electrochemical performance, and the specific capacitance reaches 242 F·g -1 at a scan rate of 5 mV·s -1 , and still maintains a specific capacitance of 168 F·g -1 at a high scan rate of 200 mV·s -1 , showing a good capacitance retention rate (70%). In addition, the constant current charge-discharge curve of the MHM shows good symmetry, indicating that it has excellent double-layer capacitance behavior and cycle stability, and the electrochemical impedance spectrum analysis further reveals that the MHM has a low charge transfer impedance and a fast ion diffusion rate.

[0034] (3) The MHM and activated carbon of the application are used as electrodes to construct an asymmetric mixed CDI device, and the results show that the desalination capacity of the MHM electrode for 500mg / L NaCl solution reaches 35mg / g at a voltage of 1.2V, which is significantly higher than that of the traditional two-dimensional Ti3C2T x -MXene sheet electrode. In addition, the MHM electrode shows good adsorption performance in the salt concentration range of 100-1500 mg / L, especially in the medium-high concentration salt solution of 500-1500 mg / L, and its desalination capacity reaches 44-51 mg / g, showing its great potential in treating low-concentration wastewater. Through long-term adsorption and desorption cycle experiments, the MHM electrode can still maintain a capacity retention rate of more than 90% after 155 cycles, showing excellent cycle stability and reversibility. In addition, the MHM electrode shows a fast ion release rate during the desorption process, further proving its high efficiency and reusability in CDI application. BRIEF DESCRIPTION OF DRAWINGS

[0035] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application, illustrate the preferred embodiments of the application and assist in

[0036] Figure 1 Figure 1 shows the SEM images of the multilayer Ti3C2T x MXene powder, (b) is the Ti3C2T x MXene, (c) is the SEM image of the PS microspheres obtained in step (1) of Example 1 at 5 μm, (d) is the SEM image of the PS microspheres obtained in step (1) of Example 1 at 1 μm;

[0037] Figure 2 Figure 2 shows the multilayer Ti3C2T x MXene powder and Ti3C2T x XRD pattern of MXene;

[0038] Figure 3 Figure 3 shows the PS microspheres (PS in the figure) obtained in step (1) of Example 1 and Ti3C2T x MXene (Ti3C2T x ) obtained in step (2) of Example 1;

[0039] Figure 4 Figure 4 shows the Ti3C2T x @PS composite at 5 μm (a) and 1 μm (b) size;

[0040] Figure 5 Figure 5 shows the SEM images of the MHM obtained in step (3) of Example 1 at 5 μm (a) and 2 μm (b) size;

[0041] Figure 6 Figure 6 shows the Ti3C2T x MXene, PS microspheres, MXene@PS and MHM obtained in Example 1;

[0042] Figure 7 Figure 7 shows the nitrogen adsorption-desorption curves of Ti3C2T x MXene (Ti3C2T x ) and MHM, (b) is the PS microspheres (PS in the figure), Ti3C2T x MXene (Ti3C2Tx ) and pore size analysis and specific surface area of ​​MHM;

[0043] Figure 8 (a) is the CV curve of MHM at different scan rates, (b) is the GCD curve at different current densities, and (c) is the electrochemical impedance spectroscopy (EIS) spectrum. The inset is the high-frequency region.

[0044] Figure 9 Ti3C2T in Example 1 x Comparison of the electrochemical performance of MXene and MHM, where (a) is 50 mV·s -1 CV curves at different scan rates; (b) 0.5 A·g -1 GCD curve under current density; (c) is the electrochemical impedance spectroscopy, the inset is the high-frequency region; (d) is the specific capacitance comparison diagram;

[0045] Figure 10 Schematic diagram of MHM / / AC electrosorption / desorption process;

[0046] Figure 11 This is a connection diagram of the CDI device adsorption test system;

[0047] Figure 12 Ti3C2T x -Saturated adsorption capacity (a) of CDI devices prepared by MXene and MHM at different voltages of 0.6-1.6 V, 100-1500 mg·L -1 (b) Test results of MHM saturated adsorption capacity at different NaCl concentrations;

[0048] Figure 13 The CDI device prepared for MHM was tested at 1.2 V, 50 mL, 500 mg·L -1 Adsorption and desorption curves under NaCl solution, desalination capacity test results;

[0049] Figure 14 The CDI device prepared for MHM is 500mg·L at 1.2V. -1 Desalination cycle stability test results of NaCl concentration adsorption for 5 min. DETAILED DESCRIPTION

[0050] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0051] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. Additionally, for a range of values of, for example, concentrations, amounts, and the like, every concentration between the upper and lower limit of that range is also specifically included within the scope of the present application. The upper and lower limits of these smaller ranges can independently be included or excluded in the range.

[0052] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned herein are incorporated by reference to disclose and describe in detail the methods and / or materials which are related to the present application. In case of conflict, the content of the present specification will control.

[0053] Many modifications and variations of the present application described in the specification are possible without departing from the scope or spirit of the application. Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application. The specification and examples are illustrative only.

[0054] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having", and the like are open-ended terms that are intended to permit but not limit the inclusion of elements or the number of elements, as well as the possibility that one or more other elements can be added or otherwise included.

[0055] The embodiment of the present application provides a MHM electrode material, which is a three-dimensional hollow microspherical structure.

[0056] The embodiment of the present application also provides a preparation method of the MHM electrode material, which comprises the following steps:

[0057] (1) polystyrene (PS) microspheres are prepared by an emulsion polymerization method, with polyvinylpyrrolidone (PVP) as an emulsifier, styrene as a monomer, and 2,2' azobis isobutyronitrile (AIBN) as an initiator;

[0058] (2) Ti3C2T2 powder is etched by hydrofluoric acid (HF), and Ti3C2T x -MXene is obtained through dimethyl sulfoxide intercalation and exfoliation;

[0059] (3) the polystyrene microspheres are treated by a poly diallyl dimethyl ammonium chloride (PDDA, a cationic polymer, commonly used as a surfactant, which can be adsorbed on the surface of particles, and adds positive charges to the surface of the object) solution, and then combined with Ti3C2T xMXene stirring reaction, freeze-drying, pyrolysis to obtain Ti3C2T x The MXene hollow microspheres are the MHM electrode material.

[0060] In step (1) of the preferred embodiment of the present application, the reaction temperature of the emulsion polymerization method is 65℃, and the time is 16h.

[0061] In step (1) of the preferred embodiment of the present application, the amount of polyvinylpyrrolidone, styrene, and 2,2' azobisisobutyronitrile used in the emulsion polymerization method is 5g: 55mL: 1g.

[0062] In step (3) of the preferred embodiment of the present application, the amount of the polystyrene microspheres and the polydiallyldimethylammonium chloride solution is 0.1 mg: 1 mL, and the concentration of the polydiallyldimethylammonium chloride solution is 5 mg / mL.

[0063] In step (3) of the preferred embodiment of the present application, the polystyrene microspheres are treated with the polydiallyldimethylammonium chloride solution for 30min.

[0064] In step (3) of the preferred embodiment of the present application, the stirring reaction time is 2h, the pyrolysis temperature is 500℃, the heating rate is 5℃ / min, and the holding time is 2h.

[0065] In step (3) of the preferred embodiment of the present application, the pyrolysis process is carried out in a nitrogen atmosphere.

[0066] In step (2) of the preferred embodiment of the present application, Ti3C2T x The MXene preparation process includes etching and exfoliation. The etching of MXene refers to removing the A group elements or other atomic layers by chemical etching of the MXene precursor material (such as titanium aluminum carbide), thereby obtaining the MXene layer. Typical etching methods include acid treatment, fluoride treatment, etc. Exfoliation refers to separating the layers of multi-layer MXene to obtain single-layer or few-layer MXene material. Exfoliation is usually achieved by mechanical exfoliation, ion intercalation exfoliation, etc. These methods can effectively exfoliate the MXene layer from the multi-layer structure to obtain the desired MXene material. Few-layer MXene has excellent mechanical properties, reduces the accumulation between MXene layers, increases the specific surface area and active site density, optimizes the electron transport path, improves the electron transport performance and conductivity, and improves its performance in electrochemistry, catalysis, and adsorption, etc.

[0067] The asymmetric capacitive deionization (CDI) device is also provided in the embodiments of the present application, the MHM electrode material is used as a negative electrode, and activated carbon (AC) is used as a positive electrode, and the above asymmetric capacitive deionization device is used for desalination of salt water, and a NaCl solution with a salt concentration of 500-1500 mg / L is treated at a voltage of 0.8-1.4 V.

[0068] The raw materials used in the embodiments of the present application are commercially available.

[0069] The technical solutions of the present application are further described through the following examples.

[0070] Example 1

[0071] A preparation method of a MHM electrode material includes the following steps:

[0072] (1) Preparation of polystyrene (PS) microspheres

[0073] 5 g of polyvinylpyrrolidone (PVP), 55 mL of styrene and 150 g of anhydrous ethanol are measured and added to a three-necked flask, a magnetic stirring bar is inserted into a constant-temperature magnetic water bath kettle, and PVP is dissolved after stirring at 500 rpm for 30 min. The water bath temperature is adjusted to 65 DEG C, 1 g of 2,2'-azobis isobutyronitrile (AIBN) and 50 g of anhydrous ethanol are measured in a beaker and stirred until they are uniformly mixed. When the water bath kettle temperature rises to 65 DEG C, it is added to the three-necked flask, a condenser tube is connected to the three-necked flask, the flask cap is tightly closed after nitrogen is filled, and the reaction is stirred at 65 DEG C for 16 h. The reaction product changes from transparent to milky white colloid. The reaction product is removed and washed with a mixture of anhydrous ethanol and water by centrifugation for three times. The volume is concentrated to 100 mL, and the PS microspheres are obtained and stored in a refrigerator.

[0074] (2) Ti3C2T x Preparation of MXene

[0075] (a) Etching: first, 20 mL of 50wt.% HF solution is measured in a polytetrafluoroethylene beaker, then 1.5 g of Ti3AlC2 (titanium aluminum carbide) powder is slowly added to the HF solution, and a large amount of bubbles is generated during the reaction. After 30 min, the reaction becomes gentle, a magnetic stirring bar is added and sealed with a sealing film, then it is moved to a 40 DEG C constant-temperature magnetic water bath kettle, and the reaction is carried out at 1000 rpm for 24 h. After the reaction is completed, the film is removed, and the mixed solution is washed by centrifugation at 6000 rpm until the pH is 6-7. The washed product is vacuum filtered with a 0.22 mu m filter membrane, the filter cake is placed in a vacuum drying box and dried, and the multilayer Ti3C2T x MXene powder is obtained.

[0076] (b) Peeling: 0.2 g of multilayer Ti3C2T x -MXene powder was poured into a glass beaker containing 15 mL of DMSO (dimethyl sulfoxide), a magnetic stirrer was added and the beaker was sealed with a sealing film. The beaker was moved to a constant temperature magnetic water bath at 25 ° C and reacted at 1000 rpm for 24 h. After the reaction, 30 mL of deionized water was added to the beaker, and the beaker was placed under a cell crusher and ultrasonically treated in an ice water bath for 30 min. Then, the beaker was centrifuged at 6000 rpm for 5-6 times to remove DMSO and vacuum filtered to obtain Ti3C2T x -MXene;

[0077] (3) MHM (Ti3C2T x - Preparation of MXene hollow microspheres) electrode materials

[0078] Take 0.5 mg of PS microspheres prepared in step (1) and disperse them in 100 mL of deionized water. Stir continuously until the dispersion is uniform. Add 5 mL of polydimethylammonium chloride solution (PDDA, concentration is 5 mg / mL) and treat for 30 min to make it easier to combine with MXene. Then calculate the mass of MXene to be added based on the mass of PS microspheres added. According to the ratio of PS microspheres: Ti3C2T x -MXene was added into deionized water with a mass ratio of 10:1 to disperse Ti3C2T x -MXene (Ti3C2T in deionized water x -MXene concentration was 1 mg / mL) and the reaction was continued with stirring for 2 h. After the reaction, the mixture was centrifuged and washed three times with deionized water and concentrated to less than 50 mL to obtain a powdered sample, which was Ti3C2T x The PS complex was placed in a polytetrafluoroethylene beaker, sealed with a film and frozen for 6 hours, then placed in a vacuum freeze dryer, freeze-dried for 48 hours, and then placed in a tube furnace under a nitrogen atmosphere at 5°C·min -1 The temperature was raised to 500℃ at a heating rate and pyrolyzed at this temperature for 2h. After the reaction was completed and the temperature was lowered, the MHM (Ti3C2T x -MXene hollow microspheres) electrode materials.

[0079] Performance Testing

[0080] 1. Material Characterization

[0081] The multilayer Ti3C2T obtained in step (2) of Example 1 x -SEM image of MXene powder Figure 1 (a) Ti3C2T after peeling x -MXene SEM image see Figure 1 (b) in the figure, XRD pattern is shown inFigure 2 It can be seen that the aluminum layer in Ti3AlC2 is etched away by acid, showing an accordion shape, proving that Ti3C2T x is successfully prepared. After stripping it with dimethyl sulfoxide, Ti3C2T x -Mxene, before and after stripping Ti3C2T x -MXene XRD pattern Figure 2 , before stripping is multi-layer Ti3C2T x -MXene powder, after stripping is Ti3C2T x -MXene) It can be seen that the 002 peak is obviously left shifted from 9.52° to 6.61°. According to Bragg's equation 2dsin(θ) = nλ (where n represents the reflection order, and λ represents the wavelength), it shows that the interlayer spacing of MXene material is expanded to 13.4 Å after stripping, which helps to improve the electrochemical performance and adsorption capacity of Ti3C2T x -MXene as CDI application embeds or adsorbs more ions, thereby improving the electrochemical performance and adsorption capacity, and also helps to dope the subsequent structure, creating more possibilities for its application.

[0082] PS microspheres obtained in step (1) of Example 1 (PS in the figure) and Ti3C2T x -MXene (Ti3C2T x layer in the figure) obtained in step (2) are mixed with the PS microspheres obtained in step (1) of Example 1 (PS in the figure) and Ti3C2T Figure 3 .

[0083] The SEM images of PS microspheres obtained in step (1) of Example 1 at 5 μm and 1 μm sizes are shown in Figure 1 (c) and (d) of FIG. 1, it can be seen that the diameter of the prepared PS microspheres is about 2 μm.

[0084] The SEM images of Ti3C2T x @PS composite obtained in step (3) of Example 1 at 5 μm and 1 μm sizes are shown in Figure 4 , combined with Figure 1 , Figure 3 and Figure 4 , it can be seen that the potential test of Ti3C2T x -MXene is -29.5 mV of negative potential, while the PS microspheres is +25.5 mV of positive potential. Under the action of electrostatic force, the MXene layer will tightly wrap around the surface of the PS microspheres, and Ti3C2T xThe SEM image of the PS composite shows a large number of small shiny balls adhering to each other, and it can be seen that the MXene layers are uniformly wrapped on the surface of the PS microspheres. It is indicated that the state of the two materials after mixing and centrifugal washing is a thick liquid mixture, which needs to be freeze-dried before the next step of pyrolysis.

[0085] The SEM images of the MHM obtained in step (3) of Example 1 at 5 μm and 2 μm sizes are shown in Figure 5 It can be seen that after 48 h of freeze-drying, the Ti3C2T x The PS composite changes from a thick liquid to a powder with the texture of aerogel, which is light and easily attracted by static electricity. It is put into an atmospheric tube furnace for high-temperature pyrolysis at 500°C. Since it is in a nitrogen atmosphere, the Ti3C2T x The pyrolysis temperature of MXene is 800-1000°C, so it will not decompose, while the PS microspheres will gradually decompose and lose weight, and the carbon in them will be enriched and converted into a spherical carbon skeleton to support the Ti3C2T x The MXene layers maintain a spherical shape. In addition, gases such as benzene, propylene, and ethylene are generated during the pyrolysis process, and these gaseous products are diluted and discharged in a nitrogen atmosphere, absorbed by the tail gas collection device, and prevented from pollution. After pyrolysis, the Ti3C2T x The PS is converted into MXene hollow microspheres (MHM), and the SEM image shows that the MHM are regular spherical shapes with holes on the surface. The gas generated during the pyrolysis of the PS microspheres escapes and causes the hole phenomenon. By carefully observing each sphere, it is found that the edge is darker in color, the middle is lighter in color, and there is a hollow feeling, indicating that the PS microspheres have been removed by pyrolysis.

[0086] The EDX energy spectrum analysis results of the MHM obtained in step (3) of Example 1 are shown in Table 1, and it can be found that the composition is only Ti3C2T x C, O, Ti, and a small part of F and Al of MXene, proving the successful synthesis of MXene hollow microspheres.

[0087] Table 1 EDX energy spectrum analysis results of MHM

[0088]

[0089] The Ti3C2T x The XRD (a) and Fourier transform infrared spectrum (FT-IR) (b) analysis results of MXene, PS microspheres, MXene@PS (i.e. the MXene@PS composite obtained in step (3)), and MHM obtained in Example 1 are shown in Figure 6The 002 characteristic peak of MXene can be seen in both MXene@PS and MHM, while the characteristic peaks of PS microspheres at 10° and 20° appear once in MXene@PS and disappear in MHM, indicating the addition and removal of PS microspheres during the synthesis of MHM. For the distribution of surface functional groups and valence bond distribution of the synthesized material, Fourier transform infrared spectroscopy (FT-IR) was used for analysis, and it was found that Ti3C2T x The peak at 559 cm -1 of MXene is the stretching vibration peak of Ti-O bond, which also appears in MXene@PS and MHM, while the C-H bond of PS microspheres at 3026, 2921, 756 and 698 cm-1, and the -OH functional group at 1492 cm -1 appear in MXene@PS and disappear in MHM, indicating that PS plays a template role in the synthesis of the material and successfully promotes the synthesis of MHM.

[0090] In the CDI system, the specific surface area and pore size distribution of the electrode directly affect the adsorption performance of the electrode. Larger surface area and appropriate pore structure can improve the adsorption capacity and rate of the electrode, thereby improving the deionization efficiency of the CDI system. Therefore, the nitrogen adsorption-desorption curves, specific surface area and pore size distribution of Ti3C2T x -MXene, PS microspheres and MHM in Example 1 were obtained by BET test, and the results are shown in Figure 7 ((a) is the nitrogen adsorption-desorption curve of MHM and Ti3C2T x -MXene, (b) is the nitrogen adsorption-desorption curve of Ti3C2T x -MXene (Ti3C2T x in the figure), PS microspheres (PS in the figure) and MHM). From the adsorption-desorption curve, it can be seen that MHM presents a typical type IV isotherm and H3 type hysteresis loop, indicating that the pore size distribution is mainly mesoporous. By constructing Ti3C2T x -MXene into a three-dimensional spherical structure, its specific surface area increases from 8.9 m 2 / g to 59 m 2 / g, the pore volume reaches 0.15 cm 3 / g, and the pore size is mainly distributed between 2-10 nm mesoporous. The mesoporous structure has a larger surface area and pore volume, which can provide more adsorption sites, meaning that the material has a larger electrochemical active specific surface area, which is beneficial to the diffusion and adsorption of ions on the electrode surface, and improves the ion transmission rate of the electrode and the desalination efficiency as a CDI electrode.

[0091] II. Electrochemical characterization

[0092] The CV, GCD and EIS methods were used for testing with a salt solution containing 1M NaCl as the electrolyte, a platinum sheet electrode and Ag / AgCl as the counter electrode and reference electrode, respectively. The properties of the material such as capacitance and resistance were analyzed through the feedback of electrochemical signals, so as to more comprehensively understand the characteristics of the material in the electrochemical environment, reveal the electrochemical behavior of the newly prepared electrode material, and provide important experimental data and theoretical basis for designing and optimizing the CDI system. The cyclic voltammetry curve (CV) was determined, the shape and characteristics of the CV curve were analyzed, and the redox behavior, electrochemical active surface area, capacitance characteristics, and electrolyte transmission and ion diffusion performance of the MHM material prepared in Example 1 were understood.

[0093] Figure 8 The electrochemical performance of the MHM was analyzed, wherein (a) is the CV curve at different scan rates; (b) is the GCD curve at different current densities; and (c) is the electrochemical impedance spectrum, and the insert is the high frequency region.

[0094] From Figure 8 It can be observed from (a) that as the scan rate increases, the curve area also increases, and the curve gradually approaches a shuttle shape from a quasi-rectangular shape. This is because as the scan rate increases, the number of current signals corresponding to the potential scanned per unit time increases, the CV curve shows polarization phenomenon, and the specific capacitance calculated also decreases. The shape of the CV curve of the MHM is basically quasi-rectangular, and the integral area of the curve is large, indicating that there is a large current density at a specific potential, which has high conductivity and is helpful for effective charge transmission. The specific capacitance was calculated through the CV curve. At a scan rate of 5 mV·s -1 , the MHM can reach a capacitance of 242 F·g -1 . Even when the scan rate gradually increases to 200 mV·s -1 , it still has a capacitance of 168 F·g -1 , with an excellent capacitance retention rate of 70%. In addition, it can be observed that the CV curve of the MHM has a pair of weak redox peaks at -0.2V and -0.6V, which indicates that at this potential, the ion transmission on the electrode material is no longer a simple electric double layer adsorption (EDL), and a pseudo-capacitance behavior involving redox phenomenon occurs, proving that Na + insertion and deintercalation behavior occurs therein, which is beneficial to the design of the future CDI device and improves the adsorption capacity and adsorption efficiency of ions in the salt solution.

[0095] Constant current charge-discharge curves have reference significance for important performance indicators such as capacitance, energy density, charge-discharge efficiency and cycle stability, which are crucial for evaluating the application potential of materials in energy storage and conversion devices such as supercapacitors and lithium ion batteries. The integral area of a single charge-discharge curve can be used to measure the capacitance and rate performance of the material, and the symmetry of the curve can be used to determine whether the material has good double-layer capacitance behavior. As can be seen from the GCD curves of MHM at different current densities (b) in FIG. 6, Figure 8 As the current density increases, the charge-discharge time becomes shorter, but the charge-discharge curve still maintains a symmetrical shape. At a current density of 0.5 A·g -1 , the single charge-discharge process lasts for 500 s, proving that the material has high capacitance and energy density, and the long charge-discharge process also indicates its strong cycle stability, which can maintain stable performance in multiple charge-discharge cycles, proving its potential as a CDI electrode in terms of stability.

[0096] Electrochemical impedance spectroscopy (EIS) is used to study the interface characteristics of electrochemical systems. By applying an alternating current signal at different frequencies and measuring the response, information about electrode surface reactions, electrolyte transport and interface impedance can be obtained, which reflects the process and resistance of ion diffusion and transport from the surface to the interior of the material, and probes the electrochemical behavior between the electrode and the electrolyte. It is an important means of studying electrode process kinetics. As can be seen from the EIS spectrum of MHM (c) in FIG. 6, Figure 8 The slope in the low-frequency region is nearly vertical, indicating rapid diffusion of ions at the electrode interface, while in the high-frequency region there are two small semicircles, which is different from the typical EIS graph with one high-frequency semicircle (charge transfer) and low-frequency slope (diffusion) curve. This may be due to the construction of the hollow microsphere structure, which creates a new charge transfer interface layer and interlayer transport process, which reveals the material's fast ion diffusion and extremely low conduction resistance, making it have lower energy loss as a CDI electrode.

[0097] The electrochemical performance comparison results of Ti3C2T x -MXene and MHM are shown in FIG. 6, Figure 9 , where (a) is the CV curve at a scan rate of 50 mV·s -1 , (b) is the GCD curve at a current density of 0.5 A·g -1 , (c) is the electrochemical impedance spectrum, and the inset is the high-frequency region, and (d) is the specific capacitance comparison chart. By comparing the various electrochemical performances of Ti3C2T x -MXene and MHM, it can be seen that after the two-dimensional sheet structure of Ti3C2T x -MXene is constructed into a three-dimensional hollow spherical structure, its various electrochemical performances are significantly improved. At a scan rate of 50 mV·s -1The CV curve of MHM has a larger integral area and more obvious redox peaks, indicating that the structural stability of MHM is better than that of Ti3C2T x -MXene. The specific capacitance value calculated from the CV curve of MHM is higher than that of Ti3C2T x -MXene. With the increase of scan rate, the capacity retention rate of MHM also has better performance. The charge and discharge time of MHM in the GCD graph is also longer than that of Ti3C2T x -MXene. In the EIS, the intercept of the high-frequency semicircle and the horizontal axis of MHM is also shorter, representing higher capacitance and energy density, and lower energy loss.

[0098] III. Assembly of CDI device and test of adsorption performance

[0099] (1) Assembly and test of CDI device

[0100] Based on the foregoing characterization of the micro-morphology, valence bond and functional group of MHM material and the detection of electrochemical performance, the comprehensive performance of MHM has been preliminarily judged: it has high specific surface area and rich surface active sites, excellent electrochemical performance, high charge capacity, fast ion transmission rate and strong structural stability. In order to detect the adsorption performance of MHM as a CDI electrode material, further polytetrafluoroethylene is used as a binder to prepare a flexible independent film (membrane electrode) of MHM as a CDI negative electrode relying on the adsorption of cations by pseudo-capacitance behavior, and an AC membrane electrode is prepared as a CDI positive electrode relying on the adsorption of anions by double-layer capacitance behavior. In order to ensure that the MHM membrane electrode can be fully utilized and its full performance can be played, the mass of AC is controlled to be 1.3 times that of MHM. An asymmetric hybrid CDI device is assembled to reduce the charge transmission resistance between the two electrodes, realize high-efficiency adsorption, and improve the desalination performance. When a voltage is applied, the MHM film negative electrode adsorbs Na + , and the AC anode corresponds to the adsorption of Cl - in salt water. On the contrary, when the positive and negative electrodes of the device are directly connected and the CDI device is in a short-circuit state, the adsorbed Na + is released from the negative electrode into the salt water, thereby realizing the electro-adsorption / desorption of Na + . The specific MHM / / AC electro-adsorption / desorption process is shown in Figure 10 .

[0101] (2) Connection and operation of CDI device

[0102] The connection diagram of the CDI device adsorption test system is shown in Figure 11 . A DC stabilized power supply is used to power the CDI equipment, and the CDI equipment is connected to a salt solution and a peristaltic pump. The peristaltic pump is set at a flow rate of 20 mL / min -1The rate target salt solution pump into the CDI device and then flow back into the salt solution beaker, the conductivity meter probe into the salt solution below the liquid level, real-time monitoring and recording the change of salt solution conductivity, reflect the process of CDI device adsorption ions.

[0103] After the CDI test system connection is completed, the CDI device needs to be "activated" before the adsorption experiment starts. First, detect whether the system is short-circuited, directly connect the device to the power supply, apply 1.2V voltage, and observe whether there is a digital display in the current area on the DC power supply display screen when there is no salt solution flowing through. If there is a digital display, it means that the CDI device has a short circuit, such as the positive and negative electrodes have slight contact. At this time, the device needs to be disassembled to eliminate the electrode contact problem and then reinstalled. Then, without applying voltage, use about 1L of deionized water to continuously flow through the CDI device, fully soak the electrodes, and make them adapt to the liquid environment, so as to prepare for the subsequent adsorption performance test.

[0104] (3) CDI adsorption performance test results

[0105] The parameters of the CDI test have certain influence on the adsorption performance, such as the size of the applied voltage, the concentration of the adsorbed salt solution, etc. Therefore, the adsorption performance of the electrodes of the CDI under different system parameters is tested. In the voltage range of 0.6-1.6V, set a gear every 0.2V, and test the adsorption capacity of the electrodes of the CDI under different NaCl concentrations in the range of 500mg·L -1 -1.5g·L x The test results of the saturated adsorption capacity of Ti3C2T x -MXene (a) and MHM (b) under different voltages are shown in the following table. x -1 The test results of the saturated adsorption capacity of MHM under different NaCl concentrations are shown in the following table. Figure 12

[0106] From Figure 12 (a), it can be found that the desalination amount of Ti3C2T x -MXene is low after the film electrode is made due to the inevitable stacking of the sheet layer, but it suddenly rises to 26mg·g -1 at 1.4V. This may be because the electrolysis of water or the redox reaction on the surface of the electrode is triggered by the increase of voltage, generating more active sites, instantaneously increasing the adsorption capacity, reaching the saturation limit, so the adsorption capacity will not increase even if the voltage continues to increase, and even the adsorption capacity will be greatly reduced due to the irreversible damage of the electrode. After the MXene is constructed into a hollow microsphere (MHM) form, the adsorption capacity is significantly improved, reaching a maximum of 35mg·g -1 at 1.2V.​The desalination capacity continues to increase with the increase of voltage, but decreases with the further increase of voltage, which may be due to the side reactions caused by the increase of voltage, such as water electrolysis (hydrogen evolution or oxygen evolution reaction), which consumes electrical energy and reduces the effective current available for ion adsorption, resulting in a decrease in capacity, and the high voltage may also destroy the double-layer structure of the electrode-solution interface, weakening the adsorption capacity of the electric field to the ions.

[0107] By Figure 12 It can be found from (b) that the overall trend of desalination capacity is to increase with the increase of concentration, and the desalination capacity basically reaches saturation at 500 mg·L -1 -1, and even if the salt solution continues to increase, there is no obvious change in the increase of desalination capacity. The ion adsorption capacity of the MHM membrane electrode is only 5.6-11.8 mg·g -1 -1 when the concentration is 100-300 mg·L -1 -1, and the ion adsorption capacity reaches 44-51 mg·g -1 -1 when the concentration reaches 500 mg·L -1 -1, and the capacity is significantly improved. The adsorption capacity of the HCDI device reaches the highest when the concentration reaches 500 mg·L -1 -1, and there is no change with the further increase of concentration, indicating that the CDI device prepared by MHM (i.e. HCDI device) has great potential in treating low-concentration wastewater.

[0108] The adsorption and desorption of the HCDI device can be intuitively represented by the change curve of the solution conductivity, and the adsorption and desorption curve of the HCDI device under the condition of 20 min adsorption time, 1.2 V voltage, 50 mL, 500 mg·L -1 NaCl solution is recorded, and the desalination capacity test results are shown in Figure 13 It can be seen from the limit desalination capacity test that the SAC (salt saturation adsorption capacity) of the HCDI device gradually increases with the test, indicating that the adsorption performance of the CDI device gradually releases during the test. In addition, it can be observed that the adsorption speed changes from fast to slow, and the fastest adsorption rate can reach 2.4 mg·g -1 ·min -1 -1, and the highest desalination capacity reaches 47.5 mg·g -1 -1. The desorption curve rises almost linearly, and the Na + instantaneously desorbed from the electrode, indicating that the capacitive adsorption rate of MHM for Na + is fast, and the reversibility is strong, and the potential for recycling is huge.

[0109] Further explore its cycle stability, shorten the adsorption time to 5 min, and the desorption time to 3 min for adsorption and desorption cycle test, under the condition of 1.2 V voltage, 500 mg·L -1 NaCl concentration adsorption for 5 min, the desalination cycle stability test results are shown inFigure 14 The results show that the MHM electrode has excellent capacity retention rate after multiple cycle tests, the adsorption capacity of the MHM electrode is about 30mg·g -1 -1, and the capacity retention rate is still above 90% after 155 cycles. Overall, the MHM has good adsorption capacity and capacity retention rate.

[0110] The above merely describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method of preparing an MHM electrode material, characterized by, The method comprises the following steps: (1) preparing polystyrene microspheres by an emulsion polymerization method, using polyvinylpyrrolidone as an emulsifier, styrene as a monomer, and 2,2'-azobis isobutyronitrile as an initiator; (2) Ti3C2T2 powder is etched by hydrofluoric acid, and is intercalated and peeled by dimethyl sulfoxide to obtain Ti3C2T x -MXene; (3) The polystyrene microspheres are immersed in a polydien dimethyl ammonium chloride solution for treatment, and then combined with the Ti3C2T x MXene stirring reaction, freeze-drying and pyrolysis to obtain Ti3C2T x MXene hollow microspheres, which are the MHM electrode material.

2. The method of claim 1, wherein the MHM electrode material is prepared by the steps of: In step (3), the polystyrene microspheres are mixed with Ti3C2T x The mass ratio of MXene is 10:

1.

3. The method for preparing the MHM electrode material according to claim 1, wherein: In step (1), the reaction temperature of the emulsion polymerization method is 65 DEG C, and the time is 16 h.

4. The method of claim 1, wherein the MHM electrode material is prepared by the steps of: In step (3), the ratio of the use amount of the polystyrene microspheres to the polydiallyldimethylammonium chloride solution is 0.1 mg:1 mL, and the concentration of the polydiallyldimethylammonium chloride solution is 5 mg / mL.

5. The method for preparing the MHM electrode material according to claim 1, characterized in that: In step (3), the polystyrene microspheres are treated by the polydiallyldimethylammonium chloride solution for 30 min.

6. The method of claim 1, wherein the MHM electrode material is prepared by the steps of: In step (3), the stirring reaction time is 2 h; and the pyrolysis condition is that the temperature is raised to 500 DEG C at a temperature raising rate of 5 DEG C / min, and the temperature is kept for 2 h.

7. The method of claim 6, wherein the MHM electrode material is prepared by a process comprising: In step (3), the pyrolysis process is carried out in a nitrogen atmosphere.

8. An MHM electrode material, characterized in that, The MHM electrode material is prepared by the method of any one of claims 1-7.

9. An asymmetric capacitive deionization device, comprising: The MHM electrode material of claim 8 is used as a negative electrode, and active carbon is used as a positive electrode.

10. A method of desalinating saltwater, characterized by, The asymmetric capacitive deionization device of claim 9 is used to treat NaCl solution with a salt concentration of 500-1500 mg / L at a voltage of 0.8-1.4 V.

Images