FePO4-coated MHM composite material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of electrode materials, and particularly relates to a FePO4-coated MHM composite material and a preparation method and application thereof. In order to improve the stability and selectivity of a CDI electrode material, a pseudocapacitance material FePO4 is combined with a double-electrode-layer material MHM (MXene of a three-dimensional hollow microsphere structure) with a pseudocapacitance attribute, the FePO4-MHM composite material is successfully prepared, FePO4 particles are uniformly distributed on the surfaces of MXene-based hollow microspheres, a three-dimensional structure with a high specific surface area and rich ion adsorption sites is formed, and the stability and selectivity of the CDI electrode material are improved. The FePO4-coated MHM composite material also shows excellent desalination performance in a CDI device.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, specifically relating to a FePO4@MHM composite material, its preparation method, and its application. Background Technology

[0002] The rapid development of CDI (electrochemical desalination) technology has benefited from increasing attention to water resources and environmental pollution issues, as well as continuous progress in materials science, nanotechnology, and electrochemistry. In the development of CDI technology, material selection, electrode design, system optimization, and application research have played crucial roles.

[0003] CDI (Clean-Dip Removal) technology, as a highly efficient water treatment method, places high demands on the performance of materials. Researchers have been exploring various methods and strategies to improve the performance of CDI electrode materials. Improving the cycle stability and ion selectivity of CDI electrode materials, while enhancing adsorption performance, is also a crucial issue in the development of CDI. Improved cycle stability can extend the lifespan of the CDI system, reduce maintenance costs, and increase system reliability. Enhanced ion selectivity helps achieve more efficient ion removal and water treatment. Therefore, material design is the most critical aspect in improving the stability and selectivity of CDI electrode materials. By rationally designing and optimizing the structure and composition of electrode materials, their cycle stability and ion selectivity can be improved. For example, selecting materials with good electrochemical stability and high surface area can increase their cycle life and improve ion adsorption capacity. Surface modification of electrode materials can improve the distribution of valence bonds and functional groups, increase active sites, and achieve selective adsorption or separation of specific ions. Increasing surface charge density can regulate the ion adsorption and desorption processes, thereby improving ion selectivity. In addition, controlling the microstructure and adding functional substances to the electrode material can also improve its performance. By increasing the specific surface area and more active sites, the adsorption and desorption rates of ions can be improved, and the adsorption behavior of ions can be regulated, thereby improving its adsorption performance.

[0004] Existing CDI electrode materials are mainly double-layer materials. However, the capacitive adsorption performance of double-layer materials is limited by their specific surface area and electrochemical stability. Especially when dealing with high-salinity solutions, their adsorption capacity and efficiency often fail to meet practical requirements. Therefore, there is a need in this field for a new type of electrode material to solve the above problems. Summary of the Invention

[0005] This invention addresses the aforementioned problems by providing a FePO4@MHM composite material, its preparation method, and its application. The pseudocapacitive material FePO4 provides higher selective adsorption capacity for sodium ions, while the double-layer material MHM, constructed as a three-dimensional hollow microsphere structure, continues to provide high charge storage capacity. By preparing the composite material, the synergistic effect of the two mechanisms is achieved.

[0006] To achieve the above objectives, the present invention employs the following technical solution: In a first aspect, the present invention provides a method for preparing FePO4@MHM composite material, comprising the following steps: Step 1: After etching Ti3AlC2 powder with hydrofluoric acid, Ti3C2T is prepared by intercalation and exfoliation using dimethyl sulfoxide. x -MXene; Step 2: Polystyrene microspheres are prepared using emulsion polymerization. Step 3: Ferrous sulfate and ammonium dihydrogen phosphate undergo a displacement reaction, followed by oxidation with hydrogen peroxide to prepare ferric phosphate; Step 4, Ti3C2T x FePO4@MHM composites were prepared by electrostatic self-assembly of MXene, polystyrene microspheres, and iron phosphate.

[0007] Furthermore, step 4 specifically includes: Polystyrene microspheres were dispersed in deionized water and stirred continuously until uniformly dispersed. Then, a polydimethylammonium chloride solution was added for treatment. Finally, the treated polystyrene microspheres and ferric phosphate were added to the dispersed Ti3C2T solution. x In MXene, the reaction was continuously stirred. After the reaction was completed, the mixture was centrifuged, washed, concentrated, frozen, and then freeze-dried to obtain FePO4-Ti3C2T. x The @PS composite was then pyrolyzed to obtain the FePO4@MHM composite material.

[0008] Furthermore, the polystyrene microspheres, iron phosphate, and Ti3C2T x The mass ratio of MXene is 10:1:2.

[0009] Furthermore, the specific conditions for the pyrolysis are: under a nitrogen atmosphere, at 5 °C for 1 minute. -1 The heating rate was adjusted, and the mixture was pyrolyzed at 500℃ for 2 h. After the reaction was completed and the temperature was lowered, FePO4@MHM composite material was obtained.

[0010] Secondly, the present invention also provides a FePO4@MHM composite material, which is prepared by the above-described preparation method.

[0011] Thirdly, the present invention also provides an application of the FePO4@MHM composite material for CDI electrodes.

[0012] Fourthly, the present invention also provides a CDI device, characterized in that it comprises the FePO4@MHM composite material.

[0013] Compared with the prior art, the present invention has the following advantages: 1. By combining the pseudocapacitive material FePO4 with the pseudocapacitive double-layer material MHM (MXene, a three-dimensional hollow microsphere structure), a FePO4@MHM composite material was successfully prepared. The successful preparation of the FePO4@MHM composite material was confirmed by characterization methods such as SEM, XRD, and FT-IR. FePO4 particles are uniformly distributed on the surface of the MXene-based hollow microspheres, forming a three-dimensional structure with high specific surface area and abundant ion adsorption sites. The introduction of FePO4 significantly improves the specific surface area and pore size distribution of the material, providing more active sites for ion adsorption.

[0014] 2. The FePO4@MHM composite material also exhibited excellent desalination performance in the CDI unit. The highest desalination capacity reached 67 mg g. -1 Significantly higher than Ti3C2T x The composite material, consisting of MXene, MHM, and pure FePO4, exhibited excellent cycling stability in multiple cycle tests. After 170 cycles, the capacity retention remained above 90%, outperforming many CDI electrode materials reported in the literature. SEM, XRD, and FT-IR characterization of the FePO4@MHM film electrode before and after testing revealed that the composite structure remained stable without significant changes, demonstrating the material's strong stability.

[0015] 3. During the desorption process, the FePO4@MHM / / AC HCDI device can quickly release the adsorbed ions and generate enough current to power the LED device for a short time, demonstrating its potential in energy recovery. Attached Figure Description

[0016] Figure 1 FePO4, FePO4-Ti3C2T x SEM images of @PS and FePO4@MHM; Figure 2 The XRD pattern of FePO4; Figure 3 SEM images of FePO4@MHM with different FePO4 dosages; Figure 4 FT-IR spectra of FePO4@MHM with different FePO4 dosages; Figure 5 The particle size distribution diagram for FePO4@MHM is shown below. Figure 6 This is the elemental distribution diagram corresponding to FePO4@MHM; Figure 7 Ti3C2T x MXene, PS microspheres, FePO4, FePO4-Ti3C2T x XRD and FT-IR spectra of @PS and FePO4@MHM; Figure 8 In the middle, (a) is Ti3C2T x -MXene, MHM and FePO4@MHM nitrogen adsorption-desorption curves, (b) is Ti3C2T x - MXene, PS microspheres, MHM and FePO4@MHM pore size analysis and specific surface area; Figure 9 CV curves of FePO4@MHM with different FePO4 dosages at different scan rates; Figure 10 GCD curves of FePO4@MHM with different FePO4 dosages at different current densities; Figure 11 Electrochemical impedance spectroscopy of FePO4@MHM with different FePO4 dosages, with the inset showing the high-frequency region; Figure 12 Electrochemical performance analysis of MHM, FePO4, and FePO4@MHM with different FePO4 dosages; Figure 13 Ti3C2T x Electrochemical performance analysis of MXene, FePO4, MHM and FePO4@MHM; Figure 14 The relationship between the peak currents of the anode and cathode and the scan rate in the CV plot of FePO4@MHM, and the functional relationship between the normalized contribution ratio of the capacitor current and the diffusion control current and the scan rate. Figure 15 In the middle, (a) is Ti3C2T x -MXene, FePO4, MHM, and FePO4@MHM at different voltages ranging from 0.6 to 1.6 V for 500 mg L -1 The desalination capacity of NaCl, (b) is MHM and FePO4@MHM at 1.2 V, 100-1500 mg L -1 Desalination capacity at different NaCl concentrations, (c) is for 500 mg L -1 The amount of salt desalted after 20 minutes of NaCl adsorption; Figure 16 The physical and capacitive adsorption processes of the FePO4@MHM / / AC HCDI device; Figure 17 For MHM and FePO4@MHM at 1.2 V, 500 mg L -1 Cyclic stability test of NaCl adsorption for 5 min; Figure 18 LED name badges powered by FePO4@MHM HCDI equipment; Figure 19 SEM image of FePO4@MHM membrane electrode before testing; Figure 20 SEM images of the FePO4@MHM membrane electrode after testing; Figure 21 XRD patterns of the membrane electrode before and after FePO4@MHM powder and desalination tests; Figure 22 FT-IR images of FePO4@MHM powder and the membrane electrode before and after desalination test; Figure 23 A comparison of the desalination performance of FePO4@MHM and MXene-based CDI electrodes. Detailed Implementation

[0017] To further illustrate the technical solution of the present invention, the present invention will be further described below through embodiments. Example 1

[0018] The preparation method of FePO4@MHM composite material in this embodiment includes the following steps: Step 1: After etching Ti3AlC2 powder with hydrofluoric acid, Ti3C2T is prepared by intercalation and exfoliation using dimethyl sulfoxide. x -MXene; Step 2: Polystyrene microspheres are prepared using emulsion polymerization. Step 3: Ferrous sulfate and ammonium dihydrogen phosphate undergo a displacement reaction, followed by oxidation with hydrogen peroxide to prepare ferric phosphate; Step 4, Ti3C2T x FePO4@MHM composites were prepared by electrostatic self-assembly of MXene, polystyrene microspheres, and iron phosphate.

[0019] In this embodiment, Ti3C2T x -MXene is prepared by etching commercial titanium aluminum carbide (Ti3AlC2) powder with hydrofluoric acid (HF) followed by intercalation and lift-off using dimethyl sulfoxide (DMSO). The specific steps are as follows: (1) Etching: First, measure 20 mL of 50% (mass fraction) HF solution into a polytetrafluoroethylene beaker, then slowly add 1.5 g of Ti3AlC2 powder to the HF solution. The reaction is vigorous, accompanied by the generation of a large number of bubbles. After covering and standing for 30 min, the reaction tends to slow down. Add a magnetic stir bar and seal with a sealing film. Transfer to a 40 ℃ constant temperature magnetic water bath and etch at 1000 rpm. -1 The reaction was carried out for 24 hours. After the reaction was completed, the membrane was first perforated to remove air, then the membrane was peeled off. The mixture was washed multiple times with deionized water and centrifuged at 6000 rpm until the pH reached 6-7. The washed product was then vacuum filtered through a 0.22 μm filter membrane, and the filter cake was dried in a vacuum drying oven to obtain the multilayer Ti3C2T. x powder.

[0020] (2) Peeling: 0.2 g of multilayer Ti3C2T x Pour the powder into a glass beaker containing 15 mL of DMSO, add a magnetic stir bar, and seal with sealing film. Transfer to a 25 °C constant temperature magnetic water bath and stir at 1000 rpm for 1 min. -1 The reaction was carried out for 24 h. After the reaction was completed, 30 mL of deionized water was added to the beaker, and the beaker was placed under a cell disruptor and sonicated in an ice-water bath for 30 min. Then, the beaker was centrifuged at 6000 rpm and washed with water 5-6 times to remove DMSO. The mixture was then vacuum filtered to obtain a few-layer Ti3C2T. x -MXene.

[0021] Polystyrene microspheres were prepared by emulsion polymerization, and the specific steps are as follows: First, measure 5 g of polyvinylpyrrolidone (PVP), 55 mL of styrene, and 150 g of anhydrous ethanol into a three-necked flask, and fix it in a constant-temperature magnetic water bath with a magnetic stir bar. After stirring at 500 rpm for 30 min, the PVP dissolves, and the water bath temperature is adjusted to 65 ℃. While the water bath is heating up, measure 1 g of 2,2'-azobisisobutyronitrile (AIBN) and 50 g of anhydrous ethanol into a beaker, stir until well mixed, and add the mixture to the three-necked flask when the water bath temperature reaches 65 ℃. The three-necked flask should be connected to a condenser, filled with nitrogen, and the flask cap should be tightly sealed. After stirring at a constant temperature of 65 ℃ for 16 h, the reactant changes from transparent to a milky white colloid. Remove the reactant, wash it three times with a mixture of anhydrous ethanol and water by centrifugation, concentrate it to about 100 mL, calculate its actual concentration, and then refrigerate it for later use.

[0022] Ferric phosphate is prepared by reacting ferrous sulfate and ammonium dihydrogen phosphate through a displacement reaction, followed by oxidation with hydrogen peroxide. The specific steps are as follows: Measure 0.01 mol of ferrous sulfate heptahydrate and 0.01 mol of ammonium dihydrogen phosphate into 60 mL and 40 mL of water respectively, and stir until dissolved. Then, place the ferrous sulfate solution at 800 rpm for 1 minute. -1 Ammonium dihydrogen phosphate solution was slowly added dropwise under constant temperature stirring at 50 ℃, and the reaction was allowed to proceed for 20 min. After adding 7 mL of hydrogen peroxide and stirring continuously for 30 min, the mixture was washed by centrifugation with deionized water, filtered, and dried in an 80 ℃ forced-air oven to obtain amorphous ferric phosphate.

[0023] FePO4@MHM composite materials were prepared by electrostatic self-assembly, and the specific steps are as follows: First, the prepared PS microspheres were dispersed in 100 mL of deionized water and stirred continuously until uniformly dispersed. Then, 5 mL of polydiene dimethyl ammonium chloride solution (PDDA) was added and the mixture was treated for 30 min. Next, the mass of MXene and FePO4 to be added was calculated based on the mass of the added PS microspheres, and the mixture was added to the dispersed Ti3C2T microspheres at a ratio of PS:MXene:FePO4 of 10:1:2. x The MXene reaction was continuously stirred for 2 h. After the reaction, the mixture was washed three times with deionized water by centrifugation, concentrated to below 50 mL, placed in a polytetrafluoroethylene beaker, sealed with film, and frozen for 6 h. Then, it was sent to a vacuum freeze dryer and freeze-dried for 48 h to obtain a powdered sample, which is FePO4-Ti3C2T. x @PS composite. It was placed in a tube furnace and incubated at 5 °C for [time missing] min under a nitrogen atmosphere. -1 The heating rate was set at 500 °C for 2 h, and after the reaction was completed and the temperature was lowered, the product was taken out as FePO4@MHM.

[0024] The Ti3C2Tx-MXene hollow microspheres (MHM) in this embodiment were prepared using the template method and the principle of electrostatic self-assembly. The specific steps are as follows: First, the prepared PS microspheres were dispersed in 100 mL of deionized water and stirred continuously until uniformly dispersed. Then, 5 mL of polydimethylammonium chloride solution (PDDA) was added and treated for 30 min to facilitate binding with MXene. Next, the mass of MXene to be added was calculated based on the mass of the added PS microspheres, and the dispersed Ti3C2T was added at a PS:MXene ratio of 10:1. x The MXene reaction was continuously stirred for 2 h. After the reaction, the mixture was washed three times with deionized water by centrifugation, concentrated to less than 50 mL, placed in a polytetrafluoroethylene beaker, sealed with film, and frozen for 6 h. Then, it was sent to a vacuum freeze dryer and freeze-dried for 48 h to obtain a powdered sample, which is Ti3C2T. x @PS composite. It was placed in a tube furnace and heated at 5 °C for [time missing] min under a nitrogen atmosphere.-1 The heating rate was controlled, and the pyrolysis was carried out at 500 °C for 2 h. After the reaction was completed and the temperature was lowered, the product was taken out as Ti3C2T. x -MXene hollow microspheres (MHM).

[0025] 1. Material Characterization Depend on Figure 1 As can be seen in (a), the prepared iron phosphate particles are only about 100 nm in size. Figure 2 Only broad, dome-shaped diffraction peaks were observed, indicating that the material is an amorphous, non-crystalline iron phosphate. Then, PS microspheres, MXene, and FePO4 were synthesized in a specific ratio, and the results were observed... Figure 1 In (b), it can be found that FePO4-Ti3C2T x The @PS complex is a well-formed sphere, similar to Ti3C2T. x @PS The difference is obvious, Ti3C2T x @PS has a smooth surface, covered only by MXene layers, while FePO4-Ti3C2T x The surface of the PS microspheres is rough and uneven, covered with small iron phosphate particles. After pyrolysis to remove the PS microsphere template, the synthesized FePO4@MHM image is shown ( Figure 1 In (c), MXene-based spherical shells uniformly distributed with iron phosphate particles are clearly visible. The broken parts on the spheres are caused by the pyrolysis of PS microspheres into gaseous substances such as ethylene. The two-dimensional Ti3C2T... x -MXene sheets and amorphous FePO4 particles were successfully used to construct a three-dimensional FePO4@MHM composite material.

[0026] To ensure the composite material fully utilizes the pseudocapacitive properties of FePO4 while avoiding its low conductivity from reducing the final adsorption performance, the study needs to determine the dosage of FePO4. The material was composited according to FePO4 / FePO4@MHM mass ratios of 9wt%, 20wt%, 25wt%, 40wt%, and 47wt%. Figure 3 SEM images of the composite materials at different proportions show that as the FePO4 dosage increases, the amount of iron phosphate particles loaded on the MHM gradually increases. At 9 wt%, almost no FePO4 is visible on the hollow microspheres, while at 47 wt%, the surface is covered with FePO4 particles. Furthermore, due to the excessive FePO4 addition, a large amount of FePO4 agglomerates into clumps, compressing the MHM and causing it to lose its regular spherical shape. Observation reveals that at a dosage of 40 wt%, the FePO4@MHM exhibits the best morphology, the most regular spheres, and a relatively uniform distribution of FePO4 particles on the surface. FT-IR testing revealed (…). Figure 4As the amount of FePO4 added increases, the ratio of the peak intensity of the stretching vibration peak of Fe-PO to that of the vibration peak of Ti-O bond gradually increases, reaching the highest at 40wt%, and then slightly decreasing at 47wt%, indicating that the iron phosphate content in the composite material is the highest at 40wt%, and the composite effect is the best.

[0027] The size of FePO4@MHM was analyzed using a laser particle size analyzer. Figure 5 The material size was found to be basically the same as the particle size of the template material, PS microspheres, both around 2 μm. The elemental distribution is as follows: Figure 6 As shown, the C, O, Fe, P, and Ti elements are evenly distributed, indicating that Ti3C2T was successfully synthesized. x MXene and FePO4 were synthesized into a FePO4@MHM three-dimensional composite material. EDX analysis is shown in Table 1. It can be seen from the table that the FePO4@MHM after removing PS microspheres has a high C content, which also confirms that the carbon skeleton left after PS pyrolysis supports Ti3C2T. x -MXene maintains its microsphere structure and enhances the structural stability of the composite material.

[0028] Table 1 EDX spectral analysis of FePO4@MHM

[0029] Figure 7 (a) shows the XRD patterns of various materials in the FePO4@MHM synthesis stage. Broad, dome-shaped diffraction peaks of amorphous iron phosphate can be observed in the FePO4-Ti3C2T... x The same phenomenon occurs in @PS and FePO4@MHM, but because iron phosphate is an amorphous phase, the peak intensity is relatively weaker. Meanwhile, Ti3C2T... x The 002 characteristic peak of -MXene was consistently observed at 6.2°, and the interlayer spacing calculated using the Bragg equation was 14.24 Å, indicating that the expanded Ti3C2T x The interlayer spacing of MXene remained stable at all stages of material synthesis, without any layer stacking issues. Figure 7 The FT-IR spectrum in (b) shows that in FePO4, at 546 cm⁻¹ -1 (PO4) 3- (bending pattern of the OPO bond) and 1046 cm -1 The peak at the position of (the stretching vibration peak of Fe-PO in amorphous FePO4) shifted slightly to the left to 566 cm⁻¹ after being synthesized onto FePO4@MHM. -1 and 1076 cm -1 This may be due to FePO4 and Ti3C2T. xVan der Waals forces exist between the sheets, and FePO4 undergoes crystal structure changes during material pyrolysis, improving the structural strength of the composite material. (Compared to Ti3C2T) x -sheet, FePO4-Ti3C2T x The spectra of @PS and FePO4@MHM show that at 630 cm⁻¹ -1 The same peak appeared at the vibrational peak of the Ti-O bond, further confirming the successful synthesis of the material. (744 cm⁻¹) -1 1628 cm -1 2341 cm -1 、 and 3443 cm -1 The peaks represent CH, -OH, C=O, and -OH bonds, respectively, and these peaks were observed at various stages of material synthesis. BET analysis yielded nitrogen adsorption-desorption curves for each stage of material synthesis, and the pore size distribution and specific surface area are shown below. Figure 8 As shown, FePO4@MHM exhibits a more pronounced type IV isotherm and H3 type hysteresis loop, and the two-dimensional Ti3C2T x - The specific surface area of ​​the MXene sheets is only 8.9 m². 2 g -1 The FePO4@MHM value reached 72.8 m. 2 g -1 This represents an approximately 8-fold increase, while the composite amorphous FePO4 significantly increases the number of ion adsorption sites. Furthermore, the pore size distribution of FePO4@MHM is primarily within the mesoporous region of 2–10 nm, and the pore volume reaches 0.21 cm³. 3 g -1 This forms abundant ion diffusion channels, accelerates the charge transfer process, and increases the ion diffusion rate.

[0030] 2. Electrochemical characterization To further confirm the effect of FePO4 dosage on the performance of FePO4@MHM, and to ensure that the composite material can maximize the pseudocapacitive properties of FePO4 while avoiding its low conductivity from reducing the electrochemical performance of the material, electrochemical tests were also performed on composite materials with various FePO4 dosages, and the results were characterized using CV, GCD, and EIS. (CV plots are shown in the original text.) Figure 9 The redox peaks of FePO4 in NaCl at various scan rates are clearly visible. After combining it with MHM, as the FePO4 content increases, the initial 9 wt% FePO4 / Ti3C2T x The redox peaks were not obvious, but the shape of their CV patterns remained well-maintained as quasi-rectangular, even when increased to 40 wt% FePO4 / Ti3C2T. xAt that time, the CV plot showed a good quasi-rectangular shape while containing a set of relatively obvious redox peaks with a relatively large area, thus demonstrating a significant advantage in calculating its specific capacitance. Increasing the FePO4 / Ti3C2T content to 47wt% further improved this effect. x At that time, the CV plot showed a spindle shape, and significant polarization appeared with increasing scan rate. This may be due to excessive addition of iron phosphate, which rapidly reduced the material's conductivity. Simultaneously, the iron phosphate particles agglomerated, causing the MXene hollow microsphere structure to collapse and resulting in layer stacking problems. Similarly, in GCD testing (… Figure 10 FePO4 in the 40 wt% FePO4 / Ti3C2T also exhibits a corresponding voltage plateau due to the redox reaction, and the charge / discharge time is observed to be consistent with this. x It has the longest duration, highest energy density, and highest specific capacitance.

[0031] Although FePO4 has poor electrical conductivity, according to the EIS diagram ( Figure 11 It can be seen that FePO4 exhibits relatively low internal resistance, which is due to the addition of an appropriate amount of conductive carbon black to the slurry prepared for the test. In a series of EIS tests, each material showed two semi-circles in the high-frequency region, which correspond to the charge transfer process and the interface layer or ion transport process, respectively. (Still 40 wt% FePO4 / Ti3C2T) x The electrode exhibits the lowest internal resistance to charge transfer, the fastest electrochemical reaction kinetics on its surface, and facilitates the charge transfer process. Figure 12 (a) shows the results of adding MHM, FePO4, and FePO4@MHM with different FePO4 dosages at 50 mV s. -1 Comparing the CV curves at different scan rates, it can be found that the integral area is around 40wt% FePO4 / Ti3C2T. x The time is the largest and closest to a quasi-rectangle, corresponding to the Figure 12 The specific capacitance comparison in (b) also shows a significant advantage. This indicates that ferric phosphate, as a pseudocapacitive material with poor conductivity, significantly increases its specific surface area and provides a large number of active sites by using MXene hollow microspheres with good conductivity as a carrier. At the same time, due to the specific intercalation mechanism of sodium ions, ferric phosphate exhibits a larger specific capacitance in electrochemical tests.

[0032] To investigate the contribution of the excellent electrochemical performance of FePO4@MHM, Ti3C2T x The electrochemical characterizations of -MXene, FePO4, MHM and FePO4@MHM were compared. Figure 13 In the middle (a) and (b), the scan rate is 50 mV / s, respectively. -1 CV curves of each material at 0.5 A g -1The GCD curves of each material clearly show the redox peak and voltage plateau of FePO4, indicating that FePO4, as a common sodium-based electrochemical material, exhibits intercalation and insertion of sodium ions in NaCl solution. (Ti3C2T) x -MXene also exhibits weak redox peaks at 0.1 V and -0.6 V, and shows a larger integral area after being constructed as MHM and FePO4@MHM. The structurally advantageous MHM, combined with the redox peaks of FePO4, can adsorb sodium more efficiently. Its GCD curve also shows the longest charge-discharge time, with the charging and discharging times being essentially equal, further demonstrating its reversible ion adsorption-desorption and good coulombic efficiency.

[0033] Figure 13 (c) shows the electrochemical impedance spectra of each material, FePO4 and Ti3C2T. x It typically exhibits a high-frequency semicircle (charge transfer) and a low-frequency sloping line (diffusion), while FePO4 exhibits a smaller high-frequency semicircle, representing a lower charge transfer resistance (Rct). This may be due to the affinity of FePO4 for Na. + The intercalation reaction exhibits high catalytic activity. Simultaneously, two semicircles can be clearly observed in the high-frequency region for both MHM and FePO4@MHM. This is due to the construction of the hollow microsphere structure, which introduces new interface layers and interlayer transport processes during charge transfer, thereby significantly reducing the charge transfer resistance of FePO4@MHM, minimizing energy loss during transport, and improving charge / discharge rates and rate performance. In terms of specific capacitance comparison... Figure 13 In (d), FePO4@MHM>MHM>Ti3C2T also showed a pattern. x The pattern of FePO4: Individual FePO4 exhibits the lowest specific capacitance due to its poor conductivity and ion diffusion properties. However, FePO4@MHM combines the structural advantages of hollow microspheres with the specific intercalation mechanism of sodium ions, thus exhibiting a larger specific capacitance, even at a scan rate of 5 mV s. -1 The specific capacitance can also reach 260 F g -1 Increased to 200 mV s -1 It still maintains 182 F g -1 The high capacity and 70% capacity retention indicate that FePO4@MHM has good rate performance.

[0034] For a pair of redox peaks in the CV curve of FePO4@MHM, the relationship between the ion kinetic current (i) and the scan rate (k) was calculated. Figure 14In (a), the slope (b value) is calculated by formula (1), and a is determined by plotting log(i) against log(v). The calculated b values ​​for the anodic and cathode peaks are 0.96 and 0.93, respectively, which means that the charge storage mechanism involves diffusion-controlled ion intercalation and capacitance-controlled behavior.

[0035] Furthermore, the normalized contribution rate of a specific capacitor type to the total capacitance can be quantified according to formula (2), where k1v and k2v 1 / 2 The values ​​represent the contributions from capacitor and battery type, respectively. Calculation results show that at 5 mV s... -1 and 10 mV s -1 At that time, 82.1% and 82.4% of the total capacitance were attributed to capacitance type. This is evidence of the material's dual-mode sodium storage ( Figure 14 (b)). The high proportion of surface capacitance control originates from FePO4 and Ti3C2T, which have pseudocapacitive properties. x The introduction of this technology facilitates rapid ion storage at the electrode surface. The scan rate was increased to 100 mV / s. -1 and 200 mV s -1 The capacitive contribution reaches over 90% (90.1% and 97.4%). The high scan rate suppresses the diffusion control process during redox cycles, which means that deep charge and discharge cannot be achieved inside the composite material, thus significantly reducing the diffusion capacitance.

[0036] (1) (2). Example 2

[0037] 1. Assembly of CDI device FePO4@MHM composite material was assembled with activated carbon to form an FePO4@MHM / / AC asymmetric CDI device. The desalination performance of the composite material as a CDI electrode and its cycling stability after long-term adsorption-desorption were tested.

[0038] 2. CDI Adsorption Performance Test To test the adsorption capacity of FePO4@MHM for NaCl under different voltage conditions and to explore the contribution of its adsorption performance, the adsorption capacity of MHM and Ti3C2T was tested simultaneously. x and the adsorption capacity response of FePO4 to different voltages ( Figure 15 (a)). The results showed that FePO4 exhibited extremely poor desalination performance after being fabricated into a membrane electrode due to its poor conductivity, achieving only 8 mg g / g at 1.4 V. -1 Desalination capacity, Ti3C2T xDue to unavoidable issues with sheet stacking, the desalination capacity was relatively low after fabrication of the membrane electrode, but it suddenly increased to 26 mg g at 1.4 V. -1 This may be because the increased voltage triggers water electrolysis or redox reactions on the electrode surface, generating more active sites and instantly increasing the adsorption capacity until it reaches the saturation limit. Therefore, the adsorption capacity will not increase further with continued voltage increases. However, when MXene is constructed into hollow microspheres, its adsorption capacity is significantly improved, reaching a maximum of 35 mg g at 1.2 V. -1 The desalination capacity decreased with increasing voltage, possibly due to side reactions such as water electrolysis (hydrogen or oxygen evolution), which consumes electrical energy and reduces the effective current available for ion adsorption, leading to a decrease in capacity. Secondly, high voltage may also disrupt the double-layer structure at the electrode-solution interface, weakening the electric field's ability to adsorb ions. The FePO4@MHM composite material exhibits significantly improved adsorption performance compared to MHM. The adsorption sites provided by FePO4, combined with the high conductivity and large specific surface area of ​​MHM, achieve a capacity of 67 mg g at 1.4 V. -1 The desalination capacity is extremely high; even when the voltage is increased to 1.6 V, the desalination capacity only decreases to 50 mg g. -1 To further explore the range of solution concentrations that this HCDI device can handle, concentrations of 100, 300, 500, 700, 1000, and 1500 mg / L were set. -1 The tests were conducted in six concentration gradients, each using 50 mL of NaCl solution. Figure 15 (b) It can be observed that FePO4@MHM has a higher desalination capacity compared to MHM. This is because the material has more adsorption capacity for Na after loading FePO4. + The active sites on MHM, along with the mesoporous structure, significantly increase the contact area between the electrode and the electrolyte, promoting electrolyte penetration into the material's interior and providing a suitable environment for Na+. + Constructing rapid diffusion channels accelerates its flow in both liquid and solid phases. Although CDI technology is typically used to treat low-concentration wastewater, it is more suitable for applications in the 100–300 mg / L range. -1 At that time, the ion adsorption capacities of MHM and FePO4@MHM were only 5.6~11.8 mg g, respectively. -1 and 26~40 mg g -1 When the concentration is increased to 500 mg / L -1 At the above times, the ion adsorption capacity reached 44~51 mg g. -1 and 63~66 mg g -1The capacity is significantly improved. This is because low-concentration solutions inherently have low ion content, and secondly, the ion movement rate in low-concentration solutions is slow, limiting the adsorption capacity of the HCDI device, thus the exhibited adsorption capacity is not very high. However, when the concentration reaches 500 mg / L... -1 At this point, the adsorption capacity of the HCDI device reaches its maximum and does not change with further increases in concentration, indicating that the HCDI device has great potential in treating low-to-medium concentration wastewater. The adsorption and desorption of the HCDI device can be visually represented by the change curve of solution conductivity. Adsorption times of two HCDI devices were recorded at 20 min, 1.2 V voltage, 50 mL, and 500 mg / L. -1 Adsorption-desorption curves in NaCl solution ( Figure 15 (c)). An adsorption rate was observed to decrease from fast to slow, with the fastest adsorption rate reaching 6.13 mg·g⁻¹. -1 ·min -1 During desorption, the curve rises almost linearly, indicating that the Na adsorbed at the electrode... + Instantaneous desorption indicates that FePO4@MHM is effective against Na + It has a fast capacitive adsorption rate, strong reversibility, and great potential for recycling.

[0039] By observing the changes in conductivity during testing with an HCDI device, it was found that the conductivity of the salt solution began to decrease as soon as it flowed through the device without any voltage applied initially. When the rate of decrease in conductivity slowed down and stabilized, applying voltage again caused the conductivity to decrease rapidly once more. This process of conductivity change was recorded. Figure 16 This serves as evidence of the physical and capacitive adsorption of the HCDI device. Calculations from the figure show that the physical adsorption capacity reaches 20.8 mg·g⁻¹. -1 The instantaneous adsorption rate can reach 3.37 mg·g. -1 ·min -1 The capacitive adsorption capacity reached 45 mg·g. -1 The instantaneous adsorption rate can reach 6.13 mg·g. -1 ·min -1 The combined effect of physical adsorption and capacitive adsorption resulted in a total adsorption capacity of 65 mg·g for this HCDI device. -1 This result confirms the dual-mode sodium storage observed in the FePO4@MHM composite material during electrochemical testing. The high specific surface area structure provides high-capacity physical adsorption performance, while the Ti3C2T material itself possesses pseudocapacitive properties. xMXene, together with the pseudocapacitive material FePO4, provides a higher proportion of capacitive adsorption, ultimately achieving the high adsorption performance of this HCDI device. To further investigate its cycling stability, the adsorption time was shortened to 5 min and the desorption time to 3 min for adsorption-desorption cycle testing. Figure 17 The results showed that, like the MHM electrode, the FePO4@MHM electrode maintained excellent capacity retention after multiple cycles of testing, with an adsorption capacity of 45 mg·g⁻¹. -1 The capacity retention rate remained above 90% after approximately 170 cycles, but dropped to 75% after the 171st cycle. Finally, after 180 adsorption-desorption cycles, the capacity retention rate stabilized at 75%. After multiple cycles, ions may penetrate deep into the electrode and fail to desorb, and prolonged exposure to a humid environment may cause electrode peeling, both of which can lead to a decrease in electrode capacity. However, the overall capacity retention rate remained high. Furthermore, due to the extremely fast desorption rate of HCDI devices, the current generated during this process can power an LED badge for a short period. Figure 18 This provides a framework for the energy recycling of capacitor deionization.

[0040] 3. Characterization of CDI film electrodes To investigate the source of the high adsorption capacity and the reason for the high capacity retention of this HCDI device, SEM images of the FePO4@MHM membrane electrode were taken before and after the test. Figure 19 In the middle (a), the SEM image of FePO4@MHM after it was prepared as a flexible membrane electrode can be clearly seen. The hollow microsphere morphology of FePO4@MHM, as well as the conductive carbon black (porous granular) and PTFE in the form of fibrous tubular crosslinking agent are also clearly visible. Figure 19 Images (b) and (c) show the SEM images and high-magnification images of the cross-section of the FePO4@MHM film electrode with graphite paper as the substrate. The thickness was measured to be approximately 155 μm. The hollow microsphere structure and fine granular conductive carbon black were clearly visible after magnification. Figure 20 This is a SEM image of the FePO4@MHM membrane electrode after the test. No significant changes were observed in the electrode cross-section image after the test compared to before. However, some hollow microspheres showed collapse and fragmentation. The overall structure changed from a loose, fluffy appearance to a more compact one, with slight blockiness. This is because the electrode needed to be dried before SEM analysis after the desalination test, causing the electrode structure to shrink and break into pieces.

[0041] XRD patterns of the FePO4@MHM membrane electrode after testing ( Figure 21 It was found in ( ) to belong to Ti3C2T xThe 002 characteristic peak of -MXene shows a partial rightward shift, indicating a change in the FePO4@MHM structure, which confirms the flattened and stacked Ti3C2T structure in the SEM image. x -MXene, which may be the reason for the decrease in adsorption capacity of the HCDI device at the end of the cycle stability test. The FT-IR spectrum of the FePO4@MHM membrane electrode after the test ( Figure 22 The study found that the stretching vibration peak of Fe-PO4 shifted significantly to the left after testing, which may be due to the reaction of FePO4 with Na during the desalination process. + The combination forms a Na-Fe-PO4-like complex, changing the valence state of iron ions (Fe). 2+ / Fe 3 + During the redox reaction, a large number of ions are adsorbed on the electrode surface, and the electrode material may undergo minor structural changes such as lattice expansion, resulting in a decrease in vibrational frequency and a leftward shift of the peak position. At the same time, the peak intensity of the -OH bond also increases significantly. This is because the electrode is highly hydrophilic, adsorbing a large number of water molecules. During the test, the electrolysis of water generates more -OH groups on the surface, leading to the enhancement of the -OH peak.

[0042] In current research on HCDI devices, the adsorption capacity of FePO4@MHM is comparable to that of high-level doped MXene-based materials in the CDI field, and is higher than that of many studies in the literature. Figure 23 ), such as elemental doping with N, P-Ti3C2T x (53.3 mg g) -1 ) and N-Ti3C2T x (43.5 mg g) -1 ), and various polymer backbones were used to modify GC / MXene (54.2 mg g). -1 ), HATN / MXene (57.5 mg g) -1 mPDA / MXene (37.72 mg g) -1 ), Nb2O5 / Nb2CT x -rGO (41.07 mg g) -1 ), SCNF@PAMXene (31.24 mg g -1 By modifying the 3D structure of 2D MXene and combining it with pseudocapacitive materials, the performance of HCDI electrode materials can be significantly improved. This strategy not only avoids the problem of MXene layer stacking structurally, but also enhances charge storage capacity and increases ion adsorption sites by introducing pseudocapacitive materials, greatly improving its performance as a CDI electrode and providing a better path for designing high-performance HCDI electrode materials.

[0043] In summary, this invention verifies the design concept of combining pseudocapacitive materials and double-layer materials in high-performance CDI electrode materials. By combining the advantages of both, its performance as a CDI electrode is significantly improved, expanding the potential application of CDI electrodes in energy storage.

Claims

1. A method for preparing FePO4@MHM composite material, characterized in that, Comprise the following steps: Step 1, Ti3AlC2 powder was etched by hydrofluoric acid, then dimethyl sulfoxide was used for intercalation and exfoliation to obtain Ti3C2T x -MXene; Step 2, polystyrene microspheres are prepared by emulsion polymerization; Step 3, a displacement reaction is carried out between ferrous sulfate and ammonium dihydrogen phosphate, and then hydrogen peroxide is added for oxidation to prepare iron phosphate; Step 4, Ti3C2T x FePO4@MHM composites were prepared by electrostatic self-assembly of MXene, polystyrene microspheres, and iron phosphate.

2. The preparation method of FePO4@MHM composite material according to claim 1, characterized in that, The step 4 is specifically: The polystyrene microspheres are dispersed in deionized water, continuously stirred until uniformly dispersed, treated with a polydien dimethyl ammonium chloride solution, and then the treated polystyrene microspheres and iron phosphate are added to the dispersed Ti3C2T x In the MXene, continuously stir the reaction, after the reaction is completed, centrifuge and wash the mixture, concentrate, freeze, and then freeze-dry to obtain FePO4-Ti3C2T x @PS composite, and then pyrolysis to obtain FePO4@MHM composite material.

3. The preparation method of FePO4@MHM composite material according to claim 2, characterized in that, The polystyrene microspheres, iron phosphate and Ti3C2T x The mass ratio of MXene is 10:1:

2.

4. The preparation method of FePO4@MHM composite material according to claim 2, characterized in that, The specific conditions of the pyrolysis are: pyrolysis at 500 ℃ for 2 h under a nitrogen atmosphere at a temperature increasing rate of 5 ℃ min -1 After the reaction is completed, the FePO4@MHM composite material is obtained after cooling.

5. The FePO4@MHM composite material prepared by the preparation method according to any one of claims 1-4.

6. The use of the FePO4@MHM composite material according to claim 5, characterized in that, For CDI electrode.

7. A CDI device, characterized by Containing the FePO4@MHM composite material according to claim 5. For CDI electrode.

Images