Preparation method and application of a core-shell heterostructure Prussian blue compound

Abstract

The present invention relates to the field of electrode materials for capacitive deionization desalination systems, and particularly relates to a preparation method and application of a core-shell heterostructure Prussian blue-based compound. In the present invention, Fe[Fe(CN)6] is used as the core, and a Ni3[Fe(CN)6]2 shell layer is epitaxially grown on its surface, and Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 with different shell thicknesses is obtained by regulating the concentration of the precursor. The present invention reduces the vacancy defects in Fe[Fe(CN)6] through a dual repair strategy, and increases the active sites for storing sodium ions. A stable Ni3[Fe(CN)6]2 shell layer is epitaxially grown with Fe[Fe(CN)6] as the core, which can effectively inhibit the dissolution of the inner core. At the same time, the energy required for the low-spin iron redox reaction in the inner core is reduced through the interface effect, and the electrochemical performance of the core-shell heterostructure is greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of electrode materials for capacitive deionization desalination systems, and particularly to a preparation method and application of a core-shell heterostructure Prussian blue-like compound. Background Art

[0002] The chemical general formula of Prussian blue analogues (PBAs) is A x M 1-x [R(CN)6] y · 1-y ·nH2O, where A is an alkali metal ion, M and R are transition metal ions in high-spin and low-spin states respectively, represents the R(CN)6 vacancy. Because the synthesis method of PBAs is simple, the ion storage capacity is high and it has a unique large-frame structure, it has been widely used in fields such as ion storage and capacitive deionization. In the process of pursuing high ion storage capacity, Fe[Fe(CN)6] with dual redox reaction characteristics has become the focus of many studies. However, it faces severe challenges in the structural stability during the insertion and extraction of alkali metal ions. On the one hand, due to its inherent characteristic of rapid precipitation, a large number of R(CN)6 vacancies exist, resulting in frequent crystal structure defects; on the other hand, the phase change caused by the repeated insertion and extraction of alkali metal ions generates strong internal stress, making the material prone to pulverization and dissolution, which severely restricts its further development and application in the field of ion storage.

[0003] Regarding the vacancy defect problem, a variety of repair methods have been developed by researchers, including slowing down the synthesis rate by adding complexing agents or reducing the synthesis temperature, replacing water with water-soluble organic solvents, and using solid-phase synthesis methods. However, adding complexing agents will affect the product purity and produce toxic waste, causing environmental pollution; reducing the temperature will reduce the synthesis efficiency and the reaction will be incomplete; replacing water with water-soluble organic solvents has safety hazards and is difficult to recycle; using solid-phase synthesis methods will have problems such as uneven mixing and difficulty in scale-up production. These disadvantages limit the application of these methods to a certain extent. For the dissolution phenomenon of Fe[Fe(CN)6], the following strategies are mainly adopted to deal with it: coating inorganic materials such as zinc oxide and magnesium oxide on its surface, or compounding with conductive materials such as carbon nanotubes and graphene, or using Fe[Fe(CN)6] as the core and coating conductive carbon materials on its surface. Although these methods can alleviate the dissolution problem to a certain extent, it cannot be ignored that the introduction of low Na + storage capacity materials not only reduces the overall Na +Performance, and also poses new challenges in terms of interface compatibility. Therefore, Fe[Fe(CN)6] still needs to explore convenient vacancy repair methods, find suitable coating materials, and optimize the coating process. Summary of the Invention

[0004] In order to repair the vacancies in Fe[Fe(CN)6], the present invention provides a preparation method and application of a core-shell heterostructure Prussian blue compound. The present invention proposes a "dual repair without additives" strategy to reduce the [Fe(CN)6] vacancies in Fe[Fe(CN)6] to improve the storage capacity of Na + . Different from the large particle products generated by repairing vacancies by reducing the crystallization rate in the past, this method has a fast crystallization rate, high repair efficiency, and small product particles. In addition, taking Fe[Fe(CN)6] as the core, Ni3[Fe(CN)6]2 shell layers are epitaxially grown on its surface, and Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 with different shell thicknesses is obtained by regulating the precursor concentration. Since the inner core and the outer shell have similar crystal structures, good lattice matching can be achieved during the growth process to form a stable interface.

[0005] The present invention is realized through the following technical solutions: A preparation method of a core-shell heterostructure Prussian blue compound, comprising the following steps:

[0006] (1) Add FeCl3·6H2O to the K3Fe(CN)6 solution and stir evenly, carry out hydrothermal reaction, and the precipitate is centrifuged, washed, and dried to obtain the crude product of Fe[Fe(CN)6];

[0007] (2) Add the crude product of Fe[Fe(CN)6] to the K3Fe(CN)6 solution and stir, and the precipitate is centrifuged, washed, and dried to obtain Fe[Fe(CN)6];

[0008] (3) Dissolve Fe[Fe(CN)6] to obtain a Fe[Fe(CN)6] solution, and simultaneously dropwise add the K3Fe(CN)6 solution and the NiCl2·6H2O solution into the Fe[Fe(CN)6] solution under magnetic stirring. After the reaction is fully completed, it is centrifuged, washed, and dried to obtain the core-shell heterostructure Fe[Fe(CN)6]@Ni3[Fe(CN)6]2.

[0009] As a further improvement of the technical solution of the preparation method of the present invention, in steps (1) and (2), the K3Fe(CN)6 solution is a saturated solution.

[0010] As a further improvement of the technical solution of the preparation method of the present invention, in step (3), the concentration range of the K3Fe(CN)6 solution is 2-40 mmol / L.

[0011] As a further improvement to the technical solution of the preparation method of the present invention, in step (1), the temperature of the hydrothermal reaction is 60-100°C, and the time is 10-14h.

[0012] As a further improvement to the technical solution of the preparation method of the present invention, in step (2), the temperature of the K3Fe(CN)6 solution is 25-60°C.

[0013] As a further improvement to the technical solution of the preparation method of the present invention, in step (3), the molar ratio of Fe[Fe(CN)6], K3Fe(CN)6 and NiCl2·6H2O is 3-25:2-8:3-12.

[0014] The present invention further provides the application of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 prepared by the above-mentioned preparation method of a core-shell heterostructure Prussian blue compound as an electrode material in capacitive deionization desalination.

[0015] As a further improvement to the technical solution of the application of the present invention, during capacitive deionization desalination, an electrode prepared using Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 as the electrode material is used as the Na + storage electrode, and an activated carbon electrode is used as the Cl - storage electrode. For the capacitive deionization desalination performance test, the electrode sheets are assembled into a reactor, and a constant voltage power supply is used to provide voltage to the reactor for desalination.

[0016] As a further improvement to the technical solution of the application of the present invention, the method for preparing an electrode using Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 as the electrode material is: after fully mixing Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, carbon black, and polyvinylidene fluoride in N-methylpyrrolidone, the slurry is uniformly coated on high-purity graphite paper, dried, and then sliced to obtain the Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 electrode.

[0017] As a further improvement to the technical solution of the application of the present invention, the mass ratio of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, carbon black, and polyvinylidene fluoride is 80:10:10.

[0018] The preparation method of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 provided by the present invention and its application in capacitive deionization desalination have the following advantages compared with the prior art:

[0019] The present invention reduces the vacancy defects in Fe[Fe(CN)6] through a dual repair strategy, increasing the active sites for storing sodium ions. A stable Ni3[Fe(CN)6]2 shell is epitaxially grown with Fe[Fe(CN)6] as the core, which can effectively inhibit the dissolution of the core. At the same time, the energy required for the low-spin iron redox reaction in the core is reduced through the interface effect, significantly improving the electrochemical performance of the core-shell heterostructure. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings herein are incorporated into and constitute a part of this specification, showing embodiments consistent with the present invention, and are used together with the specification to explain the principles of the present invention.

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 Scanning electron microscope images of FeHCF, Fe@Ni-1, Fe@Ni-2, Fe@Ni-3, Fe@Ni-4, Fe@Ni-5, and NiHCF obtained in Examples 1, 2, 3, 4, 5, and 6 of the present invention.

[0023] Figure 2 Transmission electron microscope image of Fe@Ni-3 obtained in Example 3 of the present invention.

[0024] Figure 3 X-ray diffraction patterns of FeHCF, Fe@Ni-1, Fe@Ni-2, Fe@Ni-3, Fe@Ni-4, Fe@Ni-5, and NiHCF obtained in Examples 1, 2, 3, 4, 5, and 6 of the present invention.

[0025] Figure 4 X-ray photoelectron spectra of FeHCF, Fe@Ni-3, and NiHCF obtained in Examples 1, 3, and 6 of the present invention.

[0026] Figure 5 Thermogravimetric curves of FeHCF, Fe@Ni-3, and NiHCF obtained in Examples 1, 3, and 6 of the present invention.

[0027] Figure 6 Electrochemical characterizations of FeHCF, Fe@Ni-1, Fe@Ni-2, Fe@Ni-3, Fe@Ni-4, Fe@Ni-5, and NiHCF obtained in Examples 1, 2, 3, 4, 5, and 6 of the present invention.

[0028] Figure 7This is the desalination performance diagram of the desalination system obtained in Example 9 of the present invention.

[0029] Figure 8 This is the desalination performance diagram of the desalination system obtained in Example 10 of the present invention. Detailed implementation manners

[0030] In order to more clearly understand the above objects, features and advantages of the present invention, the solution of the present invention will be further described below. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other.

[0031] In the following description, many specific details are set forth in order to fully understand the present invention, but the present invention can also be implemented in other ways different from those described herein; obviously, the embodiments in the specification are only a part of the embodiments of the present invention, rather than all the embodiments.

[0032] The present invention provides a specific embodiment of a preparation method of a core-shell heterostructure Prussian blue compound, including the following steps:

[0033] (1) Add FeCl3·6H2O to the K3Fe(CN)6 solution and stir evenly, perform a hydrothermal reaction, and the precipitate is centrifuged, washed and dried to obtain a crude product of Fe[Fe(CN)6];

[0034] (2) Add the crude product of Fe[Fe(CN)6] to the K3Fe(CN)6 solution and stir, and the precipitate is centrifuged, washed and dried to obtain Fe[Fe(CN)6];

[0035] (3) Dissolve Fe[Fe(CN)6] to obtain a Fe[Fe(CN)6] solution, and simultaneously dropwise add the K3Fe(CN)6 solution and the NiCl2·6H2O solution into the Fe[Fe(CN)6] solution under magnetic stirring. After the reaction is fully completed, centrifuge, wash and dry to obtain the core-shell heterostructure Fe[Fe(CN)6]@Ni3[Fe(CN)6]2.

[0036] In an embodiment provided by the present invention, in steps (1) and (2), the K3Fe(CN)6 solution is a saturated solution. In step (3), the concentration range of the K3Fe(CN)6 solution is 2-40 mmol / L.

[0037] In another embodiment provided by the present invention, in step (1), the temperature of the hydrothermal reaction is 60-100 °C, and the time is 10-14 h. Preferably, the hydrothermal reaction in step (1) is carried out in a stainless steel autoclave with a Teflon lining.

[0038] In an embodiment provided by the present invention, in step (2), the temperature of the K3Fe(CN)6 solution is 25 - 60 °C. After adding the crude Fe[Fe(CN)6], the stirring speed is 500 - 700 rpm, and the stirring time is 24 - 72 h.

[0039] In another embodiment provided by the present invention, in step (3), the molar ratio of Fe[Fe(CN)6], K3Fe(CN)6, and NiCl2·6H2O is 3 - 25:2 - 8:3 - 12.

[0040] In the preparation method of the core - shell heterostructure Prussian blue - like compound provided by the present invention, the solvent of all solutions is water, the detergent is water and ethanol, and the drying method is vacuum drying.

[0041] The present invention further provides the application of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 obtained by the above - mentioned preparation method of the core - shell heterostructure Prussian blue - like compound as an electrode material in capacitive deionization desalination.

[0042] Preferably, during capacitive deionization desalination, an electrode prepared using Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 as the electrode material is used as the Na + storage electrode, and an activated carbon electrode is used as the Cl - storage electrode. The capacitive deionization desalination performance test is carried out. The electrode sheets are assembled into a reactor, and a constant - voltage power supply is used to provide voltage to the reactor for desalination.

[0043] More preferably, the method for preparing an electrode using Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 as the electrode material is as follows: Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, carbon black, and polyvinylidene fluoride are fully mixed in N - methylpyrrolidone, and then the slurry is evenly coated on high - purity graphite paper, dried, and sliced to obtain the Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 electrode.

[0044] More preferably, the mass ratio of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, carbon black, and polyvinylidene fluoride is 80:10:10.

[0045] The specific embodiments of the present invention will be described in detail below.

[0046] Example 1

[0047] A preparation method of a core - shell heterostructure Prussian blue analogue electrode includes the following steps:

[0048] (1) Dissolve 5 mmol of FeCl3·6H2O in 60 mL of saturated K3Fe(CN)6 solution and stir evenly. Transfer it to a 100 mL Teflon-lined stainless steel autoclave and carry out hydrothermal reaction at 80 °C for 12 hours. The precipitate is centrifuged several times with deionized water and ethanol to obtain the crude product of Fe[Fe(CN)6].

[0049] (2) Take 0.5 g of the crude product of Fe[Fe(CN)6] and dissolve it in the saturated K3Fe(CN)6 solution at 40 °C and stir for 48 hours. Centrifuge to collect the precipitate and wash it several times with deionized water and ethanol. Finally, dry it overnight in a vacuum oven at 100 °C to obtain Fe[Fe(CN)6], which is named FeHCF.

[0050] (3) Take 0.2 g of FeHCF and dissolve it in 100 mL of deionized water. While stirring magnetically, simultaneously add 50 mL of a solution containing 2 mmol of K3Fe(CN)6 and 50 mL of a solution containing 3 mmol of NiCl2·6H2O dropwise into the FeHCF solution. After the reaction is complete, centrifuge to collect the precipitate and wash it several times with deionized water and ethanol. Then dry it in a vacuum at 100 °C for 12 hours to obtain the core-shell heterostructure Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, which is named Fe@Ni-1.

[0051] The elemental content of FeHCF in step (2) of this example is shown in the following table:

[0052] Table 1

[0053]

[0054] Example 2

[0055] The steps not specifically described are the same as those in Example 1. The difference is that in step (3), 50 mL of a solution containing 0.4 mmol of K3Fe(CN)6 and 50 mL of a solution containing 0.6 mmol of NiCl2·6H2O are simultaneously added dropwise into the FeHCF solution. The product prepared in Example 2 is named Fe@Ni-2.

[0056] Example 3

[0057] The steps not specifically described are the same as those in Example 1. The difference is that in step (3), 50 mL of a solution containing 0.2 mmol of K3Fe(CN)6 and 50 mL of a solution containing 0.3 mmol of NiCl2·6H2O are simultaneously added dropwise into the FeHCF solution. The product prepared in Example 3 is named Fe@Ni-3.

[0058] Example 4

[0059] The steps not specifically described are the same as those in Example 1, except that: in step (3), 50 mL of a K3Fe(CN)6 solution containing 0.13 mmol and 50 mL of a NiCl2·6H2O solution containing 0.2 mmol are simultaneously added dropwise to the FeHCF solution. The product prepared in Example 4 is named Fe@Ni-4.

[0060] Example 5

[0061] The steps not specifically described are the same as those in Example 1, except that: in step (3), 50 mL of a K3Fe(CN)6 solution containing 0.1 mmol and 50 mL of a NiCl2·6H2O solution containing 0.15 mmol are simultaneously added dropwise to the FeHCF solution. The product prepared in Example 5 is named Fe@Ni-5.

[0062] Example 6

[0063] The preparation method of NiHCF includes the following steps: 50 mL of a K3Fe(CN)6 solution containing 2 mmol and 50 mL of a NiCl2·6H2O solution containing 3 mmol are simultaneously added dropwise to 100 mL of deionized water, and after the reaction is complete, it is centrifuged, washed, and dried to obtain NiHCF.

[0064] Example 7

[0065] Na storage + The preparation method of the electrode includes the following steps:

[0066] FeHCF, Fe@Ni-1, Fe@Ni-2, Fe@Ni-3, Fe@Ni-4, Fe@Ni-5, and NiHCF are respectively mixed with carbon black and polyvinylidene fluoride in a mass ratio of 80:10:10 in N-methylpyrrolidone and stirred evenly. The slurry is evenly coated on high-purity graphite paper with a coater, and after drying overnight at 80 °C in a vacuum oven, it is sliced to obtain FeHCF electrode, Fe@Ni-1 electrode, Fe@Ni-2 electrode, Fe@Ni-3 electrode, Fe@Ni-4 electrode, Fe@Ni-5 electrode, and NiHCF electrode.

[0067] The preparation method of the activated carbon electrode includes the following steps:

[0068] Commercial activated carbon, carbon black, and polyvinylidene fluoride are mixed in a mass ratio of 80:10:10 in N-methylpyrrolidone and stirred evenly. The slurry is evenly coated on high-purity graphite paper with a coater, and after drying overnight at 80 °C in a vacuum oven, it is sliced to obtain the activated carbon electrode.

[0069] Example 8

[0070] The Na-storage prepared in Example 7 + electrode was tested for its electrochemical performance. The specific method was as follows: Using the prepared Na-storage + electrode as the working electrode, an Ag / AgCl electrode and a platinum electrode were used as the reference electrode and the counter electrode respectively, and 1 M NaCl was used as the electrolyte to test its electrochemical performance under a three-electrode system.

[0071] The calculation formula for the specific capacitance of the electrode is: , where I represents the constant current charge-discharge current (A), Δt is the discharge time (s), m is the mass of the active material on the electrode (g), ΔV is the voltage window (V).

[0072] Example 9

[0073] The Fe@Ni-3 electrode prepared in Example 7 was used as the Na-storage + electrode, and the activated carbon electrode prepared in Example 7 was used as the Cl-storage - electrode to conduct the capacitive deionization desalination performance test. The electrode sheets were assembled into a reactor, and a constant voltage power supply was used to provide voltage to the reactor for desalination. A 20 mL brine with a concentration of 500 mg / L was pumped into the reactor from the brine tank at a flow rate of 20 mL / min. After desalination in the reactor, the brine returned to the brine tank. A conductivity meter was used to monitor the conductivity change in the brine tank in real time, and the brine concentration was calculated based on the conductivity change.

[0074] The calculation formula for the desalination capacity is: , where C0 and C1 are the initial concentration and the final concentration of the NaCl solution (mg / L) respectively, V represents the volume of the salt solution (L), and m represents the total mass of the active material on the two electrodes (g).

[0075] Example 10

[0076] For comparison, the FeHCF electrode prepared in Example 7 was used as the Na-storage + electrode, and the activated carbon electrode prepared in Example 7 was used as the Cl-storage - electrode to conduct the capacitive deionization desalination performance test.

[0077] According to the elemental analysis (Table 1) and the thermogravimetric analysis results ( Figure 5 ), the chemical formula of FeHCF was calculated to be K 0.6295 Fe[Fe(CN)6] 0.8291 0.1709· 1.51H2O, and the results show that the content of vacancy defects in FeHCF synthesized by the additive-free dual repair strategy is only 17.09%.

[0078] The morphologies and structures of FeHCF, Fe@Ni-1, Fe@Ni-2, Fe@Ni-3, Fe@Ni-4, Fe@Ni-5, and NiHCF were observed by scanning electron microscopy. As Figure 1 shown, FeHCF is a nanocube with a pseudo-corner structure ( Figure 1 a), and the particle size is approximately 202.1 nm. NiHCF is a regular nanocube ( Figure 1 b), and the particle size is approximately 368.0 nm. The morphologies of the core-shell structure samples synthesized with different precursor concentrations are shown in Figure 1 c, Figure 1 d, Figure 1 e, Figure 1 f, and Figure 1 g. All the core-shell samples exhibit an irregular block structure, which is caused by the pseudo-corner structure of FeHCF. And with the decrease of the precursor concentration, the particle size of the core-shell samples gradually decreases from approximately 826.4 nm to 276.9 nm, indicating that the shell thickness gradually thins. The core-shell structure was characterized by TEM. As Figure 2 shown, it can be clearly seen that the NiHCF shell (light-colored area) completely encapsulates the FeHCF core (dark-colored area), forming a core-shell structure.

[0079] From Figure 3 the X-ray diffraction patterns, it can be seen that the main diffraction peak positions of FeHCF and NiHCF are very similar, both around 17°, 24°, 35°, 39°, 43°, 50°, 53°, and 67°, and both belong to cubic-phase Prussian blue analogues, corresponding to Fe[Fe(CN)6] (JCPDS 52-1907) and Ni3[Fe(CN)6]2 (JCPDS 82-2283) respectively. It is observed that the main diffraction peak position of NiHCF is slightly shifted to the left compared with FeHCF because the introduction of nickel ions changes the coordination environment of iron ions, resulting in an increase in the interplanar spacing. The X-ray diffraction patterns of all Fe@Ni-x (x = 1, 2, 3, 4, 5) show diffraction peaks consistent with those of FeHCF and NiHCF, indicating that the interface of the formed core-shell structure is highly stable. When the thickness of the outer shell NiHCF gradually thins, the influence of the core material FeHCF relatively increases, resulting in a decrease in the overall average interplanar spacing and a slight right shift of the diffraction peak, indirectly proving the successful synthesis of core-shell structures with different shell thicknesses.

[0080] Figure 4a is the X-ray photoelectron spectroscopy of FeHCF, NiHCF, and Fe@Ni-3, which proves the successful synthesis of FeHCF@NiHCF from the perspective of surface element composition. Figure 4 b, Figure 4 c, and Figure 4 d show the fine spectra of the elements of Fe@Ni-3. The high-resolution C 1s spectrum indicates the presence of C≡N, C-O, and C-C. The two peaks centered at 708.32 and 721.03 eV in the Fe 2p high-resolution spectrum belong to Fe 2+ of Fe II 2p 3 / 2 and Fe II 2p 1 / 2 , Fe 3+ of Fe III 2p 3 / 2 and Fe III 2p 1 / 2 The binding energy peaks are located at 709.81 and 722.75 eV, and the peaks located at 712.48 and 726.70 eV are satellite peaks caused by multiple excitation effects of Fe sat 2p 3 / 2 and Fe sat 2p 1 / 2 . In the Ni 2p high-resolution spectrum, the peaks with binding energies of 856.14, 856.99, and 873.89 eV belong to Ni 2+ of Ni II 2p 3 / 2 , Ni II 2p 3 / 2 , and Ni II 2p 1 / 2 , the peak with a binding energy of 859.98 eV belongs to Ni III 2p 3 / 2 , and the satellite peak located at 863.50 eV belongs to Ni sat 2p 3 / 2 .

[0081] In this invention, the thermal stability and water content of FeHCF, NiHCF, and Fe@Ni-3 were investigated by thermogravimetric analysis. As Figure 5 shown, it can be seen that there is weight loss in three stages. The weight loss in the first stage is the removal of adsorbed water on the sample surface, the weight loss in the second stage is the removal of coordinated water and crystal water in the sample lattice, and the weight loss in the third stage is caused by the collapse of the crystal structure. Benefiting from the FeHCF with double vacancy defect repair, Fe@Ni-3 exhibits excellent thermal stability.

[0082] In this invention, the cyclic voltammetry method was used to study the electrochemical performance of the prepared materials. From Figure 6In a, the CV curve of NiHCF only shows a pair of redox peaks (0.47 V / 0.37 V), and this pair of peaks reflects the gain and loss of electrons of low-spin iron. While FeHCF for sodium ion storage has dual active sites, and the high redox peaks at 0.23 V / 0.02 V and 1.03 V / 0.89 V correspond to high-spin iron and low-spin iron respectively. Figure 6 b shows the cyclic voltammograms of all Fe@Ni electrodes at 5 mV / s. It can be seen that except for Fe@Ni-1, Fe@Ni-5, Fe@Ni-4, Fe@Ni-3 and Fe@Ni-2 all have two pairs of redox peaks. With the increase of the encapsulation thickness of the NiHCF shell, the reaction potentials of high-spin iron and low-spin iron gradually shift to the right and left respectively, and the contribution of low-spin iron gradually decreases while the contribution of high-spin iron gradually increases. The reason for the change in redox potential may be that the core-shell heterostructure changes the ease of electron transfer to the dual active sites, greatly reducing the energy required for the redox reaction of low-spin iron. Figure 6 c and Figure 6 d shows the rate performance of all electrodes. Although FeHCF has a relatively high initial discharge capacity, it may be more prone to lattice distortion during high-rate charge and discharge, which will hinder the diffusion of ions and the transfer of charges, so the rate performance is poor. While NiHCF still has good structural stability to maintain its electrochemical performance under high-rate charge and discharge conditions, and the specific capacitance at 10 A / g can reach 109.2 F / g, much higher than 49.2 F / g of FeHCF. After encapsulating the FeHCF core with a NiHCF shell of appropriate thickness, it shows excellent rate performance. The specific capacitances of Fe@Ni-2, Fe@Ni-3 and Fe@Ni-4 at 10 A / g are 71.5, 134.6 and 63.1 F / g respectively, all higher than that of FeHCF, proving that a reasonable core-shell structure can optimize the electronic structure and ion transport channels of the material to meet the requirements of rapid sodium ion insertion and extraction.

[0083] In order to explore the structural stability of the materials, cyclic charge and discharge tests were carried out. As Figure 6 shown in e, although the initial specific capacitance of FeHCF is very high, due to severe lattice distortion and ion dissolution and other problems, only 66.5 F / g of specific capacitance remains after 500 cycles at a current density of 1 A / g. While NiHCF stores sodium ions only relying on a single redox reaction, but the stable nickel ions and small lattice strain help to maintain the stability of the electronic structure and chemical environment of the material, and the specific capacitance of 127.2 F / g can still be maintained after 500 cycles. As Figure 6 shown in f, after establishing the core-shell heterostructure, the cyclic stabilities of Fe@Ni-2, Fe@Ni-3 and Fe@Ni-4 are all better than that of FeHCF, indicating that reasonable structural design can make up for the respective defects of the core material and the shell material.

[0084] Figure 7 shows the performance of capacitive deionization desalination based on FeHCF and activated carbon. From Figure 7 a, the desalination performance is calculated according to the relationship between the conductivity and concentration of the NaCl solution. The corresponding desalination capacities at ±0.8V, ±0.9V, ±1.0V, ±1.1V, and ±1.2V are 18.34 mg / g, 27.15 mg / g, 35.44 mg / g, 42.80 mg / g, and 51.39 mg / g ( Figure 7 b), and the maximum desalination rate is 12.35 mg / g / min ( Figure 7 c). From Figure 7 d, it can be seen that the retention rate of the desalination capacity is only 73.68% after 50 cycles of the cycling test. The conductivity of the salt solution is significantly higher than the initial concentration after 25 cycles, which is caused by the dissolution of metal ions, and this phenomenon becomes more serious as the cycling process proceeds.

[0085] Figure 8 shows the performance of capacitive deionization desalination based on Fe@Ni-3 and activated carbon. From Figure 8 a, the desalination performance is calculated according to the relationship between the conductivity and concentration of the NaCl solution. The corresponding desalination capacities at ±0.8V, ±0.9V, ±1.0V, ±1.1V, and ±1.2V are 17.27 mg / g, 27.22 mg / g, 37.82 mg / g, 46.93 mg / g, and 54.61 mg / g ( Figure 8 b), and the maximum desalination rate can reach 13.68 mg / g / min ( Figure 8 c). From Figure 8 d, it can be seen that the retention rate of the desalination capacity can reach 87.53% after 50 cycles of the cycling test. This is because NiHCF, as a buffer layer, alleviates the volume expansion and contraction of the FeHCF core material during the process of sodium ion deintercalation and embedding, and greatly inhibits the dissolution of transition metal ions caused by the crystal structure distortion of FeHCF.

[0086] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Although the foregoing embodiments have been described in detail, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments, and they should all be covered by the protection scope of the claims.

Claims

1. A method for preparing a core-shell heterostructured Prussian blue compound, characterized in that: The following steps are involved: (1) Add FeCl3·6H2O to K3Fe(CN)6 solution and stir evenly to carry out hydrothermal reaction. The precipitate is centrifuged, washed and dried to obtain crude Fe[Fe(CN)6]; (2) adding crude Fe[Fe(CN)6] to a K3Fe(CN)6 solution and stirring, and washing and drying the precipitate by centrifugation to obtain Fe[Fe(CN)6]; in steps (1) and (2), the K3Fe(CN)6 solution is a saturated solution; (3) Fe[Fe(CN)6] was dissolved to obtain a Fe[Fe(CN)6] solution. K3Fe(CN)6 solution and NiCl2·6H2O solution were simultaneously added dropwise to the Fe[Fe(CN)6] solution under magnetic stirring. After the reaction was fully completed, the solution was centrifuged, washed and dried to obtain a core-shell heterostructured Fe[Fe(CN)6]@Ni3[Fe(CN)6]2.

2. The method for preparing a core-shell heterostructure Prussian blue compound according to claim 1, characterized in that: In step (3), the concentration range of the K3Fe(CN)6 solution is 2 to 40 mmol / L.

3. The method for preparing a core-shell heterostructure Prussian blue compound according to claim 1, characterized in that: In step (1), the temperature of the hydrothermal reaction is 60 to 100° C. and the time is 10 to 14 hours.

4. The method for preparing a core-shell heterostructure Prussian blue compound according to claim 1, characterized in that: In step (2), the temperature of the K3Fe(CN)6 solution is 25-60°C.

5. The method for preparing a core-shell heterostructure Prussian blue compound according to claim 2, characterized in that: In step (3), the molar ratio of Fe[Fe(CN)6], K3Fe(CN)6 and NiCl2·6H2O is 3-25:2-8:3-12.

6. Application of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 obtained by the method for preparing a core-shell heterostructure Prussian blue compound as claimed in any one of claims 1 to 5 as an electrode material in capacitive deionization and desalination.

7. The use according to claim 6, characterized in that: In capacitive deionization, the electrode prepared by using Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 as the electrode material is used as the Na storage + Electrode, using activated carbon electrode as Cl storage - The electrode is used to perform a capacitive deionization and desalination performance test. The electrode sheet is assembled into a reactor, and a constant voltage power supply is used to provide voltage to the reactor for desalination.

8. The use according to claim 7, characterized in that: The method for preparing an electrode using Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 as the electrode material is as follows: Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, carbon black, and polyvinylidene fluoride are fully mixed in N-methylpyrrolidone, the slurry is evenly coated on high-purity graphite paper, dried, and then sliced ​​to obtain a Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 electrode.

9. The use according to claim 8, characterized in that: The mass ratio of the Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, carbon black and polyvinylidene fluoride is 80:10:10.

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