Preparation method and application of prussian blue compound with core-shell heterostructure

Abstract

The invention relates to the field of electrode materials of capacitive deionization desalination systems, in particular to a preparation method and application of a core-shell heterostructure prussian blue compound, Fe [Fe (CN) 6] serves as a core, a Ni3 [Fe (CN) 6] 2 shell layer is epitaxially grown on the surface of the core, and Fe [Fe (CN) 6] and Ni3 [Fe (CN) 6] 2 with different shell thicknesses are obtained by regulating and controlling the concentration of a precursor. According to the invention, vacancy defects in Fe [Fe (CN) 6] are reduced through a dual repair strategy, and active sites for storing sodium ions are increased. A stable Ni3 [Fe (CN) 6] 2 shell layer is epitaxially grown by taking Fe [Fe (CN) 6] as a core, so that the dissolution of the inner core can be effectively inhibited, meanwhile, the energy required by the redox reaction of low-spin iron in the inner core is reduced through an interface effect, and the electrochemical performance of the core-shell heterostructure is greatly improved.

Description

Technical Field

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

[0002] The general chemical formula of Prussian blue analogs (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 R(CN)6 vacancies. PBAs have been widely used in ion storage and capacitor deionization due to their simple synthesis methods, high ion storage capacity and unique large framework structure. In the pursuit of high ion storage capacity, Fe[Fe(CN)6], which has dual redox reaction characteristics, has become the focus of many studies. However, it faces severe structural stability challenges during the embedding and extraction of alkali metal ions. On the one hand, due to its inherent characteristics of rapid precipitation, a large number of R(CN)6 vacancies exist, which in turn causes frequent crystal structure defects; on the other hand, the phase change caused by the repeated embedding and extraction of alkali metal ions will produce strong internal stress, making the material very easy to pulverize and dissolve, which seriously restricts its further development and application in the field of ion storage.

[0003] In response to the vacancy defect problem, researchers have developed a variety of repair methods, including slowing down the synthesis rate by adding complexing agents or lowering the synthesis temperature, replacing water with water-soluble organic solvents, and using solid-phase synthesis. However, adding complexing agents will affect the purity of the product and produce toxic waste, causing environmental pollution; lowering the temperature will reduce the synthesis efficiency and incomplete reaction; 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 shortcomings limit the application of these methods to a certain extent. For the dissolution phenomenon of Fe[Fe(CN)6], the following strategies are currently used to deal with it: coating its surface with inorganic materials such as zinc oxide and magnesium oxide, or compounding with conductive materials such as carbon nanotubes and graphene, or using Fe[Fe(CN)6] as the core and coating its surface with conductive carbon materials. Although these methods can alleviate the dissolution problem to a certain extent, it cannot be ignored that low storage Na + The introduction of capacity materials not only reduces the overall Na storage +Therefore, Fe[Fe(CN)6] still needs to explore convenient vacancy repair methods, find suitable coating materials and optimize coating processes. Summary of the invention

[0004] In order to repair the vacancies of Fe[Fe(CN)6], the present invention provides a preparation method and application of a core-shell heterostructured Prussian blue compound. The present invention proposes a "double repair without additives" strategy to reduce the [Fe(CN)6] vacancies in Fe[Fe(CN)6] to improve the storage capacity of Na + The capacity of the nanostructured carbon is different from the large particle products produced 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, Fe[Fe(CN)6] is used as the core to epitaxially grow a Ni3[Fe(CN)6]2 shell on its surface, and the precursor concentration is adjusted to obtain Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 with different shell thicknesses. Since the core and the 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 achieved through the following technical scheme: A method for preparing a core-shell heterostructure Prussian blue compound comprises the following steps: (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 K3Fe(CN)6 solution and stirring, the precipitate is centrifuged, washed and dried to obtain Fe[Fe(CN)6]; (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.

[0006] As a further improvement of the technical solution of the preparation method of the present invention, in step (1) and step (2), the K3Fe(CN)6 solution is a saturated solution.

[0007] 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 to 40 mmol / L.

[0008] As a further improvement of the technical solution of the preparation method of the present invention, in step (1), the temperature of the hydrothermal reaction is 60 to 100° C., and the time is 10 to 14 hours.

[0009] As a further improvement of 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.

[0010] As a further improvement of 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.

[0011] The present invention further provides the use of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 obtained by the preparation method of the above-mentioned core-shell heterostructure Prussian blue compound as an electrode material in capacitive deionization and desalination.

[0012] As a further improvement of the application technical solution of the present invention, in the case of capacitive deionization and desalination, 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. + 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.

[0013] As a further improvement of the application technical solution of the present invention, a method for preparing an electrode using Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 as an 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.

[0014] As a further improvement of the application technical solution 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.

[0015] The preparation method of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 provided by the present invention and its application in capacitive deionization and desalination have the following advantages compared with the prior art: The present invention reduces vacancy defects in Fe[Fe(CN)6] through a dual repair strategy and increases active sites for storing sodium ions. A stable Ni3[Fe(CN)6]2 shell is grown epitaxially with Fe[Fe(CN)6] as the core, which can effectively inhibit the dissolution of the core and reduce the energy required for low-spin iron redox reaction in the core through the interface effect, greatly improving the electrochemical performance of the core-shell heterostructure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] 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.

[0019] Figure 2 This is a transmission electron microscope image of Fe@Ni-3 obtained in Example 3 of the present invention.

[0020] Figure 3 These are the 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.

[0021] Figure 4 X-ray photoelectron spectra of FeHCF, Fe@Ni-3 and NiHCF obtained in Examples 1, 3 and 6 of the present invention.

[0022] Figure 5 Thermogravimetric curves of FeHCF, Fe@Ni-3 and NiHCF obtained in Examples 1, 3 and 6 of the present invention.

[0023] Figure 6 Electrochemical characterization 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.

[0024] Figure 7This is a diagram of the desalination performance of the desalination system obtained in Example 9 of the present invention.

[0025] Figure 8 This is a diagram of the desalination performance of the desalination system obtained in Example 10 of the present invention. DETAILED DESCRIPTION

[0026] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0027] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments.

[0028] The present invention provides a specific embodiment of a method for preparing a core-shell heterostructure Prussian blue compound, comprising the following steps: (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 K3Fe(CN)6 solution and stirring, the precipitate is centrifuged, washed and dried to obtain Fe[Fe(CN)6]; (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.

[0029] In one embodiment provided by the present invention, in step (1) and step (2), the K3Fe(CN)6 solution is a saturated solution. In step (3), the concentration range of the K3Fe(CN)6 solution is 2 to 40 mmol / L.

[0030] 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 hours. Preferably, the hydrothermal reaction in step (1) is carried out in a stainless steel autoclave with a Teflon lining.

[0031] In one embodiment provided by the present invention, in step (2), the temperature of the K3Fe(CN)6 solution is 25-60° C. After the crude Fe[Fe(CN)6] is added, the stirring speed is 500-700 rpm, and the stirring time is 24-72 hours.

[0032] 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.

[0033] In the preparation method of a core-shell heterostructure Prussian blue 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.

[0034] The present invention further provides the use of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 obtained by the preparation method of the above-mentioned core-shell heterostructure Prussian blue compound as an electrode material in capacitive deionization and desalination.

[0035] Preferably, in the case of capacitive deionization, Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 is used as the electrode material to prepare the electrode 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.

[0036] Further 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, 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.

[0037] Further preferably, the mass ratio of Fe[Fe(CN)6]@Ni3[Fe(CN)6]2, carbon black and polyvinylidene fluoride is 80:10:10.

[0038] The specific embodiments of the present invention are described in detail below.

[0039] Example 1

[0040] A method for preparing a core-shell heterostructure Prussian blue analog electrode comprises the following steps: (1) Dissolve 5 mmol of FeCl3·6H2O in 60 mL of saturated K3Fe(CN)6 solution and stir well. Transfer to a 100 mL Teflon-lined stainless steel autoclave and perform hydrothermal reaction at 80 °C for 12 h. The precipitate is centrifuged several times with deionized water and ethanol to obtain crude Fe[Fe(CN)6].

[0041] (2) Take 0.5 g of crude Fe[Fe(CN)6] and dissolve it in a saturated K3Fe(CN)6 solution at 40°C and stir for 48 hours. Collect the precipitate by centrifugation and wash it several times with deionized water and ethanol. Finally, dry it in a vacuum oven at 100°C overnight to obtain Fe[Fe(CN)6], which is named FeHCF.

[0042] (3) Take 0.2 g of FeHCF and dissolve it in 100 mL of deionized water. Under magnetic stirring, 50 mL of 2 mmol of K3Fe(CN)6 solution and 50 mL of 3 mmol of NiCl2·6H2O solution were added dropwise to the FeHCF solution. After the reaction was fully completed, the precipitate was collected by centrifugation and washed with deionized water and ethanol several times, and then dried in vacuum at 100 °C for 12 h to obtain Fe[Fe(CN)6]@Ni3[Fe(CN)6]2 with a core-shell heterostructure, which was named Fe@Ni-1.

[0043] The element content of FeHCF in step (2) of this embodiment is shown in the following table: Table 1

[0044] Example 2

[0045] 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 0.4 mmol and 50 mL of a NiCl2·6H2O solution containing 0.6 mmol are added dropwise to the FeHCF solution simultaneously. The product prepared in Example 2 is named Fe@Ni-2.

[0046] Example 3

[0047] 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 0.2 mmol and 50 mL of a NiCl2·6H2O solution containing 0.3 mmol are added dropwise to the FeHCF solution simultaneously. The product prepared in Example 3 is named Fe@Ni-3.

[0048] Example 4

[0049] 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 added dropwise to the FeHCF solution simultaneously. The product prepared in Example 4 is named Fe@Ni-4.

[0050] Example 5

[0051] 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 added dropwise to the FeHCF solution simultaneously. The product prepared in Example 5 is named Fe@Ni-5.

[0052] Example 6

[0053] The preparation method of NiHCF comprises 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 added dropwise into 100 mL of deionized water simultaneously, and after the reaction is fully complete, the mixture is centrifuged, washed and dried to obtain NiHCF.

[0054] Example 7

[0055] Store Na + The method for preparing an electrode comprises the following steps: FeHCF, Fe@Ni-1, Fe@Ni-2, Fe@Ni-3, Fe@Ni-4, Fe@Ni-5 and NiHCF were mixed with carbon black and polyvinylidene fluoride in a mass ratio of 80:10:10 in N-methylpyrrolidone and stirred evenly. The slurry was evenly coated on high-purity graphite paper with a coater, and then dried in a vacuum oven at 80°C overnight and 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.

[0056] The method for preparing an activated carbon electrode comprises the following steps: Commercial activated carbon, carbon black and polyvinylidene fluoride were mixed in N-methylpyrrolidone at a mass ratio of 80:10:10 and stirred evenly. The slurry was evenly coated on high-purity graphite paper with a coater, and then dried in a vacuum oven at 80°C overnight and sliced ​​to obtain an activated carbon electrode.

[0057] Example 8

[0058] The Na storage prepared in Example 7 +The electrochemical performance of the electrode was tested by using the prepared Na storage + The electrode was used as the working electrode, the Ag / AgCl electrode and the platinum electrode were used as the reference electrode and the counter electrode, respectively, and 1 M NaCl was used as the electrolyte. The electrochemical performance was tested in a three-electrode system.

[0059] The calculation formula of electrode specific capacitance is: ,in I Indicates constant current charge and discharge current (A), Δt is the discharge time (s), m is the mass of active material on the electrode (g), ΔV is the voltage window (V).

[0060] Example 9

[0061] The Fe@Ni-3 electrode prepared in Example 7 was used as a Na storage + The activated carbon electrode prepared in Example 7 was used as the Cl storage electrode. - Electrodes were used to conduct capacitive deionization and desalination performance tests. The electrode sheets were assembled into the reactor, and a constant voltage power supply was used to provide voltage to the reactor for desalination. 20 mL of brine with a concentration of 500 mg / L was pumped from the brine pool into the reactor at a flow rate of 20 mL / min. The brine was desalinated in the reactor and then returned to the brine pool. A conductivity meter was used to monitor the conductivity changes in the brine pool in real time, and the brine concentration was calculated based on the conductivity changes.

[0062] The calculation formula for desalination capacity is: , where C0 and C1 are the initial and final concentrations of the NaCl solution (mg / L), V represents the volume of the salt solution (L), and m represents the total mass of active substances on the two electrodes (g).

[0063] Example 10 As a comparison, the FeHCF electrode prepared in Example 7 was used as the Na storage + The activated carbon electrode prepared in Example 7 was used as the Cl storage electrode. - Electrode, to carry out capacitive deionization and desalination performance test.

[0064] According to the results of elemental analysis (Table 1) and thermogravimetric analysis ( Figure 5 ), the chemical formula K of FeHCF is calculated 0.6295 Fe[Fe(CN)6] 0.8291 0.1709 ·1.51H2O. The results show that the content of vacancy defects in FeHCF synthesized by the additive-free dual repair strategy is only 17.09%.

[0065] The morphology and structure 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. Figure 1 As shown, FeHCF is a nanocube with a pseudoangular structure ( Figure 1 a), the particle size is about 202.1nm. NiHCF is a regular nanocube ( Figure 1 b), the particle size is about 368.0nm. The morphology of the core-shell structure samples synthesized with different precursor concentrations is shown in Figure 2. Figure 1 c. Figure 1 d. Figure 1 e. Figure 1 f and Figure 1 As shown in Figure g, the core-shell samples all showed an irregular block structure, which was caused by the pseudoangular structure of FeHCF, and as the concentration of the precursor decreased, the particle size of the core-shell sample gradually decreased from about 826.4nm to 276.9nm, indicating that the shell thickness gradually decreased. The core-shell structure was characterized by TEM, as shown in Figure 2. Figure 2 As shown, it can be clearly seen that the NiHCF shell (light area) completely encapsulates the FeHCF core (dark area) to form a core-shell structure.

[0066] from Figure 3 It can be seen from the X-ray diffraction pattern that the main diffraction peak positions of FeHCF and NiHCF are very similar, all around 17°, 24°, 35°, 39°, 43°, 50°, 53° and 67°, all of which belong to cubic phase Prussian blue analogs, 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. This is because the introduction of nickel ions changes the coordination environment of iron ions, resulting in a larger interplanar spacing. All X-ray diffraction patterns of 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. As the thickness of the outer shell NiHCF gradually becomes thinner, the influence of the core material FeHCF increases relatively, resulting in a decrease in the overall average crystal plane spacing and a slight shift of the diffraction peak to the right, indirectly proving that core-shell structures with different shell thicknesses were successfully synthesized.

[0067] Figure 4 a is the X-ray photoelectron spectra of FeHCF, NiHCF, and Fe@Ni-3, which proves the successful synthesis of FeHCF@NiHCF from the perspective of surface elemental composition. Figure 4 b. Figure 4 c and Figure 4d shows the fine spectrum of Fe@Ni-3 elements. The high-resolution C 1s spectrum shows the presence of C≡N, CO, and CC. The two peaks centered at 708.32 and 721.03 eV in the Fe 2p high-resolution spectrum are attributed to Fe 2+ Fe II 2p 3 / 2 and Fe II 2p 1 / 2 , Fe 3+ 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 at 712.48 and 726.70 eV are satellite peaks caused by multiple excitation effects. 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 are attributed to Ni 2+ 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 is attributed to Ni III 2p 3 / 2 , while the satellite peak at 863.50 eV belongs to Ni sat 2p 3 / 2 .

[0068] The present invention investigates the thermal stability and water content of FeHCF, NiHCF and Fe@Ni-3 by thermogravimetric analysis. Figure 5 As shown, three stages of weight loss can be seen. The first stage of weight loss is the removal of adsorbed water on the sample surface, the second stage of weight loss is the removal of coordinated water and crystal water in the sample lattice, and the third stage of weight loss is caused by the collapse of the crystal structure. Thanks to the double vacancy defect-repaired FeHCF, Fe@Ni-3 exhibits excellent thermal stability.

[0069] The present invention uses cyclic voltammetry to study the electrochemical properties of the prepared material. Figure 6 In a, we can see that the CV curve of NiHCF shows only a pair of redox peaks (0.47V / 0.37V), which reflect the gain and loss of electrons of low-spin iron. FeHCF has dual active sites for storing sodium ions, and the redox peaks at 0.23V / 0.02V and 1.03V / 0.89V correspond to high-spin iron and low-spin iron, respectively. Figure 6 b shows the cyclic voltammetry curves of all Fe@Ni electrodes at 5mV / 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. As the thickness of the NiHCF shell increases, the reaction potentials of high-spin iron and low-spin iron gradually shift to the right and left, 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 difficulty of electron transmission 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 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. NiHCF still has good structural stability under high-rate charge and discharge conditions to maintain its electrochemical performance. The specific capacitance at 10A / g can reach 109.2F / g, which is much higher than 49.2F / 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 10A / g are 71.5, 134.6 and 63.1F / g, respectively, which are all higher than 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.

[0070] In order to explore the stability of the material structure, a cyclic charge and discharge test was carried out, such as Figure 6 As shown in Figure e, although the initial specific capacitance of FeHCF is very high, due to severe lattice distortion and ion dissolution, only 66.5F / g of specific capacitance is retained after 500 cycles at a current density of 1A / g. NiHCF stores sodium ions only through a single redox reaction, but the stable nickel ions and small lattice strain help maintain the stability of the electronic structure and chemical environment of the material, and can still maintain a specific capacitance of 127.2F / g after 500 cycles. Figure 6 As shown in Figure 5, after the core-shell heterostructure is established, the cycling stability of Fe@Ni-2, Fe@Ni-3 and Fe@Ni-4 are 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.

[0071] Figure 7 The capacitive deionization performance of FeHCF and activated carbon was demonstrated. Figure 7aThe desalination performance is calculated based on 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 18.34mg / g, 27.15mg / g, 35.44mg / g, 42.80mg / g, and 51.39mg / g ( Figure 7 b), the maximum desalination rate is 12.35 mg / g / min ( Figure 7 c). From Figure 7 d It can be seen that after 50 cycles of testing, the desalination capacity retention rate is only 73.68%. After 25 cycles, the conductivity of the salt solution is significantly higher than the initial concentration. This is caused by the dissolution of metal ions, and this phenomenon becomes more serious as the cycle progresses.

[0072] Figure 8 The capacitive deionization performance of salt water based on Fe@Ni-3 and activated carbon was demonstrated. Figure 8 aThe desalination performance is calculated based on 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 17.27mg / g, 27.22mg / g, 37.82mg / g, 46.93mg / g, and 54.61mg / g ( Figure 8 b), the maximum desalination rate can reach 13.68mg / g / min ( Figure 8 c). From Figure 8 d It can be seen that the desalination capacity retention rate can reach 87.53% after 50 cycles of testing. 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 insertion and extraction, and greatly inhibits the dissolution of transition metal ions caused by the distortion of the FeHCF crystal structure.

[0073] The above is only a specific implementation of the present invention, which enables those skilled in the art to understand or implement the present invention. Although detailed descriptions are given with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions 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 K3Fe(CN)6 solution and stirring, the precipitate is centrifuged, washed and dried to obtain Fe[Fe(CN)6]; (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 (1) and step (2), the K3Fe(CN)6 solution is a saturated solution.

3. 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.

4. 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 60-100° C. and the time is 10-14 hours.

5. 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.

6. The method for preparing a core-shell heterostructure Prussian blue compound according to claim 3, 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.

7. 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 6 as an electrode material in capacitive deionization and desalination.

8. The use according to claim 7, 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.

9. The use according to claim 8, 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.

10. The use according to claim 9, 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.

Images