A three-dimensional layered conductive MOF / LDH / CNF composite electrode material and its preparation method and application

Abstract

The present invention discloses a three-dimensional layered conductive MOF / LDH / CNF composite electrode material and a preparation method and application thereof, belonging to the technical field of electrode materials. The present invention utilizes the design concept of composite materials, firstly grows LDHs on an oriented carbon fiber cloth to construct a tightly coupled and evenly distributed lamellar LDH nanoarray, then adds a multidentate organic ligand to partially convert the surface LDHs into conductive MOFs with fast charge migration and strong redox activity, and finally constructs a MOF / LDH / CNF electrode material with a three-dimensional layered nanoheterostructure. The three-dimensional heterostructure designed by the present invention perfectly inherits the flexible structure and conductivity of two-dimensional conductive MOFs, retains the diversity and intrinsic catalytic activity of LDH, and at the same time has a fast charge transfer capability under the support of carbon fibers. The prepared three-dimensional layered conductive MOF / LDH / CNF composite electrode can not only stably, efficiently and quickly realize the CDI desalination cycle, but also significantly reduces energy consumption compared with traditional reverse osmosis, electrodialysis and thermal evaporation methods.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode materials, and specifically relates to a three-dimensional layered conductive MOF / LDH / CNF composite electrode material and a preparation method and application thereof. Background Art

[0002] For many developing countries and regions, rapid industrialization, increasing urbanization, growing populations, and uncontrolled use of natural resources are causing irreversible damage to the water environment. Methods to control water shortages and improve water quality are gaining increasing attention. Using seawater desalination technology to convert undrinkable seawater into fresh water is an effective way to address the problem of freshwater shortages. Currently, the main seawater desalination technologies used include reverse osmosis, electrodialysis, and thermal evaporation. These technologies are relatively mature and can meet the needs of specific conditions and regions. However, these technologies also have some problems, such as high cost, high energy consumption, and the possibility of secondary pollution.

[0003] Capacitive deionization (CDI), also known as electrosorption technology, is a newly emerging technology that uses positive and negative electrodes to remove anions and cations from water to produce freshwater. CDI boasts low cost, low energy consumption, high efficiency, and is pollution-free, making it a water treatment technology with unlimited potential. It can not only desalinate brackish or seawater and reuse reclaimed water, alleviating water scarcity, but also remove pollutants such as heavy metals, organic matter, and inorganic matter from wastewater, deeply purifying water.

[0004] The electrode materials used in capacitive deionization (CDI) technology are crucial for its desalination efficiency. Currently, CDI technology primarily utilizes porous carbon nanomaterials and pseudocapacitive materials as electrode materials, and their desalination mechanisms can be categorized as charge double layer (CDL) and pseudocapacitive mechanisms. Porous carbon nanomaterials, one of the earliest electrode materials studied and applied to CDI, achieve desalination through adsorption and desorption via the CDL. With the advancement of nanotechnology, porous carbon nanomaterials of various forms and types have been widely used, driving the development of this technology. However, the low adsorption capacity of porous carbon nanomaterials results in low CDI desalination efficiency and a short cycle life, making them difficult to meet practical requirements. Pseudocapacitive oxide electrode materials, whose desalination mechanism is based on the Faradaic reaction, offer advantages such as low toxicity, environmental friendliness, high specific capacity, and long cycle life, holding them in high demand for CDI applications. However, their poor electrical conductivity remains a bottleneck limiting their desalination performance. Therefore, the development of new electrode material modification methods is urgently needed to promote the practical application of CDI technology.

[0005] A Chinese patent with publication number CN113881040A and application date of September 26, 2021, discloses a preparation method and application of a two-dimensional MXene / polydopamine composite desalination electrode material. The invention constructs a porous structure in the polydopamine layer, and on the basis of retaining the redox capacitance characteristics and ion selectivity, increases the specific surface area and increases the active sites, thereby improving the desalination capacity of the material. However, when making this electrode material, it is necessary to first construct an in-plane columnar mesoporous structure on the MXene / polydopamine nanosheets, so that the electrolyte can be transported along the columnar pores to the interior of the stacked two-dimensional composite material. Moreover, MXene, as a new type of two-dimensional material, is more prone to rust in a humid environment, and there is a possibility of inhibiting conductivity. The Chinese patent with publication number CN112062229A and application date of August 12, 2020 provides a Bi / MOF-derived porous carbon sphere composite material and its preparation method and application. The Bi / MOF-derived porous carbon sphere composite material of the invention anchors nano-scale metal Bi particles on MOF-derived carbon, which can effectively alleviate the volume expansion problem of Bi during the redox process, enhance the stability of the electrode material, and reduce the electrode pulverization phenomenon. However, when using this electrode material, the Bi / MOF-derived porous carbon sphere composite material needs to be ground evenly with conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone to a slurry, which is then coated on the carbon electrode and vacuum-dried before being used as the CDI negative electrode, which is more complicated in application.

[0006] In summary, developing a new CDI electrode material with high desalination efficiency and good cycle stability to promote CDI technology is of great practical significance and industrial application value. Summary of the Invention

[0007] To address the deficiencies in the prior art, the present invention provides a three-dimensional layered conductive MOF / LDH / CNF composite electrode material, a preparation method, and applications thereof. The prepared three-dimensional layered conductive MOF / LDH / CNF composite electrode can not only stably, efficiently, and rapidly realize the CDI desalination cycle, but also significantly reduce energy consumption compared with traditional reverse osmosis, electrodialysis, and thermal evaporation methods.

[0008] The technical solutions of the present invention are as follows:

[0009] One of the purposes of the present invention is to provide a method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material, comprising the following steps:

[0010] (1) Pretreatment of carbon fiber substrate: soak the raw carbon fiber cloth in an acidic solution for oxidation and heat reaction;

[0011] (2) Directed anchoring of multi-component LDH: a transition metal inorganic salt and the raw carbon fiber cloth pretreated in step (1) were added to a reactor, urea was added to adjust the pH to 9-11, and then methanol was used as a solvent to stir and heat the reaction by a solvothermal method. After the reaction was completed, the mixture was repeatedly washed with deionized water and ethanol, and then dried to obtain an LDH / CNF composite material;

[0012] (3) Preparation of three-dimensional layered conductive MOF / LDH / CNF composite electrode material: The LDH / CNF composite material was immersed in deionized water, and a multidentate organic ligand was added thereto. The reaction was slowly carried out under hydrothermal conditions at a fixed heating rate. The material was then centrifuged and repeatedly washed with deionized water and ethanol. Finally, the material was dried to obtain a three-dimensional layered conductive MOF / LDH / CNF composite electrode material.

[0013] Furthermore, the acidic solution in step (1) is one or a combination of nitric acid and sulfuric acid, and the concentration of the acidic solution is 0.3-2M.

[0014] Furthermore, the temperature of the heating reaction in step (1) is 60-100° C., and the reaction time is 3-12 hours.

[0015] Furthermore, the transition metal inorganic salt in step (2) is a combination of any two or three of Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Cu(NO3)2·3H2O, and FeCl2·4H2O.

[0016] Furthermore, the molar ratio of urea added in step (2) to the total amount of transition metal inorganic salts is (1-5):1.

[0017] Furthermore, the stirring and heating reaction time in step (2) is 8 to 15 hours, and the temperature is 80 to 160°C.

[0018] Furthermore, the multidentate organic ligand added in step (3) is 2,3,6,7,10,11-hexahydroxytriphenylenebenzene HHTP or 2,3,6,7,10,11-hexaaminotriphenylenebenzene HITP; and the molar ratio of the multidentate organic ligand to the total amount of transition metal inorganic salts in the LDH / CNF composite material is (1 to 1.8):1.

[0019] Furthermore, in step (3), the reaction heating rate is 1-5°C / min, the reaction temperature is 70-110°C, and the reaction time is 12-48h.

[0020] A second object of the present invention is to provide a three-dimensional layered conductive MOF / LDH / CNF composite electrode material.

[0021] The third object of the present invention is to provide a three-dimensional layered conductive MOF / LDH / CNF composite electrode material for use in capacitive deionization (CDI) desalination.

[0022] Furthermore, the three-dimensional layered conductive MOF / LDH / CNF composite electrode material was used as the cathode, the activated carbon sheet as the anode, and the metal titanium plate as the current collector. The CDI desalination experiment of NaCl aqueous solution was carried out in a constant pressure batch circulation mode. The conductivity change during the adsorption and desorption process was recorded at any time using a conductivity meter to indirectly determine the desalination effect.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. This invention pioneers a novel MOF / LDH / CNF composite electrode material with a three-dimensional layered nanoheterostructure. LDHs are first grown on an oriented carbon fiber cloth to construct a tightly coupled and evenly distributed lamellar LDH nanoarray. Specific organic ligands are then added to partially convert the surface LDHs into conductive MOFs with rapid charge transfer and strong redox activity, ultimately creating a three-dimensional layered nanoheterostructure. This designed three-dimensional heterostructure perfectly inherits the flexible structure and conductivity of two-dimensional conductive MOFs while retaining the diversity and intrinsic catalytic activity of LDHs. Furthermore, it possesses rapid charge transfer capabilities supported by carbon fibers.

[0025] 2. The present invention utilizes the design concept of composite electrode materials, uses carbon nanofibers with a fixed orientation as a substrate, and uniformly anchors the sheet-like LDH material on its surface. The introduction of columnar carbon nanofibers not only reduces the agglomeration of LDHs, but also shortens the charge transfer distance between LDHs layers during the CDI process due to the high conductivity of carbon nanofibers, thereby increasing the conductivity of LDHs and improving the desalination performance. In addition, the present invention further post-modifies the LDH / CNF composite material, converting the surface LDH into two-dimensional conductive MOFs in situ to provide abundant charge transfer channels, further improving the charge transfer capacity in the composite electrode material system. At the same time, thanks to the large specific surface area of ​​MOFs, the active sites in the material can be fully exposed, ultimately obtaining an efficient and stable CDI electrode material.

[0026] 3. The three-dimensional layered conductive MOF / LDH / CNF composite electrode material of the present invention can stably, efficiently and quickly realize the CDI desalination cycle. Compared with traditional reverse osmosis, electrodialysis and thermal evaporation methods, the energy consumption of the CDI desalination system using this electrode material is significantly reduced, which is beneficial to energy conservation and environmental protection. According to CDI and electrochemical tests, the composite electrode material prepared by the present invention has a voltage of 1.4V and an initial NaCl concentration of 1000mg L -1 Flow rate: 20 mL min-1 After 30 min of electrosorption, the salt adsorption capacity of the system reached 119.8 mg g -1 , and can stably cycle desalination for more than 20 rounds, demonstrating high desalination capacity, good rate performance and high cycle stability, showing excellent electrochemical and pseudocapacitive properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 These are the common optical images of the pretreated carbon fiber substrate, NiCoCu-LDH / CNF, and NiCoCu-CAT / LDH / CNF in Example 1 of the present invention;

[0028] Figure 2 The scanning electron micrographs of NiCoCu-LDH / CNF, NiCoCu-CAT / LDH / CNF (transition state), and NiCoCu-CAT / LDH / CNF materials in Example 1 of the present invention are shown;

[0029] Figure 3 The powder X-ray diffraction patterns of NiCoCu-CAT / / CNF, NiCoCu-LDH / CNF, and NiCoCu-CAT / CNF materials in Example 1 of the present invention are as follows;

[0030] Figure 4 The full spectrum and fine spectrum of the X-ray photoelectron spectrum of the NiCoCu-CAT / LDH / CNF material in Example 1 of the present invention;

[0031] Figure 5 This is the synchrotron radiation pattern of the NiCoCu-CAT / LDH / CNF material in Example 1 of the present invention;

[0032] Figure 6 Cyclic voltammetry curves of the capacitive deionization electrodes prepared from NiCoCu-LDH / CNF, NiCoCu-CAT / LDH / CNF, and NiCoCu-CAT / CNF in the performance test of the present invention;

[0033] Figure 7 The constant current charge-discharge diagram of the capacitive deionization electrode prepared by NiCoCu-LDH / CNF, NiCoCu-CAT / LDH / CNF, and NiCoCu-CAT / CNF in the performance test of the present invention;

[0034] Figure 8 The electrochemical impedance spectroscopy of the capacitive deionization electrodes prepared from NiCoCu-LDH / CNF, NiCoCu-CAT / LDH / CNF, and NiCoCu-CAT / CNF in the performance test of the present invention is shown;

[0035] Figure 9The following are actual photos of the test device and the desalination mechanism used in the performance test of the present invention;

[0036] Figure 10 The graphs are a graph showing the electrical adsorption capacity versus time variation, an electrical adsorption rate versus electrical adsorption capacity variation curve, and a graph showing the effects of initial salt concentration and applied voltage on desalination capacity (SAC) when a capacitive deionization device constructed with NiCoCu-LDH / CNF and NiCoCu-CAT / LDH / CNF composite electrode material desalinates a saline solution in the performance test of the present invention.

[0037] Figure 11 This is a diagram showing the cyclic electrical adsorption effect of the capacitive deionization device built with NiCoCu-CAT / LDH / CNF material for desalting a saline solution in the performance test of the present invention, as well as a comparison of the adsorption amounts of different example materials. DETAILED DESCRIPTION

[0038] The present invention will be further described below in conjunction with the accompanying drawings and preferred embodiments. The given embodiments are only for illustrating the present invention, rather than for limiting the scope of the present invention.

[0039] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0040] The quantitative tests in the following examples were repeated three times, and the results were averaged.

[0041] The experimental methods in the following examples are conventional methods unless otherwise specified.

[0042] Example 1

[0043] This embodiment provides a method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material NiCoCu-CAT / LDH / CNF, comprising the following steps:

[0044] (1) Pretreatment of carbon fiber substrate

[0045] First, the sheet-like carbon fiber substrate was cut into 3×3 cm squares, ultrasonically cleaned with ethanol for 20 minutes, then ultrasonically cleaned with deionized water for 20 minutes, and then naturally dried at room temperature. Subsequently, the cleaned carbon fiber substrate was immersed in 30 mL of dilute sulfuric acid solution (mass fraction 10%) and heated at 60°C for 12 hours. After acidification, it was repeatedly rinsed with deionized water for neutralization, and finally dried in a vacuum oven at 60°C overnight to complete the pretreatment of the carbon fiber substrate.

[0046] (2) Synthesis of NiCoCu-LDH / CNF

[0047] 1 mmol Ni(NO₃)₂·6H₂O, 1 mmol Co(NO₃)₂·6H₂O, 1 mmol Cu(NO₃)₂·3H₂O, and 3.5 mmol urea were added to 40 mL of methanol and stirred for 10 minutes to obtain a mixed solution. A pretreated 3×3 cm carbon fiber substrate was placed in the mixed solution to fully soak the substrate. The substrate was then transferred to a 50 mL Teflon-lined stainless steel autoclave and hydrothermally reacted at 140°C for 12 hours. After the reaction, the substrate was naturally cooled to room temperature, rinsed repeatedly with deionized water, and finally dried in a vacuum oven at 60°C for 12 hours to obtain the NiCoCu-LDH / CNF composite.

[0048] (3) Preparation of NiCoCu-CAT / LDH / CNF composite electrode materials

[0049] 150 mg of NiCoCu-LDH / CNF was immersed in a mixed solution containing 1.5 mmol HHTP and 30 mL of deionized water and ultrasonicated for 10 minutes. The mixture was then placed in an oven at 100°C with a heating rate of 1°C / min and heated for 48 hours. After the reaction was completed and naturally cooled to room temperature, it was repeatedly washed with deionized water and dried at 60°C for 12 hours to obtain the final product, NiCoCu-CAT / LDH / CNF composite electrode material.

[0050] As attached Figure 1 As shown, from left to right are ordinary optical photographs of pretreated carbon fiber substrate, NiCoCu-LDH / CNF and NiCoCu-CAT / LDH / CNF. It can be clearly seen from the figure that after continuous treatment, the substrate surface has obvious morphological changes.

[0051] from Figure 2 As can be seen in a, the surface of each carbon fiber is wrapped with a highly ordered nanosheet array, and its surface is composed of interconnected ultrathin NiCoCu-LDH nanosheets, indicating that the introduction of carbon fibers can effectively inhibit the agglomeration of NiCoCu-LDH nanosheets; Figure 2 b shows that after adding HHTP organic ligands for in-situ reaction, the overall structure of the material did not change significantly, but the LDH nanosheets on the surface were partially passivated, that is, the coordination reaction guided by the multidentate ligand proceeded smoothly on the material surface; Figure 2 As can be seen in c, the LDH in the outermost layer of the composite material has been almost completely replaced by the rod-shaped conductive MOF material NiCoCu-CAT, that is, the three-dimensional layered conductive NiCoCu-CAT / LDH / CNF composite electrode material was successfully prepared.

[0052] exist Figure 3 The main diffraction peak signals are obvious in the powder X-ray diffraction pattern, and the composite material NiCoCu-CAT / LDH / CNF has the main characteristic diffraction peaks of NiCoCu-LDH and NiCoCu-CAT respectively, indicating that the composite material was successfully prepared.

[0053] Figure 4 The X-ray photoelectron energy spectrum and full spectrum show that Ni, Co, Cu, C, and O elements coexist in the material, which indirectly proves that the composite material NiCoCu-CAT / LDH / CNF is obtained.

[0054] Figure 5 The local coordination structure of NiCoCu-CAT / LDH / CNF at the atomic level was determined. Normalized k-edge XANES spectra revealed that the absorption edges of Ni and Co lie between +2 and +3, consistent with the XPS results. The absorption edge of Cu lies between +1 and +2, also consistent with the XPS results. Extended X-ray absorption fine structure (EXAFS) spectroscopy further confirmed the atomic coordination model of Ni, Co, and Cu. The composite NiCoCu-CAT / LDH / CNF exhibits a strong MO peak (M = Ni, Co, Cu) for each of the three metal elements, with coordination numbers approximately 4.0. This confirms that the metals in NiCoCu-LDH undergo coordination reactions with the HHTP organic ligands, successfully forming a heterostructure.

[0055] Example 2

[0056] This embodiment provides a method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material NiCoCu-CAT / LDH / CNF, comprising the following steps:

[0057] (1) Pretreatment of carbon fiber substrate

[0058] First, the sheet-like carbon fiber substrate was cut into 3×3 cm squares, ultrasonically cleaned with ethanol for 20 minutes, then ultrasonically cleaned with deionized water for 20 minutes, and then naturally dried at room temperature. Subsequently, the cleaned carbon fiber substrate was immersed in 30 mL of dilute sulfuric acid / dilute nitric acid solution (3% by mass each) and heated at 100°C for 3 hours. After acidification, it was repeatedly rinsed with deionized water to neutralize it, and finally dried in a vacuum oven at 60°C overnight to complete the pretreatment of the carbon fiber substrate.

[0059] (2) Synthesis of NiCoCu-LDH / CNF

[0060] 1 mmol Ni(NO₃)₂·6H₂O, 1 mmol Co(NO₃)₂·6H₂O, 1 mmol Cu(NO₃)₂·3H₂O, and 15 mmol urea were added to 40 mL of methanol and stirred for 10 minutes to obtain a mixed solution. A pretreated 3×3 cm square carbon fiber substrate was placed in the mixed solution to fully soak the substrate. The substrate was then transferred to a 50 mL Teflon-lined stainless steel autoclave and hydrothermally reacted at 80°C for 15 hours. After the reaction, the substrate was naturally cooled to room temperature, rinsed repeatedly with deionized water, and finally dried in a vacuum oven at 60°C for 12 hours to obtain the NiCoCu-LDH / CNF composite.

[0061] (3) Preparation of NiCoCu-CAT / LDH / CNF-2 composite electrode materials

[0062] 150 mg of NiCoCu-LDH / CNF was immersed in a mixed solution containing 2.0 mmol HHTP and 30 mL of deionized water and ultrasonicated for 10 minutes. The mixture was then placed in an oven at 70°C with a heating rate of 5°C / min and heated for 12 hours. After the reaction was completed and naturally cooled to room temperature, it was repeatedly washed with deionized water and dried at 60°C for 12 hours to obtain the final product, NiCoCu-CAT / LDH / CNF-2 composite electrode material.

[0063] Example 3

[0064] This embodiment provides a method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material NiCoCu-CAT / LDH / CNF, comprising the following steps:

[0065] (1) Pretreatment of carbon fiber substrate

[0066] First, the sheet-like carbon fiber substrate was cut into 3×3 cm squares, ultrasonically cleaned with ethanol for 20 minutes, then ultrasonically cleaned with deionized water for 20 minutes, and then naturally dried at room temperature. Subsequently, the cleaned carbon fiber substrate was immersed in 30 mL of dilute sulfuric acid / dilute nitric acid solution (5% by mass each) and heated at 70°C for 4 hours. After acidification, it was repeatedly rinsed with deionized water to neutralize it, and finally dried in a vacuum oven at 60°C overnight to complete the pretreatment of the carbon fiber substrate.

[0067] (2) Synthesis of NiCoCu-LDH / CNF

[0068] 1 mmol Ni(NO₃)₂·6H₂O, 1 mmol Co(NO₃)₂·6H₂O, 1 mmol Cu(NO₃)₂·3H₂O, and 5 mmol urea were added to 40 mL of methanol and stirred for 10 minutes to obtain a mixed solution. A pretreated 3×3 cm square carbon fiber substrate was placed in the mixed solution to fully soak the substrate. The substrate was then transferred to a 50 mL Teflon-lined stainless steel autoclave and hydrothermally reacted at 160°C for 8 hours. After the reaction, the substrate was naturally cooled to room temperature, rinsed repeatedly with deionized water, and finally dried in a vacuum oven at 60°C for 12 hours to obtain the NiCoCu-LDH / CNF composite.

[0069] (3) Preparation of NiCoCu-CAT / LDH / CNF-2 composite electrode materials

[0070] 150 mg of NiCoCu-LDH / CNF was immersed in a mixed solution containing 2.2 mmol HHTP and 30 mL of deionized water and ultrasonicated for 10 minutes. The mixture was then placed in an oven at 90°C with a heating rate of 3°C / min and heated for 36 hours. After the reaction was completed and naturally cooled to room temperature, it was repeatedly washed with deionized water and dried at 60°C for 12 hours to obtain the final product, NiCoCu-CAT / LDH / CNF-3 composite electrode material.

[0071] Example 4

[0072] This embodiment provides a method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material NiCoCu-CAT / LDH / CNF, comprising the following steps:

[0073] (1) Pretreatment of carbon fiber substrate

[0074] First, the sheet-like carbon fiber substrate was cut into 3×3 cm squares, ultrasonically cleaned with ethanol for 20 minutes, then ultrasonically cleaned with deionized water for 20 minutes, and then naturally dried at room temperature. Subsequently, the cleaned carbon fiber substrate was immersed in 30 mL of dilute sulfuric acid solution (mass fraction 6%) and heated at 70°C for 5 hours. After acidification, it was repeatedly rinsed with deionized water to neutralize it, and finally dried in a vacuum oven at 60°C overnight to complete the pretreatment of the carbon fiber substrate.

[0075] (2) Synthesis of NiCoCu-LDH / CNF

[0076] 1 mmol Ni(NO₃)₂·6H₂O, 1 mmol Co(NO₃)₂·6H₂O, 1 mmol Cu(NO₃)₂·3H₂O, and 3 mmol urea were added to 40 mL of methanol and stirred for 10 minutes to obtain a mixed solution. A pretreated 3×3 cm square carbon fiber substrate was placed in the mixed solution and thoroughly soaked. The substrate was then transferred to a 50 mL Teflon-lined stainless steel autoclave and hydrothermally reacted at 140°C for 16 hours. After the reaction, the substrate was naturally cooled to room temperature, rinsed repeatedly with deionized water, and finally dried in a vacuum oven at 60°C for 12 hours to obtain the NiCoCu-LDH / CNF composite.

[0077] (3) Preparation of NiCoCu-CAT / LDH / CNF-3 composite electrode materials

[0078] 150 mg of NiCoCu-LDH / CNF was immersed in a mixed solution containing 2.7 mmol HITP and 30 mL deionized water and ultrasonicated for 10 minutes. The mixture was then placed in an oven at 110°C with a heating rate of 1.5°C / min and heated for 36 hours. After the reaction was completed and naturally cooled to room temperature, it was repeatedly washed with deionized water and dried at 60°C for 12 hours to obtain the final product, NiCoCu-CAT / LDH / CNF-4 composite electrode material.

[0079] Example 5

[0080] This embodiment provides a method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material NiCoFe-CAT / LDH / CNF, comprising the following steps:

[0081] (1) Pretreatment of carbon fiber substrate

[0082] First, the sheet-like carbon fiber substrate was cut into 3×3 cm squares, ultrasonically cleaned with ethanol for 20 minutes, then ultrasonically cleaned with deionized water for 20 minutes, and then naturally dried at room temperature. Subsequently, the cleaned carbon fiber substrate was immersed in 30 mL of dilute sulfuric acid / dilute nitric acid solution (3% by mass each) and heated at 85°C for 7 hours. After acidification, it was repeatedly rinsed with deionized water to neutralize it, and finally dried in a vacuum oven at 60°C overnight to complete the pretreatment of the carbon fiber substrate.

[0083] (2) Synthesis of NiCoFe-LDH / CNF

[0084] 1 mmol Ni(NO₃)₂·6H₂O, 1 mmol Co(NO₃)₂·6H₂O, 1 mmol FeCl₂·4H₂O, and 4 mmol urea were added to 40 mL of methanol and stirred for 10 minutes to obtain a mixed solution. A pretreated 3×3 cm square carbon fiber substrate was placed in the mixed solution to fully soak the substrate. The substrate was then transferred to a 50 mL Teflon-lined stainless steel autoclave and hydrothermally reacted at 135°C for 11 hours. After the reaction, the substrate was naturally cooled to room temperature, rinsed repeatedly with deionized water, and finally dried in a vacuum oven at 60°C for 12 hours to obtain the NiCoFe-LDH / CNF composite.

[0085] (3) Preparation of NiCoFe-CAT / LDH / CNF composite electrode materials

[0086] 150 mg of NiCoFe-LDH / CNF was immersed in a mixed solution containing 2.5 mmol HHTP and 30 mL of deionized water and ultrasonicated for 10 minutes. The mixture was then placed in an oven at 95°C with a heating rate of 2°C / min and heated for 28 hours. After the reaction was completed and naturally cooled to room temperature, it was repeatedly washed with deionized water and dried at 60°C for 12 hours to obtain the final product, NiCoFe-CAT / LDH / CNF composite electrode material.

[0087] Example 6

[0088] This embodiment provides a method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material NiCo-CAT / LDH / CNF, comprising the following steps:

[0089] (1) Pretreatment of carbon fiber substrate

[0090] First, the sheet-like carbon fiber substrate was cut into 3×3 cm squares, ultrasonically cleaned with ethanol for 20 minutes, then ultrasonically cleaned with deionized water for 20 minutes, and then naturally dried at room temperature. Subsequently, the cleaned carbon fiber substrate was immersed in 30 mL of dilute sulfuric acid / dilute nitric acid solution (3% by mass each) and heated at 65°C for 5 hours. After acidification, it was repeatedly rinsed with deionized water to neutralize it, and finally dried in a vacuum oven at 60°C overnight to complete the pretreatment of the carbon fiber substrate.

[0091] (2) Synthesis of NiCo-LDH / CNF

[0092] 1 mmol Ni(NO₃)₂·6H₂O, 1 mmol Co(NO₃)₂·6H₂O, and 2.7 mmol urea were added to 40 mL of methanol and stirred for 10 minutes to obtain a mixed solution. A pretreated 3×3 cm square carbon fiber substrate was placed in the mixed solution and thoroughly soaked. The substrate was then transferred to a 50 mL Teflon-lined stainless steel autoclave and hydrothermally reacted at 130°C for 9 hours. After the reaction, the substrate was naturally cooled to room temperature, rinsed repeatedly with deionized water, and finally dried in a vacuum oven at 60°C for 12 hours to obtain the NiCo-LDH / CNF composite.

[0093] (3) Preparation of NiCo-CAT / LDH / CNF composite electrode materials

[0094] 150 mg of NiCo-LDH / CNF was immersed in a mixed solution containing 1.7 mmol HHTP and 30 mL of deionized water and ultrasonicated for 10 minutes. The mixture was then placed in an oven at 95°C with a heating rate of 2°C / min and heated for 20 hours. After the reaction was completed and naturally cooled to room temperature, it was repeatedly washed with deionized water and dried at 60°C for 12 hours to obtain the final product, NiCo-CAT / LDH / CNF composite electrode material.

[0095] Comparative Example 1

[0096] This comparative example provides a method for preparing a conductive MOF / CNF electrode material NiCoCu-CAT / CNF, comprising the following steps:

[0097] (1) Pretreatment of carbon fiber substrate

[0098] First, the sheet-like carbon fiber substrate was cut into 3×3 cm squares, ultrasonically cleaned with ethanol for 20 minutes, then ultrasonically cleaned with deionized water for 20 minutes, and then naturally dried at room temperature. Subsequently, the cleaned carbon fiber substrate was immersed in 30 mL of dilute sulfuric acid solution (mass fraction 5%) and heated at 50°C for 8 hours. After acidification, it was repeatedly rinsed with deionized water to neutralize it, and finally dried in a vacuum oven at 60°C overnight to complete the pretreatment of the carbon fiber substrate.

[0099] (2) Preparation of NiCoCu-CAT / CNF electrode materials

[0100] 0.8 mmol Ni(OAc)2·4H2O, 0.4 mmol Co(OAc)2·4H2O, 0.4 mmol Cu(OAc)2·H2O, and 2.5 mmol HHTP were dispersed in 40 mL of a water / DMF (volume ratio = 1:1) mixed solvent and sonicated for 30 minutes in a 50 mL Teflon-lined stainless steel autoclave. A carbon fiber substrate (3 × 3 cm) was immersed in the reaction solution and heated at 85°C for 24 hours. The resulting sample was thoroughly washed with ultrapure water and dried at 60°C overnight to obtain the NiCoCu-CAT / CNF electrode material.

[0101] Performance Testing

[0102] 1. Electrochemical performance test

[0103] In the three-electrode system of the electrochemical workstation, electrochemical measurements were carried out on the composite electrode material NiCoCu-CAT / LDH / CNF and two reference materials NiCoCu-LDH / CNF and NiCoCu-CAT / CNF in 1 M NaCl solution.

[0104] Graphite and Ag / AgCl electrodes were used as counter and reference electrodes, respectively, and the active material was directly used as the working electrode for electrochemical testing. Cyclic voltammetry (CV), electrostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were used to test NiCoCu-CAT / LDH / CNF, NiCoCu-LDH / CNF, and NiCoCu-CAT / CNF samples. EIS recording frequencies ranged from 0.01 Hz to 100 kHz.

[0105] like Figure 6 NiCoCu-CAT / LDH / CNF, NiCoCu-LDH / CNF, and NiCoCu-CAT / CNF samples at 20 mV s -1 As shown in the cyclic voltammetry curves below, the curves show similar rectangles, and the redox peak appears at about 0.53 V, which indicates that the electrode has good synergistic effects of Faradaic pseudocapacitance and double layer. The CV curves at different scan rates are similar in shape, indicating that the NiCoCu-CAT / LDH / CNF composite material has good capacitive reversibility.

[0106] from Figure 7The charge-discharge curves show that the NiCoCu-CAT / LDH / CNF capacitor deionization electrode has a longer discharge time than the NiCoCu-LDH / CNF and NiCoCu-CAT / CNF capacitor deionization electrodes. Furthermore, the specific capacitance of NiCoCu-CAT / LDH / CNF at different current densities is superior to that of NiCoCu-LDH / CNF and NiCoCu-CAT / CNF, and these results are consistent with the CV results.

[0107] Will Figure 8 The electrochemical impedance spectroscopy is used to analyze the electrochemical resistance of the material. Compared with NiCoCu-LDH / CNF and NiCoCu-CAT / CNF, the semicircle of NiCoCu-CAT / LDH / CNF is smaller, indicating that the charge transfer resistance of NiCoCu-CAT / LDH / CNF is the smallest, indicating that the introduction of conductive MOFs greatly improves the conductivity of the composite electrode material.

[0108] 2. Capacitive deionization and desalination test

[0109] The capacitive deionization (CDI) test device was assembled as follows: a fixing plate, a titanium plate, an ion exchange membrane, and a silicone gasket were prepared. The titanium plate was used as a current collector for the cathode and anode materials. The cathode was the prepared sample electrode, and carbon fibers (3 × 3 cm) loaded with the material were embedded on the titanium plate. An activated carbon (AC) sheet (3 × 3 cm) was used as the anode.

[0110] The CDI experiment was conducted in a circulation system, which included a CDI element, a peristaltic pump, a power supply, a conductivity meter, and a reservoir. 40 mL of a 500 mg L -1 The NaCl solution was continuously pumped into the CDI element using a peristaltic pump with a flow rate set at 20 mL min -1 , the working voltage was 1.2 V, and the duration was 30 min at 25 °C. CDI experiments were conducted at different applied voltages (0.8-1.4 V) and initial concentrations (100-1000 mg L -1 ) conditions for adsorption and regeneration. The actual picture of the entire test device is as follows Figure 9 shown.

[0111] Figure 10 a is the change of desalination capacity of NiCoCu-LDH / CNF and NiCoCu-CAT / LDH / CNF with time. The equilibrium is reached in about 30 minutes and the adsorption capacity reaches saturation.

[0112] The calculated salt adsorption capacity of the NiCoCu-CAT / LDH / CNF electrode is as high as 84.26 mg g -1, significantly exceeding NiCoCu-LDH / CNF (68.70 mg g -1 ), and significantly higher than most previously reported composite electrode materials. This can be attributed to the NiCoCu-CAT / LDH / CNF composite material exposing more active sites, and the two-dimensional planar structure of the conductive MOFs greatly enhancing electron transport and significantly improving the Faradaic reaction on the material surface, thereby demonstrating its excellent ion capture performance.

[0113] Figure 10 b is the relationship between desalination capacity and desalination rate. The NiCoCu-CAT / LDH / CNF curve is located in the area to the right and above, indicating that its desalination rate is higher than that of NiCoCu-LDH / CNF. Under the same operating conditions, the fastest desalination rate can reach 6.87 mg g -1 min -1 , which is attributed to the fast charge transfer ability and good electrical conductivity of the composite material, which is consistent with the analysis results of CV and GCD.

[0114] Figure 10 c The desalination performance of NiCoCu-CAT / LDH / CNF electrode at different initial concentrations and applied voltages was studied. As the initial concentration increased from 100 to 1000 mg L -1 The salt adsorption capacity of NiCoCu-CAT / LDH / CNF was significantly improved when the concentration reached 1000 mg L -1 When the salt adsorption capacity is as high as 119.8 mg g -1 , indicating that NiCoCu-CAT / LDH / CNF has extremely high desalination ability in salt solutions with a wide concentration range.

[0115] from Figure 10 d It can be seen that when the initial salt concentration is 500 mg L -1 The salt adsorption capacity of NiCoCu-CAT / LDH / CNF electrode increased significantly with the applied voltage increasing from 0.8 V to 1.4 V, from 49.91 mg g at 0.8 V to -1 Increased to 100.81 mgg -1 The results showed that NiCoCu-CAT / LDH / CNF had higher desalination capacity and faster desalination rate at higher voltage, which may be because the higher voltage generated a stronger electrostatic force, which pushed the ions to intercalate toward the electrode.

[0116] Figure 11 a is a graph showing the change in cyclic electrical adsorption when a capacitive deionization device is used to desalinate a saline solution. The results show that the composite material has good cyclic performance and no obvious capacity decay occurs during 20 adsorption-desorption cycles.

[0117] At the same time, Figure 11 In the control experiment in b, it can be found that at an initial concentration of 500 mg L -1 At a voltage of 1.2 V, the desalination performance of the NiCoCu-CAT / LDH / CNF composite electrode material derived from NiCoCu-DH / CNF was significantly better than that of several control materials, proving that the electrode material prepared by the present invention has more excellent CDI desalination performance.

[0118] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material, characterized in that: The steps include: (1) Pretreatment of carbon fiber substrate: soak the raw carbon fiber cloth in an acidic solution for oxidation and heat reaction; (2) Directed anchoring of multi-component LDH: a transition metal inorganic salt and the raw carbon fiber cloth pretreated in step (1) were added to a reactor, urea was added to adjust the pH to 9-11, and then methanol was used as a solvent to stir and heat the reaction by a solvothermal method. After the reaction was completed, the mixture was repeatedly washed with deionized water and ethanol, and then dried to obtain an LDH / CNF composite material; (3) Preparation of three-dimensional layered conductive MOF / LDH / CNF composite electrode material: The LDH / CNF composite material was immersed in deionized water, a multidentate organic ligand was added thereto, and the reaction was slowly carried out under hydrothermal conditions at a fixed heating rate. The material was then centrifuged and repeatedly washed with deionized water and ethanol, and finally dried to obtain a three-dimensional layered conductive MOF / LDH / CNF composite electrode material. The multidentate organic ligand added in step (3) is 2,3,6,7,10,11-hexahydroxytriphenylenebenzene HHTP or 2,3,6,7,10,11-hexaaminotriphenylenebenzene HITP; the molar ratio of the multidentate organic ligand to the total amount of the transition metal inorganic salt in the LDH / CNF composite material is (1-1.8):1; In the step (3), the reaction heating rate is 1-5°C / min, the reaction temperature is 70-110°C, and the reaction time is 12-48h.

2. The method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material according to claim 1, characterized in that: The acidic solution in step (1) is one or a combination of nitric acid and sulfuric acid, and the concentration of the acidic solution is 0.3-2M.

3. The method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material according to claim 1, characterized in that: The heating reaction temperature in step (1) is 60-100° C., and the reaction time is 3-12 hours.

4. The method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material according to claim 1, characterized in that: The transition metal inorganic salt in step (2) is any two or three of Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Cu(NO3)2·3H2O, and FeCl2·4H2O.

5. The method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material according to claim 1, characterized in that: The molar ratio of urea added in step (2) to the total amount of transition metal inorganic salts is (1-5):

1.

6. The method for preparing a three-dimensional layered conductive MOF / LDH / CNF composite electrode material according to claim 1, characterized in that: The stirring and heating reaction time in step (2) is 8 to 15 hours, and the temperature is 80 to 160°C.

7. A three-dimensional layered conductive MOF / LDH / CNF composite electrode material prepared by the method according to any one of claims 1 to 6.

8. Use of a three-dimensional layered conductive MOF / LDH / CNF composite electrode material prepared by the method according to any one of claims 1 to 6 in capacitive deionization (CDI) desalination.

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