CN108328706B - Preparation and application of MOF-derived porous carbon/graphene composite electrode material - Google Patents

Preparation and application of MOF-derived porous carbon/graphene composite electrode material Download PDF

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CN108328706B
CN108328706B CN201810036535.2A CN201810036535A CN108328706B CN 108328706 B CN108328706 B CN 108328706B CN 201810036535 A CN201810036535 A CN 201810036535A CN 108328706 B CN108328706 B CN 108328706B
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施文慧
叶陈增
沈江南
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Zhejiang University of Technology ZJUT
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Abstract

The invention discloses preparation and application of an MOF-derived porous carbon/graphene composite electrode material. The preparation method comprises the following steps: (1) adding graphene or graphene oxide dispersion liquid into a clean container, then adding MOFs crystal powder into the clean container, sealing the container, placing the container on a vortex mixer for continuous mixing, repeatedly adjusting the rotating speed from zero to the highest rotating speed to ensure that a sample is fully vibrated in the vertical direction, promoting the formation of a self-assembled three-dimensional structure of graphene sheets or graphene oxide sheets and the full and uniform dispersion of MOFs crystals in a three-dimensional framework structure, and finally freezing and drying to obtain the graphene/MOF porous aerogel; (2) adding aerogel in N2Roasting at high temperature in atmosphere to obtain a carbonized product; (3) and carrying out acid treatment on the carbonized product to obtain the MOF-derived porous carbon/graphene composite electrode material. The invention provides an application of the MOF-derived porous carbon/graphene composite electrode material in CDI or MCDI desalination, and the application has a good effect.

Description

Preparation and application of MOF-derived porous carbon/graphene composite electrode material
Technical Field
The invention relates to a preparation method of a Metal Organic Framework (MOF) derived porous carbon/graphene composite electrode material and application of the material in capacitive deionization desalination and capacitive deionization desalination.
Background
The Capacitive Deionization (CDI) technology as a novel desalination technology has the advantages of high efficiency, low energy consumption, environmental friendliness and the like. The CDI process is a process of applying an electric field to two parallel and opposite porous adsorption electrodes, and when brine flows through, anions and cations move to the positive electrode and the negative electrode respectively under the action of the electric field and are adsorbed on the surfaces of the electrodes to form an Electric Double Layer (EDL). When the electrodes reach saturation, they are shorted or a reverse voltage is applied, causing the ions to desorb from the electrodes back into solution. CDI performance relies on its physical properties and internal structure. The traditional CDI materials comprise graphene, carbon nano tubes, activated carbon, carbon nano fibers, mesoporous carbon, carbon aerogel and the like. These materials with high conductivity, high specific surface area, good water wettability and narrow pore size distribution (mesopores) were extensively studied in the early work of CDI, but the higher preparation cost and lower adsorption capacity have restricted further development of these materials in the field of CDI.
Metal-Organic Frameworks (MOFs) are crystalline materials with periodic network structures formed by self-assembly of Organic ligands and Metal ions. The MOF material has the characteristics of high specific surface area, various ligands and metal centers, controllable mesoporous size, functional structure and the like, so that the MOF material plays an important role in the fields of gas separation and storage, catalysis, drug delivery, sensing and the like. When the MOF material is used as a precursor, the metal/metal oxide doped porous carbon material is obtained through carbonization treatment at high temperature, and the obtained porous carbon material can still keep high specific surface area and has rich mesoporous structure through acid treatment. MOF, such as ZIF-8 derived porous carbon materials, exhibit good CDI performance, but MOF materials have a lower degree of graphitization and self-agglomeration problems at high temperatures, which limit further improvements of MOF derived porous carbon materials in CDI.
Graphene (Graphene) is a polymer formed from carbon atoms with SP2A hexagonal honeycomb type single-layer planar material consisting of hybrid rails. In 2004, Geim et al separated graphite to obtain single-layer graphene. Graphene has a high specific surface area (2600 m)2g-1) Outstanding thermal and mechanical properties and electrical conductivity (7200S m)-1) Therefore, the method is widely applied to the fields of nano composite material preparation, electronic devices, energy storage, biosensing, water treatment and the like. Graphene oxide is another form of graphene that is dispersed in solution. The surface of the graphene oxide has a large number of hydroxyl, carboxyl and epoxy functional groups, and the functional groups facilitate further modification and simultaneously facilitate modificationThe graphene is convenient to be compounded with other materials to prepare a novel material.
In the CDI field, the characteristics of high specific surface area and abundant mesoporous structure make MOF-derived porous carbon a hot spot which is of great interest in recent years. But self-agglomeration and poor electrical conductivity limit its further development. And the graphene has high conductivity and specific surface area, and is an ideal material for solving self aggregation and conductivity of the MOF-derived porous carbon.
Disclosure of Invention
The invention aims to provide a preparation method of an MOF-derived porous carbon/graphene composite electrode material, which is simple to operate, mild in synthesis conditions and easy for mass production, and can effectively prevent the self-aggregation of the MOF and the aggregation between graphene sheets, so that the composite material has a high specific surface area, high conductivity and a rich mesoporous structure.
The second purpose of the invention is to provide the application of the MOF-derived porous carbon/graphene composite electrode material in Capacitive Deionization (CDI) desalination, which shows good performance.
The third purpose of the invention is to provide the application of the MOF-derived porous carbon/graphene composite electrode material in capacitive deionization (MCDI) desalination, which shows good performance.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention provides a preparation method of an MOF-derived porous carbon/graphene composite electrode material, which is carried out according to the following steps:
(1) adding graphene or graphene oxide dispersion liquid into a clean container, adding MOFs crystal powder into the clean container, enabling the feeding mass ratio of the MOFs crystal powder to the graphene or graphene oxide to be 0.5-50: 1, sealing the container, placing the sealed container on a vortex mixer for continuous mixing, adjusting the rotating speed from zero to the highest rotating speed in the mixing process, keeping the rotating speed for a certain time, adjusting the rotating speed from the highest rotating speed to 0, adjusting the rotating speed from 0 to the highest rotating speed, keeping the rotating speed for a certain time, repeating the speed adjusting process, enabling a sample to fully vibrate in the vertical direction, promoting the formation of a self-assembled three-dimensional structure of graphene sheets or graphene oxide sheets and the full and uniform dispersion of MOFs crystals in a three-dimensional framework structure, and obtaining graphene/MOF porous hydrogel, finally freezing and drying to obtain the graphene/MOF porous aerogel, wherein the graphene/porous MOF aerogel has a self-supporting porous structure, the integrity of the graphene or graphene oxide and MOFs structures is reserved;
(2) mixing graphene/MOF porous composite aerogel in N2Keeping for 1-2 h after the temperature rises to 600-800 ℃ at the rate of 5-10 ℃/min in the atmosphere, and naturally cooling to obtain a carbonized product;
(3) and (3) carrying out acid treatment on the carbonized product to remove metal and metal oxide, and finally obtaining the MOF-derived porous carbon/graphene composite electrode material.
In the graphene oxide dispersion liquid adopted in the step (1), the graphene oxide achieves balanced and stable dispersion by virtue of pi-pi stacking and repulsive force between van der Waals attractive force and oxygen-containing functional groups on the surface of the graphene oxide. According to the invention, a sol-gel method is adopted, MOFs crystals are added into a stably dispersed graphene oxide solution, the balance is broken, graphene oxide is orderly stacked to form hydrogel, and the aerogel is obtained through freeze drying. Therefore, according to the gel formation mechanism analysis of the present invention, no matter what method is used to obtain the graphene oxide dispersion, the gel can be obtained as long as the MOFs crystal is added to break the equilibrium state. Similarly, regardless of the method (even if it is a dispersion of stably dispersed graphene obtained by some auxiliary method such as a surfactant, etc.), a gel can be obtained as long as the equilibrium state thereof can be broken by adding MOFs crystals. Particularly, the MOFs crystal powder and the graphene or graphene oxide dispersion liquid adopted by the invention are mixed in a mode that the sample is fully vibrated in the vertical direction, so that the MOFs crystal can be fully and uniformly dispersed in a three-dimensional frame structure, and a solid foundation can be laid for maintaining the structure after subsequent roasting and acid treatment. In the invention, preferably, in the graphene or graphene oxide dispersion liquid, the transverse size of the graphene or graphene oxide is controlled within the range of 0.1-100 μm, and more preferably between 1-10 μm; the concentration of the graphene or the graphene oxide is controlled within the range of 0.1-100 mg/mL, and more preferably 1-10 mg/mL.
The invention is almost suitable for all MOFs crystals, and one MOFs or more than two MOFs can be added in the preparation process (no special requirement is made on the MOF types). MOFs crystals can be prepared by literature-reported methods, such as Ni-MOF, Fe-MOF, ZIF-8, MOF-5, Co-MOF and [ K ]2Sn2(bdc)3](H2O)XThe crystal can be synthesized by solvothermal method. Specifically, the solvothermal method can be performed as follows: mixing metal salt or metal salt hydrate, organic ligand and solvent in proportion, then carrying out solvothermal reaction on the mixture to obtain MOFs crystal precipitate, and further carrying out centrifugation or standing treatment and vacuum drying to obtain MOFs crystal powder. The selection of the metal center in the metal salt or metal salt hydrate covers almost all metals, including main group elements, transition elements, lanthanide metals and the like, wherein Zn, Cu, Fe, Ni, Co, Sn and the like are more applied. The organic ligand may be selected from one of the following chemically pure or analytically pure drugs: carboxylic acids, imidazoles, pyridines, porphyrins, and the like. The solvent is selected from one of the following chemically pure or analytically pure reagents: methanol, ethanol, N-dimethylformamide, deionized water and the like.
Further, in the step (1), the feeding mass ratio of the MOFs crystal powder to the graphene oxide is preferably 1: 1-40: 1.
Furthermore, in the step (1), the MOFs crystal is Fe-MOF crystal, and the charging mass ratio of the Fe-MOFs crystal powder to the graphene oxide is preferably 10: 1-40: 1, and most preferably 20: 1.
Furthermore, in the step (1), the MOFs crystal is a ZIF-8 crystal, and the feeding mass ratio of the ZIF-8 crystal powder to the graphene oxide is preferably 1: 1-8: 1, and most preferably 4: 1.
Further, in step (1), the whole mixing process time generally lasts for 1-10 min. The preferred mixing process is: and in the mixing process, the rotating speed knob is adjusted from 0 to the highest rotating speed (2800r/min), stays for 5-15 s (preferably 10s) and then is adjusted to 0, then is adjusted from 0 to the highest rotating speed, stays for 5-15 s (preferably 10s), the speed adjusting process is repeated, and the whole mixing process lasts for 1-10min (preferably 4-5min, most preferably 5 min).
Further, in the step (1), the dispersion mode of the MOFs crystals in the three-dimensional frame structure includes but is not limited to: the MOFs crystal is uniformly attached to the surface of a graphene sheet or a graphene oxide sheet, and the MOFs crystal is coated by the graphene sheet or the graphene oxide sheet. In a graphene/MOF porous hydrogel or aerogel, the composite modes can exist singly or in combination, and are determined according to the properties and the adding amount of the MOFs.
Further, in the step (2), the calcination temperature is preferably 800 ℃. Mixing graphene/MOF porous composite aerogel in N2The temperature is raised to 800 ℃ at a heating rate of 5 ℃/min in the atmosphere and then kept for 1 h.
In the step (3), the acid used for the acid treatment can be hydrochloric acid, nitric acid and the like, wherein the concentration range of the hydrochloric acid is 1-5 mol L-1The concentration of nitric acid is 1mol L-1. The acid treatment method is generally to fully soak the carbonized product in acid to remove metal and metal oxide, and heating can be adopted to accelerate the etching process and completely remove the metal and the metal oxide. For example, 1mol of L is used-1Hydrochloric acid or nitric acid, heating at 60 deg.C (such as 24 hr).
The invention further provides application of the MOF-derived porous carbon/graphene composite electrode material in Capacitive Deionization (CDI) desalination. The MOF-derived porous carbon/graphene composite electrode material can be prepared into a CDI electrode by a conventional method, such as: a MOF-derived porous carbon/graphene composite electrode material: conductive carbon black: mixing the PVDF three according to a certain proportion (such as the mass ratio of 8: 1: 1), dropwise adding an N-methylpyrrolidone (NMP) solution, grinding, stirring the mixture into uniform slurry, coating the slurry on a graphite sheet, and finally drying the electrode to remove the solvent to obtain the CDI electrode.
The invention further provides application of the MOF-derived porous carbon/graphene composite electrode material in capacitive deionization (MCDI) desalination. The MOF-derived porous carbon/graphene composite electrode material can be prepared into MCDI electrodes by conventional methods, such as: a MOF-derived porous carbon/graphene composite electrode material: conductive carbon black: mixing the PVDF, the N-methylpyrrolidone (NMP) solution and the N-methylpyrrolidone (NMP) solution according to a certain ratio (the mass ratio is 8: 1: 1), grinding, stirring the mixture into uniform slurry, coating the slurry on a graphite sheet, and finally drying the electrode to remove the solvent to obtain the MCDI electrode.
Compared with the prior art, the invention has the following outstanding advantages and beneficial effects:
(1) compared with a common mixing mode of MOFs crystal powder and graphene oxide, the mixing mode of the MOFs crystal powder and the graphene oxide selected by the invention can obtain the graphene/MOF porous composite material aerogel with a self-supporting porous structure through a controllable self-assembly process, the integrity of the graphene oxide and the MOFs structure is maintained, the MOFs crystal is fully and uniformly dispersed in a three-dimensional frame structure, and a solid foundation can be laid for the structure to be kept after subsequent roasting and acid treatment. The graphene/MOF porous composite material aerogel prepared under the condition can still keep a three-dimensional porous structure after high-temperature carbonization and acid treatment, effectively prevents self-aggregation of the MOF and aggregation between graphene sheets, and enables the finally obtained composite electrode material to have a high specific surface area, high conductivity and a rich mesoporous structure.
(2) Due to the synergistic effect (conductivity, specific surface area and the like) between the MOF and the graphene, the MOF-derived porous carbon/graphene composite electrode material has greatly improved performance compared with MOF-derived porous carbon and graphene individual bodies, and has good application effect in Capacitive Deionization (CDI) desalination, especially capacitive deionization (MCDI) desalination.
Drawings
FIG. 1: photograph of Fe-MOF/GO-20 aerogel.
FIG. 2: scanning electron micrographs (a, b) of Fe-MOF crystals and Fe-MOF/GO-20 aerogel, wherein the auxiliary graph in the b graph shows the morphology of MOF encapsulated by GO.
FIG. 3: PC/rGO-20 scanning electron micrograph (a) and high resolution scanning electron micrograph (b).
FIG. 4: transmission electron microscopy images of Fe-MOF/GO-20 after high temperature calcination (a), PC/rGO-20 (b, c).
FIG. 5: scanning electron micrographs (a, b, c, d) of PC/rGO-10, PC/rGO-20, PC/rGO-30, and PC/rGO-40.
FIG. 6: PC/rGO-20, rGO, XRD pattern (a) of PC, Fe-MOF/GO-20, Raman pattern (b) of PC/rGO-20.
FIG. 7: BET plot of PC/rGO-10, PC/rGO-20, PC/rGO-30, PC/rGO-40.
FIG. 8: SEM pictures (a, b) of Fe-MOF/GO-1, SEM pictures (c, d) of Fe-MOF/GO-2, and SEM pictures of Fe-MOF/GO-4(e, f).
FIG. 9: SEM pictures (a, b, c) of PC/rGO-20-600, PC/rGO-20-700 and PC/rGO-20, and SEM pictures (a, b, c) of PC/rGO-20-600, PC/rGO-20-700 and PC/rGO-20 at 5mV s-1Cv (d) at scan rate.
FIG. 10: SEM images of ZIF-8/GO-1, ZIF-8/GO-2, ZIF-8/GO-4 and ZIF-8/GO-8.
FIG. 11: SEM images (a, b) of samples after ZIF-8, ZIF-8 calcination and etching, with the auxiliary image in the b-image being a high resolution SEM image. SEM images (c, d) of samples after calcining and etching ZIF-8/GO-4 and ZIF-8/GO-4, wherein an auxiliary image in the d image is the morphology of uniformly dispersed ZIF-8 derived porous carbon on monolithic graphene.
FIG. 12: XRD patterns (a) of ZIF-8 and ZIF-8/GO-4. XRD patterns (b) of PC (Zn) and PC (Zn)/rGO-4.
FIG. 13: schematic of an MCDI apparatus.
FIG. 14: PC/rGO-20 at 500mg L-1And testing by using CDI and MCDI devices respectively under the working voltage of 1.2V to obtain an electro-adsorption-desorption curve diagram.
FIG. 15: PC/rGO-20 at 500mg L-1The graph (a) of the electro-adsorption-desorption and the corresponding instantaneous current graph (b) are respectively under the working voltages of 1.0V, 1.2V, 1.4V and 1.6V.
FIG. 16: the electro-adsorption-desorption curve diagram (a) of the PC/rGO-20 after 22 cycles, wherein the auxiliary graph of a figure is the electro-adsorption capacity retention rate of the PC/rGO-20; PC/rGO-20 at 125mg L-1,250mg L-1,500mg L-1,1000mg L-1And adding 1.0V, 1.2V, 1.4V and 1.6V to obtain an electric adsorption capacity graph.
FIG. 17: PC/rGO-10, PC/rGO-30, PC/rGO-40 at 500mg L-1Working voltages of 1.0V, 1.2V, 1.4V and 1.6V are respectivelyThe following electrosorption-desorption graphs (a, b, c) and transient current graphs (d, e, f).
FIG. 18: PC/rGO-10, PC/rGO-20, PC/rGO-30, PC/rGO-40 at 500mg L-1The electric adsorption capacity under the working voltage of 1.0V, 1.2V, 1.4V and 1.6V respectively.
FIG. 19: PC/rGO-20, PC, rGO at 500mg L-1And an electric adsorption-desorption curve chart under the working voltage of 1.2V.
Detailed Description
The technical solution of the present invention is further illustrated by the following specific examples, but the scope of the present invention is not limited thereto:
examples 1 to 13
1. Preparation of samples
(1) Example 1
The preparation of the sample was carried out as follows:
a250 mL reaction flask was placed in an ice-water bath, and 2.4mL of concentrated sulfuric acid, 0.5g K, was added to 0.3g of graphite powder under magnetic stirring2S2O8And 0.5g P2O5The mixture was reacted at 80 ℃ for 4.5h, and the pre-oxidized product was washed with water and dried. The pre-oxidized product is then added to 12ml of concentrated sulfuric acid, and 6g of potassium permanganate (A) are slowly added>99.6%), the reaction temperature was controlled not to exceed 20 ℃, 25mL of deionized water was added, and the mixture was stirred for 2 hours in a water bath at 35 ℃. After 2h, 70mL of deionized water was added to dilute the solution, and 2mL of 30% by mass hydrogen peroxide was slowly injected. The bubbles gradually bubble out and the solution is bright yellow. Stirring for 2h, filtering, and washing with 10% hydrochloric acid solution and deionized water for multiple times until the solution is neutral. The obtained graphene oxide is dispersed into GO aqueous phase dispersion liquid with the concentration of 10mg/mL and stored for later use.
(2) Example 2
The preparation of the sample was carried out as follows:
a50 mL beaker A was taken, and 0.6488g of ferric chloride and 10mL of water were added thereto and stirred well. Another 50mL beaker B was added with 0.4643g of fumaric acid and 10mL of DMF and stirred well. And mixing and stirring the A and the B uniformly, transferring the A and the B into a reaction kettle at 80 ℃ for reaction for 4 hours, collecting the precipitate, washing the precipitate with DMF (dimethyl formamide) for 2 times, washing the precipitate with deionized water twice, and drying the precipitate in a vacuum oven at 60 ℃ for 12 hours to finally obtain Fe-MOF crystals.
(3) Example 3
The preparation of the sample was carried out as follows:
sequentially adding graphene oxide aqueous phase dispersion liquid and Fe-MOF crystal powder into a centrifugal tube with the size of 5mL, wherein the raw material feeding ratio is as follows: 1mL of graphene oxide solution with the concentration of 4 mg/mL; 40mg Fe-MOF powder (m)MOF﹕mGO10: 1); and mixing the obtained mixture on a QL-901 type vortex mixer, adjusting a rotating speed knob from 0 to the highest rotating speed (2800r/min), staying for 10s, then adjusting to 0, then adjusting from 0 to the highest rotating speed, staying for 10s, and repeating the speed adjusting process, wherein the time of the whole mixing process lasts for 5min, so that the sample is fully vibrated in the vertical direction, and the Fe-MOF/GO hydrogel is obtained. And after freeze drying for 24h, obtaining Fe-MOF/GO aerogel. Is named as: Fe-MOF/GO-10.
(4) Examples 4 to 6
Examples 4-6 the samples were prepared in the same manner as in example 3, except that the Fe-MOF powders in examples 4-6 each had a mass of 80mg (m)MOF﹕mGO=20﹕1),120mg(mMOF﹕mGO=30﹕1),160mg(mMOF﹕mGO40: 1). Respectively named as: Fe-MOF/GO-20, Fe-MOF/GO-30, Fe-MOF/GO-40.
(5) Example 7
The preparation of the sample was carried out as follows:
the graphene oxide dispersion of 10mg/mL in example 1 was diluted to 4 mg/mL. Adding 1mL of 4mg/mL graphene oxide aqueous phase dispersion liquid into a centrifugal tube with the size of 5mL, mixing on a QL-901 type vortex mixer, adjusting a rotating speed knob to the highest rotating speed (2800r/min) from 0 in the mixing process, staying for 10s, adjusting to 0 again, adjusting to the highest rotating speed from 0 again, staying for 10s, repeating the speed adjusting process all the time, keeping the time of the whole mixing process for 5min, fully oscillating the sample in the vertical direction, and freeze-drying for 24h to obtain the graphene oxide aerogel.
(6) Example 8
The preparation of the sample was carried out as follows:
Fe-MOF from example 2 was treated as follows in N2Keeping the temperature for 1h after the temperature rises to 800 ℃ at the temperature rise rate of 5 ℃/min in the atmosphere, and then naturally cooling. And immersing the product in 5M HCl for 24h to remove metals and metal oxides, and finally obtaining the MOF derived porous carbon material named as PC.
(7) Example 9
The preparation of the sample was carried out as follows:
the sample of example 9 was prepared according to the same procedure as in example 8, except that the sample of example 9 was prepared from the graphene oxide aerogel of example 7 and was named rGO.
(8) Examples 10 to 13
The preparation of the sample was carried out as follows:
the preparation steps of the samples 10 to 13 are the same as those of the sample 8, except that the samples 10 to 13 are prepared from the Fe-MOF/GO aerogels in the examples 3 to 6, and are named as follows: PC/rGO-10, PC/rGO-20, PC/rGO-30, and PC/rGO-40.
(9) Comparative example 1
The preparation of the sample was carried out as follows:
the graphene oxide dispersion liquid of 10mg/mL in the example 1 is diluted into a dispersion liquid of 4mg/mL, then Fe-MOF (1mg) in the example 2 is added into the graphene oxide dispersion liquid of 1mL of 4mg/mL, the mixture is placed in a QL-901 vortex mixer, the rotating speed is adjusted to be 180r/min, and the mixture is shaken for 24 hours.
(10) Comparative examples 2 to 3
The preparation of the sample was carried out as follows:
comparative examples 2 to 3 the samples were prepared in the same manner as in comparative example 1 except that the amount of Fe-MOF added was 2mg and 4 mg. The samples of comparative examples 1-3 were named Fe-MOF/GO-1, Fe-MOF/GO-2, Fe-MOF/GO-4, respectively.
(11) Comparative examples 4 to 5
The preparation of the sample was carried out as follows:
comparative examples 4-5 the samples were prepared according to the same procedure as in example 11, except that the calcination temperature was changed to 600 ℃ and 700 ℃ and the samples were named PC/rGO-20-600 and PC/rGO-20-700.
2. Characterization and testing
(1) SEM analysis
SEM test was performed on a HITACHI-8010 scanning electron microscope, and the sample preparation method was as follows: samples of examples 2, 4, 10-13 and comparative examples 1-5 were taken, and a small amount of sample was applied to the surface of a support base to which a conductive adhesive was applied.
(2) High resolution TEM analysis
The TEM test was performed on a JEOL 2010F transmission electron microscope using the following sample preparation method: taking a trace amount of the above example 11, placing the trace amount of the above example in a small bottle filled with 1mL of deionized water, then placing the small bottle in a water bath ultrasonic pool with the ultrasonic power of 250W, continuously performing ultrasonic treatment for 0.5h at the constant temperature of 25 ℃ to obtain a porous composite carbon material aqueous phase dispersion liquid, taking a small amount of the dispersion liquid to drop on the surface of a TEM (containing a microporous carbon support film) with a micro-grid copper mesh, and then naturally drying at the room temperature to obtain the porous composite carbon material aqueous phase dispersion liquid.
(3) Wide angle XRD analysis
The XRD test was carried out on an X' PertPro X-ray diffractometer, and the samples to be tested were prepared as follows: the PC/rGO powder of example 10-13 was spread flat in a frosted recess of a quartz plate and then pressed flat with a glass slide until no significant protrusions and voids were present and tested.
(4) Raman spectral analysis
The Raman tests were performed on a Renishaw invia type Raman spectrometer, and the samples were prepared as follows: the samples of examples 2 and 11 were collected and placed on a clean glass slide and pressed flat.
(5)BET
The BET test was performed on a Micromeritics TristarII-3020 model specific surface area and porosity analyzer, and samples were prepared as follows: a small amount of the sample of example 10-13 was placed in a sample tube, degassed at 120 ℃ for 6 hours, and then tested.
(6) CV testing
The CV test was carried out on an electrochemical workstation model CHI 760, taking 8mg of each of the samples of example 11, comparative examples 4 and 5, and was specifically prepared as follows: sample preparation: conductive carbon black: the mass ratio of PVDF is 8: 1:1 after mixing, N-methylpyrrolidone (NMP) solution was added dropwise and then ground. Stirring the mixture to form a uniform slurryAt 2X 1cm2Coated with 1X 1cm2The area of (a). Finally, the electrode was dried at 60 ℃ for 12h to remove the solvent.
1. Comparison and analysis of test results
FIG. 1 is a photograph of Fe-MOF/GO-20 aerogel. The photograph shows the aerogel appearance of Fe-MOF/GO.
FIG. 2(a) is an SEM image of Fe-MOF. It can be seen that the Fe-MOF microstructure is a uniform rod-like structure with the length of 6-8 μm. FIG. 2(b) is an SEM image of Fe-MOF/GO-20 aerogel, which shows the morphology of a three-dimensional graphene framework and the rod-shaped Fe-MOF dispersed on the framework, and it can be observed that the three-dimensional structure of the aerogel is built up by GO sheets, and the rod-shaped MOF is uniformly dispersed in the framework. The auxiliary graph of FIG. 2(b) reveals that the structure of part of the rod-like Fe-MOF is wrapped by graphene, and the wrapped structure is more favorable for reducing the agglomeration of Fe-MOF.
FIG. 3(a) is an SEM image after calcination of Fe-MOF/GO-20. By comparing with the sample after acid treatment in fig. 3(b), it can be determined that the Fe-MOF surface particles are metal oxides formed by combining metal and oxygen in the ligand through thermal reduction, and the morphology of graphene is still intact through high-temperature calcination. The SEM image of FIG. 3(b) is that of PC/rGO-20, and it can be clearly observed that the three-dimensional structure of the graphene remains intact and the Fe-MOF is uniformly dispersed after the high-temperature calcination and acid treatment of the Fe-MOF/GO-20, confirming the expected MOF-derived porous carbon/graphene composite structure. The auxiliary graph is a structure of a single rod-shaped Fe-MOF wrapped by graphene and a structure with porous surface. The micron-sized macropores after the metal and the oxide thereof are removed enable the material to have a larger specific surface area, and electrolyte can be stored better.
FIG. 4(a) is a TEM image of Fe-MOF/GO-20 after calcination. The white voids in the rod-like structure show the porous structure after high temperature calcination, while the large particles correspond to the SEM in fig. 3(a) and are metals and their oxides. FIG. 4(b, c) is a TEM image of PC/rGO-20. FIG. 4(b) shows the porous structure of Fe-MOF/GO-20 after high temperature calcination and acid treatment, which is compared with the porous structure of FIG. 4(a) after calcination, and shows that the gas generated during acid treatment has no significant effect (such as collapse, etc.) on the porous structure. The auxiliary figures are further enlarged for the porous structure. Fig. 4(c) shows the structure of the surface-coated graphene in the structure of fig. 4(b), and the auxiliary figure is a further enlargement of the graphene structure, which proves the integrity of the graphene structure.
Fig. 5(a, b, c, d) shows SEM images of aerogels composited with four different Fe-MOF to GO ratios after high temperature calcination and acid treatment. From (a) to (d), the presence of graphene three-dimensional structure was observed, but in the PC/rGO-10 sample, the MOF-derived porous carbon was less, which resulted in partial agglomeration of the graphene itself. In the PC/rGO-30 and PC/rGO-40 samples, although the three-dimensional structure of graphene can be observed, the partial agglomeration of the MOF is caused in the preparation process of the precursor due to the excessive amount of the MOF. While PC/rGO-20 exhibited the most suitable recombination state, with little agglomeration between graphene sheets and between MOF-derived porous carbon structures.
FIG. 6(a) is an XRD pattern of PC/rGO-20, rGO, PC. There is a peak package at 26 °, typical of graphitization (002), and partial graphitization of Fe-MOF during high temperature annealing results in 26 °. The 43 ° peak is another peak (100) of carbon. The corresponding diffraction peak of PC can be found out on PC/rGO-20, which proves that the structure of Fe-MOF is complete. And the result analysis of the graphene is the same as that of Fe-MOF, and the structural integrity of the graphene is proved. It also shows that the combination of Fe-MOF and graphene is a physical effect and no reaction occurs. FIG. 6(b) is a Raman plot of Fe-MOF/GO-20, PC/rGO-20. A D peak and a G peak of graphene in the PC/rGO-20 slightly shift towards the left relative to the Fe-MOF/GO-20, which shows that the surface defects of the graphene are reduced in the high-temperature reduction process. It is clear that the graphene structure remains intact despite the high temperature calcination and acid treatment from Fe-MOF/GO-20 aerogel to PC/rGO-20 composite.
FIG. 7 is a BET plot of PC/rGO-10, PC/rGO-20, PC/rGO-30, and PC/rGO-40. It is clearly observed that the curves for the four samples are typical type IV adsorption curves. The result is dominated by mesopores at pressures ranging from 0.4 to 0.9, whereas macropores are present at pressures ranging from 0.9 to 1, which is the location where the original metal/metal oxide is present, as is directly evident in fig. 4(a, b, c).
FIG. 8(a) is a SEM image of Fe-MOF/GO-1, which shows the three-dimensional structure of graphene and also shows the aggregation phenomenon of Fe-MOF. FIG. 8(b) is a representation of Fe-MOF/GO-1 vs. dispersion of Fe-MOF on graphene sheets, showing the structure of a Fe-MOF/GO sample different from that obtained in example 1. FIG. 8(c) is an SEM image of Fe-MOF/GO-2, showing that the graphene three-dimensional structure becomes more compact as the amount of Fe-MOF added increases. FIG. 8(d) is a representation of Fe-MOF/GO-2 dispersion of Fe-MOF over graphene sheets, showing that the Fe-MOF agglomeration is still severe despite some dispersion of Fe-MOF over graphene sheets. FIG. 8(e) is an SEM image of Fe-MOF/GO-4, which shows a compact graphene three-dimensional structure, and from FIG. 8(f), the morphology of Fe-MOF encapsulated by graphene sheets can be observed, but the dispersion of Fe-MOF on the graphene sheets is limited by the lower mixing strength, so that the strategy for preparing Fe-MOF/GO composite material precursors by repeatedly adjusting the rotating speed is more reasonable.
FIG. 9(a, b, c) shows SEM images of Fe-MOF/GO-20 after calcination etching at 600, 700, 800 ℃. Under the condition of calcination at 600 ℃, the obtained composite material is hardly changed, which indicates that the Fe-MOF is not completely converted. Under the condition of 700 ℃ calcination, although a small amount of Fe-MOF is converted into a porous structure similar to that of the Fe-MOF in PC/rGO-20, a large amount of Fe-MOF still exists and is not converted, and the auxiliary graph shows that the Fe-MOF is partially converted into the porous structure. FIG. 9(d) shows that PC/rGO-20-600, PC/rGO-20-700, and PC/rGO-20 are at 5mV s-1The CV diagram at the scan rate has a specific capacitance of 99.80F g-1、146.96F g-1、194.22F g-1. The specific capacitance in CV has direct relation with the electro-adsorption capacity in CDI, so that the optimal conditions for converting Fe-MOF into porous structure at 600, 700 and 800 ℃ can be obtained by taking the specific capacitance as a criterion.
Examples 14 to 21
1. Preparation of samples
(1) Example 14
A50 mL beaker A was taken, and 0.2463g of 2-methylimidazole and 30mL of methanol were added thereto and stirred well. Another 50mL beaker B was added with 0.4462g of zinc nitrate hexahydrate and 15mL of methanol and stirred well. And mixing and stirring the A and the B uniformly, transferring the A and the B into a 100mL beaker for reaction for 24h, collecting the precipitate, washing the precipitate for 3 times by using methanol, and then drying the precipitate for 24h in a vacuum drying oven in vacuum, thereby finally obtaining the ZIF-8 crystal.
(2) Example 15
The preparation of the sample was carried out as follows:
sequentially adding graphene oxide aqueous phase dispersion liquid and ZIF-8 crystal powder into a centrifugal tube with the size of 5mL, wherein the raw material feeding ratio is as follows: 1mL of graphene oxide solution with the concentration of 4 mg/mL; 4mg ZIF-8 powder (m)MOF﹕mGO1: 1); and mixing the obtained mixture on a QL-901 type vortex mixer, adjusting a rotating speed knob from 0 to the highest rotating speed (2800r/min), staying for 10s, then adjusting to 0, then adjusting from 0 to the highest rotating speed, staying for 10s, and repeating the speed adjusting process, wherein the time of the whole mixing process lasts for 5min, so that a sample is fully vibrated in the vertical direction, and the ZIF-8/GO hydrogel is obtained. And (5) freezing and drying for 24h to obtain the ZIF-8/GO aerogel. Is named as: ZIF-8/GO-1.
(3) Examples 16 to 18
The preparation of the sample was carried out as follows:
examples 16 to 18 were carried out in the same manner as in example 15 except that the mass of the ZIF-8 powder in each of examples 16 to 18 was 8mg (m)MOF﹕mGO=2﹕1),16mg(mMOF﹕mGO=4﹕1),32mg(mMOF﹕mGO8: 1). Respectively named as: ZIF-8/GO-2, ZIF-8/GO-4, and ZIF-8/GO-8.
(4) Example 19
The preparation of the sample was carried out as follows:
ZIF-8 from example 14 was treated as follows in N2Keeping the temperature for 1h after the temperature rises to 800 ℃ at the temperature rise rate of 5 ℃/min in the atmosphere, and then naturally cooling. And immersing the product in 5M HCl for 24h to remove metals and metal oxides, and finally obtaining the ZIF-8 derived porous carbon material named PC (Zn).
(5) Example 20
The preparation of the sample was carried out as follows:
the sample of example 20 was prepared according to the same procedure as in example 19, except that the sample of example 20 was prepared from the graphene oxide aerogel of example 7 and was named rGO.
(6) Examples 21 to 24
The preparation of the sample was carried out as follows:
the preparation steps of the samples in the embodiments 21 to 24 are the same as those in the embodiment 19, except that the samples in the embodiments 21 to 24 are prepared from the ZIF-8/GO aerogels in the embodiments 15 to 18, and are named as: PC (Zn)/rGO-1, PC (Zn)/rGO-2, PC (Zn)/rGO-4, and PC (Zn)/rGO-8.
2. Characterization and testing
(1) SEM analysis
SEM test was performed on a HITACHI-8010 scanning electron microscope, and the sample preparation method was as follows: samples of examples 14 to 19 and 23 were collected, and a small amount of the samples were placed on the surface of a support to which a conductive adhesive was applied.
(2) Wide angle XRD analysis
The XRD test was carried out on an X' PertPro X-ray diffractometer, and the samples to be tested were prepared as follows: the test was performed after taking examples 14, 17, 19, 23, laying flat in a frosted recess of a quartz plate, and then flattening with a glass slide until no significant protrusions and voids were present.
3. Comparison and analysis of test results
FIG. 10(a, b, c, d) is an SEM image of ZIF-8/GO composite showing the dispersion of ZIF-8 in GO three-dimensional structure with increasing addition of ZIF-8. With the increase of the addition amount of the ZIF-8, the three-dimensional structure of the graphene is kept complete from a to c, and the ZIF-8 is dispersed on the graphene sheet to a certain degree. Among them, it can be observed in FIG. 10(a) that the ZIF-8 particles are dispersed on the graphene sheets, and the granular ZIF-8 can be found on almost every graphene sheet, but a small amount of ZIF-8 is agglomerated during the mixing process, i.e., as shown in the middle region of FIG. 10 (a). In FIG. 10(b), the amount of ZIF-8 on the graphene sheet was significantly increased, but a ZIF-8/GO composite material with uniformly dispersed ZIF-8 was still not obtained. Fig. 10(c) shows that a large amount of ZIF-8 is loaded on the graphene sheet, and the morphology can be uniformly dispersed, and the three-dimensional structure of the graphene is intact, so that the graphene sheet is an ideal precursor. Fig. 10(d) shows that excess ZIF-8 was stacked on the graphene sheet, resulting in collapse of the graphene three-dimensional structure, resulting in destruction of the overall well-conductive three-dimensional structure.
FIG. 11(a) is an SEM image of ZIF-8 showing a dodecahedral structure of uniform size at the nano-scale under the microstructure. FIG. 11(b) is an SEM image after ZIF-8 calcination etching with no significant dimensional change compared to FIG. 11(a), the auxiliary image being a high magnification SEM image showing the morphology of the dodecahedral structure undergoing shrinkage, which may be caused by ligand decomposition during high temperature calcination. FIG. 11(c) is an SEM image of ZIF-8/GO-4, which shows the three-dimensional structure built by graphene, and also shows the morphology of ZIF-8 uniformly dispersed in the graphene three-dimensional structure. The ZIF-8 structure and the morphology are not changed, which can show that the chemical change does not occur in the compounding process of the ZIF-8 and the graphene. FIG. 11(d) is an SEM image of the sample after ZIF-8/GO-4 calcination etching. Through high-temperature calcination and acid treatment, the three-dimensional structure of the graphene is not obviously changed, the three-dimensional structure is the basis for improving the conductivity of the composite material, the auxiliary graph is the shape of the uniformly dispersed ZIF-derived porous carbon on the single graphene, and compared with the ZIF-8-derived porous carbon in the graph (b) in FIG. 10, the shape is not obviously changed.
FIG. 12(a) is an XRD pattern of ZIF-8, ZIF-8/GO-4, and XRD peaks of the ZIF-8/GO composite material can correspond to ZIF-8 one by one, which illustrates that the ZIF-8 and graphene composite process does not undergo chemical changes, and corresponds to FIG. 11 (c). FIG. 12(b) is an XRD pattern after the ZIF-8 and ZIF-8/GO-4 calcination etching treatments, with a peak at 26 °, typical of the graphitized peak (002), and the ZIF-8 partial structure graphitization resulted in the generation of 26 ° during the high temperature calcination process. The 43 ° peak is another peak (100) of carbon. The corresponding diffraction peaks of PC (Zn) can be found on PC (Zn)/rGO-4, which proves that the ZIF-8 structure is complete.
Examples 25 to 38
There are two major problems in the CDI process: (1) during the adsorption process, the oppositely charged ions are repelled from the electrodes, causing a reduction in desalination efficiency. (2) After several adsorption-desorption cycles, the ions are very difficult to return from the electrode to the solution again, i.e. the reproducibility is poor. Lee et al, with scientific evidence in 2006, proposed Membrane Capacitive Deionization (MCDI) to solve the above-mentioned problems. Despite the abundance of mesoporous structure, MOF-derived porous carbon still has a large number of microporous structures that make the ion transfer rate far inferior to the transport rate in mesopores, directly resulting in poor reproducibility of MOF-derived porous carbon as a CDI electrode. The addition of the ion exchange membrane can block counter charge ions from entering and exiting the electrode and greatly improve the regeneration capability. The subsequent test therefore uses the MCDI test.
1. Preparation of MCDI electrode
(1) Example 25
PC/rGO-10 samples: conductive carbon black: the mass ratio of PVDF is 8: 1:1 after mixing, N-methylpyrrolidone (NMP) solution was added dropwise and then ground. The mixture was stirred to a uniform slurry and coated at 5X 5cm2The graphite sheet of (2). Finally, the electrode was dried at 60 ℃ for 12h to remove the solvent. The mass loading was approximately 52 mg.
(2) Examples 26 to 30
The preparation steps of the sample were the same as in example 25, except that the samples were changed to PC/rGO-20, PC/rGO-30, PC/rGO-40, PC, rGO, respectively.
2. MCDI testing
(1) Example 31
After the instrument is assembled as shown in fig. 13, the anion-exchange membrane and the cation-exchange membrane have no special requirements on the anion-exchange membrane, the selected anion-exchange membrane is a heterogeneous ion-exchange membrane 1 type, and the cation-exchange membrane is a polyethylene heterogeneous ion-exchange membrane 1 type and is purchased from Hangzhou green environmental protection technology, Inc. Assembling the electrode made of PC/rGO-10 as shown in the figure, and preparing TDS 500mg L-150mL of NaCl solution and transferred to a beaker. Controlling the rotating speed of the peristaltic pump to be 30mL min-1After the conductivity of the solution is not changed, the power supply is switched on, and four groups of working voltages of 1.0V, 1.2V, 1.4V and 1.6V are respectively added. And (3) monitoring the conductivity in real time by using a conductivity meter, recording the conductivity once every 20s by using a universal meter, recording the instantaneous current by using the universal meter, and recording the current once every 2s by using the universal meter.
(2) Examples 32 to 36
The steps are the same as those in example 31, except that the MCDI electrodes are respectively changed to PC/rGO-20, PC/rGO-30, PC/rGO-40, PC, rGO.
(3) Example 37
The difference from the step 31 is that the initial TDS concentration is changed to 125mg L-1,250mg L-1,1000mg L-1Three independent experiments were performed.
(4) Example 38
The difference is that the anion-cation exchange membrane is removed from the solution 31.
3. Comparison and analysis of test results
FIG. 14 is PC/rGO-20 at 500mg L-1And testing by using CDI and MCDI devices respectively under the working voltage of 1.2V to obtain an electro-adsorption-desorption curve diagram. The MOF-derived porous carbon/graphene composite material has high specific surface area, high conductivity and abundant mesoporous structure, so that the composite material shows good performance in CDI (18.10mg g)-1) However, the large amount of microporous structure results in slow ion transport rate during desorption. The decrease in conductivity of the latter half of the curve during desorption may be the diffusion of ions from the pores into the solution, creating new vacancies at the sites of the pores, allowing the opportunity for ions in the solution to diffuse into them. In the MCDI process, the addition of the ion exchange membrane can block the counter charge ions from entering and exiting the electrode and greatly improve the regeneration capacity, and the electric adsorption capacity (30.30mg g) of the composite material-1) Compared with the CDI process, the current efficiency is also improved from 36.5 percent (CDI) to 66.7 percent (MCDI).
FIG. 15(a) is PC/rGO-20 at 500mg L-1The electrosorption-desorption graphs at the working voltages of 1.0V, 1.2V, 1.4V and 1.6V respectively show that ions in the solution are transferred to the surface of the porous material through electrostatic adsorption to form a double electron layer, so that the conductivity is reduced. When the electrode adsorption reaches saturation, reverse voltage is applied to the two ends of the electrode, and ions gradually return to the solution from the porous material. As the voltage increases, the adsorbed ions also increase significantly. Fig. 15(b) is a corresponding instantaneous current diagram showing the change in current in the circuit and the change in voltage across it over time, with the current showing a rising trend as the applied voltage rises.
FIG. 16(a) is a drawingAn electric adsorption-desorption curve chart of the PC/rGO-20 after 22 cycles, and an auxiliary graph is the electric adsorption capacity retention rate of the PC/rGO-20, and the capacity is hardly reduced after 22 cycles, which indicates that the PC/rGO-20 has good regeneration performance. FIG. 16(b) is a 125mg L PC/rGO-20, respectively-1,250mg L-1,500mg L-1,1000mg L-1And adding 1.0V, 1.2V, 1.4V and 1.6V to obtain an electric adsorption capacity graph. At the same voltage, the electric adsorption capacity is increased along with the increase of the initial concentration; at the same concentration, as the voltage increases, the electrosorption capacity increases. The test results indicate that the initial concentration and voltage are important factors affecting the electrosorption capacity.
FIG. 17(a, b, c) is PC/rGO-10, PC/rGO-30, PC/rGO-40 at 500mg L-1The electric adsorption-desorption curves are respectively under the working voltages of 1.0V, 1.2V, 1.4V and 1.6V. The results are similar to those in FIG. 15 (a). FIG. 17(d, e, f) is a corresponding instantaneous current plot, similar to the results of FIG. 15 (b).
FIG. 18 shows PC/rGO-10, PC/rGO-20, PC/rGO-30, PC/rGO-40 at 500mg L-1The electric adsorption capacity under the working voltage of 1.0V, 1.2V, 1.4V and 1.6V respectively. It was observed that the electrosorption capacity increased and then decreased with increasing amounts of Fe-MOF, peaking at PC/rGO-20. The specific surface area and conductivity of the composite material are directly affected by the self-aggregation of the MOF and the aggregation between graphene sheets, and the performance of the MCDI is indirectly affected. In the PC/rGO-10 sample, the graphene itself is heavily agglomerated, and the MCDI performance is greatly different than that of the PC/rGO-20. Too much of the MOF-derived porous carbon structure in the PC/rGO-30, PC/rGO-40 samples agglomerated by itself and therefore also did not perform as well as the PC/rGO-20, but since the PC/rGO-30 samples agglomerated less than the PC/rGO-40, it showed a second only performance in the MCDI test than the PC/rGO-20, which is also demonstrated in the BET results. In addition, in the MOF-derived porous carbon/graphene composite material structure with the PC/rGO-20 ratio, the SEM image (5) can also directly observe that the MOF is hardly agglomerated, and the three-dimensional structure of the graphene is regular and ordered and keeps complete.
FIG. 19 is a graph of the electro-adsorption-desorption curves for PC/rGO-20, PC, rGO at 1.2V operating voltage. Calculated adsorption of PC/rGO-20 (30mg g)-1) Higher than PC (23.72mg g)-1) And rGO (14.58mg g)-1). The performance of the PC/rGO-20 is improved because the self three-dimensional structure of graphene in a sample limits self agglomeration and the MOF is uniformly dispersed in the three-dimensional structure of the graphene, so that the specific surface area (BET) is greatly improved by the structurePC/rGO-20=749m2 g-1,BETPC=558m2 g-1) The number of mesopores capable of effectively transferring the electrolyte is increased. Meanwhile, the three-dimensional structure of the graphene endows good conductivity, and the factors are synergistic to enable the PC/rGO-20 to show MCDI performance exceeding that of other proportion samples, PC and rGO.

Claims (12)

1. A preparation method of an MOF-derived porous carbon/graphene composite electrode material comprises the following steps:
(1) adding graphene or graphene oxide dispersion liquid into a clean container, adding MOFs crystal powder into the clean container, enabling the feeding mass ratio of the MOFs crystal powder to the graphene or graphene oxide to be 0.5-50: 1, sealing the container, placing the sealed container on a vortex mixer for continuous mixing, adjusting the rotating speed from zero to the highest rotating speed in the mixing process, keeping the rotating speed for a certain time, adjusting the rotating speed from the highest rotating speed to 0, adjusting the rotating speed from 0 to the highest rotating speed, keeping the rotating speed for a certain time, repeating the speed adjusting process, enabling a sample to fully vibrate in the vertical direction, promoting the formation of a self-assembled three-dimensional structure of graphene sheets or graphene oxide sheets and the full and uniform dispersion of MOFs crystals in a three-dimensional framework structure, and obtaining graphene/MOF porous hydrogel, finally freezing and drying to obtain the graphene/MOF porous aerogel, wherein the graphene/porous MOF aerogel has a self-supporting porous structure, the integrity of the graphene or graphene oxide and MOFs structures is reserved;
(2) mixing graphene/MOF porous composite aerogel in N2Keeping for 1-2 h after the temperature rises to 600-800 ℃ at the rate of 5-10 ℃/min in the atmosphere, and naturally cooling to obtain a carbonized product;
(3) and (3) carrying out acid treatment on the carbonized product to remove metal and metal oxide, and finally obtaining the MOF-derived porous carbon/graphene composite electrode material.
2. The method of claim 1, wherein: in the step (1), the feeding mass ratio of the MOFs crystal powder to the graphene oxide is 1: 1-40: 1.
3. The method of claim 2, wherein: in the step (1), the MOFs crystal is Fe-MOF crystal, and the charging mass ratio of Fe-MOFs crystal powder to graphene oxide is 10: 1-40: 1.
4. The method of claim 3, wherein: in the step (1), the feeding mass ratio of Fe-MOFs crystal powder to graphene oxide is 20: 1.
5. The method of claim 2, wherein: in the step (1), the MOFs crystal is a ZIF-8 crystal, and the feeding mass ratio of ZIF-8 crystal powder to graphene oxide is 1: 1-8: 1.
6. The method of claim 5, wherein: in the step (1), the feeding mass ratio of the ZIF-8 crystal powder to the graphene oxide is 4: 1.
7. The method according to any one of claims 1 to 6, wherein: in the step (1), in the mixing process, the rotating speed knob is adjusted from 0 to 2800r/min, stays for 5-15 s and then is adjusted to 0, then is adjusted from 0 to the highest rotating speed, stays for 5-15 s, the speed adjusting process is repeated all the time, and the time of the whole mixing process lasts for 1-10 min.
8. The method of claim 7, wherein: the whole mixing process lasts for 4-5 min.
9. The method according to any one of claims 1 to 6, wherein: in the step (1), the dispersion mode of the MOFs crystals in the three-dimensional frame structure is selected from one or a combination of two of the following modes: the MOFs crystal is uniformly attached to the surface of the graphene sheet or the graphene oxide sheet, and the MOFs crystal is coated by the graphene sheet or the graphene oxide sheet.
10. The method according to any one of claims 1 to 6, wherein: in the step (2), the calcination temperature is 800 ℃.
11. The method according to any one of claims 1 to 6, wherein: in the step (2), the acid used for the acid treatment is hydrochloric acid or nitric acid.
12. The MOF-derived porous carbon/graphene composite electrode material prepared by the preparation method according to claim 1 is applied to capacitive deionization desalination or capacitive deionization desalination.
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