CN115725084B - Flake nickel-cobalt bimetallic organic framework crystal material and preparation method thereof - Google Patents

Abstract

The invention discloses a flaky nickel-cobalt bimetallic organic framework crystal material and a preparation method thereof. The preparation method comprises the following steps: dissolving soluble salts of nickel and cobalt, terephthalic acid and polyvinyl pyrrolidone in a dimethyl imidazole solution, soaking an activated carbon cloth in the mixed solution after being uniformly dissolved, and performing a hydrothermal reaction in a closed reactor; after the hydrothermal reaction, the mixture is naturally cooled to room temperature, and after washing and drying, a flaky nickel-cobalt bimetallic organic framework crystal loaded on the surface of the activated carbon cloth is obtained. The method has simple process, convenient operation and low cost, and the obtained flaky crystals are evenly distributed. The flaky structure increases the specific surface area of the crystal particles, and the increase in the surface area can expand the contact area with the electrolyte and accelerate the transmission of electrolyte ions, and has broad application prospects in the field of new energy materials.

Description

A flaky nickel-cobalt bimetallic organic framework crystal material and preparation method thereof

Technical Field

The invention belongs to the technical field of supercapacitor electrode material preparation, and specifically relates to a flaky nickel-cobalt bimetallic organic framework crystal material and a preparation method thereof.

Background technique

Metal-organic framework (MOF) materials have attracted extensive attention in the field of supercapacitor electrode materials due to their high surface area, rich active sites, diverse structures and easy control [Wang Y, et al.ACS Applied Energy Materials, 2019, 2(3)]. Studies have found that sheet-like metal-organic framework materials can increase the specific surface area of the material, expand the contact area with the electrolyte, and accelerate the transmission of electrolyte ions. The synergistic effect between the two metal ions has a stronger electron storage capacity than that of a single metal [Chen C, et al.2018,47,5639-5645], showing better electrochemical performance.

The existing synthesis routes of metal organic framework materials usually use hydrothermal or solvothermal methods. Thanks to the high temperature and high pressure characteristics of hydrothermal reactions, the obtained metal organic framework materials have uniform morphological characteristics. Gao et al. synthesized dandelion-shaped NiCo-MOF by hydrothermal method. Cobalt ions partially replaced nickel ions in the Ni-MOF structure, which not only reduced the crystallinity but also changed the structure of MOFs from bayberry-shaped to dandelion-shaped. Lower crystallinity enhances conductivity, and the nanorods dispersed on the surface of dandelion-shaped balls are conducive to ion transport, which enhances the capacitance performance [Gao S, et al. Journal of Colloid and Interface Science, 2018, 531: 83-90]. Zhang et al. synthesized a sheet-like nickel-cobalt bimetallic organic framework. The prepared sheet-like NiCo-MOF electrode has high specific capacitance, good rate performance and long-term cycle stability [Zhang X, et al. Journal of Materials Science: Materials in Electronics, 2020, 27 (4)]; Habib Gholipour-Ranjbar et al. used pyrazine solution as both solvent and ligand to synthesize a nickel-cobalt-based bimetallic organic framework by hydrothermal method. The prepared bimetallic organic framework has good rate performance and cycle retention rate. Compared with Ni-MOF, NiCo-MOF has lower charge transfer resistance and ion diffusion barrier [Gholipour-Ranjbar H et al. New Journal of Chemistry, 2016: 10.1039.C6NJ01449F].

Currently, most of the research focuses on the nickel-cobalt bimetallic organic frameworks prepared by single ligands, and there are few reports on the nickel-cobalt bimetallic organic frameworks combined with dual ligands. In addition, most of the prepared bimetallic organic framework materials and their composite materials are in powder form, and binders are required to prepare electrode materials when used, which greatly reduces the conductivity of the electrode system.

Summary of the invention

In order to solve the problems in the background technology, the present invention provides a sheet-like nickel-cobalt bimetallic organic framework crystal material and a preparation method thereof. The preparation method makes full use of the bidirectional synergistic effect between bimetallic nickel ions and cobalt ions and between the biligands, and can directly grow sheet-like nickel-cobalt bimetallic organic framework crystals on the surface of activated carbon cloth through the high temperature and high pressure effect of hydrothermal reaction and the regulation of various reaction parameters. The obtained sheet-like composite material has low density, high specific surface area and good conductivity, and has a good advantage in the application of supercapacitor electrode materials.

The technical solution adopted by the present invention is as follows:

1. A sheet-like nickel-cobalt bimetallic organic framework crystal material

Using dimethylimidazole and terephthalic acid as mixed organic ligands, nickel nitrate hexahydrate and cobalt nitrate hexahydrate as reactants, flaky nickel-cobalt bimetallic organic framework crystals loaded on the surface of activated carbon cloth are obtained through hydrothermal reaction; the nickel-cobalt bimetallic organic framework crystals present a uniform flaky structure with an average thickness of 0.15-0.20μm.

2. A method for preparing a sheet-like nickel-cobalt bimetallic organic framework crystal material

The following steps are involved:

1) dissolving nickel nitrate hexahydrate and cobalt nitrate hexahydrate in a dimethylimidazole solvent, and stirring to obtain a mixed solution A;

2) Add terephthalic acid and polyvinyl pyrrolidone to the mixed solution A, and continue stirring to obtain a mixed solution B; the amount of polyvinyl pyrrolidone used is 1.5 g;

3) Soak the treated activated carbon cloth in mixed solution B and perform ultrasonic treatment for 1 hour;

4) The solution of step 3) and the activated carbon cloth were subjected to a hydrothermal reaction in a 100 mL reactor;

5) After the reaction in step 4) is completed, the reactor is naturally cooled to room temperature, and is repeatedly washed three times with anhydrous ethanol and deionized water. After drying, flaky nickel-cobalt bimetallic organic framework crystals uniformly grown on the surface of the activated carbon cloth are obtained.

In the step 1):

The molar mass ratio of nickel nitrate hexahydrate to cobalt nitrate hexahydrate is 1:1;

The molar concentration of the dimethylimidazole solvent is 0.2 mol/L, and the amount used is 30 mL.

The total molar concentration of nickel ions and cobalt ions in the mixed solution A is 0.06 mol/L-0.30 mol/L.

The volume ratio of the terephthalic acid in step 2) to the dimethylimidazole in step 1) is 1:1.

The total molar ratio of the total amount of metal salt ions to the mixed ligands is 1:1-5:1; the metal salt ions are nickel ions and cobalt ions; and the mixed ligands are terephthalic acid and dimethylimidazole.

In the step 3), the activated carbon is treated by treating the activated carbon cloth with acetone and ethanol to remove surface grease.

In the step 3), the activated carbon cloth is a 1 cm×2 cm hydrophilic commercial carbon cloth with a thickness of 0.36 mm and a longitudinal resistance of less than 0.12×10 ^-2 Ω. It is made of pre-oxidized polyacrylonitrile fiber fabric by carbonization and has the characteristics of good conductivity, flexibility and large specific surface area.

In the step 4), the reaction temperature of the hydrothermal reaction is 150° C.-180° C., and the reaction time is 12 h-18 h.

In the step 5), the drying temperature is 80° C.-100° C., and the drying time is 12 h-24 h.

The present invention adopts a mild, low-energy, fast and safe hydrothermal synthesis method, selects soluble nickel nitrate hexahydrate and cobalt nitrate hexahydrate as reactants, uses dimethylimidazole and terephthalic acid as mixed organic ligands, and obtains a nickel-cobalt bimetallic organic framework crystal material that grows self-supportingly on the surface of activated carbon cloth by adjusting reaction parameters.

The beneficial effects of the present invention are:

1) The wavy flake structure crystals prepared by the present invention have uniform size distribution, good crystallinity, and uniform chemical composition. When the scanning window is 0-0.6V and the current density is 2mA/ ^cm2 , the specific capacitance of the prepared NiCo-MOF composite electrode material reaches 834F/g, which has high specific capacity and rate performance.

2) The present invention uses carboxylic acid compounds terephthalic acid and dimethylimidazole as mixed organic ligands:

The carboxylic acid compound terephthalic acid, also known as p-phthalic acid, is a dibasic aromatic carboxylic acid formed by two carboxyl groups connected to two opposite carbon atoms in the benzene ring. It has the following two functions: First, it reacts with metal ions as the first organic ligand. Terephthalic acid contains two diagonal carboxylic acid groups. The carboxylic acid groups can form a uniform network with nickel and cobalt ions. The uniform connection between metal ions and carboxylic acid groups leads to longer chain lengths and increased surface area and porosity. Second, it reacts synergistically with the second ligand dimethylimidazole to form a mixed network structure. The carboxylic acid groups and carbon-containing skeletons in terephthalic acid can bridge with the nitrogen-containing active sites in dimethylimidazole, which can effectively increase the coordination sites of organic ligands and metal ions, thereby increasing the redox active sites.

Dimethylimidazole has the following three functions: first, as a reaction solvent, it promotes the full dissolution of nickel and cobalt metal salts and organic ligands; second, as a second organic ligand to react with metal ions, the dimethylimidazole molecule has two functional (nitrogen) sites that can coordinate with metal ions to form a network; third, it synergistically reacts with the first organic ligand terephthalic acid, and the carboxylic acid group in terephthalic acid and the functional nitrogen group in dimethylimidazole can form a mixed coordination with metal ions to guide the microscopic arrangement of atoms. Therefore, a uniform network structure can be formed by adjusting the molar ratio of terephthalic acid and dimethylimidazole. The bimetallic organic framework crystal obtained by coordination is affected by the synergistic effect of the two ligands and has a controllable molecular morphology and structure. The synergistic effect of the two ligands in the present invention forms a network structure, inducing the generation of a two-dimensional sheet morphology. The sheet structure can increase the contact area between the electrode material and the electrolyte solution, accelerate the transmission of ions, and thus improve the performance. The preparation method is simple and easy to operate, and is expected to be applied in the fields of new electrode materials, catalysis, and sensing.

3) The present invention uses activated carbon cloth as the base material. Activated carbon cloth is an inert material. Compared with other conductive substrates such as nickel foam, it is more flexible, non-corrosive, non-rusting, and not easily corroded by harmful media. It has good durability and has advantages in current conduction. The present invention makes full use of the porous structure on the surface of activated carbon cloth fibers to increase the contact area with the electrolyte, increase the pressure on the contact surface, and reduce its contact resistance and other special functions, providing good conditions for the crystallization of metal organic framework crystals on the surface of carbon cloth, while using the excellent conductive properties of activated carbon cloth to improve the disadvantage of low conductivity of metal organic framework materials.

4) The present invention uses polyvinyl pyrrolidone as a surfactant. Since NiCo-MOF nanoparticles have a small particle size, high surface energy and are easy to agglomerate, adding polyvinyl pyrrolidone and wrapping them on the surface of activated carbon cloth can hinder agglomeration and control the shape of the material. At the same time, polyvinyl pyrrolidone plays a role in dispersing ions and stabilizing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an XRD pattern of the product obtained in Example 1.

FIG. 2 is an electron microscope photograph of the product obtained in Example 1.

FIG. 3 is an electron microscope photograph of the product obtained in Example 2.

FIG. 4 is an electron microscope photograph of the product obtained in Example 3.

FIG5 is an electron microscope photograph of the product obtained in Comparative Example 1.

FIG6 is an electron microscope photograph of the product obtained in Comparative Example 2.

FIG. 7 is an electron microscope photograph of the product obtained in Comparative Example 3.

FIG8 is an electron microscope photograph of the product obtained in Comparative Example 4.

FIG. 9 is an electron microscope photograph of the product obtained in Comparative Example 5.

Detailed ways

The present invention prepares a bimetallic organic framework crystal material that grows self-supportingly on the surface of activated carbon cloth by using a dual ligand. Activated carbon cloth is a common conductive base that has the function of a current collector material and has the characteristics of flexibility and a large specific surface area. The self-supporting generation of a metal organic framework on its surface can construct a composite electrode material, effectively improving the electrochemical performance of the electrode material. The synergistic effect of the dual ligands, the flaky morphology of the crystal material, the self-supporting structure design and the construction of a binder-free system are all conducive to improving the electrochemical performance.

The nickel-cobalt bimetallic organic framework crystal prepared by the present invention presents a uniform lamellar structure with an average thickness of 0.15-0.20 μm. The lamellar structure increases the contact area with the electrolyte, accelerates the ion transmission rate, and thus improves the performance.

Embodiment 1:

0.2910g of cobalt nitrate hexahydrate, 0.2910g of nickel nitrate hexahydrate and 0.1500g of terephthalic acid were dissolved in 30mL of dimethylimidazole solution. 1.5000g of polyvinyl pyrrolidone was added to the mixed solution and magnetically stirred for 30min to obtain a uniform solution. The treated activated carbon cloth was then immersed in the mixed solution for 1h, and the mixed uniform solution and the activated carbon cloth were poured into a 100mL reactor and reacted at 150°C for 12h. After the reaction, the reactor was naturally cooled to room temperature. The activated carbon cloth was repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove impurities such as surface adsorbed ions and attachments, and dried at 80°C for 12h to obtain a sheet-like nickel-cobalt bimetallic organic framework crystal loaded on the activated carbon cloth.

Figure 1 is an XRD spectrum of the product of Example 1, which matches the NiCO-MOF (No. 638866) simulated in the CCDC crystal library, proving that the in-situ growth on the surface of the activated carbon cloth is a nickel-cobalt bimetallic organic framework crystal, and the diffraction peaks appearing between 2θ of 20° and 30° correspond to the (001) and (110) crystal planes of the graphite crystal, indicating that the hydrophilic commercial carbon cloth is a graphite-like structure; Figure 2 is an electron microscope photograph of the product of Example 1, showing a uniform lamellar structure formed on the surface of the activated carbon cloth with a lamellar thickness of 100-200nm.

Embodiment 2:

0.5810g of cobalt nitrate hexahydrate, 0.5810g of nickel nitrate hexahydrate and 0.1500g of terephthalic acid were dissolved in 30mL of dimethylimidazole solution. 1.5000g of polyvinyl pyrrolidone was added to the mixed solution and magnetically stirred for 30min to obtain a uniform solution. The treated activated carbon cloth was then immersed in the mixed solution for 1h, and the mixed uniform solution and the activated carbon cloth were poured into a 100mL reactor and reacted at 150°C for 16h. After the reaction, the reactor was naturally cooled to room temperature. The obtained activated carbon cloth was repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove impurities such as surface adsorbed ions and attachments, and dried at 80°C for 12h to obtain a sheet-like nickel-cobalt bimetallic organic framework crystal loaded on the activated carbon cloth.

FIG3 is an electron microscope photograph of the product of Example 2. A uniform flaky structure is formed on the surface of the activated carbon cloth, and the flaky structure has a thickness of 100-150 nm.

Embodiment 3:

1.4530g of cobalt nitrate hexahydrate, 1.4530g of nickel nitrate hexahydrate and 0.1500g of terephthalic acid were dissolved in 30mL of dimethylimidazole solution. 1.5000g of polyvinyl pyrrolidone was added to the mixed solution and magnetically stirred for 30min to obtain a uniform solution. The treated activated carbon cloth was then immersed in the mixed solution for 1h, and the mixed uniform solution and the activated carbon cloth were poured into a 100mL reactor and reacted at 180°C for 18h. After the reaction, the reactor was naturally cooled to room temperature. The obtained activated carbon cloth was repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove impurities such as surface adsorbed ions and attachments, and dried at 80°C for 12h to obtain flaky nickel-cobalt bimetallic organic framework crystals loaded on the activated carbon cloth.

FIG4 is an electron microscope photograph of the product of Example 3. A uniform flaky structure is formed on the surface of the activated carbon cloth, and the flaky structure has a thickness of 200-300 nm.

Comparative Example 1:

0.5810 g of cobalt nitrate hexahydrate, 0.9880 g of nickel nitrate hexahydrate and 0.1500 g of terephthalic acid were dissolved in 30 mL of dimethylimidazole solution. 1.5000 g of polyvinyl pyrrolidone was added to the mixed solution and magnetically stirred for 30 min to obtain a uniform solution. The treated activated carbon cloth was then immersed in the mixed solution for 1 h of ultrasound, and the mixed uniform solution and the activated carbon cloth were poured into a 100 mL reactor and reacted at 180 ° C for 18 h. After the reaction was completed, the reactor was naturally cooled to room temperature, and the obtained activated carbon cloth was repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove surface adsorbed ions, attachments and other impurities, and dried at 80 ° C for 12 h to obtain the final product.

FIG5 is an electron microscope photograph of the product of Example 1. The surface of the activated carbon cloth does not form a flaky structure but is amorphous.

Comparative Example 2:

Dissolve 0.5810g of cobalt nitrate hexahydrate, 0.5810g of nickel nitrate hexahydrate and 0.5000g of terephthalic acid in 30mL of dimethylimidazole solution. Add 1.5000g of polyvinyl pyrrolidone to the mixed solution and stir magnetically for 30min to obtain a uniform solution. Then immerse the treated activated carbon cloth in the mixed solution for 1h, pour the mixed uniform solution together with the activated carbon cloth into a 100mL reactor, react at 150℃ for 12h, and after the reaction, let the reactor cool naturally to room temperature. The obtained activated carbon cloth is repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove impurities such as surface adsorbed ions and attachments, and dried at 80℃ for 12h to obtain the final product.

FIG6 is an electron microscope photograph of the product of Example 2. The surface of the activated carbon cloth does not form a flaky structure but is amorphous.

Comparative Example 3:

0.5810 g of cobalt nitrate hexahydrate, 0.581 g of nickel nitrate hexahydrate and 0.15 g of terephthalic acid were dissolved in 30 mL of dimethyl imidazole solution, 1.5000 g of polyvinyl pyrrolidone was added to the mixed solution, and magnetic stirring was performed for 30 min to obtain a uniform solution. Subsequently, the treated activated carbon cloth was immersed in the mixed solution for 1 h of ultrasound, and the mixed uniform solution and the activated carbon cloth were poured into a 100 mL reactor, and reacted at 120 ° C for 8 h. After the reaction, the reactor was naturally cooled to room temperature, and the obtained activated carbon cloth was repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove surface adsorbed ions, attachments and other impurities, and dried at 80 ° C for 12 h to obtain the final product.

FIG7 is an electron microscope photograph of the product of Example 3. The surface of the activated carbon cloth does not form a flaky structure but is amorphous.

Comparative Example 4:

0.5810 g of cobalt nitrate hexahydrate, 0.581 g of nickel nitrate hexahydrate and 0.15 g of terephthalic acid were dissolved in 30 mL of dimethyl imidazole solution and magnetically stirred for 30 min to obtain a uniform solution. The treated activated carbon cloth was then immersed in the mixed solution for 1 h of ultrasound, and the mixed uniform solution and the activated carbon cloth were poured into a 100 mL reactor and reacted at 150 ° C for 12 h. After the reaction, the reactor was naturally cooled to room temperature, and the obtained activated carbon cloth was repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove impurities such as surface adsorbed ions and attachments, and dried at 80 ° C for 12 h to obtain the final product.

FIG8 is an electron microscope photograph of the product of Example 4. The surface of the activated carbon cloth does not form a flaky structure but is amorphous.

Comparative Example 5:

0.5810 g of cobalt nitrate hexahydrate and 0.581 g of nickel nitrate hexahydrate were dissolved in 30 mL of dimethyl imidazole solution, 1.5000 g of polyvinyl pyrrolidone was added to the mixed solution, and magnetic stirring was performed for 30 min to obtain a uniform solution. Subsequently, the treated activated carbon cloth was immersed in the mixed solution for 1 h of ultrasound, and the mixed uniform solution and the activated carbon cloth were poured into a 100 mL reactor, and reacted at 150 ° C for 12 h. After the reaction, the reactor was naturally cooled to room temperature, and the obtained activated carbon cloth was repeatedly centrifuged and washed three times with anhydrous ethanol and deionized water to remove surface adsorbed ions, attachments and other impurities, and dried at 80 ° C for 12 h to obtain the final product.

FIG9 is an electron microscope photograph of the product of Example 5. The surface of the activated carbon cloth does not form a flaky structure but is amorphous.

Claims (6)

1. A preparation method of a flaky nickel-cobalt bimetallic organic frame crystal material is characterized in that dimethyl imidazole and terephthalic acid are used as mixed organic ligands, nickel nitrate hexahydrate and cobalt nitrate hexahydrate are used as reactants, and the flaky nickel-cobalt bimetallic organic frame crystal loaded on the surface of activated carbon cloth is obtained through hydrothermal reaction; the nickel cobalt bimetal organic framework crystal presents a uniform flaky structure, and the average thickness of the nickel cobalt bimetal organic framework crystal is 0.15-0.20 mu m;

The preparation method comprises the following steps:

1) Dissolving nickel nitrate hexahydrate, cobalt nitrate hexahydrate and terephthalic acid in a dimethyl imidazole solvent, and uniformly stirring to obtain a mixed solution A;

2) Adding polyvinylpyrrolidone into the mixed solution A, and continuously stirring to obtain a mixed solution B;

3) Immersing the treated activated carbon cloth in the mixed solution B for ultrasonic treatment of 1 h;

4) Carrying out hydrothermal reaction on the solution obtained in the step 3) and the activated carbon cloth in a reaction kettle;

5) Naturally cooling the reaction kettle after the reaction in the step 4) to normal temperature, repeatedly cleaning for three times by absolute ethyl alcohol and deionized water, and drying to obtain flaky nickel-cobalt bimetal organic framework crystals uniformly growing on the surface of the activated carbon cloth;

in the step (1) of the above-mentioned process,

The molar mass ratio of the nickel nitrate hexahydrate to the cobalt nitrate hexahydrate is 1:1;

The molar concentration of the dimethylimidazole solvent is 0.2 mol/L, and the dosage is 30 mL;

The amount of terephthalic acid used was 0.15g;

In the step 4), the reaction temperature of the hydrothermal reaction is 150-180 ℃ and the reaction time is 12 h-18 h.

2. The method according to claim 1, wherein in the step 1), the total molar concentration of nickel ions and cobalt ions in the mixed solution A is 0.06 mol/L to 0.30 mol/L.

3. The method of claim 1, wherein the total molar ratio of the total mass of metal salt ions to the mixed ligand is 1:1 to 5:1;

the metal salt ions are nickel ions and cobalt ions;

The mixed ligand is terephthalic acid and dimethyl imidazole.

4. The method according to claim 1, wherein in the step 3), the activated carbon cloth is treated by: and treating the activated carbon cloth with acetone and ethanol to remove surface grease.

5. The method according to claim 1, wherein in the step 3), the activated carbon cloth has a longitudinal resistance of less than 0.12×10 ^-2 Ω and is formed by carbonizing a pre-oxidized polyacrylonitrile fiber fabric.

6. The method according to claim 1, wherein in the step 5), the drying temperature is 80 ℃ to 100 ℃ and the drying time is 12 h to 24 h.