Mof@pom composite electrocatalyst and preparation method thereof

Abstract

The application relates to a MOF@POM composite electrocatalyst and a preparation method thereof. The preparation method of the MOF@POM composite electrocatalyst comprises the following steps: (1) after adding an alkali metal salt into a water solution rich in POM anions, cooling crystallization is carried out to obtain a water-soluble POM powder material; (2) the obtained water-soluble POM powder material is dissolved in water, cooling recrystallization and ion exchange are carried out, and then washing and drying are carried out to obtain a POM material dissolved in an organic phase; (3) the obtained POM material dissolved in the organic phase is dissolved in a mixed solvent, then a MOF precursor material and foamed nickel are added, and then a solvothermal reaction is carried out to obtain the MOF@POM composite electrocatalyst.

Description

Technical Field

[0001] This invention relates to a catalyst, specifically to a method for preparing an electrocatalyst of MOF@POM composite grown in situ on nickel foam and its application in water oxidation, belonging to the field of chemical engineering. Background Technology

[0002] In recent years, due to rapid economic development, energy consumption has been increasing, and the overconsumption of fossil fuels as the primary energy source has caused various environmental problems. Faced with these emerging environmental issues, there is an urgent need to develop and explore new energy sources, transforming traditionally non-renewable energy systems into sustainable energy storage and supply systems. Since water, a byproduct of the complete combustion of hydrogen, does not cause any environmental pollution, hydrogen energy can be used to replace increasingly depleted fossil fuels such as coal, oil, and natural gas. Therefore, using electricity to decompose water to produce hydrogen is an important way to solve the current energy and environmental crisis facing humanity. However, the low hydrogen conversion efficiency severely restricts the industrialization of hydrogen production through water electrolysis. Therefore, developing a highly efficient water electrolysis system is of great significance, laying a solid foundation for the eventual industrial application of water electrolysis. In the process of hydrogen evolution through water electrolysis, the oxygen evolution reaction (OER) at the anolyte presents significant thermodynamic and kinetic challenges, greatly limiting the progress of the entire reaction.

[0003] Polyoxometalates (POMs) are typically discrete anionic metal oxide clusters composed of group V or VI transition metals in their highest oxidation state. Due to their high solubility, low specific surface area, and instability under reaction conditions, POMs generally exhibit poor processability. To overcome these undesirable properties, significant efforts have been made to improve the stability of POMs under heterogeneous catalytic conditions. Metal-organic frameworks (MOFs) provide a crystalline, multifunctional platform for depositing unstable POMs to improve catalytic performance while balancing desired properties such as porosity, substrate diffusion, or stability. MOFs combined with polyacids, i.e., polyacid-based hybrid materials, have a wider range of applications than MOFs or polyacids alone. However, the synthesis of MOFs combined with polyacids faces numerous challenges, such as complex synthesis methods, low yields, and unsatisfactory long-term stability, hindering practical applications. Therefore, exploring simple, economical, mild synthetic conditions and easily implemented synthetic processes is of great significance for MOF@POM materials. Summary of the Invention

[0004] In order to solve the technical problems of low catalytic activity, high price, poor stability and complex process of the catalyst in the prior art, the application provides a MOF@POM composite electrocatalyst and a preparation method and application thereof.

[0005] In one aspect, the application provides a preparation method of a MOF@POM composite electrocatalyst, comprising the following steps:

[0006] (1) cooling crystallization after adding an alkali metal salt in a water solution rich in POM anions (at this time, ion exchange is not performed), to obtain a water-soluble POM powder material;

[0007] (2) dissolving the obtained water-soluble POM powder material in water, performing cooling recrystallization and ion exchange, and then performing washing and drying, to obtain a POM material dissolved in an organic phase (wherein the precipitated cation of the water-soluble POM is generally an alkali metal K or Na, and the solubility in an organic solvent is very small; the cation of the POM after cation exchange is generally 4-butyl ammonium bromide, which is easily soluble in an organic solvent);

[0008] (3) dissolving the obtained POM material dissolved in an organic phase in a mixed solvent, then adding a MOF precursor material and foamed nickel, and then performing a solvothermal reaction, to obtain the MOF@POM composite electrocatalyst.

[0009] Preferably, the alkali metal salt is at least one of a sodium salt and a potassium salt, and is preferably at least one of sodium chloride and potassium chloride; the cooling crystallization temperature is 0-40 DEG C; the concentration of the POM material water solution is 50-200 g / L; and the ratio of the alkali metal salt to the POM material water solution is (100-1000) g: 1 L.

[0010] Preferably, the POM material is selected from one of Ni-POM, Co-POM and Fe-POM; and the preparation method of the water solution rich in POM anions comprises the following steps: weighing tungstate, phosphate and transition metal salt, dissolving them in water, using an acid solution to adjust the pH to 6-8, to obtain a mixed solution; transferring the obtained mixed solution into a flask, and using an oil bath reflux device to perform reflux reaction, to obtain the water solution rich in POM anions.

[0011] Further preferably, the transition metal salt is selected from at least one of a nitrate of a transition metal element and a nitrate of a transition metal element, and is preferably at least one of nickel nitrate hexahydrate, cobalt nitrate hexahydrate, copper nitrate nonahydrate, manganese nitrate tetrahydrate, nickel acetate, cobalt acetate, copper acetate and manganese acetate;

[0012] The tungstate is at least one of sodium tungstate dihydrate, potassium tungstate and zinc tungstate;

[0013] The phosphate is at least one of disodium hydrogen phosphate, sodium dihydrogen phosphate and potassium dihydrogen phosphate.

[0014] The acid solution is hydrochloric acid or nitric acid; the concentration of the acid solution is 1-12M.

[0015] Preferably, the mass ratio of the tungstate, phosphate and transition metal salt is (100-800):(10-100):(5-100).

[0016] Preferably, the temperature of the reflux reaction is 80-150℃, and the time is 1-5 hours.

[0017] Preferably, the MOF precursor comprises a cobalt salt, an iron salt and an organic ligand; the mixed solvent is at least two of DMF, ethanol, DMA, acetone and water.

[0018] Preferably, the ratio of the POM material dissolved in the organic phase and the mixed solvent is (1-5)g:1L.

[0019] The cobalt salt is cobalt chloride hexahydrate or / and cobalt nitrate hexahydrate, and the concentration of the cobalt salt and the mixed solvent is (1-5)g:1L.

[0020] The iron salt is ferric chloride hexahydrate or / and ferric nitrate nonahydrate, and the ratio of the concentration of the iron salt and the mixed solvent is (6.76-10.14)g:1L.

[0021] The organic ligand is at least one of 1,4-terephthalic acid, benzoic acid, trimesic acid and 2-amino terephthalic acid, and the ratio of the organic ligand and the mixed solvent is (3-10)g:1L.

[0022] Preferably, the temperature of the water-soluble POM powder material dissolved in water is 80-100℃. The temperature of the cooling recrystallization and ion exchange is 0-40℃. The temperature of the solvothermal reaction is 80-150℃, and the reaction time is 10-15 hours.

[0023] In another aspect, the present application provides a MOF@POM composite electrocatalyst prepared according to the above preparation method, wherein the BET of the MOF material in the MOF@POM composite electrocatalyst is <600m 2 / g, and the MOF material has a micropore adsorption characteristic curve in the adsorption-desorption curve.

[0024] In still another aspect, the present application further provides a use of a MOF@POM composite electrocatalyst in water oxidation.

[0025] Advantages:

[0026] (1) The present application adopts the synthesis of POM materials under normal pressure, and ion exchange is carried out, so that it can be dissolved in the synthesis solvent of MOF materials, and then react with metal salt and ligand in the synthesis process of MOF to generate MOF@POM composite electrocatalyst with two-dimensional morphology, which has the characteristics of simple process, mild reaction condition, cheap raw material, strong repeatability, etc.

[0027] (2) The MOF@POM composite electrocatalyst prepared by the present application has a large specific surface area, and the active sites are directly exposed to the surface, which reduces the mass transfer resistance, and thus exhibits higher oxygen evolution activity than commercial IrO2, and can maintain stable performance under long-time large current, and has wide application prospect in the field of electrocatalysis. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The scanning electron microscope photo of the MOF@POM composite electrocatalyst prepared in Example 1 can be clearly seen that the MOF@POM composite electrocatalyst has uniform morphology, and from the figure it can be seen that the micro-morphology of the MOF@POM electrocatalyst is a two-dimensional nanocluster composed of secondary nanosheets.

[0029] Figure 2 The scanning electron microscope photo of the MOF@POM composite electrocatalyst prepared in Example 1 can be clearly seen that the MOF@POM composite electrocatalyst has uniform morphology, and from the figure it can be seen that the micro-morphology of the MOF@POM electrocatalyst is a two-dimensional nanocluster composed of secondary nanosheets.

[0030] Figure 3 The polarization curve graph of the MOF@POM composite electrocatalyst prepared in Example 1 can be seen from the figure that compared with the used substrate material, the MOF@POM catalyst shows excellent catalytic activity, and only 1.47V is required to reach the current density of 100mAcm -2 ;

[0031] Figure 4 The stability graph of the MOF@POM composite electrocatalyst prepared in Example 1 can be seen from the figure that in the stability test of large current, the MOF@POM catalyst can maintain the stability of activity in the catalytic process of at least 1000h. DETAILED DESCRIPTION

[0032] The present application is further illustrated by the following embodiments, which should be understood as merely illustrating the present application, but not limiting the present application.

[0033] The present application provides a specific kind of metal-organic framework (Metal Organic Frameworks, MOF) and polyoxometalate (polyoxometalate, POM) material composite water oxidation catalyst and its preparation method.

[0034] The innovation of the present application is that: by means of POM dissolution to create an acidic environment, the number of effective active sites can be increased after combining with MOF, and the POM molecules can play a role in stabilizing the MOF molecules, so that the stability of the MOF in the catalytic process is improved. The method has the characteristics of simple process, low price, mild reaction conditions, strong repeatability and the like. At the same time, the catalytic material shows high oxygen evolution activity and super-high stability, and has wide application prospect in the field of electrocatalysis.

[0035] The following exemplary describes the preparation method of a specific kind of MOF@POM composite catalyst.

[0036] Preparation of POM material soluble in organic phase.

[0037] Specifically, weigh tungstate such as sodium tungstate dihydrate, phosphate such as disodium hydrogen phosphate, and nickel nitrate hexahydrate (or cobalt nitrate hexahydrate, copper nitrate nonahydrate, manganese nitrate tetrahydrate, or use acetate instead of transition metal salt) are dissolved in water (20ml-100ml), and hydrochloric acid (nitric acid can be used instead, and the concentration of hydrochloric acid or nitric acid is 1-12M) is used to adjust the pH to neutral pH=6-8 to obtain a solution. Preferably, the concentration of sodium tungstate dihydrate in the obtained solution can be 100-800g / L, the concentration of disodium hydrogen phosphate can be 10-100g / L, and the concentration of nickel nitrate hexahydrate can be 5-100g / L or other metal salts of the same molar ratio are added. Then the solution is transferred to a flask, and an oil bath reflux device is used for reflux reaction to obtain an aqueous solution rich in POM anions. Alkali metal salt such as sodium chloride (potassium chloride can be used instead) is added to the aqueous solution rich in POM anions after reaction and cooled to crystallize (the temperature is preferably 0-30℃), to obtain a crystalline product (water-soluble POM powder material). The amount of sodium chloride added can be 100-1000g / L, or the same molar ratio of potassium chloride. The crystalline product is taken out, washed with water, and then added to a flask to dissolve in water at 80-100℃, and then cooled to 0-30℃, recrystallized and ion exchanged to obtain a precipitate. The precipitate is washed with ethanol and water several times and dried to obtain a POM material soluble in organic phase.

[0038] A mixed solution of cobalt salt and iron salt in a certain proportion is configured. Then, the POM material soluble in the organic phase is added, and after stirring at room temperature, solution 2 is obtained. The POM concentration in solution 2 can be 1-5 g / L. The cobalt salt is cobalt chloride hexahydrate or cobalt nitrate hexahydrate, and the concentration can be 1-5 g / L. The iron salt is ferric chloride hexahydrate or ferric nitrate nonahydrate, and the concentration can be 6.76-10.14 g / L. The obtained solution 2 is transferred to a reaction kettle, and foamed nickel is added as a substrate and current collector. Subsequently, the organic ligand is added to coordinate with the metal ions precipitated from the metal salt, and the reaction is carried out at a temperature of 80-150°C for 10-15 h. The specific reaction process and mechanism are as follows: the organic ligand is deprotonated in this process and periodically assembled with the metal sites, and the in-situ growth of the catalyst is carried out on the foamed nickel substrate. The generated product is washed with ethanol and water, and dried at room temperature to obtain the MOF and POM composite material (MOF@POM). The organic ligand can be at least one of 1,4-terephthalic acid, benzoic acid, trimesic acid, and 2-amino terephthalic acid. The amount of organic ligand added and the ratio of mixed solvent can be (3-10) g: 1 L.

[0039] In an optional embodiment, the foamed nickel is sequentially ultrasonically washed in 3M hydrochloric acid, a V1:1 acetone-ethanol mixed solution, ethanol, and deionized water for 15 min, and dried for use. The porosity of the foamed nickel can be 85-98%, and the pore size distribution can be 50 nm-100 pm.

[0040] In the present application, the MOF@POM composite electrocatalyst comprises a foamed nickel substrate and a MOF@POM formed in-situ on the surface of the foamed nickel substrate.

[0041] In an optional embodiment, the specific MOF in the obtained MOF@POM composite electrocatalyst should have one or more of the following characteristics: (1) BET < 600 m 2 / g; (2) the MOF has a micropore adsorption characteristic curve in the adsorption-desorption curve.

[0042] The following examples are further illustrated to explain the present application in detail. It should also be understood that the following examples are only used to further illustrate the present application, and cannot be understood as limiting the protection scope of the present application. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present application all belong to the protection scope of the present application. The specific process parameters in the following examples are only one example in the appropriate range, i.e., those skilled in the art can make appropriate selection within the range through the description herein, and are not limited to the specific values in the following examples.

[0043] Example 1:

[0044] The preparation method of the MOF@POM composite electrocatalyst involved in the embodiment includes the following two steps:

[0045] Step 1): POM was prepared by condensation reflux at normal pressure: sodium tungstate dihydrate (18 g), sodium phosphate dibasic (1.8 g), and nickel nitrate hexahydrate (3.98 g) were dissolved in water (50 mL), and the pH was adjusted to neutral. The solution was transferred to a flask and refluxed for 2 h using an oil bath reflux device to obtain a water solution rich in POM anions. Sodium chloride (24 g) was added to the water solution of the POM material after reaction, and the crystallization was carried out at 0-30℃ to obtain a crystalline product (water-soluble POM powder material). Then the crystalline product was taken out, washed with 15 ml of water, and then added to the flask. After being dissolved in deionized water at 100℃, the recrystallization was carried out at 30℃, and the product was ion exchanged to obtain a precipitate. Finally, the precipitate was washed with ethanol and water 4 times respectively and dried to obtain a POM material that can be dissolved in an organic phase.

[0046] Step 2): The POM material (0.3 g) that can be dissolved in an organic phase obtained in step 1) was dissolved in a mixed solvent of DMF, ethanol and water (total volume 17 mL, volume ratio of each component 14:1:1), and cobalt chloride hexahydrate (0.098 g) and iron chloride hexahydrate (0.293 g, molar ratio of cobalt ions to iron ions 0.38:1) were added and stirred for 1 h, then terephthalic acid (0.25 g) was added and stirred for 30 min to obtain a mixed solution. Finally, the mixed solution was transferred to a hydrothermal reaction kettle, pre-treated nickel foam (NF) was added, and the temperature was raised to 120℃ and reacted for 12 h. After the reaction was completed, the nickel foam was taken out, washed with water and ethanol more than twice, and dried at room temperature to obtain the MOF@POM composite electrocatalyst. It is recorded as MIL-53@TBA-Ni4P2.

[0047] Example 2:

[0048] Referring to step 1) in Example 1, the metal salt used for synthesizing POM was changed to cobalt nitrate hexahydrate, and referring to step 2) in Example 1, the addition amount of POM material was 1 / 2 of that in step 2), and the mass of iron chloride hexahydrate was 2 / 3 of that in step 2). It is recorded as MIL-53@TBA-Co4P2.

[0049] The above examples of the application are illustrative of the application and should not be used to limit the application. Any variations within the meaning and scope of the claims of the application should be considered to be included within the scope of the claims.

Claims

1. A method for preparing a MOF@POM composite electrocatalyst, characterized in that, The application relates to a MOF@POM composite electrocatalyst and a preparation method thereof. The application comprises the following steps: (1) cooling crystallization after adding an alkali metal salt into a water solution rich in POM anions to obtain a water-soluble POM powder material; (2) dissolving the obtained water-soluble POM powder material in water, performing cooling recrystallization and ion exchange, and then performing washing and drying to obtain a POM material dissolved in an organic phase; and (3) dissolving the obtained POM material dissolved in the organic phase in a mixed solvent, then adding a MOF precursor material and foamed nickel, and then performing a solvothermal reaction to obtain the MOF@POM composite electrocatalyst. The alkali metal salt is a sodium salt or a potassium salt; the cooling crystallization temperature is 0-40 DEG C; the concentration of the water solution rich in POM anions is 50-200 g / L; and the ratio of the alkali metal salt to the water solution rich in POM anions is (100-1000) g:1 L. The alkali metal salt is at least one of sodium chloride and potassium chloride.

2. The production method according to claim 1, characterized by, The POM material is selected from one of Ni-POM, Co-POM and Fe-POM; and the preparation method of the water solution rich in POM anions comprises the following steps: weighing tungstate, phosphate and transition metal salt, dissolving the three in water, using an acid solution to adjust the pH value to 6-8 to obtain a mixed solution; transferring the mixed solution into a flask, and using an oil bath reflux device to perform reflux reaction to obtain the water solution rich in POM anions.

3. The preparation method according to claim 2, characterized in that, The transition metal salt is selected from nitrate of a transition metal element or / and acetate of a transition metal element; 4. The method of claim 1, wherein, The tungstate is at least one of sodium tungstate dihydrate, potassium tungstate and zinc tungstate; 5. The preparation method according to claim 4, characterized in that, The phosphate is at least one of disodium hydrogen phosphate, sodium dihydrogen phosphate and potassium dihydrogen phosphate; The acid solution is hydrochloric acid or nitric acid; and the concentration of the acid solution is 1-12 M. The transition metal salt is at least one of nickel nitrate hexahydrate, cobalt nitrate hexahydrate, copper nitrate nonahydrate, manganese nitrate tetrahydrate, nickel acetate, cobalt acetate, copper acetate and manganese acetate. The mass ratio of the tungstate, the phosphate and the transition metal salt is (100-800):(10-100):(5-100).

6. The preparation method according to claim 5, characterized in that, The reflux reaction temperature is 80-150 DEG C, and the reflux reaction time is 1-5 hours.

7. The preparation method according to claim 4, characterized in that, The MOF precursor comprises a cobalt salt, an iron salt and an organic ligand; and the mixed solvent is at least two of DMF, ethanol, DMA, acetone and water.

8. The preparation method according to claim 4, characterized in that, The ratio of the POM material dissolved in the organic phase to the mixed solvent is (1-5) g:1 L; 9. The preparation method according to claim 1, characterized in that, The cobalt salt is cobalt chloride hexahydrate or / and cobalt nitrate hexahydrate; and the ratio of the cobalt salt to the mixed solvent is (1-10) g:1 L; 10. The method of claim 9, wherein, The iron salt is iron chloride hexahydrate or / and iron nitrate nonahydrate; and the ratio of the iron salt to the mixed solvent is (6.76-20.23) g:1 L; The organic ligand is at least one of 1,4-benzenedicarboxylic acid, benzoic acid, trimesic acid and 2-amino-benzenedicarboxylic acid; and the ratio of the organic ligand to the mixed solvent is (5-20) g:1 L. The temperature for dissolving the water-soluble POM powder material in water is 80-100 DEG C; The cooling recrystallization and ion exchange temperature is 0-40 DEG C; 11. The method of claim 1, wherein, The solvothermal reaction temperature is 80-150 DEG C, and the reaction time is 10-15 hours. ​ ​ 12. The MOF@POM composite electrocatalyst prepared according to the preparation method of any one of claims 1-11, characterized in that, The BET of the MOF material in the MOF@POM composite electrocatalyst is < 600 m 2 / g, and the MOF material has a micropore adsorption characteristic curve in the adsorption-desorption curve.

13. Use of the MOF@POM composite electrocatalyst of claim 12 in water oxidation.

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