Method for rapidly synthesizing MOFs derivative sulfide CoS / Co-MOF

Abstract

The invention discloses a method for rapidly synthesizing MOFs (Metal-Organic Frameworks) derived sulfide CoS / Co-MOF (Metal-Organic Frameworks). The synthesis method comprises the following steps that a Co-MOF precursor is obtained through a solvothermal method, then the Co-MOF precursor is mixed with thioacetamide, absolute ethyl alcohol and dimethylacetamide, and the MOFs derivative sulfide CoS / Co-MOF composite material is obtained through the solvothermal method. Compared with a traditional pyrolysis method, the method is easy and convenient to operate and mild in condition, the reaction time is greatly shortened, and in addition, a series of electrochemical tests (LSV, CV, EIS and CP) prove that the CoS / Co-MOF composite material shows excellent OER performance and is an excellent OER catalyst.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrocatalysis, and particularly relates to a method for rapidly synthesizing MOF-derived sulfide CoS / Co-MOF. Background Art

[0002] Metal-organic frameworks (MOFs) are a class of porous crystalline materials composed of metal ions / clusters and organic ligands. As highly ordered coordination polymers, they possess the characteristics of both homogeneous and heterogeneous catalysts. Such special crystalline materials have excellent physicochemical properties such as: high specific surface area (which is beneficial for exposing more active sites), low mass density, rich pore structure, stable porosity (allowing for rapid mass transport), diverse metal centers in composition (which can serve as catalytic active sites for OER), and a clear host structure with periodicity, and their physicochemical properties can be changed by adjusting organic ligands. These characteristics enable MOFs to exhibit great application potential in the field of electrocatalysis.

[0003] Although MOFs have advantages such as a large number of active atoms and a high specific surface area, their inherent low conductivity and poor electrochemical stability severely limit their application in the field of electrocatalysis. To overcome these limitations, a strategy of pyrolyzing MOFs in an inert atmosphere is adopted to use them as templates and precursors for preparing various carbon-based nanoelectrocatalysts. In addition, MOF-derived hybrid materials can not only inherit the porous structural characteristics of the precursors to provide rich OER active sites for the catalysts, but also the organic ligand-derived nano-carbonaceous matrix can enhance the conductivity of the materials. Thanks to the diverse linking units of MOFs, during the preparation process, MOF materials are prone to coordinate with various heteroatoms (such as P, S, and other metal ions) to transform them into corresponding MOF derivatives (such as: MOF-derived transition metal phosphides, sulfides, hydroxides, bimetallic MOFs, etc.). Among MOF-derived transition metal sulfides, heteroatom (S) doping will form (M-S) bonds with strong covalent components, and usually, the (M-S) bonds will couple with (S-S) with strong covalent components as well, effectively improving the conductivity of the sulfide catalyst and increasing its electrocatalytic reaction active sites, thereby effectively enhancing the catalytic activity of the catalyst.

[0004] At present, the preparation of MOFs-derived metal sulfides mainly adopts the pyrolysis method. Compared with the solvothermal method, the pyrolysis method has a long reaction time and consumes a large amount of energy during the reaction, which is contrary to the concepts of green chemistry and sustainable development. In addition, the pyrolysis method requires a relatively high reaction temperature and has a certain degree of danger. In contrast, the solvothermal method has a shorter reaction time and a lower reaction temperature, effectively reducing the energy consumption and time consumption during the preparation process, simplifying the synthesis steps, shortening the synthesis time, reducing pollution, and being of great significance for the industrialization of materials. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for rapidly synthesizing MOFs-derived sulfide CoS / Co-MOF, aiming to rapidly synthesize MOFs-derived sulfide CoS / Co-MOF with excellent OER performance by the solvothermal method.

[0006] The raw materials of the present invention are cobalt nitrate hexahydrate, biphenyldicarboxylic acid, absolute ethanol, dimethylacetamide, and thioacetamide, and a CoS / Co-MOF composite material with excellent OER performance can be synthesized.

[0007] The purpose of the present invention is achieved through the following technical solutions.

[0008] A method for rapidly synthesizing MOFs-derived sulfide CoS / Co-MOF includes the following steps:

[0009] (1) Add cobalt nitrate hexahydrate and biphenyldicarboxylic acid to a mixed solution composed of absolute ethanol and dimethylacetamide, stir, transfer the mixed solution to a hydrothermal reaction kettle for heating and reaction, wait for it to cool to room temperature, wash it repeatedly with a mixed solution composed of absolute ethanol and dimethylacetamide, and dry it with a vacuum drying oven to obtain Co-MOF.

[0010] (2) Add thioacetamide and the Co-MOF prepared in step (1) to a mixed solution composed of absolute ethanol and dimethylacetamide, stir, transfer the mixed solution to a hydrothermal reaction kettle for heating and reaction, wait for it to cool to room temperature, wash it repeatedly with a mixed solution composed of absolute ethanol and dimethylacetamide, and dry it with a vacuum drying oven to obtain CoS / Co-MOF.

[0011] Preferably, the molar ratio of cobalt nitrate hexahydrate to biphenyldicarboxylic acid in step (1) is (0.5 - 1.5):(0.5 - 1.5); the stirring time is 45 - 75 min; the reaction time at 125 - 175 °C is 2 - 4 h.

[0012] Preferably, the molar ratio of Co-MOF to thioacetamide in step (2) is (0.5 - 1.5):(15 - 25); the stirring time is 45 - 75 min; the reaction time at 125 - 175 °C is 2 - 4 h.

[0013] Compared with the prior art, the present invention has the following advantages and effects:

[0014] (1) The present invention selects the solvothermal method as the preparation method of the MOFs-derived sulfide CoS / Co-MOF composite material. Compared with the traditional pyrolysis method, the reaction time is shorter and the reaction temperature is lower, effectively reducing the energy consumption and time consumption in the preparation process, which is in line with the concept of green chemistry and sustainable development, and provides the possibility for the industrialization of the material.

[0015] (2) The synthesis method of the present invention is convenient and fast. Compared with the pyrolysis method with a reaction time of 6 - 12 h and a reaction temperature of 600 - 800 °C, it only needs to react at 125 - 175 °C for 2 - 4 h to synthesize. At the same time, a series of electrochemical tests (LSV, CV, EIS, CP) prove that the CoS / Co-MOF composite material has a lower overpotential, faster electrochemistry reaction kinetics, a larger electrochemically active area, a lower charge transfer resistance, and lasting electrochemical stability, showing excellent OER performance and having good application prospects in the field of OER technology. Description of the Drawings

[0016] Figure 1 It is the X-ray diffraction pattern of the Co-MOF and CoS / Co-MOF composite materials prepared in Example 1.

[0017] Figure 2 It is the scanning electron microscope photo of the CoS / Co-MOF composite material with the best ratio prepared in Example 1.

[0018] Figure 3 It is the linear sweep LSV diagram of the Co-MOF and CoS / Co-MOF composite materials prepared in Example 1. Detailed Embodiments

[0019] The present invention will be further described below with reference to the drawings and embodiments, but the scope of protection required by the present invention is not limited to the scope described in the embodiments.

[0020] Example 1

[0021] 0.3 mmol of cobalt nitrate hexahydrate and 0.3 mmol of biphenyldicarboxylic acid were added to a mixed solution of 6 mL of absolute ethanol and 10 mL of dimethylacetamide. The obtained mixed solution was stirred for 60 min. After stirring, the above mixed solution was transferred to a high-pressure reaction kettle, sealed and reacted at 150 °C for 3 h. After it was naturally cooled to room temperature, it was centrifugally washed three times with a mixed solution composed of 10 mL of absolute ethanol and 10 mL of dimethylacetamide, and then vacuum dried at 50 °C for 12 h. The obtained product was labeled as Co-MOF.

[0022] 50 mg (0.3 mmol) of Co-MOF and 450 mg (6 mmol) of TAA were added to a mixed solution composed of 6 mL of absolute ethanol and 10 mL of dimethylacetamide. The obtained mixed solution was stirred for 60 min. After stirring, the above mixed solution was transferred to a high-pressure reaction kettle, sealed and reacted at 150 °C for 3 h. After it was naturally cooled to room temperature, it was centrifugally washed three times with a mixed solution composed of 10 mL of absolute ethanol and 10 mL of dimethylacetamide, and then vacuum dried at 50 °C for 12 h. The obtained product was labeled as CoS / Co-MOF.

[0023] Example 2

[0024] 0.3 mmol of cobalt nitrate hexahydrate and 0.3 mmol of biphenyldicarboxylic acid were added to a mixed solution of 6 mL of absolute ethanol and 10 mL of dimethylacetamide. The obtained mixed solution was stirred for 60 min. After stirring, the above mixed solution was transferred to a high-pressure reaction kettle, sealed and reacted at 125 °C for 4 h. After it was naturally cooled to room temperature, it was centrifugally washed three times with a mixed solution composed of 10 mL of absolute ethanol and 10 mL of dimethylacetamide, and then vacuum dried at 50 °C for 12 h. The obtained product was labeled as Co-MOF.

[0025] 50 mg of Co-MOF and 400 mg of TAA were added to a mixed solution composed of 6 mL of absolute ethanol and 10 mL of dimethylacetamide. The obtained mixed solution was stirred for 45 min. After stirring, the above mixed solution was transferred to a high-pressure reaction kettle, sealed and reacted at 125 °C for 4 h. After it was naturally cooled to room temperature, it was centrifugally washed three times with a mixed solution composed of 10 mL of absolute ethanol and 10 mL of dimethylacetamide, and then vacuum dried at 50 °C for 12 h. The obtained product was labeled as CoS / Co-MOF-400.

[0026] Example 3

[0027] 0.3 mmol of cobalt nitrate hexahydrate and 0.3 mmol of biphenyldicarboxylic acid were added to a mixed solution of 6 mL of absolute ethanol and 10 mL of dimethylacetamide. The obtained mixed solution was stirred for 75 min. After stirring, the above mixed solution was transferred to a high-pressure reactor, sealed and reacted at 175 °C for 4 h. After it was naturally cooled to room temperature, it was centrifugally washed three times with a mixed solution composed of 10 mL of absolute ethanol and 10 mL of dimethylacetamide, and then vacuum dried at 50 °C for 12 h. The obtained product was labeled as Co-MOF.

[0028] 50 mg of Co-MOF and 500 mg of TAA were added to a mixed solution composed of 6 mL of absolute ethanol and 10 mL of dimethylacetamide. The obtained mixed solution was stirred for 75 min. After stirring, the above mixed solution was transferred to a high-pressure reactor, sealed and reacted at 175 °C for 2 h. After it was naturally cooled to room temperature, it was centrifugally washed three times with a mixed solution composed of 10 mL of absolute ethanol and 10 mL of dimethylacetamide, and then vacuum dried at 50 °C for 12 h. The obtained product was labeled as CoS / Co-MOF-500.

[0029] Taking the CoS / Co-MOF composite material prepared in Example 1 as a representative for analysis, the analysis results of the CoS / Co-MOF composite materials prepared in other examples are basically the same as those in Example 1, and will not be provided one by one.

[0030] (I) Crystal structure properties of the rapidly synthesized MOF-derived sulfide CoS / Co-MOF composite material

[0031] The crystal structure of Example 1 of the present invention was characterized by an X-ray diffractometer of model D8-ADVANCE produced by Bruker Company, Germany.

[0032] Figure 1The XRD patterns of CoS / Co-MOF composite and Co-MOF are shown. It can be seen from the figure that compared with Co-MOF, CoS / Co-MOF treated by TAA sulfidation maintains similar diffraction peaks, but the intensities of some peaks (6.1°, 12.4°, 14.1°, 15.3° and 18.8°) are significantly weakened. This indicates that the sulfidation treatment will cause partial dissociation of the framework structure of Co-MOF. However, the characteristic peaks of some Co-MOF can be observed at 12.1°, 13.6°, 18.3°, 29.2° and 30.8°. This result shows that after the sulfidation treatment, the structural characteristics of Co-MOF are still partially retained, enabling the CoS / Co-MOF composite to inherit the structural advantages of MOFs materials (such as high specific surface area and rich electrochemically reactive sites). The synergistic effect of these structural advantages significantly improves the OER performance of the composite material.

[0033] (2) SEM images of the rapidly synthesized MOF-derived sulfide CoS / Co-MOF composite

[0034] The product obtained in Example 1 was characterized by a Czech TESCAN MIRALMS scanning electron microscope. As Figure 2 shown, compared with Co-MOF, the sulfide-treated CoS / Co-MOF composite presents a granular morphology, and the particle size is significantly reduced (Co-MOF: 1 μm, CoS / Co-MOF: 200 nm). This is because after adding TAA, in a high-temperature and high-pressure environment, TAA decomposes thermally to generate sulfur atoms, and the released sulfur atoms react with the metal sites in Co-MOF, which not only causes the reduction of the size of two-dimensional nanosheets and the increase of specific surface area, but also promotes the formation of amorphous metal sulfide composites (such as CoS, etc.). The unique chemical bond characteristics of metal sulfides (covalent component metal-metalloid bonds (M-S) and (S-S)) bring multiple advantages to the composite material: (1) The strongly covalent M-S bond enhances the adsorption and activation ability of the catalyst for reaction intermediates; (2) The highly covalent S-S bond promotes the electron interaction between metalloid elements and improves the material stability. In addition, the inherent excellent conductivity and rich redox active sites of metal sulfides further improve the OER performance of the CoS / Co-MOF composite material.

[0035] The above embodiments are the relatively ideal implementation manners of the present invention, but the implementation manners of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

[0036] (3) Electrochemical performance

[0037] By constructing a three - electrode system on a CS235OH electrochemical workstation (Wuhan Kost Instrument Co., Ltd., Wuhan, China), 4 mg of the electrocatalyst was dispersed in a mixed solution made of 25 μL of Nafion solution (5 wt%) and 475 μL of isopropanol, and ultrasonicated for 10 min to form a homogeneous catalyst ink. Finally, it was dropped onto the glassy carbon electrode, so that the glassy carbon electrode was uniformly loaded with 1 mg cm -2 of the catalyst as the working electrode, and electrochemical tests were carried out in 1 M KOH electrolyte, using a mercury / mercuric oxide electrode and a carbon rod electrode as the reference electrode and the counter electrode respectively. The measured electrode potential was converted through a reversible hydrogen electrode (RHE): E RHE = E( Hg / HgO ) + 0.098 + 0.059×pH. The test results are as Figure 3 shown. After being treated with thioacetamide sulfidation, at a current density of 20 mA cm -2 , compared with Co - MOF (overpotential of 414 mV), the CoS / Co - MOF composite material treated with thioacetamide sulfidation exhibited excellent OER performance. At a current density of 20 mA cm -2 , the overpotential was only 274 mV. This result indicates that at the same potential, the composite material CoS / Co - MOF has more excellent OER performance. The improvement of its OER performance is mainly attributed to the layered structure induced by the sulfidation treatment.

Claims

1. A method for rapidly synthesizing MOF-derived sulfide CoS / Co-MOF, characterized in that, It includes the following steps: (1) Add cobalt nitrate hexahydrate and biphenyl dicarboxylic acid into a mixed solution composed of absolute ethanol and dimethylacetamide, stir, transfer the mixed solution to a hydrothermal reaction kettle for heating reaction, wait for it to cool to room temperature, wash it repeatedly with the mixed solution composed of absolute ethanol and dimethylacetamide, and dry it with a vacuum drying oven to obtain Co-MOF; (2) Add thioacetamide and the Co-MOF prepared in step (1) into a mixed solution composed of absolute ethanol and dimethylacetamide, stir, transfer the mixed solution to a hydrothermal reaction kettle for heating reaction, wait for it to cool to room temperature, wash it repeatedly with the mixed solution composed of absolute ethanol and dimethylacetamide, and dry it with a vacuum drying oven to obtain CoS / Co-MOF.

2. The preparation method of Co-MOF according to claim 1, characterized in that: In step (1), the molar ratio of cobalt nitrate hexahydrate to biphenyl dicarboxylic acid is (0.5-1.5):(0.5-1.5); the stirring time is 45-75 min; the reaction time at 125-175 °C is 2-4 h.

3. The preparation method of Co-MOF according to claim 1, wherein: In step (2), the molar ratio of Co-MOF to thioacetamide is (0.5-1.5):(15-25); the stirring time is 45-75 min; the reaction time at 125-175 °C is 2-4 h.

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