CN114808027B - N-MoS with high-efficiency electrocatalytic hydrogen evolution performance2/COF-C4N composite catalyst and preparation method thereof - Google Patents

Abstract

The present invention relates to the technical field of electrocatalytic hydrogen evolution and the technical field of catalyst preparation. The present invention discloses a N- _MoS2 /COF- _C4N composite catalyst with high-efficiency electrocatalytic hydrogen evolution performance and a preparation method thereof. The material first synthesizes a covalent organic framework material (COF- _C4N ) connected by a phenazine bond by a solvothermal method, and then synthesizes nitrogen-doped molybdenum disulfide (N- _MoS2 ) in situ on the surface of COF- _C4N by an in-situ synthesis method, thereby forming a N- _MoS2 /COF- _C4N heterogeneous structure. The in-situ synthesis method not only allows N to be doped into the _MoS2 skeleton, but also allows N- _MoS2 to be more evenly dispersed on the surface of COF- _C4N , exposing more hydrogen evolution active sites and improving charge separation and transmission capabilities. Therefore, the N- _MoS2 /COF- _C4N composite catalyst has a unique structure and can be used as an excellent electrocatalytic water decomposition hydrogen production catalyst material.

Description

A N-MoS2/COF-C4N composite catalyst with high-efficiency electrocatalytic hydrogen evolution performance and its preparation method

Technical Field

The invention relates to an N-MoS ₂ /COF-C ₄ N composite catalyst with high-efficiency electrocatalytic hydrogen evolution performance and a preparation method thereof, belonging to the technical field of electrocatalytic hydrogen evolution and catalyst preparation.

technical background

Since the Industrial Revolution, people have become increasingly dependent on fossil fuels to rapidly advance industrialization and urbanization, and the resulting environmental pollution and energy crisis have begun to emerge. Therefore, people are in urgent need of developing clean energy and sustainable energy. The production of hydrogen and oxygen by water electrolysis is an effective strategy for producing clean energy and renewable resources. How to improve the efficiency of hydrogen and oxygen production by water electrolysis and reduce production costs are important challenges facing the promotion and production of water electrolysis, and the core factor is the development of efficient catalysts. Although commercial Pt/C catalysts have the advantages of high catalytic activity and good stability, their high cost and limited reserves limit their large-scale application. Therefore, the development of efficient and inexpensive electrocatalysts is of great significance;

Two-dimensional covalent organic framework (COF) is an emerging graphene-like material. It is a typical porous crystalline material connected by covalent bonds and has a rigid skeleton with strong π-π stacking. In recent years, there have been a large number of reports on the electrocatalytic decomposition of water by covalent organic frameworks to produce oxygen. It is a potential excellent material for electrocatalytic water decomposition. Molybdenum disulfide (MoS ₂ ) in transition metal dihalides (TMDs) is basically composed of two-dimensional S-Mo-S layers. It is considered to be a substitute for traditional Pt group metals in the electrocatalytic decomposition of water to produce hydrogen. It is low-cost, but its active sites are not exposed enough, and its small specific surface area limits its hydrogen production efficiency. Therefore, designing molybdenum disulfide materials with more active sites is an effective way to improve its electrocatalytic hydrogen production efficiency. The composite of molybdenum disulfide and two-dimensional covalent organic framework materials to form a heterostructure can greatly improve the hydrogen production efficiency of molybdenum disulfide.

Summary of the invention

The present invention aims to provide a nitrogen-doped molybdenum disulfide and COF-C ₄ N composite catalyst (N-MoS ₂ /COF-C ₄ N) synthesized in situ and a preparation method, and the electrocatalytic hydrogen evolution performance of MoS ₂ is greatly improved by forming a heterogeneous structure with N-doped MoS ₂ (N-MoS ₂ ) and COF-C ₄ N.

To achieve the above purpose, the technical solution adopted by the present invention is: N-MoS ₂ is synthesized in situ on the surface of the prepared COF-C ₄ N by a hydrothermal method, thereby forming a heterojunction between N-MoS ₂ and COF-C ₄ N. The following raw materials and components are included: triphenylene-2,3,6,7,10,11-hexamine hexahydrochloride, hexaketone cyclohexane octahydrate, the mass ratio of which is 25.5mg:25.0mg; 1,4-dioxane, 1,3,5-mesitylene, acetic acid, the volume ratio of which is 1.5mL:1.5mL:0.5mL; COF-C ₄ N, thiourea, molybdenum trioxide, the mass ratio of which is: 10mg:19.7mg:19.0mg.

The N-MoS ₂ /COF-C ₄ N composite catalyst is prepared by the following steps:

(1) Preparation of COF-C ₄ N:

At room temperature, triphenylene-2,3,6,7,10,11-hexamine hexahydrochloride and hexaketone cyclohexane octahydrate were fully ground, and the mixture was added to an organic solvent mixture of 1,4-dioxane and 1,3,5-mesitylene, and ultrasonically treated at 25°C for 30 minutes, and then 4 mol/L acetic acid was added to obtain a dispersion; the dispersion was nitrogen-vacuumed, and then degassed by freezing-thawing, and this operation was repeated three times; the degassed dispersion was placed in a 150°C oven for reaction for 72 hours, and the test tube was taken out after the oven temperature dropped to room temperature to obtain a crude reaction product; the crude reaction product was filtered and washed with tetrahydrofuran, and a brown-black solid product was obtained after natural drying; the solid product was Soxhlet extracted with tetrahydrofuran until the outflowing liquid was colorless; the obtained solid product was vacuum dried at 100°C for 24 hours to obtain a two-dimensional covalent organic framework material (COF-C ₄ N) with a crystal structure.

(2) Preparation of N-MoS ₂ /COF-C ₄ N composite catalyst:

At room temperature, 59.1 mg of thiourea and 57.2 mg of molybdenum trioxide were added to 60 mL of distilled water and ultrasonically treated for 30 min to uniformly disperse the ligands. Subsequently, 30 mg of the pure product obtained in step 1 was added to the dispersion and stirred for 1 h to form a uniform dispersion. The dispersion was placed in a 200°C oven for reaction for 24 h. After the oven temperature dropped to room temperature, the reactor was taken out to obtain a crude reaction product. The crude product was filtered using a large amount of distilled water, and the obtained solid product was vacuum dried at 100°C for 12 h to obtain the N-MoS ₂ /COF-C ₄ N composite catalyst.

The N-MoS ₂ /COF-C ₄ N was treated and coated on a carbon cloth, and the carbon cloth was used as a working electrode. The linear voltammetric scanning diagram of the N-MoS ₂ /COF-C ₄ N composite catalyst was measured to measure its ability to electrocatalyze water decomposition and hydrogen evolution. It can be concluded that the overpotential of the present invention at 10 mA is only 106 mV, which is nearly 6 times higher than the overpotential of 629 mV of the original molybdenum disulfide at 10 mA.

The benefits of the present invention are:

(1) The preparation method of the present invention is simple, efficient, low-cost, and has practical application significance;

(2) The present invention synthesizes N-MoS ₂ /COF-C ₄ N composite catalyst in situ, and the obtained material has excellent electrocatalytic water decomposition and hydrogen production performance;

(3) The present invention uses the covalent organic framework COF-C ₄ N as a carrier for synthesizing N-MoS _2. The porous structure and ultra-high surface area of COF-C ₄ N reduce the agglomeration of N-MoS ₂ and achieve a high degree of dispersion, thereby exposing more active sites of N-MoS _2. In addition, the excellent conductivity and abundant pores of COF-C ₄ N provide good conditions for charge separation and transmission in electrocatalytic hydrogen evolution. The electrocatalytic hydrogen evolution performance of the N-MoS ₂ /COF-C ₄ N composite catalyst is greatly improved compared with that of the single MoS ₂ material.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a comparison of X-ray powder diffraction patterns of two-dimensional covalent organic framework materials (COF-C ₄ N), molybdenum disulfide (MoS ₂ ), nitrogen-doped molybdenum disulfide (N-MoS ₂ ) and N-MoS ₂ /COF-C ₄ N composite catalysts.

Figure 2 shows (a) SEM image of COF-C ₄ N at a scale of 1 μm; (b) SEM image of N-MoS ₂ at a scale of 1 μm; (c) SEM image of N-MoS ₂ /COF-C ₄ N composite catalyst at a scale of 1 μm; (d), (e) and (f) are element mapping diagrams of N, Mo and S elements in N-MoS ₂ /COF-C ₄ N composite catalyst, respectively.

Figure 3 is a comparison of transmission electron microscopy images of (a) COF-C ₄ N (b) N-MoS ₂ (c) N-MoS ₂ /COF-C ₄ N composite catalysts at a 5 nm scale.

Figure 4 shows the X-ray photoelectron spectroscopy (XPS) spectrum of the N-MoS ₂ /COF-C ₄ N composite catalyst.

Figure 5 is the XPS spectra of each element in the N-MoS ₂ /COF-C ₄ N composite catalyst

Figure 6 shows the linear voltammetric scanning graphs (LSV) of pure MoS ₂ , pure COF-C ₄ N and N-MoS ₂ /COF-C ₄ N composite catalysts.

Detailed ways

The present invention is further described below in conjunction with examples, but is not limited to the following embodiments:

(1) Preparation of COF-C ₄ N

At room temperature, triphenylene-2,3,6,7,10,11-hexamine hexahydrochloride and hexaketone cyclohexane octahydrate were fully ground, and the mixture was added to an organic solvent mixture of 1,4-dioxane and 1,3,5-mesitylene, and ultrasonically treated at 25°C for 30 minutes, and then 4 mol/L acetic acid was added to obtain a dispersion; the dispersion was nitrogen-vacuumed, and then degassed by freezing-thawing, and this operation was repeated three times; the degassed dispersion was placed in a 150°C oven for reaction for 72 hours, and the test tube was taken out after the oven temperature dropped to room temperature to obtain a crude reaction product; the crude reaction product was filtered and washed with tetrahydrofuran, and a brown-black solid product was obtained after natural drying; the solid product was Soxhlet extracted with tetrahydrofuran until the outflowing liquid was colorless; the obtained solid product was vacuum dried at 100°C for 24 hours to obtain a two-dimensional covalent organic framework material (COF-C ₄ N) with a crystal structure;

(2) Preparation of N-MoS ₂ /COF-C ₄ N composite catalyst

At room temperature, 59.1 mg of thiourea and 57.2 mg of molybdenum trioxide were added to 60 mL of distilled water and ultrasonically treated for 30 min to uniformly disperse the ligands. Subsequently, 30 mg of the pure product obtained in step 1 was added to the dispersion and stirred for 1 h to form a uniform dispersion. The dispersion was placed in a 200°C oven for reaction for 24 h. After the oven temperature dropped to room temperature, the reactor was taken out to obtain a crude reaction product. The crude product was filtered using a large amount of distilled water, and the obtained solid product was vacuum dried at 100°C for 12 h to obtain a N-MoS ₂ /COF-C ₄ N composite catalyst;

The N-MoS ₂ /COF-C ₄ N composite catalyst and carbon black were added to a mortar in a mass ratio of 1:1, and after being fully ground, they were placed in a small centrifuge tube. Distilled water, glacial acetic acid, and naphthalene were added to the centrifuge tube to form a dispersion, and the dispersion was ultrasonicated for 1 hour to obtain a uniform dispersion. The dispersion was evenly coated on a carbon cloth with an area of 1 cm ² , and the carbon cloth was used as a working electrode, Hg/Hg ₂ SO ₄ was used as a reference electrode, a Pt wire was used as a counter electrode, and a 0.5 mol/l sulfuric acid solution was used as an electrolyte. The linear voltammetric scanning diagram of the N-MoS ₂ /COF-C ₄ N composite catalyst was measured to measure its ability to release hydrogen in electrocatalytic decomposition of water, as shown in FIG6 ;

Figure 1 is a comparison of X-ray powder diffraction of two-dimensional covalent organic framework material (COF-C ₄ N), molybdenum disulfide (MoS ₂ ), nitrogen-doped molybdenum disulfide (N-MoS ₂ ) and N-MoS ₂ /COF-C ₄ N composite catalyst. It can be seen from the figure that the characteristic peaks of nitrogen-doped molybdenum disulfide N-MoS ₂ disappear compared with pure MoS ₂ , indicating that N is doped into the MoS ₂ framework, and the peaks at 14.4, 32.6 and 39.5 correspond to the 002, 100, and 103 crystal planes of N-MoS ₂ . The peaks of pure COF-C ₄ N at 2θ=7.2° and 27° correspond to the 100 and 001 crystal planes, which correspond to regular pores and layered structures, respectively. After compounding, the N-MoS ₂ /COF-C ₄ N composite catalyst retains part of the characteristic peak at 7.2° and shifts slightly, and the peak at 27° decreases significantly, indicating that with COF-C ₄ N as the substrate, the in-situ synthesized N-MoS ₂ grows well and disperses on the material based on COF-C ₄ N;

Figure 2 shows (a) SEM images of COF-C ₄ N at a scale of 1 μm, (b) SEM images of N-MoS ₂ at a scale of 1 μm, and (c) SEM images of N-MoS ₂ /COF-C ₄ N composite catalyst at a scale of 1 μm; (d), (e) and (f) are element mappings (SEM and EDS) of N, Mo and S elements in N-MoS ₂ /COF-C ₄ N composite catalyst, respectively. By comparison, it can be clearly seen that N-MoS ₂ is well dispersed on COF-C ₄ N, and the N element is evenly distributed on the surface of the material;

Figure 3. Comparison of transmission electron microscopy (TEM) images of (a) COF-C ₄ N (b) N-MoS ₂ (c) N-MoS ₂ /COF-C ₄ N composite catalysts at 5 nm scale. By comparison, it can be seen that the two-dimensional layered N-MoS ₂ is uniformly dispersed on the surface of COF-C ₄ N;

Figure 4 is the X-ray photoelectron spectroscopy (XPS) spectrum of the N-MoS ₂ /COF-C ₄ N composite catalyst; the XPS spectrum confirms the presence of C, N, Mo, S and O elements in the prepared sample. The presence of O may be due to the oxidation of the sample or the absorption of oxygen-containing substances by the sample;

Figure 5 shows the XPS spectra of each element in the N-MoS ₂ /COF-C ₄ N composite catalyst. In the S 2p spectrum, the two peaks at 161.46eV and 161.97eV correspond to the S 2p _3/2 and S 2p _1/2 orbitals, respectively, which are typical characteristics of the divalent sulfide ion (S ^2- ) of molybdenum disulfide; in the N1s spectrum, the peak at 395.41eV can be attributed to the Mo-N bond, and the peak at 401.60eV proves the existence of the C＝NC bond; in the C1s spectrum, 284.57eV corresponds to the CC bond; in the 3d spectrum of Mo, 229.16eV is also attributed to the Mo-N bond. In summary, it can be seen that N is successfully doped into the MoS ₂ skeleton, and COF-C ₄ N basically maintains its original structure. We have successfully prepared the N-MoS ₂ /COF-C ₄ N composite catalyst;

Figure 6 shows the linear voltammetric scanning diagram (LSV) of pure MoS ₂ , pure COF-C ₄ N and N-MoS ₂ /COF-C ₄ N composite catalyst. It can be seen that the overpotential of the composite N-MoS ₂ /COF-C ₄ N composite catalyst at 10mA is only 106mV, which is 6 times higher than the overpotential of 629mV of the original MoS ₂ ;

In summary, we have successfully prepared the N-MoS ₂ /COF-C ₄ N composite catalyst by in situ synthesis, and the material has excellent performance in electrocatalytic decomposition of water to produce hydrogen.

Claims (1)

1. A N-MoS ₂/COF-C ₄ N composite catalyst with high-efficiency electrocatalytic hydrogen evolution performance is characterized in that,

(1) N-MoS ₂ is synthesized on the surface of the prepared COF-C ₄ N in situ by a hydrothermal method, so that a heterojunction of N-MoS ₂ and the COF-C ₄ N is formed, and the heterojunction is composed of the following raw materials: triphenylene-2, 3,6,7,10, 11-hexamine hexahydro-chloride and hexaketocyclohexane octahydrate in a mass ratio of 25.5mg to 25.0mg;1, 4-dioxane, 1,3, 5-mesitylene and acetic acid, wherein the volume ratio is 1.5mL to 0.5mL; c ₄ N, thiourea and molybdenum trioxide, wherein the mass ratio is as follows: 10mg:19.7mg:19.0mg;

(2) The nitrogen doped molybdenum disulfide (N-MoS ₂) containing Mo-N bonds is uniformly dispersed on the two-dimensional COF-C ₄ N nanosheets, and the successful doping of N on the surface of MoS ₂ and the uniform dispersion of N-MoS ₂ on the COF-C ₄ N nanosheets increase the surface hydrogen evolution active sites;

the N-MoS ₂/COF-C ₄ N composite catalyst is specifically prepared by the following steps:

(1) Preparation of COF-C ₄ N:

Fully grinding triphenylene-2, 3,6,7,10, 11-hexamine hexahydrochloride and hexaketocyclohexane octahydrate at room temperature, adding the mixture into an organic solvent mixed solution of 1, 4-dioxane and 1,3, 5-mesitylene, carrying out ultrasonic treatment at 25 ℃ for 30min, and adding 4mol/L acetic acid to obtain a dispersion; introducing nitrogen into the dispersion liquid, vacuumizing, and then degassing by freezing and thawing, wherein the operation is repeated for three times; placing the degassed dispersion liquid into an oven to react for 72 hours at 150 ℃, and taking out a test tube after the temperature of the oven is reduced to normal temperature to obtain a crude reaction product; filtering and washing the crude reaction product with tetrahydrofuran, and naturally airing to obtain a brown-black solid product; carrying out Soxhlet extraction on the solid product by tetrahydrofuran until effluent liquid is colorless; vacuum drying the obtained solid product for 24 hours at 100 ℃ to obtain a two-dimensional covalent organic framework material (COF-C ₄ N) with a crystal structure;

(2) Preparation of N-MoS ₂/COF-C ₄ N composite catalyst:

Adding 59.1mg of thiourea and 57.2mg of molybdenum trioxide into 60mL of distilled water at room temperature, carrying out ultrasonic treatment for 30min to uniformly disperse a ligand, then adding 30mg of the pure product obtained in the step one into the dispersion liquid, fully stirring for 1h to form uniform dispersion liquid, putting the dispersion liquid into a baking oven to react for 24h at 200 ℃, and taking out a reaction kettle after the baking oven temperature is reduced to the room temperature to obtain a crude reaction product; filtering the crude product by using a large amount of distilled water, vacuum drying the obtained solid product for 12 hours at 100 ℃ to obtain the N-MoS ₂/COF-C ₄ N composite catalyst,

Adding the N-MoS ₂/COF-C ₄ N composite catalyst and carbon black into a mortar according to a mass ratio of 1:1, fully grinding, then filling into a small centrifuge tube, adding distilled water, glacial acetic acid and naphthol into the centrifuge tube to form a dispersion liquid, carrying out ultrasonic treatment on the dispersion liquid for 1h to obtain a uniform dispersion liquid, uniformly coating the dispersion liquid on carbon cloth with an area of 1cm ², taking the carbon cloth as a working electrode, hg/Hg ₂SO ₄ as a reference electrode, taking Pt wires as a counter electrode and taking a sulfuric acid solution with a concentration of 0.5mol/l as an electrolyte, and measuring the hydrogen evolution capacity of the N-MoS ₂/COF-C ₄ N composite catalyst in electrocatalytic decomposition water by measuring a linear voltammetry scanning graph of the N-MoS ₂/COF-C ₄ N composite catalyst.