A porous carbon quantum dot, its preparation method and application

Abstract

This invention relates to porous carbon quantum dots, their preparation method, and applications, belonging to the technical field of carbon quantum dots. The method for preparing porous carbon quantum dots uses coal powder as raw material, which is inexpensive and readily available. The coal powder is then subjected to pyrolysis, acid washing, and solid-liquid separation to obtain a first solution. A pore-forming agent is added to the first solution, specifying the type and weight ratio of the pore-forming agent to the first solution. At least one of ammonium chloride, zinc chloride, and potassium hydroxide is mixed with the first solution, catalyzing the dehydration of hydroxyl groups in the solid residue and the breaking of C-O bonds in phenols / ethers. This substance itself has swelling and volatility, and is released during subsequent carbonization and heat treatment. The carbon quantum dots form a large number of porous structures, thus obtaining porous carbon quantum dots.

Description

Technical Field

[0001] This invention relates to the technical field of carbon quantum dots, specifically to a porous carbon quantum dot, its preparation method, and its applications. Background Technology

[0002] Coal is composed of an ordered aromatic core and a disordered structure of surrounding alkyl side chains / functional groups. Its molecular structure naturally resembles that of carbon quantum dots, making it an excellent precursor for carbon quantum dots. Among various carbon-based material precursors, coal has a very high carbon yield. my country has abundant coal resources, but lignite has high moisture content and low calorific value. Traditional direct combustion of lignite is wasteful and causes environmental pollution. Using lignite as a source of anode materials for sodium-ion batteries and developing high-quality coal-based carbon materials can not only reduce environmental pollution during coal utilization but also increase the utilization value of coal resources, bringing significant economic benefits and possessing major practical significance. However, the disadvantages of coal-based carbon materials as anode materials for sodium-ion batteries include low initial coulombic efficiency, low sodium storage capacity, poor rate performance, and poor cycle performance. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a porous carbon quantum dot, its preparation method and application. Carbon quantum dots are prepared using coal as raw material. By utilizing its small size effect and abundant functional groups on the surface, the carbon material is structurally constructed and its surface is physicochemically modified at the nanoscale. By controlling the pore structure of the material and the occurrence state of heteroatoms on the surface, porous carbon quantum dots with high coulombic efficiency and sodium storage capacity, excellent rate performance and cycling performance are prepared.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] In a first aspect, the present invention provides a method for preparing porous carbon quantum dots, comprising the following steps:

[0006] (1) Pyrolysis of coal powder to obtain pyrolysis material, acid washing of the pyrolysis material, and then solid-liquid separation by filtration to obtain a first solution;

[0007] (2) Add a pore-forming agent to the first solution, stir evenly, and then carry out a hydrothermal reaction to obtain a second solution; the pore-forming agent includes at least one of ammonium chloride, zinc chloride, and potassium hydroxide; the mass ratio of the first solution to the pore-forming agent is 1:(0.01~0.05);

[0008] (3) The second solution is centrifuged, and the separated solution is then dialyzed and dried to obtain carbon quantum dots;

[0009] (4) Carbonize the obtained carbon quantum dots to obtain porous carbon quantum dots.

[0010] The method for preparing porous carbon quantum dots of the present invention uses coal powder as raw material, and performs pyrolysis, acid washing and solid-liquid separation treatment in sequence to obtain a first solution. Then, a pore-forming agent is added to the first solution, and the type and weight ratio of the pore-forming agent to the first solution are specified. At least one of ammonium chloride, zinc chloride and potassium hydroxide is used. After mixing with the first solution, it will catalyze the dehydration of alcohol hydroxyl groups in the solid residue and the breaking of CO bonds in phenol / ether. At the same time, the substance itself has swelling and volatility, and will be released after heat treatment during subsequent carbonization. The carbon quantum dots form a large number of pore structures, thereby obtaining porous carbon quantum dots.

[0011] Preferably, the mass ratio of the first solution to the pore-forming agent is 1:(0.02-0.03).

[0012] Preferably, the pore-forming agent is ammonium chloride, zinc chloride, and potassium hydroxide in a mass ratio of (0-5):1:(0-3).

[0013] Preferably, the volatile matter content of the pulverized coal is ≥5wt%, the ash content is ≤3wt%, and the particle size Dv is [missing information]. 50 The size is 1–10 μm;

[0014] Preferably, in step (1), the pyrolysis is carried out under an inert atmosphere, the temperature of which is 250-400°C and the time is 2-10 hours; wherein the inert atmosphere is at least one of nitrogen and argon.

[0015] Preferably, in step (1), the acid washing involves mixing the pyrolysis material, acid, and water; wherein the mass ratio of the pyrolysis material, acid, and water is 1:(0.5-2):(2-10); the acid includes at least one of concentrated hydrochloric acid, concentrated nitric acid, and concentrated sulfuric acid. Since some carbon quantum dots are trapped by metal oxides in the biochar and not exposed on the surface, simply soaking the biochar in water results in a slightly low separation rate of carbon quantum dots. Therefore, this invention employs acid washing of the carbon quantum dots to dissolve the metal oxides in the biochar framework or pores, thereby releasing more carbon quantum dots. Impurities such as inorganic salts dissolved in the acid solution are removed in subsequent dialysis steps.

[0016] Preferably, in step (2), the temperature of the hydrothermal reaction is 120-180°C and the reaction time is 4-8 hours.

[0017] Preferably, in step (3), the centrifugation speed is 7000-12000 r / min and the time is 5-10 min.

[0018] Preferably, in step (3), the dialysis cutoff is 1000-3500 Da and the dialysis time is 10-24 h; the drying is done by freeze drying, the drying temperature is -84 to -94 °C and the drying time is 24-36 h.

[0019] Preferably, in step (4), the carbonization is carried out under an inert atmosphere, the carbonization temperature is 800-1200℃, and the time is 1-8h; wherein the inert atmosphere includes at least one of nitrogen and argon.

[0020] Secondly, the present invention provides a porous carbon quantum dot, which is prepared by the method described above.

[0021] Thirdly, the present invention provides the application of the porous carbon quantum dots in the preparation of batteries.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] (1) The method for preparing porous carbon quantum dots of the present invention uses coal powder as raw material, which is inexpensive and readily available. The coal powder is then subjected to pyrolysis, acid washing, and solid-liquid separation treatment in sequence to obtain a first solution. A pore-forming agent is added to the first solution, and the type and weight ratio of the pore-forming agent to the first solution are specified. At least one of ammonium chloride, zinc chloride, and potassium hydroxide is mixed with the first solution, which will catalyze the dehydration of alcohol hydroxyl groups in the solid residue and the breaking of CO bonds in phenol / ether. At the same time, the substance itself has swelling and volatility, and will be released after heat treatment during subsequent carbonization. The carbon quantum dots form a large number of pore structures, thereby obtaining porous carbon quantum dots.

[0024] (2) Compared with traditional conductive agents, this porous carbon quantum dot has more sodium storage sites. When applied to sodium batteries, it can balance the energy density and power density of the cell. Moreover, due to the good conductivity of carbon quantum dots, it can also effectively improve the power density of the battery. Attached Figure Description

[0025] Figure 1 This is a scanning electron microscope image of the porous carbon quantum dots from Example 1;

[0026] Figure 2 The pore size distribution diagrams are for porous carbon quantum dots in Examples 1-2 and Comparative Example 1. Detailed Implementation

[0027] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0028] Example 1

[0029] A method for preparing porous carbon quantum dots includes the following steps:

[0030] (1) 6 wt% of volatile matter, D v50 coal powders with a particle size of 6μm and an ash content of 3wt% were heated and pyrolyzed at 250℃ for 6 hours under a nitrogen atmosphere to obtain pyrolysis material. After cooling, the pyrolysis material was placed in a hydrochloric acid solution for acid washing. The mass ratio of pyrolysis material, concentrated hydrochloric acid and water was 1:1:4. After filtration and separation, the resulting liquid was denoted as the first solution.

[0031] (2) Add a pore-forming agent to the first solution. The pore-forming agent is a combination of ammonium chloride, zinc chloride and potassium hydroxide in a mass ratio of 1:1:1. The mass ratio of the first solution to the pore-forming agent is 1:0.03. Stir evenly and then transfer to a polytetrafluoroethylene-lined reactor for hydrothermal reaction at 120°C for 8 hours to obtain the second solution.

[0032] (3) The second solution was centrifuged at 8000 rpm for 10 min. The separated solution was then dialyzed with a dialysis bag with a cutoff of 2000 Da for 10 h, and then freeze-dried at -84℃ for 24 h to obtain carbon quantum dots.

[0033] (4) Carbon quantum dots were carbonized at high temperature for 4 hours under nitrogen atmosphere and 1000℃ to obtain porous carbon quantum dots, which were denoted as Sample 1.

[0034] Example 2

[0035] A method for preparing porous carbon quantum dots includes the following steps:

[0036] (1) Add 5 wt% volatile matter and D v 50 coal powders with a particle size of 6μm and an ash content of 1wt% were heated and pyrolyzed at 250℃ for 6 hours under a nitrogen atmosphere to obtain pyrolyzed material. After cooling, the pyrolyzed material was placed in a hydrochloric acid solution for acid washing. The mass ratio of pyrolyzed material, concentrated hydrochloric acid and water was 1:1:4. After filtration and separation, the resulting liquid was denoted as the first solution.

[0037] (2) Add a pore-forming agent to the first solution. The pore-forming agent is a combination of ammonium chloride and zinc chloride with a mass ratio of 1:1. The mass ratio of the first solution to the pore-forming agent is 1:0.01. Stir evenly and then transfer to a polytetrafluoroethylene-lined reactor for hydrothermal reaction at 120°C for 4 hours to obtain the second solution.

[0038] (3) The second solution was centrifuged at 8000 rpm for 10 min. The separated solution was then dialyzed with a dialysis bag with a cutoff of 1000 Da for 24 h, and then freeze-dried at -84℃ for 30 h to obtain carbon quantum dots.

[0039] (4) The obtained carbon quantum dots are then carbonized at 1000℃ for 6 hours under a nitrogen atmosphere to obtain porous carbon quantum dots, which are denoted as sample 2.

[0040] Example 3

[0041] A method for preparing porous carbon quantum dots includes the following steps:

[0042] (1) Volatile matter 12wt%, D v 50 coal powders with a particle size of 8μm and an ash content of 2wt% were heated and pyrolyzed at 400℃ for 2 hours under a nitrogen atmosphere to obtain pyrolyzed material. After cooling, the pyrolyzed material was placed in a sulfuric acid solution for acid washing. The mass ratio of pyrolyzed material, concentrated sulfuric acid and water was 1:2:6. After filtration and separation, the resulting liquid was designated as the first solution.

[0043] (2) Add ammonium chloride, a pore-forming agent, to the first solution. The mass ratio of solution A to zinc chloride is 1:0.05. Stir evenly and then transfer to a polytetrafluoroethylene-lined reactor for hydrothermal reaction at 180°C for 8 hours to obtain the second solution.

[0044] (3) The second solution was centrifuged at 8000 rpm for 5 min. The separated solution was then dialyzed with a dialysis bag with a cutoff of 3500 Da for 15 h, and then freeze-dried at -84℃ for 30 h to obtain carbon quantum dots.

[0045] (4) The obtained carbon quantum dots are then carbonized at high temperature of 800℃ for 8 hours in a nitrogen atmosphere to obtain porous carbon quantum dots, which are denoted as sample 3.

[0046] Example 4

[0047] A method for preparing porous carbon quantum dots includes the following steps:

[0048] (1) Volatile matter 6wt%, D v 50 coal powders with a particle size of 2μm and an ash content of 3wt% were heated and pyrolyzed at 250℃ for 6 hours under a nitrogen atmosphere to obtain pyrolysis material. After cooling, the pyrolysis material was placed in a hydrochloric acid solution for acid washing. The mass ratio of pyrolysis material, concentrated hydrochloric acid and water was 1:0.5:6. After filtration and separation, the resulting liquid was denoted as the first solution.

[0049] (2) Add potassium hydroxide, a pore-forming agent, to the first solution. The mass ratio of solution A to zinc chloride is 1:0.03. Stir evenly and then transfer to a polytetrafluoroethylene-lined reactor for hydrothermal reaction at 120°C for 6 hours to obtain the second solution.

[0050] (3) The second solution was centrifuged at 12000 rpm for 10 min. The separated solution was then dialyzed with a dialysis bag with a cutoff of 2000 Da for 18 h and then dried at -94℃ for 30 h to obtain carbon quantum dots.

[0051] (4) The obtained carbon quantum dots are then carbonized at high temperature for 4 hours under nitrogen atmosphere and 1000℃ to obtain porous carbon quantum dots, which are denoted as sample 4.

[0052] Example 5

[0053] A method for preparing porous carbon quantum dots includes the following steps:

[0054] (1) Volatile matter 6wt%, D v 50 coal powders with a particle size of 6μm and an ash content of 3wt% were heated and pyrolyzed at 280℃ for 8 hours under a nitrogen atmosphere. The resulting pyrolysis material was cooled and then placed in a nitric acid solution for acid washing. The mass ratio of the pyrolysis material, concentrated nitric acid and water was 1:1:10. After filtration and separation, the resulting liquid was designated as the first solution.

[0055] (2) Add ammonium chloride, a pore-forming agent, to the first solution. The mass ratio of the first solution to zinc chloride is 1:0.02. Stir evenly and then transfer to a polytetrafluoroethylene-lined reactor for hydrothermal reaction at 160°C for 8 hours to obtain the second solution.

[0056] (3) The second solution was centrifuged at 8000 rpm for 8 min. The separated solution was then dialyzed with a dialysis bag with a cutoff of 2500 Da for 18 h and then dried at -94℃ for 36 h to obtain carbon quantum dots.

[0057] (4) The obtained carbon quantum dots are then carbonized at high temperature for 4 hours under nitrogen atmosphere and 1200℃ to obtain porous carbon quantum dots, which are denoted as sample 5.

[0058] Example 6

[0059] A method for preparing porous carbon quantum dots includes the following steps:

[0060] (1) Volatile matter 6wt%, D v 50 coal powders with a particle size of 10μm and an ash content of 3wt% were heated and pyrolyzed at 300℃ for 10h under an argon atmosphere to obtain pyrolysis material. After cooling, the pyrolysis material was placed in a hydrochloric acid solution for acid washing. The mass ratio of pyrolysis material, concentrated hydrochloric acid and water was 1:2:6. After filtration and separation, the resulting liquid was denoted as the first solution.

[0061] (2) Add zinc chloride, a pore-forming agent, to the first solution. The mass ratio of the first solution to zinc chloride is 1:0.03. Stir evenly and then transfer to a polytetrafluoroethylene-lined reactor for hydrothermal reaction at 150°C for 8 hours to obtain the second solution.

[0062] (3) The second solution was centrifuged at 10,000 rpm for 10 min. The separated solution was then dialyzed with a dialysis bag with a cutoff of 2000 Da for 18 h and then dried at -94℃ for 30 h to obtain carbon quantum dots.

[0063] (4) The carbon quantum dots are then carbonized at high temperature for 1 hour under nitrogen atmosphere and 1000℃ to obtain porous carbon quantum dots, which are denoted as sample 6.

[0064] Comparative Example 1

[0065] Acetylene black, brand name Li-100, purchased from Mitsubishi Japan.

[0066] Comparative Example 2

[0067] The difference between Comparative Example 2 and Example 1 is that no pore-forming agent is added to the porous carbon quantum dots in Comparative Example 2 in step (2).

[0068] Comparative Example 3

[0069] The difference between Comparative Example 3 and Example 1 is that the porous carbon quantum dots in Comparative Example 3 are made of calcium carbonate and ammonium bicarbonate in a mass ratio of 6:1.

[0070] Comparative Example 4

[0071] The difference between Comparative Example 4 and Example 1 is that the mass ratio of the first solution to the pore-forming agent in step (2) of the porous carbon quantum dots in Comparative Example 4 is 1:0.005.

[0072] Comparative Example 5

[0073] The difference between Comparative Example 5 and Example 1 is that the mass ratio of the first solution to the pore-forming agent in step (2) of the porous carbon quantum dots in Comparative Example 5 is 1:0.06.

[0074] Performance testing

[0075] 1. Powder conductivity test

[0076] The powder conductivity of the porous carbon quantum dots in Examples 1-6 was tested using a Nitto Seiko UMCP-PD51 powder analyzer under a pressure of 20 kN.

[0077] 2. Powder compaction test

[0078] The compaction density of the porous carbon quantum dots in Examples 1-6 was tested using a Shenzhen Sansi UTM7305 powder compactor under a pressure of 5T.

[0079] 3. Electrochemical performance testing

[0080] The porous carbon quantum dots obtained in Examples 1-6, the acetylene black of Comparative Example 1, and the carbon quantum dots of Comparative Example 2 were used as conductive agents for coin cells. They were prepared as negative electrodes in the following ratio (hard carbon: SBR: CMC: conductive agent = 95.5:2:1:1.5), with a sodium metal sheet as the positive electrode. The electrolyte was 1 mol / L NaPF6 / EC+DEC (1:1), and the cells were assembled into CR2032 coin cells. Battery testing was conducted using the Blue Battery Testing System, employing constant current charge-discharge testing at a current density of C / 10. The test results are shown in Table 1, under the conditions of a discharge cutoff voltage of 0V and a charge cutoff voltage of 2.0V.

[0081] 4. Scanning electron microscopy test

[0082] The porous carbon quantum dots of Example 1 were subjected to scanning electron microscopy (SEM) testing. The SEM morphology is shown in the figure below. Figure 1 .

[0083] 5. Pore volume testing of carbon quantum dots

[0084] The porous carbon quantum dots of Examples 1 and 2 and the carbon quantum dots of Comparative Example 2 were tested for pore volume. Specifically, the pore size of the samples was measured using a JW-BK200C fully automated specific surface area and micropore analyzer. The data are shown below. Figure 2 .

[0085] Table 1 Performance data results of each group of carbon quantum dots

[0086]

[0087] like Figure 1 As shown, the porous carbon quantum dots in Example 1 achieved quantum dot-level dimensions.

[0088] Table 1 shows that the compaction density of the porous carbon quantum dots in Examples 1-6 is not significantly different from that of the acetylene black in Comparative Example 1 and the carbon quantum dots in Comparative Example 2, indicating that the capacity of sodium batteries made from these materials is also not significantly different. However, the powder conductivity of the porous carbon quantum dots in Examples 1-6 is much higher than that of Comparative Example 1 and Comparative Example 2. The reversible specific capacity and rate charging capability of sodium batteries made from the porous carbon quantum dots in Examples 1-6 are also significantly higher than those of Comparative Example 1 and 2. Comparative Example 1 uses acetylene black as a conductive agent, but it suffers from poor conductivity and limited functionality. Comparative Example 2's carbon quantum dots do not contain a pore-forming agent, and therefore... Figure 2 It can be seen that the carbon quantum dots in Comparative Example 2 do not have a porous structure. Therefore, it is shown that the porous carbon quantum dots in Examples 1 and 2 can store more sodium sites because of their porous structure, which can improve the energy density of the battery. At the same time, the good conductivity of carbon quantum dots also improves the power density of the battery.

[0089] Comparative Example 3 used commercially available pore-forming agents calcium carbonate and ammonium bicarbonate, but the porosity and pore size of the porous carbon quantum dots obtained were lower than those of the pore-forming agents in Examples 1-6. This indicates that the type of pore-forming agent has a significant impact on the porous structure of carbon quantum dots. At least one of zinc chloride, zinc chloride, and potassium hydroxide can catalyze the dehydration of alcohol hydroxyl groups in solid residues and the breaking of CO bonds in phenols / ethers. At the same time, this substance itself has swelling and volatility, and will be released during subsequent carbonization and heat treatment. Carbon quantum dots enable the formation of a large number of porous structures.

[0090] In Comparative Examples 4 and 5, the mass ratios of the first solution to the pore-forming agent were 1:0.005 and 1:0.06, respectively. Neither ratio falls within the 1:(0.01–0.05) range specified in this invention. Too low a ratio results in insufficient pore-forming agent dosage, leading to less contact with the pore-forming substrate and thus less noticeable pore formation. Too high a concentration of the pore-forming agent causes excessive pore formation, resulting in overly large pores and damage to the material's morphology and structure. Therefore, this invention specifies a mass ratio of 1:(0.01–0.05) for the first solution to the pore-forming agent to obtain carbon quantum dots with a superior porous structure, thereby improving the performance of sodium batteries.

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing porous carbon quantum dots, characterized in that, Includes the following steps: (1) Pyrolysis of coal powder to obtain pyrolysis material, acid washing of the pyrolysis material, and solid-liquid separation by filtration to obtain a first solution; the acid washing is to mix the pyrolysis material, acid and water; wherein the mass ratio of pyrolysis material, acid and water is 1:(0.5-2):(2-10); (2) Add a pore-forming agent to the first solution, stir evenly, and then carry out a hydrothermal reaction to obtain a second solution; the pore-forming agent is ammonium chloride, zinc chloride, and potassium hydroxide in a mass ratio of (0~5):1:(0~3); The mass ratio of the first solution to the pore-forming agent is 1:(0.01~0.05); (3) The second solution is centrifuged, and the separated solution is then dialyzed and dried to obtain carbon quantum dots; (4) Carbonize the obtained carbon quantum dots to obtain porous carbon quantum dots.

2. The method for preparing porous carbon quantum dots as described in claim 1, characterized in that, The mass ratio of the first solution to the pore-forming agent is 1:(0.02~0.03).

3. The method for preparing porous carbon quantum dots as described in claim 1, characterized in that, The volatile matter content of the pulverized coal is ≥5wt%, the ash content is ≤3wt%, and the particle size is Dv. 50 It ranges from 1 to 10 μm.

4. The method for preparing porous carbon quantum dots as described in claim 1, characterized in that, In step (1), the pyrolysis is carried out under an inert atmosphere at a temperature of 250-400°C for 2-10 hours; wherein the inert atmosphere is at least one of nitrogen or argon. And / or, in step (1), the acid includes at least one of concentrated hydrochloric acid, concentrated nitric acid, and concentrated sulfuric acid.

5. The method for preparing porous carbon quantum dots as described in claim 1, characterized in that, In step (2), the temperature of the hydrothermal reaction is 120~180℃ and the reaction time is 4~8h.

6. The method for preparing porous carbon quantum dots as described in claim 1, characterized in that, In step (3), the dialysis cutoff is 1000~3500 Da and the dialysis time is 10~24 h; And / or, in step (3), the drying is carried out by freeze drying, the drying temperature is -84~-94℃, and the drying time is 24~36h; And / or, in step (3), the centrifugation speed is 7000~12000 r / min and the time is 5~10 min.

7. The method for preparing porous carbon quantum dots as described in claim 1, characterized in that, In step (4), the carbonization is carried out in an inert atmosphere, the carbonization temperature is 800~1200℃, and the time is 1~8h; wherein, the inert atmosphere includes at least one of nitrogen and argon.

8. A porous carbon quantum dot, characterized in that, The porous carbon quantum dots are prepared by the method for preparing porous carbon quantum dots according to any one of claims 1 to 7.

9. The application of the porous carbon quantum dots of claim 8 in the preparation of batteries.

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