Preparation method of carbon nanofiber

Abstract

The invention discloses a preparation method of carbon nanofibers. The preparation method comprises the following steps: (1) mixing and stirring ZIF-8, ferrite, copper salt and hydrazine hydrate in a solvent for a certain time to obtain a precursor; (2) uniformly mixing the precursor and polyacrylonitrile in a solvent to obtain a spinning solution; and (3) carrying out electrostatic spinning and carbonization on the spinning solution to obtain the carbon nanofiber. Compared with the prior art, the carbon nanofiber prepared by the method disclosed by the invention has abundant pore structures, uniform morphology and uniform distribution of elements such as iron, copper and nitrogen, so that the carbon nanofiber has ORR catalytic performance far superior to that of a commercial Pt / C catalyst, and can be used for a fuel cell cathode catalyst.

Description

Technical Field

[0001] The present invention belongs to the field of cathode catalysts, and in particular relates to a method for preparing carbon nanofibers used as cathode catalysts for fuel cells. Background Art

[0002] With the increasing use of flexible devices in daily life, flexible zinc-air batteries (Zn-air batteries) are attracting increasing attention due to their low cost, high energy density, excellent foldability, and lightweight and convenient design. However, the slow kinetics of the oxygen reduction reaction (ORR) at the cathode of flexible Zn-air batteries have hindered their application. Although commercial platinum-based catalysts, such as precious metals, can effectively accelerate the ORR, their high cost and poor stability limit their large-scale application.

[0003] Transition metal and nitrogen-doped carbon materials obtained by pyrolysis are considered to be one of the most promising catalytic materials. For example, CN115810765A discloses a carbon nanofiber catalyst, the preparation process of which includes (1) preparing ZnM-ZIF powder using a coprecipitation method using a zinc salt and a transition metal salt; (2) mixing a polymer source, an organic solvent, and the ZnM-ZIF powder obtained in step (1), and stirring them uniformly to obtain a spinning solution; (3) using the spinning solution obtained in step (2) to prepare a fiber precursor by an electrospinning method; (4) heat-treating the fiber precursor obtained in step (3) to obtain a carbon nanofiber catalyst; wherein M in the ZnM-ZIF powder is a transition metal, including any one of Co, Fe, or Ni, or a combination of at least two thereof. This type of material has the advantages of low cost, wide availability, and environmental friendliness. Summary of the Invention

[0004] In order to improve the ORR catalytic performance of the material, the present invention provides an improved method for preparing carbon nanofibers.

[0005] The technical solutions adopted by the present invention to achieve the above-mentioned purpose are as follows: A method for preparing carbon nanofibers comprises the following steps: (1) ZIF-8, ferrous salt, copper salt and hydrazine hydrate are mixed and stirred in a solvent for a certain period of time to obtain a precursor; (2) mixing the precursor and polyacrylonitrile in a solvent to obtain a spinning solution; (3) The spinning solution is subjected to electrospinning and carbonization to obtain the carbon nanofibers.

[0006] Preferably, the ratio of the total molar amount of the ferrous salt and the copper salt to the mass of ZIF-8 is 0.1-10 mmol / g, and the amount of hydrazine hydrate used is 15-25 times the mass of ZIF-8.

[0007] More preferably, the ratio of the total molar amount of the ferrous salt and the copper salt to the mass of ZIF-8 is 0.2-1 mmol / g, and the amount of hydrazine hydrate used is 18-22 times the mass of ZIF-8.

[0008] Most preferably, the ratio of the total molar amount of the ferrous salt and the copper salt to the mass of ZIF-8 is 0.42 mmol / g, and the amount of hydrazine hydrate used is 20 times the mass of ZIF-8.

[0009] Preferably, the Fe / Cu molar ratio of the ferrous salt and the cupric salt is 1-1.5:1.

[0010] More preferably, the Fe / Cu molar ratio is 1-1.1:1.

[0011] Most preferably, the Fe / Cu molar ratio is 1.06:1.

[0012] Preferably, the ferrous salt is ferrous sulfate or ferrous nitrate.

[0013] Preferably, the copper salt is copper sulfate or copper nitrate.

[0014] Preferably, the stirring time in step (1) is 10 to 30 hours.

[0015] More preferably, the stirring time is 24 hours.

[0016] Preferably, stirring is carried out at room temperature.

[0017] Preferably, the solvent in step (1) is methanol.

[0018] Preferably, the mass ratio of the precursor to polyacrylonitrile is 1-1.5:1.

[0019] More preferably, the mass ratio of the precursor to polyacrylonitrile is 1.2:1.

[0020] Preferably, in step (2), sodium chloride is also added to the spinning solution.

[0021] The precursor plays the role of introducing catalytically active atoms and also acts as a pore-forming agent, while the added sodium chloride plays the role of conducting the porous structure.

[0022] More preferably, the amount of sodium chloride used is 1 / 10 to 1 / 20 of the mass of the precursor.

[0023] Most preferably, the amount of sodium chloride used is 1 / 15 of the mass of the precursor.

[0024] Preferably, the solvent in step (2) is N,N-dimethylformamide.

[0025] Preferably, the voltage of the electrospinning is 15-25 kV, the flow rate is 0.1-1 mL / h, the temperature is 20-30° C., and the distance between the spinning nozzle and the receiving plate is 10-20 cm.

[0026] More preferably, the voltage is 22 kV, the flow rate is 0.5 mL / h, and the distance is 15 cm.

[0027] Preferably, the carbonization temperature is 800-1000° C., and the time is 1-3 hours.

[0028] More preferably, the temperature is 900° C. and the time is 2 hours.

[0029] Preferably, the heating rate is 5°C / min.

[0030] Preferably, the temperature is first raised to 250-300° C. in an air atmosphere for heat treatment for a certain time, and then the temperature is raised to 800-1000° C. in an inert atmosphere.

[0031] More preferably, the inert atmosphere is nitrogen or argon.

[0032] More preferably, the heat treatment is performed at 250-300° C. for 1-3 hours.

[0033] A carbon nanofiber prepared according to the method.

[0034] The carbon nanofibers are used as cathode catalysts in the preparation of fuel cells.

[0035] Preferably, the fuel cell is a zinc-air battery.

[0036] Beneficial effects: Compared with the prior art, the carbon nanofibers prepared by the method of the present invention have a rich pore structure and a uniform morphology, and elements such as iron, copper, and nitrogen are evenly distributed.

[0037] Compared with the existing technology, the carbon nanofibers prepared by the method of the present invention have better ORR catalytic performance, which is not only far superior to carbon nanofibers doped with iron and copper single atoms, but also far superior to commercial Pt / C catalysts.

[0038] Compared with the prior art, the method of the present invention can perform spinning when the amounts of precursor and polyacrylonitrile are relatively high, thereby achieving high-concentration doping of carbon nanofibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the morphology and element distribution diagram of the carbon nanofiber in Example 1.

[0040] Figure 2 LSV curves of different carbon nanofibers under saturated oxygen.

[0041] Figure 3 is the open circuit voltage of the flexible zinc-air battery prepared using the carbon nanofibers of Example 1 as the cathode catalyst.

[0042] Figure 4 This is a diagram of the power density of the flexible zinc-air battery prepared using the carbon nanofibers of Example 1 as the cathode catalyst. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is further described in detail below with reference to the embodiments.

[0044] Synthesis of ZIF-8: Dissolve 2-methylimidazole (10.5097 g) in methanol (144 mL) and sonicate for 20 minutes to create Solution A. Separately, dissolve zinc nitrate hexahydrate (4.7606 g) in methanol (144 mL) and sonicate for 20 minutes to create Solution B. Use a pipette to slowly add Solution A to Solution B while stirring (6 minutes). Stir at 35°C for 4 hours. Filter, wash three times with methanol, and dry at 60°C overnight.

[0045] Example 1 (1) Synthesis of FeCu@ZIF-8: Disperse 500 mg of ZIF-8 in 50 mL of methanol and sonicate for 30 minutes. Dissolve 30 mg of ferrous sulfate heptahydrate and 30 mg of copper nitrate hexahydrate in 10 mL of methanol and stir for 20 minutes. Combine the two solutions, then add 10 mL of hydrazine hydrate and stir for 24 hours. Filter using a vacuum pump and rinse three times with methanol. Dry the filtered sample at 60°C overnight.

[0046] (2) Preparation of spinning solution: 0.25 g of polyacrylonitrile, 0.02 g of sodium chloride, and 0.3 g of FeCu@ZIF-8 were dissolved in 0.28 mL of N,N-dimethylformamide (DMF) and stirred for 30 h to obtain the spinning solution.

[0047] (3) The spinning solution was placed in an electrospinning apparatus and electrospun at a temperature of 20-30°C, a voltage of 22 kV, and a flow rate of 0.5 mL / h, with the distance between the spinning nozzle and the receiving plate maintained at 15 cm. The resulting nanofibers were vacuum dried at 80°C overnight.

[0048] (4) The dried nanofibers were then heated to 280 °C in an air atmosphere at a heating rate of 5 °C / min and maintained for 2 h, and then heated to 900 °C in a nitrogen atmosphere at a heating rate of 5 °C / min and maintained for 2 h, finally obtaining carbon nanofibers (Fe-Cu@CNFs).

[0049] Scanning electron microscopy images of carbon nanofibers Figure 1 A. Figure 1 As shown in B, the transmission electron microscope image is as follows Figure 1 As shown in Figure C, it can be seen from the electron microscope image that the prepared carbon nanofibers have rich pore structure, uniform morphology and good dispersion.

[0050] Figure 1 D is the high-angle annular dark field and element distribution electron microscope image of carbon nanofibers, from which it can be seen that the elements are evenly distributed.

[0051] Comparative Example 1 (1) Preparation of spinning solution: 0.25 g of polyacrylonitrile was dissolved in 0.28 mL of DMF and stirred for 30 h to obtain the spinning solution; (2) The spinning solution was placed in an electrospinning apparatus and electrospun at a temperature of 20-30°C, a voltage of 22 kV, and a flow rate of 0.5 mL / h, with the distance between the spinning nozzle and the receiving plate maintained at 15 cm. The resulting nanofibers were vacuum dried at 80°C overnight.

[0052] (3) The dried nanofibers were then heated to 280 °C in an air atmosphere at a heating rate of 5 °C / min and maintained for 2 h, and then heated to 900 °C in a nitrogen atmosphere at a heating rate of 5 °C / min and maintained for 2 h, finally obtaining carbon nanofibers.

[0053] Comparative Example 2 (1) Synthesis of Fe@ZIF-8: Disperse 500 mg of ZIF-8 in 50 mL of methanol and sonicate for 30 minutes. Dissolve 30 mg of ferrous sulfate heptahydrate in 10 mL of methanol and stir for 20 minutes. Combine the two solutions, then add 10 mL of hydrazine hydrate and stir for 24 hours. Filter using a vacuum pump and rinse three times with methanol. Dry the filtered sample at 60°C overnight.

[0054] (2) Preparation of spinning solution: 0.25 g of polyacrylonitrile, 0.02 g of sodium chloride, and 0.30 g of Fe@ZIF-8 were dissolved in 0.28 mL of DMF and stirred for 30 h to obtain the spinning solution.

[0055] (3) The spinning solution was placed in an electrospinning apparatus and electrospun at a temperature of 20-30°C, a voltage of 22 kV, and a flow rate of 0.5 mL / h, with the distance between the spinning nozzle and the receiving plate maintained at 15 cm. The resulting nanofibers were vacuum dried at 80°C overnight.

[0056] (4) The dried nanofibers were then heated to 280 °C in an air atmosphere at a heating rate of 5 °C / min and maintained for 2 h, and then heated to 900 °C in a nitrogen atmosphere at a heating rate of 5 °C / min and maintained for 2 h, finally obtaining carbon nanofibers.

[0057] Comparative Example 3 (1) Synthesis of Cu@ZIF-8: Disperse 500 mg of ZIF-8 in 50 mL of methanol and sonicate for 30 minutes. Dissolve 30 mg of copper nitrate hexahydrate in 10 mL of methanol and stir for 20 minutes. Combine the two solutions, then add 10 mL of hydrazine hydrate and stir for 24 hours. Filter using a vacuum pump and rinse three times with methanol. Dry the filtered sample at 60°C overnight.

[0058] (2) Preparation of spinning solution: 0.25 g of polyacrylonitrile, 0.02 g of sodium chloride, and 0.30 g of Cu@ZIF-8 were dissolved in 0.28 mL of DMF and stirred for 30 h to obtain the spinning solution.

[0059] (3) The spinning solution was placed in an electrospinning apparatus and electrospun at a temperature of 20-30°C, a voltage of 22 kV, and a flow rate of 0.5 mL / h, with the distance between the spinning nozzle and the receiving plate maintained at 15 cm. The resulting nanofibers were vacuum dried at 80°C overnight.

[0060] (4) The dried nanofibers were then heated to 280 °C in an air atmosphere at a heating rate of 5 °C / min and maintained for 2 h, and then heated to 900 °C in a nitrogen atmosphere at a heating rate of 5 °C / min and maintained for 2 h, finally obtaining carbon nanofibers.

[0061] Comparative Example 4 (1) Synthesis of Fe@ZIF-8: Disperse 500 mg of ZIF-8 in 50 mL of methanol and sonicate for 30 minutes. Dissolve 60 mg of ferrous sulfate heptahydrate in 10 mL of methanol and stir for 20 minutes. Combine the two solutions, then add 10 mL of hydrazine hydrate and stir for 24 hours. Filter using a vacuum pump and rinse three times with methanol. Dry the filtered sample at 60°C overnight.

[0062] (2) Preparation of spinning solution: 0.25 g of polyacrylonitrile, 0.02 g of sodium chloride, and 0.30 g of Fe@ZIF-8 were dissolved in 0.28 mL of DMF and stirred for 30 h to obtain the spinning solution.

[0063] (3) The spinning solution was placed in an electrospinning apparatus and electrospun at a temperature of 20-30°C, a voltage of 22 kV, and a flow rate of 0.5 mL / h, with the distance between the spinning nozzle and the receiving plate maintained at 15 cm. The resulting nanofibers were vacuum dried at 80°C overnight.

[0064] (4) The dried nanofibers were then heated to 280 °C in an air atmosphere at a heating rate of 5 °C / min and maintained for 2 h, and then heated to 900 °C in a nitrogen atmosphere at a heating rate of 5 °C / min and maintained for 2 h, finally obtaining carbon nanofibers.

[0065] Comparative Example 5 (1) Synthesis of Cu@ZIF-8: Disperse 500 mg of ZIF-8 in 50 mL of methanol and sonicate for 30 minutes. Dissolve 60 mg of copper nitrate hexahydrate in 10 mL of methanol and stir for 20 minutes. Combine the two solutions, then add 10 mL of hydrazine hydrate and stir for 24 hours. Filter using a vacuum pump and rinse with methanol three times. Dry the filtered sample at 60°C overnight.

[0066] (2) Preparation of spinning solution: 0.25 g of polyacrylonitrile, 0.02 g of sodium chloride, and 0.30 g of FeCu@ZIF-8 were dissolved in 0.28 mL of DMF and stirred for 30 h to obtain the spinning solution.

[0067] (3) The spinning solution was placed in an electrospinning apparatus and electrospun at a temperature of 20-30°C, a voltage of 22 kV, and a flow rate of 0.5 mL / h, with the distance between the spinning nozzle and the receiving plate maintained at 15 cm. The resulting nanofibers were vacuum dried at 80°C overnight.

[0068] (4) The dried nanofibers were then heated to 280 °C in an air atmosphere at a heating rate of 5 °C / min and maintained for 2 h, and then heated to 900 °C in a nitrogen atmosphere at a heating rate of 5 °C / min and maintained for 2 h, finally obtaining carbon nanofibers.

[0069] Comparative Example 6 (1) Synthesis of FeCu@ZIF-8: Disperse 500 mg of ZIF-8 in 50 mL of methanol and sonicate for 30 minutes. Dissolve 30 mg of ferrous sulfate heptahydrate and 30 mg of copper nitrate hexahydrate in 10 mL of methanol and stir for 20 minutes. Combine the two solutions and stir for 24 hours. Filter using a vacuum pump and rinse with methanol three times. Dry the filtered sample at 60°C overnight.

[0070] (2) Preparation of spinning solution: 0.25 g of polyacrylonitrile, 0.02 g of sodium chloride, and 0.30 g of FeCu@ZIF-8 were dissolved in 0.28 mL of DMF and stirred for 30 h to obtain the spinning solution.

[0071] (3) When the spinning solution is placed in the electrospinning device, there is no obvious Taylor cone. Spinning cannot be performed.

[0072] The results of Comparative Example 6 show that the addition of hydrazine hydrate during the synthesis of the precursor is the key to the successful spinning of the precursor and polyacrylonitrile at a high dosage ratio.

[0073] ORR catalytic performance Catalyst samples: the carbon nanofibers of Example 1 and Comparative Examples 1-5, and a commercially available 20 wt% Pt / C catalyst.

[0074] The ORR activity of the catalysts was tested on a CHI 760 electrochemical workstation using a three-electrode system. 2 ) as the working electrode, graphite electrode as the counter electrode, and Ag / AgCl electrode as the reference electrode. The test was carried out at 25°C and O2-saturated 0.1 mol·L -1 The scan was performed in KOH solution at a scan rate of 10 mV·s -1 , rotation speed 1600 rpm, potential range 0.16~1.16 V vs . RHE. All potentials were converted to the RHE scale using the Nernst equation: E (RHE) = E (Ag / AgCl) + 0.0591×pH + 0.197. The LSV curve obtained by the test is as follows Figure 2 As shown, the catalytic performance of the comparative example 1 without metal doping is poor, and its half-wave potential ( E 1 / 2 ) is only 0.64 V vs . RHE. In contrast, Fe or Cu metal doping significantly improved the oxygen reduction reaction (ORR) activity of the catalyst. Among them, Cu-doped comparative example 3 showed better performance, with an onset potential ( E onset ) and half-wave potential reached 0.96 V, respectively vs . RHE and 0.87 V vs . RHE, slightly higher than that of Fe-doped comparative example 2 ( E onset = 0.94 V, E 1 / 2 = 0.84 V vs. RHE). This performance improvement is mainly attributed to the Cu-N x and Fe-N x It is worth noting that Example 1 exhibits the best ORR catalytic performance among all tested samples, with an onset potential as high as 0.99 V. vs . RHE, half-wave potential reaches 0.89 V vs . RHE, and outperformed the commercial 20% Pt / C catalyst ( E onset = 0.98 V, E 1 / 2 = 0.86 V vs . RHE), fully demonstrated the superiority of this catalyst in oxygen reduction reaction.

[0075] Flexible zinc-air battery Flexible zinc-air batteries (FZABs) were assembled using the carbon nanofibers prepared in Example 1 as the air cathode, zinc foil as the anode, and a PVA-based solid gel as the electrolyte. Before testing, the assembled batteries were allowed to stand at a constant temperature of 25°C for 30 minutes to achieve stability. The PVA-based solid gel was prepared as follows: 0.5 g of polyvinyl alcohol (PVA) was dissolved in 50 mL of ultrapure water and stirred at 90°C for 2 hours until completely dissolved. Then, 10 mL of a mixed solution containing 0.2 M zinc acetate (Zn(Ac)2) and 2 M potassium hydroxide (KOH) was added. Stirring and heating were continued for 1 hour to ensure uniform mixing. The solution was then refrigerated at 4°C for 12 hours and thawed at room temperature to form a stable gel electrolyte.

[0076] The open circuit voltage test directly measures the stable potential difference between the two electrodes of the battery through the electrochemical workstation and the multimeter. The test results are as follows: Figure 3 The power density test was performed by linear sweep voltammetry in the voltage range of 0.20-1.55 V at a scan rate of 5mV / s. The test results are shown as follows: Figure 4 shown.

[0077] The test results show that the battery exhibits excellent electrochemical performance, with an open circuit voltage of 1.47 V and a maximum power density of 286.5 mW / cm 2 .

[0078] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing carbon nanofibers, comprising the following steps: (1) ZIF-8, ferrous salt, copper salt and hydrazine hydrate are mixed and stirred in a solvent for a certain period of time to obtain a precursor; (2) mixing the precursor and polyacrylonitrile in a solvent to obtain a spinning solution; (3) The spinning solution is subjected to electrospinning and carbonization to obtain the carbon nanofibers.

2. The preparation method according to claim 1, wherein: The ratio of the total molar amount of the ferrous salt and the copper salt to the mass of ZIF-8 is 0.1-10 mmol / g, and the amount of hydrazine hydrate used is 15-25 times the mass of ZIF-8; Preferably, the ratio of the total molar amount of the ferrous salt and the copper salt to the mass of ZIF-8 is 0.2 to 1 mmol / g, and the amount of hydrazine hydrate used is 18 to 22 times the mass of ZIF-8; More preferably, the ratio of the total molar amount of the ferrous salt and the copper salt to the mass of ZIF-8 is 0.42 mmol / g, and the amount of hydrazine hydrate used is 20 times the mass of ZIF-8.

3. The preparation method according to claim 1, wherein: The Fe / Cu molar ratio of the ferrous salt and the copper salt is 1 to 1.5:1; Preferably, the Fe / Cu molar ratio is 1 to 1.1:1; More preferably, the Fe / Cu molar ratio is 1.06:1; Preferably, the ferrous salt is ferrous sulfate or ferrous nitrate, and the copper salt is copper sulfate or copper nitrate.

4. The preparation method according to claim 1, wherein: The stirring time in step (1) is 10 to 30 hours; Preferably, the stirring time is 24 hours; Preferably, stirring is carried out at room temperature.

5. The preparation method according to claim 1, wherein: The mass ratio of the precursor to polyacrylonitrile is 1-1.5:1; Preferably, the mass ratio of the precursor to polyacrylonitrile is 1.2:

1.

6. The preparation method according to claim 1, wherein: In step (2), sodium chloride is also added to the spinning solution; Preferably, the amount of sodium chloride used is 1 / 10 to 1 / 20 of the mass of the precursor; More preferably, the amount of sodium chloride used is 1 / 15 of the mass of the precursor.

7. The preparation method according to claim 1, wherein: The electrospinning voltage is 15-25 kV, the flow rate is 0.1-1 mL / h, the temperature is 20-30 ° C, and the distance between the spinning nozzle and the receiving plate is 10-20 cm; Preferably, the voltage is 22 kV, the flow rate is 0.5 mL / h, and the distance is 15 cm.

8. The preparation method according to claim 1, wherein: The carbonization temperature is 800-1000°C and the time is 1-3 hours; Preferably, the temperature is 900°C and the time is 2 hours; Preferably, the heating rate is 5°C / min; Preferably, the temperature is first raised to 250-300° C. in an air atmosphere for heat treatment for a certain time, and then the temperature is raised to 800-1000° C. in an inert atmosphere.

9. Carbon nanofibers prepared according to the method of any one of claims 1 to 8.

10. Use of the carbon nanofiber according to claim 9 as a cathode catalyst in the preparation of a fuel cell; Preferably, the fuel cell is a zinc-air battery.

Images