Method for preparing Fe / Co-N-C catalyst

Abstract

The invention relates to a method for preparing a Fe / Co-N-C catalyst through a subject-object strategy. The preparation method comprises the following steps: adding zinc nitrate hexahydrate, cobalt nitrate hexahydrate, F127, 2-methylimidazole and 30nm Fe2O3 nanoparticles into a methanol solution, uniformly stirring, carrying out reflux reaction, centrifuging, drying to obtain a Fe2O3 (at) Co-ZIF-8 precursor, and carrying out high-temperature pyrolysis calcination on the Fe2O3 (at) Co-ZIF-8 precursor to obtain the Fe / Co-N-C catalyst. The method is simple in synthesis steps, and the prepared catalyst is suitable for efficiently catalyzing an acidic oxygen reduction reaction.

Description

Technical Field

[0001] This invention relates to a method for preparing Fe / Co-NC catalysts, belonging to the field of catalyst preparation technology. Background Technology

[0002] With the energy crisis and environmental pollution becoming increasingly severe, the development of sustainable clean energy technologies has become a global consensus. Proton exchange membrane fuel cells (PEMFCs) are considered one of the most promising clean energy solutions due to their high energy conversion efficiency and environmental friendliness. However, the oxygen reduction reaction (ORR) at the cathode of PEMFCs suffers from slow kinetics, heavily relying on highly efficient catalysts to improve reaction efficiency. Currently, while platinum-based catalysts exhibit excellent ORR activity, their high cost and scarcity limit the widespread commercial application of PEMFCs. Therefore, developing low-cost, highly active, and stable non-precious metal ORR catalysts has become a research hotspot.

[0003] In recent years, transition metal-nitrogen-carbon (MNC) catalysts have attracted widespread attention due to their excellent objective response rate (ORR) performance under acidic conditions. Among them, Fe-NC catalysts exhibit high ORR activity, but during high-temperature synthesis, Fe atoms tend to aggregate into particles, leading to a reduction in active sites and decreased stability. In contrast, Co-NC catalysts possess better stability, but their ORR activity is relatively low. To overcome the limitations of single-metal catalysts, bimetallic doping strategies (such as Fe / Co-NC) have been proposed, aiming to combine the high activity of Fe-NC with the excellent stability of Co-NC. However, existing Fe / Co-NC catalyst preparation processes are complex, and it is difficult to effectively suppress Fe aggregation under high-temperature conditions, resulting in uneven distribution of active sites and limiting further performance improvements. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a simple and efficient method for preparing Fe / Co-NC diatomic catalysts. This method utilizes a Co-ZIF-8 metal-organic framework structure to coat Fe₂O₃ nanoparticles, forming a Fe₂O₃@Co-ZIF-8 precursor. During pyrolysis, the introduction of cobalt ions effectively inhibits iron atom aggregation, promotes uniform iron dispersion, and coordinates with nitrogen atoms to form FeN₄ active sites. Simultaneously, cobalt ions and nitrogen atoms form a coordinated coating on the exterior of the FeN₄ sites, further optimizing the catalyst's electronic structure. This method not only solves the problem of iron agglomeration under high-temperature conditions but also significantly improves the catalyst's ORR activity and stability.

[0005] The implementation process of this invention is as follows: A method for preparing an Fe / Co-NC catalyst, characterized by comprising the following steps: (1) After stirring zinc nitrate, cobalt nitrate and 5~30 nm Fe2O3 nanoparticles in methanol solution evenly, add methanol solution containing surfactant F127 block copolymer (PEO106-PPO70-PEO106), mix evenly, add methanol solution containing 2-methylimidazole, wherein the molar ratio of zinc nitrate to 2-methylimidazole is 1:3, the molar ratio of zinc nitrate to cobalt nitrate is (60~150):1, the molar ratio of cobalt nitrate to Fe2O3 is (5~30):1, and the mass ratio of F127 to 2-methylimidazole is 1:(2~5). (2) The well-mixed solution is refluxed at 40~60℃; (3) After the reaction is complete, centrifugation and drying are performed to obtain the Fe2O3@Co-ZIF-8 precursor; (4) The Fe2O3@Co-ZIF-8 precursor was pyrolyzed and calcined at 900~1100 °C in an inert atmosphere to obtain the Fe / Co-NC catalyst.

[0006] In step (1) above, the molar ratio of zinc nitrate to cobalt nitrate is (80~120):1, the molar ratio of cobalt nitrate to Fe2O3 is (10~20):1, and the mass ratio of F127 to 2-methylimidazole is 1:(2~4).

[0007] In step (4) above, the inert atmosphere is argon or nitrogen.

[0008] In step (4) above, the calcination and heat preservation time is 1~3 h, and the heating rate is 5~30 ℃ / min.

[0009] The Fe / Co-NC catalyst prepared by the above method exhibits excellent catalytic activity in acidic ORR, with a half-wave potential of 0.840 V.

[0010] The Fe / Co-NC catalyst prepared by the above method is used as an oxygen reduction catalyst in proton exchange membrane fuel cells. Specifically, the oxygen reduction reaction catalyzed by the Fe / Co-NC catalyst is carried out under 0.5 M H₂SO₄ conditions.

[0011] The beneficial effects of this invention are as follows: This invention successfully prepares a high-performance Fe / Co-NC diatomic catalyst by employing a simple host-guest strategy. The process is concise, has a short cycle time, and is highly operable. Specifically, by coating Fe2O3 nanoparticles with Co-ZIF-8, cobalt ions are introduced during pyrolysis, effectively inhibiting the aggregation of Fe atoms, promoting uniform dispersion of Fe, and forming FeN4 active sites with nitrogen atoms. Simultaneously, the coordination of cobalt ions and nitrogen atoms around the FeN4 sites optimizes the electronic structure of the catalyst. This method solves the problem of Fe's tendency to aggregate at high temperatures, providing a new approach for the design of non-noble metal catalysts. The prepared Fe / Co-NC catalyst exhibits excellent catalytic performance in ORR under acidic conditions, with a half-wave potential reaching 0.840 V, providing a simple and efficient solution for the preparation of acidic oxygen reduction catalysts, with broad application potential. Attached Figure Description

[0012] Figure 1 This is a TEM image of the Fe2O3@Co-ZIF-8 precursor in Example 1; Figure 2 The image shows a TEM image of the Fe / Co-NC catalyst in Example 1. Figure 3 The image shows a TEM image of the Fe2O3-Co-ZIF-8 precursor in Example 4. Figure 4 The image shows a TEM image of the Fe-NC catalyst in Example 5. Figure 5 The X-ray diffraction (XRD) pattern of the Fe / Co-NC catalyst in Example 1; Figure 6 The linear sweep voltammetry curves of the Fe / Co-NC catalyst in Example 1 are shown below. Figure 7 The linear sweep voltammetry curve for Example 2; Figure 8 The linear sweep voltammetry curve for Example 3; Figure 9 Linear sweep voltammetry curves of the catalysts in Examples 1 and 4; Figure 10 Linear sweep voltammetry curves of the catalysts in Examples 1 and 5 are shown. Detailed Implementation

[0013] The technical solution of the present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto. For parts of the embodiments where specific experimental steps or conditions are not specified, they can be performed according to general experimental methods in the art or conventional operations described in the literature. Unless otherwise specified, the reagents or instruments used are all conventional products that can be obtained through commercial channels. Example 1

[0014] 3.8675 g of zinc nitrate hexahydrate, 43.65 mg of cobalt nitrate hexahydrate, and 1.5 mg of Fe₂O₃ particles with a particle size of 5 nm were dissolved in 25 mL of methanol. After thorough mixing, 25 mL of methanol solution containing 1 g of F127 was added, and the mixture was sonicated for 5 minutes. Next, 50 mL of methanol solution containing 3.2 g of 2-methylimidazole was added. The resulting solution was transferred to a 60 °C water bath and refluxed for 30 minutes. After the reaction was complete, the solution was allowed to stand at room temperature for 12 hours, then repeatedly washed with ethanol and centrifuged. After washing, the product was placed in a vacuum drying oven and dried at 60 °C for 12 hours. After drying, the sample was ground uniformly to obtain Fe₂O₃@Co-ZIF-8 precursor powder for subsequent preparations.

[0015] 100 mg of Fe₂O₃@Co-ZIF-8 precursor powder was placed in a quartz boat and then placed in a tube furnace. Heating was carried out under an argon atmosphere at a flow rate of 60 sccm and a heating rate of 30 °C·min⁻¹. After reaching 1100 °C, the temperature was maintained for 1 hour, and then slowly cooled to room temperature. Finally, the obtained product was ground to obtain the Fe / Co-NC catalyst.

[0016] Figure 1 The image shows a TEM image of the Fe2O3@Co-ZIF-8 precursor of this invention, which shows that Fe2O3 is successfully encapsulated inside the grains.

[0017] Figure 2 This is a TEM image of the Fe / Co-NC catalyst of the present invention. It can be seen that the catalyst has uniform crystal grains and no obvious particles are present.

[0018] Figure 5 The XRD pattern of the Fe / Co-NC catalyst of this invention shows that only two main diffraction peaks are observed at approximately 26.5° and 44.6°, which correspond to the (002) and (101) crystal planes of carbon, respectively. No XRD peaks associated with any Co or Fe crystal phases were observed in the catalyst, indicating that Co or Fe elements exist in single-atom form throughout the nitrogen-doped carbon material.

[0019] Figure 6The LSV curve of the Fe / Co-NC catalyst of the present invention shows that the half-wave potential of the Fe / Co-NC diatomic catalyst is 0.840 V, which means that the diatomic catalyst has high oxygen reduction activity. Example 2

[0020] The preparation process of Example 1 is used, except that the content of Fe2O3 is 0~4 mg.

[0021] Figure 7 The LSV curve of the Fe / Co-NC catalyst of this invention shows that the catalyst performance is optimal when the Fe2O3 content is 1.5 mg. Example 3

[0022] The preparation process of Example 1 is used, except that the content of cobalt ions is 0~2 mmol.

[0023] Figure 8 The LSV curve of the Fe / Co-NC catalyst of this invention shows that the catalyst performance is optimal when the cobalt ion content is 0.15 mmol. Example 4

[0024] To demonstrate the effectiveness of the host-guest strategy of this invention, the raw materials were used in the same amounts as in Example 1, only the synthesis method of the precursor was changed. The specific steps are as follows: 3.8675 g of zinc nitrate hexahydrate and 43.65 mg of cobalt nitrate hexahydrate were dissolved in 25 mL of methanol solution and stirred until homogeneous. Then, 25 mL of methanol solution containing 1 g of F127 was added, and the mixture was sonicated for 5 minutes. Next, 50 mL of methanol solution containing 3.2 g of 2-methylimidazole was added. The solution was then transferred to a 60 °C water bath and refluxed for 30 minutes. After the reaction was complete, the solution was allowed to stand at room temperature for 12 hours, then repeatedly washed with ethanol solution and centrifuged. After washing, the solution was placed in a vacuum drying oven and dried for 12 hours at 60 °C. After complete drying, the solution was ground to obtain ZIF-8 powder. 1.5 mg of Fe2O3 nanoparticles were then thoroughly ground with ZIF-8 to obtain the Fe2O3-Co-ZIF-8 precursor.

[0025] Weigh 100 mg of Fe2O3-Co-ZIF-8 precursor powder and place it in a quartz boat. Place the boat in a tube furnace, maintain an Ar atmosphere, a flow rate of 60 sccm, and a heating rate of 30 ºC·min. -1 The temperature was raised to 1100 ºC and held for 1 h. After cooling to room temperature, the catalyst was ground to obtain Fe-Co-NC catalyst.

[0026] Figure 3The TEM image of the Fe2O3-Co-ZIF-8 precursor of this invention shows that Fe2O3 is located outside the grains.

[0027] Figure 9 The graph shows a performance comparison between the Fe-Co-NC catalyst of this invention and the Fe / Co-NC catalyst of Example 1. It can be seen that the half-wave potential of the Fe / Co-NC catalyst is much higher than that of the Fe-Co-NC catalyst, which also proves the advantages of the host-guest strategy in preparing diatomic catalysts. Example 5

[0028] To verify the role of cobalt ions, a single-atom catalyst was prepared using the aforementioned host-guest strategy, with ZIF-8 as the host and Fe2O3 nanoparticles as the guest, without the addition of cobalt ions. The specific steps are as follows: 3.8675 g of zinc nitrate hexahydrate and 1.5 mg of Fe₂O₃ nanoparticles were dissolved in 25 mL of methanol and stirred thoroughly. Then, 25 mL of methanol solution containing 1 g of F127 was added. The mixture was then sonicated for 5 minutes. Next, 50 mL of methanol solution containing 3.2 g of 2-methylimidazole was added. The resulting solution was transferred to a 60 °C water bath and refluxed for 30 minutes. After the reaction was complete, the solution was allowed to stand at room temperature for 12 hours, then repeatedly washed with ethanol and centrifuged. After washing, the product was placed in a vacuum drying oven and dried at 60 °C for 12 hours. After drying, the sample was ground uniformly to obtain Fe₂O₃@ZIF-8 precursor powder for later use.

[0029] 100 mg of Fe2O3@ZIF-8 precursor powder was placed in a quartz boat and then placed in a tube furnace. The furnace was heated under an argon atmosphere at a flow rate of 60 sccm and a heating rate of 30 °C·min⁻¹. After reaching 1100 °C, the temperature was maintained for 1 hour, and then slowly cooled to room temperature. Finally, the product was ground to obtain the Fe-NC catalyst.

[0030] Figure 4 The image shows a TEM image of the Fe-NC catalyst of the present invention, in which obvious particles can be observed.

[0031] Figure 10 This is a performance comparison graph between the Fe-NC catalyst of the present invention and the Fe / Co-NC catalyst in Example 1. The results show that the half-wave potential of the Fe / Co-NC diatomic catalyst in Example 1 is significantly higher than that of the Fe-NC catalyst, indicating that the introduction of cobalt ions effectively improves the oxygen reduction activity of the catalyst.

Claims

1. A method for preparing an Fe / Co-NC catalyst, characterized in that... Includes the following steps: (1) After stirring zinc nitrate, cobalt nitrate and 5~30 nm Fe2O3 nanoparticles in methanol solution evenly, add methanol solution containing surfactant F127 block copolymer, mix evenly, and then add methanol solution containing 2-methylimidazole. The molar ratio of zinc nitrate to 2-methylimidazole is 1:3, the molar ratio of zinc nitrate to cobalt nitrate is (60~150):1, the molar ratio of cobalt nitrate to Fe2O3 is (5~30):1, and the mass ratio of F127 to 2-methylimidazole is 1:(2~5). (2) The well-mixed solution is refluxed at 40~60℃; (3) After the reaction is complete, centrifugation and drying are performed to obtain the Fe2O3@Co-ZIF-8 precursor; (4) The Fe2O3@Co-ZIF-8 precursor was pyrolyzed and calcined at 900~1100 °C in an inert atmosphere to obtain the Fe / Co-NC catalyst.

2. The method for preparing the Fe / Co-NC catalyst according to claim 1, characterized in that: In step (1), the molar ratio of zinc nitrate to cobalt nitrate is (80~120):1, the molar ratio of cobalt nitrate to Fe2O3 is (10~20):1, and the mass ratio of F127 to 2-methylimidazole is 1:(2~4).

3. The method for preparing the Fe / Co-NC catalyst according to claim 1, characterized in that: In step (4), the inert atmosphere is argon or nitrogen.

4. The method for preparing the Fe / Co-NC catalyst according to claim 1, characterized in that: In step (4), the calcination holding time is 1~3 h, and the heating rate is 5~30 ℃ / min.

5. The Fe / Co-NC catalyst prepared by the method described in claim 1.

6. The application of the Fe / Co-NC catalyst of claim 5 as an oxygen reduction catalyst in a proton exchange membrane fuel cell.

7. The application according to claim 6, characterized in that: The oxygen reduction reaction catalyzed by the Fe / Co-NC catalyst was carried out under the condition of 0.5 MH2SO4.

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