A carbon layer coated NiFe nanoparticle bifunctional composite electrocatalyst and its preparation method and application

Abstract

This invention provides a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst, its preparation method, and its application, belonging to the field of new energy materials technology. The invention uses dimethylimidazole and transition metal salts to synthesize transition metal-organic framework compounds with specific morphologies and sizes. These compounds are then pyrolyzed to prepare nitrogen-doped carbon materials (Fe-N-C) containing transition metal iron, exhibiting specific morphology and high oxygen reduction activity (ORR). The Fe-N-C material is then physically mixed with nickel and iron precursors, and under microwave heating conditions, a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst with high ORR and oxygen evolution reaction (OER) performance is obtained. This catalyst can be used as a cathode catalyst in metal-air batteries.

Description

Technical Field

[0001] This invention relates to the field of new energy materials technology, and in particular to a bifunctional composite electrocatalyst of NiFe nanoparticles coated with a carbon layer, its preparation method, and its application. Background Technology

[0002] With the increasing depletion of fossil fuels leading to the energy crisis and the growing environmental problems caused by overconsumption, the search for and development of clean, efficient, and sustainable energy supply systems has become urgent. Renewable energy will become an important component of the future energy structure, and rechargeable air batteries, due to their high theoretical energy density, low cost, and safety, have significant application value in future energy utilization and energy storage. The charging and discharging reaction at the air cathode of the air battery is as follows: During discharge, the oxygen reduction reaction (ORR) shifts to the right, while during charging, the oxygen evolution reaction (OER) shifts to the left. The catalysts for both reactions (ORR and OER) are crucial components of the cathode in rechargeable metal-air batteries. However, both ORR and OER involve complex 4e⁻ reactions. - The slow kinetics of the coupling reaction limit the overall energy conversion efficiency of metal-air batteries. Therefore, developing high-performance ORR and OER electrocatalysts to improve their conversion efficiency is crucial for the development of metal-air batteries. Currently, Pt-based catalysts, based on noble metals, are considered the most ideal ORR catalysts, while Ru and Ir-based catalysts are the most efficient OER catalysts. However, the scarcity, high cost, and poor stability of these noble metal catalysts severely hinder their large-scale industrial application. Therefore, designing and developing a cheap and efficient non-noble metal, bifunctional catalyst to replace noble metal Pt, Ru, and Ir-based catalysts is of great significance for advancing the commercialization and practical application of air batteries.

[0003] Among numerous non-precious metal catalysts, transition metal-nitrogen-doped carbon-based (MNC) ORR catalysts have attracted widespread attention from research institutions both domestically and internationally due to their high catalytic efficiency, good stability, and low cost. In recent years, significant progress has been made in the application of MNC catalysts to ORR. For example, Dodelet et al. (M. Lefevre et al., Science, 2009, 324.) prepared a highly active Fe-NC catalyst by pyrolyzing a precursor composed of o-phenanthroline, iron salts, and zeolite imidazole ester framework materials. Its catalytic performance is close to that of commercial Pt / C catalysts, marking a significant advancement in the activity and stability of non-precious metal ORR catalysts. In 2011, Wu et al. (G. Wu et al., Science, 2011, 332.) used polyaniline (PANI) coated carbon black and transition metal salts as precursors to prepare a highly active PANIFeCo-C catalyst via pyrolysis, whose ORR catalytic performance is close to that of Pt-based ORR catalysts. Zhao et al. (J. Mater. Chem. A, 2016, 4, 3858) used o-phenylenediamine-coated carbon black and ferric chloride as precursors, and obtained Fe-Nx / C-1 catalysts after pyrolysis. Under alkaline conditions, its oxygen reduction activity was even higher than that of commercial Pt / C catalysts. Guan et al. (Z. Guan. et al., Chem. Commun., 2018, 54, 12073) used poly(o-phenylenediamine) as a raw material, silica gel as a template, and ammonium persulfate and ferric chloride as initiators and dopants to obtain a polymer as a precursor. After pyrolysis, they obtained Fe, S, N co-doped carbon (m-FeSNC) ORR catalysts, which were applied to zinc-air batteries, achieving a peak power density of 0.22 W / cm³. -2 Their performance is superior to that of Pt / C catalysts. However, the catalytic performance of this type of MNC catalyst for OER is not ideal.

[0004] In fact, as early as 1952, Edison and Jungner discovered the poisoning effect of Fe impurities on nickel-based alkaline batteries (RL Tichenor, Ind. Eng. Chem. 1952, 44, 973). Subsequent studies have shown that NiFe alloys (especially Ni3Fe) are extremely effective OER catalysts. Metallic Ni is converted to NiOOH through activation treatment, which accelerates the absorption of OH-. - The oxidation of species into molecular oxygen, through coupling with iron to adjust the electronic structure of the catalyst, significantly improves the catalytic activity of OER (X. Zhang, et al., ACS Catal. 2015, 6, 580.), but NiFe alloy has no activity for ORR, which affects its widespread application. Summary of the Invention

[0005] The purpose of this invention is to provide a bifunctional composite electrocatalyst for carbon-coated NiFe nanoparticles, its preparation method, and its application. This bifunctional composite electrocatalyst has high oxygen reduction (ORR) and oxygen evolution reaction (OER) activity.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] This invention provides a method for preparing a bifunctional composite electrocatalyst of NiFe nanoparticles coated with a carbon layer, comprising the following steps:

[0008] Dimethylimidazole, zinc salt, iron salt and organic solvent were mixed and self-assembled to obtain transition metal-organic framework compounds;

[0009] The transition metal-organic framework compound was heat-treated in a protective atmosphere to obtain Fe-NC material;

[0010] The Fe-NC material was physically mixed with iron and nickel precursors and then microwave-heated to obtain a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst.

[0011] Preferably, the iron salt includes ferric ammonium sulfate, ferrous sulfate heptahydrate, ferric chloride, or ferric chloride hexahydrate; the zinc salt includes zinc nitrate hexahydrate.

[0012] Preferably, the molar ratio of the dimethylimidazole, zinc salt and iron salt is 30:8:1 to 5.

[0013] Preferably, the self-assembly temperature is 20–35°C and the time is 24–36 h.

[0014] Preferably, the heat treatment temperature is 900–1000°C and the time is 2–4 hours.

[0015] Preferably, the iron precursor comprises iron acetylacetonate or iron dodecylcarbonyl; the nickel precursor comprises nickel acetylacetonate or nickel chloride.

[0016] Preferably, the ratio of Fe-NC material, iron precursor to nickel precursor is 30-40 mg: 30 mmol: 60-90 mmol.

[0017] Preferably, the microwave heating time is 10 to 60 seconds; the microwave heating is carried out under a protective atmosphere, and the gas used in the protective atmosphere includes nitrogen or argon.

[0018] This invention provides a bifunctional composite electrocatalyst for carbon-coated NiFe nanoparticles prepared by the preparation method described above.

[0019] This invention provides the application of the carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst described above as a cathode material for metal-air batteries.

[0020] This invention provides a method for preparing a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst. The method involves synthesizing a transition metal-organic framework compound with specific morphology and size using dimethylimidazole and a transition metal salt. This compound is then pyrolyzed to obtain a nitrogen-doped carbon material (Fe-NC) containing transition metal iron with a specific morphology. The Fe-NC material is then combined with nickel and iron precursors under microwave heating to obtain the carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst. The preparation principle of this invention is as follows: under the action of a microwave field, the iron and nickel precursors are rapidly reduced to NiFe nanoparticles; simultaneously, the localized high temperature generated by microwave heating causes a strong interaction between the carbon material and the NiFe nanoparticles, resulting in the NiFe nanoparticles being coated with a carbon layer. This invention loads NiFe alloy nanoparticles with high OER activity onto a Fe-NC carbon support with high ORR activity, thereby preparing a bifunctional composite electrocatalyst with high oxygen reduction (ORR) and oxygen evolution reaction (OER) activity, which can be used as a cathode catalyst in metal-air batteries.

[0021] Compared with the prior art, the present invention has the following advantages or positive effects:

[0022] This invention is the first to utilize a simple microwave heating technique to efficiently and rapidly prepare a highly active bifunctional composite catalyst for metal-air batteries.

[0023] This invention rapidly reduces nickel and iron precursors into NiFe nanoparticles under a microwave field, while simultaneously achieving carbon coating of the NiFe particles. Compared to existing technologies, the reduction time of NiFe nanoparticles is extremely short. The strong interaction between the NiFe nanoparticles and the Fe-NC material surface forms a core-shell structure of carbon coating on the NiFe nanoparticles, which effectively protects the NiFe nanoparticles from dissolution in the electrolyte and improves the stability of the catalyst.

[0024] This invention uses dimethylimidazole, which is stable and inexpensive, as well as transition metal salts and transition metal organometallic precursors, which are low-cost and abundant in Earth's resources, as raw materials to replace precious metals (Pt, Ru, Ir), thus reducing the cost of raw materials used in the entire preparation process and significantly reducing catalyst costs.

[0025] The method of this invention is simple and efficient, the raw materials used are readily available and low in cost, the preparation process is simple, and the equipment requirements are low, making it suitable for large-scale production. Attached Figure Description

[0026] Figure 1Here is a SEM image of the transition metal-organic framework compound prepared in Example 1 of this invention;

[0027] Figure 2 SEM image of the iron-nitrogen-doped carbon material (Fe-NC) prepared in Example 1 of this invention;

[0028] Figure 3 The LSV polarization curves of ORR and OER of the Fe-NC material prepared in Example 1 of this invention in O2-saturated 0.1M KOH solution are compared with those of commercial Pt / C (20 wt.% Pt) and commercial RuO2 catalyst (Pt / C-RuO2).

[0029] Figure 4 The image shows the SEM image (a) and the corresponding backscattered electron scanning electron microscopy image (BS-SEM, b) of the Fe-NC@NiFe-1 catalyst in Example 1 of this invention.

[0030] Figure 5 The images shown are TEM (a) and HR-TEM (b) images of the Fe-NC@NiFe-1 catalyst in Example 1 of this invention.

[0031] Figure 6 This is an HR-TEM image of a single NiFe nanoparticle coated with a carbon layer in the Fe-NC@NiFe-1 catalyst of Example 1 of the present invention;

[0032] Figure 7 This is a comparison of the ORR / OER LSV polarization curves of the Fe-NC and Fe-NC@NiFe-1 catalysts prepared in Example 1 of this invention and the commercial Pt / C-RuO2 catalyst in O2-saturated 0.1M KOH solution.

[0033] Figure 8 This is a comparison of the ORR / OER LSV polarization curves of the Fe-NC@NiFe-2 catalyst prepared in Example 2 of the present invention and the commercial Pt / C-RuO2 catalyst in O2-saturated 0.1M KOH solution;

[0034] Figure 9 This is a comparison of the ORR / OER LSV polarization curves of the Fe-NC@NiFe-3 catalyst prepared in Example 3 of the present invention and the commercial Pt / C-RuO2 catalyst in O2-saturated 0.1M KOH solution;

[0035] Figure 10 This is a comparison of the ORR / OER LSV polarization curves of the Fe-NC@NiFe-4 catalyst prepared in Example 4 of this invention and the commercial Pt / C-RuO2 catalyst in O2-saturated 0.1M KOH solution. Detailed Implementation

[0036] This invention provides a method for preparing a bifunctional composite electrocatalyst of NiFe nanoparticles coated with a carbon layer, comprising the following steps:

[0037] Dimethylimidazole, zinc salt, iron salt and organic solvent were mixed and self-assembled to obtain transition metal-organic framework compounds;

[0038] The transition metal-organic framework compound was heat-treated in a protective atmosphere to obtain Fe-NC material;

[0039] The Fe-NC material was physically mixed with iron and nickel precursors and then microwave-heated to obtain a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst.

[0040] In this invention, unless otherwise specified, the raw materials or reagents required for preparation are all commercially available products well known to those skilled in the art.

[0041] This invention involves mixing dimethylimidazole, zinc salt, iron salt, and an organic solvent to perform self-assembly, thereby obtaining a transition metal-organic framework compound.

[0042] In this invention, the iron salt preferably includes ferric ammonium sulfate, ferrous sulfate heptahydrate, ferric chloride, or ferric chloride hexahydrate; the zinc salt preferably includes zinc nitrate hexahydrate.

[0043] In this invention, the molar ratio of dimethylimidazole, zinc salt and iron salt is preferably 30:8:1 to 5, and more preferably 30:8:3 to 5.

[0044] In this invention, the organic solvent is preferably methanol; the amount of the organic solvent used is not particularly limited, and can be adjusted according to actual needs to ensure the smooth progress of the reaction.

[0045] In this invention, dimethylimidazole, zinc salt, and iron salt are dissolved in organic solvents respectively, and the resulting three solutions are mixed and self-assembled under continuous stirring.

[0046] In this invention, the self-assembly temperature is preferably 20-35°C, more preferably 25-30°C, and the time is preferably 24-36h, more preferably 24-30h.

[0047] After the self-assembly is completed, the present invention preferably centrifuges the obtained product, washes the obtained precipitate repeatedly with anhydrous ethanol three times, and then transfers it to a vacuum drying oven at 80°C for 12 hours. The obtained product is then ground in a mortar for 20-30 minutes to obtain a transition metal-organic framework compound.

[0048] After obtaining the transition metal-organic framework compound, the present invention heat-treats the transition metal-organic framework compound in a protective atmosphere to obtain Fe-NC material.

[0049] The present invention preferably places the transition metal-organic framework compound in a quartz boat and performs heat treatment in a tube furnace.

[0050] In this invention, the heat treatment temperature is preferably 900-1000℃, more preferably 950-1000℃, and the time is preferably 2-4h, more preferably 2-3h; the protective atmosphere preferably includes argon or nitrogen, to obtain an iron-nitrogen-doped carbon material: denoted as Fe-NC material.

[0051] After obtaining the Fe-NC material, the present invention physically mixes the Fe-NC material with iron precursor and nickel precursor, and then microwaves it to obtain a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst.

[0052] In this invention, the iron precursor preferably includes iron acetylacetonate or iron dodecylcarbonyl; the nickel precursor includes nickel acetylacetonate or nickel chloride.

[0053] In this invention, the preferred ratio of Fe-NC material, iron precursor and nickel precursor is 30-40 mg:30 mmol:60-90 mmol, more preferably 40 mg:30 mmol:90 mmol.

[0054] In this invention, iron precursor, nickel precursor and Fe-NC material are preferably ground in a mortar for 20-30 minutes to mix evenly, then placed in a quartz bottle and a protective gas (Ar or N2) is introduced for 10 minutes to remove residual air from the quartz bottle. The quartz bottle is then placed in a microwave heating device for microwave heating to obtain a highly active bifunctional composite electrocatalyst with carbon layer coated NiFe nanoparticles.

[0055] In this invention, the microwave heating time is preferably 10-60 seconds, more preferably 20-40 seconds, and even more preferably 25-30 seconds; the microwave heating is preferably carried out under a protective atmosphere, and the gas used in the protective atmosphere preferably includes nitrogen or argon; the output power of the microwave device used for microwave heating is preferably 700W, and the frequency is preferably 2.45GHz. This invention does not impose any special limitation on the microwave device used for microwave heating; a microwave oven well-known in the art is acceptable. In the embodiments of this invention, a Midea microwave oven with an output power of 700W and a frequency of 2.45GHz is specifically used.

[0056] This invention utilizes microwave heating to reduce Ni and Fe precursors into NiFe nanoparticles (2-5 nm) in a very short time under a microwave field. Figure 5-6Loading NiFe nanoparticles onto the surface of Fe-NC material particles with high ORR activity can effectively prevent Ostwald ripening. Simultaneously, the strong interaction between NiFe nanoparticles and the carbon material surface under a microwave field promotes the formation of a NiFe core-carbon shell (4-6 carbon layers) structure by coating NiFe nanoparticles with carbon layers. The carbon shell can improve catalyst durability by protecting the NiFe core from corrosion or detachment, demonstrating originality and innovation. This process yields a high-performance non-precious metal bifunctional carbon-based catalyst for ORR and OER.

[0057] This invention provides a bifunctional composite electrocatalyst for carbon-coated NiFe nanoparticles prepared by the preparation method described above.

[0058] This invention provides the application of the carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst described above as a cathode material in a metal-air battery. This invention does not impose any particular limitation on the method of application; any method well-known in the art can be used.

[0059] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0060] Example 1

[0061] (1) 30 mmol of dimethylimidazole, 8 mmol of zinc nitrate hexahydrate and 5 mmol of ferrous sulfate heptahydrate were dissolved in 50 mL of methanol respectively. The three solutions were mixed and stirred continuously at room temperature (25 °C) for 24 h. After centrifugation, the precipitate was washed three times with anhydrous ethanol and then transferred to a vacuum drying oven at 80 °C for 12 h. The product was ground in a mortar for 20 min to obtain a transition metal organoframe compound.

[0062] (2) Weigh 500 mg of the transition metal-organic framework compound from step (1) and place it in a quartz boat. Under Ar protection, heat-treat it at 1000 °C for 2 h in a tube furnace to obtain iron-nitrogen-doped carbon material (Fe-NC).

[0063] (3) Take 30 mmol of iron acetylacetone, 90 mmol of nickel acetylacetone and 40 mg of Fe-NC material obtained in step (2) respectively, grind them in a mortar for 30 min and mix them evenly, place them in a quartz bottle and pass Ar through for 10 min to remove the residual air in the quartz bottle, then place the quartz bottle in a microwave oven (Midea microwave oven, output power: 700W, 2.45GHz) for microwave heating for 20s to obtain a bifunctional composite electrocatalyst, denoted as Fe-NC@NiFe-1.

[0064] Example 2

[0065] (1) 30 mmol of dimethylimidazole, 8 mmol of zinc nitrate hexahydrate and 5 mmol of ferrous sulfate heptahydrate were dissolved in 50 mL of methanol respectively. The three solutions were mixed and stirred continuously at room temperature (25 °C) for 24 h. After centrifugation, the precipitate was washed three times with 80% ethanol and then transferred to a vacuum drying oven at 80 °C for 12 h. The product was ground in a mortar for 20 min to obtain a transition metal organoframe compound.

[0066] (2) Weigh 500 mg of the transition metal-organic framework compound from step (1) and place it in a quartz boat. Under Ar protection, heat-treat it in a tube furnace at 1000 °C for 2 h to obtain iron-nitrogen-doped carbon material (Fe-NC).

[0067] (3) Take 30 mmol of iron acetylacetone, 90 mmol of nickel acetylacetone and 40 mg of Fe-NC material obtained in step (2) respectively, grind them in a mortar for 30 min and mix them evenly, place them in a quartz bottle and pass Ar through for 10 min to remove the residual air in the quartz bottle, then place the quartz bottle in a microwave oven (Midea microwave oven, output power: 700W, 2.45GHz) for microwave heating for 30s to obtain a bifunctional composite electrocatalyst, denoted as Fe-NC@NiFe-2.

[0068] Example 3

[0069] (1) 30 mmol of dimethylimidazole, 8 mmol of zinc nitrate hexahydrate and 5 mmol of ferrous sulfate heptahydrate were dissolved in 50 mL of methanol respectively. The three solutions were then mixed and stirred continuously at room temperature (25 °C) for 24 h. After centrifugation, the precipitate was washed three times with anhydrous ethanol and then transferred to a vacuum drying oven at 80 °C for 12 h. The product was ground in a mortar for 20 min to obtain a transition metal organoframe compound.

[0070] (2) Weigh 500 mg of the organometallic framework compound obtained in step (1) and place it in a quartz boat. Under Ar protection, heat treat it at 1000 °C for 2 h in a tube furnace to obtain iron-nitrogen-doped carbon material (Fe-NC).

[0071] (3) Take 30 mmol of iron acetylacetone, 90 mmol of nickel acetylacetone and 30 mg of Fe-NC material obtained in step (2) respectively, grind them in a mortar for 30 min and mix them evenly, place them in a quartz bottle and pass N2 through it for 10 min to remove the residual air in the quartz bottle, then place the quartz bottle in a household microwave oven (Midea microwave oven, output power: 700W, 2.45GHz) for microwave heating for 40s to obtain a bifunctional composite electrocatalyst, denoted as Fe-NC@NiFe-3.

[0072] Example 4

[0073] (1) Weigh 30 mmol of dimethylimidazole, 8 mmol of zinc nitrate hexahydrate and 5 mmol of ferrous sulfate heptahydrate and dissolve them in 50 mL of methanol respectively. Then mix the three solutions and stir continuously at room temperature (25°C) for 24 h. After centrifugation, the precipitate was washed three times with anhydrous ethanol and then transferred to a vacuum drying oven at 80°C for 12 h. The product was ground in a mortar for 20 min to obtain a transition metal organoframe compound.

[0074] (2) Weigh 500 mg of the transition metal-organic framework compound obtained in step (1) and place it in a quartz boat. Under Ar protection, heat treat it at 1000 °C for 2 h in a tube furnace to obtain iron-nitrogen-doped carbon material (Fe-NC).

[0075] (3) Take 30 mmol of dodecyltriferrocarbonyl, 90 mmol of nickel acetylacetonate and 30 mg of Fe-NC material obtained in step (2) respectively, grind them in a mortar for 30 min and mix them evenly. Then place them in a quartz bottle and pass Ar through it for 10 min to remove the residual air in the quartz bottle. Then place the quartz bottle in a household microwave oven (Midea microwave oven, output power: 700W, 2.45GHz) for microwave heating for 30s to obtain a bifunctional composite electrocatalyst, denoted as Fe-NC@NiFe-4.

[0076] Characterization and performance testing

[0077] 1) Figure 1 This is a SEM image of the transition metal-organic framework compound prepared in Example 1 of the present invention; it can be observed that these particles have a regular octahedral morphology, uniform particle size, and good dispersibility.

[0078] 2) Figure 2 The image shows a SEM image of the iron-nitrogen-doped carbon material (Fe-NC) prepared in Example 1 of this invention; as shown. Figure 2 As shown, its particles still maintain the original octahedral structure, with uniform particle size and good dispersibility, and the particle size is 300-500 nm.

[0079] 3) The ORR activity of the Fe-NC material prepared in Example 1 was compared with that of a commercial Pt / C (20 wt.% Pt) catalyst and the OER activity of a commercial RuO2 catalyst by electrochemical testing (LSV polarization curve test conditions: in O2-saturated 0.1 M KOH solution, rotation speed 1600 rpm, scan rate 5 mV / s); the results are shown in […]. Figure 3 .

[0080] Figure 3This is a comparison of the LSV polarization curves of the ORR and OER of the Fe-NC material prepared in Example 1 of this invention in O2-saturated 0.1M KOH solution with those of commercial Pt / C (20 wt.% Pt) and commercial RuO2 catalyst (Pt / C-RuO2). Figure 3 As can be seen, the half-wave potential (E) of the Fe-NC material 1 / 2 =0.90V) is significantly better than the half-wave potential (E) of commercial Pt / C catalysts. 1 / 2 =0.86V). However, in the OER activity test, it can be seen that at 10mA / cm 2 (η 10 At the limiting current, the overpotential of the Fe-NC material is 0.77V, which is higher than that of the commercial RuO2 catalyst (0.38V), indicating that its OER performance is worse than that of RuO2. This demonstrates that the Fe-NC material has higher ORR activity than commercial Pt / C, but lower OER activity than commercial RuO2. The ORR / OER potential difference of the Fe-NC material (ΔE = E) 10,OER -E 1 / 2,ORR The potential difference (ΔE) of Fe-NC material is 1.1V, which is higher than that of Pt / C-RuO2 catalyst (ΔE = 0.75V). This indicates that the bifunctional performance of Fe-NC material is inferior to that of Pt / C-RuO2 catalyst.

[0081] 4) Figure 4 The images show the SEM image (a) and the corresponding backscattered electron scanning electron microscopy image (BS-SEM, b) of the Fe-NC@NiFe-1 catalyst in Example 1 of this invention. Through (a) and (b), it can be observed that a large number of NiFe nanoparticles (white bright spots) are uniformly distributed on the surface of the carbon particles of the Fe-NC material.

[0082] 5) Figure 5 These are TEM images (a) and high-resolution TEM images (HR-TEM, b) of the Fe-NC@NiFe-1 catalyst in Example 1 of this invention; from Figure 5 As can be seen, a large number of uniformly sized NiFe particles are evenly distributed on the surface of Fe-NC carbon particles, and the size of NiFe nanoparticles is 2-5 nm.

[0083] 6) Figure 6 This is a high-resolution transmission electron microscope (HR-TEM) image of a single NiFe nanoparticle coated with a carbon layer in the Fe-NC@NiFe-1 catalyst of Example 1 of this invention; from Figure 6As can be seen, the NiFe nanoparticles are coated with a carbon layer, which is about 4-6 carbon atom layers thick. The lattice spacing of the NiFe nanoparticles was measured to be about 0.204 nm, which is consistent with the lattice spacing of the Ni3Fe(111) plane. This confirms that the alloy nanoparticles formed in the Fe-NC@NiFe-1 catalyst are Ni3Fe.

[0084] 7) Comparison of LSV polarization curves of ORR / OER of Fe-NC and Fe-NC@NiFe-1 catalysts prepared in Example 1 and commercial Pt / C-RuO2 catalyst by electrochemical testing (test conditions: O2 saturated 0.1M KOH solution, rotation speed 1600 rpm, scan rate 5 mV / s). The results are shown in [Figure number missing]. Figure 7 ;Depend on Figure 7 As can be seen, the potential difference ΔE(E) between Fe-NC, Fe-NC@NiFe-1 catalysts and ORR / OER is... 10,OER -E 1 / 2,ORR The order is: Fe-NC (1.1V) > Pt / C-RuO2 (0.75V) > Fe-NC@NiFe-1 (0.66V), indicating that Example 1 successfully prepared the Fe-NC@NiFe-1 bifunctional catalyst, and its performance is superior to that of the commercial Pt / C-RuO2 catalyst.

[0085] 8) Comparison of LSV polarization curves of ORR / OER of Fe-NC@NiFe-2 prepared in Example 2 and commercial Pt / C-RuO2 catalyst by electrochemical testing (test conditions: in O2-saturated 0.1 MkOH solution, rotation speed 1600 rpm, scan rate 5 mV / s). The results are shown in [Figure number missing]. Figure 8 ;Depend on Figure 8 It can be seen that the potential difference of the commercial Pt / C-RuO2 catalyst (ΔE = 0.75V) is higher than that of Fe-NC@NiFe-2 (ΔE = 0.68V), indicating that the Fe-NC@NiFe-2 bifunctional catalyst was successfully prepared in Example 2, and its performance is better than that of the commercial Pt / C-RuO2 catalyst.

[0086] 9) Comparison of LSV polarization curves of ORR / OER of Fe-NC@NiFe-3 prepared in Example 3 and commercial Pt / C-RuO2 catalyst by electrochemical testing (test conditions: in O2-saturated 0.1 MkOH solution, rotation speed 1600 rpm, scan rate 5 mV / s). The results are shown in [Figure number missing]. Figure 9 ;Depend on Figure 9It can be seen that the potential difference of the commercial Pt / C-RuO2 catalyst (ΔE = 0.75V) is higher than that of Fe-NC@NiFe-3 (ΔE = 0.69V), indicating that the Fe-NC@NiFe-3 bifunctional catalyst was successfully prepared in Example 3, and its performance is better than that of the commercial Pt / C-RuO2 catalyst.

[0087] 10) Comparison of LSV polarization curves of ORR / OER of Fe-NC@NiFe-4 prepared in Example 4 and commercial Pt / C-RuO2 catalyst by electrochemical testing (test conditions: in O2-saturated 0.1 MkOH solution, rotation speed 1600 rpm, scan rate 5 mV / s). The results are shown in [Figure number missing]. Figure 10 ;Depend on Figure 10 As can be seen, the potential difference of the commercial Pt / C-RuO2 catalyst (ΔE = 0.75V) is slightly lower than that of Fe-NC@NiFe-4 (ΔE = 0.76V), indicating that the Fe-NC@NiFe-4 bifunctional catalyst was successfully prepared in Example 4, and its performance is similar to that of the commercial Pt / C-RuO2 catalyst.

[0088] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst for use as a cathode material in metal-air batteries, characterized in that, Includes the following steps: Dimethylimidazole, zinc salt, iron salt and organic solvent were mixed and self-assembled to obtain transition metal-organic framework compounds; The transition metal-organic framework compound was heat-treated in a protective atmosphere to obtain Fe-NC material; The Fe-NC material was physically mixed with iron and nickel precursors and then microwave heated to obtain a carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst. The molar ratio of the dimethylimidazole, zinc salt and iron salt is 30:8:1~5; The self-assembly temperature is 20~35℃, and the time is 24~36h; The heat treatment temperature is 900~1000℃, and the time is 2~4h; The iron precursor includes iron acetylacetonate or iron dodecylcarbonyl; the nickel precursor includes nickel acetylacetonate or nickel chloride. The Fe-NC material, iron precursor, and nickel precursor are used in a ratio of 30-40 mg: 30 mmol: 60-90 mmol. The microwave heating time is 10~60s; The microwave heating is performed under a protective atmosphere, the gas used in which nitrogen or argon is used.

2. The preparation method according to claim 1, characterized in that, The iron salts include ferric ammonium sulfate, ferrous sulfate heptahydrate, ferric chloride, or ferric chloride hexahydrate; the zinc salts include zinc nitrate hexahydrate.

3. The bifunctional composite electrocatalyst of carbon-coated NiFe nanoparticles prepared by the preparation method according to any one of claims 1 to 2.

4. The application of the carbon-coated NiFe nanoparticle bifunctional composite electrocatalyst of claim 3 as a cathode material for metal-air batteries.

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