Preparation and application of Fe-N-CNFs catalyst based on Fe-MIL

Abstract

The present invention provides a preparation and application of Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL, wherein the method is to first react ferric chloride hexahydrate and terephthalic acid in N, N-dimethylformamide to generate a Fe-MIL precursor, then mix the Fe-MIL precursor with a nitrogen-containing binder in N, N-dimethylformamide for electrostatic spinning to obtain nanofibers, and obtain Fe-N-CNFs (Carbon nanofibers) catalyst after pre-calcination and pyrolysis. The catalyst has a large active area, strong electron transfer ability, and strong mass transfer ability, so it has a strong ability to catalyze oxygen reduction reaction. In addition, the synthetic material is cheap and can reduce the catalyst cost, so the catalyst has a large application prospect in proton exchange membrane fuel cells.

Description

Technical Field

[0001] The invention belongs to the field of fuel cells, and in particular relates to a preparation method of a Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL and an application thereof in an oxygen reduction reaction of a fuel cell. Background Art

[0002] As energy and environmental issues worsen, the demand for clean energy increases. Proton exchange membrane fuels have attracted widespread attention due to their advantages such as high energy conversion efficiency, high power density and zero pollution emissions. However, the use of expensive Pt / C catalysts in the cathode hinders the large-scale application of proton exchange membrane fuel cells. Therefore, the development of low-cost, highly active non-precious metal catalysts has become an urgent matter. Fe-NC catalysts, as the most likely non-precious metal catalysts to replace Pt / C catalysts, have made great progress. Among them, iron-nitrogen-carbon nanofiber catalysts have relatively excellent performance (JL Shui, C. Chen, et al. PNAS, 2015, 112 (34), 10629-10634).

[0003] Iron-nitrogen-carbon nanofiber catalysts have been widely studied as cathode oxygen reduction catalysts for proton exchange membrane fuel cells due to their high graphitization degree, high specific surface area, and hierarchical mesoporous structure. In the prior art, Fe-NC catalysts are usually prepared by directly pyrolyzing a mixture of Fe precursors, conventional carbon materials (such as activated carbon), and other raw materials. The Fe-NC catalysts prepared by the above method have problems such as low Fe content, low porosity, and low specific surface area, which leads to a low number of catalyst active sites, limited mass transport capacity, and limited electron conduction capacity. Summary of the invention

[0004] In view of the problems of small porosity, poor conductivity and low specific surface area of ​​Fe-NC catalysts prepared by existing methods, the present invention uses Fe-MIL as the Fe source (Fe-MIL is a type of metal organic framework material) and adopts electrospinning technology to prepare Fe-N-CNFs catalysts, which are applied to proton exchange membrane fuel cells to improve the activity, mass transfer capacity and conductivity of the catalytic layer.

[0005] The technical solution of the present invention is as follows:

[0006] The preparation method of Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL comprises the following steps:

[0007] (1) dissolving Fe-MIL in solvent 1, then adding an electrospinning nitrogen-containing binder, and stirring at 10-24° C. for 12-24 h to prepare a mixed solution;

[0008] (2) electrospinning the mixed solution to obtain nanofibers;

[0009] (3) The nanofibers are pre-sintered at 200-300°C for 2-5h and then pyrolyzed at 700-1000°C for 2-5h to obtain Fe-N-CNFs (Carbon nanofibers) catalyst.

[0010] In the step (1), the mass ratio of Fe-MIL to the electrospinning nitrogen-containing binder is 1-2.5:1; the mass fraction of the electrospinning nitrogen-containing binder in the mixed solution is 10-16%. The nitrogen content of the electrospinning nitrogen-containing binder should be no less than 12.61wt%.

[0011] As a preferred technical solution, the solvent 1 is N,N-dimethylformamide.

[0012] As a preferred technical solution, the electrospinning nitrogen-containing binder is polyacrylonitrile (PAN) or polyvinyl pyrrolidone (PVP), etc. The mass fraction of Fe in the Fe-MIL is 20-30%.

[0013] As a preferred technical solution, the dissolution process of step (1) is to add Fe-MIL into solvent 1, ultrasonicate for 20-30 minutes, and then add the electrospinning nitrogen-containing binder.

[0014] As a preferred technical solution, in step (2), the voltage of electrospinning is 15-30 kV, the liquid flow rate is 0.4-0.6 ml / h, and the spinning time is 4-10 h.

[0015] As a preferred technical solution, the pre-burning process in step (3) is carried out under air conditions, and the pyrolysis process is carried out under argon conditions.

[0016] As a preferred technical solution, the Fe-MIL is prepared by the following steps:

[0017] (a) dissolving ferric chloride hexahydrate and terephthalic acid in solvent 2 by ultrasonication to prepare a mixed solution A;

[0018] (b) reacting the mixed solution A at 140-160° C. for 12-24 h, preferably at 150° C. for 12 h, centrifuging, washing, and drying to obtain Fe-MIL;

[0019] The molar ratio of the ferric chloride hexahydrate to terephthalic acid is 1:1; and the solvent 2 is N,N-dimethylformamide.

[0020] As a preferred technical solution, in step (a), the ultrasonic time is 5-10 minutes; and in step (b), the drying temperature is 80° C. and the drying time is 6-12 hours.

[0021] The present invention also provides a Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared by any of the above methods.

[0022] As a preferred technical solution, the catalyst has an average pore radius of 3.0-4.0 nm and a specific surface area of ​​330-410 m 2 / g, the mass fraction of Fe in the catalyst is 30-40%. Compared with Fe-MIL, the Fe content is increased. This is because pyrolysis will cause carbon loss in the Fe-MIL precursor, so the Fe content will increase after pyrolysis.

[0023] The present invention also provides the use of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL as a cathode catalyst in a fuel cell.

[0024] Beneficial Effects

[0025] (1) The present invention uses Fe-MIL precursor as Fe source, which can increase the iron content in the catalyst and reduce the curling of Fe particles during pyrolysis compared with other Fe sources; a large amount of Fe in the Fe-MIL precursor can be used to form Fe-N4 active sites, thereby improving the catalytic activity of the catalyst. Therefore, the Fe-N-CNFs catalyst based on Fe-MIL has more active sites, high conductivity and high mass conversion efficiency. The material is applied to the cathode oxygen reduction reaction of proton exchange membrane fuel cells: the catalyst has high activity, large specific surface area, high mass transfer capacity, and good electron transfer capacity, which improves the oxygen reduction catalytic performance of the catalyst.

[0026] (2) The present invention uses the nitrogen-containing binder used in electrospinning as a nitrogen source, which can eliminate the use of nitrogen-containing organic matter and increase the Fe content of the catalyst. At the same time, the nanofiber catalyst obtained by the electrospinning method replaces the conventional carbon material as a carrier, which can improve the mass transport capacity, electron conduction capacity and the number of active sites; at the same time, it can also increase the porosity of the catalyst, which can increase the contact between the active sites and the reactants, thereby improving the catalytic activity of the catalyst.

[0027] (3) The mixing temperature of Fe-MIL precursor and nitrogen-containing binder in solvent is 10-24°C. Above this temperature, dripping will occur during spinning, and below this temperature, the needle will be blocked. The nitrogen content of nitrogen-containing binder should not be less than 12.61wt%. Below this value, the nitrogen content in the nanofibers formed by electrospinning is not high. Finally, the catalyst Fe-N4 synthesized by pyrolysis has fewer active sites and the performance of the catalyst is not high.

[0028] (4) The present invention uses polyacrylonitrile as a nitrogen source and Fe-MIL as an iron source. It is inexpensive and has simple reaction conditions and can be widely used. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the SEM image of the Fe-MIL precursor prepared in Example 1.

[0030] Figure 2 This is the SEM image of the Fe-MIL precursor electrospun nanofibers prepared in Example 1.

[0031] Figure 3 This is the SEM image of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 1.

[0032] Figure 4 The oxygen reduction polarization curves of the Fe-N-CNFs (Carbon nanofibers) catalysts based on Fe-MIL prepared for Examples 1, 2, 3 and 4 were obtained in 0.1M KOH solution by rotating disk electrode (RDE) testing.

[0033] Figure 5 This is the XRD pattern of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2.

[0034] Figure 6 This is the Raman graph of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2.

[0035] Figure 7 XPS spectrum of the N element of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2.

[0036] Figure 8 XPS spectrum of the Fe element of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2.

[0037] Fig. 9 Nitrogen adsorption-desorption curve of Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2.

[0038] Fig.10 This is the pore size distribution diagram of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2.

[0039] Fig.11 The oxygen reduction polarization curves of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2, the catalysts prepared in Comparative Examples 1, 2 and 3 and the commercial 20% Pt / C catalyst were obtained by rotating disk electrode (RDE) testing in 0.1 M KOH solution.

[0040] Fig.12 Methanol resistance test of Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2 and commercial 20% Pt / C catalyst.

[0041] Fig.13 This is a graph showing the change in current during the constant voltage decay process of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2 and the commercial 20% Pt / C catalyst.

[0042] Fig.14 Oxygen reduction polarization curves of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2 and the commercial 20% Pt / C catalyst obtained by rotating ring disk electrode (RRDE) testing.

[0043] Fig.15 This is a cell polarization curve and power density curve of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 2 as a cathode catalyst.

[0044] Fig.16 This is the XRD pattern of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 3.

[0045] Fig.17 This is the Raman graph of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 3.

[0046] Fig.18 XPS spectrum of the N element of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 4.

[0047] Fig.19 XPS spectrum of the Fe element of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 4.

[0048] Fig. 20 This is the nitrogen adsorption-desorption curve of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 4.

[0049] Fig.21 This is the pore size distribution diagram of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Example 4.

[0050] Fig. 22 This is the SEM image of the catalyst prepared in Comparative Example 3.

[0051] Fig.23 The oxygen reduction polarization curves of the Fe-N-CNFs (Carbon nanofibers) catalysts based on Fe-MIL prepared for Examples 3, 5, 6 and 7 were obtained by rotating disk electrode (RDE) testing in 0.1M KOH solution.

[0052] Fig.24 This is the nitrogen adsorption-desorption curve of the MIL-NC-1.5-800 catalyst prepared in Comparative Example 3.

[0053] Fig.25 The oxygen reduction polarization curves of the Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL prepared in Comparative Example 4 and Example 2 were obtained by rotating disk electrode (RDE) testing in 0.1 M KOH solution.

[0054] Fig.26 The catalysts prepared for Comparative Example 5 and Example 2 were tested for oxygen reduction polarization curves in 0.1 M KOH solution using a rotating disk electrode (RDE).

[0055] Fig. 27 The catalysts prepared for Comparative Example 6 and Example 2 were tested for oxygen reduction polarization curves in 0.1 M HClO 4 solution by rotating disk electrode (RDE). DETAILED DESCRIPTION

[0056] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL comprises the following steps:

[0057] (1) dissolving ferric chloride hexahydrate and terephthalic acid in N,N-dimethylformamide by ultrasonication, wherein the molar ratio of ferric chloride hexahydrate to terephthalic acid is 1:1, and the ultrasonication time is 5-10 minutes;

[0058] (2) reacting the mixed solution at 150° C. for 12 h, centrifugally filtering, washing, and drying at 80° C. for 6-12 h to obtain a precursor Fe-MIL;

[0059] (3) The precursor Fe-MIL is first added to N,N-dimethylformamide, ultrasonicated for 20-30 min, and then polyacrylonitrile (PAN) or polyvinyl pyrrolidone (PVP) is added and stirred at room temperature for 12-24 h to obtain a mixed solution, wherein the mass ratio of the precursor Fe-MIL to polyacrylonitrile (PAN) or polyvinyl pyrrolidone (PVP) is 1-2.5:1, and the mass fraction of polyacrylonitrile (PAN) or polyvinyl pyrrolidone (PVP) is 10-16%;

[0060] (4) electrospinning the mixed solution at a voltage of 15-30 kV and a liquid flow rate of 0.4-0.6 ml / h for 4-10 h to obtain nanofibers;

[0061] (5) The nanofibers were pre-sintered at 200-300°C for 2-5 h in an air atmosphere and then pyrolyzed at 700-1000°C for 2-5 h in an argon atmosphere to obtain Fe-N-CNFs (Carbon nanofibers) catalyst.

[0062] Example 1

[0063] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL comprises the following steps:

[0064] (1) 1.084 g of ferric chloride hexahydrate and 0.664 g of terephthalic acid were dissolved in 197 ml of N,N-dimethylformamide and ultrasonically dispersed for 5 min to prepare a mixed solution;

[0065] (2) The mixed solution was stirred at a constant temperature of 150°C for 12 hours. After the reaction was completed, it was centrifuged, washed, and dried at 80°C for 6 hours to obtain a brown Fe-MIL precursor.

[0066] (3) 0.5 g of Fe-MIL precursor was first added to 5 ml of N,N-dimethylformamide, ultrasonicated for 30 min, and then 0.5 g of polyacrylonitrile (PAN) was added and stirred at 20 °C for 24 h to obtain a mixed solution;

[0067] (4) electrospinning the mixed solution at a voltage of 25 kV and a liquid flow rate of 0.5 ml / h for 5 h to obtain nanofibers;

[0068] (5) The nanofibers were pre-sintered at 250°C for 2 h at a heating rate of 5°C / min in an air atmosphere, and then pyrolyzed at 800°C for 2 h at a heating rate of 5 min / °C in an argon atmosphere to obtain Fe-N-CNFs (Carbon nanofibers) catalyst.

[0069] Figure 1 It can be seen that the Fe-MIL precursor is a bipyramidal hexagonal prism.

[0070] Figure 2 It can be seen that the Fe-MIL precursor nanofibers are very uniform, with a radius between 100 and 200 nm.

[0071] Figure 3 It can be seen that the radius of the nanofiber catalyst is also between 100-200nm.

[0072] Figure 4 From the oxygen reduction polarization curves obtained by the rotating disk electrode (RDE) test of the Fe-N-CNFs catalysts prepared in Examples 1-4, it can be seen that the catalyst synthesized in Example 2 has the largest half-wave potential, which is higher than that of Example 1, Example 3 and Example 4, indicating that the optimal addition amount of Fe-MIL is 0.75 g, that is, the optimal mass ratio of Fe-MIL:PAN is 1.5:1.

[0073] Example 2

[0074] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this example is similar to that in Example 1, the difference is that 0.75 g of Fe-MIL precursor is added in step (3), and the mass ratio of Fe-MIL precursor to polyacrylonitrile (PAN) is 1.5:1.

[0075] Figure 5 It can be seen that the catalyst contains Fe and Fe3C crystals, and the Fe and Fe3C particles coated with nitrogen-doped carbon nanofibers can serve as active sites of the catalyst.

[0076] Figure 6 It can be seen that the graphitization degree of the catalyst is high. G / I D =1.06. A high degree of graphitization can improve the catalyst's ability to conduct electrons, thereby improving the catalyst's catalytic activity.

[0077] Figure 7 It can be seen that the active components of the catalyst, graphitic nitrogen and pyridine nitrogen, account for a total of 48.7%. Graphitic nitrogen can increase the limiting current density, and pyridine nitrogen can increase the starting potential.

[0078] Figure 8 It can be seen that the catalyst contains Fe-N 2P1 / 2 ,Fe(III)2P 1 / 2 、Fe(III)2P 3 / 2 、Fe-N 2P 3 / 2 、Fe2P 3 / 2 , indicating that Fe is successfully doped into the structure of carbon nanofibers to form Fe-N active sites.

[0079] Fig. 9 It can be seen that the pore specific surface area of ​​the catalyst is 401.97 m 2 / g, Fig.10 It can be seen that the average pore size is 3.932, and the large specific surface area can improve the mass transfer capacity of the catalyst. The hierarchical structure of micropores, mesopores and macropores shortens the mass transfer distance between the reactants and the active sites and increases the active site density of the catalyst.

[0080] Fig.11 It can be seen that the half-wave potential of the Fe-N-CNFs (Carbon nanofibers) catalyst prepared in Example 2 is close to that of commercial Pt / C, with a difference of about 50 mV, but is much higher than the half-wave potential of the catalysts synthesized in Comparative Examples 1, 2 and 3, indicating that the catalyst synthesized by electrospinning is more active.

[0081] Fig.12 It can be seen that compared with the Pt / C catalyst, the Fe-N-CNFs (Carbon nanofibers) catalyst is almost unaffected by methanol, indicating that the synthesized catalyst has a strong ability to resist methanol.

[0082] Fig.13 It can be seen that under constant voltage decay conditions, the current of the Pt / C catalyst decayed by 30.61%, while the current of the Fe-N-CNFs (Carbon nanofibers) catalyst only decayed by 13.78%, indicating that the catalyst has a strong ability to resist decay.

[0083] Fig.14 It can be seen that the ring current half-wave potential of the Pt / C catalyst is slightly higher than that of the Fe-N-CNFs (Carbon nanofibers) catalyst, indicating that the H2O2 yield of this catalyst will be slightly higher, but not very large, and the ORR reaction is mainly carried out via a 4-electron reaction pathway.

[0084] Fig.15 It can be seen that the maximum power density of the proton exchange membrane fuel cell with Fe-N-CNFs (Carbon nanofibers) as the cathode catalyst can reach 167.34mW / cm 2 , indicating that the synthesized catalyst has good performance.

[0085] Example 3

[0086] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this example is similar to that in Example 1, the difference is that 1.0 g of Fe-MIL precursor is added in step (3), and the mass ratio of Fe-MIL precursor to polyacrylonitrile (PAN) is 2:1.

[0087] Fig.16 It can be seen that the catalyst contains Fe and Fe3C crystals, and the Fe and Fe3C particles coated with nitrogen-doped carbon nanofibers can serve as active sites of the catalyst, which is the same as Example 2.

[0088] Fig.17 It can be seen that the graphitization degree of the catalyst is high. G / I D =0.98, a high degree of graphitization can improve the electron conductivity of the catalyst, thereby improving the catalytic activity of the catalyst, which is slightly higher than the degree of graphitization in Example 2.

[0089] Example 4

[0090] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this example is similar to that in Example 1, the difference is that 1.25 g of Fe-MIL precursor is added in step (3), and the mass ratio of Fe-MIL precursor to polyacrylonitrile (PAN) is 2.5:1.

[0091] Fig.18 It can be seen that the active components of the catalyst, graphitic nitrogen and pyridinic nitrogen, account for a total of 26.2%, which is much lower than that of Example 2, so the performance is worse than that of the catalyst synthesized in Example 2.

[0092] Fig.19 It can be seen that the catalyst contains Fe(III)2P 1 / 2 、Fe(III)2P 3 / 2 、Fe-N 2P 3 / 2 、Fe 2P 3 / 2 , indicating that Fe is successfully doped into the structure of carbon nanofibers to form Fe-N active sites, which is basically consistent with Example 2.

[0093] Fig. 20 It can be seen that the pore specific surface area of ​​the catalyst is 329.674 m 2 / g, Fig.21 It can be seen that the average pore size is 3.929, the specific surface area is lower than that of Example 2, and the average pore size is basically the same, indicating that the mass transfer capacity of the catalyst synthesized in Example 4 is slightly poor.

[0094] Example 5

[0095] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this embodiment refers to Example 1, except that 1.0 g of Fe-MIL precursor is added in step (3), the mass ratio of Fe-MIL precursor to polyacrylonitrile (PAN) is 2:1, and the pyrolysis temperature in step (5) is 700°C.

[0096] Fig.23 The oxygen reduction polarization curves of the Fe-N-CNFs catalysts prepared in Examples 3, 5, 6, and 7 were obtained by rotating ring disk electrode (RDE) testing. The catalyst synthesized in Example 3 had the largest half-wave potential, which was higher than that of Examples 5-7, indicating that the optimal temperature for pyrolysis was 800°C.

[0097] Example 6

[0098] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this embodiment refers to Example 1, except that 1.0 g of Fe-MIL precursor is added in step (3), the mass ratio of Fe-MIL precursor to polyacrylonitrile (PAN) is 2:1, and the pyrolysis temperature in step (5) is 900°C.

[0099] Example 7

[0100] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this embodiment refers to Example 1, except that 1.0 g of Fe-MIL precursor is added in step (3), the mass ratio of Fe-MIL precursor to polyacrylonitrile (PAN) is 2:1, and the pyrolysis temperature in step (5) is 1000°C.

[0101] Comparative Example 1

[0102] Comparative Example 1 The method for preparing N-CNFs (Carbon nanofibers) catalyst is similar to that of Example 1, except that no Fe-MIL precursor is added in step (3).

[0103] Comparative Example 2

[0104] Comparative Example 2 The method for preparing the Fe-MIL-800 based catalyst is similar to that of Example 1, except that the Fe-MIL precursor is pre-calcined and then pyrolyzed, and steps (3) and (4) are not performed.

[0105] Comparative Example 3

[0106] The method for preparing the MIL-NC-1.5-800 catalyst in Comparative Example 3 is the same as that in Example 1, except that Fe-MIL and polyacrylonitrile are ground and mixed, but not mixed in a solution and then spun.

[0107] Depend on Fig. 22 It can be seen that the catalyst synthesized in Comparative Example 3 has a blocky morphology, which is not conducive to mass transfer and exposure of active sites, and thus has poor performance.

[0108] Fig.24 It can be seen that the pore specific surface area of ​​the catalyst synthesized in Comparative Example 3 is 291.039 m 2 / g, much lower than the catalyst synthesized in Example 2. Fig.11 From the comparison of oxygen reduction activity, it can be seen that the catalytic activity of Comparative Example 3 is lower than that of Example 2, which indicates that the nanofiber structure can increase the porosity, shorten the mass transfer distance between reactants and products, increase the number of active sites, and thus improve the catalytic activity.

[0109] Comparative Example 4

[0110] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this comparative example refers to Example 1, except that 0.1 g of Fe-MIL precursor is added in step (3), the mass ratio of Fe-MIL precursor to polyacrylonitrile (PAN) is 1:5, and the pyrolysis temperature in step (5) is 800°C.

[0111] Fig.25 It can be seen that the performance of the catalyst synthesized in Comparative Example 4 is much lower than that in Example 2, indicating that the ratio of Fe-MIL to PAN is lower than 1:1, and the performance of the synthesized catalyst is extremely poor. When the ratio is higher than 2.5:1, the viscosity of the slurry is too low and spinning cannot be performed.

[0112] Comparative Example 5

[0113] The method for preparing Fe-CNFs (Carbon nanofibers) catalyst based on Fe-MIL in this comparative example is based on Example 1, except that the binder is polyethylene oxide (PEO) instead of polyacrylonitrile (PAN).

[0114] Fig.26 It can be seen that the performance of the catalyst synthesized in Comparative Example 5 is much lower than that in Example 2, indicating that the nitrogen content has a significant effect on the catalyst performance. The nitrogen content of PEO is 0 and Fe-MIL does not contain nitrogen. Therefore, the performance of the catalyst synthesized using PEO as a binder is extremely poor. In order to improve the catalyst performance, the nitrogen content of the binder must be increased.

[0115] Comparative Example 6

[0116] The method for preparing Fe-N-CNFs (Carbon nanofibers) catalyst based on ZIF-8(Fe) in this comparative example refers to Example 1, except that the precursor is ZIF-8(Fe) instead of Fe-MIL, and the pyrolysis temperature is 1050°C, which is the optimal pyrolysis temperature of ZIF-8(Fe) precursor. The preparation method of ZIF-8(Fe) is as follows: 2.05g 2-methylimidazole and 1.83g butylamine are dissolved in 200ml anhydrous methanol, 2.97g Zn(NO3)2·6H2O and 0.2g FeCl2·4H2O are dissolved in 200ml anhydrous methanol, and then the two solutions are mixed and allowed to stand for 1 hour, followed by centrifugation, washing, and drying to obtain a ZIF-8(Fe) precursor.

[0117] Fig. 27 It can be seen that the performance of the catalyst synthesized in Comparative Example 6 is lower than that in Example 2. The Fe content in Fe-MIL is 20-30%, while the Fe content in ZIF-8(Fe) is less than 5%, indicating that the catalyst synthesized by precursor spinning pyrolysis with a high Fe content has better performance.

Claims

1. An application of a Fe-N-CNFs catalyst as a cathode catalyst in a fuel cell, characterized in that: The catalyst preparation method comprises the following steps: (1) dissolving Fe-MIL in solvent 1, then adding an electrospinning nitrogen-containing binder, and stirring at 10-24° C. for 12-24 h to prepare a mixed solution; (2) electrospinning the mixed solution to obtain nanofibers; (3) pre-calcining the nanofibers at 200-300°C for 2-5h, and then pyrolyzing them at 700-1000°C for 2-5h to obtain Fe-N-CNFs catalyst; The mass ratio of the Fe-MIL to the electrospinning nitrogen-containing binder is 1-2.5:1; the mass fraction of the electrospinning nitrogen-containing binder in the mixed solution is 10-16%; the nitrogen content of the electrospinning nitrogen-containing binder is not less than 12.61wt%; the mass fraction of Fe in the Fe-MIL is 20-30%.

2. The use according to claim 1, characterized in that: The solvent 1 is N,N-dimethylformamide; the electrostatic spinning nitrogen-containing binder is polyacrylonitrile (PAN) or polyvinyl pyrrolidone (PVP).

3. The use according to claim 1, characterized in that: In step (1), Fe-MIL is added to solvent 1, ultrasonicated for 20-30 minutes, and then a nitrogen-containing electrospinning binder is added.

4. The use according to claim 1, characterized in that: In step (2), the voltage of electrospinning is 15-30 kV, the liquid flow rate is 0.4-0.6 ml / h, and the spinning time is 4-10 h.

5. The use according to claim 1, characterized in that: The pre-burning process of step (3) is carried out under air conditions, and the pyrolysis process is carried out under argon conditions.

6. The use according to claim 1, characterized in that: The Fe-MIL is prepared by the following steps: (a) dissolving ferric chloride hexahydrate and terephthalic acid in solvent 2 by ultrasonication to prepare a mixed solution A; (b) reacting the mixed solution A at 140-160° C. for 12-24 h, centrifuging, washing, and drying to obtain Fe-MIL; The molar ratio of the ferric chloride hexahydrate to terephthalic acid is 1:1; and the solvent 2 is N,N-dimethylformamide.

7. The use according to claim 6, characterized in that: In step (a), the ultrasonic time is 5-10 minutes; in step (b), the drying temperature is 80° C. and the drying time is 6-12 hours.

8. The use according to claim 1, characterized in that: The average pore radius of the catalyst is 3.0-4.0 nm; the specific surface area is 330-410 m 2 / g, and the mass fraction of Fe in the catalyst is 30-40%.

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