High-durability Fe-N-C catalyst capable of being used for fuel cell as well as preparation method and application of high-durability Fe-N-C catalyst

Abstract

The invention belongs to the technical field of fuel cell catalysts, and particularly relates to a high-durability Fe-N-C catalyst for a fuel cell as well as a preparation method and application of the high-durability Fe-N-C catalyst. The preparation method comprises the following steps: (1) sequentially dissolving iron oxide nanoparticles, zinc nitrate hexahydrate, polyvinylpyrrolidone (PVP) and 2-methylimidazole in a methanol solution, carrying out heating treatment, cooling, carrying out solid-liquid separation, collecting a solid, and drying to obtain a Fe2O3 ZIF-8 precursor; (2) calcining the Fe2O3 (at) ZIF-8 precursor, so as to obtain a Fe-N-C intermediate; and (3) mixing the Fe-N-C intermediate with ammonium chloride to obtain a mixture, and carrying out heat treatment on the mixture to obtain the high-durability Fe-N-C catalyst for the fuel cell. The method provided by the invention is helpful for improving the activity and stability of the Fe-N-C catalyst.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cell catalysts, and in particular relates to a high-durability Fe-NC catalyst that can be used in fuel cells, and a preparation method and application thereof. Background Art

[0002] With the depletion of energy resources and the worsening of environmental pollution, hydrogen fuel cells have attracted widespread attention due to their efficient energy conversion and environmental protection. Despite the important milestones achieved in proton exchange membrane fuel cells (PEMFCs), the reliance on Pt-based catalysts for the oxygen reduction reaction (ORR) still hinders its practical application. In extremely challenging acidic environments, atomically dispersed and nitrogen-coordinated FeN4 sites on carbon (Fe-NC) are the most promising catalysts to replace Pt for the cathode oxygen reduction reaction (ORR) (Nat. Energy 2022, 7(7), 652-663; ACS Catal. 2019, 9(11), 10126-10141.). However, over the past few decades, the improvement of the activity and durability of Fe-NC catalysts has remained limited due to the lack of understanding of the active sites, low active site density, poor stability, and activity-stability trade-offs (Nat. Catal. 2021, 4(1), 6-7.). As with the Pt active sites, the adsorption strength of O2 and ORR intermediates on the FeN4 active sites is also strong. - For the transfer pathway, the activation barrier for OO bond cleavage is too high (J.Phys.Chem.Lett.2020,11(8),2896-2901.). Small carbon faces with local defects are conducive to OO bond breakage (J.Am.Chem.Soc.2019,141(15),6254-6262.). However, during the ORR process, oxidation of this defect-rich carbon substrate may trigger demetallization of FeN4 sites on the catalyst (Adv.Mater.2019,31(31),e1807615.). Graphitized carbon structures have higher corrosion / oxidation resistance to stabilize FeN4 active sites (Science 2011,332(6028),443-7). However, they lack sufficient defects and nitrogen dopants to carry sufficient FeN4 site density. Therefore, the coordination environment and local carbon structure are crucial to their intrinsic activity and stability (Adv. Energy Mater. 2019, 10(11), 1902844).

[0003] Generally speaking, the FeN4 sites embedded in the carbon plane of Fe-NC catalysts are supported in two connection environments containing pyrrolic and pyridinic N, respectively, and these two coordination environments can be The experiment was effectively identified and quantified. One is FeN4C 12 One part, usually denoted as the S1 site, has a pyrrolic N ligand coordination environment, and the other is FeN4C with a pyridinic N ligand coordination environment. 10 part, usually represented by the S2 site. The S1 site has stronger intrinsic activity, but it will irreversibly degrade into inactive iron oxides during the ORR process, resulting in loss of activity (Adv. Energy Mater. 2024, 14 (12), 2303733.). The S2 site is more stable to demetallization, but due to its stronger adsorption of oxygen and intermediates, its activity is lower (Nat. Energy 2022, 7 (7), 652-663). (Nat. Energy 2022, 7 (7), 652-663) Therefore, it is challenging to achieve both high activity and stability of Fe-NC catalysts. It is a very high requirement for low-cost transition metals to have high activity and long-term durability in PEMFCs. There is an urgent need for a controllable strategy to prepare a cheap Fe-NC catalyst that can ensure high activity while having special durability.

[0004] Therefore, it is necessary to provide an improved technical solution to address the above-mentioned deficiencies in the prior art. Summary of the invention

[0005] The object of the present invention is to provide a high-durability Fe-NC catalyst that can be used in fuel cells, and a preparation method and application thereof, so as to help solve or improve the problems of low catalytic activity and poor stability of transition metal catalysts in the prior art.

[0006] In order to achieve the above-mentioned object, the present invention provides the following technical scheme: a method for preparing a high-durability Fe-NC catalyst that can be used for a fuel cell, comprising the following steps: (1) dissolving iron oxide nanoparticles, zinc nitrate hexahydrate, polyvinyl pyrrolidone (PVP) and 2-methylimidazole in a methanol solution in sequence, heating, cooling, collecting the solid by solid-liquid separation and drying to obtain a Fe2O3@ZIF-8 precursor; (2) calcining the Fe2O3@ZIF-8 precursor to obtain a Fe-NC intermediate; (3) mixing the Fe-NC intermediate with ammonium chloride to obtain a mixture, and heat-treating the mixture to obtain the high-durability Fe-NC catalyst that can be used for a fuel cell.

[0007] Preferably, in step (1), the mass ratio of iron oxide nanoparticles to zinc nitrate hexahydrate is (0.01-0.1):(3.5-6), the mass ratio of polyvinyl pyrrolidone to the iron oxide nanoparticles is (0.02-0.1):(0.01-0.1); and the dosage ratio of zinc nitrate hexahydrate, 2-methylimidazole and methanol is (3.5-6) ​​g:(3-7) g:(100-300) mL.

[0008] Preferably, in step (1), the temperature of the heating treatment is 50-80°C, and the time of the heating treatment is 12-36 hours.

[0009] Preferably, in step (2), the calcination temperature is 800-1000° C., and the calcination time is 1-3 h; and the calcination is carried out under an inert atmosphere.

[0010] Preferably, in step (3), the heat treatment temperature is 1000-1200° C., and the heat treatment time is 0.5-3 h; and the heat treatment is carried out under an inert atmosphere.

[0011] Preferably, the heating rate of the calcination in step (2) and / or the heat treatment in step (3) is 8-15° C. / min.

[0012] Preferably, in step (3), the mass ratio of the Fe-NC intermediate to ammonium chloride is 1:(1-5).

[0013] The present invention also provides a high-durability Fe-NC catalyst that can be used in fuel cells, which adopts the following technical scheme: a high-durability Fe-NC catalyst that can be used in fuel cells, wherein the high-durability Fe-NC catalyst that can be used in fuel cells is prepared by the method as described above.

[0014] The present invention also provides a proton membrane exchange fuel cell cathode, which adopts the following technical solution: a proton membrane exchange fuel cell cathode, wherein the proton membrane exchange fuel cell cathode contains the high-durability Fe-NC catalyst that can be used in fuel cells as described above.

[0015] The present invention also provides a proton membrane exchange fuel cell, which adopts the following technical solution: a proton membrane exchange fuel cell, the proton membrane exchange fuel cell containing the high-durability Fe-NC catalyst that can be used for fuel cells as described above or the proton membrane exchange fuel cell cathode as described above.

[0016] Beneficial effects:

[0017] (1) The present invention provides a new approach to prepare atomically dispersed Fe-NC catalysts. Compared with chemical vapor deposition, template method and other methods, the method provided by the present invention utilizes a simple secondary annealing method to prepare Fe-NC catalysts with high specific surface area and high active site density. The preparation process is simple and easy to achieve large-scale preparation.

[0018] (2) The present invention prepares the Fe-NC catalyst through the spatial confinement effect of the metal organic framework with an ordered porous structure and the chemical coordination effect of the additionally introduced pyrrole N ligand polyvinyl pyrrolidone. The two-way confinement strategy not only greatly increases the density of active sites and improves the proportion of high-activity coordination configuration sites (S1), but also achieves high stability while improving the activity. The preparation method is simple and the structure is easy to control.

[0019] (3) The present invention uses NH4Cl to further atomize and etch the synthesized Fe-NC-950. NH4Cl decomposes into NH3 and HCl gas at high temperature, which will produce great internal stress and etch carbon, thereby generating a large number of micro / mesopores and carbon defects. HCl may further dissociate at high temperature to form H2 and Cl2, which reacts with the residual Fe aggregates in the catalyst to promote the formation of atomically dispersed Fe sites, and finally obtain an atomically dispersed Fe-NC catalyst. This method uses the confinement strategy of composite carbon materials to effectively anchor more metal single atoms, greatly increase the specific surface area of ​​the catalyst, and expose more active sites. At the same time, the secondary pyrolysis treatment also improves the degree of graphitization and enhances the stability of the coordination structure. At a certain current density, the overpotential of the catalyst is significantly reduced, the reaction kinetics are accelerated, and it can be successfully applied to proton exchange membrane fuel cells (PEMFCs), effectively improving the activity and durability of transition metal catalysts in fuel cells. At the same time, the cost of the catalyst is greatly reduced due to the wide source and low price of the synthetic raw materials.

[0020] (4) The Fe-NC catalyst of the present invention was applied to PEMFCs. After 50,000 and 100,000 square wave cycles, the -2 The voltage can only lose 7mV (1.1%) and 22mV (3.5%) at the current density. Further fuel cell life test at a constant voltage of 0.6V proved that the Fe-NC catalyst has excellent long-term stability (>230h). The Fe-NC catalyst developed in this work has great prospects for replacing Pt-based catalysts in PEMFCs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. Among them:

[0022] Figure 1 The transmission electron microscopy (TEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersion spectrum of the Fe-NC catalyst prepared in Example 1, wherein (a) and (b) are TEM images at different resolutions, with scales of 200 nm and 20 nm, respectively; (c) is a HAADF-STEM image with a scale of 5 nm; and (d) is an energy dispersion spectrum.

[0023] Figure 2 These are N2 adsorption-desorption curves of the Fe-NC catalyst prepared in Example 1, the Fe-NC-0 catalyst prepared in Comparative Example 1, and the Fe-NC-PVP catalyst prepared in Comparative Example 2.

[0024] Figure 3 The left figure (a) is the Fe-NC catalyst prepared in Example 1. 57 Fe Mössbauer spectrum, right (b) is the Fe-NC catalyst prepared in Example 1 57 Relative contents of S1 and S2 sites resolved by Fe Mössbauer spectra.

[0025] Figure 4 In the figure, Figure (a) is the LSV curve of the Fe-NC catalyst prepared in Example 1, the Fe-NC-0 catalyst prepared in Comparative Example 1, and the Fe-NC-PVP catalyst prepared in Comparative Example 2 tested in an O2-saturated 0.1M HClO4 solution at 1600rpm; Figure (b) is the LSV curve of the Fe-NC catalyst prepared in Example 1 before and after 100,000 accelerated cycles in an O2-saturated 0.1M HClO4 solution; Figure (c) is the LSV curve of the Fe-NC-PVP catalyst prepared in Comparative Example 2 after 100,000 accelerated cycles in an O2-saturated 0.1M HClO4 solution; Figure (d) is the LSV curve of the Fe-NC-0 catalyst prepared in Comparative Example 1 after 100,000 accelerated cycles in an O2-saturated 0.1M HClO4 solution.

[0026] Figure 5 In the figure, the left figure (a) shows the polarization curves and power density measured under H2-O2 conditions when the Fe-NC catalyst prepared in Example 1 and the Fe-NC-0 catalyst prepared in Comparative Example 1 are assembled into proton exchange membrane fuel cell cathodes; the right figure (b) shows the polarization curves and power density measured under H2-O2 conditions when the Fe-NC catalyst prepared in Example 1 is assembled into a proton exchange membrane fuel cell cathode after accelerated cycles of 50,000 and 100,000 times, respectively.

[0027] Figure 6The Fe-NC catalyst prepared in Example 1 and the Fe-NC-0 catalyst prepared in Comparative Example 1 were respectively assembled into cathodes of proton exchange membrane fuel cells, and the long-term durability data were measured under a H2-O2 environment and a constant voltage of 0.6V. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0029] The present invention will be described in detail below in conjunction with embodiments. It should be noted that the embodiments and features in the embodiments of the present invention can be combined with each other without conflict.

[0030] Due to the current problems of low activity and poor stability of transition metal catalysts, it is urgent to synthesize a catalyst: the inventors thought that the catalyst should contain a large number of increased S1 and S2 sites to help enhance the intrinsic activity and stability of the catalyst at the same time. However, it is almost impossible to achieve this goal using traditional synthesis methods based on catalysts previously reported in the literature (Nat. Catal. 2023, 6, 1215-1227). In addition, the inventors found in the study that the ratio of the presence of S1 and S2 sites in the catalyst is regulated, which may help to improve the catalytic performance of the catalyst, but there is no relevant research on the relationship between the ratio of the presence of S1 and S2 sites in the catalyst and the catalytic performance.

[0031] The present invention provides a method for preparing a high-durability Fe-NC catalyst that can be used in a fuel cell. The method for preparing a high-durability Fe-NC catalyst that can be used in a fuel cell according to an embodiment of the present invention comprises the following steps: (1) dissolving iron oxide nanoparticles, zinc nitrate hexahydrate, polyvinyl pyrrolidone (PVP) and 2-methylimidazole in a methanol solution in sequence, heating, cooling, collecting solids by solid-liquid separation and drying to obtain a Fe2O3@ZIF-8 precursor; (2) calcining the Fe2O3@ZIF-8 precursor to obtain a Fe-NC intermediate; (3) mixing the Fe-NC intermediate with ammonium chloride to obtain a mixture, and heat-treating the mixture to obtain a high-durability Fe-NC catalyst that can be used in a fuel cell.

[0032] The present invention adds pyrrole N ligand PVP to the precursor. As the thermal activation temperature increases, the pyrrole N in PVP helps anchor more active sites. At the same time, combined with the heat treatment of NH4Cl, the density of metal active sites is significantly increased, the proportion of high-activity coordination sites (S1 sites) is increased, the activity-stability constraint is broken, and an accurate and optimal S1 site and high-stability coordination site (S2 site) existence ratio (3:1) is provided. The Fe-NC catalyst with the optimal site ratio exhibits excellent activity and stability.

[0033] The present invention prepares the Fe-NC catalyst through the spatial confinement effect of the metal organic framework of the ordered porous structure and the chemical coordination effect of the additionally introduced pyrrole N ligand polyvinyl pyrrolidone. The two-way confinement strategy not only greatly increases the density of active sites and improves the proportion of high-activity coordination configuration sites (S1), but also achieves high stability while improving the activity. The preparation method is simple and the structure is easy to control.

[0034] The present invention uses NH4Cl to further atomize and etch the synthesized Fe-NC intermediate. NH4Cl decomposes into NH3 and HCl gas at high temperature, which will produce great internal stress and etch carbon, thereby generating a large number of micro / mesopores and carbon defects. HCl may further dissociate at high temperature to form H2 and Cl2, which reacts with the Fe aggregates remaining in the catalyst to promote the formation of atomically dispersed Fe sites, and finally obtain an atomically dispersed Fe-NC catalyst. The method uses the composite carbon material confinement strategy to effectively anchor more metal single atoms, greatly increase the specific surface area of ​​the catalyst, expose more active sites, and at the same time, the secondary pyrolysis treatment also improves the degree of graphitization and enhances the stability of the coordination structure. At a certain current density, the overpotential of the catalyst is significantly reduced, the reaction kinetics are accelerated, and it can be successfully applied to proton exchange membrane fuel cells (PEMFCs), effectively improving the activity and durability of transition metal catalysts in fuel cells. At the same time, the cost of the catalyst is greatly reduced due to the wide source and low price of the synthetic raw materials.

[0035] In a preferred embodiment of the method for preparing a high-durability Fe-NC catalyst that can be used for a fuel cell of the present invention, in step (1), the mass ratio of the iron oxide nanoparticles to the zinc nitrate hexahydrate is (0.01-0.1):(3.5-6) ​​(for example, 0.01:3.5, 0.01:4.5, 0.01:6, 0.05:3.5, 0.05:5, 0.05:6, 0.1:3.5, 0.1:4 or 0.01:6; preferably (0.015- 0.09):(3.8-5.5); more preferably (0.02-0.07):(4-5)), the mass ratio of polyvinyl pyrrolidone to the iron oxide nanoparticles is (0.02-0.1):(0.01-0.1) (for example, 0.02:0.01, 0.02:0.05, 0.02:0.1, 0.06:0.01, 0.06:0.05, 0.06:0.1, 0.1:0.01, 0.1:0.05 or 0. 1:0.1; preferably (0.03-0.08):(0.015-0.09)); more preferably (0.04-0.06):(0.02-0.07)); the usage ratio of zinc nitrate hexahydrate, 2-methylimidazole and methanol is (3.5-6) ​​g:(3-7) g:(100-300) mL (for example, 3.5 g:3 g:100 mL, 3.5 g:5 g:100 mL, 3.5 g:7 g:100 mL, 3.5 g:3 g:200mL, 3.5g:3g:300mL, 4.8g:5g:150mL, 6g:3g:100mL, 6g:7g:100mL, 6g:3g:160mL, 6g:3g:300mL, 6g:5g:160mL, etc.; preferably (3.8-5.5)g:(4-6)g:(150-280)mL; more preferably (4-5)g:(4.5-5.5)g:(180-220)mL). Among them, if the amount of iron oxide nanoparticles used is too large, Fe nanoparticles will appear in the final product instead of uniformly dispersed single-atom catalysts; if the amount of iron oxide nanoparticles used is too small, the single-atom loading is low and the catalytic activity is low; in addition, the present invention adopts a "top-down" synthesis method, and the iron oxide nanoparticles are preferably α-Fe2O3 or γ-Fe2O3 with a particle size of less than 30nm; the main role of polyvinyl pyrrolidone in the present invention is to serve as a pyrrole N ligand to increase the highly active S1 site in the catalyst, so its amount determines the ratio of S1 and S2 sites in the catalyst.

[0036] In a preferred embodiment of the method for preparing a high-durability Fe-NC catalyst that can be used for a fuel cell of the present invention, in step (1), the temperature of the heating treatment is 50-80°C (for example, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C; preferably 50-70°C; more preferably 55-65°C), and the time of the heating treatment is 12-36h (for example, 12h, 15h, 18h, 20h, 22h, 24h, 26h, 28h, 30h, 32h, 34h or 36h; preferably 15-30h; more preferably 20-26h). The heating in step (1) is to accelerate the reaction (synthesis of Zn-ZIF and confined anchoring of iron oxide nanoparticles); if the heating temperature is too low, it will not play the corresponding role; if the heating temperature is too high, the methanol evaporates too quickly, the reaction is not complete, and the metal particles are unevenly dispersed; if the heating time is too short, it will not play the role of fully allowing the above reaction to proceed; if the heating time is too long, the metal salt will precipitate as the methanol evaporates.

[0037] In a preferred embodiment of the method for preparing a high-durability Fe-NC catalyst that can be used for a fuel cell of the present invention, in step (1), iron oxide nanoparticles, zinc nitrate hexahydrate and polyvinyl pyrrolidone are first dispersed in methanol in sequence and ultrasonically dispersed at room temperature (preferably, the ultrasonic dispersion time is 30 minutes), and then 2-methylimidazole is added and magnetic stirring is performed until the mixed solution is no longer clear, and heating treatment is started.

[0038] In a preferred embodiment of the method for preparing a high-durability Fe-NC catalyst that can be used for a fuel cell of the present invention, in step (1), after the heating treatment is completed, the reaction product is preferably washed and dried in sequence, wherein the washing adopts methanol centrifugal washing, and the rotation speed of the centrifugal washing is 6000-10000rpm (for example, 6000rpm, 7000rpm, 8000rpm, 9000rpm or 10000rpm; preferably 7000-9000rpm; more preferably 8000rpm); the centrifugal washing time is 3-10min (for example, 3min, 4min, 5min, 6min, 7min, 8min, 9min or 10min; preferably 4-8min; more preferably 5-6min); the drying environment is vacuum, and the drying temperature is 50-80°C (for example, 50°C, 60°C, 70°C or 80°C; preferably 60°C).

[0039] Preferably, in step (1), the particle size of the iron oxide nanoparticles is 20-30 nm (preferably 20 nm). In a preferred embodiment of the method for preparing a highly durable Fe-NC catalyst for a fuel cell of the present invention, in step (2), the calcination temperature is 800-1000°C (e.g., 800°C, 850°C, 900°C, 950°C or 1000°C; preferably 900-1000°C; more preferably 900-950°C), and the calcination time is 1-3h (e.g., 1h, 1.5h, 2h, 2.5h or 3h; preferably 1-2h; more preferably 1.5h); the calcination is carried out under an inert atmosphere (e.g., nitrogen). Among them, if the calcination temperature in step (2) is lower than 800°C, the Zn ions cannot be completely volatilized and removed; if the calcination temperature is higher than 1000°C, the single atoms produced by atomization may further agglomerate into particles; if the calcination time is too short, the iron oxide nanoparticles cannot be completely atomized; if the calcination time is too long, single atoms may agglomerate; the "inert atmosphere" can be a rare gas such as nitrogen or argon.

[0040] In a preferred embodiment of the method for preparing a highly durable Fe-NC catalyst for a fuel cell of the present invention, in step (3), the temperature of the heat treatment is 1000-1200°C (for example, 1000°C, 1050°C, 1100°C, 1150°C or 1200°C; preferably 1000-1150°C; more preferably 1050-1100°C), the time of the heat treatment is 0.5-3h (for example, 0.5h, 1h, 1.5h, 2h, 2.5h or 3h; preferably 1-2.5h; more preferably 1.5h); the heat treatment is carried out under an inert atmosphere (for example, nitrogen). If the temperature of the heat treatment is too low, NH4Cl cannot be decomposed quickly and the degree of graphitization of carbon cannot be increased; if the temperature of the heat treatment is higher than 1200°C, firstly, the energy consumption is high, and secondly, it is easy to cause single atoms to agglomerate; the "inert atmosphere" can be a rare gas such as nitrogen or argon.

[0041] In a preferred embodiment of the method for preparing a highly durable Fe-NC catalyst for a fuel cell of the present invention, the heating rate of the calcination in step (2) and / or the heat treatment in step (3) is 8-15°C / min (for example, 8°C / min, 10°C / min, 12°C / min, 14°C / min or 15°C / min; preferably 15°C / min). If the heating rate is too fast, incomplete carbonization or agglomeration of single atoms may occur; if the heating rate is too slow, the porosity of the carbon may be affected, reducing the specific surface area.

[0042] In a preferred embodiment of the method for preparing a highly durable Fe-NC catalyst for a fuel cell of the present invention, in step (3), the mass ratio of the Fe-NC intermediate to ammonium chloride is 1:(1-5) (for example, 1:1, 1:2, 1:2.5, 1:3, 1:3.5, 1:4 or 1:5; preferably 1:(1-4); more preferably 1:(2-3.5)). If the amount of ammonium chloride used is too small, the gas products in stages at high temperature are small, and the carbon carrier cannot be effectively etched and the atomic-level dispersion of the Fe sites cannot be promoted; if the amount of ammonium chloride used is too large, a macroporous structure or too many defects may be caused, resulting in the collapse of the carbon carrier and a reduction in specific surface area.

[0043] Preferably, the Fe-NC intermediate and ammonium chloride are mixed and ground for 0.5-3 h (eg, 0.5 h, 0.8 h, 1 h, 1.5 h, 2 h, 2.5 h or 3 h; preferably 0.8-2.5 h; more preferably 1-2 h).

[0044] The present invention also provides a high-durability Fe-NC catalyst that can be used in a fuel cell. The high-durability Fe-NC catalyst that can be used in a fuel cell in an embodiment of the present invention is prepared by the method described above.

[0045] The present invention also provides a proton membrane exchange fuel cell cathode. The proton membrane exchange fuel cell cathode of the embodiment of the present invention contains the high-durability Fe-NC catalyst that can be used in fuel cells as described above.

[0046] The present invention also provides a proton membrane exchange fuel cell. The proton membrane exchange fuel cell of the embodiment of the present invention contains the high-durability Fe-NC catalyst applicable to fuel cells as described above or the proton membrane exchange fuel cell cathode as described above.

[0047] The highly durable Fe-NC catalyst for fuel cells and its preparation method and the proton membrane exchange fuel cell of the present invention are described in detail below through specific examples.

[0048] All raw materials used in the following examples can be purchased commercially; the sources of the main raw materials are as follows: α-Fe2O3 nanoparticles (20nm, 99.5%), 2-methylimidazole (98%) and polyvinyl pyrrolidone (analytical grade) were purchased from Beijing Inokai Technology Co., Ltd.; zinc nitrate hexahydrate (analytical grade) and anhydrous methanol (analytical grade) were purchased from Sinopharm Chemical Reagents.

[0049] Example 1

[0050] The preparation method of the high-durability Fe-NC catalyst that can be used for a fuel cell in this embodiment comprises the following steps:

[0051] (1) α-Fe2O3 nanoparticles with a size of 20 nm (0.02 g), Zn(NO3)2·6H2O (4.5 g) and polyvinyl pyrrolidone (0.05 g) were dissolved in anhydrous methanol solution (200 mL). After ultrasonic dispersion for 30 min, 2-methylimidazole (5.28 g) was added under magnetic stirring. When the mixed solution was no longer clear, it was placed in a 60°C water bath and heated for 24 h. After natural cooling, the precipitate was collected by centrifugation washing three times with anhydrous methanol, wherein the speed of centrifugation washing was 8000 rpm and the time was 5 min. Then, it was vacuum dried at 60°C to obtain a pink Fe2O3@ZIF-8 precursor powder.

[0052] (2) calcining the pink Fe2O3@ZIF-8 precursor powder under nitrogen atmosphere at a temperature of 950°C for 1.5 h at a heating rate of 15°C / min to obtain Fe-NC intermediate (abbreviated as Fe-NC-950);

[0053] (3) After being fully ground with NH4Cl at a mass ratio of 1:3 for 1 h, Fe-NC-950 was heat treated at 1100°C for 1.5 h in a nitrogen flow, with a heating rate of 15°C / min. After natural cooling, a highly durable Fe-NC catalyst with ultra-high specific surface area and abundant active sites that can be used in fuel cells was obtained.

[0054] Example 2

[0055] The preparation method of the high-durability Fe-NC catalyst that can be used for a fuel cell in this embodiment comprises the following steps:

[0056] (1) α-Fe2O3 nanoparticles with a size of 20 nm (0.05 g), Zn(NO3)2·6H2O (5 g) and polyvinyl pyrrolidone (0.08 g) were dissolved in anhydrous methanol solution (300 mL). After ultrasonic dispersion for 30 min, 2-methylimidazole (6 g) was added under magnetic stirring. When the mixed solution was no longer clear, it was placed in a 60°C water bath and heated for 30 h. After natural cooling, the precipitate was collected by centrifugation washing three times with anhydrous methanol at a speed of 10,000 rpm for 8 min. The precipitate was then vacuum dried at 70°C to obtain a pink Fe2O3@ZIF-8 precursor powder.

[0057] (2) calcining the pink Fe2O3@ZIF-8 precursor powder under nitrogen atmosphere at a temperature of 1000°C, a calcination holding time of 2 h, and a heating rate of 15°C / min to obtain Fe-NC-1000;

[0058] (3) After Fe-NC-1000 and NH4Cl were fully ground at a mass ratio of 1:5 for 2 h, the mixture was heat treated at 1200°C for 2 h in a nitrogen flow, wherein the heating rate during the heat treatment was 15°C / min. After natural cooling, the catalyst Fe-NC of this embodiment that can be used for fuel cells and has an ultra-high specific surface area and abundant active sites was obtained.

[0059] Example 3

[0060] The preparation method of the high-durability Fe-NC catalyst that can be used for a fuel cell in this embodiment comprises the following steps:

[0061] (1) α-Fe2O3 nanoparticles with a size of 20 nm (0.01 g), Zn(NO3)2·6H2O (3.8 g) and polyvinyl pyrrolidone (0.03 g) were dissolved in anhydrous methanol solution (160 mL). After ultrasonic dispersion for 30 min, 2-methylimidazole (4 g) was added under magnetic stirring. When the mixed solution was no longer clear, it was placed in a 60°C water bath and heated for 20 h. After natural cooling, the precipitate was collected by centrifugation washing three times with anhydrous methanol, wherein the speed of centrifugation washing was 6000 rpm and the time was 3 min. Then, it was vacuum dried at 50°C to obtain a pink Fe2O3@ZIF-8 precursor powder.

[0062] (2) calcining the pink Fe2O3@ZIF-8 precursor powder under nitrogen atmosphere at a temperature of 900°C, a calcination holding time of 1 h, and a heating rate of 10°C / min to obtain Fe-NC-900;

[0063] (3) After Fe-NC-900 and NH4Cl were fully ground at a mass ratio of 1:2 for 0.5 h, the mixture was heat treated at 1000°C for 1 h in a nitrogen flow, wherein the heating rate during the heat treatment was 15°C / min. After natural cooling, the catalyst Fe-NC of this embodiment that can be used for fuel cells and has an ultra-high specific surface area and abundant active sites was obtained.

[0064] Comparative Example 1

[0065] The difference between this comparative example and Example 1 is that the polyvinyl pyrrolidone in step (1) is omitted, and the rest is consistent with Example 1.

[0066] The product of this comparative example is abbreviated as Fe-NC-0 catalyst.

[0067] Comparative Example 2

[0068] The difference between this comparative example and Example 1 is that the amount of polyvinyl pyrrolidone added in step (1) is half of the amount of α-Fe2O3 nanoparticles, that is, 0.01 g, and the rest is consistent with Example 1.

[0069] The product of this comparative example is abbreviated as Fe-NC-PVP catalyst.

[0070] Experimental example

[0071] 1. Transmission electron microscopy, high-angle annular dark field scanning transmission electron microscopy and energy dispersion spectrum detection:

[0072] Transmission electron microscopy characterization (TEM) was carried out on a Tecnai G220 transmission electron microscope from FEI, USA, with an accelerating voltage of 200 kV. Sample preparation method: First, grind the sample to be tested evenly with a mortar until no particles are present, then take a small amount of the ground sample powder to be tested and add it to ultrapure water or anhydrous ethanol, ultrasonicate for 20-30 minutes until it is evenly dispersed, take an appropriate amount of liquid and drop it onto the carbon support film or microgrid or copper mesh, and dry it at low temperature in a vacuum drying oven for testing.

[0073] Figure 1 The transmission electron microscopy (TEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersion spectrum of the Fe-NC catalyst prepared in Example 1, wherein (a) and (b) are TEM images at different resolutions, with scales of 200 nm and 20 nm, respectively; (c) is a HAADF-STEM image with a scale of 5 nm; and (d) is an energy dispersion spectrum.

[0074] Figure 1 The characterization results show that through the spatial confinement of the metal organic framework and the chemical confinement strategy of polyvinyl pyrrolidone, combined with the secondary pyrolysis treatment of NH4Cl, an atomically dispersed Fe-NC catalyst with a rich pore structure was obtained by calcination in an argon atmosphere.

[0075] 2. Specific surface area test: Nitrogen adsorption-desorption analysis is mainly used to characterize the BET specific surface area and pore size distribution of the sample. It is carried out on the American Micromeritics fully automatic adsorption instrument APSP 2020. Test requirements: According to the estimated specific surface area of ​​the sample to be tested (the sample specific surface area in this invention is estimated to be 1000m 2 g -1 Approximately 100 mg of sample was weighed and degassed at 200 °C for 12 h. Subsequently, a BET fully automatic specific surface area analyzer was used to perform N2 adsorption / desorption test on the sample under 77 K liquid nitrogen conditions. The isothermal adsorption / desorption curve was obtained after the instrument analysis was completed.

[0076] Figure 2 These are N2 adsorption-desorption curves of the Fe-NC catalyst prepared in Example 1, the Fe-NC-0 catalyst prepared in Comparative Example 1, and the Fe-NC-PVP catalyst prepared in Comparative Example 2. Figure 2 The results of N2 adsorption-desorption curves show that: on the one hand, the specific surface area of ​​the carbon composite material is effectively increased by the secondary pyrolysis treatment of NH4Cl, which is beneficial to expose more active sites; on the other hand, the introduced pyrrole N ligand polyvinyl pyrrolidone can not only anchor more active sites through the chemical confinement strategy, but also the volatilization of polyvinyl pyrrolidone at high temperature can create more mesoporous structures and defects to increase the specific surface area.

[0077] 3. Detection of relative contents of different coordination structures (S1 and S2 sites) in the catalyst: 57 Fe Mössbauer Spectroscopy ( 57 Fe ) is mainly used to detect the coordination structure, valence state, symmetry and other information of Fe in the sample. Mössbauer spectroscopy was performed on the MFD-500AV instrument of Topologic Systems in Japan in constant acceleration mode. Sample preparation requirements: The powder sample to be tested is ground to 200-400 mesh, and the sample amount is generally 50-100 mg. When the total content of Fe in the sample to be tested is less than 20%, it is necessary to provide as much sample as possible, but not too much, which will block the signal.

[0078] Figure 3 The Fe-NC catalyst prepared in Example 1 57 Fe Mössbauer spectrum (a); Fe-NC catalyst prepared in Example 1 57 The relative contents of S1 and S2 sites analyzed by Fe Mössbauer spectrum (b).

[0079] Figure 3 of 57 The results of Fe Mössbauer spectroscopy detection and analysis showed that the relative content ratio of S1 sites and S2 sites in the prepared atomically dispersed Fe-NC catalyst was about 3:1, which proved the controllability of the synthesis strategy.

[0080] 4. Redox performance test:

[0081] The oxygen reduction reaction test adopts a three-electrode test system, that is, a carbon rod is used as a counter electrode, a saturated calomel electrode is used as a reference electrode, and a glassy carbon electrode is used as a working electrode, and the performance is tested using a three-electrode system. Specifically, the catalyst materials prepared in Example 1 and Comparative Example 1 are prepared into a slurry and loaded onto a glassy carbon electrode. The method for experimentally loading the catalyst slurry onto a glassy carbon electrode is as follows: take 5.0 mg of the catalyst sample, and disperse it in a solution prepared by low-temperature ultrasonic dispersion in 336.0 μL of ethanol, 144.0 μL of deionized water, and 20 μL of Nafion117 (~5%); after ultrasonically mixing the prepared solution for 30 minutes, use a pipette to measure 5.0 μL of the mixed solution, vertically drip it on the glassy carbon electrode, and after it is naturally dried, it can be used as a working electrode. The steps for preparing the working electrode of Comparative Example 1 are the same as those of Example 1.

[0082] Figure 4 LSV curves (a) of the atomically dispersed Fe-NC catalyst prepared in Example 1, the Fe-NC-0 catalyst prepared in Comparative Example 1, and the Fe-NC-PVP catalyst prepared in Comparative Example 2 tested in an O2-saturated 0.1M HClO4 solution at 1600rpm; LSV curve of the Fe-NC catalyst prepared in Example 1 before and after 100,000 accelerated cycles in an O2-saturated 0.1M HClO4 solution (b); LSV curve of the Fe-NC-PVP catalyst prepared in Comparative Example 2 before and after 100,000 accelerated cycles in an O2-saturated 0.1M HClO4 solution (c); LSV curve of the Fe-NC-0 catalyst prepared in Comparative Example 1 before and after 100,000 accelerated cycles in an O2-saturated 0.1M HClO4 solution (d).

[0083] Figure 4 The oxygen reduction performance results show that the prepared atomically dispersed Fe-NC catalyst has significantly better catalytic activity than the catalysts prepared in Comparative Examples 1 and 2, and has excellent durability. After 100,000 cycles of accelerated durability test, the half-wave potential (E 1 / 2 ) only decreased by 5 mV, which was better than the Fe-NC-0 catalyst prepared in Comparative Example 1 (E 1 / 2 The Fe-NC-PVP catalyst prepared in Example 2 (E 1 / 2 53mV drop).

[0084] 5. Polarization curve and power density test: The test is carried out according to the standard test scheme provided by the U.S. Department of Energy (DOE).

[0085] Figure 5The Fe-NC catalyst prepared in Example 1 and the Fe-NC-0 catalyst prepared in Comparative Example 1 were respectively assembled into cathodes of proton exchange membrane fuel cells, and polarization curves and power densities measured under H2-O2 conditions (a); the Fe-NC catalyst prepared in Example 1 was assembled into cathodes of proton exchange membrane fuel cells, and polarization curves and power densities after accelerated cycles of 50,000 and 100,000 times under H2-O2 conditions (b).

[0086] Figure 5 The test results of proton exchange membrane fuel cells in the experiment showed that the prepared Fe-NC catalyst had a significantly excellent power density, at 2117 mA cm -2 The peak power density can reach 0.88Wcm -2 In the kinetic region of 0.8 V, the current density of Fe-NC reaches 130 mA cm -2 After 100,000 square wave cycles, the peak power density of the catalyst decayed by only 10.3%. After 50,000 and 100,000 square wave cycles, the peak power density of the catalyst decreased by only 10.3% at 0.8 A cm -2 The voltage loss at the current density was only 7 mV (1.1%) and 22 mV (3.5%), respectively. Further fuel cell life test at a constant voltage of 0.6 V demonstrated that Fe-NC has excellent long-term stability (> 230 h). These results are significantly better than the US Department of Energy's target (at 0.8 V, current density above 60 mA cm -2 ; After 30,000 cycles of accelerated durability testing, at 0.8A cm -2 The voltage attenuation under the condition is less than 30mV).

[0087] 6. Long-term durability test:

[0088] The fuel cell performance of Fe-NC catalyst as cathode was tested on a fuel cell test system. The active area of ​​2.0 cm was prepared by conventional GDE membrane method. 2The membrane electrode is specifically as follows: before preparing the membrane electrode, the Nafion212 proton exchange membrane (purchased by DuPont) is treated with 5wt% H2O2 and 0.5M H2SO4 solutions in an 80°C water bath for 1 hour, and then washed with deionized water; the diffusion layer is ultrasonically treated with acetone, ultrapure water and ethanol for 5 minutes, and then dried in low-temperature vacuum. Secondly, the catalyst powder, isopropanol and Nafion 117 (~5wt%) solution are mixed by low-temperature ultrasound for 3 hours to form a slurry. Finally, the slurry is sprayed on one side of the pretreated diffusion layer to form a cathode catalyst layer. A commercially available Pt / C (JM 40wt%) catalyst is used as the anode. The anode catalyst slurry is prepared by a similar method and sprayed on one side of another diffusion layer to form an anode catalyst layer. The mass ratio of the catalyst and dry Nafion in the anode and cathode catalyst layers is 1:1. The Pt loading on the anode is 0.2mg cm -2 , the Fe-NC catalyst loading on the cathode was maintained at 3 mg cm -2 For comparative example 1, the Fe-NC-0 catalyst loading is 3 mg cm -2 .

[0089] Figure 6 The atomically dispersed catalysts Fe-NC and Fe-NC-0 prepared in Example 1 and Comparative Example 1 are assembled into cathodes of proton exchange membrane fuel cells, and the long-term durability data are measured in a H2-O2 environment at a constant voltage of 0.6V.

[0090] Figure 6 The long-term durability test results of the proton exchange membrane fuel cell in the present invention show that the prepared catalyst Fe-NC catalyst has excellent durability and can operate stably for more than 230 hours with a current density decay rate of no more than 20%, while the prepared catalyst Fe-NC-0 catalyst has a current density decay rate of more than 50% after operating for nearly 20 hours; at the same time, the durability of the Fe-NC catalyst is also significantly better than the vast majority of transition metal-based catalysts reported so far, proving that the preparation method provided by the present invention can obtain a highly durable non-precious metal Fe-NC catalyst.

[0091] As can be seen from the above embodiments, the present invention provides a controllable preparation method and application of a high-durability fuel cell catalyst Fe-NC. The present invention introduces pyrrole N-rich ligand polyvinyl pyrrolidone into the Fe2O3@ZIF-8 precursor, and then carbonizes the pink precursor under an inert atmosphere to obtain a Fe-NC intermediate sample. The additionally introduced pyrrole N-ligand polyvinyl pyrrolidone will assist the spatial confinement effect of MOF to anchor more active sites through the chemical coordination effect during the calcination process, which not only greatly increases the density of active sites, but also increases the proportion of highly active coordination configuration sites (S1), and obtains high stability while improving activity. Finally, the present invention grinds and mixes the Fe-NC intermediate and NH4Cl, and then heats the mixture under an inert atmosphere, and obtains an efficient catalyst atomic-level dispersed Fe-NC after natural cooling. NH4Cl decomposes into NH3 and HCl gas at high temperature, which will produce great internal stress and etch the carbon material, thereby producing a large number of micropores and carbon defects. HCl may further dissociate at high temperature to form H2 and Cl2. Cl2 reacts with the residual Fe aggregates in the catalyst to promote the formation of atomically dispersed Fe sites, and finally obtains an atomically dispersed Fe-NC catalyst. This method uses the confinement strategy of carbon composite materials to effectively anchor more metal single atoms, greatly increasing the specific surface area of ​​the catalyst and exposing more active sites. At the same time, the secondary pyrolysis treatment also improves the degree of graphitization and enhances the stability of the coordination structure. At a certain current density, the overpotential of the catalyst is significantly reduced, the reaction kinetics are accelerated, and it can be successfully applied to PEMFCs, effectively improving the activity and durability of transition metal catalysts in fuel cells. At the same time, the cost of the catalyst is greatly reduced due to the wide source and low price of synthetic raw materials.

[0092] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a high-durability Fe-NC catalyst that can be used in a fuel cell, characterized in that: The steps include: (1) dissolving iron oxide nanoparticles, zinc nitrate hexahydrate, polyvinyl pyrrolidone and 2-methylimidazole in a methanol solution in sequence, heating, cooling, solid-liquid separation, collecting the solid and drying it to obtain a Fe2O3@ZIF-8 precursor; (2) calcining the Fe2O3@ZIF-8 precursor to obtain a Fe-NC intermediate; (3) The Fe-NC intermediate is mixed with ammonium chloride to obtain a mixture, and the mixture is heat-treated to obtain the high-durability Fe-NC catalyst that can be used in fuel cells.

2. The method for preparing a highly durable Fe-NC catalyst that can be used in a fuel cell according to claim 1, characterized in that: In step (1), the mass ratio of the iron oxide nanoparticles to the zinc nitrate hexahydrate is (0.01-0.1):(3.5-6), and the mass ratio of the polyvinyl pyrrolidone to the iron oxide nanoparticles is (0.02-0.1):(0.01-0.1); The dosage ratio of zinc nitrate hexahydrate, 2-methylimidazole and methanol is (3.5-6) ​​g: (3-7) g: (100-300) mL.

3. The method for preparing a highly durable Fe-NC catalyst that can be used in a fuel cell according to claim 1, characterized in that: In step (1), the heating treatment temperature is 50-80°C, and the heating treatment time is 12-36h.

4. The method for preparing a highly durable Fe-NC catalyst that can be used in a fuel cell according to claim 1, characterized in that: In step (2), the calcination temperature is 800-1000° C. and the calcination time is 1-3 h; The calcination is performed under an inert atmosphere.

5. The method for preparing a highly durable Fe-NC catalyst that can be used in a fuel cell according to claim 1, characterized in that: In step (3), the heat treatment temperature is 1000-1200° C., and the heat treatment time is 0.5-3 h; The heat treatment is performed under an inert atmosphere.

6. The method for preparing a highly durable Fe-NC catalyst that can be used in a fuel cell according to claim 1, characterized in that: The calcination in step (2) and / or the heat treatment in step (3) has a heating rate of 8-15°C / min.

7. The method for preparing a highly durable Fe-NC catalyst for fuel cells according to claim 1, characterized in that: In step (3), the mass ratio of the Fe-NC intermediate to ammonium chloride is 1:(1-5).

8. A high-durability Fe-NC catalyst that can be used in a fuel cell, characterized in that: The high-durability Fe-NC catalyst that can be used in fuel cells is prepared by the method according to any one of claims 1 to 7.

9. A cathode of a proton membrane exchange fuel cell, characterized in that: The proton membrane exchange fuel cell cathode contains the high-durability Fe-NC catalyst applicable to fuel cells as claimed in claim 8.

10. A proton membrane exchange fuel cell, characterized in that: The proton membrane exchange fuel cell contains the high-durability Fe-NC catalyst applicable to fuel cells as claimed in claim 8 or the proton membrane exchange fuel cell cathode as claimed in claim 9 .

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