A heterogeneous nano-flower electrocatalyst and preparation method thereof

Abstract

The invention discloses a heterogeneous nanoflower electrocatalyst and a preparation method thereof. The electrocatalyst is a heterogeneous nanoflower constructed by cross-assembly of ultrathin palladium-based alloy nanosheets and ferroferric oxide nanosheets. The heterogeneous nanoflower contains two physical phases, palladium-based alloy and ferroferric oxide. The palladium-based alloy nanosheets and ferroferric oxide nanosheets cross each other to form a clear interface. The preparation method is simple and feasible, and the process is controllable. The heterogeneous nanoflower electrocatalyst has abundant interface sites, can significantly enhance the electrocatalytic performance, is a new alloy / oxide coupling material system, and is of great significance for improving the electrocatalytic activity and durability.

Description

Technical Field

[0001] The invention belongs to the technical field of nanomaterial electrocatalysis, and specifically relates to a heterogeneous nanoflower electrocatalyst and a preparation method thereof. Background Art

[0002] Using electricity generated by clean energy to convert water, carbon dioxide, nitrogen, etc. into fuels such as hydrogen, methanol, ammonia, etc. for storage, and then releasing and applying chemical energy through electrochemical devices such as metal-air batteries, water electrolysis, and fuel cells is a key measure to solve the current climate and energy crisis. The above energy conversion technologies all involve electrocatalytic processes occurring at the interface between electrodes and electrolytes, which can accelerate the reaction rate of charge transfer, thereby achieving efficient energy conversion. Among them, the electrocatalyst is the "heart" of the electrocatalytic reaction, so the development of high-performance, low-cost electrocatalysts is crucial for the next generation of energy conversion technology.

[0003] In the periodic table, palladium is in the same group as platinum and exhibits very similar physical and chemical properties to platinum, making it one of the best candidate electrocatalysts to replace platinum. In addition, palladium is more abundant on the earth, and its historical price is only 1 / 2 to 1 / 3 of that of platinum. Therefore, reducing the cost of palladium-based electrocatalysts while maximizing catalytic activity and durability is a key scientific issue that needs to be urgently addressed in the field of electrocatalysis. Morphology regulation is an effective strategy to enhance the catalytic performance of electrocatalysts, especially to design electrocatalysts with larger electrochemical active areas. Among electrocatalysts of various morphologies, ultrathin two-dimensional structures with atomic-level thickness and three-dimensional nanoflowers constructed from ultrathin nanosheets can maximize the exposure of surface atoms, which can not only provide higher catalytic activity, but also help maximize the utilization of precious metals and reduce the cost of catalysts. The advantages of this electrocatalyst composed of ultrathin two-dimensional structures mainly come from the low-coordination vertices or slightly changed edge sites caused by the reduction in thickness, as well as the quantum size effect and / or strain effect caused by the reduction in nanosheet thickness, which helps to enhance the electrocatalytic performance.

[0004] By utilizing the synergistic effect between multiple components, interface engineering in electrocatalysts can optimize electrocatalytic activity, durability, selectivity, etc. by balancing the adsorption and / or desorption of intermediates and regulating the transport of electrons and protons. Constructing the interface between alloys and metal compounds can provide an unobstructed interface for electron transport, which has a great significance for enhancing the performance of electrocatalytic reactions such as oxygen reduction and alcohol oxidation. Combining ultrathin nanosheets of palladium-based alloys with interface structures to construct ultrathin metal / oxide heterogeneous nanoflowers may provide new opportunities for further enhancing electrocatalytic performance, but the controllable preparation of this structure has not yet been achieved. Summary of the invention

[0005] To this end, the present invention intends to provide a heterogeneous nanoflower electrocatalyst and a preparation method thereof. The electrocatalyst is a heterogeneous nanoflower constructed by cross-assembly of ultrathin palladium-based alloy nanosheets and ferroferric oxide nanosheets. The heterogeneous nanoflower contains two physical phases, palladium-based alloy and ferroferric oxide. The palladium-based alloy nanosheets and ferroferric oxide nanosheets cross each other to form a clear interface. The preparation method is simple and feasible, and the process is controllable.

[0006] In one aspect, the present invention provides a heterogeneous nanoflower electrocatalyst, which is a heterogeneous nanoflower constructed by cross-assembly of palladium-based alloy nanosheets and ferroferric oxide nanosheets, the thickness of the palladium-based alloy nanosheets and the ferroferric oxide nanosheets are both 1.3 to 1.7 nm, and the diameter of the heterogeneous nanoflower is 90 to 150 nm; wherein the palladium-based alloy is a PdM alloy, and the metal M includes but is not limited to copper, iridium, manganese, chromium, platinum or ruthenium.

[0007] As a preferred embodiment of the present invention, the molar percentage of each metal element in the above-mentioned heterogeneous nanoflowers is: 30% to 50% palladium, 10% to 35% metal M, and 10% to 35% iron.

[0008] On the other hand, the present invention further provides a preparation method for preparing the above-mentioned heterogeneous nanoflower electrocatalyst, which comprises the following steps: S1, continuously ventilating to form an inert atmosphere to protect the reaction environment, then taking a certain amount of palladium precursor salt and putting it into a certain amount of oleylamine, and dissolving it by magnetic stirring at room temperature to form a uniform solution; S2, maintaining the reaction environment in step S1, heating the uniform solution formed in step S1 to 95-115°C, and then adding a certain amount of iron precursor salt, and continuing to keep warm for 30-60 minutes; S3, taking another certain amount of A certain amount of metal M precursor salt is put into a certain amount of oleylamine, and ultrasonic stirring is used to dissolve and form a uniform solution; S4, maintaining the reaction environment in step S1, adding a certain amount of the uniform solution formed in step S3 to the uniform solution in step S2, heating to 150-210°C and continuing the reaction for 10-60 minutes, then stopping heating and cooling to room temperature; S5, stopping ventilation, adding a certain amount of ethanol to the solution after the reaction in step S4, centrifuging and washing with a cyclohexane / ethanol mixture, the heterogeneous nanoflower electrocatalyst can be obtained.

[0009] The above-mentioned preparation method provided by the present invention is essentially based on wet chemistry-sequential reduction / oxidation method, which promotes the sequential reduction and oxidation of metals by gradient temperature increase. Specifically, palladium precursor salt is dissolved in oleylamine, and iron precursor salt is added after heating to preferentially reduce palladium, and then metal M precursor salt is added, and alloyed with palladium after heating to form palladium-based alloy nanosheets, while iron element is oxidized to form ferroferric oxide nanosheets, and after washing with cyclohexane / ethanol, heterogeneous nanoflowers self-assembled from palladium-based alloy and ferroferric oxide nanosheets can be obtained.

[0010] It can be understood by those skilled in the art that, based on the chemical reaction mechanism between the above-mentioned components, the "certain amount" referred to in the above-mentioned steps is just the amount required for the reaction. A larger amount means a larger amount of the final product heterogeneous nanoflower electrocatalyst, and a smaller amount means a smaller amount of the final product heterogeneous nanoflower electrocatalyst. Those skilled in the art can flexibly control this according to their needs.

[0011] In addition, those skilled in the art can understand that, based on the chemical reaction mechanism between the above-mentioned components, the "certain amount" referred to in the above-mentioned steps, for different amounts of different components, can affect the molar percentage of each metal element in the final product heterogeneous nanoflower electrocatalyst.

[0012] As a preferred embodiment of the present invention, in the above step S1, the palladium precursor salt is palladium acetylacetonate; in the above step S2, the iron precursor salt is hexacarbonyl iron; in the above step S3, the metal M precursor salt includes but is not limited to acetylacetonate, chloride, formate or acetate.

[0013] Furthermore, in the above step S1, the certain amount of the palladium precursor salt is 0.04 to 0.10 mmol of palladium acetylacetonate, and the certain amount of the oleylamine is 5 to 15 mL of oleylamine; in the above step S2, the certain amount of the iron precursor salt is 10 to 40 μL of hexacarbonyl iron; in the above step S3, the concentration of the formed uniform solution is 0.02 to 0.05 mol / L; in the above step S4, the certain amount of the uniform solution formed in step S3 is 1 to 3 mL of a uniform solution; in the above step S5, the certain amount of the ethanol is 10 to 30 mL of ethanol.

[0014] Preferably, in the above step S2, the heating rate is 2-10°C / min, and / or, in the above step S4, the heating rate is 2-5°C / min.

[0015] Preferably, in the above step S1, the inert atmosphere is nitrogen and / or argon.

[0016] Preferably, in the above step S2, the adding of a certain amount of iron precursor salt is specifically: injecting a certain amount of iron precursor salt with a microinjector.

[0017] Preferably, in the above step S5, the rotation speed of the centrifugal separation is 3000-8000 rpm / min, and the time is 3-6 minutes; the volume ratio of cyclohexane / ethanol is 1 / 1-9 / 1, and the number of washing times is 1-4 times.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] (1) The heterogeneous nanoflower electrocatalyst prepared by the present invention is a palladium-based alloy / iron tetroxide heterogeneous nanoflower electrocatalyst formed by self-assembly of two-dimensional ultra-thin nanosheets with a stable structure. The heterogeneous nanoflower is constructed by cross-self-assembly of palladium-based alloy nanosheets and iron tetroxide nanosheets. The thickness of the nanosheet is nanometer and / or sub-nanometer size, and the alloy and oxide nanosheets are combined with each other to form a clear interface;

[0020] (2) The heterogeneous nanoflower electrocatalyst prepared by the present invention can regulate the electronic structure of the metal due to the strong interface coupling interaction between the alloy and the metal oxide, which can not only improve the electrocatalytic activity but also improve the durability;

[0021] (3) The heterogeneous nanoflower electrocatalyst prepared by the present invention has abundant interface sites, which can significantly enhance the electrocatalytic performance. It is a new alloy / oxide coupling material system, which is of great significance for improving the electrocatalytic activity and durability;

[0022] (4) The preparation method of the present invention has a simple process and can control the size, thickness, composition, etc. of the product by controlling the concentration of the metal precursor salt, reaction temperature and time. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings. In the accompanying drawings, unless otherwise specified, the same reference numerals throughout the multiple drawings represent the same or similar components or elements. These drawings are not necessarily drawn to scale. It should be understood that these drawings only depict some embodiments disclosed in the present invention, are used to provide a further understanding of the present invention, constitute a part of this application, and should not be regarded as limiting the scope of the present invention. Among them:

[0024] Figure 1 This is the X-ray diffraction spectrum of the palladium copper / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared by the present invention;

[0025] Figure 2 This is a low-magnification transmission electron microscope image of the palladium manganese / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared by the present invention;

[0026] Figure 3 This is a high-magnification transmission electron microscope image of the palladium manganese / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared by the present invention;

[0027] Figure 4 This is a transmission electron microscope image of the palladium platinum / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared by the present invention;

[0028] Figure 5This is the element surface distribution diagram of the palladium-iridium / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared by the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and cannot be understood as limiting the present invention.

[0030] First, the present invention provides a heterogeneous nanoflower electrocatalyst, which is a heterogeneous nanoflower constructed by cross-assembly of palladium-based alloy nanosheets and ferroferric oxide nanosheets, the thickness of the palladium-based alloy nanosheets and the ferroferric oxide nanosheets are both 1.3-1.7 nm, and the diameter of the heterogeneous nanoflower is 90-150 nm; wherein the palladium-based alloy is a PdM alloy, and the metal M includes but is not limited to copper, iridium, manganese, chromium, platinum or ruthenium.

[0031] As a preferred embodiment of the present invention, the molar percentage of each metal element in the above-mentioned heterogeneous nanoflowers is: 30% to 50% palladium, 10% to 35% metal M, and 10% to 35% iron.

[0032] On the other hand, the present invention further provides a preparation method for preparing the above-mentioned heterogeneous nanoflower electrocatalyst, which comprises the following steps: S1, continuously ventilating to form an inert atmosphere to protect the reaction environment, then taking a certain amount of palladium precursor salt and putting it into a certain amount of oleylamine, and magnetically stirring and dissolving it at room temperature to form a uniform solution; S2, maintaining the reaction environment in step S1, heating the uniform solution formed in step S1 to 95-115°C, and then adding a certain amount of iron precursor salt, and continuing to keep warm for 30-60 minutes; S3, taking another certain amount of The metal M precursor salt is placed in a certain amount of oleylamine, and ultrasonically stirred to dissolve to form a uniform solution; S4, maintaining the reaction environment in step S1, adding a certain amount of the uniform solution formed in step S3 to the uniform solution in step S2, heating to 150-210°C and continuing the reaction for 10-60 minutes, then stopping heating and cooling to room temperature; S5, stopping ventilation, adding a certain amount of ethanol to the solution after the reaction in step S4, centrifuging and washing with a cyclohexane / ethanol mixture, the heterogeneous nanoflower electrocatalyst can be obtained.

[0033] Furthermore, in the above step S1, the palladium precursor salt is palladium acetylacetonate; in the above step S2, the iron precursor salt is hexacarbonyl iron; in the above step S3, the metal M precursor salt includes but is not limited to acetylacetonate, chloride, formate or acetate.

[0034] Further, in the above step S1, the certain amount of palladium precursor salt is 0.04-0.10 mmol of acetylacetonate palladium, and the certain amount of oleylamine is 5-15 mL of oleylamine; in the above step S2, the certain amount of iron precursor salt is 10-40 μL of hexacarbonyl iron; in the above step S3, the concentration of the generated uniform solution is 0.02-0.05 mol / L; in the above step S4, the certain amount of the uniform solution generated in step S3 is 1-3 mL of a uniform solution; in the above step S5, the certain amount of ethanol is 10-30 mL of ethanol

[0035] In this embodiment, the experimental equipment used in the above preparation method includes a four-necked flask, a splash-proof ball, a heating jacket, a programmable temperature controller, a micro-injector, etc.

[0036] Embodiment 1

[0037] (1) Nitrogen was continuously introduced to form an inert atmosphere to protect the reaction environment, 0.06 mmol of palladium acetylacetonate and 10 mL of oleylamine were weighed and placed in a four-necked flask, and magnetically stirred at room temperature until dissolved;

[0038] (2) heating the uniform solution obtained in step (1) to 105° C. at a heating rate of 5° C. / min, injecting 20 μL of iron hexacarbonyl using a microinjector, and keeping the temperature for 30 minutes;

[0039] (3) dissolving copper chloride in oleylamine by ultrasonication to obtain a uniform solution with a concentration of 0.02 mol / L;

[0040] (4) injecting 3 mL of the homogeneous solution obtained in step (3) into the reaction system of step (2), heating the mixture to 160° C. at a heating rate of 5° C. / min, and keeping the mixture warm for 30 minutes. Then, remove the heating jacket and cool the reaction system to room temperature.

[0041] (5) Stop ventilation, add 20 mL of ethanol to the product obtained in step (4), centrifuge at a speed of 5000 rpm / min for 4 min, wash the collected product four times with a cyclohexane / ethanol mixture with a volume ratio of 3 / 1, and obtain a palladium copper / ferroferric oxide heterogeneous nanoflower electrocatalyst.

[0042] in, Figure 1 The composition of the palladium-copper / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared in this embodiment is disclosed, indicating that the heterogeneous nanoflower contains two physical phase structures of face-centered cubic palladium-copper alloy and spinel ferroferric oxide, thereby forming a heterogeneous nanoflower structure.

[0043] Embodiment 2

[0044] (1) Nitrogen was continuously introduced to form an inert atmosphere to protect the reaction environment, 0.06 mmol of palladium acetylacetonate and 10 mL of oleylamine were weighed and placed in a four-necked flask, and magnetically stirred at room temperature until dissolved;

[0045] (2) heating the uniform solution obtained in step (1) to 105° C. at a heating rate of 5° C. / min, injecting 20 μL of iron hexacarbonyl using a microinjector, and keeping the temperature for 30 minutes;

[0046] (3) dissolving manganese acetylacetonate in oleylamine by ultrasonication to obtain a uniform solution with a concentration of 0.03 mol / L;

[0047] (4) injecting 2 mL of the homogeneous solution obtained in step (3) into the reaction system of step (2), heating the mixture to 160° C. at a heating rate of 5° C. / min, and keeping the temperature for 30 minutes. Then, remove the heating jacket and cool the reaction system to room temperature.

[0048] (5) Stop ventilation, add 10 mL of ethanol to the product obtained in step (4), centrifuge at a speed of 4000 rpm / min for 5 min, wash the collected product three times with a cyclohexane / ethanol mixture with a volume ratio of 2 / 1, and obtain a palladium manganese / ferroferric oxide heterogeneous nanoflower electrocatalyst.

[0049] in, Figure 2-3 The morphological characteristics of the palladium manganese / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared in this example are revealed. As can be seen from the figure, the prepared product presents a uniform nanoflower morphology with an average diameter of 120 nm and an average thickness of the nanosheets of 1.4 nm.

[0050] Embodiment 3

[0051] (1) Nitrogen was continuously introduced to form an inert atmosphere to protect the reaction environment, 0.04 mmol of palladium acetylacetonate and 8 mL of oleylamine were weighed and placed in a four-necked flask, and magnetically stirred at room temperature until dissolved;

[0052] (2) heating the uniform solution obtained in step (1) to 110° C. at a heating rate of 10° C. / min, injecting 15 μL of iron hexacarbonyl using a microinjector, and keeping the temperature for 30 minutes;

[0053] (3) dissolving platinum acetylacetonate in oleylamine by ultrasonication to obtain a uniform solution with a concentration of 0.03 mol / L;

[0054] (4) injecting 2 mL of the homogeneous solution obtained in step (3) into the reaction system of step (2), heating the mixture to 160° C. at a heating rate of 5° C. / min, and keeping the temperature for 30 minutes. Then, remove the heating jacket and cool the reaction system to room temperature.

[0055] (5) Stop ventilation, add 10 mL of ethanol to the product obtained in step (4), centrifuge at a speed of 6000 rpm / min for 5 min, wash the collected product four times with a cyclohexane / ethanol mixture with a volume ratio of 2 / 1, and obtain a palladium platinum / ferroferric oxide heterogeneous nanoflower electrocatalyst.

[0056] in, Figure 4 The morphological characteristics of the palladium platinum / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared in this example are revealed, and the prepared product also exhibits a uniform nanoflower morphology.

[0057] Embodiment 4

[0058] (1) Continuously introduce argon gas to form an inert atmosphere to protect the reaction environment, weigh 0.08 mmol of palladium acetylacetonate and 15 mL of oleylamine into a four-necked flask, and stir magnetically at room temperature until dissolved;

[0059] (2) heating the uniform solution obtained in step (1) to 100° C. at a heating rate of 5° C. / min, injecting 35 μL of iron hexacarbonyl using a microinjector, and keeping the temperature for 30 minutes;

[0060] (3) dissolving iridium acetylacetonate in oleylamine by ultrasonication to obtain a uniform solution with a concentration of 0.03 mol / L;

[0061] (4) inject 3 mL of the homogeneous solution obtained in step (3) into the reaction system of step (2), heat the mixture to 210° C. at a heating rate of 3° C. / min, keep the mixture warm for 30 minutes, remove the heating jacket, and cool the reaction system to room temperature;

[0062] (5) Stop ventilation, add 30 mL of ethanol to the product obtained in step (4), centrifuge at 3000 rpm / min for 6 min, wash the collected product three times with a cyclohexane / ethanol mixture with a volume ratio of 3 / 1, and obtain a palladium-iridium / ferroferric oxide heterogeneous nanoflower electrocatalyst.

[0063] in, Figure 5 The element distribution of the palladium-iridium alloy / ferroferric oxide heterogeneous nanoflower electrocatalyst prepared in Example 4 is shown. It can be seen that the two metal elements of palladium and iridium and the two elements of iron and oxygen are cross-distributed in the heterogeneous nanoflowers and form a clear interface.

[0064] Embodiment 5

[0065] (1) Nitrogen was continuously introduced to form an inert atmosphere to protect the reaction environment, 0.05 mmol of palladium acetylacetonate and 10 mL of oleylamine were weighed and placed in a four-necked flask, and magnetically stirred at room temperature until dissolved;

[0066] (2) heating the uniform solution obtained in step (1) to 100° C. at a heating rate of 3° C. / min, injecting 30 μL of iron hexacarbonyl using a microinjector, and keeping the temperature for 45 minutes;

[0067] (3) dissolving chromium formate in oleylamine by ultrasonication to obtain a uniform solution with a concentration of 0.05 mmol / mL;

[0068] (4) injecting 1 mL of the homogeneous solution obtained in step (3) into the reaction system of step (2), heating the mixture to 200°C at a heating rate of 3°C / min, and keeping the temperature for 20 minutes. Then, remove the heating jacket and cool the reaction system to room temperature.

[0069] (5) Stop ventilation, add 10 mL of ethanol to the product obtained in step (4), centrifuge at a speed of 5000 rpm / min for 5 min, wash the collected product three times with a cyclohexane / ethanol mixture with a volume ratio of 2 / 1, and obtain a palladium chromium / ferroferric oxide heterogeneous nanoflower electrocatalyst.

[0070] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and intent of the present invention.

Claims

1. A heterogeneous nanoflower electrocatalyst, characterized in that: The heterogeneous nanoflower electrocatalyst is a heterogeneous nanoflower constructed by cross-assembly of palladium-based alloy nanosheets and ferroferric oxide nanosheets, the thickness of the palladium-based alloy nanosheets and the ferroferric oxide nanosheets are both 1.3-1.7 nm, and the diameter of the heterogeneous nanoflower is 90-150 nm; wherein the palladium-based alloy is a PdM alloy, and the metal M is copper, iridium, manganese, chromium, platinum or ruthenium.

2. The heterogeneous nanoflower electrocatalyst according to claim 1, characterized in that: The molar percentages of the metal elements in the heterogeneous nanoflowers are: 30% to 50% of palladium, 10% to 35% of metal M, and 10% to 35% of iron.

3. A method for preparing the heterogeneous nanoflower electrocatalyst according to claim 1 or 2, characterized in that: The preparation method comprises the following steps: S1. Continuously ventilate to form an inert atmosphere to protect the reaction environment, then take a certain amount of palladium precursor salt and put it into a certain amount of oleylamine, and dissolve it under magnetic stirring at room temperature to form a uniform solution; S2, maintaining the reaction environment in step S1, heating the uniform solution formed in step S1 to 95-115° C., then adding a certain amount of iron precursor salt, and continuing to keep warm for 30-60 minutes; S3, taking another certain amount of metal M precursor salt and putting it into a certain amount of oleylamine, and dissolving it by ultrasonic stirring to form a uniform solution; S4, maintaining the reaction environment in step S1, adding a certain amount of the uniform solution formed in step S3 to the uniform solution in step S2, heating to 150-210° C. and continuing the reaction for 10-60 minutes, then stopping heating and cooling to room temperature; S5. Stop ventilation, add a certain amount of ethanol to the solution after the reaction in step S4, centrifuge and wash with a cyclohexane / ethanol mixture to obtain the heterogeneous nanoflower electrocatalyst.

4. The preparation method according to claim 3, characterized in that: In the above step S1, the palladium precursor salt is palladium acetylacetonate; in the above step S2, the iron precursor salt is hexacarbonyl iron; in the above step S3, the metal M precursor salt is acetylacetonate, chloride, formate or acetate.

5. The preparation method according to claim 4, characterized in that: In the above step S1, the certain amount of the palladium precursor salt is 0.04-0.10 mmol of acetylacetonate palladium, and the certain amount of the oleylamine is 5-15 mL of oleylamine; in the above step S2, the certain amount of the iron precursor salt is 10-40 μL of hexacarbonyl iron; in the above step S3, the concentration of the generated uniform solution is 0.02-0.05 mol / L; in the above step S4, the certain amount of the uniform solution formed in step S3 is 1-3 mL of a uniform solution; in the above step S5, the certain amount of the ethanol is 10-30 mL of ethanol.

6. The preparation method according to any one of claims 3 to 5, characterized in that: In the above step S2, the heating rate is 2-10°C / min.

7. The preparation method according to any one of claims 3 to 5, characterized in that: In the above step S4, the heating rate is 2-5°C / min.

8. The preparation method according to any one of claims 3 to 5, characterized in that: In the above step S1, the inert atmosphere is nitrogen and / or argon.

9. The preparation method according to any one of claims 3 to 5, characterized in that: In the above step S5, the rotation speed of the centrifugal separation is 3000-8000 rpm, and the time is 3-6 minutes; the volume ratio of cyclohexane / ethanol is 1 / 1-9 / 1, and the number of washing times is 1-4 times.

10. The preparation method according to any one of claims 3 to 5, characterized in that: In the above step S2, the adding of a certain amount of iron precursor salt is specifically: injecting a certain amount of iron precursor salt with a microinjector.

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