阅读说明：本技术 一种用于电化学合成氨的四氧化三铁催化剂及其制备方法与应用 (Ferroferric oxide catalyst for electrochemically synthesizing ammonia and preparation method and application thereof ) 是由 李钟号 英昊 于 2021-01-22 设计创作，主要内容包括：本发明提供了一种用于电化学合成氨的四氧化三铁催化剂及其制备方法与应用。本发明四氧化三铁催化剂的制备方法,先以氯化胆碱和草酸为原料制备低共熔溶剂；然后将四氧化三铁溶于低共熔溶剂中,微波加热进行反应,之后经离心、洗涤、干燥,得到前驱体；在氮气保护下,将前驱体进行煅烧得到四氧化三铁催化剂。本发明制备方法简便快捷,原料价格低廉,来源广泛；所得四氧化三铁催化剂是由纳米颗粒构筑成的二维多孔纳米片,具有较好的电催化氮还原性能。(The invention provides a ferroferric oxide catalyst for electrochemically synthesizing ammonia, and a preparation method and application thereof. The preparation method of the ferroferric oxide catalyst comprises the steps of firstly preparing a eutectic solvent by taking choline chloride and oxalic acid as raw materials; then dissolving ferroferric oxide in a eutectic solvent, carrying out microwave heating for reaction, and then centrifuging, washing and drying to obtain a precursor; and calcining the precursor under the protection of nitrogen to obtain the ferroferric oxide catalyst. The preparation method is simple, convenient and quick, and the raw materials are low in price and wide in source; the obtained ferroferric oxide catalyst is a two-dimensional porous nano-sheet constructed by nano-particles, and has good electrocatalytic nitrogen reduction performance.)

1. A ferroferric oxide catalyst for electrochemically synthesizing ammonia is characterized in that the microscopic morphology of the ferroferric oxide catalyst is a nano sheet constructed by nano particles, and the particle size of the nano particles is 9-10 nm; the ferroferric oxide catalyst is obtained by dissolving ferroferric oxide in a eutectic solvent, and then heating and calcining by microwave.

2. The ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 1, wherein the eutectic solvent is prepared by the reaction of choline chloride and oxalic acid.

3. The process for preparing ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 1 or 2, comprising the following steps:

(1) mixing choline chloride and oxalic acid at 80 ℃ for reaction to obtain a eutectic solvent;

(2) dissolving ferroferric oxide in a eutectic solvent, carrying out microwave heating for reaction, and then centrifuging, washing and drying to obtain a precursor;

(3) and (3) calcining the precursor obtained in the step (2) to obtain the ferroferric oxide catalyst.

4. The preparation method of the ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 3, wherein the molar ratio of the choline chloride to the oxalic acid in the step (1) is 1: 0.5-2, preferably 1: 1-1.5; the preparation method of the eutectic solvent comprises the following steps: mixing choline chloride and oxalic acid at 80 ℃ under stirring, and reacting until colorless transparent liquid is formed, namely the eutectic solvent.

5. The preparation method of the ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 3, wherein in the step (2), the ferroferric oxide is dissolved in a eutectic solvent at the temperature of 20-80 ℃; preferably, the dissolution temperature is 30 to 70 ℃, and more preferably 50 ℃.

6. The preparation method of the ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 3, wherein the ratio of the mass of the ferroferric oxide to the volume of the eutectic solvent in the step (2) is 20-50 mg:1mL, preferably 25-40 mg:1mL, and more preferably 30mg:1 mL.

7. The preparation method of the ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 3, wherein the microwave heating power in the step (2) is 50-150W, preferably 100W; the microwave heating reaction time is 5-20 s, preferably 10 s.

8. The preparation method of the ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 3, wherein in the step (2), the washing is carried out 3-4 times by using absolute ethyl alcohol; the drying is carried out for 12-24 h under vacuum at room temperature.

9. The preparation method of the ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 3, wherein the calcination temperature in the step (3) is 300-400 ℃, preferably 300 ℃; the calcination time is 2-3 h, preferably 2 h.

10. Use of the ferroferric oxide catalyst for the electrochemical synthesis of ammonia according to claim 1 or 2 as a nitrogen reduction catalyst for electrocatalytic nitrogen reduction.

Technical Field

The invention relates to a ferroferric oxide catalyst for electrochemically synthesizing ammonia, a preparation method and application thereof, and belongs to the technical field of energy materials.

Background

Ammonia gas is widely used in the synthesis of pharmaceuticals, synthetic fibers, fertilizer production and other industrial chemicals as an essential chemical for the survival and development of human society. Ammonia gas is also an ideal hydrogen storage material because of its higher hydrogen content per unit mass and lower liquefaction pressure. At present, the large-scale industrial production of ammonia mainly depends on the traditional Harper-Bosch method, namely, N is reacted under the conditions of high temperature (400-500 ℃) and high pressure (150-250 atm)₂And H₂Conversion to ammonia, H required for the reaction₂Mainly comes from reforming natural gas, the process causes a great deal of energy consumption and environmental pollution, and the synthesis process has complex process flow and equipment requirementThe piece is high. Therefore, it is necessary to explore a green, efficient and sustainable synthetic ammonia technology to replace the traditional haber-bosch process.

The electrochemical synthesis of ammonia is N rich in air₂And H widely occurring in nature₂O is used as a raw material, ammonia is mildly synthesized under the action of a catalyst, and the required electric energy belongs to renewable energy sources, can be derived from wind energy, solar energy, tidal energy and the like, so that the large consumption of fossil fuels and the emission of greenhouse gases are avoided, and the method is an environment-friendly ammonia synthesis technology. The catalyst is the core of the electrochemical synthesis of ammonia, so that the synthesis of cheap and high-performance electrocatalyst is very important. The currently reported ferroferric oxide catalyst has higher nitrogen reduction performance, for example, in a document' Nanoscale,2018,10,14386-₃·6H₂O is used as a raw material, and Fe is obtained by adopting a hydrothermal method and high-temperature calcination₃O₄Nanorods, applied to electrocatalytic nitrogen reduction, having an average ammonia yield of 5.6X 10 at-0.4V^-11mol s^-1cm^-2The Faraday efficiency is 2.6%, which indicates that the ferroferric oxide nanorod has certain nitrogen reduction performance, but the ferroferric oxide nanorod still has certain defects in the aspects of mass transfer, exposed active sites, morphology regulation and the like, and the catalytic performance of the ferroferric oxide nanorod needs to be further improved.

Therefore, the preparation of the ferroferric oxide catalyst with higher electrocatalytic nitrogen reduction performance has important significance. The invention is therefore proposed.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a ferroferric oxide catalyst for electrochemically synthesizing ammonia and a preparation method and application thereof. The preparation method provided by the invention is simple, convenient and quick, and is easy to operate; the obtained ferroferric oxide catalyst is of a nanosheet structure constructed by nanoparticles, and has good electrocatalytic nitrogen reduction performance.

The invention also provides application of the ferroferric oxide catalyst for electrochemically synthesizing ammonia in electrocatalytic nitrogen reduction.

Description of terms:

room temperature: having a meaning well known in the art, meaning 25 ℃. + -. 5 ℃.

The technical scheme of the invention is as follows:

a ferroferric oxide catalyst for electrochemically synthesizing ammonia is disclosed, wherein the microscopic morphology of the ferroferric oxide catalyst is a nano sheet constructed by nano particles, and the particle size of the nano particles is 9-10 nm; the ferroferric oxide catalyst is obtained by dissolving ferroferric oxide in a eutectic solvent, and then heating and calcining by microwave.

According to the invention, the eutectic solvent is prepared by the reaction of choline chloride and oxalic acid.

The preparation method of the ferroferric oxide catalyst for electrochemically synthesizing ammonia comprises the following steps:

(1) mixing choline chloride and oxalic acid at 80 ℃ for reaction to obtain a eutectic solvent;

(2) dissolving ferroferric oxide in a eutectic solvent, carrying out microwave heating for reaction, and then centrifuging, washing and drying to obtain a precursor;

(3) and (3) calcining the precursor obtained in the step (2) to obtain the ferroferric oxide catalyst.

According to the invention, the mol ratio of the choline chloride to the oxalic acid in the step (1) is 1: 0.5-2, and the mol ratio is 1: 1-1.5.

According to the present invention, the eutectic solvent in step (1) is preferably prepared as follows: mixing choline chloride and oxalic acid at 80 ℃ under stirring, and reacting until colorless transparent liquid is formed, namely the eutectic solvent.

Preferably, in the step (2), ferroferric oxide is dissolved in the eutectic solvent at the temperature of 20-80 ℃; preferably, the dissolving temperature is 30-70 ℃, and more preferably 50 ℃; the ferroferric oxide is a common commercial product.

According to the invention, the ratio of the mass of the ferroferric oxide to the volume of the eutectic solvent in the step (2) is preferably 20-50 mg:1mL, more preferably 25-40 mg:1mL, and even more preferably 30mg:1 mL.

According to the invention, the power of the microwave heating in the step (2) is preferably 50-150W, and more preferably 100W; the reaction time of microwave heating is 5 to 20s, and more preferably 10 s.

Preferably, the washing in the step (2) is washing with absolute ethyl alcohol for 3-4 times; the drying is carried out for 12-24 h under vacuum at room temperature.

According to the invention, the calcination temperature in the step (3) is preferably 300-400 ℃, and more preferably 300 ℃; the calcination time is 2-3 h, and preferably 2 h.

The application of the ferroferric oxide catalyst for electrochemically synthesizing ammonia is used as a nitrogen reduction catalyst for electrocatalytic nitrogen reduction.

The invention has the following technical characteristics and beneficial effects:

(1) according to the invention, commercial ferroferric oxide is dissolved by a eutectic solvent formed by choline chloride and oxalic acid, then a ferroferric oxide precursor is obtained by microwave heating, and a regenerated ferroferric oxide nanosheet is obtained by high-temperature calcination, wherein the nanosheet is constructed by nanoparticles, so that the nanosheet has a porous structure. The unique sheet structure has a larger specific surface area, so that the sheet structure has a larger active area, more catalytic active sites can be exposed, and the porous structure can promote electron transmission and is beneficial to the diffusion of electrolyte, so that the sheet structure has higher electrocatalytic nitrogen reduction performance. According to the invention, the solubility of a specific eutectic solvent to a metal oxide is utilized to dissolve commercial ferroferric oxide so as to precipitate and regenerate the commercial ferroferric oxide, thus obtaining the ferroferric oxide nano sheet with specific morphology and excellent performance. The preparation method of the eutectic solvent is simple, the eutectic solvent serving as a reaction solvent can be subjected to microwave heating and then high-temperature calcination to synthesize the two-dimensional porous ferroferric oxide nanosheet, and the raw materials are cheap and easily available, so that the method is favorable for large-scale production.

(2) The two-dimensional porous ferroferric oxide nano sheet obtained by the invention has special microscopic morphology, larger active area, more catalytic sites and the like due to the special preparation method, so that the two-dimensional porous ferroferric oxide nano sheet is more catalytic than the ferroferric oxide nano sheet prepared by other methodsThe catalyst has better electrocatalytic nitrogen reduction performance, higher ammonia yield, good catalytic selectivity and stability. At 0.1mol/L Na₂SO₄In neutral electrolyte, the average ammonia yield at-0.1V (relative to a reversible hydrogen electrode) can reach 12.09 mu g h^-1mg_cat. ^-1And no byproduct hydrazine is generated, the Faraday efficiency is 34.38%, and the current density of electrocatalytic nitrogen reduction for 24h is kept stable.

Drawings

Fig. 1 is an X-ray diffraction (XRD) pattern of the ferroferric oxide catalyst prepared in example 1.

Fig. 2 is a Transmission Electron Microscope (TEM) image of the ferroferric oxide catalyst prepared in example 1, wherein fig. 2a is a TEM image at a low magnification, and fig. 2b is a TEM image at a high magnification.

FIG. 3 shows the concentration of Na in 0.1mol/L of the ferroferric oxide catalyst prepared in example 1₂SO₄Electrocatalytic nitrogen reduction performance in neutral electrolytes, where figure 3a is the average ammonia yield at different voltages and figure 3b is the faradaic efficiency at different voltages.

Fig. 4 is a Transmission Electron Microscope (TEM) image of the ferroferric oxide nanoparticles prepared in comparative example 1.

Detailed Description

The present invention is further illustrated by, but not limited to, the following examples.

Meanwhile, the experimental methods described in the following examples are all conventional methods unless otherwise specified; the reagents and materials are commercially available, unless otherwise specified.

Example 1

A preparation method of a ferroferric oxide catalyst for electrochemically synthesizing ammonia comprises the following steps:

(1) 2.79g of choline chloride and 2.52g of oxalic acid were added to a round bottom flask, and placed in an oil bath at 80 ℃ with continuous stirring, and the reaction was carried out until a colorless transparent liquid was formed to obtain a eutectic solvent.

(2) 30mg of Fe in a 50 ℃ water bath₃O₄Dissolving in the above 1mL eutectic solvent, completely dissolving, and transferring to micro-eutectic solventAnd heating for 10s in a wave oven under the condition of 100W power, cooling to room temperature, centrifuging, washing the obtained solid for 3 times by using absolute ethyl alcohol, then placing the product in a vacuum drying oven, and drying for 16h at room temperature to obtain the ferroferric oxide precursor.

(3) And (3) placing the ferroferric oxide precursor obtained in the step (2) into a tubular furnace, and calcining for 2 hours at 300 ℃ under the protection of nitrogen to obtain black solid powder two-dimensional porous ferroferric oxide nanosheets, namely the ferroferric oxide catalyst.

The composition of the catalyst for the electrochemical synthesis of ammonia obtained in this example was characterized by X-ray diffraction (XRD), and as shown in fig. 1, it was shown that the black solid powder obtained in this example was ferroferric oxide.

The material morphology of the ferroferric oxide catalyst for electrochemically synthesizing ammonia prepared in this embodiment is characterized by a Transmission Electron Microscope (TEM), and a TEM characterization picture is shown in fig. 2, and as can be seen from fig. 2, the microstructure of the ferroferric oxide prepared in this embodiment is: the nano-sheets constructed by nano-particles with the particle size of 9-10nm show that the two-dimensional porous ferroferric oxide nano-sheets are successfully prepared by the method.

The ferroferric oxide catalyst for electrochemically synthesizing ammonia, which is prepared in the embodiment, is applied to electrocatalytic nitrogen reduction, and the specific application method is as follows:

the electrocatalytic nitrogen reduction experiment is carried out in an H-shaped electrolytic cell of two chambers, carbon cloth modified by a ferroferric oxide catalyst is used as a working electrode, a silver/silver chloride electrode is used as a reference electrode, a platinum sheet is used as a counter electrode, and the voltages shown in the invention are all voltages converted into a relatively reversible hydrogen electrode.

The preparation method of the working electrode comprises the following steps:

2mg of the ferroferric oxide catalyst prepared in the embodiment and 40 mu L of 5 wt% Nafion solution are dispersed in 960 mu L of absolute ethyl alcohol, ultrasonic treatment is carried out for 1h to form a uniform ink shape, 100 mu L of dispersion liquid is dripped into a solution with the area of 1cm²On carbon cloth (1X 1 cm)²) The loading capacity of the ferroferric oxide catalyst is 0.2mg cm^-2And then placing the modified carbon cloth in a vacuum drying oven for drying for later use.

At 0.1mol/L Na₂SO₄The electrocatalytic nitrogen reduction performance of the electrolyte is tested in a neutral electrolyte. The test is carried out under different voltages, and no by-product N is generated₂H₄The generated result shows that the ferroferric oxide catalyst prepared by the invention has good selectivity; at-0.1V, the ammonia yield reached a maximum of 12.09. mu. g h^-1mg_cat. ^-1And the current density can be kept stable in the electrolysis process of 24h under-0.1V, which shows that the catalyst has good catalytic stability.

The electrocatalytic nitrogen reduction performance of the ferroferric oxide catalyst obtained in the embodiment is shown in figure 3; as shown in FIG. 3a, the average ammonia yield gradually increased with increasing voltage from 0 to-0.1V, and gradually decreased beyond-0.1V. As shown in fig. 3b, the faraday efficiency gradually decreases with increasing voltage. Comprehensively considering, -0.1V is the optimal voltage for electrochemically synthesizing ammonia by using the two-dimensional ferroferric oxide nano-sheets, and the first-pass efficiency is 34.38% under the voltage of-0.1V. The above shows that the catalyst prepared by the invention has excellent electro-catalysis ammonia production performance.

Example 2

A preparation method of a ferroferric oxide catalyst for electrochemically synthesizing ammonia comprises the following steps:

(1) 2.79g of choline chloride and 2.52g of oxalic acid were added to a round bottom flask, and placed in an oil bath at 80 ℃ with continuous stirring, and the reaction was carried out until a colorless transparent liquid was formed to obtain a eutectic solvent.

(2) 20mg of Fe was added in a 50 ℃ water bath₃O₄Dissolving in the 1mL of eutectic solvent, transferring to a microwave oven after complete dissolution, heating for 10s under the condition of 100W power, cooling to room temperature, centrifuging, washing the obtained solid with absolute ethyl alcohol for 3 times, then placing the product in a vacuum drying oven, and drying for 16h at room temperature to obtain the ferroferric oxide precursor.

(3) And (3) placing the ferroferric oxide precursor obtained in the step (2) into a tubular furnace, and calcining for 2 hours at 300 ℃ under the protection of nitrogen to obtain black solid powder two-dimensional porous ferroferric oxide nanosheets, namely the ferroferric oxide catalyst.

Example 3

A preparation method of a ferroferric oxide catalyst for electrochemically synthesizing ammonia comprises the following steps:

(1) 2.79g of choline chloride and 2.52g of oxalic acid were added to a round bottom flask, and placed in an oil bath at 80 ℃ with continuous stirring, and the reaction was carried out until a colorless transparent liquid was formed to obtain a eutectic solvent.

(2) 50mg of Fe was added in a 50 ℃ water bath₃O₄Dissolving in the 2mL eutectic solvent, transferring to a microwave oven after complete dissolution, heating for 10s under the condition of 100W power, cooling to room temperature, centrifuging, washing the obtained solid with absolute ethyl alcohol for 3 times, then placing the product in a vacuum drying oven, and drying for 16h at room temperature to obtain the ferroferric oxide precursor.

(3) And (3) placing the ferroferric oxide precursor obtained in the step (2) into a tubular furnace, and calcining for 2 hours at 300 ℃ under the protection of nitrogen to obtain black solid powder two-dimensional porous ferroferric oxide nanosheets, namely the ferroferric oxide catalyst.

Comparative example 1

A preparation method of ferroferric oxide nanoparticles comprises the following steps:

(1) 2.79g of choline chloride and 2.52g of oxalic acid were added to a round bottom flask, and placed in an oil bath at 80 ℃ with continuous stirring, and the reaction was carried out until a colorless transparent liquid was formed to obtain a eutectic solvent.

(2) 30mg of Fe in a 50 ℃ water bath₃O₄Dissolving in the 1mL of eutectic solvent, transferring to a microwave oven after complete dissolution, heating for 10s under the condition of 100W power, cooling to room temperature, centrifuging, washing the obtained solid with absolute ethyl alcohol for 3 times, then placing the product in a vacuum drying oven, and drying for 16h at room temperature to obtain the ferroferric oxide precursor.

(3) And (3) placing the ferroferric oxide precursor obtained in the step (2) into a tubular furnace, and calcining for 2 hours at 500 ℃ under the protection of nitrogen to obtain the ferroferric oxide nano-particles.

A Transmission Electron Microscope (TEM) of the ferroferric oxide nanoparticles prepared in the comparative example is shown in fig. 4, and it can be seen from fig. 4 that the microstructure of the ferroferric oxide nanoparticles prepared in the comparative example is sintered nanoparticles, and the sheet structure is not maintained; compared with the figure 2 of the invention, the nano-sheet morphology does not exist, and the obvious pore structure does not exist, so that the calcination temperature adopted by the invention plays an important role in regulating and controlling the morphology of the ferroferric oxide nano-sheet.

The ferroferric oxide nano-particles prepared by the comparative example are applied to electrocatalytic nitrogen reduction, and the specific application method is as described in example 1. The catalyst has an ammonia yield of 4.38 mu g h at-0.1V^-1mg_cat. ^-1The Faraday efficiency is 10.89%, and the catalyst has no good catalytic performance, which shows the importance of the calcination temperature adopted by the invention.

Comparative example 2

A preparation method of ferroferric oxide nanoparticles comprises the following steps:

(1) 2.79g of choline chloride and 2.52g of oxalic acid were added to a round bottom flask, and placed in an oil bath at 80 ℃ with continuous stirring, and the reaction was carried out until a colorless transparent liquid was formed to obtain a eutectic solvent.

(2) 30mg of Fe in a 50 ℃ water bath₃O₄Dissolving in the 1mL of eutectic solvent, transferring to a microwave oven after complete dissolution, heating for 10s under the condition of 100W power, cooling to room temperature, centrifuging, washing the obtained solid with absolute ethyl alcohol for 3 times, then placing the product in a vacuum drying oven, and drying for 16h at room temperature to obtain the ferroferric oxide precursor.

(3) And (3) placing the ferroferric oxide precursor obtained in the step (2) into a tubular furnace, and calcining for 4 hours at 300 ℃ under the protection of nitrogen to obtain the ferroferric oxide nano-particles.

The ferroferric oxide nano-particles prepared by the comparative example are applied to electrocatalytic nitrogen reduction, and the specific application method is as described in example 1. The catalyst has an ammonia yield of 3.87 mu g h at-0.1V^-1mg_cat. ^-1The Faraday efficiency is 9.5%, which is not as good as the catalytic performance of the ferroferric oxide nano-sheet prepared in the embodiment 1 of the invention, and the importance of the calcination time adopted by the invention is illustrated.

Comparative example 3

Mixing commercial tri (oxy) tetraoxideThe iron nanoparticles were used in electrocatalytic nitrogen reduction, the specific application method being as described in example 1. The catalyst has an ammonia yield of 3.68 mu g h at-0.1V^-1mg_cat. ^-1The Faraday efficiency is 8.05%, which is not as good as the catalytic performance of the ferroferric oxide nano-sheet prepared in the embodiment 1 of the invention, and the importance of preparing the ferroferric oxide catalyst by using the eutectic solvent is demonstrated.

Comparative example 4

A preparation method of ferroferric oxide nanoparticles comprises the following steps:

(1) 2.79g of choline chloride and 2.52g of oxalic acid were added to a round bottom flask, and placed in an oil bath at 80 ℃ with continuous stirring, and the reaction was carried out until a colorless transparent liquid was formed to obtain a eutectic solvent.

(2) 30mg of Fe in a 50 ℃ water bath₃O₄Dissolving in the 1mL of eutectic solvent, transferring to a microwave oven after complete dissolution, heating for 30s under the condition of 100W power, cooling to room temperature, centrifuging, washing the obtained precipitate for 3 times by using absolute ethyl alcohol, then placing the product in a vacuum drying oven, and drying for 16h at room temperature to obtain the ferroferric oxide precursor.

(3) And (3) placing the ferroferric oxide precursor obtained in the step (2) into a tubular furnace, and calcining for 2 hours at 300 ℃ under the protection of nitrogen to obtain the ferroferric oxide nano-particles.

The ferroferric oxide nano-particles prepared by the comparative example are applied to electrocatalytic nitrogen reduction, and the specific application method is as described in example 1. The catalyst has an ammonia yield of 4.15 mu g h at-0.1V^-1mg_cat. ^-1The Faraday efficiency is 8.9%, which is not as good as the catalytic performance of the ferroferric oxide nano-sheet prepared in the embodiment 1 of the invention, and the importance of the microwave heating time is illustrated.

Comparative example 5

A preparation method of ferroferric oxide nanoparticles comprises the following steps:

(1) 2.79g of choline chloride and 2.52g of oxalic acid were added to a round bottom flask, and placed in an oil bath at 80 ℃ with continuous stirring, and the reaction was carried out until a colorless transparent liquid was formed to obtain a eutectic solvent.

(2) In a 50 ℃ water bath, 15mg of Fe₃O₄Dissolving in the 1mL of eutectic solvent, transferring to a microwave oven after complete dissolution, heating for 10s under the condition of 100W power, cooling to room temperature, centrifuging, washing the obtained precipitate for 3 times by using absolute ethyl alcohol, then placing the product in a vacuum drying oven, and drying for 16h at room temperature to obtain the ferroferric oxide precursor.

(3) And (3) placing the ferroferric oxide precursor obtained in the step (2) into a tubular furnace, and calcining for 2 hours at 300 ℃ under the protection of nitrogen to obtain the ferroferric oxide nano-particles.

The ferroferric oxide nano-particles prepared by the comparative example are applied to electrocatalytic nitrogen reduction, and the specific application method is as described in example 1. The catalyst has an ammonia yield of 3.72 mu g h at-0.1V^-1mg_cat. ^-1The Faraday efficiency is 7.5%, which is not as high as the catalytic performance of the ferroferric oxide nano-sheet prepared in the embodiment 1 of the invention, and the importance of the ratio of the volume of the eutectic solvent used in the invention to the mass of the ferroferric oxide is illustrated.