CN115763845B - A method for preparing a chromium-based inorganic coupled transition metal nitrogen-doped carbon catalyst - Google Patents

Abstract

A preparation method of a chromium-based inorganic matter coupled transition metal nitrogen doped carbon catalyst, belonging to the field of electrocatalysis. The method takes a prepared metal M-bipyridine solution as a starting point, then sodium chloride, chromium salt and organic ammonium salt are sequentially added into the solution, and the solution is stirred to dissolve solids and evaporated to dryness to obtain mixed powder; and then annealing, template removing, acid washing, suction filtering and drying are carried out to obtain the catalyst. Has the following advantages: introducing a chromium salt inorganic carrier into an M-N-C atomic-level dispersion catalyst to replace a conventional carbon carrier by a molten salt template method, wherein the method is applicable to various metal-nitrogen co-doped carbon catalysts (such as Fe, cu, ni and the like); the catalyst is ultrathin two-dimensional sheet-shaped formed by interconnecting nanocrystals, so that the mass transfer capacity can be effectively improved; the introduction of the chromium-based inorganic salt can improve Faraday efficiency, catalytic activity and durability under high-current and long-time working conditions, and is obviously superior to commercial platinum carbon catalysts and transition metal nitrogen doped carbon.

Description

A method for preparing a chromium-based inorganic coupled transition metal nitrogen-doped carbon catalyst

Technical field

The invention belongs to the field of new energy materials. Specifically, two-dimensional flaky chromium-based inorganic materials are used as carriers to couple transition metal nitrogen-doped carbon-type atomic-level dispersion materials (M-N-C, M is Fe, Co, Cu and Ni). One or more) are prepared into composite catalysts, which can be used in fuel cells, cathode oxygen reduction reaction (ORR) of metal-air batteries, and oxygen evolution reaction (OER) of electrolyzed water.

Background technique

Electrocatalyst is one of the core components of many new energy conversion devices such as fuel cells, metal-air batteries, and water electrolysis projects. At present, electrocatalysts are still dominated by precious metal catalytic materials, such as Pt, Pd, Ru and Ir, which are the most advanced oxygen electrode catalysts. However, its high price and limited reserves have greatly restricted the development of the new energy industry. Therefore, it is necessary to develop non-precious metal catalysts with high catalytic performance to replace noble metal catalysts.

In catalysts that do not contain platinum group metals, transition metal nitrogen-doped carbon-type atomically dispersed materials (MNC), that is, MN _x sites with atomic dispersion and nitrogen coordination embedded in the carbon plane, are used in oxygen reduction and oxygen evolution ( OER) and carbon dioxide reduction (CO ₂ RR) and other fields have shown good performance and good application prospects. The extremely high surface-to-volume ratio of MNCs increases the catalytic efficiency of each atom, improves the uniformity of active sites, and enables tuning of the guest environment, all of which are not only attractive for industrial applications but also for in-depth understanding of atomic-level catalytic mechanisms. Also attractive. Despite tremendous progress in their activity and site structure, there is an urgent need to improve durability due to insufficient understanding of their degradation mechanisms during operation. Further research and improvements on MNC catalysts are still needed to facilitate practical applications, especially at high currents, as well as the long-term stability of the catalysts in harsh battery tests.

Contents of the invention

Aiming at the deficiencies in catalytic activity and stability of the existing transition metal nitrogen-doped carbon catalyst M-N-C (M is one or more of Fe, Co, Cu and Ni), a two-dimensional flaky chromium-based inorganic material is provided. Coupling M-N-C as a supporting carrier as a catalyst, the catalytic durability of the resulting catalyst is significantly higher than that of traditional M-N-C catalysts and the latest commercial 20% Pt/C catalyst.

In order to achieve the above objects, the technical solutions adopted by the present invention are as follows:

A method for preparing a chromium-based inorganic coupled transition metal nitrogen-doped carbon catalyst. The method steps are:

Step 1: Preparation of metal M-bipyridine solution:

At room temperature, weigh the metal M salt and bipyridine and dissolve them into ultrapure water respectively. After they are completely dissolved, mix the two solutions to obtain an orange-yellow solution. Stir at room temperature to allow the M ions and bipyridine to fully coordinate and grow;

Step 2: Preparation of M-Cr-organic ammonium salt mixed precursor:

Add sodium chloride, chromium salt and organic ammonium salt in sequence to the orange-yellow solution obtained in step 1 and stir to completely dissolve the solid; place the above solution on a heating platform and remove the water in the solution by rotary evaporation to dryness to obtain The solid mixture is placed in a vacuum oven, vacuum heated and dried to remove the remaining moisture, and an M-Cr-organic ammonium salt mixed precursor is obtained;

Step 3: Preparation of chromium-based inorganic coupled M-N-C composite catalyst:

Grind the precursor obtained in step 2 fully in a mortar, then put it into a tube furnace for calcination under a protective atmosphere; the calcined product is dispersed into an aqueous solution, and washed with water multiple times to remove the sodium chloride template; in order to remove the possible Existing metal M clusters, disperse the template-removed product into a dilute H ₂ SO ₄ aqueous solution, and stir it continuously for 5 hours at a temperature of 80°C. After washing with water - suction filtration - vacuum drying, chromium-based inorganic coupling is obtained. Transition metal nitrogen-doped carbon catalysts.

Further, in step one, the molar ratio of the metal M salt and bipyridine is 1:1-4.

Further, in step one, the metal M salt is ferric nitrate, ferric chloride, ferric acetate, ferrous nitrate, ferrous chloride, ferrous acetate, cobalt nitrate, cobalt chloride, cobalt acetate, copper nitrate, chlorine One or more of copper chloride, copper acetate, nickel nitrate, nickel chloride, and nickel acetate, and the bipyridine is 2,2'-bipyridine or 4,4'-bipyridine.

Further, in step 2, the input amount of sodium chloride is determined according to the volume of the M-bipyridine solution. Each milliliter of the solution corresponds to 0.5g sodium chloride. The molar ratio of the chromium salt to the metal M salt is 1 to 8. : 1, the mass ratio of the organic ammonium salt and chromium salt is 3:1.

Further, in step two, the chromium salt is one of chromium chloride, chromium sulfate, and chromium nitrate, and the organic ammonium salt is one of urea, melamine, dicyandiamide, and ammonium citrate.

Further, in step three, the protective atmosphere is Ar or N ₂ , the calcination temperature is 650-800°C, the time is 1-3h, and the heating rate is 2-10°C/min.

A chromium-based inorganic coupled transition metal nitrogen-doped carbon catalyst prepared by the above method.

A chromium-based inorganic coupled transition metal nitrogen-doped carbon catalyst prepared by the above method is used in fuel cells, metal-air battery cathode oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of electrolyzed water.

The beneficial effects of the present invention compared with the prior art are:

(1) The present invention can introduce chromium-based inorganic carriers into M-N-C atomically dispersed catalysts through a simple and effective molten salt template method to replace conventional carbon carriers, thereby synthesizing a variety of ultra-stable catalysts with different compositions and structures. It can be used by changing the organic The choice of ammonium nitrogen salt controls the form of chromium-based inorganic substances, and the nanoscale size of the catalyst can also be controlled by changing the input amount of sodium chloride template. The method is green, gentle, simple, and has great versatility.

(2) The chromium-based inorganic coupling M-N-C catalyst of the present invention consists of an ultra-thin two-dimensional sheet structure composed of many interconnected nanocrystals. This two-dimensional structure helps to fully contact the active sites and reactants during the catalytic process. The open structure on both sides of the nanosheet can also effectively improve the efficiency of the mass transfer process and simplify the transportation pathways of reaction intermediates and products. . Since there are a large number of pores between the crystals, electrolytes and ions are allowed to be transported through the sheets, which can also greatly improve the mass transfer capacity of the catalyst.

(3) The chromium-based inorganic carrier introduced in the present invention can improve the degree of graphitization of M-N-C and enhance the conductivity of the catalyst. The introduction of chromium-based inorganics can also induce electron spin polarization in the carbon layer where M-N-C is located, improve catalytic selectivity and Faradaic efficiency, and reduce the generation of by-products. Chromium-based inorganic substances are more thermodynamic and corrosion-resistant than carbon supports, ensuring the long-term stability of the catalyst under high current and harsh battery tests.

In summary, the catalyst of the present invention is an ultra-thin two-dimensional sheet composed of many interconnected nanocrystals. The introduction of chromium-based inorganic carriers into M-N-C atomically dispersed catalysts aims to replace conventional carbon carriers, improve the degree of graphitization of M-N-C, improve catalytic selectivity and Faradaic efficiency, and improve the durability of the catalyst under high current and long-term working conditions. sex.

Description of the drawings

Figure 1 is a flow chart of the preparation of Examples 1-3 of the present invention;

Figure 2 is an SEM image of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention;

Figure 3 is an SEM image of FeCN@Cr ₂ O ₃ prepared in Example 2 of the present invention;

Figure 4 is an SEM image of NiCN@CrN prepared in Example 3 of the present invention;

Figure 5 is a TEM image of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention;

Figure 6 is an HRTEM image of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention;

Figure 7 is the XPS pattern of N1s in CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention and CoCN prepared in Comparative Example 3;

Figure 8 is the XPS pattern of Co 2p in CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention and CoCN prepared in Comparative Example 3;

Figure 9 is the Raman spectrum of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention and CoCN prepared in Comparative Example 3;

Figure 10 is the ORR polarization diagram of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention, Pt/C prepared in Comparative Example 1, CoCN prepared in Comparative Example 3, and Cr ₂ O ₃ prepared in Comparative Example 4 ( fuel cell field);

Figure 11 is a catalytic efficiency test chart of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention and CoCN prepared in Comparative Example 3, including electron transfer number and by-product H ₂ O ₂ yield (fuel cell field);

Figure 12 is a continuous 10-hour it stability test chart of the half-cell of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention, Pt/C in Comparative Example 1, and CoCN prepared in Comparative Example 3 (fuel cell and zinc- air battery field);

Figure 13 is a 50-hour it stability test chart of the CoCN@Cr ₂ O ₃ half-cell prepared in Example 1 of the present invention (in the field of fuel cells and zinc-air batteries);

Figure 14 is a 10,000-cycle aging test chart of the CoCN@Cr ₂ O ₃ half-cell prepared in Example 1 of the present invention (in the field of fuel cells and zinc-air batteries);

Figure 15 is a long-term charge and discharge cycle performance diagram of CoCN@Cr ₂ O ₃ prepared in Example 1 of the present invention and the mixture of Comparative Examples 1 and 2 of Pt/C+Ir/C applied in ZABs (zinc-air battery field);

Figure 16 is the ORR polarization diagram (fuel cell field) of FeCN@Cr ₂ O ₃ prepared in Example 2 of the present invention and Pt/C in Comparative Example 1;

Figure 17 is the OER polarization diagram of NiCN@CrN prepared in Example 3 of the present invention and Ir/C of Comparative Example 2 (in the field of water electrolysis);

Figure 18 is a stability test chart of NiCN@CrN electrocatalytic OER prepared in Example 3 of the present invention (in the field of water electrolysis).

Detailed ways

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings and examples, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered. within the protection scope of the present invention.

In the present invention, a metal M-bipyridine solution is prepared at room temperature and stirred at room temperature to fully coordinate the growth of M ions and bipyridine; then sodium chloride solid, chromium salt solid and organic ammonium salt are added to the above solution and stirred Completely dissolve the solid; place the above solution on a heating platform, remove the water in the solution by rotary evaporation to obtain a solid mixture, and then dry the collected solid mixture in a vacuum oven to obtain a mixed powder; finally, The mixed powder is annealed under a protective atmosphere, and the sodium chloride template is removed by washing with water, and then undergoes acid washing-suction filtration-drying procedures to obtain a chromium-based inorganic coupled transition metal nitrogen-doped carbon ultra-stable catalyst.

The catalyst designed in the present invention is an ultra-thin two-dimensional lamellar chromium-based inorganic substance composed of many interconnected nanocrystals as a carrier. This two-dimensional structure helps the active sites to fully contact the reactants during the catalytic process. Nano The open structure on both sides of the sheet can also effectively improve the efficiency of the mass transfer process and simplify the transportation pathways of reaction intermediates and products. There are a large number of pores between the crystals, allowing electrolyte and ions to be transported through the sheets, which can also greatly improve the mass transfer capacity of the catalyst. In addition, the introduced chromium-based inorganic carrier can improve the degree of graphitization of MNC and enhance the conductivity of the catalyst. It can also induce electron spin polarization in the carbon layer of MNC, improve catalytic selectivity and Faradaic efficiency, and reduce by-products. generation, effectively inhibiting the damage of active oxygen species to the active sites of the catalyst. Chromium-based inorganic carriers have higher thermodynamic stability and corrosion resistance than traditional carbon materials, so that chromium-based inorganic coupled MNCs not only have higher catalytic activities such as ORR and OER, but also have the ability to operate at high current, long-term Ultra-high durability under time loops. In addition, the present invention can introduce inorganic carriers into different types of MNC (M = Co, Fe, Cu, Ni, etc.) atomically dispersed catalysts through the green and easy sodium chloride molten salt template method, which is widely used in ORR, Many catalytic fields and energy conversion devices such as OER and carbon dioxide reduction (CO ₂ RR) further enhance the practical value of the present invention.

Example 1:

This embodiment provides a preparation method for CoCN@Cr ₂ O ₃ catalyst, including the following steps:

1. Preparation of Co-bipyridine solution: Weigh 24 mg CoCl ₂ ·6H ₂ O and 48 mg 2,2'-bipyridine respectively and dissolve them in 20 mL ultrapure water. After complete dissolution, mix the two solutions to obtain an orange-yellow solution. Stir for 24 hours at room temperature to fully coordinate the growth of Co ions and bipyridine;

2. Preparation of Co-Cr-organic ammonium salt mixed precursor: Add 20g NaCl, 24mg CrCl ₂ and 72mg organic ammonium salt to the solution prepared in step 1 and stir to completely dissolve the solid. Then, the above solution is placed on a heating platform at 65°C, and the moisture in the solution is removed by rotary evaporation to dryness to obtain a solid mixture. The collected solid mixture is then heated and dried in a vacuum oven to remove the remaining moisture;

3. Preparation of Cr ₂ O ₃ coupled Co-NC composite catalyst: Grind the precursor obtained in step 2 fully in a mortar, then put it into a tube furnace for calcination in an inert atmosphere. The protective atmosphere used is Ar or _N2 inert gas, the carbonization temperature is 700°C, the carbonization time is 2h, and the heating rate is 2°C/min. Subsequently, the calcined product was dispersed into an aqueous solution, and the sodium chloride template was removed through multiple water washings. In order to remove possible Co clusters, the template-removed product was dispersed into 0.5 mol L ^-1 dilute H ₂ SO _{4 aqueous} solution, and allowed to stir continuously for 5 h at a temperature of 80°C, followed by water washing-suction filtration-vacuum The final flake Cr ₂ O ₃ coupled Co-NC composite ultra-stable catalyst (CoCN@Cr ₂ O ₃ ) is obtained in a dry manner;

The process of this embodiment is shown in Figure 1. The SEM, TEM and HRTEM morphology of CoCN@Cr ₂ O ₃ prepared under this process are shown in Figure 2, Figure 5 and Figure 6 respectively. The ICP test Co content is 1.02wt.%. Figure 7 and Figure 8 are CoCN@ XPS patterns of Co 2p and N1s in Cr ₂ O _3. The results show that Co in the catalyst exists as single atom Co-N _x . Figure 9 shows the Raman pattern before and after the introduction of inorganic carriers. The introduction of Cr ₂ O ₃ improves the performance of CoCN. Degree of graphitization. The ORR polarization test and catalytic efficiency test of the CoCN@Cr ₂ O ₃ catalyst are shown in Figure 10 and Figure 11 respectively. The electron transfer technique and the by-product H ₂ O ₂ yield indicate that the catalytic efficiency is significantly improved after the introduction of Cr ₂ O ₃ . The stability test of the catalyst in RDE is shown in Figure 12, Figure 13, and Figure 14. The chromium salt inorganic substance greatly improved the stability of the catalyst so that the current retention rate was 100% in the 10-hour it test. Significantly surpassing precious metal and MNC catalysts, the long-term cycle stability performance in ZABs batteries is shown in Figure 15, which can provide continuous and stable charging and discharging cycles for the battery for more than 1500 hours.

Example 2:

This embodiment provides a preparation method of FeCN@Cr ₂ O ₃ catalyst, including the following steps:

1. Preparation of Fe-bipyridine solution: Weigh 28 mg FeCl ₃ ·6H ₂ O and 48 mg 2,2'-bipyridine and dissolve them in 20 mL of ultrapure water. After complete dissolution, mix the two solutions to obtain an orange-yellow solution. Stir for 24 hours at room temperature to fully coordinate the growth of iron ions and bipyridine;

2. Preparation of Fe-Cr-organic ammonium salt mixed precursor: Add 20g NaCl, 24mg CrCl ₂ and 72mg urea to the solution prepared in step 1 and stir to completely dissolve the solid. Then, the above solution is placed on a heating platform at 65°C, and the moisture in the solution is removed by rotary evaporation to dryness to obtain a solid mixture. The collected solid mixture is then heated and dried in a vacuum oven to remove the remaining moisture;

3. Preparation of Cr ₂ O ₃ coupled Fe-NC composite catalyst: Grind the precursor obtained in step 2 fully in a mortar, then put it into a tube furnace for calcination under an inert atmosphere. The protective atmosphere used was Ar inert gas, the carbonization temperature was 700°C, the carbonization time was 2h, and the heating rate was 2°C/min. Subsequently, the calcined product was dispersed into an aqueous solution, and the sodium chloride template was removed through multiple water washings. In order to remove possible Fe clusters, the template-removed product was dispersed into 0.5 mol L ^-1 dilute H ₂ SO _{4 aqueous} solution, and allowed to stir continuously for 5 h at a temperature of 80°C, followed by water washing-suction filtration-vacuum The final flake Cr ₂ O ₃ coupled Fe-NC composite ultra-stable catalyst (FeCN@Cr ₂ O ₃ ) is obtained in a dry manner;

The process of this embodiment is shown in Figure 1. The SEM morphology of FeCN@Cr ₂ O ₃ prepared under this process is shown in Figure 3. The ICP test Fe content is 1.52wt.%. The ORR polarization test of FeCN@Cr ₂ O ₃ catalyst is shown in Figure 16. The catalytic performance It is significantly higher than the precious metal catalyst of Comparative Example 1, which means that it has broad practical prospects in the field of fuel cells.

Example 3:

This embodiment provides a preparation method for NiCN@CrN catalyst, including the following steps:

1. Preparation of Ni-bipyridine solution: Weigh 25 mg NiCl ₂ ·6H ₂ O and 48 mg 2,2'-bipyridine respectively and dissolve them in 20 mL of ultrapure water. After complete dissolution, mix the two solutions to obtain an orange-yellow solution. Stir for 24 hours at room temperature to fully coordinate the growth of Co ions and bipyridine;

2. Preparation of Co-Cr-organic ammonium salt mixed precursor: Add 20g NaCl, 24mg CrCl ₂ and 72mg melamine to the solution prepared in step 1 and stir to completely dissolve the solid. Then, the above solution is placed on a heating platform at 65°C, and the moisture in the solution is removed by rotary evaporation to dryness to obtain a solid mixture. The collected solid mixture is then heated and dried in a vacuum oven to remove the remaining moisture;

3. Preparation of CrN-coupled Ni-NC composite catalyst: Grind the precursor obtained in step 2 fully in a mortar, and then put it into a tube furnace for calcination in an inert atmosphere. The protective atmosphere used was Ar inert gas, the carbonization temperature was 650°C, the carbonization time was 2h, and the heating rate was 5°C/min. Subsequently, the calcined product was dispersed into an aqueous solution, and the sodium chloride template was removed through multiple water washings. In order to remove possible Ni clusters, the template-removed product was dispersed into 0.5 mol L ^-1 dilute H ₂ SO _{4 aqueous} solution, and allowed to stir continuously for 5 h at a temperature of 80°C, followed by water washing-suction filtration-vacuum The final flake CrN coupled Ni-NC composite ultra-stable catalyst (NiCN@CrN) is obtained in a dry manner;

The process of this embodiment is shown in Figure 1. The SEM morphology of NiCN@CrN prepared under this process is shown in Figure 4. The Ni content measured by ICP is 1.34wt.%. The OER polarization test of the NiCN@CrN catalyst is shown in Figure 17. The catalytic performance far exceeds that of the precious metal comparative example 2. In addition, the stability test of the catalyst in RDE is shown in Figure 18. The catalytic performance is still excellent after 10,000 cycles of aging, which means that it has broad practical prospects in the field of fuel cells.

Comparative Example 1:

Purchase the latest commercial 20% Pt/C (Comm. 20% Pt/C) from a company and use it directly for testing without any processing.

The ORR polarization test of Comm.20%Pt/C in this embodiment is shown in Figure 7. The RDE stability test is shown in Figure 10, and the catalytic stability is far less than that of CoCN@Cr ₂ O ₃ .

Comparative Example 2:

Purchase the latest commercial Ir/C (Comm.20% Ir/C) from a company and use it directly for testing without any processing.

The OER polarization test of Comm. 20% Ir/C in this embodiment is shown in Figure 17.

Comparative Example 3:

CoCN without introducing inorganic carriers is prepared by the sodium chloride molten salt template method. The steps are as follows:

1. Preparation of Co-bipyridine solution: Weigh 24 mg CoCl ₂ ·6H ₂ O and 48 mg 2,2'-bipyridine respectively and dissolve them in 20 mL ultrapure water. After complete dissolution, mix the two solutions to obtain an orange-yellow solution. Stir for 24 hours at room temperature to fully coordinate the growth of Co ions and bipyridine;

2. Preparation of Co-organic ammonium salt mixed precursor: Add 20g NaCl and 75mg urea to the solution prepared in step 1 and stir to completely dissolve the solid. Then, the above solution is placed on a heating platform at 65°C, and the moisture in the solution is removed by rotary evaporation to dryness to obtain a solid mixture. The collected solid mixture is then heated and dried in a vacuum oven to remove the remaining moisture;

3. Preparation of CoCN catalyst without Cr ₂ O ₃ coupling: Grind the precursor obtained in step 2 fully in a mortar, and then put it into a tube furnace for calcination under an inert atmosphere. The protective atmosphere used is Ar or _N2 inert gas, the carbonization temperature is 700°C, the carbonization time is 2h, and the heating rate is 2°C/min. Subsequently, the calcined product was dispersed into an aqueous solution, and the sodium chloride template was removed through multiple water washings. In order to remove possible Co clusters, the template-removed product was dispersed into 0.5 mol L ^-1 dilute H ₂ SO _{4 aqueous} solution, and allowed to stir continuously for 5 h at a temperature of 80°C, followed by water washing-suction filtration-vacuum The final sheet-like Cr ₂ O _3- free coupled CoCN is obtained in a dry manner;

The element valence state analysis (XPS test) and graphitization degree (Raman test) of CoCN prepared in this example are shown in Figure 7, Figure 8 and Figure 9 respectively; the ORR polarization test, electron transfer number and H ₂ of CoCN O ₂ yield test and RDE stability test are shown in Figure 10, Figure 11, and Figure 12 respectively. The ORR catalytic performance and electron transfer of CoCN are significantly lower than those of CoCN@Cr ₂ O ₃ in Example 1, and the yield of by-product H ₂ O ₂ is higher, proving that its catalytic efficiency is low and its catalytic stability is far inferior to that of CoCN. @Cr ₂ O ₃ .

Comparative Example 4:

Cr ₂ O ₃ carrier prepared by sodium chloride molten salt template method, the steps are as follows:

1. Preparation of Cr-organic ammonium salt mixed precursor: Add 20g NaCl, 24mg CrCl ₂ and 72mg urea respectively to 40ml ultrapure water and stir to completely dissolve the solid. Then, the above solution is placed on a heating platform at 65°C, and the moisture in the solution is removed by rotary evaporation to dryness to obtain a solid mixture. The collected solid mixture is then heated and dried in a vacuum oven to remove the remaining moisture;

2. Preparation of Cr ₂ O ₃ inorganic matter: Grind the precursor obtained in step 2 fully in a mortar, and then put it into a tube furnace for calcination in an inert atmosphere. The protective atmosphere used is Ar or _N2 inert gas, the carbonization temperature is 700°C, the carbonization time is 2h, and the heating rate is 2°C/min. Subsequently, the calcined product is dispersed into an aqueous solution, washed with water multiple times to remove the sodium chloride template, and the final flake Cr ₂ O ₃ is obtained through suction filtration and vacuum drying.

The ORR polarization tests of Cr ₂ O ₃ prepared in this example are shown in Figure 7. The ORR catalytic performance of Cr ₂ O ₃ is not as good as that of CoCN@Cr ₂ O ₃ .

Claims (8)

1. A preparation method of a chromium-based inorganic matter coupled transition metal nitrogen doped carbon catalyst is characterized by comprising the following steps of: the method comprises the following steps:

step one: preparing a metal M-bipyridine solution:

weighing metal M salt and bipyridine, respectively dissolving in ultrapure water at room temperature, mixing the two solutions after complete dissolution to obtain orange-yellow solution, and stirring at room temperature to enable M ions and bipyridine to fully coordinate and grow; the metal M salt is one or more of ferric nitrate, ferric chloride, ferric acetate, ferrous nitrate, ferrous chloride, ferrous acetate, cobalt nitrate, cobalt chloride, cobalt acetate, copper nitrate, copper chloride, copper acetate, nickel nitrate, nickel chloride and nickel acetate;

step two: preparation of M-Cr-organic ammonium salt mixed precursor:

sequentially adding sodium chloride, chromium salt and organic ammonium salt into the orange-yellow solution obtained in the step one, and stirring to completely dissolve the solid; placing the solution on a heating platform, removing water in the solution by a rotary evaporation method to obtain a solid mixture, and placing the solid mixture in a vacuum oven for vacuum heating and drying to remove residual water to obtain an M-Cr-organic ammonium salt mixed precursor; the input amount of the sodium chloride is determined according to the volume of the M-bipyridine solution, and each milliliter of the solution corresponds to 0.5g sodium chloride; the organic ammonium salt is one of urea, melamine, dicyandiamide and ammonium citrate;

step three: preparation of chromium-based inorganic matter coupled M-N-C composite catalyst:

putting the precursor obtained in the second step into a mortar for full grinding, and then putting the mortar into a tube furnace for calcination treatment under a protective atmosphere; dispersing the calcined product into an aqueous solution, and removing a sodium chloride template through multiple times of water washing; dispersing the template-removed product into dilute H ₂ SO ₄ And (3) in the aqueous solution, continuously stirring at 80 ℃ for 5h, and performing water washing, suction filtration and vacuum drying to obtain the chromium-based inorganic matter coupling transition metal nitrogen doped carbon catalyst.

2. The method for preparing the chromium-based inorganic matter-coupled transition metal nitrogen-doped carbon catalyst according to claim 1, which is characterized in that: in the first step, the molar ratio of the metal M salt to the bipyridine is 1: 1-4.

3. The method for preparing the chromium-based inorganic matter-coupled transition metal nitrogen-doped carbon catalyst according to claim 1, which is characterized in that: in the first step, the bipyridine is 2,2 '-bipyridine or 4,4' -bipyridine.

4. The method for preparing the chromium-based inorganic matter-coupled transition metal nitrogen-doped carbon catalyst according to claim 1, which is characterized in that: in the second step, the molar ratio of the chromium salt to the metal M salt is 1-8: 1, the mass ratio of the organic ammonium salt to the chromium salt is 3:1.

5. the method for preparing the chromium-based inorganic matter-coupled transition metal nitrogen-doped carbon catalyst according to claim 1, which is characterized in that: in the second step, the chromium salt is one of chromium chloride, chromium sulfate and chromium nitrate.

6. The method for preparing the chromium-based inorganic matter-coupled transition metal nitrogen-doped carbon catalyst according to claim 3, wherein the method comprises the following steps: in the third step, the protective atmosphere is Ar or N ₂ The calcining temperature is 650-800 ℃, the time is 1-3h, and the heating rate is 2-10 ℃/min.

7. A chromium-based inorganic coupled transition metal nitrogen-doped carbon catalyst prepared by the method of any one of claims 1-6.

8. Use of a chromium-based inorganic coupled transition metal nitrogen-doped carbon catalyst prepared by the method of any one of claims 1-6 in fuel cells, metal-air cell cathode oxygen reduction reactions, and electrolytic water oxygen evolution reactions.