Phosphorus-doped porous carbon metal-free oxygen reduction catalyst, preparation method and application thereof

Abstract

The application discloses a phosphorus-doped porous carbon metal-free oxygen reduction catalyst and a preparation method and application thereof, and belongs to the field of electrochemical catalytic materials. In the application, alizarin and tetraol diphosphite are dissolved in anhydrous ethanol to perform chemical doping, a porous brown precursor dry gel is obtained through a sol-gel method, and then the phosphorus-doped porous carbon metal-free oxygen reduction catalyst is obtained after carbonization under an inert atmosphere. The phosphorus doping can effectively regulate the electron concentration of adjacent carbon atoms, reduce the energy barrier, and thus promote the catalytic reaction. The porous structure is beneficial to exposing more active sites. The raw materials used in the application are cheap, the preparation method is simple, and the application is easy to produce on a large scale. Compared with commercial platinum-carbon, the prepared catalyst has good activity and shows more excellent methanol tolerance.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical catalytic materials, specifically relating to a phosphorus-doped porous carbon metal-free oxygen reduction catalyst, its preparation method, and its application in fuel cell cathodes. Background Technology

[0002] Fuel cells have broad application prospects and are key equipment for clean energy conversion due to their high efficiency and environmental friendliness. Fuel cells use H2 and O2 as raw materials to generate pollution-free H2O and release energy. According to the Carnot cycle, the overall hydrogen combustion conversion efficiency is no longer limited, offering advantages such as high efficiency, strong performance, and good environmental performance. However, its cathode oxygen reduction reaction (ORR) kinetics are slow and require a catalyst. These catalysts mainly use precious metals, especially platinum-based materials, which are expensive, have limited sources, and are susceptible to carbon monoxide and methanol poisoning, hindering their commercialization.

[0003] Existing Fe-NC catalysts exhibit excellent resistance to poisoning, but in alkaline environments, small-molecule alcohols (such as methanol and ethanol) significantly inhibit the ORR process of microporous Fe-NC catalysts. Studies on the size, polarity, and inhibitory ability of organic molecules have revealed that low-polarity, low-molecular-weight organic molecules have a more pronounced inhibitory effect on catalyst performance. This inhibition is attributed to the small molecules filling the microporous structure of the catalyst, leading to a decrease in mass transfer capacity. Chinese patent CN111416130A discloses a phosphorus-nitrogen co-doped porous carbon cathode oxygen reduction iron-based catalyst, its preparation, and its application. This invention uses phosphoric acid and melamine as phosphorus and nitrogen sources, respectively, to synthesize precursors in multiple steps. Doping with phosphorus atoms increases the amorphous region, which can accelerate lithium-ion transport. However, this method is complex and has poor performance. Summary of the Invention

[0004] Technical problem solved: In view of the technical problems existing in the background art, the present invention provides a phosphorus-doped porous carbon metal-free oxygen reduction catalyst, its preparation method and application, which has the advantages of simple preparation method, low raw material price and environmental friendliness. Moreover, the use of phosphorus doping makes the catalyst have better oxygen reduction performance and methanol tolerance (compared to commercial platinum carbon), solving the problems of high cost and poor poisoning resistance of platinum-based materials.

[0005] Technical solution: The preparation method of a phosphorus-doped porous carbon metal-free oxygen reduction catalyst according to the present invention includes the following steps:

[0006] Step 1: Dissolve alizarin in anhydrous ethanol and stir at 40°C until completely dissolved; then add tetraol diphosphite and continue stirring to form a homogeneous mixed solution;

[0007] Step 2: Add HCl solution dropwise to the mixed solution to adjust the pH to 3-4, heat to 60℃-90℃ and stir for 6 hours to form a dark red viscous gel;

[0008] Step 3: Let the gel age at room temperature for 4-12 hours, and then vacuum dry for 6-12 hours to obtain a porous brown precursor dry gel.

[0009] Step 4: Under an inert atmosphere, the dry gel is heated to 750-950℃ and carbonized for 1-4 hours at a heating rate of 3-10℃ / min to obtain a phosphorus-doped porous carbon metal oxygen-free reduction catalyst.

[0010] Preferably, in step 1, the mass ratio of alizarin to tetraol diphosphite is (1-5):(1-5).

[0011] Preferably, the concentration of the HCl solution in step 2 is 0.05–0.1 mol·L⁻¹. -1 .

[0012] Preferably, the stirring temperature in step 2 is 80℃ (under this reaction temperature condition, the obtained dark red viscous gel can obtain a good carbonized structural framework in the subsequent carbonization process; if the reaction temperature is too low, it is not conducive to the reaction; if the reaction temperature is too high, it is easy to cause the framework to collapse, which is not conducive to the formation of a multi-mesoporous structure).

[0013] Preferably, the aging time in step 3 is 12 hours (the gel aging time has a significant impact on the chemical structure. As the aging time increases, the crosslinking density of the precursor colloid gradually increases, the pore meshes connect with each other, and the specific surface area and pore volume are greatly improved).

[0014] Preferably, in step 4, the heating rate is 3℃ / min, the carbonization temperature is 900℃, and the holding time is 2h.

[0015] Preferably, the phosphorus-doped porous carbon metal-free oxygen reduction catalyst has a PC-specific coordination structure, and its XPS phosphorus characteristic peak is located at 130-132 eV.

[0016] This invention also discloses a phosphorus-doped porous carbon metal-free oxygen reduction catalyst, prepared by the above method, which has a three-dimensional mesoporous structure and a specific surface area ≥800 m². 2 / g, in 0.1M KOH solution, oxygen reduction initiation potential ≥0.875V, half-wave potential ≥0.770V, methanol tolerance test current retention rate ≥90%.

[0017] The present invention also discloses the application of the above-mentioned catalyst in the oxygen reduction reaction at the cathode of a fuel cell.

[0018] Compared with the prior art, the present invention achieves the following technical effects:

[0019] 1. The preparation method of this invention obtains a precursor dry gel via a sol-gel method, followed by carbonization to prepare a phosphorus-doped porous carbon metal-free oxygen reduction catalyst. Alizarin is used as the carbon source and structure directing agent; the aromatic ring of alizarin is carbonized to form a graphitized three-dimensional mesoporous carbon framework. Tetraol diphosphite is used as the phosphorus source; during the pyrolysis of tetraol diphosphite, gases (such as PH3, CO2, etc.) are released, generating mesopores / macropores, allowing methanol molecules to pass through without clogging the pores, while oxygen can still diffuse to the active sites. During the reaction, tetraol diphosphite crosslinks with carbon to form chelates, enhancing thermal stability. Compared with ordinary phosphorus sources, these phosphorus-doped sites form stable PC bonds. The release of gas creates pores, effectively doping phosphorus atoms into the carbon framework. Phosphorus, with its large atomic radius and low electronegativity, can modulate the electronic structure of carbon, promoting oxygen adsorption while reducing methanol adsorption on the catalyst surface. Phosphorus doping can regulate electrocatalyst activity, enhance electronic asymmetry through PC bonds, lower the ORR barrier, promote mass transfer through the three-dimensional mesoporous structure, expose more active sites, and improve catalytic activity. Its metal-free properties avoid methanol poisoning, combining high ORR activity with methanol tolerance, making it suitable for methanol fuel cells. It can solve the problems of complex preparation steps and the easy formation of unstable PO bonds in conventional phosphorus-doped carbon materials.

[0020] 2. The raw materials used, alizarin and tetraol diphosphite, are inexpensive and the preparation process is simple, green, and pollution-free, making them suitable for large-scale production.

[0021] 3. The catalyst prepared by the method of the present invention has a three-dimensional mesoporous structure (pore size 2-50 nm), a specific surface area ≥800 m2 / g; forms PC bonds (XPS peak 130-132 eV), and has no metal residue; in 0.1 M KOH solution, the oxygen reduction initiation potential is ≥0.875 V, the half-wave potential is ≥0.770 V, and the methanol tolerance test current retention rate is ≥90% (superior to 83.89% of commercial platinum carbon). Attached Figure Description

[0022] Figure 1 This is a flowchart of the preparation method of the phosphorus-doped porous carbon metal-free oxygen reduction catalyst of the present invention;

[0023] Figure 2 This is a scanning electron microscope image of the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of the present invention;

[0024] Figure 3 The pore size distribution diagram is shown for the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of this invention.

[0025] Figure 4The linear scan curves of the phosphorus-doped porous carbon metal oxygen-free reduction catalyst prepared in Example 1 at 1600 rpm are compared with those of commercial platinum carbon and Comparative Example 1.

[0026] Figure 5 The graph shows the percentage of current density loss in Example 1, commercial platinum carbon, and Comparative Example 1 before and after adding methanol at a constant voltage of -0.35V and 1600rpm.

[0027] Figure 6 The CV diagram is shown for the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of this invention.

[0028] Figure 7 The image shows the XPS spectrum of phosphorus in the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of this invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will be described in conjunction with the accompanying drawings. Figures 1-7 The technical solutions of the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0030] like Figure 1 As shown, this invention discloses a method for preparing a phosphorus-doped porous carbon metal-free oxygen reduction catalyst, comprising the following steps:

[0031] Step 1: Dissolve alizarin in anhydrous ethanol and stir at 40°C until completely dissolved; then add tetraol diphosphite and continue stirring to form a homogeneous mixed solution; wherein the mass ratio of alizarin to tetraol diphosphite is (1-5):(1-5);

[0032] Step 2: Add HCl solution dropwise to the mixed solution to adjust the pH to 3-4, heat to 60℃-90℃ and stir for 6 hours to form a dark red viscous gel; wherein the concentration of the HCl solution is 0.05-0.1 mol·L⁻¹. -1 ;

[0033] Step 3: Let the gel age at room temperature for 4-12 hours, and then vacuum dry for 6-12 hours to obtain a porous brown precursor dry gel.

[0034] Step 4: Under an inert atmosphere, the dry gel is heated to 750-950℃ and carbonized for 1-4 hours at a heating rate of 3-10℃ / min to obtain a phosphorus-doped porous carbon metal oxygen-free reduction catalyst.

[0035] This invention also discloses a phosphorus-doped porous carbon metal-free oxygen reduction catalyst, prepared by the above method. The prepared phosphorus-doped porous carbon metal-free oxygen reduction catalyst has a PC-specific coordination structure, and its XPS phosphorus characteristic peak is located at 130-132 eV; it has a three-dimensional mesoporous structure (pore size 2-50 nm) and a specific surface area ≥800 m². 2 / g; forms PC bonds (XPS peak 130-132 eV), with no metal residue; oxygen reduction onset potential ≥0.875 V, half-wave potential ≥0.770 V in 0.1 M KOH solution, and methanol tolerance test current retention rate ≥90% (superior to 83.89% for commercial platinum-carbon). This invention also discloses the application of the above catalyst in the oxygen reduction reaction at the cathode of a fuel cell.

[0036] The following examples further illustrate the preparation method of this invention.

[0037] The linear sweep voltammetry curves of the samples in this embodiment were measured using an Autolab PGSTAT302N telephone workstation. The test conditions were as follows: a three-electrode system with Ag / AgCl as the reference electrode, a platinum sheet as the counter electrode, a rotating disk electrode loaded with the catalyst as the working electrode, and 0.1M KOH as the electrolyte solution. The scan rate was 10 mV / s. The working electrode was prepared as follows: 6 mg of the catalyst sample was dissolved in a mixture of 10 mL ethanol and 30 μL Nafion (5% solution, purchased from DuPont). After ultrasonic dispersion, 10 μL was added dropwise to the rotating disk electrode and dried at room temperature to obtain the working electrode. During the linear sweep voltammetry curve test, nitrogen and oxygen were separately introduced into the 0.1M KOH electrolyte solution to create a nitrogen / oxygen atmosphere. Before the test, the electrode was cycled 30 times in the electrolyte solution to activate it. When testing under an oxygen atmosphere, the linear sweep voltammetry curves of the electrode were measured under different rotation speeds (400-2000 rpm).

[0038] The chronoamperometry curves of the samples in the embodiments of the present invention were measured in an Autolab PGSTAT302N telephone workstation. The test conditions were as follows: a three-electrode system with Ag / AgCl as the reference electrode, a platinum sheet as the counter electrode, and a rotating disk electrode loaded with catalyst as the working electrode; 0.1M KOH as the electrolyte solution; and a scan rate of 10mV / s. The working electrode was prepared by dissolving 6mg of catalyst sample in a mixture of 10mL ethanol and 30μL Nafion (5% solution, purchased from DuPont, USA), ultrasonically dispersing it, and then adding 10μL dropwise to the rotating disk electrode. After drying at room temperature, this became the working electrode. The chronoamperometry curves were measured under the following conditions: a constant potential of -0.35V, a rotation speed of 1600rpm, a test time of 1000s, and the change in current over time after adding 10ml of methanol at 500s.

[0039] Example 1: 2g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40°C until completely dissolved; then 2g tetraol diphosphite was added, and stirring was continued to form a homogeneous solution; then 0.1mol·L⁻¹ was added dropwise. -1 The pH was adjusted to 3-4 with HCl solution, the temperature was raised to 80℃ and stirred for 6 hours to form a dark red viscous gel. The gel was allowed to stand at room temperature for 12 hours for aging, and then transferred to a vacuum drying oven and vacuum dried at 80℃ for 12 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 900℃ for 2 hours under an inert atmosphere at a heating rate of 3℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0040] Example 2: 5g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40℃ until completely dissolved; then 1g tetraol diphosphite was added, and stirring continued to form a homogeneous solution; then 0.1mol·L⁻¹ was added dropwise. -1 The pH was adjusted to 3-4 with HCl solution, the temperature was raised to 80℃ and stirred for 6 hours to form a dark red viscous gel. The gel was allowed to stand at room temperature for 12 hours for aging, and then transferred to a vacuum drying oven and vacuum dried at 80℃ for 12 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 900℃ for 2 hours under an inert atmosphere at a heating rate of 3℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0041] Example 3: 1g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40℃ until completely dissolved; then 5g tetraol diphosphite was added, and stirring was continued to form a homogeneous solution; then 0.1mol·L⁻¹ was added dropwise. -1The pH was adjusted to 3-4 with HCl solution, the temperature was raised to 80℃ and stirred for 6 hours to form a dark red viscous gel. The gel was allowed to stand at room temperature for 12 hours for aging, and then transferred to a vacuum drying oven and vacuum dried at 80℃ for 12 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 900℃ for 2 hours under an inert atmosphere at a heating rate of 3℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0042] Example 4: 2g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40℃ until completely dissolved; then 2g tetraol diphosphite was added and stirred to form a homogeneous solution; then 0.05mol·L-1 HCl solution was added dropwise to adjust the pH to 3-4, the temperature was raised to 80℃ and stirred for 6h to form a dark red viscous gel; the gel was allowed to stand at room temperature for 12h for aging, and then transferred to a vacuum drying oven and vacuum dried at 80℃ for 12h to obtain a porous brown precursor dry gel; the dry gel was placed in a tube furnace and heated to 900℃ for 2h under an inert atmosphere at a heating rate of 3℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0043] Example 5: 2g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40°C until completely dissolved; then 2g tetraol diphosphite was added, and stirring was continued to form a homogeneous solution; then 0.1mol·L⁻¹ was added dropwise. -1 The pH was adjusted to 3-4 with HCl solution, the temperature was raised to 60℃ and stirred for 6 hours to form a dark red viscous gel. The gel was allowed to stand at room temperature for 8 hours to age, and then transferred to a vacuum drying oven and dried under vacuum at 80℃ for 12 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 750℃ under an inert atmosphere at a heating rate of 3℃ / min for 4 hours to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0044] Example 6: 2g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40°C until completely dissolved; then 2g tetraol diphosphite was added, and stirring was continued to form a homogeneous solution; then 0.1mol·L⁻¹ was added dropwise. -1 The pH was adjusted to 3-4 with HCl solution, the temperature was raised to 90℃ and stirred for 6 hours to form a dark red viscous gel. The gel was allowed to stand at room temperature for 4 hours to age, and then transferred to a vacuum drying oven and dried under vacuum at 80℃ for 6 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 900℃ for 2 hours under an inert atmosphere at a heating rate of 3℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0045] Example 7: 2g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40°C until completely dissolved; then 2g tetraol diphosphite was added, and stirring was continued to form a homogeneous solution; then 0.1mol·L⁻¹ was added dropwise. -1 The pH was adjusted to 3-4 with HCl solution, and the mixture was heated to 80℃ and stirred for 6 hours to form a dark red viscous gel. The gel was aged at room temperature for 12 hours, then transferred to a vacuum drying oven and dried under vacuum at 80℃ for 12 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 900℃ for 2 hours under an inert atmosphere at a heating rate of 10℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal-free oxygen reduction catalyst. Studies showed that the heating rate is related to the pore size. Compared with Example 1, a higher heating rate is not conducive to the formation of mesopores, resulting in increased edge defects and a decrease in catalyst product performance.

[0046] Example 8: 2g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40°C until completely dissolved; then 2g tetraol diphosphite was added, and stirring was continued to form a homogeneous solution; then 0.1mol·L⁻¹ was added dropwise. -1 The pH was adjusted to 3-4 with HCl solution, the temperature was raised to 80℃ and stirred for 6 hours to form a dark red viscous gel. The gel was allowed to stand at room temperature for 12 hours to age, and then transferred to a vacuum drying oven and dried under vacuum at 80℃ for 12 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 950℃ for 1 hour under an inert atmosphere at a heating rate of 3℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0047] Example 9: 2g alizarin was dissolved in 50ml anhydrous ethanol and stirred at 40℃ until completely dissolved; then 2g tetraol diphosphite was added and stirred to form a homogeneous solution; then 0.1mol·L-1 HCl solution was added dropwise to adjust the pH to 3-4, the temperature was raised to 80℃ and stirred for 6h to form a dark red viscous gel; the gel was allowed to stand at room temperature for 12h for aging, and then transferred to a vacuum drying oven and vacuum dried at 80℃ for 9h to obtain a porous brown precursor dry gel; the dry gel was placed in a tube furnace and heated to 900℃ for 2h under an inert atmosphere at a heating rate of 6℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen-free reduction catalyst.

[0048] Comparative Example 1: Dissolve 2g alizarin in 50ml anhydrous ethanol and stir at 40℃ until completely dissolved; then add 2g tetraol diphosphite and continue stirring to form a homogeneous solution; then add 0.1mol·L⁻¹ dropwise. -1The pH was adjusted to 3-4 with HCl solution, the temperature was raised to 80℃, and ferric nitrate nonahydrate was added and stirred continuously for 6 hours to form a dark red viscous gel. The gel was allowed to stand at room temperature for 12 hours for aging, and then transferred to a vacuum drying oven and dried under vacuum at 80℃ for 12 hours to obtain a porous brown precursor dry gel. The dry gel was placed in a tube furnace and heated to 900℃ for 2 hours under an inert atmosphere at a heating rate of 3℃ / min to obtain a phosphorus-doped three-dimensional porous carbon metal oxygen reduction catalyst.

[0049] Figure 2 This is a scanning electron microscope image of the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of the present invention; Figure 3 This is a pore size distribution diagram of the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of this invention; Figure 2 and Figure 3 It can be seen that the phosphorus-doped three-dimensional porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of the present invention has a good mesoporous structure.

[0050] Figure 4 The image shows a comparison of the linear scanning curves of the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 with those of commercial platinum-carbon and Comparative Example 1 at a rotation speed of 1600 rpm. The catalyst has an onset potential of 0.921 V and a half-wave potential of 0.837 V, indicating that the catalyst has good oxygen reduction performance.

[0051] Figure 5 This is a comparison chart showing the percentage of current density loss in Example 1, commercial platinum-carbon, and Comparative Example 1 before and after the addition of methanol at a constant voltage of -0.35V and 1600rpm. Figure 5 It can be seen that the current density of the catalyst in Example 1 of the present invention was retained at 96.17% before and after the addition of methanol, while that of the commercial platinum-carbon catalyst was 83.89%, and that of the iron-based catalyst in Comparative Example 1 was 90.45%, indicating that the catalyst has better methanol tolerance than the commercial platinum-carbon catalyst.

[0052] Figure 6 The CV (cyclic voltammetry) diagram of the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of this invention; Figure 6 The presence of an oxygen reduction peak indicates that the metal-free oxygen reduction catalyst prepared in Example 1 possesses catalytic activity. Linear sweep voltammetry yielded... Figure 4 Based on the data in Table 1, the specific performance values ​​of the embodiments can be obtained, and... Figure 6 Adaptable.

[0053] Figure 7 The image shows the XPS spectrum of phosphorus in the phosphorus-doped porous carbon metal-free oxygen reduction catalyst prepared in Example 1 of this invention; Figure 7 The XPS peak position is between 130 and 132 eV, indicating the formation of a stable PC coordination structure.

[0054] Table 1 shows the comparison results of electrochemical data between Examples 1-9 and commercial platinum-carbon and Comparative Example 1. It can be seen that the phosphorus-doped porous carbon metal oxygen reduction catalyst prepared in the examples of this application can achieve high ORR activity and anti-toxicity through electronic structure regulation.

[0055] Table 1 shows the electrochemical data comparison results between Examples 1-9 and commercial platinum-carbon and Comparative Example 1:

[0056]

[0057] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a phosphorus-doped porous carbon metal-free oxygen reduction catalyst, characterized in that, Includes the following steps: Step 1: Dissolve alizarin in anhydrous ethanol and stir at 40°C until completely dissolved; then add tetraol diphosphite and continue stirring to form a homogeneous mixed solution; Step 2: Add HCl solution dropwise to the mixed solution to adjust the pH to 3-4, heat to 60℃-90℃ and stir for 6 hours to form a dark red viscous gel; Step 3: Let the gel age at room temperature for 4-12 hours, and then vacuum dry for 6-12 hours to obtain a porous brown precursor dry gel. Step 4: Under an inert atmosphere, the dry gel is heated to 750-950℃ and carbonized for 1-4 hours at a heating rate of 3-10℃ / min to obtain a phosphorus-doped porous carbon metal oxygen-free reduction catalyst.

2. The method for preparing the phosphorus-doped porous carbon metal-free oxygen reduction catalyst according to claim 1, characterized in that, In step 1, the mass ratio of alizarin to tetraol diphosphite is (1-5):(1-5).

3. The method for preparing the phosphorus-doped porous carbon metal-free oxygen reduction catalyst according to claim 1, characterized in that, In step 2, the concentration of the HCl solution is 0.05–0.1 mol·L⁻¹. -1 .

4. The method for preparing the phosphorus-doped porous carbon metal-free oxygen reduction catalyst according to claim 1, characterized in that, The stirring temperature in step 2 is 80℃.

5. The method for preparing the phosphorus-doped porous carbon metal-free oxygen reduction catalyst according to claim 1, characterized in that, The aging time in step 3 is 12 hours.

6. The method for preparing the phosphorus-doped porous carbon metal-free oxygen reduction catalyst according to claim 1, characterized in that, In step 4, the heating rate is 3℃ / min, the carbonization temperature is 900℃, and the holding time is 2h.

7. The method for preparing the phosphorus-doped porous carbon metal-free oxygen reduction catalyst according to claim 1, characterized in that, The prepared phosphorus-doped porous carbon metal-free oxygen reduction catalyst has a PC-specific coordination structure, and its XPS phosphorus characteristic peak is located at 130-132 eV.

8. A phosphorus-doped porous carbon metal-free oxygen reduction catalyst, characterized in that: Prepared by the method described in any one of claims 1 to 7, it has a three-dimensional mesoporous structure and a specific surface area ≥ 800 m². 2 / g, in 0.1M KOH solution, oxygen reduction initiation potential ≥0.875V, half-wave potential ≥0.770V, methanol tolerance test current retention rate ≥90%.

9. The application of the catalyst as described in claim 8 in the oxygen reduction reaction at the cathode of a fuel cell.

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