Preparation method and application of a uniform COF microsphere

Abstract

A method for preparing uniform COF microspheres and its application, which relates to a method for preparing COF microspheres and its application. The present invention aims to solve the technical problems that the solvothermal method cannot synthesize monodisperse and uniform COF microspheres and it is difficult to effectively control the morphology of COF materials. The present invention proposes to introduce a regulator based on benzaldehyde into the solvothermal system. The regulator occupies the polymerization sites and undergoes dynamic exchange with 4,4'-biphenyldicarboxaldehyde to slow down the rate of the rapid Schiff base reaction between 1,3,5-tris(4-aminophenyl)benzene and 4,4'-biphenyldicarboxaldehyde, and the particle size of the uniform COF microspheres can be controlled by temperature regulation. Then, uniform COF carbon microspheres are generated by sintering. Using the COF carbon microspheres as the core, COF microspheres are synthesized again by the solvothermal method. Due to the COF carbon microspheres as the core, the core-shell COF microspheres exhibit excellent electrocatalytic oxygen reduction performance.

Description

Technical Field

[0001] The present invention relates to a preparation method and application of COF microspheres. Background Art

[0002] Covalent organic frameworks (COFs) have received extensive attention due to their high specific surface area, permanent porosity, low density, and multidimensional molecular structure. Their unique properties can be easily adjusted by reticular chemistry to regulate pore size, functionality, and robust framework modification. Covalent organic frameworks (COFs) are composed of organic building blocks with precise symmetry combined by covalent bonds. The transformation of morphology and size from molecules (0.1 - 5 nm) to crystals (20 - 100 nm) can effectively control the crystallinity and porosity of COFs. These two properties directly reflect the applications of COFs in adsorption, storage, separation, ion exchange, transport, heterogeneous catalytic sensing, water harvesting, etc. However, after synthesis, COF crystals integrate through uncontrolled covalent self-assembly at different length scales, resulting in their precipitation as polycrystalline powders. Therefore, as bulk polycrystalline powder materials, COFs often cannot meet the specific requirements of device and membrane morphology-related applications. The crystallization of covalent organic frameworks mainly relies on dynamic covalent chemistry, which allows the simultaneous formation of bonds and the phase change from amorphous to crystalline, ensuring the healing of structural defects. However, the synthesis of COF microcrystals with a uniform size distribution is still difficult to achieve. For the synthesis methods of COF microspheres, there are room temperature method, solvothermal method, microwave method, ultrasonic method, photocatalytic induction method, grinding method, etc. However, for the solvothermal method, which is the earliest and most used to synthesize COF microspheres, it is impossible to synthesize monodisperse and uniform COF microspheres.

[0003] Covalent organic framework (COF) is a kind of crystalline porous material, mainly composed of light elements such as C / H / O / N. COF has rich monomers, flexible bonding forms, and clear reaction pathways, and has advantages such as large specific surface area, high porosity, structural tunability, and various functionalization methods. In particular, due to the firmness of the covalent bonds connecting the structural units, COF has an inherent advantage in terms of stability. COF is formed by the ordered building units connected by Π-Π bonds. Therefore, elements such as N that can be coordinated with metals are evenly distributed on the COF material, resulting in its excellent potential in uniformly anchoring single metal atoms, clusters, and nanoparticles, and then showing excellent performance in the field of electrocatalysis. However, the effective regulation of the morphology and size of COF materials has an obvious impact on electrocatalytic performance. Similarly, in-depth understanding of morphology and applications will help to produce better engineered COFs, which has practical significance. Therefore, the regulation of COF morphology is particularly important. Summary of the Invention

[0004] The present invention aims to solve the technical problems that currently, the solvothermal method cannot synthesize monodisperse and uniform COF microspheres and it is difficult to effectively control the morphology and size of COF materials, and provides a method for preparing uniform COF microspheres and its application.

[0005] The method for preparing uniform COF microspheres of the present invention is carried out according to the following steps:

[0006] 1,3,5-Tris(4-aminophenyl)benzene dissolved in 1,4-dioxane and 4,4'-biphenyldicarboxaldehyde dissolved in absolute ethanol are uniformly mixed, then an acetic acid solution is added as a catalyst, and 2,4,6-trimethylbenzaldehyde is added as a reaction regulator. After stirring at room temperature for 30 min to 35 min, the mixture is transferred to a high-pressure autoclave lined with polytetrafluoroethylene and heated at 120 °C to 180 °C for 12 h to 13 h. After the autoclave is cooled to room temperature, the product is centrifuged for 10 min to 15 min to obtain a yellow precipitate, and the product is washed successively with DMF and absolute ethanol to remove excessive regulators and soluble fragments. Finally, it is dried at 45 °C to 50 °C for 6 h to 7 h to obtain uniform COF microspheres;

[0007] The molar amount of 1,3,5-tris(4-aminophenyl)benzene and the volume ratio of 1,4-dioxane are 0.02 mol:(1 L to 1.5 L);

[0008] The molar amount of 4,4'-biphenyldicarboxaldehyde and the volume ratio of absolute ethanol are 0.03 mol:(1 L to 1.5 L);

[0009] The molar ratio of 1,3,5-tris(4-aminophenyl)benzene to 4,4'-biphenyldicarboxaldehyde is 1:(1.5 to 2);

[0010] The concentration of the acetic acid solution is 6 mol / L to 7 mol / L;

[0011] The volume ratio of the acetic acid solution to 1,4-dioxane is 1:(5 to 6);

[0012] The molar ratio of 1,3,5-tris(4-aminophenyl)benzene to 2,4,6-trimethylbenzaldehyde is 1:(40 to 42).

[0013] The present invention proposes to introduce a benzaldehyde-based regulator, 2,4,6-trimethylbenzaldehyde (TBA), into a solvothermal system and utilize dynamic covalent bonds to achieve control over the synthesis of COF microspheres. The regulator TBA used can slow down the rate of the rapid Schiff base reaction between 1,3,5-tris(4-aminophenyl)benzene and 4,4'-biphenyldicarboxaldehyde by occupying the polymerization sites and undergoing dynamic exchange with 4,4'-biphenyldicarboxaldehyde, and the particle size of uniform COF microspheres can be regulated by simple temperature control. In addition, the COF microspheres are synthesized from continuous organic building units and exhibit excellent performance in the ORR test that depends on continuous charge carrier transport.

[0014] The uniform COF microspheres prepared by the present invention are used as nuclei to prepare core-shell structured COF microspheres, and the specific process is as follows:

[0015] I. Put the uniform COF microspheres into a muffle furnace, introduce nitrogen, and then calcine at 750 - 800 °C for 2 - 3 h to obtain uniform COF carbon microspheres;

[0016] II. Preparation of core-shell COF microspheres: Dissolve 1,3,5-tris(4-aminophenyl)benzene and 4,4'-biphenyldicarboxaldehyde in 1,4-dioxane and mix evenly, then add an acetic acid solution as a catalyst and 2,4,6-trimethylbenzaldehyde as a reaction regulator; add the uniform COF carbon microspheres prepared in step I, stir the mixture at room temperature for 30 - 35 min, then transfer the mixture to a polytetrafluoroethylene-lined autoclave, and heat at 120 - 180 °C for 12 - 13 h. After the autoclave is cooled to room temperature, centrifuge for 10 - 15 min to obtain a yellow precipitate, and then wash the product successively with DMF and absolute ethanol to remove excessive regulator and soluble fragments, and finally dry at 45 - 50 °C for 6 - 7 h to obtain core-shell COF microspheres;

[0017] The molar amount of the 1,3,5-tris(4-aminophenyl)benzene and the volume ratio of 1,4-dioxane is 0.02 mol:(1 L - 1.5 L);

[0018] The molar amount of the 4,4'-biphenyldicarboxaldehyde and the volume ratio of 1,4-dioxane is 0.03 mol:(1 L - 1.5 L);

[0019] The concentration of the acetic acid solution is 6 mol / L - 7 mol / L;

[0020] The volume ratio of the acetic acid solution and 1, four-dioxane is 1:(5 - 6);

[0021] The molar ratio of the described 1,3,5-tris(4-aminophenyl)benzene to 2,4,6-trimethylbenzaldehyde is 1:(40 - 42);

[0022] The molar ratio of the mass of the uniform COF carbon spheres prepared in the first step to 1,3,5-tris(4-aminophenyl)benzene is 0.4 g:(1 mmol - 2 mmol).

[0023] The present invention proposes a synthesis method of core-shell structured COF microspheres. Utilizing the reversibility of imine bonds, a large amount or an excess of 2,4,6-trimethylbenzaldehyde is used for dynamic exchange with 4,4'-biphenyldicarboxaldehyde to control the nucleation and growth of COF, thereby generating uniform COF microspheres. Then, uniform COF carbon spheres are generated through sintering. Using the COF carbon spheres as the core, COF microspheres are synthesized again by the solvothermal method. Due to the COF carbon spheres being the core, the core-shell COF microspheres exhibit excellent electrocatalytic oxygen reduction performance. Description of the Drawings

[0024] …… Figure 1 is the SEM image of the COF microspheres prepared in Experiment 2;

[0025] Figure 2 is the SEM image of the COF microspheres prepared in Experiment 1;

[0026] Figure 3 is the TEM image of the COF microspheres prepared in Experiment 1;

[0027] Figure 4 is the low-magnification SEM image of the COF microspheres prepared in Experiment 3;

[0028] Figure 5 is the high-magnification SEM image of the COF microspheres prepared in Experiment 3;

[0029] Figure 6 is the low-magnification SEM image of the COF microspheres prepared in Experiment 1;

[0030] Figure 7 is the high-magnification SEM image of the COF microspheres prepared in Experiment 1;

[0031] Figure 8 is the low-magnification SEM image of the COF microspheres prepared in Experiment 4;

[0032] Figure 9 is the high-magnification SEM image of the COF microspheres prepared in Experiment 4;

[0033] Figure 10 is the low-magnification SEM image of the COF microspheres prepared in Experiment 5;

[0034] Figure 11 is the high-magnification SEM image of the COF microspheres prepared in Experiment 5;

[0035] Figure 12 It is the LSV test curve of COF at different rotation speeds in Experiment 6;

[0036] Figure 13 It is the graph of the number of electron transfers of COF at different rotation rates in Experiment 6;

[0037] Figure 14 It is the CV test curve graph in nitrogen and oxygen atmospheres in Experiment 6;

[0038] Figure 15 It is the Tafel test curve graph of COF in Experiment 6;

[0039] Figure 16 It is the EIS test curve graph of COF in Experiment 6;

[0040] Figure 17 It is the I-T test curve graph of COF for a long time of 1200 min in Experiment 6;

[0041] Figure 18 It is the low-magnification SEM image of the COF carbon spheres prepared in Step 1 of Experiment 7;

[0042] Figure 19 It is the high-magnification SEM image of the COF carbon spheres prepared in Step 1 of Experiment 7;

[0043] Figure 20 It is the low-magnification SEM image of the core-shell COF microspheres prepared in Step 2 of Experiment 7;

[0044] Figure 21 It is the high-magnification SEM image of the core-shell COF microspheres prepared in Step 2 of Experiment 7;

[0045] Figure 22 It is the TEM image of the COF carbon spheres prepared in Step 1 of Experiment 7;

[0046] Figure 23 It is the TEM image of the core-shell COF microspheres prepared in Step 2 of Experiment 7;

[0047] Figure 24 It is the CV test curve graph in nitrogen and oxygen atmospheres in Experiment 8;

[0048] Figure 25 It is the LSV test curve graph in Experiment 8;

[0049] Figure 26 It is the Tafel test curve graph in Experiment 8;

[0050] Figure 27 It is the I-T test curve graph in Experiment 8. Detailed implementation mode

[0051] Detailed implementation mode 1: This implementation mode is a method for preparing uniform COF microspheres, which is specifically carried out according to the following steps:

[0052] 1,3,5-Tris(4-aminophenyl)benzene dissolved in 1,4-dioxane and 4,4'-biphenyldicarboxaldehyde dissolved in absolute ethanol are uniformly mixed, then acetic acid solution is added as a catalyst, and 2,4,6-trimethylbenzaldehyde is added as a reaction regulator. After stirring at room temperature for 30 min to 35 min, the mixture is transferred to a high-pressure autoclave lined with polytetrafluoroethylene and heated at 120 °C to 180 °C for 12 h to 13 h. After the high-pressure autoclave is cooled to room temperature, the product is centrifuged for 10 min to 15 min to obtain a yellow precipitate, and the product is washed successively with DMF and absolute ethanol to remove excessive regulator and soluble fragments. Finally, it is dried at 45 °C to 50 °C for 6 h to 7 h to obtain uniform COF microspheres;

[0053] The molar amount of the 1,3,5-tris(4-aminophenyl)benzene and the volume ratio of 1,4-dioxane is 0.02 mol:(1 L to 1.5 L);

[0054] The molar amount of the 4,4'-biphenyldicarboxaldehyde and the volume ratio of absolute ethanol is 0.03 mol:(1 L to 1.5 L);

[0055] The molar ratio of the 1,3,5-tris(4-aminophenyl)benzene to the 4,4'-biphenyldicarboxaldehyde is 1:(1.5 to 2);

[0056] The concentration of the acetic acid solution is 6 mol / L to 7 mol / L;

[0057] The volume ratio of the acetic acid solution to 1,4-dioxane is 1:(5 to 6);

[0058] The molar ratio of the 1,3,5-tris(4-aminophenyl)benzene to the 2,4,6-trimethylbenzaldehyde is 1:(40 to 42).

[0059] Detailed implementation mode 2: The difference between this implementation mode and detailed implementation mode 1 is that the rotation speed of the centrifugation is 2000 rpm. Others are the same as detailed implementation mode 1.

[0060] Detailed implementation mode 3: The difference between this implementation mode and detailed implementation mode 1 or 2 is that it is heated at 140 °C for 12 h. Others are the same as detailed implementation mode 1 or 2.

[0061] Embodiment 4: This embodiment is about the application of uniform COF microspheres in Embodiment 1. The uniform COF microspheres are used as nuclei to prepare core-shell structured COF microspheres. The specific process is as follows:

[0062] 1. Put the uniform COF microspheres into a muffle furnace, introduce nitrogen, and then calcine at 750 - 800 °C for 2 - 3 hours to obtain uniform COF carbon microspheres;

[0063] 2. Preparation of core-shell COF microspheres: Dissolve 1,3,5-tris(4-aminophenyl)benzene and 4,4'-biphenyldicarboxaldehyde in 1,4-dioxane and mix them evenly. Then add acetic acid solution as a catalyst and 2,4,6-trimethylbenzaldehyde as a reaction regulator; add the uniform COF carbon microspheres prepared in step 1. After stirring at room temperature for 30 - 35 minutes, transfer the mixture to a high-pressure autoclave lined with polytetrafluoroethylene, and heat at 120 - 180 °C for 12 - 13 hours. After the autoclave is cooled to room temperature, centrifuge for 10 - 15 minutes to obtain a yellow precipitate. Then wash the product successively with DMF and absolute ethanol to remove excessive regulator and soluble fragments. Finally, dry at 45 - 50 °C for 6 - 7 hours to obtain core-shell COF microspheres;

[0064] The molar amount of the described 1,3,5-tris(4-aminophenyl)benzene to the volume of 1,4-dioxane is 0.02 mol:(1 L - 1.5 L);

[0065] The molar amount of the described 4,4'-biphenyldicarboxaldehyde to the volume of 1,4-dioxane is 0.03 mol:(1 L - 1.5 L);

[0066] The concentration of the described acetic acid solution is 6 mol / L - 7 mol / L;

[0067] The volume ratio of the described acetic acid solution to 1,4-dioxane is 1:(5 - 6);

[0068] The molar ratio of the described 1,3,5-tris(4-aminophenyl)benzene to 2,4,6-trimethylbenzaldehyde is 1:(40 - 42);

[0069] The mass ratio of the uniform COF carbon microspheres prepared in step 1 to the molar amount of 1,3,5-tris(4-aminophenyl)benzene is 0.4 g:(1 mmol - 2 mmol).

[0070] Embodiment 5: The difference between this embodiment and Embodiment 4 is that in step 2, it is heated at 140 °C for 12 hours. Others are the same as Embodiment 4.

[0071] Embodiment Six: The difference between this embodiment and Embodiment Five is that the rotation speed of centrifugation in Step 2 is 2000 rpm. Others are the same as Embodiment Five.

[0072] The present invention was verified by the following experiments:

[0073] Experiment One: This experiment is a method for preparing uniform COF microspheres, which is specifically carried out according to the following steps:

[0074] 1,3,5-Tris(4-aminophenyl)benzene (0.02 mmol) dissolved in 1 mL of 1,4-dioxane was uniformly mixed with 4,4'-biphenyldicarboxaldehyde (0.03 mmol) dissolved in 1 mL of absolute ethanol, then 0.2 mL of acetic acid solution was added as a catalyst, 0.8 mmol of 2,4,6-trimethylbenzaldehyde was added as a reaction regulator. After stirring at room temperature for 30 min, the mixture was transferred to a high-pressure autoclave lined with polytetrafluoroethylene and heated at 140 °C for 12 h. After the autoclave was cooled to room temperature, the product was centrifuged at a rotation speed of 2000 rpm for 10 min to obtain a yellow precipitate. The product was washed successively with DMF and absolute ethanol to remove excessive regulators and soluble fragments, and finally dried at 45 °C for 6 h to obtain uniform COF microspheres; the concentration of the acetic acid solution was 6 mol / L.

[0075] Experiment Two: This experiment is a comparative experiment. The difference from Experiment One is that 2,4,6-trimethylbenzaldehyde was not added. Others are the same as Experiment One.

[0076] Figure 1 is the SEM image of the COF microspheres prepared in Experiment Two, Figure 2 is the SEM image of the COF microspheres prepared in Experiment One. It can be clearly seen by comparison that due to the addition of TBA, monodisperse and uniform COF microspheres were successfully prepared.

[0077] Figure 3 is the TEM image of the COF microspheres prepared in Experiment One. It can be seen from the figure that the COF microspheres synthesized using the TBA regulator are solid microsphere structures and have uniformity.

[0078] Experiment Three: The difference between this experiment and Experiment One is that it was heated at 120 °C for 12 h. Others are the same as Experiment One.

[0079] Experiment Four: The difference between this experiment and Experiment One is that it was heated at 160 °C for 12 h. Others are the same as Experiment One.

[0080] Experiment Five: The difference between this experiment and Experiment One is that it was heated at 180 °C for 12 h. Others are the same as Experiment One.

[0081] Figure 4 This is a low-magnification SEM image of the COF microspheres prepared in experiment three. Figure 5 This is a high-magnification SEM image of the COF microspheres prepared in experiment three. Figure 6 This is a low-magnification SEM image of the COF microspheres prepared in experiment 1. Figure 7 This is a high-magnification SEM image of the COF microspheres prepared in experiment 1. Figure 8 This is a low-magnification SEM image of the COF microspheres prepared in experiment 4. Figure 9 This is a high-magnification SEM image of the COF microspheres prepared in experiment 4. Figure 10 This is a low-magnification SEM image of the COF microspheres prepared in experiment 5. Figure 11 This is a high-magnification SEM image of the COF microspheres prepared in Experiment 5. It can be seen that, while TBA regulates the uniformity of the COF microspheres, simple temperature control can effectively regulate the size of the microspheres. Furthermore, as the reaction temperature increases, the average particle size of the microspheres increases.

[0082] Experiment 6: This experiment was conducted on the COF microspheres prepared in Experiment 1 for electrocatalytic oxygen reduction reaction (ORR). The experiment was conducted at room temperature using a CHI 760E electrochemical workstation (Chenhua, Shanghai, China) in a conventional three-electrode system. A 5 mm diameter glassy carbon electrode (GC) and a 5 mm diameter rotating disk electrode (RDE) were used as working electrodes. The counter electrode and reference electrode were carbon rods and Ag / AgCl (1 M KCl), respectively. The reversible hydrogen electrode (RHE) potential was converted relative to the E(Ag / AgCl) potential according to the Nernst equation:

[0083] E(RHE)=E Ag / AgCl +0.0592(pH)+E 0 Ag / AgCl (E 0 Ag / AgCl =0.2224V)

[0084] Preparation of ink: 5 mg of the uniform COF microspheres prepared in Experiment 1 were dispersed in a mixed solution of 50 μL of Nafion solution (0.5 wt.%), 0.7125 mL of DI water, and 0.2375 mL of anhydrous ethanol to obtain a slurry. Then, 10 μL of the slurry was coated on a glassy carbon electrode for the corresponding test.

[0085] Figure 12 These are the LSV test curves of COF at different rotation speeds in Test 6. The rotation speed of curve 1 is 400 rpm, the rotation speed of curve 2 is 625 rpm, the rotation speed of curve 3 is 900 rpm, the rotation speed of curve 4 is 1225 rpm, and the rotation speed of curve 5 is 1600 rpm. Figure 13It is the graph of the number of electron transfers of COF at different rotation rates in Experiment 6. The reversible hydrogen electrode (RHE) potential of Curve 1 is 0.5V, the reversible hydrogen electrode (RHE) potential of Curve 2 is 0.45V, the reversible hydrogen electrode (RHE) potential of Curve 3 is 0.4V, the reversible hydrogen electrode (RHE) potential of Curve 4 is 0.35V, and the reversible hydrogen electrode (RHE) potential of Curve 5 is 0.3V. It can be seen that testing COF at different rates yields the corresponding number of electron transfers, indicating that COF approaches four-electron transfer.

[0086] Figure 14 It is the CV test curve graph of COF in nitrogen and oxygen atmospheres in Experiment 6. It can be seen that in the test atmosphere relative to nitrogen, the oxygen atmosphere test shows obvious redox peaks, indicating that COF has good ORR catalytic activity.

[0087] Figure 15 It is the Tafel test curve graph of COF in Experiment 6. It can be seen that the Tafel slope of the COF microspheres is 88.98 mV / dec.

[0088] Figure 16 It is the EIS test curve graph of COF in Experiment 6. It can be seen that the COF microspheres have a small resistance.

[0089] Figure 17 It is the I-T test curve graph of COF for a long time of 1200 min in Experiment 6. It can be seen that it shows excellent stability under the long-term test of 1200 min.

[0090] All the above data prove that the COF microspheres prepared by the present invention are an excellent ORR material.

[0091] Experiment Seven; This experiment is the application of the uniform COF microspheres prepared in Experiment One. The uniform COF microspheres are used as the core to prepare core-shell structured COF microspheres. The specific process is as follows:

[0092] I. Put the uniform COF microspheres into a muffle furnace, introduce nitrogen, and then calcine at 750 °C for 2 h to obtain uniform COF carbon spheres;

[0093] II. Preparation of core-shell COF microspheres: 0.02 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.03 mmol of 4,4'-biphenyldicarboxaldehyde were both dissolved in 1 mL of 1,4-dioxane and evenly mixed. Then, 0.2 mL of acetic acid solution was added as a catalyst, and 0.8 mmol of 2,4,6-trimethylbenzaldehyde was added as a reaction regulator; 0.008 g of the uniform COF carbon spheres prepared in the first step was added. After stirring at room temperature for 30 min, the mixture was transferred to a polytetrafluoroethylene-lined autoclave and heated at 140 °C for 12 h. After the autoclave was cooled to room temperature, it was centrifuged for 10 min (the centrifugation speed was 2000 rpm) to obtain a yellow precipitate. Then, the product was washed successively with DMF and absolute ethanol to remove excessive regulators and soluble fragments. Finally, it was dried at 45 °C for 6 h to obtain core-shell COF microspheres; the concentration of the acetic acid solution was 6 mol / L.

[0094] Figure 18 It is the low-magnification SEM image of the COF carbon spheres prepared in the first step of Experiment VII. Figure 19 It is the high-magnification SEM image of the COF carbon spheres prepared in the first step of Experiment VII. Figure 20 It is the low-magnification SEM image of the core-shell COF microspheres prepared in the second step of Experiment VII. Figure 21 It is the high-magnification SEM image of the core-shell COF microspheres prepared in the second step of Experiment VII. Figure 22 It is the TEM image of the COF carbon spheres prepared in the first step of Experiment VII. Figure 23 It is the TEM image of the core-shell COF microspheres prepared in the second step of Experiment VII. It can be seen that due to the presence of TBA, uniform COF microspheres were synthesized, and then uniform solid COF carbon spheres were synthesized. The COF material grown on the COF carbon spheres still formed a spherical morphology, and core-shell COF microspheres were obtained.

[0095] Experiment VIII: This experiment was the electrocatalytic oxygen reduction reaction (ORR) test of the core-shell COF microspheres prepared in Experiment VII: It was carried out at room temperature using a CHI 760E electrochemical workstation (Chenhua, Shanghai, China) in a traditional three-electrode system; a glassy carbon electrode (GC) with a diameter of 5 mm and a rotating disk electrode (RDE) with a diameter of 5 mm were used as the working electrodes. The counter electrode and the reference electrode were a carbon rod and an Ag / AgCl (1 M KCl) electrode, respectively. The reversible hydrogen electrode (RHE) potential was converted relative to the E(Ag / AgCl) potential according to the Nernst equation:

[0096] E(RHE) = E Ag / AgCl + 0.0592(pH) + E 0 Ag / AgCl (E 0 Ag / AgCl= 0.2224 V)

[0097] Preparation of ink: 5 mg of the core-shell COF microspheres prepared in Experiment 7 were dispersed in a mixed solution of 50 μL of Nafion solution (0.5 wt.%), 0.7125 mL of DI water, and 0.2375 mL of absolute ethanol to obtain a slurry. Then, 10 μL of the slurry was coated onto a glassy carbon electrode for corresponding tests.

[0098] Figure 24 It is the CV test curve graph in nitrogen and oxygen atmospheres in Experiment 8. The dashed line is nitrogen and the solid line is oxygen; Figure 25 It is the LSV test curve graph in Experiment 8; Figure 26 It is the Tafel test curve graph in Experiment 8. Curve 1 is the core-shell COF microspheres and Curve 2 is the COF microspheres in Experiment 6; Figure 27 It is the I-T test curve graph in Experiment 8. Curve 1 is the COF microspheres in Experiment 6 and Curve 2 is the core-shell COF microspheres.

[0099] From Figure 24 It can be seen that in the tests of core-shell COF microspheres and original COF microspheres in the atmospheres of N2 and O2, although both COF and core-shell COF show oxygen reduction ability, the core-shell COF has a more positive potential compared to COF, indicating that its ORR catalytic activity is superior to that of COF. This is also the same as the LSV test results of COF and core-shell COF at 1600 rpm (as Figure 25 ), and the core-shell COF shows a higher initial potential and half-wave potential than COF. From Figure 26 It can be seen that a smaller Tafel also provides evidence for the better performance of the core-shell COF.

[0100] From Figure 27 It can be seen that under a long-term test of 1200 min, the core-shell COF maintains a higher original current density than COF, indicating that the core-shell COF has better stability.

[0101] The above results all show that the core-shell COF material has improved the ORR performance due to the construction of the core-shell structure, indicating that the adjustment of morphology has a significant impact on the performance of the material.

Claims

1. A preparation method of uniform COF microspheres, characterized in that The preparation method of uniform COF microspheres is carried out according to the following steps: 1,3,5-Tris(4-aminophenyl)benzene dissolved in 1,4-dioxane and 4,4'-biphenyldicarboxaldehyde dissolved in absolute ethanol are uniformly mixed, then an acetic acid solution is added as a catalyst, and 2,4,6-trimethylbenzaldehyde is added as a reaction regulator. After stirring at room temperature for 30 min to 35 min, the mixture is transferred to a high-pressure autoclave lined with polytetrafluoroethylene and heated at 120 °C to 180 °C for 12 h to 13 h. After the autoclave is cooled to room temperature, the product is centrifuged for 10 min to 15 min to obtain a yellow precipitate. The product is washed successively with DMF and absolute ethanol to remove excessive regulators and soluble fragments, and finally dried at 45 °C to 50 °C for 6 h to 7 h to obtain uniform COF microspheres; The molar amount of 1,3,5-tris(4-aminophenyl)benzene to the volume of 1,4-dioxane is 0.02 mol:(1 L to 1.5 L); The molar amount of 4,4'-biphenyldicarboxaldehyde to the volume of absolute ethanol is 0.03 mol:(1 L to 1.5 L); The molar ratio of 1,3,5-tris(4-aminophenyl)benzene to 4,4'-biphenyldicarboxaldehyde is 1:(1.5 to 2); The concentration of the acetic acid solution is 6 mol / L to 7 mol / L; The volume ratio of the acetic acid solution to 1,4-dioxane is 1:(5 to 6); The molar ratio of 1,3,5-tris(4-aminophenyl)benzene to 2,4,6-trimethylbenzaldehyde is 1:(40 to 42).

2. The preparation method of a uniform COF microsphere according to claim 1, characterized in that The rotation speed of the centrifugation is 2000 rpm.

3. The preparation method of a uniform COF microsphere according to claim 1, characterized in that Heat at 140 °C for 12 h.

4. The application of a uniform COF microsphere as described in claim 1, characterized in that Uniform COF microspheres are used as cores to prepare core-shell structured COF microspheres, and the specific process is as follows: I. Put the uniform COF microspheres into a muffle furnace, introduce nitrogen, and then calcine at 750 °C to 800 °C for 2 h to 3 h to obtain uniform COF carbon microspheres; II. Preparation of core-shell COF microspheres: 1,3,5-Tris(4-aminophenyl)benzene and 4,4'-biphenyldicarboxaldehyde are both dissolved in 1,4-dioxane and uniformly mixed, then an acetic acid solution is added as a catalyst, and 2,4,6-trimethylbenzaldehyde is added as a reaction regulator; add the uniform COF carbon microspheres prepared in step I. After stirring at room temperature for 30 min to 35 min, the mixture is transferred to a high-pressure autoclave lined with polytetrafluoroethylene and heated at 120 °C to 180 °C for 12 h to 13 h. After the autoclave is cooled to room temperature, it is centrifuged for 10 min to 15 min to obtain a yellow precipitate, and then the product is washed successively with DMF and absolute ethanol to remove excessive regulators and soluble fragments, and finally dried at 45 °C to 50 °C for 6 h to 7 h to obtain core-shell COF microspheres; In step II, the molar amount of 1,3,5-tris(4-aminophenyl)benzene to the volume of 1,4-dioxane is 0.02 mol:(1 L to 1.5 L); In step two, the molar amount of 4,4'-biphenyldicarboxaldehyde to the volume of 1,4-dioxane is 0.03 mol:(1 L - 1.5 L); In step two, the concentration of the acetic acid solution is 6 mol / L - 7 mol / L; In step two, the volume ratio of the acetic acid solution to 1,4-dioxane is 1:(5 - 6); In step two, the molar ratio of 1,3,5-tris(4-aminophenyl)benzene to 2,4,6-trimethylbenzaldehyde is 1:(40 - 42); In step two, the mass ratio of the uniform COF carbon spheres prepared in step one to 1,3,5-tris(4-aminophenyl)benzene is 0.4 g:(1 mmol - 2 mmol).

5. The application of a uniform COF microsphere according to claim 4, characterized in that In step two, heat at 140 °C for 12 h.

6. The application of a uniform COF microsphere according to claim 4, characterized in that In step two, the rotation speed of centrifugation is 2000 rpm.

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