Preparation method of MOF (at) COF core-shell heterostructure doped Nafion-based proton exchange membrane

Abstract

The invention relates to the field of fuel cells, and discloses a preparation method of an MOF (at) COF core-shell heterostructure doped Nafion-based proton exchange membrane, and the preparation method comprises the following steps: step 1, preparing MOF-NH2 powder; step 2, preparing MOF-NH2 (at) COF-SO3H powder, and preparing a COF-SO3H powder; and step 3, dispersing the MOF-NH2 (at) COF-SO3H powder into a 5% Nafion dispersion liquid, and drying to form the film. The proton exchange membrane is based on Nafion, COF and MOF, the MOF and the COF form an MOF-coated COF core-shell heterostructure, pore channels of the COF are modified by sulfonic acid, so that water molecules can be effectively confined, the water retention rate is increased, and a rigid MOF framework provides enough strength to prevent the proton exchange membrane from swelling.

Description

Technical Field

[0001] The present invention relates to the field of fuel cells, and in particular to a method for preparing a Nafion-based ion exchange membrane doped with a MOF@COF core-shell heterostructure. Technical Background

[0002] Nafion (perfluorosulfonic acid polymer) is a proton exchange membrane developed by DuPont in the 1960s and is still a core component of the hydrogen fuel cell industry.

[0003] However, as fuel cells evolve toward high power density, high-temperature, low-humidity operating environments, and long lifespans, the flaws of traditional Nafion membranes have gradually become apparent in recent years. For example, the proton conduction of Nafion membranes is highly dependent on hydration, but Nafion membranes have a high degree of swelling and poor water retention, and their internal water is easily lost at high temperatures. As a result, under low-humidity conditions, ion clusters in Nafion membranes collapse due to dehydration, resulting in a sudden drop in proton conductivity. Furthermore, dry-wet cycles under dynamic conditions can lead to periodic swelling stress, inducing microcracks and accelerating free radicals, reducing service life.

[0004] In summary, it is of positive significance to develop a highly stable Nafion-based ion exchange membrane with good water retention and low swelling. Summary of the Invention

[0005] To address these issues, the present invention provides a method for preparing a Nafion-based proton exchange membrane doped with a MOF@COF core-shell heterostructure. The proposed proton exchange membrane is based on Nafion, COF, and MOF. The MOF and COF form a MOF@COF core-shell heterostructure. The COF pores, modified with sulfonic acid, effectively trap water molecules, increasing water retention. The rigid MOF backbone provides sufficient strength to prevent swelling of the proton exchange membrane.

[0006] The specific technical solution of the present invention is: a method for preparing a Nafion-based ion exchange membrane doped with a MOF@COF core-shell heterostructure, which comprises the following steps:

[0007] Step 1: Dissolve 2-aminoterephthalic acid and ZrCl4 in an organic solvent, add acetic acid to disperse them evenly, undergo hydrothermal reaction, wash and dry to obtain MOF-NH2 powder.

[0008] Step 2: 1,3,5-triformylphloroglucinol, 2,5-diaminobenzenesulfonic acid, mesitylene, dioxane, and MOF-NH2 powder were mixed uniformly. The resulting solution was degassed by freeze-pump-thaw cycles and then heated to react. The product was washed and dried to obtain MOF-NH2@COF-SO3H powder with a core-shell heterostructure. The mass ratio of 2,5-diaminobenzenesulfonic acid to MOF-NH2 was 90-110:54.

[0009] Step 3: Disperse MOF-NH2@COF-SO3H powder in 5% Nafion dispersion and dry it into a film to prepare a Nafion-based ion exchange membrane doped with a MOF@COF core-shell heterostructure.

[0010] Aiming at the technical problems that pure Nafion membrane has high swelling degree, poor water retention performance and easy loss of internal water at high temperature, the proton exchange membrane of the present invention is a composite based on Nafion, COF and MOF. Among them, MOF is modified with amino group and can undergo Schiff base reaction with COF-SO3H-COF, and MOF-NH2@COF-SO3H powder with a core-shell heterogeneous structure can be formed through this reaction (the amino-modified MOF material can produce Schiff base reaction with the hydroxyl group in COF, anchored on the MOF surface, and form a core-shell structure). The pores of COF can be effectively improved by sulfonic acid modification, and the amino group in MOF-NH2 and the sulfonic acid group in COF-SO3H can form an acid-base pair. The protons can jump along the donor (acid group) and the acceptor (base) through the Grotthuss mechanism, thereby increasing the efficiency of proton conduction.

[0011] In the present invention, it is crucial to prepare COF using sulfonic acid-modified p-phenylenediamine as raw material. COF prepared with unmodified p-phenylenediamine as raw material cannot effectively improve the water retention effect of Nafion membrane, but if the modification is too much, it is easy to clog the pores. Therefore, the amount of 2,5-diaminobenzenesulfonic acid is more critical.

[0012] Preferably, in step 1, the mass ratio of 2-aminoterephthalic acid to ZrCl4 is 180:200-260.

[0013] Preferably, in step 1, the organic solvent is DMF.

[0014] Preferably, in step 1, the temperature of the hydrothermal reaction is 110-130° C., and the time is 20-30 h.

[0015] Preferably, in step 1, the washing is to wash the product with DMF and methanol; the drying is to use a vacuum drying oven at a temperature of 50-70° C. for 10-15 h.

[0016] Preferably, in step 2, the heating reaction temperature is 110-130° C., and the time is 2.5-3.5 days.

[0017] Preferably, in step 2, the washing is washing the product with tetrahydrofuran and acetone; and the drying is vacuum drying at a temperature of 90-110° C. for 10-15 h.

[0018] Preferably, in step 3, the content of Nafion in the Nafion dispersion is 3-7 wt %; and the usage ratio of the MOF-NH2@COF-SO3H powder and the Nafion dispersion is 35-65 mg / 10 mL.

[0019] Excessive use of MOF-NH2@COF-SO3H powder can cause stress concentration and reduce mechanical properties. Furthermore, the sulfonic acid group network of the Nafion membrane is the core pathway for proton conduction. When the powder dosage exceeds a critical value, the particles occupy the free volume of the ion clusters, leading to blockage of the proton transport channel. Using too little powder will not achieve the desired modification effect, so the appropriate amount of MOF-NH2@COF-SO3H powder is crucial.

[0020] Preferably, in step three, the temperature for drying the film is 50-70° C., and the time is 4-8 hours.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The proton exchange membrane of the present invention is a composite membrane based on Nafion, COF, and MOF, wherein the MOF and COF form a MOF@COF core-shell heterostructure. The sulfonic acid-modified pores of the COF not only effectively trap water molecules, increasing the water retention of Nafion, but also provide a low-barrier transport path (acid-base pair); while the rigid MOF skeleton provides sufficient strength to prevent the proton exchange membrane from swelling.

[0023] (2) The proton exchange membrane of the present invention has excellent thermal stability, which enables the fuel cell to maintain the proton exchange rate of the proton exchange membrane under operating conditions above 100°C. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to the embodiments.

[0025] Example 1

[0026] (1) 180 mg of 2-aminoterephthalic acid and 240 mg of ZrCl4 were dissolved in 24 mL of DMF solution and 2.4 mL of acetic acid was added for ultrasonic dispersion for 10 min. The solution was placed in a 100 mL reactor and heated at 120 °C for 24 h. The product was washed three times with DMF and methanol and dried in a vacuum drying oven at 60 °C for 12 h to obtain MOF-NH2 powder.

[0027] (2) 126 mg of 1,3,5-triformylphloroglucinol, 96 mg of 2,5-diaminobenzenesulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 powder were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and then heated at 120 °C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100 °C for 12 h to obtain MOF-NH2@COF-SO3H powder with a core-shell heterostructure.

[0028] (3) 50 mg of MOF-NH2@COF-SO3H powder was dissolved in 10 ml of 5% Nafion dispersion and ultrasonically dispersed for 10 min. The solution was placed in a mold and dried at 60 °C to form a film for 6 h.

[0029] Comparative Example 1 (MOF-NH2@COF-SO3H powder content in step (3) is too low)

[0030] (1) 180 mg of 2-aminoterephthalic acid and 240 mg of ZrCl4 were dissolved in 24 mL of DMF solution and 2.4 mL of acetic acid was added for ultrasonic dispersion for 10 min. The solution was placed in a 100 mL reactor and heated at 120 °C for 24 h. The product was washed three times with DMF and methanol and dried in a vacuum drying oven at 60 °C for 12 h to obtain MOF-NH2 powder.

[0031] (2) 126 mg of 1,3,5-triformylphloroglucinol, 96 mg of 2,5-diaminobenzenesulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 powder were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and then heated at 120 °C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100 °C for 12 h to obtain MOF-NH2@COF-SO3H powder with a core-shell heterostructure.

[0032] (3) 30 mg of MOF-NH2@COF-SO3H powder was dissolved in 10 ml of 5% Nafion dispersion and ultrasonically dispersed for 10 min. The solution was placed in a mold and dried at 60 °C to form a film for 6 h.

[0033] Comparative Example 2 (MOF-NH2@COF-SO3H powder content in step (3) is too high)

[0034] (1) 180 mg of 2-aminoterephthalic acid and 240 mg of ZrCl4 were dissolved in 24 mL of DMF solution and 2.4 mL of acetic acid was added for ultrasonic dispersion for 10 min. The solution was placed in a 100 mL reactor and heated at 120 °C for 24 h. The product was washed three times with DMF and methanol and dried in a vacuum drying oven at 60 °C for 12 h to obtain MOF-NH2 powder.

[0035] (2) 126 mg of 1,3,5-triformylphloroglucinol, 96 mg of 2,5-diaminobenzenesulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 powder were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and then heated at 120 °C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100 °C for 12 h to obtain MOF-NH2@COF-SO3H powder with a core-shell heterostructure.

[0036] (3) 70 mg of MOF-NH2@COF-SO3H powder was dissolved in 10 ml of 5% Nafion dispersion and ultrasonically dispersed for 10 min. The solution was placed in a mold and dried at 60 °C to form a film for 6 h.

[0037] Comparative Example 3 (the amount of 2,5-diaminobenzenesulfonic acid used in step (2) is too small)

[0038] (1) 180 mg of 2-aminoterephthalic acid and 240 mg of ZrCl4 were dissolved in 24 mL of DMF solution and 2.4 mL of acetic acid was added for ultrasonic dispersion for 10 min. The solution was placed in a 100 mL reactor and heated at 120 °C for 24 h. The product was washed three times with DMF and methanol and dried in a vacuum drying oven at 60 °C for 12 h to obtain MOF-NH2 powder.

[0039] (2) 126 mg of 1,3,5-triformylphloroglucinol, 80 mg of 2,5-diaminobenzenesulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 powder were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and heated at 120 °C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100 °C for 12 h to obtain MOF-NH2@COF-SO3H powder with a core-shell heterostructure.

[0040] (3) 50 mg of MOF-NH2@COF-SO3H powder was dissolved in 10 ml of 5% Nafion dispersion and ultrasonically dispersed for 10 min. The solution was placed in a mold and dried at 60 °C to form a film for 6 h.

[0041] Comparative Example 4 (Excessive use of 2,5-diaminobenzenesulfonic acid in step (2))

[0042] (1) 180 mg of 2-aminoterephthalic acid and 240 mg of ZrCl4 were dissolved in 24 mL of DMF solution and 2.4 mL of acetic acid was added for ultrasonic dispersion for 10 min. The solution was placed in a 100 mL reactor and heated at 120 °C for 24 h. The product was washed three times with DMF and methanol and dried in a vacuum drying oven at 60 °C for 12 h to obtain MOF-NH2 powder.

[0043] (2) 126 mg of 1,3,5-triformylphloroglucinol, 120 mg of 2,5-diaminobenzenesulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 powder were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and heated at 120 °C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100 °C for 12 h to obtain MOF-NH2@COF-SO3H powder with a core-shell heterostructure.

[0044] (3) 50 mg of MOF-NH2@COF-SO3H powder was dissolved in 10 ml of 5% Nafion dispersion and ultrasonically dispersed for 10 min. The solution was placed in a mold and dried at 60 °C to form a film for 6 h.

[0045] Comparative Example 5 (Step (2) using p-phenylenediamine instead of 2,5-diaminobenzenesulfonic acid)

[0046] (1) 126 mg of 1,3,5-triformylphloroglucinol, 96 mg of 2,5-diaminobenzenesulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and heated at 120°C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100°C for 12 h to obtain MOF-NH2 powder.

[0047] (2) 126 mg of 1,3,5-triformylphloroglucinol, 96 mg of p-phenylenediamine, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 powder were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and heated at 120°C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100°C for 12 h.

[0048] (3) 50 mg of MOF-NH2@COF powder was dissolved in 10 ml of 5% Nafion dispersion and ultrasonically dispersed for 10 min. The solution was placed in a mold and dried at 60 °C to form a film for 6 h.

[0049] Comparative Example 6 (Step (2) uses 2,5-diaminobenzene-1,4-disulfonic acid instead of 2,5-diaminobenzenesulfonic acid)

[0050] (1) 126 mg of 1,3,5-triformylphloroglucinol, 96 mg of 2,5-diaminobenzenesulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and heated at 120°C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100°C for 12 h to obtain MOF-NH2 powder.

[0051] (2) 126 mg of 1,3,5-triformylphloroglucinol, 96 mg of 2,5-diaminobenzene-1,4-disulfonic acid, 3 ml of mesitylene, 3 ml of dioxane, and 54 mg of MOF-NH2 powder were added to a reaction tube and sonicated for 10 min. The solution was degassed by three freeze-pump-thaw cycles and heated at 120°C for 3 days. The product was washed three times with tetrahydrofuran and acetone and dried in a vacuum drying oven at 100°C for 12 h.

[0052] (3) 50 mg of MOF-NH2@COF-SO3H powder was dissolved in 10 ml of 5% Nafion dispersion and ultrasonically dispersed for 10 min. The solution was placed in a mold and dried at 60 °C to form a film for 6 h.

[0053] Comparative Example 7 (using pure Nafion membrane)

[0054] Use a pipette to draw 3 ml of 5% Nafion solution and evenly drop it onto the center of the glass substrate. Use a spatula to evenly spread the solution. Dry it in an oven at 80°C for 12 hours with a vacuum of ≤0.1 MPa to form a proton exchange membrane.

[0055] Performance Testing

[0056] (1) Proton exchange capacity (IEC): Take a 5 cm × 5 cm dry proton exchange membrane sample and weigh the sample using an electronic balance to obtain the weight W. dPlace the proton exchange membrane sample in a saturated sodium chloride solution for 12 hours, remove the sample and rinse and soak it in deionized water. Mix the washing solution and soaking solution, and titrate the mixture with 0.1M sodium hydroxide solution. Add phenolphthalein as an indicator during the titration process and observe the change in solution color. Titrate until the solution is completely neutralized and record the volume of sodium hydroxide consumed (V). NaOH .

[0057] The ion exchange capacity (IEC) of the sample membrane can be calculated using the following formula:

[0058]

[0059] Among them, V NaOH is the volume of NaOH consumed, W d The quality of the dry film.

[0060] Table 1: Proton Exchange Capacity (IEC)

[0061] Case Proton exchange capacity (IEC) Example 1 1.19meq / g Comparative Example 1 1.10meq / g Comparative Example 2 1.28meq / g Comparative Example 3 1.08meq / g Comparative Example 4 1.21meq / g Comparative Example 5 0.98meq / g Comparative Example 6 1.44meq / g Comparative Example 7 1.03meq / g

[0062] Proton exchange capacity is closely related to the density of sulfonic acid groups. Higher sulfonic acid content generally results in higher IEC values. Comparison of the data in Table 1 shows that the proton exchange capacity of the proton exchange membrane obtained in Example 1 significantly increases compared to the pure Nafion membrane in Comparative Example 7. The use of unsulfonated COF in Comparative Example 5 results in a lower proton exchange capacity. Furthermore, the IEC value in Comparative Example 1 is lower than that in Example 1 due to the insufficient amount of MOF-NH2@COF-SO3H powder and the reduced number of sulfonic acid sites. The lower IEC value in Comparative Example 3 is due to the fact that the amount of 2,5-diaminobenzenesulfonic acid used does not meet the stoichiometric ratio of COF, resulting in an imbalance and structural defects, which reduces the orderliness of the pores and the number of sulfonic acid sites.

[0063] (2) Proton Exchange Rate (PC): A 5 cm × 5 cm dry proton exchange membrane sample was soaked in deionized water for 24 h. A test platform was constructed using an AC impedance spectrometer. The hydrated sample was sandwiched between two electrodes to ensure good contact between the electrodes and the membrane. The AC impedance spectrometer was set to a test frequency range of 0.01 Hz to 100 Hz with an amplitude of 10 mV. Based on the impedance data, an equivalent circuit model was established. The membrane resistance R was obtained by fitting the impedance data. The proton conductivity was then calculated according to the following formula.

[0064]

[0065] where L is the distance between the electrodes, A is the cross-sectional area of ​​the membrane, and R is the membrane resistance characterized by electrochemical impedance spectroscopy (EIS).

[0066] Table 2: Proton exchange rate (PC) (120°C, 100% RH)

[0067] Case Proton exchange rate (PC) Example 1 0.2256S / cm Comparative Example 1 0.1991S / cm Comparative Example 2 0.1873S / cm Comparative Example 3 0.1779S / cm Comparative Example 4 0.1831S / cm Comparative Example 5 0.1121S / cm Comparative Example 6 0.2042S / cm Comparative Example 7 0.0992S / cm

[0068] Comparison of the data in Table 2 shows that in Comparative Example 1, the COF was not functionalized with sulfonic acid groups, thus failing to provide a proton-conducting donor, resulting in a lower PC value than in Example 1. However, the porous structure of the COF improved water retention to a certain extent, resulting in a higher proton exchange rate than the pure Nafion membrane in Comparative Example 7. In Comparative Example 6, when preparing MOF-NH2@COF-SO3H powder, the COF was modified with 2,5-diaminobenzene-1,4-disulfonic acid. Excessive sulfonic acid groups clogged the COF pores, disrupting the proton conduction pathway, resulting in a lower PC value.

[0069] (3) Membrane water absorption and surface expansion rate test:

[0070] The acid-treated membrane samples were mixed and cut into 1 cm 2 After the film is fully hydrated, wipe off the surface moisture with filter paper and weigh it (W wet ), then measure its side length and calculate its area (A wet ), after vacuum drying for 24 h, the weight and area of ​​the membrane were measured and recorded as W dry and A dry .

[0071] Water retention rate (%) = (W wet -W dry ) / W dry ×100%

[0072] Area expansion ratio (%) = (A wet -A dry ) / A dry ×100%

[0073] Table 3: Water retention (%) (120°C, 100% RH)

[0074] Case Water retention rate Example 1 36％ Comparative Example 1 44％ Comparative Example 2 61％ Comparative Example 3 32％ Comparative Example 4 39％ Comparative Example 5 36％ Comparative Example 6 71％ Comparative Example 7 12％

[0075] From the comparison of the data in Table 3, it can be seen that: due to the strong hydrophilicity of the sulfonic acid group, the water retention rate of Example 1 and Comparative Examples 1 and 2 is significantly improved; in Comparative Example 5, the COF is not modified with sulfonic acid groups, but the porous structure of COF can improve the water retention rate to a certain extent, so the water retention rate is higher than that of pure Nafion in Comparative Example 7. In Comparative Example 6, due to the modification of the disulfonic acid site, the water retention capacity of the pores is greatly improved, so the water retention rate is the highest.

[0076] Table 4: Area expansion rate (%) (120°C, 100% RH)

[0077] Case Area expansion rate Example 1 12％ Comparative Example 1 16％ Comparative Example 2 13％ Comparative Example 3 23％ Comparative Example 4 27％ Comparative Example 5 14％ Comparative Example 6 56％ Comparative Example 7 42％

[0078] From the comparison of the data in Table 4, it can be seen that: due to the addition of the MOF skeleton, the membrane area expansion rate in Example 1 is significantly reduced compared with the pure Nafion membrane in Comparative Example 7, while the COF in Comparative Example 5 is not modified with sulfonic acid groups and lacks the effect of acid-base pairing, resulting in a slightly higher volume expansion rate. Comparative Example 6 is too hydrophilic and absorbs a large amount of water, resulting in excessive volume expansion, making it unsuitable as a proton exchange membrane; the amount of core-shell material used in Comparative Example 1 is too small, and it cannot fully contact with the Nafion matrix, resulting in an expansion rate higher than that of Example 1. In Comparative Example 2, the amount of MOF-NH2@COF-SO3H powder used is too much, and the acid-base pairing is insufficient, resulting in a higher expansion rate. Comparative Examples 3 and 4 destroy the stoichiometric ratio of COF, resulting in framework collapse and structural defects, resulting in a higher expansion rate.

[0079] Unless otherwise specified, the raw materials and equipment used in the present invention are commonly used in the art; the methods used in the present invention are conventional methods in the art unless otherwise specified.

[0080] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing a Nafion-based ion exchange membrane doped with a MOF@COF core-shell heterostructure, characterized in that include: Step 1: Dissolve 2-aminoterephthalic acid and ZrCl4 in an organic solvent, add acetic acid to disperse them evenly, undergo hydrothermal reaction, wash, and dry to obtain MOF-NH2 powder; Step 2: 2,5-diaminobenzenesulfonic acid, 1,3,5-triformylphloroglucinol, mesitylene, dioxane, and MOF-NH2 powder are uniformly mixed, the resulting solution is subjected to freeze-pump-thaw cycle degassing, and then heated to react. The product is washed and dried to obtain MOF-NH2@COF-SO3H powder with a core-shell heterostructure; wherein the mass ratio of 2,5-diaminobenzenesulfonic acid to MOF-NH2 is 90-110:54; Step 3: Disperse MOF-NH2@COF-SO3H powder in Nafion dispersion and dry it into a film to prepare a Nafion-based ion exchange membrane doped with a MOF@COF core-shell heterostructure.

2. The preparation method according to claim 1, wherein: In step 1, the mass ratio of 2-aminoterephthalic acid to ZrCl4 is 180:200-260.

3. The preparation method according to claim 1 or 2, characterized in that: In step 1, the organic solvent is DMF.

4. The preparation method according to claim 1, wherein: In step 1, the temperature of the hydrothermal reaction is 110-130° C., and the time is 20-30 h.

5. The preparation method according to claim 1, wherein: In step 1, the washing is to wash the product with DMF and methanol.

6. The preparation method according to claim 1, wherein: In step 1, the drying is carried out in a vacuum drying oven at a temperature of 50-70° C. for 10-15 hours.

7. The preparation method according to claim 1, wherein: In step 2, the heating reaction temperature is 110-130° C. and the time is 2.5-3.5 days.

8. The preparation method according to claim 1, wherein: In step 2, The cleaning comprises cleaning the product with tetrahydrofuran and acetone; The drying is vacuum drying at a temperature of 90-110° C. for 10-15 hours.

9. The preparation method according to claim 1, wherein: In step three, The content of Nafion in the Nafion dispersion is 3-7 wt%; The dosage ratio of the MOF-NH2@COF-SO3H powder and Nafion dispersion is 35-65 mg / 10 mL.

10. The preparation method according to claim 1 or 9, characterized in that: In step 3, the drying temperature for film formation is 50-70° C. and the drying time is 4-8 hours.

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