Proton exchange membrane with gradient functional layer and preparation method thereof

Abstract

The invention discloses a proton exchange membrane with a gradient functional layer and a preparation method of the proton exchange membrane. The proton exchange membrane comprises a polybenzimidazole base layer, an Al2O3 layer, a carbon nanotube array vertically arranged and grown on the surface, a sulfonated covalent organic framework hydrogen-resistant layer with gradient pore size distribution, and a hydrophobic modified SiO2 surface passivation layer from bottom to top in sequence, phosphoric acid molecules are immobilized in covalent organic framework channels of the sulfonated covalent organic framework hydrogen resistance layer and on the surfaces of the carbon nanotubes. A vertical carbon nanotube array is grown on the surface of perfluorinated sulfonic acid resin through PECVD, an Al2O3 intermediate layer is deposited in combination with ALD to protect a substrate, a hydrogen blocking layer is formed by sequentially dipping a large-aperture SA-TFP COF solution and a small-aperture SO3H-COF-300 / SO3H-COF-316 solution, hydrogen permeation is inhibited by using a size screening effect, and segmented treatment is carried out through phosphoric acid solution activation and a gradient annealing process, so that the performance of the hydrogen barrier layer is improved. The proton conductivity and the interface chemical crosslinking density are effectively enhanced, the hydrogen permeability is reduced, and the high-temperature stability is high.

Description

Technical Field

[0001] This invention relates to the fields of proton exchange membrane water electrolysis and fuel cells, and particularly to a proton exchange membrane with a gradient functional layer and its preparation method. Background Technology

[0002] The proton exchange membrane (PEM) is a core component of proton exchange membrane fuel cells (PEMFC) and water electrolysis for hydrogen production (PEMWE), and it needs to have properties such as high proton conductivity, low gas permeability, excellent chemical stability, high mechanical strength and long life.

[0003] Traditional perfluorosulfonic acid membranes, typified by Nafion membranes, while meeting some application requirements under normal conditions, also have significant drawbacks. Their performance is particularly poor at high temperatures (>100°C). This is because proton conduction relies primarily on hydrogen bonds formed by water molecules within the membrane. When the temperature exceeds 100°C, the water within the membrane evaporates rapidly, leading to dehydration. Once dehydrated, the hydrogen bond network is disrupted, obstructing proton conduction channels and causing a sharp drop in proton conductivity, severely impacting the efficiency of fuel cells and water electrolysis for hydrogen production. Furthermore, traditional perfluorosulfonic acid membranes also suffer from high hydrogen permeability. High hydrogen permeability leads to hydrogen waste and reduces energy conversion efficiency. Moreover, hydrogen permeating to the cathode may react unnecessarily with oxygen, generating additional energy loss and potentially affecting membrane stability and lifespan. To address these issues, existing technologies involve doping perfluorosulfonic acid membranes with inorganic materials or activating them with phosphoric acid. For example, in the paper "Phosphoric Acid Dynamics in High Temperature Polymer Electrolyte Membranes" published in the Journal of The Electrochemical Society, Aili, David, Becker, Hans, et al. proposed that phosphoric acid doping can create more proton conduction sites within the membrane, thereby increasing the proton migration rate and maintaining or even improving proton conduction performance at high temperatures. However, problems such as weak interfacial bonding and poor long-term stability still exist. Summary of the Invention

[0004] The purpose of this invention is to provide a proton exchange membrane with a gradient functional layer and its preparation method. Its advantages are that it has long-term high-temperature stability while having good hydrogen permeability and proton conductivity.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: This invention provides a method for preparing a proton exchange membrane, comprising the following steps: Step 1: An Al2O3 layer is formed on the surface of a polybenzimidazole substrate using atomic layer deposition (ALD). Then, a Fe-Mo bimetallic catalyst solution is spin-coated onto the surface of the Al2O3 layer. After annealing at 200-250°C, Fe-Mo nanoparticles are formed on the surface of the Al2O3 layer, providing nucleation sites for carbon nanotube growth. Subsequently, using C2H6 as the carbon source, a vertically aligned array of carbon nanotubes is grown on the Fe-Mo nanoparticles using PECVD. Step 2: The membrane material obtained in Step 1 is sequentially immersed in Solution A and Solution B and then dried; Solution A is a dimethyl sulfoxide solution of SA-TFPCOF, and Solution B is an N,N-dimethylformamide solution of SO3H-COF-300 or an N,N-dimethylformamide solution of SO3H-COF-316. Step 3: The membrane material obtained in Step 2 is subjected to phosphoric acid solution activation treatment and two-stage high-temperature annealing treatment in sequence; Step 4: Prepare a SiO2 layer on the surface of the film material obtained in Step 3 using atomic layer deposition technology, and then perform hydrophobic modification treatment on the SiO2 layer; The Fe to Mo molar ratio in the Fe-Mo bimetallic catalyst solution is (4-5):(0.5-1).

[0006] The present invention is further configured such that: in step 1, the parameters for depositing an Al2O3 layer on the surface of a polybenzimidazole substrate using atomic layer deposition technology include: deposition temperature of 120-150℃, TMA pulse time of 0.1-0.3s, H2O pulse time of 0.1-0.3s, purging with N2 for 15-30s / cycle, and the thickness of the Al2O3 layer of 30-50nm.

[0007] The present invention is further configured such that: in step 1, the Fe-Mo bimetallic catalyst solution is prepared by dissolving Fe(NO3)3·9H2O and ammonium molybdate in a solvent at a Fe to Mo molar ratio of (4-5):(0.5-1), then adding polyethylene glycol, and stirring magnetically for 30-50 min at a speed of 500-600 rpm to form a homogeneous solution. The concentration of polyethylene glycol in the homogeneous solution is 0.3-0.6 wt%. Finally, the pH of the solution is adjusted to 3-3.5 with dilute ammonia. The solvent is a 1:1 volume ratio mixture of ethanol and deionized water. The spin coating is a two-stage spin coating: the first stage spin coating speed is 400-600 rpm and the time is 5-10s, and the second stage spin coating speed is 2000-3000 rpm and the time is 25-40s.

[0008] The present invention is further configured such that step 1, the step of growing a vertically aligned carbon nanotube array using the PECVD process, includes: 1) Chamber pretreatment: Place the membrane in the CVD reaction chamber and evacuate to 3×10⁻⁶. -3 -6×10 -3 Torr; 2) Reduction activation: Introduce a mixture of Ar and H2 gas with a volume ratio of Ar:H2=3:1, heat to 250-280°C, and maintain for 5-10 min to activate the catalyst; 3) Carbon nanotube growth: Apply radio frequency power in pulse mode at a frequency of 20-40 Hz and a duty cycle of 50%-60%. The plasma radio frequency power is 150-250 W. Introduce a mixture of C2H2 and Ar gas with a volume ratio of Ar:C2H2=3:1 and adjust the flow rate to 40-60 sccm. Control the substrate temperature to 250-300°C to start carbon nanotube growth. The carbon nanotube growth time is 10-20 min to obtain a vertical array with a height of 5-10 μm. 4) Cooling: Turn off C2H2 and plasma, and cool to room temperature in a pure Ar atmosphere at a cooling rate of ≤5°C / min.

[0009] The present invention is further configured such that: in step 2, the concentration of the DMSO solution of SA-TFP COF is 2-3 wt%, and the immersion time is 5-8 min; the concentration of the DMF solution of SO3H-COF-300 or the DMF solution of SO3H-COF-316 is 4-5 wt%, and the immersion time is 10-15 min; The SA-TFP COF has a pore size of 1.2-1.8 nm and a sulfonic acid group density of 1.2-2.0 mmol / g; the SO3H-COF-300 and SO3H-COF-316 have a pore size of 0.65-1.0 nm and a sulfonic acid group density of 2.8-4.2 mmol / g; SA-TFPCOF, SO3H-COF-300, and SO3H-COF-316 are prepared by a solvothermal method: the monomers are reacted in NMP solvent at 120-150℃ for 24-48 h, and then SO3H groups are introduced by sulfonation with concentrated sulfuric acid.

[0010] The present invention is further configured such that: in step 3, the phosphoric acid solution activation treatment is: the membrane material that has completed step 2 is immersed in an 85wt% phosphoric acid solution at 70-85℃ for 1-3 hours, so that the phosphoric acid doping amount is 180-220wt%.

[0011] The present invention is further configured such that, in step 3, the two-stage high-temperature annealing treatment is: the first stage is treated at 100-120℃ for 1-2 hours, and the second stage is treated at 150-160℃ for 1-2 hours.

[0012] The present invention is further configured such that step 4 is specifically as follows: a) Place the membrane material that has completed step 3 into the ALD chamber and evacuate it to 0.1-0.5 Torr; b) A SiO2 layer is deposited at 120-150℃, with BTBAS and O3 as precursors, wherein: BTBAS pulse for 1.0-2.0s, followed by 20-30s purging, and O3 pulse for 0.5-1s, followed by 20-30s purging; the thickness of the SiO2 layer is 10-15nm. c) On the surface of the SiO2 layer deposited by ALD, vaporized heptadecafluorodecyltrimethoxysilane is introduced and reacted at 120°C under N2 carrier gas for 20-30 min to form a fluorinated hydrophobic layer. After modification, the residue is removed by purging with N2 for 5 min, and then unreacted heptadecafluorodecyltrimethoxysilane is removed by purging with N2 for 5 min.

[0013] Another aspect of the present invention provides a proton exchange membrane prepared by the above-mentioned preparation method, comprising, from bottom to top, a polybenzimidazole base layer, an Al2O3 layer, a carbon nanotube array grown vertically on the surface, a sulfonated covalent organic framework hydrogen barrier layer with a gradient pore size distribution, and a hydrophobically modified SiO2 surface passivation layer, wherein phosphate molecules are immobilized in the covalent organic framework pores of the sulfonated covalent organic framework hydrogen barrier layer and on the surface of the carbon nanotubes.

[0014] The present invention is further configured such that: the thickness of the polybenzimidazole base layer is 50-100 μm; and the thickness of the Al2O3 layer is 30-50 nm; The carbon nanotube array has a tube height of 5-10 μm and an array density of 10. 8 -10 10 root / cm 2 ; The thickness of the hydrophobic modified SiO2 surface passivation layer is 10-15 nm.

[0015] The beneficial effects of this invention are as follows: 1. Regarding proton conductivity: The vertically arranged carbon nanotubes of this invention fix phosphate molecules through nano-confinement, forming directional transport channels, which can effectively alleviate the problem of phosphate loss caused by high doping in traditional PBI films; In addition, the gradient sulfonated COFs layer set in this invention, through pore size classification and synergistic effect of sulfonic acid groups, can retain water in low humidity using rigid channels, maintain the continuity of the proton carrier, and thus ensure proton conductivity; Furthermore, the hydrophobic SiO2 layer set in this invention can suppress water vapor escape, and combined with the Al2O3 bottom layer to achieve sealing, so that this composite film still maintains high conductivity during long-term operation at high temperature; 2. Regarding hydrogen permeability: The vertical CNT array of this invention is bonded to the PBI base film through an Al2O3 layer, forming a dense interface that fills the intrinsic pores of the PBI; gradient sulfonation of COF SSmaller pore size and high sulfonic acid group density COF in the layer S By repelling hydrogen molecules with negative charges, hydrogen permeation is inhibited. Simultaneously, the rigid framework of COFs can suppress membrane swelling and maintain pore size stability. Furthermore, gradient sulfonation of COFs... S The bottom layer of the layer has a larger pore size SA-TFP COF S It will preferentially adsorb hydrated protons and accelerate proton transport through size sieving. The small-pore SO3H-COF on the surface generates a negatively charged interface through high sulfonic acid group density, polarizing and repelling hydrogen molecules. The gas barrier properties of the SiO2 hydrophobic layer are superimposed to effectively ensure that the hydrogen permeation path is blocked by multiple levels. 3. Regarding interfacial bonding: The carbon nanotubes vertically arranged on the surface of the polybenzimidazole base layer of this invention can be embedded in the polybenzimidazole matrix, effectively enhancing the interfacial bonding force between the two through mechanical interlocking. The hydroxyl groups of Al2O3 form Al-OC covalent bonds with the imidazole ring nitrogen atoms of PBI and the defect sites of CNTs, respectively. At the same time, Al2O3 bridges CNTs and the overlying sulfonated COF layer, further enhancing the interfacial strength through protonation. In addition, this invention uses high-temperature tiered annealing to optimize the crystallinity of COF and the chain arrangement of PBI. Combined with the thermal expansion coefficient buffering effect of the Al2O3 layer, the interfacial peeling force of this composite film is effectively improved during thermal cycling. 4. Regarding high-temperature stability: Firstly, CNTs, acting as a rigid framework, restrict the high-temperature mobility of PBI segments, effectively inhibiting the swelling, cracking, and creep of the polybenzimidazole substrate at high temperatures. Secondly, SO3H-COF exhibits good crystal retention at high temperatures, and the pore size further shrinks after phosphoric acid doping, thus further improving the high-temperature hydrogen barrier performance and maintaining stability. Finally, the synergistic effect of phosphoric acid activation and annealing promotes the formation of Al-OC bonds between PBI and CNTs, improving the interfacial peeling force. Phosphoric acid is anchored to sulfonic acid groups through confinement. Therefore, the proton exchange membrane prepared in this invention can maintain good proton conduction, as well as excellent mechanical properties such as swelling and cracking at high temperatures, while also exhibiting excellent interfacial thermal stability. 5. This invention employs an Fe-Mo / Al2O3 catalyst system and a low-temperature PE-CVD process. On the one hand, Mo, as an additive, can reduce the carbon dissolution activation energy of Fe, promote carbon source decomposition, and lower the growth temperature of CNTs to below 350°C, thus avoiding thermal decomposition of the PBI substrate. On the other hand, Mo doping can change the surface energy of Fe particles, inducing carbon atoms to deposit along specific crystal planes and form a vertical array. The Al2O3 support provides a high specific surface area to anchor Fe-Mo nanoparticles, inhibit high-temperature agglomeration, and ensure catalyst stability. Furthermore, the lattice matching degree between the Al2O3 crystal plane and Fe-Mo can promote directional bonding at the root of CNTs, ensuring verticality. In addition, the Al2O3 intermediate layer can also block the thermal shock of the Fe-Mo catalyst to the PBI substrate film at high temperatures. Detailed Implementation

[0016] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0017] Unless otherwise specified, the materials used in the embodiments of the present invention can be obtained commercially or prepared according to conventional methods known to those skilled in the art.

[0018] Example 1 Step a: Growing vertical carbon nanotube arrays on the surface of polybenzimidazole substrate (1) An Al2O3 layer with a thickness of 100 μm was deposited on the surface of a polybenzimidazole substrate using atomic layer deposition (ALD) technology. The deposition parameters were: temperature 150°C, TMA pulse 0.3s-N2 purge 30s-H2O pulse 0.3s-N2 purge 30s, cycled 450 times to obtain a 50 nm thick Al2O3 layer. (2) Prepare 500 mL of a mixture of ethanol and deionized water at a volume ratio of 1:1. Dissolve 6.72 g of Fe(NO3)3·9H2O and 0.65 g of ammonium molybdate in the mixture of ethanol and deionized water. Add 3 g of polyethylene glycol and stir magnetically at 600 rpm for 50 min. Then add 0.1 M dilute ammonia water dropwise until the pH reaches 3.5 to obtain the Fe-Mo bimetallic catalyst solution. (3) The Fe-Mo bimetallic catalyst solution was dropped onto the surface of the Al2O3 layer. The spin coater was started, and the spin coater was first spin coated at 550 rpm for 10 s, then at 3000 rpm for 40 s. This spin coating step was repeated twice. (4) Under Ar atmosphere in a tube furnace, the temperature is increased to 250℃ at 7℃ / min, held for 50min, and then cooled to room temperature at 5℃ / min to complete the annealing treatment, forming Fe-Mo nanoparticles on the surface of the Al2O3 layer. (5) Place the substrate film loaded with Fe-Mo nanoparticles in the CVD reaction chamber and evacuate to 6×10⁻⁶. -3 Torr was used to introduce a mixture of Ar and H2 gas (volume ratio Ar:H2 = 3:1), and the temperature was raised to 280°C and held for 10 min to activate the catalyst. The plasma was then activated in pulse mode with a frequency of 40 Hz, a duty cycle of 60%, and a plasma RF power of 250 W. A mixture of C2H2 and Ar gas (volume ratio Ar:C2H2 = 3:1) was introduced, and the flow rate was adjusted to 60 sccm. The substrate temperature was controlled at 300°C, and carbon nanotube growth began. The growth time was 20 min, yielding nanotubes with a height of 10 μm and a density of 10⁻⁶. 10 root / cm 2 After the vertically arranged carbon nanotube array was grown, C2H2 and plasma were turned off, and the array was naturally cooled to room temperature at a cooling rate of 5°C / min in a pure Ar atmosphere. Step b: Form a hydrogen barrier layer of sulfonated covalent organic framework with a gradient pore size distribution on the polybenzimidazole substrate. 1. Dissolve 3 g of SA-TFP COF (sulfonic acid group density 2.0 mmol / g, pore size 1.2 nm) in 97 g of DMSO to obtain a 3 wt% SA-TFP COF DMSO solution; dissolve 5 g of SO3H-COF-300 (sulfonic acid group density 4.2 mmol / g, pore size 0.65 nm) in 95 g of DMF to obtain a 5 wt% SO3H-COF-300 DMF solution. 2. Immerse the polybenzimidazole-based film on which the carbon nanotube array is grown in a 3 wt% SA-TFP COF DMSO solution and keep it for 8 min to allow the large-pore SA-TFP COF to preferentially fill the bottom of the CNT array. 3. Remove the base film and immediately transfer it to a 5 wt% DMF solution of SO3H-COF-300. Immerse it for 15 min to induce the self-assembly of small-pore SO3H-COF-300 on the surface. 4. After impregnation, the membrane material is naturally dried for 2 hours. The COFs are driven by solvent evaporation and concentration gradient to form a continuous gradient structure of pore size from the bottom layer to the surface layer, resulting in a hydrogen barrier layer of sulfonated covalent organic framework with pore size gradient distribution. Step c: Activation treatment with phosphoric acid solution and two-stage high-temperature annealing treatment Heating an 85 wt% phosphoric acid solution to 85°C and maintaining the temperature, immersing the membrane from step b into the phosphoric acid solution for 3 hours to achieve a phosphoric acid doping amount of 220 wt%, rinsing it with deionized water until neutral, drying it with nitrogen, and then placing the membrane in a vacuum oven, first heating it to 120°C at a rate of 2°C / min and holding it for 2 hours to release internal stress, then continuing to heat it to 160°C at a rate of 3°C / min and holding it for 2 hours to enhance the chemical crosslinking density of COFs and CNTs; Step d: Surface passivation layer treatment a) Place the membrane material that has completed step c into the ALD chamber and evacuate it to 0.5 Torr; b) A SiO2 layer was deposited at 150℃ with BTBAS and O3 as precursors. The BTBAS pulse was 2.0s, and the O3 pulse was 30s, and the O3 pulse was 1s, and the O3 pulse was 30s. The SiO2 layer with a thickness of 15nm was formed by 107 cycles. c) On the surface of the SiO2 layer deposited by ALD, vaporized heptadecafluorodecyltrimethoxysilane is introduced and reacted at 120°C under N2 carrier gas for 30 min to form a fluorinated hydrophobic layer. After modification, the residue is removed by purging with N2 for 5 min, and then unreacted heptadecafluorodecyltrimethoxysilane is removed by purging with N2 for 5 min.

[0019] Example 2 Step a: Growing vertical carbon nanotube arrays on the surface of polybenzimidazole substrate (1) An Al2O3 layer was deposited on the surface of a polybenzimidazole substrate with a thickness of 80 μm using ALD technology. The deposition parameters were: temperature 130℃, TMA pulse 0.3s-N2 purge 20s-H2O pulse 0.3s-N2 purge 20s, cycled 350 times to obtain an Al2O3 layer with a thickness of 40 nm. (2) Prepare 500 mL of a mixture of ethanol and deionized water at a volume ratio of 1:1. Dissolve 2.02 g of Fe(NO3)3·9H2O and 0.18 g of ammonium molybdate in the mixture of ethanol and deionized water, then add 2.25 g of polyethylene glycol. Stir magnetically at 550 rpm for 40 min, then add 0.1 M dilute ammonia dropwise until the pH reaches 3.25 to obtain the Fe-Mo bimetallic catalyst solution. (3) The Fe-Mo bimetallic catalyst solution was dropped onto the surface of the Al2O3 layer. The spin coater was started, and the spin coater was first spin coated at 500 rpm for 8 s, then at 2500 rpm for 30 s. This spin coating step was repeated 3 times. (4) In a tube furnace under an Ar atmosphere, the temperature was increased to 230°C at 6°C / min and held for 40 min, and then decreased to room temperature at 3°C / min to form Fe-Mo nanoparticles on the surface of the Al2O3 layer. (5) Place the substrate film loaded with Fe-Mo nanoparticles in the CVD reaction chamber and evacuate to 4×10⁻⁶. -3 Torr was used to introduce a mixture of Ar and H2 gas (volume ratio Ar:H2 = 3:1), and the temperature was raised to 280°C and held for 8 min to activate the catalyst. The plasma was then activated in pulse mode with a frequency of 30 Hz, a duty cycle of 55%, and a plasma RF power of 200 W. A mixture of C2H2 and Ar gas (volume ratio Ar:C2H2 = 3:1) was introduced, and the flow rate was adjusted to 50 sccm. The substrate temperature was controlled at 280°C, and carbon nanotube growth began. The growth time was 15 min, yielding nanotubes with a height of 8 μm and a density of 10⁻⁶. 9 root / cm 2 The vertically arranged carbon nanotube array was grown, and after the C2H2 and plasma were turned off, it was naturally cooled to room temperature at a cooling rate of 2.5°C / min in a pure Ar atmosphere. Step b: Form a hydrogen barrier layer of sulfonated covalent organic framework with a gradient pore size distribution on the polybenzimidazole substrate. 1. Dissolve 2.5 g of SA-TFP COF (sulfonic acid group density 1.6 mmol / g, pore size 1.5 nm) in 97.5 g of DMSO to obtain a 2.5 wt% SA-TFP COF DMSO solution; dissolve 4.5 g of SO3H-COF-300 (sulfonic acid group density 3 mmol / g, pore size 0.8 nm) in 95.5 g of DMF to obtain a 4.5 wt% SO3H-COF-300 DMF solution. 2. Immerse the polybenzimidazole-based film on which the carbon nanotube array is grown in a 2.5 wt% SA-TFP COF DMSO solution and keep it for 6 min to allow the large-pore SA-TFP COF to preferentially fill the bottom of the CNT array. 3. Remove the base film and immediately transfer it to a 4.5 wt% SO3H-COF-300 DMF solution. Immerse it for 12 min to induce the self-assembly of small-pore SO3H-COF-300 on the surface. 4. After impregnation, the membrane material is naturally dried for 2 hours. The COFs are driven by solvent evaporation and concentration gradient to form a continuous gradient structure of pore size from the bottom layer to the surface layer, resulting in a hydrogen barrier layer of sulfonated covalent organic framework with pore size gradient distribution. Step c: Activation treatment with phosphoric acid solution and two-stage high-temperature annealing treatment Heating an 85 wt% phosphoric acid solution to 80°C and maintaining the temperature, immersing the membrane from step b into the phosphoric acid solution for 2 hours to achieve a phosphoric acid doping concentration of 200 wt%, rinsing with deionized water until neutral, drying with nitrogen, and then placing the membrane in a vacuum oven, first heating it to 110°C at a rate of 2°C / min and holding it for 1.5 hours to release internal stress, then continuing to heat it to 155°C at a rate of 3°C / min and holding it for 1.5 hours to enhance the chemical crosslinking density between COFs and CNTs; Step d: Surface passivation layer treatment a) Place the membrane material obtained from step c into the ALD chamber and evacuate it to 0.1-0.5 Torr; b) A SiO2 layer was deposited at 130℃ with BTBAS and O3 as precursors. The BTBAS pulse was 1.5s followed by a 25s purge, and the O3 pulse was 0.75s followed by a 25s purge. After 118 cycles, a SiO2 layer with a thickness of 13nm was formed. c) On the surface of the SiO2 layer deposited by ALD, vaporized heptadecafluorodecyltrimethoxysilane is introduced and reacted at 120°C under N2 carrier gas for 25 min to form a fluorinated hydrophobic layer. After modification, the residue is removed by purging with N2 for 5 min, and then unreacted heptadecafluorodecyltrimethoxysilane is removed by purging with N2 for 5 min.

[0020] Example 3 Step a: Growing vertical carbon nanotube arrays on the surface of polybenzimidazole substrate (1) An Al2O3 layer was deposited on the surface of a 50 μm thick polybenzimidazole substrate using ALD technology. The deposition parameters were: temperature 120℃, TMA pulse 0.3s-N2 purge 20s-H2O pulse 0.3s-N2 purge 20s, 500 cycles, to obtain a 30 nm thick Al2O3 layer. (2) Prepare 500 mL of a mixture of ethanol and deionized water at a volume ratio of 1:1. Dissolve 1.83 g of Fe(NO3)3·9H2O and 0.18 g of ammonium molybdate in the mixture of ethanol and deionized water, add 1.5 g of polyethylene glycol, stir magnetically at 500 rpm for 30 min, and then add 0.1 M dilute ammonia water dropwise to pH 3 to obtain the Fe-Mo bimetallic catalyst solution. (3) The Fe-Mo bimetallic catalyst solution was dropped onto the surface of the Al2O3 layer. The spin coater was started, and the spin coater was first spin coated at 400 rpm for 5 s, then at 2000 rpm for 25 s. This spin coating step was repeated twice. (4) In a tube furnace under an Ar atmosphere, the temperature is increased to 200℃ at 5℃ / min, held for 30min, and then cooled to room temperature at 2℃ / min to form Fe-Mo nanoparticles on the surface of the Al2O3 layer. (5) Place the substrate film loaded with Fe-Mo nanoparticles in the CVD reaction chamber and evacuate to 3×10⁻⁶. -3 Torr was used to introduce a mixture of Ar and H2 gas (volume ratio Ar:H2 = 3:1), and the temperature was raised to 250°C and held for 5 min to activate the catalyst. The plasma was then activated in pulse mode with a frequency of 20 Hz, a duty cycle of 50%, and a plasma RF power of 150 W. A mixture of C2H2 and Ar gas (volume ratio Ar:C2H2 = 3:1) was introduced, and the flow rate was adjusted to 40 sccm. The substrate temperature was controlled at 200°C, and carbon nanotube growth began. The growth time was 10 min, yielding nanotubes with a height of 5 μm and a density of 102. 8 root / cm 2 After the vertically arranged carbon nanotube array was grown, C2H2 and plasma were turned off, and the array was naturally cooled to room temperature at a cooling rate of 2°C / min in a pure Ar atmosphere. Step b: Form a hydrogen barrier layer of sulfonated covalent organic framework with a gradient pore size distribution on the polybenzimidazole substrate. 1. Dissolve 2 g of SA-TFP COF (sulfonic acid group density 1.2 mmol / g, pore size 1.8 nm) in 98 g of DMSO to obtain a 2 wt% SA-TFP COF DMSO solution; dissolve 4 g of SO3H-COF-316 (sulfonic acid group density 4.2 mmol / g, pore size 0.65 nm) in 96 g of DMF to obtain a 4 wt% SO3H-COF-316 DMF solution. 2. Immerse the polybenzimidazole-based film on which the carbon nanotube array is grown in a 2wt% SA-TFP COF DMSO solution and keep it for 5 min to allow the large-pore SA-TFP COF to preferentially fill the bottom of the CNT array. 3. Remove the base film and immediately transfer it to a 4 wt% SO3H-COF-316 DMF solution. Immerse it for 10 min to induce the self-assembly of small-pore SO3H-COF-316 on the surface. 4. After impregnation, the membrane material is naturally dried for 2 hours. The COFs are driven by solvent evaporation and concentration gradient to form a continuous gradient structure of pore size from the bottom layer to the surface layer, resulting in a hydrogen barrier layer of sulfonated covalent organic framework with pore size gradient distribution. Step c: Activation treatment with phosphoric acid solution and two-stage high-temperature annealing treatment Heat an 85 wt% phosphoric acid solution to 70°C and maintain the temperature. Immerse the membrane from step b in the phosphoric acid solution for 1 hour to achieve a phosphoric acid doping concentration of 180 wt%. After removal, rinse with deionized water until neutral and dry with nitrogen. Then place the membrane in a vacuum oven and heat it to 100°C at a rate of 2°C / min for 1 hour to release internal stress. Then heat it to 150°C at a rate of 3°C / min for 1 hour to enhance the chemical crosslinking density between COFs and CNTs. Step d: Surface passivation layer treatment a) Place the membrane material obtained from step c into the ALD chamber and evacuate it to 0.1 Torr; b) A SiO2 layer was deposited at 120℃ with BTBAS and O3 as precursors. The BTBAS pulse was 1.0s followed by a 20s purge, and the O3 pulse was 0.5s followed by a 20s purge. After 100 cycles, a SiO2 layer with a thickness of 10nm was obtained. c) On the surface of the SiO2 layer deposited by ALD, vaporized heptadecafluorodecyltrimethoxysilane is introduced and reacted at 120°C under N2 carrier gas for 20 min to form a fluorinated hydrophobic layer. After modification, the residue is removed by purging with N2 for 5 min, and then unreacted heptadecafluorodecyltrimethoxysilane is removed by purging with N2 for 5 min.

[0021] Comparative Example 1 The preparation steps and parameter formulation are the same as in Example 1, except that step 1 in step a is omitted, that is, the atomic layer deposition (ALD) process is not performed, and Fe-Mo catalyst is directly loaded on the surface of polybenzimidazole substrate before carbon nanotube growth.

[0022] Comparative Example 2 The preparation steps and parameter formulations of Example 1 are the same, except that step a is omitted, and a gradient COFs hydrogen barrier layer is directly prepared on the surface of the polybenzimidazole substrate, along with the remaining steps.

[0023] Comparative Example 3 The preparation steps and parameter formulation are the same as in Example 1, except that step c is omitted, that is, after completing step b, high-temperature annealing and surface passivation layer treatment are performed directly.

[0024] Comparative Example 4 The preparation steps and parameter formulation are the same as in Example 1, except that step d is changed and only one annealing treatment is performed, that is: the film material is placed in a vacuum oven and heated to 120°C at 2°C / min and held for 2 hours to release internal stress.

[0025] Comparative Example 5 The preparation steps and parameter formulation are the same as in Example 1, except that step e is omitted, that is, the process is ended after high-temperature annealing after completing step c.

[0026] The membranes of Examples 1-3 and Comparative Examples 1-5 were subjected to performance tests: Proton conductivity was tested using an electrochemical impedance spectroscopy (EIS) system at a test temperature of 120°C and a humidity of 80% RH; gas permeability was measured using a gas permeability tester; peel strength was tested using a universal testing machine; and the proton conductivity decay rate was monitored after 500 hours of operation under high temperature and constant humidity (120°C, 80% RH). The test data results are shown in Table 1.

[0027] Table 1

[0028] In Comparative Example 1, the absence of the Al2O3 layer leads to several issues. Firstly, the hydrogen barrier layer absorbs water and swells, diluting the phosphate support and hindering proton migration. Secondly, the CNT array density decreases, and its vertical orientation deteriorates, resulting in reduced proton conductivity. Furthermore, the absence of the Al2O3 layer, a dense barrier layer, also allows hydrogen to diffuse through macropores. Additionally, the direct contact between the base film and the hydrogen barrier layer causes interfacial microcracks due to differences in thermal expansion coefficients, resulting in poor peel strength. The directly supported Fe-Mo catalyst exhibits poor interfacial adhesion with the perfluorosulfonic acid resin, making CNTs prone to detachment under mechanical stress, thus weakening the interfacial bond. In the PE-CVD process, the base film without the Al2O3 layer lacks a thermal barrier, leading to the decomposition of the perfluorosulfonic acid resin when local temperatures become excessively high, significantly reducing its stability.

[0029] In Comparative Example 2, there is no vertical CNT array. Due to the lack of continuous axial channels for vertically arranged CNTs, proton transport depends on the randomly distributed CNT interface, which increases the interface resistance. Therefore, the conductivity decreases at high temperatures. Furthermore, the randomly arranged CNTs cannot form dense sub-nanometer channels, thus increasing the hydrogen permeability. Vertical CNTs enhance interfacial bonding by embedding into the resin matrix and suppress high-temperature swelling. Without vertical CNTs, COFs and the base film rely solely on hydrogen bonds or chemical bonds, increasing the risk of interfacial delamination. Furthermore, without vertical CNTs, the base film is prone to swelling or decomposition of sulfonic acid groups at high temperatures, leading to further attenuation of proton conductivity.

[0030] Comparative Example 3 was not activated with phosphoric acid solution. Due to the lack of phosphoric acid doping, i.e., the lack of modification of proton jumping sites on the COFs pores and CNTs surface, continuous proton transport channels could not be formed, resulting in extremely low proton conductivity. Phosphoric acid immobilization can suppress the water absorption and swelling of sulfonic acid groups. The unactivated membrane has an increased swelling rate when humidity changes, which leads to increased porosity.

[0031] In Comparative Example 4, insufficient annealing resulted in low crosslinking density and insufficient chemical bonding density between COFs and CNTs. The polymer chains loosened at high temperatures, and the pores were prone to collapse. The collapsed pores prevented the phosphoric acid from filling evenly, resulting in poor proton conductivity. At the same time, insufficient crosslinking also reduced the density of the hydrogen barrier layer, leading to higher hydrogen permeability. Furthermore, the structure with insufficient crosslinking was prone to creep during long-term operation, thus reducing stability.

[0032] In Comparative Example 5, the lack of a SiO2 passivation layer led to phosphoric acid hydrolysis, causing the proton carrier to fail and resulting in extremely low proton conductivity. The absence of an atomically dense SiO2 layer to seal surface defects significantly increased hydrogen permeability. Without a SiO2 layer, the base film was in direct contact with the hydrogen barrier layer.

[0033] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A method for preparing a proton exchange membrane, characterized in that, Includes the following steps: Step 1: An Al2O3 layer is formed on the surface of a polybenzimidazole substrate using atomic layer deposition (ALD). Then, a Fe-Mo bimetallic catalyst solution is spin-coated onto the surface of the Al2O3 layer. After annealing at 200-250℃, Fe-Mo nanoparticles are formed on the surface of the Al2O3 layer. Subsequently, using C2H6 as the carbon source, a vertically aligned array of carbon nanotubes is grown on the Fe-Mo nanoparticles using PECVD. Step 2: The membrane material obtained in Step 1 is sequentially immersed in Solution A and Solution B and then dried; Solution A is a dimethyl sulfoxide solution of SA-TFP COF, and Solution B is an N,N-dimethylformamide solution of SO3H-COF-300 or an N,N-dimethylformamide solution of SO3H-COF-316. Step 3: The membrane material obtained in Step 2 is subjected to phosphoric acid solution activation treatment and two-stage high-temperature annealing treatment in sequence; Step 4: Prepare a SiO2 layer on the surface of the film material obtained in Step 3 using atomic layer deposition technology, and then perform hydrophobic modification treatment on the SiO2 layer; The Fe to Mo molar ratio in the Fe-Mo bimetallic catalyst solution is (4-5):(0.5-1).

2. The preparation method according to claim 1, characterized in that: In step 1, the parameters for depositing an Al2O3 layer on the surface of a polybenzimidazole substrate using atomic layer deposition technology include: deposition temperature of 120-150℃, TMA pulse time of 0.1-0.3s, H2O pulse time of 0.1-0.3s, purging with N2 for 15-30s / cycle, and the thickness of the Al2O3 layer of 30-50nm.

3. The preparation method according to claim 1, characterized in that: In step 1, the Fe-Mo bimetallic catalyst solution is prepared as follows: Fe(NO3)3·9H2O and ammonium molybdate are dissolved in a solvent at a Fe to Mo molar ratio of (4-5):(0.5-1). Then, polyethylene glycol is added, and the mixture is magnetically stirred for 30-50 minutes at a speed of 500-600 rpm to form a homogeneous solution. The concentration of polyethylene glycol in the homogeneous solution is 0.3-0.6 wt%. Finally, the pH of the solution is adjusted to 3-3.5 with dilute ammonia. The solvent is a 1:1 volume ratio mixture of ethanol and deionized water. The spin coating is a two-stage spin coating: the first stage spin coating speed is 400-600 rpm and the time is 5-10s, and the second stage spin coating speed is 2000-3000 rpm and the time is 25-40s.

4. The preparation method according to claim 1, characterized in that: Step 1, the steps of growing a vertically aligned carbon nanotube array using the PECVD process include: 1) Chamber pretreatment: Place the membrane in the CVD reaction chamber and evacuate to 3×10⁻⁶. -3 -6×10 -3 Torr; 2) Reduction activation: Introduce a mixture of Ar and H2 gas with a volume ratio of Ar:H2=3:1, heat to 250-280°C, and maintain for 5-10 min to activate the catalyst; 3) Carbon nanotube growth: Apply radio frequency power in pulse mode at a frequency of 20-40 Hz and a duty cycle of 50%-60%. The plasma radio frequency power is 150-250 W. Introduce a mixture of C2H2 and Ar gas with a volume ratio of Ar:C2H2=3:1 and adjust the flow rate to 40-60 sccm. Control the substrate temperature to 250-300°C to start carbon nanotube growth. The carbon nanotube growth time is 10-20 min to obtain a vertical array with a height of 5-10 μm. 4) Cooling: Turn off C2H2 and plasma, and cool to room temperature at a rate of ≤5°C / min in a pure Ar atmosphere.

5. The preparation method according to claim 1, characterized in that: In step 2, the concentration of the DMSO solution for SA-TFP COF is 2-3 wt%, and the impregnation time is 5-8 min; the concentration of the DMF solution for SO3H-COF-300 or SO3H-COF-316 is 4-5 wt%, and the impregnation time is 10-15 min. The SA-TFP COF has a pore size of 1.2-1.8 nm and a sulfonic acid group density of 1.2-2.0 mmol / g; the SO3H-COF-300 and SO3H-COF-316 have a pore size of 0.65-1.0 nm and a sulfonic acid group density of 2.8-4.2 mmol / g.

6. The preparation method according to claim 1, characterized in that: In step 3, the phosphoric acid solution activation treatment is as follows: the membrane material that has completed step 2 is immersed in an 85wt% phosphoric acid solution at 70-85℃ for 1-3 hours to make the phosphoric acid doping amount 180-220wt%.

7. The preparation method according to claim 1, characterized in that: In step 3, the two-stage high-temperature annealing process is as follows: the first stage is treated at 100-120℃ for 1-2 hours, and the second stage is treated at 150-160℃ for 1-2 hours.

8. The preparation method according to claim 1, characterized in that: The specific steps of step 4 are as follows: a) Place the membrane material that has completed step 3 into the ALD chamber and evacuate it to 0.1-0.5 Torr; b) A SiO2 layer is deposited at 120-150℃, with BTBAS and O3 as precursors, wherein: BTBAS pulse for 1.0-2.0s, followed by 20-30s purging, and O3 pulse for 0.5-1s, followed by 20-30s purging; the thickness of the SiO2 layer is 10-15nm. c) On the surface of the SiO2 layer deposited by ALD, vaporized heptadecafluorodecyltrimethoxysilane is introduced and reacted at 120°C under N2 carrier gas for 20-30 min to form a fluorinated hydrophobic layer. After modification, the residue is removed by purging with N2 for 5 min, and then unreacted heptadecafluorodecyltrimethoxysilane is removed by purging with N2 for 5 min.

9. A proton exchange membrane prepared by the method according to any one of claims 1-8, characterized in that: From bottom to top, the layers are: a polybenzimidazole base layer, an Al2O3 layer, a carbon nanotube array grown vertically on the surface, a sulfonated covalent organic framework hydrogen barrier layer with a gradient pore size distribution, and a hydrophobically modified SiO2 surface passivation layer. Phosphoric acid molecules are immobilized in the covalent organic framework channels of the sulfonated covalent organic framework hydrogen barrier layer and on the surface of the carbon nanotubes.

10. The proton exchange membrane according to claim 9, characterized in that: The thickness of the polybenzimidazole base layer is 50-100 μm; the thickness of the Al2O3 layer is 30-50 nm; The carbon nanotube array has a tube height of 5-10 μm and an array density of 10. 8 -10 10 root / cm 2 ; The thickness of the hydrophobic modified SiO2 surface passivation layer is 10-15 nm.

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