A proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets and its preparation method

Abstract

The invention discloses a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets and a preparation method thereof. The invention comprises the following steps: mixing a heat-treated MMT sample with ultrapure water, stirring at high speed until the heat-treated MMT sample is fully rehydrated, and ultrasonically treating the mixture to obtain an MMT nanosheet dispersion having uniform diameters; adding DA powder and fully stirring the mixture to obtain a DA and MMT dispersion system; adjusting the pH value and fully stirring the mixture to obtain a dark brown PDA-MMT nanosheet dispersion; vacuum filtering the dark brown PDA-MMT nanosheet dispersion, rinsing the mixture with additional ultrapure water, and naturally drying the mixture to obtain a PDA-MMT film; screening to obtain an xDA-MMT film having stable water phase; characterizing the xDA-MMT film having stable water phase and testing its surface proton conductivity, screening the xDA-MMT film having the best proton conductivity under water-stable conditions, and preparing a VAPMM proton exchange membrane having uniform thickness by polymer encapsulation and precise cutting in a direction perpendicular to the nanosheets. The VAPMM film prepared by the present invention has lower cost than a commercial Nafion film, is environmentally friendly, and has good water phase stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and in particular relates to a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets and a preparation method thereof. Background Art

[0002] As a "hydrogen-to-electricity" conversion device on the "hydrogen-using" side, the independent research and development of the key components of the proton exchange membrane fuel cell (PEMFC) (including bipolar plates, gas diffusion layers, catalysts and proton exchange membranes) is a fundamental solution to breaking through the patent restrictions of hydrogen energy in Europe, the United States, Japan and other countries.

[0003] As the core component of PEMFCs, the proton exchange membrane (PEM) must possess high proton conductivity, low fuel permeability, and excellent thermal and chemical stability. Currently, commercial PEM systems primarily utilize perfluorosulfonic acid (PFSA) polymer structures, exemplified by Nafion, manufactured by Dupont Corporation in the United States. Its polymer backbone is polytetrafluoroethylene (PTFE). Through a fluorine chemical synthesis process, hydrophilic sulfonic acid groups are grafted onto the PTFE backbone as side chains, resulting in Nafion's excellent proton conductivity, achieving a proton conductivity of ~0.1 S / cm at room temperature.

[0004] However, for the PEMs mentioned above, which are more commonly used, although increasing the number of sulfonic acid groups / high sulfonation in the polymer's internal structure can improve the membrane's proton conductivity, it will inevitably cause the PEM to absorb too much water, thereby triggering the gradual separation of the hydrophilic and hydrophobic phases within the PEM, reducing the membrane's dimensional stability and mechanical properties, and ultimately causing degradation of the membrane structure. At the same time, due to the monopoly of commercial PEMs and the involvement of expensive and highly toxic perfluorinated preparation technology, the fields that currently require the use of PEMs are currently facing high costs. Faced with such difficulties, the key to achieving further widespread application of PEMFCs is to develop low-cost, water-stable PEMs with high proton conductivity.

[0005] The microstructured, two-dimensional (MMT) clay mineral is a typical, abundant, low-cost, chemically and thermally stable, and environmentally friendly clay mineral. It is composed of two silicon-oxygen tetrahedral sheets and one aluminum-oxygen octahedral sheet. Frequent isomorphous substitution endows MMT with excellent adsorption, cation exchange, and swelling properties. Previously, MMT was used as just one of several inorganic nanofillers in organic PEMs for proton conduction. However, exfoliated MMT nanosheets can be readily reassembled into stacked thin films by vacuum filtration, resulting in continuous interstitial spaces with tunable interlayer spacing. Furthermore, MMT possesses excellent water-retention and acid-binding properties, suggesting its potential as a next-generation PEM using inorganic structures as proton transport media. Self-assembled layered membranes from exfoliated vermiculite nanosheets have been reported to possess the ability to construct 2D nanofluidic proton channels with excellent proton conductivity. However, the use of 2D nanochannel arrays formed by MMT nanosheets as a primary proton transport medium has not received much attention. This is likely due to the excessive swelling rate of intrinsic MMT membranes in aqueous environments, which inevitably leads to a decrease in aqueous stability and loss of molecular selectivity. If the interlayer spacing of layered MMT films can be limited to a certain extent through molecular intercalation or pillaring, a novel membrane structure using 2D MMT nanochannel arrays for rapid proton conduction can be realized.

[0006] Previous studies have reported that introducing certain rigid aromatic groups into PEM can bring about aqueous phase stability. Inspired by the adhesive proteins secreted by mussels, Lee et al. first discovered that polydopamine (PDA) with good hydrophilicity formed by the self-polymerization of dopamine (DA) can adhere to almost all types of material surfaces, including SiO2 crystals, Al2O3, polycarbonate (PC), polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS) and polyetheretherketone (PEEK), regardless of their hydrophilicity / hydrophobicity. Due to this unique property, PDA is used in many scenarios, such as hydrogels, metal deposition and energy storage devices. Yoon et al. used PDA as an interfacial glue to achieve strong adhesion between PTFE and PFSA polymers, thereby preparing a highly durable PEM. Therefore, the introduction of PDA is conducive to achieving good dimensional stability of the modified object. Therefore, the present invention uses PDA to modify MMT to prepare a PDA-MMT film with good surface proton conductivity and good aqueous phase stability.

[0007] On the other hand, it is important to consider that the sub-nanoscale channels with high proton conductivity generated by the PDA-MMT membrane prepared by vacuum filtration are perpendicular to the membrane surface, which greatly increases the tortuosity and total length of the ion / molecule transport path, thereby limiting the conduction of ions / polymers. To fully utilize the high along-plane proton conductivity of the PDA-MMT film, the horizontal nanochannels must be arranged in the vertical direction without losing the structural integrity of the membrane. Zhang et al. prepared long-range vertically aligned graphene sheet membranes using antifreeze-assisted freezing technology. Wong's team created a hierarchical graphene surface structure through extreme mechanical deformation of specific thermoplastic materials. In 2017, Abraham et al. reported a strategy using manually stacked multiple micron slits to achieve a new device with millimeter-scale diffusion path length for nanofluid transport.

[0008] In recent years, research on PEMs has continued unabated. In the area of ​​PFSA-based PEMs, researchers have achieved improvements in specific PEM properties through chemical modification, polymer blending, and nanodoping. Beyond fluorine chemistry, sulfonated hydrocarbon polymers have become an important tool for achieving high-proton-conducting PEMs, and related research has provided multiple options for the continued development of PEMs. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to address the deficiencies in the above-mentioned prior art and provide a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets and a preparation method thereof, so as to solve the technical problems of foreign monopoly of proton exchange membranes, high costs, and poor membrane stability under fluorine-containing and high humidity conditions.

[0010] The present invention adopts the following technical solutions:

[0011] A method for preparing a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets comprises the following steps:

[0012] S1, heat treating MMT powder to obtain MMT sample;

[0013] S2, mixing the MMT sample heat-treated in step S1 with ultrapure water, stirring at high speed until the heat-treated MMT sample is fully rehydrated, and ultrasonically treating to obtain a dispersion of MMT nanosheets with uniform size;

[0014] S3, adding DA powder to the MMT nanosheet dispersion prepared in step S2 and stirring thoroughly to obtain a DA and MMT dispersion system;

[0015] S4, adjusting the pH value of the DA and MMT dispersion system, and stirring thoroughly to obtain a dark brown PDA-MMT nanosheet dispersion;

[0016] S5. Vacuum filter the dark brown PDA-MMT nanosheet dispersion obtained in step S4, rinse with additional ultrapure water, and air dry to obtain a PDA-MMT film;

[0017] S6. Performing a water phase stability test on the PDA-MMT film obtained in step S5 to screen out a water phase stable xDA-MMT film;

[0018] S7. The water-stable xDA-MMT film obtained in step S6 is characterized and its along-surface proton conductivity is tested. The xDA-MMT film with the best proton conductivity under water-stable conditions is selected, and a VAPMM proton exchange membrane with uniform thickness is obtained by polymer encapsulation and precise cutting in a direction perpendicular to the nanosheets.

[0019] Preferably, step S1 is specifically:

[0020] The heating rate was set to 10°C / min, and the MMT powder raw material was heated at 150±5°C for 5-10 minutes and then naturally cooled to perform a partial dehydration treatment.

[0021] Preferably, in step S2, the mass volume ratio of the MMT sample to ultrapure water is (0.029-0.031 g):300 mL, and the concentration of the MMT nanosheet dispersion is 0.97-1.03×10 -1 g / L, the stirring time is 6 to 8 hours; the ultrasonic treatment time is 30 to 35 minutes, and the ultrasonic treatment power is 60 to 80W.

[0022] Preferably, in step S3, the mass ratio of DA powder to MMT is (0.6-1):1.

[0023] Preferably, the stirring time is 4 to 5 hours.

[0024] Preferably, in step S4, the pH value of the DA and MMT dispersion liquid is 8.5-9, and the sufficient stirring time is 2-2.5 hours.

[0025] Preferably, in step S5, the vacuum filtration time is 14 to 16 hours.

[0026] Preferably, in step S6, x is used as the relative mass ratio of DA to MMT, and PDA-MMT films with different mass ratios are named xDA-MMT films, and water-phase stable xDA-MMT films are screened.

[0027] Preferably, in step S7, the xDA-MMT thin films obtained by screening are cut and stacked, encapsulated with epoxy resin, and the encapsulated blocks are cut to obtain VAPMM proton exchange membranes with uniform thickness.

[0028] Another technical solution of the present invention is a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets.

[0029] Compared with the prior art, the present invention has at least the following beneficial effects:

[0030] A method for preparing a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets. A PDA-MMT membrane structure with good aqueous phase stability and high proton conductivity is developed based on low-cost and environmentally friendly two-dimensional (2D) montmorillonite (MMT) materials. Based on this membrane structure, a new proton exchange membrane VAPMM with high power density and high durability is invented through steps such as vertical arrangement stacking, epoxy support packaging, and precision instrument cutting, which greatly reduces the production cost of PEM.

[0031] Furthermore, heating the MMT powder to a specific temperature can achieve a "starvation treatment" effect, partially dehydrating the MMT powder, thereby enhancing its own hydrophilicity and facilitating its subsequent full rehydration.

[0032] Furthermore, for 10 -1 High-speed stirring of MMT dispersion near the concentration of 0.5 g / L for 6 to 8 hours is conducive to the full dispersion and rehydration process of MMT, and the expansion of the MMT interlayer spacing. Assisted by 30 to 35 minutes of low-power weak ultrasonic action of 60 to 80 W, the MMT layered structure can be promoted to maintain the original aspect ratio while peeling off, thereby obtaining uniformly dispersed MMT nanosheets.

[0033] Furthermore, DA is easily soluble in water and has an adhesive effect. After sufficient stirring, DA can be fully adsorbed on the surface of MMT nanosheets, providing the prerequisite for their subsequent surface self-polymerization into PDA. Experiments have shown that a DA&MMT mass ratio of 0.6:1 to 1:1 can directly lead to the subsequent PDA-MMT film having good aqueous phase stability.

[0034] Furthermore, the weakly alkaline pH environment can promote the self-polymerization of DA adsorbed on the surface of MMT nanosheets into PDA. After 2 to 2.5 hours of stirring self-polymerization process, DA can be fully converted into PDA and partially coated on the surface of MMT nanosheets to form PDA-MMT nanosheets with good adhesion effect, providing a good premise for further PDA-MMT films.

[0035] Furthermore, the vacuum filtration process can make the PDA-MMT single layer / several layers of nanosheets completely stacked in parallel, realizing the transformation of powdered MMT particles into PDA-MMT films with good aqueous phase stability. Additional ultrapure water can wash away excess small molecules and ions in the PDA-MMT film, obtaining a PDA-MMT film with higher purity.

[0036] Furthermore, since the proton exchange membrane is used in a high-humidity environment and liquid water is often produced, it is necessary to ensure the good water phase stability of the proton exchange membrane. Therefore, it is necessary to conduct a water phase stability test on the PDA-MMT film and screen out the xDA-MMT film with good water phase stability, so as to provide a stable core functional material for the invented new proton exchange membrane.

[0037] Furthermore, in step S7, while ensuring good aqueous phase stability of the proton exchange membrane, more attention should be paid to the key performance indicator of the proton exchange membrane - proton conductivity. Screening out the xDA-MMT film with the highest proton conductivity under aqueous phase stability conditions can achieve the optimal battery performance of the new proton membrane invented subsequently.

[0038] Furthermore, in step S8, the screened xDA-MMT films are uniformly cut, which can facilitate the calculation of the active area of ​​the film during application. Stacking and encapsulating with epoxy resin can fix the required proton conduction direction to the surface direction of the xDA-MMT film, maximize the excellent proton conduction performance along the membrane of the xDA-MMT film, reduce the tortuosity of proton transmission, and improve the performance of the proton exchange membrane. At the same time, the good thermochemical stability of epoxy resin can also provide good support for the stacked xDA-MMT films.

[0039] A high proton conductivity membrane based on partially coated montmorillonite with polydopamine has achieved a 0.58Scm -1 The surface proton conductivity is higher than that of Nafion film with the same thickness (0.16S cm -1 The membrane electrode (MEA) assembled with the VAPMM proton exchange membrane invented based on the PDA-MMT film has a maximum power density of 480.60 mW cm at 75°C and 100% RH. -2 , is the same thickness Nafion film (116.17mW cm -2 ) is 4.14 times; at the same time, the VAPMM membrane assembled MEA of the present invention can achieve a stable output current of at least 120h.

[0040] In summary, the present invention adopts cheap and environmentally friendly MMT as the raw material for preparing the new proton exchange membrane VAPMM. The resulting VAPMM film has a lower cost than the commercial Nafion film. Since the entire preparation process does not involve the introduction of Nafion and fluorine, it has good environmental friendliness. At the same time, the membrane has good aqueous phase stability. This new idea has completely opened up a new proton exchange membrane development idea with inorganic MMT film with high proton conductivity along the membrane as the core proton conduction material. This strategy can be simultaneously applied to a variety of new nanomaterials with good proton conductivity along the membrane, and contributes to breaking the situation where proton exchange membrane-related technologies are monopolized by Europe and the United States.

[0041] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Flow chart for VAPMM film preparation;

[0043] Figure 2 This is the experimental diagram of the aqueous phase stability of PDA-MMT films with different DA to MMT mass ratios;

[0044] Figure 3 The data of the proton conductivity performance of xDA-MMT film are shown in Figure 2, where (a) is the ToF-SIMS analysis of the Na content of xDA-MMT film before and after proton exchange. + / Si + Relative intensity change; (b) 0.6DA-MMT rectangular film samples of different sizes used for packaging testing; (c) Schematic diagram of the test platform for proton conductivity along the surface of xDA-MMT film; (d) IV curve of 0.6DA-MMT film after stabilization under different acid electrolyte concentrations; (e) Proton conductivity of xDA-MMT film after stabilization under different acid electrolyte concentrations; (f) Proton conductivity durability of xDA-MMT film and Nafion NR211 film;

[0045] Figure 4 Figure 3 is a graph showing the proton conductivity performance of xDA-MMT films, where (a) is the Zeta potential distribution of xDA-MMT nanosheets; (b) is the 0.8DA-MMT rectangular film samples of different sizes used for packaging tests; (c) is the 1DA-MMT rectangular film samples of different sizes used for packaging tests; (d) is the IV curve of the 0.8DA-MMT film after stabilization under different acid electrolyte concentrations; (e) is the IV curve of the 1DA-MMT film after stabilization under different acid electrolyte concentrations; (f) is the IV curve obtained by the drift-diffusion experiment with asymmetric acid electrolyte solutions loaded on both sides of the xDA-MMT film (the inset is a schematic diagram of the drift-diffusion experiment principle);

[0046] Figure 5 The solidification map was generated for the reaction of E51 and DA;

[0047] Figure 6 The MEA morphology and active area calculation diagram based on VAPMM membrane, where (a) is the MEA morphology based on VAPMM membrane, and (b) is the active area calculation diagram of VAPMM membrane;

[0048] Figure 7 Figures 2 and 3 show the performance correlation of MEA cells based on VAPMM membranes, where (a) shows the performance of MEA cells based on VAPMM membranes at 30°C to 75°C under 100% RH, and (b) shows the performance of MEA cells based on Nafion N117. (c) shows the performance of cycle tests after the VAPMM membranes have adapted to the internal environment of the battery. (d) shows the peak power density of MEA cells based on VAPMM membranes after 20 consecutive cycle tests. (e) shows the durability test of MEA cells based on VAPMM membranes under 75°C and 100% RH conditions. DETAILED DESCRIPTION

[0049] The technical solution of the present invention will be described clearly and completely below. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0050] In the present invention, unless otherwise specified, all the embodiments and preferred implementation methods mentioned herein can be combined with each other to form a new technical solution.

[0051] In the present invention, unless otherwise specified, all technical features and preferred features mentioned herein can be combined with each other to form a new technical solution.

[0052] In the present invention, unless otherwise specified, percentages (%) or parts refer to percentages by weight or parts by weight relative to the composition.

[0053] In the present invention, unless otherwise specified, the components involved or their preferred components can be combined with each other to form a new technical solution.

[0054] In this disclosure, unless otherwise specified, the numerical range "a-b" is an abbreviation for any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "6-22" indicates that all real numbers between "6-22" are listed herein, and "6-22" is merely an abbreviation for these numerical combinations.

[0055] The "range" disclosed in the present invention is in the form of lower limit and upper limit, which can be one or more lower limits, and one or more upper limits respectively.

[0056] In the present invention, the term "and / or" used herein refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0057] In the present invention, unless otherwise specified, each reaction or operation step can be carried out sequentially or in accordance with the order. Preferably, the reaction method herein is carried out sequentially.

[0058] Unless otherwise indicated, the professional and scientific terms used herein are the same as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to the described content can also be applied to the present invention.

[0059] The present invention provides a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets and a preparation method thereof. By stacking PDA-MMT films, vertically encapsulating them with epoxy, and cutting them with precision instruments, a novel proton exchange membrane VAPMM is prepared, which fully utilizes the high along-plane proton conductivity of the PDA-MMT films.

[0060] See also Figure 1 The present invention provides a method for preparing a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets, comprising the following steps:

[0061] S1. Place a small amount of MMT powder in a clean crucible and transfer it to a muffle furnace. Set the heating rate to 10°C / min. Heat the sample at 150±5°C for 5-10 minutes and then cool it naturally to partially dehydrate it.

[0062] S2, the heat-treated MMT sample was mixed with ultrapure water in a certain ratio (0.029 g: 300 mL to 0.031 g: 300 mL, and the concentration of the MMT dispersion was 0.97 to 1.03 × 10 -1 g / L) were mixed, stirred at high speed at room temperature for 6 to 8 hours to ensure that the heat-treated MMT sample was fully rehydrated, and then ultrasonicated at 60 to 80 W ultrasonic power for 30 to 35 minutes to promote the full exfoliation of MMT, thereby obtaining a dispersion of MMT nanosheets with uniform size;

[0063] S3, taking a certain mass ratio of DA powder to MMT and adding it to the prepared MMT nanosheet dispersion at a mass ratio of DA powder to MMT of (0.6-1):1, and stirring thoroughly for 4-5 hours to achieve adsorption of DA on the surface of MMT nanosheets;

[0064] S4, adding 20 to 25 drops of Tris-HCl buffer to the DA and MMT dispersion system, adjusting the pH of the dispersion to 8.5 to 9, and stirring for 2 to 2.5 hours to achieve self-polymerization of DA on the surface of the MMT nanosheets, so that PDA is partially coated on the surface of the MMT nanosheets to obtain PDA-MMT nanosheets;

[0065] S5. The PDA-MMT nanosheet dispersion that has turned dark brown is vacuum filtered for 14 to 16 hours to promote the self-assembly film formation process of the PDA-MMT nanosheets, and ultrapure water is used to clean the excess ions and molecules in the wet film. After natural drying, the PDA-MMT film is obtained.

[0066] S6. Test the water phase stability of the PDA-MMT film, using x as the relative mass ratio of DA to MMT, and name the PDA-MMT films with different mass ratios as xDA-MMT films, and screen out the water-phase stable xDA-MMT films;

[0067] S7. Characterize the water-stable xDA-MMT film and test its surface proton conductivity, screening the xDA-MMT film with the best proton conductivity under water-stable conditions; cut and stack the screened xDA-MMT films, and encapsulate them with epoxy resin. During encapsulation, vacuum the encapsulated block multiple times to ensure that there are no bubbles in the encapsulated block, and cut the encapsulated block with a precision wire cutting machine to obtain a VAPMM proton exchange membrane with uniform thickness;

[0068] S9. Treat the defect-free VAPMM membrane and perform membrane electrode (MEA) hot pressing at 160°C, 4 MPa, 5 min, and perform fuel cell single cell performance testing.

[0069] A proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets prepared by the method of the present invention can achieve a power output of 400-480.60 mW cm at 75°C and 100% RH after being installed in a battery. -2 The peak power density is much higher than that of commercial Nafion films of the same thickness.

[0070] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0071] Example 1

[0072] S1. Place a small amount of MMT powder in a clean crucible and transfer it to a muffle furnace. Set the heating rate to 10°C / min, heat the sample at 150°C for 5 minutes, and then cool it naturally to partially dehydrate it.

[0073] S2, the heat-treated MMT sample was mixed with ultrapure water in a certain ratio (0.03 g: 300 mL, the concentration of MMT dispersion was 1 × 10 -1 g / L) were mixed and stirred at high speed at room temperature for 6 h to ensure that the heat-treated MMT sample was fully rehydrated, and then ultrasonicated at 60 W ultrasonic power for 30 min to promote the full exfoliation of MMT, thereby obtaining a dispersion of MMT nanosheets with uniform size;

[0074] S3, adding DA powder with a mass ratio of 0.6:1 to MMT to the prepared MMT nanosheet dispersion, and stirring thoroughly for 4 h to achieve adsorption of DA on the surface of MMT nanosheets;

[0075] S4, adding 20 drops of Tris-HCl buffer to the DA and MMT dispersion system, adjusting the pH value of the dispersion to 8.5, and stirring thoroughly for 2 hours to achieve self-polymerization of DA on the surface of the MMT nanosheets, so that PDA is partially coated on the surface of the MMT nanosheets to obtain PDA-MMT nanosheets;

[0076] S5. The PDA-MMT nanosheet dispersion that has turned dark brown is vacuum filtered for 15 hours to promote the self-assembly film formation process of the PDA-MMT nanosheets, and ultrapure water is used to clean the excess ions and molecules in the wet film. After natural drying, the PDA-MMT film is obtained.

[0077] S6. The 0.6DA-MMT film (PDA-MMT film is renamed as 0.6DA-MMT film here with x as the relative mass ratio of DA to MMT) was tested for its water phase stability. The experiment showed that the 0.6DA-MMT film was water phase stable.

[0078] S7. The 0.6DA-MMT film was characterized and its surface proton conductivity was tested. It was found that its average surface proton conductivity was 0.58S cm -1 ;

[0079] S8. Cut and stack the 0.6DA-MMT film, and encapsulate it with epoxy resin. During the encapsulation, vacuum is drawn multiple times to ensure that there are no bubbles in the encapsulated block. The encapsulated block is cut with a precision wire cutting machine to obtain a VAPMM proton exchange membrane with uniform thickness based on the 0.6DA-MMT film.

[0080] Example 2

[0081] S1. Place a small amount of MMT powder in a clean crucible and transfer it to a muffle furnace. Set the heating rate to 10°C / min. Heat the sample at 155°C for 7 minutes and then cool it naturally to partially dehydrate it.

[0082] S2, the heat-treated MMT sample was mixed with ultrapure water in a certain ratio (0.031 g: 300 mL, the concentration of MMT dispersion was 1.03 × 10 -1 g / L) were mixed and stirred at high speed at room temperature for 8 h to ensure that the heat-treated MMT sample was fully rehydrated, and then ultrasonicated at 80 W ultrasonic power for 35 min to promote the full exfoliation of MMT, thereby obtaining a dispersion of MMT nanosheets with uniform size;

[0083] S3, adding DA powder with a mass ratio of 1:1 to MMT powder into the prepared MMT nanosheet dispersion, and stirring thoroughly for 5 h to achieve adsorption of DA on the surface of MMT nanosheets;

[0084] S4, adding 25 drops of Tris-HCl buffer to the DA and MMT dispersion system, adjusting the pH value of the dispersion to 9, and stirring thoroughly for 2.5 hours to achieve self-polymerization of DA on the surface of the MMT nanosheets, so that PDA is partially coated on the surface of the MMT nanosheets to obtain PDA-MMT nanosheets;

[0085] S5. The PDA-MMT nanosheet dispersion that has turned dark brown is vacuum filtered for 16 hours to promote the self-assembly film formation process of the PDA-MMT nanosheets, and ultrapure water is used to clean the excess ions and molecules in the wet film. After natural drying, the PDA-MMT film is obtained.

[0086] S6. Perform an aqueous phase stability test on the 1DA-MMT film (using x as the relative mass ratio of DA to MMT, the PDA-MMT film is renamed and referred to as 1DA-MMT film here). If the film is stable, proceed to the next step.

[0087] S7. Characterize the 1DA-MMT film and test its surface proton conduction performance;

[0088] S8. Cut and stack the 1DA-MMT film, and encapsulate it with epoxy resin. During the encapsulation, vacuum is drawn multiple times to ensure that there are no bubbles in the encapsulated block. The encapsulated block is cut with a precision wire cutting machine to obtain a VAPMM proton exchange membrane with uniform thickness based on the 1DA-MMT film.

[0089] Example 3

[0090] S1. Place a small amount of MMT powder in a clean crucible and transfer it to a muffle furnace. Set the heating rate to 10°C / min. Heat the sample at 145°C for 10 minutes and then cool it naturally to partially dehydrate it.

[0091] S2, the heat-treated MMT sample was mixed with ultrapure water in a certain ratio (0.029 g: 300 mL, the concentration of MMT dispersion was 0.97 × 10 -1 g / L) were mixed and stirred at high speed at room temperature for 7 h to ensure that the heat-treated MMT sample was fully rehydrated, and then ultrasonicated at 70 W ultrasonic power for 32 min to fully exfoliate the MMT, thereby obtaining a dispersion of MMT nanosheets with uniform size;

[0092] S3, adding DA powder with a mass ratio of 0.8:1 to MMT powder into the prepared MMT nanosheet dispersion, and stirring thoroughly for 4.5 h to achieve adsorption of DA on the surface of MMT nanosheets;

[0093] S4, adding 23 drops of Tris-HCl buffer to the DA and MMT dispersion system, adjusting the pH value of the dispersion to 8.8, and stirring thoroughly for 2.1 hours to achieve self-polymerization of DA on the surface of the MMT nanosheets, so that PDA is partially coated on the surface of the MMT nanosheets to obtain PDA-MMT nanosheets;

[0094] S5. The PDA-MMT nanosheet dispersion that has turned dark brown is vacuum filtered for 14 hours to promote the self-assembly film formation process of the PDA-MMT nanosheets, and ultrapure water is used to clean the excess ions and molecules in the wet film. After natural drying, the PDA-MMT film is obtained.

[0095] S6. Perform an aqueous phase stability test on the 0.8DA-MMT film (using x as the relative mass ratio of DA to MMT, the PDA-MMT film is renamed and here is 0.8DA-MMT film). If it is stable, proceed to the next step;

[0096] S7. Characterize the 0.8DA-MMT film and test its surface proton conduction performance;

[0097] S8. Cut and stack the 0.8DA-MMT film, and encapsulate it with epoxy resin. During the encapsulation, vacuum is drawn multiple times to ensure that there are no bubbles in the encapsulated block. The encapsulated block is cut with a precision wire cutting machine to obtain a VAPMM proton exchange membrane with uniform thickness based on the 0.8DA-MMT film.

[0098] Preliminary experiments have shown that the aqueous phase stability of PDA-MMT films is related to the mass ratio of DA to MMT. Therefore, the experiment first prepared black paper-like PDA-MMT films with different mass ratios of DA to MMT. For easy distinction, the PDA-MMT films are labeled as xDA-MMT, where x represents the mass ratio of DA to MMT (0.1, 0.2, 0.4, 0.6, 0.8, 1). Figure 2 As shown, a small rectangular piece was cut from each xDA-MMT film and subjected to an aqueous stability test (i.e., immersion in water for 24 hours followed by a "lift-and-drop" operation with tweezers). The results showed that the 0.1DA-MMT and 0.2DA-MMT films disintegrated in water without any external force after immersion for 24 hours. The 0.4DA-MMT film, while only slightly dissolved after 24 hours of water treatment, quickly disintegrated upon the "lift-and-drop" operation. Only xDA-MMT films with an x ​​value greater than or equal to 0.6 maintained their dimensional stability and mechanical integrity under this rigorous testing procedure. Furthermore, the 0.6DA-MMT, 0.8DA-MMT, and 1DA-MMT films remained intact even after immersion in water for an additional 14 days. Therefore, incorporating a certain amount of PDA into MMT membranes can effectively improve their aqueous stability.

[0099] Zeta potential often provides important information about the stability of colloidal systems. Figure 4 As shown in (a), during the experimental study of the present invention, the zeta potential distributions of an MMT nanosheet dispersion and an xDA-MMT nanosheet dispersion adjusted to pH 8.5 for 2 hours were measured. The zeta potential peak of the pristine MMT nanosheets is located at -52.2 mV, reflecting the inherent electronegativity of MMT. Furthermore, the absolute value of the zeta potential is greater than 30 mV, indicating that MMT can be stably present in the aqueous dispersion. For the xDA-MMT nanosheet dispersion, the zeta potential fluctuates around -41 mV, indicating that the dispersion is also stable. Furthermore, the decrease in the absolute value of the zeta potential (sliding plane potential) of the xDA-MMT nanosheet dispersion compared to the native MMT nanosheet dispersion further demonstrates that PDA can be stably coated on the surface of the MMT nanosheets. Furthermore, the negative zeta potential of the xDA-MMT nanosheet dispersion indicates that the xDA-MMT nanosheets can still attract various cations, including protons.

[0100] like Figure 3 As shown in (a), Na + / Si + The relative intensity of the xDA-MMT films was recorded and analyzed. The study found that as the relative mass ratio of DA increased, the sodium ion content in the xDA-MMT film decreased. This means that when DA was added to the MMT dispersion, DA was not only absorbed to the MMT surface as a monomer, but also exchanged some sodium ions in the form of cation exchange, and then formed PDA on the surface. Therefore, for xDA-MMT nanosheets, Na + It is bound to the original MMT area on its surface by electrostatic force, but is almost absent in the PDA-coated area. After the xDA-MMT film was proton exchanged (immersed in 1M HCl solution for several days), the Na + / Si + The relative intensity of the Na + The effective exchange of protons also indirectly confirms that PDA is combined with MMT nanosheets in a partially coated form.

[0101] Therefore, based on the previously reported studies, the present invention studied the proton conductivity of xDA-MMT films. Figure 3 (b) and Figure 4 As shown in (b) and (c), the experiment first cut rectangular film samples of xDA-MMT films of different sizes and encapsulated them into 2D nanofluidic devices using PDMS. A surface proton conductivity (σ) test platform was established, as shown in the figure. Figure 3 (c) As shown. At the beginning of each test, the study found that the xDA-MMT film sample needed to be immersed in the HCl electrolyte solution for at least 4 days to achieve sufficient proton exchange and thus obtain a stable σ value. Here, after stabilization, the surface proton conductivity of the xDA-MMT film was also tested for stability for 1 month. The final stable I–V curves of the xDA-MMT film in different concentrations of HCl electrolyte solution are shown as follows: Figure 3 (d) and Figure 4 (d), (e) The results show that the current increases linearly with the change of voltage, and the stable σ value of the xDA-MMT film is positively correlated with the HCl electrolyte concentration, as shown in Figure 3(e) This means that as long as there are enough protons, the water-stable xDA-MMT film can provide excellent proton conductivity. In other words, a "proton transport highway" is formed inside the xDA-MMT film, allowing protons to move freely within the two-dimensional nanochannels. When 1M HCl solution is continuously used as the liquid electrolyte for 1 month, the average σ values ​​of the 0.6DA-MMT film and the 1DA-MMT film are 0.58S cm -1 and 0.45S cm -1 , and the fluctuation is very small within 1 month, such as Figure 3 (f) These results indicate that the invented 0.6DA-MMT membrane can achieve a balance between high proton conductivity and aqueous phase stability. Although the surface proton conductivity of the intrinsic MMT film is about half, the average σ value of the xDA-MMT film is still higher than the maximum σ value of the NafionNR211 film tested by the same process (Nafion NR211 thickness is 25.4μm, close to that of the 0.6DA-MMT film, with a maximum σ value of 0.16Scm -1 ). In addition, if Figure 4 As shown in Figure (f), the present invention conducted drift-diffusion experiments on water-stable xDA-MMT films (injecting HCl electrolyte solutions of varying concentrations on both sides of the 2D nanofluidic device). The results show that in the presence of an HCl concentration gradient, the I–V curves remain linear, and for different xDA-MMT films, a negative zero-bias current and a positive zero-current potential of approximately 76.9 mV are observed. This value is close to the Nernst value of 59.1 mV, further confirming the cation-selective transport properties of the water-stable xDA-MMT films. In this system, proton-selective transport is also observed.

[0102] In summary, the prepared xDA-MMT film with good hydrophilicity and water phase stability has excellent surface proton conductivity performance, showing great potential for further forming a new generation of PEM.

[0103] In the present invention, the 0.6DA-MMT membrane having both water phase stability and excellent surface proton conductivity is also effectively applied to the newly developed VAPMM, such as Figure 5As shown. During the preparation process, multiple 0.6DA-MMT films after proton exchange are first cut into the same rectangle and stacked. Then, bisphenol A epoxy resin E51 is used as the matrix and 4,4'-diaminodiphenylmethane DDM is used as the amine hardener to encapsulate the stacked micron-thick 0.6DA-MMT films into blocks. DDM-cured E51 has been shown to have good thermochemical stability, low cost and low toxicity. At the same time, because the PDA structure contains abundant amino groups (-NH2) and hydroxyl groups (-OH), these groups provide a large amount of active hydrogen to react with unconsumed epoxy groups. Figure 6 The experiments shown also confirmed the relevant curing reaction between E51 and DA, which also made the interface between epoxy resin and 0.6DA-MMT film well compatible. Finally, by cutting the obtained epoxy resin encapsulated block using a precision wire cutting instrument, a new type of PEM with a vertically aligned water-stabilized 0.6DA-MMT membrane as the main proton transport medium was obtained, namely VAPMM membrane, as shown in Figure 2. Figure 5 As shown in the actual picture. The present invention cuts out a 150μm thick VAPMM membrane, which can achieve a proton transport path of less than 200 microns in length. The present invention prepares MEA by a method of coating a substrate with a catalyst, that is, combining a gas diffusion electrode (GDE) with VAPMM, but due to the thermosetting nature of the epoxy resin and the inorganic properties of the 0.6DA-MMT membrane, the interface between the VAPMM membrane and the GDE needs to be well resolved. Since phosphoric acid (PA) can soften the interface between the PEM and the electrode by increasing ion contact, the present invention uses PA to pretreat VAPMM and GDE, and then prepares MEA by a hot pressing process. The resulting MEA is as shown Figure 6As shown in (a). After the 0.6DA-MMT film is hydrated in an aqueous environment, PA can easily enter the interlayer of 0.6DA-MMT. At the same time, due to the bioadhesion effect of PDA and the electrostatic attraction of its amino (-NH2) and imino (-NH-) groups, acidic groups can be easily enriched on the PDA-modified 2D nanosheets. Therefore, the PA molecules that enter the 0.6DA-MMT interlayer are very likely to adsorb around PDA and form stable acid-base pairs, while also providing additional proton carrier sites for the interlayer proton transport channel. Therefore, for the 0.6DA-MMT film, its interlayer contains PDA's -OH and -NH2, H3PO4, MMT hydrophilic surface and H2O molecules, which interact to construct a complex hydrogen bond network, and then there must be a situation in which protons are transported by a hopping mechanism within the VAPMM. Compared to the intrinsic MMT film, the 1.43 nm interlayer spacing of the 0.6DA-MMT membrane after hydration is clearly more restricted. While some proton transport may still occur via the onboard mechanism in regions of higher water density, it is certainly no longer dominant. Therefore, for VAPMMs based on 0.6DA-MMT membranes, proton transport should occur via both the onboard and hopping mechanisms, with the hopping mechanism predominantly higher.

[0104] For this new type of PEM, the epoxy encapsulation part only plays a supporting role and is ineffective for proton transport, so it is necessary to Figure 6 (b) The active area of ​​the VAPMM membrane is calculated by the process shown in FIG. Based on this, the present invention evaluates the performance of a MEA cell based on a VAPMM membrane at 30°C to 75°C under 100% RH conditions. Figure 7 As shown in (a). Due to the excellent surface proton conductivity of the 0.6DA-MMT film, the maximum peak power density of the MEA based on the VAPMM membrane reached 237.55 mW cm at only 30°C. -2 As the temperature increases, the activity of the Pt catalyst and water vapor gradually increases. Surprisingly, the maximum peak power density of the MEA based on the VAPMM membrane soars to 480.60 mW cm at 75 °C. -2 This is one of the best PEMFC performances achieved to date using Nafion-free PEM. In comparison, a MEA fabricated from commercial Nafion N117 film (183 μm thick) exhibited a power output of 68.04 mW cm at 30 °C. -2 The maximum peak power density is 116.17 mW cm at 75°C. -2 ,like Figure 7(b) Compared to Nafion, the performance of VAPMM membranes of comparable thickness at 30°C and 75°C was improved by 3.49 times and 4.14 times, respectively. Therefore, VAPMM membranes, which are Nafion-free and utilize 2D nanochannel arrays formed by PDA-MMT nanosheets as the primary proton transport medium, have pioneered a new generation of PEMs.

[0105] Considering that the typical operating conditions of PEMFC are 60℃ to 80℃, after the VAPMM membrane has adapted to the cell for a period of time, the present invention further studies the cycle stability of VAPMM-based MEA at 75℃ and 100% RH. Figure 7 (c) and Figure 7 (d) The MEA can undergo 20 consecutive polarization curve cycles and shows good stability, during which all peak power density values ​​are higher than 400 mW cm -2 The peak power density at the beginning of the cycle is 401.92 mW cm -2 The peak power density at the end of the cycle is 400.70 mW cm -2 , the performance retention rate reached 99.7%, and almost no fluctuation was observed.

[0106] Based on the above results, the present invention further conducted a durability test of MEA based on VAPMM at 75°C and 100% RH. Figure 7(e) shows. The durability curve can be divided into three parts. The first section is 0-43h. During this stage, the battery voltage dropped from 0.66V to 0.48V, but the high-frequency resistance (HFR) seemed to remain relatively stable. This is attributed to the loss of residual free PA inside the MEA, which led to a reduction in proton carrier sites. The voltage dropped by about 27% during this stage. The second section is 43-60h. During this stage, the voltage reached a plateau (the overall voltage dropped by only about 2%), indicating that various reactions and processes within the MEA have reached equilibrium. This also proves that under high humidity conditions below 140°C, a stable adsorption is formed between PDA-MMT and PA, thereby preventing PA loss. It is well known that the total ohmic impedance measured by the battery is not only attributed to the PEM, but also to the ohmic resistance of the catalyst layer and its respective interface. In the first two stages, despite the presence of fin-like protrusions in the curves and the significantly higher HFR of 0.70Ω for the VAPMM-based MEA compared to the approximately 0.12Ω for the N117-based MEA, the overall MEA maintained stable current output and a voltage plateau due to the relatively moderate interface between the VAPMM membrane and the Nafion-containing GDE. However, in the third stage, from 60 to 120 hours, the HFR showed an overall upward trend, with more frequent fin-like protrusions and a strong negative correlation observed between the voltage and HFR curves. The HFR reached a maximum of 2.05Ω, and the voltage finally dropped to 0.28V at 120 hours. This indicates that the performance of the VAPMM-based MEA gradually deteriorated over time, driven by interfacial breakdown between the VAPMM membrane and the GDE. However, the VAPMM membrane, a new generation PEM that does not contain Nafion, showed good fuel cell durability, achieving a constant current output for 120 hours. This durability is even better than the m-PBI / PA membrane, which experienced a sharp drop in voltage after only 4 hours.

[0107] In summary, the present invention presents a proton exchange membrane based on vertically aligned polydopamine-intercalated montmorillonite nanosheets and its preparation method. The VAPMM membrane, prepared from the aqueous-stable and highly proton-conductive PDA-MMT membrane, is a pioneer in the next generation of low-cost PEMs and could be a promising alternative to traditional PFSA polyelectrolytes (such as Nafion) or sulfonated hydrocarbon polymers (such as SPEEK). Furthermore, this vertical encapsulation and cutting strategy can be applied to various inorganic layered structures with ultrahigh in-plane proton conductivity, such as other clay materials or covalent organic frameworks (COFs).

[0108] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a proton exchange membrane based on vertically arranged polydopamine intercalated montmorillonite nanosheets, characterized in that: The following steps are involved: S1, heat treating MMT powder to obtain MMT sample; S2, mixing the MMT sample heat-treated in step S1 with ultrapure water, stirring at high speed until the heat-treated MMT sample is fully rehydrated, and ultrasonically treating to obtain a dispersion of MMT nanosheets with uniform size; S3, adding DA powder to the MMT nanosheet dispersion prepared in step S2 and stirring thoroughly to obtain a DA and MMT dispersion system; S4, adjusting the pH value of the DA and MMT dispersion system, and stirring thoroughly to obtain a dark brown PDA-MMT nanosheet dispersion; S5. Vacuum filter the dark brown PDA-MMT nanosheet dispersion obtained in step S4, rinse with additional ultrapure water, and air dry to obtain a PDA-MMT film; S6, the PDA-MMT film obtained in step S5 is subjected to a water phase stability test. x As the relative mass ratio of DA to MMT, the water-stable x DA-MMT film; S7, the aqueous phase obtained in step S6 is stabilized x DA-MMT films were characterized and their surface proton conductivity was tested to screen for the best proton conductivity under stable water phase conditions. x The DA-MMT film is encapsulated with a polymer and precisely cut along the direction perpendicular to the nanosheet to produce a VAPMM proton exchange membrane with uniform thickness.

2. The method for preparing a proton exchange membrane based on vertically aligned polydopamine intercalated montmorillonite nanosheets according to claim 1, characterized in that: Step S1 is specifically as follows: The heating rate was set at 10 °C / min, and the MMT powder raw material was heated at 150±5 °C for 5-10 min and then naturally cooled to perform a partial dehydration treatment.

3. The method for preparing a proton exchange membrane based on vertically aligned polydopamine intercalated montmorillonite nanosheets according to claim 1, characterized in that: In step S2, the mass volume ratio of MMT sample to ultrapure water is (0.029-0.031 g): 300 mL, and the concentration of MMT nanosheet dispersion is 0.97-1.03×10 -1 g / L, the stirring time was 6~8 h; the ultrasonic treatment time was 30~35 min, and the ultrasonic treatment power was 60~80 W.

4. The method for preparing a proton exchange membrane based on vertically aligned polydopamine intercalated montmorillonite nanosheets according to claim 1, characterized in that: In step S3, the mass ratio of DA powder to MMT is (0.6~1):

1.

5. The method for preparing a proton exchange membrane based on vertically aligned polydopamine intercalated montmorillonite nanosheets according to claim 4, characterized in that: The stirring time is 4 to 5 hours.

6. The method for preparing a proton exchange membrane of vertically aligned polydopamine intercalated montmorillonite nanosheets according to claim 1, characterized in that: In step S4, the pH value of the DA and MMT dispersion liquid is 8.5-9, and the stirring time is 2-2.5 h.

7. The method for preparing a proton exchange membrane of vertically aligned polydopamine intercalated montmorillonite nanosheets according to claim 1, characterized in that: In step S5, the vacuum filtration time is 14 to 16 hours.

8. The method for preparing a proton exchange membrane of vertically aligned polydopamine intercalated montmorillonite nanosheets according to claim 1, characterized in that: In step S7, the screened x The DA-MMT film is cut, stacked, and encapsulated with epoxy resin. The encapsulated block is cut along the direction perpendicular to the nanosheet to obtain a VAPMM proton exchange membrane with uniform thickness. 9 . A proton exchange membrane of vertically aligned polydopamine intercalated montmorillonite nanosheets prepared according to the method of any one of claims 1 to 8 .

Images