Surface-modified mofs synergized mass transfer nanocomposite flow battery separator and preparation method

Abstract

This invention belongs to the field of flow battery membrane technology and provides a surface-modified MOFs synergistic mass transfer nanocomposite flow battery membrane and its preparation method. The surface-modified MOFs synergistic mass transfer nanocomposite flow battery membrane includes a polymer matrix, surface-modified MOFs functional fillers, and interconnecting channels. The surface-modified MOFs form a dual bond of "Si-O covalent bonds + hydrogen bonds" with the polymer matrix through a modifier, constructing a synergistic mass transfer structure of "50-200nm polymer interconnecting channels + 0.7-0.8nm MOFs precise sieved channels". The polymer matrix is ​​selected from PVDF, PP, or PVDF / PP blends, and the MOFs are ZIF-8 or UiO-66-NH2, with an addition amount of 5%-15% of the polymer matrix mass. The surface modifier is KH550 or MDI, and the pore-forming agent is PEG-2000 / 4000. In this invention, the diaphragm has an ion selectivity ≥99%, an ionic conductivity ≥0.02S / cm, a performance decay of ≤5% after 2000 hours of immersion, a tensile strength ≥25MPa, and a controllable cost. It is suitable for zinc-bromine, vanadium redox flow batteries, etc., and solves the core problems of existing diaphragm sieving and mass transfer imbalance, interface instability, and short lifespan.

Description

Technical Field

[0001] This invention belongs to the field of flow battery membrane technology, and particularly relates to surface-modified MOFs synergistic mass transfer nanocomposite flow battery membrane and its preparation method. Background Technology

[0002] As a core technology for large-scale electrochemical energy storage, the energy conversion efficiency, cycle life, and operating cost of flow batteries are highly dependent on the dual functions of the separator: "efficiently conducting target ions and strictly isolating active materials." However, current mainstream separator technologies and related composite modification schemes all face insurmountable core bottlenecks, specifically in the following aspects:

[0003] On the one hand, existing technologies for porous polymer composite separators fail to balance ion transport efficiency and sieving precision. For example, patent CN115207566B (published as "PMMA / PVDF composite separator and its preparation method and application") discloses a PMMA / PVDF composite separator for batteries. It improves the adhesion between the separator and the electrode and reserves storage space by agglomerating PMMA and PVDF to form composite aggregates and dot-coating them on the surface of the heat-resistant layer. However, this technology is designed for lithium batteries and does not meet the core requirements of flow batteries: the secondary particle size PVDF aggregates used severely hinder ion migration, resulting in low ion conductivity; and it does not construct a precise sieving structure, making it unable to distinguish hydrated ions with a size difference of <0.5nm in the flow battery (such as Zn²⁺ with a hydration radius of 0.41nm and Br3⁻ with a hydration radius of 0.82nm), resulting in insufficient ion selectivity. Similarly, traditional PVDF and PP porous membranes generally suffer from a wide pore size distribution (50-500nm), resulting in an ion selectivity of only about 82.5% and a high cross-migration rate of active materials as high as 18.3%. This directly leads to a battery cycle life of only 800 cycles, making it difficult to meet the requirements for long-life energy storage. In addition, some traditional composite membranes use random pore fillers, which have poor interfacial compatibility. After long-term immersion, the fillers are prone to detachment and aggregation, and the selectivity decreases by more than 20% after 2000 hours, raising concerns about stability.

[0004] On the other hand, ion exchange membranes and MOFs composite membrane technologies suffer from limitations in both cost and performance. For example, patent CN108695534A (published as "Amphoteric Nafion Ion Exchange Membrane for Vanadium Batteries and its Preparation Method") discloses a modification scheme based on Nafion membranes, which grafts anionic and cationic monomers through ATRP reaction to balance conductivity and ion permeability. However, it still relies on high-cost Nafion substrates, with a price of 1200 yuan per square meter, which restricts large-scale application. Moreover, the highly cross-linked structure leads to high ion transport resistance, with an ion conductivity of only 0.015 S / cm, failing to achieve a synergistic effect of "high selectivity and low mass transfer resistance". While existing MOFs composite membranes attempt to utilize the pore size advantage of MOFs, they have key drawbacks: the MOF addition amount is as high as 20%-30%, which easily leads to agglomeration and pore blockage; and the filler and matrix are only bonded by a single hydrogen bond, resulting in a shedding rate of 15%. After 2000 hours, the ion selectivity drops to 78%, failing to fully realize the precise sieving potential of MOFs.

[0005] In summary, existing technologies generally suffer from the core contradictions of "imbalance between screening efficiency and mass transfer rate, unstable interfacial bonding, high cost, and insufficient long-term stability," which cannot meet the comprehensive requirements of flow batteries for "high selectivity, high conductivity, long life, and low cost" in terms of separators for large-scale energy storage.

[0006] Therefore, a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator and its preparation method are needed to solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to provide a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator and its preparation method, so as to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator, comprising a polymer matrix, surface-modified MOFs functional fillers, surface modifiers, and interconnected pores formed by pore-forming agents;

[0009] The surface-modified MOFs functional filler achieves dual bonding with the polymer matrix through a surface modifier—the ethoxy group of the modifier condenses with the hydroxyl group on the MOFs surface to form a Si-O covalent bond, and the amino group forms a hydrogen bond with the fluorine atom of the polymer matrix, so that the MOFs are uniformly dispersed in the matrix, constructing a synergistic mass transfer structure of "polymer matrix interconnected channels (50-200nm) + MOFs precisely sieved channels (0.7-0.8nm)";

[0010] The MOFs functional filler has a pore size of 0.7-0.8 nm (matching Zn). 2+ Hydrated ions and Br3 -(Difference in size of hydrated ions), particle size 50-100nm (to avoid agglomeration and pore blockage), the addition amount is 5%-15% of the polymer matrix mass (to balance the sieving effect and mechanical strength).

[0011] The surface modifier is a silane coupling agent or an isocyanate modifier, and the addition amount is 5%-8% of the mass of MOFs; the membrane thickness is 20-40μm, the porosity is 35%-45%, the tensile strength is ≥25MPa, the ion selectivity is ≥99%, the ionic conductivity is ≥0.02S / cm, and the performance degradation after 2000 hours of immersion is ≤5%.

[0012] In a further technical solution, the polymer matrix is ​​polyvinylidene fluoride (PVDF), polypropylene (PP), or a PVDF / PP blend, wherein the PVDF has a molecular weight of 530,000, and the mass ratio of PP to PVDF in the PVDF / PP blend is 6:4.

[0013] In a further technical solution, the MOFs functional filler is zinc-based MOFs (ZIF-8) or zirconium-based MOFs (UiO-66-NH2), with a specific surface area of ​​1500-1800 m² / g.

[0014] In a further technical solution, the surface modifier is γ-aminopropyltriethoxysilane (KH550) or isocyanate modifier (MDI).

[0015] In a further technical solution, the pore-forming agent is polyethylene glycol (PEG-2000) or polyethylene glycol (PEG-4000), and the amount added is 20%-30% of the polymer matrix mass.

[0016] A method for preparing a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator, applicable to any of the surface-modified MOFs synergistic mass transfer nanocomposite flow battery separators described above, includes the following steps:

[0017] S1. MOFs surface modification: MOFs raw powder was dispersed in anhydrous ethanol to prepare a 5wt% suspension. After ultrasonic dispersion for 10 minutes, a surface modifier was added. The mixture was refluxed at 65-75℃ for 3.5-4.5 hours. After centrifugation at 8000rpm for 10 minutes, washing with anhydrous ethanol 3 times, and drying at 80-85℃ for 5-6 hours, surface-modified MOFs filler with a particle size of 50-100nm was obtained.

[0018] S2. Preparation of casting solution: Dissolve the polymer matrix in N,N-dimethylformamide (DMF) and stir at 70°C for 2 hours to form a 15wt% solution. Add 20%-30% of the polymer matrix mass of pore-forming agent and stir for 1 hour. Then add surface-modified MOFs filler and ultrasonically disperse at 280-320W for 25-35 minutes. Then mechanically stir at 500rpm for 2 hours and let stand for 2 hours to remove bubbles to obtain a uniform casting solution.

[0019] S3. Phase separation film formation: Cast the casting solution onto the substrate, control the wet film thickness to 90-110μm, immerse it in a deionized water non-solvent bath at 25℃±2℃ for 50-70 minutes, solidify the film and wash off the pore-forming agent.

[0020] S4. Post-treatment: Peel off the cured diaphragm, wash until the washing solution is neutral, dry at 60°C for 4 hours, and cut to obtain the finished diaphragm.

[0021] In a further technical solution, in step S1, when the surface modifier is KH550, the amount added is 6%-7% of the mass of MOFs; when it is MDI, the amount added is 5%-6% of the mass of MOFs.

[0022] In a further technical solution, in step S2, the ultrasound device is a probe-type ultrasound cell disruptor with a probe insertion depth of 2cm and a working mode of 3s ultrasound / 3s intermittent.

[0023] In a further technical solution, in step S3, the ratio of wet film thickness to dry film thickness is 2.5-5:1, and the dry film thickness is controlled at 20-40μm.

[0024] A further technical solution is that when the MOF functional filler is UiO-66-NH2, the reflux reaction temperature in step S1 is adjusted to 75-80℃ and the reflux time is adjusted to 4.5-5 hours; when the polymer matrix is ​​a PVDF / PP blend, the dissolution temperature in step S2 is adjusted to 75℃ and the stirring time is extended to 2.5 hours.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] This invention constructs a dual-layer pore system consisting of "polymer matrix interconnected channels (50-200 nm) + MOFs precise sieving channels (0.7-0.8 nm)" to achieve synergistic optimization of "low-resistance mass transfer" and "high-selectivity sieving." The interconnected channels, acting as the "main channels" for ion transport, significantly shorten ion migration distance, avoiding the surge in ohmic losses caused by ion detours, and maintaining a stable ionic conductivity of ≥0.023 S / cm, comparable to pure PVDF membranes and significantly superior to traditional MOFs composite membranes (0.018 S / cm). The precise pore size of the MOFs filler (0.7-0.8 nm) forms a precise size match with the target ions and active material ions in the flow battery, achieving ion selectivity ≥99%, a significant improvement over existing technologies. Simultaneously, this synergistic structure reduces the cross-migration rate of active materials to <3%, a reduction of over 70% compared to traditional membranes (15%-18%), fundamentally suppressing electrolyte cross-contamination and resolving the core contradiction in existing technologies where "high sieving efficiency leads to high mass transfer resistance."

[0027] This invention constructs a dual interfacial bonding mechanism of "Si-O covalent bonds + hydrogen bonds" through surface modifiers (such as KH550 modification), enabling MOF functional fillers to form a firm connection with the polymer matrix, thus completely solving the defects of traditional composite membranes where "the filler and the matrix rely only on weak van der Waals forces and are prone to falling off after long-term immersion".

[0028] This invention designs diversified material systems for different application scenarios. The polymer matrix can be selected from polyvinylidene fluoride (PVDF), polypropylene (PP), or a PVDF / PP blend (mass ratio 6:4): PVDF matrix, with its excellent film-forming properties and chemical inertness, is suitable for high-end energy storage scenarios with extremely high stability requirements; the PVDF / PP blend, while maintaining a tensile strength ≥25MPa (even up to 32MPa), reduces costs by 40% compared to pure PVDF matrix, making it suitable for mid-to-low-end energy storage scenarios and breaking the notion that "high performance necessarily means high cost". The industry limitations; the MOFs functional filler uses ZIF-8 or UiO-66-NH2 with a specific surface area of ​​1500-1800m² / g. The high specific surface area increases the ion contact sites, which improves the ion mass transfer rate by 15%-20%. Moreover, the large-scale preparation process of the two MOFs materials is mature and the cost is controllable; the pore-forming agent is polyethylene glycol (PEG-2000 / 4000). The addition amount of 20%-30% can form uniform and interconnected pores, and the elution is complete (residual amount <0.1wt%), avoiding residual impurities from affecting the stability of the electrolyte;

[0029] This invention utilizes existing non-solvent phase separation equipment, eliminating the need for a dedicated production line. Standardized parameter design ensures performance differences between batches are <4%, achieving a production efficiency of 600 m² / day, meeting industrial-grade mass production requirements. Specifically, the MOF surface modification step employs precise parameters such as a 5wt% suspension concentration, 70℃ reflux for 4 hours, and 8000 rpm centrifugation to ensure thorough modification and uniform dispersion. The casting solution preparation process utilizes a combination of ultrasonic dispersion and mechanical stirring to avoid residual bubbles and uneven composition, reducing pinholes and defects in the membrane. The phase separation and film formation stage uses a non-solvent bath temperature control of 25℃±2℃ to ensure uniform pore size distribution (porosity 35%-45%) and complete elution of the pore-forming agent (elution rate ≥99.9%). In addition, process parameters can be flexibly adapted to different MOF types and polymer matrices (e.g., the reflux temperature of UiO-66-NH2 modification is adjusted to 75-80℃, and the dissolution temperature of PVDF / PP blend is adjusted to 75℃), forming an industrialization technology system of "core solution + branch adaptation", which is compatible with the upgrading and transformation of existing diaphragm production lines and lowers the threshold for mass production.

[0030] This invention provides a separator with comprehensive advantages such as high selectivity, high conductivity, long lifespan, and low cost, which can be widely adapted to mainstream flow battery systems such as zinc-bromine and vanadium redox flow batteries, solving the problem of existing separators having "limited adaptability and difficulty in meeting the needs of different energy storage scenarios".

[0031] To more clearly illustrate the structural features and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the overall process of the present invention;

[0033] Figure 2 This is a schematic diagram of the overall structure of the present invention.

[0034] In the figure: 1. Polymer matrix; 2. Surface-modified MOFs functional filler; 3. Connecting channels. Detailed Implementation

[0035] The present invention will be further described below with reference to embodiments.

[0036] The following embodiments are used to illustrate the present invention, but should not be used to limit the scope of protection of the present invention. The conditions in the embodiments can be further adjusted according to specific conditions, and simple improvements to the method of the present invention under the premise of the concept of the present invention are all within the scope of protection claimed by the present invention.

[0037] Example 1: ZIF-8 / PVDF / PEG-2000 / KH550 composite membrane;

[0038] Raw material preparation:

[0039] Polymer matrix 1: PVDF (molecular weight 530,000, purity ≥99%, Shanghai Sanaifu).

[0040] MOF functional filler: ZIF-8 raw powder: 2.97g Zn(NO3)2・6H2O (CAS7779-88-6, Sinopharm Group) and 6.5g 2-methylimidazole (CAS693-98-1, Aladdin) were dissolved in 50mL methanol, stirred at 25℃ for 24 hours, centrifuged at 8000rpm for 10 minutes, washed 3 times with anhydrous methanol, and dried at 80℃ for 12 hours (particle size 80-120nm, specific surface area 1650m² / g).

[0041] Surface modifier: KH550 (purity ≥98%, Sinopharm Group), added at 6% of the mass of MOFs;

[0042] Pore-forming agent: PEG-2000 (purity ≥99%, Aladdin), added at 30% of the PVDF mass;

[0043] Solvent: DMF (analytical grade, Sinopharm Group).

[0044] Preparation steps:

[0045] S1, MOF surface modification: 10g of ZIF-8 raw powder was dispersed in 190g of anhydrous ethanol to prepare a 5wt% suspension. A JY92-IIN probe-type ultrasonic cell disruptor (Ningbo Xinzhi) was used, with a probe insertion depth of 2cm, operating mode of 3s ultrasonic / 3s intermittent, and ultrasonic dispersion at 300W for 10 minutes. 0.6g of KH550 was added, and the mixture was refluxed at 70℃ for 4 hours. After centrifugation at 8000rpm for 10 minutes, the mixture was washed three times with anhydrous ethanol and dried at 80℃ for 6 hours to obtain modified ZIF-8 with a particle size of 50-80nm (a Si-O characteristic peak appeared at 1080cm⁻¹ in the infrared spectrum).

[0046] S2. Preparation of casting solution: Dissolve 15g PVDF in 85g DMF and stir at 70℃ for 2 hours to form a 15wt% solution; add 4.5g PEG-2000 and mechanically stir at 500rpm for 1 hour; then add 1.5g modified ZIF-8, ultrasonically disperse at 300W for 30 minutes, mechanically stir at 500rpm for 2 hours, and let stand for 2 hours to remove bubbles;

[0047] S3. Phase separation and film formation: The casting solution was cast onto the glass substrate using RKK ControlCoater (RK, UK), with the wet film thickness controlled at 100μm and the corresponding dry film thickness at 30μm; the film was then immersed in a 25℃ deionized water non-solvent bath for 60 minutes.

[0048] S4. Post-treatment: Peel off the diaphragm, wash with deionized water until pH=7.0 (neutral), dry at 60℃ for 4 hours, and cut to obtain the finished diaphragm.

[0049] Performance testing (according to national and industry standards):

[0050] Tensile strength: 32 MPa, tested according to GB / T1040.3-2006; Porosity: 42%; Ion selectivity: 99.5%; Ionic conductivity: 0.025 S / cm; 2000-hour immersion: performance degradation 4.2%, MOF shedding rate 1.8%; Battery cycle life: 5200 cycles.

[0051] In this embodiment

[0052] Strictly adhering to the core parameter range of the claims, MOFs are uniformly dispersed without agglomeration through the dual combination of PVDF matrix and KH550 modified ZIF-8 (Si-O bond + hydrogen bond);

[0053] PEG-2000 forms 50-100nm interconnected channels, which work in conjunction with ZIF-8’s 0.75nm sieving channels to ensure both rapid ion transport (conductivity 0.025S / cm) and precise sieving (selectivity 99.5%).

[0054] The preparation process is standardized, with a batch-to-batch performance difference of 3.2% (<4%), a production efficiency of 600m² / day, and a cost of 210 yuan per square meter (≤220 yuan). This represents an 82.5% reduction in cost compared to Nafion membranes and a 91% improvement in stability compared to traditional MOFs composite membranes.

[0055] Example 2: UiO-66-NH2 / PVDF-PP / PEG-4000 / MDI composite membrane;

[0056] Raw material preparation:

[0057] Polymer matrix 1: PVDF / PP blend (mass ratio 6:4, PVDF molecular weight 530,000, PP melt index 2.5 g / 10 min).

[0058] MOF functional filler: UiO-66-NH2 raw powder (prepared according to the method in the Journal of Inorganic Chemistry, Vol. 38, No. 5, 2022, with a specific surface area of ​​1780 m² / g).

[0059] Surface modifier: MDI (purity ≥99%, Wanhua Chemical), added at 5% of the mass of MOFs;

[0060] Pore-forming agent: PEG-4000 (purity ≥99%, Aladdin), added at 20% of the blend mass;

[0061] Solvent: DMF (analytical grade, Sinopharm Group).

[0062] Preparation steps:

[0063] S1, MOF surface modification: 10g of UiO-66-NH2 raw powder was dispersed in 190g of anhydrous ethanol, 5wt% suspension, and ultrasonically dispersed at 300W for 10 minutes; 0.5g of MDI was added, and the mixture was refluxed at 78℃ for 4.8 hours; centrifuged at 8000rpm for 10 minutes, washed 3 times with anhydrous ethanol, and dried at 85℃ for 5.5 hours to obtain modified UiO-66-NH2 with a particle size of 70-100nm;

[0064] S2. Preparation of casting solution: Dissolve 15g of PVDF-PP blend in 85g of DMF and stir at 75℃ for 2.5 hours to form a 15wt% solution; add 3g of PEG-4000 and stir at 500rpm for 1 hour; then add 0.75g of modified UiO-66-NH2, ultrasonically disperse at 320W for 25 minutes, stir at 500rpm for 2 hours, and let stand for 2 hours to remove bubbles;

[0065] S3. Phase separation and membrane formation: Cast a wet membrane with a thickness of 110 μm, corresponding to a dry membrane thickness of 40 μm; immerse in 27℃ deionized water for 50 minutes; S4. Post-treatment: Wash until pH=6.8, dry at 60℃ for 4 hours, and cut to obtain the finished membrane.

[0066] Performance testing:

[0067] Tensile strength: 28 MPa; Porosity: 38%; Ion selectivity: 99.2%; Ionic conductivity: 0.023 S / cm; 2000-hour immersion: performance degradation 4.8%, MOFs shedding rate 1.5%; Battery cycle life: 5050 cycles.

[0068] In this embodiment, for the PVDF-PP blend matrix and UiO-66-NH2, the process parameters are adjusted according to claim 10 to ensure that the blend is completely dissolved and the MOFs are sufficiently modified.

[0069] MDI forms covalent bonds with the methylene groups of the PP matrix, resulting in a 33% improvement in interfacial peel strength compared to KH550 modification, making it suitable for high-stability scenarios.

[0070] PEG-4000 forms 100-200nm interconnected channels, which work in synergy with the 0.78nm pore size of UiO-66-NH2 to achieve a balance between ion transport resistance and sieving efficiency;

[0071] The cost per square meter is 198 yuan (40% lower than that of pure PVDF membrane), the performance difference between batches is 3.8% (<4%), it is suitable for low-to-mid-range energy storage scenarios, and the production efficiency is 600m² / day, which meets the needs of industrialization.

[0072] Example 3: ZIF-8 / PP / PEG-2000 / KH550 composite membrane;

[0073] Raw material preparation:

[0074] Polymer matrix 1: PP (melt index 2.5 g / 10 min, Yanshan Petrochemical);

[0075] MOFs functional filler: ZIF-8 raw powder (same as Example 1, specific surface area 1650m² / g);

[0076] Surface modifier: KH550, added at 7% of the mass of MOFs;

[0077] Pore-forming agent: PEG-2000, added at 25% of the PP mass;

[0078] Solvent: DMF (analytical grade, Sinopharm Group).

[0079] Preparation steps:

[0080] S1, MOFs surface modification: 10g ZIF-8 raw powder was used to prepare a 5wt% suspension, which was ultrasonicated at 300W for 10 minutes; 0.7g KH550 was added, and the mixture was refluxed at 65℃ for 4.5 hours; the mixture was centrifuged at 8000rpm for 10 minutes, washed 3 times, and dried at 82℃ for 5 hours to obtain modified ZIF-8 with a particle size of 60-90nm.

[0081] S2. Preparation of casting solution: Dissolve 15g PP in 85g DMF and stir at 70℃ for 2 hours to form a 15wt% solution; add 3.75g PEG-2000 and stir at 500rpm for 1 hour; then add 2.25g modified ZIF-8, ultrasonically disperse at 280W for 35 minutes, stir at 500rpm for 2 hours, and let stand for 2 hours to remove bubbles;

[0082] S3. Phase separation and film formation: wet film thickness 90μm, corresponding to dry film thickness 20μm; immerse in 23℃ deionized water for 70 minutes.

[0083] S4. Post-treatment: Wash until pH=7.2, dry at 60℃ for 4 hours, and cut to obtain the finished diaphragm.

[0084] Performance testing:

[0085] Tensile strength: 26 MPa; Porosity: 45%; Ion selectivity: 99.1%; Ionic conductivity: 0.024 S / cm; 2000-hour immersion: performance degradation 4.5%, MOF shedding rate 1.7%; Battery cycle life: 5100 cycles.

[0086] In this embodiment, a PP matrix is ​​used, and a stable bond is formed with MOFs through the hydrogen bonding of KH550. Even with an MOF addition of 15%, there is no aggregation, which verifies the rationality of the 5%-15% addition amount in claim 1.

[0087] The 20μm ultrathin membrane reduces ion transport distance and achieves a conductivity of 0.024S / cm, which is 41% higher than that of traditional PP membranes.

[0088] With a cost of 185 yuan per square meter, it falls within the lowest cost range of existing technologies, while maintaining performance without degradation, achieving an ultimate balance between "low cost + high performance" and making it suitable for cost-sensitive scenarios of large-scale energy storage.

[0089] Working principle of the invention

[0090] The core working principle of this invention is based on four interconnected mechanisms: "interface bonding - synergistic mass transfer - precise sieving - standardized preparation," forming a complete logical chain from molecular interactions, ion transport, structural stabilization to industrialization.

[0091] Strong interface integration mechanism:

[0092] The surface modifier (KH550 / MDI) acts as a "molecular bridge," enabling MOFs and the polymer matrix to function in a dual way: the ethoxy group of KH550 undergoes a condensation reaction with the hydroxyl groups on the surface of MOFs (2RO-Si-(OC2H5)3+3MOFs-OH→RO-Si-(O-MOFs)3+3C2H5OH), forming a stable Si-O covalent bond; at the same time, the amino group (-NH2) forms a hydrogen bond (-NH2…FC-) with the -CF2- group of PVDF, constructing a dual interface of "covalent bond + hydrogen bond"; the isocyanate group (-NCO) of MDI undergoes an addition reaction with the methylene group (-CH2-) of PP, forming a urea bond (-NH-CO-NH-), which strengthens the interfacial bonding strength;

[0093] This mechanism completely solves the defect of traditional MOFs composite membranes that rely solely on weak van der Waals forces / single hydrogen bonds, resulting in a MOFs shedding rate of <2% and a selectivity retention rate of ≥98.5% after 2000 hours, laying the foundation for long-term stability.

[0094] Collaborative mass transfer mechanism:

[0095] The porogen (PEG-2000 / 4000) was completely eluted during phase separation, forming interconnected pores of 50-200 nm. Figure 2 ), serving as the "main channel" for ion transport, enabling the target ion (Zn) 2+ ) at 1.2×10 -5 With a rapid migration rate of cm² / s, the transmission distance is shortened by 60% compared to traditional diaphragms, significantly reducing ohmic losses.

[0096] MOF functional fillers are uniformly dispersed within the polymer matrix, with a precise pore size of 0.7-0.8 nm and targeting ions (Zn). 2+ Hydration radius 0.41 nm), active substance ions (Br3) - A hydration radius of 0.82 nm forms a size match, acting as a "sieving node" to achieve physical blocking;

[0097] Ion transport path: target ions → connecting channels (rapid migration) → MOFs sieving channels (precise passage) → opposite electrode; active material ions → blocked by MOFs channels → unable to cross the membrane, achieving synergistic optimization of "low resistance mass transfer + high selectivity sieving", with a dual breakthrough of ion conductivity ≥0.023S / cm and selectivity ≥99%;

[0098] Precise screening and low aggregation mechanism:

[0099] MOFs selected are ZIF-8 (pore size 0.74 nm) or UiO-66-NH2 (pore size 0.78 nm), with a specific surface area of ​​1500-1800 m² / g, increasing ion contact sites and improving mass transfer rate by 15%-20%; the 0.7-0.8 nm pore size precisely matches the commonly used ion size in flow batteries, allowing only target ions to pass through while completely blocking active material ions, with a cross-mobility of <3%;

[0100] During the preparation process, a triple process of 5wt% MOF suspension concentration, ultrasonic dispersion at 280-320W (3s / 3s intervals), and reflux reaction at 65-80℃ is used to synergistically break down MOF agglomerates, thereby stabilizing the particle size at 50-100nm (<50nm is prone to agglomeration, >100nm is prone to pore blockage). At the same time, the MOF addition amount is only 5%-15% (far lower than the traditional 20%-30%), thus avoiding the pore blockage problem caused by agglomeration from the source.

[0101] Standardized preparation mechanism:

[0102] Standardized process parameters: parameters such as 5wt% MOFs suspension, 65-80℃ reflux, 280-320W ultrasonication, and 90-110μm wet film thickness are all necessary technical features to achieve "uniform MOFs dispersion, sufficient modification, and stable film structure". The parameter range is compatible with the errors of equipment from different manufacturers (±5℃ / ±20W).

[0103] Scenario-based adaptation design: Adjust the reflux temperature (75-80℃) for UiO-66-NH2 and the dissolution temperature (75℃) for PVDF-PP blends to form a "core solution + branch adaptation" system, which ensures process uniformity and meets the needs of different material combinations.

[0104] Industrial compatibility: All steps are based on existing solvent-free phase separation equipment, without the need for additional dedicated production lines. The production efficiency reaches 600m² / day, with batch-to-batch performance differences of <4%, making it suitable for industrial-grade mass production. The cost per square meter is ≤220 yuan, making large-scale applications possible.

[0105] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator, characterized in that, It includes a polymer matrix (1), surface-modified MOF functional fillers (2), and interconnected pores formed by surface modifiers and pore-forming agents (3). The surface-modified MOFs functional filler (2) achieves dual bonding with the polymer matrix (1) through the surface modifier - the ethoxy group of the modifier condenses with the hydroxyl group on the surface of MOFs to form Si-O covalent bonds, and the amino group forms hydrogen bonds with the fluorine atoms of the polymer matrix (1), so that MOFs are uniformly dispersed in the matrix and a synergistic mass transfer structure of "polymer matrix interconnected pores (50-200nm) + MOFs precise sieve pores (0.7-0.8nm)" is constructed. The MOFs functional filler has a pore size of 0.7-0.8 nm (matching Zn). 2+ Hydrated ions and Br3 - (size difference of hydrated ions), particle size 50-100nm (to avoid agglomeration and pore blockage), the amount added is 5%-15% of the mass of polymer matrix (1) (to balance the sieving effect and mechanical strength); The surface modifier is a silane coupling agent or an isocyanate modifier, and the addition amount is 5%-8% of the mass of MOFs; the membrane thickness is 20-40μm, the porosity is 35%-45%, the tensile strength is ≥25MPa, the ion selectivity is ≥99%, the ionic conductivity is ≥0.02S / cm, and the performance degradation after 2000 hours of immersion is ≤5%.

2. The surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 1, characterized in that, The polymer matrix (1) is polyvinylidene fluoride (PVDF), polypropylene (PP) or PVDF / PP blend, wherein the PVDF has a molecular weight of 530,000 and the mass ratio of PP to PVDF in the PVDF / PP blend is 6:

4.

3. The surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 1, characterized in that, The MOFs functional filler is zinc-based MOFs (ZIF-8) or zirconium-based MOFs (UiO-66-NH2), with a specific surface area of ​​1500-1800 m² / g.

4. The surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 1, characterized in that, The surface modifier is γ-aminopropyltriethoxysilane (KH550) or isocyanate modifier (MDI).

5. The surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 1, characterized in that, The pore-forming agent is polyethylene glycol (PEG-2000) or polyethylene glycol (PEG-4000), and the amount added is 20%-30% of the mass of the polymer matrix (1).

6. A method for preparing a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator, applicable to the surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to any one of claims 1-5, characterized in that, Includes the following steps: S1. MOFs surface modification: MOFs raw powder was dispersed in anhydrous ethanol to prepare a 5wt% suspension. After ultrasonic dispersion for 10 minutes, a surface modifier was added. The mixture was refluxed at 65-75℃ for 3.5-4.5 hours. After centrifugation at 8000rpm for 10 minutes, washing with anhydrous ethanol 3 times, and drying at 80-85℃ for 5-6 hours, surface-modified MOFs filler with a particle size of 50-100nm was obtained. S2. Preparation of casting solution: Dissolve the polymer matrix (1) in N,N-dimethylformamide (DMF), stir at 70°C for 2 hours to form a 15wt% solution, add 20%-30% of the pore-forming agent of the polymer matrix (1) and stir for 1 hour, then add surface-modified MOFs filler, use 280-320W ultrasonic dispersion for 25-35 minutes, then mechanically stir at 500rpm for 2 hours, and let stand for 2 hours to degas to obtain a uniform casting solution; S3. Phase separation film formation: Cast the casting solution onto the substrate, control the wet film thickness to 90-110μm, immerse it in a deionized water non-solvent bath at 25℃±2℃ for 50-70 minutes, solidify the film and wash off the pore-forming agent. S4. Post-treatment: Peel off the cured diaphragm, wash until the washing solution is neutral, dry at 60°C for 4 hours, and cut to obtain the finished diaphragm.

7. The method for preparing a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 6, characterized in that, In step S1, when the surface modifier is KH550, the amount added is 6%-7% of the mass of MOFs; when it is MDI, the amount added is 5%-6% of the mass of MOFs.

8. The method for preparing a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 6, characterized in that, In step S2, the ultrasound device is a probe-type ultrasound cell disruptor with a probe insertion depth of 2cm and a working mode of 3s ultrasound / 3s intermittent.

9. The method for preparing a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 6, characterized in that, In step S3, the ratio of wet film thickness to dry film thickness is 2.5-5:1, and the dry film thickness is controlled at 20-40 μm.

10. The method for preparing a surface-modified MOFs synergistic mass transfer nanocomposite flow battery separator according to claim 6, characterized in that, When the MOF functional filler is UiO-66-NH2, the reflux reaction temperature in step S1 is adjusted to 75-80℃ and the reflux time is adjusted to 4.5-5 hours; when the polymer matrix (1) is a PVDF / PP blend, the dissolution temperature in step S2 is adjusted to 75℃ and the stirring time is extended to 2.5 hours.

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