PI-in situ functionalized bimetallic MOF / phosphoric acid proton exchange membrane and preparation method, aqueous organic liquid flow battery

Abstract

The present disclosure provides a PI-in-situ functionalized bimetallic MOF / phosphoric acid proton exchange membrane and a preparation method and an aqueous organic flow battery. The preparation method comprises: sequentially adding diamine monomers and dianhydride monomers in a polar solvent and stirring to form a viscous polyamide acid spinning solution; electrospinning the spinning solution to obtain a PAA fiber membrane; gradient temperature imidization of the PAA fiber membrane to obtain a PI electrospun fiber membrane; plasma pretreatment of the PI electrospun fiber membrane to obtain a surface-activated PI electrospun fiber membrane; placing a sulfonated modified bimetallic MOF reaction solution and the PI electrospun fiber membrane in a reaction kettle by a solvothermal in-situ growth method to obtain a PI@ sulfonated modified bimetallic MOF composite fiber membrane; soaking the PI@ sulfonated modified bimetallic MOF composite fiber membrane in a phosphoric acid solution containing a silane coupling agent for soaking treatment, and then performing heat pressing densification and drying treatment to obtain a PI-in-situ functionalized bimetallic MOF / phosphoric acid composite proton exchange membrane.

Description

Technical Field

[0001] This disclosure belongs to the field of aqueous organic flow battery technology, specifically relating to a PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane and its preparation method, and an aqueous organic flow battery. Background Technology

[0002] With the continuous increase in the installed capacity of new energy power, the demand for grid frequency regulation is becoming increasingly urgent. As a traditional baseload power source, thermal power's energy storage frequency regulation retrofit is a key path to ensure the safe and stable operation of the power grid. The core requirements for energy storage batteries in thermal power energy storage frequency regulation systems include: rapid response capability (adapting to the millisecond-level frequency regulation needs of the power grid), long cycle life (meeting the long-term operation needs of thermal power plants and reducing operation and maintenance costs), stable operation over a wide temperature range (adapting to seasonal temperature fluctuations in thermal power plants), and high safety (avoiding safety risks in thermal power scenarios). Aqueous organic flow batteries, due to their advantages such as high electrolyte safety, low cost, and flexible structure, have become one of the preferred technologies for thermal power energy storage frequency regulation. However, the proton exchange membrane, as the core component of the battery, directly determines the battery's power density, cycle stability, and self-discharge characteristics, and is a key bottleneck restricting the adaptation of aqueous organic flow batteries to thermal power energy storage frequency regulation.

[0003] Existing proton exchange membranes for aqueous organic flow batteries mainly include perfluorosulfonic acid membranes (such as Nafion membranes), pure polymer membranes (such as PI membranes and sulfonated polyether ether ketone membranes), and physically mixed composite membranes (such as MOF / polymer hybrid membranes). However, all of them have significant drawbacks in the context of thermal power energy storage frequency regulation: Firstly, although perfluorosulfonic acid membranes have high proton conductivity, they are expensive and easily swell in aqueous organic electrolytes, leading to severe cross-permeation of active materials (such as anthraquinones and ionogens), resulting in high battery self-discharge rates. This fails to meet the requirements of thermal power energy storage frequency regulation for long-term energy storage and low self-discharge. Secondly, pure polymer membranes (such as pure PI membranes) have excellent mechanical and chemical stability, but extremely low proton conductivity and slow battery response, making them difficult to adapt to the rapid power regulation requirements of thermal power frequency regulation. Thirdly, physical hybrid composite membranes improve conductivity by adding functional fillers such as MOFs, but the interface between MOFs and polymer matrices is poor. Under the high-frequency charge-discharge cycles and temperature fluctuations of thermal power energy storage frequency regulation, MOFs are prone to agglomeration and interface peeling, leading to membrane structure damage and a significant reduction in battery cycle life (usually less than 100 cycles), which cannot meet the long-term operation requirements of thermal power.

[0004] In addition, in the scenario of frequency regulation for thermal power energy storage, batteries often need to operate for a long time in an environment of 40~80℃. Existing proton exchange membranes generally have the problem of severe leakage of proton carriers (such as phosphoric acid) in high-temperature water environments, which further aggravates the degradation of battery performance.

[0005] Therefore, developing a proton exchange membrane that combines high proton conductivity, low active material permeability, excellent high-temperature stability, and long cycle life has become a core technological requirement for adapting aqueous organic flow batteries to thermal power energy storage frequency regulation scenarios, and is of great significance for promoting the industrialization of thermal power energy storage frequency regulation. Summary of the Invention

[0006] This disclosure aims to at least solve one of the technical problems existing in the prior art of proton exchange membranes for aqueous organic flow batteries, namely the difficulty in balancing "proton conduction-active material barrier-structural stability", rapid performance degradation in high-temperature aqueous environments, and severe phosphoric acid leakage. It provides a polyimide (PI)-in-situ grown bimetallic MOF / phosphoric acid composite proton exchange membrane and its preparation method for use in fire storage frequency modulation scenarios, as well as an aqueous organic flow battery.

[0007] One aspect of this disclosure provides a method for preparing a PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane, the method comprising:

[0008] An inert gas is introduced into a polar solvent and the solvent is placed in a constant temperature water bath. Diamine monomer and dianhydride monomer are added sequentially and stirred to form a viscous polyamic acid spinning solution.

[0009] The viscous polyamic acid spinning solution was subjected to electrospinning treatment to obtain a PAA fiber membrane.

[0010] The PAA fiber membrane was subjected to gradient heating imidization to obtain a PI electrospun fiber membrane.

[0011] The PI electrospun fiber membrane was subjected to plasma pretreatment to obtain a surface-activated PI electrospun fiber membrane.

[0012] The sulfonated modified bimetallic MOF reaction solution and the surface-activated PI electrospun fiber membrane were reacted in a reaction vessel using a solvothermal in-situ growth method to obtain a PI@sulfonated modified bimetallic MOF composite fiber membrane.

[0013] The PI@sulfonated modified bimetallic MOF composite fiber membrane was immersed in a phosphoric acid solution containing silane coupling agent for soaking treatment, followed by hot pressing densification treatment and drying treatment to obtain a PI-in-situ functionalized bimetallic MOF / phosphoric acid composite proton exchange membrane.

[0014] Optionally, the molar ratio of the diamine monomer to the dianhydride monomer is 1:(0.8~1.2); wherein,

[0015] The diamine monomer is at least one selected from 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, and p-phenylenediamine;

[0016] The dianhydride monomer is at least one of pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, and biphenyl dianhydride.

[0017] Optionally, the temperature of passing an inert gas into the polar solvent and placing it in a constant temperature water bath is 20~30℃;

[0018] The reaction time after adding the diamine monomer and dianhydride monomer is 1.5–3 h; and / or,

[0019] The solid content of the viscous polyamic acid (PAA) spinning solution is 12~18 wt.%.

[0020] Optionally, the electrospinning treatment of the viscous polyamic acid spinning solution is performed at a spinning voltage of 15~25kV, a receiving distance of 12~18cm, a feed speed of 0.3~0.8mL / h, an ambient temperature of 20~30℃, a humidity of 30~50%, and a spinning time of 1.5~3h.

[0021] Optionally, the PAA fiber membrane is subjected to gradient temperature imidization, comprising:

[0022] Dry at 60~100℃ for 1~3 hours;

[0023] Dry at 120~180℃ for 1~3 hours;

[0024] Dry at 220~280℃ for 0.5~2 hours;

[0025] Dry at 280~320℃ for 0.5~2 hours.

[0026] Optionally, the parameters for plasma pretreatment are: plasma power 30~60W, treatment time 5~15min, treatment atmosphere argon or oxygen, and vacuum degree 10~50Pa. Plasma pretreatment can introduce a large number of active functional groups such as hydroxyl and carboxyl groups onto the PI fiber surface, while simultaneously increasing the fiber surface roughness, thereby increasing the nucleation sites for subsequent MOF growth, significantly improving the interfacial bonding strength between the MOF and the PI matrix, and preventing MOF detachment during cycling.

[0027] Optionally, the sulfonated modified bimetallic MOF reaction solution comprises: a Zr source, an Fe source, an amino-modified organic ligand, a sulfonating modifier, and an organic solvent; wherein,

[0028] The molar ratio of the Zr source to the Fe source is (2~5):1, the Zr source is ZrCl4 or zirconium oxychloride, and the Fe source is FeCl3·6H2O or Fe(NO3)3·9H2O;

[0029] The bimetallic system combines the excellent water stability of Zr-based MOFs with the good proton conduction potential of Fe-based MOFs. Through synergistic effects, the thermal decomposition temperature of MOFs is increased to above 380℃, while enhancing the adsorption and confinement ability of phosphoric acid.

[0030] The amino-modified organic ligand is 2-aminoterephthalic acid or 2-aminonaphthalenedicarboxylic acid, and the sulfonating modifier is chlorosulfonic acid or aminosulfonic acid, the amount of which is added is 5-15% of the mass of the organic ligand;

[0031] Sulfonation modification can introduce sulfonic acid groups (-SO3H) onto the surface of MOF channels, forming a synergistic proton transport system with phosphoric acid, which significantly improves proton conductivity.

[0032] The organic solvent is at least one of DMF and NMP.

[0033] Optionally, the reaction of the sulfonated modified bimetallic MOF reaction solution with the surface-activated PI electrospun fiber membrane in a reaction vessel is carried out at a temperature of 100-150°C for 6-24 hours, with a programmed temperature increase at a rate of 2-5°C / min. Programmed temperature increase avoids the aggregation problem caused by rapid MOF nucleation, allowing the MOF to grow uniformly on and within the PI fiber surface, forming a three-dimensional interpenetrating network structure.

[0034] Optionally, in the phosphoric acid solution containing the silane coupling agent, the mass fraction of the silane coupling agent is 0.3-1.0% of the phosphoric acid solution content, and the silane coupling agent is γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane; the silane coupling agent can simultaneously form chemical bonds with the amino group of PI, the hydroxyl group of MOF and phosphoric acid to construct a stable "PI-MOF-phosphoric acid" triadic structure, further reducing the phosphoric acid leakage rate;

[0035] The concentration of the phosphoric acid solution is 5~12 mol / L;

[0036] The soaking temperature is 20~30℃, and the time is 2~6 hours;

[0037] The hot pressing densification process is carried out at a temperature of 60~100℃, a pressure of 3~8MPa, and a time of 20~40min.

[0038] The drying process is carried out at a temperature of 50-70°C for 4-8 hours.

[0039] In another aspect of this disclosure, a PI-functionalized bimetallic MOF / phosphate proton exchange membrane is provided, wherein the PI-functionalized bimetallic MOF / phosphate proton exchange membrane is prepared using the preparation method described above.

[0040] In another aspect of this disclosure, an aqueous organic flow battery is proposed, the aqueous organic flow battery comprising the PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane described above.

[0041] This disclosure presents a polyimide (PI)-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane and its preparation method for frequency regulation in fire-storage applications, as well as an aqueous organic flow battery, which has the following advantages compared to existing technologies:

[0042] First, an innovative synergistic strategy of "plasma pretreatment-sulfonation modified bimetallic MOF in situ growth" was introduced: plasma pretreatment constructs highly active sites on the surface of PI fibers, increasing the MOF loading by more than 30% and enhancing the interfacial bonding strength by 50%. Through synergistic effects, the bimetallic MOF (Zr-Fe) forms a stable three-dimensional framework with its Zr-O bonds and Fe-O bonds, maintaining the thermal decomposition temperature above 380℃, while providing multiple adsorption sites for phosphoric acid. The sulfonic acid groups introduced by sulfonation modification form a "-SO3H-H3PO4" synergistic proton transport system with phosphoric acid, significantly improving the proton conductivity and achieving synergistic optimization of proton conduction and structural stability.

[0043] Second, it pioneered the "phosphoric acid impregnation process containing silane coupling agent": the silane coupling agent acts as a bridging molecule, and at the same time forms chemical bonds with PI, MOF and phosphoric acid to build a stable chemical cross-linking network, which reduces the phosphoric acid leakage rate (48h) to 2.1%~3.1%, and specifically solves the technical pain point of severe phosphoric acid leakage in high-temperature water system environment under the frequency regulation scenario of thermal power energy storage.

[0044] Third, the solvothermal in-situ growth method disclosed herein completes MOF synthesis and composite in one step, reducing process steps by 60% and improving MOF loading uniformity by 40%. Simultaneously, programmed temperature control of MOF growth prevents agglomeration and ensures membrane structure uniformity. Compared to the "blending-coating" process, this disclosure achieves chemical bonding between MOF and PI through in-situ growth, resulting in tighter interfacial bonding and reducing MOF detachment during high-frequency charge-discharge cycles, making it more suitable for the long-cycle requirements of frequency regulation in thermal power energy storage. Furthermore, the technical solution disclosed herein is precisely designed to address the proton conduction, active material barrier properties, and high-temperature stability requirements of aqueous organic redox flow batteries.

[0045] Fourth, the raw materials used (PI monomer, bimetallic source, ligand, phosphoric acid, silane coupling agent) are all conventional industrial reagents. The solvent DMF (or NMP) can be recovered and reused through vacuum distillation (recovery rate of over 85%). The preparation process does not require toxic catalysts, and the process parameters are mild (maximum hot pressing temperature 100℃, MOF growth temperature 150℃). No special high-pressure equipment is required, which reduces energy consumption and pollution from the mechanism, lowers production costs, and is more suitable for large-scale industrial production. Attached Figure Description

[0046] Figure 1 This is a flowchart illustrating the preparation method of PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane according to a specific embodiment of this disclosure.

[0047] Figure 2 The stress-strain curves of Embodiment 12 and Comparative Example 2 of this disclosure are shown.

[0048] Figure 3 This is a graph showing the change in discharge capacity over 150 cycles for Embodiment 3 and Comparative Example 3 of this disclosure;

[0049] Figure 4 The diagrams show the 240-hour self-discharge voltage decay of Embodiment 10 and Comparative Example 2 of this disclosure.

[0050] Figure 5 Examples 1 and 5 of this disclosure are performed at 60 mA·cm -2 The charge / discharge curves are shown below. Detailed Implementation

[0051] To enable those skilled in the art to better understand the technical solutions of this disclosure, the disclosure will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain this disclosure and represent a part of the embodiments of this disclosure, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the protection scope of this disclosure.

[0052] like Figure 1 As shown, one aspect of this disclosure provides a method S100 for preparing a PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane, specifically including the following steps S110~S160:

[0053] S110: Pour the polar solvent into a dry four-necked flask, introduce an inert gas for protection, and place it in a constant temperature water bath at 20~30℃. Add the diamine monomer and dianhydride monomer in batches and stir for 1.5~3 hours to form a viscous polyamic acid spinning solution.

[0054] In some preferred embodiments, the polar solvent may be at least one of N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), or dimethyl sulfoxide (DMSO), for example, DMF is preferred. Secondly, the content of the polar solvent is preferably 80-120 mL.

[0055] In some preferred embodiments, the inert gas may be one of inert gases such as nitrogen or argon. For example, nitrogen may be preferred to prevent the oxidation of diamine monomers.

[0056] In some preferred embodiments, the molar ratio of diamine monomer to dianhydride monomer is 1:(0.8~1.2), preferably 1:1. Furthermore, the content of diamine monomer is preferably 3.5~5.0g, and the content of dianhydride monomer is preferably 3.8~4.8g.

[0057] In some preferred embodiments, the diamine monomer is 4,4'-diaminodiphenyl ether (ODA), or at least one of 4,4'-diaminodiphenylmethane (MDA) and p-phenylenediamine (PDA).

[0058] In some preferred embodiments, the dianhydride monomer is pyromellitic dianhydride (PMDA), or at least one of benzophenone dianhydride (BTDA) and biphenyl dianhydride (BPDA).

[0059] It should be noted that adding the diamine monomer and dianhydride monomer in batches and slowly means adding both components multiple times, rather than adding them all at once. For example, the diamine monomer can be added slowly to the flask in 2-3 batches, with an interval of 10-20 minutes (preferably 15 minutes) between each batch. After adding all the diamine monomer in batches to the flask, stir for 20-40 minutes (preferably 30 minutes) until completely dissolved. Then, add the dianhydride monomer slowly to the flask in the same way in 2-3 batches, and continue stirring to react for 1.5-3 hours (preferably 2 hours) to form a viscous polyamic acid (PAA) spinning solution with a solid content of 12-18 wt.% (preferably 15 wt.%).

[0060] In step S110, polyamic acid (PAA) is prepared by low-temperature solution polymerization, which avoids monomer decomposition or side reactions caused by high temperature and ensures the length and structural regularity of polymer chains.

[0061] S120. Transfer the PAA spinning solution to the syringe of the electrospinning equipment for electrospinning treatment to obtain a PAA fiber membrane.

[0062] In some preferred embodiments, the electrospinning process parameters are as follows: spinning voltage 15~25kV (preferably 20kV), receiving distance 12~18cm (preferably 15cm), feed speed 0.3~0.8mL / h (preferably 0.5mL / h), and spinning is carried out for 1.5~3h (preferably 2h) under ambient temperature of 20~30℃ (preferably 25℃) and humidity of 30~50% (preferably 40%) to obtain PAA fiber membrane.

[0063] In step S120, electrospinning can be used to directly prepare a three-dimensional network structure formed by the interlacing of nanofibers, which has a continuous and interconnected pore network.

[0064] S130. The PAA fiber membrane is placed in a vacuum drying oven for gradient temperature imidization to obtain a PI electrospun fiber membrane.

[0065] In some preferred embodiments, the process parameters for gradient imidization are as follows: drying at 60~100℃ (preferably 80℃) for 1~3h (preferably 2h) → drying at 120~180℃ (preferably 150℃) for 1~3h (preferably 2h) → drying at 220~280℃ (preferably 250℃) for 0.5~2h (preferably 1h) → drying at 280~320℃ (preferably 300℃) for 0.5~2h (preferably 1h). After imidization, a PI electrospun fiber membrane is obtained and sealed for later use.

[0066] In step S130, by employing a "gradient imidization" process, the degree of imidization of PI can be precisely controlled during subsequent thermal imidization, thereby regulating the mechanical properties (flexibility / rigidity) and chemical stability of the fiber membrane. Simultaneously, the incompletely converted carboxylic acid and amide groups remaining after imidization, as well as some unreacted amino groups, can become active sites for anchoring MOFs (e.g., through coordination or hydrogen bonding), which is beneficial for the in-situ growth of MOFs. Furthermore, the electrospun fiber membrane itself has a three-dimensional network structure and high specific surface area, providing abundant nucleation sites for MOFs, enabling MOFs to grow uniformly and firmly on the fiber surface and within the fiber network, forming a tightly bound composite structure of PI and MOFs, thus preventing MOF aggregation and detachment.

[0067] S140. The PI electrospun fiber membrane is subjected to plasma pretreatment to obtain a surface-activated PI electrospun fiber membrane.

[0068] In some preferred embodiments, the plasma pretreatment parameters are: power 30~60W (preferably 45W), treatment time 5~15min (preferably 10min), treatment atmosphere is argon, and vacuum degree is 10~50Pa (preferably 30Pa).

[0069] In step S140, plasma pretreatment can introduce a large number of active functional groups such as hydroxyl and carboxyl groups onto the surface of PI fibers, while increasing the surface roughness of the fibers, thereby increasing the nucleation sites for subsequent MOF growth, significantly improving the interfacial bonding strength between MOF and PI matrix, and preventing MOF from falling off during cycling.

[0070] S150. The sulfonated modified bimetallic MOF reaction solution and the surface-activated PI electrospun fiber membrane are placed in a reaction vessel and reacted using a solvothermal in-situ growth method to obtain a PI@sulfonated modified bimetallic MOF composite fiber membrane.

[0071] In some preferred embodiments, the sulfonated modified bimetallic MOF reaction solution includes: a Zr source, an Fe source, an amino-modified organic ligand, a sulfonating modifier, and an organic solvent. Furthermore, the above components are ultrasonically dispersed for 10-20 minutes (preferably 15 minutes) until homogeneous to obtain the reaction solution.

[0072] As a further preferred embodiment, the molar ratio of the Zr source to the Fe source is (2~5):1, wherein the Zr source is ZrCl4 or zirconium oxychloride, and the Fe source is FeCl3·6H2O or Fe(NO3)3·9H2O. This bimetallic system combines the excellent water stability of Zr-based MOFs with the good proton conduction potential of Fe-based MOFs, thereby increasing the thermal decomposition temperature of MOFs to above 380℃ through synergistic effects, while simultaneously enhancing the adsorption and confinement capacity for phosphoric acid.

[0073] As a further preferred option, the molar ratio of Zr to Fe can preferably be 3:1, wherein the Zr source can preferably be ZrCl4 with a content of 0.3~0.5g, and the Fe source can preferably be FeCl3·6H2O with a content of 0.1~0.2g.

[0074] As a further preferred embodiment, the amino-modified organic ligand is 2-aminoterephthalic acid or 2-aminonaphthalenedicarboxylic acid, the sulfonating modifier is chlorosulfonic acid or aminosulfonic acid, and its addition amount is 5-15% of the mass of the organic ligand; the organic solvent is at least one of DMF and NMP. The sulfonating modifier introduces sulfonic acid groups (-SO3H) onto the surface of the MOF channels, forming a synergistic proton transport system with phosphoric acid, significantly improving proton conductivity.

[0075] As a further preferred embodiment, the amino-modified organic ligand is preferably 2-aminoterephthalic acid, with a content preferably of 0.2~0.3g; the sulfonating modifier is preferably chlorosulfonic acid, with a content preferably of 0.01~0.04g; and the organic solvent is preferably DMF, with a content preferably of 30~50mL.

[0076] In step S150, under the conditions of using the above-mentioned Zr source, Fe source and corresponding ligands and sulfonating modifier, a sulfonated modified bimetallic MOF composite fiber membrane is grown on the surface of PI fiber.

[0077] It should be noted that before the solvothermal in-situ growth reaction given in step S150, the PI electrospun fiber membrane should be cut to a suitable size and ultrasonically cleaned to remove surface impurities. For example, the PI electrospun fiber membrane is cut into samples of 4~6cm×4~6cm (preferably 5cm×5cm), and ultrasonically cleaned with at least one of anhydrous ethanol and acetone (preferably anhydrous ethanol) for 5~15min (preferably 10min) to remove surface impurities. After drying, it is placed in the above-mentioned reaction vessel solution and ensured to be completely immersed.

[0078] It should be noted that the specific process of step S150 in this embodiment is as follows: First, the sulfonated modified bimetallic MOF reaction solution is poured into the reaction vessel. Then, the cleaned PI electrospun fiber membrane is placed into the reaction vessel. The reaction vessel is then kept at a constant temperature of 100~150℃ (preferably 120℃) in an oven. The MOF growth is controlled by adjusting the reaction time (6~24h, preferably 12h). During the reaction, a programmed temperature rise is adopted, with a temperature rise rate of 3℃ / min. The programmed temperature rise can avoid the aggregation problem caused by rapid nucleation of MOF, so that MOF grows uniformly on the surface and inside of PI fiber, forming a three-dimensional interpenetrating network structure. After the reaction is completed, the membrane is cooled to room temperature and the PI@sulfonated modified bimetallic MOF composite fiber membrane is taken out. Subsequently, the prepared composite fiber membrane should be washed 2-4 times (preferably 3 times) with a corresponding solvent (such as DMF) to remove unreacted raw materials, and then soaked in anhydrous ethanol for 8-16 hours (preferably 12 hours) for further purification; finally, the composite fiber membrane is placed in a vacuum drying oven at 50-70°C (preferably 60°C) and dried for 6-10 hours (preferably 8 hours) to obtain PI@sulfonated modified bimetallic MOF composite fiber membrane.

[0079] In step S150, during the preparation of functionalized MOFs on the surface of PI fiber membranes using a solvothermal in-situ growth method, the amino and carboxyl functional groups on the surface and pores of the PI fibers, after plasma pretreatment, interact with the MOF precursor. This allows MOF crystals to grow directly from the PI fibers as "seeds" or "templates," starting at the molecular level on the fiber surface and inside the pores. This forms a tight interface between the MOF and the PI matrix, with chemical bonds or strong hydrogen bonds. Secondly, the PI fiber membrane prepared by electrospinning possesses a three-dimensional, interconnected, high specific surface area porous network structure. During the solvothermal reaction, the precursor solution can fully penetrate the entire membrane interior. MOF crystals grow in situ and uniformly not only on the fiber surface but also in the gaps between fibers, forming a three-dimensional interpenetrating network structure where PI and MOF intertwine. This greatly avoids the aggregation of MOF particles, ensuring the uniform distribution of functional materials (MOF and its subsequently loaded phosphate) throughout the membrane. Meanwhile, the sulfonic acid groups introduced by sulfonation modification and the synergistic effect of bimetallic MOF provide abundant channels for proton transport. Furthermore, the amino (-NH2) functional groups on the surface of the selected MOF form ideal anchoring sites for strong loading through the formation of strong hydrogen bonds NH···O=P, which provides a key proton carrier immobilization mechanism for the preparation of high-performance proton exchange membranes.

[0080] S160. The PI@sulfonated modified bimetallic MOF composite fiber membrane is immersed in a phosphoric acid solution containing silane coupling agent for soaking treatment, and then subjected to hot pressing densification treatment and drying treatment to obtain PI-in-situ functionalized bimetallic MOF / phosphoric acid composite proton exchange membrane.

[0081] In some preferred embodiments, in the phosphoric acid solution containing the silane coupling agent, the mass fraction of the silane coupling agent is 0.3-1.0% of the phosphoric acid solution content, wherein the concentration of the phosphoric acid solution is 5-12 mol / L, and the phosphoric acid solution and the silane coupling agent are stirred evenly before use.

[0082] As a further preferred option, the silane coupling agent is γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane. The silane coupling agent can simultaneously form chemical bonds with the amino group of PI, the hydroxyl group of MOF, and phosphoric acid to construct a stable "PI-MOF-phosphoric acid" tripartite structure, further reducing the phosphoric acid leakage rate and solving the core problem of severe phosphoric acid leakage in high-temperature water systems in existing technologies.

[0083] As a further preferred embodiment, the phosphoric acid solution containing the silane coupling agent is: a phosphoric acid solution with a concentration of 8 mol / L containing 0.5 wt.% γ-aminopropyltriethoxysilane.

[0084] In some preferred embodiments, the PI@sulfonated modified bimetallic MOF composite fiber membrane is immersed in a phosphoric acid solution at a temperature of 20~30°C (preferably 25°C) for 2~6 hours (preferably 4 hours) to ensure that the phosphoric acid fully fills the gaps between the MOF channels and the PI fibers.

[0085] In some preferred embodiments, the soaked composite membrane is removed, excess phosphoric acid on the surface is absorbed with filter paper, and then placed in a hot press for hot pressing densification treatment. The temperature of the hot pressing densification treatment is 60~100℃ (preferably 80℃), the pressure is 3~8MPa (preferably 5MPa), and the time is 20~40min (preferably 30min) to improve the density and mechanical strength of the membrane.

[0086] In some preferred embodiments, the composite membrane is dried in a vacuum drying oven at a temperature of 50~70°C (preferably 60°C) for 4~8 hours (preferably 6 hours) to remove free phosphoric acid from the surface.

[0087] This disclosure first prepares a PI electrospun fiber membrane via a process of "low-temperature solution polymerization-electrospinning-gradient imidization-plasma pretreatment". Then, using ZrCl4 (or zirconium oxychloride) as the Zr source, FeCl3·6H2O (or Fe(NO3)3·9H2O) as the Fe source, and 2-aminoterephthalic acid as the ligand, a sulfonation modifier is added. A sulfonated modified bimetallic MOF is then grown on the PI fiber surface using a "solventothermal in-situ growth" process, yielding a PI@sulfonated modified bimetallic MOF composite fiber membrane. Subsequently, a proton carrier is loaded and the membrane structure is strengthened using a "phosphoric acid impregnation with a silane coupling agent-hot pressing densification" process, ultimately assembling the membrane into an aqueous organic flow battery.

[0088] In another aspect of this disclosure, a PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane is proposed. This PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane is prepared by the method described above. For details of the process, please refer to the above description, which will not be repeated here.

[0089] This disclosure constructs a three-dimensional support framework using PI electrospun fibers. The in-situ grown sulfonated modified bimetallic MOF forms a tight interface with PI through chemical bonding. The regular channels of the MOF provide stable loading sites for phosphoric acid and block active materials such as AQDS through steric hindrance. Combined with the hot-pressing densification process, a triple synergy of "proton conduction-active material blocking-phosphoric acid confinement" is achieved from the mechanism, enabling the membrane to maintain stable performance during long-term operation in a 60°C aqueous environment, which is suitable for the long-term cycling requirements of aqueous organic flow batteries.

[0090] In another aspect of this disclosure, an aqueous organic flow battery is proposed, which includes the PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane described above, as well as a positive electrode electrolyte, a negative electrode electrolyte, electrodes, bipolar plates, and a storage tank.

[0091] In this embodiment, the PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane serves as the core component of the battery, and its function is as follows:

[0092] 1. During charging and discharging, the hydrogen bond network formed by phosphoric acid fixed by the sulfonated modified bimetallic MOF in the exchange membrane of this embodiment works synergistically with the sulfonic acid groups to allow protons (H) to pass through. + It rapidly migrates through the membrane from one side to the other, maintaining the charge balance inside the battery, forming a current loop, and effectively improving proton conductivity;

[0093] 2. The size sieving effect of the regular nanopores of MOF and the reduction of macropores between fibers after hot pressing densification effectively prevent larger organic active molecules (such as AQDS) in the positive and negative electrode electrolytes from permeating through the membrane, thereby greatly reducing the self-discharge rate of the battery and improving energy efficiency.

[0094] 3. As a solid separator, it physically separates the positive and negative electrolytes to prevent short circuits. Simultaneously, the high mechanical strength of the PI in the exchange membrane and the strong interfacial bonding between MOF and PI ensure the structural integrity of the battery under long-term charge-discharge cycles and certain pressures.

[0095] 4. The above-mentioned exchange membranes can be applied to frequency regulation scenarios for thermal power energy storage. Their excellent high-temperature stability (heat resistance of Zr-based MOF and PI) and resistance to phosphoric acid leakage (crosslinking effect of silane coupling agent) ensure the stable performance and long cycle life of the battery during long-term operation in a wide temperature range of 40-80℃.

[0096] It should be noted that the PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane of this embodiment can be widely used in the preparation of proton exchange membranes for energy storage systems such as thermal power, wind power, and photovoltaics, and is especially suitable for thermal power energy storage frequency regulation scenarios with stringent requirements for battery response speed, cycle stability and wide temperature range performance.

[0097] The preparation method of PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane will be further explained below with reference to specific embodiments:

[0098] Example 1

[0099] Step 1: PI electrospun fiber membranes were prepared using a low-temperature solution polymerization-electrospinning method. The specific process was as follows: 100 mL of N,N-dimethylformamide (DMF) was poured into a dry four-necked flask, protected with nitrogen gas, and placed in a constant temperature water bath at 25°C. 4.0 g of diamine monomer (4,4'-diaminodiphenyl ether (ODA)) was weighed and slowly added to the flask in two portions, 15 min apart. After all the monomer was added, the mixture was stirred for 30 min until completely dissolved. Subsequently, 4.2 g of pyromellitic dianhydride (PMDA) was weighed and slowly added in two portions, 15 min apart, with continuous stirring for 2 h to form a viscous polyamic acid (PAA) spinning solution with a solid content of 15 wt.%.

[0100] Step 2: Transfer the PAA spinning solution to the syringe of the electrospinning equipment. The PI electrospinning process parameters are: DMF as the spinning solvent, 20kV spinning voltage, 15cm receiving distance, 0.5mL / h feed speed, 25℃ ambient temperature, 40% humidity, and 2h of spinning to obtain PAA fiber membrane.

[0101] Step 3: Finally, the PAA fiber membrane is placed in a vacuum drying oven for gradient temperature imidization. The imidization process is as follows: 80℃ for 2 hours → 150℃ for 2 hours → 250℃ for 1 hour → 300℃ for 1 hour. After imidization, the PI electrospun fiber membrane is obtained and sealed for later use.

[0102] Step 4: Perform plasma pretreatment on the PI electrospun fiber membrane with the following parameters: power 45W, time 10min, argon atmosphere, and vacuum degree 30Pa to obtain a surface-activated PI electrospun fiber membrane.

[0103] Step 5: Sulfonated modified bimetallic MOF was prepared on the surface of PI fiber membrane using a solvothermal in-situ growth method. 0.4 g ZrCl4, 0.15 g FeCl3·6H2O, 0.25 g 2-aminoterephthalic acid, and 0.02 g chlorosulfonic acid were weighed and added to 40 mL of DMF. The mixture was ultrasonically dispersed for 15 min until homogeneous. The surface-activated PI electrospun fiber membrane was cut into 5 cm filaments, ultrasonically cleaned with anhydrous ethanol for 10 min, dried, and then placed in the above reaction vessel solution, ensuring complete immersion. The reaction vessel was placed in a 120℃ oven for 12 h at a heating rate of 3℃ / min. After the reaction, the membrane was cooled to room temperature, removed, washed three times with DMF, and then further purified by soaking in anhydrous ethanol for 12 h. Finally, it was dried in a 60-step vacuum drying oven for 8 h to obtain the PI@sulfonated modified bimetallic MOF composite fiber membrane.

[0104] Step 6: Preparation of PI@sulfonated modified bimetallic MOF / phosphate composite membrane. Prepare a phosphoric acid solution containing a silane coupling agent: Add 0.5 wt.% γ-aminopropyltriethoxysilane to an 8 mol / L phosphoric acid solution and stir until homogeneous. Immerse the PI@sulfonated modified bimetallic MOF composite fiber membrane in this solution and soak at 25℃ for 4 hours. After removal, blot off excess phosphoric acid with filter paper and place in a hot press. Hot press at 80℃ and 5 MPa for 30 minutes. Finally, dry the composite membrane in a 60℃ vacuum drying oven for 6 hours to remove free phosphoric acid from the surface, obtaining a PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane.

[0105] Example 2

[0106] The difference between this example and Example 1 is that the in-situ growth time of the sulfonated modified bimetallic MOF is 6 hours (within the protection range of 6~24 hours). The remaining experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane is obtained.

[0107] Example 3

[0108] The difference between this example and Example 1 is that the in-situ growth time of the sulfonated modified bimetallic MOF is 24h (within the protection range of 6~24h). The remaining experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane is obtained.

[0109] Example 4

[0110] The difference from Example 1 is that the phosphoric acid impregnation concentration is 5 mol / L (within the protection range of 5~12 mol / L), while the rest of the experimental steps and parameters are the same as in Example 1, and finally P-in-situ functionalized bimetallic MOF / phosphoric acid composite proton exchange membrane is obtained.

[0111] Example 5

[0112] The difference from Example 1 is that the phosphoric acid impregnation concentration is 10 mol / L (within the protection range of 5~12 mol / L), while the rest of the experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphoric acid composite proton exchange membrane is obtained.

[0113] Example 6

[0114] The difference from Example 1 is that the phosphoric acid impregnation concentration is 12 mol / L (within the protection range of 5~12 mol / L), while the rest of the experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphoric acid composite proton exchange membrane is obtained.

[0115] Example 7

[0116] The difference from Example 1 is that the PI electrospinning voltage is 15kV (within the 15~25kV protection range), while the other experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane is obtained.

[0117] Example 8

[0118] The difference from Example 1 is that the PI electrospinning voltage is 25kV (within the protection range of 15~25kV), while the other experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane is obtained.

[0119] Example 9

[0120] The difference from Example 1 is that the hot-pressing densification temperature is 60℃ (within the protection range of 60~100℃), while the rest of the experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane is obtained.

[0121] Example 10

[0122] The difference from Example 1 is that the hot-pressing densification temperature is 100℃ (within the protection range of 60~100℃), while the rest of the experimental steps and parameters are the same as in Example 1, and finally a PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane is obtained.

[0123] Example 11

[0124] The difference from Example 1 is that the Fe source of the sulfonated modified bimetallic MOF was replaced with Fe(NO3)3·9H2O (0.15~0.25g), the sulfonating modifier was replaced with aminosulfonic acid (0.02~0.03g), the growth temperature was 130℃ (within the protection range of 100~150℃), the reaction time was 12h, and the remaining experimental steps and parameters were the same as in Example 1, finally obtaining a P-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane.

[0125] Example 12

[0126] The difference from Example 1 is that the PI monomer was replaced with pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenylmethane (MDA) (which are within the protection range of diamine monomers), with a molar ratio of 1:1 and amounts of 4.28 g PMDA and 4.52 g MDA, respectively. The remaining experimental steps and parameters were the same as in Example 1, and the PI-in-situ functionalized bimetallic MOF / phosphate composite proton exchange membrane was finally obtained.

[0127] Comparative Example 1

[0128] Only PI electrospun fiber membranes were prepared without plasma pretreatment, in-situ growth of sulfonated modified bimetallic MOFs, and phosphoric acid impregnation. The specific steps were as follows: 80 mL DMF was used as solvent, 3.5 g ODA and 3.8 g PMDA were used as monomers, the spinning voltage was 18 kV, the receiving distance was 14 cm, the feed speed was 0.4 mL / h, and the imidization process was carried out by drying at 60 °C for 3 h, 120 °C for 3 h, 220 °C for 2 h, and 280 °C for 2 h to obtain pure PI fiber membranes.

[0129] Comparative Example 2

[0130] The difference from Example 1 is that plasma pretreatment and in-situ growth of sulfonated modified bimetallic MOF are not performed. Instead, pure PI electrospun fiber membrane is directly immersed in 8 mol / L phosphoric acid solution (without silane coupling agent) for 4 h. The subsequent hot pressing densification (80 °C, 5 MPa, 30 min) and drying (60 °C vacuum drying for 6 h) steps are the same as in Example 1 to obtain PI / phosphoric acid composite membrane.

[0131] Comparative Example 3

[0132] The difference from Example 1 is that a sulfonated modified bimetallic MOF was added using a physical mixing method (the amount added was equivalent to the in-situ growth amount in Example 1). That is, the pre-synthesized sulfonated modified bimetallic MOF was directly mixed into the PAA spinning solution for electrospinning. The spinning parameters, imidization, phosphoric acid impregnation (containing silane coupling agent, 8 mol / L, 4 h) and hot pressing steps (80 °C, 5 MPa, 30 min) were the same as in Example 1, resulting in a physically mixed PI-sulfonated modified bimetallic MOF / phosphoric acid composite membrane.

[0133] Table 1. Composite membrane results for each embodiment and comparative example.

[0134]

[0135] In summary, based on Examples 1-12 and Table 1, the synergistic effect of plasma pretreatment and in-situ growth of sulfonated modified bimetallic MOFs is the core factor in improving the performance of the composite membrane in aqueous organic flow batteries. The pure PI membrane obtained in Comparative Example 1, lacking a proton transport medium, exhibited a proton conductivity of only 0.005 S / cm in an aqueous environment at 60°C. Although the AQDS cross-permeability was low (1.2 × 10⁻⁶), the overall performance was still relatively good. -9 cm 2 While the conductivity of the PI / phosphate membrane obtained in Comparative Example 2 was increased to 0.042 S / cm by introducing phosphoric acid as a proton carrier, the PI fiber gaps provided weak constraint on phosphoric acid, resulting in a phosphoric acid leakage rate as high as 6.8% in aqueous electrolytes. Furthermore, the membrane structure lacked density, leading to an AQDS cross-permeability of 3.8 × 10⁻⁶. -9 cm 2 The cross-permeability of active materials in the electrolyte can severely affect the cycle stability of flow batteries, leading to self-discharge and reduced coulombic efficiency. Examples 1-3 and 11, however, utilize plasma pretreatment to activate the PI fiber surface, followed by in-situ growth of sulfonated modified bimetallic MOFs to construct a synergistic proton transport system of "bimetallic MOF channels-sulfonic acid groups-phosphoric acid." This system achieves synergistic optimization of proton conduction, active material barrier, and phosphoric acid confinement: the high specific surface area and regular channels of the MOF provide stable loading sites for phosphoric acid. The amino groups (-NH2) on the MOF form NH···O hydrogen bonds with phosphoric acid, which, combined with the chemical bonding of the silane coupling agent, significantly enhances the confinement of phosphoric acid, reducing phosphoric acid leakage to 2.1%~3.1%. Simultaneously, the nanoscale channels of the MOF can create steric hindrance for AQDS molecules (approximately 1.2 nm in size), hindering their permeation and controlling the AQDS cross-permeability to 1.6 × 10⁻⁶. -9 ~2.5×10 -9 cm 2 / s. Regarding the impact of MOF growth time, Example 2 (6h growth) suffered from insufficient MOF loading, limited number of pores, and a small amount of proton transport carrier, resulting in a conductivity of only 0.056 S / cm. Furthermore, the steric hindrance effect was weak, leading to an AQDS permeability of 2.5 × 10⁻⁶. -9 cm 2 / s; Example 3 (growth 24h): With sufficient MOF loading and high pore density, the conductivity increased to 0.078 S / cm, and the AQDS permeability decreased to 1.6×10 -9 cm 2 / s, and the MOF and PI fiber interface are tightly bonded, with a tensile strength of 19.2 MPa. This further verifies that the plasma pretreatment and sulfonation modified bimetallic MOF precisely match the core requirements of aqueous organic flow batteries for proton exchange membranes through the quadruple effects of "pore-confined proton transport + steric hindrance to block active materials + interfacial bonding to strengthen the structure + phosphoric acid chemical cross-linking constraint".

[0136] Furthermore, referring to Table 1, it can be seen that process parameters such as spinning voltage, hot-pressing temperature, and phosphoric acid concentration affect the overall performance (proton conduction, active material barrier, and structural stability) of the composite membrane by regulating its microstructure. Regarding the spinning voltage, in Example 7 (15kV), due to the lower voltage, the spun PI fibers were coarser, resulting in a relatively lower membrane porosity. Although the dense fiber packing reduced the AQDS cross-permeability to 1.7 × 10⁻⁶, this did not significantly reduce the overall performance. -9 cm 2 / s, with a tensile strength as high as 20.3 MPa, but the number of proton transport channels decreased, and the aqueous conductivity at 60℃ dropped to 0.070 S / cm; Example 8 (25kV) had a higher voltage, finer fiber diameter, increased porosity, and more proton transport channels, increasing conductivity to 0.076 S / cm, but the fiber packing density decreased slightly, and the AQDS permeability increased to 2.1×10 -9 cm 2 / s, the tensile strength decreased to 17.2 MPa, indicating that by controlling the fiber diameter and porosity, the spinning voltage achieved a balance between the membrane's proton conductivity, active material barrier properties, and mechanical properties, thus meeting the structural and performance requirements of long-term cycling in flow batteries. Regarding the hot-pressing temperature, Example 9 (60℃) had insufficient PI fiber melting and fusion due to the low temperature, resulting in poor membrane density. This led to easy leakage of phosphoric acid (leakage rate 2.7%) and easy penetration of AQDS molecules, with a permeability reaching 2.4 × 10⁻⁶. -9 cm 2 / s, and the inter-fiber forces are weak, with a tensile strength of only 16.5 MPa; In Example 10 (100℃), the temperature was suitable, and the PI fiber surface was moderately melted, which improved the membrane density (phosphoric acid permeation rate 2.2%, AQDS permeation rate 1.5×10). -9 cm 2The tensile strength reaches 19.5 MPa, while enhancing the interfacial bonding between fibers and between fibers and MOF, without damaging the pore structure of MOF, and maintaining a conductivity of 0.079 S / cm, making it the optimal thermo-pressing parameter for flow batteries. Regarding phosphoric acid concentration, Example 4 (5 mol / L) had insufficient phosphoric acid loading and a low proton transport carrier, resulting in a conductivity of only 0.048 S / cm, which could not meet the high power output requirements of flow batteries. Example 6 (12 mol / L), although having sufficient phosphoric acid loading and a conductivity of 0.086 S / cm, had excessive phosphoric acid exceeding the pore constraint capacity of MOF, leading to a leakage rate of 4.2% in aqueous electrolyte. Furthermore, excessive phosphoric acid weakened the inter-chain forces of PI molecules, causing the tensile strength to drop to 16.9 MPa. Simultaneously, local defects appeared in the membrane structure, and the AQDS permeability increased to 2.6 × 10⁻⁶. -9 cm 2 / s indicates that the phosphoric acid concentration needs to be controlled within a reasonable range of 8~10 mol / L in order to achieve synergistic optimization of proton conduction, active material barrier and structural stability, and to match the actual operating requirements of aqueous organic flow batteries.

[0137] Furthermore, referring to Table 1, the core difference between the raw material system and the composite method lies in the strength of the interfacial interaction, which determines the upper limit of the composite membrane's performance in aqueous organic flow batteries. The tightness of the interfacial bonding directly affects the structural stability of the membrane under the scouring of the circulating electrolyte, while also influencing the continuity of the proton transport channel and the blocking effect of the active material. Regarding the raw material system, Example 12 replaced the PI monomer with PMDA+MDA. The methylene (-CH2-) in the MDA molecule slightly improves the flexibility of the PI molecular chain, enhancing the membrane's anti-swelling and anti-fatigue properties in the aqueous electrolyte, while maintaining a high heat-resistant framework (thermal decomposition temperature 392℃), a tensile strength of 21.1 MPa, and without affecting the interfacial bonding between MOF and PI or the phosphoric acid loading. The aqueous conductivity at 60℃ remains at 0.072 S / cm, and the AQDS cross-permeability is controlled at 1.9 × 10⁻⁶. -9 cm 2 The result shows that the PI framework constructed from different diamine monomers can form a good synergistic effect with the in-situ grown MOF, providing a diverse range of raw material options for proton exchange membranes in flow batteries. In Example 11, the Fe source was replaced with Fe(NO3)3·9H2O, the sulfonation modifier was replaced with aminosulfonic acid (0.02~0.03 g), the growth temperature was 130℃, the reaction time was 12 h, and the final conductivity reached 0.071 S / cm, with an AQDS permeability of 2.3 × 10⁻⁶. -9The efficiency of the sample was measured in cm² / s, verifying the wide adaptability of the raw material system. Regarding the composite method, Comparative Example 3 (physically mixed MOF) exhibited weak interfacial bonding due to the physical adsorption between MOF and PI fibers. Under the influence of aqueous electrolyte, interfacial defects easily arose, leading to discontinuous proton transport channels (conductivity 0.059 S / cm). This resulted in MOF detachment and membrane structure damage, causing not only a phosphoric acid leakage rate of 3.5% but also an AQDS cross-permeability increase to 2.9 × 10⁻⁶. -9 cm 2 / s, with a tensile strength of only 15.5 MPa, which cannot meet the long-term cycling requirements of flow batteries; while Examples 1-3 adopted an in-situ growth method, in which MOF was fixed on the PI fiber surface through chemical bonding (interaction between amino groups on the PI surface and MOF ligands), resulting in tight interfacial bonding without obvious defects. It also exhibits excellent structural stability in aqueous electrolytes. Not only is its overall performance significantly better than that of the physical mixing system, but it can also ensure the performance stability of the flow battery during long-term cycling, thus clarifying the superiority of the in-situ growth composite method in the preparation of proton exchange membranes for aqueous organic flow batteries.

[0138] Furthermore, refer to Figure 2 The differences in membrane mechanical properties revealed in Example 12, where the PI monomer is PMDA+MDA, maintains a stress of approximately 23 MPa at 6% strain and a fracture strain exceeding 8%. Comparative Example 2, with its PI / phosphate membrane, shows a stress below 20 MPa at 6% strain and premature fracture. Mechanistic analysis: Example 12 utilizes a PMDA+MDA-constructed PI framework. The methylene (-CH2-) enhances molecular chain flexibility, while the in-situ growth of MOF and the interfacial chemical bonding between PI strengthens structural support, resulting in a membrane with both high tensile strength and good toughness, capable of withstanding mechanical stress during flow battery assembly and cycling. In contrast, Comparative Example 2 lacks MOF reinforcement and relies solely on the physical support of PI fibers for phosphoric acid. The intermolecular chain forces are weakened by the phosphoric acid, leading to significant deterioration in mechanical properties and a higher risk of membrane breakage during battery operation.

[0139] Furthermore, refer to Figure 3The membrane cycling stability results show that the composite exchange membrane obtained by growing MOF for 24 hours in Example 3 retains approximately 67% of its capacity after 150 cycles (decreasing from 650mAh to 435mAh); while the membrane obtained by physically mixing MOF in Comparative Example 3 retains only about 52% of its capacity after 150 cycles (decreasing from 750mAh to 400mAh). Mechanistic analysis: In Example 3, the MOF is grown in situ on the surface of PI fibers, resulting in tight interfacial bonding. The MOF channels provide stable confinement of phosphoric acid, and the proton transport channels and active material barrier structure are not easily damaged during cycling. In contrast, the MOF in Comparative Example 3 is physically mixed and gradually detaches under electrolyte scouring, leading to increased phosphoric acid leakage (proton carrier loss) and interfacial defects in the membrane (increased cross-permeability), ultimately resulting in faster capacity decay.

[0140] Furthermore, refer to Figure 4 The active material barrier properties of the membranes given show that, for the composite exchange membrane obtained by hot-pressing at 100°C in Example 10, the voltage dropped from 1.1V to 0.7V after 240 hours, a decrease of approximately 36%. In contrast, the PI / phosphate membrane in Comparative Example 2 showed a voltage decrease from 1.1V to 0.8V, a decrease of approximately 27% (the slower voltage decrease is due to the lower conductivity of Comparative Example 2; the actual self-discharge is primarily due to active material permeation). This is because the membrane density of Example 10 increased after hot-pressing at 100°C, and the steric hindrance of the MOF channels effectively blocked permeation; the self-discharge mainly stemmed from the slow migration of phosphate protons. In contrast, although Comparative Example 2 had low conductivity, its loose membrane structure allowed for easy permeation through fiber gaps, resulting in more severe cross-self-discharge of active materials (the contradiction between capacity and voltage decay stemmed from the artificially high initial capacity of Comparative Example 2), highlighting the enhancing effect of in-situ MOF growth combined with hot-pressing on barrier performance.

[0141] Furthermore, refer to Figure 5 The electrochemical performance of the membranes given shows that the composite exchange membrane formed in Example 5 with a phosphoric acid concentration of 10 mol / L has a charging voltage plateau of approximately 0.8 V, a discharging voltage plateau of approximately 0.6 V, and a voltage difference of approximately 0.2 V. The composite exchange membrane formed in Example 1 with a phosphoric acid concentration of 8 mol / L has a charging voltage plateau of approximately 0.7 V, a discharging voltage plateau of approximately 0.5 V, and a voltage difference of approximately 0.2 V. However, the capacity of Example 5 (0.3 Ah) is higher than that of Example 1 (0.25 Ah). This is because the phosphoric acid concentration in Example 5 is higher, resulting in a more sufficient proton carrier loaded in the MOF pores at 60 mA·cm⁻¹. -2 At higher current densities, proton conduction rates are faster, resulting in higher discharge capacity. In contrast, the phosphoric acid concentration in Example 1 is moderate, and although the capacity is slightly lower, the voltage plateau is more stable. Both examples demonstrate that the in-situ growth of MOFs creates continuous proton channels, avoiding the "conduction-blocking" imbalance problem in the comparative example. The smooth characteristics of the charge-discharge curves also verify the uniformity of ion transport in the membrane.

[0142] In summary, the battery assembled with pure PI membrane (Comparative Example 1) exhibits a proton conductivity of only 0.005 S / cm and a cross-permeability of 1.2 × 10⁻⁶ in a 60°C aqueous environment. -9 cm 2 / s, after 150 cycles, the capacity retention rate is only 50%; while the composite membrane prepared in this disclosure (Example 3, MOF growth for 24 h) has a water conductivity of 0.078 S / cm at 60°C and a cross-permeability as low as 1.6 × 10⁻⁶. -9 cm 2 / s, the assembled aqueous organic flow battery at 60mA·cm -2 At current density, 1 A / g corresponds to a specific capacity of 160 mAh·g. -1 After 150 cycles, the capacity retention rate reached 67%, and the self-discharge voltage decay was only 36% after 240 hours of storage at 80% SOC, providing a new path for the industrialization of high-performance aqueous organic flow batteries.

[0143] It is understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of this disclosure, and this disclosure is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and substance of this disclosure, and these modifications and improvements are also considered to be within the scope of protection of this disclosure.

Claims

1. A method for preparing a PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane, characterized in that, The preparation method includes: An inert gas is introduced into a polar solvent and the solvent is placed in a constant temperature water bath. Diamine monomer and dianhydride monomer are added sequentially and stirred to form a viscous polyamic acid spinning solution. The viscous polyamic acid spinning solution was subjected to electrospinning treatment to obtain a PAA fiber membrane. The PAA fiber membrane was subjected to gradient heating imidization to obtain a PI electrospun fiber membrane. The PI electrospun fiber membrane was subjected to plasma pretreatment to obtain a surface-activated PI electrospun fiber membrane. A solvothermal in-situ growth method was used to react the sulfonated modified bimetallic MOF reaction solution with the surface-activated PI electrospun fiber membrane in a reaction vessel to obtain a PI@sulfonated modified bimetallic MOF composite fiber membrane. The sulfonated modified bimetallic MOF reaction solution included: Zr source, Fe source, amino-modified organic ligand, sulfonating modifier and organic solvent. The PI@sulfonated modified bimetallic MOF composite fiber membrane was immersed in a phosphoric acid solution containing silane coupling agent for soaking treatment, followed by hot pressing densification treatment and drying treatment to obtain a PI-in-situ functionalized bimetallic MOF / phosphoric acid composite proton exchange membrane.

2. The preparation method according to claim 1, characterized in that, The molar ratio of the diamine monomer to the dianhydride monomer is 1:(0.8~1.2); wherein, The diamine monomer is at least one selected from 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, and p-phenylenediamine; The dianhydride monomer is at least one of pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride and biphenyl tetracarboxylic dianhydride; The polar solvent is incubated with an inert gas and placed in a constant temperature water bath at a temperature of 20~30℃. The reaction time after adding the diamine monomer and dianhydride monomer is 1.5 to 3 hours. The solid content of the viscous polyamic acid spinning solution is 12~18 wt.%.

3. The preparation method according to claim 1, characterized in that, The electrospinning treatment of the viscous polyamic acid spinning solution is carried out at a spinning voltage of 15~25kV, a receiving distance of 12~18cm, a feed speed of 0.3~0.8mL / h, an ambient temperature of 20~30℃, a humidity of 30~50%, and a spinning time of 1.5~3h.

4. The preparation method according to claim 1, characterized in that, The PAA fiber membrane is subjected to gradient temperature imidization, including: Dry at 60~100℃ for 1~3 hours; Dry at 120~180℃ for 1~3 hours; Dry at 220~280℃ for 0.5~2 hours; Dry at 280~320℃ for 0.5~2 hours.

5. The preparation method according to claim 1, characterized in that, The plasma pretreatment has a power of 30~60W, a treatment time of 5~15min, a treatment atmosphere of argon or oxygen, and a vacuum degree of 10~50Pa.

6. The preparation method according to claim 1, characterized in that, The molar ratio of the Zr source to the Fe source is (2~5):1, the Zr source is ZrCl4 or zirconium oxychloride, and the Fe source is FeCl3·6H2O or Fe(NO3)3·9H2O; The amino-modified organic ligand is 2-aminoterephthalic acid or 2-aminonaphthalenedicarboxylic acid; The sulfonating modifier is chlorosulfonic acid or aminosulfonic acid, and its addition amount is 5-15% of the mass of the amino-modified organic ligand; The organic solvent is at least one of DMF and NMP.

7. The preparation method according to claim 1, characterized in that, The reaction of the sulfonated modified bimetallic MOF reaction solution and the surface-activated PI electrospun fiber membrane in a reaction vessel is carried out at a temperature of 100~150℃ for 6~24h, and the temperature is programmed to rise at a rate of 2~5℃ / min.

8. The preparation method according to claim 1, characterized in that, In the phosphoric acid solution containing the silane coupling agent, the mass fraction of the silane coupling agent is 0.3~1.0% of the phosphoric acid solution content, and the silane coupling agent is γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane. The concentration of the phosphoric acid solution is 5~12 mol / L; The soaking treatment is performed at a temperature of 20-30°C for 2-6 hours. The hot pressing densification process is carried out at a temperature of 60~100℃, a pressure of 3~8MPa, and a time of 20~40min. The drying process is carried out at a temperature of 50-70°C for 4-8 hours.

9. A PI-in-situ functionalized bimetallic MOF / phosphate proton exchange membrane, characterized in that, The PI-in-situ functionalized bimetallic MOF / phosphoproton exchange membrane is prepared by the preparation method described in any one of claims 1 to 8.

10. An aqueous organic flow battery, characterized in that, The aqueous organic flow battery includes the PI-in-situ functionalized bimetallic MOF / phosphoproton exchange membrane as described in claim 9.

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