Phosphoric acid type covalent organic framework proton exchange membrane based on local flexibility of connecting bonds as well as preparation method and application of phosphoric acid type covalent organic framework proton exchange membrane

Abstract

The invention discloses a preparation method of a phosphoric acid type covalent organic framework proton exchange membrane based on connecting bond local flexibility, which comprises the following steps: preparing a phosphoric acid type amine monomer, synthesizing a covalent organic framework nanosheet and preparing a phosphoric acid type covalent organic framework membrane, according to the preparation method, 2, 5-diaminobenzene phosphoric acid and 2, 5-dihydrazinobenzene phosphoric acid are added, the controllable local flexibility of the phosphoric acid type iCOF is regulated and controlled by changing the types and bond angles of connecting bonds, and a conduction unit is endowed with higher local motion ability, so that the flexible iCOF nanosheet maintains the continuity of a short hydrogen bond network through local motion under a low-humidity condition; the ionic covalent organic framework membrane (iCOFMs) self-assembled by the nanosheets shows excellent proton conductivity under high temperature and high humidity, and maintains high proton conductivity under low humidity. Meanwhile, the local flexibility endows the iCOFMs with higher mechanical strength, and the working stability in the battery device is ensured.

Description

Technical Field

[0001] This invention belongs to the field of proton exchange membrane material preparation technology. Specifically, it relates to the preparation of a phosphoric acid-based covalent organic framework proton exchange membrane. Background Technology

[0002] To effectively address the dual challenges of dwindling traditional fossil fuel resources and increasingly severe environmental pollution, the development and utilization of renewable energy has become one of the core strategic directions for future development globally. Hydrogen energy, with its high energy density, zero carbon emissions, and pollution-free characteristics, has become a key candidate for future energy systems. Among the diverse utilization pathways of hydrogen energy, proton exchange membrane fuel cells (PEMFCs) have emerged as one of the most promising application technologies due to their high energy conversion efficiency and near-zero emission clean characteristics. In fuel cells, catalysts tend to operate at high temperatures to accelerate redox reaction kinetics, while high temperatures also simplify hydrothermal management and improve fuel tolerance. However, the decrease in humidity caused by increased temperature places higher demands on the proton exchange membrane. Currently, the proton exchange membrane materials commonly used in PEMFCs are mostly polymers, such as Nafion and SPEEK. Under low humidity conditions, the ion channel contraction and breakage lead to increased conduction resistance and a significant decrease in proton conductivity.

[0003] Covalent organic frameworks (COFs) are a class of crystalline porous polymers formed by strong covalent bonds connecting organic structural units composed of light elements (H, B, C, N, O, etc.). They possess advantages such as tunable structural dimensions and easily modifiable functional groups. The designable crystalline framework structure allows researchers to select specific monomer types to synthesize COFs with predictable structures and physicochemical properties. Based on these characteristics, COFs have become ideal materials for preparing highly efficient proton transport channels.

[0004] Currently, there is an increasing number of research reports on the use of COF materials in proton exchange membranes, including studies on the preparation of pure COF membranes by different methods and the preparation of composite membranes by doping COF into polymers (Nafion, SPEEK, etc.).

[0005] A patent document with publication number CN117276610A and publication date of December 22, 2023, discloses a high-toughness COFs / PTFE composite proton exchange membrane and its preparation method, solving the key problems of poor strength and toughness of covalent organic framework (COF) self-supported membranes and the difficulty in improving the performance of COFs blended membranes. COFs are grown in situ into the interconnected channels of a porous polytetrafluoroethylene (PTFE) network to form a dense composite proton exchange membrane with a continuous microstructure of COFs and PTFE. In the composite membrane, the supporting effect of the PTFE network significantly improves the mechanical strength, toughness, and dimensional stability of the membrane, while sulfonated COFs provide continuous and ordered sulfonic acid functional groups and hydrogen bond networks for rapid proton conduction, thus achieving excellent overall performance. However, the proton conduction of the composite membrane depends on the water-mediated hydrogen bond network. Under low humidity conditions, the continuous aqueous phase will transform into isolated water clusters, causing the hydrophilic channels to shrink and leading to a decrease in membrane proton conductivity.

[0006] A patent document with publication number CN114773550A and publication date July 22, 2022, discloses a rigid-flexible covalent organic framework material and its preparation method. This method introduces a flexible structure into the COF framework, resulting in a rigid-flexible COF material that not only possesses good crystallinity like rigid COFs but also releases a large number of active sites in the flexible structure through a simple dissociation reaction, without the need for additional functional groups introduced through post-functionalization. However, the rigid-flexible COF in this patent document has a certain number of disulfide bonds, giving it a large degree of spatial freedom and torsion. Furthermore, the dissociation reaction causes the breaking of some bonds, creating certain defects that negatively impact the pore order and crystallinity of the COF, hindering the construction of efficient proton transport channels.

[0007] Reference [1] with DOI 10.1002 / adma.202005565 developed a bottom-up method to synthesize intrinsic proton-conducting COF (IPC-COF) nanosheets (NUS-9) in aqueous solution by co-regulating diffusion and solvent, achieving controllable nucleation and in-plane dominant IPC-COF growth. These nanosheets can be conveniently used to prepare IPC-COF films. IPC-COF films with crystalline, rigid ion nanochannels exhibit weak humidity-dependent conductivity over a wide humidity range (30-98%). Although the COF film prepared in this reference achieves water retention through capillary forces, the large spacing between sulfonic acid groups leads to the isolation of each hydrated proton domain, thus forming a discrete short hydrogen bond network.

[0008] Reference [2] with DOI number 10.1038 / s41467-022-33868-8 reported a confined short hydrogen bond (SHB) network that achieves proton transfer in a nearly barrier-free manner. Based on the idea of ​​confined short hydrogen bond (SHB) networks, it reported a SHB network confined on the surface of an ionic covalent organic framework (COF) film modified with dense and uniformly distributed hydrophilic ligands. Combined with experimental and theoretical evidence, the water molecules allocated to each ligand are confined, realizing the local enrichment of hydrated hydrogen ions, accompanied by the formation of SHBs in the water-hydrated hydrogen ion structural domains. These overlapping water-hydrated hydrogen ion structural domains form an interconnected SHB network, realizing rapid proton transfer. However, when the humidity decreases, the continuous aqueous phase will transform into isolated water clusters, the water-mediated continuous short hydrogen bond network will break, resulting in discontinuous proton transport paths and a decrease in proton conductivity.

[0009] Therefore, developing proton exchange membranes with high proton conductivity, especially high proton conductivity under low humidity conditions, has become a key research direction for improving the environmental adaptability and commercial application of batteries.

[0010] [References]

[0011] [1] L.Cao, H.Wu, Y.Cao, C.Fan, R.Zhao, X.He, P.Yang, B.Shi, X.You, Z.Jiang, Advanced Materials, 32(52), 2005565; December 2020.

[0012] [2]B.Shi, X.Pang, S.Li, H.Wu, J.Sheng, X.Wang, C.Fan, L.Cao, T.Zhu, M.Qiu, Z.Yin, Y.Kong, Y.Liu, M.Zhang, Y.Liu, F.Pan, Z.Jiang, Nature Communications, 13(1), 6666; November 5, 2022. Summary of the Invention

[0013] In view of the above-mentioned prior art, the advantages of short hydrogen bonds and flexible linkages in the hydrated proton domain around the phosphate group to enhance the local mobility of the conduction unit make the construction of phosphate-based covalent organic framework materials with local flexibility of linkages a possible way to prepare weakly humidity-dependent proton exchange membranes. Therefore, the present invention provides a method for preparing phosphate-based covalent organic framework proton exchange membranes with local flexibility of linkages.

[0014] To address the aforementioned technical problems, this invention proposes a method for preparing a phosphoric acid-based covalent organic framework proton exchange membrane with localized flexibility based on linkage bonds, comprising the following steps:

[0015] Step 1: Synthesis of phosphoric acid-type covalent organic framework nanosheets: Phosphoric acid-type amine monomers were weighed and dissolved in a 5% trimethylamine aqueous solution to obtain nanosheets with a concentration of 0.00075~0.075 mol / L. -1 A amine monomer solution; trialdehyde phloroglucinol was dissolved in octanoic acid and ultrasonically dispersed to obtain a concentration of 0.0005~0.05 mol L. -1 An aldehyde monomer solution was prepared; the aldehyde monomer solution was slowly transferred to the amine monomer solution using a dropper at a volume ratio of 1:1, and reacted at 16°C for 7-28 days; the lower aqueous phase solution was removed, placed in a dialysis bag, and dialyzed in deionized water for 3-7 days; the dialyzed aqueous solution was removed and diffused in a diffusion cell for 7-14 days to obtain a phosphoric acid covalent organic framework nanosheet dispersion.

[0016] Step 2, Preparation of phosphate-based covalent organic framework proton exchange membranes: The membrane area is related to the amount of phosphate-based covalent organic framework nanosheet dispersion used in a ratio of 0.75-16.7 mL / cm². 2 The phosphoric acid-based covalent organic framework nanosheet dispersion prepared in step one was added to a filtration cup, and the nanosheets were filtered onto a polyacrylonitrile substrate using a water pump. The substrate was then treated with N,N-dimethylformamide and a solution of 0.1 mol / L... -1 After soaking in a sulfuric acid solution for 1 day, a phosphoric acid-type covalent organic framework proton exchange membrane with a thickness of 10~60μm was obtained.

[0017] Furthermore, in the preparation method described in this invention, wherein:

[0018] In step one, the phosphoric acid amine monomer is 3,5-dihydrazide carbonyl phenyl phosphate, 2,5-diaminophenyl phosphate, or 2,5-dihydrazide carbonyl phenyl phosphate.

[0019] The synthesis of the 3,5-dihydrazide carbonyl phenylphosphonic acid comprises: weighing dimethyl 5-aminoisophthalate and sodium tetrafluoroborate at a mass ratio of 1:0.89 and dispersing them in deionized water, wherein the concentration of sodium tetrafluoroborate is 0.093 mg / mL. -1 Stir for 3 hours to obtain solution A; then, add concentrated hydrochloric acid and sodium nitrite solution in a volume ratio of 1.7:1 to solution A, wherein the volume ratio of sodium nitrite solution to solution A is 1:25, stir for 1 hour, filter, wash, and dry to obtain a yellow powder; prepare a mixed solution of phosphorus trichloride and ethyl acetate in a volume ratio of 1:4, denoted as solution B, and weigh the above-prepared yellow powder and cuprous bromide in a mass ratio of 1:0.6 and disperse them in solution B to obtain solution C, wherein the mass concentration of cuprous bromide in solution C is 0.045 g / mL. -1After stirring for 3 hours, deionized water was added, with a volume ratio of deionized water to phosphorus trichloride of 1:0.2. The reaction was continued for 3 hours, followed by filtration, vacuum distillation, and recrystallization to obtain a white solid. This white solid was dispersed in a mixed solution of hydrazine hydrate and ethanol at a volume ratio of 4:1 to obtain a solid concentration of 0.6 g / mL. -1 Solution D was reacted at 80℃ for 36 h, and then filtered, washed and dried to obtain the phosphate-type amine monomer 3,5-dihydrazide carbonylphenylphosphoric acid.

[0020] The synthesis of 2,5-diaminophenylphosphonic acid includes: preparing a mixed solution of diethyl phosphonite, triethylamine, and toluene in a volume ratio of 2.24:2.2:1, denoted as solution E; weighing 2-bromophenyl-1,4-diamine and tetrakis(triphenylphosphine)palladium in a mass ratio of 1:0.25 and dispersing them in solution E to obtain a concentration of 0.18 g / mL of 2-bromophenyl-1,4-diamine. -1 Solution F was reacted at 90℃ for 48 h. After vacuum evaporation and chromatography purification, a purple liquid was obtained; a solution with a concentration of 0.02 g / mL was prepared. -1 A mixture of a purple liquid and anhydrous acetonitrile, denoted as solution G, was prepared by adding trimethylbromosilane to solution G, wherein the volume ratio of trimethylbromosilane to anhydrous acetonitrile was 1:9.1. The mixture was stirred in an N2 environment for 12 h to form a white suspension. Anhydrous methanol was then added to the white suspension, wherein the volume ratio of anhydrous methanol to anhydrous acetonitrile was 1:6, to produce an orange solid. After extraction, recrystallization, filtration, and drying, the phosphoric acid amine monomer 2,5-diaminophenylphosphoric acid was obtained.

[0021] The phosphoric acid covalent organic framework proton exchange membrane obtained according to the above preparation method of the present invention has a proton conductivity of 1111.5 ~ 2399 mS / cm at a temperature of 90°C and a humidity of 100% RH. -1 The membrane exhibits a proton conductivity of 50.8 ~ 594.2 mS / cm at a temperature of 90℃ and a humidity of 40% RH. -1 The mechanical tensile strength of the membrane is 55.8 ~ 61.1 MPa.

[0022] The phosphoric acid-based covalent organic framework proton exchange membrane prepared in this invention was assembled into an H2-O2 fuel cell. At a temperature of 60°C and a humidity of 100% RH, the highest power density reached 265.7~578.4 mW / cm³. -2 .

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] In the preparation method of this invention, the phosphate-type amine monomer is selected, and the controllable local flexibility of the phosphate-type iCOF (covalent organic framework) is regulated by changing the type of linkage (imine bond and hydrazone bond) and the bond angle (120° and 180°). This endows the conductive units with stronger local mobility at high temperatures, enabling the flexible iCOF nanosheets to ensure the continuity of the short hydrogen bond network through the local movement of the conductive units under low humidity conditions. This achieves short hydrogen bond network self-adaptation, and the iCOFMs (ionic covalent organic framework membranes) self-assembled from the nanosheets exhibit excellent proton conductivity (up to 2399 mS cm⁻¹) under high temperature and high humidity conditions. -1 (90℃, 100% RH)) It maintains a high proton conductivity (up to 594.2 mS cm⁻¹) even at low humidity. -1 (90℃, 40% RH). Simultaneously, the localized flexibility endows iCOFMs with stronger mechanical strength (up to 61.1 MPa), ensuring operational stability in battery devices. The performance of this flexible membrane in a proton exchange membrane fuel cell was tested under low humidity conditions, exhibiting high power density. The highest power density measured when this membrane was assembled into an H2-O2 fuel cell was 578.4 mWcm². -2 (60℃, 100% RH). Attached Figure Description

[0025] Figure 1 The graph shows the change in proton conductivity of the membranes prepared in Examples 1, 2, and 3 as a function of temperature.

[0026] Figure 2 The graph shows the change in proton conductivity of the membranes prepared in Examples 1, 2, and 3 as a function of humidity.

[0027] Figure 3 This is a stress-strain curve of the membrane prepared in Example 1;

[0028] Figure 4 This is a graph showing the performance of the fuel cell under high humidity conditions for the membrane prepared in Example 1;

[0029] Figure 5 This is a graph showing the performance of the fuel cell under low humidity conditions using the membrane prepared in Example 1.

[0030] Figure 6 The graph shows the performance of the fuel cell under high humidity conditions in Example 2, where the membrane was prepared.

[0031] Figure 7 This is a graph showing the performance of the fuel cell under high humidity conditions for the membrane prepared in Example 3; Detailed Implementation

[0032] This invention discloses a method for preparing a phosphoric acid-based covalent organic framework (COF) proton exchange membrane (COF) based on the local flexibility of its connecting bonds. The design concept is as follows: Phosphate groups (-PO3H2) are commonly used ionic groups due to their strong acidity, ease of dissociation to provide protons, and strong water retention capacity. In phosphoric acid-based COF materials, the high concentration of hydrated hydrogen ions within the hydrated proton domains formed around the phosphate groups induces water molecule rearrangement, thereby forming short hydrogen bonds and accelerating proton transfer within the hydrated proton domains. During the preparation process of this invention, by selecting specific phosphoric acid amine monomers to control the type and bond angle of the connecting bonds, controllable local flexibility can be imparted to the COF framework, enabling the conductive units on the framework to have localized movement capabilities under thermal stimulation. Regarding enhanced conductivity, introducing phosphate groups into the COF framework can improve the material's water retention and proton conductivity. Adjusting the spacing between phosphate groups to ensure overlap of adjacent hydrated proton domains, thereby constructing a continuous short hydrogen bond network, can further accelerate proton conduction.

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0034] Example 1

[0035] A phosphoric acid-based covalent organic framework proton exchange membrane with localized flexibility based on linkages was prepared. In Example 1, the phosphoric acid amine monomer was 3,5-dihydrazide carbonyl phenylphosphonic acid (Mbh-PO3H2), and the aldehyde monomer was trialdehyde phloroglucinol (Tp). The membrane preparation steps are as follows:

[0036] Step 1, the synthesis of Mbh-PO3H2, includes: weighing 20.92 mg of dimethyl 5-aminoisophthalate and 18.62 mg of sodium tetrafluoroborate and dispersing them in 200 mL of deionized water, stirring for 2 h; then adding 13.6 mL of concentrated hydrochloric acid and 8 mL of sodium nitrite solution, stirring for 1 h, filtering, washing, and drying to obtain a yellow powder. Weighing 3 g of the yellow powder and 1.8 g of cuprous bromide and dispersing them in a mixed solution of 8 mL of phosphorus trichloride and 32 mL of ethyl acetate, stirring for 3 h, then adding 40 mL of deionized water and reacting for 3 h; after filtering, vacuum distillation, and recrystallization, a white solid is obtained; dispersing 3 g of the white solid in a mixed solution of 4 mL of hydrazine hydrate and 1 mL of ethanol, and reacting at 80 °C for 36 h. After the reaction is complete, filtering, washing, and drying yields Mbh-PO3H2.

[0037] Step 2, Synthesis of Phosphoric Acid Covalent Organic Framework Nanosheets: 2.1 mg of Mbh-PO3H2 obtained in Step 1 was dissolved in 10 mL of 5% trimethylamine aqueous solution to prepare an amine monomer solution. 1.1 mg of Tp was dissolved in 10 mL of n-octanoic acid and ultrasonically dispersed to prepare an aldehyde monomer solution. The aldehyde monomer solution was slowly transferred above the amine monomer solution using a dropper and reacted at 16°C for 7 days. The lower aqueous phase solution was removed and placed in a dialysis bag for dialyzing in deionized water for 7 days. The dialyzed aqueous solution was then diffused in a diffusion cell for 14 days to obtain a phosphoric acid covalent organic framework nanosheet dispersion, denoted as TpMbh-PO3H2.

[0038] Step 3: Preparation of a phosphoric acid-based covalent organic framework proton exchange membrane: 30 mL of the TpMbh-PO3H2 nanosheet solution obtained in Step 2 was added to a filtration cup. The nanosheets were then filtered using a water pump into a membrane with an area of ​​1.8 cm². 2 On a polyacrylonitrile (PAN) substrate, N,N-dimethylformamide (DMF) and 0.1 mol L... -1 After soaking in sulfuric acid solution for one day, a phosphoric acid-type covalent organic framework membrane was obtained, denoted as TpMbh-PO3H2 membrane. The thickness of this membrane was 10 μm.

[0039] After testing, such as Figure 1 and Figure 2 As shown, the proton conductivity of the TpMbh-PO3H2 membrane prepared in Example 1 under high temperature and high humidity conditions is 2399 mS / cm. -1 (90℃, 100% RH), the proton conductivity under low humidity is 594.2 mS cm⁻¹. -1 (90℃, 40% RH). For example... Figure 3 As shown, the mechanical tensile strength is 61.1 MPa, and the deformation is 20.2%. Figure 4 and Figure 5 As shown, the highest power density of 578.4 mW / cm² was measured when the TpMbh-PO3H2 membrane was assembled into an H2-O2 fuel cell at 60℃ and 100% RH. -2 The highest power density at 60℃ and 40% RH is 503.8 mW / cm³. -2 .

[0040] Example 2

[0041] A phosphoric acid-based covalent organic framework proton exchange membrane with localized flexibility based on linkages was prepared. In Example 2, the phosphoric acid amine monomer was 2,5-dihydrazide carbonyl phenylphosphonic acid (Pbh-PO3H2), and the aldehyde monomer was trialdehyde phloroglucinol (Tp). The membrane preparation steps are as follows:

[0042] Step 1: Synthesis of phosphoric acid-based covalent organic framework nanosheets, including: dissolving 41.1 mg of Pbh-PO3H2 in 10 mL of 5% trimethylamine aqueous solution to obtain an amine monomer solution; dissolving 21 mg of Tp in 10 mL of n-octanoic acid to obtain an aldehyde monomer solution; slowly transferring the aldehyde monomer solution above the amine monomer solution using a dropper and reacting at 16°C for 16 days; removing the lower aqueous phase solution and placing it in a dialysis bag for dialyzing in deionized water for 5 days; removing the dialyzed aqueous solution and diffusing it in a diffusion cell for 10 days to obtain a phosphoric acid-based covalent organic framework nanosheet dispersion, denoted as TpPbh-PO3H2.

[0043] Step 2, Phosphoric acid covalent organic framework proton exchange membrane: Add 20 mL of the TpPbh-PO3H2 nanosheet solution obtained in Step 1 to a filtration cup, and filter the nanosheets using a water pump to form a membrane with an area of ​​1.8 cm². 2 On a PAN substrate, after treatment with DMF and 0.1 mol L... -1 After soaking in sulfuric acid solution, a phosphoric acid-type covalent organic framework membrane was obtained, denoted as TpPbh-PO3H2 membrane. The thickness of this membrane was 15 μm.

[0044] After testing, such as Figure 1 and Figure 2 As shown, the proton conductivity of the TpPbh-PO3H2 membrane prepared in Example 2 under high temperature and high humidity conditions is 1428.6 mS / cm. -1 (90℃, 100% RH), the proton conductivity under low humidity is 85.5 mS cm⁻¹. -1 (90℃, 40%RH), the mechanical tensile strength is 54.7 MPa. For example... Figure 6 As shown, the highest power density of the H2-O2 fuel cell using the TpPbh-PO3H2 membrane is 400.1 mW / cm². -2 (60℃, 100% RH).

[0045] Example 3

[0046] A phosphoric acid-based covalent organic framework proton exchange membrane with localized flexibility based on linkages was prepared. In Example 2, the phosphoric acid amine monomer was 2,5-diaminophenylphosphonic acid (Pa-PO3H2), and the aldehyde monomer was trialdehyde phloroglucinol (Tp). The membrane preparation steps are as follows:

[0047] Step 1, the synthesis of Pa-PO3H2, includes: weighing 2 g of 2-bromophenyl-1,4-diamine and 0.5 g of tetrakis(triphenylphosphine)palladium and dispersing them in a mixed solution of 4.48 mL diethyl phosphonite, 4.4 mL triethylamine, and 2 mL toluene, and reacting at 90 °C for 48 h. After the reaction, the mixture was purified by vacuum evaporation and chromatography to obtain a purple liquid; 0.06 g of the purple liquid and 3 mL of anhydrous acetonitrile were added to a reaction tube, along with 0.33 mL of trimethylbromosilane, and stirred in a N2 environment for 12 h to form a white suspension. 0.5 mL of anhydrous methanol was added to the white suspension to produce an orange solid. After extraction, recrystallization, filtration, and drying, the obtained product is Pa-PO3H2.

[0048] Step 2, Synthesis of Phosphoric Acid Covalent Organic Framework Nanosheets: 141.1 mg of Pa-PO3H2 obtained in Step 1 was dissolved in 10 mL of 5% trimethylamine aqueous solution to obtain an amine monomer solution. 105.1 mg of Tp was dissolved in 10 mL of n-octanoic acid to obtain an aldehyde monomer solution. The aldehyde monomer solution was slowly transferred above the amine monomer solution using a dropper, and the reaction was carried out at 16℃ for 4 weeks. The lower aqueous phase solution was removed and placed in a dialysis bag for dialyzing in deionized water for 3 days. The dialyzed aqueous solution was then diffused in a diffusion cell for 1 week to obtain a phosphoric acid covalent organic framework nanosheet dispersion, denoted as TpPa-PO3H2.

[0049] Step 3, Preparation of a phosphoric acid-based covalent organic framework proton exchange membrane: 15 mL of the TpPa-PO3H2 nanosheet solution obtained in Step 2 was added to a filtration cup, and the nanosheets were filtered through a water pump into a membrane with an area of ​​1.8 cm². 2 On a PAN substrate, after treatment with DMF and 0.1 mol L... -1 After soaking in sulfuric acid solution, a phosphoric acid-type covalent organic framework membrane was obtained, denoted as TpPa-PO3H2 membrane. The thickness of this membrane was 20 μm.

[0050] After testing, such as Figure 1 and Figure 2 As shown, the proton conductivity of the TpPa-PO3H2 membrane prepared in Example 3 under high temperature and high humidity conditions is 1111.5 mS / cm. -1 (90℃, 100% RH), the proton conductivity under low humidity is 50.8 mS / cm. -1 (90℃, 40%RH), the mechanical tensile strength is 55.8 MPa. For example... Figure 7 As shown, the highest power density of the H2-O2 fuel cell using the TpPa-PO3H2 membrane is 265.7 mW / cm². -2 (60℃, 100% RH).

[0051] Comparative Example

[0052] The amine monomer 4,4'-diamino-3,3'-biphenyl disulfonic acid (BD-(SO3H)2) was dissolved in dimethyl sulfoxide to obtain an amine monomer solution with a concentration of 56.25 mmol / L. The aldehyde monomer trialdehyde phloroglucinol (Tp) was dissolved in N-methylpyrrolidone to obtain an aldehyde monomer solution with a concentration of 37.5 mmol / L. Equal volumes of the above amine and aldehyde monomer solutions were mixed and stirred thoroughly, then sonicated for 15 min. 3 mL of the mixture was cast onto a glass slide coated with a porous PTFE substrate (80% porosity, 8 μm thickness). The slide was then placed in an 80°C oven for in-situ polymerization and drying for 4 days to obtain a BD-(SO3H)2-COFs / PTFE membrane, referred to as the control membrane. This control membrane has a thickness of 20 μm, a COFs mass fraction of 82.4%, a tensile strength of 14.3 MPa at room temperature, and a proton conductivity of 135.0 mS / cm. -1 (90℃, 100% RH). See patent document CN117276610A.

[0053] Table 1 Test data of Examples 1-3 and Comparative Examples

[0054]

[0055] The preparation processes of Examples 1-3 mainly include: synthesizing amine monomer (Pa-PO3H2, Pbh-PO3H2, and Mbh-PO3H2) powders under mild conditions; synthesizing a series of iCOF nanosheets (TpPa-PO3H2, TpPbh-PO3H2, and TpMbh-PO3H2) via phase transfer interfacial polymerization; and preparing a series of iCOFMs (TpPa-PO3H2 film, TpPbh-PO3H2 film, and TpMbh-PO3H2 film) via vacuum-assisted self-assembly. Based on the preparation conditions of Examples 1-3 and the comparative examples, and Table 1, it can be concluded that from TpPa-PO3H2 film to TpPbh-PO3H2 film and then to TpMbh-PO3H2 film, as the COF transitions from rigid to flexible, the proton conductivity gradually increases. This is attributed to the increased frequency of decreased phosphate group spacing due to more pronounced localized movement of the flexible COF after heating, which enhances the continuity of the short hydrogen bond network and improves proton conductivity. Even under low humidity conditions, the flexible TpMbh-PO3H2 membrane can still maintain excellent proton conductivity, and when applied to fuel cells under low humidity conditions, the maximum power density can still reach 503.8 mW / cm². -2 (60℃, 40%RH). Meanwhile, the local flexibility of the connecting bonds endows the membrane with excellent tensile strength.

[0056] In summary, the series of phosphate nanosheets (TpPa-PO3H2, TpPbh-PO3H2, and TpMbh-PO3H2) involved in the preparation method of this invention are synthesized by phase transfer interfacial polymerization, and then a series of phosphate covalent organic framework proton exchange membranes are prepared by vacuum-assisted self-assembly. The preparation method is simple to operate and easy to implement. This invention enhances proton conduction by modifying the type and bond angle of the COF linkages, thereby controlling the local flexibility of the COF framework, and thus adjusting the spacing between phosphate groups and the continuity of the short hydrogen bond network. Compared with traditional polymer membranes (such as Nafion, SPEEK, etc.) and COF membranes, the flexible iCOFMs based on the localized flexibility of the linkage bonds of the present invention provide stronger local mobility and construct an adaptive short hydrogen bond network. This enables the flexible iCOFMs to exhibit excellent proton conductivity and mechanical properties under low humidity conditions, effectively solving the problem of limited proton exchange membrane performance under low humidity conditions.

[0057] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many improvements and changes under the guidance of the present invention without departing from the spirit of the present invention, and these improvements and changes are all within the protection scope of the present invention.

Claims

1. A method for preparing a phosphoric acid-based covalent organic framework proton exchange membrane with localized flexibility based on linkage bonds, characterized in that, Includes the following steps: Step 1: Synthesis of phosphate-based covalent organic framework nanosheets: Weigh out the phosphate-type amine monomer and dissolve it in a 5% trimethylamine aqueous solution to obtain a concentration of 0.00075~0.075 mol L. -1 A amine monomer solution; trialdehyde phloroglucinol was dissolved in octanoic acid and ultrasonically dispersed to obtain a concentration of 0.0005~0.05 mol L. -1 An aldehyde monomer solution was prepared; the aldehyde monomer solution was slowly transferred to the amine monomer solution using a dropper at a volume ratio of 1:1, and reacted at 16°C for 7-28 days; the lower aqueous phase solution was removed, placed in a dialysis bag, and dialyzed in deionized water for 3-7 days; the dialyzed aqueous solution was removed and diffused in a diffusion cell for 7-14 days to obtain a phosphoric acid covalent organic framework nanosheet dispersion. Step 2, Preparation of phosphoric acid-based covalent organic framework proton exchange membranes: The relationship between membrane area and the amount of phosphoric acid-based covalent organic framework nanosheet dispersion is 0.75-16.7 mL / cm². 2 The phosphoric acid-based covalent organic framework nanosheet dispersion prepared in step one was added to a filtration cup, and the nanosheets were filtered onto a polyacrylonitrile substrate using a water pump. The substrate was then treated with N,N-dimethylformamide and a solution of 0.1 mol / L... -1 After soaking in a sulfuric acid solution for 1 day, a phosphoric acid-type covalent organic framework proton exchange membrane with a thickness of 10~60 μm was obtained.

2. The method for preparing a phosphoric acid-type covalent organic framework proton exchange membrane according to claim 1, characterized in that, In step one, the phosphoric acid amine monomer is 3,5-dihydrazide carbonyl phenyl phosphate, 2,5-diaminophenyl phosphate, or 2,5-dihydrazide carbonyl phenyl phosphate.

3. The method for preparing a phosphoric acid-type covalent organic framework proton exchange membrane according to claim 2, characterized in that, The synthesis of the 3,5-dihydrazide carbonyl phenylphosphonic acid includes: Dimethyl 5-aminoisophthalate and sodium tetrafluoroborate were weighed at a mass ratio of 1:0.89 and dispersed in deionized water, wherein the concentration of sodium tetrafluoroborate was 0.093 mg / mL. -1 Stir for 3 hours to obtain solution A; then, add concentrated hydrochloric acid and sodium nitrite solution in a volume ratio of 1.7:1 to solution A, wherein the volume ratio of sodium nitrite solution to solution A is 1:25, stir for 1 hour, filter, wash and dry to obtain yellow powder; A mixed solution of phosphorus trichloride and ethyl acetate with a volume ratio of 1:4 was prepared, denoted as solution B. The yellow powder obtained above and cuprous bromide were weighed and dispersed in solution B at a mass ratio of 1:0.6 to obtain solution C. The mass concentration of cuprous bromide in solution C was 0.045 g / mL. -1 After stirring for 3 hours, deionized water was added, with a volume ratio of deionized water to phosphorus trichloride of 1:0.

2. After reacting for 3 hours, the mixture was filtered, distilled under reduced pressure, and recrystallized to obtain a white solid. The white solid obtained above was dispersed in a mixed solution of hydrazine hydrate and ethanol at a volume ratio of 4:1 to obtain a solid concentration of 0.6 g / mL. -1 Solution D was reacted at 80℃ for 36 h, and then filtered, washed and dried to obtain the phosphate-type amine monomer 3,5-dihydrazide carbonylphenylphosphoric acid.

4. The method for preparing a phosphoric acid-type covalent organic framework proton exchange membrane according to claim 2, characterized in that, The synthesis of the 2,5-diaminophenylphosphonic acid includes: A mixed solution of diethyl phosphonite, triethylamine, and toluene in a volume ratio of 2.24:2.2:1 was prepared and denoted as solution E. 2-Bromophenyl-1,4-diamine and tetrakis(triphenylphosphine)palladium were weighed and dispersed in solution E at a mass ratio of 1:0.25, yielding a concentration of 0.18 g / mL for 2-bromophenyl-1,4-diamine. -1 Solution F was reacted at 90℃ for 48 h, and after vacuum evaporation and chromatography purification, a purple liquid was obtained. Prepare a concentration of 0.02 g / mL -1 A mixture of a purple liquid and anhydrous acetonitrile, denoted as solution G, is added to solution G. Trimethylbromosilane is added to solution G, wherein the volume ratio of trimethylbromosilane to anhydrous acetonitrile is 1:9.

1. The mixture is stirred in an N2 environment for 12 h to form a white suspension. Anhydrous methanol was added to the above white suspension, wherein the volume ratio of anhydrous methanol to anhydrous acetonitrile was 1:6, and an orange solid was generated. After extraction, recrystallization, filtration and drying, the phosphoric acid amine monomer 2,5-diaminophenylphosphoric acid was obtained.

5. A phosphate-based covalent organic framework proton exchange membrane with locally flexible linkages, characterized in that, The phosphoric acid-type covalent organic framework membrane prepared according to any one of claims 1-4 has a proton conductivity of 1111.5 ~ 2399 mS / cm at a temperature of 90°C and a humidity of 100% RH. -1 The membrane exhibits a proton conductivity of 50.8 ~ 594.2 mS / cm at a temperature of 90℃ and a humidity of 40% RH. -1 The mechanical tensile strength of the membrane is 55.8 ~ 61.1 MPa.

6. An application of a phosphate-based covalent organic framework proton exchange membrane with locally flexible linkages, characterized in that, When the phosphoric acid-based covalent organic framework membrane described in claim 5 is assembled into an H2-O2 fuel cell, the highest power density is 265.7~578.4 mWcm³ at a temperature of 60°C and a humidity of 100% RH. -2 .

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