Wood nanocellulose bonded COF composite film and preparation method thereof

Abstract

The invention discloses a wood nanocellulose bonded COF composite film and a preparation method thereof. The preparation method comprises the following steps: firstly, preparing a COF nanosheet dispersion liquid by adopting an interfacial polymerization method; then carrying out suction filtration on the wood nanocellulose dispersion liquid and the COF nanosheet dispersion liquid according to a certain solute mass ratio in a vacuum-assisted self-assembly manner to obtain a wood nanocellulose / COF composite film; and finally, carrying out hot pressing treatment on the composite membrane to obtain the wood nanocellulose bonded COF composite proton exchange membrane. Lignin particles in the wood nanocellulose can serve as an adhesive in the composite membrane in hot pressing treatment to further enhance the stability of the composite membrane. The rich sulfonic acid groups on the synthesized COF and the hydrogen bond network at the interface ensure the efficient proton conduction capability of the composite membrane. The preparation method is simple, efficient, mild and controllable, and the prepared composite membrane is beneficial to promoting development and application of the hydrogen energy technology.

Description

Technical Field

[0001] The present invention belongs to the field of proton exchange membranes for fuel cells, and particularly relates to a COF composite membrane bonded with lignin nanocellulose and a preparation method thereof. Background Art

[0002] Fuel cells using hydrogen energy as fuel and proton exchange membranes as solid electrolytes have the characteristics of high energy density, fast startup response, easy modular installation, and convenient operation, and are considered the best alternative energy sources for devices such as electric vehicles, drones, and stationary power supplies. The proton exchange membrane (PEM) is the core component of electrochemical devices such as proton exchange membrane fuel cells (PEMFCs), and mainly undertakes the tasks of transporting protons and isolating the anode and cathode inside the fuel cell. For proton exchange membranes, excellent proton conduction ability and outstanding stability are the keys to ensuring their efficient and stable operation inside fuel cells. So far, the most widely commercially used PEM material is the perfluorosulfonic acid (Nafion) polymer material produced by DuPont in the United States. However, there is a difficult-to-balance Trade-off effect between the proton conductivity and mechanical strength of the Nafion membrane, which severely restricts the further popularization and application of fuel cells. Therefore, developing new proton conduction materials to break through the barriers existing in the Nafion membrane is an important measure to promote the development of the proton exchange membrane and fuel cell industries.

[0003] Covalent organic framework (COF) materials have become an ideal platform for proton conductors due to their regular and ordered nanochannels, pre-designed functional groups, all-organic nature, and excellent chemical / thermal stability. Compared with synthesis methods such as solvothermal, ionothermal, microwave synthesis, and mechanical synthesis, the interfacial polymerization strategy can directly obtain two-dimensional COF nanosheet materials without relying on high temperature, high pressure, and complex equipment. For proton exchange membranes, two-dimensional COF nanosheets with a two-dimensional structure can give full play to their structural advantages to overcome the Trade-off effect existing in the Nafion membrane. However, due to the rigid framework structure, assembling COF nanosheets into pure COF membranes often exhibits problems such as insufficient mechanical strength and poor toughness.

[0004] Chinese Patent Publication No. CN116655972A discloses a tannic acid-bonded covalent organic framework composite proton exchange membrane and a preparation method thereof. The tannic acid nanoaggregates and covalent organic framework nanosheets prepared by thermal polymerization are vacuum-assisted co-assembled into a composite membrane with excellent mechanical strength and proton conductivity.

[0005] Chinese Patent Publication No. CN118522928A discloses a polybenzimidazole / ionic covalent organic framework composite proton exchange membrane, in which an ionic covalent organic framework material is synthesized in-situ in a polybenzimidazole matrix, and then a composite proton exchange membrane is obtained by solvent evaporation. This composite membrane also exhibits excellent mechanical strength and proton conduction performance.

[0006] Therefore, introducing an interlayer material between COF nanosheets to regulate the interaction force between nanosheets is expected to enhance the stability of the COF membrane and fully exert the application potential of the COF membrane in hydrogen energy technology. Summary of the Invention

[0007] In view of the PEM application requirements mentioned in the above background technology, the present invention proposes a lignocellulose-bonded COF composite membrane and its preparation method. Taking COF nanosheets as the main body, lignocellulose is used to regulate the assembly behavior of COF, and a proton exchange membrane material with high stability is obtained through vacuum-assisted self-assembly and hot pressing methods; First, COF nanosheets are synthesized by an interfacial polymerization method; Then, the obtained COF nanosheets and lignocellulose are mixed evenly in a mass ratio of 1:1 to 4:1 and a preliminary lignocellulose / COF composite membrane is obtained by vacuum-assisted self-assembly; Finally, the obtained lignocellulose / COF composite membrane is hot-pressed to obtain a lignocellulose-bonded COF composite proton exchange membrane. The specific steps are as follows: Step 1: Preparation of COF nanosheet dispersion: 2,5-diaminobenzenesulfonic acid and phloroglucinol trialdehyde are dissolved in deionized water and n-octanoic acid solution respectively according to a molar ratio of 3:2 to obtain an amine monomer aqueous phase and an aldehyde monomer oil phase; The aldehyde monomer oil phase is slowly dropped onto the amine monomer aqueous phase according to a volume ratio of 2:3, and the reaction is allowed to polymerize at room temperature for 3-7 days; After the reaction, the oil phase is removed, and at the same time, the dark red aqueous solution is dialyzed and purified to obtain a COF nanosheet dispersion. Step 2: Preparation of lignocellulose / COF composite membrane: The COF nanosheet dispersion obtained in Step 1 and the lignocellulose dispersion are dispersed evenly according to a solute mass ratio of 1:1 to 4:1; The mixed solution is subjected to vacuum-assisted self-assembly on a polyacrylonitrile substrate to obtain a lignocellulose / COF composite membrane. Step 3: Preparation of lignocellulose-bonded COF composite proton exchange membrane: The composite membrane obtained in Step 2 is hot-pressed to obtain a lignocellulose-bonded COF composite proton exchange membrane; The composite membrane is peeled off from the polyacrylonitrile substrate and immersed in a dilute acid solution for standby to obtain a self-supporting composite membrane.

[0008] Furthermore, in the preparation method of the present invention, Preferably, the reaction time for preparing the COF nanosheet dispersion by interfacial polymerization in step 1 is 5 days.

[0009] Preferably, the operating conditions for hot pressing in step 3 are a hot pressing temperature of 100 °C, a pressure of 10 MPa, and a hot pressing time of 1 h.

[0010] Compared with the COF composite membranes prepared by the prior art, the present invention uses green, low-carbon, and inexpensive lignocellulose nanofibers as adhesives for COF nanosheets to prepare a proton exchange membrane material with high mechanical strength and proton conduction performance. In particular, lignocellulose nanofibers can not only increase the interaction force between COF nanosheets but also act as an adhesive during the hot pressing process, thereby further enhancing the mechanical strength and toughness of the formed COF composite membrane. The preparation method of the present invention is mild, controllable, and easy to operate. The prepared composite membrane has excellent stability and proton conduction performance, and is expected to well overcome the Trade-off effect of Nafion membranes. The prepared composite membrane exhibits outstanding application potential in devices such as fuel cells, flow batteries, and electrochemical hydrogen compression. Description of the Drawings

[0011] Figure 1 It is the surface morphology diagram of the scanning electron microscope of composite membranes 1-4 in the examples; Figure 2 It is the cross-sectional morphology diagram of the scanning electron microscope of composite membranes 1-4 in the examples; Figure 3 It is the comparison diagram of the mechanical strength and toughness of composite membranes 1-4 in the examples; Figure 4 It is the comparison diagram of the proton conduction performance of composite membranes 1-4 in the examples; Figure 5 It is the surface morphology diagram of the COF membrane without lignocellulose nanofiber adhesion in the examples. Detailed Embodiments

[0012] The design concept of the wood-derived nanocellulose-bonded COF composite proton exchange membrane proposed by the present invention is as follows: aiming at the problem of weak interaction between COF nanosheets, wood-derived nanocellulose is used to enhance the interaction between COF nanosheets and regulate the assembly behavior of nanosheets. A composite proton exchange membrane material with excellent stability and proton conduction performance is prepared by vacuum-assisted self-assembly and hot pressing methods. Its preparation process mainly includes: preparation of COF nanosheets, preparation of wood-derived nanocellulose / COF composite proton exchange membrane, and preparation of wood-derived nanocellulose-bonded COF composite proton exchange membrane. The thickness of the composite membrane can be flexibly controlled by controlling the mass of wood-derived nanocellulose and COF nanosheets; the lignin contained in wood-derived nanocellulose can melt during the hot pressing process and enhance the adhesion between multi-dimensional components through the formation of an intermolecular hydrogen bond network. The preparation method of the present invention is simple, efficient, mild and controllable, which not only helps to expand the preparation strategy of COF composite membranes, but also helps to realize the high-value utilization of forest biomass materials.

[0013] The following further illustrates the present invention in conjunction with the accompanying drawings and specific examples. However, the following examples are for illustrative purposes only and should not be considered as any limitation to the present invention.

[0014] Example 1 To prepare a wood-derived nanocellulose-bonded COF composite proton exchange membrane, the steps are as follows: Step 1: Preparation of COF nanosheet dispersion: 42.0 mg of phloroglucinol trialdehyde and 28.2 mg of 2,5-diaminobenzenesulfonic acid are respectively dissolved in 40 mL of n-octanoic acid and 50 mL of deionized water to obtain an aldehyde monomer oil phase and an amine monomer aqueous phase; the aldehyde monomer aqueous phase is slowly dropped onto the surface of the amine monomer oil phase and reacted at room temperature for 5 days. After the reaction, the oil phase is removed, and at the same time, the dark red aqueous solution is dialyzed and purified to obtain a COF nanosheet dispersion; the dispersion is freeze-dried, weighed and calibrated to obtain its mass concentration of about 1.2 mg / mL.

[0015] Step 2: Preparation of wood-derived nanocellulose / COF composite proton exchange membrane: 5 mL of the COF nanosheet dispersion prepared in Step 1 and 3.75 g of a wood-derived nanocellulose dispersion with a mass fraction of 0.16% (i.e., the mass ratio of the solutes is 1:1) are fully mixed and uniformly dispersed, and a wood-derived nanocellulose / COF composite proton exchange membrane is prepared by vacuum-assisted self-assembly.

[0016] Step 3: Preparation of wood-derived nanocellulose-bonded COF composite proton exchange membrane: The composite membrane obtained in Step 2 is subjected to hot pressing treatment at a temperature of 100 °C, a pressure of 10 MPa and a hot pressing time of 1 h to obtain a wood-derived nanocellulose-bonded COF composite proton exchange membrane; the composite membrane is peeled off from the substrate to obtain a self-supporting composite membrane 1.

[0017] Example 2 Prepare a wood nanocellulose-bonded COF composite proton exchange membrane. The preparation process is basically the same as that of Example 1, except that in Step 2, 10 mL of the COF nanosheet dispersion prepared in Step 1 and 3.75 g of a wood nanocellulose dispersion with a mass fraction of 0.16% (i.e., the solute mass ratio of the two is 2:1) are fully mixed and homogenized. The finally obtained composite membrane is denoted as Composite Membrane 2.

[0018] Example 3 Prepare a wood nanocellulose-bonded COF composite proton exchange membrane. The preparation process is basically the same as that of Example 1, except that in Step 2, 15 mL of the COF nanosheet dispersion prepared in Step 1 and 3.75 g of a wood nanocellulose dispersion with a mass fraction of 0.16% (i.e., the solute mass ratio of the two is 3:1) are fully mixed and homogenized. The finally obtained composite membrane is denoted as Composite Membrane 3.

[0019] Example 4 Prepare a wood nanocellulose-bonded COF composite proton exchange membrane. The preparation process is basically the same as that of Example 1, except that in Step 2, 20 mL of the COF nanosheet dispersion prepared in Step 1 and 3.75 g of a wood nanocellulose dispersion with a mass fraction of 0.16% (i.e., the solute mass ratio of the two is 4:1) are fully mixed and homogenized. The finally obtained composite membrane is denoted as Composite Membrane 4.

[0020] Comparative Example 1 This comparative example provides a COF proton exchange membrane without adding wood nanocellulose. The preparation process is as follows: Step 1: Preparation of the COF nanosheet dispersion: Dissolve 42.0 mg of trialdehyde phloroglucinol and 28.2 mg of 2,5-diaminobenzenesulfonic acid in 40 mL of n-caprylic acid and 50 mL of deionized water respectively to obtain an aldehyde monomer oil phase and an amine monomer aqueous phase; slowly drop the aldehyde monomer aqueous phase onto the surface of the amine monomer oil phase and place it in a room temperature environment for reaction for 5 days. After the reaction, remove the oil phase, and at the same time dialyze and purify the dark red aqueous solution to obtain a COF nanosheet dispersion; perform freeze-drying and weighing calibration on this dispersion to obtain its mass concentration of about 1.2 mg / mL.

[0021] Step 2: Preparation of the COF proton exchange membrane: Prepare a COF proton exchange membrane from 20 mL of the COF nanosheet dispersion prepared in Step 1 by the method of vacuum-assisted self-assembly, and dry it at ambient temperature.

[0022] Table 1 Preparation process conditions and properties of the obtained composite proton exchange membranes in Examples 1-4 In Table 1, A refers to the COF nanosheet dispersion, and B refers to the lignocellulose nanofiber dispersion.

[0023] By comparing Examples 1-4, it can be concluded that Composite Films 1-4 exhibit a flat morphology and a layered cross-section ( Figure 1-2 ). The mechanical strength of the composite films shows a trend of first increasing and then decreasing with the increase in the doping amount of COF nanosheets ( Figure 3 ). When the solute mass ratio of the COF nanosheet dispersion to the lignocellulose nanofiber dispersion is 2:1, the highest mechanical strength of the composite film is 95.38 MPa. Continuing to increase the doping amount of COF nanosheets will cause the mechanical properties of the composite film to decline, which is mainly due to the insufficient adhesion of lignocellulose nanofibers caused by excessive COF nanosheets. By comparing Examples 1-4, it can be concluded that the proton conductivity of the composite film increases with the increase in the addition amount of COF nanosheets. When the solute mass ratio of the COF nanosheet dispersion to the lignocellulose nanofiber dispersion is 4:1, the proton conductivity of the composite film is 348 mS cm -1 ( Figure 4 ). The COF film directly assembled from COF nanosheets is in a broken state after drying and cannot be tested further, which also further proves the excellent adhesion of lignocellulose nanofibers between COF nanosheets ( Figure 5 ).

[0024] In summary, the present invention uses COF nanosheets as the main body, and with the help of lignocellulose nanofibers, regulates the assembly behavior of COF nanosheets, and obtains a proton exchange membrane material with high stability through vacuum-assisted self-assembly and hot pressing methods. By adjusting the mass ratio and dosage of lignocellulose nanofibers and COF materials, the physical and chemical properties of the formed composite film can be flexibly regulated and optimized to better meet the requirements of energy devices such as fuel cells.

[0025] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many improvements and changes without departing from the purpose of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A wood-derived nanocellulose-bonded COF composite film and a preparation method thereof, characterized in that, It includes the following steps: Step 1: By means of interfacial polymerization, using deionized water and octanoic acid as the aqueous phase and the oil phase at normal temperature and pressure, dissolve 2,5-diaminobenzenesulfonic acid and phloroglucinol trialdehyde in the aqueous phase and the oil phase respectively according to a certain molar ratio. After the reaction is completed, remove the oil phase. After dialysis purification of the obtained aqueous solution containing COF, obtain an aqueous solution of COF nanosheets; Step 2: Blend the aqueous solution of COF nanosheets obtained in Step 1 with the aqueous solution of lignocellulose according to a certain solid mass ratio, and stir well to make it pre-assembled; Step 3: Filter the mixed solution obtained in Step 2 by vacuum-assisted self-assembly to obtain a composite membrane; Step 4: Thermally press the composite membrane obtained in Step 3 to obtain a COF composite proton exchange membrane bonded by lignocellulose, and then place it in a dilute acid solution for acidification for standby.

2. The method according to claim 1, wherein In Step 1, the volume ratio of the aqueous phase to the oil phase is 1:1 to 3:

1.

3. The method according to claim 1, wherein In Step 1, the molar ratio of 2,5-diaminobenzenesulfonic acid to phloroglucinol trialdehyde is 1:1 to 1:

3.

4. The method according to claim 1, characterized in that In Step 2, the lignin content in the lignocellulose solution is about 2 to 5 wt%.

5. The method according to claim 1, characterized in that, In Step 2, the mass ratio of COF nanosheets to lignocellulose is 1:1 to 4:

1.

6. The method according to claim 1, characterized in that In Step 2, the mixing method includes one of shaking, ultrasonic, and stirring.

7. The method according to claim 1, wherein In Step 4, the thermal pressing temperature of the composite membrane is 100 to 120 °C, the pressure is 10 to 15 MPa, and the thermal pressing time is 1 to 2 hours.

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