Preparation method of laminated composite electrostatic spinning film and application thereof in photolysis water hydrogen production

Abstract

This invention relates to the technical field of photocatalytic water splitting for hydrogen evolution, and discloses a method for preparing a kelp-like composite electrospun membrane and its application in photocatalytic water splitting for hydrogen evolution. By adding LiCl to the PVDF spinning solution, the charge distribution during the electrospinning process is altered, causing the spinning jet to be electrostatically repelled in the radial direction, resulting in asymmetric stretching and forming flat cross-section fibers with a high β phase. Simultaneously, the PDA interlayer formed by in-situ polymerization on the surface of the flat PVDF-LiCl composite fiber membrane exhibits photoconductive properties under sunlight irradiation, enabling rapid conduction of the potential in the piezoelectric field induced by PVDF deformation to the g-C3N4@Pt surface. Therefore, the kelp-like composite electrospun membrane prepared by this invention can utilize the piezoelectric effect generated only under water flow impact to drive the directional separation of photocatalyst photogenerated carriers, thereby achieving highly efficient photocatalytic water splitting for hydrogen evolution and comprehensive utilization of solar and water flow energy.

Description

Technical Field

[0001] This invention belongs to the technical field of photocatalytic water splitting for hydrogen evolution, and more specifically relates to a method for preparing a kelp-like composite electrospun membrane and its application in photocatalytic water splitting for hydrogen evolution. Background Technology

[0002] Hydrogen (H2), as a typical green and renewable energy source, possesses extremely high energy density and plays a crucial role in solving the carbon emission problem and alleviating the global energy crisis. Solar-driven water splitting, as a typical solar-chemical energy conversion system, provides a promising technological path for efficiently utilizing abundant solar energy and producing clean and sustainable hydrogen fuels.

[0003] Semiconductor-based photocatalysts can utilize solar energy to split water into hydrogen, a green hydrogen production technology that has attracted much attention in recent years. Semiconductor photocatalysts, such as g-C3N4, have been widely explored in the photocatalytic water splitting and hydrogen evolution system due to their suitable electronic band structure and good mechanical stability. However, their photogenerated carriers are prone to recombination in the bulk phase and on the surface, therefore, improving their energy conversion efficiency is urgent. The basic mechanism of the photocatalytic process can be summarized as three ordered steps: light capture, charge separation / transfer, and surface catalytic reaction. Based on this basic understanding of the photocatalytic process, it is easy to see that improving solar energy utilization, promoting the separation and migration efficiency of photogenerated carriers, and accelerating the kinetics of surface redox reactions all contribute to improving photocatalytic activity.

[0004] Wind energy, hydroelectric energy, and tidal energy are all renewable mechanical energy sources inherent in the natural environment. Effectively integrating these clean energy sources and using them to improve the efficiency of photocatalytic water splitting for hydrogen evolution is undoubtedly a technological path that combines environmental friendliness and sustainability. In summary, there is an urgent need to develop a new method that focuses on improving photocatalytic efficiency while efficiently capturing and converting solar and hydroelectric energy from the natural environment, thereby achieving efficient hydrogen evolution from composite materials. Summary of the Invention

[0005] The main objective of this invention is to address the above-mentioned problems by providing a method for preparing a kelp-like composite electrospun membrane and its application in photocatalytic water splitting for hydrogen evolution. This invention achieves high-efficiency piezoelectric / photocatalytic water splitting for hydrogen evolution by increasing the contact area between piezoelectric fibers and g-C3N4 and optimizing the electron transport path.

[0006] This invention modifies the charge distribution of the jet during electrospinning by adding LiCl to the PVDF spinning solution, inducing uneven charge distribution and utilizing electrostatic repulsion to induce asymmetric stretching of the nanofibers, thereby forming a flattened fiber structure. On one hand, the addition of LiCl promotes the formation of more β-phase in the PVDF fibers, thus enhancing the piezoelectric effect and promoting the directional separation of photogenerated carriers. On the other hand, the flattened fiber structure significantly expands the contact area between the fibers and g-C3N4@Pt nanosheets, helping to improve the interaction between the piezoelectric and photocatalytic interfaces. This allows the piezoelectric effect generated during PVDF deformation to more efficiently drive the separation of photogenerated carriers in the g-C3N4@Pt nanosheets, thereby improving the photocatalytic hydrogen evolution performance. Simultaneously, the PVP component added to the spinning solution dissolves after the flattened fiber structure is formed, roughening the surface of the resulting PVDF-LiCl fiber membrane, which is beneficial for subsequent in-situ polymerization and uniform loading of PDA.

[0007] Subsequently, a charge transfer pathway was constructed by in-situ polymerization of a PDA photoconductive interlayer on the surface of the flat PVDF-LiCl composite fiber membrane. This interlayer exhibits photoconductive properties under sunlight irradiation, enabling rapid conduction of the piezoelectric potential induced by PVDF deformation to the g-C3N4@Pt surface. This further promotes the separation of photogenerated carriers and effectively hinders their recombination. Therefore, by constructing a flat fiber structure and a PDA photoconductive layer to enhance the piezoelectric effect and electron transport efficiency of the composite electrospun membrane, the recombination rate of photogenerated carriers can be slowed down, ultimately achieving highly efficient photocatalytic water splitting and hydrogen evolution performance.

[0008] Finally, after filtration and deposition of g-C3N4@Pt, a layer of PVDF fiber membrane is electrospun. The top PVDF fiber membrane adopts a circular cross-section fiber design, which has higher porosity and incident light transmittance compared to flat fibers, ensuring sufficient absorption of incident light by g-C3N4@Pt. This results in a kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite electrospun membrane. Its principle for photocatalytic water splitting and hydrogen evolution is as follows: the kelp-like composite electrospun membrane is placed in a flowing water environment. The piezoelectric effect generated by the water flow impacting the membrane deformation drives the directional separation of photogenerated carriers. Simultaneously, the prepared PDA photoconductive layer constructs a charge transfer pathway at the fiber-catalyst interface. Through the synergistic effect of the piezoelectric effect and the charge transfer pathway, a high hydrogen evolution efficiency is ultimately achieved.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0010] One of the technical solutions of the present invention: a method for preparing a kelp-like composite electrospun film, comprising the following steps:

[0011] (1) Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) and anhydrous lithium chloride (LiCl) were added to a mixed solvent of N,N'-dimethylformamide (DMF) and acetone, and heated and stirred to obtain spinning solution I; polyvinylidene fluoride (PVDF) was added to a mixed solvent of N,N'-dimethylformamide (DMF) and acetone, and heated and stirred to obtain spinning solution II;

[0012] (2) The composite fiber membrane was prepared by electrospinning of spinning solution I. The composite fiber membrane was immersed in water to dissolve polyvinylpyrrolidone (PVP) to obtain a PVDF-LiCl fiber membrane with a flat fiber cross-section structure.

[0013] (3) The PVDF-LiCl fiber membrane was immersed in a dopamine solution for in-situ polymerization to obtain a PVDF-LiCl / PDA composite fiber membrane;

[0014] (4) The g-C3N4@Pt nanosheet suspension was filtered through vacuum to filter the dispersed g-C3N4@Pt onto the surface of the PVDF-LiCl / PDA composite fiber membrane to obtain the PVDF-LiCl / PDA / g-C3N4@Pt composite fiber membrane.

[0015] (5) A PVDF fiber membrane was prepared on the surface of the PVDF-LiCl / PDA / g-C3N4@Pt composite fiber membrane by electrospinning of spinning solution II, and finally a kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite electrospun membrane was obtained.

[0016] Further preferred, in step (1), the mass ratio of polyvinylidene fluoride, polyvinylpyrrolidone and anhydrous lithium chloride in spinning solution I is (1~5):(1~5):(0.01~0.1); and the concentration of polyvinylidene fluoride in spinning solution II is 8~12wt%.

[0017] Because the flattened fiber membrane affects the fiber stretching by adding LiCl to change the charge distribution during electrospinning, a low LiCl addition results in an insignificant flattened fiber structure and limited increase in contact area. A high LiCl addition causes the fiber morphology to revert to a circular shape, reducing the contact area. When the LiCl addition is moderate, a composite fiber membrane with a better flattened structure is formed, increasing the contact area with the photocatalyst and thus improving hydrogen production performance.

[0018] Further preferred, in step (1), the volume ratio of N,N'-dimethylformamide to acetone in the mixed solvent is (1~5):(1~5); the heating and stirring temperature is 35-55℃, and the time is 4-10 h.

[0019] Further preferred, in step (2), the parameters of the electrospinning are: voltage 12-20 kV, speed 0.02-0.08 mL / h, distance between needle tip and collector 10-15 cm; the composite fiber membrane is immersed in water at 70-90 ℃ for 1-5 h to dissolve polyvinylpyrrolidone.

[0020] Further preferred, in step (2), the thickness of the PVDF-LiCl fiber membrane is 20-35 μm.

[0021] Further preferred, in step (3), the preparation of the dopamine solution is as follows: dopamine hydrochloride is added to a buffer solution (mainly sodium tetraborate) with pH = 8.5, and the ratio of the amount of dopamine hydrochloride to the amount of buffer solution added is 1-6 mg: 1-10 mL, to obtain the dopamine solution; the in-situ polymerization temperature is room temperature, and the polymerization time is 12-36 h, more preferably 20-30 h.

[0022] Different polymerization times result in different PDA loadings. Shorter polymerization times result in a thinner PDA layer that doesn't fully cover the fiber surface. Moderate polymerization times lead to a uniform and continuous coating layer on the fiber surface. This layer acts as a photoelectron transport channel, enhancing charge conduction in the piezoelectric field and driving the separation of photogenerated carriers, significantly improving the hydrogen production performance of the composite fiber membrane. Excessive polymerization times result in an overly thick PDA layer, extending the electron transport path and causing some photogenerated electrons to recombine before reaching the reaction sites, thus reducing hydrogen production performance.

[0023] Further preferred, in step (4), the preparation of the g-C3N4@Pt nanosheet suspension is as follows: g-C3N4@Pt nanosheets are added to an ethanol solution, the ratio of the amount of g-C3N4@Pt nanosheets to the amount of ethanol solution added is 1-10 mg: 1-5 mL, and the mixture is stirred and dispersed to obtain the g-C3N4@Pt nanosheet suspension.

[0024] Further preferred, in step (4), the loading amount of the g-C3N4@Pt nanosheets is 14-140 wt% of the mass of the PVDF-LiCl / PDA composite fiber membrane.

[0025] g-C3N4@Pt nanosheets are embedded into the pores of the PVDF-LiCl / PDA composite fiber membrane through a filtration process. The loading of g-C3N4@Pt has a certain impact on the photocatalytic hydrogen production performance of the composite fiber membrane. When the loading is too low, there are insufficient active sites and the light absorption utilization rate is low; when the loading is too high, catalyst agglomeration and mass transfer are hindered. An appropriate loading can maintain good interfacial charge transport and piezoelectric synergy while ensuring light absorption and active sites, thereby maximizing the photocatalytic water splitting and hydrogen evolution performance.

[0026] Further preferred, in step (5), the parameters of the electrospinning are: voltage 12-20 kV, speed 0.02-0.08 mL / h, and distance between the needle tip and the collector 10-15 cm.

[0027] Further preferred, in step (5), the thickness of the PVDF fiber membrane is 1-5 μm; the fiber cross-section of the PVDF fiber membrane is circular, and the fiber diameter is 500-1000 nm.

[0028] The diameter of the top PVDF fiber membrane also affects the hydrogen production performance of the composite fiber membrane. When the diameter is too small, the porosity between the fibers increases, causing the photocatalyst to fall off. When the diameter is too large, the porosity between the fibers decreases, hindering the incident light from contacting the photocatalyst and thus reducing the photocatalytic activity. When the diameter is moderate, it can prevent the photocatalyst from falling off and allow the incident light to contact the photocatalyst well, thereby improving the hydrogen production performance of the composite fiber membrane.

[0029] The second technical solution of the present invention: the application of a kelp-like composite electrospun membrane in photolysis of water and hydrogen evolution in a flowing water environment.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) This invention alters the charge distribution during electrospinning by adding LiCl to the PVDF spinning solution, causing the spinning jet to be electrostatically repelled in the radial direction, resulting in asymmetric stretching and forming a flat cross-section fiber with a high β phase. On the one hand, the high β phase content enhances the piezoelectric properties of the fiber membrane and promotes the generation and separation of photogenerated carriers. On the other hand, the flat structure increases the contact area between the fiber and the catalyst, enhancing the effect of the fiber's piezoelectricity on the catalyst.

[0032] (2) The present invention uses the photoconductive properties of PDA to construct a charge transfer pathway on the fiber surface by in-situ polymerization of PDA layer on the surface of seaweed fiber, thereby delaying the recombination of photogenerated carriers and further improving the hydrogen production efficiency of photocatalyst. Attached Figure Description

[0033] Figure 1The images show SEM images of the composite fiber membranes prepared in Examples 1, 2, and 6 of this invention, as well as photographs of kelp. Specifically, (a) is an SEM image of the PVDF fiber membrane of Example 1; (b) is an SEM image of the flat PVDF-LiCl composite fiber membrane of Example 2; (c) is a photograph of kelp; (d) is an SEM image of the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6; (e) is a cross-sectional SEM image of the flat PVDF-LiCl composite fiber membrane of Example 2; and (f) is a cross-sectional SEM image of the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6.

[0034] Figure 2 The attenuated total reflectance Fourier transform infrared spectra of the photocatalyst (g-C3N4@Pt nanosheets) and the composite fiber membranes prepared in Examples 1, 2, 3, and 6 of this invention.

[0035] Figure 3 The UV-Vis diffuse reflectance spectra and band gap diagrams of the composite fiber membranes prepared by the present invention as photocatalysts (g-C3N4@Pt nanosheets) in Examples 2, 5, and 6 are shown below; where (a) is the UV-Vis absorption curve and (b) is the band gap curve.

[0036] Figure 4 The photoluminescence spectra of the composite fiber membranes prepared in Examples 5 and 6 of this invention are shown.

[0037] Figure 5 The output voltage diagrams are for the composite fiber membranes prepared in Examples 1, 2, and 3 of this invention.

[0038] Figure 6 The XRD patterns of the composite fiber membranes prepared in Examples 1 and 2 of this invention are shown.

[0039] Figure 7 XPS spectra of the composite nanofiber membranes prepared in Examples 3 and 6 of this invention;

[0040] Figure 8 The graphs show the hydrogen production curves and hydrogen production rate diagrams of the photocatalyst (g-C3N4@Pt nanosheets) and the composite nanofiber membranes prepared in Examples 1, 5, and 6 of this invention; wherein, (a) is the hydrogen production curve and (b) is the hydrogen production rate diagram.

[0041] Figure 9 The hydrogen production rate diagrams are for the composite fiber membranes prepared in Examples 5 and 6 of the present invention; wherein, (a) is the hydrogen production rate diagram of Example 5; and (b) is the hydrogen production rate diagram of Example 6. Detailed Implementation

[0042] The following description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0043] Measurement of hydrogen production via photocatalytic water splitting:

[0044] The composite fiber membrane was immersed in a glass tube containing a mixed solution of sacrificial agent triethanolamine (TEOA, 8 mL) and deionized water (20 mL). After sealing and pumping with nitrogen for 30 min, the glass tube loaded with the seaweed-like composite electrospun membrane was exposed to a xenon lamp for photocatalytic water splitting and hydrogen evolution measurement. The distance between the glass tube and the xenon lamp was set to 10 cm, and the light intensity was measured to be 637 W·m using a laser power meter. -2 The intensity is close to that of sunlight. 400 μL of gas is drawn from the sealed glass tube every 1 hour using a syringe (1 mL). The drawn gas is measured by a TCD gas chromatograph, and the hydrogen production rate (HER) is calculated using formula (1):

[0045] (1)

[0046] Where m represents the mass of the photocatalyst (mg), t represents the illumination time (h), and the calibration coefficient is equal to 0.001238968.

[0047] Example 1

[0048] (1) 0.2 g PVDF and 0.2 g PVP were added to a mixed solvent containing 1.5 mL DMF and 0.5 mL acetone and dissolved at 45 °C and 450 rpm for 6 h to obtain a uniform spinning solution.

[0049] (2) The prepared spinning solution was electrospun at a voltage of 12 kV and a speed of 0.3 mL / h, with the receiver and needle distance of 13 cm. Finally, the electrospun membrane was soaked in distilled water at 80°C for 3 h to remove the hydrophilic PVP component, and a PVDF composite fiber membrane with a thickness of 32.5 ± 2.5 μm was obtained.

[0050] Example 2

[0051] (1) 0.2 g PVDF, 0.2 g PVP and LiCl were added to a mixed solvent containing 1.5 mL DMF and 0.5 mL acetone and dissolved at 45 °C and 450 rpm for 6 h to obtain a uniform spinning solution.

[0052] (2) The prepared spinning solution was electrospun at a voltage of 12 kV and a speed of 0.02 mL / h, with the receiver and needle distance being 13 cm. Finally, the obtained electrospun composite membrane was soaked in distilled water at 80°C for 3 h to remove the hydrophilic PVP component, resulting in a PVDF-LiCl fiber membrane with a flat fiber cross-section structure and a thickness of 32.5 ± 2.5 μm.

[0053] In Example 2, three parallel groups were set up to control the amount of LiCl added. A 2 mL precursor solution was prepared (containing 0.2 g PVDF, 0.2 g PVP, and a mixed solvent containing 1.5 mL DMF and 0.5 mL acetone). The DMF and acetone mixed solvent was 2 mL, and its mass was set to 2 g based on its density (approximately 1 g / mL). Based on this solvent mass, 0.5 wt%, 1 wt%, and 2 wt% LiCl, equivalent to the solvent mass, were added respectively. The amounts of LiCl added were 10 mg, 20 mg, and 40 mg, respectively, resulting in spinning solutions. PVDF-LiCl (0.5 wt%), PVDF-LiCl (1 wt%), and PVDF-LiCl (2 wt%) composite fiber membranes were then prepared.

[0054] Example 3

[0055] 6 mg of dopamine hydrochloride was added to 3 mL of buffer solution with a pH of 8.5 to obtain a dopamine solution. Subsequently, the PVDF-LiCl (1 wt%) composite fiber membrane obtained in Example 2 was immersed in the dopamine solution and placed on a shaker at a speed of 150 rpm for in-situ polymerization. Then, unreacted dopamine hydrochloride on the surface was removed with deionized water to obtain a PVDF-LiCl / PDA composite fiber membrane with in-situ polymerization on both the upper and lower surfaces.

[0056] In Example 3, three parallel groups were set up for the immersion time of the dopamine solution, with immersion times of 12 h, 24 h, and 36 h, respectively, resulting in PVDF-LiCl / PDA (12 h) composite fiber membrane, PVDF-LiCl / PDA (24 h) composite fiber membrane, and PVDF-LiCl / PDA (36 h) composite fiber membrane.

[0057] Example 4

[0058] (1) 2 mg of g-C3N4@Pt nanosheets were ultrasonically treated in 2 mL of 75% ethanol solution for 10 min to form a g-C3N4@Pt nanosheet suspension. Subsequently, the g-C3N4@Pt nanosheet suspension was pumped by a vacuum pressure pump for filtration, and the dispersed g-C3N4@Pt was filtered onto the surface of the PVDF composite fiber membrane in Example 1 to obtain a PVDF / g-C3N4@Pt composite fiber membrane;

[0059] (2) 0.2 g of PVDF powder was added to a mixed solvent containing 1.5 mL of DMF and 0.5 mL of acetone and heated and stirred at 45 °C and 450 rpm for 6 h to obtain a spinning solution with a PVDF concentration of 10 wt%.

[0060] (3) The prepared spinning solution was electrospun on the surface of the PVDF-LiCl / PDA / g-C3N4@Pt composite fiber membrane (with the g-C3N4@Pt nanosheets facing upwards) at a voltage of 12 kV, a speed of 0.3 mL / h, and a distance of 13 cm between the receiver and the needle. The thickness of the resulting PVDF fiber membrane was 2.5 μm, and the PVDF / g-C3N4@Pt / PVDF composite fiber membrane was finally obtained.

[0061] Example 5

[0062] (1) 2 mg of g-C3N4@Pt nanosheets were ultrasonically treated in 2 mL of 75% ethanol solution for 10 min to form a g-C3N4@Pt nanosheet suspension. Subsequently, the g-C3N4@Pt nanosheet suspension was pumped by a vacuum pressure pump for filtration, and the dispersed g-C3N4@Pt was filtered onto the surface of the PVDF-LiCl (0.5 wt%) composite fiber membrane, PVDF-LiCl (1 wt%) composite fiber membrane, and PVDF-LiCl (2 wt%) composite fiber membrane in Example 2, respectively, to obtain PVDF-LiCl / g-C3N4@Pt (0.5 wt%) composite fiber membrane, PVDF-LiCl / g-C3N4@Pt (1 wt%) composite fiber membrane, and PVDF-LiCl / g-C3N4@Pt (2 wt%) composite fiber membrane;

[0063] (2) 0.2 g of PVDF powder was added to a mixed solvent containing 1.5 mL of DMF and 0.5 mL of acetone, and heated and stirred at 45°C and 450 rpm for 6 h to obtain a spinning solution with a PVDF concentration of 10 wt%.

[0064] (3) The prepared spinning solution was electrospun on the surface of PVDF-LiCl / g-C3N4@Pt (0.5 wt%) composite fiber membrane, PVDF-LiCl / g-C3N4@Pt (1 wt%) composite fiber membrane, and PVDF-LiCl / g-C3N4@Pt (2 wt%) composite fiber membrane (with the g-C3N4@Pt nanosheets facing upwards). The voltage was 12 kV, the speed was 0.3 mL / h, and the distance between the receiver and the needle was 13 cm. The thickness of the obtained PVDF fiber membrane was 2.5 μm. Finally, PVDF-LiCl / g-C3N4@Pt / PVDF composite fiber membrane, PVDF-LiCl / g-C3N4@Pt / PVDF (1 wt%) composite fiber membrane, and PVDF-LiCl / g-C3N4@Pt / PVDF (2 wt%) composite fiber membrane were obtained.

[0065] Example 6

[0066] (1) 2 mg of g-C3N4@Pt nanosheets were ultrasonically treated in 2 mL of 75% ethanol solution for 10 min to form a g-C3N4@Pt nanosheet suspension. Subsequently, the g-C3N4@Pt nanosheet suspension was pumped by a vacuum pressure pump for filtration. The dispersed g-C3N4@Pt was filtered onto the surface of the PVDF-LiCl / PDA (12 h) composite fiber membrane, PVDF-LiCl / PDA (24 h) composite fiber membrane, and PVDF-LiCl / PDA (36 h) composite fiber membrane in Example 3, respectively, to obtain PVDF-LiCl / PDA / g-C3N4@Pt (12 h) composite fiber membrane, PVDF-LiCl / PDA / g-C3N4@Pt (24 h) composite fiber membrane, and PVDF-LiCl / PDA / g-C3N4@Pt (36 h) composite fiber membrane;

[0067] (2) 0.2 g of PVDF powder was added to a mixed solvent containing 1.5 mL of DMF and 0.5 mL of acetone, and heated and stirred at 45°C and 450 rpm for 6 h to obtain a spinning solution with a PVDF concentration of 10 wt%.

[0068] (3) Electrospinning was performed on the surface of the PVDF-LiCl / PDA / g-C3N4@Pt (12 h) composite fiber membrane, PVDF-LiCl / PDA / g-C3N4@Pt (24 h) composite fiber membrane, and PVDF-LiCl / PDA / g-C3N4@Pt (36 h) composite fiber membrane (with the g-C3N4@Pt nanosheets facing upwards) of the prepared spinning solution. The voltage was 12 kV, the speed was 0.3 mL / h, the distance between the receiver and the needle was 13 cm, and the thickness of the obtained PVDF fiber membrane was 2.5 μm. Finally, the PVDF-LiCl / PDA / g-C3N4@Pt / PVDF (12 h) composite fiber membrane, PVDF-LiCl / PDA / g-C3N4@Pt / PVDF (24 h) composite fiber membrane, and PVDF-LiCl / PDA / g-C3N4@Pt / PVDF (36 h) composite fiber membrane were obtained.

[0069] Performance testing

[0070] (1) SEM images of the composite fiber membranes prepared in Examples 1, 2, and 6 of this invention are shown below. Figure 1 As shown. Under the combined action of high-voltage polarization and mechanical stretching, a uniform charge distribution can produce a pure PVDF fiber membrane with a regular circular fiber cross-section. Figure 1 (a) Figure 1 Image (d) is a SEM image of the composite fiber membrane in Example 6, showing that the fibers in the uppermost PVDF membrane also exhibit a regular circular cross-section. Introducing conductive lithium chloride into the PVDF causes charge accumulation in the conductive region, leading to uneven charge distribution during electrospinning. This, in turn, causes the spinning jet to be subjected to asymmetric stretching by electrostatic repulsion in the radial direction, ultimately resulting in a flattened fiber cross-section. Figure 1 Images (b) and (e) are SEM images of the PVDF-LiCl composite fiber membrane with a flat fiber cross-section in Example 2. The height and width of the fiber cross-section of the prepared flat PVDF-LiCl composite fiber membrane are 250 nm and 1000 nm, respectively. The fiber cross-section of the top PVDF membrane is circular with a fiber diameter of 600 nm. The flat structure can significantly increase the contact area between PVDF nanofibers and g-C3N4@Pt. Figure 1 (f) shows the cross-section of the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane in Example 6, which is compared with... Figure 1 The kelp in (c) exhibits a similar morphological appearance. Meanwhile, Figure 1The dashed box in (f) shows that the PDA interlayer achieves sufficient contact with the photocatalyst. Therefore, this invention successfully prepared a flat fibrous structure by adding LiCl. This structure increases the contact area with the catalyst, and the formed PDA interlayer also achieves sufficient contact with the photocatalyst, which is beneficial to improving hydrogen production efficiency.

[0071] (2) The attenuated total reflection Fourier transform infrared spectra of the photocatalyst (g-C3N4@Pt nanosheets) of the present invention and the composite fiber membranes prepared in Examples 2, 3, and 6 are as follows: Figure 2 As shown. For g-C3N4@Pt, at 800 cm -1 1200-1600 cm -1 and 3000-3300 cm -1 Characteristic peaks of the triazine ring backbone and residual NH groups were observed at 840 cm⁻¹. For the flat PVDF-LiCl composite fiber membrane of Example 2, characteristic peaks were observed at 840 cm⁻¹. -1 and 1250 cm -1 The characteristic absorption peak of the β phase is visible at 3000-3500 cm⁻¹. For the flat PVDF-LiCl / PDA composite fiber membrane of Example 3, after in-situ polymerization of PDA, the characteristic absorption peak of the β phase is visible at 3000-3500 cm⁻¹. -1 Additional vibrational peaks were observed within the range, corresponding to NH and OH groups. (1500 cm⁻¹) -1 A weak absorption peak at the position originates from the stretching vibration of C=C in the aromatic ring. These characteristic absorption peaks indicate that PDA has successfully achieved self-polymerization on the surface of the fiber membrane. In the spectrum of the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6, all characteristic absorption peaks from the g-C3N4@Pt nanosheets, PVDF, and PDA components can be found, which further confirms the successful preparation of the material.

[0072] also, Figure 2 Figure (b) shows the ATR-FTIR spectra of the α and β phases in the PVDF composite fiber membrane of Example 1 and the flat PVDF-LiCl composite fiber membrane of Example 2. The α phase at 761 cm⁻¹ is obtained by integrating the spectrum. -1 The characteristic peak area at 839 cm⁻¹ and the β phase at 839 cm⁻¹ -1 The β phase content can be calculated using formula (1) based on the characteristic band area at the location. The β phase contents of the composite fiber membranes in Examples 1 and 2 (1 wt%) were 77.3% and 82.2%, respectively. This finding indicates that the addition of LiCl can also enhance the polarization of PVDF nanofibers, thereby forming more β phase, which is also beneficial to the piezoelectric effect of the membrane under the action of water flow.

[0073] (3) The attenuated total reflection Fourier transform infrared spectra of the photocatalyst (g-C3N4@Pt nanosheets) of the present invention and the composite fiber membranes prepared in Examples 2, 5, and 6 are as follows: Figure 3 As shown. The UV-Vis absorption performance and band gap of the obtained films were studied using a spectrophotometer equipped with an integrating sphere. The absorption efficiency of g-C3N4@Pt nanosheets in the UV region was improved, indicating that the photocatalyst has a high UV responsiveness. In Example 5, the visible and UV absorption of the kelp-like PVDF-LiCl / g-C3N4@Pt / PVDF composite fiber film was significantly improved compared to the photocatalyst. This is because the photocatalyst is uniformly dispersed on the surface of the fiber film, increasing the contact area of ​​the photocatalyst. In Example 6, the visible and UV absorption of the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber film was further enhanced. This is because: firstly, the catechol groups in PDA and the amino groups in g-C3N4@Pt nanosheets have strong adhesion through hydrogen bonds, which allows g-C3N4@Pt / nanosheets to be uniformly anchored; secondly, the photoconductivity of PDA is conducive to accepting more electrons. When incident light irradiates the surface, the PDA interlayer establishes a charge transfer channel between the flat PVDF-LiCl / PDA composite fiber membrane and the photocatalyst through a redox coupling mechanism involving two electrons and two protons, thereby promoting the photoactivation of the photocatalyst. Therefore, the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6, possessing a flat fiber structure and an additional PDA interlayer, exhibits superior light absorption and capture performance.

[0074] (4) The photoluminescence spectra of the composite fiber membranes prepared in Examples 5 and 6 of the present invention are as follows: Figure 4 As shown. The effect of the PDA interlayer on the charge separation and transfer efficiency in the prepared thin film was studied by photoluminescence spectroscopy. Generally, a higher PL intensity indicates a faster recombination rate of photogenerated carriers. Compared with the kelp-like PVDF-LiCl / g-C3N4@Pt / PVDF composite fiber membrane of Example 5, the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6, after in-situ synthesis of the PDA interlayer on the surface of the PVDF electrospun membrane, showed a significant decrease in PL intensity. This indicates that the PDA interlayer has excellent photoconductivity after light irradiation, which can transfer charge and effectively suppress the recombination of photogenerated carriers. Therefore, the photoconductivity of the PDA interlayer constructs a charge transfer channel between the flat PVDF-LiCl composite fiber membrane of Example 2 and the photocatalyst, which is a key factor in improving photocatalytic activity.

[0075] (5) The output voltage diagrams of the composite fiber membranes prepared in Examples 1, 2, and 3 of the present invention are as follows: Figure 5As shown. Besides the interlayer effect, the piezoelectric effect is another key factor in improving photocatalytic performance in this study. Therefore, the piezoelectric properties of the obtained films were tested. In Example 1, the PVDF fiber membrane had the lowest output voltage, at 0.6 ± 0.1 V. After adding conductive LiCl to the electrospun PVDF solution, the polarization of the PVDF nanofibers was enhanced, which was beneficial to the formation of the β phase. Therefore, in Example 2 (0.5 wt%), the output voltage of the PVDF-LiCl (0.5 wt%) composite fiber membrane slightly increased to 0.7 ± 0.2 V. Notably, when the mass ratio of LiCl increased to 1 wt%, the PVDF-LiCl (1 wt%) composite fiber membrane in Example 2 (1 wt%) exhibited the best piezoelectric performance, with the corresponding output voltage rising to 3.0 ± 0.2 V, which is 5 times that of the pure PVDF electrospun membrane. This significant improvement can be attributed to two reasons: first, the addition of extra LiCl in the spinning solution led to the formation of more piezoelectric β phases in the PVDF nanofibers; second, the contact area between the nanofibers and the copper foil electrode increased when the cross-section of the flat PVDF-LiCl composite fiber membrane in Example 2 changed from circular to flat. This finding also confirms that the piezoelectric field generated after the deformation of the flat PVDF-LiCl composite fiber membrane in Example 2 can better act on the photocatalyst. When the LiCl mass ratio was further increased to 2 wt%, the PVDF-LiCl (2 wt%) composite fiber membrane in Example 2 (2 wt%) returned to a circular shape, and its corresponding output voltage dropped to 2.0 ± 0.2 V. After in-situ synthesis of PDA on PVDF nanofibers, no significant change was observed in the output voltage of the flat PVDF-LiCl / PDA composite fiber membrane in Example 3. This indicates that the additional PDA interlayer does not affect the piezoelectric properties of the PVDF nanofibers.

[0076] (6) The XRD patterns of the composite nanofiber membranes prepared in Examples 1 and 2 of the present invention are as follows: Figure 6 As shown, XRD was used to measure the crystal structure of the PVDF fibers of Example 1 and the flat PVDF-LiCl composite fiber membrane of Example 2. The peaks of the α and β phases in the pure PVDF ionized membrane appeared at 18.5° and 20.5°, respectively. A more significant peak related to the β phase appeared in the flat PVDF-LiCl composite fiber membrane of Example 2. This finding indicates that additional LiCl can enhance the polarization of PVDF nanofibers and form a higher β phase within the nanofibers, thereby contributing to improved piezoelectric properties of the fiber membrane.

[0077] (7) The XPS spectra of the composite nanofiber membranes prepared in Examples 3 and 6 of the present invention are as follows: Figure 7As shown. XPS was used to further investigate the chemical bonding between PDA and PVDF or g-C3N4@Pt. In the C 1s spectrum, the peaks of C-H2, CO / CN, C=O, and CF bonds in the flat PVDF-LiCl / PDA composite fiber membrane of Example 3 were located at 284.6, 285.7, 287.4, and 290.3 eV, respectively. These peaks remained almost unchanged in the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6. In the N 1s spectrum, the peak positions of CN=C and N-(C)3 bonds in the flat PVDF-LiCl / PDA composite fiber membrane of Example 3 and the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6 also showed no significant changes. It is noteworthy that the peak position of the CNH bond in the kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane of Example 6 (400.9 eV) was positively shifted compared to the peak position of the flat PVDF-LiCl / PDA composite fiber membrane of Example 3 (400.4 eV). This is related to the hydrogen bonds formed between the -OH groups in PDA and the -NH2 groups in g-C3N4@Pt nanosheets.

[0078] (8) The photocatalyst of the present invention (g-C3N4@Pt nanosheets), the composite fiber membranes prepared in Examples 4, 5, and 6, and the hydrogen production graphs and hydrogen production rate graphs for water splitting are shown below. Figure 8 As shown. The corresponding photocatalytic hydrogen evolution rate was determined by gas chromatography. During the test, a magnetic stirrer was used to simulate water flow, and the stirring speed was set to 1500 rpm. In Example 4, the HER value of the PVDF / g-C3N4@Pt / PVDF composite fiber membrane was 25600 μmol g after 4 hours. -1 Its average hydrogen evolution rate is 6400 μmol·g -1 ·h -1 It is g-C3N4@Pt nanosheets (1485 μmol·g -1 ·h -1 The efficiency of photogenerated carriers was 4.3 times that of the PVDF nanofibers. This improvement is attributed to the efficient separation of photogenerated carriers facilitated by the water flow-induced piezoelectric field, consistent with the behavior observed in our previous studies. Importantly, the addition of conductive LiCl to the PVDF nanofibers not only increased the β-phase content and enhanced the piezoelectric effect, but also transformed the PVDF nanofibers into a flattened structure. Therefore, the contact area between the PVDF nanofibers and the photocatalyst was increased, further strengthening the piezoelectric effect and increasing the AHER of the kelp-like PVDF-LiCl / g-C3N4@Pt / PVDF composite fiber membrane in Example 5 to 10453 μmol·g⁻¹. -1 ·h -1The efficiency was improved by 63.3% compared to the film without a flat structure. Further in-situ synthesis of PDA interlayers constructed a charge transfer pathway, and the corresponding AHER value in Example 6 significantly increased to 17500 μmol·g⁻¹. -1 ·h -1 This is equivalent to 11.8 times that of a photocatalyst. Therefore, the obtained kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite fiber membrane exhibits excellent photocatalytic performance based on the water flow-triggered piezoelectric effect.

[0079] like Figure 9 As shown, the hydrogen production rate (AHER) was compared for different LiCl addition amounts and different PDA polymerization times. Figure 9 As shown in Figure (a), when the amount of LiCl added is 0.5 wt%, the AHER of Example 5 (0.5 wt%) is 7254 μmol·g. -1 ·h -1 The AHER was slightly improved compared to the sample without LiCl. This is because the amount of LiCl added under these conditions was low, which had a limited effect on increasing the polarization of the PVDF molecular chain and the content of the β phase, and the fiber flattening structure was not obvious, resulting in a limited increase in contact area. When the amount of LiCl added was further increased to 2 wt%, the AHER of Example 5 (2 wt%) was 8440 μmol·g. -1 ·h -1 The concentration decreased compared to the 1 wt% sample. This is because excess LiCl caused the fiber morphology to gradually revert to a circular shape, reducing the contact area and decreasing the piezoelectric output, thus weakening the piezoelectric-photocatalytic synergistic effect. Figure 9 As shown in (b), when the PDA polymerization time is 12 h, the AHER of Example 6 (12 h) is 15177 μmol·g. -1 ·h -1 At this point, the PDA layer was relatively thin and had not yet completely covered the fiber surface. When the PDA polymerization time was increased to 36 h, the AHER of Example 6 (36 h) was 10394 μmol·g. -1 ·h -1 The value decreased compared to the 24-hour sample. This is because when the PDA layer is too thick, its electron transport path is lengthened, the charge conduction of the piezoelectric field slows down, and some photogenerated carriers recombine.

[0080] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the field; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a sea-weed-shaped composite electrospun membrane, characterized in that, Includes the following steps: (1) Polyvinylidene fluoride, polyvinylpyrrolidone and anhydrous lithium chloride were added to a mixed solvent of N,N'-dimethylformamide and acetone, and heated and stirred to obtain spinning solution I; polyvinylidene fluoride was added to a mixed solvent of N,N'-dimethylformamide and acetone, and heated and stirred to obtain spinning solution II; (2) The composite fiber membrane was prepared by electrospinning of spinning solution I. The composite fiber membrane was immersed in water to dissolve polyvinylpyrrolidone and obtain a PVDF-LiCl fiber membrane with a flat fiber cross-section structure. (3) The PVDF-LiCl fiber membrane was immersed in a dopamine solution for in-situ polymerization to obtain a PVDF-LiCl / PDA composite fiber membrane; (4) The g-C3N4@Pt nanosheet suspension was filtered through vacuum to filter the dispersed g-C3N4@Pt onto the surface of the PVDF-LiCl / PDA composite fiber membrane to obtain the PVDF-LiCl / PDA / g-C3N4@Pt composite fiber membrane. (5) A PVDF fiber membrane was prepared on the surface of the PVDF-LiCl / PDA / g-C3N4@Pt composite fiber membrane by electrospinning of spinning solution II, and finally a kelp-like PVDF-LiCl / PDA / g-C3N4@Pt / PVDF composite electrospun membrane was obtained.

2. The method of claim 1, wherein the kelp-shaped composite electrospun membrane is prepared by the steps of: In step (1), the mass ratio of polyvinylidene fluoride, polyvinylpyrrolidone and anhydrous lithium chloride in spinning solution I is (1~5):(1~5):(0.01~0.1); the concentration of polyvinylidene fluoride in spinning solution II is 8~12 wt%.

3. The method for preparing the kelp-like composite electrospun film as described in claim 1 or 2, characterized in that, In step (1), the volume ratio of N,N'-dimethylformamide to acetone in the mixed solvent is (1~5):(1~5); the heating and stirring temperature is 35-55℃ and the time is 4-10 h.

4. The method for preparing the kelp-like composite electrospun film as described in claim 1, characterized in that, In step (2), the parameters of electrospinning are: voltage 12-20 kV, speed 0.02-0.08 mL / h, distance between needle tip and collector 10-15 cm; the composite fiber membrane is immersed in water at 70-90℃ for 1-5 h to dissolve polyvinylpyrrolidone.

5. The method for preparing the kelp-like composite electrospun membrane as described in claim 1 or 4, characterized in that, In step (2), the thickness of the PVDF-LiCl fiber membrane is 20-35 μm.

6. The method for preparing the kelp-like composite electrospun film as described in claim 1, characterized in that, In step (3), the preparation of the dopamine solution is as follows: dopamine hydrochloride is added to a buffer solution with pH = 8.5, and the ratio of the amount of dopamine hydrochloride to the amount of buffer solution added is 1-6 mg: 1-10 mL to obtain the dopamine solution; the in-situ polymerization temperature is room temperature and the polymerization time is 12-36 h.

7. The method for preparing the kelp-like composite electrospun membrane as described in claim 1 or 6, characterized in that, In step (4), the preparation of the g-C3N4@Pt nanosheet suspension is as follows: g-C3N4@Pt nanosheets are added to an ethanol solution, the ratio of g-C3N4@Pt nanosheets to ethanol solution is 1-10 mg: 1-5 mL, and the mixture is stirred and dispersed to obtain the g-C3N4@Pt nanosheet suspension.

8. The method for preparing the kelp-like composite electrospun membrane as described in claim 1, characterized in that, In step (5), the parameters of the electrospinning are: voltage 12-20 kV, speed 0.02-0.08 mL / h, and distance between the needle tip and the collector 10-15 cm.

9. The method for preparing the kelp-like composite electrospun membrane as described in claim 1 or 8, characterized in that, In step (5), the thickness of the PVDF fiber membrane is 1-5 μm; the fiber cross-section of the PVDF fiber membrane is circular, and the fiber diameter is 500-1000 nm.

10. The application of a kelp-like composite electrospun membrane prepared by the preparation method according to any one of claims 1-9 in the photolysis of water for hydrogen evolution in a flowing water environment.

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