Preparation method of PVA / chitin porous nanosheet layered film and salinity gradient power generation application

Abstract

The invention discloses a preparation method of a PVA / chitin porous nanosheet layered film for salinity gradient power generation, and belongs to the technical field of bio-based material functionalization. The preparation method comprises the following operation steps: firstly, preparing a chitin porous nanosheet by utilizing an acid steam combined ultrasonic strategy; modifying the surface of the chitin porous nanosheet with rich positive charges through quaternization reaction, and performing suction filtration and drying to obtain a porous nanosheet layered film; and finally, circularly freezing, thawing and packaging the porous nanosheet layered film by using polyvinyl alcohol hydrogel to prepare the PVA / chitin porous nanosheet layered film. According to the PVA / chitin porous nanosheet layered film, by means of combination of chitin nanosheet layer gap channels and micro-nano pore channels in nanosheet surfaces, ion transmission paths are increased, ion transmission resistance is reduced, the microstructure of the layered film is effectively stabilized jointly by means of the interlayer weak force effect and the hydrogen bond bonding effect between polyvinyl alcohol molecules, and therefore the film has the advantages of being capable of being used for preparing the composite film. And the optimal salinity gradient power generation output power density reaches 7.33 W / m < 2 >.

Description

Technical Field

[0001] The present invention belongs to the technical field of bio-based functional polymers, and specifically relates to a method for preparing a PVA / chitin porous nanosheet layered membrane and its application in salt gradient power generation. Background Art

[0002] The osmotic energy released by mixing seawater with river water across a salinity gradient (approximately 2.4 TW) is a sustainable and renewable "blue" energy source. Nanofluids have attracted considerable attention in the past decade for osmotic energy harvesting, as the emergence of various new nanomaterials has made it possible to controllably tune nanofluidic channels.

[0003] When a fluid passes through a charged nanochannel, due to the overlapping of the double electrical layers on the inner wall of the channel, a unique ion-selective permeability behavior occurs. Based on this principle, salt solutions of different concentrations are placed on both sides of the charged nanofluidic system. Due to the existence of osmotic pressure, the spontaneous directional transport of ions is induced, which can generate an osmotic current. Currently, the building blocks of nanofluidic channels include one-dimensional nanowires, two-dimensional nanosheets, and three-dimensional network structures. These materials can be constructed into membrane-based, gel-based, and fiber-based nanofluidic channel forms through vacuum filtration, casting, wet processing, or microfluidic spinning, and have applications in osmotic energy collection, self-powered intelligent sensing, water treatment, biomedicine, and other fields.

[0004] In the most widely studied area of ​​osmotic energy harvesting, layered membranes formed by stacking two-dimensional materials through simple vacuum filtration have proven to be a promising approach for harvesting osmotic energy. However, the interlamellar spacing or pores between adjacent layers assembled using this strategy are typically subnanometer, comparable to the molecular size range. This presents a significant challenge for layered membranes in osmotic energy harvesting applications, as such narrow interlamellar spacing significantly hinders ion transport through the interlamellar space, significantly slowing ion flux. One strategy involves introducing various charged nanofibers between two-dimensional nanosheets to expand the interlamellar space between adjacent nanosheets and generate space charge to increase ion flux. However, this strategy obviously requires the additional preparation of nanofibers and the introduction of charge modification, complicating the overall membrane fabrication process. Another strategy involves etching nanopores into the surface of assembled microelement nanosheets. These in-plane nanopores interconnect the nanocapillaries between the layered membranes, forming a network for ion passage. This approach, while increasing the number of nanochannels, reduces the tortuosity of ion migration pathways, shortening the migration path and thereby improving ion flux, has proven to be an effective and promising strategy. However, the strategy of etching holes in nanosheets typically involves first preparing the nanosheets and then processing and etching the nanopores, resulting in a complex process. Furthermore, both in the initial preparation of the nanosheets and in the etching of the holes, commonly used two-dimensional materials, such as graphene, graphene oxide, boron nitride, carbon nitride, two-dimensional transition metal carbides / nitrides (MXenes), and two-dimensional metal-organic frameworks, have high preparation costs and poor environmental friendliness. Therefore, the development of a low-cost, surface-rich, and sustainable high-ion flux two-dimensional nanosheet material, which can then be assembled into a two-dimensional layered membrane, is of considerable significance in the field of osmotic energy harvesting. Summary of the Invention

[0005] In order to solve the problem of low osmotic energy collection output power density due to dense structure and few ion channels of bio-based nanofluid membranes prepared by existing technology, the present invention provides a preparation method of PVA / chitin porous nanosheet layered membrane and its application in salt difference power generation.

[0006] The preparation steps of a PVA / chitin porous nanosheet layer membrane are as follows:

[0007] (1) Preparation of porous chitin nanosheets

[0008] (1.1) Add 20 g of purified chitosan to 200 mL of sodium hydroxide solution, heat to 60-100°C in an oil bath, and stir under magnetic stirring for 2-6 h for deacetylation. Wash the reactant with deionized water until neutral, filter, and dry for later use to obtain pretreated chitosan.

[0009] (1.2) In a sealed container, spray a 1M-5M hydrochloric acid solution on the pretreated chitin in an amount 2-5 times the mass of the pretreated chitin; acid hydrolysis is performed with acid vapor at a temperature of 60-120°C for 1-8 hours. After the reaction is complete, the product is neutralized and washed with deionized water, and freeze-dried to obtain fluffy chitin;

[0010] (1.3) Prepare a 1 wt% suspension of fluffy chitosan in deionized water and ultrasonicate it in a cell disruptor to produce porous chitosan nanosheets.

[0011] (2) Armored chitin porous nanosheet layer membrane

[0012] After the surface of the chitin porous nanosheets is modified with abundant positive charges by quaternization reaction, vacuum filtration is performed to prepare a porous nanosheet layer membrane; the porous nanosheet layer membrane is formed by stacking the chitin porous nanosheets in parallel;

[0013] (3) Preparation of PVA / chitin porous nanosheet membranes by PVA hydrogel encapsulation

[0014] (3.1) Add 5 mL of a 15 wt% aqueous solution of polyvinyl alcohol to a glass dish and scrape it into a first thin layer using a glass slide.

[0015] (3.2) Place the dried porous nanosheet membrane on the first thin layer of the glass culture dish. Once the porous nanosheet membrane is completely soaked and spread out, quickly pour a 15 wt% polyvinyl alcohol (PVA) aqueous solution onto the porous nanosheet membrane. Use a glass slide to scrape the poured PVA solution into a second thin layer, positioning the porous nanosheet membrane between the first and second thin layers.

[0016] (3.3) Freezing at -20°C for 12 hours; thawing at room temperature for 2 hours; and repeating the freeze-thaw operation four times to obtain a polyvinyl alcohol hydrogel-encapsulated PVA / chitin porous nanosheet membrane.

[0017] The PVA / chitin porous nanosheet layered membrane is a sandwich structure, wherein the central inner layer is a porous nanosheet layered membrane layer, and the two outer sides of the central inner layer are polyvinyl alcohol hydrogel layers;

[0018] At room temperature, in a potassium chloride (KCl) solution with a molar concentration of 50 times 0.01 M to 0.5 M, the resistance of the PVA / chitin porous nanosheet layered membrane is 10 to 40 kΩ, and the maximum output power density is 2.59 to 7.33 W / m2.

[0019] Further preparation technology scheme is as follows:

[0020] In step (1.1), the concentration of the sodium hydroxide solution is 20wt%-50wt%.

[0021] In step (1.2), the sealed container is a hydrothermal reactor or a high-temperature resistant high-density polyethylene plastic bottle.

[0022] In step (1.3), ultrasonic conditions are: power 600W to 900W, time 1 to 10 minutes.

[0023] In step (2), the quaternization modification reagent is 3-chloro-2-hydroxypropyltrimethylammonium chloride solution;

[0024] The quaternization reaction was carried out as follows: 1 g of chitosan was added to 100 mL of a 2 wt% sodium hydroxide (NaOH) solution, followed by the addition of 12 g of a 65 wt% 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHMAC) solution; the mixture was stirred at 65°C for 10 hours, washed with deionized water to a pH of 7, vacuum filtered, and dried at room temperature to obtain a positively charged chitosan porous nanosheet membrane.

[0025] In step (3.3), the thickness of the PVA / chitin porous nanosheet layered membrane is 20-80 μm.

[0026] The beneficial technical effects of the present invention are embodied in the following aspects:

[0027] (1) The present invention utilizes two-dimensional nanochannels between the layers of the chitin porous nanosheets and nanopores within the surface. The in-plane pores significantly increase the ion transport path and reduce the ion transport resistance, thereby improving the output power density of the layered membrane. The interlayer weak force and the hydrogen bonding of the polyvinyl alcohol (PVA) hydrogel effectively stabilize the microstructure of the layered membrane, resulting in high process controllability, stable product performance, and good environmental protection. As shown in Table 1, the optimal output power density of the salinity difference power generation reached 7.33W / m2, which ranks among the top output power densities in pure natural material-based nanofluid systems.

[0028] (2) Currently, the two-dimensional materials commonly used to construct two-dimensional layered membranes include graphene, graphene oxide, boron nitride, carbon nitride, two-dimensional transition metal carbides / nitrides (MXenes), and two-dimensional metal-organic frameworks. These materials are expensive to prepare or have poor environmental friendliness. This invention develops a new all-biomass chitosan-based nanosheet that can replace the reported inorganic two-dimensional materials and apply it to the field of osmotic energy harvesting, achieving similar salinity-based power generation performance at a low cost and in a more environmentally friendly manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more apparent.

[0030] Figure 1 Schematic diagram of the strategy for preparing chitin nanosheets by combining acid vapor with ultrasound and the preparation of hydrogel-encapsulated PVA / chitin porous nanosheet layered membranes;

[0031] Figure 2 This is a morphology diagram of the porous chitin nanosheet in Example 1;

[0032] Figure 3 This is a diagram showing the morphological changes of the PVA / chitin porous nanosheet laminar membrane hydrogel before and after encapsulation in Example 1;

[0033] Figure 4 Schematic diagram of the penetration energy testing device;

[0034] Figure 5 The IV curve of the hydrogel-encapsulated PVA / chitin porous nanosheet layered membrane at different electrolyte concentrations and the graph showing the change of the layered membrane conductivity with the electrolyte concentration in Example 1;

[0035] Figure 6 Schematic diagram of the output power density and current density testing device of the hydrogel-encapsulated PVA / chitin porous nanosheet membrane of Example 1 and its test results;

[0036] Figure 7 The morphology of the cross section of the hydrogel-encapsulated PVA / chitin porous nanosheet layer membrane of Example 2;

[0037] Figure 8 Graph showing the output power density and current density test results of the hydrogel-encapsulated PVA / chitin porous nanosheet membrane of Example 2 under a 50-fold potassium chloride concentration gradient;

[0038] Figure 9 Graph showing the output power density and current density test results of the hydrogel-encapsulated PVA / chitin porous nanosheet membrane of Example 3 under a 50-fold potassium chloride concentration gradient;

[0039] Figure 10 Graphs showing the output power density and current density test results of the hydrogel-encapsulated PVA / chitin porous nanosheet membrane of Example 2 under different electrolyte conditions, i.e., 100-fold (Example 4), 300-fold (Example 5), and 500-fold (Example 6) potassium chloride concentration gradients;

[0040] Figure 11 This is a morphology of the chitosan dense nanosheets of Comparative Example 1;

[0041] Figure 12 This is a morphology of the chitosan dense nanosheet layer membrane of Comparative Example 1;

[0042] Figure 13 The IV curves of the hydrogel-encapsulated PVA / chitin dense nanosheet layered membrane at different concentration gradients, the corresponding open circuit voltage (Voc) and short circuit current (Isc) graphs, and the output power density and current density test results of Comparative Example 1 are shown;

[0043] Figure 14 The rate curves of the fluorescent dyes sulfonylrhodamine B (Dye (-), green line) and rhodamine 6G (Dye (+), yellow line) penetrating through the PVA / chitin porous nanosheet layer membrane and PVA / chitin dense nanosheet layer membrane, respectively, and the molecular structural formulas of the corresponding fluorescent dyes. DETAILED DESCRIPTION

[0044] The present invention will be further described in detail below with reference to specific embodiments.

[0045] Polyvinyl alcohol raw material was from Anhui Wanwei High-tech Materials Co., Ltd., model 1799.

[0046] Example 1

[0047] See also Figure 1 a and Figure 1 b, a PVA / chitin porous nanosheet layered membrane preparation process is as follows:

[0048] (1) Preparation of porous chitin nanosheets

[0049] (1.1) Deacetylate 20 g of purified chitin in 200 mL of 20 wt% sodium hydroxide solution. Heat to 80°C in an oil bath with magnetic stirring at 800 rpm for 4 h. Wash the reaction mixture with deionized water until neutral, filter it, and dry it at 60°C for later use. This yields pretreated chitin.

[0050] (1.2) In a hydrothermal reactor, evenly spray 1 g of 3 M hydrochloric acid solution onto 0.4 g of pretreated chitin using an acid-resistant spray bottle. Hydrochloric acid vapor hydrolysis is performed at 80°C for 8 h. The reaction mixture is washed with deionized water until neutral and freeze-dried to obtain fluffy chitin.

[0051] (1.3) Prepare 25 ml of a 1 wt% chitosan suspension in deionized water. Ultrasonicate the suspension in a cell disruptor at 650 W for 2 minutes, using a 2-second on / 2-second off mode. This yields porous chitosan nanosheets with hierarchical micro- and nano-scale pores.

[0052] See also Figure 2 a and Figure 2 b, the surface of the porous chitin nanosheets prepared in Example 1 has abundant micro-nano pores, see Figure 2 In c, the thickness of the porous chitosan nanosheets prepared in Example 1 is about 1.55 nm.

[0053] (2) Assembly of porous nanosheet membranes

[0054] After the surface of the porous chitin nanosheets is modified with abundant positive charges through a quaternization reaction, vacuum filtration is performed to produce a porous nanosheet layered membrane; the porous nanosheet layered membrane is composed of parallel stacks of the porous chitin nanosheets. The specific operation of the quaternization reaction is as follows:

[0055] (2.1) 1 g of porous chitin nanosheets was added to 100 g of a 2 wt% sodium hydroxide solution and stirred evenly. Then, 12 g of a 65 wt% 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHMAC) solution was added and stirred at 65°C for 10 h.

[0056] (2.2) After the reaction is completed, 300 g of 2 wt% acetic acid solution is added to neutralize most of the alkali solution in the reaction solution. The mixed solution is then centrifuged and washed with deionized water until neutral, thereby obtaining quaternized chitin porous nanosheets.

[0057] (2.3) A 0.4 mg / mL suspension of quaternary ammonium-modified chitin porous nanosheets was prepared using deionized water. A membrane was obtained by vacuum filtration of 30 mL of the suspension through a polyethersulfone filter membrane (0.22 µm pore size and 47 mm diameter). The membrane was air-dried for 24 h and peeled off from the substrate for later use, yielding a porous nanosheet layered membrane.

[0058] (3) Preparation of PVA / chitin porous nanosheet membranes by PVA hydrogel encapsulation

[0059] (3.1) Add 15 g of 1799 polyvinyl alcohol powder to 85 mL of deionized water and stir at 95°C for 2 h to prepare a 15 wt % polyvinyl alcohol solution.

[0060] (3.2) Pour 5 mL of 15 wt% polyvinyl alcohol solution into a glass dish with a diameter of 8.5 cm and scrape it into a first thin layer using a glass slide.

[0061] (3.3) Place the dried porous nanosheet membrane on the first thin layer of the glass culture dish. When the porous nanosheet membrane is completely soaked and unfolded, quickly pour a 15 wt% polyvinyl alcohol (PVA) aqueous solution on the porous nanosheet membrane. Use a glass slide to scrape the poured PVA solution into a second thin layer, so that the porous nanosheet membrane is located between the first and second thin layers.

[0062] (3.4) Freeze at -20°C for 6 h and thaw at room temperature for 6 h. Repeat the freeze-thaw operation four times to obtain a polyvinyl alcohol hydrogel-encapsulated PVA / chitin porous nanosheet membrane with good water stability.

[0063] The thickness of the PVA / chitin porous nanosheet layered membrane prepared in this Example 1 is 50 μm. The PVA / chitin porous nanosheet layered membrane is a sandwich-like structure, wherein the central inner layer is a porous nanosheet layered membrane layer, and the two outer sides of the central inner layer are polyvinyl alcohol hydrogel layers.

[0064] See also Figure 1 b, the hydrogel-encapsulated layered membrane prepared in Example 1 has good uniformity and bendability, that is, the membrane surface is smooth and has no defects and is flexible and can be bent at a large angle, which is conducive to application in actual osmotic energy collection application environments.

[0065] The morphology of the PVA / chitin porous nanosheet membrane prepared in Example 1 was characterized. Figure 3 As shown in Figure a, the surface of the porous nanosheet membrane is smooth and translucent. Figure 3 As shown in Figure b, the cross section of the layered membrane shows a distinct layered structure with clear boundaries between the layers. Figure 3 As shown in Figure c, the PVA / chitin porous nanosheet layered membrane after hydrogel encapsulation presents a typical sandwich-like structure.

[0066] See also Figure 4 In the test device, the layered film prepared in Example 1 was placed under different potassium chloride (KCl) concentrations to test the IV curve. Figure 5 In (a), the typical IV line shows the symmetrical structure of the layered membrane. The slope is used to calculate the ionic conductivity of the composite membrane. That is, at high concentrations (≥10−2 mol / L), the ionic conductivity of the composite membrane is close to that of the bulk solution (black dashed line). Figure 5In figure (b), as the potassium chloride electrolyte concentration decreases, the membrane conductivity gradually deviates from the bulk solution ionic conductivity and stabilizes in the low-concentration region. This is because the surface of the porous chitin nanosheets is positively charged, and anionic chloride (Cl-) will be enriched in the channel, while cationic potassium (K+) will be excluded. In this case, the surface charge determines the ion transport in the channel, showing higher ionic conductivity than the bulk solution at low concentrations.

[0067] See also Figure 6 In (a), the layered film prepared in Example 1 was loaded with different resistances to test the output power and current density, see Figure 6 In (b), the hydrogel-encapsulated chitin porous nanosheet layered membrane has an internal resistance of about 20 kΩ, and a maximum output power of 2.59 W / m2 under a 50-fold potassium chloride concentration gradient. Due to the presence of abundant micro-nanopores within its surface, the ion transmission channels are greatly increased, making its output power density greater than the output power of most all-natural material-based nanofluid systems. However, due to the relatively thick membrane (~80 μm), the power is lower than the minimum standard for industrial applications (5 W / m2).

[0068] Example 2

[0069] A method for preparing a PVA / chitin porous nanosheet membrane for salinity gradient power generation:

[0070] (1) The preparation of porous chitin nanosheets is the same as in Example 1;

[0071] (2) Armored chitin porous nanosheet layer membrane

[0072] Steps (2.1) and (2.2) are the same as those in Example 1;

[0073] (2.3) The steps are as follows: quaternary ammonium-modified chitosan nanosheets are prepared into a 0.4 mg / mL suspension using deionized water, 10 mL of the suspension is vacuum filtered on a polyethersulfone filter membrane to obtain a membrane, the membrane is air-dried for 24 h and peeled off from the substrate for later use, thereby obtaining a chitosan porous nanosheet layered membrane.

[0074] (3) PVA hydrogel encapsulation The preparation of PVA / chitin porous nanosheet layered membrane is the same as in Example 1.

[0075] The PVA / chitin porous nanosheet layered membrane with good water stability was obtained. The layered membrane prepared in Example 2 was observed by SEM. Figure 7 In (a), the cross section of the layered membrane shows a clear layered structure, see Figure 7 In (b), the boundaries between the layers are obvious. Due to the reduction of the volume of the filtered suspension, the membrane thickness becomes thinner, about 40μm.

[0076] The layered membrane prepared in Example 2 was tested for output power and current density under different potassium chloride (KCl) concentration gradients and different resistances. Figure 8 It can be seen that the internal resistance of the layered membrane is about 10kΩ, and the maximum output power is 6.19W / m2 under a 50-fold potassium chloride concentration gradient, indicating that reducing the thickness of the composite membrane by reducing the suspension filtration volume reduces the membrane internal resistance and improves the output power density.

[0077] Example 3

[0078] This Example 3 is basically the same as Example 1, except that the volume of the suspension used for filtration is reduced, which reduces the thickness of the final membrane. Specifically, the volume of the 0.4 mg / mL chitosan porous nanosheet suspension filtered by vacuum is 8 mL, resulting in a final chitosan porous nanosheet layered membrane with a thickness of 25 μm. Figure 9 By further reducing the membrane thickness, the output power density increases to 7.33 W / m² at a 50-fold potassium chloride concentration gradient. As shown in Table 1, the output power density is among the highest in natural material-based nanofluid systems, thanks to the abundant micro- and nanochannels within the porous chitin nanosheets, which form numerous ion diffusion channels.

[0079] Table 1 Comparative statistics of output power density of osmotic energy harvesting of natural material-based nanofluid systems

[0080] Natural material-based nanofluidic systems concentration gradient Output power (W / m2) References Negatively charged (N-) wood film 60 times sodium chloride 0.00514 Adv. Energy Mater. 2020, 10 (1), 1902590 Positively charged (P-) nanoclay film 50 times sodium chloride 1.38 J. Membr. Sci. 2024, 695, 122456 N-cellulose membrane 50 times sodium chloride 1.49 Carbohydr. Polym. 2024, 327, 121656 N-cellulose membrane 50 times sodium chloride 1.3 Nano Energy 2023, 115, 108693 N-2DMMT Membrane 50 times potassium chloride 0.15-1.25 Nano Energy 2020, 76, 105113; Nano Energy 2022, 100, 107526 N-bacterial cellulose membrane 50 times potassium chloride 0.84 Nano Energy 2022, 101, 107548 N-bacterial cellulose composite membrane 50 times potassium chloride 0.82 Carbohydr. Polym. 2023, 305, 120556. N-bacterial cellulose membrane 50 times potassium chloride 0.79 Nano Energy 2022, 103, 107786 P-bacterial cellulose membrane 50 times potassium chloride 0.64 Nano Energy 2022, 103, 107786 P-cellulose membrane 50 times sodium chloride 0.63 Carbohydr. Polym. 2024, 327, 121656 N-cellulose nanocrystal / PVA film 50 times potassium chloride 0.5 Small 2023, 19 (50), 2304603 P-bacterial cellulose membrane 50 times potassium chloride 0.5 Nano Energy 2022, 101, 107548 P-bacterial cellulose membrane 50 times sodium chloride 0.39 Nano Energy 2021, 88, 106275 N-cellulose nanofiber membrane 50 times potassium chloride 0.24 Carbohydr. Polym. 2022, 280, 119023 N-bacterial cellulose membrane 50 times sodium chloride 0.23 Nano Energy 2021, 80, 105554 N-bacterial cellulose hydrogel film 50 times potassium chloride 0.125 Carbohydr. Polym. 2023, 305, 120556 N-regenerated cellulose membrane 50 times sodium chloride 0.05 ACS Energy Lett. 2024, 9, 2092-2100 N-Luffa Base Membrane 60 times sodium chloride 0.00183 Small 2022, 18 (2), 2104320 N-silk fibroin nanofiber membrane 10,000 times sodium chloride 0.00059 ACS Nano2021, 15 (3),5649–5660 P-PVA / chitin porous nanosheet membrane 50 times potassium chloride 2.59 Example 1 P-PVA / chitin porous nanosheet membrane 50 times potassium chloride 6.19 Example 2 P-PVA / chitin porous nanosheet membrane 50 times potassium chloride 7.33 Example 3

[0081] Example 4

[0082] At room temperature, the chitosan porous nanosheet membrane prepared in Example 2 was placed in a 100 (0.01 / 1 M) times KCl solution to test its output power density and current density at different resistances. The results are as follows. Figure 10 As shown in the 0.01 / 1M curve (black) in (a), when the external resistance is 4kΩ, the output power density reaches the maximum, 9.56W / m2, indicating that the output power density of the layered membrane can be increased by increasing the electrolyte concentration difference, and the current density change during the test is shown in Figure 10 The black curve in (b).

[0083] Example 5

[0084] At room temperature, the chitosan porous nanosheet layered membrane prepared in Example 2 was placed in a 300 (0.01 / 3 M) times KCl solution to test its output power density and current density at different resistances. The results are as follows. Figure 10As shown in the 0.01 / 3M curve (blue) in (a), when the external resistance is 5kΩ, the output power density reaches the maximum, 16.35 W / m2, indicating that the output power density of the layered membrane can be increased by increasing the electrolyte concentration difference, and the current density change during the test is shown in Figure 10 The blue curve in (b).

[0085] Example 6

[0086] At room temperature, the chitosan porous nanosheet layered membrane prepared in Example 2 was placed in a 500 (0.01 / 5 M) times KCl solution to test its output power density and current density at different resistances. The results are as follows. Figure 10 As shown in the 0.01 / 5M curve (red) in (a), when the external resistance is 2kΩ, the output power density reaches the maximum, 33.63W / m2, indicating that the output power density of the layered membrane can be increased by increasing the electrolyte concentration difference, and the current density change during the test is shown in Figure 10 The red curve in (b).

[0087] Comparative Example 1

[0088] This comparative example 1 is a method for preparing a PVA / chitin dense nanosheet layered membrane for salinity gradient power generation, comprising the following steps:

[0089] (1) Preparation of dense chitin nanosheets;

[0090] (1.1) Evenly spray 0.4 g of purified chitin with 1 g of 3 M hydrochloric acid solution using an acid-resistant spray bottle. Hydrochloric acid vapor is then hydrolyzed in a hydrothermal reactor at 80°C for 8 h. Wash with deionized water until neutral and freeze-dry to obtain the acid-decomposed chitin.

[0091] (1.2) Prepare 25 ml of a 1 wt% chitosan suspension with deionized water. Ultrasonicate the chitosan in a 650 W cell disruptor at 100% power for 20 minutes, using a 2-second on / 2-second off cycle. This results in dense, nonporous chitosan nanosheets.

[0092] (2) Formation of dense chitin nanosheet membranes;

[0093] (2.1) 1 g of chitosan nanosheets was added to 100 g of a 2 wt% sodium hydroxide solution, and 20 g of a 65 wt% 3-chloro-2-hydroxypropyltrimethylammonium chloride solution was added thereto. The mixture was stirred at 65°C for 12 h.

[0094] (2.2) After the reaction is completed, 300 g of 2 wt% acetic acid solution is added to neutralize most of the alkali solution in the reaction solution, and the mixed solution is then centrifuged and washed with deionized water until it is neutral to obtain quaternized ammonium-modified dense chitin nanosheets;

[0095] (2.3) A 0.4 mg / mL suspension of quaternary ammonium-modified dense chitin nanosheets was prepared using deionized water. 30 mL of the suspension was vacuum filtered through a polyethersulfone filter membrane (0.22 µm pore size and 47 mm diameter) to obtain a membrane. The membrane was air-dried for 24 h and peeled off from the substrate for later use, yielding a chitin dense nanosheet lamellar membrane.

[0096] (3) PVA hydrogel encapsulation The steps for preparing PVA / chitin porous nanosheet laminar membrane are the same as those in Example 1, except that the encapsulated membrane is replaced with a chitin dense nanosheet laminar membrane.

[0097] The dense chitin nanosheets of Comparative Example 1 were observed by SEM and AFM. Figure 11 (a) in the figure shows that the surface of the dense chitin nanosheets has no obvious pore structure, see Figure 11 (b) in the figure has a thickness of approximately 155.15 nm. This is because the acid vapor combined with ultrasonic process peels off purified chitin, and the raw material has not been pre-treated with deacetylation. In other words, the repulsive effect of the surface positive charge is insufficient to fully peel off the intrinsic Bouligand structure of chitin. The cross-section of the polyvinyl alcohol hydrogel-encapsulated PVA / chitin porous nanosheet layered membrane of Example 1 was observed by SEM. Figure 12 (b) The cross section of the PVA / chitin porous nanosheet membrane encapsulated by polyvinyl alcohol hydrogel also shows a clear layered structure with obvious boundaries between the layers, but see Figure 12 In (c), the enlarged image shows that the surface of the dense chitin nanosheet is dense and non-porous, which means that ion transmission can only pass through the pores between the sheets, which has an adverse effect on ion transmission. The PVA / chitin porous nanosheet layered membrane encapsulated by the polyvinyl alcohol hydrogel of Comparative Example 1 was tested for current and output power density at different resistances. Figure 13 It can be seen that the maximum output power density of the nanofilm obtained in Comparative Example 1 is only 1.03W / m2, which is far from reaching the industrial level and is significantly lower than that of Examples 1, 2 and 3 of the present invention. Figure 14It can be seen that the concentrations of two fluorescent dyes with similar chemical structures but opposite charges (i.e., sulforhodamine B as Dye (-) and rhodamine 6G as Dye (+)) passing through the two membranes change over time. The results show that the PVA / chitin porous nanosheet lamellar membrane exhibits faster and more effective selective permeation of Dye (-). After 30 minutes of testing, the concentration of Dye (-) permeating through the PVA / chitin porous nanosheet lamellar membrane was 300.43 nM, and the concentration ratio of Dye (-) / Dye (+) was as high as 70 times, significantly higher than the 69.41 nM and 16 times that passing through the PVA / chitin porous nanosheet lamellar membrane. Figure 13 (a) and (b) in Fig. 3, which can be explained by the fact that the dense structure of the surface of the dense chitin nanosheets reduces the number of ion channels, which in turn leads to a decrease in ion channels. Under the same concentration gradient, the number of ions diffusing through the membrane is less, resulting in a short-circuit current; see Figure 13 (c) in the figure shows a smaller output power.

[0098] It will be easily understood by those skilled in the art that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a PVA / chitin porous nanosheet film, characterized in that: The steps are as follows: (1) Preparation of porous chitin nanosheets (1.1) Add 20 g of purified chitosan to 200 mL of sodium hydroxide solution, heat to 60-100°C in an oil bath, and stir under magnetic stirring for 2-6 h for deacetylation. Wash the reactant with deionized water until neutral, filter, and dry for later use to obtain pretreated chitosan. (1.2) In a sealed container, spray a 1M-5M hydrochloric acid solution on the pretreated chitin in an amount 2-5 times the mass of the pretreated chitin; acid hydrolysis is performed with acid vapor at a temperature of 60-120°C for 1-8 hours. After the reaction is complete, the product is neutralized and washed with deionized water, and freeze-dried to obtain fluffy chitin; (1.3) Prepare a 1 wt% suspension of fluffy chitosan in deionized water and ultrasonicate it in a cell disruptor to produce porous chitosan nanosheets. (2) Armored chitin porous nanosheet layer membrane The surface of the chitin porous nanosheets is modified with abundant positive charges by quaternization reaction, and vacuum filtration is performed to prepare a porous nanosheet layer membrane; the porous nanosheet layer membrane is formed by stacking the chitin porous nanosheets in parallel; (3) Preparation of PVA / chitin porous nanosheet membranes by PVA hydrogel encapsulation (3.1) Add 15 g of 1799 polyvinyl alcohol (PVA) powder to 85 mL of deionized water and stir at 95°C for 2 h to prepare a 15 wt% PVA solution. (3.2) Add 5 mL of a 15 wt% polyvinyl alcohol aqueous solution to a glass Petri dish and scrape it into a first thin layer using a glass slide. (3.3) Place the dried porous nanosheet membrane on the first thin layer of the glass culture dish. Once the porous nanosheet membrane is completely soaked and spread out, quickly pour a 15 wt% PVA aqueous solution onto the porous nanosheet membrane. Use a glass slide to scrape the poured PVA aqueous solution into a second thin layer, positioning the porous nanosheet membrane between the first and second thin layers. (3.4) Freezing at -20°C for 12 hours; thawing at room temperature for 2 hours; and repeating the freeze-thaw operation four times to produce a PVA / chitin porous nanosheet membrane. The PVA / chitin porous nanosheet layered membrane has a sandwich-like structure, wherein the central inner layer is a porous nanosheet layered membrane layer, and the two outer sides of the central inner layer are PVA hydrogel layers. At room temperature, in a 50-fold (0.01 M / 0.5 M) potassium chloride (KCl) solution, the resistance of the PVA / chitin porous nanosheet layered membrane is 10 to 40 kΩ, and the maximum output power density is 2.59 to 7.33 W / m2.

2. The method for preparing a PVA / chitin porous nanosheet membrane according to claim 1, characterized in that: In step (1.1), the concentration of the sodium hydroxide solution is 20wt%-50wt%.

3. The method for preparing a PVA / chitin porous nanosheet membrane according to claim 1, characterized in that: In step (1.2), the sealed container is a hydrothermal reactor or a high-temperature resistant high-density polyethylene plastic bottle.

4. The method for preparing a PVA / chitin porous nanosheet membrane according to claim 1, wherein: In step (1.3), ultrasonic conditions are: power 600W to 900W, time 1 to 10 minutes.

5. The method for preparing a PVA / chitin porous nanosheet membrane according to claim 1, characterized in that: In step (2), the quaternization modification reagent is 3-chloro-2-hydroxypropyltrimethylammonium chloride solution; The specific operation of the quaternization reaction is as follows: 1g of chitosan is added to 100mL of a 2wt% sodium hydroxide (NaOH) solution, and then 12g of a 65wt% 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHMAC) solution is added; the mixture is stirred at 65°C for 10h, washed with deionized water to a pH of 7, vacuum filtered, and dried at room temperature to obtain a chitosan porous nanosheet membrane.

6. The method for preparing a PVA / chitin porous nanosheet membrane according to claim 1, characterized in that: In step (3.4), the thickness of the PVA / chitin porous nanosheet layered membrane is 20-80 μm.

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