A photothermal evaporator with gradient pore structure and a preparation method and application thereof

Abstract

This invention relates to the field of seawater desalination technology, providing a photothermal evaporator with a gradient pore structure, its preparation method, and its application. The photothermal evaporator comprises an upper hydrophobic porous photothermal layer and a lower hydrophilic gel water-transporting layer; wherein the water-transporting layer has a gradient pore structure with gradually changing pore size along its thickness direction, with smaller pores near the photothermal layer to enhance capillary water supply, and larger pores further away from the photothermal layer to promote reverse diffusion of salt ions. This invention constructs interconnected pores in the photothermal layer using a sacrificial template method to promote steam escape; utilizes a hydrogel prepolymer to infiltrate and solidify into the pores, forming a spatially interlocked interface between the photothermal layer and the water-transporting layer; and induces the hydrogel to form a gradient pore structure through directional semi-crosslinking treatment. This invention achieves synergistic regulation of water upward transport and salt downward discharge, effectively inhibits salt crystallization blockage, significantly improves the long-term operational stability of the evaporator in high-salt environments, and is suitable for solar-powered seawater desalination.

Description

Technical Field

[0001] This invention relates to the field of seawater desalination technology, and in particular to a photothermal evaporator with a gradient pore structure, its preparation method, and its application. Background Technology

[0002] The global freshwater crisis is becoming increasingly severe, making seawater desalination a key way to alleviate the supply-demand imbalance. Traditional technologies such as reverse osmosis (RO) and multi-stage flash evaporation (MSF) rely on energy-intensive equipment, limiting their widespread adoption. Solar-driven interfacial evaporation technology, however, directly utilizes renewable energy and offers both low cost and high energy efficiency, making it a highly promising alternative.

[0003] The core technological challenge of current photothermal evaporators lies in their long-term operational stability. During seawater evaporation, salt continuously crystallizes and accumulates at the evaporation interface, leading to pore blockage and a sharp drop in evaporation efficiency. Existing homogeneous hydrogel evaporators, characterized by integrated structure, uniform heat / mass transfer, and simple fabrication processes, can improve initial efficiency through localized heating, but lack directional salt removal capabilities. They fail after only a few hours of operation due to salt crystallization blockage, severely limiting practical applications. While the three-dimensional network structure of hydrogels can regulate water transport, their single pore size distribution makes it difficult to coordinate the contradiction between rapid water supply and salt diffusion. Small-pore structures facilitate capillary water supply but exacerbate salt retention; large-pore structures promote salt removal but weaken capillary forces, leading to water supply interruptions. Current technologies have not yet achieved coordinated control of water-salt transport within the evaporation layer, necessitating a breakthrough in structural design bottlenecks.

[0004] Inspired by the gradient vascular bundle structure of redwood trees in nature (large pores for water transport in the roots and small pores for transpiration in the leaves), a biomimetic gradient pore photothermal evaporator is constructed, which is expected to simultaneously solve the problems of efficient water supply and resistance to salt crystallization. However, how to accurately design pore parameters such as gradient scale, connectivity, and surface wettability to achieve stable integration with photothermal components remains a technical challenge. Therefore, this invention aims to provide a photothermal evaporator with a gradient pore structure that has a reasonable structural design and feasible fabrication process, as well as its fabrication method, providing a reliable technical solution for solving the aforementioned problem of synergistic regulation of water-salt transport. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a photothermal evaporator with a gradient pore structure, its preparation method, and its application. This invention solves the core defect of existing photothermal evaporators, which suffer from poor long-term stability due to salt crystallization blockage after a period of operation. Through gradient pore design combined with a photothermal layer pore-forming process, a synergistic mechanism is achieved, enabling rapid water transport through gradient pores, reverse diffusion of salt ions along the gradient pores, and steam escape, thus suppressing crystallization blockage. Simultaneously, the photothermal conversion layer and the water transport layer are integrated, achieving a high water evaporation rate.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a photothermal evaporator with a gradient pore structure, comprising a porous photothermal layer and a gel water-transporting layer disposed vertically. The porous photothermal layer is a polydimethylsiloxane porous matrix (PDMS-CNT) with dispersed carbon nanotubes, exhibiting hydrophobicity and a permeable microporous structure. The gel water-transporting layer, located below the photothermal layer, is a hydrophilic sodium alginate-poly(acrylamide-acrylic acid) hydrogel (P(AM-CO-AA)-SA). The gel water-transporting layer has a gradient pore structure with gradually changing pore size along its thickness direction, with smaller pores on the side closer to the porous photothermal layer and larger pores on the side farther away from the porous photothermal layer. The porous photothermal layer and the gel water-transporting layer are spatially interlocked through the sodium alginate-poly(acrylamide-acrylic acid) hydrogel wetting and solidifying the permeable pores of the porous photothermal layer. The thickness ratio of the porous photothermal layer to the gel water-transporting layer is 2:3.

[0007] The present invention also provides a method for preparing the photothermal evaporator described in the above technical solution, comprising the following steps: (1) Mix polydimethylsiloxane prepolymer (PDMS) with curing agent to obtain mixed solution a; mix mixed solution a with acetone solution, add carbon nanotubes (CNTs), and perform ultrasonic treatment to obtain mixed solution b; (2) Add a pore-forming agent to the mixed solution b in step (1), stir, pour into a mold and heat to cure, to obtain a hydrophobic photothermal layer; the heating and curing temperature is 60℃ and the time is 3 hours; (3) Peel the hydrophobic photothermal layer obtained in step (2) from the mold, put it into deionized water to dissolve the pore-forming agent, form pores, and obtain a porous photothermal layer (PDMS-CNT), and put the porous photothermal layer back into the mold; (4) Acrylamide is added to deionized water to obtain an acrylamide solution, and dimethyl sulfoxide and acrylic acid are added and mixed to obtain a mixed solution c.

[0008] (5) Sodium alginate is added to deionized water to obtain a sodium alginate solution, wherein the mass-volume fraction of sodium alginate is 1.86% w / v; (6) Mix the mixed solution c with tetramethylethylenediamine, N,N-methylenebisacrylamide and ammonium persulfate to obtain mixed solution d; mix the mixed solution d with the sodium alginate solution from step (5) to obtain the prepolymer solution; (7) Pour the prepolymer solution from step (6) into the mold containing the porous photothermal layer obtained in step (3) and heat it to obtain sodium alginate-poly(acrylamide-acrylic acid) hydrogel (P(AM-CO-AA)-SA); the heating temperature is 60℃ and the time is 30min; (8) Take out the sodium alginate-poly(acrylamide-acrylic acid) hydrogel obtained in step (7) together with the lower porous photothermal layer from the mold, place the porous photothermal layer at the bottom and put it into a container, add calcium chloride solution to the container, and carry out a semi-crosslinking reaction on the sodium alginate-poly(acrylamide-acrylic acid) hydrogel to obtain a photothermal evaporator combining the gel water transport layer and the porous photothermal layer.

[0009] Preferably, in step (1), the mass ratio of the polydimethylsiloxane prepolymer to the curing agent is 10:1; the curing agent is a vinyl-terminated polydimethylsiloxane crosslinking agent containing a platinum catalyst.

[0010] Preferably, in step (1), the ultrasound is performed under the condition of 10 minutes in an ice bath.

[0011] Preferably, in step (1), the mass ratio of the polydimethylsiloxane prepolymer, acetone solution and carbon nanotubes is 1:1:0.005.

[0012] Preferably, in step (2), the pore-forming agent is NaCl particles with a particle size of 100 μm; the volume ratio of the pore-forming agent to the mixed solution b is 1:1.2.

[0013] Preferably, the mass volume fraction of each component in the mixed solution c in step (4) is: acrylamide 11.2% w / v, dimethyl sulfoxide 5.12% w / v, and acrylic acid 2.35% w / v.

[0014] Preferably, the mass volume fraction of each component in the mixed solution d in step (6) is: tetramethylethylenediamine 0.029% w / v, N,N-methylenebisacrylamide 0.075% w / v, and ammonium persulfate 0.056% w / v.

[0015] Preferably, the calcium chloride solution in step (8) has a mass-volume fraction of 2.5% w / v, the immersion height of the calcium chloride solution is 30% to 70% of the thickness of the hydrogel, and the immersion time is 1 to 3 hours.

[0016] Preferably, the thickness of the photothermal evaporator is 4-6 mm.

[0017] The present invention also provides an application of the photothermal evaporator described in the above technical solution or the photothermal evaporator prepared by the preparation method described in the above technical solution in seawater desalination.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention induces the formation of a gradient pore structure through semi-crosslinking, which accelerates the downward diffusion of water and salt ions. It also works in conjunction with the hydrophobic micropores of PDMS-CNT (contact angle > 110°) to inhibit salt crystal adhesion, thus solving the problem of clogging in photothermal evaporators and enabling continuous operation for 7 days without clogging. 2. This invention employs a "fill-then-pore" process to construct interconnected micropores in PDMS, allowing vapor to escape; simultaneously achieving a synergistic effect of 94% light absorption rate under a high load of 0.5 wt% CNTs and a high temperature of 54°C (1 sun) on the PDMS surface. Under 1 solar irradiation, the evaporation rate can reach 2.25 kg (m²·h), the ion removal rate is 99.99% (concentration reduced by 3-4 orders of magnitude), and the produced water meets WHO standards. 3. The preparation process of this invention is simple. The semi-crosslinking reaction replaces the precision mold, which simplifies the construction of the gradient structure. Combined with biodegradable raw materials sodium alginate and acrylamide, the raw materials are environmentally friendly, the overall cost is reduced, and the seawater desalination efficiency and operational stability of the photothermal evaporator are significantly improved, which is conducive to the industrial application of photothermal evaporators. Attached Figure Description

[0019] Figure 1 Scanning electron microscope (SEM) image of the gradient pore structure of the gel water transport layer; Figure 2 The curves showing the effect of different CNT contents on the evaporation rate; Figure 3 Evaporation rate comparison curves of the photothermal evaporator prepared in the example for 20 wt% concentrated brine and the fully cross-linked random pore structure photothermal evaporator prepared in the comparative example; Figure 4 Evaporation rate curve of the photothermal evaporator prepared for the example, which was continuously operated in 3.5 wt% brine for seven days. Detailed Implementation

[0020] This invention provides a photothermal evaporator with a gradient pore structure, comprising a porous photothermal layer and a gel water-transporting layer disposed vertically. The porous photothermal layer is a polydimethylsiloxane porous matrix (PDMS-CNT) with dispersed carbon nanotubes, exhibiting hydrophobicity and a permeable microporous structure. The gel water-transporting layer, located below the photothermal layer, is a hydrophilic sodium alginate-poly(acrylamide-acrylic acid) hydrogel (P(AM-CO-AA)-SA). The gel water-transporting layer has a gradient pore structure with gradually changing pore size along its thickness direction, with smaller pores on the side closer to the porous photothermal layer and larger pores on the side farther away from the porous photothermal layer. The porous photothermal layer and the gel water-transporting layer are spatially interlocked through the sodium alginate-poly(acrylamide-acrylic acid) hydrogel wetting and solidifying the permeable pores of the porous photothermal layer. The thickness ratio of the porous photothermal layer to the gel water-transporting layer is 2:3.

[0021] In this invention, the gel water transport layer has a gradient pore structure with gradually changing pore size along the thickness direction. Specifically, the small pore size on the side closer to the porous photothermal layer is 15~33μm, and the large pore size on the side farther away from the porous photothermal layer is 170~200μm. The pore size of the entire gel water transport layer shows a gradient and continuous variation law along the thickness direction.

[0022] In this invention, the micropores of the porous photothermal layer are uniform in size, with a pore size of 28~55μm.

[0023] In this invention, the thickness of the porous photothermal layer is preferably 2 mm, and the thickness of the gel water-conducting layer is preferably 3 mm.

[0024] In this invention, the thickness of the photothermal evaporator is preferably 4-6 mm, more preferably 5 mm.

[0025] The photothermal evaporator with the preferred thickness of the present invention can ensure that heat is transferred to the evaporation interface in a timely manner and that the vapor escape path is optimized, while achieving a good evaporation rate.

[0026] This invention also provides a method for preparing a photothermal evaporator with a gradient pore structure, comprising the following steps: (1) Mix PDMS prepolymer with curing agent to obtain mixed solution a; mix mixed solution a with acetone solution, add carbon nanotubes, and perform ultrasonic treatment to obtain mixed solution b; (2) Add a pore-forming agent to the mixed solution b in step (1), stir, pour into a mold and heat to cure, to obtain a hydrophobic photothermal layer; the heating and curing temperature is 60℃ and the time is 3 hours; (3) Peel the hydrophobic photothermal layer obtained in step (2) from the mold, put it into deionized water to dissolve the pore-forming agent to form pores, obtain PDMS-CNT, and put the PDMS-CNT back into the mold; (4) Acrylamide is added to deionized water to obtain an acrylamide solution, and dimethyl sulfoxide and acrylic acid are added and mixed to obtain a mixed solution c.

[0027] (5) Sodium alginate is added to deionized water to obtain a sodium alginate solution, wherein the mass-volume fraction of sodium alginate is 1.86% w / v; (6) Mix the mixed solution c with tetramethylethylenediamine, N,N-methylenebisacrylamide and ammonium persulfate to obtain mixed solution d; mix the mixed solution d with the sodium alginate solution from step (5) to obtain the prepolymer solution; (7) Pour the prepolymer solution from step (6) into the mold containing the porous photothermal layer obtained in step (3) and heat it to obtain sodium alginate-poly(acrylamide-acrylic acid) hydrogel (P(AM-CO-AA)-SA); the heating temperature is 60℃ and the time is 30min; (8) Take out the P(AM-CO-AA)-SA obtained in step (7) together with the lower PDMS-CNT from the mold, place the porous photothermal layer at the bottom and put it into a container, add calcium chloride solution to the container, and carry out a semi-crosslinking reaction of P(AM-CO-AA)-SA to obtain a photothermal evaporator combining the gel water transport layer and the porous photothermal layer.

[0028] Unless otherwise specified, the raw materials used in this invention are preferably commercially available products.

[0029] The present invention does not specifically limit the mixing method; any mixing method well known to those skilled in the art can be used, as long as the mixture is uniform.

[0030] In this invention, the mass ratio of the PDMS prepolymer to the curing agent is preferably 10:1.

[0031] In this invention, the curing agent is preferably a vinyl-terminated polydimethylsiloxane crosslinking agent containing a platinum catalyst.

[0032] In this invention, the PDMS prepolymer raw material and the curing agent raw material can be selected separately from commercially available products and mixed in proportion, or the Sylgard 184 product sold by Dow Corning can be used directly. The Sylgard 184 product contains the prepolymer and curing agent raw materials of this invention, and the mixing ratio of the prepolymer and the curing agent is 10:1.

[0033] In this invention, the ultrasound conditions are 10 minutes of ultrasound in an ice bath.

[0034] In this invention, the preferred mass ratio of the PDMS prepolymer, acetone solution and CNT is 1:1:0.005.

[0035] This invention involves mixing a certain proportion of PDMS prepolymer with acetone and stirring, then adding CNTs and subjecting the mixture to ultrasonic treatment. This ensures the uniform dispersion of CNTs in the PDMS matrix, providing a foundation for forming a porous photothermal layer with high light absorption.

[0036] In this invention, the pore-forming agent in step (2) is preferably NaCl particles with a particle size of 100 μm; the volume ratio of the pore-forming agent to the mixed solution b is preferably 1:1.2.

[0037] In this invention, a pore-forming agent is added to a mixed solution b and then heated to cure. Acetone evaporates upon heating, and PDMS crosslinks to form a hydrophobic layer, in which CNTs are embedded, thus imparting photothermal responsiveness.

[0038] The porous photothermal layer provided by this invention employs a "fill-then-pore" preparation method: CNTs are pre-dispersed in a PDMS / acetone solution, and then NaCl particles are added as a sacrificial template; after heating and curing, the layer is washed with water to form a continuous microporous structure. The combination of PDMS and CNT materials results in a light absorption rate of 94%, and the hydrophobicity (contact angle >110°) inhibits salt crystal adhesion, while the microporous structure allows water vapor to escape during evaporation.

[0039] In this invention, the preferred mass-volume fractions of each component in the mixed solution c in step (4) are: acrylamide 11.2% w / v, dimethyl sulfoxide 5.12% w / v, and acrylic acid 2.35% w / v.

[0040] In this invention, the preferred mass-volume fractions of each component in the mixed solution d in step (6) are: tetramethylethylenediamine 0.029% w / v, N,N-methylenebisacrylamide 0.075% w / v, and ammonium persulfate 0.056% w / v.

[0041] In this invention, the preferred mass-volume fraction of the calcium chloride solution in step (8) is 2.5% w / v, the preferred immersion height of the calcium chloride solution is 30% to 70% of the thickness of the gel water transport layer, and the immersion time is 1 to 3 hours.

[0042] The porous photothermal layer of PDMS-CNT produced by this invention possesses a permeable microporous structure, allowing the subsequently cast P(AM-CO-AA)-SA hydrogel to fully wet and penetrate into the pores, forming a robust spatial interlocking structure (or mechanical interlocking) after heating and curing. This mechanism effectively solves the technical problem of weak interfacial bonding and easy delamination between hydrophobic PDMS and hydrophilic hydrogel, thus ensuring the structural integrity and stability of the double-layer photothermal evaporator during long-term operation. Simultaneously, the porous photothermal layer prepared by this invention enhances multiple reflections, achieving a light absorption rate of 94%.

[0043] The gel water-transporting layer of this invention utilizes in-situ polymerization to synthesize P(AM-CO-AA)-SA hydrogel, followed by semi-immersion cross-linking in calcium chloride solution, resulting in a hydrogel with a gradient pore size structure. This gel water-transporting layer is hydrophilic, providing a continuous water supply, and simultaneously promotes water transport through the capillary effect of the gradient pores. The biomimetic design of this gradient pore structure replicates the transport mechanism of tree roots, namely, rapid water supply through dense micropores on the surface (contact angle 21°), and back diffusion of salt ions through macropores in the lower layer. Figure 1As shown, the gel water-transporting layer prepared by this invention exhibits a pore structure with a gradient change in pore size along its thickness direction. Specifically, the pore structure transitions from a dense micropore structure near the porous photothermal layer to a loose macropore structure away from the porous photothermal layer, clearly demonstrating that the gel water-transporting layer forms a gradient pore structure with a gradually changing pore size, providing a structural basis for the upward transport of water and the downward diffusion of salt ions.

[0044] When calcium chloride solution comes into contact with P(AM-CO-AA)-SA hydrogel, a cross-linking reaction occurs. The semi-cross-linking of the present invention involves immersing the portion of P(AM-CO-AA)-SA hydrogel near the porous photothermal layer in calcium chloride solution to undergo a cross-linking reaction and shrink, forming small pores at the top. Meanwhile, the portion of P(AM-CO-AA)-SA hydrogel far from the porous photothermal layer does not come into contact with the calcium chloride solution and can maintain its original relatively loose state, retaining larger pores. This creates a gradient change in pore size.

[0045] The present invention also provides the application of the photothermal evaporator with the gradient pore structure and the photothermal evaporator prepared by the preparation method in seawater desalination.

[0046] The photothermal evaporator provided by this invention features a hydrophobic PDMS-CNT porous photothermal layer with excellent light absorption characteristics and salt resistance. The hydrophilic P(AM-CO-AA)-SA gel water-transporting layer undergoes non-uniform shrinkage induced by semi-immersion cross-linking with calcium chloride solution. The uncross-linked regions of the gel water-transporting layer form macroporous structures to accelerate water transport and salt dissolution, while the cross-linked regions maintain dense pores to ensure strong capillary water supply, thus constructing a biomimetic gradient structure. The preparation method of the photothermal evaporator provided by this invention significantly simplifies the traditional process of constructing porous structures using high-precision complex gradient molds. The raw materials are environmentally friendly and biodegradable, improving the seawater desalination efficiency of the photothermal evaporator. By controlling the CNT loading (0.5 wt%) and the thickness of the porous photothermal layer, this invention utilizes a sacrificial template method for pore formation combined with a controllable semi-cross-linking process to prepare the photothermal evaporator, which can effectively achieve rapid water vapor escape and reverse salt diffusion. Under 1 unit of solar irradiation, the evaporation rate can reach 2.25 kg (m²·h).

[0047] The innovation and advantages of this invention: By employing a preparation process combining sacrificial template pore-forming and directional semi-crosslinking, a double-layer photothermal evaporator with a biomimetic gradient pore structure was successfully fabricated. This breakthrough overcomes the technical contradictions of "high evaporation rate and high salt resistance" and "high performance and low cost," achieving a balance between high evaporation rate, high salt resistance, and low cost. Therefore, compared with traditional homogeneous photothermal evaporators, the photothermal evaporator provided by this invention not only has high evaporation efficiency but also exhibits excellent long-term stability, remaining crystallized for 7 days continuously in 3.5 wt% high-salt water. The produced water quality meets WHO drinking water standards, demonstrating broad prospects for engineering applications.

[0048] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0049] The method for determining the evaporation rate of the photothermal evaporator prepared in this embodiment of the invention under solar irradiation can be carried out using the gravimetric method commonly used in the field, namely the photothermal-vapor conversion rate gravimetric determination method.

[0050] The experimental lighting conditions in this embodiment of the invention include 1.0 kW / m² simulated sunlight. Example

[0051] Weigh 10 g of PDMS prepolymer and 1 g of curing agent, mix with 10 g of acetone solution, stir until homogeneous, then add 0.05 g of CNT, and sonicate in an ice bath for 10 min to form a homogeneous mixed solution. Add 29.2 g of NaCl particles with a particle size of 100 μm to this homogeneous mixed solution, stir until homogeneous, pour into a mold, and heat to cure at 60 °C for 3 hours to obtain a hydrophobic photothermal layer. Peel the obtained hydrophobic photothermal layer from the mold, dissolve the NaCl particles in deionized water to form pores, and obtain PDMS-CNT. Place the PDMS-CNT back into the mold. Weigh 11.2 g of acrylamide and dissolve it in 100 mL of deionized water, stir to obtain an acrylamide solution, and then mix it with 5.12 g of dimethyl sulfoxide and 2.35 g of acrylic acid to obtain mixed solution c. Separately, add 1.86 g of sodium alginate to 100 mL of deionized water, stir to dissolve, and obtain a sodium alginate solution. The aforementioned mixed solution c, 0.023 g tetramethylethylenediamine, 0.075 g N,N-methylenebisacrylamide, and 0.056 g ammonium persulfate were mixed and then poured into a sodium alginate solution. The mixture was stirred until homogeneous to obtain a prepolymer solution. This prepolymer solution was poured into a mold containing PDMS-CNTs and heated at 60°C for 30 minutes until gelation occurred, yielding P(AM-CO-AA)-SA hydrogel. The P(AM-CO-AA)-SA hydrogel, along with the underlying PDMS-CNT layer, was removed from the mold. The PDMS-CNT layer was placed at the bottom in a container, and a 2.5% w / v calcium chloride solution was added to the container. The solution was soaked until the gel water-conducting layer reached 50% of its thickness. A semi-crosslinking reaction was then carried out on the P(AM-CO-AA)-SA hydrogel for 2 hours to obtain a photothermal evaporator combining a gel water-conducting layer and a porous photothermal layer. The porous photothermal layer has a thickness of 2 mm, and the gel water-conducting layer has a thickness of 3 mm.

[0052] The photothermal evaporator prepared in this embodiment of the invention achieves a synergistic effect of 94% light absorption rate and PDMS surface high temperature of 54℃ (1 sun) under a high load of 0.5 wt% CNT. Under 1 solar irradiation, the evaporation rate can reach 2.25 kg / (m²・h), the ion removal rate is 99.99% (concentration reduced by 3 to 4 orders of magnitude), and the produced water meets WHO standards.

[0053] The preparation method of the comparative example differs in that calcium chloride solution is added to the container to completely immerse the gel water-conducting layer, thus preparing a fully cross-linked random pore structure photothermal evaporator.

[0054] In the comparative example, due to complete immersion, the hydrogel shrinks uniformly, forming a uniform small pore size distribution with no gradient change in pore size.

[0055] Figure 1 The image shows a scanning electron microscope (SEM) image of the gradient pore structure of the gel water transport layer in the photothermal evaporator prepared according to an embodiment of the present invention. Figure 1 It can be seen that the pore structure in the gel water transport layer has the characteristic of pore size variation, specifically showing a gradient change in pore size.

[0056] Figure 2 The curves showing the effect of different CNT contents on evaporation rate and surface temperature in the photothermal evaporator prepared according to embodiments of the present invention are shown. Figure 2 It can be seen that under simulated sunlight of 1.0 kW / m², the surface temperature and evaporation rate of the evaporator continue to rise with the increase of CNT content, and tend to stabilize when the CNT content reaches 0.5 wt%, at which point the surface temperature reaches a maximum of 54℃. This indicates that the introduction of CNTs effectively enhances the photothermal conversion performance of the material. When the CNT loading reaches a certain level, its photothermal conversion capacity is fully released and enters a plateau period, thereby maintaining the evaporation rate at the highest level, with an evaporation rate of 2.25 kg / (m²・h).

[0057] Figure 3 The comparison curves of the evaporation rates of the semi-crosslinked gradient structure photothermal evaporator prepared in this embodiment and the fully crosslinked random pore structure photothermal evaporator prepared in the comparative example in 20 wt% concentrated brine are shown. Figure 3It can be seen that the initial evaporation rate of the random pore structure evaporator is lower than that of the gradient structure evaporator, and the increase is smaller, only reaching about 1.5 kg / (m²·h). In the middle stage, the evaporation rate of the gradient structure evaporator remains at a relatively high level of 1.7~1.9 kg / (m²·h), with only a slow and slight decrease. The evaporation rate of the random pore structure evaporator fluctuates around 1.5 kg / (m²·h) before gradually decreasing, with a significantly larger decrease than that of the gradient structure evaporator. This shows that in the middle stage, the evaporation rate of the gradient structure evaporator is significantly higher than that of the random pore structure evaporator, and its stability is stronger. In the later stage, the evaporation rate of the gradient structure evaporator slowly decreases to about 1.6 kg / (m²·h), but still remains in a relatively high range; the evaporation rate of the random pore structure evaporator continues to decrease to about 1.0 kg / (m²·h), with a significantly larger decrease. This shows that in the later stage, the evaporation rate advantage of the gradient structure evaporator further expands, and its long-term stability is superior.

[0058] Figure 4 The evaporation rate curve of the photothermal evaporator prepared for an embodiment of the present invention, operating continuously for seven days in 3.5 wt% brine, is based on... Figure 4 As can be seen, from Day 1 to Day 7, the evaporation rate of the photothermal evaporator of this invention remained consistently within the high range of 2.0~2.5 kg / (m²·h), without any significant downward trend; only minor fluctuations occurred (fluctuation range approximately ±0.5 kg / (m²·h)), which is within the normal error range of the experiment. During the continuous 7-day test period, the evaporation rate did not decrease with time (no continuous downward trend). In a 3.5% NaCl (simulated seawater) environment, the photothermal evaporator did not experience performance degradation due to salt accumulation or other issues, indicating that the photothermal evaporator prepared by this invention can continuously maintain high evaporation efficiency in a seawater environment and has good long-term operational stability.

[0059] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention, and is merely a preferred embodiment of the invention. Those skilled in the art can readily make various modifications to the above embodiments. For those skilled in the art, several improvements and refinements can be made without departing from the principles of the invention, and the general principles described herein can be applied to other embodiments without creative effort. Therefore, the invention is not limited to the above embodiments. Improvements and refinements made by those skilled in the art based on the teachings of the invention, without departing from the scope of the invention, should be considered within the protection scope of the invention.

Claims

1. A photothermal evaporator with a gradient pore structure, characterized in that, The system comprises a porous photothermal layer and a gel-based water-transporting layer arranged vertically. The porous photothermal layer is a polydimethylsiloxane porous matrix (PDMS-CNT) with dispersed carbon nanotubes, exhibiting hydrophobicity and a permeable microporous structure. The gel-based water-transporting layer, located below the photothermal layer, is a hydrophilic sodium alginate-poly(acrylamide-acrylic acid) hydrogel (P(AM-CO-AA)-SA). This gel-based water-transporting layer has a gradient pore structure with gradually changing pore sizes along its thickness, with smaller pores closer to the porous photothermal layer and larger pores further away. The porous photothermal layer and the gel-based water-transporting layer are spatially interlocked through a sodium alginate-poly(acrylamide-acrylic acid) hydrogel that wets and solidifies the permeable pores of the porous photothermal layer. The thickness ratio of the porous photothermal layer to the gel-based water-transporting layer is 2:

3.

2. A method for preparing the photothermal evaporator according to claim 1, characterized in that, Includes the following steps: (1) Mix polydimethylsiloxane prepolymer with curing agent to obtain mixed solution a; mix mixed solution a with acetone solution, add carbon nanotubes, and perform ultrasonic treatment to obtain mixed solution b; (2) Add a pore-forming agent to the mixed solution b in step (1), stir, pour into a mold and heat to cure, to obtain a hydrophobic photothermal layer; the heating and curing temperature is 60℃ and the time is 3 hours; (3) Peel the hydrophobic photothermal layer obtained in step (2) from the mold, put it into deionized water to dissolve the pore-forming agent, form pores, and obtain a porous photothermal layer (PDMS-CNT), and put the porous photothermal layer back into the mold; (4) Acrylamide was added to deionized water to obtain an acrylamide solution, and then dimethyl sulfoxide and acrylic acid were added and mixed to obtain a mixed solution c; (5) Sodium alginate is added to deionized water to obtain a sodium alginate solution, wherein the mass-volume fraction of sodium alginate is 1.86% w / v; (6) Mix the mixed solution c with tetramethylethylenediamine, N,N-methylenebisacrylamide and ammonium persulfate to obtain mixed solution d; mix the mixed solution d with the sodium alginate solution from step (5) to obtain the prepolymer solution; (7) Pour the prepolymer solution from step (6) into the mold containing the porous photothermal layer obtained in step (3) and heat it to obtain sodium alginate-poly(acrylamide-acrylic acid) hydrogel (P(AM-CO-AA)-SA); the heating temperature is 60℃ and the time is 30min; (8) Take out the sodium alginate-poly(acrylamide-acrylic acid) hydrogel obtained in step (7) together with the lower porous photothermal layer from the mold, place the porous photothermal layer at the bottom and put it into a container, add calcium chloride solution to the container, and carry out a semi-crosslinking reaction on the sodium alginate-poly(acrylamide-acrylic acid) hydrogel to obtain a photothermal evaporator combining the gel water transport layer and the porous photothermal layer.

3. The preparation method according to claim 2, characterized in that, In step (1), the mass ratio of the polydimethylsiloxane prepolymer to the curing agent is 10:1; the curing agent is a vinyl-terminated polydimethylsiloxane crosslinking agent containing a platinum catalyst; and the ultrasonic conditions are ice bath ultrasonication for 10 min.

4. The preparation method according to claim 2, characterized in that, In step (1), the mass ratio of the polydimethylsiloxane prepolymer, acetone solution and carbon nanotubes is 1:1:0.

005.

5. The preparation method according to claim 2, characterized in that, In step (2), the pore-forming agent is NaCl particles with a particle size of 100 μm; the volume ratio of the pore-forming agent to the mixed solution b is 1:1.

2.

6. The preparation method according to claim 2, characterized in that, The mass volume fractions of each component in the mixed solution c in step (4) are: acrylamide 11.2% w / v, dimethyl sulfoxide 5.12% w / v, and acrylic acid 2.35% w / v.

7. The preparation method according to claim 2, characterized in that, The mass-volume fractions of each component in the mixed solution d in step (6) are: tetramethylethylenediamine 0.029% w / v, N,N-methylenebisacrylamide 0.075% w / v, and ammonium persulfate 0.056% w / v.

8. The preparation method according to claim 2, characterized in that, In step (8), the mass volume fraction of the calcium chloride solution is 2.5% w / v, the immersion height of the calcium chloride solution is 30%~70% of the thickness of the gel water transport layer, and the immersion time is 1~3h.

9. The preparation method according to claim 2, characterized in that, The thickness of the photothermal evaporator is 4~6mm.

10. The application of the photothermal evaporator according to claim 1 or the photothermal evaporator prepared by the preparation method according to any one of claims 2 to 9 in seawater desalination.

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