High-salt-tolerance asymmetric solar interface evaporator and preparation method

Abstract

The invention relates to a high-salt-tolerance asymmetric solar interface evaporator and a preparation method. Nano-crystalline cellulose with low thermal conductivity is used as a base material, carbon black with good photothermal conversion performance is used as a photothermal layer, methyltrimethoxysilane is used as a hydrophobic modifier, and the high-salt-tolerance asymmetric solar interface evaporator with a vertical pore channel structure is constructed through a directional freezing technology. The evaporator is composed of a cellulose hydrophilic lower layer and a carbon black hydrophobic upper layer, water can be rapidly transported to an evaporation interface through the capillary action of cellulose, the carbon black hydrophobic upper layer can improve the light absorption capacity and the heat positioning capacity of the evaporator, and continuous and efficient water evaporation is achieved. Cellulose cation modification and an asymmetric structure can reduce the generation of salt crystals, can significantly improve the salt resistance of the evaporator, has a good application prospect in the field of solar seawater desalination, and has wide practical application potential due to easy preparation, enlargement and low cost.

Description

Technical Field

[0001] This invention relates to the field of solar interfacial water evaporation, and in particular to a highly salt-tolerant asymmetric solar interfacial evaporator and its preparation method. Background Technology

[0002] In recent years, with rapid population growth and industrial development, freshwater scarcity has become one of the major challenges facing the world. Traditional seawater desalination technologies, due to their low efficiency, complex processes, and high energy consumption, cannot be used on a large scale in remote and impoverished areas. Given the advantages of solar energy—cleanliness, zero pollution, and sustainability—solar seawater desalination technology has attracted widespread attention. Unlike traditional solar seawater desalination technologies, solar interface evaporators are composite materials that float on water. They absorb solar energy through photothermal conversion materials and convert it into heat energy. The generated heat is localized at the evaporation interface, where water reaches the surface of the photothermal conversion material through capillary action, absorbs heat, and produces water vapor. A high-efficiency solar interface evaporator should possess good light absorption capacity, excellent thermal management capabilities, and rapid water transport capabilities.

[0003] Currently, most evaporators use hydrophilic surfaces for their photothermal conversion materials. During long-term seawater desalination, this leads to heat loss and salt deposition. Salt crystallization reduces the light absorption area and affects the material's photothermal conversion efficiency. It also obstructs water supply channels and blocks steam overflow channels, resulting in reduced evaporation efficiency. Conventional methods for addressing salt deposition include physical removal (repeated rinsing or washing) and natural diffusion. However, both methods hinder continuous evaporation, increasing production costs and reducing efficiency. Therefore, developing sustainable, highly salt-tolerant solar evaporators is crucial for achieving efficient solar interfacial evaporation.

[0004] This invention first modifies low thermal conductivity cellulose cationically, using the cationically modified cellulose as raw material to construct a highly salt-resistant asymmetric solar interface evaporator with a vertical pore structure and a wettability gradient. The evaporator consists of two parts: a hydrophilic cellulose lower layer and a hydrophobic carbon black upper layer. The hydrophilic cellulose lower layer with a vertical pore structure is prepared using directional freezing technology. The numerous vertically arranged pores can rapidly transport water to the evaporation interface through capillary action. The hydrophobic carbon black upper layer with a random pore structure is prepared using a secondary freezing technique. The carbon black enhances the evaporator's light absorption capacity, and the random pores provide complex optical paths, facilitating light absorption and reflection, promoting light absorption and heat conversion of the photothermal material, and achieving continuous and efficient water evaporation. The cationic modification and hydrophobic modification of the hydrophilic layer improve the evaporator's salt resistance. The cation-modified hydrophilic layer can effectively reduce salt deposition by utilizing the Donnan effect, while the hydrophobic modification enables the evaporator to float on its own. It can also prevent salt ions from entering the light absorber with the water flow. Most of the salt ions are blocked in the lower layer of the hydrophobic light absorber and can be quickly dissolved and crystallized through convection, thus achieving sustainable and efficient water production. Summary of the Invention

[0005] This invention addresses the problems of low water evaporation rate and salt deposition in existing interfacial evaporators by proposing a highly salt-resistant asymmetric solar interfacial evaporator and its preparation method. First, quaternary ammonium salt functionalized nanocellulose is prepared through cation modification. This nanocellulose is then crosslinked with polyvinyl alcohol using a crosslinking agent to obtain a hydrophilic suspension. Carbon black (CB) is used as the photothermal material, and methyltrimethoxysilane (MTMS) is added for hydrophobic modification to obtain a hydrophobic suspension. An integrated, highly salt-resistant asymmetric solar interfacial evaporator with vertical channels is constructed using directional freezing and secondary freezing techniques. The evaporator designed and prepared in this invention exhibits excellent water evaporation efficiency and achieves continuous salt resistance, showing promising application prospects in the field of seawater desalination.

[0006] The technical solution of this invention to solve the technical problem of the product preparation method is as follows: First, the hydrophilic layer suspension is frozen into an ice gel using directional freezing technology. Then, the hydrophobic layer suspension is poured onto the ice gel and placed in a refrigerator for secondary freezing, thereby designing and preparing a highly salt-resistant asymmetric solar interface evaporator with vertical channels. The preparation process is as follows:

[0007] (1) Preparation of quaternary ammonium salt functionalized cellulose: Under alkaline conditions, quaternary ammonium salt functionalized cellulose nanoparticles (EPTMAC-CNF) were prepared by chemical reaction with 2,3-epoxypropyltrimethylammonium chloride (EPTMAC). Subsequently, the reaction product was dialyzed to neutral by distilled water (DW), and the dialyzed cellulose was freeze-dried to obtain epioxypropyltrimethylammonium nanoparticles (EPTMAC-CNF).

[0008] (2) Preparation of hydrophilic layer suspension: The quaternary ammonium salt functionalized cellulose prepared in step 1 is mixed with polyvinyl alcohol solution in a certain proportion and stirred evenly. Sulfuric acid solution and glutaraldehyde solution are added dropwise to it. After thermal crosslinking at 75℃ for 3h, a hydrophilic layer suspension is obtained.

[0009] (3) Preparation of hydrophobic layer suspension: Unmodified cellulose and polyvinyl alcohol solution are mixed and stirred evenly in a certain proportion. Sulfuric acid solution and glutaraldehyde solution are added dropwise. After thermal crosslinking at 75°C for 3 hours, nano carbon black (CB) and methyltrimethoxysilane solution (MTMS) are added and stirred for 3 hours to obtain hydrophobic modification and hydrophobic layer suspension.

[0010] (4) Preparation of Janus-structured aerogel: The hydrophilic layer suspension was poured into a mold and pre-frozen in a refrigerator at 4°C. After pre-cooling, the mold was placed on a copper plate and liquid nitrogen was injected for directional freezing to form an ice gel. The hydrophobic layer suspension was poured onto the ice gel. The sample was then left to stand at room temperature (about 20°C) for a period of time to fuse the interface between the two layers, and then transferred to a refrigerator for freezing. Janus aerogel was obtained after freeze-drying for 60 hours.

[0011] This invention relates to a high salt-resistant asymmetric solar interfacial evaporator and its preparation method. An EPTMAC-CNF and PVA solution are used to form a hydrophilic layer. A directional freezing technique is used to obtain an ice gel with vertical channels. A hydrophobic suspension is poured onto the ice gel and subjected to a secondary freezing technique to obtain an integrated Janus interfacial evaporator. Its advantages are: (1) The vertical channels in the hydrophilic layer improve water transport efficiency, allowing water to be quickly transported to the evaporator surface. (2) The high light absorption capacity of the carbon black material in the photothermal layer and the rough surface structure of the evaporator synergistically enhance light utilization, effectively improving the evaporation efficiency of the evaporator. (3) The quaternary ammonium salt cation-modified hydrophilic layer has a large number of positively charged N2 on the cellulose surface. + It is possible to effectively separate Na through the Donnan effect. + With Cl - This fundamentally avoids the formation of solid salt crystals, giving the evaporator long-term salt resistance. (4) The Janus structure enables the evaporator to float on its own. The hydrophobic properties of the upper layer can effectively prevent salt from rising. Even if a small amount of salt scale is generated in the hydrophilic lower layer, it can be re-infused into the water, thereby avoiding the salt deposition problem and extending the service life of the evaporator. Attached Figure Description

[0012] Figure 1 This invention provides a fabrication process for a highly salt-resistant asymmetric solar interface evaporation device.

[0013] Figure 2 The image shows the UV-Vis-NIR absorption spectrum of JPC23 in Example 1.

[0014] Figure 3 The images show infrared thermal images and surface temperature change curves of JPC23 and pure water under 1 Sun illumination intensity in Example 1.

[0015] Figure 4 This is a photograph of the water absorption performance of JPC23 in Example 1.

[0016] Figure 5 The evaporation rate and solar energy conversion efficiency of the solar interface evaporators in Examples 1 and 4-7 under 1 Sun irradiation intensity are given.

[0017] Figure 6 The illustration shows the evaporation rates of the JPC23 solar interface evaporators in Example 1 and JPC23-0 in Example 2 after continuous evaporation for 5 hours at a salt concentration of 15 wt%. The inset shows images of JPC23-0 (left) and JPC23 (right) after continuous evaporation for 5 hours.

[0018] Figure 7 The illustration shows the evaporation rate of the PC23 solar interface evaporator in Example 1 (JPC23) and Example 3 after continuous evaporation for 5 hours at a 15% salt concentration. The inset shows images of PC23 (left) and JPC23 (right) after continuous evaporation for 5 hours.

[0019] Figure 8 The mass loss and evaporation rate of JPC23 after 6 cycles in a 15wt% salt solution in Example 1 are shown.

[0020] Figure 9 The JPC23 in Example 1 is used to measure the change in ion concentration before and after seawater desalination.

[0021] Figure 10 This is a schematic diagram illustrating the working principle of a highly salt-resistant asymmetric solar interface evaporator according to the present invention. Detailed Implementation

[0022] This invention relates to a highly salt-resistant asymmetric solar interface evaporator and its preparation method. The evaporator uses cellulose as the substrate, carbon black (CB) as the photothermal material, and methyltrimethoxysilane (MTMS) as the hydrophobic modifier. The integrated highly salt-resistant asymmetric solar interface evaporator with vertical channels is constructed by directional freezing and secondary freezing technology.

[0023] The following examples further illustrate the high salt-resistant asymmetric solar interface evaporator and its preparation method described in this invention.

[0024] Example 1

[0025] (1) Preparation of quaternary ammonium salt functionalized cellulose: First, NaOH solution (1 wt%) was added to CNF suspension (2 wt%) and stirred until homogeneous. Then, 7.278 g of epipropyltrimethoxyammonium chloride (EPTMAC) was added to the system, and the mixture was magnetically stirred in an oil bath at 85 °C for 6 h. Subsequently, the reaction product was dialyzed through distilled water (DW) until neutral (MWCO: 8000-14000MD). The dialyzed cellulose was freeze-dried for 42 h to obtain epipropyltrimethylammonium nanocellulose (EPTMAC-CNF).

[0026] (2) Preparation of polyvinyl alcohol solution: Dissolve 5.0g PVA in 100mL deionized water, stir continuously in an 85℃ water bath for 4 hours to prepare PVA solution (0.05g / mL), let stand for 2h to remove air bubbles, seal and store in a 4℃ refrigerator for later use.

[0027] (3) Preparation of the hydrophilic layer suspension: A CNF solution (0.94 wt%) was prepared using the PEPTMAC-CNF obtained in step (1). The CNF solution and the PVA solution (0.05 g / mL) obtained in step (2) were mixed and stirred for 1 h until a homogeneous suspension was formed, wherein the mass ratio of CNF to PVA was 2:3. Then, sulfuric acid solution (volume fraction 1.0%) was gradually added dropwise to the PVA / CNFs mixed suspension until the pH of the suspension was 3-4. Glutaraldehyde solution was added to the prepared mixed suspension and stirred for 1 h. Then, the suspension was heated to 75 °C for 3 h for thermal crosslinking to obtain the hydrophilic layer suspension.

[0028] (4) Preparation of the hydrophobic layer suspension: A CNFs solution (0.65 wt%) was prepared using unmodified CNFs. 8 g of CNFs solution was mixed with 1 mL of the PVA solution (0.05 g / mL) obtained in step (2) and stirred for 1 h. Then, a 1.0% sulfuric acid solution was gradually added dropwise to the PVA / CNFs mixed suspension until the pH of the suspension was 3-4. Glutaraldehyde solution was added to the suspension and stirred for 1 h. Then, the suspension was heated to 75 °C and subjected to thermal crosslinking for 3 h. After crosslinking, nano-carbon black (0.1 wt%) and methyltrimethoxysilane solution (MTMS) were added and stirred for 3 h to perform hydrophobic modification and obtain the hydrophobic layer suspension.

[0029] (5) Preparation of Janus interfacial solar evaporator: First, the hydrophilic layer suspension was poured into a polytetrafluoroethylene mold and pre-frozen in a refrigerator at 4°C. After pre-cooling, the mold was placed on a copper plate and injected with liquid nitrogen for directional freezing to form an ice gel. The hydrophobic layer suspension was poured onto the ice gel. The sample was then left to stand at room temperature (about 20°C) for a period of time to fuse the interface between the two layers. Then, it was transferred to a refrigerator for freezing. After freeze-drying at -50°C for 60 hours using a freeze dryer, Janus aerogel was obtained, named JPC23. The preparation process is as follows: Figure 1 As shown.

[0030] Example 2

[0031] A highly salt-resistant asymmetric solar interfacial evaporator and its preparation method are basically the same as in Example 1, except that: in step 3, unmodified cellulose is used to prepare CNF solution, and the evaporator is named JPC23. - 0.

[0032] Example 3

[0033] A highly salt-resistant asymmetric solar interface evaporator and its preparation method are basically the same as those in Example 1, except that: MTMS solution is not added in step 4, and the evaporator is named PC32.

[0034] Example 4

[0035] A highly salt-resistant asymmetric solar interface evaporator and its preparation method are basically the same as those in Example 1, except that the mass ratio of CNF to PVA in step 3 is 1:2, and the evaporator is named JPC12.

[0036] Example 5

[0037] A highly salt-resistant asymmetric solar interface evaporator and its preparation method are basically the same as those in Example 1, except that the mass ratio of CNF to PVA in step 3 is 1:1, and the evaporator is named JPC11.

[0038] Example 6

[0039] A highly salt-resistant asymmetric solar interface evaporator and its preparation method are basically the same as those in Example 1, except that the mass ratio of CNF to PVA in step 3 is 2:1, and the evaporator is named JPC21.

[0040] Example 7

[0041] A highly salt-resistant asymmetric solar interface evaporator and its preparation method are basically the same as those in Example 1, except that the mass ratio of CNF to PVA in step 3 is 3:2, and the evaporator is named JPC32.

[0042] Experimental Example 1

[0043] The JPC23 prepared in Example 1 was subjected to ultraviolet-visible-near-infrared absorption spectroscopy, and the results are shown in the figure. Figure 2 Test results show that the JPC23 prepared in Example 1 has a light absorption rate of over 95% across the entire solar spectrum. This significant light absorption capacity is attributed to the addition of carbon black, which possesses a wide-bandwidth and highly efficient light absorption capability and can enhance the light absorption capacity of the aerogel.

[0044] Experimental Example 2

[0045] The surface temperature of JPC23 prepared in Example 1 was observed to change over time, with pure water used as a control group. The results are shown in [Figure 1]. Figure 3 The test results show that the JPC23 prepared in Example 1 has excellent thermal positioning ability. Under 1 Sun light intensity, the temperature of pure water increased from 24℃ to 35.1℃ by only 11.1℃, while the surface temperature of JPC23 can rapidly rise from 24.8℃ to 69.6℃ within 15 minutes. As the light exposure time continues to increase, the surface temperature of the sample tends to reach equilibrium and stabilizes at 74.7℃ after 60 minutes.

[0046] Experimental Example 3

[0047] The JPC23 prepared in Example 1 was placed in a petri dish containing red ink solution. A clean, lint-free paper was placed on the top surface, and the color change of the paper was observed to monitor the capillary absorption of water from the bottom to the top of the evaporator. The results are as follows. Figure 4 As shown, clean paper was completely stained red after 25 seconds, a phenomenon that visually confirms that the evaporator has good capillary water transport capability, which can quickly and effectively transport water from the bottom to the top of the evaporator.

[0048] Experimental Example 3

[0049] This experiment demonstrates water evaporation performance. The testing procedure is as follows: JPC11, JPC12, JPC21, JPC23, and JPC32 were selected as the test group. The aerogel was encased in insulating polystyrene foam and then placed in a beaker containing bulk water. A solar simulator (xenon lamp light source and AM1.5G filter) was used to simulate sunlight for the water evaporation experiment. The distance between the sample and the simulated light source was fixed, and the light intensity of the simulated light source was changed by adjusting the voltage to maintain the light power density at 1 Sun (1 kWm). -2 An electronic balance connected to a computer monitors the mass change of the device in real time. The computer software records the mass loss every 30 seconds, and the test duration is 3600 seconds. An infrared thermal imager is used to measure the changes in surface temperature and water temperature of the sample during the evaporation process. The test results are as follows: Figure 5 As shown, the evaporation rate and evaporation efficiency of the samples increased with increasing cellulose content. When the PVA / CNF mass ratio was 2:3, the evaporation rate was 1.81 kg m³ / s. -2 h -1 The evaporation efficiency is 80.85%, and both the evaporation rate and evaporation efficiency are at a relatively high level.

[0050] Experiment Example 4

[0051] This experimental example was used to test the effect of cationic modification on the salt resistance of the evaporator. JPC23 prepared in Example 1 and JPC23-0 prepared in Example 2 were placed in a 15% sodium chloride solution and continuously evaporated for 5 hours. The water evaporation rate was then tested, and the results are as follows: Figure 6 As shown, the water evaporation rate of JPC23 remained stable after 5 hours of operation, with an evaporation rate of 1.8 kg m³. -2 h -1 No scale was formed on the evaporator surface. After 5 hours of operation, the evaporation rate of the JPC23-0 increased from the original 1.72 kg / m³. -2 h -1 Decreased to 1.64 kgm -2 h -1 This is because a small amount of scale is generated in the evaporator channels, and the surface of the cation-modified cellulose carries a large number of positively charged N+ ions, which can effectively separate Na+ through the Donnan effect. + With Cl - This fundamentally avoids the formation of solid salt crystals, giving the evaporator long-term salt resistance.

[0052] Experimental Example 5

[0053] This experiment investigated the effect of the Janus structure on the salt resistance of the evaporator. JPC23 prepared in Example 1 and PC23 prepared in Example 3 were continuously evaporated in a 15% sodium chloride solution for 5 hours. The results are as follows: Figure 7 As shown, the evaporation rate of PC23 gradually decreased with the extension of evaporation time. After 5 hours of continuous evaporation, a large amount of salt was deposited on the surface of the evaporator. In contrast, the JPC23 evaporator showed more stable evaporation performance under the same experimental conditions, and its evaporation rate did not decrease significantly. The experimental results show that the Janus structure interface evaporator can achieve continuous salt resistance. The hydrophobic properties of the upper layer can effectively prevent salt from rising. Even if a small amount of salt scale is generated in the hydrophilic lower layer, it can quickly dissolve into the water, thereby avoiding the salt deposition problem.

[0054] Experimental Example 5

[0055] This experimental example is a salt resistance test of the evaporator. The JPC23 prepared in Example 1 was continuously run in a high-concentration salt solution with a salt concentration of 15 wt% for 6 hours, and the formation of salt crystals on the evaporator surface was observed. The results are as follows: Figure 8 As shown. After 6 hours of continuous operation, the evaporation rate stabilized at 1.82 kg m³. -2 h -1 Furthermore, no salt crystals were observed on the evaporator surface, indicating that JPC23 has excellent salt resistance.

[0056] Experimental Example 6

[0057] This experimental example tests the seawater desalination performance of an evaporator. JPC23 prepared in Example 1 was placed in simulated seawater for evaporation experiments. The condensate obtained after evaporation from the simulated seawater and the solar evaporator was measured using a cation chromatography instrument. The experimental results are as follows: Figure 9 As shown in the figure, the Na in the collected freshwater can be observed. + Mg 2+ K + Ca 2+ The concentrations were 0.54 mg / L. -1 0.08 mg / L -1 0.21 mg L -1 and 0.42 mg L -1 The ion concentration in the desalinated seawater is far below the standards set by the World Health Organization. Solar evaporators demonstrate excellent seawater desalination performance in practical applications.

[0058] This invention presents a highly salt-resistant asymmetric solar interface evaporator and its preparation method, which have been described through examples. Modifications or appropriate alterations and combinations of the content described herein can achieve the present invention without departing from its scope, spirit, and content. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included within the spirit, scope, and content of this invention.

Claims

1. A highly salt-resistant asymmetric solar interfacial evaporator and its preparation method, characterized in that... Includes the following steps: Step 1: Preparation of quaternary ammonium salt functionalized cellulose: First, 144 ml of NaOH solution (1 wt%) was added to 32.4 g of CNF suspension (2 wt%) and stirred until homogeneous. Then, 1.2–7.28 g of epichlorohydrin trimethoxyammonium chloride (EPTMAC) was added to the system, and the mixture was magnetically stirred in an oil bath at 85 °C for 6 h. Subsequently, the reaction product was dialyzed through distilled water (DW) until neutral. The dialyzed cellulose was freeze-dried for 42 h to obtain epichlorohydrin trimethylammonium nanocellulose (EPTMAC-CNF). Step 2, Preparation of polyvinyl alcohol solution: Dissolve 5.0g PVA powder in 100mL deionized water, react at 85℃ for 4 hours, let stand for 2 hours to remove bubbles, seal and store in a refrigerator at 4℃ for later use. Step 3, Preparation of the hydrophilic layer suspension: Using the PEPTMAC-CNF obtained in step (1), prepare a CNF solution (0.94 wt%). Mix 14-16 g of CNF solution and 2-3.9 mL of the PVA solution obtained in step (2) and stir for 1 h until a homogeneous suspension is formed. Then, gradually add sulfuric acid solution (volume fraction 1.0%) to the PVA / CNFs mixed suspension until the pH of the suspension is 3-4. Add glutaraldehyde (50 wt%) to the prepared mixed suspension and stir for 1 h. Then heat to 75 °C for 3 h for thermal crosslinking. Step 4: Prepare a CNFs solution (0.65 wt%) using unmodified CNFs. Stir 8 g of CNF solution with 1 mL of the PVA solution (0.05 g / mL) obtained in step (2) for 1 h. Then, gradually add 1.0% sulfuric acid solution dropwise to the PVA / CNFs mixed suspension until the pH of the suspension is 3-4. Add glutaraldehyde to the suspension and stir for 1 hour, then heat to 75 °C for 3 hours for thermal crosslinking. After crosslinking, 0.009 g of nano carbon black (0.1 wt%) and 0.156 g of methyltrimethoxysilane solution (MTMS) were added and stirred for 3 h. Step 5: Preparation of the Janus interfacial solar evaporator: First, the hydrophilic layer suspension was poured into a polytetrafluoroethylene mold and pre-frozen in a refrigerator at 4°C. After pre-cooling, the mold was placed on a copper plate and liquid nitrogen was injected for directional freezing into an ice gel. The hydrophobic layer suspension was poured onto the ice gel. The sample was then allowed to stand at room temperature (approximately 20°C) for 2–15 minutes to fuse the interface between the two layers, and then transferred to a refrigerator for freezing. After freeze-drying at -50°C for 60 hours using a freeze dryer, the asymmetric interfacial evaporator was obtained.

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