Light-steam conversion composite hydrogel sponge and preparation method and application thereof

Abstract

The present application relates to the technical field of solar interface evaporation, in particular to a light-vapor conversion composite hydrogel sponge and a preparation method and application thereof.The present application provides a preparation method of a light-vapor conversion composite hydrogel sponge, comprising: mixing carbonized coffee powder, sodium alginate and a solution containing lignin-coated nano-silver particles to obtain a mixed solution, soaking polyurethane sponge in the mixed solution, taking out the polyurethane sponge after absorbing the mixed solution, adding calcium chloride, and drying to obtain the light-vapor conversion composite hydrogel sponge.The present application combines biomass waste coffee grounds and lignin-reduced silver to construct a light-vapor conversion composite hydrogel sponge with low water evaporation enthalpy, high thermal efficiency, stable water supply and cyclic stability on the double network of polyurethane and hydrogel.

Description

Technical Field

[0001] This invention relates to the field of solar interface evaporation technology, specifically to a light-vapor conversion composite hydrogel sponge, its preparation method, and its application. Background Technology

[0002] With global population growth and accelerated industrialization, freshwater scarcity has become a severe challenge hindering the sustainable development of human society. Extracting freshwater from the abundant seawater is one of the core approaches to solving this problem. Among numerous desalination technologies, solar-driven interfacial evaporation (SDIE) technology has attracted significant attention due to its low energy consumption, environmental friendliness, and sustainability. This technology converts solar energy into heat energy using photothermal materials and confines the heat at the gas-liquid interface, thereby driving the phase change desalination of water molecules.

[0003] An efficient SDIE system typically requires excellent light absorption, high photothermal conversion efficiency, continuous water transport capability, and low heat loss. Currently, hydrogels, with their unique three-dimensional hydrophilic polymer network, good biocompatibility, and tunable physicochemical properties, have become ideal substrates for preparing interfacial evaporators. Among them, alginate extracted from brown algae, as a natural anionic polymer, can construct stable three-dimensional network structures through various methods such as ionic crosslinking and covalent crosslinking, and is widely used in the field of water treatment.

[0004] To further improve evaporation performance and reduce costs, researchers have begun to explore incorporating biomass waste into hydrogel systems. These biomass wastes are typically rich in carbon and, after carbonization, can be used as excellent photothermal materials.

[0005] However, existing hydrogel-based solar evaporators still face many challenges in practical applications: First, in high-salinity environments, salt crystals easily accumulate at the evaporation interface, clogging micro- and nano-pores and water transport channels, leading to a sharp decline in evaporation rate that is difficult to self-repair. Second, how to uniformly and stably anchor powdered coffee residue in the alginate network while ensuring efficient photothermal conversion without damaging the original water transport channels of the hydrogel remains a challenge. Finally, the long-term mechanical stability and anti-swelling properties of existing materials in complex marine environments still need improvement.

[0006] Therefore, developing a composite solar interface evaporation material that combines high evaporation rate, high photothermal conversion capacity, excellent salt resistance and good mechanical strength is of great scientific research value and practical application significance for achieving low-cost and high-efficiency seawater desalination. Summary of the Invention

[0007] The present invention aims to overcome the above-mentioned defects by providing a light-vapor conversion composite hydrogel sponge, its preparation method, and its application.

[0008] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a photo-vapor conversion composite hydrogel sponge, comprising: mixing carbonized coffee powder, sodium alginate and a solution containing lignin-encapsulated silver nanoparticles to obtain a mixture, immersing a polyurethane sponge in the mixture, removing it after it is fully saturated with the mixture, adding calcium chloride, and drying to obtain the photo-vapor conversion composite hydrogel sponge.

[0009] This invention selects carbonized coffee powder as the biochar source. Coffee residue, a byproduct of coffee production, is primarily composed of lignin and contains numerous functional groups such as phenolic hydroxyl, alcoholic hydroxyl, and carbonyl groups, making it a potential low-cost photothermal material. Carbonized coffee powder possesses a broad light absorption band, ensuring high absorbance across a wide spectrum. Furthermore, lignin-encapsulated silver nanoparticles are introduced. A lignin reduction method is used to prepare the silver nanoparticles, forming a protective layer on their surface to prevent aggregation. The strong photothermal effect of the silver nanoparticles is then utilized to enhance the light absorption and photothermal effect of the photo-vapor conversion composite hydrogel sponge.

[0010] Furthermore, using a polyurethane sponge with large pores as the matrix, a hydrogel with a three-dimensional network structure is formed by crosslinking sodium alginate and calcium chloride onto the polyurethane sponge. This hydrogel with a three-dimensional network structure has micropores smaller than those of the polyurethane sponge, allowing for pore size classification of the supporting framework containing the polyurethane sponge and hydrogel three-dimensional network structure, facilitating the rise of water and the escape of water vapor. Simultaneously, the rigid polyurethane sponge and flexible hydrogel ensure the anti-swelling and mechanical stability of the photo-vapor conversion composite hydrogel sponge under long-term immersion, which is the basis for achieving recyclability. Moreover, during crosslinking, carbonized coffee powder and silver nanoparticles are uniformly dispersed and locked within the three-dimensional network and the polyurethane sponge, fully utilizing the photothermal conversion capacity of the photothermal materials (carbonized coffee powder and silver nanoparticles), ultimately resulting in extremely strong photothermal conversion capacity and evaporation rate.

[0011] Ultimately, a photo-steam conversion composite hydrogel sponge with low water evaporation enthalpy, high thermal efficiency, stable water supply, and circulation stability was obtained.

[0012] Preferably, the solution containing lignin-coated silver nanoparticles is prepared by mixing ammonia and silver nitrate, adding alkaline lignin, and then adding sodium chloride to remove oxidized silver ions to obtain a solution containing lignin-coated silver nanoparticles.

[0013] At this time, silver ions (Ag) + It forms a stable, soluble complex cation [Ag(NH3)2] with ammonia molecules (NH3). + This is more than free Ag +It is gentler and has a lower reduction potential, making the reduction reaction easier to control and avoiding Ag... + It is quickly reduced to large silver particles or a silver mirror.

[0014] Alkaline lignin acts as a reducing agent, Ag + Silver nanoparticles are generated by reducing lignin by gaining an electron from its phenolic hydroxyl groups. Adding excess sodium chloride solution removes oxidized silver ions, resulting in lignin-coated silver nanoparticles. The carbonyl groups and ether bonds in the lignin molecules are adsorbed onto the surface of the silver nanoparticles through physical or chemical processes, forming a coating. This lignin protective layer creates a strong steric hindrance effect; when two lignin-coated silver nanoparticles approach each other, the mutual repulsion of the lignin layers prevents direct contact and aggregation between the particles, thus ensuring the long-term stability of the silver nanoparticles.

[0015] Preferably, the ratio of ammonia, silver nitrate and alkaline lignin is (3~8) mL: (0.1~0.3) g: (0.4~0.5) g.

[0016] Preferably, the carbonized coffee powder is prepared by carbonizing coffee grounds at 300-500℃ for 1-3 hours. More preferably, the carbonized coffee powder is sieved through a 0.0075 mm sieve.

[0017] Preferably, the mass ratio of the carbonized coffee powder to sodium alginate is 1:(1.5~3); and / or, the ratio of the sum of the masses of the carbonized coffee powder and sodium alginate to the amount of the solution containing lignin-coated silver nanoparticles is (5~6) g:(40~60) mL.

[0018] Preferably, the carbonized coffee powder, sodium alginate, and solution containing lignin-coated silver nanoparticles are mixed by stirring at 10-20 rpm for 4-6 hours at 60-80°C; and / or, the drying conditions are drying at 60-80°C for at least 8 hours.

[0019] This invention provides a photo-vapor conversion composite hydrogel sponge, comprising a three-dimensional porous network framework formed by the cross-linking reaction of sodium alginate and calcium chloride on a polyurethane sponge, and carbonized coffee powder particles and lignin-encapsulated silver nanoparticles locked onto the three-dimensional porous network framework by cross-linking of sodium alginate and calcium chloride.

[0020] The porous structure of the polyurethane sponge itself, combined with the three-dimensional porous network structure of the hydrogel formed by the cross-linking of sodium alginate and calcium chloride, forms a three-dimensional porous network framework. Two photothermal materials with photothermal capabilities—carbonized coffee powder particles and lignin-encapsulated silver nanoparticles—are uniformly dispersed in the three-dimensional porous network framework, forming a photo-vapor conversion composite hydrogel sponge.

[0021] The present invention provides a solar interface evaporator, which includes a light-steam conversion composite hydrogel sponge.

[0022] When used as a solar interface evaporator, the porous structure of the polyurethane sponge, combined with the three-dimensional porous network structure of the hydrogel formed by the cross-linking of sodium alginate and calcium chloride, creates a three-dimensional porous network framework. Capillary force continuously pumps water from the bottom to the surface of the photo-vapor conversion composite hydrogel sponge. Subsequently, when sunlight shines on the surface of the photo-vapor conversion composite hydrogel sponge, carbonized coffee powder and nano-silver absorb photons and undergo thermal vibration, rapidly raising the temperature of the surface of the photo-vapor conversion composite hydrogel sponge. Because the heat is confined to a thin layer on the surface of the photo-vapor conversion composite hydrogel sponge, the water is heated and turns into steam, completing the photo-vapor conversion.

[0023] This invention provides the application of light-steam conversion composite hydrogel sponge in the preparation of photothermal conversion devices or seawater desalination.

[0024] Therefore, the present invention has the following beneficial effects: (1) This invention constructs a light-steam conversion composite hydrogel sponge with low water enthalpy, high thermal efficiency, stable water supply, and stable circulation by combining biomass waste coffee grounds with lignin-reduced silver on a double network of polyurethane and hydrogel.

[0025] (2) In this invention, carbonized coffee powder and nano silver are selected as photothermal conversion materials. By utilizing the broad spectrum absorption of carbonized coffee powder and the strong photothermal effect of nano silver, the solar energy capture ability of the photo-steam conversion composite hydrogel sponge is significantly enhanced.

[0026] (3) In this invention, the powdered carbonized coffee powder and nano-silver are uniformly and firmly locked onto a three-dimensional porous network framework through the cross-linking effect of sodium alginate and calcium chloride. This not only ensures the uniformity of photothermal conversion, but also prevents the photothermal material from falling off and being lost during water treatment, effectively ensuring the stability of photothermal conversion.

[0027] (4) The photo-steam conversion composite hydrogel sponge of the present invention uses macroporous polyurethane sponge as a skeleton and grows microporous alginate hydrogel in situ, forming a hierarchical structure of macroporous and microporous structures. This structure utilizes capillary force to achieve continuous and stable water supply, while providing a smooth channel for the rapid escape of water vapor; this stable water transport channel can quickly dissolve and remove the salt accumulated at the evaporation interface, effectively preventing salt crystallization from clogging the pores, solving the problem of a sudden drop in evaporation rate under high salinity, and ensuring the long-term cyclic use of the photo-steam conversion composite hydrogel sponge.

[0028] (5) This invention utilizes the interpenetrating support of rigid polyurethane sponge and flexible hydrogel to overcome the shortcomings of traditional pure hydrogel materials that are prone to swelling and loose structure under long-term immersion; thus enabling the photo-vapor conversion composite hydrogel sponge to exhibit excellent mechanical strength in complex marine environments, ensuring the performance stability of the device in multiple cycles of use. Attached Figure Description

[0029] Figure 1 This is an SEM image.

[0030] Figure 2 This is a Fourier transform infrared spectrum.

[0031] Figure 3 The image shows a SEM-EDS plot, where a represents SA, b represents CPCM / SA, and c represents AgNPs-CPCM / SA.

[0032] Figure 4 This is an absorption curve.

[0033] Figure 5 This is a graph showing the temperature change process of heating and cooling under sunlight over 6 minutes.

[0034] Figure 6 This is a thermal image of a temperature rise and fall over 6 minutes under sunlight.

[0035] Figure 7 The graph shows the temperature changes after eight heating-cooling cycles of AgNPs-CPCM / SA.

[0036] Figure 8 This is the water contact angle.

[0037] Figure 9 The process of absorbing 2 mL of water into AgNPs-CPCM / SA and the process of 2 mL of water being transported from the bottom of AgNPs-CPCM / SA to the surface interface are shown in the diagram.

[0038] Figure 10 This is a schematic diagram of the water transport process of AgNPs-CPCM / SA under illumination.

[0039] Figure 11 This is a graph showing the mass change of brine evaporated under sunlight for 4 hours.

[0040] Figure 12 The graph shows the evaporation rate of brine under sunlight for 4 hours.

[0041] Figure 13 The graph shows the mass change of AgNPs-CPCM / SA prepared by three different methods under sunlight for 4 hours of evaporating brine.

[0042] Figure 14Evaporation rate graphs of brine evaporation under one sun for 4 h for AgNPs-CPCM / SA prepared by three different methods.

[0043] Figure 15 The graph shows the evaporation rate and mass change of AgNPs-CPCM / SA at 0, 5, 10, 15, and 20% wt NaCl.

[0044] Figure 16 The graph shows the changes in evaporation rate and light quality of AgNPs-CPCM / SA under solar light intensities of 0, 0.5, 1, 1.5, and 2.

[0045] Figure 17 This is a schematic diagram of the seawater desalination device of the present invention.

[0046] Figure 18 The diagram shows the situation where AgNPs-CPCM / SA is wetted and can withstand a load of 500 g.

[0047] Figure 19 This is a schematic diagram of the water evaporation process in AgNPs-CPCM / SA.

[0048] Figure 20 The graphs show the effects of AgNPs-CPCM / SA at pH=1 and pH=12 for 0 and 12 h, respectively.

[0049] Figure 21 This is a diagram showing the process of AgNPs-CPCM / SA growing with plants for 5 days.

[0050] Figure 22 This image shows the process of water evaporation in AgNPs-CPCM / SA under natural sunlight for 8 hours and a thermal image.

[0051] Figure 23 The graph shows the evaporation rate and mass change of AgNPs-CPCM / SA under natural sunlight for 8 hours.

[0052] Figure 24 The graph shows the surface center temperature change and surrounding ambient temperature of AgNPs-CPCM / SA after 8 hours of water evaporation under natural sunlight. Detailed Implementation

[0053] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0054] In this section, the raw materials are sourced from the following sources: alkaline lignin, silver nitrate, ammonia, calcium chloride, sodium chloride, dilute hydrochloric acid, and sodium hydroxide, all of which are from Xilong Scientific.

[0055]

Example

[0056] S2. Lignin-Silver Nanoparticle Solution: Add 0.5 g of alkaline lignin to 25 mL of deionized water and stir at room temperature until the lignin and aqueous solution are fully mixed into mixture A. Then take 20 mL of deionized water, add 200 mg of AgNO3 and 5 mL of ammonia solution to obtain mixture B. Add mixture A dropwise to mixture B, and then add excess sodium chloride solution to remove oxidized silver ions, obtaining the lignin-silver nanoparticle solution.

[0057] S3. 2 g of carbonized coffee powder and 4 g of sodium alginate were added to 50 mL of lignin-nano silver solution, followed by 50 mL of deionized water. The mixture was stirred at 15 rpm for 4 h at 65 °C using a magnetic stirrer to obtain mixture C. A polyurethane sponge was completely immersed in mixture C, and after repeated squeezing to ensure full saturation, it was removed and allowed to stand. Calcium chloride was added to initiate an ionic crosslinking reaction. The sponge was then dried at 60 °C for 12 h until the surface was no longer damp, yielding AgNPs-CPCM / SA composite hydrogel sponge, denoted as AgNPs-CPCM / SA.

[0058] Comparative example 1CPCM / SA S1. Carbonized coffee powder: Coffee grounds are carbonized at 400℃ for 2 hours, then ground and passed through a sieve with a pore size of 0.0075 mm to obtain carbonized coffee powder.

[0059] S2. 2 g of carbonized coffee powder and 4 g of sodium alginate were added to 100 mL of deionized water and stirred at 15 rpm for 4 h at 65°C using a magnetic stirrer to obtain a mixture. The polyurethane sponge was completely immersed in the mixture, and after repeated squeezing to ensure full saturation, it was removed and allowed to stand. Calcium chloride was added to initiate an ionic crosslinking reaction, and the sponge was dried at 60°C for 12 h until the surface was no longer damp, yielding a CPCM / SA composite hydrogel sponge, denoted as CPCM / SA.

[0060] Comparative Example 2AgNPs / SA S1. Lignin-Silver Nanoparticle Solution: Add 0.5 g of alkaline lignin to 25 mL of deionized water and stir at room temperature until the lignin and aqueous solution are fully mixed into mixture A. Then take 20 mL of deionized water, add 200 mg of AgNO3 and 5 mL of ammonia solution to obtain mixture B. Add mixture A dropwise to mixture B, and then add excess sodium chloride solution to remove oxidized silver ions, obtaining the lignin-silver nanoparticle solution.

[0061] S2. 4 g of sodium alginate was added to 50 mL of lignin-nano silver solution, followed by 50 mL of deionized water. The mixture was stirred at 15 rpm for 4 h at 65 °C using a magnetic stirrer to obtain mixture C. A polyurethane sponge was completely immersed in mixture C, and after repeated squeezing to ensure full saturation, it was removed and allowed to stand. Calcium chloride was added to initiate an ionic crosslinking reaction. The sponge was then dried at 60 °C for 12 h until the surface was no longer damp, yielding AgNPs-CPCM / SA composite hydrogel sponge, denoted as AgNPs / SA.

[0062] Comparative Example 3SA 4 g of sodium alginate was added to 100 mL of deionized water and stirred at 15 rpm for 4 h at 65 °C using a magnetic stirrer to obtain a mixture. A polyurethane sponge was completely immersed in the mixture, and after repeated squeezing to ensure full saturation, it was removed and allowed to stand. Calcium chloride was added to initiate an ionic crosslinking reaction. The sponge was then dried at 60 °C for 12 h until the surface was no longer damp, yielding an SA composite hydrogel sponge, denoted as SA.

[0063] Comparative Example 4CP Unprocessed coffee grounds are designated as CP.

[0064] Comparative example 5CPCM Coffee grounds were carbonized at 400℃ for 2 hours, then ground and passed through a sieve with a pore size of 0.0075 mm to obtain carbonized coffee powder, denoted as CPCM.

[0065] Comparative Example 6: Photoreduction Method S1. Carbonized coffee powder: Coffee grounds are carbonized at 400℃ for 2 hours, then ground and passed through a sieve with a pore size of 0.0075 mm to obtain carbonized coffee powder.

[0066] S2. 2 g of carbonized coffee powder and 4 g of sodium alginate were added to 100 mL of deionized water and stirred at 15 rpm for 4 h at 65°C using a magnetic stirrer to obtain a mixture. The polyurethane sponge was completely immersed in the mixture and repeatedly squeezed to ensure full saturation. After being removed and allowed to stand, calcium chloride was added to initiate an ionic crosslinking reaction. The sponge was then dried at 60°C for 12 h until the surface of the polyurethane sponge was no longer damp, thus obtaining the CPCM / SA composite hydrogel sponge.

[0067] S3.CPCM / SA composite hydrogel sponge was immersed in 0.05 mol / L silver nitrate solution for 10 min, placed under natural light for 2 h for reduction, rinsed with deionized water, and dried at 60℃ for 12 h until the surface of the polyurethane sponge was no longer damp, thus obtaining the composite hydrogel sponge.

[0068] Comparative Example 7: Glucose Reduction Method S1. Carbonized coffee powder: Coffee grounds are carbonized at 400℃ for 2 hours, then ground and passed through a sieve with a pore size of 0.0075 mm to obtain carbonized coffee powder.

[0069] S2. 2 g of carbonized coffee powder and 4 g of sodium alginate were added to 100 mL of deionized water and stirred at 15 rpm for 4 h at 65°C using a magnetic stirrer to obtain a mixture. The polyurethane sponge was completely immersed in the mixture and repeatedly squeezed to ensure full saturation. After being removed and allowed to stand, calcium chloride was added to initiate an ionic crosslinking reaction. The sponge was then dried at 60°C for 12 h until the surface of the polyurethane sponge was no longer damp, thus obtaining the CPCM / SA composite hydrogel sponge.

[0070] S3.CPCM / SA composite hydrogel sponge was immersed in 0.01 mol / L silver nitrate solution for 15 min, then removed and immersed in 0.1 mol / L glucose solution for 10 min for reduction. After washing with deionized water, it was dried at 60℃ for 12 h until the surface of the polyurethane sponge was no longer damp, thus obtaining the composite hydrogel sponge.

[0071] [Performance Testing] The measurement methods involved in this section are as follows: ① Calculation of water evaporation rate The capillary effect intensity of the evaporator was estimated by measuring the water evaporation rate. The seawater evaporation experiment used a xenon lamp source to simulate sunlight (the light intensity was adjusted to 1 kW / m²). 2 Prepare a 35 g / L NaCl solution to simulate seawater. Then, construct a simple seawater desalination device (such as...). Figure 17 The evaporator (as shown) was placed under a xenon lamp light source for irradiation, and the light intensity was measured using a light power density meter. During the experiment, the ambient temperature was 24℃, the humidity was 77%, and the irradiation was continuous for 4 hours. The evaporator was weighed every hour using an electronic analytical balance. The evaporation rate was calculated by recording the changes in mass loss and irradiation time. In calculating the evaporation efficiency of the evaporator, the evaporation phenomenon of water under natural conditions needs to be considered. Therefore, the water evaporation rate under conditions without sunlight irradiation was measured and subtracted from the overall water evaporation rate calculation. The evaporator was placed under a xenon lamp light source for irradiation, and the light intensity was measured using a light power density meter. The changes in mass loss and irradiation time were recorded to calculate the evaporation rate. Evaporation rate (kg·m) -2 ·h -1 It can be calculated using formula (1): (1); In equation (1), m is the water mass loss of the evaporator, in kg; S is the evaporation area of ​​the evaporator, in m². 2 ; t is the evaporation time, in hours; Evaporation rate, kg·m -2 ·h -1 .

[0072] ② Calculation of water evaporation efficiency To evaluate the water evaporation efficiency of the photothermal conversion material, the solar-steam conversion efficiency (η) is calculated using equation (2): (2); In equation (2), Evaporation rate, kg·m -2 ·h -1 h lv The total enthalpy change during the transition from liquid to gas phase, kJ / kg; W least Q represents the theoretical minimum energy required to remove salt from a salt solution and produce pure water, 10.39 kJ / kg; solar The illuminance of a xenon lamp light source per unit time is 1 kW·m. -2 ·h -1 =3600 kJ·m -2 ·h -1 η is the water evaporation efficiency, %.

[0073] Note: For a 35 g / L NaCl solution, the energy required to separate it into pure water and solid salt is 10.75 kJ. This is equivalent to the energy required for each kilogram of pure water being 10.39 kJ. We take 10.39 kJ / kg as the approximate energy consumption value for a 35 g / L NaCl solution, which is the theoretical minimum energy required to remove salt from the salt solution and produce pure water.

[0074] ③ Calculation of the enthalpy of vaporization of water To determine the equivalent enthalpy of vaporization of the photothermal conversion material, a pure water evaporator and a 35 g / L NaCl evaporator were constructed and placed together in a dark, sealed environment. By monitoring the mass loss of both over 12 hours, and applying the principle of energy conservation, the equivalent enthalpy of vaporization of the hydrogel was calculated using the following formula: , , (3); In equation (3), h lv The total enthalpy change from liquid to gas phase, kJ / kg; h ee,water,T1and h ee,eva,T1 The enthalpy of vaporization of pure water and the evaporator, respectively, in kJ / kg; h sh,eva,T1 Sensible heat of evaporation refers to the amount of heat absorbed by a liquid as it rises from its initial temperature to its boiling point, expressed in kJ / kg. water and eva Dark-field evaporation rates for pure water and 35 g / L NaCl evaporators, respectively, are given in kg·m². -2 ·h -1 ; The specific heat capacity of water is 4.186 kJ / kg; It is the difference between the boiling point of water and the initial temperature of water. The boiling point of water is 100℃, and the initial temperature of water is 25℃.

[0075] 1. Morphology and structure The microstructure of SA, CPCM / SA, and AgNPs-CPCM / SA was observed and analyzed using SEM. Figure 1 The SEM images reveal particulate matter on the three-dimensional porous network framework, preserving the pore characteristics of the substrate material and increasing surface roughness. It is speculated that sodium ions in the sodium alginate structure successfully undergo an ionization displacement reaction with calcium in aqueous solution, generating cross-linking sites and forming a stable gel loaded onto the PU sponge framework.

[0076] Depend on Figure 1 It can be observed that the composite hydrogel formed by the physical blending of sodium alginate hydrogel substrate and biomass-based compound CPCM enters the pores of the three-dimensional network matrix. After loading with CPCM, the macroscopic morphology of the sponge matrix changes significantly. CPCM powder is uniformly covered in the sponge channels, forming a modification layer, which provides more binding sites for subsequent loading of AgNPs. Simultaneously, localized gelation occurs, rheological properties decrease, and the loaded material is less prone to loss. 2+ Calcium chloride plays a crucial role in improving the mechanical properties of alginate hydrogels, and the Na in sodium alginate... + Ionic cross-linking occurs, Ca 2+Adsorption occurs due to electrostatic attraction between the alginate and carboxyl groups on the sodium alginate framework, forming an insoluble gel structure. This gelation process lacks covalent bonding; the hydrogel's internal cross-linking primarily relies on relatively weak interactions (electrostatic interactions, hydrogen bonding, and hydrophobic interactions), potentially leading to low mechanical strength and easy depolymerization. CPCM addresses these issues of alginate hydrogels and, since no chemical cross-linking agents are introduced during preparation, it is non-toxic. Compared to AgNPs, the resulting framework surface is rougher, exhibiting a dense particle packing with no obvious large pores observed under micron-level SEM. This dense structure reduces porosity, thus decreasing the specific surface area, but the increased surface roughness enhances the light-harvesting ability of the composite hydrogel sponge. The presence of numerous hydroxyl and carboxyl groups in the three-dimensional porous network structure of the hydrogel facilitates the gentle embedding and fixation of silver nanoparticles. The uniform dispersion of AgNPs within the three-dimensional network framework may be due to repulsive forces between adjacent lignin-encapsulated AgNP particles, preventing them from easily bonding. CPCM / SA and AgNPs-CPCM / SA contain nanoscale pores, which facilitates the rapid transport of water molecules during photothermal evaporation and makes it less likely for water molecules to aggregate, thus reducing the time of the water-vapor phase change process.

[0077] The structures of the monomer, matrix sponge, and composite hydrogel sponge were characterized by Fourier transform infrared spectroscopy, with wavelengths ranging from 500 to 4000 cm⁻¹. -1 The mid-infrared region belongs to the fundamental vibrational spectrum of molecules, and the fundamental absorption bands of most organic and inorganic substances appear in this region. For example... Figure 2 The functional groups of SA and CP are significantly different. SA exhibits the typical infrared characteristic curves of organic compounds containing carboxyl and hydroxyl groups. The CP curve shows differences in the 2843–3000 cm⁻¹ range. -1 A significant CH bond stretching vibration was observed at the site, which weakened after carbonization and in the composite hydrogel sponge. This may be because the low bond energy of the CH bonds during carbonization makes them prone to breakage, and because organic functional groups containing CH bonds (such as aliphatic chains and unsaturated hydrocarbon groups) degrade, resulting in a significant weakening of various CH absorption peaks in the infrared spectrum, indicating the removal of heteroatoms such as hydrogen and oxygen from the coffee grounds. Furthermore, CH bonds readily produce light reflection and scattering; with fewer CH bonds, the CPCM surface is primarily composed of a C=C and C=C carbon skeleton. This, combined with the rough material surface, helps reduce heat loss and improve light utilization. This is consistent with... Figure 1 Corresponding to the densification of the skeleton surface structure observed in the study, the increased carbon content on the material surface gives it better adsorption performance and can prevent the shedding of its own organic groups, which would lead to water pollution.

[0078] SA and CP appear at 3200~3400 cm -1The OH structure peak at [value] weakens in intensity after carbonization, which is due to the destruction of hydroxyl and carboxyl groups by the dehydration reaction. The CP and CPCM curves are similar to the SA curves, located at 2190 cm⁻¹. -1 The peak position indicates a significant vibration in CP, while the intensities of other peaks decrease. The weakening or even disappearance of the OH and C=O peaks in SA and CPCM / SA indicate that carboxyl and hydroxyl groups were consumed or coordinated during the recombination process. The presence of the CH peak suggests that the organic framework was not completely destroyed. The shift of the carboxyl peak directly proves that nano-silver interacted with the carboxyl groups of sodium alginate. This interaction is related to Ca... 2+ Cross-linking synergy enhances the cross-linking density of the three-dimensional backbone network. The mid-infrared spectrum is divided into fingerprint regions (500~1330 cm⁻¹). -1 ) and characteristic frequency region (1330~4000 cm) -1 The fingerprint region is not highly characteristic, therefore its absorption energy can sensitively reveal any subtle changes in molecular structure. The slight differences in the absorption peaks of the fingerprint region between CPCM / SA and AgNPs-CPCM / SA demonstrate that loading AgNPs alters the molecular structure.

[0079] Further compositional characterization of the composite hydrogel sponge was performed, and the surface elemental distribution and content of SA, CPCM / SA, and AgNPs-CPCM / SA were analyzed by SEM-EDS. Figure 3 The 'a' in the figure indicates that the substrate material mainly contains three elements: C, H, and O, with Na introduced by SA. CPCM / SA and AgNPs-CPCM / SA contain five characteristic elements: C, H, O, Na, and Ag. Figure 3 In samples b and c, Na and Ag elements changed from being dispersed in small amounts to being uniformly dispersed. The uniform dispersion and significant increase in Ag element content indicate that AgNPs were successfully decorated on CPCM particles and exhibited a uniform distribution in the sponge matrix. Therefore, enrichment of the carbon framework enhances light absorption, densification reduces heat loss, and reduction of CH bonds strengthens stability; all three factors contribute to the performance optimization of coffee grounds-based photothermal materials.

[0080] 2. Photothermal performance The photothermal conversion process of photothermal materials mainly involves light absorption and light conversion. By improving light absorption capacity and capturing more photons, electron and atomic nucleus modifications are initiated to achieve photothermal conversion and obtain more heat. Light absorption capacity is positively correlated with absorbance. The light absorption properties of SA, CPCM / SA, and AgNPs-CPCM / SA composite hydrogel sponges were studied using a UV / Vis / NIR spectrophotometer. Figure 4As shown, with only SA, the absorbance drops significantly after 500 nm, and the absorbance performance in the mid-to-long wavelength range is poor. After SA is blended with CPCM, the absorbance of the material is stable across the entire wavelength range and remains at a high level of light absorption (85-95%), which is consistent with the characteristic of carbon-based materials having broad-spectrum high light absorption. After AgNPs loading, the light absorption decreases slightly, but still remains stably at a high absorbance (80-90%). Figure 2 The 3600-3200 cm⁻¹ Fourier transform infrared absorption spectrum -1 The stretching of OH within a certain range proves that CPCM contains phenolic hydroxyl groups and aromatic rings, and the π-π conjugated system... Transitions can strongly absorb visible light (400~760 nm), and the metal elements contained within can form trace amounts of metal oxides or ions, which absorb specific wavelengths of light through dd transitions. Figure 1 The SEM images show increased surface roughness in CPCM / SA. When sunlight shines on the surface, this roughness promotes light scattering and reflection, making it easier for the interior of CPCM / SA to absorb light, thus increasing its light absorption rate. This indicates that the loading of CPCM compensates for the light absorption capacity of the matrix material SA, possibly reducing phase separation at the interface within the SA matrix, and through its own dispersion characteristics, making the overall light absorption performance of CPCM / SA more stable. Subsequent modification with AgNPs had no significant effect on the material's absorbance and stability; both CPCM / SA and AgNPs-CPCM / SA exhibited high levels of light absorption performance.

[0081] Figure 5 The temperature rise-fall curves of the composite hydrogel sponge within 6 minutes are shown. Figure 6 For the corresponding Figure 5 Thermal imaging. Observation Figures 5-6 It can be seen that the temperature rises rapidly in the first 4 minutes, and then decreases after the light is removed in the 5-6 minute interval. The surface temperature of AgNPs-UCCP / SA / PU rapidly rises to 69.4℃ within 4 minutes, with a heating rate of 10.38℃ / min, indicating that light energy can be converted into heat energy more efficiently. Notably, the surface temperature of AgNPs-CPCM / SA is significantly higher than that of CPCM / SA, further verifying that the bonding of AgNPs particles provides an excellent photothermal effect. These results demonstrate that AgNPs-CPCM / SA possesses excellent water absorption performance, outstanding solar light absorption capacity, and efficient thermal localization characteristics, which are crucial for achieving efficient solar-driven interfacial reactions.

[0082] In actual seawater desalination processes, repeated heating and cooling cycles generate continuous thermal stress, which can damage the physical morphology and structure of materials, contributing to performance degradation and directly determining the actual usable lifespan of photothermal conversion materials. Therefore, AgNPs-CPCM / SA underwent eight cycles of heating and cooling to evaluate the stability of its photothermal capabilities. The results are as follows: Figure 7 As shown, under the same illumination conditions, the heating and cooling rates and the maximum achievable temperature of AgNPs-CPCM / SA showed no significant difference, exhibiting excellent repeatability in temperature response and maintaining good photothermal properties. This rules out the possibility of the composite hydrogel sponge showing excellent initial performance followed by rapid failure. It is possible that the synergy between AgNPs and CPCM provides interfacial bonding to the material surface, effectively resisting thermal stress caused by thermal expansion and contraction.

[0083] 3. Water transport performance Water transport rate is crucial for adequate water supply to the evaporation interface and is a prerequisite for efficient water phase change. In photo-vapor conversion systems, the hydrophilic / hydrophobic properties of materials affect water molecule transport. When a material has high hydrophilicity, strong interactions form between water molecules and the material, promoting water molecule transport within the material. Therefore, water is used as the test liquid to measure the contact angle of the sample, thereby determining the hydrophilicity of the photothermal conversion material. The wetting performance of different composite hydrogel sponges is evaluated by the time required for droplet full wetting and spreading. Figure 8 As shown, the wetting properties of the matrix sponge (SA) and the composite hydrogel sponge (CPCM / SA and AgNPs-CPCM / SA) differ significantly. Complete spread of droplets on the SA surface takes 80 ms, while complete wetting and spread on the CPCM / SA and AgNPs-CPCM / SA surfaces takes only 40 ms, a substantial reduction in time. This is mainly because the water contact angle of SA is less than 30° (…). Figure 8 AgNPs exhibit significant hydrophilic properties. The combination with CPCM further enhances surface roughness, increases specific surface area, expands the contact area with droplets, and further increases hydrophilicity. Furthermore, the introduction of AgNPs may increase the number of nanoscale pores, resulting in higher capillary pressure for AgNPs-CPCM / SA, allowing water droplets to wet and spread more rapidly on the material surface.

[0084] The AgNPs-CPCM / SA material, exhibiting the best hydrophilicity, was selected for further verification of its water transport performance. The water absorption time and the time it took for water to be transported from the bottom to the upper surface interface were tested to evaluate its water absorption capacity and the water transport rate within the AgNPs-CPCM / SA material. The results are as follows: Figures 9-10 As shown. Figure 9In the experiment, water appeared at the interface at 38 s and was completely distributed after 300 s; this demonstrates that CPCM alters the three-dimensional polymer network structure of the matrix material, affecting the state of water and further reducing the enthalpy of water vaporization. The aforementioned photothermal performance test results show that CPCM / SA is an excellent solar energy absorber, forming a hydrophilic polymer network. In this photo-vapor conversion system, light energy is absorbed and converted into heat energy, which can be utilized in situ to drive water molecules transported from below through water transport channels to the polymer network, achieving a liquid-gas phase change. The microstructure formed by the interaction between AgNPs and CPCM reduces heat loss and ensures sufficient water transport of evaporated water. Figure 10 ).

[0085] 4. Water evaporation performance In the laboratory stage, a simulated light exposure experiment was conducted on the composite hydrogel sponge under a solar intensity. The water evaporation rate and mass loss over time were calculated, and the temperature at the center of the light absorption layer interface of the material was measured by a handheld thermal imager during the heating and cooling process within 6 minutes to evaluate its light conversion performance and water evaporation performance. Figure 12 In the middle, the average water evaporation rates of SA and CPCM / SA were 1.195 kg·m³. 2 ·h 1 and 1.917 kg·m 2 ·h 1 All of these are higher than the evaporation rate of pure water (0.45 kg·m³). 2 ·h 1 AgNPs-CPCM / SA experienced a mass decrease of 10.17 g within 4 h, with an average water evaporation rate of 2.023 kg·m⁻². 2 ·h 1 . Figure 11 The magnitude of the quality change curve and Figure 12 The water evaporation rate curves corroborate each other, indicating that the composite of AgNPs and CPCM / SA significantly enhances the water evaporation rate. The high evaporation rate of AgNPs-CPCM / SA is attributed not only to its geometric advantages but also to its unique low enthalpy of vaporization. Within the hydrophilic three-dimensional network framework, water molecules exist in three different forms: free water, intermediate water, and bound water. Figure 2 The Fourier transform infrared spectrum shows that SA contains hydroxyl (-OH) and carboxyl (-COO) groups. -Hydrogen bonds and electrostatic interactions can bind to water molecules, influencing their state distribution. Intermediate water, through hydrogen bonds and weak electrostatic forces, forms weak bonds with the polymer chains and evaporates more easily than bound water. Water evaporation is determined by its state within the hydrogel polymer network structure; the micro- and nano-sized pores hinder water aggregation, lowering the enthalpy of vaporization and enabling the AgNPs-CPCM / SA evaporator to achieve a more efficient water evaporation rate. Furthermore, photothermal conversion efficiency is also affected by hydrophilicity. AgNPs-CPCM / SA, with its better hydrophilicity, has a stronger binding capacity with water, allowing the solar energy absorbed at the interface to be converted into heat energy more efficiently and transferred to water molecules for liquid-gas phase change, reducing heat loss and achieving a photothermal conversion efficiency of 92.6% for the AgNPs-CPCM / SA evaporator.

[0086] To verify the impact of different methods of loading AgNPs on the performance of AgNPs-CPCM / SA, the following was conducted: Figures 13-14 The experiment shown indicates that, under one 4-hour period of sunlight, the mass change, from largest to smallest, is determined by lignin reduction, photoreduction, and then glucose reduction; similarly, the water evaporation rate, also under one 4-hour period of sunlight, is determined by lignin reduction, photoreduction, and then glucose reduction. Based on these results, lignin reduction exhibits the largest total mass change and the highest real-time water evaporation rate, demonstrating the superiority of AgNPs-CPCM / SA materials prepared by lignin reduction under dynamic operating conditions. The advantages of AgNPs-CPCM / SA materials prepared by lignin reduction in maintaining high evaporation rates and mass changes prove their higher energy utilization rate in practical solar interfacial evaporation applications, thus making them the optimal loading method.

[0087] To verify the photothermal conversion performance of AgNPs-CPCM / SA in different application scenarios, experiments were conducted with salt concentration gradients ranging from 5 to 20 wt%. Figure 15 The evaporation rate of AgNPs-CPCM / SA reached as high as 1.76 kg·m³ at 5 wt%. 2 ·h 1 This indicates that AgNPs-CPCM / SA is suitable for nearshore seawater desalination. It maintains a salinity of 1.47–1.89 kg·m³ at a salt concentration of 10–20 wt%. 2 ·h 1The results demonstrate the ability to perform seawater desalination even in high-salinity environments such as concentrated brine and salt lake brines, covering most application scenarios. When the salt concentration exceeds 10 wt%, the water evaporation rate decreases, possibly because the surface tension and thermal conductivity of water increase under high salt concentrations, leading to increased heat loss. Figure 16 The figure shows the evaporation rate and light quality changes of AgNPs-CPCM / SA under solar light intensities of 0, 0.5, 1, 1.5, and 2. As can be seen from the figure, the water evaporation rate and light quality change increase with the increase of solar light intensity.

[0088] 5. Practical Applications Considering the possibility of corrosion or biological compression on the surface of solar interface evaporation materials over time in practical applications, including changes in the pH of the environment, seawater pressure, and compression by marine organisms, we evaluated the mechanical properties of AgNPs-CPCM / SA by testing its pressure resistance. Figure 18 It can be seen that no deformation or damage occurred under a pressure of 500 g. The physical-mechanical strength of hydrogels is one of the key characteristics determining their applications. The supporting role of the AgNPs-CPCM / SA porous framework enhances mechanical properties, preventing structural collapse due to external forces and thus ensuring functionality. Figure 19 Chemical stability is a key indicator for its application in complex aquatic environments. The inventors tested the stability of AgNPs-CPCM / SA under strong acid and strong alkali conditions for 12 h and explored the changes in the morphology, dispersibility and integrity of the materials.

[0089] Figure 20 It was observed that after 12 hours, the physical morphology of AgNPs-CPCM / SA did not change significantly, with no dissolution, delamination, or damage, and the overall structural integrity remained good. The covalent bond between AgNPs and CPCM / SA enhanced the chemical stability of the material, enabling it to resist the erosion of strong acids and alkalis. To further verify the biocompatibility of AgNPs-CPCM / SA, AgNPs-CPCM / SA was placed in a hydroponic plant culture bottle to compare the plant growth status. Figure 21 The results show that the hydroponic plants maintained vibrant green leaves and exhibited good overall growth throughout the cultivation period, without any wilting or yellowing. This demonstrates that AgNPs-CPCM / SA achieves efficient seawater desalination without harming plants, and is safe for organisms during actual seawater desalination processes.

[0090] Next, an 8-hour water evaporation experiment was conducted under natural light, recording the water evaporation rate, weather conditions, surface temperature, and ambient temperature every hour. Figure 22Infrared thermal images and physical images under natural outdoor sunlight were used to observe the temperature response characteristics of the photothermal material. From 9:00 to 14:00, the high-temperature region on the AgNPs-CPCM / SA interface gradually expanded with increasing solar irradiance, and the temperature continuously increased, reaching a peak temperature of 44.4℃ at 14:00, corresponding to the maximum value of solar irradiance at noon. From 14:00 to 18:00, as solar irradiance decreased, the proportion of the high-temperature region on the material surface decreased, and the maximum temperature decreased. This indicates that AgNPs-CPCM / SA has excellent temperature response to natural solar irradiance, effectively increasing the surface temperature through photothermal conversion, providing thermal energy support for the seawater desalination process. Furthermore, based on the material state after the experiment, no obvious salt crystallization was observed on the sponge interface after 8 hours of continuous operation. Figure 23 The changes in water evaporation rate and mass show that in practical applications, the water evaporation rate of AGNPS-CPCM / SA depends on solar irradiance intensity, and it can achieve high-efficiency water evaporation performance under natural light conditions. The peak period of its evaporation rate coincides with the period of strongest solar irradiance. Figure 24 The material surface temperature always follows the same trend as the ambient temperature, indicating that AgNPs-CPCM / SA maintains excellent thermal localization in practical applications, which can concentrate heat in the evaporation area, reduce heat loss to the environment, and improve overall energy utilization efficiency.

[0091] In summary, a photo-vapor conversion composite hydrogel sponge capable of simultaneously meeting the requirements of low water evaporation enthalpy, high thermal efficiency, stable water supply, and stable circulation was prepared by depositing nano-silver particles onto a porous carbon-based framework system using a simple physical adsorption method for solar interfacial water evaporation. The evaporator contains an ion-crosslinked three-dimensional network structure SA hydrogel as the matrix material, ensuring the repulsion of multivalent ions based on the distribution of carboxylate anions. The superhydrophilic properties and micro / nano-scale pores of AgNPs-CPCM / SA provide convenient channels for water molecule transport and water vapor dissipation. The combination of CPCM and AgNPs enhances the light absorption rate and photothermal conversion of AgNPs-CPCM / SA, providing a stable heat energy supply for water phase change. This resulted in an interfacial water evaporation rate of 2.023 kg·m³ for AgNPs-CPCM / SA. 2 ·h 1 With a water evaporation efficiency of 92.6%, it demonstrates great application potential in seawater desalination. Furthermore, practical application tests have proven that AgNPs-CPCM / SA possesses mechanical stability and biological safety, making it a promising photo-vapor conversion composite material with broad application prospects.

Claims

1. A method for preparing a light-vapor conversion composite hydrogel sponge, characterized in that, include: Carbonized coffee powder, sodium alginate and a solution containing lignin-encapsulated silver nanoparticles were mixed to obtain a mixture. A polyurethane sponge was soaked in the mixture, and after it was fully saturated, it was removed, calcium chloride was added, and it was dried to obtain a photo-vapor conversion composite hydrogel sponge. The method for preparing the solution containing lignin-coated silver nanoparticles is as follows: after mixing ammonia and silver nitrate, alkaline lignin is added, and then sodium chloride is added to remove oxidized silver ions to obtain a solution containing lignin-coated silver nanoparticles; wherein, the ratio of ammonia, silver nitrate and alkaline lignin is (3~8) mL: (0.1~0.3) g: (0.4~0.5) g; The mass ratio of the carbonized coffee powder to sodium alginate is 1:(1.5~3); and / or, the sum of the masses of the carbonized coffee powder and sodium alginate to the amount of the solution containing lignin-coated silver nanoparticles is (5~6) g:(40~60) mL.

2. The preparation method according to claim 1, characterized in that, The method for preparing the carbonized coffee powder is as follows: carbonize coffee grounds at 300~500℃ for 1~3 h to obtain carbonized coffee powder.

3. The preparation method according to claim 1, characterized in that, Carbonized coffee powder, sodium alginate, and a solution containing lignin-coated silver nanoparticles are mixed by stirring at 10-20 rpm for 4-6 hours at 60-80°C; and / or, the drying conditions are drying at 60-80°C for at least 8 hours.

4. The photo-vapor conversion composite hydrogel sponge prepared by any one of claims 1 to 3, characterized in that, It includes a three-dimensional porous network framework formed by the cross-linking reaction of sodium alginate and calcium chloride on polyurethane sponge, and carbonized coffee powder particles and lignin-encapsulated silver nanoparticles locked on the three-dimensional porous network framework by the cross-linking of sodium alginate and calcium chloride.

5. A solar interface evaporator, characterized in that, The solar interface evaporator includes a photo-vapor conversion composite hydrogel sponge prepared by any one of the preparation methods described in claims 1 to 3, or a photo-vapor conversion composite hydrogel sponge as described in claim 4.

6. The application of the photo-steam conversion composite hydrogel sponge prepared by any one of the preparation methods described in claims 1 to 3, or the photo-steam conversion composite hydrogel sponge as described in claim 4, in the preparation of photothermal conversion devices or seawater desalination.

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