Preparation method and application of light absorber with low evaporation enthalpy and self-water conveying characteristic

Abstract

The invention discloses a preparation method and application of a light absorber with low evaporation enthalpy and a self-water-conveying characteristic, and aims to solve the problems that the evaporation efficiency is low, fresh water is difficult to collect, and condensed water drops and water vapor influence light absorption in traditional seawater desalination. Three-dimensional disordered porous foamy copper is selected as a substrate and is converted into copper oxide foam with super hydrophilicity through a thermal oxidation method, so that the copper oxide foam has the self-guiding water conveying capacity. Then, Zn-Co MOF grows on the nanowire copper oxide in situ through a simple hydrothermal method, and the evaporation enthalpy of water is reduced through the micropore structure of the Zn-Co MOF. And finally, through high-temperature carbonization, under the catalysis of metal nanoparticles, a wrinkled carbon material is obtained, so that the light absorber has excellent light absorption capability, self-water absorption characteristic, water activation capability and cycling stability. The method is applied to the field of seawater desalination.

Description

Technical Field

[0001] The invention relates to a preparation method and application of a light absorber with low evaporation enthalpy and self-water transport characteristics. Background Art

[0002] The global shortage of freshwater resources has become a major challenge threatening the sustainable development of human society. As an effective means to alleviate this problem, seawater desalination technology has received widespread attention. However, traditional seawater desalination technologies (such as reverse osmosis and multi-stage flash evaporation) rely on high-energy phase change processes, which not only consume a lot of energy, but may also have a negative impact on the environment. In recent years, solar-driven interfacial evaporation (SDIE) technology has become a research hotspot due to its clean and sustainable characteristics. This technology achieves seawater evaporation by locally heating the water-vapor interface through photothermal materials, but in practical applications, there are still problems such as low evaporation efficiency, difficulty in collecting condensed water, and condensed water droplets and water vapor affecting light absorption.

[0003] Many scientific research teams have made remarkable achievements in achieving high evaporation rates and high solar-steam conversion efficiencies by studying new photothermal materials, exploring heat and mass balance, and combining principles such as the water skin effect and the Donnan effect. In traditional SDIE systems, a large amount of solar energy absorbed by photothermal materials needs to be used to overcome the high phase change latent heat required for liquid water to transform into steam (the evaporation enthalpy of pure water is about 2450 J / g). In recent years, many studies have focused on material modification to reduce the evaporation enthalpy. In addition, existing SDIE systems mostly use fixed-angle transparent covers (such as glass or polymers) to collect condensed water, but are limited by the lack of synergy between surface tension and gravity, resulting in a large number of water droplets being retained or lost by sliding. At the same time, existing multi-stage evaporation systems mostly rely on external energy sources and have complex structures, resulting in large equipment size and high maintenance costs, limiting their applicability in remote areas. Therefore, developing light absorbers with low evaporation enthalpy and evaporators with efficient latent heat recovery is an effective solution to improve the practicality of SDIE technology. Summary of the invention

[0004] The purpose of the present invention is to solve the problems of low evaporation efficiency, difficulty in collecting fresh water, and influence of condensed water droplets and water vapor on light absorption in traditional seawater desalination, and proposes a preparation method and application of a light absorber with low evaporation enthalpy and self-water transport characteristics.

[0005] The preparation method of a light absorber with low evaporation enthalpy and self-water transport characteristics of the present invention is as follows:

[0006] 1. Put the pretreated raw copper foam into a muffle furnace, heat it to 480-500°C, treat it for 3h 20min-3h 40min, and then cool it to room temperature to obtain a CuO foam with a nanowire structure;

[0007] 2. Dissolve 2-methylimidazole in ethanol solution and stir it ultrasonically to obtain solution A; dissolve polyvinyl pyrrolidone in ethanol solution and stir it ultrasonically, then add Zn(NO 3 ) 2 6H 2 O and Co(NO 3 ) 2 6H 2 O continues ultrasonication to obtain solution B;

[0008] 3. Mix solution A and solution B to obtain a mixed solution; then immerse the CuO foam with nanowire structure into the mixed solution, transfer it to a stainless steel autoclave for heating, take out the stainless steel autoclave after the reaction is completed, cool it naturally to room temperature, then wash the substrate, and then dry it to obtain Zn-Co-MOF@CuO;

[0009] Fourth, Zn-Co-MOF@CuO is placed in a high-temperature tube furnace, evacuated, and then annealed under Ar gas protection to obtain the Zn-Co / C@CuONWs light absorber.

[0010] The present invention uses three-dimensional disordered porous copper foam as the substrate of the light absorber, which is converted into super-hydrophilic copper oxide foam through thermal oxidation, and then in-situ grown Zn-Co MOF by hydrothermal method and carbonized at high temperature to obtain a light absorber with excellent light absorption capacity (97.8%), self-absorption characteristics (2.23cm / s), water activation capacity (873J / g) and cyclic stability. A multi-stage solar steam gap evaporator (MSSGD) is obtained by using the light absorber, which aims to use the saturated vapor pressure difference as a new driving force for water vapor transportation in a closed environment, thereby avoiding the problem of difficult collection of fresh water in seawater desalination, condensed water droplets and water vapor affecting light absorption, and improving the efficiency of solar seawater desalination. By optimizing the air gap thickness and the total number of stages, the maximum evaporation rate of MSSGD under 1sun can reach 4.64kg / m 2 h, the water collection rate is 83%, and the solar-steam efficiency is 201%. In addition, MSSGD shows good evaporation performance in seawater of different salinities, and the fresh water produced meets WHO standards, showing its potential for application in remote water treatment or seawater desalination, and providing new ideas for the development of solar seawater desalination technology.

[0011] The beneficial effects of the present invention are:

[0012] (1) The light absorber has an ultra-low water evaporation enthalpy of 873 J·g -1 .

[0013] (2) The evaporator uses the saturated vapor pressure difference as a new driving force for water vapor transport, thereby avoiding the problem of fresh water being difficult to collect in seawater desalination and the condensation of water droplets and water vapor affecting light absorption.

[0014] (3) The evaporator has excellent evaporation performance, and the maximum evaporation rate can reach 4.64kg / m at 1sun. 2 ·h, the water collection rate is 83%, and the solar-steam efficiency is 201%. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The images are scanning electron microscopy images; (ad) original copper foam; (eh) copper oxide foam; (il) CM; (mp) TCM;

[0016] Figure 2 The superhydrophilic properties of the materials; (a) water contact angle of the original Cu foam; (b) water contact angle of CuO foam; (c) water contact angle of CM; (d) water contact angle of TCM;

[0017] Figure 3 To test the salt tolerance of CuO and TCM;

[0018] Figure 4 Schematic diagram of water evaporation in a light absorber and the three states of existence of water molecules;

[0019] Figure 5 DSC curves of different samples - evaporation enthalpy;

[0020] Figure 6 Schematic diagram of heat and mass transfer of MSSGD device;

[0021] Figure 7 is the evaporation rate of interfacial photothermal water evaporation under standard sunlight conditions;

[0022] Figure 8 It is the temperature simulation result during water vapor transmission in a confined space;

[0023] Fig. 9 It is the concentration simulation result of water vapor transmission process in a confined space;

[0024] Fig.10 The interfacial water evaporation of CuO, CM and TCM under standard sunlight conditions, as well as the water evaporation rate and energy efficiency of the MSSGD device with five, eight and 10X concentrators;

[0025] Fig.11 Simulate the evaporation performance of seawater in different salinities for MSSGD;

[0026] Fig.12 It is a schematic diagram of the structure of a multi-stage solar steam gap evaporation device. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is not limited to the specific implementation modes listed below, but also includes any combination of the specific implementation modes.

[0028] Specific implementation method 1: In this implementation method, a method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics is as follows:

[0029] First, the pretreated original copper foam was placed in a muffle furnace, heated to 490°C, treated for 3h 30min, and then cooled to room temperature to obtain a CuO foam with a nanowire structure;

[0030] 2. Dissolve 2-methylimidazole in ethanol solution and stir it ultrasonically to obtain solution A; dissolve polyvinyl pyrrolidone in ethanol solution and stir it ultrasonically, then add Zn(NO 3 ) 2 6H 2 O and Co(NO 3 ) 2 6H 2 O continues ultrasonication to obtain solution B;

[0031] 3. Mix solution A and solution B to obtain a mixed solution; then immerse the CuO foam with nanowire structure into the mixed solution, transfer it to a stainless steel autoclave for heating, take out the stainless steel autoclave after the reaction is completed, cool it naturally to room temperature, then wash the substrate, and then dry it to obtain Zn-Co-MOF@CuO;

[0032] Fourth, Zn-Co-MOF@CuO is placed in a high-temperature tube furnace, evacuated, and then annealed under Ar gas protection to obtain the Zn-Co / C@CuONWs light absorber.

[0033] Specific embodiment 2: This embodiment is different from specific embodiment 1 in that: the pretreatment method of the original copper foam in step 1 is: under ultrasonic conditions, the copper foam is washed with acetone, ethanol, and deionized water in sequence, and then dried in a vacuum chamber, and the dried copper foam is immersed in a 2.0M hydrochloric acid solution for 10 minutes to remove the surface oxide, and finally washed with deionized water and dried. Others are the same as specific embodiment 1.

[0034] Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that the heating rate in step 1 is 5°C / min. The rest is the same as specific implementation method 1 or 2.

[0035] Specific embodiment 4: This embodiment is different from specific embodiments 1 to 3 in that: the ethanol solution is prepared by mixing 35 mL of deionized water and 5 mL of ethanol. The rest is the same as specific embodiments 1 to 3.

[0036] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the ratio of 2-methylimidazole to ethanol solution is 11.9 mmol:40 mL. Other aspects are the same as those of specific embodiments 1 to 4.

[0037] Specific embodiment 6: This embodiment is different from specific embodiments 1 to 5 in that the ratio of polyvinyl pyrrolidone to ethanol solution is 1.7 mmol: 40 mL. The rest is the same as specific embodiments 1 to 5.

[0038] Specific embodiment 7: This embodiment differs from the specific embodiments 1 to 6 in that polyvinyl pyrrolidone, Zn(NO 3 ) 2 6H 2 O and Co(NO 3 ) 2 6H 2 The molar ratio of O is 1.7:2.4:1.2. The rest is the same as that of the first to sixth embodiments.

[0039] Specific embodiment 8: This embodiment is different from specific embodiments 1 to 7 in that the drying in step 3 is carried out in an oven at 60° C. for 12 hours. The rest is the same as specific embodiments 1 to 7.

[0040] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that: step 4 annealing refers to heating to 850° C. at a heating rate of 5° C. / min and maintaining for 10 minutes. The rest is the same as specific embodiments 1 to 8.

[0041] Specific implementation method 10: The application of the light absorber with low evaporation enthalpy and self-water transport characteristics in a multi-stage solar steam gap evaporator in this implementation method.

[0042] Specific embodiment eleven: This embodiment differs from specific embodiment ten in that the multi-stage solar steam gap evaporator is composed of an optically transparent acrylic plate insulation layer 1 and n-stage evaporation chambers, each stage of the evaporation chamber includes a light absorber 2, a condensation plate 3, a seawater storage 4 and a fresh water collector 5, the light absorber 2 and the condensation plate 3 are arranged oppositely on both sides of the evaporation chamber, the light absorber 2 is connected to the seawater storage 4, and the condensation plate 3 is connected to the fresh water collector 5. Other specific embodiments are the same as embodiment ten.

[0043] The embodiments of the present invention are described in detail below. The following embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation schemes and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0044] Embodiment 1:

[0045] The preparation method of the light absorber with low evaporation enthalpy and self-water transport characteristics in this embodiment is as follows:

[0046] The pretreated original copper foam (2×3 cm) was placed in a muffle furnace, and the temperature program was set to 5°C / min, the equilibrium temperature was 490°C, and the high temperature treatment was performed for 3h 30min, and then cooled to room temperature to obtain a CuO foam with a nanowire structure. Subsequently, 11.9mmol 2-methylimidazole was dissolved in 40mL (35mL deionized water and 5mL ethanol) solvent and stirred with ultrasound to obtain solution A, 1.7mmol polyvinyl pyrrolidone was dissolved in 40mL (35mL deionized water and 5mL ethanol) solvent and ultrasonicated for 30min, and then 2.4mmol Zn(NO 3 ) 2 6H 2 O and 1.2 mmol Co(NO 3 ) 2 6H 2 O was ultrasonicated for 20 min to obtain solution B, then solution A and solution B were quickly mixed, and the prepared CuO foam with nanowire structure was immersed in the obtained solution, then placed in a stainless steel autoclave lined with polytetrafluoroethylene and stored in an oven at 60 ° C for 10 h. After the reaction was completed, the stainless steel autoclave was taken out and cooled to room temperature naturally, and then the substrate was washed with deionized water and ethanol for multiple times, and dried in an oven at 60 ° C for 12 h to obtain Zn-Co-MOF@CuO(CM). Finally, Zn-Co-MOF@CuO was placed in a porcelain boat, placed in a quartz tube, and annealed in a high-temperature tube furnace. After the device was connected, the tube was evacuated for 30 min, and then Ar gas was slowly introduced for protection, and the temperature was slowly increased to 850 ° C at a heating rate of 5 ° C / min, and maintained for 10 minutes, and then slowly cooled to room temperature. Turn off the vacuum pump and Ar gas to obtain black TCM foam, namely Zn-Co / C@CuONWs(TCM) light absorber.

[0047] Microscopic morphology characterization of the light absorber: The three-dimensional porous structure of the original copper foam ( Figure 1 ad) can significantly increase the specific surface area of ​​the material, and its pore size distribution (50-500μm) provides space for the orderly growth of copper oxide nanowires and forms efficient water transport channels. Copper oxide nanowires (diameter 100-150nm, length 4-8μm) grow uniformly and densely on the foam support ( Figure 1 eh), the tip is sharp and vertically arranged, combined with a three-dimensional porous structure to reduce light reflection through multiple scattering effects, and the wide spectrum high absorption characteristics of black copper oxide ( Figure 1il) further enhances the light capture and photothermal conversion performance. Zn-Co-MOF is uniformly loaded on the copper oxide surface in the form of a regular dodecahedron. Its microporous structure promotes the clustering of water molecules in the nanochannel, increasing the proportion of intermediate water to reduce the evaporation enthalpy. In view of the corrosion risk in the seawater environment, the carbonized TCM ( Figure 1 mp) retains copper oxide nanowires, while Zn-Co-MOF is transformed into a carbon material with a corrugated sheet structure. In this process, the in-situ generated Zn / Co nanoparticles catalyze the conversion of amorphous carbon into highly thermally conductive graphitized carbon, significantly improving the corrosion resistance and thermal conductivity of the material.

[0048] The super hydrophilicity and salt resistance of the light absorber were tested: Figure 2 As shown, the original copper foam has natural hydrophobicity (water contact angle 133.59°), which is converted into superhydrophilic copper oxide foam (water contact angle 0°) after modification, and further loaded with Zn-Co-MOF and carbonized, and still maintains superhydrophilicity (water contact angle 0°). Therefore, the light absorber can quickly absorb water from simulated seawater through its three-dimensional porous structure, with an average flow rate of 2.23 cm / s. This property not only provides a basis for efficient water transport in the multi-stage solar steam gap evaporation device (MSSGD), but also significantly enhances the salt tolerance of the light absorber ( Figure 3 ). In the salt tolerance test, 1.0gNaCl takes 15 minutes to dissolve on the surface of the CuO light absorber, while it only takes 8 minutes on the surface of the TCM light absorber, indicating that TCM has excellent salt tolerance. The principle is that the pore structure of the light absorber promotes the upward absorption of water and the downward transport of salt ions through the diffusion mechanism driven by capillary force and concentration gradient. The rapid water transport further accelerates the diffusion of salt ions, effectively avoiding the accumulation of salt on the evaporation surface. When applied to MSSGD, the salt ions will flow back to the seawater storage tank, achieving the long-term stable desalination effect of MSSGD.

[0049] Low evaporation enthalpy of light absorber: e.g. Figure 4 As shown in Figure 2, the existence form of water molecules (related to the strength of hydrogen bonds) determines the size of the evaporation enthalpy, which can be divided into three categories: bound water (BW), intermediate water (IW) and free water (FW). The endothermic peak area obtained by DSC test is calculated ( Figure 5), where the DSC test conditions and steps are as follows: the sample (3-6 mg) and the reference material (empty crucible) are placed in a nitrogen atmosphere (flow rate 50 mL / min), and the temperature is increased from 30°C to 130°C at a heating rate of 10°C / min, and the instrument is calibrated using an indium standard. The evaporation enthalpy is calculated by measuring the heat flow difference between the sample and the reference material (integrated endothermic peak area, accuracy ±2%). The test was repeated 3 times and the average value was taken. The data was derived after baseline correction. The evaporation enthalpies of CuO, CM and TCM were calculated to be 1535, 1023 and 873 J / g, respectively, which are significantly lower than the 2450 J / g of pure water. This is attributed to the formation of hydrogen bonds between the oxygen atoms on the CuO surface and the water molecules, and the high porosity and large specific surface area of ​​the MOF material promote the formation of water molecule clusters, increase the IW ratio, and thus reduce the evaporation enthalpy. TCM further reduces the evaporation enthalpy because the porous carbon structure after carbonization provides more adsorption sites and transmission channels.

[0050] In the experiment, a xenon lamp was used to simulate sunlight. Under 1 standard sun, the evaporation rates of CuO, CM and TCM were 1.61 kg / m 2 h, 1.82kg / m 2 h and 2.12kg / m 2 h( Figure 7 ), which is 6.23 times that of pure water. The reasons for TCM's outstanding performance include: higher light absorption and light-to-heat conversion performance, reduced water evaporation enthalpy, and efficient heat transfer capacity of wrinkled carbon materials. Despite this, the solar-to-steam efficiency of existing light absorbers has not yet broken through the theoretical limit.

[0051] The light absorber is applied to a multi-stage solar steam gap evaporation device, the structural schematic diagram is shown in Fig.12 As shown, the multi-stage solar steam gap evaporator is mainly composed of an optically transparent acrylic plate insulation layer 1 and n-stage evaporation chambers, and each stage of the evaporation chamber includes a light absorber 2, a condensation plate 3, a seawater storage 4 and a fresh water collector 5. The light absorber 2 and the condensation plate 3 are arranged on both sides of the evaporation chamber opposite to each other, and the light absorber 2 is connected to the seawater storage 4, and the condensation plate 3 is connected to the fresh water collector 5. When used specifically, seawater is introduced into the seawater storage 4, and the super-hydrophilic light absorber absorbs water from the seawater storage and forms a thin water layer on the surface; the sunlight irradiated on the surface of the light absorber through the highly transparent acrylic plate is converted into heat energy to directionally heat the water layer; the generated steam passes through the air gap driven by the saturated vapor pressure difference, reaches the condensation plate 3 for condensation and enters the fresh water collector 5 to obtain clean water. The air gap between each stage of the light absorber and the condensation plate is 5mm, ensuring that there is a sufficient saturated vapor pressure gradient at each stage without causing excessive steam transmission resistance.

[0052] In order to optimize the performance of a multi-stage solar steam gap evaporation device (MSSGD), heat and mass transfer analysis ( Figure 6 ) and COMSOL simulation ( Figure 8 and 9 ), two key parameters were determined: air gap thickness (b) and total number of stages (n). The air gap thickness b = 5 mm, which can avoid energy loss caused by too large an air gap and prevent problems caused by too small an air gap. For the total number of stages n, although theoretically increasing the number of stages can improve evaporation performance, in practice, due to heat loss from the side walls and simulated seawater, the efficiency improvement brought by increasing the number of stages gradually weakens. Calculations show that the convection and radiation heat losses of the eight-stage device are 77 W / m 2 and 0.073W / m 2 , and the average heat loss for each additional level is 18.99W / m 2 Therefore, the total number of stages n = 8 is chosen to balance performance and cost.

[0053] In the laboratory, the light absorber was applied to five-stage, eight-stage and eight-stage solar steam gap evaporation devices equipped with a 10X concentrator. The simulated seawater concentration was 3.5wt%, and the evaporation rates were 3.63kg / m 2 h, 4.64kg / m 2 h and 6.19kg / m 2 h, and the solar-steam efficiency reached 157%, 201% and 268% respectively ( Fig.10 ). This shows that the efficiency can exceed 100% through condensation latent heat recovery. In addition, in order to demonstrate the wide applicability of MSSGD in seawater of different salinities, the simulated seawater concentration was adjusted to 3.5wt%, 10wt% and 20wt%. The evaporation rates of the eight-stage MSSGD were 4.64kg / m 2 h, 4.25kg / m 2 h and 3.88kg / m 2 h, and solar-steam efficiencies of 201%, 190%, and 177% respectively ( Fig.11 ), which shows that MSSGD exhibits good evaporation performance in seawater of different salinities.

Claims

1. A method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics, characterized in that: The preparation method is:

1. Put the pretreated raw copper foam into a muffle furnace, heat it to 480-500°C, treat it for 3h 20min-3h 40min, and then cool it to room temperature to obtain a CuO foam with a nanowire structure; 2. Dissolve 2-methylimidazole in an ethanol solution and stir it ultrasonically to obtain solution A; dissolve polyvinyl pyrrolidone in an ethanol solution and stir it ultrasonically, then add Zn(NO3)2·6H2O and Co(NO3)2·6H2O and continue to stir it ultrasonically to obtain solution B; 3. Mix solution A and solution B to obtain a mixed solution; then immerse the CuO foam with nanowire structure into the mixed solution, transfer it to a stainless steel autoclave for heating, take out the stainless steel autoclave after the reaction is completed, cool it naturally to room temperature, then wash the substrate, and then dry it to obtain Zn-Co-MOF@CuO; Fourth, Zn-Co-MOF@CuO is placed in a high-temperature tube furnace, evacuated, and then annealed under Ar gas protection to obtain the Zn-Co / C@CuONWs light absorber.

2. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: The pretreatment method of the original copper foam in step 1 is: under ultrasonic conditions, the copper foam is washed with acetone, ethanol and deionized water in sequence, then dried in a vacuum chamber, immersed in a 2.0M hydrochloric acid solution for 10 minutes to remove surface oxides, and finally washed with deionized water and dried.

3. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: The heating rate in step 1 is 5°C / min.

4. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: The ethanol solution is prepared by mixing 35 mL of deionized water and 5 mL of ethanol.

5. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: The ratio of 2-methylimidazole to ethanol solution is 11.9 mmol:40 mL.

6. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: The ratio of polyvinyl pyrrolidone to ethanol solution is 1.7 mmol:40 mL.

7. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: The molar ratio of polyvinyl pyrrolidone, Zn(NO3)2·6H2O and Co(NO3)2·6H2O is 1.7:2.4:1.

2.

8. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: Step three: drying is carried out in an oven at 60°C for 12 hours.

9. The method for preparing a light absorber with low evaporation enthalpy and self-water transport characteristics according to claim 1, characterized in that: Step 4: annealing refers to heating the temperature to 850°C at a heating rate of 5°C / min and maintaining it for 10 minutes.

10. Use of the light absorber with low evaporation enthalpy and self-water transport characteristics prepared as claimed in claim 1 in a multi-stage solar steam gap evaporator.

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