Evaporative cooling composite material and method of making and use thereof

Abstract

The application provides an aqueous composite material comprising CaCl2 nanoparticles and a MOF-801 matrix, a preparation method thereof and application to evaporative cooling. The aqueous composite material of the application has the advantages of both MOF-801 and CaCl2, and can have high water absorption capacity in a wide humidity range, and still maintain a solid-like form when absorbing water to saturation. The cooling performance of a cooling coating made of the CaCl2@MOF-801 aqueous composite material of the application does not decay over time under specific working conditions, thereby being able to meet the needs of long-time, high-power cooling.

Description

TECHNICAL FIELD

[0001] The present application relates to evaporative cooling materials, in particular, to a CaCl2@MOF-801 aqueous composite, a method for preparing the same and its use as a cooling coating. BACKGROUND

[0002] Cooling is not only a key requirement for maintaining human thermal comfort in buildings (1-2), but also for maintaining high efficiency in a wide range of industrial equipment such as data centers (3-4), solar panels (5-6), and many other fields (7-8). In the United States, heating and cooling accounts for about 20% of total energy demand, with the country’s energy costs exceeding $215 billion per year (9). For example, in 2020, data centers in the United States consumed 73 TWh of electricity and 660 billion liters of water to meet cooling demand (10), which is more than the residential demand in Hong Kong, China in 2020 (11).

[0003] Generally, according to the second law of thermodynamics (12), cooling is more challenging than heating. Although traditional cooling technologies such as air conditioning systems based on refrigeration are very powerful in terms of heat dissipation for buildings, electronic devices, or vehicles, the huge power consumption (13-14), high cost (15), and greenhouse gas generation (7, 16) can prevent people from using these cooling technologies. In addition, the power system of traditional cooling technologies is mainly powered by fossil fuel-based thermal power plants, which are considered to be the main culprit of carbon emissions. Therefore, it is necessary to develop efficient, environmentally friendly and sustainable cooling strategies to improve the working efficiency of buildings, data centers and photovoltaics, reduce carbon emissions, and meet the growing energy demand (15, 17-18).

[0004] Evaporative cooling based on the liquid-vapor phase change of water (19-20) is a very promising alternative to air conditioning, as it has an ultra-high enthalpy of about 2450 J / g, and cooling high-temperature roads by spraying water on the surface is a widespread practice worldwide (21-22). Water is also widely used as a coolant in the steel industry and nuclear power plants (23). Although evaporative cooling is considered a cheap, environmentally friendly and effective method, the need for a large amount of cooling water greatly limits its application.

[0005] Inspired by the breathing process, there have been related studies that propose evaporative cooling through the adsorption-desorption or “breathing” process of atmospheric water (24-26). Atmospheric water is a resource that accounts for about 10% of all freshwater on Earth, i.e., about 13,000 trillion liters (27). Therefore, evaporative cooling through the “breathing” process of atmospheric water in a desiccant shows great potential to solve the above cooling problems.

[0006] One of the key factors for evaporative cooling technology is the selection of the desiccant used in the "breathing" process. Conventional desiccants such as silica gel, zeolite, and activated alumina generally have a wide atmospheric moisture adsorption window. However, the water desorption process requires high temperature, making such desiccants less suitable as atmospheric water adsorbents (28-29). Recently, certain metal-organic frameworks (MOFs) such as MOF-801, MOF-303, and MOF-804 have been proven to be ideal "breathing" materials due to their good water adsorption capacity at low relative humidity (RH) (e.g., MOF-801 adsorbs water 0.25 g / g at 20% RH and 25°C), thermal stability, high specific surface area, and high mechanical strength (30-31). However, these MOFs have relatively low water adsorption at high humidity, limiting their cooling capacity to a lower level. On the other hand, hygroscopic salt desiccants such as calcium chloride (CaCl2) have a high adsorption capacity for atmospheric water (32-33). However, when the adsorbed atmospheric water is saturated, the hygroscopic salt desiccant dissolves to form an aqueous solution, causing operational and engineering problems for atmospheric water adsorbents (34-36).

[0007] Recent advances in passive evaporative cooling technology using atmospheric water have significantly improved cooling performance at relative humidity (RH) higher than 60%. However, due to the poor adsorption capacity of conventional adsorbents for atmospheric water at low RH, experimental results using atmospheric water for passive evaporative cooling at a wide range of relative humidity still perform poorly.

[0008] Therefore, there is a need to develop a new cooling material to achieve high cooling capacity and improved operation of the "breathing" process. SUMMARY

[0009] As previously described, there is a need in the art for an improved evaporative cooling material. The inventors of the present invention combined the hygroscopic salt calcium chloride (CaCl2) with MOF, not only solving the problem of relatively low water adsorption of MOF at high relative humidity (e.g., RH higher than 60%), but also avoiding the saturation of hygroscopic salt to become a freely flowing aqueous solution after adsorption by reasonably controlling the ratio of MOF to hygroscopic salt CaCl2 in the composite material. Thus, the present invention is achieved.

[0010] Therefore, in a first aspect of the present invention, there is provided an aqueous composite material comprising: CaCl2 nanoparticles and a MOF-801 matrix.

[0011] In a second aspect, there is provided a method of producing an aqueous composite material, comprising:

[0012] providing MOF-801; and

[0013] The MOF-801 is mixed with the CaCl2 nanoparticle solution under ultrasonic to obtain a CaCl2@MOF-801 aqueous composite material.

[0014] In a third aspect, an evaporative cooling coating is provided, which is made of the aqueous composite material of the first aspect or the aqueous composite material prepared by the method of the second aspect.

[0015] The CaCl2@MOF-801 aqueous composite material of the present application has the advantages that by reasonably controlling the weight / volume ratio of MOF-801 and the nanoparticle solution of the hygroscopic salt CaCl2 in the composite material, the advantages of both MOF-801 and CaCl2 are combined, i.e., high water absorption capacity in a wide humidity range, and when water absorption reaches the saturation level, the solution can still maintain a non-free flowing form similar to a solid due to the obstruction of MOF-801 particles. The cooling performance of the cooling coating made of the CaCl2@MOF-801 aqueous composite material of the present application does not decay over time under certain working conditions, thereby meeting the needs of long-term and high-power cooling. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the drawings needed in the specific embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0017] Figure 1 A schematic diagram of the adsorption-desorption-evaporation process of the CaCl2@MOF-801 aqueous composite material-based cooling coating according to an embodiment of the present application is shown.

[0018] Figure 2 A-2H is a preparation process and characterization schematic diagram of the CaCl2@MOF-801 aqueous composite material according to an embodiment of the present application, wherein Figure 2 A shows the preparation process of the CaCl2@MOF-801 aqueous composite material-based cooling coating, Figure 2 B shows the SEM photo of MOF-801, Figure 2 C and Figure 2 D shows the CaCl2@MOF-801 aqueous composite material-based coating, Figure 2 E shows a digital image of the CaCl2@MOF-801 aqueous composite material coating on a commercial PV panel by the doctor blade coating method (inset), Figure 2 F shows the use of an energy dispersive X-ray spectrometer to Figure 2elemental mapping of C, Ca, CI and O in the area indicated by D, Figure 2 G shows the XRD pattern of MOF-801 and CaCI2@MOF-801 aqueous composite after drying adsorption, Figure 2 H shows the weight change of the coating based on CaCI2@MOF-801 aqueous composite at room temperature and different relative humidity (RH) levels of 28%, 50%, 70% and 90%.

[0019] Figures 3A-3D shows the cooling performance of the coating based on CaCI2@MOF-801 aqueous composite at ambient temperature of about 25°C at 28% and 70% RH according to one embodiment of the present application, wherein Figure 3A and Figure 3B shows the cooling performance of uncoated and coated photovoltaic (PV) panels (coating thickness of 5 mm) at 1000 W / m 2 of solar irradiance at RH of 28% and 70% respectively; wherein Figure 3C and Figure 3D shows the maximum cooling temperature, cooling time and weight of the coating as a function of different thicknesses at 500 W / m 2 , 1000 W / m 2 and 1300 W / m 2 of solar irradiance when RH is 28% and 70% respectively.

[0020] Figures 4A-4E shows the infrared (IR) images of the coated PV panels and the cooling performance of the composite coating according to one embodiment of the present application, wherein Figure 4A shows the time-dependent IR images of uncoated and coated PV panels of different thicknesses at 1000 W / m 2 of solar irradiance, Figure 4B shows the “UST” pattern coated on top of a PMMA substrate using a commercial white putty powder and a coating based on CaCI2@MOF-801 aqueous composite, Figure 4C shows Figure 4B the corresponding infrared images of the “UST” pattern, Figure 4D shows the weight change of the coating at different solar irradiance at RH of 28% and 70% respectively, and Figure 4E shows the calculated cooling power of the coating based on CaCI2@MOF-801 aqueous composite at different solar irradiance for different thicknesses (3 mm, 4 mm, 5 mm).

[0021] Figure 5A-5L shows the cooling performance of the coating according to one embodiment of the present application under three different workload conditions: (I) pre-adsorption for 3 hours, solar radiation for 20 minutes, and off for 60 minutes; (II) pre-adsorption for 12 hours, solar radiation for 20 minutes, and off for 60 minutes; (III) pre-adsorption for 12 hours, solar radiation for 40 minutes, and off for 60 minutes; wherein, in all tests, the solar radiation was 500 W / m 2 at about 25 °C, and relative humidity of about 28% or about 70%; the temperature profile of the uncoated (baseline) and coated PV panels under workload condition (I) at RH @ 28% is shown in Figure 5 A, the temperature profile under workload condition (II) is shown in Figure 5 B, the temperature profile under workload condition (III) is shown in Figure 5 C, Figure 5 D-5F shows the corresponding weight change; the temperature profile of the uncoated and coated PV panels under workload condition (I) at RH @ 70% is shown in Figure 5 G, the temperature profile under workload condition (II) is shown in Figure 5 H, the temperature profile under workload condition (III) is shown in Figure 5 I, and Figure 5 J-5L shows the corresponding weight change.

[0022] Figures 6A-6D shows the outdoor cooling performance of the coating based on CaCl2@MOF-801 aqueous composite according to one embodiment of the present application, wherein Figure 6A shows a photo of the outdoor experimental setup of the HKUST campus building roof; Figure 6B shows the time-dependent temperature of the uncoated and coated PV panels, Figure 6C shows the ambient temperature and relative humidity acquired by the weather station, Figure 6D shows the corresponding weight change and solar radiation of the samples measured on the test day. DETAILED DESCRIPTION

[0023] Embodiments of the present application relate to composites for sustainable evaporative cooling coatings for a wide range of relative humidity and methods of producing the composites.

[0024] Many terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the invention. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items. As used herein, singular forms of nouns are intended to include both plural and singular forms unless the context clearly indicates otherwise. It will be further understood that many embodiments use the expressions “comprising,” “including,” or “basically / mainly composed of.” The expressions “comprising,” “including,” or “basically / mainly composed of” are generally understood to be open-ended, meaning that they include not only the elements, components, parts, method steps, etc., specifically listed after the expression, but also other elements, components, parts, method steps. Additionally, in this document, the expressions “comprising,” “including,” or “basically / mainly composed of” may in some cases be understood to be closed-ended, meaning that they include only the elements, components, parts, method steps specifically listed after the expression, and exclude any other elements, components, parts, method steps. In this case, the expression is equivalent to the expression “composed of.”

[0025] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter pertains. It will be further understood that terms such as those defined in common dictionaries should be interpreted as having the same meaning as they have in the context of the relevant art and this disclosure, and should not be interpreted in an idealized or overly formalized manner, unless expressly defined herein.

[0026] When the term “about” is used in conjunction with a numerical value in this document, it should be understood that the numerical value can be a range of 90% to 110% of the specified value, that is, the numerical value can be + / - 10% of the specified value. For example, “about 1 kg” means 0.90 kg to 1.1 kg, and “a temperature of about 100°C” can be a temperature of 90°C to 110°C.

[0027] To better understand this teaching and without limiting its scope, all figures and other numerical values ​​used in the specification and claims to express quantities, percentages, or proportions should, in all cases, be understood to be modified by the term "about." Therefore, unless otherwise stated, the numerical parameters set forth in the following specification and appended claims are approximate values ​​that may vary depending on the desired properties sought. At a minimum, each numerical parameter should be interpreted based at least on the reported significant figures and by applying common rounding techniques.

[0028] It should be understood that in describing the application, numerous techniques and steps are disclosed. Each of these has individual benefit and each can also be used in combination with one or more, or in some cases all, of the other disclosed techniques. Accordingly, for the sake of clarity, the description will refrain from repeating every possible combination of the individual steps in this description. However, it should be understood that the full disclosure of each of these combinations is entirely contemplated by this application and its claims.

[0029] In a first aspect of the application, there is provided an aqueous composite material comprising: CaCl2nanoparticles and a MOF-801 matrix.

[0030] In one specific embodiment, the MOF-801 matrix is comprised of a plurality of polycrystalline MOF-801.

[0031] In one preferred embodiment, the polycrystalline MOF-801 has:

[0032] a diameter of 200 nm to 400 nm;

[0033] a surface area of 500 m 2 g -1 to 1500 m 2 g -1 ; and / or

[0034] an average pore size of 1 nm to 4 nm.

[0035] In one more preferred embodiment, the polycrystalline MOF-801 has:

[0036] a diameter of 250 nm to 350 nm;

[0037] a surface area of 800 m 2 g -1 to 1200 m 2 g -1 ; and / or

[0038] an average pore size of 1 nm to 3 nm.

[0039] In one further preferred embodiment, the polycrystalline MOF-801 has:

[0040] a diameter of 280 nm to 300 nm;

[0041] a surface area of 950 m 2 g -1 to 1000 m 2 g -1 ; and / or

[0042] an average pore size of 1 nm to 2 nm.

[0043] In one exemplary embodiment, the polycrystalline MOF-801 has:

[0044] a diameter of 290 ± 5 nm, for example about 292 nm;

[0045] 980 ± 5 m 2 g -1 , for example about 982.6 m 2 g -1 of surface area; and / or

[0046] an average pore size of 1.7 ± 0.05 nm, for example about 1.75 nm.

[0047] In yet another specific embodiment, the CaCl2nanoparticles can be connected to the polycrystalline MOF-801adjacent in the MOF-801matrix, for example by covalent bonds, intermolecular interactions, etc.

[0048] In yet another specific embodiment, a major portion of the CaCl2nanoparticles encapsulates the plurality of polycrystalline MOF-801and another portion is embedded in the cages of the plurality of polycrystalline MOF-801. Referring to Figure 2 D and Figure 2 F, it can be seen from the contrast that the Cl element and the Ca element are generally present in the composite material, with similar distribution, but the content intensity of the two elements in different regions is not the same. This means that a major portion of CaCl2encapsulates all MOF-801particles and a small portion is embedded in the inner cages of MOF-801. Figure 2 The XRD spectrum of G confirms that a portion of CaCl2is embedded in the inner cages of MOF-801.

[0049] In yet another specific embodiment, the aqueous composite material is obtained by mixing the MOF-801matrix with a CaCl2nanoparticle solution at a mass / volume ratio m MOF-801 :V CaCl2 of 0.5 g / ml to 2 g / ml.

[0050] In a preferred embodiment, the aqueous composite material is obtained by mixing the MOF-801matrix with a CaCl2nanoparticle solution at a mass / volume ratio m MOF-801 :V CaCl2 of 0.5 g / ml to 1 g / ml.

[0051] In a more preferred embodiment, the aqueous composite material is obtained by mixing the MOF-801matrix with a CaCl2nanoparticle solution at a mass / volume ratio m MOF-801 :V CaCl2The ratio of 0.7±0.1 g / ml is obtained.

[0052] In yet another specific embodiment, the concentration of the CaCh nanoparticles in the CaCh nanoparticle solution is 2 mol / L to 6 mol / L, for example 4 mol / L.

[0053] It is observed that the CaCh@MOF-801 aqueous composite of the present application adsorbs atmospheric water at low temperature, and the adsorbed atmospheric water desorbs and evaporates at high temperature, for example in sunlight, in which case the CaCh@MOF-801 aqueous composite can increase the temperature to effect desorption of the adsorbed water, the desorption process taking away heat, thereby cooling the target object. The composite of the present application adsorbs atmospheric water at low temperature, and the adsorbed atmospheric water desorbs and evaporates at high temperature, for example in sunlight, thereby effecting evaporative cooling.

[0054] Thus, in yet another specific embodiment, the CaCh@MOF-801 aqueous composite can be configured to adsorb atmospheric water at a first temperature and desorb the adsorbed atmospheric water at a second temperature, and wherein the first temperature is lower than the second temperature.

[0055] In a second aspect, there is provided a method of producing an aqueous composite, comprising:

[0056] providing MOF-801; and

[0057] mixing the MOF-801 with a CaCh nanoparticle solution under ultrasonication to obtain a CaCh@MOF-801 composite.

[0058] In one specific embodiment, the CaCh nanoparticle solution is obtained by ultrasonication of CaCh dissolved in deionized water.

[0059] In one preferred embodiment, the concentration of the CaCh nanoparticles is 2 mol / L to 6 mol / L, for example 4 mol / L.

[0060] The CaCh@MOF-801 aqueous composite of the present application, by judiciously controlling the weight / volume ratio m MOF-801 :V CaCl2 of MOF 801 to the hygroscopic salt CaCh nanoparticle solution in the composite, not only has high water absorption capacity over a wide humidity range, but also maintains a non-free flowing solution form similar to a solid when saturated with water. Thus, in yet another specific embodiment, the grain boundaries of the plurality of polycrystalline MOF-801 inhibit the formed CaCh hydrates from becoming a freely flowing aqueous solution when the composite adsorbs water.

[0061] In one specific embodiment, the weight / volume ratio (m MOF-801 :V CaCl2 ) of the MOF-801 matrix to the CaCl2nanoparticle solution in the aqueous composite is configured such that the CaCl2@MOF-801 saturated with water is inhibited from becoming a freely flowing solution.

[0062] In one preferred embodiment, the weight / volume ratio (m MOF-801 :V CaCl2 ) is from 0.5 g / ml to 2 g / ml.

[0063] In one more preferred embodiment, the weight / volume ratio (m MOF-801 :V CaCl2 ) is from 0.5 g / ml to 1 g / ml.

[0064] In one most preferred embodiment, the weight / volume ratio (m MOF-801 :V CaCl2 ) is 0.7 ± 0.1 g / ml, for example 0.7 g / ml.

[0065] In yet another specific embodiment, the mixing is for 1 to 3 hours at a temperature of 30 °C to 50 °C.

[0066] In one preferred embodiment, the mixing is for about 1.5 hours at a temperature of about 40 °C.

[0067] In yet another specific embodiment, the MOF-801 is prepared by the following steps:

[0068] An equivalent amount of fumaric acid and ZrOCl2·8H2O are dissolved in a solvent having N,N-dimethylformamide and formic acid to make a mixture;

[0069] The mixture is heated to a temperature of about 130 °C for about 6 hours;

[0070] The mixture is cooled to room temperature to obtain a MOF-801 precipitate.

[0071] In one further embodiment, the method of preparing MOF-801 can further comprise separating the MOF-801 precipitate via a vacuum filter, for example a filter having a pore size of about 0.45 μιη.

[0072] In one preferred embodiment, the method of preparing MOF-801 can further comprise drying the separated MOF-801 to activate the MOF-801. The drying can preferably comprise about 24 hours at a temperature of about 150 °C under vacuum conditions.

[0073] In a further particular embodiment, the aqueous composite material is the aqueous composite material of the first aspect.

[0074] In a third aspect, there is provided an evaporative cooling coating made of the aqueous composite material of the first aspect or the aqueous composite material prepared by the method of the second aspect.

[0075] The CaCl2@MOF-801 aqueous composite material of the present application can be applied to the surface of an object and form a coating via drying by methods well known to those skilled in the art, such as spin coating, doctor blade coating, etc., but not limited thereto. In the context of the present application, the object includes buildings, roads, electronic devices, vehicles, photovoltaic materials, but is not limited thereto. The drying method can employ methods well known to those skilled in the art, such as heating, but is not limited thereto.

[0076] In a particular embodiment, the thickness of the coating can be 3 mm to 7 mm.

[0077] In a preferred embodiment, the thickness of the coating can be 4 mm to 6 mm.

[0078] In a more preferred embodiment, the thickness of the coating can be 5 ± 0.5 mm, for example 5 mm.

[0079] As those skilled in the art will know, the performance of evaporative cooling coatings depends on their atmospheric water adsorption capacity (AWAC). Although the 20% atmospheric water adsorption capacity (AWAC) of MOF-801 measured at very low RH is superior to that of conventional porous materials (37-38), 1 g of MOF-801 can only adsorb 0.38 g of atmospheric water at 90% RH and 25°C. Therefore, MOF-801 is far from meeting the needs of high-power, long-time cooling. The inventors of the present application have creatively combined CaCl2 with MOF, and the equilibrium water adsorption capacity of CaCl2 at 25°C exceeds 1 g / 1 g, which is much higher than that of conventional desiccants such as silica gel, and thus the CaCl2@MOF-801 aqueous composite material and the coating based on the CaCl2@MOF-801 aqueous composite material obtained by compounding the CaCl2 hygroscopic salt with super-high atmospheric water adsorption capacity in a wide RH range in the MOF-801 matrix.

[0080] The AWAC of the water-based composite and coating of the present application decreases with increasing temperature, in which case the water-based composite and coating can increase the temperature to achieve desorption of the absorbed water, and the desorption process takes away heat, thereby cooling the target object. The coating of the composite of the present application absorbs atmospheric water at low temperature, and the absorbed atmospheric water desorbs and evaporates at high temperature (e.g. under sunlight), thereby achieving evaporative cooling. Thus, the coating of the present application absorbs atmospheric water via three processes. The first process is the atmospheric water absorption process of MOF-801; in the second process, this is a chemical reaction process, CaCl2 hydration forms CaCl2·6H2O; then in the third process, CaCl2·6H2O deliquesces, finally forming a water solution similar to a solid that cannot flow freely due to the presence of MOF-801. The CaCl2@MOF-801-based coating that cools objects through the absorption-desorption process similar to the “breathing” process is shown in Figure 1 .

[0081] The water-based composite and coating of the present application have the advantages of both MOF-801 and CaCl2 hygroscopic salt, and can exhibit excellent atmospheric water absorption capacity under both low humidity conditions and high humidity conditions (e.g. RH higher than 60%). The water-based composite and coating of the present application have excellent atmospheric water absorption performance in a wide range of RH by compounding hygroscopic salt CaCl2 nanoparticles in the MOF-801 matrix. As an example, as shown in Figure 2 H, the CaCl2@MOF-801 water-based composite coating has an AWAC of about 0.22 g / g at a low relative humidity of 28% and a temperature of about 25°C for an absorption time of about 1300 minutes, and the atmospheric water absorption capacity (AWAC) of CaCl2 is as high as about 0.80 g / g at a high relative humidity of 70% and a temperature of about 25°C for an absorption time of about 1300 minutes.

[0082] The atmospheric water absorption and desorption process similar to “breathing” enables the CaCl2@MOF-801-based coating to naturally and sustainably cool objects. As an example, in an environment with a relative humidity of 28% and a temperature of 25°C, a CaCl2@MOF-801-based coating with a thickness of 5 mm is coated on a commercial PV panel, and the temperature of the coated PV panel can be reduced by as much as 9.5°C compared to the uncoated PV panel under one solar irradiation (1000 W / m 2 ) for an effective cooling time of 112 minutes; in an environment with a relative humidity of 70% and a temperature of 25°C, the temperature of the coated PV panel can be reduced by as much as 14°C compared to the uncoated PV panel under one solar irradiation for an effective cooling time of about 140 minutes.

[0083] Furthermore, the skilled person would know that the passive recovery ability resulting from the spontaneous adsorption process of the cooling coating is another key point for practical application, which determines whether the cooling coating can work continuously and naturally. The cooling performance of the cooling coating made of the CaCh@MOF-801 aqueous composite material of the present application does not decay over time under certain working conditions, thus being able to meet the needs of long-term cooling. Therefore, it is crucial to select appropriate working load conditions to achieve stable and sustainable cooling performance.

[0084] In Example 3 below, a relevant example of selecting a critical solar radiation intensity to ensure stable cooling performance of the coating based on working load conditions is given, and the skilled person can select appropriate working load conditions and critical solar radiation intensity based on the description of Example 3 to ensure that the applied coating can stably and sustainably exert its cooling performance.

[0085] As an example, the working load conditions are: pre-adsorption for 3 hours, solar radiation for 20 minutes, and off for 60 minutes. At 28% RH, the critical solar radiation intensity is about 590 W / m 2 , i.e., when the solar radiation is less than 590 W / m 2 , the coating can have stable cooling performance without degradation. At 70% RH, the critical solar radiation intensity is about 805 W / m 2 .

[0086] In other aspects, the present application provides a method for evaporative cooling of an object, which comprises using the aqueous composite material of the first aspect or the aqueous composite material prepared by the method of the second aspect.

[0087] In a specific embodiment, the aqueous composite material is applied to the surface of the object as a coating. The method of applying the coating can be a method well known to the skilled person, and the present application is not particularly limited thereto.

[0088] In a specific embodiment, the thickness of the coating can be 3 mm to 7 mm.

[0089] In a preferred embodiment, the thickness of the coating can be 4 mm to 6 mm.

[0090] In a more preferred embodiment, the thickness of the coating can be 5 ± 0.5 mm, for example 5 mm.

[0091] As previously stated, the AWAC of the waterborne composite material and coating of the present invention decreases with increasing temperature. In this case, the waterborne composite material and coating of the present invention can adsorb atmospheric water at low temperatures, and the adsorbed atmospheric water desorbs and evaporates at high temperatures (e.g., under sunlight). The desorption process carries away heat, thereby achieving evaporative cooling. Therefore, in yet another specific embodiment, the object is evaporatively cooled by the coating of the composite material on the surface of the object adsorbing atmospheric water at a first temperature and desorbing the adsorbed atmospheric water at a second temperature, wherein the first temperature is lower than the second temperature.

[0092] The CaCl2@MOF-801 waterborne composite coating of this invention can achieve a temperature reduction of up to 14°C compared to traditional coatings. As an example, in an environment with a relative humidity of 28% and a temperature of 25°C, a 5mm thick CaCl2@MOF-801-based coating of this invention was applied to a commercial PV panel. The temperature of the coated PV panel compared to the uncoated PV panel under solar irradiation (1000W / m²) was significantly lower. 2 Under normal conditions, the temperature can be reduced by up to 9.5°C, with an effective cooling time of 112 minutes. In an environment with a relative humidity of 70% and a temperature of 25°C, the temperature of the coated PV panel can be reduced by up to 14°C compared to the uncoated PV panel under one sun exposure, with an effective cooling time of approximately 140 minutes.

[0093] The coating based on CaCl2@MOF-801 aqueous composite material of this invention can meet the requirements of high-power cooling. As an example, when the solar radiation intensity is 1000 W / m²... 2 At that time, a 5mm thick coating made of CaCl2@MOF-801 composite material could achieve a high W / m² under solar irradiation at 28% relative humidity and 25°C. 2 The cooling power, under conditions of 70% relative humidity and 25℃, can reach up to 315W / m under solar radiation. 2 Furthermore, when solar radiation is 1300 W / m² 2 At 70% relative humidity and 25°C, the cooling power under solar radiation can increase to 344 W / m. 2 Therefore, the cooling power of the CaCl2@MOF-801 coating of the present invention can be 136 W / m. 2 Up to 344W / m 2 .

[0094] Example

[0095] Unless otherwise specified, all experimental methods used herein are conventional methods, and all experimental materials used in the following examples were purchased from conventional reagent stores. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0096] It should be noted that the terminology used in this specification is for the purpose of describing specific embodiments only and is not intended to limit the invention. The foregoing summary section and the following detailed description are for illustrative purposes only and are not intended to limit the invention in any way. The scope of the invention is defined by the appended claims without departing from its spirit and intent.

[0097] Materials and methods

[0098] Synthesis of MOF-801

[0099] To synthesize MOF-801, 3.48 g (30 mmol) of fumaric acid (e.g., purchased from Shanghai Maclean Biochemical Co., Ltd.) and 9.66 g (30 mmol) of ZrOCl2·8H2O (e.g., purchased from Aladdin Bio-Chem Technology, Shanghai) were dissolved in a solution containing 120 ml of N,N-dimethylformamide (e.g., purchased from DMF, RCI Labscan Limited) and 40 ml of formic acid (e.g., purchased from Anala R... The solution was dissolved in a solvent (ACS, Reag. Ph. Eur.). The mixture was then placed in a 500 mL beaker and stirred at room temperature for approximately 1 hour to ensure complete dissolution. Next, the mixture was transferred to a 500 mL screw-cap wide-mouth flask and heated in an oven at 130 °C for 6 hours. The flask was cooled to room temperature, yielding a white precipitate of MOF-801. This precipitate was separated by suction filtration using a nylon membrane filter with a pore size of, for example, 0.45 μm (e.g., purchased from Tianjin Jinteng), and washed three times with deionized (DI) water. Finally, the prepared MOF-801 solid was dried in a vacuum oven at 150 °C for 24 hours to activate the sample.

[0100] Preparation of CaCl2@MOF-801 waterborne composite materials and coatings

[0101] To synthesize the CaCl2@MOF-801 aqueous composite, first, 8.88 g of CaCl2(such as purchased from Shenzhen DIECKMANN) solid was dissolved in 20 ml of deionized (DI) water and the solution was sonicated for 5 minutes. Then, a certain amount of activated MOF-801 powder prepared as described above was mixed with the CaCl2nanoparticle solution under sonication at about 40 °C for about 1.5 hours to obtain a homogeneous mixture. In all examples of the present invention, an optimized ratio of 1 : 1 was chosen When the synthesized aqueous composite is saturated with adsorbed water, it is rendered aqueous. The prepared composite was coated on a substrate by a doctor blade coater, and the thickness of the coating layer can be precisely controlled. Next, the coated sample was dried at 100 °C for about 1 hour to desorb the adsorbed water during the synthesis process.

[0102] Characterization of the composite

[0103] The surface morphology of MOF-801 and CaCl2@MOF-801 composite was characterized by scanning electron microscopy (SEM) (e.g., obtained from FEI QUANTA450 and JSM-7100F Jeol, respectively), and EDS mapping was obtained from JSM-7100F Jeol. Energy dispersive X-ray spectroscopy (EDS) was obtained by transmission electron microscopy (TEM) (e.g., obtained from JEM-2010F, Jeol). Nitrogen adsorption of activated MOF-801 at 77 K was recorded by Brunauer-Emmett-Teller (Belsorp X mini) with a temperature pre-degassing at 130 °C for 16 hours, and the data collected during the process were used to analyze the surface area and pore size. Spectroscopic information of MOF-801 powder was obtained by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (e.g., obtained from Vertex 70 Hyperion 1000 Bruker). Raman spectra were measured with a Raman spectrometer (e.g., obtained from RAMANMICRO 300, Perkin Elmer, USA, laser wavelength = 785 nm). Thermogravimetric analysis (TGA) measurements were performed on a TA Instruments Q5000 series thermogravimetric analyzer in N2atmosphere from 20 °C to 800 °C at a heating rate of 5 °C / min. The chemical composition and valence state of MOF-801 were detected by X-ray photoelectron spectroscopy (XPS) (e.g., purchased from Axis Ultra DLD).

[0104] Water absorption capacity measurement

[0105] The water uptake performance of the coating based on CaCl2@MOF-801 aqueous composite at different relative humidity (RH) was measured in a controlled room and a humidity-controlled cabinet. The 28% and 50% RH experiments were performed in a controlled room at about 25 °C, where the relative humidity was controlled by dry bulb temperature (DBT) and wet bulb temperature (WBT) simultaneously. For example, the DBT and WBT were set to 25 °C and 17.91 °C, respectively. It is noted that the WBT is 0.03 °C higher than the value calculated using air enthalpy and humidity chart, which is due to the error of the controlled room. The CaCl2@MOF-801 aqueous composite was then coated on a glass plate and placed on a precision balance (e.g., OHAUS, PR223ZH / E) connected to a computer via RS232 communication cable and the weight change was recorded in situ. For the experiments performed at 70% and 90% RH, the precision balance was placed in a humidity cabinet and a humidifier was used to adjust the RH. The in-cabinet RH was monitored by a humidity sensor and controlled by a humidity controller with an accuracy of ± RH@3%.

[0106] Cooling performance test under laboratory conditions

[0107] First, the CaCl2@MOF-801 aqueous composite was coated on the back of a commercial PV panel. The coated PV panel and the uncoated PV panel were tested under various conditions for comparison. Prior to the test, the coated PV panel was heated to 100 °C to form the coating and evaporate most of the water, and then subjected to a water uptake process for 12 hours or 17 hours at 70% RH and 25 °C. The coated and uncoated PV panels were fixed on foam stands at the same height. Then, the system was placed on a precision balance. These samples were exposed to a solar simulator (e.g., CHX-2000, purchased from Guangzhou Xingchuang Electronics Co., Ltd.). The power density was adjusted by a solar controller. The solar radiation was measured by a solar power meter (e.g., ISM 410) with a spectral response range of 400 nm to 1100 nm and an accuracy of 10 W / m2. 2 .

[0108] A K-type thermocouple with a thermometer was used to record the temperature of the coated and uncoated PV panels during the test. In addition, the temperature and relative humidity of the ambient atmosphere were measured using a humidity and temperature sensor about 75 centimeters away from the PV. The obtained data were recorded by an autonomous development code based on LabVIEW 2019. IR images and “UST” patterns of different thickness samples were captured by InfRec R550 PRO.

[0109] For outdoor measurement, coated and uncoated PV panels were measured on the rooftop of a building at the Hong Kong University of Science and Technology (HKUST, 22.3364.N, 114.2655.E) in September using a self-made instrument. The test instrument frame of 14.5 cm x 17 cm x 7 cm (length x width x height) was made of foam material and fixed on a balance. The experiment started at 19:00 and lasted for a whole day. During the test, the wind speed, solar radiation intensity, ambient temperature, and relative humidity were recorded by a weather station with sensors located near the samples. The temperature change during the test was obtained by a K-type thermocouple, and the weight change of the coated PV panel was recorded. Another thermometer and hygrometer were also used to monitor the changes in ambient temperature and relative humidity. The data were recorded by a LabVIEW program.

[0110] The following are examples illustrating procedures for practicing the present application. These examples should not be construed as limiting. All percentages are by weight and all solvent mixture proportions are by volume unless otherwise indicated.

[0111] Example 1: Design, Synthesis, and Characterization

[0112] Although the 20% MOF-801 has a better atmospheric water adsorption capacity (AWAC) than conventional porous materials (37-38) measured at very low RH, 1 g MOF-801 can only adsorb 0.38 g atmospheric water at 90% RH and 25°C. As mentioned above, the performance of evaporative cooling coatings depends on their AWAC. Therefore, MOF-801 is far from meeting the needs of high-power, long-time cooling. Therefore, CaCl2 hygroscopic salt with super-high atmospheric water adsorption capacity in a wide range of RH was compounded in the MOF-801 matrix. The equilibrium water adsorption capacity of CaCl2 at 25°C exceeds 1 g / 1 g, which is much higher than conventional desiccants such as silica gel. By reasonably controlling the proportion of MOF-801 in the composite material, it is possible to avoid the saturated adsorbed CaCl2@MOF-801 from becoming a water solution that can flow freely. The synthesized CaCl2@MOF-801 aqueous composite material adsorbs atmospheric water at low temperature, and the adsorbed atmospheric water desorbs and evaporates at high temperature. Here, the CaCl2@MOF-801 aqueous composite material adsorbs atmospheric water via three processes. The first process is the atmospheric water adsorption process of MOF-801; in the second process, it is a chemical reaction process, CaCl2 hydration to form CaCl2·6H2O; then in the third process, CaCl2·6H2O deliquesces, finally forming a water solution similar to a solid that cannot flow freely due to the presence of MOF-801. The CaCl2@MOF-801 aqueous composite material-based coating for cooling objects by the “breathing” process is shown in FIG. 1. Figure 1

[0113] Figure 2 ​A illustrates the preparation process of the cooling coating based on CaCh@MOF-801. First, fumaric acid and ZrOCh-8H2O are dissolved in a mixed solvent of N,N-dimethylformamide and formic acid, followed by chemical hydrothermal treatment. Then, the polycrystalline MOF-801 is separated from the mixture by suction filtration and dried in a vacuum oven. As shown in Figure 2 B, the diameter of the synthesized polycrystalline MOF-801 is determined to be about 292 nm. The Brunauer-Emmett-Teller (BET) test results show that the surface area and pore size of the polycrystalline MOF-801 are about 982.6 m2 / g 2 g -1 and about 1.75 nm, respectively. The characterization results obtained from Fourier transform infrared spectroscopy (FTIR), Raman shift, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) show that the polycrystalline MOF-801 has good quality. Next, the polycrystalline MOF-801 powder is immersed in a CaCh solution (e.g., 4 mol / L) and mixed under ultrasonication for about 1.5 hours. As a result, a homogeneous CaCh@MOF-801 colloid is formed. The scanning electron microscopy (SEM) results show that the coating has high porosity, uniform micrometer-sized pores, and uniform particle accumulation, which is beneficial for water storage during the water adsorption process, as shown in Figure 2 C-2D. As shown in Figure 2 E, the CaCh@MOF-801 colloid can be coated on the target object by spin coating or doctor blade coating, and the coating is shaped after drying. Figure 2 The distribution of C and O elements shown in the energy dispersive X-ray spectroscopy (EDS) map of F indicates the clear boundaries of the MOF-801 particles. Although Ca and Cl elements are widely distributed, the higher proportion observed in the connecting areas between the MOF-801 particles indicates that the CaCh components mainly exist in these areas and can be considered as the binder of the MOF-801 particles, as shown in Figure 2 D. These boundaries of the MOF-801 particles can inhibit the CaCh hydrates from forming a freely flowing solution. Therefore, the fully adsorbed CaCh@MOF-801 does not become a freely flowing aqueous solution due to the presence of the MOF-801 particles. At the same time, CaCh nanoparticles can wrap the MOF-801 and embed in the cages of the MOF-801, which is further demonstrated by the XRD results. The good consistency of the peaks of the XRD spectra of the MOF-801 obtained from the experimental measurements and simulation results indicates that the MOF-801 has high-quality crystallinity, as shown in Figure 2G. The two peaks below 10° are (111) and (200) orientations of MOF-801. However, no distinct peaks of CaCl2were observed in the saturated CaCl2@MOF-801, which might be caused by the hydration of the hygroscopic salt. It is worth noting that all the peaks of MOF-801 shift to the left, indicating that the lattice constant of MOF-801 swells due to the addition of CaCl2nanoparticles.

[0114] As mentioned above, the adsorption capacity of CaCl2@MOF-801 for atmospheric water is the most critical factor in determining the cooling performance of the resulting coating. The AWAC of CaCl2@MOF-801 at various RH and 25 °C was measured using an artificial humidity-controlled chamber for high RH (e.g., 70% and 90%) or a controlled chamber for low RH (e.g., 28% and 50%). The weight change of CaCl2@MOF-801 samples at different RH of 28%, 50%, 70%, and 90% was recorded using a balance with a precision of 1 mg, as shown in Figure 2 The corresponding instantaneous adsorption speed calculated by the derivative of the weight change is also plotted in Figure 2 The lower graph of Fig. H. At low RH of 28%, the weight change or the adsorption speed of atmospheric water starts large and drops to zero after about 600 minutes. The weight of atmospheric water adsorbed by CaCl2@MOF-801 after 1100 minutes is about 22% of the weight of CaCl2@MOF-801. At RH of 50%, 70%, and 90%, the CaCl2@MOF-801 coating exhibits similar trends of atmospheric water adsorption in addition to the adsorption speed and AWAC. At 50% RH, CaCl2@MOF-801 adsorbed about 45% of atmospheric water of its weight after 1100 minutes. On the other hand, at high RH of 70% and 90%, CaCl2@MOF-801 adsorbed about 80% and 120% of atmospheric water of its weight after 1300 minutes. It is observed that CaCl2@MOF-801 adsorbs more atmospheric water at higher RH because CaCl2and MOF-801 have good adsorption capacity for atmospheric water at high RH.

[0115] Example 2: Cooling performance of CaCl2@MOF-801 under laboratory conditions

[0116] The CaCl2@MOF-801-based cooling coating was coated on the back of a commercial PV panel with dimensions of 5.5 x 5.5 x 0.24 cm 3 by a doctor blade coating method, as shown in Figure 2The coated PV panel was then dried at 100 °C for 1 hour. It is worth noting that the thickness of the dried coating can be slightly reduced. All the thicknesses mentioned later refer to the initial thickness of the coating before drying. The thickness of the coating was 3-5 mm with 1 mm interval. Although the thickness of the coating can be further increased, the cooling performance (e.g. cooling time and cooling power) can not increase linearly due to the inherent high thermal resistance of the coating. Then, all the samples were tested by experimentally setting in a controlled chamber with precisely adjustable temperature and RH. For all the tests, the temperature was set at 25 °C and the relative humidity was adjusted from 28% to 70%. The samples were first placed in the controlled chamber for 12 hours to adsorb atmospheric water. The weight change of the coating and the environmental information were monitored by a precision balance and a thermo-hygrograph, respectively.

[0117] Figure 3A and 3B shows the temperature of the sample with 5 mm coating under 28% and 70% RH provided by the solar simulator at 1000 W / m2for 140 minutes. The temperature of the uncoated PV panel rapidly increased to a steady state temperature of about 76 °C within about 20 minutes at RH of 28% and 70%, as shown in 2 Figure 3A and 3B Figure 3A shows the temperature of the coated PV panel slowly increased to about 76 °C within about 112 minutes at RH of 28%. As shown in Figure 3B Figure 3B Figure 3B shows the temperature of the coated PV panel was lower than that of the uncoated sample throughout the radiation period of 140 minutes when RH was 70%. The results show that the coating based on CaCl2@MOF-801 can reduce the temperature of the PV plane by as much as 9.5 °C at RH of 28% and by as much as 14.1 °C at RH of 70%, as shown by the blue line in Figure 3A and Figure 3B When most of the adsorbed atmospheric water evaporates after a certain time, it is found that the temperature of the coated PV panel is slightly higher than that of the uncoated PV panel, which is due to the low thermal conductivity of the coating based on CaCl2@MOF-801. For the sample with 5 mm thick coating, the solar simulator in the experiment was turned off at about 140 minutes (for the samples with 3 mm and 4 mm thick coating, it takes about 133 minutes), and then the temperature of the PV panel with / without cooling coating suddenly dropped to the ambient temperature, as shown in Figure 3A and 3BThe effective cooling time at 28% RH will decrease to about 65 minutes and about 85 minutes, and at 70% RH will decrease to about 87 minutes and about 101 minutes, respectively, when the coating thickness is reduced to 3 mm and 4 mm. The maximum cooling temperature for samples with 3 mm and 4 mm thick coatings is about 8.2 °C and about 9.3 °C at 28% RH, and about 8.7 °C and about 10.8 °C at 70% RH, respectively.

[0118] The effect of the intensity of solar radiation on the cooling performance of CaCl2@MOF-801 coatings was further determined, with solar radiation of 500 W / m 2 and 1300 W / m 2 . The results at 28% RH are shown in Figure 3C and the results at 70% RH are shown in Figure 3D . The maximum cooling temperature of the coated samples increases with the increase of the intensity of solar radiation, and the maximum cooling time of the coated samples decreases with the increase of the intensity of solar radiation, due to the higher evaporation rate of water under stronger solar radiation. Among all these samples, the maximum cooling temperature of the CaCl2@MOF-801 composite coating with a thickness of 5 mm is about 10.8 °C at a solar radiation intensity of 500 W / m 2 and about 15.9 °C at a solar radiation intensity of 1300 W / m 2 , when the environmental RH and temperature are 70% and 25 °C, respectively. The corresponding cooling time exceeds 140 minutes at a solar radiation intensity of 1300 W / m 2 .

[0119] To further understand the cooling performance of the coating, the spatial temperature distribution of the PV panels with / without cooling coatings was captured by an infrared (IR) camera, as shown in Figure 4A . All samples were first placed in a controlled room with a relative humidity of 28% and a temperature of 25 °C for 12 hours to adsorb atmospheric water. Then the PV panels with / without cooling coatings were placed under solar radiation for 30 minutes. It was found that the PV panels without cooling coatings were rapidly heated to about 75 °C. In contrast, the temperature of the coated PV panels slowly increased due to the cooling effect of the coatings of the present application, and the cooling coating with a thickness of 5 mm achieved the best cooling performance among all samples, thanks to its highest atmospheric water adsorption capacity, as shown in Figure 4A . As shown in Figure 4B , a pattern of “UST” was painted on a PMMA substrate using a commercial white putty powder and a CaCl2@MOF-801 coating, and then it was left in the indoor environment overnight before testing. The infrared camera can clearly distinguish “UST” when the corresponding sample is heated for a few minutes, as shown in Figure 4C .

[0120] The weight change of the 5 mm thick coating was monitored in situ under different solar irradiation to reveal the cooling process. As shown in the upper graph of Fig. Figure 4D , the evaporation rate of the adsorbed water (indicated by the slope of the curve) decreases with time after 12 hours of adsorption at 28% RH and strongly depends on the solar intensity. Higher solar intensity triggers faster water desorption, which corresponds to a larger water loss in the "water loss" region. It was determined that the water evaporated in the "water loss" region was about 1.02 g, 1.41 g and 1.42 g for solar irradiation intensities of 500, 1000 and 1300 W / m 2 , respectively. When the solar simulator was turned off at 140 min, the water in the coating continued to evaporate for a while due to the residual heat and then re-adsorbed atmospheric water again ("recovery" region). As shown in the lower graph of Fig. Figure 4D , similar behavior was observed for the sample that absorbed water at 70% RH for 17 hours. However, due to the higher water absorption rate, the corresponding desorption rate became larger. For solar intensities of 500 W / m 2 , 1000 W / m 2 and 1300 W / m 2 , the weight loss in the "water loss" region was 2.64 g, 3.26 g and 3.57 g, respectively. Correspondingly, the average evaporation cooling power Pe caused by water evaporation was calculated by the following equation: P e = (Am x h e ) / (t x A) (24, 26), where Am is the weight loss of water during cooling, t is the cooling time, h is the evaporation enthalpy of water, and A is the surface area of the coating (the side surface is neglected due to its much smaller size compared to the upper and lower surfaces).

[0121] Figure 4E The average cooling power calculated from Figure 3C and Figure 3D is shown. The results show that Pe is closely related to the solar intensity and the coating thickness. The highest cooling power of a 5 mm thick coating under one sun irradiation is about 136 W / m 2 when RH is 28%, and the highest cooling power of a 5 mm thick coating under one sun irradiation can reach about 297 W / m 2 when RH is 70%. To compare with previous work (24), a PV panel coated with a 5 mm thick coating was placed in a controlled room with a temperature of 25 °C and RH of 70% to adsorb atmospheric water for 17 hours. When the solar irradiation intensity is 1000 W / m 2 , the cooling power can be increased to 315 W / m 2 , and when the solar irradiation intensity is 1300 W / m 2 , the cooling power can be further increased to 344 W / m2 which is comparable to the results of Li et al. (24) and Wang et al. (25). The system of Li et al. (24) used a 0.5 cm PAM-CNT-CaCl2coating to cool a PV panel at 22 °C and 60% RH, achieving an average cooling power of 295 W / m 2 under one sun irradiation. The system of Wang et al. (25) used a MIL-101(Cr)-based coating with a thickness of 516 pm to cool an electronic device at 25 °C and 60% RH, achieving a cooling power of about 281 W / m 2 . The experiments of the present invention were performed at 51% RH under one sun irradiation, and the corresponding cooling power is shown to be 236 W / m 2 . Therefore, the passive evaporative cooling power of the present invention has a cooling limit comparable to or better than conventional techniques such as radiative cooling.

[0122] Example 3: Recovery ability test of CaCl2@MOF-801-based cooling coating

[0123] Meanwhile, the passive recovery ability generated by the spontaneous adsorption process of the cooling coating is another key point for practical application, which determines whether the cooling coating can work continuously and naturally. Here, the PV panel with a coating thickness of 5 mm was subjected to intermittent working conditions of three different solar irradiations of 500 W / m 2 , 1000 W / m 2 and 1300 W / m 2 , to demonstrate its recovery ability. All experiments were performed in a controlled room at 25 °C and RH of about 28% or about 70%. The real-time temperature and weight change of the coating during four consecutive cycles were recorded and shown in Figure 5 A-5L. The settings of the three periodic working loads are as follows: (I) pre-adsorption for 3 hours, solar irradiation for 20 minutes, off for 60 minutes, (II) pre-adsorption for 12 hours, solar irradiation for 20 minutes, off for 60 minutes, (III) pre-adsorption for 12 hours, solar irradiation for 40 minutes, off for 60 minutes. Figure 5 All results plotted in A-5L were obtained under a solar irradiation intensity of 500 W / m 2 .

[0124] Figure 5 A-5C shows that the temperature curves in each cycle are similar when the solar irradiation intensity is 500 W / m 2 and the RH is 28%. Under working load (I), the highest cooling temperature achieved in the first cycle is 5.81 °C (denoted as T1), which slightly rises in the following cycles, reaching 6.19 °C (denoted as T4) in the last cycle, as Figure 5A. The increase in the maximum cooling temperature is attributed to the rapid spontaneous water adsorption of the coating when the solar simulator is off. As shown in Figure 5 B, the maximum cooling temperature of the workload (II) decreases from 6.66 °C in the first cycle to 5.83 °C in the last cycle, as shown in Figure 5 C, the maximum cooling temperature of the workload (III) increases from 6.01 °C in the first cycle to 6.12 °C in the last cycle.

[0125] Similar results are obtained when RH is 70%, as shown in Figure 5 G-5I. The maximum cooling temperature of the workload (I) slightly increases from 4.88 °C to 5.19 °C, the maximum cooling temperature of the workload (II) decreases from 7.79 °C to 6.92 °C, and the maximum cooling temperature of the workload (III) decreases from 9.43 °C to 6.55 °C.

[0126] These findings can be explained by the weight change of the adsorbed atmospheric water during the cycles, as shown in Figure 5 D-5F and Figure 5 J-5I. The weight change of the coating during "solar on" and "solar off" can be approximately linear. The adsorption speed a and the desorption speed b are defined as the slope of the weight change of the coating during "solar on" and "solar off", respectively. The "solar on" region of the third cycle and the "solar off" region of the second cycle are selected to linearly fit a and b. For the workload condition (I), the coating weight at the end point of each adsorption cycle increases with the increase of the cycle, as shown by the black dots in Figure 5 J. It is found that the adsorbed atmospheric water in the coating of the present application increases at the start point of each desorption cycle, and the maximum cooling temperature increases accordingly. Then a degradation number D = (t a a) / (t d b) is defined to describe the recovery ability and the degree of degradation of the cooling performance, where t a and t d are the adsorption and desorption times, respectively. When D > 1, more water is adsorbed during the cycle test, because the water adsorbed during "solar on" cannot be completely desorbed, and the cooling performance is thus continuously enhanced, as shown in Figure 5 J. On the other hand, D = 1 indicates that the adsorbed water is equal to the desorbed water, and the maximum cooling temperature is almost constant after multiple cycles, and the cooling performance can be completely recovered. However, the degradation of the cooling performance occurs when D = 1, which indicates that the water content decreases, resulting in a decrease in the maximum cooling temperature, as shown in Figure 5 E, 5F, 5K and 5L. For 1000 W / m 2 and 1300 W / m 2The higher the solar radiation below, the weight is always found to decrease. Therefore, it is crucial to choose the right working load condition to achieve stable and sustainable cooling performance. Here, the difference AT = T4-T1 is configured to find the optimal solar radiation under a specific working load, as AT should be positively correlated with D. For the working load condition (I) of RH@28%, AT = 0 when the solar radiation intensity is about 590 W / m 2 . Therefore, under the working load condition (I), the coating can have stable cooling performance without degradation when the solar radiation is less than 590 W / m 2 . Under the working load condition (I) of 70% RH, the critical solar radiation intensity is about 805 W / m 2 . For other working load conditions, there is no critical solar radiation intensity to ensure the stable cooling performance of the coating, which can be avoided by applying the coating under weaker solar radiation or prolonging the adsorbed water time of the coating within the cooling cycle.

[0127] Example 4: Field test of cooling performance of CaCl2@MOF-801-based cooling coating

[0128] To further investigate the practicability of the CaCl2@MOF-801-based coating of the present application, cooling tests were conducted under real outdoor conditions. The tests were carried out on the roof of an academic building at the Hong Kong University of Science and Technology (HKUST, 22.3364.N, 114.2655.E). The wind speed, solar radiation intensity, ambient temperature, and relative humidity were monitored in real time by a commercial weather station. The weight change of the coating and the temperature of the PV panel were recorded by a precision balance and a thermocouple, respectively, as shown in Figure 6A .

[0129] At 19:00 on September 2, 2021, after sunset, the PV panel with / without the cooling coating was exposed to the air, and the test lasted for one day until 19:00 on September 3, 2021, and the results are shown in Figure 6B . The average RH and temperature during the night when atmospheric water adsorption occurred were about 88% and 27°C, respectively, as shown in Figure 6C . During the day, the RH dropped to about 66%, and the temperature slightly increased to 30°C. During this period, the adsorbed water began to evaporate as the solar radiation intensity increased. Figure 6D The coating shows that the coating adsorbs atmospheric water from 19:00 to around 8:00 the next day (called the loading zone in Figure 6B ), and the temperature of the coated PV panel is slightly higher than that of the uncoated PV panel due to the exothermic phenomenon during the adsorption process. When the solar radiation starts to increase after 8:00, the temperature of the coated PV panel is lower than that of the uncoated sample due to the cooling effect of the coating. The solar radiation intensity at about 11:00 is about 900 W / m 2At this time, the maximum cooling temperature was about 10 °C. Overall, from 8:00 to 12:30, the temperature of the coated PV panel was lower than that of the uncoated PV panel, indicating a stable cooling period of 4.5 hours, as shown in FIG. 4B. From 12:30 to 17:30, the temperature difference between the coated and uncoated PV panels fluctuated around zero, as shown in FIG. 4C. After sunset at around 17:30, the temperature of the coated PV panel began to be slightly higher than that of the uncoated PV panel. Notably, the final weight of the coating was greater than its original weight, indicating that the cooling performance of the cooling coating did not decrease over time. The results of the study demonstrated the feasibility and versatility of applying the CaCl2@MOF-801 coating to integrated cooling operations. Figure 6B Figure 6D The field test showed that the coating could cool the PV panel in a natural environment for 4.5 hours, with a maximum cooling temperature of about 10 °C.

[0130] All patents, patent applications, provisional applications, and publications mentioned herein or referenced by the text are incorporated herein by reference in their entirety (including all drawings and tables) to the extent that the same are not inconsistent with the explicit teachings of the present specification.

[0131] It should be understood that the embodiments and implementations described herein are for illustrative purposes only, and various modifications or changes can be made thereto by those skilled in the art and that such modifications or changes are intended to be included within the spirit and scope of the present application and the scope of the appended claims. Moreover, any element or limitation of any application or implementation disclosed herein can be combined with any and / or all other elements or limitations (alone or in any combination), of any and / or all other applications or implementations disclosed herein, or of any other application or implementation, and all such combinations are intended to be included within the scope of the present application, but are not limited thereto.

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Claims

1. An aqueous composite material comprising: CaCl2nanoparticles and MOF-801 matrix; wherein the aqueous composite is obtained by mixing the MOF-801 matrix with a CaCl2nanoparticle solution in a mass / volume ratio m MOF-801 : V CaCl2 of 0.5 g / ml to 2 g / ml; and wherein the concentration of the CaCl2nanoparticles in the CaCl2nanoparticle solution is 2 mol / L to 6 mol / L.

2. The aqueous composite of claim 1, wherein, The MOF-801 matrix is composed of a plurality of polycrystalline MOF-801.

3. The aqueous composite of claim 2, wherein, The polycrystalline MOF-801 has: a diameter of 200 nm to 400 nm; 500 m 2 g -1 up to 1500 m 2 g -1 surface area; and / or an average pore size of 1 nm to 4 nm.

4. The aqueous composite of claim 3, wherein, The polycrystalline MOF-801 has: a diameter of 250 nm to 350 nm; 800 m 2 g -1 up to 1200 m 2 g -1 surface area; and / or an average pore size of 1 nm to 3 nm.

5. The aqueous composite of claim 4, wherein, The polycrystalline MOF-801 has: a diameter of 280 nm to 300 nm; 950 m 2 g -1 up to 1000 m 2 g -1 surface area; and / or an average pore size of 1 nm to 2 nm.

6. The aqueous composite of claim 5, wherein, The polycrystalline MOF-801 has: a diameter of 290 ± 5 nm; 980 ± 5 m 2 g -1 surface area; and / or an average pore size of 1.7 ± 0.05 nm.

7. The aqueous composite of any one of claims 2-6, wherein, The CaCl2nanoparticles are connected to the polycrystalline MOF-801 adjacent in the MOF-801 matrix.

8. The aqueous composite of claim 7, wherein, The CaCl2nanoparticles are connected to the polycrystalline MOF-801 adjacent in the MOF-801 matrix by covalent bond, intermolecular interaction.

9. The aqueous composite of any one of claims 2-6, wherein, A major portion of the CaCl2nanoparticles encapsulates the plurality of polycrystalline MOF-801 and another portion is embedded in the cages of the plurality of polycrystalline MOF-801.

10. The aqueous composite of any one of claims 1-6, wherein, The aqueous composite is obtained by mixing the MOF-801 matrix with a solution of CaCl2nanoparticles in a mass / volume ratio m MOF-801 : V CaCl2 ranging from 0.5 g / ml to 1 g / ml.

11. The aqueous composite of claim 10, wherein, The mass / volume ratio m of the MOF-801 matrix to the CaCI2 nanoparticles solution was 0.7 ± 0.1 g / ml. MOF-801 :V CaCl2 was 0.7 ± 0.1 g / ml.

12. The aqueous composite of claim 10, wherein, The concentration of the CaCl2nanoparticles in the CaCl2nanoparticle solution is 4 mol / L.

13. A method of producing an aqueous composite material, comprising: providing MOF-801; and mixing MOF-801 with a CaCl2nanoparticle solution under ultrasonication to obtain a CaCl2@MOF-801 aqueous composite material; wherein the concentration of the CaCl2nanoparticles is 2 mol / L to 6 mol / L; and wherein the mass / volume ratio of the MOF-801 matrix to CaCl2nanoparticle solution in the aqueous composite material is configured to be 0.5 g / ml to 2 g / ml.

14. The method of claim 13, wherein, The CaCl2nanoparticle solution is obtained by ultrasonication of CaCl2 dissolved in deionized water.

15. The method of claim 14, wherein, The concentration of the CaCl2nanoparticles is 4 mol / L.

16. The method of any one of claims 13-15, wherein, The mass / volume ratio of the MOF-801 matrix to CaCl2nanoparticle solution in the aqueous composite material is configured to be 0.5 g / ml to 1 g / ml.

17. The method of claim 16, wherein, The mass / volume ratio of the MOF-801 matrix to CaCl2nanoparticle solution is 0.7 ± 0.1 g / ml.

18. The method of any one of claims 13-15, wherein, The mixing is at a temperature of 30 °C to 50 °C for 1 to 3 hours.

19. The method of claim 18, wherein, The mixing is at a temperature of about 40 °C for about 1.5 hours.

20. The method of any one of claims 13-15, wherein, MOF-801 is prepared by the following steps: equivalents of fumaric acid and ZrOCl2·8H2O are dissolved in a solvent having N,N-dimethylformamide and formic acid to make a mixture; the mixture is heated to a temperature of about 130 °C for about 6 hours; the mixture is cooled to room temperature to obtain a MOF-801 precipitate.

21. The method of claim 20, wherein, The MOF-801 precipitate is separated by a vacuum filter.

22. The method of claim 21, wherein, The pore size of the vacuum filter is about 0.45 μm.

23. The method of claim 21 or 22, wherein, The separated MOF-801 is dried to activate the MOF-801.

24. The method of claim 23, wherein, The drying is under vacuum conditions at a temperature of about 150 °C for about 24 hours.

25. The method of any one of claims 13-15, wherein, The aqueous composite is the aqueous composite of any one of claims 1-12.

26. An evaporative cooling coating made from the aqueous composite of any one of claims 1-12 or the aqueous composite made by the method of any one of claims 13-25.

27. The coating of claim 26, wherein, The coating has a thickness of 3 mm to 7 mm.

28. The coating of claim 27, wherein, The coating has a thickness of 4 mm to 6 mm.

29. The coating of claim 28, wherein, The coating has a thickness of 5 ± 0.5 mm.

30. The coating of any one of claims 26-29, wherein, The coating is applied to a surface of an object, the object comprising a building, a road, an electronic device, a vehicle, a photovoltaic material.

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