Hierarchical porous carbon-calcium chloride composite heat storage material and preparation method thereof

Abstract

The application discloses a kind of hierarchical porous carbon calcium chloride composite heat storage materials and preparation method thereof, comprising: multi-walled carbon nanotube is treated to obtain carboxyl functionalized multi-walled carbon nanotube;Thickening agent is dispersed into graphene oxide, carboxyl functionalized multi-walled carbon nanotube is added, and ink is obtained;The ink is formed wet three-dimensional structure by DIW process printing;Wet three-dimensional structure is dried, carbonized, and three-dimensional hierarchical porous carbon skeleton is obtained;Three-dimensional hierarchical porous carbon skeleton is immersed in calcium chloride aqueous solution and heat treated, and hierarchical porous carbon calcium chloride composite heat storage material is obtained.The hierarchical porous carbon calcium chloride composite heat storage material of the application has high salt limit ability, fast water absorption and desorption kinetics, high energy storage density and excellent cycle stability, and still maintains structural integrity after multiple adsorption / desorption cycles, without salt migration, leakage or pore collapse phenomenon, suitable for low-temperature thermochemical energy storage, solar thermal utilization and thermal management system, with significant engineering application prospect.

Description

Technical Field

[0001] This invention belongs to the field of thermal storage materials and 3D printing manufacturing technology. More specifically, this invention relates to a graded porous calcium chloride carbon composite thermal storage material and its preparation method. Background Technology

[0002] Hydrated salt thermochemical energy storage is considered an important technology for future thermal energy utilization due to its advantages such as high energy density, long-term storage, and near-zero heat loss. Among them, calcium chloride has attracted much attention due to its low reaction temperature, low cost, and good cycle stability. However, pure calcium chloride is prone to problems such as deliquescence, poor fluidity, volume expansion caused by crystallization, high thermal resistance, and deterioration of cycle performance during adsorption / desorption.

[0003] To address the aforementioned issues, existing technologies generally employ porous materials as carriers to disperse hydrated salts within the pores. However, existing calcium chloride-based thermal storage materials commonly suffer from the following technical problems: (1) Deliquescence, migration and aggregation occur during water absorption / dehydration; (2) It has weak mechanical properties and is prone to collapse during the cycle, leading to salt leakage; (3) Traditional carrier pore structures are difficult to achieve macro, meso, and micro multi-scale synergy, which limits the mass transfer rate; (4) Insufficient salt load limits energy storage density.

[0004] Direct-write technology (DIW) offers a new approach to constructing programmable porous structures, but conventional direct writing struggles with using low-viscosity, high-solids inks and maintaining shape at the macroscopic level. Embedded direct writing (DIW)... EM It can be deposited and shaped in a medium, and is suitable for preparing complex three-dimensional porous structures.

[0005] Therefore, there is an urgent need for a new type of composite thermal storage material that can achieve controllable structure, high stability, high salt load, rapid mass transfer and high energy density. Summary of the Invention

[0006] One object of the present invention is to solve at least the above-mentioned problems and / or defects, and to provide at least the advantages described below.

[0007] To achieve these and other advantages according to the present invention, the present invention provides a hierarchical porous carbon-calcium chloride composite thermal storage material, comprising a three-dimensional hierarchical porous carbon skeleton and calcium chloride impregnated and cured on the three-dimensional hierarchical porous carbon skeleton, wherein the calcium chloride loading is 50~65 wt%; the hierarchical porous carbon-calcium chloride composite thermal storage material has a multi-scale interconnected pore structure of macropores, mesopores and micropores, and a porosity of 50%~80%.

[0008] Preferably, the macropore diameter is 0.5~50 μm, the mesopore diameter is 20~500 nm, and the micropore diameter is <20 nm.

[0009] A method for preparing a hierarchical porous calcium chloride composite thermal storage material includes the following steps: Step 1: Add multi-walled carbon nanotubes to a mixture of sulfuric acid and hydrogen peroxide, stir at room temperature to introduce carboxyl functional groups onto the surface, and obtain carboxyl-functionalized multi-walled carbon nanotubes. Step 2: Disperse the thickener into graphene oxide and add carboxyl-functionalized multi-walled carbon nanotubes, mix evenly to obtain ink; Step 3: Place the ink into the embedded printing medium and print a three-dimensional structure using the DIW process. After printing, the wet three-dimensional structure is obtained by non-solvent-induced phase separation and curing. Step 4: Dry and carbonize the wet three-dimensional structure to obtain a three-dimensional hierarchical porous carbon framework; Step 5: Immerse the three-dimensional hierarchical porous carbon skeleton in an aqueous calcium chloride solution and perform heat treatment to allow calcium chloride to enter the pore network and solidify. Then, after rinsing and drying, a hierarchical porous carbon-calcium chloride composite thermal storage material is obtained.

[0010] Preferably, in step one, the ratio of the amount of multi-walled carbon nanotubes to the mixed solution of sulfuric acid and hydrogen peroxide is 0.5~1 g: 200~400 mL; the volume ratio of sulfuric acid to hydrogen peroxide in the mixed solution is 3:1; and the stirring time at room temperature is 10~20 h.

[0011] Preferably, in step two, the mass ratio of graphene oxide to carboxyl-functionalized multi-walled carbon nanotubes in the ink is 1:1 to 2:1; the mass ratio of the thickener to the total mass of graphene oxide and carboxyl-functionalized multi-walled carbon nanotubes is 6 to 8:1.

[0012] Preferably, in step two, the thickener includes N , N -Dimethylpyrrolidone, polyethersulfone and polyvinylpyrrolidone; N , N The mass ratio of dimethylpyrrolidone, polyethersulfone, and polyvinylpyrrolidone is 10~20:2~5:1~2.

[0013] Preferably, in step three, the embedded printing medium is a liquid medium capable of non-solvent-induced phase separation, and the liquid medium capable of non-solvent-induced phase separation includes deionized water.

[0014] Preferably, in step four, the drying temperature is 60~120℃ and the drying time is 15~18 h; the carbonization temperature is 650~750℃, the carbonization heating rate is 1~5℃ / min, and the carbonization time is 1~4 h.

[0015] Preferably, in step five, the heat treatment temperature is 200~300℃ and the heat treatment time is 1~5h; the drying temperature is 100~200℃ and the drying time is 12~15h.

[0016] An application of a graded porous calcium chloride composite thermal storage material, wherein the graded porous calcium chloride composite thermal storage material is used in low-temperature thermochemical energy storage, solar thermal utilization and thermal management system.

[0017] The present invention has at least the following beneficial effects: 1. The hierarchical porous calcium chloride composite thermal storage material prepared by this invention has a controllable pore structure and multi-level interconnection across scales, which improves the diffusion efficiency of water molecules.

[0018] 2. The graded porous carbon-calcium chloride composite thermal storage material achieves high salt loading and uniform dispersion, significantly improving energy storage density.

[0019] 3. This invention achieves pore wall confinement and carbon nanotube reinforcement mechanisms, effectively suppressing salt migration and aggregation.

[0020] 4. The hierarchical porous calcium chloride composite thermal storage material prepared by this invention exhibits a significantly improved thermal response rate through rapid water absorption and dehydration kinetics.

[0021] 5. The hierarchical porous calcium chloride composite thermal storage material prepared by this invention has high cycle stability and maintains its structural integrity after multiple adsorption / desorption cycles.

[0022] 6. The graded porous calcium chloride composite thermal storage material prepared by this invention has enhanced mechanical properties and can withstand repeated stress during hydration / dehydration.

[0023] The hierarchical porous calcium chloride composite thermal storage material prepared by this invention possesses high salt confinement capability, rapid adsorption-desorption kinetics, high energy storage density, and excellent cycling stability. It maintains structural integrity even after multiple adsorption / desorption cycles, exhibiting no salt migration, leakage, or pore collapse. This material is suitable for low-temperature thermochemical energy storage, solar thermal utilization, and thermal management systems, demonstrating significant engineering application prospects.

[0024] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0025] Figure 1Infrared spectra of MWCNTs prepared in Example 1 before and after carboxyl functionalization; Figure 2 Raman spectra of MWCNTs prepared in Example 1 before and after carboxyl functionalization; Figure 3 Thermogravimetric analysis of MWCNTs prepared in Example 1 before and after carboxyl functionalization; Figure 4 The rheological properties diagram is shown for the ink prepared in Example 1. Figure 5 Optical electron microscope image of the three-dimensional hierarchical porous carbon framework GMF-H prepared in Example 1; Figure 6 Stress-strain diagram of the porous material GH prepared for Comparative Example 1 after 50 cycles; Figure 7 The dynamic rheological behavior of the porous material GH in Comparative Example 1 is shown. Figure 8 The stress-strain diagram of the three-dimensional hierarchical porous carbon framework GMF-H of Example 1 after 50 cycles. Figure 9 The dynamic rheological behavior of the three-dimensional hierarchical porous carbon framework GMF-H in Example 1; Figure 10 This is a SEM image of the cross-section of the three-dimensional hierarchical porous carbon framework GMF-H in Example 1; Figure 11 This is a SEM image of the cross-section of the graded porous calcium chloride carbon-chloride composite thermal storage material GMF-H / CaCl2 in Example 1; Figure 12 EDS image of the cross section of the graded porous calcium chloride-carbohydrate composite thermal storage material GMF-H / CaCl2 in Example 1; Figure 13 The pore size distribution diagrams are for the three-dimensional hierarchical porous carbon framework GMF-H and the hierarchical porous carbon-calcium chloride composite thermal storage material GMF-H / CaCl2 in Example 1. Figure 14 The water absorption rate of the graded porous calcium chloride carbon composite thermal storage material GMF-H / CaCl2 in Example 1 under different temperatures (25℃, 30℃) and different relative humidities (40% RH, 60% RH, 80% RH); Figure 15 The degree of hydration of the graded porous calcium chloride carbon composite thermal storage material GMF-H / CaCl2 in Example 1 under different temperatures (25℃, 30℃) and different relative humidities (40% RH, 60% RH, 80% RH); Figure 16Isothermal desorption weight loss curves of graded porous calcium chloride carbon-chloride composite thermal storage material GMF-H / CaCl2 at 90℃, 120℃ and 150℃; Figure 17 The isothermal desorption hydration level variation curves of graded porous calcium chloride carbon-chloride composite thermal storage material GMF-H / CaCl2 at 90℃, 120℃ and 150℃ are shown. Figure 18 XRD patterns of graded porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2 before and after 25 cycles of testing; Figure 19 The results of the cyclic water absorption performance of the graded porous calcium chloride carbon-chloride composite thermal storage material GMF-H / CaCl2 in 25-cycle tests; Figure 20 The degree of cyclic hydration of the graded porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2 during 25 cycles of testing.

[0026] Figure 21 DSC curves for graded porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2; Figure 22 The mass energy storage density and volumetric energy storage density values ​​are for the graded porous calcium chloride carbon-chloride composite thermal energy storage material GMF-H / CaCl2. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0028] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof. Example 1 A method for preparing a hierarchical porous calcium chloride composite thermal storage material includes the following steps: Step 1: Add 0.5g of multi-walled carbon nanotubes to 200mL of sulfuric acid / hydrogen peroxide mixture (including 150mL of sulfuric acid and 50mL of hydrogen peroxide) and stir for 18h. After repeated washing until neutral, freeze dry to obtain carboxyl-functionalized multi-walled carbon nanotubes. Step 2, N , N A thickener was prepared by mixing dimethylpyrrolidone (NMP), polyethersulfone (PES), and polyvinylpyrrolidone (PVP) in a mass ratio of 16:3:1, with a total mass of 10.5g. 1g of graphene oxide was added to the thickener and dispersed, and then 0.5g of carboxyl-functionalized multi-walled carbon nanotubes were added and stirred to obtain an ink with shear-thinning properties. Step 3: Load the ink into the syringe and use DIW EM (Ink Direct Writing 3D Printing) A mesh-like three-dimensional structure is printed in a deionized water medium and cured for 4 hours to obtain a wet three-dimensional structure; Step 4: Dry the wet three-dimensional structure at 80℃, and then heat it to 700℃ in an inert atmosphere for 3 h to obtain a three-dimensional hierarchical porous carbon framework, denoted as GMF-H, with a porosity of 76%, macropore diameter of 1.04 μm, mesopore diameter of 29.6 nm, and micropore diameter of 1.75 nm.

[0029] Step 5: Immerse the three-dimensional hierarchical porous carbon framework in an aqueous calcium chloride solution and heat at 280℃ for 3 hours to allow calcium chloride to enter the pore network. After cleaning and drying at 150℃ for 12 hours, the final hierarchical porous carbon-calcium chloride composite thermal storage material is obtained, denoted as GMF-H / CaCl2.

[0030] Comparative Example 1 A method for preparing a hierarchical porous calcium chloride composite thermal storage material includes the following steps: Step 1: N , N A thickener was prepared by mixing dimethylpyrrolidone (NMP), polyethersulfone (PES), and polyvinylpyrrolidone (PVP) in a mass ratio of 16:3:1, with a total mass of 10.5g. 1g of graphene oxide was added to the thickener and dispersed, and the mixture was stirred to obtain an ink with shear-thinning properties. Step 2: Load the ink into the syringe and use DIW... EM A mesh-like three-dimensional structure was printed in a deionized water medium and cured for 4 hours to obtain a wet three-dimensional structure. Step 3: Dry the wet three-dimensional structure at 80℃, and then heat it to 700℃ in an inert atmosphere for 3 hours to obtain a porous material, denoted as GH. The porosity of the porous material is 77%, with a macropore diameter of 1.61 μm, a mesopore diameter of 18.4 nm, and a micropore diameter of 1.74 nm.

[0031] Step 4: Immerse the porous material in an aqueous calcium chloride solution and heat at 280℃ for 3 hours to allow calcium chloride to enter the pore network. After washing and drying at 150℃ for 12 hours, the final GH / CaCl2 is obtained.

[0032] Figure 1 The images shown are infrared spectra of the multi-walled carbon nanotubes before and after carboxyl functionalization obtained in step one of Example 1. Based on the infrared test results, the values ​​at 3420 cm⁻¹ are... −1 The peaks around 1629 cm⁻¹ correspond to the infrared absorption of the H-bonded hydroxyl groups in the carboxyl group. −1The peaks around the left and right are attributed to the stretching vibration of C=O in the carboxyl group. Compared to unfunctionalized multi-walled carbon nanotubes (MWCNTs), carboxyl-functionalized carbon nanotubes (MWCNTs-F) show a higher peak around 3420 cm⁻¹. −1 and 1629cm −1 The peak is stronger.

[0033] Figure 2 The images show Raman spectroscopy results of multi-walled carbon nanotubes (MWCNTs) before and after carboxyl functionalization in Example 1. Based on the Raman spectroscopy results, compared to the initial MWCNTs, the 1335 cm⁻¹… −1 The peaks around the left and right, i.e., the D peak, indicate the presence of amorphous or disordered carbon in the MWCNTs sample, 1580 cm⁻¹. −1 The peaks on the left and right are the G peaks, representing an ideal graphite lattice, I D / I G The intensity ratio of D and G bands is considered by many researchers to be one of the main indicators for quickly and roughly estimating defects; a higher I value... D / I G This implies a higher degree of covalent functionalization.

[0034] Figure 3 The thermogravimetric analysis (TGA) results for multi-walled carbon nanotubes (MWCNTs) before and after carboxyl functionalization in Example 1 are shown. According to the TGA results, sample MWCNTs-F exhibited the greatest TGA loss. In the first stage, at 150°C, a weight loss of approximately 1% was detected in the H2SO4 / H2O2 treated MWCNTs, corresponding to the evaporation of adsorbed water. In the second stage, from 150°C to 350°C, weight loss occurred due to the decarboxylation of carboxyl groups on the MWCNT walls. Removing the hydroxyl functional groups attached to the MWCNT surface resulted in weight loss between 350°C and 500°C.

[0035] Combination Figures 1-3 As a result, it was demonstrated that carboxylic acid groups were successfully introduced onto the surface of MWCNTs.

[0036] Figure 4 The rheological properties of the ink prepared in step two of Example 1 were analyzed. According to the rheological test results, the ink is a non-Newtonian body with shear-thinning properties, meaning that the slurry can be extruded from the nozzle under a certain pressure. When the angular frequency is 150 rad / s, the storage modulus is equivalent to the loss energy, indicating that the slurry is in a semi-solid state. After 150 rad / s, the storage modulus exceeds the loss modulus, and the slurry mainly undergoes elastic deformation, which enhances its formability. The slurry can maintain a stable shape after extrusion.

[0037] Figure 5The image shown is an optical electron microscope image of the wet three-dimensional structure obtained by printing in step three of Example 1. By controlling parameters such as the extrusion pressure and extrusion speed of the syringe, the printing width of the sample can be controlled. In addition, the spacing between samples during printing can be adjusted, thereby printing a regular printing material with micron-sized macropores. These macropores help water molecules to be quickly transported to the skeleton of the printed sample or to be quickly lost from the skeleton to the outside world.

[0038] The mechanical properties and dynamic rheological behavior of the three-dimensional hierarchical porous carbon framework (GMF-H) of Material Example 1 and the porous material (GH) of Comparative Example 1 were characterized using a dynamic mechanical analyzer.

[0039] Figure 6 The stress-strain diagram of the porous material GH prepared in Comparative Example 1 after 50 cycles is shown. The results show that the compressive modulus of the porous material GH is 0.015 MPa.

[0040] Figure 7 To illustrate the dynamic rheological behavior of the porous material GH in Comparative Example 1, the material was characterized in the frequency range of 0–50 Hz. The results show that the storage modulus and loss modulus of the porous material GH remain relatively stable with frequency variation.

[0041] Figure 8 The stress-strain diagram of the three-dimensional hierarchical porous carbon framework GMF-H in Example 1 after 50 cycles is shown. The results show that the compressive modulus of the three-dimensional hierarchical porous carbon framework GMF-H is 0.020 MPa.

[0042] Figure 9 The dynamic rheological behavior of the three-dimensional hierarchical porous carbon framework GMF-H in Example 1 is shown. The storage modulus of GMF-H exhibits obvious frequency independence, and its damping ratio is between 0.13 and 0.30, indicating that the material is dominated by elastic response.

[0043] Combination Figures 6-9 The mechanical properties and dynamic rheological behavior of porous materials GH and three-dimensional hierarchical porous carbon skeleton GMF-H were compared. The addition of carboxyl functionalized MWCNTs-F further significantly enhanced the mechanical properties of the materials.

[0044] Figures 10-12 This is a SEM image of the cross-section of the three-dimensional hierarchical porous carbon framework GMF-H in Example 3. The results show that the oxygen-containing functional groups on the surface of the carboxyl-functionalized MWCNTs-F interact with the functional groups on the surface of graphene oxide (GO), causing them to be uniformly dispersed on the GO substrate and forming a layer of "fluffy" covering structure.

[0045] Figure 13The images show the pore size distribution of the three-dimensional hierarchical porous carbon framework GMF-H and the hierarchical porous carbon-calcium chloride composite thermal storage material GMF-H / CaCl2 in Example 1. The evolution of the pore structure and density characteristics of the three-dimensional hierarchical porous carbon framework GMF-H before and after loading with CaCl2 were studied using MIP. The results show that the peak value of the pore size distribution curve represents the most probable pore size (i.e., the highest frequency pore size) of the material. The morphology of the distribution curves of the three-dimensional hierarchical porous carbon framework GMF-H remains similar before and after loading with CaCl2, while the peak intensity of the hierarchical porous carbon-calcium chloride composite thermal storage material GMF-H / CaCl2 is significantly reduced. However, both materials possess a hierarchical porous structure ranging from nanometer to micrometer scale, which helps to promote the rapid transport of water molecules into the composite material or their efficient dissipation to the outside. GMF-H: The highest peak value was 3.04 mL / g (pore size 1.04 μm). GMF-H / CaCl2: The highest peak value decreased to 1.03 mL / g (pore size 0.83 μm). This phenomenon indicates that CaCl2 impregnation into the matrix pores leads to a reduction in effective pore channels, causing pore size shrinkage and a decrease in pore volume. The porosity of the hierarchical porous material GMF-H decreased from 77.8% to 48.6% after loading CaCl2. The mass of the hierarchical porous material GMF-H before and after loading was weighed, and its calcium chloride loading reached 50~65 wt%.

[0046] The test was conducted using a constant temperature and humidity chamber under two temperature conditions (25℃, 30℃) and three humidity conditions (40%, 60%, 80% RH). Taking 25℃ / 40% RH as an example: 1) Sample pretreatment: The sample was vacuum dried at 150℃ for 24 h and then cooled to room temperature in a desiccator; 2) Kinetic test: The dehydrated sample was rapidly transferred to the target environment (25℃, 40% RH), and the mass change was continuously monitored for 300 min. The other conditions were the same.

[0047] Figure 14 The image shows the water adsorption curves of the graded porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2 in Example 1 at different temperatures (25℃, 30℃) and different relative humidities (40% RH, 60% RH, 80% RH). Figure 15 The degree of hydration of the graded porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2 in Example 1 is shown in Table 1 at different temperatures (25℃, 30℃) and relative humidities (40% RH, 60% RH, 80% RH). The results obtained by fitting the water adsorption curves of the composite material using a first-order kinetic equation are summarized in Table 1.

[0048] Table 1. Hydration fitting results of graded porous calcium chloride composite thermal storage materials under different temperatures and relative humidities. The time for the composite material to reach 90% equilibrium adsorption can be derived from the first-order kinetic equation. t 90 ), t 90 The value is obtained according to the following formula: In the formula k The water absorption reaction rate of the composite material is expressed in minutes. -1 .

[0049] Depend on Figure 14 , Figure 15 The fitting results in Table 1 show that the hierarchical porous calcium chloride carbon composite thermal storage material GMF-H / CaCl2 exhibits temperature and relative humidity dependence. On the one hand, when the temperature is constant (e.g., 25℃), the water absorption capacity of the hierarchical porous calcium chloride carbon composite thermal storage material increases as the relative humidity increases from 40% to 80%. k The value ranges from 0.00606 min. -1 Increased to 0.00706 min -1 Water adsorption capacity ( w e ) and hydration level ( X e The relative humidity increased from 0.56 g / g to 1.07 g / g and from 6.8 mol / mol to 13.0 mol / mol, respectively. On the other hand, when the relative humidity was constant, the water absorption performance of the graded porous calcium chloride composite thermal storage material also increased when the temperature changed from 25℃ to 30℃. When the relative humidity remained at 40%, its... k The value ranges from 0.00606 min. -1 Increased to 0.00689 min -1 The water adsorption capacity and hydration level increased from 0.56 g / g to 0.64 g / g and from 6.8 mol / mol to 7.8 mol / mol, respectively. Furthermore, the variation in water absorption performance of the hierarchical porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2 shows that relative humidity has a much stronger effect on improving water absorption performance than temperature. Moreover, under conditions of 30℃ and 80% RH, its... k The value is 0.00762 min. -1 ,That t 90 With a value of 302.2 min, the GMF-H / CaCl2 composite material exhibits rapid mass transfer kinetics during water absorption.

[0050] 1) Controllable hydration pretreatment: The three composite materials were placed in a constant temperature and humidity chamber and the mass change was continuously monitored under the conditions of 20℃ / 50% RH until CaCl2 reached the hexahydrate state (hydration degree n=6 mol H2O / mol CaCl2).

[0051] 2) Programmed dehydration test: The hydration-treated composite material was rapidly transferred to a vacuum drying oven, and three gradient dehydration temperatures were set (90℃, 120℃, 150℃). After the chamber temperature stabilized (fluctuation less than ±0.5℃) and was maintained for 30 min, the sample was placed in the vacuum drying oven and the dehydration mass loss was recorded in real time.

[0052] Figure 16 The isothermal desorption weight loss curves of graded porous calcium chloride carbon composite thermal storage material GMF-H / CaCl2 at 90℃, 120℃, and 150℃ are shown. Figure 12 To obtain the isothermal desorption hydration level variation curves of graded porous calcium chloride carbon composite thermal storage material GMF-H / CaCl2 at 90 °C, 120 °C and 150 °C, a first-order reaction kinetic model was used to fit the dehydration process at three temperatures of 90 °C, 120 °C and 150 °C. The obtained kinetic parameters are summarized in Table 2.

[0053] Table 2. Fitting results of dehydration performance tests of graded porous calcium chloride composite thermal storage materials at different temperatures. Depend on Figure 16 , Figure 17 The fitting results in Table 2 show the correlation between the change in hydration level and the rate constant: the desorption rate constant directly characterizes the rate of decrease in the hydration degree of CaCl2. A higher rate constant indicates a faster decrease in hydration level, suggesting a more rapid dehydration reaction kinetic. At 150 °C, the GMF-H / CaCl2 composite material... k The value ranges from 0.05845 min. -1 , t 90 The value was 39.4 min, indicating that the GMF-H / CaCl2 composite material exhibited rapid mass transfer kinetics during dehydration.

[0054] The moisture absorption and dehydration cycle tests were conducted using an alternating constant temperature and humidity chamber and a vacuum drying chamber. Before the experiment, the samples were first dried in a 150℃ vacuum drying chamber for 24 h to ensure complete dehydration. After dehydration, the samples were cooled to room temperature under vacuum and then sealed for storage. The sealed samples were then transferred to a constant temperature and humidity chamber (30℃, 80% RH) for water absorption testing, which lasted for 300 min. Subsequently, the samples were placed in a vacuum drying chamber at 150℃ and desorbed until the samples reached constant weight, completing the dehydration test. The above water absorption-dehydration process was defined as one cycle, and 25 cycles were performed continuously. The cyclic stability was evaluated by monitoring and recording the changes in water absorption of the composite material.

[0055] Figure 18 The XRD patterns of the hierarchical porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2 before and after 25 cycles of testing are shown. Figure 19 The results show the cyclic water absorption performance of the graded porous calcium chloride carbon composite thermal storage material GMF-H / CaCl2 in a 25-cycle test. Figure 20 The degree of cyclic hydration of the graded porous calcium chloride-carbon composite thermal storage material GMF-H / CaCl2 during 25-cycle testing was measured. Figure 18 , Figure 19 and Figure 20 The results show that typical carbon diffraction peaks appear in the three porous carbon matrices around 2θ ≈ 26 ° and 43 °, but their intensities and full width at half maximum (FWHM) vary due to differences in matrix structure. After loading CaCl2, porous carbon and... γ The diffraction peaks of -CaCl2·4H2O indicate that CaCl2 rapidly absorbs water to form a tetrahydrate in the air under the influence of ambient humidity. The composite material exhibits diffraction peaks at 21°, 28°, and 39°. γThe characteristic peak of -CaCl2·4H2O is observed, while the diffraction peaks at 26° and 43° are those of the GMF-H porous material. Compared with the single component, the intensity of the diffraction peaks in the composite is weakened, indicating that CaCl2 is successfully embedded in the porous matrix without destroying the support framework structure. Confining CaCl2 in porous carbon materials has the following advantages: 1) Effective suppression of agglomeration: Dispersed CaCl2 avoids agglomeration caused by deliquescence and recrystallization during adsorption / desorption cycles, improving structural stability; 2) Improved reaction kinetics: The huge dispersed surface area and nano-effect of the GMF-H porous material allow water molecules to contact and react with CaCl2 more quickly, increasing the rate and capacity of water adsorption / desorption. Moreover, after 25 cycles, the XRD pattern of the composite material did not change significantly, indicating that the composite material has good cycling stability, that is, no leakage of hydrated salts occurred after multiple cycles. The final adsorption capacity and hydration level of the hierarchical porous calcium chloride-carbohydrate composite thermal storage material GMF-H / CaCl2 were 1.16 g / g and 4.07 mol / mol, respectively. The fluctuations in the curves may be due to experimental errors or local inhomogeneities in the material.

[0056] Experiments were conducted using differential scanning calorimetry (DSC) to acquire heat flux signals of three hydrated, graded porous calcium chloride-carbon composite thermal storage materials. To eliminate the influence of system background, the measured heat flux signals were corrected using a blank baseline (tested under identical experimental conditions using an empty crucible, with its heat flux signal subtracted). The corrected heat flux signals were linearly integrated to determine the total heat of the exothermic process, thereby obtaining the material's mass energy density and volumetric energy density.

[0057] Figure 21 , Figure 22 The DSC curves, gravimetric energy storage density, and volumetric energy storage density of the graded porous calcium carbide-carbon chloride composite thermal energy storage material GMF-H / CaCl2 are presented. Figure 21 The results show that the hierarchical porous calcium chloride composite thermal storage material exhibits two distinct endothermic peaks. The first endothermic peak is concentrated around 110°C, while the second endothermic peak is located around 161°C. The composite material... ESD g and ESD v The value is: 1124.10 kJ·kg -1 and 2.19 GJ·m -3 .

[0058] The energy storage performance of the hierarchical porous calcium chloride composite thermal energy storage material GMF-H / CaCl2 prepared in Example 1 of this invention was compared with that of representative advanced thermochemical energy storage (TCES) materials reported in the literature.

[0059] As summarized in Table 3, Reference 1 (T. Spietz, R. Fryza, J. Lasek, J. Zuwała, Thermochemical Energy Storage Based on Salt Hydrates: A Comprehensive Review) Energies , 2025, 18, 2643. doi: 10.3390 / en18102643.) and Document 2 (J. Aarts, H.Fischer, O. Adan, H. Huinink. Towards stable performance of salt hydrates inthermochemical energy storage: A review, J. Energy Storage The mass storage density of strontium bromide (SrBr2)-based hydrated salts reported in (doi: 10.1016 / j.est.2025.115726.) is... ESD g Typically, it ranges from 900 to 1200 kJ·kg. -1 Within the range, volumetric energy storage density ( ESD v The concentration is approximately 1.8~2.5 GJ·m⁻¹. -3 However, these materials suffer from problems such as high cost, strong corrosivity, and significant volume expansion, which limit their practical application.

[0060] Literature 3 (SY Li, YJ Huo, T. Yan, H. Zhang, LW Wang, WG Pan, Preparation and thermalproperties of zeolite / MgSO4composite sorption material for heat storage, Renewal. Energy, 2024, 224, 120166. doi: 10.1016 / j.renene.2024.120166.), Document 4 (X. Liu, HM Wang, X. Liu, FM Yang, L. Guan, S. Sani, CG Sun, YP Wu, Development of MgSO4 / mesoporous silicacomposites for thermochemical energy storage: the role of porous structureonwater adsorption, Energy Rep. , 2022, 8, 4913-4921. doi: 10.1016 / j.egyr.2022.03.137.), Document 5 (T. Yan, H. Zhang, A critical review of salthydrates asthermochemical sorption heat storage materials: Thermophysicalproperties and reaction kinetics, Sol. Energy The magnesium sulfate (MgSO4)-based porous composite materials reported in [Journal Name], 2022, 242, 157-183. doi:10.1016 / j.solener.2022.07.002.] generally have low actual energy storage densities (mass energy storage density approximately 400~1000 kJ·kg⁻¹) due to slow reaction kinetics and incomplete dehydration at low temperatures. -1 The volumetric energy storage density is approximately 0.3~2.0 GJ·m³. -3 ).

[0061] Document 1 (T. Spietz, R. Fryza, J. Lasek, J. Zuwała, Thermochemical EnergyStorage Based on Salt Hydrates: A Comprehensive Review, Energies, 2025, 18,2643. doi: 10.3390 / en18102643.), Document 6 (XK Tian, ​​SJ Guo, XJ Lv, SCLin, CY Zhao, Progress in multiscale research on calcium-looping forthermochemical energy storage: From materials to systems, Prog. Energy Combust. Sci., 2025, 106, 101194. doi: 10.1016 / j.pecs.2024.101194.), Document 7 (JQ Lin, Q. Zhao, HT Huang, HZ Mao, YX Liu, YM Xiao, Applications of low-temperature thermochemical energy storage systems for salt hydratesbased on material classification: A review, Sol. Energy (2021, 214, 149-178. doi:10.1016 / j.solener.2020.11.055.) The theoretical volumetric energy storage density of the calcium oxide / calcium hydroxide (CaO / Ca(OH)2) system can reach a relatively high level (approximately 1.6~3.0 GJ·m). -3 However, this system requires operation at high temperatures above 450 °C and will experience severe sintering during the cycling process.

[0062] In contrast, reference 8 (QY Zhang, YF Wu, SH Dong, JK Zhuo, XRSun, Q. Yao, Development of activated carbon / CaCl2 compposites for seasonal thermochemical energy storage: Effect of pore structure, J. Energy Storage, 2024, 97, 112697. doi: 10.1016 / j.est.2024.112697.) and literature 9 (N. Gao, LS Deng, J. Li, T. Zeng, HY Huang, N.Kobayashi, M. Kubota, XH Yang, Effects of porous carbon materials on heat storage performance of CaCl2hydrate for low-grade thermal energy, RSC Adv. The energy storage performance of calcium chloride (CaCl2)-based hydrated salts under low-temperature conditions reported in the paper (2023, 13, 32567. doi: 10.1039 / d3ra04859d.) is typically: a mass energy storage density of approximately 700~1100 kJ·kg⁻¹. -1 The volumetric energy storage density is approximately 1.5~2.1 GJ·m³. -3 .

[0063] The hierarchical porous calcium chloride composite thermal storage material GMF-H / CaCl2 prepared in this invention achieves a thermal efficiency of 1124.10 kJ·kg⁻¹. -1 The mass storage energy density is 2.19 GJ·m -3 With its high volumetric energy density, its energy storage performance ranks among the top reported calcium chloride-based thermochemical energy storage materials, while also exhibiting excellent low-temperature operation and cycle stability.

[0064] Table 3 Performance comparison of the GMF-H / CaCl2 of the present invention with existing energy storage materials This invention constructs a hierarchical porous carbon structure and achieves highly stable confined calcium chloride loading. CaCl2 undergoes rapid hydration and dehydration reactions within the porous carbon confinement environment, significantly enhancing the heat storage / release power of the composite material. The multi-level interconnected pore structure effectively limits the leakage, liquefaction, and aggregation of hydrated salts, significantly improving the mass transfer performance, reaction rate, energy density, and long-term cycling performance of the heat storage material. It is suitable for various thermochemical energy storage and thermal management applications and has significant industrial application prospects.

[0065] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0066] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A graded porous calcium chloride-carbon composite thermal storage material, characterized in that, It includes a three-dimensional hierarchical porous carbon framework and calcium chloride impregnated and cured on the three-dimensional hierarchical porous carbon framework, with a calcium chloride loading of 50~65wt%; the hierarchical porous carbon-calcium chloride composite thermal storage material has a multi-scale interconnected pore structure of macropores, mesopores and micropores, with a porosity of 50%~80%.

2. The graded porous calcium chloride composite thermal storage material according to claim 1, characterized in that, The macropores have a diameter of 0.5~50μm, the mesopores have a diameter of 20~500nm, and the micropores have a diameter of <20nm.

3. A method for preparing a graded porous calcium chloride composite thermal storage material according to any one of claims 1-2, characterized in that, Includes the following steps: Step 1: Add multi-walled carbon nanotubes to a mixture of sulfuric acid and hydrogen peroxide, stir at room temperature to introduce carboxyl functional groups onto the surface, and obtain carboxyl-functionalized multi-walled carbon nanotubes. Step 2: Disperse the thickener into graphene oxide and add carboxyl-functionalized multi-walled carbon nanotubes, mix evenly to obtain ink; Step 3: Place the ink into the embedded printing medium and print a three-dimensional structure using the DIW process. After printing, the wet three-dimensional structure is obtained by non-solvent-induced phase separation and curing. Step 4: Dry and carbonize the wet three-dimensional structure to obtain a three-dimensional hierarchical porous carbon framework; Step 5: Immerse the three-dimensional hierarchical porous carbon skeleton in an aqueous calcium chloride solution and perform heat treatment to allow calcium chloride to enter the pore network and solidify. Then, after rinsing and drying, a hierarchical porous carbon-calcium chloride composite thermal storage material is obtained.

4. The preparation method of the hierarchical porous calcium chloride composite thermal storage material according to claim 3, characterized in that, In step one, the ratio of multi-walled carbon nanotubes to the mixed solution of sulfuric acid and hydrogen peroxide is 0.5~1g:200~400mL; the volume ratio of sulfuric acid to hydrogen peroxide in the mixed solution is 3:1; and the stirring time at room temperature is 10~20h.

5. The preparation method of the hierarchical porous calcium chloride composite thermal storage material according to claim 3, characterized in that, In step two, the mass ratio of graphene oxide to carboxyl-functionalized multi-walled carbon nanotubes in the ink is 1:1 to 2:1; the mass ratio of thickener to the total mass of graphene oxide and carboxyl-functionalized multi-walled carbon nanotubes is 6 to 8:

1.

6. The method for preparing the graded porous calcium chloride composite thermal storage material according to claim 3, characterized in that, In step two, the thickener includes N , N -Dimethylpyrrolidone, polyethersulfone and polyvinylpyrrolidone; N , N The mass ratio of dimethylpyrrolidone, polyethersulfone, and polyvinylpyrrolidone is 10~20:2~5:1~2.

7. The preparation method of the hierarchical porous calcium chloride composite thermal storage material according to claim 3, characterized in that, In step three, the embedded printing medium is a liquid medium that enables non-solvent-induced phase separation, and the liquid medium that enables non-solvent-induced phase separation includes deionized water.

8. The method for preparing the hierarchical porous calcium chloride composite thermal storage material according to claim 3, characterized in that, In step four, the drying temperature is 60~120℃ and the drying time is 15~18h; the carbonization temperature is 650~750℃, the carbonization heating rate is 1~5℃ / min, and the carbonization time is 1~4h.

9. The preparation method of the hierarchical porous calcium chloride composite thermal storage material according to claim 3, characterized in that, In step five, the heat treatment temperature is 200~300℃ and the heat treatment time is 1~5h; the drying temperature is 100~200℃ and the drying time is 12~15h.

10. An application of the graded porous calcium chloride composite thermal storage material according to any one of claims 1-2, characterized in that, The graded porous calcium chloride composite thermal storage material is used in low-temperature thermochemical energy storage, solar thermal utilization, and thermal management systems.

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