1. The phase change energy storage composite material is characterized by comprising porous carbon foam and a phase change material, wherein the porous carbon foam has a layered cavity structure, and a multi-stage porous structure is arranged in a carbon layer; and the phase change material is adsorbed and encapsulated by the porous carbon foam.

2. The phase change energy storage composite material as claimed in claim 1, wherein the porous carbon foam has a lamellar cavity with a pore size of 50-600 um;

further, the hierarchical porous structure refers to a hierarchical porous structure containing micropores, mesopores and macropores in the carbon layer.

3. The phase change energy storage composite according to claim 1 or 2, wherein the phase change material is a solid-liquid phase change material, preferably at least one of polyethylene glycol, fatty acid and eutectic thereof, fatty acid ester, polyol or sliced paraffin;

further, the porous carbon foam is prepared by adopting the following method: firstly, preparing polyacrylonitrile hydrogel, then freeze-drying the hydrogel to form polyacrylonitrile aerogel with a porous framework, and finally cyclizing and carbonizing the obtained aerogel to prepare the porous carbon foam.

4. The phase change energy storage composite material according to claim 3, wherein the polyacrylonitrile hydrogel is prepared by the following method: firstly, introducing a hydrophilic monomer, a cross-linking agent, an initiator and an inorganic salt aqueous solution into acrylonitrile to initiate the polymerization of the monomer and cross-link to prepare polyacrylonitrile hydrogel;

further, in the method for preparing the polyacrylonitrile hydrogel, the hydrophilic monomer is at least one of 2-acrylamide-2 methyl-1-propane sulfonic acid, acrylamide or acrylic acid; 2-acrylamido-2-methyl-1-propanesulfonic acid is preferred;

the inorganic salt water solution is a zinc chloride solution;

the cross-linking agent is selected from: n, N' -methylenebisacrylamide or polyethylene glycol dimethacrylate;

the initiator is selected from 2-hydroxy-2-methyl propiophenone or phenyl bis (2, 4, 6-trimethyl benzoyl) phosphine oxide.

5. The phase change energy storage composite material according to claim 3 or 4, wherein the mass ratio of acrylonitrile to the hydrophilic monomer is: 9: 1-1.5: 1;

further, the content of the cross-linking agent is 0.2-0.8% of the total mass of the acrylonitrile and the hydrophilic monomer, and the content of the initiator is 1% of the total mass of the monomer;

further, the mass ratio of the acrylonitrile monomer to the inorganic salt aqueous solution is 7-10%, and the concentration of the inorganic salt aqueous solution is 50-55 wt%.

6. The phase change energy storage composite according to any one of claims 3 to 5,

the polyacrylonitrile aerogel is prepared by the following method: fully soaking the polyacrylonitrile hydrogel in deionized water, and freeze-drying to obtain polyacrylonitrile aerogel after inorganic salts in the gel are completely soaked;

further, the method for pre-oxidation cyclization comprises the following steps: oxidizing and cyclizing the freeze-dried polyacrylonitrile aerogel at the temperature of 200-300 ℃ for 3-6 hours;

further, the carbonization method comprises the following steps: and (3) performing pyrolysis on the polyacrylonitrile aerogel at the high temperature of 700-800 ℃ for 1-3 hours after cyclization.

7. A preparation method of the phase change energy storage composite material as claimed in any one of claims 1 to 6, characterized in that the method comprises the following steps: and completely soaking the porous carbon foam into the phase-change material.

8. A preparation method of porous carbon foam is characterized by comprising the following steps: firstly, introducing a hydrophilic monomer, a cross-linking agent, an initiator and an inorganic salt aqueous solution into acrylonitrile to initiate the polymerization of the monomer and cross-link to prepare polyacrylonitrile hydrogel; then placing the obtained hydrogel in deionized water for fully soaking, and freezing and drying to obtain polyacrylonitrile aerogel after inorganic salts in the gel are completely soaked; finally, cyclizing and carbonizing the freeze-dried polyacrylonitrile aerogel to obtain black porous carbon foam;

further, the hydrophilic monomer is at least one of 2-acrylamide-2 methyl-1-propane sulfonic acid, acrylamide or acrylic acid; 2-acrylamido-2-methyl-1-propanesulfonic acid is preferred; the inorganic salt water solution is a zinc chloride solution; the cross-linking agent is selected from: n, N' -methylenebisacrylamide or polyethylene glycol dimethacrylate; the initiator is selected from 2-hydroxy-2-methyl propiophenone or phenyl bis (2, 4, 6-trimethyl benzoyl) phosphine oxide.

9. A porous carbon foam, characterized in that it is produced by the method of claim 8;

further, the porous carbon foam has a layered cavity structure, and forms a microporous, mesoporous and macroporous multilevel porous structure in the carbon layer;

further, the aperture of the layered cavity in the porous carbon foam is 50-600 um.

10. The porous carbon foam is used for preparing a phase change energy storage material, and the porous carbon foam is used for adsorbing and encapsulating the phase change material to prepare the phase change energy storage material; characterized in that the porous carbon foam is produced by the method according to claim 8 or is a porous carbon foam according to claim 9.

Background

With the rapid development of social industrialization and life facilitation, petrochemical resources are continuously consumed, environmental pollution and global warming are continuously aggravated, and energy problems and environmental problems gradually become key factors restricting future social development. Therefore, the development of renewable energy sources and the solution of the contradiction between energy supply and demand through the heat storage technology become important approaches for solving the energy and environmental problems. The phase change energy storage materials (PCMs) have biological phase change under certain conditions, can realize effective storage and release of energy, and become an important material basis in the research of energy storage technology. The phase change modes of PCMs mainly comprise three forms of solid-liquid, liquid-gas and solid-solid phase change, wherein the solid-liquid phase change energy storage material is most widely researched and applied due to relatively low phase change temperature and relatively stable performance, and the working principle is as follows: when the ambient temperature is higher than the phase transition temperature, the material is converted from a solid state into a liquid state and absorbs heat; and when the ambient temperature is lower than the phase change point, the material is transformed from a liquid state to a solid state to release heat, so that the ambient temperature is maintained at a proper level. Solid-liquid phase change materials currently in wide use include: inorganic water and salts, organic paraffin and polyethylene glycol, and the like. The main drawback of solid-liquid phase change materials in applications is the problem of easy leakage in the molten state, and therefore the prevention of leakage by physical or chemical encapsulation is a technical key.

Polyethylene glycol (PEG) is used as a high-molecular solid-liquid phase change material, and has certain advantages in the aspect of preparing high-performance multifunctional composite PCMs. However, PEG has the disadvantages of melt leakage, low thermal conductivity, electrical insulation, weak light absorption, etc., which limits its wide application and development in energy storage technology. The effective method for solving the problem of melt leakage mainly comprises the following steps: 1) converting the solid-liquid phase change material into a solid-solid phase change material by using a physical or chemical method; 2) physical encapsulation methods include both closed and non-closed types. The closed type is represented by a microcapsule method, a layer of polymer film (such as EP, PS, PU and the like) is wrapped on the surface of the solid-liquid organic PCM to form a microcapsule phase change material, so that leakage volatilization can be prevented, toxicity is reduced, and the large-surface-area and small-volume microcapsules can increase contact during heat exchange and improve heat conductivity; the non-closed type includes surface grafting, intercalation, fusion infiltration, porous adsorption and other methods. The porous adsorption is to use a light material with a micropore/mesopore structure, a large specific surface area and high thermal conductivity as an adsorption carrier, and encapsulate the organic PCM therein by utilizing capillary force, and when the external temperature rises and phase change occurs, the liquid PCM is encapsulated in the pores under the action of the capillary force, thereby avoiding the leakage problem of the phase change material. In addition, the carrier with high thermal conductivity and internal porosity and network crosslinking is adopted, so that the thermal conductivity of the composite phase change material can be increased, and the thermal conversion efficiency is improved. Common porous carriers are expanded vermiculite, expanded graphite, activated carbon, diatomaceous earth, and the like. At present, the adsorption and encapsulation of the porous material on the phase-change material mainly focuses on the capillary adsorption effect of the porous material on the phase-change material, however, the structural modification of the porous material, which is beneficial to encapsulating the phase-change energy storage material, and the synergistic regulation and control of the chemical effect and the physical adsorption effect of the porous material and the phase-change material still need to be researched systematically.

In addition, polyacrylonitrile is an important carbon material precursor, and compared with other carbon precursor materials, polyacrylonitrile has high thermal stability, high carbon residue rate and is rich in N element, and thus has attracted much attention in the research of porous carbon electrodes and functional porous carbon materials. The method for preparing carbon foam from polyacrylonitrile mainly applies emulsion polymerization or a method of reversed-phase solution precipitation to prepare polyacrylonitrile microspheres at present, and then prepares porous carbon through high-temperature carbonization. However, the methods have the disadvantages of complex preparation process, high cost, high density of the prepared carbon foam, and low adjustability of the carbonization process and components of the porous carbon. Therefore, the application of polyacrylonitrile carbon foam in the field of phase change energy storage materials is limited.

In the prior art, no relevant report that a porous carbon foam material with a specific structure prepared from polyacrylonitrile is used for preparing a phase change energy storage material exists.

Disclosure of Invention

The invention aims to provide a multifunctional phase change energy storage composite material with stable shape and photo-thermal and electric-thermal conversion, which is prepared by preparing polyacrylonitrile copolymer hydrogel through high-temperature carbonization, and adsorbing and encapsulating a phase change material by using a multistage porous carbon material with adjustable chemical composition and graphitization degree. The phase change energy storage material prepared by the invention has high encapsulation efficiency (the loading capacity of PEG is about 6 times of the mass of carbon foam), latent heat is up to 160kJ/mol, and the energy storage efficiency is up to 100%.

The technical scheme of the invention is as follows:

the first technical problem to be solved by the invention is to provide a phase change energy storage composite material, which comprises porous carbon foam and a phase change material, wherein the porous carbon foam has a layered cavity structure, and a multi-stage porous structure is arranged in a carbon layer; and the phase change material is adsorbed and encapsulated by the porous carbon foam.

Further, the aperture of the layered cavity in the porous carbon foam is 50-600 um.

Further, the hierarchical porous structure refers to a hierarchical porous structure in which micropores (with a pore diameter of less than 2nm), mesopores (5-50 nm) and macropores (with a pore diameter of more than 100nm) are contained in the carbon layer.

Further, the phase change material is a solid-liquid phase change material, preferably at least one of polyethylene glycol, fatty acid and eutectic mixture thereof, fatty acid ester, polyhydric alcohol or sliced paraffin.

Further, the porous carbon foam is prepared by adopting the following method: firstly, preparing polyacrylonitrile hydrogel, then freeze-drying the hydrogel to form polyacrylonitrile aerogel with a porous framework, and finally cyclizing and carbonizing the obtained aerogel to prepare the porous carbon foam.

Further, the polyacrylonitrile hydrogel is prepared by the following method: hydrophilic monomer, cross-linking agent, initiator and inorganic salt water solution are first introduced into acrylonitrile to initiate the polymerization of monomer and cross-linking to prepare polyacrylonitrile hydrogel.

Further, in the method for preparing the polyacrylonitrile hydrogel, the hydrophilic monomer is at least one of 2-acrylamide-2 methyl-1-propane sulfonic acid, acrylamide or acrylic acid; 2-acrylamido-2-methyl-1-propanesulfonic acid is preferred; the hydrophilic structural unit with amido bond is helpful for the oxidation cyclization process of polyacrylonitrile;

the inorganic salt water solution is a zinc chloride solution;

the cross-linking agent is selected from: n, N' -methylenebisacrylamide or polyethylene glycol dimethacrylate;

the initiator is selected from 2-hydroxy-2-methyl propiophenone (1173) or phenyl bis (2, 4, 6-trimethylbenzoyl) phosphine oxide (819).

Further, the mass ratio of acrylonitrile to the hydrophilic monomer is as follows: 9: 1-1.5: 1. in the monomer formula, the content of polyacrylonitrile is greater than that of a hydrophilic monomer, the content of the hydrophilic monomer can regulate and control the hydrophilicity of the hydrogel, but too high content of the hydrophilic monomer can reduce the carbon formation rate and the mechanical strength of a carbon foam framework; therefore, the high polyacrylonitrile content can ensure high carbon formation rate and stability of the carbon foam framework; the crosslinker content can also adjust the swelling ratio of the hydrogel, thereby adjusting the porous structure of the aerogel and carbon foam.

Further, the content of the cross-linking agent is 0.2-0.8% of the total mass of the acrylonitrile and the hydrophilic monomer, and the content of the initiator is 1% of the total mass of the monomer. The content of the cross-linking agent can influence the network density of the hydrogel, and further influence the swelling degree, and the concentration of the cross-linking agent can ensure that the hydrogel has proper strength and proper swelling behavior.

Further, the mass ratio of the acrylonitrile monomer to the inorganic salt aqueous solution is 7-10%, and the concentration of the inorganic salt aqueous solution is 50-55 wt%.

Furthermore, in the method for preparing the polyacrylonitrile hydrogel by polymerization and crosslinking of the initiation monomer, initiation modes such as photo initiation, thermal initiation and the like can be adopted.

Further, the polyacrylonitrile aerogel is prepared by adopting the following method: and (3) fully soaking the obtained polyacrylonitrile hydrogel in deionized water, and freezing and drying to obtain the polyacrylonitrile aerogel after inorganic salts in the gel are completely soaked.

Further, in the above method for producing a porous carbon foam, the method for cyclizing is: and oxidizing and cyclizing the freeze-dried polyacrylonitrile aerogel at the temperature of 200-300 ℃ for 3-6 hours.

Further, in the above method for producing a porous carbon foam, the carbonization method is: and (3) performing pyrolysis on the polyacrylonitrile aerogel at the high temperature of 700-800 ℃ for 1-3 hours after cyclization.

The second technical problem to be solved by the present invention is to provide a method for preparing the phase change energy storage composite material, wherein the method comprises: and completely soaking the porous carbon foam into the phase-change material.

The third technical problem to be solved by the invention is to provide a preparation method of porous carbon foam, which comprises the following steps: firstly, introducing a hydrophilic monomer, a cross-linking agent, an initiator and an inorganic salt aqueous solution into acrylonitrile to initiate the polymerization of the monomer and cross-link to prepare polyacrylonitrile hydrogel; then placing the obtained hydrogel in deionized water for fully soaking, and freezing and drying to obtain polyacrylonitrile aerogel after inorganic salts in the gel are completely soaked; and finally, cyclizing and carbonizing the freeze-dried polyacrylonitrile aerogel to obtain the black porous carbon foam. The porous carbon material reserves a 50-600 um layered macroporous structure left after hydrogel freeze-drying, a hierarchical porous structure is arranged in the layer, and Raman spectrum proves that the porous carbon material has a graphitized structure in a certain proportion.

Further, the cyclization method comprises the following steps: and oxidizing and cyclizing the freeze-dried polyacrylonitrile aerogel at the temperature of 200-300 ℃ for 3-6 hours.

Further, the carbonization method comprises the following steps: and (3) performing pyrolysis on the polyacrylonitrile aerogel at the high temperature of 700-800 ℃ for 1-3 hours after cyclization.

The fourth technical problem to be solved by the present invention is to provide a porous carbon foam which is produced by the above method.

Further, the porous carbon foam has a layered cavity structure, and a microporous, mesoporous, macroporous multilevel porous structure is formed in the carbon layer.

Further, the aperture of the layered cavity in the porous carbon foam is 50-600 um.

The fifth technical problem to be solved by the invention is to provide the application of the porous carbon foam in the preparation of the phase change energy storage material, namely, the phase change energy storage material is prepared by encapsulating the phase change material with the obtained porous carbon foam.

The invention has the beneficial effects that:

1. the carbon foam and the phase-change composite material thereof prepared by the invention have the characteristics of environmental protection, the precursor hydrogel is prepared by taking water as a solvent, and the carbon foam prepared by carbonization after freeze-drying has a multi-level pore structure and high porosity.

2. The carbon foam and the phase-change composite material thereof prepared by the invention have adjustability of elements and pore structures, and the element composition and the porous structure of the carbon foam can be effectively regulated and controlled by changing the content of the comonomer and regulating the carbonization temperature.

3. The phase change composite material prepared by the invention has higher phase change material (such as PEG) load, the PEG load is 6 times of the self mass of the carbon foam, the heat conductivity is 12 times of that of the PEG, and the melting enthalpy is>160J g^-1Enthalpy of crystallization>150Jg^-1The melting point is about 53 ℃ and the crystallization temperature is about 40 ℃.

4. The phase-change composite material prepared by the invention has stable thermal cycle performance, and the enthalpy value and the characteristic phase-change temperature of the phase-change composite material are basically unchanged after 300 times of thermal cycle.

5. Compared with the existing preparation method, the preparation method for the polyacrylonitrile hydrogel precursor by one-step photopolymerization simplifies the test steps, has simple operation process, good controllability and reproducibility, does not need any expensive equipment or complex chemical treatment process, and has good application prospect.

6. The invention adopts equal-simple water as a precursor solvent, and has the characteristics of safety, environmental protection and the like; the phase-change composite material such as PEG has the characteristics of low price, stability, good compatibility with porous carbon foam and the like.

7. The phase change energy storage material prepared by the invention has good shape stability, no leakage in melting, and photo-thermal and electric-thermal conversion performance brought by the porous carbon skeleton with high heat conductivity, and can be applied to heat storage equipment.

Description of the drawings:

FIG. 1: FT-IR chart of polyacrylonitrile aerogel sample prepared in example 1 of the present invention.

FIG. 2: scanning electron microscope photographs of polyacrylonitrile aerogel samples prepared in example 1 of the present invention.

FIG. 3: scanning electron micrographs of the layered macroporous cavities of the porous carbon foam prepared in example 1 of the invention.

FIG. 4: FIG. 3 is a scanning electron microscope photomicrograph showing an enlargement of the layered macroporous cavity structure of the porous carbon foam.

FIG. 5: scanning electron microscope photographs of the inner pore structure of the porous carbon foam prepared in example 1 of the present invention.

FIG. 6: raman spectrum of the porous carbon foam prepared in example 1 of the present invention.

FIG. 7: XPS spectra of the porous carbon foam prepared in example 1 of the present invention.

FIG. 8: the scanning electron microscope picture of the polyethylene glycol/porous carbon composite phase change material prepared in the embodiment 1 of the invention.

FIG. 9: FT-IR diagram of the polyethylene glycol/porous carbon composite phase change material and pure PEG prepared in the embodiment 1 of the invention.

FIG. 10: the X-ray diffraction pattern of the polyethylene glycol/porous carbon composite phase change material, pure PEG and porous carbon prepared in the embodiment 1 of the invention.

FIG. 11: the thermogravimetric curves of the porous carbon, the polyethylene glycol/porous carbon composite phase change material and the polyethylene glycol prepared in the embodiment 1 of the invention are shown in the specification.

FIG. 12: the thermal conductivity comparison graph of the polyethylene glycol/porous carbon composite phase change material prepared in the embodiment 1 of the invention and polyethylene glycol.

FIG. 13: the DSC rise/fall temperature curves of the polyethylene glycol/porous carbon composite phase change material, the polyethylene glycol and the porous carbon prepared in the embodiment 1 of the invention are shown in the specification.

FIG. 14: fig. 14a, 14b and 14c are DSC temperature rise/fall curves of the polyethylene glycol/porous carbon composite phase change material prepared in example 1 of the present invention before cycling, 100 times of cold/hot cycling and 300 times of cold/hot cycling, respectively.

FIG. 15: the shape change and leakage condition photos of the polyethylene glycol/porous carbon composite phase change material and pure PEG prepared in the embodiment 1 of the invention in the heating process of a heating table are shown.

FIG. 16: the time-temperature curve of the polyethylene glycol/porous carbon composite phase change material prepared in the embodiment 1 of the invention under simulated sunlight irradiation.

FIG. 17: the time-temperature curve of the polyethylene glycol/porous carbon composite phase change material prepared in the embodiment 1 of the invention under an applied voltage of 2.5-5V is shown.

FIG. 18: scanning electron microscope photographs of the intralayer pore structure of the porous carbon foam HPC2 prepared in example 2 of the present invention.

FIG. 19: the DSC rise/fall temperature curve of the polyethylene glycol/porous carbon composite phase change material prepared in the embodiment 2-7 of the invention.

Detailed Description

The multifunctional phase change energy storage material is prepared by encapsulating the phase change material by the carbon foam with a multi-stage porous structure and a layered cavity structure, the prepared porous carbon has a layered cavity structure formed in a hydrogel freeze-drying process besides a conventional porous structure, and contains polar oxygen-containing and nitrogen-containing groups, so that the material has high impregnation efficiency and circulation stability to phase change materials such as PEG and the like; the prepared phase change energy storage material has high thermal conductivity, so the phase change energy storage material has low supercooling degree, good thermal conversion efficiency and good photo-thermal and electric heat conversion capability.

In the preparation method of the porous carbon foam, an organic solvent is not required to be added, so that the preparation method is green and environment-friendly; and a small amount of the AMPS can be added as a comonomer, the amide bond of the AMPS is beneficial to the cyclization process of polyacrylonitrile, and the cyclization temperature is reduced; the obtained porous carbon has a hierarchical porous structure, the pore structure can be regulated and controlled through the water content of the initial hydrogel, and the residual functional groups such as C-N, C ═ O and the like of the carbonized product can be regulated through the content of the comonomer and the carbonization temperature; these polar functional groups have favorable chemisorption effects on phase change materials such as PEG.

The invention prepares the light foamy carbon with a hierarchical porous structure by green synthesis of polyacrylonitrile hydrogel and simple and easy high-temperature carbonization, and further prepares the multifunctional driving phase-change composite material with stable shape by a simple and easy melt impregnation method, and the invention has important potential application value in the fields of environmental waste heat treatment, development and application of renewable clean energy and the like.

The following examples are given to further illustrate the embodiments of the present invention and are not intended to limit the scope of the present invention.

The preparation method of the carbon foam and the photothermal conversion phase change energy storage material thereof by using the polyacrylonitrile hydrogel precursor is further described below with reference to the embodiment and the accompanying drawings.

Example 1

(1) Polyacrylonitrile (PAN) hydrogel is prepared at room temperature by a photo-initiation one-step in-situ polymerization method. Acrylonitrile (AN) has poor solubility in water due to the strong dipolar interaction of CN-CN. Therefore, we chose to synthesize PAN hydrogels in aqueous zinc chloride. First, 10g ZnCl was added to 8g deionized water₂Magnetically stirring at room temperature for 10 min to obtain ZnCl₂An aqueous solution. Then 1.6g of AN, 0.4g of the hydrophilic monomer 2-acrylamido-2-methylpropanesulfonic Acid (AMPS) and 0.008g of the crosslinker Bis are dissolved in ZnCl₂In aqueous solution. Then 0.02g of photoinitiator 1173 is added to the mixture. After stirring for 2 minutes, the solution was transferred to a glass mould and finally set at an intensity of 36mw cm^-2Polymerizing for 4 hours under the ultraviolet light (365nm) to obtain PAN-ZnCl₂A hydrogel. ZnCl is substituted by solvent₂The solution was replaced into deionized water to restore the CN-CN dipolar interaction to give PAN hydrogel. Adding PAN-ZnCl₂The hydrogel was soaked in deionized water for 3 days with water changes every 4 hours. Then freezing at-20 deg.C overnight, and freeze-drying for 2 days to obtain PAN aerogel as precursor of foam carbon. FIG. 1 is a FT-IR plot of PAN gel, in which 2244cm^-1、1453cm^-1The characteristic absorption peaks of nitrile and methylene groups of PAN, respectively, indicate the successful preparation of PAN gel. Figure 2 is a scanning electron micrograph of PAN gel demonstrating the layered and fibrous matrix and the porous structure.

(2) Similar to the preparation process of carbon fiber, PAN gel is prepared at 240 deg.C for 2 deg.C min^-1Is pre-oxidized for 5 hours at a temperature rise rate of 800 ℃ and then is heated for 10 ℃ min^-1Is disclosedThe porous carbon foam (porous carbon material, noted HPC) was prepared by carbonization at a warm rate for 1 hour. It was confirmed by scanning electron microscopy to have a layered macroporous cavity, an intralayer hierarchical pore structure, as shown in fig. 3-5. Fig. 6 is a raman spectrum showing that the porous carbon has some degree of graphitization. FIG. 7 is an X-ray photoelectron spectrum characterizing the elemental composition of carbon foam, which contains a certain amount of N, O elements in addition to carbon elements. The porous carbon material has good adsorption and encapsulation capacity on the phase change energy storage material PEG due to the capillary adsorption effect of the multi-stage porous structure and the adsorption effect of polar groups such as C-N and C-O.

(3) The PEG/HPC composite phase-change material is prepared by a vacuum melting impregnation method. The HPC support was immersed in molten PEG for 2 days in a vacuum oven at 60 ℃. And then placing the PEG/HPC composite phase-change material on filter paper, and removing residual PEG on the surface at 60 ℃ to obtain the final composite phase-change material.

The PEG/HPC composite phase change material prepared in example 1 confirmed by the scanning electron microscopy experiment of fig. 8 that polyethylene glycol infiltrated the layered cavities and internal pores filled with the porous carbon support. In the FT-IR chart of FIG. 9, 3450, 1279 and 1240cm^-1The peak is a characteristic peak of-OH, 2882 and 1466cm^-1Corresponds to-CH₂Characteristic peak of (1), 1146, 1095, 1059cm^-1Is a characteristic peak of-C-O-C-, wherein 1095cm of the composite material is compared with pure PEG^-1The peak is shifted to 1097cm^-1Here, it is indicated that there is some interaction. FIG. 10 shows the XRD orientation of PEG, HPC, and composite material, wherein the two main peaks of PEG are located at 19.2 deg. and 23.4 deg. respectively representing the (120) crystal plane and (032) crystal plane; the XRD profile of HPC shows two diffraction peaks at 25.7 ° and 43.8 °, corresponding to the diffraction plane indices (002) and (101), respectively, indicating a degree of graphitization of the porous carbon. The main peak position of PEG is almost unchanged on the XRD curve of PEG/HPC, which shows that the existence of HPC hardly influences the crystal structure of PEG. According to weighing and thermogravimetric analysis (TGA, figure 11), the content of PEG in the composite phase-change material is up to 85 wt%, namely the loading amount of PEG is up to 6 times of the mass of the carbon foam, and the composite material has high phase-change energy storage capacity due to the high loading amount.

Examples1, the thermal conductivity of the composite phase change material prepared showed a 10-fold increase in thermal conductivity of the composite material after the introduction of HPC compared to pure PEG, as shown in fig. 12. The melting enthalpy of the PEG/HPC composite material analyzed by the DSC curve of FIG. 13 is 161.7J g^-1Enthalpy of crystallization 151.3J g^-1And has high energy storage density. The high loading capacity and excellent thermal conductivity enable the composite material to have high energy storage density and low supercooling degree. After 100 and 300 times of cold/hot cycles, DSC curves are basically overlapped (figure 14), which shows that the composite phase change material has good thermal cycle stability. The PEG, PEG/HPC composite phase-change material is placed on filter paper, and the shape stability is observed by heating (figure 15), the PEG starts to melt when being heated to 40 ℃, is completely melted when being heated to 80 ℃, and flows and diffuses on the filter paper; and the PEG/HPC composite phase-change material has no PEG fusion leakage in the whole heating process and has good shape stability. FIG. 16 under the irradiation of simulated solar light source, the temperature of PEG/HPC composite phase-change material rises rapidly to-45 ℃ and a melting platform appears to realize the storage of energy; the light source was turned off, the temperature dropped rapidly to-45 ℃ and a crystallization plateau appeared to achieve energy release. Pure PEG has only a slow and slight change in temperature in the whole process, and energy conversion and storage do not occur. As shown in FIG. 17, after a voltage of 2.5-5.0V is applied to the PEG/HP composite phase-change material, the temperature of the composite material is rapidly increased; after the voltage is turned off, the temperature drops rapidly, and melting and crystallization temperature platforms appear respectively, which shows that the material has good electrothermal conversion performance. Therefore, the PEG/HPC composite phase change material can realize rapid energy conversion, storage and release and has good photo-thermal and electro-thermal conversion performance.

Examples 2 to 7

(1) 10g ZnCl was added to 8g deionized water₂Magnetically stirring at room temperature for 10 min to obtain ZnCl₂An aqueous solution. Then dissolving a series of AN, hydrophilic monomer 2-acrylamide-2-methylpropanesulfonic Acid (AMPS) and cross-linking agent Bis in ZnCl at different ratios₂In aqueous solution (the specific ratio is shown in table 1). Then 0.02g of photoinitiator 1173 is added to the mixture. After stirring for 2 minutes, the solution was transferred toIn a glass mold, finally at a strength of 36mw cm^-2Polymerizing for 4 hours under the ultraviolet light (365nm) to obtain PAN-ZnCl₂A hydrogel. ZnCl is substituted by solvent₂The solution was replaced into deionized water to restore the CN-CN dipolar interaction to give PAN hydrogel. Adding PAN-ZnCl₂The hydrogel was soaked in deionized water for 3 days with water changes every 4 hours. Then freezing at-20 deg.C overnight, and freeze-drying for 2 days to obtain PAN aerogel as precursor of foam carbon.

TABLE 1 monomer ratio for the Synthesis of Polyacrylonitrile hydrogels

(2) Similar to the preparation process of carbon fiber, PAN gel is prepared at 240 deg.C for 2 deg.C min^-1Is pre-oxidized for 5 hours at a temperature rise rate of 800 ℃ and then is heated for 10 ℃ min^-1The temperature rise rate of (3) was carbonized for 1 hour to prepare a hierarchical porous carbon material (HPC 2-7). By comparing the scanning electron microscope images of HPC2 (fig. 18) and example 1, it can be concluded that different monomer ratios affect the micro-pore structure of HPC. With the increase of the content of the AN monomer, the pores are more compact, the number of the communicating holes is increased, and the PEG adsorption and crystallization are facilitated.

(3) The PEG/HPC composite phase-change material is prepared by adopting a vacuum melting impregnation method: the HPC support was immersed in molten PEG for 2 days in a vacuum oven at 60 ℃. And then placing the PEG/HPC composite phase-change material on filter paper, and removing residual PEG on the surface at 60 ℃ to obtain the final composite phase-change material. The DSC curves of FIG. 19 compare thermal parameters of PEG/HPC1-7, such as melting point, crystallization temperature, supercooling degree, and enthalpy of phase change. Comparing the DSC test results of PEG/HPC1-4, it can be seen that, when the content of the cross-linking agent is constant, the supercooling degree of the composite phase-change material decreases and then increases, the phase-change enthalpy increases and then decreases with the increase of the AN monomer content, and the thermal performance of the composite material of example 1 reaches AN extreme value. This can be attributed to the change of the pore structure and the content of C ═ C bonds, the more dense the pore structure is, the more interconnected pores are, the more favorable the storage and phase transition of PEG; meanwhile, the more C ═ C bonds, the better the thermal conductivity, the lower the degree of supercooling, and the faster the melting and crystallization rates. Analysis of the DSC results of PEG/HPC4-7 shows that varying the amount of cross-linking agent at a given monomer level also affects the thermal properties of the material. The cross-linking agent also affects the thermal properties of the phase change material, since it affects the pore structure of PAN-gel, and thus the layered cavity and multilevel pore structure within the HPC.