Preparation method of electrostatic self-assembly carbon aerogel phase change material composite system based on shaddock peel

Abstract

The invention discloses a preparation method of a grapefruit peel-based electrostatic self-assembly carbon aerogel phase change material composite system, and overcomes the defects of low thermal conductivity, long-period instability and the like of a grapefruit peel-based composite phase change material. The invention provides the preparation method of the electrostatic self-assembled shaddock peel aerogel skeleton composite phase change material with enhanced heat conduction, high enthalpy value and long-period stability. Compared with the prior art, the shaddock peel-based aerogel composite phase change material prepared by the invention has high heat-conducting property, the heat conductivity of 1.18 Wm <-1 > k <-1 > is realized, and compared with a pure phase change material with the heat conductivity of 0.25 Wm <-1 > k <-1 >, the heat conductivity coefficient of the composite phase change material is increased by 3.48 times; the leakage loss rate of the phase change material is low, the performance is stable after 100 cycles, and the leakage rate is only 0.52%; the proportion of the carbon aerogel is low, and the proportion of the carbon aerogel in the composite phase change material system is 10-20% according to the reduction of the proportion of the carbon nanotubes.

Description

Technical Field

[0001] The present invention belongs to the field of thermal energy storage technology, and specifically relates to the technical field of constructing a grapefruit peel aerogel skeleton composite phase change material through electrostatic self-assembly to improve thermal performance, especially to the regulation of thermal conductivity and energy storage performance of phase change materials. Background Art

[0002] In order to deal with technical problems such as low thermal conductivity, easy leakage and poor cycle stability of pure phase change materials, researchers have developed a series of composite phase change materials by combining porous carbon materials, metal particles and other functional carriers with phase change materials to improve the thermal conductivity, morphological stability and cycle durability of phase change materials. Among them, the three-dimensional support network structure aerogel composite phase change material structure based on porous framework materials (such as porous carbon, metal organic framework, silica, etc.) can effectively prevent the leakage of phase change while improving the thermal conductivity of composite phase change materials, improve the stability of its overall performance, and improve the performance of phase change materials. However, existing porous aerogels still face many practical problems. For example, it is difficult to achieve high thermal conductivity and maximize energy storage performance in the regulation of the pore structure of traditional carbon materials; the framework material prepared by the traditional high-temperature carbonization aerogel method has been finalized and cannot achieve good structural matching with practical application containers of various shapes, which seriously restricts its large-scale production and application; high-performance materials such as metal organic frameworks are expensive, which is not conducive to environmental friendliness and commercial applications.

[0003] Porous carbon materials from biomass have the advantages of low cost, wide sources, renewable and environmentally friendly. Through pyrolysis and activation processes, agricultural and forestry wastes can be efficiently converted into porous carbon materials with high specific surface area and adjustable pore structure for application in the preparation of composite phase change materials, which has received widespread attention in recent years. Through pyrolysis and activation processes, agricultural and forestry wastes can be efficiently converted into porous carbon materials with high specific surface area and adjustable pore structure, which have the advantages of low cost, wide sources, renewable and environmentally friendly. Grapefruit peel, as a biological waste generated in the process of grapefruit processing, has almost no economic value, but the annual output is huge. According to statistics, the annual output of grapefruit in China alone is as high as 5 million tons, of which 40-50% is grapefruit peel.

[0004] Although existing research has been able to improve the loading capacity of grapefruit peel-based composite phase change materials, and has significant potential in thermal performance, stability and morphology retention, it still faces key technical bottlenecks. The primary problem is the contradiction between thermal conductivity and energy storage capacity: improving thermal conductivity requires increasing the proportion of thermally conductive skeletons, which will inevitably reduce the loading amount of phase change materials, thereby reducing the energy storage capacity of the system; and increasing the loading amount of phase change materials will weaken the continuity of the thermal conductive network, which not only reduces the thermal conductivity efficiency, but also increases the risk of leakage during the phase change process. Secondly, in practical applications, the thermal conductivity of existing materials is still insufficient, and there are stability problems during long-term cyclic use, which seriously restricts their engineering applications. Therefore, how to break through this technical difficulty of performance trade-off and achieve the coordinated optimization of thermal conductivity efficiency and energy storage capacity has become a key scientific problem that needs to be solved in this field. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the shortcomings of grapefruit peel-based composite phase change materials such as low thermal conductivity and long-term instability, and provide a preparation method of an electrostatically self-assembled grapefruit peel aerogel skeleton composite phase change material with enhanced thermal conductivity, high enthalpy value and long-term stability.

[0006] To achieve the above object, the technical solution of the present invention comprises the following steps:

[0007] The first object of the present invention is to provide a method for preparing a pomelo peel-based electrostatic self-assembled carbon aerogel phase change material composite system, comprising the following steps:

[0008] s1. Preparation of high specific surface area biochar: The white peel powder of the inner layer of grapefruit peel was carbonized at 450°C for 1 hour under a nitrogen atmosphere to obtain a primary carbonization product; the product was then pre-activated by stirring with potassium hydroxide in a mass ratio of 1:2 in a 0.1 mol / L potassium hydroxide solution for 12 hours, and after vacuum filtration and drying at 80°C for 24 hours, it was activated at 900°C for 2 hours under nitrogen protection; the activated product was washed with a hydrochloric acid solution until neutral, and the final activated porous biochar was obtained after drying.

[0009] S2. Preparation of hydrogel: Ultrasonic dispersion of activated biochar and 1% sodium dodecyl sulfate in deionized water for 1 hour to obtain biochar dispersion, and ultrasonic dispersion of amino carbon nanotubes and 1% polyvinyl pyrrolidone in aqueous solution for 30 minutes to obtain carbon nanotube dispersion; chitosan and polyacrylamide are dissolved in a 10:10 ratio in a glacial acetic acid solution with a pH value of 4 to 5 and stirred for 30 minutes to form a colloid; then the above two dispersions are gradually added to the chitosan polyacrylamide solution, and the temperature is strictly controlled below 30°C during the mixing process. Stir for 30 minutes to ensure that the materials are evenly mixed, and the biochar and carbon nanotubes achieve electrostatic self-adsorption and remain stable. After uniform mixing, the prepared hydrogel is inverted in the mold and placed in a 5-10°C environment for 12-24 hours for gelation.

[0010] S3. Preparation of carbon aerogel skeleton: The obtained hydrogel was frozen at -70°C for 5 hours, and then vacuum-freeze-dried for 24 hours to obtain carbon aerogel.

[0011] S4. Preparation of aerogel skeleton composite phase change material: In a vacuum environment of 0.08 MPa, immerse the carbon aerogel in the molten phase change material and maintain it for 30 minutes to ensure that the phase change material fully penetrates and fills the three-dimensional porous network structure of the carbon aerogel.

[0012] Furthermore, the purity of the amination carbon nanotubes is greater than 95%, and the degree of amination is 0.45 wt %.

[0013] Furthermore, the mass ratio of the activated biochar to the amino carbon nanotubes, chitosan and polyacrylamide is 55 to 80:0 to 25:10:10.

[0014] Furthermore, the viscosity of the chitosan is 100 to 200 mPa·s, the molecular weight of the polyacrylamide is 5,000,000, and the molecular weight of the polyvinyl pyrrolidone is 58,000.

[0015] Furthermore, the preparation method and particle size requirements of the white peel powder of the inner layer of the grapefruit peel are as follows: the inner layer of the white peel of the grapefruit peel is peeled off and dried, and then mechanically crushed and sieved to obtain a powder with a particle size of 26-74 μm (corresponding to 200-600 mesh) after sieving; the particle size distribution of the grapefruit peel powder is uniform, which is conducive to the formation of a uniform pore structure in the subsequent carbonization process.

[0016] Furthermore, the amount of the aminated carbon nanotubes added is 0 to 25% of the carbon aerogel. Preferably, the amount of the aminated carbon nanotubes added is 20% of the mass of the activated biochar.

[0017] Furthermore, the phase change material is an organic phase change material with a phase change temperature of 0-100°C. Including but not limited to organic phase change materials with phase change latent heat such as alkanes, fatty acids, alcohols, lipids, etc. Among them: 1) Paraffin phase change materials include normal alkanes, isoalkanes and mixtures thereof, with a phase change enthalpy range of 180-240 J / g; 2) Fatty acid phase change materials include lauric acid, myristic acid, palmitic acid, stearic acid and their ester derivatives, with a phase change enthalpy range of 150-200 J / g; 3) Alcohol phase change materials include normal alcohols, polyols and their derivatives, with a phase change enthalpy range of 120-180 J / g; 4) Lipid phase change materials include unsaturated fatty acids such as oleic acid and linoleic acid and their glycerides, with a phase change enthalpy range of 140-190 J / g.

[0018] Furthermore, the charge characteristics of each component in the electrostatic self-assembly process are as follows: the activated biochar is negatively charged after being modified by sodium dodecyl sulfate, and the amination carbon nanotubes are positively charged under acidic conditions with a pH value of 4-5; strong electrostatic attraction is generated between the negatively charged modified biochar and the positively charged amination carbon nanotubes, forming stable pores and effective carbon heat conduction paths.

[0019] Furthermore, the mold in step S2 is a container of any shape for macro-encapsulating aerogel composite phase change material.

[0020] Beneficial effects of the present invention:

[0021] Efficient utilization of biomass waste: The high-value utilization of waste grapefruit peels is achieved, the preparation process is simple, the cost is low, it is suitable for large-scale commercial applications, and it is in line with the concept of green and sustainable development. The activated biochar has a porous structure, which can absorb more phase change materials and effectively reduce the risk of leakage during the melting process.

[0022] Innovative electrostatic self-assembly mechanism: The electrostatic self-assembly strategy based on surface charge regulation proposed in the present invention can realize the directional self-assembly of surface negatively charged biochar and surface positively charged amino-modified carbon nanotubes in the gel solution to form a stable three-dimensional network structure.

[0023] Compared with the prior art, the grapefruit peel-based aerogel composite phase change material prepared by the present invention has high thermal conductivity and achieves 1.18Wm- 1 k- 1 The thermal conductivity is 0.25Wm- 1 k- 1 Pure phase change material, the thermal conductivity of the composite phase change material is increased by 3.48 times; the leakage rate of the phase change material is low, the performance is stable after 100 cycles, and the leakage rate is only 0.52%; the proportion of carbon aerogel is low, according to the reduction of the proportion of carbon nanotubes, the proportion of carbon aerogel in the composite phase change material system is 10-20%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 SEM comparison of biochar aerogel phase change materials, where Figure 1 (a) is the biochar image prepared in step S1, Figure 1 (b)-(d) are SEM comparison images of biochar aerogels of specific examples 1-3.

[0025] Figure 2 The leakage performance test comparison between Example 1 and pure paraffin is shown in FIG.

[0026] Figure 3 It is a comparison chart of thermal conductivity of specific embodiments 1-3 and pure paraffin. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the invention.

[0028] Example 1

[0029] S1 Preparation of high specific surface area biochar: The white peel powder of the inner layer of grapefruit peel was carbonized at 450℃ for 1 hour under a nitrogen atmosphere to obtain a primary carbonization product; the product was then pre-activated by stirring with potassium hydroxide in a mass ratio of 1:2 in a 0.1 mol / L potassium hydroxide solution for 12 hours, and after vacuum filtration and drying at 80℃ for 24 hours, it was activated at 900℃ for 2 hours under nitrogen protection; the activated product was washed with hydrochloric acid solution until neutral, and the final activated porous biochar was obtained after drying.

[0030] S2 Preparation of hydrogel: 1.2g activated biochar and 1% sodium dodecyl sulfate were ultrasonically dispersed in 12g deionized water for 1 hour, and 0.4g amino carbon nanotubes and 1% polyvinyl pyrrolidone were ultrasonically dispersed in 5g aqueous solution for 30 minutes; in a glacial acetic acid solution with a pH value of 4 to 5, chitosan and polyacrylamide were dissolved in a ratio of 0.2g:0.2g in 10g aqueous solution and stirred for 30 minutes to form a colloid; then the above two dispersions were gradually added to the chitosan polyacrylamide solution, and the temperature was strictly controlled below 30°C during the mixing process. Stir for 30 minutes to ensure that the materials are evenly mixed, and the biochar and carbon nanotubes achieve electrostatic self-adsorption and remain stable. After mixing evenly, place it in a 4°C environment and let it stand for 24 hours for gelation. In this embodiment, the amount of carbon nanotubes is 20%, and the biochar accounts for 60%.

[0031] S3 Preparation of carbon aerogel skeleton: The obtained hydrogel was placed in a -70°C freezer for 5 hours, and then vacuum freeze-dried for 24 hours to obtain a carbon aerogel.

[0032] S4 Preparation of aerogel skeleton composite phase change material: In a vacuum environment of 0.08MPa, immerse the carbon aerogel in the molten phase change material and keep it for 30 minutes to ensure that the phase change material fully penetrates and fills the three-dimensional porous network structure of the carbon aerogel, thereby obtaining the carbon aerogel skeleton phase change material.

[0032] Furthermore, in order to compare with Example 1, carbon aerogel composite phase change materials with different ratios were prepared for comparison. In the specific implementation, only step S2 was different. In the subsequent examples, only step S2 was changed, and the other steps remained unchanged, which will not be described in detail here.

[0034] Example 2

[0035] S2 Preparation of hydrogel: 1.4g activated biochar and 1% sodium dodecyl sulfate were ultrasonically dispersed in 14g deionized water for 1 hour, and 0.2g amino carbon nanotubes and 1% polyvinyl pyrrolidone were ultrasonically dispersed in 5g aqueous solution for 30 minutes; in a glacial acetic acid solution with a pH value of 4 to 5, chitosan and polyacrylamide were dissolved in a ratio of 0.2g:0.2g in 10g aqueous solution and stirred for 30 minutes to form a colloid; then the above two dispersions were gradually added to the chitosan polyacrylamide solution, and the temperature was strictly controlled below 30°C during the mixing process. Stir for 30 minutes to ensure that the materials are evenly mixed, and the biochar and carbon nanotubes achieve electrostatic self-adsorption and remain stable. After mixing evenly, place it in a 4°C environment and let it stand for 24 hours for gelation. In this embodiment, the amount of carbon nanotubes is 10%, and the biochar accounts for 70%.

[0036] Example 3

[0037] S2 Preparation of hydrogel: Ultrasonic dispersion of 1.6g activated biochar and 1% sodium dodecyl sulfate of activated biochar in 16g deionized water for 1 hour; In a glacial acetic acid solution with a pH value of 4 to 5, chitosan and polyacrylamide are dissolved in a ratio of 0.2g:0.2g in 10g and stirred for 30 minutes to form a colloid; Then gradually add the above biochar solution to the chitosan polyacrylamide solution, strictly control the temperature below 30°C during the mixing process, and stir for 30 minutes to ensure that the substances are evenly mixed. After mixing evenly, place it in a 4°C environment and let it stand for 12 hours for gelation. In this embodiment, the amount of carbon nanotubes is 0%, and the biochar accounts for 80%. This embodiment will be used as a comparative example to compare with Example 1 and Example 2.

[0038] 1. SEM characterization analysis

[0039] Figure 1 The SEM images of the carbon aerogels prepared in Example 1 and Example 2 are shown in FIG. Figure 1(a) is the characterization result of activated biochar obtained according to the preparation step S1. It can be seen that a large number of uniform mesopores are formed on the surface of biochar powder after KOH activation, which is conducive to the capillary adsorption of PCM; Figure 1 (b)-(d) are the SEM characterization results of biochar aerogels of Examples 1-3, respectively. Obviously, the electrostatic adsorption between carbon nanotubes and biochar can make the carbon aerogel form a more uniform three-dimensional network structure, which plays a key role in the formation of the overall thermal conductivity path. In Comparative Example 3, since no amino carbon nanotubes were added, the carbon aerogel without electrostatic self-assembly effect had a disordered pore structure and could not form an effective thermal conductivity path.

[0040] 2. Leakage performance of composite phase change materials

[0041] Figure 2 The leakage performance test of Example 1 and pure phase change material showed that after 1 hour of high-temperature baking, the pure paraffin completely melted within 25 minutes, while the aerogel phase change material of Example 1 had stable performance, with a leakage rate of only 0.18%, and no obvious change in appearance before and after heating. The 100-cycle test showed good repeatability of experimental data.

[0042] 3. Thermal conductivity of composite phase change materials

[0043] The thermal conductivity of the aerogel composite phase change materials of Examples 1-3 was tested, and the test results showed that the thermal conductivity of the composite aerogel phase change materials in Examples 1-3 was increased by 386% and 34.8% respectively compared with the pure paraffin material. This shows that the prepared electrostatic self-assembled aerogel can achieve an effective heat conduction path and improve the overall thermal conductivity of the composite phase change material. Since the electrostatic self-assembly of biochar and carbon nanotubes was not performed in Comparative Example 3, the thermal conductivity was limited and the thermal performance was poor.

[0044] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a composite system of electrostatically self-assembled carbon aerogel phase change material based on grapefruit peel, characterized in that: The following steps are involved: S1. Preparation of high specific surface area biochar: The white peel powder of the inner layer of grapefruit peel was carbonized at 450℃ for 1 hour under nitrogen atmosphere to obtain a primary carbonization product; the product was then stirred with potassium hydroxide in a 0.1 mol / L potassium hydroxide solution at a mass ratio of 1:2 for 12 hours for pre-activation, vacuum filtered, dried at 80℃ for 24 hours, and then activated at 900℃ for 2 hours under nitrogen protection; The activated product is washed with a hydrochloric acid solution until it becomes neutral, and then dried to obtain the final activated porous biochar; S2. Preparation of hydrogel: Ultrasonic dispersion of activated biochar and 1% sodium dodecyl sulfate in deionized water for 1 hour to obtain biochar dispersion, and ultrasonic dispersion of amino carbon nanotubes and 1% polyvinyl pyrrolidone in aqueous solution for 30 minutes to obtain carbon nanotube dispersion; in glacial acetic acid solution with a pH value of 4 to 5, dissolve chitosan and polyacrylamide in a 1:1 ratio and stir for 30 minutes to form a colloid; then gradually add the above two dispersions to the chitosan polyacrylamide solution, strictly control the temperature below 30°C during the mixing process, stir for 30 minutes to ensure uniform mixing of the substances, and electrostatic self-adsorption of biochar and carbon nanotubes and maintain stability. After uniform mixing, place in a 5-10°C environment and let stand for 12-24 hours for gelation; S3. Preparation of carbon aerogel skeleton: The obtained hydrogel was placed in a -70°C freezer for 5 hours, and then vacuum freeze-dried for 24 hours to obtain a carbon aerogel; S4. Preparation of composite phase change material: In a vacuum environment of 0.08 MPa, immerse the carbon aerogel into the molten phase change material and keep it for 30 minutes to ensure that the phase change material fully penetrates and fills the three-dimensional porous network structure of the carbon aerogel.

2. The method for preparing the pomelo peel-based electrostatic self-assembled carbon aerogel phase change material composite system according to claim 1, characterized in that: The purity of the amino-modified carbon nanotubes is greater than 95%, and the degree of amino-modification is 0.45 wt %.

3. The method for preparing the pomelo peel-based electrostatic self-assembled carbon aerogel phase change material composite system according to claim 1, characterized in that: The mass ratio of the activated biochar to the amino carbon nanotubes, chitosan and polyacrylamide is 55-80:0-25:10:

10.

4. The method for preparing the pomelo peel-based electrostatic self-assembled carbon aerogel phase change material composite system according to claim 1, characterized in that: The viscosity of the chitosan is 100 to 200 mPa·s, the molecular weight of the polyacrylamide is 5,000,000, and the molecular weight of the polyvinyl pyrrolidone is 58,000.

5. The method for preparing the pomelo peel-based electrostatic self-assembled carbon aerogel phase change material composite system according to claim 1, characterized in that: The amount of the amino-modified carbon nanotubes added is 0 to 25% of the mass of the activated biochar.

6. The method for preparing the pomelo peel-based electrostatic self-assembled carbon aerogel phase change material composite system according to claim 1, characterized in that: The phase change material is an organic phase change material with a phase change temperature of 0-100°C.

7. An electrostatic self-assembled carbon aerogel / phase change material composite system based on grapefruit peel, characterized in that: An electrostatic self-assembled carbon aerogel / phase change material composite system based on grapefruit peel prepared according to the method according to any one of claims 1 to 5.

Images