Graphene / lithium salt eutectic composite phase change material as well as preparation method and application thereof

Abstract

The invention discloses a graphene / lithium salt eutectic composite phase change material as well as a preparation method and application thereof. The material is prepared by the following steps: mixing lithium hydroxide and lithium carbonate, heating, melting and preserving heat, quickly cooling by liquid nitrogen, and refining particles to obtain eutectic salt; the preparation method comprises the following steps: dissolving hexadecyl trimethyl ammonium bromide in water, adding graphene, carrying out ultrasonic treatment to obtain a few-layer graphene dispersion liquid, and adjusting the pH value of the few-layer graphene dispersion liquid; mixing eutectic salt and the pH-adjusted few-layer graphene dispersion liquid, adjusting the pH, continuously stirring to form sol, adding pre-hydrolyzed tetraethoxysilane into the sol, adjusting the pH, continuously stirring until the sol is uniformly milky white, stopping stirring, and standing to form gel; and directionally freezing the gel liquid nitrogen, and then performing vacuum freeze drying, calcining, ultrasonic cleaning and vacuum heat treatment to obtain the composite material. The material has the advantages of high heat conductivity, leakage resistance and strong light absorption, and is applied to a heat storage medium of a heat absorber, a high-temperature waste gas waste heat recovery system of an iron and steel plant and an energy storage module of a building outer wall or a roof.

Description

Technical Field

[0001] The present invention belongs to the technical field of molten salt phase change energy storage materials, and specifically relates to a graphene / lithium salt eutectic composite phase change material and a preparation method and application thereof. Background Art

[0002] With the rapid development of concentrated solar power (CSP) technology, high-temperature phase-change thermal storage materials, as core components of the absorber, have become a key factor in determining the energy conversion efficiency and economic viability of power plants. Molten salts have become a mainstream choice due to their high phase-change enthalpy and wide operating temperature range, but they still face three major technical bottlenecks: extremely low thermal conductivity (conventional molten salts are approximately 0.6 W / (m·K)), resulting in slow heat storage and release rates, which limits the peak-shaving capacity of power plants; high fluidity in the high-temperature molten state, making it prone to leakage over long cycles (mass loss >15% after >30 cycles); and low absorption across the entire solar wavelength range (<70%), requiring external heat-absorbing coatings, which increase costs.

[0003] To address the above issues, developing a molten salt composite phase change material with high thermal conductivity, anti-leakage and strong light absorption has become the key to breaking through the bottleneck of CSP technology. Summary of the Invention

[0004] To address the issues raised in the above background technology, the present invention aims to provide a graphene / lithium salt eutectic composite phase change material, its preparation method, and its application. This invention utilizes the collaborative innovation of cetyltrimethylammonium bromide-assisted graphene dispersion, SiO2 network confinement encapsulation, and directional freezing channel regulation to simultaneously improve photothermal conversion efficiency and cycle stability, achieve interface optimization and structural strengthening of molten salt / graphene at the nanoscale, and ultimately enable the material to possess high thermal conductivity, anti-leakage, strong light absorption, and high-temperature resistance, providing a technical foundation for the large-scale application of photothermal energy storage.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows: On the one hand, the present invention provides a method for preparing a graphene / lithium salt eutectic composite phase change material, comprising the following steps:

[0006] (1) Lithium hydroxide (LiOH) and lithium carbonate (Li2CO3) are mixed, then heated to a certain temperature under the protection of inert gas to melt and keep the temperature for a period of time, and then rapidly cooled with liquid nitrogen and the particles are refined to obtain a eutectic salt;

[0007] (2) dissolving cetyltrimethylammonium bromide (CTAB) in water to form a cetyltrimethylammonium bromide solution, then adding graphene to the cetyltrimethylammonium bromide solution, ultrasonically treating and centrifuging to remove aggregates to obtain a few-layer graphene dispersion, and adjusting the pH of the few-layer graphene dispersion to 7.5-8.5;

[0008] (3) mixing the eutectic salt obtained in step (1) and the pH-adjusted few-layer graphene dispersion obtained in step (2), adjusting the pH to 9.5-10.0 to inhibit salt decomposition, and continuing to stir for a period of time to form a sol; then slowly adding pre-hydrolyzed tetraethyl orthosilicate (TEOS) to the sol under stirring, adjusting the pH to 10.8-11.2, and continuing to stir until it becomes uniform milky white without particles or phase separation, stopping stirring and standing at a certain temperature for a period of time to form a composite gel wrapped in a three-dimensional SiO2 network;

[0009] (4) The composite gel wrapped in the three-dimensional SiO2 network obtained in step (3) is subjected to liquid nitrogen directionally freezing to form vertically oriented through-holes, and then vacuum freeze-dried, and then heated to a certain temperature at a certain rate in an inert atmosphere and calcined for a period of time. After ultrasonic cleaning to remove residual silicate and vacuum heat treatment to strengthen interface bonding, the graphene / lithium salt eutectic composite phase change material is obtained.

[0010] Furthermore, the method further includes the following steps: after calcination in step (4), and before ultrasonic cleaning with nitric acid, etching with hydrofluoric acid vapor is performed, wherein the volume percentage of the hydrofluoric acid vapor is 0.3-0.7% (i.e., the volume of the hydrofluoric acid vapor is 0.3-0.7% of the total volume of the hydrofluoric acid vapor and the inert gas), and the etching time is 12-18 minutes. The hydrofluoric acid vapor etching improves the comprehensive performance (thermal conductivity, leakage prevention, and strong light absorption) of the material by optimizing the pores, strengthening the interface, and removing impurities.

[0011] Furthermore, the molar ratio of lithium hydroxide to lithium carbonate in step (1) is (50-55):(45-50);

[0012] The heating temperature in step (1) is 360-400° C., and the melting and holding time is 1.5-3 hours;

[0013] The liquid nitrogen rapid cooling rate in step (1) is ≥50°C / s to inhibit grain coarsening;

[0014] The particle size of the eutectic salt in step (1) is 150-300 mesh.

[0015] Furthermore, the liquid nitrogen rapid cooling in step (1) forms a metastable eutectic structure.

[0016] Furthermore, the particle refinement method in step (1) includes ball milling, grinding, and air flow milling.

[0017] Furthermore, the weight percentage of cetyltrimethylammonium bromide in the cetyltrimethylammonium bromide solution in step (2) is 0.4%-0.6%;

[0018] The mass ratio of the graphene to the hexadecyltrimethylammonium bromide solution in step (2) is 1:(6-10);

[0019] The ultrasonic treatment in step (2) has an ultrasonic frequency of 35-45 kHz, an ultrasonic power of 550-650 W, and an ultrasonic time of 40-50 min.

[0020] Furthermore, in the ultrasonic treatment in step (2), the immersion depth of the ultrasonic probe is 1 / 3 of the liquid surface height to prevent cavitation bubbles from escaping.

[0021] Furthermore, the mass ratio of the eutectic salt to the pH-adjusted few-layer graphene dispersion in step (3) is 1:(4-6) to ensure the continuity of the thermal conductive network;

[0022] The stirring time in step (3) is 3.5-5h;

[0023] The preparation method of the pre-hydrolyzed ethyl orthosilicate in step (3) is as follows: ethyl orthosilicate is mixed with anhydrous ethanol to obtain a mixed solution, the pH of the mixed solution is adjusted to 2.0-3.0 and heated to a certain temperature, water is added and stirred for a period of time, and then immediately cooled to 20-30°C, and the pH is adjusted to 7.0-7.5 to obtain pre-hydrolyzed ethyl orthosilicate; the heating temperature is 35-45°C, the molar ratio of water to ethyl orthosilicate is 1.8:1-2.2:1, and the stirring reaction time is 25-35 minutes;

[0024] The amount of pre-hydrolyzed ethyl orthosilicate added in step (3) is 4-6% of the volume of the sol. Too little will result in incomplete encapsulation, while too much will reduce the molten salt loading rate.

[0025] The standing temperature in step (3) is 45-55° C., and the standing time is 18-24 h.

[0026] Furthermore, the process conditions of the directional freezing in step (4) satisfy any of the following:

[0027] Liquid nitrogen bottom-up directional freezing: cooling rate of 15-25℃ / min (cooling rate of 15-25℃ / min is a key parameter that directly affects the pore structure and directly affects the thermal conductivity and cyclic stability of the material), the bottom liquid nitrogen contact surface temperature is ≤-150℃, the top is naturally cooled, and the freezing time is 15-25min;

[0028] Liquid nitrogen gradient freezing: freezing in stages in the following order: -20℃→-50℃→liquid nitrogen environment, with each stage freezing time of 10-20 minutes;

[0029] Liquid nitrogen vertical directional freezing: advance the freezing interface in the vertical direction at a rate of 5-15 mm / min, with a total freezing time of 20-30 minutes;

[0030] The vacuum freeze drying in step (4) is performed at a temperature of -60°C to 50°C, a pressure of 0.05 to 0.2 mbar, and a time of 36 to 60 hours;

[0031] The heating rate in step (4) is 3-8°C / min, preferably 5°C / min. A heating rate that is too fast may cause microcracks or collapse of the pore structure inside the material, reducing the mechanical strength and thermal conductivity. A heating rate that is too slow may increase energy consumption. The calcination temperature is 550-620°C, and the calcination time is 1.5-3h.

[0032] The vacuum heat treatment in step (4) is carried out at a temperature of 160-200° C. and for a time of 2.5-4 hours.

[0033] On the other hand, the present invention provides a graphene / lithium salt eutectic composite phase change material, which is prepared by any of the above-mentioned methods for preparing the graphene / lithium salt eutectic composite phase change material.

[0034] On the other hand, the present invention provides an application of the above-mentioned graphene / lithium salt eutectic composite phase change material as a heat storage medium for a heat absorber.

[0035] On the other hand, the present invention provides an application of the above-mentioned graphene / lithium salt eutectic composite phase change material in a high-temperature exhaust gas waste heat recovery system of a steel plant.

[0036] In another aspect, the present invention provides an application of the above-mentioned graphene / lithium salt eutectic composite phase change material in an energy storage module on a building exterior wall or roof.

[0037] On the other hand, the present invention provides a high-temperature exhaust gas waste heat recovery system for a steel plant, comprising the above-mentioned graphene / lithium salt eutectic composite phase change material.

[0038] In another aspect, the present invention provides a building exterior wall or roof energy storage module, comprising the graphene / lithium salt eutectic composite phase change material described above.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The graphene / lithium salt eutectic composite phase-change material of the present invention uses Li2CO3-LiOH eutectic salt as the core energy storage medium. The monomolecular layer dispersion of graphene in the lithium salt melt is achieved through the directional adsorption of CTAB molecules, which effectively inhibits the agglomeration of nanosheets and ensures that the graphene is evenly dispersed to form an efficient thermal conductive network. The material has high thermal conductivity (thermal conductivity coefficient ≥1.1W / (m·K)). The synergistic encapsulation of the SiO2 network and molten salt / graphene (graphene sheets fill the gaps in the silica network, and SEM shows that the graphene is embedded in the pore walls, forming a double barrier) solves the problem of molten salt leakage (the molten salt leakage rate is less than 2% after 50 cycles). The vertically oriented through-holes formed by directional freezing with liquid nitrogen give the material a wide-spectrum light absorption capability (near-infrared light absorption rate ≥93%). Its core advantage lies in the integrated design of "heat storage-heat transfer-photothermal conversion", which solves the industry problems of low thermal conductivity, easy leakage and poor light absorption of high-temperature molten salt materials, and provides an efficient and long-life solution for concentrated solar thermal power stations and industrial waste heat recovery. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] To more clearly illustrate the technical solutions of the embodiments of this application, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some of the embodiments described in this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

[0042] Figure 1 This is a scanning electron microscope (SEM) image of the graphene / lithium salt eutectic composite phase change material prepared in Example 1 of the present invention at a magnification of 10,000 times;

[0043] Figure 2 This is a scanning electron microscope (SEM) image of the graphene / lithium salt eutectic composite phase change material prepared in Example 1 of the present invention at a magnification of 100,000 times;

[0044] Figure 3 This is a cycle performance diagram of the graphene / lithium salt eutectic composite phase change material prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0045] In order to better understand the content of the present invention, the content of the present invention is further described below in conjunction with specific implementation methods, but the protection content of the present invention is not limited to the following embodiments.

[0046] Example 1

[0047] The preparation of graphene / lithium salt eutectic composite phase change material includes the following steps:

[0048] (1) Lithium hydroxide and lithium carbonate were mixed (the molar ratio of lithium hydroxide to lithium carbonate was 52:48, and the total mass was 10 g), placed in an argon-protected tube furnace, heated to 380°C for 2 hours, and rapidly cooled by liquid nitrogen (the liquid nitrogen rapid cooling rate was 50°C / s) to form a metastable eutectic structure, and then ball milled in a planetary ball mill to a 200-mesh powder (particle size distribution D50 = 15 ± 3 μm) to obtain a eutectic salt. The purity of the lithium salt raw material was ≥99.5%, and the particle size was ≤50 μm; the ball milling medium was zirconia beads, the ball-to-material ratio was 10:1, and the rotation speed was 300 rpm.

[0049] (2) 0.85 g of hexadecyltrimethylammonium bromide was dissolved in 200 mL of 60°C deionized water, magnetically stirred until clear, and then 0.1 g of graphene powder was added. The mixture was ultrasonically treated at 40 kHz and 600 W for 45 minutes, and then centrifuged at 8000 rpm for 10 minutes to remove undispersed aggregates >1 μm. The few-layer graphene dispersion was collected at 12000 rpm, and the pH of the few-layer graphene dispersion was adjusted to 8.0.

[0050] (3) The eutectic salt (10 g) obtained in step (1) and the few-layer graphene dispersion obtained in step (2) after adjusting the pH are mixed in a mass ratio of 1:5, 0.1 mol / L NaOH is added dropwise to adjust the pH to 9.5, and magnetic stirring is performed for 4 hours to form a sol; then, pre-hydrolyzed tetraethyl orthosilicate (TEOS) is slowly added dropwise to the sol under stirring (the amount of pre-hydrolyzed tetraethyl orthosilicate added is 5% of the volume of the sol), 0.1 mol / L NH3·H2O is added dropwise to pH = 11, and stirring is continued until it becomes uniform milky white without particles or phase separation. After stopping stirring, the mixture is allowed to stand at 50°C for 24 hours to form a composite gel wrapped in a three-dimensional SiO2 network. The preparation method of pre-hydrolyzed ethyl orthosilicate is as follows: ethyl orthosilicate is mixed with anhydrous ethanol to obtain a mixture, 0.1 mol / L HCl is added dropwise to adjust the pH to 2.0, deionized water is added in a constant temperature water bath at 40°C at a molar ratio of H2O:TEOS = 2:1, and the mixture is reacted with magnetic stirring (500 rpm) for 30 minutes. The density of the sample is measured by a precision hydrometer and compared with the standard curve, and the density is 1.12 g / cm 3 , then the degree of hydrolysis is ≈55%. After completion, it is immediately cooled to 25°C in an ice bath, and ammonia water is added dropwise to neutralize to pH = 7.0 to obtain pre-hydrolyzed ethyl orthosilicate.

[0051] (4) The composite gel wrapped with the three-dimensional SiO2 network obtained in step (3) was poured into a polytetrafluoroethylene mold, placed in liquid nitrogen and directionally frozen from bottom to top (cooling rate 20°C / min) for 20 minutes to form vertically oriented through-channels, and then freeze-dried at -50°C and 0.1 mbar vacuum for 48 hours to obtain a porous aerogel, and the porous aerogel was placed in an argon atmosphere tubular furnace, heated to 600°C at 5°C / min and calcined for 2 hours; then ultrasonically cleaned with 0.1 mol / L HNO3 for 30 minutes, and finally vacuum heat treated at 180°C for 3 hours to obtain a graphene / lithium salt eutectic composite phase change material.

[0052] Example 2

[0053] The preparation of graphene / lithium salt eutectic composite phase change material includes the following steps:

[0054] (1) Lithium hydroxide and lithium carbonate are mixed (the molar ratio of lithium hydroxide to lithium carbonate is 52:48, and the total mass is 15 g), placed in an argon-protected tube furnace, melted at 400° C. and kept warm for 1.5 hours, rapidly cooled by liquid nitrogen (the liquid nitrogen rapid cooling rate is 50° C. / s), and then ground into a 150 mesh powder to obtain a eutectic salt. The purity of the lithium salt raw material is ≥99.5%, and the particle size is ≤50 μm.

[0055] (2) 0.6 g of hexadecyltrimethylammonium bromide was dissolved in 135 mL of 65°C deionized water, and 0.1 g of graphene powder was added after magnetic stirring until it was clear. The mixture was ultrasonically treated at 45 kHz and 650 W for 50 min, and then centrifuged at 8000 rpm for 10 min to remove undispersed agglomerates larger than 1 μm. The few-layer graphene dispersion was collected at 12000 rpm, and the pH of the few-layer graphene dispersion was adjusted to 8.0.

[0056] (3) The eutectic salt (15 g) obtained in step (1) and the pH-adjusted few-layer graphene dispersion obtained in step (2) were mixed in a mass ratio of 1:6, the pH was adjusted to 10.0, and magnetic stirring was performed for 5 hours to form a sol; then, pre-hydrolyzed tetraethyl orthosilicate (TEOS) was slowly added dropwise to the sol under stirring (the amount of pre-hydrolyzed tetraethyl orthosilicate added was 6% of the volume of the sol), and 0.2 mol / L NH3·H2O was added dropwise to pH = 11.2, and stirring was continued until it was uniform milky white without particles or phase separation. After stopping stirring, the mixture was allowed to stand at 55°C for 20 hours to form a composite gel wrapped in a three-dimensional SiO2 network. The preparation method of pre-hydrolyzed ethyl orthosilicate is as follows: ethyl orthosilicate is mixed with anhydrous ethanol to obtain a mixture, 0.1 mol / L HCl is added dropwise to adjust the pH to 2.5, deionized water is added in a constant temperature water bath at 40°C at a molar ratio of H2O:TEOS = 2:1, and the mixture is reacted with magnetic stirring (500 rpm) for 30 minutes. The density of the sample is measured by a precision hydrometer and compared with the standard curve, and the density is 1.12 g / cm3 , then the degree of hydrolysis is ≈55%. After completion, it is immediately cooled to 25°C in an ice bath, and ammonia water is added dropwise to neutralize to pH = 7.3 to obtain pre-hydrolyzed ethyl orthosilicate.

[0057] (4) The composite gel wrapped with the three-dimensional SiO2 network obtained in step (3) was poured into a polytetrafluoroethylene mold, and gradient freezing (-20°C → -50°C → liquid nitrogen) was performed for 15 minutes / stage to form vertically oriented through-channels, and then freeze-dried at -60°C and 0.05 mbar vacuum for 60 hours to obtain a porous aerogel, and the porous aerogel was placed in an argon atmosphere tubular furnace, and the temperature was increased to 550°C at 5°C / min and calcined for 3 hours; hydrofluoric acid vapor was used for etching, the volume percentage of hydrofluoric acid vapor was 0.5 vol%, and the etching time was 15 minutes; then it was ultrasonically cleaned with 0.2 mol / L HNO3 for 30 minutes, and finally vacuum heat treated at 200°C for 4 hours to obtain a graphene / lithium salt eutectic composite phase change material.

[0058] Example 3

[0059] The preparation of graphene / lithium salt eutectic composite phase change material includes the following steps:

[0060] (1) Lithium hydroxide and lithium carbonate are mixed (the molar ratio of lithium hydroxide to lithium carbonate is 52:48, and the total mass is 20 g), placed in an argon-protected tube furnace, melted at 360° C. and kept warm for 3 hours, rapidly cooled by liquid nitrogen (the liquid nitrogen rapid cooling rate is 50° C. / s), and then air flow pulverized into a 300-mesh powder to obtain a eutectic salt. The purity of the lithium salt raw material is ≥99.5%, and the particle size is ≤50 μm.

[0061] (2) 0.45 g of hexadecyltrimethylammonium bromide was dissolved in 100 mL of 55°C deionized water, magnetically stirred until clear, and then 0.05 g of graphene powder was added. The mixture was ultrasonically treated at 35 kHz and 550 W for 40 min, and then centrifuged at 8000 rpm for 10 min to remove undispersed aggregates >1 μm. The few-layer graphene dispersion was collected at 12000 rpm, and the pH of the few-layer graphene dispersion was adjusted to 8.0.

[0062] (3) The eutectic salt (20 g) obtained in step (1) and the pH-adjusted few-layer graphene dispersion obtained in step (2) were mixed in a mass ratio of 1:4, the pH was adjusted to 9.8, and magnetic stirring was performed for 3.5 hours to form a sol; then, pre-hydrolyzed tetraethyl orthosilicate (TEOS) was slowly added dropwise to the sol under stirring (the amount of pre-hydrolyzed tetraethyl orthosilicate added was 4% of the volume of the sol), and 0.15 mol / L NH3·H2O was added dropwise to pH = 10.8, and stirring was continued until it was uniform milky white without particles or phase separation. After stopping stirring, the mixture was allowed to stand at 45°C for 18 hours to form a composite gel wrapped in a three-dimensional SiO2 network. The preparation method of pre-hydrolyzed ethyl orthosilicate is as follows: ethyl orthosilicate is mixed with anhydrous ethanol to obtain a mixture, 0.1 mol / L HCl is added dropwise to adjust the pH to 3.0, deionized water is added in a constant temperature water bath at 40°C at a molar ratio of H2O:TEOS = 2:1, and the mixture is reacted with magnetic stirring (500 rpm) for 30 minutes. The density of the sample is measured by a precision hydrometer and compared with the standard curve, and the density is 1.12 g / cm 3 , then the degree of hydrolysis is ≈55%. After completion, it is immediately cooled to 25°C in an ice bath, and ammonia water is added dropwise to neutralize to pH = 7.5 to obtain pre-hydrolyzed ethyl orthosilicate.

[0063] (4) The composite gel wrapped with the three-dimensional SiO2 network obtained in step (3) was poured into a polytetrafluoroethylene mold, and vertically oriented frozen with liquid nitrogen (rate 10 mm / min) for 25 minutes to form vertically oriented through-channels, and then freeze-dried at -55°C and 0.2 mbar vacuum for 36 hours to obtain a porous aerogel, and the porous aerogel was placed in an argon atmosphere tubular furnace, heated to 620°C at 5°C / min and calcined for 1.5 hours; then ultrasonically cleaned with 0.15 mol / L HNO3 for 30 minutes, and finally vacuum heat treated at 160°C for 2.5 hours to obtain a graphene / lithium salt eutectic composite phase change material.

[0064] Example 4

[0065] The preparation of graphene / lithium salt eutectic composite phase change material includes the following steps:

[0066] (1) Lithium hydroxide and lithium carbonate were mixed (the molar ratio of lithium hydroxide to lithium carbonate was 52:48, and the total mass was 10 g), placed in an argon-protected tube furnace, heated to 380°C for 2 hours, and rapidly cooled by liquid nitrogen (the liquid nitrogen rapid cooling rate was 50°C / s) to form a metastable eutectic structure, and then ball milled in a planetary ball mill to a 200-mesh powder (particle size distribution D50 = 15 ± 3 μm) to obtain a eutectic salt. The purity of the lithium salt raw material was ≥99.5%, and the particle size was ≤50 μm; the ball milling medium was zirconia beads, the ball-to-material ratio was 10:1, and the rotation speed was 300 rpm.

[0067] (2) 0.8 g of hexadecyltrimethylammonium bromide was dissolved in 135 mL of 60°C deionized water, and magnetically stirred until clear. 0.1 g of graphene powder was added, and the mixture was ultrasonically treated at 40 kHz and 600 W for 45 minutes. The mixture was then centrifuged at 8000 rpm for 10 minutes to remove undispersed aggregates larger than 1 μm. The few-layer graphene dispersion was collected at 12000 rpm, and the pH of the few-layer graphene dispersion was adjusted to 8.0.

[0068] (3) The eutectic salt (10 g) obtained in step (1) and the few-layer graphene dispersion obtained in step (2) after adjusting the pH are mixed in a mass ratio of 1:5, 0.1 mol / L NaOH is added dropwise to adjust the pH to 9.5, and magnetic stirring is performed for 4 hours to form a sol; then, pre-hydrolyzed tetraethyl orthosilicate (TEOS) is slowly added dropwise under stirring (the amount of pre-hydrolyzed tetraethyl orthosilicate added is 5% of the volume of the sol), 0.1 mol / L NH3·H2O is added dropwise to pH = 11, and stirring is continued until it becomes uniform milky white without particles or phase separation. After stopping stirring, the mixture is allowed to stand at 50°C for 24 hours to form a composite gel wrapped in a three-dimensional SiO2 network. The preparation method of pre-hydrolyzed ethyl orthosilicate is as follows: ethyl orthosilicate is mixed with anhydrous ethanol to obtain a mixture, 0.1 mol / L HCl is added dropwise to adjust the pH to 2.3, deionized water is added in a constant temperature water bath at 40°C at a molar ratio of H2O:TEOS = 2:1, and the mixture is reacted with magnetic stirring (500 rpm) for 30 minutes. The density of the sample is measured by a precision hydrometer and compared with the standard curve, and the density is 1.12 g / cm 3 , then the degree of hydrolysis is ≈55%. After completion, it is immediately cooled to 25°C in an ice bath, and ammonia water is added dropwise to neutralize to pH = 7.2 to obtain pre-hydrolyzed ethyl orthosilicate.

[0069] (4) The composite gel wrapped with the three-dimensional SiO2 network obtained in step (3) was poured into a polytetrafluoroethylene mold, placed in liquid nitrogen and directionally frozen from bottom to top (cooling rate 20°C / min) for 20 minutes to form vertically oriented through-channels, and then freeze-dried at -50°C and 0.1 mbar vacuum for 48 hours to obtain a porous aerogel, and the porous aerogel was placed in an argon atmosphere tubular furnace, heated to 600°C at 5°C / min and calcined for 2 hours; then ultrasonically cleaned with 0.1 mol / L HNO3 for 30 minutes, and finally vacuum heat treated at 180°C for 3 hours to obtain a graphene / lithium salt eutectic composite phase change material.

[0070] Comparative Example 1

[0071] The preparation of SiO2 encapsulated lithium salt eutectic composite phase change material includes the following steps:

[0072] (1) Lithium hydroxide (LiOH) and lithium carbonate (Li2CO3) were mixed in a molar ratio of 52:48 (total mass 10 g), placed in an argon-protected tube furnace, heated to 380°C for melting and kept warm for 2 hours, and rapidly cooled by liquid nitrogen (the liquid nitrogen rapid cooling rate was 50°C / s) to form a metastable eutectic structure. The product was ball-milled to a 200-mesh powder (particle size distribution D50 = 15 ± 3 μm) by a planetary ball mill to obtain a eutectic salt.

[0073] (2) Dissolve 0.8 g of hexadecyltrimethylammonium bromide (CTAB) in 200 mL of 60°C deionized water and stir magnetically until the solution becomes clear to obtain a uniform CTAB solution.

[0074] (3) The eutectic salt (10 g) obtained in step (1) and the CTAB solution obtained in step (2) were mixed in a mass ratio of 1:5, 0.1 mol / L NaOH was added dropwise to adjust the pH to 9.5, and magnetic stirring was performed for 4 hours to form a sol; then, pre-hydrolyzed tetraethyl orthosilicate (TEOS) was slowly added dropwise to the sol under stirring (the amount of pre-hydrolyzed tetraethyl orthosilicate added was 5% of the sol volume), and 0.1 mol / L NH3·H2O was added dropwise to pH = 11, and stirring was continued until it became uniform milky white without particles or phase separation. After stopping stirring, the mixture was allowed to stand at 50°C for 24 hours to form a composite gel wrapped in a three-dimensional SiO2 network. The preparation method of pre-hydrolyzed ethyl orthosilicate is as follows: ethyl orthosilicate is mixed with anhydrous ethanol to obtain a mixture, 0.1 mol / L HCl is added dropwise to adjust the pH to 2.0, deionized water is added in a constant temperature water bath at 40°C at a molar ratio of H2O:TEOS = 2:1, and the mixture is reacted with magnetic stirring (500 rpm) for 30 minutes. The density of the sample is measured by a precision hydrometer and compared with the standard curve, and the density is 1.12 g / cm 3 , then the degree of hydrolysis is ≈55%. After completion, it is immediately cooled to 25°C in an ice bath, and ammonia water is added dropwise to neutralize to pH = 7.0 to obtain pre-hydrolyzed ethyl orthosilicate.

[0075] (4) The composite gel wrapped with the three-dimensional SiO2 network obtained in step (3) was poured into a polytetrafluoroethylene mold, placed in liquid nitrogen and directionally frozen from bottom to top (cooling rate 20°C / min) for 20 minutes, and then freeze-dried at -50°C and 0.1 mbar vacuum for 48 hours to obtain a porous aerogel, and the porous aerogel was placed in an argon atmosphere tubular furnace, heated to 600°C at 5°C / min and calcined for 2 hours; then ultrasonically cleaned with 0.1 mol / L HNO3 for 30 minutes, and finally vacuum heat treated at 180°C for 3 hours to obtain a SiO2-encapsulated lithium salt eutectic composite phase change material.

[0076] Comparative Example 2

[0077] The preparation of graphene / lithium salt eutectic composite phase change material (without SiO2 encapsulation) includes the following steps:

[0078] (1) Lithium hydroxide (LiOH) and lithium carbonate (Li2CO3) were mixed in a molar ratio of 52:48 (total mass 10 g), placed in an argon-protected tube furnace, heated to 380°C for melting and kept warm for 2 hours, and rapidly cooled by liquid nitrogen (the liquid nitrogen rapid cooling rate was 50°C / s) to form a metastable eutectic structure. The product was ball-milled to a 200-mesh powder (particle size distribution D50 = 15 ± 3 μm) by a planetary ball mill to obtain a eutectic salt.

[0079] (2) 0.85 g of hexadecyltrimethylammonium bromide (CTAB) was dissolved in 200 mL of 60°C deionized water, magnetically stirred until clear, and then 0.1 g of graphene powder was added. The mixture was ultrasonically treated at 40 kHz and 600 W for 45 minutes, and then centrifuged at 8000 rpm for 10 minutes to remove undispersed agglomerates larger than 1 μm. The mixture was then collected at 12000 rpm to obtain a few-layer graphene dispersion, and the pH of the few-layer graphene dispersion was adjusted to 8.0.

[0080] (3) The eutectic salt (10 g) obtained in step (1) was mixed with the pH-adjusted few-layer graphene dispersion obtained in step (2) in a mass ratio of 1:5, 0.1 mol / L NaOH was added dropwise to adjust the pH to 9.5, and magnetic stirring was performed for 4 hours to form a uniform sol;

[0081] (4) The sol obtained in step (3) was poured into a polytetrafluoroethylene mold, placed in liquid nitrogen and directionally frozen from bottom to top (cooling rate 20°C / min) for 20 minutes, and then freeze-dried at -50°C and 0.1 mbar vacuum for 48 hours to obtain a porous aerogel. The porous aerogel was placed in an argon atmosphere tubular furnace, heated to 600°C at 5°C / min and calcined for 2 hours, and then ultrasonically cleaned with 0.1 mol / L HNO3 for 30 minutes, and finally vacuum heat treated at 180°C for 3 hours to obtain a graphene / lithium salt eutectic composite phase change material (without SiO2 encapsulation).

[0082] The concentration of graphene in the few-layer graphene dispersions in Examples 1-4 was calculated, a certain amount of the few-layer graphene dispersion was dried and weighed, and the mass concentration was calculated. The calculation results showed that the graphene concentrations in the few-layer graphene dispersions in Examples 1-4 were 0.9 g / L, 0.8 g / L, 1.0 g / L, and 1.0 g / L, respectively. The high concentration ensured that the graphene content in the composite material was sufficient to form a continuous thermal conductive network.

[0083] The thickness of the graphene in the few-layer graphene dispersions in Examples 1-4 was measured by TEM, the lateral size was statistically analyzed by SEM, and the particle size distribution was measured by DLS. The results showed that the thickness of the graphene in the few-layer graphene dispersions in Examples 1-4 was 0.85 nm, 0.92 nm, 0.78 nm, and 0.75 nm, respectively, and the lateral sizes were 2.5 μm, 3.0 μm, 1.8 μm, and 1.5 μm, respectively. The particle size distributions were D90480 nm, D90 520 nm, D90 450 nm, and D90 420 nm, respectively. The large lateral size enhances the thermal conduction path, and the small D90 inhibits agglomeration.

[0084] The SiO2 network cross-section of the composite gel wrapped by the three-dimensional SiO2 network in Examples 1-4 was observed by TEM, and the pore wall thickness was measured. The test results showed that the SiO2 network thickness of the composite gel wrapped by the three-dimensional SiO2 network in Examples 1-4 was 8nm, 10nm, 6nm, and 5nm, respectively. The thin and continuous SiO2 layer takes into account both confined encapsulation and high porosity.

[0085] The mercury intrusion porosimetry (MIP) method was used to calculate the pore volume ratio of the composite gel wrapped in the three-dimensional SiO2 network in Examples 1-4. The calculation results showed that the porosity of the composite gel wrapped in the three-dimensional SiO2 network in Examples 1-4 was 74%, 76%, 75% and 75%, respectively. The high porosity provides molten salt storage space and reduces thermal expansion stress.

[0086] The pore size distribution of the material after calcination in step (4) of Example 1-4 was analyzed by SEM image analysis, and the pore sizes were 120nm, 150nm, 100nm, and 80nm, respectively. The mercury intrusion method was used to calculate the pore size ratio of the material after calcination in step (4) of Example 1-4, and the results showed that the pore size distribution concentration of the material after calcination was 82%, 78%, 85%, and 88%, respectively. The uniform pore size distribution avoids local stress concentration and prolongs the cycle life. The total specific surface area of the material after calcination in step (4) of Example 1-4 was calculated by BET nitrogen adsorption method, and the results showed that the specific surface areas of the material after calcination were 310m 2 / g、295m 2 / g、330m 2 / g、350m 2 The high specific surface area enhances molten salt wettability and improves thermal storage density. The material maintains a high specific surface area and uniform pores after high-temperature calcination, indicating that the material does not sinter or structurally collapse at extreme temperatures. This indicates that the prepared graphene / lithium salt eutectic composite phase change material is suitable for use in CSP plants.

[0087] The graphene / lithium salt eutectic composite phase change material prepared in Example 1 was scanned by electron microscope, and the results were as follows: Figure 1 and Figure 2 As shown, from Figure 1It can be seen that the graphene / lithium salt eutectic composite phase change material presents a rod-like or needle-like structure. These structures are intertwined and stacked, which helps to form an efficient heat conduction channel. Some slender crystal structures can be clearly seen in the material, which may be lithium salt crystals. There is a certain interaction between them and the graphene sheets, which can limit the growth and movement of crystals to a certain extent, enhance the structural stability of the material, and prevent the material from undergoing large volume changes during the phase change cycle, which may lead to structural damage, thereby improving the cycle stability and service life of the material. Figure 2 The material exhibits a dense, columnar microstructure that is relatively uniform in size and appears to have some rough texture on the surface. These columnar structures may be the specific morphology formed by the growth of lithium salts under the template of graphene. This columnar structure with a rough surface can significantly increase the specific surface area of the material. A larger specific surface area facilitates heat exchange with the external environment, allowing for faster absorption and release of heat from the outside world during the phase change process, thereby improving the thermal response speed of the phase change material.

[0088] The thermal conductivity of the graphene / lithium salt eutectic composite phase change materials prepared in Examples 1-4, the SiO2-encapsulated lithium salt eutectic composite phase change material prepared in Comparative Example 1, and the graphene / lithium salt eutectic composite phase change material (without SiO2 encapsulation) prepared in Comparative Example 2 was measured using a laser flash method. The measurement results showed that the thermal conductivity of Example 1 was 1.2 W / (m·K), the thermal conductivity of Example 2 was 1.1 W / (m·K), the thermal conductivity of Example 3 was 1.3 W / (m·K), and the thermal conductivity of Example 4 was 1.4 W / (m·K). The thermal conductivity of Comparative Example 1 was 0.7 W / (m·K), and the thermal conductivity of Comparative Example 2 was 0.9 W / (m·K). In the graphene / lithium salt eutectic composite phase change material of the present invention, graphene provides a heat conduction path, and SiO2 inhibits the flow of molten salt. The two work together to improve the thermal conductivity compared to the thermal conductivity of pure molten salt (0.6W / (m·K)). The directional freezing process further improves the thermal conductivity by optimizing the pore structure, providing an efficient "heat storage-heat transfer" integrated solution for high-temperature heat storage systems.

[0089] The graphene / lithium salt eutectic composite phase change materials prepared in Examples 1-4, the SiO2-encapsulated lithium salt eutectic composite phase change materials prepared in Comparative Example 1, and the graphene / lithium salt eutectic composite phase change materials (without SiO2 encapsulation) prepared in Comparative Example 2 were subjected to 50 cycles of simulated use. After the cycle, the amount of molten salt leaked was accurately measured and compared with the initial total amount of molten salt to calculate the molten salt leakage rate. The measurement results showed that the molten salt leakage rate was 1.3% for Example 1, 1.6% for Example 2, 1.0% for Example 3, and 0.8% for Example 4. The molten salt leakage rate was 3.4% for Comparative Example 1 and 5.1% for Comparative Example 2. The molten salt leakage rates of Examples 1-4 were all low, all below 2%, indicating that under their respective preparation conditions, the materials had good encapsulation of the molten salt or internal structural stability, effectively preventing molten salt leakage. Figure 1 SEM shows that graphene sheets are embedded in the pore walls of the SiO2 network, forming a nanoscale double barrier structure, which effectively blocks the molten salt penetration path. The molten salt leakage rates of Examples 1-4 are all less than 2%, while the leakage rates of Comparative Examples 1 (without graphene) and 2 (without SiO2 encapsulation) are as high as 3.4% and 5.1%, respectively, indicating that the physical-chemical synergistic encapsulation mechanism of graphene and SiO2 is the core factor in inhibiting molten salt leakage, and the physical barrier of graphene and the confinement of the SiO2 network are indispensable. In addition, the directional freezing process optimizes the pore uniformity and further reduces the leakage rate, confirming that the synergistic optimization of structural regulation and interface bonding is the key to improving the cyclic stability of heat storage materials.

[0090] The initial phase change enthalpy of the graphene / lithium salt eutectic composite phase change material prepared in Example 4 and the phase change enthalpy after 10, 20, 30, 40 and 50 cycles were measured. The results are as follows: Figure 3 As shown, from Figure 3 It can be seen that the initial phase change enthalpy of the graphene / lithium salt eutectic composite phase change material prepared in Example 4 is 450 J / g. After 50 cycles, the phase change enthalpy of the material is 405 J / g. Comparing the phase change enthalpy after the cycle with the initial phase change enthalpy, the calculated enthalpy retention rate is 90%. The enthalpy retention rate results show that the graphene / lithium salt eutectic composite phase change material has good thermal stability and energy storage stability, and can well maintain its energy storage capacity during multiple cycles. Graphene dispersion reduces interfacial thermal resistance, and SiO2 network confinement inhibits phase separation. After 50 cycles, the enthalpy retention rate is 90%.

[0091] The low leakage rate and high enthalpy retention rate of the graphene / lithium salt eutectic composite phase change material prepared in Examples 1-4 indicate that the material does not undergo significant phase separation or structural degradation during long-term high-temperature cycling and has excellent thermal stability.

[0092] The graphene / lithium salt eutectic composite phase change materials prepared in Examples 1-4, the SiO2-encapsulated lithium salt eutectic composite phase change materials prepared in Comparative Example 1, and the graphene / lithium salt eutectic composite phase change materials (without SiO2 encapsulation) prepared in Comparative Example 2 were tested for absorptivity in the 300-2500nm band using an ultraviolet-visible-near-infrared spectrophotometer (UV-Vis-NIR, PerkinElmer Lambda1050). The test results were as follows: the near-infrared light absorptivity of Example 1 was 94%, the near-infrared light absorptivity of Example 2 was 93%, the near-infrared light absorptivity of Example 3 was 95%, the near-infrared light absorptivity of Example 4 was 96.5%, the near-infrared light absorptivity of Comparative Example 1 was 70% (without graphene, only SiO2 network), and the near-infrared light absorptivity of Comparative Example 2 was 75% (without SiO2 encapsulation).

[0093] The graphene / lithium salt eutectic composite phase change material of the present invention is used as the core heat storage medium of the heat absorber to achieve efficient photothermal conversion and stable energy storage in a tower solar thermal power station. Focusing sunlight heats the material to 380-400°C, triggering the solid-liquid phase transition of the Li2CO3-LiOH eutectic salt, converting light energy into latent heat for storage. At night or on rainy days, the molten salt undergoes a reverse phase transition, releasing heat energy to drive a steam turbine to generate heat and generate electricity, ensuring 24-hour continuous power supply to the grid and maintaining the stability of power output. This can significantly reduce electricity consumption, achieve effective energy management and energy conservation and emission reduction, and significantly improve photothermal conversion efficiency. The SiO2 network encapsulation reduces the molten salt leakage rate during the material's operating cycle, extending the system life and reducing operation and maintenance costs. This technology breaks through the bottleneck of low thermal storage efficiency and short life of traditional molten salt, providing feasible support for the large-scale application of solar thermal power stations.

[0094] In the steel smelting scenario, the graphene / lithium salt eutectic composite phase change material of the present invention is integrated into the high-temperature exhaust gas (500-600°C) heat exchange system to recover industrial waste heat to drive the molten salt phase change heat storage. Subsequently, when thermal energy is required in the production process, the stored heat energy is used to preheat the steelmaking furnace raw materials or drive the low-temperature waste heat generator set, reducing the need for direct combustion of fuel. It not only significantly improves the recovery rate of industrial waste heat, but also reduces energy costs for the factory and realizes energy recycling. By adjusting the LiOH / Li2CO3 ratio, the phase change temperature can be accurately matched to the industrial heat demand of 420±5°C. The rapid storage / release characteristics of the composite material significantly reduce fuel consumption, help the steel industry achieve "zero-carbon steelmaking", and promote the resource utilization of industrial high-temperature waste heat.

[0095] The graphene / lithium salt eutectic composite phase change material prepared in Example 3 is embedded in the building exterior wall or roof energy storage module to absorb solar energy and store heat during the day, and release heat energy at night to drive heating or absorption cooling. The graphene / SiO2 composite lightweight aerogel matrix (composite gel wrapped in a three-dimensional SiO2 network) and high thermal conductivity improve the storage / release efficiency and reduce building energy consumption. The vertical pore structure accelerates the molten salt phase change rate and quickly completes more thermal energy storage; it adapts to the structural requirements of high-rise buildings, and the modular design realizes seamless integration with the building exterior. In summer, the heat energy stored during the day drives the refrigeration system, which can reduce electricity consumption; in winter, it provides a stable heat source for building heating, which can reduce the annual operating cost of the system, reduce carbon emissions, and promote the popularization of green buildings and the transformation of urban energy structure.

[0096] The above description is only a specific embodiment of the present invention, not all embodiments. Any equivalent transformation of the technical solution of the present invention made by ordinary technicians in this field after reading the specification of the present invention is covered by the claims of the present invention.

Claims

1. A method for preparing a graphene / lithium salt eutectic composite phase change material, characterized in that: The following steps are involved: (1) lithium hydroxide and lithium carbonate are mixed, then heated to a certain temperature under the protection of inert gas, melted and kept warm for a period of time, and then rapidly cooled with liquid nitrogen and then refined to obtain a eutectic salt; (2) dissolving cetyltrimethylammonium bromide in water to form a cetyltrimethylammonium bromide solution, then adding graphene to the cetyltrimethylammonium bromide solution, ultrasonically treating and centrifuging to remove aggregates to obtain a few-layer graphene dispersion, and adjusting the pH of the few-layer graphene dispersion to 7.5-8.5; (3) mixing the eutectic salt obtained in step (1) and the pH-adjusted few-layer graphene dispersion obtained in step (2), adjusting the pH to 9.5-10.0, and continuing to stir for a period of time to form a sol; then slowly adding pre-hydrolyzed ethyl orthosilicate dropwise to the sol under stirring, adjusting the pH to 10.8-11.2, continuing to stir until it becomes uniform milky white, stopping stirring and standing at a certain temperature for a period of time to form a composite gel wrapped in a three-dimensional SiO2 network; (4) The composite gel wrapped in the three-dimensional SiO2 network obtained in step (3) is subjected to directionally freezing in liquid nitrogen and then freeze-dried in vacuum, and then heated to a certain temperature at a certain rate in an inert atmosphere and calcined for a period of time, and the graphene / lithium salt eutectic composite phase change material is obtained after ultrasonic cleaning and vacuum heat treatment.

2. The method for preparing the graphene / lithium salt eutectic composite phase change material according to claim 1, characterized in that: The following steps are also included: After calcination in step (4), hydrofluoric acid vapor etching is performed before nitric acid ultrasonic cleaning. The volume percentage of the hydrofluoric acid vapor is 0.3-0.7%, and the etching time is 12-18 minutes.

3. The method for preparing the graphene / lithium salt eutectic composite phase change material according to claim 1, characterized in that: The molar ratio of lithium hydroxide to lithium carbonate in step (1) is (50-55):(45-50); The heating temperature in step (1) is 360-400° C., and the melting and holding time is 1.5-3 hours; The liquid nitrogen rapid cooling rate in step (1) is ≥50°C / s; The particle size of the eutectic salt in step (1) is 150-300 mesh.

4. The method for preparing the graphene / lithium salt eutectic composite phase change material according to claim 1, characterized in that: The weight percentage of cetyltrimethylammonium bromide in the cetyltrimethylammonium bromide solution in step (2) is 0.4%-0.6%; The mass ratio of graphene to hexadecyltrimethylammonium bromide in step (2) is 1:(6-10); The ultrasonic treatment in step (2) has an ultrasonic frequency of 35-45 kHz, an ultrasonic power of 550-650 W, and an ultrasonic time of 40-50 min.

5. The method for preparing the graphene / lithium salt eutectic composite phase change material according to claim 1, wherein: The mass ratio of the eutectic salt to the pH-adjusted few-layer graphene dispersion in step (3) is 1:(4-6); The stirring time in step (3) is 3.5-5h; The preparation method of the pre-hydrolyzed ethyl orthosilicate in step (3) is as follows: ethyl orthosilicate is mixed with anhydrous ethanol to obtain a mixed solution, the pH of the mixed solution is adjusted to 2.0-3.0 and heated to a certain temperature, water is added and stirred for a period of time, and then immediately cooled to 20-30°C, and the pH is adjusted to 7.0-7.5 to obtain pre-hydrolyzed ethyl orthosilicate; the heating temperature is 35-45°C, the molar ratio of water to ethyl orthosilicate is 1.8:1-2.2:1, and the stirring reaction time is 25-35 minutes; The amount of pre-hydrolyzed ethyl orthosilicate added in step (3) is 4-6% of the volume of the sol; The standing temperature in step (3) is 45-55° C., and the standing time is 18-24 h.

6. The method for preparing the graphene / lithium salt eutectic composite phase change material according to claim 1, characterized in that: The process conditions for liquid nitrogen directional freezing in step (4) meet any of the following: Liquid nitrogen directional freezing from bottom to top: cooling rate is 15-25℃ / min, the temperature of the liquid nitrogen contact surface at the bottom is ≤-150℃, the top is naturally cooled, and the freezing time is 15-25min; Liquid nitrogen gradient freezing: freezing in stages in the following order: -20℃→-50℃→liquid nitrogen environment, with each stage freezing time of 10-20 minutes; Liquid nitrogen vertical directional freezing: advance the freezing interface in the vertical direction at a rate of 5-15 mm / min, with a total freezing time of 20-30 minutes; The vacuum freeze drying in step (4) is performed at a temperature of -60°C to 50°C, a pressure of 0.05 to 0.2 mbar, and a time of 36 to 60 hours; The heating rate in step (4) is 3-8°C / min, the calcination temperature is 550-620°C, and the calcination time is 1.5-3h; The vacuum heat treatment in step (4) is carried out at a temperature of 160-200° C. and for a time of 2.5-4 hours.

7. A graphene / lithium salt eutectic composite phase change material, characterized in that: The graphene / lithium salt eutectic composite phase change material is prepared by the preparation method of any one of claims 1-6.

8. Use of the graphene / lithium salt eutectic composite phase change material according to claim 7 as a heat storage medium for a heat absorber.

9. Use of the graphene / lithium salt eutectic composite phase change material according to claim 7 in a high-temperature exhaust gas waste heat recovery system in a steel plant.

10. Use of the graphene / lithium salt eutectic composite phase change material according to claim 7 in an energy storage module for a building exterior wall or roof.

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