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

Abstract

The application discloses a graphene / lithium salt eutectic composite phase change material and a preparation method and application thereof. The material is prepared as follows: lithium hydroxide and lithium carbonate are mixed, heated, melted and kept warm, and then eutectic salt is obtained through liquid nitrogen rapid cooling and particle refinement; cetyltrimethylammonium bromide is dissolved in water, graphene is added, ultrasonic treatment is carried out, a few-layer graphene dispersion liquid is obtained, and the pH value thereof is adjusted; the eutectic salt and the few-layer graphene dispersion liquid with the adjusted pH value are mixed, the pH value is adjusted, and stirring is continuously carried out to form sol; pre-hydrolyzed tetraethyl orthosilicate is added into the sol, the pH value is adjusted, and stirring is continuously carried out until the sol is uniform and white; after the stirring is stopped, the gel is formed by standing; and the gel is prepared through directional freezing in liquid nitrogen, vacuum freeze drying, calcination, ultrasonic cleaning and vacuum heat treatment. The material has the advantages of high heat conduction, leakage resistance and strong light absorption, and is applied to heat absorber heat storage medium, a high-temperature waste gas waste heat recovery system of a steel plant and an energy storage module of a building outer wall or roof.

Description

TECHNICAL FIELD

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

[0002] With the rapid development of concentrated solar power (CSP) technology, high-temperature phase change heat storage materials, as the core component of heat absorbers, directly determine the energy conversion efficiency and economy of power stations. Molten salt has become the mainstream choice due to its high phase change enthalpy and wide working temperature range, but still faces three major technical bottlenecks: extremely low thermal conductivity (traditional molten salt is about 0.6 W / (m·K)) leads to slow heat storage / release rate, which restricts the peak shaving capacity of power stations; high flowability in molten state, easy to leak after long-term circulation (>30 times, mass loss >15%); low full-waveband solar absorption rate (<70%), relying on external heat-absorbing coating to increase cost.

[0003] In view of the above problems, it is necessary to develop a molten salt composite phase change material with high thermal conductivity, anti-leakage and strong light absorption, which is the key to breaking through the technical bottlenecks of CSP. SUMMARY

[0004] In order to solve the problems proposed in the background art, the purpose of the present application is to provide a graphene / lithium salt eutectic composite phase change material and a preparation method and application thereof. The present application realizes the interface optimization and structure strengthening of molten salt / graphene at the nanoscale through the synergistic innovation of cetyltrimethylammonium bromide-assisted graphene dispersion, SiO2 network limited encapsulation and directional frozen pore regulation, while improving the light-heat conversion efficiency and cycle stability, so as to make the material have the characteristics of high thermal conductivity, anti-leakage, strong light absorption and high temperature resistance, and provide a technical basis for the large-scale application of photo-thermal energy storage.

[0005] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows: on the one hand, the present application provides a preparation method of a graphene / lithium salt eutectic composite phase change material, comprising the following steps:

[0006] (1) mixing lithium hydroxide (LiOH) and lithium carbonate (Li2CO3), then heating to a certain temperature under inert gas protection and keeping for a period of time, and then performing particle refinement after liquid nitrogen rapid cooling to obtain eutectic salt;

[0007] (2) dissolving cetyltrimethylammonium bromide (CTAB) in water to form a cetyltrimethylammonium bromide solution, then adding graphene into the cetyltrimethylammonium bromide solution, and after ultrasonic treatment and centrifugation to remove agglomerates, a few-layer graphene dispersion liquid is obtained, and the pH of the few-layer graphene dispersion liquid is adjusted 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, continuing stirring 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, continuing stirring until uniform milky white, no particles or phase separation, stopping stirring and standing at a certain temperature for a period of time to form a three-dimensional SiO2 network-wrapped composite gel;

[0009] (4) subjecting the three-dimensional SiO2 network-wrapped composite gel obtained in step (3) to directional freezing in liquid nitrogen to form vertically oriented through channels, then performing vacuum freeze-drying, subsequently heating to a certain temperature at a certain rate in an inert atmosphere and calcining for a period of time, removing residual silicic acid by ultrasonic cleaning, and then performing vacuum heat treatment to strengthen the interface bonding to obtain the graphene / lithium salt eutectic composite phase change material.

[0010] Further, the following step is included: after calcination in step (4), before ultrasonic cleaning with nitric acid, steam etching with hydrofluoric acid is performed, the volume percentage of the hydrofluoric acid steam is 0.3-0.7% (i.e. the volume of the hydrofluoric acid steam is 0.3-0.7% of the total volume of the hydrofluoric acid steam and the inert gas), and the etching time is 12-18 min. The hydrofluoric acid steam etching optimizes the channels, strengthens the interface, and removes impurities, thereby improving the overall performance (thermal conductivity, leakage prevention, and strong light absorption) of the material.

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

[0012] In step (1), the heating temperature is 360-400℃, and the melting holding time is 1.5-3h;

[0013] In step (1), the liquid nitrogen rapid cooling rate is ≥50℃ / s to inhibit grain coarsening;

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

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

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

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

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

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

[0020] Further, the immersion depth of the ultrasonic probe in the ultrasonic treatment in step (2) is 1 / 3 of the liquid level height to avoid cavitation bubbles escaping.

[0021] Further, the mass ratio of the eutectic salt to the pH-adjusted few-layer graphene dispersion in step (3) is 1: (4-6), which ensures the continuity of the heat conduction network.

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

[0023] The preparation method of the pre-hydrolyzed tetraethyl orthosilicate in step (3) is as follows: mixing tetraethyl orthosilicate and anhydrous ethanol to obtain a mixed solution, adjusting the pH of the mixed solution to 2.0-3.0 and heating to a certain temperature, then adding water and stirring for a period of time, immediately cooling to 20-30℃, adjusting the pH to 7.0-7.5 to obtain the pre-hydrolyzed tetraethyl orthosilicate; the heating temperature is 35-45℃, the molar ratio of water to tetraethyl orthosilicate is 1.8:1-2.2:1, and the stirring reaction time is 25-35 min.

[0024] The addition amount of the pre-hydrolyzed tetraethyl orthosilicate in step (3) is 4-6% of the volume of the sol, too little leading to incomplete packaging, and too much reducing the molten salt loading rate.

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

[0026] Further, the process conditions of the directional freezing in step (4) meet any one of the following:

[0027] Liquid nitrogen directional freezing from bottom to top: cooling rate is 15-25℃ / min (cooling rate 15-25℃ / min is a key parameter directly affecting the pore structure, directly affecting the thermal conductivity and cycle stability of the material), bottom liquid nitrogen contact surface temperature ≤-150℃, top natural cooling, freezing time 15-25 min;

[0028] Liquid nitrogen gradient freezing: sequentially frozen in -20℃→-50℃→liquid nitrogen environment, each stage freezing time 10-20 min;

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

[0030] The temperature of the vacuum freeze drying in step (4) is -60-50 DEG C, the pressure is 0.05-0.2 mbar, and the time is 36-60 h;

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

[0032] The temperature of the vacuum heat treatment in step (4) is 160-200 DEG C, and the time is 2.5-4 h.

[0033] In another aspect, the application provides a graphene / lithium salt eutectic composite phase change material prepared by the preparation method of any of the graphene / lithium salt eutectic composite phase change materials described above.

[0034] In another aspect, the application provides the use of the graphene / lithium salt eutectic composite phase change material described above as a heat storage medium for a heat absorber.

[0035] In another aspect, the application provides the use of the graphene / lithium salt eutectic composite phase change material described above in a high-temperature waste gas waste heat recovery system of a steel plant.

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

[0037] In another aspect, the application provides a high-temperature waste gas waste heat recovery system of a steel plant, comprising the graphene / lithium salt eutectic composite phase change material described above.

[0038] In another aspect, the application 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 application has the following beneficial effects:

[0040] The graphene / lithium salt eutectic composite phase change material takes Li2CO3-LiOH eutectic salt as a core energy storage medium, realizes monolayer dispersion of graphene in the lithium salt melt through directional adsorption of CTAB molecules, effectively inhibits agglomeration of nanosheets, ensures uniform dispersion of graphene to form an efficient heat conduction network, and has high thermal conductivity (thermal conductivity coefficient ≥ 1.1 W / (m·K)); the synergistic encapsulation of the SiO2 network and the melt salt / graphene (graphene sheets fill the gaps in the SiO2 network, SEM shows that graphene is embedded in the pore wall to form a double barrier) solves the problem of melt salt leakage (melt salt leakage rate < 2% after 50 cycles); the vertical oriented through channels formed by directional freezing of liquid nitrogen endow the material with wide spectrum light absorption capacity (near infrared light absorption rate ≥ 93%). The core advantage lies in the integrated design of "heat storage-heat conduction-light-heat conversion", which solves the industry problems of low thermal conductivity, easy leakage and poor light absorption of high-temperature melt salt materials, and provides an efficient and long-life solution for concentrated solar power stations and industrial waste heat recovery. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments described in the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0042] Figure 1 A scanning electron microscope (SEM) image of the graphene / lithium salt eutectic composite phase change material prepared in Example 1 of the present application under 10000 times magnification;

[0043] Figure 2 A scanning electron microscope (SEM) image of the graphene / lithium salt eutectic composite phase change material prepared in Example 1 of the present application under 100000 times magnification;

[0044] Figure 3 A cycle performance graph of the graphene / lithium salt eutectic composite phase change material prepared in Example 4 of the present application. DETAILED DESCRIPTION

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

[0046] Example 1

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

[0048] (1) Lithium hydroxide and lithium carbonate were mixed (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 melting and heat preservation for 2 hours, and then formed a metastable eutectic structure by liquid nitrogen rapid cooling (the rate of liquid nitrogen rapid cooling was 50°C / s). Then, the eutectic salt was obtained by ball milling in a planetary ball mill to a 200-mesh powder (particle size distribution D50 = 15 ± 3 μm). 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 hexadecyl trimethyl ammonium bromide was dissolved in 200 mL of 60°C deionized water, and after magnetic stirring until clear, 0.1 g of graphene powder was added. The mixture was treated by ultrasonic wave at 40 kHz and 600 W for 45 minutes. Then, >1 μm undispersed agglomerates were removed by centrifugation at 8000 rpm for 10 minutes, and a few-layer graphene dispersion liquid was collected by centrifugation at 12000 rpm. The pH of the few-layer graphene dispersion liquid was adjusted to 8.0.

[0050] (3) The eutectic salt (10 g) obtained in step (1) and the pH-adjusted few-layer graphene dispersion liquid 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 addition amount of pre-hydrolyzed TEOS was 5% of the volume of the sol). 0.1 mol / L NH3·H2O was added dropwise to adjust the pH to 11. The stirring was continued until a uniform milky white color was obtained without particles or phase separation. After stopping the stirring, a three-dimensional SiO2 network-encapsulated composite gel was formed by standing at 50°C for 24 hours. The preparation method of the pre-hydrolyzed TEOS was as follows: TEOS and anhydrous ethanol were mixed to obtain a mixed solution. 0.1 mol / L HCl was added dropwise to adjust the pH to 2.0. Deionized water was added in a molar ratio of H2O:TEOS = 2:1 in a 40°C constant temperature water bath. Magnetic stirring (500 rpm) was performed for 30 min. The density of the sample was measured by a precision hydrometer, and the density was 1.12 g / cm 3 , and the hydrolysis degree was approximately 55%. After completion, the sample was immediately cooled to 25°C in an ice bath, and ammonia water was added to neutralize the pH to 7.0 to obtain the pre-hydrolyzed TEOS.

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

[0052] Example 2

[0053] The graphene / lithium salt eutectic composite phase change material was prepared by the following steps:

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

[0055] (2) 0.6 g of hexadecyl trimethyl ammonium bromide was dissolved in 135 mL of 65 °C deionized water, and after magnetic stirring until clear, 0.1 g of graphene powder was added. The mixture was ultrasonically treated at 45 kHz and 650 W for 50 min, followed by centrifugation at 8000 rpm for 10 min to remove >1 μm undispersed agglomerates, and then collected at 12000 rpm to obtain a few-layer graphene dispersion. 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 few-layer graphene dispersion with adjusted pH 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 h to form a sol. Then, pre-hydrolyzed tetraethyl orthosilicate (TEOS) was slowly added dropwise to the sol under stirring (the amount of pre-hydrolyzed TEOS added was 6% of the volume of the sol), 0.2 mol / L NH3·H2O was added dropwise until the pH was 11.2, and stirring was continued until a uniform milky white color was obtained without particles or phase separation. After stopping the stirring, the three-dimensional SiO2 network-encapsulated composite gel was formed by standing at 55 °C for 20 h. The method for preparing the pre-hydrolyzed TEOS was as follows: TEOS and anhydrous ethanol were mixed to obtain a mixed solution, 0.1 mol / L HCl was added dropwise to adjust the pH to 2.5, deionized water was added in a molar ratio of H2O:TEOS=2:1 in a 40 °C constant temperature water bath, and magnetic stirring (500 rpm) was performed for 30 min. The density of the sample was measured by a precision hydrometer, and the standard curve was used for comparison. The density was 1.12 g / cm3.3 The hydrolysis degree is about 55%, and the pre-hydrolyzed tetraethyl orthosilicate is cooled to 25°C in an ice bath immediately after completion, and ammonia water is added dropwise to neutralize to pH = 7.3.

[0057] (4) The three-dimensional SiO2 network-wrapped composite gel obtained in step (3) is poured into a polytetrafluoroethylene mold, gradient freezing (-20°C→-50°C→liquid nitrogen) for 15 minutes per stage to form vertically oriented through-pores, and then freeze-drying at -60°C, 0.05 mbar vacuum for 60 hours to obtain a porous aerogel. The porous aerogel is placed in an argon atmosphere tube furnace, heated to 550°C at a rate of 5°C / min, and calcined for 3 hours; hydrogen fluoride acid vapor etching is performed with a volume percentage of hydrogen fluoride acid vapor of 0.5 vol%, and the etching time is 15 min; then ultrasonic cleaning with 0.2 mol / L HNO3 for 30 min, and finally vacuum heat treatment at 200°C for 4 hours to obtain a graphene / lithium salt eutectic composite phase change material.

[0058] Example 3

[0059] The preparation of the 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 protection tube furnace, melted at 360°C for 3 hours, and then air-grounded to 300 mesh powder after liquid nitrogen rapid cooling (the rate of liquid nitrogen rapid cooling is 50°C / s) to obtain a eutectic salt. The purity of the lithium salt raw material is ≥99.5%, and the particle size is ≤50μm.

[0061] (2) 0.45g of cetyltrimethylammonium bromide is dissolved in 100mL of 55°C deionized water, and 0.05g of graphene powder is added after magnetic stirring to clarify, and then ultrasonic treatment is performed at 35kHz, 550W for 40 minutes, followed by centrifugation at 8000rpm for 10 minutes to remove >1μm undispersed agglomerates, and 12000rpm to collect a few-layer graphene dispersion liquid, and the pH of the few-layer graphene dispersion liquid is adjusted to 8.0.

[0062] (3) The eutectic salt obtained in step (1) (20 g) 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 TEOS added was 4% of the volume of the sol), 0.15 mol / L NH3·H2O was added dropwise to a pH of 10.8, and stirring was continued until a uniform milky white color was obtained without particles or phase separation; after stopping the stirring, the three-dimensional SiO2 network- wrapped composite gel was formed by standing at 45°C for 18 hours. The preparation method of the pre-hydrolyzed TEOS was as follows: tetraethyl orthosilicate was mixed with anhydrous ethanol to obtain a mixed solution, 0.1 mol / L HCl was added dropwise to adjust the pH to 3.0, deionized water was added in a molar ratio of H2O:TEOS = 2:1 in a 40°C constant temperature water bath, and magnetic stirring (500 rpm) was performed for 30 min; the density of the sample was measured by a precision hydrometer, and the density was 1.12 g / cm 3 , then the degree of hydrolysis was approximately 55%, and immediately after completion, the pre-hydrolyzed TEOS was cooled to 25°C in an ice bath and neutralized to pH = 7.5 by adding ammonia.

[0063] (4) The three-dimensional SiO2 network-wrapped composite gel obtained in step (3) was poured into a polytetrafluoroethylene mold, and vertical directional freezing was performed in liquid nitrogen (rate 10 mm / min) for 25 minutes to form vertically oriented through channels, followed by freeze-drying at -55°C and 0.2 mbar vacuum for 36 hours to obtain a porous aerogel; the porous aerogel was placed in an argon atmosphere tube furnace, heated to 620°C at a rate of 5°C / min, and calcined for 1.5 hours; then the porous aerogel was 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 the graphene / lithium salt eutectic composite phase change material included 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 protection tube furnace, heated to 380°C for melting and heat preservation for 2 hours, and then formed a metastable eutectic structure by liquid nitrogen rapid cooling (the rate of liquid nitrogen rapid cooling was 50°C / s); then the eutectic salt was obtained by planetary ball milling to a 200 mesh powder (particle size distribution D50 = 15 ± 3 μm). 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 cetyltrimethylammonium bromide was dissolved in 135 mL of deionized water at 60°C, and after magnetic stirring until clear, 0.1 g of graphene powder was added, and ultrasonic treatment was performed at 40 kHz and 600 W for 45 minutes, followed by centrifugation at 8000 rpm for 10 minutes to remove >1 μm undispersed agglomerates, and a 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 obtained in step (1) (10 g) and the few-layer graphene dispersion with adjusted pH 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 a sol was formed after magnetic stirring for 4 hours; then pre-hydrolyzed tetraethyl orthosilicate (TEOS) was slowly added dropwise under stirring (the amount of pre-hydrolyzed TEOS added was 5% of the volume of the sol), 0.1 mol / L NH3·H2O was added dropwise to pH = 11, and stirring was continued until a uniform milky white color was obtained without particles or phase separation, and after stopping stirring, a three-dimensional SiO2 network-encapsulated composite gel was formed by standing at 50°C for 24 hours. The preparation method of the pre-hydrolyzed TEOS was as follows: tetraethyl orthosilicate was mixed with anhydrous ethanol to obtain a mixed solution, 0.1 mol / L HCl was added dropwise to adjust the pH to 2.3, deionized water was added at 40°C in a constant temperature water bath according to H2O:TEOS = 2:1 (molar ratio), and magnetic stirring (500 rpm) was performed for 30 min. The density of the sample was measured by a precision hydrometer, and the density was 1.12 g / cm3 3 , then the degree of hydrolysis was approximately 55%, and immediately after completion, the sample was cooled to 25°C in an ice bath, and ammonia was added dropwise to neutralize to pH = 7.2 to obtain pre-hydrolyzed TEOS.

[0069] (4) The three-dimensional SiO2 network-encapsulated composite gel obtained in step (3) was poured into a polytetrafluoroethylene mold, and vertical orientation was performed by freezing from the bottom up in liquid nitrogen (cooling rate 20°C / min) for 20 minutes to form vertically oriented through channels, followed by freeze-drying at -50°C and 0.1 mbar vacuum for 48 hours to obtain a porous aerogel, and the porous aerogel was placed in a tube furnace in an argon atmosphere, and heated to 600°C at a rate of 5°C / min and calcined for 2 hours; then the porous aerogel was 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 the SiO2-encapsulated lithium salt eutectic composite phase change material included 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) and placed in a tube furnace under argon protection. The temperature was raised to 380℃ and held for 2 hours. The metastable eutectic structure was formed by liquid nitrogen rapid cooling (the rate of liquid nitrogen rapid cooling was 50℃ / s). The product was ground into a 200 mesh powder (particle size distribution D50=15±3μm) by a planetary ball mill to obtain the eutectic salt.

[0073] (2) 0.8 g of hexadecyl trimethyl ammonium bromide (CTAB) was dissolved in 200 mL of 60℃ deionized water, and stirred magnetically until clear to obtain a uniform CTAB solution.

[0074] (3) The eutectic salt obtained in step (1) (10 g) was mixed with the CTAB solution obtained in step (2) in a mass ratio of 1:5, and 0.1 mol / L NaOH was added dropwise to adjust the pH to 9.5. The sol was formed by magnetic stirring for 4 hours. Then pre-hydrolyzed tetraethyl orthosilicate (TEOS) was slowly added dropwise to the sol under stirring (the amount of pre-hydrolyzed TEOS added was 5% of the volume of the sol), and 0.1 mol / L NH3·H2O was added dropwise to pH=11. The stirring was continued until a uniform milky white color was obtained without particles or phase separation. After stopping the stirring, the three-dimensional SiO2 network-encapsulated composite gel was formed by standing at 50℃ for 24 hours. The preparation method of the pre-hydrolyzed TEOS was as follows: TEOS was mixed with anhydrous ethanol to obtain a mixed solution, 0.1 mol / L HCl was added dropwise to adjust the pH to 2.0, deionized water was added in a molar ratio of H2O:TEOS=2:1 in a 40℃ constant temperature water bath, and magnetic stirring (500 rpm) was carried out for 30 min. The density of the sample was measured by a precision hydrometer, and the density was 1.12 g / cm3. Therefore, the degree of hydrolysis ≈55%, and the pre-hydrolyzed TEOS was obtained by cooling to 25℃ in an ice bath immediately after completion and neutralizing with ammonia water to pH=7.0. 3

[0075] (4) The three-dimensional SiO2 network-encapsulated composite gel obtained in step (3) was poured into a polytetrafluoroethylene mold and directionally frozen from bottom to top in liquid nitrogen (cooling rate 20℃ / min) for 20 minutes. Then the porous aerogel was obtained by freeze-drying at -50℃ and 0.1 mbar vacuum for 48 hours. The porous aerogel was placed in a tube furnace under argon atmosphere, and the temperature was raised to 600℃ at a rate of 5℃ / min and calcined for 2 hours. The porous aerogel was then ultrasonically cleaned with 0.1 mol / L HNO3 for 30 minutes, and finally vacuum heat-treated at 180℃ for 3 hours to obtain the 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) included 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) and placed in a tube furnace under argon protection. The temperature was raised to 380°C and held for 2 hours. The product was rapidly cooled in liquid nitrogen (the rate of liquid nitrogen cooling 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) using a planetary ball mill to obtain the eutectic salt.

[0079] (2) 0.85 g of cetyltrimethylammonium bromide (CTAB) was dissolved in 200 mL of deionized water at 60°C. After magnetic stirring until clear, 0.1 g of graphene powder was added. The mixture was treated with ultrasonic waves at 40 kHz and 600 W for 45 minutes. Then, the mixture was centrifuged at 8000 rpm for 10 minutes to remove >1 μm undispersed agglomerates. The resulting dispersion of few-layer graphene was collected at 12000 rpm and the pH of the dispersion of few-layer graphene was adjusted to 8.0.

[0080] (3) The eutectic salt obtained in step (1) (10 g) was mixed with the pH-adjusted dispersion of few-layer graphene 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. The mixture was stirred magnetically for 4 hours to form a uniform sol.

[0081] (4) The sol obtained in step (3) was poured into a polytetrafluoroethylene mold and directionally frozen from bottom to top in liquid nitrogen (cooling rate 20°C / min) for 20 minutes. Then, the mixture was freeze-dried at -50°C and 0.1 mbar for 48 hours to obtain a porous aerogel. The porous aerogel was placed in a tube furnace under argon atmosphere and heated to 600°C at a rate of 5°C / min and calcined for 2 hours. The calcined product was ultrasonically washed with 0.1 mol / L HNO3 for 30 minutes. Finally, the product was 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 dispersion of few-layer graphene in Examples 1-4 was calculated by drying and weighing a certain amount of the dispersion of few-layer graphene and calculating the mass concentration. The results showed that the concentration of graphene in the dispersion of few-layer graphene in Examples 1-4 was 0.9 g / L, 0.8 g / L, 1.0 g / L and 1.0 g / L, respectively. The high concentration ensured that the content of graphene in the composite material was sufficient to form a continuous heat conduction network.

[0083] The thickness of the graphene in the few-layer graphene dispersion in Examples 1-4 was measured by TEM, the lateral size was counted by SEM, and the particle size distribution was measured by DLS. The results were that the thickness of the graphene in the few-layer graphene dispersion in Examples 1-4 was 0.85 nm, 0.92 nm, 0.78 nm, and 0.75 nm, respectively, the lateral size was 2.5 μm, 3.0 μm, 1.8 μm, and 1.5 μm, respectively, and the particle size distribution was D90 480 nm, D90 520 nm, D90 450 nm, and D90 420 nm, respectively. The large lateral size enhances the heat conduction path, and the small D90 inhibits agglomeration.

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

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

[0086] The pore size distribution of the calcined material in step (4) of Examples 1-4 was analyzed by SEM image. The pore size was 120 nm, 150 nm, 100 nm, and 80 nm, respectively. The pore size ratio of the calcined material in step (4) of Examples 1-4 was counted by mercury intrusion method. The results were that the pore size distribution concentration of the calcined material 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 calcined material in step (4) of Examples 1-4 was calculated by BET nitrogen adsorption method. The results were that the specific surface area of the calcined material was 310 m 2 / g, 295 m 2 / g, 330 m 2 / g, 350 m 2 / g, respectively. The high specific surface area enhances the molten salt wettability and improves the heat storage density. The material still maintains a high specific surface area and uniform pore channel after high-temperature calcination, indicating that the material does not sinter or structure collapse at extreme temperatures, and indicating that the graphene / lithium salt eutectic composite phase change material prepared is suitable for a solar thermal power station.

[0087] The graphene / lithium salt eutectic composite phase change material prepared in Example 1 was subjected to electron microscopy scanning, and the results are shown in Figure 1 and Figure 2 . Figure 1It can be seen that the graphene / lithium salt eutectic composite phase change material presents a rod-like or needle-like structure, which is interlaced and stacked with each other, which helps to form an efficient heat conduction channel. There are some elongated crystal structures in the material, which are likely to be lithium salt crystals. There is a certain interaction between the lithium salt crystals and the graphene layers, which can limit the growth and movement of the crystals to a certain extent, enhance the structural stability of the material, prevent the material from being damaged due to the large volume change during the phase change cycle, and thus improve the cycle stability and service life of the material. From the SEM images of the material after 1000 cycles, it can be seen that the material still maintains a relatively complete structure, and the graphene layers are still relatively uniform and orderly arranged, which indicates that the material has a good cycle stability. Figure 2 It can be seen that the material presents a relatively dense columnar microstructure, and the size of the columnar structure is relatively uniform, and the surface seems to have some rough texture. These columnar structures are likely to be specific morphologies of lithium salt grown under the template action of graphene. Such columnar and rough surface structure can significantly increase the specific surface area of the material. The larger specific surface area is beneficial to heat exchange with the external environment, and can more quickly absorb or release heat from the external environment during the phase change process, thereby improving the thermal response speed of the phase change material.

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

[0089] The graphene / lithium salt eutectic composite phase change material prepared in Example 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 prepared in Comparative Example 2 (without SiO2 encapsulation) were subjected to 50 cycles of use simulation. After the cycles, the amount of leaked molten salt was accurately measured, and compared with the total amount of the initial molten salt to calculate the molten salt leakage rate. The results of the determination were as follows: the molten salt leakage rate of Example 1 was 1.3%, the molten salt leakage rate of Example 2 was 1.6%, the molten salt leakage rate of Example 3 was 1.0%, the molten salt leakage rate of Example 4 was 0.8%, the molten salt leakage rate of Comparative Example 1 was 3.4%, and the molten salt leakage rate of Comparative Example 2 was 5.1%. The molten salt leakage rates of Examples 1-4 were all low, and were all below 2%, indicating that under the respective preparation conditions, the materials had good stability of encapsulation or internal structure of the molten salt, and could effectively prevent leakage of the molten salt. Figure 1 The SEM showed that the graphene sheets were embedded in the pore walls of the SiO2 network to form a nanoscale double barrier structure, effectively blocking the penetration path of the molten salt. The molten salt leakage rates of Examples 1-4 were all below 2%, while the leakage rates of Comparative Examples 1 (without graphene) and 2 (without SiO2 encapsulation) were as high as 3.4% and 5.1% respectively, indicating that the physical-chemical synergistic encapsulation mechanism of graphene and SiO2 was the key factor for inhibiting leakage of the molten salt, and the physical barrier of graphene and the confinement of the SiO2 network were indispensable. In addition, the directional freezing process optimized the uniformity of the pores, further reducing the leakage rate, and confirmed that structure regulation and interface bonding synergistic optimization were the key to improving the cycle stability of the heat storage material.

[0090] The initial phase change enthalpy value of the graphene / lithium salt eutectic composite phase change material prepared in Example 4 and the phase change enthalpy values after 10, 20, 30, 40 and 50 cycles were measured, and the results are shown in Table 1. Figure 3 As can be seen from Table 1, Figure 3 The initial phase change enthalpy value of the graphene / lithium salt eutectic composite phase change material prepared in Example 4 was 450 J / g, and the phase change enthalpy value after 50 cycles was 405 J / g. The enthalpy retention rate was calculated by comparing the phase change enthalpy value after the cycles with the initial phase change enthalpy value, and the enthalpy retention rate was 90%. It can be seen from the enthalpy retention rate that the graphene / lithium salt eutectic composite phase change material had good thermal stability and energy storage stability, and could well maintain its energy storage capacity during multiple cycles. The graphene dispersion reduced the interfacial thermal resistance, and the SiO2 network confinement inhibited phase separation, and the enthalpy retention rate was 90% after 50 cycles.

[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 indicated that the material did not undergo significant phase separation or structural degradation during long-term high-temperature cycles, and had excellent thermal stability.

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

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

[0094] In the steel smelting scene, the graphene / lithium salt eutectic composite phase change material of the present application is integrated into a high-temperature waste gas (500-600°C) heat exchange system to recover industrial waste heat and drive molten salt phase change heat storage. Subsequently, when heat energy is needed during the production process, the stored heat energy is used to preheat the raw materials of the steel smelting furnace or drive a low-temperature waste heat generator set, reducing the need for direct fuel combustion. This not only significantly improves the recovery rate of industrial waste heat, but also reduces the energy cost for the factory, achieving 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 fast heat storage / release characteristics of the composite material significantly reduce fuel consumption, helping the steel industry to achieve "zero-carbon steelmaking" and promoting 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 a building outer wall or roof energy storage module, absorbs solar energy and stores heat during the day, and releases heat energy to drive heating or absorption refrigeration at night. The graphene / SiO2 composite light-weight aerogel matrix (three-dimensional SiO2 network wrapped composite gel) has high thermal conductivity, improves heat storage and release efficiency, and reduces building energy consumption. The vertical pore structure accelerates the phase change rate of the molten salt, quickly completes more heat energy storage, adapts to the structural requirements of high-rise buildings, and simultaneously realizes seamless integration with the appearance of the building through modular design. In summer, the stored heat energy during the day drives the refrigeration system, which can reduce power 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 is only a specific embodiment of the present application, not all embodiments, any equivalent transformation of the technical solution of the present application by a person of ordinary skill in the art through reading the present application is covered by the claims of the present application.

Claims

1. A method for preparing a graphene / lithium salt eutectic composite phase change material, characterized in that, Includes the following steps: (1) Mix lithium hydroxide and lithium carbonate, then heat to 360-400℃ under inert gas protection and hold for 1.5-3h. After rapid cooling with liquid nitrogen, refine the particles to obtain eutectic salt. (2) Dissolve hexadecyltrimethylammonium bromide in water to form a hexadecyltrimethylammonium bromide solution, then add graphene to the hexadecyltrimethylammonium bromide solution, and after ultrasonic treatment and centrifugation to remove agglomerates, obtain a few-layer graphene dispersion, and adjust the pH of the few-layer graphene dispersion to 7.5-8.5; (3) Mix the eutectic salt obtained in step (1) and the pH-adjusted few-layer graphene dispersion obtained in step (2), adjust the pH to 9.5-10.0, and continue stirring for 3.5-5h to form a sol; then slowly add pre-hydrolyzed tetraethyl orthosilicate to the sol while stirring, adjust the pH to 10.8-11.2, continue stirring until it is uniformly milky white, stop stirring and let it stand at 45-55℃ for 18-24h to form a composite gel wrapped with a three-dimensional SiO2 network; (4) The composite gel wrapped by the three-dimensional SiO2 network obtained in step (3) is directionally frozen with liquid nitrogen and then freeze-dried in vacuum. Then it is calcined in an inert atmosphere at a rate of 3-8℃ / min to 550-620℃ for 1.5-3h. After ultrasonic cleaning and vacuum heat treatment, the graphene / lithium salt eutectic composite phase change material is obtained.

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

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 liquid nitrogen rapid cooling rate described in step (1) is ≥50℃ / s; The eutectic salt in step (1) has a particle size of 150-300 mesh.

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

5. The method for preparing the graphene / lithium salt eutectic composite phase change material according to claim 1, characterized in that, The mass ratio of the eutectic salt to the pH-adjusted few-layer graphene dispersion in step (3) is 1:(4-6); The method for preparing the pre-hydrolyzed tetraethyl orthosilicate in step (3) is as follows: tetraethyl 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, then water is added and the mixture is stirred for a period of time, and then immediately cooled to 20–30℃, and the pH is adjusted to 7.0–7.5 to obtain the pre-hydrolyzed tetraethyl orthosilicate; the heating temperature is 35–45℃, the molar ratio of water to tetraethyl orthosilicate is 1.8:1–2.2:1, and the stirring reaction time is 25–35 min; The amount of pre-hydrolyzed tetraethyl orthosilicate added in step (3) is 4-6% of the sol volume.

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) satisfy any one of the following: Liquid nitrogen directional freezing from bottom to top: cooling rate is 15-25℃ / min, bottom liquid nitrogen contact surface temperature ≤-150℃, top natural cooling, freezing time 15-25min; Liquid nitrogen gradient freezing: freezing in stages sequentially at -20℃ → -50℃ → liquid nitrogen environment, with each stage freezing time being 10-20 minutes; Vertical directional freezing with liquid nitrogen: The freezing interface is advanced along the vertical direction at a rate of 5-15 mm / min, with a total freezing time of 20-30 min; The vacuum freeze-drying process in step (4) is carried out at a temperature of -60℃ to 50℃, a pressure of 0.05 to 0.2 mbar, and a time of 36 to 60 h. The temperature of the vacuum heat treatment in step (4) is 160-200℃ and the time is 2.5-4h.

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

8. The application of the graphene / lithium salt eutectic composite phase change material as described in claim 7 as a heat storage medium in a heat absorber.

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

10. The application of the graphene / lithium salt eutectic composite phase change material according to claim 7 in building exterior walls or rooftop energy storage modules.

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