Multi-element nano carbon-based heat storage composite material and preparation method thereof

Abstract

The invention provides a multi-element nano carbon-based heat storage composite material and a preparation method thereof, the multi-element nano carbon-based heat storage composite material is a heat storage material loaded by a nano mesoporous carbon material, the heat storage material is magnesium oxide and lithium hydroxide and / or lithium chloride, and relative to 100 wt% of the multi-element nano carbon-based heat storage composite material, the content of the magnesium oxide and the lithium hydroxide and / or lithium chloride is 100 wt%. The content of magnesium oxide is 30wt%-80wt%, the total content of the heat storage material is 30.6 wt%-92wt%, and the balance is the nano mesoporous carbon material. Due to the addition of lithium chloride and / or lithium hydroxide, the dehydration temperature of magnesium hydroxide is reduced, the dehydration / hydration rate is increased, and the heat storage dynamics and reversibility of magnesium oxide are greatly improved.

Description

Technical Field

[0001] The present invention relates to the field of heat storage, and in particular to a method for preparing a multi-element nano-carbon-based heat storage composite material. Background Art

[0002] Climate warming and resource depletion have become global concerns, and the need to reduce carbon dioxide emissions is imminent and has received widespread attention. Studies have shown that the effective utilization rate of industrial energy in my country is only about 33%, and less than 55% of industrial waste heat is recycled, resulting in a large amount of medium and low-grade thermal energy (150-300°C) loss. Therefore, waste heat recovery is of great significance for improving energy efficiency and reducing energy consumption. At present, thermal energy storage has become a research hotspot. According to the heat storage method, it is divided into three main types: latent heat storage, sensible heat storage and chemical heat storage. Among them, chemical heat storage has received widespread attention because of its high energy density and the fact that it does not have the problems of overcooling and phase separation of latent heat storage and sensible heat storage. In chemical heat storage technology, MgO / PbO and Mg(OH) are used. 2 / Pb(OH) 2 The reversible hydration / dehydration reaction of NH4Cl2 to release / store thermal energy is considered to be one of the most promising thermal storage systems.

[0003] However, Mg(OH) 2 In the low temperature range of 100-300℃, effective dehydration reaction cannot occur. This is mainly because the dehydration of thermal storage materials requires high activation energy. Modifying thermal storage materials to reduce their desorption temperature is an effective way to overcome the above defects. A large number of studies have shown that in Mg(OH) 2 and Pb(OH) 2 Adding highly hydrophilic adsorbent salts to the heat storage material can effectively reduce the dehydration activation energy and increase the hydration rate (Li et.Al., Tremendous enhancement of heat storage efficiency for Mg(OH) 2 -MgO-H 2 O thermochemiPbl system with addition of Ce(NO 3 ) 3 and LiOH, Nano Energy, Volume 81, 2021). The results showed that these additives reduced the Mg(OH) 2 The dehydration temperature is 76℃. Mg(OH) 2 The dehydration rate and heat storage rate of Mg(OH) increased significantly at 270℃. The activation energy of the dehydration reaction was greatly reduced, which proved that Mg(OH) 2 -Ce(NO 3 ) 3-LiOH composite materials are promising heat storage materials. CN111684039A proposes a chemical heat storage material and a preparation method thereof. The composite heat storage material is prepared by mixing lead or magnesium hydroxide and lithium compounds in a certain proportion, and adding at least one metal compound from the group consisting of nickel, cobalt, copper, aluminum, iron and zinc. The composite heat storage material has a higher reaction rate and can achieve heat storage at a lower temperature, but the material is prone to agglomeration during the hydration / dehydration process. CN111978922A proposes a hydrated salt-based medium- and low-temperature chemical heat storage material and a preparation method. Three types of hydrated salts with different properties are loaded in a porous carbon material according to a specific weight ratio. By proportioning heat storage materials with different desorption temperatures, the heat storage temperature of the composite material can be regulated. However, this composite method is only for water and salt heat storage materials with low heat storage temperatures (≤150°C) and that are easily soluble in water. It is not suitable for heat storage materials that are difficult to dissolve in water in the medium temperature region (such as Mg(OH) 2 and Pb(OH) 2 CN201811560036.X uses graphene and / or expanded graphite or expanded graphite-graphene composite carbon materials as a matrix to load chemical thermal storage materials such as calcium sulfate, calcium chloride or magnesium sulfate. Because the carbon material has a high specific surface area, the active components of the chemical thermal storage material are dispersed and not easy to agglomerate, which is beneficial to the improvement of the thermal storage performance of the thermal storage material.

[0004] However, the dehydration temperature of the heat storage materials in the prior art is still relatively high, and the dehydration / hydration rate needs to be further improved. Summary of the invention

[0005] The invention discloses a preparation method of a multi-element nano carbon-based heat storage composite material, wherein magnesium hydroxide and lithium hydroxide or chloride are selected as heat storage materials, and acid ion magnesium salt soluble in water is used as a heat storage material precursor, and the mesoporous material is preferably a nano-mesoporous carbon material. Acid ion magnesium salt and lithium chloride or lithium hydroxide crystals are dissolved in deionized water, a porous carbon material is added to the above solution, a hydrothermal method is used for high pressure impregnation, and then the suspended solid-liquid mixture is rotated and dried, and the dried material is placed in an inert environment and calcined at 330-450°C for 3h to obtain a multi-element nano carbon-based heat storage composite material. The invention selects two heat storage materials and a carbon material for coupling, and the heat storage temperature of the composite material can be regulated, the heat storage temperature range is wide, and low-grade thermal energy is fully utilized; the solvent hydrothermal-calcination method makes the heat storage material not only uniformly loaded inside the carbon material pores, but also the heat storage material particles have a small particle size, so that the hydration reaction rate is greatly improved, the heat storage density is high, and the cycle stability is good.

[0006] In view of the problems and defects in the prior art, the purpose of the present invention is to provide a method for preparing a multi-element nano-carbon-based thermal storage composite material, and a multi-element nano-carbon-based thermal storage composite material prepared by the method.

[0007] According to a first aspect of the present invention, a multi-component nano-carbon-based thermal storage composite material is provided, which is a thermal storage material loaded with a nano-mesoporous carbon material, wherein the thermal storage material is magnesium oxide, and lithium hydroxide and / or lithium chloride, relative to 100wt% of the multi-component nano-carbon-based thermal storage composite material, the content of magnesium oxide is 30wt% to 80wt%, and the total content of the thermal storage material is 30.6wt% to 92wt%, preferably 75wt% to 92wt%, and the rest is nano-mesoporous carbon material.

[0008] Preferably, the content of lithium hydroxide or lithium chloride is 2 wt% to 15 wt% of the mass of the added magnesium oxide.

[0009] Preferably, the heat storage density of the multi-element nano-carbon-based heat storage composite material is 420-1053 kJ / kg.

[0010] Preferably, the pore size of the nano-mesoporous carbon material is 2 to 50 nm, and the BET specific surface area is 200 m 2 / g or above.

[0011] Preferably, the nano-mesoporous carbon material is NCP or CMK3 nano-mesoporous carbon material.

[0012] According to a second aspect of the present invention, there is provided a method for preparing the multi-element nano-carbon-based thermal storage composite material according to the present invention, which comprises the following steps:

[0013] 1) completely dissolving a water-soluble salt of magnesium, lithium hydroxide and / or lithium chloride in deionized water, and then adding nano-mesoporous carbon material thereto;

[0014] 2) adding the mixture obtained in step 1) into a high-pressure reactor for hydrothermal reaction, and then drying by rotary evaporation;

[0015] 3) calcining the dried product obtained in step 2) under an inert atmosphere, and grinding it after cooling to obtain a multi-element nano-carbon-based thermal storage composite material;

[0016] The weight ratio of the water-soluble salt of magnesium converted into magnesium oxide: lithium hydroxide and / or lithium chloride: nano-mesoporous carbon material is 30-80: 0.6-12: 8-69.4.

[0017] In the present invention, water-soluble salts of magnesium, and lithium hydroxide and / or lithium chloride are used as precursors of thermal storage materials. After impregnation, these precursors enter the nano-mesoporous material, and the thermal storage material is in-situ formed and loaded in the nano-mesoporous material through a solvent hydrothermal-calcination method.

[0018] Preferably, the pore size of the nano-mesoporous carbon material is 2 to 50 nm, and the BET specific surface area is 200 m 2 / g or above.

[0019] Preferably, the nano-mesoporous carbon material is NCP or CMK nano-mesoporous carbon material.

[0020] Preferably, the nano-mesoporous carbon material is ground before use.

[0021] Preferably, the water-soluble salt of magnesium is selected from magnesium nitrate, magnesium acetate and magnesium oxalate; more preferably magnesium nitrate or magnesium acetate.

[0022] Preferably, the hydrothermal reaction conditions in step 2) are: temperature 90-110° C.; pressure 0.1-2 MPa; and time 6-12 h.

[0023] Preferably, the drying conditions in step 2) are: first remove moisture, and then dry at a temperature of 90-100°C.

[0024] Preferably, the calcination conditions in step 3) are: nitrogen atmosphere; calcination temperature is 330-450° C.; and calcination time is 3-4 hours.

[0025] Preferably, in step 3), the product is ground to a particle size of 75 to 100 μm.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. The addition of lithium chloride and / or lithium hydroxide reduces the dehydration temperature of magnesium hydroxide and increases the dehydration / hydration rate, greatly improving the heat storage kinetics and reversibility of magnesium oxide.

[0028] 2. The water-soluble salt of magnesium is used as a precursor of the thermal storage material. The thermal storage material is evenly loaded in the porous medium through the solvent hydrothermal-calcination method, with high loading capacity and good thermal conductivity.

[0029] 3. Selecting nano-mesoporous carbon materials as the matrix can effectively inhibit the agglomeration of the heat storage material during the reaction process, solve the problem of the slow hydration reaction rate of the pure active component, and significantly improve the heat and mass transfer efficiency.

[0030] 4. By adjusting the mass ratio of binary thermal storage materials, the thermal storage temperature can be regulated and the waste heat can be fully recovered. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a synchronous thermal analysis structure diagram of MgO, and the multi-element nano-carbon-based thermal storage composite materials prepared according to Examples 1 to 4 and the nano-carbon-based thermal storage composite materials prepared in Comparative Example 1.

[0032] Figure 2 The SEM images are of the carbon-based thermal storage composite material prepared according to Comparative Example 1, and the multi-element nano-carbon-based thermal storage composite materials prepared according to Example 1, Example 2 and Example 4.

[0033] Figure 3 It is a hydration rate curve diagram of MgO, the multi-element nano-carbon-based thermal storage composite materials prepared according to Examples 1 to 4, and the nano-carbon-based thermal storage composite materials prepared in Comparative Example 1 at a reaction temperature of 110° C. and a water vapor partial pressure of 57.8 kPa.

[0034] Figure 4 This is a hydration rate curve of the multi-element nano-carbon-based thermal storage composite material prepared according to Example 6 at a reaction temperature of 110° C. and a water vapor partial pressure of 57.8 kPa.

[0035] Figure 5 Schematic diagram of the structure of a thermogravimetric balance used to test the hydration rate of thermal storage materials. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0037] Material

[0038] NCP-10: purchased from Nanjing Jicang Nano, pore size 10nm, BET specific surface area 200~600m 2 / g;

[0039] CMK-3: purchased from Nanjing Jicang Nano, pore size 3.5nm, BET specific surface area 500m 2 / g.

[0040] method

[0041] Thermal storage density measurement method: Thermal storage density is determined by differential scanning calorimetry, which is a thermal analysis method that can directly measure the enthalpy change of the sample during heating. The differential scanning calorimeter is composed of a heating furnace, a program temperature control system, an atmosphere control system and a signal recording system. The principle is to place the sample and the reference in a mutually insulated heating system, give the same thermal power for heating, and then in order to keep the temperature of the sample and the reference consistent, when the sample changes, in order to maintain isothermal with the reference, the microheater installed at the bottom of the sample and the reference will provide energy to keep the two at the same temperature. The energy provided by the microheater will be converted into an electrical signal and recorded, which is the origin of the DSC curve. By integrating the area of ​​the peaks appearing in the DSC curve. The heat required for the reaction of the material corresponding to the DSC peak can be calculated, and this part of the heat corresponds to the chemical thermal storage density of the material. The TGA / DSC produced by Mettler-Toledo of Switzerland is used here. 3+ Thermogravimetric analyzer.

[0042] The hydration rate of the thermal storage material is measured by Figure 5 The thermogravimetric balance shown in the figure is used for measurement, which includes an electronic balance, a vertical tube furnace, a water vapor generator and a quartz boat. The quartz boat is applied to the electronic balance through a hook to detect the weight change of the sample in the quartz boat. The electronic balance and the vertical tube furnace are in a closed cavity. The vertical tube furnace has an air inlet located at the upper end and an air outlet located at the bottom. Feed the purge gas and water vapor from the air inlet, and discharge the gas from the air outlet. 20mL / min of nitrogen is introduced into the closed cavity of the electronic balance to prevent the high-temperature water vapor from moving upward, avoid large fluctuations in the balance, and reduce data acquisition errors. The test process is as follows: a multi-element nano-carbon-based thermal storage composite material as a sample is placed in the quartz boat, and an inert purge inert gas (nitrogen) is circulated at a rate of 100mL / min, and the temperature in the vertical tube furnace is set to 110°C; record the weight of the electronic balance, then turn on the water vapor generator, feed water vapor, so that the water vapor partial pressure is 57.8kPa, and record the reading change of the electronic balance. When the reading of the electronic balance no longer changes, stop feeding water vapor, maintain the temperature of the vertical tube furnace at 110°C for 5 minutes to remove the adsorbed water, and record the reading of the electronic balance.

[0043] Calculation of hydration conversion rate: The weight recorded before starting the steam generator is taken as the starting weight and the reverse hydration conversion rate is set to 0%, and the weight recorded on the electronic balance at the end is taken as the weight at the end of the reaction, and the hydration conversion rate is set to 100%. In this way, the hydration conversion rate during the reaction is calculated based on the recorded electronic balance reading.

[0044] The weight recorded before turning on the steam generator is used as the starting weight. The weight of the electronic balance will change as the reaction proceeds.

[0045] The recorded weight change Δm was substituted into the following formula to calculate the hydration conversion rate.

[0046]

[0047] In the formula is the hydration conversion rate, is the sample weight, unit: g, is the molecular weight of water, unit: g / mol, m is the sample mass, unit: g, ε is the magnesium oxide loading rate, M MgO is the molecular weight of magnesium oxide, unit: g / mol.

[0048] The measurement method of the relevant parameters of reversibility: Repeating the heat storage and hydration experiments for many times is called the cycle experiment, and analyzing the heat storage / hydration performance before and after is called the reversibility analysis.

[0049] Example 1

[0050] The multi-element nano-carbon-based thermal storage composite material is prepared by the following steps:

[0051] S1) Weigh 267.5 g of magnesium acetate tetrahydrate crystals (1.247 mol) and 2.5 g of LiCl crystals (0.059 mol), and completely dissolve them in 1000 g of deionized water at room temperature; then add 10 g of nano-mesoporous carbon material (NCP-10) to the mixed solution;

[0052] S2) the solid-liquid mixture is then poured into a high pressure reactor, immersed at 110°C and 1 MPa for 6 hours, and then rotary evaporated at a drying temperature of 90°C;

[0053] S3) The dried product is calcined in a tubular furnace for 3 hours under a nitrogen atmosphere, the calcination temperature is 400° C., and the nitrogen flow rate is 50 ml / min; finally, the calcined composite material is ground and sieved to 100 μm to obtain a multi-element nano-carbon-based thermal storage composite material.

[0054] The MgO loading rate of the heat storage material in the composite material (the loading rate is the percentage of the mass of MgO in the composite material to the mass of the composite material, calculated as: mass of magnesium oxide / mass of the composite material) is 80%, and the mass ratio of LiCl to MgO is 5wt%. The thermal storage density of the composite material measured in the experiment can reach 891kJ / kg.

[0055] Example 2

[0056] The multi-element nano-carbon-based thermal storage composite material was prepared in the same manner as in Example 1, except that the mass ratio of LiCl to MgO was adjusted to 10wt%, and the amount of nano-mesoporous carbon material (NCP-10) was reduced accordingly. The MgO loading rate of the thermal storage material in the composite material was 80%, and the mass ratio of LiCl to MgO was 10wt%. The thermal storage density of the composite material was measured to be 866kJ / kg.

[0057] Example 3

[0058] The multi-element nano-carbon-based thermal storage composite material was prepared in the same manner as in Example 1, except that the mass ratio of LiCl to MgO was adjusted to 15wt%, and the amount of nano-mesoporous carbon material (NCP-10) was reduced accordingly. The MgO loading rate of the thermal storage material in the composite material was 80%, and the mass ratio of LiCl to MgO was 15wt%. The thermal storage density of the composite material was measured to be 830kJ / kg.

[0059] Example 4

[0060] The multi-element nano-carbon-based thermal storage composite material was prepared in the same manner as in Example 1, except that the mass ratio of LiCl to MgO was adjusted to 2wt%, and the amount of nano-mesoporous carbon material (NCP-10) was increased accordingly. The MgO loading rate of the thermal storage material in the composite material was 80%, and the mass ratio of LiCl to MgO was 2wt%. The thermal storage density of the composite material was measured to be 897 kJ / kg.

[0061] Example 5

[0062] The multi-element nano-carbon-based thermal storage composite material is prepared by the following steps:

[0063] S1) Weigh 426.7 g of magnesium nitrate hexahydrate crystals (1.664 mol) and 6.67 g of LiOH crystals (0.2785 mol), and completely dissolve them in 2000 g of deionized water at room temperature; then add 10 g of nano-mesoporous carbon material (NCP-10) to the mixed solution;

[0064] S2) the solid-liquid mixture is then poured into a high pressure reactor, immersed at 100°C and 1.5 MPa for 6 hours, and then rotary evaporated at a drying temperature of 90°C;

[0065] S3) calcining the dried solid in a tubular furnace for 3 hours under a nitrogen atmosphere, the calcination temperature is 450° C., and the nitrogen flow rate is 50 ml / min; finally, the calcined composite material is ground and sieved to 100 μm to obtain a multi-element nano-carbon-based thermal storage composite material.

[0066] The MgO loading rate of the heat storage material in the composite material is 80%, and the mass ratio of LiOH to MgO is 10wt%. The heat storage density of the composite material measured experimentally is 871 kJ / kg.

[0067] Example 6

[0068] In addition to replacing NCP-10 in Example 5 with CMK3, a multi-element nano-carbon-based thermal storage composite material was prepared in the same manner as in Example 5. The MgO loading rate of the thermal storage material in the composite material was 80%, and the mass ratio of LiOH to MgO was 10wt%. The thermal storage density of the composite material was measured to be 878kJ / kg.

[0069] Comparative Example 1

[0070] Except that LiCl was not added, a nano-carbon-based thermal storage composite material was prepared in the same manner as in Example 1. The thermal storage density of the composite material was measured to be 946.5 kJ / kg.

[0071] Figure 1 The multi-element nano-carbon-based thermal storage composite materials prepared in Examples 1 to 4 are thermally stored (i.e., the MgO therein reacts with water to form Mg(OH) 2 ) of the synchronous thermal analysis data, from Figure 1 It can be seen that with the gradual increase of LiCl concentration, the initial decomposition temperature of the composite material tends to gradually decrease, as shown in the following table.

[0072] Table 1: Initial decomposition temperature of samples after thermal storage

[0073]

[0074]

[0075] Figure 2 The SEM images of the multi-element nano-carbon-based thermal storage composite materials prepared in Comparative Example 1 (MgO-NCP), Example 4 (2% LiCl + MgO-NCP), Example 1 (5% LiCl + MgO-NCP) and Example 2 (10% LiCl + MgO-NCP) are shown. Figure 2 It can be seen that the particle size of the thermal storage composite material becomes significantly smaller after doping with LiCl. This is because LiCl can effectively inhibit the growth and agglomeration of particles, which is beneficial to increasing the hydration rate of the composite thermal storage material and improving the reaction rate and reversibility of the thermal storage material.

[0076] Figure 3The hydration conversion rates of MgO, the multi-nano carbon-based thermal storage composite materials prepared according to Example 1 (NCP-MgO-LiCl-5), Example 2 (NCP-MgO-LiCl-10), Example 3 (NCP-MgO-LiCl-15) and Example 4 (NCP-MgO-LiCl-2) and the nano-carbon-based thermal storage composite materials prepared in Comparative Example 1 (NCP-MgO) at a reaction temperature of 110°C and a water vapor partial pressure of 57.8 kPa show that the hydration rate of the composite material after the nano-mesoporous carbon material is loaded with MgO is significantly faster than that of pure MgO; after adding lithium chloride, the hydration rate is significantly accelerated as the content gradually increases.

[0077] Figure 4 This is the hydration rate diagram of CMK3-loaded MgO and LiOH composite material at a reaction temperature of 110°C and a water vapor partial pressure of 57.8 kPa, which also has a faster hydration rate.

Claims

1. A multi-element nano-carbon-based thermal storage composite material, which is a thermal storage material loaded with nano-mesoporous carbon material, in, The heat storage material is magnesium oxide, lithium hydroxide and / or lithium chloride. Relative to 100wt% of the multi-element nano-carbon-based heat storage composite material, the content of magnesium oxide is 30wt% to 80wt%, and the total content of the heat storage material is 30.6wt% to 92wt%, and the rest is nano-mesoporous carbon material.

2. The multi-element nano-carbon-based thermal storage composite material according to claim 1, in, The content of lithium hydroxide or lithium chloride is 2wt% to 15wt% of the mass of the added magnesium oxide.

3. The multi-element nano-carbon-based thermal storage composite material according to claim 1 or 2, in, The nano-mesoporous carbon material has a pore size of 2 to 50 nm and a BET specific surface area of ​​200 m 2 / g or more; Preferably, the heat storage density of the multi-element nano-carbon-based heat storage composite material is 420-1053 kJ / kg.

4. A method for preparing the multi-element nano-carbon-based thermal storage composite material according to any one of claims 1 to 3, comprising the following steps: 1) completely dissolving a water-soluble salt of magnesium, lithium hydroxide and / or lithium chloride in deionized water, and then adding nano-mesoporous carbon material thereto; 2) adding the mixture obtained in step 1) into a high-pressure reactor for hydrothermal reaction, and then drying by rotary evaporation; 3) calcining the dried product obtained in step 2) under an inert atmosphere, and grinding it after cooling to obtain a multi-element nano-carbon-based thermal storage composite material; in, The weight ratio of the water-soluble salt of magnesium converted into magnesium oxide: lithium hydroxide and / or lithium chloride: nano-mesoporous carbon material is 30-80: 0.6-12: 8-69.

4.

5. The method for preparing a multi-element nano-carbon-based thermal storage composite material according to claim 4, in, The nano-mesoporous carbon material has a pore size of 2 to 50 nm and a BET specific surface area of ​​200 m 2 / g or above.

6. The method for preparing a multi-element nano-carbon-based thermal storage composite material according to claim 4 or 5, in, The nano-mesoporous carbon material is ground before use.

7. The method for preparing a multi-element nano-carbon-based thermal storage composite material according to any one of claims 4 to 6, in, The water-soluble salt of magnesium is selected from magnesium nitrate, magnesium acetate and magnesium oxalate; preferably magnesium nitrate or magnesium acetate.

8. The method for preparing a multi-element nano-carbon-based thermal storage composite material according to any one of claims 4 to 7, in, The hydrothermal reaction conditions in step 2) are: temperature 90-110° C.; pressure 0.1-2 MPa; time 6-12 h; Preferably, the drying conditions in step 2) are: first remove moisture, and then dry at a temperature of 90-100°C.

9. The method for preparing a multi-element nano-carbon-based thermal storage composite material according to any one of claims 4 to 8, in, The calcination conditions in step 3) are: nitrogen atmosphere; calcination temperature is 330-450° C.; and calcination time is 3-4 hours.

10. The method for preparing a multi-element nano-carbon-based thermal storage composite material according to any one of claims 4 to 9, in, In step 3), the product is ground to a particle size of 10-100 μm.

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