Photo-thermally renewable carbon nanocage confinement lithium chloride low-temperature energy storage composite material as well as preparation and application thereof

Abstract

The invention discloses a photo-thermal regenerative carbon nanocage confinement lithium chloride low-temperature energy storage composite material, and preparation and application thereof, and belongs to the technical field of energy storage materials. The composite material takes a carbon nanocage with a micropore-mesopore-macropore hierarchical structure as a carrier, lithium chloride is loaded in a nano cavity and a pore channel of the carbon nanocage in a limited range to form a stable composite structure, and the problems of leakage and agglomeration of lithium chloride can be effectively inhibited; during preparation, the carbon nanocage is prepared through a chemical vapor deposition method, and the carbon nanocage can be obtained through drying, vacuum impregnation, suction filtration and freeze-drying shaping, the process is simple and controllable, and the graded pore mass transfer advantage of the carbon nanocage can be reserved; the composite material can stably work under the environment with the temperature of 20-40 DEG C and the relative humidity of 30%-60%, and the fit lt is achieved; the device can capture, store and release 100 DEG C low-grade heat energy, can be directly driven by solar energy to dehydrate and regenerate, has the advantages of high cycle stability and green regeneration, is suitable for adsorption type heat storage, solar photo-thermal energy storage and industrial waste heat recovery devices, and has remarkable industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of energy storage materials technology, specifically relating to a photothermal regenerable carbon nanocage confined lithium chloride low-temperature energy storage composite material, as well as the preparation method of the composite material and its application in low-temperature thermochemical energy storage, solar energy utilization and industrial waste heat recovery. Background Technology

[0002] Thermal energy storage is one of the core technologies for solving the problem of the mismatch between energy supply and demand in time and space, especially in regions with significant diurnal and seasonal temperature fluctuations, where it plays a crucial role in improving energy utilization efficiency and ensuring the stability of energy supply. Among various thermal energy storage technologies, adsorption thermochemical energy storage, with its outstanding advantages such as high energy density, low heat loss during storage, and flexible and controllable operation, has shown broad application prospects in the capture and storage of low-grade thermal energy (<100℃, such as solar energy and industrial waste heat), and has become a research hotspot in the current energy field.

[0003] The adsorbent is the core component of adsorption-based thermochemical energy storage systems, and its comprehensive performance directly determines key indicators such as system energy density, operational stability, and power output. Among numerous candidate working fluids, hydrate systems have become a research focus due to their combination of high water absorption activity and controllable phase change characteristics. Lithium chloride (LiCl), with its water absorption capacity far exceeding that of similar materials and its excellent theoretical energy density, is widely recognized as a potential preferred material in the field of thermal energy storage. However, the inherent defects of pure LiCl make it difficult to directly apply to practical engineering. Three major technological bottlenecks severely restrict its large-scale deployment: First, the risk of deliquescence and leakage is prominent. LiCl has an extremely low critical humidity for deliquescence, easily forming flowing salt solutions under normal operating conditions. This not only causes loss of the working fluid but also leads to equipment corrosion, pipeline blockage, and ultimately, the failure and shutdown of the energy storage system. Second, its cycle stability is extremely poor. During repeated hydration-dehydration charge-discharge cycles, LiCl is prone to particle agglomeration and solidification, resulting in blockage of heat and mass transfer channels. Its performance degrades sharply with the number of cycles, failing to meet long-term operating requirements. Third, its kinetic response is slow. The adsorption-desorption reaction rate of pure LiCl is slow, making it difficult to achieve rapid energy storage and release, directly limiting the power density of the energy storage system and making it unsuitable for efficient energy dispatch scenarios. These problems, combined, become the main obstacles to the practical application of pure LiCl-based thermal storage materials.

[0004] To address these issues, a common technical approach in the industry is to encapsulate LiCl and other salt hydrates within a porous matrix to construct composite adsorbents, leveraging the physical confinement of the porous support to enhance material stability. Currently, silica gel, zeolites, and various carbon-based framework materials have been widely explored for this application. While these composite systems alleviate the leakage and aggregation problems of pure LiCl to some extent, they still face several insurmountable limitations: First, the preparation process is complex and cumbersome, with redundant steps leading to high production costs, hindering industrialization. Second, there is a significant performance balancing issue; ensuring the stability of the support structure often requires sacrificing salt loading, resulting in a substantial reduction in the composite material's energy density. Third, the pore structure design of the support is mismatched with the rapid mass transfer requirements under high salt loading, resulting in significant internal diffusion resistance and limited improvement in adsorption kinetics, failing to fundamentally address the inherent shortcomings of LiCl-based materials. For example, the literature “Composite sorbents “Li / Cahalogenides inside multi-wall carbon nano-tubes” for thermal energy storage, Sol. Energy Mater. Sol. Cells 155 (2016) 176-183” proposes a lithium chloride / multi-wall carbon nanotube (LiCl / MWCNT) composite water-absorbing material. Its core advantage lies in improving energy storage capacity, and therefore it has been proposed for use in adsorption thermal storage (SHS) systems. However, it should be noted that the one-dimensional tubular structure of MWCNT has inherent limitations, making it impossible to construct a multi-dimensional, interconnected mass transfer network. Its synergistic guiding effect on water vapor transport is limited, resulting in no fundamental improvement in the internal diffusion resistance of the material and no significant improvement in adsorption kinetics performance. It is still difficult to meet the requirements of high-efficiency energy charging. In addition, this scheme does not specifically design the photothermal performance of the material and does not consider the energy input efficiency of the regeneration process, making it unable to meet the requirements of green and efficient regeneration, and thus falling short of the design goals of an ideal energy storage system.

[0005] The regeneration (desorption) process of an energy storage system is a crucial link in ensuring its cyclical operation, and its efficiency directly determines the overall performance of the system. Traditional heat-driven regeneration methods generally rely on high-temperature heat sources heated by fossil fuels or additional auxiliary heat transfer media, which not only suffers from slow regeneration speed and low energy utilization efficiency, but also leads to complex system structures and increased operating costs. An ideal energy storage system should not only possess excellent energy storage capacity, stability, and dynamic performance, but also integrate a fast, efficient, green, and convenient energy input mechanism.

[0006] In summary, current LiCl-based composite thermal storage materials generally face challenges such as difficulty in achieving performance balance, low mass transfer efficiency, and outdated regeneration methods. Therefore, there is an urgent need to develop a novel LiCl-based composite thermal storage material with precise structural design, comprehensive performance optimization, and the ability to achieve efficient regeneration using low-grade thermal energy such as solar energy. This would overcome existing technological limitations and provide a feasible solution for the efficient capture, storage, and recycling of low-grade thermal energy. This has significant scientific and engineering value for alleviating the spatial and temporal imbalance between energy supply and demand and promoting the industrialization of energy storage technology. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies. Specifically, it addresses the core problems of current lithium chloride-based low-temperature thermochemical energy storage composite materials, such as an imbalance between salt loading and structural stability, poor kinetic performance due to unreasonable porous carrier mass transfer network design, reliance on external high-temperature heat sources or auxiliary heat transfer media for regeneration, and inability to achieve direct drive from low-grade thermal energy like solar energy. This invention provides a carbon nanocage-confined lithium chloride low-temperature energy storage composite material that combines high energy density, excellent cycle stability, rapid mass transfer kinetics, and efficient photothermal regeneration characteristics. Furthermore, this invention discloses a green and simple preparation method for this composite material, as well as its practical applications in low-grade thermal energy storage, solar photothermal energy storage, and industrial waste heat recovery.

[0008] The technical solution of the present invention is: a photothermal regenerable carbon nanocage confined lithium chloride low-temperature energy storage composite material, which uses carbon nanocage as carrier and lithium chloride as active component. The lithium chloride is confined and loaded in the nanocavities and channels of the carbon nanocage to form a stable lithium chloride@carbon nanocage composite structure.

[0009] Furthermore, the composite material possesses broadband solar energy absorption and photothermal conversion capabilities, enabling it to complete dehydration and regeneration via solar energy drive, and is suitable for low-temperature thermochemical energy storage conditions where low-grade thermal energy at temperatures <100℃ serves as the regeneration driving heat source.

[0010] Furthermore, the carbon nanocage possesses a hierarchical porous structure with micropores, mesopores, and macropores coexisting, and its specific surface area is 2000~2200 m². 2 ·g -1 The pore volume is 3.8~4.3 cm. 3 ·g -1 The lithium chloride loading in the composite material is 45-95 wt%, and the specific surface area of ​​the composite material is 50-650 m². 2 ·g -1 The pore volume is 0.4~1.3cm. 3 ·g -1 .

[0011] The preparation method of the above-mentioned photothermal regenerable carbon nanocage confined lithium chloride low-temperature energy storage composite material is as follows: 1) Preparation and drying of carbon nanocages: Carbon nanocages were prepared by chemical vapor deposition and then dried in a drying device to obtain pretreated carbon nanocages. 2) Prepare an aqueous solution of lithium chloride; 3) Vacuum impregnation and shaping: The dried carbon nanocages are placed in a sealed container, and a vacuum is drawn until the pressure inside the container is <10 Pa. The vacuum state is maintained for 0.5~1 h to remove the air in the cavity. Then, the prepared lithium chloride aqueous solution is injected into the container and vacuum impregnated at room temperature for 8~16 h to allow the lithium chloride aqueous solution to fully and uniformly penetrate into the nanocavities and channels of the carbon nanocages. After filtering to remove excess solution, the product is freeze-dried to obtain the carbon nanocage confined lithium chloride low-temperature energy storage composite material.

[0012] Further, in step 2), the mass fraction of the prepared lithium chloride aqueous solution is 5~20 wt%, and the liquid-solid ratio of the lithium chloride aqueous solution to the carbon nanocage is 50:(0.1~1) mL / g. Preferably, the mass fraction of the lithium chloride aqueous solution is 10 wt%, and the liquid-solid ratio of the lithium chloride aqueous solution to the carbon nanocage is 50:0.1 mL / g.

[0013] Furthermore, in step 1), the drying temperature is 80~120℃ and the drying time is 4~16 h to effectively remove moisture from the carbon nanocage pores.

[0014] The aforementioned photothermal regenerable carbon nanocage confined lithium chloride cryogenic energy storage composite material can be used in cryogenic thermochemical energy storage systems under different humidity environments to capture, store, and release low-grade thermal energy at temperatures <100℃. The composite material can adsorb and store water in an environment with a temperature range of 20~40℃ and a relative humidity of 30%~60%, and complete thermal energy storage through a hydration reaction. It does not require external high-temperature heat sources and auxiliary heat transfer media and can be directly dehydrated and regenerated by solar energy.

[0015] Furthermore, the low-grade thermal energy includes one or more of solar energy, industrial waste heat, and geothermal energy.

[0016] Furthermore, the composite material has an equilibrium adsorption capacity ≥0.5 g / g in a low humidity environment of 30℃ and 30% relative humidity; it can be applied to one or more of the following: adsorption thermal storage devices, solar thermal energy storage systems, and industrial waste heat recovery devices.

[0017] The beneficial effects of this invention are as follows: 1. This application constructs a LiCl@hCNC composite thermal storage material by confining lithium chloride (LiCl) within a carbon nanocage (hCNC) matrix. The carbon nanocage, with its large internal pore volume, high specific surface area, and defect-rich surface structure, provides ample loading space and binding sites for LiCl. Combined with a dedicated loading process involving vacuum pretreatment and solution filling, air within the nanocavities of the carbon nanocage can be eliminated, allowing the LiCl solution to uniformly and efficiently penetrate the nanocavities under negative pressure. This prevents LiCl from agglomerating only on the carrier surface, achieving efficient and high loading of LiCl. This allows more active LiCl to participate in the hydration reaction, directly increasing the thermal energy storage per unit mass / volume of the material. Under conditions of 30℃ and 43% relative humidity, the equilibrium adsorption capacity of the sample CLi10 reaches 1.36 g / g, and the energy storage density reaches 1671.5 kJ / kg. This approach overcomes the traditional problem of the mutual constraint between high loading capacity and high adsorption rate in porous materials, achieving high energy storage density while maintaining excellent adsorption kinetics performance. 2. The porous structure of carbon nanocages provides precise spatial physical confinement for LiCl. On the one hand, it can effectively inhibit the leakage of LiCl due to swelling and dissolution during repeated hydration-dehydration cycles. On the other hand, it can prevent the migration, aggregation and agglomeration of LiCl particles, ensuring that the reactive sites of LiCl are always exposed. After 80 consecutive adsorption-desorption cycle tests, the adsorption capacity of the material remains stable at 1.3-1.4 g / g, with a decay rate of less than 3%. The adsorption performance is basically undegraded, which significantly improves the reliability and service life of the material in long-term operation. 3. This application uses freeze drying instead of traditional oven drying, which avoids the collapse of the carbon nanocage hierarchical pore structure caused by capillary forces during the drying process, and completely preserves the efficient hierarchical mass transfer network constructed by the layered porous structure. Among them, the macropores serve as rapid diffusion channels for water vapor, accelerating the penetration of ambient water vapor into the material interior; the micropores achieve precise contact between water vapor and LiCl inside the cage, greatly improving the reaction contact efficiency between the two, and increasing the water adsorption rate constant by nearly 40 times compared with pure LiCl. At the same time, the hierarchical pores themselves have physical adsorption capacity, which can enrich trace amounts of water vapor in low relative humidity environments, enabling ClI10 to still achieve a high water absorption rate of 1.1 g / g in a 30% RH environment, breaking through the bottleneck of weak hydration capacity of pure LiCl in low humidity, and expanding the application range of the material in low humidity environments. 4. This application utilizes the excellent broadband solar energy absorption characteristics and efficient photothermal conversion capabilities of carbon nanocages to achieve solar-driven regeneration of thermal storage materials, eliminating the need for fossil fuels or external heaters, thus aligning with the technological development trend of energy conservation and environmental protection. The carbon nanocages can rapidly absorb solar energy across the entire wavelength range and convert it into heat energy, efficiently transferring it to the LiCl hydrate within the cage. This provides sufficient activation energy for the dehydration reaction, causing the LiCl hydrate to undergo endothermic dehydration and decomposition, regenerating LiCl and water vapor. The heat absorbed during dehydration is released into the environment, completing the heat release process. Simultaneously, the hierarchical channels can quickly discharge the water vapor generated during dehydration, preventing water vapor accumulation that could lead to a reverse reaction and further enhancing desorption kinetics. This regeneration method constructs a closed-loop cycle of heat storage-heat release-regeneration. The dehydrated LiCl remains stably confined within the channels and can repeatedly participate in the hydration reaction, achieving green and efficient recycling of the material. 5. This application achieves comprehensive optimization of the composite material in terms of loading, energy storage density, adsorption kinetics, cycle stability, regeneration method, and application scenario adaptability through the synergistic effect of carbon nanocages on LiCl loading, confinement, mass transfer enhancement, and photothermal drive. The material can be directly regenerated through low-grade thermal energy such as solar irradiation without the need for auxiliary heat transfer media, thus constructing an efficient and compact solar-thermal energy storage path. It is particularly suitable for scenarios such as low-temperature thermochemical energy storage, solar energy utilization, and industrial waste heat recovery, and has extremely high industrial application potential and economic value. Attached Figure Description

[0018] Figure 1 In the image, a is a scanning electron microscope (SEM) image of hCNC; b is a SEM image of CLI5; and c is a SEM image of CLI20. Figure 2 In the image, a is a transmission electron microscope (TEM) image of hCNC; b is a TEM image of CLI5; and c is a TEM image of CLI20. Figure 3 In the diagram, a represents the N2 adsorption-desorption isotherms of hCNC, CLI5, CLI10, and CLI20; b represents the pore size distribution of hCNC, CLI5, CLI10, and CLI20. Figure 4 In the figure, a is the dynamic water absorption curve of samples with different LiCl loading at 30℃ and 43% RH; b is the component separation bar chart of adsorption capacity of different samples. Figure 5In the figures, a) shows the equilibrium water absorption of CLI5, CLI10, and CLI20 as a function of temperature at 43% RH; b) shows the equilibrium water absorption of CLI5, CLI10, and CLI20 as a function of relative humidity at 30℃; c) shows the solute mass fraction of CLI5, CLI10, and CLI20 as a function of temperature at 43% RH; and d) shows the solute mass fraction of CLI5, CLI10, and CLI20 as a function of relative humidity at 30℃. Figure 6 In the figure, a is the TG-DSC curve of CLI5; b is the TG-DSC curve of CLI10; c is the TG-DSC curve of CLI20; d is the XRD pattern of hCNC, CLI5, CLI10 and CLI20 after complete dehydration. Figure 7 In the table, a represents the reflectance spectra of hCNC, pure LiCl, CLI5, CLI10, and CLI20; b represents the photothermal regeneration performance test results of samples with different lithium chloride loadings under one indoor sunlight irradiation; and c represents the photothermal conversion performance test results of CLI10 under outdoor conditions. Figure 8 The figure shows the capacity stability test results of CLI10 in 80 adsorption-desorption cycles. Detailed Implementation

[0019] To better understand the content of this invention patent, the technical solution of this invention is further illustrated below through specific embodiments and accompanying drawings. However, these examples do not limit the invention. Modifications and substitutions made to the methods, steps, or conditions of this invention without departing from the essence of this invention are all within the scope of this invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0020] Example 1

[0021] 1) Synthesis of carbon nanocages (hCNC)

[0022] 4 g of basic magnesium carbonate (MgCO3·Mg(OH)2·5H2O) was weighed and placed in a quartz reaction tube of a horizontal tube furnace. After sealing the reaction tube, high-purity nitrogen gas was introduced to purge the air inside. The furnace temperature was then raised to 800℃ at a heating rate of 10℃ / min and kept constant. Benzene was continuously fed into the reaction tube at a flow rate of 0.06 mL / min using a constant flow pump. After 30 min of feeding and reaction, the constant flow pump was turned off, and the reaction system was allowed to cool naturally to room temperature. The obtained carbon / magnesium oxide composite product was placed in a 6 mol / L sulfuric acid solution and magnetically stirred at room temperature until the magnesium oxide template was fully dissolved. After filtration, the product was washed three times each with deionized water and anhydrous ethanol. The washed solid was placed in a vacuum drying oven and dried at 120℃ for 12 h to obtain carbon nanocage material, denoted as hCNC.

[0023] 2) Preparation of LiCl@hCNC composite material (CLi5) by vacuum impregnation method

[0024] Weigh a certain mass of lithium chloride and dissolve it in deionized water to prepare 50 mL of 5 wt% lithium chloride aqueous solution. Weigh 100 mg of the dried hCNC prepared in step 1) and place it in a vacuum impregnation device. Vacuum the air in its channels to remove air (vacuum time ≥ 30 min to ensure complete degassing). Maintain a vacuum environment (vacuum degree < 10 Pa). Quickly inject the above lithium chloride aqueous solution into the impregnation device so that the hCNC is completely immersed in the solution. Vacuum impregnate at room temperature for 12 h to ensure that lithium chloride fully enters the hCNC channels to achieve uniform loading.

[0025] After impregnation, the solid-liquid mixture was filtered to remove excess unloaded lithium chloride aqueous solution; the filter cake was then freeze-dried in a freeze dryer (to avoid the collapse of the porous carbon nanocage structure due to conventional drying), and finally LiCl@hCNC composite adsorbent material was obtained, which was denoted as CLI5 according to the mass fraction of the lithium chloride aqueous solution used for impregnation.

[0026] Example 2

[0027] The only difference between this embodiment and Example 1 is that the mass fraction of the lithium chloride aqueous solution prepared in step 2) is adjusted to 10 wt%. The remaining preparation steps and process parameters are consistent with those in Example 1. The obtained LiCl@hCNC composite adsorbent material is denoted as CLI10.

[0028] Example 3

[0029] The only difference between this embodiment and Example 1 is that the mass fraction of the lithium chloride aqueous solution prepared in step 2) is adjusted to 20 wt%. The remaining preparation steps and process parameters are consistent with those in Example 1. The obtained LiCl@hCNC composite adsorbent material is denoted as CLI20.

[0030] Related performance tests

[0031] 1. Determination of lithium chloride loading in different composite materials

[0032] Table 1 lists the actual salt content and adsorption kinetic parameters of pure lithium chloride and different LiCl@hCNC composite adsorbents. The salt content data shows that a ClI5 sample with an actual lithium chloride loading of up to 48.50 wt% can be obtained by vacuum impregnation using only a 5 wt% lithium chloride aqueous solution. With increasing impregnation solution concentration, the LiCl loading of ClI10 and ClI20 further increases to 65.57 wt% and 91.70 wt%, respectively. These results clearly demonstrate that hCNC possesses an extremely strong adsorption loading capacity for lithium chloride. This characteristic can significantly reduce the preparation cost of the composite material, as high loading can be achieved without relying on high-concentration lithium chloride aqueous solutions, reducing both raw material costs and the operational difficulty of preparing high-concentration solutions.

[0033] Table 1. Salt content and adsorption rate of pure LiCl and LiCl@hCNC composite material

[0034]

[0035] Note: Pure LiCl in the table is the control sample, used as a benchmark to compare the adsorption kinetics of the composite material.

[0036] 2. Characterization of LiCl@hCNC composite material

[0037] 2.1 The morphological features and microstructure of the samples were systematically characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Pure hCNC exhibits a unique hierarchical structure, with hollow carbon nanocages interconnected to form carbon nanosheets, which then self-assemble into a three-dimensional mesoscopic network structure. Figure 1 (a) Compared with the SEM image of pure hCNC, CLI5 ( Figure 1 b) and CLi20 ( Figure 1 c) still maintains the regular spherical morphology of hCNC, and there are no obvious LiCl agglomerates on the outer surface of the carbon cage. This indicates that the vacuum impregnation method can effectively confine and fill the hollow cavity of the carbon matrix with LiCl, effectively avoiding the agglomeration of salt particles on the support surface.

[0038] 2.2 Transmission electron microscopy (TEM) characterization provides direct microscopic evidence for the confined distribution of LiCl. In Cl5 (… Figure 1 b) and CLi20 ( Figure 1In the TEM images of c), the lattice fringes of the corresponding crystalline LiCl (111) crystal plane can be clearly observed. This clear crystal phase characterization is due to the compressed air-assisted dry dispersion technique used in this experiment. This method replaces the traditional solvent-based dispersion method, which can effectively avoid premature deliquescence of LiCl due to contact with solvents or humid air during TEM sample preparation, thus completely preserving the crystal morphology and crystal phase information of LiCl. As can be seen from the TEM results, LiCl in Cl5 is completely confined inside the hollow pores of the carbon nanocage ( Figure 1 (b); while in the high-load ClI20 sample ( Figure 1 (c) As the LiCl loading increases, the salt particles not only fill the internal pores of the carbon cage, but are also partially distributed on the outer surface of the carbon cage.

[0039] 2.3. Nitrogen adsorption-desorption isotherms from hCNC, CLI5, CLI10, and CLI20 ( Figure 3 As shown in a), the isotherm of pure hCNC exhibits typical Type IV isotherm characteristics, accompanied by a significant hysteresis loop, reflecting its hierarchical porous structure (coexistence of micropores, mesopores, and macropores): rapid adsorption in the low relative pressure region corresponds to micropore filling; the adsorption plateau and hysteresis loop in the medium pressure region correspond to mesopore adsorption; and the sharp increase in adsorption in the high pressure region originates from capillary condensation in macropores. Among them, mesopores mainly correspond to the hollow cavities of carbon nanocages. With the increase of LiCl loading, the overall adsorption capacity of the sample gradually decreases, especially the adsorption capacity in the mesopore and macropore regions decreases more significantly, directly reflecting the filling effect of salt particles on the pores.

[0040] The pore size distribution of the samples further verified this conclusion. Pure hCNC showed obvious signals in the micropore (<2 nm), mesopore (2~50 nm) and macropore (>50 nm) ranges. The strong peak in the mesopore range corresponds to the pore cavity of carbon nanocages. As the LiCl loading increased, the signal intensity in the mesopore range decreased significantly, indicating that LiCl preferentially filled the mesopore cavity.

[0041] 2.4 Although the specific surface area and total pore volume of the samples decreased with increasing LiCl loading (Table 2), CLI5 and CLI10 still retained 1.2469 cm³ of pore volume, respectively. 3 ·g -1 and 0.9155 cm 3 ·g -1 The composite material exhibits a considerable pore volume. This high residual porosity provides ample liquid-holding space during the adsorption-desorption cycle, ensuring its excellent mass transfer efficiency and cycle stability.

[0042] Table 2. Specific surface area and pore volume of hCNC, CLI5, CLI10 and CLI20

[0043]

[0044] 3. Water vapor adsorption isotherm and adsorption kinetics test

[0045] All samples underwent rigorous in-situ pretreatment before testing: approximately 10 mg of sample was weighed and placed in a sample tube, and then subjected to high vacuum (<10) conditions. -5 The sample was activated at 150°C for 6 hours under a certain temperature (Pa) to completely remove pre-adsorbed impurities and moisture from the sample surface and pores, ensuring the accuracy of the test results.

[0046] Water vapor adsorption isotherm tests were conducted at 30℃, and the equilibrium adsorption capacity of the samples was collected under 43% relative humidity (RH) conditions to characterize their adsorption capacity and humidity response characteristics under typical operating conditions. Adsorption kinetics tests were conducted through a humidity step experiment: the relative humidity of the sample environment was rapidly switched from 0% to 43% at 30℃, and the change in sample mass over time was monitored in real time to evaluate its adsorption rate and mass transfer kinetics performance.

[0047] Lithium chloride deliquesces at a relative humidity of approximately 11% RH. Therefore, under test conditions of 43% RH, the LiCl in the sample will inevitably deliquesce. The formula for calculating the water absorption α is as follows:

[0048]

[0049] In the formula, m0 and m t These represent the mass of the sample before the test and at time t, respectively.

[0050] Figure 4 Figure 'a' shows the dynamic water absorption curves of samples with different LiCl loadings under 30℃ and 43% RH conditions, clearly demonstrating the synergistic effect of loading on adsorption kinetics and equilibrium capacity: CLI5 exhibits the fastest water absorption kinetics, reaching over 90% of its equilibrium adsorption capacity within 20 min and approaching complete equilibrium around 100 min, serving as a reference sample with rapid response characteristics; CLI10 completes rapid adsorption within 130 min, while its equilibrium adsorption capacity (approximately 1.36 g / g) is significantly higher than CLI5, demonstrating the optimal balance between adsorption capacity and kinetic rate, confirming that the carbon nanocage structure provides a balance range for capacity and kinetics in the composite material; CLI20 has the highest equilibrium adsorption capacity, but its adsorption kinetics are significantly slower, failing to reach complete equilibrium even after 200 min, reflecting that when the salt loading exceeds the effective confinement capacity of the carbon matrix, excess LiCl hinders water vapor transport, leading to a decrease in mass transfer efficiency; pure LiCl has the slowest water absorption rate, with less than 0.6 g of water absorbed after 500 min. g / g, further highlighting the key promoting role of the carbon matrix framework in adsorption kinetics.

[0051] Based on the kinetic fitting results in Table 1, the apparent rate constant of CLI5 is nearly 40 times higher than that of bulk LiCl. This result indicates that the hierarchical porous structure of carbon nanocages constructs a hierarchical water vapor transport network, which can effectively shorten the water vapor diffusion distance and alleviate the kinetic bottleneck of traditional salt-based adsorbents, rather than simply relying on salt content to improve performance. This kinetic improvement is particularly significant under moderate salt loading (such as CLI10), demonstrating the synergistic optimization effect of structural confinement and salt content.

[0052] To clarify the contribution ratio of different water absorption pathways, the total adsorption capacity was decomposed into three components: physical adsorption on the carbon matrix surface, LiCl chemisorption (initial stage), and LiCl liquid-phase absorption (dominant stage). The results showed that physical adsorption on the carbon matrix contributed only a small portion of the total water absorption, while LiCl liquid-phase absorption was the dominant mechanism, especially in ClI₂O, where liquid-phase absorption contributed over 80% of the total water absorption. As the LiCl loading increased, the proportion of liquid-phase absorption gradually increased, indicating that under medium-high humidity conditions (43% RH), the gas-solid adsorption-liquid-phase absorption coupling mechanism triggered by the rapid deliquescence of LiCl is the core reason for the high adsorption capacity of the composite material. Figure 4 (b) in the middle.

[0053] The premise of this coupling mechanism is that the carbon nanocage has sufficient internal pores to effectively confine the salt solution generated by deliquescence, which not only prevents the loss of salt solution but also ensures the rapid transport of water vapor, thus achieving a balance between high capacity and fast kinetics.

[0054] 4. The regulation of adsorption performance by temperature and humidity

[0055] To assess the material’s adaptability under actual working conditions, tests were conducted systematically within a temperature range of 20–40°C and a relative humidity (RH) range of 30%–60%.

[0056] At a constant humidity of 43% RH, the equilibrium water absorption of all LiCl@hCNC composites decreased linearly with increasing temperature. This trend is consistent with the exothermic characteristics of the LiCl hydration reaction: increasing temperature inhibits the exothermic hydration process, thereby reducing the adsorption capacity (a in Figure 5).

[0057] At a constant temperature of 30℃, driven by the increase in water vapor partial pressure, the water absorption of the composite material showed a monotonically increasing trend with increasing RH. Notably, even in a low-humidity environment of 30% RH, all samples maintained considerable water absorption (CLi20≈1.7 g / g, CLI10≈1.1 g / g, CLI5≈0.5 g / g), indicating that confined LiCl still possesses water-absorbing activity under relatively dry conditions. Figure 5(b) This property is significantly superior to traditional porous adsorbents (whose adsorption performance typically decreases significantly under low humidity), proving that the LiCl@hCNC composite material is suitable for adsorption-based thermal storage applications in low humidity environments.

[0058] Based on the measured water absorption, the solute mass fraction of the confined LiCl aqueous solution was further calculated to reflect the degree of hydration of LiCl:

[0059] At a constant humidity of 43% RH, the mass fraction of solute increases with increasing temperature. This trend indicates that increasing temperature reduces the hydration degree of LiCl, thus increasing the solution concentration. Figure 5 (c in the text)

[0060] At a constant temperature of 30℃, the solute mass fraction decreased with increasing RH, reflecting a deeper degree of LiCl hydration and a lower solution concentration under high humidity. Notably, under high humidity conditions of 60% RH, the solute mass fraction of CLI20 showed an anomalously high level. Combined with previous characterization of insufficient pore volume, this phenomenon indicates that under high load, the pores of CLI20 cannot completely confine the deliquescent salt solution, resulting in partial solution loss and a reduction in the effective LiCl involved in water absorption. Consequently, the solute mass fraction of the remaining solution is higher, directly reflecting the failure of the material's confinement ability. Figure 5 (d in the text)

[0061] Figure 5 Systematic characterization showed that the LiCl@hCNC composite material exhibited stable adsorption behavior over a wide temperature and humidity range, maintaining effective water absorption even in low humidity environments. Compared to all samples, CLI10 demonstrated the best overall stability through its balanced salt loading and sufficient confinement capability, achieving stable operation without salt solution leakage, thus providing a reliable material choice for adsorption thermal storage applications.

[0062] 5. Characterization of thermochemical behavior and energy storage performance

[0063] 5.1 To evaluate the thermochemical reversibility and regeneration characteristics of LiCl@hCNC composite materials, this study conducted thermogravimetric-differential scanning calorimetry (TG-DSC) analysis on samples with different LiCl loadings. Figure 6 (ac in the text). All samples reached adsorption equilibrium at 30℃ and 43% RH before testing, at which point LiCl in the composite material existed in the form of a confined salt solution.

[0064] During the heating process, the sample undergoes three distinct thermal evolutions:

[0065] Salt solution concentration stage: Free water in the salt solution evaporates at low temperature, and the solution concentration gradually increases until the solution reaches the saturation concentration of lithium chloride monohydrate (LiCl·H2O);

[0066] Monohydrate crystallization stage: Further increase in temperature triggers the crystallization phase transition of the saturated solution, generating LiCl·H2O crystals; the complete hydration temperature marked by the red dashed line in the figure (such as 76.8℃ for CLi5) is the end point of this stage, representing the complete transformation of the solution into monohydrate;

[0067] Monohydrate dehydration stage: After heating to the monohydrate decomposition temperature, LiCl·H2O undergoes a dehydration reaction to generate anhydrous LiCl.

[0068] As can be seen from the TG-DSC curves, all samples exhibit a clear and reproducible three-stage evolution characteristic. Among them, the weight loss and endothermic peaks in the monohydrate dehydration stage are the most significant, indicating that the hydration-dehydration phase transition of LiCl@hCNC has good thermochemical reversibility. This result confirms that after LiCl is confined inside the carbon nanocage, its intrinsic thermochemical properties remain unchanged; only the composition of the composite material (the proportion of carbon matrix) affects the phase transition temperature by regulating the heat transfer efficiency.

[0069] As shown in the figure, the temperature at which the composite material is completely converted into lithium chloride monohydrate (LiCl·H2O) increases systematically with increasing LiCl loading: approximately 76.8℃ for CLI5, approximately 78.9℃ for CLI10, and approximately 91.2℃ for CLI20. This trend reflects the decrease in carbon matrix content and heat transfer efficiency under high loading, rather than a change in the essential pathway of the LiCl hydration reaction, indicating that the hydration mechanism is consistent across samples with different loadings.

[0070] The temperature differences for the complete dehydration of lithium chloride monohydrate to form anhydrous LiCl are even more significant: ClI5 is approximately 90.3℃, ClI10 is approximately 91.9℃, and ClI20 rises to approximately 113.2℃. Among them, the complete dehydration temperature of ClI10 (91.9℃) is within the typical operating temperature range (80~120℃) of low-grade heat sources such as flat-plate solar collectors and industrial waste heat. Combined with its excellent adsorption capacity, this fully demonstrates that ClI10 is suitable for low-temperature thermochemical energy storage scenarios.

[0071] from Figure 6The curve morphology also confirms this pattern: CLi5 has the narrowest temperature range for thermal evolution, the most compact connection between monohydrate crystallization and dehydration stages, and the earliest appearance of the DSC endothermic peak, indicating excellent heat transfer efficiency when the carbon matrix content is high, and an early and fast regeneration process; CLi20 has the widest temperature range for thermal evolution, and the DSC endothermic peak shifts significantly later, reflecting that the carbon matrix content is low and the heat transfer efficiency is insufficient under high load, and there is additional heat loss in the regeneration process, requiring a higher temperature to complete dehydration; CLi10's thermal evolution range and endothermic peak position are both between the two, reflecting the optimal balance between salt load and heat transfer efficiency.

[0072] By integrating the endothermic peak of the complete dehydration stage in the DSC curve, the endothermic heat of water desorption (i.e., desorption enthalpy) was calculated, which directly characterizes the mass energy storage density of the material. Under conditions of 30℃ and 43% RH, the LiCl@hCNC composite material exhibits a high energy storage density, with CLi5 reaching 1477.3 kJ·kg⁻¹. -1 The CLi10 value is 1671.5 kJ·kg. -1 The CLi20 concentration is 2027.3 kJ·kg⁻¹. -1 Although CLi20 has the highest energy storage density, its dehydration temperature of 113.2℃ exceeds the achievable range of conventional low-grade heat sources; while CLi10 boasts a mild regeneration temperature (91.9℃) and excellent energy storage density (1671.5 kJ·kg⁻¹). -1 It achieves an optimal balance between ) and is an ideal choice for low-temperature thermochemical energy storage.

[0073] 5.2 The XRD results of the sample after complete dehydration are as follows: Figure 6As shown in d in the figure, for the low-load ClI5 and ClI10 samples, their XRD curves are highly consistent with the amorphous dispersion characteristics of the support hCNC, and no sharp characteristic diffraction peaks of lithium chloride (LiCl) are observed. This indicates that under low-load conditions, the low mass proportion of LiCl allows it to be uniformly dispersed in the nanocavities and channels of hCNC in an amorphous or nanocrystalline form (under low load, LiCl is mainly confined and loaded in the cavities of carbon nanocages, with a small amount adsorbed in the channels), and forms strong hydrogen bonds with the hydroxyl groups on the hCNC surface. In this dispersed state, the dehydration process only needs to break the weak hydrogen bonds and hydration bonds, requiring less energy. However, the high-load ClI20 sample shows a significant difference, with its XRD curve exhibiting clear and sharp characteristic diffraction peaks of lithium chloride. This indicates that when the LiCl loading exceeds the pore volume and surface adsorption threshold of hCNC, the excess LiCl will agglomerate outside the channels and undergo confined crystallization, forming a crystalline LiCl phase with a regular lattice structure. The ion-water coordination of crystalline LiCl hydrate is stronger, and the dehydration process requires higher energy to break the lattice stability. At the same time, the dehydration activation energy of crystalline LiCl is significantly higher than that of amorphous LiCl, which directly leads to its complete dehydration temperature jumping to 113.2℃.

[0074] 6. Photothermal charging and solar-driven regeneration

[0075] 6.1 Characterization of light absorption properties

[0076] As can be seen from the reflectance spectrum ( Figure 7 (a) Pure LiCl exhibits high reflectivity across the entire solar spectrum, while the reflectivity of hCNC and the LiCl@hCNC composite material remains consistently at an extremely low level below 10%. This result demonstrates that the excellent near-full-spectrum strong absorption characteristics of carbon nanocages are well preserved after LiCl impregnation modification, laying the foundation for efficient photothermal conversion.

[0077] 6.2 Evaluation of Indoor Photothermal Regeneration Performance

[0078] To quantitatively evaluate the regenerative performance driven by solar energy, all samples were first fully hydrated at 30°C and 43% RH, and then placed in a sealed quartz chamber using a calibrated light intensity of 100 mW / cm². 2 Irradiation was performed using a solar simulator at 1 solar radiation intensity, and the mass change (water content) and surface temperature of the sample were monitored simultaneously. Results are shown below. Figure 7As shown in b: CLI5 exhibits the fastest heating rate due to its higher carbon content and stronger light absorption capacity, with its surface temperature reaching approximately 90°C within 5 minutes; CLI20, due to its high salt loading leading to a lower hCNC ratio and reduced photothermal conversion efficiency, has the slowest heating rate and a lower steady-state temperature; simultaneously, its insufficient pore confinement capacity significantly prolongs the complete dehydration time; CLI10 demonstrates photothermal response characteristics that balance performance and practical application value, reaching a complete dehydration temperature of 91.9°C in just 385 seconds under 1 solar irradiance, and completing the complete dehydration process within 580 seconds, significantly outperforming CLI20; its superior performance stems from the synergistic optimization of the three core properties of solar energy absorption, heat transfer, and salt phase confinement achieved by the carbon nanocage framework: it retains efficient photothermal conversion capacity while ensuring heat transfer efficiency and confinement effect through a moderate salt loading, avoiding performance degradation caused by high loading.

[0079] 6.3 Feasibility Verification of Outdoor Recycling

[0080] To verify the regeneration capability in practical applications, CLI10 was tested in an outdoor environment (ambient temperature 9℃, solar irradiance 100mW / cm², with natural fluctuations). The results showed that hydrated CLI10 could still reach its complete dehydration temperature of 91.9℃ within 445 seconds and complete the entire dehydration process within 12 minutes. This result demonstrates that the LiCl@hCNC composite material can achieve direct regeneration through solar irradiation alone, without the need for auxiliary heat transfer media or external heaters, thus constructing an efficient and compact solar-thermal energy storage pathway and providing strong support for its application in practical low-temperature thermochemical energy storage systems.

[0081] 7. Cyclic stability test

[0082] To evaluate the long-term operational reliability of CLi10, this study conducted 80 consecutive adsorption-desorption cycle tests. The standard procedure for each cycle was as follows:

[0083] Adsorption stage: Adsorption was carried out at 30℃ and 43% RH for 60 min to make the sample saturated with water.

[0084] Desorption and regeneration stage: Irradiate the sample with a solar intensity of 100 mW / cm² for 10 min in a warm environment to drive the sample to the complete dehydration temperature and complete the regeneration.

[0085] The results are as follows Figure 8As shown in the figure, during 80 consecutive cycles, the adsorption and desorption capacities of CLI10 remained stable within the range of 1.3-1.4 g / g, with a capacity decay rate of less than 3% and no significant downward trend. This result indicates that the composite material can maintain structural integrity and adsorption function stability during repeated complete hydration-dehydration cycles.

[0086] This excellent cycling stability stems from the robust confinement of LiCl by the hierarchical channels of carbon nanocages: the vacuum impregnation process ensures that LiCl is uniformly immobilized inside the nanopores, effectively inhibiting the migration, aggregation, and agglomeration of salt particles during cycling. This preserves the adsorption active sites and short-range diffusion paths of the material, ensuring that the composite material maintains stable adsorption capacity and mass transfer kinetics performance during long-term operation.

[0087] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the protection scope of the present invention.

Claims

1. A photothermally regenerable carbon nanocage-confined lithium chloride cryogenic energy storage composite material, characterized in that, The composite material uses carbon nanocages as a carrier and lithium chloride as an active component. Lithium chloride is confined and loaded within the nanocavities and channels of the carbon nanocages to form a lithium chloride@carbon nanocage composite structure.

2. The photothermally regenerable carbon nanocage-confined lithium chloride cryogenic energy storage composite material as described in claim 1, characterized in that, The composite material has broadband solar energy absorption and photothermal conversion capabilities, and can complete dehydration and regeneration through solar energy drive. It is suitable for low-temperature thermochemical energy storage conditions where low-grade thermal energy with a temperature of <100℃ is used as the heat source for regeneration drive.

3. The photothermally regenerable carbon nanocage-confined lithium chloride cryogenic energy storage composite material as described in claim 1, characterized in that, The carbon nanocages possess a hierarchical porous structure with micropores, mesopores, and macropores coexisting, and the specific surface area of ​​the carbon nanocages is 2000~2200 m². 2 ·g -1 The pore volume is 3.8~4.3 cm. 3 ·g -1 The lithium chloride loading in the composite material is 45–95 wt%, and the specific surface area of ​​the composite material is 50–650 m². 2 ·g -1 The pore volume is 0.4~1.3cm. 3 ·g -1 .

4. The method for preparing the photothermally regenerable carbon nanocage-confined lithium chloride low-temperature energy storage composite material as described in any one of claims 1-3, characterized in that, Includes the following steps: 1) Carbon nanocages were prepared by chemical vapor deposition and then dried; 2) Prepare an aqueous solution of lithium chloride; 3) Place the dried carbon nanocages into a sealed container, evacuate to a pressure <10 Pa, maintain the vacuum state for 0.5~1h to remove the air in the cavity, inject lithium chloride aqueous solution, and vacuum impregnate at room temperature for 8~16h to allow the lithium chloride aqueous solution to fully penetrate into the nanocavities and channels of the carbon nanocages, filter, freeze dry, and obtain carbon nanocage confined lithium chloride low-temperature energy storage composite material.

5. The preparation method of the photothermally regenerable carbon nanocage confined lithium chloride low-temperature energy storage composite material as described in claim 4, characterized in that, In step 2), the mass fraction of the prepared lithium chloride aqueous solution is 5~20 wt%, and the liquid-solid ratio of the lithium chloride aqueous solution to the carbon nanocage is 50: (0.1~1) mL / g.

6. The preparation method of the photothermally regenerable carbon nanocage confined lithium chloride low-temperature energy storage composite material as described in claim 4, characterized in that, In step 1), the drying temperature is 80~120℃ and the drying time is 4~16 h.

7. The preparation method of the photothermally regenerable carbon nanocage confined lithium chloride low-temperature energy storage composite material as described in claim 5, characterized in that, The mass fraction of the lithium chloride aqueous solution was 10 wt%, and the liquid-solid ratio of the lithium chloride aqueous solution to the carbon nanocage was 50:0.1 mL / g.

8. The application of the photothermally regenerable carbon nanocage-confined lithium chloride cryogenic energy storage composite material as described in any one of claims 1-3, characterized in that, The composite material is used in low-temperature thermochemical energy storage systems under different humidity environments, and is adapted to capture, store and release low-grade heat energy at <100℃. The composite material can adsorb and store water in an environment with a temperature range of 20~40℃ and a relative humidity of 30%~60%, and then complete the heat energy storage through hydration reaction, and can be directly dehydrated and regenerated by solar energy.

9. The application as described in claim 8, characterized in that, The low-grade thermal energy includes one or more of solar energy, industrial waste heat, and geothermal energy.

10. The application as described in claim 9, characterized in that, The composite material has an equilibrium adsorption capacity of ≥0.5 g / g in a low humidity environment of 30℃ and 30% relative humidity; it can be applied to one or more of the following: adsorption thermal storage devices, solar thermal energy storage systems, and industrial waste heat recovery devices.

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