Performance investigation of LiCl·H₂O-γ-Al₂O₃ composite materials for low-grade heat storage

Abstract

In this study, the influences of nano γ-Al₂O₃ on the thermal storage performance of LiCl were experimentally investigated. The XRD results show that a complex of lithium aluminium oxychloride (LiAlOCl₂) was formed through the LiCl·H₂O and γ-Al₂O₃ composites preparation process. The in situ diffuse reflectance infrared Fourier transform spectroscopy measurement reveals that the addition of γ-Al₂O₃ accelerated the hydration rate of LiCl composites, concentrated the spectrum utilization range, and promoted the desorption rate of physical adsorbed H₂O and low-frequency structural –OH in the materials. The highest specific surface area of the composite is 34.5 times higher than that of pure LiCl. The addition of γ-Al₂O₃ can increase the conversion rate of LiCl·H₂O to approximately 100% at the hydration time of 1 h and the addition content of γ-Al₂O₃ at 15 wt%. A maximum heat storage density (HSD) for the LiCl·H₂O-γ-Al₂O₃ composite can reach 714.7 kJ kg_LiCl·H2O⁻¹ in 1 h when the addition content of γ-Al₂O₃ was 15%_wt and its water uptake can reach 0.26 g g⁻¹ in 1 h. It also can be found that the addition of Al₂O₃ in LiCl resulted in a decrease of the activation energy from 90.89 kJ mol⁻¹ to 79.76 kJ mol⁻¹. However, the thermal conductivity of the LiCl·H₂O-γ-Al₂O₃ composite slightly decreased with the increase of nano γ-Al₂O₃ content.

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1 Introduction

Plenty of waste heat, about 66% being low-grade heat (below 200 °C), is discharged into the environment directly during industrial processes.^1,2 Therefore, the efficient use of low-grade waste heat can reduce energy consumption, which has been attracting much attention during recent decades.^3–5 Unfortunately, low-grade waste heat is difficult to recover and utilize due to its low exergy.⁶ Thermal energy storage (TES) technologies are some of the most hopeful methods for realizing continuous and stable utilization of waste heat,⁷ which is classified into three types: sensible,^8,9 latent^10,11 and thermochemical heat storage (TCS).^12–14 TCS utilizing reversible chemical reactions to store and release energy with high HSD^15–17 and lossless heat storage¹⁸ makes a promising and useful contribution to the storage of low-grade thermal energy. Various TCS materials have been studied, such as metal salts with water,^19,20 ammonia,^21–23 methanol,^24,25 or metal alloys with hydrogen.^26,27 In a relevant research article, Cot-Gores et al. reported that water as an environmentally friendly adsorbate could meet the requirements of safety system, and crystalline hydrate as a heat storage material had a good development prospect.²⁸ Hygroscopic salts like LiCl, LiBr, KBr, and CaCl₂ can be used as suitable heat storage materials.²⁹

Lithium chloride hydrate (LiCl·nH₂O) was selected as a promising candidate for low-grade heat storage.³⁰ The reversible chemical reaction³¹ for LiCl at normal temperatures and pressures is as follows:

LiCl·H₂O(s) ↔ LiCl(s) + H₂O(g) (1)

The deliquescence relative humidity (DRH) value for LiCl at 30 °C is 11%. The complete water absorption process of inorganic salts can be considered as a three-phase process involving chemisorption (mainly energy storage part), gas/liquid/solid three-phase liquid hydrolysis/crystallization, liquid/gas absorption and other processes. Most scholars mainly pay attention to the total amount of water vapor adsorption of hygroscopic salt materials to increase the HSD,^32–34 but rarely pay attention to the adsorption amount per unit time (sorption kinetics) on materials' use. In some industrial waste heat recycle, a large amount of waste heat needs to be treated or recovered promptly. For the short-term TCS, the proportion of chemical heat storage should be enhanced to increase the HSD per unit time.³⁵ To improve the sorption kinetics and solve limitations (deliquescence and agglomeration) of LiCl, composites are usually advanced by adding porous media with high specific surface area to increase the contact abilities with water.^30,36–39 Zhao et al. modified LiCl with zeolite, and the water uptake capacity within 1 h was less than 0.15 g g⁻¹ at 25 °C and 80% RH.⁴⁰ Zhang et al. chose activated alumina spherical grain (3.29–5.30 mm) as the matrix and LiCl as the inserted salt and the maximum water uptake capacity within 1 h was about 0.2 g g⁻¹ at 20 °C and 80% RH and its adsorption capacity per unit time has been greatly improved compared with other materials.⁴¹ Existing studies mainly focus on the thermochemical characterizations of the prepared composite materials, there is a lack of further theoretical analyses and in-depth study on the reaction kinetics of hydration and dehydration process, which is of exceptional importance for deeper understanding of the heat storage mechanism. Moreover, to motivate advancement of TCE in the field short-term energy storage, the sorption performances of composite materials must be further improved. Powdered nano aluminum oxide (Al₂O₃) attracts the attention of many scholars to choose it to selective adsorption for gases, water vapor and water in some liquids as the additive due to its high porosity, high activity, and low cost.^42–46 And it was also proven that a small amount of transition metals and their oxides have a large effect on the thermodynamic properties of MgH₂ (ref. 47) and the cations had a strong influence on the hydration energy.^48–50

To further improve the water sorption kinetics of materials for short-term TCS, this paper selected powdered nano γ-Al₂O₃ as the additive which can increase the contact opportunities and binding capacity with water, and the proportion of LiCl is greatly increased to strengthen the hydration reactants. The structure and properties of LiCl-γ-Al₂O₃ TCS composites were analyzed by in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray diffraction (XRD), and other micro characterization methods. Then, a further study on how γ-Al₂O₃ affected the sorption kinetics and thermal storage performance of LiCl from the aspect of reaction mechanism was investigated.

2 Experimental methods

2.1 Materials and preparation procedure

All the raw materials are purchased from Aladdin Reagent Co., Ltd. The LiCl (purity, >99.99%) was used as hydrated salt. Nano γ-Al₂O₃ (purity, >99.9%) was selected as the porous matrix. Fig. 1 schematically illustrates the preparation process of LiCl-γ-Al₂O₃ composites. Firstly, pretreat LiCl and γ-Al₂O₃ with an oven at 100 °C for 2 h and prepare four LiCl solutions which concentration are 10% and volume is 90 ml respectively. Then different mass (526.3 mg, 1111.1 mg, 1764.7 mg, 2500 mg) of γ-Al₂O₃ was impregnated with the LiCl solution at a stirring rate of 600 rpm for 12 h to form the composite solution. Finally, the composite solutions were dried at 100 °C for 12 h, and then at 200 °C for 6 h in the oven, to obtain the anhydrous composites (LiCl-Al₂O₃). The hydrous composites (LiCl·H₂O-Al₂O₃) were acquired by the hydration of the anhydrous composites in a constant temperature and humidity chamber (Labonce-60CH, ±0.5 °C, ±3% RH) at 30 °C and 60% RH. The masses were measured by an analytical balance (MS105DU, Mettler-Toledo, ±0.01 mg). The resulting materials were sequentially named LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15% and LiCl·H₂O-Al₂O₃-20%.

Fig. 1 Preparation flow chart of LiCl-γ-Al₂O₃ composites.

2.2 Characterization and thermal storage performance evaluation of composites

The transmission electron micrographs (TEM) equipped with a FEI Tecnai G212 and a JEOL JEM-2100F was used to record microstructure pictures. The crystal structures of the samples were analyzed and obtained on a D8-advance X-ray diffractometer with a scanning step of 0.0167° and counting time of 10.160 s per step. The thermal conductivity was measured by a hot disk thermal constant analyzer (TPS 500 (S), Hot Disk AB Company, Sweden, ±5%). The Brunauer–Emmett–Teller (BET) specific surface area and pore volume were evaluated by the nitrogen physical adsorption method with the Quantachrome QDS-30 analyzer (±1%). After obtaining hydrates, the thermogravimetric and differential thermal analyzer (TG-DSC, STA-200, Nanjingdazhan, ±0.1 °C) was used to measure the endothermic heat and weight change during the dehydration step. The TG-DSC curves were described by heating the samples from 30 °C to 200 °C at the heating rate of 10 °C min⁻¹ under the nitrogen atmosphere.

The uncertainty of the composite content was mainly caused by the mass measurements of the LiCl and Al₂O₃. Therefore, the content of LiCl in composite (z) is calculated from:

where, the masses of LiCl and Al₂O₃ are respectively denoted as m₁ and m₂; u₁ and u₂ are the uncertainties associated with m₁ and m₂, respectively. The accuracy of the electronic balance is ±0.01 mg. Therefore, the uncertainties of the composite content of 95%, 90%, 85%, 80% are ±0.00045%, ±0.00041%, ±0.00037%, and ±0.00033%, respectively.

The uncertainty analysis of the hydration conversion of LiCl (α_LiCl) can be evaluated by the following equation.

where u_w is the uncertainty of the relative mass change (w) measurement. The accuracies of the electronic balance in TG-DSC, is ±0.01 mg. Finally, the uncertainty of α_LiCl is about 1.11–1.61%.

2.3 Presentation of in situ DRIFT spectroscopy

KBr (99.99% spectroscopy grade), obtained from Aladdin Reagent Co., Ltd, was used as the background material and was dried at 200 °C for 6 h before testing. The effect of nano γ-Al₂O₃ on the hydration/dehydration reaction of LiCl·H₂O in a quasi environmental state was studied by in situ DRIFT spectroscopy Nicolet 6700 and thermogravimetric analysis of TA tgaq50, USA. Prior to the acquisition of in situ DRIFT spectra, pure LiCl and the LiCl-γ-Al₂O₃ composites were dried at 200 °C for 8 h in the oven, and the reactor was purged with N₂ for 20 min. Then, the samples were placed in the in situ reactor immediately. The reactor was purged using an N₂ flow with water vapor (100 ml min⁻¹). The condition for hydration was at 26 °C and around 60% RH. After the hydration reaction, the LiCl·H₂O was heated at a rate of 10 °C min⁻¹ under the control of temperature-programmed technology with a dry N₂ flow.

3 Results and discussion

XRD patterns of pure LiCl and LiCl·H₂O-Al₂O₃ composites were recorded at room temperature as shown in Fig. 2. Diffraction peaks at around 23.08°, 32.97°, 34.97°, 40.71°, 47.46° and 58.67° are attributed to LiCl·H₂O, while peaks at 30.18°, 34.97°, 50.24°, 54.51° and 59.12° belong to LiAlOCl₂, a complex formed by LiCl·H₂O and γ-Al₂O₃.⁵¹ In the XRD pattern of the composites, even if the mass fraction of γ-Al₂O₃ is as high as 20%, there is no corresponding peak of γ-Al₂O₃. This indicates that Li⁺ with small radius is likely to enter into the lattice of γ-Al₂O₃ and react in bulk.⁵⁰ Both LiCl and Al₂O₃ are Lewis acids. In the process of obtaining dehydrated composite materials by impregnation, the following equation mainly occurs:

LiCl·H₂O(s) → LiCl(s) + H₂O(g) (4)

Fig. 2 XRD patterns of pure LiCl·H₂O, γ-Al₂O₃, LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15%, LiCl·H₂O-Al₂O₃-20% samples with different nano γ-Al₂O₃ contents.

However, in the process of heating, hydrolysis will also occur,⁵² as follows:

LiCl·H₂O(s) → LiOH(s) + HCl(g) (5)

Al₂O₃ is an amphoteric substance which can react both with base (LiOH) or acid (HCl).

6LiOH + Al₂O₃ → 2Al(OH)₃ + 3Li₂O (6)

6HCl + Al₂O₃ → 2AlCl₃ + 3H₂O (7)

Meanwhile, HCl can react with Al(OH)₃ as follows.

6HCl + 2Al(OH)₃ → 2AlCl₃ + 3H₂O (8)

At this time, LiCl and AlCl₃ may form composite salt LiAlCl₄.⁵³

AlCl₃ + LiCl → LiAlCl₄ (9)

Then, LiAlOCl₂ may be generated by the following equation:⁵¹

Li₂O + LiAlCl₄ → LiAlOCl₂ + 2LiCl (10)

Compared to the XRD patterns of pure LiCl, there was a clear reduction of the relative intensity of the Bragg diffraction peaks at ambient temperature with the increase of γ-Al₂O₃ content. In addition, there are many diffraction peaks of LiCl·H₂O in the composites, which are 25.87°, 47.02°, 68.99°, and 78.94° respectively. It is speculated that –AlCl bond is formed after the reaction of LiCl with γ-Al₂O₃, which reduces the activation energy of hydration reaction.⁵⁴ It can be seen from XRD patterns that the diffraction intensity become lower and the diffraction peak range become wider. According to Scherrer formula, the reason for the above phenomenon may be that the addition of γ-Al₂O₃ provides more loading area for LiCl, so that LiCl can absorb more water vapor in the same test environment and time, resulting in the decrease of diffraction intensity; and the other reason is that the particles size of the composites become smaller and more dispersed than that of pure LiCl which can be seen in the TEM images.

The TEM images of pure LiCl, LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15%, and LiCl·H₂O-Al₂O₃-20% are shown in Fig. 3a–j. It can be seen that after contacting with water vapor during the TEM experiment, the pure LiCl generates agglomeration as a whole shown in Fig. 3a with strip crystal structure (about 7.3 nm, the blue part in Fig. 3b). This is due to the rapid reaction of LiCl with water vapor in the air during the preparation process before the test. When TEM shooting, there will be small moving layers (about 30 nm, the red part in Fig. 3b) on the edge and surface of the sheet structure, and the small moving layers and strip crystal structure will be significantly reduced after adding γ-Al₂O₃ (Fig. 3c–j). In addition, it can be found that with the increase of γ-Al₂O₃ content, the transparency of TEM images increases gradually which is mainly because the distribution of Al₂O₃ becomes uniform. When the content of γ-Al₂O₃ reaches 20% (Fig. 3i and j), the average particle size of γ-Al₂O₃ nanoparticles can be reduced to around 11.07 nm, and the distribution of Al₂O₃ is more consistent than that observed in Fig. 3c–h (with lower γ-Al₂O₃ content).

Fig. 3 TEM images of samples: (a and b) pure LiCl; (c and d) LiCl·H₂O-Al₂O₃-5%, (e and f) LiCl·H₂O-Al₂O₃-10%; (g and h) LiCl·H₂O-Al₂O₃-15%; (i and j) LiCl·H₂O-Al₂O₃-20% with different nano γ-Al₂O₃ contents; SEM images of samples; (k) pure LiCl·H₂O; (l) LiCl·H₂O-Al₂O₃-20%.

The SEM images of pure LiCl and LiCl·H₂O-Al₂O₃-20% are shown in Fig. 3k and l. The pure LiCl generates agglomeration as a whole (≫500 μm) shown in Fig. 3k. While the particles size range of LiCl·H₂O-Al₂O₃-20% is 2.5–10 μm which is much smaller than that of pure LiCl. In summary, the results show that the increase of γ-Al₂O₃ content is beneficial to the dispersion of LiCl, effectively inhibit the agglomeration of LiCl and LiCl·H₂O, and facilitate the hydration process.

Fig. 4 can be used to observe the whole process of hydration and dehydration of pure LiCl·H₂O and composites. The in situ DRIFT spectroscopy of the hydration reaction of LiCl and H₂O are showed in Fig. 4a. The materials were dried at 200 °C for 8 h in the oven before hydration, while the weak peaks standing for structural –OH and physical adsorbed H₂O can be observed in Fig. 4a, demonstrating that the materials partially hydrated during the sample placement due to the hygroscopic property of LiCl. The stretching vibrations of structural –OH and physical adsorbed H₂O always assign to the bands in the range of 3800–2200 cm⁻¹.⁵⁰ The bands at 3630–2950 cm⁻¹ and at 1700–1580 cm⁻¹ which the peak was centered at around 1673 cm⁻¹ in the spectra can be respectively attributed to the stretching vibrations (ν_OH-s) and bending vibrations (β_OH-s) of the structural –OH in LiCl·H₂O.^54,55 Moreover, the broad peak in the 3423–2842 cm⁻¹ range centered around 3400 cm⁻¹ and the peak at 1640 cm⁻¹, respectively, belonged to the stretching vibrations (ν_OH-p) and bending vibrations (β_OH-p) of the physical adsorbed H₂O. The band between ν_OH-p and β_OH-p (2500–1900 cm⁻¹ centered at 2125 cm⁻¹) was called the sum frequency absorption H₂O band.^50,54 As the hydration reaction proceeds, the bandwidth of ν_OH-s markedly increased from 10 min to 70 min, indicating the continuous reaction of LiCl and H₂O after 70 min, the hydration reaction rate gradually decreased. In addition, it can be observed that the peak intensities of β_OH-s gradually decreased while the β_OH-p gradually appeared due to the deliquescence of LiCl. At the same time, the sum frequency absorption H₂O band gradually became wider.

Fig. 4 The in situ DRIFT spectroscopy of (a) the hydration reaction of LiCl; (b) dehydration reaction of LiCl·H₂O; (c) hydration reaction of LiCl·H₂O-Al₂O₃-10%; (d) dehydration reaction of LiCl·H₂O-Al₂O₃-10%.

Fig. 4b shows the dehydration reaction process of LiCl·H₂O obtained from 1.5 h hydration of LiCl in situ DRIFT spectroscopy reactor. Because the reactor required a certain time to reach the set temperature, the ending and initial curves in Fig. 4a and b are slightly different. When heated above 75 °C, the ν_OH-s was getting narrower and the peak intensities of β_OH-s started to rise and the trend became more evident over 100 °C. With the increase of temperature, structural –OH and physical adsorbed H₂O gradually disappeared.

Fig. 4c shows the hydration reaction process of LiCl-Al₂O₃-10% and H₂O in situ DRIFT spectroscopy reactor. The ν_OH-s bands showed an obvious broadening to 3630–2950 cm⁻¹ within 10 min hydration because of the existing metal coordination bond (Al³⁺), which formed the complex compound LiAlOCl₂. It took 40 min that the ν_OH-s bands were virtually no longer wider, shorter than that of pure LiCl (70 min), indicating that the nano γ-Al₂O₃ addition enhanced the hydration reaction rate of LiCl.

Fig. 4d presents the in situ DRIFT spectroscopy of dehydration reaction of LiCl·H₂O-Al₂O₃-10%. When LiCl·H₂O-Al₂O₃-10% was heated over 75 °C, the peak intensities of ν_OH-s and β_OH-s became sharper and narrower; beyond 125 °C, the dehydration of the composite gradually slowed down. The hydrogen bonding with Al³⁺ on the surface of γ-Al₂O₃ started to decompose. The peak intensities of –OH in structural H₂O did not change until 175 °C, which was lower than that of pure LiCl·H₂O (225 °C). It can be found that the addition of γ-Al₂O₃ accelerated the hydration rate of LiCl, concentrated the utilization range of the spectrum, and promoted the desorption rate of physical adsorbed H₂O and low-frequency structural –OH in the materials that are consistent with our XRD data. The main reason is that the addition of Al³⁺ can enhance the stretching vibration of structural –OH (3630–2950 cm⁻¹), lead to the formation of coordination bonds between structural –OH and metal ions and promote the formation of intermolecular hydrogen bonds that further strengthens the binding of water molecules.^50,51

The nitrogen adsorption isotherms and pore size distributions of LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15% and LiCl·H₂O-Al₂O₃-20% were carried out by the nitrogen adsorption method and shown in Fig. 5. It can be seen from Fig. 5a that in the low pressure region (p/p₀ < 0.2), there is a weak interaction between the adsorbate and the pore wall, and the adsorption curve rises slowly. With the increase of relative pressure, that is, when 0.2 < p/p₀ < 1, the adsorption curve rises gradually. This is a typical type III adsorption isotherm, indicating that the pores are mainly mesoporous. With the increase of Al₂O₃ concentration, the increasing range of adsorption curve will be higher, and the hysteresis loop appears, which is due to the occurrence of mesoporous capillary condensation in the adsorption process, resulting in irreversible desorption process. This is a typical isotherm of type III combined with type V. In addition, a small amount of micropores gradually appear with the increase of the proportion of Al₂O₃. According to Fig. 5b, avery small amount of 5–10 nm pores were present in the pure LiCl which may result from the stacking or the voids between of LiCl particles. However, the cumulative pore volume of pure LiCl can be ignored as compared to the γ-Al₂O₃, as indicated in Table 1. In contrast, the pore size distribution of LiCl-Al₂O₃ changed significantly (Fig. 5c) and mainly concentrated within 8–15 nm, which indicated that the addition of nano γ-Al₂O₃ formed a large number of nano-scale pores. As shown in Table 1, the specific surface area of pure LiCl is low (0.4 m² g⁻¹), and the biggest specific surface area (15.18 m² g⁻¹) of the composite (LiCl·H₂O-Al₂O₃-20%) is 34.5 times higher than that of pure LiCl and the specific surface area (calculated by BET method), the pore volume and their proportion (microporous and mesoporous ratio) of the composites increase with the increase of the ratio of Al₂O₃. However, it has been pointed out that the mechanical mixing of additive (nano γ-Al₂O₃) and reactive salt (LiCl) could also lead to a increase of BET surface area compared to the reactive salt, due to the relatively high specific surface area of the additive.⁵⁶ For the purpose of assess the real change of surface area caused by the addition of nano γ-Al₂O₃, it is proposed that the surface area should be normalized relative to the additive weight in the composite rather than the composite weight. The normalized surface (NSA) was defined as:

where SA is the specific surface area of the parent Al₂O₃ or LiCl-Al₂O₃ composite, z is the percentage of LiCl content in composites, which is 95%, 90%, 85%, 80% for LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15%, LiCl·H₂O-Al₂O₃-20%, respectively. Thus the NAS for LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15%, LiCl·H₂O-Al₂O₃-20% is 0.25, 0.38, 0.40, 0.43. It can be found that the NSA of composites increases with the increase of Al₂O₃ content, which is consistent with the changes of BET surface area and pore volume as shown in Table 1. However, all of these values are much less than 1, indicating that LiCl particles with the size less than the pore diameter are at least partially located inside the pore while substantial pore blocking by large LiCl particles (whose sizes are comparable or higher than Al₂O₃ pore diameter) exist.^56,57

Fig. 5 (a) Nitrogen adsorption isotherms of LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15%, LiCl·H₂O-Al₂O₃-20%; pore size distributions of; (b) LiCl and Al₂O₃; (c) LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10%, LiCl·H₂O-Al₂O₃-15%, LiCl·H₂O-Al₂O₃-20%.

Table 1 Texture parameters of pure LiCl and composites with different nano γ-Al₂O₃ contents

Samples	BET Surface area (m^2 g^−1)	Pore volume (cc g^−1)	Average pore size (nm)
γ-Al2O3	176.3	0.7	15.8
LiCl·H2O-Al2O3-20%	15.2	0.04	10.1
LiCl·H2O-Al2O3-15%	10.7	0.03	10.6
LiCl·H2O-Al2O3-10%	6.8	0.02	12.1
LiCl·H2O-Al2O3-5%	2.2	0.009	14.2
Pure LiCl	0.4	0.001	12.6

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The thermal storage performance and reaction mechanism of LiCl chemical thermal storage materials were studied by TG-DSC curve. The calculation quality of material HSD is based on the quality after material hydration. Fig. 6 is the TG-DSC curve of LiCl·H₂O-Al₂O₃ composites, which have been hydrated for 1 h at 30 °C and 60% RH. The hydration conversion of LiCl (α_LiCl) was calculated according to the relative mass change (w) and the percentage of LiCl content in composites (z). The calculation formula is as followed:

Fig. 6 TG-DSC curves of: (a) LiCl·H₂O; (b) LiCl·H₂O-Al₂O₃-5%; (c) LiCl·H₂O-Al₂O₃-10%; (d) LiCl·H₂O-Al₂O₃-15%.

Due to the slow hydration rate of pure LiCl, only 51.70% of LiCl is hydrated after 1 h and its HSD is only about 402.04 kJ kg⁻¹ (Fig. 6a). Compared with the pure LiCl, the LiCl conversion in composites can achieve 78.28%, 82.63% and 97.34% after 1 h hydration corresponding to LiCl·H₂O-Al₂O₃-5%, LiCl·H₂O-Al₂O₃-10% and LiCl·H₂O-Al₂O₃-15% respectively, demonstrating that nano γ-Al₂O₃ is beneficial to enhance the LiCl hydration reaction rate. By changing the content of nano γ-Al₂O₃, the overall HSD of composites can reach 565.75 kJ kg⁻¹ (5%), 577.75 kJ kg⁻¹ (10%), and 607.5 kJ kg⁻¹ (15%) respectively, and the highest HSD of LiCl·H₂O monomer can be increased to 714.7 kJ kg⁻¹ (LiCl·H₂O-Al₂O₃-15%). Table 2 shows the comparison of water uptake amount for 1 h of the prepared composite with other reported composites for TCS. It can be found that the water uptake amount of composite prepared in this work can reach 0.26 g g⁻¹ after 1 h hydration, exhibiting superior performance compared to other composites. Therefore, although some pores of Al₂O₃ are blocked by LiCl particles as discussed above, the hydration performance of reactive salt is improved by the addition of Al₂O₃.

Table 2 Comparison of water uptake capacity at 1 h hydration between the prepared composite and composites from literature

Composite	LiCl (or LiOH) content [%]	Temperature (°C)	Relative humidity (%)	Water uptake value (g g^−1)
LiCl/ENG-TSA/silica^32	33.3%LiCl	30	60	0.14
LiCl/zeolite^40	47.1%LiCl	25	80	0.15
LiCl/alumina spherical grain^46	14.7%LiCl	20	80	0.20
LiCl/HKUST-1 (ref. 58)	52.4%LiCl	35	50	0.21
This study	85.0%LiCl	30	60	0.26

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Meanwhile, the decomposition temperature of composites were reduced compared with that of the pure material. There are two possible reasons why the hydration rate and HSD of LiCl·H₂O-Al₂O₃ composites have been greatly increased. First, the addition of Al³⁺ forms coordination bonds between structural –OH and metal ions and forms more intermolecular hydrogen bonds, further strengthening the binding of water molecules and enhancing the adsorption capacity of materials on water molecules to improve the hydration reaction rate. Second, when nano γ-Al₂O₃ compounded with LiCl·H₂O, the size of LiCl·H₂O particles reaches nano-scale and the surface atomic quantity increases significantly.⁵⁹

Fig. 7 illustrates the thermal conductivity (on left-hand side) and HSD (for 1 h hydration on right-hand side) of pure LiCl and LiCl-γ-Al₂O₃ composites after a drying period of 8 h at 200 °C. It can be seen from Fig. 7 that the thermal conductivity decreases with the increase of nano γ-Al₂O₃ content due to the low thermal conductivity of nano γ-Al₂O₃, which is about 0.3 W (m K)⁻¹. The porosity of the initial material is the largest under the same reaction environment and the thermal conductivity is the lowest. Due to the thermal conductivity measurement was conducted immediately after the drying treatment, it can be considered that there was no water present in the composites. Therefore, the data obtained can reflect the thermal conductivity of composite materials during the initial hydration reaction. On the other hand, the HSD of composites (30 °C and 80% RH for 1 h hydration) increases along with the γ-Al₂O₃ ratio, demonstrating that the addition of nano γ-Al₂O₃ promotes the HSD of composites. The heat and mass transfer are two factors that need to be considered comprehensively for chemical heat storage. From the perspective of heat transfer, it can be seen from Fig. 7 that the optimal γ-Al₂O₃ addition ratio should be less than 5%. However, it also can be found that the rising ratio of HSD and the falling ratio of thermal conductivity become slow with the γ-Al₂O₃ addition ratio of more than 5%. For this study, the optimum addition ratio of γ-Al₂O₃ would be 5–10%. Consequently, there is a trade-off between thermal conductivity and HSD for practical applications.

Fig. 7 Variations of thermal conductivity and HSD of the composites with the Al₂O₃ mass content.

Activation energy refers to the energy required for a molecule to change from normal to an active state which is prone to chemical reactions. The chemical reaction rate is closely related to the size of the activation energy. The lower the activation energy, the faster the reaction rate. Therefore, studying the value of the activation energy can also effectively reflect the speed of the reaction.⁶⁰ The activation energy of dehydration performance of LiCl and LiCl·H₂O-Al₂O₃-15% were calculated by Ozawa method⁶¹ for computing the activation energy of thermal decomposition and dehydration reaction.⁶² According to Ozawa method, the following equation can be obtained:

E is activation energy [kJ mol⁻¹], the value of heating rate (β) is 3, 5, 7 and 10 [K (min)⁻¹], T is temperature [K], the value of R is 8.314 [J (mol K)⁻¹], A is the pre-exponential factor, α is the dehydration conversion and f(α) is the function of dehydration conversion. During the calculation of activation energy, the dehydration conversion kept constant at 70%.^61,62 As shown in Fig. 8, the activation energy of dehydration reaction for LiCl and LiCl·H₂O-Al₂O₃-15% is 90.89 kJ mol⁻¹ and 79.76 kJ mol⁻¹, respectively. It can be found that the addition of Al₂O₃ is useful for decreasing the activation energy of composites. It is speculated that the addition of γ-Al₂O₃ provides more loading area for LiCl, so that LiCl can absorb more water vapor in the same test environment and time to promote hydration reaction. And –AlCl bond is formed after the reaction of LiCl with γ-Al₂O₃, which reduces the activation energy of dehydration reaction.⁵⁴

Fig. 8 The activation energy of dehydration reaction of (a) LiCl·H₂O, (b) LiCl·H₂O-Al₂O₃-15%.

4 Conclusions

In this study, the LiCl·H₂O-Al₂O₃ composites were successfully constructed and characterized. The structural properties of these composites was experimentally investigated by XRD, SEM, BET, in situ DRIFT spectroscopy and the thermal performance were obtained by TG-DSC and a hot disk thermal constant analyzer. The following findings can be addressed:

(1) Results of XRD show that a complex of LiAlOCl₂ was formed by LiCl·H₂O and Al₂O₃. Moreover, with the increase of γ-Al₂O₃ content, the particle size of LiCl·H₂O-Al₂O₃ composites gradually decreased and became uniform through TEM graphs.

(2) According to the in situ DRIFT spectroscopy results, the addition of Al₂O₃ accelerated the hydration rate of the material, concentrated the utilization range of spectrum, and promoted the desorption of physical adsorbed H₂O and low-frequency structural –OH in the materials. Furthermore, LiCl-Al₂O₃-20% exhibited the highest specific surface area (15.18 m² g⁻¹), which is 34.5 times higher than that of pure LiCl.

(3) Compared with pure LiCl·H₂O, LiCl·H₂O-Al₂O₃ composites exhibited better thermal storage performance, higher HSD of LiCl·H₂O monomer (714.7 kJ kg⁻¹) and water uptake (0.26 g g⁻¹). Meanwhile, the desorption temperature of composites decreased compared with that of the pure LiCl and the addition of Al₂O₃ is useful for decreasing the activation energy of composites (90.89 kJ mol⁻¹ to 79.76 kJ mol⁻¹). However, the thermal conductivity of composite decreased with the increase of nano γ-Al₂O₃ content. Further research should be undertaken to improve this aspect.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (no. 52176091 and no. 52006158), and the Open Fund of Science and Technology on Thermal Energy and Power Laboratory (no. TPL2020A02). We would also like to acknowledge Juan Fu from GuangZhou Institute of Energy Conversion for the experimental guidance.

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