Regular Article
Dehydration Reactivity of Mg(OH)2 Containing Low Amounts of Li-additives for Thermochemical Energy Storage
2022 Volume 62 Issue 12 Pages 2551-2558
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2022 Volume 62 Issue 12 Pages 2551-2558
Mg(OH)2 is a chemical heat storage material suitable for the utilization of unused heat at 300–400°C. It has been reported that the addition of Li compounds to Mg(OH)2 promotes the dehydration of Mg(OH)2. However, the demand for Li compounds has increased in recent years and the price of Li compounds is relatively high. Therefore, the purpose of this study is to enhance the dehydration reactivity of Mg(OH)2 with a small amount of Li. In this study, several alkali metal chlorides and hydroxides such as LiCl and NaOH or KOH were added to Mg(OH)2, and the dehydration reactivity and composition of the corresponding mixtures were investigated. The samples prepared using 5 mol% LiCl and 10 mol% NaOH (LiCl-5NaOH-10) or 2.5 mol% KOH (LiCl-5KOH-2.5) showed excellent dehydration reactivity and were dehydrated below 300°C with comparatively lower amounts of Li compounds than those reported in previous studies. These results indicate that these samples have great potential as low-cost chemical heat storage materials. However, the stabilities of these samples in air are quite different. Based on X-ray diffraction analysis of the data, the results are associated with the composition of Cl−, OH−, Li+, Na+, and K+.
In recent years, the high consumption of fossil fuels has caused global environmental problems such as global warming and the consequent depletion of energy sources. Renewable energy sources have been utilized as alternative energy sources to fossil fuels, but they have some problems, including inefficient energy generation and unstable energy supply because of their dependence on weather, time, and place.
Therefore, the utilization of waste heat, such as from industrial waste and surplus solar heat, has drawn attention. Heat storage techniques are classified into three types: sensible, latent, and chemical heat storage techniques. Sensible and latent heat storage techniques use temperature and phase changes, respectively. However, the period of energy storage using these techniques is limited by the large energy loss. In contrast, chemical heat storage techniques are characterized by relatively higher energy densities (0.5–1 kWh kg−1), and the period of energy storage is theoretically unlimited.1,2) These properties of chemical heat storage techniques could be favorable in solving the problems of renewable energy sources mentioned above.
Chemical heat storage materials can utilize waste heat via chemical reactions, many of which have been researched.1,2,3) Ca(OH)2 is a commonly used chemical heat storage material. The advantages of Ca(OH)2 include low cost, comparatively high energy density, and reversibility of heat storage and output operation.3,4) However, the dehydration (heat storage stage) of Ca(OH)2 occurs at temperatures above 390°C and should be decreased to expand the utility of Ca(OH)2 in chemical heat storage systems. Shkatulov and Aristov investigated the effect of Ca(OH)2 doped with different salts on the onset temperature of dehydration. KNO3 as a dopant decreased the onset temperature of Ca(OH)2 dehydration by approximately 40°C.3) Maruyama et al. reported that the addition of one or two kinds of Li compounds to Ca(OH)2 reduced the dehydration temperature by up to 59°C.4) Additionally, the thermal properties of materials play a crucial role in the development of chemical heat storage systems. Kariya et al. investigated the effect of expanded graphite (EG) on the heat transfer and reactivity of Ca(OH)2. The addition of EG significantly enhanced the reactivity and moldability of Ca(OH)2.5)
Mg(OH)2 is a type of chemical heat storage material that has similar advantages as Ca(OH)2, including a relatively high energy density, nontoxicity of the materials in the Mg(OH)2/MgO system, and low cost.3,6) The reaction of Mg(OH)2, which is used as a chemical heat storage material, is represented by Eq. (1).7)
| (1) |
Dehydration (heat storage step) of Mg(OH)2 usually occurs in the temperature range of 300–400°C,7) which is lower than that of Ca(OH)2. However, the dehydration reaction rate at temperatures below 300°C, which constitutes a large part of the industrial waste heat in Japan,8) is markedly lower.6) Therefore, high dehydration rates are required for practical application of the materials.
Several studies have been conducted to increase the dehydration reactivity of Mg(OH)2. Ryu et al. reported that Mg(OH)2 mixed with Co(OH)2 or Ni(OH)2 exhibited higher dehydration reactivity at a lower temperature compared to pure Mg(OH)2.9) Additionally, the Mg(OH)2 dehydration temperature decreased by 44°C when using 10 mol% LiCl.10) A low dehydration temperature is beneficial for the application of Mg(OH)2/MgO systems. LiCl-containing Mg(OH)2 has been investigated in previous studies. Ishitobi et al. investigated the effect of 105 dehydration/hydration cycles for LiCl-containing Mg(OH)2.11) The hydration temperature, water vapor pressure, dehydration rate,12) and LiCl mixing ratio were investigated for Li-containing Mg(OH)2.13) The addition of LiCl lowered the dehydration temperature of the aforementioned Mg–Co mixed hydroxides.14) Additives other than LiCl were also investigated to enhance the performance of Mg(OH)2-based chemical heat storage materials. The addition of EG improved the thermal conductivity of Mg(OH)2.15) To recover thermal energy from the iron-making process, the dehydration/hydration behavior of composites containing Mg(OH)2, EG, and LiBr was investigated.16) Based on previous research, the dehydration temperature of Mg(OH)2 can be lowered using 7.0 mol% LiNO3,3) 7.0 mol% LiOOCCH3,3) 20 mol% LiOH,17,18) 5 mol% C6H5Na3O7,19) 1.7 mol% Ce(NO3)3 and 17 mol% LiOH,6) or, 3.2 mol% ZrO(NO3)2 and 31 mol% LiOH.20) Kurosawa et al. reported that doping 10 mol% LiCl and 10 mol% LiOH can lower the dehydration temperature of Mg(OH)2 by 66°C.18) In addition, LiCl and LiOH have been proposed to enhance the dehydration of Mg(OH)2.21)
In most of these investigations, Li compounds are used. However, the demand for Li compounds will increase in the future,22) and the price of Li compounds is high because of the cost of refining Li compounds from their raw materials such as ore and brine.23) Therefore, reducing the amount of Li compounds used as additives is desirable. In addition, alkali metal hydroxides, including LiOH, are deleterious substances and unstable in air because of their high reactivity with CO2;24) thus, decreasing the amount of alkali metal hydroxides is also preferable.
Accordingly, the goal of this study is to lower the dehydration temperature of Mg(OH)2 by decreasing the amount of Li compounds. Specifically, the temperature at which the dehydration of Mg(OH)2 proceeds is less than 300°C, while the amount of Li compounds is limited to 10 mol% or less. In addition, we attempted to reduce the amount of alkali metal hydroxide. Mg(OH)2/MgO systems with fewer Li compounds and alkali metal hydroxide additives show potential for thermochemical energy storage in terms of their cost, safety, and stability.
The materials used as precursors were Mg(OH)2 (99.9%, 0.07 μm, FUJIFILM Wako Pure Chemical Corporation) and additives, LiCl·H2O (99.9%, Wako Pure Chemical Industries, Ltd.), NaCl (>99.5%, Wako Pure Chemical Industries, Ltd.), KCl (99.9%, FUJIFILM Wako Pure Chemical Corporation), LiOH·H2O (98.0–102.0%, Wako Pure Chemical Industries, Ltd.), NaOH (>97.0%, Wako Pure Chemical Industries, Ltd.), and KOH (>85.0%, FUJIFILM Wako Pure Chemical Corporation).
All samples were prepared using the impregnation method. First, aqueous additives were prepared using ultrapure water and additives, such as LiCl·H2O and NaCl. Thereafter, Mg(OH)2 powder was impregnated into each solution, and the mixtures were stirred for 30 min. Subsequently, water was evaporated using a rotary evaporator at 40°C under low pressure. Finally, the specimens were dried at 120°C overnight, and white powder samples were obtained. Additives-added Mg(OH)2 with a Mg(OH)2/additives mole ratio of 100:X is referred to as additives-X; for example, adding both LiCl and KOH to Mg(OH)2 with Mg(OH)2/LiCl/KOH = 100:5:2.5, is referred to as LiCl-5KOH-2.5. For comparison, Mg(OH)2 without additives, referred to as Mg(OH)2-W, was prepared using the same method. The mixing ratios of all prepared samples are listed in Table 1. In Table 1(a), alkali metal chlorides (LiCl, NaCl, and KCl) and hydroxides (LiOH, NaOH, and KOH) with singly added Mg(OH)2 are listed. It has been reported that the addition of LiCl and/or LiOH enhances the dehydration reactivity of Mg(OH)2;18) therefore, LiCl, NaCl, KCl, LiOH, NaOH, and KOH, which are typical alkali metal chlorides and hydroxides, were chosen as additives. The samples listed in Table 1(b) are discussed in a later section.
| (a) | sample | mixing ratio [mole ratio] |
|---|---|---|
| Mg(OH)2-W | Mg(OH)2 without additives | |
| LiCl-10 | Mg(OH)2/LiCl = 100:10 | |
| NaCl-10 | Mg(OH)2/NaCl = 100:10 | |
| KCl-10 | Mg(OH)2/KCl = 100:10 | |
| LiOH-10 | Mg(OH)2/LiOH = 100:10 | |
| NaOH-10 | Mg(OH)2/NaOH = 100:10 | |
| KOH-10 | Mg(OH)2/KOH = 100:10 |
| (b) | sample | mixing ratio [mole ratio] |
|---|---|---|
| Mg(OH)2-W | Mg(OH)2 without additives | |
| LiCl-10NaOH-10 | Mg(OH)2/LiCl/NaOH = 100:10:10 | |
| LiCl-10NaOH-5 | Mg(OH)2/LiCl/NaOH = 100:10:5 | |
| LiCl-10KOH-10 | Mg(OH)2/LiCl/KOH = 100:10:10 | |
| LiCl-10KOH-5 | Mg(OH)2/LiCl/KOH = 100:10:5 | |
| LiCl-5NaOH-10 | Mg(OH)2/LiCl/NaOH = 100:5:10 | |
| LiCl-5NaOH-5 | Mg(OH)2/LiCl/NaOH = 100:5:5 | |
| LiCl-5NaOH-2.5 | Mg(OH)2/LiCl/NaOH = 100:5:2.5 | |
| LiCl-2NaOH-1 | Mg(OH)2/LiCl/NaOH = 100:2:1 | |
| LiCl-5KOH-10 | Mg(OH)2/LiCl/KOH = 100:5:10 | |
| LiCl-5KOH-5 | Mg(OH)2/LiCl/KOH = 100:5:5 | |
| LiCl-5KOH-2.5 | Mg(OH)2/LiCl/KOH = 100:5:2.5 | |
| LiCl-2KOH-1 | Mg(OH)2/LiCl/KOH = 100:2:1 |
The dehydration reactivities of all samples listed in Table 1 were investigated using a TGD-9600 series thermobalance (ADVANCE RIKO, Inc.). The samples (~10 mg) were placed in Pt cells and subsequently heated from room temperature (15–25°C) to 600°C at a rate of 10°C min−1 under an Ar flow of 100 mL min−1.
To evaluate the dehydration reactivity of the samples, the weight change of the samples, measured using the thermobalance, was transformed to the mole fraction of Mg(OH)2, as shown below.
| (2) |
| (3) |
Figure 1 shows the TG curve of Mg(OH)2-W heated to 600°C. The first weight loss was caused by the removal of physically adsorbed water. The sample weight did not change at 200°C; therefore, wini is the weight of Mg(OH)2 at 200°C.
Thermogravimetric (TG) curve for Mg(OH)2-W heated to 600°C under 100 mL min−1 Ar flow. (Online version in color.)
To investigate the crystal structure and composition of the samples, XRD measurements were conducted using an Ultima IV XRD instrument (Rigaku Corp.) in air at room temperature in the 2θ range of 10–150° at a scan rate of 10° min−1 and a scan width of 0.01° using Cu Kα radiation at a voltage of 40 kV and current of 40 mA. Because the 2θ range of 10–80° could sufficiently characterize the samples, the data are shown in this range. Before the measurements, the prepared samples were dried at 120°C for approximately 30 min to remove any physically adsorbed water.
Figure 2 shows the dehydration behavior of the samples listed in Table 1(a) when heated to 600°C. The y-axis represents the mole fraction of Mg(OH)2, based on Eq. (2), and the x-axis represents the temperature. The dehydration of Mg(OH)2 with added NaCl or KCl proceeded at temperatures as low as those of Mg(OH)2-W (Fig. 2(a)). In contrast, the dehydration of Mg(OH)2 with the addition of LiCl, LiOH, NaOH, or KOH progressed at relatively lower temperatures (Fig. 2(b)). The differential thermogravimetric (DTG) curve of these data are shown in Figs. 2(c) and 2(d).
Dehydration behavior of Mg(OH)2-W, (a) LiCl-10, NaCl-10, and KCl-10, (b) LiOH-10, NaOH-10, and KOH-10, and differential thermogravimetry (DTG) curve of Mg(OH)2-W, (c) LiCl-10, NaCl-10, and KCl-10, (d) LiOH-10, NaOH-10, and KOH-10 heated to 600°C under 100 mL min−1 Ar flow. (Online version in color.)
Mg(OH)2 in NaOH or KOH was dehydrated in two steps. In a previous study, Mg(OH)2 with 20 mol% LiOH added into it presented a two-step reaction,18) which constitutes the reactions of two different sites (the surface and bulk) of Mg(OH)2 at lower and higher temperatures.18,21) Therefore, the addition of NaOH or KOH should have a similar effect on the enhancement of the dehydration reactivity of Mg(OH)2 to that of LiOH. Further research is required to elucidate these effects in detail.
Table 2 shows the peak temperature for the dehydration of the samples derived from the differential thermogravimetric (DTG) curve based on Figs. 2(c) and 2(d). The peak temperatures of 10 mol% LiCl and 10 mol% LiOH co-added with Mg(OH)2 (LiCl-10LiOH-10) are also shown in Table 2.18) The peak temperature was the temperature at which the dehydration reaction rate was the highest. The dehydration reaction rate is the sample weight, which is differentiated by the temperature. A lower peak temperature indicates that the Mg(OH)2 in the sample can be dehydrated at lower temperatures, that is, the sample is suitable for low-temperature heat storage. The peak temperature of LiCl-10 was 45°C lower than that of Mg(OH)2-W, which had the lowest peak temperature of all the samples shown in Fig. 2; therefore, the addition of LiCl was the most effective for the enhanced dehydration reactivity of Mg(OH)2. The peak temperatures of KOH-10 and NaOH-10 were lowered by 30°C and 27°C, respectively, than that of Mg(OH)2-W. However, the reduction in the peak temperatures of LiCl-10, KOH-10, and NaOH-10 was lower than that of LiCl-10LiOH-10.18)
| sample | peak temperature [°C] |
|---|---|
| Mg(OH)2-W | 364 |
| LiCl-10 | 319 |
| NaCl-10 | 358 |
| KCl-10 | 361 |
| LiOH-10 | 357 |
| NaOH-10 | 298, 337 (maximum rate) |
| KOH-10 | 285, 334 (maximum rate) |
| LiCl-10LiOH-10 18) | 305 |
Previous research indicates that the addition of LiOH increases the dehydration of Mg(OH)2 owing to the substitution of Mg2+ and Li+ ions.21) In this study, similar results were obtained with the addition of NaOH or KOH to Mg(OH)2. Thus, NaOH and KOH are hypothesized to play the similar role as LiOH in the dehydration of Mg(OH)2. Future research should investigate the function of Na+ and K+ ions.
3.2. Dehydration Reactivity of Mg(OH)2 with Both LiCl and NaOH or KOH AddedFrom Section 3.1, it was found that LiCl, NaOH, and KOH had relatively better effects in lowering the dehydration temperature of Mg(OH)2 than the other additives used in the samples shown in Fig. 2. Moreover, Mg(OH)2, with the addition of LiCl and LiOH (alkali metal hydroxides), showed better dehydration reactivity than pure Mg(OH)2.18) Therefore, the addition of LiCl and NaOH or KOH (alkali metal hydroxides) to Mg(OH)2 is worth investigating. Based on Section1, the mole ratios of LiCl, NaOH, and KOH in the samples were limited to 10 mol% or less for Mg(OH)2. The samples prepared using LiCl, NaOH, and KOH are listed in Table 1(b).
Figures 3, 4, 5 show the dehydration behavior of the samples and the differential thermogravimetric (DTG) curve listed in Table 1(b) when heated to 600°C. All samples prepared using LiCl, NaOH, or KOH were dehydrated at relatively lower temperatures than Mg(OH)2-W. Similar to the dehydration of NaOH-10 and KOH-10, the dehydration of LiCl-5NaOH-10 and LiCl-5KOH-10 proceeded in a two-step reaction. The reason for this two-step reaction is discussed in a subsequent section.
(a) Dehydration behavior, (b) differential thermogravimetry (DTG) curve of Mg(OH)2-W, and both 10 mol% LiCl and NaOH or KOH-added Mg(OH)2 heated to 600°C under 100 mL min−1 Ar flow. (Online version in color.)
(a) Dehydration behavior, (b) differential thermogravimetry (DTG) curve of Mg(OH)2-W, and both 5 mol% LiCl and NaOH-added Mg(OH)2 heated to 600°C under 100 mL min−1 Ar flow. (Online version in color.)
(a) Dehydration behavior, (b) differential thermogravimetry (DTG) curve of Mg(OH)2-W, and both 5 mol% LiCl and KOH-added Mg(OH)2 heated to 600°C under 100 mL min−1 Ar flow. (Online version in color.)
Table 3 shows the peak temperature for the dehydration of the samples derived from the DTG curve based on Figs. 3(b), 4(b), and 5(b). The peak temperature of LiCl-10LiOH-10 is also shown in Table 3.18) The peak temperatures of LiCl-5NaOH-10 and LiCl-5KOH-2.5 were 84°C and 67°C lower than those of Mg(OH)2-W, respectively, which were the lowest and second lowest in the samples listed in Tables 2 and 3.18) In addition, the amount of Li used in LiCl-5NaOH-10 and LiCl-5KOH-2.5 is a quarter of the amount of Li used in LiCl-10LiOH-10.18) These results indicate that LiCl-5NaOH-10 and LiCl-5KOH-2.5 have great potential as chemical heat storage materials for industrial waste heat recovery at a lower cost.
| sample | peak temperature [°C] |
|---|---|
| Mg(OH)2-W | 364 |
| LiCl-10NaOH-10 | 319 |
| LiCl-10NaOH-5 | 304 |
| LiCl-10KOH-10 | 351 |
| LiCl-10KOH-5 | 299 |
| LiCl-5NaOH-10 | 280 (maximum rate), 321 |
| LiCl-5NaOH-5 | 350 |
| LiCl-5NaOH-2.5 | 306 |
| LiCl-2NaOH-1 | 362 |
| LiCl-5KOH-10 | 278, 335 (maximum rate) |
| LiCl-5KOH-5 | 352 |
| LiCl-5KOH-2.5 | 297 |
| LiCl-2KOH-1 | 343 |
| LiCl-10LiOH-10 18) | 305 |
However, LiCl-5NaOH-10 and LiCl-5KOH-2.5 might not be stable in air because alkali metal hydroxides that react with CO2 in air were added to them. Therefore, to discuss the stability of LiCl-5NaOH-10 and LiCl-5KOH-2.5 in air, the dehydration behavior of the samples, which are LiCl-5NaOH-10 and LiCl-5KOH-2.5, were exposed to air for about one week from the date when the dehydration reaction tests of LiCl-5NaOH-10 and LiCl-5KOH-2.5 were conducted (LiCl-5NaOH-10-air and LiCl-5KOH-2.5-air, respectively).
Figure 6 shows the dehydration behavior and the differential thermogravimetric (DTG) curve of LiCl-5NaOH-10, LiCl-5NaOH-10-air, LiCl-5KOH-2.5, and LiCl-5KOH-2.5-air when heated to 600°C, and Table 4 shows the peak temperature for dehydration of the samples derived from the DTG curve based on Fig. 6(b). The dehydration of LiCl-5NaOH-10-air proceeded as a nearly one-step reaction, whereas that of LiCl-5NaOH-10 proceeded in a two-step reaction, and the peak temperature of LiCl-5NaOH-10-air was 60°C higher than that of LiCl-5NaOH-10. These results indicate that the effect of additives, that is, 5 mol% LiCl and 10 mol% NaOH, on lowering the dehydration temperature of Mg(OH)2 is reduced by LiCl-5NaOH-10 exposed to air. In contrast, the dehydration behavior and peak temperature of LiCl-5KOH-2.5-air are similar to those of LiCl-5KOH-2.5. Thus, the effect of additives used in LiCl-5KOH-2.5 for lowering the dehydration temperature of Mg(OH)2 hardly changes or changes extremely slowly, even if LiCl-5KOH-2.5 is exposed to air. Therefore, in the case when chemical heat storage materials are treated in air and with respect to their dehydration reactivity, LiCl-5KOH-2.5 is better than LiCl-5NaOH-10 as a chemical heat storage material. The reason why LiCl-5NaOH-10 was affected by air is discussed in a subsequent section.
(a) Dehydration behavior, (b) differential thermogravimetry (DTG) curve of LiCl-5NaOH-10, LiCl-5KOH-2.5, and LiCl-5NaOH-10 of LiCl-5KOH-2.5 exposed to air for about one week and heated to 600°C under 100 mL min−1 Ar flow. (Online version in color.)
| sample | peak temperature [°C] |
|---|---|
| LiCl-5NaOH-10 | 280 (maximum rate), 321 |
| LiCl-5NaOH-10-air | 340 |
| LiCl-5KOH-2.5 | 297 |
| LiCl-5KOH-2.5-air | 298 |
Figures 7, 8, 9, 10 show the XRD patterns of all samples before the dehydration reaction tests, as shown in Figs. 2, 3, 4, 5. The peaks of the brucite structure of Mg(OH)2 were identified in all the samples in Figs. 7, 8, 9, 10, indicating that the brucite structure of Mg(OH)2 is dominant in all the samples in Figs. 7, 8, 9, 10. No peaks corresponding to LiCl were detected for LiCl-10 (Fig. 7(a)). This is probably because LiCl species were incorporated into the Mg(OH)2 lattice, and is well dispersed throughout the Mg(OH)2 particles, converted into solution, or partially formed a solid solution with Mg(OH)2.13,18,21)
XRD patterns of the samples shown in Fig. 3 before the dehydration reaction tests. (Online version in color.)
XRD patterns of the samples shown in Fig. 4 before the dehydration reaction tests. (Online version in color.)
XRD patterns of the samples shown in Fig. 5 before the dehydration reaction tests. (Online version in color.)
No peaks corresponding to LiOH, NaOH, or KOH were detected in LiOH-10, NaOH-10, or KOH-10 (Fig. 7(b)). This is probably because LiOH, NaOH, and KOH species dissolve in the Mg(OH)2 phase or are well dispersed throughout the Mg(OH)2 particles.17) Instead, in LiOH-10, NaOH-10, and KOH-10, peaks of Li2CO3, Na2CO3, and K2CO3 were detected. It has been found that LiOH is carbonated within 6 h when exposed to air, based on a previous study,24) and the basicity of NaOH and KOH is stronger than that of LiOH; LiOH, NaOH, and KOH might partially react with CO2 in air during preparation, measurement, or storage. Namely, it is implied that alkali metal hydroxides probably exist in the sample with the addition of alkali metal hydroxides when carbonates based on the alkali metal hydroxides are detected in the sample in XRD measurements.
The peaks of NaCl or KCl were detected in all samples with the addition of LiCl and NaOH or KOH, although NaCl and KCl were not used at the time of sample preparation. The solubilities of the substances used in this study are shown in Fig. 11.25,26) When the samples were prepared, the substances used as additives were ionized (Section 2.1). That is, materials consisting of ions based on additives (not Mg(OH)2 because the solubility of Mg(OH)2 is much lower than that of the others) and characterized by lower solubility should be well deposited in the samples. As shown in Fig. 11, the solubilities of NaCl and KCl were lower than those of LiCl, NaOH, and KOH; therefore, in the samples co-added with LiCl and NaOH or KOH, NaCl or KCl consisting of Na+ or K+ based on NaOH or KOH and Cl− based on LiCl were present.
Solubility of the substances used in this study at 25°C.
The peaks of the carbonates based on NaOH and KOH were only detected in LiCl-5NaOH-10 and LiCl-5KOH-10, respectively (Figs. 9 and 10).27) Moreover, the dehydration of these samples proceeded in two steps, which was similar to that of NaOH-10 and KOH-10 (Figs. 2(b), 4, and 5). From these results and the discussion about the XRD patterns of LiCl-10, NaOH-10, and KOH-10 (Fig. 7(b)), NaOH and KOH could exist in the LiCl-5NaOH-10 and LiCl-5KOH-10, which is not contradictory to the quantitative relationships of the ions based on the additives.
In Section 3.2, the dehydration behavior of LiCl-5NaOH-10 changed when LiCl-5NaOH-10 was exposed to air for approximately one week (Fig. 6). Based on the discussion hereinbefore, this change was likely caused by the carbonation of NaOH and the change in composition of the sample. In contrast, the dehydration behavior of LiCl-5KOH-2.5 hardly changed under similar conditions (Fig. 6). In addition, no peaks of carbonates based on alkali metal hydroxides were detected in LiCl-5KOH2.5 (Fig. 10). Namely, the ratio of alkali metal hydroxides based on the additives in LiCl-5KOH-2.5 could be relatively small; therefore, the sample has difficulty reacting with CO2 and is relatively more stable in air.
The relationship between the composition and stability of the samples in air can be summarized as follows: When LiCl and NaOH or KOH were simultaneously added to Mg(OH)2 using the impregnation method, NaCl or KCl was deposited in the prepared samples for solubility. If the ratio of the additives is similar to that in this study, i.e., if the amount of LiCl is higher than that of NaOH or KOH (for example, LiCl-5KOH-2.5), the ratio of alkali metal hydroxides deposited in the sample is relatively low, and NaOH or KOH, which shows higher basicity than LiOH, is almost non-existent in the sample; therefore, it is relatively harder for the prepared samples to react with CO2 and is more stable. On the other hand, if the amount of LiCl is less than that of NaOH or KOH (for example, LiCl-5NaOH-10), the ratio of alkali metal hydroxides deposited in the sample is relatively high, and NaOH or KOH is non-negligible in the sample, so the prepared samples react relatively easily with CO2 and are more unstable.
In future studies, the combination of LiOH and NaCl or KCl as additives should be investigated to determine such relationships. In addition, since we were not able to clearly identify the presence of LiCl, LiOH, NaOH, and KOH, we must perform different characterizations or experiments.
To lower the dehydration temperature of Mg(OH)2 and decrease the amount of Li compound additives for Mg(OH)2 for application as a low-cost chemical heat storage material, Mg(OH)2 was singularly added with typical alkali metal chlorides or hydroxides such as LiCl and NaOH or KOH. The dehydration reactivity of all samples was investigated. Moreover, the stabilities of the two samples, LiCl/NaOH/Mg(OH)2 (LiCl-5NaOH-10) and LiCl/KOH/Mg(OH)2 (LiCl5KOH-2.5), whose dehydration temperatures were particularly low in air, were also investigated, and the causes for this stability in air were estimated based on XRD characterization.
LiCl/Mg(OH)2 (LiCl-10) showed the lowest dehydration temperature of all samples using alkali metal chlorides or hydroxides, and KOH/Mg(OH)2 (KOH-10) and NaOH/Mg(OH)2 (NaOH-10) had the second- and third-lowest dehydration temperatures, respectively (Table 2). Therefore, the addition of LiCl, KOH, and NaOH had a greater effect on the enhancement of the dehydration reactivity of Mg(OH)2 than that by the addition of LiOH, NaCl, and KCl. However, further studies are required to clearly reveal the effect of the addition of NaOH and KOH. For example, NaOH-10 and KOH-10 were dehydrated in a two-step reaction (Fig. 2(b)).
LiCl-5NaOH-10 and LiCl-5KOH-2.5 showed the largest- and second-largest enhancements in the dehydration reactivity of Mg(OH)2, and their peak temperatures are shown below as 300°C (280°C and 297°C, respectively) (Table 3). In addition, the ratio of Li compounds in these samples was lower than those in previous studies,3,6,10,18,19,20,21) so LiCl-5NaOH-10 and LiCl-5KOH-2.5 should have great potential as chemical heat storage materials with low amounts of Li compounds required.
After LiCl-5NaOH-10 and LiCl-5KOH-2.5 were exposed to air for approximately one week, the effect of the additives in LiCl-5NaOH-10 on lowering the dehydration temperature was weakened, whereas that of LiCl-5KOH-2.5 hardly changed (Fig. 6 and Table 4). Based on the XRD data, these results may be attributed to the NaOH or KOH added. In detail, if the ratio of added NaOH or KOH is less than that of LiCl, the prepared samples show high stability in air because most of the Na+ or K+ is deposited as NaCl or KCl in the samples in terms of solubility, that is, little NaOH or KOH is present in the samples. Therefore, considering the ease of handling, LiCl-5KOH-2.5 is a better chemical heat storage material than LiCl-5NaOH-10.
The combination of LiCl-NaOH and LiCl-KOH was effective in reducing the dehydration temperature of Mg(OH)2. However, the relationship between the additive amount and the peak dehydration temperature was not clear. Therefore, future studies should entail the optimization of additive amounts for practical use as thermochemical energy storage. Additionally, the role of Na+ and K+ ions as additives in Mg(OH)2 dehydration and other combinations thereof (LiOH-NaCl and LiOH-KCl), should be investigated in future work.
In this study, we investigated the dehydration of Mg(OH)2. However, no hydration tests were performed on the samples. An investigated on the hydration behavior of these samples will be conducted in future research.
This study was supported by Tanigawa Foundation and Steel Foundation for Environmental Protection Technology. The authors gratefully acknowledge these foundation.
