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. 2020 Apr 21;5(17):9820–9829. doi: 10.1021/acsomega.9b04444

Effect of Lithium Compound Addition on the Dehydration and Hydration of Calcium Hydroxide as a Chemical Heat Storage Material

Aya Maruyama 1, Ryo Kurosawa 1, Junichi Ryu 1,*
PMCID: PMC7203692  PMID: 32391469

Abstract

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Many studies on calcium hydroxide [Ca(OH)2] as a chemical heat storage material have been conducted. Generally, calcium hydroxide undergoes a dehydration reaction (heat storage operation) efficiently at about 400 °C or higher. In this study, we aimed to lower the dehydration reaction temperature and increase the dehydration reaction rate to expand the applicability of calcium hydroxide as a chemical heat storage material. For the purpose of improving the dehydration reactivity, calcium hydroxide with added lithium compounds was prepared, and the dehydration/hydration reactivities were evaluated. From the results, it was confirmed that the addition of the lithium compounds lowered the dehydration reaction temperature of calcium hydroxide and enhanced the reaction rate. The dehydration reaction of Ca(OH)2 with Li compounds proceeded efficiently even at 350 °C, and the reversibility of the dehydration/hydration reaction was confirmed. The reason for the improvement of the calcium hydroxide dehydration reactivity upon the addition of a lithium compound was examined from the viewpoint of its crystal structure. It was presumed that when lithium ions enter the calcium hydroxide crystals, the crystals became fragile and the dehydration reaction was accelerated.

1. Introduction

In the past decades, various energy issues, such as global warming, increased CO2 emissions, and the depletion of energy resources, have been attracting significant attention. The effective use of energy is necessary to solve these issues, and the studies on the solar energy utilization and recycling of industrial waste heat are important. However, these thermal energies have gaps in time and space between recovery and usage, and their use is limited. Therefore, heat storage/output systems are essential for solar energy utilization and recycling of industrial waste heat.

There are three techniques for heat storage: sensible heat storage, latent heat storage, and chemical heat storage. Although sensible heat storage and latent heat storage techniques are simple and easy to apply practically, they have disadvantages, such as their low heat storage density (sensible heat storage: ∼0.02–0.03 kW h/kg-material, latent heat storage: ∼0.05–0.1 kW h/kg-material).1 Also, they are inappropriate for long-term storage. In contrast, chemical heat storage has a high heat storage density (∼0.5–1 kW h/kg-reactant) and can store heat for long periods without any insulation.1−6 However, the drawbacks of chemical heat storage include its high capital cost, technical complexity, and limited temperature range available for heat storage operation.

Chemical heat storage technology uses endothermic/exothermic reactions associated with reversible chemical reactions, particularly gas–solid reactions.1 Several reaction systems including CaCO3/CaO,7−10 MgSO4·7H2O/MgSO4,11−13 and Mg(OH)2/MgO,14−22 have been studied as chemical heat storage systems. Material modifications aimed at improving the performance of chemical heat storage systems have been reported in a number of publications. A CaCO3/CaO system has high heat storage density. Zheng et al. designed dark CaCO3 particles capable of full-spectrum solar absorption. Binary-doped particles, comprising Cu and Mn, achieved an energy storage density of 1952 kJ/kg after 20 cycles, which is 84% higher than that of pure CaCO3 particles.10 However, the heat storage operation of the system requires a high temperature (850 °C or higher). The MgSO4·7H2O/MgSO4 system can store heat at temperatures below 200 °C, although it experiences challenges with the reversibility of the reaction. In order to overcome the problems with the kinetics of the hydration and dehydration reactions, Posern et al. prepared porous composites with MgSO4. Although the hydration rate improved, the dehydration rate did not improve.13 Although the Mg(OH)2/MgO system is suitable for heat storage in the temperature range of 300–350 °C, a high water vapor partial pressure (57.8 kPa) is required for heat output operations.14

In this research, we focused on the Ca(OH)2/CaO heat storage system.23−33Equation 1 shows the chemical reaction formula for the Ca(OH)2/CaO system.23

1. 1

The heat is stored by an endothermic reaction when a calcium hydroxide is dehydrated, and the heat is evolved by an exothermic reaction when a calcium oxide is hydrated. Calcium hydroxide is inexpensive and has a high energy storage density (approximately 1400 kJ/kg). Also, it has a high reversibility of heat storage/output operation compared with other materials mentioned above. Thus far, calcium hydroxide has been studied as a chemical heat storage material. Ogura et al. studied heat and mass transfers in a dehydration/hydration chemical heat pump reactor using the CaO/Ca(OH)2 system experimentally and theoretically.24 Schaube et al. reported on the physical properties, such as heat capacity, thermodynamic equilibrium, reaction enthalpy, and kinetics, of Ca(OH)2 under high H2O partial pressures.23

The advantage of the Ca(OH)2/CaO system is that the hydration reaction (heat output operation) proceeds even at low water vapor partial pressures. Rammelberg et al. reported that the hydration of calcium oxide could be proceeded even at a water vapor partial pressure of 2.0 kPa.25 Contrarily, the temperature range available for the heat storage operation is limited. The temperature for dehydration reaction of Ca(OH)2 is 400 °C or higher, and the reaction proceeds with great difficulty at temperatures lower than that. To expand the usage range as a chemical heat storage material and store heat efficiently, it is necessary to lower the dehydration reaction temperature and increase the dehydration reaction rate of Ca(OH)2.

Three methods have been studied to improve the performance of Ca(OH)2 or Mg(OH)2 as a chemical heat storage material.

The first method involves the design of a new type of reactor. Azpiazu et al. reported a prototype reactor designed for the dehydration/hydration cycle of Ca(OH)2/CaO for thermal energy storage. In this study, liquid water was supplied for CaO hydration. The selected chemical reaction did not exhibit complete reversibility because complete Ca(OH)2 dehydration was not achieved. However, the system could be satisfactorily used for at least 20 cycles.26

The second method involves making a hydroxide composite material, pellets, or microcapsules. These techniques improve the thermal conductivity and increase the volumetric energy density by increasing the bulk density of the chemical thermal storage material. Moreover, by obtaining mechanical strength, stable dehydration/hydration reactivity is achieved over several cycles. Sakellariou et al. prepared nearly spherical structured formulations using kaolinite as a binder to develop CaO-based materials with enhanced mechanical properties.27 Criado et al. developed composite materials synthesized using sodium silicate to bind fine CaO/Ca(OH)2 particles and to improve mechanical properties for fluidized bed or fixed bed applications. By keeping the hydration conversion below 40%, mechanical stability was maintained over several cycles.28 Piperopoulos et al. investigated the effect of cationic surfactant cetyl trimethyl ammonium bromide (CTAB) addition during the precipitation of Mg(OH)2 on the structural, morphological, and physical properties of the resulting hydroxide. Mg(OH)2 prepared at the optimum CTAB concentration exhibits the highest volumetric stored/released heat capacity, ∼560 MJ/m3, almost two times higher than that measured over Mg(OH)2 prepared in the absence of CTAB. Cyclic experiments evidenced an excellent stability of the sample up to thirteen dehydration/hydration reactions.19 They also inserted Ca2+, Co2+, and Ni2+ ions into the Mg(OH)2 matrix to investigate the influence of metal doping in Mg(OH)2 synthesis on its thermochemical behavior. It increased the volumetric stored/released heat capacity (between 400 and 725 MJ/m3), reaching almost three times the undoped Mg(OH)2 value.20 Haruki et al. measured the effective thermal conductivities of pelletized magnesium hydroxide/expanded graphite (EG) and magnesium oxide/EG composite heat storage materials with high packing densities.21 Mejia et al. encapsulated CaO as a chemical heat storage material to address the low thermal conductivity and cohesiveness of the powder bulk material. This study showed that encapsulated materials were stable under operation in reactors. Additionally, the volumetric energy density of compacted material was higher compared to powder bulk.29

The third method involves adding salts to the hydroxides. Various salts have been added to Ca(OH)2 or Mg(OH)2 to improve the dehydration reactivity. Ryu et al. studied the dehydration behaviors of metal salt-added magnesium hydroxide as chemical heat storage media.14 Ishitobi et al. showed that the dehydration temperature of magnesium hydroxide was lowered by the addition of lithium chloride.15,16 Yan and Zhao determined the energy required for the dehydration of calcium hydroxide with added lithium using computational and experimental methods.30,31 Shkatulov and Aristov conducted extensive screening of additives that could control the dehydration temperature of calcium hydroxide and magnesium hydroxide. They also reported improvement in the dehydration reactivity of calcium hydroxide with added KNO3.32,33 Kurosawa et al. reported that Mg(OH)2 with co-added LiOH and LiCl improved dehydration and hydration reactivity compared to Mg(OH)2 by singly adding them individually.18 Li et al. reported about LiNO3-doped Mg(OH)2. LiNO3 doping significantly reduced the onset temperature of dehydration. It was also reported that LiNO3 doping improves the dehydration rate of Mg(OH)2.22 However, there are no studies that apply two or more additives to calcium hydroxide as long as we known. Moreover, the reason for the improvement of the dehydration reactivity of metal hydroxide by additives has not been clarified yet.

In this study, calcium hydroxide with added single or two lithium compounds was prepared, and the dehydration reactivity was evaluated. We also investigated the reversibility of the dehydration/hydration reaction, which is indispensable if calcium hydroxide is to be used as a heat storage material. The influence of additives on the dehydration of calcium hydroxide was examined from the viewpoint of side reactions during sample preparation and the crystal structure of the prepared samples.

2. Results and Discussion

2.1. Dehydration Reaction Behavior of the Samples

Figure 1a,b shows the dehydration behavior of the sample in the dehydration tests heated from room temperature to 600 °C. The red line shows the dehydration behavior of Ca(OH)2-w. The dehydration reaction of Ca(OH)2 with added Li compounds proceeded at a lower temperature compared to Ca(OH)2-w without additive. In this study, furthermore, we evaluated the reactivity of Ca(OH)2 with co-added Li compounds. For LO5LC5 (Ca(OH)2/LiOH/LiCl = 100:5:5) and LC10LCO5 (Ca(OH)2/LiCl/Li2CO3 = 100:10:5), the dehydration proceeded at dramatically lower temperatures. Calcium hydroxide was dehydrated at a lower temperature with the co-addition of Li compounds, compared to the case of adding individual Li compounds. Conversely, it was found that the samples with LiCl (LC10, LO5LC5, LC10LCO5) exhibited dehydration behaviors different from those of other samples. In particular, at the high temperatures above 420 °C, the dehydration reaction rate of Ca(OH)2 with LiCl was decreased (Figure 1b). This indicates that LiCl or byproducts during sample preparation can inhibit the dehydration at high temperatures. The reason for this phenomenon is discussed in detail in Section 2.3.

Figure 1.

Figure 1

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Mole fraction change in the dehydration test heated from room temperature to 600 °C. The y axis shows change in the molar fraction of calcium hydroxide based on eq 4, and the x axis shows temperature: (a) the dehydration behavior at 200–600 °C and (b) expanded (a) between 350 and 500 °C.

Table 1 shows the dehydration peak temperature derived from the differential thermogravimetry (DTG) curve (Figure S1) based on the dehydration behavior, as shown in Figure 1. Here, the dehydration peak temperature is the temperature at which the dehydration reaction rate is maximum in the dehydration tests. A decrease in the dehydration peak temperature means that the dehydration reaction of calcium hydroxide proceeds at a relatively low temperature. In other words, the applicability of calcium hydroxide as a chemical heat storage material is expanded. All the sample of Ca(OH)2 with added Li compounds showed that the temperatures lowered compared with Ca(OH)2-w. The dehydration peak temperature of LO5LC5 and LC10LCO5 was 60 °C lower than that of Ca(OH)2-w. Similarly, it has been reported that chemical modification of Ca(OH)2 lowers its dehydration reaction. Shkatulov and Aristov reported that the addition of 5 wt % KNO3 to Ca(OH)2 reduced the onset temperature of dehydration from 494 °C to 459 °C.33 Yan and Zhao tested a sample of Ca(OH)2 with added LiOH at 15 °C/min. The time required for the dehydration to proceed by 40% was reduced by 106 s for Ca(OH)2 with added LiOH as compared to that for pure Ca(OH)2 (when converted to temperature, it dropped by about 26 °C).31 Because the experimental conditions and analytical methods were different, they could not be compared unconditionally. However, in this study, the dehydration temperature of Ca(OH)2 was lowered by the addition of salt, as in previous studies. Hence, it is evident that the temperature, where heat storage operation is possible for calcium hydroxide as a chemical heat storage material, was shifted by the addition of lithium compounds. It indicates that Ca(OH)2 with Li compounds is more suitable for heat storage operation at a lower temperature than pure Ca(OH)2. Additionally, this effect was enhanced more by the co-addition of Li compounds than single addition of Ca(OH)2.

Table 1. Peak Temperatures of all the Samples.

sample peak temperature [°C]
Ca(OH)2-w 459
LO10 427
LC10 422
LCO10 431
LO5LC5 400
LO10LCO5 429
LC10LCO5 402
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Figure 2 shows the dehydration behavior of the sample in the dehydration tests performed at 350 °C. Compared to Ca(OH)2-w indicated by the red line, Ca(OH)2 with added Li compounds completed the dehydration in a shorter time. The dehydration of Ca(OH)2 with added Li compounds progressed more rapidly than that of Ca(OH)2-w upon commencement of the reaction. An increase in the reaction rate implies an improvement in the heat storage efficiency. These results showed that the addition of Li compounds to calcium hydroxide was able to store heat more efficiently.

Figure 2.

Figure 2

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Mole fraction change in the dehydration test at 350 °C of all samples. The x axis shows time, the left y axis shows temperature, and the right y axis shows the molar fraction of calcium hydroxide. The time when the decrease in the mole fraction was less than 1% in 10 min was defined as the measurement finish time.

We calculated the rate constant of the dehydration reaction for the samples at 350 °C from the dehydration behavior in the isothermal test.15,17 When the dehydration reaction is considered as a zero-order reaction, the reaction rate equation is expressed by eq 2. When the dehydration reaction is considered as a first-order reaction, the reaction rate equation is expressed by eq 3.

2.1. 2
2.1. 3

Here, x is the mole fraction of Ca(OH)2 [-], t is the reaction time [s], and k0 and k1 are reaction rate constant [s–1].

When the model fitting was performed on the assumption that all dehydration reactions were zero-order reactions or first-order reactions, a large error was observed between the measured values and calculated values, as shown in Figure S2. Assuming that all dehydration reactions of Ca(OH)2-w are zero-order reactions, the model and the measured values agree well at the initial stage of the reaction, but the error increased as the reaction progressed. In this case, the coefficient of determination between the actually measured value and the calculated value was 0.9957. Similarly, when all dehydration reactions were assumed to be first-order reactions, the error between the measured value and the calculated value was large, and the coefficient of determination was 0.9606. In previous studies on the dehydration kinetics of Mg(OH)2, the rate constant was calculated by assuming that the dehydration reaction of Mg(OH)2 is a two-step reaction, comprising a zero-order reaction and a first-order reaction.15,17 In this study, assuming that the dehydration reaction is a two-step reaction, we calculated the reaction rate constant at 350 °C. We assumed that the dehydration reactions for the molar fractions of 0.9–0.4 and 0.3–0.2 were zero-order (eq 2) and first-order (eq 3), respectively. Assuming the above, when model fitting was performed, the error between the measured and calculated values became smaller. The coefficient of determination of the measured and calculated values for zero-order and first-order reactions was 0.9991 and 0.9971, respectively. We considered that the rate-limiting step of the dehydration reaction changed its behavior during the course of the reaction. In the dehydration of Ca(OH)2, zero-order reaction was dominant in the initial stage of the reaction, and the first-order reaction becomes dominant as the reaction proceeds. Figure S3 shows the measured and calculated mole fractions of Ca(OH)2.

Table 2 shows the rate constants for the zero-order (molar fraction of 0.9–0.4) and first-order (molar fraction of 0.3–0.1) reactions of each sample. The dehydration rate constants of Ca(OH)2 with added Li compounds and Ca(OH)2-w were compared. The larger the reaction rate constant, the faster is the dehydration. The reaction rate constant k0 of the zero-order reaction for all Ca(OH)2 with added Li compounds exceeded that of Ca(OH)2-w. This is consistent with the finding that the dehydration of Ca(OH)2 with added Li compounds progresses more rapidly than that of Ca(OH)2-w upon commencement of the reaction, as is evident from their dehydration behavior, depicted in Figure 2. On the other hand, the reaction rate constant k1 of the first-order reaction was larger or smaller for Ca(OH)2-w compared to that for Ca(OH)2 with added Li compounds. At 350 °C, the rate constants k1 of LO10, LCO10, LO10LCO5, and LC10LCO5 exceeded that of Ca(OH)2-w while LC10 and LO5LC5 exhibited lower k1 values. It was found that for Ca(OH)2 with added LiCl, the rate constant k1 tended to be smaller than that of Ca(OH)2-w. Li compound addition led to enhancement of the reaction rate in the initial stages of the dehydration of calcium hydroxide. However, as the dehydration proceeded further, LiCl addition tended to slow it down. It can be said that the effect of LiCl on the dehydration of calcium hydroxide differs from that of LiOH and Li2CO3. In this study, the rate constant was calculated assuming that the dehydration of calcium hydroxide was a two-step reaction, comprising a zero-order and a first-order reaction. However, further analysis of the reaction mechanism and exploration of the rate equations is required, this will be addressed in the future.

Table 2. Reaction Rate Constant of the Dehydration of Calcium Hydroxide at 350 °C (k0, k1) and the Coefficient of Determination of the Measured and Calculated Values (R2)a.

  initial stage of dehydration (0.9 > x > 0.4): zero-order reaction
 late stage of dehydration (0.3 > x > 0.2): first-order reaction
sample k0 [s–1] R2 k1 [s–1] R2
Ca(OH)2-w 6.26 × 10–5 0.9991 1.89 × 10–4 0.9971
LO10 2.19 × 10–4 0.9998 5.07 × 10–4 0.9023
LC10 1.62 × 10–4 0.9661 1.15 × 10–4 0.9557
LCO10 2.05 × 10–4 0.9995 3.60 × 10–4 0.8924
LO5LC5 2.17 × 10–4 0.9686 1.61 × 10–4 0.9580
LO10LCO5 2.03 × 10–4 0.9992 6.06 × 10–4 0.9731
LC10LCO5 2.33 × 10–4 0.9842 2.09 × 10–4 0.9549
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a

We assumed that the dehydration reactions for the molar fractions of 0.9–0.4 and 0.3–0.1 were zero-order and first-order, respectively. R2 was compared with the measured value and a two-step reaction model.15,17

2.2. Hydration Reaction Behavior of the Samples

Figure 3 shows the dehydration/hydration behavior of the samples. Table 3 shows the dehydration conversion and hydration conversion of each sample, derived from Figure 3. First, the dehydration was carried out at 350 °C for 30 min under Ar flow. At 350 °C, the dehydration of Ca(OH)2-w, indicated by the red line, did not show notable progress. Contrarily, Ca(OH)2 with added Li compounds exhibited about 40–50% dehydration. This confirms the acceleration of the dehydration of calcium hydroxide by the addition of Li compounds, as shown in Figure 2. Subsequently, the hydration was carried out at 110 °C with a water vapor partial pressure of 7.4 kPa for 80 min. For Ca(OH)2 with added Li compounds, the hydration reaction was almost completed similar to the case in Ca(OH)2-w. This indicates that the dehydration of calcium hydroxide and hydration of calcium oxide proceed reversibly with or without Li compounds as additives. We also investigated the dehydration/hydration behavior of magnesium hydroxide under the same experimental conditions. The black line shows the reaction behavior of Mg(OH)2-w. For Mg(OH)2-w, the dehydration reaction proceeded easily while the hydration reaction hardly proceeded. When high water vapor partial pressure was applied, the hydration of magnesium oxide can be proceeded.16 For the practical application of chemical heat storage materials, it is necessary to show the reversibility of dehydration/hydration under mild reaction conditions. Therefore, it is significant advantage that the Ca(OH)2/CaO system can be hydrated efficiently under much lower water vapor pressure (7.4 kPa; equivalent to a 40 °C saturated water vapor pressure).

Figure 3.

Figure 3

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Mole fraction change in the dehydration and hydration tests: Td = 350 °C, Th = 110 °C, and PH2O = 7.4 kPa. The x axis shows time, the left y axis shows temperature, and the right y axis shows the molar fraction of calcium hydroxide.

Table 3. Dehydration/Hydration Conversion for all Samples: Td = 350 °C, Th = 110 °C, PH2O = 7.4 kPa.

sample Xd [%] Xh [%]
Ca(OH)2-w 9 8
LO10 41 36
LC10 38 34
LCO10 41 37
LO5LC5 52 50
LO10LCO5 54 50
LC10LCO5 49 42
Mg(OH)2-w 89 6
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Figure 4 shows the heat output density by hydration reaction for each sample.14,16,18 From the hydration conversion shown in Figure 3 and Table 3, we calculated how much heat could be evolved per kg of the sample. For all combinations of Ca(OH)2 with added Li compounds, the heat output density increased compared to the case in Ca(OH)2-w as the dehydration conversion and hydration conversion were improved. Ca(OH)2 with added Li compounds has a higher heat output density than Mg(OH)2-w. Among them, Ca(OH)2 with added LiOH and LiCl (LO5LC5) was able to output 670 kJ heat per kg. For practical use of chemical heat storage materials, it is necessary to evaluate the temperature and pressure dependence of the hydration reaction rate. This will be an issue for the future.

Figure 4.

Figure 4

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Heat output density: Td = 350 °C, Th = 110 °C, and PH2O = 7.4 kPa.

2.3. Characterization of the Samples

Figure 5 shows the X-ray diffraction (XRD) pattern of each sample before the reaction test. This figure suggests that in some samples, side reactions may have occurred between calcium hydroxide and the additives during sample preparation. For Ca(OH)2 with added LiOH and/or LiCl, no peaks indicating LiOH and LiCl were detected. This indicates that Li+ was taken into the calcium hydroxide crystals.34,35 Conversely, the characteristic peak of Li2CO3 was detected for all combinations of Ca(OH)2 with added Li2CO3 (LCO10, LO10LCO5, and LC10LCO5). This is because of the lower solubility of Li2CO3 compared to those of LiOH and LiCl. In this study, the sample was prepared using the impregnation method. It is assumed that many of the Li+ were not taken into the calcium hydroxide and were reprecipitated as Li2CO3. Furthermore, CaCO3 peaks were detected in Ca(OH)2-w, LCO10, and LO10LCO5. Ca(OH)2 is slightly soluble in water. CO2 dissolved from air or CO32– derived from Li2CO3 was probably precipitated together with Ca2+. CaCl(OH) was detected in Ca(OH)2 with added 10 mol % LiCl (LC10, LC10LCO5). These byproducts might inhibit the dehydration of calcium hydroxide. In particular, CaCl(OH) may reduce the dehydration rate of Ca(OH)2 with added LiCl at high temperatures, as shown in Figure 1.

Figure 5.

Figure 5

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XRD patterns of all the samples.

Figure 6a,b shows enlarged views of the (001) and (101) planes of calcium hydroxide. The black dotted line indicates the peak position of Ca(OH)2-w. In all Ca(OH)2 with added Li compounds, the peak positions of both the (001) plane and (101) plane shifted to the lower angle side. The observed peak shifts presumed the insertion of Li+ ions into the Ca(OH)2 lattice. If the added Li compound existed solely on the Ca(OH)2 surface, and no substitution or insertion of Li+ had occurred, peak shifts would not be observed. The lattice constant and lattice volume of each sample were calculated using all the measured XRD peak positions (2θ = 10–150°). Table 4 shows the lattice constant and lattice volume of all the samples. The lattice volume of calcium hydroxide was larger for Ca(OH)2 combined with Li compounds than for Ca(OH)2-w. We hypothesized on the phenomenon that actually occurred in the calcium hydroxide crystals. The comparison of Ca2+ and Li+ ionic radii reveals that Ca2+ is larger (Ca2+: 1.00 Å, Li+: 0.76 Å).36 When the lithium compounds were added to calcium hydroxide by the impregnation method, the possible phenomena are as follows: (1) Li+ replaces Ca2+ (as shown in Figure S5a), and (2) Li+ locates at the interlayer of the Ca(OH)2 structure (as shown in Figure S5b).18,34,35 Yan et al. reported that Li+ had a small replacement energy for Ca2+ and could be easily replaced.30 However, if Li+ replaces Ca2+, the lattice volume should decrease. We considered that phenomenon (2) contributed greatly to the widening of the interplanar spacing, as inferred from Figure 6, and the increase in the lattice volume, as shown in Table 4. These phenomena may render the calcium hydroxide crystals more fragile and promote the dehydration. No significant correlation was found between the lattice volume and the dehydration peak temperature; the same was true for the relationship between the lattice volume and the dehydration kinetic constant, as shown in Figures S6 and S7, respectively. Peak shifts and increases in lattice volumes could have arisen from errors in the measurement and require further investigation. However, the qualitative trends appear to be consistent.

Figure 6.

Figure 6

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Expanded XRD patterns of all the samples. The dotted line shows the peak position of Ca(OH)2-w: (a) (001) plane and, (b) (101) plane.

Table 4. Lattice Parameter and Lattice Volume of all Samples Calculated from the Measured Peak Position.

sample lattice parameter: a [Å] lattice parameter: c [Å] lattice volume: V [Å3]
Ca(OH)2-w 3.597 ± 0.001 4.914 ± 0.005 55.05 ± 0.10
LO10 3.604 ± 0.013 4.919 ± 0.001 55.34 ± 0.37
LC10 3.597 ± 0.002 4.922 ± 0.001 55.15 ± 0.06
LCO10 3.598 ± 0.000 4.919 ± 0.009 55.16 ± 0.09
LO5LC5 3.597 ± 0.000 4.915 ± 0.002 55.09 ± 0.01
LO10LCO5 3.595 ± 0.004 4.924 ± 0.035 55.11 ± 0.27
LC10LCO5 3.598 ± 0.000 4.919 ± 0.011 55.16 ± 0.13
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Figure 1 shows that the dehydration behavior of Ca(OH)2 with added LiCl is different from those of other samples. The effect of each additive on the dehydration reaction of calcium hydroxide must be evaluated. Each sample was heated to various temperatures, and XRD studies (qualitative analysis) were performed on the sample after the tests.18

Figure 7a shows the XRD patterns recorded after heating Ca(OH)2-w to various temperatures. The black dotted line indicates the calcium hydroxide peak position, and the red dotted line indicates the calcium oxide peak position. When Ca(OH)2-w was heated to 425 °C, the characteristic peak of calcium oxide that was not observed at 400 °C was detected. This indicates that calcium oxide production began between 400 and 425 °C. When Ca(OH)2-w was heated to 475 °C, the peak derived from calcium hydroxide disappeared. This indicates that the decomposition of calcium hydroxide was completed between 450 and 475 °C.

Figure 7.

Figure 7

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XRD patterns of each sample after heating from room temperature to various temperatures. Once the sample had been heated to the specified temperature, it was cooled to room temperature and subjected to XRD analysis. The black dotted line shows the peak positions of Ca(OH)2, and the red dotted line shows the peak positions of CaO: (a) Ca(OH)2-w, (b) LO10, (c) LC10, (d) LCO10 (e) LO5LC5, (f) LO10LCO5, and (g) LC10LCO5.

Same experiments were performed on other samples. Figure 7b–g shows these XRD patterns. Table 5 shows the temperature at which the calcium oxide peak appeared and the temperature at which the calcium hydroxide peak disappeared for each sample. The peak of calcium oxide appeared at a lower temperature than that of Ca(OH)2-w for all Ca(OH)2 with added Li compounds. In particular, for the sample with 10 mol % of LiCl (LC10, LC10LCO5), the temperature at which the peak of calcium oxide appeared was significantly reduced compared to the other samples. LiCl presumably improves the reaction rate in the early stages of the dehydration of calcium hydroxide and promotes CaO production. Contrarily, for some samples, the hydroxide peak disappeared at higher temperatures than Ca(OH)2-w. For LC10, the peak indicating calcium hydroxide did not disappear even when the temperature was raised to 800 °C. This indicates that the dehydration reaction was not completed. For LO5LC5, the calcium hydroxide peak disappeared at 525–550 °C. This temperature is higher by 50 °C or more than that of Ca(OH)2-w. For other combinations of Ca(OH)2 with added Li compounds, the temperature at which the calcium hydroxide peak disappears decreased. Based on the above, it is possible that CaCl(OH) (produced by Ca(OH)2 and LiCl), or LiCl inhibits the complete dehydration of calcium hydroxide. This is consistent with the decrease in the dehydration reaction rate on the high-temperature side shown in Figure 1. It can be said that the effect of LiCl on the dehydration of calcium hydroxide differs from that of LiOH and Li2CO3.

Table 5. Temperature of the Disappearance of the Ca(OH)2 Peak and Appearance of the CaO Peak for all Samples.

sample temp. of Ca(OH)2 disappearance [°C] temp. of CaO appearance [°C]
Ca(OH)2-w 450–475 400–425
LO10 425–450 350–375
LC10 not disappeared even at 800 °C 325–350
LCO10 425–450 350–375
LO5LC5 525–550 350–375
LO10LCO5 425–450 375–400
LC10LCO5 425–450 325–350
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3. Conclusions

We investigated the effect of Li compound addition on the dehydration reaction temperature, dehydration reaction rate, and reversibility of the dehydration/hydration of calcium hydroxide. The addition of Li compounds lowered the calcium hydroxide dehydration reaction temperature. In particular, the dehydration peak temperature of LiOH/LiCl/Ca(OH)2 and LiCl/Li2CO3/Ca(OH)2 decreased by about 60 °C compared to that of pure Ca(OH)2. Li compound co-addition is the effective way for expanding the applicability of calcium hydroxide as a heat storage material. The dehydration of Ca(OH)2 with added Li compounds could proceed efficiently even at 350 °C. Li compound addition led to enhancement of the reaction rate in the initial stages of the dehydration of Ca(OH)2. As a future issue, it is necessary to conduct isothermal tests at different temperatures to determine the activation energy of the dehydration reaction. A hydration test was conducted after the dehydration test, and the reversibility of the dehydration/hydration reaction was confirmed regardless of the presence or absence of additives. In addition, the evaluation of temperature and pressure dependence of the hydration reaction rate is required for practical applications as a chemical heat storage material and would be investigated in the future.

XRD analysis before thermogravimetry suggested that side reactions occurred during the sample preparation. Byproducts can negatively affect the dehydration reactivity of calcium hydroxide. Contrarily, it was presumed the insertion of Li+ ions into the Ca(OH)2 lattice. We assume that this phenomenon has a positive effect on the dehydration reactivity of calcium hydroxide. As a future issue, it is necessary to study, in detail, the effect of Li compounds on the dehydration of calcium hydroxide using spectroscopic methods such as FT-IR and XAFS.

4. Experimental Section

4.1. Sample Preparation

LiOH·H2O (Wako Pure Chemical Industries, Ltd.), LiCl·H2O (99.9%, Wako Pure Chemical Industries, Ltd.), Li2CO3 (Wako Pure Chemical Industries, Ltd.), and Ca(OH)2 (99.9%, FUJIFILM Wako Pure Chemical Corporation) were used to prepare Ca(OH)2 with Li compounds being added. LiOH/Ca(OH)2, LiCl/Ca(OH)2, Li2CO3/Ca(OH)2, LiOH/LiCl/Ca(OH)2, LiCl/Li2CO3/Ca(OH)2, and LiCl/Li2CO3/Ca(OH)2 were prepared by the impregnation method.14−18,32,33 First, an aqueous solution of Li compounds was prepared from LiOH and/or LiCl and/or Li2CO3 and ultrapure water. The solution was impregnated with pure Ca(OH)2 powder and stirred for 30 min. After this, water was evaporated at 40 °C using a rotary evaporator. Finally, the samples were dried overnight at 120 °C. All the samples were obtained as a white powder. For comparison, Ca(OH)2 and Mg(OH)2 without Li compound addition were also prepared in the same way. Ca(OH)2 and Mg(OH)2 without added Li compounds are expressed as Ca(OH)2-w and Mg(OH)2-w, respectively. Table 6 shows the prepared samples and the ratio of the additive.

Table 6. Preparation of Samples and Mixing Ratio.

sample mixing ration [mole ratio]
Ca(OH)2-w Ca(OH)2 without additives
LO10 Ca(OH)2/LiOH = 100:10
LC10 Ca(OH)2/LiCl = 100:10
LCO10 Ca(OH)2/Li2CO3 = 100:10
LO5LC5 Ca(OH)2/LiOH/LiCl = 100:10:10
LO10LCO5 Ca(OH)2/LiOH/Li2CO3 = 100:10:5
LC10LCO5 Ca(OH)2/LiCl/Li2CO3 = 100:10:5
Mg(OH)2-w Mg(OH)2 without additives
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4.2. Evaluation of Reactivity

A thermobalance (TGD-9600 series, ADVANCE RIKO, Inc.) was used to evaluate the reactivity of all the samples. The samples were charged in a Pt cell and heated under a constant flow of Ar gas. After the thermogravimetric measurement by the thermobalance, the weight change was converted into a mole fraction change of calcium hydroxide as shown in eq 4.15−18

4.2. 4

Here, X is the mole fraction of Ca(OH)2 [-], w0 is the initial weight of the sample [mg], Δw is the weight change of the sample [mg], MH2O is the molecular weight of H2O [g/mol], and MCa(OH)2 is the molecular weight of Ca(OH)2 [g/mol]. In this study, it was assumed that Li compounds in the sample did not react each other. Therefore, the weight of the Li compounds in the sample was subtracted from the total weight of the sample.

Figure 8 shows the weight change after dehydration at 350 °C and hydration at 110 °C. The first weight loss corresponds to the removal of physically adsorbed water. Assuming that the evaporation of physically adsorbed water is completed at 200 °C, the sample weight at 200 °C is the initial weight.

Figure 8.

Figure 8

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Thermogravimetric curve in the dehydration and hydration tests: dehydration at 350 °C under 100 mL/min Ar flow and hydration at 110 °C with a water vapor partial pressure of 7.4 kPa.

The measurement conditions for the dehydration test are as follows: first, the samples were heated from room temperature to various temperatures at a rate of 10 °C/min under Ar flow at 100 mL/min. Once the sample had been heated to the specified temperature, it was cooled to room temperature and subjected to XRD analysis. Second, dehydration tests were conducted at a constant temperature of 350 °C at a rate of 20 °C/min under Ar flow at 100 mL/min. The time when the decrease in the mole fraction was less than 1% in 10 min was defined as the measurement finish time.

The measurement conditions for the hydration test are as follows: first, to remove physically adsorbed water, the sample was heated at 120 °C for 30 min at a rate of 20 °C/min under Ar flow at 100 mL/min. Subsequently, the samples were dehydrated at 350 °C for 30 min at a heating rate of 20 °C/min under Ar flow at 100 mL/min. Thereafter, the samples were hydrated at 110 °C for 80 min at a heating rate of −20 °C/min by introducing a gas mixture of water vapor at 7.4 kPa and Ar gas. Finally, the supply of water vapor was halted, and the sample was dried at 110 °C for 30 min.

Figure 9 shows the dehydration and hydration behavior of LC10LCO5. X0 is the mole fraction of Ca(OH)2 before the hydration reaction [-], X2 is that of Ca(OH)2 upon completion of hydration [-], X3 is that of Ca(OH)2 at 10 min after completion the end of hydration [-], ΔXd is the dehydration reaction conversion [%], ΔXh is the hydration reaction conversion [%], and ΔXc is the reaction conversion by physical water adsorption [%]; ΔXd, ΔXh, and ΔXc are calculated as follows

4.2. 5
4.2. 6
4.2. 7

Figure 9.

Figure 9

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Mole fraction of Ca(OH)2 in the dehydration and hydration tests: dehydration at 350 °C under 100 mL/min Ar flow and hydration at 110 °C with water a vapor partial pressure of 7.4 kPa.

The heat output densities of all the samples were calculated from hydration conversion (eq 6) and the enthalpy change of the hydration reaction (eq 1). Here, the heat output density is expressed by how much heat can be evolved per kg of the sample.15,16,18 The heat output densities were calculated as follows

4.2. 8
4.2. 9
4.2. 10
4.2. 11

Here, Qr is the heat output density of the hydration reaction [kJ/kg], and Mhyd is the weight of calcium hydroxide per kg of the sample [kg], ΔHr is the enthalpy change of the reaction and per kg of calcium hydroxide [kJ/kg], MCa(OH)2 is the molecular weight of Ca(OH)2 [g/mol], MLiOH is the molecular weight of LiOH [g/mol], MLiCl is the molecular weight of LiCl [g/mol], and MLi2CO3 is the molecular weight of Ca(OH)2 [g/mol]. a, b, c, and d are constants representing the composition of the sample. For example, for LO10LCO5 (Ca(OH)2/LiOH/Li2CO3 = 100:10:5), a = 100, b = 10, c = 0, and d = 5.

4.3. Sample Characterization

To investigate the crystal structure of the samples, XRD analysis was carried out in triplicate to reduce the error using an Ultima IV (Rigaku Corp.) X-ray diffractometer. The 2θ range was 10–150°, the scan rate was 10.0° min–1, and the scan width was 0.01°. Cu Kα radiation was used with a generator voltage of 40 kV and 40 mA current.

To analyze the structures of pure Ca(OH)2 and the combinations of Ca(OH)2 with added Li compounds, the lattice parameters and volumes of all of the samples were calculated by the least-squares method. These values were adjusted to match the measured peak positions by this method.18,34 The following equations were used to calculate the lattice constant.

4.3. 12
4.3. 13

Here, d is the plane spacing [Å]; λ is the X-ray wavelength [Å]; θ is the diffraction angle [°]; a and, c are lattice parameters [Å]; h, k, and l are miller indices [-]. Using the actual value of θ measured by XRD, the plane distance d was calculated from eq 12. The lattice constants a and c were determined so that the sum of squares of the error between the plane spacing d obtained from eq 12 and the plane spacing d obtained from eq 13 was minimized (least-squares method).18

The lattice volume V of calcium hydroxide was calculated using eq 14.

4.3. 14

The 90% confidence interval for the calculations was estimated by Student’s t-distribution, based on eq 15

4.3. 15

where N is the number of measurements (in this study, N = 3), and u is the standard deviation.

Acknowledgments

This research was supported by the Frontier Science Program of Chiba University, VBL Program of Chiba University, and Strategic Innovation Program for Energy Conservation Technologies from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors greatly thank these foundations. The authors would also like to thank Editage (www.editage.com) for English language editing.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04444.

  • DTG curve heated to 600 °C: (a) Ca(OH)2-w, (b) LO10, (c) LC10, (d) LCO10 (e) LO5LC5, (f) LO10LCO5, (g) LC10LCO5, measured and calculated mole fraction of Ca(OH)2 for Ca(OH)2-w at 350 °C; (a) calculated as zero-order reaction, (b) calculated as first-order reaction, (c) calculated as a two-step reaction (0.4 < x < 0.9: zero-order reaction, 0.1 < x < 0.3: first-order reaction), measured and calculated mole fraction of Ca(OH)2 for samples at 350 °C; (a) Ca(OH)2-w, (b) LO10, (c) LC10, (d) LCO10 (e) LO5LC5, (f) LO10LCO5, (g) LC10LCO5, XRD patterns of all the samples (2θ = 10–150°), schematic diagram of Ca(OH)2 crystal structure: (a) shows phenomenon (1) and (b) shows phenomenon (2), relationship between lattice volume and dehydration peak temperature, relationship between lattice volume and dehydration rate constant: (a) lattice volume versus rate constant of zero-order reaction (k0), (b) lattice volume versus rate constant of first-order reaction (k1), and XRD pattern of each sample after TG measurement: (a) Ca(OH)2-w, (b) LO10, (c) LC10, (d) LCO10 (e) LO5LC5, (f) LO10LCO5, and (g) LC10LCO5 (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b04444_si_001.pdf (1.3MB, pdf)

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