Measurements of the dissociation heats of tetrabutylammonium acetate and tetrabutylphosphonium hydroxide ionic semiclathrate hydrates

doi:10.21203/rs.3.rs-2358907/v1

Ionic semiclathrate hydrates mainly consist of water typically together with tetrabutylammonium and tetrabutylphosphonium salts. Since ionic semiclathrate hydrates have the large dissociation heat under ambient pressure and temperature conditions, various ionic semiclathrate hydrates have been studied as safety and eco-friendly phase change materials. In this study, tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates were proposed as thermal energy storage media for air conditioning and cooling lithium-ion batteries. The dissociation heat, which was a significant thermophysical property to design thermal energy storage systems, were measured at various mass fractions. The largest dissociation heats of tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates were 212.9 ± 0.9 kJ/kg and 200.4 ± 2.2 kJ/kg. As a result of the comparison of the dissociation heats of tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates with those of other ionic semiclathrate hydrates, it was found that tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates had the promising thermophysical properties as thermal energy storage media for air conditioning and cooling lithium-ion batteries respectively.

Ionic semiclathrate hydrate

Thermal energy storage

Dissociation heat

Tetrabutylammonium acetate

Tetrabutylammonium hydroxide

Hydrates are crystalline compounds made up of cage structure of water molecules and other compounds encapsulated in the cage, called guest compounds. Ionic semiclathrate hydrates are formed by the substitution of the anions of guest compounds for the cage of water molecules [1, 2]. Various quaternary ammonium and phosphonium salts have been investigated as guest compounds of ionic semiclathrate hydrates. These compounds crystallize and dissociate under ambient pressure and temperature conditions. Since the main component of ionic semiclathrate hydrate is water, there are advantages in terms of safety and environmental protection in industrial application. Thermophysical properties of a variety of ionic semiclathrate hydrates have been reported for industrial application, such as thermal energy storage [3–6] and carbon dioxide separation [7–10]. Thermal energy storage is a technology storing surplus power and waste heat as thermal energy. Phase change materials which dissociate at room temperature and pressure are necessary for thermal energy storage for general air conditioning, data center, and cooling lithium-ion batteries. Dissociation heat is one of the essential thermophysical properties to estimating thermal energy storage density. Besides, ionic semiclathrate hydrate have often been studied as a medium to separate carbon dioxide. Carbon dioxide separation technology is based on the guest compound selectivity of hydrate [11, 12]. The pressure required for hydrate formation is decreased with addition of the guest compounds which form ionic semiclathrate hydrate, such as quaternary ammonium and phosphonium salts. These hydrates have been studied as media for gas separation in the preceding studies.

Tetrabutylammonium acetate and hydroxide were investigated as the guest compounds of ionic semiclathrate hydrate in the present study. The equilibrium temperature of tetrabutylammonium hydroxide hydrate was reported to be 27.0 ˚C by Karimi et al. [13]. This temperature would be useful for thermal energy storage medium for cooling lithium-ion batteries and data center. Lots of ionic semiclathrate hydrates were studied for application as thermal energy storage media for air conditioning, required for the temperature range from 5 ˚C to 15 ˚C. The study about a medium for application of ionic semiclathrate hydrate at higher temperature, such as data center (15 ˚C – 32 ˚C) [14] and cooling lithium-ion batteries (25 ˚C – 40 ˚C) [15], is comparatively limited. Tetrabutylammonium hydroxide hydrate was also investigated as a medium for carbon dioxide separation in the previous study [16]. There is no study of relationship between the dissociation heat and mass fraction of tetrabutylammonium hydroxide hydrate. On the other hand, the equilibrium temperature of tetrabutylammonium acetate ionic semiclathrate hydrate was experimentally determined to be 14.8 ˚C by Nakayama and Torigata [17]. This result indicates the potential for a media for general air conditioning. Tetrabutylammonium bromide, which is a halogenated compound, was greatly studied as a thermal energy storage medium, meanwhile fatty acids, such as acetic acids and butyric acids, have less environmental impacts than halogenated compounds. The relationship between the mass fraction and the dissociation heat of tetrabutylammonium acetate hydrate has not been investigated. In this study, the dissociation heats of tetrabutylammonium hydroxide and acetate ionic semiclathrate hydrate were measured at various mass fractions.

2.1 Materials

The compounds used in the experiments were summarized in Table 1. To obtain an aqueous solution of any mass fractions, the compounds were mixed with the distilled water made in laboratory. The mass measurements were performed with an electronic balance (AS ONE Co., IUW-200D sefi). The expanded uncertainty of the mass measurements was ± 0.1 mg (k = 2). Approximately 1.5 g of the aqueous solutions of tetrabutylammonium acetate were prepared at 10 mass fractions. Tetrabutylammonium hydroxide aqueous solutions were prepared at 16 mass fractions. The uncertainties of the mass fractions were estimated from those of the mass measurements of the distilled water and the compounds.

Table 1

Compounds used in the dissociation heat measurements.
Compounds	Chemical formula	Purity	Supplier
Tetrabutylammonium acetate	(CH₃CH₂CH₂CH₂)₄NOOHCH₃	1.003 in powder ¹	Sigma Aldrich
Tetrabutylammonium hydroxide	(CH₃CH₂CH₂CH₂)₄NOH	0.551 in aqueous solution ¹	Sigma Aldrich
Distilled water	H₂O	Electrical conductivity 0.5 mS cm^− 1	Laboratory made

¹ The values of purity were provided by the supplier.

2.2 Dissociation heat measurements

The dissociation heat measurements were conducted with a differential scanning calorimetry (DSC) (DSC25, TA Instruments). Two aluminum pans were prepared: one contained approximately 15 mg of the sample and the other was filled with air. The difference in the heat flow rate which was added to raise the two pans simultaneously to the same temperature was detected as the dissociation heat of hydrates. Since tetrabutylammonium acetate and hydroxide hydrates dissociated at different temperature, the different control of temperature was conducted for the dissociation heat measurements of tetrabutylammonium acetate and hydroxide hydrates as displayed in Fig. 1. The inside of DSC was filled with dry nitrogen gas at 0.1 MPa to avoid the influence of water vapor on the heat flow rate at 20 ˚C in the measurements of tetrabutylammonium acetate. In the measurements of tetrabutylammonium hydroxide, the inside was filled with dry nitrogen at 40 ˚C. The temperature was decreased at 5 ˚C/min and kept at − 10 or − 20 ˚C for 10 min to form each hydrate. To dissociate hydrate completely, the temperature was increased at 2 ˚C/min.

The validity of the temperature increasing rate was demonstrated by measuring the dissociation heat of ice and various hydrates in the previous studies [5, 6]. In this study, the reliabilities of the apparatus and procedure used were confirmed by measuring the latent heat of ice. The hydrate formation was confirmed by the heat of formation. The measurement of dissociation heat was performed at least four times with the different samples at each mass fraction.

3.1 Measurements of the latent heat of ice

The reliability of the dissociation heat measurements was demonstrated by measuring the latent heat of ice and comparing the obtained values with the well-acknowledged literature data [18]. The temperature control in the measurements of the latent heat of ice was the same as that in the measurements of the dissociation heat of tetrabutylammonium acetate hydrates. The result of measurements of the latent heat of ice were presented in Table 2. ΔH_d stands for the latent heat of ice. ΔH_d,ave is the average value of the five times measurements. The expanded uncertainties (k = 2) were calculated from the five times measurements. The latent heat of ice obtained in these measurements agreed with the literature data [18] within the expanded uncertainty.

Table 2

Measurements of the dissociation heat of ice.
Run No.	ΔH_d / kJཥkg^{− 1 1}	ΔH_d,ave / kJཥkg^{− 1 2}
1	331.2	334.5 ± 4.0
2	336.5
3	338.0
4	337.7
5	329.3
Literature data [17]	333.427 ± 0.2	-

¹ ΔH_d indicates the latent heat of ice.

² ΔH_d,ave is the average value of the five times measurements. The expanded uncertainty was calculated from the five times measurements (k = 2).

3.2 Dissociation heats of tetrabutylammonium acetate hydrates

The dissociation heats of tetrabutylammonium acetate hydrates were measured at 10 mass fractions as presented in Table 3. w_Ace and x_Ace represent the mass fraction and the mole fraction of tetrabutylammonium acetate aqueous solutions. ΔH_d,Ace means the dissociation heat of tetrabutylammonium acetate hydrates. The expanded uncertainties of the dissociation heats were estimated from the at least four times measurements. The largest value of the dissociation heat of tetrabutylammonium acetate hydrates was 212.9 ± 0.9 kJ/kg at w_Ace = 0.36. As the mass fractions of tetrabutylammonium acetate aqueous solutions was approaching to 0.36, the values of the dissociation heat increased. The dissociation heat was previously reported to be 208 kJ/kg at w_Ace = 0.351 [24]. The dissociation heat obtained in this study agreed with that in the previous study within the expanded uncertainty (k = 2).

Table 3

Dissociation heats of tetrabutylammonium acetate hydrates at 10 mass fractions.
w_Ace ¹	x_Ace ²	ΔH_d,Ace / kJཥkg^{− 1 3}	U (ΔH_d,Ace) ³
0.20	0.0147	111.2	1.0
0.30	0.0250	155.2	2.9
0.35	0.0312	208.4	1.7
0.36	0.0326	212.9	0.9
0.37	0.0339	208.1	5.7
0.38	0.0354	205.3	1.7
0.39	0.0369	203.7	1.8
0.40	0.0384	189.2	5.1
0.45	0.0468	149.1	4.1
0.50	0.0566	105.4	1.5

¹ w_Ace indicates the mass fraction of the samples of tetrabutylammonium acetate. U (w_Ace) was ± 5.0 × 10^− 4. (k = 2).

² x_Ace expresses the mole fraction of the samples of tetrabutylammonium acetate. U (x_Ace) was ± 4.0 × 10^− 5 (k = 2).

³ ΔH_d,Ace indicates the dissociation heat of tetrabutylammonium acetate hydrates.

⁴ U (ΔH_d,Ace) was estimated from the at least 4 times measurements (k = 2).

The heat flow rates of tetrabutylammonium acetate hydrates at w_Ace ≤ 0.37 and at w_Ace ≥ 0.38 were displayed in Figs. 2 and 3. The depth of the heat flow peaks increased by increasing the mass fraction of the samples in Fig. 2. The peaks moved to the higher temperature with the increase in the mass fraction of the samples at w_Ace ≤ 0.37. The peaks were detected at the highest temperature at 0.35 ≤ w_Ace ≤ 0.37. The depth of the peaks decreased by increasing the mass fraction of the samples in Fig. 3. Previously, the highest equilibrium temperature was reported at 14.8 ± 0.1 ˚C at w_Ace = 0.358 [17], which was the congruent point of tetrabutylammonium acetate hydrate. The peaks would move with the change in the equilibrium temperature of the hydrates at each mass fraction, and the area of the peak would become largest near the congruent point.

The temperature at which the peaks were detected was approximately 1˚C higher than the equilibrium temperature reported in the previous study [17]. The same phenomenon was observed in the measurements of the dissociation heat of other hydrates by the same method, and the reason why the peak was detected at higher temperature had been discussed in the previous studies [6, 23]: The difference in the temperature at which the hydrates dissociated may be caused by the difference in the temperature increasing rate between this study (2 ˚C/min) and the previous study (0.5 ˚C/h [17]).

3.3 Dissociation heats of tetrabutylammonium hydroxide hydrates

The dissociation heats of tetrabutylammonium hydroxide hydrates at 16 mass fractions were shown in Table 4. w_OH is the mass fraction of the samples of tetrabutylammonium hydroxide, while x_OH expresses the mole fraction of the samples of tetrabutylammonium hydroxide. ΔH_d,OH indicates the dissociation heat of tetrabutylammonium hydroxide hydrates. The largest dissociation heat of tetrabutylammonium hydroxide hydrates was 200.4 ± 2.2 kJ/kg at w_OH = 0.33. Tetrabutylammonium hydroxide – water system formed the two hydrate structures in the previous work [1]: the more stable one (w_OH = 0.337) melted at 27.4 ˚C and the other (w_OH = 0.308) melted at 19.0 ˚C. The values of the dissociation heat in Table 4 contained the heat flow peaks of the two types of hydrate. Since the heat flow peak of tetrabutylammonium hydroxide hydrate would be largest near w_OH = 0.337, which was the congruent point, the experimental results agreed with the previous study within the expanded uncertainties of the measurements.

Table 4

Dissociation heats of tetrabutylammonium hydroxide hydrates at 16 mass fractions.
w_OH ¹	x_OH ²	ΔH_d,OH / kJཥkg^{− 1 3}	U (ΔH_d,OH) ⁴
0.20	0.0171	98.5	8.1
0.28	0.0263	164.0	6.0
0.29	0.0276	171.3	4.7
0.30	0.0289	186.0	3.2
0.31	0.0303	192.0	1.5
0.32	0.0316	198.8	1.3
0.33	0.0331	200.4	2.2
0.34	0.0345	195.0	3.6
0.35	0.0360	184.3	8.8
0.36	0.0376	176.4	3.2
0.37	0.0392	167.0	3.7
0.38	0.0408	158.4	1.2
0.39	0.0425	150.4	1.4
0.40	0.0443	142.5	1.7
0.45	0.0538	100.8	4.7
0.50	0.0649	68.1	1.3

¹ w_OH stands for the mass fraction of the samples of tetrabutylammonium hydroxide. U (w_Ace) was ± 1.0 × 10^− 3. (k = 2).

² w_OH means the mole fraction of the samples of tetrabutylammonium hydroxide. U (x_Ace) was ± 3.0 × 10^− 4 (k = 2).

³ ΔH_d,OH indicates the latent heat of tetrabutylammonium hydroxide hydrates.

⁴ U (ΔH_d,OH) was estimated from the at least four times measurements (k = 2).

The heat flow rates obtained in this study were displayed in Figs. 4 and 5. As shown in Fig. 4, the two types of heat flow peak were confirmed at w_OH = 0.20, w_OH = 0.30, and w_OH = 0.32; the one located at 20 ˚C and the other was obtained at 30˚C. In the preceding study [1], the two types of hydrate dissociated at 27.4 ˚C at w_OH = 0.337 and 19.0 ˚C at w_OH = 0.308, respectively. The composition of hydrates would change with the change in the concentration of the samples of tetrabutylammonium hydroxide aqueous solutions. The peaks at 20 ˚C and 30 ˚C moved to the higher temperature side with increasing mass fraction at w_OH ≤ 0.34. The areas of the peaks decreased with the increase in the mass fraction of the samples at w_OH ≥ 0.35 in Fig. 5. The peaks at w_OH ≥ 0.35 moved to the lower temperature side with increasing mass fraction. The tendency of the change in the location at which the hydrates dissociated agreed with the equilibrium temperature of tetrabutylammonium hydroxide hydrates investigated in the preceding study [13]. The two peaks at w_OH = 0.34, w_OH = 0.35, and w_OH = 0.37 overlapped at 30 ˚C as presented in Figs. 4 and 5. The hydrate different from the two types of hydrate reported in the previous study would form. The reason why the temperature at which the peaks were observed was higher than the equilibrium temperature in the literature was discussed in Section 3.2.

3.4 Evaluations of the dissociation heats of tetrabutylammonium acetate and tetrabutylammonium hydroxide hydrates.

The dissociation heat of tetrabutylammonium acetate hydrates was compared with that of other ionic semiclathrate hydrates as presented in Table 5. Tetrabutylphosphonium bromide hydrates had the largest dissociation heat of all the hydrates which had been studied as thermal energy storage media for general air conditioning. The experimental results indicated that tetrabutylammonium acetate hydrates have the large dissociation heat comparable to tetrabutylphosphonium bromide hydrates. Tetrabutylphosphonium bromide is a kind of halogen compound. Considering environmental impact and corrosion of the process in the thermal energy storage system, it is desirable to use non-halogen compounds. Tetrabutylphosphonium butylate hydrates was known to have the largest dissociation heat among all the hydrates formed by non-halogen compounds. As shown in Table 5, the measurements in the present study demonstrated that the dissociation heat of tetrabutylammonium acetate hydrate is larger than that of tetrabutylphosphonium butylate hydrates, which indicated that tetrabutylammonium acetate hydrates had a great potential as thermal energy storage media.

Tetrabutylammonium hydroxide hydrates dissociated at higher temperature than the other ionic semiclathrate hydrates which had been studied as thermal energy storage media. The studies of thermophysical properties of ionic semiclathrate hydrates which dissociated at the temperatures exceeding 25 ˚C were extremely limited, while thermal energy storage media for cooling lithium-ion batteries require the temperature range of 25 ˚C to 40˚C. Tetrabutylammonium fluoride hydrates were the only ionic semiclathrate hydrate which dissociated at the temperatures exceeding 25˚C as indicated in Table 5. Tetrabutylammonium hydroxide hydrates may be one of the hopeful thermal energy storage media for cooling lithium-ion batteries.

Table 5

Dissociation heats of ionic semiclathrate hydrates investigated in the previous studies.
Guest compounds	Dissociation heat / kJ kg^− 1	Equilibrium temperature / ˚C
Tetrabutylammonium acetate	212.9 ± 0.9 (this study)	14.8 [17]
Tetrabutylammonium hydroxide	200.4 ± 2.2 (this study)	27.0 [13]
Tetrabutylphosphonium bromide	214 ± 5 [19]	9.3 [19]
Tetrabutylphosphoium butylate	204 ± 5 [20]	13.9 [20]
Tetrabutylammonium fluoride	204.8 ± 2.3 [21]	27.2 [22]

The dissociation heats of tetrabutylammonium acetate hydrates and tetrabutylammonium hydroxide hydrates were measured at the various mass fractions to investigate the relationship between the mass fractions and the dissociation heats of the two hydrates. The largest dissociation heat of tetrabutylammonium acetate and tetrabutylammonium hydroxide hydrates was 212.9 ± 0.9 kJ/kg at w_Ace = 0.36 and 200.4 ± 2.2 kJ/kg at w_OH = 0.33 respectively. The two hydrates were compared with the other ionic semiclathrate hydrates from a viewpoint of dissociation heats and equilibrium temperatures. Tetrabutylammonium acetate hydrates had the largest dissociation heat among all the hydrates formed by non-halogen compounds. As tetrabutylammonium hydroxide hydrates had the large dissociation heats at the temperatures exceeding 25 ˚C, the hydrates had a great potential as thermal energy storage media for cooling lithium-ion batteries.

Competing Interests

The authors declare no competing interests.

Acknowledgements

This study was supported by a Keirin-racing-based research-promotion fund from the JKA Foundation (2022M-270).

Author Contributions

Conceptualization: Taro Iwai, Ryo Ohmura; Methodology: Taro Iwai, Shuhei Takamura; Formal analysis and investigation: Taro Iwai, Shuhei Takamura, Ryo Ohmura; Writing - original draft preparation: Taro Iwai; Writing - review and editing: Ryo Ohmura; Funding acquisition: Ryo Ohmura; Resources: Atsushi Hotta, Ryo Ohmura; Supervision: Ryo Ohmura

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No competing interests reported.

Measurements of the dissociation heats of tetrabutylammonium acetate and tetrabutylphosphonium hydroxide ionic semiclathrate hydrates

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Method

2.1 Materials

2.2 Dissociation heat measurements

3. Results And Discussions

3.1 Measurements of the latent heat of ice

3.2 Dissociation heats of tetrabutylammonium acetate hydrates

3.3 Dissociation heats of tetrabutylammonium hydroxide hydrates

3.4 Evaluations of the dissociation heats of tetrabutylammonium acetate and tetrabutylammonium hydroxide hydrates.

4. Conclusion

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1