Electrolyte for thermochemical battery

Abstract

To provide an electrolyte for a thermochemical battery capable of dissolving an electrolyte at a high concentration.SOLUTION: An electrolyte for a thermochemical battery includes an anion forming a redox pair, a crown ether, and an organic solvent, and the anion forming the redox pair is [Fe(CN)6]3- and [Fe(CN)6]4-.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrolyte for a thermochemical battery. [Background technology]

[0002] Thermochemical batteries, a type of thermoelectric conversion element that can convert thermal energy into electrical energy, have been attracting attention. Thermochemical batteries can be used to obtain electrical energy from temperature differences.

[0003] Non-Patent Document 1 describes that while conventional aqueous thermochemical batteries using K3[Fe(CN)6] / K4[Fe(CN)6] as an electrolyte are limited to an operating temperature of less than 100°C, it has been confirmed that they can operate at a temperature of approximately 150°C by using a deep eutectic solvent (ethaline) with high thermal stability instead of water.

[0004] Non-Patent Document 2 describes that when K3[Fe(CN)6] / K4[Fe(CN)6] was used as the electrolyte, the solubility in organic solvents was low, and that by changing the electrolyte from potassium salt to tetraethylammonium salt, the solubility in organic solvents was improved.

[0005] Patent Document 1 describes the addition of a capture compound to an electrolyte, which selectively captures only one of the redox species at a low temperature and releases it at a high temperature. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 155046 [Non-patent literature]

[0007] [Non-Patent Document 1] Antariksa, NF, Yamada, T. & Kimizuka, N. High seebeck coefficient in middle-temperature thermocell with deep eutectic solvent. Sci Rep 11, 11929 (2021) [Non-Patent Document 2] Taheri, A., MacFarlane, DR, Pozo-Gonzalo, C., & Pringle, JM The Effect of Solvent on the Seebeck Coefficient and Thermocell Performance of Cobalt Bipyridyl and Iron Ferri / Ferrocyanide Redox Couples. Australian Journal of Chemistry, 72(9), 709-716 Summary of the Invention [Problem to be solved by the invention]

[0008] In order to improve the performance of thermochemical batteries, it is desirable to dissolve an electrolyte in the electrolytic solution at a high concentration.

[0009] In Non-Patent Document 1, K3[Fe(CN)6] / K4[Fe(CN)6] is used as the electrolyte. However, K3[Fe(CN)6] / K4[Fe(CN)6] has low solubility in organic solvents and a large increase in viscosity when dissolved, so the output reaches a maximum at around 25 mW, and performance improvement by dissolving the electrolyte at a high concentration cannot be expected.

[0010] In Non-Patent Document 2, tetraethylammonium cation and ammonium cation are used in combination as the cation. When the cation is tetraethylammonium cation, there is a problem that the solubility of the tetraethylammonium salt is insufficient. Also, when the cation is ammonium cation, there is a problem that the ammonium salt is thermally decomposed at around 100°C.

[0011] In Patent Document 1, the scavenging compound undergoes a host-guest reaction with a redox species, but if the affinity of the counter ion (such as potassium ion) for the organic solvent is low, there is a problem that the solubility improving effect is insufficient.

[0012] The present invention has been made to solve the above problems, and has an object to provide an electrolyte for a thermochemical battery capable of dissolving an electrolyte at a high concentration. [Means for solving the problem]

[0013] The present inventors conducted extensive research to solve the above problems and arrived at the present invention. The present invention relates to a method for producing a redox pair comprising the steps of: (a) preparing a redox pair of an anion, (b) preparing a crown ether, and (c) preparing an organic solvent; 3- and [Fe(CN)6] 4- The present invention relates to an electrolyte for a thermochemical battery, characterized in that Effect of the Invention

[0014] According to the present invention, it is possible to provide an electrolyte solution for a thermochemical battery capable of dissolving an electrolyte at a high concentration. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram of an apparatus for evaluating the performance of a thermochemical battery. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present invention relates to a method for producing a redox pair comprising the steps of: (a) preparing a redox pair of an anion, (b) preparing a crown ether, and (c) preparing an organic solvent; 3- and [Fe(CN)6] 4- The present invention relates to an electrolyte for a thermochemical battery, characterized in that

[0017] The electrolyte for a thermochemical battery of the present invention contains an anion that forms a redox pair, a crown ether, and an organic solvent. The electrolytic solution for a thermochemical battery of the present invention usually contains a cation which is a counter ion of the above anion.

[0018] The anion in the redox pair is [Fe(CN)6] 3- and [Fe(CN)6] 4- (ferricyanide ion and ferrocyanide ion). In addition, the electrolyte for a thermochemical battery of the present invention may contain [Fe(CN)6] within a range that does not affect the effects of the present invention. 3- and [Fe(CN)6] 4- It may contain anions other than the above.

[0019] As the cation, an inorganic cation can be suitably used, and examples thereof include an alkali metal ion, an alkaline earth metal ion, etc. Examples of the alkali metal ion include a lithium ion, a sodium ion, and a potassium ion, and the potassium ion is preferred. In addition, organic cations can also be used, such as EMIM (1-ethyl-3-methylimidazolium). Only one type of cation may be used, or two or more types may be used in combination. Only inorganic cations may be used, or inorganic cations and organic cations may be used in combination.

[0020] Inorganic cations are trapped by the crown ether and organic cations are not trapped by the crown ether. When inorganic cations and organic cations are used in combination, one of the redox pair is provided with an inorganic cation in an amount corresponding to the maximum solubility of the crown ether, and the other is provided with an organic cation, thereby making it possible to increase the concentration of the redox species.

[0021] Examples of crown ethers include 12 crown 4 ether, 15 crown 5 ether, 18 crown 6 ether, dibenzo 18 crown 6 ether, etc. It is preferable to use a combination of crown ethers that interact with cations depending on the type of cation. The crown ether may be used alone or in combination of two or more kinds.

[0022] Examples of combinations of cations and crown ethers include a combination of lithium ion and 12-crown 4 ether, a combination of sodium ion and 15-crown 5 ether, a combination of 18-crown 6 ether and potassium ion, and a combination of dibenzo-18-crown 6 ether and potassium ion. Among these, the combination of 18-crown-6 ether and potassium ion is preferred.

[0023] The electrolytic solution for a thermochemical battery of the present invention has an effect of being able to dissolve an electrolyte at a high concentration by containing a crown ether in the electrolytic solution. The reason why the inclusion of crown ether improves the solubility of the electrolyte is presumably because an interaction occurs between the cation and the crown ether, in which the cation is captured by the crown ether. In addition, when the cation is captured by the crown ether, the interaction between the cation and the anion is weakened. By weakening the interaction between the cation and the anion, the solubility of the redox species can be improved and the ionic conductivity of the anion can be increased. As a result, the performance (current extraction ability) of the thermochemical battery is improved.

[0024] The crown ether concentration [mol / kg] in the electrolytic solution for a thermochemical battery is preferably equal to or higher than the cation concentration [mol / kg] in the electrolytic solution for a thermochemical battery. The cation concentration referred to here means the concentration of a cation that is a counter ion of an anion that forms a redox pair. In particular, it is preferable that the concentration [mol / kg] of the crown ether in the electrolytic solution for a thermochemical battery is equal to or higher than the concentration of inorganic cations that can be captured by the crown ether. When the crown ether concentration is equal to or higher than the cation concentration, there is no shortage of crown ether molecules relative to the cations, and all the cations can interact with the crown ether.

[0025] The ratio of the crown ether concentration to the cation concentration (crown ether concentration / cation concentration) is preferably 1.0 to 2.0, and more preferably 1.2 to 1.5.

[0026] The concentration [mol / kg] of the crown ether in the electrolytic solution for a thermochemical battery is, for example, preferably 0.3 to 1.0 [mol / kg], and more preferably 0.4 to 0.8 [mol / kg].

[0027] Examples of the organic solvent include ethaline (a 2:1 molar mixture of ethylene glycol (EG) and choline chloride (ChCl)), DMSO (dimethyl sulfoxide), and 1-ethyl-3-methylimidazolium dicyanamide.

[0028] The electrolytic solution for a thermochemical battery of the present invention can contain a high concentration of redox species, and even when the redox species is contained at a high concentration, the ion mobility of the redox species can be improved. The concentration of the redox species (molar number per 1 kg of electrolyte) is preferably 0.2 mol / kg or more, more preferably 0.4 mol / kg or more, and is preferably 1.0 mol / kg or less.

[0029] A thermochemical battery using the electrolytic solution for thermochemical batteries of the present invention includes a thermochemical battery comprising the electrolytic solution for thermochemical batteries of the present invention, a first electrode on the low-temperature side, and a second electrode on the high-temperature side in contact with the electrolytic solution for thermochemical batteries of the present invention. The electrode material may be platinum or a carbon material. By positioning the first electrode on the low-temperature side of the thermochemical battery at a low temperature relative to the installation location of the thermochemical battery and the second electrode on the high-temperature side at a high temperature relative to the installation location of the thermochemical battery, a temperature difference is generated between the first and second electrodes. This temperature difference causes an oxidation-reduction reaction at the first and second electrodes, generating a potential difference between the first and second electrodes, allowing the battery to function as a thermochemical battery.

[0030] The performance of a thermochemical battery can be evaluated by the Seebeck coefficient (Se) as follows: Se = ΔE / ΔT ΔE = potential difference between the hot and cold sides ΔT = temperature difference between the hot side and the cold side The higher the Seebeck coefficient, the higher the electromotive force of the thermochemical battery.

[0031] The present specification discloses the following:

[0032] The present disclosure (1) relates to a redox pair anion and Crown ether and An organic solvent, The anion that forms the redox pair is [Fe(CN)6] 3- and [Fe(CN)6] 4- The electrolytic solution for a thermochemical battery is characterized in that

[0033] The present disclosure (2) is the electrolyte for a thermochemical battery according to the present disclosure (1), in which the cation that is the counter ion of the redox pair is a potassium ion, and the crown ether is 18-crown-6 ether.

[0034] The present disclosure (3) is the electrolyte for a thermochemical battery according to the present disclosure (1) or (2), in which the concentration [mol / kg] of the crown ether in the electrolyte for a thermochemical battery is equal to or higher than the concentration [mol / kg] of a cation in the electrolyte for a thermochemical battery. EXAMPLES

[0035] The present invention will now be described in detail with reference to examples. However, the present invention is not limited to these examples as long as it does not depart from the gist of the present invention.

[0036] In each of the Examples and Comparative Examples, as shown in Table 1, a solvent, a redox species, and a counter cation of the redox pair were selected, and the presence or absence of a crown ether and the concentration of the crown ether were changed to prepare an electrolyte solution.

[0037] The substances used in each of the Examples and Comparative Examples shown in Table 1 are as follows. solvent Ethylene glycol (EG) and choline chloride (ChCl) in a 2:1 molar ratio mixture Counter cation EMIM: 1-ethyl-3-methylimidazolium K: Potassium ion Redox species [Fe(CN)6] 3- : Ferricyanide ion [Fe(CN)6] 4- : Ferrocyanide ion Crown Ether 18 Crown 6 Ether

[0038] [Table 1]

[0039] [Performance evaluation as a thermochemical battery] FIG. 1 is a schematic diagram of an apparatus for evaluating the performance of a thermochemical battery. Fig. 1 shows a thermochemical battery performance evaluation device 1. In this device, an electrolyte 20 is placed in an H-shaped glass tube 10, and the low-temperature side 30 of the H is immersed in a water bath at a specified temperature, and the high-temperature side 40 of the H is heated to a temperature higher than that of the low-temperature side 30, creating a temperature difference between both ends of the H. Two platinum wires are inserted into the H-shaped glass tube 10 with the temperature difference created, and the platinum wire on the low-temperature side 30 is used as the first electrode 50 and the platinum wire on the high-temperature side is used as the second electrode 60, and the thermoelectromotive force and the extracted current value are measured using a current / voltage meter. Stirrers 70 were placed at the bottoms of the low temperature side 30 and high temperature side 40 of the H-shape to perform stirring. The temperature difference (ΔT) between the low temperature side and the high temperature side was set to approximately 100°C. The temperature on the low side is about 40°C, and the temperature on the high side is about 140°C.

[0040] Table 1 shows the Seebeck coefficient (Se coefficient) and the extraction current value (μA) for each example. The Seebeck coefficient (Se coefficient) was calculated from the slope when the temperature difference (ΔT) between the low and high sides is plotted on the horizontal axis and the potential difference (value monitored by a current-voltage meter) generated between the terminals at that temperature difference is plotted on the vertical axis. The output current value (μA) was measured when the maximum temperature difference (approximately 100°C in this experimental system) was reached and the voltage between the terminals was set to 0 mV (forced short circuit) using a current-voltage meter.

[0041] In addition, in order to confirm the effect of improving durability by increasing the redox species concentration, the extraction current value was measured at 0 seconds and 60 seconds after the start of the test, and the retention rate (%) of the extraction current value was calculated.

[0042] In Examples 1 to 4, by adding a crown ether to the electrolyte, an electrolyte in which the redox species was dissolved could be obtained even when the total redox species concentration was increased to 0.1 to 0.4 mol / kg. These electrolytes were able to increase the extraction current value. The retention rate of the output current was also increased.

[0043] In Comparative Example 1, which does not contain crown ether, the solubility of the redox species is low, so the total redox species concentration is set to 0.03 mol / kg as the maximum concentration at which the redox species can dissolve. In this case, the redox species dissolves, but the concentration is low, so the extraction current value is low. Furthermore, the retention rate of the output current was also low. In Comparative Example 2, an attempt was made to set the total redox species concentration in an electrolyte solution containing no crown ether to 0.4 mol / kg, the same as in Example 3, but almost no redox species were dissolved. [Explanation of symbols]

[0044] 1. Thermochemical battery performance evaluation device 10 H-shaped glass tube 20 Electrolyte 30 Low temperature side 40 High temperature side 50 1st electrode 60 2nd electrode 70 Stirrer

Claims

1. An anion that serves as a redox pair; Crown ether and An organic solvent, The anion forming the redox pair is [Fe(CN) ６ ] ３－ and [Fe(CN) ６ ] ４－ 1. An electrolyte for a thermochemical battery, comprising:

2. 2. The electrolytic solution for a thermochemical battery according to claim 1, wherein the cation which is the counter ion of the redox pair is a potassium ion, and the crown ether is 18-crown 6 ether.

3. 3. The electrolytic solution for thermochemical batteries according to claim 1, wherein the concentration [mol / kg] of the crown ether in the electrolytic solution for thermochemical batteries is equal to or higher than the concentration [mol / kg] of the cation in the electrolytic solution for thermochemical batteries.

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