Electrolyte for thermoelectric conversion, thermoelectric conversion element including the same, thermochemical battery, temperature control device, and thermoelectric sensor

Abstract

To provide an electrolyte for thermal conversion capable of improving a dimensionless figure of merit (ZT), a thermochemical battery comprising the same, a temperature control device, and a thermoelectric sensor.SOLUTION: An electrolyte for thermoelectric conversion according to an embodiment of the present invention includes a redox pair and an acidic aqueous solvent, and the redox pair is an oxo aqua vanadium complex.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an electrolyte for thermoelectric conversion, a thermoelectric conversion element, a thermochemical battery, a temperature control device, and a thermoelectric sensor each including the electrolyte. [Background technology]

[0002] Thermoelectric conversion materials have been attracting attention in recent years for their ability to convert minute amounts of energy, such as waste heat, into electricity. In particular, expectations are high for thin, highly efficient thermoelectric conversion materials as energy sources for mobile devices. However, while conventional alloy-based thermoelectric conversion materials have high thermal conductivity, they have low Seebeck coefficients (S e ) is a problem.

[0003] Therefore, thermochemical batteries using ion solutions capable of oxidation and reduction have been attracting attention in recent years (Non-Patent Document 1). The Seebeck coefficient (S e ) is about 0.2 mV / K, whereas thermochemical cells have a Seebeck coefficient (S e ) is expected to be an order of magnitude larger and can be produced at low cost. For example, in a thermochemical battery consisting of ferrocyanide ions and ferricyanide ions, the equilibrium potential formed by oxidation-reduction equilibrium shifts between high and low temperatures, generating a potential. Its Seebeck coefficient is approximately 1.43 mV / K, which is known to be higher than that of alloy-based thermoelectric conversion materials. Furthermore, it has recently been reported that a high Seebeck coefficient can be obtained by using the proton-coupled electron transfer reaction of a ruthenium complex (Non-Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Non-Patent Document 1] Theodore J. Abraham et al., "High Seebeck coefficient redox ionic liquid electrolytes for thermal energy harvesting," Energy & Environmental Science, (UK), 2013, Vol. 6, pp. 2639-2645 [Non-patent document 2] Suguru Kobayashi, Teppei Yamada, Makoto Tadokoro, and Nobuo Kimizuka, "A Novel Thermocell System Using Large Solvation Entropy of Proton," Chemistry A European Journal, (Germany), 2020 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when a ruthenium complex is used, the solubility of the ruthenium complex in water is low, and the ionic conductivity of the electrolyte is low, resulting in a problem of a low dimensionless figure of merit (ZT) expressed by the following formula (1).

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[0006] The present invention has been made to solve the above problems, and aims to provide an electrolyte for heat conversion that can improve the dimensionless figure of merit (ZT), a thermochemical battery, a temperature control device, and a thermoelectric sensor that include the electrolyte. [Means for solving the problem]

[0007] [1] An electrolyte solution for thermoelectric conversion comprising a redox couple and an acidic aqueous solvent, wherein the redox couple is an oxoaqua vanadium complex.

[0008] [2] The electrolyte solution for thermoelectric conversion according to the above [1], wherein the pH of the electrolyte solution is 1 to 4.

[0009] [3] The electrolyte solution for thermoelectric conversion according to the above [1], wherein the pH of the electrolyte solution is 2 to 4.

[0010] [4] The electrolyte solution for thermoelectric conversion according to any one of the above [1] to [3], wherein the redox couple is an oxoaqua vanadium(IV) complex and an oxoaqua vanadium(V) complex.

[0011] [5] The redox couple is [VO(H2O)5] 2+ and [VO(OH)3(H2O)2].

[0012] [6] The electrolyte solution for thermoelectric conversion according to any one of [1] to [5] above, further comprising a non-aqueous solvent.

[0013] [7] A thermoelectric conversion element comprising the electrolytic solution for thermoelectric conversion according to any one of [1] to [6] above, and a pair of electrodes in contact with the electrolytic solution for thermoelectric conversion.

[0014] [8] A thermochemical battery comprising the thermoelectric conversion element according to [7] above.

[0015] [9] A temperature control device comprising the thermoelectric conversion element according to [7] above.

[0016]

[10] A thermoelectric sensor comprising the thermoelectric conversion element according to [7] above. [Effects of the Invention]

[0017] The electrolyte for thermoelectric conversion according to the present invention, and the thermoelectric conversion element, thermochemical battery, temperature control device, and thermoelectric sensor each including the electrolyte can improve the dimensionless figure of merit. [Brief explanation of the drawings]

[0018] [Figure 1] Figure 1 shows the potential-pH diagram of a V4+ / 5+ aqueous solution. [Figure 2] FIG. 2 is a conceptual diagram of the oxidation-reduction reaction that occurs in the electrolyte solution for thermoelectric conversion having a pH of 3 to 4 according to the embodiment. [Figure 3] FIG. 3 is a conceptual diagram of an oxidation-reduction reaction that occurs in a liquid electrolyte for thermoelectric conversion having a pH of less than 3 according to an embodiment. [Figure 4] FIG. 4 is a schematic diagram of a thermochemical battery according to an embodiment. [Figure 5] FIG. 5 is a schematic diagram of a temperature control device according to an embodiment. [Figure 6] FIG. 6 is a schematic diagram of another temperature control device according to the embodiment. [Figure 7] FIG. 7 is a graph showing the relationship between the closed circuit voltage and the temperature difference according to the seventh embodiment. [Figure 8] FIG. 8(a) is a graph showing the pH dependence of the Seebeck coefficient Se and ionic conductivity σ in the electrolyte solutions according to Examples 1 to 7, and FIG. 8(b) is a graph showing the pH dependence of σSe2 in the electrolyte solutions according to Examples 1 to 7. [Figure 9] 9(a) is a graph showing the relationship between current and voltage in the electrolytic solution according to Example 7, and FIG. 9(b) is a graph showing the relationship between power and voltage in the electrolytic solution according to Example 7. In FIG. [Figure 10] FIG. 10 is a graph showing the Seebeck coefficients for various pH values ​​in the electrolyte solutions (mixed solutions) according to Examples 8 to 30. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, a description will be given of an electrolyte solution for thermoelectric conversion according to an embodiment of the present invention, a thermoelectric conversion element, a thermochemical battery, a temperature control device, and a thermoelectric sensor each including the electrolyte solution. 4+ / 5+ 2 is a conceptual diagram of an oxidation-reduction reaction occurring in an electrolyte solution according to this embodiment; FIG. 3 is a conceptual diagram of an oxidation-reduction reaction occurring in an electrolyte solution having a pH of less than 3; FIG. 4 is a schematic diagram of a thermochemical battery according to this embodiment; FIG. 5 is a schematic diagram of a temperature control device according to this embodiment; and FIG. 6 is a schematic diagram of another temperature control device according to this embodiment.

[0020] <<Electrolyte for Thermoelectric Conversion>> The electrolytic solution for thermoelectric conversion contains a redox couple and an acidic aqueous solvent. In this specification, the term "electrolytic solution for thermoelectric conversion" refers to an electrolytic solution used in an element that directly converts heat into electricity or electricity into heat. The pH of the electrolytic solution is preferably 1 to 4. By adjusting the pH of the electrolytic solution to this range, the following 1 electron 3 proton (1e - / 3H + ) type proton-coupled electron transfer reactions and one-electron two-proton (1e - / 2H + The lower limit of the pH of the electrolyte is preferably 1.5 or more, 2.0 or more, 2.5 or more, 3.0 or more, 3.2 or more, or more preferably 3.5 or more. - / 3H + From the viewpoint of causing a proton-coupled electron transfer reaction of the type 2, the ionic liquid preferably has a pH of 3.0 or more, 3.2 or more, or 3.5 or more. The electrolytic solution may further contain a non-aqueous solvent.

[0021] Seebeck coefficient S of electrolyte for thermoelectric conversion e It is preferable that the absolute value of the Seebeck coefficient of the electrolyte for thermoelectric conversion is 2 mV / K or more and the ionic conductivity σ is 10 mS / cm or more. e In addition, the ionic conductivity σ is also high, which improves the dimensionless figure of merit ZT. Seebeck coefficient S eis expressed by the following equation (2).

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[0022] Seebeck coefficient S of electrolyte for thermoelectric conversion e The lower limit of the absolute value of σ is more preferably 3 mV / K or more, or 3.5 mV / K or more. The lower limit of the ionic conductivity σ is more preferably 50 mS / cm or more, or 100 mS / cm or more, and the upper limit may be 1000 mS / cm or less.

[0023] <Aqueous acidic solvent> The acidic aqueous solvent is included to dissolve the redox couple and to adjust the pH of the electrolytic solution to a range of 1 to 4. The acidic aqueous solvent is not particularly limited, but examples thereof include an aqueous sulfuric acid solution, an aqueous phosphoric acid solution, an aqueous citric acid solution, and an aqueous hydrochloric acid solution. Among these, it is preferable to use an aqueous sulfuric acid solution from the viewpoint of buffering action in the pH range.

[0024] <Non-aqueous solvent> Examples of non-aqueous solvents include nitrile solvents such as acetonitrile and methoxypropionitrile, propylene carbonate, N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). Among these, acetonitrile is preferred from the viewpoint of increasing the output voltage. Specifically, water is present around the protons, and when the orientation of the water aligns, the entropy change before and after the redox reaction increases. When acetonitrile is present, the orientation of water even at more distant locations aligns, increasing the entropy change before and after the redox reaction, decreasing the dielectric constant of the solvent and increasing the output voltage.

[0025] <Redox pair> The redox couple is an oxoaquavanadium complex. Examples of oxoaquavanadium complexes include oxoaquavanadium(IV) complexes and oxoaquavanadium(V) complexes. Specifically, when the pH of the electrolyte is in the range of 3 to 4, [VO(HO)] 2+ and [VO(OH)3(H2O)2] are produced (see Figures 1 and 2), so the oxoaqua vanadium(IV) complex is [VO(H2O)5] 2+ The oxoaqua vanadium(V) complex is [VO(OH)3(H2O)2]. If the pH of the electrolyte is less than 3, the oxoaqua vanadium(V) complex is [VO(H2O)5]. 2+ and [VO2(H2O)4] + (See Figures 1 and 3) and the oxoaquavanadium(IV) complex is [VO(H2O)5] 2+ and the oxoaqua vanadium(V) complex is [VO2(H2O)4] + The potential-pH diagram in Figure 1 was reported in Del Carpio, E. et al. Vanadium: History, chemistry, interactions with α-amino acids and potential therapeutic applications. Coord. Chem. Rev. 372, 117-140 (2018).

[0026] The oxoaquavanadium(IV) complex acts as a reductant, and the oxoaquavanadium(V) complex acts as an oxidant. [VO(H2O)5] 2+ The valence of vanadium is tetravalent, and it is expressed as [VO(OH)3(H2O)2] or [VO2(H2O)4] + The valence of vanadium in this is pentavalent.

[0027] Oxoaqua vanadium(IV) complexes (e.g., [VO(H2O)5] 2+ ) can be obtained, for example, by dissolving vanadium(IV) oxide sulfate hydrate in an acidic aqueous solvent. Also, oxoaqua vanadium(IV) complexes (e.g., [VO(OH)3(H2O)2] and [VO2(H2O)4] +) can be obtained by dissolving vanadium(V) oxide in an acidic aqueous solvent.

[0028] The concentration of the oxoaqua vanadium complex in the electrolyte (the total concentration of the oxoaqua vanadium complex (IV) and the oxoaqua vanadium complex (V)) is preferably 1 mol / L or more. If the concentration of the oxoaqua vanadium complex is 1 mol / L or more, when the electrolyte is used in a thermochemical battery, for example, the solubility in an acidic aqueous solvent is high, allowing for a large current. The concentration is more preferably 2 mol / L or more, or 3 mol / L or more.

[0029] Oxoaqua vanadium complex(IV) ([VO(H2O)5] 2+ ) and oxoaqua vanadium(V) complexes ([VO(OH)3(H2O)2] and [VO2(H2O)4] + ) is preferably 10:1 to 1:10. If the concentration ratio is within this range, the active material can be efficiently diffused.

[0030] <<Thermoelectric conversion element>> The thermoelectric conversion element includes the above-described electrolyte and a pair of electrodes in contact with the electrolyte. The material of the electrode portions in contact with the electrolyte can be a conductive material such as platinum or carbon. Among these, platinum is preferred because of its high dissolution potential and excellent corrosion resistance. Furthermore, carbon such as graphite is preferred because of its low manufacturing cost and large surface area.

[0031] In a thermoelectric conversion element, when the pH of the electrolyte is 3 to 4 and a temperature difference is generated between the electrodes, the oxidation-reduction equilibrium of the following formula (3) occurs in the electrolyte (see FIG. 2). [ka]

[0032] In the above formula (3), the entropy-driven reaction is dominant at the high-temperature electrode. Therefore, the elimination of three protons increases the number of ions, the valence of the oxoaqua vanadium complex decreases, and the reaction proceeds in the direction of alleviating solvation. As a result, the oxidation reaction proceeds preferentially at the high-temperature electrode. On the other hand, the enthalpy-driven reaction is dominant at the low-temperature electrode, and the reduction reaction proceeds preferentially by the opposite mechanism. This results in one electron and three protons (1e - / 3H + ) type of proton-coupled electron transfer reaction occurs.

[0033] On the other hand, if the pH of the electrolyte is less than 3, the oxidation-reduction equilibrium of the following formula (4) occurs (see FIG. 3). [ka]

[0034] The reaction (4) above is one electron and two protons (1e - / 2H + ) type proton-coupled electron transfer reaction, (1e - / 3H + Since one proton is transferred less than in the case of proton-coupled electron transfer reactions of the type (1e - / 3H + ) type proton-coupled electron transfer reaction occurs more frequently than (1e - / 2H + The dimensionless figure of merit (ZT) is higher than when a proton-coupled electron transfer reaction of the type 2 occurs.

[0035] As a result of extensive research, the present inventors have found that the dimensionless figure of merit (ZT) can be improved by using an electrolyte containing an oxoaqua vanadium complex as a redox couple. This is because, with this redox couple, - / 3H + ) type and (1e - / 2H + ) type proton-coupled electron transfer reaction can occur, resulting in a high Seebeck coefficient (S e) can be obtained, and since this redox couple has high solubility in acidic aqueous solvents, the ionic conductivity (σ) can be improved. This leads to a Seebeck coefficient (S e ) and can improve the ionic conductivity (σ). Therefore, according to this embodiment, since an oxoaqua vanadium complex is used as the redox couple, the dimensionless figure of merit (ZT) can be significantly improved. In particular, when [VO(HO)] is used as the redox couple, 2+ and [VO(OH)3(H2O)2], the Seebeck coefficient (S e ) and can improve the ionic conductivity (σ), the dimensionless figure of merit (ZT) can be further improved.

[0036] <Thermochemical battery> A thermochemical battery includes the thermoelectric conversion element. Specifically, a thermochemical battery 10 shown in FIG. 4 includes a battery container 11, an electrolyte 12 stored in the battery container 11, a pair of electrodes 13 in contact with the electrolyte 12, and a temperature difference generating unit 14 for applying a temperature difference to the electrolyte 12. The battery container 11 may be, for example, an H-tube. The electrolyte 12 is the above-described electrolyte for thermoelectric conversion. The temperature difference generating unit 14 is not particularly limited, but may be, for example, an ice bath or a water bath. The temperature difference depends on the application of the thermochemical battery, but is preferably, for example, 0.5°C or more. In the case of waste heat recovery, the temperature difference may be, for example, 10°C or more.

[0037] When a temperature difference is applied between the pair of electrodes 13 by the temperature difference generating unit 14, a reduction reaction occurs at the low-temperature electrode 13A and an oxidation reaction occurs at the high-temperature electrode 13B. As a result, a potential difference occurs due to the temperature difference between the low-temperature electrode 13A and the high-temperature electrode 13B, and an electromotive force is generated between the low-temperature electrode 13A and the high-temperature electrode 13B.

[0038] <Temperature control device> The temperature control device includes the thermoelectric conversion element. Specifically, the temperature control device 20 shown in Fig. 5 includes a battery container 21, an electrolytic solution 22 stored in the battery container 21, a pair of electrodes 23 in contact with the electrolytic solution 22, and a power source 24 electrically connected to the pair of electrodes 23. The temperature control device 20 is a device having at least one of a cooling function and a heating function.

[0039] When a current is applied between the pair of electrodes 23 from the power source 24, a reduction reaction occurs at the cathode 23A and an oxidation reaction occurs at the anode 23B. This causes a temperature difference due to the potential difference between the cathode 23A and the anode 23B, making it possible to cool or warm an object (such as a human body).

[0040] Applications of such a temperature control device include, but are not limited to, cooling of the human body, equipment, virus specimens, blood, etc. Specifically, the temperature control device can be incorporated into wearable devices, thermal cyclers, insulated boxes or transport boxes for virus specimens or blood, or small mass spectrometers. Furthermore, because such a temperature control device is capable of localized cooling, it can be used to cool people working in high temperatures, such as in steel mills or working outdoors in midsummer, or in plant factories (cooling only the area around the plants rather than the entire greenhouse).

[0041] When cooling the human body for thermoregulation purposes, it is suitable for use in regulating the temperature of sick people, the elderly, and infants. When considering use in elderly care or for infants, a cooling effect that is too strong can be problematic, and it is preferable to be able to provide continuous cooling at a stable temperature. In particular, in elderly care settings, it is important to be able to wear the device continuously, using electricity, which requires less manpower. A temperature control device makes this type of cooling possible.

[0042] In a temperature control device, it is preferable to circulate the electrolyte. For example, the temperature control device 30 shown in Fig. 6 is an example of a cooling device, and includes a first cell 31, a second cell 32, an electrolyte 33 stored in the first cell 31 and the second cell 32, a cathode 34 disposed in the first cell 31 and in contact with the electrolyte 33, an anode 35 disposed in the second cell 32 and in contact with the electrolyte, a circulation system 36 connected to the first cell 31 and the second cell 32 and for circulating the electrolyte 33, a cooler 37 (e.g., a fan or a radiator) for cooling the electrolyte 33 in the second cell 32, and a power source 38 electrically connected to the cathode 34 and the anode 35.

[0043] The electrolyte 33 is the thermoelectric conversion electrolyte. The circulation system 36 includes a pump 36A for pumping the electrolyte 33 and a pipe 36B connecting the first cell 31 and the second cell 32. When a voltage is applied between the cathode 34 and the anode 35, the temperature at the cathode 34 drops and the temperature at the anode 35 rises. Therefore, when an object to be cooled is brought into contact with the first cell 31, the object can be cooled. Furthermore, the electrolyte 33 warmed by the anode 35 is cooled in a cooler 37 and then pumped toward the first cell 31 by the pump 36A. Circulating the electrolyte 33 dramatically reduces the solution resistance and improves heat transfer efficiency. These effects cannot be achieved with solid-state Peltier elements. Air conditioners can efficiently transfer heat by circulating a liquid, but they require a compressor for heat exchange, making them unsuitable for miniaturization. In contrast, such a temperature control device is capable of circulating liquid, thereby achieving an efficient cooling function.

[0044] <Thermoelectric sensor> A thermoelectric sensor is a device that uses the thermoelectric conversion element to detect the temperature difference between electrodes. Thermoelectric conversion elements have a high Seebeck coefficient, making them highly sensitive as temperature sensors. For example, when using a thermoelectric conversion element as a thermoelectric sensor, a temperature difference of just 1°C between the electrodes can generate a voltage of 2 mV. By connecting a device that measures potential differences, such as a high-precision voltmeter, a sensor can be created that can detect differences of as little as 0.000001°C. This level of temperature difference detection allows, for example, the heat from a person's body temperature on the other side of a wall to heat one electrode, making it possible to detect the person. It can also detect weak infrared radiation radiated from a heat source. Furthermore, if a material that can absorb specific light, such as ultraviolet light, and generate heat is placed on one electrode, it can be used as a thermoelectric sensor that detects weak light. [Example]

[0045] In order to explain the present invention in detail, the following examples are given, but the present invention is not limited to these. Fig. 7 is a graph showing the relationship between the closed circuit voltage and the temperature difference in the electrolyte solution of Example 7, and Fig. 8(a) shows the Seebeck coefficient S e 8(b) is a graph showing the pH dependence of ionic conductivity σ and σSe in the electrolyte solutions according to Examples 1 to 7. 2 9(a) is a graph showing the relationship between current and voltage in the electrolyte solution of Example 7, FIG. 9(b) is a graph showing the relationship between power and voltage in the electrolyte solution of Example 7, and FIG. 10 is a graph showing the Seebeck coefficient for various pH values ​​in the electrolyte solutions (mixed solutions) of Examples 8 to 30.

[0046] <Examples 1 to 7> Sodium hydrogen sulfate monohydrate (molecular weight: 138.07, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), which served as both a buffer and a supporting electrolyte, was dissolved in distilled water at a concentration of 3 M. To this solution, 50 mM of vanadium(IV) oxide sulfate n-hydrate (molecular weight: 163.00, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as a reductant, and 25 mM of vanadium(V) oxide (molecular weight: 181.88, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as an oxidant to obtain an electrolyte solution. The pH of the electrolyte solution was adjusted appropriately by adding 12.5 M sodium hydroxide aqueous solution dropwise. The pH of the electrolyte solutions in Examples 1 to 7 was as shown in Table 1. [Table 1]

[0047] <Comparative Example 1> 1590 mg of RuCl3·3H2O (7.7 mmol) and 8930 mg of imidazole (131 mmol) were placed in a recovery flask and refluxed for 1 hour. The mixture was then allowed to cool to room temperature and dissolved in 150 mL of warm water at 55 °C. The mixture was then filtered through a Celite pad. The solvent was removed under reduced pressure, and the resulting solid was redissolved in 60 mL of water and 40 mL of acetone was added. The resulting solution was filtered again, and the filter cake was washed twice with 5 mL of acetone and dried under vacuum for 4 hours. The resulting solid was then dissolved again in an imidazole solution containing 1 g of imidazole in 85 mL of water at 55 °C. The solution was allowed to cool to room temperature and then left to stand overnight in a refrigerator. After filtration, the mixture was washed three times with 2 mL of cold water and 2.5 mL of acetone. The filtrate was recrystallized to obtain a gray-green solid. 1 The solid obtained was confirmed by H-NMR and elemental analysis to be [Ru(Him)6]Cl2. (Note: Him stands for imidazole.) This solid (1 mM) and nitrosyl tetrafluoroborate (NOBF4) (0.3 mM) as an oxidant to partially generate the oxidized product in situ were dissolved in an aqueous solution of a mixed solvent of acetonitrile (MeCN) and water (mixing ratio 2:1) to obtain an electrolyte solution (pH 10.8) containing a ruthenium hexamidazole complex.

[0048] <Comparative Example 2> Prepare K3[Fe(CN)6] and K4[Fe(CN)6] by dissolving K3[Fe(CN)6] and K4[Fe(CN)6] in water. 3- / 4- (200 mM each) to obtain an electrolyte solution.

[0049] <Seebeck coefficient S e and ionic conductivity σ measurement> For the electrolyte solutions of Examples 1 to 7 and Comparative Examples 1 and 2, the Seebeck coefficient S e The temperature difference (ΔT) was generated by pouring the electrolyte into an H-tube and immersing the low-temperature electrode and the high-temperature electrode in an ice bath and a water bath, respectively. Two platinum rods were immersed in each of these baths as electrodes to create a thermochemical battery. The open circuit voltage (ΔV OC ) was monitored for various ΔT. The results were then compared to ΔV OC and ΔT, and from the slope, S e 1 electron 3 protons (1e - / 3H + The measurement results for the electrolyte solution according to Example 7 in which a PCET reaction of type 2 occurs are shown in FIG. OC is proportional to ΔT, and S e The negative value of -3.2 mV / K was relatively large.

[0050] The ionic conductivity σ was measured using an ionic conductivity meter (model number "F-74", manufactured by Horiba, Ltd.). Although the ionic conductivity σ of the electrolyte solution decreased with increasing pH (decreasing proton concentration), the S e Even at around pH 3.2, where the coefficient is at its maximum, it maintains a high value of approximately 100 mS / cm. As a result, the σS that contributes to the dimensionless figure of merit (ZT) e 2 As shown in Figure 8(b), the term showed the highest value at pH 3.2.

[0051] In addition, for the electrolyte solutions of Examples 1 to 7 and Comparative Examples 1 and 2, Se The results shown in Figure 8(a) were obtained. e The potential gradually increased from -1 to -2 mV / K with increasing pH up to pH 3, and then increased sharply to a maximum of -3.2 mV / K above pH 3. In the former pH region, the potential of 1 electron and 2 protons (1e) as shown in the above formula (4) - / 2H + This is thought to be due to the dominance of the 1 electron 3 proton (1e) type PCET reaction. - / 3H + ) type PCET reaction, the number of protons transferred is one less and the entropy change is smaller, so S e It is thought that the value remained at -1 to -2 mV / K.

[0052] Current and power measurements When the current and power versus voltage were measured using a thermochemical battery using the electrolyte of Example 7, the plot for the electrolyte of Example 7 was linear, and the maximum output was approximately 180 nW, as shown in Figures 9(a) and 9(b). This value is more than 10 times the output when the ruthenium hexaimidazole complex of Comparative Example 1 was used, indicating that the output was also significantly improved by improving the ionic conductivity σ.

[0053] <Thermal conductivity κ measurement> The thermal conductivity κ of the electrolyte solutions according to Example 7 and Comparative Examples 1 and 2 was measured using a thermal property measuring device (model number "TPS-500S", Kyoto Electronics Manufacturing Co., Ltd.) The thermal conductivity κ of the electrolyte solution according to Example 7 was 0.62 W / mK.

[0054] The following are the measured values ​​for the electrolyte solutions according to Example 7 and Comparative Examples 1 and 2. The dimensionless figure of merit ZT is the value at the temperature shown in Table 2. [Table 2]

[0055] As shown in Table 2, the 1 electron 3 proton (1e - / 3H + The ZT of the electrolyte of the oxoaqua vanadium complex showing the PCET reaction of type 4.7 × 10 at 293 K is -2 This value is approximately 25 times the ZT value exhibited by the electrolyte containing the ruthenium hexamidazole complex according to Comparative Example 1, and is approximately 25 times the ZT value exhibited by the [Fe(CN)6] according to Comparative Example 2, which was the benchmark for redox species in conventional thermochemical batteries. 3- / 4- The ZT was approximately 2.5 times larger than that of the electrolyte containing the oxoaqua vanadium complex. - / 3H + ) type PCET reaction, high S e This enabled us to achieve both and σ, and as a result, we succeeded in improving the ZT of the thermochemical battery.

[0056] <Examples 8 to 30> S in oxoaqua vanadium complexes e The change in the electrolytic activity due to the solvent was evaluated using a mixed solvent of water and acetonitrile (MeCN). The compositions of the electrolyte solutions used in Examples 8 to 30 are shown below. Note that, from the viewpoint of solubility, the concentration of redox species was changed for each sample solution with a different mixing ratio.

[0057] The mixed solvent was a mixture of 100 mM aqueous sulfuric acid and MeCN. Three types of mixed solvents were used, with the ratio (volume ratio) of MeCN to water being 2:1 (MeCN: 67%), 1:1 (MeCN: 50%), and 0:1 (MeCN: 0%).

[0058] The electrolyte solution, which used a mixed solvent containing 67% MeCN, contained 3.3 mM each of vanadium(IV) oxide sulfate n-hydrate (molecular weight: 163.00, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and vanadium(V) oxide (molecular weight: 181.88, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0059] The electrolyte solution, which used a mixed solvent containing 50% MeCN, contained 5.0 mM each of vanadium(IV) oxide sulfate n-hydrate (molecular weight: 163.00, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and vanadium(V) oxide (molecular weight: 181.88, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0060] The electrolyte solution using a solvent containing 0% MeCN contained 10 mM each of vanadium(IV) oxide sulfate n-hydrate (molecular weight: 163.00, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and vanadium(V) oxide (molecular weight: 181.88, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0061] In each electrolyte, while changing the pH, e was measured. e The coefficient was measured in the same manner as in Example 1.

[0062] The measurement results are shown in Table 3 and FIG. [Table 3]

[0063] As shown in Fig. 10, for all electrolyte mixture ratios, the S e The S e It was observed that the S e In particular, the electrolyte solution of Example 15 showed a more significant improvement in S e The value was −3.9 mV / K, which is a very high value comparable to that of the electrolyte containing the ruthenium hexamidazole complex according to Comparative Example 6. From these results, it can be seen that the S e It can be seen that the contribution to the change in is largely influenced by the solvent used or the number of protons transferred during the PECT reaction, regardless of whether the ligand of the redox molecule is an organic ligand or hydrated water. [Explanation of symbols]

[0064] 10...Thermochemical battery 11, 21…battery tank 12, 22…electrolyte 13, 23…electrode 20, 30...temperature control device 31…1st battery tank 32…Second battery tank 36…Circulation system

Claims

1. An electrolyte solution for thermoelectric conversion comprising a redox couple and an acidic aqueous solvent, The electrolyte for thermoelectric conversion, wherein the redox couple is an oxoaqua vanadium complex.

2. 2. The electrolyte for thermoelectric conversion according to claim 1, wherein the pH of the electrolyte is 1 to 4.

3. 2. The electrolyte for thermoelectric conversion according to claim 1, wherein the pH of the electrolyte is 2 to 4.

4. 4. The electrolyte solution for thermoelectric conversion according to claim 1, wherein the redox couple is an oxoaqua vanadium(IV) complex and an oxoaqua vanadium(V) complex.

5. The redox couple is [VO(H ２ O) ５ ] ２＋ and [VO(OH) ３ (H ２ O) ２ The electrolyte solution for thermoelectric conversion according to claim 1 , wherein

6. The electrolyte solution for thermoelectric conversion according to claim 1 , further comprising a non-aqueous solvent.

7. The electrolytic solution for thermoelectric conversion according to any one of claims 1 to 6, a pair of electrodes in contact with the electrolytic solution for thermoelectric conversion; A thermoelectric conversion element comprising:

8. A thermochemical battery comprising the thermoelectric conversion element according to claim 7.

9. A temperature control device comprising the thermoelectric conversion element according to claim 7.

10. A thermoelectric sensor comprising the thermoelectric conversion element according to claim 7.

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