Electrolyte and thermoelectric conversion device

Abstract

The present invention provides an electrolyte with an improved Seebeck coefficient, and a thermoelectric conversion device using the said electrolyte. [Solution] The electrolyte 3 consists of an aqueous solution containing an active material 5 and an electrolyte 6, wherein the electrolyte 6 generates hydrate or decomposes hydrate upon temperature change, and the oxidation-reduction potential of the active material 5 changes depending on the concentration of the hydrate. Preferably, the electrolyte 6 contains at least one of a quaternary ammonium salt and a quaternary phosphonium salt. Preferably, the active material 6 is a hexacyanoferrate ion.

Description

Technical Field

[0001] The present invention relates to an electrolytic solution and a thermoelectric conversion device.

Background Art

[0002] A thermochemical battery that generates an electromotive force by applying a temperature difference between electrodes is known. The electromotive force of a thermochemical battery is represented by the product of the change in redox potential per unit temperature [V / K] (hereinafter, the Seebeck coefficient) and the temperature difference between electrodes [K]. Since the electromotive force is directly related to the output density, it is extremely important to improve the Seebeck coefficient. In Patent Document 1, an electrolytic solution with a high Seebeck coefficient is used, but an electrolytic solution with an even higher Seebeck coefficient and a thermochemical battery using the same are desired. Note that such an improvement in the Seebeck coefficient is desired not only for thermochemical batteries but also for general devices that perform thermoelectric conversion using an electrolytic solution.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In view of such circumstances, an object of the present invention is to provide an electrolytic solution with an improved Seebeck coefficient and a thermoelectric conversion device using the same.

Means for Solving the Problems

[0005] An aspect for achieving the above object is an electrolytic solution comprising an aqueous solution containing an active material and an electrolyte, wherein the electrolyte generates a hydrate or decomposes a hydrate due to a temperature change, and the active material has an oxidation-reduction potential that changes depending on the concentration of the hydrate.

[0006] Another embodiment for achieving the above objective is a thermoelectric conversion device characterized by comprising the electrolyte of the above embodiment and a pair of electrodes. [Effects of the Invention]

[0007] According to the present invention, an electrolyte with an improved Seebeck coefficient and a thermoelectric conversion device using the electrolyte are provided. [Brief explanation of the drawing]

[0008] [Figure 1] This is a phase diagram relating to the concentration and temperature of the electrolyte contained in an aqueous solution containing the electrolyte according to Embodiment 1. [Figure 2] This figure shows the relationship between the oxidation-reduction potential and the temperature of the electrolyte according to Embodiment 1. [Figure 3] This is a schematic diagram of the thermoelectric conversion device according to Embodiment 2 and its state transitions. [Figure 4] This figure shows the relationship between discharge capacity and voltage in each state according to Embodiment 2. [Figure 5] This is a schematic diagram of the thermoelectric conversion device according to Embodiment 3 and its state transitions. [Figure 6] This figure shows the relationship between discharge capacity and voltage in each state according to Embodiment 3. [Figure 7] This is a schematic diagram of a thermochemical cell according to Embodiment 4. [Figure 8] This is a schematic diagram of a thermoelectric conversion device according to Embodiment 5. [Modes for carrying out the invention]

[0009] <Embodiment 1> The electrolyte according to the present invention will now be described. The electrolyte consists of an aqueous solution containing an active material and an electrolyte.

[0010] Electrolytes have the property of generating hydrates or decomposing hydrates in response to temperature changes. Specific examples of such electrolytes include quaternary ammonium salts and quaternary phosphonium salts. Examples of quaternary ammonium salts include tetrabutylammonium salt and tetraisobutylammonium salt, while an example of a quaternary phosphonium salt is tetrabutylphosphonium salt. The electrolyte may contain only one type of these salts, or it may contain multiple types.

[0011] When we say that an electrolyte generates hydrate or decomposes hydrate due to a change in temperature, it means that some or all of the electrolyte becomes hydrate due to a change in temperature, or that the hydrate decomposes and returns to its original electrolyte state. Electrolyte hydrates refer to crystals (clathrate hydrates) formed when the electrolyte is enclosed within a cage-like structure created by hydrogen bonds between water molecules, or crystals (semi-clathrate hydrates) formed when the electrolyte participates in the hydrogen bond network of water molecules.

[0012] Figure 1 illustrates a phase diagram relating the concentration of an electrolyte in an aqueous solution to temperature. The case using tetrabutylammonium fluoride (TBAF) as the electrolyte is explained below. In the figure, the horizontal axis represents the concentration of TBAF in the aqueous solution, and the vertical axis represents temperature.

[0013] For example, suppose the concentration of the TBAF aqueous solution is Ra and the temperature is Ta. Ta is higher than the boundary temperature T0. The boundary temperature T0 is the upper limit temperature at which hydrate can be formed in the TBAF aqueous solution, and it is a temperature that depends on the concentration of the TBAF aqueous solution.

[0014] Since the temperature Ta is higher than the boundary temperature T0, no hydrate is formed in the TBAF aqueous solution. When this TBAF aqueous solution is cooled to a temperature Tb (Tb < T0), a part of the TBAF becomes a hydrate, and the TBAF aqueous solution becomes a slurry in which the liquid phase and the hydrate are mixed. Since a part of the TBAF becomes a hydrate, the concentration of the TBAF in the liquid phase decreases to Rb. Conversely, if the slurry-like TBAF aqueous solution containing the hydrate is heated, the hydrate decomposes and the concentration of the TBAF in the liquid phase increases. When the temperature exceeds T0, the hydrate disappears and the TBAF aqueous solution returns to a single liquid phase. Thus, an aqueous solution containing an electrolyte such as TBAF has the property that the concentration of the TBAF in the liquid phase (in other words, the concentration of the hydrate) changes due to the formation or decomposition of the hydrate caused by a temperature change.

[0015] The active material is a substance having the property that its redox potential changes depending on the concentration of the hydrate. Further, in the active material, at least one of the oxidized form (Ox) and the reduced form (Red) generated during the redox process by the transfer of electrons in the aqueous solution is a substance soluble in the above aqueous solution.

[0016] Specific examples of the active material include hexacyanoferrate ions (ferrocyanide [Fe(CN)6] 4- / ferricyanide [Fe(CN)6] 3- ).

[0017] Fig. 2 shows the relationship between the redox potential of hexacyanoferrate ions and the temperature of the electrolyte. The horizontal axis in the figure is the difference in the temperature applied to the aqueous solution, and the vertical axis shows the amount of potential change due to the temperature difference. This amount of potential change was obtained by measuring the potential difference when two platinum electrodes were inserted into the above-described electrolyte and a temperature difference was applied to the insertion positions of the two platinum electrodes. The square marks show the relationship between the redox potential and the temperature in the electrolyte containing TBAF according to the present invention. The round marks show the relationship between the redox potential and the temperature in the case where TBAF is not contained as a comparative example.

[0018] As shown in the figure, when an electrolyte containing TBAF is used, the oxidation-reduction potential changes in the positive direction when cooled from a certain temperature (zero). Conversely, when heated to approach the reference temperature, the oxidation-reduction potential changes in the negative direction. On the other hand, when TBAF is not present, the change in oxidation-reduction potential due to heating or cooling is small. Thus, it can be seen that a change in the temperature of the electrolyte, that is, a change in the TBAF concentration in the liquid phase accompanying this temperature change, leads to a change in the oxidation-reduction potential (electromotive force).

[0019] Furthermore, the Seebeck coefficient of the hexacyanoferrate ion in an aqueous solution containing TBAF is approximately -8 mVK on average at 282-297 K. -1 ] and on average over 292-297[K], it is approximately -14[mVK]. -1 The Seebeck coefficient of the hexacyanoferrate ion when water is used as the solvent (without TBAF) is approximately -1.5 [mVK]. -1 The Seebeck coefficient when methanol aqueous solution is used as the solvent is approximately -3 [mVK]. -1 This shows that the TBAF aqueous solution according to the present invention has a higher Seebeck coefficient than conventional electrolytes.

[0020] The electrolyte described above consists of an aqueous solution containing an active material and an electrolyte. The electrolyte, such as TBAF, has the property of generating hydrates or decomposing hydrates upon temperature changes, and the active material has the property of changing its oxidation-reduction potential depending on the concentration of hydrates. Such an electrolyte has a much higher Seebeck coefficient than conventional electrolytes. Therefore, by applying the electrolyte of the present invention to a thermoelectric conversion device such as a thermochemical cell, it is possible to obtain a device that can generate a high electromotive force even with a relatively small temperature difference.

[0021] The electrolyte in the above-mentioned electrolyte preferably contains at least one of a quaternary ammonium salt and a quaternary phosphonium salt. These electrolytes generate hydrates or decompose hydrates in response to temperature changes, and an electrolyte with a high Seebeck coefficient can be obtained.

[0022] The active material of the electrolyte described above is preferably iron hexacyanoate ions (ferrocyanide and ferricyanide). Since the oxidation-reduction potential of iron hexacyanoate ions changes significantly depending on the concentration of the electrolyte (in other words, the concentration of the hydrate), it can be expected that the Seebeck coefficient of the electrolyte will be greatly improved.

[0023] <Embodiment 2> A thermoelectric conversion device using the electrolyte described in Embodiment 1 will now be described. Figure 3 is a schematic diagram of the thermoelectric conversion device of this embodiment and its state transitions. Figure 4 is a diagram showing the relationship between discharge capacity and voltage in each state.

[0024] As shown in Figure 3, the thermoelectric converter 1 comprises a pair of electrodes 2a and 2b, electrolytes 3a and 3b, and a temperature control means (not shown). Specifically, electrode 2a is located on one side of a container separated by a diaphragm 4, and electrode 2b is located on the other side. On the electrode 2a side of the container, electrolyte 3a, which consists of an aqueous solution containing the active material 5a and an electrolyte 6 such as TBAF as described in Embodiment 1, is stored and in contact with electrode 2a. On the electrode 2b side of the container, electrolyte 3b, which contains the active material 5b and has a different Seebeck coefficient than electrolyte 3a, is filled. When the electrolyte of Embodiment 1 is used as electrolyte 3a, electrolyte 3a has a large Seebeck coefficient in the negative direction (the potential changes in the negative direction as the temperature rises), so the Seebeck coefficient of electrolyte 3b is set to have a value in the positive direction that is greater than that of electrolyte 3a. In addition, electrolyte 3b is selected to have a small difference in oxidation-reduction potential with electrolyte 3a, that is, to have the potential of electrolyte 3a and electrolyte 3b reversed during the cooling and heating processes described later. An example is an electrolyte containing water as the solvent and hexacyanoferrate ions as the active material (but not electrolyte 6 such as TBAF).

[0025] The electrodes 2a and 2b are not limited to any particular material and can be formed using known materials. Similarly, the diaphragm 4 is not limited to any particular material and can be made from known materials.

[0026] The temperature control means consists of a device capable of adjusting the temperatures of electrolytes 3a and 3b. The temperature control device can be any device capable of arbitrarily setting the temperature within the range in which a change in the oxidation-reduction potential of electrolyte 3a occurs, as shown in Figure 2. Examples include electric heaters, heat exchangers, and air-cooled coolers that cool by blowing air. For example, when using an electrolyte containing TBAF, the temperature control means is configured to be able to set the electrolyte temperature to 273[K] as the low-temperature state and 303[K] as the high-temperature state. Note that at the high-temperature state, it is not necessary for the entire amount of hydrate to be decomposed; it is sufficient if it is less than the amount of hydrate at the low-temperature state.

[0027] The operation of the thermoelectric converter of this embodiment will be explained using Figures 3 and 4. The operation consists of the following four steps. 1. Discharge at high temperature. 2. Cool it down. 3. Discharge at low temperatures. 4. Heat.

[0028] Step 1 (Discharge at high temperature) The electrolytes 3a and 3b are kept at a high temperature by the temperature control means. In this high-temperature state, in this embodiment, electrode 2a has a lower potential than electrode 2b. As shown in Figure 4, with the potential of electrode 2b as the reference, the voltage V1 between electrode 2a and electrode 2b is negative. While maintaining this high-temperature state, discharge is performed to the load 7 connected between electrode 2a and electrode 2b. Discharge to the load 7 continues until the potentials of electrode 2a and electrode 2b are almost the same, that is, until the voltage V0 is almost zero.

[0029] Step 2 (Hydrate generation by cooling) The system is cooled to a low temperature using a temperature control device. During cooling, the load 7 is disconnected from the electrodes. Hydrate 8 is generated in the electrolyte 3a due to the temperature change. The generation of hydrate 8 reduces the TBAF concentration in the liquid phase of electrolyte 3a, and consequently, the oxidation-reduction potential of the active material 5a increases (see Figure 2). As a result, as shown in Figure 4, the potentials of electrolyte 3a and electrolyte 3b are reversed, and the voltage between electrodes 2a and 2b becomes a positive voltage V2.

[0030] Step 3 (Discharging at low temperatures) The electrolytes 3a and 3b are kept at a low temperature by the temperature control means. While maintaining this low temperature, discharge is performed to the load 7 connected between electrodes 2a and 2b. The current is in the opposite direction to the discharge in the high-temperature state in step 1. In this low-temperature state, discharge is performed to the load 7 until the voltage between electrodes 2a and 2b becomes approximately zero, voltage V0.

[0031] Step 4 (Decomposition of hydrate by heating) The system is heated to a high temperature using a temperature control device. During heating, the load 7 is disconnected from the electrodes. Due to the temperature change, the hydrate 8 in the electrolyte 3a decomposes. The decomposition of hydrate 8 increases the concentration of TBAF, and consequently the oxidation-reduction potential of the active material decreases (see Figure 2). Therefore, as shown in Figure 4, the voltage between electrodes 2a and 2b becomes a negative voltage V1.

[0032] By repeating steps 1-4, the thermoelectric converter 1 can repeatedly discharge to the load 7. It is not necessary to start from step 1; the process can begin at any step depending on the state of the electrodes. Furthermore, while steps 1-4 above are an example where the electrolyte 3a has a negative Seebeck coefficient, an electrolyte 3a that exhibits a positive Seebeck coefficient due to hydrate formation and decomposition may also be used. In that case, electrolyte 3b should be selected to have a Seebeck coefficient that is more negative than that of electrolyte 3a.

[0033] According to the thermoelectric converter 1 described above, repeated discharges can be performed due to temperature changes. Furthermore, since an electrolyte with an improved Seebeck coefficient is used, sufficient electromotive force can be provided even with relatively small temperature differences. In the example using TBAF, a considerable electromotive force is obtained between 283[K] and 297[K]. This temperature range is roughly equivalent to the temperature difference between day and night, although this depends on the region and climate, making it possible to perform repeated discharges using the temperature changes throughout the day. Note that electrode 2a corresponds to one electrode of the claim, and electrode 2b corresponds to the other electrode of the claim.

[0034] <Embodiment 3> A thermoelectric conversion device using the electrolyte described in Embodiment 1 will now be described. Figure 5 is a schematic diagram of the thermoelectric conversion device of this embodiment and its state transitions. Figure 6 is a diagram showing the relationship between discharge capacity and voltage in each state. Components identical to those in Embodiment 2 are denoted by the same reference numerals, and redundant explanations are omitted.

[0035] As shown in Figure 5, similar to Embodiment 2, the thermoelectric converter 1A includes a pair of electrodes 2a and 2b, electrolytes 3a and 3b, a power supply 9, and a temperature control means (not shown). Specifically, electrode 2a is placed on one side of a container separated by a diaphragm 4, and electrode 2b is placed on the other side. On the electrode 2a side of the container, electrolyte 3a, which consists of an aqueous solution containing the active material 5a and an electrolyte 6 such as TBAF as described in Embodiment 1, is stored and in contact with electrode 2a. On the electrode 2b side of the container, electrolyte 3b, which contains the active material 5b and has a different Seebeck coefficient than electrolyte 3a, is filled. When the electrolyte of Embodiment 1 is used as electrolyte 3a, since electrolyte 3a has a large Seebeck coefficient in the negative direction (the potential changes in the negative direction as the temperature rises), the Seebeck coefficient of electrolyte 3b is set to have a value in the positive direction that is greater than that of electrolyte 3a. In addition, as a difference from Embodiment 2, electrolyte 3b is selected to have a large difference in oxidation-reduction potential from electrolyte 3a, that is, to prevent the potential of electrolyte 3a and electrolyte 3b from being reversed by the cooling and heating processes described later.

[0036] The operation of the thermoelectric converter of this embodiment will be explained using Figures 5 and 6. The operation consists of the following four steps. 1. Charge the device in a high-temperature environment. 2. Cool it down. 3. Discharge at low temperatures. 4. Heat.

[0037] Step 1 (Discharge at high temperature) The electrolytes 3a and 3b are kept at a high temperature by the temperature control means. While maintaining this high temperature, the power supply 9 is connected between electrodes 2a and 2b and charging is performed. In this high temperature state, as shown in Figure 6, the voltage V2 is higher than the voltage V1 before charging.

[0038] Step 2 (Hydrate generation by cooling) The system is cooled to a low temperature using a temperature control device. During cooling, the power supply 9 is disconnected from the electrodes. Hydrate 8 is generated in the electrolyte 3a due to the temperature change. The generation of hydrate 8 reduces the TBAF concentration in the liquid phase, and consequently, the oxidation-reduction potential of the active material 5a increases (see Figure 2). Therefore, as shown in Figure 6, the voltage between electrodes 2a and 2b becomes a voltage V3, which is higher than voltage V2.

[0039] Step 3 (Discharging at low temperatures) The electrolytes 3a and 3b are kept at a low temperature by the temperature control means. While maintaining this low temperature state, discharge is applied to the load 7 connected between electrodes 2a and 2b. In this low temperature state, discharge is carried out from electrode 2a to load 7 until the voltage reaches V4.

[0040] Step 4 (Decomposition of hydrate by heating) The system is heated to a high temperature using a temperature control device. During heating, the load 7 is disconnected from the electrodes. Due to the temperature change, the hydrate 8 in the electrolyte 3a decomposes. The decomposition of hydrate 8 increases the concentration of TBAF, and consequently the oxidation-reduction potential of the active material 5a decreases (see Figure 2). Therefore, as shown in Figure 6, the voltage between electrodes 2a and 2b becomes a voltage V1, which is lower than the voltage V4.

[0041] By repeating steps 1-4, the thermoelectric converter 1A recovers its electromotive force through charging by the power supply 9, but the electromotive force obtained from the temperature change in step 2 is also added. It is not necessary to start from step 1, and it is possible to start from any step depending on the state of the electrodes. Furthermore, the above steps 1-4 are for the case where the electrolyte 3a has a negative Seebeck coefficient, but an electrolyte 3a that exhibits a positive Seebeck coefficient due to the generation and decomposition of hydrate may also be used, in which case an electrolyte 3b with a negative value than the Seebeck coefficient of electrolyte 3a should be selected.

[0042] According to the thermoelectric converter 1A described above, the electromotive force can be recovered not only by charging with a power source but also by temperature changes. Although an external power source 9 is required, the charging voltage from the power source 9 can be made lower compared to the discharge voltage compared to a typical secondary battery. Furthermore, since an electrolyte with an improved Seebeck coefficient is used, the electromotive force can be recovered significantly even with a relatively small temperature difference. Note that electrode 2a corresponds to one electrode of the claim, and electrode 2b corresponds to the other electrode of the claim.

[0043] <Embodiment 4> A thermochemical cell, which is an example of a thermoelectric conversion device using the electrolyte described in Embodiment 1, will now be described. Figure 7 is a schematic diagram of the thermochemical cell of this embodiment. Components identical to those in Embodiment 2 are denoted by the same reference numerals, and redundant explanations are omitted.

[0044] As shown in Figure 7, similar to Embodiment 2, the thermochemical battery 1B comprises a negative electrode 2a, a positive electrode 2b, an electrolyte 3, and heating and cooling means (not shown). In this embodiment, the container is not separated by a diaphragm and stores the electrolyte 3, which consists of an aqueous solution containing the active material 5 and an electrolyte 6 such as TBAF as described in Embodiment 1. The negative electrode 2a and the positive electrode 2b are arranged in this container, and both are in contact with the electrolyte 3.

[0045] The heating means consists of a device that brings the negative electrode 2a to a high temperature. Bringing the negative electrode 2a to a high temperature does not only mean directly heating the negative electrode 2a to a high temperature, but also includes indirectly heating the negative electrode 2a and the surrounding electrolyte 3 by heating the negative electrode 2a side of the container from the outside. Heating means include electric heaters, heat exchangers that heat the negative electrode 2a side of the container using a heat transfer medium, the walls of pipes and structures, and body surfaces.

[0046] The cooling means consists of a device that brings the positive electrode 2b to a low temperature. Bringing the positive electrode 2b to a low temperature does not only mean directly lowering the temperature of the positive electrode 2b, but also includes indirectly lowering the temperature of the positive electrode 2b and the surrounding electrolyte 3 by lowering the temperature of the positive electrode 2b side of the container from the outside. Cooling means include a heat exchanger that lowers the temperature of the positive electrode 2b side of the container using a heat transfer medium, an air-cooled cooler that cools by blowing air, or the walls of pipes and structures, or the surface of a body.

[0047] The high temperature set by the heating means is the upper limit of a temperature range arbitrarily selected within the range in which a change in oxidation-reduction potential occurs, as shown in Figure 2. The constant temperature set by the cooling means is the lower limit of that temperature range.

[0048] The operation of thermochemical battery 1B is described below. The heating means raises the negative electrode 2a to a high temperature. The cooling means raises the positive electrode 2b to a low temperature. As a result, hydrate is generated in the electrolyte 3 on the positive electrode 2b side, and the hydrate is decomposed on the negative electrode 2a side. Thus, the concentration of electrolytes such as TBAF in the liquid phase decreases near the positive electrode 2b due to hydrate generation, and the concentration of electrolytes such as TBAF in the liquid phase increases near the negative electrode 2a due to hydrate decomposition. In other words, by creating a temperature difference between the negative electrode 2a and the positive electrode 2b, a difference in electrolyte concentration occurs in the liquid phase of the electrolyte 3 near the negative electrode 2a and the positive electrode 2b.

[0049] As described in Embodiment 1, the oxidation-reduction potential of the active material changes depending on the hydrate concentration (i.e., the concentration of the electrolyte in the liquid phase). Therefore, a reduction reaction occurs near the positive electrode 2b where the hydrate concentration is high (the concentration of the electrolyte in the liquid phase is low), and an oxidation reaction occurs near the negative electrode 2a where the hydrate concentration is low (the concentration of the electrolyte in the liquid phase is high). By maintaining the temperature so that the negative electrode 2a is at a high temperature and the positive electrode 2b is at a low temperature, current flows to the load 7. In this way, the thermochemical battery 1B can convert heat into electricity and supply it to the load 7.

[0050] According to the thermochemical cell 1B described above, since an electrolyte with an improved Seebeck coefficient is used, sufficient electromotive force can be provided even with a relatively small temperature difference. The negative electrode 2a corresponds to one electrode of the claim, and the positive electrode 2b corresponds to the other electrode of the claim.

[0051] <Embodiment 5> A thermoelectric conversion device using the electrolyte described in Embodiment 1 will now be described. Figure 8 is a schematic diagram of the thermoelectric conversion device of this embodiment.

[0052] As shown in Figure 8, the thermoelectric converter 1C comprises a battery cell 30, a supply and recovery means 40, and a temperature adjustment means (not shown).

[0053] The battery cell 30 has a cell container 31. The cell container 31 is partitioned by a separator 32, forming a positive electrode chamber 33 and a negative electrode chamber 34. The positive electrode chamber 33 houses the positive electrode 35, and the negative electrode chamber 34 houses the negative electrode 36. The positive electrode 35 and negative electrode 36 can be the positive and negative electrodes of a general battery. The separator 32 is designed to suppress the mixing of the electrolytes in the positive electrode chamber 33 and the negative electrode chamber 34 while ensuring ionic conductivity, and a general battery separator 32 can also be used. Examples include solid electrolytes, gel-like electrolytes, and porous membranes.

[0054] The supply and recovery means 40 consists of devices and equipment for supplying and recovering electrolyte to and from the battery cell 30. Specifically, the supply and recovery means 40 includes a high-temperature tank 11 (the first container of the claim), a low-temperature tank 12 (the second container of the claim), and first to fifth pipes 41 to 45 for forming a flow path through which the electrolyte flows. The first pipe 41 connects the high-temperature chamber 11 and the negative electrode chamber 34. The second pipe 42 connects the negative electrode chamber 34 and the low-temperature chamber 12. The third pipe 43 connects the low-temperature chamber 12 and the positive electrode chamber 33. The fourth pipe 44 connects the positive electrode chamber 33 and the high-temperature chamber 11. The fifth pipe 45 connects the low-temperature tank 12 and the high-temperature tank 11. Although not specifically shown in the figures, the supply and recovery means 40 has a pump. The electrolyte is circulated by the pump through the flow path from the first pipe 41 to the fourth pipe 44, in the order of high-temperature tank 11, negative electrode chamber 34, low-temperature tank 12, positive electrode chamber 33, and back to high-temperature tank 11.

[0055] The supply and recovery means 40 supplies electrolyte to the battery cell 30 and recovers the electrolyte from the battery cell 30, but does not supply hydrate to the battery cell 30. That is, the hydrate is separated from the liquid phase in the low-temperature chamber 12, and the phase containing hydrate at high density (hydrate phase) is not supplied to the battery cell 30 but is supplied to the high-temperature chamber 11 via the fifth pipe 45. The third pipe 43 transports the liquid phase, after being separated from the hydrate phase, from the low-temperature chamber 12 to the positive electrode chamber 33.

[0056] The temperature control means consists of a device capable of adjusting the temperature of the electrolyte. The temperature control device can be any device capable of arbitrarily setting the temperature within a range in which a change in the oxidation-reduction potential of the electrolyte is observed, as shown in Figure 2. Examples include electric heaters, heat exchangers, and air-cooled coolers that cool by blowing air. In this embodiment, the temperature control means is configured to heat the electrolyte inside the high-temperature bath 11 and release heat from the electrolyte inside the low-temperature bath 12. In other words, the electrolyte in the high-temperature bath 11 is at a higher temperature than the electrolyte in the low-temperature bath 12.

[0057] The operation of the thermoelectric converter with the above configuration will now be explained. [High temperature tank 11] In the high-temperature bath 11, the electrolyte is heated by a temperature control means. This causes the hydrate to decompose, increasing the TBAF concentration in the electrolyte. The increase in TBAF concentration causes the oxidation-reduction potential of the active material to change in the negative direction.

[0058] [Positive electrode chamber 33, positive electrode 35] An electrolyte with a relatively higher oxidation-reduction potential than the electrolyte in the negative electrode chamber 34 is supplied to the positive electrode chamber 33 from the low-temperature bath 12. As a result, a reduction reaction occurs at the positive electrode 35.

[0059] [Cryogenic chamber 12] In the low-temperature bath 12, the electrolyte is cooled by the temperature control means. As a result, some of the TBAF separates as hydrate, and the TBAF concentration in the electrolyte decreases. This decrease in TBAF concentration causes the oxidation-reduction potential of the active material to change in the positive direction.

[0060] [Negative electrode chamber 34, negative electrode 36] An electrolyte with a relatively lower oxidation-reduction potential than the electrolyte in the positive electrode chamber 33 is supplied to the negative electrode chamber 34 from the high-temperature bath 11. As a result, an oxidation reaction occurs in the negative electrode 36.

[0061] Such a thermoelectric converter can create a potential difference between the active material at the positive electrode 35 and the negative electrode 36 by heating and dissipating heat from the electrolyte in the high-temperature chamber 11 and the low-temperature chamber 12, and can pass an electric current through the load between the positive electrode 35 and the negative electrode 36. In other words, the thermoelectric converter functions as a battery that converts heat into electricity.

[0062] The example in Figure 8 is a thermoelectric conversion device that uses an electrolyte in which the TBAF concentration in the electrolyte increases as the temperature rises, causing the oxidation-reduction potential of the active material to change in the negative direction. When an active material is used in which the oxidation-reduction potential changes in the positive direction as the TBAF concentration increases, the relative positions of the high-temperature chamber 11 and the low-temperature chamber 12 are reversed.

[0063] According to the thermoelectric conversion device described above, electricity can be generated by heating and dissipating heat in the high-temperature chamber 11 and the low-temperature chamber 12. Furthermore, since an electrolyte with an improved Seebeck coefficient is used, sufficient electromotive force can be provided even with a relatively small temperature difference.

[0064] <Other Embodiments> While the above-described embodiments 2-5 illustrate thermoelectric conversion devices, the invention is not limited to such devices. For example, a thermoelectric sensor can be cited as an example of a thermoelectric conversion device. A thermoelectric sensor is a device that senses the temperature difference between electrodes. A highly sensitive temperature sensor can be obtained because it has a high Seebeck coefficient. [Explanation of symbols]

[0065] 1, 1A, 1B... Thermoelectric converter, thermochemical cell; 2a... One electrode (negative electrode); 2b... The other electrode (positive electrode); 3... Electrolyte; 4... Ion exchange membrane; 5... Active material; 6... Electrolyte; 7... Load; 8... Hydrate; 9... Power supply

Claims

1. An electrolyte comprising an aqueous solution containing an active material and an electrolyte, The electrolyte generates hydrate due to temperature changes, or the hydrate decomposes. The oxidation-reduction potential of the active material changes depending on the concentration of the hydrate. An electrolyte characterized by the following features.

2. In the electrolyte according to claim 1, The electrolyte comprises at least one of a quaternary ammonium salt and a quaternary phosphonium salt. An electrolyte characterized by the following features.

3. In the electrolyte according to claim 1, The active material is a hexacyanoferrate ion. An electrolyte characterized by the following features.

4. The electrolyte according to any one of claims 1 to 3, A pair of electrodes, A thermoelectric conversion device characterized by being equipped with the following features.

5. In the thermoelectric conversion device according to claim 4, The system includes a temperature control means capable of adjusting the temperature of the electrolyte, The electrolyte is in contact with one or both of the pair of electrodes. The temperature control means is capable of bringing the electrolyte to a relatively high temperature state and a relatively low temperature state. By inducing a discharge reaction in which electrons move from one electrode to the other at a high temperature, and then lowering the temperature, a discharge reaction in which electrons move from the other electrode to the first becomes possible. By inducing a discharge reaction in which electrons move from one electrode to the other at a low temperature, and then raising the temperature, a discharge reaction in which electrons move from one electrode to the other becomes possible. A thermoelectric conversion device characterized by the following features.

6. In the thermoelectric conversion device according to claim 4, A temperature control means capable of adjusting the temperature of the electrolyte, Equipped with a rechargeable power supply for a pair of electrodes, The electrolyte is in contact with one or both of the pair of electrodes. The temperature control means is capable of bringing the electrolyte to a relatively high temperature state and a relatively low temperature state. By inducing a charging reaction in which electrons move from one electrode to the other electrode using the power supply at a high temperature, and then lowering the temperature, a discharge reaction becomes possible in which electrons move from the other electrode to the first electrode at a higher voltage than the charging reaction. By inducing a discharge in which electrons move from one electrode to the other at a low temperature, followed by a high-temperature state, a charging reaction becomes possible in which electrons move from one electrode to the other at a voltage lower than the discharge voltage. A thermoelectric conversion device characterized by the following features.

7. In the thermoelectric conversion device according to claim 4, A heating means for heating one of a pair of electrodes, A cooling means for cooling the other of a pair of electrodes is provided. A thermoelectric conversion device characterized by the following features.

8. In the thermoelectric conversion device according to claim 4, Battery cell and A supply and recovery means for supplying and recovering the electrolyte to and from the battery cell, A temperature control means is provided, The aforementioned battery cell is A pair of electrodes, the positive electrode and the negative electrode, A positive electrode chamber for housing the positive electrode, A negative electrode chamber for housing the aforementioned negative electrode, It has a separator that separates the positive electrode chamber and the negative electrode chamber, The aforementioned supply and recovery means is The first container and The second container and The electrolyte has a flow path configured to circulate through the first container, the positive electrode chamber, the second container, the negative electrode chamber, and the first container. The aforementioned temperature control means is The electrolyte inside the first container is heated to a higher temperature than the electrolyte inside the second container. A thermoelectric conversion device characterized by the following features.

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