Humidity-variable battery

Abstract

To provide a humidity variable battery for suppressing drop in an output voltage over time and providing stable operation.SOLUTION: A humidity variable battery obtains electromotive force by utilizing humidity variations in environment. An aqueous electrolytic solution consisting of an aqueous solution of an ionic compound having deliquescence is provided together with an electrode on both sides of an ion permeable membrane made of fluorine resin. One first side is opened into environment and the other second side is cut off from environment on the ion permeable membrane. The aqueous electrolyte solution contains a water-soluble polymer to suppress permeation of water through the ion permeable membrane.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a humidity-variable battery that utilizes humidity fluctuations in the atmosphere, and particularly to a humidity-variable battery having a structure in which an aqueous electrolyte composed of a deliquescent material is provided with an ion-exchange membrane interposed therebetween.

Background Art

[0002] A humidity-variable battery is a battery that attempts to generate electricity by utilizing humidity fluctuations in the atmosphere, and is composed of an open tank open to the atmosphere, a closed tank sealed, an ion-exchange membrane separating both tanks, and electrodes. And, an aqueous electrolyte containing a deliquescent material is provided in the open tank and the closed tank, and the concentration of the electrolyte in the open tank changes according to the humidity in the air, and electricity is generated from the concentration difference generated between the open tank and the closed tank (Non-Patent Document 1).

[0003] By the way, in the humidity-variable battery having a structure in which an aqueous electrolyte composed of a deliquescent material is provided with an ion-exchange membrane interposed therebetween as described above, a fluororesin-based ion-exchange membrane made of a linear polymer and having high chemical stability can be used for the ion-exchange membrane.

[0004] For example, Patent Document 1 discloses a humidity-sensitive dew condensation element in which electrodes are provided with a fluororesin-based ion-exchange membrane having a trade name: Naflon N-110 manufactured by DuPont. In such an element, the electrical resistance value measured between the electrodes decreases monotonically as the relative humidity in the atmosphere increases, and the change from the electrical resistance value at 100% humidity to the electrical resistance value associated with dew condensation is rapid and large, and it is said that it can function as a switching element.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Non-Patent Documents

[0006]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] Humidity-variable batteries can generate electricity on-site by utilizing humidity variations in the environment and can omit the wiring for power transmission. Therefore, for example, as a power source for small sensors for IoT, it is expected to operate maintenance-free for a long period of time. On the other hand, a tendency has been observed that the output voltage from the humidity-variable battery decreases over time.

[0008] The present invention has been made in view of the above-described situation, and an object thereof is to provide a humidity-variable battery that suppresses a decrease in output voltage over time and provides stable operation.

Means for Solving the Problems

[0009] As a result of intensive studies by the inventors of the present application on the tendency of the output voltage to decrease over time in a humidity-variable battery, it has been found that it is caused by a decrease in the transport rate of the ion-exchange membrane and water movement through the ion-exchange membrane, leading to the present invention.

[0010] That is, the humidity-variable battery according to the present invention is a humidity-variable battery that obtains an electromotive force by utilizing humidity variations in the environment, and on both sides sandwiching an ion-permeable membrane made of a fluororesin, together with electrodes, an aqueous electrolyte solution composed of an ionic compound having deliquescence is provided. One first side is opened to the environment, and the other second side is blocked from the environment on the ion-permeable membrane. The aqueous electrolyte solution contains a water-soluble polymer, and is characterized in that the permeation of water through the ion-permeable membrane is suppressed.

[0011] According to such a feature, in the process of causing ions derived from an ionic compound to move on the first side and the second side through an ion permeable membrane due to humidity fluctuations in the environment to generate an ion concentration difference and extracting the generated electromotive force, the transport rate of ions through the ion permeable membrane can be improved, the voltage obtained as the electromotive force can be increased, and the decrease over time can be suppressed.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0013] Hereinafter, a humidity fluctuation battery which is an example according to the present invention will be described with reference to FIG. 1.

[0014] As shown in FIG. 1, in the humidity-variable battery 10, aqueous electrolytes 9a and 9b are provided on both sides sandwiching an ion-permeable membrane 3. As the ion-permeable membrane 3, a fluororesin which is a hydrophilic linear polymer having no crosslinked structure is used. The aqueous electrolyte 9a on one side sandwiching the ion-permeable membrane 3 is accommodated in an open tank 1 and communicates with the outside of the humidity-variable battery 10. The aqueous electrolyte 9b on the other side is accommodated in a closed tank 2 and is sealed from the outside. That is, the open tank 1 is open to the environment, and the closed tank 2 is blocked from the environment. Note that, since the open tank 1 only needs to be able to exchange moisture with the outside, a lid made of a film body or the like that allows water vapor to permeate but does not allow the aqueous electrolyte 9a to permeate may be provided. The outflow of the aqueous electrolyte 9a in the open tank 1 to the outside can be suppressed, and the handling of the humidity-variable battery 10 can be facilitated.

[0015] Further, the open tank 1 is provided with an electrode 4a which is in contact with the aqueous electrolyte 9a. Similarly, the closed tank 2 is provided with an electrode 4b which is in contact with the aqueous electrolyte 9b. The electrodes 4a and 4b are connected to wiring or the like for taking out the electromotive force generated between them to the outside.

[0016] The aqueous electrolytes 9a and 9b are aqueous electrolytes of ionic compounds having deliquescence properties. By communicating with the outside in the open tank 1, the aqueous electrolyte 9a absorbs or discharges moisture due to humidity fluctuations in the environment, thereby changing the ion concentration of the aqueous electrolyte 9a. Further, as the ionic compound having deliquescence properties, for example, halides such as chlorides and bromides can be preferably used. For example, when lithium chloride is used as the ionic compound, a relatively high voltage can be obtained relatively easily, which is suitable. Note that, when chlorides such as lithium chloride are used as the ionic compound, silver-silver chloride electrodes can be preferably used as the electrodes 4a and 4b. In this case, the internal resistance of the humidity-variable battery 10 can be made 10 ohms or less. Further, although details will be described later, the aqueous electrolytes 9a and 9b contain a water-soluble polymer.

[0017] Here, a cell for obtaining a humidity-variable battery 10 was formed by sequentially stacking a second plate body 12 having a recess for forming a closed tank 2, an ion-permeable membrane 3, and a first plate body 11 having a through-hole for forming an open tank 1. The space between the first plate body 11 and the second plate body 12 was sealed with a gasket 14 such as an adhesive, and the outer periphery of the ion-permeable membrane 3 was fixed to the first plate body 11 and the second plate body 12.

[0018] The humidity-variable battery 10 generates electricity as follows. For example, when a cation-exchange membrane is used as the ion-permeable membrane, the cation-exchange membrane has a porous structure and is provided with a large number of fixed charge groups having negative charges in the membrane. Due to the presence of the negative fixed charge groups, cations derived from ionic compounds can easily penetrate into the membrane, but the same anions cannot penetrate into the membrane due to electrostatic repulsive forces, and ion selective permeability is obtained.

[0019] When the humidity outside the humidity-variable battery 10 is lowered, the aqueous electrolyte 9a in the open tank 1 releases moisture into the environment, increasing the concentration of cations due to the dissolution of the ionic compound. Then, the cations permeate through the ion-permeable membrane 3 toward the aqueous electrolyte 9b in the relatively low-concentration closed tank 2 with the concentration difference as the driving force.

[0020] As a result, in the open tank 1, the anions in the aqueous electrolyte 9a become excessive compared to the cations and react with the electrode 4a to generate electrons in order to obtain equilibrium. On the other hand, in the closed tank 2, the anions in the aqueous electrolyte 9b become fewer than the cations, and while giving electrons to the electrode 4b, they are decomposed to generate anions. Thus, an electromotive force can be extracted in an external circuit connecting the electrode 4a and the electrode 4b. That is, the humidity variation in the environment gives a concentration difference of ions, i.e., a concentration difference of the electrolytes, to the aqueous electrolytes 9a and 9b, thereby obtaining an electromotive force. When the humidity outside the humidity-variable battery 10 is increased, conversely, cations permeate through the ion-permeable membrane 3 from the aqueous electrolyte 9b in the closed tank 2 toward the aqueous electrolyte 9a in the open tank 1, and an electromotive force in the reverse direction can be obtained.

[0021] When obtaining an electromotive force by utilizing the concentration difference of the electrolytic solution in this way, if the concentration difference of the electrolytic solution is reduced other than during the discharge as the operation of the battery, this is what is called self-discharge, which results in energy loss. Such self-discharge has also been confirmed in the humidity-variable battery. In the humidity-variable battery, the reduction of the concentration difference of the electrolytic solution that does not depend on the discharge as the operation of the battery mainly occurs due to the movement of water from the side with a lower concentration of ions in the aqueous electrolytic solution to the side with a higher concentration. That is, water permeates through the ion-permeable membrane due to osmotic pressure. As described above, the ion-permeable membrane has selective permeability to ions, but it cannot exert an electrostatic force on water molecules without a charge and allows movement through the porous pores. Therefore, by giving the ion-permeable membrane water permeability, the concentration difference of ions is reduced without depending on the discharge as the operation of the battery. As a result, there is a tendency for the output voltage from the humidity-variable battery to decrease over time.

[0022] Therefore, in the humidity-variable battery 10 of the present embodiment, a water-soluble polymer is included in the aqueous electrolytic solutions 9a and 9b as described above. The water-soluble polymer can suppress the permeation of water through the ion-permeable membrane. For example, it is presumed that the water-soluble polymer suppresses the permeation of water by increasing the apparent molecular weight of water by taking in water, increasing the viscosity of the aqueous electrolytic solution, or blocking the pores of the ion-permeable membrane with the water-soluble polymer. Therefore, it is considered preferable for the water-soluble polymer to have an appropriate size (molecular weight) corresponding to the pores of the ion-permeable membrane.

[0023] Furthermore, since the ion-permeable membrane 3 has selective permeability to ions by having a fixed charge, it is preferable that the water-soluble polymer does not affect the action of this charge. Specifically, it is preferably a polymer that is electrically neutral, rather than a polycation having a positive charge or a polyanion having a negative charge. As such a water-soluble polymer, for example, polyethylene glycols or crown ethers can be preferably used.

[0024] As described above, according to the humidity-variable battery 10, in the process of generating an ion concentration difference between the open tank 1 and the closed tank 2 by utilizing the humidity variation in the environment and extracting the electromotive force, the permeation of the water ion permeable membrane 3 can be suppressed, and the transport rate of ions derived from the ionic compound can be improved. As a result, the voltage obtained as the electromotive force can be increased, and the decrease over time can be suppressed.

[0025] [Manufacturing Test] The results of actually manufacturing such a humidity-variable battery and investigating its performance will be described with reference to FIGS. 2 to 5.

[0026] [Test 1] As shown in FIGS. 2(a) to 2(d), a humidity-variable battery using PEG (polyethylene glycol) as a water-soluble polymer was manufactured, and the change in the electromotive force obtained when the addition amount and the molecular weight were changed was investigated.

[0027] Specifically, Nafion (registered trademark) 117, which is a film made of a fluororesin, was used as the ion permeable membrane, and a 20% lithium chloride aqueous solution was used as the aqueous electrolyte. 0.75 mL of such an aqueous electrolyte was placed in each of the open tank and the closed tank, and a water-soluble polymer was added. The added water-soluble polymers were five types with average molecular weights of 200 (PEG200), 400 (PEG400), 1000 (PEG1000), 2000 (PEG2000), and 4000 (PEG4000), and the addition amounts were four types in mass %, 1%, 2%, 5%, and 10% (wt% in the figure has the same meaning as mass %). In this way, humidity-variable batteries were manufactured. These humidity-variable batteries were placed in a thermo-hygrostat chamber, and the open-circuit voltage was measured by applying a humidity variation that alternated between 30% and 90% every 4 hours.

[0028] At a molecular weight of 200, it was confirmed that it was almost equivalent to that without the addition of the water-soluble polymer measured separately. Also, as shown in the figure, at a molecular weight of 400 or more, an improvement in voltage was observed, and the larger the molecular weight, the greater the improvement in voltage. On the other hand, even when the addition amount of the water-soluble polymer was increased, the improvement width of the voltage was small. The effect of voltage improvement is considered to have been almost saturated already when the addition amount of the water-soluble polymer was 2% by mass.

[0029] [Test 2] As shown in Fig. 3, a humidity-variable battery was manufactured using (a) polyethylene glycol dimethyl ether (PEGDME500; average molecular weight: 500) and (b) 18-crown-6 ether (18-C-6; molecular weight: 264) as the water-soluble polymer, and the open-circuit voltage was measured in the same way. For both (a) and (b), 10% by mass of the water-soluble polymer was added to the water-soluble electrolyte. The others were the same as in Test 1. As a result, the open-circuit voltage in both cases was equal to or higher than that of PEG2000 and PEG4000 in Test 1. That is, for these polyethylene glycol derivatives, a high voltage improvement effect was obtained even at a molecular weight lower than that of polyethylene glycol.

[0030] [Test 3] As shown in Fig. 4, for (a) a material with 10% by mass of polyethylene glycol (PEG20000; average molecular weight 20000) added as a water-soluble polymer, (b) a material with 2% by mass of polyvinyl alcohol (PVA; average molecular weight 66000 - 79200) added, (c) a material with 3.5% by mass of poly(diallyldimethylammonium chloride) (PDDA; average molecular weight 100000 or less) added, and (d) a material with 10% by mass of polyacrylic acid (PAA; average molecular weight 5000) added, humidity-variable batteries were similarly manufactured and the open-circuit voltage was measured. However, the humidity variation was applied every 8 hours. While a voltage of 90 mV or more was obtained with PEG20000, only a voltage of about 20 mV was obtained with PDDA, which is a polycation, and PAA, which is a polyanion. Similarly, in PVA, which generally is not regarded as a polycation but has a large number of OH groups, only a voltage of about 20 mV was obtained. That is, it was considered preferable that the water-soluble polymer be an electrically neutral polymer in order to obtain a high voltage.

[0031] [Test 4] In the above test, the water-soluble polymer was changed, but here the test was conducted using a hydrocarbon-based cation exchange membrane, Neocept CSE (manufactured by Asahi Kasei Corporation), as the ion-permeable membrane. Otherwise, it was the same as Test 1.

[0032] As shown in Fig. 5, in both cases of (a) adding 10% by mass of PEG2000 and (b) adding 10% by mass of PEG4000 as the water-soluble polymer, only a voltage of about 30 mV was obtained. In the case of using a fluororesin for the ion-permeable membrane in Test 1, a voltage of about 80 - 90 mV was obtained, but the voltage obtained decreased because the ion-permeable membrane was a hydrocarbon-based cation exchange membrane. That is, it is useful to use a fluororesin as the ion-permeable membrane.

[0033] As described above, representative examples according to the present invention have been explained, but the present invention is not necessarily limited thereto, and those skilled in the art will be able to find various alternative examples and modification examples without departing from the gist of the present invention or the scope of the appended claims.

Description of Symbols

[0034] 1 Open tank 2 Closed tank 3 Ion-permeable membrane 4a, 4b Electrodes 9a, 9b Aqueous electrolytes 10 Humidity-variable battery

Claims

Claim 1 A humidity-variation battery that obtains electric power by utilizing humidity variations in the environment, comprising: on both sides sandwiching an ion-permeable membrane made of a fluororesin, providing an aqueous electrolyte solution composed of an ionic compound having deliquescence property together with electrodes, opening one first side to the environment, and blocking the other second side from the environment on the ion-permeable membrane, wherein the aqueous electrolyte solution contains a water-soluble polymer, and the humidity-variation battery is characterized in that permeation of water through the ion-permeable membrane is suppressed. Claim 2 The humidity-variation battery according to claim 1, wherein the water-soluble polymer is polyethylene glycols or crown ethers. Claim 3 The humidity-variation battery according to claim 1 or 2, wherein the ionic compound is a chloride. Claim 4 The humidity-variation battery according to claim 3, wherein the chloride is lithium chloride. Claim 5 The humidity-variation battery according to claim 3 or 4, wherein the electrode is a silver-silver chloride electrode. Claim 6 The humidity-variation battery according to any one of claims 3 to 5, characterized in that the internal resistance is 10 ohms or less.

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