JPS6313310B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION This invention relates to zinc-bromine storage battery systems containing bromine anodes and to improved techniques for reversibly complexing bromine in these systems. [Prior Art] Conceptually, the zinc-bromine storage battery can be considered the simplest electrochemical system.
Among them, only zinc, bromine and zinc bromide actively participate in the basic cell reactions. In its most basic form, a rechargeable zinc-bromine storage battery can be constructed of two inert, electronically conductive electrode substrates and an aqueous solution of zinc bromide. Thus, the charge/discharge cycle involves electrodeposition of zinc at the cathode and subsequent dissociation of the zinc and liberation of bromine into the aqueous solution at the anode, followed by its reduction. This inherent simplicity, along with the fact that bielectrode reactions occur with good electrochemical reversibility, was recognized rather early on. This early incentive led to the development of simpler alternatives to lead-acid batteries. Strong commercial interest in this system is now driven by the need for large scale batteries for use in off-peak energy storage. The requirements for this application are quite different from those for other reversible battery applications.
The most important criteria that these batteries must meet are basic cost, cycle life, safety efficiency and maintainability. To compete in this load averaging application, storage batteries face very stringent cost and cycle life requirements. Early research into zinc-bromine battery systems pinpointed two major problems with the system. First of all, this type of system has a relatively high self-discharge rate (i.e., low Coulombic efficiency), which is due to the reaction of zinc electrodepositing in a tree pattern, which eventually ,
This will cause a short circuit in the battery. Subsequent attempts using the zinc-bromine system were focused on developing solutions to the two problems mentioned above. Most of these efforts have been devoted to and expanded upon various attempts to improve the Coulombic efficiency by lowering the concentration of free bromine in the zinc bromide aqueous electrolyte. In early attempts, for example, in U.S. Patent No. 1006494,
The use of porous carbon as an anode structure has been proposed. The usefulness of this technique was limited by the extremely large amounts of carbon required to absorb the bromine and the material's reluctance to liberate bromine for cell discharge. Further research has involved the use of complexing agents incorporated into the electrode structure. The use of quaternary ammonium bromide, such as tetramethylammonium bromide, is described in US Pat. No. 2,566,114. Such materials have a maximum level of bromination Br ₉
A series of polybromides are produced. The level of bromination here refers to the level of bromination of the quaternary ammonium salt.
It is expressed as twice the number of moles of Br ₂ present per mole plus one. In other words, four bromine molecules can be used per molecule of organic compound. Varying levels of bromination produce both solid and liquid polybromides, making it difficult to retain complexed compounds in the electrode structure and resulting in excessive electrode polarization. It is then claimed that the performance of bromine electrodes can be significantly improved if only a single solid bromine complex is formed in the electrode structure, as described in U.S. Pat. No. 3,738,870. . In order to arrive at a single solid bromine complex, it is clearly necessary to add bromine extremely slowly to a solution containing bromine ions and a fine suspension of particulate complexing agent. Rapid addition of bromine therefore results in the formation of what is called a mixed polybromide-bromine mass. More recently, aprotic dipoles in electrolytes such as propylene carbonate have been studied.
dipole) with selected quaternary ammonium bromides results in the formation of insoluble polybromide oils over a range of levels of bromination. This is described in US Pat. No. 3,816,177. More recently, U.S. Pat.
A number of compounds are disclosed that are added to complex the halogen. The said U.S. No. 4,038,459 patent is
A variety of alcohols and nitriles are disclosed that form insoluble oil complexes with halogens, and these compounds may also contain a tetraalkylammonium component. Said U.S. No.
The '4038460 patent shows a variety of ethers that complex with halogens, which may also contain a tetraalkylammonium component, and these compounds complex with halogens in an insoluble oil. form a compound. A major drawback of these alcohols, ethers, and nitriles is that in the presence of bromine and active bromine/bromide electrodes, these compounds rapidly degrade chemically and electrochemically, which makes them makes the compound completely unsuitable for use in zinc-bromine systems. [Problems to be Solved by the Invention] The present invention solves the problems of conventional zinc-bromine batteries, namely the formation of solid or highly viscous polybromide.
This method is intended to eliminate problems such as no deterioration of the complexing agent and the need to add an additional complexing agent. Therefore, the main object of the present invention is to provide a zinc-bromine storage battery system characterized by a relatively high coulombic efficiency. In connection with this objective, and for more specific purposes, a reliable means of storing bromine is provided in order to maintain the bromine concentration in the electrolyte at an acceptable level. Another object is to provide complexing agents for bromine that form water-insoluble, low viscosity oils over a significant range of levels of bromination. Yet another object of the present invention is to provide a bromine complexing agent that is stable in the environment of zinc-bromine storage batteries. Yet another object of the present invention is to provide a complexing agent for bromine that forms a polybromide oil of sufficiently low viscosity that the transfer of bromine between the oil and the electrolyte is at a satisfactory rate. The goal is to make it possible to do so. A further object of the invention is to provide a zinc-bromine solution which is less corrosive to the system provided by the invention and thus allows the use of a wide variety of materials in the construction of battery storage systems of this type. The aim is to provide storage batteries. Yet another object of the present invention is to provide a battery system with less potential toxicity due to the lower bromine activity achieved. Other objects and advantages of the invention will become apparent from the description below. [Means and effects for solving the problems] The zinc-bromine battery of the present invention includes: 1. A zinc-bromine battery comprising a bromine electrode, an aqueous electrolyte, and a bromine complexing agent, wherein the bromine complexing agent a quaternary ammonium salt having an asymmetric quaternary tetraalkyl moiety selected from ethyltrimethyl, diethyldimethyl, ethyldimethylpropyl, dimethyldipropyl, methyltriethyl, diethylmethylpropyl, and ethylmethyldipropyl, and The asymmetric tetraalkyl quaternary compound contains one member selected from a mixture of diethyldipropyl quaternary ammonium salt and tetraethyl quaternary ammonium salt, and has a bromination level of at least 3 to 7. The ammonium salt and bromine directly form a liquid polybromide oil, and this liquid oil has a viscosity of about 30 centipoise or less in the temperature range in which the battery is used. be. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof will now be described in detail. However, it is not the intention to limit the invention to the particular forms disclosed below, but rather all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined in the claims. It covers the following. Thus, for example, although the present invention has been described primarily with respect to a zinc-bromine rechargeable battery or battery system, it is understood that the present invention has equal application to any system in which a bromine anode is used. You should understand. As one illustration, the present invention has been used in a variety of redox systems. The present invention may simply be utilized as a facile technique to complex bromine for purposes outside the field of electrochemistry, such as storage of bromine prior to transport, etc. Still further, the invention will be described in connection with the use of certain tetraalkylammonium compounds. However, while the use of such ammonium compounds is preferred, it should be understood that similar phosphonium or sulfonium compounds may also be used.
Of course, the use of such alternative compounds must meet the criteria for ammonium compounds described herein. In general, the present invention provides that certain asymmetric tetraalkylammonium salts form polybromide oils that are essentially insoluble in aqueous solution and that bromine transfer between an electrode and the oil is effective. The compound (oil) has a sufficiently low viscosity such that it can be carried out at a satisfactory rate, is stable in the environment of a zinc-bromine storage battery, and, in addition, It is based on the discovery that zinc bromide has properties suitable for reaching and maintaining the bromine concentration in an aqueous electrolyte at a desired value. The particular salts used and the method of producing the polybromide oil will be discussed as the description progresses. The general construction of the particular battery used is well known. That is, basically,
The cell includes an anode (bromine electrode), a cathode (zinc electrode), a separator therebetween, and a suitable electrolyte. During charging, zinc metal is plated onto the surface of the cathode substrate and remains there until discharged. On the other hand, the anode is a redox electrode. The separator used must generally be able to fulfill two functions. The separator must be capable of minimizing the rate of bromine migration from the anode to the cathode. Furthermore, the material selected for the separator must be chemically inert in the battery environment and have low resistance to the passage of ions. A variety of microporous cation-exchange materials may be utilized and are known in the art. As one illustrative example, a conventional Daramic microporous polyethylene separator (WRGrace) has been found suitable. To minimize the formation of zinc dendrites,
Preferably, a battery system using some type of electrolyte circulation system is used. The electrolyte on the cathode side of the separator must normally have a much lower bromine concentration than the electrolyte in the anode side, so it can be provided with two separate annular channels. desirable. Generally, in this type of system, the electrolyte is in constant circulation, which creates multiple paths through the battery during a single charge-discharge cycle. Any technique may be used for the complexation of bromine to form an oil in accordance with the present invention, but with external storage of the complexing agent and access to the liquid. Preferably, a liquid contacting device is used. In this format, bromine is distributed between the electrolyte and the bromine oil formed. The electrolyte solution, which has been freed of excess bromine by the oil formed according to the invention, may be sent to a reservoir or pumped back to the cell. The complexing agent used in accordance with the present invention consists of asymmetric tetraalkyl quaternary ammonium salts that form a liquid polybromide oil on contact with bromine over a significant and continuous range of levels of bromination. . The salts used must therefore form a liquid oil at the operating conditions encountered at levels of bromination of at least 3 to 7, preferably 3 to 9, and at the operating temperature of the cell ( For zinc-bromine cells, forming a liquid oil having a viscosity of less than about 30 centipoises, preferably less than about 20 centipoises, and most preferably less than 10 to 15 centipoises at temperatures below about 50 to 60°C. It has to be something. In the present invention, it has been discovered that over the desired level range of bromination, a single complexing agent, including certain asymmetric quaternary tetraalkylammonium salts, and bromine directly form a liquid polybromide oil. . The formation of this liquid polybromide oil does not require the addition of any additional complexing agent in addition to the asymmetric tetraalkylammonium salt described above. These salts and the resulting polybromide oils have the necessary requirements for use in zinc-bromine battery systems and may find particular utility when electrolyte circulation is used. has been done. Asymmetric tetraalkyl moieties that form liquids at room temperature (e.g. 23°C) with levels of bromination ranging from 3 to 9 are diethyldimethyl, ethyltrimethyl, ethyldimethylpropyl, diethylmethylpropyl, ethylmethyldipropyl. , and dimethyldipropyl.
Additionally, methyltriethylammonium salts can also be used as they form polybromide oils at bromination levels ranging from 3 to 7. Further, a mixture of tetraethylammonium salt and diethyldipropylammonium salt is also effective in the present invention. The bromine partition achieved with the aforementioned complexing agents is satisfactory and this can be achieved by using a single complexing agent as described above. However, it may be desirable in some cases to increase the bromine concentration in the electrolyte, and more usually to decrease it. Of course, this can be achieved to some extent by using a complexing agent consisting of a mixture of two or more of the asymmetric ammonium salts. However, in addition to an additional feature of the invention, by forming a mixture of one or more asymmetric ammonium salts as described above and a suitable amount of a co-complexing agent,
The bromine allocation may be effectively changed. This extends the possibility of obtaining the desired result by using a mixture containing any of the tetraalkylammonium groups, whether or not it produces the required liquid oil by itself. There is. over the required range of bromination levels.
The small number of such components associated with seven species that form liquid oils on their own illustrates the versatility of this study. Furthermore, according to another feature of the invention, the polybromide oils described herein may be combined with salts of the tetraalkylammonium component that do not by themselves form satisfactory liquid polybromide oils. They may also be formed by using mixtures with specific ammonium salts of the invention. The flexibility involved here is clear. In either embodiment, when a salt (or salts) of an asymmetric tetraalkylammonium component is used which itself forms a liquid polybromide oil, in a mixture with the other salts or salts used. Of course, when used in certain amounts, a liquid polybromide oil is formed. The suitable amount can then be most easily determined by simply complexing the bromine and ascertaining the bromine distribution, concentration and desired mixture properties. It should be noted that in all embodiments, the tetraalkylammonium moiety used need not be unsubstituted. Accordingly, the term "tetraalkyl" as used herein should be considered to include both substituted and unsubstituted moieties. Any substituents must make the material stable in the desired environment. Thus chlorine and bromine can be used as substituents on the tetraalkyl moiety. The viscosity of the polybromide oil phase in zinc-bromine storage battery systems designed for high efficiency is a critical factor in the selection of the compound or compounds for complexing the bromine. an aqueous electrolyte phase;
In the transfer of bromine to and from the polybromide oil phase, a rate-limiting step usually occurs in the oil phase due to its high viscosity. This is also clear from the equation defining the mass transfer coefficient in liquid-liquid extraction operations. Under normal operating conditions, the mass transfer coefficient for the oil phase becomes small, similar to that for the aqueous phase, as the viscosity of the oil phase approaches zero. Therefore, the performance of zinc-bromine accumulators (under given operating conditions) is significantly better the lower the viscosity of the oil phase. A more important consideration with respect to the viscosity of the oil phase is the power required to achieve effective mixing of the oil and water phases. For example, in a contact tower in which an oil phase is circulated by a pump, the pump power required to keep the flow rate of the oil phase constant is linearly proportional to the viscosity of the oil. For higher viscosity oils, more power is required to operate the pump, resulting in lower overall efficiency of the battery. It has therefore been found that it is highly preferable to maintain the viscosity of the oil phase below 10 to 15 centipoise over the range of operating temperatures of the battery or accumulator. As the viscosity rises above 30 centipoise, performance decreases significantly resulting in low efficiency. As the viscosity increases further, performance deteriorates further, sometimes reaching a point where the accumulator or battery is barely operational. In the present invention, the importance of low viscosity oils can be similarly recognized when considering the functions involved in the charge-discharge cycle. Therefore, in the charging phase, an increase in the viscosity of the oil phase will excessively reduce the rate of bromine migration into the oil phase, causing self-discharge and thus reducing the coulombic efficiency. On the other hand, in the discharge phase, the transfer of bromine from the oil phase to the electrolyte is too slow, resulting in a constant voltage not being maintained. Regarding the salt used, conceptually any anion can be used that does not oxidize at a lower potential than bromide and does not form insoluble species in the electrochemical environment. Although the use of brominated compounds is preferred for this purpose, chlorinated compounds can be used as well.
Furthermore, when the environment does not contain potassium ions,
The salt may be in the form of perchlorate. The polybromide oils of the present invention may be prepared in any desired manner. For example, this oil can be dissolved in water with the desired bromide or other salt;
It may then be prepared by adding at least the same molar amount of liquid bromine. This oil forms immediately and is collected with a pipette at the bottom of the container or with a separator funnel. Otherwise, if desired, bromine may be added directly to the salt or to an aqueous solution containing the salt. The maximum yield is usually
Obtained when an oil forms in water. Organic complexation of bromine allows independent control of its aqueous phase concentration. The relative amounts of oil or salt to be used can be determined by calculating backwards from the bromine distribution to the polybromide oil having the highest bromination level desired for the application. Depending on the number of moles of bromine produced in the particular size cell being used and the level of bromination of the oil or salt desired for use at the start of operation, the oil or
The amount of salt to be added is defined. Therefore, if the bromine distribution and other properties afforded by an oil with a bromination level of 7 are desired to be the end goal, and theoretically 10 moles of bromine are produced, then , at bromination level 3, 5 moles of oil should be used. To determine a specific concentration range, consider keeping the bromine concentration as low as possible.
It is therefore desirable to minimize the degree of material degradation caused by added bromine and, of course, to increase the coulombic efficiency. On the other hand, a lower operational limit arises from the requirement for mass transfer of dissolved bromine to the anode substrate during discharge. As is recognized, slowing this migration results in unacceptable concentration polarization. Similarly, the bromine concentration on the cathode side of the separator must be kept very low (eg, about 0.005 molar), thereby minimizing non-Falade zinc-bromine reactions. According to Fyck's law of diffusion, the transfer of bromine from the anode side to the cathode side depends on the diffusion coefficient of bromine in the separator and also on the relative magnitude of the bromine concentration at each pole side, i.e., the concentration gradient. Depends on. In theory, the diffusion coefficient of bromine in a separator is zero, but in practice each particular type of separator has a limit to the concentration gradient that it can see before allowing excessive bromine flux. have. For this reason, there is a clear operational upper limit for the bromine concentration on the anode side, based on the performance of the separator. A 1 molar aqueous zinc bromide electrolyte is porous,
When used with a flow through electrode, it is preferred to maintain the bromine concentration in the range of about 5 to 15 grams per liter of electrolyte solution, with 5 to 10 grams being most preferred. This type of electrode is generally preferred as it maximizes Coulombic efficiency. however,
When flow-through electrodes are not used, the bromine concentration in the electrolyte, which is held constant, must necessarily increase due to differences in mass transfer properties. Generally, the pH of the electrolyte is kept within certain limits,
It will thereby be found desirable to provide satisfactory performance at the cathode. Thus, as is already known, at pHs above about 4, the zinc plated during the charging phase is somewhat mossy and does not adhere very well. The resulting low adhesion restricts electrolyte flow and
Similarly, the coulombic efficiency is lowered. However, on the other hand, at very low pH (eg, below 1), acid corrosion of the zinc plate poses problems and also reduces the coulombic efficiency. For these reasons, it is preferable to utilize a moderate pH in the range of 1 to 3, as this range provides a strongly adherent galvanizing. Producing polybromide oils over any or preferred range of bromination levels6
The density and viscosity properties of the oils produced by using the seed complexing agents are shown in Tables 1 and 2 below.

【table】

TABLE As can be seen from the table above, the liquid oil produced has a sufficiently low viscosity to provide the necessary transfer properties of bromine between the oil and the electrolyte. Table 3 below shows the oils in equilibrium with the present invention.
This shows the concentration of bromine dissolved in a 1.0M zinc bromide electrolyte.

[Table] As can be seen from the above table, the various oils obtained from the asymmetric tetraalkyl quaternary ammonium salts of the present invention achieve sufficiently low bromine concentrations in the electrolyte in an equilibrium state. As can be seen from the above, activated zinc-bromine batteries, especially cells of the size required for load-balancing applications, generate heat and thus
requires operation at temperatures up to 60°C, or
Operations are carried out at such temperatures. Tables 4 to 6 show the properties of the ethyltrimethyl component in the temperature range 25°C to 60°C.

【table】

【table】

Table: As is evident from the table above, the properties of this species at elevated temperatures are satisfactory. The significant decrease in viscosity as temperature increases is particularly important since the rate of bromine migration will increase. Tables 7 through 9 show the properties of the diethyldimethyl component over a temperature range of 25°C to 60°C.

【table】

【table】

〔Example〕

The following examples are merely illustrative of the use of the invention and are not intended to limit it. EXAMPLE 1 This example describes a rechargeable 32 watt hour zinc-
This figure shows the use of ethyltrimethylammonium-bromine sp. as a complexing agent in a bromine storage battery. The cathode substrate is a flat, non-porous titanium plate.
The anode substrate is a flat, porous sheet of titanium catalytically activated with a ruthenium-containing coating. A microporous Daramik separator (WRGrace) was used, consisting of a flat sheet with vertical ribs at predetermined intervals on each side to form flow passages for the electrolyte. The composition of the electrolyte in a fully discharged state is as follows. Zinc bromide concentration 1.5 mol Potassium chloride concentration 4.0 mol Lead chloride concentration 8.0×10 ^-4 mol PH 1.0-3.0 The bromine oil at full discharge condition is 57 grams of ethyltrimethylammonium-bromine oil,
It contains 51 grams of bromine and has a bromination level of 3. During charging, the amount of complexed bromine and the total volume of the oil increases. The battery performed satisfactorily when subjected to the charge-discharge cycles described below. (a) Charging at 3 amperes (galvanic static electricity) for 2 hours. The charging voltage should initially be about 1.90 volts at 27°C, and rise by about 50 millivolts during charging. (b) Leave the circuit open for 15 minutes. (c) Discharge the battery with 3 amps (galvanic static) until the voltage drops sharply, and (d) short circuit the battery for 30 minutes. Example 2 This example demonstrates the formation of a liquid polybromide oil by using a mixture of two salts, each of which forms a solid complex with bromine by itself. A mixture of equal molar amounts of tetraethylammonium bromide and diethyldipropylammonium bromide, approx.
0.02 mol with 100ml of 1 mol _ZnBr2 solution
ml beaker. Four additions of 0.02 mole each of bromine were made. The solution was stirred to equilibrate between the additives. The results shown in Table 10 compare the properties of equivalent molar mixtures with those obtained using each salt separately.

〔Effect of the invention〕

As can be seen from the foregoing, the present invention is stable in chemical and electrochemical environments and sufficiently increases coulombic efficiency by reducing and maintaining the level of bromine at a desired concentration. This provides a single complexing agent that is highly complex. Over a useful range of bromination levels, certain complexing agents of the present invention function as polybromide oils formed to remain as low viscosity liquids, ranging from liquids to solids, depending on the level of bromination. It provides a wide degree of freedom in operation without the fear of insufficient functionality as is the case when using complexing agents that change to . The oil of the invention has a relatively low viscosity, which is important in order to achieve a sufficiently rapid transfer of bromine between the oil phase and the aqueous phase.

Claims (1)

[Scope of Claims] 1. A zinc-bromine battery comprising a bromine electrode, an aqueous electrolyte, and a bromine complexing agent, wherein the bromine complexing agent is ethyltrimethyl, diethyldimethyl, ethyldimethylpropyl, dimethyldipropyl. , a quaternary ammonium salt having an asymmetric tetraalkyl component selected from methyltriethyl, diethylmethylpropyl, and ethylmethyldipropyl, and a mixture of diethyldipropyl quaternary ammonium salt and tetraethyl quaternary ammonium salt. The asymmetric tetraalkyl quaternary ammonium salt and bromine directly form a liquid polybromide oil containing at least one selected member and having a bromination level of at least 3 to 7. , a zinc-bromine battery characterized in that the liquid oil has a viscosity of about 30 centipoise or less in the temperature range in which the battery is used. 2. The cell of claim 1, wherein the complexing agent is liquid at a level of bromination of 3 to 9. 3. The battery according to claim 1, wherein the quaternary tetraalkyl component is an ethyltrimethyl group. 4. The battery according to claim 1, wherein the quaternary component is a diethyldimethyl group. 5. The battery of claim 1, wherein the liquid oil has a viscosity of about 20 centipoise or less. 6. The battery of claim 5, wherein the liquid oil has a viscosity of about 15 centipoise or less.