The invention relates to a method for generating elemental bromine in electrolyte solutions used for operating metal- bromine cells, such as zinc-bromine batteries.

Zinc-bromine rechargeable cell contains two chemically non- reactive electrodes and a suitable separator located between the electrodes (e.g., an ion exchange membrane); The electrolyte used in the cell is an aqueous solution of zinc bromide, which is generally fed to the two compartments of the cell from two separate external reservoirs, utilizing a suitable circulation system. The term "anode" is used herein to indicate the electrode where metal zinc is formed (during charge) and oxidized (during discharge) . The term "cathode" is used herein to indicate the electrode where elemental bromine evolves (during charge) and reduced (during discharge) . The charge and discharge states of zinc-bromine battery will now be described in more detail.

During charge, an electric current is supplied to the cell from an external source, causing the deposition of zinc metal onto the anode and the concurrent generation of elemental bromine at the cathode, as shown by the following reaction :

Zn ²⁺ _(aq) + 2Br ^~ _(aq) → Zn, _s , + Br ₂₍ i,

The aqueous electrolyte solution which circulates through the cathodic side during the cell charge contains a complexing agent which is capable of readily forming a liquid phase upon complexing with elemental bromine. Thus, the elemental bromine generated at the cathodic side during cell charge reacts almost instantaneously with the complexing agent, to form an oily phase. The dense bromine- containing oily phase tends to settle at the bottom of the reservoir used for holding the catholyte. The recirculation of the bromine-containing medium is prevented using suitable mechanical means, thus allowing the accumulation of elemental bromine in the catholyte reservoir. In this way, bromine is produced and stored in a reservoir outside the electrode .

During discharge, the reverse chemical reaction takes place and an electric current is drawn from the cell. The bromine- containing liquid, which forms part of the catholyte, is brought to the cathodic side of the cell, while the anolyte is simultaneously circulated through the anodic side. This results in the dissolution of the zinc anode to give zinc ions and the reduction of elemental bromine to form bromide ions (and the generation of electrical current) . The chemical reaction is represented by the following equation:

Zn _(s , + Br ₂₍ ij → Zn < _aq ) + 2Br

Figure 1 provides a schematic illustration of an example of a zinc-bromine cell, wherein numerals la and lc indicate the anode and cathode, respectively, and numeral 2 represents the separator positioned between the electrodes. A reservoir for accommodating an aqueous solution of zinc bromide, used as the anolyte, is indicated by numeral 3a. Similarly, a reservoir 3c contains the catholyte, which consists of two liquid phases: an upper, aqueous solution of zinc bromide and a lower, dense organic phase comprising the elemental bromine in a form of a complex. The flow paths allowing the circulation of the anolyte and catholyte are respectively indicated by arrows (the streams are driven by pumps Pa, Pc) . A suitable valve (v) allows injection of bromine into the flow path of the catholyte on discharge only.

As explained in US 5,702,842, on cell discharge, zinc fragments may detach from the surface of the electrode. The presence of these zinc fragments in the electrolyte may interfere with the efficient operation of the cell (zinc will react with water to produce the undesirable hydrogen gas) . For this reason, it is proposed in US 5,702, 842 to introduce, at the end of the discharge process, bromine- containing electrolyte into the electrode space where zinc is deposited, namely, at .the anodic side, in order to chemically dissolve the undesired zinc fragments in the solution. The addition of a small amount of bromine to the anolyte or catholyte may serve other useful purposes, e.g., polishing the zinc surface of the anode or allowing full discharge of the cell, respectively.

Accordingly, the introduction of a small amount of bromine to the anolyte, the catholyte or both, e.g., between about 0.05% and 2%, and more specifically between 0.3% and 0.7% by w/w (relative to the weight of the anolyte or the catholyte) is considered to be beneficial. For example, a moderate capacity unit operating at lOOkW'h contains about one ton of an electrolyte solution, and therefore, a few kilograms of bromine are to be added to the anodic half-cell prior to charging. Similarly, for industrial units operating at 0.5-2 M ^■ h capacity, the initial amount of bromine reguired prior to starting a new unit charge cycle is up to 100 kg. However, elemental bromine is an easily volatile liquid with a strong, disagreeable odor an irritating effect. Therefore, the transportation and storage of elemental bromine must satisfy stringent requirements, and employing liquid bromine in populated areas requires the application of stringent safety measures and trained personal.

The present invention provides a safe method for generating elemental bromine in-situ in a bromide-containing electrolyte solution suitable for use in a metal bromine cell, and more specifically in a zinc bromine cell, which method comprises chemically oxidizing bromide (Br ^" ) in said electrolyte solution in an acidic environment, to produce elemental bromine. The in-situ generation of elemental bromine according to the method of the present invention may facilitate the operation of various zinc-bromine rechargeable cells, including the zinc bromine cell having separate streams of anolyte and catholyte circulating in the cell, as shown in Figure 1 (known as "flow battery") .

Thus, in another aspect, the present invention provides a method for operating a metal bromine cell (e.g., zinc bromine cell) containing an electrolyte solution, comprising generating elemental bromine in-situ by means of chemically oxidizing bromide (Br ^" ) in an acidic environment, thereby supplying elemental bromine to the electrolyte solution of said cell. The term "in-situ" refers to the situation in which the oxidation reaction takes place either in the electrolyte, or in a separate reaction vessel, followed by the addition of the bromine to the electrolyte. In either case, elemental bromine is supplied to the electrolyte. More specifically, the present invention provides a method for operating a zinc-bromine rechargeable cell having an anolyte and catholyte circulating therein, comprising generating elemental bromine in-situ by means of chemically oxidizing bromide (Br-) in an acidic environment, thereby supplying elemental bromine to said anolyte, catholyte or both, and charging or discharging the cell. Preferably, the elemental bromine is generated at a concentration in the range from 0.05 to 2.0% by weight relative to the weight of the anolyte, catholyte or both.

An electrolyte solution which is suitable for use according to the invention is an aqueous, concentrated solution of zinc bromide, as commonly employed for operating zinc bromine rechargeable batteries. The concentration of the zinc bromide in the aqueous electrolyte solution is not less than 1.0M, and preferably between 2.0 and 3.0M (prior to cell charge) . In addition to zinc bromide, the electrolyte solution may optionally contain one or more other halide salts, such as zinc chloride (zinc ions source usually 0.5M), sodium chloride or potassium chloride , and also sulfate salts (both are conductivity enhancers up to 3M) . The total concentration of these secondary water-soluble salts, which may be optionally present in the electrolyte solution, can be up to 3.5 M, e.g., between 0.5-3.5 M.

As already indicated before, the electrolyte solution further comprises at least one water soluble complexing agent which is capable of forming a liquid phase upon complexing with elemental bromine. Quaternary ammonium salts, especially halide salts and specifically bromide salts, are suitable for use as complexing agents. The cationic portion of said salts contains a nitrogen atom, which is bonded to four organic groups, (e.g., alkyl groups which may be the same or different). The tetracoordinate nitrogen may also be a member of a ring, namely, a heterocyclic ring, which heterocyclic ring may optionally contain a further heteroatom other than said tetracoordinate nitrogen. The cationic portion of said salts may also contain a positively charged nitrogen atom which is a member of a heteroaromatic ring. Tetra-alkyl ammonium bromides, and the bromide salts of Ν,Ν-dialkyl morpholinium, N,N-d.ialkyl pyrrolidinium and N-alkyl pyridinium salts are suitable for use in the method provided by the present invention, wherein the alkyl groups are C1-C7 straight or branched alkyl groups, which may be the same or different from one another. Specific examples of quaternary ammonium bromide salts include N-methyl-N-ethyl morpholinium bromide (MEM) , N- methyl-N-ethyl pyrrolidinium bromide (MEP) , or their mixtures. Other complexing agents, or mixtures thereof, may also be used. The concentration of the one or more complexing agents in the electrolyte solution may be in the range between 0.4 and 1.0 M.

A suitable electrolyte solution which may be used in zinc bromine batteries has the following composition: from 2.0 to 3.0 M ZnBr ₂ , from 0.5 to 1.0 M ZnCl ₂ and from 0.5 to 1.0 M total concentration of N-methyl-N-ethyl pyrrolidinium bromide (MEP) and N-methyl-N-ethyl morpholinium bromide (MEM) as the complexing agent. In addition, one or more water soluble salts may be present in the electrolyte solution at a concentration ranging from 0.5 to 3 M. The method according to the invention involves the chemical oxidation of bromide in the electrolyte solution in an acidic environment. Accordingly, a bromide source, an oxidant and an acid are combined in the electrolyte described above in order to accomplish the reaction.

Although the bromide ion is of course already available in the electrolyte solution in the form of the zinc bromide salt, an auxiliary bromide source may be added to the solution in order to supply the bromide. A useful auxiliary bromide source may be, for example, hydrobromic acid, which may be applied in the form of an aqueous solution (e.g., of 48% w/w concentration) . One or more water soluble bromide salts may also be used as the auxiliary bromide source. Suitable examples of such salts include - but are not limited to - sodium bromide (NaBr) , potassium bromide (KBr) and ammonium bromide (NH ₄ Br) . The salt is added to the electrolyte in an amount sufficient for generating the required concentration of elemental bromine. As noted above, this concentration is preferably from about 0.05 up to 2.0 percent by weight relative to the anolyte or catholyte weight. The weight concentration of the auxiliary bromide source (either the alkali or ammonium salt) added to the electrolyte solution is in the range between 0.5 to 10% relative to the weight of the anolyte or catholyte (the exact amount is dictated by the stoichiometry of the chemical reactions which are presented below) . If zinc bromide is used as the bromide source for the oxidation reaction, then a slight excess of said salt should be used over the amount intended for the normal operation of the cell . Useful oxidants include various peroxide compounds. For example, hydrogen peroxide can be used as an oxidation agent to produce bromine from bromide in acidic medium according to the following chemical equation:

H ₂ 0 ₂ + 2HBr → Br ₂ +2H ₂ 0 (1)

The amount of hydrogen peroxide in the electrolyte can be in the range between 0.1 and 0.3% (w/w relative to the anolyte or catholyte) , e.g., about 0.2% w/w. Hydrogen peroxide is commonly provided in the form of a commercially available 52% solution.

The peroxide of metals are also useful oxidizers in the method of the present invention. Zinc peroxide (Zn0 ₂ ) has been found to be especially useful in the oxidation of bromide to form elemental bromine in the electrolyte solution of a zinc bromine cell. The oxidation reaction proceeds rather smoothly, exhibiting a moderate exothermic profile, which can be conveniently controlled. In addition, the use of zinc peroxide as the oxidation agent results in the in-situ formation of zinc bromide as a by-product in the electrolytic solution. Zinc peroxide may be also provided in a form of a mixture with zinc oxide (ZnO); the mixture Zn0 ₂ /ZnO is commercially available (e.g., from Aldrich) . The aforementioned oxidizers may be used in the electrolyte solution in the following weight concentration ranges: from 0.1 to 5% of Zn0 ₂ , e.g., about 0.3%, or from 0.2 to 10.0% of Zn0 ₂ /ZnO (about 1:1 mixture or any mixture compositions), e.g., about 0.6% of said mixture. The relevant chemical reactions are as follows:

Zn0 ₂ + 2HBr → ZnBr ₂ + H ₂ 0 ₂

H ₂ 0 ₂ + 2HBr -> Br ₂ + 2H ₂ 0 (2)

ZnO + 2HBr → ZnBr ₂ + 2H ₂ 0

Another class of utilizable oxidants includes bromate salts. In its most general form, the chemical oxidation of bromide using bromate as an oxidizing agent in an acidic environment is represented by the following chemical equation (3) :

BrO, - (aq) + 5 Br (aq) + 6 H ⁺ (aq) - 3 Br ₂ (aq) + 3 H ₂ 0 (I) (3)

Bromate salts which can be used as oxidizing agents in the practice of the present invention may be selected from the group consisting of potassium bromate (KBr0 ₃ ) , sodium bromate (NaBrC>3) and zinc bromate (Zn(Br03) ₂ ) . For the case in which one of the aforementioned bromate salts is used as the oxidizing agent in conjunction with hydrobromic acid as the bromide source, then the general equation (3) is reduced to the following specific forms:

KBr0 ₃ + 6HBr → 3Br ₂ + KBr + 3H ₂ 0 (3a)

NaBr0 ₃ + 6HBr -» 3Br ₂ + NaBr + 3H ₂ 0 (3b)

Zn(Br0 ₃ ) ₂ + 12HBr → 6Br ₂ + ZnBr ₂ + 6H ₂ 0 (3c)

The weight concentration of the bromate salt oxidizer in the electrolyte solution can be in the following ranges: from 0.1 to 5% KBr0 ₃ , e.g., about 0.2%; from 0.1 to 10% NaBr0 ₃ , e.g., about 0.3%; or from 0.1 to 10% Zn(Br0 ₃ ) ₂ , e.g., about 0.3%. Other useful oxidants include hypohalites. Specific hypohalite salts which can be used as oxidizing agents in the practice of the present invention may be selected from the group consisting of hypochlorites, e.g., NaClO.

As already noted above, the chemical oxidation of the bromide ion to generate elemental bromine is carried out in an acidic environment. The pH of the electrolyte solution is preferably adjusted within the range between 1.5 and 3.5, more preferably between 2.3 and 3.3, using either a monoprotic or a polyprotic acid (e.g., HBr, HCl, H ₂ S0 ₄ ) or a mixture thereof. Hydrohalide acid, especially HBr, is preferred. Of course, if an acid other than HBr is used, then an auxiliary bromide source, in the form of a bromide salt, is added to the electrolyte. For example, in the specific case where bromate is used as the oxidizer, and both the bromide and bromate reactants are provided in the form of their alkali salts, the general equation (3) reduces to the following form (4):

5MBr + MBr0 ₃ + nH _p A → 3Br ₂ + nM _p A + 3H ₂ 0 (4) wherein M represents a cation of an alkali metal, A is the anion of the acid, and the product of the coefficients n and p equals 6. The reaction by-product is the salt M _P A.

Having fed the oxidizer, the acid, and optionally an auxiliary bromide source into the electrolyte solution according to the combinations and quantities set forth above, the oxidation reaction proceeds at room temperature (in the range between 20 and 30°C) under stirring, and the desired amount of elemental bromine is generally formed after 1 to 24 hours. The measurement of the bromine content of the electrolyte solution can be carried out using acceptable titration techniques. For example, the reaction mixture may be periodically sampled and subjected to iodometric titration. Spectroscopy techniques may also be employed for monitoring the progress of the reaction and for measuring the amount of bromine formed, since the absorption of the reaction mixture correlates nicely with the concentration of bromine. Thus, calibration solutions containing different concentrations of elemental bromine can be prepared, against which the absorption of a sample taken from the reaction mixture is compared. Absorption spectroscopy can be used for low bromine concentration solutions, up to 1.5% w/w. At bromine concentrations higher than 1.5% iodometric titration can be used.

It should be understood that the method for generating elemental bromine provided by the present invention may be carried out in bromide-containing electrolytes used in various metal-bromine cell, e.g., vanadium-bromine cell, and is not limited to zinc bromine cells.

Furthermore, the method of the present invention may be used for the in-situ generation of elemental bromine at the discharge or charge state of various zinc-bromine batteries utilizing flowing electrolyte, including batteries arranged in the form of serially connected bipolar electrodes (a stack arrangement, in which a plurality of bipolar electrodes and separators interposed therebetween are positioned between two terminal electrodes is described, for example, in US 4,615,108). Once the desired bromine level is reached at the anolyte or catholyte by employing the method of the invention, the battery may be subsequently charged or discharged according to methods known in the art (e.g., US 5,459,390 and US 6,036,937).

Examples

Preparation 1

An electrolyte solution was prepared by charging into an Erlenmeyer flask the following ingredients:

1) Zinc bromide brine (672 g of 76% w/w aqueous ZnBr2 solution, commercially available from ICL-IP) .

2) Zinc chloride brine (74 g of 50% w/w aqueous ZnC12 solution, commercially available from ICL-IP) .

3) MEP (commercially available from ICL-IP as 65% w/w aqueous solution) ; the concentration of the MEP in the electrolyte solution was 0.5M.

4) MEM (commercially available from ICL-IP as 65% w/w aqueous solution) ; the concentration of the MEP in the electrolyte solution was 0.5M.

4) An aqueous solution of hydrogen bromide (about 7-8 g of 48% w/w solution, commercially available from ICL-IP.

5) Deionized water, up to 1000 g.

The procedure set forth above was repeated to produce electrolyte solutions which were used in the following examples. The pH values of the resultant electrolyte solutions were about 2.4 (±0.5). Example 1

Bromide source and acid: HBr

Oxidizer: H ₂ 0 ₂

To 1000 g of an electrolyte solution containing ZnBr ₂ , ZnCl ₂ , HBr and MEP/MEM mixture in an Erlenmeyer flask, were added consecutively under stirring at 22°C hydrobromic acid (20 g of an aqueous 48% HBr solution) and hydrogen peroxide (3.28 g of 52% H ₂ 0 ₂ solution). The electrolyte solution was stirred in the closed flask at said temperature for 24 hours. The electrolyte solution was sampled in order to measure the bromine concentration at 2, 4, 18 and 24 hours after the beginning of the reaction. The concentration of bromine, which was measured in the electrolyte solution 24 hour after the beginning of the reaction, was about 0.8% (w/w) . The measurements were carried out using either UV-vis absorption spectroscopy (against a calibration graph) or iodometric titration.

Example 2

Bromide source and acid: HBr

Oxidizer: zinc peroxide (as Zn0 ₂ /ZnO mixture)

To 1000 g of an electrolyte solution containing ZnBr ₂ , ZnCl ₂ , HBr and MEP/MEM in an Erlenmeyer flask, were added consecutively under stirring at 22°C hydrobromic acid (7 g of an aqueous 48% HBr solution) and a 1:1 mixture of Zn0 ₂ /ZnO (1.3 g) . The electrolyte solution was stirred in the closed flask at said temperature for four hours. The electrolyte solution was sampled 1, 2, and 4 hours after the beginning of the reaction in order to measure the concentration of elemental bromine. The bromine content of the electrolyte solution after four hours was -0.1% (measured using the techniques set forth in Example 1).

Example 3

Bromide source and acid: HBr

Oxidizer: potassium bromate

To 1000 g of an electrolyte solution containing ZnBr ₂ , ZnCl ₂ , HBr and EP /MEM in an Erlenmeyer flask, were added consecutively under stirring at 22°C hydrobromic acid (63 g of an aqueous 48% HBr solution) and potassium bromate (9.5 g) . The electrolyte solution was stirred in the closed flask at said temperature for four hours. Samples of the electrolyte solution were taken to analysis in order to measure the bromine concentration 1, 2, and 4 hours after the beginning of the reaction. Four hours after the initiation of the reaction, the concentration of the bromine in the electrolyte solution was about 2.5% (w/w) , as determined by the techniques set forth in Example 1.

Example 4

Operation of Zinc-bromine rechargeable cell Bromide source and acid: HBr

Oxidizer: zinc peroxide (as Zn0 ₂ /ZnO mixture)

To 300g of an electrolyte solution in a catholyte reservoir (3c in Figure 1) of a circulated (0.5 to 3 ml/sec) zinc bromine cell containing 2.25M ZnBr ₂ , 0.5M ZnCl ₂ , 1M KC1 and 0.8M MEP, were added consecutively under stirring at 25°C (at the middle of discharge state) 5.5 g 1:1 mixture of Zn0 ₂ /ZnO (Aldrich) and 34 g hydrobromic acid (aqueous 49.5% HBr solution) . The electrolyte solution was stirred in the catholyte reservoir at said temperature for four hours. During the period of bromine formation, the cell was under no load. Samples of the electrolyte solution were periodically analyzed in order to measure the bromine concentration before the reaction and 1, 2, and 4 hours after the beginning of the reaction. Four hours after the initiation of the reaction, the concentration of the bromine in the aqueous phase of catholyte solution was 1.14% (w/w) , as determined by the techniques set forth in Example 1. After four hours, the cell discharge was restarted .