Processes for preparing 1-alkyl-3-alkyl-pyridinium bromide and uses thereof as additives in electrochemical cells

Abstract

The invention relates to the use of at least one 1-alkyl-3-alkyl-pyridinium halide, in particular 1-alkyl-3-methyl-pyridinium bromide, as an additive in bromine-generating electrochemical cells, such as zinc/bromine cells. Processes for preparing 1-alkyl-3-methyl-pyridinium bromide and concentrated aqueous solutions comprising same for use as additives in the aforementioned cells, are also disclosed.

Claims

1. An electrolyte solution suitable for use in bromine-generating electrochemical cells, comprising an aqueous solution of zinc bromide and a liquid complex composed of at least one 1-alkyl-3-methyl-pyridinium halide combined with one or more bromine molecules, wherein said alkyl at position 1 is a C3-C10 alkyl group which may be either straight-chain or branched, wherein said electrolyte solution is configured for circulation in a zinc/bromine cell.

2. The electrolyte solution according to claim 1, wherein the C3-C10 alkyl group is selected from the group consisting of n-propyl, n-butyl, n-pentyl and n-hexyl.

3. The electrolyte solution according to claim 2, comprising 1-n-butyl-3-methyl-pyridinium bromide.

4. The electrolyte solution according to claim 3, further comprising at least one compound selected from the group consisting of 1-n-propyl-3-methyl-pyridinium bromide, 1-n-pentyl-3-methyl-pyridinium bromide and 1-n-hexyl-3-methyl-pyridinium bromide.

5. The electrolyte solution according to claim 1, wherein the electrolyte solution further comprises one or more compounds of Formulas I, II or III: ##STR00019## wherein: in Formula (I), R.sub.3 is hydrogen or alkyl group and R.sub.4 is independently an alkyl group; in Formula (II), X is null, —CH.sub.2— or —O—, and R.sub.1 and R.sub.2 are independently alkyl groups, with at least one of R.sup.1 and R.sup.2 being an alkyl group comprising not less than three carbon atoms; and in Formula (III), R.sub.5 and R.sub.6 are independently an alkyl group.

6. The electrolyte solution according to claim 5, wherein the compound of Formula (I) is selected from the group consisting of N-alkyl pyridinium bromide and 1-alkyl-2-methyl pyridinium bromide; the compound of Formula (II) is selected from the group consisting of N-methyl-N-alkyl pyrrolidinium bromide, wherein said alkyl group attached to the pyrrolidinium ring comprises not less than four carbon atoms; and the compound of Formula (III) is 1-alkyl-3-methyl imidazolium bromide.

7. The electrolyte solution according to claim 6, wherein the compound of Formula (I) is selected from the group consisting of N-ethyl pyridinium bromide and 1-ethyl-2-methyl pyridinium bromide; the compound of Formula (II) is selected from the group consisting of N-methyl-N-butyl pyrrolidinium bromide, N-methyl-N-hexyl pyrrolidinium bromide and N-methyl-N-isooctyl pyrrolidinium bromide; and the compound of Formula (III) is 1-n-butyl-3-methyl imidazolium bromide.

8. The electrolyte solution according to claim 1, wherein the liquid complex does not solidify at a temperature above 0° C.

9. A method of operating a bromine-generating electrochemical cell which is a zinc/bromine cell, comprising adding to the electrolyte of said cell at least one 1-alkyl-3-methyl-pyridinium halide, wherein the alkyl at position 1 is a C3-C10 alkyl group, which may be either straight-chain or branched; and charging and/or discharging said cell.

10. The method according to claim 9, which further comprises adding to the electrolyte a compound of Formulas (I), (II) or (III): ##STR00020## wherein: in Formula (I), R.sub.3 is hydrogen or alkyl group and R.sub.4 is independently an alkyl group; in Formula (II), X is null, —CH.sub.2— or —O—, and R.sub.1 and R.sub.2 are independently alkyl groups, with at least one of R.sup.1 and R.sup.2 being an alkyl group comprising not less than three carbon atoms; and in Formula (III), R.sub.5 and R.sub.6 are independently an alkyl group.

11. The method according to claim 9, comprising forming a liquid complex composed of at least one 1-alkyl-3-methyl-pyridinium bromide combined with one or more bromine molecules, wherein said alkyl at position 1 is C3-C10 alkyl group which may be either straight-chain or branched, wherein the liquid complex does not solidify at a temperature above 0° C.

12. A process for preparing an aqueous solution of one or more 1-alkyl-3-methyl-pyridinium bromide, wherein the alkyl at position 1 is C3-C10 alkyl group, which may be either straight-chain or branched, comprising reacting 3-picoline and one or more bromoalkanes in a reaction vessel at a temperature above the melting point of the reaction mixture, in the absence of a solvent, combining the reaction product with water, wherein said reaction product consists essentially of 1-alkyl-3-methyl-pyridinium bromide in a liquid form, and recovering an aqueous solution of said 1-alkyl-3-methyl-pyridinium bromide.

13. The process according to claim 12, which does not involve the formation, isolation and purification of the 1-alkyl-3-methyl-pyridinium bromide in a solid form.

14. The process according to claim 13, wherein the solvent-free reaction mixture is heated to a temperature of not less than 70° C., maintaining the progressively formed 1-alkyl-3-methyl-pyridinium bromide in a liquid form and providing a stirrable reaction mass, and combining the liquid 1-alkyl-3-methyl-pyridinium bromide directly with water to form a clear aqueous solution.

15. A concentrated aqueous solution comprising at least one 1-alkyl-3-methyl-pyridinium bromide, wherein the alkyl at position 1 is C3-C10 alkyl group, which may be either straight-chain or branched, wherein the concentration of the solution is from 60 wt % to 90 wt %.

16. The concentrated aqueous solution according to claim 15, comprising at least one 1-alkyl-3-methyl-pyridinium bromide selected from the group consisting of 1-n-propyl-3-methyl-pyridinium bromide, 1-n-butyl-3-methyl-pyridinium bromide, 1-n-pentyl-3-methyl-pyridinium bromide and 1-n-hexyl-3-methyl-pyridinium bromide.

17. The concentrated aqueous solution according to claim 16, further comprising 1-ethyl-3-methyl pyridinium bromide.

18. The concentrated aqueous solution according to claim 15, wherein the 1-alkyl-3-methyl-pyridinium bromide was isolated in a non-solid form.

Description

EXAMPLES

Example 1

Preparation of 1,3-Dimethylpyridinium bromide (3-MMPy)

(1) ##STR00008##

(2) A three neck round bottom flask (1 L) was equipped with a magnetic stirrer, a thermocouple well, a dip tube connected to a methyl bromide generator (from SBr.sub.6 and methanol) and a condenser assembled with methyl bromide absorbing system. The flask was charged with 3-picoline (311 g), acetonitrile (207 g) and DIW (81 mL). The reaction mixture was cooled in an ice bath and bromomethane (gas) was fed during 1.6 hours. The volatiles were removed by rotavapor and the residue was diluted with small volume of DIW. Final product, 558.4 g, 87.9 weight % (argentometric titration); yield, 84%.

Example 2

Preparation of 1-n-propyl-3-methyl-pyridinium bromide (3-MPrPy)

(3) ##STR00009##

(4) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was purged with nitrogen during the whole procedure. The reactor was charged with 3-picoline (372 g) and heated to 80° C. 1-Bromopropane (500 g) was added drop-wise during 3 hours. The reaction mixture was heated at 80° C. for 1.5 hours. DIW (500 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor, 400 mL distillate).

(5) Another 500 mL DIW was added and the mixture was re-evaporated (550 mL distillate). Finally, the mixture was diluted with small volume of DIW. Final product, 1137 g, 73.1 weight % (argentometric titration); yield, 96%.

Example 3

Preparation of 1-n-butyl-3-methyl pyridinium bromide (3-MBPy)

(6) ##STR00010##

(7) A double surface reactor was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was purged with nitrogen during the whole procedure. The reactor was charged with 3-picoline (465 g) and heated to 79° C. n-Butyl bromide (718 g) was then added drop-wise during 4 hours. The reaction mixture was heated at 80° C. for 0.5 hours. DIW (500 mL) was added, the mixture was cooled and the volatiles evaporated in a rotavapor. Additional DIW (500 g) was added and the mixture was re-evaporated. DIW was added to correct dilution. Final product, 1431 g, 77.5 weight % (argentometric titration); yield, 96.5%.

Example 4

Preparation of 1-n-pentyl-3-methyl-pyridinium bromide (3-MPePy)

(8) ##STR00011##

(9) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was purged with nitrogen during the whole procedure. The reactor was charged with 3-picoline (370.5 g) and heated to 80° C. 1-Bromopentane (610 g) was added drop-wise during 2 hours. The reaction mixture was heated at 82-84° C. for 2.5 hours. DIW (500 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor). Another 500 mL DIW was added and the mixture was re-evaporated. Finally, the mixture was diluted with small volume of DIW. Final product, 1115 g, 82.7 weight % (argentometric titration); yield, 95%.

Example 5

Preparation of 1-n-hexyl-3-methyl-pyridinium bromide (3-MHePy)

(10) ##STR00012##

(11) A four neck round bottom flask (500 mL) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The flask was purged with nitrogen during the whole procedure. The flask was charged with 3-picoline (83 g) and heated to 80° C. 1-Bromohexane (150 g) was added drop-wise during 3 hours. The reaction mixture was heated at 87-90° C. for 2.5 hours. DIW (100 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor). Another 100 mL DIW was added and the mixture was re-evaporated. Finally, the mixture was diluted with small volume of DIW. Final product, 292 g, 77.1 weight % (argentometric titration); yield, 98%.

Example 6

Preparation of 1-iso-butyl-3-Methyl-pyridinium bromide (3-MiBuPy)

(12) ##STR00013##

(13) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was purged with nitrogen during the whole procedure. The reactor was charged with 3-picoline (422.7 g) and heated to 80° C. 1-Bromo-2-methylpropane (630 g) was added drop-wise during 3.5 hours. The reaction mixture was heated at 100° C. for 21 hours. DIW (200 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor). Another 500 mL DIW was added and the mixture was re-evaporated (510 mL distillate). Finally, the mixture was diluted with small volume of DIW. Final product, 1226 g, 74.1 weight % (argentometric titration); yield, 87%.

Example 7

Preparation of 1-iso-octyl-3-Methyl-pyridinium bromide (3-MiOcPy)

(14) ##STR00014##

(15) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was purged with nitrogen during the whole procedure. The reactor was charged with 3-picoline (237 g) and heated to 80° C. 3-(Bromomethyl)heptane (500 g) was added drop-wise during 1.5 hours. The reaction mixture was heated at 100-105° C. for 26 hours. DIW (300 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor, 325 g distillate). Another 300 mL DIW was added and the mixture was re-evaporated (450 g distillate). Finally, the mixture was diluted with small volume of DIW. Final product, 864 g, 65 weight % (argentometric titration); yield, 77%.

Example 8

Preparation of a mixture of 1-ethyl-3-methyl-pyridinium bromide and 1-n-butyl-3-methyl-pyridinium bromide (1:3)

(16) ##STR00015##

(17) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was purged with nitrogen during the whole procedure. The reactor was charged with 3-picoline (465.6 g) and 1-bromoethane (136.2 g). The reaction mixture was gradually heated to 87° C. during 1.3 hours. 1-Bromobutane (520.7 g) was added drop-wise during 4 hours. The mixture was further heated at 90° C. for 1.25 hours. DIW (754 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor). Finally, the mixture was diluted with small volume of DIW. Final product, 1367 g, 79.7 weight % (argentometric titration); yield, 97.7%.

Example 9

Preparation of a mixture of 1-ethyl-3-methyl pyridinium bromide and 1-n-butyl-3-methyl-pyridinium bromide (1:3)

(18) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was purged with nitrogen during the whole procedure. The dropping funnel was charged with a mixture of 1-bromoethane (136.2 g) and 1-bromobutane (520.7 g) and the reactor was charged with 3-picoline (465.6 g). The reactor was heated to 85° C. and the alkyl bromide mixture was added drop-wise via dip tube during 3.5 hours. The mixture was further heated at 88-90° C. for 3 hours. DIW (550 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor). Finally, the mixture was diluted with small volume of DIW. Final product, 1367 g, 79.7 weight % (argentometric titration); yield, 97.7%.

Examples 10-19

Zinc bromide electrolyte Solutions which Contain 1-alkyl-3-methyl pyridinium bromide

(19) In the next set of Examples, zinc bromide electrolyte solutions were prepared and tested, in order to demonstrate the ability of 1-alkyl-3-methyl pyridinium bromide compounds, either alone or in combinations with additional complexing agents, to form bromine-containing complexes in such solutions. To this end, 24 ml samples were prepared, with electrolyte compositions corresponding to distinct states of charge (SOC) defined by the concentrations of zinc bromide and elemental bromine. Each sample contains, in addition to the aqueous solution of zinc bromide and elemental bromine (which were present in the sample in suitable amounts as tabulated below, in order to match the state of charge investigated), also zinc chloride at a concentration of up to 0.5M and optionally potassium chloride at 1.0M concentration. The samples were stored at 25° C. for 24-hours after preparation before any measurement was conducted. One or more following properties of the samples were determined: the temperature at which a solid phase is formed in the electrolyte, free bromine concentration in the aqueous phase, solution conductivity and viscosity of the aqueous and organic phases, using the following methods:

(20) 1) The specific conductivity of the zinc bromide solutions containing the complexing agents was measured at room temperature after the addition of bromine to the samples using InnoLab 740 instrument with graphite conductivity cell.

(21) 2) The temperature at which the formation of a solid phase takes place in the electrolyte solution was determined by gradually cooling the samples from room temperature (RT, approximately 25° C.) to −5° C. The cooling regime was as follows: the temperature was decreased from RT down to 15° C. with a cooling rate of 0.2° C./min, and kept at 15° C. for 4 hours and so forth down to −5° C. At each of the following temperatures: 15° C., 10° C., 5° C., 0° C. and −5° C., the solution was maintained at a constant temperature for four hours. The cooling test was performed in polyethylene glycol solution, until the formation of crystals was observed.
3) The bromine concentration in the aqueous phase above the polybromide complex-oily phase was determined by a conventional iodometric titration technique. Each vial was sampled two times at room temperature.
4) The electrolyte solution was allowed to stand for 24 hours at 25° C. and then separated into aqueous and organic phases in a separating funnel for 2 hours at 25° C. The density of each phase was measured. Viscosity measurements were done with Zeitfuchs cross-arm viscometer and/or Cannon-Fenske opaque viscometer.

(22) In view of the fact that the composition of an electrolyte solution varies while the charge process is in progress (the amount of zinc bromide decreases while the amount of elemental bromine correspondingly increases), the utility of the additives under consideration was tested at different compositions which match different states of charge. In the experiments, the composition of the electrolyte solutions was adjusted to correspond to the beginning, middle and end of charge process (SOC of 0%, 50 and 100%, respectively). The letters A, B and C next to the Example's number indicate these three SOC that were investigated, respectively.

(23) In the first sub-set of Examples, different 1-alkyl-3-methyl-pyridinium bromides were tested separately as complexing agents for zinc/bromine cells. The results are tabulated in Table 1 (con. is the abbreviation for conductivity).

(24) TABLE-US-00002 TABLE 1 Physical state of % Br.sub.2, [additive] polybrornide % Br.sub.2, Con., Ex. SOC, ZnBr.sub.2(M) M additive [M] complex aq. mS/cm 10A 0 2.25 0.2 3-MPrPy 0.8M Liquid at 0° C. 0.10 128 10B 50 1.125 1.0 Liquid at 0° C. 0.14 139 10C 100 0.25 2.0 Liquid at 0° C. 0.09 121 11A 0 2.25 0.2 3-MBPy 0.8M Liquid at −5° C. 0.065 130 11B 50 1.125 1.0 Liquid at −5° C. 0.080 149 11C 100 0.25 2.0 Liquid at −5° C. 0.028 131 12A 0 2.25 0.2 3-MPePy 0.8M Liquid at 0° C. 0.02 131 12B 50 1.125 1.0 Liquid at 0° C. 0.36 140 12C 100 0.25 2.0 Liquid at 0° C. 0.07 122

(25) In the second sub-set of Examples, mixtures consisting of two different 1-alkyl-3-methyl-pyridinium bromides were tested as complexing agents for zinc/bromine cells. The results are tabulated in Table 2.

(26) TABLE-US-00003 TABLE 2 Physical state of % Br.sub.2, [additive] polybromide % Br.sub.2, Con, Ex. SOC, ZnBr.sub.2(M) M additive [M] complex aq. mS/cm 13A 0 2.25 0.2 3-MBPy/ 0.8M Liquid at −5° C. 0.051 122 13B 50 1.125 1.0 3-MEPy Liquid at −5° C. 0.070 143 13C 100 0.25 2.0 3:1 Liquid at −5° C. 0.027 129

(27) In the third sub-set of Examples, mixtures consisting of one 1-alkyl-3-methyl-pyridinium bromide and one 1-alkyl-2-methyl-pyridinium bromide were tested as complexing agents for zinc/bromine cells. The results are tabulated in Table 3.

(28) TABLE-US-00004 TABLE 3 Physical state of % Br.sub.2, [additive] polybromide % Br.sub.2, Con, Ex. SOC, ZnBr.sub.2(M) M additive [M] complex aq. mS/cm 14A 0 2.25 0.2 3-MBPy/ 0.8M Liquid at −5° C. 0.064 120 14B 50 1.125 1.0 2-MEPy Liquid at −5° C. 0.088 143 14C 100 0.25 2.0 3:1 Liquid at −5° C. 0.020 124

(29) In the fourth sub-set of Examples, the additive tested was a mixture consisting of 1-alkyl-3-methyl-pyridinium bromide together with a compound of Formula (I), i.e., N-alkyl pyridinium bromide. The results are tabulated in Table 4.

(30) TABLE-US-00005 TABLE 4 Physical state of % Br.sub.2, [additive] polybromide % Br.sub.2, Con, Ex. SOC, ZnBr.sub.2(M) M additive [M] complex aq. mS/cm 15A 0 2.25 0.2 3-MBPy/ 0.8M Liquid at −5° C. 0.056 115 15B 50 1.125 1.0 EPy Liquid at −5° C. 0.077 138 15C 100 0.25 2.0 3:1 Liquid at −5° C. 0.025 128

(31) In the fifth sub-set of Examples, the additive tested was a mixture consisting of 1-alkyl-3-methyl-pyridinium bromides together with a compound of Formula (I), i.e., N-alkyl pyridinium bromide and 1-alkyl-2-methyl pyridinium bromide, and the property measured was the viscosity of each of the separate phases. The results are tabulated in Table 5 (vis. is the abbreviation of viscosity).

(32) TABLE-US-00006 TABLE 5 Vis. of Vis. of Volumetric aqueous organic ratio % Br.sub.2, [additive] % Br.sub.2, phase phase aqu./org Ex. SOC , ZnBr.sub.2(M) M additive [M] aq. (cP) (cP) phases 16A 0 2.25 0.2 3-MBPy/ 0.8M 0.134 23.9 56.9 4.3 16B 50 1.125 1.0 EPy 0.251 17.2 18.7 2.6 16C 100 0.25 2.0 (1:1) 0.591 12.3 12.1 2.0 17A 0 2.25 0.2 3-MBPy/ 0.8M 0.093 23.3 57.8 2.9 17B 50 1.125 1.0 2-MEPy 0.183 15.8 21.7 2.3 17C 100 0.25 2.0 (1:1) 0.702 11.9 12.8 2.6

(33) In the sixth sub-set of Examples, the additive tested was a mixture consisting of 1-alkyl-3-methyl-pyridinium bromide together with a compound of Formula (III), i.e., 1-n-butyl-3-methyl imidazolium bromide (BMIBr). The results are tabulated in Table 6.

(34) TABLE-US-00007 TABLE 6 Physical state of % Br.sub.2, [additive] polybromide % Br.sub.2, Con, Ex. SOC, ZnBr.sub.2(M) M additive [M] complex aq. mS/cm 18A 0 2.25 0.2 BMIBr/ 0.8M Liquid at −5° C. 0.105 114 18B 50 1.125 1.0 3-MEPy Liquid at −5° C. 0.100 140 18C 100 0.25 2.0 3:1 Liquid at −5° C. 0.178 140 19A 0 2.25 0.2 BMIBr/ 0.8M Liquid at −5° C. 0.075 111 19B 50 1.125 1.0 3-MEPy Liquid at −5° C. 0.075 137 19C 100 0.25 2.0 1:1 Liquid at −5° C. 0.080 139

Examples 20-21

(35) An experimental set-up which is schematically illustrated in FIG. 2 was used to evaluate the effect of the presence of various bromine complexing agents on the efficacy of the operation of zinc/bromine membraneless cell. A characteristic property of the cell which was chosen for a quantitative study is the efficiency of zinc plating formed onto the anode surface, when the cell was charged at current density of 60 mA/cm.sup.2.

(36) During charge, zinc metal is increasingly formed on the anode and elemental bromine is increasingly generated in the electrolyte. In the set of experiments described below, various bromine-complexing agents were added to zinc bromide aqueous electrolyte which was recirculated in a membraneless electrochemical cell configuration during charge, and the bromine-complexing agents were tested for their ability to capture and hold the elemental bromine in the form of water-immiscible phase, minimizing the dissolution of elemental bromine in the aqueous phase of the electrolyte and correspondingly decreasing the direct chemical oxidation of the zinc by elemental bromine present in the aqueous phase. Thus, in membraneless cells, in the absence of physical membrane separating between the zinc and bromine electrodes, the plating efficiency of the zinc critically depends on the efficacy of the bromine-complexing agent.

(37) Experimental Set-Up

(38) Referring to FIG. 2, the experimental set-up comprises a pair of graphite electrodes 21 and 22 which serve as zinc and bromine electrodes, respectively. The electrode plates are made of compressed graphite particles, are rectangular in shape and are about 5 mm thick. The electrodes are mounted horizontally, in parallel with one another, and are spaced 2 mm apart. As shown in FIG. 2, the zinc electrode is placed on top of the bromine electrode. It is noted that no membrane is interposed in the space between the electrodes.

(39) Viton® gaskets 25 are applied onto the sides of the electrodes which face each other, i.e., the lower and upper faces of electrodes 21 and 22, respectively, are covered with the gasket, except for a central region which is left exposed on each of said electrodes faces. The non-coated central regions of the electrodes are hence available for the electrochemical reactions. The electrochemically-reactive central regions on the lower and upper faces of electrodes 21 and 22, respectively, coincide with one another with respect to position, geometric shape and size. Each of the two opposed electrochemically-reactive central regions has the shape of a square with an area of 10 cm.sup.2.

(40) A flow distributor provided in the form of a Teflon® frame corresponding in shape and size to the rectangular electrodes 21 and 22 is positioned in the space between said electrodes, such that the central open area of the frame coincides with the non-coated active regions of the electrodes with respect to position, geometric shape and size. FIG. 3 provides a top view of the relevant elements, i.e., the electrode plates 21, 22, Viton® gasket 25 and Teflon® flow distributor 26 which were used in the experimental set-up of FIG. 2. The electrode plates are perforated to allow the access and exit of electrolyte flow.

(41) The Compositions of the Tested Solutions

(42) The aqueous electrolyte solutions that were tested contain zinc bromide, elemental bromine and zinc chloride, the latter at a constant concentration of 0.4M. It should be noted that the electrolyte solutions prepared fall into two groups, A and B, which differ from one another in respect to the initial concentrations of the zinc bromide and elemental bromine:
[ZnBr.sub.2]=˜1.7 M, [Br.sub.2]=0.5 wt %  Group A
[ZnBr.sub.2]=˜1.0 M, [Br.sub.2]=˜1.0M.  Group B

(43) The composition of solutions of group A corresponds to a state of charge of 0%, i.e., it represents a composition of an electrolyte solution at the beginning of the charging process (a small amount of elemental bromine is present to avoid over potential). The composition of the solutions of group B is representative of a state of charge of 60%. During the experiments, while the electrolysis is in progress, the composition of the solutions gradually varies, with the concentrations of zinc bromide and elemental bromine decreasing and increasing, respectively, such that the final compositions of the solutions of groups A and B match states of charges of 25%-30% and -90%, respectively. Thus, the activity of the bromine-complexing agents (BCA) was investigated at two distinct “windows” of the cell charge: from 0 to 30% SOC (Group A), and from 60 to 90% SOC (Group B). The BCA tested were either 3-MBPy alone or a mixture of 3-MBPy and 3-MEPy (1:1 molar mixture).

(44) The Experiments

(45) All the experiments were carried out at room temperature, with the cell being charged at current density of 60 mA/cm2.

(46) Each experiment is run as follows. The electrolyte solution under study is held in a reservoir 23. The electrolyte volume is 90-100 ml (110-130 g). Peristaltic pump 24, operating at 30 rpm up to 100 rpm, drives the electrolyte solution through the cell, causing the solution to flow in the space between electrodes 21 and 22. The flow path of the electrolyte is schematically indicated by means of arrows in FIG. 2. The electrolyte solution is drawn from the upper (aqueous) part of the electrolyte volume and returned to the bottom of reservoir 23, where the dense (organic) phase accumulates. The experiments lasted about 4.5-5.5 hours. At the end of the experiment, the cell was opened and washed in water and NaHSO.sub.3 solution. The anode with Zn deposited thereon was washed several times with deionized water, dried and carefully removed and weighted, to determine the mass of zinc formed through the electrolysis. Plating efficiency was calculated as follows:

(47) $\mathrm{Plating}\mathrm{efficiency}=\frac{M}{\left(\frac{I*t}{F}\right)*\left(\frac{M_w}{z}\right)}*100$
M—mass of zinc deposited on the electrode
I—electrical current (0.6 A)
t—time during which the current passed through the cell (sec)
F—Faraday constant (96485 C/mol)
Mw—molecular weight (g/mol)
z—metal valence (2)

(48) The details and the results are tabulated in Table 7.

(49) TABLE-US-00008 TABLE 7 t ZnBr.sub.2 Br.sub.2 BCA [BCA] Plating Ex. (h) % SOC (M) (M) Additive M efficiency % 20A 5.5  0.fwdarw.30 ~1.7.fwdarw.1.2  0.2-0.6 3-MBPy 0.8M 94% 20B 5.0 60.fwdarw.92 ~1.0.fwdarw.0.14 1.0.fwdarw.1.6 97% 21A 4.5  0.fwdarw.25 ~1.7.fwdarw.1.28 0.2-0.5 3-MBPy + 3-MEPy 0.8M 89%

(50) The results in Table 7 demonstrate the efficacy of 3-MBPy and mixtures thereof in minimizing the amount of free bromine in the aqueous phase, thereby lessening the direct reaction between bromine in the electrolyte and the plated Zn (i.e., the undesired self-discharge), as shown by the high level of plating efficiency maintained in the presence of said additive.

Examples 22-23

Zinc bromide electrolyte Solutions which Contain N-methyl-N-isooctyl pyrrolidinium bromide

(51) To demonstrate the effect of mixtures of N-methyl-N-isooctyl pyrrolidinium bromide and 1-butyl 3-methyl imidazolium bromide together (at molar ratios of 1:3 and 1:1), 24 ml samples were prepared with electrolyte compositions corresponding to three distinct states of charge (SOC) defined by the concentrations of zinc bromide and elemental bromine. In addition to zinc bromide and elemental bromine (which were present in the samples in suitable amounts as set out in Table 8 below in order to match the state of charge investigated), each sample also contained zinc chloride and potassium chloride at constant concentrations of 0.5M and 1.0M, respectively. The samples were stored at 25° C. for 24-48 hours after preparation before any measurement was conducted. The samples were tested for one or more of the following properties: the temperature at which a solid phase is formed in the electrolyte, free bromine concentration in the aqueous phase and conductivity. The results are set out in Table 8.

(52) TABLE-US-00009 TABLE 8 Physical state of % Br.sub.2, [additive] polybromide % Br.sub.2, Con, Ex. SOC, ZnBr.sub.2(M) M additive [M] complex aq. mS/cm 22A 0 2.25 0.2 BMIBr/MiOP 0.8M Liquid at −5° C. <0.001 127 22B 50 1.125 1.0 3:1 Liquid at −5° C. 0.094 145 22C 100 0.25 2.0 Liquid at −5° C. 0.047 139 23A 0 2.25 0.2 BMIBr/MiOP 0.8M Liquid at −5° C. <0.001 125 23B 50 1.125 1.0 1:1 Liquid at −5° C. 0.055 148 23C 100 0.25 2.0 Liquid at −5° C. 0.062 145

(53) Table 8 illustrates that the mixed bromine-containing complex is highly effective, as it did not undergo solidification even at a temperature as low as −5° C., the amount of bromine measured in the aqueous phase is very low and the electrolyte solution exhibits good conductivity.

Examples 24 (Comparative) and 25 (of the Invention)

Zinc bromide electrolyte Solutions which Contain N-methyl-N-ethyl pyrrolidinium bromide and N-methyl-N-butyl pyrrolidinium bromide

(54) The procedures set forth in previous examples were repeated, but this time the complexing agent used was N-methyl-N-ethyl pyrrolidinium bromide (MEP; Example 24) and N-methyl-N-butyl pyrrolidinium bromide (MBP; Example 25). The results are given in Table 9.

(55) TABLE-US-00010 TABLE 9 Physical state of % Br.sub.2, [additive] polybromide % Br.sub.2, Con, Ex. SOC, ZnBr.sub.2(M) M additive [M] complex aq. mS/cm 24A 0 2.25 0.2 MEP 0.8M Liquid at 0° C. 0.44 88 24B 50 1.125 1.0 Soild at 0° C. 0.60 95 24C 100 0.25 2.0 Soild at 5° C. 0.98 90 25A 0 2.25 0.2 MBP 0.8M Liquid at −5° C. 0.07 68 25B 50 1.125 1.0 Liquid at −5° C. 0.12 111 25C 100 0.25 2.0 Liquid at −5° C. 0.65 90

(56) Table 9 illustrates that the bromine-containing complex based on MEP solidifies already at 5° C. and the conductivity of the electrolyte solution is relatively low, clearly indicating the inferiority of MEP in comparison to the additives of the invention. The amount of ‘free’ bromine in the aqueous phase is relatively high.

Preparations 1-3

Preparation of N-ethyl pyridinium bromide (EPy)

(57) ##STR00016##
1) Preparation of EPy in an Aqueous Medium:

(58) A stirred pressure reactor was equipped with a thermocouple well and a dosing pump. The reactor was charged with pyridine (450 g) and de-ionized water (DIW) (330 mL), sealed and heated to 95° C. Ethyl bromide (600 g) was continuously added during 1 hour; afterwards heating was continued for additional 1 hour. The reactor was cooled to ambient temperature, the pressure was released and distillation apparatus installed. The reaction mass was diluted with DIW (200 mL) and distilled under vacuum until 200 mL of distillate were collected. Final product: 1340 g; 72% w (argentometric titration); yield, 93%.

(59) 2) Preparation of EPy in Aqueous Medium:

(60) A stirred pressure reactor was equipped with a thermocouple well and a dosing pump. The reactor was charged with pyridine (475 g) and de-ionized water (DIW) (282 mL), sealed and heated to 95° C. Ethyl bromide (674 g) was continuously added during 1 hour; afterwards heating was continued for additional 1 hour. The reactor was cooled to ambient temperature, the pressure was released and distillation apparatus installed. The reaction mass was diluted with DIW (200 mL) and distilled under vacuum until 200 mL of distillate were collected. Final product: 1384 g; 77% w (argentometric titration); yield, 95%.

(61) 3) Preparation of EPy without a Solvent

(62) A stirred pressure reactor was equipped with a thermocouple well and a dosing pump. The reactor was charged with pyridine (475 g), sealed and heated to 90° C. Ethyl bromide (667 g) was continuously added during 1 hour; afterwards heating was continued for additional 1 hour. The reactor was cooled to ambient temperature, the pressure was released and distillation apparatus installed. Initial distillation was applied under vacuum for 15 minutes. The reaction mass was diluted with DIW (300 mL) and further distilled under vacuum until 150 mL of distillate were collected. Final product: 1230 g; 88% w (argentometric titration); yield, 96%.

Preparations 4-6

Preparation of N-ethyl-2-methyl pyridinium bromide (2-MEPy)

(63) ##STR00017##
4) Preparation of 2-MEPy in an Aqueous Medium

(64) A pressure reactor was equipped with a mechanical stirrer with a magnetic relay and a thermocouple well. The reactor was charged with 2-picoline (101.3 g) and de-ionized water (DIW) (20 mL), sealed and the mixture was heated to 92° C. Ethyl bromide (97.9 g) was slowly added during 3 hours, at 92-100° C. The mixture was heated at 94-100° C. for additional 2 hours, then cooled, and the pressure was released. The crude solution was diluted with DIW (24 mL) and excess 2-picoline was distilled-off as aqueous azeotrope, under reduced pressure. Finally, the residue was diluted with DIW. Final product: 251 g; 66.1 weight % (argentometric titration); yield, 91.5%.

(65) 5) Preparation of 2-MEPy in Acetonitrile as a Solvent

(66) A pressure reactor was equipped with a mechanical stirrer with a magnetic relay and a thermocouple well. The reactor was charged with 2-picoline (57.9 g), ethyl bromide (69 g) and acetonitrile (69 g). The reactor was sealed and the mixture heated to 97° C. Heating at 97° C. was continued for 6 hours. Distillation of the solvent was controlled by the upper valve of the reactor followed by vacuum distillation (without cooling). DIW (31 mL) was added to dissolve the crude mixture and vacuum was applied to remove residual acetonitrile. Finally, the solution was diluted with DIW (10.5 g). Final product: 149 g; 80.0 weight % (argentometric titration); yield, 95%.

(67) 6) Preparation of 2-MEPy with Excess of Ethyl Bromide

(68) A pressure reactor was equipped with a mechanical stirrer with a magnetic relay and a thermocouple well. The reactor was charged with 2-picoline (95 g) and ethyl bromide (145 g). The reactor was sealed and the mixture heated to 97° C. Heating at 97° C. was continued for 18 hours. Distillation of excess ethyl bromide was controlled by the upper valve of the reactor followed by vacuum distillation (without cooling). Finally, the solution was diluted with DIW (47 g). Final product: 250 g; 79.3 weight % (argentometric titration); yield, 96%.

Preparations 7-8

Preparation of N-methyl-N-isooctyl pyrrolidinium bromide (MiOP)

(69) ##STR00018##
7) Preparation of MiOP in Acetonitrile as a Solvent:

(70) A four neck round bottom flask was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The flask was purged with nitrogen during the whole procedure. The flask was charged with 1-methylpyrrolidine (49 g) and acetonitrile (63 g) and heated to 80° C. 2-Ethylhexylbromide (100 g) was added drop-wise during 1.5 hours. The reaction mixture was heated at 80-104° C. for 3 hours. The mixture was cooled and the volatiles evaporated in a rotavapor. DIW (100 mL) and evaporation was applied twice. Finally, DIW was added to correct dilution. Final product, 184 g, 63.1 weight % (argentometric titration); yield, 80.5%.

(71) 8) Preparation of MiOP without a Solvent:

(72) A four neck round bottom flask was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The flask was purged with nitrogen during the whole procedure. The flask was charged with 1-methylpyrrolidine (681 g) and 2-ethylhexylbromide (1400 g). The reaction mixture was heated to 100° C. during 2 hours and kept at that temperature for 4 hours. DIW (1 L) was added, the mixture was cooled and the volatiles evaporated in a rotavapor (900 mL distillate were collected). Additional DIW (200 mL) was added and the mixture was re-evaporated. DIW was added to correct dilution. Final product, 2551 g, 77.8 weight % (argentometric titration); yield, 98.4%.

(73) The entitled compound was identified as follows: M.P. by DSC: 85.6° C. (peak). .sup.1H NMR: (D.sub.2O, TMS) δ ppm 3.56-3.42 (4H, m), 3.29 (1H, dd, J.sub.1=14 Hz, J.sub.2=5 Hz), 3.25 (1H, dd, J.sub.1=14 Hz, J.sub.2=5 Hz), 3.03 (3H, s), 2.24-2.15 (4H, m), 1.90-1.82 (1H, m), 1.52-1.37 (4H, m), 1.33-1.25 (4H, m), 0.89 (3H, t, J=7.3 Hz), 0.87 (3H, t, J=7.1 Hz).