Additives for a flow battery

Abstract

The invention relates to an electrolyte solution suitable for use in a zinc-bromine battery, comprising zinc bromide and a mixture of at least two complexing agents selected from the group consisting of 1-R.sup.2-2-methyl pyridinium bromide and 1-R.sup.3-3-methyl pyridinium bromide salts, wherein each of R.sup.2 and R.sup.3 is independently an alkyl group having not less than five carbon atoms.

Claims

1. An electrolyte solution suitable for use in a zinc-bromine battery, comprising zinc bromide and a mixture of at least two complexing agents selected from the group consisting of 1-R.sup.2-2-methyl pyridinium bromide and 1-R.sup.3-3-methyl pyridinium bromide salts, wherein each of R.sup.2 and R.sup.3 is independently an alkyl group having not less than five carbon atoms.

2. The electrolyte solution according to claim 1, wherein R.sup.2 and R.sup.3 are independently selected from the group consisting of C5, C7 and C8 straight-chain and branched alkyl groups.

3. The electrolyte solution according to claim 2, wherein at least one of R.sup.2 and R.sup.3 is C5 alkyl group.

4. The electrolyte solution according to claim 1, wherein the mixture comprises two complexing agents selected from the group consisting of 1-R.sup.2-2-methyl pyridinium bromide and 1-R.sup.3-3-methyl pyridinium bromide salts, wherein the molar ratio between the components of the mixture is in the range from 1:5 to 5:1.

5. The electrolyte solution according to claim 3, comprising 1-n-pentyl-2-methyl pyridinium bromide (2-MPePy) and 1-n-pentyl-3-methyl pyridinium bromide (3-MPePy).

6. The electrolyte solution according to claim 3, comprising 1-n-pentyl-2-methyl pyridinium bromide (2-MPePy) and 1-isopentyl-3-methyl pyridinium bromide (3-MiPePy).

7. The electrolyte solution according to claim 3, comprising 1-n-pentyl-3-methyl pyridinium bromide (3-MPePy) and 1-isopentyl-3-methyl pyridinium bromide (3-MiPePy).

8. The electrolyte solution according to claim 1, which further comprises tetraalkyl ammonium halide salt.

9. The electrolyte solution according to claim 8, comprising 1-R.sup.2-2-methyl pyridinium bromide, 1-R.sup.3-3-methyl pyridinium bromide and a tetraalkyl ammonium halide salt.

10. The electrolyte solution according to claim 9, wherein R.sup.2 is C5 straight or branched alkyl; R.sup.3 is C7-C8 straight or branched alkyl; and the tetraalkyl ammonium salt is selected from the group consisting of tetraethyl ammonium bromide and tetrabutyl ammonium bromide.

11. The electrolyte solution according to claim 10, comprising 1-n-pentyl-2-methyl pyridinium bromide, 1-iso-octyl-3-methyl pyridinium bromide and tetraethyl ammonium bromide.

12. The electrolyte solution according to claim 8, comprising a first 1-R.sup.3-3-methyl pyridinium bromide, a second 1-R.sup.3-3-methyl pyridinium bromide and a tetraalkyl ammonium salt.

13. The electrolyte solution according to claim 12, wherein the first R.sup.3 is C5 straight or branched alkyl and the second R.sup.3 is C7-C8 straight or branched alkyl.

14. The electrolyte solution according to claim 13, comprising 1-n-pentyl-3-methyl pyridinium bromide, 1-iso-octyl-3-methyl pyridinium bromide and tetrabutyl ammonium bromide.

15. A method of using a mixture comprising at least two complexing agents selected from the group consisting of 1-R.sup.2-2-methyl pyridinium bromide and 1-R.sup.3-3-methyl pyridinium bromide salts, wherein each of R.sup.2 and R.sup.3 is independently an alkyl group having not less than five carbon atoms, said method comprising adding the mixture as an additive to the electrolyte solution of zinc bromine flow cells operating at a temperature above 35° C.

16. The method according to claim 15, wherein R.sup.2 and R.sup.3 are independently selected from the group consisting of C5, C7 and C8 normal and branched alkyl groups.

17. A method of operating a zinc-bromine battery, comprising adding to the electrolyte of said battery a mixture of at least two complexing agents selected from the group consisting of 1-R.sup.2-2-methyl pyridinium bromide and 1-R.sup.3-3-methyl pyridinium bromide salts, wherein each of R.sup.2 and R.sup.3 is independently an alkyl group having not less than five carbon atoms, and charging or discharging said cell at a temperature above 35° C.

18. The method according to claim 17, wherein R.sup.2 and R.sup.3 are independently selected from the group consisting of C5, C7 and C8 normal and branched alkyl groups.

Description

(1) In the drawings:

(2) FIG. 1 provides a schematic illustration of a zinc/bromine cell.

(3) FIG. 2 is a graph showing the concentration of elemental bromine dissolved in the aqueous phase in the electrolyte solution at 55° C. in the presence of the tested additive mixture, against the state of charge (SOC).

(4) FIG. 3 shows conductivity versus SOC curves for the additive mixtures of the invention.

(5) FIG. 4 is a graph showing the concentration of elemental bromine dissolved in the aqueous phase in the electrolyte solution at 55° C. in the presence of one preferred additive mixture (BCAm10), against the SOC. Results are also shown for the individual components.

(6) FIG. 5 is a graph showing the concentration of elemental bromine dissolved in the aqueous phase in the electrolyte solution at 55° C. in the presence of one preferred additive mixture (BCAm9), against the SOC. Results are also shown for one of the components.

EXAMPLES

Examples 1-5

Properties of Zinc Bromide Electrolyte Solutions at Elevated

Temperature (55° C.) in the Presence of Additives of the Invention

(7) To test the utility of different mixtures of bromine-complexing agents (BCA) in zinc-bromine batteries, 100 ml samples of zinc bromide electrolyte solutions were prepared, with varying amounts of zinc bromide and elemental bromine as tabulated in Table 1, to match different states of charge. Each sample contains, in addition to the aqueous solution of zinc bromide, elemental bromine and the BCA mixture at concentrations tabulated below, also zinc chloride at a concentration of ˜0.4M. The samples were stored under stirring at 55° C. for 24-48 hours after preparation before any measurement was conducted.

(8) The following properties of interest were measured at 55° C.:

(9) (i) the bromine concentration in the aqueous phase was determined by a conventional iodometric titration technique. Each vial was sampled two times.

(10) (ii) the viscosity of the complex containing-oily phase was measured using Cannon-Fenske Opaque Viscometer.

(11) (iii) the conductivity was measured using InoLab 740 conductivity meter with TetraCon 325 standard conductivity cell.

(12) The foregoing properties were measured for mixtures of additives under consideration at different compositions of the electrolyte solution, matching different states of charge (the end points and the midpoint of the SOC scale were investigated, i.e., three compositions corresponding to 0%, 50% and 100% SOC). The results are set out in Table 1.

(13) TABLE-US-00001 TABLE 1 [Free Br.sub.2] Viscosity BCA in the Of organic [1M aqueous phase phase Conductivity Ex. total] % SOC [ZnBr.sub.2] M [Br.sub.2] M (M) (cP) (mS/cm) 1A 3-MPePy + 0 1.6 0.5 0.0057 38 149 1B 3-MiPePy 50 0.9 1.1 0.0090 28 150 1C (1:1) 100 0.2 1.7 0.0195 17 130 BCAm3* 2A 2-MPePy + 0 1.6 0.5 0.0034 33 154 2B 3-MiPePy 50 0.9 1.1 0.0062 19 158 2C (1:1) 100 0.2 1.7 0.0144 12 133 BCAm9* 3A 2-MPePy + 0 1.6 0.5 0.0038 31 153 3B 3-MPePy 50 0.9 1.1 0.0070 14 157 3C (1:1) 100 0.2 1.7 0.0147 12 136 BCAm10* 4A 2-MPePy + 0 1.6 0.5 0.0050 91 149 4B 3-MiOPy + 50 0.9 1.1 0.0070 41 152 4C TEA 100 0.2 1.7 0.0092 17 139 (1:1:0.2) BCAm13* 5A 3-MPePy + 0 1.6 0.5 0.0017 28 137 5B 3-MiOPy + 50 0.9 1.1 0.0046 ND 157 5C TBP 100 0.2 1.7 0.0067 ND 141 (1:1:0.2) BCAm14*

(14) Some results are also shown graphically in FIGS. 2 and 3.

(15) The concentration of bromine dissolved in the aqueous phase at 55° C. in the presence of the tested additive mixture is plotted in FIG. 2. The abscissa indicates the composition of the electrolyte solutions, which correspond to the 0%, 50% and 100% states of charge, as set forth above. The results show that the tested mixtures display high complexing ability towards elemental bromine at high temperature, keeping the concentration of free bromine in the electrolyte solution low at 55° C.

(16) The concentration of the additive in the aqueous phase is low; this in turn leads to high conductivity of the solution (or lower resistance), as seen in the conductivity versus SOC curves which are plotted in the graph of FIG. 3.

Example 6

(17) To illustrate the advantage of a mixture of complexing agents over its separate components, the experimental procedure set forth above was used to produce the results shown in FIG. 4.

(18) The concentration of bromine dissolved in the aqueous electrolyte at 55° C. in the presence of the additive of Example 3 is plotted in FIG. 4. The additive is a mixture consisting of 1-n-pentyl-2-methyl pyridinium bromide and 1-n-pentyl-3-methyl pyridinium bromide (1:1). The abscissa indicates the compositions of the electrolyte solutions tested, which correspond to the 0%, 50% and 100% state of charge, as set forth above (therefore the curve represents the same information as in Table 3 for the mixture named “BCAm10”). The concentration of bromine dissolved in the aqueous electrolyte at 55° C. in the presence of each of the individual components is also plotted (the curves which correspond to 1-n-pentyl-2-methyl pyridinium bromide and 1-n-pentyl-3-methyl pyridinium bromide are marked with triangles and “X”, respectively). The results indicate that the mixture is more effective than the individual components in keeping the aqueous phase bromine concentration low at 55° C.

Example 7

(19) 1-iso-pentyl-3-methyl-pyridinium bromide, one of the components of the mixtures “BCAm3” and “BCAm9” tested in Examples 1 and 2, respectively, produces a thick gel or a solid phase when incorporated as a single additive into a zinc bromide electrolyte solution at high temperature. On the other hand, a mixture of 1-iso-pentyl-3-methyl-pyridinium bromide with either 1-n-pentyl-3-methyl pyridinium bromide (Example 1) or with 1-n-pentyl-2-methyl pyridinium bromide (Example 2), gives good results in that no solids are formed in the electrolyte solution and the concentration of the aqueous bromine is fairly low at a temperature of 55° C.

(20) In the graph of FIG. 5, which refers to the mixture named BCAm9, the abscissa indicates the composition of the electrolyte solutions tested corresponding to three different states of charge: discharged, half charge and full charge (therefore the curve marked with circles represents the same information as in Table 3 for the mixture BCAm9). In addition, the concentration of bromine dissolved in the aqueous electrolyte at 55° C. in the presence of a single compound, that is, 1-n-pentyl-2-methyl pyridinium bromide (marked with squares) is also plotted against the state of charge.

(21) The results indicate that the mixture is more effective than its individual components in keeping the aqueous phase bromine concentration low at 55° C. (as explained above, the second component of the mixture, i.e., 1-iso-pentyl-3-methyl-pyridinium bromide, solidifies under the experimental conditions).

Preparation 1

Preparation of 1-n-pentyl-3-methyl-pyridinium bromide

3-MPePy

(22) ##STR00002##

(23) 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%.

Preparation 2

Preparation of 1-iso-octyl-3-methyl-pyridinium bromide

3-MiOPy

(24) ##STR00003##

(25) 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%.

Preparation 3

Preparation of 1-iso-pentyl-3-methyl-pyridinium bromide

3-MiPePy

(26) ##STR00004##

(27) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was charged with 3-picoline (364.5 g) and heated to 90° C. iso-Bromopentane (1-bromo-3-methylbutane, 600 g) was added drop-wise during 5 hours. The reaction mixture was heated at 90-99° C. for 2.5 hours. DIW (50 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor). Additional DIW was added and the mixture was re-evaporated. Finally, the mixture was diluted with small volume of DIW. Final product, 1010 g, 91.6 weight % (argentometric titration); yield, 95.3%.

Preparation 4

Preparation of 1-pentyl-2-methyl-pyridinium bromide

2-MPePy

(28) ##STR00005##

(29) A double surface reactor (1 L) was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was charged with 2-picoline (303.7 g) and heated to 100° C. 1-Bromopentane (500 g) was added drop-wise during 6 hours. The reaction mixture was heated at 100-108° C. for 2.5 hours. DIW (100 mL) was added; the mixture was cooled and the volatiles were evaporated (rotavapor). Additional DIW was added and the mixture was re-evaporated. Finally, the mixture was diluted with small volume of DIW. Final product, 745 g, 90.8 weight % (argentometric titration); yield, 91.1%.