Additives for zinc-bromine membraneless flow cells

Abstract

The invention relates to the use of nitrogen-containing compounds belonging to the classes of N-alkyl pyridinium halide, N-alkyl-2-alkyl pyridinium halide and 1-alkyl-3-alkyl imidazolium halide, as additives in electrolyte solutions for zinc bromine membraneless flow cells. The invention also provides electrolyte solutions comprising such additives and processes for operating said cells.

Claims

1. An energy storage device comprising at least one electrochemical cell and an electrolyte recirculation system, wherein the at least one electrochemical cell contains spaced apart zinc and bromine electrodes, mounted in parallel with one another, wherein the space between the electrodes is devoid of a membrane or separator, where the energy storage device further comprises a recirculating electrolyte suitable for use in a zinc bromine membraneless flow cell, said electrolyte comprising aqueous zinc bromide solution and a liquid complex composed of bromine-complexing agent combined with one or more bromine molecules, wherein the bromine-complexing agent is selected from the group consisting of: (i) N-alkyl pyridinium halide; (ii) N-alkyl-2-alkyl pyridinium halide; (iii)1-alkyl-3-alkyl imidazolium halide; and mixtures thereof, wherein the alkyl groups in each of compounds (ii) and (iii) may be the same or different.

2. An energy storage device according to claim 1, wherein the bromine-complexing agent is selected from the group consisting of: (i) N-ethyl pyridinium bromide; (ii) N-ethyl-2-methyl pyridinium bromide; (iii)1-butyl-3-methyl imidazolium bromide; and mixtures thereof.

3. An energy storage device according to claim 1, wherein the electrolyte comprises a mixture of 1-alkyl-3-alkyl imidazolium halide and at least one of N-alkyl pyridinium halide and N-alkyl-2-alkyl pyridinium halide.

4. An energy storage device according to claim 3, wherein the electrolyte comprises a mixture of 1-alkyl-3-methyl imidazolium bromide and at least one of N-alkyl pyridinium bromide and N-alkyl-2-methyl pyridinium bromide.

5. An energy storage device according to claim 4, wherein the electrolyte comprises a mixture of 1-butyl-3-methyl imidazolium bromide and at least one of N- ethyl pyridinium bromide and N-ethyl-2-methyl pyridinium bromide.

6. A process of operating a zinc bromine membraneless flow cell, comprising adding to an electrolyte of said cell at least one bromine-complexing agent selected from the group consisting of: (i) N-alkyl pyridinium halide; (ii) N-alkyl-2-alkyl pyridinium halide; and (iii) 1-alkyl-3-alkyl imidazolium halide, wherein the alkyl groups in each of compounds (ii) and (iii) may be the same or different; and charging and/or discharging said cell.

7. A process according to claim 6, wherein the at least one bromine compelxing agent is selected from the group consisting of: (i) N-ethyl pyridinium bromide; (ii) N-ethyl-2-methyl pyridinium bromide; and (iii) 1-butyl 3-methyl imidazolium bromide.

8. A process according to claim 6, wherein a mixture of bromine complexing agents is added to the cell, said mixture comprising 1-alkyl-3-alkyl imidazolium halide and at least one of N-alkyl pyridinium halide and N-alkyl-2-alkyl pyridinium halide.

9. A process according to claim 8, wherein the mixture comprises 1-alkyl-3-methyl imidazolium bromide and at least one of N-alkyl pyridinium bromide and N-alkyl-2-methyl pyridinium bromide.

10. A process according to claim 9, wherein the mixture comprises 1-butyl 3-methyl imidazolium bromide and at least one of N-ethyl pyridinium bromide and N-ethyl-2-methyl pyridinium bromide.

Description

IN THE DRAWINGS

(1) FIG. 1 schematically illustrates the structure of a conventional zinc/bromine cell with a membrane positioned in the space between the electrodes.

(2) FIG. 2 schematically illustrates an experimental set-up representative of a membraneless cell configuration.

(3) FIG. 3 provides a top view of some of the elements used in the experimental set-up of FIG. 2.

EXAMPLES

(4) Methods

(5) 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. 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 vapor pressure above the electrolyte solutions containing the complexing agents was measured at 20-26° C. according to “Vapor pressures of bromine-quaternary ammonium salt complexes for zinc-bromine battery applications” Satya N. Bajpal J. Chem. Eng. Data, 26, 2-4 (1981).

(6) In the examples that follow, bromine-complexing agent and mixtures thereof are sometimes abbreviated “BCA”.

Preparations 1-3

Preparation of N-ethyl pyridinium bromide (EPy)

(7) ##STR00006##
1) Preparation of EPy in Aqueous Medium:

(8) 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%.

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

(10) 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%.

(11) 3) Preparation of EPy without a Solvent

(12) 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-methylpyridinium bromide (2-MEPy)

(13) ##STR00007##
4) Preparation of 2-MEPy in Aqueous Medium

(14) 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%.

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

(16) 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%.

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

(18) 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%.

Examples 1-5

Preparing and Measuring the Properties of Zinc Bromide Electrolyte Solutions which Contain 1-butyl 3-methyl Imidazolium Bromide (BMIBr) as a Bromine-Complexing Agent

(19) To demonstrate the effect of 1-butyl 3-methyl imidazolium bromide, 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 the aqueous solution of zinc bromide and elemental bromine (which were present in the samples in suitable amounts as set out in Table 1 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, conductivity and vapor pressure. The results are tabulated in Table 1. The letters A, B and C next to the Example's number indicate the SOC that was investigated ((at the beginning, middle and end of charge process (0%, 50 and 100%, respectively)).

(20) TABLE-US-00001 TABLE 1 Vapor Br.sub.2, BMIBr % Br.sub.2, Pressure, Conductivity, Example % SOC ZnBr.sub.2 (M) M (M) aq. mm Hg mS/cm 1A 0 2.25 0.2 0.8M 0.04 135 1B 50 1.125 1 0.13 140 1C 100 0.25 2 0.13 <25 138 2A 0 2.25 0.2 0.7M 0.04 130 2B 50 1.125 1 0.1 142 2C 100 0.25 2 0.18 <25 128 3A 0 2.25 0.2 0.6M 0.05 136 3B 50 1.125 1 0.14 143 3C 100 0.25 2 1.05 121 4A 0 2.25 0.2 0.5M 0.06 133 4B 50 1.125 1 0.28 140 4C 100 0.25 2 0.96 <25 133 5A 0 2.25 0.2 0.4M 0.07 136 5B 50 1.125 1 0.47 140 5C 100 0.25 2 1.80 N/A 123

(21) Table 1 illustrates that the bromine vapor pressure measured is low, the amount of bromine measured in the aqueous phase is low and the electrolyte solution exhibits good conductivity.

Examples 6-8

Preparing and Measuring the Properties of Zinc Bromide Electrolyte Solutions which Contain N-ethyl Pyridinium Bromide (EPy) as Bromine-Complexing Agent

(22) To demonstrate the effect of N-ethyl pyridinium bromide, 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 2 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 and the results are summarized in Table 2:

(23) TABLE-US-00002 TABLE 2 Physical Br2, state of Br.sub.2 EPy aq. , polybromide Conductivity, Example % SOC ZnBr.sub.2 (M) (M) (M) % complex mS/cm 6A 0 2.25 0.2 0.8M 0.38 Solid at 116 −5° C.* 6B 50 1.125 1.0 0.60 Liquid at − −5° C. 6C 100 0.25 2.0 0.45 Liquid at 140 −5° C. 7A 0 2.25 0.2 0.7M 0.32 Liquid at 121 −5° C. 7B 50 1.125 1.0 0.53 Liquid at − −5° C. 7C 100 0.25 2.0 0.45 Liquid at 140 −5° C. 8A 0 2.25 0.2 0.6M 0.40 Liquid at 120 −5° C. 8B 50 1.125 1.0 0.40 Liquid at − −5° C. 8C 100 0.25 2.0 0.62 Liquid at 132 −5° C. *liquid at −1° C.

(24) Table 2 illustrates that in general, the bromine-containing complex did not undergo solidification even at a temperature as low as −5° C., the amount of bromine measured in the aqueous phase is low and the electrolyte solution exhibits good conductivity.

Examples 9-10

Preparing and Measuring the Properties of Zinc Bromide Electrolyte Solutions which Contain Mixtures of EPy and BMIBr as Bromine-Complexing Agents

(25) To demonstrate the effect of mixtures of N-ethyl pyridinium bromide and 1-butyl 3-methyl imidazolium bromide (at molar ratios of 3:1 and 1:3), 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 3 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, conductivity and vapor pressure. The results are set out in Table 3.

(26) TABLE-US-00003 TABLE 3 Physical state of % Br.sub.2, BCA [BCA] polybromide % Br.sub.2, Conductivity, Example SOC ZnBr.sub.2 (M) M mixture M complex aq. mS/cm  9A 0 2.25 0.2 EPy:BMIBr 0.8M Liquid at 0.07 133 1:3 −5° C.  9B 50 1.125 1.0 Liquid at 0.1 151 −5° C.  9C 100 0.25 2.0 Liquid at 0.15 149 −5° C. 10A 0 2.25 0.2 EPy:BMIBr 0.8M Liquid at 0.24 127 3:1 −5° C. 10B 50 1.125 1.0 Liquid at 0.25 151 −5° C. 10C 100 0.25 2.0 Liquid at 0.30 150 −5° C.

(27) Table 3 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 low and the electrolyte solution exhibits very good conductivity.

Example 11 (Comparative)

(28) The procedures set forth in previous examples were repeated, but this time the complexing agent used was N-methyl-N-ethyl pyrrolidinium bromide (MEP). The results are given in Table 4.

(29) TABLE-US-00004 TABLE 4 Physical state of Br.sub.2 MEP polybromide Br.sub.2, Conductivity, Example % SOC ZnBr.sub.2 (M) (M) complex aq., % mS/cm 11A 0 2.25 0.2 0.8M Liquid at 0.44 88 0° C. 11B 50 1.125 1.0 Solid at 0.60 95 0° C. 11C 100 0.25 2.0 Solid at 0.98 90 5° C.

(30) Table 4 illustrates that the bromine-containing complex 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.

Examples 12-14

Preparing and Measuring the Properties of Zinc Bromide Electrolyte Solutions which Contain Mixtures of 2-MEPy and BMIBr as Bromine-Complexing Agents

(31) To demonstrate the effect of mixtures of N-ethyl-2-methyl pyridinium bromide and 1-butyl 3-methyl imidazolium bromide (at molar ratios of 3:1, 1:1 and 1:3), 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 5 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 5.

(32) TABLE-US-00005 TABLE 5 Physical state of % Br.sub.2, [BCA] polybromide % Br.sub.2, Conductivity, Example soc, ZnBr.sub.2 (M) M BCA mixture M complex aq. mS/cm 12A 0 2.25 0.2 2-MEPy:BMIBr 0.8M Liquid at 0.18 98 3:1 0° C. 12B 50 1.125 1.0 Liquid at 0.20 109 0° C. 12C 100 0.25 2.0 Liquid at 0.14 91 0° C. 13A 0 2.25 0.2 2-MEPy:BMIBr 0.8M Liquid at 0.13 101 1:1 0° C. 13B 50 1.125 1.0 Liquid at 0.08 109 0° C. 13C 100 0.25 2.0 Liquid at 0.17 91 0° C. 14A 0 2.25 0.2 2-MEPy:BMIBr 0.8M Liquid at 0.16 95 1:3 0° C. 14B 50 1.125 1.0 Liquid at 0.14 108 0° C. 14C 100 0.25 2.0 Liquid at 0.13 92 0° C.

(33) Table 5 illustrates that the mixed bromine-containing complex is highly effective, as it did not undergo solidification even at a temperature as low as 0° C., the amount of bromine measured in the aqueous phase is very low and the electrolyte solution exhibits good conductivity.

Examples 15-18 (of the Invention) and 19-20 (Comparative)

(34) 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 40 mA/cm.sup.2.

(35) 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.

(36) Experimental Set-Up

(37) 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 5 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.

(38) 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.

(39) 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.

(40) The Compositions of the Tested Solutions

(41) The aqueous electrolyte solutions that were tested contain zinc bromide, elemental bromine and zinc chloride, the latter at a constant concentration of 0.4M. In addition, the following bromine-complexing agents were present in the tested solutions (the BCA's were added at two different concentrations, of 0.8 M and 1.2M): 3:1 mixture of BMIBr and EPy (of the invention) 3:1 mixture of BMIBr and 2-MEPy (of the invention) MEP (comparative).

(42) The electrolyte solutions prepared fall into two groups, A and B, which differ from one another in respect to the concentrations of the zinc bromide and elemental bromine: Group A: [ZnBr.sub.2]=1.64 M, [Br.sub.2]=0.2M. Group B: [ZnBr.sub.2]=0.74 M, [Br.sub.2]=1.1M.

(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. 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 30% and 90%, respectively. Thus, the activity of the bromine-complexing agents 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).

(44) The Experiments

(45) All the experiments were carried out at room temperature, with the cell being charged at current density of 40 mA/cm.sup.2.

(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 which corresponds to a flow rate of around 60 ml per minute, 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. Each experiment lasted about 4.5 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) $\mathrm{Plating}\mathrm{efficiency}=\frac{M}{\frac{I*t}{F}*\frac{\mathrm{Mw}}{z}}*100$ M—mass of zinc deposited on the electrode I—electrical current (0.4 A) t—time during which the current passed through the cell (16200s) F—Faraday constant (96485 C/mol) Mw—molecular weight (g/mol) —metal valence (2)

(48) The experimental details and the results are tabulated in Table 6.

(49) TABLE-US-00006 TABLE 6 [BCA ZnBr.sub.2 Br.sub.2 additive] Plating Example % SOC (M) (M) BCA Additive M efficiency % 15A  0-30 1.64-1.19 0.2-0.65 BMIBr:EPy 0.8M 48% 15B 60-90 0.74-0.29 1.1-1.55 3:1 37% 16A  0-30 1.64-1.19 0.2-0.65 1.2M 91-93% 16B 60-90 0.74-0.29 1.1-1.55 88-90% 17A  0-30 1.64-1.19 0.2-0.65 BMIBr:2-MEPy 0.8 68-70% 17B 60-90 0.74-0.29 1.1-1.55 3:1 65% 18A  0-30 1.64-1.19 0.2-0.65 1.2 94% 18B 60-90 0.74-0.29 1.1-1.55 90% 19A  0-30 1.64-1.19 0.2-0.65 MEP 0.8 <20% 19B 60-90 0.74-0.29 1.1-1.55 <20% 20A  0-30 1.64-1.19 0.2-0.65 1.2 <20% 20B 60-90 0.74-0.29 1.1-1.55 <20%

(50) The results in Table 6 show that the lower the free bromine concentration in the aqueous phase is (see Tables 3, 4 and 5 for relevant data), the lesser the direct reaction between bromine in the electrolyte and the plated Zn (self-discharge). As a result, the plating efficiency is higher for electrolyte with more bromine complexed in the oily phase.