Additive for a flow battery

Abstract

The invention relates to a method of operating a zinc-bromine battery, especially at a high temperature, comprising adding 1-n-butyl-2-methyl-pyridinium bromide to the electrolyte of said battery, and charging or discharging said cell. Also provided is the use of 1-n-butyl-2-methyl-pyridinium bromide as an additive in a zinc-bromine battery operating at a temperature above 30 C., and an aqueous concentrate with high content of 1-n-butyl-2-methyl-pyridinium bromide.

Claims

1. An electrolyte solution suitable for use in a zinc-bromine battery, comprising zinc bromide and a liquid complex composed of 1-n-butyl-2-methyl-pyridinium bromide combined with one or more bromine molecules.

2. A method of operating a zinc-bromine battery, the method comprising adding the electrolyte solution of claim 1 to an electrolyte of the zinc-bromine battery, and operating the zinc-bromide battery at a temperature above 30 C.

Description

(1) In the drawings:

(2) FIG. 1 is a schematic description of zinc-bromine cell.

(3) FIG. 2 is a graph where the concentration of elemental bromine in the aqueous phase is plotted against SOC at 25 C. for 2-MBPy and 3-MBPy.

(4) FIG. 3A is a graph where the concentration of elemental bromine in the aqueous phase is plotted against SOC at 35 C. for 2-MBPy, 3-MBPy and 2-MEPy.

(5) FIG. 3B is a graph where the concentration of elemental bromine in the aqueous phase is plotted against SOC (the 0-40% interval only) at 35 C. for 2-MBPy and 3-MBPy.

(6) FIGS. 4 and 5 illustrate an experimental set-up used.

(7) FIGS. 6A and 6B are photos of the bromine electrode and zinc electrode, respectively, taken immediately on completion of the an experiment with 3-MBPy.

(8) FIGS. 7A and 7B are photos of the bromine electrode and zinc electrode, respectively, taken immediately on completion of an experiment with 2-MBPy.

(9) FIGS. 8A and 8B are graphs where the concentration of aqueous bromine is plotted against SOC for 3-MBPy and 2-MBPY-containing electrolyte solutions, respectively, at three different temperatures.

(10) FIGS. 9A and 9B are graphs where the aqueous bromine concentration is plotted against temperature, for 3-MBPy and 2-MBPY-containing electrolyte solutions, respectively, at three different SOC.

EXAMPLES

Examples 1 (of the Invention) and 2-3 (Comparative)

Properties of Zinc Bromide Electrolyte Solutions with Different Complexing Agents

(11) To test the utility of 2-MBPy, 3-MBPy and 2-MEPy as bromine-complexing agents 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 and elemental bromine, also zinc chloride at a concentration of 0.4M-0.5M. The samples were stored at 25 C. or 35 C. for 24-48 hours after preparation before any measurement was conducted. The samples which were tested at 25 C. contain also potassium chloride at a concentration of 1.0M.

(12) The following properties of interest were measured:

(13) (i) 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 (measurements were carried out at 25 c. and 35 C.)

(14) (ii) Viscosity of the complex containing-oily phase was measured at 35 C., using Cannon-Fenske Opaque Viscometer.

(15) (iii) Constant-current electrolysis experiments were run and electrode voltage was measured on oxidizing bromide to elemental bromine at 35 C. The experimental set-up consists of a glassy carbon rotating-disc electrode (having diameter of 3 mm and area of 0.07 cm.sup.2), which was set up to rotate at 2400 rpm; a counter electrode (a titanium electrode having much larger active area, of about 5 cm.sup.2) and a standard calomel electrode as a reference electrode.

(16) The foregoing properties were measured for each of the additives under consideration at different compositions of the electrolyte solution, matching different states of charge (the 0% to 100% SOC scale was investigated at 20% increments, i.e., at six discrete points). The results are set out in Table 1.

(17) TABLE-US-00001 TABLE 1 [aq. [aq. Vis- Addi- Br.sub.2] Br.sub.2] cosity Voltage % ZnBr.sub.2 Br.sub.2, tive M M cP V Ex. SOC M M M (25 C.) (35 C.) (35 C.) (35 C.) 1A 0 1.6 0.16 2- 0.0039 0.004 50 1.064 1B 20 1.28 0.33 MBPy 0.0043 0.004 55 1.069 1C 40 0.96 0.65 0.8M 0.0044 0.005 48 1.070 1D 60 0.64 0.99 0.0080 0.007 ND ND 1E 80 0.32 1.35 0.0116 0.015 ND ND 1F 100 0.08 1.7 0.0123 0.017 ND ND 2A 0 1.6 0.16 3- 0.0001 0.001 65-70 1.079 2B 20 1.28 0.33 MBPy 0.0019 0.004 65 1.081 2C 40 0.96 0.65 0.8M 0.0034 0.006 60 1.098 2D 60 0.64 0.99 0.0049 0.007 ND ND 2E 80 0.32 1.35 0.0107 0.013 ND ND 2F 100 0.08 1.7 0.0123 0.016 ND ND 3A 0 1.6 0.16 2- ND 0.030 ND ND 3B 20 1.28 0.33 MEPy ND 0.020 ND ND 3C 40 0.96 0.65 0.8M ND 0.018 ND ND 3D 60 0.64 0.99 ND 0.019 ND ND 3E 80 0.32 1.35 ND 0.020 ND ND 3F 100 0.08 1.7 ND 0.018 ND ND

(18) In FIGS. 2, 3A and 3B, the concentration of elemental bromine in the aqueous phase is plotted against the SOC (measured at 25 C.FIG. 2; and 35 C.FIGS. 3A and 3B). The curves shown in the Figures illustrate the ability of 2-MBPy to maintain a workable concentration of aqueous bromine from the very beginning of the charge process, all the way round to the fully charged state.

(19) Regarding 3-MBPy, the advantage of 2-MBPy over 3-MBPy is observed at the low state of charge both at 25 C. and 35 C., in the range from 0% to 20% SOC (FIG. 3B provides an enlarged representation of the interval of interest). The increased concentration of bromine in the aqueous phase of the electrolyte solution, together with consistently lower viscosity and lower charge voltage measured across the 0% SOC to 40% SOC interval for the 2-MBPy-containing samples vis--vis the 3-MBPy containing samples, indicate that 2-MBPy is suitable for use as bromine-complexing agent in zinc-bromine batteries.

(20) As to 2-MEPy, the concentration of elemental bromine in the aqueous phase of the electrolyte solution when 2-MEPy is the added BCA is too high for membraneless cell configuration. This high concentration would lead to self-discharge and decreased current efficiency, namely, the efficiency of zinc plating formed onto the anode surface, when the cell is charged at a fixed current density, would be unsatisfactory.

Examples 4 to 6 (Comparative) and 7 (of the Invention)

Testing Zinc Bromide Electrolyte Solutions with Different Bromine-Complexing Agents (BCA) in an Electrochemical Experimental Set-Up

(21) An experimental set-up schematically illustrated in FIG. 4 was used to evaluate the effect of the presence of various bromine complexing agents on the efficiency of the operation of zinc/bromine membraneless cell. A characteristic property of the cell which was chosen for the quantitative study is the efficiency of zinc plating formed onto the anode surface, when the cell is charged at current density of 60 mA/cm.sup.2.

(22) 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 lessening the direct chemical oxidation of the zinc by the 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.

(23) The Experimental Set-Up

(24) Referring to FIG. 4, 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, with a trapezoidal section laterally extending from one of the long sides of the rectangular electrodes, as shown in FIG. 5, which provides a top view of the electrodes. Electrodes 21 and 22 are mounted horizontally, in parallel to one another, and are spaced 2 mm apart, with zinc electrode 21 being positioned on top of bromine electrode 22.

(25) Within the space between the two electrodes, 1 mm thick Viton gasket frame 25 is placed over the upper face of bromine electrode 22, and 1 mm thick Viton flow frame 26 is disposed on top of gasket 25. A top view of elements 25 and 26 is presented in FIG. 5, showing the open central regions thereof (note that the open regions are not equal, and are not in scale). Therefore, the lower and upper faces of electrodes 21 and 22, respectively, are covered, 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 an area of 10 cm.sup.2. It is noted that no membrane is interposed in the space between the electrodes.

(26) The electrode plates are perforated to allow the access and exit of electrolyte flow.

(27) The Compositions of the Tested Solutions

(28) 100 ml samples of aqueous electrolyte solutions were prepared, with the following compositions:

(29) [ZnBr.sub.2]=2.0 M, [Br.sub.2]=0.25 wt %, [BCA]=0.8M and [ZnCl.sub.2]=0.4 M

(30) A typical composition of an electrolyte solution at the beginning of the charging process, i.e., an electrolyte solution corresponding to 0% SOC, has the concentrations set forth above (note that a small amount of elemental bromine is normally added to avoid overpotential).

(31) 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. In the present experiments, the electrolysis was allowed to proceed until the composition of the solution reached a state of charge of 40%. Thus, the activity of the bromine-complexing agents (BCA) was investigated across the to 40% SOC range. The BCA tested in Examples 4 to 7 were 3-MBPy, 2-MEPy, MEP (N-methyl-n-ethyl-pyrrolidinium bromide, a commonly used bromine-complexing agent) and the BCA of the invention, 2-MBPy, respectively.

(32) The Experiments

(33) All the experiments were carried out at temperature of 35 C., with the cell being charged at 0.6 A, i.e., current density of 60 mA/cm.sup.2. Before use, the electrolyte solution was kept at 35 C. for at least twenty four hours.

(34) Each experiment is run as follows. The electrolyte solution is held in a reservoir 23, where it is stirred with the aid of a magnetic stirrer at a rate of 150 rpm. The electrolyte volume is 100 ml (110-130 g). Reservoir 23 is kept at a temperature of 360.1 C. Peristaltic pump 24, operating at a constant rate, 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. 4; note the single inlet opening and the pair of outlet openings, to prevent accumulation of the complex inside the cell. 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.

(35) Each experiment lasted about 4.0-4.5 hours (or less, due to short circuit caused by dendrite growth inside the cell). At the end of the experiment, the cell was opened and photos of the two electrodes were taken immediately. The cell components were then washed in a soap solution and distilled water, and were allowed to dry. The weight of Zn deposit was determined. Plating efficiency was calculated as follows:

(36) $\mathrm{Plating}\mathrm{efficiency}=\frac{M}{\left(\frac{I*t}{F}\right)*\left(\frac{\mathrm{Mw}}{z}\right)}*100$
Mmass of zinc deposited on the electrode
Ielectrical current (0.6 A)
ttime during which the current passed through the cell (sec)
FFaraday constant (96485 C/mol)
Mwmolecular weight (g/mol)
zmetal valence (2)

(37) The experimental details and the results are tabulated in Table 2.

(38) TABLE-US-00002 TABLE 2 Time ZnBr.sub.2 [BCA] plating Ex. (h) % SOC (M) BCA (M) efficiency % 4 4.0 0.fwdarw.40 2.0.fwdarw. 1.2 3-MBPy 0.8 80 5 4.5 0.fwdarw.20 2.0.fwdarw. 1.6 2-MEPy 0.8 25 6 4.5 0.fwdarw.15 2.0.fwdarw. 1.8 MEP 0.8 <20 7 4.0 0.fwdarw.40 2.0.fwdarw. 1.2 2-MBPy 0.8 89

(39) The results indicate that 2-MBPy is an excellent bromine-complexing agent, achieving the highest plating efficiency during the electrolysis (cell charging).

(40) FIGS. 6A and 6B are photos of the bromine electrode and zinc electrode, respectively, taken immediately on completion of the experiment with 3-MBPy. It may be seen that a growth of zinc dendrites took place on the face of the zinc anode, in the region which corresponds in position to the region of the face of the parallel-opposed bromine cathode in which the complex accumulated.

(41) FIGS. 7A and 7B are photos of the bromine electrode and zinc electrode, respectively, taken immediately on completion of the experiment with 2-MBPy. It may be seen that the zinc plating deposited onto the anode surface is more uniform, with significantly lesser dendrites formation observed.

Examples 8 to 13

8, 10 and 12 of the Invention; 9, 11, 13 Comparative

Testing the Physic-Chemical Properties of Zinc Bromide Electrolyte Solutions with Different Bromine-Complexing Agents (BCA) at High Temperatures Range

(42) To test the utility of 2-MBPy and 3-MBPy as bromine-complexing agents at high operating temperature 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 3 below, to match different states of charge. Each sample contains, in addition to the aqueous solution of zinc bromide and elemental bromine, also zinc chloride at a concentration of 0.3M-0.5M. The samples were stored at 40 C., 50 C. or 55 C. for 24 hours after preparation before any measurement was conducted. All measurements and sampling were carried out at the storing temperatures.

(43) The following properties of interest were measured:

(44) (i) 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 (measurements were carried out at 40 C., 50 C. and 55 C.)

(45) (ii) Viscosity of the complex containing-oily phase was measured at 40 C., 50 C. and 55 C., using Cannon-Fenske Opaque Viscometer.

(46) (iii) Concentration of the BCA in the aqueous phase was measured with chromatographic techniques. Aqueous phase samples were taken at 40 C., 50 C. and 55 C.

(47) (iv) Specific conductivity of the aqueous phase was measured using a bi-plate conductivity meter at 40 C., 50 C. and 55 C.

(48) The foregoing properties were measured for each of the additives under consideration at different compositions of the electrolyte solution, matching different states of charge (the 0% to 100% SOC scale was investigated at the endpoints and midpoint, i.e., at 0%, 50% and 100% SOC). The results are set out in Table 3.

(49) TABLE-US-00003 TABLE 3 Specific [aq. Br.sub.2] Viscosity [aq. BCA] Cond. % ZnBr.sub.2 Br.sub.2, Additive M cP M mS/cm Ex. SOC M M M (40 C.) (40 C.) (40 C.) (40 C.) 8A 0 1.6 0.5 2-MBPy 0.0062 42 0.1209 124 8B 50 0.9 1.12 1M 0.0085 26 0.0509 131 8C 100 0.2 1.7 0.0168 15 0.0081 114 9A 0 1.6 0.5 3-MBPy 0.0050 42 0.0810 129 9B 50 0.9 1.12 1M 0.0071 22 0.0424 132 9C 100 0.2 1.7 0.0162 14 0.0063 114 Specific [aq. Br.sub.2] Viscosity [aq. BCA] Cond. % ZnBr.sub.2 Br.sub.2, Additive M cP M mS/cm Ex. SOC M M M (50 C.) (50 C.) (50 C.) (50 C.) 10A 0 1.6 0.5 2-MBPy 0.0071 24 0.1278 139 10B 50 0.9 1.12 1M 0.0092 18 0.0535 144 10C 100 0.2 1.7 0.0171 11 0.0083 125 11A 0 1.6 0.5 3-MBPy 0.0075 27 0.1042 142 11B 50 0.9 1.12 1M 0.0108 15 0.0469 144 11C 100 0.2 1.7 0.0203 10 0.0072 125 Specific [aq. Br.sub.2] Viscosity [aq. BCA] Cond. % ZnBr.sub.2 Br.sub.2, Additive M cP M mS/cm Ex. SOC M M M (55 C.) (55 C.) (55 C.) (55 C.) 12A 0 1.6 0.5 2-MBPy 0.0072 20 0.1407 143 12B 50 0.9 1.12 1M 0.0095 15 0.0582 149 12C 100 0.2 1.7 0.0181 11 0.0094 129 13A 0 1.6 0.5 3-MBPy 0.0081 21 0.1304 150 13B 50 0.9 1.12 1M 0.0114 13 0.0546 151 13C 100 0.2 1.7 0.0225 9 0.0081 129

(50) The results set forth in Table 3 indicate that electrolyte solutions, which contain either 2-MBPy or 3-MBPy, display comparable conductivities and viscosities. However, the complexation abilities of 2-MBPy and 3-MBPy show different dependence on temperature. In the case of 2-MBPy, the concentration of aqueous bromine increases only slightly with increasing temperature. 3-MBPy, on the other hand, shows stronger temperature dependence, seeing that the concentration of the aqueous bromine increases more sharply in the temperature range under consideration. Thus, 2-MBPy is more effective in keeping the aqueous phase bromine concentration low at high temperatures.

(51) To better appreciate these findings, the experimental results are also presented graphically.

(52) In the graphs shown in FIGS. 8A and 8B, the concentration of aqueous bromine is plotted against the state of charge for the 3-MBPy and 2-MBPY-containing electrolyte solutions, respectively. In each graph, three separate curves are plotted, which correspond to measurements conducted at 40 C., 50 C. and 55 C. (the curves are not labeled; an upward pointing arrow in the graph indicates the 40 C..fwdarw.50 C..fwdarw.55 C. temperature increase). It is seen that in the case of 2-MBPy, the three individual curves are only very slightly separated from each other.

(53) The same experimental results are also shown in the graphs of FIGS. 9A and 9B, where the aqueous bromine concentration is plotted against temperature, for the 3-MBPy and 2-MBPY-containing electrolyte solutions, respectively. In each graph three distinct concentration vs. temperature curves are shown, corresponding to the three distinct state of charge investigated (0%, 50% and 100%; the individual curves are not labeled; an upward pointing arrow in the graph indicates the 0%.fwdarw.50%.fwdarw.100% change in the state of charge). In the case of 2-MBPy, the weak temperature dependence of the aqueous bromine concentration is clearly evident in each of the states of charge investigated; steeper slopes are observed for the curves produced for 3-MBPy.

Preparation 1

Preparation of 1-n-butyl-2-methyl pyridinium bromide (2-MBPy)

(54) A double surface pressure reactor was equipped with a mechanical stirrer, a thermocouple well and a dosing pump. The reactor was charged with 2-picoline (250 g) and heated to 100-104 C. n-butyl bromide (358.8 g) was then continuously fed during 3 hours. During feeding the temperature rose up to 124-129 C. Maximal pressure during feeding stage was 1.5 barg. After feeding was completed the reaction mixture was heated for 2 hours (110-105 C.). DIW (100 mL) was added, the pressure was released and the mixture removed from the reactor through the bottom valve. The mixture was evaporated using a rotavapor. Additional DIW (100 g) was added and the mixture was re-evaporated. DIW was added for dilution. Final product: 685 g, 82 weight % (argentometric titration); 93% yield.

Preparation 2

Preparation of 1-n-butyl-2-methyl pyridinium bromide (2-MBPy)

(55) A double surface reactor was equipped with a mechanical stirrer, a thermocouple well and two dropping funnels. The reactor was charged with 2-picoline (250 g) and heated to 85-90 C. n-butyl bromide (358.8 g) was added drop-wise during hours. DIW was added in small portions (15 ml in total). During feeding the temperature rose to 92-104 C. After feeding was completed the reaction mixture was heated for 3 hours (105 C.). DIW (100 ml) was added. The mixture was withdrawn from the reactor, and was evaporated using a rotavapor. Additional DIW (100 g) was added and the mixture was re-evaporated. DIW was added for dilution. Final product: 666 g, 81 weight % (argentometric titration); 89% yield.

Preparation 3

Preparation of 1-n-butyl-2-methyl pyridinium bromide (2-MBPy)

(56) A double surface reactor was equipped with a mechanical stirrer, a condenser, a thermocouple well and a dropping funnel. The reactor was charged with 2-picoline (341 g) and acetonitrile (374 g) and heated to 78 C. n-butyl bromide (528 g) was added drop-wise during 2.5 hours. The reaction mixture was kept at 78-79 C. for 4 hours. The mixture was cooled and the solvent evaporated using a rotavapor. DIW (150 ml) was added and another evaporation was applied. Finally, the mixture was diluted with DIW. Final product: 559 g, 81% w; 54% yield.