JP2003257467A - Method of operating redox flow battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of operating a redox flow battery having superior battery efficiency, a less generation amount of gas and lower cost. <P>SOLUTION: The redox flow battery comprises positive and negative electrodes separated from each other with a separating membrane, a positive electrode electrolyte storage tank for storing a positive electrode electrolyte to be circulated and supplied to the positive electrode, and a negative electrode electrolyte storage tank for storing a negative electrode electrolyte to be circulated and supplied to the negative electrode. The electrolyte is stored in the same amount in each tank at starting charge/discharge. When the rate of an amount change of the electrolyte moved through the separating membrane with the repetition of charge/discharge cycles is within a range of -15% to +17%, the electrolytes in both tanks are restored into the same amount. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for operating a redox flow battery. In particular, the present invention relates to a method of operating a redox flow battery having higher battery efficiency and a small amount of gas generation.

[0002]

2. Description of the Related Art It has been proposed to use a redox flow battery for load leveling applications, voltage sag / blackout countermeasures, and the like.

Particularly, the vanadium redox flow battery has a high electromotive force, a large energy density, and since the electrolytic solution is a single element system, it can be regenerated by charging even if the positive electrode electrolytic solution and the negative electrode electrolytic solution are mixed. It has many advantages that it can.

In such a vanadium redox flow battery, when the charge and discharge are repeated, various ions and solvents in the electrolytic solution move through the diaphragm, and the amount of the electrolytic solution in the positive electrode and the negative electrode increases or decreases. For example, when an anion diaphragm is used,
Liquid transfer occurs from the positive electrode side to the negative electrode side, and the amount of the electrolytic solution becomes unbalanced, so that one of the electric capacities significantly decreases.

In order to solve the problem associated with such liquid transfer, conventionally, the liquid amount is adjusted by connecting or mixing the positive electrode electrolytic solution and the negative electrode electrolytic solution every fixed number of charge / discharge cycles. As a conventional technique relating to this liquid amount adjustment, JP 2001-167787 A, JP-A 4-124754 JP, JP 1
The ones described in 1-204124 and JP-A-2-195657 are known.

[0006] For example, in the technique described in Japanese Patent Laid-Open No. 2001-167787, the electrolytic solution is returned from a tank having an increased liquid amount to a tank having an decreased liquid amount through a pipe, so that the change in the liquid amount in each tank is kept constant. Holds in range.

[0007]

However, heretofore, detailed examination has not been made in detail as to how much the liquid amount should be changed and how much the liquid amount should be moved. Therefore, it is difficult for the conventional technique to further improve the battery efficiency, for example.

Further, in the technique disclosed in Japanese Patent Laid-Open No. 2001-167787, as a setting condition before charging / discharging, the liquid amount of a tank whose liquid amount decreases due to repeated charging / discharging cycles is set in advance from the liquid amount of the other tank. Most of the cases are mainly evaluated. However, if the liquid amount of the tank whose liquid amount is reduced is made larger than the liquid amount of the other tank in advance, the electrolytic solution that is not used for charging / discharging is additionally included, resulting in an increase in cost.

Further, conventionally, almost no evaluation has been carried out on the evolved gas required for battery evaluation. Gas is generated as a side reaction of charge and discharge. However, in the above-mentioned conventional techniques, no clear knowledge about the type and amount of generated gas has been obtained. Therefore, the evaluation items of the battery are only efficiency (power, voltage, current) and discharge power amount (compared with the initial state), and sufficient battery evaluation has not been performed.

Therefore, a main object of the present invention is to provide a method of operating a redox flow battery which produces less gas, is more excellent in battery efficiency, and is lower in cost.

[0011]

According to the present invention, at the start of charging / discharging, the amount of the electrolytic solution stored in the electrolytic solution storage tanks of both positive and negative electrodes is set to the same amount, and the solution of the electrolytic solution accompanying the repetition of the charging / discharging cycle. The above object is achieved by equalizing the liquid amounts in both tanks before the rate of change in volume exceeds the specified range.

That is, according to the present invention, a positive electrode and a negative electrode separated by a diaphragm, a positive electrode electrolyte storage tank for storing a positive electrode electrolytic solution circulated and supplied to the positive electrode, and a negative electrode electrolytic solution circulated and supplied to the negative electrode. A method for operating a redox flow battery, comprising: a negative electrode electrolyte storage tank for storing The same amount of electrolytic solution is stored in each of the tanks at the start of charging / discharging. Then, the rate of change in the amount of the electrolytic solution that has moved through the diaphragm with the repetition of the charge / discharge cycle is -15% or more +1.
When it is within the range of 7% or less, return the same amount of electrolyte in each tank. In particular, when the rate of change in the liquid amount is within the range of more than 0% and less than + 5%, it is optimal to return the electrolytic solution to the same amount.
In the present invention, the rate of change in the liquid amount of the electrolytic solution is a liquid amount difference obtained by subtracting the negative electrode electrolytic solution amount from the positive electrode electrolytic solution amount with respect to the average liquid amount obtained by dividing the sum of both electrolytic solutions at the start of charge / discharge by 2. Is the ratio.

According to the present invention having the above-mentioned structure, the cost is reduced by making the same amount of the electrolytic solution stored in each tank at the start of charging / discharging, that is, under the set condition. In addition, the rate of change in the amount of electrolytic solution is specified, and in order to equalize the amounts of electrolytic solution in both tanks before this specified range is exceeded, the amount of liquid that has increased due to liquid transfer is moved to the tank on the side that has decreased. As a result, the generation of gas is reduced and the battery efficiency is further improved.

In the present invention, the reason why the rate of change in the amount of the electrolytic solution is specified and the amounts of the electrolytic solution in both tanks are made equal before the specified range is exceeded will be explained. Conventionally, at the start of charging / discharging, the amount of electrolytic solution in both tanks was set to the same amount to perform charging / discharging, and in order to improve work efficiency, the amount of liquid was often adjusted every charging / discharging period of time. The frequency of this liquid volume adjustment is at most 1 day a day.
It is about the number of times, and about how much change in liquid volume can be moved to improve battery efficiency,
There was no clear guideline. Further, as a conventional technique, there is a technique in which the liquid supply pressure in one tank is increased and the liquid supply pressure in the other is decreased so that the liquid amounts in both tanks are always kept the same. That is, in this technique, the flow rate of the electrolytic solution that flows in one cell is made smaller than the flow rate of the electrolytic solution that flows in the other cell to make the flow rate difference. However, due to the limitation of withstand voltage of the cell,
Especially when the area of the cell is large, the flow rate may have to be reduced. Then, the energy density is reduced by decreasing the flow rate, and as a result, the battery performance may be deteriorated. Therefore, as a result of examination by the present inventors, before the liquid amount change exceeds a certain range, the increased liquid is returned from the liquid-increased tank to the liquid-depleted tank so that the liquid amounts of both tanks are the same. It has been found that, when the above is satisfied, more excellent battery efficiency is obtained and the amount of gas generated is relatively small. In particular, it was also found that in this case, the battery efficiency and the energy density are better than in the case where the flow rate is adjusted and the liquid amount in each tank is always maintained at the same amount.

Further, the increase / decrease in the amount of liquid in each tank due to the repetition of the charge / discharge cycle is determined by the type of diaphragm and the difference in the pressure of liquid fed to the positive and negative electrodes. However, when it is recognized that the liquid amount increases or decreases only by the type of diaphragm as in JP-A-2001-167787, it is not possible to clearly grasp the increase or decrease in the liquid amount.

Based on the above matters and findings, according to the present invention, under the setting conditions in which the amount of the electrolytic solution in each tank is the same, the ratio of the change in the amount of the electrolytic solution reaches the specified range and before it exceeds the specified range. Return the amount of electrolyte in the tank to the same amount.

Next, the reason for reducing the amount of gas generation in the present invention will be described. In the vanadium redox flow battery, gas is generated as a side reaction of charge / discharge. For example, if the positive electrode active material is overcharged, oxygen is generated as a side reaction due to the decomposition reaction of water, which leads to oxidative deterioration of the electrode, which in turn lowers the voltage efficiency.

The present inventors have also found that gas such as hydrogen is generated from the negative electrode and carbon dioxide is generated from the positive electrode.
In the worst case, hydrogen may cause ignition or explosion.
In the case of carbon dioxide, the electrode is decomposed, and the deterioration is progressing before it appears as a decrease in battery efficiency.

Further, if the gas is generated and accumulated for a long period of time, there arises a problem in the tank pressure resistance. Therefore, it is preferable to reduce the generation of gas.

A known technique may be applied to the method of adjusting the amount of the electrolytic solution in the present invention. For example, both tanks may be connected by communication pipes, and both ends of the communication pipes may be connected at a position above the liquid level of the electrolyte stored in each tank (actual open flat 4-124754
(See Japanese Patent Publication No.), the both ends of the communication tube is connected at a position below the liquid surface of the electrolytic solution (see Japanese Patent Laid-Open No. 11-204124), and the electrolytic solution is passed through the diaphragm by adjusting at least one of the liquid sending pressures. It may be moved in the opposite direction (see Japanese Patent Laid-Open No. 2-195657). Further, in the present invention, in order to return the amount of the electrolytic solution to the same amount, for example, it is preferable that both tanks are connected by a communication pipe and the electrolytic solution can be moved from one tank to the other tank.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. At the start of charging / discharging, the redox flow battery was charged / discharged with the same amount of the positive electrode electrolyte solution and the negative electrode electrolyte solution, and the battery efficiency, liquid energy density, and gas generation amount were measured.

An outline of the redox flow battery used in this test is shown in FIG. This battery has a diaphragm 4 that allows the passage of ions.
The cell 1 is divided into a positive electrode cell 1A and a negative electrode cell 1B. Each of the positive electrode cell 1A and the negative electrode cell 1B has a positive electrode 5 and a negative electrode 6 built therein. A positive electrode electrolyte storage tank 2 for supplying and discharging a positive electrode electrolyte is provided in the positive electrode cell 1A.
Connected via 7,8. Similarly, in the negative electrode cell 1B,
Negative electrolyte storage tank for supplying and discharging negative electrolyte
3 are connected via conduits 10, 11. Each electrolyte is
An aqueous solution of ions whose valence changes, such as vanadium ions, is circulated by pumps 9 and 12, and charging / discharging is performed along with the valence change reaction of the ions in the positive electrode 5 and the negative electrode 6.

In this test, a method of moving the electrolytic solution to make the amounts of the electrolytic solution in both tanks the same will be described. First, the same amount of electrolytic solution is stored in the bipolar electrolytic solution storage tank. A liquid level sensor is provided on the side wall of each tank so that the amount of electrolytic solution in the tank can be grasped. Both tanks are connected by a communication pipe via a pump. Then, when the amount of electrolyte in each tank reaches the amount shown in Table 1 with charging and discharging, only the amount increased with charging and discharging from this tank is used from one tank to the other tank through this communicating pipe. And move it to adjust the amount of electrolyte in both tanks to the same amount. The amount (l) of the bipolar electrolytic solution and the rate of change in the electrolytic solution amount shown in Table 1 were measured before adjusting to the same amount. Next, a method for adjusting the amount of electrolyte in this test will be described. In this test, the liquid amount was changed by adjusting the pump 12 shown in FIG. 1 and adjusting the liquid feeding pressure of the electrolytic solution. Specifically, in all the samples, the flow rate of the positive electrode electrolyte was 7 l / min, and the sending pressure was 0.6.
The flow rate of the negative electrode electrolytic solution and the solution sending pressure were made different by making them equal to × 10 ⁵ Pa. Sample Nos. 1 to 4, which increase the electrolytic solution on the positive electrode side,
The flow rate of the negative electrode electrolytic solution was 14 l / min, the solution sending pressure was 1.4 × 10 ⁵ Pa, and the amount of solution in each electrode was changed to the amount shown in Table 1.
For sample Nos. 6 to 8 in which the amount of electrolyte on the negative electrode side is increased, the flow rate of the negative electrode electrolyte is 7 l / min, the liquid sending pressure is 0.7 × 10 ⁵ Pa, and similarly, the liquid amount of each electrode is the amount shown in Table 1. Changed. In sample 5 in which the electrolytes of both electrodes were maintained at the same amount, the flow rate of the negative electrode electrolyte was 10 l / min, and the sending pressure was 1.0 × 10 ⁵ Pa.

[0024]

【table 1】

The test conditions are shown below. (Battery specifications) Electrode reaction area: 1000 cm ² × 10 Cell diaphragm: Anion (anion) exchange membrane Cathode electrolyte: Tetravalent vanadium ion 1.7 mol / l Sulfuric acid 2.6
mol / l electrolyte 25l negative electrode electrolyte: trivalent vanadium ion 1.7mol / l sulfuric acid 2.6
25 l mol / l electrolyte

(Amount of Electrolyte Solution in this Test) The positive electrode electrolyte solution and the negative electrode electrolyte solution are mixed to prepare 50 liters of an electrolyte solution having a valence balance of 3.5. The same amount of this mixed electrolyte is stored in each of the positive and negative electrode electrolyte storage tanks.
Add 25 liters.

(Charging Method) Constant current charging is started at a current density of 100 mA / cm ² , and then constant voltage charging is performed under the condition of an upper limit charging voltage of 1.60 V / cell until an open circuit voltage of 1.55 V / cell is reached.

(Discharging method) Constant current discharging is performed at a current density of 100 mA / cm ² . The discharge is terminated when the lower limit discharge voltage of 1.0 V / cell is reached.

(Evaluation method) An operation of returning the amount of electrolytic solution in both tanks to the same amount when the amount of liquid in each tank changes to the amount shown in Table 1 after starting charging / discharging using the battery having the above specifications repeat. The operation of repeating such charging / discharging and liquid amount adjustment is continuously performed for one week. Measure the battery efficiency and liquid energy density around one week. After the end of continuous charge / discharge for one week, the amount of gas (hydrogen and carbon dioxide) generated is measured.

The battery efficiency is {discharge voltage (V) × discharge current (A)
X discharge time (h)} / {charge voltage (V) x charge current (A) x charge time (h)}.

As for gas, carbon dioxide was generated at the positive electrode and hydrogen was measured at the negative electrode. Gas analysis was carried out by a gas chromatography method.

The test results are shown in Table 2. In addition, Table 3 shows the evaluation criteria for the reduction amount of battery efficiency in this test, Table 4 shows the evaluation criteria for the reduction amount of the same liquid energy density, and Table 5 shows the evaluation criteria for the same gas generation amount. Furthermore, the relationship between the battery efficiency and the ratio of the change in the amount of electrolyte solution (positive electrode electrolyte amount-negative electrode electrolyte amount) / {(positive electrode electrolyte amount + negative electrode electrolyte amount) / 2}, and (positive electrode electrolyte amount amount
The graph of FIG. 2 shows the relationship between the negative electrode electrolytic solution amount) / {(positive electrode electrolytic solution amount + negative electrode electrolytic solution amount) / 2} and the liquid energy density.

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

[Table 5]

As is clear from Table 2 and the graph of FIG.
(Amount of positive electrode electrolyte-amount of negative electrode electrolyte) / {(amount of positive electrode electrolyte + amount of negative electrode electrolyte) / 2} is less than -0.15 (less than -15%) or +0.17 (+17
It can be seen that Sample Nos. 3, 4, and 6 in which the amount of the electrolytic solution in both tanks was returned to the same amount before becoming (more than%)), the cell efficiency and the liquid energy density are high, and the amount of gas generated is small.
In particular, (amount of positive electrode electrolyte-amount of negative electrode electrolyte) / {(amount of positive electrode electrolyte +
Anode electrolyte amount) / 2} is more than 0 to less than +0.05 (more than 0% to + 5%
Sample No. 4 in which the electrolytic solution in each tank was returned to the same amount before exceeding (below), the battery efficiency and the liquid energy density were higher, and the amount of gas generated was extremely small. Also, conventionally,
When operating so as to flow the electrolytic solution sufficiently and maintain the same amount, that is, (amount of positive electrode electrolytic solution-amount of negative electrode electrolytic solution) / {(amount of positive electrode electrolytic solution + amount of negative electrode electrolytic solution) / 2} is 0 When driving so that
(Sample No. 5) was considered to have the best battery efficiency. However, within the range where the liquid volume change is greater than 0 (0%), more specifically, within the range from 0 (0%) to +0.05 (+ 5%) or less, the liquid volumes in both tanks are returned to the same volume. It was found that the sample No. 4 in which the operation was performed gave better results. Further, the sample No. 4 in which the operation for returning the liquid amount to the same amount was performed was superior in energy density to the sample No. 5 in which this operation was not performed.

In this example, a mechanism for moving the electrolytic solution to adjust the amounts of the electrolytic solution at both electrodes to the same amount will be specifically described. In this example, a mechanism is used in which a pump is installed in the communication pipe and the electrolytic solution can be moved from either of the positive and negative electrode tanks. FIG. 3 is a schematic diagram showing a state in which each tank is connected by a communication pipe. The positive electrode electrolyte storage tank 20 and the negative electrode electrolyte storage tank 21 are connected by two communication pipes 22 and 23, and the communication pipes 22 and 23 are connected by a connection pipe 25 including a pump 24. .

In this case, in order to move the electrolytic solution from the positive electrode electrolytic solution storage tank 20 to the negative electrode electrolytic solution storage tank 21,
The valves 30 and 31 may be opened, the valves 32 and 33 may be closed, and the pump 24 may be used as appropriate. On the other hand, in order to move the electrolytic solution from the negative electrode electrolytic solution storage tank 21 to the positive electrode electrolytic solution storage tank 20, open the valve 32 and the valve 33,
It is preferable that the valve 30 and the valve 31 are closed and the pump 24 is appropriately used. With this configuration, the electrolytic solution can be more efficiently moved from one tank to the other tank.

In this example, the pump 24 is provided in the communication pipe, but the pump 24 is not provided and only the communication pipe and the valve are provided.
The liquid may move according to gravity by opening the valve.

[0041]

As described above, according to the redox flow battery of the present invention, the amount of electrolytic solution of both electrodes is returned to the same amount before the rate of change in the amount of electrolytic solution due to charging and discharging exceeds the specified value. Further, it is possible to achieve the excellent effects of less gas generation and higher battery efficiency than in the past. In addition, since the amount of the electrolytic solution at the start of charging / discharging is the same in each tank, it is economical without including an extra electrolytic solution.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the operating principle of a redox flow battery.

FIG. 2 is a graph showing the relationship between the rate of change in the amount of electrolytic solution and battery efficiency, and the relationship between the rate of change in the amount of electrolytic solution and liquid energy density.

FIG. 3 is a schematic view showing a state in which a positive electrode electrolyte solution storage tank and a negative electrode electrolyte solution storage tank are connected by a communication pipe.

[Explanation of symbols]

1 Cell 1A Positive Cell 1B Negative Cell 2 Positive Electrolyte Storage Tank 3 Negative Electrolyte Storage Tank 4 Diaphragm 5 Positive Electrode 6
Negative electrode 7, 8, 10, 11 Conduit 9, 12 Pump 20 Positive electrolyte storage tank 21 Negative electrolyte storage tank 22, 23 Communication pipe 24 Pump 25 Connection pipe 30, 31, 32, 33 Valve

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Seiji Ogino             1-3-3 Shimaya, Konohana-ku, Osaka Sumitomo Electric             Ki Industry Co., Ltd. Osaka Works (72) Inventor Nobuyuki Tokuda             3-3-22 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture             Kansai Electric Power Co., Inc. F term (reference) 5H026 AA10 CX04 HH05 RR01                 5H027 AA10 BE07

Claims (2)

[Claims] 1. A positive electrode and a negative electrode that are separated by a diaphragm, a positive electrode electrolyte storage tank that stores a positive electrode electrolyte solution that is circulated and supplied to the positive electrode, and a negative electrode that stores a negative electrode electrolyte solution that is circulated and supplied to the negative electrode. In a method of operating a redox flow battery comprising a tank for storing an electrolyte solution, in each of the tanks, an equal amount of electrolyte solution is stored at the start of charging / discharging, and the electrolyte solution that has moved through the diaphragm as the charging / discharging cycle is repeated. The method for operating a redox flow battery, which comprises returning the electrolytic solution in both tanks to the same amount when the rate of change in the liquid amount is within the range of -15% to + 17%. However, the rate of change in the liquid volume should be the sum of both electrolytes at the start of charging / discharging.
The ratio is the ratio of the liquid amount difference obtained by subtracting the negative electrode electrolyte amount from the positive electrode electrolyte amount with respect to the average liquid amount divided by. 2. The redox flow battery according to claim 1, wherein the electrolytic solution in both tanks is returned to the same amount when the rate of change in the liquid amount is within the range of more than 0% and + 5% or less. how to drive.