JPH01115068A - Operation of redox-flow cell - Google Patents

Abstract

PURPOSE:To safely operate a cell in a uniform charge condition even if charge- discharge cycles are repeated for a long time by taking out a given quantity of each of a positive solution and a negative solution, mixing them, measuring the quantity of the oxidation-reduction electricity of the mixture, detecting the overcharge quantity of the solution, and adjusting the reduced quantity of ferric compound in the positive solution. CONSTITUTION:A system consists of a cell main body 10, a positive solution tank 12, a negative solution tank 11, circulation pumps 13, 14, a rebalance cell 15 for eliminating the overcharge of a positive solution, a detector 17, and an open circuit voltage measuring cell 18. The detector 17 detects the quantity of overcharge of a positive solution or a negative solution by measuring the oxidation-reduction state of a mixed solution by cloumetry. By using detected results, the reduced quantity of ferric compound in the positive solution is adjusted and both solutions are kept in a uniform charge state. Both solutions are kept in a uniform charge state even if charge-discharge cycles are repeated for a long time.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method of operating a redox flow battery, which is a secondary battery for power storage, and more specifically, to an operation method that is optimal for maintaining bipolar fluids in an even state of charge. It is about the method.

(Prior technology) A redox flow battery, which is a secondary battery for power storage, is a room-temperature battery that reversibly converts electrical energy into chemical energy by utilizing the change in valence of iron and chromium ions due to redox. It is.

As shown in Figure 1, in this redox flow battery,
When the positive and negative electrode liquids, each consisting of an iron compound and a chromium compound, are fed into the flow-through electrolytic cell 2 by a pump 8.9, the reaction shown by the following formula occurs on the electrodes according to the charging and discharging process, and energy is converted. .

In other words, the electricity left over at night is converted into AC/DC through the inverter 1, and then supplied to the redox flow battery. At the positive electrode, ferrous compounds become ferric compounds through the reaction in the charging direction of equation (1), and at the negative electrode. Then, the secondary chromium compound is electrolyzed into the primary chromium compound by the reaction in the charging direction of equation (2).

At the same time, cations such as hydrogen ions or anions such as chloride ions pass through the diaphragm 5 such as an ion exchange membrane and move between the two electrodes. The ferric compounds and chromium compounds obtained by this charging are stored in tanks 6.7, respectively.

Next, when there is a shortage of electricity during the day, the above (1)
It is discharged according to the reaction in the discharge direction of equation (2), and after orthogonal conversion is performed by an inverter, it is supplied to the power system. This is a power storage system using redox flow batteries.

In order for this redox flow battery to operate stably as a secondary battery for a long time, the coulombic efficiencies of the charging and discharging reactions of both electrolytes must be the same. However, the charging and discharging reaction of iron compounds, which is a positive electrode reaction, proceeds with a coulombic efficiency of approximately 100%, but the charging reaction of chromium compounds, which is a negative electrode reaction, involves a reduction side reaction of hydrogen ions as shown in equation (3) below. .

2H" + 2e-+H, ↑ (3
) Therefore, at the end of charging, the amount of chromium compounds formed in the negative electrode liquid is relatively smaller than the ferric compounds formed in the positive electrode liquid, causing the so-called positive electrode liquid to overcharge the negative electrode liquid. In addition, in the presence of an electron conductor such as an electrode, the oxidation-reduction reaction of formula (4) also proceeds, resulting in a further overcharged state of the catholyte.

2Cr" + 2H"-42Cr" + H, ↑
(4) Therefore, if charging and discharging are repeated in this state, the power storage capacity will decrease due to overcharging of the positive electrode liquid, and eventually a state will arise in which charging and discharging are impossible.

For this reason, in a redox flow battery, an operation called rebalancing is required in which the catholyte is reduced in a separate system rather than in a battery reaction to maintain an even state of charge of both electrolytes.

The conventional method for this purpose is to detect the state of charge of both electrodes using the open circuit voltage or charge/discharge voltage of the battery, and to eliminate overcharging of the catholyte, use hydrogen generated from the negative electrode or, if necessary, a hydrogen gas cylinder, etc. Method for reducing ferric compounds in catholyte by supplying external hydrogen, and JP-A-60-70672
The concentration of ferric and ferric compounds in the catholyte is measured, and the overcharge state of the cathode is detected from the measured value and the open circuit voltage of the battery, A method of reducing an iron compound to a ferrous compound using a reducing agent such as hydrogen gas has been proposed.

However, since the redox a-battery has a structure in which the positive and negative electrodes are separated by a diaphragm such as an ion exchange membrane, when charging and discharging cycles are repeated, iron ions in the catholyte pass through this diaphragm and transfer to the anode solution. Chromium ions gradually move into the positive electrode solution, causing a phenomenon in which the concentration of the active material in the positive and negative electrode solutions gradually decreases.

For this reason, among the conventional methods, when the former method is operated for a long time and the concentration of the active material decreases, it is difficult to accurately detect the state of charge of the bipolar liquid by measuring the open circuit voltage or charge/discharge voltage. It is.

In addition, in the latter method, the depth of the positive electrode solution (the ratio of ferric iron concentration to the total iron concentration) and the depth of the negative electrode solution are determined from the measured values, and the ferric iron in the positive electrode solution is reduced so that they match. However, as shown above, the active material concentration in the positive and negative electrode fluids gradually decreases.

The rate of decline may also vary, and the depth changes for reasons other than hydrogen generation. Therefore, in order to match the depths, there is a drawback that in addition to the generated hydrogen, it is necessary to introduce a reducing agent such as hydrogen from the outside. In addition, when the active material concentration differs, the depth of the bipolar liquid must be measured in a fixed state of charge, such as before the start of charging or after the end of charging, but when the battery is operated in an irregular pattern, etc. It may take a long operating time to reach this fixed state of charge, and there is also the drawback that overcharged state cannot be detected during this period.

(Objective of the Invention) The object of the present invention is to provide a method for operating a redox flow battery that eliminates the above-mentioned drawbacks of the prior art and maintains the charged state of the bipolar liquid evenly even after repeated charging and discharging cycles for a long period of time. be.

(Structure of the Invention) The present invention relates to a redox flow battery that uses an iron compound and a chromium compound as battery active materials for charging and discharging. The overcharge amount of the positive electrode liquid or negative electrode liquid is detected by measuring the state using coulometry, and the amount of reduction of the ferric compound in the positive electrode liquid is determined based on the detection result.
This is a method of operating a redox flow battery characterized by maintaining both electrolytes in a uniform state of charge.

The battery active materials used in the present invention include ferrous compounds and ferric compounds such as iron chloride, iron bromide, and ferrous sulfate for the positive electrode, and ferrous compounds such as chromium chloride, chromium bromide, and chromium sulfate for the negative electrode. In the present invention, in which a chromium compound and a nichrome compound are used, certain amounts of these two electrolytes are extracted and mixed, and the fermentation and reduction state of the mixed solution is measured by coulometry. The measurement method is to measure the mixed liquid at 0. IV
Constant potential electrolysis is performed at a potential of (vs. NHE). Here 0. I
V (vs. NHE) is the potential of complete discharge of iron and chromium compounds.

In this case, the ferric and ferrous chromium in the mixed solution become ferrous and nichrome in a discharged state through the reaction of equation (5), and if the positive electrode is overcharged, ferric and ferrous chromium are present in the mixed solution. In the case of overcharging, the amount of chromium chloride remaining in the battery remains as much as the amount of overcharging.

Fe” + Cr” →Fe” + Cr”
(5) This remaining ferric iron or ferrous chromium is removed at O, tV.
Constant potential electrolysis is performed at a potential of You can find the quantity. By adjusting the amount of reduction of the ferric compound in the positive electrode liquid using the overcharge amount detected in this way, it becomes possible to maintain the bipolar liquid in an evenly charged state very easily. In this case, the amount of reduction of the ferric compound in the positive electrode solution can be adjusted by, for example, changing the current value of the rebalance cell.

(Effect of the invention) The operating method of the present invention extracts and mixes a certain amount of positive and negative electrode liquids, detects the amount of overcharge of the electrode liquid based on the measurement of the amount of redox electricity of the mixed liquid, and detects the amount of overcharge of the electrode liquid. Based on the detection results, calculate the reduction amount of ferric compounds in the catholyte! 11w1, even if the active material concentration in the positive and negative electrode liquids decreases due to a longer charge/discharge cycle compared to conventional methods.

(1) Regardless of the operating state of the battery, the amount of overcharge of the electrolyte can be accurately determined, and based on this, the ferric compound in the catholyte can be reduced to match the amount of charge of both electrolytes. . (2) It has the first effect of making maximum use of the hydrogen gas generated from the negative electrode for reducing the ferric compound in the positive electrode liquid, and enables stable operation in an evenly charged state.

(Example) Examples of the present invention are shown below.

Example A solution containing 1 mol IQ of iron chloride and 4 mol/Ω of hydrochloric acid as the positive electrode liquid, 1 mol/Q of chromium chloride and 4 mol/Q of hydrochloric acid as the negative electrode liquid.
Using a solution containing mol/Q, charging/discharging operation was performed in a 100 W class battery device shown in FIG. The device shown in FIG. 2 includes a battery main body 10 in which 13 cells with an electrode area of 216i are stacked with bipolar plates made of carbon plastic, and carbon cloth is used as the positive and negative electrodes, and a cation exchange membrane is used as the diaphragm that partitions the two electrodes. , catholyte tank 12. a negative electrode liquid tank 11, a circulation pump I3 for sending liquid to the battery main body,
14, a balance cell 15 for eliminating overcharge of the positive electrode liquid, a detector 17 and an open circuit voltage measuring cell 18 shown in FIG. In addition, in the figure.

Reference numerals 19 and 20 are a line that leads hydrogen gas generated from the negative electrode to the rebalance cell, and a hydrogen gas line that leads from the hydrogen cylinder to the balance cell, respectively.

An example of the detector 17 is shown in FIG. Figure 3 shows a coulometry cell 22 using carbon cloth as an electrode and a cation exchange membrane as a diaphragm, and electromagnetic valves 25 and 26 for positive electrode liquid sampling.
, a solenoid valve 27, 28 for sampling the negative electrode liquid, a counter electrode liquid tank 23 for coulometry, and a pump 2 for sending liquid to the cell 22.
It is mainly composed of 4. After sampling a certain amount of positive electrode liquid and negative electrode liquid from the positive electrode liquid line 30 and negative electrode liquid line 31 using electromagnetic valves, the pump 24 circulates them in the coulometric cell, o, iv (vs. N) IB).
If constant potential electrolysis is carried out, the amount of overcharge can be calculated from the amount of electricity obtained. In addition, a solution containing 0.5 mol/Q ferrous chloride, 0.5 mol/Ω ferric chloride, and 4N hydrochloric acid was used as a reference potential solution in the Coulometry cell counter-electrode tank.

According to the overcharge amount of the positive electrode liquid or negative electrode liquid obtained by this detector 17, the amount of reduction of the positive electrode liquid in the rebalance cell 15 is adjusted.

In the 10011 class battery device with such a configuration, charging and discharging was performed once with a current of 8.6 A with the rebalance cell 15 stopped, and hydrogen gas generated from the negative electrode was measured. A continuous charge/discharge experiment was conducted by operating the battery main body 10 and the rebalance cell 15 at the same time, which was found to be 1%. In this case, the current of the rebalance cell 15 was initially set as an amount corresponding to 1% of the amount of charged electricity. The detector 17 was operated once every two charge/discharge cycles, and the current of the rebalance cell was adjusted based on the results. Results of a charge/discharge test over 100 cycles.

The iron ion concentration in the positive electrode solution decreased to 0.7 mol/Ω, and the chromium ion concentration in the negative electrode solution decreased to 0.8 mol/Ω, but the amount of overcharge of the positive or negative electrode solution was always 2 of the amount of charged electricity. % or less, and the battery was operated in a normal state of charge.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the principle of a redox flow battery, FIG. 2 is an explanatory diagram of the configuration of a redox flow battery device according to an embodiment of the present invention, and FIG. 3 is an explanatory diagram of the configuration of the overcharge amount detector shown in FIG. 2. be. DESCRIPTION OF SYMBOLS 1... Inverter, 2... Electrolytic cell, 3, 4... Electrode, 5... Diaphragm, 10... Battery body, 11... Negative electrode liquid tank, 12... Positive electrode liquid tank, 15... Rebalance cell, 17... Overcharge amount detector, 18... Open circuit voltage measurement cell, 22... Coulometry cell, 23
...Counter electrode tank for coulometry. Patent applicant Chiyoda Corporation

Claims (1)

[Claims] (1) In a redox flow battery that uses iron compounds and chromium compounds as battery active materials and charges and discharges, after extracting and mixing a certain amount of positive and negative electrode fluids,
By measuring the redox state of the mixed liquid using coulometry, the amount of overcharge of the positive or negative electrode liquid is detected, and based on the detection result, the amount of reduction of the ferric compound in the positive electrode liquid is adjusted to reduce the amount of ferric compound in the positive electrode liquid. A method of operating a redox flow battery characterized by maintaining the cells in an even state of charge.