JPH0534785B2 - - Google Patents

Description

[Detailed description of the invention]

OBJECTS OF THE INVENTION INDUSTRIAL APPLICATION FIELD OF THE INVENTION This invention relates to improvements in battery structures used for power storage, such as redox flow batteries, and more particularly to batteries with improved electrolytes and diaphragms within the cells. BACKGROUND OF THE INVENTION Electrical energy is difficult to store in its raw form, so it must be converted into a storable energy form. On the other hand, in order to provide stable power supply, it is necessary to supply or generate power in accordance with the power demand. For this reason, electric power companies always construct power generation facilities that meet the maximum demand and generate power in response to demand. However, as shown by the power demand curve P in FIG. 4, there is a large difference in power demand during the day and at night. Similar phenomena occur across weeks, months, and seasons. Therefore, if it is possible to store electricity efficiently, it is possible to store surplus electricity during off-peak hours (corresponding to the part indicated by X in Figure 1) and release it during peak hours. It is possible to cover the portion indicated by Y, and it is possible to respond to fluctuations in demand.
This means that it is necessary to always generate only approximately constant electric power (an amount corresponding to the broken line Z in FIG. 1). If such load leveling can be achieved,
It becomes possible to reduce the amount of power generation equipment, and it can also greatly contribute to saving energy and fuel such as oil. Therefore, various power storage methods have been proposed in the past. For example, pumped storage power generation has already been implemented, but in pumped storage power generation facilities are installed far away from consumption areas, which results in transmission and substation losses, and there are also environmental constraints on location. There are some problems. Therefore, there is a desire to develop a new power storage technology to replace pumped storage power generation, and the development of redox flow batteries is currently underway as one of these technologies. FIG. 5 is a schematic configuration diagram showing an example of a conventional redox flow battery. This redox flow battery 1 includes a cell 2, a positive electrode liquid tank 3, and a negative electrode liquid tank 4, and is called a two-tank system because it uses two tanks 3 and 4. The inside of the cell 2 is partitioned by a diaphragm 5 made of an ion exchange membrane, and one side is the positive electrode cell 2.
a and the other side constitute a negative electrode cell 2b, respectively. A positive electrode 6 and a negative electrode 7 are arranged as electrodes in the positive electrode cell 2a and the negative electrode cell 2b, respectively. In the redox flow battery 1 shown in FIG.
For example, an aqueous solution of ions with varying valences, such as iron ions and chromium ions, is stored in tanks 3 and 4, and pumps P ₁ and P ₂ send this to the flow-through electrolysis cell 2, where it undergoes a redox reaction. Charging and discharging is performed. For example, when Fe ³⁺ /Fe ²⁺ hydrochloric acid solution is used as the positive electrode liquid and Cr ²⁺ /Cr ³⁺ hydrochloric acid solution is used as the negative electrode liquid, the electrochemical reaction at both electrodes 6 and 7 of each redox system is as follows: , and the electromotive force is approximately 1V. Positive electrode: Fe ³⁺ +e discharge――→ ←―― Charge Fe ²⁺ E=0.6V Negative electrode: Cr ²⁺ discharge――→ ←―― Charge Cr ³⁺ +eE=−0.4V Problems to be solved by the invention However, unlike a normal battery, the redox flow battery 1 as described above has a two-component battery configuration in which the positive electrode liquid and the negative electrode liquid have different configurations.
It is necessary to prevent mixing of both electrolytes, and therefore a highly selective ion exchange membrane is used as the diaphragm 5. However, as a result, the internal resistance of the battery cannot be reduced, resulting in high charge/discharge rate. The problem was that efficiency could not be achieved. In addition, the diaphragm 5 is a highly selective ion exchange membrane as described above, that is, it has a special function of being able to prevent active materials from passing through, while also allowing ion transfer carriers such as H ⁺ and Cl ^- to pass through. Therefore, there was a drawback that the cost of the material constituting the diaphragm 5 was high. Furthermore, even if an ion exchange membrane with excellent selectivity was used as the diaphragm 5 as described above, mixing of the active materials could not be avoided. That is, FIGS. 6 and 7 are partially cutaway front views showing the reaction state inside the battery during charging and discharging in the conventional redox flow battery shown in FIG. In Fig. 7, arrows A...D
As shown in the figure, in the conventional redox flow battery, as the charging and discharging operations are repeated, the positive electrode active material and the negative electrode active material permeate through the diaphragm 5, and as a result, the amount of active material in the positive and negative electrode liquids decreases. As a result, the amount of stored power decreases, resulting in a decrease in charging and discharging efficiency. Therefore, an object of the present invention is to provide a relatively inexpensive battery that exhibits extremely small decreases in power storage capacity and charging/discharging efficiency even when used for a long period of time. Means for Solving the Constituent Problems of the Invention To summarize, the present invention is a battery in which a positive electrode and a negative electrode are separated by a diaphragm in a cell, and a positive electrode liquid is supplied to the positive electrode and a negative electrode liquid is supplied to the negative electrode to perform charging and discharging. The structure is characterized in that the positive electrode liquid and the negative electrode liquid before charging are composed of electrolytic solutions of the same composition containing approximately equal moles of positive electrode active material and negative electrode active material, and a porous membrane is used as a diaphragm. , a battery. As the electrolyte before charging, for example, CrCl ₃ −
In addition to the FeCl _{dihydrochloric} acid solution, any active material conventionally used as an electrolyte in battery structures can be used. Further, as the "porous membrane", for example, a membrane made of a material such as a cellulose resin, a polyolefin resin, or a fluororesin can be used.
However, considering that strong acidic solutions such as HCl solutions are often used and high-temperature operations are taken into consideration, fluorine resins are preferred because of their excellent chemical resistance and high-temperature resistance. Furthermore, in consideration of the reduction in internal resistance and the mobility of the ion transfer carrier, a membrane having an electrical resistance of 2.0 Ωcm ² or less and a pore diameter of 0.5 μm or less is preferable. Effect In this invention, since the cathode liquid and the anode liquid before charging contain approximately equal moles of the cathode active material and the anode active material, the active material is placed between both sides of the diaphragm in the cell, that is, between the cathode cell and the anode cell. Almost no concentration gradient occurs, and almost no mass transfer of the active material occurs through the diaphragm. Furthermore, as is clear from the description of the examples below, since the positive and negative electrode liquids are mixed after the discharge operation, even if there is an unreacted active material during the discharge operation, the active material on the other side As a result, the concentrations of the positive electrode active material and the negative electrode active material remain the same as at the beginning of use. That is,
In this invention, even if mixing of active materials occurs through the diaphragm, the efficiency of charging and discharging electricity within one cycle only decreases, and the problem of long-term capacity decrease does not occur. Therefore, it becomes possible to use a porous membrane in which mixing of electrode active materials occurs more easily than an ion exchange membrane. On the other hand, using a porous membrane that tends to cause mixing of electrode active materials causes mixing of the electrolyte through the cell diaphragm, and its self-discharge increases battery efficiency (battery efficiency is
Evaluated by current efficiency. ), leading to a decrease in However, on the other hand, from the viewpoint of membrane resistance, porous membranes are sufficiently smaller, and voltage loss (voltage loss is evaluated by voltage efficiency) is greater in ion exchange membranes. Total energy efficiency is expressed as the product of current efficiency and voltage efficiency. FIG. 8 is a diagram showing the relationship between electrical resistance and each efficiency (current efficiency, voltage efficiency, energy efficiency). In the figure, the black circles indicate the characteristics of the ion exchange membrane.
The white circles indicate the characteristics of the porous membrane. The reason why porous membranes have low electrical resistance is that they are more porous than ion exchange membranes, and ions can more easily permeate through them. As is clear from Figure 8, by using a porous layer as a battery diaphragm, the internal resistance of the diaphragm can be lowered and the voltage efficiency (ratio of voltages during charging and discharging operations) can be increased. This makes it possible to improve the total energy efficiency (ratio of charging and discharging electric power). Also, referring to Fig. 8, if a porous membrane with a large pore diameter is used, it will fall into the area indicated by the white triangle mark, and 2
Mixing of liquids is significant, resulting in a significant decrease in current efficiency. For this reason, the pore diameter is preferably 0.5 μm or less. Furthermore, if the electrical resistance is too high, the advantages of using a porous membrane cannot be utilized. Therefore, the electrical resistance is preferably 2.0Ωcm ² or less. DESCRIPTION OF EMBODIMENTS FIG. 1 is a schematic diagram showing an embodiment of the present invention. In FIG. 1, reference numeral 31 indicates a flow-through electrolytic cell, which has the same structure as cell 2 in the conventional redox flow battery shown in FIG. In this embodiment, the electrolyte tanks connected to the cell 31 are a common tank 41 and a positive electrode tank 42.
and three tanks 41 and 4 of the negative electrode liquid tank 43.
2,43. common tank 41
is charged with an electrolytic solution 44 which is a hydrochloric acid solution containing equal moles of CrCl ₃ and FeCl ₂ . The common tank 41 is connected to the positive electrode cell 31a and the negative electrode cell 31b of the cell 31, and is connected to the pumps 45, 46.
The electrolytic solution 44 is supplied or discharged by the pump. On the other hand, the positive electrode cell 31a and the negative electrode cell 31b have a positive electrode liquid tank 42 and a negative electrode liquid tank 4, as in the conventional redox flow battery shown in FIG.
3 is connected. Now, as described above, CrCl ₃ − is used as the electrolyte 44.
When FeCl ₂ hydrochloric acid solution is used, during charging operation, the electrolyte 44 is pumped through pumps 45 and 46, respectively.
As a result, the positive electrode liquid and the negative electrode liquid are supplied to the positive electrode cell 31a and the negative electrode cell 31b, an electrode reaction occurs, and the reaction-completed positive electrode liquid and negative electrode liquid are discharged into the positive electrode liquid tank 42 and the negative electrode liquid tank 43, respectively. On the other hand, during the discharging operation, the positive electrode liquid and the negative electrode liquid in the positive electrode liquid tank 42 and the negative electrode liquid tank 43 are
The positive electrode cells 31a are transported in the direction of the arrow Y, respectively.
and causes an electrode reaction in the negative electrode cell 31b,
It is collected again into the common tank 41. The electrode reaction in the embodiment shown in FIG. 1 is carried out at the positive electrode and the negative electrode, respectively, according to the following formula. Positive electrode: Fe ³⁺ +Ce ³⁺ charge――→ ←―― Discharge/Fe ³⁺ +Cr ³⁺ +e Negative electrode: Fe ²⁺ +Ce ³⁺ charge――→ ←―― Discharge/Fe ²⁺ +Cr ²⁺ −e By the way In the redox flow battery of the embodiment ^shown in ^FIG . diaphragm 3
No concentration gradient across 2 occurs. Therefore, diaphragm 3
The mass transfer through the positive electrode cell 31a and the negative electrode cell 31b can therefore be effectively prevented from decreasing in concentration of the respective active materials. That is, FIG. 2 shows a schematic front view of the inside of the cell 31 during charging operation, and in this case arrows A and B
This means that the mass transfer shown by is almost non-existent. Similarly, during the discharge operation, almost no mass transfer occurs across the diaphragm 32, and therefore, as shown in the schematic front view in FIG. . Further, in the embodiment shown in FIG. 1, the positive electrode liquid and the negative electrode liquid that have completed the electrode reaction in the charging operation are discharged and stored in the positive electrode liquid tank 42 and the negative electrode liquid tank 43, but in this case, as shown in FIG. As shown in the figure, Fe ³⁺ and Cr 3+ are present in the positive electrode liquid tank 42, and Fe ²⁺ and Cr ³⁺ are present in the negative electrode liquid tank 43.
Cr2 ⁺ is present. On the other hand, during the discharging operation, the positive electrode liquid and negative electrode liquid in the positive electrode liquid tank 42 and negative electrode liquid tank 43 are supplied to the cell 31 again to cause an electrode reaction, but all the active materials in the positive electrode liquid and negative electrode liquid are does not necessarily respond. For example, in the positive electrode cell 31a, the supplied Fe ³⁺
A portion of Cr 2+ may not undergo an electrode reaction and may be discharged as Fe ³⁺ into the common tank 41, and similarly on the negative electrode side, a portion of Cr ²⁺ may remain unreacted and be discharged into the common tank 4.
1 may be ejected. However, even in this case, unreacted Fe ³⁺ and Cr ²⁺ react within the common tank 41 and are converted into Fe ²⁺ and Cr ³⁺ . Therefore, even if unreacted Fe ³⁺ and
Even if Cr ²⁺ is present, the reaction within the common tank 41 causes all the electrolytes stored in the common tank 41 to have the same composition as the initial electrolyte containing equal moles of Fe ²⁺ and Cr ³⁺ It is possible.
Therefore, it can be seen that even if the charging/discharging operation is repeated over a long period of time, the charging/discharging efficiency does not decrease at all. Furthermore, in the embodiment shown in FIG.
Porous membranes are used as The porous membrane is
As mentioned above, there is a problem in that active materials are more likely to mix than with conventionally used ion exchange membranes. However, even if the active materials are mixed as described above, when the electrolyte is returned to the common tank 41, the reaction within the common tank 41 causes the electrolyte to have the same composition as at the beginning. Therefore, the disadvantage that active material cases are more likely to occur than with ion exchange membranes is that
This problem is solved by the above configuration. In addition, since the electrical resistance of the porous membrane is lower than that of the ion exchange membrane, the internal resistance of the redox flow battery shown in FIG. 1 is effectively reduced and the voltage efficiency is increased. In addition, in the above embodiment, an example using three tanks was explained, but the present invention is not limited to this, and a two-tank system with one positive electrode liquid tank and one negative electrode liquid tank, or a system using four or more tanks is also applicable. Needless to say, it can also be applied. To explain this in more detail, the two-tank system has two tanks 3 and 4 for 21 cells, as shown in FIG. As shown in Figure 9, the 4-tank system means that for 21 cells,
It refers to one that has four tanks 3a, 3b, 4a, 4b. All of these are simply changes to the way the electrolyte circulates, and are batteries in which the positive and negative electrodes are separated by a diaphragm within the cell, and charging and discharging is performed by supplying the positive electrode to the positive electrode and the negative electrode to the negative electrode. In some respects, this embodiment is similar to the embodiment described above using three tanks. The above-mentioned implementation can be carried out by forming the above-mentioned positive electrode solution and negative electrode solution before charging with an electrolytic solution having the same composition containing approximately equimolar amounts of positive electrode active material and negative electrode active material, and using a porous membrane as the above-mentioned diaphragm. Achieve the same effect as the example. Furthermore, in the above example, FeCl ₂ was used as the active material.
Although the case where CrCl 3 and CrCl ₃ are used has been described, it goes without saying that the present invention is not limited to this and that various active materials as shown in the table below can be used.

【table】

[Table] Next, an example of specific experimental results of the present invention will be explained. In a small flow-through electrolysis cell with an electrode area of 1 x 10 cm ² , a 1 x 10 cm ² carbon cloth was used as the electrode.
As a diaphragm, the pore diameter is 0.1μm, the porosity is approximately 60%, and the film thickness is 70μm.
A polypropylene membrane of m was used. As the electrolyte before charging, 1 mol of FeCl ₂ and CrCl ₃ was added to 2N
A battery was constructed using the material dissolved in HCl. As a comparative example, a conventional battery was constructed under the same conditions as described above, except that a cation exchange membrane (sulfonic acid group) having a thickness of about 200 μm was used as the diaphragm.
Charging and discharging experiments were conducted using these batteries.
The charge/discharge experiment was conducted by constant current charging/discharging at a current density of 40 mA/cm ² . The average energy efficiency of a battery using the above porous membrane as a diaphragm is
It was around 83%. The average energy efficiency of the above-mentioned conventional batteries using cation exchange membranes was about 70%. In the case of the example, no decrease in capacity was observed even after several tens of cycles of charging and discharging operations. Effects of the Invention As described above, according to the present invention, in a battery structure in which a positive electrode and a negative electrode are separated by a diaphragm in a cell, and charging and discharging are performed by supplying a positive electrode liquid to the positive electrode and a negative electrode liquid to the negative electrode, The positive and negative electrode solutions in the cell are composed of electrolytes with the same composition containing approximately equal moles of positive and negative active materials, and a porous membrane is used as the diaphragm. In addition, the electrolyte returned to the common tank after the discharge operation has the same composition as the initial one, which prevents a decrease in power storage capacity and charge/discharge efficiency. It becomes possible to realize an extremely small battery structure. Furthermore, since a porous membrane is used as the diaphragm, it is possible to lower the internal resistance and increase the energy efficiency of the entire battery, and the characteristics of the porous membrane do not change even at high temperatures. It is also possible to increase energy efficiency at high temperatures. Furthermore, since porous membranes are relatively inexpensive compared to ion exchange membranes, which are conventional diaphragm materials, it is also possible to reduce costs. This invention is not limited to redox flow batteries.
It should be pointed out that the present invention can be applied to secondary batteries for power storage in general.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an embodiment of the present invention. 2 and 3 are diagrams showing reaction states during charging and discharging operations within the cell in the embodiment shown in FIG. 1. FIG. 4 is a diagram showing a power demand curve. FIG. 5 is a schematic configuration diagram showing an example of a conventional redox flow battery.
6 and 7 are partially cutaway front views showing reaction states during charging and discharging operations in the cells of the conventional redox flow battery shown in FIG. 5. FIG. Figure 8 shows electrical resistance and each efficiency (current efficiency,
FIG. FIG. 9 is a schematic diagram showing an example of a four-tank redox flow battery. In the figure, 31 is a cell, 32 is a diaphragm, 33 is a positive electrode, 34 is a negative electrode, 41 is a common tank, 42 is a positive electrode tank, 43 is a negative electrode tank, and 44 is an electrolyte.

Claims (1)

[Claims] 1. A positive electrode and a negative electrode are separated by a diaphragm in a cell,
In a battery that is charged and discharged by supplying a positive electrode liquid to the positive electrode and a negative electrode liquid to the negative electrode, the positive electrode liquid and the negative electrode liquid before charging are made of an electrolytic solution of the same composition containing approximately equal moles of the positive electrode active material and the negative electrode active material. What is claimed is: 1. A battery comprising: a porous membrane as the diaphragm; 2. The battery according to claim 1, wherein the electrolytic solution before charging is a CrCl ₃ -FeCl ₂ hydrochloric acid solution. 3 The porous membrane has a pore diameter of 0.5 μm or less and an electrical resistance.
The battery according to claim 1 or ² , which is a membrane made of a material having a characteristic of 2.0 Ωcm 2 or less.