JP2011233372A - Redox flow battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a redox flow battery which can improve energy density.SOLUTION: A redox flow battery 100 is charged and discharged by supplying a positive electrode electrolyte solution and a negative electrode electrolyte solution to a positive electrode cell 102, and to a negative electrode cell 103 respectively. Both the positive electrode electrolyte solution and the negative electrode electrolyte solution contain vanadium ions as an active material. In addition to the vanadium ions, the negative electrode electrolyte solution contains other metal ions such as chromium ions that have a lower potential than that of vanadium ions. The redox flow battery 100 can suppress a side reaction such as generation of hydrogen gas owing to reduction, which occurs at the end of charging, of the other metal ions contained with vanadium ions, even where a state of charge of the battery 100 is close to 100%. Accordingly, since the utilization efficiency of vanadium ions in the electrolyte solutions can be increased by enhancing the state of charge, the redox flow battery 100 can have improved energy density compared with conventional redox flow batteries.

Description

  The present invention relates to a redox flow battery containing vanadium ions as an active material. In particular, the present invention relates to a redox flow battery capable of improving the energy density as compared with a conventional vanadium redox flow battery.

  In recent years, introduction of new energy such as solar power generation and wind power generation has been promoted worldwide as a countermeasure against global warming. Since these power generation outputs are affected by the weather, it is predicted that there will be a problem in the operation of the electric power system such that it becomes difficult to maintain the frequency and voltage when the mass introduction is advanced. As one of the countermeasures against this problem, it is expected to install a large-capacity storage battery to smooth the output fluctuation, save surplus power, and level the load.

  One of the large-capacity storage batteries is a redox flow battery. A redox flow battery performs charge / discharge by supplying a positive electrode electrolyte and a negative electrode electrolyte to battery cells in which a diaphragm is interposed between a positive electrode and a negative electrode. As the electrolytic solution, typically, an aqueous solution containing a water-soluble metal ion whose valence is changed by oxidation-reduction is used, and the metal ion is used as an active material. Recently, the most widely studied form is a vanadium redox flow battery using vanadium (V) ions as active materials of both electrodes (for example, Patent Document 1). Vanadium redox flow batteries have been put into practical use and are expected to be used in the future.

JP2003-157884

  However, in the conventional vanadium redox flow battery, it is difficult to further improve the energy density.

  Generally, a battery is desired to have a higher energy density. In order to increase the energy density, for example, it is conceivable to increase the solubility of the active material in the electrolytic solution or increase the utilization rate of the electrolytic solution, that is, the utilization rate of metal ions contained as an active material in the electrolytic solution. . Here, the utilization rate refers to the battery capacity (discharge capacity) that can actually be used with respect to the theoretical battery capacity (Ah) of the metal ion, that is, the battery capacity at the lower limit of charge depth (SOC: State of Charge). And the difference between the battery capacity at the upper limit charging depth.

  However, when charging is performed with the utilization rate increased as much as possible, in other words, when the charging depth is increased, the following problems occur. Specifically, since a typical form of a redox flow battery uses an aqueous solution as an electrolyte as described above, a side reaction such as generation of hydrogen due to water decomposition occurs at the negative electrode at the end of charging.

  The above side reactions are (1) current loss (a part of the amount of electricity (Ah) used during charging is not used for battery reaction (change in valence), but used for other reactions such as decomposition of water). Loss), the battery efficiency is reduced, (2) the charging depth of the positive and negative electrodes is varied, and the usable battery capacity is reduced due to the difference in charging depth. Therefore, in actual battery operation, a charge stop voltage (upper limit charge voltage) is determined so as to be used within a range where the side reaction does not occur. In order to suppress the side reaction, for example, Patent Document 1 proposes to continue charging so that divalent V ions in the negative electrode active material become 94% or less. However, since the cell resistance increases in long-term use, the charge state (charge depth) at the beginning of use cannot be maintained if the charge stop voltage is set to a constant value without changing from the beginning. Therefore, over time, the charge stop voltage is increased in order to ensure a predetermined state of charge. As a result, it has been difficult to ensure a high state of charge without releasing hydrogen gas over a long period of time.

  As described above, from the viewpoint of suppressing side reactions, at present, the charge depth of vanadium ions in the electrolyte cannot be increased to 90% or more over a long period of time, and vanadium ions cannot be sufficiently used. Therefore, in the conventional vanadium redox flow battery, it is difficult to improve the utilization ratio of vanadium ions to 90% or more, and further, and there is a limit to improvement of energy density.

  Then, the objective of this invention is providing the redox flow battery which can improve an energy density.

  In the conventional vanadium redox flow battery, the metal ion serving as the active material is only vanadium ion. On the other hand, the inventors of the present invention have an electrolyte solution containing vanadium ions as an active material, for example, a lower potential (oxidation-reduction potential; hereinafter referred to simply as vanadium ions such as chromium (Cr) ions) As a result of the inclusion of vanadium ions with metal ions having an electric potential), a surprising finding has been obtained that the utilization of vanadium ions can be greatly improved compared to conventional vanadium redox flow batteries. It was. The reason is considered as follows.

In a redox flow battery using an electrolytic solution containing vanadium ions in the active material, the following reaction occurs at the negative electrode when charged. Also shown is the standard potential at which this reaction occurs.
Charging (negative ^{electrode): V 3+ + e - →} V 2+ potential: about -0.26V (V ³⁺ / V ²⁺⁾

In addition, the following side reaction may occur at the end of charging. The standard potential when the following reduction reaction occurs is also shown.
Charge (positive _{electrode) H 2 O → 1 / 2O} 2 + 2H + + 2e -
Charge ^{(negative) H + + e - → 1} / 2H 2 potential: about 0V (the actual potential: approximately -0.5 V)

As described above, hydrogen gas may be generated on the negative electrode side. Here, in actual operation, the potential at the actual side reaction tends to be lower than the standard value depending on the electrode material used. For example, when the electrode material is a carbon material, the hydrogen overvoltage is large, so that the potential when hydrogen is generated is lower than the potential at the time of battery reaction (about -0.26 V) generated at the negative electrode: about- It becomes about 0.5V. Therefore, at the time of charging, although the reduction reaction of vanadium ions (V ³⁺ → V ²⁺ ) mainly occurs at the time of charging, when the charging voltage is increased at the end of charging and the potential of the negative electrode becomes more base, Along with the reduction reaction of vanadium ions, hydrogen gas can be generated.

On the other hand, consider a case where the negative electrode electrolyte contains metal ions having a lower potential than vanadium ions in addition to vanadium ions. For example, the potential of Cr ³⁺ / Cr ²⁺ is about −0.42 V, which is lower than the potential of V ³⁺ / V ²⁺ (about −0.26 V), but side reactions such as the generation of hydrogen occur. It exists on the noble side from the actual potential (about -0.5V). Therefore, for example, in the case of containing trivalent chromium ions (Cr ³⁺ ), first, a reduction reaction of Cr ³⁺ occurs before the side reaction occurs. That is, at the end of charging, the reduction reaction of Cr ³⁺ occurs as part of the battery reaction together with the reduction reaction of V ³⁺ which is the main reaction of the battery. The side reaction can be suppressed by a reduction reaction of a metal ion different from the vanadium ion.

  By containing metal ions having a lower potential than vanadium ions together with vanadium ions in the negative electrode electrolyte as described above, for example, even when charging with a charging depth exceeding 90% is performed, the side reaction described above is performed. It is hard to occur or does not occur substantially. Therefore, in the form containing the above metal ions, it is considered that the vanadium ions in the electrolytic solution can be used repeatedly and stably as compared with the conventional vanadium redox flow battery. Thus, energy density can be improved by raising the utilization factor of vanadium ion. The present invention is based on the above findings.

  The present invention relates to a redox flow battery that charges and discharges by supplying a positive electrode electrolyte and a negative electrode electrolyte to a battery cell. Both the positive electrode electrolyte and the negative electrode electrolyte contain vanadium ions. Furthermore, at least the negative electrode electrolyte contains metal ions having a lower potential than vanadium ions.

  According to the above configuration, for example, even when the charge depth of the negative electrode electrolyte is charged to nearly 100%, another metal ion contained together with vanadium ions is reduced at the end of the charge, and thus the above-described configuration is performed. Side reactions such as generation of hydrogen gas can be suppressed. Therefore, conventionally, the charging depth can be increased only to about 90% due to the side reaction occurring at the end of charging, whereas the charging depth can be increased to nearly 100% in the above configuration. Since the utilization rate of vanadium ions in the electrolytic solution can be increased by increasing the charging depth, in the above configuration, the energy density of the redox flow battery can be improved as compared with the conventional case.

  Moreover, according to the said structure, since a side reaction can be suppressed as mentioned above, the various malfunction (a battery efficiency fall, a battery capacity fall) accompanying a side reaction can also be suppressed effectively. Therefore, according to the said structure, it can have the outstanding battery characteristic over a long term.

  A typical form of the present invention includes a form in which the negative electrode electrolyte contains vanadium ions and metal ions having a lower potential than the vanadium ions, and the positive electrode electrolyte contains vanadium ions. The presence of the metal ions in at least the negative electrode electrolyte effectively suppresses side reactions at the end of charging as described above and increases the utilization rate of vanadium ions.

  In addition, both positive and negative electrode electrolytes include vanadium ions and metal ions having a lower potential than vanadium ions. Typically, the metal ion species in both electrode electrolytes are the same. be able to. When the metal ion species in the electrolyte solution of both electrodes are the same, (1) the metal ion at the base potential in the negative electrode electrolyte moves to the counter electrode (here, the positive electrode), and the metal ions that originally react with the negative electrode are relatively (2) Liquid transfer over time with charge / discharge (electrolyte from one electrode moves to the other) Even if there is a variation in the amount of electrolyte solution in both electrodes, the variation can be easily corrected by mixing the electrolyte solution in both electrodes, etc. (3) Excellent electrolyte productivity It is possible to achieve a unique effect. In this embodiment, metal ions having a lower potential than the vanadium ions present in the positive electrode electrolyte exist mainly for the purpose of overlapping the metal ion species of the electrolyte solutions of both electrodes, and do not act positively as an active material. Therefore, the concentration of the base potential metal ions in the cathode electrolyte can be different from the concentration of the base potential metal ions in the anode electrolyte, but if equal, the effects of (1) to (3) above Easy to get.

The above low potential metal ions are water-soluble like vanadium ions or dissolved in acid aqueous solution, and are present on the noble side from the actual potential (about -0.5V) when side reactions occur. Those that do are preferred. Specifically, at least one metal ion of chromium ion and zinc ion can be mentioned. The standard potential of chromium is Cr ³⁺ / Cr ²⁺ : about −0.42 V, which is lower than the potential of V ³⁺ / V ²⁺ (about −0.26 V), and the potential of the side reaction described above ( It is more noble than about -0.5V). On the other hand, the standard potential of zinc is Zn ²⁺ / Zn (metal): about -0.76 V, which is less basic than the potential of V ^{3 +} / V ²⁺ (about -0.26 V), but the above side reaction It is lower than the potential. However, zinc has a sufficiently large hydrogen overvoltage and can cause a battery reaction. In addition to vanadium ions, either a form containing one kind of the above metal ions or a form containing a combination of a plurality of kinds of metal ions having different potentials may be used.

As the above metal ion, a reversible oxidation-reduction reaction is possible, and at least when it functions as a negative electrode active material, the amount of vanadium ion necessary for storing a predetermined amount of power (kWh) is reduced in practice. it can. Therefore, in this case, it is expected that the metal ions used for the active material can be stably supplied at a lower cost. The above-described chromium ions and zinc ions reversibly undergo a redox reaction in a sulfuric acid solution. That is, Cr ²⁺ or Zn (metal) reduced during charging is used for the discharge reaction of the battery (Cr ²⁺ → Cr ³⁺ + e ⁻ , Zn → Zn ²⁺ + 2e ⁻ ) during discharge, and is used as an active material. Can be used repeatedly. Therefore, the metal ions preferably include chromium ions and zinc ions.

When chromium ions are contained as the low-potential metal ions, a more specific form is a form containing at least one kind of chromium ions of divalent chromium ions and trivalent chromium ions. By containing any of the above chromium ions, during discharge: trivalent chromium ions (Cr ³⁺ ) are present, during charging: divalent chromium ions (Cr ²⁺ ) are present, and repeated charge and discharge Thus, both chromium ions are present. Chromium is easy to handle because it always exists stably as an ion.

  As one form of the present invention, a form in which the total concentration of the above-mentioned low potential metal ions is 0.1 M or more and 5 M or less (M: molar concentration) can be mentioned.

  When the total concentration of the metal ions is less than 0.1M, the reduction reaction of the metal ions is hardly caused and it is difficult to obtain the effect of suppressing the side reaction described above by this reduction reaction. As a result, it is difficult to sufficiently improve the energy density. As the total concentration of the metal ions is higher, the side reaction suppression effect and the energy density are increased, but the solubility of vanadium ions tends to decrease due to the increase of metal ions. In the case of ensuring sufficient solubility, the side reaction suppression effect and the like can be obtained even when the total concentration of the metal ions is 1 M or less, and even 0.5 M or less. Further, when the solvent of the electrolytic solution is an aqueous solution of acid, the increase in the acid concentration of the electrolytic solution causes a decrease in the solubility of metal ions.

As one form of the present invention, a form in which the electrolyte solution of both electrodes contains a sulfate anion (SO ₄ ²⁻ ) can be mentioned.

An aqueous solution containing at least one of a sulfate anion (SO ₄ ²⁻ ), a phosphate anion (PO ₄ ³⁻ ), and a nitrate anion (NO ₃ ⁻ ) can be suitably used as the solvent for the electrolyte solution of both electrodes. . These acid aqueous solutions may (1) improve the stability and reactivity of vanadium ions or the above metal ions in the electrolyte, and may improve the solubility, (2) high ionic conductivity, battery A plurality of effects can be expected, such as (3) unlike the case of using hydrochloric acid (HCl), and no chlorine gas is generated. In particular, a form containing a sulfate anion (SO ₄ ²⁻ ) is preferable because stability and reactivity of vanadium ions and the above metal ions can be improved as compared with the case of containing a phosphate anion and a nitrate anion. In order for the electrolyte solution of both electrodes to contain a sulfate anion, for example, use of a sulfate containing vanadium ions or the above metal ions can be mentioned.

As one form of the invention, the solvent of the electrolytic solution of the poles include the form of an aqueous solution of H ₂ SO _4. At this time, the sulfuric acid concentration of the electrolyte solution of both electrodes is preferably 5M or less.

In addition to using sulfate as described above, if the electrolyte solvent is an aqueous solution of H ₂ SO ₄ (sulfuric acid aqueous solution), as described above, the stability and reactivity of vanadium ions and metal ions are improved. The internal resistance can be reduced. However, if the sulfuric acid concentration is too high, the solubility of metal ions such as vanadium ions and chromium ions may decrease and the viscosity of the electrolyte may increase, so the sulfuric acid concentration is preferably 5M or less, and 1M to 4M are used. Easy to do.

  As one form of this invention, the form operated so that the charge depth of the said negative electrode electrolyte solution may exceed 90% is mentioned.

  In the present invention, in addition to vanadium ions, the negative electrode electrolyte contains metal ions with a lower potential than vanadium ions, so that side reactions are suppressed as described above even when the charging depth is increased to more than 90%. can do. Thus, the utilization rate of vanadium ion can be effectively raised by raising a charge depth.

  The redox flow battery of the present invention can improve energy density.

FIG. 1 is an explanatory diagram showing the operating principle of a battery system including a redox flow battery. FIG. 2 is a graph showing the relationship between the charge / discharge cycle time (sec) and the battery voltage (V) in the system of the example produced in Test Example 1. FIG. 3 is a graph showing the relationship between the charging time (sec) of the comparison system (I) produced in Test Example 1 and the battery voltage (V). FIG. 4 is a graph showing the relationship between the charge / discharge cycle time (sec) and the battery voltage (V) of the comparative system (II) produced in Test Example 1.

  Hereinafter, an outline of a battery system including the redox flow battery of the embodiment will be described with reference to FIG. The ions shown in FIG. 1 are exemplary.

  The redox flow battery 100 is typically connected to a power generation unit (e.g., a solar power generator, a wind power generator, other general power plants, etc.), an electric power system, and a consumer via an AC / DC converter. Connected, charging is performed using the power generation unit as a power supply source, and discharging is performed using a power system or a consumer as a power supply target. In performing the charging / discharging, the following battery system including a redox flow battery 100 and a circulation mechanism (tank, conduit, pump) for circulating an electrolyte in the battery 100 is constructed.

  The redox flow battery 100 includes a positive electrode cell 102 incorporating a positive electrode 104, a negative electrode cell 103 incorporating a negative electrode 105, and a diaphragm 101 that separates the cells 102 and 103 and appropriately transmits ions. A positive electrode electrolyte tank 106 is connected to the positive electrode cell 102 via conduits 108 and 110. A negative electrode electrolyte tank 107 is connected to the negative electrode cell 103 via conduits 109 and 111. The conduits 108 and 109 include pumps 112 and 113 for circulating the electrolyte solution of each electrode. The redox flow battery 100 uses the conduits 108 to 111 and the pumps 112 and 113 to form the positive electrode electrolyte in the tank 106 and the negative electrode electrolyte in the tank 107 in the positive electrode cell 102 (positive electrode 104) and the negative electrode cell 103 (negative electrode 105), respectively. Is charged and discharged along with the valence change reaction of the metal ions that become the active material in the electrolyte solution of each electrode.

  The redox flow battery 100 typically uses a form called a cell stack in which a plurality of the cells 102 and 103 are stacked. The cells 102 and 103 have a bipolar plate (not shown) in which the positive electrode 104 is disposed on one side and the negative electrode 105 is disposed on the other side, a liquid supply hole for supplying an electrolytic solution, and a drainage hole for discharging the electrolytic solution. A typical configuration is a cell frame including a frame (not shown) formed on the outer periphery of the bipolar plate. By laminating a plurality of cell frames, the liquid supply hole and the drainage hole constitute a flow path for the electrolytic solution, and the flow path is appropriately connected to the conduits 108 to 111. The cell stack is configured by repeatedly stacking a cell frame, a positive electrode 104, a diaphragm 101, a negative electrode 105, a cell frame,. In addition, a well-known structure can be utilized suitably for the basic structure of a redox flow battery system.

  In particular, in the present invention, both of the positive electrode electrolyte and the negative electrode electrolyte contain vanadium ions, and the negative electrode electrolyte further contains metal ions having a lower potential than vanadium ions in addition to vanadium ions. Contained (FIG. 1 shows chromium ions as an example). Hereinafter, it will be described more specifically with reference to test examples.

[Test Example 1]
The redox flow battery system shown in FIG. 1 was constructed and charged and discharged under various conditions using various electrolytes containing vanadium ions on both the positive and negative electrodes.

The following was prepared as an example system.
As a positive electrode electrolyte, an electrolyte with a concentration of vanadium ions (tetravalent) of 1.7 M is obtained by dissolving sulfate: vanadium sulfate (tetravalent) in a sulfuric acid aqueous solution (H ₂ SO ₄ aq) having a sulfuric acid concentration of 2.6 M. 9 ml (9 cc) was prepared.
Dissolve sulfate: vanadium sulfate (trivalent) and chromium sulfate (trivalent) in sulfuric acid aqueous solution (H ₂ SO ₄ aq) with a sulfuric acid concentration of 1.75M as the negative electrode electrolyte, and the concentration of vanadium ions (trivalent) 6 ml (6 cc) of an electrolyte solution having a concentration of 1.7 M and a chromium ion (trivalent) concentration of 0.1 M was prepared. By making the amount of the positive electrode electrolyte larger than the amount of the negative electrode electrolyte, the battery reaction on the negative electrode side (including not only the reduction reaction of vanadium ions but also the reduction reaction of chromium ions) can be sufficiently performed during charging. (This also applies to Test Example 2 described later).

Carbon felt was used for each electrode, and an ion exchange membrane was used for the diaphragm. The constituent material of an electrode or a diaphragm can be selected as appropriate. The electrode composed of carbon felt has the following effects: (1) Oxygen gas is hardly generated on the positive electrode side and hydrogen gas is not generated on the negative electrode side, (2) Large surface area, (3) Excellent electrolyte circulation . The ion exchange membrane has effects such as (1) excellent separability of metal ions that are active materials of the positive and negative electrodes, and (2) excellent permeability of H ⁺ ions (charge carriers inside the battery).

In Test Example 1, a small cell having an electrode reaction area of 9 cm ² was prepared, and using the prepared electrolyte solution for each electrode, a constant current of 630 mA (current density: 70 mA / cm ² ) was used. Was charged. More specifically, the negative electrode electrolyte was charged until the vanadium ion charging depth corresponded to 109%. The charging depth is a numerical value when the vanadium ion only is used as an active material, and the charging depth is more than 100%. In addition to the charging depth of vanadium ions being almost 100%, This shows that Cr ³⁺ is changed to Cr ²⁺ to be charged. Then, it switched to discharge and discharged, and also charge / discharge was repeated on the same charging conditions as the above. Fig. 2 shows the relationship between the charge / discharge cycle time and the battery voltage.

As a comparison system, an all-vanadium redox flow battery system was constructed. The basic configuration of the comparison system is the same as that of the above-described embodiment system, and is the same as that of the above-described embodiment system except that the electrolytic solution and the operating conditions are different. In Test Example 1, as the positive electrode electrolyte and the negative electrode electrolyte, the positive electrode: sulfuric acid aqueous solution having a sulfuric acid concentration of 2.6 M (H ₂ SO ₄ aq), vanadium sulfate (tetravalent), negative electrode: sulfuric acid aqueous solution having a sulfuric acid concentration of 1.75 M Dissolve vanadium sulfate (trivalent) in (H ₂ SO ₄ aq), and use a 1.7M electrolyte solution for both the positive electrode: vanadium ion (tetravalent) concentration and the negative electrode: vanadium ion (trivalent) concentration. Produced.

Then, in the comparison system (I), a small cell having an electrode reaction area of 9 cm ² was produced, and the above-described vanadium electrolyte was used for each 10 ml (10 cc) positive and negative, current: 540 mA (current density: 60 mA / Charging was performed at a constant current of cm ² ). In Comparative System (I), charging was continued for a while even when the charging depth of vanadium ions in the positive electrode electrolyte exceeded 100%. Fig. 3 shows the relationship between charging time and battery voltage.

On the other hand, the comparative system (II) was the same as the comparative system (I) except that the amount of electrolyte and the operating conditions were different. Specifically, charging was performed at a constant current of 630 mA (current density: 70 mA / cm ² ) using 7 ml (7 cc) of each of the above vanadium electrolyte solutions. In the comparison system (II), when the voltage reached 1.6 V (vanadium ion charging depth: 78%), the charging was stopped and switched to discharging, and charging and discharging were repeated in the same manner. FIG. 4 shows the relationship between the charge / discharge cycle time and the battery voltage.

  As a result, in the comparison system (I), as shown in FIG. 3, the voltage rose rapidly from near 1.6V to 2.6V or higher. When charging was continued, generation of hydrogen gas was observed from the negative electrode. After discharging from such a state, repeated charging and discharging several times under the same conditions (charging: until the charging state exceeds 100%), the internal resistance of the battery gradually increased, and the battery capacity also increased. There was a tendency to decrease.

  On the other hand, in the comparative system (II), generation of hydrogen gas was not observed because the upper limit voltage of charging was 1.6V. Moreover, although charging / discharging was repeated several times, an increase in the internal resistance of the battery and a decrease in the battery capacity were not observed, and the battery could operate stably repeatedly. However, in the comparative system (II), the battery capacity that was actually available was 20.4 minutes compared to the theoretical capacity (vanadium ion concentration: 1.7M, 7ml, converted from 630mA to discharge time): 30.4 minutes. The utilization rate of ions is 67% (≦ 90%).

  On the other hand, in the example system, although a voltage increase is observed from around 1.6 V as shown in FIG. 2, this increase is not steep and is slower than the comparison system (I). In addition, from the voltage characteristics after the voltage becomes 1.6 V or more, at the time of charging, at the negative electrode, there is a further reduction reaction of vanadium ions and a reduction reaction of chromium ions (trivalent). Observed. Furthermore, unlike the comparative system (I), in the example system, even when the charging depth of the negative electrode exceeded 100%, the increase in battery voltage was suppressed and it was at most about 2V. In addition, generation of hydrogen gas was not observed in the example system. And, the discharge time (discharge capacity) of the example system is 25.9 minutes, 99.6% with respect to the theoretical capacity (vanadium ion concentration: 1.75 M, 6 ml, value converted to discharge time from 630 mA: 26 minutes), almost 100%. A close capacity was obtained, with utilization exceeding 90%. Moreover, although charging / discharging was performed repeatedly, it was confirmed that there is no decrease in battery capacity and the operation is stable.

  From Test Example 1 above, at least the negative electrode electrolyte contains, in addition to vanadium ions, metal ions having a lower potential than vanadium ions on the negative electrode side, effectively increasing the utilization of vanadium ions, It can be said that the density can be improved.

[Test Example 2]
The redox flow battery system shown in FIG. 1 was constructed, and charging / discharging was performed using an electrolytic solution containing vanadium ions and chromium ions as the electrolytic solution for the positive and negative electrodes.

  In Test Example 2, as the positive electrode electrolyte, the same raw materials as in the system of Example 1 of Test Example 1, further prepared sulfate: chromium sulfate (trivalent), vanadium ion (tetravalent) concentration: 1.7M, Chromium ion (trivalent) concentration: 9 ml (9 cc) of 0.1M electrolyte was prepared. The negative electrode electrolyte solution was the same as that of the example system of Test Example 1 (vanadium ion (trivalent) concentration: 1.7 M, chromium ion (trivalent) concentration: 0.1 M, 6 ml (6 cc)). Other configurations were the same as those of the example system of Test Example 1.

Then, a small cell similar to Test Example 1 (reaction area: 9 cm ² ) was prepared, and using the prepared electrolyte solution for each electrode, the same as in Test Example 1, current: 630 mA (current density: 70 mA / cm ² ) At a constant current of) until the point where the vanadium ion charge depth corresponds to 110%. Then, the behavior of the voltage characteristic of the system of Test Example 2 showed almost the same behavior as that of the Example system of Test Example 1. In addition, the discharge time of the system of Test Example 2 is 25 minutes, and it is confirmed that the battery capacity is nearly 100%, which is 98% of the theoretical capacity (26 minutes), and the utilization rate can be over 90%. It was done. Further, the system of Test Example 2 was repeatedly charged and discharged, but could be operated stably and no generation of hydrogen gas was observed.

  Therefore, from Test Example 2, in addition to vanadium ions, the positive and negative electrode electrolytes contain metal ions having a lower potential than the negative vanadium ions, thereby effectively increasing the utilization of vanadium ions. It can be said that the energy density can be improved.

  The above-described embodiment can be appropriately changed without departing from the gist of the present invention, and is not limited to the above-described configuration. For example, the type and concentration of metal ions, the concentration of the solvent of the electrolytic solution, and the like can be changed as appropriate.

  The redox flow battery of the present invention has a large capacity for the purpose of stabilizing fluctuations in power generation output, storing electricity when surplus of generated power, load leveling, etc., for power generation of new energy such as solar power generation and wind power generation. It can utilize suitably for a storage battery. In addition, the redox flow battery of the present invention can be suitably used as a large-capacity storage battery that is provided in a general power plant and is intended for measures against instantaneous voltage drop / power outage and load leveling.

100 Redox flow battery 101 Diaphragm 102 Positive electrode cell 103 Negative electrode cell
104 Positive electrode 105 Negative electrode 106 Tank for positive electrolyte
107 Tank for negative electrolyte 108,109,110,111 Conduit 112,113 Pump

Claims (7)

A redox flow battery for charging and discharging by supplying a positive electrode electrolyte and a negative electrode electrolyte to a battery cell,
The positive electrode electrolyte and the negative electrode electrolyte both contain vanadium ions,
At least the negative electrode electrolyte further contains metal ions having a lower potential than vanadium ions.   2. The redox flow battery according to claim 1, wherein each of the positive electrode electrolyte and the negative electrode electrolyte further contains metal ions having a base potential lower than that of vanadium ions. The base potential metal ion is at least one metal ion of chromium ion and zinc ion,
3. The redox flow battery according to claim 1, wherein a total concentration of the metal ions is 0.1 M or more and 5 M or less.   The redox flow battery according to any one of claims 1 to 3, wherein the metal ions having a low potential are at least one kind of chromium ions of divalent chromium ions and trivalent chromium ions.   5. The redox flow battery according to claim 1, wherein the electrolyte solution of both electrodes contains a sulfate anion. The solvent of the electrolyte solution of both electrodes is an aqueous solution of H ₂ SO ₄ ,
6. The redox flow battery according to any one of claims 1 to 5, wherein the sulfuric acid concentrations of the electrolyte solutions of both electrodes are all 5M or less.   The redox flow battery according to any one of claims 1 to 6, wherein the redox flow battery is operated such that a charging depth of the negative electrode electrolyte exceeds 90%.

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