Patent Publication Number: US-2021167411-A1

Title: Redox flow battery

Description:
TECHNICAL FIELD 
     The present invention relates to a redox flow battery. 
     The present application claims priority from Japanese Patent Application No. 2017-121744 filed on Jun. 21, 2017, the entire contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     PTL 1 discloses a redox flow battery including, as main components, battery cells each including: a positive electrode to which a positive electrolyte is supplied; a negative electrode to which a negative electrolyte is supplied; and a membrane interposed between the positive electrode and the negative electrode. During charge/discharge of the redox flow battery, the positive and negative electrolytes are supplied to their respective electrodes. In each battery cell, a layered body composed of the positive electrode, the membrane, and the negative electrode is sandwiched between a pair of cell frames. Each of the cell frames includes: a bipolar plate with positive and negative electrodes disposed on its front and back sides, respectively; and a frame body disposed on the outer circumference of the bipolar plate. The bipolar plate is formed of an electrically conductive carbon plastic containing graphite. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2015-210849 
     SUMMARY OF INVENTION 
     The redox flow battery according to the present disclosure includes: 
     a battery cell including a positive electrode, a negative electrode, and a membrane interposed between the positive electrode and the negative electrode; 
     a pair of cell frames each including a bipolar plate and a frame body surrounding a circumferential edge of the bipolar plate, the pair of cell frames holding the battery cell therebetween; 
     a positive electrolyte supplied to the positive electrode; and 
     a negative electrolyte supplied to the negative electrode, 
     wherein the bipolar plate is formed of pure titanium or a titanium alloy, and 
     wherein the negative electrolyte has an oxidation-reduction potential of 0.0 V or higher relative to a standard hydrogen electrode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an illustration showing the principle of operation of a redox flow battery according to embodiment 1. 
         FIG. 2  is a schematic structural diagram of the redox flow battery according to embodiment 1. 
         FIG. 3  is a schematic structural diagram of a cell stack included in the redox flow battery according to embodiment 1. 
         FIG. 4  is a schematic plan view of a bipolar plate included in a redox flow battery according to embodiment 2 when the bipolar plate is viewed from one side. 
         FIG. 5  is a schematic cross-sectional view showing bipolar plates included in the redox flow battery according to embodiment 2. 
         FIG. 6  is a schematic plan view of a bipolar plate included in a redox flow battery according to embodiment 3 when the bipolar plate is viewed from one side. 
         FIG. 7  is a schematic perspective view showing the bipolar plate included in the redox flow battery according to embodiment 3. 
         FIG. 8  is a schematic diagram showing a state in which a frame body of a cell frame included in a redox flow battery according to embodiment 4 is attached to a bipolar plate of the cell frame. 
         FIG. 9  is a schematic diagram showing a state in which a frame body of a cell frame included in a redox flow battery according to embodiment 5 is attached to a bipolar plate of the cell frame. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Problems to be Solved by Present Disclosure 
     There is a need for a redox flow battery that can maintain its performance stably for a long time. 
     Accordingly, it is one object to provide a redox flow battery that can maintain its performance stably for a long time. 
     Advantageous Effects of Present Disclosure 
     The redox flow battery according to the present disclosure can maintain its performance stably for a long time. 
     Description of Embodiments of Present Invention 
     It has been found that, when a bipolar plate made of an electrically conductive carbon plastic is used, the bipolar plate may undergo oxidative deterioration in an electrolyte during long-term operation of a redox flow battery (hereinafter may be referred to as an RF battery). When the bipolar plate undergoes oxidative deterioration, the thickness of the bipolar plate may be reduced, so that its mechanical strength may deteriorate. Moreover, a hole may be formed in the bipolar plate, so that the function of the battery itself may not be maintained. 
     The present inventors have conducted studies on the use of a titanium-based bipolar plate in order to prevent the oxidative deterioration of the bipolar plate during long-term used of an RF battery. However, the inventors have found that, even when the titanium-based bipolar plate is used, the titanium-based bipolar plate undergoes oxidative deterioration (dissolution) on the negative electrode side, depending on the type of electrolyte. Therefore, the inventors have conducted studies on a negative electrolyte that can prevent dissolution of the titanium-based bipolar plate. The inventors have found that the use of an electrolyte having a specific potential as the negative electrolyte can prevent the dissolution of the titanium-based bipolar plate, so that an RF battery capable of maintaining its performance stably for a long time can be obtained. Thus, the present invention has been completed. Next, the details of embodiments of the present invention will be enumerated and described. 
     (1) A redox flow battery according to an embodiment of the present invention includes: 
     a battery cell including a positive electrode, a negative electrode, and a membrane interposed between the positive electrode and the negative electrode; 
     a pair of cell frames each including a bipolar plate and a frame body surrounding a circumferential edge of the bipolar plate, the pair of cell frames holding the battery cell therebetween; 
     a positive electrolyte supplied to the positive electrode; and 
     a negative electrolyte supplied to the negative electrode, 
     wherein the bipolar plate is formed of pure titanium or a titanium alloy, and 
     wherein the negative electrolyte has an oxidation-reduction potential of 0.0 V or higher relative to a standard hydrogen electrode. 
     Since the titanium-based bipolar plate formed of pure titanium or a titanium alloy is used as the bipolar plate, oxidative deterioration of the bipolar plate can be less than that when a bipolar plate made of an electrically conductive carbon plastic is used. Since the negative electrolyte used is an electrolyte having an oxidation-reduction potential of 0.0 V or higher relative to the standard hydrogen electrode, the oxidative deterioration (dissolution) of the bipolar plate caused by the negative electrolyte can be reduced even when the titanium-based bipolar plate is used on the negative electrode side, so that the durability of the bipolar plate can be improved. This is because, when the titanium-based bipolar plate is used, a passivation film is formed on the surface of the bipolar plate, and the inner side of the passivation film (i.e., the bipolar plate) can be protected. Moreover, since the negative electrolyte used is an electrolyte having a specific oxidation-reduction potential, the dissolution of the passivation film can be prevented. Since the durability of the bipolar plate can be improved, the RF battery can maintain its performance stably for a long time. 
     (2) In one aspect of the redox flow battery, the titanium alloy may contain 95% by mass or more of titanium and may further contain at least one element selected from platinum, palladium, ruthenium, nickel, and chromium in a total amount of 0.4% by mass or more and 5% by mass or less. 
     When the bipolar plate is formed of the titanium alloy, if any of the above listed elements is contained, titanium is unlikely to be brought to an active state, so that the dissolution of the bipolar plate can be further reduced. 
     (3) In one aspect of the redox flow battery, the negative electrolyte may contain at least one active material selected from titanium ions, iron ions, manganese ions, and cerium ions. 
     When the negative electrolyte containing any of the above-listed active materials is used, titanium is unlikely to be brought to an active state, so that the dissolution of the bipolar plate can be further reduced. 
     (4) In one aspect of the redox flow battery, the concentration of hydrogen ions in the negative electrolyte may be 0.1 mol/L or more. 
     When the concentration of hydrogen ions in the negative electrolyte is 0.1 mol/L or more, the surface of the bipolar plate is unlikely to be brought to an active state, so that the dissolution of the bipolar plate can be further reduced. 
     (5) In one aspect of the redox flow battery, the negative electrolyte may contain sulfate radicals, and the concentration of the sulfate radicals in the negative electrolyte may be 2 mol/L or more. 
     When the negative electrolyte contains the sulfate radicals, the stability and reactivity of metal ions serving as the active material in the negative electrolyte can be improved. When the concentration of the sulfate radicals is 2 mol/L or more, the dissolution of the passivation film formed on the surface of the bipolar plate can be easily reduced, and the dissolution of the bipolar plate can thereby be reduced. 
     (6) In one aspect of the redox flow battery, the negative electrolyte may contain at least one metal selected from iron, copper, antimony, and platinum in a total amount of 0.01 mmol/L or more and 0.1 mol/L or less. 
     When the negative electrolyte containing any of the above-listed metals is used, the surface of the bipolar plate is unlikely to be brought to an active state, so that the dissolution of the bipolar plate can be further reduced. 
     (7) In one aspect of the redox flow battery, the bipolar plate may include grooves through which the positive electrolyte or the negative electrolyte circulates, the grooves being provided on at least one of a positive electrode-side surface in contact with the positive electrode and a negative electrode-side surface in contact with the negative electrode. 
     When the bipolar plate includes the grooves, the circulation of the electrolyte is facilitated as compared to that when no grooves are provided, so that the flow of the electrolyte circulating through the electrode can be controlled. By controlling the flow of the electrolyte, the pressure loss of the electrolyte can be reduced. 
     (8) In one aspect of the redox flow battery, the grooves may be formed on both the positive electrode-side surface and the negative electrode-side surface by bending the bipolar plate. 
     When the titanium-based bipolar plate is used, the grooves can be easily formed by bending the bipolar plate. Since the grooves can be formed by bending, the bipolar plate can have a uniform thickness, so that a reduction in the mechanical strength of the bipolar plate can be easily prevented. Since the grooves can be formed by bending, the grooves can be easily formed on both the positive electrode-side surface and negative electrode-side surface of the bipolar plate by bending the bipolar plate into, for example, a wavy shape. 
     (9) In one aspect of the redox flow battery, the bipolar plate may be formed by stacking two bipolar plate pieces, and the grooves may be provided on both the positive electrode-side surface and the negative electrode-side surface. 
     When the bipolar plate is formed by stacking the two bipolar plate pieces, the grooves on the positive electrode-side surface of the bipolar plate and the grooves on the negative electrode-side surface can be easily formed to be symmetric to each other. 
     (10) In one aspect of the redox flow battery, the distance between a positive electrode-side surface of the bipolar plate that is in contact with the positive electrode and a negative electrode-side surface in contact with the negative electrode-side surface may be 3 mm or more and 7 mm or less. 
     When the distance in the bipolar plate is 3 mm or more, a reduction in the mechanical strength of the bipolar plate can be prevented. When the distance is 7 mm or less, an increase in size due to the thickness of the bipolar plate can be prevented. 
     (11) In one aspect of the redox flow battery, the redox flow battery may further include: an engaging protrusion that is provided in one of the bipolar plate and the frame body; and an engaging recess that is provided in the other one of the bipolar plate and the frame body and engages the engaging protrusion. 
     When the bipolar plate and the frame body engage each other through the engaging protrusion and the engaging recess, the internal pressure of the electrolyte acting on the frame body can be shared by the frame body and the bipolar plate, so that the frame body can be prevented from undergoing excessive stress. When the titanium-based bipolar plate is used, the engaging protrusion and the engaging recess can be easily formed in the bipolar plate. 
     DETAILS OF EMBODIMENTS OF PRESENT INVENTION 
     Referring next to the drawings, redox flow batteries (RF batteries) according to embodiments of the present invention will be described in detail. In the drawings, the same numerals denote components with the same names. In the drawings, the thickness of each of the components (such as a bipolar plate and electrodes) is exaggerated. Therefore, the thickness etc. of each component may differ from its actual thickness etc. 
     Embodiment 1 
     As shown in  FIG. 1 , an RF battery  1  according to embodiment 1 includes: battery cells  2 ; and circulation mechanisms (a positive electrolyte circulation mechanism  200 P and a negative electrolyte circulation mechanism  200 N) that supply electrolytes to the battery cells  2  in a circulating manner. Each battery cell  2  is separated by a membrane  21  into a positive cell  22  and a negative cell  23 . A positive electrode  24  to which a positive electrolyte is supplied is installed in the positive cell  22 , and a negative electrode  25  to which a negative electrolyte is supplied is installed in the negative cell  23 . As shown in  FIG. 3 , the battery cell  2  is sandwiched between a pair of cell frames  3 ,  3 . Each cell frame  3  includes: a bipolar plate  4  in which a positive electrode  24  and a negative electrode  25  are disposed on its front and back side, respectively; and a frame body  5  surrounding the circumferential edges of the bipolar plate  4 . 
     One feature of the RF battery  1  in embodiment 1 is that it has a structure that can reduce oxidative deterioration (dissolution) of the bipolar plates  4  during long-term operation of the RF battery  1  and allows its performance to be maintained stably for a long time. Specifically, this feature means that each bipolar plate  4  used is a titanium (Ti)-based bipolar plate and that an electrolyte with a specific potential is used. The basic structure of the RF battery  1  will be described, and then the features of the RF battery  1 , i.e., the bipolar plate and the electrolyte will be described in detail. 
     [Basic Structure of RF Battery] 
     Typically, as shown in  FIG. 1 , the RF battery  1  is connected to a power generation unit and a load such as a power system or a consumer through an alternating current/direct current converter, a transformer facility, etc. The RF battery  1  is charged using the power generation unit as an electric power supply source and discharged using the load as a power consumer. Examples of the power generation unit include a solar photovoltaic power generator, a wind power generator, and other general power stations. 
     The positive electrode  24  and the negative electrode  25  installed in each battery cell  2  serve as reaction fields in which active materials contained in the supplied electrolytes undergo battery reactions. The membrane  21  is a separation member that separates the positive electrode  24  and the negative electrode  25  from each other and allows specific ions to pass through. Each bipolar plate  4  is formed of a conductive material that allows a current to pass through but does not allow the electrolytes to pass through. The bipolar plate  4  is disposed such that a positive electrode  24  is in contact with its one surface and a negative electrode  25  is in contact with the other surface. Each frame body  5  forms an inner region serving as a battery cell  2 . Specifically, the thickness of the frame body  5  is larger than the thickness of the bipolar plate  4 , and the circumferential edges of the bipolar plate  4  are surrounded by the frame body  5 . In this manner, a step that forms a space in which the positive electrode  24  (the negative electrode  25 ) is disposed is formed between the front (back) surface of the bipolar plate  4  and the front (back) surface of the frame body  5 . 
     The positive electrolyte circulation mechanism  200 P that supplies the positive electrolyte to the positive cells  22  in a circulating manner includes: a positive electrolyte tank  202  that stores the positive electrolyte; pipes  204  and  206  that connect the positive electrolyte tank  202  to the positive cells  22 ; and a pump  208  disposed in the pipe  204  on the upstream side (supply side). The negative electrolyte circulation mechanism  200 N that supplies the negative electrolyte to the negative cells  23  in a circulating manner includes: a negative electrolyte tank  203  that stores the negative electrolyte; pipes  205  and  207  that connect the negative electrolyte tank  203  to the negative cells  23 ; and a pump  209  disposed in the pipe  205  on the upstream side (supply side). 
     The positive electrolyte is supplied from the positive electrolyte tank  202  through the pipe  204  on the upstream side to the positive electrodes  24  and returned from the positive electrodes  24  through the pipe  206  on the downstream side (discharge side) to the positive electrolyte tank  202 . The negative electrolyte is supplied from the negative electrolyte tank  203  through the pipe  205  on the upstream side to the negative electrodes  25  and returned from the negative electrodes  25  through the pipe  207  on the downstream side (discharge side) to the negative electrolyte tank  203 . In  FIGS. 1 and 2 , manganese (Mn) ions and titanium (Ti) ions shown in the positive electrolyte tank  202  and the negative electrolyte tank  203 , respectively, are examples of the types of ions contained in the positive electrolyte and the negative electrolyte as active materials. In  FIG. 1 , solid arrows indicate charging, and broken arrows indicate discharging. The positive electrolyte and the negative electrolyte are circulated. Therefore, while the positive electrolyte is supplied to the positive electrode  24  in a circulating manner and the negative electrolyte is supplied to the negative electrode  25  in a circulating manner, charge/discharge is performed through valence change reactions of the active material ions in the electrolytes for the electrodes. 
     Typically, the RF battery  1  is used in the form of a cell stack  9  including a plurality of battery cells  2  stacked one on another. As shown in  FIG. 3 , the cell stack  9  includes: a layered body prepared by repeatedly stacking a first cell frame  3 , a positive electrode  24 , a membrane  21 , a negative electrode  25 , and a second cell frame  3 ; a pair of end plates  91  and  92  holding the layered body therebetween; connecting members  93  such as long bolts connecting the end plates  91  and  92 ; and tightening members such as nuts. When the end plates  91  and  92  are tightened with the tightening members, the stacked state of the layered body is maintained by the tightening force in the stacking direction. A prescribed number of battery cells  2  form a substack, and the cell stack  9  used includes a plurality of substacks  9 S stacked one on another. In cell frames  3  at opposite ends, with respect to the stacking direction of the battery cells, of each of the substacks  9 S and the cell stack  9 , supply/drainage plates (not shown) are disposed instead of the bipolar plates  4 . 
     The electrolytes are supplied to the respective electrodes, i.e., the positive electrode  24  and the negative electrode  25 , for each cell frame  3  through liquid supply manifolds  51  and  52 , liquid supply guide grooves  51   s  and  52   s , and liquid supply rectifying portions (not shown) that are formed on one of a pair of sides facing each other (a liquid supply side, the lower side in the drawing sheet of  FIG. 3 ) of the frame body  5 . The electrolytes are discharged from the respective electrodes, i.e., the positive electrode  24  and the negative electrode  25 , through liquid discharge rectifying portions (not shown), liquid discharge guide grooves  53   s  and  54   s , and liquid discharge manifolds  53  and  54  that are formed on the other one of the pair of sides facing each other (a liquid discharge side, the upper side in the drawing sheet of  FIG. 3 ) of the frame body  5 . The positive electrolyte is supplied to the positive electrode  24  from the liquid supply manifold  51  through the liquid supply guide groove  51   s  formed on a first surface (the front surface in the drawing sheet) of the frame body  5 . The positive electrolyte flows from the lower side of the positive electrode  24  to the upper side as shown by arrows in the upper part of  FIG. 3  and then discharged to the liquid discharge manifold  53  through the liquid discharge guide groove  53   s  formed on the first surface (the front surface in the drawing sheet) of the frame body  5 . The negative electrolyte is supplied and discharged in the same manner as that of the positive electrolyte except that the negative electrolyte is supplied and discharged on a second surface (the back surface in the drawing sheet) of the frame body  5 . Ring-shaped sealing members  6  such as O-rings or flat packings ( FIGS. 2 and 3 ) are disposed between frame bodies  5  to prevent leakage of the electrolytes from the battery cells  2 . Sealing grooves (not shown) for placing the ring-shaped sealing members  6  are formed circumferentially in the frame bodies  5 . 
     A well-known structure may be appropriately used for the basic structure of the RF battery  1  described above. 
     [Bipolar Plates] 
     As shown in  FIG. 3 , each bipolar plate  4  is a rectangular flat plate. One feature of the RF battery  1  in embodiment 1 is that each bipolar plate  4  is formed from pure titanium or a titanium alloy. When the titanium-based bipolar plate  4  is used, a passivation film is formed on the surface of the bipolar plate  4 , and the inner side of the passivation film (i.e., the bipolar plate  4 ) can be protected. 
     Examples of the pure titanium include pure titanium type 1 to pure titanium type 4 specified in JIS H 4600:2012. 
     The titanium alloy is a titanium-based alloy containing titanium (Ti) in an amount of 95% by mass or more, preferably 97% by mass or more, and 98% by mass or more and further containing an additive element other than titanium. The additive element in the titanium alloy is at least one selected from platinum (Pt), palladium (Pd), ruthenium (Ru), nickel (Ni), and chromium (Cr). When the additive elements are contained, titanium is unlikely to be brought to an active state, and the passivation film is easily formed on the surface of the bipolar plate  4 . The total amount of the additive elements is 0.4% by mass or more and 5% by mass or less and 0.5% by mass or more and 2% by mass or less. The titanium alloy may further contain unavoidable impurities. 
     The bipolar plate  4  may have a thickness of 0.3 mm or more and 0.7 mm or less. When the thickness of the bipolar plates  4  is small, the thickness of the battery cells  2  of the RF battery  1  in the stacking direction can be reduced accordingly, and the RF battery  1  can be reduced in size. Therefore, the thickness of the bipolar plates  4  is preferably 0.5 mm or less and 0.4 mm or less. When the bipolar plates  4  are formed of a titanium-based material, the thickness of the bipolar plates  4  can be easily reduced. When the thickness of the bipolar plates  4  is 0.3 mm or more, the bipolar plates  4  resist damage when the battery cells  2  sandwiched between cell frames  3  are tightened in the stacking direction of the battery cells  2 . 
     Each bipolar plate  4  may have a rough surface or grooves on at least part of at least one of the positive electrode-side surface in contact with the positive electrode  24  and the negative electrode-side surface in contact with the negative electrode  25 . When the bipolar plate  4  has a rough surface, friction easily occurs between the bipolar plate  4  and the positive electrode  24  or the negative electrode  25 , and positional displacement of the positive electrode  24  or the negative electrode  25  relative to the bipolar plate  4  can be easily prevented. When the bipolar plate  4  has the rough surface, the flow velocity of an electrolyte is easily changed to cause turbulence, and the electrolyte can be forcedly diffused to the positive electrode  24  or the negative electrode  25  in contact with the bipolar plate  4  with ease, so that the reactivity of the battery can be easily improved. When the bipolar plate  4  has grooves, the flow of an electrolyte flowing to the positive electrode  24  or the negative electrode  25  can be controlled, and the pressure loss of the electrolyte can be easily reduced. The manner of forming the grooves in the bipolar plate  4  will be described in detail in embodiment 2 described later. 
     [Electrolytes] 
     One feature of the RF battery  1  in embodiment 1 is that the negative electrolyte has an oxidation-reduction potential of 0.0 V or higher relative to the standard hydrogen electrode. When an electrolyte with a specific potential is used as the negative electrolyte, dissolution of the passivation film formed on the surface of each bipolar plate  4  can be reduced. The positive electrolyte is not limited to the electrolyte with the specific potential. 
     In this example, the negative electrolyte contains, as the active material, metal ions whose valence is changed by oxidation/reduction. When the active material is metal ions, the negative electrolyte may contain at least one type of active material ions selected from titanium (Ti) ions, iron (Fe) ions, manganese (Mn) ions, and cerium (Ce) ions. The ions of a metal element are present in the negative electrolyte as at least one type of ions with a specific valence, and a plurality of types of ions of a single element with different valences may be present. Among the metal ions listed above, one type of metal ions may be contained, or a plurality of types of metal ions may be contained. When a plurality of types of metal ions are contained, they are combined such that the oxidation-reduction potential of the negative electrolyte relative to the standard hydrogen electrode is 0.0 V or higher. 
     The concentration of the above metal ions used as the active material (the total concentration when a plurality of types of metal ions are contained) may be 0.3 mol/L or more and 5 mol/L or less. When the concentration is 0.3 mol/L or more, the RF battery  1  can have an energy density (e.g., about 10 kWh/m 3 ) large enough for a high-capacity storage battery. The higher the concentration, the higher the energy density. Therefore, the concentration is 0.5 mol/L or more, 1.0 mol/L or more, 1.2 mol/L or more, and 1.5 mol/L or more. In consideration of solubility in a solvent, the concentration is 5 mol/L or less and 2 mol/L or less in terms of ease of use. In this case, high electrolyte productivity is obtained. 
     The oxidation-reduction potential of the negative electrolyte relative to the standard hydrogen electrode can be changed by changing the type and concentration of metal ions contained as the active material. For example, when the negative electrolyte contains titanium ions in an amount of 0.5 mol/L or more and 2 mol/L or less, the oxidation-reduction potential relative to the standard hydrogen electrode can be 0.0 V or more and 0.2 V or less. The higher the oxidation-reduction potential relative to the standard hydrogen electrode, the further the dissolution of the passivation film formed on the surface of the bipolar plate  4  can be reduced. Therefore, the oxidation-reduction potential may be 0.02 V or higher and 0.1 V or higher. The oxidation-reduction potential relative to the standard hydrogen electrode satisfies the above values irrespective of charging depth. 
     The concentration of hydrogen ions in the negative electrolyte may be 0.1 mol/L or more. When the concentration of hydrogen ions is 0.1 mol/L or more, the surface of the bipolar plate  4  is unlikely to be brought to an active state, and the dissolution of the bipolar plate  4  can be easily reduced. The higher the concentration of hydrogen ions, the less likely the surface of the bipolar plate  4  is to be brought to an active state. Therefore, the concentration of hydrogen ions may be 1 mol/L or more and 1.5 mol/L or more. 
     The negative electrolyte may further contain at least one metal selected from iron (Fe), copper (Cu), antimony (Sb), and platinum (Pt) in a total amount of 0.01 mmol/L or more and 0.1 mol/L or less. When any of these metals are contained in a total amount of 0.01 mmol/L or more, the surface of the bipolar plate  4  is unlikely to be brought to an active state, so that the dissolution of the bipolar plate  4  can be easily reduced. The larger the amount of the metals contained, the less likely the surface of the bipolar plate  4  is to be brought to an active state. Therefore, the total amount may be 0.03 mmol/L or more, 0.05 mmol/L or more, and 0.1 mmol/L or more. In consideration of solubility in a solvent, the total content of the metals is 10 mmol/L or less and preferably 5 mmol/L or less. 
     The metal ions listed for the active material are water-soluble ions. Therefore, an aqueous solution using water as a solvent can be preferably used for the negative electrolyte. In particular, when the negative electrolyte is an aqueous acid solution containing sulfuric acid or a sulfate, the following effects can be expected. (1) The reactivity of the metal ions may be improved, and solubility may be improved. (2) Even when metal ions with a high potential are used, a side reaction is unlikely to occur (decomposition is unlikely to occur). (3) High ionic conductance is obtained, and the internal resistance of the battery is reduced. (4) Unlike the case where hydrochloric acid is used, chlorine gas is not generated. (5) The electrolyte can be easily obtained using water and, for example, a sulfate, and productivity is high. In the aqueous acid solution (electrolyte) produced using sulfuric acid or a sulfate, sulfate radicals (SO 4   2− ) are present. The concentration of the sulfate radicals may be 2 mol/L or more. When the concentration is 2 mol/L or more, the dissolution of the passivation film formed on the surface of the bipolar plate  4  can be easily reduced. The negative electrolyte used may be not only an aqueous solution produced using sulfuric acid or a sulfate but also an aqueous solution produced using a well-known acid or a well-known salt. 
     The electrolyte may contain, as the active material, organic molecules that undergo a reduction-oxidation reaction. Examples of such organic molecules include organic molecules having an unshared electron pair. With these organic molecules, donation or acceptance of electrons causes binding or unbinding of protons and the organic molecules, so that charge and discharge can be performed through the reduction-oxidation reaction. Examples of the organic molecules include quinone derivatives and viologen derivatives. 
     The positive electrolyte used may be an electrolyte with an oxidation-reduction potential of 0.0 V or higher relative to the standard hydrogen electrode, as is the negative electrolyte. The positive electrolyte is not limited to the electrolyte with a specific potential, and a well-known configuration may be used appropriately. 
     [Effects] 
     In the RF battery  1  in embodiment 1, since the titanium-based bipolar plates  4  are used, the passivation film is formed on the surface of each bipolar plate  4 , and the passivation film can reduce oxidative deterioration of the bipolar plate  4 . Since the negative electrolyte with a specific potential is used, the dissolution of the passivation film formed on the surface of the bipolar plate  4  can be reduced. With the above configuration, the dissolution of the bipolar plates  4  can be prevented during long-term operation of the RF battery  1 , and a reduction in thickness of the bipolar plates  4  can be prevented. For example, the rate of reduction in the thickness of the bipolar plates  4  after one year relative to the thickness at the beginning of the operation of the RF battery  1  can be 1% or less and 0.1% or less. Since the dissolution of the bipolar plates  4  can be reduced, the RF battery  1  can maintain its performance stably for a long time. 
     [Applications] 
     The RF battery  1  in the embodiment can be used as a high-capacity storage battery used for the purpose of stabilizing fluctuations in power generation output, storing excess electricity generated, load leveling, etc. for power generation using natural energy such as solar photovoltaic power generation and wind power generation. The RF battery  1  in the embodiment can be preferably used as a high-capacity storage battery placed in a general power plant and used for the purpose of load leveling and for measures against momentary voltage drop and power failure. 
     Embodiment 2 
     In embodiment 2, a description will be given of an RF battery including titanium-based bipolar plates  4 A having grooves  41  on their surface, as shown in  FIGS. 4 and 5 . The RF battery according to embodiment 2 differs from that in embodiment 1 in that each bipolar plate  4 A has grooves  41 , and the other components are the same as those in embodiment 1. 
     In each bipolar plate  4 A, a plurality of grooves  41  and rib portions  42  each located between adjacent grooves  41 ,  41  are provided on both the positive electrode-side surface in contact with a corresponding positive electrode  24  and the negative electrode-side surface in contact with a corresponding negative electrode  25 . The plurality of grooves  41  function as flow channels through which the electrolytes flow. The positive electrolyte circulates through the grooves  41  disposed on the positive electrode-side surface, and the negative electrolyte circulates through the grooves  41  disposed on the negative electrode-side surface. The flows of the electrolytes in each battery cell  2  can be controlled by changing the shape and dimensions of the grooves  41 . In  FIG. 4 , the rib portions  42  are hatched for the ease of understanding. 
     In this example, the grooves  41  are formed as vertical grooves extending in a direction from one side of a frame body  5  ( FIG. 3 ) to another side (the vertical direction from the lower side toward the upper side in the drawing sheet of  FIG. 3 ). The rib portions  42  are located between respective adjacent grooves  41 ,  41  and form most of the outermost surface of the bipolar plate  4 A. Therefore, when the battery cell  2  is assembled, the rib portions  42  are in contact with a positive electrode  24  or a negative electrode  25 . 
     In each bipolar plate  4 A, an electrolyte introduced from the liquid supply manifold  51 ( 52 ) of the frame body  5  ( FIG. 3 ) through the liquid supply guide groove  51   s  ( 52   s ) and a liquid supply rectifying portion (not shown) is distributed to grooves  41  and spreads over the entire bipolar plate  4 A. The flows of the electrolyte flowing through the grooves  41  are merged at a liquid discharge rectifying portion (not shown) of the frame body  5  ( FIG. 3 ) and discharged through the liquid discharge guide groove  53   s  from the liquid discharge manifold  53 . The flows of the electrolytes in the bipolar plate  4 A include flows along the grooves  41  (in the direction shown by solid arrows in  FIG. 4 ) and flows (in directions shown by broken arrows in  FIG. 4 ) flowing across the rib portions  42  located between adjacent grooves  41 ,  41  in the width direction (the horizontal direction in  FIG. 4 ). The electrolytes in the bipolar plates  4 A penetrate and diffuse into the electrodes facing the respective bipolar plates  4 A and undergo the battery reactions in the electrodes. 
     In this example, as shown in  FIG. 5 , the grooves  41  are formed by bending the bipolar plates  4 A into a wavy shape. In each bipolar plate  4 A sandwiched between a corresponding positive electrode  24  and a corresponding negative electrode  25 , the grooves  41  include positive-side grooves  41   p  formed so at to protrude toward the negative electrode  25  and negative-side grooves  41   n  formed so as to protrude toward the positive electrode  24 , and the positive-side grooves  41   p  and the negative-side grooves  41   n  are arranged alternately. Therefore, in the bipolar plate  4 A disposed between the positive electrode  24  and the negative electrode  25 , the positive-side grooves  41   p  serve as grooves  41  when viewed from the positive electrode  24  side and serve as rib portions  42  when viewed from the negative electrode  25  side. Similarly, the negative-side grooves  41   n  serve as grooves  41  when viewed from the negative electrode  25  side and serve as rib portions  42  when viewed from the positive electrode  24  side. 
     Each of the positive-side grooves  41   p  is composed of a bottom surface and side surfaces that connect the bottom surface to the positive electrode-side surface of the bipolar plate  4 A and has a rectangular cross-sectional shape. Similarly, each of the negative-side grooves  41   n  is composed of a bottom surface and side surfaces connecting the bottom surface to the negative electrode-side surface of the bipolar plate  4 A and has a rectangular cross-sectional shape. Preferably, the cross-sectional shape of each of the positive-side grooves  41   p  and the negative-side grooves  41   n  has a flat bottom surface. This is because of the following reason. Each positive-side groove  41   p  (each negative-side groove  41   n ) serves as a rib portion  42  when viewed from the negative electrode  25  side (the positive electrode  24  side). Therefore, when the positive-side groove  41   p  (the negative-side groove  41   n ) has a flat bottom surface, the rib portion  42  can be in surface contact with the negative electrode  25  (the positive electrode  24 ). When the rib portions  42  are in surface contact with the positive electrode  24  or the negative electrode  25 , an electrolyte flowing across the rib portions  42  can easily penetrate and diffuse into the positive electrode  24  or the negative electrode  25 . The cross section of each of the positive-side grooves  41   p  and the negative-side grooves  41   n  may have a semicircular shape, a U shape, a V shape, etc. 
     The opening width of the grooves  41  may be appropriately selected according to the cross-sectional area of the grooves  41  and may be, for example, 0.1 mm or more and 10 mm or less and 0.7 mm or more and 2.5 mm or less. 
     The depth of the grooves  41  (the positive-side grooves  41   p  and the negative-side grooves  41   n ) may be 2.4 mm or more and 6 mm or less. When the depth of the grooves  41  is 2.4 mm or more, the grooves  41  can have a sufficient volume, and the circulatability of the electrolytes can be improved. When the depth of the grooves  41  is 6 mm or less, the thickness of the battery cells  2  in the RF battery  1  in the stacking direction can be reduced, and the RF battery  1  can be reduced in size. The depth of the grooves  41  may be 3 mm or more and 5.5 mm or less and particularly 3.5 mm or more and 5 mm or less. 
     In the bipolar plate  4 A having the grooves  41  formed by bending the bipolar plate  4 A into a wavy shape, the distance between the positive electrode-side surface in contact with the positive electrode  24  and the negative electrode-side surface in contact with the negative electrode  25  may be 3 mm or more and 7 mm or less. When the distance is 3 mm or more, the grooves  41  are easily formed by bending. When the distance is 7 mm or less, the thickness of the battery cells  2  in the RF battery  1  in the stacking direction can be reduced, and the RF battery  1  can be reduced in size. The distance may be 3 mm or more and 6 mm or less and particularly 4 mm or more and 5 mm or less. 
     The bipolar plate  4 A having the grooves  41  formed by bending the bipolar plate  4 A into a wavy shape can be produced by subjecting the material forming the bipolar plate  4 A to press forming. Since the grooves  41  are formed by press forming, the thickness of the bipolar plate  4 A is substantially uniform over the entire area. 
     In this example, since the bipolar plate  4 A is formed of the titanium-based material, the grooves  41  can be easily formed by bending the bipolar plate  4 A. In particular, by bending the bipolar plate  4 A into a wavy shape, the grooves  41  can be easily formed on both the positive electrode-side surface and the negative electrode-side surface of the bipolar plate  4 A. 
     Embodiment 3 
     In embodiment 3, a description will be given of an RF battery including titanium-based bipolar plates  4 B having grooves  41  with a non-communicating shape on their surface, as shown in  FIGS. 6 and 7 . In the RF battery according to embodiment 3, the shape of the grooves  41  provided in the bipolar plates  4 B differs from that in embodiment 2, and the other components are the same as those in embodiment 2. 
     The grooves  41  are formed as vertical grooves having a non-communicating shape and extending in a direction from one side of a frame body  5  ( FIG. 3 ) toward another side (the vertical direction from the lower side toward the upper side in the drawing sheet of  FIG. 3 ). Specifically, the grooves  41  include introduction-side grooves  41   i  for introducing an electrolyte into an electrode and discharge-side grooves  41   o  for discharging the electrolyte from the electrode. The introduction-side grooves  41   i  and the discharge-side grooves  41   o  are not in communication with each other, are independent of one another, and are arranged alternately at prescribed intervals. The introduction-side grooves  41   i , the discharge-side grooves  41   o , liquid supply rectifying portions (not shown), and liquid discharge rectifying portions (not shown) of the frame body  5  form an interdigitated structure. A rib portion  42  is formed between an introduction-side groove  41   i  and a discharge-side grooves  41   o  adjacent to each other. 
     In each bipolar plate  4 B, an electrolyte introduced from the liquid supply manifold  51 ( 52 ) of the frame body  5  ( FIG. 3 ) through the liquid supply guide groove  51   s  ( 52   s ) and a liquid supply rectifying portion (not shown) is distributed to the introduction-side grooves  41   i  and spreads over the entire bipolar plate  4 B. The electrolyte flowing through the introduction-side grooves  41   i  penetrates into an electrode facing the bipolar plate  4 B, passes across the rib portions  42  of the bipolar plate  4 B, and flows into the discharge-side grooves  41   o  adjacent to the introduction-side grooves  41   i . The flows of the electrolyte flowing through the discharge-side grooves  41   o  are merged at a liquid discharge rectifying portion (not shown) of the frame body  5  ( FIG. 3 ) and discharged through the liquid discharge guide groove  53   s  from the liquid discharge manifold  53 . The flows of the electrolyte in the bipolar plate  4 B include flows along the introduction-side grooves  41   i  and the discharge-side grooves  41   o  (in the direction shown by solid arrows in  FIG. 6 ) and flows (in directions shown by broken arrows in  FIG. 6 ) flowing across the rib portions  42  located between the introduction-side grooves  41   i  and the discharge-side grooves  41   o  in the width direction (the horizontal direction in  FIG. 6 ). The electrolytes in the bipolar plates  4 B penetrate and diffuse into the respective electrodes facing the bipolar plates  4 B and undergo the battery reactions in the electrodes. 
     In this example, as shown in  FIG. 7 , each bipolar plate  4 B is formed by stacking two bipolar plate pieces  40 . Each of the bipolar plate pieces  40  is bent such that the vertical grooves have a non-communicating shape. In each bipolar plate  4 B, the two bipolar plate pieces  40  are stacked, so that the grooves  41  having a non-communicating shape are provided on both the positive electrode-side surface in contact with a corresponding positive electrode  24  and the negative electrode-side surface in contact with a corresponding negative electrode  25 . In this case, the bipolar plate pieces  40  are stacked such that the grooves  41  are arranged symmetric, and therefore the grooves  41  on the positive electrode-side surface and the grooves  41  on the negative electrode-side surface can have the same shape. The distance between adjacent grooves  41 ,  41 , i.e., the width of the rib portions  42 , may be 100% or more and 700% or less of the width of the grooves  41  and 200% or more and 500% or less of the width of the grooves  41 . 
     The bipolar plate  4 B having the grooves  41  with a non-communicating shape and formed by stacking the two bipolar plate pieces  40  can be produced by subjecting the material forming the bipolar plate pieces  40  to press forming to form the grooves  41 , stacking the bipolar plate pieces  40  with the back surfaces of the grooves  41  facing each other, and then laser-welding the bipolar plate pieces  40  together. 
     A bipolar plate having grooves with a non-communicating shape can be produced by using one bipolar plate and forming grooves and rib portions on the front and back surfaces of the bipolar plate when the material forming the bipolar plate is subjected to press forming. The grooves can be formed by subjecting the front and back surfaces of a flat plate with no grooves to cutting. In these cases, the thickness of portions of the bipolar plate in which the grooves are formed differs from the thickness of portions in which the rib portions are formed. 
     Embodiment 4 
     In embodiment 4, a description will be given of an RF battery including cell frames  3 α each having an engagement structure in which a bipolar plate  4 α engages a frame body  5 α, as shown in  FIG. 8 . The engagement structure includes engaging protrusions  56  provided in one of the bipolar plate  4 α and the frame body  5 α and engaging recesses  46  provided in the other one of the bipolar plate  4 α and the frame body  5 α. The RF battery according to embodiment 4 differs from embodiment 1 in that each cell frame  3 α includes the engagement structure, and the other components are the same as those in embodiment 1. 
     Each cell frame  3 α in this example is formed by fitting the bipolar plate  4 α into an inner circumferential recess  55   c  of the frame body  5 α. 
     The frame body  5 α has an opening  55   w  passing through the frame body  5 α in its thickness direction, and the bipolar plate  4 α is disposed so as to fill the opening  55   w . In the frame body  5 α, its circumferential edge portion surrounding the entire opening  55   w  is thinner than the other portions of the frame body  5 α, and the thin portion forms the inner circumferential recess  55   c  to which the bipolar plate  4 α is fitted. In this example, the inner circumferential recess  55   c  is formed only on a first side (the right side in  FIG. 8 ) of the frame body  5 α. Specifically, a second side (the left side in  FIG. 8 ) of the inner circumferential recess  55   c  is flush with the other portions of the frame body  5 α. 
     The bipolar plate  4 α has a thin portion  45  that is thinner than the other portions of the bipolar plate  4 α and located in a portion that engages the inner circumferential recess  55   c  of the frame body  5 α. The thin portion  45  of the bipolar plate  4 α faces the inner circumferential recess  55   c  of the frame body  5 α, and portions other than the thin portion  45  are fitted into the opening  55   w  of the frame body  5 α. Therefore, the fitting state of the bipolar plate  4 α with respect to the frame body  5 α is easily stabilized. 
     In  FIG. 8 , a positive electrode  24  is disposed on the left side of the bipolar plate  4 α, and a negative electrode  25  is disposed on the right side. Since the inner circumferential recess  55   c  is formed in the frame body  5 α, the length (the vertical length in  FIG. 8 ) of the negative electrode  25  is longer than the length of the positive electrode  24 . The thickness of the bipolar plate  4 α (except for the thin portion  45 ) is smaller than the thickness of the frame body  5 α (except for the inner circumferential recess  55   c ), and the thickness of the thin portion  45  of the bipolar plate  4 α is smaller than the depth of the inner circumferential recess  55   c  of the frame body  5 α. Therefore, the total thickness of the bipolar plate  4 α, the positive electrode  24 , and the negative electrode  25  is substantially equal to the thickness of the frame body  5 α (except for the inner circumferential recess  55   c ). Specifically, with the positive electrode  24  and the negative electrode  25  disposed on the frame body  5 α and the bipolar plate  4 α, the surfaces of the frame body  5 α are substantially flush with the surface of the positive electrode  24  and the surface of the negative electrode  25 . To prevent the electrolytes from flowing between the first and second sides of the bipolar plate  4 α, a sealing member  58   s  is disposed between the frame body  5 α and the bipolar plate  4 α. In this example, a sealing groove  58  is formed in the inner circumferential recess  55   c  of the frame body  5 α, and the sealing member  58   s  is disposed in the sealing groove  58 . The sealing groove may be formed on the bipolar plate  4 α side. 
     In this example, the frame body  5 α has the engaging protrusions  56 , and the bipolar plate  4 α has the engaging recesses  46 . Specifically, the engaging protrusions  56  are disposed in the inner circumferential recess  55   c  of the frame body  5 α and located inward of the sealing groove  58 , and the engaging recesses  46  are disposed in the thin portion  45  of the bipolar plate  4 α so as to correspond to the engaging protrusions  56 . In this example, the engaging protrusions  56  are formed as elongated protrusions, and the engaging recesses  46  are formed as grooves. 
     The engaging protrusions  56  and the engaging recesses  46  may be disposed continuously on one of opposite sides (the liquid supply side, the lower side in the drawing sheet of  FIG. 3 ) of the frame body  5 α and the other one of the opposite sides (the liquid discharge side, the upper side of the drawing sheet of  FIG. 3 ) so as to extend in the longitudinal direction or may be disposed intermittently in a plurality of portions. 
     The depth of the engaging recesses  46  may be 10% or more and 50% or less of the thickness of the member forming the engaging recesses  46  (the thin portion  45  of the bipolar plate  4 α in this example). When the depth of the engaging recesses  46  is 10% or more of the thickness of the member forming the engaging recesses  46 , the state in which the engaging protrusions  56  engage the engaging recesses  46  can be firmly maintained. When the depth of the engaging recesses  46  is 50% or less of the thickness of the member forming the engaging recesses  46 , the strength of the member itself can be sufficiently maintained. The protruding length of the engaging protrusions  56  formed may be set according to the depth of the engaging recesses  46 . When a space that allows each engaging recess  46  and a corresponding engaging protrusions  56  to move in the width direction of the engaging recess  46  (the vertical direction in  FIG. 8 ) is provided between the engaging recess  46  and the engaging protrusion  56 , the engaging protrusion  56  can be easily disposed in the engaging recess  46 . 
     In this example, since the bipolar plate  4 α is formed of the titanium-based material, the strength of the bipolar plate  4 α itself is high even when the engaging recesses  46  are provided in the bipolar plate  4 α, and the frame body  5 α is prevented from undergoing excessive stress. The engaging protrusions may be provided in the bipolar plate  4 α, and the engaging recesses may be provided in the frame body  5 α. Even in this case, the strength of the engaging protrusions is high, and the frame body  5 α is prevented from undergoing excessive stress, and damage of the engaging protrusions due to the stress can be prevented. 
     Embodiment 5 
     In embodiment 5, a description will be given of an RF battery including cell frames  3 β each having a different engagement structure in which a bipolar plate  4 β and a frame body  5 β engage each other, as shown in  FIG. 9 . In each cell frame  3 β in this example, the frame body  5 β is formed by joining a pair of frame pieces  59  together such that a thin portion  45  of the bipolar plate  4 β is sandwiched between inner circumferential recesses  55   c  of the frame pieces  59 . A sealing member  59   s  is interposed between the pair of frame pieces  59 . The RF battery according to embodiment 5 differs from embodiment 4 in that the frame body  5 β is composed of the pair of frame pieces  59  and that the engaging recesses  46  are formed on both side of the bipolar plate  4 β. The other components are the same as those in embodiment 4. 
     In this example, each frame piece  59  has engaging protrusions  56 , and the engaging recesses  46  are provided in the thin portion  45  of the bipolar plate  4 β so as to correspond to the engaging protrusions  56 . Since the thin portion  45  of the bipolar plate  4 β is sandwiched between the pair of frame pieces  59 , the engaging recesses  46  are formed on both sides of the thin portion  45 . The engaging recesses  46  are disposed at positions opposed to each other. In this case also, since the bipolar plate  4 β is formed of the titanium-based material in this example, the strength of the bipolar plate  4 β itself is high, and the frame body  5 β is prevented from undergoing excessive stress. 
     The present invention is not limited to the above examples but is defined by the claims. The present invention is intended to cover any modifications within the scope of the claims and meaning equivalent to the scope of the claims. For example, in a titanium-based bipolar plate having grooves on its surface, the grooves may be horizontal grooves. When the grooves include introduction-side grooves and discharge-side grooves, the introduction-side grooves and the discharge-side grooves may not be disposed alternately in an intermeshing manner but may be disposed so as to face each other with a spacing in the circulating direction of the electrolytes. Engagement structures may be provided in frame bodies and titanium-based bipolar plates having grooves on their surface. 
     Test Example 1 
     A titanium-based bipolar plate was immersed in an electrolyte (immersion solution) with a specific oxidation-reduction potential relative to the standard hydrogen electrode, and the rate of reduction in the thickness of the bipolar plate was examined as temporal changes. In this example, the electrolyte contains no active material, and the state in which the active material was contained was simulated by applying a potential between the titanium-based bipolar plate and the standard hydrogen electrode. 
     Sample No. 1-1 
     A bipolar plate (size: 25 mm×50 mm, thickness: 0.5 mm) formed of pure titanium and 2 mol/L sulfuric acid (immersion solution) were prepared. 
     The bipolar plate was immersed in the immersion solution, and a potential was applied between the titanium-based bipolar plate and the standard hydrogen electrode. The potential was maintained such that the oxidation-reduction potential relative to the standard hydrogen electrode was 0.0 V. 
     Sample No. 1-2 
     A bipolar plate (size: 25 mm×50 mm, thickness: 0.5 mm) formed of a titanium alloy (Ti-0.15 Pd) and 2 mol/L sulfuric acid (immersion solution) were prepared. 
     The bipolar plate was immersed in the immersion solution, and a potential was applied between the titanium-based bipolar plate and the standard hydrogen electrode and maintained such that the oxidation-reduction potential relative to the standard hydrogen electrode was 0.0 V, as in sample No. 1-1.
         Sample No. 1-3       

     A bipolar plate (size: 25 mm×50 mm, thickness: 0.5 mm) formed of a titanium alloy (Ti-0.4 Ni-0.015 Pd-0.025 Ru-0.14 Cr) and 2 mol/L sulfuric acid (immersion solution) were prepared. 
     The bipolar plate was immersed in the immersion solution, and a potential was applied between the titanium-based bipolar plate and the standard hydrogen electrode and maintained such that the oxidation-reduction potential relative to the standard hydrogen electrode was 0.0 V, as in sample No. 1-1. 
     Sample No. 1-11 
     A bipolar plate (size: 25 mm×50 mm, thickness: 0.5 mm) formed of pure titanium and 2 mol/L sulfuric acid (immersion solution) were prepared. 
     The bipolar plate was immersed in the immersion solution, and a potential was applied between the titanium-based bipolar plate and the standard hydrogen electrode and maintained such that the oxidation-reduction potential relative to the standard hydrogen electrode was −0.1 V. 
     For each of the samples described above, the amount of dissolution of the materials forming the bipolar plate was measured after one month, and the amount of dissolution was converted to the rate of reduction in the thickness of the bipolar plate. The results are shown in Table 1. In Table 1, the rate of reduction in the thickness of the bipolar plate after one month is multiplied by 12, and the rate of reduction after one year is shown. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Oxidation-reduction potential 
                 Rate of reduction 
               
               
                 Sample 
                   
                 relative to standard hydrogen 
                 in thickness 
               
               
                 No. 
                 Bipolar plate 
                 electrode (V) 
                 (μm/year) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1-1 
                 Pure titanium 
                 0.0 
                 50 
               
               
                 1-2 
                 Ti—0.15Pd 
                 0.0 
                 0.5 
               
               
                 1-3 
                 Ti—0.4Ni—0.015Pd—0.025Ru—0.14Cr 
                 0.0 
                 5 
               
               
                 1-11 
                 Pure titanium 
                 0.1 
                 160 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 1, in samples Nos. 1-1 to 1-3 in which the oxidation-reduction potential relative to the standard hydrogen electrode is 0.0 V, the rate of reduction in the thickness is reduced by about 70% as compared with that of sample No. 1-11 in which the oxidation-reduction potential relative to the standard hydrogen electrode is −0.1 V. This may be because of the following reason. Since a passivation film was formed on the surface of the bipolar plate, dissolution of the passivation film was prevented, so that oxidative deterioration of the bipolar plate was reduced. In particular, when the bipolar plate is formed of a titanium alloy, the rate of reduction in thickness can be further reduced. 
     Test Example 2 
     An RF battery was produced using bipolar plates (size: 25 mm×50 mm, thickness: 0.5 mm) formed of pure titanium. A charge and discharge test was performed using different electrolytes, and the rate of reduction in the thickness of the bipolar plates was examined as temporal changes. 
     Sample No. 2-1 
     A stack of a positive electrode, a negative electrode, and a membrane was sandwiched between a pair of cell frames to produce an RF battery having a single cell structure. Carbon paper (10AA manufactured by SGL CARBON JAPAN Co., Ltd.) was used for the positive electrode and the negative electrode, and Nafion (registered trademark) 212 manufactured by DuPont was used for the membrane. The positive electrolyte used was an aqueous sulfuric acid solution electrolyte containing manganese ions as the active material and further containing titanium ions as metal ions not serving as the active material. Their concentrations are manganese ions: 1 mol/L, titanium ions: 1 mol/L, and sulfuric acid: 5 mol/L. The oxidation-reduction potential of the positive electrolyte relative to the standard hydrogen electrode was 1.5 V. The negative electrolyte used was an aqueous sulfuric acid solution electrolyte containing titanium ions as the active material and further containing manganese ions as metal ions not serving as the active material. Their concentrations are titanium ions: 1 mol/L, manganese ions: 1 mol/L, and sulfuric acid: 5 mol/L. The oxidation-reduction potential of the negative electrolyte relative to the standard hydrogen electrode was 0.1 V. 
     Sample No. 2-2 
     An RF battery having a layered structure including a plurality of stacked battery cells was produced using the positive electrode, the negative electrode, and the membrane in sample No. 2-1. The positive electrolyte and negative electrolyte used were the same as those in sample No. 2-1. 
     Sample No. 2-3 
     An RF battery having the same single cell structure as that in sample No. 2-1 was produced. The positive electrolyte was prepared by adding copper (Cu) to the positive electrolyte in sample No. 2-1 in an amount of 1 mmol/L. The negative electrolyte was prepared by adding copper (Cu) to the negative electrolyte in sample No. 2-1 in an amount of 1 mmol/L. The oxidation-reduction potentials of the positive and negative electrolytes relative to the standard hydrogen electrode were the same as those of sample No. 2-1. 
     Sample No. 2-11 
     An RF battery having the same single cell structure as in sample No. 2-1 was produced. An aqueous sulfuric acid solution containing vanadium ions as the active material was used for both the positive electrolyte and the negative electrolyte. Their concentrations were vanadium ions: 1 mol/L and sulfuric acid: 5 mol/L. The oxidation-reduction potential of the negative electrolyte relative to the standard hydrogen electrode was −0.3 V. 
     For each of the samples described above, each battery cell was charged and discharged using a constant current with a current density of 70 mA/cm 2 . In this test, a continuous charge/discharge test in which charging was changed to discharging when a prescribed switching voltage was reached was performed for one month. The rate of reduction in the bipolar plates after one month was measured. To measure the rate of reduction in the bipolar plates, part of the bipolar plates were masked in advance, and the step height between the masked portion and the unmasked portion was measured. The step height was used as the reduction in the thickness of the bipolar plates. The results are shown in Table 2. In Table 2, the rate of reduction in the thickness of the bipolar plates after one month is multiplied by 12, and the rate of reduction after one year is shown. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Oxidation-reduction potential 
                 Rate of reduction 
               
               
                 Sample 
                   
                 relative to standard hydrogen 
                 in thickness 
               
               
                 No. 
                 Electrolyte 
                 electrode (v) 
                 (μm/year) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 2-1 
                 TiMn (1 mol/L, 1 mol/L, sulfuric acid 5 mol/L) 
                 0.1 
                 10 
               
               
                 2-2 
                 TiMn (1 mol/L, 1 mol/L, sulfuric acid 5 mol/L) 
                 0.1 
                 10 
               
               
                 2-3 
                 TiMn (1 mol/L, 1 mol/L, sulfuric acid 5 mol/L) 
                 0.1 
                 0 
               
               
                 2-11 
                 V (1 mol/L, sulfuric acid 5 mol/L) 
                 −0.3 
                 500 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 2, in samples Nos. 2-1 to 2-3 in which the negative electrolyte containing titanium ions as the active material was used, the oxidation-reduction potential was 0.1 V, and the rate of reduction in thickness can be reduced by about 98% as compared with that of sample No. 2-11 in which vanadium ions were used as the active material and the oxidation-reduction potential was −0.3 V. This may be because of the following reason. Since a passivation film was formed on the surface of the bipolar plates, dissolution of the passivation film was prevented, so that oxidative deterioration (dissolution) of the bipolar plates was reduced. In particular, when the electrolytes contain Cu, the rate of reduction can be further reduced. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  redox flow battery (RF battery) 
               2  battery cell 
               21  membrane,  22  positive cell,  23  negative cell,  24  positive electrode 
               25  negative electrode,  200 P positive electrolyte circulation mechanism,  200 N negative electrolyte circulation mechanism 
               202  positive electrolyte tank,  203  negative electrolyte tank 
               204 ,  205 ,  206 ,  207  pipe 
               208 ,  209  pump 
               3 ,  3 α,  3 β cell frame 
               4 ,  4 A,  4 B,  4 α,  4 β bipolar plate 
               40  bipolar plate piece,  41  groove,  41   p  positive-side groove 
               41   n  negative-side groove,  41   i  introduction-side groove,  41   o  discharge-side groove 
               42  rib portion,  45  thin portion,  46  engaging recess 
               5 ,  5 α,  5 β frame body 
               51 ,  52  liquid supply manifold 
               53 ,  54  liquid discharge manifold 
               51   s ,  52   s  liquid supply guide groove 
               53   s ,  54   s  liquid discharge guide groove 
               55   c  inner circumferential recess,  55   w  opening,  56  engaging protrusion 
               58  sealing groove,  58   s  sealing member 
               59  frame piece,  59   s  sealing member 
               6  sealing member 
               9  cell stack,  9 S sub stack 
               91 ,  92  end plate,  93  connecting member