Patent Description:
Patent Literature <NUM> describes a cell stack in which a cell frame, a positive electrode, a membrane, a negative electrode, and a cell frame are repeatedly stacked, and the resulting stack body is sandwiched between supply/drainage plates; and a redox flow battery including the cell stack. The cell frames each include a bipolar plate disposed between the positive electrode and the negative electrode, and a frame body that supports the bipolar plate from the outer periphery of the bipolar plate. In this configuration, a single cell is formed between the bipolar plates of adjacent cell frames.

Patent Literature <NUM> and <NUM> describe a redox flow battery including a side electrolyte leakage channel as a gap formed between a frame body and a bipolar plate.

A redox flow battery according to the present disclosure includes.

In the redox flow battery, a gap between, among outer peripheral edge surfaces of the electrode, a side edge surface parallel to a direction in which an electrolyte flows and an inner wall surface of the fitting recess, the inner wall surface facing the side edge surface, is <NUM> or more and <NUM> or less.

In recent years, redox flow batteries have attracted attention as means for storing electricity of renewable energy, and there has been a requirement for the development of redox flow batteries having a high discharge capacity. To meet such a requirement, the inventors of the present invention focused on the fact that a leakage channel is formed between an outer peripheral edge surface of an electrode and an inner wall surface of a fitting portion of the electrode. The leakage channel is a gap between an electrode and a member facing an outer peripheral edge surface of the electrode. An electrolyte flowing in this leakage channel is drained from a cell substantially without contacting the electrode. Therefore, with an increase in the amount of electrolyte flowing through the leakage channel, the discharge capacity of the redox flow battery decreases. Thus, it is believed to be important to appropriately control the size of the leakage channel.

An object of the present disclosure is to provide a redox flow battery having a good discharge capacity by controlling the size of a leakage channel to an appropriate value.

The redox flow battery according to the present disclosure has a good battery performance.

Features of embodiments according to the invention of the present application will be first listed and described.

In the configuration in which an electrode is fitted in a fitting recess of a cell frame, leakage channels are formed between outer peripheral edge surfaces of the electrode and corresponding inner wall surfaces of the fitting recess. When, among the leakage channels, a portion parallel to a direction in which an electrolyte flows, that is, a side leakage channel formed between a side edge surface of the electrode and an inner wall surface of the fitting recess, the inner wall surface facing the side edge surface, has a small width, the amount of electrolyte flowing through the side leakage channel can be reduced. As a result, a decrease in the discharge capacity of the redox flow battery can be suppressed. Specifically, when the side leakage channel has a width of <NUM> or less, a decrease in the discharge capacity of the redox flow battery can be efficiently suppressed. A decrease in the width of the side leakage channel enables the amount of electrolyte flowing through the side leakage channel to be reduced. Therefore, the width of the side leakage channel is <NUM> or less. Herein, the term "direction in which an electrolyte flows" refers to a direction from a frame piece of the frame body having a liquid-supplying manifold toward a frame piece of the frame body having a liquid-draining manifold.

With a decrease in the width of the side leakage channel, the amount of electrolyte flowing through the side leakage channel can be reduced. However, when the width of the side leakage channel is excessively small, there is a concern that a membrane that faces the electrode may be damaged. This is because when the width of the side leakage channel is excessively small, during the compression of a cell or during flow of an electrolyte, an outer peripheral edge of the electrode may protrude from the fitting recess, and the protruding portion may apply an excessive surface pressure to the membrane. Therefore, the width of the side leakage channel is set to <NUM> or more, specifically, the electrode is made slightly smaller than the fitting recess. As a result, protrusion of the electrode from the fitting recess is suppressed, and application of an excessive surface pressure to the membrane can be suppressed. In order to reliably suppress protrusion of the electrode, the width of the side leakage channel is <NUM> or more.

In the above configuration, the outline shape of the frame body on the inner peripheral side forms the outline shape of an opening of the fitting recess. That is, a step portion formed by the frame body and the bipolar plate, the step portion being originally provided in the cell frame, functions as the fitting recess. With this configuration, the electrode is easily fitted in the fitting recess.

The frame body is a member to which a stress of a tightening mechanism that tightens members constituting a cell is applied. Therefore, there is a concern that when an electrode is sandwiched between adjacent frame bodies, an electrolyte leaks from the cell. According to the above configuration in which a fitting recess is formed in a bipolar plate, the possibility that an electrode is sandwiched between frame bodies can be significantly decreased.

When the gap is <NUM> or more and <NUM> or less, the amount of electrolyte flowing through the side leakage channel can be reduced while effectively suppressing application of an excessive surface pressure to the membrane. As a result, the battery performance of the redox flow battery can be improved.

Hereinafter, redox flow batteries (RF batteries) according to embodiments of the present disclosure will be described. The present invention is not limited to the configurations described in the embodiments. The present invention is defined by the claims described below and is intended to cover all the modifications within the meaning and scope of equivalents of the claims.

A redox flow battery (hereinafter, an RF battery) according to an embodiment will be described on the basis of <FIG>.

The RF battery is one of electrolyte-circulation storage batteries, and is used for storage of electricity of new energy from solar photovoltaic power generation and wind power generation. As illustrated in <FIG>, which is a view illustrating the principle of operations of an RF battery <NUM>, the RF battery <NUM> is a battery configured to be charged and discharged by means of the difference between the oxidation-reduction potential of active material ions contained in a positive electrode electrolyte, and the oxidation-reduction potential of active material ions contained in a negative electrode electrolyte. The RF battery <NUM> includes a cell <NUM>, which is divided into a positive electrode cell <NUM> and a negative electrode cell <NUM> by a membrane <NUM> permeable to hydrogen ions.

The positive electrode cell <NUM> includes a positive electrode <NUM> therein, and a positive electrode electrolyte tank <NUM> storing a positive electrode electrolyte is connected to the positive electrode cell <NUM> via ducts <NUM> and <NUM>. The duct <NUM> is equipped with a pump <NUM>. These members <NUM>, <NUM>, <NUM>, and <NUM> constitute a positive electrode circulation mechanism 100P configured to circulate the positive electrode electrolyte. Similarly, the negative electrode cell <NUM> includes a negative electrode <NUM> therein, and a negative electrode electrolyte tank <NUM> storing a negative electrode electrolyte is connected to the negative electrode cell <NUM> via ducts <NUM> and <NUM>. The duct <NUM> is equipped with a pump <NUM>. These members <NUM>, <NUM>, <NUM>, and <NUM> constitute a negative electrode circulation mechanism 100N configured to circulate the negative electrode electrolyte. The electrolytes stored in the tanks <NUM> and <NUM> are circulated through the cells <NUM> and <NUM> with the pumps <NUM> and <NUM> during charging and discharging. When charging or discharging is not performed, the pumps <NUM> and <NUM> are stopped and the electrolytes are not circulated.

The cell <NUM> is usually formed within a structure referred to as a cell stack <NUM>, which is illustrated in <FIG> and <FIG>. The cell stack <NUM> is constituted by sandwiching a stack structure referred to as a sub-stack <NUM> (<FIG>), from its both sides with two end plates <NUM> and <NUM>, and by tightening the sub-stack <NUM> with a tightening mechanism <NUM> (in the configuration provided as an example in <FIG>, a plurality of sub-stacks <NUM> are used).

Such a sub-stack <NUM> (<FIG>) has a configuration in which a cell frame <NUM>, a positive electrode <NUM>, a membrane <NUM>, and a negative electrode <NUM> are repeatedly stacked, and the resulting stack body is sandwiched between supply/drainage plates <NUM> and <NUM> (refer to the lower drawing in <FIG>, omitted in <FIG>).

Such a cell frame <NUM> includes a frame body <NUM> having a through window and a bipolar plate <NUM>, which covers the through window. Specifically, the frame body <NUM> supports the bipolar plate <NUM> on an outer peripheral side of the bipolar plate <NUM>. The positive electrode <NUM> is disposed so as to be in contact with one of the surfaces of the bipolar plate <NUM>, and the negative electrode <NUM> is disposed so as to be in contact with the other surface of the bipolar plate <NUM>. In this configuration, a single cell <NUM> is formed between the bipolar plates <NUM> fitted in adjacent cell frames <NUM> (refer to the upper drawing in <FIG>).

Flow of electrolytes in the cell <NUM> through the supply/drainage plates <NUM> and <NUM>, which are illustrated in the lower drawing in <FIG>, is performed with liquid-supplying manifolds <NUM> and <NUM> and liquid-draining manifolds <NUM> and <NUM> formed in the frame body <NUM> of the cell frame <NUM> (also refer to <FIG>). The positive electrode electrolyte is supplied from the liquid-supplying manifold <NUM>, then through an inlet slit <NUM> (<FIG>) formed in one surface of the cell frame <NUM> (the surface illustrated as being exposed in the drawing), to the positive electrode <NUM>; and the positive electrode electrolyte is drained through an outlet slit <NUM> (<FIG>) formed in an upper portion of the cell frame <NUM> to the liquid-draining manifold <NUM>. Similarly, the negative electrode electrolyte is supplied from the liquid-supplying manifold <NUM>, then through an inlet slit <NUM> (<FIG>) formed in the other surface of the cell frame <NUM> (the surface illustrated as being hidden in the drawing), to the negative electrode <NUM>; and the negative electrode electrolyte is drained through an outlet slit <NUM> (<FIG>) formed in an upper portion of the cell frame <NUM> to the liquid-draining manifold <NUM>. Loop-shaped seal members <NUM> (<FIG>) such as O-rings or flat gaskets are individually disposed between the cell frames <NUM> to suppress leakage of electrolytes from the sub-stack <NUM>. In this embodiment, as illustrated in <FIG>, a seal groove <NUM> into which an O-ring is inserted is formed in the cell frame <NUM> (when a flat gasket is used, the seal groove <NUM> may be omitted). In addition, seal members may be disposed so as to surround the outer peripheries of the manifolds <NUM>, <NUM>, <NUM>, and <NUM>, though not illustrated in the figure. The direction in which overall electrolytes flow (flow direction) in such a cell frame <NUM> is a direction from a frame piece of the frame body <NUM> having the liquid-supplying manifolds <NUM> and <NUM> toward a frame piece of the frame body <NUM> having the liquid-draining manifolds <NUM> and <NUM>, that is, the upward direction in the drawing of <FIG>.

As illustrated in <FIG>, which is a sectional view taken along line V-V in <FIG>, the frame body <NUM> of this embodiment is formed by bonding two frame-shaped divided bodies 22A and 22B whose sectional shapes are symmetric with respect to a stacking direction (the up-down direction in the drawing). The frame-shaped divided bodies 22A and 22B are formed so as to have thin portions on the through window side thereof (on the center side in the drawing). When the two frame-shaped divided bodies 22A and 22B are bonded to each other, a space for housing an outer peripheral edge portion of the bipolar plate <NUM> is formed between the thin portions of the two frame-shaped divided bodies 22A and 22B.

The material of the frame body <NUM> preferably has a good insulating property and more preferably also has acid resistance. Examples of the material of the frame body <NUM> include vinyl chloride, chlorinated polyethylene, and chlorinated paraffin.

As illustrated in the sectional view of <FIG>, the bipolar plate <NUM> is a member having one surface side that contacts the positive electrode <NUM> and the other surface side that contacts the negative electrode <NUM>. The bipolar plate <NUM> of this embodiment is a plate member having a substantially uniform thickness.

As illustrated in <FIG>, the outer peripheral edge portion of the bipolar plate <NUM> is sandwiched between the two frame-shaped divided bodies 22A and 22B that constitute the frame body <NUM>. With this sandwiched structure, the bipolar plate <NUM> is integrally fixed to the frame body <NUM>. The outer peripheral edge portion of the bipolar plate <NUM> is provided with grooves. O-rings (seal members) <NUM> are disposed in the grooves. These seal members <NUM> suppress flow of an electrolyte between the one surface side and the other surface side of the bipolar plate <NUM>.

The material of the bipolar plate <NUM> preferably has good conductivity and more preferably also has acid resistance and flexibility. An example of the material is a conductive material containing a carbon material. Specifically, examples thereof include conductive plastics formed of graphite and a chlorinated organic compound.

Alternatively, part of the graphite of the conductive plastics may be replaced by at least one of carbon black and diamond-like carbon. Examples of the chlorinated organic compound include vinyl chloride, chlorinated polyethylene, and chlorinated paraffin. When the bipolar plate <NUM> is formed of such a material, the bipolar plate <NUM> can have low electrical resistance, good acid resistance, and good flexibility.

As illustrated in <FIG>, the positive electrode <NUM> and the negative electrode <NUM> are respectively disposed on one surface side (on the upper side of the drawing) and the other surface side (on the lower side of the drawing) of the bipolar plate <NUM>. More specifically, the positive electrode <NUM> (negative electrode <NUM>) is fitted in a fitting recess <NUM> (<NUM>) constituted by an inner peripheral edge surface 22i of the frame body <NUM> (refer to the circled enlarged views) and a surface of the bipolar plate <NUM>, the surface facing the positive electrode <NUM> (negative electrode <NUM>). With regard to the fitting recess <NUM>, also refer to <FIG>. In <FIG>, the fitting recess <NUM> (<FIG>) is not illustrated. However, the fitting recess <NUM> has the same configuration as the fitting recess <NUM>.

In the configuration in which the electrodes <NUM> and <NUM> are respectively fitted in the fitting recesses <NUM> and <NUM> of the cell frame <NUM>, leakage channels <NUM> are formed between outer peripheral edge surfaces 4o and 5o of the electrodes <NUM> and <NUM> and the inner wall surfaces 24i and 25i (inner peripheral edge surfaces 22i) of the fitting recesses <NUM> and <NUM>. Among the leakage channels <NUM>, in particular, portions parallel to a direction in which electrolytes flow (in <FIG>, the direction from the surface illustrated as being exposed in the drawing toward the surface illustrated as being hidden in the drawing) are referred to as side leakage channels <NUM>. Specifically, the side leakage channels <NUM> are formed between side edge surfaces 4os and 5os of the electrodes <NUM> and <NUM> and the corresponding inner wall surfaces 24i and 25i of the fitting recesses <NUM> and <NUM>, the inner wall surfaces 24i and 25i facing the corresponding side edge surfaces 4os and 5os. The width of each of the side leakage channels <NUM> affects the discharge capacity of the RF battery <NUM> (<FIG> and <FIG>). This is because when the width of the side leakage channel <NUM> is increased, the amounts of electrolytes that are drained to the outside of the cell <NUM> (<FIG> and <FIG>) substantially without contacting the electrodes <NUM> and <NUM> increase. From this viewpoint, it is believed that when the width (the left-right direction in the drawing) of the side leakage channel <NUM> is decreased, the amount of electrolyte flowing through the side leakage channel <NUM> can be reduced, and a decrease in the discharge capacity of the RF battery <NUM> can be suppressed. In this embodiment, the width of the side leakage channel <NUM> is <NUM> or less. As a result, a decrease in the discharge capacity of the RF battery <NUM> is effectively suppressed. A decrease in the width of the side leakage channel <NUM> enables the amount of electrolyte flowing through the side leakage channel <NUM> to be reduced. Therefore, the width of the side leakage channel <NUM> is <NUM> or less.

With a decrease in the width of the side leakage channel <NUM>, the amount of electrolyte flowing through the side leakage channel <NUM> can be reduced. However, when the width of the side leakage channel <NUM> is excessively small, there is a concern that the membrane <NUM> that directly faces the electrodes <NUM> and <NUM> (refer to the upper drawing in <FIG>) may be damaged. This is because when the width of the side leakage channel <NUM> is excessively small, during the compression of the cell <NUM> (<FIG>) or during flow of electrolytes, outer peripheral edge portions of the electrodes <NUM> and <NUM> may protrude from the fitting recesses <NUM> and <NUM>, and the protruding portions may extend on the frame body <NUM> and apply an excessive surface pressure to the membrane <NUM>. Therefore, the width of the side leakage channel <NUM> is set to <NUM> or more, specifically, the electrodes <NUM> and <NUM> are respectively made slightly smaller than the fitting recesses <NUM> and <NUM>, thereby suppressing protrusion of the electrodes <NUM> and <NUM> from the fitting recesses <NUM> and <NUM> to suppress application of an excessive surface pressure to the membrane <NUM>. In order to reliably suppress protrusion of the electrodes <NUM> and <NUM>, the width of the side leakage channel <NUM> is <NUM> or more.

Herein, the electrodes <NUM> and <NUM> are formed of porous bodies. Even when the electrodes <NUM> and <NUM> are compressed between adjacent cell frames <NUM>, the sizes of the electrodes <NUM> and <NUM> in the planar direction do not substantially change. Accordingly, when the cell stack <NUM> (<FIG>) is disassembled and the width of the side leakage channel <NUM> between the fitting recess <NUM> (<NUM>) and the electrode <NUM> (<NUM>) in <FIG> is measured, the measured value is considered to be equal to the width of the side leakage channel <NUM> in the cell stack <NUM>. Specifically, the width of the side leakage channel <NUM> measured in a state where an uncompressed electrode <NUM> (<NUM>) is fitted in the fitting recess <NUM> (<NUM>) prior to assembly of the cell stack <NUM>, the width of the side leakage channel <NUM> in the cell stack <NUM>, and the width of the side leakage channel <NUM> measured after disassembly of the cell stack <NUM> are considered to be substantially equal to each other.

The material of the electrodes <NUM> and <NUM> preferably has good conductivity and more preferably also has acid resistance. For example, a woven fabric or a non-woven fabric formed of fibers of a carbon material may be used to form the electrodes <NUM> and <NUM>. Alternatively, carbon paper or the like may also be used as the electrodes <NUM> and <NUM>.

A plurality of RF batteries <NUM> (testing samples A to G) having different widths of side leakage channels <NUM> were prepared. A charge-discharge test was conducted with each of the testing samples A to G, and the cell resistivities of the testing samples A to G were compared. Regarding conditions for the charge-discharge test, the end-of-discharge voltage was <NUM> V, the end-of-charge voltage was <NUM> V, and the current was <NUM> mA/cm<NUM>. In the evaluation of the discharge capacity/current efficiency, a charge-discharge curve was prepared on the basis of the charge-discharge test, and the evaluation of the discharge capacity/current efficiency of the third cycle was conducted by using the charge-discharge curve.

After the charge-discharge test, the testing samples A to G were disassembled. According to the results, in the testing sample A, in which the width of the side leakage channel <NUM> was substantially <NUM>, breakage occurred in a portion of the membrane <NUM>, the portion corresponding to the fitting recess <NUM> (<NUM>). It is believed that the breakage of the membrane <NUM> occurred because the electrodes <NUM> and <NUM> protruded from the fitting recesses <NUM> (<NUM>) during compression of the cell <NUM>, and the protruding portions extended on the frame body <NUM>, resulting in a stress concentration on the membrane <NUM>. On the other hand, in the testing sample G, in which the width of the side leakage channel <NUM> was very wide, namely, <NUM>, elongation of a membrane <NUM> was observed in a portion of the membrane <NUM>, the portion being close to the fitting recess <NUM> (<NUM>). It is believed that the elongation of the membrane <NUM> of the testing sample G occurred because the membrane <NUM> was elongated by receiving the pressure difference generated between the positive electrode <NUM> and the negative electrode <NUM> or the repulsive force between the electrodes <NUM> and <NUM> in the portion of the wide side leakage channel <NUM>. In the other testing samples B, C, D, E, and F, defects such breakage or the formation of elongation of the membrane <NUM> were not observed.

In the evaluation of the charge-discharge test, the evaluation of the testing sample A could not be performed. According to the results of disassembly of the cell <NUM> of the testing sample A after the test, breakage of the membrane <NUM> was confirmed. The evaluation of the testing samples B to G could be performed. The testing samples C and D had the highest discharge capacity. The discharge capacities of the other testing samples B, E, F, and G were lower than the discharge capacity of the testing samples C and D by -<NUM>%, -<NUM>%, -<NUM>%, and -<NUM>%, respectively. The testing samples C, D, and E had the highest current efficiency of <NUM>% The other testing samples B, F, and G had current efficiencies of <NUM>%, <NUM>%, and <NUM>%, respectively. Thus, a decrease in the current efficiency was observed.

The results of Test Example described above showed that when the width of the side leakage channel <NUM> was <NUM> or more and <NUM> or less, defects such as breakage and elongation were unlikely to be generated in the membrane <NUM>, and a decrease in the discharge capacity of the RF battery <NUM> was suppressed. It also became clear that, from the viewpoint of suppressing a decrease in the discharge capacity of the RF battery <NUM>, the width of the side leakage channel <NUM> is <NUM> or less.

In Embodiment <NUM>, a configuration in which fitting recesses <NUM> and <NUM> of electrodes <NUM> and <NUM> are provided in a bipolar plate <NUM> will be described on the basis of <FIG> and <FIG>. <FIG> is a plan view of a cell frame <NUM> when viewed from the positive electrode <NUM> side, and <FIG> is a sectional view taken along line VII-VII in <FIG>.

As illustrated in <FIG>, the cell frame <NUM> of this embodiment has a fitting structure in which an outer peripheral edge portion 21c of a bipolar plate <NUM>, the outer peripheral edge portion 21c being formed so as to have a small thickness, is engaged with a step portion 22c formed in an inner peripheral edge portion (a portion close to a through window) of a frame body <NUM>. The step portion 22c is configured so that the thickness of a peripheral portion of the frame body <NUM>, the peripheral portion surrounding the entire circumference of the through window of the frame body <NUM>, is smaller than the thickness of the other portion of the frame body <NUM>. In addition, the outer peripheral edge portion 21c of the bipolar plate <NUM> locally has a small thickness so as to engage with the step portion 22c of the frame body <NUM>. The surface of the outer peripheral edge portion 21c is substantially flush with two surfaces of portions other than the step portion 22c in the frame body <NUM> when the outer peripheral edge portion 21c of the bipolar plate <NUM> is fitted in the step portion 22c of the frame body <NUM>. On the other hand, a surface of the bipolar plate <NUM>, the surface being adjacent to the negative electrode <NUM>, is disposed at a position recessed with respect to a surface of the frame body <NUM> when the outer peripheral edge portion 21c of the bipolar plate <NUM> is fitted in the step portion 22c of the frame body <NUM>.

By fitting the bipolar plate <NUM> in the step portion 22c, the step portion 22c of the frame body <NUM> and the outer peripheral edge portion 21c of the bipolar plate <NUM> are engaged with each other over the entire circumference in the thickness direction of the frame body <NUM>. As a result, the through window of the frame body <NUM> is covered with the bipolar plate <NUM>. Here, as illustrated in <FIG>, in the case where a fitting structure is used, it is necessary to seal a gap between the frame body <NUM> and the bipolar plate <NUM> so that an electrolyte does not flow between one surface side and the other surface side of the bipolar plate <NUM>. In this embodiment, a loop-shaped groove is formed in a part of the outer peripheral edge portion 21c of the bipolar plate <NUM>, the part facing the step portion 22c, and an O-ring (seal member) <NUM> is disposed in the groove. When a plurality of cell frames <NUM> are stacked and tightened, the O-ring <NUM> is compressed and functions as a seal. Alternatively, the gap between the step portion 22c of the frame body <NUM> and the outer peripheral edge portion 21c of the bipolar plate <NUM> may be sealed with a flat gasket, an adhesive, or the like.

In addition, the fitting recess <NUM> in which the positive electrode <NUM> is fitted is formed in a portion of the bipolar plate <NUM> of this embodiment, the portion facing the positive electrode <NUM> (also refer to <FIG>). In this case, a side leakage channel <NUM> is formed between an inner wall surface 24i of the fitting recess <NUM> formed in the bipolar plate <NUM> and a side edge surface 4os of the positive electrode <NUM>. The width of this side leakage channel <NUM> is also <NUM> or more and <NUM> or less as in Embodiment <NUM>. Consequently, a decrease in the discharge capacity of the RF battery <NUM> (<FIG> and <FIG>) can be suppressed while suppressing damage of the membrane <NUM> (<FIG>) that directly faces the positive electrode <NUM>. The upper limit of the width of the side leakage channel <NUM> is <NUM> or less. The lower limit of the width of the side leakage channel <NUM> is <NUM> or more.

On the other hand, the fitting recess <NUM> in which the negative electrode <NUM> is fitted is constituted by an inner peripheral edge surface 22i of the frame body <NUM> and a surface of the bipolar plate <NUM>, the surface facing the negative electrode <NUM>, as in Embodiment <NUM>. Accordingly, a side leakage channel <NUM> is formed between the inner peripheral edge surface 22i and a side edge surface 5os of the negative electrode <NUM>. The width of this side leakage channel <NUM> on the negative electrode <NUM> side is preferably determined as in Embodiment <NUM>. In such a case, a decrease in the discharge capacity of the RF battery <NUM> (<FIG> and <FIG>) can be suppressed while suppressing damage of the membrane <NUM> (<FIG>) that directly faces the negative electrode <NUM>.

As illustrated in <FIG>, in addition to the formation of a fitting recess <NUM> in a surface of a bipolar plate <NUM>, the surface being adjacent to the positive electrode <NUM>, a fitting recess <NUM> may also be formed in a surface of the bipolar plate <NUM>, the surface being adjacent to the negative electrode <NUM>. With this configuration, the size of the positive electrode <NUM> in the planar direction can be made same as the size of the negative electrode <NUM> in the planar direction.

Claim 1:
A redox flow battery (<NUM>) comprising:
an electrode (<NUM>, <NUM>);
a cell frame (<NUM>) including a frame body (<NUM>) and a bipolar plate (<NUM>) and having a fitting recess (<NUM>, <NUM>) in which the electrode (<NUM>, <NUM>) is fitted; and
a membrane (<NUM>) disposed so as to sandwich the electrode (<NUM>, <NUM>) between the bipolar plate (<NUM>) and the membrane (<NUM>),
wherein a gap between, among outer peripheral edge surfaces (4o, 5o) of the electrode (<NUM>, <NUM>), a side edge surface (4os, 5os) parallel to a direction in which an electrolyte flows and an inner wall surface (24i, 25i) of the fitting recess (<NUM>, <NUM>), the inner wall surface (24i, 25i) facing the side edge surface (4os, 5os), is <NUM> or more and <NUM> or less.