Patent Publication Number: US-2022238904-A1

Title: Redox flow battery

Description:
TECHNICAL FIELD 
     The present invention relates to a redox flow battery. 
     BACKGROUND ART 
     Conventionally, as a secondary battery for energy storage, a redox flow battery is known which is charged and discharged through a redox reaction of active materials contained in an electrolyte solution. The redox flow battery has features such as easy increase in capacity, long life, and accurate monitoring of its state of charge. Because of these features, in recent years, the redox flow battery has attracted a great deal of attention, particularly for application in stabilizing the output of renewable energy whose power production fluctuates widely or leveling the electric load. 
     To obtain a predetermined voltage, a redox flow battery generally includes a cell stack having a plurality of cells that are stacked. Further, by installing a plurality of cell stacks, high power requirements ranging from several MW to several tens of MW can be met (see, for example, Non-Patent Literature 1). On the other hand, focusing on a cost reduction effect due to economies of scale, for the purpose of meeting the high power requirements, it is also conceivable to increase the size of each cell in the cell stack, instead of increasing the number of cell stacks (see, for example, Non-Patent Literature 2). 
     CITATION LIST 
     Non-Patent Literature 
     Non-Patent Literature 1: Keiji Yano et al., “Development and demonstration of redox flow battery system”, SEI Technical Review, January 2017, No. 190, p. 15-20 Non-Patent Literature 2: Puiki Leung et al., “Progress in redox flow batteries, remaining challenges and their applications in energy storage”, RSC Advances, Royal Society of Chemistry, 2012, Vol. 2, p. 10125-10156 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The increase in size of the cell requires increasing the sizes of a frame body and a bipolar plate that constitute the cell. However, the bipolar plate is generally made of a hard and brittle material, and when the size of the bipolar plate is increased, it is difficult to ensure sufficient mechanical strength. As a result, the bipolar plate may be broken to mix the positive and negative electrolyte solutions, resulting in failure such as self-discharge. 
     It is therefore an object of the present invention to provide a redox flow battery that achieves an increase in size of a cell while maintaining its mechanical strength. 
     Solution to Problem 
     To achieve the above object, according to an aspect of the present invention, a redox flow battery includes a cell frame including a frame body and a bipolar plate, the frame body having a rectangular opening divided into a plurality of small openings along a first direction parallel to a longitudinal direction of the opening, the bipolar plate divided into a plurality of regions, each of the regions disposed within each of the small openings to form a plurality of recesses, and an electrode divided into a plurality of regions, each of the regions received in each of the recesses, wherein each of the small openings has a rectangular shape whose longitudinal direction is parallel to the first direction. 
     According to another aspect of the present invention, a redox flow battery includes a housing an electrode housed in the housing and held in a plate shape, a fluid flow mechanism for allowing flow of a fluid containing an active material through the electrode, wherein the fluid is supplied to a first surface of the electrode and collected from a second surface opposite to the first surface, or the fluid is supplied into the electrode and collected from the first or second surface, and a conductive member provided outside the housing and electrically connected to the electrode. 
     Advantageous Effects of Invention 
     As described above, according to the present invention, an increase in size of the cell can be achieved while maintaining its mechanical strength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic configuration diagram of a redox flow battery according to a first embodiment; 
         FIG. 1B  is a schematic configuration diagram of a cell stack that constitutes the redox flow battery according to the first embodiment; 
         FIG. 2  is an exploded plan view of the cell according to the first embodiment; 
         FIG. 3A  is a plan view showing an additional example of an uneven flow prevention mechanism according to the first embodiment; 
         FIG. 3B  is a perspective view of the uneven flow prevention mechanism shown in  FIG. 3A ; 
         FIG. 3C  is an exploded perspective view of the uneven flow prevention mechanism shown in  FIG. 3A ; 
         FIG. 4  is a plan view showing another example of the cell frame according to the first embodiment; 
         FIG. 5  is a schematic configuration diagram of the cell stack that constitutes the redox flow battery according to a second embodiment; 
         FIG. 6A  is a perspective view and a cross-sectional view of an electrode holder and a distribution plate according to the second embodiment; 
         FIG. 6B  is a cross-sectional view taken along line A-A in  FIG. 6A ; 
         FIG. 6C  is a cross-sectional view taken along line B-B in  FIG. 6A ; 
         FIG. 6D  is a cross-sectional view taken along line C-C in  FIG. 6A ; 
         FIG. 7A  is a diagram showing an exemplary configuration of the uneven flow preventing mechanism according to the second embodiment; 
         FIG. 7B  is a diagram showing an exemplary configuration of the uneven flow prevention mechanism according to the second embodiment; 
         FIG. 8  is a schematic configuration diagram of the cell that constitutes the redox flow battery according to a third embodiment; 
         FIG. 9A  is a cross-sectional view taken along line D-D in  FIG. 8 ; 
         FIG. 9B  is a cross-sectional view taken along line E-E in  FIG. 8 ; and 
         FIG. 9C  is a cross-sectional view taken along line F-F in  FIG. 8 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. 
     First Embodiment 
       FIG. 1A  is a schematic configuration diagram of a redox flow battery according to a first embodiment of the present invention.  FIG. 1B  is a schematic configuration diagram of a cell stack that constitutes the redox flow battery of this embodiment. 
     Redox flow battery  1  is configured to be charged and discharged through a redox reaction of positive- and negative-electrode active materials in cell  10 , and includes cell stack  2  having a plurality of stacked cells  10 . Cell stack  2  is connected to positive electrode-side tank  3  for storing a positive electrolyte solution through positive electrode-side incoming pipe L 1  and positive electrode-side outgoing pipe L 2 . Positive electrode-side incoming pipe L 1  is provided with positive electrode-side pump  4  for circulating the positive electrolyte solution between positive electrode-side tank  3  and cell stack  2 . Cell stack  2  is connected to negative electrode-side tank  5  for storing a negative electrolyte solution through negative electrode-side incoming pipe L 3  and a negative electrode-side outgoing pipe L 4 . Negative electrode-side incoming pipe L 3  is provided with negative electrode-side pump  6  for circulating the negative electrolyte solution between negative electrode-side tank  5  and cell stack  2 . As the electrolyte solution, any fluid containing an active material may be used, such as a slurry formed by suspending and dispersing a granular active material in a liquid phase, or a liquid active material itself. Therefore, the electrolyte solution described herein is not limited to a solution of an active material. 
     Cells  10  are formed by alternately stacking a cell frame and a membrane unit, both of which will be described below. Detailed configurations of the cell frame and the membrane unit will be described below. Although four cells  10  are shown in  FIG. 1B , the number of cells  10  in cell stack  2  is not limited thereto. As will be described in detail below, each cell  10  is divided into three regions in a direction perpendicular to stacking direction Z of cell stack  2  (i.e. in an X direction). 
     Each of cells  10  includes positive cell  12  that houses positive electrode  11 , negative cell  14  that houses negative electrode  13 , and membrane  15  that separates positive cell  12  and negative cell  14 . Positive cell  12  is connected to positive electrode-side incoming pipe L 1  through individual supply flow channel P 1  and common supply flow channel C 1 , and is connected to positive electrode-side outgoing pipe L 2  through individual return flow channel P 2  and common return flow channel C 2 . This allows positive cell  12  to be supplied with the positive electrolyte solution containing the positive-electrode active material from positive electrode-side tank  3 . Thus, in positive cell  12 , an oxidation reaction occurs during a charge process in which the positive-electrode active material changes from a reduced state to an oxidized state, and a reduction reaction occurs during a discharge process in which the positive-electrode active material changes from the oxidized state to the reduced state. On the other hand, negative cell  14  is connected to negative electrode-side incoming pipe L 3  through individual supply flow channel P 3  and common supply flow channel C 3 , and is connected to negative electrode-side outgoing pipe L 4  through individual return flow channel P 4  and common return flow channel C 4 . This allows negative cell  14  to be supplied with the negative electrolyte solution containing the negative-electrode active material from negative electrode-side tank  5 . Thus, in negative cell  14 , a reduction reaction occurs during the charge process in which the negative-electrode active material changes from an oxidized state to a reduced state, and an oxidation reaction occurs during the discharge process in which the negative-electrode active material changes from the reduced state to the oxidized state. 
       FIG. 2  is an exploded plan view of the cell of this embodiment, showing a plane viewed from the stacking direction of the cell stack. Here, a case is shown where the longitudinal directions of the cell frame and the membrane unit that constitute the cell are oriented horizontally, but this does not limit the position of the cell when used. 
     As described above, cells  10  are formed by alternately stacking cell frame  20  and membrane unit  30 . Cell frame  20  separates adjacent cells  10  from each other and includes rectangular frame body  21 . Frame body  21  has substantially rectangular opening  22 , and opening  22  is divided into three small openings  22   a - 22   c  along its longitudinal direction (first direction) X. Specifically, opening  22  is divided into three rectangular small openings  22   a - 22   c  such that the longitudinal direction of each of small openings  22   a - 22   c  is parallel to longitudinal direction X of opening  22 . Cell frame  20  includes rectangular bipolar plate  23 . Bipolar plate  23  is divided into three regions  23   a - 23   c , which are respectively disposed within small openings  22   a - 22   c  of opening  22 . Thus, bipolar plate  23  includes three recesses formed on one surface thereof (i.e. on a side facing out of the page), and in these three recesses, three divided regions  11   a - 11   c  of positive electrode  11  are respectively received in contact with bipolar plate  23 . Bipolar plate  23  also includes three recesses formed on the other surface thereof (i.e. on a side facing into the page), and in these three recesses, three divided regions (not shown) of negative electrode  13  are respectively received in contact with bipolar plate  23 . 
     Membrane unit  30  includes membrane  15  divided into three regions  15   a - 15   c  and support frame  31  supporting membrane  15 . Membrane unit  30  is stacked on cell frame  20  such that three regions  15   a - 15   c  of membrane  15  respectively face three regions  23   a - 23   c  of bipolar plate  23  and close the three recesses as described above. Thus, positive cell  12  divided into three regions is formed between one surface of bipolar plate  23  and membrane  15 , and negative cell  14  divided into three regions is formed between the other surface of bipolar plate  23  and membrane  15 . As a result, cell  10  is divided into three regions in longitudinal direction X of frame body  21 . 
     Frame body  21  includes through-holes  24   a - 24   d  that are formed near four corners thereof and that penetrate respectively frame body  21  in thickness direction Z thereof. Similarly, support frame  31  includes through-holes  32   a - 32   d  that are formed near four corners thereof and that penetrate respectively support frame  31  in thickness direction Z thereof. Once cell frame  20  and membrane unit  30  are alternately stacked to form cell stack  2 , through-holes  24   a - 24   d ,  32   a - 32   d  constitute common flow channels C 1 -C 4  as described above, through which the electrolyte solution flows. Specifically, through-holes  24   a ,  32   a  on the lower left corner constitute common supply flow channel C 1  for the positive electrolyte solution, and through-holes  24   b ,  32   b  on the upper right corner constitute common return flow channel C 2  for the positive electrolyte solution. Through-holes  24   c ,  32   c  on the lower right corner constitute common supply flow channel C 3  for the negative electrolyte solution, and through-holes  24   d  and  32   d  on the upper left corner constitute common return flow channel C 4  for the negative electrolyte solution. 
     Further, frame body  21  includes two flow channel grooves  25 ,  26  formed on one surface thereof (i.e. on a side facing out of the page). Two flow channel grooves  25 ,  26  are adjacent to both sides of opening  22  in width direction (second direction) Y perpendicular to longitudinal direction X of opening  22 , and extend in longitudinal direction X of opening  22 . First flow channel groove  25  constitutes individual supply flow channel P 1  for the positive electrolyte solution, connecting through-hole  24   a  (common supply flow channel C 1 ) to the recess of positive cell  12  that receives positive electrode  11 . Second flow channel groove  26  constitutes individual return flow channel P 2  for the positive electrolyte solution, connecting the recess of positive cell  12  that receives positive electrode  11  to through-hole  24   b  (common return flow channel C 2 ). Although not shown, frame body  21  also includes two flow channel grooves formed on the other surface thereof (i.e. on a side facing into the page). One of the flow channel grooves constitutes individual supply flow channel P 3  for the negative electrolyte solution, connecting through-hole  24   c  (common supply flow channel C 3 ) to the recess of negative cell  14  that receives negative electrode  13 . The other of the flow channel grooves constitutes individual return flow channel P 4  for the negative electrolyte solution, connecting the recess of negative cell  14  that receives negative electrode  13  to through-hole  24   d  (common return flow channel C 4 ). 
     As described above, in this embodiment, opening  22  of frame body  21  is divided into three small openings  22   a - 22   c , and accordingly bipolar plate  23  is also divided into three regions  23   a - 23   c . Therefore, by maintaining the size of regions  23   a - 23   c  equal to that of the conventional bipolar plate, a reduction in the overall mechanical strength of bipolar plate  23  can be prevented even when the total size of bipolar plate  23  is increased. Further, frame body  21  includes beam-like portions  22   d ,  22   e , each of which extends across opening  22  in width direction Y to divide opening  22  into three small openings  22   a - 22   c , and these beam-like portions  22   d ,  22   e  function as a reinforcement to enhance the rigidity of frame body  21 . This also can minimize the strength reduction associated with the increase in size of frame body  21 . As a result, an increase in size of cell  10  can be achieved while maintaining the mechanical strength of cell  10  or cell frame  20 . 
     In the illustrated embodiment, three regions  23   a - 23   c  of bipolar plate  23  are not electrically connected to each other, and thus the three divided regions of electrode cell  10  are also not electrically connected to each other. However, if there is a concern that the potential difference between the divided regions of cell  10  becomes large which degrades the charge/discharge performance, three regions  23   a - 23   c  of bipolar plate  23  may be electrically connected to each other. For that purpose, for example, frame body  21  may include conductive elements provided inside beam-like portions  22   d ,  22   e  that electrically connect three regions  23   a - 23   c  of bipolar plate  23 . The number of each of opening  22  and bipolar plate  23  of frame body  21  is three in the illustrated embodiment, but is not limited thereto. Depending on the desired size of cell  10 , opening  22  and bipolar plate  23  can each be divided into an appropriate number of regions. In other words, when it is desired to further increase the size of cell  10 , opening  22  and bipolar plate  23  can each be divided into four or more regions. 
     Bipolar plate  23  must be liquid-tightly attached to opening  22  to prevent leakage of the electrolyte solution from the gap between opening  22  and bipolar plate  23 . The fact that bipolar plate  23  is divided into the multiple regions is also preferable because it can improve the workability during such attachment. From the standpoint of resistance to the electrolyte solution (chemical resistance, acid resistance, or the like) as well as mechanical strength, a carbon-containing conductive material is generally used as a material of bipolar plate  23 . However, if higher mechanical strength is required, bipolar plate  23  that is a carbon-plated metal plate may be used. On the other hand, frame body  21  is made of an insulating material. As the material of frame body  21 , a material may be used that has an appropriate rigidity, that does not react with an electrolyte solution, and that has resistance to it. Such materials include, for example, vinyl chloride, polyethylene, and polypropylene. 
     Membrane  15  may not necessarily be divided into multiple regions, and for example may be provided on the entire surface of frame body  21 . However, an area of frame body  21  other than opening  22  does not come into contact with the electrolyte solution, and thus does not function as cell  10  even when membrane  15  that is an ion exchange membrane is provided on that area. This results in waste of expensive ion exchange membrane. Further, there is also a concern that an increase in size of membrane  15  may lead to insufficient strength or deterioration of handleability. Thus, membrane  15  is also preferably divided into multiple regions  15   a - 15   c . In addition, as shown, each of regions  15   a - 15   c  of membrane  15  is more preferably divided into a matrix of small regions. The number of divisions of membrane  15  may not be the same as the number of divisions of opening  22  or bipolar plate  23 . On the other hand, support frame  31  is preferably formed of a material having a higher strength than that of membrane  15 . Such materials include, for example, plastics. 
     As materials of electrodes  11 ,  13 , a carbon material is preferably used, and its forms include felt-like and sheet-like. However, from the standpoint of ease and cost of uniformly installing the required amount of electrode materials in cells  12 ,  14 , a pellet-like carbon material may also be used. Specific forms of the pellet, for example, include forms such as spherical, granular, tablet-shaped, and ring-shaped, and an extruded form having a multilobed cross section. 
     In the meantime, if the length of opening  22  in longitudinal direction X increases with increasing the size of frame body  21 , the length of cell  10  in longitudinal direction X may also increases, and the electrolyte solution may flow unevenly through cell  10 . Such uneven flow may be prevented to some extent by beam-like portions  22   d ,  22   e  formed between small openings  22   a - 22   c , but its effect is limited. For that reason, in this embodiment, first communication section  27  is formed between first flow channel groove  25  and opening  22 , which consists of a plurality of grooves communicating first flow channel groove  25  with opening  22 . Further, second communication section  28  is also formed between second flow channel groove  26  and opening  22 , which consists of a plurality of grooves communicating second flow channel groove  26  with opening  22 . The grooves constituting each of communication sections  27 ,  28  are arranged in longitudinal direction X of opening  22  between each of flow channel grooves  25 ,  26  and opening  22 . Since communication sections  27 ,  28  thus provided supplies the electrolyte solution to cell  10  so as to distribute it in longitudinal direction X of opening  22 , the occurrence of uneven flow as described above can be prevented and the charge/discharge performance can be maximized. To more effectively prevent the uneven flow, communication sections  26 ,  27  are preferably formed throughout the length of opening  22  in longitudinal direction X. Therefore, flow channel grooves  25 ,  26  also preferably extend throughout the length of opening  22  in longitudinal direction X. 
     An uneven flow prevention mechanism for preventing the electrolyte solution from flowing unevenly through cell  10  is not limited to communication section  27 ,  28  as described above, and other configurations may be additionally employed.  FIG. 3A  is a plan view showing such an additional uneven flow prevention mechanism installed in the cell frame.  FIG. 3B  is a perspective view of the uneven flow prevention mechanism shown in  FIG. 3A , and  FIG. 3C  is an exploded perspective view thereof. 
     Referring to  FIG. 3A , each of regions  11   a - 11   c  of positive electrode  11  is further divided into three in longitudinal direction X of opening  22  and two in width direction Y thereof, i.e., six small regions (electrode pieces)  11   d . Perforated sheet  16  having a plurality of holes is provided on a side of each electrode piece  11   d  into which the electrolyte solution flows, i.e., on a side facing first flow channel groove  25 . In addition, flow directing sheet  17  is provided on two sides adjacent to the side of each electrode piece  11   d , on which perforated sheet  16  is provided. Perforated sheet  16  facilitates distribution of the electrolyte solution in longitudinal direction X of opening  22 , and flow directing sheet  17  prevents diffusion of the electrolyte solution in longitudinal direction X of opening  22 . Thus, uneven flow of the electrolyte solution through cell  10  can be further prevented. To prevent the electrolyte solution from passing between adjacent flow directing sheets  17 , adjacent flow directing sheets  17  are preferably joined to each other. As materials of perforated sheet  16  and flow directing sheet  17 , a material may be used that has flexibility adaptable to the internal shape of cell  10  and has resistance to the electrolyte solution. Such materials include, for example, plastics. 
     The installation position and the number of perforated sheets  16  are not particularly limited as long as they are arranged along longitudinal direction X of opening  22  in cell  10 . Therefore, perforated sheet  16  may be provided only on an end surface of each of regions  11   a - 11   c  of positive electrode  11  that faces first flow channel groove  25 . In this case, each of regions  11   a - 11   c  of positive electrode  11  may not be necessarily divided in width direction Y of opening  22 . On the other hand, flow directing sheet  17  can provide desired effects as long as it is arranged along width direction Y of opening  22  in cell  10 . However, for this purpose, each of regions  11   a - 11   c  of positive electrode  11  must be divided into two or more small regions (electrode pieces) in longitudinal direction X of opening  22 . 
     In the above embodiment, while the length of opening  22  in the flow direction of the electrolyte solution (i.e. in a Y direction) is maintained equal to that in the conventional case, the length of opening  22  in a direction perpendicular to the flow direction (i.e. in the X direction) is increased, which can lead to the increase in size of cell  10 . With this configuration, the occurrence of problems that may occur with the increase in size of cell  10  can also be prevented. Specifically, an increase in height (i.e. length in the Y direction) of electrodes  11 ,  13  may lead to an increase in pressure drop when the electrolyte solution passes through electrodes  11 ,  13 , and an increase in thickness (i.e. length in the Z direction) of electrodes  11 ,  13  may lead to an increase in internal resistance of cell  10 , but such increases in both the pressure drop and the internal resistance can be prevented. On the other hand, by forming a plurality of openings  22  in frame body  21  along the flow direction of the electrolyte solution (i.e. along the Y direction), the size of cell  10  in the flow direction can be increased while preventing the increase in pressure drop and internal resistance as described above.  FIG. 4  is a plan view showing an exemplary configuration of the cell frame having the frame body with such openings. 
     Referring to  FIG. 4 , openings  22  are arranged along width direction Y of opening  22  such that longitudinal directions X of openings  22  are parallel to each other. First flow channel groove  25  is composed of first common flow channel groove  25   a  extending in arrangement direction Y of openings  22 , and a plurality of first individual flow channel grooves  25   b  each extending in longitudinal direction Y of opening  22 . Similarly, second flow channel groove  26  is composed of second common flow channel groove  26   a  extending in arrangement direction Y of opening  22 , and a plurality of second individual flow channel grooves  26   b  each extending in longitudinal direction Y of opening  22 . First common flow channel groove  25   a  extends upward from through-hole  24   a  on the lower left corner, and second common flow channel groove  26   a  extends downward from through-hole  24   b  on the upper right corner. First individual flow channel grooves  25   b  and second individual flow channel grooves  26   b  are alternately arranged between openings  22  adjacent to each other in arrangement direction Y, and are each connected to adjacent openings  22 . 
     As described above, cell frame  20  shown in  FIG. 4  is not configured to increase the size of electrodes  11 ,  13  by increasing the size of opening  22  in the flow direction of the electrolyte solution (i.e. in the Y direction), but to increase the number of electrodes  11 ,  13  by increasing the number of openings  22 . As a result, a high output power can be achieved by increasing the total size of cell  10 , while preventing an increase in size of electrodes  11 ,  13 . Thus, even in cell frame  20  shown in  FIG. 4 , the occurrence of the above-described problems that may occur with the increase in size of cell  10  can be prevented. Specifically, since the length of flow channel in electrodes  11 ,  13  through which the electrolyte solution flows in height direction Y is not increased, its pressure drop can be prevented from increasing. Further, since the thickness (i.e. the length in the Z direction) of electrodes  11 ,  13  is not also increased, the internal resistance of electrodes  11 ,  13  can be prevented from increasing. In cell frame  20  shown in  FIG. 4 , four openings  22  are formed in frame body  21 , each of which is divided into four small openings, but the number of openings  22  is not particularly limited and the number of small openings is also not particularly limited. Frame body  21  may therefore include two, three, or five or more openings  22 , and each opening  22  may also be divided into two, three, or five or more small openings. 
     Second Embodiment 
       FIG. 5  is a schematic configuration diagram of the cell stack which constitutes the redox flow battery according to a second embodiment of the present invention. This embodiment is a variation of the first embodiment, and differs from the first embodiment in that no bipolar plate is provided. Hereinafter, components identical to those of the first embodiment will be denoted by the same reference numerals in the drawings, description thereof will be omitted, and only components that are different from those of the first embodiment will be described. 
     In this embodiment, cell  10  is composed of a flattened cuboid-shaped cell case (housing)  40 . Therefore, cell stack  2  is formed by stacking a plurality of cell cases  40 . Cell case  40  includes a pair of bulkheads  41 ,  42  which are opposed to each other in stacking direction Z of cell stack  2  and between which membrane  15  is disposed. Therefore, positive cell  12  is formed between first bulkhead  41  and membrane  15 , and negative cell  14  is formed between second bulkhead  42  and membrane  15 . As a material of cell case  40 , a material is preferably used that has an appropriate rigidity, that does not react with an electrolyte solution, and that has resistance to it. Such a material may be, for example, an insulating material that is similar to that of frame body  21  of the first embodiment. The number of cells  10  in cell stack  2  is not limited to the illustrated one. 
     Positive electrode  11  is housed in positive cell  12  while being held in a plate shape by an electrode holder as described below. Positive electrode  11  is spaced apart from and faces first bulkhead  41  on one side of two opposite surfaces (first and second surfaces) thereof, and is spaced apart from and faces membrane  15  on the other side. Thus, positive cell  12  includes space S 1  formed between first bulkhead  41  and one surface of positive electrode  11 , and space S 2  formed between the other surface of positive electrode  11  and membrane  15 . Negative electrode  13  is also housed in negative cell  14  while being held in a plate shape by an electrode holder as described below. Negative electrode  13  is spaced apart from and faces second bulkhead  42  on one side of two opposite surfaces (first and second surfaces) thereof, and is spaced apart from and faces membrane  15  on the other side. Thus, negative cell  14  includes space S 3  formed between second bulkhead  42  and one surface of negative electrode  13 , and space S 4  formed between the other surface of negative electrode  13  and membrane  15 . As materials of electrodes  11 ,  13 , not only a felt-like or sheet-like carbon material but also a pellet-like carbon material may be used, as in the first embodiment. 
     Individual flow channels P 1 -P 4 , each of which is configured as an independent piping member, are connected to cell case  40  and communicate with the interior of cell  10 . Individual supply flow channel P 1  for the positive electrolyte solution is connected to space S 1  in positive cell  12 , and individual return flow channel P 2  is connected to space S 2  in positive cell  12 . Therefore, the positive electrolyte solution is supplied from individual supply flow channel P 1  to positive electrode  11  through the space S 1 , flows through positive electrode  11  in thickness direction Z, and then is returned from space S 2  to individual return flow channel P 2 . In other words, space S 1  functions as a fluid supply for supplying the positive electrolyte solution to positive electrode  11 , and space S 2  functions as a fluid collector for collecting the positive electrolyte solution from positive electrode  11 , which constitute a fluid flow mechanism for allowing flow of the positive electrolyte solution through positive electrode  11 . Individual supply flow channel P 3  for the negative electrolyte solution is connected to space S 3  in negative cell  14 , and individual return flow channel P 4  is connected to space S 4  in negative cell  14 . Therefore, the negative electrolyte solution is supplied from individual supply flow channel P 3  to negative electrode  13  through space S 3 , flows through negative electrode  13  in thickness direction Z, and then is returned from space S 4  to individual return flow channel P 4 . In other words, space S 3  functions as a fluid supply for supplying the negative electrolyte solution to negative electrode  13 , and space S 4  functions as a fluid collector for collecting the negative electrolyte solution from negative electrode  13 , which constitute a fluid flow mechanism for allowing flow of the negative electrolyte solution through negative electrode  13 . In this embodiment, similarly to individual flow channels P 1 -P 4 , each of common flow channels C 1 -C 4  is also configured as a separate piping member that is independent of cell case  40 . 
     In the first embodiment, the electrical connection between positive and negative electrodes  11 ,  13  is established by bipolar plate  23 , but in this embodiment, conductive member  18  is provided instead of such a bipolar plate. Conductive member  18  is disposed outside cell case  40  and functions to electrically connect positive and negative electrodes  11 ,  13  of adjacent cells  10 . Specifically, conductive member  18  is connected through an opening (not shown) formed on a side of cell case  40  to a current collecting portion of an electrode holder as described below, so as to be electrically connected to positive electrode  11  or negative electrode  13 . The use of conductive member  18  is not desirable because its electrical path length is longer and its cross-sectional area is smaller as compared with the case of using bipolar plate  23 , but is advantageous in that the resistance to the electrolyte solution need not be taken into account because of no contact with the electrolyte solution. Therefore, as a material of conductive member  18 , a metal material having high conductivity may be used. On the other hand, unlike bipolar plate  23 , conductive member  18  does not require so high mechanical strength, and therefore a highly conductive carbon material may also be selected as a material of conductive member  18 . Conductive member  18  may be provided on up to four sides of cell case  40 , so as to further reduce the electrical resistance between positive and negative electrodes  11 ,  13 . 
     Thus, in this embodiment, there does not exist a bipolar plate which may cause a problem of mechanical strength reduction when the size of cell  10  is increased. As a result, an increase in size of cell  10  can be achieved without a large reduction in mechanical strength. In addition, the supply and return of the electrolyte solution with respect to cell  10  are performed by separate piping members C 1 -C 4 , P 1 -P 4  that are independent of cell case  40 . Therefore, there is no need to form a groove serving as a flow channel of the electrolyte solution in cell case  40  itself, and a cost reduction effect due to economies of scale can be further expected. Further, since the electrolyte solution flows through electrodes  11 ,  13  in thickness direction Z, a large increase in pressure drop when the electrolyte solution passes through electrodes  11 ,  13  can also be prevented even if the size of cell  10  is increased. As described above, there is also a concern that an increase in size of membrane  15  may lead to insufficient strength or deterioration of handleability. For that reason, as in the first embodiment, membrane  15  of this embodiment may be divided into a plurality of regions, and alternatively or in addition, it may be divided into a plurality of small regions. In this case, the regions or the small regions may be supported on a support frame made of, for example, plastic. 
     If the plane size of electrodes  11 ,  13  (i.e. the size of it in the XY plane) increases with increasing the size of cell  10 , the electrolyte solution may flow unevenly through electrodes  11 ,  13  in thickness direction Z. For that reason, in this embodiment, distribution plate  19  is provided in supply spaces S 1 , S 3  to face electrodes  11 ,  13 . Distribution plate  19  has a matrix of holes as described below. Thus, the electrolyte solution that has been supplied into supply spaces S 1 , S 3  is uniformly distributed on the surfaces of electrodes  11 , 13 . As a result, the occurrence of uneven flow as described above can be prevented and the charge/discharge performance can be maximized. Distribution plate  19  may also be provided in collection spaces S 2 , S 4 . 
     The direction in which the electrolyte solution passes through each of electrodes  11 ,  13  may be opposite to the illustrated direction. Specifically, in positive cell  12 , the positive electrolyte solution may flow from space S 2  adjacent to membrane  15  toward space S 1  adjacent to bulkhead  41 . In other words, individual supply flow channel P 1  may be connected to space S 2  adjacent to membrane  15 , and individual return flow channel P 2  may be connected to space S 1  adjacent to bulkhead  41 . Further, in negative cell  14 , the negative electrolyte solution may flow from space S 4  adjacent to membrane  15  toward space S 3  adjacent to bulkhead  41 . In other words, individual supply flow channel P 3  may be connected to space S 4  adjacent to membrane  15 , and individual return flow channel P 4  may be connected to space S 3  adjacent to bulkhead  42 . In this case, distribution plate  19  is preferably provided in spaces S 2 , S 4  adjacent to membrane  15 . 
     The direction in which the electrolyte solution passes through each of electrodes  11 ,  13  may be different between the charge and discharge processes. As an example, a pipe switching device may be provided between positive electrode-side incoming pipe L 1  and positive electrode-side outgoing pipe L 2 , as well as between negative electrode-side incoming pipe L 3  and negative electrode-side outgoing pipe L 4 , so as to change the flow direction of the electrolyte solution when switching between the charge and discharge processes. In this case, distribution plate  19  is preferably provided not only in spaces S 1 , S 3  adjacent bulkheads  41 ,  42  but also in spaces S 2 , S 4  adjacent to membrane  15 . 
     The configuration of an electrode holder housed in the cell case and holding each electrode in a plate shape will be described here. The electrode holder holding the positive electrode and the electrode holder holding the negative electrode have the same configuration. Therefore, only the configuration of the electrode holder holding the positive electrode will be described below.  FIG. 6A  is a perspective view of the electrode holder holding the positive electrode and the distribution plate provided in conjunction therewith.  FIGS. 6B-6D  are cross-sectional views of a current collecting portion and a reinforcement portion which constitute the electrode holder,  FIG. 6B  being a cross-sectional view taken along line A-A in  FIG. 6A ,  FIG. 6C  being a cross-sectional view taken along line B-B in  FIG. 6A , and  FIG. 6D  being a cross-sectional view taken along line C-C in  FIG. 6A . 
     Electrode holder  43  is formed in a flat rectangular parallelepiped shape, and includes frame member  44  constituting four sides of the rectangular parallelepiped and grid member  45  constituting the remaining two sides of the rectangular parallelepiped. Electrode holder  43  houses positive electrode  11  therein, and is housed in cell case  40  such that a pair of opposite grid members  45  faces first bulkhead  41  and membrane  15 . This allows the positive electrolyte solution to flow into positive electrode  11  through one of grid members  45 , flow through positive electrode  11  in thickness direction Z, and then flow out of positive electrode  11  through the other of grid members  45 . 
     Frame member  44  and grid member  45  are each composed of current collecting portion  46  and reinforcement portion  47 . Current collecting portion  46  is made of a conductive material and forms the inner surfaces, i.e. surfaces facing and contacting positive electrode  11 , of frame member  44  and grid member  45 . As a material of current collecting portion  46 , a carbon material having high conductivity is preferably used. Reinforcement portion  47  functions to reinforce current collecting portion  46  and is preferably formed of a material having a higher strength than that of membrane  15  Such materials include, for example, plastics. Reinforcement portion  47  forms the outer surfaces of frame member  44  and grid member  45 , but is not provided on a portion of the outer surface of frame member  44 . Therefore, current collecting portion  46  is exposed on the outer surface of frame member  44  through that portion, and conductive member  18  is connected to the portion thus exposed. This allows electrical connection between connect conductive member  18  and positive electrode  11 . The location where current collecting portion  46  is exposed is not limited to the illustrated one as long as current collecting portion  46  is exposed to the outside through at least one portion of frame member  44 . When a material having a certain level of mechanical strength, such as a carbon-plated metal plate, is used as a material of current collecting portion  46 , reinforcement portion  47  is not necessarily provided. 
     As described above, distribution plate  19  has a matrix of holes  19   a  and is provided to face grid member  45  of electrode holder  43 . Such distribution plate  19  can uniformly distribute the positive electrolyte solution that has passed through holes  19   a  onto the surface of positive electrode  11 , preventing the electrolyte solution from flowing unevenly through positive electrode  11  in thickness direction Z. However, the uneven flow prevention mechanism for the electrolyte solution in this embodiment is not limited to such distribution plate  19 , and other configurations may be employed.  FIGS. 7A and 7B  are perspective views showing other examples of such uneven flow prevention mechanism. 
     In the example shown in  FIG. 7A , distribution plate  19  is not provided, but instead electrode holder  43  itself is provided with the uneven flow prevention mechanism. Specifically, electrode holder  43  includes distribution plate member  48  provided on a side thereof facing bulkhead  41 . Distribution plate member  48  includes a matrix of holes  48   a , which can produce the same effects as those produced by distribution plate  19 . Like the frame member  44 , distribution plate member  48  is composed of current collecting portion  46  forming the inner surface of electrode holder  43  and reinforcement portion  47  forming the outer surface thereof. Distribution plate member  48  may also be provided on a side of electrode holder  43  that faces membrane  15 . 
     On the other hand, in the example shown in  FIG. 7B , a plurality of electrolyte solution introduction pipes (fluid introduction pipes)  50  each having a plurality of supply ports  50   a  are provided instead of distribution plate  19 . Electrolyte solution introduction pipes  50  are connected to individual supply flow channel P 1  and function as a fluid supply for supplying the positive electrolyte solution to positive electrode  11  through supply ports  50   a . On the other hand, since supply ports  50   a  of each electrolyte solution introduction pipe  50  open toward bulkhead  41  (i.e. in the negative direction of the Z-axis), electrolyte solution introduction pipes  50  also function to distribute the positive electrolyte solution uniformly over positive electrode  11 . Thus, also in this example, the same effects as those produced by distribution plate  19  can be produced. 
     In this embodiment, even if the number of stacked cells  10  is the same as in the first embodiment, the size of cell stack  2  in stacking direction Z is larger than that in the first embodiment due to the structural difference between cell frame  20  and cell case  40 . Therefore, in the first embodiment, as a method of securing cell stack  2 , a method is generally used where stacked bodies each composed of cell frame  20  and membrane unit  30  are secured together, but in this embodiment, each adjacent pair of cell cases  40  may be individually secured. When it is desired to further increase the size of cell  10 , from the standpoint of maintaining mechanical strength, cell case  40  may be composed of two half cases each constituting positive cell  12  and negative cell  14 . Also in this case, each pair of the two half cases, that are adjacent to each other with membrane  15  interposed therebetween, may be individually secured, and each cell case  40  thus secured may be individually secured to adjacent cell case  40 . Such a method is preferable because cell stack  2  can be assembled more easily, as compared with the method of entirely securing cell stack  2  as in the first embodiment. 
     Third Embodiment 
       FIG. 8  is a schematic side view showing a portion of the cell which constitutes the redox flow battery according to a third embodiment of the present invention, specifically a schematic side view of the positive cell.  FIG. 9A  is a cross-sectional view taken along line D-D in  FIG. 8 ,  FIG. 9B  is a cross-sectional view taken along line E-E in  FIG. 8 , and  FIG. 9C  is a cross-sectional view taken along line F-F in  FIG. 8 . This embodiment is a variation of the second embodiment, and differs from the second embodiment in terms of the fluid flow mechanism for allowing flow of the electrolyte solution through the electrode. Hereinafter, components identical to those of the second embodiment will be denoted by the same reference numerals in the drawings, description thereof will be omitted, and only components that are different from those of the second embodiment will be described. It should be noted that since the positive cell and the negative cell have substantially the same configuration, the following description for the positive cell applies to the negative cell as well. 
     From the standpoint of preventing an increase in internal resistance of cell  10 , the distance between positive electrode  11  and membrane  15  is preferably as short as possible. For that reason, in this embodiment, electrode holder  43  is configured to bring positive electrode  11  housed therein into contact with membrane  15 . Specifically, electrode holder  43  has an open side facing membrane  15 , and is housed in cell case  40  such that positive electrode  11  housed therein is brought into contact with membrane  15 . Accordingly, space S 2  is not formed between positive electrode  11  and membrane  15 . Therefore, individual return flow channel P 2  is connected to space S 1  formed between positive electrode  11  and first bulkhead  41 . In addition, in this embodiment, electrolyte solution introduction pipes  50  similar to the second embodiment are provided as a fluid supply for supplying the positive electrolyte solution to positive electrode  11 . However, electrolyte solution introduction pipes  50  are not inserted into space S 1  formed between positive electrode  11  and first bulkhead  41 , but into the inside of positive electrode  11 . Accordingly, supply ports  50   a  of each electrolyte solution introduction pipe  50  open toward the side of positive electrode  11  (i.e. in the positive or negative direction of the X-axis). In addition, electrode holder  43  includes distribution plate member  48 , which is similar to the second embodiment except for the shape and arrangement of holes  48   a , provided on a side facing first bulkhead  41 . Holes  48   a  of distribution plate member  48  are disposed between electrolyte solution introduction pipes  50  when viewed in stacking direction Z of cell stack  2 . 
     With this configuration, the positive electrolyte solution flows from individual supply flow channel P 1  into positive electrode  11  through holes  50   a  of each electrolyte solution introduction pipe  50 . Then, the positive electrolyte solution flows through positive electrode  11  in a direction perpendicular to thickness direction Z (i.e. in the positive or negative direction of the X-axis), flows into space S 1  through holes  48   a  of distribution plate member  48 , and then is returned from space S 1  to individual return flow channel P 2 . Therefore, in this embodiment, space S 1  functions as a fluid collector for collecting the positive electrolyte solution from positive electrode  11   
     As described above, according to this embodiment, the distance between positive electrode  11  and membrane  15  can be significantly shortened, and therefore, in addition to the effects obtained in the second embodiment, the internal resistance of cell  10  can be reduced. The positive electrolyte solution that has been supplied from electrolyte solution introduction pipe  50  initially flows through positive electrode  11  in the direction perpendicular to thickness direction Z (i.e. in the X direction), but finally flows through positive electrode  11  in thickness direction Z and is returned to space S 1 . Therefore, as compared with the second embodiment, a pressure drop which occurs when the positive electrolyte solution passes through positive electrode  11  does not significantly increase. As in the first embodiment, membrane  15  of this embodiment may be divided into a plurality of regions, and alternatively or in addition, it may be divided into a plurality of small regions. In this case, the regions or the small regions may be supported on a support frame made of, for example, plastic. 
     REFERENCE SIGNS LIST 
     
         
           1  Redox flow battery 
           10  Cell 
           11 ,  11   a - 11   c  Positive electrode 
           12  Positive cell 
           13  Negative electrode 
           14  Negative cell 
           15 ,  15   a - 15   c  Membrane 
           16  Perforated sheet 
           17  Flow directing sheet 
           18  Conductive member 
           19  Distribution plate 
           20  Cell frame 
           21  Frame body 
           22  Opening 
           22   a - 22   c  Small opening 
           22   d ,  22   e  Beam-like portion 
           23 ,  23   a - 23   c  Bipolar plate 
           25 ,  26  Flow channel groove 
           27 ,  28  Communication section 
           30  Membrane unit 
           31  Support frame 
           40  Cell case 
           41 ,  42  Bulkhead 
           43  Electrode holder 
           44  Frame member 
           45  Grid member 
           46  Current collecting portion 
           47  Reinforcement portion 
           48  Distribution plate member 
           50  Electrolyte solution introduction pipe 
           50   a  Supply port 
         S 1 -S 4  Space 
         X Longitudinal direction (of the opening) 
         Y Width direction (of the opening)