Redox flow battery cell, cell stack, and redox flow battery system

Abstract

A redox flow battery cell of the present disclosure is provided with an electrode and a separation membrane. The electrode is provided with a fiber aggregate including a plurality of carbon fibers. The plurality of carbon fibers include soft carbon fibers having a tensile elastic modulus of 200 GPa or less. The soft carbon fibers include first soft carbon fibers. There are a plurality of folds provided to the surface of the first soft carbon fibers. The plurality of carbon fibers in the electrode, when not compressed, have a first orientation tensor of 0.1 to 0.5. The first orientation tensor is an orientation tensor indicating a state of orientation of the plurality of carbon fibers, and represents the extent to which the plurality of carbon fibers are oriented in the thickness direction of electrode. A ratio L1 / L2 of a length L1 to length L2 in the first soft carbon fibers is over 1. The length L1 is the perimeter of a cross-section of the first soft carbon fibers. The length L2 is the circumference of an imaginary rectangle circumscribed around the cross-section of the first soft carbon fibers.

Description

Redox flow battery cells, cell stacks, and redox flow battery systems

[0001] This disclosure relates to a redox flow battery cell, a cell stack, and a redox flow battery system. This application claims priority to Japanese Patent Application No. 2023-185301, filed on October 30, 2023, and incorporates by reference all of the contents of said Japanese application.

[0002] Patent Document 1 discloses an electrode for a redox flow battery having a soft fiber layer mainly made of soft carbon fibers with a Young's modulus of 200 GPa or less. When this electrode for a redox flow battery is incorporated into a redox flow battery cell, the soft fiber layer and the diaphragm are arranged to face each other.

[0003] International Publication No. 2017 / 068944

[0004] The redox flow battery cell of the present disclosure comprises an electrode and a diaphragm. The electrode comprises a fiber assembly including a plurality of carbon fibers. The plurality of carbon fibers include soft carbon fibers having a tensile modulus of 200 GPa or less. The soft carbon fibers include first soft carbon fibers. The first soft carbon fibers have a surface with a plurality of folds. A first orientation tensor of the plurality of carbon fibers in the electrode in an uncompressed state is 0.1 or more and 0.5 or less. The first orientation tensor is an orientation tensor that indicates the orientation state of the plurality of carbon fibers and represents the degree to which the plurality of carbon fibers are oriented in the thickness direction of the electrode. The ratio L1 / L2 of the length L1 to the length L2 of the first soft carbon fiber is greater than 1. The length L1 is the perimeter of a cross section of the first soft carbon fiber. The length L2 is the perimeter of an imaginary rectangle circumscribing the cross section of the first soft carbon fiber.

[0005] Fig. 1 is a schematic diagram of a redox flow battery system according to an embodiment. Fig. 2 is a schematic diagram of a cell stack according to an embodiment. Fig. 3 is a schematic diagram of an electrode incorporated in a redox flow battery cell according to an embodiment. Fig. 4 is an enlarged view schematically showing an example of an internal region surrounded by a dashed line in Fig. 3. Fig. 5 is an enlarged view schematically showing another example of an internal region surrounded by a dashed line in Fig. 3. Fig. 6 is an explanatory diagram of angle θ used in calculating the orientation tensor of carbon fibers. Fig. 7 is an explanatory diagram of angle φ used in calculating the orientation tensor of carbon fibers. Fig. 8 is a cross-sectional view schematically showing an example of a cross section of a first soft carbon fiber.

[0006] In the electrode for a redox flow battery of Patent Document 1, the soft fiber layer reduces the penetration of the carbon fiber into the diaphragm, but there is a possibility that the cell resistance can be further reduced.

[0007] One object of the present disclosure is to provide a redox flow battery cell in which the carbon fibers are less likely to penetrate into the diaphragm and the cell resistance is small.

[0008] In the redox flow battery cell of the present disclosure, the carbon fibers are less likely to penetrate into the diaphragm, resulting in low cell resistance.

[0009] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0010] (1) A redox flow battery cell according to an embodiment of the present disclosure includes an electrode and a diaphragm. The electrode includes a fiber assembly including a plurality of carbon fibers. The plurality of carbon fibers include soft carbon fibers having a tensile modulus of 200 GPa or less. The soft carbon fibers include first soft carbon fibers. The first soft carbon fibers have a surface with a plurality of folds. A first orientation tensor of the plurality of carbon fibers in the electrode in an uncompressed state is 0.1 or more and 0.5 or less. The first orientation tensor is an orientation tensor that indicates the orientation state of the plurality of carbon fibers and represents the degree to which the plurality of carbon fibers are oriented in the thickness direction of the electrode. The ratio L1 / L2 of the length L1 to the length L2 of the first soft carbon fiber is greater than 1. The length L1 is the perimeter of a cross section of the first soft carbon fiber. The length L2 is the perimeter of an imaginary rectangle circumscribing the cross section of the first soft carbon fiber.

[0011] The orientation tensor indicates the degree to which the carbon fibers are oriented in the first, second, and third directions. The first, second, and third directions are perpendicular to one another. The first direction is the thickness direction of the electrode. That is, the first direction is perpendicular to the surface of the electrode on which the diaphragm is placed. The second direction is any direction in the electrode that is perpendicular to the first direction. The third direction is a direction that is perpendicular to both the first and second directions. The first orientation tensor indicates the degree to which the carbon fibers are oriented in the first direction. The second orientation tensor indicates the degree to which the carbon fibers are oriented in the second direction. The third orientation tensor indicates the degree to which the carbon fibers are oriented in the third direction. The sum of the first orientation tensor, the second orientation tensor, and the third orientation tensor is 1. The closer the first orientation tensor is to 1, the more the carbon fibers are oriented in the first direction. The closer the second orientation tensor is to 1, the more the carbon fibers are oriented in the second direction. The closer the third orientation tensor is to 1, the more the carbon fibers are oriented in the third direction.

[0012] When an electrode contains soft carbon fibers and the first orientation tensor of the multiple carbon fibers in the uncompressed electrode is 0.5 or less, fewer carbon fibers are likely to penetrate the diaphragm. Fewer carbon fibers penetrate the diaphragm, making it less likely for holes to form in the diaphragm. If holes form in the diaphragm, the positive electrode electrolyte and the negative electrode electrolyte may mix, electrically connecting the positive electrode and the negative electrode, i.e., causing a short circuit between the positive electrode and the negative electrode. Therefore, by reducing the penetration of carbon fibers into the diaphragm, the current efficiency of the redox flow battery cell is less likely to decrease.

[0013] If the first orientation tensor of the carbon fibers in the uncompressed electrode is 0.1 or greater, there is a high possibility that the carbon fibers are oriented in the first direction. Carbon fibers oriented in the first direction have excellent electronic conductivity. The more carbon fibers oriented in the first direction there are, the easier it is to reduce the cell resistance.

[0014] The redox flow battery cell includes first soft carbon fibers having a plurality of pleats. The first soft carbon fibers are unlikely to pierce the diaphragm even when oriented in a first direction. The first soft carbon fibers are likely to be oriented in the first direction, and the first orientation tensor of the plurality of carbon fibers in an uncompressed electrode is likely to be 0.1 or greater. The reason for this is as follows: The first soft carbon fibers are likely to be crimped when subjected to a crimping treatment during the manufacturing process. Therefore, it is thought that when an entanglement treatment is performed after the crimping treatment, the carbon fibers are likely to maintain an entangled state. Therefore, the entangled carbon fibers are likely to be oriented in the first direction.

[0015] (2) In the redox flow battery cell of (1) above, the electrode may be a single layer of the fiber assembly.

[0016] The single-layer fiber assembly electrode helps maintain the electrode's flexibility, which reduces the risk of the carbon fibers piercing the diaphragm when the electrode is assembled into a redox flow battery cell, regardless of whether the electrode's front or back surface faces the diaphragm.

[0017] (3) In the redox flow battery cell of (1) or (2), the average diameter of the plurality of carbon fibers may be 5 μm or more and 50 μm or less.

[0018] The average diameter of the plurality of carbon fibers is the average value of the circle-equivalent diameters of the carbon fibers. The circle-equivalent diameter of the carbon fiber is the diameter of a perfect circle having an area equal to the cross-sectional area of ​​the carbon fiber. The cross-section of the carbon fiber is exposed by cutting the electrode. The method for determining the average diameter of the plurality of carbon fibers will be described later. When the average diameter of the plurality of carbon fibers is 5 μm or more, the cross-sectional area and strength of each carbon fiber are sufficiently large. When the average diameter of the plurality of carbon fibers is 50 μm or less, the carbon fibers are likely to be entangled with each other, and the entangled carbon fibers are likely to be oriented in the first direction. When the average diameter of the plurality of carbon fibers is 50 μm or less, the first orientation tensor of the plurality of carbon fibers in an uncompressed electrode is likely to be 0.1 or more. When the average diameter of the plurality of carbon fibers is 50 μm or less, the surface area of ​​each carbon fiber is sufficiently large, and the contact area between the electrode and the electrolyte is sufficiently large.

[0019] (4) In the redox flow battery cell according to any one of (1) to (3), the porosity of the electrode in an uncompressed state may be 60% or more and 95% or less.

[0020] When the porosity of the electrode in an uncompressed state is 60% or more, the flowability of the electrolyte within the electrode is excellent even when the electrode is in a compressed state. When the porosity of the electrode in an uncompressed state is 95% or less, the battery reactivity of the electrode with the electrolyte is excellent even when the electrode is in a compressed state.

[0021] (5) In the redox flow battery cell according to any one of (1) to (4), the diaphragm may have a thickness of 100 μm or less.

[0022] When the thickness of the diaphragm is 100 μm or less, the cell resistance of the redox flow battery cell is easily reduced. In the redox flow battery cell, as described above, the electrode contains soft carbon fibers, and the first orientation tensor of the multiple carbon fibers in the electrode in an uncompressed state is 0.5 or less. Therefore, there are few carbon fibers that tend to pierce the diaphragm. Therefore, even if the thickness of the diaphragm is 100 μm or less, holes are unlikely to form in the diaphragm.

[0023] (6) In the redox flow battery cell according to any one of (1) to (5), the first orientation tensor of the electrode in a state compressed at a pressure of 0.8 MPa may be 0.02 or more and 0.20 or less.

[0024] When the first orientation tensor of the electrode in the compressed state is 0.02 or more, the cell resistance is easily reduced, and when the first orientation tensor of the electrode in the compressed state is 0.20 or less, the penetration of the carbon fibers into the diaphragm can be reduced.

[0025] (7) A cell stack according to an embodiment of the present disclosure includes the redox flow battery cell according to any one of (1) to (6) above.

[0026] In the cell stack, the carbon fibers are less likely to penetrate into the diaphragm, and the cell resistance is small.

[0027] (8) A redox flow battery system according to an embodiment of the present disclosure includes the cell stack described above in (7).

[0028] In the redox flow battery system, the carbon fibers are less likely to penetrate into the diaphragm, and the cell resistance is small.

[0029] [Details of the embodiments of the present disclosure] Specific examples of the redox flow battery cell, cell stack, and redox flow battery system of the present disclosure will be described with reference to the drawings. The same reference numerals in the drawings indicate the same or equivalent parts. In the drawings, some components may be exaggerated or simplified for ease of explanation. The dimensional ratios of each part in the drawings may differ from the actual dimensional ratios. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. Hereinafter, the redox flow battery may be referred to as an "RF battery."

[0030] As shown in Figures 1 and 2, the RF battery system 1 includes a cell stack 3. The cell stack 3 includes a plurality of RF battery cells 2. The RF battery cell 2 includes a positive electrode 22, a negative electrode 23, and a diaphragm 21. At least one of the positive electrode 22 and the negative electrode 23 is an electrode 7. One of the features of the RF battery cell 2 is that the electrode 7 (see Figure 3) includes soft carbon fibers 80, and the orientation tensor of the plurality of carbon fibers 8 satisfies a specific condition. Below, an overview of the RF battery system 1 and the electrode 7 will be described.

[0031] <RF Battery System> An RF battery system 1 will be described with reference to Figures 1 and 2. The RF battery system 1 is one type of electrolyte circulation type storage battery system. The RF battery system 1 includes RF battery cells 2 and a circulation mechanism 4 that circulates electrolyte through the RF battery cells 2. The RF battery system 1 charges and discharges while supplying electrolyte to the RF battery cells 2.

[0032] The RF battery system 1 is typically connected to an AC / DC converter 50, which is connected to a substation 52. The substation 52 is connected to a power generation unit 51 and a load 53. The RF battery system 1 charges the power generation unit 51 as a power supply source and discharges the power from the load 53. The power generation unit 51 is, for example, a solar power generator, a wind power generator, or another generator installed in a general power plant. The load 53 is, for example, a power grid or a power consumer. The RF battery system 1 can be used for load leveling, instantaneous voltage drop compensation, or output smoothing of natural energy power generation, for example. The redox flow battery system 1 can also be used as an emergency power source.

[0033] <RF Battery Cell> As shown in FIG. 1 , in the RF battery cell 2, a positive electrode cell 2P and a negative electrode cell 2N are separated by a diaphragm 21. A positive electrode 22 is built into the positive electrode cell 2P. A positive electrode electrolyte circulates through the positive electrode cell 2P. The positive electrode 22 is a reaction field where the active material contained in the positive electrode electrolyte undergoes a battery reaction. A negative electrode 23 is built into the negative electrode cell 2N. A negative electrode electrolyte circulates through the negative electrode cell 2N. The negative electrode 23 is a reaction field where the active material contained in the negative electrode electrolyte undergoes a battery reaction. At least one of the positive electrode 22 and the negative electrode 23 is the electrode 7.

[0034] The diaphragm 21 is, for example, an ion exchange membrane. The thickness of the diaphragm 21 is, for example, 100 μm or less. When the thickness of the diaphragm 21 is 100 μm or less, the cell resistance of the RF battery cell 2 is easily reduced. The thickness of the diaphragm 21 is, for example, 3 μm or more. The thickness of the diaphragm 21 is, for example, 3 μm to 100 μm, 5 μm to 100 μm, 4 μm to 50 μm, 6 μm to 50 μm, 3 μm to 30 μm, 5 μm to 30 μm, or 8 μm to 30 μm.

[0035] The RF battery cell 2 comprises a stack of a cell frame 25, a positive electrode 22, a diaphragm 21, a negative electrode 23, and another cell frame 25 stacked in this order. Each cell frame 25 comprises a bipolar plate 26 and a frame 27. The frame 27 surrounds the outer periphery of the bipolar plate 26. One RF battery cell 2 is disposed between the bipolar plates 26 of adjacent cell frames 25. The positive electrode 22 is disposed on a first surface of the bipolar plate 26. The negative electrode 23 is disposed on a second surface of the bipolar plate 26. As shown in the upper diagram of FIG. 2 , the frame 27 is formed with liquid supply manifolds 271 and 272, liquid supply slits 271s and 272s, liquid drainage manifolds 273 and 274, and liquid drainage slits 273s and 274s. A seal groove 28 is provided on the outer periphery of the frame 27. An annular seal member 29 is disposed between the frame bodies 27. The seal member 29 is disposed in the seal groove .

[0036] <Cell Stack> RF battery cells 2 are typically arranged inside a structure called a cell stack 3. As shown in the lower diagram of FIG. 2 , the cell stack 3 includes a substack 3S, two end plates 32, and a clamping mechanism 33. The cell stack 3 includes, for example, multiple substacks 3S. Each substack 3S includes multiple stacks described above and two supply / discharge plates 31. As shown in the lower diagram of FIG. 2 , the supply / discharge plates 31 are arranged on both ends of the multiple stacks. A supply pipe 44 and a discharge pipe 46 of the positive electrode circulation mechanism 4P and a supply pipe 45 and a discharge pipe 47 of the negative electrode circulation mechanism 4N are connected to the supply / discharge plates 31. The two end plates 32 sandwich the multiple substacks 3S. The clamping mechanism 33 clamps the multiple substacks 3S and the two end plates 32 together so that the two end plates 32 approach each other.

[0037] 1, the circulation mechanism 4 includes a positive electrode circulation mechanism 4P and a negative electrode circulation mechanism 4N. The positive electrode circulation mechanism 4P circulates the positive electrode electrolyte through the positive electrode cell 2P. The negative electrode circulation mechanism 4N circulates the negative electrode electrolyte through the negative electrode cell 2N.

[0038] The positive electrode circulation mechanism 4P includes a positive electrode electrolyte tank 42, a supply pipe 44, a discharge pipe 46, and a pump 48. The positive electrode electrolyte tank 42 stores positive electrode electrolyte. The supply pipe 44 and the discharge pipe 46 connect the positive electrode electrolyte tank 42 and the positive electrode cell 2P. The pump 48 is provided midway along the supply pipe 44 and pressure-feeds the positive electrode electrolyte in the positive electrode electrolyte tank 42 to the positive electrode cell 2P. The positive electrode electrolyte is supplied from the positive electrode electrolyte tank 42 through the supply pipe 44 to the positive electrode cell 2P, and is returned from the positive electrode cell 2P through the discharge pipe 46 to the positive electrode electrolyte tank 42. Specifically, the positive electrode electrolyte that has passed through the supply pipe 44 is supplied to the positive electrode 22 from a liquid supply manifold 271 through a liquid supply slit 271s shown in the upper diagram of FIG. 2 . The positive electrode electrolyte supplied to the positive electrode 22 flows from the bottom to the top of the positive electrode 22, as shown by the arrows in the upper diagram of Fig. 2. The positive electrode electrolyte that has flowed from the positive electrode 22 passes through the drain slit 273s and is discharged from the drain manifold 273 to the discharge pipe 46.

[0039] The anode circulation mechanism 4N includes an anode electrolyte tank 43, a supply pipe 45, a discharge pipe 47, and a pump 49. The anode electrolyte tank 43 stores the anode electrolyte. The supply pipe 45 and the discharge pipe 47 connect the anode electrolyte tank 43 and the anode cell 2N. The pump 49 is provided midway along the supply pipe 45 and pressure-feeds the anode electrolyte in the anode electrolyte tank 43 to the anode cell 2N. The anode electrolyte is supplied from the anode electrolyte tank 43 through the supply pipe 45 to the anode cell 2N and is returned from the anode cell 2N through the discharge pipe 47 to the anode electrolyte tank 43. Specifically, the anode electrolyte that has passed through the supply pipe 45 is supplied to the anode electrode 23 from a liquid supply manifold 272 and through a liquid supply slit 272s shown in the upper diagram of FIG. 2 . The negative electrode electrolyte supplied to the negative electrode 23 flows from below to above the negative electrode 23, as indicated by the arrows in the upper diagram of Fig. 2. The negative electrode electrolyte that has flowed from the negative electrode 23 passes through the drain slit 274s and is discharged from the drain manifold 274 to the discharge pipe 47.

[0040] The positive electrode electrolyte circulates through the positive electrode cell 2P, and the negative electrode electrolyte circulates through the negative electrode cell 2N. The RF battery cell 2 is charged and discharged in accordance with the valence change reaction of the active material ions in the positive electrode electrolyte and the negative electrode electrolyte.

[0041] The active material contained in the positive electrode electrolyte, the solvent for the positive electrode electrolyte, the active material contained in the negative electrode electrolyte, and the solvent for the negative electrode electrolyte are not particularly limited. The active material for the positive electrode electrolyte is, for example, one or more selected from the group consisting of vanadium ions, manganese ions, iron ions, polyacids, quinone derivatives, amines, and organometallic complexes. The active material for the negative electrode electrolyte is, for example, one or more selected from the group consisting of vanadium ions, titanium ions, chromium ions, polyacids, quinone derivatives, amines, and organometallic complexes. In this embodiment, both the positive electrode electrolyte and the negative electrode electrolyte contain vanadium ions. The solvent for the positive electrode electrolyte or the negative electrode electrolyte is, for example, an aqueous solution containing one or more acids or acid salts selected from the group consisting of sulfuric acid, phosphoric acid, nitric acid, and hydrochloric acid.

[0042] <Electrode> The electrode 7 provided in the RF battery cell 2 will be described with reference to Fig. 3 to Fig. 8. Fig. 3 is a schematic diagram of the electrode 7. Figs. 4 and 5 are enlarged views schematically showing an example of the internal region surrounded by the dashed dotted line in Fig. 3. The orientation tensor of the carbon fibers 8 in Fig. 4 is different from the orientation tensor of the carbon fibers 8 in Fig. 5. Figs. 6 and 7 are views for explaining the orientation tensor of the carbon fibers 8. Fig. 8 is a cross-sectional view schematically showing an example of a transverse section of a first soft carbon fiber 81.

[0043] The electrode 7 is, for example, a positive electrode 22. The electrode 7 may also be a negative electrode 23.

[0044] <<Basic Configuration>> The electrode 7 includes a fiber assembly 70 including a plurality of carbon fibers 8. The electrode 7 may include the carbon fibers 8 as a main component. "Includes the carbon fibers 8 as a main component" means that the ratio of the mass of the carbon fibers 8 to the mass of the electrode 7 is greater than 50%. The ratio of the mass of the carbon fibers 8 to the mass of the electrode 7 may be 60% or more, or 70% or more.

[0045] In addition to the fiber assembly 70, the electrode 7 may include at least one of a binder, a catalyst, and carbon particles. The binder, catalyst, and carbon particles are not shown. The binder binds the carbon fibers 8 together or fixes the catalyst or carbon particles to the carbon fibers 8. The binder includes, for example, one or more materials selected from the group consisting of a resin, a metal, a carbide, and a metal oxide. The resin is, for example, phenol, polytetrafluoroethylene, or polyvinylidene fluoride. The metal is, for example, titanium or tungsten. The carbide is, for example, titanium carbide, manganese carbide, or tungsten carbide. The metal oxide is, for example, alumina. The catalyst promotes the battery reaction. The catalyst is, for example, a non-carbon-based material. The non-carbon-based material includes, for example, one or more of an oxide and a carbide. The elements constituting the oxide or carbide are, for example, one or more selected from the group consisting of tungsten, silicon, titanium, cerium, manganese, iron, cobalt, nickel, tin, molybdenum, indium, antimony, lead, bismuth, tantalum, niobium, ruthenium, iridium, palladium, rhodium, rhenium, and barium. The carbon particles increase the surface area of ​​the electrode 7. The carbon particles are, for example, fine particles with an average particle size of 0.01 μm or more and 15 μm or less. The carbon particles may include secondary particles formed by agglomeration of fine particles. The average particle size of the carbon particles is determined as follows: The cross section of the electrode 7 is observed under a microscope, and the diameter of a perfect circle having an area equal to the cross-sectional area of ​​each carbon particle is measured for all carbon particles within the field of view. The average of these diameters is the average particle size of the carbon particles.

[0046] The fiber aggregate 70 has a three-dimensional mesh structure in which multiple carbon fibers 8 are entangled with each other. Figures 4 and 5 show hypothetical and schematic examples of multiple carbon fibers 8 entangled with each other. In the fiber aggregate 70, there are many contact points between the carbon fibers 8, which makes it easy to increase conductivity. In the fiber aggregate 70, voids are easily formed within the electrode 7, which makes it easy to increase the flow of electrolyte. The fiber aggregate 70 is, for example, a nonwoven fabric or a woven fabric. In a nonwoven fabric, independent carbon fibers 8 are entangled. In a woven fabric, the warp and weft of the carbon fibers 8 are woven alternately. The fiber aggregate 70 may be carbon paper in which multiple carbon fibers 8 are fixed with a binder.

[0047] The shape of the fiber assembly 70, that is, the shape of the electrode 7, is typically a thin plate as shown in FIG.

[0048] <Soft Carbon Fiber> The plurality of carbon fibers 8 include soft carbon fibers 80. The soft carbon fibers 80 are carbon fibers with a tensile modulus of elasticity of 200 GPa or less. Because the soft carbon fibers 80 have a tensile modulus of elasticity of 200 GPa or less, they have excellent flexibility. The soft carbon fibers 80 are less likely to pierce the diaphragm 21. Therefore, in an electrode 7 including soft carbon fibers 80, there are fewer carbon fibers 8 that easily pierce the diaphragm 21. Because the carbon fibers 8 are less likely to pierce the diaphragm 21, holes are less likely to form in the diaphragm 21. Therefore, the positive electrode electrolyte and the negative electrode electrolyte are less likely to mix. Furthermore, the positive electrode 22 and the negative electrode 23 are less likely to be electrically connected, that is, a short circuit between the positive electrode 22 and the negative electrode 23 is less likely to occur. Because the positive electrode electrolyte and the negative electrode electrolyte are less likely to mix and the positive electrode 22 and the negative electrode 23 are less likely to short circuit, the current efficiency of the RF battery cell 2 is less likely to decrease. In the electrode 7 including the soft carbon fiber 80, holes are unlikely to form in the diaphragm 21 even if the thickness of the diaphragm 21 is 100 μm or less.

[0049] The tensile modulus of the soft carbon fiber 80 may be 150 GPa or less, 100 GPa or less, 75 GPa or less, or 50 GPa or less. The lower limit of the tensile modulus of the soft carbon fiber 80 is not particularly limited, but in practical use it is, for example, 1 GPa. The tensile modulus of the soft carbon fiber 80 may be 10 GPa or more, or 25 GPa or more. The tensile modulus is determined by performing a tensile test on the carbon fiber 8 removed from the electrode 7.

[0050] The plurality of carbon fibers 8 may contain soft carbon fibers 80 as a main component. "Containing soft carbon fibers 80 as a main component" means that the ratio of the mass of the soft carbon fibers 80 to the mass of the plurality of carbon fibers 8 is greater than 50%. The greater this ratio, the fewer carbon fibers 8 are likely to pierce the diaphragm 21. The ratio of the mass of the soft carbon fibers 80 to the mass of the plurality of carbon fibers 8 may be 10% or more, 30% or more, 50% or more, 70% or more, 80% or more, or 100%. All of the carbon fibers 8 may be soft carbon fibers 80. The ratio is calculated as follows: 100 or more carbon fibers 8 are taken from any position on the electrode 7, and a tensile test is performed on all of the taken carbon fibers 8. The number of soft carbon fibers 80 is determined from the results of the tensile test. The ratio can be calculated from the number of carbon fibers 8 whose tensile modulus was measured and the number of soft carbon fibers 80. When performing a tensile test, the length of the carbon fibers 8 is adjusted according to the gauge length of the test piece to be placed in the tensile tester. For example, the length of the carbon fibers 8 is adjusted to a length that can be measured using a tensile tester described in JIS R 7606:2000. In this embodiment, the ratio of the number of soft carbon fibers 80 to the number of carbon fibers 8 whose tensile modulus is measured is considered to be the ratio of the mass of the soft carbon fibers 80 to the mass of the plurality of carbon fibers 8. In this embodiment, since all carbon fibers 8 are soft carbon fibers 80, the average value of the tensile modulus of the plurality of soft carbon fibers 80 may be used as the tensile modulus of the electrode 7. At least one carbon fiber 8 among the plurality of carbon fibers 8 may be a soft carbon fiber 80. Of all the carbon fibers 8, 10% or more of the carbon fibers 8 may be soft carbon fibers 80, 30% or more of the carbon fibers 8 may be soft carbon fibers 80, 50% or more of the carbon fibers 8 may be soft carbon fibers 80, more than 50% of the carbon fibers 8 may be soft carbon fibers 80, 70% or more of the carbon fibers 8 may be soft carbon fibers 80, or 80% or more of the carbon fibers 8 may be soft carbon fibers 80. In one carbon fiber 8, only a portion of the carbon fibers 8 may be soft carbon fibers 80. In this embodiment, whether or not this carbon fiber 8 is a soft carbon fiber is determined based on the tensile modulus of one carbon fiber 8. A method for measuring the tensile modulus of carbon fiber described in JIS R 7606:2000 will be described.The tensile tester is a constant-speed tension tester that can automatically record the relationship between load and elongation. The length of the test specimen is equal to or greater than the diameter of the slot in the mount, i.e., 25 mm ± 0.5 mm or more. The test specimen is adhesively fixed to the mount with a slot. After placing the test specimen in the tester, the mount is removed and measurements are taken on the remaining portion. The crosshead speed of the tensile tester is set to 1 mm / min or more and 5 mm / min or less. The mount is attached to the grip so that the test specimen is aligned with the load axis of the tester. Then, without applying a load to the mount, both sides of the mount are cut and removed with scissors, or both sides of the mount are burned with a flame. After starting the recorder, the tester is started. A load is applied to the test specimen, and the tensile modulus of the test specimen is measured.

[0051] The electrode 7 is, for example, a single-layer fiber assembly 70. The fiber assembly 70 includes a plurality of carbon fibers 8. The plurality of carbon fibers 8 may include soft carbon fibers 80 as a main component. A single-layer electrode 7 can easily maintain the flexibility of the electrode 7. When the electrode 7 is assembled into the RF battery cell 2, piercing of the carbon fibers 8 into the diaphragm 21 can be reduced regardless of whether the front or back surface of the electrode 7 faces the diaphragm 21. In the single-layer electrode 7, for example, the ratio of the mass of the soft carbon fibers 80 to the mass of a total of 100 or more carbon fibers 8 taken from three specific locations exceeds 50%. The three specific locations are any one location on the first surface of the electrode 7, any one location on the second surface of the electrode 7, and one location on the side surface of the electrode 7. The first and second surfaces constitute the front and back surfaces of the electrode 7. The one location on the side surface of the electrode 7 is a location located near the center of the electrode 7 in the thickness direction. At least 35 carbon fibers 8 are taken from each of the first and second surfaces, and at least 30 carbon fibers 8 are taken from the side surfaces, for a total of at least 100 carbon fibers 8. The length of the carbon fibers 8 taken from each location is adjusted according to the gauge length of the test piece to be placed in the tensile tester, as described above. The ratio of the number of soft carbon fibers 80 to the number of carbon fibers 8 whose tensile modulus is measured is considered to be the ratio of the mass of the soft carbon fibers 80 to the mass of the plurality of carbon fibers 8. The ratio of the mass of the soft carbon fibers 80 to the mass of the 100 or more carbon fibers 8 taken from three specific locations may be 10% or more, 30% or more, 50% or more, more than 50%, 70% or more, 80% or more, or 100%. At least one carbon fiber 8 of the 100 or more carbon fibers 8 taken from the three specific locations may be a soft carbon fiber 80.

[0052] The soft carbon fibers 80 include first soft carbon fibers 81 shown in FIG. 8 . A plurality of pleats 810 are provided on the surface of the first soft carbon fibers 81. A pleat-like unevenness is provided on the surface of the first soft carbon fibers 81. The first soft carbon fibers 81 are flexible and tend to be oriented in the first direction D1. By orienting the carbon fibers 8 in the first direction D1, the first orientation tensor of the plurality of carbon fibers 8 in the electrode 7 in an uncompressed state tends to be 0.1 or greater.

[0053] Referring to FIG. 8 , the length L1 is the perimeter of the cross section of the first soft carbon fiber 81. The length L2 is the perimeter of an imaginary rectangle 9 circumscribing the cross section of the first soft carbon fiber 81. In the first soft carbon fiber 81, the ratio L1 / L2 of the length L1 to the length L2 is greater than 1. When the ratio L1 / L2 is greater than 1, the flexibility of the first soft carbon fiber 81 is improved, and the reaction area where the electrode 7 comes into contact with the electrolyte is sufficiently large. Therefore, the reactivity between the electrode 7 and the electrolyte is improved. The ratio L1 / L2 may be 1.1 or greater. When the ratio L1 / L2 is too large, the number of pleats 810 increases, which may cause adjacent pleats 810 to stick together or the gaps between the pleats 810 to become too narrow. The ratio L1 / L2 may be 2 or less. When the ratio L1 / L2 is 2 or less, it is easy to form sufficient gaps between the pleats 810. The ratio L1 / L2 may be 1.8 or less, 1.6 or less, or 1.4 or less. The ratio L1 / L2 is, for example, greater than 1 and less than 2, greater than 1 and less than 1.8, greater than 1 and less than 1.6, or 1.1 or more and less than 1.4. The length L1 of the first soft carbon fiber 81 is determined by image analysis of a cross-sectional observation image of the transverse section of the first soft carbon fiber 81.

[0054] The pleats 810 are arranged, for example, along the longitudinal axis of the first soft carbon fiber 81. In one first soft carbon fiber 81, all of the plurality of pleats 810 may be arranged continuously along the longitudinal axis of the first soft carbon fiber 81. In one first soft carbon fiber 81, all of the plurality of pleats 810 may be arranged discontinuously along the longitudinal axis of the first soft carbon fiber 81. In one first soft carbon fiber 81, pleats 810 arranged continuously along the longitudinal axis of the first soft carbon fiber 81 may be mixed with pleats 810 arranged discontinuously along the longitudinal axis of the first soft carbon fiber 81. The plurality of pleats 810 may be arranged regularly. "The plurality of pleats 810 are arranged regularly" includes all of the pleats 810 having the same length and all of the pleats 810 being arranged in parallel along the longitudinal axis of the first soft carbon fiber 81. The plurality of pleats 810 may be arranged randomly. "Multiple pleats 810 are arranged randomly" means at least one of the following: pleats 810 of different lengths are mixed together; and pleats 810 of different inclination angles relative to the longitudinal axis of the first soft carbon fiber 81 are mixed together.

[0055] <Orientation Tensor> The orientation tensor indicates the orientation state of the multiple carbon fibers 8 in the electrode 7. In this embodiment, the orientation tensor satisfies specific conditions. The orientation tensor indicates the degree to which the carbon fibers 8 are oriented with respect to the first direction D1, the second direction D2, and the third direction D3. The first direction D1 is the thickness direction of the electrode 7. In other words, the first direction D1 is perpendicular to the surface of the electrode 7 on which the diaphragm 21 is arranged. The second direction D2 is any direction perpendicular to the first direction D1 in the electrode 7. The third direction D3 is a direction perpendicular to both the first direction D1 and the second direction D2. The second direction D2 and the third direction D3 are parallel to the surface of the electrode 7 on which the diaphragm 21 is arranged.

[0056] The first orientation tensor is the degree to which the carbon fibers 8 are oriented in the first direction D1. "The carbon fibers 8 are oriented in the first direction D1" means that the axes of the carbon fibers 8 are parallel to the first direction D1, or that the angle between the axes of the carbon fibers 8 and an axis along the first direction D1 is 10° or less. The second orientation tensor is the degree to which the carbon fibers 8 are oriented in the second direction D2. "The carbon fibers 8 are oriented in the second direction D2" means that the axes of the carbon fibers 8 are parallel to the second direction D2, or that the angle between the axes of the carbon fibers 8 and an axis along the second direction D2 is 10° or less. The third orientation tensor is the degree to which the carbon fibers 8 are oriented in the third direction D3. "The carbon fibers 8 are oriented in the third direction D3" means that the axes of the carbon fibers 8 are parallel to the third direction D3, or that the angle between the axes of the carbon fibers 8 and an axis along the third direction D3 is 10° or less. The sum of the first orientation tensor, the second orientation tensor, and the third orientation tensor is 1. The closer the first orientation tensor is to 1, the more the carbon fibers 8 are oriented in the first direction D1. The closer the second orientation tensor is to 1, the more the carbon fibers 8 are oriented in the second direction D2. The closer the third orientation tensor is to 1, the more the carbon fibers 8 are oriented in the third direction D3.

[0057] The first orientation tensor of the multiple carbon fibers 8 in the electrode 7 in an uncompressed state is 0.1 or more and 0.5 or less. When this first orientation tensor is 0.1 or more, there is a high possibility that multiple carbon fibers 8 are oriented in the first direction D1. Carbon fibers 8 oriented in the first direction D1 have excellent electronic conductivity because the electrons travel a short distance to the bipolar plate 26. A large number of carbon fibers 8 oriented in the first direction D1 and having excellent electronic conductivity facilitates reducing the cell resistance of the RF battery cell 2. When the first orientation tensor is 0.5 or less, there are fewer carbon fibers 8 oriented in a direction that penetrates the diaphragm 21. Reducing the penetration of the carbon fibers 8 into the diaphragm 21 reduces the likelihood of holes forming in the diaphragm 21. Therefore, the positive electrode electrolyte and the negative electrode electrolyte are less likely to mix. Furthermore, the positive electrode 22 and the negative electrode 23 are less likely to be electrically connected, i.e., a short circuit is less likely to occur. The positive electrode electrolyte and the negative electrode electrolyte are less likely to mix, and short circuits between the positive electrode 22 and the negative electrode 23 are reduced, so the current efficiency of the RF battery cell 2 is less likely to decrease.

[0058] The larger the first orientation tensor, the more carbon fibers 8 are oriented in the direction of piercing the diaphragm 21. Even if the first orientation tensor is large and the carbon fibers 8 are aligned along the first direction D1, if the carbon fibers 8 are soft carbon fibers 80, the carbon fibers 80 (soft carbon fibers 80) are less likely to pierce the diaphragm 21. The first orientation tensor of the multiple carbon fibers 8 in the electrode 7 in an uncompressed state may be 0.1 or more and 0.45 or less, or 0.1 or more and 0.4 or less.

[0059] The orientation tensor is calculated as follows. A three-dimensional image of a sample taken from the electrode 7 is obtained using an X-ray CT (Computed Tomography) device. The sample is taken so as to include two surfaces of the electrode 7 that face each other along an axis parallel to the first direction D1. A diaphragm 21 is placed on one of these two surfaces. The sample has a size of 4 mm in a plane perpendicular to the first direction D1 on the electrode 7. ２ Collect the sample so that the amount is equal to or greater than the above.

[0060] In the three-dimensional image, the first orientation tensor, the second orientation tensor, and the third orientation tensor of each carbon fiber 8 are calculated by the following equations: First orientation tensor = sin 2 θ sin 2 φ Second orientation tensor = sin 2 θ cos 2 φ Third orientation tensor = cos 2 θ

[0061] As shown in Figure 6, the angle θ is the angle of inclination of the carbon fiber 8 with respect to a straight line along the third direction D3. As shown in Figure 7, the angle φ is the angle of inclination of the carbon fiber 8 with respect to a straight line along the second direction D2. In Figures 6 and 7, some of the carbon fibers 8 are shown as straight lines. In reality, the entire carbon fiber 8 is not straight. Therefore, the angles θ and φ are calculated as follows: The carbon fiber 8 is divided into predetermined ranges along the longitudinal axis. The carbon fiber 8 in each range is considered to be a straight line. The average value of the angles θ over all ranges is the angle θ of the carbon fiber 8, and the average value of the angles φ over all ranges is the angle φ of the carbon fiber 8.

[0062] The average value of the first orientation tensors of all carbon fibers 8 included in the sample is the first orientation tensor of the carbon fibers 8 in the electrode 7. The average value of the second orientation tensors of all carbon fibers 8 included in the sample is the second orientation tensor of the carbon fibers 8 in the electrode 7. The average value of the third orientation tensors of all carbon fibers 8 included in the sample is the third orientation tensor of the carbon fibers 8 in the electrode 7.

[0063] The orientation tensor of the carbon fibers 8 obtained from the three-dimensional image can be automatically and easily obtained using, for example, image analysis software Avizo (manufactured by Thermo Fisher Scientific).

[0064] 4 and 5 show a virtual schematic representation of a plurality of carbon fibers 8 to explain a method for calculating the first orientation tensor of a plurality of carbon fibers 8. In FIG. 4 , of the ten carbon fibers 8, the first orientation tensor of each of three carbon fibers 8 is 1, and the first orientation tensor of each of seven carbon fibers 8 is 0. In this case, the average value of the first orientation tensors calculated for the ten carbon fibers 8 is 0.3. Therefore, the first orientation tensor of the plurality of carbon fibers 8 in the electrode 7 shown in FIG. 4 is 0.3. In FIG. 5 , the first orientation tensors of all of the plurality of carbon fibers 8 are 0.3. In this case, the average value of the first orientation tensors calculated for the plurality of carbon fibers 8 is 0.3. Therefore, the first orientation tensor of the plurality of carbon fibers 8 in the electrode 7 shown in FIG. 5 is 0.3.

[0065] When the electrode 7 is assembled into the RF battery cell 2, it is compressed in the first direction D1. The orientation tensor of the multiple carbon fibers 8 changes before and after compression. When the electrode 7 is compressed, the carbon fibers 8 tend to align in a plane perpendicular to the first direction D1. Therefore, the first orientation tensor of the multiple carbon fibers 8 in the electrode 7 after compression tends to be smaller than the first orientation tensor before compression.

[0066] The electrode 7 in a compressed state is the electrode 7 in a compressed state at a pressure of 0.8 MPa.

[0067] The first orientation tensor of the multiple carbon fibers 8 in the electrode 7 in a compressed state is, for example, 0.02 or more and 0.20 or less. When the first orientation tensor is 0.02 or more in the electrode 7 in a compressed state, the cell resistance of the RF battery cell 2 is easily reduced. When the first orientation tensor is 0.20 or less in the electrode 7 in a compressed state, the piercing of the carbon fibers 8 into the diaphragm 21 can be reduced. The first orientation tensor of the multiple carbon fibers 8 in the electrode 7 in a compressed state may be 0.05 or more and 0.20 or more, or 0.10 or more and 0.20 or less. The first orientation tensor of the multiple carbon fibers 8 in the electrode 7 in a compressed state is determined in the same manner as the first orientation tensor of the multiple carbon fibers 8 in the electrode 7 in an uncompressed state. A sample taken from the electrode 7 is sandwiched between two plates and compressed at a pressure of 0.8 MPa. If the plate material is a material that is transparent to X-rays, the orientation tensor can be determined for the compressed sample.

[0068] <Average Diameter of Carbon Fibers> The average diameter of the plurality of carbon fibers 8 is, for example, 5 μm or more and 50 μm or less. When the average diameter of the plurality of carbon fibers 8 is 5 μm or more, the cross-sectional area and strength of the carbon fibers 8 are sufficiently large. When the average diameter of the plurality of carbon fibers 8 is 50 μm or less, the carbon fibers 8 are likely to be entangled with each other, and the entangled carbon fibers 8 are likely to be oriented in the first direction D1. When the average diameter of the plurality of carbon fibers 8 is 50 μm or less, the first orientation tensor of the plurality of carbon fibers 8 in the electrode 7 in an uncompressed state is likely to be 0.1 or more. When the average diameter of the plurality of carbon fibers 8 is 50 μm or less, the surface area of ​​each carbon fiber 8 is large, and the contact area between the electrode and the electrolyte is sufficiently large.

[0069] The average diameter of the plurality of carbon fibers 8 may be 5 μm or more and 30 μm or less, 5 μm or more and 20 μm or less, or 5 μm or more and 15 μm or less. As described above, the average diameter of the plurality of carbon fibers 8 is the average value of the circle-equivalent diameters of the carbon fibers 8. The average diameter of the plurality of carbon fibers 8 is determined as follows. The electrode 7 is cut to expose the cross sections of the plurality of carbon fibers 8, and the cross sections of the plurality of carbon fibers 8 are observed under a microscope. The number of observation fields is 5 or more. The circle-equivalent diameters of three or more carbon fibers 8 per field are measured. The average value of the circle-equivalent diameters is the average diameter of the plurality of carbon fibers 8.

[0070] <Porosity of Electrode> The porosity of the electrode 7 in an uncompressed state is, for example, 60% or more and 95% or less. The electrode 7 has voids inside it, and therefore has excellent electrolyte flowability. If the porosity of the electrode 7 in an uncompressed state is 60% or more, the electrolyte flowability within the electrode 7 is excellent even when the electrode 7 is in a compressed state. If the porosity of the electrode 7 in an uncompressed state is 95% or less, the battery reactivity of the electrode with the electrolyte is excellent even when the electrode 7 is in a compressed state.

[0071] The porosity of the electrode 7 in an uncompressed state may be 65% or more and 95% or less, or 70% or more and 95% or less. The porosity is the ratio of the volume of voids inside the electrode 7 to the volume of the electrode 7 including the voids. For example, if the true density of the carbon fibers 8 in the electrode 7 is uniform, the porosity is calculated as follows: 1 - {(basis weight (g / m)} ２ ) / thickness of electrode 7 (m) / true density of carbon fiber 8 (g / m ３)}" and is the percentage of the value obtained by the following method. The basis weight is the mass of the electrode 7 per unit area. The true density of the carbon fiber 8 is determined by measurement in accordance with Method A: Liquid Displacement Method of JIS R 7603:1999. The method for measuring the density of carbon fiber (liquid displacement method) described in JIS R 7603:1999 will be explained. The test specimen holder is a stainless steel wire with a diameter of 0.4 mm or less. The mass of the test specimen holder and the mass of the test specimen attached to the test specimen holder are measured in air. The test specimen holder and the test specimen attached to the test specimen holder are immersed in immersion liquid in a borosilicate glass beaker. The liquid level in the beaker is adjusted so that the distance from the top of the immersed object to the liquid level is 10 mm. Air bubbles are removed from the immersion liquid. With the test specimen holder and the test specimen immersed in the immersion liquid, the mass of the test specimen holder and the mass of the test specimen are measured. The density ρθ (g / cm ３ ) is calculated by the formula ρθ = {(m3 - m1) / [(m3 - m1) - (m4 - m2)]} × ρL, where m1 is the mass (g) of the test specimen holder in air, m2 is the mass (g) of the test specimen holder in the immersion liquid, m3 is the mass (g) of the test specimen holder and test specimen in air, m4 is the mass (g) of the test specimen holder and test specimen in the immersion liquid, and ρL is the density (g / cm) of the immersion liquid. ３ )

[0072] <Electrode Thickness> The thickness of the electrode 7 in an uncompressed state is, for example, 0.5 mm or more and 3.0 mm or less. If the thickness of the electrode 7 in an uncompressed state is 0.5 mm or more, the reaction area of ​​the electrode 7 with the electrolyte is sufficiently large even when the electrode 7 is in a compressed state. If the thickness of the electrode 7 in an uncompressed state is 3.0 mm or less, the electrolyte can easily penetrate and diffuse sufficiently throughout the entire interior of the electrode 7 when the electrode 7 is in a compressed state. The thickness of the electrode 7 in an uncompressed state may be 0.5 mm or more and 2.5 mm or less, 0.5 mm or more and 2.0 mm or less, or 0.5 mm or more and 1.5 mm or less.

[0073] <<Manufacturing Method>> The electrode 7 can be obtained, for example, by preparing base fibers that form the basis of the carbon fibers 8, crimping the base fibers to entangle the base fibers, and carbonizing and graphitizing the entangled base fiber aggregate. Crimping involves winding and shrinking the base fibers. Methods for entangling the base fibers include, for example, needle punching and spunlacing. In the needle punching method, a sheet called a web composed of multiple base fibers is prepared. Needles are repeatedly pierced into this sheet in the thickness direction of the sheet. The needle piercing causes the base fibers to entangle. In the spunlacing method, a sheet called a web is prepared and a high-pressure water stream is sprayed onto this sheet. The pressure of the water stream causes the base fibers to entangle. The entangled base fibers are likely to be oriented in the first direction D1. The carbonized and graphitized base fibers are carbon fibers.

[0074] The first soft carbon fibers 81 having a plurality of pleats 810 are easily crimped when subjected to a crimping process during the manufacturing process. By subjecting the first soft carbon fibers 81 to an entanglement process after the crimping process, the carbon fibers 8 are easily maintained in an entangled state.

[0075] The conditions for the crimping treatment and the entanglement treatment may be appropriately selected so that the first orientation tensor of the plurality of carbon fibers 8 in the electrode 7 in an uncompressed state is 0.1 or more and 0.5 or less.

[0076] The electrode 7 may be subjected to a hydrophilic treatment. In an electrode that has been subjected to a hydrophilic treatment, the amount of oxygen functional groups is, for example, 1% or more. The amount of oxygen functional groups is the percentage ratio A / B, where A is the number of bonded oxygen atoms on the electrode surface to B is the total number of carbon atoms on the electrode surface. The amount of oxygen functional groups in the electrode may be 2% or more, or 3% or more. The amount of oxygen functional groups in the electrode may be 15% or less, or 10% or less. The number of bonded oxygen atoms A and the total number of carbon atoms B can be measured using a known measurement method, for example, X-ray photoelectron spectroscopy. Known methods can be used for the hydrophilic treatment.

[0077] [Test Example] An electrode was fabricated as a fiber assembly containing a plurality of carbon fibers. In the electrode, the first orientation tensor of the plurality of carbon fibers, the penetration of the carbon fibers into the diaphragm, and the cell resistivity were examined.

[0078] <Samples> <Sample No. 1-1 to Sample No. 1-4> For Sample No. 1-1 to Sample No. 1-4, a base fiber serving as the base of the carbon fiber was prepared, and the base fiber was subjected to a crimping treatment to entangle the base fibers. The entangled aggregate of base fibers was carbonized and graphitized to produce an electrode. The electrodes for Sample No. 1-1 to Sample No. 1-4 were subjected to a hydrophilization treatment. For Sample No. 1-1 to Sample No. 1-4, all carbon fibers were soft carbon fibers. The soft carbon fibers in Sample No. 1-1 to Sample No. 1-4 have the tensile modulus shown in Table 1.

[0079] In Sample No. 1-1 to Sample No. 1-4, all of the soft carbon fibers are first soft carbon fibers. The first soft carbon fibers have a plurality of pleats on their surfaces. The first soft carbon fibers in Sample No. 1-1 to Sample No. 1-4 have the ratio L1 / L2 shown in Table 1. The ratio L1 / L2 is the ratio of the length L1 to the length L2. The length L1 is the perimeter of the cross section of the first soft carbon fiber. The length L2 is the perimeter of an imaginary rectangle circumscribing the cross section of the first soft carbon fiber. The ratio L1 / L2 was determined by image analysis of a cross-sectional observation image of the cross section of the first soft carbon fiber.

[0080] The average diameters of the multiple carbon fibers of Samples No. 1-1 to 1-4 are shown in Table 1. The average diameter of the multiple carbon fibers was determined as follows. The electrode was cut to expose the cross section of the carbon fiber, and the cross section of carbon fiber 8 was observed under a microscope. The number of observation fields was 5 or more. The circle-equivalent diameters of three or more carbon fibers per field were measured. The average circle-equivalent diameter was the average diameter of the multiple carbon fibers.

[0081] Table 1 shows the porosity of the electrodes in the uncompressed state for Samples No. 1-1 to 1-4.

[0082] <Samples No. 1-11 to 1-15> In Samples No. 1-11 to 1-15, a base fiber serving as the base of the carbon fiber was prepared, and the base fiber was subjected to a crimping treatment to entangle the base fibers. The entangled aggregate of base fibers was then carbonized and graphitized to produce an electrode. All of the electrodes in Samples No. 1-11 to 1-15 were subjected to a hydrophilization treatment. In Samples No. 1-11 to 1-13, all of the carbon fibers were rigid carbon fibers having a tensile modulus of greater than 200 GPa. In Samples No. 1-14 and 1-15, all of the carbon fibers were flexible carbon fibers. The carbon fibers in Samples No. 1-11 to 1-15 had the tensile modulus shown in Table 1.

[0083] In Sample No. 1-14 and Sample No. 1-15, all of the carbon fibers are first soft carbon fibers. The first soft carbon fibers have a plurality of pleats on their surfaces. The carbon fibers in Sample No. 1-14 and Sample No. 1-15 have the L1 / L2 ratio shown in Table 1. The L1 / L2 ratio is determined as described above. In Sample No. 1-11 to Sample No. 1-13, all of the carbon fibers have a circular cross-sectional shape. In Sample No. 1-11 to Sample No. 1-13, none of the carbon fibers have pleats on their surfaces. In Sample No. 1-11 to Sample No. 1-13, the L1 / L2 ratio is 0.79.

[0084] The average diameters of the carbon fibers of Samples No. 1-11 to 1-15 are shown in Table 1. The method for determining the average diameters of the carbon fibers is as described above.

[0085] The porosity of the electrodes in an uncompressed state for Sample No. 1-11 to Sample No. 1-15 is shown in Table 1. The porosity was determined as described above.

[0086] <First Orientation Tensor of Multiple Carbon Fibers in an Electrode in an Uncompressed State> For each sample, the first orientation tensor of multiple carbon fibers in the electrode in an uncompressed state was determined. An electrode in an uncompressed state is an electrode in which no load other than gravity acts in the first direction (thickness direction of the electrode). Three-dimensional images of the samples taken from the electrodes in an uncompressed state were obtained using an X-ray CT device. The X-ray CT device used was a ZEISS Xradia 520 Versa. The tube voltage was 60 kV. The pixel size was 2.0 μm / pixel. The samples were taken so as to include two surfaces of the electrode facing each other along an axis parallel to the first direction. The sample had a plane perpendicular to the first direction that was 4 mm in size. ２ The carbon fibers were collected so that the above results were obtained. The first orientation tensor of each carbon fiber was determined from the three-dimensional image using image analysis software Avizo. The results are shown in Table 1 as "First orientation tensor in the uncompressed state."

[0087] <First orientation tensor of multiple carbon fibers in an electrode in a compressed state> For each sample, the first orientation tensor of multiple carbon fibers in an electrode in a compressed state was determined. An electrode in a compressed state is an electrode maintained in a compressed state at a pressure of 0.8 MPa. For each sample, a sample was taken from the electrode in an uncompressed state in the same manner as the above sample. For each sample, the sample was sandwiched between two plates and compressed at a pressure of 0.8 MPa. X-rays are transmitted through the plates. The first orientation tensor of multiple carbon fibers in an electrode in a compressed state was determined in the same manner as for an electrode in an uncompressed state. The results are shown in Table 1 as "first orientation tensor in a compressed state."

[0088] <Penetration of Carbon Fiber into Diaphragm> The penetration of the carbon fiber constituting the electrode into the diaphragm was examined. The method for evaluating the penetration of the carbon fiber into the diaphragm was as follows. For each sample, two electrodes of the same size were prepared. The diaphragm was sandwiched between these two electrodes and compressed at a pressure of 0.8 MPa. The diaphragm had a thickness of 30 μm. The resistance between the two electrodes was measured to evaluate whether the two electrodes were electrically connected. If the two electrodes were electrically connected, it was determined that a hole had formed in the diaphragm. In other words, if the two electrodes were electrically connected, it was determined that the carbon fiber had penetrated the diaphragm. For each sample, the evaluation of the penetration of the carbon fiber into the diaphragm was performed 10 times. If the two electrodes were not electrically connected in all 10 evaluations, the evaluation result was A. If the two electrodes were electrically connected at least once, the evaluation result was B. The results are shown in Table 1 as "Penetration of Carbon Fiber into Diaphragm."

[0089] <Cell Resistance> A single-cell RF battery system was assembled using the electrodes of each sample, and a charge / discharge test was performed. The same sample was used for a pair of positive and negative electrodes. For Sample No. 1-1 to Sample No. 1-4 and Sample No. 1-14, the electrodes were compressed at a pressure of 0.8 MPa. For Sample No. 1-11 to Sample No. 1-13 and Sample No. 1-15, the electrodes were compressed at a pressure of 0.1 MPa. The reason for compressing the electrodes at a pressure of 0.1 MPa for Sample No. 1-11 to Sample No. 1-13 and Sample No. 1-15 is that when the electrodes were compressed at a pressure of 0.8 MPa, holes were formed in the diaphragm in the evaluation of "piercing of the carbon fiber into the diaphragm." For Sample No. 1-1 to Sample No. 1-4 and Sample No. In the case of No. 1-14, no holes were formed in the diaphragm even when the electrode was compressed at a pressure of 0.8 MPa.

[0090] A vanadium sulfate aqueous solution was used as the positive electrode electrolyte and the negative electrode electrolyte. The vanadium concentration was 1.7 mol / L. The charge-discharge test was performed at a current density of 70 mA / cm. ２The charging and discharging were performed at a constant current of 1000 kJ / s, and when the battery voltage reached a preset switching voltage, charging and discharging were switched. After three cycles of charging and discharging, the cell resistance was determined. The cell resistance was calculated as follows: the difference between the charging midpoint voltage and the discharging midpoint voltage was divided by 2, and this value was further divided by the current value to determine the resistance value. The cell resistance was calculated by multiplying the resistance value by the area of ​​the electrode. The "midpoint voltage" is the voltage at the point when half the time from the start to the end of charging or discharging has elapsed since the start of charging or discharging. The results are shown in Table 1.

[0091]

[0092] As shown in Table 1, in Samples No. 1-1 to 1-4, the carbon fibers did not penetrate the diaphragm, and the cell resistance was low. In Samples No. 1-1 to 1-4, the carbon fibers were soft carbon fibers, and the first orientation tensor in the uncompressed state was 0.5 or less. Therefore, it is believed that there were few carbon fibers that easily penetrated the diaphragm. In Samples No. 1-1 to 1-4, the first orientation tensor in the uncompressed state was 0.1 or more, and it is highly likely that there were multiple carbon fibers oriented in the first direction. Therefore, it is believed that the electronic conductivity was excellent and the cell resistance was low. It is believed that because the carbon fibers were soft carbon fibers, they did not penetrate the diaphragm even though they were oriented in the first direction. In Samples No. 1-1 to 1-4, the first orientation tensor in the uncompressed state was 0.1 or more, and it is highly likely that there were multiple carbon fibers oriented in the first direction. In Sample No. 1-4, the carbon fibers were compressed at a pressure of 0.8 MPa, but the first orientation tensor in the compressed state was 0.03 or more and 0.2 or less, and the carbon fibers did not penetrate the diaphragm. The cell resistance of Sample No. 1-1 to Sample No. 1-4 was small, at 0.64 Ω cm ２ and less than 0.6 Ω cm ２ The following is the result.

[0093] In Sample No. 1-1 to Sample No. 1-4, in which the soft carbon fiber is the first soft carbon fiber, the tensile modulus increases as the average diameter of the plurality of carbon fibers increases.

[0094] In Sample No. 1-11 to Sample No. 1-13, the carbon fibers pierced the diaphragm. It is believed that, because Sample No. 1-11 to Sample No. 1-13 used rigid carbon fibers, a plurality of carbon fibers oriented in the first direction pierced the diaphragm, forming holes. In Sample No. 1-11 to Sample No. 1-13, the electrodes were compressed at a pressure of 0.1 MPa, and the electrode thickness was greater than that of Sample No. 1-1 to Sample No. 1-4. Therefore, it is believed that the cell resistance was higher in Sample No. 1-11 to Sample No. 1-13.

[0095] In Sample No. 1-14, the carbon fibers did not penetrate the diaphragm, but the cell resistance was high. It is believed that Sample No. 1-14 contained soft carbon fibers, and therefore few carbon fibers easily penetrated the diaphragm. In Sample No. 1-14, the first orientation tensor in the uncompressed state was 0.05, and the first orientation tensor in the compressed state was 0.01. It is believed that these small first orientation tensors and the small amount of carbon fibers oriented in the first direction resulted in poor electronic conductivity and high cell resistance.

[0096] In Sample No. 1-15, the carbon fibers pierced the diaphragm. Although Sample No. 1-15 contained soft carbon fibers, the first orientation tensor in the uncompressed state was large, and it is believed that too many carbon fibers were oriented in a direction that made them more likely to pierce the diaphragm. In Sample No. 1-15, the first orientation tensor in the uncompressed state was large, which is thought to have caused the first orientation tensor in the compressed state to become large.

[0097] REFERENCE SIGNS LIST 1 RF battery system (redox flow battery system) 2 RF battery cell (redox flow battery cell) 2P Positive electrode cell 2N Negative electrode cell 21 Diaphragm 22 Positive electrode 23 Negative electrode 25 Cell frame 26 Bipolar plate 27 Frame 271, 272 Liquid supply manifold 271s, 272s Liquid supply slit 273, 274 Liquid discharge manifold 273s, 274s Liquid discharge slit 28 Seal groove 29 Seal member 3 Cell stack 3S Substack 31 Supply and discharge plate 32 End plate 33 Fastening mechanism 4 Circulation mechanism 4P Positive electrode circulation mechanism 4N Negative electrode circulation mechanism 42 Positive electrode electrolyte tank 43 Negative electrode electrolyte tank 44, 45 Supply pipe 46, 47 Discharge pipe 48, 49 Pump 50 AC / DC converter 51 power generation unit 52 substation equipment 53 load 7 electrode 70 fiber assembly 8 carbon fiber 80 soft carbon fiber 81 first soft carbon fiber 810 pleat 9 imaginary rectangle θ, φ angles D1 first direction D2 second direction D3 third direction

Claims

1. A redox flow battery cell comprising an electrode and a diaphragm, wherein the electrode comprises a fiber assembly including a plurality of carbon fibers, the plurality of carbon fibers including soft carbon fibers having a tensile modulus of 200 GPa or less, the soft carbon fibers including first soft carbon fibers, and a surface of the first soft carbon fibers having a plurality of folds, a first orientation tensor of the plurality of carbon fibers in the electrode in an uncompressed state is 0.1 or more and 0.5 or less, the first orientation tensor is an orientation tensor that indicates the orientation state of the plurality of carbon fibers and represents the degree to which the plurality of carbon fibers are oriented in the thickness direction of the electrode, a ratio L1 / L2 of a length L1 to a length L2 of the first soft carbon fiber is greater than 1, the length L1 is the perimeter of a cross section of the first soft carbon fiber, and the length L2 is the perimeter of an imaginary rectangle circumscribing the cross section of the first soft carbon fiber.

2. The redox flow battery cell according to claim 1, wherein the electrode is a single layer of the fiber assembly.

3. The redox flow battery cell according to claim 1 or 2, wherein the average diameter of the plurality of carbon fibers is 5 μm or more and 50 μm or less.

4. A redox flow battery cell according to any one of claims 1 to 3, wherein the porosity of the electrode in an uncompressed state is 60% or more and 95% or less.

5. A redox flow battery cell according to any one of claims 1 to 4, wherein the thickness of the diaphragm is 100 μm or less.

6. A redox flow battery cell according to any one of claims 1 to 5, wherein the first orientation tensor in the electrode when compressed at a pressure of 0.8 MPa is 0.02 or more and 0.20 or less.

7. A cell stack comprising the redox flow battery cell according to any one of claims 1 to 6.

8. A redox flow battery system comprising the cell stack according to claim 7.

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