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

Abstract

To provide a redox flow battery electrode with low reaction resistance.SOLUTION: A redox flow battery electrode comprises a fiber assembly including carbon fibers, where the carbon fibers each include: a plurality of groove portions that are provided along the length of the carbon fiber; a ridge portion that is between the groove portions and relatively protrudes beyond the groove portions; and a cross-section in which the perimeter of the carbon fiber is larger than the perimeter of a virtual circle. The cross-section is a section made by cutting the carbon fiber by a plane orthogonal to the length of the carbon fiber. The virtual circle is a perfect circle having area equal to area of the cross-section.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an electrode for a redox flow battery, a redox flow battery cell, and a redox flow battery system. [Background technology]

[0002] Patent Document 1 discloses an electrode for a redox flow battery formed of a carbon fiber aggregate containing a plurality of carbon fibers. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2019 / 167283 Summary of the Invention [Problem to be solved by the invention]

[0004] It is desirable to reduce the reaction resistance of electrodes for redox flow batteries. The technology of Patent Document 1 leaves room for further improvement in terms of reducing reaction resistance.

[0005] An object of the present disclosure is to provide an electrode for a redox flow battery that has low reaction resistance. [Means for solving the problem]

[0006] The electrode for a redox flow battery of the present disclosure comprises a fiber assembly containing carbon fibers, the carbon fibers comprising a plurality of grooves provided along the length of the carbon fibers, ridges located between the grooves and protruding relatively more than the grooves, and a cross section of the carbon fibers whose perimeter is greater than the perimeter of an imaginary circle, the cross section being a cross section of the carbon fibers cut along a plane perpendicular to the length of the carbon fibers, and the imaginary circle being a perfect circle having an area equal to the area of ​​the cross section. [Effects of the Invention]

[0007] The electrode for a redox flow battery according to the present disclosure has a low reaction resistance. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic perspective view showing an electrode for a redox flow battery according to an embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an example of a cross section of a carbon fiber contained in an electrode for a redox flow battery according to an embodiment. [Figure 3] FIG. 3 is a schematic side view showing an example of carbon fibers contained in the redox flow battery electrode of the embodiment. [Figure 4] FIG. 4 is a schematic side view showing another example of carbon fibers contained in the redox flow battery electrode of the embodiment. [Figure 5] FIG. 5 is a schematic side view showing another example of carbon fibers contained in the redox flow battery electrode of the embodiment. [Figure 6] FIG. 6 is a schematic side view showing another example of carbon fibers contained in the redox flow battery electrode of the embodiment. [Figure 7] FIG. 7 is a schematic configuration diagram of a redox flow battery system according to an embodiment. [Figure 8] FIG. 8 is a schematic configuration diagram of a cell stack provided in the redox flow battery system of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

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

[0010] (1) An electrode for a redox flow battery according to an embodiment of the present disclosure comprises a fiber assembly containing carbon fibers, the carbon fibers comprising a plurality of grooves provided along the length of the carbon fibers, ridges located between the grooves and protruding relatively beyond the grooves, and a cross section of the carbon fibers having a perimeter greater than the perimeter of an imaginary circle, the cross section being a cross section of the carbon fibers cut along a plane perpendicular to the length of the carbon fibers, and the imaginary circle being a perfect circle having an area equal to the area of ​​the cross section.

[0011] The length of a carbon fiber is the length along the axis of the carbon fiber, connecting the two ends of the carbon fiber. The carbon fiber may extend linearly or curvedly. The axis of a carbon fiber is the line connecting the geometric centers of gravity of each cross section of the carbon fiber. "Along the length of the carbon fiber" means that the groove extends non-orthogonally to the length of the carbon fiber. "Along the length of the carbon fiber" includes both the groove length being parallel to the length of the carbon fiber and the groove length being non-parallel to the length of the carbon fiber. The length of the groove is a length perpendicular to both the width and depth of the groove. "The length of the groove is parallel to the length of the carbon fiber" means that the groove extends along the length of the carbon fiber. "The length of the groove is non-parallel to the length of the carbon fiber" means that the groove extends at a predetermined angle to the length of the carbon fiber. The groove may extend linearly or curvedly. Curved lines include meandering and spiral lines.

[0012] Carbon fibers with grooves and ridges have a longer circumference and a larger surface area than carbon fibers without grooves or ridges and having the same cross-sectional area. When grooves are provided along the length of the carbon fiber, the surface area of ​​the carbon fiber is large over a wide area. Redox flow battery electrodes containing carbon fibers with a large surface area have a larger reaction area in contact with the electrolyte and lower reaction resistance. Carbon fibers with grooves promote agitation of the electrolyte around the carbon fiber, increasing reactivity compared to carbon fibers without grooves.

[0013] (2) In the redox flow battery electrode of (1) above, at least one of the plurality of grooves may have a length perpendicular to both the width and depth of the groove that is five times or more the diameter of the imaginary circle.

[0014] If the length of the groove is large, the surface area of ​​the carbon fiber becomes large over a wider range, and the reaction resistance of the redox flow battery electrode tends to become small.

[0015] (3) In the redox flow battery electrode of (1) or (2), at least one of the plurality of grooves may have a rounded bottom having a radius of 2 μm or less.

[0016] When the radius of the roundness of the bottom of the groove is small, the depth of the groove tends to be deep. When the depth of the groove is deep, stirring of the electrolyte around the carbon fiber tends to be promoted. When the radius of the roundness of the bottom of the groove is small, a large number of grooves can be ensured when the cross-sectional area of ​​the carbon fiber and the depth of each groove are constant. When a large number of grooves is ensured, the surface area of ​​the carbon fiber increases.

[0017] (4) In the electrode for a redox flow battery according to any one of (1) to (3) above, the carbon fiber may have a pseudo-flattening ratio of 0.15 or more.

[0018] When the carbon fiber is flat, the circumferential length of the carbon fiber increases and the surface area of ​​the carbon fiber per weight of the carbon fiber increases, provided that the cross-sectional area of ​​the carbon fiber is constant.

[0019] (5) In the electrode for a redox flow battery according to any one of (1) to (4) above, the carbon fibers may have a standard deviation of pseudo-flatness of 0.1 or more.

[0020] When the pseudo-flattening ratio of the carbon fiber varies, the carbon fiber may include a portion with a large pseudo-flattening ratio. If the pseudo-flattening ratio is large, the perimeter of the carbon fiber increases and the surface area of ​​the carbon fiber increases when the cross-sectional area of ​​the carbon fiber is constant. When the pseudo-flattening ratio of the carbon fiber varies, stirring of the electrolyte around the carbon fiber is likely to be promoted.

[0021] (6) In the electrode for a redox flow battery according to any one of (1) to (5) above, the diameter of the imaginary circle may be 1 μm or more and 50 μm or less.

[0022] When the diameter of the imaginary circle is 1 μm or more, the circumferential length of the carbon fiber is large, and a sufficient surface area of ​​the carbon fiber is ensured. When the diameter of the imaginary circle is 1 μm or more, a sufficient cross-sectional area of ​​the carbon fiber is ensured, and sufficient strength of the carbon fiber is ensured. When the diameter of the imaginary circle is 50 μm or less, the cross-sectional area of ​​the carbon fiber is not too large, and a sufficient number of carbon fibers is ensured in the fiber assembly. When a large number of carbon fibers is ensured, a large reaction area per unit volume in the redox flow battery electrode is easily ensured.

[0023] (7) In the electrode for a redox flow battery according to any one of (1) to (6) above, the fiber assembly may be a nonwoven fabric or a woven fabric.

[0024] Fiber assemblies made of nonwoven or woven fabrics have gaps between the carbon fibers and a relatively high porosity. Redox flow battery electrodes made of fiber assemblies with a high porosity can easily ensure the flow of electrolyte and allow the electrolyte to diffuse throughout the electrode.

[0025] (8) A redox flow battery cell according to an embodiment of the present disclosure includes any one of the redox flow battery electrodes described above in (1) to (7).

[0026] The redox flow battery cell of the present disclosure has excellent battery performance because it includes the redox flow battery electrode of the present disclosure.

[0027] (9) The redox flow battery system according to an embodiment of the present disclosure includes the redox flow battery cell of the above (8).

[0028] Since the redox flow battery system of the present disclosure includes the redox flow battery cell of the present disclosure, it has excellent battery performance.

[0029] [Details of Embodiments of the Present Disclosure] Specific examples of the electrode for a redox flow battery, the redox flow battery cell, and the 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 corresponding parts. In each drawing, for convenience of explanation, a part of the configuration may be shown exaggeratedly or simplified. The dimensional ratios of each part in the drawings may also be different from the actual ones. Note that 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] <Electrode for RF Battery> Referring to FIGS. 1 to 6, the electrode 1 for an RF battery according to an embodiment will be described. The electrode 1 for an RF battery is a component of an RF battery cell 10C described later with reference to FIG. 7, and is a reaction field where the active material contained in the electrolyte performs a battery reaction. The electrode 1 for an RF battery includes a fiber aggregate containing carbon fibers 3. One of the features of the electrode 1 for an RF battery according to the embodiment is that, as shown in FIG. 2, the carbon fibers 3 are provided with a plurality of groove portions 4 and ridge portions 5.

[0031] FIG. 1 is an overall view of the electrode 1 for an RF battery. FIG. 2 is a cross-sectional view of the carbon fibers 3 included in the electrode 1 for an RF battery cut along a plane orthogonal to the length of the carbon fibers 3. FIGS. 3 to 6 are side views of the carbon fibers 3. FIGS. 3 to 6 schematically show the carbon fibers 3. For easy understanding, the groove portions 4 provided in the carbon fibers 3 are shown by cross-hatching in FIGS. 3 to 6.

[0032] ≪Basic Configuration≫ The RF battery electrode 1 comprises a fiber aggregate containing carbon fibers 3. The RF battery electrode 1 contains the carbon fibers 3 as a main component. "Containing the carbon fibers 3 as a main component" means that the ratio of the mass of the carbon fibers 3 to the mass of the RF battery electrode 1 is greater than 50%. The ratio of the mass of the carbon fibers 3 to the mass of the RF battery electrode 1 may be 60% or more, or 70% or more.

[0033] In addition to the fiber aggregate, the RF battery electrode 1 may contain at least one of a binder, a catalyst, and carbon particles (not shown). The binder binds the carbon fibers 3 together or fixes the catalyst or carbon particles to the carbon fibers 3. The binder includes, for example, one or more selected from the group consisting of a resin, a metal, a carbide, and a metal oxide. Examples of the resin include phenol, polytetrafluoroethylene, and polyvinylidene fluoride. Examples of the metal include titanium and tungsten. Examples of the carbide include titanium carbide, manganese carbide, and tungsten carbide. Examples of the metal oxide include alumina. The catalyst promotes the battery reaction. The catalyst is, for example, made of a non-carbon-based material. Examples of the non-carbon-based material include, for example, one or more of an oxide and a carbide. Examples of elements constituting the oxide or carbide include 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 RF battery electrode 1. 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 also include secondary particles formed by agglomeration of the fine particles. The average particle size of the carbon particles can be determined by observing the cross section of the RF battery electrode 1 under a microscope, measuring the diameter of a perfect circle having an area equal to the cross-sectional area of ​​each carbon particle for all carbon particles within the field of view, and calculating the average diameter.

[0034] The fiber aggregate has a three-dimensional mesh structure in which multiple carbon fibers 3 are entangled with each other. The fiber aggregate has many contact points between the carbon fibers 3, making it easy to ensure high conductivity. The fiber aggregate makes it easy to ensure voids within the RF battery electrode 1, making it easy to improve the flow of electrolyte. The fiber aggregate is, for example, a nonwoven fabric or a woven fabric. A nonwoven fabric is made by entangling independent carbon fibers 3. A woven fabric is made by alternately weaving warp and weft threads of carbon fibers 3. The fiber aggregate may be paper. Paper is made by fixing multiple carbon fibers 3 with a binder.

[0035] As described above, the RF battery electrode 1 has excellent electrolyte flowability due to the inclusion of voids within the electrode. For example, the porosity of the RF battery electrode 1 measured when no surface pressure is applied to the RF battery electrode 1, i.e., when it is in an uncompressed state, is 50% or more. If the porosity in the uncompressed state is 50% or more, even when the RF battery electrode 1 is compressed, some voids are maintained within the electrode. From the viewpoint of electrolyte flowability, the porosity in the uncompressed state may be 60% or more, 65% or more, or 70% or more. From the viewpoint of ensuring battery reactivity, the porosity in the uncompressed state may be, for example, 99% or less, 97% or less, or 95% or less. The porosity in the uncompressed state may be 60% or more and 99% or less, 65% or more and 97% or less, or 70% or more and 95% or less. The porosity is the volume fraction of voids contained within the RF battery electrode 1. The porosity is defined as 100% of the volume of the RF battery electrode 1 including the voids. For example, if the true density of the carbon fibers 3 constituting the RF battery electrode 1 is uniform, the porosity is calculated as "1 - {basis weight (g / m 2 ) / RF battery electrode 1 thickness (m) / carbon fiber 3 true density (g / m 3 )}" The true density of the carbon fiber 3 is determined by measurement in accordance with Method A: liquid displacement method of JIS R 7603:1999.

[0036] The shape of the fiber assembly, that is, the shape of the RF battery electrode 1, is typically a thin plate as shown in FIG.

[0037] The weight of the RF battery electrode 1 is, for example, 20 g / m2 More than 600g / m 2 The weight is 20g / m or less. 2 When the weight is 600 g / m or more, the number of contact points between the carbon fibers 3 is likely to increase, and high conductivity is likely to be secured. 2 When the weight of the RF battery electrode 1 is 25 g / m or less, it is easy to secure voids in the RF battery electrode 1 and to improve the flowability of the electrolyte. 2 More than 500g / m 2 Below 30g / m 2 More than 450g / m 2 The basis weight can be determined by measuring the mass per unit area.

[0038] <Furrows and ridges> As shown in Figures 2 to 6, the carbon fiber 3 has multiple grooves 4 and multiple ridges 5. The grooves 4 and ridges 5 are arranged alternately clockwise and counterclockwise on the surface of the carbon fiber 3 in the cross section 6 shown in Figure 2. The grooves 4 and ridges 5 form a relatively large uneven shape on the surface of the carbon fiber 3. The relatively large uneven shape means that the depth of the grooves 4 is on the order of microns and significantly contributes to the external shape of the carbon fiber 3. The depth of the grooves 4 is also the height of the ridges 5. The carbon fiber 3 having multiple grooves 4 promotes stirring of the electrolyte around the carbon fiber 3. The grooves 4 and ridges 5 increase the perimeter of the carbon fiber 3 and increase the surface area of ​​the carbon fiber 3. A large surface area of ​​the carbon fiber 3 increases the reaction area in contact with the electrolyte in the RF battery electrode 1, thereby reducing the reaction resistance of the RF battery electrode 1. For example, even if nano-order minute irregularities are formed on the surface of at least one of the groove portion 4 and the ridge portion 5, these minute irregularities are not included in the groove portion 4 and the ridge portion 5. The nano-order minute irregularities do not substantially affect the circumferential length of the carbon fiber 3.

[0039] A relatively large uneven shape is, for example, one in which the difference in height between the groove portions 4 and the ridge portions 5 is 0.1 μm or more and 10 μm or less. The difference in height between the groove portions 4 and the ridge portions 5 is the length between a first line segment connecting adjacent ridge portions 5 across the groove portion 4, and a second line segment that is parallel to the first line segment and contacts the bottom portion 40 of the groove portion 4. The difference in height between the groove portions 4 and the ridge portions 5 is the depth of the groove portions 4. If the difference in height between the groove portions 4 and the ridge portions 5 is 0.1 μm or more, the surface area of ​​the carbon fibers 3 is likely to be large. If the difference in height between the groove portions 4 and the ridge portions 5 is 0.1 μm or more, stirring of the electrolyte around the carbon fibers 3 is likely to be promoted. When the difference in height between the groove portions 4 and the ridge portions 5 is 10 μm or less, the central portion of the carbon fiber 3 is sufficiently secured and the strength of the carbon fiber 3 is sufficiently secured when the cross-sectional area of ​​the carbon fiber 3 is constant. The difference in height between the groove portions 4 and the ridge portions 5 may be 0.2 μm or more and 8 μm or less, or 0.3 μm or more and 5 μm or less.

[0040] As shown in FIGS. 3 to 6 , each groove 4 is provided along the length of the carbon fiber 3. When the groove 4 is provided along the length of the carbon fiber 3, the surface area of ​​the carbon fiber 3 is large over a wide range. When the groove 4 is provided along the length of the carbon fiber 3, stirring of the electrolyte around the carbon fiber 3 is likely to be promoted over a wide range. For ease of understanding, FIGS. 3 to 6 illustrate an example in which the carbon fiber 3 extends linearly and each groove 4 extends linearly parallel to the carbon fiber 3. The carbon fiber 3 may extend in a curved manner. The groove 4 may extend while intersecting the length of the carbon fiber 3 at a predetermined angle. The predetermined angle is, for example, 20° or less, or 10° or less. Regardless of how the groove 4 extends along the length of the carbon fiber 3, it extends non-orthogonally to the length of the carbon fiber 3. The groove 4 may extend linearly non-orthogonal to the carbon fiber 3. The groove 4 may extend in a curved manner.

[0041] The length 4L of the groove 4 (FIG. 3) is, for example, five times or more the diameter of the imaginary circle 7 shown in FIG. 2. The imaginary circle 7 will be described later. The length 4L of the groove 4 is a length perpendicular to both the width and depth of the groove 4. The length 4L of the groove 4 shown in FIG. 3 is a length parallel to the length of the carbon fiber 3 in the groove 4. When the groove 4 extends in a curved shape, the length 4L of the groove 4 is a length along the curve. When the length 4L of the groove 4 is large, the surface area of ​​the carbon fiber 3 tends to be large over a wider area. When the length 4L of the groove 4 is large, stirring of the electrolyte around the carbon fiber 3 tends to be promoted over a wider area. The length 4L of the groove 4 is preferably 10 times or more, 15 times or more, or 20 times or more the diameter of the imaginary circle 7. The length 4L of at least one of the multiple grooves 4 is preferably five times or more the diameter of the imaginary circle 7. The lengths 4L of all of the multiple grooves 4 are preferably five times or more the diameter of the imaginary circle 7.

[0042] The length 4L of the groove portion 4 is determined by image analysis of the side surface of the carbon fiber 3. The length 4L of the groove portion 4 is determined by arbitrarily selecting five or more groove portions 4 and averaging the lengths of the respective groove portions 4. The number of groove portions 4 whose lengths are measured may be five or more taken from a single carbon fiber 3, or may be a total of five or more selected from a plurality of carbon fibers 3. The image analysis exemplified in this specification can be performed using, for example, images observed with a scanning electron microscope (SEM), but is not limited to this.

[0043] When viewing a single carbon fiber 3, as shown in FIGS. 3 and 4, all of the multiple grooves 4 may be discontinuously arranged along the length of the carbon fiber 3. In this case, multiple groups of grooves 4 discontinuously arranged along the length of the carbon fiber 3 are arranged clockwise or counterclockwise on the surface of the carbon fiber 3. The multiple grooves 4 may be arranged regularly, as shown in FIG. 3. Regularly means that all grooves 4 have the same length 4L, that all grooves 4 are parallel, and that adjacent groups of grooves 4 are aligned in the same position in a clockwise or counterclockwise direction. The multiple grooves 4 may be arranged randomly, as shown in FIG. 4. Randomly means that grooves 4 with different lengths 4L are mixed, that grooves 4 with different angles relative to the length of the carbon fiber 3 are mixed, and that adjacent groups of grooves 4 are aligned in a clockwise or counterclockwise direction but are offset along the length of the carbon fiber 3. In the carbon fiber 3 shown in FIG. 4, all grooves 4 are parallel, but long grooves 4 and short grooves 4 are mixed, and the clockwise or counterclockwise alignment positions of adjacent groups of grooves 4 are offset along the length of the carbon fiber 3. When multiple grooves 4 are arranged regularly, all ridges 5 are arranged regularly, as shown in FIG. 3. When multiple grooves 4 are arranged randomly, the ridges 5 are also arranged randomly, as shown in FIG. 4. For example, when multiple cross sections 6 including grooves 4 are taken from the carbon fiber 3 shown in FIG. 4, the shapes of the cross sections 6 differ from one another. The sizes and shapes of the grooves 4 and ridges 5 may also differ from one another. When multiple grooves 4 are arranged discontinuously along the length of the carbon fiber 3, the length 4L of each discontinuous groove 4 is preferably at least five times the diameter of the imaginary circle 7.

[0044] When viewing one carbon fiber 3, as shown in Fig. 5, all of the plurality of grooves 4 may be provided continuously along the length of the carbon fiber 3. Both ends of each groove 4 are located at both ends of the carbon fiber 3. In this case, the length of each groove 4 is at least five times the diameter of the imaginary circle 7.

[0045] When viewing a single carbon fiber 3, as shown in FIG. 6, the multiple grooves 4 may include both grooves 4 that are discontinuously provided along the length of the carbon fiber 3 and grooves 4 that are continuously provided along the length of the carbon fiber 3. In FIG. 6, the multiple grooves 4 are provided regularly. Among the multiple grooves 4, a group of grooves 4 that are discontinuously provided along the length of the carbon fiber 3 may include a mixture of grooves 4 with different lengths 4L. The group of grooves 4 may include a mixture of grooves 4 with different angles relative to the length of the carbon fiber 3. The groups of grooves 4 may be offset along the length of the carbon fiber 3 in the clockwise or counterclockwise alignment positions of adjacent groups of grooves 4.

[0046] The fiber assembly includes one or more carbon fibers 3 having grooves 4. All of the carbon fibers constituting the fiber assembly may be carbon fibers 3 having grooves 4. When the fiber assembly includes a plurality of carbon fibers 3 having grooves 4, each of the plurality of carbon fibers 3 is the carbon fiber 3 shown in FIG. 3, the carbon fiber 3 shown in FIG. 4, the carbon fiber 3 shown in FIG. 5, or the carbon fiber 3 shown in FIG. 6. All of the plurality of carbon fibers 3 may be the carbon fiber 3 shown in FIG. 3, the carbon fiber 3 shown in FIG. 4, the carbon fiber 3 shown in FIG. 5, or the carbon fiber 3 shown in FIG. 6. The plurality of carbon fibers 3 may be configured by combining some of the carbon fibers 3 shown in FIG. 3, the carbon fiber 3 shown in FIG. 4, the carbon fiber 3 shown in FIG. 5, and the carbon fiber 3 shown in FIG. 6.

[0047] The cross-sectional shape of the grooves 4 can be selected arbitrarily. The cross-sectional shape of the grooves 4 is the shape of a cross section of the grooves 4 cut along a plane perpendicular to the length of the carbon fibers 3. The cross-sectional shape of the grooves 4 is, for example, a V-shape in which the bottom 40 is the narrowest. The V-shape includes having at least a portion of the bottom 40 and the side portions rounded. The radius of the roundness of the bottom 40 is, for example, 2 μm or less. The radius of the roundness of the bottom 40 is the radius of a circle inscribed in the bottom surface and both side surfaces connected to the bottom surface. When the radius of the roundness of the bottom 40 is 2 μm or less, the depth of the grooves 4 is likely to be deep. When the depth of the grooves 4 is deep, stirring of the electrolyte around the carbon fibers 3 is likely to be promoted. When the radius of the roundness of the bottom 40 is 2 μm or less, a large number of grooves 4 can be ensured when the cross-sectional area of ​​the carbon fibers 3 and the depth of each groove 4 are constant. A large number of grooves 4 facilitates the stirring of the electrolyte around the carbon fibers 3. A large number of grooves 4 increases the surface area of ​​the carbon fibers 3. The radius of the roundness of the bottom 40 is preferably 1 μm or less, or 0.5 μm or less.

[0048] As shown in Fig. 2, the ridge portions 5 protrude relatively more than the groove portions 4. Any cross-sectional shape can be selected for the ridge portions 5. The cross-sectional shape of the ridge portions 5 is the shape of a cross section obtained by cutting the ridge portions 5 along a plane perpendicular to the length of the carbon fibers 3. The cross-sectional shape of the ridge portions 5 is, for example, an arc shape.

[0049] <Cross Section> As shown in Figure 2, the carbon fiber 3 having the grooves 4 has a specific cross section 6. The specific cross section 6 is a cross section in which the perimeter of the carbon fiber 3 is greater than the perimeter of an imaginary circle 7. The cross section 6 is a cross section of the carbon fiber 3 cut along a plane perpendicular to the length of the carbon fiber 3. The imaginary circle 7 is a perfect circle having an area equal to the area of ​​the cross section 6. In Figure 2, the imaginary circle 7 is indicated by a two-dot chain line.

[0050] The ratio L1 / L2 of the circumferential length L1 of the carbon fiber 3 to the circumferential length L2 of the imaginary circle 7 is greater than 1. When the value of the circumferential length L2 is constant, the larger the ratio L1 / L2, the larger the surface area of ​​the carbon fiber 3. The ratio L1 / L2 is preferably 1.1 or greater, or 1.2 or greater. The ratio L1 / L2 is, for example, 2 or less. When the ratio L1 / L2 is 2 or less, when the cross-sectional area of ​​the carbon fiber 3 is constant, the number of groove portions 4 and ridge portions 5 is not too large, and the size of each groove portion 4 is ensured to be sufficient. When the size of each groove portion 4 is ensured to be sufficient, the electrolyte can easily enter each groove portion 4, and a sufficient contact area between the carbon fiber 3 and the electrolyte can easily be ensured. When the electrolyte can easily enter each groove portion 4, stirring of the electrolyte around the carbon fiber 3 is easily promoted. The ratio L1 / L2 is preferably greater than 1 and 2 or less, 1.1 to 2, 1.2 to 2, or 1.2 to 1.8.

[0051] The ratio L1 / L2 is determined by image analysis of the cross section 6. The ratio L1 / L2 is the average value of the ratios L1 / L2 determined for any cross section 6. The ratio L1 / L2 is determined as follows: For five or more cross sections 6 taken from one or more carbon fibers 3, the perimeter L1 and area of ​​the cross section 6 are measured, and the perimeter L2 of a perfect circle having an area equal to the area of ​​the cross section 6 is determined. The ratio L1 / L2 of each cross section 6 is calculated, and the average value of the ratios L1 / L2 is defined as the ratio L1 / L2 of the carbon fiber 3. The number of cross sections taken may be five or more taken from a single carbon fiber 3, or a total of five or more taken from multiple carbon fibers 3.

[0052] The diameter of the imaginary circle 7 is, for example, 1 μm or more and 50 μm or less. When the diameter of the imaginary circle 7 is 1 μm or more, the circumferential length L1 of the carbon fibers 3 is large, and the surface area of ​​the carbon fibers 3 is sufficiently ensured. When the diameter of the imaginary circle 7 is 1 μm or more, the area of ​​the cross section 6 is sufficiently ensured, and the strength of the carbon fibers 3 is sufficiently ensured. When the diameter of the imaginary circle 7 is 50 μm or less, the area of ​​the cross section 6 does not become too large, and a sufficient number of carbon fibers 3 occupy the fiber aggregate. When a large number of carbon fibers 3 are ensured, a large reaction area per unit volume in the RF battery electrode 1 is easily ensured. The diameter of the imaginary circle 7 may be 2 μm or more and 40 μm or less, 3 μm or more and 30 μm or less, 5 μm or more and 25 μm or less, or 7 μm or more and 20 μm or less.

[0053] The diameter of the imaginary circle 7 is determined by performing image analysis on the cross section 6. The diameter of the imaginary circle 7 is determined by averaging the diameters of perfect circles calculated from the areas of any five or more cross sections 6 measured for each cross section 6. The number of cross sections 6 whose areas are measured may be five or more taken from a single carbon fiber 3, or may be five or more in total taken from multiple carbon fibers 3.

[0054] The carbon fiber 3 has a pseudo flattening ratio of, for example, 0.15 or more. The pseudo flattening ratio is calculated by 1-(b / a), where a is the length of the long side 61 that overlaps the cross section 6 and b is the length of the short side 62. The long side 61 is the longest line segment that overlaps the cross section 6. The short side 62 is a line that intersects the long side 61 at right angles and is the longest line segment that overlaps the cross section 6. If the pseudo flattening ratio of the carbon fiber 3 is 0.15 or more, when the cross-sectional area of ​​the carbon fiber 3 is constant, the perimeter of the carbon fiber 3 increases, and the surface area of ​​the carbon fiber 3 also increases. The pseudo flattening ratio of the carbon fiber 3 may be 0.2 or more, 0.25 or more, or 0.3 or more. The pseudo flattening ratio of the carbon fiber 3 is, for example, 0.9 or less. When the pseudo flattening of the carbon fibers 3 is 0.9 or less, the carbon fibers 3 tend to be in point contact rather than in plane contact, and it is easy to secure many contact points between the carbon fibers 3 while securing gaps between the carbon fibers 3. The pseudo flattening of the carbon fibers 3 may be 0.15 or more and 0.9 or less, 0.2 or more and 0.9 or less, 0.25 or more and 0.8 or less, or 0.3 or more and 0.75 or less.

[0055] The pseudo flattening of the carbon fiber 3 is determined by image analysis of the cross section 6. The pseudo flattening of the carbon fiber 3 is determined as follows. Any cross section 6 of the carbon fiber 3 is collected, the length a of the long side 61 and the length b of the short side 62 of the cross section 6 are measured, and 1-(b / a) is calculated. Five or more arbitrary cross sections 6 are collected, the pseudo flattening of each is determined, and the average value of the pseudo flattenings is taken as the pseudo flattening of the carbon fiber 3.

[0056] When looking at a single carbon fiber 3, a portion of the carbon fiber 3 may have a pseudo flattening of 0.15 or more, or the entire carbon fiber 3 may have a pseudo flattening of 0.15 or more. When the fiber aggregate includes a plurality of carbon fibers 3 having groove portions 4, for example, the average pseudo flattening of any of the plurality of carbon fibers 3 is 0.15 or more. When the fiber aggregate includes a plurality of carbon fibers 3 having groove portions 4, it is preferable that at least one of the plurality of carbon fibers 3 has a pseudo flattening of 0.15 or more. All of the plurality of carbon fibers 3 may have a pseudo flattening of 0.15 or more.

[0057] The standard deviation of the pseudo flattening of the carbon fibers 3 is, for example, 0.1 or more. When the standard deviation of the pseudo flattening of the carbon fibers 3 is 0.1 or more, stirring of the electrolyte around the carbon fibers 3 is easily promoted. The standard deviation of the pseudo flattening of the carbon fibers 3 may be 0.12 or more, or 0.15 or more. The standard deviation of the pseudo flattening of the carbon fibers 3 is a value determined from the pseudo flattening calculated in the same manner as in the method for determining the pseudo flattening described above.

[0058] ≪Manufacturing method≫ The above carbon fiber 3 is manufactured by firing and carbonizing an organic fiber having a relatively large concavo-convex shape on the surface. The organic fiber is, for example, a polyacrylonitrile fiber, a pitch fiber, or a rayon fiber. An organic fiber having a relatively large concavo-convex shape on the surface is manufactured, for example, by extruding a raw material solution of the fiber from a die having concavo-convex holes. The shape and size of the concavo-convex formed in the holes of the die may be appropriately adjusted and set according to the shape and size of the plurality of groove portions 4 and the plurality of ridge portions 5 in the obtained carbon fiber 3. The pseudo aspect ratio of the obtained carbon fiber 3 can also be adjusted according to the pseudo aspect ratio of the holes of the die. The extrusion conditions can be appropriately selected.

[0059] It is also possible to compress the periphery of the carbon fiber 3 extruded from the nozzle. The carbon fiber 3 extruded from the nozzle may be discontinuously compressed. The compression ratio may be changed when compressing the carbon fiber 3 extruded from the nozzle. When discontinuously compressing or changing the compression ratio, regions with different pseudo aspect ratios can be formed along the length of the carbon fiber 3. The carbon fiber 3 extruded from the nozzle may be compressed over the entire circumference of the carbon fiber 3, or a part of the periphery of the carbon fiber 3 may be compressed. When compressing a part of the periphery of the carbon fiber 3, the portions facing each other across the center of the virtual circle 7 shown in FIG. 2 on the outer peripheral surface of the carbon fiber 3 may be compressed so as to approach each other. By changing the compression position, for example, the height difference between the groove portion 4 and the ridge portion 5 can be changed.

[0060] <RF battery system> Referring to FIGS. 7 and 8, the RF battery system 10 of the embodiment will be described. The RF battery system 10 is one of the electrolyte circulation type battery systems. The RF battery system 10 includes an RF battery cell 10C and a circulation mechanism 20 that circulates an electrolyte to the RF battery cell 10C. The RF battery system 10 performs charging and discharging while supplying an electrolyte to the RF battery cell 10C.

[0061] The RF battery system 10 is typically connected to the power generation unit 210 and the load 230 with the transformer equipment 220 and the AC / DC converter 200 interposed therebetween. The RF battery system 10 charges using the power generation unit 210 as a power supply source and discharges with the load 230 as the power supply target. The power generation unit 210 is, for example, a solar power generator, a wind power generator, or other general power plants. The load 230 is, for example, a power grid or a power consumer. The RF battery system 10 is used, for example, for load leveling, momentary voltage drop compensation, emergency power supply, or output smoothing of natural energy power generation.

[0062] <RF battery cell> As shown in FIG. 7, the RF battery cell 10C is separated into a positive electrode cell 10P and a negative electrode cell 10N by a separator 11. The positive electrode cell 10P incorporates a positive electrode 12. A positive electrode electrolyte is circulated through the positive electrode cell 10P. The negative electrode cell 10N incorporates a negative electrode 13. A negative electrode electrolyte is circulated through the negative electrode cell 10N. At least one of the positive electrode 12 and the negative electrode 13 is the above-described electrode 1 for RF battery. Both the positive electrode 12 and the negative electrode 13 may be the above-described electrode 1 for RF battery. The separator 11 is, for example, an ion exchange membrane.

[0063] The RF battery cell 10C is usually configured inside a structure commonly called a cell stack 100. As shown in the lower diagram of FIG. 8, the cell stack 100 includes sub-stacks 100S, two end plates 120, and a clamping mechanism 130. The cell stack 100 includes, for example, a plurality of sub-stacks 100S. Each sub-stack 100S includes a laminate and two supply / discharge plates 110. The laminate is configured by laminating a plurality of cell frames 15, positive electrodes 12, separators 11, and negative electrodes 13 in this order, as shown in the upper diagrams of FIGS. 7 and 8. The supply / discharge plates 110 are arranged at both ends of the laminate, as shown in the lower diagram of FIG. 8. The supply pipe 24 and the discharge pipe 26 of the positive electrode circulation mechanism 20P and the supply pipe 25 and the discharge pipe 27 of the negative electrode circulation mechanism 20N, which will be described later, are connected to the supply / discharge plates 110. The two end plates 120 sandwich the plurality of sub-stacks 100S. The clamping mechanism 130 clamps both end plates 120.

[0064] As shown in the upper diagrams of Figures 7 and 8, the cell frame 15 includes a bipolar plate 151 and a frame body 152. The frame body 152 surrounds the outer periphery of the bipolar plate 151. One RF battery cell 10C is configured between the bipolar plates 151 of adjacent cell frames 15. A positive electrode 12 is disposed on a first surface of the bipolar plate 151. A negative electrode 13 is disposed on a second surface of the bipolar plate 151. The frame body 152 is formed with liquid supply manifolds 153 and 154, liquid supply slits 153s and 154s, liquid drainage manifolds 155 and 156, and liquid drainage slits 155s and 156s. An annular seal member 157 is disposed between each frame body 152.

[0065] <Circulation mechanism> 7, the circulation mechanism 20 includes a positive electrode circulation mechanism 20P and a negative electrode circulation mechanism 20N. The positive electrode circulation mechanism 20P circulates the positive electrode electrolyte through the positive electrode cell 10P. The negative electrode circulation mechanism 20N circulates the negative electrode electrolyte through the negative electrode cell 10N.

[0066] The positive electrode circulation mechanism 20P includes a positive electrode electrolyte tank 22, a supply pipe 24, a discharge pipe 26, and a pump 28. The positive electrode electrolyte tank 22 stores a positive electrode electrolyte. The supply pipe 24 and the discharge pipe 26 connect the positive electrode electrolyte tank 22 and the positive electrode cell 10P. The pump 28 is provided midway along the supply pipe 24 and pressure-feeds the positive electrode electrolyte in the positive electrode electrolyte tank 22 to the positive electrode cell 10P. The positive electrode electrolyte is supplied from the positive electrode electrolyte tank 22 through the supply pipe 24 to the positive electrode cell 10P, and is returned from the positive electrode cell 10P through the discharge pipe 26 to the positive electrode electrolyte tank 22. Specifically, the positive electrode electrolyte that has passed through the supply pipe 24 is supplied to the positive electrode 12 from a liquid supply manifold 153 shown in the upper diagram of FIG. 8 through a liquid supply slit 153s. The positive electrode electrolyte supplied to the positive electrode 12 flows from the bottom to the top of the positive electrode 12, as shown by the arrows in the upper diagram of Fig. 8. The positive electrode electrolyte that has flowed through the positive electrode 12 passes through the drainage slit 155s and is discharged from the drainage manifold 155 to the discharge pipe 26.

[0067] The negative electrode circulation mechanism 20N includes a negative electrode electrolyte tank 23, a supply pipe 25, a discharge pipe 27, and a pump 29. The negative electrode electrolyte tank 23 stores a negative electrode electrolyte. The supply pipe 25 and the discharge pipe 27 connect the negative electrode electrolyte tank 23 and the negative electrode cell 10N. The pump 29 is provided midway along the supply pipe 25 and pressure-feeds the negative electrode electrolyte in the negative electrode electrolyte tank 23 to the negative electrode cell 10N. The negative electrode electrolyte is supplied from the negative electrode electrolyte tank 23 through the supply pipe 25 to the negative electrode cell 10N, and is returned from the negative electrode cell 10N through the discharge pipe 27 to the negative electrode electrolyte tank 23. Specifically, the negative electrode electrolyte that has passed through the supply pipe 25 is supplied to the negative electrode 13 from a liquid supply manifold 154 and through a liquid supply slit 154s shown in the upper diagram of FIG. 8 . The negative electrode electrolyte supplied to the negative electrode 13 flows from the lower end to the upper end of the negative electrode 13 as shown by the arrows in the upper diagram of Fig. 8. The negative electrode electrolyte that has flowed through the negative electrode 13 passes through the drainage slit 156s and is discharged from the drainage manifold 156 to the discharge pipe 27.

[0068] By circulating the positive electrode electrolyte to the positive electrode cell 10P and the negative electrode electrolyte to the negative electrode cell 10N, the RF battery cell 10C is charged and discharged in accordance with the valence change reaction of the active material ions in the electrolyte of each electrode.

[0069] The electrolyte is, for example, a solution containing active material ions. The positive electrode active material ions contained in the positive electrode electrolyte are, for example, manganese ions, vanadium ions, or iron ions. The positive electrode active material may be one or more selected from the group consisting of polyacids, quinone derivatives, and amines. The negative electrode active material ions contained in the negative electrode electrolyte are, for example, titanium ions, vanadium ions, or chromium ions. The negative electrode active material may be one or more selected from the group consisting of polyacids, quinone derivatives, and amines. An example of an electrolyte is a vanadium-based electrolyte in which both the positive electrode active material and the negative electrode active material are vanadium ions. The solvents for the positive electrode electrolyte and the negative electrode electrolyte are, for example, aqueous solutions containing one or more acids or acid salts selected from the group consisting of sulfuric acid, phosphoric acid, nitric acid, and hydrochloric acid.

[0070] The kinematic viscosity of the electrolyte at 35°C is, for example, 8.0 x 10-3 cm 2 / s or more, 1.0×10 -2 cm 2 / s or more, or 1.2 × 10 -2 cm 2 / s or more. When the kinetic viscosity of the electrolyte is high, the reactivity of the electrolyte with the positive electrode 12 or the negative electrode 13 tends to be high. If the kinetic viscosity of the electrolyte is high and at least one of the positive electrode 12 and the negative electrode 13 is the RF battery electrode 1 described above, the reactivity of the RF battery electrode 1 can be further increased. The kinetic viscosity of the electrolyte at 35°C is, for example, 20 × 10 -2 cm 2 / s or less.

[0071] [Test example] An electrode for RF batteries was fabricated from a fiber assembly containing multiple carbon fibers, and the reaction resistance of the electrode for RF batteries was investigated.

[0072] <Sample> Samples No. 1 to No. 11 were prepared as fiber assemblies containing a plurality of carbon fibers having a plurality of grooves and ridges on their surfaces. The fiber assemblies of Samples No. 1 to No. 11 were nonwoven fabrics made by processing a plurality of organic fibers having relatively large irregularities on their surfaces into a felt-like shape and then firing them. Sample No. 101 was prepared as a fiber assembly made of a plurality of carbon fibers having a circular cross-sectional shape. The fiber assembly of Sample No. 101 was a nonwoven fabric made by processing a plurality of organic fibers having a circular cross-sectional shape into a felt-like shape and then firing them. All samples were prepared under the same conditions except for the composition of the carbon fibers. The porosity of the fiber assemblies of all samples was 88%.

[0073] <Carbon fiber circumference> For each sample, the cross section was observed using a scanning electron microscope (SEM), and the perimeter L1 and area of ​​the cross section of five randomly selected carbon fibers were measured by image analysis. For each carbon fiber, the perimeter L2 of a perfect circle, i.e., a virtual circle, with an area equal to the area of ​​the cross section was determined. The average ratio L1 / L2 of the perimeter L1 to the perimeter L2 was calculated. Table 1 shows the average perimeter L1, average perimeter L2, and average ratio L1 / L2. For Samples No. 1 to No. 11, the carbon fibers had multiple grooves and multiple ridges, so the ratio L1 / L2 was greater than 1. For Sample No. 101, the carbon fiber had a circular cross section, so the ratio L1 / L2 was 1. Table 1 also shows the diameter L4 of the virtual circle. Table 1 also shows the average value of the bottom radius of the grooves in the cross section. Sample No. 101 does not have a groove, so the rounded radius of the bottom is indicated by "-".

[0074] <Groove length> For each sample, five carbon fibers were randomly selected, and one groove portion was selected from each carbon fiber, and the length L3 of each groove portion for all carbon fibers was measured. The length L3 of each groove portion is the length perpendicular to both the width and depth of the groove portion. The average value of the groove lengths L3 was calculated. The average value of the ratio L3 / L4 of the groove length L3 to the diameter L4 of the imaginary circle was calculated. The average value of the groove length L3 and the average value of the ratio L3 / L4 are shown in Table 1. Since sample No. 101 has no groove portions, the groove length L3 and the ratio L3 / L4 are indicated by "-".

[0075] <Pseudo flatness> For each sample, one random carbon fiber was selected, and multiple cross sections were collected. The lengths of the long and short sides of each cross section were measured, and 1-(b / a) was calculated for each cross section. The long side is the longest line segment overlapping the cross section. The short side is the line segment perpendicular to the long side and the longest line segment overlapping the cross section. The average pseudo flattening of the multiple cross sections was taken as the pseudo flattening of the carbon fiber. The standard deviation of the pseudo flattening was calculated from the average pseudo flattening and the pseudo flattening of each cross section. The pseudo flattening and standard deviation of the pseudo flattening of the carbon fiber are shown in Table 1. For Sample No. 101, the carbon fiber had a circular cross section, but the pseudo flattening was 0.12 and the standard deviation of the pseudo flattening was 0.07. Even when multiple organic fibers with circular cross sections were used, it is believed that variation in the cross section shape occurred during manufacturing.

[0076] <Reaction Resistance> A single-cell RF battery system was assembled using RF battery electrodes made from the fiber aggregates of each sample, and a charge-discharge test was performed. A vanadium sulfate aqueous solution was used for both the positive and negative electrode electrolytes. The vanadium concentration was 1.7 mol / L. The charge-discharge test was performed at a current density of 70 mA / cm. 2 The charging and discharging were switched at a constant current of 100 kJ / s, and when a preset switching voltage was reached, charging and discharging were switched. The cell resistance was calculated after three cycles of charging and discharging. The cell resistance was calculated by dividing the difference between the midpoint voltage during charging and discharging by 2, dividing this value by the current value, and multiplying the resulting resistance by the area of ​​the RF battery electrode. "Midpoint voltage" refers to the voltage value at the midpoint between the start and end of charging or discharging. The electrode reaction resistance was calculated from the cell resistance measured for each sample single-cell battery. The reaction resistance is the resistance obtained by subtracting the conductive resistance from the cell resistance and is calculated using the following formula. The conductive resistance was measured using a Battery HiTester. The reaction resistance of the RF battery electrodes for each sample is shown in Table 1. In Table 1, the term "electrode" is used. Reaction resistance (Ω cm 2 ) = cell resistance (Ω cm 2 ) - Conductive resistance (Ω cm 2 )

[0077] [Table 1]

[0078] As shown in Table 1, the RF battery electrodes of Samples 1 to 11 have lower cell resistance, i.e., lower reaction resistance, than the RF battery electrode of Sample 101. It is believed that the grooves in the RF battery electrodes of Samples 1 to 11 promote stirring of the electrolyte around the carbon fibers, increasing reactivity and lowering cell resistance compared to the RF battery electrode of Sample 101. The RF battery electrodes of Samples 6 and 7 have a longer cross-sectional perimeter and a larger surface area than the RF battery electrode of Sample 101, which likely increases the reaction area in contact with the electrolyte and lowers cell resistance. The RF battery electrodes of Samples 1 to 5 and Samples 8 to 11 have slightly shorter perimeters than the RF battery electrode of Sample 101. However, as described above, it is believed that the RF battery electrodes of Samples No. 1 to No. 5 and Samples No. 8 to No. 11 had lower cell resistance because the grooves promoted stirring of the electrolyte around the carbon fibers.

[0079] Looking at samples No. 1 to No. 7, the longer the groove length, the lower the cell resistance. The longer the groove length, the larger the surface area of ​​the carbon fiber, which is thought to have increased the contact area with the electrolyte. It is also thought that the longer the groove length, the more widely the surface area of ​​the carbon fiber was, promoting the stirring of the electrolyte around the carbon fiber over a wider area. For example, the RF battery electrode of sample No. 7 contains more relatively long grooves than the RF battery electrode of sample No. 1, which is thought to have reduced the cell resistance. The RF battery electrode of sample No. 1 contains more relatively short grooves than the RF battery electrode of sample No. 7, which is thought to have increased the ridge area across the entire carbon fiber, resulting in a smaller reduction in cell resistance. Although sample No. 1 contains many relatively short grooves, if the grooves are aligned closely along the length of the carbon fiber, the number of ridges will be reduced, which is expected to result in a larger reduction in cell resistance.

[0080] The RF battery electrodes of samples No. 3 to No. 7 have lower cell resistance than the RF battery electrode of sample No. 2. This is thought to be because the rounded radius of the bottom of the grooves in the RF battery electrodes of samples No. 3 to No. 7 is smaller, which makes the grooves deeper and promotes better stirring of the electrolyte around the carbon fibers. The RF battery electrode of sample No. 8 has lower cell resistance than the RF battery electrode of sample No. 1. The rounded radius of the grooves differs between sample No. 1 and sample No. 8. A comparison of sample No. 1 and sample No. 8 shows that the rounded radius of the grooves contributes to reducing cell resistance.

[0081] The RF battery electrodes of samples No. 4 to No. 7 have lower cell resistance than the RF battery electrode of sample No. 3. The RF battery electrodes of samples No. 4 to No. 7 have a pseudo-flattening ratio of 0.2 or greater, which is thought to increase the perimeter of the carbon fibers and the surface area of ​​the carbon fibers. The RF battery electrode of sample No. 9 has lower cell resistance than the RF battery electrode of sample No. 1. The RF battery electrode of sample No. 10 has higher cell resistance than the RF battery electrode of sample No. 1. Samples No. 1, No. 9, and No. 10 have different pseudo-flattening ratios and standard deviations of the pseudo-flattening ratios. The pseudo-flattening ratio of sample No. 1 is smaller than that of sample No. 9 and larger than that of sample No. 10. Comparing sample No. 1 with sample No. 9 and sample No. 1 with sample No. 10 reveals that the pseudo-flattening ratio contributes to reducing cell resistance.

[0082] The RF battery electrodes of samples No. 5 to No. 7 have lower cell resistance than the RF battery electrode of sample No. 4. This is thought to be because the standard deviation of the pseudo flatness of the RF battery electrodes of samples No. 5 to No. 7 is 0.1 or more, which promotes better stirring of the electrolyte around the carbon fibers. The RF battery electrode of sample No. 11 has lower cell resistance than the RF battery electrode of sample No. 1. The standard deviation of the pseudo flatness of samples No. 1 and No. 11 is different. A comparison of sample No. 1 and No. 11 shows that the standard deviation of the pseudo flatness contributes to reducing cell resistance. [Explanation of symbols]

[0083] 1. RF battery electrodes (redox flow battery electrodes) 3. Carbon fiber 4 grooves, 40 bottoms 5 ridges 6 cross section, 61 long side, 62 short side 7 Virtual Circle 4L,a,b length 10 RF battery system (redox flow battery system) 10C RF battery cell (redox flow battery cell) 10P positive electrode cell, 10N negative electrode cell, 11 diaphragm 12 positive electrode, 13 negative electrode 15 cell frame, 151 bipolar plate, 152 frame 153,154 Liquid supply manifold, 153s, 154s Liquid supply slit 155,156 Drainage manifold, 155s, 156s Drainage slit 157 Sealing material 20 Circulation mechanism 20P positive electrode circulation mechanism, 20N negative electrode circulation mechanism 22 positive electrode electrolyte tank, 23 negative electrode electrolyte tank 24,25 supply pipe, 26,27 discharge pipe 28,29 Pump 100 cell stack, 100S sub-stack 110 supply / discharge plate, 120 end plate, 130 tightening mechanism 200 AC / DC converter, 210 power generation unit, 220 transformer equipment, 230 load

Claims

1. A fiber assembly including carbon fibers is provided, The carbon fiber is a plurality of grooves disposed along the length of the carbon fiber; a ridge portion located between the groove portions and protruding relatively more than the groove portions; A cross section of the carbon fiber having a circumferential length greater than the circumferential length of an imaginary circle, the cross section is a cross section of the carbon fiber cut along a plane perpendicular to the length of the carbon fiber, The imaginary circle is a perfect circle having an area equal to the area of ​​the cross section. Electrodes for redox flow batteries.

2. 2. The electrode for a redox flow battery according to claim 1, wherein at least one of the plurality of grooves has a length perpendicular to both the width and depth of the groove that is five times or more the diameter of the imaginary circle.

3. 3. The electrode for a redox flow battery according to claim 1, wherein at least one of the plurality of grooves has a rounded bottom having a radius of 2 μm or less.

4. 3. The electrode for a redox flow battery according to claim 1, wherein the carbon fiber has a pseudo-flattening ratio of 0.15 or more.

5. 3. The electrode for a redox flow battery according to claim 1, wherein the carbon fiber has a standard deviation of pseudo-flatness of 0.1 or more.

6. 3. The electrode for a redox flow battery according to claim 1, wherein the diameter of the imaginary circle is 1 μm or more and 50 μm or less.

7. 3. The electrode for a redox flow battery according to claim 1, wherein the fiber assembly is a nonwoven fabric or a woven fabric.

8. The electrode for a redox flow battery according to claim 1 is provided. Redox flow battery cell.

9. A redox flow battery cell according to claim 8, Redox flow battery system.

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