Patent Description:
A redox flow battery (hereinafter, may be referred to as "RF battery") is one of the known large-capacity storage batteries (e.g., see PTLs <NUM> and <NUM>). RF batteries commonly include a cell stack constituted by a plurality of sets of a cell frame, a positive electrode, a membrane, and a negative electrode which are stacked on top of one another. The cell frame includes a bipolar plate interposed between the positive electrode and the negative electrode and a frame body disposed on the periphery of the bipolar plate. In the cell stack, two bipolar plates of a pair of adjacent cell frames and positive and negative electrodes interposed between the bipolar electrodes with a membrane being interposed between the electrodes form a cell. An RF battery performs charge and discharge by circulating electrolytes through the cells that include the electrodes.

The electrodes of a redox flow battery serve as a reaction field that promotes the cell reaction of an active material (metal ions) included in the electrolytes. Carbon fiber aggregates that include carbon fibers (e.g., carbon felt) have been commonly used for producing the electrodes for redox flow batteries. The carbon fibers are typically PAN-based carbon fibers produced using polyacrylonitrile (PAN) fibers as a raw material (e.g., see PTLs <NUM> to <NUM>).

An electrode for redox flow batteries according to the present invention is defined in claim <NUM>, the electrode comprising.

A redox flow battery cell according to the present invention is defined in claim <NUM>, and includes
the above-described electrode for redox flow batteries according to the present disclosure.

A redox flow battery according to the present invention is defined in claim <NUM>, and includes
the above-described redox flow battery cell according to the present disclosure.

There has been a demand for improvement in the battery performance of redox flow batteries.

The carbon fiber aggregate used for producing an electrode for redox flow batteries have been commonly composed of circular cross-section carbon fibers having a substantially circular cross section (the cross section of the fibers which is orthogonal to the longitudinal direction of the fibers).

One of the methods for improving the battery performance of a redox flow battery is to reduce the reaction resistance of electrodes. Since the carbon fibers that have been used for producing electrodes formed of the carbon fiber aggregate have a circular cross section, the reaction area of electrodes at which the electrodes come into contact with electrolytes is small. Therefore, it has been difficult to reduce the reaction resistance of the electrodes.

Accordingly, it is an object of the present invention to provide an electrode for redox flow batteries with which the reaction resistance of electrodes can be reduced. Another object of the present invention is to provide a redox flow battery cell and a redox flow battery that have excellent battery performance.

The present invention provides an electrode for redox flow batteries capable of reducing the reaction resistance of electrodes. The present invention also provides a redox flow battery cell and a redox flow battery that have excellent battery performance.

The inventors of the present invention proposes using modified cross-section carbon fibers each of which having a plurality of pleats formed in the surface thereof for producing an electrode formed of the carbon fiber aggregate. This increases the specific surface area of the fibers and the reaction area of the electrode at which the electrode comes into contact with an electrolyte and, consequently, enhances the reactivity of the electrode with the electrolyte. As a result, the reaction resistance of the electrode can be reduced, and the battery performance can be enhanced.

Since the electrode for redox flow batteries according to the present invention is formed of a carbon fiber aggregate, the electrode has voids formed therein, which enable an electrolyte to pass through the electrode and permeate and diffuse into the electrode. This increases the reaction region in which the electrode reacts with the electrolyte and makes it easy to maintain the reaction field. Moreover, since the carbon fibers constituting the carbon fiber aggregate have a plurality of pleats formed in the surfaces thereof, the fibers have a large surface area. This increases the reaction area of electrode at which the electrode comes into contact with an electrolyte and, consequently, improves the reactivity of the electrode with the electrolyte. Specifically, since the ratio L<NUM>/L<NUM> of the peripheral length (L<NUM>) of a cross section of the carbon fibers to the peripheral length (L<NUM>) of a virtual rectangle circumscribing the cross section of the carbon fibers is more than <NUM>, the fibers have a large specific surface area. This enables a sufficiently large reaction area of the electrode, at which the electrode comes into contact with an electrolyte, to be maintained and enhances the reactivity of the electrode with the electrolyte. Accordingly, the electrode for redox flow batteries is capable of reducing the reaction resistance of electrodes. As for the upper limit for the ratio L<NUM>/L<NUM>, the ratio L<NUM>/L<NUM> is limited to be, for example, <NUM> or less.

The term "cross section of the carbon fibers" used herein refers to a cross section of the fibers which is orthogonal to the longitudinal direction of the fibers. The virtual rectangle circumscribing a cross section of carbon fibers is determined as follows. The outline of a cross section of a carbon fiber <NUM> is determined as illustrated in <FIG>. A pair of parallel lines between which the outline of the fiber <NUM> is sandwiched such that the interval between the parallel lines is minimum is determined. Subsequently, another pair of parallel lines that are perpendicular to the above pair of the parallel lines and between which the outline of the fiber <NUM> is sandwiched such that the interval between the other pair of parallel lines is maximum is determined. A rectangle defined by the two pairs of the parallel lines is a virtual rectangle R. The length of the minor axis of the carbon fiber <NUM> is the length a of the short side of the virtual rectangle R. The length of the major axis of the carbon fiber <NUM> is the length b of the long side of the virtual rectangle R.

In a preferred embodiment of the electrode according to the present invention, the ratio of S<NUM> to S<NUM>, that is, S<NUM>/S<NUM>, may be <NUM> or more and <NUM> or less, where S<NUM> is the area of a cross section of the carbon fibers and S<NUM> is the area of a virtual rectangle circumscribing the cross section of the carbon fibers.

When the ratio S<NUM>/S<NUM> of the area (S<NUM>) of a cross section of the carbon fibers to the area (S<NUM>) of a virtual rectangle circumscribing the cross section of the carbon fibers is <NUM> or more, a sufficiently high strength of the carbon fibers is readily maintained and a reduction in the strength of electrodes can be limited. The higher (the closer to <NUM>) the ratio S<NUM>/S<NUM>, the larger the proportion of the cross section of the fiber to the virtual rectangle and the higher the strength of the fiber. However, an increase in the ratio S<NUM>/S<NUM> narrows the gaps formed between the pleats and reduces the likelihood of an electrolyte entering the gaps formed between the pleats. Consequently, it becomes difficult to increase the reaction area of the electrode at which the electrode comes into contact with the electrolyte. When the ratio S<NUM>/S<NUM> is <NUM> or less, sufficiently large gaps may be readily maintained between the pleats and the reduction in the likelihood of an electrolyte entering the gaps formed between the pleats may be limited. Accordingly, when the ratio S<NUM>/S<NUM> is limited to be <NUM> or more and <NUM> or less, the reaction area of electrodes at which the electrodes come into contact with an electrolyte can be maintained while the strength of the fibers is maintained.

According to the present invention, see again appended claim <NUM>, the length of the major axis of the carbon fibers may be <NUM> or more and <NUM> or less.

When the length of the major axis of the carbon fibers is <NUM> or more, the strength of the fibers can be readily maintained and a reduction in the strength of electrodes can be limited. When the length of the major axis of the carbon fibers is <NUM> or less, the fibers are thin and flexible. When the fibers are flexible, the likelihood of the fibers sticking into a membrane included in the redox flow battery cell can be reduced. The term "the length of the major axis of the carbon fibers" used herein refers to, as described above, the length b of the long side of the virtual rectangle R circumscribing a cross section of the fibers (see <FIG>).

(<NUM>) In a further preferred embodiment of the electrode according to the present invention, the carbon fibers may have a Young's modulus of <NUM> GPa or more and <NUM> GPa or less.

When the Young's modulus of the carbon fibers is <NUM> GPa or more, the fibers have high flexural rigidity, which reduces the likelihood of the electrode becoming damaged when the electrode is deformed by compression. When the Young's modulus of the carbon fibers is <NUM> GPa or less, the likelihood of the fibers sticking into a membrane included in the redox flow battery cell can be reduced.

(<NUM>) In another further preferred embodiment of the electrode according to the present invention, the carbon fiber aggregate may be carbon felt or carbon cloth.

Specific examples of the carbon fiber aggregate include carbon felt (nonwoven fabric made of carbon fibers) and carbon cloth (woven fabric made of carbon fibers), which are produced using carbon fibers; and carbon paper (carbon fiber-carbon composite material) synthesized using carbon fibers and carbon. Among these, carbon felt and carbon cloth have gaps formed between the fibers and a relatively high porosity. Therefore, using carbon felt or carbon cloth as an electrode material facilitates the circulation of an electrolyte and permeation and diffusion of the electrolyte into the electrode. It is particularly preferable to use carbon felt in order to readily diffuse the electrolyte to the entirety of the electrode, because carbon felt is composed of randomly oriented carbon fibers.

Specific examples of the carbon fibers include PAN-based carbon fibers produced using PAN fibers, pitch-based carbon fibers produced using pitch fibers, and rayon-based carbon fibers produced using rayon fibers.

(<NUM>) A redox flow battery cell according to the present invention is defined in claim <NUM>.

Since the redox flow battery cell includes the above-described electrode for redox flow batteries according to an embodiment, the redox flow battery cell is capable of reducing the reaction resistance of electrodes and enhancing the battery performance.

(<NUM>) A redox flow battery according to the present invention is defined in claim <NUM>.

Since the redox flow battery includes the redox flow battery cell according to the above embodiment, the redox flow battery is capable of reducing the reaction resistance of electrodes and enhancing the battery performance. Thus, the redox flow battery has excellent battery performance.

Specific examples of the electrode for redox flow batteries (hereinafter, may be referred to simply as "electrode"), the redox flow battery cell (hereinafter, may be referred to simply as "cell"), and the redox flow battery (RF battery) according to an embodiment of the present invention are described below with reference to the attached drawings. In the drawings, the same or equivalent components are denoted by the same reference numeral. It is intended that the scope of the present invention be not limited by the following examples, but determined by the appended claims, and include all modifications and variations that are equivalent to or fall within the scope of the invention.

First, an example of an RF battery <NUM> and an example of a cell <NUM> included in the RF battery <NUM> according to an embodiment are described below with reference to <FIG>. The RF battery <NUM> illustrated in <FIG> and <FIG> includes an electrolyte containing an active material that is a metal ion the valence of which varies due to oxidation and reduction, the electrolyte serving as a positive electrolyte and a negative electrolyte, and performs charge and discharge by utilizing the difference in oxidation reduction potential between the ion contained in the positive electrolyte and the ion contained in the negative electrolyte. The example of the RF battery <NUM> illustrated in the drawings is a vanadium-based RF battery including a vanadium electrolyte containing V ions which serves as a positive electrolyte and a negative electrolyte. In <FIG>, the solid arrows in the cell <NUM> show the charge reaction, while the dashed arrows show the discharge reaction. The RF battery <NUM> is connected to a power system P with an alternating current/direct current converter C and used for, for example, load levelling purpose, instantaneous voltage drop compensation, or as an emergency power source. The RF battery <NUM> may be used also for power levelling purpose in natural energy power generation, such as photovoltanic power generation or wind power generation.

The RF battery <NUM> includes a cell <NUM> that performs charge and discharge, tanks <NUM> and <NUM> that store an electrolyte, and circulation channels 100P and 100N through which the electrolyte is circulated between the tank <NUM> or <NUM> and the cell <NUM>.

As illustrated in <FIG>, the cell <NUM> includes a positive electrode <NUM>, a negative electrode <NUM>, and a membrane <NUM> interposed between the electrodes <NUM> and <NUM>. As for the structure of the cell <NUM>, the cell <NUM> is divided into a positive electrode cell <NUM> and a negative electrode cell <NUM> by the membrane <NUM>. The positive electrode cell <NUM> includes a positive electrode <NUM> disposed therein. The negative electrode cell <NUM> includes a negative electrode <NUM> disposed therein.

Each of the electrodes, that is, the positive electrode <NUM> and the negative electrode <NUM>, is formed of a carbon fiber aggregate including carbon fibers which is any of carbon felt, carbon cloth, carbon paper, and the like. Examples of the carbon fibers include PAN-based carbon fibers, pitch-based carbon fibers, and rayon-based carbon fibers. The membrane <NUM> is formed of, for example, an ion-exchange membrane permeable to hydrogen ions. The carbon fibers constituting the carbon fiber aggregate are detailed below.

Electrolytes (a positive electrolyte and a negative electrolyte) circulate through the cell <NUM> (the positive electrode cell <NUM> and the negative electrode cell <NUM>) and the circulation channels 100P and 100N. The positive electrode cell <NUM> is connected to a positive electrolyte tank <NUM> that stores the positive electrolyte with the positive electrolyte circulation channel 100P. Similarly, the negative electrode cell <NUM> is connected to a negative electrolyte tank <NUM> that stores the negative electrolyte with the negative electrolyte circulation channel 100N. The circulation channel 100P includes a supply pipe <NUM> through which the electrolyte is transported from the tank <NUM> to the cell <NUM> and a return pipe <NUM> through which the electrolyte is returned from the cell <NUM> to the tank <NUM>. The circulation channel 100N includes a supply pipe <NUM> through which the electrolyte is transported from the tank <NUM> to the cell <NUM> and a return pipe <NUM> through which the electrolyte is returned from the cell <NUM> to the tank <NUM>. The supply pipe <NUM> is provided with a pump <NUM> disposed therein, which is used for pressure feed of the electrolyte stored in the tank <NUM>. The supply pipe <NUM> is provided with a pump <NUM> disposed therein, which is used for pressure feed of the electrolyte stored in the tank <NUM>. The electrolytes are circulated through the cell <NUM> with the pumps <NUM> and <NUM>.

The cell <NUM> may have a single-cell structure that includes only one cell <NUM> or a multi-cell structure that includes a plurality of cells <NUM>. Normally, the cell <NUM> is used in the form of "cell stack" <NUM> as illustrated in <FIG>, which includes a plurality of the cells <NUM> stacked on top of one another. As illustrated in the lower diagram of <FIG>, the cell stack <NUM> includes substacks <NUM>, two end plates <NUM> between which the substacks <NUM> are sandwiched, and a clamping mechanism <NUM> with which the end plates disposed at the respective ends are clamped together. <FIG> illustrates a cell stack <NUM> including a plurality of substacks <NUM> as an example. Each of the substacks <NUM> includes a plurality of sets of a cell frame <NUM>, a positive electrode <NUM>, a membrane <NUM>, and a negative electrode <NUM> that are stacked on top of one another in this order (see the upper diagram of <FIG>) and two supply and discharge plates <NUM> disposed on the respective ends of the multilayer body (see the lower diagram of <FIG>; not illustrated in <FIG>). The supply and discharge plates <NUM> are connected to the supply pipes <NUM> and <NUM> and the return pipes <NUM> and <NUM> of the circulation channels 100P and 100N (see <FIG> and <FIG>).

The cell frame <NUM> includes a bipolar plate <NUM> interposed between the positive electrode <NUM> and the negative electrode <NUM> and a frame body <NUM> disposed on the periphery of the bipolar plate <NUM>, as illustrated in the upper diagram of <FIG>. The positive electrode <NUM> is disposed on one of the surfaces of the bipolar plate <NUM>. The negative electrode <NUM> is disposed on the other surface of the bipolar plate <NUM>. The bipolar plate <NUM> is disposed inside the frame body <NUM>. The bipolar plate <NUM> and the frame body <NUM> define recesses 32o. The recesses 32o are formed on the respective sides of the bipolar plate <NUM>. Each of the recesses 32o houses the positive electrode <NUM> and the negative electrode <NUM> with the bipolar plate <NUM> being interposed therebetween. Each of the recesses 32o forms a cell space, that is, the positive electrode cell <NUM> or the negative electrode cell <NUM> (see <FIG>).

The bipolar plate <NUM> is composed of, for example, plastic containing carbon. The frame body <NUM> is composed of, for example, a plastic such as a vinyl chloride resin (PVC), polypropylene, polyethylene, a fluororesin, or an epoxy resin. The cell frame <NUM> is produced by forming the frame body <NUM> on the periphery of the bipolar plate <NUM> by injection molding or the like in an integrated manner.

In the cell stack <NUM> (the substacks <NUM>), one of the surfaces of the frame body <NUM> of a cell frame <NUM> and the other surface of the frame body <NUM> of an adjacent cell frame <NUM> are brought into contact with each other so as to face each other to form a cell <NUM> between the two bipolar plates <NUM> of the adjacent cell frames <NUM>. When the assembly of the cell <NUM> is completed, the electrodes <NUM> and <NUM> are housed in each of the recesses 32o of the frame bodies <NUM> while being compressed in the thickness direction. The depth of the recesses 32o determines the thickness of the electrodes <NUM> and <NUM> that are in compression. Ringshaped sealing members <NUM>, such as O-rings or flat gaskets, are interposed between the frame bodies <NUM> of each pair of adjacent cell frames <NUM> in order to prevent the leakage of the electrolyte. The frame body <NUM> is provided with sealing grooves (not illustrated) formed therein, in which the sealing members <NUM> are to be disposed.

The electrolytes are circulated through the cell <NUM> by using liquid-supply manifolds <NUM> and <NUM> and liquid-discharge manifolds <NUM> and <NUM> that are formed in the frame body <NUM> of the cell frame <NUM> so as to penetrate through the frame body <NUM> and liquid-supply slits <NUM> and <NUM> and liquid-discharge slits <NUM> and <NUM> formed in the frame body <NUM>. In the cell frame <NUM> (the frame body <NUM>) illustrated in this example, the positive electrolyte is fed from the liquid-supply manifold <NUM> formed in the lower portion of the frame body <NUM> to the positive electrode <NUM> through the liquid-supply slit <NUM> formed in one of the surfaces of the frame body <NUM> and then discharged to the liquid-discharge manifold <NUM> through the liquid-discharge slit <NUM> formed in the upper portion of the frame body <NUM>. Similarly, the negative electrolyte is fed from the liquid-supply manifold <NUM> formed in the lower portion of the frame body <NUM> to the negative electrode <NUM> through the liquid-supply slit <NUM> formed in the other surface of the frame body <NUM> and then discharged to the liquid-discharge manifold <NUM> through the liquid-discharge slit <NUM> formed in the upper portion of the frame body <NUM>. The liquid-supply manifolds <NUM> and <NUM> and the liquid-discharge manifolds <NUM> and <NUM> form channels for electrolytes when a plurality of the cell frames <NUM> are stacked on top of one another. The channels are communicated with the supply pipes <NUM> and <NUM> and the return pipes <NUM> and <NUM> of the circulation channels 100P and 100N (see <FIG> and <FIG>) through the supply and discharge plates <NUM> (see the lower diagram of <FIG>) and thereby enable the electrolytes to circulate through the cell <NUM>.

In the cell <NUM> illustrated in this example, the electrolytes are fed from the lower parts of the positive electrode <NUM> and the negative electrode <NUM> and discharged from the upper parts of the electrodes <NUM> and <NUM>. That is, the electrolytes flow in the direction from the lower ends to the upper ends of the electrodes <NUM> and <NUM>. In <FIG> and the upper diagram of <FIG>, the arrows in the electrodes <NUM> and <NUM> show the direction in which the electrolyte generally flows.

A channel (not illustrated) that includes a plurality of grooves through which the electrolyte flows may be formed in the surfaces of the bipolar plate <NUM> which face the electrode <NUM> or <NUM>. In such a case, the resistance to the flow of the electrolyte through the cell <NUM> can be reduced and, accordingly, the pressure loss of the electrolyte in the cell <NUM> can be reduced. The width and the depth of the grooves may be selected adequately in accordance with the size and thickness of the bipolar plate <NUM> and are not limited.

The electrodes (the positive electrode <NUM> and the negative electrode <NUM>) according to the embodiment are formed of a carbon fiber aggregate that includes a plurality of carbon fibers. Since the electrodes formed of the carbon fiber aggregate is porous and have voids formed in the electrodes, the electrolyte can pass through the electrodes and permeate and diffuse into the electrodes. This increases the reaction region of the electrodes in which the electrodes react with the electrolyte and enables the reaction field to be readily maintained. Typical examples of the carbon fiber aggregate include carbon felt, carbon cloth, and carbon paper. Among these, carbon felt and carbon cloth have a relatively high porosity. Therefore, using carbon felt or carbon cloth as an electrode material facilitates the circulation of the electrolyte and permeation and diffusion of the electrolyte into the electrodes. It is particularly preferable to use carbon felt in order to readily diffuse the electrolyte to the entirety of the electrodes, because carbon felt is composed of randomly oriented carbon fibers. Typical examples of the carbon fibers include PAN-based carbon fibers, pitch-based carbon fibers, and rayon-based carbon fibers.

One of the features of the electrodes according to the embodiment is that a carbon fiber <NUM> constituting the carbon fiber aggregate has a plurality of pleats <NUM> formed in the surface thereof, as illustrated in <FIG>.

The carbon fiber <NUM> is a modified cross-section fiber having a plurality of pleats <NUM> formed in the surface thereof and has pleat-like irregularities formed in the surface. Since the carbon fiber <NUM> has a plurality of the pleats <NUM> formed in the surface, the fiber <NUM> has a large surface area. This increases the reaction area of electrodes at which the electrodes come into contact with the electrolyte and, consequently, improves the reactivity of the electrodes with the electrolyte.

The ratio of L<NUM> to L<NUM>, that is, L<NUM>/L<NUM>, is more than <NUM>, where L<NUM> is the peripheral length of a cross section of the carbon fiber <NUM> and L<NUM> is the peripheral length of a virtual rectangle R circumscribing the cross section of the carbon fiber <NUM>.

When the peripheral length ratio L<NUM>/L<NUM> is more than <NUM>, a sufficiently large reaction area of electrodes, at which the electrodes come into contact with the electrolyte, can be maintained and, consequently, the reactivity of the electrodes with the electrolyte can be enhanced. Since the surface area of the carbon fiber <NUM> is proportional to the peripheral length L<NUM>, the higher the ratio L<NUM>/L<NUM>, the larger the specific surface area of the fiber <NUM> and the larger the reaction area of electrodes at which the electrodes come into contact with the electrolyte. Accordingly, the ratio L<NUM>/L<NUM> is preferably <NUM> or more. However, if the ratio L<NUM>/L<NUM> is excessively high, the number of the pleats <NUM> becomes excessively large. In such a case, the pleats <NUM> may adhere to one another, and the gaps between the pleats <NUM> may be reduced disadvantageously. As a result, it becomes difficult for the electrolyte to enter the gaps between the pleats <NUM> and the reaction area of electrodes at which the electrodes react with the electrolyte may fail to be increased. As for the upper limit for the ratio L<NUM>/L<NUM>, the ratio L<NUM>/L<NUM> is limited to be, for example, <NUM> or less. This enables sufficiently large gaps to be maintained between the pleats <NUM> and increases the likelihood of the electrolyte entering the gaps between the pleats <NUM>. As for the upper limit for the ratio L<NUM>/L<NUM>, the ratio L<NUM>/L<NUM> is preferably <NUM> or less, is further preferably <NUM> or less, and is particularly preferably <NUM> or less.

The method for determining the virtual rectangle R circumscribing a cross section of the carbon fiber <NUM> is described below specifically with reference to <FIG>. First, a cross section of the electrode is inspected with an optical microscope, a scanning electrophotographic microscope (SEM), or the like. The outline of a cross section of the carbon fiber <NUM> (a cross section orthogonal to the longitudinal direction of the fiber) is extracted from the cross-sectional image. A pair of parallel lines between which the outline of the fiber <NUM> is sandwiched such that the interval between the parallel lines is minimum is determined. Subsequently, another pair of parallel lines that are perpendicular to the above pair of the parallel lines and between which the outline of the fiber <NUM> is sandwiched such that the interval between the other pair of parallel lines is maximum is determined. A rectangle defined by the two pairs of the parallel lines is the virtual rectangle R. The length of the minor axis of the carbon fiber <NUM> is the length a of the short side of the virtual rectangle R. The length of the major axis of the carbon fiber <NUM> is the length b of the long side of the virtual rectangle R.

The peripheral length L<NUM> of a cross section of the carbon fiber <NUM> can be determined by analyzing the cross-sectional image. In this embodiment, the ratio of the peripheral length L<NUM> of a cross section of the carbon fiber <NUM> to the peripheral length L<NUM> of a virtual rectangle R circumscribing the cross section of the carbon fiber <NUM>, that is, peripheral length ratio L<NUM>/L<NUM>, is determined as follows. The peripheral length L<NUM> of a cross section of each of a plurality of carbon fibers is measured. The virtual rectangle R of the cross section of each of the fibers is determined, and the peripheral length L<NUM> of the virtual rectangle R is measured. The ratios L<NUM>/L<NUM> of the fibers are calculated and averaged. The number of fibers to be measured is, for example, <NUM> or more and is further preferably <NUM> or more.

It is preferable that the carbon fiber <NUM> satisfy the following conditions.

The ratio of S<NUM> to S<NUM>, that is, S<NUM>/S<NUM>, is <NUM> or more and <NUM> or less, where S<NUM> is the area of a cross section of the carbon fiber <NUM> and S<NUM> is the area of a virtual rectangle R circumscribing the cross section of the carbon fiber <NUM>.

When the area ratio S<NUM>/S<NUM> is <NUM> or more, a sufficiently high strength of the carbon fiber <NUM> can be maintained readily and, consequently, a reduction in the strength of the electrodes can be limited. The higher (the closer to <NUM>) the ratio S<NUM>/S<NUM>, the larger the proportion of the cross section of the fiber <NUM> to the virtual rectangle R and, therefore, the higher the strength of the fiber. However, an increase in the ratio S<NUM>/S<NUM> may narrow the gaps formed between the pleats <NUM> and reduces the likelihood of the electrolyte entering the gaps formed between the pleats <NUM>. Consequently, it may become difficult to increase the reaction area of electrodes at which the electrodes come into contact with the electrolyte. When the ratio S<NUM>/S<NUM> is <NUM> or less, sufficiently large gaps may be maintained between the pleats <NUM> and the reduction in the likelihood of the electrolyte entering the gaps formed between the pleats <NUM> may be limited. Accordingly, when the ratio S<NUM>/S<NUM> is limited to be <NUM> or more and <NUM> or less, the reaction area of electrodes at which the electrodes come into contact with the electrolyte may be maintained while the strength of the fiber is maintained. The ratio S<NUM>/S<NUM> is further preferably <NUM> or more and <NUM> or less and is particularly preferably <NUM> or less. The area S<NUM> of the cross section of the carbon fiber <NUM> is, for example, <NUM><NUM> or more and <NUM><NUM> or less, is further preferably <NUM><NUM> or more and <NUM><NUM> or less, and is particularly preferably <NUM><NUM> or less.

The area S<NUM> of the cross section of the carbon fiber <NUM> can be measured by analyzing the cross-sectional image. In this embodiment, the ratio of the area S<NUM> of a cross section of the carbon fiber <NUM> to the area S<NUM> of the virtual rectangle R, that is, the area ratio S<NUM>/S<NUM>, is determined as follows. The area S<NUM> of a cross section of each of a plurality of carbon fibers is measured. The virtual rectangle R of the cross section of each of the fibers is determined, and the area S<NUM> of the virtual rectangle R is measured. The ratios S<NUM>/S<NUM> of the fibers are calculated and averaged. The number of fibers to be measured is, for example, <NUM> or more and is further preferably <NUM> or more.

The length of the major axis of the carbon fiber <NUM> is <NUM> or more and <NUM> or less.

When the length of the major axis of the carbon fiber <NUM> (corresponds to the length b of the long side of the virtual rectangle R) is <NUM> or more, the strength of the fiber <NUM> can be readily maintained and a reduction in the strength of the electrodes can be limited. When the length of the major axis of the carbon fiber <NUM> is <NUM> or less, the fiber is thin and flexible, which reduces the likelihood of the fiber sticking into the membrane <NUM> (see the upper diagram of <FIG>) included in the cell <NUM>. When the length of the major axis of the carbon fiber <NUM> is <NUM> or less, the reaction area of the electrodes per unit volume is increased and the efficiency with which the electrodes react with the electrolyte is increased. The length of the major axis of the carbon fiber <NUM> is further preferably <NUM> or less. The length of the minor axis of the carbon fiber <NUM> (corresponds to the length a of the short side of the virtual rectangle R) is substantially equal to or less than the length of the major axis and is, for example, <NUM> or more and <NUM> or less.

The lengths of the minor and major axes of the carbon fiber <NUM> are the lengths a and b of the short and long sides of the above-described virtual rectangle R, respectively. In this embodiment, the lengths of the minor and major axes of the carbon fiber <NUM> are measured as follows. The virtual rectangle R of a cross section of each of a plurality of carbon fibers is determined, and the lengths a and b of the short and long sides of the virtual rectangle R are measured. The lengths a and b of the short and long sides of the virtual rectangle R are considered to be the lengths of the minor and major axes of the corresponding fiber. The average of the lengths of the minor axes of the fibers and the average of the lengths of the major axes of the fibers are calculated. The number of fibers to be measured is, for example, <NUM> or more and is further preferably <NUM> or more.

The Young's modulus of the carbon fiber <NUM> is <NUM> GPa or more and <NUM> GPa or less.

When the Young's modulus of the carbon fiber <NUM> is <NUM> GPa or more, the fiber <NUM> has high flexural rigidity, which reduces the likelihood of the electrodes becoming damaged when the electrodes are deformed by compression. When the Young's modulus of the carbon fiber <NUM> is <NUM> GPa or less, the likelihood of the fiber sticking into the membrane <NUM> (see the upper diagram of <FIG>) can be reduced. The Young's modulus of the carbon fiber <NUM> can be adjusted by changing, for example, the type of the carbon fiber and firing conditions (e.g., firing temperature) under which the raw materials, that is, organic fibers, are carbonized.

The Young's modulus of the carbon fiber <NUM> can be measured by, for example, subjecting carbon fibers taken from the electrodes to a tensile test.

The carbon fiber <NUM> is produced by carbonizing an organic fiber, such as a PAN fiber, a pitch fiber, or a rayon fiber, by firing. The carbon fiber <NUM> having a plurality of the pleats <NUM> formed in the surface thereof can be prepared by firing a modified cross-section organic fiber having a plurality of pleats formed in the surface thereof. The shape of a cross section of an organic fiber can be changed by changing the shape of the hole of a spinneret (nozzle) through which a raw-material solution is extruded into a fiber. The above-described, modified cross-section organic fiber can be prepared by forming a plurality of pleats in the surface of the fiber by using a nozzle having a plurality of irregularities formed in the inner periphery of the nozzle hole in the circumferential direction.

The electrodes (the positive electrode <NUM> and the negative electrode <NUM>) according to the embodiment may have the following features.

The thickness of the electrodes may be, for example, <NUM> or more and <NUM> or less. When the thickness of the electrodes is <NUM> or more, a sufficiently large reaction region (reaction field) of electrodes, in which the electrodes react with the electrolyte, can be readily maintained. When the thickness of the electrodes is <NUM> or less, the electrolyte can readily permeate and diffuse into the entirety of the electrodes to a sufficient degree. When the thickness of the electrodes is <NUM> or less, the thickness of the cell <NUM> (see the upper diagram of <FIG>) can be further reduced.

The thickness of the electrodes described above is not the thickness of the electrodes that are included in the cell and in compression but the thickness of the electrodes that are not in compression, that is, in the natural state in which no external force is applied to the electrodes.

The compression ratio of the electrodes is, for example, <NUM>% or more and <NUM>% or less. When the compression ratio of the electrodes is <NUM>% or more, the reaction area of the electrodes per unit volume is increased and, accordingly, the efficiency with which the electrodes react with the electrolyte is increased. When the compression ratio of the electrodes is <NUM>% or less, voids formed in the electrodes are maintained and the circulation of the electrolyte can be facilitated at a sufficient level. When the compression ratio of the electrodes is <NUM>% or less, the likelihood of the electrodes becoming damaged when the electrodes are excessively deformed can be reduced. The compression ratio of the electrodes is further preferably <NUM>% or more and <NUM>% or less. The compression ratio of the electrodes can be adjusted by changing, for example, the thickness of the electrodes and the depth of the cell spaces (the recesses 32o of the cell frame <NUM> illustrated in <FIG>) in which the electrodes are housed.

The compression ratio of an electrode can be calculated by {(T<NUM> - T<NUM>)/T<NUM>)} × <NUM> (%), where T<NUM> is the thickness of the electrode that is in compression, and T<NUM> is the thickness of the electrode that is not in compression.

Since the electrodes according to the embodiment (the positive electrode <NUM> and the negative electrode <NUM>) are formed of the carbon fiber aggregate and the carbon fiber <NUM> constituting the carbon fiber aggregate has a plurality of pleats <NUM> formed in the surface thereof, the reaction area of the electrode at which the electrodes come into contact with the electrolyte is large and, consequently, the reactivity of the electrodes with the electrolyte can be improved. In addition, since the ratio L<NUM>/L<NUM> of the peripheral length (L<NUM>) of a cross section of the carbon fiber <NUM> to the peripheral length (L<NUM>) of a virtual rectangle R circumscribing the cross section of the carbon fiber <NUM> is more than <NUM>, a sufficiently large reaction area of the electrodes, at which the electrodes come into contact with the electrolyte, can be maintained and the reactivity of the electrodes with the electrolyte is enhanced. As a result, the electrode according to the embodiment is capable of reducing the reaction resistance of electrodes and enhancing the battery performance.

The cell <NUM> according to an embodiment, which includes the electrodes according to the above-described embodiment, is capable of reducing the reaction resistance of electrodes and enhancing the battery performance.

The RF battery <NUM> according to an embodiment, which includes the cell <NUM> according to the above-described embodiment, is capable of reducing the reaction resistance of electrodes and enhancing the battery performance. Thus, the RF battery <NUM> has excellent battery performance.

Carbon fiber aggregates composed of carbon fibers having different cross-sectional shapes were prepared. Single-cell RF batteries were assembled using the carbon fiber aggregates as electrodes. The RF batteries were evaluated.

In Test example <NUM>, various types of carbon fiber aggregates (carbon felt) were prepared by working raw materials that were modified cross-section organic fibers having a plurality of pleats formed in the surfaces thereof into a felt-like form and firing the organic fibers, and a plurality of single cells (Sample Nos. <NUM> to <NUM>) were prepared using the carbon fiber aggregates as electrodes. The carbon fibers that constituted the electrodes of the single cells of Sample Nos. <NUM> to <NUM> had a plurality of pleats formed in the surfaces thereof. Carbon fibers were taken from each of the electrodes and subjected to a tensile test in order to determine the Young's modulus of the carbon fibers. The carbon fibers had a Young's modulus of <NUM> to <NUM> GPa.

For comparison, carbon felt was prepared by working circular cross-section organic fibers into a felt-like form and firing the organic fibers, and a single cell (Sample No. <NUM>) was prepared using the carbon felt as electrodes. The carbon fibers that constituted the electrodes of the single cell of Sample No. <NUM> had a Young's modulus of about <NUM> GPa.

In each of the single cell samples, carbon felt electrodes composed of the same carbon fibers were used as positive and negative electrodes, and the area of the electrodes was set to <NUM><NUM>. The thicknesses and compression ratios of the electrodes of the single cell samples were set to be substantially equal to one another. The thickness of the electrodes of the single cell samples was set to <NUM>. The compression ratio of the electrodes of the single cell samples was set to <NUM>%.

A cross section of the electrodes of each of the samples was inspected with an SEM. By image analysis, the peripheral lengths L<NUM> of cross sections of three carbon fibers were measured, the peripheral lengths L<NUM> of virtual rectangles R of the cross sections of the fibers were measured, and the average of the peripheral length ratios L<NUM>/L<NUM> of the carbon fibers constituting the electrodes was calculated. The areas S<NUM> of cross sections of the three carbon fibers were measured, the areas S<NUM> of virtual rectangles R of the cross sections of the fibers were measured, and the average of the area ratios S<NUM>/S<NUM> of the carbon fibers constituting the electrodes was calculated. The average of the areas S<NUM> of cross sections of the carbon fibers was calculated. Table <NUM> summarizes the peripheral length ratio L<NUM>/L<NUM>, the area ratio S<NUM>/S<NUM>, and the area of cross section S<NUM> of the carbon fibers constituting the electrodes of each sample.

The lengths b of the long sides of the virtual rectangles R of the cross sections of the fibers were measured and averaged to determine the length of the major axis of the carbon fibers constituting each electrode. Table <NUM> summarizes the length of the major axis of the carbon fibers constituting the electrodes of each sample.

Single-cell RF batteries were each assembled using a specific one of the single cell samples and subjected to a charge-discharge test. An aqueous vanadium sulfate solution (vanadium concentration: <NUM> mol/L) was used as positive and negative electrolytes. The charge-discharge test was conducted at a current density of <NUM> mA/cm<NUM> under a constantcurrent condition. Upon the predetermined switch voltage being reached, charge and discharge were switched. The cell resistance was measured after three cycles of charge and discharge. The cell resistance was calculated by dividing the difference in intermediate voltage between charge and discharge by two, further diving the quotient by the current value to obtain a resistance value, and multiplying the resistance value with the area of the electrode. The term "intermediate voltage" used herein refers to the voltage measured at the midpoint between the start and end of charge or discharge.

The reaction resistance of the electrodes was determined from the cell resistance measured using each of the single-cell battery samples. The reaction resistance was considered to be the cell resistance minus the conduction resistance and calculated using the following formula. The conduction resistance was measured using a BATTERY HiTESTER. Table <NUM> summarizes the reaction resistance of the electrodes of each sample.

Claim 1:
An electrode for redox flow batteries, the electrode being formed of a carbon fiber aggregate including a plurality of carbon fibers,
and being characterised in that:
each of the carbon fibers having a plurality of pleats formed in the surface thereof,
the ratio of L<NUM> to L<NUM>, that is, L<NUM>/L<NUM>, being more than <NUM>, where L<NUM> is the peripheral length of a cross section of the carbon fibers and L<NUM> is the peripheral length of a virtual rectangle (R) circumscribing the cross section of the carbon fibers, wherein the method for determining the virtual rectangle R circumscribing a cross section of the carbon fiber is defined in the description, and
wherein the length of the major axis of the carbon fibers is <NUM> or more and <NUM> or less, wherein the length of the major axis of the carbon fiber corresponds to the length (b) of the long side of the virtual rectangle (R).