JP7775643B2 - Electrode material for redox flow battery and redox flow battery including the same - Google Patents

Description

The present invention relates to an electrode material used in redox flow batteries.

Redox flow batteries utilize the oxidation-reduction reaction of redox ions in an aqueous solution while circulating (flowing) an electrolyte within the cell. Because the reaction is mild and occurs only in the liquid phase, they are highly safe, large-capacity storage batteries. The electrolyte used in redox flow batteries can be an aqueous solution of elements such as iron or chromium, whose valence can change through oxidation-reduction. Typically, an acidic aqueous solution of vanadium oxysulfate is widely used for the positive electrode electrolyte, and an acidic aqueous solution of vanadium sulfate is widely used for the negative electrode electrolyte.

Redox flow batteries are highly safe and offer excellent scalability benefits, but reducing the cost of batteries is highly desirable for widespread adoption. Generally, the selection of inexpensive, general-purpose materials can reduce the cost of batteries. Meanwhile, the development of high-performance cell components (electrodes, ion exchange membranes, current collector plates, etc.) can reduce the amount of components used and running costs, contributing to cost reductions. In particular, electrode materials for redox flow batteries are extremely important cell components, as they not only provide the ion reaction field but also the electrolyte flow path. Giving electrode materials low resistance can improve battery and system efficiency, which in turn will significantly contribute to reducing the cost of redox flow batteries.

For example, Patent Document 1 discloses that low-crystallinity carbon particles can be supported by a phenolic resin to reduce resistance. Patent Document 2 discloses a method of adding carbon filler to create unevenness and a method of adding erosion particles to form voids. Patent Documents 3 and 4 disclose a method of supporting high-specific surface area carbon particles with a highly crystalline carbonized binder to reduce resistance.

JP 2017-33758 A JP 2018-147595 A International Publication WO2019/049755 International Publication WO2015/032667

To further promote the widespread use of redox flow batteries, there is a demand for lower-resistance, cheaper electrode materials. Generally, lower resistance in electrode materials for redox flow batteries is achieved by increasing the surface area, but it is believed that there is an optimal pore size and shape to increase contact efficiency with the electrolyte.

However, the technology in Patent Document 1 simply applies a commercially available dispersion liquid, and although it has the effect of reducing resistance, it cannot be said that the design takes into account optimal pore size and shape. Furthermore, the void formation method in Patent Document 2 is limited to particles and does not take into account optimal shape. Furthermore, the technologies in Patent Documents 3 and 4 also do not include a method for actively forming voids to improve contact efficiency with the electrolyte, and it cannot be said that the design takes into account optimal pore size and shape.

The present invention was made in consideration of the above-mentioned problems, and its purpose is to provide an electrode material for a redox flow battery and a redox flow battery that can reduce cell resistance during charging and discharging and improve the system efficiency of the battery. The inventors have conducted extensive research to provide a carbon electrode material that is particularly suitable for use in vanadium-based redox flow batteries. In conventional vanadium-based redox flow batteries, low resistance is important from the perspective of reducing costs.

The electrode material for a redox flow battery according to the present invention, which has been able to solve the above problems, has the following configuration.
1. An electrode material for a redox flow battery, comprising carbon fibers, a carbonized binder, and carbon particles, wherein the carbonized binder contains recesses having a width of 1 to 50 μm and a length of 100 μm or more.
2. The electrode material for a redox flow battery according to 1 above, wherein the ratio (ID/IG) of the peak intensity ID at 1350 cm ⁻¹ to the peak intensity IG at 1590 cm ⁻¹ of the carbonized binder determined by laser Raman spectroscopy is 0.3 to 1.5.
3. The electrode material for a redox flow battery according to the above 1 or 2, wherein ID/IG of the carbonized binder is smaller than ID/IG of the carbon fiber.
4. The electrode material for a redox flow battery according to any one of 1 to 3 above, wherein the carbon fibers have a fiber diameter of 5 to 30 μm.
5. A redox flow battery using the electrode material according to any one of 1 to 4 above.

The present invention provides an electrode material suitable for use in vanadium-based redox flow batteries, which reduce cell resistance during charging and discharging, resulting in excellent battery energy efficiency and, as a result, excellent system efficiency. The electrode material of the present invention is suitable for use in flow-type and non-flow-type redox flow batteries, or redox flow batteries combined with lithium, capacitor, and fuel cell systems.

FIG. 1 is a scanning electron microscope (SEM) surface view (100x magnification) of Example No. 1. 2 is a scanning electron microscope (SEM) surface view (1000x magnification) of Example No. 1. FIG. 3 is a schematic diagram of a redox flow battery in which the electrode material of the present invention is used. FIG. 4 is a diagram showing a single cell of a redox flow battery in which the electrode material of the present invention is used.

The present invention will be described in detail for each component with reference to Figures 3 and 4. Note that the present invention is not limited to the following, and appropriate modifications may be made within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present invention.

Figure 3 is a schematic diagram of a redox flow battery suitable for use in the present invention. Figure 4 is a diagram showing a single cell, in which an ion exchange membrane 3 is disposed between two opposing current collector plates 1, 1, and electrolyte passages 4a, 4b are formed on both sides of the ion exchange membrane 3 along the inner surfaces of the current collector plates 1, 1 by spacers 2. An electrode material 5 is disposed in at least one of the passages 4a, 4b. The current collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolyte. As shown in Figure 4, the electrode is composed of the electrode material 5 and the current collector plate 1, and the structure allows the electrolyte to pass through the electrode material 5 (a three-dimensional electrode structure).

Electrode material 5 of the present invention is an electrode material that uses a carbon fiber sheet as a base material, supports carbon particles with a carbonized binder, and actively forms spaces (recesses) that improve contact efficiency with the electrolyte in order to achieve low resistance. Details of each requirement are as follows:

[Carbon fiber]
The carbon fiber used in the electrode material 5 of the present invention is a fiber obtained by subjecting an organic fiber precursor to a thermal carbonization treatment (details will be described later), and refers to a fiber composed of 90% or more by mass of carbon (JIS L 0204-2). Examples of organic fiber precursors that can be used as raw materials for carbon fibers include acrylic fibers such as polyacrylonitrile; phenolic fibers; PBO fibers such as polyparaphenylene benzobisoxazole (PBO); aromatic polyamide fibers; pitch fibers such as isotropic pitch fibers, anisotropic pitch fibers, and mesophase pitch; and cellulose fibers. Among these, from the viewpoint of excellent strength and elastic modulus, acrylic fibers, phenolic fibers, cellulose fibers, isotropic pitch fibers, and anisotropic pitch fibers are preferred as organic fiber precursors, with acrylic fibers being more preferred. The acrylic fiber is not particularly limited as long as it contains acrylonitrile as the main component. However, the acrylonitrile content of the raw material monomers forming the acrylic fiber is preferably 95% by mass or more, and more preferably 98% by mass or more.

The average fiber diameter of the carbon fibers is preferably 5 to 30 μm. If the average fiber diameter is smaller than 5 μm, liquid permeability will deteriorate. On the other hand, if the average fiber diameter is larger than 30 μm, the surface area will decrease, and even if carbon particles are supported, cell resistance will increase. Considering the balance between liquid permeability and reaction surface area, a diameter of 8 to 20 μm is more preferable.

The electrode material 5 of the present invention preferably uses a carbon fiber structure as the substrate, which improves strength and facilitates handling and processability. Specific examples of such structures include spun yarns and bundled filament yarns made of carbon fibers, nonwoven fabrics, knitted fabrics, and woven fabrics that are sheet-like carbon fiber materials, as well as the special knitted and woven fabrics described in JP-A-63-200467. Of these, carbon fiber nonwoven fabrics are more preferred in terms of handling, processability, manufacturability, etc.

When a carbon fiber sheet is used as the substrate, its thickness is preferably 1.5 mm or more and 8.0 mm or less. If the thickness is less than 1.5 mm, the spacer thickness must be reduced to reduce contact resistance, significantly reducing the liquid permeability within the electrode. On the other hand, if the thickness exceeds 8.0 mm, although liquid permeability improves, it becomes difficult to achieve the desired resistance-lowering effect.

When using nonwoven fabric, the average fiber length is preferably 30 to 100 mm. By keeping it within this range, a uniform fiber structure can be obtained.

Nonwoven fabrics are defined in JIS L 0222, and depending on the manufacturing method (e.g., entanglement, fusion, or bonding), examples include spunbonded nonwoven fabrics, spunlaced nonwoven fabrics, needlepunched nonwoven fabrics, resinbonded nonwoven fabrics, and thermalbonded nonwoven fabrics. Any of these may be used.

As mentioned above, carbon fiber is obtained by subjecting an organic fiber precursor to a thermal carbonization treatment, and the thermal carbonization treatment preferably includes at least a flame-proofing step and a carbonization (baking) step. However, the carbonization step does not necessarily have to be performed after the flame-proofing step as described above; the carbonization step may be performed after particles are attached to the flame-proofed fiber, in which case the carbonization step after the flame-proofing step can be omitted.

The flame-resistant process involves heating an organic fiber precursor in an air atmosphere, preferably at a temperature of 180°C or higher and 350°C or lower, to obtain flame-resistant organic fibers. The heat treatment temperature is more preferably 190°C or higher, and even more preferably 200°C or higher. It is also preferably 330°C or lower, and even more preferably 300°C or lower. Heating within the above temperature range reduces the nitrogen and hydrogen contents in the organic fibers while maintaining the carbon fiber morphology without thermal decomposition, thereby improving the carbonization rate. During the flame-resistant process, the organic fibers may thermally shrink, causing the molecular orientation to collapse and reducing the conductivity of the carbon fibers. Therefore, it is preferable to perform the flame-resistant treatment on the organic fibers under tension or stretching, and more preferably under tension.

The carbonization process involves heating the flame-resistant organic fiber obtained as described above in an inert atmosphere (preferably a nitrogen atmosphere) at a temperature of preferably 1000°C or higher and 2000°C or lower to obtain carbon fiber. The heating temperature is more preferably 1100°C or higher, and even more preferably 1200°C or higher. Furthermore, it is more preferably 1900°C or lower. By performing the carbonization process within the above temperature range, the carbonization of the organic fiber progresses, and carbon fiber having a pseudo-graphite crystal structure can be obtained.

Since organic fibers have different crystallinity, the heating temperature in the carbonization process can be selected depending on the type of organic fiber used as the raw material. For example, when using acrylic resin (preferably polyacrylonitrile) as the organic fiber, the heating temperature is preferably 800°C or higher and 2000°C or lower, and more preferably 1000°C or higher and 1800°C or lower.

The flame-proofing process and carbonization process described above are preferably carried out continuously, and the temperature rise rate when raising the temperature from the flame-proofing temperature to the carbonization temperature is preferably 20°C/min or less, and more preferably 15°C/min or less. By keeping the temperature rise rate within this range, it is possible to obtain carbon fibers that retain the shape of the organic fibers and have excellent mechanical properties. Taking into account mechanical properties, etc., the lower limit of the temperature rise rate is preferably 5°C/min or more.

[Space-forming fibers]
In the electrode material 5 of the present invention, space-forming fibers are used to more actively form spaces that improve contact efficiency with the electrolyte. Space-forming fibers refer to fibers that exist as fibers during one of the manufacturing processes of the electrode material 5, but after manufacturing, the fiber form is no longer retained, and spaces (depressions) where the fibers were present remain. The fiber form of the space-forming fibers is destroyed, for example, by melting or decomposition during carbonization treatment, and spaces that improve contact efficiency with the electrolyte are formed in the electrode material 5 at the locations where the space-forming fibers were present. In particular, by forming depressions in the carbonized binder as spaces with a width of 1 to 50 μm and a length of 100 μm or more, a higher level of low resistance can be achieved.

The raw materials used for the space-forming fibers are not particularly limited, as long as they do not retain their fibrous form after carbonization. Examples include aliphatic polymers such as polypropylene, polyethylene, and polyvinyl chloride during carbonization. Furthermore, grades that melt upon heat treatment, such as polyethylene terephthalate, nylon, aromatic polyamide, polyethersulfone, polyetherketone, and poly-p-phenylene ether, can also be used. Among these, polyethylene terephthalate, nylon, and poly-p-phenylene ether are particularly preferred.

The carbon content of the space-forming fiber after heating to 1000°C is preferably 10 to 50%. If it is below 10%, there is a risk that the binding strength of the carbonized binder described below will be reduced. If it exceeds 50%, the molten material from the lost fiber may excessively coat the carbon particles, which may lead to high resistance. A more preferable carbon content is 15 to 40%.

The fiber diameter of the fibers used for space-forming fibers is preferably 1 to 50 μm. If it is less than 1 μm, the size of the spaces formed will be small, making it difficult to improve the efficiency of contact with the electrolyte. On the other hand, if it exceeds 50 μm, the amount of space formed will be significantly reduced, making it difficult to improve the efficiency of contact with the electrolyte. A more preferable fiber diameter is 3 to 30 μm.

The space-forming fibers are formed into a fiber structure together with the carbon fiber precursor fibers and used as a fiber substrate, but in the final electrode material 5, the space-forming fibers disappear and recesses are formed. Here, the weight fraction of the space-forming fibers relative to the carbon fiber precursor fibers is preferably 10 to 80%. If it is less than 10%, it becomes difficult to achieve the effect of high liquid permeability. On the other hand, if it exceeds 80%, the amount of carbon fiber is too small, significantly impairing springiness and making it impossible to maintain the structure when compressed. Furthermore, excessive shrinkage during carbonization treatment can cause problems such as significant deformation.

[Carbon particles]
In the electrode material of the present invention, the carbon particles are useful for increasing the reaction surface area and achieving both low resistance and high liquid permeability.

The particle size of the carbon particles used in the present invention is 0.01 to 20 μm, preferably 0.05 to 10 μm. If the particle size exceeds 20 μm, the reactive surface area becomes small, making it difficult to achieve a low-resistance effect. On the other hand, if the particle size is 0.01 μm or less, the proportion of particles buried in the binder increases, or in very small spaces, surface tension and other factors make it difficult for the electrolyte to contact, potentially reducing the proportion of particles effective for the reaction. Here, "particle size" refers to the average particle size (D50) at the median 50% diameter in a particle size distribution obtained by dynamic light scattering or other methods. In this case, even if the primary particle size is 0.01 μm or less, it is sufficient as long as the secondary particle size of these aggregates is 0.01 μm or greater. In this way, by appropriately selecting the particle size of the carbon particles, the supported carbon particles can be effectively utilized.

The BET specific surface area calculated from the nitrogen adsorption amount of the carbon particles used in the present invention is preferably 20 m ² /g or more, more preferably 30 m ² /g or more, and even more preferably 40 m ² /g or more. If the BET specific surface area is less than 20 m ² /g, the edge exposure of the carbon particles is reduced and the contact area with the electrolyte is also reduced, making it impossible to obtain the desired low resistance. While the upper limit is not particularly limited from the above viewpoint, taking into consideration that particles with a large surface area and high bulkiness tend to increase the viscosity of the dispersion solution and deteriorate processability into sheets, etc., it is preferably approximately 2000 m ² /g or less. Here, the above-mentioned "BET specific surface area calculated from the nitrogen adsorption amount" means the specific surface area calculated from the amount of gas molecules adsorbed when nitrogen molecules are adsorbed onto solid particles.

The carbon particles used in the present invention are often those with high reactivity, high specific surface area, and low crystallinity, such as carbon blacks such as acetylene black (acetylene soot), oil black (furnace black, oil soot), ketjen black, and gas black (gas soot). Other examples include carbon nanotubes (CNTs), carbon nanofibers, carbon aerogels, mesoporous carbon, graphites, graphene, graphene oxide, N-doped CNTs, boron-doped CNTs, and fullerenes. From the perspective of raw material cost, carbon blacks are preferred.

The content of carbon particles used in the present invention is preferably 5% or more, and more preferably 10% or more, in terms of mass ratio relative to the total amount of the aforementioned carbon fiber, carbon particles, and carbonized binder described below. This ensures sufficient efficacy of the supported carbon particles and reduces resistance. However, if the amount of carbon particles is excessive, it becomes difficult to achieve both improved packing density and liquid permeability. Therefore, the upper limit is preferably approximately 90% or less. Note that the carbon fiber content used to calculate the above content is the content of the structure when a structure such as a nonwoven fabric is used as the substrate.

In the present invention, the mass ratio of the carbonized binder (described below) to the carbon particles is preferably 0.2 or more and 20 or less, and more preferably 0.3 or more and 10 or less. If the ratio is less than 0.2, the carbon particles will fall off more frequently and will not be sufficiently bound to the carbonized binder. On the other hand, if the ratio exceeds 20, the carbon edge surfaces of the carbon particles, which are the reaction sites, will be coated, and the desired low resistance will not be achieved.

[Carbonized binder]
The carbonized binder used in the present invention is electrically conductive and is added as a binder to strongly bond carbon fibers and carbon particles, which would not normally be bonded together. The carbonized binder used in the present invention preferably has a ratio (ID/IG) of the peak intensity ID at 1360 cm ⁻¹ to the peak intensity IG at 1580 cm ⁻¹ , as determined by laser Raman spectroscopy, of 0.3 to 1.5. By using a highly crystalline carbonized binder with a ratio of 0.3 to 1.5, the electronic conduction resistance between the carbon particles and the carbon fibers is reduced, resulting in a smooth electronic conduction path between the carbon particles and the carbon fibers. Furthermore, because the carbonized binder strongly bonds the carbon fibers via the carbon particles, an efficient conductive path can be formed, further enhancing the resistance-reducing effect of adding the carbon particles.

In the present invention, the ID/IG ratio of the carbonized binder is preferably 0.3 or more, and more preferably 0.8 or more, in consideration of the balance of affinity with the electrolyte. On the other hand, in consideration of electronic conductivity and other factors from the above viewpoints, a ratio of approximately 1.5 or less is generally preferred.

In the present invention, it is preferable that the ID/IG ratio of the carbonized binder is smaller than the ID/IG ratio of the carbon fiber. As a result, even if the carbon fiber has low crystallinity, it can be coated with a highly crystalline carbonized binder, ensuring oxidation resistance and high conductivity.

The content of the carbonized binder relative to the total amount of the carbon fiber, carbon particles, and carbonized binder is preferably 5% or more, more preferably 10% or more, and even more preferably 15% or more. By increasing the carbonized binder content in this way, both the carbon fiber and the carbon particles can be sufficiently bound together. Taking into consideration the permeability of the electrolyte and the need to suppress surface coating of the carbon particles, the upper limit is preferably approximately 60% or less.

Furthermore, the mass ratio of the total content of carbon particles and carbonized binder to the total amount of the above-mentioned carbon fibers, carbon particles, and carbonized binder is not particularly limited as long as the above requirements are satisfied, but is, for example, 20 to 65%. By keeping it in this range, it is possible to achieve both liquid permeability and low resistance.

The starting material for the carbonized binder used in the present invention may be any material capable of binding carbon fibers and carbon particles. Specifically, there are no particular restrictions on the type of material as long as it exhibits binding properties during carbonization during the production of the electrode material of the present invention. Examples of such materials include pitches such as coal tar pitch and coal-based pitch; resins such as phenolic resin, benzoxazine resin, epoxide resin, furan resin, vinyl ester resin, melamine-formaldehyde resin, urea-formaldehyde resin, resorcinol-formaldehyde resin, cyanate ester resin, bismaleimide resin, polyurethane resin, and polyacrylonitrile; furfuryl alcohol; and rubbers such as acrylonitrile-butadiene rubber. Commercially available products may also be used.

Among these, pitches such as coal tar pitch and coal-based pitch, which are particularly easily crystalline, are preferred because they can produce highly crystalline carbonized binders at low firing temperatures. Other resins are also preferably used because the desired carbonized binders can be obtained by increasing the firing temperature. Pitches are particularly preferred.
According to a preferred embodiment of the present invention, no phenolic resin is used, and therefore the adverse effects associated with phenolic resin (e.g., generation of formaldehyde and the odor of formaldehyde at room temperature) do not occur, and there are advantages such as no odor being generated at room temperature, etc. In contrast, in the above-mentioned Patent Document 2, a phenolic resin is used in the carbonized binder precursor, and in addition to the above adverse effects, there are disadvantages in terms of cost and work, such as the need for separate equipment to control the formaldehyde concentration in the work area to below the control concentration.

Here, particularly preferred pitches are described in detail. The mesophase (liquid crystal phase) content of the aforementioned coal tar pitch and coal-based pitch can be controlled by adjusting the temperature and time of the infusibility treatment. A low mesophase content results in a pitch that melts at a relatively low temperature or remains liquid at room temperature. On the other hand, a high mesophase content results in a pitch that melts at a high temperature and achieves a high carbonization yield. When pitches are used as carbonization binders, a high mesophase content (i.e., a high carbonization rate) is preferred; for example, 30% or more is preferred, and 50% or more is more preferred. This suppresses fluidity during melting and allows carbon fibers to be bonded together via the carbon particles without excessively coating the surfaces of the carbon particles. Taking into account factors such as the expression of binding properties, the upper limit is preferably 90% or less.

For the same reasons as above, the melting point of pitches is preferably 100°C or higher, and more preferably 200°C or higher. This not only achieves the above-mentioned effects, but also suppresses odor during the impregnation process, making it preferable in terms of processability. Taking into account factors such as the development of binding properties, the upper limit is preferably 350°C or lower.

(Characteristics of electrode materials)
The electrode material of the present invention comprises carbon fibers, a carbonized binder, and carbon particles, and the carbonized binder has recesses with a width of 1 to 50 μm and a length of 100 μm or more. By intentionally forming the recesses, which are spaces that increase the efficiency of contact with the electrolyte, a low-resistance electrode material for redox flow batteries can be obtained. According to the present invention, spaces (recesses with a width of 1 to 50 μm and a length of 100 μm or more) that increase the efficiency of contact with the electrolyte are intentionally formed.

In conventional technologies, high specific surface area, primarily consisting of micro- to mesopores several nanometers in size, is sought to achieve low resistance, and activation treatments such as oxidation are typically performed. Also, as in Patent Document 2, voids of several micrometers are sometimes designed. In contrast, in the present invention, the recesses described above, which are micrometer spaces with minimal surface tension, are formed to ensure contact of the electrolyte with the carbon surface. Furthermore, continuous recesses with widths of 1 to 50 μm and lengths of 100 μm or more are believed to enhance the resistance-lowering effect, as the electrolyte passes smoothly through the recesses. This is believed to be due to the promotion of electrolyte diffusion near the micro- to mesopores used in the electrode's redox reaction. Therefore, a width of less than 1 μm not only prevents efficient contact due to the effects of surface tension on the electrolyte, but also creates a large difference with the size of the space through which the electrolyte preferentially passes (several tens of micrometers), resulting in uneven flow and preventing the liquid diffusion effect near the micro- to mesopores. On the other hand, a width of more than 50 μm is believed to reduce the number of recesses, resulting in a failure to achieve low resistance. Furthermore, if the length of the continuous recesses is less than 100 μm, the electrolyte flow path will often be interrupted, and it is thought that the low resistance will not be achieved.

The recesses of the present invention, measuring 1 to 50 μm in width and 100 μm or longer, can be confirmed by observing the surface of the electrode material with a scanning electron microscope. The recesses of the present invention refer to the presence of semicircular, semielliptical, slit, or tunnel-shaped spaces in the area where the binder is continuously present. In this case, defects or fine holes may be partially formed in the recesses. By forming recesses of 1 to 50 μm in width and 100 μm or longer in the binder portion, the electrolyte can pass smoothly through the recesses, thereby achieving low resistance in the electrode material. The width of the recesses is preferably in the range of 1 to 50 μm, more preferably 4 to 30 μm. A width of less than 1 μm prevents efficient contact with the electrolyte due to the influence of surface tension. On the other hand, a width of more than 50 μm significantly reduces the number of recesses, preventing sufficient low resistance. The width of the recesses can be controlled by changing the diameter of the fibers used as the space-forming fibers described above. The length of the recesses is preferably 100 μm or more; if it is 100 μm or more, the electrolyte flow path will not be disrupted and low resistance can be achieved. The length of the recesses can be controlled by changing the fiber length and shape (crimped or straight) of the fibers used as the space-forming fibers.

In the electrode material of the present invention, it is preferable that two or more recesses having a width of 1 to 50 μm and a length of 100 μm or more can be confirmed in a field of view (unit area 2.3 mm ² ) of a scanning electron microscope at 100x magnification. Two or more recesses per 2.3 mm ² are preferable for achieving the effect of reducing resistance. However, if the width exceeds 50 μm, the number decreases significantly, making it difficult to have two or more recesses per 2.3 mm ² .

The basis weight of the electrode material of the present invention is preferably 100 to 700 g/m², more preferably 150 to 400 g/ ^m² , when the thickness of the spacer 2 sandwiched between the current collector plate 1 and the ion exchange membrane ³ (hereinafter referred to as the "spacer thickness") is 1 to 4 mm. By controlling the basis weight within the above range, it is possible to prevent damage to the ion exchange membrane 3 while ensuring liquid permeability. In particular, in recent years, the thickness of the ion exchange membrane 3 has tended to decrease in order to reduce resistance, and measures and usage methods that reduce damage to the ion exchange membrane 3 are extremely important. From the above perspective, it is also more preferable to use a nonwoven fabric or paper that has been flattened on one side as the substrate for the electrode material of the present invention. Any known flattening method can be used, for example, a method in which a slurry is applied to one side of carbon fiber and dried; or a method in which the slurry is impregnated on a smooth film such as PET and dried.

The thickness of the electrode material of the present invention is preferably at least greater than the spacer thickness. For example, it is preferably 1.5 to 6.0 times the spacer thickness. However, if the thickness is too large, the compressive stress of the sheet-like material may break through the ion exchange membrane ³ , so it is preferable to use an electrode material of the present invention with a compressive stress of 9.8 N/cm2 or less. The electrode material of the present invention can also be used in a stack of two or three layers. Alternatively, it can be combined with an electrode material of another form.

The following describes each step in manufacturing the electrode material of the present invention. The following describes an example in which the substrate of the electrode material is a nonwoven fabric of carbon fibers.

(Non-woven fabric process)
First, space-forming fibers are added to carbon fiber precursor fibers such as flame-retardant polyacrylonitrile as organic fibers, and the resulting fibers are processed into a nonwoven fabric by any known method. For example, after defibration, a web is produced using a carding machine, and the webs are laminated in cross layers or parallel layers, and then processed into a nonwoven fabric by needle punching, calendaring, water jetting, or the like. The web may be of a single composition, or may have a layer structure with different compositions, with each layer assigned the functions of low resistance and high liquid permeability.

(attaching process)
Carbon particles and a carbonizing binder precursor are impregnated onto the nonwoven fabric obtained in the nonwoven fabric formation process. To impregnate the carbon particles and the carbonizing binder precursor, the carbonizing binder precursor and the carbon particles are dispersed in a solvent such as water or alcohol to which a binder (temporary adhesive) that disappears upon carbonization, such as polyvinyl alcohol, has been added. The structure is then immersed in this dispersion and heated to dry. Any excess liquid remaining after immersion of the structure can be removed by passing it through nip rollers with a predetermined clearance, or by scraping the surface of the excess dispersion liquid after immersion in the dispersion with a doctor blade or the like, or by squeezing it with nip rollers.

Then, dry in an air atmosphere, for example, at 80 to 150°C.

(Carbonization process)
The carbonization step is carried out to calcinate the product obtained after the impregnation step. This allows the carbon particles, carbon fibers, and carbonized binder to be bound together. In the carbonization step, it is preferable to thoroughly remove decomposition gases generated during carbonization. For example, heating is carried out in an inert atmosphere (preferably a nitrogen atmosphere) at a temperature of 500°C or higher and lower than 2000°C. The heating temperature is preferably 600°C or higher, more preferably 800°C or higher, even more preferably 1000°C or higher, and even more preferably 1200°C or higher; more preferably 1400°C or lower, and even more preferably 1300°C or lower.
The heating time in the inert atmosphere is preferably, for example, 1 to 2 hours. This short treatment time sufficiently promotes bonding between the carbon fibers and removal of decomposition gases generated during carbonization.

(Graphitization process)
The graphitization step is a step carried out to sufficiently increase the crystallinity of the carbonaceous material, thereby improving electronic conductivity and oxidation resistance to sulfuric acid solutions in the electrolyte. Heating is preferably performed in an inert atmosphere (preferably a nitrogen atmosphere) at a temperature of 1300°C or higher, which is higher than the heating temperature in the carbonization step, and more preferably 1500°C or higher. The upper limit is preferably 2000°C or lower, in order to impart high electrolyte affinity to the carbonaceous material.

(oxidation process)
In the present invention, a dry oxidation treatment may be performed after the graphitization step. By further performing an oxidation treatment after the graphitization step, oxygen functional groups such as hydroxyl groups, carbonyl groups, quinone groups, lactone groups, and free radical oxides are introduced onto the surface of the electrode material. As a result, the aforementioned O/C ratio of ≥ 1% can be achieved. These oxygen functional groups significantly contribute to the electrode reaction, resulting in a sufficiently low resistance.

The oxidation treatment process can be performed using various treatment processes such as wet chemical oxidation, electrolytic oxidation, and dry oxidation. In the present invention, however, dry oxidation treatment is performed from the viewpoint of workability and manufacturing costs. The oxidation treatment is preferably performed in an air atmosphere. The heating temperature is controlled to a range of 500°C or higher and 900°C or lower. This allows oxygen functional groups to be introduced onto the surface of the electrode material, thereby effectively achieving the above-mentioned effects. The heating temperature is preferably 600°C or higher, more preferably 650°C or higher. Furthermore, it is preferably 800°C or lower, more preferably 750°C or lower.
Similarly to the primary oxidation treatment described above, the oxidation treatment is preferably carried out for, for example, 5 minutes to 1 hour. If the primary oxidation treatment solution is used for less than 5 minutes, there is a risk that the entire carbon electrode material will not be oxidized uniformly. On the other hand, if the primary oxidation treatment solution is used for more than 1 hour, there is a risk that the strength of the carbon electrode material will decrease, resulting in a decrease in productivity.

Furthermore, in the oxidation treatment step, from the viewpoint of maintaining the mechanical strength of the electrode material, it is preferable to adjust the mass yield of the electrode material before and after the oxidation treatment (i.e., the ratio of the mass of the electrode material after the secondary oxidation treatment solution to the mass of the electrode material before the secondary oxidation treatment) to 85% or more and 96% or less. The above mass yield can be adjusted, for example, by appropriately adjusting the treatment time and heating temperature of the dry air oxidation.

The present invention will be explained in more detail below with reference to examples and comparative examples. However, the present invention is not limited to the following examples. In the following, % means "% by mass" unless otherwise specified.

In the examples and comparative examples, the following items were measured. Details of the measurement methods are as follows:

(1) ID/IG by laser Raman spectroscopy
For the carbon fiber and the carbonized binder, the ratio (ID/IG) of the peak intensity ID at 1360 cm ⁻¹ to the peak intensity IG at 1580 cm ⁻¹ was measured as follows.
The carbon fiber and carbonized binder constituting the electrode material were focused using a laser Raman microscope, and then Raman spectroscopy was measured. The Raman spectrum was measured using a laser Raman microscope (Raman-11 manufactured by Nanophoton Inc.) with a 532 nm laser. A diffraction grating of 600 gr/mm was used, and the range of 500 to 2300 cm ^-1 was measured. The obtained Raman spectrum was peak-separated using a Lorentz function, and the peak intensities of the D band near 1350 cm ^-1 and the G band near 1590 cm ^-1 were evaluated.

(2) Recesses An electrode material sample was cut to a size of ⁵ mm2 and, with the surface facing up, attached to a microscope stand with conductive tape. This was subjected to Au vapor deposition treatment, and then set in a scanning electron microscope (SU3800) manufactured by Hitachi High-Technologies Corporation, and the surface was observed. At this time, the width of recesses formed in the carbonized binder portion containing particles was measured at 1000x magnification, and the length was measured at 100x magnification. The length was measured as the linear distance between the start and end points of the recesses. Several areas where two or more recesses could be confirmed were selected within a 100x magnification field of view (2.3 ^mm2 ), and the above-mentioned measurements were performed on any 10 recesses from these, and the average values were used as the width and length of the recesses.

(3) BET specific surface area (m ² /g)
Approximately 50 mg of a sample was weighed out and dried in vacuum for 24 hours at 130° C. The amount of nitrogen adsorption of the dried sample was measured by a gas adsorption method using nitrogen gas using an automatic specific surface area analyzer (Shimadzu Corporation, GEMINI VII), and the BET specific surface area (m ² /g) was determined by a multipoint method based on the BET method.

(4) Total Cell Resistance Each electrode material sample obtained by the method described below was cut into a piece measuring 10 cm in the vertical direction (direction of liquid flow) and 1.6 cm in the width direction, with an electrode area of 16 ^cm2 , to assemble the cell shown in Figure 4. An APS4 membrane manufactured by Asahi Glass Co., Ltd. was used as the ion exchange membrane, and the spacer thickness was 1.5 mm.
The total cell resistance was calculated from the voltage curve after 10 cycles at 100 mA/ ^cm² in a voltage range of 1.70 to 1.00 V using the following formula. The positive electrode electrolyte was a 2.5 mol/L aqueous sulfuric acid solution containing 1.7 mol/L vanadium oxysulfate, and the negative electrode electrolyte was a 2.5 mol/L aqueous sulfuric acid solution containing 1.7 mol/L vanadium sulfate. The amount of electrolyte was in large excess relative to the cell and piping. The flow rate was 20 mL per minute, and measurements were conducted at 30°C. The charge voltage (V _{C50 )} and discharge voltage (V _{D50 )} , corresponding to the quantity of electricity at a charge rate of 50%, were determined from the voltage curve. The current density was defined as I (mA/ ^cm² ), and the total cell resistance (Ω· ^cm² ) was calculated using the following formula:

Total cell resistance (Ω・cm ² ) = (V _C50 - V _D50 )/(2×I)
where:
V _C50 is the charging voltage for the amount of electricity when the charging rate is 50%, calculated from the electrode curve.
V _D50 is the discharge voltage for the amount of electricity when the charge rate is 50% calculated from the electrode curve.
I=current density (mA/cm ² ),
is.

(5) Average Fiber Diameter Five fibers were randomly selected and observed under a scanning electron microscope (1000x magnification) to measure their cross-sectional area. The cross-sectional area was considered to be the cross-sectional area of a fiber with a round cross section, and the fiber diameter was calculated using the following formula. The average value of the diameters of the five fibers was calculated, and this was used as the average fiber diameter of the carbonized binder.
Fiber diameter (μm) = √ (4 x cross-sectional area (μm ² )/3.14)

(6) Basis Weight A sample was cut into a size of 1.6 cm x 10 cm, and its weight was measured using an electronic balance. The same procedure was repeated for three samples, and the basis weight was calculated using the average weight according to the following formula.
Weight per unit area (g/m ² )=average weight of 3 samples (g)/0.0016 (m ² )

(7) Thickness A sample was cut into a size of 1.6 cm x 10 cm, and the thickness was measured at three points using a thickness meter with a φ20 mm measuring terminal under a load of 7 g/cm ² , and the average value was taken as the thickness.

[Example]
In the examples, granular carbon black having a specific surface area of 800 m ² /g was used as the carbon particles.
(No. 1)
80% flame-resistant polyacrylonitrile cotton (carbon fiber precursor fiber) with an average fiber diameter of 16 μm and 20% nylon cotton (space-forming fiber) with an average fiber diameter of 8 μm were mixed and defibrated using a carding machine to form a web. The resulting web was needle-punched at a punching density of 100 g/cm ² to obtain a nonwoven fabric (basis weight 250 g/m ² , thickness 4.4 mm), which served as the fiber substrate. Next, 2.0% of Kao Corporation's Rheodor TWL-120 (nonionic surfactant), 1.5% of polyvinyl alcohol (temporary adhesive), 9.6% of JFE Chemical Corporation's MCP100 (melting point 100°C, particle size 10 μm) as a carbonized binder precursor, and 3.4% of the aforementioned carbon black were added to ion-exchanged water. 3 mm diameter zirconia beads were added to this mixture, and the mixture was pulverized using a bead mill to prepare a dispersion.
The fiber substrate obtained above was immersed in the dispersion thus obtained, and then passed through nip rollers to remove excess dispersion, followed by drying in an air atmosphere at 120° C. for 20 minutes.
Next, the electrode material was hot-pressed at 150°C and a gauge pressure of 10 kg/ ^cm2 . The electrode material was then heated to 1000°C at a heating rate of 5°C/min in nitrogen gas and held at this temperature for 1 hour for carbonization (firing). The electrode material was then heated to 1500°C at a heating rate of 5°C/min in nitrogen gas and held at this temperature for 1 hour for graphitization. Finally, the electrode material was oxidized at 650°C for 5 minutes in an air atmosphere to obtain No. 1 electrode material (basis weight 223 g/ ^m2 , thickness 2.4 mm).
Fig. 1 is a scanning electron microscope (SEM) surface view (magnification: 100x) of the sample of Example No. 1, and Fig. 2 is a scanning electron microscope (SEM) surface view (magnification: 1000x) of the sample of Example No. 1.

(No. 2)
70% flame-resistant polyacrylonitrile cotton (carbon fiber precursor fiber) with an average fiber diameter of 21 μm and 30% polyethylene terephthalate cotton (space-forming fiber) with an average fiber diameter of 8 μm were mixed and defibrated using a carding machine to form a web. The resulting web was needle-punched at a punching density of 100 g/cm ² to obtain a nonwoven fabric (basis weight 280 g/m ² , thickness 4.4 mm) as a fiber substrate. Thereafter, an electrode material was prepared in the same manner as No. 1, and No. 2 electrode material (basis weight 203 g/m ² , thickness 3.3 mm) was obtained.

(No. 3)
As the fiber base material, 80% flame-resistant polyacrylonitrile cotton (carbon fiber precursor fiber) with an average fiber diameter of 21 μm and 20% polyphenylene ether cotton (space-forming fiber) with an average fiber diameter of 43 μm were mixed, and No. 3 electrode material (basis weight 231 g/m ² , thickness 3.0 mm) was obtained in the same manner as No. 1.

(No. 4)
As the fiber substrate, 80% flame-resistant polyacrylonitrile cotton (carbon fiber precursor fiber) with an average fiber diameter of 21 μm and 20% polyethylene terephthalate cotton (space-forming fiber) with an average fiber diameter of 45 μm were mixed, and No. 4 electrode material (basis weight 231 g/m ² , thickness 3.0 mm) was obtained in the same manner as No. 1.

[Comparative Example]
(No. 5)
Felt (350 g/m ² , thickness 3.7 mm) made of flame-resistant polyacrylonitrile fibers with an average fiber diameter of 21 μm was used as the fiber substrate, and carbonization, graphitization, and air oxidation treatments similar to those for No. 1 were carried out, except that the impregnation and drying procedures were omitted, to obtain No. 5 electrode material (basis weight 220 g/m ² , thickness 4.3 mm).

(No. 6)
An electrode material No. 6 (basis weight 191 g/m ² , thickness 2.8 mm) was obtained in the same manner as No. 1, except that a felt (220 g/m ² , thickness 3.1 mm) made of flame-resistant polyacrylonitrile fibers with an average fiber diameter of 21 μm was used as the fiber substrate.

Table 1 shows the measurement results for various items No. 1 to 6 above.

Examples No. 1 to No. 4 have recesses in the carbonized binder measuring 1 to 50 μm in width and 100 μm or longer. Example Nos. 1 to No. 4 support carbon particles while forming recesses, which are spaces that increase contact efficiency with the electrolyte, resulting in low-resistance electrode materials. This is thought to be because the active formation of recesses increases the surface area available for electrode reactions.

Comparative Example No. 5 is an example in which no carbon particles are supported. The lack of carbon particles results in an insufficient specific surface area, resulting in high resistance. Additionally, No. 6 is an example in which carbon particles are supported, but no recesses like those in Nos. 1 to 4 are formed. While the resistance is lower than No. 5, which does not contain carbon particles, the effect is less than that of electrode materials Nos. 1 to 4.

The present invention provides a carbon electrode material that can reduce cell resistance during charge and discharge and has excellent battery energy efficiency, making it useful, for example, as an electrode material for redox flow batteries that use vanadium-based electrolytes. The carbon electrode material of the present invention is suitable for use in flow-type and non-flow-type redox flow batteries, as well as redox flow batteries combined with lithium, capacitor, and fuel cell systems.

REFERENCE SIGNS LIST 1 Current collector plate 2 Spacer 3 Ion exchange membrane 4a, 4b Liquid passage 5 Electrode material 6 Positive electrode electrolyte tank 7 Negative electrode electrolyte tank 8, 9 Pump 10 Liquid inlet 11 Liquid outlet 12, 13 External flow path

Claims (5)

An electrode material comprising carbon fibers, a carbonized binder, and carbon particles,
The carbonized binder has two or more recesses per unit area of 2.3 ^mm2 , each having a width of 1 to 50 μm and a length of 100 μm or more. 2. The electrode material for a redox flow battery according to claim 1, wherein the carbonized binder has a ratio (ID/IG) of a peak intensity ID at 1350 ^cm to a peak intensity IG at 1590 ^cm , as determined by laser Raman spectroscopy, of 0.3 to 1.5. 3. The electrode material for a redox battery according to claim 2 , wherein the ID/IG of the carbonized binder is smaller than the ID/IG of the carbon fiber. The electrode material for a redox flow battery according to any one of claims 1 to 3, characterized in that the carbon fibers have an average fiber diameter of 5 to 30 μm. A redox flow battery using the electrode material described in any one of claims 1 to 4.