Electrode material for redox flow battery and redox flow battery including the same

Abstract

To provide a carbon electrode material for a redox flow battery with excellent battery energy efficiency capable of reducing cell resistance during charging and discharging.SOLUTION: An electrode material for a redox flow battery includes carbon fibers, a carbonizing binder, and carbon particles. The carbonizing binder has recesses of 1 to 50 μm in width and 100 μm or more in length.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrode material for use in a redox flow battery. [Background technology]

[0002] 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 be changed by 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.

[0003] Redox flow batteries are highly safe and offer excellent benefits for scaling up, 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. On the other hand, 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. If low resistance can be imparted to electrode materials, battery efficiency and system efficiency can be improved, which will ultimately contribute significantly to reducing the cost of redox flow batteries.

[0004] 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 a 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-33758 [Patent Document 2] Japanese Patent Application Publication No. 2018-147595 [Patent Document 3] International Publication WO2019 / 049755 [Patent Document 4] International Publication WO2015 / 032667 Summary of the Invention [Problem to be solved by the invention]

[0006] To further promote the widespread use of redox flow batteries, there is a need for inexpensive electrode materials with lower resistance. Generally, lower resistance in electrode materials for redox flow batteries is achieved by increasing the surface area, but it is thought that there exists an optimal pore size and shape to increase contact efficiency with the electrolyte.

[0007] However, the technology of Patent Document 1 simply applies a commercially available dispersion liquid, and although it has the effect of reducing resistance, it is difficult to say that the design takes into consideration the optimal pore size and shape. Furthermore, the void formation method of Patent Document 2 is limited to particles and does not take into consideration the optimal shape. Furthermore, the technologies of Patent Documents 3 and 4 also do not include a method for actively forming voids to improve contact efficiency with the electrolyte, and it is difficult to say that the design takes into consideration the optimal pore size and shape.

[0008] The present invention has been made in view of the above problems, and its object is to provide an electrode material for a redox flow battery and a redox flow battery that can reduce cell resistance during charge and discharge 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 cost reduction. [Means for solving the problem]

[0009] The configuration of the electrode material for a redox flow battery according to the present invention, which has been able to solve the above problems, is as follows. 1. An electrode material for a redox flow battery, comprising carbon fiber, a carbonized binder, and carbon particles, characterized in that the carbonized binder contains recesses having a width of 1 to 50 μm and a length of 100 μm or more. 2. The 1350 cm peak of the carbonized binder determined by laser Raman spectroscopy -1 Peak intensities of I.D. and 1590 cm -1 2. The electrode material for a redox flow battery according to 1 above, wherein the ratio of the peak intensity of the peak to the peak intensity of the peak IG (ID / IG) is 0.3 to 1.5. 3. The electrode material for a redox flow battery according to 1 or 2 above, wherein the ID / IG ratio of the carbonized binder is smaller than the ID / IG ratio 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. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an electrode material suitable for use in vanadium-based redox flow batteries, which have excellent battery energy efficiency due to reduced cell resistance during charge and discharge, 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. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a scanning electron microscope (SEM) surface view (magnification: 100 times) of Example No. 1. [Figure 2] FIG. 2 is a scanning electron microscope (SEM) surface view (magnification: 1000 times) of Example No. 1. [Figure 3]FIG. 3 is a schematic diagram of a redox flow battery in which the electrode material of the present invention is used. [Figure 4] 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. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be described in detail for each constituent element with reference to Figures 3 and 4. Note that the present invention is not limited to the following and can be implemented by making appropriate modifications within the scope of the above and below-described aims, and all such modifications are included in the technical scope of the present invention.

[0013] 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 by spacers 2 along the inner surfaces of the current collector plates 1, 1. 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).

[0014] The electrode material 5 of the present invention is an electrode material that uses a carbon fiber sheet as a substrate, 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.

[0015] [Carbon fiber] The carbon fiber used in the electrode material 5 of the present invention is a fiber obtained by thermal carbonization of an organic fiber precursor (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 to make carbon fibers include acrylic fibers such as polyacrylonitrile; phenolic fibers; PBO fibers such as polyparaphenylenebenzobisoxazole (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.

[0016] The average fiber diameter of the carbon fibers is preferably 5 to 30 μm. If the average fiber diameter is smaller than 5 μm, the liquid permeability will be reduced. On the other hand, if the average fiber diameter is larger than 30 μm, the surface area will be reduced, and even if carbon particles are supported, the cell resistance will be high. Considering the balance between liquid permeability and reaction surface area, the diameter is more preferably 8 to 20 μm.

[0017] 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 the structure include spun yarns and bundled filament yarns made of carbon fibers, nonwoven fabrics, knitted fabrics, and woven fabrics that are carbon fiber sheet-like materials, and 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.

[0018] When a carbon fiber sheet is used as the substrate, its thickness is preferably 1.5 mm to 8.0 mm. 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 the liquid permeability improves, it becomes difficult to achieve the desired resistance reduction effect.

[0019] When a nonwoven fabric is used, the average fiber length is preferably 30 to 100 mm. By setting the average fiber length within the above range, a uniform fiber structure can be obtained.

[0020] Nonwoven fabrics are defined in JIS L 0222, and depending on the manufacturing method (e.g., entanglement, fusion, adhesion), examples include spunbonded nonwoven fabrics, spunlace nonwoven fabrics, needle-punched nonwoven fabrics, resin-bonded nonwoven fabrics, and thermal-bonded nonwoven fabrics. Any of these may be used.

[0021] As described above, carbon fibers are obtained by subjecting organic fiber precursors to a thermal carbonization treatment, and the thermal carbonization treatment preferably includes at least a flame-proofing step and a carbonization (baking) step. However, among these, the carbonization step does not necessarily have to be performed after the flame-proofing step as described above, and the carbonization step may be performed after particles are attached to the flame-proofed fibers, in which case the carbonization step after the flame-proofing step can be omitted.

[0022] Among these, the flame-resistant process refers to a process of heating an organic fiber precursor in an air atmosphere at a temperature of preferably 180°C or higher and 350°C or lower to obtain a flame-resistant organic fiber. 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 fiber while maintaining the carbon fiber morphology without thermal decomposition of the organic fiber, thereby improving the carbonization rate. During the flame-resistant process, the organic fiber may thermally shrink, causing the molecular orientation to collapse and reducing the conductivity of the carbon fiber. Therefore, it is preferable to perform the flame-resistant treatment on the organic fiber under tension or stretching, and more preferably under tension.

[0023] The carbonization step refers to a step of 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 step within the above temperature range, carbonization of the organic fiber progresses, and carbon fiber having a pseudo-graphite crystal structure can be obtained.

[0024] Since organic fibers have different crystallinity, the heating temperature in the carbonization step can be selected depending on the type of organic fiber used as the raw material. For example, when an acrylic resin (preferably polyacrylonitrile) is used 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.

[0025] The flame-proofing step and carbonization step 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, more preferably 15°C / min or less. By setting the temperature rise rate within the above range, it is possible to obtain carbon fibers that retain the shape of the organic fibers and have excellent mechanical properties. Note that the lower limit of the temperature rise rate is preferably 5°C / min or more, taking into account the mechanical properties, etc.

[0026] [Space-forming fibers] The electrode material 5 of the present invention uses space-forming fibers 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 for the electrode material 5, but that do not retain their fibrous form after manufacturing, leaving spaces (depressions) where the fibers were present. The space-forming fibers are destroyed in fiber form by melting or decomposition during, for example, 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.

[0027] The raw material used for the space-forming fiber is not particularly limited as long as it does not retain its fibrous form after carbonization. Examples include aliphatic polymers such as polypropylene, polyethylene, and polyvinyl chloride. 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.

[0028] The residual carbon ratio of the space-forming fiber after heating at 1000°C is preferably 10 to 50%. If it is less than 10%, the binding strength of the carbonized binder described below may be reduced. If it is more than 50%, the molten material derived from the lost fiber may excessively coat the carbon particles, which may lead to high resistance. A more preferable residual carbon ratio is 15 to 40%.

[0029] The fiber diameter of the fibers used for the space-forming fibers is preferably 1 to 50 μm. If the fiber diameter is less than 1 μm, the size of the spaces formed will be small, making it difficult to obtain the effect of improving the contact efficiency with the electrolyte. On the other hand, if the fiber diameter is more than 50 μm, the amount of spaces formed will be significantly reduced, making it difficult to obtain the effect of improving the contact efficiency with the electrolyte. A more preferred fiber diameter is 3 to 30 μm.

[0030] 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 finally produced 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, which significantly impairs springiness and makes it impossible to maintain the structure shape during compression. Furthermore, excessive shrinkage during carbonization treatment causes problems such as large deformation.

[0031] [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.

[0032] 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 obtain 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, the electrolyte is less likely to contact due to surface tension, etc., which may reduce the proportion of particles effective for the reaction. Here, "particle size" refers to the average particle size (D50) at the median 50% diameter in the 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, which is the aggregate of these particles, is 0.01 μm or more. In this way, by appropriately selecting the particle size of the carbon particles, the supported carbon particles can be effectively utilized.

[0033] The BET specific surface area of ​​the carbon particles used in the present invention, calculated from the amount of nitrogen adsorption, is 20 m 2 / g or more is preferable, and 30m 2 / g or more is more preferable, and 40m 2 / g or more is more preferable. 2 If the surface area is less than 2000 m / 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. Although 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, which deteriorates the processability into sheets, etc., the upper limit is generally set to 2000 m / g. 2 The "BET specific surface area determined from the amount of nitrogen adsorption" means the specific surface area calculated from the amount of gas molecules adsorbed when nitrogen molecules are adsorbed onto solid particles.

[0034] The carbon particles used in the present invention are often carbon blacks, such as acetylene black (acetylene soot), oil black (furnace black, oil soot), ketjen black, and gas black (gas soot), which have high reactivity and specific surface area and low crystallinity. Other examples include carbon nanotubes (CNTs), carbon nanofibers, carbon aerogels, mesoporous carbon, graphites, graphene, graphene oxide, N-doped CNTs, boron-doped CNTs, and fullerenes. Carbon blacks are preferred from the standpoint of raw material cost.

[0035] 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 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.

[0036] In the present invention, the mass ratio of the carbonized binder (described below) to the carbon particles is preferably 0.2 to 20, more preferably 0.3 to 10. If the ratio is less than 0.2, the carbon particles tend to fall off, and the carbon particles are not 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, are covered, and the desired low resistance cannot be obtained.

[0037] [Carbonized binder] The carbonized binder used in the present invention is a binder that has electrical conductivity itself and is added as a binder to strongly bind carbon fibers and carbon particles, which are not originally capable of binding together. The carbonized binder used in the present invention has a 1360 cm -1Peak intensities at I.D. and 1580 cm -1 Preferably, the ratio (ID / IG) of peak intensities I to I of the carbon particles is 0.3 to 1.5. By using a highly crystalline carbonized binder with a crystallinity of 0.3 to 1.5, the electronic conduction resistance between the carbon particles and carbon fibers is reduced, resulting in a smooth electronic conduction path between the carbon particles and carbon fibers. Furthermore, since the carbonized binder strongly bonds the carbon fibers together via the carbon particles, an efficient conductive path can be formed, and the resistance-reducing effect of adding the carbon particles described above is more effectively exerted.

[0038] In the present invention, the ID / IG of the carbonized binder is preferably 0.3 or more, more preferably 0.8 or more, in consideration of the balance of affinity with the electrolyte, while in consideration of the above-mentioned electronic conductivity, it is preferably about 1.5 or less.

[0039] In the present invention, the ID / IG ratio of the carbonized binder is preferably smaller than the ID / IG ratio of the carbon fiber, so that even if the carbon fiber has low crystallinity, the carbonized binder coats the carbon fiber with high crystallinity, thereby ensuring oxidation resistance and high electrical conductivity.

[0040] The content of the carbonized binder relative to the total amount of the carbon fibers, 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 content of the carbonized binder in this way, both the carbon fibers and the carbon particles can be sufficiently bound. Note that the upper limit is preferably approximately 60% or less, taking into consideration the permeability of the electrolyte and the suppression of surface coating of the carbon particles.

[0041] The mass ratio of the total content of the carbon particles and the carbonized binder to the total amount of the 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 setting it in this range, both liquid permeability and low resistance can be achieved.

[0042] The type of starting material for the carbonized binder used in the present invention is not particularly limited as long as it can bind carbon fibers and carbon particles. Specifically, it is not particularly limited as long as it exhibits binding properties during carbonization during the preparation 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 resins, benzoxazine resins, epoxide resins, furan resins, vinyl ester resins, melamine-formaldehyde resins, urea-formaldehyde resins, resorcinol-formaldehyde resins, cyanate ester resins, bismaleimide resins, polyurethane resins, and polyacrylonitrile; furfuryl alcohol; and rubbers such as acrylonitrile-butadiene rubber. Commercially available products may also be used.

[0043] 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 (formaldehyde generation and formaldehyde odor at room temperature) do not occur, and there are advantages such as no odor being generated at room temperature. In contrast, in the above-mentioned Patent Document 2, a phenolic resin is used in the carbonization binder precursor, and in addition to the adverse effects described above, 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.

[0044] 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. If the mesophase content is low, the pitch will melt at a relatively low temperature or will be in a liquid state at room temperature. On the other hand, if the mesophase content is high, the pitch will melt at a high temperature and a high carbonization yield will be obtained. When pitches are used as carbonization binders, it is preferable that the mesophase content is high (i.e., the carbonization rate is high), 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 via the carbon particles without excessively coating the surfaces of the carbon particles. Note that the upper limit is preferably, for example, 90% or less, taking into consideration the expression of binding properties.

[0045] From the same viewpoint as above, the melting point of the pitch is preferably 100°C or higher, more preferably 200°C or higher. This not only achieves the above-mentioned effects, but also suppresses odor during the impregnation process, which is preferable in terms of processability. Note that the upper limit is preferably 350°C or lower, for example, in consideration of the development of binding properties.

[0046] (Electrode material characteristics) 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, in the electrode material, 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.

[0047] In conventional technologies, efforts have been made to increase the specific surface area, primarily through micro- to mesopores of several nanometers in size, to achieve low resistance. This is typically achieved through activation treatments such as oxidation. Also, as in Patent Document 2, voids of several micrometers in size are sometimes designed. In contrast, in the present invention, the recesses, which are micrometer spaces with minimal surface tension, are formed to ensure contact of the electrolyte with the carbon surface. Furthermore, continuous recesses with a width of 1 to 50 μm and a length of 100 μm or more allow the electrolyte to pass smoothly through the recesses, presumably resulting in a significant resistance-lowering effect. This is presumably 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 with the electrolyte due to the influence of surface tension, but also significantly increases the difference in size with the space through which the electrolyte preferentially passes (several tens of micrometers), resulting in a biased flow that inhibits the liquid diffusion effect near the micro- to mesopores. On the other hand, a width of more than 50 μm reduces the number of recesses, presumably resulting in a reduced resistance. Furthermore, if the length of the continuous recesses is less than 100 μm, the flow path of the electrolyte is likely to be interrupted, and it is thought that the resistance cannot be reduced.

[0048] The recesses having a width of 1 to 50 μm and a length of 100 μm or more according to the present invention can be confirmed by observing the surface of the electrode material with a scanning electron microscope. The recesses according to the present invention refer to a state in which spaces, such as semicircular, semielliptical, slit, or tunnel-shaped spaces, exist in a portion where the binder is continuously present. In this case, defects or fine holes may be partially formed in the recesses. By forming recesses having a width of 1 to 50 μm and a length of 100 μm or more in the binder portion, the electrolyte can pass smoothly through the recesses, thereby achieving low resistance of the electrode material. The width of the recesses is preferably in the range of 1 to 50 μm, more preferably 4 to 30 μm. If the width is less than 1 μm, efficient contact with the electrolyte cannot be achieved due to the influence of surface tension. On the other hand, if the width exceeds 50 μm, the number of recesses is significantly reduced, 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, and when it is 100 μm or more, the flow path of the electrolyte is not interrupted, and low resistance can be achieved. The length of the recesses can be controlled by changing the fiber length and shape (crimp or straight) of the fibers used as the space-forming fibers.

[0049] In the electrode material of the present invention, recesses with a width of 1 to 50 μm and a length of 100 μm or more are formed in a field of view of 100x magnification (unit area 2.3 mm ) of a scanning electron microscope. 2 It is preferable that two or more recesses can be confirmed. 2 It is preferable to have two or more per unit area in order to obtain the effect of reducing resistance. 2 It becomes difficult for more than two to exist in a hit.

[0050] The weight of the electrode material of the present invention is 100 to 700 g / m when the thickness of the spacer 2 sandwiched between the current collector plate 1 and the ion exchange membrane 3 (hereinafter referred to as "spacer thickness") is 1 to 4 mm. 2 is preferable, and 150 to 400 g / m 2is more preferable. 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 therefore, treatments and usage methods for reducing damage to the ion exchange membrane 3 are extremely important. From the above viewpoint, 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, and examples include 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.

[0051] The thickness of the electrode material of the present invention is preferably at least greater than the thickness of the spacer. For example, it is preferably 1.5 to 6.0 times the thickness of the spacer. However, if the thickness is too large, the compressive stress of the sheet-like material may break through the ion exchange membrane 3. Therefore, it is preferable that the compressive stress of the electrode material of the present invention is 9.8 N / cm. 2 The following are preferably used. The electrode material of the present invention can also be used in a laminate of two or three layers. Alternatively, it can be combined with an electrode material of another form.

[0052] Each step of producing the electrode material of the present invention will be described below, taking as an example a case where the substrate of the electrode material is a nonwoven fabric of carbon fibers.

[0053] (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.

[0054] (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, is 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.

[0055] Thereafter, the mixture is dried in an air atmosphere at, for example, 80 to 150°C.

[0056] (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 bind together. In the carbonization step, it is preferable to thoroughly remove decomposition gases generated during carbonization. For example, the carbonization step is carried out at a temperature of 500°C or higher and lower than 2000°C in an inert atmosphere (preferably a nitrogen atmosphere). 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.

[0057] (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 carried out 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.

[0058] (oxidation process) In the present invention, a dry oxidation treatment may be performed after the graphitization process. By performing an oxidation treatment after the graphitization process, 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 O / C ratio of ≥ 1% can be achieved. These oxygen functional groups significantly contribute to the electrode reaction, resulting in a sufficiently low resistance.

[0059] The oxidation treatment process can be performed using various processes such as wet chemical oxidation, electrolytic oxidation, and dry oxidation. In the present invention, dry oxidation is used 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, 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.

[0060] 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. [Example]

[0061] The present invention will be described 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.

[0062] In the examples and comparative examples, the following items were measured. Details of the measurement methods are as follows.

[0063] (1) ID / IG by laser Raman spectroscopy For carbon fiber and carbonized binder, 1360 cm -1 Peak intensities at I.D. and 1580 cm -1 The ratio of the peak intensity ID to the peak intensity IG (ID / IG) was measured as follows. The carbon fiber and carbonized binder that make up the electrode material were focused on by 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) with a wavelength of 532 nm. A diffraction grating of 600 gr / mm was used, and the wavelength was measured from 500 to 2300 cm. -1 The Raman spectrum was measured in the region of 1350 cm. -1 Nearby D band, 1590cm -1 The peak intensity of the G band near

[0064] (2) Recess The electrode material sample is 5 mm 2The sample was cut to size and attached to a microscope stand with conductive tape with the surface facing up. Au deposition was then performed on the sample, and it was 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 the depressions formed in the carbonized binder containing the 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 depressions. The field of view of 100x magnification (2.3 mm) 2 ) were selected from several areas where two or more recesses could be confirmed, and the above-mentioned measurements were carried out on 10 recesses selected from these, and the average values ​​were taken as the width and length of the recesses.

[0065] (3) BET specific surface area (m 2 / g) Approximately 50 mg of sample was weighed out and vacuum dried at 130°C for 24 hours. The amount of nitrogen adsorption was measured for the dried sample using an automatic specific surface area measuring device (Shimadzu Corporation, GEMINI VII) by a gas adsorption method using nitrogen gas, and the BET specific surface area (m 2 / g) was determined.

[0066] (4) Total cell resistance Each electrode material sample obtained by the method described below was placed in a 16cm electrode area measuring 10cm in the vertical direction (flow direction) and 1.6cm in the width direction. 2 The cell shown in Figure 4 was assembled using an APS4 membrane manufactured by Asahi Glass Co., Ltd. as the ion exchange membrane, and the spacer thickness was 1.5 mm. 100mA / cm 2 The total cell resistance was calculated from the voltage curve after 10 cycles in the 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 liquid flow rate was 20 mL per minute, and measurements were taken at 30°C. The charge voltage V corresponding to the amount of electricity at a charge rate of 50% is C50 , discharge voltage V D50 are calculated from the voltage curve, and the current density is calculated as I (mA / cm2 ) and the total cell resistance (Ω cm 2 ) was sought.

[0067] 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.

[0068] (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 diameter of the five fibers was calculated, and this was used as the average fiber diameter of the carbonized binder. Fiber diameter (μm) = √(4 × cross-sectional area (μm 2 ) / 3.14)

[0069] (6) Metsuke The 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 average weight was used to calculate the basis weight using the following formula. Weight (g / m 2 ) = average weight of 3 samples (g) / 0.0016(m 2 )

[0070] (7) Thickness A sample was cut to 1.6cm x 10cm, and a load of 7g / cm was applied using a thickness gauge with a φ20mm measuring terminal. 2 The thickness was determined as the average value of measurements taken at three points.

[0071] [Example] In the example, carbon particles with a specific surface area of ​​800 m2 / g of granular carbon black was used. (No.1) 80% flame-retardant 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 in a carding machine to form a web. The resulting web was punched to a punching density of 100 g / cm. 2 The nonwoven fabric (weight 250 g / m) was needle punched. 2 A fiber substrate (4.4 mm thick) was obtained. 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-exchange water. 3 mm diameter zirconia beads were added to this mixture, and the mixture was pulverized using a bead mill to create 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, at 150°C and a gauge pressure of 10 kg / cm 2 The electrode was then heated to 1000°C at a rate of 5°C / min in nitrogen gas and held at this temperature for 1 hour for carbonization (sintering). The electrode was then heated to 1500°C at a rate of 5°C / min in nitrogen gas and held at this temperature for 1 hour for graphitization. Finally, the electrode was oxidized at 650°C for 5 minutes in an air atmosphere to produce No. 1 electrode material (basis weight 223 g / m 2 , 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.

[0072] (No.2) 70% flame-retardant 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 in a carding machine to form a web. The resulting web was punched to a punching density of 100 g / cm. 2 The nonwoven fabric (weight 280 g / m) was needle punched. 2 , thickness 4.4 mm) was obtained as a fiber substrate. Subsequently, electrode materials were prepared in the same manner as No. 1, and No. 2 electrode material (basis weight 203 g / m 2 , thickness 3.3 mm).

[0073] (No.3) As the fiber base material, 80% flame-retardant 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) was prepared in the same manner as No. 1. 2 , thickness 3.0 mm).

[0074] (No.4) As the fiber base material, 80% flame-retardant 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) was prepared in the same manner as No. 1. 2 , thickness 3.0 mm).

[0075] [Comparative Example] (No.5) Felt (350 g / m) made of flame-retardant polyacrylonitrile fibers with an average fiber diameter of 21 μm 2 The No. 5 electrode material (weight 220 g / m2) was produced by carbonizing, graphitizing, and air-oxidizing the same process as No. 1, except that the impregnation and drying processes were omitted. 2 , thickness 4.3 mm).

[0076] (No.6) Felt (220 g / m) made of flame-retardant polyacrylonitrile fibers with an average fiber diameter of 21 μm 2The electrode material No. 6 (191 g / m2, thickness 3.1 mm) was used as the fiber substrate in the same manner as No. 1. 2 , thickness 2.8 mm).

[0077] Table 1 shows the measurement results for various items Nos. 1 to 6 above.

[0078] [Table 1]

[0079] Examples 1 to 4 have recesses in the carbonized binder with a width of 1 to 50 μm and a length of 100 μm or more. Examples 1 to 4 support carbon particles while forming recesses that are spaces that increase the contact efficiency with the electrolyte, resulting in low-resistance electrode materials. This is thought to be because the active formation of the recesses increases the surface area effective for the electrode reaction.

[0080] Comparative Example No. 5 is an example in which no carbon particles were supported. The absence of carbon particles resulted in an insufficient specific surface area, resulting in high resistance. Furthermore, No. 6 is an example in which carbon particles were supported but no recesses like those in Nos. 1 to 4 were formed. While the resistance was lower than No. 5, which did not contain carbon particles, the effect was smaller than that of the electrode materials Nos. 1 to 4. [Industrial Applicability]

[0081] According to the present invention, a carbon electrode material can be provided that can reduce cell resistance during charge and discharge and has excellent battery energy efficiency, and is therefore useful as an electrode material for, for example, redox flow batteries that use vanadium-based electrolytes. The carbon electrode material of the present invention is suitably used in flow-type and non-flow-type redox flow batteries, and redox flow batteries combined with lithium, capacitor, and fuel cell systems. [Explanation of symbols]

[0082] 1 Current collector plate 2 spacers 3. Ion exchange membrane 4a,4b Liquid passage 5 Electrode material 6 Positive Electrolyte Tank 7. Anode electrolyte tank 8,9 Pump 10 Liquid inlet 11 Liquid outlet 12,13 External flow path

Claims

1. An electrode material comprising carbon fibers, a carbonized binder, and carbon particles, The electrode material for a redox flow battery is characterized in that the carbonized binder has two or more recesses per unit area of ​​2.3 mm 2 , each recess having a width of 1 to 50 μm and a length of 100 μm or more.

2. The carbonized binder has a surface roughness of 1350 cm determined by laser Raman spectroscopy. -1 Peak strength Degree ID and 1590cm -1 2. The electrode material for a redox flow battery according to claim 1, wherein the ratio (ID / IG) of the peak intensity I to the peak intensity I of

3. 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.

4. 4. The electrode material for a redox flow battery according to claim 1, wherein the carbon fibers have an average fiber diameter of 5 to 30 μm.

5. A redox flow battery using the electrode material according to any one of claims 1 to 4.

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