JP2020149858A - Positive electrode material for redox battery and redox battery including the same - Google Patents

Abstract

To provide a positive electrode material for a redox battery which achieves both low resistance and long life.SOLUTION: A positive electrode material for a redox battery according to the present invention is made of carbonaceous fiber, the number of bonded oxygen atoms is 0.2 to 0.6% of the total number of carbon atoms on the surface of the electrode material, and the BET specific surface area determined from the amount of nitrogen adsorbed is 0.5 m2/g or more. Further, the carbonaceous fiber contains active particles bound by a binder, and the active particles and the binder are less than 20% of the total amount of the electrode material.SELECTED DRAWING: None

Description

The present invention relates to an electrode material made of carbonaceous fibers or an electrode material made of carbonaceous fibers and active particles bound thereto, which is used as a positive electrode of a redox battery using an aqueous electrolyte solution, and a redox battery provided with the same. It is a thing.

Conventionally, electrodes have been mainly developed as having an influence on the performance of a battery. There is a type of electrode that does not become an active material by itself but acts as a reaction field that promotes the electrochemical reaction of the active material. Carbon materials are often used for this type because of its conductivity and chemical resistance. Be done. In particular, carbonaceous fiber non-woven fabrics having chemical resistance, conductivity, and liquid permeability are used for electrodes of redox batteries, which are being actively developed for power storage.

Redox batteries have changed from a type that uses an aqueous solution of iron hydrochloric acid for the positive electrode and an aqueous solution of chromium hydrochloric acid for the negative electrode to a type that uses an aqueous solution of vanadium sulfuric acid with high electromotive force for both electrodes, and the energy density has been increased. Development to increase the concentration of substances is progressing, and the energy density is further increasing.

As shown in FIG. 1, the main configuration of the redox battery is composed of external tanks 6 and 7 for storing electrolytic solutions (positive electrode electrolytic solution, negative electrode electrolytic solution) and an electrolytic cell EC. In the electrolytic cell EC, the ion exchange membrane 3 is arranged between the opposing current collector plates 1 and 1. In the redox battery, while the electrolytic solution containing the active material is sent from the external tanks 6 and 7 to the electrolytic cell EC by the pumps 8 and 9, electrochemical energy conversion, that is, charging is performed on the electrode 5 incorporated in the electrolytic cell EC. Discharge is performed.

Generally, during charging and discharging, the electrolytic cell is circulated between the external tank and the electrolytic cell, so that the electrolytic cell has a liquid flow type structure as shown in FIG. The liquid flow type electrolytic cell is referred to as a single cell, and is used alone or in multiple stages with this as the smallest unit. Since the electrochemical reaction in the liquid flow type electrolytic cell is a heterogeneous phase reaction that occurs on the electrode surface, it generally involves a two-dimensional electrolytic reaction field. If the electrolytic reaction field is two-dimensional, there is a drawback that the amount of reaction per unit volume of the electrolytic cell is small.

Therefore, the electrochemical reaction field has come to be three-dimensionalized in order to increase the reaction amount per unit area, that is, the current density. FIG. 2 is an exploded perspective view of a liquid flow type electrolytic cell having a three-dimensional electrode. In the electrolytic cell, an ion exchange membrane 3 is arranged between two opposing current collector plates 1, 1, and an electrolytic solution along the inner surface of the current collector plates 1, 1 is provided by spacers 2 on both sides of the ion exchange membrane 3. Flow paths 4a and 4b are formed. An electrode material 5 made of a non-woven fabric of carbonaceous fibers or the like is disposed in at least one of the flow passages 4a and 4b, and a three-dimensional electrode is formed in this way. The current collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolytic solution.

In the case of a redox type battery in which vanadium oxysulfate is used as the positive electrode electrolyte and vanadium sulfate is used as the negative electrode electrolytic solution, the electrolytic solution containing V ²⁺ is supplied to the liquid flow path 4a on the negative electrode side during discharge, and the positive electrode is positive. An electrolytic solution containing V ⁵⁺ (actually an ion containing oxygen) is supplied to the side flow path 4b. In the flow path 4a on the negative electrode side, V ²⁺ emits electrons in the three-dimensional electrode 5 and is oxidized to V ³⁺ . The emitted electrons reduce V ⁵⁺ to V ⁴⁺ (actually oxygen-containing ions) in the three-dimensional electrode on the positive electrode side through an external circuit. The redox reaction SO ₄ ^2-of the negative electrode electrolytic solution is insufficient with the, for SO ₄ ^2-becomes excessive in the positive electrolyte, negative electrode SO ₄ ^2-is from the positive electrode side through the ion-exchange membrane 3 It moves to the side and the charge balance is maintained. Alternatively, the charge balance can be maintained by moving H ⁺ from the negative electrode side to the positive electrode side through the ion exchange membrane. When charging, the reaction opposite to discharging proceeds.

The electrode materials for redox batteries are particularly required to have the following performances.

1) Do not cause side reactions other than the desired reaction (high reaction selectivity), specifically, high current efficiency (η _I ).
2) The electrode reaction activity is high, specifically, the cell resistance (R) is small. That is, the voltage efficiency (η _V ) is high.
3) High battery energy efficiency (η _E ) related to 1) and 2) above.
η _E = η _I × η _V
4) Small deterioration due to repeated use (long life), specifically, small decrease in battery energy efficiency (η _E ).

As for the cell resistance (R), the contact resistance between the electrode material such as the carbonaceous fiber aggregate and the current collector plate and the contact resistance between the carbonaceous fibers constituting the electrode material contribute to a large proportion. The contact resistance and its change with time have a great influence on the battery energy efficiency and its change with time.

For example, Patent Document 1 discloses a carbonaceous material having a specific pseudographite microcrystal structure with high crystallinity as an electrode material of an Fe—Cr battery capable of increasing the total energy efficiency of the battery. Specifically, there are pseudographite microcrystals in which the <002> plane spacing obtained by X-ray wide-angle analysis is 3.70Å or less on average, and the crystallite size in the c-axis direction is 9.0Å or more on average. However, it has been proposed to use a carbonaceous material having a total acidic functional group amount of at least 0.01 meq / g as an electrode material for a redox battery.

Further, Patent Document 2 describes a carbonaceous fiber made of polyacrylonitrile fiber as a raw material as an electrode for an electric field layer of an iron-chromium redox battery or the like that enhances the energy efficiency of the battery and improves the charge / discharge cycle life. It has a pseudographite crystal structure with a <002> plane spacing of 3.50 to 3.60Å determined by X-ray wide-angle analysis, and the number of bonded oxygen atoms on the surface of the carbonaceous material is 10 to 25% of the number of carbon atoms. It has been proposed to use a carbonaceous material as an electrode material for a redox battery.

Japanese Unexamined Patent Publication No. 60-232669 Japanese Unexamined Patent Publication No. 5-234612

However, in the techniques of Patent Documents 1 and 2, the total amount of acidic functional groups is 0.01 meq / g or more, or the carbonaceous material is used in order to develop effective wettability between the surface of the carbonaceous material and the electrolytic solution. The number of bonded oxygen atoms on the surface must be 10% or more of the total number of carbon atoms. Therefore, it has been found that there are too many functional groups on the surface of the carbonaceous material and the contact resistance between the single fibers becomes high, and as a result, the cell resistance becomes high and high battery energy efficiency cannot be obtained.

Therefore, an object of the present invention is to provide an electrode material which can improve battery energy efficiency and has little deterioration even after repeated use in view of such circumstances.

The configuration of the electrode material according to the present invention that has solved the above problems is as follows.
1. 1. In a positive electrode material for a redox battery made of carbonaceous fiber, the number of bound oxygen atoms on the surface of the electrode material is 0.2 to 0.6% of the total number of carbon atoms on the surface of the electrode material, and it is obtained from the amount of nitrogen adsorbed. An electrode material having a BET specific surface area of 0.5 m ² / g or more.
2. 2. The electrode material according to 1, wherein the electrode material contains active particles bound to the carbonaceous fiber by a binder, and the active particles and the binder are less than 20% of the total amount of the electrode material.
A redox battery comprising the electrode material according to 3.1 or 2.
A vanadium-based redox battery comprising the electrode material according to 4.1 or 2.

With the above configuration, the electrode material of the present invention can improve the energy efficiency of the battery and has little deterioration even after repeated use.

The electrode material of the present invention is preferably used for the positive electrode of a redox battery, particularly a vanadium-based redox battery. In vanadium-based redox batteries, the reaction rate between the active material and the surface of the electrode material is faster than that of the iron-chromium-based electrolyte, and the contact resistance of the electrode material is relative to the resistance (reaction resistance) associated with the reaction with the electrode material. Tends to be higher. Therefore, the contact resistance between the fibers constituting the electrode material tends to be a problem, and the carbon electrode material of the present invention having the above-mentioned action and effect is particularly useful.

FIG. 1 is a schematic view of a redox battery. FIG. 2 is an exploded perspective view of a liquid flow type electrolytic cell having a three-dimensional electrode preferably used in the present invention.

The electrode material of the present invention consists of carbonaceous fibers or active particles bonded to the carbonaceous fibers by a binder. The carbonaceous fiber preferably has a shape as a non-woven fabric (carbonaceous fiber non-woven fabric) from the viewpoint of handling, processability, manufacturability, etc., but may have a different shape. The non-woven fabric is made by opening infusible or flame-resistant short fibers before firing (carbonization), applying them to a curd, first creating a web consisting of several layers, and then applying it to a needle punching machine. Therefore, it is suitably produced.

The basis weight of the carbonaceous fiber non-woven fabric used for the electrode material of the present invention is preferably 100 to 1000 g / m ² when the thickness of the packed state sandwiched between the diaphragm and the current collector plate is 2 to 3 mm, and particularly 100. ~ 600 g / m ² is desirable. In addition, one with a concave groove on one side is preferably used because of its liquid permeability. In that case, the groove width and groove depth are preferably at least 0.3 mm, particularly 0.5 mm or more. The thickness of the carbonaceous fiber nonwoven fabric is at least larger than the thickness in the filled state, preferably about 1.5 times the thickness in the filled state. However, if the thickness is too thick, the film will be pierced by the compressive stress, so it is preferable to design the compressive stress to be 1 kgf / cm ² or less.

The average fiber diameter of the carbonaceous fibers is preferably about 5 to 20 μm, and the average length is preferably about 30 to 100 mm.

The carbonaceous fiber non-woven fabric is pressure-welded and incorporated into the battery, and the highly viscous electrolytic solution flows through the thin gaps. Therefore, in order to prevent the carbonaceous fiber non-woven fabric from falling off and maintain its shape, the tensile strength should be 0.1 kg / cm or more. It is desirable to do. In addition, in order to improve the contact resistance with the current collector plate, the density of the packing layer sandwiched between the diaphragm and the current collector plate should be 0.05 g / cm ³ or more, and the repulsive force against the electrode surface should be 0.1 kgf / cm ² or more. Is preferable.

Such a carbonaceous fiber nonwoven fabric is premised on the nonwoven fabric structure having specific voids and compressive properties. Specific voids and compression properties are obtained by controlling the conditions of the needle punch in the previous step. That is, the density of the needle punch is set to 150 to 300 / cm ² , preferably 200 to 300 / cm ² , and the needles of the needle punch are easily entangled with infusible fibers or flame resistant fibers alternately, so that the fibers come into contact with each other. In particular, it is preferable to use SB # 40 (Foster Needle), for example, which increases the contact between intersecting fibers.

In the electrode material of the present invention, active particles may be bound to the carbonaceous fibers described above.

The active particles are for obtaining reaction activity and increasing conductivity in the electrode material. Conventionally known ones can be used.

The binder serves as a binder that binds the carbonaceous fibers and the active particles. Here, it refers to a binder material after being manufactured as an electrode material, that is, after carbonization.
The type of binder used in the present invention may be any as long as it can bind carbonaceous fibers and active particles, and specifically, it exhibits binding properties during carbonization during the production of the electrode material of the present invention. It is not particularly limited as long as it is a thing. Examples of such are pitches such as coal tar pitch and coal pitch; phenol resin, benzoxazine resin, epoxide resin, furan resin, vinyl ester resin, melanin-formaldehyde resin, urea-formaldehyde resin, resorcinol-formaldehyde. Examples thereof include resins such as resins, cyanate ester resins, bismaleimide resins, polyurethane resins and polyacrylonitrile; furfuryl alcohols; rubbers such as acrylonitrile-butadiene rubber. Commercially available products may be used for these.

When the electrode material of the present invention has active particles bound to carbonaceous fibers by a binder, the active particles and the binder are preferably less than 20% of the total amount of the electrode material. When it is less than 20%, the energy efficiency of the battery can be increased and the resistance can be reduced, the deterioration of the electrode material due to repeated use can be suppressed, and the life can be extended.

(Characteristics of electrode material)
In the electrode material of the present invention, the number of bonded oxygen atoms on the surface of the carbon electrode material is 0.2% to 0.6% of the total number of carbon atoms on the surface of the carbon electrode material. Hereinafter, the ratio of the number of bound oxygen atoms to the total number of carbon atoms may be abbreviated as O / C. O / C can be measured by surface analysis such as X-ray photoelectron spectroscopy (XPS) and fluorescent X-ray analysis. By using an electrode material having an O / C of 0.2% to 0.6%, high durability can be obtained and low resistance can be maintained. On the other hand, if an electrode material having a large O / C is used, the durability cannot be improved and the life may be shortened.

Further, the electrode material of the present invention satisfies a BET specific surface area of 0.5 m ² / g or more, which is determined from the amount of nitrogen adsorbed. If the BET specific surface area is less than 0.5 m ² / g, the desired low resistance cannot be obtained. The upper limit of the BET specific surface area is not particularly limited from the above viewpoint, but it is preferably about 150 m ² / g or less in consideration of oxidation resistance and the like.

The basis weight of the electrode material of the present invention is 50 to 50 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 0.3 to 3 mm. preferably ^{500g / m 2, 100~400g / m} 2 is 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, from the viewpoint of reducing resistance, the thickness of the ion exchange membrane 3 tends to be thin, and treatment and usage methods for reducing damage to the ion exchange membrane 3 are extremely important. From the above viewpoint, it is more preferable to use a non-woven fabric or paper having a flat surface processed on one side as a base material as the electrode material of the present invention. Any known method can be applied to the flattening method, and examples thereof include a method of applying a slurry to one side of a carbonaceous fiber and drying it; a method of impregnating and drying it on a smooth film such as PET.

The thickness of the electrode material of the present invention is preferably at least larger than the spacer thickness. For example, when a carbonaceous fiber having a low density such as a non-woven fabric is used and graphite particles or a binding carbonaceous material used for the electrode material of the present invention are carried therein, the spacer thickness is 1.5 to 1.5 to. 6.0 times is preferable. However, if the thickness is too thick, the ion exchange membrane 3 may be pierced by the compressive stress of the sheet-like material. Therefore, it is recommended to use the electrode material of the present invention having a compressive stress of 9.8 N / cm ² or less. preferable. In order to adjust the compressive stress and the like according to the basis weight and thickness of the electrode material of the present invention, it is also possible to use the electrode material of the present invention in a laminated manner such as two layers or three layers. Alternatively, it can be combined with another form of electrode material.

(Manufacturing method of electrode material)
Next, a method for producing an electrode material made of carbonaceous fiber will be described. Carbonized fibers having an internal structure and surface properties suitable for redox batteries include polyacrylonitrile, isotropic pitch, mesophase pitch, cellulose, etc., which have undergone initial air oxidation at 200 to 300 ° C. under tension, or phenol, polyparaphenylene benzo. It is obtained by dry oxidation treatment of a carbon material having a pseudo-graphite crystal structure that is fired (carbonized) at 1000 to 3000 ° C. in an inert atmosphere using bisoxazole (PBO) or the like as a raw material. When the active particles are bound to the carbonaceous fiber, the active particles may be bound with a binder before carbonization.

In the above, the carbonization temperature is not limited to the temperature because the crystallinity differs depending on the raw material, and it is preferable to optimize the carbonization temperature according to the raw material. A known method may be used for dry oxidation, but it is desirable to adjust the weight yield after oxidation to 90 to 96% in consideration of the mechanical strength of the material. However, the treatment method is not limited to this, and the same effect can be obtained even if electrolytic oxidation is performed instead of this dry oxidation treatment, for example. Further, in the present invention, the acid-containing functional groups are partially removed by firing (carbonizing) at 900 to 2000 ° C., preferably 900 to 1500 ° C. in an inert atmosphere thereafter, and the contact resistance is increased while maintaining the wettability. A low surface can be obtained.

(battery)
The electrode material of the present invention is used for the positive electrode of a redox battery. In the redox battery, as described above, for example, a diaphragm is arranged between a pair of current collector plates arranged so as to face each other with a gap between them, and at least one of the current collector plates and the diaphragm is provided. The electrode material is pressure-welded and sandwiched, and the electrode material includes an electrolytic cell having a structure containing an electrolytic solution composed of an aqueous solution containing an active material.

Examples of the aqueous electrolytic solution include iron-chromium-based, titanium-manganese-chromium-based, chromium-chromium-based, iron-titanium-based, and the like, in addition to the vanadium-based electrolytic solution as described above, but vanadium-based electrolytic solution is preferable. .. The carbon electrode material aggregate of the present invention uses, in particular, a vanadium-based electrolytic solution having a viscosity of 0.005 Pa · s or more at 25 ° C. or a vanadium-based electrolytic solution containing vanadium ions of 1.5 mol / l or more. It is useful to use for redox batteries.

〔Example〕
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. The present invention is not limited to the following examples. In the following,% means "mass%" unless otherwise specified.

In the examples and comparative examples, the following items were measured. The details of the measurement method are as follows.

1. 1. Number of bound oxygen atoms on the surface of the electrode material O / C with respect to the total number of carbon atoms on the surface of the electrode material
The ULVAC-PHI 5801MC device was used for the measurement of X-ray photoelectron spectroscopy, which is abbreviated as ESCA or XPS.
First, the sample was fixed on the sample holder with an Mo plate, sufficiently exhausted in the preliminary exhaust chamber, and then charged into the chamber of the measurement chamber. A monochrome AlKα ray was used as the radiation source, the output was 14 kV, 12 mA, and the degree of vacuum inside the apparatus was 10-8 torr.
All element scans were performed to examine the composition of surface elements, and narrow scans were performed on the detected and expected elements to evaluate the abundance ratio.
The ratio of the number of surface-bonded oxygen atoms to the total number of surface carbon atoms was calculated as a percentage (%), and O / C was calculated.

2. 2. About 100 mg of a BET specific surface area sample was collected, vacuum dried at 120 ° C. for 12 hours, weighed 90 g, and the BET specific surface area was measured using a specific surface area / pore distribution measuring device Gemini2375 (manufactured by Micromeritics). Specifically, the amount of nitrogen gas adsorbed at the boiling point (-195.8 ° C.) of liquid nitrogen was measured in the range of relative pressure of 0.02 to 0.95, and an adsorption isotherm of the sample was prepared. Based on the results in the range of relative pressure 0.02 to 0.15, the BET specific surface area per weight (unit: m ² / g) was determined by the BET method.

3. 3. Electrode characteristics Each electrode material obtained by the method described later was cut into an electrode area of 10 cm ² having an electrode area of 10 cm in the vertical direction (liquid flow direction) and 1 cm in the width direction, and the cell of FIG. 1 was assembled. This cell was used as the positive electrode. As the negative electrode, the electrode material of Comparative Example 1 described later was cut out in the same manner as described above and assembled into a cell. Using this reddock battery having a positive electrode and a negative electrode, charging and discharging were repeated at a constant current density, and electrode characteristics were measured. A 3 mol / l sulfuric acid aqueous solution of 2 mol / l vanadium oxysulfate was used as the positive electrode electrolytic solution, and a 3 mol / l sulfuric acid aqueous solution of 2 mol / l vanadium sulfate was used as the negative electrode electrolytic solution. The amount of electrolyte was too large for the cell and piping. The liquid flow rate was 6.2 ml per minute, and the measurement was performed at 30 ° C.

(A) Current efficiency: η _I
Beginning in charge, the 1-cycle test of ending the discharge, the current density of the electrode geometry area per 40mA ^/ cm 2 (400mA), the amount of electricity required to charge up to 1.7V _{Q 1} coulombs, up to 1.0V Let Q ₂ and Q ₃ Coulomb be the amounts of electricity taken out by the constant current discharge and the subsequent constant voltage discharge at 1.2 V, and calculate the current efficiency η _I by Equation 1.

(B) Cell resistance: R
The V ³⁺ in negative electrode solution on the theoretical quantity of electricity Q _th required to completely reduce the V ^2+, the electric quantity ratios began taking by discharge and charging rate, determine the charging rate in Equation 2.

The charge voltage _VC50 and the discharge voltage V _D50 corresponding to the amount of electricity when the charge rate is 50% are obtained from the amount of electricity-voltage curve, respectively, and the cell resistance R (Ω · cm ² ) with respect to the geometric area of the electrode is obtained from Equation 3. ..

(C) Voltage efficiency: η _V
Using the cell resistance R obtained by the above method, the voltage efficiency η _V is obtained by the simple method of Equation 4.

Here, E is the cell open circuit voltage 1.432V (measured value) when the charging rate is 50%, and I is the current value 0.4A in constant current charging / discharging.

(D) Energy efficiency: η _E
Using the above-mentioned current efficiency η _I and voltage efficiency η _V , the energy efficiency η _E is obtained by Equation 5.

(E) Changes over time in the charge / discharge cycle After measuring (a), (b), (c), and (d), the cell is subsequently used, and the cell voltage is 1.0 to 1.0 at a constant current density of 40 mA / cm ^2. Charging and discharging are repeated between 1.7V. After the lapse of the specified cycle, the measurements (a), (b), (c), and (d) are performed again to obtain η _E and the amount of change Δη _E from the initial stage.

The characteristics of the electrodes for electrolytic cells such as redox batteries are mainly the above-mentioned current efficiency η _I , voltage efficiency η _V (cell resistance R) and energy efficiency η _E (product of η _I and η _V ). It is expressed by the charge / discharge cycle stability (life) of efficiency.

(Example 1)
After making polyacrylonitrile fibers with an average fiber diameter of 16 μm flame-resistant in air at 200 to 300 ° C., felt needle SB # 40 (Foster Needle) and punching density using the short fibers (length of about 80 mm) of the flame-resistant fibers. A non-woven fabric having a grain size of 600 g / m ² and a thickness of 5.0 mm was prepared by felting at 250 lines / cm ² . Next, the temperature is raised to 1500 ° C. in nitrogen gas at a heating rate of 10 ° C./min, held at this temperature for 1 hour, carbonized and cooled, and the weight yield is 95% at 700 ° C. in air. The treatment gave a carbonized fiber non-woven fabric. Further, the temperature was raised to 1300 ° C. in nitrogen gas at a heating rate of 10 ° C./min, and the temperature was maintained at this temperature for 1 hour.

(Example 2)
After making polyacrylonitrile fibers with an average fiber diameter of 16 μm flame-resistant in air at 200 to 300 ° C., felt needle SB # 40 (Foster Needle) and punching density using the short fibers (length of about 80 mm) of the flame-resistant fibers. A non-woven fabric having a grain size of 600 g / m ² and a thickness of 5.0 mm was prepared by felting at 250 lines / cm ² . Next, the temperature is raised to 1500 ° C. in nitrogen gas at a heating rate of 10 ° C./min, held at this temperature for 1 hour, carbonized and cooled, and the weight yield is 95% at 700 ° C. in air. The treatment gave a carbonized fiber non-woven fabric. Further, the temperature was raised to 1100 ° C. in nitrogen gas at a heating rate of 10 ° C./min, and the temperature was maintained at this temperature for 1 hour.

(Example 3)
After making polyacrylonitrile fibers with an average fiber diameter of 16 μm flame-resistant in air at 200 to 300 ° C., felt needle SB # 40 (Foster Needle) and punching density using the short fibers (length of about 80 mm) of the flame-resistant fibers. A non-woven fabric having a grain size of 600 g / m ² and a thickness of 5.0 mm was prepared by felting at 250 lines / cm ² . Next, the temperature is raised to 1500 ° C. in nitrogen gas at a heating rate of 10 ° C./min, held at this temperature for 1 hour, carbonized and cooled, and the weight yield is 95% at 700 ° C. in air. The treatment gave a carbonized fiber non-woven fabric. Further, the temperature was raised to 900 ° C. in nitrogen gas at a heating rate of 10 ° C./min, and the temperature was maintained at this temperature for 1 hour.

(Example 4)
For the non-woven fabric produced in Example 1, expanded graphite particles having a particle size of 5 μm and a BET specific surface area of 10 m ² / g were used as active particles, and pitches of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. were used as binders. These were wet-coated. The amount of active particles after drying was 9 g / m ² , and the amount of binder was 26 g / m ² . The treatment from carbonization was carried out in the same manner as in Example 2.

(Example 5)
For the non-woven fabric produced in Example 1, carbon black particles having a particle size of 0.04 μm and a BET specific surface area of 1800 m ² / g were used as active particles, and pitches of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. were used as binders. These were used and adhered wet. The active particles after drying were 5 g / m ² , and the binder was 31 g / m ² . The treatment from carbonization was carried out in the same manner as in Example 2.

(Example 6)
For the non-woven fabric produced in Example 1, carbon black particles having a particle size of 0.04 μm and a BET specific surface area of 1800 m ² / g were used as active particles, and pitches of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. were used as binders. These were used and adhered wet. Active particles after drying 10 g / ^{m 2,} binder was 62 g / ^{m 2.} The treatment from carbonization was carried out in the same manner as in Example 2.

(Comparative Example 1)
After making polyacrylonitrile fibers with an average fiber diameter of 16 μm flame-resistant in air at 200 to 300 ° C., felt needle SB # 40 (Foster Needle) and punching density using the short fibers (length of about 80 mm) of the flame-resistant fibers. A non-woven fabric having a grain size of 600 g / m ² and a thickness of 5.0 mm was prepared by felting at 250 lines / cm ² . Next, the temperature is raised to 1500 ° C. in nitrogen gas at a heating rate of 10 ° C./min, held at this temperature for 1 hour, carbonized and cooled, and the weight yield is 95% at 700 ° C. in air. The treatment gave a carbonized fiber non-woven fabric.

(Comparative Example 2)
After making polyacrylonitrile fibers with an average fiber diameter of 16 μm flame-resistant in air at 200 to 300 ° C., felt needle SB # 40 (Foster Needle) and punching density using the short fibers (length of about 80 mm) of the flame-resistant fibers. A non-woven fabric having a grain size of 600 g / m ² and a thickness of 5.0 mm was prepared by felting at 250 lines / cm ² . Next, the temperature was raised to 1500 ° C. in nitrogen gas at a heating rate of 10 ° C./min, held at this temperature for 1 hour, carbonized and cooled to obtain a carbonaceous fiber non-woven fabric.

(Comparative Example 3)
After making polyacrylonitrile fibers with an average fiber diameter of 16 μm flame-resistant in air at 200 to 300 ° C., felt needle SB # 40 (Foster Needle) and punching density using the short fibers (length of about 80 mm) of the flame-resistant fibers. A non-woven fabric having a grain size of 600 g / m ² and a thickness of 5.0 mm was prepared by felting at 250 lines / cm ² . Next, the temperature is raised to 1500 ° C. in nitrogen gas at a heating rate of 10 ° C./min, held at this temperature for 1 hour, carbonized and cooled, and the weight yield is 95% at 700 ° C. in air. The treatment gave a carbonized fiber non-woven fabric. Further, the temperature was raised to 700 ° C. in nitrogen gas at a heating rate of 10 ° C./min, and the temperature was maintained at this temperature for 1 hour.

(Comparative Example 4)
For the non-woven fabric produced in Example 1, carbon black particles having a particle size of 0.04 μm and a BET specific surface area of 1800 m ² / g were used as active particles, and pitches of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. were used as binders. These were used and adhered wet. The active particles after drying were 5 g / m ² , and the binder was 31 g / m ² . The treatment from carbonization was carried out in the same manner as in Comparative Example 1.

(Comparative Example 5)
For the non-woven fabric produced in Example 1, carbon black particles having a particle size of 0.04 μm and a BET specific surface area of 1800 m ² / g were used as active particles, and pitches of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. were used as binders. These were used and adhered wet. The active particles after drying were 5 g / m ² , and the binder was 31 g / m ² . The treatment from carbonization was carried out in the same manner as in Comparative Example 2.

(Comparative Example 6)
For the non-woven fabrics produced in Examples 1 to 3, carbon black particles having a particle size of 0.04 μm and a BET specific surface area of 1800 m ² / g were used as active particles, and a pitch of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. was used as a binder. Classes were used and these were wetted. The active particles after drying were 5 g / m ² , and the binder was 31 g / m ² . The treatment from carbonization was carried out in the same manner as in Comparative Example 3.

(Comparative Example 7)
For the non-woven fabric produced in Example 1, carbon black particles having a particle size of 0.04 μm and a BET specific surface area of 1800 m ² / g were used as active particles, and pitches of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. were used as binders. These were used and adhered wet. Active particles after drying 33 g / ^{m 2,} binder was 207 g / ^{m 2.} The treatment from carbonization was carried out in the same manner as in Example 2.

Table 1 shows the basis weight, thickness, BET specific surface area, and O / C ratio by XPS analysis of the carbonaceous fiber nonwoven fabrics obtained in the above Examples and Comparative Examples together with the production conditions.

Table 1 shows the results of measuring the electrode performance (second cycle and 100th cycle of the charge / discharge cycle) of all the above-mentioned processed products with a spacer thickness of 2.0 mm.

As is clear from the results in Table 1, the carbonaceous fiber nonwoven fabrics of Examples 1 to 6 had high voltage efficiency and excellent energy efficiency. Moreover, an increase in contact resistance between fibers during long-term use, that is, a decrease in conductivity can be suppressed, and a change over time in energy efficiency during a long-term charge / discharge cycle is also suppressed.

On the other hand, in Comparative Examples 1, 3, 4, and 6 in which the acidic functional group is not controlled or insufficiently controlled, the voltage efficiency is low, the energy efficiency changes significantly over a long period of time, and the acidic functional group is not controlled. In Comparative Examples 2 and 5 in which the grouping process was not performed, the voltage efficiency was extremely low. In Comparative Example 7 in which the amount of additives was large with respect to the carbonaceous fiber, the flow of the liquid was poor, the voltage efficiency was low, and the change in energy efficiency was large due to dropping off even in the long term.

According to the present invention, it is possible to provide an electrode material having both low resistance and long life, which is useful as a positive electrode material for a redox battery. The electrode material of the present invention is suitably used for flow type and non-flow type redox flow batteries, redox batteries combined with lithium, capacitor, and fuel cell systems.

1 ... current collector plate, 2 ... spacer, 3 ... ion exchange membrane, 4a, b ... liquid passage, 5 ... electrode material, 6 ... positive electrode liquid tank, 7 ... negative electrode liquid tank, 8, 9 ... pump, 10 ... liquid Inlet,
11 ... Liquid outlet

Claims (4)

In the positive electrode material for redox batteries made of carbonaceous fiber
The number of bonded oxygen atoms on the surface of the electrode material is 0.2 to 0.6% of the total number of carbon atoms on the surface of the electrode material, and
An electrode material having a BET specific surface area of 0.5 m ² / g or more, which is determined from the amount of nitrogen adsorbed. Contains active particles bound to the carbonaceous fiber by a binder,
The electrode material according to claim 1, wherein the active particles and the binder are less than 20% of the total amount of the electrode material. A redox battery comprising the electrode material according to claim 1 or 2. A vanadium-based redox battery comprising the electrode material according to claim 1 or 2.