JP2000357521A - Carbon electrode material for redox flow battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon electrode material for a redox flow battery capable of reducing internal resistance (cell resistance) of an electrolytic container and enhancing the overall efficiency of the battery. SOLUTION: The carbon electrode material for a redox flow battery using an aqueous electrolyte has a crystallite size in the direction of (a) axis as determined by X-ray wide angle diffraction of 30-80 Å. The carbon electrode material preferably has an amount of surface acidic functional groups as determined by XPS surface analysis of 0.2% or more and 1.2% or less of the number of carbon atoms on the whole surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon electrode material for a redox flow battery using an aqueous electrolyte solution, and is particularly useful for a vanadium redox flow battery.

[0002]

2. Description of the Related Art Conventionally, electrodes have been developed with emphasis on the performance of batteries. Some electrodes do not become active materials themselves, but work as a reaction field to promote the electrochemical reaction of the active material.For this type, carbon materials are often used due to their conductivity and chemical resistance. Can be In particular, a carbon fiber aggregate having chemical resistance, conductivity, and liquid permeability is used for an electrode of a redox flow battery which is actively developed for power storage.

[0003] Redox flow batteries have a higher energy density from a type using an aqueous hydrochloric acid solution of iron for the positive electrode and an aqueous solution of chromium hydrochloric acid for the negative electrode, to a type using a high-electromotive force aqueous solution of vanadium sulfuric acid for both electrodes. Recently, developments for further increasing the concentration of the active material have been advanced, and the energy density has been further increased.

The main structure of a redox flow type battery is, as shown in FIG. 1, composed of external tanks 6 and 7 for storing an electrolytic solution and an electrolytic cell EC, and pumps 8 and 9 for supplying an electrolytic solution containing an active material to the external tank. While being sent from 6, 7 to the electrolytic cell EC, electrochemical energy conversion, that is, charge / discharge is performed on the electrodes incorporated in the electrolytic cell EC.

In general, during charging and discharging, an electrolytic solution is circulated between an external tank and an electrolytic bath, so that the electrolytic bath has a liquid flow type structure as shown in FIG. The liquid flow type electrolytic cell is referred to as a single cell, which is used as a minimum unit and is used alone or in a multi-layered structure. Since the electrochemical reaction in the liquid flowing type electrolytic cell is a heterogeneous phase reaction occurring on the electrode surface, it generally involves a two-dimensional electrolytic reaction field. When the electrolytic reaction field is two-dimensional, there is a disadvantage that the reaction amount per unit volume of the electrolytic cell is small.

In order to increase the amount of reaction per unit area, that is, the current density, three-dimensional electrochemical reaction fields have been used. FIG. 2 is an exploded perspective view of a liquid flow type electrolytic cell having three-dimensional electrodes. In the electrolytic cell, an ion exchange membrane 3 is arranged between two opposing current collector plates 1 and 1, and the current collector plates 1 and 1 are disposed on both sides of the ion exchange membrane 3 by spacers 2.
Are formed along the inner surface of the cell. An electrode material 5 such as a carbon fiber aggregate is provided in at least one of the flow passages 4a and 4b, and thus a three-dimensional electrode is formed. The current collector 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolytic solution.

[0007] In the case of a redox flow battery using a sulfuric acid aqueous solution of vanadium oxysulfate as the positive electrode electrolyte and vanadium sulfate as the negative electrode electrolyte, during discharge, the electrolyte containing V ²⁺ is ^supplied to the liquid flow path 4a on the negative electrode side. And an electrolyte containing V ⁵⁺ (actually, ions containing oxygen) is supplied to the flow path 4b on the positive electrode side. 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 pass through an external circuit and enter V ⁵⁺ in the three-dimensional electrode on the positive electrode side.
To V ⁴⁺ (actually an ion containing oxygen). 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 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. At the time of charging, a reaction reverse to that of discharging proceeds.

As the characteristics of the electrode material for a vanadium-based redox flow battery, the following performance is particularly required.

1) No side reaction other than the intended reaction should occur (high reaction selectivity), specifically, high current efficiency (η _I ). 2) High electrode reaction activity, specifically 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) Deterioration due to repeated use is small (long life), and specifically, the amount of decrease in battery energy efficiency (η _E ) is small.

For example, Japanese Patent Application Laid-Open No. 60-232669 discloses that the <002> plane spacing determined by X-ray wide angle analysis is 3.70 ° or less on average, and the crystallite size in the c-axis direction is It has been proposed to use a carbonaceous material having pseudographite crystallites of 9.0 ° or more and having a total acidic functional group content of at least 0.01 meq / g as an electrode material for an electrolytic cell of a redox flow battery.

Japanese Unexamined Patent Publication (Kokai) No. 5-234612 discloses a carbonaceous fiber made of polyacrylonitrile-based fiber having a <002> plane spacing of 3.0 obtained by X-ray wide-angle analysis.
A carbonaceous material having a pseudographite crystal structure of 50 to 3.60 ° and having the number of bonded oxygen atoms on the surface of the carbonaceous material of 10 to 25% of the number of carbon atoms is used as an electrode material for an electrolytic cell of a redox flow battery. It has been proposed to use.

[0012]

However, JP-A-60-232669 and JP-A-5-234612 disclose that in order to exhibit effective wettability between the carbonaceous material surface and the electrolyte, Total acidic functional group content is 0.01me
q / g or more, or determined by X-ray wide-angle analysis <00
2> Since the plane spacing was 3.50 ° or more and the number of bonded oxygen atoms on the surface of the carbonaceous material was required to be 10% or more of the number of carbon atoms, carbonization at a low temperature had to be performed to satisfy this condition. Thus, there was a problem that the conductivity of carbon could not be increased. Further, the contact resistance between the surface of the carbonaceous material and the current collector plate is increased due to too many functional groups. As a result, the cell resistance is increased, and a problem that high energy efficiency cannot be obtained is also a problem. Further, for the above-mentioned reasons, since the crystallinity of carbon cannot be increased, oxidation resistance is not sufficient, particularly in a redox flow battery in which the electrolyte contains 1.5 mol / l or more of vanadium ions, and the charge / discharge cycle is repeated. It was found that the cell resistance increased and the change (decrease rate) in energy efficiency was large.

In view of the above, an object of the present invention is to provide a carbon electrode material for a redox flow battery capable of reducing the internal resistance (cell resistance) of an electrolytic cell and increasing the overall efficiency of the battery. is there.

[0014]

Means for Solving the Problems The present inventors have made intensive studies to achieve the above object, and found that the above object can be achieved by controlling the crystallite size in the a-axis direction to a specific range. The present inventors have further found that it is effective to suppress the surface acidic functional group content of the carbon electrode material to be lower than before, and have completed the present invention.

That is, the carbon electrode material of the present invention is a carbon electrode material for a redox flow battery using an aqueous electrolyte, and has a crystallite size of 30 in the a-axis direction obtained by X-ray wide-angle analysis. Å80 °. According to the carbon electrode material of the present invention, the crystallites of carbon have an appropriate size, the conductivity of the carbon electrode material itself is improved, and thereby the cell resistance can be reduced and the energy efficiency can be improved,
Good application of surface acidic functional groups to the edge surface around the crystallites is possible, and good reactivity of the active material is obtained. As a result, it is possible to reduce the internal resistance of the electrolytic cell by increasing the conductivity of the electrode material itself and obtaining moderate wettability, thereby increasing the voltage efficiency and the battery energy efficiency.

In the above, it is preferred that the amount of surface acidic functional groups determined by XPS surface analysis be 0.2% or more and 1.2% or less of the total number of surface carbon atoms. Thereby, the wettability with the aqueous electrolyte solution can be appropriately given while the contact resistance on the electrode material surface is kept low, and the above-described effects can be obtained more reliably.

The carbon electrode material of the present invention is preferably used for a vanadium redox flow battery.
In a vanadium-based redox flow battery, the wettability with the above-mentioned electrolyte is relatively good, so that the above-described effects are more remarkable. Further, in the battery, since the contact resistance between the fibers constituting the electrode material and the surface of the electrode material with respect to the current collector plate tends to be particularly problematic, the carbon electrode material of the present invention having the above-mentioned effects is particularly useful.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The carbon electrode material for a redox flow battery of the present invention is made of a carbonaceous material, and its structure and microstructure are not particularly limited, but those capable of increasing the electrode surface area are preferable. Specifically, spun yarn, filament bundled yarn, non-woven fabric, knitted fabric, woven fabric, special knitted fabric (Japanese Unexamined Patent Publication No.
No. 200447), or a carbonaceous fiber aggregate having a hybrid structure thereof, a porous carbon body, a carbon-carbon composite, a particulate carbon material, and the like. Among these, a sheet-like material made of carbonaceous fiber is preferable from the viewpoint of handling, workability, manufacturability and the like.

The basis weight of the sheet-like material or the like depends on the structure thereof, but the thickness of the filled state sandwiched between the diaphragm and the current collector is 2 to 3 times.
When used in mm, 100~1000g / m ^2, in the case of non-woven tissue 200 to 600 g / m ² is desirable. A nonwoven fabric or the like having a groove on one side is preferably used because of its liquid permeability. In this case, the groove width and the groove depth are at least 0.3 mm, preferably 0.5 mm or more. It is desirable that the thickness of the carbonaceous fiber sheet is at least larger than the thickness in the above-described filled state, and that the thickness of the carbonized fiber sheet in the case of a low-density nonwoven fabric is about 1.5 times the thickness in the filled state. However, if the thickness is too thick, it will break through the film with compressive stress,
Preferably, the compressive stress is designed to be 1 kgf / cm ² or less.

The average fiber diameter of the carbonaceous fibers is 5
About 20 μm is preferable, and the average length is 30 to 100 m.
m is preferable.

The carbonaceous fiber sheet is assembled by being pressed into the battery, and the electrolyte flows through the thin gap. However, since the viscosity of the electrolyte may be high, the tensile strength is set to 0.1 kg so as not to fall off. / Cm ² or more is desirable for shape retention. In order to improve the contact resistance with the current collector plate, the density of the packed layer sandwiched between the diaphragm and the current collector plate is set to 0.05 g / cm ³ or more and the repulsive force against the electrode surface is set to 0.1 kgf / cm. cm ² or more.

Furthermore, the carbon electrode material of the present invention was obtained by X-ray wide angle analysis in order to achieve both the originally required conductivity as an electrode material and the cell bonding property as an electrode for a pressure-contact type electrolytic layer.
The crystallite size (La) in the axial direction is 30 to 80 °, and the amount of surface acidic functional groups determined by XPS surface analysis is adjusted to 0.2% to 1.2% of the total number of surface carbon atoms. .
Preferably, La is 35 to 80 °, and the amount of surface acidic functional groups is 0.2 to 1.1%, and more preferably, La is 4 to 80%.
0 to 79 ° and the amount of surface acidic functional groups is 0.3 to 1.0
%.

The crystallite size in the a-axis direction means the spread of the net plane in the carbon crystallites. When the crystallite size in the a-axis direction is less than 30 °, the internal resistance of the battery (cell resistance) Of the electrode material cannot be ignored, as a result, the cell resistance increases (voltage efficiency decreases) and energy efficiency decreases. On the other hand, if it is larger than 80 °, it becomes impossible to provide a hydrophilic group which affects the wettability of the electrolytic solution, and the reactivity of the active material is significantly reduced, so that the internal resistance of the battery is increased. The size of the crystallite in the a-axis direction is X
It is calculated from the half width of the <10> plane diffraction peak obtained by line wide angle analysis.

When the amount of the surface acidic functional group is less than 0.2%, the wettability of the electrolytic solution is poor and the cell resistance is significantly increased. This is because the carbon atoms themselves are hydrophobic,
It is considered that when the number of the acidic functional groups of the hydrophilic group is small, water is easily repelled. On the other hand, when the content is 1.2% or more, the presence of the functional group greatly affects the contact between the fibers of the electrode material and the fibers.
The conductivity between the current collectors is undesirably hindered. The above-mentioned surface acidic functional group amount refers to the amount of hydroxyl groups or carboxyl groups that can be replaced with silver ions by silver nitrate treatment among the oxygen-containing functional groups, and indicates the surface carbon ion amount of the surface carbon ions detected by XPS surface analysis. Ratio to the number of atoms (percentage)
Expressed as

Carbonaceous materials having such crystallinity and surface characteristics include polyacrylonitrile, isotropic pitch, mesophase pitch, cellulose, phenol, and polyparaphenylene that have been subjected to initial air oxidation (flame resistance) at 200 to 300 ° C. under tension. It is manufactured using benzobisoxazole (PBO) or the like as a raw material. Among them, particularly, the weight average molecular weight (Mw) of polyacrylonitrile is set to 20000 to 500.
Those adjusted to 000 are preferred as raw materials for flame resistance. Lower firing (carbonization) for higher molecular weights
Although a predetermined La can be obtained even at a temperature, the orientation of crystallites tends to be relatively high at the same time, the bending elastic modulus of the single fiber increases, the compressive elastic modulus at the time of cell joining decreases, and When the molecular weight is small, crystallization of the single fiber does not proceed during firing, and a turbostratic structure tends to develop and the conductivity does not tend to improve. Such polyacrylonitrile fibers having a predetermined molecular weight are flame-resistant by a known method.

The flame-resistant raw material is 100
It is fired at 0 to 1800 ° C. to become a carbon material having a pseudo-graphite crystal structure. Since the crystallinity differs depending on the raw material and its molecular weight, the carbonization temperature is not particularly limited to the temperature, and it is necessary to optimize the carbonization temperature according to the raw material. That is, La is
It can be controlled by a balance between the molecular weight of the raw material and the carbonization temperature.

Further, dry oxidation treatment is performed at a predetermined oxygen concentration,
If necessary, the functional groups may be partially reduced in the presence of hydrogen gas. For the dry oxidation, a known method can be adopted, but in order to obtain an appropriate amount of surface acidic functional groups in the electrode material, it is desirable to adjust the weight yield after the oxidation treatment to 90 to 96%.

Next, the respective methods of measuring the crystallite size (La) in the a-axis direction, the XPS surface analysis, the total amount of acidic functional groups, the conductive resistance of a single fiber, and the electrode performance employed in the present invention will be described. .

1. Crystallite size in the a-axis direction (La) The electrode material is pulverized in an agate mortar to a particle size of about 10 μm, and about 5% by weight of the sample, high purity silicon powder for X-ray standard is used as an internal standard material. , And packed in a sample cell, and a wide angle X-ray is measured by a diffractometer method using CuKα radiation as a radiation source.

For the correction of the curve, the following simple method is used without performing correction for the so-called Lorentz factor, polarization factor, absorption factor, atomic scattering factor and the like. That is, the actual intensity from the base line of the peak corresponding to the <10> diffraction is plotted again to obtain the <10> corrected intensity curve. A line parallel to the angle axis drawn to half the peak height of this curve is expressed by the following equation (1) from the length of the line segment (half width β) that intersects the corrected intensity curve.
To determine the size of the crystallite.

[0031]

(Equation 1) Here, the wavelength λ = 1.5418 °, the structure coefficient k2 = 1.
84 and θ indicate the <10> diffraction angle, and β indicates the half value width of the <10> diffraction peak.

2. XPS Surface Analysis A device used for measurement of X-ray photoelectron spectroscopy, which is abbreviated as ESCA or XPS, is Shimadzu ESCA750, and ESCAPAC760 is used for analysis.

Each sample was immersed in a solution of silver nitrate in acetone,
The proton of the acidic functional group is completely replaced with silver, washed with acetone and water, punched out to a diameter of 6 mm, attached to a heated sample stand with a conductive paste, and subjected to analysis. Before the measurement, the sample is heated to 120 ° C. and vacuum degassed for 3 hours or more. MgKα radiation (1253.6 eV) is used as the radiation source, and the degree of vacuum in the apparatus is set to 10 ⁻⁷ torr.

The measurement was performed on the Cls and Ag3d peaks, and each peak was measured using ESCAPAC760 (JH Sco
(based on the field correction method) to determine the respective peak areas. The ratio of the obtained area multiplied by the relative intensity of 1.00 for Cls and 10.68 for Ag3d is the atomic number ratio, and the amount of surface acidic functional groups to the total number of surface carbon atoms is (the number of surface silver atoms) / Number of surface carbon atoms) is calculated as a percentage (%).

3. Specific Resistance of Single Fiber Measured according to “6.7 Volume Resistivity” described in JIS R7601 (1986).

4. Electrode performance A small cell having an electrode area of 10 cm ² of 1 cm in the vertical direction (liquid flow direction) and 10 cm in the width direction is made, and charge and discharge are repeated at a constant current density to test the electrode performance. 3 m of 2 mol / l vanadium oxysulfate was used for the positive electrode electrolyte.
ol / l sulfuric acid aqueous solution, and 2 mol / l
A 3 mol / l sulfuric acid solution of 1 vanadium sulfate is used.
The amount of electrolyte was set to a large excess with respect to the cells and piping. The liquid flow rate is 6.2 ml per minute, and the measurement is performed at 30 ° C.

(A) Current efficiency: η _{I In a} one-cycle test starting from charging and ending with discharging, the current density was set to 40 mA / cm ² per electrode geometric area.
(400 mA), the quantity of electricity required for charging up to 1.7 V is Q ₁ coulomb, the quantity of electricity taken out by constant current discharge up to 1.0 V, and the subsequent quantity of electricity taken out by constant voltage discharge at 1.2 V are Q ₂ , Q ₃ coulombs, and the current efficiency η _I is obtained by equation (2).

[0038]

(Equation 2) (B) Cell resistance: R The ratio of the amount of electricity taken out by discharging to the theoretical amount of electricity Q _th required to completely reduce V ³⁺ in the negative electrode solution to V ²⁺ is defined as a charging rate. The charging rate is determined in step 3.

[0039]

(Equation 3) Charging voltage V corresponding to the amount of electricity when the charging rate is 50%
_C50 and discharge voltage V _D50 are obtained from the electric quantity-voltage curve, respectively.
・ Calculate cm ² ).

[0040]

(Equation 4) Here, I is a current value of 0.4 A in constant current charging and discharging.

(C) Voltage efficiency: η _V Using the cell resistance R obtained by the above method, the voltage efficiency η _V is obtained by a simple method of Expression 5.

[0042]

(Equation 5) Here, E is the cell open circuit voltage when the charging rate is 50%.
432 V (actual measurement value), and I is a current value of 0.4 A in constant current charging and discharging.

(D) Energy efficiency: η_E Current efficiency η described above_I And voltage efficiency η_V And using Equation 6
More energy efficiency η _E Ask for.

[0044]

(Equation 6) The higher the current efficiency and the voltage efficiency, the higher the energy efficiency. Therefore, the energy loss in charging and discharging is small, and it is determined that the electrode is an excellent electrode.

The carbon electrode material of the present invention is used for a redox flow battery using an aqueous electrolyte. As described above, the redox flow battery has, for example, a diaphragm disposed between a pair of current collectors disposed to face each other with a gap therebetween, and at least one of the diaphragms disposed between the current collector and the diaphragm. The electrode material is provided with 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-based, manganese-chromium-based, chromium-chromium-based, iron-titanium-based and the like, in addition to the vanadium-based electrolyte described above. Vanadium-based electrolytes are preferred. In particular, the carbon electrode material of the present invention is a redox flow using a vanadium-based electrolyte having a viscosity of at least 0.005 Pa · s at 25 ° C. or a vanadium-based electrolyte containing 1.5 mol / l or more of vanadium ions. Useful for batteries.

[0047]

EXAMPLES Examples and the like that specifically show the structure and effects of the present invention will be described below.

Example 1 98 mol% of acrylonitrile
-Polyacrylonitrile fibers having a weight average molecular weight of 75,000 and an average fiber diameter of 16 [mu] m composed of 2 mol% of methyl methacrylate are oxidized in air at 200 to 300 [deg.] C., and then short fibers of the oxidized fibers (about 80 mm in length). It is made into felt by using and the basis weight is 400 g / m ² and the thickness is 4.0 mm.
Was prepared. 100 ° C. while continuously blowing argon gas at 600 cc / min / m ² onto the nonwoven fabric.
/ Min at a heating rate of 1 / min.
Hold for a time, perform carbonization and cool, then 700 ° C in air
To give a weight yield of 93% to obtain a carbonaceous fiber nonwoven fabric. Table 1 shows the results of the XPS surface analysis, the specific resistance of the single fiber, and the electrode performance.

Example 2 98 mol% of acrylonitrile
-Polyacrylonitrile fibers having a weight average molecular weight of 75,000 and an average fiber diameter of 16 μm composed of 2 mol% of methyl methacrylate are flame-resistant in air at 200 to 300 ° C., and then the short fibers (length: about 80 mm) of the flame-resistant fibers are removed. This was used to make a felt to produce a nonwoven fabric having a basis weight of 400 g / m ² and a thickness of 4.0 mm. Argon gas is continuously applied to the nonwoven fabric.
The temperature was raised to 1400 ° C. at a rate of 100 ° C./min while spraying at a rate of 100 cc / min / m ² , held at this temperature for 1 hour, carbonized and cooled, and then weight-collected at 700 ° C. in air. The treatment was carried out until the ratio became 90% to obtain a carbonaceous fiber nonwoven fabric. Table 1 shows the results of the XPS surface analysis, the specific resistance of the single fiber, and the electrode performance.

Example 3 98 mol% of acrylonitrile
-Polyacrylonitrile fibers having a weight average molecular weight of 75,000 and an average fiber diameter of 16 [mu] m composed of 2 mol% of methyl methacrylate are oxidized in air at 200 to 300 [deg.] C., and then short fibers of the oxidized fibers (about 80 mm in length). It is made into felt by using and the basis weight is 400 g / m ² and the thickness is 4.0 mm.
Was prepared. Argon gas was continuously sprayed at 600 cc / min / m ² onto the nonwoven fabric at 100 ° C. /
The temperature was raised to 1600 ° C. at a heating rate of 1 minute, kept at this temperature for 1 hour, carbonized and cooled, and subsequently treated in air at 700 ° C. until the weight yield became 93% to obtain a carbonaceous fiber nonwoven fabric. Obtained. Table 1 shows the results of the XPS surface analysis, the specific resistance of the single fiber, and the electrode performance.

Example 4 97 mol% of acrylonitrile
Weight average molecular weight 2 composed of 3 mol% of vinyl acetate
After oxidizing polyacrylonitrile fibers having an average fiber diameter of 16 μm in air at 200 to 300 ° C. in air, felting is performed using short fibers (about 80 mm in length) of the oxidized fibers to obtain a basis weight of 400 g / m ² and a thickness of 400 g / m ² . A 4.0 mm nonwoven fabric was prepared. Argon gas is continuously applied to the nonwoven fabric for 600 c.
The temperature was raised to 1400 ° C. at a rate of 100 ° C./min while spraying c / min / m ² , kept at this temperature for 1 hour, carbonized and cooled, and subsequently weight gained at 700 ° C. in air. 9
The treatment was continued until the content became 3% to obtain a carbonaceous fiber nonwoven fabric. XPS
Table 1 shows the results of the surface analysis, the specific resistance of the single fiber, and the electrode performance.

Example 5 97 mol% of acrylonitrile
Weight average molecular weight 2 composed of 3 mol% of vinyl acetate
After oxidizing a polyacrylonitrile fiber having a flat fiber diameter of 16 μm in air at 200 to 300 ° C. in air, it is felted using short fibers (about 80 mm in length) of the oxidized fiber, and a basis weight of 400 g / m ² and a thickness of 400 g / m ² . A 4.0 mm nonwoven fabric was prepared. 600cc of argon gas continuously into the non-woven fabric
/ Min / m ² , the temperature is raised to 1600 ° C. at a rate of 100 ° C./min while spraying, maintained at this temperature for 1 hour, carbonized and cooled, and subsequently weight gained at 700 ° C. in air. 9
The treatment was carried out until the content became 5% to obtain a carbonaceous fiber nonwoven fabric. XPS
Table 1 shows the results of the surface analysis, the specific resistance of the single fiber, and the electrode performance.

Comparative Example 1 98 mol% of acrylonitrile
After oxidizing polyacrylonitrile fiber having a weight average molecular weight of 10000 and having an average fiber diameter of 16 μm composed of 2 mol% of methyl methacrylate in air at 200 to 300 ° C. in air, short fibers of the oxidized fiber (about 80 mm in length) It is made into felt by using and the basis weight is 400 g / m ² and the thickness is 4.0 mm.
Was prepared. Argon gas was continuously sprayed at 600 cc / min / m ² onto the nonwoven fabric at 100 ° C. /
The temperature was raised to 1600 ° C. at a heating rate of 1 minute, kept at this temperature for 1 hour, carbonized and cooled, and subsequently treated in air at 700 ° C. until the weight yield became 93% to obtain a carbonaceous fiber nonwoven fabric. Obtained. Table 1 shows the results of the XPS surface analysis, the specific resistance of the single fiber, and the electrode performance.

Comparative Example 2 98 mol% of acrylonitrile
After oxidizing polyacrylonitrile fiber having a weight average molecular weight of 10000 and having an average fiber diameter of 16 μm composed of 2 mol% of methyl methacrylate in air at 200 to 300 ° C. in air, short fibers of the oxidized fiber (about 80 mm in length) It is made into felt by using and the basis weight is 400 g / m ² and the thickness is 4.0 mm.
Was prepared. Argon gas was continuously sprayed at 600 cc / min / m ² onto the nonwoven fabric at 100 ° C. /
The temperature was raised to 1400 ° C. at a heating rate of 1 minute, kept at this temperature for 1 hour, carbonized and cooled, and then treated in air at 700 ° C. until the weight yield became 97% to obtain a carbonaceous fiber nonwoven fabric. Obtained. Table 1 shows the results of the XPS surface analysis, the specific resistance of the single fiber, and the electrode performance.

Comparative Example 3 98 mol% of acrylonitrile
After oxidizing polyacrylonitrile fibers having a weight average molecular weight of 1,000,000 and having an average fiber diameter of 16 μm, which is composed of 2 mol% of methyl methacrylate, at 200 to 300 ° C. in air, short fibers of the oxidized fibers (about 80 mm in length) To give a felt weight of 400 g / m ² and a thickness of 4.0 m
m was prepared. 100 ° C. while continuously blowing argon gas at 600 cc / min / m ² onto the nonwoven fabric.
/ Min at a heating rate of 1 / min.
Hold for a time, perform carbonization and cool, then 700 ° C in air
To give a weight yield of 93% to obtain a carbonaceous fiber nonwoven fabric. Table 1 shows the results of the XPS surface analysis, the specific resistance of the single fiber, and the electrode performance.

[0056]

[Table 1]

The use of the electrode material of the present invention makes it possible to increase the conductivity of the single fiber of the electrode material and obtain a moderate wettability in the field of using an aqueous electrolytic cell to reduce the internal resistance of the electrolytic cell. As a result, voltage efficiency can be increased, and battery energy efficiency can be increased. This is particularly effective for vanadium-based redox flow batteries.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a vanadium-based redox flow battery.

FIG. 2 is an exploded perspective view of an electrolytic cell of a vanadium-based redox flow battery having a three-dimensional electrode.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Current collection plate 2 Spacer 3 Ion exchange membrane 4a, 4b Liquid passage 5 Electrode material 6 External liquid tank (positive electrode side) 7 External liquid tank (negative electrode side) 8, 9 Pump 10 Liquid inlet 11 Liquid outlet

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Satoshi Takase 2-1-1 Katata, Otsu-shi, Shiga F-Term in Toyobo Co., Ltd. Research Laboratory 5H018 AA08 AS07 CC06 DD01 DD06 EE05 HH03 HH05 5H026 AA10 CX03 CX05 EE05 HH03 HH05 RR01

Claims (3)

[Claims] 1. A carbon electrode material for a redox flow battery using an aqueous electrolyte solution, wherein the size of crystallites in the a-axis direction obtained by X-ray wide-angle analysis is 30 to 80 °. Carbon electrode material. 2. The carbon electrode material according to claim 1, wherein the amount of surface acidic functional groups determined by XPS surface analysis is from 0.2% to 1.2% of the total number of surface carbon atoms. 3. The carbon electrode material according to claim 1, which is used for a vanadium redox flow battery.