JP2017027920A - Electrode material for redox battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive carbon electrode material for redox flow battery having high activity.SOLUTION: In an electrode material for redox battery, conductive carbon fine particles are bonded, by an ion conductive binder, onto carbon fibers composed of a carbonaceous material having a pseudo-graphite crystal structure where the<002>plane interval obtained by X-ray wide angle analysis is 3.43-3.60 Å, the size of a crystallite in the c-axis direction is 15-50 Å, the size of a crystallite in the a-axis direction is 30-75 Å, and the number of bound oxygen atoms obtained by XPS surface analysis is 1.0% or more of the number of carbon atoms of the entire surface.SELECTED DRAWING: None

Description

  The present invention relates to a porous electrode material having active ability suitable for a redox battery.

  Conventionally, electrodes have been intensively developed as affecting the performance of batteries. There are electrode types that do not become active materials themselves but act as reaction fields that promote the electrochemical reaction of the active materials. Carbon materials are often used for this type because of their electrical conductivity and chemical resistance. It is done. In particular, the electrode of a redox flow battery, which has been actively developed for power storage, uses a carbon fiber non-woven fabric having chemical resistance, conductivity, and liquid permeability.

  The redox flow battery has been changed from a type that uses 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 that uses an aqueous solution of vanadium sulfuric acid with a high electromotive force for both electrodes. Development to increase the active material concentration is progressing, and energy density is further increased.

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, the electrolyte containing V ²⁺ is supplied to the negative electrode side during discharge, Is supplied with an electrolyte containing V ⁵⁺ (actually ions containing oxygen). In the negative channel, V ²⁺ emits electrons in the electrode and is oxidized to V ³⁺ . The emitted electrons pass through an external circuit and reduce V ⁵⁺ to V ⁴⁺ (actually ions containing oxygen) in the positive electrode. With this oxidation-reduction reaction, SO ₄ ^2- in the negative electrode electrolyte becomes insufficient, and SO ₄ ^2- becomes excessive in the positive electrode electrolyte, so that SO ₄ ^2- passes through the ion exchange membrane from the positive electrode side to the negative electrode side. The charge balance is maintained. Alternatively, the charge balance can be maintained by moving H ⁺ through the ion exchange membrane from the negative electrode side to the positive electrode side. During charging, a reaction opposite to discharging proceeds.

  As the characteristics of the redox battery electrode material, the following performance is particularly required.

1) No side reaction other than the intended reaction is caused (high reaction selectivity), more specifically, high current efficiency (η _I ).
2) High electrode reaction activity, more specifically, low cell resistance (R). That is, the voltage efficiency (η _V ) is high.
3) Battery energy efficiency (η _E ) related to 1) and 2) above is high.
η _E = η _I × η _V
4) Deterioration against repeated use is small (long life), more specifically, the decrease in battery energy efficiency (η _E ) is small.

  And regarding cell resistance (R), between fibers, such as a carbonaceous fiber aggregate, the contact resistance of an electrode material and a current collecting plate, the conductive resistance which the electroconductivity of electrode material itself contributes, and ion at the time of charging / discharging It is roughly classified into charge transfer reaction resistance contributed by electrode activity when changing the valence, and diffusion resistance contributed by diffusion of ions and electrolyte.

  Regarding the charge transfer reaction resistance, heat treatment is generally performed at about 400 to 700 ° C. in the air, and activated by introducing acidic functional groups such as hydroxyl groups and carboxyl groups on the surface of the graphite crystallized carbon fiber. Is reduced.

  However, the above-described conductive resistance and charge transfer reaction resistance have a so-called trade-off relationship in which when one is increased, the other characteristic is remarkably impaired. Therefore, various methods have been proposed in order to express both characteristics in a well-balanced manner. For example, Patent Document 1 discloses that a pseudo-graphite fine particle whose <002> plane spacing obtained by X-ray wide-angle analysis is 3.70 mm or less on average and the crystallite size in the c-axis direction is 9.0 mm or more on average. It has been proposed to use a carbonaceous material having crystals and a total acidic functional group amount of at least 0.01 meq / g as an electrode material for a redox flow battery.

  Further, Patent Document 2 is a carbonaceous fiber made from polyacrylonitrile-based fiber, and has a pseudo-graphite crystal structure with a <002> plane spacing of 3.50 to 3.60 mm determined by X-ray wide angle analysis, It has been proposed to use, as an electrode material for a redox flow battery, a carbonaceous material in which the number of bonded oxygen atoms on the surface of the carbonaceous material is 10 to 25% of the number of carbon atoms.

  Further, Patent Document 3 discloses that the <002> plane spacing obtained by X-ray wide angle analysis is 3.43 to 3.60 mm, the crystallite size in the c-axis direction is 15 to 33 mm, and the crystallite in the a-axis direction. Having a pseudo-graphite crystal structure with a size of 30 to 75 mm, the amount of surface acidic functional groups determined by XPS surface analysis is 0.2 to 1.0% of the total surface carbon atoms, and surface-bound nitrogen atoms It has been proposed to use a carbonaceous material having a number of 3% or less of the total surface carbon atoms as an electrode material for an electrolytic cell of a vanadium redox flow battery.

JP-A-60-232669 JP-A-5-234612 JP 2000-357520 A Japanese Patent Laid-Open No. Sho 63-200467

  However, even in the techniques of Patent Documents 1 to 3, it is not possible to escape from the trade-off described above, and it is difficult to further reduce the cell resistance, and high battery energy efficiency cannot be obtained.

  Therefore, in view of such circumstances, an object of the present invention is to introduce a carbon electrode material that can reduce the cell resistance of a redox battery and increase energy efficiency by introducing an ion conductive binder in addition to the conventional acidic functional group. It is to provide an aggregate.

  The present inventors have found that the <002> plane spacing determined by X-ray wide angle analysis is 3.43 to 3.60 mm, the crystallite size in the c-axis direction is 15 to 50 mm, and the crystallite size in the a-axis direction is On a carbon fiber made of a carbonaceous material having a pseudo-graphite crystal structure with a size of 30 to 75 mm, and the number of bonded oxygen atoms on the surface determined by XPS surface analysis being 1.0% or more of the total surface carbon atoms. Furthermore, the inventors have found that the above object can be achieved by adhering carbon fine particles having conductivity with an ion conductive binder, and have completed the present invention.

The present invention has the following configuration.
1. The <002> plane spacing determined by X-ray wide angle analysis is 3.43 to 3.60 mm, the crystallite size in the c-axis direction is 15 to 50 mm, and the crystallite size in the a-axis direction is 30 to 75 mm. It has conductivity on carbon fibers made of a carbonaceous material having a pseudographite crystal structure of which the number of bonded oxygen atoms on the surface determined by XPS surface analysis is 1.0% or more of the total number of carbon atoms on the surface. An electrode material for a redox battery, wherein carbon fine particles are bonded with an ion conductive binder.
2. 2. The electrode material for a redox battery according to 1, wherein the ion conductive binder is a fluorine-based resin or a heterocyclic polymer having a sulfonic acid group.
3. The redox battery using the electrode material in any one of said 1-2.

  The carbon electrode material for a redox battery in which the carbon fiber aggregate of the present invention and carbon fine particles are bonded with an ion conductive binder has excellent activity, and as a result, a low-resistance electrode material was obtained. Thereby, the material which shows outstanding performance as a carbon electrode material for redox flow batteries can be provided.

FIG. 1 shows (a) a schematic diagram of a vanadium redox flow battery and (b) an exploded perspective view of an electrolytic cell of a vanadium redox flow battery.

Hereinafter, the present invention will be described in detail. The carbon electrode material for a redox battery of the present invention has a <002> plane spacing determined by X-ray wide-angle analysis of 3.43 to 3.60 mm, a crystallite size in the c-axis direction of 15 to 50 mm, and an a-axis Carbonaceous material having a pseudographite crystal structure with a crystallite size of 30 to 75 mm in the direction, and the number of bonded oxygen atoms on the surface determined by XPS surface analysis being 1.0% or more of the total surface carbon atoms Conductive carbon fine particles are bonded onto a carbon fiber made of an ion conductive binder. By using ion conductive binder and conductive carbon fine particles, the hydrophilicity and reaction activity of the electrode material can be remarkably enhanced. The adhesion amount on the carbon electrode is preferably 30 wt% or less with respect to the carbon fiber aggregate. The carbon electrode material of the present invention is suitably used for flow type and non-flow type Redox batteries, or redox batteries that are combined with lithium, capacitor, and fuel cell systems. Thereby, the characteristic which the conventional acidic functional group introduction | transduction material does not have can be expressed, As a result, the carbon electrode material for redox flow batteries with low resistance can be provided.

That is, the carbon electrode material for a redox battery of the present invention is characterized in that the carbon fiber aggregate and the carbon fine particles are bonded by an ion conductive binder as follows.

  An electrolytic cell using the carbon electrode material for a redox battery of the present invention has a structure shown in FIG. 1 as an example. In the electrolytic cell, an ion exchange membrane 3 is disposed between two opposing current collector plates 1 and 1, and an electrolytic solution is provided along the inner surface of the current collector plates 1 and 1 by spacers 2 on both sides of the ion exchange membrane 3. Is formed. At least one of the liquid passages is provided with the carbon electrode material 5 for a redox battery of the present invention, and thus an electrolytic cell is configured. The current collector plate 1 is provided with a liquid inlet and a liquid outlet for the electrolyte.

  The carbon electrode material 5 for a redox battery of the present invention is made of a carbonaceous material, and the constitutional structure thereof is not particularly limited, but is preferably one that can increase the electrode surface area. Specifically, a spun yarn, a filament bundle yarn, a nonwoven fabric, a knitted fabric, a woven fabric, a special knitted fabric (for example, Patent Document 4) made of carbonaceous fibers, or a carbonaceous fiber assembly or a porous carbon body made of a hybrid structure thereof. , Carbon-carbon composites, particulate carbon materials, and the like. Among these, a carbonaceous fiber aggregate is preferable, and in particular, a non-woven fabric, a knitted fabric, a woven fabric, a special woven or knitted fabric composed of carbonaceous fibers, which is a sheet-shaped material composed of carbonaceous fibers, or a carbonaceous fiber assembly composed of a hybrid structure thereof The body is more preferable in terms of handling, processability, manufacturability and the like.

Although the basis weight of the carbonaceous material depends on the structure, the thickness of the spacer 2 sandwiched between the current collector plate 1 and the ion exchange membrane 3 in FIG. when used in 3～3Mm, preferably 50 to 1000 g / m ^2, when construction organization of knitting 50 to 1000 g / m ^2, in the case of textile 50 to 800 g / m ^2, in the case of non-woven fabric 50～600G / m ² is preferred. Moreover, it is more preferable from a liquid-permeable property to use the nonwoven fabric by which the concave groove process was given to the single side | surface as a carbonaceous material. In that case, the groove width and groove depth are preferably at least 0.1 mm.

The thickness of the carbonaceous material is preferably at least larger than the thickness of the spacer 2 and 1.5 to 6.0 times the thickness of the spacer 2 in the case of a low-density material such as a nonwoven fabric. However, if the thickness is too thick, it may break through the ion exchange membrane 3 due to the compressive stress of the sheet material, so it is preferable to use a sheet material having a compressive stress of 9.8 N / cm ² or less. . Depending on the carbonaceous material, two or three layers of carbonaceous materials can be used to adjust the basis weight, thickness, and compressive stress, and combinations with other forms of carbonaceous materials Is also possible.

The carbonaceous fiber assembly used for the carbon electrode of the present invention is composed of carbonaceous fibers. The carbonaceous fiber means a fiber in which 90% or more is composed of carbon by a mass ratio obtained by heating and carbonizing an organic fiber precursor (JIS L 0204-2). Organic fiber precursors used as raw materials for carbonaceous fibers include acrylic fibers such as polyacrylonitrile; phenol fibers; PBO fibers such as polyparaphenylenebenzobisoxazole (PBO); aromatic polyamide fibers; isotropic pitch and mesophase pitch. Pitch fibers such as cellulose fibers, etc. can be used. Among these, from the viewpoint of excellent strength and elastic modulus of the carbonaceous fiber and easy formation of the carbonaceous fiber aggregate, the organic fiber precursor is preferably an acrylic fiber or a pitch fiber, and more preferably an acrylic fiber. The acrylic fiber is not particularly limited as long as it contains acrylonitrile as a main component, but in the raw material monomer that forms the acrylic fiber, the content of acrylonitrile is preferably 95% by mass or more, and 98% by mass. More preferably.
The mass average molecular weight of the organic fiber is not particularly limited, but is preferably 10,000 or more and 100,000 or less, more preferably 15000 or more and 80000 or less, and further preferably 20,000 or more and 50000 or less.

When carbonaceous fibers are used as the carbonaceous material, the average fiber diameter is preferably 0.5 to 20 μm, and the average fiber length is preferably 30 to 100 mm.

  In particular, the heat carbonization treatment preferably includes at least a flameproofing step and a carbonization step.

  The flameproofing step means a step of obtaining a flameproof organic fiber by heating a precursor of an organic fiber at a temperature of 180 ° C. or higher and 350 ° C. or lower in an air atmosphere. The heat treatment temperature is more preferably 190 ° C. or higher, and further preferably 200 ° C. or higher. Moreover, it is preferable that it is 330 degrees C or less, and it is more preferable that it is 300 degrees C or less. By heating in the said temperature range, the content rate of nitrogen and hydrogen in an organic fiber can be reduced and the carbonization rate can be improved, maintaining the form of a carbonaceous fiber, without organic fiber thermally decomposing. During the flameproofing step, the organic fibers may be thermally shrunk, the molecular orientation may be collapsed, and the conductivity of the carbonaceous fibers may be reduced. Therefore, it is preferable to flameproof the organic fibers under tension or stretching. More preferably, the flameproofing treatment is performed under tension.

The carbonization step means a step of heating the flame-resistant organic fiber at a temperature of 1000 ° C. or higher and 2000 ° C. or lower in an inert atmosphere (preferably in a nitrogen atmosphere) to obtain a carbonaceous fiber. The heating temperature is more preferably 1100 ° C. or higher, and further preferably 1200 ° C. or higher. Moreover, it is 1900 degrees C or less more preferably. By performing the carbonization step in the above temperature range, carbonization of the organic fiber proceeds, and a carbonaceous fiber having a pseudographite crystal structure can be obtained.
Since the organic fibers have different crystallinity, the heating temperature can be selected according to the type of organic fiber used as a raw material.
For example, when an acrylic resin (preferably polyacrylonitrile) is used as the organic fiber, the heating temperature is preferably 2000 ° C. or lower, and more preferably 1800 ° C. or lower.

  The flameproofing treatment step and the carbonization step are preferably carried out continuously, and the rate of temperature rise when raising the temperature from the flameproofing temperature to the carbonization temperature is preferably 20 ° C./min or less, more Preferably, it is 15 ° C./min. By setting the heating rate within the above range, it is possible to obtain a carbonaceous fiber that maintains the shape of the organic fiber and is excellent in mechanical properties.

The heating carbonization treatment preferably further includes a dry oxidation treatment step. The dry oxidation treatment step means a step of heating the carbonaceous fiber at 500 ° C. or more and 900 ° C. or less in an air atmosphere. The dry oxidation treatment temperature is more preferably 600 ° C. or higher, and further preferably 650 ° C. or higher. Moreover, it is more preferable that it is 800 degrees C or less, and it is preferable that it is 750 degrees C or less. By subjecting the carbonaceous fiber to a dry oxidation treatment within the above temperature range, the low crystalline portion in the carbonaceous fiber is oxidized and consumed, and a carbonaceous fiber having excellent crystallinity can be obtained.
In the dry oxidation treatment step, the mass yield before and after oxidation is preferably adjusted to 90% or more and 96% or less from the viewpoint of maintaining the mechanical strength of the carbonaceous fiber.

The carbonaceous material is assembled by being pressed into the battery, and a high-viscosity electrolyte flows through the thin gap. Therefore, in order for the carbonaceous material not to fall off, the tensile strength of the carbonaceous material is 0.49 N / cm. It is preferable to make it ² or more. Further, in order to improve the contact resistance with the current collector plate, when the carbonaceous material is a nonwoven fabric structure, the density should be 0.01 g / cm ³ or more and the repulsive force against the electrode surface should be 0.98 N / cm ² or more. Is preferred.

  The crystal structure of the carbon fiber of the present invention is such that the <002> plane spacing obtained by X-ray wide angle analysis is greater than 3.60 mm, the crystallite size in the c-axis direction is less than 15 mm, or the crystal in the a-axis direction. When the size of the child is smaller than 50 mm, the electrode material conductive resistance component in the battery internal resistance (cell resistance) cannot be ignored, resulting in an increase in cell resistance (decrease in voltage efficiency) and energy efficiency. descend.

  Further, in the crystal structure of the carbon fiber of the present invention, the <002> plane spacing obtained by X-ray wide angle analysis is smaller than 3.43 mm, the crystallite size in the c-axis direction is larger than 50 mm, or the a-axis When the size of the crystallite in the direction is larger than 75 mm, the cell resistance increases and the energy efficiency decreases due to repeated charge / discharge cycles. This is because the carbonaceous material having the crystal structure as described above has a distortion in the crystal structure or has a structure close to that of graphite, so that it is likely to cause decomposition by sulfuric acid used in an electrolyte solution of a redox battery, for example. it is conceivable that.

  The carbonaceous material of the present invention requires that the number of bonded oxygen atoms on the surface of the carbonaceous material determined by XPS (X-ray photoelectron spectroscopy) surface analysis is 1.0% or more of the total number of surface carbon atoms. The electrode reaction rate can be remarkably increased by using a carbon-based material having the number of bonded oxygen atoms of 1.0% or more of the total number of surface carbon atoms for the electrode material. When a carbonaceous material having a low oxygen concentration, in which the number of bonded oxygen atoms on the surface of the carbonaceous material obtained by XPS surface analysis is less than 1.0% of the total number of carbon atoms, the electrode reaction rate during discharge is small, and the electrode reaction The activity cannot be increased. The reason why the electrode reaction activity, in other words, the voltage efficiency is increased by using a carbonaceous material having many oxygen atoms bonded to the surface of the material as described above is not clear, but the affinity between the carbonaceous material and the electrolyte is not clear. It is considered that oxygen atoms on the surface are effectively working in the properties, electron transfer, desorption of complex ions from carbon materials, complex exchange reactions, and the like.

  The carbon electrode of the present invention is characterized in that the carbonaceous fibers and the carbonaceous fine particles are adhered to each other by an ion conductive binder such as a compound having a sulfonic acid group. By using an ion conductive binder, the affinity between the electrode material and the electrolytic solution can be increased, and the electrochemical reaction can be performed smoothly. Moreover, the carbonaceous material surface which is an electrochemical reaction field can be raised by the effect of carbonaceous fine particles, and activity can also be improved. As a method for applying the carbon fine particles, a conventionally known method may be used. For example, a method in which carbon fine particles are dispersed in a solution and impregnated into carbon fibers, or a coating method using a bar coater or the like can be preferably used.

  The ion conductive binder is not particularly limited as long as it is a polymer containing an ionic group such as a sulfonic acid group, a phosphonic acid group, or a carboxylic acid group. Examples of these polymers include polyesters such as polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate, polyamides such as nylon 6, nylon 6,6, nylon 6,10 and nylon 12, polymethyl methacrylate and polymethacrylate. Acrylate resins such as polymethyl acrylate, polyacrylic acid esters, polyacrylic acid resins, polymethacrylic acid resins, polyethylene, polypropylene, various polyolefins including polystyrene and diene polymers, polyurethane resins, cellulose acetate, Cellulosic resins such as ethyl cellulose, polyarylate, aramid, polycarbonate, polyphenylene sulfide, polyphenylene oxide, polysulfone, polyether Aromatic hydrocarbon polymers such as sulfone, polyether ether ketone, polyether imide, polyimide, polyamide imide, polybenzimidazole, polybenzoxazole, polybenzthiazole, fluorine such as polytetrafluoroethylene, polyvinylidene fluoride There is no particular limitation as long as it is a polymer in which an ionic group as described above is introduced into a resin, an epoxy resin, a phenol resin, a novolac resin, a benzoxazine resin, or the like. Of the ionic groups, sulfonic acid groups having the highest degree of acid dissociation are preferred. In these compositions, if necessary, for example, antioxidants, heat stabilizers, lubricants, tackifiers, plasticizers, crosslinking agents, viscosity modifiers, antistatic agents, antibacterial agents, antifoaming agents. Further, various additives such as a dispersant and a polymerization inhibitor may be included.

More preferably, the polymer is excellent in oxidation resistance. Specifically, polyimide, polybenzimidazole, polybenzoxazole, polybenzthiazole and fluororesin are preferably exemplified. A polymer or composition obtained by sulfonating these polymers is more preferable.

  The adhesion amount of the carbon fine particles is preferably 1 to 50 wt% of the carbon fiber, and more preferably 1 to 20 wt%. If it is 50 wt% or more, the liquid permeability of the electrolytic solution is deteriorated.

The drying method after the impregnation or coating is not particularly limited, and a conventionally known drying method can be employed. For example, a method of heating to 80 to 120 ° C. (more preferably 90 to 110 ° C.) and drying can be preferably employed.

As the carbon fine particles, acetylene black, ketjen black, graphite, carbon black, or those obtained by air oxidation of these can be preferably used. The graphite may be either artificial graphite or natural graphite, but natural graphite is preferred from the viewpoints of performance stability and availability. The average primary particle diameter of the carbon particles is preferably 30 nm to 5 μm, more preferably 30 nm to 100 μm. When the particle diameter of the carbon particles is within the above range, the adhesive can efficiently fix only the contact point between the fiber assembly and the carbon fine particles without covering the fiber surface as an electrochemical reaction field.

  <002> spacing (d002) employed in the present invention, crystallite size in the c-axis direction (Lc), crystallite size in the a-axis direction (La), XPS surface analysis, mercury intrusion method, current Each measuring method of efficiency, voltage efficiency (cell resistance R), energy efficiency, and change with time of charge / discharge cycle will be described.

(1) <002> spacing (d002), crystallite size (Lc), crystallite size in the a-axis direction (La)
The electrode material is pulverized in an agate mortar until the particle size becomes about 10 μm, and about 5% by weight of X-ray standard high-purity silicon powder is mixed as an internal standard substance, packed in a sample cell, and CuKα rays As a radiation source, wide-angle X-rays are measured by a diffractometer method.

  For the correction of the curve, the following simple method is used without correcting the so-called Lorentz factor, polarization factor, absorption factor, atomic scattering factor and the like. That is, the actual intensity from the baseline of the peak corresponding to <002> diffraction is re-plotted to obtain a <002> corrected intensity curve. Find the midpoint of the line segment where the line parallel to the angle axis drawn to 2/3 of the peak height of this curve intersects the correction intensity curve, and correct the midpoint angle with the internal standard. The <002> plane spacing is obtained from the Bragg equation of Formula 1 from the wavelength λ of CuKα, which is twice the folding angle.

Here, the wavelength λ = 1.5418Å, θ represents the <002> diffraction angle.

  Further, from the length of the line segment intersecting with the correction intensity curve (half-value width β), the line parallel to the angle axis drawn to ½ the height of the peak height, the value of the crystallite in the c-axis direction is calculated according to Equation 2. The size Lc is obtained.

Here, wavelength λ = 1.5418４, structure coefficient k1 = 0.9, θ represents the <002> diffraction angle, and β represents the half width of the <002> diffraction peak.

  Also, the actual intensity from the baseline of the peak corresponding to <10> diffraction is re-plotted to obtain a <10> corrected intensity curve. The crystallite size La in the a-axis direction is calculated by Equation 3 from the length (half-value width β) of the line segment where the line parallel to the angle axis drawn to ½ the peak height intersects the correction intensity curve. Ask.

Here, wavelength λ = 1.5418４, structure coefficient k2 = 1.84, θ represents the <10> diffraction angle, and β represents the half width of the <10> diffraction peak.

(2) XPS surface analysis The apparatus used for the measurement of the X-ray photoelectron spectroscopy abbreviated as ESCA or XPS uses ULVAC-PHI 5801MC.
The sample was fixed on the sample holder with a Mo plate, exhausted sufficiently in the preliminary exhaust chamber, and then put into the chamber of the measurement chamber. A monochromatic AlKα ray is used as the radiation source, the output is 14 kV, 12 mA, and the vacuum in the apparatus is 10 ⁻⁸ torr.
A full element scan is performed to examine the composition of the surface elements, a narrow scan is performed on the detected and expected elements, and the abundance ratio is evaluated.
The ratio of the number of surface-bound oxygen atoms to the total number of surface carbon atoms is calculated as a percentage (%).

Example 1
A felt made of flame-resistant polyacrylonitrile (thickness 4.5 mm, basis weight 600 g / m ² ) was heated to 1500 ° C. at a temperature rising temperature of 100 ° C./h or less in a nitrogen atmosphere. Then, after calcination for 1 hour and carbonization, air oxidation was performed at 700 ° C. for 10 minutes and air-cooled to room temperature. Thereafter, 20 wt% of ketjen black was dispersed in N-methyl-2-pyrrolidone, 3 wt% of polybenzimidazole introduced with a sulfonic acid group was dissolved in this solution, the above electrode material was immersed in this solution, Defoaming was performed by sonication. The excess solution adhering to the electrode material after immersion was wiped off and dried at 200 ° C. for 2 hours to obtain the intended electrode material. The obtained electrode material had a <002> plane spacing of 3.51 mm, Lc of 36 mm, La of 45 mm, and as a result of XPS surface analysis, the number of oxygen atoms was 8%.

(Example 2)
The same procedure as in Example 1 was performed except that a 3 wt% Nafion dispersion was used instead of polybenzimidazole.

(Comparative Example 1)
A felt made of flame-resistant polyacrylonitrile (thickness 4.5 mm, basis weight 600 g / m ² ) was heated to 1500 ° C. at a temperature rising temperature of 100 ° C./h or less in a nitrogen atmosphere. Then, after baking for 1 hour and carbonizing, air oxidation was performed at 700 ° C. for 10 minutes, and air cooling was performed to room temperature to obtain a target electrode material.

(Test Example 1)
The electrode materials obtained in Examples 1 and 2 and Comparative Example 1 were cut into an electrode area of 16 cm ² of 10 cm in the vertical direction (liquid passing direction) and 1.6 cm in the width direction, and the cell as shown in FIG. Assembled. The ion exchange membrane was a Nafion 212 membrane, and the spacer thickness was 2.5 mm. The battery was charged to 1.45 V at 70 mA / cm ² , and the conductive resistance, diffusion resistance, and charge transfer reaction resistance were each separated using an AC impedance device manufactured by Solartron. The results are shown in Table 1. Also, a 2.5 mol / l sulfuric acid aqueous solution of 1.7 mol / l vanadium oxysulfate was used for the positive electrode electrolyte, and a 2.5 mol / l sulfuric acid aqueous solution of 1.7 mol / l vanadium sulfate was used for the negative electrode electrolyte. It was. The amount of the electrolytic solution was excessively large with respect to the cell and the piping. The liquid flow rate was 6.2 ml per minute, and the measurement was performed at 30 ° C.

As is clear from Table 1, Examples 1-2, in which a conductive additive is adhered to the carbon fiber surface, exhibited a very low charge transfer reaction resistance, resulting in a low overall cell resistance. It was. This is probably because the introduction of pyridine-type nitrogen atoms can significantly increase the hydrophilicity and reaction activity of the electrode material, resulting in lower resistance. On the other hand, in Comparative Example 1, although the conductive resistance and the diffusion resistance were the same as those in Examples 1 and 2, the total cell resistance was large because the charge transfer reaction resistance was large.

The carbon electrode material for a redox battery of the present invention can improve the hydrophilicity and electrode reaction activity of the electrode material by using an ion conductive binder and conductive carbon fine particles. As a result, cell resistance during initial charge / discharge can be reduced, and battery energy efficiency can be improved. The carbon electrode material of the present invention is suitably used for flow type and non-flow type Redox batteries, or redox batteries that are combined with lithium, capacitor, and fuel cell systems to improve battery performance. It becomes possible and contributes greatly to the industry.

DESCRIPTION OF SYMBOLS 1 ... Current collecting 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 (3)

  The <002> plane spacing determined by X-ray wide angle analysis is 3.43 to 3.60 mm, the crystallite size in the c-axis direction is 15 to 50 mm, and the crystallite size in the a-axis direction is 30 to 75 mm. It has conductivity on carbon fibers made of a carbonaceous material having a pseudographite crystal structure of which the number of bonded oxygen atoms on the surface determined by XPS surface analysis is 1.0% or more of the total number of carbon atoms on the surface. An electrode material for a redox battery, wherein carbon fine particles are bonded with an ion conductive binder.   The electrode material for a redox battery according to claim 1, wherein the ion conductive binder is a fluororesin or a heterocyclic polymer having a sulfonic acid group.   A redox battery using the electrode material according to claim 1.