JPWO2017022564A1 - Carbon electrode material for redox batteries - Google Patents

Abstract

A carbon electrode material for a redox battery capable of reducing cell resistance during initial charge / discharge and improving battery energy efficiency is provided. The crystal structure has a <002> plane spacing determined by X-ray wide-angle analysis of 3.43 to 3.60 mm, the crystallite size in the c-axis direction is 15 to 35 mm, and the crystallite in the a-axis direction. Is a pseudo-graphite crystal structure having a size of 30 to 75 mm, the number of surface bonded oxygen atoms determined by XPS surface analysis is 2.5% or more of the total surface carbon atoms, and is obtained by a mercury intrusion method. The carbon electrode material for redox batteries which consists of a carbonaceous material which has the pore in the range whose pore diameter is 0.2-2 micrometers in the pore distribution measurement result on the surface. [Selection figure] None

Description

The present invention relates to a carbon electrode material used for a redox battery, and more particularly to a carbon electrode material excellent in energy efficiency in the entire battery system.

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, an electrode of a redox flow battery, which has been actively developed for power storage, uses a carbon fiber aggregate having chemical resistance, conductivity, and liquid permeability.

The redox flow battery is changed from a type using an aqueous hydrochloric acid solution of iron as a positive electrode and an aqueous solution of chromium hydrochloric acid as a negative electrode to a type using a vanadium sulfuric acid aqueous solution having a high electromotive force at both electrodes, and has a high energy density. And as what has a higher electromotive force and can be stably supplied at a low cost, for example, a type using manganese for the positive electrode and chromium, vanadium, and titanium for the negative electrode as in Patent Document 1 has been developed. High energy density is progressing.

The main structure of a redox flow battery is comprised from the external tanks 6 and 7 which store electrolyte solution, and an electrolytic cell, as shown in FIG. In the redox flow battery, an electrolytic solution containing an active material is sent from the external tanks 6 and 7 to the electrolytic cell by the pumps 8 and 9, and electrochemical energy conversion, that is, charge and discharge is performed on the electrodes incorporated in the electrolytic cell. Done.

In general, when charging / discharging, an electrolytic solution is circulated between an external tank and an 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 as a minimum unit alone or in a multi-layered manner. 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 problem that the reaction amount per unit volume of the electrolytic cell is small.

In order to increase the reaction amount per unit area, that is, the current density, the electrochemical reaction field is three-dimensionalized. 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 disposed between two opposing current collector plates 1 and 1, and an electrolyte 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. Liquid passages 4a and 4b are formed. An electrode material 5 such as a carbon fiber aggregate is disposed in at least one of the liquid passages 4a and 4b, and thus a three-dimensional electrode is configured. The current collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolyte.

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, an electrolyte containing V ²⁺ is supplied to the liquid passage 4a on the negative electrode side during discharge. An electrolytic solution containing V ⁵⁺ (actually an ion containing oxygen) is supplied to the liquid passage 4b on the positive electrode side. In the liquid passage 4a on the negative electrode side, V ²⁺ releases electrons in the three-dimensional electrode 5 and is oxidized to V ³⁺ . The emitted electrons pass through an external circuit and reduce V ⁵⁺ to V ⁴⁺ (actually oxygen-containing ions) in the three-dimensional electrode on the positive electrode side. 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 3 from the positive electrode side to the negative electrode. 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 flow battery electrode material, the following performance is particularly required.

(1) No side reaction other than the intended reaction occurs (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) The battery energy efficiency (η _E ) related to the above (1) and (2) is high. η _E = η _I × η _V (4) Deterioration with repeated use is small (long life), specifically, the amount of decrease in battery energy efficiency (η _E ) is small. These are required.

Here, in Patent Document 2, the pseudo-graphite having a <002> plane spacing obtained by X-ray wide-angle analysis of 3.70 mm or less on average and a crystallite size in the c-axis direction of 9.0 mm or more on average. It has been proposed to use a carbonaceous material having microcrystals and a total acidic functional group amount of at least 0.01 meq / g as an electrode material for an electrolytic cell of an iron-chromium redox flow battery.

Further, Patent Document 3 is a carbonaceous fiber made from polyacrylonitrile-based fiber, 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 a carbonaceous material whose number of bonded oxygen atoms on the surface of the porous material is 10 to 25% of the number of carbon atoms as an electrode material for an electrolytic cell of an iron-chromium redox flow battery.

Further, Patent Document 4 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.

However, it was confirmed that the carbonaceous material proposed in Patent Document 4 has few reaction active points, increases the reaction resistance of the battery, and decreases the battery energy efficiency.

Japanese Patent Publication "JP 2012-204135" Japanese Patent Publication “JP-A-60-232669” Japanese Patent Publication “JP-A-5-234612” Japanese Patent Publication “JP 2000-357520”

When a carbonaceous material is used as an electrode material for an electrolytic cell of a redox battery, the reaction active point is small and the resistance due to the reaction is still large. Then, this invention makes it a subject to provide the carbon electrode material for redox batteries which can reduce cell resistance at the time of initial stage charge / discharge, and can improve battery energy efficiency.

As a result of intensive studies by the present inventors in order to solve the above problems, the present invention has finally been completed. That is, the present invention is as follows.

1. 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 35 mm, and the crystallite size in the a-axis direction is Pore distribution measurement result obtained by mercury porosimetry, which has a pseudo-graphite crystal structure of 30 to 75%, and the number of bonded oxygen atoms on the surface determined by XPS surface analysis is 2.5% or more of the total surface carbon atoms. The carbon electrode material for redox batteries which consists of a carbonaceous material which has the pore in the range whose pore diameter is 0.2-2 micrometers in the surface. 2. The carbonaceous material in the spectrum obtained by laser Raman spectroscopy of the excitation wavelength 532 nm, the intensity ratio R (ID / IG of 1360 cm ^-1 vicinity of the peak intensity (ID) and 1580 cm ^-1 vicinity of the peak intensity (IG) ) is 1.0 to 2.5, 1580 cm ^-1 near the peak half width (.DELTA.G) carbon electrode material for a redox battery according to claim 1 is 70cm ^-1 or less. 3. 3. The carbon electrode material for a redox battery according to 1 or 2 above, wherein the total volume of pores having a pore diameter of 0.2 to 2 μm on the surface of the carbonaceous material is 0.003 cc / g or more. 4). 4. The carbon electrode material for a redox battery according to any one of 1 to 3, wherein the carbonaceous material is a fiber structure. 5. In the method for producing a carbon electrode material for a redox battery, the raw material is first baked at 600 to 1250 ° C. in an inert gas or nitrogen gas atmosphere, and then the first dry oxidation treatment is performed. And a step of performing a second dry oxidation treatment after a second baking at 1300 to 2300 ° C. in an inert gas or nitrogen gas atmosphere.

The carbon electrode material for a redox battery of the present invention is a cell at the time of initial charge / discharge by enhancing electrode reaction activity while not containing acetylene black, ketjen black, etc., which are said to be conductive graphite powder or conductive additive. It is possible to reduce resistance and improve battery energy efficiency. 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.

It is a main block diagram of a redox flow battery. It is a disassembled perspective view of the liquid circulation type electrolytic cell which has a three-dimensional electrode. 2 is a pore size distribution curve showing a pore size distribution obtained by a mercury intrusion method for carbon electrode materials obtained in Example 1 and Comparative Example 1. FIG. 3 is a pore size distribution curve showing a pore size distribution obtained by a mercury intrusion method for carbon electrode materials obtained in Example 2 and Comparative Example 2. 6 is a pore size distribution curve showing the pore size distribution obtained by mercury porosimetry for the carbon electrode materials obtained in Example 3 and Comparative Example 3.

An electrolytic cell using the carbon electrode material for a redox battery of the present invention has a structure shown in FIG. 2 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. Liquid passages 4a and 4b are formed. The carbon electrode material 5 for a redox battery of the present invention is disposed in at least one of the liquid passages 4a and 4b. In this way, the electrolytic cell is configured. The current collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolyte.

The carbon electrode material 5 for a redox battery according to the present invention is made of a carbonaceous material, and the constitutional structure thereof is not particularly limited. Specifically, a spun yarn, a filament bundle yarn, a nonwoven fabric, a knitted fabric, a woven fabric, a special knitted fabric (for example, see Japanese Patent Laid-Open Publication No. Sho 63-200467), or a mixture thereof, made of carbonaceous fibers Examples thereof include carbonaceous fiber aggregates composed of tissues, porous carbon bodies, carbon-carbon composites, and particulate carbon materials. 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.

The basis weight of the carbonaceous material depends on the structure, but 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, the ion exchange membrane 3 may be broken by the compressive stress of the sheet-like material, so it is preferable to use a sheet-like 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.

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.

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 carbonaceous material 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 35 mm, and an a-axis A carbon electrode material having a pseudo-graphite crystal structure with a crystallite size in the direction of 30 to 75 mm, preferably the <002> plane spacing is 3.45 to 3.53 mm and the crystal in the c-axis direction This is a carbonaceous material having a pseudo-graphite crystal structure having a child size of 20 to 35 mm and a crystallite size in the a-axis direction of 45 to 75 mm.

It is widely known that the <002> plane spacing obtained from X-ray wide-angle analysis of carbonaceous materials takes various values from 3.35% to over 3.70% of amorphous carbon, and the characteristics are also greatly different. ing.

The crystal structure of the carbonaceous material of the present invention has a <002> plane spacing determined by X-ray wide-angle analysis of greater than 3.60 mm, a c-axis direction crystallite size of less than 15 mm, or an a-axis direction When the size of the crystallite is smaller than 30 mm, the electrode material conductive resistance component in the battery internal resistance (cell resistance) cannot be ignored, and as a result, the cell resistance increases (voltage efficiency decreases), and energy efficiency Decreases.

Further, in the crystal structure of the carbonaceous material of the present invention, the <002> plane spacing determined by X-ray wide angle analysis is smaller than 3.43 mm, or the crystallite size in the c-axis direction is larger than 35 mm, Alternatively, when the crystallite size in the a-axis direction is larger than 75%, the growth of the basal surface in the a-axis direction is large, so that there are few reaction points into which oxygen functional groups are introduced, and the reaction activity becomes low.

In the carbonaceous material of the present invention, the number of bonded oxygen atoms on the surface of the carbonaceous material determined by XPS (X-ray photoelectron spectroscopy) surface analysis needs to be 2.5% or more of the total surface carbon atoms. By using a carbon-based material having the number of bonded oxygen atoms of 2.5% or more of the total number of surface carbon atoms as the electrode material, the electrode reaction rate, that is, the conductivity can be remarkably increased. 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 2.5% of the total number of surface carbon atoms is used, the electrode reaction rate at the time of 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.

Carbonaceous materials of the present invention, the intensity ratio R (ID / IG) of a peak intensity in the vicinity of 1360 cm ^-1 was determined by Raman spectroscopy (ID) and 1580 cm ^-1 vicinity of the peak intensity (IG) is 1.0 or more 2.5 or less, it is preferable 1580 cm ^-1 vicinity of the peak half width (ΔνG) is 70cm ^-1 or less. The carbonaceous material of the present invention has a small peak half-value width ΔνG and a high strength ratio R when compared with a carbonaceous material treated at the same carbonization temperature as in Patent Document 4. That is, compared with the prescription described in Patent Document 4, it is considered that the carbon material of the present invention has an increased defect structure of the carbon crystal due to the introduction of the oxygen functional group, and the reaction active point is increased.

As the carbonaceous material of the present invention, it is necessary to use a carbonaceous material having pores with pore diameters in the range of 0.2 to 2 μm on the surface in the pore distribution measurement result obtained by mercury porosimetry. By having the pores in the carbonaceous material, the surface described in Patent Document 4 has an outer surface area larger than that of the carbonaceous material having no pores. Therefore, the surface area of reaction with ions that are active materials in the electrolytic solution is low. Increases reaction activity.

Such a carbonaceous material can be obtained by the following manufacturing method. Inactive gas (or nitrogen gas) using polyacrylonitrile, isotropic pitch, mesophase pitch, cellulose, phenol, polyparaphenylene benzobisoxazole (PBO), etc. that have undergone initial air oxidation at 200 to 300 ° C. under tension After a first firing (carbonization) at 600 to 1250 ° C. in an atmosphere to obtain a carbon material having a pseudographite crystal structure, the weight yield is 45 to 95% in a gas atmosphere having an oxygen concentration of 1 to 10%. Preferably, the first dry oxidation treatment is carried out so as to be in the range of 50 to 90%. The first dry oxidation treatment temperature is preferably 350 to 900 ° C, more preferably 450 to 750 ° C. Further, after baking (carbonization) for the second time at 1300 to 2300 ° C. in an inert gas (or nitrogen gas) atmosphere, the weight yield is 80 to 99% in a gas atmosphere having an oxygen concentration of 1 to 10%, preferably A second dry oxidation treatment is carried out so as to be in the range of 93 to 99%. The second dry oxidation treatment temperature is preferably 500 to 900 ° C, and more preferably 650 to 750 ° C. However, the method of oxidation treatment is not limited to dry oxidation. For example, the same effect can be obtained by performing electrolytic oxidation. In the second firing, it was found that the growth of the crystal structure of carbon is promoted by firing in an atmosphere in which the concentration of an inert gas such as argon is 90% or more even in the vicinity of the processed material. . The mechanism of this reaction is currently being elucidated, but it may be because a highly reactive cracked gas such as HCN, NH ₃ , and CO is generated when the temperature is raised, so that surface reforming by the cracked gas is suppressed. I guess. An atmosphere in which the inert gas concentration is 90% or more even in the vicinity of the object to be processed can be obtained by, for example, constantly spraying 1 cc / min or more of inert gas per 1 g.

In Patent Document 4, a carbon material having a pseudo-graphite crystal structure is obtained by firing (carbonization) in an inert gas (or nitrogen gas) atmosphere at 1000 to 1800 ° C., and then dry oxidation treatment to obtain a carbonaceous material. However, in this treatment method, the surface of the carbonaceous material becomes non-porous. In the carbonaceous material of the present invention, by performing the first firing and the first dry oxidation treatment at 600 to 1250 ° C. before the second firing in a temperature range exceeding 1300 ° C., the surface of the carbonaceous material is increased. Prior to crystallization, pores are formed on the surface, and the pores can be retained even after the second firing at 1300 to 2300 ° C. in an inert gas (or nitrogen gas) atmosphere.

In the present invention, before the second baking in the temperature range exceeding 1300 ° C., the first baking is performed at 600 to 1250 ° C. to create a state in which low-developed crystallites are present randomly. By performing the dry oxidation process of the second time, a part of the amorphous carbon covering the crystallites that developed lowly was gasified, and it developed to a low degree by roughening until it had pores on the surface. A carbonaceous material having crystallites exposed on the fiber surface is obtained. After that, even if the second-stage firing is performed in a temperature range exceeding 1300 ° C. with the low-developed crystallites exposed on the fiber surface, the exposed state of the crystallites is maintained. It is considered that a carbonaceous material having an enhanced reaction activity with the active material in the electrolytic solution is obtained by positively introducing an oxygen functional group into the crystallite portion exposed by the oxidation treatment. On the other hand, in the carbonaceous material as disclosed in Patent Document 4, since there is no pore and the exposure of the crystallite is small, the amount of oxygen functional groups introduced is not so large, and the reaction activity with the active material is the carbonaceous material of the present invention. It is considered to be inferior to the material.

In the above manufacturing method, the <002> plane spacing and the crystallite size in the a-axis direction and the c-axis direction mainly adjust the temperature, the heating rate, the time, etc. during the second firing (carbonization). Can be controlled. The number of bonded oxygen atoms on the surface depends on the crystallinity (crystal growth degree) of the pseudographite crystal structure, but is mainly controlled by adjusting the oxygen concentration, temperature, etc. of the first and second dry oxidation treatments. it can.

<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 the charge / discharge cycle will be described.

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

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 (%).

(3) Mercury Intrusion Method The pore size distribution is obtained by measuring the pore size distribution by the mercury intrusion method. The measurement by the mercury intrusion method uses a pore master manufactured by Quantachrome. The mercury intrusion method is a method of injecting mercury while applying pressure to the pores of samples such as porous particles, and obtaining information such as specific surface area and pore size distribution from the relationship between the pressure and the amount of mercury injected It is. Specifically, first, the container containing the sample is evacuated and then filled with mercury. Since mercury has a high surface tension, mercury does not press into the pores on the sample surface as it is. However, when pressure is applied to the mercury and the pressure is gradually increased, mercury gradually pressurizes into the pores from the pores with the largest diameter to the pores with the smaller diameter. If a change in the mercury liquid level is detected while the pressure is continuously increased, a mercury intrusion curve representing the relationship between the pressure applied to mercury and the amount of mercury intrusion can be obtained.

Here, assuming that the shape of the pore is cylindrical, the diameter is D (nm), the surface tension of mercury is σ (dyn / cm), the contact angle of mercury with respect to the sample is θ (°), and the pressure is P. Then, the magnitude | size to the direction which extrudes mercury from a pore is represented by Numerical formula 4 which is a mathematical formula of Washburn.

[Equation 4] P · D = −4σcos θ

In the case of mercury, a surface tension δ = 480 dyn / cm and a contact angle of mercury with respect to a sample are generally used. Therefore, in Examples 1 to 3 and Comparative Examples 1 to 3 described later, the surface tension δ = 480 dyn / cm and the contact angle of mercury with respect to the sample was set to contact angle θ = 140 °. Based on the obtained mercury intrusion curve, a pore distribution curve representing the relationship between the size of the pore diameter of the sample and its volume can be obtained. Further, the total volume of pores having a pore diameter in the range of 0.2 to 2 μm was calculated from the pore diameter distribution data obtained by the mercury intrusion method.

(4) Raman spectroscopic measurement The Raman spectroscopic measurement uses Raman-11 manufactured by Nanophoton Co., Ltd., the objective lens uses 100 times (NA = 0.9), the grating of 600 gr / mm, and the excitation laser wavelength is 532 nm. It was. The measurement is carried out under the condition that the laser intensity is weakened using an ND filter and the structural change of carbon does not occur by laser irradiation. Maximum peak around 1580 cm ^-1 is a peak derived from a graphite crystalline structure, the maximum peak in the vicinity of 1360 cm ^-1 is a peak derived from reduced carbon atoms symmetry by structural defects. 1360 cm ^-1 vicinity of the peak intensity and (ID) "is the peak intensity of the D band appearing in the vicinity of 1360 cm ^-1, and the" 1580 cm ^-1 vicinity of the peak intensity (IG) ", appeared in the vicinity of 1580 cm ^-1 The peak intensity of the G band. The intensity ratio R (ID / IG) is a value obtained by dividing the peak intensity of the D band by the peak intensity of the G band. 1580 cm ^-1 vicinity of the peak half width (ΔνG) is, D band near 1360 cm ^-1 peak obtained by the Raman spectrometry of the, G band near 1580 cm ^-1, 1620 cm ^-1 near D'bands and other The peak is separated into two peaks and fitting is performed using the Lorentz function. The full width at half maximum is calculated from the G band peak obtained from the peak separation.

(5) Electrode characteristics Referring to Patent Document 4, a small cell having an electrode area of 10 cm ² of 1 cm in the vertical direction (liquid passing direction) and 10 cm in the width direction is made, and charge and discharge are repeated at a constant current density. Do the test. As the electrolytic solution, a vanadium-based electrolytic solution is used. As the vanadium-based electrolyte, a mixture of a positive electrode electrolyte and a negative electrode electrolyte with 2.0 mol / l vanadium oxysulfate and a 3 mol / l sulfuric acid aqueous solution is used with reference to Patent Document 4. The amount of the electrolyte is very large relative 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: In a one-cycle test starting with η _I charging and ending with discharging, assuming that the current density is 100 mA / cm ² (1000 mA) per electrode geometric area, the amount of electricity required for charging up to 1.5 V is Q ₁ , Q ₂ is the amount of electricity taken out at a constant voltage discharge up to 1.0 V, and the current efficiency η _I is obtained by Equation 5.

(B) Cell resistance: R The ratio of the quantity of electricity taken out by discharge to the theoretical quantity of electricity Q _th required to completely reduce V ³⁺ of the vanadium electrolyte in the negative electrode solution to V ^2+. The charging rate is obtained by Equation 6 using the charging rate.

The charging voltage V _C50 and the discharging voltage V _D50 corresponding to the amount of electricity when the charging rate is 50% are obtained from the amount of electricity-voltage curve, and the cell resistance R (Ω · cm ² ) with respect to the electrode geometric area is obtained from Equation 7. .

Here, I is a current value 1A in constant current charging / discharging.

(C) Voltage Efficiency: obtaining a voltage efficiency eta _V by simplified method of Equation 8 using cell resistance (R) obtained in eta _V above method. Here, I is a current value of 0.4 A in constant current charge / discharge.

Here, E is a cell open circuit voltage of 1.432 V (measured value) when the charging rate is 50%.

(D) Energy efficiency: η _E Energy efficiency η _E is obtained from Equation 9 using the current efficiency η _I and voltage efficiency η _V described above.

The higher the current efficiency and voltage efficiency, the higher the energy efficiency. Therefore, the energy loss during charging and discharging is small, and it is judged that the electrode is an excellent electrode.

(Example 1) A polyacrylonitrile fiber having an average fiber diameter of 16 µm was flame-resistant in air at 200 to 300 ° C. Thereafter, the flame-resistant short fibers (about 80 mm in length) are used to make a felt with a felt needle SB # 40 (Foster Needle) at a punching density of 250 / cm ² , and a basis weight of 300 g / m ² and a thickness of 3. A 2 mm nonwoven fabric was produced. The nonwoven fabric was adjusted to a thickness of 0.9 ± 0.1 mm at a pressure of 45 kgf / cm ² at 210 ± 10 ° C. and then increased to 950 ± 50 ° C. at a rate of 5 ° C./min in nitrogen gas. It was heated, held at this temperature for 1 hour, carbonized, cooled, and further subjected to dry oxidation at 550 ± 50 ° C. until a mass yield of 50 to 95% was obtained to obtain a carbonaceous fiber nonwoven fabric A. The obtained carbon fiber non-woven fabric A was heated to 1500 ± 50 ° C. at a temperature rising rate of 5 ° C./min in nitrogen gas, kept at this temperature for 1 hour, carbonized, cooled, and further 700 ±± 7 in air. Dry oxidation treatment was performed at 50 ° C. until a mass yield of 90 to 95% was obtained, so that a carbonaceous fiber nonwoven fabric B having a basis weight of 101 g / m ² was obtained. Table 1 shows the interplanar spacing, crystallite size, XPS surface analysis result, Raman spectroscopic measurement result, mercury intrusion measurement result, and electrode performance of the obtained carbon fiber nonwoven fabric B. The pressure used in the mercury intrusion measurement was 0.9 to 25,000 psia. The following Examples 2 and 3 and Comparative Examples 1 to 3 are the same. In the electrode performance evaluation, the spacer thickness was set to 0.6 mm, and the carbon fiber nonwoven fabric B was evaluated as a single layer using a V-based electrolyte. Further, FIG. 3 shows a pore size distribution curve showing the pore size distribution obtained by the mercury intrusion method. It was 0.0044 cc / g when the total volume of the pores having a pore diameter in the range of 0.2 to 2 μm was calculated from the pore diameter distribution data obtained by the mercury intrusion method.

(Example 2) A polyacrylonitrile fiber having an average fiber diameter of 16 µm was flame-resistant in air at 200 to 300 ° C. Thereafter, the flame-resistant short fibers (about 80 mm in length) are used to make a felt with a felt needle SB # 40 (Foster Needle) at a punching density of 250 / cm ² , and a basis weight of 300 g / m ² and a thickness of 3. A 2 mm nonwoven fabric was produced. The nonwoven fabric was adjusted to a thickness of 0.9 ± 0.1 mm at a pressure of 45 kgf / cm ² at 210 ± 10 ° C. and then increased to 950 ± 50 ° C. at a rate of 5 ° C./min in nitrogen gas. It was heated, held at this temperature for 1 hour, carbonized, cooled, and further subjected to dry oxidation at 550 ± 50 ° C. until a mass yield of 50 to 95% was obtained to obtain a carbonaceous fiber nonwoven fabric A. The obtained carbon fiber non-woven fabric A was heated to 1800 ± 50 ° C. at a rate of temperature increase of 5 ° C./min in nitrogen gas, kept at this temperature for 1 hour, carbonized, cooled, and further 700 ± 700 in air. Dry oxidation treatment was performed until the mass yield became 90 to 95% at 50 ° C. to obtain a carbonaceous fiber nonwoven fabric C having a basis weight of 98 g / m ² . Table 1 shows the interplanar spacing, crystallite size, XPS surface analysis result, Raman spectroscopic measurement result, mercury intrusion method measurement result, and electrode performance of the obtained carbon fiber nonwoven fabric C. In the electrode performance evaluation, the spacer thickness was set to 0.6 mm, and the carbon fiber nonwoven fabric C was evaluated as a single layer using a V-based electrolyte. In addition, FIG. 4 shows a pore size distribution curve showing the pore size distribution obtained by the mercury intrusion method. The total volume of pores having a pore diameter in the range of 0.2 to 2 μm was calculated from the pore diameter distribution data obtained by the mercury intrusion method and found to be 0.0039 cc / g.

(Example 3) A polyacrylonitrile fiber having an average fiber diameter of 16 µm was flame-resistant at 200 to 300 ° C in air. Thereafter, the flame-resistant short fibers (about 80 mm in length) are used to make a felt with a felt needle SB # 40 (Foster Needle) at a punching density of 250 / cm ² , and a basis weight of 300 g / m ² and a thickness of 3. A 2 mm nonwoven fabric was produced. The nonwoven fabric was adjusted to a thickness of 0.9 ± 0.1 mm at a pressure of 45 kgf / cm ² at 210 ± 10 ° C. and then increased to 950 ± 50 ° C. at a rate of 5 ° C./min in nitrogen gas. It was heated, held at this temperature for 1 hour, carbonized, cooled, and further subjected to dry oxidation at 550 ± 50 ° C. until a mass yield of 50 to 95% was obtained to obtain a carbonaceous fiber nonwoven fabric A. The obtained carbon fiber non-woven fabric A was heated to 2000 ± 50 ° C. at a temperature rising rate of 5 ° C./min in nitrogen gas, kept at this temperature for 1 hour, carbonized and cooled, and further 700 ± 700 in air. Dry oxidation treatment was performed until the mass yield was 90 to 95% at 50 ° C. to obtain a carbonaceous fiber nonwoven fabric D having a basis weight of 100 g / m ² . Table 1 shows the interplanar spacing, crystallite size, XPS surface analysis result, Raman spectroscopic measurement result, mercury intrusion method measurement result, and electrode performance of the obtained carbon fiber nonwoven fabric D. In the electrode performance evaluation, the spacer thickness was set to 0.6 mm, and the carbon fiber nonwoven fabric D was evaluated as a single layer using a V-based electrolyte. Further, FIG. 5 shows a pore size distribution curve showing the pore size distribution obtained by the mercury intrusion method. It was 0.004 cc / g when the total volume of the pores having a pore diameter in the range of 0.2 to 2 μm was calculated from the pore diameter distribution data obtained by the mercury intrusion method.

(Comparative example 1) The polyacrylonitrile fiber with an average fiber diameter of 16 micrometers was flame-resistant at 200-300 degreeC in the air. Thereafter, the flame-resistant short fibers (about 80 mm in length) are used to make a felt with a felt needle SB # 40 (Foster Needle) at a punching density of 250 / cm ² , and a basis weight of 300 g / m ² and a thickness of 3. A 2 mm nonwoven fabric was produced. The nonwoven fabric was adjusted to a thickness of 0.9 ± 0.1 mm at a pressure of 45 kgf / cm ² at 210 ± 10 ° C. and then increased to 1500 ± 50 ° C. at a rate of 5 ° C./min in nitrogen gas. Warm, hold at this temperature for 1 hour, perform carbonization, cool, and dry-oxidize in air at 700 ± 50 ° C. to a mass yield of 90-95%, with a basis weight of 161 g / m ² carbonaceous fiber nonwoven fabric E was obtained. Table 1 shows the interplanar spacing, crystallite size, XPS surface analysis result, Raman spectroscopic measurement result, mercury intrusion method measurement result, and electrode performance of the obtained carbon fiber nonwoven fabric E. In the electrode performance evaluation, the spacer thickness was set to 0.6 mm, and the carbon fiber nonwoven fabric E was evaluated as a single layer using a V-based electrolyte. Further, FIG. 3 shows a pore size distribution curve showing the pore size distribution obtained by the mercury intrusion method. In the pore size distribution data obtained by the mercury intrusion method, there were no pores having a pore size in the range of 0.2 to 2 μm (the total volume was 0.0000 cc / g).

(Comparative example 2) The polyacrylonitrile fiber with an average fiber diameter of 16 micrometers was flame-resistant at 200-300 degreeC in the air. Thereafter, the flame-resistant short fibers (about 80 mm in length) are used to make a felt with a felt needle SB # 40 (Foster Needle) at a punching density of 250 / cm ² , and the basis weight is 300 g / m.
^2. A nonwoven fabric having a thickness of 3.2 mm was produced. The nonwoven fabric was adjusted to a thickness of 0.9 ± 0.1 mm at a pressure of 45 kgf / cm ² at 210 ± 10 ° C. and then increased to 1800 ± 50 ° C. at a rate of 5 ° C./min in nitrogen gas. Warm, hold at this temperature for 1 hour, perform carbonization, cool, and dry-oxidize in air at 700 ± 50 ° C. until the mass yield is 90-95%, and has a basis weight of 153 g / m ^2. F was obtained. Table 1 shows the interplanar spacing, crystallite size, XPS surface analysis result, Raman spectroscopic measurement result, mercury intrusion measurement result, and electrode performance of the obtained carbon fiber nonwoven fabric F. In the electrode performance evaluation, the spacer thickness was set to 0.6 mm, and the carbon fiber nonwoven fabric F was evaluated as a single layer using a V-based electrolyte. In addition, FIG. 4 shows a pore size distribution curve showing the pore size distribution obtained by the mercury intrusion method. In the pore size distribution data obtained by the mercury intrusion method, there were no pores having a pore size in the range of 0.2 to 2 μm (the total volume was 0.0000 cc / g).

(Comparative example 3) The polyacrylonitrile fiber with an average fiber diameter of 16 micrometers was flame-resistant at 200-300 degreeC in the air. Thereafter, the flame-resistant short fibers (about 80 mm in length) are used to make a felt with a felt needle SB # 40 (Foster Needle) at a punching density of 250 / cm ² , and a basis weight of 300 g / m ² and a thickness of 3. A 2 mm nonwoven fabric was produced. The nonwoven fabric was adjusted to a thickness of 0.9 ± 0.1 mm at 210 ± 10 ° C. with a pressing pressure of 45 kgf / cm ² and then increased to 2000 ± 50 ° C. at a rate of temperature increase of 5 ° C./min in nitrogen gas. Warm, hold at this temperature for 1 hour, carbonize, cool, and dry-oxidize in air at 700 ± 50 ° C. to a mass yield of 90-95%. Carbonaceous fiber nonwoven fabric with a basis weight of 157 g / m ² G was obtained. Table 1 shows the interplanar spacing, crystallite size, XPS surface analysis result, Raman spectroscopic measurement result, mercury intrusion measurement result, and electrode performance of the obtained carbon fiber nonwoven fabric G. In the electrode performance evaluation, the spacer thickness was set to 0.6 mm, and the carbon fiber nonwoven fabric G was evaluated as a single layer using a V-based electrolyte. Further, FIG. 5 shows a pore size distribution curve showing the pore size distribution obtained by the mercury intrusion method. In the pore size distribution data obtained by the mercury intrusion method, there were no pores having a pore size in the range of 0.2 to 2 μm (the total volume was 0.0000 cc / g).

The carbon electrode material for a redox battery of the present invention is a cell at the time of initial charge / discharge by enhancing electrode reaction activity while not containing acetylene black, ketjen black, etc., which are said to be conductive graphite powder or conductive additive. It is possible to reduce resistance and improve battery energy efficiency. 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 can contribute greatly to the industry.

DESCRIPTION OF SYMBOLS 1 Current collecting plate 2 Spacer 3 Ion exchange membrane 4a, 4b Liquid passage 5 Electrode material 6 Positive electrode electrolyte tank 7 Negative electrode electrolyte tank 8, 9 Pump 10 Liquid inlet 11 Liquid outlet 12, 13 External liquid path

Claims (5)

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 35 mm, and the crystallite size in the a-axis direction is Pore distribution measurement result obtained by mercury porosimetry, which has a pseudo-graphite crystal structure of 30 to 75%, and the number of bonded oxygen atoms on the surface determined by XPS surface analysis is 2.5% or more of the total surface carbon atoms. The carbon electrode material for redox batteries which consists of a carbonaceous material which has the pore in the range whose pore diameter is 0.2-2 micrometers in the surface. The carbonaceous material in the spectrum obtained by laser Raman spectroscopy of the excitation wavelength 532 nm, the intensity ratio R (ID / IG of 1360 cm ^-1 vicinity of the peak intensity (ID) and 1580 cm ^-1 vicinity of the peak intensity (IG) ) is 1.0 to 2.5, a carbon electrode material for a redox battery according to claim 1 1580 cm ^-1 vicinity of the peak half width (.DELTA.G) is 70cm ^-1 or less. The carbon electrode material for a redox battery according to claim 1 or 2, wherein a total volume of pores having a pore diameter of 0.2 to 2 µm on the surface of the carbonaceous material is 0.003 cc / g or more. The carbon electrode material for a redox battery according to any one of claims 1 to 3, wherein the carbonaceous material comprises a fiber structure. In the method for producing a carbon electrode material for a redox battery, the raw material is first fired at 600 to 1250 ° C. in an inert gas or nitrogen gas atmosphere, and then the first dry oxidation treatment is performed. And a step of performing a second dry oxidation treatment after a second baking at 1300 to 2300 ° C. in an inert gas or nitrogen gas atmosphere.