JP2023100374A - Electrode, battery cell, cell stack, and redox flow battery system - Google Patents

Abstract

Kind Code: A1 A battery cell having both good flowability of an electrolytic solution and good battery reactivity is provided. An electrode for use in a redox flow battery system, comprising: a first electrode including first carbon fibers and voids; and a second electrode including second carbon fibers and voids; The electrode and the second electrode are laminated, and the first median diameter based on the pore volume of the first electrode obtained by mercury porosimetry is the first mode diameter of the first electrode obtained by mercury porosimetry. is 1.025 times or more, and the second median diameter based on the pore volume of the second electrode obtained by mercury porosimetry is 1.02 of the second mode diameter of the second electrode obtained by mercury porosimetry The electrode, which is less than double. [Selection drawing] Fig. 2

Description

The present disclosure relates to electrodes, battery cells, cell stacks, and redox flow battery systems.

As one type of storage battery, there is a redox flow battery in which an electrolytic solution is supplied to electrodes to cause a battery reaction. For example, in the redox flow battery of Patent Document 1, carbon paper is used for electrodes.

Japanese Patent Publication No. 2015-505148

There is a demand for the development of an electrode that has both good electrolyte flowability and good battery reactivity.

One of the objects of the present disclosure is to provide an electrode that has both good electrolyte flowability and good battery reactivity. An object of the present disclosure is to provide a battery cell including the electrode. An object of the present disclosure is to provide a cell stack including the battery cells described above. An object of the present disclosure is to provide a redox flow battery system including the battery cell or the cell stack.

The electrodes of the present disclosure are
An electrode for use in a redox flow battery system, comprising:
a first electrode comprising a first carbon fiber and a void;
a second electrode comprising a second carbon fiber and a void;
The first electrode and the second electrode are laminated,
The pore volume-based first median diameter of the first electrode obtained by mercury porosimetry is 1.025 times or more the first mode diameter of the first electrode obtained by mercury porosimetry,
The pore volume-based second median diameter of the second electrode determined by mercury porosimetry is 1.02 times or less of the second mode diameter of the second electrode determined by mercury porosimetry.

The battery cell of the present disclosure is
A battery cell used in a redox flow battery system,
An electrode of the present disclosure is provided.

The cell stack of the present disclosure is
A cell stack used in a redox flow battery system,
A plurality of battery cells of the present disclosure are provided.

A redox flow battery system of the present disclosure comprises a battery cell of the present disclosure or a cell stack of the present disclosure.

The electrodes, battery cells, cell stacks, and redox flow battery systems of the present disclosure combine good electrolyte fluidity and good battery reactivity.

FIG. 1 is a schematic perspective view showing electrodes provided in the redox flow battery system of the embodiment. FIG. 2 is a schematic cross-sectional view taken along line II-II of FIG. FIG. 3 is a schematic enlarged view showing an enlarged area A1 in FIG. FIG. 4 is a schematic sectional view showing an enlarged region A2 in the IV-IV section of FIG. FIG. 5 is a graph illustrating the Log differential pore volume distribution of the first electrode in the electrodes provided in the redox flow battery system of the embodiment. FIG. 6 is a schematic enlarged view showing an enlarged area B1 in FIG. FIG. 7 is a schematic cross-sectional view showing an enlarged region B2 in the VII-VII cross section of FIG. FIG. 8 is a diagram showing a graph explaining the Log differential pore volume distribution of the second electrode in the electrodes provided in the redox flow battery system of the embodiment. FIG. 9 is a schematic configuration diagram of the redox flow battery system of the embodiment. FIG. 10 is a schematic configuration diagram of a cell stack included in the redox flow battery system of the embodiment. FIG. 11 is a schematic configuration diagram of a measurement system used to measure the pressure loss of the redox flow battery system in the test example.

<<Description of Embodiments of the Present Disclosure>>
First, the embodiments of the present disclosure are listed and described.

(1) The electrode of one aspect of the present disclosure is
An electrode for use in a redox flow battery system, comprising:
a first electrode comprising a first carbon fiber and a void;
a second electrode comprising a second carbon fiber and a void;
The first electrode and the second electrode are laminated,
The pore volume-based first median diameter of the first electrode obtained by mercury porosimetry is 1.025 times or more the first mode diameter of the first electrode obtained by mercury porosimetry,
The pore volume-based second median diameter of the second electrode determined by mercury porosimetry is 1.02 times or less of the second mode diameter of the second electrode determined by mercury porosimetry.

A first electrode having a first median diameter of 1.025 times or more the first mode diameter has many large voids compared to an electrode having a first median diameter of less than 1.025 times the first mode diameter. This first electrode is excellent in electrolyte solution flowability. A second electrode having a second median diameter less than or equal to 1.02 times the second mode diameter has more small voids than an electrode having a second median diameter greater than 1.02 times the second mode diameter. This second electrode is excellent in battery reactivity. Therefore, since the above embodiment includes the first electrode having excellent electrolyte fluidity and the second electrode having excellent battery reactivity, it has both good electrolyte fluidity and good battery reactivity. Therefore, the above configuration facilitates reducing the diffusion resistance and the charge transfer resistance.

(2) As one form of the electrode,
The first mode diameter is 80 μm or less,
The second mode diameter may be 60 μm or less.

A first electrode having a first mode diameter of 80 μm or less has excellent battery reactivity. A second electrode having a second mode diameter of 60 μm or less is also excellent in battery reactivity.

(3) As one form of the electrode,
The first median diameter is 80 μm or less,
The second median diameter may be 60 μm or less.

A first electrode having a first median diameter of 80 μm or less has excellent battery reactivity. A second electrode having a second median diameter of 60 μm or less also has excellent battery reactivity.

(4) As one form of the electrode,
In the Log differential pore volume distribution of the first electrode obtained by mercury porosimetry, the first symmetry coefficient S _0.1h of the first curve having the maximum peak is 1.5 or more,
In the log differential pore volume distribution of the second electrode obtained by mercury porosimetry, the second symmetry coefficient _S0.1h of the second curve having the maximum peak may be 1.5 or more.

As will be described later in detail with reference to FIG. 5, the first symmetry coefficient S _0.1h is obtained by first width W _0.1h /(2×first width f _0.1h ). The first width _W0.1h is the difference between the first pore diameter _D11 and the second pore diameter _D12 . The first width _f0.1h is the difference between the first pore diameter _D11 and the third pore diameter _D13 . The first pore diameter _D11 is the smaller one of the pore diameters at a height of 10% of the height of the maximum peak in the first curve. The second pore diameter _D12 is the larger one of the pore diameters at a height of 10% of the height of the maximum peak in the first curve. The third pore diameter _D13 is the pore diameter of the maximum peak of the first curve.

As will be described later in detail with reference to FIG. 8, the second symmetry coefficient S _0.1h is obtained by second width W _0.1h /(2×second width f _0.1h ). The second width _W0.1h is the difference between the first pore diameter _D21 and the second pore diameter _D22 . The second width f _0.1h is the difference between the first pore diameter D ₂₁ and the third pore diameter D ₂₃ . The first pore diameter _D21 is the smaller one of the pore diameters at a height of 10% of the height of the maximum peak in the second curve. The second pore diameter _D22 is the larger one of the pore diameters at a height of 10% of the height of the maximum peak in the second curve. The third pore diameter _D23 is the pore diameter of the maximum peak of the second curve.

A first electrode having a first symmetry coefficient S _0.1h of 1.5 or more has many large gaps compared to an electrode having a first symmetry coefficient S _0.1h of less than 1.5. This first electrode is excellent in electrolyte solution flowability. Similarly, the second electrode having a second symmetry coefficient S _0.1h of 1.5 or more has many large gaps compared to the electrode having a second symmetry coefficient S _0.1h of less than 1.5. This second electrode is excellent in fluidity of the electrolytic solution.

(5) As one form of the electrode,
(Log differential pore volume at the second mode diameter x 1/10 diameter obtained by mercury porosimetry)/(Log differential pore volume of the second mode diameter obtained by mercury porosimetry) is 0.002 or more may be Hereinafter, (Log differential pore volume at the mode diameter x 1/10 diameter obtained by mercury porosimetry) / (Log differential pore volume of the mode diameter obtained by mercury porosimetry) Log differential pore volume It is sometimes called a ratio. The Log differential pore volume ratio is obtained by dividing the Log differential pore volume at the second mode diameter x 1/10 diameter obtained by the mercury intrusion method by the Log differential pore volume of the second mode diameter obtained by the mercury intrusion method. It is the ratio required by

A second electrode with a Log differential pore volume ratio of 0.002 has more small voids than an electrode with a Log differential pore volume ratio of less than 0.002. This second electrode is excellent in battery reactivity.

(6) As one form of the electrode,
The number of the first carbon fibers and the number of the second carbon fibers are plural,
The average diameter of the plurality of first carbon fibers and the average diameter of the plurality of second carbon fibers may be 2 μm or more and 30 μm or less.

When the average diameter of the plurality of first carbon fibers and the average diameter of the plurality of second carbon fibers are 2 μm or more, the strength of the first electrode and the second electrode is increased. When the average diameter of the plurality of first carbon fibers and the average diameter of the plurality of second carbon fibers are 30 μm or less, the surface area of the first carbon fibers and the surface area of the second carbon fibers per unit weight are increased. Therefore, the first electrode and the second electrode can perform sufficient battery reaction.

(7) As one form of the electrode,
The first electrode may have a porosity of 60% or more as determined by mercury porosimetry.

A first electrode with a porosity of 60% or more has more voids than an electrode with a porosity of less than 60%. This first electrode is excellent in electrolyte solution flowability.

(8) As one form of the electrode,
The second electrode is
carbon particles;
a binder that fixes the carbon particles to the second carbon fibers.

The second electrode has carbon particles fixed to the second carbon fibers by a binder, and thus has excellent battery reactivity.

(9) A battery cell according to one aspect of the present disclosure is
A battery cell used in a redox flow battery system,
The electrode according to any one of (1) to (8) above is provided.

Since the above-described form includes an electrode having both good electrolyte flowability and good battery reactivity, it is easy to reduce the diffusion resistance and the charge transfer resistance.

(10) The cell stack of one aspect of the present disclosure is
A cell stack used in a redox flow battery system,
A plurality of battery cells according to (9) are provided.

Since the above embodiment includes a plurality of battery cells, it has both good flowability of the electrolytic solution and good battery reactivity. Therefore, the above configuration facilitates reducing the diffusion resistance and the charge transfer resistance.

(11) The redox flow battery system of one aspect of the present disclosure includes
The battery cell according to (9) above or the cell stack according to (10) above is provided.

Since the above embodiment includes the battery cell or the cell stack, it has both good fluidity of the electrolytic solution and good battery reactivity. Therefore, the above configuration facilitates reducing the diffusion resistance and the charge transfer resistance.

(12) As one form of the redox flow battery system,
A positive electrode electrolyte and a negative electrode electrolyte supplied to the battery cell,
The positive electrode electrolyte contains manganese ions,
The negative electrode electrolyte may contain titanium ions.

The above forms have a high electromotive force.

<<Details of the embodiment of the present disclosure>>
Details of the redox flow battery system of embodiments of the present disclosure are described below. Hereinafter, the redox flow battery system may be referred to as an RF battery system. The same reference numerals in the drawings indicate the same names.

<<Embodiment>>
[RF battery system]
An RF battery system 100 according to an embodiment will be described with reference to FIGS. 1 to 10 . The RF battery system 100 includes battery cells 4 and a circulation mechanism 6, as shown in FIG. The battery cell 4 has a diaphragm 4M, a positive electrode 4P and a negative electrode 4N. The diaphragm 4M is arranged between the positive electrode 4P and the negative electrode 4N. At least one of the positive electrode 4P and the negative electrode 4N is composed of the electrode 1 shown in FIG. The circulation mechanism 6 circulates the electrolyte to the battery cells 4 . One of the features of the RF battery system 100 of this embodiment is that the electrode 1 has a structure in which a specific first electrode 2 and a specific second electrode 3 are laminated, as shown in FIG. The following description will be given in order of the overview and basic configuration of the RF battery system 100 and details of each configuration of the RF battery system 100 of the present embodiment.

[Overview of RF battery system]
The RF battery system 100 shown in FIG. 9 charges and stores power generated by the power generation unit 310 , discharges the stored power, and supplies the load 330 . RF battery system 100 is typically connected between power generation section 310 and load 330 via AC/DC converter 300 and transformer equipment 320 . An example of the power generation unit 310 is a solar power generator, a wind power generator, or any other general power plant. An example of load 330 is a power consumer. A solid line arrow extending from the substation equipment 320 toward the AC/DC converter 300 means charging. A dashed arrow extending from the AC/DC converter 300 toward the substation 320 means discharge. The RF battery system 100 uses a positive electrolyte and a negative electrolyte. The positive electrode electrolyte and the negative electrode electrolyte contain, as active materials, metal ions whose valences change due to oxidation-reduction. Charging and discharging of the RF battery system 100 is performed using the difference between the oxidation-reduction potential of ions contained in the positive electrode electrolyte and the oxidation-reduction potential of ions contained in the negative electrode electrolyte. An example of an application for the RF battery system 100 is load leveling, voltage sag compensation, emergency power, or output smoothing of solar or wind-generated renewable energy.

[Basic configuration of RF battery]
A battery cell 4 provided in the RF battery system 100 is separated into a positive electrode cell and a negative electrode cell by a diaphragm 4M. The diaphragm 4M is an ion exchange membrane that is impermeable to electrons but permeable to hydrogen ions, for example. The positive cell incorporates a positive electrode 4P. The negative cell incorporates a negative electrode 4N. The circulation mechanism 6 provided in the RF battery system 100 includes a positive electrode circulation mechanism 6P and a negative electrode circulation mechanism 6N. The positive electrode circulation mechanism 6P circulates the positive electrode electrolyte to the positive electrode cells. The negative electrode circulation mechanism 6N circulates the negative electrode electrolyte to the negative electrode cells.

[electrode]
As described above, the electrode 1 of this embodiment shown in FIG. 1 constitutes at least one of the positive electrode 4P and the negative electrode 4N shown in FIG. Electrode 1 contributes to the cell reaction. The electrode 1 has a first electrode 2 and a second electrode 3, as shown in FIG. The shape of the first electrode 2 and the shape of the second electrode 3 are sheet-like. The number of the first electrode 2 and the second electrode 3 may be singular or plural. Although illustration is omitted, the electrode 1 may include a third electrode in addition to the first electrode 2 and the second electrode 3 . Either the first electrode or the second electrode 3 may be arranged to face either the diaphragm 4M shown in FIG. 9 or the bipolar plate 51 described later.

(first electrode)
The first electrode 2 contains first carbon fibers 21 as a main component, as shown in FIG. Having the first carbon fibers 21 as the main component means that the ratio of the weight of the first carbon fibers 21 to the weight of the first electrode 2 is more than 40%. The ratio of the weight of the first carbon fibers 21 to the weight of the first electrode 2 may be 50% or more, 60% or more, and particularly 70% or more. The first electrode 2 includes a plurality of first carbon fibers 21 in this embodiment. A plurality of first carbon fibers 21 form a three-dimensional network structure. Voids 25 are provided in the mesh gaps. The voids 25 are typically provided between the plurality of first carbon fibers 21 . The first electrode 2 may contain at least one of binder, carbon particles, catalyst, hydrophilic material and hydrophobic material. The binder fixes the first carbon fibers 21 together or fixes the carbon particles to the first carbon fibers 21 . Carbon particles increase the surface area of the first electrode 2 . Catalysts promote cell reactions. In the first electrode 2, as shown in FIG. 4, unlike the second carbon fibers 31 of the second electrode 3 shown in FIG. 7, carbon particles are not fixed to the first carbon fibers 21 in this embodiment. The first electrode 2 satisfies the pore size relationship that the first median diameter based on the pore volume is 1.025 times or more the first mode diameter. That is, in the first electrode 2, the difference between the first median diameter and the first mode diameter satisfies 2.5% or more of the first mode diameter. The difference between the first median diameter and the first mode diameter is (first median diameter−first mode diameter).

The pore volume-based first median diameter is obtained from the cumulative pore volume curve of the cumulative pore distribution based on the mercury intrusion method. In the cumulative pore size distribution, the X-axis is the pore diameter and the Y-axis is the pore volume. The pore volume-based first median diameter is a pore diameter value corresponding to the middle between the minimum and maximum pore volumes in the cumulative pore volume curve. Note that the measurement range of the pore diameter on the X-axis is the range of the upper limit and the lower limit that can be measured by the mercury intrusion method. The upper limit is the minimum value of the pore volume on the Y axis. The lower limit is the maximum pore volume on the Y axis. The upper limit is usually about 500 μm. The lower limit is usually about 0.003 μm. The minimum value of the pore volume on the Y-axis is usually 0 (zero). Hereinafter, the first median diameter based on the pore volume may be simply referred to as the first median diameter. The first mode diameter is the pore diameter at which the differential value is maximized in the differential pore size distribution based on the mercury porosimetry.

The first electrode 2 in which the first median diameter is 1.025 times or more the first mode diameter has a larger gap 25 than the electrode in which the first median diameter is less than 1.025 times the first mode diameter. many. This first electrode 2 is excellent in electrolyte solution flowability.

The first median diameter may further be 1.03 times or more the first mode diameter, particularly 1.05 times or more the first mode diameter. An example of the first median diameter is 1.5 times or less the first mode diameter, further 1.3 times or less the first mode diameter, and particularly 1.2 times or less the first mode diameter. That is, the first median diameter is 1.025 to 1.5 times the first mode diameter, further 1.03 to 1.3 times the first mode diameter, particularly 1.05 times the first mode diameter. It is more than twice and less than 1.2 times. The difference between the first median diameter and the first mode diameter may be 3% or more of the first mode diameter, particularly 5% or more of the first mode diameter. An example of the difference between the first median diameter and the first mode diameter is 50% or less of the first mode diameter, further 30% or less of the first mode diameter, particularly 20% or less of the first mode diameter. That is, the difference between the first median diameter and the first mode diameter is 2.5% or more and 50% or less of the first mode diameter, further 3% or more and 30% or less of the first mode diameter, particularly 5% of the first mode diameter. % or more and 20% or less.

An example of the first median diameter is 80 μm or less. The first electrode 2 having a first median diameter of 80 μm or less has excellent battery reactivity. The first median diameter is also 75 μm or less, 60 μm or less, 55 μm or less, especially 50 μm or less. An example of the lower limit of the first median diameter is 3 μm. That is, the first median diameter is 3 μm or more and 80 μm or less, further 10 μm or more and 75 μm or less, and particularly 20 μm or more and 60 μm or less.

An example of the first mode diameter is 80 μm or less. The first electrode 2 having a first mode diameter of 80 μm or less has excellent battery reactivity. The first mode diameter is further 75 μm or less, 60 μm or less, 55 μm or less, especially 50 μm or less. An example of the lower limit of the first mode diameter is 2 μm. That is, the first mode diameter is 2 μm or more and 80 μm or less, further 10 μm or more and 75 μm or less, and particularly 20 μm or more and 60 μm or less.

An example of the first symmetry coefficient S _0.1h of the first curve is 1.5 or more. The first symmetry coefficient _S0.1h of the first curve C1 will be described with reference to FIG. A first curve C1 in FIG. 5 is a graph of the Log differential pore volume distribution of the first electrode 2 obtained by mercury porosimetry. The horizontal axis of FIG. 5 is the pore size (μm) of the first electrode 2 . The horizontal axis means that the pore size increases toward the right side of the paper. The vertical axis in FIG. 5 is the Log differential pore volume (cm ³ /g) of the first electrode 2 . The vertical axis means that the log differential pore volume increases toward the upper side of the paper. FIG. 5 is expressed for the purpose of clarifying the explanation of the first symmetry coefficient _S0.1h of the first curve C1, and does not necessarily represent the actual first curve C1. The first curve C1 of the first electrode 2 has one peak, as shown in FIG.

The first symmetry coefficient S _0.1h is obtained by “first width W _0.1h /(2×first width f _0.1h )”. The first width _W0.1h is the difference between the first pore diameter _D11 and the second pore diameter _D12 . The first width _f0.1h is the difference between the first pore diameter _D11 and the third pore diameter _D13 . The first pore diameter _D11 is the smaller one of the pore diameters at 10% of the height of the maximum peak in the first curve C1. The second pore diameter _D12 is the larger one of the pore diameters at 10% of the height of the maximum peak in the first curve C1. The third pore diameter _D13 is the pore diameter of the maximum peak of the first curve C1.

The first electrode 2 having a first symmetry coefficient S _0.1h of 1.5 or more has more large gaps 25 than the electrodes having a first symmetry coefficient S _0.1h of less than 1.5. This first electrode 2 is excellent in electrolyte solution flowability. The first symmetry factor S _0.1h is also 1.6 or more, especially 1.7 or more. An example of the first symmetry coefficient S _0.1h is 15 or less, and further 10 or less. That is, the first symmetry coefficient _S0.1h is 1.5 or more and 15 or less, further 1.6 or more and 10 or less, and particularly 1.7 or more and 10 or less.

An example of the average diameter of the plurality of first carbon fibers 21 is 2 μm or more and 30 μm or less. Since the average diameter of the plurality of first carbon fibers 21 is 2 μm or more, the strength of the first electrode 2 is increased. Since the average diameter of the plurality of first carbon fibers 21 is 30 μm or less, the surface area of the first carbon fibers 21 per unit weight is increased. Therefore, the first electrode 2 can perform sufficient battery reaction. The average diameter of the plurality of first carbon fibers 21 is further 5 μm or more and 25 μm or less, particularly 7 μm or more and 20 μm or less.

The average diameter of the first carbon fibers 21 is the average diameter of circles having the same area as the cross-sectional area of the first carbon fibers 21, and is obtained as follows. The first electrode 2 is cut to expose the cross section of the first carbon fiber 21 . The first electrode 2 is cut in the stacking direction of the electrode 1 . Five or more observation fields are taken under a microscope from the cross section of the first electrode 2 . A scanning electron microscope (SEM) is used for the microscope. Magnification shall be 500 times or more and 3000 times or less. For three or more first carbon fibers 21 per one observation field of view, the diameter of a circle having the same area as the cross-sectional area is obtained. Average the diameters of the circles obtained over the entire observation field.

An example of the basis weight of the first electrode 2 is 20 g/m ² or more and 400 g/m ² or less. The first electrode 2 having a basis weight of 20 g/m ² or more tends to increase contact points between the first carbon fibers 21 . Therefore, the conductivity of the first electrode 2 is easily increased. The first electrode 2 having a basis weight of 400 g/m ² or less easily secures the voids 25 . Therefore, the first electrode 2 is excellent in fluidity of the electrolytic solution. The basis weight of the first electrode 2 is further 25 g/m ² or more and 300 g/m ² or less, particularly 30 g/m ² or more and 200 g/m ² or less. The basis weight is obtained by measuring the weight per unit area.

An example of the porosity of the first electrode 2 is 60% or more. Porosity is determined by mercury porosimetry. The porosity is the ratio of the pore volume to the volume including the pores of the first electrode 2 . The first electrode 2 with a porosity of 60% or more has more voids 25 than an electrode with a porosity of less than 60%. This first electrode 2 is excellent in electrolyte solution flowability. An example of the upper limit of the porosity of the first electrode 2 is 99%. The first electrode 2 having a porosity of 99% or less has excellent battery reactivity. The porosity of the first electrode 2 is 60% or more and 99% or less, 60% or more and 95% or less, further 65% or more and 93% or less, particularly 75% or more and 90% or less.

The first electrode 2 is one selected from the group consisting of non-woven fabric, woven fabric and paper. The nonwoven fabric is obtained by entangling independent first carbon fibers 21 . The woven fabric is obtained by alternately weaving the warp and weft of the first carbon fibers 21 . The paper has a plurality of first carbon fibers 21 and a binder that fixes the first carbon fibers 21 . An example of the material of the paper binder is the same material as the binder 33 described later.

The 1st electrode 2 of this form is a nonwoven fabric. The first electrode 2 of this embodiment is obtained by entangling the first carbon fibers 21 by, for example, a needle punching method, a water punching method, or a stitch bonding method. The first electrode 2 of this embodiment can be obtained, for example, by appropriately adjusting the average diameter of the first carbon fibers 21, the average length of the first carbon fibers 21, the weight per unit area, and the entangling conditions.

(Second electrode)
The second electrode 3 contains second carbon fibers 31 as a main component, as shown in FIG. Having the second carbon fibers 31 as the main component means that the ratio of the weight of the second carbon fibers 31 to the weight of the second electrode 3 is more than 40%. The ratio of the weight of the second carbon fibers 31 to the weight of the second electrode 3 may be 50% or more, 60% or more, and particularly 70% or more. The second electrode 3 includes a plurality of second carbon fibers 31 in this embodiment. The multiple second carbon fibers 31 form a three-dimensional network structure. Voids 35 are provided in the mesh gaps. The voids 35 are typically provided between the plurality of second carbon fibers 31 . The second electrode 3 may contain at least one of binder, carbon particles, catalyst, hydrophilic material and hydrophobic material. The binder fixes the second carbon fibers 31 to each other or fixes the carbon particles to the second carbon fibers 31 . Carbon particles increase the surface area of the second electrode 3 . Catalysts promote cell reactions. The binder and carbon particles will be described later. The second electrode 3 satisfies the pore size relationship that the second median diameter based on the pore volume is 1.02 times or less the second mode diameter. That is, the second electrode 3 satisfies that the difference between the second median diameter and the second mode diameter is 2% or less of the second mode diameter. The difference between the second median diameter and the second mode diameter is (second median diameter - second mode diameter).

The definitions of the pore volume-based second median diameter and the second mode diameter are the same as the definitions of the pore volume-based first median diameter and the first mode diameter described above. That is, the pore volume-based second median diameter is obtained from the cumulative pore volume curve of the cumulative pore distribution based on the mercury intrusion method. In the cumulative pore size distribution, the X-axis is the pore diameter and the Y-axis is the pore volume. The pore volume-based second median diameter is a pore diameter value corresponding to the middle between the minimum and maximum pore volumes in the cumulative pore volume curve. Note that the measurement range of the pore diameter on the X-axis is the range of the upper limit and the lower limit that can be measured by the mercury intrusion method. The upper limit is the minimum value of the pore volume on the Y axis. The lower limit is the maximum pore volume on the Y axis. The upper limit is usually about 500 μm. The lower limit is usually about 0.003 μm. The minimum value of the pore volume on the Y axis is usually 0 (zero). Hereinafter, the second median diameter based on the pore volume may simply be referred to as the second median diameter. The second mode diameter is the pore diameter at which the differential value is maximized in the differential pore size distribution based on the mercury porosimetry.

The second electrode 3 whose second median diameter is 1.02 times or less the second mode diameter has many small voids 35 as compared with the electrode whose second median diameter is more than 1.02 times the second median diameter. This second electrode 3 is excellent in battery reactivity.

The second median diameter may be 1.015 times or less the second mode diameter, further 1.01 times or less the second mode diameter, particularly 1.00 times or less the second mode diameter. An example of the second median diameter is 0.5 times or more the second mode diameter, 0.6 times or more the second mode diameter, further 0.7 times or more the second mode diameter, particularly 0.7 times the second mode diameter. 8 times or more. That is, the second median diameter is 0.5 to 1.02 times the second mode diameter, 0.6 to 1.015 times the second mode diameter, and further 0.7 times the second mode diameter. 0.8 times or more and 1.00 times or less of the second mode diameter. The difference between the second median diameter and the second mode diameter may be 1.5% or less of the second mode diameter, further 1% or less of the second mode diameter, particularly 0% or less of the second mode diameter.

An example of the second median diameter is 60 μm or less. The second electrode 3 having a second median diameter of 60 μm or less has excellent battery reactivity. The second median diameter is also 55 μm or less, especially 50 μm or less. An example of the lower limit of the second median diameter is 3 μm. That is, the second median diameter is 3 μm or more and 60 μm or less, further 10 μm or more and 55 μm or less, and particularly 20 μm or more and 50 μm or less.

An example of the second mode diameter is 60 μm or less. The second electrode 3 having a second mode diameter of 60 μm or less has excellent battery reactivity. The second mode diameter is also 55 μm or less, especially 50 μm or less. An example of the lower limit of the second mode diameter is 2 μm. That is, the second mode diameter is 2 μm or more and 60 μm or less, further 10 μm or more and 55 μm or less, and particularly 20 μm or more and 50 μm or less.

An example of the second symmetry coefficient S _0.1h of the second curve is 1.5 or more. The second symmetry coefficient _S0.1h of the second curve C2 will be described with reference to FIG. A second curve C2 in FIG. 8 is a graph of the Log differential pore volume distribution of the second electrode 3 obtained by mercury porosimetry. The horizontal axis of FIG. 8 is the pore size (μm) of the second electrode 3 . The horizontal axis means that the pore size increases toward the right side of the paper. The vertical axis in FIG. 8 is the Log differential pore volume (cm ³ /g) of the second electrode 3 . The vertical axis means that the log differential pore volume increases toward the upper side of the paper. FIG. 8 is expressed for the purpose of clarifying the description of the second symmetry coefficient _S0.1h of the second curve C2, and does not necessarily represent the actual second curve C2. The second curve C2 of the second electrode 3 has multiple peaks, as shown in FIG. FIG. 8 shows an example where there are three peaks. The second symmetry coefficient _S0.1h of the second curve C2 refers to the symmetry coefficient of the curve having the maximum peak among the plurality of peaks.

The second symmetry coefficient S _0.1h is obtained by “second width W _0.1h /(2×second width f _0.1h )”. The second width _W0.1h is the difference between the first pore diameter _D21 and the second pore diameter _D22 . The second width f _0.1h is the difference between the first pore diameter D ₂₁ and the third pore diameter D ₂₃ . The first pore diameter _D21 is the smaller one of the pore diameters at 10% of the height of the maximum peak in the second curve C2. The second pore diameter _D22 is the larger one of the pore diameters at 10% of the height of the maximum peak in the second curve C2. The third pore diameter _D23 is the pore diameter of the maximum peak of the second curve C2.

The second electrode 3 having a second symmetry coefficient S _0.1h of 1.5 or more has more large gaps 35 than the electrode having a second symmetry coefficient S _0.1h of less than 1.5. This second electrode 3 is excellent in electrolyte solution flowability. The second symmetry factor S _0.1h is also 1.6 or more, especially 1.7 or more. An example of the second symmetry coefficient S _0.1h is 15 or less, and further 10 or less. That is, the second symmetry coefficient _S0.1h is 1.5 or more and 15 or less, further 1.6 or more and 10 or less, and particularly 1.7 or more and 10 or less.

For example, a plurality of peaks may be present in the Log differential pore volume distribution of the second electrode 3 determined by mercury porosimetry. The plurality of peaks includes a second mode diameter peak. The peak intensity of the second mode diameter is the maximum peak intensity among the multiple peaks. That is, the plurality of peaks includes peaks having peak intensities smaller than the peak intensity of the second mode diameter. One or more peaks having a peak intensity smaller than the peak intensity of the second mode diameter may exist in the range of pore diameters smaller than the second mode diameter. In this case, the ratio of the second peak intensity to the first peak intensity may be 10 ⁻¹⁰ or more. The first peak intensity is the maximum peak intensity, that is, the peak intensity of the second mode diameter. The second peak intensity is the next largest peak intensity after the peak intensity of the second mode diameter among the peak intensities of the one or more peaks. That is, the second peak intensity is smaller than the peak intensity of the second mode diameter and is the highest peak intensity among the peak intensities of the one or more peaks. The second electrode 3 may have such a distribution.

For example, as shown in FIG. 8, in the log differential pore volume distribution of the second electrode 3 obtained by mercury porosimetry, there are multiple peaks within the pore diameter range of 0.01 μm or more and 200 μm or less on the second curve C2. May be present. FIG. 8 shows an example in which three peaks exist within the pore diameter range of 0.01 μm or more and 200 μm or less. Two of the three peaks are in the range of pore sizes smaller than the maximum peak. In FIG. 8, the maximum peak is the rightmost peak and the peak of the third pore diameter _D23 . In FIG. 8, the second largest peak is the second peak from the right. In this case, an example of the ratio of the second largest peak intensity to the maximum peak intensity in the plurality of peaks may be 10 ⁻¹⁰ or more. The second electrode 3 may have such a distribution.

An example of (second mode diameter×Log differential pore volume at diameter of 1/10)/(Log differential pore volume at second mode diameter) is 0.002 or more. The Log differential pore volume at the second mode diameter×1/10 diameter and the Log differential pore volume at the second mode diameter are obtained by mercury porosimetry. The second electrode 3 having the Log differential pore volume ratio of 0.002 or more has more small voids 35 than the electrode having the Log differential pore volume ratio of less than 0.002. This second electrode 3 is excellent in battery reactivity. The above Log differential pore volume ratio is 0.010 or more, more preferably 0.020 or more. An example of the above Log differential pore volume ratio is 0.5 or less, or even 0.3 or less. The above Log differential pore volume ratio is 0.002 or more and 0.5 or less, 0.010 or more and 0.3 or less, and further 0.020 or more and 0.3 or less.

An example of the average diameter of the plurality of second carbon fibers 31 is 2 μm or more and 30 μm or less. The reasons for the upper and lower limits of the average diameter of the plurality of second carbon fibers 31 are the same as the reasons for the upper and lower limits of the average diameter of the plurality of first carbon fibers 21 . The average diameter of the plurality of second carbon fibers 31 is further 5 μm or more and 25 μm or less, particularly 7 μm or more and 20 μm or less. The method of obtaining the average diameter of the plurality of second carbon fibers 31 is the same as the method of obtaining the average diameter of the first carbon fibers 21 described above.

The second electrode 3 of this embodiment includes carbon particles 32 and a binder 33, as shown in FIG. Unlike this embodiment, the second electrode 3 may not contain the carbon particles 32 and the binder 33 shown in FIG. The binder 33 fixes the carbon particles 32 to the second carbon fibers 31 . The binder 33 firmly fixes the carbon particles 32 in contact with the second carbon fibers 31 . Therefore, the carbon particles 32 are less likely to fall off from the second carbon fibers 31 over a long period of time. The second electrode 3 has carbon particles 32 fixed to the second carbon fibers 31 by a binder 33, and thus has excellent battery reactivity. The number of carbon particles 32 is plural in this embodiment.

An example of the average particle size of the plurality of carbon particles 32 is 0.01 μm or more and 30 μm or less, and further 0.03 μm or more and 20 μm or less. The average particle size of the plurality of carbon particles 32 is the average diameter of circles having the same area as the cross-sectional area of the carbon particles 32, and is obtained as follows. The second electrode 3 is cut so that cross sections of the plurality of carbon particles 32 are exposed. Five or more observation fields are taken from the cross section of the second electrode 3 under a scanning electron microscope. The diameter of a circle having the same area as the cross-sectional area of three or more carbon particles 32 is obtained for one observation field of view. Average the diameters of the circles obtained over the entire observation field.

An example of the material of the binder 33 is a carbon-based material. The material of the binder 33 is, for example, pitch, resin, alcohol, or rubber. An example of pitch is coal pitch or petroleum pitch. Examples of resins include phenolic resins, benzoxazine resins, epoxide resins, furan resins, vinyl ester resins, melamine-formaldehyde resins, urea-formaldehyde resins, resorcinol-formaldehyde resins, cyanate ester resins, bismaleimide resins, polyurethane resins, or poly Acrylonitrile resin. An example of an alcohol is furfuryl alcohol. One example of a rubber is acrylonitrile-butadiene rubber.

An example of the abundance ratio of the carbon particles 32 in the electrode 1 is 3% by mass or more and 60% by mass or less, and further 5% by mass or more and 55% by mass or less. The abundance ratio of the carbon particles 32 is the mass ratio of the total content of the elements forming the carbon particles 32 when the total content of the second carbon fibers 31, the carbon particles 32, and the binder 33 is 100% by mass. The mass ratio of the carbon particles 32 is determined by thermogravimetry.

An example of the abundance ratio of the binder 33 in the electrode 1 is 3% by mass or more and 60% by mass or less, and further 5% by mass or more and 55% by mass or less. The existence ratio of the binder 33 is the mass ratio of the total content of the elements constituting the binder 33 when the total content of the second carbon fibers 31, the carbon particles 32, and the binder 33 is 100% by mass. The mass ratio of the binder 33 is determined by thermogravimetry.

An example of the ratio of the mass of the binder 33 to the mass of the carbon particles 32 is 0.1 or more and 10.0 or less, and further 0.3 or more and 8.0 or less.

An example of the basis weight of the second electrode 3 is 20 g/m ² or more and 600 g/m ² or less. The second electrode 3 having a basis weight of 20 g/m ² or more tends to increase contact points between the second carbon fibers 31 . Therefore, the conductivity of the second electrode 3 is easily increased. The second electrode 3 having a basis weight of 600 g/m ² or less easily secures the voids 35 . Therefore, the second electrode 3 is excellent in flowability of the electrolytic solution. The basis weight of the second electrode 3 is further 25 g/m ² or more and 500 g/m ² or less, particularly 30 g/m ² or more and 450 g/m ² or less.

The second electrode 3 is one selected from the group consisting of nonwoven fabric, woven fabric, and paper. The second electrode 3 of this embodiment is a nonwoven fabric. The second electrode 3 of this embodiment is obtained by entangling the second carbon fibers 31 by a hydroentanglement method. The second electrode 3 of this embodiment includes, for example, the average diameter of the second carbon fibers 31, the average length of the second carbon fibers 31, the average particle diameter of the carbon particles 32, the abundance ratio of the carbon particles 32, the material of the binder 33, It can be obtained by appropriately adjusting the abundance ratio of the binder 33, the mass ratio of the carbon particles 32 and the binder 33, the basis weight, and the entangling conditions.

(third electrode)
As described above, the electrode 1 may include a third electrode (not shown). The third electrode meets the following requirements (a) and (b): different from
(a) The pore volume-based third median diameter is more than 1.02 times and less than 1.025 times the third mode diameter.
(b) An example of the third symmetry coefficient S _0.1h of the third curve is less than 1.5.
The concept of the pore volume-based third median diameter, third mode diameter, and third symmetry coefficient S _{0.1 h} is based on the pore volume-based first median diameter, first mode diameter, and first symmetry coefficient S _{0 .1h} . When the electrode 1 has a third electrode, the stacking order of the first electrode 2, the second electrode 3 and the third electrode does not matter.

[Cell stack]
Battery cells 4 are usually formed inside a structure called a cell stack 200 as shown in FIGS. 9 and 10 . The cell stack 200 includes a substack 201 , two end plates 220 and a tightening mechanism 230 . The cell stack 200 exemplifies a form including a plurality of sub-stacks 201 in the lower diagram of FIG. 10 . Each sub-stack 201 comprises a laminate and two feeding/discharging plates 210, as shown in the lower diagram of FIG. As shown in FIGS. 9 and 10, the laminate is constructed by stacking a plurality of cell frames 5, positive electrodes 4P, diaphragms 4M, and negative electrodes 4N in this order. The supply/discharge plates 210 are arranged at both ends of the laminate, as shown in the lower diagram of FIG. A supply pipe 63 and a discharge pipe 65 of the positive electrode circulation mechanism 6P and a supply pipe 64 and a discharge pipe 66 of the negative electrode circulation mechanism 6N, which will be described later, are connected to the supply/discharge plate 210 . The two end plates 220 sandwich the plurality of substacks 201 from the outside of the substacks 201 at both ends. A tightening mechanism 230 tightens both end plates 220 .

[Cell Frame]
The cell frame 5 includes a bipolar plate 51 and a frame 52, as shown in the upper diagram of FIG. The frame 52 surrounds the outer peripheral edge of the bipolar plate 51 . One battery cell 4 is formed between the bipolar plates 51 of adjacent cell frames 5 . A positive electrode 4P is arranged on one side of the bipolar plate 51 so as to face each other. A negative electrode 4N is arranged on the other surface of the bipolar plate 51 so as to face each other. The frame 52 is formed with liquid supply manifolds 53 and 54, liquid supply slits 53s and 54s, liquid discharge manifolds 55 and 56, and liquid discharge slits 55s and 56s, which will be described later. An annular seal member 57 is arranged in an annular seal groove between the frames 52 .

[Positive electrode circulation mechanism/negative electrode circulation mechanism]
The positive electrode circulation mechanism 6P includes a positive electrode electrolyte tank 61, a supply pipe 63, a discharge pipe 65, and a pump 67, as shown in FIG. A positive electrode electrolyte is stored in the positive electrode electrolyte tank 61 . The positive electrode electrolyte flows through the supply pipe 63 and the discharge pipe 65 . The supply pipe 63 connects the positive electrode electrolyte tank 61 and the positive electrode cell. The discharge pipe 65 connects the positive electrode cell and the positive electrode electrolyte tank 61 . The pump 67 pressure-feeds the positive electrode electrolyte in the positive electrode electrolyte tank 61 . The pump 67 is provided in the middle of the supply pipe 63 .

The negative electrode circulation mechanism 6</b>N includes a negative electrode electrolyte tank 62 , a supply pipe 64 , a discharge pipe 66 and a pump 68 . A negative electrode electrolyte is stored in the negative electrode electrolyte tank 62 . A negative electrode electrolyte is circulated through the supply pipe 64 and the discharge pipe 66 . A supply pipe 64 connects the negative electrode electrolyte tank 62 and the negative electrode cell. The discharge pipe 66 connects the negative electrode cell and the negative electrode electrolyte tank 62 . A pump 68 pumps the negative electrode electrolyte in the negative electrode electrolyte tank 62 . A pump 68 is provided in the middle of the supply pipe 64 .

During charge/discharge operation, the flow of the positive electrode electrolyte and the negative electrode electrolyte is as follows. The positive electrode electrolyte is supplied from the positive electrode electrolyte tank 61 through the supply pipe 63 to the positive electrode cell by the pump 67 . The positive electrode electrolyte is supplied from the liquid supply manifold 53 shown in the upper diagram of FIG. 10 to the positive electrode 4P through the liquid supply slit 53s. The positive electrode electrolyte supplied to the positive electrode 4P flows from the lower side to the upper side of the positive electrode 4P as indicated by the arrows in the upper diagram of FIG. The positive electrode electrolyte that has flowed through the positive electrode 4</b>P flows through the drain slit 55 s and is discharged to the drain manifold 55 . The positive electrode electrolyte flows from the positive electrode cell through the discharge pipe 65 and is discharged to the positive electrode electrolyte tank 61 . A pump 68 supplies the negative electrode electrolyte from the negative electrode electrolyte tank 62 through the supply pipe 64 to the negative electrode cell. The negative electrode electrolyte flows from the liquid supply manifold 54 through the liquid supply slit 54s and is supplied to the negative electrode 4N. The negative electrode electrolyte supplied to the negative electrode 4N flows from the lower side to the upper side of the negative electrode 4N as indicated by the arrows in the upper diagram of FIG. The negative electrode electrolyte that has flowed through the negative electrode 4N flows through the drain slit 56s and is discharged to the drain manifold 56. FIG. The negative electrode electrolyte flows from the negative electrode cell through the discharge pipe 66 and is discharged to the negative electrode electrolyte tank 62 . By this supply and discharge, the positive electrode electrolyte and the negative electrode electrolyte are circulated to the positive electrode cell and the negative electrode cell. The pumps 67 and 68 are stopped during standby when charging and discharging are not performed. That is, the positive electrolyte and the negative electrolyte are not circulated.

[Electrolyte]
The positive electrode active material contained in the positive electrode electrolyte is one or more selected from the group consisting of manganese ions, vanadium ions, iron ions, polyacids, quinone derivatives, and amines. The negative electrode active material contained in the negative electrode electrolyte is one or more selected from the group consisting of titanium ions, vanadium ions, chromium ions, polyacids, quinone derivatives, and amines. When the positive electrode electrolyte contains manganese ions and the negative electrode electrolyte contains titanium ions, the RF battery system 100 having a high electromotive force is likely to be obtained. In FIG. 9, manganese (Mn) ions are exemplified as ions contained in the positive electrode electrolyte, and titanium (Ti) ions are exemplified as ions contained in the negative electrode electrolyte. The solvent for the positive electrode electrolyte and the negative electrode electrolyte is, for example, an aqueous solution containing one or more acids or acid salts selected from the group consisting of sulfuric acid, phosphoric acid, nitric acid, and hydrochloric acid.

The RF battery system 100 of the present embodiment includes an electrode 1 having a first electrode 2 having excellent electrolyte flowability and a second electrode 3 having excellent battery reactivity, thereby easily reducing diffusion resistance and charge transfer resistance. .

<<Test example>>
A plurality of electrodes were prepared, and the battery reactivity and electrolyte flowability were examined.

[Sample No. 1 to sample no. 3]
Sample no. 1 to sample no. The electrode of the battery cell No. 3 was an electrode having a laminated structure in which the first electrode and the second electrode were laminated.

The first electrode of each sample includes a plurality of first carbon fibers and voids. The first electrode of each sample was free of carbon particles and binder. The average diameter of the first carbon fibers of each sample is within the range of 2 μm or more and 30 μm or less. The average diameter of the first carbon fibers was obtained by the same method as described in the embodiment. Sample no. The basis weight of the first electrode of No. 1 is 110 g/m ² . Sample no. The weight per unit area of the first electrode No. 2 is 80 g/m ² . Sample no. The basis weight of the first electrode of No. 3 is 90 g/m ² . The basis weight was determined by the method described in the embodiment.

A second electrode of each sample includes a plurality of second carbon fibers, a plurality of carbon particles, a binder and voids. The average diameter of the second carbon fibers of each sample is within the range of 2 μm or more and 30 μm or less. The average diameter of the second carbon fibers was obtained by the same method as for the first carbon fibers. Sample no. The basis weight of the second electrode of 1 is 120 g/m ² . Sample no. The basis weight of the second electrode No. 2 is 150 g/m ² . Sample no. The basis weight of the second electrode of No. 3 is 110 g/m ² . The basis weight was obtained by the same method as for the first electrode. The average particle size of the carbon particles of each sample is within the range of 0.01 μm or more and 30 μm or less. The abundance ratio of the carbon particles in each sample is within the range of 5% by mass or more and 55% by mass or less. The material of the binder of each sample is pitch-based. The abundance ratio of the binder in each sample is 5% by mass or more and 55% by mass or less. The mass ratio of binder to carbon particles in each sample is in the range of 0.3 to 8.0. Sample no. 1 and sample no. The mass ratio of the binder to the carbon particles of Sample No. 2 is greater than the binder to carbon particle weight ratio of 3. Sample no. 1, sample no. 2, and sample no. In the order of 3, the existence ratio of the binder is small. Sample no. 1, sample no. 2, and sample no. In the order of 3, the existence ratio of carbon particles is large. The average particle size and the ratio of both were obtained by the method described in the embodiment.

Table 1 shows the pore volume-based median diameter, mode diameter, symmetry coefficient, Log differential pore volume ratio, and porosity of the first and second electrodes of each sample. The Log differential pore volume ratio is (mode diameter×Log differential pore volume at 1/10 diameter)/(Log differential pore volume of mode diameter). These values were measured by mercury porosimetry. For the measurement, a pore distribution analyzer Autopore IV 9520 manufactured by Shimadzu Corporation-Micromeritics was used. Table 1 also shows the value of mode diameter×1.025 and the value of mode diameter×1.02 in the electrode of each sample. The mode diameter used to calculate the value of mode diameter×1.025 is the mode diameter of the first electrode. The mode diameter used to calculate the value of mode diameter×1.02 is the mode diameter of the second electrode. These values are rounded to the third decimal place.

A strip piece obtained by cutting each electrode into a size of about 12.5 mm×25 mm was used as a measurement sample. The weight of the measurement sample was about 0.03g to 0.24g. A measurement sample was placed in a 5 cc powder cell. The stem volume of the powder cell is 0.4 cc. It was measured under the condition of an initial pressure of about 3.7 kPa. About 3.7 kPa corresponds to about 0.5 psia (pound-force per square inch absolute), which corresponds to a pore diameter of about 340 μm. Mercury parameters were set to instrument default mercury contact angle of 130 degrees and mercury surface tension of 485 dynes/cm (485 mN/m).

[Sample No. 101]
Sample no. The electrode of the battery cell of 101 was made into the electrode of one sheet. Sample no. The electrodes at 101 contain carbon fibers. Sample no. The pore volume-based median diameter, mode diameter, symmetry coefficient, Log differential pore volume ratio, and porosity of 101 electrodes are shown in Table 1. Table 1 shows sample no. Values of mode diameter x 1.025 and mode diameter x 1.02 for 101 electrodes are also shown. These values are rounded to the third decimal place.

[Evaluation of battery reactivity]
A single cell battery was produced using the battery cell of each sample, and various resistivities described later were measured. A single cell battery is a battery having one positive electrode, one diaphragm, and one negative electrode. A single cell battery was constructed by stacking a first cell frame, a positive electrode, a diaphragm, a negative electrode, and a second cell frame in this order. The diaphragm is sandwiched between the positive electrode and the negative electrode. The first cell frame is arranged such that the bipolar plate provided in the first cell frame and the positive electrode are in contact with each other. The second cell frame is arranged such that the bipolar plate provided in the second cell frame and the negative electrode are in contact with each other. Carbon paper was used for the positive electrode. Sample No. 1 was used as the negative electrode. 1 to sample no. 3, and sample no. 101 individual electrodes were used. Sample no. 1 and sample no. In No. 2, as shown in Table 1, the first electrode was arranged on the diaphragm side and the second electrode was arranged on the bipolar plate side. Sample no. In 3, as shown in Table 1, the first electrode was arranged on the bipolar plate side and the second electrode was arranged on the diaphragm side. A manganese sulfate solution containing manganese ions as an active material was used as the positive electrode electrolyte. A titanium sulfate solution containing titanium ions as an active material was used as the negative electrode electrolyte.

The battery cell of each sample was charged and discharged at a constant current with a current density of 100 mA/cm ² . In this test, 3 cycles of charging and discharging were performed. In this test, charging was switched to discharging when a preset switching voltage was reached, and discharging was switched to charging when a preset switching voltage was reached. The switching voltage from charge to discharge was set to 1.62V. The switching voltage from discharging to charging was set to 1.0V. As a result, all sample RF batteries could be charged and discharged.

After charging and discharging at the above constant current, the following charging was performed again, and the conductive resistivity (Ω·cm 2 ), charge transfer resistivity (Ω·cm ² ), diffusion resistivity (Ω·cm ² ), and diffusion resistivity (Ω·cm ² ) was obtained by the following method. The results are shown in Table 2 together with the cell resistivity (Ω·cm ² ). Cell resistivity is the sum of conductive resistivity, charge transfer resistivity and diffusion resistivity. The charging here was performed until the midpoint SOC was reached. The midpoint SOC is the average sum of the maximum SOC value and the minimum SOC value in charging and discharging at the constant current.

[Conductive resistivity]
Conductive resistivity was defined as impedance at a measurement frequency of 1 kHz in the AC impedance method.

[Charge transfer resistivity]
Charge transfer resistivity was measured by the AC impedance method. The charge transfer resistivity was measured using a commercially available measuring device, applying a bias centered on an open circuit voltage, with a voltage amplitude of 10 mV and a measurement frequency range of 10 kHz to 10 mHz.

[Diffusion resistivity]
Diffusion resistivity was also measured by the AC impedance method in the same manner as charge transfer resistivity.

[Evaluation of flowability of electrolytic solution]
Using the measurement system 600 shown in FIG. 11, the pressure loss ΔP of each sample was measured. The measuring system 600 comprises a measuring cell 610 , a fluid bath 620 , a pipe 630 , a pump 640 , a flow meter 650 and a differential pressure gauge 660 .

The measurement cell 610 is a single cell battery having the same structure as the single cell batteries used for the various resistivity measurements described above. Fluid reservoir 620 stores fluid 622 to be supplied to the electrodes in measurement cell 610 . Fluid 622 is, for example, water. A pipe 630 connects between the fluid bath 620 and the measurement cell 610 . Pump 640 is provided in piping 630 to pump fluid 622 in fluid reservoir 620 to measurement cell 610 . Fluid 622 discharged from measurement cell 610 is returned to fluid reservoir 620 via piping 630 . Thus, the pump 640 and the pipe 630 circulate the fluid 622 in the fluid bath 620 to the measurement cell 610 . The dashed-dotted arrows shown in FIG. 11 indicate the flow direction of the fluid 622 . Flow meter 650 is provided downstream of pump 640 in pipe 630 and upstream of measurement cell 610 . Flow meter 650 measures the flow rate of fluid 622 discharged from pump 640 . A branch pipe 632 bypassing the measurement cell 610 is provided downstream of the flow meter 650 in the pipe 630 . A differential pressure gauge 660 is provided on the branch pipe 632 . Also, the differential pressure gauge 660 is provided in parallel with the measurement cell 610 .

Differential pressure gauge 660 measures the difference (P ₀ −P ₁ ) between the pressure P ₀ of fluid 622 supplied to measurement cell 610 and the pressure P ₁ of fluid 622 discharged from measurement cell 610 . The pressure loss ΔP is the above pressure difference (P ₀ −P ₁ ). It can be said that the smaller the pressure loss ΔP, the more excellent the circulation of the electrolyte in the measurement cell 610 . Table 2 shows the pressure loss results for each sample. The pressure loss results in Table 2 are for sample no. It is shown as a relative value with the pressure loss ΔP of 1 being 1.0.

As shown in Table 2, sample no. 1 to sample no. The conductive resistivity of each of Sample No. 3 is Equivalent to 101. Sample no. 1 to sample no. The charge transfer resistivity and diffusion resistivity of each of sample Nos. smaller than 101. As a result, sample no. 1 to sample no. The cell resistivity of each of sample nos. smaller than 101. Sample no. 1 to sample no. The pressure loss of each sample No. 3 is smaller than 101.

Sample no. 1 to sample no. It was found that No. 3 has both good battery reactivity and good fluidity of the electrolyte.

The present invention is not limited to these exemplifications, but is indicated by the scope of the claims, and is intended to include all modifications within the meaning and scope of equivalents of the scope of the claims.

100 RF battery system 1 electrode 2 first electrode 21 first carbon fiber 25 void 3 second electrode 31 second carbon fiber 32 carbon particles 33 binder 35 void 4 battery cell 4M diaphragm 4P positive electrode 4N Negative Electrode 5 Cell Frame 51 Bipolar Plate 52 Frame 53, 54 Liquid Supply Manifold 53s, 54s Liquid Supply Slit 55, 56 Drainage Manifold 55s, 56s Drainage Slit 57 Seal Member 6 Circulation Mechanism, 6P Positive Electrode Circulation Mechanism , 6N negative electrode circulation mechanism 61 positive electrode electrolyte tank 62 negative electrode electrolyte tank 63, 64 supply pipe 65, 66 discharge pipe 67, 68 pump 200 cell stack 201 sub stack 210 supply and discharge plate 220 end plate 230 tightening Auxiliary Mechanism 300 AC/DC Converter 310 Power Generation Unit 320 Substation Equipment 330 Load 600 Measurement System 610 Measurement Cell 620 Fluid Tank 622 Fluid 630 Piping 632 Branch Pipe 640 Pump 650 Flow Meter 660 Differential Pressure Gauge C1 One curve, C2 second curve

Claims (12)

An electrode for use in a redox flow battery system, comprising:
a first electrode comprising a first carbon fiber and a void;
a second electrode comprising a second carbon fiber and a void;
The first electrode and the second electrode are laminated,
The pore volume-based first median diameter of the first electrode obtained by mercury porosimetry is 1.025 times or more the first mode diameter of the first electrode obtained by mercury porosimetry,
The pore volume-based second median diameter of the second electrode obtained by mercury porosimetry is 1.02 times or less of the second mode diameter of the second electrode obtained by mercury porosimetry,
electrode. The first mode diameter is 80 μm or less,
2. The electrode of claim 1, wherein the second mode diameter is 60 [mu]m or less. The first median diameter is 80 μm or less,
3. The electrode according to claim 1, wherein said second median diameter is 60 [mu]m or less. In the Log differential pore volume distribution of the first electrode obtained by mercury porosimetry, the first symmetry coefficient S _0.1h of the first curve having the maximum peak is 1.5 or more,
Claims 1 to 3, wherein the second symmetry coefficient _S0.1h of the second curve having the maximum peak in the Log differential pore volume distribution of the second electrode obtained by mercury porosimetry is 1.5 or more. The electrode according to any one of . (Log differential pore volume at the second mode diameter x 1/10 diameter obtained by mercury porosimetry)/(Log differential pore volume of the second mode diameter obtained by mercury porosimetry) is 0.002 or more 5. The electrode according to any one of claims 1 to 4, wherein The number of the first carbon fibers and the number of the second carbon fibers are plural,
The electrode according to any one of claims 1 to 5, wherein the average diameter of the plurality of first carbon fibers and the average diameter of the plurality of second carbon fibers are 2 µm or more and 30 µm or less. 7. The electrode according to claim 1, wherein said first electrode has a porosity of 60% or more as determined by mercury porosimetry. The second electrode is
carbon particles;
and a binder fixing the carbon particles to the second carbon fibers. A battery cell used in a redox flow battery system,
An electrode according to any one of claims 1 to 8,
battery cell. A cell stack used in a redox flow battery system,
A plurality of battery cells according to claim 9,
cell stack. The battery cell according to claim 9 or the cell stack according to claim 10,
Redox flow battery system. A positive electrode electrolyte and a negative electrode electrolyte supplied to the battery cell,
The positive electrode electrolyte contains manganese ions,
12. The redox flow battery system of claim 11, wherein said negative electrode electrolyte comprises titanium ions.

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