JP2020077540A - Redox flow cell - Google Patents

Abstract

To provide a redox flow cell capable of improving degree-of-freedom in a layout of a collector plate while preventing contact failure between the collector plate and a porous member, and damage in the porous member.SOLUTION: A redox flow cell includes: a carbon foam 15; and a collector plate 14. The carbon foam 15 is made of a carbon fiber having a liner part and a coupling part that couples the liner part. The collector plate 14 has at least one contact surface. The contact surface is laminated onto the carbon foam so as to be surface-contact. A height position in a lamination direction of the surface-contact is different as a position of a virtual flat surface vertical to a lamination direction.SELECTED DRAWING: Figure 2

Description

  The present invention relates to redox flow batteries.

  It is known that a cell is formed by laminating a porous member as an electrode between a diaphragm and a current collector (see Patent Document 1). By roughening the contact surface of the current collector plate with the porous member, providing a step, or inclining it, it is possible to prevent the displacement of the porous member with respect to the current collector plate (see Patent Document 2). It is expected that various functions will be exhibited depending on the structure of the current collector plate.

JP, 2001-085028, A International publication 2017/208570 gazette

  However, in order to use it as a cell, it is required to prevent poor contact between the porous member and the current collector and the contact plate and damage to the porous member. For example, in Patent Document 2, if the contact surface of the current collector plate with the porous member is roughened, a step is provided, or inclined, the current collector plate may damage the porous member when assembled into a cell. There is. Further, in this structure, poor contact between the current collector plate and the porous member may occur, the surface area that functions as an electrode becomes insufficient, and the cell resistance increases.

  In order to prevent contact failure and damage, it is conventionally necessary to accurately match the height of the contact surface of the current collector plate with the porous member. However, although various functions are expected to be exhibited depending on the structure of the current collector, the degree of freedom in designing the current collector was low due to the restriction of the contact surface.

  Therefore, an object of the present invention is to provide a redox flow battery that improves the degree of freedom in designing a current collector while preventing poor contact between the current collector and the porous member and damage to the porous member.

  The present inventors diligently studied how to solve the above problems. As a result, by applying a highly flexible porous member, it was found that contact failure can be prevented by the porous member following and contacting the contact surfaces of the current collecting plate having different heights, and The invention was completed.

That is, the present invention is as follows.
[1]
A carbon foam made of carbon fiber having a linear part and a bonding part for bonding the linear part;
A current collector having at least one contact surface that is laminated on the carbon foam and is in surface contact, and a height position of the contact surface in a stacking direction varies depending on a position of an imaginary plane perpendicular to the stacking direction. Redox flow battery.
[2]
The redox flow battery according to [1], wherein the current collector plate has a plurality of ridges each including the contact surface.
[3]
The redox flow battery according to [1] or [2], wherein the height position of the contact surface in the stacking direction decreases from one end of the current collector plate toward the other end.
[4]
The redox flow battery according to [3], wherein the one end is an end on the discharge side of the electrolytic solution, and the other end is an end on the supply side of the electrolytic solution.
[5]
The redox flow battery according to [3], wherein at least a part of the contact surface is inclined with respect to the virtual plane.
[6]
The redox flow battery according to any one of [1] to [4], wherein at least a part of the contact surface is a curved surface.
[7]
The redox flow battery according to any one of [1] to [6], wherein the ratio of the number of the linear portions to the number of the coupling portions is 1.4 or more and 1.6 or less.
[8]
The redox flow battery according to any one of [1] to [7], wherein the carbon fiber has a fiber diameter of 1 μm or more and 5 μm or less.
[9]
An average value of orientation angles of the linear portions included in the region of 300 μm × 300 μm × 300 μm of the carbon foam with respect to the stacking direction, and an average value of orientation angles with respect to two directions perpendicular to the stacking direction and perpendicular to each other. The redox flow battery according to any one of [1] to [8], wherein the difference is 3 ° or more.
[10]
The redox flow battery according to any one of [1] to [9], wherein the difference between the maximum height and the minimum height of the contact surface is 0.01 mm or more and 0.5 mm or less.
[11]
The carbon foam is a melamine carbon foam obtained by carbonizing a melamine resin foam. The redox flow battery according to any one of [1] to [10].

  According to the present invention, it is possible to provide a redox flow battery in which the degree of freedom in designing a current collector is improved while preventing poor contact between the current collector and the porous member.

It is a typical block diagram of the redox flow battery which concerns on 1st Embodiment. FIG. 2 is a cross-sectional view along a stacking direction showing a schematic configuration of a charging / discharging unit of FIG. 1. It is an external appearance perspective view seen from the contact surface side of the current collector plate of FIG. It is sectional drawing along the lamination direction which shows the typical structure of the charging / discharging part of 2nd Embodiment.

  Hereinafter, embodiments for carrying out the present invention will be described in detail, but the present invention is not limited to the following description, and various modifications can be carried out within the scope of the gist thereof.

(Redox flow battery)
As shown in FIG. 1, the redox flow battery 10 according to the first embodiment includes tanks 11 on the anode side and the anode side, pumps 12 on the anode side and the anode side, and a charging / discharging unit 13. The tank 11 stores the electrolyte solutions on the anode side and the anode side, respectively. The pump 12 circulates the electrolytic solution between the tank 11 and the charging / discharging unit 13.

(Charging / discharging part)
As shown in FIG. 2, the charging / discharging unit 13 includes a collector plate 14 on the anode side and the anode side, a carbon foam 15 on the anode side and the anode side, an electrolyte membrane 16, and a frame 17. In the charging / discharging unit 13, the current collector plate 14, the carbon foam 15, the electrolyte membrane 16, the carbon foam 15, and the current collector plate 14 are sequentially stacked. The electrolyte membrane 16 and the carbon foam 15 are sandwiched by two current collector plates 14 separated by a frame 17 at a predetermined interval.

  In the first embodiment, as will be described later, the first flow path of the current collector plate 14 is connected to the discharge pipe of the pump 12. The second flow path of the current collector plate 14 is connected to a pipe for supplying the tank 11. In the first embodiment, the electrolytic solution supplied to the first flow path is discharged from the charging / discharging section 13 via the second flow path while passing through the carbon foam 15 with which the current collector plate 14 abuts. To be done. The electrolytic solution discharged from the pump 12 circulates between the tank 11 and the charging / discharging unit 13. The redox flow battery 10 charges and discharges by causing electrochemical energy conversion on the carbon foam 15 while circulating an electrolytic solution between the tank 11 and the charging / discharging unit 13 by a pump 12.

(Current collector)
As shown in FIG. 3, the current collector plate 14 has at least one contact surface (see the hatched portion) that makes surface contact with the carbon foam 15. In the specification of the present application, two directions perpendicular to the thickness direction of the main surface of the current collector plate 14 and perpendicular to each other are referred to as a first direction and a second direction. The current collector plate 14 may have the contact surface on one main surface side.

  In the current collector plate 14, the height position of the contact surface in the thickness direction differs depending on the position on the virtual plane perpendicular to the thickness direction. The difference in the height position of the contact surface in the thickness direction is relatively small, and in FIG. 3, the height positions are illustrated equal for convenience.

  As described later, the current collector plates 14 are stacked in the charging / discharging unit 13 such that the thickness direction of the current collector plates 14 and the stacking direction are parallel to each other. Therefore, the height position of the contact surface in the stacking direction differs depending on the position on the virtual plane perpendicular to the stacking direction.

  In the first embodiment, the current collector plate 14 has a plurality of ridges including a contact surface. The plurality of ridges may include a first ridge 18 extending along the first direction and a second ridge 19 extending along the second direction. The first ridge portion 18 may be connected to the second ridge portion 19 at at least one of both ends in the direction parallel to the first direction. The second ridge portion 19 may be connected to the first ridge portion 18 at at least one end of both ends in the direction parallel to the second direction. With such a connection, the first ridge portion 18 and the second ridge portion 19 define a comb-like first flow channel fp1 and second flow channel fp2 that mesh with each other when viewed from the contact surface side. You can do it.

  The first flow path fp1 and the second flow path fp2 may not communicate with each other and may be separated by the first ridge portion 18 and the second ridge portion 19. The first flow path fp1 may be opened on the side surface of the current collector plate 14 on the first direction side. The second flow path fp2 may be opened on the side surface of the current collector plate 14 on the opposite side to the first direction. As described above, the first flow path fp1 is connected to the discharge pipe of the pump 12. Further, the second flow path fp2 is connected to the supply pipe of the tank 11.

  The height position of the contact surface in the thickness direction may become lower from one end of the current collector plate 14 toward the other end. For example, in the first ridge portion 18, the height (height position) of the contact surface may increase as it goes in the first direction. Further, in the second ridge portion 19, the height of the contact surface may decrease as it goes in the second direction. As described above, in FIG. 3, for convenience, the height positions of the first ridge portion 18 and the second ridge portion 19 are drawn to be the same. The current collector plate 14 is installed in the redox flow battery 10 so that the end in the first direction faces the supply side of the electrolytic solution from the tank 11 and the end in the opposite direction of the first direction faces the discharge side of the electrolytic solution. It may have been done.

(Method of measuring the height position of the contact surface)
In the present embodiment, the height position of the contact surface of the current collector plate 14 in the stacking direction is measured using a reading microscope. The height of the bottom of the groove, that is, the height of the flat portion sandwiched between two adjacent ridges is measured at a predetermined interval over the front surface of the current collector plate, and the average value thereof is obtained. A value obtained by subtracting the average value of the heights of the bottoms of the grooves from the height of the contact surface measured in the same manner is defined as the height position in the stacking direction.

  The difference between the maximum height and the minimum height of the contact surface is preferably 0.5 mm or less, more preferably 0.4 mm or less, and further preferably 0.3 mm or less from the viewpoint of improving current efficiency. .. From the viewpoint of efficiently utilizing the electrode surface in the cell plane and reducing the cell resistance, the difference is preferably 0.01 mm or more, more preferably 0.03 mm or more, and further preferably 0.05 mm or more. Is.

  Further, the difference in height of the contact surface is preferably 50% or less, more preferably 30% or less, and further preferably, with respect to the thickness of the electrode used, from the viewpoint of reducing the liquid passage resistance. Is 20% or less. The difference is preferably 1% or more, more preferably 5% or more, and further preferably 8% or more, from the viewpoint of efficiently utilizing the electrode surface in the cell surface and reducing the cell resistance.

  The current collector plate 14 may be made of a material having conductivity, and may be, for example, resin-impregnated graphite. The current collector plate 14 may be formed by cutting resin-impregnated graphite or by molding a compound of graphite and resin.

(Carbon foam)
The carbon foam 15 is a carbon foam made of carbon fiber. The carbon foam 15 is a carbon foam having a linear portion and a connecting portion that connects the linear portion, and has a three-dimensional network structure. In the first embodiment, the carbon foam 15 may have a size that covers the entire first flow passage fp1 and second flow passage fp2 from the contact surface side.

  From the viewpoint of resistance, the thickness of the carbon foam 15 before being incorporated into the charging / discharging portion 13 may be 1.0 mm or less, preferably 0.8 mm or less, and 0.6 mm or less. Is more preferable. The thickness may be 0.1 mm or more, preferably 0.15 mm or more, and preferably 0.2 mm or more, from the viewpoint of ensuring contact at the interface between the carbon foam 15 and the current collector plate 14. Is more preferable.

<Ratio of the number of linear parts to the number of bonded parts>
In the carbon foam 15, the ratio of the number of linear parts to the number of bonding parts may be 1.4 or more and 1.6 or less. In other words, the ratio is half of the average number of branches branched at the joint. When the ratio is 1.4 or more, the linear portion does not have a three-dimensional network structure branched at the bonding portion, and the structure in which the unbonded linear portions are in contact with each other like a non-woven fabric is carbon. It can be excluded from Form 15. In addition, since the ratio is 1.6 or less, a complicated structure in which branching due to branching cannot be accurately detected, or a porous structure having a wall-shaped linear portion having a band-like shape, for example, Can be excluded from carbon foam 15. The ratio is preferably 1.42 or more and 1.58 or less, more preferably 1.44 or more and 1.56 or less.

<Fiber diameter of carbon fiber>
In the carbon foam 15, the average fiber diameter of the carbon fibers forming the linear portion may be 0.1 μm or more and 5.0 μm or less. The fiber diameter of the carbon fiber is the thickness of the linear part connecting the connecting parts. When the average fiber diameter of the carbon fibers is 0.1 μm or more, physical strength and conductivity can be secured. The average fiber diameter is preferably 0.8 μm or more, more preferably 1.2 μm or more, even more preferably 1.5 μm or more. Further, if the average fiber diameter of the carbon fibers is 5 μm or less, the deformability and the resilience during compression behavior can be secured. Further, it is possible to suppress sticking into the electrolyte membrane and obtain high current efficiency. The average fiber diameter is preferably 4 μm or less, more preferably 3.5 μm or less.

<Fiber length of carbon fiber>
The average fiber length of the linear portions (carbon fibers) that form the carbon foam 15 may be 20 μm or more and 200 μm or less. The average fiber length of the linear portion is specifically the length between any adjacent bonding portions. When the average fiber length of the linear portion is 20 μm or more, the flexibility of the carbon foam 15 can be secured. The average fiber length is preferably 30 μm or more, more preferably 40 μm or more. If the average fiber length of the linear part is 200 or less, the conductivity of the carbon foam 15 can be secured. The average fiber length is preferably 150 μm or less, more preferably 100 μm or less.

  In addition, the average fiber diameter and the average fiber length of the linear part (carbon fiber) which comprises the carbon foam 15 are calculated | required by image-analyzing a scanning electron microscope (Scanning Electron Microscope, SEM) image. Specifically, the carbon foam 15 is observed with a scanning electron microscope at a magnification of 10,000 times. From the obtained observation image, the thickness of the carbon fiber is randomly measured at 20 locations. This average thickness is defined as the average fiber diameter, assuming that the cross-sectional shape is circular. Further, the length of the carbon fiber is randomly measured at 20 points from the obtained observed image. Let this average length be the said average fiber length.

<Alignment angle of linear part>
The carbon foam 15 before being incorporated into the charging / discharging part 13 is an isotropic material in which carbon fibers constituting the skeleton of the carbon foam are spread evenly in all directions when, for example, melamine resin foam is heat-treated and carbonized in a heat treatment furnace. Has a general structure. In such a carbon foam 15, the difference θ between the average value of the orientation angle with respect to one direction and the average value of the orientation angle with respect to the other direction is the average value of the orientation angle with respect to each of the three directions of the linear portion orthogonal to each other. Usually, it is 1 ° or less.

  However, when a compressive stress is applied to the resin foam that is a raw material of the carbon foam 15 when the melamine resin foam is heat-treated and carbonized, the carbon foam 15 having a skeleton structure having anisotropy in the spread of the carbon fibers is obtained. .. In such a carbon foam 15, even when a compressive load is applied, breakage of carbon fibers (linear portions) can be suppressed, powder falling can be reduced, and high resilience can be realized. In order to obtain this effect, in the carbon foam 15, in the charging / discharging portion 13, the average value of the orientation angle with respect to the stacking direction and the orientation angle with respect to two directions perpendicular to the thickness direction and perpendicular to each other in at least a partial region The difference from the average value may be 3 ° or more. The difference is preferably 5 ° or more, more preferably 8 ° or more.

  The region where the difference between the average value of the orientation angle with respect to the stacking direction and the average value of the orientation angle with respect to the two directions perpendicular to the stacking direction and perpendicular to each other is 3 ° or more is 300 μm × 300 μm of the carbon foam 15. It may be a portion of × 300 μm. The region is preferably 50% by volume or more of the carbon foam 15, more preferably 75% by volume or more, and further preferably an arbitrary part of the carbon foam 15. ..

<Density of joint part>
The density of the bonded portion of the carbon foam 15 before being incorporated into the charging / discharging portion 13 may be 8,000 pieces / mm ³ or more, preferably 15, from the viewpoint of the resilience when a compressive load is applied. It is 000 pieces / mm ³ or more, more preferably 30,000 pieces / mm ³ or more, and further preferably 50,000 pieces / mm ³ or more. Further, the density of the bonding portion may be 5,000,000 pieces / mm ³ or less, preferably 3,000,000 pieces / mm ³ or less, and more preferably from the viewpoint of flexibility of the carbon foam 15. It is 2,000,000 pieces / mm ³ or less.

  It is preferable that at least a part of the carbon foam 15 before being incorporated into the charging / discharging portion 13 has a portion that satisfies the density of this bonding portion, and it is more preferable that the density range is 50% by volume, and if it is 75% by volume. It is more preferable that the density range is satisfied, and it is particularly preferable that the carbon foam 15 satisfies the density range at any position.

  In the present specification, the number of the bonding portions, the number of linear portions, the density of the bonding portions, and the orientation angle are the X-ray CT (Computed Tomography) apparatus, and the carbon foam 15 before incorporation is photographed. From the obtained tomographic image data, after using the Median filter as preprocessing, Otsu's binarization algorithm (Nobuyuki Otsu, "Automatic threshold selection method based on discrimination and least squares criterion"), electronic information communication The three-dimensional image of the structure including the inside of the carbon foam 15 is obtained by segmenting the structure into a space by using the academic journal D, Vol. J63-D, No. 4, pp. 346-356 (1980)). It is a value obtained by using structural analysis software from the obtained three-dimensional image.

  Specifically, the number of joints and the number of linear portions are obtained by detecting the joints and linear portions included in the three-dimensional image obtained as described above and counting the numbers. From the number of joints and the number of linear portions thus obtained, the ratio of the number of linear portions to the number of joints can be obtained.

  Furthermore, the orientation angle of the linear portion is an angle between a straight line connecting the connecting portions at both ends of the linear portion and each direction, and is obtained for each of the three directions orthogonal to each other in the three-dimensional image. For the direction, the average value of the orientation angle of the linear portion is obtained.

  As a CT apparatus used for the structural analysis of the carbon foam 15, a CT apparatus using low energy high brightness X-rays, for example, a high resolution 3DX ray microscope nano3DX manufactured by Rigaku Corporation can be used. Further, for the image processing and the structural analysis, for example, Centerline editor of software simpleware manufactured by JSOL Co., Ltd. can be used.

<Ratio of oxygen atoms>
The proportion of oxygen atoms in the carbon foam 15 measured by surface analysis by fluorescent X-ray analysis may be 0.03 mass% or more, and 0.04 mass% or more, from the viewpoint of wettability with an electrolytic solution. It is preferably present, and more preferably 0.06 mass% or more. From the viewpoint of the resistance of the electrode, the proportion of oxygen atoms may be 10% by mass or less, preferably 5% by mass or less, and more preferably 3% by mass or less.

<Carbon content>
From the viewpoint of conductivity, the carbon content of the carbon foam 15 is preferably 51% by mass or more, 60% by mass or more, 65% by mass or more, 70% by mass or more, 75% by mass or more, 80% by mass or more, 85% by mass. % Or more and 90% by mass or more. The upper limit is not particularly limited, but the carbon content of the carbon foam 15 may be 100% by mass or less, 99% by mass or less, or 98% by mass or less. The carbon content of the carbon foam 15 can be determined by fluorescent X-ray measurement.

<Concentration of surface functional groups>
In the carbon foam 15, the proportion of graphite in the carbon atoms measured by X-ray photoelectron spectroscopy may be 70 at% or more and 80 at% or less. When the ratio is 70 at% or more, the resistance can be stably kept low against long-term charge / discharge. Further, when the ratio is 80 at% or less, the wettability with the electrolytic solution may be good.

  Further alternatively or additionally, in the carbon foam 15, the ratio of carbon atoms having a hydroxy group to carbon atoms measured by X-ray photoelectron spectroscopy may be 5 at% or more and 15 at% or less. When the ratio is 5 at% or more, the wettability with the electrolytic solution may be good. Moreover, when the said ratio is 15 at% or less, resistance can be stably maintained low with respect to long-term charge / discharge.

  Further alternatively or additionally, in the carbon foam 15, the proportion of carbon atoms constituting the carbonyl group in the carbon atoms measured by X-ray photoelectron spectroscopy may be 10 at% or more and 15 at% or less. When the ratio is 10 at% or more, the wettability with the electrolytic solution may be good. Moreover, when the said ratio is 15 at% or less, resistance can be stably maintained low with respect to long-term charge / discharge.

  Further alternatively or additionally, in the carbon foam 15, the proportion of carbon atoms constituting the carboxy group in the carbon atoms measured by X-ray photoelectron spectroscopy may be 0.1 at% or more and 5 at% or less. .. When the ratio is 0.1 at% or more, the wettability with the electrolytic solution may be good. Moreover, when the said ratio is 5 at% or less, resistance can be stably maintained low with respect to long-term charge / discharge.

<Method of measuring functional group concentration>
In the present specification, surface analysis of carbon foam 15 by X-ray photoelectron spectroscopy is performed as follows. The oxygen-containing functional group concentration on the carbon foam surface can be measured using an X-ray photoelectron spectrometer (Perkin Elmer, ESCA-5500MT). The obtained C1s peak was fitted by four Gaussian distributions having binding energies of 284.4 eV (graphite), 285.6 eV (C-OH), 287.0 eV (C = O) and 288.6 eV (COOH). Then, the surface functional group concentration can be measured by calculating the ratio of the area of each peak to the total area of the four peaks.

<Porosity>
The porosity of the carbon foam 15 may be 70% or more from the viewpoint of flexibility, preferably 80% or more, and more preferably 85% or more. The upper limit is not particularly limited, but the carbon foam 15 porosity may be 99.5% or less, preferably 99% or less, and more preferably 95% or less.

  In the present specification, the porosity is a value obtained from bulk density (described later) and true density (described later). The bulk density is a density based on the volume including the voids included in the carbon foam 15. On the other hand, the true density is a density based on the volume occupied by the material of the carbon foam 15.

[Measurement of bulk density]
First, the dimensions of the carbon foam 15 are measured using a caliper or the like, and the bulk volume V _bulk of the carbon foam 15 is determined from the obtained dimensions. Next, the mass M of the carbon foam 15 is measured using a precision balance. From the obtained mass M and bulk volume V _bulk , the bulk density ρ _bulk of the carbon foam 15 can be obtained by using the following formula (1).
ρ _bulk = M / V _bulk (1)

The bulk density may be from the viewpoint of reducing the resistance when used as an electrode 3.0Kgm ^-3 or more, preferably 3.5Kgm ^-3 or more, more preferably 4.0Kgm ^-3 or more. From the viewpoint of flexibility of the carbon foam 15, the bulk density may be 400 kgm ^-3 or less, preferably 300 kgm ^-3 or less, and more preferably 200 kgm ^-3 or less.

[Measurement of true density]
The true density ρ _real of the carbon foam 15 can be determined by the float-sink method using a mixed solution of n-heptane, carbon tetrachloride and ethylene dibromide. Specifically, first, a carbon foam 15 of an appropriate size is put in a stoppered test tube. Next, the three kinds of solvents are appropriately mixed, added to a test tube, and immersed in a constant temperature bath at 30 ° C. If the sample piece floats, low density n-heptane is added. On the other hand, when the test piece sinks, high density ethylene dibromide is added. Repeat this operation so that the test piece floats in the liquid. Finally, the density of the liquid is measured using a Gehrusack pycnometer.

[Calculation of porosity]
From the _bulk density ρ _bulk and the true density ρ _real obtained as described above, the porosity V _{f, pore} can be obtained using the following equation (2).
V _{f, pore} = ((1 / ρ _bulk ) − (1 / ρ _real )) / (1 / ρ _bulk ) × 100 (%) (2)

<Crystal size>
From the viewpoint of conductivity, the crystallite size of the carbon foam 15 may be 1.1 nm or more, preferably 1.5 nm or more. From the viewpoint of physical fragility, the crystallite size may be 4.0 nm or less, preferably 3.0 nm or less.

<Compressive stress>
Further, the compressive stress σ _{60 at} a compressive strain of 60% of the carbon foam 15 measured by a uniaxial compression test may be 400 kPa or less, preferably 100 kPa or less, and more preferably 40 kPa or less. Since the compressive stress σ ₆₀ is 400 kPa or less, the carbon foam 15 becomes more flexible even in a state where it is strongly sandwiched between the current collector plates 14 and a large compressive stress is applied, and the electrolyte membrane 16 and the carbon foam are not formed. It is possible to improve the anti-peeling performance with respect to 15. The compressive stress σ ₆₀ may be 10 kPa or more, preferably 20 kPa or more. When the compressive stress σ ₆₀ is 10 kPa or more, the handling property of the membrane electrode assembly described later becomes good, and the cell assembly becomes easy.

[Measurement method of compressive stress]
The compressive stress σ ₆₀ of the carbon foam 15 at a compressive strain of 60% can be obtained from a stress-strain curve in uniaxial compression measured using a universal material testing machine TG-1KN manufactured by Minebea. Specifically, a sample piece obtained by cutting the carbon foam 15 into a rectangular parallelepiped having a length and width of 20 mm and a height of 5 to 20 mm is uniaxially compressed in the height direction at a speed of 5 mm / min. The stress at 60% strain of the obtained stress-strain curve is defined as compressive stress σ ₆₀ .

<Method for producing carbon foam>
The carbon foam 15 can be formed, for example, by carbonizing a melamine resin foam at 800 to 2500 ° C. under an inert atmosphere. As the melamine resin foam, for example, a melamine / formaldehyde condensation foam produced by the method disclosed in JP-A-4-349178 can be used.

  According to the above method, first, an aqueous solution or dispersion containing a melamine / formaldehyde precondensate, an emulsifier, a vaporizable foaming agent, a curing agent, and, if necessary, a well-known filler is subjected to foaming treatment and then cured. By performing the treatment, a melamine / formaldehyde condensation foam can be obtained.

  In the above method, as the melamine / formaldehyde precondensate, for example, one having melamine: formaldehyde = 1: 1.5 to 1: 4 and an average molecular weight of 200 to 1000 can be used. Further, as the emulsifier, for example, 0.5 to 5% by mass of a sodium salt of alkylsulfonic acid or arylsulfonic acid (based on melamine / formaldehyde precondensate, hereinafter the same), and as the vaporizable foaming agent, for example, pentane or hexane 1 to 50% by mass, and as the curing agent, hydrochloric acid, sulfuric acid, and the like are 0.01 to 20% by mass. The foaming treatment and the curing treatment may be performed by heating the solution containing the above components to a temperature set according to the type of the vaporizable foaming agent used.

  Next, this melamine resin foam is carbonized. Carbonization can be performed in an inert gas stream or in an inert atmosphere such as a vacuum. The lower limit of the heat treatment temperature may be 800 ° C. or higher, preferably 900 ° C. or higher, and more preferably 1000 ° C. or higher, from the viewpoint of enhancing conductivity. On the other hand, the upper limit of the heat treatment temperature may be 2500 ° C. or lower, preferably 2400 ° C. or lower, and more preferably 2200 ° C. or lower, from the viewpoint of maintaining the flexibility of the electrode.

  The skeleton structure having anisotropy in the spread of the carbon fibers is obtained by performing the carbonization step and the temperature raising step of raising the temperature to the carbonization step while applying a compressive load to the resin foam of the raw material. Carbon foam 15 of As described above, the carbon foam 15 having anisotropy can suppress the breakage of the carbon fibers to reduce the powder falling even when a compressive load is applied, and can realize high resilience. it can.

  The compression load can be applied by placing a weight such as a graphite plate on the resin foam as the raw material. The applied compressive load is preferably 50 Pa or higher, more preferably 200 Pa or higher. Further, it is preferably 2000 Pa or less, more preferably 1500 Pa or less.

(Electrolyte membrane)
The electrolyte membrane 16 is a polymer electrolyte membrane. The type of polymer electrolyte membrane is not particularly limited, but from the viewpoint of use in a redox flow battery, a membrane having proton conductivity is preferable.

  As the membrane having proton conductivity, a porous membrane-based membrane such as a PTFE (polytetrafluoroethylene resin) porous membrane, a polyolefin-based porous membrane, or a polyolefin-based nonwoven fabric described in JP-A-2005-158383, JP-B-6- A composite membrane in which a porous membrane described in 105615 is combined with a water-containing polymer, a membrane of cellulose or ethylene-vinyl alcohol copolymer described in JP-B-62-226580, and a disclosure of JP-A-6-188005. Polysulfone-based membrane Anion exchange membrane, Fluorine-based or polysulfone-based ion exchange membrane described in JP-A-5-242905, Hydrophilic pores of porous membrane formed of polypropylene described in JP-A-6-260183 A membrane provided with a resin, a membrane in which both surfaces of a polypropylene porous membrane are thinly coated with a fluorine-based ion exchange resin (Nafion (registered trademark)) having a thickness of several μm, and an anion having a pyridium group described in JP-A-10-208767 A membrane composed of a cross-linked polymer obtained by copolymerizing an ion exchange type with styrene type and divinylbenzene, and a cation type ion exchange membrane (fluorine type polymer or hydrocarbon type polymer) described in JP-A No. 11-260390. A membrane having a structure in which anion-based ion exchange membranes (polysulfone-based polymers and the like) are alternately laminated, a vinyl heterocyclic compound having two or more hydrophilic groups on a porous substrate described in JP-A-2000-235849 ( Examples thereof include an anion exchange membrane formed by complexing a cross-linked polymer having an amine group and a repeating unit such as vinylpyrrolidone. Among these, a film made of a perfluorosulfonic acid (PFSA) resin is preferable from the viewpoint of long-term durability.

<Perfluorosulfonic acid type resin>
Examples of the perfluorosulfonic acid-based resin include a polymer containing a repeating unit represented by the following general formula (1) and a repeating unit represented by the following general formula (2).
^{- [CX 1 X 2 -CX 3} X 4] - ··· (1)
(In formula (1), X ¹ , X ² , X ³ , and X ⁴ are each independently a hydrogen atom, a halogen atom, or a perfluoroalkyl group having 1 to 10 carbon atoms, and X ¹ , X ² , and X ^At least one of ³ and X ⁴ is a fluorine atom or a perfluoroalkyl group having 1 to 10 carbon atoms.)
^{_{- [CF 2 -CF (- (}} O a -CF 2 - (CFX 5) b) c -O d - (CF 2) e -SO 3 R)] - ··· (2)
(In the formula (2), X ⁵ is a halogen atom or a perfluoroalkyl group having 1 to 4 carbon atoms, R is a hydrogen atom, a lithium atom, a sodium atom, or an alkali metal atom such as a potassium atom, NH ₄ , _{^{NH 3 R 1, NH 2 R}} 1 R 2, NHR 1 R 2 R 3, or ^{NR 1 R 2 R 3 R 4} (R 1 R 2 R 3 R 4 are each independently of 1 to 10 carbon atoms An alkyl group or an aryl group), a is 0 or 1, b is 0 or 1, c is an integer of 0 to 8, and d is 0 or 1. , E are integers from 0 to 8, provided that b and e are not 0 at the same time.)

  When the perfluorosulfonic acid-based resin contains a plurality of repeating units represented by the general formula (2) and / or a plurality of repeating units represented by the general formula (2), each repeating unit is It may be the same or different.

As the perfluorosulfonic acid-based resin, compounds having one or more repeating units represented by the following general formulas (3) to (7) are preferable.
^{_{- [CF 2 -CX 3 X 4}} ] f - [CF 2 -CF (-O-CF 2 -CFX 5) c -O d - (CF 2) e -SO 3 R)] g - ··· (3 )
_{- [CF 2 -CF 2] f} - [CF 2 -CF (-O-CF 2 -CF (CF 3)) c -O- (CF 2) e -SO 3 R)] g - ··· (4 )
_{- [CF 2 -CF 2] f} - [CF 2 -CF-O- (CF 2) e -SO 3 R)] g - ··· (5)
_{- [CF 2 -CF 2] f} - [CF 2 -CF (-O-CF 2 -CFX 5) c -O d - (CF 2) e -SO 3 H] g ··· (6)
_{- [CF 2 -CF 2] f} - [CF 2 -CF- (CF 2) e -SO 3 R)] g - ··· (7)
(In the formulas (3) to (7), X ³ , X ⁴ , X ⁵ , and R are the same as those in the formulas (1) and (2). Further, c, d, and e are the formula (1), Similar to (2), 0 ≦ f <1, 0 <g ≦ 1, f + g = 1, provided that e is not 0 in formulas (5) and (7).

  The perfluorosulfonic acid-based resin may further contain other constitutional units other than the repeating units represented by the general formulas (1) and (2). Examples of the other structural unit include the structural unit represented by the following general formula (I) and the like.

（式（Ｉ）中、Ｒ^１は、単結合または炭素数１〜６の２価のパーフルオロ有機基（例えば、炭素数１〜６のパーフルオロアルキレン基等）であり、Ｒ^２は、炭素数１〜６の２価のパーフルオロ有機基（例えば、炭素数１〜６のパーフルオロアルキレン基、等）である。）

(In the formula (I), R ¹ is a single bond or a divalent perfluoroorganic group having 1 to 6 carbon atoms (for example, a perfluoroalkylene group having 1 to 6 carbon atoms), and R ² is carbon. It is a divalent perfluoroorganic group having 1 to 6 carbon atoms (for example, a perfluoroalkylene group having 1 to 6 carbon atoms).

  As the above-mentioned perfluorosulfonic acid-based resin, a resin having a repeating unit represented by the formula (4) or (5) is preferable from the viewpoint that a proton-permeable resin is easily obtained and a polymer electrolyte membrane having lower resistance is obtained. A resin consisting only of the repeating unit represented by the formula (5) is more preferable.

  The equivalent weight EW of the PFSA resin (dry weight grams of the PFSA resin per equivalent of the proton exchange group) may be adjusted to 300 to 3000. The equivalent mass EW of the PFSA resin in the present embodiment is preferably 700 to 2000, more preferably 800 to 1500, and further preferably 850 to 1200 from the viewpoint of resistance.

  The thickness of the electrolyte membrane 16 may be 5 μm or more, preferably 10 μm or more, and more preferably 20 μm or more. Thereby, short circuit between the positive and negative electrodes can be suppressed, and at the same time, crossover of the positive and negative electrode electrolyte solutions can be suppressed. The thickness of the electrolyte membrane 16 may be 150 μm or less, preferably 100 μm or less, and more preferably 75 μm or less. Thereby, the battery size can be reduced, and at the same time, the internal resistance can be reduced.

(Membrane electrode complex)
In the charging / discharging section 13, the electrolyte membrane 16 and the two carbon foams 15 are joined together to form a membrane electrode assembly. For forming the membrane electrode assembly, a hot pressing method, a method of laminating with a polymer solution of the same kind as the membrane, or the like may be applied. Among these, the hot pressing method is preferable from the viewpoint of workability. In the formation of the membrane electrode assembly by the hot pressing method, specifically, first, the electrolyte membrane 16 is sandwiched between two carbon foams 15 and is placed between pressure plates of a hot pressing machine together with spacers having an appropriate thickness. Next, the pressure plate is heated to a predetermined temperature and then pressed. After holding for a predetermined time, the pressure plate is opened and the membrane electrode assembly is taken out and cooled to room temperature. The membrane electrode assembly may thus be formed.

  In the above hot press method, the heating temperature is preferably 80 ° C. or higher and 200 ° C. or lower, and more preferably 120 ° C. or higher and 160 ° C. or lower. As a result, the softened electrolyte membrane 16 adheres tightly to the carbon foam 15 without thermally deteriorating the electrolyte membrane 16, so that a strong bond can be achieved. Further, the thickness of the spacer is preferably 10% or more and 50% or less, more preferably 20% or more and 40% or less, with respect to the total thickness of the electrolyte membrane 16 and the two carbon foams 15. .. The holding time after pressing is preferably 5 minutes or more and 30 minutes or less, and more preferably 10 minutes or more and 20 minutes or less. Thereby, the electrolyte membrane 16 and the carbon foam 15 can be firmly bonded without causing a short circuit between the carbon foam 15.

(flame)
The frame 17 separates the two current collector plates 14 from each other at a predetermined interval while making the thickness direction parallel to the stacking direction in the charging / discharging unit 13. The frame 17 may be a frame that covers the side surfaces of the carbon foam 15 and the electrolyte membrane 16. The inner circumference of the frame 17 may have the same shape and the same size as the outer circumference of the electrolyte membrane 16. The entire outer circumference of the electrolyte membrane 16 may be joined to the inner wall of the frame 17. The height of the frame 17 is shorter than the thickness of the membrane electrode assembly before being assembled. Therefore, when the frame 17 is sandwiched between the two collector plates 14, the collector plate 14 presses the membrane electrode assembly in the thickness direction.

  The frame 17 may be formed of a material that is insulative and resistant to the electrolytic solution used in the redox flow battery 10.

  As described above, in the redox flow battery 10 according to the first embodiment, the height position of the contact surface of the current collector plate 14 in the stacking direction differs depending on the position of the virtual plane perpendicular to the stacking direction. With such a configuration, in the redox flow battery 10, compared with the current collector plate 14 having a flat contact surface, the degree of freedom in designing the current collector plate 14 is improved so as to exert various functions. Furthermore, the redox flow battery 10 according to the first embodiment uses the carbon foam 15 as an electrode in contact with the current collector plate 14. With such a configuration, in the redox flow battery 10, while using the current collector plate 14 in which the height position of the contact surface in the stacking direction differs depending on the position of the virtual plane, the contact failure between the contact surface and the carbon foam 15 and the carbon foam 15 are caused. 15 damage is prevented.

  Further, in the redox flow battery 10 according to the first embodiment, the current collector plate 14 has a plurality of ridges 18 and 19 each including a contact surface. With such a configuration, in the redox flow battery 10, flow passages capable of passing the electrolytic solution, such as the first flow passage fp1 and the second flow passage fp2, can be defined. In the configuration in which the electrolytic solution is simply passed through the pores of the carbon foam 15, a deviation may occur in the location where the electrolytic solution is passed. On the other hand, in the redox flow battery 10 using the current collector plate 14 having the flow passage, the electrolytic solution can pass through the carbon foam 15 in the entire region covering the flow passage while passing through the flow passage. Therefore, in the redox flow battery 10, the region of the carbon foam 15 used for the redox reaction is expanded, and the charging / discharging efficiency is improved.

  Further, in the redox flow battery 10 according to the first embodiment, the height position of the contact surface in the stacking direction becomes lower from one end of the current collector plate 14 toward the other end. Further, in the redox flow battery 10, the height position of the contact surface in the stacking direction becomes lower from the discharge side end of the electrolytic solution toward the supply side end. In the flow path to which the electrolytic solution is supplied, the pressure of the electrolytic solution decreases due to pressure loss as it goes from the supply side to the discharge side. Therefore, in a flow path having the same cross-sectional area, the amount of the electrolytic solution flowing from the flow path to the carbon foam 15 decreases as it goes from the supply side to the discharge side. On the other hand, in the redox flow battery 10 configured as described above, the cross-sectional area of the flow path covered with the carbon foam 15 increases from the supply side toward the discharge side. Therefore, in the redox flow battery 10, a decrease in the amount of liquid passing to the carbon foam 15 on the discharge side in the flow path is suppressed, and the entire carbon foam 15 can be efficiently used for the redox reaction.

  Next, the redox flow battery according to the second embodiment will be described. In the redox flow battery according to the second embodiment, the structure of the charging / discharging unit is different from that of the first embodiment. The configuration of the charging / discharging unit of the second embodiment will be described below. It should be noted that parts having the same configurations as those in the first embodiment are designated by the same reference numerals.

  As shown in FIG. 4, in the second embodiment, the charging / discharging unit 130 is similar to the first embodiment in that the current collectors 140 on the anode side and the anode side, the carbon foam 15 on the anode side and the anode side, The electrolyte membrane 16 and the frame 170 are provided. The configurations of the carbon foam 15 and the electrolyte membrane 16 are the same as those in the first embodiment.

  Unlike the first embodiment, in the second embodiment, the discharge pipe of the pump 12 is connected to the hole formed in the frame 170. The pipe for supplying the tank 11 is connected to a hole formed in the frame 170. In the second embodiment, the electrolytic solution supplied to the charging / discharging unit 130 passes through the carbon foam 15 and is discharged from the charging / discharging unit 13.

  In the second embodiment, unlike the first embodiment, the current collector 140 has a single contact surface on the entire main surface. As in the first embodiment, in the current collector plate 140, the height position of the contact surface in the thickness direction differs depending on the position on the virtual plane perpendicular to the thickness direction. For example, the current collector 140 may have a surface inclined with respect to the virtual plane as at least a part of the contact surface, and may have a curved surface as at least a part of the contact surface. In other words, in the current collector 140, at least a part of the contact surface may be distorted. In the second embodiment, the other configuration of the current collector plate 140 is the same as that of the first embodiment.

  In the second embodiment, unlike the first embodiment, the frame 170 is for passing the electrolyte solution for the anode and the electrolyte solution for the cathode to both sides with the electrolyte membrane 16 bonded to the inner periphery as a boundary. Holes are drilled. The holes for passing the liquid are formed on both sides of the supply side and the discharge side. In the second embodiment, the other configuration of the frame 170 is similar to that of the first embodiment.

  As described above, in the redox flow battery 10 according to the second embodiment, at least a part of the contact surface is inclined with respect to the virtual plane and / or is a curved surface. With such a configuration, the redox flow battery 10 can employ the current collector plate 140 having relatively low dimensional accuracy. Since the current collector plate 140 having relatively low dimensional accuracy can be adopted, it is possible to apply, for example, a molding method or the like, which can form various shapes and has low manufacturing cost, to the manufacture of the current collector plate 140.

  Hereinafter, the present invention will be described with reference to specific examples and comparative examples, but the present invention is not limited thereto.

<Production of carbon foam>
(Example 1)
First, melamine resin foam (dimensions: 400 mm × 400 mm × 20 mm) was prepared as a material for carbon foam. Then, the sample was placed on a 450 mm square graphite plate, and a 400 mm × 20 mm SUS plate having a thickness of 0.6 mm was placed as a spacer on each side of the sample, and a graphite plate was further placed from above to sandwich the sample and the spacer. While being sandwiched between the graphite plates, the graphite plate was introduced into a vacuum press machine and pressed at a set pressure of 3.0 MPa while decompressing with a vacuum pump. While continuing to reduce the pressure, the temperature was raised to 370 ° C. at a temperature rising rate of 5 ° C./minute, held for 10 minutes, and then cooled to take out a sample. Subsequently, the pressed sample is placed on a 400 mm square graphite plate, the graphite plate is placed on the graphite plate, a compressive load of 280 Pa is applied, and the sample is placed in a high temperature heat treatment furnace with the compressive load applied. Introduced. Then, the inside of the furnace was evacuated by a vacuum pump to reduce the degree of vacuum in the furnace to less than 1 Pa. Then, while evacuation was carried out under reduced pressure, while supplying nitrogen gas into the furnace at a flow rate of 2 L / min, the temperature inside the furnace was raised to 800 ° C. at a temperature rising rate of 5 ° C./min, and then nitrogen gas was introduced. The temperature was stopped, the temperature was further raised to 1500 ° C. at a temperature rising rate of 5 ° C./minute, the temperature was maintained for 2 hours, the temperature was cooled to room temperature, and then the sample was taken out. Then, the obtained carbon foam was heat-treated at 500 ° C. for 1 hour in a dry air stream to obtain a carbon foam whose surface was oxidized. The dry air flow rate was 1 L / min. Thus, the carbon foam according to Example 1 was produced. Details of the obtained carbon foam are shown in Table 1.

(Example 2)
A carbon foam according to Example 2 was produced in the same manner as in Example 1. However, the temperature was raised to 2000 ° C. in the high temperature heat treatment furnace and kept for 2 hours. Further, the oxidation temperature under a dry air stream was changed to 600 ° C. All other conditions are the same as in Example 1. Details of the obtained carbon foam are shown in Table 1.

(Example 3)
A carbon foam according to Example 3 was produced in the same manner as in Example 2. However, a melamine resin foam (dimensions: 400 mm × 400 mm × 40 mm) was prepared as a material, and the thickness of the spacer used during pressing with a vacuum press machine was 1.1 mm. . All other conditions are the same as in Example 2. Details of the obtained carbon foam are shown in Table 1.

(Example 4)
A carbon foam according to Example 4 was produced in the same manner as in Example 2. However, the thickness of the spacer used at the time of pressing with a vacuum press was 1.1 mm. All other conditions are the same as in Example 2. Details of the obtained carbon foam are shown in Table 1.

(Example 5)
A carbon foam according to Example 5 was produced in the same manner as in Example 4. However, in the cell configuration, a current collector having a flow path difference, which will be described later, different from that in Example 4 was used. Details of the obtained carbon foam are shown in Table 1.

(Example 6)
A carbon foam according to Example 6 was prepared in the same manner as in Example 3. However, in the cell structure, a current collector having a flow path step difference described later was used. Details of the obtained carbon foam are shown in Table 1.

(Comparative Example 1)
Air-oxidized carbon fiber paper was obtained by air-oxidizing carbon fiber paper (SGL CARBON Co. Ltd, SIGRACET GDL39AA) according to the method shown in Example 1.

(Comparative example 2)
A carbon foam was prepared in the same manner as in Example 4. However, the current collector plate used had no step in the flow path.

<Current collector>
The current collector plate was made by cutting resin-impregnated graphite (Owada Carbon, P-3100) using a milling machine. Comb-shaped channels were formed parallel to the longitudinal direction in a range of 33 mm in length and 30 mm in width on one surface of a resin-impregnated graphite plate having a thickness of 5 mm. The width of the ridge was 1 mm, and the interval between two adjacent ridges was 1 mm. The height of the contact surface, that is, the difference between the height of the contact surface and the average value of the heights of the flat parts sandwiched between two adjacent ridges, is 1 mm at the inlet side of the electrolyte, It was designed to decrease linearly toward the side. The difference in height between the contact surfaces on the electrolyte inlet side and the outlet side is called a flow path step, and a plurality of current collector plates having different values are prepared. The height of the contact surface was measured using a reading microscope manufactured by Nihon Koki Seisakusho, as described above.

<Electrolyte membrane>
Nafion (registered trademark) 211 manufactured by Kemers Co., Ltd. was used and cut into 40 mm × 150 mm for use. In addition, when the redox flow battery was evaluated, the film peeled from the substrate in advance in a 2M sulfuric acid aqueous solution at room temperature was used by immersing it for 1 hour.

<Redox flow battery>
After the Nafion 211, the carbon foam, the frame plate, the current collector, and the gasket were stacked in a predetermined order, screw cells were fastened with a predetermined torque to produce a cell. As the positive and negative electrode electrolyte, a sulfuric acid aqueous solution having a vanadium ion concentration of 1.6 M / L, a sulfate ion concentration of 4.5 M / L and a vanadium ion valence of 3.5 was used. The amount of electrolyte was 30 ml for both positive and negative electrodes. The electrolytic solution flow rate per electrode unit area was set to 2 ml / (min cm 2). The charge / discharge test was performed using a charge / discharge tester (PFX2011) manufactured by Kikusui Electronics. The voltage range was 1.00 to 1.55 V and the current density was 160 mA / cm 2. The current efficiency was calculated from the ratio of the discharge capacity to the charge capacity at the 10th cycle. The cell resistance was obtained by dividing the difference between the average charge voltage and the average discharge voltage at the 10th cycle by twice the current density.

<SEM observation>
SEM images were taken at a magnification of 10,000 times using a scanning electron microscope at a total of 20 points on the surface of the carbon foam of Example 1. The fiber diameter of any one carbon fiber in the carbon fiber image included in the SEM image of each location was measured and regarded as the fiber diameter of each location. The fiber diameters at 20 locations were averaged and the average value was calculated. The obtained average values are shown in Table 1. For each of the carbon foams of Examples 2 to 6 and Comparative Examples 1 and 2, SEM images were taken in the same manner as in Example 1 and the average value of fiber diameter was obtained.

<Structural analysis by X-ray CT>
Structural analysis by X-ray CT was performed on the carbon foams of Examples 1 to 6 and Comparative Examples 1 and 2. Specifically, in order to make it easier to capture an X-ray image, electroless copper plating is performed on each of the examples and comparative examples, and then a test piece is sampled and a high-resolution 3D X-ray microscope nano3DX (manufactured by Rigaku Corporation). Was used to perform structural analysis on the collected test pieces. The specific electroless plating conditions and X-ray CT analysis conditions are as follows.

[Electroless plating conditions]
The sample was immersed in OPC Condy Clean MA (Okuno Pharmaceutical Co., Ltd., diluted with distilled water to 100 mL / L) at 70 ° C. for 5 minutes and then washed with distilled water for 1 minute. Subsequently, it was immersed in OPC pre-dip 49L (Okuno Pharmaceutical Co., Ltd., 10 mL / L diluted with distilled water, 98% sulfuric acid added 1.5 mL / L) at 70 ° C. for 2 minutes, and then washed with distilled water for 1 minute. .. Then, mix OPC Inducer 50AM (Okuno Pharmaceutical Co., Ltd., diluted to 100 mL / L with distilled water) and OPC Inducer 50CM (Okuno Pharmaceutical Co., Ltd., diluted to 100 mL / L with distilled water) at a ratio of 1: 1. After dipping in the above solution at 45 ° C. for 5 minutes, it was washed with distilled water for 1 minute. Subsequently, it was immersed in OPC-150 Crysta MU (Okuno Pharmaceutical Co., Ltd., diluted to 150 mL / L with distilled water) at room temperature for 5 minutes, and then washed with distilled water for 1 minute. Then, it was immersed in OPC-BSM (Okuno Pharmaceutical Co., Ltd., diluted to 125 mL / L with distilled water) at room temperature for 5 minutes. Subsequently, a chemical copper 500A (Okuno Pharmaceutical Co., Ltd., diluted to 250 mL / L with distilled water) and a chemical copper 500B (Okuno Pharmaceutical Co., Ltd., diluted to 250 mL / L with distilled water) were mixed at a ratio of 1: 1. It was immersed in the solution at room temperature for 10 minutes, and then washed with distilled water for 5 minutes. Then, vacuum drying was performed at 90 ° C. for 12 hours to dry the water.

[X-ray condition]
X-ray target: Cu
X-ray tube voltage: 40 kV
X-ray tube current: 30 mA

[Shooting conditions]
Number of projections: 1500 sheets Rotation angle: 180 °
Exposure time: 20 seconds / sheet Spatial resolution: 0.54 μm / pixel

[X-ray CT analysis conditions]
The obtained three-dimensional image was processed by 1 pixel adjacent to the Median filter, and binarized by using Otsu's algorithm. Subsequently, using the software Centerware editor (Ver.7) of JSOL Corporation with default setting values, lines of 2.16 μm or less are removed as noise, and then the binding within the measurement visual field of 300 μm × 300 μm × 300 μm is performed. The number of parts and linear parts were detected.

By the above structural analysis, the average values of the numbers N _n and N _{l of} the bonding portions and the linear portions included in the test piece, the bonding portion density, and the orientation angles with respect to three directions (x, y, z) orthogonal to each other were obtained. .. Furthermore, the minimum value θ _d of the difference between the orientation angle with respect to the x direction and the orientation angle with respect to the y direction or the z direction was obtained. The results obtained are shown in Table 1.

From Table 1, the ratio R of the number N ₁ of the linear parts to the number N _n of the bonding parts to the carbon foams of Examples 1 to 6 is in the range of 1.4 to 1.51. As another material, for example, in a structure such as a carbon fiber nonwoven fabric, the ratio R of the number N ₁ of linear portions to the number N _n of coupling portions is 1.29 or less, which is 1.4 to 1.51. It is not included in the range, but this range is a characteristic value due to the structure of the carbon foam of the present application.

<Measurement of bulk density>
The dimension of the carbon foam was measured using a caliper, and the bulk volume V _bulk of the carbon foam was determined from the obtained dimension. Next, the mass M of the carbon foam was measured using a precision balance. From the obtained mass M and the bulk volume V _bulk , the bulk density ρ _bulk (kgm ⁻³ ) of the carbon foam was obtained by using the above formula (2). The results obtained are shown in Table 1.

<Carbon content>
The carbon content of the carbon foam was determined by fluorescent X-ray measurement using a fluorescent X-ray analyzer ZSX-100E (wavelength dispersion type, Rh tube) manufactured by Rigaku Corporation. The sample area used was 20 mmφ or more. The results obtained are shown in Table 1.

<Oxygen atom concentration>
The oxygen content of carbon foam was determined by fluorescent X-ray measurement. For the fluorescent X-ray measurement, a fluorescent X-ray analyzer ZSX-100E (wavelength dispersion type, Rh tube) manufactured by Rigaku Corporation was used. A sample having a size of 20 mmφ or more was used. The results obtained are shown in Table 1.

<Flow path step>
The flow path steps of the current collectors used in Examples 1 to 6 and Comparative Examples 1 and 2, that is, the difference in height of the contact surface between the electrolyte inlet side and the outlet side was measured using a reading microscope manufactured by Nihon Koki Seisakusho. It was measured. The results obtained are shown in Table 1.

<Current efficiency>
The current efficiency in Examples 1 to 6 and Comparative Examples 1 and 2 was measured. Specifically, the current efficiency was calculated from the ratio of the discharge capacity to the charge capacity at the 10th cycle. The results obtained are shown in Table 1.

<Cell resistance>
The cell resistance in Examples 1 to 6 and Comparative Examples 1 and 2 was measured. Specifically, the cell resistance was determined by dividing the difference between the average charge voltage and the average discharge voltage at the 10th cycle by twice the current density. The results obtained are shown in Table 1.

  According to the present invention, a carbon foam having flexibility as an electrode is brought into contact with a current collector plate having contact surfaces having different height positions, which is useful in a redox flow battery in which contact failure at the contact surface is prevented. ..

10 Redox Flow Battery 11 Tank 12 Pump 13 Charge / Discharge Body 14, 140 Current Collector Plate 15 Carbon Foam 16 Electrolyte Membrane 17, 170 Frame 18 First Ridge Part 19 Second Ridge Part fp1 First Flow Path fp2 Second Channel

Claims (11)

A carbon foam made of carbon fiber having a linear part and a bonding part for bonding the linear part;
A current collector having at least one contact surface that is laminated on the carbon foam and is in surface contact, and a height position of the contact surface in a stacking direction varies depending on a position of an imaginary plane perpendicular to the stacking direction. Redox flow battery. The redox flow battery according to claim 1, wherein the current collector plate has a plurality of ridges each including the contact surface. The redox flow battery according to claim 1, wherein a height position of the contact surface in the stacking direction becomes lower from one end of the current collector plate toward the other end. The redox flow battery according to claim 3, wherein the one end is an end on the discharge side of the electrolytic solution, and the other end is an end on the supply side of the electrolytic solution. The redox flow battery according to claim 1, wherein at least a part of the contact surface is inclined with respect to the virtual plane. The redox flow battery according to claim 1, wherein at least a part of the contact surface is a curved surface. The redox flow battery according to any one of claims 1 to 6, wherein a ratio of the number of the linear portions to the number of the coupling portions is 1.4 or more and 1.6 or less. The redox flow battery according to claim 1, wherein a fiber diameter of the carbon fiber is 0.1 μm or more and 5 μm or less. In at least a part of the carbon foam, the difference between the average value of the orientation angles of the linear portion with respect to the stacking direction and the average value of the orientation angles with respect to two directions perpendicular to the stacking direction and perpendicular to each other is 3 ° or more. The redox flow battery according to any one of claims 1 to 8. The redox flow battery according to any one of claims 1 to 9, wherein a difference between a maximum height and a minimum height position of the contact surface is 0.01 mm or more and 0.5 mm or less. The carbon foam is a melamine carbon foam obtained by carbonizing a melamine resin foam.
The redox flow battery according to any one of claims 1 to 10.

Images