JP2018101541A - Membrane-electrode assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a membrane-electrode assembly capable of suppressing deterioration in performance by inhibiting rupture of a diaphragm and peeling from an electrode due to electrolysis and repetition of charge/discharge or the like.SOLUTION: Disclosed is a membrane-electrode assembly 1 which includes: a pair of electrodes 3 for performing an oxidation-reduction reaction; and a diaphragm 2 which is provided between the pair of electrodes 3 and separates both electrodes 3. Each electrode 3 is a porous body formed by a fibrous structure, and at least a part of the fibrous structure is embedded into the diaphragm 2.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a membrane-electrode assembly including a diaphragm and an electrode.

  In recent years, the introduction of power generation using renewable energy such as sunlight and wind power has been promoted as a countermeasure against global warming. These power generations have a problem of unstable voltage and frequency of the system power supply because the output is greatly affected by the weather. Therefore, when introducing large-scale power generation, it is necessary to smooth the output and level the load by combining with a large-capacity storage battery.

  Examples of the large-capacity storage battery include a lead storage battery, a sodium-sulfur battery, a metal-halogen battery, and a redox flow battery. Among these, the redox flow battery is superior to other batteries in terms of reliability and economy, and is one of the batteries most likely to be put to practical use.

  FIG. 1 shows a schematic diagram of a general structure of a redox flow battery. As shown in this figure, the redox flow battery 100 includes an electrolytic cell 101, tanks 102 and 103 for storing an electrolytic solution, and pumps 104 and 105 for circulating the electrolytic solution between the tank and the electrolytic cell. Further, the electrolytic cell 101 includes a pair of electrodes 112 that perform an oxidation-reduction reaction, and an electrolyte membrane 111 that is provided between the electrodes 112 and separates the electrodes 112. The electrode 112 is connected to a power source 106. ing.

  In the redox flow battery 100, by causing the pumps 104 and 105 to circulate the electrolyte between the tanks 102 and 103 and the electrolytic cell 101, electrochemical energy conversion is performed on the electrode 112 of the electrolytic cell 101. Charge and discharge.

  In the redox flow battery 100 shown in FIG. 1, the electrolyte membrane 111 and the electrode 112 are separated from each other. However, in order to increase the current density per unit area of the electrode, as shown in FIG. Further, the membrane electrode assembly 10 is formed by laminating the electrolyte membrane 11 and the electrode 12, and the membrane electrode assembly 10 is sandwiched between the current collector plates 14 held by the frame 13, whereby FIG. In many cases, a cell (cell stack) is formed by providing a plurality of cells 20.

  In the cell 20 shown in FIG. 2 (b), as the electrode 12, it is necessary to ensure conductivity, electrochemical stability, and flowability of the electrolytic solution, and therefore a carbon fibrous structure is generally used. . For example, Patent Document 1 uses carbon fiber paper, Patent Document 2 uses a carbon fiber nonwoven fabric, and Patent Document 3 uses a carbonized porous organic matrix (carbon foam) as an electrode material. It is described.

Special Table 2014-1414697 gazette JP 2001-85028 A JP-A-9-167621

  However, when the redox flow battery configured by providing a plurality of the cells 20 is repeatedly charged and discharged, the electrolyte membrane 11 in the membrane-electrode laminate 10 may be peeled off from the electrode 12 and the battery performance may be significantly reduced. is there.

  Such peeling of the electrolyte membrane may occur in other applications. That is, the membrane-electrode laminate as described above is used not only in redox flow batteries but also in various applications such as solid polymer membrane water electrolysis devices and direct methanol fuel cells. When discharge or the like is repeatedly performed, the electrolyte membrane may be peeled off from the electrode, thereby reducing the performance.

  Accordingly, an object of the present invention is to provide a membrane-electrode assembly capable of suppressing degradation of performance by suppressing the electrolyte membrane from repeatedly swelling and shrinking due to repetition of electrolysis, charge and discharge, etc., and peeling from the electrode. .

  The present inventors diligently studied a membrane-electrode assembly that can solve the above-mentioned problems. Therefore, the cause of peeling of the electrolyte membrane 11 in the membrane-electrode laminate 10 was investigated in detail. The electrolyte membrane 11 swells and contracts due to the entry / exit of the electrolyte accompanying charging / discharging. This swelling and shrinkage is suppressed by sandwiching the electrodes 12 on both surfaces of the electrolyte membrane 11 by the current collector 14. However, when the bonding between the electrolyte membrane 11 and the electrode 12 or the pressure between the electrodes 12 is not sufficient with respect to the degree of swelling and shrinkage, local peeling caused by charging / discharging progresses with repeated charging / discharging, Eventually, peeling of the entire interface is caused.

  Therefore, as a result of intensive studies on a method for increasing the bonding strength between the electrolyte membrane and the electrode, the present inventors have configured the electrode of the membrane-electrode laminate as a porous body formed of a fibrous structure, and has a fibrous shape. It has been found that it is extremely effective to bury at least a part of the structure in the diaphragm, and the present invention has been completed.

That is, the present invention is as follows.
[1]
A pair of electrodes that perform a redox reaction;
A diaphragm provided between the electrodes and separating both electrodes;
A membrane-electrode assembly comprising:
The electrode is a porous body formed of a fibrous structure, and at least a part of the fibrous structure is buried in the diaphragm.

[2]
The fibrous structure of the electrode according to [1], wherein the fibrous structure of the electrode has a portion buried in five or more locations per 1 cm in the film length direction in a cross section in the film thickness direction of the membrane-electrode assembly. Joined body.

[3]
The membrane-electrode assembly according to [1] or [2], wherein a thickness of a portion of the diaphragm in which the fibrous structure of the electrode is not buried is 2 μm or more.

[4]
The membrane-electrode assembly according to [3] above, wherein the thickness of the diaphragm portion in which the fibrous structure of the electrode is not buried is specified by energy dispersive X-ray analysis.

[5]
The membrane-electrode assembly according to any one of [1] to [4], wherein the diaphragm has a thickness of 30 μm or less.

[6]
In a portion of a square membrane-electrode assembly having a side of 30 mm, a first cross section cut in parallel to the film thickness direction on the diagonal of the square,
On the first cross section, the number of the buried portions is evaluated with three different visual fields that do not overlap with each other in the film length direction of 1 cm, and the average number of the buried portions is 5 or more per cm [2 ] The membrane-electrode assembly according to any one of [5] to [5].

  ADVANTAGE OF THE INVENTION According to this invention, it can suppress that an electrolyte membrane peels from an electrode by repetition, such as electrolysis and charging / discharging, and can provide the membrane-electrode assembly which can suppress a performance fall.

It is a schematic diagram of the general structure of a redox flow battery. It is a schematic diagram of a general cell structure. It is a figure which shows the membrane-electrode assembly by this invention. It is a figure explaining the definition of "burial" in the present invention. It is a figure which shows the specific example of the part which is buried of the fibrous structure, and the part which is not buried. It is a schematic diagram explaining the hot press method. 3 is an analysis image used for calculating the thickness of a diaphragm portion in which the fibrous structure in the membrane-electrode assembly according to Example 1 is not buried. FIG. 3 is a diagram showing strength profiles in the film thickness direction of carbon and fluorine with respect to the membrane-electrode assembly according to Example 1.

  Hereinafter, although the form for implementing this invention (henceforth "this embodiment") is demonstrated in detail, this invention is not limited to the following description, In the range of the summary Various modifications can be made.

(Membrane-electrode assembly)
FIG. 3 shows a membrane-electrode assembly according to the present invention. The membrane-electrode assembly 1 shown in this figure includes a pair of electrodes 3 that perform an oxidation-reduction reaction, and a diaphragm 2 that is provided between the electrodes 3 and separates both electrodes 3. Here, the electrode 3 is a porous body formed of a fibrous structure, and at least a part of the fibrous structure is buried in the diaphragm 2.

  A membrane-electrode assembly 1 according to the present invention is a joined body in which an electrode 3 is joined to at least one surface of a diaphragm 2. Here, the state in which the diaphragm 2 and the electrode 3 are “joined” refers to a state in which the electrode 3 does not fall off due to its own weight when the diaphragm 2 is gripped. In the present specification, a material that does not satisfy the above-described bonding requirements is called a “membrane-electrode laminate”, and is clearly distinguished from a “membrane-electrode assembly”.

  In the present specification, the state in which the fibrous structure of the electrode 3 is “buried” in the diaphragm 2 means that an arbitrary cross section of the membrane-electrode assembly 1 cut parallel to the film thickness direction is an electron. When observed with a microscope, as shown schematically in FIG. 4A, all of the outer periphery S of the cross section of the portion E of the fibrous structure constituting the electrode 3 is the substance M constituting the diaphragm 2. Refers to the coated state. As shown in FIG. 4B, when only a part of the outer periphery S is covered with the substance M constituting the diaphragm 2, it is not considered to be buried. More specifically, in the electron microscope image of the cross section of the membrane-electrode assembly illustrated in FIG. 5, it is assumed that the portions E1 to E4 of the fibrous structure constituting the electrode are buried in the diaphragm. On the other hand, the portions E5 and E6 are not considered to be buried because part of the outer periphery is exposed from the diaphragm.

  In addition, when it is difficult to discriminate the outer periphery S that is the boundary between the diaphragm 2 and the fibrous structure of the electrode 3, qualitative characteristics such as energy-dispersive X-ray spectroscopy (EDX) are used. Analysis may be used. Specifically, if a distribution image of the constituent elements of the diaphragm 2 and the electrode 3 is acquired by surface analysis, the contrast between the part of the diaphragm 2 and the part of the electrode 3 in the image is caused by the difference in the constituent elements of the diaphragm 2 and the electrode 3. Therefore, by determining the boundary as the outer periphery S, the outer periphery S is determined by the EDX method, and it is determined whether or not the portion of the fibrous structure constituting the electrode 3 is buried in the diaphragm 2. it can.

-Diaphragm-
The diaphragm 2 in this embodiment is a polymer electrolyte membrane. The type of polymer electrolyte membrane is not particularly limited, but a membrane having proton conductivity is preferable from the viewpoint of use in a redox flow battery.

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

--Perfluorosulfonic acid resin--
Examples of the perfluorosulfonic acid 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 the formula ^(1), X ^1, X ^2, X 3, ^{X 4} are each independently a hydrogen atom, a halogen atom or a perfluoroalkyl group having 1 to 10 carbon ^atoms, X ^{1, X} 2, X ³ and at least one of X ⁴ is a fluorine atom or a C 1-10 perfluoroalkyl group.)
^{_{- [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 perfluoroalkyl group having 1 to 4 carbon halogen atom or carbon, R represents a hydrogen atom, an alkali metal atom such as lithium atom, sodium atom, or 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 , and R ⁴ each independently represent an amine such as a C 1-10 alkyl group or aryl group. Moreover, a is 0 or 1, b is 0 or 1, c is an integer of 0-8, d is 0 or 1, and e is an integer of 0-8. However, b and e are not 0 at the same time. )
In addition, when the perfluorosulfonic acid resin includes a plurality of repeating units represented by the general formula (1) and / or a plurality of repeating units represented by the general formula (2), each repeating unit is It may be the same or different.

The perfluorosulfonic acid resin is preferably a compound having one or more repeating units represented by the following general formulas (3) to (7).
^{_{- [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). Also, c, d and e are the formulas (1), Same as (2), 0 ≦ f <1, 0 <g ≦ 1, and f + g = 1, where e is not 0 in formulas (5) and (7).

The perfluorosulfonic acid resin may further contain other structural units other than the repeating units represented by the general formulas (1) and (2). Examples of the other structural units include structural units represented by the following general formula (I).
(In Formula (I), ^{R 1} is a single bond or a divalent perfluorinated organic group having 1 to 6 carbon atoms (e.g., such as perfluoroalkylene group having 1 to 6 carbon atoms), ^{R 2} is carbon A divalent perfluoro organic group having 1 to 6 carbon atoms (for example, a perfluoroalkylene group having 1 to 6 carbon atoms).

  As the perfluorosulfonic acid resin, a resin having a repeating unit represented by the formula (4) or the formula (5) is preferable from the viewpoint of obtaining a polymer electrolyte membrane that easily transmits protons and has a lower resistance. More preferably, the resin consists only of the repeating unit represented by the formula (5).

  The equivalent mass EW of the PFSA resin (the dry mass in grams of PFSA resin per equivalent of proton exchange groups) is preferably adjusted to 300 to 1300. The equivalent mass EW of the PFSA resin in the present embodiment is more preferably 350 to 1000, still more preferably 400 to 900, and most preferably 450 to 750.

  Moreover, it is preferable that the thickness of the part of the diaphragm 2 in which the fibrous structure of the electrode 3 is not buried is 2 μm or more. More preferably, it is 5 μm or more, and further preferably 10 μm or more. Furthermore, it is most preferable that it is 20 micrometers or more. The short circuit between the positive and negative electrodes can be suppressed, and at the same time, the crossover of the positive and negative electrode electrolytes can be suppressed. In the present specification, the thickness of the portion of the diaphragm 2 where the fibrous structure is not buried is calculated by the following three procedures.

1. Selection of quantitative target element About the fibrous structure and the diaphragm 2 which comprise the electrode 3, an element with many ratios which comprises each is selected as a quantitative target element. However, the quantitative target element X of the fibrous structure and the quantitative target element Y of the diaphragm 2 are different elements. The method for obtaining the ratio of the elements constituting the fibrous structure or the diaphragm 2 is not particularly limited. For example, it may be obtained from composition information or composition analysis may be performed. As a method of composition analysis, various methods such as inductively coupled plasma mass spectrometry (ICP-MS) and EDX method can be used. Among them, it is preferable to use the EDX method in that the same position as the part observed with the electron microscope image can be easily analyzed.

2. Measurement of bulk element ratio In the cross-section of the joined body cut in parallel with the film thickness direction, elements X and Y are quantitatively analyzed, for example, by the EDX method, and the element ratio Rm is calculated by the following formula (8).
Element ratio Rm of diaphragm part
= Quantitative value of element X in the diaphragm part ÷ Quantitative value of element Y in the diaphragm part (8)
At this time, when the quantitative value of the element X is less than 1/100 of the quantitative value of Y, Rm is set to 0.01.

3. Calculation of the thickness of the unfilled diaphragm part In the field of view including the diaphragm of the cross section of the joint cut parallel to the film thickness direction, quantitative analysis of elements X and Y by EDX is performed in the film thickness direction so as to cross the diaphragm. The change in the film thickness direction of the element ratio R is calculated by the following formula (9).
Element ratio R
= Quantitative value of element X at the calculation point ÷ Quantitative value of element Y at the calculation point (9)
At that time, in order to obtain the element ratio R averaged in the film length direction (film extending direction), the analysis region is 180 μm in the film length direction for line analysis.

About the calculated element ratio R, the following formula (10), that is, 1.5 Rm>R> 0.7 Rm (10)
The range that satisfies the condition is defined as a diaphragm portion where the fibrous structure is not buried, and the thickness thereof is defined as the thickness of the diaphragm 2 where the fibrous structure constituting the electrode 3 is not buried.

  Moreover, the thickness of the diaphragm 2 (thickness of the entire diaphragm 2) is preferably 150 μm or less, more preferably 100 μm or less, further preferably 75 μm or less, and most preferably 30 μm or less. Thereby, the battery size can be reduced, and at the same time, the internal resistance can be reduced.

-Electrode-
The electrode 3 in this embodiment is a porous body formed of a fibrous structure. Here, it is important that at least a part of the fibrous structure is buried in the diaphragm. Thereby, the dimensional change by swelling of the diaphragm 2 can be suppressed by an anchor effect, and it can suppress that the diaphragm 2 peels from the electrode 3 by repetition, such as electrolysis and charging / discharging, and a performance falls.

  The fibrous structure constituting the electrode 3 in the present embodiment preferably has carbon. Examples of carbon fibrous structures include carbon fiber nonwoven fabric, carbon fiber paper, and carbon foam. Hereinafter, requirements in the case of using a carbon fiber nonwoven fabric, carbon fiber paper, and carbon foam as the electrode 3 will be specifically described.

--- Carbon fiber nonwoven fabric--
<Fiber diameter of carbon fiber>
When the fibrous structure which comprises the electrode 3 is a carbon fiber nonwoven fabric, it is preferable that the fiber diameter of the carbon fiber which comprises a carbon fiber nonwoven fabric is 1 micrometer or more, It is more preferable that it is 3 micrometers or more, It is 5 micrometers or more. More preferably it is. Thereby, the handleability at the time of processing a fiber into a nonwoven fabric can be improved. The fiber diameter is preferably 20 μm or less, more preferably 15 μm or less, and further preferably 10 μm or less. Thereby, when using as an electrode, the surface area per unit mass can be enlarged.

<Bulk density>
The bulk density of the carbon fiber nonwoven fabric is preferably at 0.01 g / cm ³ or more, more preferably 0.02 g / cm ³ or more, further preferably 0.05 g / cm ³ or more . Thereby, when using as an electrode, the surface area can be enlarged. The bulk density of the carbon fiber nonwoven fabric is preferably 0.3 g / cm ³ or less, more preferably 0.2 g / cm ³ or less, and further preferably 0.1 g / cm ³ or less. . Thereby, the porosity can be increased and the circulation of the electrolyte can be facilitated when used as an electrode.

<Thickness>
The thickness of the carbon fiber nonwoven fabric is preferably 1 mm or more, more preferably 1.5 mm or more, and further preferably 2 mm or more. Thereby, sufficient electrode surface area can be ensured and the resistance accompanying an electrochemical reaction can be reduced. Moreover, the thickness of the carbon fiber nonwoven fabric is preferably 15 mm or less, more preferably 10 mm or less, and further preferably 8 mm or less. Thereby, the resistance accompanying the movement of electrons in the electrode can be reduced, and the cell can be miniaturized.

--- Carbon fiber paper--
<Fiber diameter of carbon fiber>
Moreover, when the fibrous structure which comprises the electrode 3 is carbon fiber paper, it is preferable that the fiber diameter of the carbon fiber which comprises carbon fiber paper is 1 micrometer or more, It is more preferable that it is 3 micrometers or more, 5 micrometers More preferably, it is the above. Thereby, the handleability at the time of processing a fiber into paper can be improved. The fiber diameter is preferably 20 μm or less, more preferably 15 μm or less, and further preferably 10 μm or less. Thereby, when using as an electrode, the surface area per unit mass can be enlarged.

<Average length of carbon fiber>
Further, the average length of the carbon fibers in the carbon fiber paper is preferably 5 mm or more, more preferably 10 mm or more, and further preferably 15 mm or more. Thereby, mechanical strength can be raised by the increase in the contact density between fibers. The average length of the carbon fibers is preferably 50 mm or less, more preferably 40 mm or less, and further preferably 30 mm or less. Thereby, the opening in the paper making process can be facilitated.

<Bulk density>
The bulk density of the carbon fiber paper is preferably 0.1 g / cm ³ or more, more preferably 0.15 g / cm ³ or more, and further preferably 0.2 g / cm ³ or more. . Thereby, when using as an electrode, the surface area can be enlarged. The bulk density of the carbon fiber paper is preferably 0.6 g / cm ³ or less, more preferably 0.5 g / cm ³ or less, and further preferably 0.4 g / cm ³ or less. . Thereby, the porosity can be increased and the circulation of the electrolyte can be facilitated when used as an electrode.

<Thickness>
The thickness of the carbon fiber paper is preferably 50 μm or more, more preferably 100 μm or more, and further preferably 200 μm or more. Thereby, sufficient electrode surface area can be ensured and the resistance accompanying an electrochemical reaction can be reduced. Further, the thickness of the carbon fiber paper is preferably 600 μm or less, more preferably 500 μm or less, and further preferably 400 μm or less. Thereby, the resistance accompanying the movement of electrons in the electrode can be reduced, and the cell can be miniaturized.

--- Carbon foam--
<Porosity>
Furthermore, when the fibrous structure constituting the electrode 3 is a carbon fiber foam, the porosity of the carbon foam is preferably 90% or more, more preferably 95% or more, and 99% or more. Is more preferable. When the porosity is less than 90%, when used as an electrode of a redox flow battery, the surface area of the electrode cannot be sufficiently increased, and the cell resistance may be increased.

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

[Measurement of bulk density]
First, the dimensions of the carbon foam are measured using calipers or the like, and the bulk volume V _bulk of the carbon foam is determined from the obtained dimensions. Next, the mass M of the carbon foam 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 can be obtained using the following formula (11).
ρ _bulk = M / V _bulk (11)

[Measurement of true density]
The true density ρ _real of the carbon foam can be determined by a flotation method using a mixed solution composed of n-heptane, carbon tetrachloride and ethylene dibromide. Specifically, first, a carbon foam of an appropriate size is put in a stoppered test tube. Next, three kinds of solvents are mixed as appropriate and added to a test tube, which is then immersed in a constant temperature bath at 30 ° C. If the sample piece floats, add n-heptane which is low density. On the other hand, when the test piece sinks, high-density ethylene dibromide is added. This operation is repeated so that the specimen floats in the liquid. Finally, the density of the liquid is measured using a Geryusack specific gravity bottle.

[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 (12).
V _{f, pore} = ((1 / ρ _bulk ) − (1 / ρ _real )) / (1 / ρ _bulk ) × 100 (%)
(12)

  When the carbon foam is formed by heating, for example, a melamine resin foam, the fibrous carbon constituting the skeleton of the carbon foam has a structure in which the carbon is uniformly spread in all directions. When the pressure is applied in a predetermined direction, a skeletal structure having anisotropy in the expansion of the fibrous carbon is obtained. In the present embodiment, the carbon foam has an anisotropic skeletal structure having anisotropy in the expansion of the fibrous carbon, even if the carbon foam has an isotropic skeleton structure in which the fibrous carbon isotropically expanded. You may have.

  Carbon foams with an isotropic skeletal structure do not apply pressure during formation, so the porosity of the formed carbon foam is high (for example, 99% or more) and is more flexible than can be compressed with small stress Therefore, it is possible to respond more flexibly to swelling and shrinkage of the diaphragm during electrolysis and battery charge / discharge. In contrast, a carbon foam having an anisotropic skeletal structure has a lower porosity than that having an isotropic skeletal structure (for example, 95% or more) because a pressure is applied during formation. ) When a cell is formed and strongly compressed by a current collector plate and a large compressive stress is applied, deterioration due to breakage of the fibrous structure and powder falling can be suppressed.

<Average diameter>
Further, the average diameter d _{ave of} the fibrous carbon (linear portion) constituting the carbon foam, measured by scanning electron microscope observation, is preferably 10 μm or less, more preferably 5 μm or less, and more preferably 3 μm or less. More preferably it is. Thereby, the joint strength between the diaphragm 2 and the carbon foam can be improved by increasing the specific surface area.

[Measuring method of average diameter]
The average diameter d _ave of the fibrous carbon constituting the carbon foam is obtained by image analysis of a scanning electron microscope image. Specifically, the carbon foam is observed at a magnification of 10,000 times using a scanning electron microscope. From the obtained observation image, the thickness of fibrous carbon is measured at 20 random locations. Assuming that the cross-sectional shape is circular, this average thickness is _defined as an average diameter d _ave .

<Specific surface area>
The specific surface area S determined from the true density of the carbon foam and the average diameter of the fibrous carbon constituting the carbon foam is preferably 0.5 m ² / g or more, and more preferably 1 m ² / g or more. preferable. As a result, a sufficient bonding area with the diaphragm 2 can be secured, and the peel resistance can be improved.

[Calculation of specific surface area]
The specific surface area S of the carbon foam is calculated using the following (13) from the true density ρ _real and the average diameter d _ave obtained as described above, assuming that the shape of the fibrous carbon constituting the column is cylindrical. Can be sought.
S = 4 / (ρ _real × d _ave ) (13)

  The carbon foam 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 known filler is foamed and then cured. By performing the treatment, a melamine / formaldehyde condensed foam can be obtained.

  In the above method, as the melamine / formaldehyde precondensate, for example, melamine: formaldehyde = 1: 1.5 to 1: 4 and an average molecular weight of 200 to 1000 can be used. Further, as an emulsifier, for example, sodium salt of alkyl sulfonic acid or aryl sulfonic acid is 0.5 to 5% by mass (based on melamine / formaldehyde precondensate, the same shall apply hereinafter), and as a vaporizable foaming agent, for example, pentane or hexane. 1 to 50% by mass, and examples of the curing agent include 0.01 to 20% by mass of hydrochloric acid and sulfuric acid. The foaming treatment and the curing treatment may be performed by heating the solution composed of 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 atmosphere such as in an inert gas stream or in a vacuum. Moreover, about the minimum of heat processing temperature, it is preferable that it is 800 degreeC or more from a viewpoint of improving electroconductivity. On the other hand, the upper limit of the heat treatment temperature is preferably 2500 ° C. or lower from the viewpoint of maintaining the flexibility of the electrode.

  The thickness of the carbon foam is preferably 20 μm or more, more preferably 100 μm or more, and further preferably 200 μm or more. Thereby, sufficient electrode surface area can be ensured and the resistance accompanying an electrochemical reaction can be reduced. The thickness of the carbon foam is preferably 2000 μm or less, more preferably 1000 μm or less, and further preferably 500 μm or less. Thereby, the resistance accompanying the movement of electrons in the electrode can be reduced, and the cell can be miniaturized.

  The shape of the diaphragm 2 and the electrode 3 is not particularly limited, but is preferably a sheet from the viewpoint of easy handling and processing.

(Method for forming membrane-electrode assembly)
In order to join the diaphragm 2 and the electrode 3 to form the membrane-electrode assembly 1, there are a hot press method and a method of bonding using a polymer solution of the same type as the membrane. Among these, the hot press method is preferable from the viewpoint of workability. FIG. 6 is a schematic diagram for explaining the hot press method. Specifically, in the formation of the membrane-electrode assembly 1 by the hot press method, first, as shown in FIG. 6 (a), the diaphragm 2 is sandwiched between two electrodes 3 (for example, carbon fiber nonwoven fabric). It is placed between pressure plates 24 of a hot press machine together with a spacer 23 having a thickness. Next, as shown in FIG. 6B, the pressure plate 24 is heated to a predetermined temperature and then pressed. After holding for a predetermined time, the pressure plate 24 is opened and cooled to room temperature. Thus, the membrane-electrode assembly 1 can be formed.

  In the 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 diaphragm 2 is brought into close contact with the electrode 3 without causing the diaphragm 2 to be thermally deteriorated, so that the diaphragm 2 can be firmly bonded. The thickness of the spacer 23 is preferably 10% or more and 50% or less, and more preferably 20% or more and 40% or less with respect to the total thickness of the diaphragm 2 and the two electrodes 3. The holding time after pressing is preferably 5 min or more and 30 min or less, and more preferably 10 min or more and 20 min or less. Thereby, the diaphragm 2 and the electrode 3 can be firmly joined without causing a short circuit between the electrodes 3.

  The fibrous structure of the electrode 3 preferably has 5 or more buried portions per 1 cm in the film length direction in the cross section in the film thickness direction of the membrane-electrode assembly 1. Thereby, the joining between the diaphragm 2 and the electrode 3 can be made stronger. In addition, “in the cross section in the film thickness direction of the membrane-electrode assembly 1, the portion having 5 or more buried portions per 1 cm in the film length direction” means that in the portion of the square membrane-electrode assembly having a side of 30 mm, The first cross section cut in parallel to the film thickness direction on the square diagonal line is defined as the film thickness direction cross section, and the number of buried portions in three different non-overlapping visual fields of 1 cm in the film length direction on the first cross section. This means that the average number of the buried portions is 5 or more per 1 cm. In addition, it is more preferable that the square-shaped portion having an arbitrary side of 30 mm on the membrane-electrode assembly has five or more buried portions per 1 cm in the film length direction.

  Hereinafter, although a specific Example and a comparative example are given and demonstrated, this invention is not limited to these.

Example 1
<Preparation of membrane-electrode assembly>
First, two carbon fiber nonwoven fabrics (SIGLELL (registered trademark) GFA6EA manufactured by SGL CARBON) having dimensions: 30 mm × 30 mm × 6 mm as electrodes, and electrolyte membranes (Nafion 212 manufactured by Dupon) having dimensions of 25 mm × 25 mm × 55 μm as diaphragms. Each sheet was prepared. Next, the carbon fiber nonwoven fabric was arrange | positioned on both surfaces of the diaphragm, and the carbon fiber nonwoven fabric and the diaphragm were joined by the hot press method. Specifically, the electrolyte membrane was sandwiched between two carbon fiber nonwoven fabrics and placed between the pressure plates of a hot press machine together with a spacer having a thickness of 4 mm. Subsequently, the pressure plate was heated to 200 ° C. and pressed. After holding for 20 minutes in this state, the pressure plate was opened and cooled to room temperature. Thus, a membrane-electrode assembly according to Example 1 was produced.

(Example 2)
A membrane-electrode assembly according to Example 2 was produced in the same manner as Example 1. However, the heating time at the time of hot pressing was set to 7 minutes. The other conditions are all the same as in Example 1.

(Example 3)
A membrane-electrode assembly according to Example 3 was produced in the same manner as Example 1. However, the heating time during hot pressing was 3 min. The other conditions are all the same as in Example 1.

Example 4
A membrane-electrode assembly according to Example 4 was produced in the same manner as Example 1. However, the heating temperature was 180 ° C. and the heating time was 15 minutes. The other conditions are all the same as in Example 1.

(Example 5)
A membrane-electrode assembly according to Example 5 was produced in the same manner as Example 1. However, the heating temperature was 160 ° C. and the heating time was 30 minutes. The other conditions are all the same as in Example 1.

(Comparative Example 1)
In Example 1, the membrane-electrode laminated body by the comparative example 1 was produced, without joining a carbon fiber nonwoven fabric with an electrode and a diaphragm by the hot press method.

(Comparative Example 2)
As in Example 1, a membrane-electrode assembly according to Comparative Example 2 was produced. However, the thickness of the spacer was 8 mm, the heating temperature was 80 ° C., and the heating time was 10 min. The other conditions are all the same as in Example 1.

<Measurement of the number of buried portions of the fibrous structure>
About Example 1, the number of the embedded parts of the fibrous structure which comprises an electrode in the following procedure was investigated. First, a substantially square sample joined body having a side of 30 mm was produced. Next, it cut | disconnected in parallel with the film thickness direction on the diagonal of a sample joining body, and three different samples which are 1 cm in length and which do not overlap mutually on the cut surface were produced. And the buried part was evaluated about each of the obtained three samples. In this evaluation, specifically, the cut surface of each sample was scanned with a scanning electron microscope S4700 manufactured by Hitachi High-Technologies Corporation, with an acceleration voltage of 1.5 kV, a working distance WD of 12 mm, and a magnification of 250 times. Observed. At that time, in order to observe a sample having a length of 1 cm, the stage on which the sample was placed was moved for observation. The number of buried portions of each sample is as follows.

  The average value of the number of buried portions in the three samples is defined as the number of buried portions of the fibrous structure with respect to Example 1. The same evaluation was performed for Examples 2 to 5 and Comparative Examples 1 and 2. The obtained results are shown in Table 2.

<Calculation of the thickness of the diaphragm portion where the fibrous structure is not buried>
About Example 1, the thickness of the diaphragm part in which the fibrous structure which comprises an electrode is not buried in the following procedure was computed. First, an analysis image shown in FIG. 7 was obtained with a scanning electron microscope S4700 manufactured by Hitachi High-Technologies Corporation at an acceleration voltage of 7.0 kV, a working distance WD of 12 mm, and a magnification of 250 times. With this analysis image, quantitative analysis was performed with EMAX-7000 manufactured by Horiba, Ltd. Spot analysis was performed on the diaphragm portion of carbon (hereinafter, “C”) as the quantitative target element X and fluorine (hereinafter, “F”) as the quantitative target element Y. At that time, the measurement time was 60 seconds. From the obtained strength ratio of C and F, Rm was set to 0.25.

  Subsequently, quantitative line analysis of C and F was performed in the film thickness direction for a range of 180 μm width in the analysis image shown in FIG. At that time, the measurement time was 30 seconds / line, and the number of integrations was 10 times. FIG. 8 shows the obtained intensity profiles of C and F in the film thickness direction. The intensity ratio of C and F was calculated from the intensity profile shown in FIG. 8, and the range satisfying the above formula (10) was examined. As a result, since Expression (10) was satisfied within the range of 81 μm to 109 μm from the left end of the analysis image, the thickness of the diaphragm portion where the fibrous structure was not buried was calculated as 109 μm−81 μm = 28 μm. Similarly, for Examples 2 to 5 and Comparative Example 2, the thickness of the diaphragm portion where the fibrous structure was not buried was calculated. The obtained results are shown in Table 2. In Comparative Example 1, the above calculation was not performed because the diaphragm and the electrode were not joined.

<Evaluation of peeling resistance>
The peel resistance of the membrane-electrode assemblies according to Examples 1 to 5 and Comparative Examples 1 and 2 (Comparative Example 1 is a membrane-electrode laminate) was evaluated. Specifically, peeling when a membrane-electrode assembly (Comparative Example 1 is a membrane-electrode laminate) was immersed in ethylene glycol and ethanol was evaluated. Hereinafter, each evaluation will be described.

[Evaluation by immersion in ethylene glycol]
Regarding each of the membrane-electrode assembly according to Examples 1 to 5 and Comparative Example 2 and the membrane-electrode laminate according to Comparative Example 1, does peeling occur between the electrode and the diaphragm when immersed in ethylene glycol? Evaluated whether or not. Specifically, the membrane-electrode assembly (Comparative Example 1 is a membrane-electrode laminate) was immersed in ethylene glycol while ethylene glycol was supplied into the container and the temperature of ethylene glycol was maintained at 25 ° C. At the time point after 20 minutes, whether or not peeling occurred between the electrode and the diaphragm was evaluated by visual observation. The evaluation results are shown in Table 2. In Table 2, “◯” indicates that no peeling occurred, and “X” indicates that the diaphragm and the electrode were completely peeled off. Moreover, how much the dimension of the membrane-electrode assembly (Comparative Example 1 is a membrane-electrode laminate) according to Examples and Comparative Examples was changed after immersion in ethylene glycol. The obtained results are shown in Table 2.

[Evaluation by ethanol immersion]
Evaluation similar to the evaluation by the ethylene glycol immersion was performed using ethanol. Specifically, in a state where ethanol was supplied into the container and the temperature of ethanol was maintained at 25 ° C., the membrane-electrode assemblies of Comparative Examples and Examples (Comparative Example 1 was a membrane-electrode laminate) were replaced with ethanol. Soaked. At the time point after 20 minutes, whether or not peeling occurred between the electrode and the diaphragm was evaluated by visual observation. The evaluation results are shown in Table 2. In Table 2, “◯” indicates that no peeling occurred, and “X” indicates that the diaphragm and the electrode were completely peeled off. Moreover, how much the dimension of the membrane-electrode assembly (Comparative Example 1 is a membrane-electrode laminate) according to Examples and Comparative Examples was changed after ethanol immersion. The obtained results are shown in Table 2.

  As is apparent from Table 2, the membrane-electrode assemblies according to Examples 1 to 4 did not cause peeling in any of the peeling evaluations by ethylene glycol immersion and ethanol immersion. Moreover, about the membrane-electrode assembly by Example 5, although peeling did not arise in peeling evaluation by ethylene glycol immersion, peeling generate | occur | produced in peeling evaluation by ethanol immersion. However, the peeling evaluation by ethanol immersion is an evaluation in an environment more severe than the environment in normal electrolysis or battery, and there is no problem if no peeling occurs in the peeling evaluation by ethylene glycol immersion. On the other hand, in Comparative Examples 1 and 2, exfoliation occurred in both exfoliation evaluation by ethylene glycol immersion and ethanol immersion.

<Evaluation of occurrence of short circuit>
About Examples 1-5 and the membrane-electrode assembly of Comparative Example 2 and the membrane-electrode laminate of Comparative Example 1, a short circuit occurs between the electrodes by measuring the resistance between the electrodes facing each other across the diaphragm. It was evaluated whether or not. Specifically, a custom digital multimeter CDM-11D needle was applied to each of the electrodes facing each other across the diaphragm, and the resistance between the electrodes was measured. And when resistance between electrodes was 300 ohms or more, it evaluated that a short circuit did not generate | occur | produce. The obtained results are shown in Table 2. As shown in Table 2, it was evaluated that no short circuit occurred in all of the membrane-electrode assemblies of Examples 1 to 5, Comparative Example 2 and the membrane-electrode laminate of Comparative Example 1.

  According to the present invention, it is possible to provide a membrane-electrode assembly that can suppress the separation of the diaphragm from the electrode by repetition of electrolysis, charge / discharge, etc., and can suppress the deterioration of the performance. Useful.

1 Membrane-electrode assembly 2, 11, 111 Diaphragm (electrolyte membrane)
3, 12, 112 Electrode 10 Membrane-electrode stack 13 Frame 14 Current collector 20 Cell 23 Spacer 24 Pressure plate 100 Redox flow battery 101 Electrolyzer 102, 103 Tank 104, 105 Pump 106 Power supply

Claims (6)

A pair of electrodes that perform a redox reaction;
A diaphragm provided between the electrodes and separating both electrodes;
A membrane-electrode assembly comprising:
The electrode is a porous body formed of a fibrous structure, and at least a part of the fibrous structure is buried in the diaphragm.   2. The bonding according to claim 1, wherein the fibrous structure of the electrode has five or more buried portions per 1 cm in the film length direction in the film thickness direction cross section of the membrane-electrode assembly. body.   The membrane-electrode assembly according to claim 1 or 2, wherein a thickness of a portion of the diaphragm in which the fibrous structure of the electrode is not buried is 2 µm or more.   The membrane-electrode assembly according to claim 3, wherein the thickness of the portion of the diaphragm in which the fibrous structure of the electrode is not buried is specified by energy dispersive X-ray analysis.   The membrane-electrode assembly according to any one of claims 1 to 4, wherein the diaphragm has a thickness of 30 µm or less. The film thickness direction cross section
In a portion of a square membrane-electrode assembly having a side of 30 mm, a first cross section cut in parallel to the film thickness direction on the diagonal of the square,
The number of locations of the buried portion is evaluated from three different visual fields that do not overlap each other in the film length direction of 1 cm on the first cross section, and an average value of the number of locations of the buried portion is 5 or more per 1 cm. The membrane-electrode assembly according to any one of -5.