JP2008077956A - Varnish for forming catalyst electrode for fuel cell, catalytic electrode using it, membrane electrode assembly using it, fuel cell using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a varnish for forming catalyst electrode for fuel cell in which a dispersion process of a noble-metal catalyst carrying carbon in the varnish for forming catalyst electrode for fuel cell is less than a conventional one and a formation time of the varnish for forming catalyst electrode is shortened, and a catalyst electrode for fuel cell improved in a noble-metal catalyst utilization rate and having high output characteristics. <P>SOLUTION: The catalyst electrode, a membrane electrode assembly (MEA), and a fuel cell are manufactured using a varnish for forming catalyst electrode for fuel cell comprising a noble-metal catalyst-carrying electron conductive substance, a proton conductive polymer electrolyte, amorphous carbon introduced with a sulfonic acid group, and a solvent. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a varnish for forming a catalyst electrode of a fuel cell used as a power source for in-vehicle use, home use, and portable equipment, a catalyst electrode using the same, a membrane electrode assembly using the same, and a fuel cell using the same. .

The fuel cell has been attracting attention as a power generation system that has essentially no adverse effects on the environment because its reaction product is essentially only water.
In recent years, a solid polymer fuel cell using an ion exchange membrane having hydrogen ion conductivity as an electrolyte among fuel cells has a low operating temperature, a high output density, and can be easily downsized. It is considered promising as a power source for in-vehicle use, home use, and portable equipment.

A polymer electrolyte fuel cell is configured by stacking a large number of unit cells.
As shown in FIG. 1, the unit cell includes an anode-side separator 1, an anode-side catalyst electrode 2, a hydrogen ion conductive polymer electrolyte membrane 3, a cathode-side catalyst electrode 4, and a cathode-side separator 5 in this order. It is configured by stacking.

The catalyst electrode 2 on the anode side is composed of an electrode base material 21 and an electrode catalyst layer 22 laminated on this surface.
The cathode-side catalyst electrode 4 includes an electrode base material 41 and an electrode catalyst layer 42 laminated on the surface.

The anode side electrode base material 21 and the cathode side electrode base material 41 are both made of a material having gas diffusibility and conductivity. For example, carbon paper or carbon cloth is used.

The anode-side electrode catalyst layer 22 and the cathode-side electrode catalyst layer 42 are both formed into particles by supporting a noble metal catalyst on carbon particles, and this is made of electrode base materials 21 and 41 with a hydrogen ion conductive polymer electrolyte. It is fixed and configured.

The anode-side separator 1 is provided with a reaction gas channel 1a, which supplies a fuel gas to a membrane electrode assembly (MEA) composed of a hydrogen ion conductive polymer electrolyte membrane and a catalyst electrode.

On the other hand, the reaction gas flow path 5a is also provided in the cathode-side separator 5, whereby oxygen gas is supplied to the MEA.

An electromotive force is generated between the electrodes 2 and 4 by reacting hydrogen and oxygen gas in the fuel gas in the presence of a noble metal catalyst.

As the fuel gas sent to the anode, it has been studied to use a hydrogen gas-rich reformed gas obtained by steam reforming methane, methanol, or the like.

The steam reforming of methanol is performed using a two-stage reaction as follows using a catalyst such as a Cu—Zn system at a temperature of 250 to 300 ° C.

First, methanol is reacted with water vapor using a reformer to convert it into hydrogen and carbon monoxide (CO).
_{CH 3 OH = 2H 2 + CO} -90kJ / mol ··· (1)

Next, a shift converter is used to shift-react CO with water vapor to obtain a reformed gas rich in hydrogen gas.
CO + H ₂ O = H ₂ + CO ₂ +40 kJ / mol (2)

As solid polymer fuel cells become practically used, reduction of manufacturing cost is required.

A fuel cell catalyst electrode forming varnish comprising a noble metal catalyst-supporting carbon, a proton conductive polymer electrolyte, and a solvent is prone to agglomeration of the noble metal catalyst-supporting carbon, so when producing a fuel cell catalyst electrode forming varnish, A varnish for forming a catalyst electrode for a fuel cell must be prepared by previously dispersing a noble metal catalyst-supporting carbon in a solvent and then mixing a proton conductive polymer electrolyte.

If this is neglected, the dispersibility of the noble metal catalyst-supported carbon in the varnish for forming a catalyst electrode for a fuel cell is remarkably deteriorated.

Accordingly, when the fuel cell catalyst electrode forming varnish is generated, there are problems that the dispersion process has to be performed in a plurality of stages, the number of processes is increased, and time is required.

In addition, the noble metal catalyst-supported carbon is likely to be aggregated in the fuel cell catalyst electrode forming varnish thus dispersed.

If the catalyst electrode is formed using a varnish for forming a catalyst electrode for a fuel cell in which the noble metal catalyst-supported carbon easily aggregates, the formation of a proton supply path is hindered, causing a problem that the noble metal catalyst-supported carbon is not effectively used.

As a result, the battery performance obtained per unit noble metal catalyst amount is lowered, and accordingly, there is a problem that a large amount of noble metal catalyst, which is an expensive and limited resource, must be used.

As a method for solving this problem, a method of adding a surfactant to a varnish for forming a catalyst electrode for a fuel cell is disclosed. (For example, see Patent Document 1)

Japanese Patent Laid-Open No. 10-284086

However, when a surfactant is added to the fuel cell catalyst electrode forming varnish, the dispersibility of the noble metal catalyst-supported carbon in the fuel cell catalyst electrode forming varnish is improved, but the dispersion step cannot be reduced. .

In addition, the surfactant remaining in the catalyst electrode causes a new problem that stable battery performance cannot be obtained.

A first object of the present invention is to provide a catalyst electrode for a fuel cell in which the time required for generating the varnish for forming a catalyst electrode for a fuel cell is reduced as compared with the prior art. It is to provide a method for producing a forming varnish.

The second object of the present invention is to provide a fuel cell catalyst electrode having improved output characteristics by improving the utilization rate of the noble metal catalyst.

The third object of the present invention is to provide a membrane electrode assembly and a fuel cell using the fuel cell catalyst electrode.

The invention according to claim 1 is a varnish for forming a catalyst electrode for a fuel cell, comprising a noble metal catalyst-supported electron conductive material, a proton conductive polymer electrolyte, amorphous carbon into which a sulfonic acid group has been introduced, and a solvent. It is.

The amorphous carbon into which the sulfonic acid group is introduced may be any substance as long as it has a sulfonic acid group and exhibits properties as an amorphous carbon.

Here, “amorphous carbon” refers to a substance made of carbon and does not have a clear crystal structure such as diamond or graphite, and more specifically, a clear peak in powder X-ray diffraction. Means a substance in which no or a broad peak is detected.

The proton conductivity of the amorphous carbon into which the sulfonic acid group is introduced is not particularly limited, but is preferably 0.01 S / cm or more.
If the proton conductivity of amorphous carbon is 0.01 S / cm or more, protons generated in the electron conductive material can be efficiently conducted.
If it is 0.01 S / cm or less, the proton conductivity is too low to reduce the amount of platinum catalyst.
(The proton conductivity is a value measured by the AC impedance method under conditions of a temperature of 80 ° C. and a humidity of 100%.)

Amorphous carbon into which a sulfonic acid group has been introduced may be subjected to chemical treatment such as fluorination.

As the electron-conducting substance, carbon black such as oil carb black, channel black, lamp black, thermal black, acetylene black, ketjen black and the like having a large electronic conductivity and specific surface area can be used.

The proton conductive polymer electrolyte is preferably used as long as it is a polymer electrolyte that conducts protons.
Among them, the sulfonated proton conductive polymer electrolyte has particularly good proton conductivity. In particular, a fluorine-based cation exchange resin such as perfluorocarbon sulfonic acid is preferable.

The solvent is not particularly limited, and a volatile liquid organic solvent that does not react with amorphous carbon into which a noble metal catalyst-supported electron conductive material, proton conductive polymer electrolyte, and sulfonic acid groups are introduced may be used. desirable.

The invention according to claim 2 is characterized in that the noble metal catalyst is one or more metals selected from Au, Pt, Pd, Rh, Ru, Os and Ir, or Mo. It is a varnish for catalyst electrode formation for fuel cells of Claim 1.

The noble metal catalyst may contain W, Sn, Re, etc. as an additive in the Pt alloy.
When the additive is contained, the CO poisoning resistance increases.

The additive metal may exist as an intermetallic compound of a Pt alloy or may form an alloy.

The invention according to claim 3 is the varnish for forming a catalyst electrode for a fuel cell according to claim 1 or 2, wherein the proton conductive polymer electrolyte is perfluorocarbon sulfonic acid.

Invention of Claim 4 is a manufacturing method of the varnish for catalyst electrode formation for fuel cells of any one of Claim 1 thru | or 3, Comprising:
A fuel characterized by having a one-step processing step of mixing a noble metal catalyst-supported electron conductive material, a proton conductive polymer electrolyte, and amorphous carbon into which a sulfonic acid group has been introduced, and dispersing using a disperser It is a manufacturing method of the varnish for battery catalyst electrode formation.

The invention according to claim 5 is the method for producing a varnish for forming a catalyst electrode for a fuel cell according to claim 4, wherein the disperser is a planetary ball mill, a homogenizer, or a nanomizer.

A sixth aspect of the present invention is directed to a catalyst electrode comprising a conductive porous substrate coated with the varnish for forming a catalyst electrode for a fuel cell according to any one of the first to third aspects. It is a manufacturing method.

The invention according to claim 7 is a membrane electrode assembly (MEA) in which a hydrogen ion conductive polymer electrolyte membrane is sandwiched between the catalyst electrodes.

The invention according to claim 8 is a fuel cell in which a membrane electrode assembly (MEA) is sandwiched between separators.

According to the present invention, it is possible to reduce the number of steps for producing a fuel cell catalyst electrode forming varnish and to reduce the production cost by shortening the time for producing a fuel cell catalyst electrode forming varnish.
Further, the utilization rate of the noble metal catalyst of the fuel cell can be improved by the present invention.

A fuel cell catalyst electrode forming varnish of the present invention, a catalyst electrode using the same, a membrane electrode assembly using the same, and a method for producing a fuel cell using the same will be described.

(Preparation of noble metal catalyst-supported electron conductive material)
First, the electron conductive material is oxidized to form acidic groups on the surface of the electron conductive material.

As the electron conductive substance, acetylene black having a specific surface area of 240 to 700 m ² / g can be used.

When the specific surface area is 240 m ² / g or more, high discharge characteristics (battery voltage) can be obtained.
Moreover, even if the specific surface area exceeds 700 m ² / g, the discharge characteristics (battery voltage) do not change, but the cost increases as the surface area is increased.

A carboxyl group can be used as the acidic group.
By forming a carboxyl group on the surface of the electron conductive material, the catalyst can be easily and uniformly supported on the surface of the electron conductive material.

As a method for forming an acidic group, a method of reacting an electron conductive substance with an aqueous potassium permanganate solution having a concentration of 0.4 to 0.6 mol / liter at 70 to 80 ° C. can be used.

Next, ions contained in the acidic group on the surface of the electron conductive material are ion-exchanged with a complex cation having a catalyst, and then reduced.

Platinum can be used as the catalyst.
Since platinum has high oxygen reduction ability and hydrogen oxidation ability, a solid polymer fuel cell with high output can be produced by using platinum.

The average particle diameter (volume average particle diameter (Dv)) of platinum (catalyst) is preferably 1 nm or more and 5 nm or less.

By setting the average particle size (volume average particle size (Dv)) of platinum (catalyst) to 1 nm or more and 5 nm or less, the fuel cell has a very large surface area per unit weight of the catalyst and excellent output characteristics with high catalyst efficiency. Can be produced.

It is difficult to make the average particle diameter (volume average particle diameter (Dv)) of platinum (catalyst) less than 1 nm, which increases the cost.
In addition, when the average particle size (volume average particle size (Dv)) of platinum (catalyst) exceeds 5 nm, the surface area per unit weight of the catalyst is reduced, the catalyst efficiency is deteriorated, and the electromotive force of the fuel cell is lowered. End up.

As the complex cation, hexaammine platinum (IV) chloride ([Pt (IV) (NH ₃ ) ₆ ] Cl ₄ ) is preferably used.

As a method of ion exchange, a method of immersing an electron conductive material having an acidic group formed on the surface in a solution containing a complex cation having a catalyst as a dissolved species can be used.

The weight ratio of the catalyst and the electron conductive material is preferably 1: 2 to 2: 1.
If the catalyst weight ratio is smaller than this range, sufficient discharge characteristics (battery voltage) cannot be obtained.
If the catalyst weight ratio is larger than this range, the catalyst particles cannot be supported on the electron conductive material powder with good dispersibility.

As a method of reducing, a method of reducing at 180 ° C. in a hydrogen stream can be used.

(Production of amorphous carbon with sulfonic acid group introduced)
First, the aromatic compound is dissolved in sulfonic acid at a temperature of 240 to 260 ° C.

Anthracene or a derivative thereof can be used as the aromatic compound.
Anthracene has many polyene structures and is most easily substituted with sulfonic acid groups among aromatic compounds.

As the sulfonic acid, sulfuric acid can be used.
By using sulfuric acid as the sulfonic acid, a sulfo group can be introduced into the aromatic compound by an electrophilic substitution reaction.

If the temperature is less than 240 ° C, the reaction rate is slow, and if the temperature exceeds 260 ° C, the sulfone group may be decomposed, and therefore the temperature is preferably 240 to 260 ° C.

Next, the sulfonic acid is removed by filtration, and then the residue is washed with water to obtain an aromatic compound substituted with a sulfonic acid group.

The sulfonic acid group contributes to proton conduction, and the higher the sulfonic acid group density, the higher the proton conductivity.

(Production of catalyst electrode for fuel cell)
First, a pressure nozzle type emulsifier such as a ball mill, a homogenizer, or a nanomizer is prepared by dissolving a noble metal catalyst-supported electron conductive material, proton conductive polymer electrolyte, and amorphous carbon into which a sulfonic acid group has been introduced in a solvent. A varnish for forming a catalyst electrode for a fuel cell is produced by carrying out a dispersion treatment using the above.

As the proton conductive polymer electrolyte, perfluorocarbon sulfonic acid can be used.
Perfluorocarbon sulfonic acid can withstand the operating environment of the fuel cell, has excellent heat resistance, oxidation deterioration resistance, reduction deterioration resistance, dimensional stability, impact resistance, and proton conductivity, and as a proton conductive polymer electrolyte, Is optimal.

As the solvent, N-methyl-1-pyrrolidone, isopropyl alcohol, normal propyl alcohol, or the like can be used.

Next, the catalyst electrode for fuel cells is obtained by laminating | stacking the varnish for catalyst electrode formation for fuel cells on a conductive porous base material.

As the conductive porous substrate, carbon paper or carbon cloth having a porosity of 65 to 85% can be used.

Carbon paper or carbon cloth has good reaction gas permeability and high electrical conductivity, and is optimal as an electrode base material for catalyst electrodes for fuel cells.

If the porosity is less than 65%, the fuel gas and the oxidant gas cannot be sufficiently supplied. If the porosity exceeds 85%, the strength may be insufficient.

As a lamination method, a brush coating method, a brush coating method, a bar coater coating method, a knife coater coating method, a die coater coating method, a screen printing method, a spray coating method, or the like can be used.

(Production of membrane electrode assembly (MEA))
First, the hydrogen ion conductive polymer electrolyte membrane is sandwiched with the catalyst layers of the two catalyst electrodes facing each other, and thermocompression bonded.

As the heating temperature, a temperature exceeding the softening temperature of the resin of the catalyst layer and the hydrogen ion conductive polymer electrolyte membrane or the glass transition temperature can be used.
When the resin of the catalyst layer and the hydrogen ion conductive polymer electrolyte membrane is perfluorocarbon sulfonic acid, use the thermocompression bonding conditions of a temperature of 100 ° C. to 300 ° C., a pressure of 20 to 50 kgf / cm ² , and a time of 5 seconds to 700 seconds. Can do.

Finally, the fuel cell is obtained by sandwiching the membrane electrode assembly (MEA) with the gas flow paths of the two separators facing each other and fastening with a bolt and a nut.

The pressure at the time of fastening with a bolt and a nut is preferably a surface pressure of 1 to ² kg / cm ² with respect to the thickness direction of the membrane electrode assembly (MEA).

First, 25 g of acetylene black (specific surface area 257 m ² / g) (manufactured by Cabot, Vulcan XC-72R) was put into 10 liter of 0.5 mol / liter potassium permanganate aqueous solution and reacted at 75 ° C. for 5 hours. .

Next, the reaction product is filtered off, washed with distilled water at 75 ° C., dried at 105 ° C., and then hexaammineplatinum (IV) chloride having a platinum content of 10 g / liter ([Pt (IV ) (NH ₃ ) ₆ ] Cl ₄ ) Ion exchange was performed by immersion in an aqueous solution at room temperature.

Next, the reaction product is filtered off from the reaction solution, and then the reaction product is washed with distilled water at 75 ° C., then dried at 105 ° C., and then in a hydrogen stream at a temperature of 180 ° C. By carrying out reduction, platinum-supported carbon (Pt: 45 mass% supported) was obtained.

Next, 40 g of anthracene was dissolved in 600 ml of concentrated sulfuric acid (96%) and heated at 240 ° C. for 15 hours, after which the concentrated sulfuric acid was removed by filtration, and then the residue was washed with water to introduce sulfonic acid groups. Amorphous carbon (sulfonic acid group density = 4.4 mmol / g) was obtained.

Amorphous carbon into which a sulfonic acid group has been introduced is uniaxially molded using a mold, and then cylindrical (diameter 10 mm × height 0) using a hydrostatic pressure press (CIP) at a pressure of 2000 kg / cm ^2. .7 mm) and then forming a measurement electrode by vapor-depositing Pt on both sides of the cylindrical disk, and then connecting the measurement electrode to the terminal of the impedance analyzer via a conductor, After the columnar disk was brought to 100 ° C. and humidity 0% (measured by the Karl Fischer method), the ohmic resistance value of the columnar disk was measured using a known alternating current impedance method at a measurement frequency of 20 kHz. Then, proton conductivity of amorphous carbon into which sulfonic acid groups were introduced from the ohmic resistance value (corrected for the lead resistance component in the measuring apparatus) and the dimensions of the cylindrical disk Was calculated, proton conductivity of the amorphous carbon having sulfonate group introduced therein was ^{1.44 × 10 -1 S · cm -1} .

Next, 0.1 g of amorphous carbon having sulfonic acid groups introduced, 1.5 g of platinum-supported carbon (Pt: 45 mass% supported), 4 g of water, 6 g of 2-propanol, and 20 w% solution of perfluorocarbon sulfonic acid (solvent; Propanol) (manufactured by DuPont, Nafion Solution (registered trademark)) in an amount of 3.5 g was mixed in this order, and treated with a planetary ball mill at 200 rpm for 1 hour to produce a fuel cell catalyst electrode forming varnish.

It was 63 cPs (25 degreeC) when the viscosity of the varnish for catalyst electrode formation for fuel cells was measured with the viscometer (the Yamaichi Denki company make, Viscomate VM-1A-MH ((trademark)).

Next, carbon paper (TGP-H-030, manufactured by Toray Industries, Inc.) having a porosity of 80% and a thickness of 110 μm was cut into a size of 110 mm × 110 mm.

Next, after removing dust attached to the cut carbon paper using an air gun,
Set on a screen printer.

Next, the prepared catalyst ink was placed on a screen of # 250SUS mesh × emulsion thickness 20 μm × wire diameter 0.05 mm.

Next, the catalyst ink was extruded onto the carbon paper (screen printing) under the conditions of a squeegee pressure of 5 kgf / cm ² , a squeegee speed of 100 mm / second, and a clearance between the screen and the carbon paper of 2.5 mm, and then in a nitrogen atmosphere Was subjected to heat treatment at 80 ° C. for 10 minutes and then allowed to cool for 30 minutes to form a catalyst electrode.

The amount of platinum catalyst supported on the catalyst electrode was 0.4 mg / cm ² .

Next, the catalyst layers of the two fuel cell catalyst electrodes face each other, and a 50 μm thick perfluorocarbon sulfonic acid membrane (hydrogen ion conductive polymer electrolyte membrane) (DuPont (registered trademark) Nafion 117) is placed between them. The membrane electrode assembly (MEA) was produced by thermocompression bonding under conditions of a temperature of 140 ° C., a pressure of 40 kgf / cm ² , and a time of 10 minutes.

Next, the membrane electrode assembly (MEA) is sandwiched between two carbon separators, further sandwiched between two titanium current collectors, and further sandwiched between two heaters, thereby providing an effective area of 5 cm. ^Two fuel cells were assembled.

The temperature of the fuel cell was kept at 80 ° C., hydrogen and oxygen were supplied at a humidity of 100% RH, and the current density (A / cm 2) at 0.7 V was measured. Further, the catalyst mass activity (Ag-1) was calculated using the current density at 0.9 V (after IR resistance correction).
(Catalyst mass activity is specific activity per mass of catalyst used and is a measure of catalyst utilization.)
The calculated values of catalyst mass activity (Ag-1) are shown in Table 1.

<Comparative Example 1>
A fuel cell was produced and evaluated in the same manner as in Example 1 except that the amorphous carbon into which the sulfonic acid group was introduced was not added.

<Comparative example 2>
A fuel cell was produced in the same manner as in Example 1 except that amorphous carbon introduced with a sulfonic acid group was not added and the perfluorocarbon sulfonic acid was treated with a planetary ball mill at 150 rpm for 0.5 hour before mixing. And evaluated.
(The viscosity of the varnish for forming a catalyst electrode for a fuel cell was 700 cPs (25 ° C. when measured with a viscometer manufactured by Yamaichi Electronics Co., Ltd., Viscomate VM-1A-MH (registered trademark)).

<Comparative Example 3>
Example 1 was repeated except that 0.2 g of a polymer dispersant (manufactured by Kao Corporation) (Dimor N (registered trademark)) was added instead of 0.1 g of amorphous carbon having a sulfonic acid group introduced therein. A fuel cell was manufactured and evaluated.
(The viscosity of the fuel cell catalyst electrode forming varnish was 80 cPs (25 ° C. when measured with a viscometer manufactured by Yamaichi Denki Co., Ltd., Viscomate VM-1A-MH (registered trademark)).

From Table 1, it was possible to significantly reduce the dispersion treatment time by adding amorphous carbon into which sulfonic acid groups were introduced.
In addition, the fuel cell to which amorphous carbon introduced with a sulfonic acid group was added had a current density at 0.7 V of 1.0 A / cm ² , a catalyst mass activity of 13.0 (Ag-1), and sulfone. It showed better characteristics than the fuel cell without adding amorphous carbon with acid group.

It is sectional drawing of the conventional fuel cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Anode side separator 2 ... Anode side catalyst electrode 3 ... Hydrogen ion conductive polymer electrolyte membrane 4 ... Cathode side catalyst electrode 5 ... Cathode side separator 21 ... Anode-side electrode substrate 22 ... Anode-side electrode catalyst layer 41 ... Cathode-side electrode substrate 42 ... Cathode-side electrode catalyst layer

Claims (8)

A varnish for forming a catalyst electrode for a fuel cell, comprising a noble metal catalyst-supported electron conductive material, a proton conductive polymer electrolyte, amorphous carbon into which a sulfonic acid group has been introduced, and a solvent. 2. The fuel cell according to claim 1, wherein the noble metal catalyst is one or more metals selected from Au, Pt, Pd, Rh, Ru, Os, and Ir, or Mo. Catalyst electrode forming varnish. The varnish for forming a catalyst electrode for a fuel cell according to claim 1 or 2, wherein the proton conductive polymer electrolyte is perfluorocarbon sulfonic acid. A method for producing a varnish for forming a catalyst electrode for a fuel cell according to any one of claims 1 to 3,
A fuel characterized by having a one-step treatment process in which a noble metal catalyst-supported electron conductive material, a proton conductive polymer electrolyte, and amorphous carbon having sulfonic acid groups introduced therein are mixed and dispersed using a disperser. The manufacturing method of the varnish for battery catalyst electrode formation. The method for producing a varnish for forming a catalyst electrode for a fuel cell according to claim 4, wherein the disperser is a planetary ball mill, a homogenizer, or a nanomizer. A method for producing a catalyst electrode, comprising coating a conductive porous substrate with the varnish for forming a catalyst electrode for a fuel cell according to any one of claims 1 to 3. A membrane electrode assembly (MEA) comprising a hydrogen ion conductive polymer electrolyte membrane sandwiched between the catalyst electrodes. A fuel cell comprising a membrane electrode assembly (MEA) sandwiched between separators.

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