JP4660082B2 - Fuel cell separator - Google Patents

Description

  The present invention relates to a fuel cell separator.

  In recent years, there is an increasing demand for fuel cells that directly convert chemical energy of fuel into electrical energy. In general, a fuel cell has a structure in which a large number of unit cells each having an electrode plate sandwiched between electrolyte membranes and a separator disposed outside the electrode plate are stacked.

  FIG. 1 is a schematic view showing the external appearance of a general fuel cell separator, which is formed by establishing a plurality of partition walls 7 at predetermined intervals on both surfaces of a flat plate portion 6. In order to obtain a fuel cell, a large number of fuel cell separators 5 are stacked in the protruding direction of the partition walls (vertical direction in the figure). And by this lamination | stacking, it becomes the structure which distribute | circulates reaction gas (hydrogen and oxygen) to the channel 8 formed of a pair of adjacent partition 7. Therefore, the fuel cell separator 5 needs to be excellent in gas impermeability so that both reaction gases are not mixed. In addition, since the unit cells are stacked, the fuel cell separator 5 is required to have high conductivity and excellent strength.

  For example, in a field that requires a high voltage like an automobile, a stack is formed by stacking several hundred unit cells. In addition, since this stack is generally used under conditions of about 80 ° C., thermal dimensional stability (low thermal expansion) is required. When the thermal expansion is high, the entire stack becomes large due to heat, and therefore the fuel cell separator body and the electrolyte membrane are damaged due to an increase in tightening load. Further, when assembling the stack, it may be difficult to assemble or break due to the strength, dimensional accuracy, warpage, etc. of the separator. In particular, the warpage of the separator for the fuel cell not only makes it difficult to assemble the stack, but also the adhesion of each cell after assembly becomes insufficient, resulting in uneven contact electrical resistance, resulting in a decrease in power generation performance and further unevenness. The fuel cell separator may be damaged by the load.

  Since the separator for a fuel cell is advantageous in terms of productivity, it can be obtained by press-molding a conductive resin composition in which a conductive filler such as expanded graphite or carbon fiber is dispersed in a resin into a predetermined shape. It is common. Therefore, if the dispersion of the composition in the conductive resin composition is not sufficient, the composition is unevenly distributed in the fuel cell separator, causing warpage and undulation due to local thermal expansion difference, and further insufficient local strength Also happens. Therefore, the orientation of the conductive filler in the thickness direction (FIG. 1, vertical direction) and the horizontal direction (FIG. 1, horizontal direction) due to the difference in shape between the front and back surfaces (current collection surface, water channel surface) of the fuel cell separator. A difference occurs in the state, and as a result, a difference in elongation occurs locally due to a difference in thermal expansion, and warping and undulation occur.

  In order to suppress warpage, for example, a fuel cell separator in which the content of the resin contained in the surface in contact with the electrode portion is less than the content of the resin contained in the surface not in contact with the electrode portion (see Patent Document 1) In addition, a fuel cell separator (see Patent Document 2) in which at least a part of the outer periphery or outer surface is reinforced with a reinforcing material made of a metal material, a fiber-reinforced resin material, or the like is known.

In addition, it is also carried out to suppress the difference in thermal expansion coefficient by specifying the resin and conductive filler to be used, for example, graphite powder of a specific aspect ratio and average particle size, novolac type phenol resin, and carbon fiber There is also known a fuel cell separator (see Patent Document 3) in which a density difference is provided between a partition wall and a groove.
JP 2003-151574 A JP 2002-358773 A JP 2002-25572 A

  However, Patent Document 1 and Patent Document 2 have not studied the difference in thermal expansion that is the cause of warping, and have not yet reached a fundamental solution. Moreover, although the fuel cell separator of Patent Document 3 also suppresses the coefficient of thermal expansion of the separator as a whole, the difference in the coefficient of thermal expansion between the separator parts has not been studied.

  The present invention has been made in view of such a situation, and has less warpage than conventional products, excellent assemblability, no breakage, and excellent adhesion between cells, and contact electrical resistance. An object is to provide a high-performance fuel cell separator that is free from unevenness.

In order to achieve the above object, the present invention provides a fuel cell separator described below.
(1) A) The dry graphite is mixed at a normal temperature in a ratio of 20 to 60% by mass of expanded graphite, 20 to 40% by mass of thermosetting resin powder, 15 to 30% by mass of spherical filler, and 5 to 10% by mass of carbon fiber. Obtaining a powder;
B) A step of melting and mixing the mixed powder at a temperature at which the thermosetting resin is not completely cured to obtain a molten mixture;
C) After the molten mixture is naturally cooled and solidified, the obtained solidified product is pulverized and classified to obtain a powder composed of a molten mixture having a particle size of 500 μm or less;
D) Filling a molding die with powder made of a molten mixture, and temporarily forming the sheet by press molding at a temperature at which the thermosetting resin is not completely cured;
E) A step of accommodating the obtained sheet-like temporary molded article in a molding die for a fuel cell separator and performing a main molding at a temperature at which the thermosetting resin is completely cured;
Obtained by the process comprising the difference between the thermal expansion coefficient in the direction perpendicular to the thermal expansion coefficient and the thickness in the thickness direction 20 × 10 -6 ^· K ^-1 or less, flexural strength 40MPa or more and flexural modulus A fuel cell separator having a rate of 12 GPa or less .
(2) The fuel cell separator as described in (1) above, wherein the spherical filler contains one or more of spherical silica and spherical graphite .
(3) The average particle size of the spherical filler is 75% or less with respect to the thickness of the thinnest portion of the obtained fuel cell separator, for the fuel cell according to the above (1) or (2) Separator.

  According to the present invention, it is possible to obtain a high-performance fuel cell separator that has low warpage, excellent assemblability, no breakage, excellent adhesion between cells, and no uneven contact electric resistance.

  Hereinafter, the fuel cell separator of the present invention will be described in detail.

The separator for a fuel cell of the present invention is formed by molding a conductive resin composition into a predetermined shape as shown in FIG. 1, for example, and there is a difference between the thermal expansion coefficient in the thickness direction and the thermal expansion coefficient in the horizontal direction. It is 20 × 10 ⁻⁶ · K ⁻¹ or less. As long as it can satisfy such a thermal expansion coefficient difference, but there is no limitation on its shape, as the conductive resin composition, the expanded graphite 20 to 60 wt%, a thermosetting resin 20 to 40 wt% , the conductive resin composition comprising 15 to 30 wt% spherical filler and carbon fiber 5 to 10 wt%. The expanded graphite , thermosetting resin, spherical filler and carbon fiber will be described in detail below.

  Here, the epoxy resin includes a structure formed by a reaction between a polyfunctional epoxy compound and a curing agent, and all of the epoxy compound and the curing agent that give the structure. Hereinafter, the epoxy compound before the reaction may be referred to as an epoxy resin precursor, and the structure produced by the reaction may be referred to as an epoxy compound. The amount of epoxy resin is equal to the mass of the epoxy cured product.

  Various known compounds can be used as the epoxy resin precursor. For example, bifunctional epoxy compounds such as bisphenol A diglycidyl ether type, bisphenol F diglycidyl ether type, bisphenol S diglycidyl ether type, bisphenol AD diglycidyl ether type, resorcinol diglycidyl ether type; phenol novolac type, cresol novolac type Polyfunctional epoxy compounds such as: linear aliphatic epoxy compounds such as epoxidized soybean oil, cyclic aliphatic epoxy compounds, heterocyclic epoxy compounds, glycidyl ester epoxy compounds, glycidyl amine epoxy compounds, etc. However, it is not limited to these. A compound having a substituent such as halogen or a compound in which an aromatic ring is hydrogenated can also be used. Moreover, there is no restriction | limiting in particular also in the epoxy equivalent, molecular weight, the number of epoxy groups. However, when an epoxy compound having an epoxy equivalent of about 400 or more, particularly about 700 or more is mainly used as the epoxy resin precursor, the pot life can be extended. Moreover, since these compounds are solid at normal temperature, handling becomes easy when performing powder molding. It is also possible to use a plurality of epoxy compounds in combination. For example, an epoxy resin precursor that gives a cured product having a high network density with an epoxy equivalent of about 200 is mixed with a precursor with an epoxy equivalent of about 900 that has a long usable time, and is used as a powder or a liquid with a slightly long usable time. It can be handled as a thing.

These epoxy resin precursors react with a curing agent to produce an epoxy cured product. Various known compounds can also be used for the cured product. For example, dimethylenetriamine, triethylenetetramine, tetraethylenepentamine, mensendiamine, isophoronediamine, and other aliphatic, alicyclic, and aromatic polyamines or carbonates thereof; phthalic anhydride, methyltetrahydrophthalic anhydride, trihydric anhydride Acid anhydrides such as merit acid; polyphenols such as phenol novolac; polymercaptan; anionic polymerization catalysts such as tris (dimethylaminomethyl) phenol, imidazole, ethylmethylimidazole; cationic polymerization catalysts such as BF ₃ and complexes thereof; Includes, but is not limited to, a latent curing agent that generates the above compound by thermal decomposition or photolysis. A plurality of curing agents can be used in combination.

Polyimide includes all polymers having an imide group ((—CO—) ₂ N—) in the molecule. Examples include thermoplastic polyimide such as polyamideimide and polyetherimide; non-thermoplastic polyimide such as (all) aromatic polyimide; thermosetting polyimide such as nadic acid type polyimide such as bismaleimide polyimide and allyl nadiimide, acetylene type Although polyimide etc. are mentioned, it is not limited to these. A plurality of polyimides can be used in combination. Of these, the use of thermosetting polyimide is particularly preferred. Thermosetting polyimide has the advantage that it is easier to process than thermoplastic polyimide or non-thermoplastic (aromatic) polyimide. Although the high temperature characteristics are inferior to those of non-thermoplastic polyimides, it is a very good class among various organic polymers. In addition, since voids and cracks hardly occur during curing, it is suitable as a component of the conductive resin composition of the present invention.

  Moreover, the compounding ratio of the epoxy resin and the polyimide resin is preferably 5 to 95 parts by weight for the epoxy resin and 95 to 5 parts by weight for the polyimide resin. In any resin, when the blending ratio is less than 5% by mass, the advantages produced by using both resins in combination are slight. The compounding ratio of epoxy resin: polyimide resin is more preferably 95: 5 to 30:70, and still more preferably 85:15 to 60:40.

The blending amount of the thermosetting resin is 20 to 40% by mass with respect to the total amount of the separator for the fuel cell. However, if it is less than 20% by mass, shape molding becomes difficult due to a decrease in material fluidity, and the effect as a binder is thin. Thus, the amount of restoration of the thickness of the separator for the fuel cell increases, and there arises a problem that a desired thickness cannot be obtained. On the other hand, when the blending amount of the thermosetting resin exceeds 40% by mass, problems such as an increase in the amount of burrs during molding due to insufficient strength, a decrease in electrical conductivity, and fluidity, and a problem with the mold are caused. Considering these points, the blending amount of the thermosetting resin is preferably 20 to 30% by mass.
( Expanded graphite )
The expanded graphite is used as the conductive filler in view of the forming shapes and economy. Expanded graphite is obtained by expanding the layers of the graphite crystal structure and is extremely bulky. The expanded graphite preferably has a bulk specific gravity of about 0.3 or less, more preferably about 0.1 or less, and particularly preferably about 0.05 or less. When these expanded graphites are used, the strength, conductivity and lubricity are particularly good.

The blending amount of the conductive filler is 20 to 60% by mass with respect to the total amount of the separator for the fuel cell. However, if it is less than 20% by mass, satisfactory conductivity cannot be obtained. Problems arise. Considering these, the blending amount of the conductive filler is preferably 25 to 60% by mass, and more preferably 30 to 60% by mass.
(Spherical filler)
In general, when a conductive resin composition containing expanded graphite is molded into a plate shape, expanded graphite is oriented in the horizontal direction of the molded body, resulting in a difference in thermal expansion coefficient between the horizontal direction and the thickness direction. It becomes a factor of occurrence. Therefore, a spherical filler is added to reduce such anisotropy.

  Examples of the spherical filler include spherical silica, hollow silica, and spherical graphite (artificial graphite), which are low thermal expansion materials, and these can be used alone or in combination. Among these, in view of conductivity, spherical graphite that can be used as a conductive filler is more preferable than silica. When the spherical filler is mixed in the conductive resin composition, the expanded graphite is easily oriented in the thickness direction of the molded body around the spherical filler, and there is a difference in the coefficient of thermal expansion between the horizontal direction and the thickness direction. Less. Further, the larger the particle size of the spherical filler, the easier the expanded graphite is oriented in the thickness direction.

  However, when the particle size of the spherical filler is too large, part of the spherical filler is exposed from the surface of the fuel cell separator, which may cause a decrease in contact resistance. Therefore, the particle size of the spherical filler is desirably 75% or less of the thickness of the thinnest part of the fuel cell separator. For example, when the thinnest part of the fuel cell separator is 0.5 mm, the spherical filler preferably has a particle size of 125 μm or less, more preferably 50 μm or less.

The blending amount of the spherical filler is 15 to 30% by mass with respect to the total amount of the separator for the fuel cell, but if it is less than 5% by mass, there is no reduction in thermal expansion because satisfactory orientation control of the expanded graphite is not possible. Warpage occurs in the fuel cell separator. When it exceeds 30 mass%, it may protrude from the surface of the fuel cell separator, which may cause a decrease in contact resistance and a decrease in strength and gas impermeability of the molded article. Considering these, the blending amount of the spherical filler is preferably 15 to 25% by mass.
(Carbon fiber)
Examples of the carbon fiber include PAN-based carbon fiber, pitch-based carbon fiber, rayon-based carbon fiber, and the like, and these can be used alone or in combination. By adding carbon fiber, the strength of the fuel cell separator, particularly impact resistance, can be improved, and the strength can be improved with little influence on conductivity and thermal expansion. Although there is no restriction | limiting in particular in the shape of carbon fiber, When using it as an electroconductive resin composition, Preferably, the fiber length is about 0.01-100 mm, Especially 0.1-20 mm is used. If the fiber length exceeds 100 mm, it is difficult to mold and the surface is difficult to smooth, and if it is less than 0.01 mm, the reinforcing effect cannot be expected.

  The blending amount of the carbon fiber is 5 to 10% by mass with respect to the total amount of the separator for the fuel cell, but if it is less than 5% by mass, satisfactory impact resistance cannot be obtained, and if it exceeds 10% by mass, there is a problem in molding. Arise. Considering these, the blending amount of the carbon fiber is preferably 7 to 9% by mass.

  The fuel cell separator of the present invention can be produced, for example, as follows.

First, expanded graphite , thermosetting resin, spherical filler, and carbon fiber are dry-mixed at room temperature using a Henschel mixer, shaker, or the like at the above blending ratio to obtain a mixed powder. Next, the mixed powder is melt-mixed at a temperature at which the thermosetting resin is not completely cured. A mixer such as a pressure kneader, a brabender, a short shaft or a twin screw can be used for this melt mixing. By melting and mixing, the expanded graphite , spherical filler and carbon fiber are crushed or cut into fine particles, and the thermosetting resin softens and flows into the gap between the expanded graphite , spherical filler and carbon fiber, and the conductive resin. The dispersibility of the blend in the composition is improved, and in particular, the anisotropy of expanded graphite which is a conductive filler can be reduced. Next, the obtained molten mixture is naturally cooled and solidified, and then pulverized by a Henschel mixer, a mixer, a ball mill, or the like so that the solidified product can be easily filled in a mold. The powder of the molten mixture preferably has an average particle size of 500 μm or less in consideration of ease of filling into the mold and moldability.

  The molten mixture powder can be filled in a mold for a fuel cell separator as it is and press-molded at a temperature at which the thermosetting resin is completely cured to obtain a fuel cell separator. The body is temporarily formed into a sheet by press molding at a temperature at which the thermosetting resin is not completely cured, and the produced sheet-like temporary molded body is accommodated in a mold for a fuel cell separator, and the thermosetting resin is completely cured. It is preferable to perform the main molding at a temperature.

In the fuel cell separator of the present invention, the difference between the coefficient of thermal expansion in the thickness direction and the coefficient of thermal expansion in the horizontal direction is 20 × 10 ⁻⁶ · K ⁻¹ or less, which is much smaller than conventional fuel cell separators. As shown in the examples described later, the amount of warpage is 2 mm or less and excellent in dimensional accuracy, and the bending strength is 40 MPa or more and the bending elastic modulus is 12 GPa or less and the mechanical strength is also excellent.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.
(Sample preparation)
Using expanded graphite, epoxy resin, polyimide resin, carbon fiber having an average fiber diameter of 13 μm and an average fiber length of 370 μm, spherical silica having an average particle diameter of about 50 μm and spherical graphite, the mixture ratio shown in Table 1 is put into a Henschel mixer. And dry mixed at room temperature. The obtained mixed powder was put into a pressure kneader, melted and mixed at 100 ° C., and then naturally cooled to solidify. Next, the solidified product was pulverized to obtain a molten mixture powder having an average particle size of 100 μm. Then, the molten mixed powder is filled into a mold and press-molded at 100 ° C. to produce a sheet-like temporary molded product having a thickness of 3 mm, followed by main molding at a mold temperature of 200 ° C. and a press pressure of 100 MPa for 10 minutes. A sheet-like sample having a thickness of 2 mm was produced (Examples 1 to 4 and Comparative Example 1).

Moreover, using the said mixture, it injected | thrown-in to the Henschel mixer with the compounding ratio shown in Table 1, and dry-mixed at room temperature. Then, the obtained mixed powder is filled into a mold and press-molded at room temperature to form a sheet, which is then molded at a mold temperature of 200 ° C. and a press pressure of 100 MPa for 10 minutes to form a sheet-like sample having a thickness of 2 mm. (Comparative Examples 2 to 4).
(Measurement of sample physical properties)
The coefficient of thermal expansion was 5 mm square and 10 mm thick test piece cut out from the obtained sample, and using a “MA8310” made by Rigaku in the thickness direction and horizontal direction of the test piece, a 0.1 N load with a φ3 mm probe The coefficient of thermal expansion at 28 to 100 ° C. was measured when the temperature was raised at 1 ° C./min.

Bending strength and flexural modulus were obtained by cutting a test piece having a width of 20 mm, a length of 100 mm, and a thickness of 2 mm from the obtained sample, and using “Autograph AG-100kND” manufactured by Shimadzu Corporation at 100 ° C. in accordance with JIS K7171. Measured with
(Measurement of warpage of fuel cell separator)
A fuel cell separator having the shape shown in FIG. 1 was prepared in accordance with the sample preparation procedure and molding conditions. The shape and dimensions of each part are such that the flat plate part has a total length of 300 mm and a total width of 250 mm, 60 cooling water channel grooves (width 2 mm, depth 0.5 mm) on one side, and gas flow on the other side. 120 road grooves (width 1 mm, depth 0.5 mm) are provided, the thickness of the flat plate portion, which is the thinnest part, is 0.5 mm, and the upper end surface of the partition wall on one surface to the upper end surface of the partition wall on the other surface (Total thickness) was 1.5 mm.

  Then, using a three-dimensional laser measuring machine (manufactured by COMS), the fuel cell separator was placed on a plate, the surface was measured at 35 points, and the difference between the maximum value and the minimum value was obtained as the amount of warpage.

  Table 1 shows the above measurement results together with the mixing method and the mixing ratio.

From Table 1, if the blending material and blending ratio specified in the present invention are satisfied, and the difference between the thermal expansion coefficient in the thickness direction and the thermal expansion coefficient in the horizontal direction is 20 × 10 ⁻⁶ · K ⁻¹ or less, It can be seen that the amount of warpage is suppressed to 2 mm or less, the bending strength is 40 MPa or more, and the bending elastic modulus is 12 GPa or less.
(Verification of dispersibility of compounding materials)
About the sample produced in Example 1 and Comparative Example 4, the cross section was image | photographed with the optical microscope. FIG. 2 shows an optical micrograph of the sample of Example 1 and FIG. 3 shows the sample of Comparative Example 4. In the sample of Example 1, fine expanded graphite, carbon fiber, and spherical silica are uniformly dispersed. On the other hand, in the sample of Comparative Example 4, it can be seen that the carbon fibers remain the original long fibers, and the dispersibility of the expanded graphite and spherical silica is also poor. It can be said that the dispersibility of such a compounded material reflects the difference in thermal expansion coefficient and appears as a warp amount.

It is the schematic which shows an example of the separator for this invention and the conventional fuel cell. 2 is an optical micrograph showing a cross section of a sample manufactured in Example 1. FIG. 6 is an optical micrograph of a cross section of a sample produced in Comparative Example 4.

Explanation of symbols

5 Fuel Cell Separator 6 Flat Plate 7 Bulkhead 8 Channel

Claims (3)

A) The mixed powder was dry-mixed at room temperature at a ratio of 20 to 60% by mass of expanded graphite, 20 to 40% by mass of thermosetting resin powder, 15 to 30% by mass of spherical filler and 5 to 10% by mass of carbon fiber. Obtaining a step;
B) A step of melting and mixing the mixed powder at a temperature at which the thermosetting resin is not completely cured to obtain a molten mixture;
C) After the molten mixture is naturally cooled and solidified, the obtained solidified product is pulverized and classified to obtain a powder composed of a molten mixture having a particle size of 500 μm or less;
D) Filling a molding die with powder made of a molten mixture, and temporarily forming the sheet by press molding at a temperature at which the thermosetting resin is not completely cured;
E) A step of accommodating the obtained sheet-like temporary molded article in a molding die for a fuel cell separator and performing a main molding at a temperature at which the thermosetting resin is completely cured;
Obtained by the process comprising the difference between the thermal expansion coefficient in the direction perpendicular to the thermal expansion coefficient and the thickness in the thickness direction 20 × 10 -6 ^· K ^-1 or less, flexural strength 40MPa or more and flexural modulus A fuel cell separator having a rate of 12 GPa or less . 2. The fuel cell separator according to claim 1, wherein the spherical filler contains one or more of spherical silica and spherical graphite . The fuel cell separator according to claim 1 or 2, wherein the spherical filler has an average particle size of 75% or less with respect to the thickness of the thinnest portion of the obtained fuel cell separator.