CA1288470C

CA1288470C - Process for producing an electrode substrate and the thus produced electrode substrate which is uniform in physical properties

Info

Publication number: CA1288470C
Application number: CA000541919A
Authority: CA
Inventors: Masayuki Funabashi; Masayosi Sibuya
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1986-07-14
Filing date: 1987-07-13
Publication date: 1991-09-03
Anticipated expiration: 2008-09-03
Also published as: JPS6321753A; GB2193714B; FR2603424A1; GB8716571D0; DE3723146A1; JPH07118323B2; FR2603424B1; GB2193714A; US4814307A; DE3723146C2

Abstract

ABSTRACT OF THE DISCLOSURE:
Disclosed herein are a process for producing an electrode substrate, which process comprises the steps of kneading a raw material mixture comprising from 30 to 60 %
by weight of short carbon fibers, from 20 to 50 % by weight of a phenol resin binder and from 20 to 50 % by weight of a molding additive, extruding the thus kneaded raw material mixture and after press-molding the thus extruded material by rolling or stamping, calcining the thus press-molded material in an inert atmosphere and/or under a reduced pressure and the electrode substrate produced by the above-mentioned process.

Description

347(~

TITLE OF THE INVENTION:
A PROCESS FOR PRODUCING AN ELECTRODE SUBSTRATE
AND THE THUS PRODUCED ELECTRODE SU~STRATE WHICH
IS UNIFORM IN PHYSICAL PROPERTIES

BACKGROUND OF THE INVENTION:
The present invention relates to a process for producing an electrode substrate, and more in detail; relates to a process for continuously producing an electrode substrate having uniform physical properties, particularly an electrode substrate for fuel cells at a low cost and a favorable productivity, and the electrode substrate obtained by the above-mentioned process.
Hitherto, as tha process for producing an electrode substrate for fuel cells, etc., various processes have been proposed. For instance, a process of subjecting dispersed carbon fibers to paper-manufacturing(refer to U.S. Patent No. 3,998,689) and a process of chemical vapor depositing thermally decomposed carbon onto a web of carbon fibers(refer to U.S. Patent No. 3,829,327~ have been ~proposed.
As the other prGcess, there is a process wherein an alcohol having a boiling point of higher than 150C is used as a preparatory binder for forming a mat o~ pitch fibers and then the mat of pitch fibers is subjected to ~F .

9 ~88470 carbonization and heat-treatment in a non-oxidative at~ospher~
(refer to U.S. Patent No. 3,991,169).
Furthermore, as the other process, there is a process wherein a web comprising pitch fibers produced by blow-spinning are subjected to infusibilization and carbonization, thereby obtaining the web of carbon fibers~refer to U.S. Patent No. 3,960,601~.
Still more, a process for producing an electrode substrate for the fuel cell of monopolar type~ which process comprises the steps of press-molding a mixture comprisina short carbon fibers as the base, a carbonaceous resin binder such as a phenol resin and organic granules as the pore regulator, such as polyvinyl alcohol, polyethylene and polypropylene and calcining the thus press-molded body, has been proposed(refer to U.S. Patent No. 4,506,02 and U.S. Patent No. 4,666!755).

Although various electrode substrates have ~een produced by the abo~e~mentioned processes, it is very difficult to have the uniform physical properties all over the electrode substrate.
- Namely, although the electrode substrate takes a thinplate form in general, the values of physical proper ties measured on the various points of the flat surface of the electxode substrate show fluctuation.

~ ~i8~7V

For instance, in the case where the compound containing the carbon fibers as the base material is supplied to a metal mold and is subjected to press-molding or roll-molding, the occurrence of the uneven supply of the compound is inevitable, and as a result, the physical properties of the thus obtained electrode substrate become uneven.
Particularly, in the case where the value of the bending strength of the electrode substrate fluctuates t there is a fear of breakages in the handling of the electrode substrate, and in the case where the bulk density of the electrode substrate is uneven f the parts of large thermal resistance and electric resistance are ~enerated, and parti-cularly, in the case where the value of thermal resistance fluctuates, the parts heated to high temperatures appear locally in the electrode substrate to accelerate the sintering of the catalyst thereby reducing the life of the electrode.
Still more ! in the case where the gas-permeability of the electrode substrate is uneven~ since the resistance to the diffusion of reactant gas becomes uneven, there is a problem that the output specificity varies locally.
Furthermore, since there is a limit in the produc-tivity of the electrode substrate by the non-continuous process such as the above-mentioned press-process, the development of the process for continuously producing the electrode substrate at a favorable productivity has been desired.

~ ~8~47(~

As the above-mentioned process for continuously producing the electrode substrate, the extruding process is considered, however, the conventional compound for press-molding, which comprises carbon fibers and a binder, is poor in fluidity and accordingly cannot be extruded.
In consideration of the above-mentioned situations, the present inventors have studied the process for con-tinuously producing the electrode substrate of the uniform physical properties, and as a result of their studies, it has been found out by the present inventors that both the kneadability and the fluidity of the compound consisting of short carbon fibers and a binder are improved by mixing a molding additive with the compound and accordingly, it becomes possible to extrude the thus treated compound by an extruder. On the basis of their findings, the present inventors have arrived at the present invention.
Accordingly, the first object of the present invention is to provide a process for continuously producing an electrode substrate having uniform physical properties at a favorable productivity.
Further, the second object of the present invention is to provide an electrode substrate which can be continuously produced and accordingly, can be produced at a remarkably reduced cost and which has unif,orm and favorable physical properties.

~X~847(3 SUMMARY OF THE INVENTION:
In a first aspect of the present invention, there is provided a process for producing an electrode substrate, which process comprises the steps of kneading a raw material mixture comprising from 30 to 60 ~ by weight of short carbon fibers, from 20 to 50 % by weight of a phenol resin ~inder and from 20 to 50 % by weight of a molding additive, extruding the thus kneaded raw material mixture and after press-molding the thus extruded material by rolling or stamping, calcining the thus press-molded material in an inert atmos-phere and/or under a reduced pressure.
In a second aspect of the present invention, there is provided an electrode substrate having uniform physical properties, which electrode substrate has been produced by a process comprising the steps of kneading a raw material mixture comprising from 30 to 60 % by weight of short carbon fibers, from 20 to 50 ~ by weight of a phenol resin binder and from 20 to 50 % by weiqht of a molding additive, extrudina the thus kneaded raw material mixture and after press~molding the thus extruded material by rolling or stamping, calcining the thus press-molded material in an inert atmosphere and/or under a reduced pressure.

847(~

DETAILED DESCRIPTION OF THE IN~ENTION:
The present invention relates to a process for producing an electrode substrate, which process comprises the steps of kneading a raw material mixture comprising from 30 to 60 ~ by weight of short carbon fibers, from 20 to 50 ~ by weight of a phenol resin binder and from 20 to 50 ~ by weight of a molding additive, extruding the thus kneaded raw material mixture and after press-molding the thus extruded material by rolling or stamping, calcining the thus press-molded material in an inert atmosphere and/or under a reduced pressure, and an electrode substrate produced by the above process.
The process according to the present invention will be explained in detail as follows:
As the short carbon fibers used in the present invention, those having a fiber diameter of -from 5 to 30 micrometers and a fiber length of from about 0.05 to about 2 mm are desirable. In the case where the fiber length is o~er 2 mm, the fibers entwine each other during the steps up to molding to form hair ball-like bodies and it is impossible to obtain the electrode substrate having the desired bulk density and distribution of the pore diameter.
On the other hand, in the case where the fiber length is below 0.05 mm,-the necessary strength of the electrode substrate cannot be obtained.

847~) Further, in the case where the above-mentioned short carbon fibers are calcined at 2000C in an inert atmosphere and/or under a reduced pressure, the carbonizing linear shrinkage of the short carbon fiber is preferably not more than 3.0 %. In the case where the carbonizing linear shrinkage is over 3.0 %, there is a fear that the large linear shrinkage becomes one of the causes of generating the cracks in the product at the time of calcination. By using the short carbon fibers shown as above, it is possible to produce an electrode substrate of particularly large in size.
The binder used in the present in~ention is useful after carbonization thereof as the carbonaceous binding material for binding the carbon fibers one another, and in order to obtain the desired bulk density of the electrode substrate, a phenol resin having a carbonizing yield in the range of from 30 to 75 % by weight is used for the purpose.
According to the present invention, both the knead-ability and ~luidity of the raw material mixture are improved by mixing a molding additive with a mixture of the above-mentiond short carbon fibers and phenol resin binder. As such a molding additive r a substance generally used in the field of processing plastics can be used; however, the amount of carbon fibers contained in the fiber-reinforced plastic recently commerciallized as the extrusion-molded products is at most 30 % by weight, and the extrusion-molded products ~ 8 --containing not less than 30 % by weight of the carbon fibers according to the present invention have not been yet known.
As the molding additive used according to the present invention, an organic high polymer having a carboni-zing yield of not more than 5 % by weight is preferably used.
As the organic high polymer, copolymers of ethylene and vinyl acetate (hereinafter referred to as EVA) or mixtures of EVA
and polyolefin may be exemplified. The polyolefin is preferably mixed in the range of not more than 100 parts by weight with 100 parts by weight of EVA, more preferably mixed in the range of not more than 70 parts by weight with 100 parts by weight of EVA. The carbonizing yield~of the mixture of EVA and the polyolefin is preferably not more than 5 % by weight. As the above-mentioned polyolefin, polyethylene is preferable. As the above-mentioned molding additi~e ! those which do not volatilize until the temperature reaches 100C are used. Namely, the thermal deformation and the melt flow of the above-mentioned molding additive is allowed at the extruding temperature and under the extruding pressure, however, the molding additive should not volatilize under tha thus conditions.
The above-mentioned raw material mixture comprises from 30 to 60 ~ by weight, preferably from 35 to 50 % by weight of short carbon fibers, from 20 to 50 % by weight, preferably from 25 to 40 ~ by weight of a phenol resin binder and from 20 to 50 % by weight, preferably from 25 to 40 % by weight of ~ ~3847~3 _ g _ a molding ~dditive.
The abov~-mentioned raw material mixture is supplied to an extruder and is kneaded therei~ under desirable condi-tions of a temperature of not higher than 110C and a ~neading time ~a retention time of the raw ma~erial mixture in the extruder of not longer than about 10 min. After kneadin~ the raw material mixture under the above-mentioned conditions, the thus ~neaded mixture is extruded out through a T-die.
Although the extruding speed in this case depends on the type and size of the extruder, the kneaded raw material mixtura is axtruded generally at a rate of from 10 to lO0 kg/hour.
After heating the thus extruded material to from 130 to 180C, the thus heated material is mold~d under a pressure of from ~0 to 80 kgf/cm2 by rolling or stamping.
In this case, by suitably selec~ing the shape of the roll or the stamp, it is possible to obtain the press-molded material of the desired shape. For instance, by providing concaves and convexes on the surface of the roll, it is possible to produce the ribbed electrode ~ubstrate.
The molded material obtained as above is calcined for about one hour at a temperature of rom 800 ~o 3000C
in an inert atmosphere and/or under a reduced pressure.
In this case, it is desirable that the temperature of the molded material is slowly raised up to about 700C, for instance, at a rate of 100~50 C/hour in the step of thermal ~ ~88~7~) decomposition at low temperatures, thereby preventing the generation of stress due to rapid shrinkage of the molde~
material at the time of gasification. In the case of raising rapidly the temperature of the molded material in the above-mentioned step of thermal decomposition at low temperatures, such an action becomes the cause of interlayer exfoliation and generation of cracks in the product.
The physical properties of the thus obtained electrode substrate are uniform all over the electrode substrate and moreover, the thus obtained electrode substrate has the same or the better values of physical properties as compared to those of the conventional press-molded products.
The descriptions of "uniform in physical properties"
and "little in fluctuation of the values of physical properties"
in the present invention mean that a fluctuation ~R) of the values of physical properties in the product of the present invention is less than that in the conventional product, when each of the physical properties of the electrode substrate is measured at the various points of the specimen of the electrode substrate as will be examplified in Example later !
and the difference between the maximum value and the minimum value of the physical property is considered as the fluctu-ation ~R).
The physical properties of the electrode substrate mentioned as above particularly indicate the bulk density, the gas-permeability and/or the bending stren~th.

~ ~84~

The reason of the substantial reduction of the 1uctu-ation of the values of physical properties of the thus obtained electrode substrate depends on the process according to the present invention, wherein a molding additive is mixed ~lith the raw material mixture comprising short carbon fibers and a bind~r, each component of the thus formed mixture is sufficientl~ kneaded to obtain a raw material mixture having a remarkably improved fluidity, the thus obtained raw material mixture is extruded without unevenness in supply and the thus extruded material is press-molded by rolling or stamping.
Since the above-mentioned process according to the present invention can be carried out continuously, for instance, the productivity of the electrode substrate is remarkably improved as compared to that of the non-continuous process of press-molding by metal mold, and as a result, a remarkable reduction of the production cost of the electrode substrate can be expected.
The electrode substrate having the uniform physical properties obtained according to the process of the present invention is useful ! for instance, as the electrode substrate~
for fuel cells.
In particular! since the bending strength of the thus produced electrode substrate is uniform at the various point thereof, there are little fear of damage of the electrode subst-rate in the handling thereof.

1~1847~) Further, since there are no fluctuation in the value of the bulk density of the -thus obtained electrode substrate, any local unevenness in the thermal resistance and the electri.c resistance of the electrode substrate does not occur and as a result, the life thereof as an electrode is prolonged.
Still more, since there is no fluctuation in the value of the gas-permeability of the thus obtained electrode substrate, there is an effect of showing the uniform output specificity.
The present invention will be described in detail while referring to the non-limitative Example as follows:

EXAMPLE:
Three kinds of electrode substrates (A, B and C) were produced as follows:

A raw material mixture compxising ~1) 45 ~ by weight of short carbon flbers~made by KURE~A ~AGAXU KOGYO Co., Ltd. undar the pr~duct No. of M 104, 14 microme~ers in mean fiber diameter and 0.4 mm in mean fiber length), ~2) 30 %
by weight of a phenol resin~made ~y ASAHI YU~IZAI Co., Ltd.
under the product No. of RM-210 and ~3) 25 % by weight of an EVA resin~EVA~L ~ 7050, made by MITSUI-~u Pont Co., Ltd.l was dry-blend~d and then pell~tized by a pelletizer, and the thus obtained pellets were ~upplied to an extruder ~ade by NIHON SEIXOSHO under the product No. P90-22AB) while controll-ing the t~mperature of the metering zone at 90C! and fur~her extruded from a T-die maintained at 110~C.
After preliminarily heating the thus extruded material to 150C by an infra~red heater, the thus heated material is continuously suppli~d to the rollc heated to 170C. The pressure between the rolls was made to be 20 kgf /~m2.
The thus obtained, molded body was calcined for one hour at 2000~C in an atmosphere of nitrogen to obtain an electrode substrate A.

.
. . . _ .
.. ..

~ 2~3847(~

(B):
A raw material mixture comprising ~1) 45 % by weight of the same short carbon fibers as in (A), (2) 30 by weight of the same phenol resin as in IA) and ~3) a mixtur~
of 20 % by weight of the same EVA as in ~) and 5 % by weight of a polyethylene (Hi-zex Powde ~ SlOOEP, made by MITSUI Petrochemical Co., Ltd.) was dry-blended and then pelletized by the same pelletizer as in (A) to obtain the pellets.
An electrode substrate B was produced by using the thus obtained pellets in the same manner as in (A).

(C):
As a comparative example, an electrode substrate was produced by subjecting a mixture of (1~ 45 % by weight of the same short carbon fibers as in ~A), ~2~ 30 ~ by weight of the same phenol resin as in ~A) and (3) 25 % by weight of the same polyethylene as in (B) ~which polyethylene was used as a pore regulator) to press-molding in the usual manner by a metal mold and calcining the thus obtained, molded body for one hour at 2000C in an atmosphere or nitrogen.
The physical properties of the thus produced three electrode substrates A, B and C were measured as follows, the results being shown later:
The measurement was carried out on each of the above-mentioned electrode substrates of 600 mm in leangth and width.

~8~3470 ~ amely, the bulk density(pb , g/cm3~ was measured on 25 pieces of the specimen of 50 mm in length and width, which pieces had been cut from each of the electrode substrates.
For that purpose lines parallel to the edge of the electrode substrate and to each other were at first drawn on the surfac~ of each of the electrode substrates lengthwise and widthwise at an interval of 100 mm to obtain 25 inter-secting points as the measuring points, and the above-mentioned 25 pi~ces were cut out such that each of the above measuring points is loc~ted at the centre of each of the pieces.
Before cutting out the above~mentioned pieces, the gas-permeability(ml/cm2.hour.mmA~) of each of the electrode substrates was measured by applying a cup of 80 mm in diameter on each of the measuring point t flowing a predetermined amount of air and calculating the gas-permeability from the back pressure~it is publicly known that the back pressure is in proportion linearly to the gas-permeability in that case).
The bending strength ~kgf /mm2) of each of the electrode substrate was measured by cutting out five pieces of specImen of 10 mm in length and 80 mm in width from the remaining part of each o~ the electrode substrates and carrying out the three-point bendin~ test on the thus cut pieces according to Japanese Industrial Standards(JIS) R-6911.

X~38f~7() The results of the measurement are shown in Table.
In Table, R is the difference between the maxim7lm and the minim7lm of the thus measured values, As are seen in Table, R values of the physical properties of the electrode substrate produced according to the process of the present invention(A and B) are respectively about 2/3 and about 1/2 of those of the electrode substrate produced by the conventional process(C).

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Claims

1. A process for continuously producing an electrode substrate, which process comprises the steps of:
(1) kneading, at a temperature of not higher than 110°C.
a raw material mixture comprising from 30 to 60%
by weight of short carbon fibers, from 20 to 50% by weight of a phenol resin binder, and from 20 to 50%
by weight of a molding additive of an organic high polymer having a carbonizing yield of not more than 5% by weight, said organic high polymer being selected from a copolymer resin of ethylene and vinyl acetate, and a mixture of a copolymer resin of ethylene and vinyl acetate and a polyolefin;
(2) extruding the kneaded raw material mixture through an extrusion die;
(3) thereafter heating the extruded raw material to a temperature of from 130° to 180°C., and press-molding the heated material under a pressure of from 20 to 80 kgf/cm2 by rolling or stamping, and then (4) calcining the press-molded material for about one hour at a temperature of from 800° to 3000°C. in an inert atmosphere and/or under a reduced pressure, whereby said electrode substrate is formed.

2. A process according to claim 1, wherein the kneading of step (1) is practiced for not longer than 10 minutes.

3. A process according to claim 1, wherein said short carbon fibers have diameters of from 5 to 30 micro-meters and lengths of from 0.05 to 2 mm, and wherein the carbonizing linear shrinkage of said short carbon fibers is not more than 3% when said short carbon fibers are calcined at 2000°C. in an inert atmosphere and/or under a reduced pressure.

4. A process according to claim 1, wherein the carbo-nizing yield of said phenol resin binder is from 30 to 75%
by weight.

5. A process according to claim 1, wherein the mixture of a copolymer resin of ethylene and vinyl acetate and a polyolefin comprises 100 parts by weight of said copolymer resin of ethylene and vinyl acetate and not more than 100 parts by weight of said polyolefin.

6. A process according to claim 1, wherein said polyolefin is a polyethylene.

7. A process according to claim 1, wherein said raw material mixture comprises from 35 to 50% by weight of said short carbon fibers, from 25 to 40% by weight of said phenol resin binder and from 25 to 40% by weight of said molding additive.

8. A process according to claim 1, wherein step (3) is practiced so as to select a shape of the roll or stamp to produce a press-molded material of desired shape.

9. A process according to claim 1, wherein step (2) is practiced by extruding the kneaded raw material mixture through a T-shaped extrusion die.