U.S. patent application number 10/640022 was filed with the patent office on 2005-02-17 for polymer electrolyte fuel cell, electrolyte material therefore and method for its production.
This patent application is currently assigned to Asahi Glass Company, Limited. Invention is credited to Watakabe, Atsushi.
Application Number | 20050037265 10/640022 |
Document ID | / |
Family ID | 34136002 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050037265 |
Kind Code |
A1 |
Watakabe, Atsushi |
February 17, 2005 |
Polymer electrolyte fuel cell, electrolyte material therefore and
method for its production
Abstract
An electrolyte material for a polymer electrolyte fuel cell,
which is made of a copolymer comprising repeating units based on
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3H and repeating
units based on tetrafluoroethylene and which has an ion exchange
capacity of from 0.9 to 1.5 (meq/g dry resin). This electrolyte
material has ion conductivity and durability equal to conventional
electrolyte material, is easy to synthesize, has a softening point
higher than electrolyte material heretofore widely used for
application to fuel cells and is suitable for operation of a
polymer electrolyte fuel cell at a temperature higher than the
conventional material.
Inventors: |
Watakabe, Atsushi;
(Yokohama-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Asahi Glass Company,
Limited
12-1, Yurakucho 1-chome, Chiyoda-ku
TOKYO
JP
100-8405
|
Family ID: |
34136002 |
Appl. No.: |
10/640022 |
Filed: |
August 14, 2003 |
Current U.S.
Class: |
429/309 ;
429/314 |
Current CPC
Class: |
H01M 4/92 20130101; Y02E
60/50 20130101; H01M 4/926 20130101; H01M 2300/0082 20130101; H01M
8/1039 20130101; Y02P 70/50 20151101; Y10T 29/49108 20150115; H01M
4/8605 20130101; H01M 8/1004 20130101; H01M 8/1023 20130101 |
Class at
Publication: |
429/309 ;
429/314 |
International
Class: |
H01M 006/04; H01M
006/18 |
Claims
What is claimed is:
1. An electrolyte material for a polymer electrolyte fuel cell,
which is made of a copolymer comprising repeating units based on a
monomer of the formula (1) and repeating units based on
tetrafluoroethylene and which has an ion exchange capacity of from
0.9 to 1.5 (meq/g dry resin):
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3H (1)
2. A method for producing an electrolyte material for a polymer
electrolyte fuel cell, which is made of a copolymer comprising
repeating units based on a monomer of the formula (1) and repeating
units based on tetrafluoroethylene and which has an ion exchange
capacity of from 0.9 to 1.5 (meq/g dry resin), wherein an initiator
of a perfluoro compound is used, and tetrafluoroethylene and a
monomer of the formula (2) are subjected to radical
copolymerization at from 60 to 100.degree. C.:
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3H (1)
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.2F (2)
3. The method for producing an electrolyte material for a polymer
electrolyte fuel cell according to claim 2, wherein the initiator
of a perfluoro compound is a perfluorobenzoyl peroxide.
4. The method for producing an electrolyte material for a polymer
electrolyte fuel cell according to claim 2, wherein the polymer
obtained by the copolymerization of the monomer of the formula (2)
and tetrafluoroethylene, has T.sub.Q of from 200 to 400.degree. C.
while T.sub.Q indicates a temperature at which, when melt-extrusion
of the resin is carried out using a nozzle having a length of 1 mm
and an inner diameter of 1 mm under an extrusion pressure condition
of 30 kg/cm.sup.2, the extrusion rate would be 100 mm.sup.3/sec.,
and is obtained by hydrolysis of said polymer followed by
conversion to an acid-form.
5. A method for producing an electrolyte material for a polymer
electrolyte fuel cell, which is made of a copolymer comprising
repeating units based on a monomer of the formula (1) and repeating
units based on tetrafluoroethylene and which has an ion exchange
capacity of from 0.9 to 1.5 (meq/g dry resin), wherein by means of
a hydrocarbon type initiator, tetrafluoroethylene and a monomer of
the formula (2) are subjected to radical copolymerization at a
temperature of at least 20.degree. C. and less than 50.degree. C.:
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3- H (1)
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.2F (2)
6. The method for producing an electrolyte material for a polymer
electrolyte fuel cell according to claim 5, wherein the polymer
obtained by the copolymerization of the monomer of the formula (2)
and tetrafluoroethylene, has T.sub.Q of from 200 to 400.degree. C.
while T.sub.Q indicates a temperature at which, when melt-extrusion
of the resin is carried out using a nozzle having a length of 1 mm
and an inner diameter of 1 mm under an extrusion pressure condition
of 30 kg/cm.sup.2, the extrusion rate would be 100 mm.sup.3/sec.,
and is obtained by hydrolysis of said polymer followed by
conversion to an acid-form.
7. A copolymer which comprises repeating units based on a monomer
of the formula (2) and repeating units based on tetrafluoroethylene
and which has T.sub.Q of from 200 to 400.degree. C. as an index of
the molecular weight and an ion exchange capacity of from 0.9 to
1.5 (meq/g dry resin), when converted to a --SO.sub.3H form:
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.- sub.2SO.sub.2F (2)
8. A polymer electrolyte fuel cell having an anode, a cathode and a
polymer electrolyte membrane disposed between the anode and the
cathode, wherein the polymer electrolyte membrane is made of a
copolymer comprising repeating units based on a monomer of the
formula (1) and repeating units based on tetrafluoroethylene and
has an ion exchange capacity of from 0.9 to 1.5 (meq/g dry resin)
and a membrane thickness of from 5 to 70 .mu.m:
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3H (1)
9. A polymer electrolyte fuel cell having an anode, a cathode and a
polymer electrolyte membrane disposed between the anode and the
cathode, wherein at least one of the anode and the cathode has a
catalyst layer which comprises a catalyst and a copolymer
comprising repeating units based on a monomer of the formula (1)
and repeating units based on tetrafluoroethylene and having an ion
exchange capacity of from 0.9 to 1.5 (meq/g dry resin):
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3H (1)
Description
DESCRIPTION
TECHNICAL FIELD
[0001] The present invention relates to an electrolyte material for
a polymer electrolyte fuel cell, and a polymer electrolyte fuel
cell.
BACKGROUND ART
[0002] As an electrolyte material to be used as a polymer
electrolyte membrane or a proton conductive polymer to be
incorporated in a catalyst layer of an electrode constituting a
polymer electrolyte fuel cell, it has been common to employ a
polymer obtained by hydrolyzing a copolymer of tetrafluoroethylene
(hereinafter referred to as "TFE") with a perfluorovinyl ether of
the formula (A), followed by treatment for conversion to an
acid-form to convert --SO.sub.2F groups to --SO.sub.3H groups. In
the formula (A), Y is a fluorine atom or a trifluoromethyl group, m
is an integer of from 0 to 3, n is an integer of from 1 to 12, and
p is 0 or 1, provided that (m+p)>0.
CF.sub.2.dbd.CF(OCF.sub.2CFY).sub.mO.sub.p(CF.sub.2).sub.nSO.sub.2F
(A)
[0003] Among such polymers, particularly preferably employed is one
obtained by converting a polymer obtainable by copolymerization of
TFE with a monomer represented by the formula (B) to (D), to an
acid form. In the formulae (B) to (D), q is an integer of from 1 to
8, r is an integer of from 1 to 8, and s is 2 or 3.
CF.sub.2.dbd.CFO(CF.sub.2).sub.qSO.sub.2F (B)
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub.rSO.sub.2F
(C)
CF.sub.2.dbd.CF(OCF.sub.2CF(CF.sub.3)).sub.sO(CF.sub.2).sub.2SO.sub.2F
(D)
[0004] However, although the above-mentioned conventional copolymer
was excellent in such properties as an ion conductivity to
accomplish a high cell output power and durability to make a long
term operation possible, it had a problem that the production cost
was high, and it could not be produced at low costs. As a large
factor for such a high production cost of the conventional
copolymer, it may be mentioned that, for example, in the case of a
TFE/CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)OCF.sub.2CF.sub.2SO-
.sub.3H copolymer, it is produced by copolymerizing a vinyl ether
monomer containing a --SO.sub.2F group synthesized by using, as an
intermediate, expensive hexafluoropropylene oxide, with TFE.
[0005] Whereas, U.S. Pat. No. 4,273,729 discloses a copolymer of a
monomer of the formula (2) (monomer (2)) synthesized without using
hexafluoropropylene oxide, with TFE.
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.2F (2)
[0006] However, the polymer synthesized for brine electrolysis in
an Example (UTILITY EXAMPLE Q) of this patent publication, has an
ion exchange capacity of 0.85 (meq/g dry resin) (equivalent weight:
1180), and thus, the ion exchange capacity is inadequate for a fuel
cell, whereby it has a problem that the resistance is practically
too high. Further, Journal of Applied Polymer Science, Vol. 47,
735-741 (1993) discloses a report on the results of a study of the
synthesis of the monomer (2) and copolymerization of TFE with the
monomer (2), wherein the relation between the charge composition
comprising TFE and the monomer (2) and the obtainable polymer
composition, is reported. However, also in this report, the highest
ion exchange capacity among the obtained polymers is 0.77 (meq/g
dry resin). And, it is stated that rather than the monomer (2), a
conventional vinyl ether type monomer having a structure of
CF.sub.2.dbd.CFO-- has a higher reactivity and is more advantageous
for the copolymerization.
[0007] Namely, the monomer (2) has a lower copolymerization
reactivity with TFE than the conventional vinyl ether type monomer,
whereby a polymer having a high ion exchange capacity practically
useful as an ion exchange membrane, has not heretofore been
obtained. In the case of a copolymer using as starting materials a
vinyl ether type monomer such as
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F or
CF.sub.2.dbd.CFOCF.sub.2CF(C- F.sub.3)OCF.sub.2CF.sub.2SO.sub.2F,
and TFE, it is possible to obtain a polymer having an ion exchange
capacity of at least 1.1 (meq/g dry resin) easily by means of
2,2'-azobisisobutyronitrile (AIBN), which is hydrocarbon, as the
initiator, as disclosed e.g. in Examples of JP-A-60-243292.
However, in the case of the monomer (2), polymerization will not
substantially proceed by the polymerization by means of AIBN, as
disclosed in Comparative Reference Example in this
specification.
DISCLOSURE OF THE INVENTION
[0008] The present invention has been made in view of the problems
which the above-mentioned prior art has had, and it is an object of
the present invention to provide an electrolyte material for a
polymer electrolyte fuel cell which is easy to synthesize and which
has an ion conductivity and durability equal to the above-mentioned
conventional electrolyte materials, and to provide a polymer
electrolyte fuel cell constituted by using such a material.
[0009] The present inventor has conducted an extensive research to
accomplish the above object and as a result, has found it possible
to obtain a polymer having a practically sufficient ion exchange
capacity and a sufficient molecular weight and having a higher
content of the monomer (2) than U.S. Pat. No. 4,273,729 or Journal
of Applied Polymer Science, Vol. 47, 735-741 (1993), by improving
the polymerization conditions. Further, it has been found that a
TFE/monomer (2) copolymer prepared by adjusting the ion exchange
capacity within a prescribed range, has sufficient ion conductivity
and durability at the same time, and thus, the present invention
has been accomplished.
[0010] The present invention provides an electrolyte material for a
polymer electrolyte fuel cell, which is made of a copolymer
comprising repeating units based on a monomer of the formula (1)
and repeating units based on tetrafluoroethylene and which has an
ion exchange capacity of from 0.9 to 1.5 (meq/g dry resin); and a
polymer electrolyte fuel cell wherein a polymer electrolyte
membrane made of the electrolyte material for a polymer electrolyte
fuel cell and having a membrane thickness of from 5 to 70 .mu.m, is
disposed between an anode and a cathode.
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3H (1)
[0011] Further, the present invention provides a method for
producing an electrolyte material for a polymer electrolyte fuel
cell, which is made of a copolymer comprising repeating units based
on a monomer of the formula (1) and repeating units based on
tetrafluoroethylene and which has an ion exchange capacity of from
0.9 to 1.5 (meq/g dry resin), wherein an initiator of a perfluoro
compound is used, and tetrafluoroethylene and a monomer of the
formula (2) are subjected to radical copolymerization at from 60 to
100.degree. C.:
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.3H (1)
CF.sub.2.dbd.CFCF.sub.2OCF.sub.2CF.sub.2SO.sub.2F (2)
[0012] Further, the present invention provides a method for
producing an electrolyte material for a polymer electrolyte fuel
cell, which is made of a copolymer comprising repeating units based
on a monomer of the formula (1) and repeating units based on
tetrafluoroethylene and which has an ion exchange capacity of from
0.9 to 1.5 (meq/g dry resin), wherein by means of a hydrocarbon
type initiator, tetrafluoroethylene and a monomer of the formula
(2) are subjected to radical copolymerization at a temperature of
at least 20.degree. C. and less than 50.degree. C.
[0013] The present inventor has found it preferred {circle over
(1)} to use an initiator having no hydrogen atom involved in chain
transfer, preferably an initiator of a perfluoro compound and
{circle over (2)} to carry out radical copolymerization at a
relatively high temperature of about from 60 to 100.degree. C., in
order to obtain a TFE/monomer (2) copolymer having a practically
sufficient molecular weight and having a high content of the
monomer of the formula (2) (monomer (2)) having a low reactivity.
It is preferred to adopt such conditions also in order to secure a
practically sufficient polymerization rate.
[0014] If an initiator having no hydrogen involved in chain
transfer is employed, the polymerization will be possible even at a
low temperature of less than 60.degree. C. In such a case, however,
not only the reactivity of the monomer (2) tends to be low, but
also the solubility of TFE in the monomer (2) or in the mixed
liquid of the monomer (2) and the polymerization solvent, tends to
increase due to the decrease of the polymerization temperature,
whereby the polymerization pressure tends to be remarkably low. As
the polymerization pressure lowers, the fluctuation of the ion
exchange capacity of the obtainable polymer tends to be large as
compared with the fluctuation of the polymerization pressure,
whereby it tends to be difficult to obtain a polymer having the
same ion exchange capacity in good reproducibility by controlling
the pressure. In UTILITY EXAMPLE Q in U.S. Pat. No. 4,273,729, an
initiator made of a perfluoro compound is used, but the
polymerization is carried out at 40.degree. C., since the
decomposition temperature of the initiator is low. Therefore, the
reactivity of the monomer (2) is low, and in spite of the
polymerization at a low pressure of 0.07 MPaG, a copolymer having a
high content of the monomer (2) is not obtained.
[0015] In a case where a hydrocarbon type initiator is employed, if
the polymerization temperature becomes high, the polymerization for
the polymer tends to be substantially slow, or a problem that the
molecular weight can not be increased, is likely to result. This is
considered attributable to chain transfer to the initiator from the
polymer growth terminal radicals. Accordingly, in a case where a
hydrocarbon initiator is employed, it is obliged to carry out the
polymerization at a relatively low polymerization temperature of
less than 70.degree. C., preferably less than 50.degree. C. In such
a case, the polymerization is possible, but the polymerization
pressure tends to be low, whereby it tends to be difficult to
control the ion exchange capacity, as compared with a case where
the polymerization temperature is high, as mentioned above. In
either case of the initiator having no hydrogen atom involved in
chain transfer or the initiator of a hydrocarbon type,
polymerization is required to be carried out at a temperature of at
least 10.degree. C., preferably at least 20.degree. C., in order to
secure the polymerization reactivity of the monomer (2).
[0016] The electrolyte material for a polymer electrolyte fuel cell
of the present invention can be prepared without using
hexafluoropropylene oxide as a raw material, whereby as compared
with the above-mentioned conventional electrolyte material, the
production cost can be substantially reduced. Further, the
electrolyte material for a polymer electrolyte fuel cell of the
present invention has excellent ion conductivity and durability
equal to the conventional electrolyte material, since its ion
exchange capacity (hereinafter referred to as A.sub.R) is from 0.9
to 1.5 (meq/g dry resin) (hereinafter referred to simply as
meq/g).
[0017] Here, if A.sub.R is less than 0.9, the ion conductivity
tends to be inadequate. On the other hand, if A.sub.R exceeds 1.5,
the water content tends to be so large that if a membrane is formed
by using this electrolyte material, the membrane strength tends to
be inadequate. From a similar view point, AR of this electrolyte
material is preferably from 1.0 to 1.4 meq/g, particularly
preferably from 1.1 to 1.3 meq/g.
[0018] In the present invention, the molecular weight of the
polymer as the electrolyte material can be evaluated by a value of
T.sub.Q, an index of melt-flowability, as an index of the molecular
weight of the polymer. T.sub.Q indicates the temperature, at which,
when melt-extrusion of the resin is carried out using a nozzle
having a length of 1 mm and an inner diameter of 1 mm under an
extrusion pressure condition of 30 kg/cm.sup.2, the extrusion rate
would be 100 mm.sup.3/sec. In order to avoid the influence of the
thermal decomposition of the polymer, the measurement is carried
out at a temperature of not higher than 330.degree. C. If T.sub.Q
exceeds 330.degree. C., the relation between the temperature and
the extrusion rate at not higher than 330.degree. C., is
extraporated on the high temperature side to obtain the temperature
at which the extrusion rate would be 100 mm.sup.3/sec. T.sub.Q is
measured and evaluated at a stage of the polymer having --SO.sub.2F
groups before the hydrolysis and conversion to an acid form.
T.sub.Q is a numerical value which will be an index of the
molecular weight of the resin, and usually, the molecular weight is
higher as T.sub.Q is higher. In order for the resin to have
practically sufficient strength as a membrane, T.sub.Q is usually
at least 150.degree. C., preferably at least 180.degree. C., more
preferably at least 200.degree. C.
[0019] Accordingly, the electrolyte material of the present
invention is preferably obtained by hydrolysis and conversion to an
acid form, of a TFE/monomer (2) copolymer having T.sub.Q within the
above-mentioned range.
[0020] The upper limit of the preferred range of T.sub.Q depends on
the fabricating method. In a case where melt processing is carried
out, decomposition of --SO.sub.2F groups of the polymer begins in
the vicinity of 350.degree. C., whereby T.sub.Q is preferably at
most 400.degree. C., particularly preferably at most 350.degree.
C., further preferably at most 300.degree. C. In a case where
--SO.sub.2F groups are hydrolyzed and converted to --SO.sub.3M
groups (wherein M is a monovalent cation, preferably H, an alkali
metal or NR.sup.1R.sup.2R.sup.3R.sup.4 (wherein each of R.sup.1 to
R.sup.4 which are independent of one another, is H or an alkyl
group)), followed by dispersion or dissolution in an alcohol and/or
water, and a liquid composition thereby obtained is used for
fabrication such as casting, it is not necessary to set the upper
limit for T.sub.Q. However, with a view to securing the
solubility/dispersibili- ty in the solvent, T.sub.Q is preferably
at most 400.degree. C., more preferably at most 350.degree. C.
[0021] Further, the polymer electrolyte fuel cell of the present
invention is provided with at least a polymer electrolyte membrane
made of the above-mentioned electrolyte material for a polymer
electrolyte fuel cell and accordingly has output characteristics
and cell life equal to the above-mentioned conventional polymer
electrolyte fuel cell, and yet, the monomer production step can be
shortened. Namely, for example, the compound of the formula (D)
which has heretofore been used, is prepared from
FSO.sub.2CF.sub.2COF via two steps of hexafluoropropylene oxide
addition and thermal decomposition. Whereas, the monomer (2) can be
prepared by a single step reaction from FSO.sub.2CF.sub.2COF as
shown by scheme A which will be described hereinafter. Further, in
the polymer electrolyte fuel cell of the present invention, the
above-mentioned electrolyte material for a polymer electrolyte fuel
cell of the present invention may be incorporated also in a
catalyst layer of an anode and/or a cathode.
[0022] Here, if the membrane thickness of the polymer electrolyte
membrane to be used for the polymer electrolyte fuel cell of the
present invention is less than 5 .mu.m, the strength of the
membrane tends to be inadequate. On the other hand, if the membrane
thickness exceeds 70 .mu.m, the resistance of the electrolyte tends
to be so large that no adequate cell output power tends to be
obtainable. Further, from a similar viewpoint as described above,
the membrane thickness of the polymer electrolyte membrane to be
used for the polymer electrolyte fuel cell of the present invention
is preferably from 10 to 50 .mu.m.
BRIEF DESCRIPTION OF THE DRAWING
[0023] FIG. 1 shows the temperature dependency of the elastic
modulus in one embodiment of the electrolyte material of the
present invention and the temperature dependency of the elastic
modulus of a conventional electrolyte material, as measured in
Examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0024] Now, the electrolyte material for a polymer electrolyte fuel
cell, and the polymer electrolyte fuel cell, of the present
invention, will be described in further detail.
[0025] The electrolyte material for a polymer electrolyte fuel cell
of the present invention can be obtained by copolymerizing the
monomer (2) with TFE, and hydrolyzing the obtained copolymer and
contacting it with an acid.
[0026] Further, the polymer electrolyte fuel cell of the present
invention has an anode, a cathode and a polymer electrolyte
membrane disposed between the anode and the cathode. And, the
polymer electrolyte fuel cell of the present invention is not
particularly limited with respect to the construction except that
it is made of the electrolyte material for a polymer electrolyte
fuel cell of the present invention and provided with a polymer
electrolyte membrane having a membrane thickness of from 5 to 70
.mu.m. For example, it may have a construction similar to the
conventional polymer electrolyte fuel cell. Further, as mentioned
above, the electrolyte material for a polymer electrolyte fuel cell
may be incorporated also to a catalyst layer of the anode and/or
the cathode, instead of the conventional perfluorosulfonic acid
polymer. Further, the method for producing the polymer electrolyte
fuel cell of the present invention is also not particularly
limited, and the method for producing a polymer electrolyte
membrane made of the electrolyte material for a polymer electrolyte
fuel cell of the present invention, or the method for preparing the
fuel cell from the electrodes and the polymer electrolyte membrane,
is also not particularly limited, and a conventional method may be
employed.
[0027] The monomer (2) to be used in the present invention can be
prepared by a known synthetic reaction shown by the following
scheme A as disclosed, for example, in U.S. Pat. No. 4,273,729. A
similar synthetic method is disclosed also in Journal of Applied
Polymer Science, Vol. 47, 735-741 (1993). 1
[0028] Further, the polymerization reaction of the monomer (2) with
TFE is carried out under such a condition that radicals will be
formed, for example, by a bulk polymerization method, a solution
polymerization method, a suspension polymerization method or an
emulsion polymerization method. Preferred is a bulk polymerization
method or a solution polymerization method. Further, as a method to
let radicals be formed, a method of irradiating a radiation such as
ultraviolet rays, y-rays or electron rays, or a method of adding a
radial initiator, may, for example, be mentioned.
[0029] Further, in the present invention, with respect to the
reaction temperature for the polymerization reaction of the above
monomers, the polymerization is possible within a range of from 10
to 350.degree. C. in a case where an initiator having no hydrogen
atoms involved in chain transfer, preferably an initiator of a
perfluoro compound, is used. If the reaction temperature for the
polymerization reaction exceeds 350.degree. C., the heat resistance
of the resulting copolymer tends to be inadequate. From the
viewpoint of the safety and easy control of the polymerization, it
is preferably from 40 to 200.degree. C., more preferably from 60 to
100.degree. C. In a case where an initiator of a hydrocarbon type
containing no fluorine atom, is used, the polymerization
temperature is at least 10.degree. C. and lower than 50.degree. C.,
preferably at least 200.degree. C. and lower than 50.degree. C.
[0030] Further, in a case where the above-mentioned polymerization
reaction of monomers is carried out by using a radical initiator in
the present invention, the radical initiator may, for example, be a
bis(fluoroacyl) peroxide, bis(chlorofluoroacyl) peroxide, a dialkyl
peroxydicarbonate, a diacyl peroxide, a peroxyester, an azo
compound, a persulfate, a perfluorocarbon having a tertiary
carbon-tertiary carbon, tertiary carbon-quaternary carbon or
quaternary carbon-quaternary carbon bond, or a perfluorocarbon
compound having a N--F bond. From the viewpoint of increasing the
molecular weight of the resulting copolymer, it is preferred to
employ a fluorinated initiator among the above radical initiators,
and it is more preferred to employ an initiator of a perfluoro
compound.
[0031] As the initiator of a perfluoro compound, perfluorobenzoyl
peroxide, bis(perfluoropropionyl) peroxide, bis(perfluorobutyryl)
peroxide, bis[(perfluorocyclohexyl)carbonyl] peroxide,
bis(perfluoro-2-propoxypropanoyl) peroxide,
bis(perfluoro-2,5-dimethyl-3,- 6-dioxanonanoyl) peroxide or
perfluorodi-tert-butyl peroxide may, for example, be mentioned. As
an initiator of a hydrocarbon type, diisopropyl peroxydicarbonate,
isobutyryl peroxide, t-hexyl peroxypivalate or t-butyl
peroxypivalate may, for example, be mentioned.
[0032] In the present invention, for the polymerization, a bulk
polymerization method using no solvent, may be employed. However,
in a case where the above-mentioned polymerization reaction of
monomers is carried out by a solution polymerization method, the
boiling point of the solvent to be used is usually from 20 to
350.degree. C., preferably from 40 to 150.degree. C., from the
viewpoint of the handling efficiency. Further, the useful solvent
is not particularly limited, the following may, for example, be
mentioned.
[0033] 1) A polyfluorotrialkylamine compound such as
perfluorotributylamine or perfluorotripropylamine.
[0034] 2) A fluoroalkane such as perfluorohexane, perfluorooctane,
perfluorodecane, perfluorododecane, perfluoro(2,7-dimethyloctane),
2H,3H-perfluoropentane, 1H-perfluorohexane, 1H-perfluorooctane,
1H-perfluorodecane, 1H,4H-perfluorobutane,
1H,1H,1H,2H,2H-perfluorohexane- , 1H,1H,1H,2H,2H,-perfluorooctane,
1H,1H,1H,2H,2H-perfluorodecane, 3H,4H-perfluoro(2-methylpentane) or
2H,3H-perfluoro(2-methylpentane).
[0035] 3) A chlorofluoroalkane such as
3,3-dichloro-1,1,1,2,2-pentafluorop- ropane,
1,3-dichloro-1,1,2,2,3-pentafluoropropane or
1,1-dichloro-1-fluoroethane.
[0036] 4) A polyfluorocycloalkane such as perfluorodecalin,
perfluorocyclohexane, perfluoro(1,2-dimethylcyclohexane),
perfluoro(1,3-dimethylcyclohexane),
perfluoro(1,3,5-trimethylcyclohexane) or
perfluorodimethylcyclobutane (regardless of structural
isomers).
[0037] 5) A polyfluorocyclic ether compound such as
perfluoro(2-butyltetrahydrofuran).
[0038] 6) A hydrofluoroether such as n-C.sub.3F.sub.7OCH.sub.3,
n-C.sub.3F.sub.7OCH.sub.2CF.sub.3, n-C.sub.3F.sub.7OCHFCF.sub.3,
n-C.sub.3F.sub.7OC.sub.2H.sub.5, n-C.sub.4F.sub.9OCH.sub.3,
iso-C.sub.4F.sub.9OCH.sub.3, n-C.sub.4F.sub.9OC.sub.2H.sub.5,
iso-C.sub.4F.sub.9OC.sub.2H.sub.5,
n-C.sub.4F.sub.9OCH.sub.2CF.sub.3, n-C.sub.5F.sub.11OCH.sub.3,
n-C.sub.6F.sub.130CH.sub.3, n-C.sub.5F.sub.11OC.sub.2H.sub.5,
CF.sub.3OCF(CF.sub.3)CF.sub.2OCH.sub.3,
CF.sub.3OCHFCH.sub.2OCH.sub.3, CF.sub.3OCHFCH.sub.2OC.sub.2H.sub.5
or n-C.sub.3F.sub.7OCF.sub.2CF(CF.sub.3)OCHFCF.sub.3.
[0039] 7) A fluorinated low molecular weight polyether.
[0040] Among the above solvents, it is preferred to select one
having a small number of hydrogen atoms and having a low chain
transfer property of the solvent. Further, the above solvents may
be used alone or in combination as a mixture of two or more of
them.
[0041] Further, in addition to the above solvents, a
chlorofluorocarbon such as 1,1,2-trichloro-1,2,2-trifluoroethane,
1,1,1-trichloro-2,2,2-trif- luoroethane,
1,1,1,3-tetrachloro-2,2,3,3-tetrafluoropropane or
1,1,3,4-tetrachloro-1,2,2,3,4,4-hexafluorobutane, may be
technically used, but such is not preferred from the viewpoint of
the global environment protection. Further, in the present
invention, it is also possible to carry out the polymerization
reaction by means of liquid or supercritical carbon dioxide.
[0042] Further, the copolymer of the monomer (2) with TFE may
contain repeating units based on another fluorinated monomer as a
small amount component. As such a fluorinated monomer, vinylidene
fluoride, trifluoroethylene, vinyl fluoride, ethylene,
chlorotrifluoroethylene, perfluoro(3-butenyl vinyl ether),
perfluoro(allyl ether), perfluoro(2,2-dimethyl-1,3-dioxole),
perfluoro(1,3-dioxole),
2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole,
perfluoro(2-methylene-4-m- ethyl-1,3-dioxolane),
1,1'-[(difluoromethylene)bis(oxy)]bis[1,2,2-trifluor- oethylene],
hexafluoropropylene or a perfluorovinyl ether compound represented
by CF.sub.2.dbd.CFOR.sup.f, may, for example, be mentioned. Here,
R.sup.f is a C.sub.18 perfluoroalkyl group, which may have a
branched structure and which may contain an etheric oxygen
atom.
[0043] Among copolymers thus obtained, a polymer having a perfluoro
structure is particularly preferred from the viewpoint of the
durability of the fuel cell.
[0044] The polymer may be treated with fluorine gas in order to
stabilize the unstable moieties at the terminals. In the
fluorination reaction in such a case, as the fluorine gas, it is
preferred to employ fluorine gas diluted with an inert gas. The
fluorination temperature is from 150 to 200.degree. C., preferably
from 170 to 190.degree. C.
[0045] The copolymer prepared as described above, is, in the form
of a powder or after being processed into a film by melt extrusion
or hot press, subjected to hydrolysis treatment and then to
treatment for conversion to an acid form. In the hydrolysis
treatment, for example, in a solution of a base such as NaOH or KOH
or in water or a mixture of water and an alcohol such as methanol
or ethanol, or a polar solvent such as dimethylsulfoxide,
--SO.sub.2F groups in the prepared copolymer are hydrolyzed and
converted to --SO.sub.3Na groups or --SO.sub.3K groups. In the
subsequent treatment for conversion to an acid-form, metal ions of
e.g.--SO.sub.3Na groups or --SO.sub.3K groups in the copolymer are
substituted by protons in an aqueous solution of an acid such as
hydrochloric acid, nitric acid or sulfuric acid to an acid-form,
whereby the functional groups are converted to sulfonic acid groups
(--SO.sub.3H groups). The hydrolysis treatment and the treatment
for conversion to an acid form are carried out usually at a
temperature of from 0.degree. C. to 120.degree. C.
[0046] Here, in a case where the electrolyte material for a polymer
electrolyte fuel cell of the present invention is to be used as a
material for constituting a polymer electrolyte membrane to
constitute a polymer electrolyte fuel cell, the copolymer prepared
by the polymerization reaction may be formed into a film and then
subjected to hydrolysis treatment and treatment for conversion to
an acid-form, as described above, but it may be subjected to
hydrolysis treatment and treatment for conversion to an acid form
in the state of a powder, then dissolved in a solvent and formed
into a film by a casting method. Further, in such a case, the
polymer electrolyte membrane may be reinforced by e.g. a
polytetrafluoroethylene (hereinafter referred to as PTFE) porous
substrate or PTFE fiber (fibrils).
[0047] In a case where the electrolyte material for a polymer
electrolyte fuel cell of the present invention is to be used as a
resin to be incorporated to a catalyst layer of an electrode to
constitute a polymer electrolyte fuel cell, the copolymer after
converted to sulfonic groups by the above-mentioned treatment for
conversion to an acid form, may be dissolved or dispersed in an
organic solvent, a mixed solvent of an organic solvent and water,
or water, so that it can be used in the form of a liquid
composition.
[0048] The organic solvent is not particularly limited. However,
the copolymer after converted to an acid form can be dissolved or
well dispersed in an organic solvent having a --OH group, and it is
preferred to use an organic solvent having a --OH group, and it is
more preferred to use an organic solvent having an alcoholic-OH
group. Specifically, methanol, ethanol, 1-propanol,
2,2,2-trifluoroethanol, 2,2,3,3,3-pentafluoro-1-propanol,
2,2,3,3-tetrafluoro-1-propanol, 4,4,5,5,5-pentafluoro-1-pentanol,
1,1,1,3,3,3-hexafluoro-2-propanol, 3,3,3-trifluoro-1-propanol,
3,3,4,4,5,5,6,6,6-nonafluoro-1-hexanol or
3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol may, for example,
be mentioned. Further, as an organic solvent having a --OH group,
an organic solvent having a carboxyl group such as acetic acid may
also be used in addition to the above alcohols.
[0049] A catalyst layer of an anode and/or cathode of a polymer
electrolyte fuel cell can be prepared by using the liquid
composition obtained by dissolving or dispersing the copolymer
after converted to an acid form, in water or in a solvent
containing the above-mentioned organic solvent having a --OH group.
For example, by using such a liquid composition, a cathode
excellent in a gas diffusion property can be obtained. The
concentration of the copolymer in such a liquid composition is
preferably from 1 to 50%, more preferably from 3 to 30%, based on
the total mass amount of the liquid composition. If this
concentration is less than 1%, a large amount of an organic solvent
will be required at the time of the preparation of an electrode.
Further, if this concentration exceeds 50%, the viscosity of the
liquid composition tends to be so high that the handling efficiency
tends to deteriorate.
[0050] The polymer electrolyte fuel cell of the present invention
can be prepared, for example, by either one of the following two
methods by using a uniform dispersion obtained by mixing and
dispersing electroconductive carbon black powder having fine
platinum catalyst particles supported thereon in the
above-mentioned liquid composition containing the copolymer. The
first method is a method wherein the above-mentioned dispersion is
coated and dried on both sides of a polymer electrolyte membrane,
and then a carbon cloth or carbon paper to be a gas diffusion
layer, is tightly bonded thereto. The second method is a method in
which the above-mentioned dispersion is coated and dried on a
carbon cloth or carbon paper to be a gas diffusion layer, and then
an anode thus obtained is bonded on one side of the polymer
electrolyte membrane and a cathode obtained by the same way is
bonded to the other side. In such a manner, a so-called membrane
electrode assembly having an anode disposed on one side and a
cathode disposed on the other side of the polymer electrolyte
membrane will be obtained. And, the obtained membrane electrode
assembly is, for example, sandwiched between separators having
grooves formed to constitute passages for a fuel gas or an
oxidizing gas (air, oxygen or the like) containing oxygen and
assembled into a cell to obtain a polymer electrolyte fuel cell of
the present invention.
[0051] Further, the resin (hereinafter referred to as the electrode
resin) to be incorporated to the electrodes of the polymer
electrolyte fuel cell of the present invention may be composed
solely of the solid polymer electrolyte material of the present
invention, but may be a mixture of the solid polymer electrolyte
material of the present invention with the above-mentioned
conventional electrolyte material, or may be composed solely of the
conventional electrolyte material.
[0052] Further, in the polymer electrolyte fuel cell of the present
invention, the catalyst and the electrode resin contained in the
cathode and/or the anode (hereinafter simply referred to as an
electrode unless it is required to particularly distinguish it) are
preferably in a mass ratio of the catalyst:electrode resin=from
20:80 to 95:5, from the viewpoint of the electroconductivity and
the water discharging property of the electrode. The mass of the
catalyst here includes the mass of the carrier in the case of a
supported catalyst which is supported on a carrier such as
carbon.
[0053] The polymer electrolyte fuel cell of the present invention
is useful not only for a hydrogen/oxygen type fuel cell but also
for e.g. a direct methanol type fuel cell (DMFC). Methanol or an
aqueous methanol solution to be used as a fuel for DMFC may be
supplied by a liquid feed or a gas feed.
[0054] Now, the present invention will be described in further
detail with reference to Examples and Comparative Examples.
However, it should be understood that the present invention is by
no means restricted thereto. Further, in Examples, the following
abbreviations will be used. 2
POLYMERIZATION EXAMPLE 1
[0055] Into a stainless steel autoclave having an internal capacity
of 125 cm.sup.3, 37.2 g of PFAE and, 205 mg of perfluorobenzoyl
peroxide (PFBPO) as an initiator, were introduced and cooled with
liquid nitrogen and deaerated. Then, TFE was introduced into the
autoclave, and the system was maintained at 80.degree. C. under
0.345 MPaG (gauge pressure, the same applies hereinafter) for 2
hours and 50 minutes. The autoclave was cooled, and the gas in the
system was purged to stop the polymerization. After diluting with
HCFC 225cb, the polymer was flocculated by an addition of HCFC
141b, followed by filtration. Then, the polymer was stirred in HCFC
225cb and re-flocculated by HCFC 141b, followed by vacuum drying
overnight at 80.degree. C. A.sub.R of the polymer obtained by
titration was 1.12 meq/g, and T.sub.Q measured by means of
Capillary Rheometer CFT-500D (manufactured by Shimadzu Corporation)
was 204.degree. C.
POLYMERIZATION EXAMPLES 2 to 5 and POLYMERIZATION REFERENCE
EXAMPLES 1 and 2
[0056] TFE/PFAE copolymers of polymerization Examples 2 to 5 and
Polymerization Reference Examples 1 and 2 were prepared in the
similar operation as in Polymerization Example 1 by adjusting the
polymerization conditions to the respective conditions as
identified in Table 1 (the charged amount of PFAE, the species and
amount of the initiator and its concentration to PFAE monomer, the
polymerization temperature, the polymerization pressure and the
reaction time). Further, the initiator used in Polymerization
Example 4 was charged by using a solution having 3% of PFB
dissolved in HCFC 225cb. With respect to the polymers of
Polymerization Examples 2 to 5 and Polymerization Reference
Examples 1 and 2 thus obtained, the yield, A.sub.R and T.sub.Q were
measured, and the results are shown in Table 2 together with the
results of Polymerization Example 1.
[0057] In a case where IPP or PFB was used as the initiator, the
polymerization temperature had to be set low, since the
decomposition temperature is low as compared with PFBPO, and it was
necessary to substantially reduce the polymerization pressure in
order to obtain a polymer having a proper ion exchange capacity. In
such a system, the fluctuation of the ion exchange capacity of the
resulting polymer tends to be large to the fluctuation of the
polymerization pressure. In order to obtain a polymer having the
same ion exchange capacity with good reproducibility, control is
easy when the polymerization pressure is high. Accordingly, from
the viewpoint of controlling the ion exchange capacity, it is
preferred to carry out the polymerization by means of an initiator
having a high decomposition temperature like PFBPO.
1 TABLE 1 Initiator Polymerization Polymerization PFAE Amount
Concentration temperature pressure Reaction (g) Species (mg) (ppm)
(.degree. C.) (MPaG) time (h) Polymerization 37.2 PFBPO 205 5520 80
0.345 2.83 Example 1 Polymerization 74.4 PFBPO 74 995 80 0.33 5.08
Example 2 Polymerization 74.4 PFBPO 297 3990 80 0.31 2.25 Example 3
Polymerization 74.4 PFBPO 74 1000 80 0.45 2.90 Reference Example 1
Polymerization 63.7 PFB 45 700 30 0.02 9.50 Example 4
Polymerization 63.8 IPP 123 1920 40 0.05 9.00 Example 5
Polymerization 63.8 IPP 130 2030 40 0.12 4.42 Reference Example
2
[0058]
2 TABLE 2 Yield (g) A.sub.R (meq/g) T.sub.Q (.degree. C.)
Polymerization 7.3 1.12 204 Example 1 Polymerization 15.4 1.12 405
Example 2 Polymerization 9.6 1.18 227 Example 3 Polymerization 22.8
0.84 >>400 Reference Example 1 Polymerization 2.3 1.07 271
Example 4 Polymerization 4.4 1.07 225 Example 5 Polymerization 4.5
0.80 266 Reference Example 2
[0059] COMPARATIVE REFERENCE EXAMPLE
[0060] Into a stainless steel autoclave having an internal capacity
of 125 cm.sup.3, 61.6 g of PFAE and 34 mg of AIBN were introduced
and cooled with liquid nitrogen and deaerated. Then, TFE was
introduced at 75.degree. C. until the pressure became 0.30 MPaG,
and stirring was continued for 3 hours, whereby no decrease in
pressure was observed. Then, the temperature was raised to
80.degree. C. The pressure rose to 0.32 MPaG. Stirring was
continued in this state for 50 minutes, whereby no decrease in
pressure was observed, and therefore, the autoclave was cooled, and
the gas in the system was purged, whereupon the autoclave was
opened, and 124 mg of AIBN was added. After cooling with liquid
nitrogen and deaeration, tetrafluoroethylene was introduced again
at 75.degree. C. until the pressure became 0.30 MPaG. Stirring was
continued for 1.5 hours, but no decrease in pressure was observed.
Therefore, the temperature was raised again to 80.degree. C. The
pressure rose to 0.32 MPaG. Stirring was continued for 30 minutes,
but no decrease in pressure was observed. Accordingly, the
autoclave was cooled, and the gas in the system was purged to stop
the reaction. A part of the liquid in the autoclave was sampled,
and the monomers were distilled off under reduced pressure, whereby
no substantial solid content remained. Thus, by the hydrocarbon
type initiator, polymerization tends to hardly proceed when the
polymerization temperature becomes high.
[0061] Measurement of Physical Properties
[0062] Using the TFE/PFAE copolymer of Polymerization Example 1, a
film having a thickness of about 100 .mu.m was prepared by hot
press. It was hydrolyzed with a KOH aqueous solution containing
dimethyl sulfoxide and then immersed in hydrochloric acid to
convert it to an acid-form, followed by washing with deionized
water. The obtained membrane was immersed in deionized water of
90.degree. C., whereupon the water content was 40%. Further, with
respect to the membrane, the AC resistivity was 6.0
.OMEGA..multidot.cm as measured by a four-terminal method employing
a platinum wire at 80.degree. C. under a relative humidity of 95%.
As measured in the same manner, the water content of the TFE/PSVE-H
copolymer having A.sub.R of 1.1 meq/g was 60% and the AC
resistivity was 4.5 .OMEGA..multidot.cm.
[0063] With respect to the film of the TFE/PFAE copolymer converted
to an acid-form, the temperature dependency of the elastic modulus
was measured by means of a dynamic viscoelasticity measuring
apparatus (manufactured by IT Keisoku Seigyo K. K.) at a measuring
frequency of 1 Hz at a temperature-raising speed of from 2 to
3.degree. C./min. The results of the measurement are shown in FIG.
1 together with the results of the measurement with respect to the
TFE/PSVE-H film having AR of 1.1 meq/g. If the softening
temperature where the elastic modulus starts to decrease is
compared using the temperature where the elastic modulus becomes
1.times.10.sup.8 Pa, it is evident that the TFE/PFAE-H copolymer
has a softening temperature higher by at least 30.degree. C. than
the TFE/PSVE-H copolymer, and the TFE/PFAE-H copolymer is more
suitable for operation at a high temperature than the TFE/PSVE-H
copolymer.
[0064] Evaluation as a Polymer Electrolyte Fuel Cell
[0065] The TFE/PFAE copolymer having AR of 1.1 meq/g was hot
pressed to obtain a film having a thickness of 50 .mu.m. Then, this
film was immersed in a solution of
KOH/dimethylsulfoxide/water=15/30/55 (mass ratio) and subjected to
hydrolysis at 90.degree. C. Then, the film after the hydrolysis
treatment was subjected to treatment for conversion to an acid-form
by using 1 mol/L hydrochloric acid, followed by washing with water
and drying.
[0066] Then, using an ethanol solution of a copolymer (A.sub.R=1.1
meq/g) comprising repeating units based on PSVE-H and repeating
units based on TFE, a platinum-supporting carbon is mixed to the
solution so that the mass ratio of the copolymer to the
platinum-supporting carbon (the amount of platinum supported=40
mass %) would be 3:7, to obtain a coating fluid. The coating fluid
is coated on a carbon cloth to form a gas diffusion layer, to
obtain a gas diffusion electrode having a catalyst layer with an
amount of supported platinum of 0.4 mg/cm.sup.2 formed on the gas
diffusion layer.
[0067] Then, the above-mentioned film as a solid polymer
electrolyte membrane is interposed between two sheets of the
above-mentioned gas diffusion electrodes, followed by pressing by
means of a flat plate press machine and further by hot pressing to
obtain a membrane electrode assembly. On outside of this membrane
electrode assembly, a titanium separator having a gas passage
formed is disposed. Further, on outside thereof, a gas supply
compartment made of PTFE, and further on outside thereof, a heater
is disposed, whereby a polymer electrolyte fuel cell having an
effective membrane area of 10 cm.sup.2 is assembled.
[0068] While maintaining the temperature of the above polymer
electrolyte fuel cell at 80.degree. C., oxygen is supplied to the
cathode and hydrogen is supplied to the anode, respectively, under
atmospheric pressure while humidifying at 80.degree. C., and an
electric power is generated under such conditions. When the output
power density is 0.3A/cm.sup.2, a voltage between terminals of
about 0.67 V is obtained.
[0069] Then, as a Comparative Example, a polymer electrolyte fuel
cell having the same construction as the above fuel cell except
that as the solid polymer electrolyte membrane, Nafion 112
(A.sub.R=091 meq/g) having a membrane thickness of about 50 Um is
used, is prepared. An electric power is generated under the same
conditions as the above fuel cell, whereby when the output power
density is 0.3 A/cm.sup.2, a voltage between terminals of about
0.66 V is obtained.
INDUSTRIAL APPLICABILITY
[0070] According to the present invention, an electrolyte material
for a polymer electrolyte fuel cell which has ion conductivity and
durability equal to a conventional electrolyte material and which
can be produced at a low cost in a shorter process than ever, and a
polymer electrolyte fuel cell constituted by using it, can be
presented. Further, the electrolyte material of the present
invention has a higher softening temperature than the polymer
electrolyte material which is widely used in applications to fuel
cells and is more suitable for operation at a high temperature than
the conventional material.
* * * * *