U.S. patent application number 13/174664 was filed with the patent office on 2011-10-27 for electrolyte membrane for polymer electrolyte fuel cell, process for its production and membrane-electrode assembly for polymer electrolyte fuel cell.
This patent application is currently assigned to Asahi Glass Company, Limited. Invention is credited to Eiji ENDOH, Shinji Terazono.
Application Number | 20110262832 13/174664 |
Document ID | / |
Family ID | 35510028 |
Filed Date | 2011-10-27 |
United States Patent
Application |
20110262832 |
Kind Code |
A1 |
ENDOH; Eiji ; et
al. |
October 27, 2011 |
ELECTROLYTE MEMBRANE FOR POLYMER ELECTROLYTE FUEL CELL, PROCESS FOR
ITS PRODUCTION AND MEMBRANE-ELECTRODE ASSEMBLY FOR POLYMER
ELECTROLYTE FUEL CELL
Abstract
An electrolyte membrane which comprises a cation exchange
membrane made of a polymer having cation exchange groups and
contains cerium ions is used as an electrolyte membrane for a
polymer electrolyte fuel cell. In a case where the cation exchange
membrane has sulfonic acid groups, the sulfonic acid groups are
ion-exchanged, for example, with cerium ions so that cerium ions
are contained preferably in an amount of from 0.3 to 20% of
--SO.sub.3.sup.- groups contained in the cation exchange membrane.
A membrane for a polymer electrolyte fuel cell capable of power
generation in high energy efficiency, having high power generation
performance regardless of the dew point of the feed gas and capable
of stable power generation over a long period of time, can be
provided.
Inventors: |
ENDOH; Eiji; (Yokohama-shi,
JP) ; Terazono; Shinji; (Yokohama-shi, JP) |
Assignee: |
Asahi Glass Company,
Limited
|
Family ID: |
35510028 |
Appl. No.: |
13/174664 |
Filed: |
June 30, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11615256 |
Dec 22, 2006 |
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13174664 |
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PCT/JP05/11466 |
Jun 22, 2005 |
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11615256 |
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Current U.S.
Class: |
429/480 ;
429/492; 429/493; 429/494; 521/27 |
Current CPC
Class: |
H01B 1/122 20130101;
C08J 2327/18 20130101; C08J 5/2237 20130101; H01M 8/1046 20130101;
H01M 8/1053 20130101; H01M 8/1088 20130101; H01M 8/1039 20130101;
Y02P 70/50 20151101; H01M 8/1093 20130101; H01M 8/1051 20130101;
H01M 8/1025 20130101; H01M 2300/0082 20130101; Y02E 60/50 20130101;
H01M 6/183 20130101; H01M 8/1081 20130101; H01M 2008/1095 20130101;
H01M 8/109 20130101; H01M 8/1023 20130101; H01M 2300/0094
20130101 |
Class at
Publication: |
429/480 ;
429/492; 429/493; 429/494; 521/27 |
International
Class: |
H01M 8/10 20060101
H01M008/10; C08J 5/22 20060101 C08J005/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2004 |
JP |
2004-183712 |
Aug 2, 2004 |
JP |
2004-225706 |
Sep 13, 2004 |
JP |
2004-265176 |
Apr 15, 2005 |
JP |
2005-118412 |
Claims
1. An electrolyte membrane for a polymer electrolyte fuel cell,
which comprises a cation exchange membrane made of a polymer having
cation exchange groups, characterized by containing cerium
ions.
2. An electrolyte membrane for a polymer electrolyte fuel cell,
which comprises a cation exchange membrane made of a polymer having
cation exchange groups, characterized in that some of the cation
exchange groups are ion-exchanged with cerium ions.
3. An electrolyte membrane for a polymer electrolyte fuel cell,
which comprises a cation exchange membrane having at least two
layers made of a polymer having cation exchange groups laminated,
characterized in that at least one of the at least two layers
contains cerium ions.
4. An electrolyte membrane for a polymer electrolyte fuel cell,
which comprises a cation exchange membrane having at least two
layers made of a polymer having cation exchange groups laminated,
characterized in that at least one of the at least two layers is a
cation exchange membrane in which at least some of the cation
exchange groups are ion-exchanged with cerium ions.
5. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 1, wherein the polymer having cation exchange
groups is a polymer having sulfonic acid groups.
6. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 5, wherein cerium ions are contained in an
amount of from 0.3 to 200 of the number of --SO.sub.3.sup.- groups
contained in the cation exchange membrane.
7. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 5, wherein the polymer having sulfonic acid
groups is a perfluorocarbon polymer having sulfonic acid
groups.
8. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 7, wherein the perfluorocarbon polymer is a
copolymer containing polymerized units based on a perfluorovinyl
compound represented by
CF.sub.2.dbd.CF--(OCF.sub.2CFX).sub.m--O.sub.p--(CF.sub.2).sub.n--SO.sub.-
3H (wherein m is an integer of from 0 to 3, n is an integer of from
1 to 12, p is 0 or 1, and X is a fluorine atom or a trifluoromethyl
group) and polymerized units based on tetrafluoroethylene.
9. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 5, wherein the polymer having sulfonic acid
groups has such a structure that it has an aromatic ring in the
main chain of the polymer or in the main chain and side chains, and
that sulfonic acid groups are introduced to the aromatic ring, and
has an ion exchange capacity of from 0.8 to 3.0 meq/g dry
polymer.
10. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 1, which is a reinforced electrolyte
membrane.
11. A process for producing the electrolyte membrane for a polymer
electrolyte fuel cell as defined in claim 1, which comprises
immersing a cation exchange membrane made of a polymer having
cation exchange groups in an aqueous solution containing cerium
ions.
12. The process for producing the electrolyte membrane according to
claim 11, wherein the aqueous solution containing cerium ions is an
aqueous cerium nitrate solution or an aqueous cerium sulfate
solution.
13. A membrane-electrode assembly for a polymer electrolyte fuel
cell, which comprises an anode and a cathode each having a catalyst
layer containing a catalyst and an ion exchange resin, and an
electrolyte membrane disposed between the anode and the cathode,
characterized in that the electrolyte membrane is the electrolyte
membrane as defined in claim 1.
14. A membrane-electrode assembly for a polymer electrolyte fuel
cell, which comprises an anode and a cathode each having a catalyst
layer containing a catalyst and an ion exchange resin, and an
electrolyte membrane disposed between the anode and the cathode,
characterized in that the ion exchange resin contained in at least
one of the anode and the cathode contains cerium ions.
15. The membrane-electrode assembly for a polymer electrolyte fuel
cell according to claim 13, wherein the ion exchange resin
contained in at least one of the anode and the cathode contains
cerium ions.
16. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 2, wherein the polymer having cation exchange
groups is a polymer having sulfonic acid groups.
17. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 16, wherein cerium ions are contained in an
amount of from 0.3 to 20% of the number of --SO.sub.3.sup.- groups
contained in the cation exchange membrane.
18. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 17, wherein the polymer having sulfonic acid
groups is a perfluorocarbon polymer having sulfonic acid
groups.
19. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 3, wherein the polymer having cation exchange
groups is a polymer having sulfonic acid groups, and cerium ions
are contained in an amount of from 0.3 to 20% of the number of
--SO.sub.3.sup.- groups contained in the cation exchange
membrane.
20. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 19, wherein the polymer having sulfonic acid
groups is a perfluorocarbon polymer having sulfonic acid
groups.
21. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 4, wherein the polymer having cation exchange
groups is a polymer having sulfonic acid groups, and cerium ions
are contained in an amount of from 0.3 to 20% of the number of
--SO.sub.3 groups contained in the cation exchange membrane.
22. The electrolyte membrane for a polymer electrolyte fuel cell
according to claim 21, wherein the polymer having sulfonic acid
groups is a perfluorocarbon polymer having sulfonic acid
groups.
23. A membrane-electrode assembly for a polymer electrolyte fuel
cell, which comprises an anode and a cathode each having a catalyst
layer containing a catalyst and an ion exchange resin, and an
electrolyte membrane disposed between the anode and the cathode,
characterized in that the electrolyte membrane is the electrolyte
membrane as defined in claim 2.
24. The membrane-electrode assembly for a polymer electrolyte fuel
cell according to claim 23, wherein the ion exchange resin
contained in at least one of the anode and the cathode contains
cerium ions.
25. A membrane-electrode assembly for a polymer electrolyte fuel
cell, which comprises an anode and a cathode each having a catalyst
layer containing a catalyst and an ion exchange resin, and an
electrolyte membrane disposed between the anode and the cathode,
characterized in that the electrolyte membrane is the electrolyte
membrane as defined in claim 3.
26. The membrane-electrode assembly for a polymer electrolyte fuel
cell according to claim 25, wherein the ion exchange resin
contained in at least one of the anode and the cathode contains
cerium ions.
Description
TECHNICAL FIELD
[0001] The present invention relates to an electrolyte membrane for
a polymer electrolyte fuel cell, whereby the initial output voltage
is high, and the high output voltage can be obtained over a long
period of time.
BACKGROUND ART
[0002] A fuel cell is a cell whereby a reaction energy of a gas as
a feed material is converted directly to electric energy, and a
hydrogen-oxygen fuel cell presents no substantial effect to the
global environment since its reaction product is only water in
principle. Especially, a polymer electrolyte fuel cell employing a
polymer membrane as an electrolyte, can be operated at room
temperature to provide a high power density, as a polymer
electrolyte membrane having high ion conductivity has been
developed, and thus is expected to be a prospective power source
for mobile vehicles such as electric cars or for small cogeneration
systems, along with an increasing social demand for an energy or
global environmental problem in recent years.
[0003] In a polymer electrolyte fuel cell, a proton conductive ion
exchange membrane is commonly employed as a polymer electrolyte,
and an ion exchange membrane made of a perfluorocarbon polymer
having sulfonic acid groups, is particularly excellent in the basic
properties. In the polymer electrolyte fuel cell, gas diffusion
type electrode layers are disposed on both sides of the ion
exchange membrane, and power generation is carried out by supplying
a gas containing hydrogen as a fuel and a gas (such as air)
containing oxygen as an oxidizing agent to the anode and the
cathode, respectively.
[0004] In the reduction reaction of oxygen at the cathode of the
polymer electrolyte fuel cell, the reaction proceeds via hydrogen
peroxide (H.sub.2O.sub.2), and it is worried that the electrolyte
membrane may be deteriorated by the hydrogen peroxide or peroxide
radicals to be formed in the catalyst layer. Further, to the anode,
oxygen molecules will come from the cathode through the membrane,
and it is worried that hydrogen peroxide or peroxide radicals may
be formed at the anode too. Especially when a hydrocarbon membrane
is used as the polymer electrolyte membrane, it is poor in the
stability against radicals, which used to be a serious problem in
an operation for a long period of time.
[0005] For example, the first practical use of a polymer
electrolyte fuel cell was when it was adopted as a power source for
a Gemini space ship in U.S.A., and at that time, a membrane having
a styrene/divinylbenzene polymer sulfonated, was used as an
electrolyte membrane, but it had a problem in the durability over a
long period of time. As a technique to overcome such problems, a
method of having a compound with a phenolic hydroxyl group or a
transition metal oxide capable of catalytically decomposing
hydrogen peroxide incorporated to the polymer electrolyte membrane
(see Patent Document 1) or a method of supporting catalytic metal
particles in the polymer electrolyte membrane to decompose hydrogen
peroxide (see Patent Document 2) is also known. However, such a
technique is a technique of decomposing formed hydrogen peroxide,
and is not one attempted to suppress decomposition of the ion
exchange membrane itself. Accordingly, although at the initial
stage, the effect for improvement was observed, there was a
possibility that a serious problem would result in the durability
over a long period of time. Further, there was a problem that the
cost tended to be high.
[0006] As opposed to such a hydrocarbon type polymer, an ion
exchange membrane made of a perfluorocarbon polymer having sulfonic
acid groups as a polymer remarkably excellent in the stability
against radicals, has been known. In recent years, a polymer
electrolyte fuel cell employing an ion exchange membrane made of
such a perfluorocarbon polymer is expected as a power source for
e.g. automobiles or housing markets, and a demand for its practical
use is increasing, and its developments are accelerated. In such
applications, its operation with particularly high efficiency is
required. Accordingly, its operation at higher voltage is desired,
and at the same time, cost reduction is desired. Further, from the
viewpoint of the efficiency of the entire fuel cell system, an
operation under low or no humidification is required in many
cases.
[0007] However, it has been reported that even with a fuel cell
employing an ion exchange membrane made of a perfluorocarbon
polymer having sulfonic acid groups, the stability is very high in
operation under high humidification, but the voltage degradation is
significant in operation under low or no humidification conditions
(see Non-Patent Document 1). Namely, it is considered that, also in
the case of the ion exchange membrane made of a perfluorocarbon
polymer having sulfonic acid groups, deterioration of the
electrolyte membrane proceeds due to hydrogen peroxide or peroxide
radicals in operation under low or no humidification. [0008] Patent
Document 1: JP-A-2001-118591 [0009] Patent Document 2:
JP-A-6-103992 [0010] Non-Patent Document 1: Summary of debrief
session for polymer electrolyte fuel cells research and development
achievement in 2000 sponsored by New Energy and Industrial
Technology Development Organization, page 56, lines 16 to 24
DISCLOSURE OF THE INVENTION
Object to be Accomplished by the Invention
[0011] Accordingly, for the practical application of a polymer
electrolyte fuel cell to e.g. vehicles or housing markets, it is an
object of the present invention to provide a membrane for a polymer
electrolyte fuel cell, whereby power generation with sufficiently
high energy efficiency is possible, high power generation property
is achieved, and stable power generation is possible over a long
period of time, either in its operation under low or no
humidification where the humidification temperature (dew point) of
the feed gas is lower than the cell temperature or in its operation
under high humidification where humidification is carried out at a
temperature close to the cell temperature.
Means to Achieve the Object
[0012] The present inventors have conducted extensive studies on
fuel cells employing an ion exchange membrane made of a polymer
having cation exchange groups, for the purpose of preventing
deterioration of the membrane in operation under low or no
humidification, and as a result, they have found that deterioration
of the electrolyte membrane can be remarkably suppressed by
incorporating specific ions into the membrane, and accomplished the
invention.
[0013] The present invention provides an electrolyte membrane for a
polymer electrolyte fuel cell, which comprises a cation exchange
membrane made of a polymer having cation exchange groups,
characterized by containing cerium ions. Here, the cerium ions may
be trivalent or tetravalent, but the valence is not particularly
limited in the present invention.
[0014] Further, the present invention provides an electrolyte
membrane for a polymer electrolyte fuel cell, which comprises a
cation exchange membrane having at least two layers made of a
polymer having cation exchange groups laminated, characterized in
that at least one of the at least two layers contains cerium
ions.
[0015] The cerium ions may be present in any state in the
electrolyte membrane so long as they are present as ions, and as
one embodiment, they may be present in such a state that some of
the cation exchange groups in the cation exchange membrane are
ion-exchanged with cerium ions. Thus, the present invention further
provides an electrolyte membrane for a polymer electrolyte fuel
cell, which comprises a cation exchange membrane made of a polymer
having cation exchange groups, characterized in that some of the
cation exchange groups are ion-exchanged with cerium ions, and an
electrolyte membrane for a polymer electrolyte fuel cell, which
comprises a cation exchange membrane having at least two layers
made of a polymer having cation exchange groups laminated,
characterized in that at least one of the at least two layers is a
cation exchange membrane in which at least some of the cation
exchange groups are ion-exchanged with cerium ions.
[0016] The electrolyte membrane of the present invention does not
necessarily uniformly contain cerium ions. It may be a cation
exchange membrane (laminated membrane) comprising at least two
layers, wherein some of the cation exchange groups are
ion-exchanged with cerium ions in at least one layer, not in all
the layers, i.e. the electrolyte membrane may contain cerium ions
non-uniformly in the thickness direction. Therefore, in a case
where it is required to increase durability against hydrogen
peroxide or peroxide radicals particularly on the anode side, it is
possible to employ an ion exchange membrane containing cerium ions
only for the layer closest to the anode.
[0017] In the present invention, the polymer having cation exchange
groups is preferably a polymer having sulfonic acid groups.
[0018] Further, the present invention provides a process for
producing an electrolyte membrane for a polymer electrolyte fuel
cell, which comprises immersing a cation exchange membrane made of
a polymer having cation exchange groups in an aqueous solution
containing cerium ions.
[0019] Further, the present invention provides a membrane-electrode
assembly for a polymer electrolyte fuel cell, which comprises an
anode and a cathode each having a catalyst layer containing a
catalyst and an ion exchange resin, and an electrolyte membrane
disposed between the anode and the cathode, characterized in that
the electrolyte membrane is the above-described electrolyte
membrane.
[0020] Still further, the present invention provides a
membrane-electrode assembly for a polymer electrolyte fuel cell,
which comprises an anode and a cathode each having a catalyst layer
containing a catalyst and an ion exchange resin, and an electrolyte
membrane disposed between the anode and the cathode, characterized
in that the ion exchange resin contained in at least one of the
anode and the cathode contains cerium ions.
Effects of the Invention
[0021] The electrolyte membrane obtained by the present invention
has excellent resistance to hydrogen peroxide or peroxide radicals.
The reason is not clear yet, but it is estimated as follows. By
incorporation of cerium ions in the electrolyte membrane,
particularly by ion-exchange of some of cation exchange groups with
cerium ions, the interaction between the cerium ions and a residue
after dissociation of protons from the cation exchange groups (such
as --SO.sub.3.sup.-) effectively improves the resistance of the
electrolyte membrane to hydrogen peroxide or peroxide radicals.
[0022] Since the electrolyte membrane of the present invention has
excellent resistance to hydrogen peroxide or peroxide radicals, a
polymer electrolyte fuel cell provided with a membrane-electrode
assembly having the electrolyte membrane of the present invention
is excellent in durability and capable of generating the electric
power stably over a long period of time.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023] In the present invention, the polymer having cation exchange
groups before incorporation of cerium ions is not particularly
limited so long as it has a function to produce protons by
dissociation of the cation exchange groups. Specific examples of
the cation exchange group include a sulfonic acid group, a
sulfonimide group, a phosphonic acid group, a carboxylic acid group
and a ketimide group, among which a sulfonic acid group with a
strong acidity and high chemical stability is particularly
preferred. The present invention will be described below with
reference to a polymer having sulfonic acid groups as an
example.
[0024] The method of incorporating cerium ions into a polymer
having sulfonic acid groups to obtain the electrolyte membrane of
the present invention is not particularly limited, and the
following methods may, for example, be mentioned. (1) A method of
immersing a membrane made of a polymer having sulfonic acid groups
in a solution containing cerium ions. (2) A method of adding a salt
containing a cerium ion to a dispersion of a polymer having
sulfonic acid groups to incorporate cerium ions in the dispersion,
or mixing a solution containing cerium ions and a dispersion of a
polymer having sulfonic acid groups to incorporate cerium ions, and
forming a membrane employing the obtained liquid by e.g. cast
coating. (3) A method of bringing an organic metal complex salt of
cerium into contact with a cation exchange membrane made of a
polymer having sulfonic acid groups to incorporate cerium ions.
[0025] In the electrolyte membrane obtained by the above method,
some of sulfonic acid groups are considered to be ion-exchanged
with cerium ions.
[0026] The cerium ions may be trivalent or tetravalent, and various
cerium salts are used to obtain a solution containing cerium ions.
Specific examples of a salt containing trivalent cerium ion include
cerium(III) acetate (Ce(CH.sub.3COO).sub.3.H.sub.2O), cerium(III)
chloride (CeCl.sub.3.6H.sub.2O), cerium (III) nitrate
(Ce(NO.sub.3).sub.3.6H.sub.2O), cerium(III) sulfate
(Ce.sub.2(SO.sub.4).sub.3.8H.sub.2O) and cerium(III) carbonate
(Ce.sub.2(CO.sub.3).sub.3.8H.sub.2O). Specific examples of a salt
containing tetravalent cerium ion include cerium(IV) sulfate
(Ce(SO.sub.4).sub.2.4H.sub.2O), cerium(IV) diammonium sulfate
(Ce(NH.sub.4).sub.2(NO.sub.3).sub.6) and cerium(IV) tetraammonium
sulfate (Ce(NH.sub.4).sub.4(SO.sub.4).sub.4.4H.sub.2O). In
addition, examples of an organic metal complex salt of cerium
include cerium(III) acetylacetonate
(Ce(CH.sub.3COCHCOCH.sub.3).sub.3.3H.sub.2O). Among them, cerium
nitrate and cerium sulfate, which are water soluble and easily
handled, are preferred. Further, they are preferred since when the
polymer having sulfonic acid groups is subjected to ion exchange by
an aqueous solution of either of them, the formed nitric acid or
sulfuric acid is easily dissolved in the aqueous solution and
removed.
[0027] In a case where cerium ions are trivalent for example, when
sulfonic acid groups are ion-exchanged with cerium ions, Ce.sup.3+
is bonded to three --SO.sub.3.sup.-, as shown below.
##STR00001##
[0028] In the present invention, the number of cerium ions
contained in the electrolyte membrane is preferably from 0.3 to 20%
of the number of --SO.sub.3.sup.- groups in the membrane
(hereinafter this ratio will be referred to as the "content of
cerium ions"). In a case where a cerium ion completely has the
above structure, the above content is the same as the content of
sulfonic acid groups ion-exchanged with a cerium ion of from 0.9 to
60% of the total amount of sulfonic acid groups and the sulfonic
acid groups ion-exchanged with a cerium ion (hereinafter this ratio
will be referred to as the "substitution ratio"). The content of
cerium ions is more preferably from 0.7 to 16%, furthermore
preferably from 1 to 13%, still more preferably from 1.5 to 12%,
especially preferably from 1 to 10%. In terms of the above
substitution ratio, it is preferably from 1 to 60%, more preferably
from 2 to 50%, furthermore preferably from 3 to 40%, still further
preferably from 5 to 30%.
[0029] If the content of cerium ions is lower than this range, no
adequate stability against hydrogen peroxide or peroxide radicals
may be secured. On the other hand, if the content of cerium ions is
higher than this range, no adequate conductivity of hydrogen ions
may be secured, whereby the membrane resistance may increase to
lower the power generation property.
[0030] Here, in a case where the electrolyte membrane of the
present invention is a laminated membrane, only the proportion of
cerium ions to the --SO.sub.3.sup.- groups of the entire
electrolyte membrane has to be within the above range, and the
content of cerium ions of the layer containing cerium ions itself
may be higher than the above range. Further, a method for preparing
the laminated membrane is not particularly limited, although it is
preferred to prepare a cation exchange membrane containing cerium
ions by any one of the above methods (1) to (3) and then laminate
it with a cation exchange membrane containing no cerium ions.
[0031] Further, when the preferred range of the content of cerium
ions is represented by the proportion to the mass of the
electrolyte membrane, the mass of cerium to the mass of the entire
electrolyte membrane is preferably from 0.02 to 8%, more preferably
from 0.05 to 6.6%, furthermore preferably from 0.07 to 5.3%.
[0032] In the present invention, the polymer having sulfonic acid
groups before incorporation of cerium ions is not particularly
limited, but its ion exchange capacity is preferably from 0.5 to
3.0 meq/g dry polymer, more preferably from 0.7 to 2.5 meq/g dry
polymer, particularly preferably from 1.0 to 2.5 meq/g dry polymer.
If the ion exchange capacity is too low, no satisfactory
conductivity of hydrogen ions will be secured when the sulfonic
acid groups are ion-exchanged with cerium ions, whereby the
membrane resistance will increase to lower the powder generation
property. On the other hand, if the ion exchange capacity is too
high, the water resistance or the strength of the membrane may
decrease. Further, the polymer is preferably a fluoropolymer from
the viewpoint of durability, particularly preferably a
perfluorocarbon polymer having sulfonic acid groups (which may
contain etheric oxygen atom). The perfluorocarbon polymer is not
particularly limited, but is preferably a copolymer containing
polymerized units based on a perfluorovinyl compound represented by
CF.sub.2.dbd.CF--(OCF.sub.2CFX).sub.m--O.sub.p--(CF.sub.2).sub.n--SO.sub.-
3H (wherein m is an integer of from 0 to 3, n is an integer of from
1 to 12, p is 0 or 1, and X is a fluorine atom or a trifluoromethyl
group) and polymerized units based on tetrafluoroethylene.
[0033] Specific preferred examples of the perfluorovinyl compound
include compounds represented by the following formulae (I) to
(iii). In the following formulae, q is an integer of from 1 to 8, r
is an integer of from 1 to 8, and t is an integer of from 1 to
3.
CF.sub.2.dbd.CFO(CF.sub.2).sub.q--SO.sub.3H (i)
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub.r--SO.sub.3H
(ii)
CF.sub.2.dbd.CF(OCF.sub.2CF(CF.sub.3)).sub.tO(CF.sub.2).sub.2--SO.sub.3H
(iii)
[0034] In a case where a perfluorocarbon polymer having sulfonic
acid groups is used, one obtained by fluorination treatment after
polymerization and thereby having terminals of the polymer
fluorinated may be used. When the terminals of the polymer are
fluorinated, more excellent stability against hydrogen peroxide and
peroxide radicals will be achieved, whereby the durability will
improve.
[0035] Further, the polymer having sulfonic acid groups before
incorporation of cerium ions may be one other than a
perfluorocarbon polymer having sulfonic acid groups. For example, a
polymer having such a structure that it has an aromatic ring in the
main chain of the polymer or in the main chain and side chains, and
that sulfonic acid groups are introduced to the aromatic ring, and
having an ion exchange capacity of from 0.8 to 3.0 meq/g dry
polymer, may be preferably used. Specifically, the following
polymers may, for example, be used.
[0036] Sulfonated polyarylene, sulfonated polybenzoxazole,
sulfonated polybenzothiazole, sulfonated polybenzoimidazole,
sulfonated polysulfone, sulfonated polyether sulfone, sulfonated
polyether ether sulfone, sulfonated polyphenylene sulfone,
sulfonated polyphenylene oxide, sulfonated polyphenylene sulfoxide,
sulfonated polyphenylene sulfide, sulfonated polyphenylene sulfide
sulfone, sulfonated polyether ketone, sulfonated polyether ether
ketone, sulfonated polyether ketone ketone, sulfonated polyimide,
and so on.
[0037] The polymer electrolyte fuel cell provided with the
electrolyte membrane of the present invention has, for example, the
following structure. Namely, the cell is provided with
membrane-electrode assemblies, each of which comprises an anode and
a cathode each having a catalyst layer containing a catalyst and an
ion exchange resin, disposed on both sides of the electrolyte
membrane of the present invention. The anode and the cathode of the
membrane-electrode assembly preferably have a gas diffusion layer
made of carbon cloth, carbon paper, or the like disposed outside
the catalyst layer (opposite to the membrane). Separators having
grooves formed to constitute flow paths for a fuel gas or an
oxidizing agent gas are disposed on both sides of each
membrane-electrode assembly. A plurality of membrane-electrode
assemblies are stacked with the separators to form a stack, and a
hydrogen gas is supplied to the anode side and an oxygen gas or air
to the cathode side. A reaction of H.sub.2.fwdarw.2H.sup.++2e.sup.-
takes place on the anodes, and a reaction of
1/2O.sub.2+2H.sup.++2e.sup.-.fwdarw.H.sub.2O on the cathodes,
whereby chemical energy is converted into electric energy.
[0038] Furthermore, the electrolyte membrane of the present
invention is also applicable to direct methanol fuel cells in which
methanol is supplied instead of the fuel gas to the anode side.
[0039] The above-mentioned catalyst layer may be obtained in
accordance with conventional methods, for example, as follows.
First, a conductive carbon black powder carrying particles of a
platinum catalyst or a platinum alloy catalyst, is mixed with a
solution of a perfluorocarbon polymer having sulfonic acid groups
to obtain a uniform dispersion liquid, and gas diffusion electrodes
are formed, for example, by any one of the following methods,
thereby obtaining a membrane-electrode assembly.
[0040] The first method is a method of coating the both surfaces of
the electrolyte membrane with the above-mentioned dispersion
liquid, drying it, and then attaching two sheets of carbon cloth or
carbon paper closely onto the both sides. The second method is a
method of applying the above-mentioned dispersion liquid onto two
sheets of carbon cloth or carbon paper, drying it, and then placing
the two sheets on both sides of the above ion-exchange membrane so
that the surfaces coated with the dispersion liquid is close in
contact with the ion-exchange membrane. The carbon cloth or carbon
paper herein functions as gas diffusion layers to more uniformly
diffuse the gas to the catalyst-containing layers, and functions as
current collectors. Furthermore, another available method is such
that a substrate separately prepared is coated with the
above-mentioned dispersion liquid to make a catalyst layer, such
catalyst layers are bonded to an electrolyte membrane by a method
such as transcription, then the substrate is peeled off, and the
electrolyte membrane is sandwiched between the above-mentioned gas
diffusion layers.
[0041] There are no particular restrictions on the ion-exchange
resin contained in the catalyst layer, and it is preferably a
polymer having sulfonic acid groups, more preferably a
perfluorocarbon polymer having sulfonic acid groups. The
ion-exchange resin in the catalyst layer may contain cerium ions
just like the electrolyte membrane of the present invention. Such
an ion-exchange resin containing cerium ions can be applied to both
anodes and cathodes, and decomposition of the resin can be
effectively suppressed, so as to further enhance the durability of
the polymer electrolyte fuel cell. Further, an ion-exchange resin
containing no cerium ions may be used as the electrolyte membrane
so that cerium ions are incorporated only in the ion-exchange resin
in the catalyst layer.
[0042] In a case where it is desired to incorporate cerium ions
into both the ion exchange resin in the catalyst layer and the
electrolyte membrane, it is possible, for example, to preliminarily
prepare an assembly of a catalyst layer and an electrolyte
membrane, and to immerse the assembly into a solution containing
cerium ions. Further, in a case where cerium ions are to be
contained in the catalyst layer, it is possible to form the
catalyst layer by the above method employing, as a coating liquid,
one having a catalyst dispersed in a dispersion containing cerium
ions and a polymer having sulfonic acid groups. In this case,
cerium ions may be contained in either one of the cathode and the
anode, or cerium ions may be contained in both the cathode and the
anode. Here, the cathode and the anode may be made by using
dispersions differing in the content of cerium ions so that the
cathode and the anode have different contents of cerium ions. From
the viewpoint of improvement in the durability, more preferably,
the anode contains from 10 to 30 mol % of cerium ions and the
cathode contains from 3 to 10 mol % of cerium ions, relative to the
--SO.sub.3.sup.- groups contained in the polymer having sulfonic
acid groups, whereby decomposition of the ion exchange resin in the
catalyst layer can be effectively suppressed.
[0043] The electrolyte membrane of the present invention may be a
membrane made of only a polymer having sulfonic acid groups, some
of which are replaced by cerium ions, but it may contain another
component, or it may be a membrane reinforced by e.g. fibers, woven
cloth, non-woven cloth or a porous material of another resin such
as a polytetrafluoroethylene or a perfluoroalkyl ether. Even in the
case of a reinforced membrane, the electrolyte membrane of the
present invention can be obtained by immersing a reinforced cation
exchange membrane having sulfonic acid groups in a solution
containing cerium ions. Further, a method of preparing a membrane
by using a dispersion containing a polymer ion-exchanged with
cerium ions may also be applicable. In a case where the electrolyte
membrane is reinforced, the whole membrane may be reinforced, or
the circumference of the membrane may be reinforced in a frame-like
shape with a film, a sheet or the like. If the membrane is
reinforced in a frame-like shape, the strength around the
circumference will increase whereby to improve handling efficiency.
The whole membrane may be reinforced with a reinforcing material
having a high percentage of void and only the circumference may be
reinforced with a reinforcing material having a low percentage of
void or having no void.
[0044] The polymer electrolyte fuel cell provided with the
membrane-electrode assembly of the present invention is excellent
in the durability even at high temperature, whereby it can operate
at 100.degree. C. or higher to generate the electric power. In a
case where the fuel gas is hydrogen obtained by reforming methanol,
natural gas, gasoline or the like, if carbon monoxide is contained
even in a trace amount, the electrolyte catalyst will be poisoned,
and the output of the fuel cell tends to be low. When the operation
temperature is at least 100.degree. C., it is possible to suppress
the poisoning. The operation temperature is more preferably at
least 120.degree. C., whereby the effect of suppressing the
poisoning tends to be high.
EXAMPLES
[0045] Now, the present invention will be described in further
detail with reference to Examples (Examples 1 to 5, 10, 12 to 15)
and Comparative Examples (Examples 6 to 9 and 11). However, it
should be understood that the present invention is by no means
restricted to such specific Examples.
Example 1
[0046] As a polymer electrolyte membrane, an ion exchange membrane
having a thickness of 50 .mu.m, made of a perfluorocarbon polymer
having sulfonic acid groups (Flemion, trade name, manufactured by
Asahi Glass Company, Limited, ion exchange capacity: 1.1 meq/g dry
polymer) in a size of 5 cm.times.5 cm (area 25 cm.sup.2) was used.
The weight of the entire membrane, after being left to stand in dry
nitrogen for 16 hours, was measured in dry nitrogen and found to be
0.251 g. The amount of sulfonic acid groups in this membrane is
obtained from the following formula:
0.251 (g).times.1.1 (meq/g dry polymer)=0.276 (meq)
[0047] Then, 12.0 mg of cerium nitrate
(Ce(NO.sub.3).sub.3.6H.sub.2O) was dissolved in 500 mL of distilled
water so that cerium ions (trivalent) in an amount corresponding to
30% of the amount (equivalent) of sulfonic acid groups in this
membrane were contained, and the above ion exchange membrane was
immersed in the solution, followed by stirring by a stirrer at room
temperature for 40 hours to incorporate cerium ions into the ion
exchange membrane. The cerium nitrate solution was analyzed by
inductively-coupled plasma (ICP) emission spectrometry before and
after the immersion and as a result, the content of cerium ions in
the ion exchange membrane (the proportion of cerium ions to the
number of --SO.sub.3.sup.- groups in the membrane) was found to be
9.3%.
[0048] Then, 5.1 g of distilled water was mixed with 1.0 g of a
catalyst powder (manufactured by N.E. CHEMCAT CORPORATION) in which
platinum was supported on a carbon carrier (specific surface area:
800 m.sup.2/g) so as to be contained in an amount of 50% of the
whole mass of the catalyst. With this liquid mixture, 5.6 g of a
liquid having a
CF.sub.2.dbd.CF.sub.2/CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub-
.2SO.sub.3H copolymer (ion exchange capacity: 1.1 meq/g dry
polymer) dispersed in ethanol and having a solid content
concentration of 9 mass % was mixed. This mixture was homogenized
by using a homogenizer (Polytron, trade name, manufactured by
Kinematica Company) to obtain a coating fluid for forming a
catalyst layer.
[0049] This coating fluid was applied by a bar coater on a
substrate film made of polypropylene and then dried for 30 minutes
in a dryer at 80.degree. C. to obtain a catalyst layer. Here, the
mass of the substrate film alone before formation of the catalyst
layer and the mass of the substrate film after formation of the
catalyst layer were measured to determine the amount of platinum
per unit area contained in the catalyst layer, whereupon it was 0.5
mg/cm.sup.2.
[0050] Then, using the above ion exchange membrane having cerium
ions incorporated, catalyst layers formed on the substrate film
were disposed on both sides of the membrane and transferred by hot
press method to obtain a membrane-catalyst layer assembly having an
anode catalyst layer and a cathode catalyst layer bonded to both
sides of the ion exchange membrane. The electrode area was 16
cm.sup.2.
[0051] This membrane-catalyst layer assembly was interposed between
two gas diffusion layers made of carbon cloth having a thickness of
350 .mu.m to prepare a membrane-electrode assembly, which was
assembled into a cell for power generation, and an open circuit
voltage test (OCV test) was carried out as an accelerated test. In
the test, hydrogen (utilization ratio: 70%) and air (utilization
ratio: 40%) corresponding to a current density of 0.2 A/cm.sup.2
were supplied under ordinary pressure to the anode and to the
cathode, respectively, the cell temperature was set at 90.degree.
C., the dew point of the anode gas was set at 60.degree. C. and the
dew point of the cathode gas was set at 60.degree. C., the cell was
operated for 100 hours in an open circuit state without generation
of electric power, and a voltage change was measured during the
period. Furthermore, by supplying hydrogen to the anode and
nitrogen to the cathode, amounts of hydrogen gas having leaked from
the anode to the cathode through the membrane were analyzed before
and after the test, thereby to check the degree of degradation of
the membrane. The results are shown in Table 1.
[0052] Then, a membrane-electrode assembly was prepared and
assembled into a cell for power generation in the same manner as
above, and a durability test under operation conditions under low
humidification was carried out. The test conditions were as
follows. Hydrogen (utilization ratio: 70%)/air (utilization ratio:
40%) was supplied under ordinary pressure at a cell temperature at
80.degree. C. and at a current density of 0.2 A/cm.sup.2, and the
polymer electrolyte fuel cell was evaluated as to the initial
property and durability. Hydrogen and air were so humidified and
supplied into the cell that the dew point on the anode side was
80.degree. C. and that the dew point on the cathode side was
50.degree. C., respectively, whereupon the cell voltage at the
initial stage of the operation and the relation between the elapsed
time after the initiation of the operation and the cell voltage
were measured. The results are shown in Table 2. In addition, the
cell voltage at the initial state of the operation and the relation
between the elapsed time after the initiation of the operation and
the cell voltage were also measured in the same manner as above
under the above cell evaluation conditions except that the dew
point on the cathode side was changed to 80.degree. C. The results
are shown in Table 3.
Example 2
[0053] In the same manner as in Example 1 except that an aqueous
solution having 9.8 mg of cerium sulfate
(Ce.sub.2(SO.sub.4).sub.3.8H.sub.2O) containing cerium ions
(trivalent) dissolved in 500 mL of distilled water is used instead
of the cerium nitrate aqueous solution used in Example 1, the same
commercially available ion exchange membrane used in Example 1 is
treated to obtain a membrane having a content of cerium ions of
9.3%. Using this membrane, in the same manner as in Example 1, a
membrane-catalyst layer assembly is obtained and then a
membrane-electrode assembly is obtained. The membrane-electrode
assembly is evaluated in the same manner as in Example 1, whereupon
results shown in Tables 1 to 3 are obtained.
Example 3
[0054] In the same manner as in Example 1 except that an aqueous
solution having 8.0 mg of cerium nitrate
(Ce(NO.sub.3).sub.3.6H.sub.2O) dissolved in 500 mL of distilled
water was used instead of the cerium nitrate aqueous solution used
in Example 1, the same commercially available ion exchange membrane
used in Example 1 was treated to obtain a membrane having a content
of cerium ions of 6.3%. Using this membrane, in the same manner as
in Example 1, a membrane-catalyst layer assembly was obtained and
then a membrane-electrode assembly was obtained. The
membrane-electrode assembly was evaluated in the same manner as in
Example 1, whereupon results shown in Tables 1 to 3 are
obtained.
Example 4
[0055] In the same manner as in Example 1 except that an aqueous
solution having 4.0 mg of cerium nitrate
(Ce(NO.sub.3).sub.3.6H.sub.2O) dissolved in 500 mL of distilled
water is used instead of the cerium nitrate aqueous solution used
in Example 1, the same commercially available ion exchange membrane
used in Example 1 is treated to obtain a membrane having a content
of cerium ions of 3.3%. Using this membrane, in the same manner as
in Example 1, a membrane-catalyst layer assembly is obtained and
then a membrane-electrode assembly is obtained. The
membrane-electrode assembly is evaluated in the same manner as in
Example 1, whereupon results shown in Tables 1 to 3 are
obtained.
Example 5
[0056] As a polymer electrolyte membrane, an ion exchange membrane
having a thickness of 50 .mu.m made of a polymer wherein some of
sulfonic acid groups of a polyether ether ketone having sulfonic
acid groups were ion-exchanged with cerium ions, was prepared as
follows. Namely, 60 g of commercially available granular polyether
ether ketone (PEEK-450P manufactured by British Victrex Company)
was added gradually to 1,200 g of 98% sulfuric acid at room
temperature, followed by stirring at room temperature for 60 hours
to obtain a uniform solution of a polymer in which sulfonic acid
groups were introduced into polyether ether ketone. Then, this
solution was gradually dropwise added to 5 L of distilled water
under cooling to precipitate the polyether ether ketone having
sulfonic acid groups, which was separated by filtration. Then, the
separated product was washed with distilled water until the washing
liquid became neutral. Thereafter, it was dried under vacuum at
80.degree. C. for 24 hours to obtain 48 g of polyether ether ketone
having sulfonic acid groups.
[0057] Then, about 1 g of this compound was precisely weighed and
immersed in 500 mL of a 1 N sodium chloride aqueous solution and
reacted at 60.degree. C. for 24 hours so that protons of the
sulfonic acid groups and sodium ions were ion-exchanged. This
sample was cooled to room temperature and then sufficiently washed
with distilled water, and the sodium chloride aqueous solution
after ion exchange and the distilled water used for washing were
titrated with 0.01 N sodium hydroxide to determine the ion exchange
capacity. The ion exchange capacity was 1.6 meq/g dry polymer.
[0058] Then, the polyether ether ketone having sulfonic acid groups
was dissolved in N-methyl-2-pyrrolidone (NMP) to obtain a solution
of about 10 mass %, which was applied to a substrate made of
polytetrafluoroethylene at room temperature by cast coating and
dried in a nitrogen atmosphere at 100.degree. C. for 10 hours to
evaporate NMP, thereby to obtain a membrane having a thickness of
50 .mu.m. Then, this membrane was cut into a size of 5 cm.times.5
cm (area 25 cm.sup.2), and the weight of the entire membrane was
measured in the same manner as in Example 1 and found to be 0.168
g. The amount of the sulfonic acid groups in the membrane is
obtained from the following formula:
0.168 (g).times.1.6 (meq/g dry polymer)=0.269 (meq)
[0059] The above ion exchange membrane is immersed in an aqueous
solution having 12.0 mg of cerium nitrate
(Ce(NO.sub.3).sub.3.6H.sub.2O) containing cerium ions (trivalent)
corresponding to the amount (equivalent) of about 30% of the amount
of sulfonic acid groups in the membrane dissolved in 500 mL of
distilled water, followed by stirring by a stirrer at room
temperature for 40 hours to obtain a membrane having a content of
cerium ions of 10.3%. Then, using this membrane, in the same manner
as in Example 1, a membrane-catalyst layer assembly is obtained and
then a membrane-electrode assembly is obtained. The
membrane-electrode assembly is evaluated in the same manner as in
Example 1, whereupon results shown in Tables 1 to 3 are
obtained.
Example 6
[0060] As a polymer electrolyte membrane, the same commercially
available ion exchange membrane used in Example 1 was used without
any treatment, and using this membrane, in the same manner as in
Example 1, a membrane-catalyst layer assembly was obtained and then
a membrane-electrode assembly was obtained. The membrane-electrode
assembly was evaluated in the same manner as in Example 1,
whereupon results shown in Tables 1 to 3 were obtained.
Example 7
[0061] In the same manner as in Example 1, the same commercially
available ion exchange membrane used in Example 1 is immersed in an
aqueous solution having 9.8 mg of calcium nitrate
(Ca(NO.sub.3).sub.2.4H.sub.2O) containing calcium ions (bivalent)
dissolved in 500 mL of distilled water to obtain a membrane having
a content of calcium ions of 10.3%. Then, using this membrane, in
the same manner as in Example 1, a membrane-catalyst layer assembly
is obtained and then a membrane-electrode assembly is obtained. The
membrane-electrode assembly is evaluated in the same manner as in
Example 1, whereupon results shown in Tables 1 to 3 are
obtained.
Example 8
[0062] In the same manner as in Example 1, the same commercially
available ion exchange membrane used in Example 1 is immersed in an
aqueous solution having 10.3 mg of copper sulfate
(CuSO.sub.4.5H.sub.2O) containing copper ions (bivalent) dissolved
in 500 mL of distilled water to obtain a membrane having a content
of copper ions of 9.7%. Then, using this membrane, in the same
manner as in Example 1, a membrane-catalyst layer assembly is
obtained and then a membrane-electrode assembly is obtained. The
membrane-electrode assembly is evaluated in the same manner as in
Example 1, whereupon results shown in Tables 1 to 3 are
obtained.
Example 9
[0063] In the same manner as in Example 5 except that the ion
exchange membrane made of polyether ether ketone having sulfonic
acid groups obtained in Example 5 is used without treatment with
cerium ions, a membrane-catalyst layer assembly is obtained and
then a membrane-electrode assembly is obtained. The
membrane-electrode assembly is evaluated in the same manner as in
Example 1, whereupon results shown in Tables 1 to 3 are
obtained.
Example 10
[0064] In the same manner as in Example 1 except that an aqueous
solution having 6.0 mg of cerium nitrate
(Ce(NO.sub.3).sub.3.6H.sub.2O) dissolved in 500 mL of distilled
water was used instead of the cerium nitrate aqueous solution used
in Example 1, the same commercially available ion exchange membrane
used in Example 1 was treated to obtain a membrane having a content
of cerium ions of 4.7%. Then, using this membrane, in the same
manner as in Example 1, a membrane-catalyst layer assembly was
obtained.
[0065] The membrane-catalyst layer assembly was interposed between
two gas diffusion layers made of carbon cloth having a thickness of
350 .mu.m to prepare a membrane-electrode assembly, which was
assembled into a cell for power generation, and a durability test
under operation conditions under low humidification at 120.degree.
C. was carried out as follows. Hydrogen (utilization ratio:
50%)/air (utilization ratio: 50%) were supplied to the anode and to
the cathode under an elevated pressure of 200 kPa at a cell
temperature of 120.degree. C. at a current density of 0.2
A/cm.sup.2, and the polymer electrolyte fuel cell was evaluated as
to the initial property and durability. Hydrogen and air were so
humidified and supplied into the cell that the dew point on the
anode side was 100.degree. C. and that the dew point on the cathode
side was 100.degree. C., respectively, whereupon the cell voltage
at the initial stage of the operation and the relation between the
elapsed time after the initiation of the operation and the cell
voltage were measured. The results are shown in Table 4.
Example 11
[0066] As a polymer electrolyte membrane, the same commercially
available ion exchange membrane used in Example 1 was used without
any treatment, and using this membrane, in the same manner as in
Example 1, a membrane-catalyst layer assembly was obtained and then
a membrane-electrode assembly was obtained. The membrane-electrode
assembly was evaluated in the same manner as in Example 10,
whereupon the power generation voltage suddenly decreased to about
0 V after 110 hours and power generation could be no more possible.
After the test, the membrane was taken out and examined and as a
result, a large pore was formed on the membrane, which was found to
be the cause of the sudden decrease in the voltage.
(Preparation of Solution of Perfluorocarbon Polymer Having Sulfonic
Acid Groups)
[0067] 300 g of a CF.sub.2.dbd.CF.sub.2/CF.sub.2.dbd.CFOCF.sub.2CF
(CF.sub.3)O(CF.sub.2).sub.2SO.sub.3H copolymer (ion exchange
capacity: 1.1 meq/g dry polymer), 420 g of ethanol and 280 g of
water were charged into a 2 L autoclave, sealed hermetically, and
mixed at 105.degree. C. for 6 hours by means of a double helical
blade to obtain a uniform liquid (hereinafter referred to as
"liquid A"). The solid content concentration of the liquid A was 30
mass %.
(Preparation of Solution of Perfluorocarbon Polymer Having Sulfonic
Acid Groups Containing Cerium Ions)
[0068] 100 g of the liquid A and 1.00 g of cerium carbonate hydrate
(Ce.sub.2(CO.sub.3).sub.3.8H.sub.2O) were charged into a 300 mL
round-bottomed flask made of glass and stirred at room temperature
for 8 hours by a meniscus blade made of polytetrafluoroethylene
(PTFE). Bubbles due to generation of CO.sub.2 were generated from
the start of stirring, and a uniform transparent liquid composition
(hereinafter referred to as liquid B) was finally obtained. The
solid content concentration of the liquid B was 30.2 mass %.
The content of cerium ions of the liquid B was examined as follows.
The above liquid B was applied to a 100 .mu.m
ethylene-tetrafluoroethylene copolymer (ETFE) sheet (AFLEX100N,
trade name, manufactured by Asahi Glass Company, Limited) by cast
coating with a die coater, preliminarily dried at 80.degree. C. for
10 minutes and dried at 120.degree. C. for 10 minutes and further
annealed at 150.degree. C. for 30 minutes to obtain an electrolyte
membrane having a thickness of 50 .mu.m. From this electrolyte
membrane, a membrane having a size of 5 cm.times.5 cm was cut out
and left to stand in dry nitrogen for 16 hours, and its mass was
measured. Then, it was immersed in a 0.1 N aqueous HCl solution to
obtain a liquid into which cerium ions were completely extracted.
This liquid was subjected to ICP emission spectrometry to
quantitatively determine cerium in the electrolyte membrane. As a
result, the amount of cerium ions was 1.5% based on the mass of the
membrane, and the content of cerium ions was 10% based on the
number of --SO.sub.3.sup.- groups contained in the perfluorocarbon
polymer.
Example 12
[0069] The above liquid A is applied to a 100 .mu.m ETFE sheet by
cast coating with a die coater, preliminarily dried at 80.degree.
C. for 10 minutes and dried at 120.degree. C. for 10 minutes and
further annealed at 150.degree. C. for 30 minutes to obtain an
electrolyte membrane having a thickness of 25 .mu.m. Similarly, the
above liquid B is applied on a 100 .mu.m ETFE sheet by cast coating
with a die coater, preliminarily dried at 80.degree. C. for 10
minutes and dried at 120.degree. C. for 10 minutes, and further
annealed at 150.degree. C. for 30 minutes to obtain an electrolyte
membrane having a thickness of 25 .mu.m and a content of cerium
ions of 10%. Then, these membranes are hot pressed at 150.degree.
C. to obtain a polymer electrolyte composite membrane having a
thickness of 50 .mu.m in which the content of cerium ions is
non-uniform in the thickness direction.
[0070] Then, 5.1 g of distilled water is mixed with 1.0 g of a
catalyst powder (manufactured by N.E. CHEMCAT CORPORATION) in which
platinum is supported on a carbon carrier (specific surface area:
800 m.sup.2/g) so as to be contained in an amount of 500 of the
whole mass of the catalyst. With this liquid mixture, 5.6 g of a
liquid having the above liquid A diluted with ethanol to a solid
content concentration of 9 mass % is mixed. This mixture is
homogenized by using a homogenizer to prepare a coating liquid for
forming a catalyst layer.
[0071] This coating liquid is applied by a bar coater on a
substrate film made of polypropylene and then dried for 30 minutes
in a dryer at 80.degree. C. to prepare a catalyst layer. Here, the
mass of the substrate film alone before formation of the catalyst
layer and the mass of the substrate film after formation of the
catalyst layer are measured to determine the amount of platinum per
unit area contained in the catalyst layer, whereupon it is 0.5
mg/cm.sup.2.
[0072] Then, using the above composite membrane, the catalyst layer
formed on the substrate film is disposed as an anode on the
membrane containing cerium ions and the catalyst layer formed on
the substrate film is disposed as a cathode on the membrane
containing no cerium ions, and these catalyst layers are
transferred by hot press method to prepare a membrane-catalyst
layer assembly having an anode catalyst layer and a cathode
catalyst layer bonded to both sides of the ion exchange membrane.
The electrode area is 16 cm.sup.2.
[0073] Using this membrane-catalyst layer assembly, in the same
manner as in Example 1, a membrane-electrode assembly is obtained.
The membrane-electrode assembly is subjected to an open circuit
voltage test in the same manner as in Example 1, whereupon results
are as shown in Table 1.
[0074] Then, the membrane-electrode assembly is prepared as
mentioned above and assembled into a cell for power generation, and
a durability test is carried out under operation conditions under
low humidification at high temperature in the same manner as in
Example 10. Namely, the test conditions are as follows. Hydrogen
(utilization ratio: 50%)/air (utilization ratio: 50%) are supplied
to the anode and to the cathode under an elevated pressure of 200
kPa at a cell temperature of 120.degree. C. and at a current
density of 0.2 A/cm.sup.2, and the polymer electrolyte fuel cell is
evaluated as to the initial property and durability. Hydrogen and
air are so humidified and supplied into the cell that the dew point
on the anode side is 100.degree. C. and that the dew point on the
cathode side is 100.degree. C., respectively, whereupon the cell
voltage at the initial stage of the operation and the relation
between the elapsed time after the initiation of the operation and
the cell voltage are measured. The results are shown in Table
4.
[0075] Then, the membrane-electrode assembly is further prepared as
mentioned above and assembled into a cell for power generation, and
a durability test is carried out under operation conditions under
high humidification in the same manner as in Example 1. Namely, the
test conditions are as follows. Hydrogen (utilization ratio:
70%)/air (utilization ratio: 40%) are supplied under ordinary
pressure at a cell temperature of 80.degree. C. and at a current
density of 0.2 A/cm.sup.2, and the polymer electrolyte fuel cell is
evaluated as to the initial property and durability. Hydrogen and
air are so humidified and supplied into the cell that the dew point
on the anode side is 80.degree. C. and that the dew point on the
cathode side is 80.degree. C., respectively, whereupon the cell
voltage at the initial stage of the operation and the relation
between the elapsed time after the initiation of the operation and
the cell voltage are measured. The results are shown in Table
3.
Example 13
[0076] The liquid A was applied on a 100 .mu.m ETFE sheet by cast
coating with a die coater, preliminarily dried at 80.degree. C. for
10 minutes and dried at 120.degree. C. for 10 minutes, and further
annealed at 150.degree. C. for 30 minutes to obtain an electrolyte
membrane having a thickness of 50 .mu.m and a size of 5 cm.times.5
cm.
[0077] Then, 5.1 g of distilled water was mixed with 1.0 g of a
catalyst powder (manufactured by N.E. CHEMCAT CORPORATION) in which
platinum was supported on a carbon carrier (specific surface area:
800 m.sup.2/g) so as to be contained in an amount of 50% of the
whole mass of the catalyst. With this liquid mixture, 5.6 g of a
liquid having the above liquid B diluted with ethanol to a solid
content concentration of 9 mass % was mixed. This mixture was
homogenized by using a homogenizer to obtain a coating fluid for
forming an anode catalyst layer.
[0078] This coating fluid was applied by a bar coater on a
substrate film made of polypropylene and then dried for 30 minutes
in a dryer at 80.degree. C. to prepare an anode catalyst layer
containing cerium ions in an amount of 10 mol % based on
--SO.sub.3.sup.- groups contained in the perfluorocarbon polymer in
the catalyst layer. Here, the mass of the substrate film alone
before formation of the catalyst layer and the mass of the
substrate film after formation of the catalyst layer were measured
to determine the amount of platinum per unit area contained in the
catalyst layer, whereupon it was 0.5 mg/cm.sup.2.
[0079] Separately, a cathode catalyst layer containing no cerium
ions was prepared in the same manner as preparation of the anode
catalyst layer except that the above liquid A was used instead of
the liquid B.
[0080] Then, the anode catalyst layer and the cathode catalyst
layer each formed on the substrate film were disposed on both sides
of the electrolyte membrane prepared by using the liquid A, and the
catalyst layers were transferred to the membrane by hot press
method to obtain a membrane-catalyst layer assembly having an anode
catalyst layer containing cerium ions in an amount of 10 mol % of
--SO.sub.3.sup.- groups contained in the perfluorocarbon polymer in
the catalyst layer and a cathode catalyst layer containing no
cerium ions bonded to both sides of the polymer electrolyte
membrane. The electrode area was 16 cm.sup.2.
[0081] Using the membrane-catalyst layer assembly, a
membrane-electrode assembly was obtained in the same manner as in
Example 1. The membrane-electrode assembly was subjected to an open
circuit voltage test in the same manner as in Example 1. The
results are shown in Table 1. Further, a membrane-electrode
assembly is prepared in the same manner as mentioned above and
assembled into a cell for power generation, and a durability test
under operation conditions under low humidification and under high
humidification is carried out, whereupon results are as shown in
Tables 2 and 3.
Example 14
[0082] In the same manner as the above preparation of the liquid B
except that the amount of cerium carbonate hydrate
(Ce.sub.2(CO.sub.3).sub.3.8H.sub.2O) was 2.00 g, a liquid having a
content of cerium ions of 20% based on the number of
--SO.sub.3.sup.- groups contained in the perfluorocarbon polymer
was obtained. Then, in the same manner as in Example 13 except that
this liquid was used for formation of the anode catalyst layer, a
membrane-catalyst layer assembly having an anode catalyst layer
containing cerium ions in an amount of 20 mol % of --SO.sub.3.sup.-
groups contained in the perfluorocarbon polymer in the catalyst
layer and a cathode catalyst layer containing no cerium ions bonded
to both sides of the polymer electrolyte membrane, was
obtained.
[0083] Using the membrane-catalyst layer assembly, a
membrane-electrode assembly was obtained in the same manner as in
Example 1. The membrane-electrode assembly was subjected to an open
circuit voltage test in the same manner as in Example 1. The
results are shown in Table 1. Further, a membrane-electrode
assembly is obtained in the same manner as mentioned above and
assembled into a cell for power generation, and a durability test
under operation conditions under low humidification and under high
humidification is carried out, whereupon results are as shown in
Tables 2 and 3.
Example 15
[0084] A membrane-catalyst layer assembly was obtained in the same
manner as in Example 13 except that an anode catalyst layer
containing no cerium ions was prepared by using the liquid A. This
membrane-catalyst layer assembly was immersed in an aqueous
solution containing cerium nitrate (Ce(NO.sub.3).sub.3.6H.sub.2O)
to obtain a membrane-catalyst layer assembly having some of
sulfonic acid groups in the perfluorocarbon polymer of the membrane
and the catalyst layer ion-exchanged with cerium ions. The ion
exchange was carried out as follows.
[0085] First, the weight of the entire membrane prepared by cast
coating, after being left to stand in dry nitrogen for 16 hours,
was measured in dry nitrogen and found to be 0.251 g. The amount of
sulfonic acid groups in this membrane is obtained from the
following formula:
0.251 (g).times.1.1 (meq/g dry polymer)=0.276 (meq)
[0086] Then, 12.0 mg of cerium nitrate
(Ce(NO.sub.3).sub.3.6H.sub.2O) was dissolved in 500 mL of distilled
water so that cerium ions (trivalent) in an amount corresponding to
10% of the number of sulfonic acid groups in the membrane portion
of this membrane-catalyst layer assembly were contained. The above
membrane-catalyst layer assembly was immersed in the solution,
followed by stirring by a stirrer at room temperature for 40 hours
so that some of the sulfonic acid groups in the perfluorocarbon
polymer in the membrane-catalyst layer assembly were ion-exchanged
with cerium ions and that cerium ions were incorporated in the
entire membrane-catalyst layer assembly. The cerium nitrate
solution was analyzed by ICP emission spectrometry before and after
the immersion and as a result, it was found that the
membrane-catalyst layer assembly contained cerium ions in an amount
of 9.3% of the number of --SO.sub.3.sup.- groups in the membrane
portion of the membrane-catalyst layer assembly.
[0087] Using this membrane-catalyst layer assembly, in the same
manner as in Example 1, a membrane-electrode assembly was obtained.
The membrane-electrode assembly was subjected to an open circuit
voltage test in the same manner as in Example 1. The results are as
shown in Table 1. Further, a membrane-electrode assembly is
prepared in the same manner as mentioned above and assembled into a
cell for power generation, and a durability test is carried out
under operation conditions under low humidification and under high
humidification in the same manner as in Example 1, whereupon the
results are as shown in Tables 2 and 3.
TABLE-US-00001 TABLE 1 Open circuit Hydrogen voltage (V) leak (ppm)
Initial After 100 hours Initial After 100 hours Ex. 1 0.99 0.98 710
720 Ex. 2 0.99 0.99 700 710 Ex. 3 0.98 0.96 730 780 Ex. 4 0.97 0.94
750 790 Ex. 5 0.96 0.92 850 1,100 Ex. 6 0.96 0.75 1,100 12,000 Ex.
7 0.96 0.71 850 22,000 Ex. 8 0.96 0.60 900 35,000 Ex. 9 0.94 0.51
1,300 70,000 Ex. 12 0.99 0.96 710 720 Ex. 13 0.99 0.96 710 720 Ex.
14 0.99 0.98 700 710 Ex. 15 0.99 0.99 720 720
TABLE-US-00002 TABLE 2 Durability/output Initial output voltage (V)
voltage (V) After 500 hours After 2,000 hours Ex. 1 0.77 0.77 0.76
Ex. 2 0.77 0.76 0.76 Ex. 3 0.76 0.75 0.75 Ex. 4 0.76 0.75 0.74 Ex.
5 0.75 0.73 0.72 Ex. 6 0.77 0.70 0.65 Ex. 7 0.75 0.66 0.60 Ex. 8
0.75 0.62 0.55 Ex. 9 0.73 0.58 0.50 Ex. 13 0.77 0.76 0.75 Ex. 14
0.77 0.76 0.76 Ex. 15 0.75 0.74 0.73
TABLE-US-00003 TABLE 3 Durability/output Initial output voltage (V)
voltage (V) After 500 hours After 2,000 hours Ex. 1 0.78 0.78 0.78
Ex. 2 0.78 0.78 0.77 Ex. 3 0.78 0.77 0.77 Ex. 4 0.78 0.77 0.77 Ex.
5 0.76 0.75 0.74 Ex. 6 0.77 0.73 0.70 Ex. 7 0.76 0.71 0.67 Ex. 8
0.76 0.70 0.64 Ex. 9 0.74 0.65 0.60 Ex. 12 0.78 0.77 0.76 Ex. 13
0.78 0.77 0.76 Ex. 14 0.78 0.77 0.76 Ex. 15 0.77 0.76 0.76
TABLE-US-00004 TABLE 4 Durability/output Initial output voltage (V)
voltage (V) After 500 hours After 2,000 hours Ex. 10 0.77 0.73 0.68
Ex. 11 0.76 Power generation Power generation impossible impossible
Ex. 12 0.76 0.72 0.66
[0088] It was confirmed from the above results of Examples and
Comparative Examples that the open circuit voltage test (OCV test)
under high temperature and low humidification conditions as an
acceleration test resulted in deterioration of the conventional
electrolyte membranes and increase of hydrogen leak due to hydrogen
peroxide or peroxide radials formed on the anode and the cathode,
but exhibited the dramatically excellent durability of the
electrolyte membrane of the present invention.
INDUSTRIAL APPLICABILITY
[0089] The electrolyte membrane of the present invention is very
excellent in durability against hydrogen peroxide or peroxide
radicals formed by power generation of a fuel cell. Accordingly, a
polymer electrolyte fuel cell provided with a membrane-electrode
assembly having the electrolyte membrane of the present invention
has durability over a long period of time either in power
generation under low humidification and in power generation under
high humidification and even in power generation at a high
temperature of at least 100.degree. C.
[0090] The entire disclosures of Japanese Patent Application No.
2004-183712 filed on Jun. 22, 2004, Japanese Patent Application No.
2004-225706 filed on Aug. 2, 2004, Japanese Patent Application No.
2004-265176 filed on Sep. 13, 2004 and Japanese Patent Application
No. 2005-118412 filed on Apr. 15, 2005 including specifications,
claims, drawings and summaries were incorporated herein by
reference in their entireties.
* * * * *