U.S. patent application number 09/200735 was filed with the patent office on 2002-01-03 for solid polyelectrolyte-type fuel cell having a solid polyelectrolyte membrane with varying water content.
Invention is credited to ASUKABE, MICHIO, KATOH, MICHIAKI, NEZU, SHINJI, YAMADA, CHIAKI.
Application Number | 20020001742 09/200735 |
Document ID | / |
Family ID | 26572234 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001742 |
Kind Code |
A1 |
KATOH, MICHIAKI ; et
al. |
January 3, 2002 |
SOLID POLYELECTROLYTE-TYPE FUEL CELL HAVING A SOLID POLYELECTROLYTE
MEMBRANE WITH VARYING WATER CONTENT
Abstract
A solid polyelectrolyte fuel cell has (a) a positive electrode,
(b) a negative electrode, and (c) a solid polyelectrolyte membrane
containing water, between the positive electrode and the negative
electrode, where the water content of portions of the solid
polyelectrolyte membrane adjacent to the negative electrode is
greater than the water content of portions of the solid
polyelectrolyte membrane adjacent to the positive electrode. The
cell outputs high voltage and has good properties.
Inventors: |
KATOH, MICHIAKI; (AICHI-KEN,
JP) ; ASUKABE, MICHIO; (AICHI-KEN, JP) ;
YAMADA, CHIAKI; (AICHI-KEN, JP) ; NEZU, SHINJI;
(AICHI-KEN, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
26572234 |
Appl. No.: |
09/200735 |
Filed: |
November 27, 1998 |
Current U.S.
Class: |
429/494 |
Current CPC
Class: |
H01M 8/04291 20130101;
H01M 8/1023 20130101; Y02P 70/50 20151101; H01M 8/1088 20130101;
H01M 8/1072 20130101; H01M 8/1067 20130101; Y02E 60/50 20130101;
H01M 8/1039 20130101 |
Class at
Publication: |
429/30 ;
429/33 |
International
Class: |
H01M 008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 1997 |
JP |
09-326601 |
Dec 15, 1997 |
JP |
09-345422 |
Claims
1. A solid polyelectrolyte fuel cell, comprising: (a) a positive
electrode, (b) a negative electrode, and (c) a solid
polyelectrolyte membrane containing water, between said positive
electrode and said negative electrode, wherein the water content of
a portion of said solid polyelectrolyte membrane adjacent to said
negative electrode is greater than the water content of a portion
of said solid polyelectrolyte membrane adjacent to said positive
electrode.
2. The solid polyelectrolyte fuel cell of claim 1, wherein the
water content of said solid polyelectrolyte membrane varies
continuously from said portion of said solid polyelectrolyte
membrane adjacent to said negative electrode to said portion of
said solid polyelectrolyte membrane adjacent to said positive
electrode.
3. The solid polyelectrolyte fuel cell of claim 1, wherein the
water content of said portion of said solid polyelectrolyte
membrane adjacent to said negative electrode is at least 5% by
weight larger than said portion of said solid polyelectrolyte
membrane adjacent to said positive electrode.
4. The solid polyelectrolyte fuel cell of claim 3, wherein the
water content of each portion of said solid polyelectrolyte
membrane is 30 to 200% by weight.
5. The solid polyelectrolyte fuel cell of claim 1, wherein the
water content of each portion of said solid polyelectrolyte
membrane varies in proportion to the ion-exchanging capacity of
said each portion of said solid polyelectrolyte membrane.
6. The solid polyelectrolyte fuel cell of claim 1, wherein the
water content of each portion of said solid polyelectrolyte
membrane varies in proportion to the degree of crosslinking of said
each portion of said solid polyelectrolyte membrane.
7. The solid polyelectrolyte fuel cell of claim 1, wherein said
solid polyelectrolyte membrane comprises a copolymer comprising
main chains and side chains, said main chains formed by
copolymerizating fluorocarbon vinyl monomers and hydrocarbon vinyl
monomers, and said side chain are sulfonic acid group-containing
hydrocarbon side chains.
8. The solid polyelectrolyte fuel cell of claim 1, wherein said
solid polyelectrolyte membrane comprises a copolymer comprising
main chains and side chains, said main chain formed by
copolymerizating olefinic perfluorocarbons and olefinic
hydrocarbons, and said side chains are crosslinked polymers of
sulfonic acid group-containing olefinic hydrocarbons and diolefinic
hydrocarbons.
9. The solid polyelectrolyte fuel cell of claim 7, wherein said
solid polyelectrolyte membrane is a cation-exchanging membrane, and
said cation-exchanging membrane is a laminate of at least two
layers, each of said layers having a different water content.
10. The solid polyelectrolyte fuel cell of claim 1, wherein said
solid polyelectrolyte membrane comprises a copolymer comprising
main chains and side chains, said main chains comprising groups
represented by: 3wherein R.sup.1 represents a fluorine atom, or a
fluoroalkyl group having from 1 to 3 carbon atoms; R.sup.2
represents a hydrogen atom, or an alkyl group having from 1 to 3
carbon atoms; m represents an integer of 1 or more; and n
represents an integer of 1 or more, and said side chains comprising
groups represented by: 4wherein R.sup.3, R.sup.4 and R.sup.5 each
independently represent a hydrogen atom, or an alkyl group having
from 1 to 3 carbon atoms; s represents an integer of 1 or more; and
t represents 0 or an integer of 1 or more.
11. The solid polyelectrolyte fuel cell of claim 7, wherein said
fluorocarbon vinyl monomers are tetrafluoroethylene, and said
hydrocarbon vinyl monomers are ethylene.
12. The solid polyelectrolyte fuel cell of claim 10, wherein
R.sup.1 represents a fluorine atom, and R.sup.2 represents a
hydrogen atom.
13. The solid polyelectrolyte fuel cell of claim 7, wherein said
side chains are styrene-sulfonic acid polymer.
14. The solid polyelectrolyte fuel cell of claim 9, wherein the
water content of each portion of said solid polyelectrolyte
membrane varies in proportion to previous exposure to a dose of
.gamma.-radiation or electron radiation to change the degree of
grafting on said main chains.
15. The solid polyelectrolyte fuel cell of claim 9, wherein the
water content of each portion of said solid polyelectrolyte
membrane varies in proportion to grafting temperature and the
grafting time to change the degree of grafting on said main
chains.
16. A method of making the solid polyelectrolyte fuel cell of claim
1, comprising: laminating together said positive electrode, said
negative electrode, and said solid polyelectrolyte membrane.
17. The method of claim 16, further comprising grafting side chains
onto main chains, to form said solid polyelectrolyte membrane.
18. The method of claim 17, wherein said grafting is carried out by
exposing a membrane to .gamma.-radiation or electron radiation.
19. The method of claim 17, wherein said grafting is carried out by
different portions of a membrane to different grafting temperatures
and the grafting times, to change the degree of grafting on said
main chains.
20. The method of claim 19, wherein said main chains comprising
groups represented by: 5wherein R.sup.1 represents a fluorine atom,
or a fluoroalkyl group having from 1 to 3 carbon atoms; R.sup.2
represents a hydrogen atom, or an alkyl group having from 1 to 3
carbon atoms; m represents an integer of 1 or more; and n
represents an integer of 1 or more, and said side chains comprising
groups represented by: 6wherein R.sup.3, R.sup.4 and R.sup.5 each
independently represent a hydrogen atom, or an alkyl group having
from 1 to 3 carbon atoms; s represents an integer of 1 or more; and
t represents 0 or an integer of 1 or more.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a solid
polyelectrolyte-type fuel cell.
[0003] 2. Discussion of the Background
[0004] A solid polyelectrolyte-type fuel cell is considered to be a
hopeful, small-sized lightweight power source for vehicles and
other devices which uses hydrogen and oxygen as fuel. The cell
comprises an ion-exchangeable, solid polyelectrolyte membrane, and
a positive-electrode and a negative electrode disposed to be in
contact with both sides of the membrane. The fuel hydrogen is
electrochemically oxidized at the negative electrode to give
protons and electrons. The protons pass through the polyelectrolyte
membrane toward the positive electrode, which is fed oxygen.
Electrons, having been formed at the negative electrode, travel to
the positive electrode, where the protons and the electrons react
with oxygen to form water.
[0005] The solid polyelectrolyte-type cell can operate at low
temperatures and is small, while producing a high power output
density. Therefore, many studies have been made on these cells for
use as a power source for vehicles. Generally used in the cell is a
sulfonic acid group-containing perfluorocarbon polymer membrane
(e.g., NAFION, trade name of DuPont Co.; ACIPLEX, trade name of
Asahi Chemical Co.) or the like as the polyelectrolyte membrane.
However, the conventional fuel cell is not still satisfactory,
because its output is not high enough.
[0006] In order to increase the output of the cell, the hydrogen
ion conductivity of the polyelectrolyte membrane therein must be
increased to thereby lower the internal resistance of the cell. For
this, the concentration of the ion-exchanging groups (for example,
sulfonic acid groups) existing in the polyelectrolyte membrane may
be increased and the thickness of the membrane may be reduced.
However, too great an increase in the ion-exchanging group
concentration in the membrane results in an increase in the water
content of the membrane, and is therefore problematic in that the
positive electrode at which water is formed through cell reaction
becomes too wet, lowering the cell output.
[0007] On the other hand, the reduction in the thickness of the
membrane is also problematic in that the mechanical strength of the
membrane is reduced, and that the amount of fuel, (hydrogen gas and
oxygen gas) passing through the membrane is increased, lowering the
cell-output efficiency.
[0008] One prior technique is disclosed in Japanese Patent
Application Laid-open (JP-A) Hei-6-231781. A cation-exchanging
membrane of a laminate, composed of at least two layers of a
sulfonic acid group-containing perfluorocarbon polymer, in which
the layers each have a different water content, is used as the
solid electrolyte membrane, so that the membrane may have low
electric resistance. In the solid polyelectrolyte-type fuel cell
disclosed therein, a plurality of polymer films, each having a
different water content, are so laminated to construct the
polyelectrolyte membrane that their water content varies to
increase from the positive electrode side to the negative electrode
side.
[0009] Another prior technique is disclosed in JP-A Hei-6-231782.
The polyelectrolyte-type fuel cell disclosed therein comprises a
laminate membrane of at least two, sulfonic acid group-containing
perfluorocarbon polymer films, each having a different water
content, in which the water content of the polymer film facing the
positive electrode is made larger than that of the polymer film
facing the negative electrode, in order that the laminate membrane
may have low electric resistance.
[0010] However, the former, in which the water content of the
laminate membrane greatly varies around the lamination boundaries,
suffers from the problem that some stress is generated around the
boundaries, thereby lowering the mechanical durability of the
membrane. In addition, in the former, the discontinuous variation
in the water content of the laminate membrane interferes with
efficient back diffusion of water from the positive electrode that
compensates for the reduction in the water content of the negative
electrode, whereby the cell can not produce a large output. On the
other hand, in the latter, the positive electrode becomes too wet
and the negative electrode has high resistance. In this, therefore,
the increase in the cell output could not be attained.
[0011] In order to increase the output of the fuel cell of that
type, the hydrogen ion conductivity of the polyelectrolyte membrane
in the cell must be high and the internal resistance of the
membrane must be small. The hydrogen ion conductivity and the
internal resistance of the membrane are significantly influenced by
the amount of ion-exchanging groups (for example, sulfonic acid
groups) existing in the membrane and also by the water content of
the membrane. Specifically, membranes having a higher
ion-exchanging group content and a higher water content have a
higher degree of hydrogen ion conductivity.
[0012] In the negative electrode side of the fuel cell, hydrogen
ions derived from hydrogen gas pass through the polyelectrolyte
membrane and move toward the side of the positive electrode. In the
fuel cell, therefore, the water content of the membrane adjacent to
the negative electrode is lowered, thereby causing the reduction in
the cell output. On the other hand, water is formed through cell
reaction in the membrane adjacent to the positive electrode so that
excess water exists therein. As a result, therefore, it is presumed
that such excess water will cover the catalyst, and interfere with
gas diffusion, thereby causing a reduction in the cell output.
[0013] Still another prior technique is disclosed in JP-A
Hei-6-231783. In the polyelectrolyte-type fuel cell disclosed
therein, the solid polyelectrolyte membrane is of a cation
exchanging membrane having a laminate structure of at least three
layers of a sulfonic acid group-containing perfluorocarbon polymer
each having a different water content, in which the water content
of the polymer film layers adjacent to the positive electrode and
to the negative electrode is larger than that of the interlayer
polymer film.
[0014] In the three prior techniques noted above, the solid
polyelectrolyte membrane is a sulfonic acid group-containing
perfluorocarbon polymer. In those, a plurality of such polymer
layers each having a different water content are laminated to
construct the solid polyelectrolyte membrane, in order to increase
the cell output.
[0015] However, the starting materials, such as
tetrafluoroethylene, 2-fluorosulfonyl-perfluoroethyl vinyl ether
and others for the polymer, are extremely expensive, and the high
price of these materials is fixed, and are unlikely to
significantly fall in the future. In addition, the polymer is
further problematic in that it requires complicated polymerization
steps to be followed by the final step of sheeting it into thin
films. As a result, if the polymer is used in producing fuel cells
for electric cars, the price of each fuel cell produced is high and
will be equal to the price of the car itself. For the same reasons
as above, the polymer could not be used in producing fuel cells for
leisure appliances. Therefore, using the polymer in producing
practical fuel cells is not practicable at present.
SUMMARY OF THE INVENTION
[0016] Given that situation, the present invention is to provide a
high-output, solid polyelectrolyte-type fuel cell which is
characterized in that the water content of the polyelectrolyte
membrane in the cell is not uniform, but rather continuously varies
in the direction of the thickness of the membrane in such a manner
that the water content is highest in the side of the membrane
adjacent to the negative electrode, and is lowest in the side
thereof adjacent to the positive electrode. As a result, in the
cell of the invention, the surface of the membrane repels water and
promotes the back-diffusion of water from the positive electrode,
which compensates for the water content loss in the negative
electrode, and therefore the positive electrode is prevented from
becoming to wet and the catalytic action in Cthe cell is promoted
to increase the cell output.
[0017] The invention also provides a high-output, solid
electrolyte-type fuel cell in which the polyelectrolyte membrane is
of a laminate of at least two layers each having a different water
content so as to optimize the water retentiveness of the membrane
that faces the positive electrode and the negative electrode. This
is based on the technical idea of the applicant's own invention in
a prior patent application of JP-A Hei-9-102322, hereby
incorporated by reference. As in the prior application, an
ethylene-tetrafluoroethylene copolymer (ETFE) film base is exposed
to radiation, then grafted with styrene at the resulting radicals,
and thereafter sulfonylated with chlorosulfonic acid and hydrolyzed
with an alkali or acid to thereby introduce sulfonic acid groups
into the grafted copolymer. The solid-polyelectrolyte composition
thus produced is based on the ETFE film, and is produced through
irradiation followed by grafting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0019] FIG. 1 is a graph showing the time-dependent variation in
the output voltage of the cell of Example 4;
[0020] FIG. 2 is a graph showing the time-dependent variation in
the output voltage of the cell of Comparative Example 2;
[0021] FIG. 3 is a graph showing the time-dependent variation in
the output voltage of the cell of Example 5; and
[0022] FIG. 4 is a graph showing the time-dependent variation in
the output voltage of the cell of Comparative Example 3.
DETAILED DESCRIPTION OF THE INVENTION
[0023] To solve the problems noted above, a first aspect of the
invention is a solid polyelectrolyte-type fuel cell in which the
water content of the solid polyelectrolyte membrane is controlled
so that the water content thereof adjacent to the negative
electrode is higher than the water content thereof adjacent to the
positive electrode, relative to the thickness of the membrane.
[0024] In a second aspect, the water content of the solid
polyelectrolyte membrane in the cell is controlled so that it
varies continuously from the side adjacent to the negative
electrode to the side adjacent to the positive electrode in the
direction of the thickness of the membrane.
[0025] In a third aspect, the water content of the membrane
adjacent to the negative electrode in the cell is at least 5% by
weight larger than that adjacent to the positive electrode. If the
difference in the water content is smaller than 5% by weight, the
back diffusion of water from the positive electrode to the negative
electrode is not efficient.
[0026] In a fourth aspect, the water content of the solid
polyelectrolyte membrane in the cell is controlled so that it
varies continuously from the side adjacent to the negative
electrode to the side adjacent to the positive electrode in the
direction of the thickness of the membrane, within a range falling
between 30 and 200% by weight. If the water content variation is
smaller than 30% by weight, the inner resistance of the membrane
will be high; but if it is larger than 200% by weight, the
mechanical properties of the membrane will be poor, or that is, the
membrane will be hard and brittle.
[0027] In a fifth aspect, the water content of the solid
polyelectrolyte membrane in the cell is controlled by controlling
the ion-exchanging capacity of the membrane.
[0028] In a sixth aspect, the water content of the solid
electrolyte membrane in the cell is controlled by controlling the
degree of crosslinking in the membrane.
[0029] In a seventh aspect, the solid polyelectrolyte membrane in
the cell is a copolymer comprising a main chain formed through
copolymerization of a fluorocarbon vinyl monomer and a hydrocarbon
vinyl monomer, and sulfonic acid group-containing hydrocarbon side
chains.
[0030] In an eighth aspect of the invention, the solid
polyelectrolyte membrane in the cell is a copolymer comprising a
main chain formed through copolymerization of an olefinic
perfluorocarbon and an olefinic hydrocarbon, and side chains of a
crosslinked polymer of a sulfonic acid group-containing olefinic
hydrocarbon and a diolefinic hydrocarbon.
[0031] In a ninth aspect of the invention, the solid
polyelectrolyte membrane in the cell is a cation-exchanging
membrane of a copolymer that comprises a main chain formed through
copolymerization of a fluorocarbon vinyl monomer and a hydrocarbon
vinyl monomer, and sulfonic acid group-containing hydrocarbon side
chains. In this, the cation-exchanging membrane is a laminate of at
least two layers each having a different water content, and the
water content of the membrane adjacent to the negative electrode is
higher than that adjacent to the positive electrode.
[0032] In a tenth aspect, the main chain in the copolymer for the
solid polyelectrolyte membrane in the cell is represented by
formula I: 1
[0033] wherein R.sup.1 represents a fluorine atom, or a fluoroalkyl
group having from 1 to 3 carbon atoms; R.sup.2 represents a
hydrogen atom, or an alkyl group having from 1 to 3 carbon atoms; m
represents an integer of 1 or more; and n represents an integer of
1 or more,
[0034] and the side chains are represented by formula II: 2
[0035] wherein R.sup.3, R.sup.4 and R.sup.5 each represent a
hydrogen atom, or an alkyl group having from 1 to 3 carbon atoms; s
represents an integer of 1 or more; and t represents 0 or an
integer of 1 or more.
[0036] In an eleventh aspect, the main chain in the copolymer for
the solid polyelectrolyte membrane in the cell is an
ethylene-tetrafluoroethy- lene copolymer.
[0037] In a twelfth aspect, the side chains in the copolymer for
the solid polyelectrolyte membrane in the cell are styrene-sulfonic
acid polymers.
[0038] In a thirteenth aspect, the copolymer for the
cation-exchanging membrane for the solid polyelectrolyte membrane
in the cell is exposed to a controlled dose of .gamma.-radiation or
electron radiation to change the degree of grafting on its main
chain, the main chain being a copolymer of a fluorocarbon vinyl
monomer and a hydrocarbon vinyl monomer, to thereby control the
water content of the membrane.
[0039] In a fourteenth aspect, the degree of grafting on the main
chain of the copolymer for the cation-exchanging membrane for the
solid polyelectrolyte membrane in the cell is varied by changing
the grafting temperature and the grafting time, to thereby control
the water content of the membrane. In this, the main chain is a
copolymer of a fluorocarbon vinyl monomer and a hydrocarbon vinyl
monomer.
[0040] In order to continuously vary the water content of the
polyelectrolyte membrane in the direction of the thickness of the
membrane, in the cell of the invention, the ion-exchanging group
content of the membrane and also the degree of crosslinking in the
membrane may be continuously varied. For example, where the
polyelectrolyte membrane is formed from a copolymer that comprises
a main chain formed through copolymerization of an olefinic
perfluorocarbon and an olefinic hydrocarbon, and side chains of a
crosslinked copolymer of a sulfonic acid group-containing olefinic
hydrocarbon and a diolefinic hydrocarbon, the concentration of the
sulfonic acid groups in the side chains themselves or to be added
to the side chains, is varied in the direction of the thickness of
the membrane, or alternatively, the degree of crosslinking with the
diolefinic hydrocarbon is varied, whereby the water content of the
resulting membrane may be controlled in the intended manner.
[0041] Concretely, when side chains are introduced into the film of
a main chain copolymer, the material for the side chains or the
crosslinking material is contacted with only one surface of the
film, whereby the concentration of the side chains thus formed in
the film or the degree of crosslinking in the film may be
controlled in the intended manner. Alternatively, when sulfone
groups are introduced into the side chains of the copolymer of the
film, the sulfonating agent is contacted with only one surface of
the film, whereby the concentration of the sulfonic acid group in
the film may be continuously varied in the direction of the
thickness of the film.
[0042] The water content, .DELTA.W of the polymer film (acid type)
for use in the invention is defined as follows:
.DELTA.W=(W.sub.1/W.sub.2-1).times.100 (% by weight)
[0043] wherein
[0044] W.sub.1 indicates the weight of the film having been dipped
in pure water at 80.degree. C. for 3 hours; and
[0045] W.sub.2 indicates the weight of the film having been dried
in vacuum at 100.degree. C. for 24 hours after its W.sub.1 was
measured.
[0046] An ETPE (ethylene-tetrafluoroethylene copolymer) film having
a thickness of from 5 to 500 .mu.m, which is a base for the solid
polyelectrolyte membrane in the cell of the invention, is exposed
to radiation, such as .gamma.-radiation or electron radiation, at a
dose of from 1 to 100 kGy, and the resulting radicals are contacted
with an alkenylbenzene such as styrene or the like.
[0047] In forming the graft-polymerizing side chains in the main
chain copolymer, at which the polymerizing alkenylbenzene is
grafted, the dose of the .gamma.-radiation or electron radiation to
be applied to the copolymer may be varied so as to control the
degree of the grafting reaction in the copolymer, or that is, to
control the degree of grafting therein. Noting this, the dose is
varied within a range of from 1 to 100 kGy, and at least two
membranes of the copolymer, each having a different degree of
grafting, are prepared.
[0048] Next, at least two of these membranes are laminated in order
of the dose applied thereto, and are further grafted with the
monomer noted above at a temperature falling between 40 and
100.degree. C. for 10 minutes to 50 hours.
[0049] As a result, under the same grafting condition, the membrane
which a higher dose has been applied shall have a higher degree of
grafting than that to which a lower dose has been applied. Next,
the resulting laminate is sulfonated with a sulfonating agent, such
as sulfuric acid, fuming sulfuric acid, chlorosulfonic acid or the
like, sufficiently to the depth of the laminate. For this, the
sulfonation of the laminate is effected with the sulfonating agent,
which is not diluted or diluted 500-fold with a solvent such as
1,1,2,2-tetrachloroethane, 1,2-dichloroethane or the like, capable
of swelling the ETFE films, at a temperature falling between room
temperature (20.degree. C.) and 100.degree. C., for 10 minutes to
10 hours.
[0050] As a result, the films exposed to a larger dose of radiation
have a higher degree of grafting, and have a larger amount of
sulfonyl groups introduced through the sulfonation, while those
exposed to a smaller dose of the radiation have a smaller amount of
sulfonyl groups.
[0051] Next, the laminate is hydrolyzed. For this, for example, the
laminate is dipped in an alkaline aqueous solution of 0.01 to 10 N
potassium hydroxide, sodium hydroxide or the like, at a temperature
falling between room temperature and 100.degree. C., and then
dipped in an acidic aqueous solution of sulfuric acid, hydrochloric
acid or the like at a temperature falling between room temperature
and 100.degree. C. As a result, the sulfonyl groups having been
introduced into the films through the previous sulfonation are
converted into sulfonic acid groups, and the resulting laminate has
the intended water absorbability and ion-exchangeability.
[0052] Accordingly, the films having been exposed to a larger dose
of radiations have a larger number of sulfonic acid groups and have
a higher water content. In the laminate for which a plurality of
films are so laminated, those nearer to the negative electrode are
exposed to a larger dose of radiation, the films nearer to the
negative electrode shall have a higher water content; those nearer
to the positive electrode have a lower water content.
[0053] Next, gas diffusion electrodes for the negative electrode
and the positive electrode are disposed, with the laminate, of
which the water content varies in the manner mentioned above,
sandwiched therebetween, in such a manner that the platinum-carried
carbon catalyst-coated surface of each electrode faces the surface
of the laminate membrane, and they are pressed together under heat.
In this manner, the laminate membrane is integrated with the
positive electrode and the negative electrode to construct a cell.
Next, a plurality of the thus-integrated cells are stacked up to
construct a fuel cell stack. As fuel, hydrogen is fed into the cell
stack at the side of the negative electrode, while oxygen is fed
thereinto at the side of the positive electrode as an oxidizing
agent, and the cell stack is driven to generate electric power.
[0054] In the embodiment of the invention mentioned above, the
method employed comprises exposing plural membranes to radiation at
different doses varying within a range of from 1 to 100 kGy, and
this is based on the findings that, in the formation of graft
polymer chains of a polymerizing alkenylbenzene, a variation in the
dose of .gamma. or electron radiation to be applied to the main
chain polymer membranes brings about a variation in the degree of
grafting reaction, or that is, the variation in the degree of
grafting in the resulting membranes. In place of this method, also
employable herein, is a different method in which the grafting
temperature is controlled within a range falling between 40 and
100.degree. C. and the grafting time is also controlled within a
range falling between 10 minutes and 50 hours under a predetermined
grafting condition, to thereby produce a plurality of graft polymer
membranes each having a different degree of grafting
polymerization, or that is, a different degree of grafting.
[0055] In the solid polyelectrolyte-type fuel cell of the
invention, the water content of the polyelectrolyte membrane is not
uniform but continuously varies in the direction of the thickness
of the membrane in such a manner that the water content thereof
adjacent to the negative electrode is the highest while that
adjacent to the positive electrode is the lowest. As a result, it
is believed that, in the cell of the invention, the surface of the
membrane repels water and promotes the back diffusion of water from
the positive electrode, which is to compensate for the water
content loss in the negative electrode, whereby the positive
electrode is prevented from being too wet and the catalytic action
in the cell is promoted to increase the cell output.
[0056] In the fuel cell of the invention, the plural
polyelectrolyte membranes are laminated that so the water content
of the laminate membrane adjacent to the negative electrode is
higher so as to facilitate the movement of water along with protons
from the negative electrode side to the positive electrode side.
Therefore, the fuel cell can be driven stably, and its output
capabilities are improved.
EXAMPLES
[0057] Having generally described this invention, a further
understanding can be obtained by reference to certain specific
examples which are provided herein for purposes of illustration
only and are not intended to be limiting unless otherwise
specified.
Example 1
[0058] An ethylene-tetrafluoroethylene copolymer film (thickness:
50 .mu.m) was exposed to 10 kGy of .gamma.-radiation in air at room
temperature, and then grafted with styrene at 60.degree. C. for 2
hours in such a manner that one surface of the film was contacted
with a mixture of 100 parts by volume of styrene and 20 parts by
volume of xylene while the other surface thereof was contacted with
xylene. After drying, the film was sulfonated by contacting both
surfaces of the film with a mixture of 5 parts by volume of
chlorosulfonic acid and 60 parts by volume of 1,2-dichloroethane at
room temperature for 1 hour. After drying, the film was hydrolyzed
in 1 N potassium hydroxide, and then dipped in 1 N hydrochloric
acid. Next, the film was washed with water at 90.degree. C. for 1
hour. The thus-prepared, solid polyelectrolyte membrane had an
ion-exchanging capacity of 1.69 milliequivalents/g, and a water
content at 80.degree. C. of 71% by weight. The contact angle with
water of the membrane on the surface that had been contacted with
styrene in the grafting reaction was 30.degree., and that on the
surface that had been contacted with xylene was 72.degree.. (The
contact angle with water of the starting
ethylene-tetrafluoroethylene copolymer film was 110.degree..) The
difference in the contact angle between both surfaces indicates
that the surface on which the contact angle was smaller had a
larger number of sulfonic acid groups and was therefore
hydrophilic. On the other hand, a .gamma.-radiation exposed film,
similar to the above, but having a thickness of 25 .mu.m, was
grafted in a mixture of 100 partsby volume of styrene and 20 parts
by volume of xylene and then processed in the same manner as above.
As a result, the thus-processed film had an ion-exchanging capacity
of 1.80 milliequivalents/g and a water content at 80.degree. C. of
86% by weight. From this, it is known that one surface of the solid
polyelectrolyte membrane produced herein had the highest water
content of around 86% by weight and that the water content of the
membrane gradually decreased in the direction of the thickness of
the membrane.
[0059] commercially-available carbon paper was coated with TEFLON
dispersion and then burned to make it water-repellent. One surface
of the thus-processed paper was coated with a mixture of
commercially-available, platinum-carried carbon (platinum: 40% by
weight), commercially-available NAFION solution and isopropanol, in
an amount of 0.35 mg/cm.sup.2 in terms of platinum, to prepare a
gas diffusion electrode.
[0060] The gas diffusion electrode was used as the positive
electrode and the negative electrode. The solid polyelectrolyte
membrane prepared above was sandwiched between both electrodes and
pressed under heat to construct a fuel cell. The V-I
characteristics of the fuel cell were measured at a hydrogen
pressure of 2.5 atmospheres (utilization: 80%), an air pressure of
2.5 atmospheres (utilization: 40%) and a cell temperature of
80.degree. C. As a result, the output voltage of the cell was 0.52
V at a current density of 1 A/cm.sup.2.
Example 2
[0061] A solid polyelectrolyte membrane was prepared in the same
manner as in Example 1, except that a mixture of 100 parts by
volume of styrene and 30 parts by volume of xylene and a mixture of
95 parts by volume of styrene, 5 parts by volume of divinylbenzene
and 30 parts by volume of xylene were used for the grafting
reaction. The membrane prepared herein had an ion-exchanging
capacity of 1.63 milliequivalents/g, and a water content at
80.degree. C. of 69% by weight. The contact angle with water of the
membrane on the surface that had been contacted with the
divinylbenzene-containing mixture in the grafting reaction was
52.degree., and that on the surface that had been contacted with
the divinylbenzene-free mixture was 33.degree.. On the other hand,
the same films were grafted separately with the two grafting
mixtures. One having been grafted with the
divinylbenzene-containing mixture had an ion-exchanging capacity of
1.72 milliequivalents/g and a water content at 80.degree. C. of 61%
by weight, while the other having been grafted with the
divinylbenzene-free mixture had an ion-exchanging capacity of 1.74
milliequivalents/g and a water content at 80.degree. C. of 78% by
weight. From these data, it is known that the water content of the
solid polyelectrolyte membrane produced herein gradually varies
from about 61 to 78% by weight in the direction of the thickness of
the membrane.
[0062] Using the membrane prepared herein and the same gas
diffusion electrodes as in Example 1, a fuel cell was constructed
in the same manner as in Example 1. The V-I characteristic of the
cell was measured under the same condition as in Example 1. As a
result, the output voltage of the cell was 0.50 V at a current
density of 1 A/cm.sup.2.
Example 3
[0063] A solid polyelectrolyte membrane was prepared in the same
manner as in Example 1, except that the two reactions for grafting
and sulfonation were effected in the manner mentioned below. In
this, the irradiated film was grafted by keeping both its surfaces
in contact with a mixture of 100 parts by volume of styrene and 30
parts by volume of xylene at 60.degree. C. for 2 hours. Then, after
having been dried, the grafted film was sulfonated at 40.degree. C.
for 1 hour with one surface being kept in contact with a mixture of
5 parts by volume of chlorosulfonic acid and 60 parts by volume of
1,2-dichloroethane, with the other surface being kept in contact
with 1,2-dichloroethane only. The membrane prepared herein had an
ion-exchanging capacity of 1.59 milliequivalents /g, and a water
content at 80.degree. C. of 68% by weight. The contact angle with
water of the membrane on the surface that had been contacted with
the chlorosulfonic acid-containing mixture in the sulfonation was
31.degree., and that on the surface that had been contacted with
the chlorosulfonic acid-free mixture was 71.degree.. On the other
hand, a .gamma.-radiation exposed film similar to the above, but
having a thickness of 25 .mu.m, was grafted in the same manner as
above, then sulfonated with both its surfaces in contact with a
mixture of 5 parts by volume of chlorosulfonic acid and 60 parts by
volume of 1,2-dichloroethane, and thereafter processed in the same
manner as above. As a result, the thus-processed film had an
ion-exchanging capacity of 1.76 milliequivalents/g and a water
content at 80.degree. C. of 81% by weight. From this, it is known
that one surface of the solid polyelectrolyte membrane produced
herein had the highest water content of around 81% by weight and
that the water content of the membrane gradually decreases in the
direction of the thickness of the membrane.
[0064] Using the membrane prepared herein and the same gas
diffusion electrodes as in Example 1, a fuel cell was constructed
in the same manner as in Example 1. The V-I characteristics of the
cell were measured under the same condition as in Example 1. As a
result, the output voltage of the cell was 0.48 V at a current
density of 1 A/cm.sup.2.
Comparative Example 1
[0065] A solid polyelectrolyte membrane was prepared in the same
manner as in Example 1, except that both surfaces of the film were
grafted with a mixture of 100 parts by volume of styrene and 30
parts by volume of xylene. The membrane prepared herein had an
ion-exchanging capacity of 1.71 milliequivalents/g, and a water
content at 80.degree. C. of 73% by weight. The contact angle with
water of the membrane was 28.degree..
[0066] Using the membrane prepared herein and the same gas
diffusion electrodes as in Example 1, a fuel cell was constructed
in the same manner as in Example 1. The V-I characteristics of the
cell were measured under the same condition as in Example 1. As a
result, the output voltage of the cell was 0.42 V at a current
density of 1 A/cm.sup.2.
Example 4
[0067] Two base films of ETFE each having a thickness of 25 .mu.m
and an area of 10 cm.sup.2 were, after having been washed with
acetone, exposed to .gamma.-radiation from Co60 at a dose of 6 kGy
and 10 kGy, respectively. These two irradiated films were placed
one upon another and put in a reactor tube, to which was added 50
ml of styrene, and the reactor tube was fully purged with nitrogen.
Next, the reactor tube was dipped in a bath at 60.degree. C., and
the films therein were grafted for 15 hours. Next, the films were
each washed three times with 100 ml of benzene each, and dried in a
drier. The two films both having been grafted under the same
condition but having been irradiated at different doses had
different degrees of grafting of 38% and 55%, respectively.
[0068] Next, the two films were dipped in and reacted with a
mixture of 30 parts (by weight--the same shall apply hereunder) of
chlorosulfonic acid and 70 parts of 1,1,2,2-tetrachloroethane, at
room temperature for 30 minutes in a nitrogen atmosphere, and then
washed with 1,1,2,2-tetrachloroethane to remove the remaining
chlorosulfonic acid therefrom. Then, these were further washed with
ion-exchanged water.
[0069] Next, these films were dipped in an aqueous solution of 2 N
potassium hydroxide at 100.degree. C. for 30 minutes, and then in
an aqueous solution of 1 N sulfuric acid at 100.degree. C. for 30
minutes, and thereafter fully washed with ion-exchanged water. The
ion-exchanging capacity of the resulting membranes were measured to
be 1.7 and 2.1 meq/g, respectively. These were sandwiched between
gas diffusion electrodes each having a platinum content of 0.4
mg/cm.sup.2, and set in a hot press, in which they were pressed
together at 100.degree. C. and under 20 kg/cm.sup.2 for 5 minutes,
then at 130.degree. C. and under 20 kg/cm.sup.2 for 5 minutes, then
at 165.degree. C. and under 20 kg/cm.sup.2 for 5 minutes, and
finally at 165.degree. C. and under an increased pressure of 80
kg/cm.sup.2 for 90 seconds. Thus a membrane-electrode structure was
obtained. Using this, a cell was constructed and tested for its
output characteristics. The test data are in FIG. 1, which shows
the time-dependent variation in the output voltage of the cell.
Comparative Example 2
[0070] The same process as in Example 4 was repeated, except that
an ETFE film having a thickness of 50 .mu.m was used as the base
film and was exposed to .gamma.-radiation at a dose of 10 kGy. The
degree of grafting and the ion-exchanging capacity of the film were
53% and 2.0 meq/g, respectively.
[0071] Using the single membrane prepared herein and the same
electrodes as in Example 4, a membrane-electrode structure was
produced through thermal pressing under the same condition as in
Example 4. Using the membrane-electrode structure, a cell was
constructed and tested for its output characteristics. The test
data are in FIG. 2, which shows the time-dependent variation in the
output voltage of the cell.
[0072] As is shown in FIG. 1 and FIG. 2, the output voltage of the
cell of Example 4 of the invention did not decrease with the lapse
of time, but that of the cell of Comparative Example 2 did
decrease. This supports the practical usefulness of the cell of
Example 4.
Example 5
[0073] Two base films of ETFE each having a thickness of 25 .mu.m
and an area of 10 cm.sup.2 were, after having been washed with
acetone, exposed to .gamma.-radiation from Co60 at a dose of 6 kGy.
These two irradiated films were put in different reactor tubes, to
which was added 25 ml of styrene each. Then, the reactor tubes were
fully purged with nitrogen. Next, the reactor tubes were dipped in
a bath at 60.degree. C., and the films therein were grafted for 15
hours and 25 hours, respectively. Next, the films were washed three
times with 100 ml of benzene each, and dried in a drier. The two
films thus had different degrees of grafting of 39% and 52%,
respectively.
[0074] Next, the two films were placed one upon another, and dipped
in and reacted with a mixture of 30 parts (by weight--the same
shall apply hereunder) of chlorosulfonic acid and 70 parts of
1,1,2,2-tetrachloroetha- ne, at room temperature for 30 minutes in
a nitrogen atmosphere, and then washed with
1,1,2,2-tetrachloroethane to remove the remaining chlorosulfonic
acid therefrom. Then, these films were further washed with
ion-exchanged water. Next, these films were dipped in an aqueous
solution of 2 N potassium hydroxide at 100.degree. C. for 30
minutes, and then in an aqueous solution of 1 N sulfuric acid at
100.degree. C. for 30 minutes, and thereafter fully washed with
ion-exchanged water. The ion-exchanging capacity of the resulting
membranes were measured to be 1.7 and 2.0 meq/g, respectively.
[0075] These membranes were sandwiched between gas diffusion
electrodes each having a platinum content of 0.4 mg/cm.sup.2, and
set in a hot press, in which these were pressed together at
100.degree. C. and under 20 kg/cm.sup.2 for 5 minutes, then at
130.degree. C. and under 20 kg/cm.sup.2 for 5 minutes, then at
165.degree. C. and under 20 kg/cm.sup.2 for 5 minutes, and finally
at 165.degree. C. and under an increased pressure of 80 kg/cm.sup.2
for 90 seconds. Thus was obtained a membrane-electrode structure.
Using this, a cell was constructed and tested for its output
characteristics. The test data are in FIG. 3, which shows the
time-dependent variation in the output voltage of the cell.
Comparative Example 3
[0076] The same process as in Example 4 was repeated, except that
an ETFE film having a thickness of 50 .mu.m was used as the base
film and that this was grafted at 60.degree. C. for 25 hours. The
degree of grafting and the ion-exchanging capacity of the film were
50% and 1.9 meq/g, respectively.
[0077] Using the single membrane prepared herein and the same
electrodes as in Example 4, a membrane-electrode structure was
produced through thermal pressing under the same condition as in
Example 4. Using the membrane-electrode structure, a cell was
constructed and tested for its output characteristics. The test
data are in FIG. 4, which shows the time-dependent variation in the
output voltage of the cell.
[0078] As is shown in FIG. 3 and FIG. 4, the output voltage of the
cell of Example 5 of the invention did not decrease with the lapse
of time, but that of the cell of Comparative Example 3 decreased.
This supports the practical usefulness of the cell of Example
5.
[0079] As in its construction and production method mentioned
above, the solid polyelectrolyte-type fuel cell of the invention
has the following advantages:
[0080] 1. The polyelectrolyte membranes in the cell are laminated
so that the water content of the laminate membrane adjacent to the
negative electrode is higher so as to facilitate the movement of
water along with protons from the negative electrode side to the
side of the positive electrode side while the cell is driven to
generate electric power. Therefore, the cell can be driven stably,
and its output capabilities are improved.
[0081] 2. Plural polymer films, while being placed one upon
another, can be grafted and sulfonated and are fused together along
with the reaction. In addition, while the laminate membrane is
integrated with electrodes through thermal pressing, the
adhesiveness between the laminated films is further increased.
Therefore, the laminate membrane does not require any additional
step for specifically bonding and adhering the laminated films
together.
[0082] 3. The solid polyelectrolyte-type laminate membrane can be
produced through irradiation and grafting polymerization, and the
production costs are therefore significantly reduced. Therefore,
the cell can be produced at low cost and is favorably used in
various fields of electric cars, leisure appliances, etc.
[0083] Further advantages of the solid polyelectrolyte-type cell of
the invention are mentioned below.
[0084] In the cell, the water content of the polyelectrolyte
membrane is suitably controlled relative to the adjacent positive
electrode and negative electrode. Therefore, the power output of
the cell is high, and the properties of the cell are good.
[0085] In the cell in which the solid polyelectrolyte membrane is a
cation-exchanging membrane of a copolymer that comprises a main
chain as formed through copolymerization of a fluorocarbon vinyl
monomer and a hydrocarbon vinyl monomer, and sulfonic acid
group-containing hydrocarbon side chains, the cation-exchanging
membrane is a laminate of at least two layers each having a
different water content, and the water content of the membrane
adjacent to the negative electrode is higher than that adjacent to
the positive electrode. Specifically, in the cell, the polymer
films are so laminated that the water content of the laminate
membrane adjacent to the negative electrode is higher so as to
facilitate the movement of water along with protons from the
negative electrode side to the positive electrode side while the
cell is driven to generate electric power. Therefore, the cell can
be driven stably, and its outputting capabilities are improved. In
addition, the cell can be produced at low cost.
[0086] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
[0087] The priority documents of the present application, Japanese
Patent Application Nos. 09-326601 and 09-345422, filed on Nov. 27
and Dec. 15, 1997, respectively, are hereby incorporated by
reference.
* * * * *