U.S. patent application number 13/846763 was filed with the patent office on 2013-08-22 for polyelectrolyte and process for producing the polyelectrolyte.
This patent application is currently assigned to DAIKIN INDUSTRIES, LTD.. The applicant listed for this patent is ASAHI KASEI E-MATERIALS CORPORATION, DAIKIN INDUSTRIES, LTD.. Invention is credited to Tadashi INO, Kohei KITA, Masahiro KONDO, Naoto MIYAKE, Takahiko MURAI, Masaharu NAKAZAWA, Naoki SAKAMOTO, Noriyuki SHINOKI, Takashi YOSHIMURA.
Application Number | 20130216937 13/846763 |
Document ID | / |
Family ID | 41090850 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130216937 |
Kind Code |
A1 |
KITA; Kohei ; et
al. |
August 22, 2013 |
POLYELECTROLYTE AND PROCESS FOR PRODUCING THE POLYELECTROLYTE
Abstract
The present invention provides an electrolyte having high
conductivity even under high-temperature low-humidification
conditions (e.g. at a temperature of 100 to 120.degree. C. and a
humidity of 20 to 50% RH) and thereby makes it possible to realize
a higher performance fuel cell. The present invention is a
fluoropolymer electrolyte having an equivalent weight (EW) of not
less than 250 but not more than 700 and a proton conductivity of
not lower than 0.10 S/cm as measured at a temperature of
110.degree. C. and a relative humidity of 50% RH and comprising a
COOZ group- or SO.sub.3Z group-containing monomer unit, wherein Z
represents an alkali metal, an alkaline earth metal, hydrogen atom
or NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom.
Inventors: |
KITA; Kohei; (Tokyo, JP)
; MURAI; Takahiko; (Tokyo, JP) ; SAKAMOTO;
Naoki; (Tokyo, JP) ; MIYAKE; Naoto; (Tokyo,
JP) ; INO; Tadashi; (Osaka, JP) ; SHINOKI;
Noriyuki; (Osaka, JP) ; NAKAZAWA; Masaharu;
(Osaka, JP) ; KONDO; Masahiro; (Osaka, JP)
; YOSHIMURA; Takashi; (Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASAHI KASEI E-MATERIALS CORPORATION;
DAIKIN INDUSTRIES, LTD.; |
|
|
US
US |
|
|
Assignee: |
DAIKIN INDUSTRIES, LTD.
Osaka
JP
ASAHI KASEI E-MATERIALS CORPORATION
Tokyo
JP
|
Family ID: |
41090850 |
Appl. No.: |
13/846763 |
Filed: |
March 18, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12933325 |
Sep 17, 2010 |
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PCT/JP2009/054746 |
Mar 12, 2009 |
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13846763 |
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Current U.S.
Class: |
429/494 |
Current CPC
Class: |
C08F 6/22 20130101; C08F
214/262 20130101; C08F 14/185 20130101; H01M 8/1023 20130101; C08F
2810/00 20130101; C08F 2/26 20130101; C08J 2327/18 20130101; C08J
5/2237 20130101; C08F 8/22 20130101; C08F 8/12 20130101; H01M
8/1039 20130101; C08F 20/04 20130101; C08F 28/02 20130101; C08F
2800/10 20130101; C08F 8/44 20130101; H01B 1/122 20130101; C08F
2810/50 20130101; Y02E 60/50 20130101; C08F 8/12 20130101; C08F
14/26 20130101; C08F 2/16 20130101; C08F 8/44 20130101; C08F 8/26
20130101; C08F 8/12 20130101; C08F 214/262 20130101; C08F 8/12
20130101; C08F 8/44 20130101; C08F 8/26 20130101; C08F 8/12
20130101; C08F 8/22 20130101; C08F 214/262 20130101; C08F 214/262
20130101; C08F 8/12 20130101; C08F 6/22 20130101; C08F 8/44
20130101; C08L 27/18 20130101; C08F 8/26 20130101; C08F 8/12
20130101; C08F 8/22 20130101; C08F 214/262 20130101; C08F 6/22
20130101; C08F 214/26 20130101; C08F 216/1475 20200201; C08L 27/18
20130101; C08F 214/26 20130101; C08F 216/1475 20200201 |
Class at
Publication: |
429/494 |
International
Class: |
H01M 8/10 20060101
H01M008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 19, 2008 |
JP |
2008-071245 |
Claims
1.-7. (canceled)
8. An electrolyte membrane constituted of a fluoropolymer
electrolyte having an equivalent weight (EW) of not less than 250
but not more than 700 and a proton conductivity of not lower than
0.10 S/cm as measured at a temperature of 110.degree. C. and a
relative humidity of 50% RH and which comprises a COOZ group- or
SO.sub.3Z group-containing monomer unit, wherein Z represents an
alkali metal, an alkaline earth metal, hydrogen atom or
NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom.
9. An electrolyte solution containing the fluoropolymer electrolyte
according to claim 8.
10. A membrane-electrode assembly comprising the fluoropolymer
electrolyte according to claim 8.
11. A solid polymer fuel cell comprising the membrane-electrode
assembly according to claim 10.
12. A production method for a fluoropolymer electrolyte, comprising
a step (1) of obtaining a fluoropolymer electrolyte precursor by
emulsion polymerization and a step (2) of obtaining said
fluoropolymer electrolyte by subjecting said fluoropolymer
electrolyte precursor to a chemical treatment, said fluoropolymer
electrolyte having an equivalent weight (EW) of not less than 250
but not more than 700 and a proton conductivity of not lower than
0.10 S/cm as measured at a temperature of 110.degree. C. and a
relative humidity of 50% RH and comprising a COOZ group- or
SO.sub.3Z group-containing monomer unit, wherein Z represents an
alkali metal, an alkaline earth metal, hydrogen atom or
NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom.
13. The production method according to claim 12, wherein the step
(1) comprises carrying out the emulsion polymerization at a
temperature of not lower than 0.degree. C. but not higher than
40.degree. C.
14. The production method according to claim 12, wherein the
fluoropolymer electrolyte precursor has a group convertible to COOZ
or SO.sub.3Z (wherein Z represents an alkali metal, an alkaline
earth metal, hydrogen atom or NR.sup.1R.sup.2R.sup.3R.sup.4 in
which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each independently
represents an alkyl group containing 1 to 3 carbon atoms or
hydrogen atom) by the chemical treatment, is melt-flowable and has
a melt flow rate of 0.01 to 100 g/10 minutes.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S.
application Ser. No. 12/933,325 filed Sep. 17, 2010, which is a 371
of PCT International Application No. PCT/JP2009/054746 filed Mar.
12, 2009, which claims benefit of Japanese Patent Application No.
2008-071245 filed Mar. 19, 2008. The above-noted applications are
incorporated herein by reference in their entirety.
TECHNICAL FIELD
[0002] The present invention relates to a polymer electrolyte
suited for an electrolyte membrane for a solid polymer electrolyte
fuel cell and to a production method for the same, among
others.
BACKGROUND ART
[0003] A fuel cell, in which such a fuel as hydrogen or methanol is
electrochemically oxidized to thereby convert a chemical energy of
the fuel directly to electrical energy for consumption, has
attracted attention as a clean electric energy supply source. In
particular, a solid polymer electrolyte fuel cell, which can
operate at lower temperatures as compared with other fuel cells, is
expected to be useful as or in an alternative automotive power
source, a cogeneration system for household use and a portable
electric generator, among others.
[0004] Such a solid polymer electrolyte fuel cell is at least
equipped with a membrane-electrode assembly which comprises an
electrolyte membrane and two gas diffusion electrodes joined
thereto in the manner of sandwiching the same between them, each of
the electrodes being an electrode catalyst layer-gas diffusion
layer laminate. The electrolyte membrane so referred to herein is a
material having strongly acidic groups such as sulfonic acid groups
or carboxylic acid groups within each polymer chain and having a
selective permeability for proton. Preferably used as such an
electrolyte membrane is perfluorinated proton exchange membrane,
typically Nafion (registered trademark; products of du Pont) having
high chemical stability.
[0005] On the occasion of operation of a fuel cell, a fuel (e.g.
hydrogen) is fed to the gas diffusion electrode on the anode side,
and an oxidizing agent (e.g. oxygen or air) to the gas diffusion
electrode on the cathode side, and both the electrodes are
connected to each other via an external circuit, whereby an
operation of the fuel cell is realized. More specifically, when
hydrogen is used as the fuel, hydrogen is oxidized on an anode
catalyst to give protons. The protons pass through an electrolyte
binder within an anode catalyst layer and then migrate within the
electrolyte membrane and then through an electrolyte binder within
a cathode catalyst layer to arrive at the cathode catalyst surface.
On the other hand, the electrons formed simultaneously with the
protons by the oxidation of hydrogen travel through the external
circuit to arrive at the gas diffusion electrode on the cathode
side. On the cathode catalyst, the protons react with oxygen in the
oxidizing agent to form water. And, on that occasion, electric
energy is generated.
[0006] Owing to a characteristic feature that it is of a reduced
environmental load type and can secure high energy conversion
efficiency, the solid polymer electrolyte fuel cell is expected to
be usable in a stationary cogeneration system or serve as a
vehicle-mounted power source. In the current automotive field, a
fuel cell is generally operated at about 80.degree. C. For the
spread of fuel cell vehicles, however, it is necessary to reduce a
radiator size, simplify a humidifier and attain cost reduction. To
that end, those electrolyte membranes are desired which are
appropriate for operation under high-temperature low-humidification
conditions (corresponding to an operation temperature of 100 to
120.degree. C. and a humidity of 20 to 50% RH) and can show high
performance in a wide-ranging operation environment (room
temperature to 120.degree. C./20 to 100% RH). More specifically, a
proton conductivity at 50% RH of not lower than 0.10 S/cm is
required to enable operation at a temperature of 100.degree. C.,
and a proton conductivity at 20% RH of not lower than 0.10 S/cm is
required to enable operation at a temperature of 120.degree. C., as
shown in Non-Patent Document 1.
[0007] However, a conductivity of the prior art perfluorinated
proton exchange membrane greatly depends on the humidity and
markedly drops at a humidity of 50% RH or below, in particular.
Therefore, Patent Documents 1 to 3 disclose fluorinated electrolyte
membranes having an equivalent weight (EW), namely a dry weight per
equivalent of proton exchange groups, of 670 to 776 EW (g/eq). By
lowering the EW value in this manner, namely by increasing a proton
exchange capacity, it becomes possible to produce improvements in
conductivity. Further, Patent Document 4 discloses electrolyte
membranes hardly soluble in hot water in spite of their having a
low EW value, mentioning, as an example, an electrolyte membrane
with an EW of 698. Patent Document 5 discloses a production example
for a polymer electrolyte with an EW of 564.
[0008] It is also known that a perfluorinated proton exchange
membrane becomes deteriorated after a prolonged period of use;
hence, various methods of stabilization have been proposed. Thus,
for example, Patent Document 6 discloses fluoropolymer electrolytes
obtained through a polymerization process in which the
copolymerization is carried out at a polymerization temperature of
0 to 35.degree. C. using a radical polymerization initiator which
comprises a fluorinated compound having a molecular weight of not
lower than 450. [0009] Patent Document 1: Japanese Kokai
Publication H06-322034 [0010] Patent Document 2: Japanese Kokai
Publication H04-366137 [0011] Patent Document 3: WO 2002/096983
[0012] Patent Document 4: Japanese Kokai Publication 2002-352819
[0013] Patent Document 5: Japanese Kokai Publication S63-297406
[0014] Patent Document 6: Japanese Kokai Publication 2006-173098
[0015] Non-Patent Document 1: H. Gasteiger and M. Mathias, In
Proton Conducting Membrane Fuel Cells, PV2002-31, pp. 1-22, The
Electrochemical Society Proceedings Series (2002).
DISCLOSURE OF INVENTION
Problems which the Invention is to Solve
[0016] However, the electrolyte membranes disclosed in the
above-cited Patent Documents 1 to 6 show still low conductivity
levels, far from 0.10 S/cm, at a humidity of 50% RH or below. It is
an object of the present invention to provide an electrolyte having
high conductivity even under high-temperature low-humidification
conditions and thereby make it possible to realize a higher
performance fuel cell.
Means for Solving the Problems
[0017] As a result of inventive investigations made by the present
inventors, it was found that a use of a fluorinated electrolyte
precursor prepared by a particular process for polymerization makes
it possible to control an ion cluster structure to be formed in a
fluorinated electrolyte and that by controlling the ion cluster
structure of an electrolyte membrane, it becomes possible to
realize high levels of conductivity even at low humidity levels.
Such findings have led to completion of the present invention.
[0018] The present invention thus provides a polymer electrolyte
membrane, a production method thereof, a membrane-electrode
assembly, and a solid polymer electrolyte fuel cell.
[0019] (1) A fluoropolymer electrolyte having an equivalent weight
(EW) of not less than 250 but not more than 700 and a proton
conductivity of not lower than 0.10 S/cm as measured at a
temperature of 110.degree. C. and a relative humidity of 50% RH and
which comprises a COOZ group- or SO.sub.3Z group-containing monomer
unit, wherein Z represents an alkali metal, an alkaline earth
metal, hydrogen atom or NR.sup.1R.sup.2R.sup.3R.sup.4 in which
R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each independently represents
an alkyl group containing 1 to 3 carbon atoms or hydrogen atom.
[0020] (2) The fluoropolymer electrolyte according to (1), which
comprises a repeating unit (.alpha.) derived from a COOZ group- or
SO.sub.3Z group-containing monomer represented by the general
formula (I):
CF.sub.2.dbd.CF(CF.sub.2).sub.k--O.sub.l--(CF.sub.2CFY.sup.1--O).sub.n---
(CFY.sup.2).sub.m-A.sup.1 (I)
wherein Y.sup.1 represents F, Cl or a perfluoroalkyl group; k
represents an integer of 0 to 2, l represents 0 or 1, and n
represents an integer of 0 to 8 and n atoms or groups of Y.sup.1
may be the same or different; Y.sup.2 represents F or Cl; m
represents an integer of 0 to 6 provided that when m=0, l=0 and
n=0; m atoms of Y.sup.2 may be the same or different; and A.sup.1
represents COOZ or SO.sub.3Z in which Z represents an alkali metal,
an alkaline earth metal, hydrogen atom or
NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom, and a repeating unit (.beta.)
derived from an ethylenic fluoromonomer copolymerizable with the
COOZ group- or SO.sub.3Z group-containing monomer, the content of
the repeating unit (.alpha.) being 10 to 95 mole percent and the
content of the repeating unit (.beta.) being 5 to 90 mole percent,
with the sum of the repeating unit (.alpha.) content and the
repeating unit (.beta.) content being 95 to 100 mole percent.
[0021] (3) The fluoropolymer electrolyte according to (2), wherein
k is 0, l is 1, Y.sup.1 is F, n is 0 or 1, Y.sup.2 is F, m is 2 or
4 and A.sup.1 is --SO.sub.3H.
[0022] (4) The fluoropolymer electrolyte according to (3), wherein
n is 0 and m is 2.
[0023] (5) The fluoropolymer electrolyte according to (1), (2), (3)
or (4), wherein the distance between ionic clusters at 25.degree.
C. and a relative humidity of 50% RH as calculated from the formula
(I) given below following small angle X-ray scattering measurement
is not shorter than 0.1 nm but not longer than 2.6 nm:
d=.lamda./2/sin(.theta.m) (1)
wherein d is the distance between ionic clusters, .lamda. is
wavelength of incident X ray used in small angle X-ray scattering
measurement and em is peak-showing Bragg angle.
[0024] (6) The fluoropolymer electrolyte according to (1), (2),
(3), (4) or (5), which is obtained by a chemical treatment of a
fluoropolymer electrolyte precursor, wherein the fluoropolymer
electrolyte precursor has a group convertible to COOZ or SO.sub.3Z
(wherein Z represents an alkali metal, an alkaline earth metal,
hydrogen atom or NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1,
R.sup.2, R.sup.3 and R.sup.4 each independently represents an alkyl
group containing 1 to 3 carbon atoms or hydrogen atom) upon the
chemical treatment, is melt-flowable and has a melt flow rate of
0.01 to 100 g/10 minutes.
[0025] (7) The fluoropolymer electrolyte according to (6), wherein
the chemical treatment comprises immersion in an basic reacting
liquid.
[0026] (8) An electrolyte membrane constituted of the fluoropolymer
electrolyte according to (1), (2), (3), (4), (5), (6) or (7).
[0027] (9) An electrolyte solution containing the fluoropolymer
electrolyte according to (1), (2), (3), (4), (5), (6) or (7).
[0028] (10) A membrane-electrode assembly comprising the
fluoropolymer electrolyte according to (1), (2), (3), (4), (5), (6)
or (7).
[0029] (11) A solid polymer fuel cell comprising the
membrane-electrode assembly according to (10).
[0030] (12) A production method for a fluoropolymer electrolyte,
comprising a step (1) of obtaining a fluoropolymer electrolyte
precursor by emulsion polymerization and a step (2) of obtaining
the fluoropolymer electrolyte by subjecting the fluoropolymer
electrolyte precursor to a chemical treatment, the fluoropolymer
electrolyte having an equivalent weight (EW) of not less than 250
but not more than 700 and a proton conductivity of not lower than
0.10 S/cm as measured at a temperature of 110.degree. C. and a
relative humidity of 50% RH and comprising a COOZ group- or
SO.sub.3Z group-containing monomer unit, wherein Z represents an
alkali metal, an alkaline earth metal, hydrogen atom or
NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom.
[0031] (13) The production method according to (12), wherein the
step (1) comprises carrying out the emulsion polymerization at a
temperature of not lower than 0.degree. C. but not higher than
40.degree. C.
[0032] (14) The production method according to (12) or (13),
wherein the fluoropolymer electrolyte precursor has a group
convertible to COOZ or SO.sub.3Z (wherein Z represents an alkali
metal, an alkaline earth metal, hydrogen atom or
NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom) by the chemical treatment, is
melt-flowable and has a melt flow rate of 0.01 to 100 g/10
minutes.
Effects of the Invention
[0033] The fluoropolymer electrolyte according to the invention
shows high proton conductivity even under low humidity conditions
and can provide a fuel cell having good performance
characteristics.
BEST MODES FOR CARRYING OUT THE INVENTION
[0034] Hereinafter, best modes for carrying out the present
invention (hereinafter, "modes of practice of the invention") are
described in detail. The invention is not limited to the modes of
practice described below but can be carried out in various modified
modes falling within the scope of the invention.
[0035] The fluoropolymer electrolyte according to the invention has
an equivalent weight (EW), namely a dry weight per equivalent of
ion exchange groups, of not less than 250 but not more than 700. An
upper limit to EW is preferably 650, more preferably 600, still
more preferably 550. A lower limit to EW is preferably 300, more
preferably 350, still more preferably 400. A low level of EW is
preferred in view of the resulting increased level of conductivity
but may sometimes result in increased solubility in hot water;
therefore, such an appropriate range as mentioned above is
desired.
[0036] The fluoropolymer electrolyte according to the invention has
a proton conductivity of not lower than 0.10 S/cm as measured at a
temperature of 110.degree. C. and a relative humidity (RH) of 50%.
Preferably, it has a proton conductivity at 40% RH of not lower
than 0.10 S/cm; more preferably, it has a proton conductivity at
30% RH of not lower than 0.10 S/cm; still more preferably, it has a
proton conductivity at 20% RH of not lower than 0.10 S/cm. As for
the proton conductivity of the fluoropolymer electrolyte according
to the invention, the higher, the better; however, the proton
conductivity at a temperature of 110.degree. C. and a relative
humidity of 50% RH may be, for example, not higher than 1.0
S/cm.
[0037] Further, the fluoropolymer electrolyte according to the
invention has a particular ion cluster structure. Thus, the
fluoropolymer electrolyte according to the invention preferably has
a distance between ionic clusters of not shorter than 0.1 nm but
not longer than 2.6 nm as measured at 25.degree. C. and 50% RH.
FIG. 1 is a graphic representation of a relation between the
distance between ionic clusters (abscissa) and the ionic
conductivity (ordinate) under high-temperature low-humidification
conditions as obtained by plotting the results obtained in the
examples and comparative examples given later herein; the figure
indicates that the distance between ionic clusters of not longer
than 2.6 nm result in rapid increases in the conductivity.
[0038] A more preferred upper limit to the distance between ionic
clusters is 2.5 nm. A lower limit to the distance between ionic
clusters is more preferably 0.5 nm, still more preferably 1.0 nm,
most preferably 2.0 nm.
[0039] The ion cluster is an ion channel formed as a result of
gathering of a plurality of proton exchange groups, and
perfluorinated proton exchange membranes, typically Nafion
membranes, are also considered to have such an ion cluster
structure (cf. e.g. Gierke, T. D., Munn, G. E., Wilson, F. C. J.
Polymer Sci. Polymer Phys., 1981, 19, 1687).
[0040] The distance between ionic clusters d can be determined by
the measurement and calculation methods mentioned below.
[0041] A fluoropolymer electrolyte in the form of a membrane is
subjected to small angle X-ray scattering measurement in an
atmosphere maintained at 25.degree. C. and 50% RH. Scattering
intensities obtained are plotted against Bragg angles .theta., and
the Bragg angle .theta.m at the position of the cluster
structure-due peak generally appearing at 2.theta.>1.degree. is
calculated. The distance between ionic clusters d is calculated
from em according to the following expression (1):
d=.lamda./2/sin(.theta.m) (1)
(where .lamda. is incident X ray wavelength.)
[0042] In cases where the membrane is prepared by casting, the
membrane is annealed beforehand at 160.degree. C. The fluoropolymer
electrolyte is treated so that the terminal groups thereof as
represented by COOZ or SO.sub.3Z may be converted to COOH or
SO.sub.3H groups. Prior to the measurement, the sample membrane is
maintained in an atmosphere maintained at 25.degree. C. and 50% RH
for at least 30 minutes and, then, the measurement is carried
out.
[0043] The fluoropolymer electrolyte according to the invention,
which has a short distance between ionic clusters, supposedly
facilitates the transfer of protons between ion clusters; thus, it
shows high conductivity even at low humidity levels.
[0044] The fluoropolymer electrolyte according to the invention has
a COOZ group- or SO.sub.3Z group-containing monomer unit (wherein Z
represents an alkali metal, an alkaline earth metal, hydrogen atom
or NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.4, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom).
[0045] In the above-mentioned fluoropolymer electrolyte, the COOZ
group- or SO.sub.3Z group-containing monomer units preferably
account for 10 to 95 mole percent of all monomer units. The term
"all monomer units" as used herein refers to all monomer-derived
moieties from the viewpoint of the molecular structure of the
fluoropolymer electrolyte.
[0046] The COOZ group- or SO.sub.3Z group-containing monomer unit
mentioned above is generally derived from a COOZ group- or
SO.sub.3Z group-containing monomer represented by the general
formula (I):
CF.sub.2.dbd.CF(CF.sub.2).sub.k--O.sub.l--(CF.sub.2CFY.sup.1--O).sub.n---
(CFY.sup.2).sub.m-A.sup.1 (I)
wherein Y.sup.1 represents F, Cl or a perfluoroalkyl group; k
represents an integer of 0 to 2, l represents 0 or 1, and n
represents an integer of 0 to 8 and n atoms or groups of Y.sup.1
may be the same or different; Y.sup.2 represents F or Cl; m
represents an integer of 0 to 6 provided that when m=0, l=0 and
n=0; m atoms of Y.sup.2 may be the same or different; and A.sup.1
represents COOZ or SO.sub.3Z in which Z represents an alkali metal,
an alkaline earth metal, hydrogen atom or
NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom.
[0047] Referring to the above general formula (I), it is more
preferred from the synthesis and operability viewpoint that k be
equal to 0, l to 1, and n to 0 or 1, still more preferably to 0,
and that Y.sup.2 be F and m be an integer of 2 to 6, still more
preferably Y.sup.2 be F and m be 2 or 4, most preferably Y.sup.2 be
F and m be 2.
[0048] In producing the fluoropolymer electrolyte mentioned above,
the COOZ group- or SO.sub.3H group-containing monomer mentioned
above may be used singly or in combination of two or more
species.
[0049] The fluoropolymer electrolyte according to the invention is
preferably a copolymer comprising the repeating unit (.alpha.)
derived from the COOZ group- or SO.sub.3Z group-containing monomer
mentioned above and the repeating unit (.beta.) derived from an
ethylenic fluoromonomer copolymerizable with the COOZ group- or
SO.sub.3Z group-containing monomer.
[0050] The ethylenic fluoromonomer which is to constitute the
repeating unit (.beta.) mentioned above is a monomer having vinyl
group but having no ether oxygen (--O--), and the hydrogen atoms of
the vinyl group may be partially or fully substituted by fluorine
atom or fluorine atoms.
[0051] The "ether oxygen" so referred to herein means the --O--
structure constituting the monomer molecule.
[0052] As the above ethylenic fluoromonomer, there may be
mentioned, for example, a haloethylenic fluoromonomer represented
by the general formula (II):
CF.sub.2.dbd.CF--Rf.sup.1 (II)
wherein Rf.sup.1 represents F, Cl or a straight or branched
fluoroalkyl group containing 1 to 9 carbon atoms, or a
hydrogen-containing fluoroethylenic fluoromonomer represented by
the general formula (III):
CHY.sup.3.dbd.CFY.sup.4 (III)
wherein Y.sup.3 represents H or F and Y.sup.4 represents H, F, Cl
or a straight or branched fluoroalkyl group containing 1 to 9
carbon atoms.
[0053] As the above ethylenic fluoromonomer, there may be
mentioned, for example, tetrafluoroethylene [TFE],
hexafluoropropylene [HFP], chlorotrifluoroethylene [CTFE], vinyl
fluoride, vinylidene fluoride [VDF], trifluoroethylene,
hexafluoroisobutylene and perfluorobutylethylene; among them, TFE,
VDF, CTFE, trifluoroethylene, vinyl fluoride and HFP are preferred,
TFE, CTFE and HFP are more preferred, TFE and HFP are still more
preferred, and TFE is most preferred. Such ethylenic fluoromonomers
may be used singly or in combination of two or more.
[0054] The fluoropolymer electrolyte according to the invention is
preferably a copolymer having a content of the COOZ group- or
SO.sub.3Z group-containing monomer-derived repeating unit (.alpha.)
of 10 to 95 mole percent and a content of the ethylenic
fluoromonomer-derived repeating unit (.beta.) of 5 to 90 mole
percent, with the sum of the repeating unit (.alpha.) content and
the repeating unit (.beta.) content being 95 to 100 mole
percent.
[0055] A more preferred lower limit to the content of the COOZ
group- or SO.sub.3Z group-containing monomer-derived repeating unit
(.alpha.) is 15 mole percent, a still more preferred lower limit
thereto is 20 mole percent, a more preferred upper limit thereto is
60 mole percent, and a still more preferred upper limit thereto is
50 mole percent.
[0056] A more preferred lower limit to the content of the ethylenic
fluoromonomer-derived repeating unit (.beta.) is 35 mole percent, a
still more preferred lower limit thereto is 45 mole percent, a more
preferred upper limit thereto is 85 mole percent, and a still more
preferred upper limit thereto is 80 mole percent.
[0057] The fluoropolymer electrolyte according to the invention may
further comprise, as a repeating unit derived from a third
component monomer other than those mentioned above, a repeating
unit (.gamma.) derived from a vinyl ether other than the COOZ
group- or SO.sub.3Z group-containing monomer, preferably at a
content level of 0 to 5 mole percent, more preferably not higher
than 4 mole percent, still more preferably not higher than 3 mole
percent.
[0058] The composition of the fluoropolymer electrolyte can be
calculated from the measured values obtained in melt NMR at
300.degree. C., for instance.
[0059] The vinyl ether which is other than the COOZ group- or
SO.sub.3Z group-containing monomer and constitutes the repeating
unit (.gamma.) is not particularly restricted provided that it does
not contain either COOZ group or SO.sub.3Z group; thus, it
includes, among others, fluorovinyl ethers, more preferably
perfluorovinyl ethers, represented by the general formula (IV):
CF.sub.2.dbd.CF--O--Rf.sup.2 (IV)
wherein Rf.sup.2 represents a fluoroalkyl group containing 1 to 9
carbon atoms or a fluoropolyether group containing 1 to 9 carbon
atoms, or hydrogen-containing vinyl ethers represented by the
general formula (V):
CHY.sup.5.dbd.CF--O--Rf.sup.3 (V)
wherein Y.sup.5 represents H or F and Rf.sup.3 represents a
straight or branched fluoroalkyl group containing 1 to 9 carbon
atoms, which may contain at least one ether group. Such vinyl
ethers may be used singly or two or more of them may be used in
combination.
[0060] When the fluoropolymer electrolyte according to the
invention is used as an electrolyte membrane, the membrane
preferably has a thickness of not less than 1 .mu.m but not more
than 500 .mu.m, more preferably not less than 2 .mu.m but not more
than 100 .mu.m, still more preferably not less than 5 .mu.m but not
more than 50 .mu.m. A thin membrane can lead to a reduction in
direct current resistance during power generation but may possibly
cause an increased gas permeation, so that such an appropriate
range as specified above is desirable. In certain cases, the
membrane may further comprise a porous membrane prepared from a
PTFE membrane by stretching treatment, as described in Japanese
Kokai Publication H08-162132, or a fibrillated fiber, as described
in Japanese Kokai Publication S53-149881 and Japanese Patent
Publication S63-61337.
[0061] The fluoropolymer electrolyte according to the invention can
also be used as an electrolyte binder in an electrode catalyst
layer. In such a case, the electrode catalyst layer is preferably
formed by applying an electrode ink obtained by preparing a
fluoropolymer electrolyte solution by dispersing the fluoropolymer
electrolyte according to the invention in a solvent and admixing an
electrode catalyst such as a carbon-supported Pt therewith; the ink
application is followed by drying. The amount of the fluoropolymer
electrolyte supported per unit electrode surface area, in the state
after the electrode catalyst layer formation, is preferably 0.001
to 10 mg/cm.sup.2, more preferably 0.01 to 5 mg/cm.sup.2, still
more preferably 0.1 to 1 mg/cm.sup.2.
[0062] A unit constituted of an electrolyte membrane and two,
namely an anode and a cathode, electrode catalyst layers joined
thereto in a manner of sandwiching the same therebetween is called
a membrane-electrode assembly (hereinafter sometimes abbreviated as
"MEA"). Such a unit that further comprises a pair of gas diffusion
layers opposite to each other and joined to the outside of the
respective electrode catalyst layers is also called MEA in certain
cases.
[0063] The electrode catalyst layer is constituted of fine
particles of a catalyst metal and a conductive agent for supporting
the particles and, if necessary, contains a water repellent. The
catalyst to be used in the electrode may be any metal capable of
promoting an oxidation reaction of hydrogen and a reduction
reaction of oxygen and, thus, it includes, among others, platinum,
gold, silver, palladium, iridium, rhodium, ruthenium, iron, cobalt,
nickel, chromium, tungsten, manganese, vanadium, and an alloy of
these; among them, platinum is mainly used.
[0064] The amount of the electrode catalyst supported per unit
electrode surface area, in the state after electrode catalyst layer
formation, is preferably 0.001 to 10 mg/cm.sup.2, more preferably
0.01 to 5 mg/cm.sup.2, most preferably 0.1 to 1 mg/cm.sup.2.
[0065] The MEA obtained as mentioned above or the MEA further
comprising a pair of opposing gas diffusion electrodes, as the case
may be, is further combined with those constituents which are
generally used in a solid polymer electrolyte fuel cell, such as a
bipolar plate and a packing plate, whereby a solid polymer
electrolyte fuel cell is constituted.
[0066] The term "bipolar plate" means a plate or the like made of a
graphite-resin composite material or a metal with grooves formed on
the surface thereof for such a gas as a fuel or an oxidizing agent
to flow through them. The bipolar plate has not only a function to
transfer electrons to an external load circuit but also a function
as a channel structure to feed a fuel or an oxidizing agent to the
vicinity of the electrode catalyst. A fuel cell is produced by
stacking a plurality of unit cells each constituted by inserting an
MEA between two such bipolar plates.
[0067] In the following, the production method for the
fluoropolymer electrolyte according to the invention is
described.
(Production Method for Fluoropolymer Electrolyte)
[0068] The fluoropolymer electrolyte according to the invention can
be produced, for example, by a production method comprising the
step (1) of obtaining a fluoropolymer electrolyte precursor by the
polymerization technique to be described later herein and the step
(2) of obtaining the fluoropolymer electrolyte by subjecting the
fluoropolymer electrolyte precursor to a chemical treatment.
[0069] It is preferred that the fluoropolymer electrolyte precursor
mentioned above have a group convertible to COOZ or SO.sub.3Z group
(wherein Z represents an alkali metal, an alkaline earth metal,
hydrogen atom or NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1,
R.sup.2, R.sup.3 and R.sup.4 each independently represents an alkyl
group containing 1 to 3 carbon atoms or hydrogen atom) by the
chemical treatment.
[0070] The step (1) mentioned above preferably comprises
copolymerzing an ethylenic fluoromonomer and a fluorovinyl compound
having a group convertible to a COOZ or SO.sub.3Z group (wherein Z
represents an alkali metal, an alkaline earth metal, hydrogen atom
or NR.sup.1R.sup.2R.sup.3R.sup.4 in which R.sup.1, R.sup.2, R.sup.3
and R.sup.4 each independently represents an alkyl group containing
1 to 3 carbon atoms or hydrogen atom) (hereinafter such fluorovinyl
compound being referred to as "fluorovinyl compound" for short) by
the chemical treatment, to give the fluoropolymer electrolyte
precursor.
[0071] Preferred as the above-mentioned fluorovinyl compound is a
fluorovinyl compound represented by the general formula (VI):
CF.sub.2.dbd.CF(CF.sub.2).sub.k--Ol--(CF.sub.2CFY.sup.1--O).sub.n--(CFY.-
sup.2).sub.m-A.sup.2 (VI)
wherein Y.sup.1 represents F, Cl or a perfluoroalkyl group; k
represents an integer of 0 to 2, l represents 0 or 1, and n
represents an integer of 0 to 8 and the n atoms or groups of
Y.sup.1 may be the same or different; Y.sup.2 represents F or Cl; m
represents an integer of 0 to 6 provided that when m=0, l=0 and
n=0; the m atoms of Y.sup.2 may be the same or different; and
A.sup.2 represents SO.sub.2Z.sup.1 or COZ.sup.2 in which Z.sup.1
represents a halogen element and Z.sup.2 represents an alkoxy group
containing 1 to 3 carbon atoms or a halogen element.
[0072] Referring to the above general formula (VI), it is preferred
from the viewpoint of synthesis and operation that k be 0 and l be
1. For obtaining a reduced EW value, it is more preferred that n be
0 or 1, still more preferably 0. Further, it is more preferred that
Y.sup.2 be F and m be an integer of 2 to 6, it is still more
preferred that Y.sup.2 be F and m be 2 or 4, and it is most
preferred that Y.sup.2 be F and m be 2.
[0073] As specific examples of the fluorovinyl compound represented
by the above general formula (VI), there may be mentioned, among
others, the following:
CF.sub.2.dbd.CFO(CF.sub.2).sub.p--SO.sub.2F,
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub.p--SO.sub.2F,
CF.sub.2.dbd.CF(CF.sub.2).sub.p-1--SO.sub.2F
CF.sub.2.dbd.CF(OCF.sub.2CF(CF.sub.3)).sub.p--(CF.sub.2).sub.p-1--SO.sub-
.2F,
CF.sub.2.dbd.CFO(CF.sub.2).sub.p--CO.sub.2R,
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub.p--CO.sub.2R,
CF.sub.2.dbd.CF(CF.sub.2).sub.p--CO.sub.2R, and
CF.sub.2.dbd.CF(OCF.sub.2CF(CF.sub.3)).sub.p--(CF.sub.2).sub.2--CO.sub.2-
R.
In the above formulas, p represents an integer of 1 to 8 and R
represents an alkyl group containing 1 to 3 carbon atoms.
[0074] In the step (1) mentioned above, one or a combination of two
or more of such fluorovinyl compounds as mentioned above may be
used.
[0075] As the above-mentioned ethylenic fluoromonomer, there may be
mentioned those already enumerated hereinabove. If desired, a third
component monomer other than the ethylenic fluoromonomer and the
fluorovinyl compound may be copolymerized therewith.
[0076] The following polymerization methods may be mentioned as the
method of polymerization in the step (1).
(i) A method comprising using a polymerization solvent such as a
fluorinated hydrocarbon and realizing the polymerization by
reacting the fluorovinyl compound with a gaseous ethylenic
fluoromonomer in a state charged and dissolved in that
polymerization solvent (solution polymerization). Usable as the
above-mentioned fluorinated hydrocarbon is a group of compounds
generically referred to as "flons", for example
trichlorotrifluoroethane and 1,1,1,2,3,4,4,5,5,5-decafluoropentane.
(ii) A method comprising carrying out the polymerization of the
fluorovinyl compound using the fluorovinyl compound per se as a
polymerization solvent without using such a solvent as the
fluorinated hydrocarbon (bulk polymerization). (iii) A method
comprising using an aqueous solution of a surfactant as a
polymerization solvent and carrying out the polymerization by
reacting the fluorovinyl compound with a gaseous ethylenic
fluoromonomer in a state charged and dissolved in that
polymerization solvent (emulsion polymerization).
[0077] The above emulsion polymerization may be carried out by the
method comprising using an aqueous solution of a surfactant and a
coemulsifier such as an alcohol and carrying out the polymerization
by reacting the fluorovinyl compound with a gaseous ethylenic
fluoromonomer in a state charged and emulsified in that aqueous
solution (miniemulsion polymerization, microemulsion
polymerization).
(iv) A method comprising using an aqueous solution of a suspension
stabilizer and carrying out the polymerization by reacting the
fluorovinyl compound with a gaseous ethylenic fluoromonomer in a
state charged and suspended in that aqueous solution (suspension
polymerization).
[0078] Preferred among them for efficiently obtaining polymers with
an equivalent weight (EW) of not less than 250 but not more than
700 is the method (iii), namely emulsion polymerization.
[0079] For adjusting the EW within the range of not less than 250
but not more than 700, the use of the fluorovinyl compound in an
increased proportion is preferable. Generally, however, the
reactivity of the fluorovinyl compound is markedly low as compared
with the ethylenic fluoromonomer, so that the reaction will become
slow and a prolonged period of time may possibly be required for
the polymerization.
[0080] Generally, a radical polymerization reaction involves a
mechanism of termination by coupling of two radicals. The
polymerization methods mentioned above under (i), (ii) and (iv)
cannot exclude a possibility of a molecular weight being reduced by
that coupling; for increasing the molecular weight, a radical
concentration must be markedly lowered in some instances, which
results in a further reduced rate of reaction and thus it
substantially becomes difficult to produce a desired polymer.
[0081] On the other hand, in the above-mentioned emulsion
polymerization (including the miniemulsion polymerization and
microemulsion polymerization), each minute particle constitutes
each polymerization field and a probability of two or more radicals
occurring in one and the same polymerization field is generally
considered to be low. Therefore, as compared with the other
polymerization methods, it is possible to increase the number of
growing radicals per unit volume; hence, an apparent rate of
polymerization can be increased.
[0082] The step (1) is preferably one in which a fluoropolymer
electrolyte precursor is obtained by the method (iii) mentioned
above at a polymerization temperature of not lower than 0.degree.
C. but not higher than 40.degree. C. For uncertain reasons, it
becomes possible to control the distance between ionic clusters of
the fluoropolymer electrolyte precursor within the specific range
mentioned above when the polymerization reaction is carried out at
the polymerization temperature mentioned above; thus, a high
conductivity can be realized even at low humidity levels. More
preferably, the polymerization temperature is not lower than
5.degree. C. but not higher than 35.degree. C.
[0083] The above emulsion polymerization is preferably one in which
the fluorovinyl compound and the gaseous ethylenic fluoromonomer
are subjected to radical copolymerization in an aqueous solution of
the surfactant in a pressure vessel by means of radicals generated
from a polymerization initiator. In that case, the fluorovinyl
compound may be charged and emulsified under application of a
strong shearing force in the presence of the surfactant and the
coemulsifier such as an alcohol.
[0084] For controlling the composition of a product polymer, a
manner for controlling a pressure due to the gaseous ethylenic
fluoromonomer is desired. The pressure mentioned above is
preferably not lower than -0.05 MPaG but not higher than 2.0 MPaG.
The pressure (MPaG) so referred to herein is a value of a pressure
gage (gage pressure), with atmospheric pressure being taken as 0
MPa. Generally, low pressure levels are preferred for lowering the
EW; however, an excessively low pressure will cause a prolonged
polymerization time, possibly resulting in inefficiency. Amore
preferred lower limit to the pressure in question is 0.0 MPaG, and
a still more preferred lower limit thereto is 0.1 MPaG. A more
preferred upper limit is 1.0 MPaG, and a still more preferred upper
limit is 0.7 MPaG.
[0085] Further, it is desirable to supplement the gaseous ethylenic
fluoromonomer in an appropriate manner since, otherwise, the
pressure generally falls as a result of consumption of the gaseous
ethylenic fluoromonomer with the progress of the polymerization
reaction. A manner for supplementary feeding of the fluorovinyl
compound is also preferably employed since that compound is
consumed simultaneously. The fluorovinyl compound to be
supplemented may be charged and emulsified together with the
surfactant and the coemulsifier such as an alcohol under
application of a strong shearing force. In cases where the
fluorovinyl compound is in a liquid form, use can be made of a
method for feeding under pressure by means of a metering pump or a
method for feeding under pressure by means of a pressurized inert
gas, for instance.
[0086] It is desirable that the fluoropolymer electrolyte precursor
mentioned above be melt-flowable. In this mode of practice, an
index "melt flow rate" (hereinafter abbreviated as "MFR") can be
used as an indicator of the degree of a polymerization of the
fluoropolymer electrolyte precursor. In this mode of practice, the
MFR of the fluoropolymer electrolyte precursor is preferably not
lower than 0.01, more preferably not lower than 0.1, still more
preferably not lower than 0.3. An upper limit to the MFR is
preferably set at 100 or below, more preferably at 20 or below,
still more preferably at 16 or below, particularly preferably at 10
or below. At MFR levels lower than 0.01, a molding process, such as
a membrane formation, will possibly become difficult. When the MFR
is higher than 100, a membrane obtained by molding that precursor
will possibly gain a diminished strength, so that when it is used
in a fuel cell, the cell will possibly be reduced in durability as
well.
[0087] For attaining an MFR of not lower than 0.01 but not higher
than 100, it is desirable that the emulsion polymerization be
carried out at a temperature of not lower than 0.degree. C. but not
higher than 40.degree. C. At temperatures higher than 40.degree.
C., radicals at polymer termini undergo .beta. rearrangement and a
rate of the disproportionation reaction leading to termination of
the polymerization is thereby increased, so that it may become
impossible to obtain a high molecular weight polymer. The
polymerization temperature is more preferably not higher than
35.degree. C., still more preferably not higher than 30.degree. C.
On the other hand, at temperatures lower than 0.degree. C., the
polymerization occurs very slowly, possibly leading to very low
productivity. The temperature is more preferably not lower than
5.degree. C., still more preferably not lower than 10.degree.
C.
[0088] The polymerization initiator to be used in the step (1) is
preferably a water-soluble one, for example one selected from among
an inorganic peroxide such as a persulfate compound, a perborate
compound, a perchlorate compound, a perphosphate compound and a
percarbonate compound; an organic peroxide such as disuccinyl
peroxide, tert-butyl permaleate and tert-butyl hydroperoxide; and
so forth. The inorganic peroxide mentioned above may be in the form
of an ammonium salt, a sodium salt or a potassium salt, for
instance.
[0089] The so-called redox system catalyst resulting from
combination of the above-mentioned water-soluble polymerization
initiator and a reducing agent is also suited for use. As the
reducing agent, there may be mentioned, for example, a sulfite, a
hydrogensulfite, a salt of a low-valence ion of iron, copper,
cobalt, etc., hypophosphorous acid, a hypophosphite, an organic
amine such as N,N,N',N'-tetramethylethylenediamine and, further, a
reducing sugar such as an aldose and a ketose. In particular, when
the polymerization temperature is not higher than 30.degree. C.,
the use of such a redox system initiator is preferred.
[0090] An azo compound is also the most preferred initiator in the
practice of the invention; usable is
2,2'-azobis-2-methylpropionamidine hydrochloride,
2,2'-azobis-2,4-dimethylvaleronitrile,
2,2'-azobis-N,N'-dimethyleneisobutyramidine hydrochloride,
2,2'-azobis-2-methyl-N-(2-hydroxyethyl)propionamide,
2,2'-azobis-2-(2-imidazolin-2-yl)propane and a salt thereof, and
4,4'-azobis-4-cyanovaleric acid and a salt thereof, among others.
Further, it is also possible to use two or more of such
polymerization initiators as mentioned above in combination. The
amount of the polymerization initiator is generally 0.001 to 5% by
mass.
[0091] The polymerization initiator may be charged into a pressure
vessel prior to introduction of the ethylenic fluoromonomer or,
alternatively, it may be fed into the vessel under pressure in the
form of an aqueous solution after introduction of the ethylenic
fluoromonomer.
[0092] In cases where the redox system initiator is used, a
technique is preferred which comprises successively supplementing
both or either one of the polymerization initiator and the reducing
agent.
[0093] The emulsifier to be used in the step (1) is not
particularly restricted but preferably is one low in chain
transferability, for example an emulsifier represented by
RfZ.sup.3. In the formula, Rf is an alkyl group containing 4 to 20
carbon atoms in which the hydrogen atoms have been partially or
fully substituted by a fluorine atom or atoms and which may contain
one or more ether oxygen atoms and may have an unsaturated bond
copolymerizable with the ethylenic fluoromonomer. Z.sup.3
represents a dissociable polar group, preferably --COO.sup.-B.sup.+
or --SO.sub.3.sup.-B.sup.+ in which B.sup.+ is a monovalent cation
such as an alkali metal ion, ammonium ion or hydrogen ion.
[0094] As the emulsifier represented by RfZ.sup.3, there may be
mentioned, for example, Y(CF.sub.2).sub.nCOO.sup.-B.sup.+ (n
representing an integer of 4 to 20 and Y representing fluorine or
hydrogen atom),
CF.sub.3--OCF.sub.2CF.sub.2--OCF.sub.2CF.sub.2COO.sup.-B.sup.+ and
CF.sub.3--(OCF(CF.sub.3)CF.sub.2).sub.nCOO.sup.-B.sup.+ (n
representing an integer of 1 to 3).
[0095] The level of addition of the emulsifier is not particularly
restricted but suitably is not smaller than 0.01% by mass but not
larger than 10% by mass in the aqueous solution. As the emulsifier
addition level is raised, the number of polymerized particles tends
to increase and, hence, an apparent rate of polymerization tends to
increase. At addition levels lower than 0.01% by mass, emulsified
particles may no longer be maintained stably. At levels exceeding
10% by mass, a washing procedure in an after-treatment step becomes
difficult to perform. Amore preferred lower limit is 0.05% by mass,
and a still more preferred lower limit is 0.1% by mass. A more
preferred upper limit is 5% by mass, and a still more preferred
upper limit is 3% by mass.
[0096] For increasing the number of polymerized particles in the
step (1), it is also possible to carry out the so-called "seed
polymerization" which comprises carrying out the polymerization
using a large amount of the emulsifier, then diluting the
thus-obtained dispersion and continuing the polymerization.
[0097] A polymerization time is not particularly restricted but
generally is 1 to 48 hours. A pH during polymerization is not
particularly restricted, either, but the polymerization may be
carried out while adjusting the pH according to need. A pH
adjusting agent to be used on that occasion includes, among others,
an alkalizing agent such as sodium hydroxide, potassium hydroxide
and ammonia, a mineral acid such as phosphoric acid, sulfuric acid
and hydrochloric acid, and an organic acid such as formic acid and
acetic acid.
[0098] A chain transfer agent may also be used for adjusting the
molecular weight and the molecular weight distribution. As a
preferred chain transfer agent, there may be mentioned a gaseous
hydrocarbon such as ethane and pentane, a water-soluble compound
such as methanol, and an iodine compound, among others. In
particular, an iodine compound is preferred since it makes it
possible to produce a block polymer by the so-called iodine
transfer polymerization.
[0099] From the viewpoint that a durability of the fluoropolymer
electrolyte can be improved by increasing the molecular weight
thereof, it is preferred that any chain transfer agent be not used
in the step (1).
[0100] Unstable terminal groups of the fluoropolymer electrolyte
precursor obtained in the step (1) may be subjected to a
stabilization treatment so that the durability of the fluoropolymer
electrolyte obtained by the production method according to the
invention may be improved. The unstable terminal groups of the
fluoropolymer electrolyte precursor include carboxylic acid, a
carboxylic acid salt, a carboxylic acid ester, carbonate, a
hydrocarbon and methylol group, among others, and the actually
existing species depend on the method of polymerization and the
initiator, the chain transfer agent and a polymerization terminator
species employed, among others.
[0101] When the emulsion polymerization is selected as the method
of polymerization and no chain transfer agent is used, the unstable
terminal groups are mostly carboxylic acid groups.
[0102] A method of stabilizing the unstable terminal groups of the
fluoropolymer electrolyte precursor mentioned above is not
particularly restricted but there may be mentioned, among others, a
method comprising a treatment with a fluorinating agent for
stabilization in the form of --CF.sub.3 and a method comprising
decarboxylation under heating for stabilization in the form of
--CF.sub.2H.
[0103] As for the method of stabilization in the form of --CF.sub.3
by treatment with a fluorinating agent, the use of a gaseous
fluorinating agent is preferred from the ease-of-handling
viewpoint, and F.sub.2, NF.sub.3, PF.sub.S, SF.sub.4, IF.sub.5, ClF
and ClF.sub.3 may be mentioned among others. These fluorinating
agents may be used in a form diluted with an inert gas such as
nitrogen. The treatment temperature is preferably not lower than
0.degree. C. but not higher than 300.degree. C., more preferably
not lower than 50.degree. C. but not higher than 200.degree. C.,
still more preferably not lower than 80.degree. C. but not higher
than 150.degree. C. The treatment pressure is preferably not lower
than -0.05 MPaG but not higher than 1 MPaG, more preferably not
lower than -0.02 MPaG but not higher than 0.5 MPaG, as expressed in
terms of gage pressure.
[0104] The production method according to the invention preferably
comprises the step (2) of subjecting the fluoropolymer electrolyte
precursor obtained in the step (1) to the chemical treatment to
give the fluoropolymer electrolyte. As the chemical treatment,
there may be mentioned, among others, a hydrolysis treatment and an
acid treatment, and the hydrolysis treatment may consist in
immersion in an basic reacting liquid.
[0105] The above-mentioned basic reacting liquid is not
particularly restricted but preferably is an aqueous solution of an
alkali metal or alkaline earth metal hydroxide such as sodium
hydroxide or potassium hydroxide. The content of the alkali metal
or alkaline earth metal hydroxide thereof is not restricted but is
preferably 10 to 30% by mass. The reacting liquid mentioned above
preferably contains a swelling organic compound such as methyl
alcohol, ethyl alcohol, acetone, DMSO, DMAC or DMF. The content of
the swelling organic compound thereof is preferably 1 to 50% by
mass. While a treatment temperature varies depending on a solvent
species, a solvent composition and other factors, the treatment
time can be reduced by raising the temperature. If the treatment
temperature is excessively high, the polymer electrolyte precursor
may possibly be dissolved and, in such a case, the handling becomes
difficult; hence, the treatment is preferably carried out at 20 to
160.degree. C. For obtaining a high level of conductivity, it is
preferred that all the functional groups convertible to SO.sub.3H
by hydrolysis undergo the hydrolysis treatment. Therefore, a
prolonged treatment time is preferred. Since, however, an
excessively longtime sometimes results in reduced productivity, a
treatment time of 0.5 to 48 hours is preferred.
[0106] It is also preferable to obtain the fluoropolymer
electrolyte in a protonated form by thoroughly washing the product
obtained after the above-mentioned hydrolysis treatment in the step
(2) with water, if desirable with warm water, followed by the acid
treatment. The acid to be used in the acid treatment is not
particularly restricted provided that it is such an mineral acid as
hydrochloric acid, sulfuric acid or nitric acid or such an organic
acid as oxalic acid, acetic acid, formic acid or trifluoroacetic
acid.
[0107] In cases where the fluorinated electrolyte precursor is in a
film form as a result of molding, a film obtained after such
hydrolysis treatment and acid treatment as mentioned above can be
used as a fluorinated electrolyte membrane in the fuel cell.
(Polymer Electrolyte Solution)
[0108] The fluoropolymer electrolyte according to the invention can
also be dissolved or suspended in an appropriate solvent (solvent
having good affinity for the resin). As the appropriate solvent,
there may be mentioned, for example, water, a protic organic
solvent such as ethanol, methanol, n-propanol, isopropyl alcohol,
butanol and glycerol, and an aprotic solvent such as
N,N-dimethylformamide, N,N-dimethylacetamide and
N-methylpyrrolidone. These may be used singly or two or more of
them may be used in combination. In particular, in cases where only
one solvent species is used, the use of water alone is preferred.
In cases where two or more species are used in combination, a mixed
solvent composed of water and a protic organic solvent(s) is
particularly preferred.
[0109] The method of dissolution or suspending is not particularly
restricted. For example, the polymer electrolyte is first placed in
the mixed solvent composed of water and the protic organic solvent,
for instance, under conditions such that the total solid
concentration may amount to 1 to 50% by mass. Then, the resulting
composition is placed in an autoclave, if necessary equipped with
an inner cylinder made of glass, and, after substitution of the
inside air with an inert gas such as nitrogen, the contents are
heated with stirring for 1 to 12 hours under conditions such that
the inside temperature is within the range of 50.degree. C. to
250.degree. C. A solution or suspension is thereby obtained. On
that occasion, a higher total solid concentration is preferred from
a yield viewpoint but an excessively increased concentration may
possibly allow an undissolved portion to remain; hence, the
concentration is preferably 1 to 50% by mass, more preferably 3 to
40% by mass, still more preferably 5 to 30% by mass.
[0110] In cases where the protic organic solvent is used, a mixing
ratio between water and the protic organic solvent can be properly
selected according to a dissolution method, dissolution conditions,
a polymer electrolyte species, a total solid concentration, a
dissolution temperature, and a rate of stirring, among others. As
for the mass ratio of the protic organic solvent to water, a
water-to-protic organic solvent ratio of 1:0.1 to 10 is preferred,
and a water-to-organic solvent ratio of 1:0.1 to 5 is particularly
preferred.
[0111] Such a solution or suspension comprises one or more of an
emulsion (liquid particles being dispersed in a liquid as colloidal
particles or coarser particles to give a milk-like appearance), a
suspension (solid particles being dispersed in a liquid as
colloidal particles or microscopically detectable particles), a
colloidal liquid (resulting from dispersion of macromolecules) and
a micellar liquid (a lyophilic colloidal dispersion system
resulting from intermolecular force-due association of a large
number of small molecules), among others.
[0112] Such a solution or suspension can be concentrated. The
method of concentration is not particularly restricted. Usable is,
for example, the method comprising heating to evaporate the solvent
and the method comprising concentration under reduced pressure. As
for the solid content of the resulting coating composition, an
excessively high level thereof results in an increased viscosity,
possibly making it difficult to handle the composition whereas,
when it is too low, the productivity lowers in certain instances;
therefore, it is preferred that the coating composition have a
final solid content of 0.5 to 50% by mass.
[0113] The solution or suspension obtained in the above manner is
more preferably filtered from the viewpoint of removing a coarser
particle fraction. The method of filtration is not particularly
restricted but any of those ordinary methods in conventional use is
applicable. For example, mention may typically be made of the
method comprising pressure filtration using a filter made by
processing a filtering material showing a standard filter rating
for ordinary use. As regards the filter, the use is preferred of
such a filtering material that the 90% collection particle size is
10 to 100 times the average particle size of the particles. This
filtering material may be filter paper or such a filtering material
as a sintered metal filter. In the case of filter paper, in
particular, the 90% collection particle size is preferably 10 to 50
times the average particle size of the particles. In the case of
the sintered metal filter, the 90% collection particle size is
preferably 50 to 100 times the average particle size of the
particles. By setting the 90% collection particle size at a level
of not lower than 10 times the average particle size, it becomes
possible to prevent a pressure required on the occasion of liquid
feeding from becoming excessively high and to prevent the filter
from being clogged in a short period of time. On the other hand, to
set that particle size at a level not higher than 100 times the
average particle size is preferred from the viewpoint of
efficiently removing those agglomerates of particles or undissolved
resin portions which cause the occurrence of foreign matter in the
film.
[0114] The thus-obtained fluoropolymer electrolyte solution can be
used in producing the electrode catalyst layer, as mentioned above,
and in producing the fluorinated electrolyte membrane by
solvent-casting method.
EXAMPLES
[0115] The following examples illustrate certain typical modes of
practice of the present invention more specifically. The modes of
practice of the present invention are, however, by no means limited
to these modes of practice.
[0116] The methods of evaluation and measurement as used in these
modes of practice are as follows.
(EW Measurement)
[0117] A polymer electrolyte membrane in a state in which counter
ions of the ion exchange groups are protons, about 2 to 20 cm.sup.2
in area, is immersed in 30 ml of a saturated aqueous solution of
NaCl at 25.degree. C. and allowed to remain there with stirring for
30 minutes. Then, a proton content in the saturated aqueous
solution of NaCl is determined by neutralization titration using a
0.01 N aqueous solution of sodium hydroxide with phenolphthalein as
an indicator. A polymer electrolyte membrane obtained after
neutralization and now in a state in which the counter ions of the
ion exchange groups are sodium ions is rinsed with pure water,
further dried under vacuum and weighed. The equivalent weight EW
(g/eq) is calculated according to the following expression (2) in
which M (mmol) is the mole number of sodium hydroxide as required
for neutralization and W (mg) is the weight of the polymer
electrolyte membrane in a state in which the counter ions of the
ion exchange groups are sodium ions:
EW=(W/M)-22 (2).
(Distance Between Clusters Calculation)
[0118] A plurality of polymer electrolyte membranes are stacked up
to a total thickness of about 0.25 mm and the stack is set in a
humidity-controllable cell for small angle X-ray. The test specimen
is maintained under conditions of 25.degree. C. and 50% RH for 30
minutes and then subjected to X ray irradiation for scattering
measurement. As for the measurement conditions, the X-ray
wavelength .lamda. is 0.154 nm, the camera length is 515 mm, and an
imaging plate is used as the detector. The two-dimensional
scattering pattern obtained from the imaging plate is subjected to
the empty cell scattering correction and the detector background
correction, followed by circular averaging, to give a
one-dimensional scattering profile. The Bragg angle .theta.m at the
cluster structure-due peak position occurring in the region
2.theta.>1.degree. is read out from the scattering profile
obtained by plotting the scattering intensity data against the
Bragg angle .theta., and the distance between ionic clusters d is
calculated from the following formula:
d=.lamda./2/sin(.theta.m) (1)
(Conductivity Measurement)
[0119] Measurements are made using an MSB-AD-V-FC polymer membrane
water content test apparatus manufactured by BEL Japan, Inc., as
follows. A specimen having a width of 1 cm and a length of 3 cm is
excised from a polymer electrolyte membrane molded and having a
thickness of 50 .mu.m and set in a conductivity measurement cell.
The conductivity measurement cell is then set in the chamber of the
above-mentioned test apparatus, and the chamber inside is adjusted
to 110.degree. C. and below 1% RH. Then, water vapor generated
using deionized water is introduced into the chamber to humidify
the chamber inside to 10% RH, 30% RH, 50% RH, 70% RH, 90% RH and
95% RH, in that order, and conductivity measurements are made at
the respective humidity levels mentioned above.
[0120] The humidity H at 0.10 S/cm is calculated based on the
following formula (3):
H=(H2-H1)/(.sigma.2-.sigma.1).times.(0.1-.sigma.1)+H1 (3)
wherein H2 and .sigma.2 are respectively the relative humidity and
the conductivity at the first measurement point at which the
conductivity goes over 0.10 S/cm and H1 and .sigma.1 are
respectively the highest relative humidity at which the
conductivity does not surpass 0.10 S/cm and the conductivity on
that occasion.
(Melt Flow Rate [MFR] Measurement Method)
[0121] The measurement of the MFR of each fluoropolymer is
performed according to JIS K 7210 under conditions of 270.degree.
C. and a load of 2.16 kg using a MELT INDEXER TYPE C-5059D (product
of Toyo Seiki Seisaku-Sho, Ltd., Japan). The mass of the polymer
extruded is expressed in terms of the number of grams per 10
minutes.
(Polymer Composition)
[0122] The polymer composition is calculated from the measured
values obtained in melt NMR at 300.degree. C. For the NMR, a model
AC300P Burker Fourier transform nuclear magnetic resonance
spectrometer (FT-NMR) is used. For the calculation, the intensity
of the tetrafluorothylene- and vinyl ether-due peak at about -120
ppm and the intensity of the vinyl ether-due peak at about -80 ppm
are used, and the polymer composition is determined from the
respective integrated peak values.
Example 1
[0123] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an EW of
527 was produced in the following manner.
[0124] A 6-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 2980 g of water purified by using a reverse osmosis
membrane, 60 g of C.sub.7F.sub.15COONH.sub.4 and 943 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.2 MPaG.
Temperature regulation was carried out with stirring at 400 rpm for
the inside temperature to be maintained at 38.degree. C., CF.sub.4,
as an explosion inhibitor, was introduced in an amount
corresponding to 0.1 MPaG and, then, TFE was further introduced
until arrival of the inside pressure at 0.5 MPaG. A solution of 6 g
of (NH.sub.4).sub.2S.sub.2O.sub.8 in 20 g of water was introduced
into the system to initiate the polymerization. Thereafter, the
inside pressure was maintained at 0.51 MPaG by supplementary
addition of TFE.
[0125] After 408 minutes from the start of polymerization, namely
at the point of time after additional introduction of 381 g of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (4400 g) was added to the polymerization mixture
obtained (4260 g), and nitric acid was added to cause coagulation.
The polymer coagulate was collected by filtration and, after three
repetitions of redispersion in water and filtration, dried in a
hot-air drier at 90.degree. C. for 12 hours, followed by 12 hours
of drying at 120.degree. C., to give 893 g of a polymer.
[0126] The polymer obtained showed an MFR of 16 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 29 mole percent.
[0127] The thus-obtained fluoropolymer electrolyte precursor was
kept in contact with an aqueous solution containing potassium
hydroxide (15% by mass) and methyl alcohol (50% by mass) at
80.degree. C. for 20 hours for hydrolysis treatment, followed by 5
hours of immersion in water at 60.degree. C. Then, the treatment
comprising immersing in 2 N hydrochloric acid at 60.degree. C. for
1 hour was repeated 5 times while the aqueous hydrochloric acid was
each time replaced with a fresh portion; a subsequent washing with
deionized water and drying gave a fluoropolymer electrolyte.
[0128] This fluoropolymer electrolyte, together with aqueous
ethanol (water:ethanol=50.0:50.0 (mass ratio)), was placed in a
5-liter autoclave and, after tight closure, the contents were
heated to 160.degree. C. with stirring by means of a blade, and
that temperature was maintained for 5 hours. Then, the autoclave
was allowed to cool spontaneously, whereupon a homogeneous
fluoropolymer electrolyte solution with a solid matter
concentration of 5% by mass was obtained.
[0129] This fluoropolymer solution was concentrated at 80.degree.
C. under reduced pressure, and the thus-obtained casting solution
having a solid matter concentration of 20% by mass was cast onto a
tetrafluoroethylene film using a doctor blade. The coated film was
then placed in an oven and subjected to 30 minutes of predrying at
60.degree. C., then to 30 minutes of drying at 80.degree. C. for
removing the solvent and further to 1 hour of heat treatment at
160.degree. C. to give a fluoropolymer electrolyte membrane with a
thickness of about 50 .mu.m.
[0130] This fluoropolymer electrolyte membrane had an EW of 527 and
a distance between ionic clusters of 2.4 nm. The conductivity of
this fluoropolymer electrolyte membrane as measured at 110.degree.
C. and 30% RH was 0.10 S/cm, and the conductivity measurement at
110.degree. C. and 50% RH gave a high level of conductivity of 0.14
S/cm.
Example 2
[0131] A fluoropolymer electrolyte according to the invention
comprising the repeating units derived from CF.sub.2.dbd.CF.sub.2
and CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an
EW of 578 was produced in the following manner.
[0132] A 6-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 2980 g of water purified by using a reverse osmosis
membrane, 60 g of C.sub.7F.sub.15COONH.sub.4 and 490 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.2 MPaG.
Temperature regulation was carried out with stirring at 400 rpm for
the inside temperature to be maintained at 38.degree. C., CF.sub.4,
as an explosion inhibitor, was introduced in an amount
corresponding to 0.1 MPaG and, then, TFE was further introduced
until arrival of the inside pressure at 0.59 MPaG. A solution of 6
g of (NH.sub.4).sub.2S.sub.2O.sub.8 in 20 g of water was introduced
into the system to initiate the polymerization. Thereafter, the
inside pressure was maintained at 0.59 MPaG by supplementary
addition of TFE.
[0133] After 97 minutes from the start of polymerization, namely at
the point of time after additional introduction of 179 g of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (3500 g) was added to the polymerization mixture
obtained (3500 g), and nitric acid was added to cause coagulation.
The polymer coagulate was collected by filtration and, after three
repetitions of redispersion in water and filtration, dried in a
hot-air drier at 90.degree. C. for 12 hours, followed by 12 hours
of drying at 120.degree. C., to give 350 g of a polymer.
[0134] The polymer obtained showed an MFR of 2.3 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 25 mole percent.
[0135] A fluoropolymer electrolyte, a fluoropolymer electrolyte
solution and a fluoropolymer electrolyte membrane were produced in
the same manner as in Example 1 except that the fluorinated
electrolyte precursor obtained in the above manner was used. As a
result of the EW, the distance between ionic clusters and the
conductivity measurements, the EW was 578 and the distance between
ionic clusters was 2.5 nm, and high levels of conductivity, namely
0.10 S/cm at 110.degree. C. and 40% RH and 0.12 S/cm at 110.degree.
C. and 50% RH, were obtained.
Example 3
[0136] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an EW of
662 was produced in the following manner.
[0137] A 189-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 90.6 kg of water purified by using a reverse osmosis
membrane, 1.89 g of C.sub.7F.sub.15COONH.sub.4 and 28.4 kg of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.2 MPaG.
Temperature regulation was carried out with stirring at 189 rpm for
the inside temperature to be maintained at 34.degree. C., CF.sub.4,
as an explosion inhibitor, was introduced in an amount
corresponding to 0.1 MPaG and, then, TFE was further introduced
until arrival of the inside pressure at 0.65 MPaG. A solution of
189 g of (NH.sub.4).sub.2S.sub.2O.sub.8 in 3 liter of water was
introduced into the system to initiate the polymerization.
Thereafter, the inside pressure was maintained at 0.65 MPaG by
supplementary addition of TFE.
[0138] After 188 minutes from the start of polymerization, namely
at the point of time after additional introduction of 20 kg of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (200 kg) was added to the polymerization mixture
obtained (132 kg), and nitric acid was added to cause coagulation.
The polymer coagulate was collected by centrifugation and, after
washing with flowing deionized water, dried in a hot-air drier at
90.degree. C. for 24 hours, followed by 24 hours of drying at
150.degree. C., to give 27 kg of a polymer.
[0139] The above polymer was quickly charged into a 50-liter
Hastelloy vibrating reactor (product of Okawara Mfg. Co., Ltd.) and
heated to 100.degree. C. with vibration at a frequency of 50 rpm
while the reactor was evacuated. Then, nitrogen was introduced to a
gage pressure of -0.05 MPaG. A gaseous halogenating agent prepared
by diluting F.sub.2 gas with nitrogen gas to 20% by mass was
introduced until arrival of the gage pressure at 0.0 MPaG, and the
resulting state was maintained for 30 minutes.
[0140] The gaseous halogenating agent was discharged from the
reactor and, after evacuation, a gaseous halogenating agent
prepared by diluting F.sub.2 gas with nitrogen gas to 20% by mass
was introduced until arrival of the gage pressure at 0.0 MPaG, and
the resulting state was maintained for 3 hours.
[0141] Thereafter, the reactor was cooled to room temperature, the
gaseous halogenating agent was discharged from the reactor and,
after three repetitions of evacuation and nitrogen substitution,
the autoclave was opened, and 27 kg of a polymer was obtained.
[0142] The polymer obtained showed an MFR of 0.71 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 21 mole percent.
[0143] A fluoropolymer electrolyte, a fluoropolymer electrolyte
solution and a fluoropolymer electrolyte membrane were produced in
the same manner as in Example 1 except that the fluoropolymer
electrolyte precursor obtained in the above manner was used. As a
result of the EW, the distance between ionic clusters and the
conductivity measurements, the EW was 662 and the distance between
ionic clusters was 2.6 nm, and a high level of conductivity, namely
0.10 S/cm at 110.degree. C. and 50% RH, was obtained.
Example 4
[0144] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an EW of
512 was produced in the following manner.
[0145] A 6-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 2950 g of water purified by using a reverse osmosis
membrane, 60 g of C.sub.7F.sub.15COONH.sub.4 and 920 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.2 MPaG.
Temperature regulation was carried out with stirring at 400 rpm for
the inside temperature to be maintained at 30.degree. C., CF.sub.4,
as an explosion inhibitor, was introduced in an amount
corresponding to 0.1 MPaG and, then, TFE was further introduced
until arrival of the inside pressure at 0.46 MPaG. A solution of 6
g of (NH.sub.4).sub.2S.sub.2O.sub.8 in 20 g of water was introduced
into the system and further a solution of 0.6 g of Na.sub.2SO.sub.3
in 10 g of water was fed into the system under the pressure to
initiate the polymerization. Thereafter, the inside pressure was
maintained at 0.46 MPaG by supplementary addition of TFE. The
polymerization was continued, during which, after the lapse of 120
minutes and of 240 minutes from the start of polymerization, a
solution of 0.6 g of Na.sub.2SO.sub.3 in 10 g of water was fed into
under pressure each occasion.
[0146] After 360 minutes from the start of polymerization, namely
at the point of time after additional introduction of 321 g of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (4400 g) was added to the polymerization mixture
obtained (4020 g), and nitric acid was added to cause coagulation.
The polymer coagulate was collected by filtration and, after three
repetitions of redispersion in water and filtration, dried in a
hot-air drier at 90.degree. C. for 12 hours, followed by 12 hours
of drying at 120.degree. C., to give 643 g of a polymer.
[0147] The polymer obtained showed an MFR of 2.9 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 30 mole percent.
[0148] A fluoropolymer electrolyte, a fluoropolymer electrolyte
solution and a fluoropolymer electrolyte membrane were produced in
the same manner as in Example 1 except that the fluoropolymer
electrolyte precursor obtained in the above manner was used. As a
result of the EW, the distance between ionic clusters and the
conductivity measurements, the EW was 512 and the distance between
ionic clusters was 2.4 nm, and high levels of conductivity, namely
0.10 S/cm at 110.degree. C. and 30% RH and 0.16 S/cm at 110.degree.
C. and 50% RH, were obtained.
Example 5
[0149] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an EW of
559 was produced in the following manner.
[0150] A 6-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 2950 g of water purified by using a reverse osmosis
membrane, 60 g of C.sub.7F.sub.15COONH.sub.4 and 920 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.2 MPaG.
Temperature regulation was carried out with stirring at 400 rpm for
the inside temperature to be maintained at 25.degree. C., CF.sub.4,
as an explosion inhibitor, was introduced in an amount
corresponding to 0.1 MPaG and, then, TFE was further introduced
until arrival of the inside pressure at 0.56 MPaG. A solution of 6
g of (NH).sub.2S.sub.2O.sub.8 in 20 g of water was introduced into
the system and further a solution of 0.6 g of Na.sub.2SO.sub.3 in
10 g of water was fed into the system under the pressure to
initiate the polymerization. Thereafter, the inside pressure was
maintained at 0.46 MPaG by supplementary addition of TFE. The
polymerization was continued, during which, after the lapse of 120
minutes and of 240 minutes from the start of polymerization, a
solution of 0.6 g of Na.sub.2SO.sub.3 in 10 g of water was fed into
under pressure each occasion.
[0151] After 360 minutes from the start of polymerization, namely
at the point of time after additional introduction of 350 g of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (4400 g) was added to the polymerization mixture
obtained (4020 g), and nitric acid was added to cause coagulation.
The polymer coagulate was collected by filtration and, after three
repetitions of redispersion in water and filtration, dried in a
hot-air drier at 90.degree. C. for 12 hours, followed by 12 hours
of drying at 120.degree. C., to give 680 g of a polymer.
[0152] The polymer obtained showed an MFR of 0.91 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 26 mole percent.
[0153] A fluoropolymer electrolyte, a fluoropolymer electrolyte
solution and a fluoropolymer electrolyte membrane were produced in
the same manner as in Example 1 except that the fluoropolymer
electrolyte precursor obtained in the above manner was used. As a
result of the EW, the distance between ionic clusters and the
conductivity measurements, the EW was 559 and the distance between
ionic clusters was 2.4 nm, and high levels of conductivity, namely
0.10 S/cm at 110.degree. C. and 40% RH and 0.13 S/cm at 110.degree.
C. and 50% RH, were obtained.
Comparative Example 1
[0154] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an EW of
720 was produced in the following manner.
[0155] A 189-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 90.5 kg of water purified by using a reverse osmosis
membrane, 0.945 g of C.sub.7F.sub.15COONH.sub.4 and 5.68 kg of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.2 MPaG.
Temperature regulation was carried out with stirring at 189 rpm for
the inside temperature to be maintained at 47.degree. C., CF.sub.4,
as an explosion inhibitor, was introduced in an amount
corresponding to 0.1 MPaG and, then, TFE was further introduced
until arrival of the inside pressure at 0.70 MPaG. A solution of 3
liter of (NH.sub.4).sub.2S.sub.2O.sub.8 in 47 g of water was
introduced into the system to initiate the polymerization.
Thereafter, the inside pressure was maintained at 0.7 MPaG by
supplementary addition of TFE. The polymerization was continued,
during which 0.7 kg of CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F
was fed every time the TFE additionally fed amounted to 1 kg.
[0156] After 360 minutes from the start of polymerization, namely
at the point of time after additional introduction of 24 kg of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (200 kg) was added to the polymerization mixture
obtained (140 kg), and nitric acid was added to cause coagulation.
The polymer coagulate was collected by centrifugation and, after
washing with flowing deionized water, dried in a hot-air drier at
90.degree. C. for 24 hours, followed by 24 hours of drying at
150.degree. C., to give 34 kg of a polymer.
[0157] The above polymer was quickly charged into a 28 kg Hastelloy
vibrating reactor (product of Okawara Mfg. Co., Ltd.) and heated to
100.degree. C. with vibration at a frequency of 50 rpm while the
reactor was evacuated. Then, nitrogen was introduced to a gage
pressure of -0.05 MPaG. A gaseous halogenating agent prepared by
diluting F.sub.2 gas with nitrogen gas to 20% by mass was
introduced until arrival of the gage pressure at 0.0 MPaG, and the
resulting state was maintained for 30 minutes.
[0158] The gaseous halogenating agent was discharged from the
reactor and, after evacuation, a gaseous halogenating agent
prepared by diluting F.sub.2 gas with nitrogen gas to 20% by mass
was introduced until arrival of the gage pressure at 0.0 MPaG, and
the resulting state was maintained for 3 hours.
[0159] Thereafter, the reactor was cooled to room temperature, the
gaseous halogenating agent was discharged from the reactor and,
after three repetitions of evacuation and nitrogen substitution,
the reactor was opened, and 28 kg of a polymer was obtained.
[0160] The polymer obtained showed an MFR of 3.0 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 18 mole percent.
[0161] A fluoropolymer electrolyte solution and a fluoropolymer
electrolyte membrane were produced in the same manner as in Example
1 except that the above-mentioned fluoropolymer electrolyte was
used. As a result of the EW, the distance between ionic clusters
and the conductivity measurements, the EW was 720 and the distance
between ionic clusters was 3.1 nm, and a conductivity not
satisfying desired level, namely 0.06 S/cm at 110.degree. C. and
50% RH, was obtained.
Comparative Example 2
[0162] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub.2--SO.sub.3H
and having an EW of 910 was produced in the following manner.
[0163] First, a fluorinated ion exchange resin precursor, namely a
fluorocarbon copolymer of CF.sub.2.dbd.CF.sub.2 (hereinafter, TFE)
and
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub.2--SO.sub.2F
(hereinafter, S monomer), was produced by polymerization in the
following manner.
[0164] A one-liter stainless steel autoclave was charged with 580 g
of CF.sub.2ClCFCl.sub.2 (hereinafter, CFC113) and 280 g of
CF.sub.2.dbd.CFOCF.sub.2CF(CF.sub.3)O(CF.sub.2).sub.2--SO.sub.2F,
and purged with nitrogen, followed by purging with TFE. The
temperature was adjusted to 35.degree. C. and the pressure of TFE
to 0.157 MPaG, then 0.55 g of a CFC113 solution containing 5% by
weight of (n-C.sub.3F.sub.7COO--).sub.2 was added, and the
polymerization was carried out for about 3.5 hours. During the
polymerization, TFE was fed from outside the system to maintain the
pressure of TFE at a constant level. The remaining TFE was purged
from the polymerization mixture obtained, the CFC113 was distilled
off at 90.degree. C. and ordinary pressure and, then, the remaining
S monomer was distilled off at 90.degree. C. under reduced
pressure. The residual product was further dried at 150.degree. C.
under reduced pressure for 2 days, whereupon 10.5 g of a
fluorinated ion exchange resin precursor was obtained.
[0165] The polymer obtained showed an MFR of 20 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 18 mole percent.
[0166] A fluoropolymer electrolyte solution and a fluoropolymer
electrolyte membrane were produced in the same manner as in Example
1 except that the above fluoropolymer electrolyte was used. As a
result of the EW, the distance between ionic clusters and the
conductivity measurements, the EW was 910 and the distance between
ionic clusters was 3.1 nm, and a conductivity not satisfying
desired level, namely 0.04 S/cm at 110.degree. C. and 50% RH, was
obtained.
Example 3
[0167] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an EW of
705 was produced in the following manner.
[0168] A 6-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 2950 g of water purified by using a reverse osmosis
membrane, 60 g of C.sub.7F.sub.15COONH.sub.4 and 180 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.2 MPaG.
Temperature regulation was carried out with stirring at 400 rpm for
the inside temperature to be maintained at 48.degree. C., CF.sub.4,
as an explosion inhibitor, was introduced in an amount
corresponding to 0.1 MPaG and, then, TFE was further introduced
until arrival of the inside pressure at 0.70 MPaG. A solution of
1.5 g of (NH.sub.4).sub.2S.sub.2O.sub.8 in 20 g of water was fed
into the system under pressure to initiate the polymerization.
Thereafter, the inside pressure was maintained at 0.70 MPaG by
supplementary addition of TFE. The polymerization was continued,
during which 6.5 g of CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F was
fed every time the TFE additionally fed amounted to 10 g.
[0169] After 218 minutes from the start of polymerization, namely
at the point of time after additional introduction of 774 g of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (4400 g) was added to the polymerization mixture
obtained (4400 g), and nitric acid was added to cause coagulation.
The polymer coagulate was collected by filtration and, after three
repetitions of redispersion in water and filtration, dried in a
hot-air drier at 90.degree. C. for 12 hours, followed by 12 hours
of drying at 120.degree. C., to give 1200 g of a polymer.
[0170] The polymer obtained showed an MFR of 3.5 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 19 mole percent.
[0171] A fluoropolymer electrolyte solution and a fluoropolymer
electrolyte membrane were produced in the same manner as in Example
1 except that the above-mentioned fluoropolymer electrolyte was
used. As a result of the EW, the distance between ionic clusters
and the conductivity measurements, the EW was 705 and the distance
between ionic clusters was 2.7 nm, and a conductivity not
satisfying desired level, namely 0.08 S/cm at 110.degree. C. and
50% RH, was obtained.
Example 6
[0172] A fluoropolymer electrolyte comprising the repeating units
derived from CF.sub.2.dbd.CF.sub.2 and
CF.sub.2.dbd.CF--O--(CF.sub.2).sub.2--SO.sub.3H and having an EW of
455 was produced in the following manner.
[0173] A 6-liter SUS-316 stainless steel pressure vessel equipped
with a mixing blade and a jacket for temperature adjustment was
charged with 2850 g of water purified by using a reverse osmosis
membrane, 150 g of C.sub.7F.sub.15COONH.sub.4 and 1150 g of
CF.sub.2.dbd.CFOCF.sub.2CF.sub.2SO.sub.2F, the system inside was
purged with nitrogen and then evacuated and, thereafter, TFE was
introduced until arrival of the inside pressure at 0.07 MPaG.
Temperature regulation was carried out with stirring at 400 rpm for
the inside temperature to be maintained at 10.degree. C. A solution
of 6 g of (NH.sub.4).sub.2S.sub.2O.sub.8 in 20 g of water was fed
into the system under pressure and further a solution of 0.6 g of
Na.sub.2SO.sub.3 in 10 g of water was fed into the system under the
pressure to initiate the polymerization. Thereafter, the
polymerization was continued while TFE was additionally fed to
maintain the inside pressure at 0.07 MPaG. Further, a solution of
0.6 g of Na.sub.2SO.sub.3 in 10 g of water was injected at each
1-hour interval.
[0174] After 11 hours from the start of polymerization, namely at
the point of time after additional introduction of 400 g of TFE,
the unreacted TFE was discharged and the polymerization was thus
terminated. Water (250 g) was added to the part of the
polymerization mixture obtained (200 g), and nitric acid was added
to cause coagulation. The polymer coagulate was collected by
filtration and, after three repetitions of redispersion in water
and filtration, dried in a hot-air drier at 90.degree. C. for 24
hours, followed by 5 hours of drying at 120.degree. C., to give
44.3 g of a polymer.
[0175] The polymer obtained showed an MFR of 0.4 g/10 minutes and
had an SO.sub.3H group-containing monomer-derived repeating unit
content of 34 mole percent.
[0176] A fluoropolymer electrolyte, a fluoropolymer electrolyte
solution and a fluoropolymer electrolyte membrane were produced in
the same manner as in Example 1 except that the fluorinated
electrolyte precursor obtained in the above manner was used. As a
result of the EW, the distance between ionic clusters and the
conductivity measurements, the EW was 455 and the distance between
ionic clusters was 2.3 nm, and high levels of conductivity, namely
0.10 S/cm at 110.degree. C. and 25% RH and 0.20 S/cm at 110.degree.
C. and 50% RH, were obtained.
[0177] The relationship between the distance between ionic clusters
and the conductivity at 50% RH as revealed from the data obtained
in the above-mentioned Examples 1 to 6 and Comparative Examples 1
to 3 is shown in FIG. 1. FIG. 1 is a graph obtained by plotting the
results of Examples 1 to 6 and Comparative Examples 1 to 3, with
the abscissa denoting the distance between ionic clusters and the
ordinate denoting the conductivity at 50% RH. FIG. 1 indicates that
the conductivity at 50% RH rapidly rises as the distance between
ionic clusters decreases in the range not longer than 2.6 nm.
INDUSTRIAL APPLICABILITY
[0178] The highly conductive fluoropolymer electrolyte according to
the invention makes it possible to provide a fuel cell showing high
performance even under high-temperature low-humidification
conditions. The fluoropolymer electrolyte according to the
invention can be used in various fuel cells, including a direct
methanol fuel cell, in water electrolysis, hydrohalic acid
electrolysis and brine electrolysis, and in oxygen concentrators,
humidity sensors and gas sensors, among others.
BRIEF DESCRIPTION OF THE DRAWING
[0179] FIG. 1 This figure is a graph obtained by plotting the
results of Examples 1 to 6 and Comparative Examples 1 to 3, with
the abscissa denoting the distance between ionic clusters and the
ordinate denoting the conductivity at 50% RH.
* * * * *