U.S. patent application number 10/488845 was filed with the patent office on 2004-12-02 for solid polymer membrane for fuel cell prepared by in situ polymerization.
Invention is credited to Yang, Zhen-Yu.
Application Number | 20040241518 10/488845 |
Document ID | / |
Family ID | 23285030 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040241518 |
Kind Code |
A1 |
Yang, Zhen-Yu |
December 2, 2004 |
Solid polymer membrane for fuel cell prepared by in situ
polymerization
Abstract
The present invention provides for a solid polymer electrolyte
membrane comprising a fluorinated ionomer having imbibed therein
the polymerization product of a composition comprising a
non-fluorinated, non-ionomeric monomer, wherein the fluorinated
ionomer comprises at least 6 mole % of monomer units having a
fluorinated pendant group with a terminal ionic group. Catalyst
coated membranes and fuel cells using these membranes are also
provided.
Inventors: |
Yang, Zhen-Yu; (Wilmington,
DE) |
Correspondence
Address: |
Alanson G Bowen JR
E I du Pont de Nemours & Company
Legal Patents
Wilmington
DE
19898
US
|
Family ID: |
23285030 |
Appl. No.: |
10/488845 |
Filed: |
March 4, 2004 |
PCT Filed: |
October 15, 2002 |
PCT NO: |
PCT/US02/32838 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60329361 |
Oct 15, 2001 |
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Current U.S.
Class: |
429/494 ;
429/314; 429/316; 429/317; 429/492; 429/506; 429/516; 521/27 |
Current CPC
Class: |
B01D 71/32 20130101;
B01D 69/141 20130101; H01M 8/1039 20130101; C08J 5/2293 20130101;
H01M 4/8817 20130101; C08J 2327/18 20130101; H01M 8/1044 20130101;
H01M 2300/0082 20130101; B01D 67/0088 20130101; C08J 5/2281
20130101; H01M 8/103 20130101; H01M 8/1004 20130101; H01M 4/926
20130101; C08J 5/2237 20130101; H01M 8/1023 20130101; H01M 4/8605
20130101; B01J 35/065 20130101; H01M 4/881 20130101; H01M 8/1009
20130101; H01M 8/04197 20160201; Y02E 60/50 20130101; H01M
2300/0091 20130101; H01M 4/921 20130101 |
Class at
Publication: |
429/033 ;
429/316; 429/317; 429/314; 521/027 |
International
Class: |
H01M 008/10; C08J
005/22 |
Claims
What is claimed is:
1. A solid polymer electrolyte membrane comprising a fluorinated
ionomer having imbibed therein the polymerization product of a
composition comprising a non-fluorinated, non-ionomeric monomer,
wherein the fluorinated ionomer comprises at least 6 mole % of
monomer units having a fluorinated pendant group with a terminal
ionic group.
2. The membrane of claim 1 wherein the composition further
comprises a free radical initiator.
3. The membrane of claim 2 wherein the composition further
comprises a crosslinking agent.
4. The membrane of claim 1 wherein the monomer is a polar vinyl
monomer.
5. The membrane of claim 1 wherein the polar vinyl monomer is
selected from the group consisting of vinyl acetate; vinyl
isocyanate; acrylonitrile; acrylic acid; acrylate esters; and
mixtures thereof.
6. The membrane of claim 1 wherein the fluorinated pendant group is
the radical represented by the formula
--(OCF.sub.2CFR).sub.aOCF.sub.2(CFR').-
sub.bSO.sub.2X.sup.-(H.sup.+)[YZ.sub.c].sub.d (I) wherein R and R'
are independently selected from F, Cl or a perfluoroalkyl group
having 1 to 10 carbon atoms, optionally substituted by one or more
ether oxygens; a=0, 1 or 2; b=0 to 6; X is O, C or N with the
proviso that d=0 when X is O and d=1 otherwise, and c=1 when X is C
and c=0 when X is N; when c=1, Y and Z are electron-withdrawing
groups selected from the group consisting of CN,
SO.sub.2R.sub.f,SO.sub.2R.sup.3, P(O)(OR.sup.3).sub.2,
CO.sub.2R.sup.3, P(O)R.sup.3.sub.2, C(O)R.sub.f, C(O)R.sup.3, and
cycloalkenyl groups formed therewith wherein R.sub.f is a
perfluoroalkyl group of 1-10 carbons optionally containing one or
more ether oxygens; R.sup.3 is an alkyl group of 1-6 carbons
optionally substituted with one or more ether oxygens, or an aryl
group optionally further substituted; or, when c=0, Y may be an
electron-withdrawing group represented by the formula
--SO.sub.2R.sub.f' where R.sub.f' is the radical represented by the
formula
--(R.sub.f.varies.SO.sub.2N--(H.sup.+)SO.sub.2).sub.mR.sub.f'-
"where m=0 or 1, and R.sub.f" is --C.sub.nF.sub.2n-- and R.sub.f'"
is --C.sub.nF.sub.2n+1 where n=1-10.
7. The membrane of claim 6 wherein the pendant group is a radical
represented by the formula
--OCF.sub.2CF(CF.sub.3)--OCF.sub.2CF.sub.2SO.s- ub.3H
8. The membrane of claim 6 wherein the pendant group is a radical
represented by the formula --OCF.sub.2CF.sub.2--SO.sub.3H
9. The membrane of claim 1 wherein the ionomer is
polyfluorinated.
10. The membrane of claim 9 wherein the ionomer is
perfluorinated.
11. The membrane of claim 3 wherein the monomer is present in the
composition in the amount of greater than about 6%, free radical
initiators in the amount of less than about 0.1%, and the
crosslinking agent in the amount of about 0 to about 5%,
12. A catalyst coated membrane comprising a solid polymer
electrolyte membrane having a first surface and a second surface,
an anode present on the first surface of the solid polymer
electrolyte membrane, and a cathode present on the second surface
of the solid polymer electrolyte membrane, wherein the solid
polymer electrolyte membrane comprises a fluorinated ionomer having
imbibed therein the polymerization product of a composition
comprising a non-fluorinated, non-ionomeric monomer, wherein the
fluorinated ionomer comprises at least 6 mole % of monomer units
having a fluorinated pendant group with a terminal ionic group.
13. The catalyst coated membrane of claim 12 wherein the
composition further comprises a free radical initiator.
14. The catalyst coated membrane of claim 13 wherein the
composition further comprises a crosslinking agent.
15. The catalyst coated membrane of claim 12 wherein the monomer is
a polar vinyl monomer.
16. The catalyst coated membrane of claim 12 wherein the polar
vinyl monomer is selected from the group consisting of vinyl
acetate; vinyl isocyanate; acrylonitrile; acrylic acid; acrylate
esters; and mixtures thereof.
17. The catalyst coated membrane of claim 12 wherein the
fluorinated pendant group is the radical represented by the formula
--(OCF.sub.2CFR).sub.aOCF.sub.2(CFR').sub.bSO.sub.2
X.sup.-(H.sup.+)[YZ.sub.c].sub.d (I) wherein R and R' are
independently selected from F, Cl or a perfluoroalkyl group having
1 to 10 carbon atoms, optionally substituted by one or more ether
oxygens; a=0, 1 or 2; b=0 to 6; X is O, C or N with the proviso
that d=0 when X is 0 and d=1 otherwise, and c=1 when X is C and c=0
when X is N; when c=1, Y and Z are electron-withdrawing groups
selected from the group consisting of CN,
SO.sub.2R.sub.f,SO.sub.2R.sup.3, P(O)(OR.sup.3).sub.2,
CO.sub.2R.sup.3, P(O)R.sup.3.sub.2, C(O)R.sub.f, C(O)R.sup.3, and
cycloalkenyl groups formed therewith wherein R.sub.f is a
perfluoroalkyl group of 1-10 carbons optionally containing one or
more ether oxygens; R.sup.3 is an alkyl group of 1-6 carbons
optionally substituted with one or more ether oxygens, or an aryl
group optionally further substituted; or, when c=0, Y may be an
electron-withdrawing group represented by the formula
--SO.sub.2R.sub.f' where R.sub.f' is the radical represented by the
formula
--(R.sub.f'SO.sub.2N--(H.sup.+)SO.sub.2).sub.mR.sub.f'"where m=0 or
1, and R.sub.f' is --C.sub.nF.sub.2n-- and R.sub.f'" is
--C.sub.nF.sub.2n+1 where n=1-10.
18. The catalyst coated membrane of claim 17 wherein the pendant
group is a radical represented by the formula
--OCF.sub.2CF(CF.sub.3)--OCF.sub.2CF- .sub.2SO.sub.3H.
19. The catalyst coated membrane of claim 17 wherein the pendant
group is a radical represented by the formula
--OCF.sub.2CF.sub.2--SO.sub.3H
20. The catalyst coated membrane of claim 12 wherein the ionomer is
polyfluorinated.
21. The catalyst coated membrane of claim 20 wherein the ionomer is
perfluorinated.
22. The catalyst coated membrane of claim 12 wherein the anode and
cathode comprise a catalyst, which may be supported or
unsupported.
23. The catalyst coated membrane of claim 14 wherein the monomer is
present in the composition in the amount of greater than about 6%,
free radical initiators in the amount of less than about 0.1%, and
the crosslinking agent in the amount of about 0 to about 5%,
24. A fuel cell comprising a solid polymer electrolyte membrane
having a first surface and a second surface, wherein the solid
polymer electrolyte membrane comprises a fluorinated ionomer having
imbibed therein the polymerization product of a composition
comprising a non-fluorinated, non-ionomeric monomer, wherein the
fluorinated ionomer comprises at least 6 mole % of monomer units
having a fluorinated pendant group with a terminal ionic group.
25. The fuel cell of claim 24 wherein the composition further
comprises a free radical initiator.
26. The fuel cell of claim 25 wherein the composition further
comprises a crosslinking agent.
27. The fuel cell of claim 24 wherein the monomer is a polar vinyl
monomer.
28. The fuel cell of claim 27 wherein the polar vinyl monomer is
selected from the group consisting of vinyl acetate; vinyl
isocyanate; acrylonitrile; acrylic acid; acrylate esters; and
mixtures thereof.
29. The fuel cell of claim 24 wherein the fluorinated pendant group
is the radical represented by the formula
--(OCF.sub.2CFR).sub.aOCF.sub.2(CFR').- sub.bSO.sub.2 X.sup.-(
H.sup.+)[YZ.sub.c].sub.d (I) wherein R and R' are independently
selected from F, Cl or a perfluoroalkyl group having 1 to 10 carbon
atoms, optionally substituted by one or more ether oxygens; a=0, 1
or 2; b=0 to 6; X is O, C or N with the proviso that d=0 when X is
0 and d=1 otherwise, and c=1 when X is C and c=0 when X is N; when
c=1, Y and Z are electron-withdrawing groups selected from the
group consisting of CN, SO.sub.2R.sub.f,SO.sub.2R.sup.3,
P(O)(OR.sup.3).sub.2, CO.sub.2R.sup.3, P(O)R.sup.3.sub.2,
C(O)R.sub.f, C(O)R.sup.3, and cycloalkenyl groups formed therewith
wherein R.sub.f is a perfluoroalkyl group of 1-10 carbons
optionally containing one or more ether oxygens; R.sup.3 is an
alkyl group of 1-6 carbons optionally substituted with one or more
ether oxygens, or an aryl group optionally further substituted; or,
when c=0, Y may be an electron-withdrawing group represented by the
formula --SO.sub.2R.sub.f' where R.sub.f' is the radical
represented by the formula
--(R.sub.f'SO.sub.2N--(H.sup.+)SO.sub.2).sub.m R.sub.f'"where m=0
or 1, and R.sub.f" is --C.sub.nF.sub.2n-- and R.sub.f'" is
--C.sub.nF.sub.2n+1 where n=1-10.
30. The fuel cell of claim 29 wherein the pendant group is a
radical represented by the formula
--OCF.sub.2CF(CF.sub.3)--OCF.sub.2CF.sub.2SO.s- ub.3H.
31. The fuel cell of claim 29 wherein the pendant group is a
radical represented by the formula
--OCF.sub.2CF.sub.2--SO.sub.3H
32. The fuel cell of claim 24 wherein the ionomer is
polyfluorinated.
33. The fuel cell of claim 32 wherein the ionomer is
perfluorinated.
34. The fuel cell of claim 24 further comprising an anode and a
cathode present on the first and second surfaces of the polymer
electrolyte membrane.
35. The fuel cell of claim 34 further comprising a means for
delivering a fuel to the anode, a means for delivering oxygen to
the cathode, a means for connecting the anode and cathode to an
external electrical load, methanol in the liquid or gaseous state
in contact with the anode, and oxygen in contact with the
cathode.
36. The fuel cell of claim 35 wherein the fuel is in the liquid or
vapor phase.
37. The fuel cell of claim 36 wherein the fuel is selected from the
group consisting of methanol and hydrogen.
38. The fuel cell of claim 26 wherein the monomer is present in the
composition in the amount of greater than about 6%, free radical
initiators in the amount of less than about 0.1%, and the
crosslinking agent in the amount of about 0 to about 5%,
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a solid polymer electrolyte
membrane, more particularly to a direct methanol fuel cell
containing the solid polymer electrolyte membrane.
BACKGROUND OF THE INVENTION
[0002] Direct methanol fuel cells (DMFCs), fuel cells in which the
anode is fed directly with liquid or vaporous methanol, have been
under development for a considerable period of time, and are
well-known in the art. See for example Baldauf et al, J. Power
Sources, vol. 84, Pages 161-166. One essential component in a
direct methanol, or any, fuel cell is the separator membrane.
[0003] It has long been known in the art to form ionically
conducting polymer electrolyte membranes and gels from organic
polymers containing ionic pendant groups. Well-known so-called
ionomer membranes in widespread commercial use are Nafion.RTM.
perfluoroionomer membranes available from E. I. du Pont de Nemours
and Company, Wilmington Del. Nafion.RTM. is formed by
copolymerizing tetrafluoroethylene (TFE) with
perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride), as
disclosed in U.S. Pat. No. 3,282,875. Other well-known
perfluoroionomer membranes are copolymers of TFE with perfluoro
(3-oxa-4-pentene sulfonyl fluoride), as disclosed in U.S. Pat. No.
4,358,545. The copolymers so formed are converted to the ionomeric
form by hydrolysis, typically by exposure to an appropriate aqueous
base, as disclosed in U.S. Pat. No. 3,282,875. Lithium, sodium and
potassium are all well known in the art as suitable cations for the
above cited ionomers.
[0004] Other fluorinated ionomer membranes are known in the art
such as those described in WO 9952954, WO 0024709, WO 0077057, and
U.S. Pat. No. 6,025,092.
[0005] DMFCs employing ionomeric polymer electrolyte membranes as
separators are known to exhibit high methanol cross-over--the
transport of as much as 40% of the methanol from the anode to the
cathode by diffusion through the membrane. This methanol cross-over
essentially represents a fuel leak, greatly decreasing the
efficiency of the fuel cell. In addition, the presence of methanol
at the cathode interferes with the cathode reaction, with the
methanol itself undergoing oxidation, and, in sufficient volume,
floods the cathode and shuts down the fuel cell altogether.
Methanol cross-over occurs primarily as a result of the high
solubility of methanol in the ionomeric membranes of the art.
[0006] It is of considerable interest in the art to identify ways
to reduce methanol cross-over in ionomeric membranes while
entailing as small as possible cost in conductivity.
[0007] Kyota et al, J P Sho 53(1978)-60388, describes a process for
producing modified Nafion.RTM. membranes with reduced permeability
to hydroxide ion by swelling with a solvent or liquid, diffusing a
polymerizable vinyl monomer into the swollen matrix with an
initiator, and polymerizing in situ. Also disclosed by reference is
a process for diffusing the monomers without solvent-swelling, but
the solvent-swelling process is said to be superior. Disclosed
monomers include vinyl acetate, acrylics, vinylisocyanate,
di-vinyls such as divinyl benzene, styrene, and fluorinated vinyl
monomers though not expressly TFE itself. Methanol permeability is
not discussed.
[0008] Seita et al, U.S. Pat. No. 4,200,538, disclose a cation
exchange membrane prepared by swelling a fluorinated ionomer with
an organic solvent, removing the solvent, immersing in a vinyl
monomer, adding initiators and other additives, and polymerizing
the monomer in situ. Improvements in hydroxyl ion permeability are
noted. Suitable monomers include styrene and styrene derivatives;
acrylic, methacrylic, and maleic acids and salts and esters
thereof; vinyl acetate, vinyl isocyanate, acrylonitrile, acrolein,
vinyl chloride, vinylidene chloride, vinylidene fluoride, vinyl
fluoride; and numerous others. Methanol permeability is not
discussed.
[0009] Fleischer et al, U.S. Pat. No. 5,643,689, disclose composite
membranes which include combination of ionomeric polymers and
numerous non-ionic polymers including polythyleneimine and
polyvinylpyrrolidone. Metal oxides are always present in the
composite. The composites are prepared by dissolving the respective
polymers in a common solvent and then removing the solvent, and are
said to be useful in hydrogen fuel cells.
[0010] Li et al, WO 98/42037, discloses polymer electrolyte blends
in batteries. Disclosed are blends of polybenzimidazoles with
Nafion.RTM. and other polymers in concentration ratios of ca. 1:1.
Preferred are blends of polybenzimidazoles and polyacrylamides.
Polyvinylpyrrolidone and polyethyleneimine are also disclosed.
SUMMARY OF THE INVENTION
[0011] In a first aspect, the invention provides a solid polymer
electrolyte membrane comprising a fluorinated ionomer having
imbibed therein the polymerization product of a composition
comprising a non-fluorinated, non-ionomeric monomer, wherein the
fluorinated ionomer comprises at least 6 mole % of monomer units
having a fluorinated pendant group with a terminal ionic group. The
invention also provides a composition further comprising a free
radical initiator, and optionally a crosslinking agent.
[0012] In the first aspect, the monomer is a polar vinyl monomer
that may be selected from the group consisting of vinyl acetate;
vinyl isocyanate; acrylonitrile; acrylic acid; acrylate esters; and
mixtures thereof.
[0013] In a second aspect, the invention provides a catalyst coated
membrane comprising a solid polymer electrolyte membrane having a
first surface and a second surface, an anode present on the first
surface of the solid polymer electrolyte membrane, and a cathode
present on the second surface of the solid polymer electrolyte
membrane, wherein the solid polymer electrolyte membrane comprises
a fluorinated ionomer having imbibed therein the polymerization
product of a composition comprising a non-fluorinated,
non-ionomeric monomer, wherein the fluorinated ionomer comprises at
least 6 mole % of monomer units having a fluorinated pendant group
with a terminal ionic group. The invention also provides a
composition further comprising a free radical initiator, and
optionally a crosslinking agent.
[0014] In the second aspect, the monomer is a polar vinyl monomer
that may be selected from the group consisting of vinyl acetate;
vinyl isocyanate; acrylonitrile; acrylic acid; acrylate esters; and
mixtures thereof. The anode and cathode comprise a catalyst, which
may be supported or unsupported.
[0015] In a third aspect, the invention provides a fuel cell
comprising a solid polymer electrolyte membrane having a first
surface and a second surface, wherein the solid polymer electrolyte
membrane comprises a fluorinated ionomer having imbibed therein the
polymerization product of a composition comprising a
non-fluorinated, non-ionomeric monomer, wherein the fluorinated
ionomer comprises at least 6 mole % of monomer units having a
fluorinated pendant group with a terminal ionic group. The
invention also provides a composition further comprising a free
radical initiator, and optionally a crosslinking agent.
[0016] In the third aspect, the monomer is a polar vinyl monomer
that may be selected from the group consisting of vinyl acetate;
vinyl isocyanate; acrylonitrile; acrylic acid; acrylate esters; and
mixtures thereof.
[0017] In the third aspect, the fuel cell further comprises an
anode and a cathode present on the first and second surfaces of the
polymer electrolyte membrane.
[0018] In the third aspect, the fuel cell further comprises a means
for delivering liquid or gaseous fuel to the anode, a means for
delivering oxygen to the cathode, a means for connecting the anode
and cathode to an external electrical load, methanol in the liquid
or gaseous state in contact with the anode, and oxygen in contact
with the cathode.
[0019] In the third aspect, the fuel is in the liquid or vapor
phase and comprises methanol or hydrogen.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 depicts a typical direct methanol fuel cell.
DETAILED DESCRIPTION
[0021] Following the practice of the art, in the present invention,
the term "ionomer" is used to refer to a polymeric material having
a pendant group with a terminal ionic group. The terminal ionic
group may be an acid or a salt thereof as might be encountered in
an intermediate stage of fabrication or production of a fuel cell.
Proper operation of the fuel cell of the invention requires that
the ionomer be in acid form. The term "polymeric precursor" to an
ionomer suitable for use in the present invention refers to the
non-ionic form of a polymer which when subject to hydrolysis
according to well-known methods in the art is converted into the
ionomer suitable for use in the present invention, or a salt
thereof.
[0022] The term "membrane precursor" refers to a membrane formed
from the ionomer suitable for the practice of the invention, prior
to the formation of a blend with another polymer which is not an
ionomer in order to produce the composite ionomeric polymer
electrolyte membrane of the invention. It is not necessary for the
practice of the invention that a precursor membrane first be formed
followed by incorporation of a polymer that is not an ionomer to
form the composite membrane of the invention. For example, it is
possible in some cases to melt blend the ionomeric precursor and
the polymer which is not an ionomer followed by melt casting a film
and hydrolysis. In other cases, it is possible to dissolve the
ionomer or its precursor and the other polymer that is not an
ionomer in a common solvent, and then solution cast a film.
However, it is found in the practice of the invention that it is
convenient to first fabricate a membrane precursor from the ionomer
or its precursor followed by incorporation of an other polymer that
is not an ionomer.
[0023] It is found that a solid polymer electrolyte membrane
comprising a fluorinated ionomer having imbibed therein the
polymerization product of a composition comprising a
non-fluorinated, non-ionomeric monomer, provides a membrane having
reduced methanol permeability, at relatively modest cost in
conductivity, and provides an improved fuel cell, eg. a DMFC or
hydrogen fuel cell.
[0024] One of ordinary skill in the art will understand that the
membrane having a film or sheet structure will have utility in
packaging, in non-electrochemical membrane applications. Membranes
also have application as an adhesive or other functional layer in a
multilayer film or sheet structure, and other classic applications
for polymer films and sheets that are outside electrochemistry. For
the purposes of the present invention, the term "membrane," a term
of art in common use in the fuel cell art is synonymous with the
term "film" or "sheet " which are terms of art in more general
usage but refer to the same articles. Ionomers suitable for use in
the present invention comprise at least 6 mol % of monomer units
having a fluorinated pendant group with a terminal ionic group,
preferably a sulfonic acid or sulfonate salt. A "polymeric
precursor" to an ionomer suitable for use in the present invention
preferably comprises a sulfonyl fluoride end-group, which when
subject to hydrolysis under alkaline conditions, according to
well-known methods in the art, is converted into a sulfonic acid or
sulfonate salt.
[0025] Any fuel cell, and in particular a direct methanol fuel cell
or a hydrogen fuel cell, known in the art, of the type provided
with a solid polymer electrolyte membrane may be employed in the
present invention. It is by the substitution of a membrane
comprising a fluorinated ionomer having imbibed therein the
polymerization product of a composition comprising a
non-fluorinated, non-ionomeric monomer, wherein the fluorinated
ionomer comprises at least 6 mole % of monomer units having a
fluorinated pendant group with a terminal ionic group, according to
the teachings of the present invention, for the ionomeric membrane
of the art that the benefits of the present invention may be
realized.
[0026] Ionomeric polymer electrolyte membranes have been prepared
that are particularly well-suited for use in direct methanol fuel
cells because of the surprisingly large decrease in methanol
permeability achieved at relatively small sacrifice of
conductivity.
Ionomeric Membrane Polymers
[0027] Some ionomeric polymer electrolyte membranes suitable for
use in the present invention, and methods for preparing them, are
variously described in Kyota et al, op. cit., and Fleischer et al,
op.cit.
[0028] A membrane in accordance with the invention is a mixture of
an ionomeric polymer or ionomer, and a non-ionomeric polymer
combined therewith. The ionomer suitable for the practice of the
invention has cation exchange groups that can transport protons
across the membrane. The cation exchange groups are acids
preferably selected from the group consisting of sulfonic,
carboxylic, phosphonic, imide, methide, sulfonimide and sulfonamide
groups. Various known cation exchange ionomers can be used
including ionomeric derivatives of trifluoroethylene,
tetrafluoroethylene, styrene-divinylbenzene, alpha, beta,
beta-trifluorostyrene, etc., in which cation exchange groups have
been introduced alpha, beta, beta-trifluorstyrene polymers useful
for the practice of the invention are disclosed in U.S. Pat. No
5,422,411.
[0029] In one embodiment of the invention, the ionomer comprises a
polymer backbone and recurring side chains attached to the backbone
with the side chains carrying the cation exchange groups. For
example, ionomers are formed by copolymerization of a first
fluorinated vinyl monomer and a second fluorinated vinyl monomer
having a side cation exchange group or a fluorinated cation
exchange group precursor (e.g., SO.sub.2F) which can be
subsequently hydrolyzed to sulfonic acid groups. Possible first
monomers include but are not limited to tetrafluoroethylene,
hexafluoropropylene, vinyl fluoride, vinylidine fluoride,
trifluorethylene, chlorotrifluoroethylene, perfluoro(alkyl vinyl
ether), and mixtures thereof. Possible second monomers include but
are not limited to a variety of fluorinated vinyl ethers with
fluorinated cation exchange groups or precursor groups.
[0030] In a further embodiment, the ionomer in accordance with the
invention has a backbone which is substantially fluorinated and the
ion exchange groups are sulfonic acid groups or alkali metal or
ammonium salts thereof which are readily converted to sulfonic acid
groups by ion exchange. "Substantially fluorinated" means that at
least 60% of the total number of halogen and hydrogen atoms are
fluorine atoms. In a further embodiment, the ionomer backbone and
the side chains are highly fluorinated, particularly
perfluorinated. The term "highly fluorinated" means that at least
90% of the total number of halogen and hydrogen atoms are fluorine
atoms.
[0031] Some ionomers suitable for use in the present invention are
variously described in U.S. Pat. No. 4,358,545, U.S. Pat. No.
4,940,525, WO 9945048, U.S. Pat. No. 6,025,092. Suitable ionomers
as disclosed therein comprise a highly fluorinated carbon backbone
having at least 6 mol % of a perfluoroalkenyl monomer unit having a
pendant group comprising the radical represented by the formula
--(OCF.sub.2CFR).sub.aOCF.sub.2(CFR').sub.bSO.sub.2X.sup.-(H.sup.+)[YZ.sub-
.c].sub.d (I)
[0032] wherein
[0033] R and R' are independently selected from F, Cl or a
perfluoroalkyl group having 1 to 10 carbon atoms, optionally
substituted by one or more ether oxygens;
[0034] a=0,1 or 2;
[0035] b=0 to 6;
[0036] X is O, C or N with the proviso that d=O when X is O and d=1
otherwise, and c=1 when X is C and c=0 when X is N; when c=1, Y and
Z are electron-withdrawing groups selected from the group
consisting of CN, SO.sub.2R.sub.f,SO.sub.2R.sup.3,
P(O)(OR.sup.3).sub.2, CO.sub.2R.sup.3, P(O)R.sup.3.sub.2,
C(O)R.sub.f, C(O)R.sup.3, and cycloalkenyl groups formed therewith
wherein R.sub.f is a perfluoroalkyl group of 1-10 carbons
optionally containing one or more ether oxygens; R.sup.3 is an
alkyl group of 1-6 carbons optionally substituted with one or more
ether oxygens, or an aryl group optionally further substituted;
[0037] or, when c=0, Y may be an electron-withdrawing group
represented by the formula --SO.sub.2R.sub.f' where R.sub.f' is the
radical represented by the formula
--(R.sub.f"SO.sub.2N--(H.sup.+)SO.sub.2).sub.mR.sub.f'"
[0038] where m=0 or 1, and R.sub.f" is --C.sub.nF.sub.2n-- and
R.sub.f'" is --C.sub.nF.sub.2n+1 where n=1-10
[0039] Most preferably, the ionomer comprises a perfluorocarbon
backbone and said pendant group is represented by the formula
--OCF.sub.2CF(CF.sub.3)--OCF.sub.2CF.sub.2SO.sub.3H
[0040] Ionomers of this type are disclosed in U.S. Pat. No.
3,282,875.
[0041] The equivalent weight (a term of the art defined herein to
mean the weight of the ionomer in acid form required to neutralize
one equivalent of NaOH) of the ionomer can be varied as desired for
the particular application. Where the ionomer comprises a
perfluorocarbon backbone and the side chain is represented by the
formula
--[OCF.sub.2CF(CF.sub.3)].sub.n--OCF.sub.2CF.sub.2SO.sub.3H
[0042] where n=0 or 1. The equivalent weight when n=1is preferably
800-1500, most preferably 900-1200. The equivalent weight when n=0
is preferably 600-1300.
[0043] In the manufacture of the preferred membranes wherein the
ionomer has a highly fluorinated backbone and sulfonate ion
exchange groups, a membrane precursor is conveniently initially
formed from the polymer in its sulfonyl fluoride form since it is
thermoplastic and conventional techniques for making films from
thermoplastic polymers can be used. Alternatively, the ionomer
precursor may be in another thermoplastic form such as by having
--SO.sub.2X groups where X is alkoxy such as CH.sub.3O-- or
C.sub.4H.sub.9O--, or an amine. Solution film casting techniques
using suitable solvents for the particular polymer can also be used
if desired.
[0044] The ionomer precursor polymer in sulfonyl fluoride form can
be converted to the sulfonate form (i.e, ionic form) by hydrolysis
using methods known in the art. For example, the membrane may be
hydrolyzed to convert it to the sodium sulfonate form by immersing
it in 25% by weight NaOH for about 16 hours at a temperature of
about 90.degree. C. This is followed by rinsing the film twice in
deionized 90.degree. C. water using about 30 to about 60 minutes
per rinse. Another possible method employs an aqueous solution of
6-20% of an alkali metal hydroxide and 5-40% of a polar organic
solvent such as dimethyl sulfoxide with a contact time of at least
5 minutes at 50-100.degree. C. followed by rinsing for 10 minutes.
After hydrolyzing, the membrane can be converted if desired to
another ionic form by contacting the membrane in a bath containing
a 1% salt solution containing the desired cation or, to the acid
form, by contacting with an acid and rinsing. For fuel cell use,
the membrane is usually in the sulfonic acid form.
[0045] If desired, the membrane precursor may be a laminated
membrane of two or more ionomeric precursors such as two highly
fluorinated ionomers having different ion exchange groups and/or
different ion exchange capacities. Such membranes can be made by
laminating films or co-extruding a multi-layer film. In addition,
the ionomeric component of the membrane suitable for use in the
present invention may be itself a blend of two or more ionomers,
such as two or more highly fluorinated ionomers preferred for the
practice of the invention, that have different ion exchange groups
and/or different ion exchange capacities. It is also possible to
form a multilayer structure incorporating one or more layers of the
composite membrane of the invention.
[0046] The thickness of the membrane can be varied as desired for a
particular electrochemical cell application. Typically, the
thickness of the membrane is generally less than about 250 .mu.m,
preferably in the range of about 25 .mu.m to about 150 .mu.m.
[0047] The membrane may optionally include a porous support for the
purposes of improving mechanical properties, for decreasing cost
and/or other reasons. The porous support of the membrane may be
made from a wide range of components. The porous support of the
present invention may be made from a hydrocarbon such as a
polyolefin, e.g., polyethylene, polypropylene, polybutylene,
copolymers of those materials, and the like. Perhalogenated
polymers such as polychlorotrifluoroethylene may also be used. For
resistance to thermal and chemical degradation, the support
preferably is made of a highly fluorinated polymer, most preferably
perfluorinated polymer.
[0048] For example, the polymer for the porous support can be a
microporous film of polytetrafluoroethylene (PTFE) or a copolymer
of tetrafluoroethylene with other perfluoroalkyl olefins or with
perfluorovinyl ethers. Microporous PTFE films and sheeting are
known which are suitable for use as a support layer. For example,
U.S. Pat. No. 3,664,915 discloses uniaxially stretched film having
at least 40% voids. U.S. Pat. Nos. 3,953,566, 3,962,153 and
4,187,390 disclose porous PTFE films having at least 70% voids.
[0049] Alternatively, the porous support may be a fabric made from
fibers of the support polymers discussed above woven using various
weaves such as the plain weave, basket weave, leno weave, or
others. A membrane suitable for the practice of the invention can
be made by coating the porous support fabric with a fluorinated
ionomer having imbibed therein the polymerization product of a
composition comprising a non-fluorinated, non-ionomeric monomer and
a free radical initiator. The fluorinated ionomer comprises at
least 6 mole % of monomer units having a fluorinated pendant group
with a terminal ionic group. To be effective the coating must be on
both the outside surfaces as well as distributed through the
internal pores of the support. This may be accomplished by
impregnating the porous support with a solution or dispersion of
the ionomeric polymer and drying, followed by imbibing a
composition comprising the non-ionomeric polymer, a free radical
initiator and optionally a crosslonking agent into the ionomeric
polymer and polymerizing to form the polymerization product of said
composition. It is important to use a solvent which is not harmful
to the polymer of the support under the impregnation conditions,
and that can form a thin, even coating on the support.
[0050] It is preferred for the cation exchange ionomer to be
present as a continuous phase within the membrane.
[0051] Non-ionomeric Polymers and Formation of Membranes
[0052] In accord with the present invention, the composite membrane
of the invention further comprises a non-ionomeric polymer. The
selection of non-ionomeric polymers and free radical initiators
suitable for use in the ionomeric polymer electrolyte membrane
composition is quite wide. It is desirable that the non-ionomeric
polymer be chemically and thermally stable under conditions of use
in a fuel cell. It is preferred that the non-ionomeric polymer
comprises a relatively high frequency of dipolar monomer units but
is not itself ionic. A "high frequency" of dipolar monomer units
means that the mole percentage concentration of monomer units
having a dipolar functionality should be at least 75%, and is
preferably greater than 90%. A "high frequency" of dipolar monomer
units also means that the monomer units of which the dipolar moiety
is a part should be as short as possible to increase the frequency
of occurrence of the dipolar moiety. Thus a vinyl monomer would be
preferred over, for example a butenyl monomer.
[0053] Preferred for the non-ionomeric polymer are polymers or
copolymers derived from polar vinyl monomers such as vinyl acetate,
vinyl isocyanate, acrylonitrile, acrylic acid or acrylate esters.
Preferred polymers include poly(vinyl acetate), polyacrylonitrile,
or polyacrylates Also suitable for the practice of the invention is
tetrafluoroethylene.
[0054] In one embodiment, the process for making the ionomeric
polymer electrolyte membrane composition comprises three steps
which may be performed concurrently by contacting an ionomer or
polymeric precursor thereto with a solution of a free radical
initiator; contacting the ionomer or polymeric precursor thereto
with a solution of one or more polymerizable dipolar monomers, e.g.
a dipolar vinyl monomers; and, carrying out the polymerization by
heating the thus formed composite intermediate to a temperature
sufficient to polymerize the vinyl monomers.
[0055] For monomers such as vinyl acetate or tetrafluoroethylene,
the polymerization may be effected in the presence of the ionomer
in proton or acid form. However, it is generally preferable to
employ the ionomer in the salt form. After polymerization is
completed the acid form may be regenerated by treatment in a
mineral acid such as HNO.sub.3. It has been found in the practice
of the present invention that the efficacy of the invention in
bringing about a significant decrease in methanol permeability with
relatively small sacrifice in conductivity, depends significanty
upon the cation employed to form the ionomeric salt. Alkali metal
cations, such as lithium, sodium and potassium are preferred over
rubidium or cesium. It is also acceptable to employ the
unhydrolyzed polymer precursor to the ionomer in place of the
ionomer until after a composite polymer is formed, and then to
hydrolyze the precursor to prepare the ionomer.
[0056] To enhance the rate of transport of monomers and initiators,
or, in the alternative, of non-fluorinated, non-ionomeric polymer,
into the precursor polymer or ionomer, it is useful to subject the
precursor polymer or ionomer to swelling using a swelling agent,
preferably one which also serves as a solvent for the monomers or
non-fluorinated polymers. In the case of unhydrolyzed fluorinated
polymer precursors, dimethylsulfoxide is an excellent swelling
agent as are fluorinated solvents. In the case of ionomers, polar
solvents such as MeOH, water or DMF are preferred. Usually the more
swelled the ionomers or copolymers are, the more non-fluorinated,
non-ionomeric polymer will be present in the final polymeric
compositions. Other solvents may also be employed as may be
effective for a particular polymer composition. The outcome of the
practice of the invention is not highly dependent upon the solvent
used for swelling the polymer except insofar as solvents which
result in more swelling are in general preferred over those which
result in less swelling. Extractability of the solvent after
imbibition of the monomers or non-fluorinated, non-ionomeric
polymer is also a desirable property.
[0057] In a preferred method, the initiator is conveniently added
to the ionomers or precursor polymer dissolved in a solvent which
also swells the ionomer or precursor polymer. Some preferred
initiators include liquids, such as hexafluoropropylene oxide
(HFPO) dimer peroxide; per fluoropropionyl peroxide,
(CF.sub.3CF.sub.2COO).sub.2; Lupersol.RTM. manufactured by Ashland,
Covington, Ky.; 2,2-azobis(isobutyronitrile) and di-t-butyl
peroxide. Optionally crosslinking agents may be present in the
composition to be polymerized. Some useful crosslinking agents
include divinyl benzene, dienes, fluorinated dienes, diacrylates,
divinyl esters and divinylether. If the vinyl monomer is a liquid,
it is preferably added in the same solution as the initiator.
Preferably, the temperature is maintained well-below the initiation
temperature to provide the most homogeneous composition. However,
polymerizatiom may also occur as the vinyl monomer is diffusing
into the ionomer or polymeric precursor thereto. If the vinyl
monomer is a gas, it is preferred to contact the ionomer or
polymeric precursor thereto to the gaseous vinyl monomer in a
sealed vessel after the ionomer or polymeric precursor thereto has
been treated with the initiator solution and, preferably, been
swollen thereby. If the vinyl monomer is a solid, it is preferably
first dissolved in a solvent and then the ionomer or polymeric
precursor thereto is immersed in the monomer solution.
[0058] Monomers are present in the composition to be polymerized in
the amount of greater than about 6%, more typically up to about
40%, free radical initiators are present in the amount of about
less than about 0.1%, more typically about less than about 0.05%,
and the optional crosslinking agent is present in the amount of
about 0 to about 5%, more typically about 0.01 to about 5%, and
still more typically about 0.5 to about 2%, based on the weight of
the total composition to be polymerized.
[0059] Membrane Electrode Assemblies and Electrochemical Cell
[0060] One embodiment of a fuel cell suitable for the practice of
the present invention is shown in FIG. 1. While the cell depicted
represents a single-cell assembly such as that employed in
determining some of the results herein, one of skill in the art
will recognize that all of the essential elements of a direct
methanol fuel cell are shown therein in schematic form.
[0061] A ionomeric polymer electrolyte membrane of the invention,
1, is used to form a membrane electrode assembly (MEA) by combining
it with a catalyst layer, 2, comprising a catalyst, e.g. platinum,
unsupported or supported on carbon particles, a binder such as
Nafion.RTM., and a gas diffusion backing, 3. The gas diffusion
backing may comprise carbon paper which may be treated with a
fluoropolymer and/or coated with a gas diffusion layer comprising
carbon particles and a polymeric binder to form an membrane
electrode assembly (MEA). The fuel cell is further provided with an
inlet for liquid or gaseous methanol, 4, an anode outlet, 5, a
cathode gas inlet, 6, a cathode gas outlet, 7, aluminum end blocks,
8, tied together with tie rods (not shown), a gasket for sealing,
9, an electrically insulating layer, 10, and graphite current
collector blocks with flow fields for gas distribution, 11, and
gold plated current collectors, 12.
[0062] The fuel cell utilizes a fuel source that may be in the
liquid or gaseous phase, and may comprise an alcohol or ether.
Typically a methanol/water solution is supplied to the anode
compartment and air or oxygen supplied to the cathode compartment.
The ionomeric polymer electrolyte membrane serves as an electrolyte
for proton exchange and separates the anode compartment from the
cathode compartment. A porous anode current collector, and a porous
cathode current collector are provided to conduct current from the
cell. A catalyst layer that functions as the cathode is in contact
with and between the cathode-facing surface of the membrane and the
cathode current collector. A catalyst layer that functions as the
anode is disposed between and is in contact with the anode-facing
surface of the membrane and anode current collector. The cathode
current collector is electrically connected to a positive terminal
and the anode current collector is electrically connected to a
negative terminal.
[0063] The catalyst layers may be made from well-known electrically
conductive, catalytically active particles or materials and may be
made by methods well known in the art. The catalyst layer may be
formed as a film of a polymer that serves as a binder for the
catalyst particles. The binder polymer can be a hydrophobic
polymer, a hydrophilic polymer or a mixture of such polymers.
Preferably, the binder polymer is an ionomer and most preferably is
the same ionomer as in the membrane.
[0064] For example, in a catalyst layer using a perfluorinated
sulfonic acid polymer membrane and a platinum catalyst, the binder
polymer can also be perfluorinated sulfonic acid polymer and the
catalyst can be a platinum catalyst supported on carbon particles.
In the catalyst layers the particles are preferably uniformly
dispersed in the polymer to assure that a uniform and controlled
depth of the catalyst is maintained, preferably at a high volume
density with the particles being in contact with adjacent particles
to form a low resistance conductive path through catalyst layer.
The connectivity of the catalyst particles provides the pathway for
electronic conduction and the network formed by the binder ionomer
provides the pathway for proton conduction.
[0065] The catalyst layers formed on the membrane should be porous
so that they are readily permeable to the gases/liquids that are
consumed and produced in cell. The average pore diameter is
preferably in the range of 0.01 to 50 .mu.m, most preferably 0.1 to
30 .mu.m. The porosity is generally in a range of 10 to 99%,
preferably 10 to 60%.
[0066] The catalyst layers are preferably formed using an "ink",
i.e., a solution of the binder polymer and the catalyst particles,
which is used to apply a coating to the membrane. The binder
polymer may be in the ionomeric (proton) form or in the sulfonyl
fluoride (precursor) form. When the binder polymer is in the proton
form the preferred solvent is a mixture of water and alcohol. When
the binder polymer is in the precursor form the preferred solvent
is a perfluorinated solvent (FC-40 made by 3M).
[0067] The viscosity of the ink (when the binder is in the proton
form) is preferably controlled in a range of 1 to 102 poises
especially about 102 poises before printing. The viscosity may be
controlled by:
[0068] (i) particle size selection,
[0069] (ii) composition of the catalytically active particles and
binder,
[0070] (iii) adjusting the water content (if present), or
typically
[0071] (iv) by incorporating a viscosity regulating agent such as
carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose,
and cellulose and polyethyleneglycol, polyvinyl alcohol, polyvinyl
pyrrolidone, sodium polyacrylate and polymethyl vinyl ether.
[0072] The area of the membrane to be coated with the ink may be
the entire area or only a select portion of the surface of the
membrane. The catalyst ink may be deposited upon the surface of the
membrane by any suitable technique including spreading it with a
knife or blade, brushing, pouring, metering bars, spraying and the
like. The catalyst layer may also be applied by decal transfer,
screen printing, or by application of a printing plate.
[0073] If desired, the coatings are built up to the thickness
desired by repetitive application. The desired loading of catalyst
upon the membrane can be predetermined, and the specific amount of
catalyst material can be deposited upon the surface of the membrane
so that no excess catalyst is applied. The catalyst particles are
preferably deposited upon the surface of a membrane in a range from
about 0.2 mg/cm.sup.2 to about 20 mg/cm.sup.2.
[0074] Typically a screen printing process is used for applying the
catalyst layers to the membrane with a screen having a mesh number
of 10 to 2400, more typically a mesh number of 50 to 1000, and a
thickness in the range of 1 to 500 micrometers. The mesh and the
thickness of the screen, and viscosity of the ink are selected to
give electrode thickness ranging from 1 micron to 50 microns, more
particularly 5 microns to 15 microns. The screen printing process
can be repeated as needed to apply the desired thickness. Two to
four passes, usually three passes, have been observed to produce
the optimum performance. After each application of the ink, the
solvent is preferably removed by warming the electrode layer to
about 50.degree. C. to 140.degree. C., preferably about 75.degree.
C. A screen mask is used for forming an electrode layer having a
desired size and configuration on the surface of the ion exchange
membrane. The configuration is preferably a printed pattern
matching the configuration of the electrode. The substances for the
screen and the screen mask can be any materials having satisfactory
strength such as stainless steel, poly(ethylene terephthalate) and
nylon for the screen and epoxy resins for the screen mask.
[0075] After forming the catalyst coating, it is preferable to fix
the ink on the surface of the membrane so that a strongly bonded
structure of the electrode layer and the cation exchange membrane
can be obtained. The ink may be fixed upon the surface of the
membrane by any one or a combination of pressure, heat, adhesive,
binder, solvent, electrostatic, and the like. Typically the ink is
fixed upon the surface of the membrane by using pressure, heat or a
combination of pressure and heat. The electrode layer is preferably
pressed onto the surface of the membrane at 100.degree. C. to
300.degree. C., most typically 150.degree. C. to 280.degree. C.,
under a pressure of 510 to 51,000 kPa (5 to 500 ATM), most
typically 1,015 to 10,500 kPa (10 to 100 ATM).
[0076] An alternative to applying the catalyst layer directly onto
the membrane is the so-called "decal" process. In this process, the
catalyst ink is coated, painted, sprayed or screen printed onto a
substrate and the solvent is removed. The resulting "decal" is then
subsequently transferred from the substrate to the membrane surface
and bonded, typically by the application of heat and pressure.
[0077] When the binder polymer in the ink is in the precursor
(sulfonyl fluoride) form, the catalyst coating after it is affixed
to the membrane, either by direct coating or by decal transfer, is
subjected to a chemical treatment (hydrolysis) where the binder is
converted to the proton (or ionomeric) form.
[0078] The invention is illustrated in the following examples.
EXAMPLES
[0079] Glossary:
[0080] Lupersol.RTM. 11 t-Butyl Peroxypivalate, made by Ashland,
Covington, Ky.
[0081] HFPO dimer peroxide having structure:
[0082] [CF.sub.3CF.sub.2CF.sub.2OCF(CF.sub.3)COO].sub.2
[0083] AIBN 2,2'-Azobisisobutyronitrile
EXAMPLES
[0084] In the following specific embodiments conductivity, methanol
absorption, and methanol permeability were determined, where
indicated.
[0085] Methanol absorption was determined by first pre-drying the
specimen and weighing in a closed container to get W.sub.D. The
dried sample was then immersed in methanol at room temperature for
6-8 hours. The thus immersed specimen was then removed from the
methanol, surface dried, placed in a closed container and weighed
to get W.sub.W. The methanol absorption was determined from the
equation:
% Absorption=[(W.sub.W-W.sub.D)/W.sub.D].times.100
[0086] In order to determine methanol permeability, the membrane
samples were loaded in permeation cells (316 SS, Millipore.RTM.
high-pressure, 47 mm filters modified by the addition of liquid
distribution plates). Each cell has a permeation area of 9.6
cm.sup.2. The cells (up to 4 per run) are located inside an
insulated box kept at constant temperature. The insulated box was
heated by two Chromalox.RTM., 1100 watts, and finstrips heaters.
The air within the box was mixed by a 7" diameter, 5-blade
propeller connected to a Dayton Model 4Z140 variable speed DC
motor. The insulated box temperature was controlled by a Yokogawa
UT320 Digital indicating temperature controller. 1 M methanol
solution was circulated on the top side of the membrane at a flow
rate of 5.7-9.6 cc/min (measured with Brook Instruments, Model
1355EYZQFA1G rotameters). The bottom of the membrane was swept with
nitrogen at 1,000-5,000 SCCM (measured with mass flow controllers:
2 MKS type 1179 and 2 Tylan 2900 series mass flow meters connected
by a Tylan RO-28 controller box). Both the methanol solution and
the nitrogen were heated to the cell temperature by circulating
through stainless steel coils before entering the permeation cells.
Samples of the nitrogen sweeping the permeation cells were sent to
a set of heated Valco.RTM. valves and then a 2 cc gas sample was
injected into a HP 6890 Gas Chromatograph with a Thermal
Conductivity Detector (TCD) and HP-PLOT Q GC Column to analyze the
methanol and water. The GC was controlled by HP Chem Station
software Revision A.06.03. The permeation rates (molar fluxes) of
methanol and water through the membrane were calculated as
follows:
Methanol Molar flux (mol/cm.sup.2
min)=grams.sub.MeOH*F/(V.sub.nitrogen*A.- sub.p*MW.sub.MeOH)
Water Molar flux (mol/cm.sup.2
min)=grams.sub.Water*F/(V.sub.nitrogen*A.su- b.p*MW.sub.Water)
[0087] Where:
[0088] grams.sub.MeOH=MeOH Peak Area*MeOH Response Factor=Grams
MeOH Injected in GC.
[0089] grams.sub.water=Water Peak Area*Water Response Factor=Grams
water Injected in GC.
[0090]
V.sub.nitrogen=V.sub.s-grams.sub.MeOH/pMeOH-grams.sub.Water/Pwater=-
Volume of nitrogen injected in GC (cm.sup.3)
[0091] V.sub.s=Volume Gas Sample injected into GC (cm.sup.3)
[0092] T.sub.s=Temperature of Gas sample=Temperature of sampling
valve (K)
[0093] P.sub.s=Pressure of gas sample (psia)
[0094] .sub.Pnitrogen=Density of nitrogen at T.sub.s and P.sub.s
(g/cm.sup.3)
[0095] .sub.PMeOH=Density of Methanol at T.sub.s and P.sub.s
(g/cm.sup.3)
[0096] .rho..sub.Water=Density of Water at T.sub.s and P.sub.s
(g/cm.sup.3)
[0097] A.sub.p=Permeation Area of cells (cm.sup.2)
[0098] F=Flow of nitrogen sweeping membrane at T.sub.s, P.sub.s
(cm.sup.3/min)
[0099] The methanol and water response factors were calculated by
injecting known amounts of methanol and water into the GC. The
methanol and water response factors were the ratio of grams of
components injected /Peak area of methanol and water.
[0100] Conductivity of the subject membrane was determined by
impedance spectroscopy by which is measured the ohmic (real) and
capacitive (imaginary) components of the membrane impedance.
Impedance wa determined using the Solartron model SI 1260
Impedance/Gain-phase Analyzer, manufactured by Schlumberger
Technologies Ltd., Instrument Division, Farnborough, Hampshire,
England, utilizing a conductivity cell having a cell constant of
202.09, as described in J. Phys. Chem., 1991, 95, 6040 and
available from Fuel Cell Technologies, Albuquerque, N.M.
[0101] Prior to the conductivity measurement, a membrane was boiled
in deionized water for one hour prior to testing. The conductivity
cell was submersed in water at 25.+-.1.degree. C. during the
experiment.
[0102] The impedance spectrum was determined from 10 Hz to 10.sup.5
Hz at 0 VDC, and 100 mv (rms) AC. The real impedance that
corresponded to the highest (least negative) imaginary impedance
was determined.
[0103] Conductivity was calculated from the equation:
Conductivity=cell constant/[(real impedance)*(film thickness)]
Example 1
[0104] Four dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 7.879
g) (E. I. duPont de Nemours and Co., Wilmington, Del.) were
immersed in a solution of 50 mL of acrylonitrile (AN), and 0.6 g of
Lupersol.RTM. 11, an initiator, at -5.degree. C. for 3 hrs, and
then transferred to a flask under N.sub.2 atmosphere. The sealed
flask was heated at 70.degree. C. for 18 hrs. to polymerize
acrylonitrile to form a polacrylonitrile . After polymerization,
the films were dried at 100.degree. C. in full vacuum for 3 hrs to
remove volatiles. The films weighed 11.00 g. The weight % of
polymer formed in the Nafion.RTM. 117 film, as shown in Table 1 was
determined by first weighing the dry Nafion.RTM. film, and
re-weighing it following the steps of immersion, polymerization,
and extraction of volatiles at 100.degree. C. under full vacuum.
The weight difference was divided by the initial weight to give the
weight % increase in Table 1. After drying the films were treated
with 10% HNO.sub.3 at 60.degree. C. for 2 hrs, washed with water
until pH=7was reached, and boiled in de-ionized water for 2 hrs.
Conductivity and MeOH permeability are shown in Table 1.
Example 2
[0105] Four dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 7.898
g) were immersed in a solution of 50 mL of acrylonitrile (AN), 2 mL
of divinylbenzene (DVB), a crosslinking agent, and 0.6 g of
Lupersol.RTM. 11 at -5.degree. C. for 3 hrs, and then transferred
to a flask under N.sub.2 atmosphere. The sealed flask was heated at
70.degree. C. for 20 hrs. After polymerization, the films were
dried at 100.degree. C. in full vacuum for 6 hrs to remove
volatiles. The films weighed 10.668 g. The weight % of polymer
formed in the Nafion.RTM. 117 film was determined as described in
Example 1. After drying the films were treated as in Example 1.
Conductivity and MeOH permeability are shown in Table 1.
Example 3
[0106] Three dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 5.93
g) were immersed in a solution of 50 mL of acrylonitrile (AN), 2 mL
of divinylbeneze (DVB) and 0.6 g of Lupersol.RTM. 11 at -5.degree.
C. for 1.5 hrs, and then transferred to a flask under N.sub.2
atmosphere. The sealed flask was heated at 70.degree. C. for 20
hrs. After polymerization, the films were dried at 100.degree. C.
in full vacuum for 6 hrs to remove volatiles. The films weighed
10.668 g. The weight % of polymer formed in the Nafion.RTM. 117
film was determined as described in Example 1. After drying, the
films were treated as in Example 1. Conductivity and MeOH
permeability are shown in Table 1.
Example 4
[0107] Two dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 3.977
g) were immersed in a solution of 50 mL of acrylonitrile (AN), 2 mL
of divinylbeneze (DVB) and 0.6 g of Lupersol.RTM. 11 at -5.degree.
C. for 0.5 hrs, and then transferred to a flask under N.sub.2
atmosphere. The sealed flask was heated at 75.degree. C. for 16
hrs. After polymerization, the films were dried at 100.degree. C.
in full vacuum for 6 hrs to remove volatiles. The films weighed
4.361 g. The weight % of polymer formed in the Nafion.RTM. 117 film
was determined as described in Example 1. After drying, the films
were treated as in Example 1. Conductivity and MeOH permeability
are shown in Table 1.
Example 5
[0108] Two dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 4.018
g) were immersed a solution of 12 mL of THF and 2 mL of water for
30 min. and then 20 g of acrylonitrile (AN) and kept at 10.degree.
C. for 2 hrs, followed by addition of 0.2 g of Lupersole.RTM. 11.
The films were transferred to a flask under N.sub.2 atmosphere. The
sealed flask was heated at 70-80.degree. C. for 15 hrs. After
polymerization, the films were dried at 100.degree. C. in full
vacuum for 3 hrs to remove volatiles. The films weighed 4.458 g.
The weight % of polymer formed in the Nafion.RTM. 117 film was
determined as described in Example 1. After drying, the films were
treated as in Example 1. Conductivity and MeOH permeability are
shown in Table 1.
Example 6
[0109] Three dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 5.922
g) were boiled in de-ionic water for 1 hr. After being wiped with a
paper towel, the films were immersed in a solution of 50 mL of
acrylonitrile (AN) and 0.6 g of Lupersole.RTM. 11 at -5.degree. C.
for 2 hrs, and then transferred to a flask under N.sub.2
atmosphere. The sealed flask was heated at 70.degree. C. for 16
hrs. After polymerization, the films were dried at 100.degree. C.
in full vacuum for 6 hrs to remove volatiles. The films weighed
6.188 g. The weight % of polymer formed in the Nafion.RTM. 117 film
was determined as described in Example 1. After drying, the films
were treated as in Example 1. Conductivity and MeOH permeability
are shown in Table 1.
Example 7
[0110] Four dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 8.011
g) were immersed in a solution of 50 mL of methyl acrylate (MA), 2
mL of divinylbeneze (DVB) and 0.6 g of Lupersol.RTM. 11 at
-5.degree. C. for 3 hrs, and then transferred to a flask under
N.sub.2 atmosphere. The sealed flask was heated at 75.degree. C.
for 16 hrs. After polymerization, the films were dried at
100.degree. C. in full vacuum for 6 hrs to remove volatiles. The
films weighed 10.938 g. The weight % of polymer formed in the
Nafion.RTM. 117 film was determined as described in Example 1.
After drying, the films were treated as in Example 1. Conductivity
and MeOH permeability are shown in Table 1.
Example 8
[0111] Four dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 8.005
g) were immersed in a solution of 40 mL of methyl acrylate (MA), 15
mL of divinylbeneze (DVB) and 0.6 g of Lupersol.RTM. 11 at
-5.degree. C. for 2 hrs, and then transferred to a flask under
N.sub.2 atmosphere. The sealed flask was heated at 70.degree. C.
for 11 hrs. After polymerization, the films were dried at
100.degree. C. in full vacuum for 5 hrs to remove volatiles. The
films weighed 8.858 g. The weight % of polymer formed in the
Nafion.RTM. 117 film was determined as described in Example 1.
After drying, the films were treated as in Example 1. Conductivity
and MeOH permeability are shown in Table 1.
Example 9
[0112] Three dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 5.914
g) were immersed in a solution of 50 mL of methyl acrylate (MA), 2
mL of divinylbeneze (DVB) and 0.6 g of Lupersol.RTM. 11 at
-5.degree. C. for 0.5 hr, and then transferred to a flask under
N.sub.2 atmosphere. The sealed flask was heated at 70.degree. C.
for 16 hrs. After polymerization, the films were dried at
100.degree. C. in full vacuum for 4 hrs to remove volatile. The
films weighed 6.248 g. The weight % of polymer formed in the
Nafion.RTM. 117 film was determined as described in Example 1.
After drying, the films were treated as in Example 1. Conductivity
and MeOH permeability are shown in Table 1.
Example 10
[0113] Four dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 8.069
g) were immersed in a solution of 30 mL of acrylonitrile (AN), 20
mL of methyl acrylate (MA), 4 mL of divinylbeneze (DVB) and 0.6 g
of Lupersole.RTM. 11 at -5.degree. C. for 0.5 hr, and then
transferred to a flask under N.sub.2 atmosphere. The sealed flask
was heated at 70-80.degree. C. for 16 hrs. After polymerization,
the films were dried at 100.degree. C. in full vacuum for 3 hrs to
remove volatiles. The films weighed 8.548 g. The weight % of
polymer formed in the Nafion.RTM. 117 film was determined as
described in Example 1. After drying, the films were treated as in
Example 1. Conductivity and MeOH permeability are shown in Table
1.
Example 11
[0114] Four dried Nafion.RTM. 117 films (7.62.times.7.62 cm, 5.964
g) were immersed in a solution of 40 mL of methyl methacrylate
(MMA), 7 mL of divinylbeneze (DVB) and 0.6 g of Lupersol.RTM. 11 at
-5.degree. C. for 2 hrs, and then transferred to a flask under
N.sub.2 atmosphere. The sealed flask was heated at 70.degree. C.
for 16 hrs. After polymerization, the films were dried at
100.degree. C. in full vacuum for 6 hrs to remove volatiles. The
films weighed 6.318 g. The weight % of polymer formed in the
Nafion.RTM. 117 film was determined as described in Example 1.
After drying, the films were treated as in Example 1. Conductivity
and MeOH permeability are shown in Table 1.
1TABLE 1 Mono- Immmer- % Decrease in % Decrease mer(s)* Volume
Volume sion MeOH MeOH Conductivity in Ex Type (ml) DVB (ml) solvent
Time (h) Wt % permeability Permeability (S/cm). Conductivity
Control 1.85E-05 0.1 1 AN 50 0 none 3 40 2.23E-06 88% 0.0407 59% 2
AN 50 2 none 3 35 4.92E-06 73% 0.0398 60% 3 AN 50 2 none 1.5 16.2
6.36E-06 65% 0.0856 14% 4 AN 50 2 none 0.5 9.6 0.0733 27% 5 AN 20 0
THF/H2O = 12/2 2 11 1.48E-05 20% 0.0892 11% 6 AN 50 0 none 3 4.5
8.67E-06 53% 0.0869 13% 7 MA 50 2 none 3 36.6 7.53E-06 59% 0.0594
41% 8 MA 30 7 none 2 10.6 8.15E-06 56% 0.0833 17% 9 MA 50 2 none
0.5 5.6 1.36E-05 25% 0.091 9% 10 MA/AN 20/30 4 none 0.5 5.9
1.39E-05 25% 0.0907 9% 11 MMA 40 7 none 2 5.9 1.50E-05 19% 0.0873
13% *AN is CH.sub.2.dbd.CHCN, MA is methyl acrylate, and MMA is
methyl methacrylate.
Example 12
[0115] Two Nafion.RTM. 117 films, (3.05.times.2.79 cm, 0.552 9 and
0.475 g, respectively) were treated with 0.1 M LiOH in water for 20
min. to form the lithium salt. The film was removed and washed with
water and briefly dried in air and then immersed in a solution of
7.01 g of vinyl acetate, 0.2 g of divinylbenzene and 5 drops of
Lupersol.RTM.D 11 at 0.degree. C. to 5.degree. C. for 3.5 hours.
The film was removed, wiped off and transferred in a flask in
N.sub.2 atmosphere. After being kept in an oven at 70.degree. C.
overnight, the film was converted back to the acid form by
treatment with 8% HNO.sub.3 for 4 hours and washed with water. IR
indicated that the film contained poly(vinylacetate): 2924, 2853,
1725 cm.sup.-1. Absorption of methanol was 34.1%, and conductivity
was 0.098 S/cm. Absorption of MeOH for untreated Nafion.RTM. 117
was 66%, and conductivity was 0.103 S/cm.
Example 13
[0116] A Nafion.RTM. 117 film was treated with 0.1 M LiOH in water
for 20 min. The film were removed and washed with water and briefly
dried in air and then immersed in a solution of styrene,
divinylbenzene and Lupersol.RTM. 11 in a ratio of 10.5 to 3.5 to
0.34 at 0.degree. C. to -15.degree. C. for 20 hours. The films were
removed, wiped off and transferred in a flask in N.sub.2
atmosphere. After being kept in oven at 70.degree. C. overnight,
the films were treated with 8% HNO.sub.3 for 4 hours and washed
with water. IR indicated that the films contained polystyrene.
Absorption of methanol was 73.2%, and conductivity was 0.095
S/cm.
Example 14
[0117] A Nafion.RTM. 117 film was treated in dilute LiOH for 20
minutes. It was then immersed in 6.6 weight % of HFPO dimer
peroxide in
CF.sub.3CF.sub.2CF.sub.2OCF(CF.sub.3)CF.sub.2OCFHCF.sub.3 (E-2) for
2 hours. Upon removal from the solution, the films were briefly air
dried and transferred into a dry-ice prechilled shaker tube. The
tube was evacuated and loaded with 25 g of tetrafluoroethylene
(TFE), shaken at 25.degree. C. to 35.degree. C. for 10 hrs, causing
the pressure drop from 319 psi to 160 psi. Loose Poly TFE (PTFE)
was observed on the surface of the film. Once the PTFE was wiped
off with MeOH, the film was washed with water and treated with 8%
HNO.sub.3 overnight, and then washed with water. Absorption of
methanol was 51.1% and conductivity was 0.096 S/cm.
Example 15
[0118] A Nafion.RTM. 117 film (0.42 g) was treated with 2%
Cs.sub.2CO.sub.3 in water for 18 hrs at room temperature to form
Nafion.RTM. -Cs salt. After being removed, washed with water and
dried in air at room temperature for 2 hrs, the film was immersed
in 3% HFPO dimer peroxide in E-2 at -15.degree. C. overnight. The
films were removed, wiped off and transferred in a 240 mL of shaker
tube at -10.degree. C. After charging with 40 g of TFE, the tube
was shaken at 30.degree. C. for 10 hrs. After being wiped off to
remove loose PTFE, the film was dried at 110.degree. C. in full
vacuum overnight. Weight of the film increased 2%. The film was
treated with 8% HNO.sub.3 overnight, and then washed with water.
Absorption of water and methanol were 31.8% and 50.5%,
respectively, and conductivity was 0.063 S/cm.
* * * * *