U.S. patent application number 11/295764 was filed with the patent office on 2007-06-07 for reinforced ion-conductive membranes.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Scott L. Ciliske, Ilyess Hadj Romdhane.
Application Number | 20070128425 11/295764 |
Document ID | / |
Family ID | 38119121 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070128425 |
Kind Code |
A1 |
Hadj Romdhane; Ilyess ; et
al. |
June 7, 2007 |
Reinforced ion-conductive membranes
Abstract
An ion-conductive membrane that includes a first layer
comprising a first ionomer, and a porous polymer substrate, where
at least a portion of the first ionomer is interpenetrated within
the porous polymer substrate by ionomer-induced phase
separation.
Inventors: |
Hadj Romdhane; Ilyess;
(Woodbury, MN) ; Ciliske; Scott L.; (Saint Paul,
MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
38119121 |
Appl. No.: |
11/295764 |
Filed: |
December 7, 2005 |
Current U.S.
Class: |
428/304.4 ;
427/207.1; 428/306.6; 428/309.9 |
Current CPC
Class: |
C08J 2379/08 20130101;
H01M 2300/0094 20130101; B01D 69/125 20130101; B01D 2325/26
20130101; B01D 67/0013 20130101; H01M 8/1027 20130101; H01M 8/1053
20130101; B01D 2325/14 20130101; Y02E 60/50 20130101; B01D 71/32
20130101; B01D 69/141 20130101; H01M 8/1069 20130101; Y02P 70/50
20151101; Y10T 428/24996 20150401; B01D 67/0009 20130101; H01M
8/1067 20130101; B01D 67/0016 20130101; C08J 5/2275 20130101; H01M
8/1048 20130101; H01M 8/106 20130101; C08J 2381/06 20130101; Y10T
428/249953 20150401; H01M 8/1039 20130101; H01M 8/103 20130101;
H01M 8/1032 20130101; Y10T 428/249955 20150401; H01M 8/1062
20130101; B01D 2325/42 20130101 |
Class at
Publication: |
428/304.4 ;
427/207.1; 428/306.6; 428/309.9 |
International
Class: |
B05D 5/10 20060101
B05D005/10; B32B 3/26 20060101 B32B003/26; B32B 3/06 20060101
B32B003/06; B32B 5/14 20060101 B32B005/14 |
Claims
1. An ion-conductive membrane comprising: a first layer comprising
a first ionomer; and a porous polymer substrate, wherein at least a
portion of the first ionomer is interpenetrated within the porous
polymer substrate by ionomer-induced phase separation.
2. The ion-conductive membrane of claim 1, wherein the first
ionomer is interpenetrated within the porous polymer substrate in a
substantially uniform manner.
3. The ion-conductive membrane of claim 1, wherein the first
ionomer comprises a sulfonated fluoropolymer.
4. The ion-conductive membrane of claim 1, wherein the porous
polymer substrate comprises a material selected from the group
consisting of polysulfones, polyether imides, and combinations
thereof.
5. The ion-conductive membrane of claim 1, wherein the porous
polymer substrate comprises a sulfonated porous polymer.
6. The ion-conductive membrane of claim 1, wherein the porous
polymer substrate comprises a polymer having a glass transition
temperature of at least about 180.degree. C.
7. The ion-conductive membrane of claim 1, wherein the
ion-conductive membrane has a membrane thickness of about 30
micrometers or less.
8. The ion-conductive membrane of claim 7, wherein the
ion-conductive membrane has a membrane thickness of about 15
micrometers or less.
9. The ion-conductive membrane of claim 1, further comprising a
second layer, the second layer comprising a second ionomer, wherein
at least a portion of the second ionomer is interpenetrated within
the porous polymer substrate by ionomer-induced phase
separation.
10. The ion-conductive membrane of claim 9, wherein the first
ionomer and the second ionomer have different equivalent
weights.
11. The ion-conductive membrane of claim 9, wherein the first
ionomer and the second ionomer each comprise a sulfonated
fluoropolymer.
12. A method of forming an ion-conductive membrane, the method
comprising: coating a first layer adjacent to a second layer,
wherein the first layer comprises a first ionomer dispersed in a
first solvent, and wherein the second layer comprises a
reinforcement polymer substantially dissolved in a second solvent;
and initiating an ionomer-induced phase separation of the
reinforcement polymer from the second solvent, thereby forming a
porous polymer substrate from the reinforcement polymer and
interpenetrating a portion of the first ionomer within the porous
polymer substrate.
13. The method of claim 12, wherein the second layer further
comprises a second ionomer.
14. The method of claim 12, wherein the step of coating the first
layer adjacent to the second layer comprises coextruding the first
layer and the second layer.
15. The method of claim 12, wherein the first ionomer comprises a
sulfonated fluoropolymer.
16. The method of claim 12, wherein the reinforcement polymer
comprises a material selected from the group consisting of
polysulfones, polyether imides, and combinations thereof.
17. The method of claim 12, further comprising coating the second
layer with a third layer on an opposing surface of the second layer
from the first layer, the third layer comprising a second ionomer
dispersed in a third solvent, wherein the ionomer-induced phase
separation further interpenetrates a portion of the second ionomer
within the porous polymer substrate.
18. A method of forming an ion-conductive membrane, the method
comprising the steps of: a) coating a first substrate layer having
a first surface and a second surface from a first solution, the
first solution comprising a reinforcement polymer substantially
dissolved in a first solvent; b) coating a first ionomer layer
adjacent the first surface of the substrate layer, the first
ionomer layer comprising a first ionomer; c) coating a second
ionomer layer adjacent the second surface of the substrate layer,
the second ionomer layer comprising a second ionomer; and d)
initiating an ionomer-induced phase separation of the reinforcement
polymer from the first solvent, thereby forming a porous polymer
substrate from the substrate layer and interpenetrating a portion
of the first ionomer and a portion of the second ionomer within the
porous polymer substrate.
19. The method of claim 18, wherein the first substrate layer, the
first ionomer layer, and the second ionomer layer are
coextruded.
20. The method of claim 18, further comprising the steps of: e)
coating a second substrate layer having a first surface and a
second surface from a second solution, the second solution
comprising a second reinforcement polymer substantially dissolved
in a second solvent; f) coating a third ionomer layer adjacent the
first surface of the second substrate layer, the third ionomer
layer comprising a third ionomer; g) coating a fourth ionomer layer
adjacent the second surface of the second substrate layer, the
fourth ionomer layer comprising a second ionomer; wherein third
ionomer layer is coated adjacent the second ionomer layer.
21. The method of claim 20 wherein step d) of initiating an
ionomer-induced phase separation of the reinforcement polymer from
the first solvent is carried out after step e) of coating a second
substrate layer.
22. The method of claim 20 wherein step d) of initiating an
ionomer-induced phase separation of the reinforcement polymer from
the first solvent is carried out before step e) of coating a second
substrate layer.
23. The method of claim 20, wherein the first substrate layer, the
first ionomer layer, the second ionomer layer, the second substrate
layer, the third ionomer layer, and the fourth ionomer layer are
coextruded.
24. The method of claim 20, further comprising the step of: h)
initiating an ionomer-induced phase separation of the second
reinforcement polymer from the second solvent, thereby forming a
second porous polymer substrate from the second substrate layer and
interpenetrating a portion of the third ionomer and a portion of
the fourth ionomer within the second porous polymer substrate.
25. A method comprising the step of laminating an ion-conductive
membrane made according to the method of claim 12 with a second
ion-conductive membrane.
26. A method comprising the step of laminating an ion-conductive
membrane made according to the method of claim 18 with a second
ion-conductive membrane.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to ion-conductive membranes
(ICMs) for electrochemical applications. In particular, the present
invention relates to reinforced ICMs formed with multilayer coating
and phase-inversion technologies.
BACKGROUND OF THE INVENTION
[0002] ICMs are used as electrolyte membranes in a variety of
electrochemical applications, such as electrochemical devices
(e.g., fuel cells), chloroalkali applications, and vapor
permeation/separation applications. With respect to fuel cells,
ICMs may function as electrolyte membranes capable of transferring
protons (e.g., proton-exchange membranes). ICMs are particularly
suitable in fuel cell applications because they may replace
hazardous acidic liquid electrolytes, such as those used in
phosphoric acid fuel cells.
[0003] A fuel cell such as a proton-exchange membrane fuel cell
typically contains a membrane electrode assembly (MEA), which is a
catalyst coated membrane disposed between a pair of gas diffusion
layers. The catalyst coated membrane itself typically includes an
electrolyte membrane disposed between a pair of catalyst layers,
where an ICM may function as the electrolyte membrane. The
respective sides of the electrolyte membrane are referred to as an
anode portion and a cathode portion. In a typical proton-exchange
membrane fuel cell, hydrogen fuel is introduced into the anode
portion, where the hydrogen reacts and separates into protons and
electrons. The electrolyte membrane transports the protons to the
cathode portion, while allowing a current of electrons to flow
through an external circuit to the cathode portion to provide
power. Oxygen is introduced into the cathode portion and reacts
with the protons and electrons to form water and heat.
[0004] ICMs used in the above-discussed applications typically
require adequate mechanical strengths (e.g., tear resistance).
Previous attempts to enhance the mechanical strengths of ICMs
involved increasing the thicknesses of the membranes. However,
increases in membrane thicknesses generally decrease the ion
conductivities of such membranes. Moreover, membranes that are
inherently weak at small thicknesses (e.g., less than about 50
micrometers) require reinforcement with additional materials, which
also undesirably cause the resulting membrane to have increased
thicknesses and reduced ion conductivity.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention relates to an ICM and methods of
making ICMs. The ICM includes a first layer comprising a first
ionomer, and a porous polymer substrate, where at least a portion
of the first ionomer is interpenetrated within the porous polymer
substrate by ionomer-induced phase separation. The ICM is
mechanically reinforced and exhibits good ion conductivity for use
in a variety of electrochemical applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a schematic illustration of a membrane electrode
assembly of the present invention in use with an external
electrical circuit.
[0007] FIG. 2 is a schematic sectional view of an ion-conductive
membrane of the present invention.
[0008] FIG. 3 is a schematic illustration of tri-die system for
manufacturing the ion-conductive membrane of the present
invention.
[0009] FIG. 4 is an expanded view of section 4 taken in FIG. 3.
[0010] FIG. 5 is a schematic sectional view of an alternative
ion-conductive membrane of the present invention.
[0011] FIG. 6 is an SEM micrograph of an exemplary ICM of the
present invention.
[0012] FIG. 7 is a graphical representation of tear strength
results measured for exemplary ICMs of the present invention and a
control membrane.
[0013] FIG. 8 is a graphical representation of fuel cell testing
results measured for an exemplary ICM of the present invention and
a control membrane.
[0014] While the above-identified drawing figures set forth several
embodiments of the invention, other embodiments are also
contemplated, as noted in the discussion. In all cases, this
disclosure presents the invention by way of representation and not
limitation. It should be understood that numerous other
modifications and embodiments can be devised by those skilled in
the art, which fall within the scope and spirit of the principles
of the invention. The figures may not be drawn to scale. Like
reference numbers have been used throughout the figures to denote
like parts.
DETAILED DESCRIPTION
[0015] FIG. 1 is an illustration of MEA 10 in use with external
electrical circuit 12, where MEA 10 includes ICM 14 of the present
invention. MEA 10 is suitable for use in electrochemical cells,
such as proton-exchange membrane fuel cells, and further includes
anode portion 16, cathode portion 18, catalyst layers 20 and 22,
and gas diffusion layers 24 and 26. Anode portion 16 and cathode
portion 18 generally refer to the anode and cathode sides of MEA
10.
[0016] ICM 14 is a proton-exchange membrane disposed between
catalyst layers 20 and 22, where ICM 14 and catalyst layers 20 and
22 may be a catalyst coated membrane. As discussed below, ICM 14 is
mechanically reinforced and exhibits good proton conductivity. This
allows ICM 14 to resist damage during manufacture and use of MEA
10, while also readily allowing protons to transfer between anode
portion 16 and cathode portion 18.
[0017] While the present disclosure focuses on ICM 14 as a
proton-exchange membrane in MEA 10, ICMs of the present invention
may alternatively be used as electrolyte membranes in a variety of
electrochemical applications. For example, ICMs of the present
invention may be conductive of only ions either of positive charge
(i.e., cation-exchange membrane) or negative charge (i.e.,
anion-exchange membrane), or of only one type of ion (e.g.,
proton-exchange membrane).
[0018] Catalyst layer 20 is disposed between ICM 14 and gas
diffusion layer 24, where gas diffusion layer 24 is located at
anode portion 16 of MEA 10. Similarly, catalyst layer 22 is
disposed between ICM 14 and gas diffusion layer 26, where gas
diffusion layer 26 is located at cathode portion 8 of MEA 10. Gas
diffusion layers 24 and 26 may each be any suitable electrically
conductive porous substrate, such as carbon fiber constructions
(e.g., woven and non-woven carbon fiber constructions). Gas
diffusion layers 24 and 26 may also be treated to increase or
impart hydrophobic properties.
[0019] During operation of MEA 10, hydrogen fuel (H.sub.2) is
introduced into gas diffusion layer 24 at anode portion 16. MEA 10
may alternatively use other fuel sources, such as methanol,
ethanol, formic acid, and reformed gases. The fuel passes through
gas diffusion layer 24 and over catalyst layer 20. At catalyst
layer 20, the fuel is separated into hydrogen ions (H.sup.+) and
electrons (e.sup.-). ICM 14 only permits the hydrogen ions to pass
through to reach catalyst layer 22 and gas diffusion layer 26. The
electrons generally cannot pass through ICM 14. As such, the
electrons flow through external electrical circuit 12 in the form
of electric current. This current can power an electric load, such
as an electric motor, or be directed to an energy storage device,
such as a rechargeable battery. Oxygen (O.sub.2) is introduced into
gas diffusion layer 26 at cathode portion 18. The oxygen passes
through gas diffusion layer 26 and over catalyst layer 22. At
catalyst layer 22, oxygen, hydrogen ions, and electrons combine to
produce water and heat.
[0020] FIG. 2 is a schematic sectional view of ICM 14, which
oriented perpendicularly to the view shown in FIG. 1. As shown in
FIG. 2, ICM 14 is a multilayer membrane that includes anode layer
28, substrate 30, and cathode layer 32. Substrate 30 includes first
surface 30a and second surface 30b, where anode layer 28 is
disposed adjacent first surface 30a and cathode layer 32 is
disposed adjacent second surface 30b. The references to "anode" and
"cathode" for anode layer 28 and cathode layer 32 are used for ease
of discussion to show relative locations (i.e., anode layer 28 is
disposed adjacent catalyst layer 20 shown in FIG. 1, and cathode
layer 32 is disposed adjacent catalyst layer 22 also shown in FIG.
1).
[0021] Substrate 30 is a porous polymer substrate that
compositionally includes one or more reinforcement polymers having
a microporous infrastructure (i.e., a plurality of micropores
distributed throughout the structure of the reinforcement polymer).
As discussed below, the micropores are interpenetrated with
ionomers from anode layer 28 and cathode layer 32. The ionomers
desirably interpenetrate at least a majority of the micropores of
substrate 30, and more desirably interpenetrate substantially all
of the micropores of substrate 30. Moreover, the ionomers desirably
interpenetrate substrate 30 in a substantially uniform manner,
which provides an even distribution of the ionomers throughout
substrate 30. The interpenetrated ionomers allow substrate 30 to
assist anode layer 28 and cathode layer 32 in conducting protons
through ICM 14. Otherwise, substrate 30 would effectively function
as a barrier to proton transfer through ICM 14.
[0022] The reinforcement polymer of substrate 30 is used to
mechanically reinforce ICM 14. In the absence of substrate 30, a
proton-exchange membrane formed from anode layer 28 and/or cathode
layer 32 would exhibit good proton conductivity. However, the
proton-exchange membrane would also have low structural integrity,
increasing the risk of damage during manufacture and use. The use
of substrate 30, interpenetrated with ionomers, allows ICM 14 to
have a combination of good mechanical strength and proton
conductivity.
[0023] The reinforcement polymer of substrate 30 is also desirably
thermally stable, which allows the reinforcement polymer to retain
its microporous infrastructure during drying and annealing
operations. Suitable thermally stable reinforcement polymers
desirably have glass transition temperatures of at least about
180.degree. C., more desirably of at least about 200.degree. C.,
and even more desirably of at least about 215.degree. C.
[0024] Examples of suitable materials for the reinforcement polymer
of substrate 30 include polysulfones, polyether imides,
polyethersulfones, polytetrafluoroethylenes (PTFE), ter-polymers of
PTFE, polyphenylene sulfides, polyetheretherketones, fluorinated
ethylene-propylenes, polyvinylidenedifluorides,
hexafluoropropylenes, vinylidene fluorides (THV),
ultra-high-molecular weight polyethylenes, liquid crystalline
polyesters, and combinations thereof. Such materials mechanically
reinforce ICM 14 and are thermally stable. Examples of particularly
suitable materials for the reinforcement polymer of substrate 30
include polysulfones and polyether imides which provide good
mechanical strengths and have high glass transition temperatures.
In one embodiment, the reinforcement polymer may also be sulfonated
to increase proton conductivity.
[0025] Anode layer 28 and cathode layer 32 each compositionally
include one or more ionomers, which allow anode layer 28 and
cathode layer 32 to function as proton-transfer skins at the anode
and cathode sides of ICM 14. As discussed below, during the
formation of ICM 14, ionomers from anode layer 28 and cathode layer
32 interpenetrate within substrate 30 by ionomer-induced phase
separation. This secures anode layer 28 and cathode layer 32 to
substrate 30 (reducing the risk of interlayer delamination) and
allows substrate 30 to conduct protons.
[0026] Examples of suitable ionomers for each of anode layer 28 and
cathode layer 32 include proton-exchange membrane ionomers, such as
film-forming fluoropolymers having pendant acid groups (e.g.,
phosphonyl, carbonyl, and sulfonyl groups). Examples of suitable
pendent groups include sulfonic acid groups having the formula
--R.sup.1--SO.sub.3Y, where R.sup.1 may be a branched or unbranched
perfluoroalkyl, perfluoroalkoxy, or perfluoroether group, which
contains 1-15 carbon atoms and 0-4 oxygen atoms, and where Y is a
hydrogen ion, a cation, or combinations thereof. Examples of
particularly suitable pendant groups include
--OCF.sub.2CF(CF.sub.3)OCF.sub.2CF.sub.2SO.sub.3Y,
--O(CF.sub.2).sub.4SO.sub.3Y, and combinations thereof.
[0027] The suitable film-forming fluoropolymers may also include
one or more acidic endgroups, such as sulfonyl endgroups having the
formula --SO.sub.3Y. The backbone chain of the acidic polymer may
be partially or fully fluorinated. Suitable fluorine concentrations
in the backbone chain include about 40% or more by weight, based on
the entire weight of the backbone chain. In one embodiment of the
present invention, the backbone chain of the fluoropolymer is
perfluorinated.
[0028] Examples of particularly suitable ionomer materials for each
of anode layer 28 and cathode layer 32 include perfluorosulfonic
acid copolymers with 800-1000 equivalent weights of gaseous
tetrafluoroethylene comonomer (TFE) having a formula
CF.sub.2.dbd.CF.sub.2 and a molecular weight of 100.02, and a
sulfonyl fluoride comonomer (MV4S) having a formula
CF.sub.2.dbd.CFO(CF.sub.2).sub.4SO.sup.2F and a molecular weight of
378.11, where the MV4S was prepared as described in U.S. Pat. No.
6,624,328 (in a hydrolyzed sulfonic acid form), and where the
perfluorosulfonic acid copolymer was prepared as described in U.S.
Patent Application No. 2004/0121210, and which is manufactured by
3M Corporation, St. Paul, Minn.
[0029] Anode layer 28 and cathode layer 32 may include the same
ionomers, or alternatively, may include different ionomers. For
example, anode layer 28 and cathode layer 32 may include ionomers
having different equivalent weights. Lower equivalent weight
ionomers have more acid groups, but are believed to cause swelling
in water. The use of higher equivalent weight ionomers in one of
anode layer 28 and cathode layer 32, and lower equivalent weight
ionomers in the opposing layer provides a balance of increased
proton conductivity, good mechanical strength, and low swelling
with water.
[0030] ICM 14 may be formed by initially dissolving or dispersing
the reinforcement polymer and the ionomers in solvents to form
solutions. The solutions used to form anode layer 28, substrate 30,
and cathode layer 32 are respectively referred to herein as ionomer
solution 28s, reinforcement polymer solution 30s, and ionomer
solution 32s. The term "solution" is herein defined to include full
dissolutions, partial dissolutions, and dispersions. Preferably,
the reinforcement polymer used to provide reinforcement polymer
solution 30s is substantially dissolved in the corresponding
solvent to further promote the ionomer-induced phase
separation.
[0031] Suitable solvents for each of ionomer solution 28s,
reinforcement polymer solution 30s, and ionomer solution 32s
include any carrier fluid suitable for forming solutions with the
reinforcement polymer and the ionomers. Examples of suitable
solvents include 1-methyl-2-pyrrolidinone (NMP), dimethylacetamide,
alcohols (e.g., methanol and n-propanol), and combinations thereof.
NMP is particularly suitable for polymer solution 30s because it
contributes sufficient free volume in the composition to readily
remove residual solvents, thereby reducing the risk of blistering
during a drying operation. Additionally, NMP is a good solvent for
dissolving reinforcement polymers such as polysulfones and
polyether imides, and is a good coalescing agent for ionomers.
Examples of suitable concentrations for each of the reinforcement
polymer and the ionomers in their respective solutions range from
about 5% to about 50% by weight, with particularly suitable
concentrations ranging from about 10% to about 30% by weight, based
on the entire weight of the given solution.
[0032] In one embodiment, reinforcement polymer solution 30s may be
a metastable solution, where one or more ionomers are dispersed in
the solvent with the dissolved reinforcement polymer. Such
solutions provide better coating characteristics and are believed
to enhance ionomer interpenetration. Examples of suitable
concentrations of ionomers dispersed in the metastable solution
range from about 10% to about 20% on a dry weight basis.
[0033] Reinforcement polymer solution 30s may be cast as a layer
and coated on each side with ionomer solutions 28s and 32s,
respectively. The coatings may be performed in any suitable manner,
which may include wet casting, extrusion processes, knife coating,
slide coating, curtain coating, meniscus coating, as well as
wet-on-wet (i.e. multiple coating heads with no drying in between)
coating methods including wet-on-wet slide, curtain and meniscus
coating and other multilayer processes. Upon coating, the ionomers
in ionomer solutions 28s and 32s cause the reinforcement polymer to
phase separate from the solvent of reinforcement polymer solution
30s by ionomer-induced phase separation. The phase separation
causes the reinforcement polymer to precipitate out of the solvent,
thereby forming substrate 30 with the microporous
infrastructure.
[0034] While not wishing to be bound by theory, the phase
separation is believed to proceed by a spinodal decomposition
mechanism, where ionomer solutions 28s and 32s act as coagulants
that interpenetrate the newly formed micropores from each side of
substrate 30. Upon drying, the solvents from ionomer solutions 28s
and 32s and reinforcement polymer solution 30s are evaporated. The
solvent evaporation further promotes the phase separation and locks
the interpenetrating ionomers within the micropores of substrate
30. The remaining ionomers of ionomer solutions 28s and 32s that
did not interpenetrate substrate 30 respectively form anode layer
28 and cathode layer 32.
[0035] The resulting ICM 14 is mechanically reinforced by the
reinforcement polymer of substrate 30, and exhibits good proton
conductivity from the ionomers of anode layer 28, cathode layer 32,
and the ionomers interpenetrated within substrate 30. The
ionomer-induced phase separation allows substrate 30 to be filled
with ionomer materials in a single-step process, and provides for a
more uniform filling of the micropores compared to standard
techniques of impregnating ionomers within pre-formed porous
polymers.
[0036] FIG. 3 is a schematic illustration showing tri-die system
34, which is a three-layer coextrusion system suitable for wet
casting ICM 14 in a continuous process. As shown, tri-die 34
includes drum wheel 36 and extrusion head 38. Drum wheel 36 is a
rotatable wheel that receives extruded layers from extrusion head
38. Drum wheel 36 desirably includes a low-stick liner (e.g.,
polyimide) to support ICM 14 during the coextrusion and drying
operations. Extrusion head 38 includes extrusion slots 40, 42, and
44, which are pathways for coextruding ionomer solution 28s,
reinforcement polymer solution 30s, and ionomer solution 32s,
respectively. The desired slot height is established by placing
precision metal shims of the necessary thickness between the bars
of the die, which are notched to the correct width and depth (not
shown). For example, reinforcement polymer solution 30s may be
introduced into extrusion slot 42 to subsequently form substrate
30. Similarly, ionomer solutions 28s and 30s may be respectively
introduced into extrusion slots 40 and 44 to subsequently form
anode layer 28 and cathode layer 32 on opposing surfaces of
substrate 30.
[0037] FIG. 4 is an enlarged detail view of section 4 taken in FIG.
3, which shows ionomer solution 28s, reinforcement polymer solution
30s, and ionomer solution 32s being respectively extruded from
extrusion slots 40, 42, and 44. The resulting multilayer film is
then wound around drum wheel 36 in a clock-wise direction and
supported on a liner, as discussed above. Examples of extrusion
rates of each of ionomer solution 28s, reinforcement polymer
solution 30s, and ionomer solution 32s range from about one
meter/minute to about three meters/minute.
[0038] As the layers of ionomer solution 28s, reinforcement polymer
solution 30s, and ionomer solution 32s are extruded, ionomer
solution 28s contacts reinforcement polymer solution 30s from the
top side and ionomer solution 32s contacts reinforcement polymer
solution 30s from the bottom side, as shown in FIG. 4. As a result,
ionomer-induced phase separations occur from each side of the layer
of reinforcement polymer solution 30s. lonomer solution 28s induces
a first portion of the reinforcement polymer to phase separate from
the solvent of reinforcement polymer solution 30s, thereby forming
micropores within the reinforcement polymer. lonomer solution 28s
then interpenetrates into the micropores from the top side.
[0039] In a substantially simultaneous manner, ionomer solution 32s
induces a second portion of the reinforcement polymer to phase
separate from the solvent of reinforcement polymer solution 30s,
thereby forming additional micropores within the reinforcement
polymer. lonomer solution 32s interpenetrates into the additional
micropores from the bottom side.
[0040] The extruded film may then be dried to evaporate the
solvents by passing the film along a series of drying zones that
incrementally increase in temperature. Examples of suitable zones
temperatures range from about 50.degree. C. to about 180.degree. C.
Examples of suitable line speeds during the drying operation are
generally dependent on the length of the drying oven, and range
from about one meter/minute to about three meters/minute. During
the drying operation, the solvents evaporate, thereby allowing the
ionomers to become locked within the porous infrastructure of
substrate 30. After the solvents have substantially evaporated, ICM
14 is ready for post-formation manufacturing steps, such as
annealing, cleaning, gasket sealing, and construction of MEA
10.
[0041] Tri-die system 34 is also particularly beneficial for wet
casting films that have low membrane thicknesses upon drying. Lower
membrane thicknesses for ICM 14 allow for lower internal
mass-transfer resistances and reduce material costs, without
compromising mechanical strength. In addition, because of
back-diffusion of water from cathode portion 18 to the anode
portion 16 through such thin membranes, water management during
fuel cell operation can be improved. This correspondingly improves
the proton conductivity of MEA 10. Examples of suitable membrane
thicknesses for ICM 14 include thicknesses less than about 30
micrometers, with particularly suitable membrane thicknesses
including thicknesses less than about 15 micrometers, and with even
more particularly suitable membrane thicknesses including
thicknesses less than about 10 micrometers.
[0042] Based on the relative sizes of extrusion slots 40, 42, and
44, and the extrusion rates and line speeds used, the relative
layer thicknesses of anode layer 28, substrate 30, and cathode
layer 32 may also vary. Examples of suitable layer thickness ratios
of each of anode layer 28 and cathode layer 32 compared to
substrate 30 range from about 1:1 to about 5:1, with particularly
suitable layer thickness ratios ranging from about 2:1 to about
3:1. For example, suitable layer thicknesses for anode layer 28,
substrate 30, and cathode layer 32 may respectively include 7
micrometers, 2.5 micrometers, and 7 micrometers. Additionally,
extrusion slots 40 and 44 and/or the solution flow rates may be
varied such that anode layer 28 and cathode layer 32 have differing
layer thicknesses. Initially, three slot heights were used: 250,
180 and 125 microns, with 250 microns being particularly effective
at 3 mpm for final thicknesses of less than 20 microns. All shim
notches were 125 mm in width.
[0043] Thick layers for substrate 30 yield stronger membranes
(higher tear strength), but suffer a reduced level of proton
conductivity due to interfacial delamination and/or incomplete
phase separation. Accordingly, examples of suitable layer
thicknesses for substrate 30 include thicknesses of about 5
micrometers or less, with particularly suitable layer thicknesses
including thicknesses of about 3 micrometers or less. Such layer
thicknesses allow substrate 30 to provide good mechanical strength
to ICM 14, while also providing good proton conductivity.
[0044] FIG. 5 is a schematic sectional view of ICM 114, which is an
alternative proton-exchange membrane suitable for use in MEA 10. As
shown, ICM 114 is similar to ICM 14, except that ICM 114 is a
five-layer membrane that includes anode layer 146, anode substrate
148, core layer 150, cathode substrate 152, and cathode layer 154.
ICM 114 may also be dimensionally bisected into top half 114a and
bottom half 114b, where each of top half 114a and bottom half 114b
generally resemble ICM 14. ICM 114 is beneficial where thick
proton-exchange membranes are desired. As discussed above, thick
proton-exchange membranes generally reduce the proton
conductivities of the given membranes. However, ICM 114 may attain
large membrane thicknesses while preserving good proton
conductivity.
[0045] Anode substrate 148 includes first surface 148a and second
surface 148b, where anode layer 146 is disposed adjacent first
surface 148a and core layer 150 is disposed adjacent second surface
148b. Similarly, cathode substrate 152 includes first surface 152a
and second surface 152b, where core layer 150 is disposed adjacent
first surface 152a and cathode layer 154 is disposed adjacent
second surface 152b. Similar to the discussion above, the
references to "anode" and "cathode" are used for ease of discussion
to show relative locations.
[0046] Anode substrate 148 and cathode substrate 152 are porous
polymer substrates that provide mechanical reinforcement for ICM
114. Anode substrate 148 and cathode substrate 152 compositionally
each include one or more reinforcement polymers having a
microporous infrastructure, similar to substrate 30 discussed above
in FIG. 2. Examples of suitable reinforcement polymers for anode
substrate 148 and cathode substrate 152 are the same as those
discussed above for substrate 30.
[0047] Anode layer 146 and cathode layer 154 function as
proton-transport skins at the anode and cathode sides of ICM 114.
Anode layer 146 and cathode layer 154 each compositionally include
one or more ionomers, similar to anode layer 28 and cathode layer
32 discussed above in FIG. 2. Examples of suitable ionomers for
anode layer 146 and cathode layer 154 are the same as those
discussed above for anode layer 28 and cathode layer 32.
[0048] Core layer 150 compositionally also includes one or more
ionomers, and functions as a proton-transport core region of ICM
114. This allows ICM 114 to achieve greater membrane thicknesses
without sacrificing proton conductivity. Examples of suitable
ionomers for core layer 150 are also the same as those discussed
above for anode layer 28 and cathode layer 32. A particularly
suitable arrangement for ICM 114 includes using high equivalent
weight ionomers for anode layer 146 and cathode layer 154, and a
low equivalent weight ionomer for core layer 150. This arrangement
provides a combination of high mechanical strengths and good proton
conductivity.
[0049] Examples of suitable membrane thicknesses for ICM 114
include thicknesses less than about 60 micrometers, with
particularly suitable membrane thicknesses including thicknesses
less than about 50 micrometers, and with even more particularly
suitable membrane thicknesses including thicknesses less than about
40 micrometers. Examples of suitable layer thicknesses for anode
substrate 148 and cathode substrate 152 are the same as those
discussed above for substrate 30. Similarly, examples of suitable
layer thickness ratios of each of anode layer 146 and cathode layer
154 compared to anode substrate 148 and cathode substrate 152 are
the same as discussed above for anode layer 28 and cathode layer
32.
[0050] ICM 114 may be formed with larger membrane thicknesses
because, in part, the relative amounts of ionomer-to-reinforcement
polymer are generally the same as the relative amounts used for ICM
14, discussed above. As a result, ionomers are interpenetrated
within anode substrate 148 and cathode substrate 152 in a
substantially uniform manner. This allows anode substrate 148 and
cathode substrate 152 to assist anode layer 146, core layer 150,
and cathode layer 154 in transferring protons through ICM 114.
[0051] ICM 114 may be formed in a dual-casting process using
tri-die system 34, discussed above in FIGS. 3 and 4. Bottom half
114b of ICM 114 may initially be cast from tri-die system 34 in the
same manner as discussed above for ICM 14. Bottom half 114b may
then be dried to promote ionomer-induced phase separation of the
reinforcement polymer of cathode substrate 152 with the ionomers
from half of core layer 150 and cathode layer 154. The solvents may
be fully evaporated or partially evaporated during this drying
operation. Top half 114a may then be cast from tri-die system 34 on
top of the previously cast bottom half 114b. The entire membrane of
ICM 114 may then undergo a second drying operation to promote
ionomer-induced phase separation of the reinforcement polymer of
anode substrate 148 with the ionomers from anode layer 146 and the
second half of core layer 150. Laminating top half 114a on top of
bottom half 114b allows the ionomers of each half of core layer 150
to blend together to form core layer 150. This secures top half
114a to bottom half 114b to form ICM 114. The resulting ICM 114 may
then undergo post-formation manufacturing steps, such as cleaning,
gasket sealing, and construction of MEA 10.
[0052] Alternatively, ICM 114 may be formed with a fuel cell
lamination technique, which may initially involve obtaining a pair
of ICMs 14 wet cast with tri-die system 34, as discussed above in
FIGS. 3 and 4, and releasing each of the pair of ICMs 14 from the
respective low-stick liner. This may be facilitated by
humidification or immersion in ambient water. The pair of ICMs 14
may then be placed between a pair of 50-micrometer silicone liner
sheets. The resulting assembly may then introduced between two
rolls in a hot roll laminator where the top roll is preheated at
about 140.degree. C., the bottom roll at about 132.degree. C., and
the pressure is set at about 3.4 MegaPascals (about 500
pounds/inch.sup.2), to produce ICM 114. During this lamination
procedure, enhanced pore filling with the ionomers may be achieved
by stretching the resulting ICM 114. ICM 114 may then undergo
post-formation manufacturing steps, as discussed above.
[0053] In alternative embodiments, different numbers of layers may
be used to form ICMs of the present invention from those discussed
above. For example, in one embodiment, a single ionomer layer may
be laminated on a substrate. In this embodiment, the
ionomer-induced phase separation may be allowed to continue until
ionomers interpenetrate within the substrate in a substantially
uniform manner.
EXAMPLES
[0054] The present invention is more particularly described in the
following examples that are intended as illustrations only, since
numerous modifications and variations within the scope of the
present invention will be apparent to those skilled in the art.
Unless otherwise noted, all parts, percentages, and ratios reported
in the following examples are on a weight basis, and all reagents
used in the examples were obtained, or are available, from the
chemical suppliers described below, or may be synthesized by
conventional techniques.
[0055] The following compositional abbreviations are used in the
following Examples: [0056] "Polysulfone": A polysulfone having the
repeating unit [--O-Ph-C(CH.sub.3).sub.2-Ph--O-Ph-SO.sub.2-Ph-],
where "Ph" represents a phenyl ring, and which is commercially
available from Sigma-Aldrich Company, Saint Louis, Mo. [0057] "3M
800 PFSA": A perfluorosulfonic acid copolymer with an 800
equivalent weight of gaseous tetrafluoroethylene comonomer (TFE)
having a formula CF.sub.2.dbd.CF.sub.2 and a molecular weight of
100.02, and a sulfonyl fluoride comonomer (MV4S) having a formula
CF.sub.2.dbd.CFO(CF.sub.2).sub.4SO.sub.2F and a molecular weight of
378.11, where the MV4S was prepared as described in U.S. Pat. No.
6,624,328 (in a hydrolyzed sulfonic acid form), and where the
perfluorosulfonic acid copolymer was prepared as described in U.S.
Patent Application No. 2004/0121210, and which is manufactured by
3M Corporation, St. Paul, Minn. [0058] "3M 1000 PFSA": A
perfluorosulfonic acid copolymer with a 1000 equivalent weight of
gaseous tetrafluoroethylene comonomer (TFE) having a formula
CF.sub.2.dbd.CF.sub.2 and a molecular weight of 100.02, and a
sulfonyl fluoride comonomer (MV4S) having a formula
CF.sub.2.dbd.CFO(CF.sub.2).sub.4SO.sub.2F and a molecular weight of
378.11, where the MV4S was prepared as described in U.S. Pat. No.
6,624,328 (in a hydrolyzed sulfonic acid form), and where the
perfluorosulfonic acid copolymer was prepared as described in U.S.
Patent Application No. 2004/0121210, and which is manufactured by
3M Corporation, St. Paul, Minn. [0059] "NMP":
1-methyl-2-pyrrolidinone solvent commercially available from
Sigma-Aldrich Company, Saint Louis, Mo.
Examples 1A-8
[0060] ICMs for Examples 1A-8 were prepared pursuant to the
following procedure using a Coater/Drier pilot line. The ICMs were
wet cast from a tri-die system having an upper slot (for a first
ionomer layer), a middle slot (for a substrate), and a lower slot
(for a second ionomer layer). Dispersions of 20% 3M 1000 PFSA in a
solvent blend of water and n-propanol were fed to the upper and
lower slots with infuse/withdraw syringe pumps (commercially
available under the trade designation "PHD 2000" from Harvard
Apparatus, Holliston, Mass.). A solution of 15% polysulfone in NMP
was fed to the middle slot also with an infuse/withdraw syringe
pump. The volumetric flow rates of the dispersions and solutions
through the upper, middle, and lower slots are shown below in Table
1. Table 1 also shows the coating speeds used for wet casting the
multilayer films.
[0061] The extruded multilayer films were wet cast onto a 10-inch
wide, 2-mil thick, polyimide liner (commercially available under
the trade designation "KAPTON" from E.I. DuPont de Nemours and
Company). Upon wet casting, the middle layers of the multilayer
films underwent ionomer-induced phase separation. The multilayer
films were then passed through a three-zone convection oven at a
line speed of 0.9 meters/minute (three feet/minute) to evaporate
the solvents and to lock the ionomer within the porous
infrastructure of the porous polymer substrate. The three zones
were respectively maintained at 60.degree. C., 71.degree. C., and
160.degree. C. Following the drying operation, the resulting ICMs
of Examples 1-8 were tested, as discussed below.
[0062] FIG. 6 is an SEM micrograph taken of the ICM of Example 1A.
The layers shown on the top and bottom of FIG. 6 are layers of 3M
1000 PFSA ionomer. The central layer is a porous polymer substrate
of polysulfone, which has a microporous infrastructure that is
interpenetrated with 3M 1000 PFSA ionomer. The light regions of the
central layer are the polysulfone and the darker regions are the
micropores filled with 3M 1000 PFSA ionomer. The overall membrane
thickness of the ICM of Example 1A was 16.5 micrometers, which
consisted of a 2.5 micrometer polysulfone porous substrate
sandwiched between two 7-micrometer layers of 3M 1000 PFSA ionomer.
The average pore size of the microporous infrastructure was about
98 nanometers with a standard deviation of about 30 nanometers. As
shown in FIG. 6, the 3M 1000 PFSA ionomer interpenetrates the
polysulfone porous substrate in a substantially uniform manner.
This allows the ICM of Example 1A to exhibit good proton
conductivity.
Example 9
[0063] The ICM of Example 9 was prepared in the same manner as
discussed above for Examples 1A-8, except that a dispersion of 20%
3M 800 PFSA was fed to the upper slot and a dispersion of 20% 3M
1000 PFSA was fed to the lower slot.
Examples 10 and 11
[0064] The ICMs of Examples 10 and 11 were prepared in the same
manner as discussed above for Examples 1A-8, except that a solution
of 21.8% polysulfone in NMP was fed to the middle slot instead of
the 15% polysulfone/NMP solution. Additionally, the extruded
multilayer films were wet cast onto a 6-inch wide, 2-mil thick,
polyimide liner (commercially available under the trade designation
"KAPTON" from E.I. DuPont de Nemours and Company).
Examples 12-14
[0065] The ICMs of Examples 12-14 were prepared in the same manner
as discussed above for Examples 1A-8, except that a metastable
ionomer/polysulfone solution was fed to the middle slot instead of
the 15% polysulfone/NMP solution. The metastable solution included
14% 3M 1000 PFSA and 86% polysulfone, on a dry-weight basis.
[0066] TABLE 1 provides the volumetric flow rates of the upper,
middle, and lower slots of the tri-die system, and the resulting
coating speeds of the wet cast multilayer film for ICMs of Examples
1A-14. TABLE-US-00001 TABLE 1 Middle Coating Upper slot flow slot
flow Lower slot flow speed Example rate (ml/min) rate (ml/min) rate
(ml/min) (fpm) Example 1A 23.4 4.1 23.4 10 Example 1B 23.4 4.1 23.4
10 Example 2 23.4 8.0 23.4 5 Example 3 24.0 8.0 24.0 5 Example 4
24.0 5.0 24.0 5 Example 5 24.0 5.0 24.0 10 Example 6 40.0 5.0 40.0
5 Example 7 23.4 8.1 23.4 10 Example 8 24.0 5.0 24.0 10 Example 9
24.0 5.0 24.0 10 Example 10 23.4 8.0 23.4 5 Example 11 23.4 6.0
23.4 5 Example 12 24.0 10.0 24.0 10 Example 13 24.0 10.0 24.0 5
Example 14 40.0 10.0 40.0 10
Examples 15 and 16
[0067] The ICMs of Examples 15 and 16 each involved a five-layer
ICM, which was prepared by joining two ICMs of Example 9. The ICM
of Example 15 was prepared by wet casting a second multilayer film
on top of the ICM of Example 9 with the tri-die system discussed
above for Examples 1A-8. The second multilayer film was wet cast
with a dispersion of 20% 3M 800 PFSA fed to the lower slot and a
dispersion of 20% 3M 1000 PFSA fed to the upper slot to ensure
continuity of the low equivalent weight material. The resulting
five-layer film was then re-passed through the convection oven.
[0068] The ICM of Example 16 was prepared by laminating two
pre-formed ICMs of Example 9 with a fuel cell lamination technique.
This involved releasing each of the ICM's of Example 9 from the
respective polyimide liners by immersion in ambient water. The
ICM's of Example 9 were then placed between a pair of 50-micrometer
silicone liner sheets. The resulting assembly was then introduced
between two rolls in a hot roll laminator where the top roll is
preheated at 140.degree. C., the bottom roll at 132.degree. C., and
the pressure is set at 3.4 MegaPascals (about 500
pounds/inch.sup.2), thereby forming the ICM of Example 16 disposed
between the silicone liner sheets.
[0069] The ICMs of Example 9 each had a membrane thickness of about
15 micrometers. After the secondary joining steps to form the ICMs
of Examples 15 and 16, the ICMs of Examples 15 and 16 each had
layers thicknesses of about 30 micrometers. The ICMs of Examples 15
and 16 each contained a core layer of 3M 800 PFSA ionomer, which
was sandwiched between a pair of interpenetrated polysulfone
substrates, which themselves were between a pair of outer layers of
3M 1000 PFSA ionomer. As such, the ICMs of Examples 15 and 16 each
represent a five-layer ICM that has a lower equivalent weight
ionomer in the core layer and higher equivalent weight ionomers in
the outer ionomer layers.
Tear Strength Testing of Examples 1A-7 and 10-14
[0070] The tear strengths of the ICMs of Examples 1A-7 and 10-14
were quantitatively measured pursuant to ASTM D882-95, which
included sample dimensions of a 5-centimeter (cm) length, a 2-cm
width, a 15-50 micrometer layer thickness, and 0.2-0.8-cm
ligaments. The strain rate was two millimeters/minute, and the
process conditions included a temperature of 25.degree. C., 50%
relative humidity, and a temperature of 50.degree. C., 95% relative
humidity. Table 2 provides the tear strength results in MegaPascals
(MPa) for the ICMs of Examples 1A-7 and 10-14. TABLE-US-00002 TABLE
2 Example Tear Strength (MPa) Example 1A 9.0 Example 1B 7.5 Example
2 10.9 Example 3 9.5 Example 4 7.0 Example 5 5.9 Example 6 8.0
Example 7 10.0 Example 10 10.5 Example 11 6.0 Example 12 9.1
Example 13 7.7 Example 14 5.1
[0071] FIG. 7 is a graph of tear strength profiles measured for
ICMs of Examples 1A and 1B, and a control membrane (comparative
example) that is not mechanically reinforced. The data in Table 2
and FIG. 7 illustrate the good tear strengths of the ICMs of
Examples 1A-7 and 10-14. In particular, the ICM's of Examples 2, 7,
and 10 exhibited high tear strengths. The increased tear strengths
result from the use of the reinforcement polymers in the porous
polymer substrates. The polysulfone porous substrates mechanically
reinforce the ICMs of the present invention, thereby reducing the
risk of damage to the ICMs during manufacture and use.
Fuel Cell Testing of Examples 1A, 1B, 4, 5, 8, 9, 12, 15, and
16
[0072] The ICMs of Examples 1A, 1B, 4, 5, 8, 9, 12, 15, and 16 were
quantitatively measured for conductivity pursuant to the following
procedure. Each ICM was placed in the fuel cell station, which was
maintained at 70.degree. C. and 100% relative humidity. Hydrogen
(H.sub.2) gas streams were introduced on the anode and cathode
sides, each at flow rates of 640 standard cubic centimeters per
minute, and were each maintained at ambient pressures 0 psig. A
current sweep was then performed on the ICM, which increased from
0.1 amps/cm.sup.2 to 0.7 amps/cm.sup.2 in 0.05 amp/cm2 increments
(25 seconds/point). The current sweep was repeated four times and
the voltage vs. current was plotted, where the slope of the plot
represents the resistance. Table 3 provides the current density
results of the fuel cell tests at cell potentials of 0.8 volts and
0.6 volts. TABLE-US-00003 TABLE 3 Current Density Current Density
Example (amps/cm.sup.2) at 0.8 volts (amps/cm.sup.2) at 0.6 volts
Example 1A 0.295 0.820 Example 1B 0.238 0.516 Example 4 0.159 0.505
Example 5 0.170 0.588 Example 8 0.255 0.755 Example 9 0.255 0.795
Example 12 0.058 0.235 Example 15 0.022 0.178 Example 16 0.179
0.560
[0073] The data in Table 3 illustrates the good current densities
of the ICMs of Examples 1A, 1B, 4, 5, 8, 9, 12, 15, and 16. The
current densities are attainable because the ionomers (e.g., the 3M
800 PFSA and the 3M 1000 PFSA) interpenetrate within the
polysulfone porous substrate via ionomer-induced phase separation.
This allows polysulfone porous substrates used in the ICMs of the
present invention to transfer protons, rather than functioning as
barriers to proton transfer. In particular, the ICMs of Examples
1A, 1B, 8, and 9 outperformed in conductivity with hydrogen pump
values below 100 mohm-cm.sup.2 due to enhanced ionomer
interpenetration within the polysulfone porous substrate. In
addition, an accelerated durability test using a SHIVA fuel cell
station that measures total fluoride ion release gave a lifetime of
about 350 hours for the ICM of Example 1A, slightly lower than the
lifetime on a 1.2 mil 800-equivqlent weight non-reinforced
membrane.
[0074] FIG. 8 is a graph of cell potential versus current density
for the ICM of Example 1A and a control membrane (comparative
example) that is not mechanically reinforced. As shown, the ICM of
Example 1A exhibits similar proton conductivity to the control
membrane. However, as discussed above, the ICM of Example 1A also
exhibits good tear strengths due to the reinforcement of the
polysulfone porous substrate. Because the control membrane is not
mechanically reinforced, it would have a low structural integrity.
In contrast, the ICMs of the present invention exhibit good proton
conductivity, while also being mechanically reinforced by the
polysulfone porous substrate. As a result, the ICMs of the present
invention are robust membranes suitable for use in a variety of
electrochemical devices, such as fuel cells.
[0075] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
* * * * *