U.S. patent application number 11/192822 was filed with the patent office on 2005-12-22 for composite electrolyte with crosslinking agents.
Invention is credited to Kurano, Matthew Robert, Mada Kannan, Arunachala Nadar, Panambur, Gangadhar, Taft, Karl Milton III.
Application Number | 20050282053 11/192822 |
Document ID | / |
Family ID | 34217802 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050282053 |
Kind Code |
A1 |
Kurano, Matthew Robert ; et
al. |
December 22, 2005 |
Composite electrolyte with crosslinking agents
Abstract
A covalent crosslinking of ion-conducting materials via sulfonic
acid groups can be applied to various low cost electrolyte membrane
base materials for improved fuel cell performance metrics relative
to such base material. This proposed approach is due, in part, to
the observation that many aromatic and aliphatic polymer materials
have significant potential as proton exchange membranes if a
modification can increase their physical and chemical stabilities
without sacrificing electrochemical performance or significantly
increasing the material and production costs.
Inventors: |
Kurano, Matthew Robert;
(Honolulu, HI) ; Panambur, Gangadhar; (Honolulu,
HI) ; Mada Kannan, Arunachala Nadar; (Honolulu,
HI) ; Taft, Karl Milton III; (Honolulu, HI) |
Correspondence
Address: |
DECHERT LLP
P.O. BOX 10004
PALO ALTO
CA
94303
US
|
Family ID: |
34217802 |
Appl. No.: |
11/192822 |
Filed: |
July 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11192822 |
Jul 28, 2005 |
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10653016 |
Aug 28, 2003 |
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6962959 |
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Current U.S.
Class: |
429/483 ;
429/493; 429/495; 429/535; 521/27; 525/326.1 |
Current CPC
Class: |
C08J 5/2218 20130101;
C08G 65/48 20130101; C08G 65/4056 20130101; H01M 8/1039 20130101;
H01M 8/1025 20130101; C08G 75/24 20130101; H01M 2300/0082 20130101;
Y02E 60/50 20130101; H01M 8/1072 20130101; H01M 8/1027 20130101;
H01M 2300/0091 20130101; H01M 8/1032 20130101; H01M 8/1081
20130101; H01M 8/1004 20130101; H01M 8/1051 20130101; H01M 8/1023
20130101; Y02P 70/50 20151101 |
Class at
Publication: |
429/033 ;
525/326.1; 521/027 |
International
Class: |
C08J 005/20; H01M
008/10 |
Claims
What is claimed is:
1. A fuel cell, comprising: an anode; a cathode fuel supply to the
anode; oxidant supply to the cathode; a polymer electrolyte
membrane positioned between the cathode and anode and fashioned
with crosslinking agent crosslinked into an ion-conducting base
material through hydroxyl and sulfonic acid condensation or though
amine and sulfonic acid condensation; and a membrane electrode
assembly (MEA) with the polymer electrolyte membrane.
2. A method of fabricating a polymer membrane suitable for use in
an electrochemical fuel cell, comprising: synthesizing a polymer
material of viscous nature which contains (a) crosslinked polymer
chains, (b) a solvent for dissolving the polymer chains, and (c)
any quantity of inorganic additives, spreading the synthesized
polymer material to form a uniform thickness layer on a substrate;
allowing the solvent to evaporate under controlled atmosphere from
the synthesized polymer material to yield the polymer electrolyte
membrane; and preparing the polymer electrolyte membrane for use in
a fuel cell by protonation and purification.
3. A material with tailorable microstructure, comprising: ion
conducting base material that is sulfonated; and a crosslinking
agent that is hydroxyl terminated and is crosslinked to the
sulfonated ion conducting base material via direct covalent
crosslinking characterized by
HO--R.sub.1--OH+2(HSO.sub.3)--R.sub.2.fwdarw.R.sub.2--SO.sub.2--O--R.sub.-
1--O--SO.sub.2--R.sub.2+2H.sub.2O where R.sub.1 is the hydroxyl
terminated crosslinking agent's main chain and R.sub.2 is the
sulfonated ion conducting base material.
4. A material as in claim 3, wherein the ion conducting base
material includes an inorganic cation exchange material
5. A material as in claim 4, wherein the inorganic cation exchange
material is selected from a group consisting of clay, zeolite,
hydrous oxide, and inorganic salt.
6. A material as in claim 4, wherein the inorganic cation exchange
material further includes a silica based material and a proton
conducting polymer based material.
7. A material as in claim 3, wherein the main chain includes one or
more chains selected from a group consisting of an aromatic polymer
chain, an aliphatic polymer chain, organic molecules and inorganic
molecules.
8. A material as in claim 3, wherein the sulfonated ion conducting
base material includes, one or more chains selected from a group
consisting of an aromatic polymer chain, an aliphatic polymer
chain, organic molecules and inorganic molecules.
9. A material with tailorable microstructure, comprising: ion
conducting base material that is sulfonated; and a crosslinking
agent that is amine terminated and is crosslinked to the sulfonated
ion conducting base material via direct covalent crosslinking
characterized by
H.sub.2N--R.sub.1--N--H.sub.2+2(HSO.sub.3)--R.sub.2.fwdarw.R.sub.2--SO.su-
b.2--NH--R.sub.1--NH--SO.sub.2--R.sub.2+2H.sub.2O where R.sub.1 is
the amine terminated crosslinking agent's main chain and R.sub.2 is
the sulfonated ion conducting base material.
10. A material as in claim 9, wherein the ion conducting base
material includes an inorganic cation exchange material
11. A material as in claim 10, wherein the inorganic cation
exchange material is selected from a group consisting of clay,
zeolite, hydrous oxide, and inorganic salt.
12. A material as in claim 10, wherein the inorganic cation
exchange material further includes a silica based material and a
proton conducting polymer based material.
13. A material as in claim 9, wherein the main chain includes one
or more chains selected from a group consisting of an aromatic
polymer chain, an aliphatic polymer chain, organic molecules and
inorganic molecules.
14. A material as in claim 9, wherein the sulfonated ion conducting
base material includes, one or more chains selected from a group
consisting of an aromatic polymer chain, an aliphatic polymer
chain, organic molecules and inorganic molecules.
15. A material with tailorable microstructure, comprising: ion
conducting base material that is amine or hydroxyl terminated; and
a crosslinking agent that is sulfonic acid terminated and is
crosslinked to the amine or hydroxyl terminated ion conducting base
material via direct covalent crosslinking characterized by,
respectively, HO.sub.3S--R.sub.3--SO.sub.3-
H+2(HO)--R.sub.4.fwdarw.R.sub.4--SO.sub.2--O--R.sub.3--O--SO.sub.2--R.sub.-
4+2H.sub.2O or
HO.sub.3S--R.sub.3--SO.sub.3H+2(H.sub.2N)--R.sub.4.fwdarw.R-
.sub.4--SO.sub.2--NH--R.sub.3--NH--SO.sub.2--R.sub.4+2H.sub.2O
where R.sub.3 is the sulfonic acid terminated crosslinking agent's
main and R.sub.4 is the amine or hydroxyl terminated ion conducting
base.
16. A material as in claim 15, wherein the ion conducting base
material includes an inorganic cation exchange material
17. A material as in claim 16, wherein the inorganic cation
exchange material is selected from a group consisting of clay,
zeolite, hydrous oxide, and inorganic salt.
18. A material as in claim 16, wherein the inorganic cation
exchange material further includes a silica based material and a
proton conducting polymer based material.
19. A material as in claim 15, wherein the main chain includes one
or more chains selected from a group consisting of an aromatic
polymer chain, an aliphatic polymer chain, organic molecules and
inorganic molecules.
20. A material as in claim 15, wherein the sulfonated ion
conducting base material includes, one or more chains selected from
a group consisting of an aromatic polymer chain, an aliphatic
polymer chain, organic molecules and inorganic molecules.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a divisional application claiming the benefit of
prior-filed U.S. patent application Ser. No. 10/653,016, filed Aug.
28, 2003, entitled "COMPOSITE ELECTROLYTE WITH CROSSLINKING
AGENTS."
FIELD OF THE INVENTION
[0002] The present invention relates to ionomers. More
particularly, the present invention relates to proton exchange
materials and methods for improving the physical and mechanical
properties of ion-conducting materials.
BACKGROUND OF THE INVENTION
[0003] There is considerable demand for high performance, low cost,
polymer electrolyte materials for use in hydrogen/oxygen and direct
methanol fuel cell (DMFC) applications. Several polymer types have
been used as proton exchange membranes including, but not limited
to perfluorosulfonic acid, sulfonated aromatics, acidified
imidazoles, and other organic/inorganic based composites. However,
to date, no material has met all the requirements needed to enable
fuel cells to become a viable commercial technology. High
production costs, high methanol permeability, low physical
strength, and/or poor electrochemical performance have plagued the
various candidate materials and have hindered the emergence of a
suitable material for wide spread use in fuel cell
applications.
[0004] For the last 30 years, the industry standard proton
conducting electrolyte membrane has been Nafion.RTM. (polyperfluoro
sulfonic acid) produced by DuPont (U.S. Pat. Nos. 3,282,875 and
4,330,654). While the performance of Nafion.RTM. is moderately
effective as a membrane within the context of hydrogen/ oxygen PEM
fuel cells, the polymer has a variety of limitations that have
hampered the emergence of the proton exchange membrane (PEM) fuel
cell design. Among these are Nafion's.RTM. high methanol
permeability, low thermal stability, and its high cost.
[0005] Nafion.RTM. is the most commonly incorporated material in
all low to medium temperature fuel cells although it works poorly
as a direct methanol fuel cell (DMFC) membrane. Nafion's.RTM. poor
performance within the DMFC context is primarily due to its high
methanol permeability and resulting methanol crossover. To minimize
crossover, some researchers have incorporated additives into
Nafion.RTM. as described in U.S. patent application No
2002-0094466A1 as well as volatilized the methanol before
introducing it to the anode side of the cell. However,
incorporating additives does not mitigate Nafion's.RTM. high
production cost and the volatilization of the methanol increases
fuel cell system complexity.
[0006] Other perflourinated sulfonic acid materials have been
developed to compete with Nafion.RTM.. One alternative membrane
incorporates Nafion.RTM. or a Nafion.RTM.-like polymer into a
porous polytetrafluoroethylene (TEFLON.RTM.) structure). These
membranes are available under the trade name GORE-SELECT.RTM. from
W. L. Gore & Associates, Inc. and they are described in U.S.
Pat. Nos. 5,635,041, 5,547,551 and 5,599,614. Other similar
membranes are available under the trade names ACIPLEX.RTM. from
Asahi Chemical Co. and FLEMION.RTM. from Asahi Glass. Regardless of
their developer, these alternative membranes exhibit many of the
same deficiencies as Nafion.RTM., namely, its high cost and high
fuel crossover in DMFC applications.
[0007] To address the cost and performance limitations faced with
the use of perflourinated sulfonic acid materials, recent research
has focused on the development of acid functionalized aromatic
polymers for use as proton exchange membranes in PEM fuel cells.
Thermoplastics such as polysulfone-udel (PS-Udel) or
poly-ether-ether ketone (PEEK) described in U.S. Pat. Nos.
4,625,000, 4,320,224 and 6,248,469 B1, have been extensively
studied as ion-conducting materials as described by Tchicaya et.
al, Xiao et. al, and U.S. Pat. No. 4,625,000, and U.S. patent
application US2002/0091225 (see: Tchicaya, L. Hybrid
Polyaryletherketone Membranes for Fuel Cell Applications, Fuel
Cells 2002, 2, No 1. and Xiao, G., Synthesis and Characterization
of Novel Sulfonated Poly(arylene ether ketone)s derived form
4,4'-sulfonyldiphenol, Polymer Bulletin 48, 309-315 (2002)).
Functionalizing these aromatic polymers has the potential of
meeting the cost and production challenges that face the
perfluorinated based polymers, but has two problematic properties
for fuel cell operation: excessive osmotic swelling and low
mechanical strength under hydrated conditions.
[0008] Aromatic based membranes, such as PEEK, which are described
in U.S. Pat. Nos. 4,320,224, 4,419,486, 5,122,587 and 6,355,149 B1,
use a post sulfonation process to attach sulfonic acid groups onto
the polymer backbone. The frequency of sulfonation or other acid
sites improves the electrochemical properties of the ionomer but
also increases osmotic swelling and lowers the material's
mechanical strength. While the increase in osmotic swelling can
help to increase the conductivity of the material, an over hydrated
material will become unsuitable for fuel cell applications.
[0009] Generally, all of the potentially low cost aromatic based
polymers share the same challenges as sulfonated PEEK. Increasing
sulfonation (or acid sites) increases the electrochemical
performance of the membrane but decreases its mechanical properties
after hydration. Many of the aromatic polymers such as the
polyether sulfones and polymer aryl ketones as described in U.S.
Pat. No. 4,625,000 from Union Carbide Corporation, have significant
potential if their weaknesses can be overcome such that they can
have high electrochemical properties while also retaining high
mechanical properties.
[0010] One method that has been employed to overcome the
shortcomings of acid functionalized proton exchange materials is to
incorporate crosslinking agents. Kerres et al combined sulfinated
(--SO.sub.2) polymer chains with halogenated alkanes as described
in U.S. patent application No. 2003/0032739 to reduce the osmotic
swelling and improve the mechanical strength of the polymer. The
reaction linked the sulfinated functional groups of polymer chains
via a mid-length alkane, thereby reducing the osmotic swelling of
the material. However, this process was experimentally complicated
and reduced the proton conductivity of the proton exchange membrane
product. Furthermore, the alkane (crosslinking agent) used to
crosslink the material was devoid of functionality such as acid
sites, and thereby could not add electrochemical performance
characteristics to the material.
[0011] Another crosslinking strategy that has been implemented to
improve fuel cell membrane performance entailed the
copolymerization of styrene with divinylbenzene (see Tsyurupa M.
P., Hypercrosslinked Polymers: Basic Principle of Preparing the New
Class of Polymeric Materials, Reactive and Functional Polymers,
Vol. 53, Issues 2-3, December 2002, 193-203.). In this method, the
resulting crosslinked ionomer had limited oxidative resistance
since both styrene and divinylbenzene display sensitivity to
oxidation (Assink, R. A.; Arnold C.; Hollandsworth, R. P., J. Memb.
Sci. 56, 143-151 (1993)). The crosslinking of the material did
improve its performance characteristics though the base material
was susceptible to chemical degradation and thus would limit
membrane lifetime.
[0012] Accordingly, there is a need for improved proton exchange
membrane materials with better physical and chemical properties
that have good electrochemical performance such as proton
conductivity.
SUMMARY OF THE INVENTION
[0013] The present invention is based upon the covalent
crosslinking of ion-conducting materials via sulfonic acid groups
which can be applied to alternative low cost membranes for improved
fuel cell performance metrics over the base material. The invention
is due, in part, to the observation that many aromatic and
aliphatic polymer materials have significant potential as proton
exchange membranes if a modification can increase their physical
and chemical stabilities without sacrificing electrochemical
performance or significantly increasing the material and production
costs. More specifically, non-fluorinated polymer materials such as
the aromatic poly ether ketones and poly ethersulfones show
significant potential due to their low cost and high proton
conductivity if the osmotic properties of the material and
electrochemical performance can be optimized for use in PEMFCs.
Other fluorinated materials such as the perfluorinated polymers can
be improved if greater functionality and lower methanol
permeability can be achieved (although cost is still a factor).
[0014] Crosslinking agents improve properties such as water uptake,
methanol crossover, thermal stability and mechanical strength,
without significantly decreasing the base material's positive
attributes. The incorporation of functionalized crosslinking
components or additives offers a promising technology for material
modification for fuel cell applications.
[0015] Formation of a composite material in accordance with the
present invention involves primarily (i) an ion-conducting base
material, and (ii) a functionalized crosslinking component.
Incorporation of the crosslinking agent with the base ion-exchange
material improves the physical characteristics and potentially the
overall performance of the ion-exchange material for use as a
proton exchange membranes in fuel cells. Additionally, the
crosslinking component allows for the addition of functional groups
into base materials and can improve the physical and
electrochemical performance of the resulting membranes for fuel
cell use.
[0016] In one embodiment of the invention, an electrochemical fuel
cell includes:(i) an anode; (ii) a cathode; (iii) fuel supply means
for supplying fuel toward the anode; (iv) oxidant supply means for
supplying an oxidant toward the cathode; (v) a polymer electrolyte
as defined above that is positioned between the anode and cathode;
and (vi) membrane electrode assembly (MEA).
[0017] In yet another embodiment of the invention, a method of
fabricating a polymer membrane suitable for use in an
electrochemical fuel cell includes (i) synthesizing a polymer
material (of viscous nature) which contains (a) crosslinked polymer
chains, (b) a fitting solvent which can dissolve the polymer
material, (c) any or no inorganic additives, (ii) spreading the
viscous liquid material to form a uniform thickness layer on the
substrate; (iii) evaporating the solvent under controlled
atmosphere from the viscous liquid to yield the polymer electrolyte
membrane; and (iv) preparing the membrane for use in a fuel cell by
acidification.
BRIEF DESCRIPTION OF THE FIGURES
[0018] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate several
embodiments of the invention and together with the description,
serve to explain the principles of the invention. Wherever
convenient, the same reference numbers will be used throughout the
drawings to refer to the same or like elements.
[0019] FIG. 1 illustrates tensile strength and elongation values
for sulfonated-polyether ketone (sEK) membranes with varying amount
of crosslinking components.
[0020] FIG. 2 shows graphs of methanol crossover current values for
unmodified sulfonated polyether ketone (sEK) membranes, crosslinked
sEK membranes w/a 5000 amu polyether ketone (EK) based crosslinking
agent, and Nafion.RTM.-15.
[0021] FIG. 3 shows SEM photographs of crosslinked networks for
sPEK based membranes.
[0022] FIG. 4 shows bar graphs of water uptake values of sPEK
membranes with various crosslinking percentages (5000 amu
non-sulfonated PEK crosslinking additive).
[0023] FIG. 5 shows bar graphs of water uptake values of sPEK
membranes with various crosslinking additive percentages (3000 amu
non-sulfonated PEK crosslinking additive).
[0024] FIG. 6 shows bar graphs of water uptake values of sPEK
membranes with various sulfonated crosslinking additive percentages
(2500 amu sulfonated PEK crosslinking additive).
[0025] FIG. 7 shows graphs of proton conductivity values for sPEK
membranes with varying amount of crosslinking components (2500 amu
sulfonated PEK crosslinking additive)
[0026] FIG. 8 shows bar graphs of ion exchange capacity (IEC)
values for unmodified sPEK membranes with varying amount of
crosslinking components (2500 amu sulfonated PEK crosslinking
additive)
[0027] FIG. 9 shows bar graphs of oxidation resistance values for
unmodified sPEK and crosslinked sPEK.
[0028] FIG. 10 illustrates a disassembled fuel cell.
[0029] FIG. 11 illustrates a partial cross sectional view of a
single electrochemical fuel cell.
[0030] FIG. 12 illustrates a partial cross sectional view of a
membrane electrode assembly.
DETAILED DESCRIPTION OF INVENTION
[0031] The invention provides performance enhancing crosslinking
agents, a method for their incorporation into ion-conducting
materials, and their incorporation into a fuel cell as a high
performance membrane material.
[0032] Forming an ion conducting material in accordance with the
present invention involves primarily (i) a crosslinking additive or
component, and (ii) a method for the incorporation of the
crosslinking agent into an ion-conducting base material.
Incorporation of the crosslinking agent with the base ion-exchange
material improves the physical characteristics and potentially the
overall performance of the ion-exchange material for use as a
proton exchange material in fuel cells. Furthermore, the
crosslinking component enables the easy tailoring of low cost
ion-exchange base materials into higher performing ionomers.
[0033] The specifically designed crosslinking components may have
various compositions; however, in order for the crosslinking
additive and the base ion-conducting polymer to combine, one
component must have a primary, secondary, or tertiary hydroxyl or
amine group and the other component must have a primary, secondary,
or tertiary, sulfonic acid group. These groups can be on the
crosslinking component or the base ion-exchange component. The
crosslinking agent's main chain may vary in length or structure, as
well as composition and conformation. In addition, the crosslinking
chain may include but is not limited to aromatic polymer chains,
aliphatic polymer chains, organic/ inorganic polymer networks, or
any combination thereof. The crosslinking agents do not necessarily
have to be polymeric in nature. Low molecular weight compound with
same functionality may also be used.
[0034] The crosslinking agent may have independent functionality
such as ion conducting groups or other chemical modifications such
that the agent will introduce unique characteristics to the base
material. Modifications may aid in improving mechanical, chemical,
and/or electrochemical properties, or any combinations thereof. The
crosslinking agent may have one or more amine, hydroxyl, or
sulfonic acid groups present but must have at least one functional
group which may react with the ion exchange base material's
functional group to form the covalent crosslinking bond.
[0035] The ion-conducting base material may be of inorganic,
organic, or of mixed constitution. Preferably, the ion-conducting
material is organically based, of aromatic or aliphatic in
composition, and may or may not contain structure stabilizing
inorganic units. Possible aromatic structures include, but are not
limited to poly-aryl ether ketones and poly-aryl ether sulfones
where the polymer nature of the material has a molecular weight of
at least 10,000 amu. Possible aliphatic materials include, but are
not limited to perfluorinated or styrene co-polymer types.
Generally, the preferred ion-conducting materials must have
adequate molecular weights and/or polymer structures with
functional groups that include, but are not limited to, sulfonic
acids, phosphoric acids, carboxylic acids, imidazoles, amines, and
amides. These groups help to increase the electrochemical
functionality of the material. However, regardless of the many
functional groups possible, the ion-conducting base material must
contain amine, hydroxyl, or sulfonic acid groups so that the
material may be crosslinked effectively with the crosslinking
components by the method disclosed.
[0036] The reaction scheme in EQUATION 1 is a generalization of the
direct covalent crosslinking method between a hydroxyl terminated
crosslinking agent and a sulfonated base ion-conducting
material.
HO--R.sub.1--OH+2(HSO.sub.3)--R.sub.2.fwdarw.R.sub.2--SO.sub.2--O--R.sub.1-
--O--SO.sub.2--R.sub.2+2H.sub.2O (1)
[0037] R.sub.1 is the crosslinking component's main chain including
but not limited to aromatic polymer chains, aliphatic polymer
chains, and organic/ inorganic molecules. R.sub.2 is any base
material which can be sulfonated and includes, but is not limited
to, aromatic polymer chains, aliphatic polymer chains, and organic/
inorganic molecules and structures. The functional group on the
crosslinking agent may appear at the chain end or anywhere in the
backbone. Incorporation of the crosslinking component with the
ion-conducting base material takes place in a non-aqueous
environment. The presence of water can limit the extent of the
reaction. Furthermore, the reaction temperature, duration, and
component concentrations all affect the degree of crosslinking, as
well as the dynamic nature of the reaction composition.
[0038] The reaction between an amine terminated crosslinking agent
and ion-conducting base material is generalized in the reaction
equation below:
H.sub.2N--R.sub.1--NH.sub.2+2(HSO.sub.3)--R.sub.2.fwdarw.R.sub.2--SO.sub.2-
--NH--R.sub.1--NH--SO.sub.2--R.sub.2+2H.sub.2O (2)
[0039] EQUATION 2 is a generalization for the direct covalent
crosslinking method between an aminated crosslinking agent and a
sulfonated base material. R.sub.1 and R.sub.2 represent the same
materials as listed in EQUATION 1.
[0040] Furthermore, the reaction may take place with the sulfonic
acid groups on the crosslinking additive and the amine or hydroxyl
group on the ion-conducting base material. These reactions are
generalized in EQUATION 3 and EQUATION 4 below.
HO.sub.3S--R.sub.3--SO.sub.3H+2(HO)--R.sub.4.fwdarw.R.sub.4--SO.sub.2--O---
R.sub.3--O--SO.sub.2R.sub.4+2H.sub.2O (3)
HO.sub.3S--R.sub.3--SO.sub.3H+2(H.sub.2N
)--R.sub.4.fwdarw.R.sub.4--SO.sub-
.2--NH--R.sub.3--NH--SO.sub.2--R.sub.4+2H.sub.2O (4)
[0041] Here, R.sub.3 is the crosslinking component's main chain
including but not limited to aromatic polymer chains, aliphatic
polymer chains, and organic/inorganic molecules. R.sub.4 is any
base material which can be sulfonated and includes, but is not
limited to, aromatic polymer chains, aliphatic polymer chains, and
organic/inorganic molecules and structures.
[0042] The reaction solvent used in all reaction schemes includes
but is not limited to high boiling point, non-polar solvents such
as dimethyl sulfoxide (DMSO), n-methyl pyrrolidinone (NMP),
dimethyl acetamide(DMAC) or dimethylformamide (DMF). The reaction
may proceed under azeotrophic distillation via the removal of water
by toluene to facilitate the reaction kinetics. Reaction times vary
depending on the conditions used but typically range from 1 to 12
hours and more preferably from 3-10 hours. If the crosslinking
additives are amine or hydroxyl functionalized, the reaction
mixture may consist of 0.1% to 100% crosslinking component molar
equivalents with respect to the base polymer's sulfonic acid sites,
but preferably between 0.1 and 8%. If the crosslinking additives
are sulfonic acid functionalized, the reaction mixture may consist
of 0.1% to 100% crosslinking component molar equivalents with
respect to the base polymer's amine or hydroxyl groups, but
preferably between 0.1% and 8%.
[0043] The crosslinking components disclosed improve the tensile
strength of the base material in both wet and dry conditions. This
is primarily due to the increase in molecular weight and
reinforcing linkages between adjacent polymer chains. Furthermore,
the inherent nature of the crosslinking agent imparts either
hydrophobic or hydrophilic regions affecting the overall
hydrodynanic nature of the resulting material. The use of
particular crosslinking agents of varying molecular weight allows
for tuning the elongation of the material without sacrificing
tensile strength.
[0044] In polymer chains, where the elongation of the material is
directly dependent on the coiling and orientation of the material
before sheering, covalent crosslinking affects the material's
ability to elongate. Longer crosslinking agents increase the
elongation of the material while not sacrificing the materials
tensile strength. The table in FIG. 1 highlights this phenomenon.
The crosslinked network formed by the covalent crosslinking through
sulfonic acid groups enables a significant level of control of the
physical properties of the base material. The improvements in
tensile strength, Young's modulus, and elongation at break are
direct results of the incorporation of the crosslinking agents into
the base material. As seen in FIG. 1, generally, the elongation of
the material increases as the length of the crosslinking agent
increases; while tensile strength increases as the percentage of
crosslinking agent increases.
[0045] In addition, crosslinking components may impart
microstructure changes to the base ion-conducting material, thereby
limiting the resulting material's permeability. With the
ion-conducting channel size reduced, the methanol permeability of
the material also lessens. Reducing the methanol permeability of
proton exchange membranes is critical to improving the performance
of direct methanol fuel cells (DMFCs). FIG. 2 shows the reduction
in methanol crossover that the crosslinking components add to the
base ion-conducting polymer. Incorporation of the crosslinking
agent reduces methanol crossover by 30% when normalized for
thickness and polymer type.
[0046] The microstructure network formed by the covalent
crosslinking of the material is visible as shown by the scanning
electron micrographs in FIG. 3. The scanning electron micrographs
show a base ion-conducting material crosslinked with various
crosslinking agents. The size of the web-like network visible
between the micrographs correlates to the differences of the
crosslinking agents. This visible difference in microstructure is
apparent and correlates with the increase in all physical
properties of the membrane. Other properties such as water uptake
of the material can also be manipulated with control over the
covalent crosslinking as well as the structure and amount of
crosslinking agent.
[0047] Base ion-conducting materials may have inherent water uptake
properties that are either too high or too low for their objective
purposes. Too high of a water uptake may cause the weakening of the
materials physical properties, while too low of a water uptake may
limit the ion-conducting material's ability to conduct protons at a
high efficiency. Water balance of the membrane is critical for high
fuel cell performance. To tailor the material for usage, the
covalent crosslinking can act as an ideal modifier for a polymer
electrolyte membrane. As seen in FIGS. 4-6, the incorporation of
the crosslinking component into the base material can increase or
decrease the water uptake of the base ion-conducting material.
[0048] As mentioned previously, many of the inexpensive acid
functionalized aromatic polymers with potential as proton exchange
membranes in fuel cells excessively swell in the presence of water
due to their hydrophilic nature. This excessive osmotic swelling
limits their use as ionomers in fuel cells. Previous work by Kerres
et al. showed that these materials could be crosslinked, improving
osmotic swelling, but the resulting material had low proton
conductivity (see: U.S. patent application No. 2003/0032739). The
crosslinking components disclosed in the present invention may
embody functional groups such as sulfonic acid that can minimize
electrochemical performance losses such as proton conductivity as
well as augment the oxidative resistance of the material.
[0049] The material life of a membrane in fuel cell use is largely
dependant on the stability of the material in the operationally
oxidative atmosphere. Unmodified (non-crosslinked) polymer
materials with functional groups directly bonded to the chemical
backbone may be susceptible to greater degrees of oxidation than
other materials which have the functional groups removed from their
primary backbones. In some cases, the crosslinking agents disclosed
may increase the chemical stability of the material. As seen in
FIG. 9, the stability of the base material in a highly oxidative
atmosphere is lower than the stability of the base material
incorporated with a crosslinking agent. The base material alone
shows a loss of 10% of its weight after 4 hours in a solution of 3%
H.sub.2O.sub.2 and 0.4 M iron sulfate, while the base material
incorporated with the crosslinking agent shows only a loss of 5%.
As is apparent, the incorporation of the performance enhancing
crosslinking agents improve the material in many aspects.
[0050] FIGS. 6-8 highlight the effect of the present invention on
an acid functionalized aromatic poly ether polymer (35% sPEK). In
some cases, the incorporation of the sulfonated crosslinking agent
helps to reduce osmotic swelling without reducing the proton
conductivity significantly. This result can be extremely useful in
improving the characteristics of acid functionalized aromatic
polymers for use in fuel cells. Furthermore, FIGS. 7 and 8 indicate
the inconsistencies of IEC and proton conductivity results. As the
amount of crosslinking component is increased, the IEC of the
resultant polymer decreases. This result was anticipated as the
amount of sulfonic acid sites for the resultant makeup decreased by
one for every crosslinking additive which was incorporated. This
decrease was consistent with what was observed in FIG. 8. However,
for the same materials the proton conductivity actually increased
between the 3% and 5% crosslinking agent samples. This result was
unexpected and can most likely be attributed to the microstructure
change of the resultant material after the crosslinking additive
was incorporated. The ability of the present invention to alter the
microstructure in a manner where conductivity increases while IEC
decreases suggests that there is significant potential to modify
acid functionalized aromatic polymers and others in a fashion
suitable for use as proton exchange membranes. The performance
enhancing crosslinking agents significantly change the properties
of base material as well as enable the tailoring of the material
for specific uses.
[0051] The covalently crosslinked electrolyte of the present
invention is particularly suited for use as the proton exchange
membrane in an electrochemical fuel cell that is illustrated in
FIGS. 10, 11, and 12. The electrochemical cell 10 generally
includes a membrane electrode assembly 12 flanked by the anode and
cathode flow field structures. On the anode side, the cell includes
an endplate 14, graphite block or bipolar plate 18 with openings 22
to facilitate gas distribution, gasket 26, and anode carbon cloth
current collector 30. Conversely, on the cathode side, the cell
includes stainless steel endplate 16, graphite block or bipolar
plate 20 with openings 24 to facilitate gas distribution, gasket
28, and cathode carbon cloth current collector 32. The carbon cloth
material is a porous conductive support for the diffusion
layer.
[0052] The electrochemical cell also includes a membrane electrode
assembly (MEA) 12 as shown in FIGS. 11 and 12. The MEA includes a
proton exchange membrane 46 that is flanked by anode 42 and cathode
44 electrodes. Each electrode is made of a porous electrode
material such as carbon cloth or carbon paper. The proton exchange
membrane 46, which comprises the inventive composite electrolyte,
provides proton conducting medium during operation of the fuel
cell. Anode current collector 30 and cathode current collector 32
are connected to external circuit 50 by leads 31, 33 respectively.
The external circuit can comprise any conventional electronic
device or load such as those described in U.S. Pat. Nos. 5,248,566,
5,272,017, 5,547,777, and 6,387,556, which are incorporated herein
by reference. The components, mainly the MEA, can be hermetically
sealed by known techniques.
[0053] In operation, fuel from fuel source 37 diffuses through the
anode and an oxidizer from oxidant source 39 (e.g., container or
ampoule) diffuses through the cathode of the MEA. The chemical
reactions at the MEA develop the electromotive force and the
electrons are transported though an electronic load. Hydrogen fuel
cells use hydrogen as the fuel and oxygen as the oxidizer. For
direct methanol fuel cells, the fuel is liquid methanol.
[0054] The inventive composite electrolyte can be employed in
conventional fuel cells which are described, for example, in U.S.
Pat. Nos. 5,248,566 and 5,547,777. In addition, several fuel cells
can be connected in series by conventional means to fabricate or
assemble fuel cell stacks.
[0055] Methodology
[0056] The performance enhanced crosslinked membranes of the
present invention can be fabricated by reacting the designed
crosslinking component with the ion-conducting base material. High
boiling point solvents such as the preferred solvent, DMSO, are to
be used to dissolve the reactants for the chemical reaction.
Furthermore, other solvents such NMP, DMSO, DMAC, and DMF may be
used. This is not meant to be an extensive list, but is meant to
give the reader a sense for other solvents that can be employed.
The reaction takes place in a dry atmosphere, either through
azeotrophic distillation or other forms of a water free reaction
conditions.
[0057] After the base material has been successfully crosslinked
with the desired crosslinking component, the viscous solution is
poured onto a substrate and leveled to a uniform thickness.
Alternately, the mixture can also been cast by doctor blade. The
resulting film is dried, removed from the substrate, and cut to
size before use. Heating or applying a vacuum to the membrane while
drying may also be used to facilitate evaporation.
[0058] A preferred manufacturing technique for the present
invention is a tape casting method whereby the mixture of
components in dispersant is poured onto a level sheet. A doctor
blade moving across the gel adjusts the height to the desired
thickness ranging from about 0.5 .mu.m to about 500 .mu.m and
preferably from about 50 .mu.m to about 300 .mu.m. Evaporation of
the solvent takes place in a controlled temperature and humidity
environment. Afterwards, the membrane is removed from the substrate
and conditioned for use as a proton exchange membrane. Preparation
may include, but is not limited to, hydrolysis, annealing,
protonating, and hydrating. Membrane preparation depends on the
specific formulation of the resulting film. Although tape casting
can be used for producing the membranes disclosed, other methods
such as extrusion and tray casting may be employed.
[0059] In testing, the membranes are evaluated for their water
uptake, proton conductivity, ion-exchange capacity and mechanical
properties.
[0060] Water uptake is done via percentage uptake where:
% water
uptake=(Weight.sub.wet-Weight.sub.dry)*100/(Weight.sub.dry)
[0061] For this evaluation, samples of the crosslinked membranes
are dried at 100.degree. C. in a convection oven for 24 hours and
weighed for the dry weight then placed into 90.degree. C. dH.sub.2O
for 48 hour and weight for the wet weight.
[0062] For proton conductivity measurements, the evaluation is
conducted with, for example, a Solartron frequency response
analyzer between 1 Hz-50 kHz. The resistance at the X-intercept is
taken as the resistivity then used to calculate the conductivity of
the material by: Conductivity (S/cm)=Thickness (cm)/(resistance
(ohm)*electrode area (cm.sup.2).
[0063] In measuring the ion-exchange capacity (EEC) of the
material, the EEC is determined before and after the crosslinking
reaction by wet chemistry method. To that end, the material is
first protonated by submersion in 1N H.sub.2SO.sub.4 for 12 hours
then rinsed until the rinse water is near neutral pH. The membrane
is dried and weighed, then placed into 1M NaCl solution for 12
hours. The liberated H.sup.+ in the solution is then titrated with
standardized 0.01N NaOH solution and the IEC is calculated.
[0064] The mechanical properties of the crosslinked material are
determined by testing with, for example, an Instron mechanical
testing apparatus. The materials are protonated, rinsed, and left
in 23.degree. C. dH.sub.2O for 24 hours to allow for hydrolytic
equilibration. The films are cut to standard size and then tested
out of water, hydrated at 23.degree. C., and submerged in water at
70.degree. C. The mechanical properties are observed and recorded
by Instron software under a pull rate of 50 mm/min.
[0065] Experimental
[0066] The following examples, illustrate a number of the
crosslinked membrane electrolytes that are formulated and tested as
a proton exchange membrane. They are not meant to encompass all
possible embodiments and they merely provide the reader with a
sense of the invention.
EXAMPLE 1
[0067] A first example describes the process of fabricating a
crosslinked electrolyte material containing an organic cation
exchange material. An aromatic polymer base material, di-sulfonated
poly ether ketone with a molecular weight of over 60,000 amu is
fabricated as described in U.S. patent application No. 2002/0091225
A1. The material's degree of sulfonation is calculated based upon
the ion-exchange capacity after the material is protonated via
refluxing in 0.5M sulfuric acid. The protonated polymer is then
thoroughly dried under vacuum.
[0068] A crosslinking agent of phenol terminated poly-ether ketone
with a molecular weight of 2500 amu is fabricated and dried
similarly to the method described in U.S. patent application No.
2002/0091225 A1. The dry base polymer and crosslinking agent are
then combined in a high boiling solvent such as DMSO. The solution
is azeotrophically distilled with toluene in a dean-stark apparatus
between 135.degree. C. and 175.degree. C. for four hours. The
toluene is then removed and the reaction is allowed to progress for
6 hours. The resulting viscous solution is then poured onto a level
plate and cast flat with a doctor blade apparatus. The solvent is
allowed to evaporate at 90.degree. C. for 12 hours followed by
further drying at 130.degree. for 12 hours and 150.degree. C. for
12 hours. The film is then removed from the casting surface and
refluxed in H.sub.2O for 1 hour. The hydrated film is then
protonated for 1 hour in boiling 0.5M H.sub.2SO.sub.4 then rinsed
in deionized (DI) water until the pH of the wash water is near
neutral. The membrane is then tested for its physical and
electrochemical performance properties and compared to the
ion-conducting base material.
EXAMPLE 2
[0069] A second example describes the process of fabricating a
functionalized crosslinked material: A commercially available
polymer material (PEEK from Victrex) is dissolved in concentrated
sulfuric acid and allowed to sulfonate for a given period of time
under constant temperature. The polymer is then precipitated in ice
water and rinsed until the wash water is neutral and then dried
under vacuum. The phenol sulfonated crosslinking agent is then
synthesized by copolymerization of sulfonated difluorodiphenol
ketone and bis-phenol A. The molar ratio of each monomer is used to
calculate the resulting molecular weight of the crosslinking
component as well as its sulfonation level. The resulting
crosslinking component consisting of phenol terminated polymer
chains are then rinsed and dried under vacuum.
[0070] The sulfonated polymer base material is then combined with
the sulfonated crosslinking agent and dissolved in DMSO. The ratio
of polymer base material to crosslinking agent is calculated such
that the crosslinking agent represented between 1-50 molar percent.
The reaction is azeotrophically distilled between 135.degree. C.
and 175.degree. C. with toluene in a dean-stark apparatus. The
toluene is then removed at 175.degree. C. and the reaction is
allowed to continue for 8 hours. Afterwards, one or more inorganic
additives may be added. The resulting mixture is then cast in a
tray and allowed to evaporate at 90.degree. C. for 12 hours
followed by heating under vacuum at 135.degree. C. for 24 hours.
The dried film is then protonated in 1 M H.sub.2SO.sub.4 solution
then rinsed with dH.sub.2O. The membrane is then tested for its
physical and electrochemical properties.
[0071] The inorganic additives can be those described in U.S.
patent application Ser. No. 10/219,083 filed Aug. 13, 2002, by Taft
et al. with the title "Composite Electrolyte For Fuel Cells," which
is incorporated herein by reference. Specifically, an inorganic
additive is selected, from the group that includes, for example,
clays, zeolites, hydrous oxides, and inorganic salts. A clay
includes an aluminosilicate-based exchange material selected from
the group consisting of montmorillonite, kaolinite, vermiculite,
smectite, hectorite, mica, bentonite, nontronite, beidellite,
volkonskoite, saponite, magadite, kenyaite, zeolite, alumina,
rutile.
EXAMPLE 3
[0072] A third example shows that, by comparison, sulfoamide
linkages are also formed via the direct crosslinking of sulfonic
acid functional groups to amine terminated crosslinking agents. The
crosslinking reaction may proceed identically to method detailed in
Examples 1 and 2 with no changes in reaction conditions or
compositions. The resulting gel may be cast in a manner as
described above. The sulfoamide crosslinked membrane can then be
prepared for its incorporation into a fuel cell or further physical
and electrochemical tests.
EXAMPLE 4
[0073] A fourth example describes the process of forming sulfoamide
crosslinked material via the indirect reaction of an amine
terminated crosslinking agent to a sulfonated ion conducting base
material. The reaction uses the same crosslinking agents and base
materials as those listed in Example 1 and Example 2. First, the
sulfonated base material is protonated. The material is then
dissolved into a high boiling point solvent such as
n-methyl-pyrrolidinone (10 wt % solution). Stoichiometric amounts
with respect to the sulfonic acid groups of the base material of
Carbonyl-diimidizole (CDI) is then added to the solution and
allowed to react at 60.degree. C. for 6 hours. After the
intermediate reaction of sulfonic acid sites to the CDI occurs, the
amine terminated crosslinking agent is then added. The
stoichiometric addition of amine terminated crosslinking agent is
allowed to react at 90.degree. C. for 12 hours before the film is
cast by methods similarly to that described in Example 1. The film
is then dried and protonated, and ready for testing.
[0074] In summary, the present invention provides performance
enhancing crosslinking agents, a method for their incorporation
into ion-conducting materials, and their incorporation into a fuel
cell as a high performance electrolyte membrane material.
Incorporation of the crosslinking agent with the base ion-exchange
material improves the physical characteristics and potentially the
overall performance of the ion-exchange material for use as a
proton exchange material in fuel cells. Furthermore, the
crosslinking component enables the easy tailoring of low cost
ion-exchange base materials into higher performing ionomers.
[0075] Although the present invention has been described in
accordance with the embodiments shown, variations to the
embodiments would be apparent to those skilled in the art and those
variations would be within the scope and spirit of the present
invention. Accordingly, it is intended that the specification and
embodiments shown be considered as exemplary only, with a true
scope of the invention being indicated by the following claims and
equivalents.
* * * * *