Note: Descriptions are shown in the official language in which they were submitted.
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PCT/US99/19470
NOVEL ION-CONDUCTING MATERIALS SUITABLE FOR USE IN
ELECTROCHEMICAL APPLICATIONS AND METHODS RELATED
THERETO
STATEMENT OF GOVERNMENT RIGHTS
This invention was made with Government support under Contract No.
DE-FC02-97EE50478 awarded by the Department of Energy and Contract No.
DMI-9760978 with National Science Foundation. The Government has certain
rights in this invention.
FIELD OF THE INVENTION
This invention relates to novel ion-conducting materials suitable for use as
solid polymer electrolyte membranes in electrochemical applications including
1o fuel cell systems. More specifically, these novel ion-conducting polymers
are
based on sulfonated polyaryletherketone polymers or sulfonated
polyphenylsulfone polymers, including copolymers, or blends thereof. The
present invention also describes novel processes for producing these ion-
conducting materials.
BACKGROUND OF THE INVENTION
There is a considerable need in both the military and commercial sectors
for quiet, efficient and lightweight power sources that have improved power
density. Military applications include, but are not limited to, submersibles,
surface
ships, portable/mobile field generating units, and low power units (i.e.,
battery
replacements). For example, the military has a strong interest in developing
low
range power sources (a few watts to a few kilowatts) that can function as
replacements for batteries. Commercial applications include transportation
(i.e.,
automotive, bus, truck and railway), communications, on-site cogeneration and
stationary power generation.
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Other interest exists for household applications, such as radios, camcorders
and laptop computers. Additional interest exists in larger power sources or
sources of higher power density that can be used in operating clean, efficient
vehicles. In general, there is a need for quiet, efficient, and lightweight
power
sources anywhere stationary power generation is needed.
Additionally, the use of gasoline-powered internal combustion engines has
created several environmental exhaust gas-related problems. One possible
solution to these environmental problems is the use of fuel cells. Fuel cells
are
highly efficient electrochemical energy conversion devices that directly
convert
the chemical energy derived from renewable fuel into electrical energy.
Significant research and development activity has focused on the
development of proton-exchange membranes for use in various electrochemical
applications including fuel cell systems. Proton-exchange membrane fuel cells
have a polymer electrolyte membrane disposed between a positive electrode
(cathode) and a negative electrode (anode). The polymer electrolyte membrane
is
composed of an ion-exchange polymer (i.e., ionomer). Its role is to provide a
means for ionic transport and prevent mixing of the molecular forms of the
fuel
and the oxidant.
Solid polymer electrolyte fuel cells (SPEFCs) are an ideal source of quiet,
efficient, and lightweight power. While batteries have reactants contained
within
their structure which eventually are used up, fuel cells use air and hydrogen
to
operate continuously. Their fuel efficiency is high (45 to 50 percent), they
do not
produce noise, operate over a wide power range ( 10 watts to several hundred
kilowatts), and are relatively simple to design, manufacture and operate.
Further,
SPEFCs currently have the highest power density of all fuel cell types. In
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addition, SPEFCs do not produce any environmentally hazardous emissions such
as NOX and SOX (typical combustion by-products).
The traditional SPEFC contains a solid polymer ion-exchange membrane
that lies between two gas diffusion electrodes, an anode and a cathode, each
commonly containing a metal catalyst supported by an electrically conductive
material. The gas diffusion electrodes are exposed to the respective reactant
gases, the reductant gas and the oxidant gas. An electrochemical reaction
occurs
at each of the two junctions (three phase boundaries) where one of the
electrodes,
electrolyte polymer membrane and reactant gas interface.
During fuel cell operation, hydrogen permeates through the anode and
interacts with the metal catalyst, producing electrons and protons. The
electrons
are conducted via an electrically conductive material through an external
circuit to
the cathode, while the protons are simultaneousiy transferred via an ionic
route
through the polymer electrolyte membrane to the cathode. Oxygen permeates to
the catalyst sites of the cathode, where it gains electrons and reacts with
protons to
form water. Consequently, the products of the SPEFC's reactions are water,
electricity and heat. In the SPEFC, current is conducted simultaneously
through
ionic and electronic routes. Efficiency of the SPEFC is largely dependent on
its
ability to minimize both ionic and electronic resistivity to these currents.
Ion exchange membranes play a vital role in various electrochemical
applications, including that of SPEFCs. For example, in SPEFCs, the ion-
exchange membrane has two functions: (1) it acts as the electrolyte that
provides
ionic communication between the anode and cathode; and (2) it serves as a
separator for the two reactant gases (e.g., OZ and H,). In other words, the
ion-
exchange membrane, while serving as a good proton transfer membrane, must
also have low permeability for the reactant gases to avoid cross-over
phenomena
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that reduce performance of the fuel cell. This is especially important in fuel
cell
applications in which the reactant gases are under pressure and the fuel cell
is
operated at elevated temperatures.
Fuel cell reactants are classified as oxidants and reductants on the basis of
their electron acceptor or electron donor characteristics. Oxidants include
pure
oxygen, oxygen-containing gases (e.g., air) and halogens (e.g., chlorine).
Reductants include hydrogen, carbon monoxide, natural gas, methane, ethane,
formaldehyde and methanol.
Optimized proton and water transports of the membrane and proper water
management are also crucial for efficient fuel cell application. Dehydration
of the
membrane reduces proton conductivity, and excess water can lead to swelling of
the membranes. Inefficient removal of by-product water can cause flooding of
the
electrodes hindering gas access. Both of these conditions lead to poor cell
performance.
Despite their potential for many applications, SPEFCs have not yet been
commercialized due to unresolved technical problems and high overall cost. One
major deficiency impacting the commercialization of the SPEFC is the inherent
limitations of today's leading membrane and electrode assemblies. To make the
SPEFC commercially viable (especially in automotive applications), the
membranes employed must operate at elevated/high temperatures (>120°C)
so as
to provide increased power density, and limit catalyst sensitivity to fuel
impurities. This would also allow for applications such as on-site
cogeneration
(high quality waste heat in addition to electrical power). Current membranes
also
allow excessive methanol crossover in liquid feed direct methanol fuel cells
(dependent on actual operating conditions, but is typically equivalent to a
current
density loss of about SO to 200 mA/cm' @ O.SV). In general, fuel crossover is
a
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parasitic reaction that lowers efficiency, reduces performance and generates
heat
in the fuel cell. It is highly desirable to minimize the rate of fuel
crossover.
Several polymer electrolyte membranes have been developed over the
years for application as solid polymer electrolytes for use in electrochemical
oxidation reduction-driven applications, including fuel cell systems. However,
these membranes have significant limitations when applied to liquid-feed
direct
methanol fuel cells and to hydrogen fuel cells. For example, the membranes in
today's most advanced SPEFCs do not possess the required combination of ionic
conductivity, mechanical strength, dehydration resistance, chemical stability
and
fuel impermeability (e.g., methanol crossover) to operate at elevated
temperatures.
DuPont developed a series of perfluorinated sulfonic acid membranes
known as Nafion~ membranes. The Nafion~ membrane technology is well
known in the art and is described in U.S. Patent Nos. 3,282,875 and 4,330,654.
Unreinforced Nafion~ membranes are used almost exclusively as the ion
exchange membrane in present SPEFC applications. This membrane is fabricated
from a copolymer of tetrafluoroethylene (TFE) and a perfluorovinyl
ethersulfonyl
fluoride. The vinyl ether comonomer is copolymerized with TFE to form a melt-
processable polymer. Once in the desired shape, the sulfonyl fluoride group is
hydrolyzed into the ionic sulfonate fonm.
The fluorocarbon component and the ionic groups are incompatible or
immiscible (the former is hydrophobic, the latter is hydrophilic). This causes
a
phase separation, which leads to the formation of interconnected hydrated
ionic
"clusters". The properties of these clusters determine the electrochemical
characteristics of the polymer, since protons are conducted through the
membrane
as they "hop" from one ionic cluster to another. To ensure proton flow, each
ionic
group needs a minimum amount of water to surround it and form a cluster. If
the
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ionic group concentration is too low (or hydration is insufficient) proton
transfer
will not occur. At higher ionic group concentrations (or increased hydration
levels) proton conductivity is improved, but membrane mechanical
characteristics
are sacrificed.
As the membrane temperature is increased, the swelling forces (osmotic)
become larger than the restraining forces (fluorocarbon chains). This allows
the
membrane to assume a more highly swollen state, but may eventually promote
membrane dehydration. Peroxide radicals form more quickly as the temperature
is increased; these radicals can attack and degrade the membrane. At even
higher
temperatures (230°C), the fluorocarbon phase melts and permits the
ionic domains
to "dissolve" (phase inversion of Nafion~).
There are several mechanisms that limit the performance of Nafion~
membranes in fuel cell environments and other electrochemical applications.
For
example, Nafion~ is sensitive to heat and can only be used effectively to
temperatures of about 100°C. In fact, performance-limiting phenomenon
may
begin even at temperatures of about 80°C. Mechanisms which limit the
performance of Nafion~ include membrane dehydration, reduction of ionic
conductivity, radical formation in the membrane (which can destroy the solid
polymer electrolyte membrane chemically), loss of mechanical strength via
softening, and increased parasitic losses through high fuel permeation.
Crossover problems with Nafion~ membranes are especially troublesome
in liquid feed direct methanol fuel cell applications, where excessive
methanol
transport (which reduces efficiency and power density) occurs. Methanol-
crossover not only lowers fuel utilization efficiency but also adversely
affects the
oxygen cathode performance, significantly lowering cell performance. More
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specifically, this crossover causes a mixed reaction (oxidation and reduction)
to
develop on the cathode side, reducing the reaction efficiency.
The Nafion~ membrane/electrode is also very expensive to produce, and
as a result it is not (yet) commercially viable. Reducing membrane cost is
crucial
to the commercialization of SPEFCs. It is estimated that membrane cost must be
reduced by at least an order of magnitude from the Nafion~ model for SPEFCs to
become commercially attractive.
10 Another type of ion-conducting membrane, Gore-Select~ (commercially
available from W.L. Gore), is currently being developed for fuel cell
applications.
Gore-Select0 membranes are further detailed in a series of U.S. Patents (U.S.
5,635,041, 5,547,551 and 5,599,614).
Gore discloses a composite membrane consisting of a porous Teflon~ film
filled with a Nafion~ or Nafion~-like ion-conducting solution. Although it has
been reported to show high ionic conductance and greater dimensional stability
than Nafion~ membranes, the Teflon~ and Nafion~ materials selected and
employed by Gore as the film substrate and the ion-exchange material,
20 respectively, may not be appropriate for operation in high-temperature
SPEFCs.
Teflon~ undergoes extensive creep at temperatures above 80°C, and
Nafion~ and
similar ionomers swell and soften above the same temperature. This can result
in
the widening of interconnected channels in the membrane and allow performance
degradation, especially at elevated temperatures and pressures.
Further, Gore-SelectO, as well as many other types of perfluorinated ion-
conducting membranes (e.g., Aciplex from Asahi Chemical, Flemion~ from
Asahi Glass, Japan), are just as costly as Nafion~, since these membranes
employ
a high percentage of perfluorinated ionomers.
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In an effort to reduce costs and move toward potential commercialization
of SPEFCs and improved performance in all suitable electrochemical
applications,
ion-exchange membranes that are less expensive to produce also have been
investigated for use in polymer electrolyte membrane fuel cells.
Poly(trifluorostyrene) copolymers have been studied as membranes for use
in polymer electrolyte membrane fuel cells. See e.g., U.S. Patent No.
5,422,411.
However, these membranes are suspected to have poor mechanical and film
forming properties. In addition, these membranes may be expensive due to the
inherent difficulties in processing fluorinated polymers.
Various sulfonated polymers have been studied for use in electrochemical
applications.
Sulfonated polyaromatic based systems, such as those described in U.S.
Patent Nos. 3,528,858 and 3,226,361, also have been investigated as membrane
materials for SPEFCs. However, these materials suffer from poor chemical
resistance and mechanical properties that limit their use in SPEFC
applications.
Solid polymer membranes comprising a sulfonated poly(2,6 dimethyl 1,4
phenylene oxide} alone or blended with poly(vinylidene fluoride) also have
been
investigated. These membranes are disclosed in WO 97/24777. However, these
membranes are known to be especially vulnerable to degradation from peroxide
radicals.
Sulfonated poly{aryl ether ketone) (PEK) polymers developed by Hoechst
AG are described in European Patent No. 574,891 A2. These polymers can be
cross-linked by primary and secondary amines. However, when used as
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membranes and tested in polymer electrolyte membrane fuel cells, only modest
cell performance is observed.
U.S. Patent No. 4,268,650 also describes sulfonated PEK polymers. The
process disclosed in this patent relies on the copolymerization of PEK and
polyether ether ketone (PEEK) to control the level of sulfonation in
concentrated
sulfuric acid. Sulfonation under these conditions is thought to occur
exclusively
at the ether-phenyl-ether linkages.
U.S. Patent No. 4,273,903 involves a similar process using
polyarylethersulphone copolymers. Specifically, this patent discloses the
controllable sulfonation of polyarylethersulphone polymers in concentrated
sulfuric acid using copolymers of polyetherether sulfone (sulfonatable) and
polyethersulfone (less susceptible to sulfonation). This patent also reports
that the
use of sulfuric acid/oleum, oleum or chlorosulfonic acid will completely
sulfonate
and/or degrade the polyethersulfone polymer. The sulfonation procedure
described
in this reference is carned out homogeneously (i.e., putting the polymer into
solution before addition of the sulfonating agent.)
U.S. Patent No. 5,795,496 discloses methods for producing sulfonated
polyether ether ketone (PEEK) and sulfonated poly (p-phenylene ether sulfone)
(PES) polymer materials for use in fuel cells. Target properties of the
asymmetric
membranes formed using the materials and methods of this patent include a
target
membrane thickness of 0.05-0.5 mm (equivalent to about 2-20 mils). This patent
also reports that thinner membranes, e.g., less than 0.05 mm, have poor
mechanical strength and dimensional stability. This reference also describes
cross-linking of these sulfonated polymer materials at temperatures of about
120°C for an unspecified time in order to minimize fuel crossover.
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U.S. Patent No. 4,413,106 provides methods for the heterogeneous
sulfonation (i.e., sulfonation of the precipitated polymer/solvent co-
crystals) for
polyarylethersulfone (PES) polymers in chlorinated hydrocarbon solvents with a
sulfonating agent. This patent describes aromatic PES polymers as being
"notoriously difficult to sulfonate" due to the electron withdrawing effect of
the
sulfone linkages which deactivate the adjacent aromatic rings for
electrophilic
substitution. This patent further reports that sulfonation of these polymers
with
chlorosulfonic acid or oleum at ambient temperatures requires an enormous
excess
of sulfonating agent and results in a highly degraded product with the extent
of
sulfonation being impossible to control.
This patent further reports that once easily sulfonatable PEES units are
introduced to the PES polymer (PES/PEES copolymer), polymer sulfonation can
be successfully controlled. PES polymers of formula (PH-SO~-Ph-O) are known to
crystallize from a variety of solvents, such as CHzCl2, DMF, DMAc. The
crystallization is promoted by agitation or addition of non-solvents, and is
highly
dependent upon reaction conditions. Polymer/solvent intercrystallites (< 1
micron
diameter) form as a suspension in a surplus amount of the solvent. The amount
of
sulfonating agent added controls the degree of sulfonation. Temperatures are
indicated as being preferable from -10 to 25°C.
U.S. Patent No. 5,013,765 reports the controllable sulfonation of aromatic
PES polymers using sulfur trioxide (sulfonating agent) and sulfuric acid
(solvent).
This patent indicates that the sulfuric acid content must be less than 6% wt
(based
upon the solvent) and that the temperature must be <30°C in order to
control side
reactions and degradation. Chlorosulfonic acid and oleum as sulfonating agents
are discouraged since they can lead to an excessive degree of sulfonation
and/or
polymer degradation. Citing another source, this patent indicates that
controllable
sulfonation using these agents is not possible. This patent also reports that
use of
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sulfur trioxide in concentrated sulfuric acid can produce sulfonated polymers
without by-products or degradation products, especially in the case of O-Ph-
SOZ-
Ph based polymers.
The desirability of using sulfonated poly(aryl ether) polymers in
applications such as reverse osmosis has been described in the literature (D.
Lloyd
et al., Synthetic Membranes: Desalination, pp. 327-350 (1981)). This reference
reports that membranes employing such polymers are highly desirable and merit
further investigation. Though the chemical structure corresponding to
polyphenyl
sulfone (PPSU) polymer is listed (among others) in this reference as being a
potential candidate for sulfonation, no experimental evidence is provided on
the
sulfonation of PPSU.
Sulfonation of PPSU has been reported elsewhere (see e.g., O. Savadogo,
J. New Mat. Electrochem. Systems 1,47-66 (1998)). However, this publication,
in
addition to the references cited in connection with the discussion of PPSU,
are
equally devoid of experimental data showing sulfonation of this particular
polymer.
Despite the magnitude of research that has been conducted into the use of
sulfonated aromatic polymers as an alternative to perfluorinated ionomers, it
remains highly desirable to provide novel ion-conducting materials suitable
for
use in a broad range of electrochemical applications and to develop novel
processes of producing such materials.
It also would be highly desirable to develop an improved solid polymer
electrolyte membrane employing these novel ion-conducting materials. Such
membranes would be suitable for use in a variety of electrochemical
applications,
including hydrogen or methanol fuel cell systems.
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SUMMARY OF THE INVENTION
A central object of the present invention is to provide novel ion-
conducting materials suitable for use in a broad range of electrochemical
applications, including fuel cell systems.
In one embodiment of the present invention, these novel ion-conducting
materials are based on sulfonated polyaryletherketone (PEK) homopolymers or
PEK copolymers, or blends thereof. In another embodiment, these novel ion
conducting materials are based on sulfonated polyphenylsulfone (PPSU)
homopolymers or PPSU copolymers, or blends thereof.
The class of PEK and PPSU homopolymers, copolymers, and blends
thereof disclosed herein may include any desired substituents, provided that
such
substituents do not substantially impair properties desired for the intended
use of
the polymer, as may readily be determined by one of ordinary skill in the art.
Such properties may include ionic conductivity, chemical and structural
stability,
swelling properties and so forth. Accordingly, as used herein, PEK and PPSU
homopolymers, copolymers and blends thereof, include both substituted and
unsubstituted PEK and PPSU polymers.
Another object of this invention is to provide novel processes for
producing such ion-conducting materials.
Another object of the invention is to provide an improved solid polymer
electrolyte membrane that is resistant to methanol crossover when used in a
direct
methanol fuel cell.
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Another object of the invention is to substantially lower the overall cost of
producing solid polymer electrolyte membranes to allow for commercialization
of
SPEFCs.
One preferred ion-conducting material in accordance with the present
invention comprises at least one polyaryletherketone (PEK) homopolymer or at
least one PEK copolymer, or blends thereof (sometimes referred to hereinafter
as
"PEK based ion-conducting materials"), wherein the PEK homopolymer is
sulfonated and the ion-conducting material is devoid of ether-phenyl-ether
linkages.
In one particularly preferred embodiment of the present invention, the
PEK homopolymer comprises repeating units of the formula
0
/ \ o /-\
wherein the phenyl rings may be substituted or unsubstituted.
It is preferred that the PEK based ion-conducting materials have an IEC
from at least about 0.~ meq.; ~ to at least about 4 meq./g.
In applications where stability is important, the PEK based ion-conducting
materials may further comprise sulfone crosslinkages and/or antioxidants.
The PEK based ion-conducting materials of the present invention may be
halogenated, e.g., brominated or chlorinated, in order to further enhance
stability.
The PEK based ion-conducting materials of the present invention are
useful in the preparation of solid polymer electrolyte membranes.
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One preferred method of producing ion-conducting materials devoid of
ether-phenyl-ether linkages suitable for use in electrochemical applications
in
accordance with the present invention comprises: providing a solution of at
least
one PEK homopolymer; adding a sulfonating agent to the PEK homopolymer
solution to form a sulfonated PEK homopolymer; and isolating the sulfonated
PEK homopolymer from the solution. Preferred sulfonating agents for use in
this
method comprise at Least one of sulfur trioxide, concentrated sulfuric acid
and
fuming sulfuric acid. In same methods, it is preferred that the sulfonating
agent
has a free sulfur trioxide content of about 0 (about 100% sulfuric acid) to
about 30
wt.% (about 70% sulfuric acid).
It is preferred that the PEK homopolymer solution is maintained at a
reaction temperature from about 10°C to about 60°C.
One preferred method of isolating the sulfonated PEK based ion-
conducting material comprises re-precipitating the sulfonated PEK homopolymer
into water, water saturated with sodium chloride, methanol or other non-
solvent.
In other preferred methods, antioxidants are added to the sulfonated PEK
homopolymer, either prior to or following isolation of the sulfonated PEK
homopolymer. In yet another preferred method, the PEK based ion-conducting
materials are crosslinked, particularly when the ion-conducting material is in
the
H+ (acid) form.
In yet another preferred method of the present invention, the PEK based
ion-conducting material undergoes a halogenation step.
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Another ion-conducting material in accordance with the present invention
comprises at least one sulfonated polyphenyIsulfone (PPSU) homopolymer or
PPSU copolymer or blends thereof (sometimes referred to hereinafter as "PPSU
based ion-conducting materials").
One preferred PPSU based ion-conducting material comprises repeating
units of the formula
0
~S"'~"° 0 ~ °
0
wherein the phenyi rings may be substituted or unsubstituted.
The PPSU based ion-conducting materials of the present invention have an
IEC from at least about 0.~ meq.ig to about 4 meq./g. In some embodiments,
these materials further comprise sulfone crosslinkages and/or
antioxidants.
PPSU based ion-conducting materials of the present invention may be
used in the preparation of solid polymer electrolyte membranes. Such solid
polymer electrolyte membrane may contain suIfone crosslinkages.
One method of producing PPSU based ion-conducting materials of the
present invention which are suitable for use in electrochemical applications
comprises: providing a solution of a PPSU polymer; allowing the PPSU polymer
to precipitate from the solution; adding a suifonating agent comprising sulfur
trioxide to the solution to form a sulfonated PPSU polymer, wherein the sulfur
trioxide is diluted in solvent comprising a chlorinated hydrocarbon; and
isolating
the sulfonated PPSU polymer from the solution. Methylene chloride is one
preferred chlorinated hydrocarbon solvent for use in the present invention.
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In some embodiments, the method further comprises purifying the ion-
conducting material to remove overly sulfonated or degraded fractions of the
PPSU based ion-conducting material. This is accomplished in one preferred
method by re-dissolving the sulfonated PPSU polymer in a solvent, and re-
precipitating the sulfonated PPSU polymer into water, water saturated with
sodium chloride, methanol or other non-solvent.
The method of forming PPSU based ion-conducting polymers of the
present invention may further comprise crosslinking the ion-conducting
material,
especially when the ion-conducting material is in the H+ form.
The preferred methods may also comprise adding antioxidants to the
sulfonated PPSU polymer, either before or following isolation of the
sulfonated
PPSU polymer. Yet other preferred methods include halogenating, e.g.,
chlorinating or brominating PPSU based ion-conducting materials.
The foregoing and other objects, features and advantages of the invention
will become better understood with reference to the following description and
appended claims.
DETAILED DESCRIPTION
The present invention relates to novel ion-conducting materials which may
be used to produce solid polymer electrolyte membranes (SPEMs). These
materials could also be used in composite SPEMs comprising a substrate polymer
and ion-conducting material component.
The novel ion-conducting materials and SPEMs of the present invention
are designed to address the present shortcomings of today's leading solid
polymer
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electrolyte membranes, e.g., Nafion~ and other Nafion~-like membranes, such as
Gore-Select~.
Materials and membranes of the present invention may be used in a host of
electrochemical applications, including but not limited to, polarity-based
chemical
separations; electrolysis; fuel cells and batteries; pervaporation; reverse
osmosis -
water purification, gas separation; dialysis separation; industrial
electrochemistry,
such as choralkali production and other electrochemical applications; water
splitting and subsequent recovery of acids and bases from waste water
solutions;
use as a super acid catalyst; use as a medium in enzyme immobilization, for
example; or use as an electrode separator in conventional batteries.
Innovative processes for producing ion-conducting materials of the present
invention are also disclosed. These processes can be tailored to produce solid
polymer electrolyte membranes useful over a range of operating conditions
and/or
applications.
Such processes generally comprise providing a solution of a
polyaryletherketone (PEK) homopolymer, PEK copolymer or blends thereof, or a
polyphenyl sulfone (PPSU) homopolymer, PPSU copolymer or blends thereof;
adding a sulfonating agent to the polymer solution; and isolating the
sulfonated
polymer from the solution. Post-processing steps (i.e., purification, cross-
linking,
use of antioxidants, chlorination or bromination of the aromatic polymer
backbone) may be employed to enhance or improve the stability of the ion-
conducting material and the membranes comprising these materials.
In one embodiment of the present invention, the ion-conducting material
comprises a sulfonated PEK homopolymer, PEK copolymer or a blend thereof.
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One preferred PEK homopolymer has the repeating unit shown below:
0
/ \ o /~\
The phenyl rings of the PEK homopolymer may be substituted or
unsubstituted.
It is a feature of the claimed invention that the PEK hompolymer, PEK
copolymer or blends thereof be suIfonated in the absence of ether-phenyl-ether
linkages (e.g., in the absence of PEK,PEEK copolymerization).
This is achieved, in part, by increasing the sulfonation power of the
sulfonating solution, while preventing significant degradation of the polymer.
Such innovation is in contrast to the suppositions of the prior art, e.g.,
that
~5 aromatic groups adjacent to the protonated ketones can not be sulfonated to
a
useful degree.
In one preferred embodiment, PEK homopolymer, e.g., available from
Victrex LSA, is dissolved in concentrated sulfuric acid. Slow addition of
oleum
reacts with the water present in the solution producing a more concentrated
form
of sulfuric acid solvent that eventually contains free SO, (after all the
water has
reacted). Further addition of oleum increases the sulfonating power of the
solution. (The water content is lowered, thereby increasing the sulfonating
power
of the solution.) Care must be taken to avoid excessive S03 content or
overheating of the solution in order to minimize polymer degradation. Overhead
stirring of the solution and a room temperature water bath are highly
recommended.
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Maintenance of the concentration of the reactant acid solution is critical.
As these solutions are hygroscopic, they tend to change concentration due to
moisture absorption with every exposure to air. Careful monitoring of their
density can be used to check concentration. Titration may be used to monitor
concentration.
The sulfonated PEK based ion-conducting product can be isolated by
precipitation into water provided that the extent of sulfonation has not made
it
water soluble (methanol / water or water saturated with salt may also provide
for a
high degree of sulfonation). Longer reaction times or a larger excess of
sulfur
trioxide will cause more sulfonation.
Preferable temperature ranges for the reaction are as follows: 0-
80°C
including chilling the initial PEK polymer solution before, after, or during
the
addition of the oleum. However, since pure H,SO4 solutions will freeze at
approximately 10°C, it is preferable to maintain a temperature between
10-60°C.
Other preferred reaction conditions include using a free SO, content of
between about 0 (about 100% sulfuric acid) to about 30 wt.% (about 70%
sulfuric
acid). Optimal content is dependent upon other reaction conditions (i.e.,
temperature), but generally S03 content is preferred to be between 1-25wt.%.
The final concentration of polymer depends upon the starting sulfuric acid
concentration, the free S03 level desired, and the concentration of fuming
sulfuric
acid added. Generally preferred concentration ranges include the following:
l Owt.% polymer (~S-30wt.%) in concentrated sulfuric acid (preferably 80-100%,
more preferably 90-99%).
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Oleum (SO,/H,SO~) is the preferred SO, source for sulfonating the PEK
homopolymer, PEK homopolymer, copolymer or blends thereof. A free SO3
content is preferably between 10-60wt.°,%, more preferably
approximately 20-
30wt.%. However, pure SO, also may be used under the appropriate conditions.
The afore-mentioned reaction conditions may be tailored to produce
sulfonated PEK based ion-conducting materials having IECs from at least about
0.5 to at least about 4 meq./g.
In accordance with the present invention, ion-conducting materials
comprising sulfonated PEK homopolymers, copolymers or blends thereof, can be
isolated via direct precipitation into water (or salt water) depending on the
level of
sulfonation.
'Though the peroxide stability of sulfonated PEK may not be sufficient for
long-term use in fuel cells, many electrochemical applications do not require
hydrolytic stability at this level. For example, dialysis, electrodialysis or
electrolysis systems, pervaporation or gas separation systems, or for water
splitting systems for recovering acids and bases from waste water solutions,
or as
an electrode separator in a battery.
In another preferred embodiment of the present invention, the ion-
conducting material comprises at least one of a suIfonated PPSU homopolymer,
PPSU copolymer or blends thereof. This polymer has the repeating unit shown
below:
0
s~ o~~ o
0
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The phenyl rings of the PPSU polymer may be substituted or
unsubstituted.
The prior art teaches that PEES units sulfonate readily while PES units are
very difficult to sulfonate (see e.g., U.S. Patent No. 5,013,765). Based on
the
chemical structure of the PPSU polymer, it was anticipated that sulfonation of
PPSU could be achieved using a sulfuric acid (solvent)/sulfur trioxide
(sulfonating
agent) system. Our results indicate that controllable sulfonation of PPSU is
not
possible with a sulfuric acid solvent.
Another method was investigated using a combination of DMF/sulfur
trioxide. This method also was found to be unsuccessful in producing a
sulfonated PPSU polymer.
Still another method was investigated using a sulfonation process
employing chlorosulfonic acid (solvent)/sulfur trioxide (sulfonating agent).
See
e.g., U.S. Patent No. 4,413,106. This method was also found to be
unacceptable.
It was found in the present invention that controllable sulfonation of PPSU
homopolymers, copolymers and blends thereof, can be achieved using a
methylene chloride (solvent)/ sulfur trioxide (sulfonating agent) system.
One preferred process of the present invention for preparing PPSU based
ion-conducting materials is described as follows: the polymer is first
dissolved in
a suitable solvent, preferably methylene chloride or another non-reactive
solvent,
e.g., halogenated hydrocarbons or nitrobenzene and the like. The polymer
solution is allowed to stir until the polymer precipitates and a slush-like
suspension is formed. Sulfonation is performed by admixture of the PPSU
polymer suspension and a sulfonating agent, preferably, sulfur trioxide. The
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amount of sulfonating agent added controls the degree of sulfonation.
Preferably,
temperatures are maintained between -10 to 25°C.
The methods for preparing PPSU based ion-conducting materials of the
present invention may be further optimized by additional dilution of the
polymer
solution with solvent and extending the addition time of the sulfonating
agent.
In accordance with the methods of the present invention, addition of sulfur
trioxide to the slush-like suspension did not cause agglomeration during
sulfonation. Rather, the reaction product was maintained as a suspension
(e.g., a
very finely dispersed powder) which was recovered by filtration. The product
was
then washed with solvent, e.g., methylene chloride, and air dried.
Surprisingly,
we discovered that use of CLSO,H/CH,CIz was not compatible with such a
suspension.
Generally preferred concentration ranges for the PPSU based ion-
conducting materials of the present invention include the following: S-30wt.%,
preferably 10-20wt% (initial polymer solution, before dilution of any excess
solvent).
This method can be used to produce highly sulfonated polymeric films
(IEC of at least about 0.5 meq./g to at least about 4.0 meq./g), with equally
high
conductivity {IC of at least about 0.01 to about 0.5 S/cm) and minimal
polymeric
degradation.
Some heterogeneity of the sulfonated PPSU based ion-conducting
polymers may result from the sulfonation procedure. Heterogeneity may not be
severe enough to cause the polymer to precipitate during the sulfonation
procedure, but some distribution may exist as the polymer crystallites
sulfonate
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from the outside in. That is, some portions of the sulfonated polymer may be
so
highly sulfonated as to be water soluble (possibly leaching out during fuel
cell
operation) or otherwise degraded.
Various purification methods can be employed to effectively remove
overly sulfonated or degraded fractions of these polymers. In a preferred
embodiment, the purification procedure involves re-dissolving the ion-
conducting
polymer in a suitable solvent (e.g., NMP), and re-precipitating it into water
or
saturated NaCI solution. Re-precipitation of the ion-conducting polymer into
saturated salt solution has been shown to result in a high yield of the ion-
conducting polymer, while removing polymer that has been excessively
sulfonated or degraded.
Additionally, there are several methods to improve the long term stability
(aqueous hydroperoxide radical) of the ion-conducting materials of the present
invention.
For example, the stability of the ion-conducting materials of the present
invention may be enhanced by several post-processing steps. These steps
include
the following: (i) cross-linking the ion-conducting polymer in the H+ form to
develop sulfone cross-links; (ii) addition of small amounts of antioxidants
(insoluble) into the ion-conducting polymer; and (iii) chlorination /
bromination of
the ion-conducting polymer backbone, thereby reducing degradation sites.
Crosslinking methods can provide or enhance peroxide stability.
Additionally, various procedures are described in the literature wherein
sulfonated
polymers can be crosslinked to further enhance the barner properties of the
ion-
conducting polymer. See e.g., U.S. Patent No. 5,795,496; Kerres, et al., "New
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Ionomers and their Applications in PEM Fuel Cells", ICE, Stuttgart, Germany
(1996); and Kerres, et al., J. Membrane Sci. I39:2I I-225 (1998).
For example, U.S. Patent No. 5,795,496 describes a method of
crosslinking ion-conducting polymers via the SO,H groups (sulfonic acid
groups)
to form sulfone crosslinks between polymer chains. This method entails
sulfonating the polymer (e.g., PEEK) using concentrated sulfuric acid, casting
of a
film, then heating the film to a temperature of I20°C under vacuum. It
is the
heating step which causes the crosslinking to occur.
Although the enhanced barrier properties provided by crosslinking, the
crosslinking procedure results in decreased ionic conductivity, water
adsorbtion
and swelling of the polymer. However, adjustments can be made to the
crosslinking procedures employed in order to minimize the sacrifice of ionic
conductivity.
The use of additives can also provide or enhance peroxide stability of the
PEK and PPSU based ion-conducting materials of the present invention. Polymer
additives can be used as radical scavengers within the ion-conducting
component
of the ion-conducting material or SPEM. Examples of these include Irganox I
135
(Primary Phenolic Antioxidant, commercially available from Ciba Geigy) and
DTTDP (Di(tridecyl) Thiodipropionate, Secondary Antioxidant, commercially
available from Hampshire).
PEK and PPSU based ion-conducting materials of the present invention
may be used to produce SPEMs using standard membrane casting techniques.
Such techniques are well known to the skilled artisan. For example, films can
be
made by dissolving the polymer in a suitable solvent and casting onto a glass
plate
(or other surface). More specifically, the ion conducting polymer can be
dissolved
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in n-methyl pyrrolidone (NMP), filtered, and then cast onto the substrate. The
films are slowly evaporated in a low humidity chamber, then dried in a vacuum
oven to fully densify. Once dry the films can be removed from the casting
substrate by immersion in water.
The PEK and PPSU based ion-conducting materials and SPEMs of the
present invention have limited methanol permeability (limited methanol
diffusivity and solubility) even at elevated temperatures and pressures, are
substantially chemically stable to acids and free radicals, and
thermally/hydrolyticalIy stable to temperatures of at least about
100°C.
The PEK and PPSU based ion-conducting membranes of the present
invention have an ion-exchange capacity (IEC) of >I .Omeq/g dry membrane
(preferably, 1.5 to 2.Omeq/g) and are highly ion-conducting (preferably, from
about 0.01 to about O.SS/cm, more preferably, to greater than about 0.1 S/cm
or
<IOS2cm resistivity).
Ion-conducting materials and SPEMs of the present invention are durable,
substantially defect-free, and dimensionally stable (less than about 20%
change in
dimension wet to dry), preferably even above temperatures of at least about
100°C.
Particularly preferred PEK and PPSU based ion-conducting materials have
the ability to survive operation in fuel cells (i.e., HZ/O2, methanol) for at
least
about 5000 hours (e.g., automotive applications).
The PEK and PPSU based ion-conducting materials for use in the present
invention may be substituted or unsubstituted. These materials may be present
as
homopolymers, copolymers, or other blends. The class of PEK and PPSU
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homopolymers, copolymers, and blends thereof disclosed herein may include any
desired substituents, provided that such substituents do not substantially
impair
properties desired for the intended use of the polymer, as may readily be
determined by one of ordinary skill in the art. Such properties may include
ionic
conductivity, chemical and structural stability, swelling properties and so
forth.
The utility of blending polymers to form a PEK and PPSU based ion-
conducting material of the present invention is in optimizing each of their
properties. Unlike simple mixing, blending does not create a composite
material
with two dispersed components. Rather, the blend is uniform in composition
throughout. It may be useful to blend a sulfonated polymer with an
unsulfonated
one to optimize swelling, fuel crossover resistance, conductivity, peroxide
resistance, hydrolytic stability and the like. Similarly, the blending of two
sulfonated polymers might allow improved properties over each individual
component. This concept may also be employed to improve the strength, cost,
processability or stability of the ion-conducting materials of the present
invention.
Utilizing the innovative PEK and PPSU based ion-conducting materials
and membranes of the present invention, relatively low cost ion-conducting
materials of the present invention may be produced which exhibit improved
power
density and reduced sensitivity to carbon monoxide in hydrogen fuel.
Ion-conducting materials of the present invention also alleviate water
management problems which limit the efficiency of present Nafion~ membrane-
based fuel cells.
Ion-conducting materials of the present invention exhibit resistance to
degradation and hydrolysis, as well as resistance to stress-induced creep.
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Ion-conducting materials of the present invention exhibit high ionic
conductivity, high resistance to dehydration, high mechanical strength,
chemical
stability during oxidation and hydrolysis, low gas permeability to limit
parasitic
losses, and stability at elevated temperatures and pressures.
Ion-conducting materials of the present invention are resistant to methanol
crossover when used in a direct methanol fuel cell. This results in high
efficiency
and improved cell performance.
Ion-conducting materials of the present invention utilize relatively low
cost starting materials and fabrication methods, and therefore can be produced
at a
fraction of the cost of those membranes employing Nafion~ and Nafion~-like
ionomers.
Ion-conducting materials of the present invention of the present
invention act as a barrier against reactants (Hz, O, and methanol permeation)
in fuel cell applications.
Surfactants or surface active agents having a hydrophobic portion and
hydrophilic portion may be utilized in producing ion-conducting materials of
the
present invention of the present invention. These agents are well known in the
art
and include Triton X-100 (commercially available from Rohm & Haas of
Philadelphia, PA).
Compatibilizers may also be employed in producing membranes of the
present invention. As used herein, "compatibilizers" refer to agents that aid
in the
blendability of two or more polymers that would otherwise be resistant to such
blending. Examples include block copolymers containing connecting segments of
each component.
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The PEK and PPSU based ion-conducting materials, SPEMs and methods of
the present invention will be further illustrated by the Table and Examples
below.
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C
w
o ,= o o y
;a v v
A .~..U "~ ,
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r o o c
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o F' a 4:, a'\o4-....
Ua z w ~ z '~ z
c
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C
v
U
N
M M 00
a a N O O
~ C O C
OG
~
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tn 'a O O O
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a
O O M
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U
_v~
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p f "" W o m o a
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A .- $ ~
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z v ~ H
z
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EXAMPLES OF THE INVENTION
Commercial Availability of Starting Materials
Radel~ polyphenyl sulfone polymer is commercially available from
Amoco Polymers.
Victrex~ poly (aryl ether ketone) polymer is commercially available from
Victrex USA.
General Procedures:
The following procedures were employed in the fabrication and testing of
samples that were prepared in accordance with the materials, membranes and
methods of this invention.
IEC PROCEDURE:
1. Cut out pieces of sulfonated films (target weight 0.2g, target film
thickness
2 mils).
2. Vacuum dry films at 60°C, record dry weights and note if films are
in H+
or Na+ form.
3. Boil deionized water in separate beakers on hotplate.
4. Place films into boiling water.
5. Boil films vigorously for t/2 hour.
6. Prepare I.SN HZS04.
7. Place films into HZSO, and soak for I/2 hour.
8. Remove films and rinse with deionized water.
9. Boil in deionized water again. Repeat until the films have soaked in H,S04
three times.
10. Remove films from boiling water, pat film surface with paper towel, and
rinse carefully with deionized water.
11. Place films in another beaker of water and check for pH.
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12. Continue to rinse the films with water until pH is neutral to remove any
excess acid trapped in the folds of the film.
13. Prepare saturated NaCI solution.
14. Boil the NaCI solution, pour into screw cap vials, add film and cap.
1 S. Place capped vials with film into water bath at 90°C for 3 hours.
16. Remove capped beaker from water bath and cool to room temperature.
17. Remove the films from NaCI solution by pouring the salt solution into
another beaker (save), wash the films with deionized water (save all
washings - they will be used for titration).
18. Titrate the NaCI solution with O.1N NaOH.
19. Take the films, pat with paper towel and take wet weight. (Use wet weight
to determine water content of films.)
20. Dry the films under vacuum at 60°C until constant weight.
21. Take dry weight and use this to calculate IEC.
PEROXIDE TEST PROCEDURE:
22. Place films into the H+ form by following steps 1-12.
23. Make peroxide solution by adding 4ppm Fe to 3% hydrogen peroxide
(28.Smg of ammonium iron (II) sulfate hexahydrate per liter of peroxide
obtained from Aldrich).
24. Place the peroxide solution into water bath at 68°C.
25. Add films to the peroxide solution already at 68°C.
26. Peroxide test for 8 hours and record film properties (mechanical, color,
handling etc.).
27. If film passes, remove from peroxide, rinse with water to remove all
traces
of peroxide solution.
28. Follow steps 13-21 to obtain post-peroxide test IEC.
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CROSSLINKING PROCEDURES
Ion-conducting polymeric samples can be crosslinked in the acid (H+)
form to improve ICP stability. Normally, crosslinking is performed in vacuum,
to
exclude oxygen from the system (which can cause ICP charring). The vacuum
oven should be preheated to temperatures of at least about 200°C. The
ICP
sample is then heated in the vacuum oven for a prolonged period of time. The
ICP sample should be tested before and after crosslinking for IEC and peroxide
stability in order to evaluate long term membrane stability. See e.g., Example
5
below.
FILM FABRICATION PROCEDURES:
Film Castins.
Ion-conducting materials of the present invention may be used to produce
SPEMs using standard membrane casting techniques. Such techniques are well
known to the skilled artisan. For example, films can be made by dissolving the
polymer in a suitable solvent and casting onto a glass plate (or other
surface).
More specifically, the ion conducting polymer can be dissolved in n-methyl
pyrrolidone (NMP), filtered, and then cast onto the substrate. The films are
slowly evaporated in a low humidity chamber, then dried in a vacuum oven to
fully densify. Once dry the films can be removed from the casting substrate by
immersion m water.
SULFONATION PROCEDURES:
Sulfonation Procedure I:
Aromatic PES polymers can be sulfonated to controlled degrees of
substitution with sulfonating agents. The degree of substitution is controlled
by
the choice of and mole ratio of sulfonating agent to aromatic rings of the
polymer,
by the reaction temperature and by the time of the reaction. This procedure
offers
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a method for carrying out sulfonation in a heterogeneous manner, i.e.,
sulfonation
of precipitated polymer crystals.
The polymer (preferably a polyethersulfone) is first dissolved in the
appropriate solvent (preferably methylene chloride} and then allowed to
precipitate into a fine crystalline suspension. SuIfonation is carned out by
simple
admixture of the suspension with a sulfonating agent. Suitable agents include
chorosulfonic acid and, preferably, sulfur trioxide (Allied chemicals
stabilized
Sulfan B~ in CHZCIz). The sulfonating agent used should be in sufficient
proportion to introduce a number of sulfonate groups onto the polymer that is
within the range of between 0.4:1 to 5:1 per polymer repeat unit, although
this is
not critical. The temperature at which sulfonation takes place is critical to
limiting the side reactions but varies with the type of polymer (a preferable
temperature is within the range of from -50° to 80°C, preferably
-10° to +25°C).
When the desired degree of sulfonation has been reached, the sulfonated
polymer may be separated from the reaction mixture by conventional techniques
such as by filtration, washing and drying.
The polymer products of the process of the invention may be neutralized
with the addition of a base, such as sodium bicarbonate, when desired and
converted to the alkali salts thereof. The alkali salts of the polymer
products of
the invention may be used for the same purposes as the parent acid polymers.
See e.g., U.S. Patent 4,413,106.
Sulfonation Procedure II:
Concentrated sulfuric acid is used as the solvent in this procedure. The
content of the sulfonating agent, sulfur trioxide, is based on the total
amount of
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pure (100% anhydrous) sulfuric acid present in the reaction mixture, and is
kept to
a value of less than 6% by weight throughout the entire sulfonation. The
sulfur
trioxide may be mixed in dissolved form (oleum, fuming sulfuric acid) with
concentrated sulfuric acid. The concentration of the starting sulfuric acid
and
oleum were determined by measuring their density immediately before use in the
reactions.
The temperature of the reaction mixture is kept at less than +30°C
throughout the reaction. The sulfonation procedure is stopped with the
addition of
water to the reaction mixture or by pouring the reaction mixture into water.
More specifically, the polymer is first dried in high vacuum at room
temperature to constant weight, then dissolved in concentrated sulfuric acid.
Oleum is then added drop-wise over a period of hours with constant cooling
below +30°C, and with stirring. When all of the oleum has been added,
the
reaction mixture is stirred for a further period of hours at the same
temperature.
The resultant viscous solution is then run into water and the precipitated
polymer
is filtered off. The polymer is then washed with water until the washings no
longer are acidic, and it is then dried.
If these conditions are maintained, a controllable sulfonation of aromatic
polyether sulfones is possible and polymer degradation can be substantially or
completely prevented.
Though less preferred, another variation of this procedure is to add the
sulfur trioxide either in pure solid state or in gaseous state to a solution
of the
polymer in concentrated sulfuric acid.
See e.g., U.S. Patent 5,013,765.
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ION-CONDUCTING MATERIALS - TESTING METHODS
IEC. % Water Content, Membrane DeEradation:
During this procedure, films are immersed in distilled HZO and boiled for a
period of 30 minutes. The films are then placed in a solution of 1.SN HZS04 at
room temperature and soaked for a period of 30 minutes. This is repeated three
separate times to ensure proper H+ ion exchange into the membrane. Films are
rinsed free of acid (pH of rinse water > S.0) and placed into separate
beakers, each
filled with a saturated solution of NaCI. The salt solution is boiled for a
period of
three hours. The films, which are now in the Na+ form, are removed from the
salt
solution, rinsed with distilled water and padded to remove excess water. Now a
wet weight and thickness of the sample are measured. While in the Na+ form,
the
films are dried in an air oven at a temperature of 60°C. The dry weight
and
thickness of the films are measured and the percent water content is
calculated.
The salt solutions are titrated with 0.1 N NaOH to a phenolphthalein endpoint
and
IEC dry (meq/g) values calculated.
Ionic Conductivity:
Transverse ionic conductivity measurements are performed on film
samples in order to determine the specific resistance (ohm*cm'). Prior to the
ionic
conductivity measurements, film samples are exchanged into the H+ form using
the standard procedure discussed above. To measure the ionic conductivity, the
film samples are placed in a die consisting of platinum-plated niobium plates.
The
sample size tested is 25cm'. Prior to assembling in the measuring device,
platinum black electrodes are placed on each side of the film sample to form a
membrane-electrode assembly (MEA). To insure complete contact during the
resistivity measurement, the MEA is compressed at 100 to S00 psi between two
platinum-plated niobium/stainless steel rams. The resistance of each f lm is
determined at 1000 Hz, 1 to SA, with a four point probe resistance measuring
device and converted to conductivity by using formula 1.
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( 1 ) C = t/RxA
Where: C =Conductivity (S/cm)
R =Sample Resistance (ohm)
t =Wet sample thickness (cm)
A = Sample area (cm2)
Measurements are converted to specific resistance by calculating the ratio
of thickness over conductivity (ohm*cm2).
Membrane Degradation.
Accelerated degradation testing is carned out using 3% H,O, solution with
4 ppm Fe++ added as an accelerator. The films are tested for a period of 8
hours at
a temperature of 68°C. The percent degradation of IEC was measured in
the film
samples after the test. After 8 hours, the films are removed from solution,
and re-
exchanged using 1.5 N HzS04. The IEC is recalculated, and the test result
expressed as the % loss in IEC. This test simulates long term (several
thousand
hours) of actual fuel cell operation. For H,O2, fuel cells, <10% IEC
degradation in
this test would be considered acceptable.
Example 1
Sulfonation of Radel R~ Using Sulfan B~ (100% and 150% Sulfonation)
Sulfonation Procedure I was used in the following example.
Two separate 1000 ml 3 neck resin kettles (with ribs) equipped with an N,
inlet, addition funnel, and overhead stirrer were charged with the following
reactants: 340 ml of dichloromethane and 50.00 grams of Radel R~ Polyphenyl
Sulfone Polymer (beads). These mixtures were stirred until solutions formed
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(approximately .25 hours). Once solutions formed, they were cooled in ice
baths
to about a temperature of 0°C (ice bath was maintained throughout the
duration of
the addition and reaction). (Note that Radel R~ was dried at 70°C under
full
dynamic vacuum for about 12 hours to remove any adsorbed moisture.)
While the above solutions were cooling, the following amounts of Sulfan
B~ were combined with dichloromethane in two separate 125 ml addition
funnels. In funnel #1 (100% sulfonation) 10.00 grams of Sulfan B~ was
combined with 120 ml of dichloromethane. In funnel #2 (150% sulfonation)
15.00 grams of Sulfan B~ was combined with 120 ml of dichloromethane.
As polymer solutions were cooled, the polymer precipitated from solution
to form a viscous paste. To each of these polymers approximately 350 ml of
dichloromethane was added to aid in the uniform mixing of the suspensions. The
diluted suspensions were then cooled to ice bath temperatures once again.
To the rapidly stirnng cooled and diluted polymer suspension, the Sulfan
B~ solutions were added drop-wise over a period of 3.5 to 4.0 hours.
Upon completing the addition of the Sulfan B~ /dichloromethane solution,
the reaction mixtures were permitted to stir at ice bath temperatures for
another
2.5 hours, then the reaction was stopped by adding approximately 10 ml of
deionized water to each of the reaction mixtures.
The reaction mixtures (white dispersions) were recovered by filtration
using a glass frit. Products (white powder) were washed 3X with 100 ml
portions
of dichloromethane. The washed products were then permitted to air dry in the
hood. 20% solutions of the dried products were made in NMP and cast on soda
lime glass plates. The freshly cast films were left to stand in a dry box with
a
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relative humidity of less than 5% for a period of 24 hours. The cast films
were
heated at 70°C under full dynamic vacuum for an hour prior to floating
the films
off with deionized water. The floated films were then permitted to air dry
overnight.
The 100% and I50% sulfonated products swell greatly in water and
become opaque, but when films are dry they shrink and become clear once again.
The mechanical strength of these films allows creasing while resisting
tearing.
Films of the 100% and I50% products are not soluble in boiling water, and
under
these conditions also maintain their mechanical properties.
IEC: I00% sulfonated Radel ROO unpurified = 1.39 meq./g
I 50% sulfonated Radel R~ unpurified = I .58 meq./g
These polymers were further purified by dissolving in NMP (at 20wt.%)
and then precipitated into a large excess of saturated salt water. The
resulting
polymers were soaked in sodium bicarbonate, washed several times with water,
then dried under vacuum 0100°C}. These polymers were also cast into
films as
described above for characterization.
IEC: 100% suIfonated Radel R~ purified = 1.26 meq./g
150% sulfonated Radel R~ purified = 1.44 meq./g
Water Pick-up (wt.%):
100% sulfonated Radel R~ purified = 56%
I50% sulfonated Radel R~ purified = 110%
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Example 2
Sulfonation of Radel R~ Using Sulfan B~ at 200% Stoichiometric'Sulfonation
Procedure:
This sulfonation reaction was run very similarly to those previously
described in Example 1 (100, 150% suIfonations with Sulfan B~), with the
adjustments noted below:
- After the precipitation of the polymer from the initial dichloromethane
solutions, only 200m1 of new solvent was added to enhance stirring (in
Example 1, an additional 350m1 was used).
- Previous data suggests that not all of the SO, reacts with the polymer over
6
hours at 0-S°C. Therefore, one reaction was carried out at ice bath
temperatures for 8 (or more) hours and then allowed to warm to room
temperature. Although the resulting sulfonated Radel R~ was darker in color
than the batch that was quenched after only 6 hours, the polymer was still
water insoluble and showed good film properties. The properties of these
ICPs were:
200% Sulfonated Radel R (quenched after 6 hrs at 0-5°C):
IEC = 1.67 meq/g, Water Pick-up = 144%, Conductivity = 0.073 S/cm
200% Sulfonated Radel R (quenched after allowed to warm up):
IEC = 1.90 meq/g, Water Pick-up = 174%, Conductivity = 0.091 S/cm
- A similar reaction was carried out in which the SO, was allowed to react
longer, but was kept cold (0-5°C) throughout the reaction. The product
isolated from this reaction had an IEC value (I.7lmeq./g) which was between
the two batches described above.
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Note: Percent sulfonation in terms of stoichiometry, e.g., 200% sulfonation,
refers
to the use of a particular excess per each polymeric repeat unit, e.g., two
moles in
the case of 200% sulfonation.
Example 3
Sulfonation of Radel R Using Sulfan B at 220% Stoichiometric Sulfonation
Procedure:
Two sulfonation experiments were run according to Examples 1 and 2 with the
adjustments noted below:
- After the precipitation of the polymer form the initial solution, 350m1 of
dichloromethane was added to aid in stirring.
- 22g of Sulfan B was combined with approximately 115m1 of dichloromethane
(con:esponding to 220% sulfonation of the monomer unit). This solution was
added drop wise to each polymer suspension (previously chilled in an ice bath)
over the coarse of 4 hours.
- After addition, the reactions were kept packed in ice for approximately 10
hours and then allowed to warm to room temperature.
- After quenching with a few milliliters of water, each was filtered and then
washed with dichloromethane, as well as tetrahedrofuran.
- The sulfonated polymers were dried overnight, dissolved in NMP (at 20wt.%)
and then precipitated into a large excess of distilled water, followed by
soaking in sodium bicarbonate solution (to convert to the sodium salt form).
After neutralization, the gelatinous polymer precipitate was rinsed several
times with deionized water.
- One reaction (Sample A) was filtered and allowed to dry. The second reaction
(Sample B) was added to approximately 4 liters of acetone. The acetone
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caused the collapse of the water swollen polymer which was then isolated by
filtration. It also caused a significant portion to dissolve, which was
isolated
by drying of the acetone / water mixture (Sample B-1 ). Samples B and B-1
contained approximately the same amount of polymer, but showed varying
properties. In particular, the sulfonated polymer that was not extracted into
acetone showed a slightly lower IEC, and a significantly lower water up-take.
220% Sulfonated Radel R:
Sample A: IEC = 2.08 meq./g, water pick-up = 306wt.%, IC = 0.179 (S/cm)
Sample B: IEC = 1.82 meq./g, water pick-up = 128wt.%, IC = 0. I55 (S/cm)
Sample B-1: IEC = 2. IZ meq./g, water pick-up = 432wt.%, IC = 0.132 (S/cm)
Example 4
Sulfonation of Victrex ~ Poly(Ether Ketone) Using HZS04 / SO,
Sulfonation Procedure II was used in the following example.
Procedure:
30.OOg PEK polymer was dissolved in 270g of concentrated sulfuric acid
(93.Swt.%) under nitrogen, stirred by an overhead mechanical stirrer. The
polymer was dispersed over several days to form a dark red thick solution.
176g of this solution was left in the three neck flask with overhead stirrer,
N2, etc. To the flask, 208.4g of fuming sulfuric acid (25.Swt. % free S03) was
added over the coarse of a few minutes with constant stirring to raise the
solution
to a free SO, content of 2wt. %. The resulting solution was immersed in a room
temperature water bath to control the temperature.
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Samples were removed after approx. 1 hour, 3 hours, and 16 hours, and
quenched into deionized water to precipitate.
In order to make films, the I and 3 hour products were washed several
times with deionized water, soaked overnight in approximately O.SM NaOH
solution, then washed until a neutral pH was achieved. These were blotted dry
and placed in the vacuum oven overnight at 50°C. Dried samples were
dissolved
in NMP to make a 20wt. % solution. This required heating overnight at
60°C.
Films of approx. 6 mils were cast onto a freshly cleaned glass plate. After
two
days of drying the films were removed by immersion into deionized water.
Soaking the films in water (at room temperature) caused considerable
swelling to give a hazy gel-like consistency, but the 1 hour and 3 hour
samples did
not dissolve. Film of the I hour product could be hydrated and dehydrated,
while
maintaining resistance to tearing. The I hour sulfonated PEK film IEC was
measured to be 2.3meq/g.
Example 5
Crosslinking of Sulfonated PPSU
Ion-conducting polymeric samples can be crosslinked in the acid (H+)
form to improve ICP stability. Normally, crosslinking is performed in vacuum,
to
exclude oxygen from the system (can cause ICP charring). For example, SPPSU
was crosslinked in a vacuum oven preheated to temperatures of 200, 225 and
250°C for durations of up to 8 hours. Under these conditions, samples
showed a
slight IEC Ioss (~IO%), and little improvement in Iong term stability
(peroxide
test). More severe conditions were employed by exposing samples to
250°C in
full vacuum for more than 20 hours. Peroxide testing did not show any
considerable difference between SPPSU crosslinked films and SPPSU controls
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until heated for at least 32 hours. The SPPSU films crosslinked at
250°C for 32
hours and 72 hours maintained their film integrity during the peroxide
accelerated
life test. The IEC of these test samples decreased significantly.
Specifically, a
loss of 63% (1.90 to 0.69 meg/g) for the 32 hour sample and a loss of 73%
(1.90
meg/g to 0.51 meq./g) for the 72 hour crosslinked SPPSU films was calculated.
It is anticipated that many of the SO,H acid groups form aromatic sulfone (Ar-
SO,-Ar) crosslinks between polymer chains. This trend confirms that
crosslinking
(H+ form) of sulfonated polymers can be used to improve long term membrane
stability.
