Polymer electrolyte membranes from mixed dispersions

Details Polymer electrolyte membranes from mixed dispersions Polymer electrolyte membranes from mixed dispersions

1999-06-18

2001-08-21

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S309000, C521S027000

Reexamination Certificate

active

06277512

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to ion conducting polymer electrolyte membranes (PEMs) comprising an intimate mixture of an ionomeric polymer and a structural film-forming polymer and methods of producing such membranes from mixed dispersions of the polymeric components.
BACKGROUND OF THE INVENTION
Electrochemical devices, including proton exchange membrane fuel cells, sensors, electrolyzers, chlor-alkali separation membranes, and the like, are typically constructed from a basic unit called a membrane electrode assembly (MEA). In an MEA of a typical electrochemical cell, an ion conducting polymer electrolyte membrane (PEM) is in contact with a cathode and an anode, and transports ions that are formed at the anode to the cathode, allowing current to flow in an external circuit connecting the electrodes. The PEM is a solid electrolyte which must conduct the appropriate ions and should not conduct electrons or allow passage of reactant gasses. In addition, the PEM should have the necessary structural strength under operating conditions.
Commonly used PEM materials include Nafion™ polymers (DuPont Chemicals, Inc., Wilmington, Del., available from ElectroChem, Inc., Woburn, Mass., and Aldrich Chemical Co., Inc., Milwaukee, Wis.) which are perfluorocarbon polymers having attached —SO
3
−
anion groups. However, membranes of Nafion alone are inherently weak, especially as the level of hydration increases. Higher strength may be achieved by increasing the thickness of the membrane or by raising the equivalent weight, but at the cost of higher ionic resistance.
One means of constructing a reinforced membrane is to imbibe or infuse an ion-conductive material into a porous inert reinforcing membrane to make a composite membrane. U.S. Pat. No. 5,547,551 (W. L. Gore & Associates, Inc.) describes a composite PEM composed of a porous membrane of polytetrafluoroethylene (PTFE) whose pores are fully impregnated with Nafion™ ionomer. Such composite membranes may exhibit separation between the ionomer and the material of the membrane, especially after repeated hydration or heating, and may then develop gaps.
European Patent No. 0,094,679 (Asahi Glass Co. Ltd.) discloses a PEM made by mixing fibrils such as PTFE fibrils into a fluorinated ion exchange resin, extruding the resin as a membrane, cooling the membrane and then stretching it at a specified reduced temperature.
SUMMARY OF THE INVENTION
Briefly, the present invention provides ion conducting polymer electrolyte membranes (PEMs) comprising an intimate mixture of an ionomeric polymer and a structural film-forming polymer.
In another aspect, the present invention provides methods of producing PEMs by providing a mixture of at least one ionomeric polymer and at least one structural film-forming polymer; and coalescing at least one of said polymers. Furthermore, one or both polymers may be crosslinked.
What has not been described in the art, and is provided by the present invention, is a method to provide a PEM composed of an intimate mixture of an ionomeric polymer and a structural film-forming polymer providing both strength and ionic conductivity.
In this application:
“ionomeric polymer” means a polymer containing ionic groups, such as a copolymer of an ionic monomer and a non-ionic monomer or a polymer treated (e.g., by sulfonation) so as to have ionic groups, which preferably demonstrates ionic conductivity (especially H
+
conductivity), including polymers such as Nafion™ and Flemion™;
“film-forming temperature” (T
ff
) means the minimum temperature at which a polymer coalesces to form an essentially continuous film, which may be altered by the presence of a solvent, dispersant or suspendant, a plasticiser, or a coalescing aid such as glycerol; and
“substituted” means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
It is an advantage of the present invention to provide a strong, thin, durable and sufficiently conductive PEM for use in electrochemical cells such as fuel cells.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides ion conducting polymer electrolyte membranes (PEMs) comprising an intimate mixture of an ionomeric polymer and a structural film-forming polymer.
Film-forming polymers are polymers that form free-standing films upon removal of solvent/dispersant/suspendant from a solution/dispersion/suspension of the polymer at or above a certain temperature, which is the film-forming temperature (T
ff
) of the polymer. Alternately, the solvent/dispersant/suspendant may be removed and the polymer may be subsequently heated to T
ff
or above. When heated above T
ff
, separate particles of polymer material flow together and coalesce to form an essentially continuous material. Film-forming polymers are typically those that do not have high melting points (MP) or glass transition temperatures (Tg).
Film-forming polymers may be selected from polysulfones, polyvinyl halides, polyvinylidene fluoride copolymers, polytetrafluoroethylene copolymers, nylon 6, nylon 6,6, polyether sulfones, polyamides, polyetherphenylketones, polyimides, polyepoxy compounds, polycarbonates, substituted polystyrenes, poly-alpha-olefins, polyphenylene oxides and copolymers of (meth)acrylates. Film-forming polymers useful in the present invention are preferably fully or partially fluorinated. Since PEMs for use in electrochemical cells such as fuel cells cannot be electrically conductive, preferred structural and ionomeric film-forming polymers are not electrically conductive.
Structural film-forming polymers, preferably have a melting point of 180° C. or less, preferably 170° C., more preferably 160° C. or less and most preferably 140° C. or less. Structural film-forming polymers useful in the present invention should have a T
ff
no higher than the decomposition temperature of the ionomeric polymer. Film-forming structural polymers useful in the present invention preferably have a T
ff
of 180° C. or less, more preferably 160° C. or less, even more preferably 140° C. or less, and most preferably 120° C. or less. Where the ionomeric polymer is a film-forming polymer, the T
ff
of the structural film-forming polymer is preferably no more than 20° C. higher than the T
ff
of the ionomeric film-forming polymer and more preferably no higher than the T
ff
of the ionomeric film-forming polymer. More preferably, the T
ff
of the structural film-forming polymer is at least 15° C. less than the T
ff
of the ionomeric film-forming polymer and most preferably the T
ff
of the structural film-forming polymer is less than the T
ff
of the ionomeric film-forming polymer by 30° C. or more.
In their original state prior to incorporation into a film of the present invention, one or both of the polymers may be capable of crosslinking. The polymer may be capable of radiation crosslinking, such as by UV or by electron beam, or crosslinking by use of a crosslinking agent. The polymer may have crosslinkable functions which react during crosslinking.
Film-forming structural polymers useful in the present invention are preferably fluorinated. They may be fully or partially fluorinated, but preferably are partially fluorinated. While polytetrafluoroethylene (PTFE) is not a film-former, many copolymers of tetrafluoroethylene are useful. Many copolymers of vinylidenefluoride are useful. Preferred film-forming structural fluoropolymers include Fluorel™ (Dyneon Corp., Oakdale, Minn.) and the THV series of fluoropolymers polymers also available from Dyneon Corp. THV fluoropolymers are terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride and thus are composed of —CF
2
CF
2
—, —CH
2
CF
2
—, and —CF
2
CF(CF
3
)— units.
Ionomeric polymers useful in the present invention are preferably film-forming polymers but may be non-film-forming polymers. Ionomeric polymers useful in the present invention may be fluorinated, including partially fluorinated and

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