Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ
Reexamination Certificate
1999-03-18
2001-02-13
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Reexamination Certificate
active
06187231
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the manufacture of films for use as polymer electrolytes in fuel cell applications.
BACKGROUND OF THE INVENTION
Fuel cells can be configured in numerous ways with a variety of electrolytes, fuels and operating temperatures. For example, fuels such as hydrogen or methanol can be provided directly to the fuel cell electrode. Alternatively, fuels, such as methane or methanol, can be converted to a hydrogen rich gas mixture external to the cell itself and subsequently provided to the fuel cell. Air is the source of oxygen in most fuel cells, although in some applications, the oxygen is obtained by hydrogen peroxide decomposition or from a cryogenic storage system.
Although there are theoretically a limitless number of combinations of electrolyte, fuel, oxidant, temperatures and so on, practical systems include solid polymer electrolyte systems using hydrogen or hydrazine as the fuel source and pure oxygen as the oxidant. A polybenzimidzole (PBI) which has been doped with a strong acid is an example of a suitable solid polymer for use in an electrolyte system.
See, e.g., U.S. Pat. No. 5,091,087 which discloses a process for preparing a microporous PBI membrane having a uniform pore structure by immersing fine PBI particles in a polymeric solution of a high temperature stable matrix polymer to coat the PBI with the matrix polymer, drying the coated PBI particle, and compression molding the particles to sinter the PBI. The matrix polymer is extracted from the molded PBI.
It is known in the art to imbibe polybenzimidazole (PBI) dense films with a strong acid to make a proton conducting media.
Recently, International Patent Application No. WO96/13872, published May 9, 1996, disclosed a method of doping a PBI with a strong acid, such as phosphoric acid or sulfuric acid, such that a single phase system is formed, i.e., acid is dissolved in the polymer.
Even in view of the advances in the art, the performance, high cost and processability of suitable polymeric electrolyte materials remain important considerations in fuel cell construction with respect to polymeric media for fuel cells.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an improved method of preparing polymeric films and membranes, particularly those made of a PBI, for use as electrolytes in fuel cells. In one embodiment, the method comprises forming a porous structure by coagulating a PBI dope solution in a coagulation bath containing a non-solvent alone or a mixture of a non-solvent and a solvent, both miscible in each other. The resulting membrane is then submerged into a water bath to remove any residual solvent, and then placed into an acid and water solution. The pores are then filled with the acid/water mixture. The membrane is dried to remove residual water which collapse the porous structure entrapping the acid.
In another aspect, the method comprises coagulating the polymer/solvent mixture directly in an acid/solvent/water mixture to produce the membrane directly and imbibe the acid/water solution. The membrane is then dried to remove residual water and solvent and collapse the pores to produce a dense film. The amount of acid in the resulting film can be controlled by adjusting the membrane porosity, which is determined by the solvent content in the coagulation medium, and controlling the acid concentration in the coagulation bath. Since PBIs are basic polymers, these compounds have an affinity for acids and retain them under extreme conditions. This method is advantageous due to the speed of imbibition and the resulting morphology of the film produced.
In another aspect, the present invention provides improved porous polymeric films prepared by the novel methods described herein. Such films are characterized by higher acid loadings and improved electrochemical and/or mechanical properties. For example, the films may be characterized by better retention of the acid than the films of the prior art.
Other aspects and advantages of the present invention are described further in the following detailed description of the preferred embodiments thereof.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an improvement over the art in methods of preparing a microporous polymeric film or membrane for use as an electrolyte in fuel cells. Generally, according to this process, polymeric films, preferably those made from a polybenzimidazole (PBI), are prepared in a controlled fashion with single or multiple component liquids. In one embodiment, one of these liquids, the coagulation bath, comprises a non-solvent. In another embodiment, one of these liquids, the coagulation bath, comprises a miscible mixture of a non-solvent and a solvent. In still a further embodiment, one of these liquids, the coagulation bath, comprises a miscible mixture of a non-solvent, a solvent and an acid.
According to the methods of this invention, a porous structure is formed by coagulating polymer dope solutions in a coagulation bath containing either a non-solvent alone, or a miscible mixture of a solvent and a non-solvent. The addition of a solvent to the mixture slows down the coagulation reaction and can change the morphology of the resulting membrane. The resulting membrane is then submerged into a bath containing the non-solvent to remove any residual solvent, and then placed into a solution containing an acid and a non-solvent, such as an acid/water solution or an acid/methanol solution, among other non-solvents, to fill the membrane pores with the acid solution. The membrane is oven dried to remove residual non-solvent which collapses the porous structure, entrapping the acid.
An advantage of this method is that the acid imbibition occurs over a short timespan, i.e., about 30 seconds to about 1 hour, in contrast to prior art methods which take between 10 to 72 hours. This advantage is reflected in both reduced cost and better performance of the electrolyte.
In any of the embodiments of the method which are described in the examples and in more detail below, the term “polymer dope solution” means a solution containing about 2 to about 30% of a selected polymer dissolved in a suitable solvent. Suitable solvents in which the polymers are dissolved include, without limitation, DMAC, NMP, DMF, DMSO, strong acids such as sulfuric acid, methanesulfonic acid, and trifluoroacetic acid.
The selected polymers employed to form the dopes include polymers containing basic groups that can form complexes with stable acids, or polymers containing acidic groups which can be used to form films suitable for use as a solid polymer electrolyte membrane in fuel cells. These polymers may contain a variety of functional groups. Examples of such polymers include, but are not limited to, polybenzimidazoles (PBI), poly(pyridines), poly(pyrimidines), polyimidazoles, polybenzthiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, and poly(tetrazapyrenes). The presently preferred, and exemplified polymers are PBIs.
The polybenzimidazole polymer has the known structure, wherein R and R
1
are selected from among a variety of linking or functional groups [See, e.g., U.S. Pat. Nos. 4,814,399 and 5,525,436; and International Patent Application No. WO96/13872]:
More specifically, in a desired embodiment of this method of producing a polymeric microporous film, polymer dope solution is coagulated in a coagulation bath comprising a non-solvent alone. Among such non-solvents are included, without limitation, water, methanol, acetone, other alcohols and other water miscible non-solvents. The amount of non-solvent(s) in the bath is about 100% by weight of the composition. The bath temperature is generally room temperature but can be anywhere between the freezing point and boiling point of the non-solvent.
The polymeric dope is coagulated in the non-solvent bath until it coagulates into a porous membrane, characterized by a porous structure. The coagulation time, i.e., the amount of time taken for the polymer dope to coagulate, is generally between about 5 se
French Stuart M.
Marikar Faruq
Onorato Frank J.
Sansone Michael J.
Celanese Ventures GmbH
Howson and Howson
Tentoni Leo B.
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