Process of making a composite membrane

Details Process of making a composite membrane Process of making a composite membrane

2000-10-05

2002-12-10

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Ion-exchange polymer or process of preparing

C521S031000, C521S037000, C521S038000, C521S053000, C521S134000, C521S139000

Reexamination Certificate

active

06492431

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the field of ionically conducting separators. The invention particularly relates to methods and processes of fabricating composite membranes especially useful in electrochemical devices requiring a proton conductor such as fuel cells.
BACKGROUND OF THE INVENTION
The operation of an electrochemical cell requires the occurrence of oxidation and reduction reactions which produce or consume electrons. In operation, an electrochemical cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This mechanism includes one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
One category of electrolytes particularly suitable for use in conjunction with electrochemical cells are proton exchange membranes (PEM). PEMs usually consist of a polymer matrix to which are attached functional groups capable of exchanging cations or anions. The polymer matrix generally consists of an organic polymer such as polystyrene, or other polytetrafluoroethylene (PTFE) analog. In general the material is acid with a sulfonic acid group incorporated into the matrix.
The apparent advantages of using PEMs in fuel cells are numerous. The solid electrolyte membrane is simpler and more compact than other types of electrolytes. Also, the use of a PEM instead of a liquid electrolyte offers several advantages, such as simplified fluid management and elimination of the potential of corrosive liquids. In systems using a PEM, the membrane also serves as an electronically insulating separator between the anode and cathode. However, a number of properties are desirable when using an acid ion exchange membrane as an electrolyte. These include: high ionic conductivity with zero electronic conductivity; low gas permeability; resistance to swelling; minimal water transport; high resistance to dehydration, oxidation, reduction and hydrolysis; a high cation transport number; surface properties allowing easy catalyst bonding, and mechanical strength.
Conventional proton conducting membranes for use in polymer electrolyte membrane (PEM) fuel cells consist of homogeneous polymer films.
FIGS. 1 and 2
are schematic diagrams depicting three examples of homogeneous polymer films used in polymer electrolyte membranes. The polymers depicted in
FIG. 1
were developed at DuPont and Dow Chemical Company. These polymers represent a class of compounds known as perfluorosulfonic acids (PFSA). These polymers are fully fluorinated, i.e., all of the sites occupied by hydrogen atoms in a conventional hydrocarbon polymer have been replaced by fluorine atoms. This makes the polymers extremely resistant to chemical attack.
PFSA polymers are generally synthesized by the copolymerization of a derivatized, or active, comonomer with tetrafluoroethylene, (TFE), as illustrated in FIG.
3
. After synthesis, the thermoplastic polymer, which is both hydrophobic and electrochemically inert, is converted into the active ionomer by a base hydrolysis process, as illustrated. The result of this step is an ionomer in its salt form. This can be converted to the proton form by ion-exchange with a strong acid. The sulfonate functionalities (R—SO
3
−
) act as the stationary counter charge for the mobile cations (H
+
, Li
+
, Na
+
, etc.) which are generally monovalent.
Another type of polymer, illustrated in
FIG. 2
, is a derivatized trifluorostyrene (TFS), of the type developed by Ballard. This polymer has a fully fluorinated backbone, but some of the side chains have hydrogen atoms.
The polymer is synthesized by copolymerizing derivatized and non-derivitized trifluorostyrene monomers, as illustrated in FIG.
4
. This process also produces an electrochemically inactive thermoplastic. In this system the derivatized monomers create the inert sites while the non-derivatized monomers can be sulfonated, as illustrated in FIG.
4
. The result of this process is a proton conducting polymer.
Other homogeneous proton conducting polymers are tabulated in Table I. All of these polymers tend to have poor physical properties making them difficult to handle. For example, sheets of the polymers are easily torn or punctured, thereby requiring a minimum usable thickness of about 2 mils (0.002″, 0.05 mm).
TABLE I
Other Homogeneous Polymer Electrolytes
Manufacturer
Polymer
DAIS Corp.
Sufonated styrene-butadiene block copolymer
Maxdem, Inc.
Sufonated polyparaphenylene
(Not yet commercial)
Sulfonated side chains radiation grafted to PTFE
In U.S. Pat. No. 5,547,551 Bahar et. al disclose a composite membrane fabricated by filling the void portion of a porous substantially inert polymer membrane with an ionically conducting polymer. This approach starts with a porous membrane fabricated from an inert polymer, such as polytetrafluoroethylene (PTFE) and converts it to an ion conducting membrane by filling the pores with ionomer deposited from solution. This approach leads to thinner membranes, with membranes less than 1 mil (0.001″, 0.025 mm) produced. These membranes are more conductive than pure PFSA membranes on a conductivity per unit area basis, but have lower specific conductivities. The advantage of these membranes is their strength. A 1 mil membrane produced using this technology is tougher than a conventional 5 mil homogeneous membrane.
In U.S. Pat. No. 5,654,109, Plowman et al. disclose an alternate approach to the fabrication of reinforced membranes. In this approach, a core layer of a tough membrane material is clad with surface layers of highly ionically conductive polymer. Typically all of the layers are PFSA type materials, with the core layer having a significantly higher equivalent weight than the surface layers. Although it would seem that the use of a high equivalent weight polymer would significantly impede the proton flux, it has been experimentally determined that a membrane with a core having an equivalent weight as much as 20% greater than the surface layers exhibits a conductivity equivalent to a solid membrane with the composition of the surface polymer.
While the above methods and processes may allow the fabrication of composite membranes that may present enhanced structural stability and ionic conductivity, the methods used do not allow the flexibility needed in fabricating composite membranes suitable for use in a wide range of applications. Thus there is a great need for membranes and membrane fabricating processes that allow greater flexibility in controlling the physical properties of the composite membranes.
SUMMARY OF THE INVENTION
The present invention provides a method of making a composite membrane, comprising: (a) combining a first polymer component with a second polymer component; wherein the first polymer component is a non ion-conducting precursor to an ion-conducting polymer; and (b) converting the first polymer component from the non ion-conducting precursor to the ion-conducting polymer.
The combining of the polymeric components may comprise melting and mixing the polymers, co-polymerizing two or more monomers, co-precipitating a solution of a first polymer and a suspension of a second polymer, filling the pores of a porous polymeric matrix with a solution of a second polymer, or filling the pores of a porous polymeric matrix with a melted polymer.
The invention encompasses a process wherein the step of combining the first and second polymer components comprises mixing a solution of the first component and a suspension of the second component, co-precipitating the first and second polymer components to form a gelatinous mass, and drying the gelatinous mass. The dried gelatinous mass may further be sintered and/or pressed into a sheet, optionally at temperatures of about 150° C. or about 300° C

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