The present invention relates to multi-layer polyelectrolyte membranes containing polymeric resins, for instance and more specifically to fluoropolymer and non-perfluorinated polymeric resins containing ionic and/or ionizable groups (also referred to as a “polyelectrolyte”), which are useful in a variety of products such as fuel cells and the like. The present invention further relates to methods of making these multi-layer polyelectrolyte membranes.
Perfluorocarbon ionic exchange membranes provide high cation transport, and have been extensively used as ionic exchange membranes. Polymeric ion exchange membranes can be referred to as solid polymer electrolytes or polymer exchange membranes (PEM). Because of the severe requirements for fuel cell applications, the most commonly used membranes, and commercially available, are made from perfluorosulfonated Nafion®, Flemion® and Aciplex® polymers. However, reports and literature describe these membranes as working well but show several limitations that prevent developing the technology further to commercialization. Additionally, they work better with gaseous fuels than with liquid fuels which may be mainly due to liquid fuel crossover that diminishes cell performance. A membrane's chemical resistance and mechanical strength are important properties for fuel cell applications. Indeed, the membrane is often subjected to high differential pressure, hydration-dehydration cycles, as well as other stressful conditions. Also, mechanical strength becomes important when the membrane is very thin such as less than 50 microns. Further, when used with fuel cells or battery applications, the membrane sits in a very acidic medium at temperatures that can reach 200° C., in an oxidizing and/or reducing environment due to the presence of metal ions and sometimes the presence of solvents. This environment requires that the membrane be chemically and electrochemically resistant, as well as thermally stable.
Currently, many fluorine-containing membranes can suffer from one or more of the following short comings:                i) high liquid and gas crossover through the membrane;        ii) heterogeneous blending between the fluorinated polymer and other polymers that leads to inferior properties;        iii) insufficient chemical resistance in the presence of some liquid fuels;        iv) poor electrochemical resistance;        v) lack of heterogeneous distribution of sulfonated groups;        vi) poor mechanical properties; and/or        vii) poor thermal stability.        
U.S. Pat. No. 4,295,952 to de Nora et al. relates to cationic membranes which have partly sulfonated tripolymers of styrene, divinylbenzene, and at least one of 2-vinylpyridine, 4-vinylpyridine, and/or acrylic acid.
U.S. Pat. No. 5,679,482 to Ehrenberg et al. relates to fuel cells incorporating an ion-conducting membrane having ionic groups. The polymer forming the membrane contains styrene which has been sulfonated using a sulfonation agent. The sulfonation can take place with the monomer or polymer.
U.S. Pat. No. 5,795,668 describes a fuel cell containing a MEA with a reinforced polymeric ion exchange membrane (PEM) using Nafion® type polymers. The PEM is based on a fluorinated porous support layer and a reinforced ion exchange membrane with an equivalent weight of about 500 to 2000 and a preferred ion exchange capacity of from 0.5 to 2 meq/g dry resin. The porous support layer is made of certain PTFE and PTFE copolymers. The membrane is a perfluorinated polymer with side chains containing —CF2CF2SO3H. It is known from the literature that Nafion® type polymers can have mechanical failure in methanol fuel cells as well as problems with liquid crossover.
WO 97/41168 to Rusch relates to a multi-layered ion-exchange composite membrane having ionic exchange resins, such as fluorinated or non-fluorinated polystyrene based sulfonates and sulfonated polytetrafluoroethylenes.
WO 98/20573 A1 describes a fuel cell containing a highly fluorinated lithium ion exchange polymer electrolyte membrane (PEM). The PEM is based on an ion exchange membrane which is imbibed with an aprotic solvent.
WO 98/22989 describes a polymeric membrane containing polystyrene sulfonic acid and poly(vinylidene fluoride), which provides reduced methanol crossover in direct methanol fuel cell (DMFC) use. However, the polymer blending process described does not provide an acceptable blend and the sulfonation steps are complicated.
Holmberg et al., (J. Material Chem. 1996, 6(8), 1309) describes the preparation of proton conducting membranes by irradiation grafting of styrene onto PVDF films, followed by sulfonation with chlorosulfonic acid. In the present invention, a sulfonation step is not required since the sulfonated group can be incorporated using a sulfonated monomer.
U.S. Pat. No. 6,252,000 relates to a blend of fluorinated ion exchange/non-functional polymers. Specific examples include perfluorinated sulfonyl fluoride polymer/poly(CTFE-co-perfluorodioxolane) blends.
WO 99/67304 relates to an aromatic perfluorinated ionomer prepared by the copolymerization of sulfonated aromatic perfluorinated monomer with acrylic monomers. The sulfonated group that is present is in the fluorinated aromatic chain of the polymer.
U.S. Pat. No. 6,025,092 relates to a perfluorinated ionomer wherein a VDF monomer is polymerized with a sulfonated monomer.
Moore et al., (J. Membrane Sci., 1992, 75, 7) describes a procedure for preparing a melt-processable form of perfluorosulfonate ionomers utilizing bulky tetrabutyl ammonium counterions as internal plasticizers to yield the desired melt-flow properties.
Boucher-Sharma et al., (J. Appl. Polym. Sci., 1999, 74, 47), describes the application of pervaporation of aqueous butenol solutions using a thin film composite composed of PVDF coated with a sulfonated poly(2,6-dimethyl-1,4-phenylene oxide)polymer. The polymer is then ion exchanged with quaternary ammonium cations having aliphatic substituents of varying chain lengths.
U.S. Pat. No. 6,011,074 relates to use of quaternary ammonium cations to enhance the ion-exchange properties of perfluorosulfonated ionomers.
Berezina et al. (Russian J. Electrochemistry, 2002, 38(8), 903), describes the effect of tetraalkyl ammonium salts on the transport and structural parameters of perfloronated membranes including Nafion®-117 and MF4SK. They observe that specific adsorption of organic ions makes the water clusters of the polymers disintegrate and the elasticity of side segments diminish thereby significantly decreasing the proton conductivity of the polymer films.
Pasternac et al., (J. Polym. Sci., A: Polym. Chem., 1991, 29(6), 915) relates to the application of pervaporative membranes for C2-C4 alkanes, and demonstrates that when Nafion®-117 is treated with tetraalkyl ammonium bromides, the separation factor increases with increasing counterion organic chain length.
Smith et al. in European Patent No. 143,605 A2 describes a process where the membrane is cation exchanged with tetraalkyl ammonium ions and expanded by dry stretching to yield a membrane useful for electrolysis.
Feldheim et al., (J. Polym. Sci., B: Polym. Physics, 1993, 31(8), 953) shows a strong dependence of Nafion® thermal stability on the nature of the counterion. Metal salts and alkyl ammonium salts were studied. The thermal stability of the membrane is shown to improve as the size of the counterion decreases. This inverse relationship of thermal stability with counterion size is attributed to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-counterion interaction.
The neutralization of Nafion® by tetrabutyl ammonium hydroxide was further studied in various publications by Moore et al. See, for example, Polymer Chemistry, 1992, 31(1), 1212; Polymer Chemistry, 1995, 36(2), 374, J. Polym. Sci. B: Polym. Physics, 1995, 33(7), 1065, and Macromolecules, 2000, 33, 6031.
Furthermore, sulfonated acrylic or sulfonated vinylic polymers are described for use in superabsorbents, diapers, and contact lenses, for instance. (See J. Mater. Chem., 1996, 6(a), 1309 and Ionics, 1997, 3, 214.) However, such types of products have not been described for application as membranes for polyelectrolyte membranes and the like. All patents, publications, and applications mentioned above and throughout this application are incorporated by reference in their entirety and form a part of the present application.
Thus, there is a need to overcome one or more of these limits and to develop a membrane that can be used for applications in fuel cells, such as liquid fuel cells. More particularly, there is a need to develop a polyelectrolyte to make membranes directly from aqueous or non-aqueous dispersions or solutions. Also, there is a need to provide compositions and methods of synthesis as well as methods of using water or non-aqueous dispersions of polyelectrolyte having sulfonated or other functionalities. Further, there is a need to provide a method that is easier and environmentally friendly. In addition, those skilled in the art would prefer a polyelectrolyte membrane having a higher chemical resistance and mechanical strength.
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