Patent Application
20060105215
The present invention relates to novel membrane electrode assemblies and methods for making the same. More particularly, the present invention relates to novel membrane electrode assemblies that have integrated into them novel ion exchange membranes and also relates to methods for making the same.
With the growing need for energy in the presence of limited fossil fuel supply, the demand for environmentally friendly and renewable energy sources is increasing. Fuel cell technology, a promising source of clean energy production, is the leading candidate to meet the growing need for energy. Fuel cells are efficient energy generating devices that are quiet during operation, fuel flexible (i.e., have the potential to use multiple fuel sources), and have co-generative capabilities (i.e., can produce electricity and usable heat, which may ultimately be converted to electricity). Of the various fuel cell types, the proton exchange membrane fuel cell (PEMFC) is believed to have the greatest potential. PEMFCs can be used for energy applications spanning the stationary, portable electronic equipment and automotive markets.
At the heart of the PEMFC is a fuel cell membrane (hereinafter “ion exchange membrane”), which separates the anode and cathode compartments of the fuel cell. The ion exchange membrane plays a significant role in controlling the performance, efficiency, and other major operational characteristics of the fuel cell. An ion exchange membrane should be an effective gas separator, effective ion conducting electrolyte, have a high proton conductivity in order to meet the energy demands of the fuel cell, and have a stable structure to support long fuel cell operational lifetimes. Moreover, the material used to form the membrane should be physically and chemically stable enough to allow for different fuel sources and a variety of operational conditions.
Currently, commercial fuel cell membranes are formed from perfluorosulfonic acid (PFSA) materials. A commonly known PFSA membrane is Nafion® and is available from DuPont.
Nafion® and other similar perfluorinated membrane materials manufactured by companies such as W. L. Gore and Asahi Glass (described in U.S. Pat. Nos. 6,287,717 and 6,660,818 respectively) show high oxidative stability as well as good performance. Unfortunately, these perfluorinated membrane materials are very expensive to produce and difficult to manufacture, which significantly hinder the economic viability of fuel cells. Currently, perfluorinated membranes such as Nafion(® cost as much as $500 per m2. Furthermore, these materials have a limited potential to operate at temperatures above 95° C. because they have a lower softening point (Tg).
To overcome the cost limitations and increase the operating temperature capabilities of PEMFCs, alternative polymer materials have been actively researched. For example, partially fluorinated polymer structures, such as poly(vinyldifluorides) (PVDF) and polystyrene grafted PTFE, hydrocarbon structures such as aliphatic elastomers and aromatic thermoplastics, and non-fluorinated non-hydrocarbon polymer systems like polyphosphazenes and polysiloxanes have been studied. To date, the most promising of the alternative materials have been acid finctionalized aromatic thermoplastics.
Aromatic thermoplastics, such as poly(ether ether ketone) (PEEK), poly(ether ketone) (PEK), poly(sulfone-udel) (PSU) and poly(ether sulfone) (PES) have performed well as fuel cell membranes due to their low cost and good film forming characteristics. When functionalized with sulfonic acid or ion exchange moieties, these materials can be used as fuel cell membranes, as described in the following publications: U.S. Pat. No. 6,465,136; U.S. Pat. No. 6,790,931; U.S. Patent App 20040028976; J. Polym. Sci., Part A, 34, 2421 (1996); J. Appl. Polym. Sci. 61, 1205 (1996); J. Membr. Sci. 139, 211 (1998); Macromolecules 33, 7609 (2000); Electrochem. Acta 46, 2401 (2001); J. Appl. Polym. Sci. 77, 1250 (2000); Electrochem. Syst. 3, 93 (2000); J. Polym. Sci. 70, 477 (1998); Macromolecules 25, 6495 (1992); Solid State Ionics 106, 219 (1998); Solid State Ionics 106, 219-225 (1998); and Solid State Ionics 106, 219 (1998).
Although these materials are inexpensive to produce and in many cases are more tolerable to relatively higher operating temperatures, most of them lack long term stability to offer viable commercial solutions as ion exchange membranes.
What is therefore needed are membrane electrode assemblies that enjoy the advantages of long term stability and commercial viability, without suffering the disadvantages of conventional ion exchange membranes.
To achieve the foregoing, the present invention provides inventive ion exchange membranes, inventive membrane electrode assemblies and methods for making the same. The present invention provides an ion exchange membrane for use in an electrochemical cell. The ion exchange membrane includes: (i) a first layer capable of conducting ions when placed between two electrodes of the electrochemical cell; and (ii) a second layer disposed around at least a portion of the first layer, wherein the second layer is made from a perimeter strengthening material, such that the second layer strengthens at least a portion of a perimeter boundary of the first layer.
In one embodiment, an ion exchange membrane of the present invention has a perimeter strengthening material adhering to a perimeter boundary on both sides of the ion exchange membrane. In an alternative embodiment, the ion exchange membrane of the present invention has a perimeter strengthening material adheres adjacent to the edges of the ion exchange membrane.
In another aspect, the present invention provides an ion exchange membrane for use in an electrochemical cell. The ion exchange membrane includes: (i) a first component; and (ii) a second component which includes at least one member selected from a group consisting of filler, porous matrix and a perimeter strengthening material, wherein the first component contains a polymer electrolyte composition which includes at least one type of a polymer repeat unit that has more than one property imparting unit. The property imparting unit includes at least one member selected from a group consisting of a conductivity imparting unit and a stability imparting unit.
In yet another aspect, the present invention provides a process of making an ion exchange membrane. The process includes: (i) preparing a solution of a polymer electrolyte composition including at least one type of a polymer repeat unit having attached thereto more than one property imparting unit, the property imparting unit includes at least one member selected from a group consisting of a conductivity imparting unit and a stability imparting unit; (ii) adding to the solution a performance enhancing component; (iii) casting on a support assembly the solution containing the performance enhancing component; and (iv) drying the solution to form an ion exchange membrane which has dispersed within it the performance enhancing component.
In yet another aspect, the present invention provides a process of making an ion exchange membrane. The process includes: (i) obtaining an ion exchange layer including at least one type of a polymer repeat unit that has more than one property imparting unit, which includes at least one member selected from a group consisting of a conductivity imparting unit and a stability imparting unit; (ii) fabricating around at least a portion of a perimeter of the ion exchange layer a performance enhancing layer to form the ion exchange membrane.
In yet another aspect, the present invention provides a process of making an ion exchange membrane. The process includes: (i) preparing a solution of a polymer electrolyte composition including at least one type of a polymer repeat unit having attached thereto more than one property imparting unit, which in turn includes at least one member selected from a group consisting of a conductivity imparting unit and a stability imparting unit; (ii) impregnating the solution into a porous material; and (iii) drying the solution to form an ion exchange membrane.
The present invention provides electrochemical cell designs and methods for making the same to impart that electrochemical cell with certain desired properties. These designs and methods are particularly useful because they provide the requisite long-term stability and prolong the longevity of the cell. To this end, the present invention offers inventive ion exchange membranes that form novel membrane electrode assemblies (“MEA”).
As shown in
Endplates 14 and 16 are connected to external load circuit 50 by leads 31 and 33, respectively. External circuit 50 can be comprised of any conventional electronic device or load such as those described in U.S. Pat. Nos. 5,248,566, 5,272,017, 5,547,777, and 6,387,556, which are incorporated herein by reference for all purposes. The electrical components can be hermetically sealed by techniques well known to those skilled in the art.
During operation, in fuel cell 10 of
Endplates 14 and 16 are made from a relatively dimensionally stable material. Preferably, such material includes one selected from a group consisting of metal and metal alloy. Bipolar plates 18 and 20 are typically made from any conductive material selected from a group consisting of graphite, carbon, metal, and metal alloys. Gaskets, 26 and 28 are typically made of any material selected from a group consisting of Teflon®, fiberglass, silicone, and rubber.
Ion exchange membranes of the present invention can comprise of two or more components. Preferred embodiments of the inventive ion exchange membranes include, for example, one component dispersed in another component, as shown in
A conductivity imparting unit can be any unit that imparts the polymer repeat unit or the ultimately produced ion exchange membrane 46 (which includes the polymer electrolyte) with a certain desired conductivity. In a preferred embodiment of the present invention, however, a conductivity imparting unit includes at least one member selected from a group consisting of sulfonic acid, derivatives of sulfonic acid, phosphonic acid, derivatives of phosphonic acid, carboxylic acid, derivatives of carboxylic acid, heterocycles such as imidazole, benzimidazole, pyrazole and any combination thereof.
A stability imparting unit can be any unit that imparts to the polymer repeat unit or ion exchange membrane 46 with a certain desired stability. In a preferred embodiment of the present invention, however, a stability imparting unit includes at least one member selected from a group consisting of crosslinking agents, antioxidizing agents, blocking agents and any combination thereof. Representative crosslinking agents include at least one member selected from a group consisting of acrylates, methacrylates, alkynes, epoxides, amines, amine derivatives, fumarates, maleates, maliemides and alkenes, which in turn include allyls, substituted allyls, vinyls and substituted vinyls, and any combination thereof. Representative antioxidizing agents include at least one member selected from a group consisting of metal chelating groups, radical absorbing groups, peroxide decomposition groups such as phosphates, phosphate esters, phosphonic acid, derivatives of phosphonic acid and EDTA and any combination thereof. Representative blocking agents include at least one member selected from a group consisting of branched hydrocarbon chains, long hydrocarbon chains, branched fluorocarbon chains, long fluorocarbon chains and any combination thereof.
Both conductivity and stability imparting units may or may not include a delinking agent. In those embodiments where a delinking agent is used, the delinking agent may vary in composition but include at least one member selected from a group consisting of C—C bond, CH2, CH3, CF2, CF3, Si, O, S, functional groups, aromatic residues and any combination thereof. In addition to the delinking agent, the conductivity imparting unit includes a member selected from a group consisting of sulfonic acid, derivatives of sulfonic acid, phosphonic acid, derivatives of phosphonic acid, carboxylic acid, derivatives of carboxylic acid, heterocycles, such as imidazole, benzimidazole and pyrazole, and any combination thereof. Similarly, in addition to the delinking agent, the stability imparting unit includes at least one member selected from the above described group consisting of crosslinking agents, antioxidizing agents, blocking agents and any combination thereof.
Filler 62 shown in
It is important to note that ion exchange material 46′ is not limited to include the above-described polymer electrolyte composition as in the case of ion exchange material 46 shown in
Inventive MEA embodiments in
The present invention also provides inventive methods of making the novel MEAs described above. In accordance with one embodiment of the present invention, a process of making MEA shown in
A performance enhancing component can, but need not, provide similar benefits as a property imparting unit of the polymer electrolyte composition. More importantly, when a performance enhancing component and a property imparting unit provide similar advantages, they are beneficial at different levels. A performance enhancing component augments the performance of the ion exchange membrane from a macroscopic point of view. For example, a fiber filler material may strengthen and support the ion exchange material helping it to resist chemical and mechanical degradation at a macro level. In stark contrast, a property imparting unit benefits the performance of the ion exchange membrane from a microscopic point of view, or alternatively at molecular level. For example, a stability imparting unit may intrinsically stabilize the ion exchange membrane and build resistance to attack from electrochemical cell byproducts at a molecular level.
Next, the process includes casting on a support assembly, such as carrier plate, the solution containing the performance enhancing component. Next, drying commences to form an ion exchange membrane (e.g., ion exchange membrane 46 shown in
In accordance with another embodiment of the present invention, a process of making MEA (e.g., shown in
In accordance with yet another embodiment of the present invention, a process of making the MEA shown in
The ion exchange membrane 60 of
This example describes the synthesis, according to one embodiment of the present invention, of an exemplar ionomer. 4,4′-difluoro 3,3′-di sodium sulfonate salt of phenyl sulfone (about 27.50 g, about 0.06 mol) and 4-fluorophneyl sulfone (about 35.56 g, about 0.014 mol) was reacted with 1,4-di(4-hydroxy phenyl) butane (about 7.26 g, about 0.03 mol) and 4,4′-biphenol (about 31.66 g, about 0.017 mol) in the presence of potassium carbonate (about 31.79 g, about 0.023 mol) under a dry nitrogen atmosphere in a round bottom flask equipped with nitrogen inlet and a Dean-Stark trap using DMSO (about 450 mL) and benzene. After refluxing/recycling of benzene or toluene at about 150° C. for about 4 hours, all the benzene was removed and the heating was continued for another about 10 hrs at about 160° C. The mixture was cooled and additional DMSO (about 200 mL) was added to the reaction mixture. The viscous solution was filtered and then poured into a large excess of water in order to obtain a transparent white polymer. The resulting polymer was washed, filtered and dried.
This example describes the synthesis, according to one embodiment of the present invention, of an exemplar ionomer of the present invention. 4-fluorophenyl sulfone (about 25.43 g, about 0.1 mol) was reacted with 3,3-di(sodium-3-(2-methyl propyl) sulfonate)-4,4′-biphenol (IUPAC name: di sodium salt of 3-[4,4′-dihydroxy-3′(2-methyl-3-sulfo-propyl)-biphenyl-3-yl]-2-methyl-propane-1-sulfonic acid) (about 20.01 g, about 0.04 mol) and 4,4′-biphenol (about 11.17 g, about 0.06 mol) in the presence of potassium carbonate (about 15.89 g, about 0.115 mol) under a dry nitrogen atmosphere in a round bottom flask equipped with nitrogen inlet and a Dean-Stark trap using DMSO (about 300 mL) and benzene. After refluxing/recycling of benzene or toluene at about 150° C. for about 4 hours, all the benzene was removed and the heating was continued for another about 6 hrs at about 160° C. The mixture was cooled and additional DMSO (about 100 mL) was added to the reaction mixture. The viscous solution was filtered and then poured into a large excess of water in order to obtain a transparent white polymer. The resulting polymer was washed, filtered and dried.
This example describes a method of making membranes, according to preferred embodiments of the present invention. About 50 g of polymer was dissolved in about 500 mL of an aprotic solvent such as DMSO, NMP, DMF, DMAc or mixtures thereof and the resulting solution was pressure filtered using a low micron pore size filter. If needed, the polymer concentration in the solution was then adjusted to about 20 wt. % by evaporating the aprotic solvent using a roto-evaporator. The membranes were prepared by web casting the resulting polymer solution and was dried at about 100° C. for about 3 hrs and then at about 130° C. for about 6 hrs. In other modifications of this example, the membranes are dried for longer and shorter periods of time.
This example describes an exemplar method of incorporating a filler into an ion exchange membrane. About 50 g of polymer was dissolved in about 500 mL of an aprotic solvent such as DMSO, NMP, DMF, DMAc or mixtures thereof and the resulting solution was pressure filtered using a low micron pore size filter. In preferred embodiments of this invention, the polymers in the solution are the ones mentioned in Examples 1 and 2 are used. Next, about 2.5 g of a filler such as MnO2 (particle size<about 5 microns) are added to the polymer solution and mixed for about 12 hrs at room temperature, while sonicating periodically. The resulting mixture was then adjusted to about 20 wt. % polymer in solvent by evaporating the aprotic solvent using a roto-evaporator. The resulting solution was then used to form membranes similarly to those prepared in Example 3 or the solution may be incorporated into a porous matrix (for more information on incorporation, see example 5 below).
This example describes the preparation of a reinforced membrane. A micro porous substrate made of polyethylene, PTFE, PP, or any combination thereof was stretched and held in place as it went through several treatments. The substrate was first infiltrated (or wetted) with an alcohol such as methanol, ethanol, isopropanol, or mixtures thereof. Next the substrate is treated with an aprotic solvent such as DMSO, NMP, DMF, DMAc or mixtures thereof. After such treatment, the substrate was infiltrated with a polymer solution and the resulting reinforced membrane is smoothed to a desired thickness. In preferred embodiments of the invention, the polymers in the solution are the ones mentioned in Examples 1 and 2 above. The membrane was then dried under various conditions and protonated to yield an ion exchange membrane described previously.
This example describes the preparation, according to one embodiment of the present invention of a MEA with a perimeter strengthening material. An ion exchange layer (membrane) is placed between two layers of perimeter strengthening materials. Preferred materials include<about 200 micron thick layers of PE, PP, PTFE, PVDF, PSU, PI, polyphenylene, or any combination thereof. Additionally, the perimeter strengthening material has an adhesive layer that is pressure or thermally sensitive. The perimeter reinforcing material prior to the sandwiching operation is cut to have dimensions to match the required active area of catalyst. Next, the perimeter sealing material is joined to the membrane by hot pressing for about 120° C. at about 3 minutes with about 400 kg load. The catalyst containing layers are applied directly to the ion exchange layer within the perimeter strengthening layer. Typical electrocatalyst loadings are about 0.1 to about 1 mg/cm2 on both the anode and the cathode. After applying, the catalyst layers are dried by placing in a vacuum oven at a temperature of about 90° C. for about half an hour to remove a majority of the catalyst ink solvent.
After drying, the resulting MEA is hot pressed at about 120° C. for about 3 minutes at a pressure of about 30 kg/cm2. After the hot pressing, the MEA is placed in 0.5 M sulfuric acid-water solution for about half an hour at about 60° C. The MEA is then rinsed and soaked in deionized water for about 15 minutes. Next, the MEA is placed between two current collectors (otherwise known as gas diffusion layers) and pressed at about 130° C. for about 3 minutes at about 30 kg/cm2.
This example describes the preparation, according to one embodiment of the present invention, of an MEA with a perimeter strengthening material using catalyst coated carbon layers. First, the perimeter sealing materials and ion exchange layer (membrane) are attached as described in Example 6. Next or contemporaneously, the catalyst layers are applied directly to a porous carbon layer. Typical electrocatalyst loadings are about 0.1 mg/cm2 to about 1 mg/cm2 on both the anode and the cathode. The final step in MEA assembly entails sandwiching the ion exchange layer and perimeter sealing materials between porous carbon layers containing catalyst (catalyst side adjacent to the ion exchange membrane and pressing at about 130° C. for about 3 minutes at about 30 kg/cm2.
Although the present invention is described in terms of fuel cell applications, those skilled in the art will recognize that the inventive structures and techniques described herein can be used for other electrochemical applications.
