Patent Application
20140045093
The present invention relates to methods for making fuel cell ion-conducting membranes.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel, and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). Proton exchange membrane (“PEM”) fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. Typically, the ion conductive polymer membrane includes a perfluorosulfonic acid (PFSA) ionomer.
Each catalyst layer has finely divided catalyst particles (for example, platinum particles), supported on carbon particles, to promote oxidation of hydrogen at the anode, and reduction of oxygen at the cathode. Protons flow from the anode through the ion conductive polymer membrane to the cathode where they combine with oxygen to form water which is discharged from the cell.
The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”), which in turn are sandwiched between a pair of electrically conductive flow field elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cells in stacks in order to provide high levels of electrical power.
In many fuel cell applications, electrode layers are formed from ink compositions that include a precious metal and a perfluorosulfonic acid polymer (PFSA). For example, PFSA is typically added to the Pt/C catalyst ink in electrode layer fabrication of proton exchange membrane fuel cells to provide proton conduction to the dispersed Pt nanoparticle catalyst as well as binding of the porous carbon network. Traditional fuel cell catalysts combine carbon black with platinum deposits on the surface of the carbon, along with ionomers. The carbon black provides (in part) a high surface area conductive substrate. The platinum deposits provide a catalytic behavior, and the ionomers provide a proton conductive component. The electrode is formed from an ink that contains the carbon black catalyst and the ionomer, which combine on drying to form an electrode layer. Although the current technologies for making the ion conducting membranes work reasonably well, there is still a need for improvement.
Accordingly, the present invention provides improved methods of making membranes that are useful in fuel cell applications.
The present invention solves one or more problems of the prior art by providing in at least one embodiment a method for making a metal electrode assembly for a fuel cell. The metal electrode assembly includes a cathode catalyst layer, an anode catalyst layer, and an ion conducting membrane disposed between the cathode catalyst layer and the anode catalyst layer. The ion conducting layer includes a polyphenylene sulfide mat with a first polymer imbibed therein, the polyphenylene sulfide mat includes the polyphenylene sulfide-containing structures.
In another embodiment, a method for forming an ion conducting membrane is provided. The method includes a step of combining a polyphenylene sulfide-containing resin with a water soluble carrier resin to form a resinous mixture. The resinous mixture is shaped to form a shaped resinous mixture which has polyphenylene sulfide-containing structures dispersed within the carrier resin. The shaped resinous mixture is contacted with water to separate the polyphenylene sulfide-containing structures from the carrier resin. The polyphenylene sulfide-containing structures are optionally sulfonated. The polyphenylene sulfide-containing structures is formed into a mat which is then imbibed with a first polymer.
Exemplary embodiments of the present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the term “polymer” includes “oligomer,” “copolymer,” “terpolymer,” and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; molecular weights provided for any polymers refers to weight average molecular weight unless indicated otherwise; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.
It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.
Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
With reference to
With reference to
wherein PG is —SO2X, —PO3H2, and —COX where X is an —OH, a halogen, or an ester and n is a number from about 20 to about 500 on average. In particular, the polyphenylene sulfide-containing fibers are sulfonated (SO3H) in this step.
In step f), polyphenylene sulfide-containing fibers 50 or modified polyphenylene sulfide-containing fibers 52 are formed into fibrous mat 54. In a refinement, mat 54 is formed by pressing and heating of sulfonated polyphenylene sulfide-containing fibers 50 or 52. In step g), a polymer containing composition is imbibed into mat 54 to form ion conducting membrane 12. In a variation, ion conducting membrane 12 has a thickness from about 5 microns to about 2 mm. In a refinement, ion conducting membrane 12 has a thickness from about 5 microns to about 500 microns. In another refinement, ion conducting membrane 12 has a thickness from about 5 microns to about 50 microns. Finally, ion conducting membrane 12 is incorporated into fuel cell 10.
In one variation the polymer containing composition used in step g) includes a first polymer and a solvent. Suitable solvents include, but are not limited to alcohols (methanol, ethanol, propanol, etc.), water and combinations thereof. In a refinement, the first polymer is present in an amount from about 1 weight percent to about 20 weight percent of the total weight of the polymer composition. Typically, the first polymer is an ion conducting polymer that includes protogenic groups as set forth above. Suitable solvents include alcohols (e.g., methanol, alcohol, propanol, and the like) and water. Examples for the first polymer include, but are not limited to, perfluorsulfonic acid polymers such as NAFION™, perfluorocyclobutyl-containing polymer (PFCBs), and combinations thereof. Examples of useful PFSA polymers include a copolymer containing a polymerization unit based on a perfluorovinyl compound represented by:
CF2═CF—(OCF2CFX1)m—Or—(CF2)q—SO3H
where m represents an integer of from 0 to 3, q represents an integer of from 1 to 12, r represents 0 or 1, and X1 represents a fluorine atom or a trifluoromethyl group and a polymerization unit based on tetrafluoroethylene. Suitable polymers having cyclobutyl moieties are disclosed in U.S. Pat. Pub. No. 2007/0099054, U.S. Pat. No. 7,897,691 issued Mar. 1, 2011; U.S. Pat. No. 7,897,692 issued Mar. 1, 2011; U.S. Pat. No. 7,888,433 issued Feb. 15, 2011, U.S. Pat. No. 7,897,693 issued Mar. 1, 2011; and U.S. Pat. No. 8,053,530 issued Nov. 8, 2011, the entire disclosures of which are hereby incorporated by reference. In a variation, the ion-conducting polymer having perfluorocyclobutyl moieties includes a polymer segment comprising polymer segment 1:
E0-P1-Q1-P2 1
wherein:
In a refinement, E0 is a C6-30 aromatic (i.e., aryl) containing group.
As set forth above, the method of the invention utilizes water soluble resins. Examples of suitable water-soluble resins include, but are not limited to, water-soluble polyamides (e.g., poly(2-ethyl-2-oxazoline) (“PEOX”). In a refinement, the PEOX has a number average molecular weight from about 40,000 to about 600,000. Number average molecular weights of 200,000 and 500,000 have been found to be particularly useful.
In a refinement of the present invention for the variations and embodiments set forth above, the polyphenylene sulfide fibers (with or without protogenic groups) have an average cross sectional width (i.e., diameter when the fibers have a circular cross section) from about 5 nanometers to about 30 microns. In another refinement, the fibers have an average width of about 5 nanometers to about 10 microns. In still another refinement, the fibers have an average width of from about 10 nanometers to about 5 microns. In still another refinement, the fibers have an average width of from about 100 nanometers to about 5 microns. The length of the fibers typically exceeds the width. In a further refinement, the fibers produced by the process of the present embodiment have an average length from about 1 mm to about 20 mm or more.
In a variation, the ion conducting membrane also includes a second polymer. Other ionomers such as TCT 891 (a PFSA multiblock PFCB polymer from Tetramer Technologies, LLC) can be used instead of Nafion® DE2020 ionomer solution, with or without KynarFlex® 5721 in polar aprotic solvents or in alcohol solvents.
The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims.
Polyphenylene sulfide (PPS) thermoplastic fibers are first created by dispersing PPS in 500,000 MW water soluble polymer poly(2-ethyl-2-oxazoline) (PeOX). Specifically, 5 grams of PPS is first blended in a Waring blender with 15 grams of 500,000 MW PeOX (a ratio of 1 to 3). The combined blend is added to a laboratory mixing extruder (Dynisco, LME) operated at 240° C. header and rotor temperatures with the drive motor operated at 50% of capacity, resulting in an extruded strand of the blend. This extruded strand is added to the blender to return it to granular form, and re-extruded two more times, creating a uniform extruded strand. During the final extrusion processes, the fibers are spun onto a take-up wheel (a Dynisco Take-Up System (TUS), at approximately 10 cm/second. The resulting extruded strand is washed in RO water with repeated rinses, until the PeOX has been removed, resulting in a sample of PPS nanofibers. The fibers are then rinsed in isopropyl alcohol and allowed to dry completely overnight.
Addition of Sulfonic Acid Groups Creating Functionalized Nano-Fibers.
The polyphenylene sulfide nanofibers are sulfonated in a way that does not reduce the high surface area form of the PPS back to a sheet form. Nanofibers of polyphenylene sulfide (2 grams) are suspended in methylene chloride (50 g) in a screw-cap jar with a Teflon gasketed lid. Chlorosulfonic acid is first dispersed in methylene chloride (1 gram in approximately 10 g). With vigorous stirring, chlorosulfonic acid dispersion (1 g of acid) is added to the dispensation of PPS fibers in methylene chloride and the lid is secured. The jar is roll-milled for 4 hours and then the dark green-blue fibrous mixture is added to water (1 L) and is stirred for 16 hours. The sulfonated fibers are washed extensively with water and filtered onto a polypropylene mat (SeFar America). The ion exchange capacity of the fibers is 1.03 meq H±/g. The reaction is repeated using two grams of chlorosulfonic acid and two grams of nanofibers of polyphenylene sulfide. The ion exchange capacity of the resultant fibers is 1.3 meq H±/g. The resulting fibers of polyphenylene sulfide with sulfonic acid groups is referred to as PPS-S fibers.
Dispersing the PPS Fibers:
Effectively dispersing the fibers is a vital step in the process of introducing them to an ink as a reinforcing component of an electrode. About 0.10 g of PPS nano-fibers are added to 3.33 grams of water and 6.67 grams of ethanol. The mixture is sonicated using a Misonix 3000 ultrasonic homogenizer for 5 minutes, set to a pulse mode of 10 seconds on and 10 seconds off at 18 Watts. The sample is then reduced by gentle heating to a final liquid wt of 9.12 g (9.22 grams total). The sample is then filtered onto a polycarbonate filter creating a mat of fibers (in this example, approximately 18 um thick).
Nanofiber Mat PEM Reinforcement.
A mat of PPS fibers is filtered onto a 47 mm polycarbonate filter. The mat is removed from the filter media. A dispersion of ionomer to imbibe into the mat of fibers is prepared as follows. A dispersion medium of n-propanol (10 grams) and water (5 grams) is prepared. Nafion® D2020 (7.5 grams of a 20% wt. % solids by wt. dispersion, DuPont de Nemours) is added to 7.5 g of the 2:1 n-propanol/water medium. The 47-mm mat of fibers is placed into a beaker of the preparation and allowed to be thoroughly wetted with the ionomer preparation. The mat of imbibed fibers is then removed and allowed to dry.
PPS-S Nanofiber Mat PEM Reinforcement.
A mat of PPS fibers is filtered onto a 47 mm polycarbonate filter. The mat is removed from the filter media. A dispersion of ionomer to imbibe into the mat of fibers is prepared as follows. About 10 grams of n-propanol and 5 grams of water are prepared as a dispersing media. D2020 dispersion (2.475 grams of a 20% wt. % solids dispersion) is combined with 7.5 g of n-propanol/water media. The 47-mm mat of fibers is placed into a beaker of the preparation and is allowed to thoroughly wet with the ionomer preparation. The mat of imbibed fibers is removed and allowed to dry. Elemental analysis of the PPS-S fibers demonstrates a fluoride peak, indicative of ionomer, in the cross section (center) of the fiber mat.
PPS Nanofiber Mat PEM Reinforcement.
A dispersion is prepared by suspending 1 gram of PPS nanofibers into ethanol (500 mL) and sonication using a Misonix 3000 ultrasonic homogenizer for 5 minutes, set to a pulse mode of 10 seconds on and 10 seconds off at 18 Watts. The nano-fibers in ethanol are then vacuum filtered onto a polypropylene screen (Sefar America). The nano-fiber mat is then dried, separated from the polypropylene screen, and applied to a wet, draw-bar coating of Nafion DE2020 ionomer dispersion prepared by diluting a 20 wt. % dispersion of the DE2020 ionomer to 10 wt. % solids with isopropanol. The wet draw-bar coated film is made by using a 3-mil Bird applicator on a backer film of Kapton-PTFE (American Durofilm) situated on a vacuum platen of an Erichsen coater that is operated at 12.5 mm/s. The nano-fiber mat imbibes the ionomer solution and is then dried at 80° C. After cooling to 23° C., the nano-fiber mat filled with ionomer is then surface coated with another layer of 10 wt. % DE2020 dispersion with isopropanol using a 3-mil Bird applicator adjusted with 1-mil tape shims. The ionomer filled nano-fiber mat is then dried to 80° C., followed by oven annealing at 140° C. for 16 hours. The nanofiber reinforced composite film is removed from the backer and used as a polyelectrolyte membrane in a fuel cell.
PPS-S Nanofiber Mat PEM Reinforcement.
A dispersion is prepared by suspending 1 gram of sulfonated-PPS (PPS-S) nanofibers into ethanol (500 mL) by sonication using a Misonix 3000 ultrasonic homogenizer for 5 minutes, set to a pulse mode of 10 seconds on and 10 seconds off at 18 Watts. The nano-fibers in ethanol are then vacuum filtered onto a polypropylene screen (Sefar America). The nano-fiber mat is then dried, separated from the polypropylene screen, and applied to a wet, draw-bar coating of Nafion DE2020 ionomer dispersion prepared by diluting a 20 wt. % dispersion of the DE2020 ionomer to 10 wt. % solids with isopropanol. The wet draw-bar coated film is made by using a 3-mil Bird applicator on a backer film of Kapton-PTFE (American Durofilm) situated on a vacuum platen of an Erichsen coater that is operated at 12.5 mm/second. The nano-fiber mat imbibes the ionomer and is then dried at 80° C. After cooling to 23° C., the nano-fiber mat filled with ionomer is then top coated with another layer of 10 wt. % DE2020 dispersion with isopropanol using a 3-mil Bird applicator adjusted with 1-mil tape shims. The ionomer filled nano-fiber composite mat is then dried to 80° C., followed by oven annealing at 140° C. for 16 hours. The film is removed from the backer and used as a polyelectrolyte membrane in a fuel cell.
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
