| Sulfonated-perfluorocyclobutane polyelectrolyte membranes for fuel cells -> Monitor Keywords |
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Sulfonated-perfluorocyclobutane polyelectrolyte membranes for fuel cellsRelated Patent Categories: Chemistry: Electrical Current Producing Apparatus, Product, And Process, Fuel Cell, Subcombination Thereof Or Methods Of Operating, Solid Electrolyte, Electrolyte Composition Chemically SpecifiedSulfonated-perfluorocyclobutane polyelectrolyte membranes for fuel cells description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070099054, Sulfonated-perfluorocyclobutane polyelectrolyte membranes for fuel cells. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] The present invention relates to electrochemical conversion cells, commonly referred to as fuel cells, which produce electrical energy by processing first and second reactants. For example, electrical energy can be generated in a fuel cell through the reduction (cathode reaction: O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O) of an oxygen-containing gas and the oxidation (anode reaction: 2H.sub.2.fwdarw.4H.sup.++4e.sup.-) of a hydrogenous gas. By way of illustration and not limitation, a typical cell comprises a membrane electrode assembly positioned between a pair of flow fields accommodating respective ones of the reactants. More specifically, a cathode flowfield plate and an anode flowfield plate can be positioned on opposite sides of the membrane electrode assembly. The voltage provided by a single cell unit is typically too small for useful automotive power application so it is common to arrange a plurality of cells in a conductively coupled "stack" to increase the electrical output of the electrochemical conversion assembly. [0002] By way of background, the conversion assembly generally comprises a membrane electrode assembly, an anode flowfield, and a cathode flowfield. The membrane electrode assembly in turn comprises a proton exchange membrane separating an anode and cathode. The membrane electrode assembly generally comprises, among other things, a catalyst supported by a high surface area support material and is characterized by enhanced proton conductivity under wet conditions. For the purpose of describing the context of the present invention, it is noted that the general configuration and operation of fuel cells and fuel cell stacks is beyond the scope of the present invention. Rather, the present invention is directed to particular polyelectrolyte membranes, processes for preparing polyelectrolyte membranes and polyelectrolyte membrane fuel cells. Regarding the general configuration and operation of fuel cells and fuel cell stacks, applicants refer to the vast collection of teachings covering the manner in which fuel cell "stacks" and the various components of the stack are configured. For example, a plurality of U.S. patents and published applications relate directly to fuel cell configurations and corresponding methods of operation. More specifically, FIGS. 1 and 2 of U.S. Patent Application Pub. No. 2005/0058864, and the accompanying text, present a detailed illustration of the components of a fuel cell stack--this particular subject matter is expressly incorporated herein by reference. BRIEF SUMMARY OF THE INVENTION [0003] Recently, proton exchange or polyelectrolyte membrane (PEM) fuel cells have attracted considerable interest as sources of non-polluting, high-density power for automotive propulsion. However, for widespread commercialization, low cost, high-performance PEMs with improved durability are still being sought. Presently, PEM fuel cells operate at temperatures up to 95.degree. C. with external humidification being required to maintain proton conductivity that deteriorates rapidly as the membranes dry out. Perfluorosulfonic acid membranes have been the preferred materials for PEM, but they suffer from poor mechanical integrity and they are expensive. Consequently, new alternative PEM materials are continuously being sought. [0004] The present invention is directed to a process for preparing a polymer. The process comprises sulfonating a perfluorocyclobutane polymer with a sulfonating agent to form a sulfonated perfluorocyclobutane polymer. The sulfonating agent comprises oleum or SO.sub.3. [0005] In accordance with another embodiment of the present invention, a process for preparing a proton exchange membrane is provided. The process comprises the steps of: (a) sulfonating a perfluorocyclobutane polymer with a sulfonating agent to form a sulfonated perfluorocyclobutane polymer and (b) forming the sulfonated perfluorocyclobutane polymer into a proton exchange membrane. The sulfonating agent comprises oleum or SO.sub.3. [0006] In accordance with yet another embodiment of the present invention, a fuel cell is provided. The fuel cell comprises a proton exchange membrane formed by sulfonating a perfluorocyclobutane polymer with oleum to form a sulfonated perfluorocyclobutane polymer and (b) forming the sulfonated perfluorocyclobutane polymer into a proton exchange membrane. The sulfonating agent comprises oleum or SO.sub.3. [0007] In accordance with a further embodiment of the present invention, a process for assembling a device comprises the act of preparing a membrane electrode assembly. The membrane electrode assembly comprises electrically conductive material on either side of a proton exchange membrane. The proton exchange membrane is prepared according to a process comprising the act of sulfonating a perfluorocyclobutane polymer with a sulfonating agent to form a sulfonated perfluorocyclobutane polymer, wherein the sulfonating agent comprises oleum or SO.sub.3. The device comprises an electrochemical conversion assembly comprising at least one electrochemical conversion cell configured to convert first and second reactants to electrical energy. The electrochemical conversion cell comprises the membrane electrode assembly, an anode flowfield portion and a cathode flowfield portion defined on opposite sides of the membrane electrode assembly. A first reactant supply configured to provide a first reactant to an anode side of the membrane electrode assembly via the anode flowfield portion, and a second reactant supply configured to provide a second reactant to a cathode side of the membrane electrode assembly via said cathode flowfield portion. BRIEF DESCRIPTION OF THE FIGURES [0008] The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following figures. [0009] FIG. 1 is a graph depicting that the amount of sulfonation is determined by the ratio of oleum to polymer used; [0010] FIG. 2 is a graph depicting the conductivity vs. % relative humidity of sulfonated perfluorocyclobutane-biphenyl vinyl ether (BPVE) polymers 4 with different Ion Exchange Capacities (I.E.C.s); [0011] FIG. 3 is a graph depicting conductivity vs. % relative humidity of sulfonated perfluorocyclobutane-hexafluoroisopropylidene biphenyl vinyl ether (BPVE 6F) copolymers, 6 with different I.E.C.s; [0012] FIG. 4 is a graph depicting water uptakes for various sulfonated perfluorocyclobutane polymers; [0013] FIG. 5 is a graph depicting volume swell at 25 and 100.degree. C. in water for PFCB Polymers, plotted as semi-log (A) and linear graph (B); [0014] FIG. 6 is a graph of fuel cell data of sulfonated BPVE polymers depicting cell voltage (in volts) versus current density (in Amperes/cm.sup.2), which has been IR-corrected for the experimentally measured High Frequency Resistance (HFR); and [0015] FIG. 7 is a graph of fuel cell data of sulfonated BPVE 6F copolymers depicting cell voltage (in volts) versus current density (in Amperes/cm.sup.2), which has been IR-corrected for the experimentally measured High Frequency Resistance (HFR). DETAILED DESCRIPTION [0016] The inventors have discovered a new process for preparing new proton conducting membranes made with perfluorocyclobutanes polymers (PFCBs) having sulfonic acid groups (SPFCBs,), which may be used in PEM fuel cells that can operate over a broad range of relative humidity and at temperatures around 95.degree. C. The properties of the SPFCB films are dependent on the chemical structure and the ion exchange capacity of the film, which can be tailored by the reaction conditions used. These SPFCB-films are reasonable alternatives to perfluorosulfonic acid membranes, presently being used in PEM fuel cells, because the sulfonated polymers have high intrinsic proton conductivity and inherent dimensional-, hydrolytic- and high-temperature stability. [0017] PFCBs are commercially available from Tetramer Technologies, under license agreements from Dow Chemical. Examples of PFCBs are provided with the structures 1-3: The synthesis of PFCBs is described in U.S. Pat. Nos. 5,037,917 and 5,159,037--this particular subject matter is expressly incorporated herein by reference. [0018] To form potentially useful PEMs, subsequent sulfonation of the PFCBs is required. Prior art teaches a sulfonation procedure that uses chlorsulfonic acid, which has limited synthesis utility and scope and which produces inconsistent membrane materials. The inventors have discovered a novel process for synthesizing SPFCBs. The process comprises sulfonating a perfluorocyclobutane polymer with a sulfonating agent to form a sulfonated perfluorocyclobutane polymer. The sulfonating agent comprises oleum, SO.sub.3 or a combination thereof. One skilled in the art will appreciate that various PFCBs are available for use in the present process, any of which may be employed herein. In one embodiment, the PFCB comprises the formula: wherein X is O or S; R is n is greater than about 20. In one embodiment, n is from about 20 to about 500. Specific examples of such PFCBs include, but are not limited to, structures 1-3, defined in detail above. [0019] In addition, one skilled in the art will appreciate that various concentrations of the sulfonating agent may be employed to sulfonate a PFCB polymer, any of which may be employed herein. In one embodiment, the oleum comprises 10% oleum. In another embodiment, the oleum comprises 20% oleum. In yet another embodiment, the oleum comprises 30% oleum. Moreover, one skilled in the art will appreciate that various SPFCBs may be formed from reacting a PFCB with a sulfonating agent. In one embodiment, the sulfonated polymers have between 0-2 sulfonic acids per repeat unit. Examples of such SPFCBs include, but are not limited to, structures 4-6: Furthermore, one skilled in the art will appreciate the various experimental parameters in which the process for preparing the sulfonated polymer may be performed, any of which may be employed herein. In one embodiment, the process further comprises the step of dissolving the PFCB polymer in methylene chloride prior to sulfonating the PFCB. In another embodiment, the process is performed from about -20.degree. C. to about 200.degree. C. [0020] In accordance with another embodiment of the present invention, a process for preparing a proton exchange membrane is provided. The process comprises the steps of: (a) sulfonating a perfluorocyclobutane polymer with a sulfonating agent to form a sulfonated perfluorocyclobutane polymer and (b) forming the sulfonated perfluorocyclobutane polymer into a proton exchange membrane. The sulfonating agent comprises oleum or SO.sub.3. Continue reading about Sulfonated-perfluorocyclobutane polyelectrolyte membranes for fuel cells... 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