Fluoropolymer laminated separator plate

Abstract: Electrolyte migrates inter-cell in fuel cell stacks due to an electric potential gradient across the stack. This migration can be substantially inhibited with a graphite separator plate which has been laminated to the electrolyte reservoir plates of adjacent fuel cells with a fluoropolymer resin. In such an arrangement, the fluoropolymer resin forms an electrolyte barrier between the graphite separator plate while maintaining an electrically conductive pathway between the electrolyte reservoir plate and the graphite separator plate.

Description: Technical Field The present invention relates to a separator plate and especially relates to an acid impervious separator plate for use in fuel cell and electrolysis cell stacks. Background of the Invention Fuel cell stacks, which are composed of individual fuel cells aligned electrically in series, produce electricity via an electrochemical reaction between hydrogen and oxygen. Within each fuel cell, the hydrogen contacts an anode catalyst where it is converted to hydrogen ions and free electrons. The hydrogen ions migrate across an electrolyte to a cathode catalyst where they react with oxygen and the free electrons which have passed through an external load, thereby producing electricity and forming water. During phosphoric acid fuel cell stack operation, electrolyte migrates across the cell stack, thereby decreasing the volume of electrolyte in one area of the fuel cell while increasing it in another. This migration can cause electrolyte flooding of end fuel cells in a fuel cell substack (approximately 8 fuel cells which are typically part of a larger cell stack), thereby reducing performance of the stack and stack efficiency. Prevention of electrolyte migration has been addressed in various fashions including the utilization of a graphite separator plate (see U.S. Pat. Nos. 4,360,485 and 4,301,222), a fluoropolymer barrier (see GB Pat. No. 2,236,012), and a separator plate comprised of a graphite-fluoropolymer mixture. Since individual fuel cells are typically stacked such that the first and last component of the stack is a porous graphite electrolyte reservoir plate and/or porous graphite cooler holder (herein after referred to as porous component), the migration prevention devices are located between individual cells, adjacent to the porous components of these cells. These conventional migration prevention devices were only partially successful. The graphite separator plate was meant to block inter-cell electrolyte migration. However, due to the graphite separator plate's porosity the plate eventually absorbed electrolyte, thereby allowing the electrolyte to migrate to the next cell of the stack. The fluoropolymer barrier initially functioned as a blockade to electrolyte migration. However, after about 500 hours of fuel cell operation, this barrier significantly degraded. Finally, the graphite-fluoropolymer mixture functioned similar to both the graphite plate and the fluoropolymer barrier since the porosity of the plate was reduced with fluoropolymer which blocked the passage of the electrolyte which entered the plate. This mixture initially prevented electrolyte migration. However, as thermal cycling of the fuel cell caused crevices to form in the mixture, pathways developed which allowed electrolyte migration. Referring to bars 1 - 4 of the Figure, the graphite separator plate (bars 3 and 4 ) minimized electrolyte migration to about 1,0 to about 280.0 g/m 2 /1000 hrs (0.09 to 26 g/ft 2 /1,000 hrs)., while the fluoropolymer barrier and the graphite-fluoropolymer mixture experienced electrolyte migration of about 1290 to about 26910 g/m 2 /1000 hrs. (120 to 2,500 g/ft 2 /1,000 hrs). (bar 1 ) and about 215 to about 1076 g/m 2 /1000 hrs. (20 to 100 g/ft 2 /1,000 hrs). (bar 2 ), respectively. Consequently, what is needed in the art is a separator plate capable of minimizing inter-cell electrolyte migration. Disclosure of the Invention The present invention relates to an improved separator plate for inhibiting inter-cell electrolyte migration according to annexed Claim 1. The present invention further relates to an improved fuel cell stack. This fuel cell stack has a plurality of fuel cells with an anode, a cathode, an electrolyte disposed therebetween, an anode chamber, a cathode chamber and a porous component. The improvement comprises a graphite plate laminated to the porous component of the first fuel cell with a first fluoropolymer film such that an electrically conductive pathway exists between the porous component and the graphite plate, and the fluoropolymer film forms an electrolyte barrier therebetween. The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying drawing. Brief Description of the Drawing The Figure is a bar graph of the typical electrolyte migration for the separator plate of the present invention and for prior art separator means which have been aged in a fuel cell simulated environment. This figure is meant to be exemplary and to further illustrate the present invention, and not to limit the scope of the present invention. Best Mode for Carrying Out the Invention The fuel cell stack of the present invention comprises a graphite plate located between porous components of adjacent fuel cells, with the improvement being a fluoropolymer film on at least one side of the graphite plate. This film forms an electrolyte barrier between the porous component and the graphite plate thereby inhibiting the flow of electrolyte from the porous component to the graphite plate, and if located on both sides of the graphite plate, flow of electrolyte from the graphite plate to the porous component of the next cell. The graphite plate is described in U.S. Patent No. 4,301,222. The plate is stacked, conventionally known as laid-up, to laminate the fluoropolymer film to the graphite plate and the porous component. The lay-up sequence is: porous component, fluoropolymer film, graphite plate, fluoropolymer film, porous component. After the lay-up is complete, it is laminated. This lamination is accomplished by applying heat and pressure to the lay-up for about 5 minutes to about 60 minutes. Temperatures of about 280°C (about 550°F) to about 345°C (about 650°F) and pressures of about 3,5 Kg/cm 2 to about 42 Kg/cm 2 (about 50 psig to about 600 psig) can be employed during the lamination process, with temperatures of about 305°C (about 580°F) to about 330°C (about 620°F) and pressures of about 14 Kg/cm 2 to about 28 Kg/cm 2 (about 200 psig to about 400 psig) preferred. Higher pressures improve the rate of fluoropolymer flow into the porous component, with the upper limit constrained by the compression characteristics of the porous plate. With respect to temperature, increasing temperature decreases fluoropolymer viscosity, thereby improving flow. However, temperatures above about 330°C degrade the fluoropolymer film. Essentially, during lamination, the fluoropolymer flows into the porous component, thereby forming a fluoropolymer barrier to electrolyte migration. Typically about 0,25 mm to about 0,51 mm (about 0.01 inches to about 0.02 inches) of the porous component adjacent to the fluoropolymer film becomes filled with the fluoropolymer. The fluoropolymer barrier inhibits electrolyte from passing from the porous component to the graphite plate. Consequently, the fluoropolymer barrier is sufficiently thick such that during fuel cell operation the barrier sufficiently inhibits the electrolyte flow to the graphite plate, thereby preventing the electrolyte migration through the graphite plate. This film can have a thickness up to about 0,76 mm (0.03 inches) and typically has a thickness up to about 0,25 mm (0.01 inches), with a thickness of about 0,05 mm to about 0,13 mm (0.002 inches to 0.005 inches) preferred before lamination. These film thicknesses correspond to barrier thicknesses equivalent to about three times the film thickness. Consequently, the barrier thickness typically ranges from about 0,15 mm to about 0,38 mm (0.006 inches to 0.015 inches). With the fluoropolymer film being substantially continuous, electrolyte cannot pass around the barrier or through the barrier since there are no holes. Therefore, a continuous (single sheet which covers substantially all of the porous component and graphite plate) fluoropolymer film is heated and pressurized to form the electrolyte barrier. As the film flows into the porous component and conforms to the topography of the graphite plate, graphite to graphite contact between the graphite plate and the porous component is attained, thereby forming an electrically conductive pathway from the porous component through the graphite plate to the next porous component, Typically, the electrical resistance between the porous component and the graphite plate is less than about 37,2 mV/A cm 2 (4 mV/100 ASF (millivolts per (100 amperes per square foot))), and preferably about 9,3 mV/A cm 2 to about 18,6 mV/A cm 2 (1 mV/100 ASF to 2 mV/100 ASF) or lower. In order to ensure maintenance of the electrically conductive pathway after lamination of the lay-up described above, it is preferred to cool the lay-up to ambient temperature, under pressure. The film can be any resin which withstands the fuel cell environment and can be melt processed. These resins include, but are not limited to: FEP (fluorinated ethylene-polypropylene resin), PFA (a copolymer of tetrafluoroethylene and perfluorinated vinyl ether), mixtures thereof, and others. These films are produced by Du Pont de Nemours, E. I., Co., Wilmington, Delaware as types of TEFLON®, and by others. The present invention will be further exemplified in the following illustrative example. This example is meant to be exemplary and not to limit the scope of the present invention. Example The following example has been used to form a laminated graphite separator plate having a FEP Grade TEFLON® film. The graphite plate was laid-up in the following component sequence: electrolyte reservoir plate, 0.127 mm (0.005 inches) thick TEFLON film, 0.9 mm (0.035 inches) thick graphite plate, 0.127 mm (0.005 inches) thick TEFLON film, and electrolyte reservoir plate, with all components having both a length and a width of 83,2 cm by 83,2 cm (32.75 inches). Once laid-up, the assembly was placed in a 500 ton, electrically heated press and heated to 343°C (620°F) under 28 Kg/cm 2 (400 psig) for 10 minutes. Once laminated, the lay-up was cooled down to about ambient temperature under about 28 Kg/cm 2 . The electrical conductivity measurement, taken in the thru-plane direction, was 12,1 mV/A cm 2 (1.3 mV/100 ASF) current density. The unique separator plate of the present invention is capable of significantly inhibiting inter-cell electrolyte migration. The prior art separator plates were only capable of reducing electrolyte migration to about 0,97 to about 10,76 g/m 2 /1000 hrs. (0.09 to 1.0 g/ft 2 /1000 hrs). (see the Figure, bar 4 , while the present invention reduces electrolyte migration to below about 0,13 g/m 2 /1000 hrs. (0.012 g/ft 2 /1,000 hrs). (bar 5 ). The separator plate of the present invention is a combination of fluoropolymer film and a graphite plate which when laminated to the fuel cell porous component creates an overall separator plate capable of enduring the 40,000 hour phosphoric acid fuel cell environment at about 0,215 A/cm 2 to about 0,43 A/cm 2 (200 amps/ft 2 to 400 amps/ft 2 ). Although the present invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the scope of the invention as defined in the claims.

Claims: A separator plate for inhibiting intercell electrolyte migration in a fuel cell, obtainable by a process comprising the steps of: stacking a plate comprising a graphite plate, at least one porous component and at least one fluoropolymer film placed between said graphite plate and at least one porous component; laminating said plate by applying heat and pressure, thereby forming an electrically conductive pathway and an elecrolyte barrier between said graphite plate and said at least one porous component. A separator plate according to claim 1, wherein the heat is applied at a temperature in a range from 280°C to 345°C. A separator plate according to claim 1, wherein the heat is applied at a temperature in a range from 305°C to 330°C. A separator plate according to claim 1, wherein said pressure is in a range from 3,5 kg/cm 2 to 42 kg/cm 2 . A separator plate according to claim 1, wherein said fluoropolymer film is fluorinated ethylene-polypropylene resin, a copolymer of tetrafluoroethylene, or mixtures thereof. A separator plate according to claim 1, wherein said porous component is an electrolyte reservoir plate. A separator plate according to claim 1, wherein said porous component is a graphite cooler holder. A separator plate according to claim 1, wherein said process comprises the following step of cooling said plate to ambient temperature under pressure. A separator plate according to claim 1, wherein said fluoropolymer film has a thickness of less than 0,76 mm before the lamination. A separator plate according to claim 1, wherein said fluoropolymer film has a thickness of less than 0,25 mm before the lamination. A separator plate according to claim 1, wherein said fluoropolymer film has a thickness in a range from 0,05 mm to 0,13 mm before the lamination. A separator plate according to claim 1, wherein from 0,15 mm to 0.38 mm of the porous component adjacent to the fluoropolymer film becomes filled with the fluoropolymer after the lamination. A separator plate according to claim 1, wherein the electrical resistance between the porous component and the graphite plate is less than 37,2 mV/Acm 2 after the lamination. A separator plate according to claim 1, wherein the electrical resistance between the porous component and the graphite plate is less than 18,6 mV/Acm 2 after the lamination. A fuel cell stack comprising a separator plate, for inhibiting intercell electrolyte migration, according to any of the preceeding claims.

Inventors: ROCHE ROBERT P, ROCHE, ROBERT P.
Applicants: CLC SRL, INT FUEL CELLS CORP, CLC S.R.L., INTERNATIONAL FUEL CELLS CORPORATION
Application year: 1993
Main classification: H01M2
IPC codes: H01M8