WO2007013046A2

WO2007013046A2 - Fuel cell surface activation

Info

Publication number: WO2007013046A2
Application number: PCT/IB2006/052597
Authority: WO
Inventors: Joseph Darke; Casey Johnson; Eve Steigerwalt
Original assignee: Dana Corporation
Priority date: 2005-07-28
Filing date: 2006-07-28
Publication date: 2007-02-01
Also published as: WO2007013046A3

Abstract

A method of decreasing contact resistance of a fuel cell by activating various surfaces of a fuel cell using a laser is disclosed. A surface layer of a resin binder of a homogeneous separator plate may be removed. In addition, a coating of a gas diffusion media in areas where the gas diffusion media contacts the separator plates may also be removed.

Description

FUEL CELL SURFACE ACTIVATION

Technical Field

[1] This application claims the benefit of United States Provisional Application No.

60/703,404, filed on July 28, 2005, the entirety of which is hereby incorporated by reference.

[2] The embodiments of the invention described herein are generally directed to a fuel cell and a method of manufacturing the same.

Background Art

[3] A fuel cell is an apparatus for power generation by supplying a reactant gas

(typically hydrogen) to an anode and an oxidant gas (typically air or oxygen) to a cathode. Referring now to FlG. 1, a typical prior art proton exchange membrane (PEM) fuel cell 6 is illustrated. Two separator plates 2, 3 are typically separated by a membrane electrode assembly (MEA) 1. MEA 1 is a thin, proton-conductive, polymeric membrane having anode and cathode electrodes on either side. In general, the membrane is made from ion exchange resins, and typically comprises a per- fluoronated sulfonic acid polymer. The anode electrode and cathode electrode usually include a catalyst layer which is composed mainly of carbon particles carrying a platinum metal catalyst and formed on the surface of the polymer membrane, and a diffusion layer formed on the outer surface of the catalyst layer which is gas permeable and electrically conductive.

[4] Separator plates 2, 3 provide mechanical support to MEA 1 and allow electrical connection of a plurality of MEAs in series, or in parallel in some cases. Gas flow paths 4, 5 are usually provided on a surface of each separator plate adjacent the MEAs. Gas flow paths 4, 5 supply reactant and oxidant gases to each fuel cell. As an alternative, gas flow paths 4, 5 may be provided separately from separator plates 2, 5.

[5] The separator plates generally need to have a high electrical conductivity, high corrosion resistance, and deliver the fuel and oxidant gases with a minimum of leakage. For such reasons, conventional separator plates are usually formed from carbon materials such as polymer composites, which consist of graphite (or other forms of carbon) with a polymer matrix, glassy carbon and expanded graphite. The gas flow paths may be formed by cutting the surface of the separator plate or by molding paths into the plate itself with a mold when the material is expanded graphite or a polymer composite.

[6] The anode and cathode faces typically comprise a diffusion media, finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive particles intermingled with the catalytic and carbon particles; or catalytic particles, without carbon, dispersed throughout a polytetrafluorethylene (PTFE) binder.

[7] The gas diffusion media of the MEA press against the anode and cathode faces, while the anode and cathode separator plates serve as the secondary current collectors for the fuel cell, and the mechanical support for the MEA. Suitable primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh, noble metal screen, and the like.

[8] Fuel cell 6 generates electric power and heat from the electrochemical reaction between the oxidant and reactant gases. During operation, the MEA selectively transports hydrogen ions from the reactant feed stream across the membrane, while remaining electrons travel from the anode to the cathode through an external circuit. Electricity from this external circuit can be used to operate an electric motor, as an example.

[9] A gasket is typically arranged on the peripheral portions of the electrodes with the

MEA therebetween so as to prevent reactant or oxidant gases from leaking out or mixing together. These gaskets are usually assembled into a single part together with the electrodes and MEA prior to assembly of the fuel cell itself.

[10] Since the fuel cell generates heat during operation, it may be necessary to cool the cell with water or other similar fluid coolants to control the temperature of the fuel cell stack. In general, a cooling section for feeding the cooling water is provided for every one to three cells. A cooling apparatus may be inserted between the separator plates, or, more frequently, a coolant flow path may be provided on the rear surface of the separator plate. The structure of a common cell stack is such that the MEAs, separator plates and cooling sections are placed one upon another to form a stack of 10 to 200 cells. This cell stack is sandwiched by end plates, with a current collector plate and an insulating plate between the cell stack and each end plate, and secured with a clamping bolt from both sides.

[11] MEA 1 is positioned between a pair of non-porous, electrically conductive separator plates 2, 3 which serve as secondary current collectors for conducting current between adjacent fuel cells internally of the stack (i.e. in the case of bipolar plates) and at the ends of a cell externally of the stack (i.e. in the case of monopolar or end plate). Separator plates 2, 3 each contain gas flowpaths 4, 5 that distribute the reactant and oxidant gases over the surfaces of the anode and the cathode. These flow fields generally include a plurality of lands which contact the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply header and an exhaust header located at opposite ends of the flow channels.

[12] Contact resistance between separator plates 2, 3 and MEA 1 has a negative effect on overall fuel cell performance, and should therefore be minimized to the furthest extent possible. One way to reduce contact resistance is to activate the surfaces of the fuel cell separator plate adjacent MEA 1. This may be carried out through any mechanical grinding or sanding process. However, it is difficult to achieve the tight tolerances necessary for production of separator plates 2, 3 using such mechanical processes. Grinding or sanding processes may additionally cause the thickness of the separator plates to be uneven. Finally, grinding or sanding processes may remove conductive carbon or graphite particles from the exposed separator plate surface, thus decreasing any gain in electrical conductivity which results from the grinding or sanding process.

[13] Contact resistance of the fuel cell 6 may also be reduced by removing the polymeric coating typically formed on gas diffusion media 14. However, mechanical processes such as grinding or sanding easily damage the delicate structure of gas diffusion media 14 or MEA 1. Moreover, it would be desirable to leave the coating intact in certain areas of gas diffusion media 14 where it is necessary to prevent accumulation of water, while precisely removing the coating in the areas contacting separator plates 2, 3, since separator plates 2, 3 themselves will prevent water accumulation in such areas. Mechanical processes are generally ill-suited for such precise removal of the coating in certain areas of the surface of MEA 1 while leaving the coating intact in other areas.

[14] Accordingly, it would be desirable to activate the surface layer of fuel cell separator plates to decrease the contact resistance of the separator plate while maintaining the tight tolerances necessary during manufacturing. This surface activation should also improve thermal conductivity. Additionally, it would also be desirable to economically and efficiently remove a coating from a primary current collector sheet such as a carbon material in the areas where the carbon material contacts the separator plates. Disclosure of Invention

Advantageous Effects

[15] In the embodiments and methods described, laser technology is employed for

'treating' or 'activating' a surface of a homogeneous separator plate without additional elements such as the electrically conductive fibers by burning off a resin layer material. The surface of the separator plate is subjected to a thermal surface treatment and not a deliberate attempt at decreasing thickness. This laser thermal surface treatment improves thermal conductivity. In addition, the method uses the laser treatment to remove a coating from a gas diffusion media in the areas where the gas diffusion media contacts the separator plates. Description of Drawings

[16] The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description:

[17] FlG. 1 is a top perspective view of a prior art proton exchange membrane or PEM fuel cell;

[18] FIG. 2 is a sectional view of a separator plate according to an embodiment; and

[19] FIG. 3 is a top perspective view of an MEA showing a coating and a laser activated pathway according to an embodiment.

Mode for Invention

[20] Referring now to the drawings, illustrative embodiments are shown in detail.

Although the drawings represent the embodiments, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain an innovative aspect of an embodiment. Further, the embodiments described herein are not intended to be exhaustive or otherwise limit or restrict the invention to the precise form and configuration shown in the drawings and disclosed in the following detailed description.

[21] Referring to FIG. 2, a separator plate 2 formed of a generally homogeneous material is shown according to an embodiment. Separator plate 2 is shown with gas flow pathways 4 for distributing reactant or oxidant gases across an MEA (not shown). Contact resistance from separator plate 2 is caused generally by the interaction of surfaces 8 of separator plate 2 with the MEA. During formation of homogenous separator plates such as separator plate 2, a resin layer may accumulate near the surfaces of separator plate 2. The method of treating surfaces 8 of the separator plate includes using a precision laser that reduces the contact resistance of a fuel cell surface by removing most, or more preferably, substantially all of the resin layer from surfaces 8. Additionally, the laser treatment generally leaves any conductive graphite or carbon particles intact upon a surface of separator plate 2, thereby maximizing gains in electrical conductivity realized from the removal of resin material. Preferably, the treatment is applied to any surface touching the MEA after assembly of the fuel cell. Separator plate 2 may be placed on a fixture while the laser beam scans the surfaces 8, thus activating surfaces 8. The laser beam power, energy, pulse frequency, and scan rate may be varied or manipulated to achieve the optimum result. In one embodiment, for example, a carbon-dioxide laser is employed. It should be noted that the surface of the separator plate is subjected to a thermal surface treatment by the laser and preferably at most minimally decreases the overall thickness of separator plate 2, if at all. Any decrease in thickness of the separator plate is preferably less than 10 microns, at most. Accordingly, separator plate 2 can maintain relatively small production tolerances. Laser thermal surface treatment also improves thermal conductivity of the separator plate by removing resin material which is less thermally conductive than other portions of the separator plate. In addition, the laser beam may be used in a deflashing operation to remove excess material from separator plate 2. As examples, it may be desirable to remove a flash portion from orifices of separator plate 2 which are used as fluid distribution manifolds during operation, or any portion of separator plate 2 where parting lines may appear as a result of any separator plate forming processes.

[22] Turning now to FlG. 3, a gas diffusion media 14 is shown according to an embodiment. Gas diffusion media 14 typically comprises a carbon cloth or paper. Typically, gas diffusion media 14 is produced as an integral part of MEA 1, such that MEA 1 comprises a membrane portion, commonly referred to as a catalyst coated membrane (CCM) (not shown), and gas diffusion media 14 on either side of the CCM. Alternatively, gas diffusion media 14 may be produced as a separate piece, such that gas diffusion media 14 generally rests between MEA 1 and separator plate 2 when assembled. Gas diffusion media 14 is typically formed of carbon fibers, which are coated with a hydrophobic polymer material either before or after gas diffusion media 14 is formed of the carbon fibers. The hydrophobic polymer material may further comprise electrically conductive particles to promote electrical conductivity of gas diffusion media 14. Examples of such electrically conductive particles include, but are not limited to, carbon or graphite particles. The hydrophobic polymer material thereby forms a coating 10 visible on gas diffusion media 14. This hydrophobic polymer material is removed from the fibers of gas diffusion media 14 in certain areas of the surface of gas diffusion media 14 by a laser, thereby exposing fibers of gas diffusion media 14 along a pathway 12. Coating 10 is generally hydrophobic to prevent accumulation of liquid water that is produced during operation of the fuel cell. The hydrophobic coating typically comprises a polymer material, such as TEFLON^®. However, coating 10 also increases the contact resistance of the fuel cell by impeding the electrical conductivity between MEA 1 and separator plate 2. The laser treatment is used to remove coating 10 from gas diffusion media 14 in the areas where MEA 1 would contact the separator plate 2 after assembly of the fuel cell.

[23] The method uses a laser treatment to remove coating 10 from gas diffusion media

14 in the areas where gas diffusion media 14 contacts separator plate 2, commonly known as a land area. The laser treatment removes coating 10, allowing the fibers of gas diffusion media 14 to directly contact separator plate 2. The laser treatment is also designed to minimally affect the 'bulk' portion of gas diffusion media 14 and catalyst, which is part of the MEA. The laser treatment may be applied by placing gas diffusion media 14 upon a fixture, and treating the desired surfaces with the laser. In one embodiment, a 60 Watt carbon dioxide laser is scanned along gas diffusion media 14 in areas where gas diffusion media 14 contacts separator plate 2. Preferably, coating 10 is removed only from material at or near a surface of gas diffusion media 14, allowing coating 10 to remain on inner material of gas diffusion media 14, thereby maintaining the effectiveness of coating 10 for its intended purpose of preventing accumulation of water. [24] The preceding description has been presented only to illustrate and describe exemplary embodiments of the methods and systems of the present invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. The invention may be practiced otherwise than is specifically explained and illustrated without departing from its spirit or scope. The scope of the invention is limited solely by the following claims.

Claims

[ 1 ] A method of manufacturing a fuel cell comprising : activating a surface of the fuel cell with a laser to reduce a contact resistance of said fuel cell; wherein a thickness of said fuel cell remains approximately unchanged.

[2] The method of claim 1, wherein said surface is a surface layer of a resin binder of a fuel cell separator plate, said separator plate being formed of a generally homogeneous material.

[3] The method of claim 2, further comprising forming said separator plate.

[4] The method of claim 2, further comprising removing a flash portion of said separator plate with said laser.

[5] The method of claim 1, further comprising providing a gas diffusion media proximate a fuel cell separator plate, said surface being a surface of said gas diffusion media; wherein activating said surface with said laser comprises removing at least a portion of a coating of said gas diffusion media, said removal of said at least a portion of said coating reducing a contact resistance of said fuel cell.

[6] The method of claim 5, wherein said at least a portion of said coating of said gas diffusion media is removed from at least one area where said gas diffusion media is in contact with said separator plate.

[7] The method of claim 6 wherein said gas diffusion media comprises a carbon material.

[8] The method of claim 5, wherein said gas diffusion media is formed integrally with a membrane electrode assembly.

[9] The method of claim 5, wherein said gas diffusion media is formed separately from a membrane electrode assembly.

[10] The method of claim 5, wherein said coating comprises a polymer material.

[11] The method of claim 10, wherein said coating comprises electrically conductive particles.

[12] The method of claim 5, wherein said coating comprises a hydrophobic material.

[13] A method of manufacturing a fuel cell separator plate comprising: providing a homogeneously molded separator plate; and activating a surface of said separator plate by treating said surface with a laser; wherein said laser activates a surface layer of a resin binder of said separator plate.

[14] The method of claim 13, wherein providing said separator plate comprises molding said separator plate.

[15] The method of claim 13, further comprising removing a flash portion from said separator plate with said laser.

[16] A method of manufacturing a fuel cell comprising: providing a gas diffusion media proximate a fuel cell separator plate; and removing at least a portion of a coating of said gas diffusion media by treating said gas diffusion media with a laser, said removal of said at least a portion of said coating reducing a contact resistance of the fuel cell.

[17] The method of claim 16, wherein said at least a portion of said coating of said gas diffusion media is removed from at least one area where said gas diffusion media is in contact with said separator plate.

[18] The method of claim 16, wherein said gas diffusion media comprises a carbon material.

[19] The method of claim 16, wherein said gas diffusion media is formed integrally with a membrane electrode assembly.

[20] The method of claim 16, wherein said gas diffusion media is formed separately from a membrane electrode assembly.

[21] The method of claim 16, wherein said coating comprises a polymer material.

[22] The method of claim 21, wherein said coating comprises electrically conductive particles.

[23] The method of claim 16, wherein said coating comprises a hydrophobic material.

[24] A fuel cell separator plate comprising: a separator plate comprising a generally homogeneous material, a resin layer binder being substantially removed from a surface of said separator plate, a plurality of graphite particles being intact upon said surface.