WO2008151068A1

WO2008151068A1 - Method of making fluid diffusion layers for fuel cells

Info

Publication number: WO2008151068A1
Application number: PCT/US2008/065401
Authority: WO
Inventors: Colleen F. Legzdins; Liviu Catoiu; John R. Gordon; Francesco P. Orfino; Virginia Branzea
Original assignee: Bdf Ip Holdings Ltd.; Ballard Material Products Inc.
Priority date: 2007-05-31
Filing date: 2008-05-30
Publication date: 2008-12-11

Abstract

A method of making a fluid diffusion layer for an electrochemical fuel cell, comprising: providing a porous carbon fiber substrate; forming a fibrous sublayer on a first surface of the porous carbon fiber substrate, the fibrous sublayer consisting essentially of an electrically conductive fibrous material and a polymeric binder; and forming at least one additional sublayer comprising an electrically conductive material on the fibrous sublayer; wherein the fibrous sublayer is interposed between the at least one additional sublayer and the porous carbon fiber substrate.

Description

METHOD OF MAKING FLUID DIFFUSION LAYERS FOR FUEL CELLS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U. S. C. § 119(e) of U.S. Provisional Patent Application No. 60/941 ,172 filed May 31 , 2007, which provisional application is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present invention relates to fluid diffusion layers for fuel cells and methods of making such fluid diffusion layers.

Description of the Related Art

Solid polymer electrolyte fuel cells employ a membrane electrode assembly ("MEA"), which comprises a solid polymer electrolyte or ion exchange membrane disposed between two electrodes. Each electrode comprises an appropriate catalyst, preferably located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy, or a supported metal catalyst such as platinum on carbon. The catalyst may be disposed in a catalyst layer, and the catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte (for example, Nafion®). The catalyst layer may also contain a binder, such as polytetrafluoroethylene. The electrode may also contain a fluid diffusion layer (typically a porous, electrically conductive sheet material) that may be employed for purposes of mechanical support and/or reactant distribution. In the case of gaseous reactants, these fluid diffusion layers are typically referred to as gas diffusion layers (GDL). Fluid diffusion layers have several functions, typically including: to provide access of the reactants to the catalyst; to provide a pathway for removal of reaction products; to serve as an electronic conductor between the catalyst layer and an adjacent flow field plate; to serve as a thermal conductor between the catalyst layer and an adjacent flow field plate; to provide mechanical support for the catalyst layer; and to provide mechanical support and dimensional stability for the ion-exchange membrane. Preferably, the fluid diffusion layers are thin, lightweight, inexpensive, and readily prepared using mass production techniques (for example, reel-to-reel processing techniques). Materials that have been employed in fluid diffusion layers for solid polymer electrolyte fuel cells include perforated sheets or meshes, and commercially available woven and non- woven carbonaceous substrates, including carbon fiber paper and carbon fabrics, which may be subjected to a hydrophobic treatment to impart hydrophobic properties to the fluid diffusion layers. Carbon fabrics and hydro- entangled felts tend to have more suitable mechanical and/or electrical properties, but contain a relatively large amount of carbon fibers, which is disadvantageous because of increased cost compared to carbon fiber papers. However, the mechanical and/or electrical properties of carbon fiber papers alone may not be adequate to meet all the requirements for fuel cell applications.

Consequently, appropriate fillers and/or coatings have been employed to improve one or more of these properties. For instance, carbon composites can be made from carbon fiber papers impregnated with a suitable matrix, typically containing a carbon-containing resin and optionally carbon and/or graphite particles. The resin is then cured and carbonized leaving behind a substantial amount of carbonization product, resulting in a stiffer and more conductive substrate. One method for making such carbon composites is described in U.S. Patent No. 6,667,127, wherein a fluid diffusion layer is prepared by impregnating a porous carbonaceous web with a carbonizable polymer having pyrrolidone functionality and then carbonizing the carbonizable polymer. The carbonizable polymer may also include a high carbon char yield resin, such as activated aramid fiber pulp, lignins, phenolics, benzoxazines and phthalonitriles, as described in U.S. Patent Application No. 2007/0087120. Fillers and/or coatings can also improve the surface roughness of carbon fiber papers. A low surface roughness is desired to prevent fibers from poking into the electrolyte membrane, which leads to internal transfers and decreases the durability of the MEA. For example, a perforation in the membrane may result in fluid transfer leaks across the membrane and/or electrical contact between the electrodes, causing a short-circuit. Fluid transfer leaks may also arise even where there is no perforation, such as when the membrane is so thin that it does not adequately prevent reactants from permeating through the membrane. A leak in the membrane of a fuel cell can cause the fuel and oxidant streams to fluidly communicate and chemically react, thereby degrading the electrochemical potential of the fuel cell. Fluid communication of the fuel and oxidant streams through a leak in the membrane during fuel cell operation can also result in serious degradation of the membrane due to the combustion of the fuel in the presence of catalyst and oxygen. Other methods of reducing surface roughness include calendering the fluid diffusion layers and/or compacting the surfaces of the fluid diffusion layers.

In addition, fillers and/or coatings have been used in fluid diffusion layers to control water management properties during fuel cell operation.

Published PCT WO 2005/048388 discloses a gas diffuser comprising a multilayer coating on a web, the coating being provided with fine gradients of porosity and hydrophobicity across the whole thickness. Carbon particles are essentially used to provide electric conductivity to the structure, and binders are used to impart structure properties to the coatings. The gas diffuser may consist of a variable number of coats, typically from 3 to 8, to achieve a fine gradient structure.

While significant advances have been made in this field, there remains a need for a fluid diffusion layer suitable for fuel cell applications that is amenable to large-scale continuous manufacturing processes, and which exhibits sufficient mechanical strength and stiffness, while having a low surface roughness, as well as providing a pore structure for adequate water management properties. The present invention fulfils these needs and provides further advantages.

BRIEF SUMMARY

In one embodiment, the invention relates to a method of making a fluid diffusion layer for an electrochemical fuel cell comprising: providing a porous carbon fiber substrate; forming a fibrous sublayer on a first surface of the porous carbon fiber substrate, the fibrous sublayer consisting essentially of an electrically conductive fibrous material and a polymeric binder; and forming at least one additional sublayer comprising an electrically conductive material on the fibrous sublayer; wherein the fibrous sublayer is interposed between the at least one additional sublayer and the porous carbon fiber substrate. The method may further comprise applying at least one additional sublayer on the fibrous sublayer, wherein the at least one additional sublayer comprises an electrically conductive material. In addition, the method may further comprise drying, heating, and/or compressing the fibrous sublayer.

These and other aspects of the invention will be evident in view of the attached figures and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the figures are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve figure legibility. Further, the particular shapes of the elements, as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the figures.

Figure 1 is a schematic view of a prior art electrode comprising a fluid diffusion layer for an electrochemical fuel cell. Figure 2 is a schematic view of a fluid diffusion layer according to one embodiment of the present invention.

Figure 3 is a schematic view of a fluid diffusion layer according to another embodiment of the present invention. Figure 4 is a schematic view of a fluid diffusion layer according to yet another embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with fuel cells, fuel cell stacks, and fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as "comprises" and "comprising" are to be construed in an open, inclusive sense, that is as "including but not limited to".

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

"Carbon fiber substrate" means a carbonized or graphitized non- woven carbon fiber mat. "Carbonized" and "carbonization" is defined herein as increasing the proportion of carbon in a carbonizable polymer precursor, such as polyacrylonitrile (PAN) polymers and phenolic-based resins by heating the carbonizable polymer precursor to temperatures of 600⁰C or greater in an inert atmosphere. "Graphitized" and "graphitization" is defined herein as increasing the crystallinity of carbon by heating the carbonizable polymer precursor or carbonized material to temperatures of 1600⁰C or greater in an inert atmosphere.

"A fibrous material" means one or more types of fibrous material with the same or different length-to-diameter ratios. In addition, "a polymeric binder" may refer to a single polymeric material, or a mixture of two or more polymeric materials. Furthermore, "a pore former" may refer to a single sacrificial pore forming material, or a mixture of two or more sacrificial pore forming materials.

In the present context, "loading" refers to the amount of material that is applied, and is typically expressed as the mass of material per unit surface area of the fluid diffusion layer. As used herein, "sintering" means stabilization of the hydrophobic polymer.

It has been surprisingly discovered that a fluid diffusion layer having a fibrous sublayer containing electrically conductive fibers on a porous carbon fiber substrate, and an additional sublayer on the fibrous sublayer improves fuel cell performance at both low and high relative humidities. Without being bound by theory, it is believed that the electrically conductive fibers in the fibrous sublayer penetrate into the surface pores of the porous substrate, or at least a portion thereof, while not significantly penetrating into the porous substrate, for example, through the thickness of the porous substrate. It is anticipated that the fibers "bridge" the surface pores of the porous substrate due to its substantially flat or horizontal orientation relative to the surface (i.e., x-y plane across the surface of the substrate). It is undesirable to incorporate a particulate material, such as carbon black or graphite, as part of the fibrous sublayer when forming the fibrous sublayer on the porous substrate because particulate materials will likely penetrate into the porous substrate and reduce the porosity thereof. However, such particulate materials may be formed as part of the additional sublayer after forming the fibrous sublayer because the fibrous sublayer helps to reduce penetration of the additional sublayer, and likely reduces the surface texture, such as waviness and roughness, and/or improves mechanical properties, such as flexural strength and stiffness, without significantly reducing porosity of the porous substrate. Thickness is also not substantially increased because the loading of the fibrous sublayer is typically low and penetrates into at least a portion of the surface pores of the substrate. In addition, since the fibers in the fibrous sublayer are oriented in a substantially flat or horizontal direction, the fibers will not form pinholes in the membrane.

Suitable porous carbon fiber substrates for the present fluid diffusion layer include carbonized or graphitized carbon fiber non-woven materials such as, but not limited to, TGP-H-060 and TGP-H-090 (Toray Industries Inc., Tokyo, Japan); AvCarb® P50, EP-40, and EP-60 (Ballard Material Products Inc., Lowell, MA); and GDL 24 and 25 series material (SGL Carbon Corp., Charlotte, NC). In some embodiments, the porous substrate may be made by the methods described in U.S. Patent No. 6,667,127 and U.S. Patent Application Publication No. 2007/0087120. The choice of porous substrates is not essential to the present invention and one of ordinary skill in the art will be able to select a suitable porous substrate for a given application. In some embodiments, the porous substrate may be made hydrophobic, such as by impregnating the substrate in a solution containing a polymeric binder, which is then dried and/or sintered prior to application of the fibrous sublayer, or simultaneously sintered with the fibrous sublayer.

The electrically conductive fibrous material of the fibrous sublayer may be carbonaceous or graphitic, such as, but not limited to, chopped carbon fibers, milled carbon fibers, carbon whiskers, carbon nanotubes, chopped graphite fibers, milled graphite fibers, graphite whiskers, and graphite nanotubes, or combinations thereof. In some embodiments, the length of the electrically conductive fibers is about equal to or slightly greater than the mean surface pore diameter of the porous substrate to prevent substantial penetration of the conductive fibers therein. The mean surface pore diameter may be measured by image analysis, for example, by taking a microscopy picture of the substrate surface at a suitable magnification, measuring the diameter of a number of open surface pores, and averaging the diameters measured. Additionally, the length of the conductive fibers may be less than the thickness of the porous substrate.

Suitable polymeric binders for the fibrous sublayer and substrate include hydrophobic fluorinated polymers such as polytetrafluoroethylene ("PTFE"), fluorinated ethylene propylene ("FEP"), and perfluoroalkoxy ("PFA"), or combinations thereof. In some embodiments, the polymeric binder in the fibrous sublayer and substrate are the same. In other embodiments, the polymeric binder in the fibrous sublayer and the substrate are different. In yet other embodiments, the loading of the polymeric binder in the fibrous sublayer and the substrate may be the same or may be different. As a result, the desired hydrophobicity, pore size, and other properties of the fluid diffusion layer may be controlled through the thickness thereof.

The constituents of the fibrous sublayer may first be dispersed in a suitable liquid carrier such as an alcohol, water, or combinations thereof, homogeneously blended to form a dispersion, and subsequently formed on the porous substrate. Any method known in the art for forming a sublayer dispersion on a substrate may be used, such as, but not limited to, knife- coating, screen-printing, slot die coating, microgravure coating, decal transferring, and spraying. At least a portion of the fibrous sublayer penetrates into at least a portion of the surface of the porous substrate, for example, into the surface pores without substantial penetration into the porous substrate, while a remainder portion of the fibrous sublayer may be dispersed on top of the substrate surface.

One skilled in the art will appreciate that the ratio of materials and types of materials in the fibrous sublayer, as well as the sublayer loading and application method, may be selected to control the desired characteristics of the resulting fluid diffusion layer. These characteristics may include porosity, reactant and water diffusion characteristics, mechanical strength, stiffness, and surface roughness. In one example, penetration of the fibrous sublayer into the surface of the porous substrate and/or sublayer loading may be controlled by adjusting the viscosity or surface tension of the fibrous sublayer dispersion. One skilled in the art may readily determine a desired viscosity for control of these parameters, for example, by varying the solids content of the dispersion and/or shearing rates of the dispersion when blending. Preferably, the fibrous sublayer is formed on the porous carbon fiber substrate, for example, formed on the substrate after carbonization because it is believed that penetration of the fibrous sublayer into the porous carbon fiber substrate can be reduced and/or better controlled.

After application of the fibrous sublayer, the coated substrate may be heated to sinter the hydrophobic binder therein. Sintering temperatures and time will vary for different types of polymeric binders. For example, for polytetrafluoroethylene (PTFE), suitable sintering temperatures may range from about 33O⁰C to about 420⁰C, and suitable sintering times may range from about 5 minutes to about 15 minutes.

In some embodiments, a drying step may be employed prior to heating to partially or completely remove the liquid carrier, and may be performed by any known method. One way is to allow for evaporation at ambient conditions. Another way is to employ an infrared lamp or hot plate set at a suitable temperature, for example, between 6O⁰C to 8O⁰C.

In other embodiments, the method may further include compressing the porous substrate and, optionally, the fibrous sublayer, for example, by compressing after forming the fibrous sublayer on the porous substrate. Without being bound by theory, compression may help reduce surface roughness by promoting orientation of the fibers in the fibrous sublayer in a substantially horizontal or flat direction, improve adhesion between the substrate and the fibrous sublayer, and/or control penetration of the fibrous material into the surface pores of the porous substrate without significantly reducing the porosity. Any suitable method of compression may be used, for example, by compressing between two platens or calendering. In some embodiments, the fibrous sublayer may be dried or partially dried before or after compression, or may be dried simultaneously by using heated platens or heated calendering rolls.

Additional sublayer(s) may also be formed on the fibrous sublayer before or after heating. The additional sublayer(s) should contain an electrically conductive material, which may be fibrous or particulate, and should be non- catalytic, for example, does not include a catalyst. In one example, the conductive material is carbonaceous or graphitic, such as, but not limited to, carbon blacks, graphitized carbon blacks, flake graphites, spherical graphites, as well as any of the fibrous materials mentioned above for the fibrous sublayer. In some embodiments, it may be desirable to incorporate a polymeric binder into the additional sublayer(s) to alter the water transport properties of the fluid diffusion layer and/or to improve adhesion between the layers. Without being bound by theory, it is believed that the fibrous sublayer reduces penetration of the additional sublayer(s) into the substrate and allows for a lower loading of sublayer constituents on the substrate, thereby modifying the gas diffusion properties of the fluid diffusion layer without significantly reducing porosity of the substrate.

The constituents of the additional sublayer(s) may also be dispersed in a suitable liquid carrier in a similar fashion as the fibrous sublayer, and applied to the fibrous sublayer by any of the methods described in the foregoing. The coated substrate may then be heated to sinter the polymeric binder.

The drying and heating steps of each of the sublayers may occur sequentially or simultaneously. In one example, the fibrous sublayer is dried and heated prior to application of the sublayer(s). In another example, the sublayer(s) may be applied to the fibrous sublayer after drying or partially drying the fibrous sublayer, and then heated simultaneously after application of the sublayer(s) to sinter the polymeric binder. One of ordinary skill in the art will readily determine the sequence of the applying, drying and heating steps suitable for the constituents in the additional sublayer(s).

In some embodiments, the dispersions of any of the sublayers may further include a sacrificial pore former to enhance the formation of a porous sublayer structure. Examples of suitable sacrificial pore formers include, but are not limited to, methyl cellulose, durene, styrene, camphene, camphor, and naphthalene. The sacrificial pore former is typically removed by heating to a suitable temperature, which is dependent on the type of pore former used. For example, the sacrificial pore former may be removed simultaneously during drying or heating (i.e., when sintering the polymeric binder), or may be removed in an additional heating step.

In any of the above embodiments, the sublayer loadings may be uniform or non-uniform across the surface of the porous substrate. For example, the fibrous and/or additional sublayer(s) may be applied only to the inlet and outlet regions of the porous substrate. In another example, the loading of the fibrous and/or additional sublayer(s) may be varied such that the inlet region has a higher sublayer loading than at the outlet region. This fluid diffusion layer structure may be desirable to prevent fuel cell inlet drying and/or outlet flooding problems during fuel cell operation. In yet another example, the sublayers may be applied such that they form a pattern on the surface of the substrate. Furthermore, the loading and/or constituents of each of the sublayers may be the same or may be different to form a desired gradient of diffusion properties through the thickness of the fluid diffusion layer.

In further embodiments, an ionomeric sublayer may be formed on the fluid diffusion layer (e.g., on the additional sublayer(s)) after heating. The ionomeric sublayer may comprise ionomeric materials, which are typically polymeric materials such as, but not limited to, fluorinated- and/or hydrocarbon- based ionomers. The ionomeric material may be optionally mixed with an electrically conductive material, such as a carbon or a graphite particulate or fibrous material, and applied to the fluid diffusion layer by methods such as those described above. The ionomeric sublayer may improve adhesion between the catalyst layer and the fluid diffusion layer.

In another embodiment, the present method includes steps to make a MEA. The method incorporates the foregoing steps for making a fluid diffusion layer as described previously, and further includes forming a catalyst layer on the additional sublayer of the fluid diffusion layer. For example, a catalyst mixture comprising catalyst particles along with an ionomer and/or PTFE binder may be applied in the case of an electrode suitable for a solid polymer electrolyte fuel cell. The selection of catalyst, catalyst layer components, and methods of applying it to the fluid diffusion layer are not essential, and persons of ordinary skill in the art may select suitable catalysts and application methods for a desired application. A polymer electrolyte membrane is then sandwiched between two electrodes to form a MEA.

Alternatively, the fluid diffusion layer may be incorporated with a catalyst-coated membrane (CCM) to form a MEA, where the catalyst layer is formed on the membrane rather than on the fluid diffusion layer. In this case, the CCM may be sandwiched between two fluid diffusion layers, and optionally bonded, to form an MEA.

Figure 1 illustrates an electrode 1 for a typical gas diffusion electrode that includes a prior art fluid diffusion layer. Electrode 1 comprises catalyst layer 2 and fluid diffusion layer 3. While Figure 1 shows catalyst layer 2 and fluid diffusion layer 3 as distinct layers, for the sake of illustration, they may also overlap to some extent in practice. Catalyst layer 2 comprises carbon-supported catalyst particles 4 along with ionomer 5 and PTFE binder 6 dispersed around catalyst particles 4. The use of ionomer 5 and/or binder 6 is optional. Fluid diffusion layer 3 comprises porous carbon fiber substrate 7 and filler 8, and carbonized matrix 9, which is dispersed around carbon fiber substrate 7 and filler 8. Fluid diffusion layer 3 may optionally include ionomer or PTFE binder (not shown). Further, electrode 1 may optionally include an electrically conductive sublayer (not shown) between catalyst layer 2 and fluid diffusion layer 3. Such a sublayer may also contain a carbon or graphite particulate material (for example, carbon black), and an ionomer or PTFE.

Figure 2 illustrates fluid diffusion layer 3 made according to one embodiment of the present method. Fluid diffusion layer 3 comprises a fibrous sublayer 10 containing an electrically conductive fibrous material and a polymeric binder, on surface 12 of porous carbon fiber substrate 17. At least a portion of the fibrous sublayer may penetrate into at least a portion of the surface, for example, into the surface pores, while a remainder portion of the fibrous sublayer may remain on top of the surface. The loading of fibrous sublayer 10 may be uniform or non-uniform across the surface of the porous carbon fiber substrate. In some embodiments, it may be desirable to use different types and/or different loadings of the polymeric binder in the fibrous sublayer and the substrate to control the hydrophobicity, pore size, and other properties, through the thickness of the fluid diffusion layer. In another embodiment, fluid diffusion layer 3 further comprises a sublayer 20 on fibrous sublayer 10, as shown in Figure 3, and which is on surface 12 of porous carbon fiber substrate 17. Sublayer 20 may contain an electrically conductive material, which may be fibrous or particulate, and, optionally, a polymeric material. In further embodiments, additional sublayers may be applied on sublayer 20 (not shown). The additional sublayers may also contain an electrically conductive material and, optionally, a polymeric material, such as those described above for sublayer 20, and may have the same or different composition as sublayer 20. The loading of fibrous sublayer 10, sublayer 20 and/or additional sublayers may vary across the surface of substrate 17, as discussed above.

In further embodiments, a sublayer 30 may also be formed on opposing surface 32 of porous carbon fiber substrate 17, as shown in Figure 4. The composition of sublayer 30 may be as described for sublayers 10, 20, above. The following examples are provided to illustrate certain aspects and embodiments of the invention but should not be construed as limiting in any way.

EXAMPLES

Comparative Example 1 - Anode Fluid Diffusion Layer An AvCarb™ P50T substrate from Ballard Material Products, Inc. was impregnated with PTFE (13wt%) and subsequently knife-coated with first and second sublayer dispersions of the compositions listed in Table 1 , each of which were uniformly blended with deionized water:

Table 1.

The P50T substrate was coated with the first sublayer dispersion. After partially drying to 50% moisture content, the coated carbon fiber paper was compressed at 100 PSI and substantially dried thereafter. The first sublayer loading was 12g/m². The second sublayer dispersion was then knife- coated on top of the first sublayer. After partially drying to 50% moisture content, the coated carbon fiber paper was again compressed at 100 PSI. The second sublayer loading was 8g/m². The coated P50T substrate was then sintered at 400⁰C for 10 minutes to form an anode fluid diffusion layer. Comparative Example 2 - Cathode Fluid Diffusion Layer

An AvCarb® EP40T carbon fiber substrate from Ballard Material Products, Inc. was impregnated with PTFE (10wt%). The impregnated carbon fiber substrate was then dried and knife-coated with first and second sublayer dispersions of the compositions listed in Table 2, each of which were uniformly blended with deionized water.

Table 2.

After coating the impregnated carbon fiber substrate with the first sublayer dispersion and partially drying to 50% moisture content, the coated carbon fiber substrate was compressed at 100 PSI and substantially dried thereafter. The first sublayer loading was 9g/m². The second sublayer dispersion was then knife-coated on top of the first sublayer. After partially drying to 50% moisture content, the coated carbon fiber substrate was again compressed at 100 PSI. The second sublayer loading was 3g/m². The coated carbon fiber substrate was then sintered at 400⁰C for 10 minutes to form a cathode fluid diffusion layer.

Comparative Example 3 - Cathode Fluid Diffusion Layer A second comparative cathode gas diffusion layer was prepared in the same manner as Comparative Example 2, and then knife-coated with the second sublayer dispersion to form a third sublayer. The loadings were 9g/m² and 12g/m² for the first and second layers, respectively, and 8g/m² for the third layer. Each of the layers were dried to 50% moisture content, compacted at 100PSI, and substantially dried, between each of the applications. After application of the third layer, the coated carbon fiber substrate was sintered at 400⁰C for 10 minutes.

Example 4 - Cathode Fluid Diffusion Layer

The carbon fiber substrate of Comparative Example 2 (AvCarb® EP40T) was coated with a layer of PAN carbon fibers (AGM99MF0150, Asbury Carbons, Asbury, NJ), which were about 7.4 microns in diameter and about 150 microns in length. The PAN carbon fibers were first dispersed in deionized water with 15% methyl cellulose and 12% PTFE to form a dispersion with about 14% solids content, and subsequently applied as a layer on a surface of the carbon fiber substrate. The fibrous sublayer dispersion was then partially dried to 50% moisture content and compressed at 100 PSI. After compaction and further drying, the fibrous sublayer loading was about 8 g/m². After application, compaction, and drying of the fibrous sublayer, the fibrous sublayer was then coated with the first and second sublayer dispersions as described in Comparative Example 2. After drying, the first sublayer loading was 12g/m² and the second sublayer loading was 8g/m². The coated carbon fiber substrate was then sintered at 400⁰C for 10 minutes to form a cathode fluid diffusion layer.

Comparative Example 5 - MEA Preparation An MEA was prepared by sandwiching a Gore PRIMEA Series 5510 CCM (0.4 mg Pt/cm² for each of the anode and cathode catalysts, 25 microns thickness) between the anode fluid diffusion layer of Comparative Example 1 and the cathode fluid diffusion layer of Comparative Example 2, such that the sublayers of the respective fluid diffusion layers contacted the corresponding catalysts, and sealed together to form an MEA. Comparative Example 6 - MEA Preparation An MEA was prepared in a like manner to the MEA of Comparative Example 5, but using the cathode fluid diffusion layer of Comparative Example 3.

Example 7 - MEA Preparation

MEAs were prepared in a like manner to the MEA of Comparative Example 5, but using the cathode fluid diffusion layer of Example 4.

Fuel Cell Testing - Comparative Examples 5 and 6, and Example 7

The MEAs of Comparative Examples 5 and 6, and of Example 7, were assembled in a 50cm² test cell with graphite plates, conditioned to fully hydrate and activate the catalyst-coated membrane, and tested for beginning of life performance capability. The test conditions are summarized in Table 3.

Table 3. Test Conditions

The test results for each of the MEAs of Comparative Examples 5 and 6, and Example 7, are summarized in Table 4.

Table 4. Performance Comparison

As shown in Table 4, the fuel cell performance of the Example 7 MEA (with the cathode fluid diffusion layer of Example 4) at 1 and 2 A/cm² was significantly higher than the other comparative examples, at both low and high relative humidity.

All of the above U.S. patents, U.S. patent application publications,

U.S. patent applications, foreign patents, foreign patent applications and non- patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. While particular elements, embodiments, and applications of the present invention have been shown and described, it will be understood that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings.

Claims

1. A method of making a fluid diffusion layer for an electrochemical fuel cell comprising: providing a porous carbon fiber substrate; forming a fibrous sublayer on a first surface of the porous carbon fiber substrate, the fibrous sublayer consisting essentially of an electrically conductive fibrous material and a polymeric binder; and forming at least one additional sublayer comprising an electrically conductive material on the fibrous sublayer; wherein the fibrous sublayer is interposed between the at least one additional sublayer and the porous carbon fiber substrate.

2. The method of claim 1 , wherein the electrically conductive fibrous material is selected from the group consisting of chopped carbon fibers, milled carbon fibers, carbon whiskers, carbon nanotubes, chopped graphite fibers, milled graphite fibers, graphite whiskers, and graphite nanotubes.

3. The method of claim 1 , wherein the polymeric binder is selected from the group consisting of polytetrafluoroethylene, fluorinated ethylene propylene, and perfluoroalkoxy.

4. The method of claim 1 , wherein forming the fibrous sublayer comprises: dispersing the electrically conductive fibrous material in a liquid carrier to form a dispersion; applying the dispersion to the first surface of the porous carbon fiber substrate; and removing the liquid carrier after applying to form the fibrous sublayer.

5. The method of claim 4, wherein the dispersion further comprises a pore former, the method further comprising removing the pore former after applying.

6. The method of claim 4, further comprising at least partially drying the dispersion after applying the dispersion to the porous carbon fiber substrate.

7. The method of claim 1 , wherein the fibrous sublayer is non- uniformly distributed in a planar direction over the first surface.

8. The method of claim 1 , wherein the electrically conductive material of the at least one additional sublayer is carbonaceous or graphitic.

9. The method of claim 1 , wherein the at least one additional sublayer further comprises a polymeric material.

10. The method of claim 9, wherein the polymeric material is an ionomer or a hydrophobic polymer.

11. The method of claim 1 , further comprising compressing the fibrous sublayer.

12. The method of claim 1 , further comprising heating the fibrous sublayer.