JP2006513545A - Diffusion layer and fuel cell - Google Patents

Abstract

The porous diffusion medium according to the present invention is placed opposite the catalyst layer of the membrane electrode assembly, the porous substrate includes carbon paper, and the water transport particles include carbon fibers or carbon powder. A region of relatively high water transport particle density and a region of relatively low water transport particle density alternate across the porous diffusion medium. The first major surface of the medium may be more collectively hydrophilic than the second major surface, and the second major surface may be more collectively hydrophobic than the first major surface. The diffusion medium is placed along the first major surface of the diffusion medium facing the catalyst layer and along the second major surface of the diffusion medium facing the flow field of the fuel cell. The porous diffusion medium includes a hydrophobic material disposed along the second major surface of the diffusion medium.

Description

  The present invention relates generally to diffusion media, fuel cells employing the diffusion media according to the present invention, and fuel cell powered systems utilizing such fuel cells. Specifically, the present invention relates to the use of diffusion media when addressing the difficulty of water transport under wet operating conditions in fuel cells and other types of devices.

  A porous diffusion medium is provided comprising a porous matrix that supports a distribution of water transport particles configured to address the difficulties of water transport under humid operating conditions.

  In accordance with one embodiment according to the present invention, a porous diffusion medium is provided comprising a porous matrix that supports a distribution of water transport particles. The distribution of water transport particles includes a plurality of high particle density regions characterized by relatively high density water transport particles and a plurality of low particle density regions characterized by relatively low density water transport particles. Define. The relatively high and low particle density regions alternate across the major planar dimensions of the porous diffusion medium. That is, the surface of the diffusion medium may include a checkerboard pattern of a high particle density region and a low particle density region, or another arrangement of regions where the high particle density region and the low particle density region are alternately adjacent to each other. .

According to another embodiment of the present invention there is provided an apparatus in which a porous diffusion medium according to the present invention is placed opposite the catalyst layer.
In accordance with yet another embodiment of the present invention, an apparatus is provided that includes a membrane electrode assembly inserted between an anode flow field and a cathode flow field of a fuel cell. A porous diffusion medium according to the invention is placed opposite the catalyst layer of the membrane electrode.

  According to yet another embodiment of the present invention, a porous diffusion medium according to the present invention is placed opposite a membrane electrode assembly, the porous substrate comprising carbon paper and water transport particles comprising carbon fiber or carbon powder. It consists of and. The relatively high water transport particle density region and the relatively low water transport particle density region alternate across the main two-dimensional area of the porous diffusion medium. Each characteristic of the relatively high particle density region and the relatively low particle density region varies across the cross section of the porous diffusion medium between the first major surface and the second major surface of the diffusion medium. The first major surface is collectively more hydrophilic than the second major surface, and the second major surface is collectively more hydrophobic than the first major surface. The diffusion medium is placed along the first major surface of the diffusion medium facing the catalyst layer and along the second major surface of the diffusion medium facing the flow field of the fuel cell. The porous diffusion medium includes a hydrophobic material disposed along the second major surface of the diffusion medium.

  The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the appended drawings. In the drawings, like structures are indicated by like reference numerals.

  Referring first to FIG. 1, a fuel cell 10 incorporating a porous diffusion medium 20 according to the present invention is illustrated. Specifically, the fuel cell 10 includes a membrane electrode assembly 30 sandwiched between an anode flow field 40 and a cathode flow field 50 of the fuel cell 10. It is contemplated that the flow fields 40, 50 and the membrane electrode 30 may take a variety of conventional or further forms to be developed without departing from the scope of the present invention. Although a particular shaped membrane electrode assembly 30 is beyond the scope of the present invention, in the illustrated embodiment, the membrane electrode assembly 30 includes a respective catalytic electrode layer 32 and an ion exchange membrane 34.

  Referring to FIG. 2, a porous diffusion medium 20 according to the present invention includes a porous substrate 22 that supports a distribution of water transport particles 24. The distribution of water transport particles includes a plurality of high particle density regions 26 characterized by relatively high density water transport particles 24 and a plurality of low particle densities characterized by relatively low density water transport particles 24. Region 28 is defined. The relatively high particle density region 26 and the relatively low particle density region 28 alternate across the main two-dimensional area of the porous diffusion medium parallel to the first major surface 21 and the second major surface 23 of the diffusion medium 20. To do.

  As will be described in detail later, the water transport particles 24 can be generated and distributed throughout the substrate 22 in a number of ways. For example, according to one embodiment of the present invention, the particles 24 are ground to the first major surface 21 of the diffusion medium 20 to create dust, and the dust is drawn through the substrate 22 by vacuum draw. A vacuum draw can also be configured to create alternating relatively high particle density regions 26 and relatively low particle density regions 28. The dust may or may not be bonded. A suitable binder, such as a fluorescent polymer, is configured to at least partially secure the water transport particles to the porous substrate.

  Suitable water transport particles 24 include any material that enhances the transport of water from one side of diffusion medium 20 to the other. For example, the water transport particles 24 include, but are not limited to, carbon (eg, carbon fiber or carbon powder), graphite (eg, graphite fiber or graphite powder), non-perfluorinated polymers, metal oxides, and combinations thereof. It is not a thing. A suitable non-perfluorinated polymer is polyvinylidene fluoride (PVDF). A suitable metal oxide is silicon dioxide. Of course, when the water transport particles 24 are generated from the material forming the porous substrate 22, the porous substrate 22 includes carbon paper and the water transport particles 24 include carbon fibers or carbon powder, or a combination of the two. If the porous substrate comprises carbon paper coated with a layer of hydrophobic material, such as PTFE, water transport particles are derived from the carbon paper and the hydrophobic layer. In this regard, the water transport particles 24 can also be derived from materials that are hydrophobic in one physical form but can operate as water transport particles that are hydrophilic in another physical form.

The porous substrate 22 includes a conductive material, carbon paper, graphite paper, cloth, felt, foam, carbon or graphite woven fabric, carbon or graphite nonwoven, metal mesh or foam, and combinations thereof. The dimensions of the substrate 22 depend largely on the design requirements associated with the particular application in which the porous diffusion medium 20 is to be utilized, but thicknesses of between about 20 μm and about 1000 μm, or in particular 200 μm, have been effective. Please note that it is easy to do. Similarly, without limitation by way of illustration, the porous substrate is about 50 ft ³ / min / ft ² (1.5 liters / min / cm ² ) in about 0.5 inches of water. permeometer number (measured by Gurley Permeometer, model No. 4301), or more generally about 20 ft ³ / min / ft ² (0.6 liter / min / cm ² ) in about 0.5 inches of water (about 1.3 cm). ) To about 100 ft ³ / min / ft ² (3.0 liters / min / cm ² ) to determine a porosity characterized by a Gurley permeometer number. In this regard, it should be noted that porosity is a measure of how easily air can pass through a sample of material. The Gurley test measures the time required for a given volume of air to pass through the sample.

  It should be noted that the water transport particles 24 may be selected such that the water transport particles 24 are sufficiently small that they can travel through the thickness dimension d of the porous substrate 22. In this manner, the particles 24 can be distributed throughout the diffusion media 20 by placing the media 20 and the particles 24 supported by the media 20 under a vacuum draw, as will be described in further detail below. it can. Furthermore, there may be operational advantages associated with the free movement of the water transport particles 24 within the substrate 22. For example, if the diffusion medium 20 is placed opposite the catalyst electrode layer of the fuel cell to address water transfer requirements in the catalyst layer, the free movement of the particles 24 will cause some of the particles to be in the catalyst layer. Can be transferred to the surface. If the size of the particles 24 is determined with reference to the ability of the particles to move within the substrate 22 of the diffusion media 20, is there a binder present in the diffusion media to adhere the particles into the substrate? It should be understood that such references are made whether or not. In other words, if the size of the particles is determined with reference to the movement characteristics of the particles in the diffusion medium 20, the movement characteristics are taken as if there is no binder in the diffusion medium 20. Please understand that.

  2 and 4 together, the water transport particles 24 are distributed across the cross section of the porous diffusion medium 20 between the first major surface 21 and the second major surface 23 of the diffusion medium. It is noted that for purposes of illustration and not limitation, according to one embodiment of the present invention, alternating high density region 26 and low density region 28 are characterized by a periodicity of about 0.5 cm. I want. Of course, the periodicity and relative size of the high density regions 26 and low density regions 28 are highly dependent on the design requirements associated with the particular application in which the porous diffusion medium 20 is to be utilized.

  The inventors have clearly understood the advantages, confirming that the water transport on the anode and cathode sides of the fuel cell is supplemented by gas transport across the diffusion medium 20. As shown in FIGS. 2 and 4, the alternate configuration of the high density region 26 and the low density region 28 of the present invention divides the region 26 where water transfer is emphasized and the region 28 where gas transfer is emphasized. To limit the interference between water transfer and gas transfer.

  As shown in FIG. 2, in one application, the cross-sectional dimensions of a relatively high particle density region 26 and a relatively low particle density region 28 are such that the first major surface 21 of the diffusion medium 20 Benefits can be obtained from the configuration that changes inversely across the cross section of the porous diffusion medium 20 between the two major surfaces 23. For example, creating a diffusion medium where one of the major surfaces 21 is occupied by a region 26 of relatively high particle density, while the other one of the major surfaces 23 is occupied by a region 28 of relatively low particle density. You can also. As a result, the first major surface is collectively more hydrophilic than the second major surface, and the second major surface is collectively more hydrophobic than the first major surface. These different characteristics can be useful in connection with a fuel cell, as described in more detail below with reference to FIG.

  The inventors have recognized that water transfer requirements in the catalyst layer of a fuel cell should be addressed in order to avoid problems associated with catalyst flooding. Specifically, water is generated in the cathode layer and back-dispersed from the cathode to the anode, leading to flooding on the cathode side and / or anode side of the fuel cell. As shown in FIG. 3, a first major surface 21 occupied by a relatively high particle density region 26 is placed opposite the catalyst electrode layer 32 to address water transfer requirements in the fuel cell catalyst layer 32.

  The density of the relatively high particle density region remains essentially uniform across one of the first and second major surfaces of the diffusion medium from one high particle density region to the next high particle density region. It may be. As an alternative, the density or configuration of the relatively high particle density region varies from one high particle density region to the next high particle density region across one of the first and second major surfaces of the diffusion medium. May be. This change in density across the surface of the diffusion medium can be useful in connection with fuel cells. This is because the water transfer requirements are more important near the flow field outlet area compared to the flow field inlet area, so the characteristic of increasing from the flow field inlet area of the diffusion medium to the flow field outlet area of the diffusion medium. This is because it may be preferable to provide a density value profile.

  In some embodiments of the present invention, the high particle density region 26 is further specified between about 135 ° and about 180 ° along one of the first major surface 21 and the second major surface 23 of the diffusion medium 20. For example, it may be defined as sufficiently hydrophilic to define an advancing contact angle between about 160 ° and about 168 °. With respect to the receding contact angle, the high particle density region is between about 95 ° to about 135 °, more specifically about 95 ° to about 135 ° along one of the first major surface 21 and the second major surface 23 of the diffusion medium 20. It may be defined as being sufficiently hydrophilic to define a receding contact angle between about 105 °.

As also shown in FIGS. 2 and 3, the porous diffusion medium 20 may include a hydrophobic material 25, for example in the form of a hydrophobic layer disposed along the second major surface 23 of the diffusion medium 20. Typically, the hydrophobic material 25 forms a relatively thin layer up to about 125 μm thick and may be impregnated with a load up to about 5 mg per cm ² diffusion medium area. The hydrophobic material 25 prevents liquid water droplets from accumulating on the second major surface 23 of the diffusion medium 20. Makes the hydrophobic material 25 more water repellent to water droplets, i.e. more hydrophobic than both the relatively high particle density region 26 and the relatively low particle density region 28 of the porous diffusion medium 20. It may be preferable to do this.

  The hydrophobic material 25 may include carbon, graphite, fluoropolymer, and combinations thereof. For purposes of illustration and not limitation, suitable fluoropolymers include polytetrafluoroethylene (PTFE), tetrafluoroethylene (TFE), ethylenetetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), Perfluoroalkoxy compounds and combinations thereof may be generated and suitable polymers may be selected from polyphenylene, polyvinylidene fluoride (PVDF), and combinations thereof.

Referring to FIG. 5, it should be noted that an apparatus according to the present invention may include a fuel cell powered vehicle 100 in combination with a fuel cell 10 and a fuel storage mechanism 15 according to the present invention.
Referring to a suitable method of making a diffusion medium according to the present invention, the first step of the method may be to immerse the substrate in a relatively hydrophobic material. A typical substrate is carbon fiber paper having a thickness of 100 to 400 microns, such as Toray TGPH-060 manufactured by Toray (Japan). The solution is generally a dispersion containing a hydrophobic material, such as polytetrafluoroethylene (PTFE) suspended in a solvent. A typical dispersion is a T-30 solution from duPont. The substrate is immersed in the dispersion for a time sufficient to achieve near complete saturation of the substrate with the material. Specifically, the substrate is placed in the solution for about 3 minutes. Remove the substrate slowly and at a slight angle to prevent breakage, allowing excess solution to flow out of the substrate. The paper is then allowed to dry on the grid shelf without squeezing for about 5 to about 10 minutes. The grid shelf holding the substrate is then placed in a furnace to withstand thermal cycling.

  The thermal cycle can be divided into three stages and the furnace temperature can be increased at 10 ° C./min. In the first stage, the thermal cycle can be increased from about 40 ° C. to about 96 ° C., and this temperature can be held for about 45 minutes. In the second stage, the temperature of the thermal cycle can be increased from 96 ° C. to about 300 ° C., and this temperature can be held for about 30 minutes. In the third stage, to sinter PTFE, the temperature of the thermal cycle can be increased from about 300 ° C. to about 390 ° C., and this temperature can be held for about 20 minutes. The furnace is then allowed to cool to 40 ° C. and the substrate is removed from the furnace.

  Usually about 0.1% to 25% of the mass of the dispersion medium, specifically about 7% of the mass of the dispersion medium, is composed of sintered PTFE. After sintering, PTFE is distributed substantially uniformly over the first and second sides of the substrate to form a relatively hydrophobic material layer. It should be understood that an accidental amount of a relatively hydrophobic material layer may remain in most substrates. The relatively hydrophobic layer may include a continuous layer or a discontinuous layer.

  The second side of the substrate is placed on a vacuum draw. The vacuum draw includes air holes that suck the substrate and hold the substrate against the vacuum table. The air holes are typically about 1/16 inch (about 1.59 mm) in diameter and are spaced apart from each other by about 1/4 inch (6.35 mm). The air holes are in rows about 1/4 inch (6.35 mm) apart from each other. These rows are usually in a staggered arrangement.

The first side of the substrate is then exposed to a vacuum draw and subjected to a grinding step. Grinding creates dust, which is drawn by a vacuum draw 70 through a region of the substrate, creating the high and low particle density regions described above. The vacuum draw pump draws air through the air holes at about 210 cubic feet per minute (about 5900 liters) for a substrate with an area of about 1000 cm ² .

  The substrate is ground on its first side, with about 10 to 500 microns of the substrate being worn away. The final thickness after grinding can be from about 185 to about 200 microns from a starting material of about 300 microns in thickness, so that typically about 100 microns of the substrate is worn away. In one embodiment of the invention where the hydrophobic material (eg, PTFE) is most concentrated on the outer surface of the substrate prior to grinding, the vacuum draw is performed throughout the grinding process and all of the relatively hydrophobic dust is passed through the substrate. Otherwise it can be continued for a sufficient time to draw the majority into a vacuumed waste container. Thus, the relatively hydrophobic dust generated near the first side 21 is the last part to be drawn through the substrate. When the vacuum is turned off, the dust can settle in and on the substrate. Specifically, relatively hydrophilic dust that has been drawn through the substrate but has not completely reached the waste container remains in the pores of the substrate. The remaining relatively hydrophobic dust that has not yet reached the pores of the substrate can settle over the first side of the substrate, thereby forming a hydrophilic layer. As an alternative, instead of relying on a ground where the relatively hydrophilic dust settles on the ground side of the substrate to form a relatively hydrophilic layer, an optional relatively hydrophilic material may be applied to the substrate. It may be spread or applied over the entire surface. The use of a grinding step with a precisely controlled thickness can improve the thickness uniformity of the diffusion media, resulting in improved thickness uniformity between and within the sheet. That is, it is well known that the thickness of the diffusion medium of the current technology is a troublesome problem because the thickness control without a margin is not allowed in the current production process.

  As explained above, the vacuum draw can include a series of air holes spaced at a certain distance according to a predetermined pattern. This pattern can be used to define separate active areas of the diffusion medium. As explained above, the fuel cell inlet region has different requirements than the fuel cell outlet region.

  Terms such as “preferably”, “usually”, and “generally” are used herein to limit the scope of the invention for which a patent application is filed or certain features have been patented. It should be noted that it is not utilized to imply that it is critical, essential, or more important to the structure or function of the present invention. Rather, these terms are merely used to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention.

  For purposes of describing and defining the present invention, the term “substantially” is used herein to refer to the inherent degree of uncertainty that can be attributed to any quantitative comparison, number, measurement, or other indication. Note that it is used to represent. The term “substantially” is also used herein to indicate the degree to which a quantitative expression can change from a specified criterion without changing the basic function of the subject matter in question.

  Although the invention has been described in detail with reference to specific embodiments thereof, it will be appreciated that modifications and changes can be made without departing from the scope of the invention as defined in the appended claims. Will. More specifically, although some embodiments of the present invention have been considered preferred or particularly advantageous, the present invention is not necessarily limited to these preferred embodiments of the invention.

1 is a schematic exploded view of a fuel cell incorporating a porous diffusion medium according to the present invention. FIG. 6 shows an appropriate distribution of water transport particles in a portion of a diffusion medium according to an embodiment of the invention. FIG. 3 shows a diffusion medium according to an embodiment of the present invention placed against a catalyst layer. FIG. 3 is a schematic diagram of a suitable distribution of high and low particle density regions across the surface of a diffusion medium according to an embodiment of the invention. It is a figure which shows the motor vehicle incorporating the fuel cell which employ | adopted the porous diffusion medium by this invention.

Claims (45)

A porous diffusion medium consisting of a porous matrix that supports a distribution of water transport particles,
The distribution of water transport particles includes a plurality of high particle density regions characterized by relatively high density water transport particles and a plurality of low particle density regions characterized by relatively low density water transport particles; Define
The relatively high particle density region and the relatively low particle density region alternate across a major planar dimension of the porous diffusion medium parallel to the first and second major surfaces of the diffusion medium. And
The water transport particles have a dimension that is small enough to allow movement of the water transport particles through the thickness dimension of the porous substrate defined between the first and second major surfaces of the diffusion medium. A porous diffusion medium characterized by the above.   The porous diffusion medium of claim 1, wherein the water transport particles are selected from carbon, graphite, non-perfluorinated polymers, metal oxides, and combinations thereof.   The porous diffusion medium of claim 2, wherein the non-perfluorinated polymer comprises polyvinylidene fluoride (PVDF) and the metal oxide comprises silicon dioxide.   The porous diffusion medium according to claim 1, wherein the water transport particles include a carbon / polymer composite.   The porous diffusion medium according to claim 1, wherein the water transport particles are selected from carbon fiber or carbon powder, graphite fiber or graphite powder, and combinations thereof.   The porous diffusion medium according to claim 1, wherein the water transfer particles include a material forming the porous substrate.   The porous diffusion medium according to claim 6, wherein the porous substrate includes carbon paper, and the water transfer particles include carbon fibers or carbon powder.   The porous diffusion medium according to claim 6, wherein the porous substrate includes carbon paper and a hydrophobic material layer, and the water transfer particles include car-on fiber or carbon powder.   The porous diffusion medium according to claim 1, wherein the water transfer particles include unbound dust.   The porous diffusion medium according to claim 1, wherein the water transport particles are dispersed between a first main surface and a second main surface of the diffusion medium across a cross section of the porous diffusion medium.   The porosity of claim 1, wherein the relatively high particle density region and the relatively low particle density region alternate across at least one of the first and second major surfaces of the diffusion medium. Quality diffusion medium.   The porous diffusion medium according to claim 11, wherein the alternating density region has a periodicity of about 1 cm or less.   The porous diffusion medium of claim 1, wherein the relatively high particle density region and the relatively low particle density region alternate across first and second major surfaces of the diffusion medium.   A cross-sectional dimension of each of the relatively high particle density region and the relatively low particle density region crosses a cross section of the porous diffusion medium, and the first main surface and the second main surface of the diffusion medium. The porous diffusion medium according to claim 1, wherein the porous diffusion medium changes inversely with each other.   The respective cross-sectional dimensions of the relatively high particle density region and the relatively low particle density region are such that one of the major surfaces is occupied by the relatively high particle density region. 15. The porous diffusion medium of claim 14, wherein the other one is varied to be occupied by the relatively low particle density region.   The respective properties of the relatively high particle density region and the relatively low particle density region are such that the first major surface is collectively more hydrophilic than the second major surface; The porous diffusion medium of claim 1, wherein the porous diffusion medium varies across the cross-section of the porous diffusion medium such that the major surfaces are collectively more hydrophobic than the first major surface.   The characteristic density value of the relatively high particle density region is substantially across one of the first and second major surfaces of the diffusion medium from one high particle density region to the next high particle density region. The porous diffusion medium of claim 1, which remains uniform.   The characteristic density value of the relatively high particle density region varies from one high particle density region to the next high particle density region across one of the first and second major surfaces of the diffusion medium. The porous diffusion medium according to claim 1.   19. The porous diffusion medium of claim 18, wherein the change in the characteristic density value defines a characteristic density value profile that increases from a flow field inlet region of the diffusion medium to a flow field outlet region of the flow field.   The high particle density region is sufficiently hydrophilic to define an advancing contact angle of about 135 ° to about 180 ° along one of the first and second major surfaces of the diffusion medium. The porous diffusion medium described in 1.   The high particle density region is sufficiently hydrophilic to define an advancing contact angle of about 160 ° to about 168 ° along one of the first and second major surfaces of the diffusion medium. The porous diffusion medium described in 1.   The high particle density region is sufficiently hydrophilic to define a receding contact angle of about 95 ° to about 135 ° along one of the first and second major surfaces of the diffusion medium. The porous diffusion medium described in 1.   The high particle density region is sufficiently hydrophilic to define a receding contact angle of about 95 ° to about 105 ° along one of the first and second major surfaces of the diffusion medium. The porous diffusion medium described in 1.   The porous diffusion medium of claim 1, wherein the porous substrate defines a substantially uniform porosity profile across the major planar dimension.   The porous substrate comprises a conductive material selected from carbon paper, graphite paper, cloth, felt, foam, carbon or graphite woven fabric, carbon or graphite nonwoven, metal mesh or foam, and combinations thereof. Item 2. The porous diffusion medium according to Item 1.   The porous diffusion medium of claim 1, wherein the porous substrate defines a thickness of about 20 μm to about 1000 μm.   The porous diffusion medium of claim 1, wherein the porous substrate defines a thickness of about 200 μm. The porous substrate defines a porosity characterized by a permeometer number of about 50 ft ³ / min / ft ² (1.5 liter / min / cm ² ) in about 0.5 inches of water (about 1.3 cm) The porous diffusion medium according to claim 1. The porous substrate is about 20 ft ³ / min / ft ² (0.6 liter / min / cm ² ) to about 100 ft ³ / min / ft ² (3 in about 0.5 inches (about 1.3 cm) of water. The porous diffusion medium of claim 1, wherein the porosity is characterized by a Gurley permeometer number of 0.0 l / min / cm ² .   The porous diffusion medium of claim 1, wherein the porous diffusion medium further comprises a binder configured to at least partially secure the water transfer particles to the porous substrate.   The porous diffusion medium according to claim 30, wherein the binder comprises a fluoropolymer.   The porous diffusion medium according to claim 1, wherein the porous diffusion medium further comprises a hydrophobic material.   The porous diffusion medium of claim 32, wherein a hydrophobic material is disposed along one of the first and second major surfaces of the diffusion medium.   34. The porous diffusion medium of claim 33, wherein the hydrophobic material comprises a layer.   35. The porous diffusion medium of claim 34, wherein the hydrophobic layer has a thickness of up to about 125 [mu] m. 34. The porous diffusion medium of claim 33, wherein the hydrophobic material disposed along one of the first and second major surfaces has a load of up to about 5 mg per cm < ² > of diffusion medium area.   The hydrophobic material is configured to make the hydrophobic material more water repellent to water droplets than the relatively high particle density and relatively low particle density regions of the porous diffusion medium. The porous diffusion medium according to claim 32.   34. The porous diffusion medium of claim 32, wherein the hydrophobic material comprises one of carbon, graphite, fluoropolymer, polymer, and combinations thereof.   The hydrophobic material is selected from one of polytetrafluoroethylene (PTFE), tetrafluoroethylene (TFE), ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy compounds, and combinations thereof. 40. The porous diffusion medium of claim 38, comprising the produced fluoropolymer.   40. The porous diffusion medium of claim 38, wherein the hydrophobic material comprises a polymer selected from polyphenylene, polyvinylidene fluoride (PVDF), and combinations thereof.   The porous diffusion medium of claim 32, wherein the hydrophobic material is disposed along only one of the first and second major surfaces of the diffusion medium.   The porous diffusion medium according to claim 1, which is placed opposite to the catalyst layer.   43. The apparatus of claim 42, wherein the diffusion medium is placed opposite the catalyst layer along the first major surface of the diffusion medium. The respective properties of the relatively high particle density region and the relatively low particle density region are such that the first major surface is collectively more hydrophilic than the second major surface; Changing across the cross section of the porous diffusion medium between the first and second major surfaces of the diffusion medium such that the major surfaces are collectively more hydrophobic than the first major surface. And
43. The apparatus of claim 42, wherein the diffusion medium is placed opposite the catalyst layer along the first major surface of the diffusion medium.   43. The apparatus of claim 42, further comprising an additional structure that, in combination with the catalyst layer and the porous diffusion medium, defines a membrane electrode assembly sandwiched between an anode flow field and a cathode flow field.

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