WO2008054510A2

WO2008054510A2 - Fuel cell with carbon nanotubes

Info

Publication number: WO2008054510A2
Application number: PCT/US2007/009501
Authority: WO
Inventors: Siyu Ye; Bien H. Chiem; Goran Vlajnic
Original assignee: Bdf Ip Holdings Ltd.; Ballard Material Products Inc.
Priority date: 2006-04-19
Filing date: 2007-04-18
Publication date: 2008-05-08
Also published as: WO2008054510A3

Abstract

A fuel cell comprising an anode electrode, a cathode electrode, and a polymer electrolyte membrane disposed therebetween, wherein the fuel cell comprises a first layer of carbon nanotubes on at least one surface of the polymer electrolyte membrane, wherein substantially all of the carbon nanotubes are oriented in a planar direction with respect to the at least one surface of the polymer electrolyte membrane, and at least a portion of the carbon nanotubes comprise a catalyst material. The fuel cell may further comprise a second layer of carbon nanotubes on the first layer, wherein at least a portion of the carbon nanotubes in the second layer are oriented in an orthogonal direction with respect to the at least one surface of the polymer electrolyte membrane.

Description

FUEL CELL WITH CARBON NANOTUBES

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to fuel cells and, more specifically, to fuel cells with catalysts comprising aligned carbon nanotubes.

Description of the Related Art

Electrochemical fuel cells convert fuel and oxidant into electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material (which is also known as a gas diffusion layer), such as carbon fiber paper or carbon cloth. The membrane electrode assembly comprises a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of membrane electrode assemblies are electrically coupled in series to form a fuel cell stack having a desired power output.

The membrane electrode assembly is typically interposed between two electrically conductive flow field plates, or separator plates, to form a fuel cell. Such flow field plates comprise flow fields to direct the flow of the fuel and oxidant reactant fluids to the anode and cathode electrodes of the membrane electrode assemblies, respectively, and to remove excess reactant fluids and reaction products, such as water formed during fuel cell operation.

Most catalyst layers for solid polymer fuel cells comprise a plurality of finely dispersed noble metal particles supported on a carbonaceous support, such as carbon and graphite, due to its high electrical conductivity and relatively low cost. The noble metal particles are typically supported on a catalyst support to produce a high catalytic activity while minimizing the amount of noble metal necessary to enhance the reactions of the fuel cell, thus decreasing the cost of the fuel cell. The catalyst may also comprise ionomeric materials to enhance proton conduction through the catalyst layer, as well as hydrophobic materials for removal of water from the catalyst layer. The catalyst layers are typically applied to a surface of the ion exchange membrane and/or a surface of the gas diffusion layers as an ink by methods such as knife-coating, screen- printing, painting, or spraying.

One way to increase power density and minimize cost of the fuel cell is to use ion-exchange membranes that are as thin as possible. However, when the ion- exchange membranes are thin, the fuel cell is more prone to gas crossover during operation of the fuel cell and internal leaks due to degradation of the ion-exchange membrane and/or concentrated compressive stresses.

It is well-known that the catalytic activity of the catalyst is highly dependent on the three-phase boundary contact of the reactants, typically hydrogen fuel or oxygen, the catalyst particles, typically a noble metal, and the proton-conducting electrolyte material (hereinafter referred to interchangeably with "ionomer"). Since carbon nanotubes possess a high electrical conductivity and a very high surface area because they have interior, exterior, and end surfaces, it is possible to achieve a very high concentration of proton-conducting networks and electron-conducting networks, as described in Publ. U.S. Appl. No. 2004/0197638, thereby decreasing the amount of catalyst required for the electrodes and decreasing the cost of the fuel cell. Furthermore, carbon nanotubes possess excellent chemical stability and mechanical strength due to their graphitic surface structures, which can also further enhance durability of the fuel cell.

Some methods in the art disclose growing carbon nanotubes directly on a substrate, such as the method described in Publ. U.S. Appl. No. 2004/0167014, wherein carbon nanotube arrays are grown directly on carbon paper and then subsequently platinum (Pt) is selectively electrodeposited on the carbon nanotubes. After Pt deposition, solubilized perfluorosulfonate ionomer (e.g., Nation^®) nanoparticles are introduced into the aligned arrays of multi-walled carbon nanotube films to make the catalyst layer by impregnation. Electrodes made by this method, however, require that the carbon nanotubes are grown directly on the substrate. In other methods, carbon nanotubes may be grown and subsequently applied to the substrate. For example, Publ. U.S. Appl. No. 2005/0081983 describes one method for transferring carbon nanotubes onto a conductive film substantially perpendicular to the surface of the film. Such perpendicularly oriented carbon nanotubes, however, are not desirable due their propensity to damage the ion exchange membrane, thus resulting in its premature failure.

While advances have been made with regard to fuel cell catalysts, there remains a need in the art for improved catalysts, particularly with regard to the use of catalyzed carbon nanotubes as fuel cell catalysts. The present invention addresses these issues and provides further related advantages.

BRIEF SUMMARY OF THE INVENTION

The present invention generally relates to fuel cells with catalysts comprising aligned carbon nanotubes, as well as to catalyst-coated membranes and methods for production of the same. In one embodiment, a fuel cell comprising an anode electrode, a cathode electrode, and a polymer electrolyte membrane disposed therebetween is disclosed. The fuel cell further comprises a first layer of carbon nanotubes on at least one surface of the polymer electrolyte membrane, wherein substantially all of the carbon nanotubes are oriented in a planar direction with respect to the at least one surface of the polymer electrolyte membrane, and wherein at least a portion of the carbon nanotubes comprise a catalyst material.

In a further embodiment, the fuel cell also comprises a second layer of carbon nanotubes on the first layer of carbon nanotubes, wherein at least a portion of the carbon nanotubes of the second layer are oriented in an orthogonal direction with respect to the at least one surface of the polymer electrolyte membrane.

Suitable carbon nanotubes (of either the first or second layers, or both layers) may take a variety of forms, and include single-walled carbon nanotubes, multi- walled carbon nanotubes, and combinations of the same. Further, a surface of the all or a portion of the carbon nanotubes may be further functionalized with, for example, a polymer, metal and/or surfactant. Catalyst materials are generally a metal, and typically a noble metal or alloy thereof.

In another embodiment, a catalyst-coated membrane is disclosed comprising an anode catalyst on a first surface of a polymer electrolyte membrane and a cathode catalyst on an opposing second surface of the membrane, wherein the at least one of the anode catalyst and the cathode catalyst comprises a first layer of carbon nanotubes on the respective surfaces of the polymer electrolyte membrane, and wherein substantially all of the carbon nanotubes are oriented in a planar direction with respect to the at least one surface of the polymer electrolyte membrane, and at least a portion of the carbon nanotubes comprise a catalyst material.

In a further embodiment, the catalyst-coated membrane is disclosed, comprising a second layer of carbon nanotubes on the first layer of carbon nanotubes, wherein at least a portion of the carbon nanotubes are oriented in an orthogonal direction with respect to the respective surface of the polymer electrolyte membrane. In yet a further embodiment, a method of making a catalyst-coated polymer electrolyte membrane is disclosed, the method comprising the steps of applying a first layer of solubilized carbon nanotubes to a first surface of the polymer electrolyte membrane, and orienting substantially all of the solubilized carbon nanotubes in a planar direction with respect to the first surface. In one embodiment, the step of orienting the solubilized carbon nanotubes comprises the step of subjecting the solubilized carbon nanotubes to an electric field.

In further embodiments, the method may further comprise the steps of removing the solvent from the first layer of carbon nanotubes, applying a second layer of solubilized carbon nanotubes to the first layer, and orienting at least a portion of the solubilized carbon nanotubes in the second layer in an orthogonal direction with respect to the first surface. In one embodiment, the step of orienting the solubilized carbon nanotubes of the second layer comprises the step of subjecting the solubilized carbon nanotubes to an electric field.

In another embodiment, a method of making a gas diffusion electrode is disclosed, the method comprising the steps of applying a first layer of solubilized carbon nanotubes to a first surface of a gas diffusion layer, and orienting substantially all of the solubilized carbon nanotubes in a planar direction. In one embodiment, the step of orienting the solubilized carbon nanotubes comprises the step of subjecting the solubilized carbon nanotubes to an electric field.

These and other aspects of the invention will be evident upon reference to the attached drawings and following detailed description. To this end, certain patent documents and journal articles are identified herein, all of which are hereby incorporated by reference in their entirety.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS 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 shows a fuel cell with first and second layers of carbon nanotubes on both surfaces of the polymer electrolyte membrane.

Figure 2 shows a fuel cell with a first layer of carbon nanotubes oriented in a planar direction with respect to the surface of the membrane, and a second layer of carbon nanotubes adjacent to the first layer of carbon nanotubes, wherein the second layer of carbon nanotubes is oriented in a substantially orthogonal direction with respect to the surface of the membrane.

Figure 3a shows a half-fuel cell wherein a section of the carbon nanotubes in the second layer is oriented in a different direction with respect to the remaining section of carbon nanotubes in the second layer.

Figure 3b shows a half- fuel cell wherein a plurality of sections of carbon nanotubes in the second layer are oriented in a different direction with respect to the remaining sections of carbon nanotubes in the second layer. Figure 4a, 4b, and 4c show a carbon nanotube with two open ends, a carbon nanotube with one open end and one closed end, and a carbon nanotube with two closed ends, respectively.

Figure 5 shows a carbon nanotube with a higher concentration of catalyst material on the bottom end than on the top end of the same carbon nanotube.

DETAILED DESCRIPTION OF THE INVENTION

Unless the context requires otherwise, throughout the specification and claims that 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".

Carbon nanotubes are typically grown via chemical vapor deposition (CVD), or arc discharge, on a bed of catalyst particles at a high temperature, the catalyst particles typically being nickel, cobalt, iron, or compounds thereof. They may be either grown to the desired length, or cut to the desired length. In most cases, carbon nanotubes need to be purified to remove any amorphous carbon deposits and residual catalyst particles from the end of the carbon nanotube, because these deposits and particles are typically not desirable. Furthermore, carbon nanotubes may be single- walled, multi-walled, or combinations thereof. Typically, the diameter of carbon nanotubes, either single-walled or multi-walled, ranges from 0.6 nm to 100 nm in diameter, and the length ranges from 50 nm to 5 cm or more. In addition, carbon nanotubes may have two open ends, two closed ends, or one open end and one closed end.

In one embodiment, with reference to Figure 1, membrane electrode assembly 10 (hereinafter referred to as "MEA") includes an anode electrode 12, a cathode electrode 16, and a polymer electrolyte membrane 40 (hereinafter referred to as "PEM") disposed therebetween. Anode electrode 12 contains an anode gas diffusion layer 14 and an anode catalyst 22. Cathode electrode 16 contains , a cathode gas diffusion layer 18 and a cathode catalyst 30. Anode catalyst 22 comprises a first layer 23 of carbon nanotubes 24 adjacent a first surface of PEM 40, wherein substantially all of the carbon nanotubes 24 are oriented in a planar direction with respect to the first surface of PEM 40. Likewise, cathode catalyst 30 includes a first layer 31 of carbon nanotubes 32 adjacent an opposing second surface of PEM 40, wherein substantially all of the carbon nanotubes 32 are oriented in a planar direction with respect to the second surface of PEM 40. Alternatively, only one of anode catalyst 22 and cathode catalyst 30 includes a first layer 23, 31 of carbon nanotubes 24, 32 that are oriented in a planar direction with respect to the surface of PEM 40.

As used herein, "planar" means that the longitudinal axis of the carbon nanotubes is parallel to the comparative surface, such as the first surface of the PEM as discussed above. Such a parallel relationship may also be characterized as a zero angle (0°) relative to the comparative surface, although some degree of variation is acceptable in this context (e.g., 0° ± 30°). Further, and as used herein, "substantially all of the carbon nanotubes" means that at least 75%, generally at least 85%, and typically at least 90% of the carbon nanotubes are oriented in a planar direction with respect to the comparative surface of the PEM. In another embodiment, at least one of anode catalyst 22 and cathode catalyst 30 comprises a second layer 25, 33 of carbon nanotubes 44, 46, respectively, wherein at least a portion of the carbon nanotubes are oriented in an orthogonal direction with respect to the first and second surfaces of PEM 40, respectively, as shown in Figures 1 and 2. The term "orthogonal" generally means that the longitudinal axis of the carbon nanotubes is perpendicular to the comparative surface, such as the first surface of the PEM as discussed above. As used herein, the term "orthogonal" includes such a perpendicular orientation, as well as angles ranging from perpendicular (i.e., 90°) down to about 30°, generally from 90° to 60°, and typically from 90° to 75°, in relationship to the comparative surface.

Without being bound by theory, it is believed that carbon nanotubes oriented in an orthogonal direction are desirable in the catalyst layer, for example, at the surface of the gas diffusion layer (hereinafter referred to as "GDL") to aid diffusion of the reactants and removal of products, while also providing enhanced through-plane electrical and thermal conductivity in the catalyst layer. However, carbon nanotubes oriented in an orthogonal direction are undesirable against the surface of the PEM, since they may penetrate into and/or through the PEM, causing unnecessary stress and premature failure. As a result, it is preferable to orient substantially all of the carbon nanotubes in a planar direction on the surface of the PEM. Furthermore, by providing carbon nanotubes in a planar direction adjacent the surface of the PEM, the in-plane electrical and thermal conductivity of the catalyst layer is enhanced.

In another embodiment, discrete regions of the carbon nanotubes in the second layer of at least one of anode catalyst 22 and cathode catalyst 30 are oriented differently along the length of the MEA, for example, from the inlet area of the MEA to the outlet area of the MEA. For example, and as shown in Figure 3a, a first region 52 of carbon nanotubes in the second layer 54 at the inlet area of the MEA 10, is oriented such that the longitudinal axis of the carbon nanotubes is farther from perpendicular (e.g., nearer the lower end of the 90° to 30° range) with respect to the surface of PEM 40, compared to the remaining region 56 of carbon nanotubes in the second layer 54, for example, at the outlet area of MEA 10. One of ordinary skill in the art will recognize that more than one discrete region of the carbon nanotubes 58, 60, 62 in the second layer 54 may be oriented differently with respect to the remaining regions of the carbon nanotubes in the second layer 54, as shown in Figure 3b.

Without being bound by theory, the inlet area of the MEA tends to be drier than the outlet area of the MEA during operation. Thus, by orienting the carbon nanotubes farther from perpendicular at the inlet region, the humidity from the incoming reactant(s) and/or from the product water formed during operation can prevent the inlet region from drying. On the other hand, by orienting the carbon nanotubes closer to perpendicular, the excess humidity and excess product water formed during operation may be more easily removed from the catalyst layer. Further, while Figures 3a and 3b illustrate discrete regions wherein the longitudinal axis of the carbon nanotubes is oriented in a specific orthogonal direction, it should be understood that a gradient or continuum may be employed, wherein such orientation changes gradually along the desired length of the MEA.

In another embodiment, at least a portion of the carbon nanotubes of at least one of anode catalyst 22 and cathode catalyst 30 may be a different length as compared to the remaining portion of the carbon nanotubes in the same layer. For example, the carbon nanotubes at the inlet area of the MEA may be longer than the carbon nanotubes at the outlet area of the MEA. In another example, carbon nanotubes of various lengths may be randomly dispersed in the catalyst layer.

In still another embodiment, all of the carbon nanotubes in each layer are the same structure, for example, with two open ends, and/or with one open end and one closed end, and/or with two closed ends, as shown in Figures 4a, 4b, and 4c, respectively. Alternatively, at least a portion of the carbon nanotubes may be different in comparison to the remaining portion of the carbon nanotubes in the same layer. For example, and referring again to Figures 3 a, carbon nanotubes comprising one open end and one closed end may be employed in at least a portion of the second layer 54, for example, in the first section 52, and carbon nanotubes comprising two open ends may be employed in the remaining portion of the second layer 54, for example, in the remaining section 56.

In any of the above embodiments, anode catalyst 22 and cathode catalyst 30 may comprise more than two layers of carbon nanotubes. In this case, each of the layers may have at least a portion of carbon nanotubes that are oriented at a different orientation with respect to the remaining carbon nanotubes of the same layer. In a further embodiment, the orientation of the carbon nanotubes in each of the layers of the anode catalyst 22 and cathode catalyst 30 need not be the same. Additionally, the number of layers of carbon nanotubes need not be the same.

In any of the above embodiments, at least a portion of the carbon nanotubes in anode catalyst 22 and/or cathode catalyst 30 comprises at least one catalyst material, for example, a precious metal, such as Pt, Ru, Au, and Ir, and/or a non-precious metals, such as Co, Fe, Ni, Cr, Sn, and V, and/or an oxide, sulfide and/or carbide. The catalyst material may be deposited on any of the surfaces of the carbon nanotubes, for example, their inner and outer walls and their end surfaces, to add catalytic functionality to at least a portion of the carbon nanotubes 24, 32, 44, 46 of the anode catalyst 22 and/or the cathode catalyst 30. Any of a variety of known methods may be used for depositing a catalyst material on the surface of the carbon nanotubes, such as those described in Publ. U.S. Appl. No. 2005/0220988. In this context, "at least a portion of the carbon nanotubes" means that at least 50%, generally at least 70%, and typically at least 90% of the carbon nanotubes comprise at least one catalyst material.

In one embodiment, a higher concentration of catalyst material may be deposited at one end of the carbon nanotube in comparison with the opposing end of the carbon nanotube, for any of the carbon nanotubes in anode catalyst layer 22 and cathode catalyst layer 30. In another embodiment, the dispersion of the catalyst material may be gradiented along the length of the carbon nanotube on at least one of the inner and outer walls. In one example, as shown in Figure 5, the bottom end 50 of the outer surface of carbon nanotube 54 has a higher concentration of catalyst material 48 than the top end 52. Alternatively, each layer of carbon nanotubes in anode catalyst layer 22 and/or cathode catalyst layer 30 may have a different concentration of catalyst material deposited on the surface of the carbon nanotubes. For example, the total concentration of catalyst material in the first layer of carbon nanotubes (adjacent to the PEM) is higher than the total concentration of catalyst material in the second layer of carbon nanotubes (adjacent to the first layer of carbon nanotubes).

In another embodiment, at least one of anode catalyst 22 and cathode catalyst 30 may further comprise at least one additive, such as non-catalytic carbon nanotubes (i.e., does not contain a catalyst material), carbonaceous and/or graphitic particles, catalyst materials (such as noble metals or alloys thereof), carbon- and/or graphite-supported catalyst materials (such as Pt supported on carbon), ionomeric materials (such as Nation^®), pore forming agents, or combinations thereof. In one embodiment, the at least one additive may be mixed with the carbon nanotubes in at least one of the layers of carbon nanotubes. Alternatively or additionally, the at least one additive may deposited as a separate layer between layers of carbon nanotubes. For example, the ionomeric materials may be deposited as a monolayer of colloidal particles between layers of carbon nanotubes, for example, between layers 23, 25 of anode catalyst 22 and/or between layers 31, 33 of cathode catalyst 30.

To add further functionality, the carbon nanotubes may comprise at least one functional group anywhere on its surface, such as the inner and outer sidewalls and the two ends of the carbon nanotubes. In one example, the functional groups may be those described in Publ. U.S. Appl. No. 2005/0186378 (herein incorporated by reference). In another example, the carbon nanotubes may not need to be purified after they are grown and/or cut to the desired length. In such cases, the catalyst particles used to catalyze the growth of the carbon nanotube may act as a functional group deposited on an end of the carbon nanotube. In yet another example, a supramolecular surfactant, such as sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB) may be adsorbed onto a surface of the carbon nanotubes.

By adding functionality to the inner and/or outer walls of the carbon nanotubes, and/or the ends of the carbon nanotubes, deposition and alignment of the carbon nanotubes on the surface of the PEM and/or GDLs can be controlled. For example, the functional group may be deposited such that it produces an opposite polarity on each end of the carbon nanotube, or on each wall of the carbon nanotube (i.e., the inner wall and the outer wall). In the case where the functional group is an SDS surfactant, the chemical adsorption of SDS molecules on the surface of the nanotubes creates a distribution of negative charges that prevents their aggregation and induces stable suspensions in water (see Richard et al., Science, 2003, 300, pg. 775- 778). The hydrophobic part of the SDS is adsorbed on the graphite by van der Waals interactions, likely following the carbon network, and the hydrophilic part of the surfactant is oriented toward the aqueous phase. In addition, M.Prato et al., Chemistry A European Journal, 2003, 9, pg. 4000-4008, describes a number of methods for introducing functional groups onto the surface of the carbon nanotubes to solubilize the carbon nanotubes.

Any method known in the art may be used to deposit and substantially align the carbon nanotubes in the desired orientation. In one example, hydrodynamic coating may be used for depositing a thin layer of catalyzed carbon nanotubes onto a surface of the PEM, wherein the catalyzed carbon nanotubes are functionalized such that they are suspended in a solvent prior to application. Likewise, a layer of carbon nanotubes may be applied onto a surface of the GDL, wherein the carbon nanotubes may or may not comprise a catalyst material. If the carbon nanotubes include a catalyst material and are applied to the GDL, a gas diffusion electrode (GDE) is formed. In one example for aligning the carbon nanotubes on the membrane (or the GDL), an electric field is applied to the solubilized carbon nanotubes in the desired orientation by utilizing the difference in polarity between the two ends of the carbon nanotubes. The electric field may be applied before application such that the carbon nanotubes are oriented in a substantially planar direction with respect to the membrane (or GDL) surface, or such that the carbon nanotubes are oriented at the desired orientation.

In another example for aligning the carbon nanotubes on the membrane (or the GDL), an electric field may be applied during application (i.e., simultaneously as the carbon nanotubes are coated onto the membrane or the GDL) such that the carbon nanotubes are oriented in a substantially planar direction with respect to the membrane (or GDL) surface, or such that the carbon nanotubes are oriented at the desired orientation.

In yet another example for aligning the carbon nanotubes on the membrane (or the GDL), an electric field may be applied after application (i.e., in solubilized form on the membrane or the GDL) such that the carbon nanotubes are oriented in a planar direction with respect to the membrane (or GDL) surface, or such that the carbon nanotubes are oriented at the desired orientation. Again, the PEM or the GDL should be either positively or negatively charged to attract an oppositely charged end of the carbon nanotube.

A second layer of carbon nanotubes may be applied on top of existing layer(s) of carbon nanotubes on the PEM or the GDL after removing the solvent from the first layer. The second layer may be individually applied onto the first layer of aligned carbon nanotubes by, for example, hydrodynamic coating. Since the solvent is removed from the first layer of carbon nanotubes prior to applying the second layer, the aligned carbon nanotubes of the first layer should not be affected when the second layer of carbon nanotubes are subjected to alignment by, for example, application of an electric field. Likewise, additional layers of carbon nanotubes may be applied to the aligned layers in a similar fashion, for example, after removing the solvent from the aligned layers.

In the case when a noble metal is deposited on the surface of the carbon nanotubes of the anode and cathode catalyst layers, the total loading of the noble metal in each of the anode catalyst layer and the cathode catalyst layer is typically in the range from about 0.05 mg/cm² to about 0.40 mg/cm². Similar to the first layer, the second layer of carbon nanotubes may be aligned before, during, or after application.

One of ordinary skill in the art will recognize that carbon nanofibers may be used in place of or in addition to the carbon nanotubes in any of the layers. For example, a heterogeneous mixture of carbon nanotubes and carbon nanofibers may be used in any of the layers. In another example, alternating layers of carbon nanotubes and carbon nanofibers may be used for the anode catalyst and/or the cathode catalyst.

In yet another example, at least one portion of one layer comprises carbon nanofibers while the remaining portions of the same layer comprise carbon nanotubes. The carbon nanofibers may be pretreated according to any of the embodiments disclosed in the foregoing, for example, containing functional groups and/or depositing catalyst particles on a surface thereof. .

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

CLAIMS What is claimed is:

1. A fuel cell comprising an anode electrode, a cathode electrode, and a polymer electrolyte membrane disposed therebetween, wherein the fuel cell comprises a first layer of carbon nanotubes on at least one surface of the polymer electrolyte membrane, wherein substantially all of the carbon nanotubes are oriented in a planar direction with respect to the at least one surface of the polymer electrolyte membrane, and at least a portion of the carbon nanotubes comprise a catalyst material.

2. The fuel cell of claim 1 wherein the carbon nanotubes are single- walled carbon nanotubes, multi-walled carbon nanotubes, or combinations thereof.

3. The fuel cell of claim 1 wherein the catalyst material comprises a metal.

4. The fuel cell of claim 1 wherein the catalyst material is a noble metal or alloys or compounds thereof.

5. The fuel cell of claim 1 wherein at least a portion of the carbon nanotubes further comprise at least one functional group on a surface of the carbon nanotubes.

6. The fuel cell of claim 1 wherein the at least one functional group is a polymer.

7. The fuel cell of claim 1 wherein the at least one functional group is a metal.

8. The fuel cell of claim 1 wherein the at least one functional group is a surfactant.

9. The fuel cell of claim 1 wherein the layer of carbon nanotubes further comprises an ionomer.

10. The fuel cell of claim 9 wherein the ionomer comprises a colloidal suspension.

1 1. The fuel cell of claim 1 wherein the fuel cell further comprises a second layer of carbon nanotubes on the first layer of carbon nanotubes, wherein at least a portion of the carbon nanotubes of the second layer are oriented in an orthogonal direction with respect to the at least one surface of the polymer electrolyte membrane.

12. The fuel cell of claim 1 1 wherein at least a portion of the carbon nanotubes of the second layer further comprises a catalyst material on a surface of the carbon nanotubes in the second layer.

13. The fuel cell of claim 11 wherein at least a portion of the carbon nanotubes of the second layer further comprises at least one functional group on a surface of the carbon nanotubes in the second layer.

14. A catalyst-coated membrane comprising an anode catalyst on a first surface of a polymer electrolyte membrane and a cathode catalyst on an opposing second surface of the polymer electrolyte membrane, wherein at least one of the anode catalyst and the cathode catalyst comprises a first layer of carbon nanotubes, wherein substantially all of the carbon nanotubes are oriented in a planar direction with respect to at least one of the first and second surfaces of the polymer electrolyte membrane, and at least a portion of the carbon nanotubes comprise a catalyst material.

15. A method of making a catalyst-coated membrane comprising the steps of: applying a first layer of solubilized carbon nanotubes to a first surface of a polymer electrolyte membrane; and orienting substantially all of the solubilized carbon nanotubes in a planar direction with respect to the first surface of the polymer electrolyte membrane.

16. The method of claim 15 wherein the carbon nanotubes are grown prior to application.

17. The method of claim 15 wherein the step of orienting at least a portion of the solubilized carbon nanotubes further comprises the step of subjecting the carbon nanotubes to an electric field.

18. The method of claim 15 further comprising the step of solubilizing the carbon nanotubes prior to application.

19. The method of claim 15 further comprising the steps of: removing the solvent from the first layer of carbon nanotubes; applying a second layer of solubilized carbon nanotubes to the first layer of carbon nanotubes, wherein at least a portion of the carbon nanotubes of the second layer comprises a catalyst material; and orienting at least a portion of the solubilized carbon nanotubes in the second layer in a substantially orthogonal direction with respect to the first surface of the polymer electrolyte membrane.

20. A method of making a gas diffusion electrode comprising the steps of: applying a first layer of the solubilized carbon nanotubes to a first surface of a gas diffusion layer; and orienting substantially all of the solubilized carbon nanotubes in a planar direction with respect to the first surface of the gas diffusion layer.