Patent Application
20080044722
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified.
For a better understanding of the present invention, reference will be made to the following Detailed Description, which is to be read in association with the accompanying drawings, wherein:
The invention is directed to fuel cells and methods of manufacture and use of the fuel cells. The invention is also directed to fuel cells with a diffusion element that contains carbon nanotubes, as well as the diffusion element itself and methods of manufacture and use of the fuel cells and diffusion elements.
A fuel cell can be formed using carbon nanotubes in a diffusion element through which fuel travels to a catalyst. For example, the fuel cell can utilize a hydrogen-containing fuel and an oxygen-containing fuel where each fuel is provided to a different section of the fuel cell and the two sections are separated by an ion exchange membrane. One fuel (e.g., the hydrogen-containing fuel) travels through the diffusion element and then reacts in the presence of a catalyst to produce ions (e.g., protons) that transport across the ion exchange membrane to react with the second fuel. These reactions generate and consume electrons, respectively, resulting in the generation of an electrical current that can be provided to an external circuit.
A diffusion element, containing carbon nanotubes, is disposed between the fuel inlet and the catalyst for one or both sections of the fuel cell. Optionally, the carbon nanotubes are disposed on a diffusion layer, such as an amorphous carbon layer. The diffusion element facilitates efficient transport of the fuel to the catalyst by spreading the fuel over a larger area. Carbon nanotubes can be used to provide a diffusion element with desirable properties. The carbon nanotubes are highly conductive and can carry the electrons away from the catalyst. In addition, the carbon nanotubes have relatively high surface area. The catalyst can be disposed on a portion of the carbon nanotubes to produce a catalyst layer. The high surface area can result in a more efficient catalytic reaction than arrangements where the catalyst is disposed on a relatively flat surface.
2H2→4H++4e− (oxidation half reaction)
O2+4H++4e−→2H2O (reduction half reaction)
2H2+O2→2H2O (net reaction)
In another embodiment, the hydrogen-containing fuel is ethylene, methanol or another compound from which hydrogen ions can be extracted (such as other alcohols.) As one example, methanol can be combined with water to form the hydrogen-containing fuel. Carbon dioxide and water are the typical byproducts of this fuel cell. The electrochemical reactions correspond to:
2CH3OH+2H2O→2CO2+12H++12e− (oxidation half reaction)
3O2+12H++12e−→6H2O (reduction half reaction)
2CH3OH+3O2→2CO2+4H2O (net reaction)
The current collectors 102, 104 of the fuel cell 100 can be made of any suitable conductive material, such as a metal or alloy or graphite. Examples of suitable materials include, but are not limited to, steel, cobalt, and titanium. Generally, at least a portion of the current collectors 102, 104 is in contact with the respective diffusion elements 106, 108 to permit the flow of electrons between the current collectors and diffusion elements. Typically, however, the fuel also flows between the current collector and diffusion element. The current collector (or diffusion element) can be shaped to allow the fuel flow while still maintaining contact. For example, the current collectors 102, 104 (or diffusion elements 106, 108) can be formed with one or more channels 120, 122 (preferably, a plurality of channels) through the current collector (or diffusion element) for passage of the fuel 124, 126 while maintaining contact of a portion of the current collectors 102, 104 with the respective diffusion elements 106, 108 to allow current to flow. The current collectors 102, 104 provide the connection points to the external circuit 118.
The ion exchange membrane 114 permits the flow of ions (e.g., hydrogen ions/protons) across the membrane while resisting (and, preferably, preventing) the flow of other species (particularly, the two fuels) across the membrane. One example of a suitable membrane is a NAFION™ membrane available from DuPont. This membrane provides good transport for protons but resists the flow of hydrogen and oxygen gas through the membrane. Other membranes that perform the desired function can also be used. The thickness of the membrane can be selected to provide a desired flow of ions and/or a desired resistance to the flow of other species, such as the fuels across the membrane. As an example, the ion exchange membrane can have a thickness in the range of 40 □m to 90 □m.
The catalyst layers 110, 112 contain catalytic material that catalyzes the desired reactions of the fuel cell. As an example, the catalysts layers of a hydrogen/oxygen fuel cell can both be made of platinum metal, for example, black platinum. Platinum catalyzes both the oxidation of hydrogen and the combination of protons with oxygen to form water. Other suitable catalysts can be used such as 50/50 platinum/ruthenium. The two catalyst layers 110, 112 can include the same or different catalysts. The selection of catalyst can depend on a variety of factors including, for example, the reaction to be catalyzed, the presence or absence of side reactions, the presence of contaminants in the fuel cell or in the fuels that may poison the catalyst, the desired reaction rate, other materials in the fuel cell, the manufacturing process, etc. In many conventional fuel cells and in some embodiments of the present invention, the catalyst layer is disposed on the ion exchange membrane or on another porous substrate.
The diffusion elements 106, 108 manage the flow of fuel to the catalyst layers 110, 112 to increase the efficiency of the fuel cell. In at least some instances, the diffusion elements 106, 108 facilitate the fuel flow so that the fuel spreads out and makes contact with most of, and preferably substantially all of, the surface of the respective catalyst layer. In conventional fuel cells, amorphous carbon, often disposed on some porous substrate, is used as the diffusion element.
One or both of the diffusion elements 106, 108 can be formed using carbon nanotubes instead of, or in addition to, amorphous carbon. Preferably, the carbon nanotubes are generally vertically aligned (i.e., aligned generally perpendicular to the current collector and ion exchange membrane.) The density of the carbon nanotubes, as well as the length of the nanotubes, can be selected to provide a desired rate of fuel flow. As an example, the density of carbon nanotubes in the diffusion element can be in the range of 1×108/cm2 to 5×108/cm2, preferably in the range of 2×108/cm2 to 3×108/cm2. In at least some embodiments, the carbon nanotubes have an average length of at least 0.1 mm and, preferably, in the range of 0.1 to 1 mm.
In addition to their other properties, the carbon nanotubes have relatively high electrical conductivity. In at least some instances, the electrical conductivity of carbon nanotubes can be higher than that of amorphous carbon resulting in lower ohmic resistance in the diffusion element and in the fuel cell, in general.
In addition, the carbon nanotubes have a high aspect ratio (i.e., the ratio of length to diameter of the tubes.) For example, at least some carbon nanotubes have aspect ratios of at least 3000 and preferably in the range of 3000 to 30,000 In at least some embodiments, the carbon nanotubes have an average diameter in the range of 10 nm to 50 nm. The high aspect ratio provides the carbon nanotubes with relatively high surface area. One side of the carbon nanotube diffusion element can be coated with the desired catalyst (e.g., black platinum) to form the catalyst layer on the diffusion element.
For example, the diffusion element 106 can be formed of generally vertically aligned carbon nanotubes. The catalyst layer 110 can then be formed by depositing the catalyst onto one surface of the diffusion element. The deposited catalyst will typically extend into the diffusion element 106 and coat the proximal regions of the carbon nanotubes for some distance depending on a variety of factors including, for example, the density of the carbon nanotubes; the particular catalyst chosen; the deposition method; deposition parameters such as time, temperature, concentration, and the like. In some embodiments, the catalyst extends at least 50 to 75% of the nanotube length into the diffusion element. The portion of the diffusion element that contains the catalyst forms the catalyst layer 110. Such a catalyst layer can be particularly effective because of the relatively high surface area of the catalyst-coated carbon nanotubes.
Optionally, the diffusion element can also be coupled to the ion exchange membrane by partially embedding the ends of the carbon nanotubes into the membrane. This arrangement can also be used to remove the diffusion element from the substrate upon which it was formed.
In one embodiment, the diffusion element contains carbon nanotubes that are formed or otherwise disposed on or adjacent to a diffusion layer, such as an amorphous carbon diffusion layer. Such an arrangement may provide a larger surface area for deposition of the catalyst than the diffusion layer itself. In this embodiment, the carbon nanotubes of the diffusion element may act primarily as a substrate for the catalyst.
In one example of a method for the formation of carbon nanotubes, a silicon substrate 204 is prepared. Other suitable substrates include, but are not limited to, quartz, ceramic, and glass substrates. As illustrated in
The carbon nanotubes 202 are then formed on the substrate 204 and in the presence of the catalyst layer (not shown), as illustrated in
Generally, the diameter and length of the carbon nanotubes depend on the process parameters (e.g., temperature, time, ratio of gases, etc.) and gases used in growing the nanotubes. In addition, some nanotube formation techniques grow single-walled nanotubes and others techniques grow multi-walled nanotubes. In one example, multi-walled carbon nanotubes were grown at 700° C. for 25 minutes on a silicon substrate with an iron catalyst layer. Different mixtures of gases were used including a first mixture containing 100 sccm (standard cubic centimeters per minute) hydrogen and 690 sccm ethylene and a second mixture containing 400 sccm hydrogen, 400 sccm ethylene, and 400 sccm argon. The resulting carbon nanotubes had an average height of about 150 micrometers and a diameter in the range of 20 to 40 nm. These carbon nanotubes can be removed from the substrate and used in a diffusion element.
In one example of a method of removing the carbon nanotubes, a polymer composition 208 is provided over the nanotubes after the carbon nanotubes 202 are grown, as illustrated in
Any polymer can be used in the polymer composition 208. Preferably, the polymer is soft and flexible, not brittle, upon removal of any solvent. Polymers with such characteristics often have a glass transition temperature that is no more than 25° C. or room temperature. More preferably, the polymer is glassy, tacky, and soft at room temperature or 25° C., upon removal of the solvent. In addition, the polymer is preferably soluble in water or an organic solvent that does not solvate the carbon nanotubes. Examples of suitable polymers include polyvinyl methyl ether (PVME), NAFION™. As an example, 1-3 ml of a 1.2% PVME aqueous solution can be disposed on carbon nanotubes disposed on a 1.5×2.5 inch (about 3.8×6.3 cm) substrate.
It will be understood that the term “polymer” includes, but is not limited, to mixtures or other combinations of polymeric materials, as well as copolymers and the like. In addition to the polymer and solvent, the polymer composition can also include one or more additives, such as surfactants, plasticizers, antioxidants, filler, tackifiers, and the like.
Once the polymer composition 208 is disposed over the carbon nanotubes 202, the solvent, if present, is at least partially removed to produce a solid composition 210 of the carbon nanotubes and polymer. Preferably, substantially all of the solvent is removed. Preferably, once the solvent is removed a top portion of the carbon nanotubes 202 extends out of the polymer composition.
Optionally, once the polymer composition 208 is formed (or, alternatively, prior to coating the polymer composition over the carbon nanotubes), the catalyst 210 can be disposed on the carbon nanotubes 202, as illustrated in
Once the polymer composition is formed 208 and, optionally, the catalyst 210 is disposed on the carbon nanotubes 202, the ion exchange membrane 214 (or another polymer film or membrane) can be brought into contact with the exposed ends of the carbon nanotubes, as illustrated in
The polymer composition 208 is removed prior to, or after, separating the carbon nanotubes from the substrate 204. Removal of the carbon nanotubes can be accomplished by a variety of methods including, for example, treating the polymer composition with a solvent that solvates the polymer and not the carbon nanotubes or the ion exchange membrane.
The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.
