Patent Application
20060057435
1. Field of the Invention
The present invention relates to a Direct Liquid Fuel Cell (DLFC) which uses a hydride fuel and also relates to specifically preventing or at least substantially reducing the generation of hydrogen caused by a decomposition of the hydride fuel at the anode of the fuel cell when the DLFC is under no or only a low load.
A hydride fuel decomposition reaction at the anode of the fuel cell generates hydrogen during the period where the fuel cell is under no or only a low load. The invention thus also provides a method which uses this hydrogen to provide a separation layer between the anode and the liquid fuel. In this way, the fuel is substantially prevented from contacting the anode, whereby decomposition of the fuel is prevented to at least a substantial extent.
One way in which this can be accomplished is by arranging a special membrane close to that surface of the anode which faces the fuel chamber. The initially generated hydrogen accumulates between the membrane and the anode, and pushes or forces out the liquid fuel from the space between the anode and the membrane. This causes the liquid fuel to separate from the anode.
2. Discussion of Background Information
The most commonly used liquid fuel for a DLFC is methanol. The main disadvantages of such Direct Methanol Fuel Cells (DMFCs) are the toxicity of methanol and the very poor discharge characteristics at room temperature. As a result, DMFCs are not generally used for portable electronics applications and the like.
Fuels based on (metal) hydride and borohydride compounds such as, e.g., sodium borohydride have a very high chemical and electrochemical activity. Consequently, DLFCs which use such fuels have extremely high discharge characteristics (current density, specific energy, etc.) even at room temperature.
For example, the electro-oxidation of borohydride fuels on the anode surface of a fuel cell occurs in accordance with the following equation:
BH4−+8OH−=BO2−+6H2O+8e− (1)
The main problem associated with hydride and borohydride fuels is a spontaneous decomposition of the fuel on the (active layer of the) anode surface which is accompanied by a generation of hydrogen, usually in the form of microbubbles, e.g., bubbles of from about 0.01 to about 2 mm in size. This process is particularly significant in a DLFC open circuit regime and in a stand-by (low current) regime.
The decomposition of a borohydride compound occurs according to the following equation:
BH4−+2H2O→BO2−+4H2↑(2)
Hydride and borohydride decomposition at the anode of a DLFC results in several technical problems, in particular, energy loss, destruction of the anode active layer, and decreasing safety characteristics. As a result, there is a need to develop ways to substantially prevent the fuel from decomposing while the DLFC is under no or no substantial load.
The present invention provides a liquid fuel cell for use with a liquid fuel that is prone to undergo decomposition on the surface of the anode and generates gas in the course of this decomposition. The fuel cell comprises a cathode, an anode, an electrolyte chamber which is arranged between the cathode and the anode, a fuel chamber which is arranged on that side of the anode which is opposite to the side which faces the electrolyte chamber, and a membrane which is arranged on that side of the anode which faces the fuel chamber. The membrane is structured and arranged to allow gas which is formed on or in the vicinity of the surface of the anode that faces the fuel chamber to accumulate adjacent the anode at least to a point where the gas substantially prevents a direct contact between the anode and the liquid fuel when liquid fuel is present in the fuel chamber.
According to one aspect of the invention, the fuel of the fuel cell may comprise a metal hydride and/or borohydride compound and the gas may comprise hydrogen.
In another aspect, the membrane may comprise a single layer of material and/or the membrane may comprise a layer of hydrophilic material. The hydrophilic material may comprise a metal and/or a metal alloy. By way of non-limiting example, the hydrophilic material may comprise stainless steel.
In yet another aspect of the fuel cell, the membrane may comprise a mesh, for example, a stainless steel micromesh. In another aspect, the micromesh may comprise cells which have a size of up to about 0.5 mm, e.g., of from about 0.06 μm to about 0.05 mm. In a still further aspect, the membrane (mesh) may have a thickness of from about 0.03 mm to about 0.3 mm.
In another aspect of the fuel cell of the present invention, the fuel cell may further comprise a spacer material which is arranged between the membrane and the anode. The spacer material may comprise a single layer of material and/or it may comprise a hydrophobic material such as, e.g., a layer of hydrophobic material. The hydrophobic material may comprise a polymeric material. By way of non-limiting example, the hydrophobic material may comprise an olefin homopolymer and/or an olefin copolymer, e.g., one or more of polyethylene, polypropylene and polytetrafluoroethylene.
In a still further aspect, the spacer material may comprises a net, for example, a wattled net. In another aspect, the net may comprise openings of from, e.g., about 1 mm to about 50 mm.
In another aspect, the spacer material may have a thickness of up to about 3 mm, preferably up to about 1.5 mm and/or may have a thickness of at least about 0.1 mm, preferably at least about 0.5 mm.
In yet another aspect of the fuel cell of the present invention, the fuel cell may further comprise a frame seal which is arranged on that surface of the anode which faces the membrane. The frame seal may comprise a single layer of material and/or may comprise a hydrophobic material, e.g., in the form of a layer of hydrophobic material. The hydrophobic material may comprise an olefinic polymer, for example, a fluorinated polymer. In particular, the hydrophobic material may comprise polytetrafluoroethylene. In another aspect, the frame seal may have a thickness of up to about 0.1 mm, e.g., of from about 0.02 mm to about 0.05 mm.
In a still further aspect, the fuel cell of the present invention may comprise a pressure relief device. This device is arranged to allow the gas to escape from a space between the anode and the membrane, e.g., into the fuel chamber. The pressure relief device may comprise a small diameter tube, for example a tube having an inner diameter of up to about 2 mm, preferably, of up to about 1 mm. The small diameter tube may have a length of up to about 20 mm, for example, up to about 10 mm. In one embodiment, the small diameter tube may comprise a capillary needle and/or a stainless steel tube, e.g., a tube having a length of about 7 mm and an inside diameter of about 1 mm.
In yet another aspect of the fuel cell of the present invention, the membrane and the anode may be arranged substantially in parallel. In a further aspect, the membrane and the spacer material may form an integral structure.
The present invention also provides a direct liquid fuel cell for use with a liquid fuel that is prone to undergo decomposition with generation of a gas. This fuel cell comprises a cathode, an anode, an electrolyte chamber arranged between the cathode and the anode, a fuel chamber arranged on that side of the anode which is opposite to the side which faces the electrolyte chamber, a membrane arranged on that side of the anode which faces the fuel chamber, and a spacer material which has a thickness of at least about 0.1 mm and is arranged between the anode and the membrane. The membrane and the spacer material are structured and arranged to allow gas which is formed on or in the vicinity of the surface of the anode which faces the fuel chamber to accumulate adjacent the anode at least to a point where the gas substantially prevents a direct contact between the anode and the liquid fuel when the fuel chamber contains liquid fuel.
In one aspect of this fuel cell, the membrane may comprise a hydrophilic material such as, e.g., a metal or a metal alloy. Further, the membrane may comprise a mesh such as, e.g., a stainless steel micromesh. The micromesh may comprise cells having a size of up to about 0.5 mm, e.g., of up to about 0.06 mm.
In another aspect of the fuel cell, the spacer material may comprise a hydrophobic material such as, e.g., an olefin homopolymer and/or an olefin copolymer. For example, the spacer material may comprise polypropylene. Also, the spacer material may comprise a wattled net. For example, the wattled net may comprise cells which have dimensions of from about 2 mm to about 3 mm. In another aspect, the spacer material may have a thickness of up to about 0.5 mm.
In yet another aspect of the above fuel cell, the fuel cell may further comprise a frame seal which is arranged on the surface of the anode which faces the membrane. The frame seal may comprise a hydrophobic material such as, e.g., a fluorinated polymer. For example, the frame seal may comprise polytetrafluoroethylene. In one aspect, the frame seal may have a thickness of up to about 0.1 mm.
According to another aspect of the fuel cell, the fuel cell may further comprise a pressure relief device which is arranged to allow the gas to escape from a space between the anode and the membrane, e.g., into the fuel chamber. The pressure relief device may comprise a tube having an inner diameter of up to about 1 mm and/or a length of up to about 20 mm. For example, the pressure relief device may comprise a capillary needle and/or a stainless steel tube.
The present invention further provides a direct liquid fuel cell for use with a liquid fuel that is prone to undergo decomposition with generation of a gas. The fuel cell comprises a cathode, an anode, an electrolyte chamber arranged between the cathode and the anode, a fuel chamber which is arranged on that side of the anode which is opposite to the side which faces the electrolyte chamber, a membrane which is arranged on that side of the anode which faces the fuel chamber, a spacer material which is arranged between the anode and the membrane, and a pressure relief device for allowing gas which is present between the anode and the membrane to escape into the fuel chamber. The membrane, the spacer material and the pressure relief device are structured and arranged to allow gas which is formed on or in the vicinity of that surface of the anode which faces the fuel chamber to accumulate adjacent the anode at least to a point where the gas substantially prevents a direct contact between the anode and the liquid fuel when liquid fuel is present in the fuel chamber.
In one aspect of the fuel cell, the membrane may comprise a hydrophilic material such as, e.g. a metal or a metal alloy. In another aspect, the membrane may comprise a micromesh such as, e.g., a stainless steel micromesh. The micromesh may comprise cells having a size of up to about 0.5 mm.
In another aspect of the fuel cell, the spacer material may comprise a hydrophobic material, for example, a polymeric material. A non-limiting example of the polymeric material is polypropylene. Further, the spacer material may comprise a net. The net may comprise openings of up to about 50 mm. In another aspect, the spacer material may have a thickness of up to about 1.5 mm and/or at least about 0.5 mm.
In another aspect, the fuel cell may further comprise a frame seal which is arranged on that surface of the anode which faces the membrane. The frame seal may comprise a hydrophobic material such as, e.g., polytetrafluoroethylene. In another aspect, the frame seal may have a thickness of up to about 0.05 mm.
The present invention also provides a method of reducing or substantially preventing a fuel decomposition at the anode of a direct liquid fuel cell which uses a fuel that generates a gas when undergoing said decomposition. This method comprises using the gas which is formed by the initial decomposition of the fuel to limit or substantially prevent any further contact between the fuel and the anode.
In a preferred aspect of this method, the gas may substantially prevent the fuel from further contacting the anode. In another aspect, the initially generated gas may be caused to form a substantially continuous layer of gas across substantially the entire surface of the anode that faces the fuel chamber of the fuel cell.
In another aspect of the method, the fuel may comprise a hydride and/or a borohydride compound, for example, an alkali metal borohydride. By way of non-limiting example, the fuel may comprise sodium borohydride dissolved or suspended in a liquid vehicle or carrier such as, e.g., methanol and/or water. Further, the gas preferably comprises hydrogen.
In another aspect of the method, the flow of the initially generated gas away from the anode may be restricted or substantially prevented.
In a further aspect, the fuel decomposition may occur as a result of placing the fuel cell under substantially no load.
In yet another aspect, the initial fuel decomposition may be substantially stopped within not more than about 5 minutes, e.g., within not more than about 3 minutes.
In a still further aspect of the method of the present invention, the method may comprise arranging a structure which restricts or substantially prevents the ability of the gas to flow away from the anode on that side of the anode which faces the fuel chamber. This structure may comprise a membrane and a spacer material for providing a space between the anode and the membrane, this space being capable of being substantially filled with the gas.
The present invention also provides a method of preventing or reducing fuel decomposition in a fuel cell of the present invention. This method comprises the generation of electrical energy with the fuel cell; substantially preventing the fuel cell from further generating electrical energy; and facilitating, with the membrane, an accumulation adjacent the anode of the gas generated at the anode at least to a point where the gas limits or substantially prevents a contact between the anode and the fuel.
The present invention also provides a another method of preventing or reducing fuel decomposition in a fuel cell of the present invention. This method comprises the generation of electrical energy with the fuel cell; substantially preventing the fuel cell from further generating electrical energy; and causing the gas generated at the anode to accumulate adjacent the anode at least to a point where the gas substantially prevents a contact between the anode and the fuel.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.
The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:
The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice.
As illustrated in
As illustrated in
In the DLFC according to the invention, the generated gas, usually hydrogen and usually in the form of micro-bubbles of a size of from about 0.01 to about 2 mm, accumulates into a space between a surface of the anode 3 and the special membrane 8. The bubbles will usually coalesce and/or unite to form a layer of gas which fills essentially all of the volume between anode 3 and the special membrane 8. This, in turn, causes the special membrane 8 to separate the liquid fuel from the anode 3 and to substantially prevent any further contact between the liquid fuel and the anode 3. The space between the anode 3 and membrane 8 can be between approximately 0.1 m and 3.0 mm thick, and is preferably between approximately 0.5 mm and approximately 1.5 mm, and most preferably is approximately 0.5 mm.
Any extra gas which exceeds the volume of the space between the anode 3 and special membrane 8 vents or bleeds out and into the fuel chamber 2 through the capillary needle 7. This bleeding process stops essentially automatically when the pressure in the volume between anode 3 and special membrane 8 equals the pressure in the fuel chamber 2.
The frame seal 6 extends around the perimeter of the anode 3 and is arranged between the anode 3 and the special membrane 8. The frame seal 6 preferably has the form of a thin (non-porous) film and is utilized to prevent fuel from escaping in the area of the borders or outer edges of the anode perimeter. The material of the frame seal 6 will usually be hydrophobic (at least on the surface thereof which faces the fuel chamber) and can be formed from a material such as, e.g., polytetrafluoroethylene, although other hydrophobic materials such as, e.g., olefin polymers like polyethylene and polypropylene may also be used for this purpose. In general, the frame seal will be made or at least include a fluorinated polymer such, e.g., a fluorinated or perfluorinated polyolefin. It is to be noted that the frame seal may also be made of a material that is not hydrophobic as such but has been rendered hydrophobic on the surface thereof by means of, e.g., coating with a hydrophobic material, or any other procedure which affords hydrophobicity. Preferably, the frame seal 6 has a thickness of not more than about 0.1 mm. It will usually have a thickness of at least about 0.02 mm. A thickness of about 0.05 mm is particularly preferred for the frame seal for use in the present invention. The frame seal may be mounted on the anode in many ways, e.g., with application of pressure and/or by using an adhesive. A preferred way of mounting the frame seal comprises insert molding.
The spacer material 9 is arranged between the anode 3 and the special membrane 8. The spacer material 9 also extends to the inside perimeter of the case 1 and, in the perimeter area, is also arranged between the frame seal 6 and the special membrane 8. The purpose of the spacer material 9 is to create a separation distance between the special membrane 8 and the surface of the anode 3. This separation distance forms space or volume for the gas layer. As the gas is generated, it accumulates within and fills this space. The spacer material 9 will permit the essentially free flow of gas across the surface of the anode 3, and will preferably be in the form of net such as, e.g., a wattled net material. The spacer material must be able to withstand the chemical attack by the components of the liquid fuel and will usually be hydrophobic, at least on the outer surfaces thereof. In other words, the spacer material may also be a hydrophilic material which has been made hydrophobic on the other surfaces thereof by any process suitable for this purpose such as e.g., coating with a hydrophobic material. Preferred spacer materials for use in the present invention include organic polymers such as, e.g., olefin homopolymers and olefin copolymers. Specific examples thereof include materials which may also be used for the frame seal such as, e.g., polyethylene, polypropylene, polytetrafluoroethylene, and the like. The spacer material will usually have a thickness of not more than about 3 mm, more commonly a thickness of not more than about 1.5 mm. The spacer material will usually have a thickness of at least about 0.1 mm, preferably at least about 0.5 mm. In a preferred embodiment of the present invention, the spacer material has a thickness of about 0.5 mm. In this regard, it is to be noted that the exemplary and preferred dimensions of the various elements of the DLFC described herein apply particularly to fuel cells for portable devices, e.g., for fuel cells which have dimensions of an order of magnitude which is suitable for portable devices (e.g., labtops, cell phones etc.). Examples of corresponding dimensions are given in the Examples below. For fuel cells which are considerably smaller or larger than those which are suitable for portable devices, the preferred dimensions given herein may not always afford the desired result to the fullest possible extent. One of ordinary skill in the art will, however, be able to readily ascertain the most suitable dimensions for any given size of fuel cell.
As explained above, the special membrane 8 separates the gas layer which has formed at the anode surface from liquid fuel in the fuel chamber. The special membrane is made of a material which can withstand the chemical attack by the components of the liquid fuel and will not catalyze a decomposition of the fuel or a component thereof to any appreciable extent. This material will usually be hydrophilic, at least on the outer surface(s) thereof. Accordingly, the material may be a hydrophobic material which has been rendered hydrophilic on the outer surface thereof by any suitable process, such as coating, surface treatment (e.g., oxidation) and the like. Preferred examples of suitable materials for the special membrane include metals, as such or in the form of alloys. Particularly preferred materials include corrosion-resistant metals and alloys such as nickel, steel, in particular, stainless steel, etc. The hydrophilic material will preferably be present in the form of a foam, a mesh and the like. By way of non-limiting example, the special membrane 8 may be or at least include a stainless steel micromesh with cells of a size of up to about 0.5 mm, e.g., up to about 0.1 mm, or up to about 0.06 mm. A preferred mesh cell size is from about 0.05 mm to about 0.06 mm, a size of about 0.05 mm being particularly preferred. The membrane (mesh) 8 will often have a thickness which does not substantially exceed about 0.3 mm. On the other hand, the thickness of the membrane will usually not be much smaller than about 0.03 mm.
The capillary needle is secured to the special membrane 8 and can be arranged at a convenient position thereon such as, e.g., centrally located (and, preferably, substantially perpendicular to the membrane). As explained above, the purpose of the needle 7 is to balance the pressure between gas layer and liquid fuel in the fuel chamber. The balance pressure range will usually be from about 1 atm to about 1.5 atm (absolute). The needle is made of a material which can withstand the chemical attack by the components of the liquid fuel and does not catalyze a decomposition thereof to any appreciable extent. This material will usually be selected from the materials which are suitable for making the special membrane 8, but may also be made of other materials, e.g., polymeric materials. Non-limiting examples of polymeric materials include polyolefins such as polytetrafluoroethylene and polypropylene. Preferably, the needle 7 is a stainless steel needle. While a suitable length of the needle may vary over a wide range (depending, in part on the dimensions of the spacer, the membrane, etc.) the needle will often have a length of up to about 20 mm, or even longer. The inner diameter of the needle will usually not exceed about 2 mm, preferably not exceed about 1 mm, or not exceed about 0.5 mm. The needle may be attached to the membrane 8 by any suitable method, e.g., by using a thermoadhesive, welding and mechanical attachment (the latter being a preferred method).
A conventional DLFC of the type shown in
The DLFC was filled with a borohydride fuel and tested under the following conditions:
A DLFC according to the present invention of the type shown in
The DLFC was filled with a borohydride fuel and tested under the following conditions:
As used herein, a “hydrophilic” material is a material that has an affinity for water. The term includes materials which can be wetted, have a high surface tension value and have a tendency to form hydrogen-bonds with water. It also includes materials which have high water vapor permeability.
As used herein, a “hydrophobic” material is a material which repels water. The term includes materials which allow for the passage of gas therethrough but which substantially prevent the flow therethrough of water and similar protic and/or polar liquids.
It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words that have been used are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the invention has been described herein with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed herein. Instead, the invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.
