Patent Application
20070128493
The technical field generally relates to fuel cells and in particular to fuel delivery system for liquid-type fuel cells.
A fuel cell is an electrochemical apparatus wherein chemical energy generated from a combination of a fuel with an oxidant is converted to electric energy in the presence of a catalyst. The fuel is fed to an anode, which has a negative polarity, and the oxidant is fed to a cathode, which, conversely, has a positive polarity. The two electrodes are connected within the fuel cell by an electrolyte to transmit protons from the anode to the cathode. The electrolyte can be an acidic or an alkaline solution, or a solid polymer ion-exchange membrane characterized by a high ionic conductivity. The solid polymer electrolyte is often referred to as a proton exchange membrane (PEM).
In fuel cells employing liquid fuel, such as methanol, and an oxygen-containing oxidant, such as air or pure oxygen, the methanol is oxidized at an anode catalyst layer to produce protons and carbon dioxide. The protons migrate through the PEM from the anode to the cathode. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this fuel cell are shown in the following equations:
Anode reaction (fuel side): CH3OH+H2O→6H++CO2+6e− (I)
Cathode reaction (air side): 3/2O2+6H++6e−3H2O (II)
Net: CH3OH+3/2O2→2H2O+CO2 (III)
One of the essential requirements of a fuel cell is efficient delivery of fuel to the electrodes. U.S. Pat. No. 5,631,099 describes a typical microchannel and plumbing design that facilitates the flow of fuel and removal of water during fuel cell operation. U.S. Pat. Nos. 5,766,786 and 6,280,867 describe pumping systems to accurately and reproducibly deliver the fuel to the electrodes. All these devices have complex arrangements of membrane, gaskets, channels that are difficult and expensive to fabricate and assemble, and are highly subject to catastrophic failure of the entire system if a leak develops. As can be easily appreciated, the cost of fabricating and assembling fuel cells is significant, due to the materials and labor involved. Typically, 85% of a fuel cell's cost is attributable to manufacturing costs. Thus, the complexity of prior art fuel cell structures is one of the factors preventing widespread acceptance of fuel cell technology. An improved style of fuel cell that is less complex and less prone to failure would be a significant addition to the field. With regard to fuel delivery systems in particular, there is a continuing need for a delivery system that can efficiently deliver fuels in a cost effective manner. A passive fuel delivery system with no plumbing and pumps would be highly desirable in applications such as portable fuel cells.
A method for delivering liquid fuel to a reaction surface in a fuel cell is disclosed. The liquid fuel is passively delivered to the reaction surface of an electrode by capillary force through an effective porous structure.
In an embodiment, the effective porous structure is inserted inside a fuel storage space of a fuel cell and delivers fuel to an electrode of the fuel cell through capillary effect.
In another embodiment, the effective porous structure is a part of a fuel cartridge. The fuel cartridge can be loaded into a cartridge holder in a fuel cell.
Additional advantages and novel features will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.
The detailed description will refer to the following drawings, in which like numerals refer to like elements, and in which:
A passive fuel delivery system using capillary effect to deliver fuel to a reaction surface is disclosed. Capillary effect is the spontaneous rise of a liquid in a fine tube due to adhesion of the liquid to the inner surface of the tube and cohesion of the adhered liquid to and among other liquid molecules.
Capillary rise of fuel in a foam can be represented by the following equation:
ρgh=[2σcosθe]/re=Pc
where ρ is the density of the fuel, g is the gravitational constant, and h is the height the fuel has risen above the fuel level in a container in which the foam is standing. The symbol σ represents the surface tension of the fuel, θe is the effective equilibrium wetting angle of the fuel on the surface of the foam, re is the effective pore radius of the foam, and Pc represents the capillary pressure. For any given fuel, ρ and g are both constant, and therefore h is inversely proportional to the pore radius re, i.e., the smaller the pores are, the higher the fuel rises. In addition, a reduction of the wetting angle θe of the fuel on the foam will improve or increase the height that the fuel rises in the foam, assuming all other parameters remain constant. The wetting angle θe can be reduced by increasing the surface energy of surfaces throughout the foam. The surface energy can be increased by subjecting the foam to a free radical oxidation plasma process.
Typically, the porous structure 201 is in the form of a felted piece of polyurethane foam or other suitable porous materials. The foam is thermally compressed, or felted, until the foam holds a compression set at a desired compression ratio. During a thermal compressing process, the foam is heated close to its melting point under a compression loading and allowed to thereafter cool, resulting in a denser foam with an increased porosity. When so felted, the foam achieves an effective porosity.
Alternatively, As shown in
In both configurations, the capillary force at the surface of the porous structure 201 that contacts the electrodes 211 is higher than the capillary force in the other parts of the porous structure 201, so that fuel will be drawn to the electrodes 211. The higher capillary force can be achieved by (1) reducing the pore radius by increasing foam density, (2) reducing the wetting angle by increasing the surface energy of the foam, or both. Foam density can be increased by packing the foam denser along the outside peripheral of the porous structure 201. Surface energy of the foam can be increased by diffusing a chemically active species into the interior portion of a bulk polymer foam by subjecting the foam surface to special treatments such as a gas plasma process. The smaller pores in denser foam or reduced wetting angle will ensure that the fuel is drawn to the electrodes 211 by the higher capillary force, so that in the embodiment of
As can be appreciated by one skilled in the art, the foam insert 201 is designed for easy replacement and can be configured into any shape to adapt to different fuel cell configurations.
In another embodiment, the foam insert is used as a fuel cartridge 305. As shown in
The fuel in the cartridge 305 enters the fuel cell 200 through one or more connectors 307 (
When the fuel cartridge 305 is loaded into the fuel cell 200, a needle-like receptacle 309 in the fuel cell 200 penetrates the non-permeable wrapping material at the end of the connector 307. The base of the receptacle 309 is connected to the electrodes 211 through a porous material that establishes a capillary passage way between the fuel cartridge 305 and the electrodes 211 (
Generally, the needle-like receptacle 309 is made of a porous material having a capillary force that is stronger than the capillary force in the connector 307, while the porous material in contact with the electrode 211 has a capillary force that is stronger than capillary force in receptacle 309. This capillary force gradient ensures that the fuel inside the fuel cartridge 305 flows preferentially to the connector 307, then to the receptacle 309, and finally to the electrode 211.
In another embodiment, a controller 311 is located at the bottom of the fuel cell 200 (
Alternatively, the fuel flow from the cartridge 305 to fuel cell 200 can be controlled by a conventional electromagnetic valve.
Although embodiments and their advantages have been described in detail, various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the fuel delivery system as defined by the appended claims and their equivalents.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10140934 | May 2002 | US |
| Child | 11539623 | Oct 2006 | US |
