Abstract:
Fuel delivery system and method for delivering liquid fuel to an electrode in a liquid-type fuel cell are disclosed. The liquid fuel is passively delivered to a reaction surface of an electrode by capillary force through a porous structure. The porous structure has a shape and a capillary force distribution to facilitate fuel flow, and can be part of a fuel cartridge for easy transportation and storage of fuel.

Description:
TECHNICAL FIELD  
         [0001]    The technical field generally relates to fuel cells and in particular to fuel delivery system for liquid-type fuel cells.  
         BACKGROUND  
         [0002]    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).  
           [0003]    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): CH 3 OH+H 2 O→6H + +CO 2 +6e −   (I)  
           Cathode reaction (air side): 3/2 O 2 +6H + +6e − →3H 2 O  (II)  
           Net: CH 3 OH+3/2O 2 →2H 2 O+CO 2   (III)  
           [0004]    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&#39;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.  
         SUMMARY  
         [0005]    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.  
           [0006]    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.  
           [0007]    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.  
           [0008]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The detailed description will refer to the following drawings, in which like numerals refer to like elements, and in which:  
         [0010]    [0010]FIG. 1 is a schematic showing the capillary effect.  
         [0011]    [0011]FIGS. 2A and 2B are schematics of porous structures for fuel delivery in a fuel cell.  
         [0012]    [0012]FIG. 3 depicts a porous structure as part of a fuel cartridge.  
         [0013]    [0013]FIGS. 4A, 4B and  4 C depict an embodiment of fuel flow control between a fuel cartridge and a fuel cell.  
         [0014]    [0014]FIGS. 5A and 5B depict another embodiment of fuel flow control between a fuel cartridge and a fuel cell. 
     
    
     DETAILED DESCRIPTION  
       [0015]    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. FIG. 1 shows capillary effect in tubes of different sizes. As depicted, capillary rise is related to the diameter of tubes  101 . The smaller is the tube diameter, the greater is the rise of a liquid column  103  from a liquid table  105 . When a porous structure, such as a foam, is placed into a fuel container, the capillary effect of the small-diameter pores in the foam will cause the fuel to rise above the fuel level to form a capillary fringe in the foam. Typically, the capillary fringe is composed of pores of various sizes, from macropores to micropores. At the base of the capillary fringe, all the pores are saturated by the fuel. At the top of the capillary fringe, saturation by fuel is limited to only the micropores.  
         [0016]    Capillary rise of fuel in a foam can be represented by the following equation:  
         ρ gh=[ 2σ cos θ e   ]/r   e   =P   c    
         [0017]    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, θ e  is the effective equilibrium wetting angle of the fuel on the surface of the foam, r e  is the effective pore radius of the foam, and P c  represents the capillary pressure. For any given fuel, ρ and g are both constant, and therefore h is inversely proportional to the pore radius r e , i.e., the smaller the pores are, the higher the fuel rises. In addition, a reduction of the wetting angle θ e  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 θ e  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.  
         [0018]    [0018]FIG. 2A depicts an embodiment of the fuel delivery system. In this embodiment, porous structure  201  is in the shape of a hollow tube so that the porous structure  201  can be inserted into outer cavity  207 , which serves as fuel container for a flex based fuel cell  200 . An inner surface  203  of the porous structure  201  is pressed against fuel electrodes  211  so that fuel can be delivered directly to reaction surfaces  213  of the fuel electrodes  211 .  
         [0019]    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.  
         [0020]    Alternatively, As shown in FIG. 2B, the flex based fuel cell  200 ′ may be configured in such a way that the fuel electrodes  211  face the inner cavity  209 . In this case, the porous structure  201  may be in the shape of a cylinder that can be inserted inside the inner cavity  209  of the flex based fuel cell  200 . The outer surface  205  of the porous structure  201  is pressed against the reaction surfaces  213  of the fuel electrodes  211 .  
         [0021]    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 FIG. 2B, even when the fuel inside the inner cavity  217  of the porous structure  201  starts to deplete, the fuel will still be transported to the electrodes  211  for efficient fuel utilization.  
         [0022]    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.  
         [0023]    In another embodiment, the foam insert is used as a fuel cartridge  305 . As shown in FIG. 3, fuel  302  is contained inside a sealed foam cylinder  301 , which is kept in a non-permeable container  303  or is wrapped with a non-permeable material. When needed, the cylinder  301  is taken out from the container  303  or from the wrapping material and is loaded into a cartridge holder  304  of a fuel cell  200 . In yet another embodiment, the fuel cylinder  301  is tightly wrapped with a non-permeable material to form cartridge  305 , which can be directly loaded into a fuel cell  200  without removing the wrapping thereby avoiding leakage of fuel from the cylinder  301  during the loading process.  
         [0024]    The fuel in the cartridge  305  enters the fuel cell  200  through one or more connectors  307  (FIG. 4A). The connector  307  can be in different shapes and sizes. Typically, the connector  307  is made of foam materials that provide higher capillary force than the rest of the fuel cartridge, so that fuel in the cartridge  305  will be drawn to the connector  307  by the capillary force. In one embodiment, the connector  307  is in the shape of a short tubing and is located at the bottom of the fuel cartridge  305  (FIG. 4A).  
         [0025]    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  (FIG. 4B). In this embodiment, the needle-like receptacle  309  is also made of a porous material so that the fuel flow can be controlled by the size of a contact area between the needle-like receptacle  309  and the connector  307  (FIG. 4C). As shown in FIG. 4B, the fuel flow rate between fuel cartridge  305  and fuel cell  200  is controlled by positioning the fuel cartridge  305  at the high, medium, or low mark on the side of the cartridge  305 .  
         [0026]    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 .  
         [0027]    In another embodiment, a controller  311  is located at the bottom of the fuel cell  200  (FIG. 5A). The fuel flows from the cartridge  305  to the fuel cell  200  through the contact between the connector  307  and receptacle  309 , which is connected to electrodes by porous materials. The controller  311  controls a cross sectional area of the connector  307  by applying a pressure to the connector  307  through a screw  313  (FIG. 5B). A fuel flow is restricted by advancing the screw  313  towards the connector  307 , thereby reducing the cross sectional area of the connector  307 .  
         [0028]    Alternatively, the fuel flow from the cartridge  305  to fuel cell  200  can be controlled by a conventional electromagnetic valve.  
         [0029]    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.