Abstract:
A fuel cell uses porous metal layers attached on a flex substrate for delivery of liquid fuel to the active catalytic areas on the anodic side. The flex substrate may form an enclosed package such that the liquid fuel can be contained in the enclosed volume and the air can freely exchange with the cathode side of the fuel cell without the need of microchannels and plumbing for mass transporting both fuel and oxygen to the active catalytic area. The porous metal provides a large surface are for the catalytic reaction to occur.

Description:
TECHNICAL FIELD 
     The technical field is fuel cells, with our without proton exchange membranes. 
     BACKGROUND 
     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 type of direct methanol fuel cell are shown in the following equations: 
     
       
         Anode reaction: CH 3 OH+H 2 O→6H + +CO 2 +6e −   
       
     
     
       
         Cathode reaction: 3/2 O 2 +6H + +6e − →3H 2 O  
       
     
     The essential requirements of typical fuel cells (see, e.g., FIG. 1) include: first, the fuel cell requires efficient delivery of fuel and air to the electrode, which typically requires complicated microchannels and plumbing structures. A second requirement is that the fuel cell should provide easy access to the catalyst and a large surface area for reaction. This second requirement can be satisfied by using an electrode made of an electrically conductive porous substrate that renders the electrode permeable to fluid reactants and products in the fuel cell. To increase the surface area for reaction, the catalyst can also be filled into or deposited onto a porous substrate. However, these modifications result in a fragile porous electrode that may need additional mechanical support, such as by use of a fiber matrix. Alternatively, the electrode can be made of an etched porous Vycor glass substrate or an etched-nuclear-particle-track membrane substrate to improve its toughness and strength. A third requirement is close contact between the electrode, the catalyst, and the PEM. The interface between the electrode and PEM is a discontinuity area as concerns the electric current transmission wherein the charge carriers are the electrons, on one side, and the protons on the other side. A solution to this problem has been attempted by hot pressing of the electrodes onto the PEM (U.S. Pat. No. 3,134,697). Another solution suggests the intimate contact of the catalytic particles with a protonic conductor before interfacing the electrode with the electrolyte (U.S. Pat. No. 4,876,115). Other solutions are described in U.S. Pat. Nos. 5,482,792 and 6,022,634. A fourth requirement is that the fuel cell should provide for humidity control of the electrode. The PEM requires water to be effective in conducting proton. However, since it operates at a higher temperature than its surroundings, the PEM tends to dehydrate during operation. The typical method of re-hydrating the PEM is to capture water in the exhaust stream and circulate it back to the PEM. 
     SUMMARY 
     A flex based fuel cell comprises two flex circuits assembled face-to-face with PEM layers in between. Each flex circuit includes layers of a flex substrate, a patterned conductive material, a porous material sheet with a catalyst coating, and a PEM. 
     The catalyst coating on the porous material provides a large surface area for chemical reactions to proceed and small gaps in the porous material deliver liquid fuel to the catalyst through capillary force. The support of a soggy PEM on a porous electrode may be achieved by assembling two flex substrates face-to-face with the PEM constrained between the two flex substrates. 
     The flex substrates can bend in such a fashion to form a closed structure for confining the fuel. The delivery of the fuel can then be achieved by capillary force of a porous material sheet with the fuel distributed uniformly to all active surfaces, as long as a portion of the porous material sheet is in contact with the liquid fuel. Microchannels and plumbing are not required. 
     The flex based fuel cell can confine water between the two flex substrates to provide moisture for the PEM. Since deionized water can easily conduct protons but not electrons, a fuel cell can be constructed without the PEM. Two flex substrates without the PEM layer can be bonded together face-to-face with an adhesive layer in between as ridges. 
     The flex substrate, such as one using Kapton, provides additional advantages. The electrodes can be patterned directly on the flex substrate, thereby connecting different fuel cell panels in either serial or parallel manner. 
     In an embodiment, the flex substrates are formed into a cylinder. The interior side of the cylinder would be the fuel side, and the exterior of the cylinder would be the oxygen side. The fuel cell can be sealed at the bottom of the cylinder to provide a container for the liquid fuel. The liquid fuel, such as methanol, is delivered by the porous metal to the active catalytic surface of the interior side. Protons generated in the cylinder interior then diffuse through the PEM and reach the catalytic surface on the exterior side of the cylinder, where the protons combine with oxygen. The exterior side of the fuel cell is open to the atmosphere, which serves to supply the oxygen to the cylinder and carry away the reactant water vapor. 
     The flex substrates can be manufactured by the following steps: 
     (1) Patterning the flex substrate, such as Kapton (Dupont) or Upilex (Ube), with a thin film of conductive material. The patterning of the thin film provides the flexibility to define the size of fuel cell panels, as well as to configure the fuel cell for any particular current density or voltage output by routing the thin film patterning appropriately. 
     (2) Attaching a porous material sheet to the patterned thin film on the flex. In an embodiment, the porous material may be a porous metal. Alternatively, other porous materials may be used. For example, an organo-metallic solgel material may be attached to the patterned thin film. The attachment step can be accomplished by either attaching a commercially available porous metal sheet on the flex, or sintering a thick layer of porous metal on the flex. A thick porous metal layer can also be manufactured by applying a paste of low temperature metal powder, such as zinc nano particles (Aldrich catalog #48,393-1) mixed in glycol, and baking the resulting assembly in an oven to dry out the glycol and partially melt the metal particles. The metal powder paste can be screen printed on the flex substrate so that the location and shape of the sintered porous metal can conform to the thin film electrode previously patterned on the flex substrate. 
     As an alternative to the zinc powdered metal, silver powder may be used. 
     (3) Depositing a catalytic coating on the porous material sheet. Several catalytic materials, such as Pt—Ru and Pt—Ru—Os, are effective in converting methanol to proton without poisoning platinum in the flex substrate. 
     (4) Ablating backside openings to allow access to the catalytic surfaces. The flex substrate is ablated with a laser from the backside to create openings so that fuel on the cathode side and oxygen on the anode side can reach the active catalytic surfaces through the openings and the porous metal layer. 
     (5) For those flex substrates with a PEM, the surface of the catalytic coating may then be coated with a thin layer of PEM by dipping the structure into a 5% Nafion solution. A thin layer of PEM on top of the catalyst surface helps to capture protons. The thickness of the PEM may be controlled so that the liquid fuel can readily diffuse through the thin layer. 
     Two flex substrates can then be assembled face-to-face with the PEM in between to form a flex based fuel cell. For fuel cells without the PEM, two flex circuits manufactured with only the first four steps (without the PEM coating step) are assembled face-to-face with the catalytic coating layer in between. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The detailed description will refer to the following drawings, in which like numerals refer to like elements, and in which: 
     FIG. 1 illustrates a prior art fuel cell assembly; 
     FIG. 2 shows a cross section of a flex based fuel cell having two flex circuits assembled face-to-face with a proton exchange membrane PEM in between; 
     FIGS. 3A and 3B show a top sectional view and a plan view of the fuel cell assembly; 
     FIG. 4 shows a flex based cylindrical fuel cell assembly; 
     FIGS. 5A-5E show a process for manufacturing the flex based fuel cell; and 
     FIG. 6 shows an alternative flex based fuel cell. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 is a simplified cross-section view showing an exemplary flex based fuel cell  100 . The fuel cell  100  includes a right flex circuit A and a left flex circuit B. This naming convention is purely arbitrary and is used to add greater clarity to the description of the flex based fuel cell  100 . Two flex substrates  101  and  102  are assembled face-to-face together with a PEM  103  in between. On either side of the PEM  103  are porous material and catalyst layers  104 . Adjacent to the PEM  103  is a palladium (Pd) layer  105  that prevents cross-over of the methanol fuel. Adjacent to the porous material and catalyst layers  104  are anode and cathode electrodes (conductors)  106  and  107 . A dry film adhesive  108  serves to separate portions of the fuel cell  100 . Recycled water  109  flows through the fuel cell  100 , as shown. A liquid fuel  110 , such as methanol, for example, is provided on the anode side of the fuel cell  100 . Air and water vapor  111  flow past the cathode electrode  107 . The methanol fuel  110  has direct contact with the porous material layers  104  through openings  112  in flex substrates  101 ,  102 . The methanol fuel  110  is delivered by the porous material layers  104  to an active catalytic surface  105  where CH 3 OH reacts with H 2 O (methanol) to form CO 2  and protons. The protons then diffuse through the PEM layer  103  and reach a catalytic layer  107 , where the protons combine with oxygen to form H 2 O. The left flex circuit B of the fuel cell  100  is open to the atmosphere, which serves to supply the oxygen to the fuel cell, and carry away the reactant water vapor  111 . 
     In an embodiment, the porous material layers  104  are formed of a porous metal material such as zinc or silver powder. The porous metal layers  104  deliver the liquid fuel (methanol)  110  by means of capillary action. Capillary action depends on the fact that a liquid near a solid wall will undergo curvature of the liquid surface. The amount of curvature depends on the difference between surface tension of the solid-vapor film (S LV ) and surface tension of the solid-liquid film (S JV ). Depending on the liquid and the solid, the curvature can be positive, negative or zero. In the case of a liquid such as methanol that wets the pores in the porous metal layer  104 , the methanol will rise in the pores until an equilibrium height y is reached:        y   =       2        S     L                 V          cos                 θ       ρ                 g                 r                              
     where: 
     r=radius of pores 
     θ=contact angle of the liquid methanol the pores. 
     Thus, by careful design, the liquid fuel (methanol)  110  can be mode to supply all portions of the fuel cell  100  without elaborate pumps and plumbing. Note that pores in the porous metal layer  104  may be oriented in the local plane, or substantially in the local plane defined by the flexible substrates  101 ,  102 . The pores may be further oriented such that liquid fuel will be transported in a specified direction (e.g. vertically) within the porous metal layer  104  so that liquid fuel reaches all, or substantially all, of the fuel side flex circuit A (see FIG.  2 ). 
     As shown in FIGS. 3A and 3B, two or more fuel cells  100  can be bonded together in such a way that there is an enclosed space  120  between two fuel cells  100 . The enclosed space  120  is filled with water or a water containing solution to provide moisture for the PEMs, which require water to be effective in conducting protons. 
     As shown in FIG. 4, a flex fuel cell assembly  130  can be shaped into the form of a cylinder. An interior  131  of the cylinder would be the fuel side, and an exterior  132  of the cylinder would be the oxygen side. The fuel cell can be sealed at a top  133  and a bottom  134  of the cylinder interior  131  to provide a container for the liquid fuel. Alternatively, the cylinder top may be left unsealed. In an additional embodiment, liquid fuel may be supplied to the exterior  132  of the flex fuel assembly  130 . 
     In addition to the non-planar cylindrical shape shown in FIG. 4, the flex fuel cell assembly  130  may be shaped into other non-planar and substantially non-planar shapes, including a polygon of N sides, a star having M points, where M may be the integer 5 or larger, and an oval, for example. The flex fuel cell assembly  130  may also be formed in a shape of a cross or other non-planar or substantially non-planar form. These complex shapes have the advantage of increasing the surface area for fuel cell reactions and power production. 
     Because the flex fuel assembly  130  can be molded to a variety of shapes, the flex fuel assembly  130  is ideal for power applications that are constrained in size and shape. Thus, a fuel cell system using the flex fuel assembly  130  can be shaped to fit virtually any container or enclosure, allowing the fuel cell system to be used in a wide variety of applications where prior art fuel cell systems would not be useable. 
     Wrapping a flex substrate into the cylindrical shape as shown in FIG. 4 has the added advantage of enhancing the adhesive effect between the Nafion and the flex substrates. The enhanced adhesive effect occurs primarily because of the compressive forces acting on the Nafion by the cylindrical flex substrates. 
     As noted above, the flex-based fuel cell  100  shown in FIG. 2, as well as other embodiments, such as the cylindrical fuel cell  130  shown in FIG. 4, may use capillary action to draw liquid fuel, such as methanol, to all active regions of the fuel cell. The rate of capillary action may be controlled by adjusting the pore size (diameter) of pores in the porous metal and catalytic layers  104 . Using capillary action to move the liquid fuel through the fuel cell  100  allows the fuel cell  100  to operate without expensive and bulky pumps, valves and piping, thereby making the fuel cell lighter and more desirable for portable power applications. 
     FIGS. 5A-5E depict processing steps for manufacturing the flex circuits. As shown in FIG. 5A, which includes a plan view and a side view, the first step is to metallize a flex substrate  150 , using a material such as Kapton or Upilex, to form a thin film electrode  151  in a predetermined pattern. The patterning of the thin film electrode  151  defines the size of the cell panels, and configures the fuel cell for any particular current density or voltage output. 
     The next step, as shown in FIG. 5B (which includes plan and side views), is to attach a porous metal layer  152  to the patterned thin film electrode  151  on the flex substrate  150 . The porous metal layer  152  can be a commercially available porous metal sheet. Alternatively, a thick porous metal layer can be sintered on the flex. Referring to FIG. 5B, a paste of low temperature metal powder such as zinc nano particles mixed in glycol, is applied on top of the thin film electrodes  151 . Alternatively, silver powder may be used. The flex substrate  150  is then baked in an oven to dry out the glycol and partially melt the metal particles to form a thick layer of porous metal. The metal powder paste also may be screen printed on the flex substrate  150  so that the location and shape of the sintered porous metal can conform to the thin film electrode  151  previously patterned on the flex substrate  150 . 
     The next step, as shown in FIG. 5C (including plan and side views), is to deposit a layer  153  of catalytic coating on the porous metal layer  152 . Referring to the composition of catalytic coating for fuel cells using methanol, catalytic materials such as Pt—Ru and Pt—Ru—Os, are found to be effective in converting methanol to protons without poisoning other fuel cell constituents. 
     The next step, as shown in FIG. 5D (again showing plan and side views), is to laser ablate backside openings  154  on the flex substrate  150  and the thin film electrodes  151 , so that the fuel or oxygen can reach the active catalytic layer  153  through openings in the flex substrate  150  and the porous metal layer  152 . 
     The final step, as shown in FIG. 5E (plan and side views), is to cover the surface of the catalytic layer  153  with a thin layer of PEM  155 . In a preferred embodiment, the flex structure is dipped into a 5% Nafion solution. A thickness of the PEM  155  should also be controlled so that the liquid fuel can diffuse through this thin layer. 
     FIG. 6 illustrates an alternative embodiment of a flex circuit for use in a flex based fuel cell. The flex based fuel cell includes a plurality of flex circuits. A flex circuit  200  includes a right, or fuel-side flex circuit C and a left, or air-side flex circuit D. This naming convention is purely arbitrary and is used to add greater clarity to the description of the flex circuit  200 . 
     The flex circuit  200  includes flex substrates  201  and  202 , each having openings  212 . In immediate contact with the flex substrates  201  and  202  are conductors  206  and  207 . Adjacent the conductors  206 ,  207  are porous metal and catalyst layers  204 . In an embodiment, the catalyst may be Pt—Ru or Pt—Ru—Os. The porous metal may be chosen so that the pores in the porous metal provide a capillary action that draws fuel into the flex circuit  200 . An adhesive  208  may be used to seal the space between the porous metal and catalyst layers  204 . 
     A liquid fuel  210 , such as methanol, is supplied at the right side C of the flex circuit  200 . Air and water  211  are removed at the left side D of the flex circuit  200 . 
     In a departure from other fuel cell designs, the flex circuit  200  does not use a PEM. Instead, a thin layer  209  of dionized water is maintained between the porous metal and catalyst layers  204 . By maintaining a spacing between the porous metal and catalyst layers  204 , the flex circuit  200  is able to generate protons from the liquid fuel  210  and the protons are combined with oxygen to form water. That is, the dionized water conducts protons but does not conduct electrons. Thus, by eliminating the PEM, the flex circuit  200  shown in FIG. 6 is less costly to build. 
     Although preferred 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 flex based fuel cell as defined by the appended claims and their equivalents.