Abstract:
An improved system for interconnects in a fuel cell. In one embodiment, the membranes are located in parallel with one another, and current flow between them is facilitated by interconnects. In another embodiment, all of the current flow is through the interconnects which are located on the membranes. The interconnects are located between two electrodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from Provisional application No. 60/253,423 filed Nov. 27, 2000. 
     
    
     STATEMENT AS TO FEDERALLY-SPONSORED RESEARCH  
       [0002] The invention described herein was made in the performance of work under a NASA 7-1407 contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title. 
     
    
     
       BACKGROUND  
         [0003]    Direct methanol fed fuel cells may be used as energy sources. Miniaturization of these devices may allow the devices to replace rechargeable batteries in certain applications. Such applications may include, for example, cellular telephones, laptop computers, and other small portable electronic devices.  
           [0004]    The electrode configuration of these devices may form a critical factor about the performance of such fuel cells. Resistance between interconnects may also form a factor.  
         SUMMARY  
         [0005]    The present application teaches an electrode and interconnect technique which may be more efficient than previous techniques, as well as enabling reduction of size. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    These and other aspects will now be described in detail with reference to the accompanying drawings, wherein:  
         [0007]    [0007]FIG. 1 shows a first interconnect technique; and  
         [0008]    [0008]FIG. 2 shows a second interconnect technique, in which the current passes through the membranes and the interconnects. 
     
    
     DETAILED DESCRIPTION  
       [0009]    Methanol fuel cells may be constructed as described in U.S. Pat. No. 6,146,781. However, these systems often use bipolar plates, often called “biplates”, between the stacks. These bipolar plates may be too large for effective miniaturization.  
         [0010]    A flat pack design has been disclosed. This flat pack technique may operate without biplates. Instead, the cells are connected in series on a common membrane plane, using electrical interconnects which are formed through the membrane.  
         [0011]    This technique, although compact and lightweight, may result in a higher internal resistance than that of the bipolar cell, which would have the same active electrode areas and the same number of cells.  
         [0012]    The higher internal resistance may lower the power density and also lower the efficiency.  
         [0013]    Power density that has been attained with a first generation flat pack was in the range of 2-3 mw per centimeter squared. However, it may be desirable to increase that power density, for example to as high as a 8-10 mw/cm 2 , in order to meet requirements of portable power source applications. The inventors accordingly realized that the high internal resistance of a flat pack may be an important part of increasing power density of the direct methanol fed fuel cell.  
         [0014]    The present application teaches a flat pack design which reduces the path length for the current flow, and increases the area of the interconnect. By carrying out both of these objectives substantially simultaneously, the internal resistance may be reduced.  
         [0015]    The electrodes of the first embodiment may allow use current to flow along the length of the electrodes. The effective path length for the current flow may be given by one arbitrary unit for each 1 cm×1 cm section of the electrode. Thirty units of path length may be used for the six cell pack. For example, all the current through the cells passes through an interconnect area of approximately 0.15 cm 2 . The ratio of the interconnect area to the electrode area is approximately 0.03. Moreover, the pack has a resistance of about 9.6 ohms.  
         [0016]    In the FIG. 1 embodiment, the current flow from a first unit  100  occurs in the direction of the arrow  102 . In the embodiment, a common membrane may be used, or membranes parts may be connected.  
         [0017]    A number of separate cells are formed, each having electrodes with different parts. Interconnects operate to connect between the respective interconnects. For example, interconnect  118  connects between the cell including membrane  110  and the cell including membrane  112 . Similarly, a second interconnect  122  connects between the cell including membrane  112  and the cell including membrane  114 . In this embodiment, the cells are connected through the membrane interconnect. The current flows along the length of the cells from one interconnect to the other. Overall current flows from the cathode  99  to the anode  107 .  
         [0018]    A second embodiment is shown in FIG. 2. In this embodiment, larger area interconnects are used with parallel current paths. In this embodiment, the current flows in parallel through the width of the cell, along and through the membranes and the interconnects. This system may use a catalyst layer coating and membrane electrode assembly formation of a type which has been known in the art.  
         [0019]    A first membrane  200  is planar, as conventional, and the current flow passes through the membrane. An electrode is associated with the membrane. Interconnect  202  is similarly planar, and associated with the electrode and membrane. Similarly, a second membrane  210  is connected using a second interconnect  214 . This effectively forms a strip cell in which current flows through the width of the cells, and the cells are connected through the interconnect along almost the entire area of the cell. For example, the interconnect may be over 90 percent of the area of the membrane.  
         [0020]    An advantage of this system is that the current flows across the pack. The effective path length is approximately 6/5 units, because there are five parallel segments of resistance, each about equal to six arbitrary units of length. The ratio between the interconnect area and the electrode area is approximately 0.2. Also, the pack has an internal resistance of about three ohms.  
         [0021]    In order to formulate this new layout and interconnect, the system uses new types of masks for appropriately overlapping electrodes and interconnects. The interconnect materials which are used are of a type that retain their form prior to curing. In addition, the interconnect material is applied in a controlled manner, as a 1 mm wide line segment.  
         [0022]    The masks may be formed of Kapton materials that are 3-5 mils thick. The electrodes are sized appropriately to fit the masks. An interconnect paste is formulated from a high surface area conducting carbon material such as Astbury graphite and that he curing binder in the weight ratio 20:1. Isopropanol is added in small amounts to the paste in order to achieve appropriate consistency. This formation technique results in formable interconnects which do not flow excessively, and which their retain their connection, allow optimum electrode alignment prior to membrane electrode assemblies being hot pressed. The system may be applied using a hypodermic syringe loaded with interconnect paste of an appropriate consistency.  
         [0023]    Although only a few modifications have been disclosed in detail above, other modifications are possible. All such modifications are intended to be encompassed within the following claims, in which: