Abstract:
A means for determining the concentration of a hydrogen-rich fuel in a fuel solution within the anode reservoir of a fuel cell. The fuel concentration is determined using a dye mixture responsive to fuel concentration within a fuel solution. As fuel is consumed, the fuel concentration decreases. As the fuel concentration decreases, the dye changes color. The resulting color changes occur within the anode reservoir of the fuel cell, or within a dye chamber in fluid contact with the anode reservoir, and are made visible by a window. A color strip and fuel scale may be included to facilitate fuel concentration determination based on the color of the fuel solution. Additionally, a valve responsive to dye color may act to control fuel delivery.

Description:
TECHNICAL FIELD 
     The present invention relates to fuel cells, and, in particular, to a dye-based fuel indicator system for use with fuel cells. 
     BACKGROUND OF THE INVENTION 
     Fuel cells produce electrical energy by reacting a fuel with an oxidant, usually in the presence of a catalyst. Typically, fuel cells consist of a fuel electrode, or anode, and a reducing electrode, or cathode, separated by an ion-conducting electrolyte. An external circuit conductor connects the electrodes to an electrical circuit, or load. In the conductor, current is transported by the flow of electrons. In the electrolyte, current is transported by the flow of ions. 
     Any number of hydrogen rich fuels may be used as a fuel source, such as methanol, ethanol, butane, and propane.  FIG. 1  is a diagram of a methanol fuel cell. A reservoir that includes the anode, or anode reservoir  102 , contains a methanol-water solution  104 . The methanol fuel cell generally is in a charged state when the percentage of methanol in the methanol-water solution is relatively large. As methanol is oxidized and electricity is generated by the fuel cell, the percentage of methanol in the methanol-water solution decreases and the fuel cell becomes depleted. 
     The methanol contained within the methanol-water solution is oxidized, usually in the presence of a catalyst, producing hydrogen ions  106 , electrons  108 , and carbon dioxide  116 . This oxidation reaction occurs inside the anode reservoir  102  of the fuel cell. A primary anode oxidation reaction is shown below:
 
CH 3 OH+H 2 O→CO 2 +6H + +6 e   − 
 
Note that, since the electrolyte is a relatively poor electrical conductor, electrons  108  flow away from the anode via an external circuit  110 . Simultaneously, hydrogen ions  106  travel through the electrolyte, or membrane  112 , to the cathode  114 . Commonly used membranes include Nafion 112®, Nafion 117®, and polybenzimidazole.
 
     At the cathode  114  of a fuel cell, oxygen  118  is reduced by hydrogen ions  106  migrating through the electrolyte  112  and incoming electrons  108  from the external circuit  110  to produce water  120 . The primary cathode reaction is shown below:
 
3/2O 2 +6H + +6 e   − →3H 2 O
 
The individual electrode reactions, described above as primary anode and primary cathode reactions, result in an overall methanol-fuel-cell reaction shown below:
 
2CH 3 OH+3O 2 →2CO 2 +4H 2 O+electricity
 
Additional minor chemical reactions may occur, and thermal energy is generally produced.
 
     Modern fuel cells can continuously produce electrical current for long periods of time without the need for recharging. However, fuel cells produce electrical charge only when fuel is present in the anode reservoir above a threshold concentration. Therefore, in order to ensure continuous operation of a fuel cell, an indication of the amount of fuel remaining in the fuel cell needs to be easily obtainable. Fuel cells commonly provide no convenient, cost-efficient means for reliably determining the amount of available fuel remaining in the fuel cell. Therefore, designers, manufacturers, and users of fuel cells have recognized the need for a convenient, cost-efficient means for determining the amount of fuel remaining in a fuel cell. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention provides a means for determining the concentration of methanol within an anode reservoir of a methanol-based fuel cell. The methanol concentration is determined through the use of a dye mixture that responds to the concentration of methanol in the methanol-water solution. As methanol is consumed during normal operation of the fuel cell, the dye mixture responds by changing color. Thus, different colors are produced in the fluid within the anode reservoir of the fuel cell, or within a fluid-filled chamber, or dye chamber, in fluid communication with the anode reservoir, as methanol is consumed. A color indicator bar and fuel scale may be included with the fuel cell to facilitate determination of the methanol concentration by visual comparison of the color of the fluid in the anode reservoir, or within a dye chamber in fluid communication with the anode reservoir, with a corresponding color-indicator-bar color. Additionally, a valve responsive to the color of the dye mixture may act to control fuel delivery. Alternative embodiments employ different types of dye mixtures suitable for indicating concentrations of different types of hydrogen-rich fuels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a methanol fuel cell. 
         FIG. 2A  shows a dye-based fuel indicator in direct contact with the anode reservoir. 
         FIG. 2B  illustrates a dye chamber with a dye-based fuel indicator separated from the anode reservoir by a membrane. 
         FIG. 2C  shows a dye chamber with a dye-based fuel indicator separated from the anode reservoir by a fuel channel and an optional membrane. 
         FIG. 3A  illustrates an exemplary dye molecule in a ground state. 
         FIG. 3B  shows an exemplary dye molecule in an excited state. 
         FIG. 4A  illustrates the structural formula for Acid Yellow 1. 
         FIG. 4B  shows the structural formula for Acid Red 29. 
         FIG. 5  illustrates an embodiment of the present invention with a photodiode emitter-receiver controlling fuel delivery. 
         FIG. 6  illustrates one embodiment of a fuel delivery mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a means to determine the concentration of fuel within a fuel cell. In one embodiment, a dye mixture is introduced into the methanol-water solution contained in the anode reservoir, or a dye chamber in fluid communication with the anode reservoir, and is made visible through a transparent window. Normal operation of the fuel cell lowers the concentration of methanol within the methanol-water solution in the anode reservoir. As the concentration of methanol decreases, the dye mixture changes color. Consequently, the color of the methanol-water solution corresponds to the concentration of methanol in the methanol-water solution in the anode reservoir. A color comparison bar, or color strip, and a fuel scale may be included to facilitate methanol concentration determination based on the color of the methanol-water solution. 
       FIG. 2A  illustrates one embodiment of the present invention with a dye-based fuel indicator in direct contact with the anode reservoir  202 . The anode reservoir  202  includes a long, thin horizontal window  204  visible from the exterior of the fuel cell. A color strip  206  and fuel scale  208 , both affixed to the exterior of the fuel cell, extend along the lower, horizontal length of the window  204 . The color strip  206  provides a convenient means to compare fluid color to calibrated colors displayed by the color strip  206  and aligned fuel scale  208 . The colors on the color strip  206  encompass a range of possible colors produced by the dye mixture in methanol concentrations ranging between a charged state and a depleted state. The colors contained on the color strip  206  form a color gradient corresponding to numeric fuel concentration indications on the fuel scale  208 . The fuel scale  208  is shown as a horizontal line with a series of evenly spaced marks, each mark representing a fuel concentration. 
       FIG. 2A  shows fluid of a particular color visible through a window  204  in the anode reservoir  202 . The fluid color can be compared to the colors displayed by the color strip  206  below the window  204 . In  FIG. 2A , the fluid color matches a color on the color strip  206  corresponding to a methanol concentration at which 20% of the available fuel supply remains. 
     In the above-described embodiment, shown in  FIG. 2A , a dye is introduced directly into the anode reservoir  202 . However, certain dye mixtures may interfere with operation of the fuel cell or may be too expensive to use in the bulk fuel mixture contained within the anode reservoir  202 . Therefore, a dye chamber  210  may be employed to maintain a necessary concentration of dye molecules in a smaller volume of methanol-water solution separate from the anode reservoir  202 . 
     The dye chamber  210  must be fluid-filled and in fluid contact with the anode reservoir  202 . This can be accomplished in any number of ways, depending on the physical characteristics of the fuel cell. Different designs may also be necessary to ensure that the window  204  is visible.  FIG. 2B  shows the dye chamber  210  separated from the anode reservoir  202  by a semi-permeable membrane  212 . This membrane serves to sequester the dye molecules within the dye chamber  210  while still allowing methanol and water to diffuse between the dye chamber  210  and anode reservoir  202 . Note that the methanol-water solution in  FIG. 2B  is around 50% methanol. 
       FIG. 2C  shows an alternate embodiment, in which the dye chamber  210  is separated from the anode reservoir  202  by a fuel channel  214 . The fuel channel  214  may have variable lengths and shapes, but must have a cross sectional area large enough to allow for equilibration of the methanol concentration in the anode reservoir  202  with the methanol concentration in the dye chamber  210  within a reasonable time frame.  FIG. 2C  also shows an optional semi-permeable membrane  212  as described in  FIG. 2B . Note that the anode reservoir  202  in  FIG. 2C  is in a depleted state. 
       FIGS. 3A–B  illustrate a general example of a dye molecule changing color in response to a changing condition in the dye molecule&#39;s environment. Certain classes of compounds absorb incident light. The color of the light absorbed is related, in these compounds, to a change of internal state from a lower energy state to a higher energy state. The wavelength of reflected light from a solution containing such a compound is enriched in non-absorbed wavelengths. Thus, if a compound absorbs red light, green-colored light may be reflected from the solution, and if a compound absorbs blue light, orange-colored light may be reflected from the solution. In a non-polar solvent, the energy-level difference between the states shown in  FIGS. 3A–B  is smaller than the energy-level difference in a polar solvent. This particular dye absorbs blue light in a polar solvent and red light in a non-polar solvent, appearing orange is a polar solvent and green in a non-polar solvent. 
       FIGS. 3A–B  show two different states for a Reichardts Dye molecule.  FIG. 3A  shows the dye molecule in a lower-energy, polar, zwitterionic state containing both a positive charge  302  and a negative charge  304 .  FIG. 3B  shows the same dye molecule in a higher-energy, non-polar state. In polar solvents, the energy difference between the lower-energy state shown in  FIG. 3A  and the higher-energy state shown in  FIG. 3B  is larger than in a non-polar solvent, and the dye therefore emits green-colored light in polar solvents and orange-colored light in non-polar solvents. 
     In this example, the dye responds to a change in dielectric constant of the solution by changing color. However, dyes may respond to other conditions as well, such as the concentration of metal ions or the pH of a solution. In the above-described embodiment of the present invention, the dye responds to a change in methanol concentration. 
     The dye mixture used in a dye-based fuel indicator may also comprise various different dyes. In one embodiment of the present invention, a dye mixture comprising Acid Yellow 1, or Naphthol Yellow S, and Solvent Blue 37 is employed.  FIG. 4A  illustrates the structural formula for Acid Yellow 1. This dye mixture produces a color gradient with a significant color change at 10% methanol in water. In an alternative embodiment, Acid Red 29, or Chromotrope 2R, is used in combination with Solvent Blue 37 to produce a color gradient with a significant color change occurring between 3% methanol and 1% methanol in water.  FIG. 4B  shows the structural formula for Acid Red 29. Both embodiments employ 3.5 milligrams of each respective dye combination per milliliter of methanol-water solution. In the above-described embodiments, each dye mixture is dissolved in a series of methanol-water solutions with methanol concentrations between 0.5% methanol in water and pure methanol, to produce a color gradient. Note that many different dye mixtures are possible that produce useful color changes in methanol concentrations present in fuel cells. Moreover, different dye mixtures can be used for indication of the concentration of other hydrogen-rich, liquid fuel sources in other types of fuel cells. 
       FIG. 5  illustrates an embodiment of the present invention with the dye color controlling the release of the fuel within the cell.  FIG. 5  shows a dye-based fuel indicator in direct contact with the anode reservoir  502 . The anode reservoir  502  contains a photodiode  504  and a light emitting diode (“LED”)  506 . The LED  506  shines light  508  upon the photodiode  504  through the methanol-water solution in the anode reservoir  502 . The photodiode  504  and LED  506  may employ several methods of operation. At a predetermined methanol level, the color of the methanol-water solution allows enough light  508  from the LED  506  to reach the photodiode  504 , activating the photodiode  504 . The photodiode  504  produces an electric current that triggers a mechanism to release fuel into the anode reservoir  502 . Alternately, at a predetermined methanol level the color of the methanol-water solution ceases to allow enough light  508  from the LED  506  to reach the photodiode  504 , activating the photodiode  504 . The photodiode  504  produces an electric current that triggers a mechanism to release fuel into the anode reservoir  502 . Note that  FIG. 5  omits the window, color strip and fuel scale, for clarity of illustration. Note also that  FIG. 5  shows the anode reservoir  502  as a different shape than in previous illustrations, for clarity of illustration. Alternative embodiments employ the photodiode  504  and LED  506  in the dye chamber in fluid-communication with the anode reservoir. 
     One embodiment of the fuel release mechanism, shown in  FIG. 6 , comprises a fuel reservoir  602  separated from the anode reservoir  604  by a door  606 . The fuel reservoir contains nearly pure methanol without excess water added. Two wires  608 , 610 , extending from the photodiode ( 504  in  FIG. 5 ) are in contact with a valve  612  that controls the aperture of the door  606 . At a predetermined methanol level, the color of the methanol-water solution allows enough light to reach the photodiode to activate the photodiode. An electric circuit is completed that signals the valve  612  to open the door  606 . Gravity allows the methanol in the fuel reservoir  602  to be released into the anode reservoir  604 . Alternatively, at a predetermined methanol level the color of the methanol-water solution ceases to allow enough light to reach the photodiode, activating the photodiode. 
     Although the present invention has been described in terms of a particular embodiment, it is not intended that the invention be limited to this embodiment. Modifications within the spirit of the invention will be apparent to those skilled in the art. For example, although two specific dye mixtures are described, there are many different dye mixtures that can be used to produce useful color gradients in response to changing concentrations of different types of fuels. Dye mixtures can encompass a series of dyes that create any number of different color gradients at different fuel concentrations. Dye mixtures can be employed that bring about a significant change in the color of the fuel at different predetermined fuel concentrations. Determination of fuel concentration can be based on dyes reacting with other changing environmental conditions, such as the presence of metal ions or pH to produce color gradients. Many different types of fuel-release systems are possible. The photodiode emitter-receiver can operate with many different electrical control valves or triggers used to actively or passively control the feeding of fuel into the anode reservoir. The photodiode emitter-receiver may be positioned at any location within the fuel solution. Finally, various different shapes, sizes, orientations and positions of the window, color strip and fuel scale may be used. For instance, the color strip may lie beside a vertical window, or wrap around an oblong window. Moreover, the fuel scale need not necessarily lie beneath the color strip. The fuel scale may be incorporated as part of the actual color strip with fuel concentration marks written directly over the colors. Alternatively, the dye chamber may be made entirely from a transparent material with neither a color strip nor a fuel scale. 
     The foregoing description, for purposes of explanation used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well-known portions of fuel cells are shown as diagrams in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents: