Abstract:
A fuel cell is provided that is suited for mobile and portable applications. Using an innovative anode with a layer to control the rate of diffusion of fuel to the fuel cell anode, fuel crossover through the liquid or solid electrolyte of the fuel cell is prevented Electrolyte integrity is preserved, giving a more robust and reliable fuel cell. Further, the innovative anode allows for the use of highly active fuel compositions which otherwise may be chemically oxidized, releasing heat, or give unstable electrical currents. The use of active fuel compositions with the anode, as well as a fuel composition and liquid electrolyte in which gaseous side-products dissolve allows for the design of robust, powerful portable fuel cell which can be used at room temperature with little peripheral equipment.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to electrochemical fuel cells and to an electrode for use in electrochemical fuel cells.  
           [0002]    A fuel cell is a device that converts the energy of a chemical reaction into electricity. Amongst the advantages that fuel cells have over other sources of electrical energy are high efficiency and environmental friendliness. Although fuel cells are increasingly gaining acceptance as electrical power sources, there are technical difficulties that prevent the widespread use of fuel cells in many applications, especially mobile and portable applications.  
           [0003]    A fuel cell produces electricity by bringing a fuel and an oxidant in contact with a catalytic anode and catalytic cathode, respectively. When in contact with the anode, the fuel is catalytically oxidized on the catalyst, producing electrons and ions. The electrons travel from the anode to the cathode through an electrical circuit connecting the electrodes. The ions pass through an electrolyte with which both the anode and the cathode are in contact. Simultaneously, the oxidant is catalytically reduced at the cathode, consuming the electrons and the ions generated at the anode.  
           [0004]    The common type of fuel cell uses hydrogen as a fuel and oxygen as an oxidant. Specifically, hydrogen is oxidized at the anode, releasing protons and electrons as shown in equation 1: 
           H 2 →2H + +2 e   −   (1) 
           [0005]    The protons pass through an electrolyte towards the cathode. The electrons travel from the anode, through an electrical load, to the cathode. At the cathode, the oxygen is reduced, combining with electrons and protons produced from the hydrogen to form water as shown in equation 2: 
           ½O 2 +2H + +2 e   − →H 2 O  (2) 
           [0006]    Although fuel cells using hydrogen as a fuel are simple, clean and efficient the extreme flammability and the bulky high-pressure tanks necessary for storage and transport of hydrogen mean that hydrogen powered fuel cells are inappropriate for many applications.  
           [0007]    In general, the storage, handling and transport of liquids is simpler than of gases. Thus liquid fuels have been proposed for use in fuel cells. Methods have been developed for converting liquid fuels such as methanol into hydrogen, in situ. These methods are not simple, requiring a fuel pre-processing stage and a complex fuel regulation system.  
           [0008]    Fuel cells that directly oxidize liquid fuels are the solution for this problem. Since the fuel is directly fed into the fuel cells, direct liquid-feed fuel cells are comparatively simple. Most commonly, methanol is used as the fuel in these type of cells, as it is cheap, available from diverse sources and has a high specific energy (5020 Ahl −1 ).  
           [0009]    A typical direct methanol-feed fuel cell  10  is schematically depicted in FIG. 1 Fuel contained in a chamber  12  is in contact with a catalytic anode  14 . Catalytic anode  14  is in contact with an electrolyte  16  that is in contact with cathode  18  Cathode  18  is in contact with oxygen in air  20  Anode  14  and cathode  18  are also electrically connected through circuit  22 . Electrolyte  16  can be solid or liquid.  
           [0010]    In fuel cell  10 , oxygen is reduced at cathode  18  as in equation 2 while methanol is catalytically oxidized at anode  14 , equation 3: 
           CH 3 OH→CO+4H + +4 e   −   (3) 
           [0011]    Carbon monoxide tightly bonds so the catalytic sites on anode  14 . The number of available sites for further oxidation is reduced, reducing power output.  
           [0012]    A solution to this problem is to supply a fuel composition into fuel chamber  12  as an “anolyte”, a mixture of a fuel, usually an alcohol such as methanol, with an aqueous electrolytic liquid. In the case where the fuel is methanol, and if the anolyte and electrolyte  16  are acidic or neutral, then the fuel reacts with water at anode  14  to produce carbon dioxide and hydrogen ions, equation 4: 
           CH 3 OH+H 2 O→6H + +CO 2 +6 e   −   (4) 
           [0013]    while oxygen  20  is reduced at cathode  18  as in equation 2.  
           [0014]    If the anolyte and electrolyte  16  are basic, the fuel reacts with hydroxide ions at anode  14  to produce carbon dioxide, water and electrons, equation 5: 
           CH 3 OH+6OH + →CO 2 +5H 2 O+6 e   −   (5) 
           [0015]    while at cathode  18  oxygen  20  is reduced and combines with water to produce hydroxide ions, equation 6: 
           O 2 +2H 2 O−4 c   − →4OH +   (6) 
           [0016]    In fuel cells with liquid electrolytes there exists the problem of methanol crossover. Methanol from fuel chamber  12  diffuses through anode  14  and accumulates in electrolyte  16 . If fuel comes in contact with cathode  18 , a “short-circuit” occurs as the fuel is oxidized directly on cathode  18 , generating heat instead of electricity. Furthermore, depending upon the nature of the cathode catalyst, catalyst poisoning or sintering often occurs  
           [0017]    For various reasons, basic liquid anolytes have lost popularity over the years. Acidic anolytes are often used. Unfortunately, the fuel cell must be operated at elevated temperatures at which the acidity of the anolyte can passivate or destroy the anode. Anolytes with a pH close to 7 are anode-friendly, but have an electrical conductivity that is too low for efficient electricity generation. Consequently, most direct methanol-feed fuel cells known in the art use solid polymer electrolyte (SPE) membranes.  
           [0018]    In a fuel cell using SPE membranes, the general construction is as depicted in FIG. 1, but that electrolyte  16  is a proton exchange membrane that acts both as an electrolyte and as a physical barrier preventing leakage from fuel chamber  12  wherein the anolyte is contained. One membrane often used as a solid fuel-cell electrolyte is a perfluorocarbon material sold by E. I. DuPont de Nemours of Wilmington Del. under the trademark “Nafion”. Although these membranes are expensive and not robust, SPE membrane fuel cells have superior performance to other fuel cell designs.  
           [0019]    A practical disadvantage of SPE membrane fuel cells arises from the tendency of high concentrations of methanol to dissolve the membrane and to diffuse through it. As a result, a significant proportion of methanol supplied to the cell is not utilized for generation of electricity but is lost through evaporation. Once the methanol passes the membrane, a short-circuit, as described hereinabove, can occur.  
           [0020]    The problem of membrane penetration is overcome by using anolytes with a low (up to 3%) methanol content. The low methanol content limits the efficiency of the fuel cell when measured in terms of electrical output as a function of volume of fuel consumed and raises issues of fuel transportation, dead weight and waste disposal. Further limiting the use of direct methanol-feed fuel cells, especially for mobile and portable applications, is the expense and complexity of necessary peripheral equipment for fuel circulation, replenishment heating and degassing. A typical direct methanol-feed fuel cell equipped with a solid electrolyte  11  is depicted in FIG. 2. An anolyte with 3% methanol is contained in a fuel chamber  12  and in contact with a catalytic anode  14 . Catalytic anode  14  is in contact with proton exchange membrane  16  that is in contact with cathode  18  Cathode  18  is in contact with oxygen in air  20 . Anode  14  and cathode  18  are also electrically connected through circuit  22 . Pump  24  causes the anolyte to pass through a degasser  26 , a cleaner  28 , a mixer  30 , and a heater  32  Gaseous side-products such as CO 2  escape through vent  34 . Mixer  30  continuously replenishes the methanol in the anolyte by adding methanol from vessel  36 .  
           [0021]    Mobile and especially portable direct liquid-feed fuel cells are much desired. However the fuel cells described above are generally not robust, do not have a sufficient power output, and as seen from FIG. 2, require so much peripheral equipment that they quickly become complex and bulky.  
           [0022]    As mentioned above, one limitation of fuel cells known in the art is that the methanol is rather unreactive at room temperature limiting the power output of fuel cells and requiring fuel heating. In U.S. patent application 09/752,551 a highly active fuel composition is disclosed which is suitable for use in direct liquid-feed fuel cells at room temperature. The fuel composition disclosed in U.S. patent application 09/752,551 combines a liquid fuel such as methanol and hydrogen-containing inorganic compounds such as NaBH 4  to produce high currents at low temperatures. However, due to its reactivity this fuel composition has an increased tendency to undergo chemical oxidation on contact with catalyst, producing heat and gas. This tends to an unstable current and may lead to destruction of the catalyst. Under certain conditions the fuel composition may even undergo chemical oxidation when the electrical circuit is open.  
           [0023]    There is a need for a direct liquid-feed fuel cell that is suitable for mobile and portable use. Such a fuel cell should have a high energy content per unit volume of fuel, should be mechanically simple with few components, and should be robust Furthermore, there is a need for a way to reduce or prevent fuel crossover in direct liquid-feed fuel cells with either liquid or solid electrolytes. There is a need for a way to stabilize the current of high active fuel composition in fuel cell.  
         SUMMARY OF THE INVENTION  
         [0024]    The above and other objectives are achieved by the use of the innovative electrode and the innovative fuel cell provided by the present invention.  
           [0025]    The electrode of the present invention is made up of at least two layers, a catalytic layer and diffusion control layer in contact with said catalytic layer. The electrode can also have a second diffusion control layer in contact with the catalytic layer, so that the catalytic layer is sandwiched between the two diffusion control layers.  
           [0026]    According to a feature of the present invention, the catalytic layer contains platinum, often with added ruthenium, nickel, cobalt, tin or molybdenum. The catalytic layer is preferable made to catalyze oxidation reactions, that is, the electrode is designed to serve as an anode.  
           [0027]    According to a feature of the present invention, the catalytic layer is attached to a conductive substrate. The conductive substrate can be, for example a nickel or gold mesh, or a non-conductive substrate (such as a ceramic material) coated with a conductive material.  
           [0028]    According to a feature of the present invention, the diffusion control layer is made of carbon paper, fiber fleece or a microporous film. The carbon paper may be modified to increase hydrophilicity, for example by impregnating it with polyvinyl alcohol.  
           [0029]    The invention further provides a fuel cell for the generation of electrical power, made up of a fuel composition, a cathode, and an anode as described above, that is, the anode has at least at diffusion control layer and a catalytic layer, so that the fuel composition must pass through the diffusion control layer to arrive at the catalytic layer.  
           [0030]    According to a further feature of the present invention, the fuel cell also has an electrolyte to transport ions from the anode to the cathode. The electrolyte may be solid, such as a proton exchange membrane, or the electrolyte may be a liquid, a gel or a suspension. According to a further feature of the present invention the exhaust gases produced in the fuel cell are substantially soluble in the electrolyte.  
           [0031]    According to a further feature of the present invention, the electrolyte has a pH above about 7, for example an aqueous solution of an alkali metal hydroxide such as KOH or NaOH with a concentration of around between 3 M and about 12 M, preferable around 6 M.  
           [0032]    According to a further feature of the present invention, the fuel composition i made of a fuel and an electrolyte, known in the are as an anolyte. The electrolyte may have a pH above about 7, for example, an aqueous solution of an alkali metal hydroxide such as KOH or NaOH with a concentration of around between 3 M and about 12 M, preferable around 6 M. According to a further feature of the present invention, the exhaust gases produced in the fuel cell are substantially soluble in the fuel composition. According to further feature of the present invention, the fuel in the fuel composition includes an alcohol for example methanol. According to a still further feature of the present invention, there is a viscosity-controlling component in the fuel composition. Such a viscosity-controlling component can be, for example, glycerine, ethylene glycol or polyethylene glycol.  
           [0033]    According to a further feature of the present invention the diffusion control layer is configured to allow diffusion of the fuel composition to the catalytic layer at a rate which is less than the rate of oxidation of the fuel at the catalytic layer.  
           [0034]    According to a still further feature of the present invention there is provided a value mechanism that blocks and unblocks the flow of fuel to the anode.  
           [0035]    There is also provided according to the teachings of the present invention a method to regulate power output of the fuel cell be adjusting the viscosity of the fuel composition and the permeability of a layer through which the fuel composition must diffuse to make contact with the anode in order to regulate the rate of diffusion of the fuel to the anode. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0036]    The invention is herein described, by way of example only, with reference to the accompanying drawings, where:  
         [0037]    [0037]FIG. 1 (prior art) is a schematic depiction of a direct liquid-feed fuel cell.  
         [0038]    [0038]FIG. 2 (prior art) is a schematic depiction of a direct liquid-feed fuel cell with a proton exchange membrane electrolyte;  
         [0039]    [0039]FIG. 3 is a first embodiment of the anode of the invention with one diffusion control layer;  
         [0040]    [0040]FIG. 4 is a second embodiment of the anode of the invention with two diffusion control layers;  
         [0041]    [0041]FIG. 5 is a first embodiment of the fuel cell of the invention with a polymer electrolyte membrane;  
         [0042]    [0042]FIG. 6 is a second embodiment of the fuel cell of the invention with liquid electrolyte; and  
         [0043]    [0043]FIGS. 7A and 7B depict a planar valve useful for preventing contact between anolyte and a fuel cell anode. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    The principles and operation of the anode and the fuel cell of the present invention may be better understood with reference to the figures and accompanying description.  
         [0045]    First, it is necessary to understand the relationship between the rate of diffusion and the stability of the current produced when using a highly active fuel composition, for example as described in U.S. patent application 09/752,551 by considering two cases:  
         [0046]    when 
           V   diffusion   &gt;V   electrochemical   (7) 
         [0047]    and when 
           V   diffusion   &lt;V   electrochemical   (8) 
         [0048]    where  
         [0049]    V diffusion  is the rate of diffusion of fuel to the anode; and  
         [0050]    V electrochemical  is the rate of electrochemical oxidation of the fuel at the anode.  
         [0051]    In equation 7 the rate of diffusion is higher than the rate of electrochemical oxidation of the fuel. All catalytic sites are occupied and in the immediate vicinity of every catalytic site there are many other fuel molecules. Once a fuel molecule has been oxidized the produced ions and electrons are immediately transported away, but the gaseous side-products require a finite time to be removed. This often happens non-monotonously as a gas bubble is formed and is suddenly released from the catalytic surface. Current instability results.  
         [0052]    In equation 8 the rate of diffusion is similar to or lower than the rate of electrochemical oxidation of the fuel V electrochemical  is dependent on the number of catalytic sites. When V diffusion &lt;V electrochemical  there are always catalytic sites free which are immediately available to catalyze the electrochemical oxidation of the fuel. There is a sufficient delay between the arrival of two fuel molecules at any given catalytic center for molecules of gaseous side-products to clear away. As is clear to one skilled in the art this leads to production of a stable current.  
         [0053]    In addition, it is necessary to remember that a fuel molecule at a catalytic site can undergo two reactions: the desired electrochemical oxidation and the undesired chemical oxidation. The chemical oxidation reaction has a higher energetic barrier and is therefore significantly slower then the electrochemical oxidation. It has been observed that when a high concentration of certain highly active fuel molecules is present in the vicinity of a catalyst, chemical oxidation may occur. When not wishing to be held to any theory, it is believed that standard catalysts may have sites that selectively catalyze only the chemical oxidation reaction. If a sufficiently active fuel molecule is present in the vicinity of the catalyst and no electrochemical reaction catalyzing site is available, there may be sufficient time for the chemical oxidation to occur despite the comparatively high energetic barrier.  
         [0054]    As methanol is relatively unreactive at room temperature and is ordinarily supplied only in low concentrations, chemical oxidation is not a serious problem. However, with more active fuel compositions this can lead to inefficient fuel use and an exorbitant release of heat.  
         [0055]    The anode of the present invention overcomes the problem of current instability and competing chemical oxidation by controlling the rate of diffusion of fuel molecules to the catalytic centers. The anode of the present invention consists of at least two layers. The first layer is a catalytic layer and the second is a diffusion control layer. The catalytic layer acts in a substantially usual way, presenting the catalytic centers that allow electrochemical fuel oxidation.  
         [0056]    The diffusion control layer separates the fuel composition from the catalytic layer. The primary function of the diffusion control layer is to limit the rate of arrival of fuel molecules at the catalytic layer. The diffusion control layer ensures that the rate of production of electricity by the fuel cell is diffusion-controlled and constant, without interference due to side-products and side-reactions.  
         [0057]    the anode is further configured to allow ions and electrons produced to be transported to the cathode ordinarily through an electrolyte and through an electric circuit, respectively.  
         [0058]    In the first embodiment of the present invention  40 , depicted in FIG. 3, catalytic layer  42  is a made of a conducting substrate  44  onto which a catalyst  46  has been supplied. Conducting substrate  44  can be, for example, nickel or gold mesh, or a nickel-plated or gold-plated perforated ceramic sheet. Catalyst  46  is typically a mixture of metals, for example Pt/Ru, Pt/Ni, Pt/Co, Pt/Sn or Pt/Mo and can be applied to conducting substrate  44  by methods known in the art. Ordinarily catalysts are either provided as pure metals (unsupported catalysts) or provided adsorbed or otherwise connected to a material such as carbon black (supported catalysts). The catalyst, whether supported or unsupported, are in contact with or attached to conducting substrate  44  so that electrons produced at the catalyst are conducted to conducting substrate.  
         [0059]    Diffusions control layer  48  is in contact with catalytic layer  42  in such a way that fuel molecules must diffuse through diffusion control layer  48  to make contact with catalytic layer  42 . Diffusion control layer  48  is made up of a sheet of carbon paper impregnated with PVA (polyvinyl alcohol). The PVA increases the hydrophilic properties of the carbon paper. Alternatively, a layer of wet-laid fiber fleece (for example, of PVA fibers) or microporous films (such as grafted polypropylene, polysulfone or polycarbonate) can be used. The thickness and other properties of the diffusion control layer are selected to ensure that the rate of diffusion of the fuel is sufficiently low to achieve the desired current properties.  
         [0060]    In a second preferred embodiment of an anode  50  of the present invention, depicted in Fig. 4, a catalytic layer  42  as described above is sandwiched between two diffusion control layers  48  and  52 . The thickness and other properties of the diffusion control layer  48  that separates catalytic layer  42  from the fuel are so that V diffusion &lt;V electrochemical . The thickness and other properties of diffusion control layer  52  that separates catalytic layer  42  from the electrolyte are so that the rate of diffusion of ions away from the catalytic layer is higher than V electrochemical .  
         [0061]    Beyond the regulation of the rate of diffusion, diffusion control layers  48  and  52  in both embodiments above prevent pieces of catalyst  46  from disconnecting from conductive substrate  44  and in particular, from falling into the fuel chamber.  
         [0062]    As is clear to one skilled in the art, the catalytic layer and the diffusion control layers must be chemically compatible with the fuel composition and the anolyte components.  
         [0063]    Beyond the current-stabilizing properties of the anode of the present invention, the anode can be used to prevent fuel crossover, that is, the passage of fuel through the catalytic layer that may contaminate or otherwise compromise the electrolyte and if arriving at the cathode, short-circuit the fuel cell. Use of the anode of the present invention allows the addition of higher than accepted concentration of fuel in an anolyte or other fuel composition, with the concomitant advantages thereof.  
         [0064]    Depicted in FIG. 5, is a first embodiment of fuel cell  56  of the present invention. Fuel cell  56  uses a two-layer anode  40  of the present invention as depicted in FIG. 3 with a diffusion control layer  48 , a catalytic layer  46  and a conducting substrate  44 . Fuel composition  58  is supplied as an anolyte composed of 40% methanol in an acidic solution (e.g. 0.1% H 2 SO 4  in water). Fuel cell  56  uses a proton exchange membrane  60  to transport protons from anode  40  to cathode  62 . Circuit  64  electrically connects anode  40  through conducting substrate  44  to cathode  62 .  
         [0065]    Protons produced by the electrochemical oxidation are transported by proton exchange membrane  60  to cathode  62  Simultaneously, electrons produced are transported to cathode  62  through conducting substrate  44  and circuit  64 . Oxidant  68  is oxygen from air and has free contact with cathode  62 . Oxidant  68  is reduced on cathode  62  and combines with the protons and electrons to produce water. Released CO 2  escapes through vent  34 .  
         [0066]    Diffusion control layer  48  limits the availability of methanol molecules at catalytic layer  46 . Since the rate of methanol molecules arriving is controlled to be less than the maximal amount that are potentially oxidized any methanol arriving at the anode is electrochemically oxidized before it can make contact with proton exchange membrane  60 . Thus, the integrity and lifetime of proton exchange membrane  60  is maintained and fuel cell  56  as a whole is more robust. Since such a cell has a higher concentration of fuel molecules per unit anolyte volume, it is more compact and efficient.  
         [0067]    Depicted in FIG. 6, is a second embodiment of a fuel cell of the present invention  72 . Fuel cell  72  uses a three-layer anode  50  of the present invention as depicted in FIG. 4 with a first diffusion control layer  48 , a catalytic layer  46 , a conducting substrate  44  and a second diffusion control layer  52 . Fuel composition  74  is supplied as an anolyte composed of 40% methanol in a 6 M KOH solution. A 6M KOH or electrolyte solution is contained within electrolyte chamber  76 . The electrolyte solution transports ions from anode  50  to cathode  62 . Circuit  64  electrically connects anode  50  through conducting substrate  44  to cathode  62 . Since anolyte  58  contains a high concentration of KOH, released CO 2  remains in solution and thus there is no need for venting of produced gases.  
         [0068]    The manner of usage and operation of fuel cell  72  in FIG. 6 is, an analogy to fuel cell  56  illustrated in FIG. 5, apparent to one skilled in the art. Accordingly, no further discussion relating to the manner of usage and operation will be provided. It is important to emphasize that the presence of diffusion control layer  48  prevents contamination of electrolyte  76  by methanol, as described hereinabove.  
         [0069]    A great advantage of a fuel cell of the present invention of the type depicted in FIG. 6 is that its simplicity of construction relative to a state-of-the-art fuel cell, as depicted in FIG. 2, makes it exceptionally suitable for mobile and portable applications. The use of an anolyte and an electrolyte that solvates the exhaust gases avoids the need for venting and a degassing step. The use of a liquid electrolyte allows a more robust design, removing the need to use an expensive and sensitive PEM membrane.  
         [0070]    Exceptionally advantageous is to use a fuel cell of the type exemplified by the fuel cell depicted in FIG. 6 with the highly active fuel composition described in U.S. patent application 09/752,551. The fuel composition allows an exceptionally high power and current density at room temperature for a given volume of anolyte, making a fuel cell using the highly active fuel composition ideal for portable and mobile applications. The high reactivity of the fuel composition obviates the need for fuel heating.  
         [0071]    When used with the fuel composition described in U.S. patent application 09/752,551 the anode of the present invention reduces or prevents chemical oxidation and leads to production of a more stable current when compared to a usual anode.  
         [0072]    Since the fuel composition described in U.S. patent application 09/752,551 is significantly more reactive than methanol, a situation may arise where there is fear that significant chemical oxidation of the fuel composition may occur when the electrical circuit if open. Thus may be especially problematic when it is necessary to store a charged fuel cell for an extended period of time. It may therefore be advantageous in certain cases to add a value mechanism to the fuel cell of the present invention that forms a physical barrier preventing contact between fuel composition and the diffusion control layer and which is opened when the fuel cell is in use. One ordinarily skilled in the art is well acquainted with the construction and use of suitable value mechanisms.  
         [0073]    For the purpose of clarification, a simple planar valve  78  that can be interposed between the chamber where fuel composition  74  is contained and anode  50  is depicted in FIG. 6. Planar valve  78  is made up of a perforated front plate  80  facing fuel  74 , a perforated back plate  82  facing anode  50  and a biasing mechanism  84 .  
         [0074]    Planar valve  78  is depicted in greater detail in FIGS. 7 a  and  7   b . When the fuel cell is not in use, FIG. 7 a  biasing mechanism  84  pushes back plate  82  upwards, clocking passage of fuel composition  74  through valve  78 . When the fuel cell is in use. FIG  7   b  , back plate  82  is pushed downwards  84 , aligning the respective perforations of front plate  80  and back plate  82 , allowing passage of fuel composition  74  through valve  78 .  
         [0075]    A method supplementary to the use of diffusion control layer for controlling the rate of diffusion of a fuel composition to the catalytic layer of an anode is the addition of a viscous component to the fuel composition. Such a viscous component may be, for example, glycerine or polyethylene glycol. The resulting increased viscosity of the fuel composition lowers the rate of diffusion through a diffusion control layer when this is desired.  
         [0076]    The addition of a viscous component is useful in a number of cases. For example, in a situation when a fuel cell containing an anode of the present invention optimized for use in Alaska is used in the Mojave Desert. The addition of a viscous component to the fuel composition compensates for the lowered viscosity of the fuel composition resulting from the increased ambient temperature.  
         [0077]    The addition of a viscous component is also useful for reducing current output in order to increase fuel economy. It is clear to one skilled in the art, that when an anode of the present invention is used, the maximal current produced is limited by the rate of diffusion of fuel through the diffusion control layer. If in a specific application less power is needed than the fuel cell is designed to supply, addition of a viscous compound to the fuel composition will lower the power output with no ill effect and lead to a significant saving of fuel. When higher power is again required, the fuel composition containing the viscous compound is washed out and fresh fuel composition is added.  
         [0078]    Many other embodiments of the invention can be countenanced. For example, whereas the two embodiments of fuel cells described above use oxygen from air as an oxidant, with the necessary modifications a liquid oxidant can be used, for example, an organic fluid with a high oxygen concentration (see U.S. Pat. No 5,185,218) or a solution of hydrogen peroxide.  
         [0079]    Although the description above refers to a fuel cell anode, it is clear to one skilled in the art that there may be instances where it is advantageous to realize the cathode of a fuel cell using the teachings of the present invention, namely to control the rate of arrival of the material to be reduced at the cathode.  
         [0080]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.