Patent Publication Number: US-2004053085-A1

Title: Hydrogen management system for a fuel cell

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] The current application claims the benefit of priority from U.S. provisional application filed on Sep. 12, 2002, entitled “Hydrogen Management System for a Zinc Regenerative Fuel Cell” having Serial No. 60/410,581, which is incorporated herein by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates to hydrogen management systems for electrochemical cells. In particular, in some embodiments, the invention relates to fuel containers for non-hydrogen-based fuel cells having at least one recombination catalyst suitable for catalyzing the reaction of hydrogen and oxygen to form water. Additionally, the invention pertains to methods for recombining hydrogen gas inside a fuel container or other components of a fuel cell system.  
       BACKGROUND OF THE INVENTION  
       [0003] In general, a fuel cell is an electrochemical device that can convert chemical energy stored in fuels such as hydrogen, methane, zinc, aluminum and the like, into useful energy. A fuel cell generally comprises a negative electrode, a positive electrode, and a separator within an appropriate container. Fuel cells operate by utilizing chemical reactions that occur at each electrode. In general, electrons are generated at the anode and current flows through an external circuit to the cathode where a reduction reaction takes place. The electrochemical potential difference between the two electrodes can be used to drive useful work in the external circuit. For example, in one embodiment of a fuel cell employing metal, such as zinc, iron, lithium and/or aluminum, as a fuel and potassium hydroxide as the electrolyte, the oxidation of the metal to form an oxide or a hydroxide takes place at the anode. In commercial embodiments, several fuel cells are usually arranged in series, or stacked, in order to create larger voltages. For commercially viable fuel cells, it is desirable to have electrodes that can function within desirable parameters for extended periods of time on the order of 1000 hours or greater.  
       [0004] A fuel cell is similar to a battery in that both generally have a positive electrode, a negative electrode and electrolytes. However, a fuel cell is different from a battery in the sense that the fuel in a fuel cell can be replaced without disassembling the cell to keep the cell operating. In some embodiments, a fuel cell can be coupled to, or contain, a fuel regeneration unit which can provide the fuel cell with regenerated fuels.  
       [0005] In some fuel cells, the fuel can be stored in a container that is connected to the electrochemical cell stacks. Generally, the fuel is transported to the cells in an aqueous electrolyte such as, for example, potassium hydroxide solution. Under certain conditions, the fuel can react with the water in the electrolyte solution to form hydrogen gas. This reaction is sometimes referred to as corrosion of the fuel. The corrosion reaction is undesirable because it can produce hydrogen gas within the fuel cell system and also removes water from the electrolyte solution.  
       [0006] Fuel cells are a particularly attractive power supply because they can be efficient, environmentally safe and completely renewable. MetaVair fuel cells can be used for both stationary and mobile applications, such as all types of electric vehicles. Fuel cells offer advantages over internal combustion engines, such as zero emissions, lower maintenance costs and higher specific energies. Higher specific energies associated with selected fuels can result in weight reductions. In addition, fuel cells can give vehicle designers additional flexibility to distribute weight for optimizing vehicle dynamics.  
       SUMMARY OF THE INVENTION  
       [0007] In one aspect, the invention pertains to a hydrogen management system comprising a container having therein metal in contact with an aqueous electrolyte and at least one recombination catalyst separate from the aqueous electrolyte. In some embodiments, the catalyst is suitable for catalyzing the reaction of hydrogen and oxygen to form water. The hydrogen management system can further comprises at least one oxygen source connected to the container that provides oxygen to the interior of the container. Generally, hydrogen gas present in the container can contact the recombination catalyst and combine with oxygen to from water.  
       [0008] In another aspect, the invention pertains to a fuel cell system comprising an electrochemical cell stack having at least one anode, at least one cathode, a separator, and a container having therein a non-hydrogen fuel suitable for electrochemical reactions in contact with an aqueous electrolyte. In some embodiments, the container comprises a flow inlet and a flow outlet coupled to the electrochemical cell stack to provide the non-hydrogen fuel and the electrolyte to the electrochemical cell stack. Generally, at least one recombination catalyst separate from the electrolyte is in gas communication with the container. The recombination catalyst generally is suitable for catalyzing the reaction of hydrogen and oxygen to form water. The fuel cell system can also comprise an oxygen source. Generally, hydrogen gas present in the container can contact the recombination catalyst and combine with oxygen to form water.  
       [0009] Additionally, the invention pertains to a method for managing hydrogen for an electrochemical cell fuel container, the method comprising reacting hydrogen gas and oxygen gas to form water. In this embodiment, the container comprises a non-hydrogen fuel and an aqueous electrolyte, and a recombination catalyst that catalyzes the reaction of hydrogen and oxygen to form water is in gas communication with the container. 
     
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
     [0010]FIG. 1 is a schematic side view of a container with a side wall made transparent to show the fuel, electrolyte and recombination catalyst contained within the container.  
     [0011]FIG. 2 is a side view of another embodiment of a container with a side wall made transparent to show the fuel, electrolyte and recombination catalyst.  
     [0012]FIG. 3 is a schematic view of a fuel cell system with a container having a recombination catalyst and an oxygen source.  
     [0013]FIG. 4 is a schematic view of a fuel cell system with a container having a recombination catalyst, an oxygen source and a regeneration unit.  
     [0014]FIG. 5 is a schematic view of an electrochemical cell stack designed for the continuous replenishment of a metal fuel, in which a sectional side view of an anode is shown in phantom lines.  
     [0015]FIG. 6 is a sectional view of the fuel cell of FIG. 6 showing a cathode, in which the section is taken along line  6 - 6  of FIG. 5.  
     [0016]FIG. 7 is a side view of a zinc electrolyzer, in which an exterior case is made transparent to show an anode and a cathode.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0017] Improved containers contain fuel, such as metal, in contact with an aqueous electrolyte, at least one recombination catalyst separate from the electrolyte and, in some embodiments, an oxygen source. Generally, the recombination catalyst is suitable for catalyzing the reaction of hydrogen and oxygen to form water. Due to the presence of the recombination catalyst and the oxygen, hydrogen gas present in the containers can be combined with the oxygen to form water. The elimination of hydrogen gas can improve safety. Additionally, the water formed by the hydrogen and the oxygen can be provided to the electrolyte solution, which can reduce depletion of the electrolyte as a result of corrosion that forms hydrogen. The recombination catalyst can be attached, for example, near the top of the container, located in a cavity formed in the top of the container, provided to a portion of a vent connected to the container, positioned at other locations in vapor communication with the container, or a combination thereof. In some embodiments, the oxygen source can be ambient air from outside the container or can be supplied from a suitable container.  
     [0018] There are several types of fuels, i.e. reducing agents, typically employed in electrochemical cells including, for example, hydrogen, direct methanol and metal-based fuel systems. A metal-based fuel cell is an electrochemical cell that uses a metal, such as zinc particles, as fuel in the anode. In a metal fuel cell, the fuel is generally stored, transported and used in the presence of a reaction medium or electrolyte, such as a potassium hydroxide solution. The zinc metal is generally in the form of particles to allow for sufficient flow of the zinc fuel through the fuel cell. Specifically, in metal/air batteries and metal/air fuel cells, oxygen is reduced at the cathode, and metal is oxidized at the anode. In some embodiments, oxygen is supplied as air. For convenience, air and oxygen are used interchangeably throughout unless otherwise noted. In other embodiments, the oxidizing agent supplied to the cathode may be bromine gas or other suitable oxidizing agents. In some embodiments, the fuel compositions may further include additional additives, such as stabilizers and/or discharge enhancers.  
     [0019] In general, gas diffusion electrodes are suitable for catalyzing the reduction of gaseous oxidizing agents, such as oxygen, at a cathode of a metal fuel cell or battery. In some embodiments, gas diffusion electrodes comprise an active layer associated with a backing layer. The active and backing layers of a gas diffusion electrode are porous to gases such that gases can penetrate through the backing layer and into the active layer. However, the backing layer of the electrode is generally sufficiently hydrophobic to prevent diffusion of the electrolyte solution into or through the backing layer. The active layer generally comprises catalyst particles for catalyzing the reduction of a gaseous oxidizing agent, electrically conductive particles such as, for example, conductive carbon and a polymeric binder. Gas diffusion electrodes suitable for use in metal/air fuel cells are generally described in co-pending application Ser. No. 10/364,768, filed on Feb. 11, 2003, titled “Fuel Cell Electrode Assembly,” and in co-pending application Ser. No. 10/288,392, filed on Nov. 5, 2002, titled “Gas Diffusion Electrodes,” both of which are incorporated herein by reference.  
     [0020] In metal/air fuel cells that utilize zinc as the fuel, the following reaction takes place at the anodes:  
     Zn+4OH − →Zn(OH) 4   2− +2 e   −   (1)  
     [0021] The two released electrons flow through a load to the cathode where the following reaction takes place:  
                   1   2          O   2       +     2        e   -       +       H   2        O       →     2        OH   -               (   2   )                       
 
     [0022] The reaction product is the zincate ion, Zn(OH) 4   2− , which is soluble in the reaction solution KOH. The overall reaction which occurs in the cell cavities is the combination of the two reactions (1) and (2). This combined reaction can be expressed as follows:  
               Zn   +     2        OH   -       +       1   2          O   2       +       H   2        O       →       Zn        (   OH   )       4     2   -               (   3   )                       
 
     [0023] Alternatively, the zincate ion, Zn(OH) 4   2− , can be allowed to precipitate to zinc oxide, ZnO, a second reaction product, in accordance with the following reaction:  
     Zn(OH) 4   2− →ZnO+H 2 O+2OH −   (4)  
     [0024] In this case, the overall reaction which occurs in the cell cavities is the combination of the three reactions (1), (2), and (4). This overall reaction can be expressed as follows:  
               Zn   +       1   2          O   2         →   ZnO           (   5   )                       
 
     [0025] Under ambient conditions, the oxidation of zinc and reduction of molecular oxygen yields an open-circuit voltage potential of about 1.4V. For additional information on embodiments of a zinc/air battery or fuel cell, see for example U.S. Pat. Nos. 5,952,117; 6,153,329; and 6,162,555, which are incorporated by reference herein as though set forth in full.  
     [0026] Under certain conditions, corrosion of the fuel can occur. For example, the simplified reaction pathway that leads to zinc corrosion is shown below:  
     Zn+H 2 O→ZnO+H 2   (6)  
     [0027] As shown in equation (6), the reaction involves the reduction of water to yield gaseous hydrogen and the oxidation of metallic zinc to the Zn (II) ion. The corrosion of fuels such as zinc can be undesirable because the hydrogen gas represents a safety hazard. Additionally, the generation of hydrogen within the plumbing of fuel cell systems can cause gas bubbles to accumulate in, for example, the pumps and other points where system operation may be impeded. The accumulation of gas bubbles in the pumps of the fuel cell can result in the pumps losing their pumping ability. Furthermore, the generation of hydrogen gas removes water from the electrolyte solution, which can reduce the amount of energy the fuel cell can produce. Thus, it is generally desirable to recombine or remove the hydrogen gas present in a fuel cell container and/or a fuel cell system and thereby to replace the water. As described herein, the build up of hydrogen gas in a fuel cell can be reduced by providing a fuel container with an oxygen source and a recombination catalyst suitable for catalyzing the reaction of hydrogen and oxygen to form water.  
     [0028] Fuel Containers  
     [0029] The fuel containers of the present disclosure generally comprise at least one recombination catalyst suitable for catalyzing the reaction of hydrogen and oxygen to form water, wherein the recombination catalyst is separate from the fuel and electrolyte solution present in the container. In some embodiments, a head space can be provided at the top of the container, and the catalyst can be in the head space or a space in gas communication with the head space. For example, a recombination catalyst can be located inside the container, in a vent or passage connected to the container, or a combination thereof. A fuel container can further comprise at least one oxygen source, which can provide oxygen to the container to react with any hydrogen present in the container. The oxygen source can be, for example, an opening to the ambient atmosphere, an air or oxygen tank connected to the container, a regeneration unit, or a combination thereof. In some embodiments, the container may further comprise a vent with a pressure sensitive valve for venting gases from the container if a certain pressure is exceeded.  
     [0030]FIG. 1 shows fuel container  100  having a metal fuel and electrolyte mixture  102  contained within container  100 . As shown in FIG. 1, in some embodiments, fuel and electrolyte mixture  102  fill only a portion of container  100 , which leaves a head space  104  at the top of container  100  between fuel and electrolyte mixture  102  and top surface  108 . In some embodiments, a recombination catalyst  106  separate from fuel and electrolyte mixture  102  can be located in head space  104 , such as near the top surface  108  of container  100 . In other embodiments, recombination catalyst  106  may be located on a side wall of container  100 . Additionally or alternatively, a cavity  113  can be formed adjacent head space  104 , such as along top surface  108  of container  100 , and at least one recombination catalyst  106  can be located in cavity  113 . In some embodiments, a gas permeable cover may be positioned across the entrance of cavity  113  such that the gas permeable cover defines the lower boundary of cavity  113 . In further embodiments, recombination catalyst  106  may contact electrolyte mixture  102 . In some embodiments, catalyst  106  can be sintered onto a porous ceramic structure to hold the catalyst particles in a fixed format that can be held in a desired position within container  100  by plastic mountings, such as brackets, frames and/or the like. Although FIG. 1 shows an embodiment with a single cavity located along top surface  108 , other embodiments can comprise a plurality of cavities, wherein at least a portion of the plurality of cavities contain at least one recombination catalyst  106 . The Generally, the recombination catalyst is a catalyst suitable for catalyzing the reaction of oxygen with hydrogen to form water. In some embodiments, vent  110  provides a pathway to expel gas from the container, including, for example, any hydrogen gas that does not recombine to form water. In one embodiment, vent  110  can comprise a pressure sensitive valve  111  that vents hydrogen and other gases that exceed a particular pressure.  
     [0031] Fuel container  100  can further comprise oxygen source  112 , which can provide oxygen to container  100 . In one embodiment, oxygen source  112  comprises an inlet  114  and a tank  113  or other suitable container containing oxygen, which delivers oxygen via oxygen inlet  114 . Additionally or alternatively, oxygen source may comprise a valve or the like that allows controlled introduction of ambient air to enter container  100 . In some embodiments, a pump  115  can be associated with oxygen source  112  to facilitate the flow of oxygen into container  100 . Additionally, container  100  can comprise flow outlet  116  and flow inlet  118 , which provide a pathway for circulating fuel and electrolyte mixture  102  to an electrochemical cell stack or the like.  
     [0032] Referring to FIG. 1, in this embodiment, hydrogen gas located in container  100  can accumulate in head space  104 , and oxygen gas can diffuse and/or be pumped into head space  104  from oxygen source  112 . The hydrogen and oxygen in the head space  104  can contact recombination catalyst  106  located in container  100  and recombine to from water. Additionally, hydrogen and/or oxygen molecules that do not recombine to form water can be vented through vent  110  to the ambient atmosphere. Thus, container  100  provides for both recombination and/or removal of hydrogen gas present inside the container.  
     [0033]FIG. 2 shows another embodiment of a fuel container  130  having a metal fuel and electrolyte mixture  132  contained within container  130 . In this embodiment, a recombination catalyst  134  can be located in a portion of vent  136 . Generally, metal fuel and electrolyte mixture  132  only fills a portion of container  130 , such that a head space  140  can be created along the top of container  130 . Additionally or alternatively, recombination catalyst can also be located within the container&#39;s head space  140 . In some embodiments, an oxygen source can be connected to container  130  to provide oxygen to head space  140  of container  130 . The oxygen source can comprise an oxygen tank  142  and an oxygen inlet  144 , a valve in vent  136  that permits ambient air to enter container  130 , or a combination thereof. Generally, container  130  comprises flow inlet  146  and flow outlet  148 , which can provide a pathway for circulating fuel and electrolyte mixture  132  to an electrochemical cell stack or other components of a fuel cell system. In this embodiment, hydrogen gas located in container  130 , as well as oxygen provided to container  130  from one or more oxygen sources, can accumulate in head space  140 , contact recombination catalyst  134  located in vent  136  and recombine to form water. Furthermore, hydrogen and oxygen gas that does not recombine into water can exit container  130  via vent  136 .  
     [0034] Referring to FIG. 3, an embodiment of a fuel cell system  199  is shown comprising fuel storage container  200 , electrochemical cell stack  202 , flow outlet pipe  204 , flow inlet pipe  208  and vent  210 . Fuel cell system  199  can further comprise cell stack exhaust  212 , cell stack air inlet  214  and cell stack air pump  216 . As shown in FIG. 3, electrochemical cell stack  202  is connected to fuel storage container  200  via flow pipes  204 ,  208 , which provide a flow pathway for fluids between stack  202  and fuel container  200 . In this embodiment, air can be pumped into electrochemical cell stack  202  through air inlet  214 , and exit stack  202  through exhaust pipe  212 . Vent  210  is connected to fuel storage container  200  and provides a flow pathway for gases, such as hydrogen, located within fuel container  200  to be released from the container. In some embodiments, at least one recombination catalyst  224  can be located in vent  210  suitable for catalyzing the reaction of hydrogen and oxygen to form water. Fuel cell system  199  can further comprise connection pipe  218 , which provides a flow path between vent  210  and cell stack exhaust  212 . Additionally or alternatively, at least one recombination catalyst  226  may be located in cell stack exhaust  212 , above connection pipe  218 . Generally, fuel container  200  contains a metal fuel and electrolyte solution  220  that fills only a portion of fuel container  200 , which creates a head space  222  at the top portion of fuel storage container  200 .  
     [0035] As shown in FIG. 3, fuel container  200  is connected to electrochemical cell stack  202  by pipes  204 ,  208 , however, in other embodiments fuel container  200  may be connected to a container having fuel regeneration unit or to one or more electrochemical cell stacks. A valve  211  can be positioned in vent  210  to regulate the flow of hydrogen gas and other fluids to connection pipe  218 . In this embodiment, hydrogen generated or present in container  200  can be vented though safety vent  210  to the atmosphere and/or can be channeled via connection pipe  218  to cell stack exhaust  212 , where the hydrogen can combine with air existing cell stack  202 , due to the presence of recombination catalyst  226 , to form water. Alternatively, hydrogen gas present in container  200  can contact recombination catalyst  224  in vent  210  and recombine with oxygen that diffused into catalyst  224  through an opening in vent  210 . Additionally, the flow of fuel and electrolyte between stack  202  and fuel container  200  via flow pipes  204 ,  208  may carry oxygen and hydrogen gas present in stack  202  into container  200 , where the gases can contact recombination catalyst  224  in vent  210  and recombine to form water.  
     [0036]FIG. 4 shows another embodiment of a fuel cell system  250  comprising fuel container  252  connected to electrochemical cell stack  254  by flow outlet  256  and flow inlet  258 . In some embodiments, flow pump  260  can be provided to circulate fuel and electrolyte solution  262  stored in container  252 . Generally, fuel and electrolyte solution  262  fills only a portion of container  252 , leaving a head space  264  along the top portion of container  252 . In some embodiments, cavity  266  can be located in the top surface of container  252 , adjacent head space  264 . In one embodiment, cavity  266  comprises at least one recombination catalyst  268  suitable for catalyzing the reaction of hydrogen and oxygen to from water. As shown in FIG. 5, an air supply pump  270  can pump air into head space  264  through air inlet  272 . Alternatively or additionally, as shown in FIG. 4, oxygen can provided to head space  264  by regeneration electrodes  274 ,  275 . As will be described in detail below, regeneration electrodes  274 ,  275  can regenerate molecular oxygen and metal fuel particles from oxidized metal compounds. In some embodiments, as shown in FIG. 4, regeneration electrodes  274 ,  275  can be located inside container  252 , however, in alternative embodiments regeneration electrodes  274 ,  275  can be located in a separate regeneration container and the regenerated oxygen can be provided to the fuel container  252  by pipes or the like.  
     [0037] As shown in FIG. 4, a temperature sensor  276  can be located inside container  252  to provide current temperature information to processor units connected to air pump power supply  278  and/or to regeneration electrodes power supply  280 . Generally, power supplies  278 ,  280  can regulate the delivery and/or generation of oxygen to container  252  by adjusting the voltage supply to air supply pump  270  and/or to regeneration electrodes  274 ,  275 . In some embodiments, the voltage supplied to air supply pump  270  and/or regeneration electrodes  274 ,  275  can be increased during higher temperature and discharge modes of operation, and decreased during standby and lower temperature modes of operation. As will be discussed below, regeneration electrodes  274 ,  275  can produce oxygen and fuel particles from solutions containing oxidized fuel compounds such as, for example, ZnO and/or Zn(OH) 4   2− . Increasing the voltage to the regeneration electrodes can increase the amount of oxygen generated from oxidized fuel compounds. The amount of O 2  generated can be selected to be at least sufficient to react with the quantity of H 2  predicted to be generated at a particular temperature.  
     [0038] The containers of the present disclosure can be composed of any material suitable for use in electrochemical cell applications that are chemically inert with respect to the fuel and electrolyte contained within the container. Examples of suitable materials include metals, metal alloys, polymers, and combinations thereof. Suitable polymers include, for example, polyetheretherketone (PEEK), NORYL® (polyphenylene oxide/PPO, General Electric Polymers, which can be blended with polystyrene) and/or other thermoplastic polymers, such as polyethylene, polypropylene, poly(vinyl chloride), poly(tetrafluoroethylene), poly(vinylidene fluoride), and blends and copolymers thereof. One of ordinary skill in the art will recognize that no particular container shape is required by the present disclosure. In general, the size of the container will be guided by the fuel requirements of the corresponding electrochemical cell stack, or fuel cell system, that the container is intended to supply with fuel and electrolyte.  
     [0039] As described above, flow pipes can be connected to the body of the container to provide a flow pathway for fuel and electrolyte between the container and at least one electrochemical cell stack. The flow pipes can be composed of any material, such as a polymeric material, suitable for electrochemical cell applications that are chemically inert with respect to the fuel and electrolyte solution. Suitable polymers include, for example, PEEK, NORYL®, polyethylene, polypropylene, poly(vinyl chloride), poly(tetrafluoro-ethylene), poly(vinylidene fluoride), and blends and copolymers thereof. Additionally, the flow pipes can be composed of polymers such as, for example, PEEK, NORYL and combinations thereof. In some embodiments, the flow pipes can be composed of the same material as the body of the container, while in other embodiments the flow pipes can be composed of a different material than the container. In further embodiments, the container may comprise a plurality of inlet pipes and a plurality of outlet pipes connected to the container. In general, the number and size of the flow pipes will be guided by the design of the corresponding fuel cell system.  
     [0040] As noted above, the recombination catalyst is generally suitable for catalyzing the reaction of hydrogen and oxygen to form water. In one embodiment, the recombination catalyst can be platinum, however, other metals such as, for example, nickel, palladium or a combination thereof, can also be used. As described in detail below, the recombination catalyst can comprise a particulate material, which can be a catalyst material coated onto a substrate such as carbon. Suitable catalysts include, for example, elemental metal particles, metal compositions and combinations thereof. Suitable metals broadly cover all recognized metal elements of the periodic table and alloys thereof. Exemplary metals include without limitation, Fe, Co, Ag, Ru, Mn, Zn, Mo, Cr, Cu, V, Ni, Rh, and Pt. Suitable metal compositions include, for example, permanganates (e.g., AgMnO 4  and KMnO 4 ), metal oxides (e.g., MnO 2  and Mn 2 O 3 ), decomposition products of metal heterocycles (e.g., iron tetraphenylporphyrin, cobalt tetramethoxyphenylporphyrin, cobalt complexes (e.g., tetramethoxyphenyl porphyrin (CoTMPP)), perovskites, cobalt pthalocynanine and iron pthalocynanine) and napthenates (e.g., cobalt napthenates and manganese napthenate) and combinations thereof. Elemental metals are un-oxidized metals in their zero oxidation state, i.e., M 0 . Suitable elemental metal particles include, for example, Ag, Pt, Pd, Ru, alloys thereof and combinations thereof. In general, the catalyst particles can be spherical, rod-shaped or any other suitable shape or combinations of shapes yielding an appropriate surface area.  
     [0041] Some metals for use as catalysts have a high cost. Therefore, cost savings can result from coating the elemental metal onto a less expensive particulate. For example, metals can be coated onto carbon black. In some embodiments, the catalysts comprise in the range(s) of at least about 80 weight percent carbon black and no more than about 20.0 weight percent metal, and in other embodiments from about 94.95 weight percent to about 99.9 weight percent carbon black, in the range(s) from about 0.1 weight percent to about 5.0 weight percent metal and in the range(s) from about 0.05 to about 5 weight percent nitrogen. To form the catalyst, carbon black is contacted with vapors of metal precursors and nitrogen precursors in a reducing environment. The metal may or may not be in elemental form and the carbon black may or may not be chemically bonded to metal and/or the nitrogen. The carbon black materials described above are also suitable for forming these catalyst materials. The carbon black-metal-nitrogen containing catalysts are further described in copending and commonly assigned U.S. patent application Ser. No. 09/973,490 to Lefebvre, entitled “Methods of Producing Oxygen Reduction Catalyst,” incorporated herein by reference.  
     [0042] At least one source of oxygen can be connected to the container to supply oxygen to react with any hydrogen present in the container. The term oxygen source is being used in the broad sense to include, for example, combinations of tanks, valves, regeneration electrodes, openings, piping systems and combinations thereof that permit oxygen to be introduced to, or generated in, a container. For example, in some embodiments, the oxygen source can comprise one or more valves that selectively permit ambient air from outside the fuel cell system to reach the container. In other embodiments, the oxygen source can comprise a tank of air and/or oxygen connected to the container. Additionally or alternatively, as will be described in detail below, the oxygen source can be regeneration electrodes that generate oxygen from oxidized fuel. In some embodiments, the regeneration electrodes can be located inside a fuel storage container, while in other embodiments the regeneration electrodes can be located in a separate regeneration container. The optional regeneration electrodes can be connected to a processor programmed to continuously provide oxygen to the container. Alternatively, the regeneration unit can periodically supply oxygen to the container. In one embodiment, oxygen can be pumped into the container from the oxygen source by an oxygen pump, while in other embodiments the oxygen may diffuse to the head space and/or recombination catalyst from the oxygen source.  
     [0043] As noted above, in some embodiments oxygen can be pumped into the head space of the container from one or more oxygen sources. The term pump is being used in its broad sense to include any mechanical device capable of applying a force against a fluid. Suitable pumps include, for example, fans, blowers, and the like and combinations thereof. In some embodiments, the pump can be programmed to provide oxygen approximately at the predicted rate of hydrogen evolution at a particular temperature. Alternatively, the oxygen pump can be programmed to operate continuously during discharge and higher electrolyte temperature modes of operation, and periodically during standby and lower electrolyte temperatures modes of operation.  
     [0044] The containers of the present disclosure generally contain a fuel and electrolyte mixture suitable for use in electrochemical cell applications. In some embodiments, the fuel is a particulate metal such as zinc, aluminum, iron, lithium, magnesium or a combination thereof. In one embodiment, the electrolyte can be an aqueous solution of potassium hydroxide. In some embodiments, the metal fuel can be present in a concentration of about 70 percent by weight, while in other embodiments the metal fuel can be present in a concentration from about 25 percent by weight to about 65 percent by weight. One of ordinary skill in the art will recognize that additional ranges of metal fuel concentration within these explicit ranges are contemplated and are within the scope of the present disclosure.  
     [0045] During operation, hydrogen gas produced by the corrosion reaction of a metal fuel and water in the cell stacks can be transported by the flowing electrolyte to the fuel and electrolyte container, where the hydrogen gas can accumulate in the head space of the container. Additionally, hydrogen gas produced by corrosion of the metal fuel and water in the container can also accumulate in the head space of the container. Hydrogen gas in present in the head space of the container can contact the recombination catalyst, which can be located adjacent to the head space, and recombine with oxygen provided to the container to form water. Hydrogen gas that does not recombine with oxygen to form water can optionally be expelled from the container through a vent. Water formed by the reaction of hydrogen gas and oxygen generally flows to the electrolyte within the container.  
     [0046] Fuel Cell System  
     [0047] An embodiment of an electrochemical cell stack  300  is shown in FIG. 5. As shown in FIG. 5, electrochemical cell stack  300  comprises a stack of one or more cells  302 , each generally defining a plane and coupled together in series. Metal-air fuel cell  302  interfaces with a stack container  304 . Each cell  302  includes an air positive electrode or cathode  306  that is positioned at one side of cell  302  and a zinc negative electrode or anode  308  that is positioned at the opposite side of cell  302 . The cathode and anode are separated by an electrically insulating separator. Additionally, adjacent cells in electrochemical cell stack  300  can be coupled in series by bipolar plates, which can electrically connect the anode of a first cell with the cathode of an adjacent cell.  
     [0048] Electrochemical cell stack  300  can be incorporated into a electrochemical cell system, such as the systems shown in FIGS. 3 and 4, by connecting piping system  310  of electrochemical cell stack  300  to the flow inlet(s) and flow outlet(s) of an appropriate container. For example, metal fuel and electrolyte can be fed from a container, such as the containers described above in FIGS.  1 - 4 , through piping system  310  into inlet manifold  312  of cell stack  300 , and correspondingly vented as shown in FIGS. 3 and 4. Piping system  310  can comprise one or more fluid connecting devices, e.g., tubes, conduits, elbows, and the like, for connecting the components of a system. The interface between cathode  306  and piping system  310  through inlet manifold  312  is shown in phantom lines in FIG. 5. Inlet manifold  312  can distribute fuel, such as fluidized zinc pellets, to the anode beds of the cells via filling tubes  314 . Thus, any hydrogen gas generated in fuel cell stack  300  can be reacted with a catalyst as described with respect to FIGS. 3 and 4.  
     [0049] The fuel and electrolyte can flow through a flow path  316  in each cell. In some embodiments, the method of delivering the fuel to each cell is a flow through method. For example, a dilute stream of fuel pellets in an electrolyte can be delivered to flow path  316  at the top of each cell via filling tubes  314 . The stream can flow through path  316 , across anode bed  318 , and exit on the opposite side of the cell via outlet tube  320 . In some embodiments, pumps  322  can be used to control the flow rate of fuel and electrolyte through cell stack  300 .  
     [0050]FIG. 6 displays a positive oxygen electrode/cathode  306  within one cell  302  of cell stack  300  of FIG. 5. A non-porous divider  360  separates gas inflow from air blowers  324  from air outlets  326 . Stack container  304  forms an inlet chamber  362  and an outlet chamber  364 . Inlet chamber  362  and outlet chamber  364 , respectively, form passageways from positive oxygen electrode  306  to air blowers  324  and air outlets  326 . A gas permeable membrane or backing layer  366  can be placed between air chambers  362 ,  364  and electrode  306  to reduce or prevent loss of electrolyte through flow out of the cell and/or evaporation. Fuel cell stacks are described further in co-pending application Ser. No. 10/437,481, filed on May 14, 2003, entitled “Combined Fuel Cell and Battery,” which is incorporated herein by reference.  
     [0051] In some embodiments, the metal/air fuel cell system and/or a fuel storage container can comprise a zinc electrolyzer having regeneration electrodes, which can reprocess the above noted reaction products to yield, for example, oxygen and zinc particles. In embodiments employing a zinc fuel, the reaction product Zn(OH) 4   2−  and/or possibly ZnO or other zinc compounds, can be reprocessed with the application of an external electric potential, for example, from line voltage, to yield molecular oxygen and zinc particles. As described previously, the regeneration electrodes can be located inside a fuel storage container, such as the containers described above, to provide oxygen to react with any hydrogen gas present in the container. Alternatively, the regeneration electrodes can be located in a stand alone container, and regenerated oxygen can be supplied to fuel storage container via a pipe or other suitable connection. Thus, the regeneration electrodes can primarily be used to form oxygen to react with hydrogen, or primarily to regenerate desired amounts of metal fuel with a portion of the resulting oxygen gas being available for reaction with hydrogen gas.  
     [0052] Referring to FIG. 7, container  414  is shown comprising regeneration electrodes  404 ,  406  suitable for generating molecular oxygen from oxidized metal compounds. In this embodiment, container  414  comprises an anode  404 , a cathode  406 , a pump  408  and a power supply  412 . In some embodiments, anode  404  and cathode  406  can be electrodes which are at least partially immersed in solution  410 . Generally, anode  404  and cathode  406  are aligned parallel to each other in container  414  such that adjacent surfaces of anode  404  and cathode  406  define a space where the regeneration reactions can occur. Container  414  generally holds solution  410  as well as anode  404  and cathode  406 . In some embodiments, container  414  can be a fuel and electrolyte storage container connected to a electrochemical cell stack to provide fuel and electrolyte to the cell stack. Solution  410  generally is aqueous, although organic solvents, such as alcohols, can be substituted for an aqueous solvent. In one embodiment, solution  410  can comprise zincate ions, Zn(OH) 4   2− , or dissolved/dispersed ZnO. The zincate ions can be produced by the above mentioned electrochemical reactions which, in one embodiment, can occur in a zinc/air (oxygen) cell. In alternative embodiments, container  414  can comprise a plurality of electrode pairs, which can be connected in parallel or in series.  
     [0053] Metal particles, for example, zinc particles, and molecular oxygen can be produced through electrolysis, which in one embodiment occurs in the space between a surface of anode  404  and the opposing surface of cathode  406 . During operation, anode  404  and cathode  406  can be coupled, respectively, to the positive and negative terminals of power supply  412 . In some embodiments, a pump  408  can be provided to circulate solution  410  into and out of container  414 . In general, pump  408  can be any mechanical device capable of circulating fluids. In one embodiment, as shown in FIG. 7, solution  410  can flow into container  414  through flow inlet  416 , and can flow out of container  414  through flow outlet  418 . Generally, by pumping solution  410  into and out of container  414  a flow path  420  can be created along the surface of cathode  406 .  
     [0054] Suitable regeneration units and their operation are discussed further in copending U.S. application Ser. No. 10/424,539 to Smedley et al. filed on Apr. 24, 2003, entitled “Discrete Particle Electrolyzer Cathode And Method Of Making Same,” incorporated herein by reference.  
     [0055] The embodiments above are intended to be illustrative and not limiting. Additional embodiments are within the claims. Although the present invention has been described with reference to particular embodiments, workers in the art will recognize that changes may be made in form and detail without departing form the spirit and scope of the invention.