Patent Publication Number: US-2005123815-A1

Title: Rechargeable and refuelable metal air electrochemical cell

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to metal air electrochemical cells. More particularly, the invention relates to rechargeable and refuelable metal air electrochemical cells and anodes assemblies for use therewith.  
      2. Description of the Prior Art  
      Electrochemical power sources are devices through which electric energy can be produced by means of electrochemical reactions. These devices include metal air electrochemical cells such as zinc air and aluminum air batteries. Such metal electrochemical cells employ an anode comprised of metal that is converted to a metal oxide during discharge. Certain electrochemical cells are, for example, rechargeable, whereby a current may be passed through the anode to reconvert metal oxide into metal for later discharge. Additionally, refuelable metal air electrochemical cells are configured such that the anode material may be replaced for continued discharge. Generally, metal air electrochemical cells include an anode, a cathode, and electrolyte. The anode is generally formed of metal particles immersed in electrolyte. The cathode generally comprises a bi-functional semipermeable membrane and a catalyzed layer for reducing oxygen. The electrolyte is usually a caustic liquid that is ionic conducting but not electrically conducting.  
      Metal air electrochemical cells have numerous advantages over traditional hydrogen-based fuel cells. In particular, the supply of energy provided from metal air electrochemical cells is virtually inexhaustible because the fuel, such as zinc, is plentiful and can exist either as the metal or its oxide. The fuel of the metal air electrochemical cells may be solid state, therefore, it is safe and easy to handle and store. In contrast to hydrogen based fuel cells, which use methane, natural gas, or liquefied natural gas to provide as source of hydrogen, and emit polluting gases, the metal air electrochemical cells results in zero emission. The metal air fuel cell batteries operate at ambient temperature, whereas hydrogen-oxygen fuel cells typically operate at temperatures in the range of 150° C. to 1000° C. Metal air electrochemical cells are capable of delivering higher output voltages (1-4.5 Volts) than conventional fuel cells (&lt;0.8V).  
      A desirable and convenient configuration of metal air electrochemical cells is that in which the metal fuel is in the form of rigid cards that may be replaced upon electrochemical consumption, also referred to as “mechanical recharging”.  
      However, heretofore known mechanically rechargeable, or refuelable, metal air cells have not been capable of electrical recharging in combination with the mechanical recharging.  
      There remains a need in the art for an electrically rechargeable and refuelable metal air electrochemical cell system.  
     SUMMARY OF THE INVENTION  
      The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the several methods and apparatus of the present invention, wherein a refuelable and rechargeable metal air electrochemical cell system is provided.  
      In one embodiment, a refuelable and rechargeable metal air electrochemical cell includes a removable and rechargeable metal fuel anode, and air cathode, a third electrode, and a separator in ionic communication with at least a portion of a major surface of the anode.  
      In another embodiment, a refuelable and rechargeable metal air electrochemical cell includes a discharging cell and a recharging cell. The discharging cell includes an air cathode structure adapted to receive a removable and rechargeable metal fuel anode that, when inserted in the air cathode structure, produces electrical energy during the process of electrochemical conversion of the metal fuel into a metal oxide. The recharging cell includes a charging electrode structure adapted to receive the removable and rechargeable metal fuel anode (generally after such anode has been discharged, or prior to initial usage of the anode for discharging), that, when inserted in the charging electrode structure, converts the metal oxide into metal fuel upon application of electrical energy.  
      Furthermore, various structures are provided that facilitates refueling of the anode.  
      The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A-1C  show general discharging and charging operations of a metal air cell;  
       FIG. 2A  shows a general embodiment of a refuelable and rechargeable module;  
       FIGS. 2B-2D  show exemplary components for use with a refuelable and rechargeable module;  
       FIG. 3  shows a general embodiment of a refuelable and rechargeable system including a refuelable module and a rechargeable module;  
       FIGS. 4A-4D  show a first embodiment of a refuelable and rechargeable system including a refuelable module and a rechargeable module;  
       FIGS. 5A-5D  show exemplary components for use with a refuelable and rechargeable system including a refuelable module and a rechargeable module;  
       FIGS. 6A-6D  show a fluid management system including electrolyte management and air management;  
       FIGS. 7A-7B  show a gripping structure for removing one or more anode structures;  
       FIGS. 8A-8C  show a second embodiment of a refuelable and rechargeable system including a refuelable module and a rechargeable module;  
       FIGS. 9A-9C  show exemplary components for use with a refuelable and rechargeable system including a refuelable module and a rechargeable module; and  
       FIG. 10A-10C  and  11 A and  11 B show a fluid management system including electrolyte management and air management. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS  
      Generalized Description of the Operative Components and Cell Operations  
      A refuelable and rechargeable metal air electrochemical cell is provided. In general, the refuelable and rechargeable metal air electrochemical cell includes a metal fuel anode, and air cathode, a third electrode, and one or more separators allowing ionic communication and maintaining electrical isolation between the anode and cathode. Furthermore, structures are provided that facilitate refueling of the anodes.  
      Referring now to the drawings, an illustrative embodiment of the present invention will be described. For clarity of the description, like features shown in the figures shall be indicated with like reference numerals and similar features as shown in alternative embodiments shall be indicated with similar reference numerals.  
       FIG. 1A  is a schematic representation of an electrochemical cell  100   a . Electrochemical cell  100   a  may be a metal oxygen cell, wherein the metal is supplied from a metal anode structure  112  and the oxygen is supplied to an oxygen cathode  114 . The anode  112  and the cathode  114  are maintained in electrical isolation from on another by a separator  116 . The shape of the cell and of the components therein is not constrained to be square or rectangular; it can be tubular, spherical, circular, elliptical, polygonal, or any desired shape. Further, the configuration of the cells components, i.e., vertical, horizontal, or tilted, may vary, even though the cell components are shown as substantially vertical in  FIG. 1 .  
      During discharging operations, oxygen from the air or another source is used as the reactant for the air cathode  114  of the metal air cell  100   a.  When oxygen reaches the reaction sites within the cathode  114 , it is converted into hydroxyl ions together with water. At the same time, electrons are released to flow as electricity in the external circuit. The hydroxyl travels through the separator  116  to reach the metal anode  112 . When hydroxyl reaches the metal anode (in the case of an anode  112  comprising, for example, zinc), zinc hydroxide is formed on the surface of the zinc. Zinc hydroxide decomposes to zinc oxide and releases water back to the alkaline solution. The reaction is thus completed.  
      The anode reaction is: 
 
Zn+4OH − →Zn(OH) 4   2− +2e   (1) 
 
Zn(OH) 4   2− →ZnO+H 2 O+2OH −   (2) 
 
      The cathode reaction is: 
 
½O 2 +H 2 O+2e→2OH −   (3) 
 
      Thus, the overall cell reaction is: 
 
Zn+½O 2 →ZnO   (4) 
 
      The anode  112  generally comprises a metal constituent such as metal and/or metal oxides and a current collector. For a rechargeable cell, it is known in the art to utilize a formulation including a combination of a metal oxide and a metal constituent. Optionally an ionic conducting medium is provided within the anode  112 . Further, in certain embodiments, the anode  112  comprises a binder and/or suitable additives. Preferably, the formulation optimizes ion conduction rate, capacity, density, and overall depth of discharge, while minimizing shape change during cycling.  
      The metal constituent may comprise mainly metals and metal compounds such as zinc, calcium, lithium, magnesium, ferrous metals, aluminum, oxides of at least one of the foregoing metals, or combinations and alloys comprising at least one of the foregoing metals. These metals may also be mixed or alloyed with constituents including, but not limited to, bismuth, calcium, magnesium, aluminum, indium, lead, mercury, gallium, tin, cadmium, germanium, antimony, selenium, thallium, oxides of at least one of the foregoing metals, or combinations comprising at least one of the foregoing constituents. The metal constituent may be provided in the form of powder, fibers, dust, granules, flakes, needles, pellets, or other particles. In certain embodiments, granule metal, particularly zinc alloy metal, is provided as the metal constituent. During conversion in the electrochemical process, the metal is generally converted to a metal oxide.  
      The anode current collector may be any electrically conductive material capable of providing electrical conductivity and optionally capable of providing support to the anode  112 . The current collector may be formed of various electrically conductive materials including, but not limited to, copper, brass, ferrous metals such as stainless steel, nickel, carbon, electrically conducting polymer, electrically conducting ceramic, other electrically conducting materials that are stable in alkaline environments and do not corrode the electrode, or combinations and alloys comprising at least one of the foregoing materials. The current collector may be in the form of a mesh, porous plate, metal foam, strip, wire, plate, or other suitable structure. As described herein, certain embodiments utilize extensions of the current collector as power output terminals.  
      The ionic conducting medium generally comprises alkaline media to provide a path for hydroxyl to reach the metal and metal compounds. The ionically conducting medium may be in the form of a bath, wherein a liquid electrolyte solution is suitably contained. In certain embodiments, an ion conducting amount of electrolyte is provided in anode  112 . The electrolyte generally comprises ionic conducting materials such as KOH, NaOH, LiOH, other materials, or a combination comprising at least one of the foregoing electrolyte media. Particularly, the electrolyte may comprise aqueous electrolytes having a concentration of about 5% ionic conducting materials to about 55% ionic conducting materials, preferably about 10% ionic conducting materials to about 50% ionic conducting materials, and more preferably about 30% ionic conducting materials to about 45% ionic conducting materials. Other electrolytes may instead be used, however, depending on the capabilities thereof, as will be obvious to those of skill in the art.  
      The optional binder of the anode  112  primarily maintains the constituents of the anode in a solid or substantially solid form in certain configurations. The binder may be any material that generally adheres the anode material and the current collector to form a suitable structure, and is generally provided in an amount suitable for adhesive purposes of the anode. This material is preferably chemically inert to the electrochemical environment. In certain embodiments, the binder material is soluble, or can form an emulsion, in water, and is not soluble in an electrolyte solution. Appropriate binder materials include polymers and copolymers based on polytetrafluoroethylene (e.g., Teflon® and Teflon® T-30 commercially available from E.I. du Pont Nemours and Company Corp., Wilmington, Del.), polyvinyl alcohol (PVA), poly(ethylene oxide) (PEO), polyvinylpyrrolidone (PVP), and the like, and derivatives, combinations and mixtures comprising at least one of the foregoing binder materials. However, one of skill in the art will recognize that other binder materials may be used.  
      Optional additives may be provided to prevent corrosion. Suitable additives include, but are not limited to indium oxide; zinc oxide, EDTA, surfactants such as sodium stearate, potassium Lauryl sulfate, Triton® X-400 (available from Union Carbide Chemical &amp; Plastics Technology Corp., Danbury, Conn.), and other surfactants; the like; and derivatives, combinations and mixtures comprising at least one of the foregoing additive materials. However, one of skill in the art will determine that other additive materials may be used.  
      The oxygen supplied to the cathode  114  may be from any oxygen source, such as air; scrubbed air; pure or substantially oxygen, such as from a utility or system supply or from on site oxygen manufacture; any other processed air; or any combination comprising at least one of the foregoing oxygen sources.  
      Cathode  114  may be a conventional air diffusion cathode, for example generally comprising an active constituent and a carbon substrate, along with suitable connecting structures, such as a current collector. Alternatively, the cathode  114  may comprise a bifunctional electrode, suitable for both discharging and charging. Typically, the cathode catalyst is selected to attain current densities in ambient air of at least 20 milliamperes per squared centimeter (mA/cm 2 ), preferably at least 50 mA/cm 2 , and more preferably at least 100 mA/cm 2 . Of course, higher current densities may be attained with suitable cathode catalysts and formulations. The cathode  114  may be a bi-functional, for example, which is capable of both operating during discharging and recharging.  
      The carbon used is preferably be chemically inert to the electrochemical cell environment and may be provided in various forms including, but not limited to, carbon flake, graphite, other high surface area carbon materials, or combinations comprising at least one of the foregoing carbon forms.  
      The cathode current collector may be any electrically conductive material capable of providing electrical conductivity and preferably chemically stable in alkaline solutions, which optionally is capable of providing support to the cathode  114 . The current collector may be in the form of a mesh, porous plate, metal foam, strip, wire, plate, or other suitable structure. The current collector is generally porous to minimize oxygen flow obstruction. The current collector may be formed of various electrically conductive materials including, but not limited to, copper, ferrous metals such as stainless steel, nickel, chromium, titanium, and the like, and combinations and alloys comprising at least one of the foregoing materials. Suitable current collectors include porous metal such as nickel foam metal. Further, embodiments of a cathode are shown herein whereby the cathode is essentially wrapped around a structure configured to receive the anode, wherein the current collector is provided at the crease of the wrapped cathode (see, e.g.,  FIG. 9A ).  
      A binder is also typically used in the cathode  114 , which may be any material that adheres substrate materials, the current collector, and the catalyst to form a suitable structure. The binder is generally provided in an amount suitable for adhesive purposes of the carbon, catalyst, and/or current collector. This material is preferably chemically inert to the electrochemical environment. In certain embodiments, the binder material also has hydrophobic characteristics. Appropriate binder materials include polymers and copolymers based on polytetrafluoroethylene (e.g., Teflon® and Teflon® T-30 commercially available from E.I. du Pont Nemours and Company Corp., Wilmington, Del.), polyvinyl alcohol (PVA), poly(ethylene oxide) (PEO), polyvinylpyrrolidone (PVP), and the like, and derivatives, combinations and mixtures comprising at least one of the foregoing binder materials. However, one of skill in the art will recognize that other binder materials may be used.  
      The active constituent is generally a suitable catalyst material to facilitate oxygen reaction at the cathode  114 . The catalyst material is generally provided in an effective amount to facilitate oxygen reaction at the cathode  114 . Suitable catalyst materials include, but are not limited to: manganese, lanthanum, strontium, cobalt, platinum, and combinations and oxides comprising at least one of the foregoing catalyst materials. An exemplary air cathode is disclosed in copending, commonly assigned U.S. Pat. No. 6,368,751, entitled “Electrochemical Electrode For Fuel Cell”, to Wayne Yao and Tsepin Tsai, which is incorporated herein by reference in its entirety. Other air cathodes may instead be used, however, depending on the performance capabilities thereof, as will be obvious to those of skill in the art.  
      To electrically isolate the anode  112  from the cathode  114 , the separator  116  is provided between the electrodes. The separator  116  may be disposed in physical and ionic contact with at least a portion of at least one major surface of the anode  112 , or all major surfaces of the anode  112 , to form an anode assembly. In still further embodiments, the separator  116  is disposed in physical and ionic contact with substantially the surface(s) of the cathode  114  that will be proximate the anode  112 .  
      The physical and ionic contact between the separator and the anode may be accomplished by: direct application of the separator  116  on one or more major surfaces of the anode  112 ; enveloping the anode  112  with the separator  116 ; use of a frame or other structure for structural support of the anode  112 , wherein the separator  116  is attached to the anode  112  within the frame or other structure; or the separator  116  may be attached to a frame or other structure, wherein the anode  112  is disposed within the frame or other structure.  
      Separator  116  may be any commercially available separator capable of electrically isolating the anode  112  and the cathode  114 , while allowing sufficient ionic transport between the anode  112  and the cathode  114 . Preferably, the separator  116  is flexible, to accommodate electrochemical expansion and contraction of the cell components, and chemically inert to the cell chemicals. Suitable separators are provided in forms including, but not limited to, woven, non-woven, porous (such as microporous or nanoporous), cellular, polymer sheets, and the like. Materials for the separator include, but are not limited to, polyolefin (e.g., Gelgard® commercially available from Dow Chemical Company), polyvinyl alcohol (PVA), cellulose (e.g., nitrocellulose, cellulose acetate, and the like), polyethylene, polyamide (e.g., nylon), fluorocarbon-type resins (e.g., the Nafion® family of resins which have sulfonic acid group functionality, commercially available from du Pont), cellophane, filter paper, and combinations comprising at least one of the foregoing materials. The separator  116  may also comprise additives and/or coatings such as acrylic compounds and the like to make them more wettable and permeable to the electrolyte.  
      In certain preferred embodiments, the separator  116  comprises a membrane having electrolyte, such as hydroxide conducting electrolytes, incorporated therein. The membrane may have hydroxide conducing properties by virtue of: physical characteristics (e.g., porosity) capable of supporting a hydroxide source, such as a gelatinous alkaline material; molecular structure that supports a hydroxide source, such as an aqueous electrolyte; anion exchange properties, such as anion exchange membranes; or a combination of one or more of these characteristics capable of providing the hydroxide source.  
      The electrolyte (in all variations of the separator  116  herein) generally comprises ion conducting material to allow ionic conduction between the metal anode and the cathode. The electrolyte generally comprises hydroxide-conducting materials such as KOH, NaOH, LiOH, RbOH, CsOH or a combination comprising at least one of the foregoing electrolyte media. In preferred embodiments, the hydroxide-conducting material comprises KOH. Particularly, the electrolyte may comprise aqueous electrolytes having a concentration of about 5% ionic conducting materials to about 55% ionic conducting materials, preferably about 10% ionic conducting materials to about 50% ionic conducting materials, and more preferably about 30% ionic conducting materials to about 40% ionic conducting materials.  
      Preferred embodiments of conductive membranes suitable as a separator  116  are described in greater detail in: U.S. patent application Ser. No. 09/259,068, entitled “Solid Gel Membrane”, by Muguo Chen, Tsepin Tsai, Wayne Yao, Yuen-Ming Chang, Lin-Feng Li, and Tom Karen, filed on Feb. 26, 1999; U.S. Pat. No. 6,358,651 entitled “Solid Gel Membrane Separator in Rechargeable Electrochemical Cells”, by Muguo Chen, Tsepin Tsai and Lin-Feng Li, filed Jan. 11, 2000; U.S. Ser. No. 09/943,053 entitled “Polymer Matrix Material”, by Robert Callahan, Mark Stevens and Muguo Chen, filed on Aug. 30, 2001; and U.S. Ser. No. 09/942,887 entitled “Electrochemical Cell Incorporating Polymer Matrix Material”, by Robert Callahan, Mark Stevens and Muguo Chen, filed on Aug. 30, 2001; all of which are incorporated by reference herein in their entireties. These membranes are generally formed of a polymeric material comprising a polymerization product of one or more monomers selected from the group of water soluble ethylenically unsaturated amides and acids, and optionally a water soluble or water swellable polymer, or a reinforcing agent such as PVA. Such membranes are not only desirable because of the high ionic conductivity due to the liquid electrolyte integral therein, but they also provide structural support and resistance to dendrite growth, thereby providing a suitable separator for recharging of metal air electrochemical cells.  
      The polymerized product may be formed on a support material or substrate. The support material or substrate may be, but not limited to, a woven or nonwoven fabric, such as a polyolefin, polyvinyl alcohol, cellulose, or a polyamide, such as nylon. Further, the polymerized product may be formed directly on the anode or cathode of the cell.  
      The electrolyte may be added prior to polymerization of the above monomer(s), or after polymerization. For example, in one embodiment, electrolyte may be added to a solution containing the monomer(s), an optional polymerization initiator, and an optional reinforcing element prior to polymerization, and it remains embedded in the polymeric material after the polymerization. Alternatively, the polymerization may be effectuated without the electrolyte, wherein the electrolyte is subsequently included.  
      The water soluble ethylenically unsaturated amide and acid monomers may include methylenebisacrylamide, acrylamide, methacrylic acid, acrylic acid, 1-vinyl-2-pyrrolidinone, N-isopropylacrylamide, fumaramide, fumaric acid, N,N-dimethylacrylamide, 3,3-dimethylacrylic acid, and the sodium salt of vinylsulfonic acid, other water soluble ethylenically unsaturated amide and acid monomers, or combinations comprising at least one of the foregoing monomers.  
      The water soluble or water swellable polymer, which acts as a reinforcing element, may include polysulfone (anionic), poly(sodium 4-styrenesulfonate), carboxymethyl cellulose, sodium salt of poly(styrenesulfonic acid-co-maleic acid), corn starch, any other water-soluble or water-swellable polymers, or combinations comprising at least one of the foregoing water soluble or water swellable polymers. The addition of the reinforcing element enhances mechanical strength of the polymer structure.  
      Optionally, a crosslinking agent, such as methylenebisacrylamide, ethylenebisacrylamide, any water-soluble N,N′-alkylidene-bis(ethylenically unsaturated amide), other crosslinkers, or combinations comprising at least one of the foregoing crosslinking agents.  
      A polymerization initiator may also be included, such as ammonium persulfate, alkali metal persulfates and peroxides, other initiators, or combinations comprising at least one of the foregoing initiators. Further, an initiator may be used in combination with radical generating methods such as radiation, including for example, ultraviolet light, X-ray, γ-ray, and the like. However, the chemical initiators need not be added if the radiation alone is sufficiently powerful to begin the polymerization.  
      In one method of forming the polymeric material, the selected fabric may be soaked in the monomer solution (with or without the ionic species), the solution-coated fabric is cooled,. and a polymerization initiator is optionally added. The monomer solution may be polymerized by heating, irradiating with ultraviolet light, gamma-rays, x-rays, electron beam, or a combination thereof, wherein the polymeric material is produced. When the ionic species is included in the polymerized solution, the hydroxide ion (or other ions) remains in solution after the polymerization. Further, when the polymeric material does not include the ionic species, it may be added by, for example, soaking the polymeric material in an ionic solution.  
      Polymerization of the membrane is generally carried out at a temperature ranging from room temperature to about 130° C., but preferably at an elevated temperature ranging from about 75° to about 100° C. Optionally, the polymerization may be carried out using radiation in conjunction with heating. Alternatively, the polymerization may be performed using radiation alone without raising the temperature of the ingredients, depending on the strength of the radiation. Examples of radiation types useful in the polymerization reaction include, but are not limited to, ultraviolet light, gamma-rays, x-rays, electron beam, or a combination thereof.  
      To control the thickness of the membrane, the coated fabric may be placed in suitable molds prior to polymerization. Alternatively, the fabric coated with the monomer solution may be placed between suitable films such as glass and polyethylene teraphthalate (PET) film. The thickness of the film may be varied will be obvious to those of skill in the art based on its effectiveness in a particular application. In certain embodiments, for example for separating oxygen from air, the membrane or separator may have a thickness of about 0.1 mm to about 0.6 mm. Because the actual conducting media remains in aqueous solution within the polymer backbone, the conductivity of the membrane is comparable to that of liquid electrolytes, which at room temperature is significantly high.  
      As generally discussed above, the separator may be adhered to or disposed in ionic contact with one or more surfaces of the anode and/or the cathode. For example, a separator may be pressed upon an anode or a cathode.  
      Referring now to  FIG. 1B , a recharging cell  100   b  for a metal air electrochemical cell is schematically depicted. The cell  100   b  includes an anode  112  and a charging electrode  115  in ionic contact and electrically isolated with a separator  116 . In operation, consumed anode material or a new rechargeable anode structure (i.e., including oxidized metal), which is in ionic contact with the charging electrode  115 , is converted into fresh anode material (i.e., metal) and oxygen upon application of a power source (e.g. more than 2 volts for metal-air systems) across the charging electrode  115  and the anode  112 . The charging electrode  115  may comprise an electrically conducting structure, for example a mesh, porous plate, metal foam, strip, wire, plate, or other suitable structure. In certain embodiments, the charging electrode  115  is porous to allow ionic transfer. The charging electrode  115  may be formed of various electrically conductive materials including, but not limited to, copper, ferrous metals such as stainless steel, nickel, chromium, titanium, and the like, and combinations and alloys comprising at least one of the foregoing materials. Suitable charging electrodes include porous metal such as nickel foam metal.  
      Alternatively, a bifunctional electrode  114  may be used in the cell  100   a , whereby charging is accomplished via application of a voltage across the electrodes  112  and  114 . However, this configuration is generally not preferred, since discharging lifetime and performance is typically decreased substantially when the discharging electrode doubles as the charging electrode.  
      One configuration including both a charging electrode  115  and a discharging air cathode  114  is depicted in  FIG. 1C , wherein metal air cell  100   c  is capable of both discharging and recharging. The cell  100   c  includes an anode  112  and a cathode  114  in ionic contact. Further, a charging electrode  115  is disposed in ionic contact with the anode  112 , and electrically isolated from the cathode  114  with a separator  117  and electrically isolated from the anode  112  with a separator  116 . Separators  116  and  117  may be the same or different. Since the charging electrode  115  is present, the cathode  114  may be a mono-functional electrode, e.g., formulated for discharging while the charging electrode  115  is formulated for charging. In operation, consumed anode material (i.e., oxidized metal), which is in ionic contact with the charging electrode  115 , is converted into fresh anode material (i.e., metal) and oxygen upon application of a power source (e.g. more than 2 volts for metal-air systems) across the charging electrode  115  and consumed anode material.  
      Generalized Embodiment of Integrated Refuelable and Rechargeable Metal Air Electrochemical Cell System  
      Referring now to  FIG. 2A , a schematic of a refuelable and rechargeable metal air electrochemical cell system  200  is depicted, as well as an associated series of removable and rechargeable anode structures  212  supported by a support structure  240 . In the system  200 , the plural anode structures  212  may be discharged, and then charged in the same unit (or an identical unit). The system  200  generally includes plural receiving structures each configured and dimensioned to receive the removable and rechargeable anode structures  212 , and capable of discharging and charging the anode structures.  
      Exemplary System and Structure for an Integated Refuelable and Rechargeable Metal Air Electrochemical Cell System  
      Referring now to  FIG. 2B , an exploded schematic view of an individual refuelable and rechargeable metal air electrochemical cell  210  is provided. The cell  210  is generally a monopolar cell, wherein an anode  212  is provided generally between a pair of active cathode portions  214 A and  214 B. Further, third charging electrodes  215 A and  215 B are disposed between the cathodes  214 A and  214 B, and the anode  212 , respectively. A pair of separators  216 A and  216 B are disposed in ionic communication with two major surfaces of the anode  212 . In a preferred embodiment, the separators  216 A and  216 B comprises a membrane having electrolyte incorporated therein, as described above. Such a membrane not only insulates the anode  212  from the third electrodes  215 A and  215 B, and further minimizes or prevents dendrite growth from the anode  212  toward the third electrodes  215 A and  215 B. Such dendrite formation is undesirable as it may lead to electrical shorting. The cell  210  further includes a pair of spacers  220 A and  220 B which generally serve to provide a constant distance between the third electrodes  215 A and  215 B and cathodes  214 A and  214 B, respectively.  
      Referring now to  FIG. 2C , an anode assembly  211  is depicted. The anode assembly  211  includes a section of anode material  212  generally provided within or upon a support frame  222 . In certain embodiments, a pair of separators  216 A and  216 B are provided on opposing major surfaces of the anode material  212 . Furthermore, a cap portion  224  is provided, which provides additional structural support for the anode assembly  211  and further provides a passageway  226  generally for air intake, escaping gases, and/or electrical connection. As depicted, the exemplary frame  222  includes three openings  227  to allow passage of air and escaping gases, and two openings  228  to allow for passage of electrically conducting elements for connection to the anode. A pair of spacers  220 A and  220 B are configured on opposing sides of the anode material  212 , generally for maintaining physical isolation between the anode material  212  and the cathode  214 . The depicted spacers  220 A and  220 B include a plurality of extending portions, for example, rods, which extends through the top (as viewed in  FIG. 2C ) of the spacers  220 A and  220 B. These extending portions generally mate with corresponding openings in the top portion  224  and may be secured with a fastener, for example, a nut. In a further embodiment, a plurality of openings or provided on the bottom of the spacers  220 A and  220 B to hold the spacers together. Such embodiment is particularly useful, for example, when separators  216 A and  216 B are provided, particularly when separators  216 A and  216 B comprise membranes incorporating electrolyte therein.  
      As described, the anode assembly  211  may include the anode material and separators (preferably electrolyte-containing membranes). Alternatively, the third electrodes may be included in each anode assembly  211  (rather than in the corresponding cell body  230  described further herein). For example, a charging electrode may be wrapped around the a separator disposed over the anode material  212 , wherein the anode and the charging electrode may be inserted and removed together as an integral anode assembly  211 . In this configuration, the charging electrodes  215  serve not only as charging electrodes, but also as structural support, which facilitates extended lifetime even with repeated removal and insertion of the anode assembly  211 .  
      Referring now to  FIG. 2D , an assembled refuelable and rechargeable electrochemical cell  210  is depicted, including the anode assembly  211  inserted within a cell body  230 . In certain embodiments, where an electrolyte bath is used as the ionic conducting medium, the cell body  230  is configured to contain a quantity of electrolyte. A third electrode may be incorporated within the body  230 , generally as shown in  FIG. 2B , or may be incorporated In the anode assembly  211  as described above.  
      A pair of cathodes  214 A and  214 B are disposed on opposing sides of the cell body  230 . Preferably, the cell body  230  is configured to provide an electrolyte reservoir on each side of the cell body  230  to contain sufficient electrolyte for recharging. To seal the electrolyte reservoir, the cell body  230  may include suitable sealing portions. Alternatively, one or more heat sinks may be provided on the cell body  230 , for example, to remove heat that may be generated within the cell  210 . Further, electrolyte may be circulated during discharging to remove heat.  
      Where the anode assembly  211  includes the third electrode, or a pair of third electrodes, the entire assembly can be charged electrically in a separate electrolyte tank after being removed from the cell body  230 . Therefore, cell  210  may be refueled with another anode assembly  211  while the discharged anode assembly  211  is recharging. This system facilitates regeneration of the anode assembly  211  with minimum hardware any recharging assembly.  
      Generalized Embodiment of Refuelable and Rechargeable Metal Air Electrochemical Cell System Employing Discrete Discharging and Charging Modules  
       FIG. 3  is a generalized schematic of a metal air electrochemical cell system  300 , including a cell discharging system  302  and a cell charging system  352 . Both systems  302  and  352  include one or more receiving structures configured and dimensioned to receive one or more anode structures  312 . As depicted, when the capacity of a first group of anode structures from the cell discharging system  302  is diminished, that group may be moved to a nearby cell charging system  352 , or alternatively transported to an off-site cell charging system  352 , and a fresh second group of anode structures may be inserted in the cell discharging system  302 . In this manner, power may be generated from the cell discharging system  302  with interruption limited to the time required to remove consumed metal fuel and insert fresh metal fuel, which is in contrast to systems wherein a user must wait for electrical recharging of the metal fuel.  
      This is also in contrast to conventionally known systems, wherein a removed anode could not be electrically recharged while remaining intact—known systems strip the anode and regenerate the metal fuel in a loose form, then use that material to fabricate new anodes. Therefore, convenience is afforded directly to users, allowing them to replace and electrically recharge rather than requiring substantially processing to electrically recharge.  
      Exemplary Systems and Structures for a Refuelable and Rechargeable Metal Air Electrochemical Cell System Employing Discrete Discharging and Charging Modules  
      First Embodiment of Discharging and Charging Modules  
      The discharging and charging modules used in the refuelable and rechargeable metal air electrochemical cell systems described herein may be of various structural types. In certain embodiments described herein, the discharging and charging modules are formed essentially as a plurality of individual cell structures aligned and joined together to form an integral discharging module and an integral charging modules.  
      For example, referring now to  FIGS. 4A and 4B , one embodiment of a metal air electrochemical cell discharging module  302  is depicted.  FIG. 4A  generally shows the. module  302  having the metal fuel removed there from, and  FIG. 4B  shows the module  302  having the metal fuel inserted therein.  
      The metal air electrochemical cell discharging module  302  includes a plurality of electrochemical discharging cells  310  arranged generally in a prismatic configuration. Each electrochemical discharging cell  310  includes: an air cathode structure  314  having active air cathodes (not shown) therein and a cathode electrical terminal  318 ; and a removable anode structure  320  including metal fuel anode portions (not shown) and an L-shaped bus  324  extending from a current collector (not shown), wherein the L-shaped bus fits into an anode electrical terminal  328 , which is shown mounted on a side of the cathode structure  314 . The plurality of electrochemical discharging cells  310  are assembled together and mounted on a fluid management unit  340 , which generally allows for airflow and electrolyte capture, as described in further detail herein.  
      The anode structures  320  may be removed, for example, to interrupt discharging of the electrochemical cell, for insertion into corresponding charging cells  355  in a charging system  352  (shown in  FIG. 4C ), or to replace the anode structures with fresh anode structures, charged anode structures, or reconditioned anode structures (collectively referred to herein as “refueling”).  
      Referring now to  FIG. 4C , a charging unit  352  is shown. The charging unit  352  includes a plurality of charging cells  355  (e.g., functioning as generally described above with respect to  FIG. 1B ) configured and dimensioned to hold removable and rechargeable anode structures  320 . External current is supplied to the charging electrodes through a bus  358 , and to the anodes through a bus  360 , wherein each anode terminal  324  mates in an opening  362  configured to allow electrical connection between bus  360  and anode terminal  324 .  
      Charging electrodes may be operably positioned within each cell  355  configured and positioned for ionic communication with anode assemblies  320  when inserted. Preferably, a pair of charging electrodes are provided for each anode assembly  320 , to allow charging from both major surfaces of the anode.  
      Alternatively, where charging electrodes are incorporated in the removable and rechargeable anode assemblies  320 , each charging cell  355  includes suitable electrical connection structures to allow current to be supplied to the charging electrodes when the anode assemblies  320  including such charging electrodes are inserted in the charging cells  355 .  
      In certain embodiments, charging operations are carried out in the presence of liquid electrolyte, thus the charging cells are configured and dimensioned to hold electrolyte.  
      Referring now to  FIG. 4D , the electrochemical cell discharging module  302  is shown without the fluid management unit  340 . For mechanical integrity, and to minimize or eliminate the occurrences of electrolyte leakage, a plurality of cells  310  (without the anode structures  320  therein) are assembled and cast into an integral module. The casting may be by pour casting, spin casting, or other suitable manufacturing technique. The casting provides a coating substantially around the entire structure, with the exception of apertures for electrolyte management and air management, embodiments of which are described further herein.  
      In preferred embodiments, the casting shell is allowed to polymerized in situ (as opposed to allowing a molten material to set). Monomers may be selected for in situ polymerization, thereby allowing polymerization and possibly cross-linking within, for example, the pores of the cathode to form a tight seal, thereby illuminating electrolyte leakage from the edges of the naturally porous cathode, and providing structural binding and support for all of the cell components. A preferred type of material includes polyurethane, such as TEK plastic polyurethane (TAN) commercially available from Tekcast Industries, Inc. New Rochelle N.Y. (manufactured by Alumilite Corporation, Kalamazoo Mich.). One of skill in the art will recognize that suitable plates or other molding structures are included with the cell structures to provide air passages between the cells, and centrally in the cell structures to form a pocket for electrolyte and the anode assembly.  
      First Embodiment of Individual Cathode and Anode Structures  
      Referring now to  FIGS. 5A, 5B  and  5 C, an exploded cathode structure, an exploded anode structure, and an assembled cell are depicted, respectively. Further,  FIG. 5D  depicts air and electrolyte management in a sectional cell view.  
      In general, the discharging cell  310  includes a cathode structure  314  and a removable anode structure  320 . The cathode structure  314  includes a support frame  370  including a top portion  382  configured and dimensioned generally to receive the anode structure  320 , preferably providing a gap at one or more of the edges or faces of the anode structure  320  for electrolyte (in systems wherein liquid electrolyte is used) and/or to accommodate for cell expansion during discharging operations.  
      As depicted, a pair of air cathode portions  373 ,  375  are disposed on opposing sides of the cathode structure support frame  370 . The cathode portions  373 ,  375  may be integrally formed into the frame, e.g., by molding, or adhered or otherwise secured to the frame  370 . A pair of separators  316   a  may also be included, generally to prevent electrical contact between the active cathode portions  373 ,  375  and the anode structure  320  when inserted. Further provided on the cathode support frame  370  is the cathode electrical terminal  318 , which is electrically connected to the cathode current collectors (not shown).  
      Adjacent the air cathode portion  375  is an air management structure  376 . In general, the air management structure  376  allows for controlled airflow across the air cathode portion  375 , as indicated by the arrows  377  in  FIG. 5D . Accordingly, the air management structure  376  should be tightly disposed or secured over the active cathode portion  375  to the frame  370 . An air management structure from an adjacent cell (not shown) is provided adjacent the air cathode portion  373  on the opposite side of the frame  370  when plural cells are assembled into a cell discharging system  302 . Thus, the air management structure  376  facilities airflow both for the air cathode portion  375  in the support frame  370 , as well as for the air cathode portion (equivalent to the air cathode portion  373  in the single cell depicted) in an adjacent cell.  
      Optionally, electrolyte management may be integrally included within the air management structure  376 . As depicted in  FIGS. 5A and 5D , the bottom portion of the air management structure  376  is sloped from right to left (as viewed in the  FIGS. 5A and 5D ). Accordingly, in the event that liquid electrolyte seeps through a cathode portion adjacent the air management structure  376 , the electrolyte will fall to the bottom sloped portion due to gravity, and further will exit the cell via the same outlet used for air exhaust.  
      Further, electrolyte management is also provided within the frame  370  itself. As shown in  FIG. 5D , an opening  384  is provided proximate the top of the inside compartment of the frame  370  providing access to an overflow or circulation tube  388 . The inside compartment, formed to contain liquid electrolyte, may be prefilled with electrolyte, or, as depicted, a system may be included to selectively provide electrolyte, e.g., via an inlet  368 . If the electrolyte level attains the height of the opening  384 , electrolyte will flow out of the cell via the channel and outlet  388 . The channel and outlet  388  may be integrally formed as part of the frame  370 , or alternatively may include one or more portions of tubing or other plumbing, as depicted in  FIG. 5A . The channel and outlet  388  may further serve to allow an escape (separate from the air exhaust) for evolved gasses, such as, for example, hydrogen that may evolve during certain types of metal air electrochemical reactions.  
       FIG. 5B  shows an exploded view of an exemplary anode structure  320 . The anode structure  320  generally includes a frame  390 , a pair of metal fuel support structures or grids  392 , and a top seal portion  394 . Metal fuel (schematically depicted as sheets  396 , although it is understood that the fuel may be in the form of powder, paste, fibers, or other “loose” form supported in the grids  392 ) is generally provided between the grids  392  and the frame  390 , typically with a pair of metal current collectors disposed on each side of the frame  390  (not shown). A pair of separators  316 b (or a single separator wrapped around the anode structure) is also provided on the anode structure  320 . The separator, which may be an electrolyte containing membrane as described above, may include a source of electrolyte, as well as minimizing or preventing dendrite penetration.  
      The frame  390  may optionally be an electrically conductive frame, to enhance current collection. The frame  390  is configured generally as an open rectangle having a first face and a second face, with the electrical terminal  324  extending from a portion of the open rectangle. The top seal  394 , as shown, is a wedge-shaped structure. This is particularly useful, for example, when the top seal  394  is formed of an elastic material, thus providing an air-tight seal when inserted into the cathode structure  314 .  
      Preferably, the anode structure  320  fits within the cathode structure  314  such that a space remains therebetween, which allows for ion conducting media, i.e., electrolyte, between the anode material and the cathodes, and also accommodates anode volume expansion during discharge due to the conversion from metal to metal oxide. The support grid  392  is also capable of mechanically supporting the anode material and accommodating expansion.  
      One method of assembling the anode includes: adhering foil on both sides of frame  390 ; spreading a desired quantity of metal fuel material on the foil (wherein the quantity is selected to provide the desired cell capacity while maintaining sufficient distance from the air cathode when the cell is assembled); pressing the grid over the metal fuel material; and adhering a separator to the grid. In preferred embodiments, the separator is adhered to the interconnecting portions of the grid for enhanced structural integrity, and also to provide a tight pressure fit preventing delamination of the separator if the metal fuel material expands during electrochemical reaction. In another method of assembling the anode, a solid plastic member is placed in the open portion of the frame prior to attaching the current collector foil. This generally assists in keeping liquid out of the region between the current collector, particularly if the level of the electrolyte opening  384  is higher than the level of the grids. In still another method of assembling the anode, a compressible member is placed in the open portion of the conductive frame prior to attaching the current collector foil. This provides volume accommodation if the anode material expands during electrochemical reaction.  
      To facilitate assembly of the anode structure  320 , a series of protruding portions may extend outwardly from the conductive frame  390 , which correspond to receiving portions on the metal fuel support structures  392 . These allow for rapid and accurate assembly, as well as enhance the overall structural integrity of the anode structure  320 , which may be particularly relevant if anode expansion occurs.  
      First Embodiment of Fluid (Air and Electrolyte) Management Structure  
      Referring now to  FIGS. 6A-6D , the fluid management unit  340  will be further described. In general, the fluid management unit  340  provides a structure for facilitating airflow through the air management structure  376  of the cathode structure  314 . Further, the fluid management unit  340  may optionally provide for management of excess electrolyte from the air management structure  376  (e.g., that which may gravitate down the bottom sloped portion exiting the cell via the air exhaust outlet) and/or via channel  386  and outlet  388 .  
      More particularly, the fluid management unit  340  generally includes air exhaust apertures  402  and electrolyte leakage openings  404 . Excess electrolyte, e.g., as described above as originating from the air management structure  376  and/or via channel and outlet  388 , or electrolyte circulated through the cell, may flow out of the cells into a channel  406  to the openings  404 .  
      Furthermore, air enters the cells (e.g., via the inlet of the air management structure  376 ) generally through a region  410 , which may house a fan or a blower, for example. Optionally, a scrubber system may be employed within the cell to remove carbon dioxide from the ambient air. Air flowing through the region  410  enters the cells via an opening  412 , and dispersed across the plural cells through a channel  414 . Exhaust air exits the system via channel  406  and openings  402 . Thus, the air management structure  376  is capable of delivering both exhaust air and overflow/leaked electrolyte to the same channel  406 .  
      In addition to providing fluid management, the fluid management unit  340  may also be configured to provide increased mechanical integrity to the overall cell structure. For example, as shown in  FIGS. 6A and 6B , a series of rails  416  may be provided, as well as ribs  418 . Further, the air management design allows for both the air inlet and outlet to be on the bottom of the cell, thus more material support can be applied around top of cell, where it is generally important to have a good seal.  
       FIG. 6D  shows a module  302  including a fluid management structure  340 , including tubing  342  directed to each of the cells  310 . For example, the individual cells may be provided without electrolyte, and upon demand, for example with activation of a pump or other fluid transfer device (not shown), electrolyte may be introduced into the cells from the reservoir. Alternatively, electrolyte may be continuously or intermittently circulated during cell discharge, for example, to remove heat. Also, during charging operations, a similar structure may be included to remove solids and prevent or minimize dendrite growth. Optionally, a clamping structure or valve may be included to increase control of the electrolyte flow. The length of the tubes carrying the electrolyte to each cell  310  contribute to an increased resistance, thereby eliminating or reducing the possibility of shorting conventionally encountered with a shared electrolyte source in metal air electrochemical cells.  
      Embodiment of Gripping Structure for Removing and Inserting Anode Structures  
      Referring now to  FIGS. 7A and 7B , a gripping structure  430  is depicted, generally for facilitating removal of anode structures  320 . The gripping structure  430  generally includes a support handle  432  secured or integrally formed with a support frame  438 . The edges of the support frame  438  are generally configured and dimensioned to fit over the top of a system module  302 . For example, a portion  440  of the support frame  438  is configured to fit over anode terminals  324 . Further, the gripping structure  430  includes a movable handle  434  secured to or integrally formed with a movable frame  436 , which moves up (generally bringing the movable handle  434  closer to the support handle  432 . The movable frame  436  includes a pair of sliding hook assemblies  442 , which slide as indicated by arrows  444 , of course with the range of motion restricted by the corresponding slot in the movable frame  406 . The sliding hook assemblies each include a plurality of hooks  446 , which correspond to eyes  448  (see, e.g.,  FIG. 5C ) on the anode structures  320 . While plural hooks  446  are depicted, it is understood that a single hook may also be employed, for removing anode structures one at a time. Accordingly, to facilitate removal of a plurality of anode structures  320 , the hooks  446  are aligned with the eyes  448  of the anode structure. The sliding hook assemblies  442  are then slid into position such that the hooks  446  are within the eyes  448 . The movable handle  434  is then pulled, generally by gripping the support handle  432  and the movable handle  434 , such that the connected anode structures  320  are pulled from the assembly. Of course, one skilled in the art will recognize that variations are possible, including integration with a gripping structure similar to the structure  430  into an automated anode refueling system.  
      Second Embodiment of Discharging and Charging Modules  
      Referring now to  FIGS. 8A-8C , another embodiment of a metal air electrochemical cell discharging module and charging module is shown. The metal air electrochemical cell discharging module  502  is depicted with fuel therein in  FIG. 8A , the system including removed fuel structures, a discharging module and a charging module is shown in  FIG. 8B , and a discharging module shown without the connection/sealant housing is shown in  FIG. 8C .  
      The metal air electrochemical cell discharging module  502  includes a plurality of electrochemical discharging cells  510  arranged generally in a prismatic configuration. Each electrochemical discharging cell  510  includes: air cathode structures  514  having active air cathodes (not shown) therein; and removable anode structures  520  including metal fuel anode portions (not shown).  
      An assembly  530  ( FIG. 8C ) of cathode structures  520  is generally positioned in a housing  532  having a cover  534 . The assembly  530  may be formed, e.g., by casting, generally as described above. Similarly, an assembly of charging structures or support structures (e.g., wherein charging electrodes are integral with the anode structure) are provided in a housing  562  having a cover  564  to form a charging module  552 . The module  502  is mounted on a fluid management unit  540  (and the module  552  may be mounted on a similar fluid management structure), which generally allows for airflow and electrolyte capture, as described in further detail herein.  
      An important feature of the modules  502  and  552  is the integrated sealing covers  534 ,  564 , which also provide electrical contact with the cathode or charging electrodes. Generally, the anode structures  520  include a conductor  524  extending from the top of the structures. Cathode electrical terminals  518 , mounted on the inside portion of the cover  534 , access the conductors  524  when the cover  534  is closed. The terminals  518  are connected to the cathodes via flexible conductors (not shown) to accommodate opening and closing of the cover  534 , for example, supported through apertures  536  in the assembly  530 . Accordingly, discharging (or charging) is accomplished by closing the cover  534  (or  564 ), which action both seals the system to prevent electrolyte spillage and causes electrical contact between opposite electrodes.  
      The anode structures  520  may be removed, for example, to interrupt discharging of the electrochemical cell, for insertion into corresponding charging cells  555  in a charging system  552 , or to replace the anode structures with fresh anode structures, charged anode structures, or reconditioned anode structures (collectively referred to herein as “refueling”).  
      The charging unit  552  includes a plurality of charging cells  555  (e.g., functioning as generally described above with respect to  FIG. 1B ) configured and dimensioned to hold removable and rechargeable anode structures  520 . External current is supplied to the charging electrodes through a terminal  558 , and to the anodes through a terminal  560 , wherein each anode terminal  524  mates with a corresponding charging electrode conductor in the cover  564 . Note that the terminals  558  and  560  may be reversed, depending on the operable conductor connections.  
      Second Embodiment of Individual Cathode and Anode Structures  
      Referring now to  FIGS. 9A, 9B  and  9 C, an exploded air cathode structure, an assembled air cathode structure, and an anode structure are depicted, respectively. A cathode structure  514  includes a support frame  570  including a top opening  582  configured and dimensioned generally to receive the anode structure  520 , preferably providing a gap at one or more of the edges or faces of the anode structure  520  for electrolyte (in systems wherein liquid electrolyte is used) and/or to accommodate for cell expansion during discharging operations.  
      As depicted, an air cathode  575  is wrapped around on opposing sides of the cathode structure support frame  570 . The cathode  575  may be integrally formed into the frame, e.g., by molding, or adhered or otherwise secured to the frame  570 , or may be overlaid and subsequently cast into place when the assembly  530  is formed. A pair of separators  516   a  may also be included between each side of the frame  570  and the cathode  575 , generally to prevent electrical contact between the anode structure  520  and the active cathode portions  575  when inserted. Further provided on the cathode support frame  570  is a cathode current collector  517 , which is electrically connected to the terminal  518  (not shown).  
      Adjacent the air cathode portion  575  is an air management structure  576 . In general, the air management structure  576  allows for controlled airflow across the air cathode portion  575 , as indicated by the arrows  577  in  FIG. 11A . Accordingly, the air management structure  576  should be tightly disposed or secured over the active cathode portion  575  to the frame  570 . An air management structure from an adjacent cell (not shown) is provided adjacent the air cathode portion  575  on the opposite side of the frame  570  when plural cells are assembled into a cell discharging system  502 . Thus, the air management structure  576  facilities airflow both for the air cathode portion  575  in the support frame  570 , as well as for the air cathode portion in an adjacent cell.  
       FIG. 9C  shows an exploded view of an exemplary anode structure  520 . The anode structure  520  generally includes a frame having metal fuel therein, and a pair of separators (or a single separator wrapped around the anode structure) on the major surfaces of the anode structure  520  (not shown). The separator, which may be an electrolyte containing membrane as described above, may include a source of electrolyte, as well as minimizing or preventing dendrite penetration.  
      The anode structure  520  also includes the extending terminal  524 , substantially across the top of the anode structure, for mating with the cathode terminals in the housing cover.  
      Preferably, the anode structure  520  fits within the cathode structure  514  such that a space remains therebetween, which allows for ion conducting media, i.e., electrolyte, between the anode material and the cathodes, and also accommodates anode volume expansion during discharge due to the conversion from metal to metal oxide.  
      Second Embodiment of Fluid (Air and Electrolyte) Management Structure  
      Referring now to  FIGS. 10A-10C , the fluid management unit  540  will be further described. In general, the fluid management unit  540  provides a structure for facilitating airflow through the air management structure  576  of the cathode structure  514 . Further, the fluid management unit optionally provides for management of excess electrolyte from the air management structure  576  and/or management of electrolyte circulation.  
      Referring to  FIG. 11A , air management is shown, wherein air is introduced (e.g., via a fan or blower, optionally wherein CO 2  is removed via a scrubber) and follows a flow path represented by arrows  577 . Further, a control valve  579  is provided, generally to increase or decrease electrolyte flow.  
      Further, referring to  FIG. 11B , electrolyte management is also provided within the frame  570  itself, similar to as described above with respect to frame  370 . Electrolyte inlets  568  are provided at the bottom of each cell, and electrolyte flows across a tube  569  into the main area of the cell containing the anode. The length of the tube  569  increases electrical resistance throughout the fluid, thereby eliminating shorting typically encountered when a common electrolyte reservoir is used. An opening  586  is provided proximate the top of the inside compartment of the frame providing access to an overflow or circulation tube  588 . Further, a control valve  589  is provided, generally to increase or decrease electrolyte flow.  
      Various materials may be used for the cell frame components, spacers, and other support structures described herein, which are preferably inert to the system chemicals. Such materials include, but not limited to, thermoset, thermoplastic, and rubber materials such as polycarbonate, polypropylene, polyetherimide, polysulfonate, polyethersulfonate, polyarylether ketone, Viton® (commercially available from EI DuPont de Nemours &amp; Co., Wilmington Del.), ethylenepropylenediene monomer, ethylenepropylene rubber, and mixtures comprising at least one of the foregoing materials.  
      While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.