Patent Publication Number: US-2011070506-A1

Title: Rechargeable electrochemical cell system with a charging electrode charge/discharge mode switching in the cells

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of priority from U.S. Provisional Patent Application No. 61/243,970, filed on Sep. 18, 2009, the content of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a rechargeable electrochemical cell system comprising a plurality of cells that each includes a charging electrode in addition to the fuel and oxidant electrodes. 
     BACKGROUND OF THE INVENTION 
     Electrochemical cell systems with a plurality of individual electrochemical cells connected in series are well known. Each individual cell includes an anode or fuel electrode at which a fuel oxidation reaction takes place, a cathode or oxidant electrode at which an oxidant reduction reaction takes place, and an ionically conductive medium for supporting the transport of ions. The fuel electrode of the first cell is coupled to a first terminal, the oxidant electrode of each cell within the cell system is connected to the fuel electrode of the subsequent cell, and the oxidant electrode of the last cell in the series is connected to a second terminal. Thus, a potential difference is created within each individual cell, and because these cells are coupled in series, a cumulative potential difference is generated between the first and second terminals. These terminals connect to a load, creating a potential difference that drives current. 
     There is a need in the art for a more efficient and effective architecture to enable recharging of such cell systems. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides a rechargeable electrochemical cell system for generating electrical current using a fuel and an oxidant. The cell system comprises N electrochemical cells each comprising a fuel electrode, an oxidant electrode, a charging electrode, and an ionically conductive medium communicating the electrodes, wherein N is an integer greater than or equal to two. Any number of cells may be used. 
     A plurality of switches are switchable between: 
     (1) a discharge mode coupling the oxidant electrode of each cell  1  to N−1 to the fuel electrode of the subsequent cell to couple the cells in a discharging series, such that when the fuel electrode of cell  1  and the oxidant electrode of cell N are coupled to a load, oxidation of fuel at the fuel electrodes and reduction of an oxidant at the oxidant electrodes creates a potential difference within each cell to thus create a cumulative potential difference anodic at the fuel electrode of cell  1  and cathodic at the oxidant electrode of cell N for delivering a current to the load, and 
     (2) a charge mode coupling the charging electrode of each cell  1  to N−1 to the fuel electrode of the subsequent cell to couple the cells in a charging series, such that when the fuel electrode of cell  1  and the charging electrode of cell N are coupled to a power source to receive a charging potential difference cathodic at the fuel electrode of cell  1  and anodic at the charging electrode of cell N, an incremental potential difference is created within each cell to reduce a reducible fuel species at the fuel electrode and oxidize an oxidizable oxidant species at the charging electrode. 
     The plurality of switches are switchable to a bypass mode for a cell (X) of the N electrochemical cells by coupling the charging electrode, in the charge mode, or the oxidant electrode, in the discharge mode, of a previous cell (X−1) to the fuel electrode of a subsequent cell (X+1). That is, whether the charging or oxidant electrode of the previous cell is the one coupled is dependent on whether the cell system is in charge or discharge mode, respectively. 
     In an embodiment, the cells are assembled adjacent one another with a non-conductive barrier separating the oxidant electrode and fuel electrode of each pair of adjacent cells such that the only permitted electrical connection therebetween is via the associated switch. 
     In another embodiment, each cell may be a metal-air cell with the fuel electrode comprising a metal fuel, the oxidant electrode comprising an air cathode for reducing oxygen, and the charging electrode being an oxygen evolving electrode for oxidizing an oxidizable oxygen species to oxygen. 
     The system may comprise a first terminal coupled to the fuel electrode of cell  1  and a second terminal, wherein the plurality of switches includes a switch switchable between coupling the oxidant electrode of cell N to the second terminal in the discharge mode, and coupling the charging electrode of cell N to the second terminal in the charge mode. 
     The plurality of switches may optionally be switchable to a bypass mode for each of the cells  1  to N, wherein: 
     in the bypass mode for cell  1 , the first terminal is coupled to the fuel electrode of cell  2 ; 
     in the bypass mode for any cell X of cells  2  to N−1, the charging electrode, in the charge mode, or the oxidant electrode, in the discharge mode, of a previous cell (X−1) is coupled to the fuel electrode of the subsequent cell (X+1); and 
     in the bypass mode for cell N, the charging electrode, in the charge mode, or the oxidant electrode, in the discharge mode, of cell N−1 is coupled to the second terminal. 
     The plurality of switches may include a triple throw single pole switch for each cell, wherein: 
     a static contact for the triple throw single pole switch for each of cells  1  to N−1 is connected to the fuel electrode of the subsequent cell (X+1) and a static contact for the triple throw single pole switch for cell N is connected to the second terminal, 
     a first selective contact for the triple throw single pole switch for each of cells  2  to N is connected to at least the static contact of the previous cell (X−1) and a first selective contact for the triple throw single pole switch for cell  1  is connected to at least the first terminal; 
     a second selective contact for the triple throw single pole switch for each of cells  1  to N is connected to the charging electrode of the associated cell (X); 
     a third selective contact for the triple throw single pole switch for each of cells  1  to N is connected to the oxidant electrode of the associated cell (X); and 
     a switch element for each triple pole single pole switch is switchable between (1) a bypass position coupling its static contact to its first selective contact, (2) a charging position coupling its static contact to its second selective contact, and (3) a discharging position coupling its static contact to its third selective contact, whereby the charge mode of the plurality of switches is established by the switch elements being in the charging positions thereof, the discharge mode is established by the switch elements being in the discharging positions thereof, and each cell may be bypassed by moving the switch element associated therewith to the bypass position in either the charge mode or the discharge mode of the plurality of switches. 
     As another alternative, the plurality of switches may include a pair of switches associated with each cell. The pair of switches associated with each cell may be a pair of double throw single pole switches. 
     Other objects, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a cell system constructed in accordance with the present invention; 
         FIG. 2  is a schematic view of an alternative embodiment of a cell system constructed in accordance with the present invention; 
         FIG. 3  is an exploded cross-sectional view showing two cells in a stack; 
         FIGS. 4   a - 4   d  are schematic views of an alternative embodiment of a cell system constructed in accordance with the present invention, with switches thereof in different operational states; and 
         FIGS. 5   a - 5   d  are schematic views of another alternative embodiment of a cell system constructed in accordance with the present invention, with switches thereof in different operational states. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS OF THE INVENTION 
     The Figures illustrate various embodiments of a rechargeable electrochemical cell system  10  for generating electrical current using a fuel and an oxidant. The cell system  10  may have any arrangement and architecture, and the illustrated embodiments are not intended to be limiting. 
     The cell system  10  comprises N electrochemical cells  12 . The number N is any integer greater than or equal to two, and is not limited to any particular number. Each cell  12  comprises a fuel electrode  14 , an oxidant electrode  16 , a charging electrode  18 , and an ionically conductive medium communicating the electrodes  14 ,  16 ,  18 . Each cell  12  is preferably encased to prevent leakage of the ionically conductive medium, which may be an electrolyte or any suitable medium for enabling ion transport during charging/discharging. For example, a conventional liquid or semi-solid electrolyte solution may be used, or a room temperature ionic liquid may be used, as mentioned in U.S. Patent Appln. No. 61/177,072, the entirety of which is incorporated herein.  FIG. 1  is a schematic diagram showing the basic arrangement of electrodes and cells for facilitating an understanding of the cell operation. 
     The fuel electrode  14  (also referred to as an anode during discharge) may have any construction or configuration. Preferably, it is formed of an electroconductive material, such as an electroconductive mesh screen. The fuel electrode  14  may also be formed of multiple electrode bodies, such as is taught in U.S. patent application Ser. No. 12/385,489, the entirety of which is incorporated by reference. The embodiment in  FIG. 3  shows the multiple electrode bodies  14   a - 14   c , separated by non-conductive, electrochemically inert isolators  15   a - 15   c , which are permeable by the electrolyte.  FIG. 3  shows 3 electrode bodies that form the fuel electrode  14  separated by 3 electrochemically inert isolators. However, the arrangement and architecture are not intended to be limiting. For example, the number of electrode bodies, size and spacing can be arbitrarily varied from μm to several mm. The fuel electrode  14  is the electrode at which oxidation occurs during discharge, and on which fuel is electrodeposited during recharging. Preferably, the fuel is a metal or other fuel selected such that, when deposited on the fuel electrode, it can be oxidized to liberate electrons and provide oxidized fuel ions in the electrolyte that can later be reduced and electrodeposited onto the fuel electrode  14  during recharging. 
     Preferably, the fuel is a metal, such as zinc, manganese, iron or aluminum. The fuel may also be a non-metal that is capable of being reduced and electrodeposited from its oxidized form in the electrolyte onto the fuel electrode  14 , and oxidized from its deposited form on the fuel electrode  14 . Thus, a fuel that is reversible between oxidation and reduction is preferred. 
     The oxidant electrode  16  (also referred to as the cathode during discharge) may also have any construction or configuration, and may have any type of oxidant supplied to it for the reduction reaction during discharging. In the illustrated embodiment, the oxidant electrode is an air breathing cathode (also referred to as an air cathode). With an air cathode as the oxidant electrode  16 , during discharging the oxidant electrode  16  absorbs oxygen from the ambient air, and reduces the oxygen, thus forming a reduced oxygen species, which may ultimately react with the oxidized fuel in the cell  12  in the electrolyte or at an electrode. The oxygen or other oxidant need not necessarily be derived from ambient air, and may be delivered from a contained source as well. 
     Thus, during a discharging operation, within each individual cell  12  fuel is oxidized at the fuel electrode  14  and an oxidant is reduced at the oxidant electrode  14 , thus creating a potential difference between the fuel and oxidant electrodes  12 ,  14  with the fuel electrode having an anodic potential and the oxidant electrode  16  having a cathodic potential (relative to one another). The reduced oxidant species and oxidized fuel may react within the cell  12  to form a by-product, from which the oxidized fuel may be later reduced and electrodeposited on the fuel electrode  14  as will be discussed below. 
     The oxidant electrode  16  may be made from a variety of materials, including but not limited to carbon, flouropolymers, nickel, silver, manganese oxide, pore formers and any combination thereof. The present disclosure is not intended to be limiting in that regard. 
     The charging electrode  18  is positioned between the fuel electrode  14  and the oxidant electrode  16 . However, it may be in another location, such as being on the side of the fuel electrode  14  opposite the oxidant electrode  16 . The charging electrode  18  is used only during charging of the cell, and functions as an anode (during charge) in that capacity. Specifically, during charge, an anodic potential is applied to the charging electrode  18  and a cathodic potential is applied to the fuel electrode  14 . As such, the fuel electrode  14  behaves as a cathode during charge, and serves as a reduction site for a reducible fuel species, such as the oxidized fuel species created in the cell during discharging. Similarly, the charging electrode  18  will oxidize an oxidizable oxygen species, such as the reduced oxidant species created in the cell during discharging. Thus, when the cell  12  is a metal-air cell, the reducible metal fuel species is being reduced and electrodeposited on the fuel electrode  14 , and the oxidizable oxygen species is being oxidized to oxygen gas, which may be off-gassed from the cell  12 . In this embodiment, the charging electrode  18  may be an oxygen evolving electrode (OEE). 
     The oxidizable oxidant species may be any species of the oxidant available for oxidation at the charging electrode. For example, the species may be a free ion, or an ion bonded to or coordinated with other ions or constituents in the ionically conductive medium. For example, in an aqueous electrolytic solution where oxygen is the oxidant, oxygen ions are oxidized, which may be available from an oxide of the fuel (e.g., ZnO when zinc is the fuel), hydroxide ions (OH − ), or water molecules (H 2 O). Similarly, the reducible fuel species may be any species of the fuel available for reduction at the fuel electrode. For example, the reducible fuel species may be a free ion, or an ion bonded to or coordinated with other ions or constituents in the ionically conductive medium. For example, when the fuel is a metal, ions of the metal are reduced and electrodeposited on the fuel electrode, which may be available from an oxide of the metal, a salt of the metal dissolved in the ionically conductive medium, or ions of the metal supported by or coordinated with other ions or constituents in the medium. 
     The charging electrode  18  may be made from a variety of materials, including but not limited to electroconductive mesh coated with catalyst such as nickel, nickel particles whose diameter range from few nm to several μm held together by a binder such as fluoropolymer, high surface area electrocatalyst such as nickel and its alloys (for example, Ni—Co, Ni—Pt) electrodeposited on electroconductive mesh. Further teachings in this regard as disclosed in the above-incorporated U.S. patent application Ser. No. 12/385,489, which may be referred to for more specifics. Also, reference may be made to U.S. patent application Ser. No. 12/549,617 for other relevant teachings, the entirety of which is incorporated herein by reference. 
     The individual cells  12  may have any construction or configuration. For example, they may use a flowing liquid electrolyte, such as is taught in the two above-incorporated patent applications. The electrolyte flow may run through each cell parallel to the electrodes  14 ,  16 ,  18 , and that flow may be recirculated within each cell  12 . Likewise, a flow that is generally orthogonal to the electrodes  14 ,  16 ,  18  may also be used, and it may be recirculated within each cell  12 . Any suitable pump may be used for generating the flow. It is also possible to deliver parallel flows of electrolyte to all the cells from the same flow source, such as a common pump, and re-circulate the parallel outputs of the same. It is also possible to maintain the electrolyte for each individual cell  12  on its own flow circuit isolated from the flows of the other cells to eliminate any power loss due to mixed potentials. In other embodiments, there may be no flow, and the electrolyte may simply remain within each individual cell  12 . The particular architecture for managing the oxidation and reduction reactions within the cells  12  themselves is not intended to be limiting. 
     As mentioned above,  FIG. 3  shows a representative structural arrangement for a stack of the cells  12 . This example is provided solely for illustrative purposes and is not intended to be limiting. A stack of two cells  12  is shown in exploded, cross-sectional view for illustrating the basic internal structural arrangement. A pair of outer housing bodies  40 ,  42  are provided at the ends, and these are formed of a non-conductive, electrochemically inert material, such as polymer or polymer-composite. A non-conductive, electrochemically inert barrier  19  is provided between the cells  12 , which may also be made of a polymer or polymer composite. 
     The housing body  40  and barrier  19  each have a recess  44  for receiving the fuel electrode  14 , which is shown as comprising the multiple electrode bodies  14   a - 14   c , as well as their associated isolators  15   a - 15   c . The charging electrode  18  is positioned adjacent to isolator  15   c , and thus is separated from the fuel electrode  14  in each cell  12 . Another electrochemically inert and non-conductive isolator, which is permeable to the electrolyte or other ionically conductive medium,  46  is positioned adjacent the charging electrode  18  for each cell  12 . 
     Permeable seal members  48  are bonded to sealing surfaces  50  on the housing bodies  40 ,  42  and barrier  19 . In each cell  12 , the permeable seal  48  encloses the fuel electrode  14 , the charging electrode  18 , and the various separators  15   a - 15   c  and  46  in the recesses  44 . The seal members  48  are non-conductive and electrochemically inert, and are preferably designed to be permeable to the electrolyte (or other ionically conductive medium) in the orthogonal direction (i.e., through its thickness), without permitting lateral transport of the electrolyte. This enables the electrolyte  12  to permeate through the seal members  48  for enabling ion conductivity with the oxidant electrode  16  on the opposing side to support the electrochemical reactions, without “wicking” the electrolyte  12  laterally outwardly from the cell  12 . A few non-limiting examples of a suitable material for the seal member  48  are EPDM and teflon. 
     The seal members  48  also cover a series of inlet and outlet fluid paths  52 ,  54 , respectively. These inlet and outlet fluid paths  52 ,  54  permit the electrolyte to flow into and out of the cells  12  with the flow within the cell running parallel to and between the fuel electrode  14  bodies  14   a - 14   c  and the charging electrode  18 . This encloses the flowing electrolyte within these paths. The entire configuration of these paths  52 ,  54  is not shown, as the particular configuration is not essential. Any construction or configuration may be used, and the flow paths may be coupled in series between the cells  12  or flow may be delivered to the cells in parallel. No particular flow management is limiting. 
     In each cell  12 , the oxidant electrode  16  is on the side of the seal member  48  opposite the fuel electrode  14  and the charging electrode  18 . A peripheral gasket  56  extends around the periphery of the oxidant electrode  16  and provides a seal between the oxidant electrode  18  and the adjacent structure (the opposing wall of barrier  19  or the outer housing body  42 , as illustrated). This prevents any electrolyte from leaking around the oxidant electrode  16  and into the area for air exposure. Preferably, the oxidant electrode  16  is permeable to the oxidant, but impermeable to the electrolyte or other ionically conductive medium, to thus prevent leakage of the ionically conductive medium through the oxidant electrode, but permit absorption of the oxidant. This characteristic may optionally enable the oxygen gas generated at the charging electrode  18  during re-charging to off-gas from the cell. The surfaces of the barrier  19  and housing body  42  have grooves  58  that extend to the exterior and are open to the ambient air. This enables the air to flow in and contact the oxidant electrode  16  to provide the reduction of oxygen, as discussed herein. 
     The example of  FIG. 3  is not limiting, and is provided solely for context to supplement the schematic illustrations of  FIGS. 1 and 2 . Any cell construction or configuration may be used. With an understanding of the cell system provided, attention is turned to the bypass switching aspect of the invention. 
     As will be discussed in further detail below, each of the cells  12  within the system  10  is connected in series. This is established by a plurality of switches  20  switchable between: 
     (1) A discharge mode. In the discharge mode, the switches  20  couple the oxidant electrode  16  of each cell  1  to N−1 to the fuel electrode  14  of the subsequent cell to couple the cells in a discharging series. That is, the oxidant electrode  16  of cell  1  is coupled to the fuel electrode  14  of cell  2 , the oxidant electrode  16  of cell  2  is coupled to the fuel electrode  14  of cell  3 , and so on, with the oxidant electrode  16  of cell N−1 being coupled to the fuel electrode  14  of cell N. As a result, when the fuel electrode of cell  1  and the oxidant electrode of cell N are coupled to a load, oxidation of fuel at the fuel electrodes  14  and reduction of the oxidant at the oxidant electrodes  16  creates a potential difference within each cell  12  to thus create a cumulative potential difference anodic at the fuel electrode of cell  1  and cathodic at the oxidant electrode of cell N for delivering a current to the load. The charging electrodes  18  do not have a potential applied to them, and they are not connected as part of the series circuitry. 
     (2) A charge mode. In the charge mode, the switches  20  couple the charging electrode  18  of each cell  1  to N−1 to the fuel electrode  14  of the subsequent cell to couple the cells in a charging series. As a result, when the fuel electrode  14  of cell  1  and the charging electrode  18  of cell N are coupled to a power source to receive a charging potential difference cathodic at the fuel electrode  14  of cell  1  and anodic at the charging electrode  18  of cell N, an incremental potential difference is created within each cell to reduce the reducible fuel species at the fuel electrode  14  and oxidize the oxidizable oxidant species at the charging electrode  18 . The oxidant electrodes  16  do not have a potential applied to them, and they are not connected as part of the series circuitry. 
     As mentioned above, the cells  12  are assembled adjacent one another with the non-conductive barrier  19  separating the oxidant electrode  16  and fuel electrode  14  of each pair of adjacent cells  12  such that the only permitted electrical connection therebetween is via the associated switch  20 . In the art, the non-conductive, insulating property of the barrier may be referred to as monopolar. In the illustrated embodiment, the electrodes  14 ,  16 ,  18  and barriers  19  are arranged generally parallel to one another so that the overall arrangement is that of a stack. 
     Preferably, in an embodiment where the cells  12  are metal-air cells, each barrier  19  has the series of grooves  58  formed on the surface thereof facing the adjacent oxidant electrode and opening as ports to the ambient atmosphere, thus allowing ambient air to enter through those ports and grooves for exposure to the air breathing oxidant electrode  16  (i.e., the air cathode). Other variations, including any type of port, may be used for delivering air or any other oxidant to the oxidant electrode  16 .  FIG. 1  shows the barrier  19  schematically with some spacing exaggerated to clearly depict the various electrodes, and its working structural configuration may take any suitable form, such as that shown in  FIG. 3 . 
     In the illustrated embodiment of  FIG. 1 , there are N switches  20 , meaning one switch for each cell  12 . The first switch  20  selectively couples to either the oxidant electrode  16  or charging electrode  18  of the first cell  12  and couples to the fuel electrode  14  of the second cell  12 . That is, the switch has a static contact connected to the fuel electrode  14  of the second cell, and has a switch element  26  movable between connection with two other selective contacts: one for connection to the oxidant electrode  16  of the first cell  12  and the other for connection to the charging electrode  18  of the first cell  12 . This type of switch is commonly referred to as a single pole double throw switch. For a fuel electrode  14  with multiple bodies, such as in  FIG. 3 , the connection of the fuel electrode contact may be made to all the bodies in parallel or to a terminal body, as described in the above-incorporated U.S. patent application Ser. No. 12/385,489. Movement of the switch element  26  to connect to one of those two contacts establishes the selection between the discharge and charge modes, as it establishes the connection between the oxidant or charging electrode  16 / 18  of the first cell with the fuel electrode  14  of the second cell.  FIG. 1  shows the switches  20  in the charge mode, thus coupling the fuel electrodes  14  and charge electrodes  18  of subsequent cells  12  together. The discharge mode is represented by the position of switches  20  in dashed lines. 
     The second switch  20  selectively couples to either the oxidant electrode  16  or charging electrode  18  of the second cell  12  and couples to the fuel electrode  14  of the third cell  12  in the same manner as the first switch does between the first and second cells. The third switch  20  likewise selectively couples to either the oxidant electrode  16  or charging electrode  18  of the third cell  12  and couples to the fuel electrode  14  of the fourth cell  12 . This continues on for each of cells  1  to N−1, so that the N−1th switch selectively couples either the oxidant electrode  16  or charging electrode  18  of the N−1th cell  12  and couples to the fuel electrode  14  of the Nth cell. As such, it can be generally described that within the cell system  10 , for any arbitrarily selected cell X among cells  1  to N−1, the associated switch  20  selectively couples either the oxidant or charging electrode of that cell X to the fuel electrode of the subsequent cell X+1. 
     The system  10  has a first terminal  22  and a second terminal  24 . The term terminal  10  is broadly used to describe any input/output connection for coupling the system  10  to a load (during discharging) and a power source (during charging). In the illustrated embodiment, the first terminal  22  is coupled to the fuel electrode of the first cell  12 . With regard to the second terminal  24 , it is coupled to an Nth one of the switches  20 . This Nth switch  20  functions the same as the other switches above, except that it selectively couples to either the oxidant electrode  16  or the charging electrode  18  of the Nth cell, and is coupled to the second terminal  24  instead of the fuel electrode of a subsequent cell. The switch element  26  of this Nth switch  20  is selectively moved in the same way as the other switches  20  to establish the charge and discharge modes. 
     In an alternative embodiment, the Nth switch  20  can be omitted, and the second terminal  24  can be replaced with two separate terminals, one coupled to the oxidant electrode  16  of the Nth cell, which is coupled to the load during discharging, and the other coupled to the charging electrode of the Nth cell, which is coupled to the power source during charging. Thus, a switch  20  would not be used for the Nth cell, as the connectivity of the Nth cell&#39;s oxidant and charging electrodes  16 ,  18  to the load and power source, respectively, can be managed via their respective terminals. 
     The switches  20  may be controlled by a controller, shown schematically at  30 . The controller may be of any construction and configuration. It may comprise hard-wired circuitry that simply manipulates the switches  20  based on an input determining whether the cell should be in discharge or charge mode. The controller  30  may also include a microprocessor for executing more complex decisions, as an option. The controller  30  may also function to manage connectivity between the load and the power source and the first and Nth cells (and particularly the fuel electrode  14  of the first cell, and the oxidant/discharge electrode  16 / 18  of the Nth cell). 
     In any embodiment, the switches  20  (or any other switch described herein) may be of any type, and the term switch is broadly intended to describe any device capable of switching between the modes or states described. For example, the switches  20  may be of single pole double throw type as shown. They may be of the pivoting, sliding or latching relay type. Also, semiconductor based switches may be used as well. The switches may be activated electrically (electromechanical relay) or magnetically or by other methods known to those familiar in the art. Any other suitable type of switch may be used, and the examples herein are not limiting. 
       FIG. 2  shows an alternative embodiment similar to  FIG. 1 , except that a series of bypass switches  32  have been added to the plurality of switches. Each bypass switch  32  is coupled between the fuel electrode  14  from that cell  12  to the fuel electrode  14  of the subsequent cell  12 . That is, for any given cell X, the fuel electrode  14  in cell X is connected or shunted to the fuel electrode  14  in cell X+1 when the bypass switch  32  for cell X is in closed position, thereby achieving bypass of cell X either during charge or discharge. More particularly, in the embodiment of  FIG. 2 , each bypass switch  32  has two contacts: (a) one contact connected to the static contact of the switch  20  of the previous cell  12  (or in the case of the first bypass switch  12  for cell  1 , this contact is connected to the terminal  22 ), which is also connected to the fuel electrode  14  of the cell  12  associated with the bypass switch  32 , and (b) another contact connected to the fuel electrode  14  of the subsequent cell  12  (or in the case of the bypass switch for the Nth cell, to the terminal  24 ). When the switch element  33  of any given bypass switch  32  for cells  2  to N−1 is closed, this couples the static contact of the previous cell&#39;s switch  20  (and thus the oxidant or charging electrode  16 ,  18  of that previous cell  12 ) to the fuel electrode  14  of the subsequent cell  12 . For the first cell  12 , when the switch element  33  of the bypass switch  32  is closed, the bypass switch  32  couples the terminal  22  to the fuel electrode  14  of the second cell  12 . And for the Nth cell, when the switch element  33  of the bypass switch is closed, the bypass switch  32  couples the static contact of the N−1th cell&#39;s switch  20  (and thus the oxidant or charging electrode  16 ,  18  of that N−1th cell  12 ) to the terminal  24 . 
     When cell X is in bypass mode, switch  20  in cell X is preferably in a position such that charging electrode of cell X is connected to fuel electrode of cell X+1 to avoid shorting of fuel electrode and oxidant electrode in cell X. Normally, each bypass switch  32  is in an open condition, and thus plays no role in the circuitry or operation of the cell system  10 . However, if the controller  30  detects that any given cell is not operating properly (which may jeopardize the whole system because the cells  12  are in series), the bypass switch  32  for that cell may be switched to a closed position, thus bypassing the problematic cell as a result of the connection established by the closed bypass switch  32 . In particular, the oxidant/charging electrode  16 ,  18  of the prior cell in the series (or the terminal  22  if the first cell  12  is being bypassed) can be coupled to the fuel electrode  14  of the subsequent cell in the series (or the terminal  24  if the Nth cell is being bypassed), thus bypassing the problematic cell while maintaining the series connections between the remaining operating cells  12 . 
     A cell can be bypassed for a number of reasons that affect the performance of the stack. For example, a short between charging electrode and fuel electrode in a cell during charge (detected by voltage measurement) leads to expense of parasitic power during charge. An electrical short leads to a sudden drop in voltage between the charging and fuel electrodes as the current is shunted between the charging and fuel electrodes. Another example is during discharge, where any cell that has a higher kinetic or ohmic loss affects the round trip efficiency and discharge power of the stack. Also, consumption of fuel in a cell during discharge earlier than other cells can lead to voltage reversal in the cell and stack power loss, and can be prevented by bypassing the cell when the discharge voltage falls below a critical value. Complete consumption of zinc or other fuel during discharge leads to a sudden drop in voltage between the fuel and oxidant electrodes. Any other criteria to detect the performance of cells may be used, and the examples herein are not limiting. Certain cells may not meet performance requirements (for example, maximum power during discharge) due to yield issues and problems related to fabrication and assembly of electrodes. These cells can be permanently placed in bypass mode. Other cells may meet performance requirements initially, however may have cycle life issues and can be placed in bypass mode after the performance falls below a required limit. Thus, bypass mode provides an option to increase reliability and performance of the stack. 
     The voltage or potential difference between the fuel and charging electrodes during charge and between the fuel and oxidant electrodes during discharge may be measured by techniques known in the art. For example, a voltmeter (digital or analog) or potentiometer or other voltage measuring device or devices may be coupled between each or the pairs of electrodes. The controller  30  may include appropriate logic or circuitry for actuating the appropriate bypass switch(es) in response to detecting a voltage reaching a predetermined threshold (such as drop below a predetermined threshold). 
     It is also preferable to include such a bypass switch between the fuel electrode  14  of the first cell  12 , or the first terminal  22 , so as to provide the same bypass for the first cell  12 . Likewise, the bypass switch  32  for the Nth cell  12  in the series would be provided between the fuel electrode contact of the N−1th switch  20  that couples to the fuel electrode  14  of the Nth cell and the second terminal  24 , thus enabling the Nth cell to be bypassed and couple the oxidant/charging electrode  16 / 18  of the N−1th cell to the second terminal  24 . 
       FIGS. 4   a - 4   d  schematically illustrate another embodiment using pairs of single pole double throw switches as the plurality of switches to provide the switching between charging and discharging, as well as the bypassing functionality. The cell  12  components are the same, and thus the same references numbers are used for common components. As can be seen  FIGS. 4   a - 4   d , each cell has a pair of single pole double throw switches. Switch  80  includes a switch element  82  that is statically connected to one contact, and selectively moves between connection to two other selective contacts: one that is coupled to the fuel electrode  14  of its associated cell  12 , and another that is coupled to the charging electrode  18  of its associated cell  10 . In the case of the switch  80  for the first cell  12 , the contact coupled to the first cell&#39;s fuel electrode  14  is also coupled to the terminal  22 . Switch  84  also includes a switch element  86  that is statically coupled to both the fuel electrode  14  of the subsequent cell and the contact of the subsequent switch  80  to which that subsequent fuel electrode  14  is coupled. The switch element  86  is selectively moved between connection to two other contacts: one that is coupled to the oxidant electrode  16  of its associated cell  12 , and another that is coupled to the static contact of its cell&#39;s switch  80 . In the case of the switch  84  for the Nth cell  12 , its static contact is coupled to the terminal  24 . 
     The operation of these switches  80  and  84  will now be described, and may be controlled by the controller  30  with appropriate logic and/or circuitry therein. 
     In  FIG. 4   a , the switches are in a state for normal charging. Each of the switch elements  82  and  86  are moved to positions to couple the charging electrode  18  of their associated cell  12  to the fuel electrode  14  of the subsequent cell  12  (or in the case of the switches  80 ,  84  for the Nth cell, to the terminal  24 ). Specifically, each switch element  82  is moved to connect with the contact coupled to the charging electrode  18  of its associated cell  12 , and each switch element  86  is moved to connect with the contact that is coupled to the static contact of switch  80 . Thus, the oxidant electrodes  16  are disconnected from the circuit. 
     In  FIG. 4   b , the switches are in a state for normal discharging. Each of the switch elements  86  are moved to positions to couple the oxidant electrode  16  of their associated cell  12  to the fuel electrode  14  of the subsequent cell  12  (or in the case of the switch  84  for the Nth cell, to the terminal  24 ). Specifically, each switch element  86  is moved to connect with the contact coupled to the oxidant electrode  16  of its associated cell  12 . The position of the switch elements  82  of switches  80  is irrelevant in this normal discharging state, as none of the switch elements  86  are connected to the static contacts of switches  80 , and therefore the switches  80  are disconnected from the circuit (as are the charging electrodes  18 ). 
       FIG. 4   c  shows a state for charging wherein the second cell  12 , for example, is bypassed. In this state, all the switches  80 ,  82 , except switch  80  associated with the second cell  12 , are in the same position as shown in  FIG. 4   a . However, the switch  80  for the second cell  12  is positioned differently. Specifically, the switch element  82  of the second cell&#39;s switch  80  is moved to a position connected to the contact that is coupled to the static contact of the switch  84  for the first cell  12  (in the case of cell X being bypassed, the first cell being the X−1th cell, the second cell being Xth cell). Thus, this couples the charging electrode  18  of the first (X−1th) cell  12  to the fuel electrode  14  of the subsequent third cell  12  (the X+1th cell). As such, the second or Xth cell is effectively by-passed, as current flow is established between the charging electrode  18  of the first (X−1th) cell  12  and the fuel electrode  14  of the third (X+1th) cell  12  via the switches  80 ,  82  associated with the second cell  12 . Likewise, in the situation where the Nth cell is the cell being bypassed, the current flow would be established between the charging electrode  18  of the N−1th cell  12  and the terminal  24  via the switches  80 ,  82  associated with the Nth cell. And where the first cell is the cell being bypassed, the current flow would be established between the terminal  22  and the fuel electrode  14  of the second cell via the switches  80 ,  82  associated with the first cell. 
       FIG. 4   d  shows a state for discharging wherein the second cell  12 , for example, is bypassed. In this state, all the switches  80 ,  82 , except switches  80 ,  82  associated with the second cell  12 , are in the same position as shown in  FIG. 4   b . However, the switches  80  and  82  for the second cell are positioned differently (as was noted above, the position for switch  80  for the non-bypassed cells is not important, and either position could be selected; however, in this circuit arrangement the position of second cell&#39;s switch  80  does perform part of the bypassing functionality for the second cell  12 ). Specifically, the switch element  82  of the second cell&#39;s switch  80  is moved to a position connected to the contact that is coupled to the static contact of the switch  84  for the first cell  12  (the X−1th cell, the second cell being Xth cell again). Also, the switch element  86  of the second cell&#39;s switch  84  is moved to a position connected to the static contact of the second (Xth) cell&#39;s switch  80 . Thus, this couples the oxidant electrode  16  of the first (X−1th) cell  12  to the fuel electrode  14  of the third (X+1th) cell  12 . As such, the second or Xth cell is effectively by-passed, as current flow is established between the oxidant electrode  16  of the first (X−1th) cell  12  and the fuel electrode  14  of the third (X+1th) cell  12  via the switches  80 ,  82  associated with the second cell  12 . Likewise, in the situation where the Nth cell is the cell being bypassed, the current flow would be established between the oxidant electrode  16  of the N−1th cell  12  and the terminal  24  via the switches  80 ,  82  associated with the Nth cell. And where the first cell is the cell being bypassed, the current flow would be established between the terminal  22  and the fuel electrode  14  of the second cell via the switches  80 ,  82  associated with the first cell. 
     The configuration in  FIG. 4  allows for placing cell X in by pass mode without shorting fuel electrode and charging electrode of cell X as is the case in the configuration shown in  FIG. 3 . 
       FIGS. 5   a - 5   d  schematically illustrate another embodiment using single pole triple throw switches to provide the switching between charging and discharging, as well as the bypassing functionality. Each cell  12  has such a switch  90  associated therewith. Each switch  90  has a switch element  92  that is statically connected to one contact. For the  1  to N−1th cells, the static contact of the switch  90  is coupled to the fuel electrode  14  of the subsequent cell  12 . The switch element  92  selectively moves between connection to three other selective contacts: a first one coupled to at least the static contact of the previous cell&#39;s switch  90 , as well as the fuel electrode  14  of its associated previous cell  12 , a second one coupled to the charging electrode  18  of its associated cell  12 , and a third one coupled to the oxidant electrode  16  of its associated cell. In the case of the switch  90  for the first cell  12 , the first selective contact is coupled to the terminal  22 , as well as the first cell&#39;s fuel electrode  14 . For the Nth cell, the static contact of the switch  90  is coupled to the terminal  24 . 
     The operation of these switches  90  will now be described, and may be controlled by the controller  30  with appropriate logic and/or circuitry therein. 
       FIG. 5   a  shows the switches  90  in a state for normal charging. Each of the switch elements  92  is moved to a position connected to the second selective contact that is coupled to the charging electrode  18  of its associated cell  12 . This couples the charging electrode  18  of the associated cell  12  to the fuel electrode  14  of the subsequent cell (or in the case of the Nth cell, the charging electrode  18  of the Nth cell is coupled to the terminal  24 ). Thus, the oxidant electrodes  16  are disconnected from the circuit. 
       FIG. 5   b  shows the switches in a state for normal discharging. Each of the switch elements  92  is moved to a position connected to the third selective contact that is coupled to the oxidant electrode  16  of its associated cell  12 . This couples the oxidant electrode  16  of the associated cell  12  to the fuel electrode  14  of the subsequent cell (or in the case of the Nth cell, the oxidant electrode  16  of the Nth cell is coupled to the terminal  24 ). Thus, the charging electrodes  18  are disconnected from the circuit. 
       FIG. 5   c  shows a state for charging wherein the second cell  12 , for example, is bypassed. In this state, all the switches  90 , except switch  90  associated with the second cell  12 , are in the same position as shown in  FIG. 5   a . However, the switch  90  for the second cell  12  is positioned differently. Specifically, the switch element  92  for the switch  90  of the second cell  12  (the Xth cell) is moved to a position connected to the first selective contact that is coupled to static contact of the switch  90  for the first (X−1th cell). This couples the charging electrode  18  of the first (X−1th) cell  12  to the fuel electrode  14  of the third (X+1th) cell  12 . As such, the second or Xth cell is effectively by-passed, as current flow is established between the charging electrode  18  of the first (X−1th) cell  12  and the fuel electrode  14  of the third (X+1th) cell  12  via the switch  90  associated with the second cell  12 . Likewise, in the situation where the Nth cell is the cell being bypassed, the current flow would be established between the charging electrode  18  of the N−1th cell  12  and the terminal  24  via the switch  90  associated with the Nth cell. And where the first cell is the cell being bypassed, the current flow would be established between the terminal  22  and the fuel electrode  14  of the second cell via the switch  90  associated with the first cell. 
       FIG. 5   d  shows a state for discharging wherein the second cell  12 , for example, is bypassed. In this state, similarly to  FIG. 5   c , all the switches, except switch  90  associated with the second cell  12 , are in the same position as shown in  FIG. 5   b . However, the switch  90  for the second cell  12  is positioned differently. Specifically, the switch element  92  for the switch  90  of the second cell  12  (the Xth cell) is moved to a position connected to the first selective contact that is coupled to static contact of the switch  90  for the first (X−1th cell), just as is the case in the bypass condition of  FIG. 5   c . In  FIG. 5   d , this couples the oxidant electrode  16  of the first (X−1th) cell  12  to the fuel electrode  14  of the third (X+1th) cell  12 . As such, the second or Xth cell is effectively by-passed, as current flow is established between the oxidant electrode  16  of the first (X−1th) cell  12  and the fuel electrode  14  of the third (X+1th) cell  12  via the switch  90  associated with the second cell  12 . Likewise, in the situation where the Nth cell is the cell being bypassed, the current flow would be established between the oxidant electrode  16  of the N−1th cell  12  and the terminal  24  via the switch  90  associated with the Nth cell. And where the first cell is the cell being bypassed, the current flow would be established between the terminal  22  and the fuel electrode  14  of the second cell via the switch  90  associated with the first cell, just as is the case with the state of  FIG. 5   c.    
     With any of these embodiments, the bypassing switches can also be used to bypass a group of adjacent cells, if it becomes necessary to do so. Thus, for example, if a group of three cells was being by-passed (e.g., cells X to X+2), these by-passing switches would also enable those cells to be bypassed from the cell prior to the group (i.e., cell X−1) to the cell subsequent to the group (i.e., cell X+3), as can be appreciated from circuitry depicted. Thus, broadly speaking, each of these embodiments with bypassing functionality may be generally characterized as having its switches being capable of establishing a bypass mode for a cell. In this bypass mode, the current (referring to the actual direction of electron flow) that would normally be applied to the fuel electrode  14  of that cell (cell X) during charging is re-directed or shunted so as to be applied to the fuel electrode of the subsequent cell (X+1), or the terminal  24  in the case of the Nth cell. Similarly, the current that would be drawn from to the fuel electrode  14  of that cell X during discharging would be drawn from the fuel electrode  14  of the subsequent cell (X+1), or the terminal  24  in the case of the Nth cell. Preferably, this is done with the oxidant and charging electrodes  16 ,  18  of that cell X disconnected by the switching, thus avoiding the creation of an electrochemical connection between the fuel electrode  14  and the oxidant/charging electrodes  16 / 18  of that cell X, as is shown in the embodiments of  FIGS. 4 and 5 . Any suitable switching arrangement, including but not limited to those illustrated may be used. 
     It should be appreciated that any of the embodiments of the switches described above (e.g., to enable the charge mode, discharge mode, bypass mode) may also be used with a plurality of electrochemical cells having a dynamically changing oxygen evolving electrode/fuel electrode, such as the progressive one described in U.S. Patent Application Ser. No. 61/383,510, which is incorporated in its entirety herein by reference. 
     For example, as described in the U.S. Patent Application Ser. No. 61/383,510, the fuel electrode  14  may include a plurality of permeable electrode bodies, which may be screens that are made of any formation able to capture and retain particles or ions of metal fuel from an ionically conductive medium that circulates in the cell  12 . Each of the permeable electrode bodies may be electrically isolated from each other using, for example, non-conductive and electrochemically inert spacers. In some embodiments, each cell  12  may also have its own plurality of switches associated with the electrode bodies to enable progressive fuel growth. 
     During charging, the charging electrode  18  of each cell  12  may be coupled to the fuel electrode  14  of the subsequent cell  12 . In one embodiment, during charging, a first electrode body (Y) of the fuel electrode  14  may have a cathodic potential and the rest of the electrode bodies and/or a separate charging electrode may have an anodic potential. In such an embodiment, during the progressive fuel growth of the fuel electrode  14 , the fuel may grow on the first electrode body (Y) having the cathodic potential and cause a short with the adjacent electrode body (Y+1) having the anodic potential. The adjacent electrode body (Y+1) may then be disconnected from the source of anodic potential such that through electrical connection, the adjacent electrode body (Y+1) also has the cathodic potential. This process may continue with the rest of the electrode bodies until no further growth is possible (i.e., the cathodic potential has shorted to the last electrode body having an anodic potential or a separate charging electrode). A plurality of switches may be provided to connect/disconnect the electrode bodies to one another and/or to sources of cathodic or anodic potential. Thus, in such embodiments having progressive fuel growth, the charging electrode  18  may be a separate charging electrode from the fuel electrode  14  or may be at least the adjacent electrode body, up to all other electrode bodies, having an anodic potential. In other words, the charging electrode  18  may be a separate charging electrode, an electrode body having an anodic potential located adjacent to the at least one electrode body having a cathodic potential, and/or a group of electrode bodies having an anodic potential located adjacent to the at least one electrode body having a cathodic potential. 
     Thus, the charging electrode, as that term is used in the broader aspects of this application, need not necessarily be a static or dedicated electrode that only plays the anodic charging role (although it may be), and it may at times be a body or bodies within the fuel electrode to which an anodic potential is applied. Hence, the term dynamic is used to refer to the fact that the physical element(s) functioning as the charging electrode and receiving an anodic potential during charging may vary. 
     During discharging, the oxidant electrode  16  of a cell  12  may be operatively connected to the fuel cell  14  of the subsequent cell  12  and fuel consumption would be through the electrode bodies (wherein the electrical connection between the electrode bodies are through fuel growth). If a cell  12  is not functioning properly or for other reasons, the cell  12  may also be bypassed using the bypass switching features described above. 
     Also, in some embodiments, the cells may be designed as “bi-cells.” That term refers to a pair of air electrodes that are on opposing sides of a fuel electrode. During discharge, the air electrodes are at generally the same cathodic potential and the fuel electrode is at an anodic potential. Typically, a pair of dedicated charging electrodes may be disposed in the ionically conductive medium between the air electrodes and the fuel electrode. During charging, the charging electrodes are at generally the same anodic potential, and the fuel electrode is at a cathodic potential (alternatively, the charging electrode may dynamically charge, as described above). Thus, the air electrodes may share a common terminal, and the fuel electrode has its own terminal, and the charging electrodes may also share a common terminal. As such, electrochemically speaking, such a bi-cell may be regarded as a single cell (although within the bi-cell, certain aspects of the cell, such as bi-directional fuel growth, may cause a bi-cell to be considered as two cells for certain purposes; however, at a higher level for mode discharging and connection management, those aspects are less relevant and the bi-cell can be viewed as a single cell). The pair of air electrodes correspond to the oxidant electrode  16 , the fuel electrode corresponds to the fuel electrode  14 , and the pair of charging electrodes correspond to the charging electrode  18 . 
     The foregoing illustrated embodiments have been provided solely to illustrate the structural and functional principles of the present invention, and should not be regarded as limiting. To the contrary, the present invention is intended to encompass all modification, substitutions, and alterations within the spirit and scope of the following claims.