Abstract:
An oxygen recovery system configured to recover evolved oxygen from a regenerative electrochemical cell. The electrochemical cell includes an oxygen reduction cathode, a fuel electrode configured to be a fuel anode when the cell is operated to generate electricity and a cathode for reducing fuel thereon when the cell is operated to regenerate the fuel, and an oxygen evolution anode that is configured to evolve oxygen from an electrolyte solution when the cell is operated to regenerate the fuel. The oxygen recovery system includes an oxygen separator located downstream of the oxygen evolution anode in a recharge direction of flow. The oxygen separator is configured to separate the evolved oxygen from the electrolyte solution. An oxygen recovery path is disposed between the oxygen separator and the oxygen reduction cathode. The oxygen recovery path is configured to direct the evolved oxygen separated from the electrolyte solution to the oxygen reduction cathode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of priority from U.S. Provisional Patent Application No. 61/136,330, filed on Aug. 28, 2008, the content of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present application relates to an oxygen recovery system for an electrochemical cell, and a method for recovering oxygen in an electrochemical cell 
     BACKGROUND 
     Regenerative metal-air electrochemical cells operate through the oxidation and reduction of a metal fuel. In some metal-air cells, such as that disclosed in U.S. Provisional Patent Application No. 61/054,364, filed on May 19, 2008, and U.S. patent application Ser. No. 12/385,489, filed on Apr. 9, 2009, the contents of both of which are incorporated herein by reference in their entireties, the initial discharge of the electrochemical cell occurs through the oxidation of a metal to a metal oxide on an electrode acting as a fuel anode, while an air cathode reduces an oxidizer, such as oxygen to hydroxide, in the flowing electrolyte solution. As the metal fuel is increasingly oxidized, the capacity of the cell to discharge energy to an external load is progressively reduced. By utilizing a third electrode as an oxygen evolution anode, and utilizing the depleted fuel anode as a plating cathode, the application of a power source can recharge the cell by reducing an oxidized form of the metal to metal fuel, and evolving oxygen. 
     Oxygen evolved during recharge of such electrochemical cells typically gasses out of the electrolyte and is discarded to the atmosphere. In such electrochemical cells, the virtually limitless supply of oxygen in the air ensures that the air cathode will constantly have a source of oxygen needed for the reduction of oxygen to hydroxide during discharge. One potential drawback of using air as a source of oxygen is that generally less than 21 percent of dry ambient air is composed of the oxygen used by the electrochemical cell. The other gasses present in air do not further the electrochemical reactions in the cell, and may detract from the operation of the cell over time. For example, carbon dioxide, which constitutes approximately 0.04 percent of dry air, may react adversely in the electrochemical cell to create carbonate on the air cathode. The gradual buildup of carbonate may increasingly degrade the efficiency of the electrochemical cell, thereby limiting the cell&#39;s effective lifespan. While continuously supplying pure oxygen from an external source to the air cathode during its operation would eliminate the adverse effects stemming from the presence of gasses other than oxygen in ambient air, such a supply may increase both the size and cost of the cell, making such cells inappropriate for a multitude of uses. 
     The present invention endeavors to provide various improvements over regenerative electrochemical cells utilizing an air cathode. 
     SUMMARY 
     According to an aspect of the present invention, there is provided an oxygen recovery system configured to recover evolved oxygen from a regenerative electrochemical cell. The regenerative electrochemical cell includes an oxygen reduction cathode, and a fuel electrode configured to be a fuel anode when the electrochemical cell is operated to generate electricity and a cathode for reducing fuel thereon when the electrochemical cell is operated to regenerate the fuel. The cell also includes an oxygen evolution anode that is configured to evolve oxygen from an electrolyte solution when the electrochemical cell is operated to regenerate the fuel. The oxygen recovery system includes an oxygen separator located downstream of the oxygen evolution anode in a recharge direction of flow. The oxygen separator is configured to separate the evolved oxygen from the electrolyte solution. The oxygen recovery system also includes an oxygen recovery path disposed between the oxygen separator and the oxygen reduction cathode. The oxygen recovery path is configured to direct the evolved oxygen separated from the electrolyte solution to the oxygen reduction cathode. 
     According to another aspect of the present invention, there is provided a method for recovering evolved oxygen from a regenerative electrochemical cell. The regenerative electrochemical cell is configured to operate through oxidation of a metal fuel and reduction of an oxidizer. The cell includes an oxygen reduction cathode, a fuel electrode configured to be a fuel anode when the electrochemical cell is operating to generate electricity and a cathode when the electrochemical cell is operating to regenerate the fuel, and an oxygen evolution anode that is configured to evolve oxygen from an electrolyte solution when the electrochemical cell is operating to regenerate the fuel. The method includes recharging the regenerative electrochemical cell by reducing reducible metal fuel ions from the electrolyte solution into the metal fuel onto the fuel electrode functioning as a cathode and the evolving oxygen via oxidation at the oxygen evolution anode, separating the evolved oxygen from the electrolyte solution with an oxygen separator, and directing the evolved oxygen from the oxygen separator into an oxygen recovery path disposed between the oxygen separator and the oxygen reduction cathode. 
     According to an aspect of the present invention, there is provided an electrochemical cell system that includes a regenerative electrochemical cell and an oxygen recovery system. The regenerative electrochemical cell includes an oxygen reduction cathode, and a fuel electrode configured to be a fuel anode when the electrochemical cell is operated to generate electricity and a cathode for reducing fuel thereon when the electrochemical cell is operated to regenerate the fuel. The cell also includes an oxygen evolution anode that is configured to evolve oxygen from an electrolyte solution when the electrochemical cell is operated to regenerate the fuel. The oxygen recovery system is configured to recover evolved oxygen from the regenerative electrochemical cell. The oxygen recovery system includes an oxygen separator located downstream of the oxygen evolution anode in a recharge direction of flow. The oxygen separator is configured to separate the evolved oxygen from the electrolyte solution. The oxygen recovery system also includes an oxygen recovery path disposed between the oxygen separator and the oxygen reduction cathode. The oxygen recovery path is configured to direct the evolved oxygen separated from the electrolyte solution to the oxygen reduction cathode. 
     Other aspects 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 an embodiment of a regenerative electrochemical cell with an embodiment of an evolved oxygen recovery system according to the present invention, when the regenerative electrochemical cell is configured to discharge electricity; 
         FIG. 2  is a schematic view of the regenerative electrochemical cell and evolved oxygen recovery system of  FIG. 1 , when the regenerative electrochemical cell is configured to recharge a fuel electrode of the electrochemical cell; 
         FIG. 3  is a schematic view of an embodiment of a regenerative electrochemical cell with an embodiment of an evolved oxygen recovery system, when the regenerative electrochemical cell is configured to discharge electricity; and 
         FIG. 4  is a schematic view of the regenerative electrochemical cell and evolved oxygen recovery system of  FIG. 3 , when the regenerative electrochemical cell is configured to recharge a fuel electrode of the electrochemical cell. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  and  FIG. 2  show schematic views of an embodiment of a single flow regenerative electrochemical cell  100  with an evolved oxygen recovery system  200 . The regenerative electrochemical cell  100  may have the same or substantially similar configuration as the electrochemical cell described in U.S. Provisional Patent Application No. 61/054,364, filed on May 19, 2008, and U.S. patent application Ser. No. 12/385,489, filed on Apr. 9, 2009, the contents of both of which are incorporated herein by reference in their entireties. 
       FIG. 1  shows the electrochemical cell  100  and the evolved oxygen recovery system  200  in a discharge mode, connected to an external load  300 . The regenerative electrochemical cell  100  of the illustrated single flow embodiment includes an oxygen reduction cathode  110 , an electrolyte solution  120 , an oxygen evolution anode  130  (also referred to as a charging anode), a metal fuel electrode  140 , an electrolyte path  150 , and a flow generator  160  for circulating the electrolyte solution  120 . In the illustrated embodiment, the flow generator  160  is a pump. In such a single flow embodiment, the oxygen evolution anode  130  is disposed within the electrochemical cell  100  between the oxygen reduction cathode  110  and the metal fuel electrode  140 , thereby allowing the electrochemical cell  100  to operate in discharge and recharge modes without needing to reverse the circulation of the electrolyte solution  120 . 
     Electricity is generated through an oxidation/reduction reaction that occurs between the oxygen reduction cathode  110  and the metal fuel electrode  140 . The oxygen reduction cathode  110  may be a passive or “breathing” cathode that passively accepts gasses it is exposed to. The oxygen reduction cathode  110  is generally gas permeable, but not liquid permeable, thereby allowing the intake of gasses such as oxygen, without releasing the electrolyte solution  120  from the electrochemical cell  100 . The oxygen reduction cathode  110  may include a catalyst material, such as manganese oxide, nickel, silver, pyrolized cobalt, activated carbon, platinum, or any other catalyst material or mixture of materials with high oxygen reduction activity in the electrolyte for catalyzing reduction of an oxidizer. 
     During discharge, or electricity generating, mode, as depicted in the embodiment of  FIG. 1 , the metal fuel electrode  140  acts as an anode. In an embodiment, the metal fuel electrode  140  may contain a filter body (not shown) made of any formation able to capture and retain, through filtering, electrodepositing, or otherwise, particles or ions of metal fuel from the electrolyte solution  120 . In addition, the filter body may be formed from a conductive material. Embodiments of such a filter body are discussed in greater detail in U.S. Provisional Patent Application No. 61/064,955, filed Apr. 4, 2008, and U.S. patent application Ser. No. 12/385,217, filed Apr. 1, 2009, the contents of both of which are incorporated herein by reference in their entireties. Electricity that can be drawn by the external load  300  is generated when an oxidizer at the oxygen reduction cathode  110  is reduced, while the metal fuel at the metal fuel electrode  140  is oxidized to an oxidized form. The external load  300  is electrically connected to both the oxygen reduction cathode  110  and the metal fuel electrode  140  to complete an electrical circuit. The electrical potential of the electrochemical cell  100  is depleted once the metal fuel at the metal fuel electrode  140  is entirely oxidized. 
     The electrolyte solution  120  can be circulated passively, such as through gravity, through motion of the device, or any other process that transports the oxidized metal fuel ions away from the metal fuel electrode  140  and through a gap towards the oxygen reduction cathode  110 . This “transport flow” of the electrolyte solution  120 , can also be accomplished in some embodiments with the flow generator  160 . When the flow generator  160  is a pump, it may have any suitable construction or configuration, including but not limited to piezoelectric, centrifugal, gear, flexible impeller, peristaltic, or any other type of pump. Any other method of circulating the electrolyte solution  120  from the metal fuel electrode  140  towards the oxygen reduction cathode  110  during discharge mode may be used. The electrolyte solution  120  will then enter the electrolyte path  150 , which has an entrance in the discharge direction of flow of the electrolyte solution  120  disposed between the electrolyte side of the oxygen reduction cathode  110  and the metal fuel electrode  140 , where it directs the electrolyte solution  120  back to the metal fuel electrode  140 . In some embodiments, the flow generator  160  may be disposed between the electrolyte path  150  and the metal fuel electrode  140 , or within the electrolyte path  150 . 
     While the illustrated embodiment is operating in discharge mode, as described above, the oxygen evolution anode  130  is inoperative and is configured to allow the electrolyte solution  120  to flow therethrough. The oxygen evolution anode  130 , which can also be described as a charging electrode, is depicted as a separate electrode in the illustrated embodiment because generally most electrodes suitable for function as an oxygen reduction cathode do not perform well as an anode when being used for charging purposes. The electrochemical cell is not intended to be limited, however, and it is possible for the oxygen evolution anode  130  and the oxygen reduction cathode  110  to be the same electrode, with the flow of the electrolyte solution  120  modified accordingly. In embodiments where the oxygen evolution anode  130  is separate, it should be permeable to the electrolyte solution  120 , thereby allowing for the flow of the electrolyte solution  120  from the metal fuel electrode  140  through the oxygen evolution anode  130 , and to the oxygen reduction cathode  110 . 
     In charging mode, as depicted in  FIG. 2 , a power supply  310  is connected to the metal fuel electrode  140 , which becomes a cathode, and to the oxygen evolution anode  130 , and the circuit to the external load  300  is intercepted. Details of the charging mode are provided in U.S. Provisional Patent Application No. 61/054,364, filed May 19, 2008, and U.S. patent application Ser. No. 12/385,489, filed on Apr. 9, 2009, the contents of both of which are incorporated herein by reference in their entireties. By supplying an electrical current across the metal fuel electrode  140  and the oxygen evolution anode  130 , the metal fuel of the metal fuel electrode  140 , which was converted from a metal to an oxidized form, including but not limited to spent metal fuel oxide, metal fuel hydroxide, and metal fuel hydroxide anions, during discharge mode, is reduced from the oxidized form back to the metal fuel. At the same time, the oxygen evolution anode  130  oxidizes hydroxide ions that are generated during the reduction of the oxidized metal fuel, and are present in the electrolyte solution  120 . Oxidation of the hydroxide ions evolves oxygen, which flows within the electrolyte solution  120 . 
     The evolved oxygen may be recycled by the evolved oxygen recovery system  200 . The evolved oxygen recovery system  200  contains an oxygen separator  210  that is configured to separate evolved oxygen from the electrolyte solution  120 . In the illustrated embodiments, the oxygen separator  210  is disposed within the electrolyte path  150 . The oxygen separator  210  may alternatively be located anywhere within the electrochemical cell  100  that is in contact with the electrolyte solution  120 . The oxygen separator  210  may be of any suitable construction or configuration, including but not limited to a reversed U-tube which may provide space for the evolved oxygen to bubble out of the electrolyte solution  120 . The evolved oxygen recovery system  200  additionally has an oxygen recovery path  220 , through which separated oxygen  230  is directed back to the air side of the oxygen reduction cathode  110  for use as an oxidizer during the discharge mode as discussed above. The oxygen recovery path  220  can be of any suitable construction or configuration. In an embodiment, the separated oxygen  230  is not released to the ambient air outside of the electrochemical cell  100  or the evolved oxygen recovery system  200 , but is instead fully contained within the combination of the electrochemical cell  100  and the evolved oxygen recovery system  200 . 
     In an embodiment, the evolved oxygen recovery system  200  may include a recovery flow generator  240 , configured to generate a flow of the separated oxygen  230  within the oxygen recovery path  220 . The recovery flow generator  240  can be of any suitable construction or configuration, including but not limited to a pump, a vacuum, a turbine, a fan, or any other suitable flow generating device. In an embodiment, the evolved oxygen recovery system  200  may include an accumulating vessel  250 . The accumulating vessel  250  can be of any construction or configuration configured to provide space for the separated oxygen  230  to gather without releasing the separated oxygen  230  to the ambient air outside of the electrochemical cell  100  or the evolved oxygen recovery system  200 . In an embodiment, the evolved oxygen recovery system  200  may have a flow regulator  260 , configured to selectively limit the flow of the separated oxygen  230  within the oxygen recovery path  220  to the oxygen reduction cathode  110 . The flow regulator  260  may have any suitable construction or configuration, including a valve, a pressure regulator, a gate, a clamp, or any other such device that can limit the flow of the separated oxygen  230 . In an embodiment that contains both a flow regulator  260  and a recovery flow generator  240 , there may be a controller (not shown) that can selectively control the operation of both the recovery flow generator  240  and the flow regulator  260 . Such a controller may be used to prevent excessive buildups of oxygen within the evolved oxygen recovery system  200 , such as between the flow regulator  260  and the oxygen separator  210 . 
     Although the oxidizer that the oxygen reduction cathode  110  is initially exposed to will eventually become separated oxygen  260  that is evolved during operation of the electrochemical cell  100 , the initial supply of oxidizer may come from oxygen that is sealed in the evolved oxygen recovery system  200  at the time of the manufacture or assembly of the system and electrochemical cell  100 . The evolved oxygen recovery system  200  may also be assembled with an initial supply of pure oxygen, or any other gaseous mixture containing oxygen contained within it, for use by the cell, and ultimate separation by the oxygen separator  210 . In an embodiment, air may be used during initial operation of the electrochemical cell  100  until enough oxygen is separated from the electrolyte solution  120  to support further operation of the electrochemical cell  100 , or until the metal fuel is fully oxidized. Once enough oxygen has been accumulated, the external air may be shut off from the oxygen reduction cathode  110 . 
       FIG. 3  and  FIG. 4  show schematic views of an embodiment of a reversible flow regenerative electrochemical cell  400  with an evolved oxygen recovery system  200 . As illustrated, operation of the evolved oxygen recovery system is as described above for the electrochemical cell  100 . In the reversible flow regenerative electrochemical cell  400 , the metal fuel electrode  440  is disposed between the oxygen reduction cathode  410  and the oxygen evolution anode  430 . Details of the embodiment of the regenerative electrochemical cell  400  are discussed in U.S. Provisional Patent Application No. 61/054,364, filed May 19, 2008 and U.S. patent application Ser. No. 12/385,489, filed on Apr. 9, 2009, the contents of both of which are incorporated herein by reference in their entireties.  FIG. 3  shows the electrochemical cell  400  and the evolved oxygen recovery system  200  in a discharge mode, connected to an external load  300 . The regenerative electrochemical cell  400  of the illustrated reversible flow embodiment includes an oxygen reduction cathode  410 , an electrolyte solution  420 , an oxygen evolution anode  430 , a metal fuel electrode  440 , an electrolyte path  450 , and a reversible flow generator  460  for circulating the electrolyte solution  420 . In the illustrated embodiment, the reversible flow generator  460  is a pump. 
     In such an embodiment, the metal fuel electrode  440  is disposed within the electrochemical cell  400  between the oxygen reduction cathode  410  and the oxygen evolution anode  430 , thereby making a flow reversal desirable to allow the electrochemical cell  400  to operate in discharge and recharge modes. As illustrated in  FIG. 3 , the electrolyte solution  420  flows from the metal fuel electrode  440  to the oxygen reduction cathode  410  during discharge mode. 
     Generation of electricity occurs through an oxidation/reduction reaction occurring between the oxygen reduction cathode  410  and the metal fuel electrode  440 . The oxygen reduction cathode  410  may be a passive or “breathing” cathode that passively accepts gasses it is exposed to. The oxygen reduction cathode  410  is generally gas permeable, but not liquid permeable, thereby allowing the intake of gasses such as oxygen, without releasing the electrolyte solution  420  from the electrochemical cell  400 . The oxygen reduction cathode  410  may include a catalyst material, such as manganese oxide, nickel, silver, pyrolized cobalt, activated carbon, platinum, or any other catalyst material or mixture of materials with high oxygen reduction activity in the electrolyte for catalyzing reduction of an oxidizer. 
     During discharge mode, as depicted in the embodiment of  FIG. 3 , the metal fuel electrode  440  acts as an anode. In an embodiment, the metal fuel electrode  440  may contain a filter body (not shown) made of any formation able to capture and retain, through filtering, electrodepositing, or otherwise, particles of metal fuel from the electrolyte solution  420 . Embodiments of such a filter body are discussed in greater detail in U.S. Provisional Patent Application No. 61/064,955, filed Apr. 4, 2008, and U.S. patent application Ser. No. 12/385,217, filed Apr. 1, 2009, the contents of both of which are incorporated herein by reference in their entireties. In addition, the filter body may be formed from a conductive material. Electricity that can be drawn by the external load  300  is generated when an oxidizer at the oxygen reduction cathode  410  is reduced, while the metal fuel at the metal fuel electrode  440  is oxidized to a metal oxidized form, as described above. The external load  300  is electrically connected to both the oxygen reduction cathode  410  and the metal fuel electrode  440  to complete an electrical circuit. The electrical potential of the electrochemical cell  400  is depleted once the metal fuel at the metal fuel electrode  440  is entirely oxidized. 
     The electrolyte solution  420  can be circulated passively, such as through gravity, through motion of the device, or any other process that transports the oxidized metal fuel ions away from the metal fuel electrode  440  and through a gap towards the oxygen reduction cathode  410 . This “transport flow” of the electrolyte solution  420 , can also be accomplished in some embodiments with the reversible flow generator  460 . When the reversible flow generator  460  is a pump, it may have any suitable construction or configuration, including but not limited to piezoelectric, centrifugal, gear, flexible impeller, peristaltic, or any other type of pump. Any other method of circulating the electrolyte solution  420  from the metal fuel electrode  440  towards the oxygen reduction cathode  410  during discharge mode may be used. The electrolyte solution  420  will then enter the electrolyte path  450 , which has an entrance in the discharge direction of flow of the electrolyte solution  420  disposed between the electrolyte side of the oxygen reduction cathode  410  and the metal fuel electrode  440 , where it directs the electrolyte solution  420  back to the metal fuel electrode  440 , passing through the oxygen evolution anode. In some embodiments, the reversible flow generator  460  may be disposed between the electrolyte path  450  and the metal fuel electrode  440 , or within the electrolyte path  450 . 
     While the illustrated embodiment is operating in discharge mode, as described above, the oxygen evolution anode  430  is inoperative. The oxygen evolution anode  430 , which can also be described as a charging electrode, is depicted as a separate electrode in the illustrated embodiment because generally most electrodes suitable for function as an oxygen reduction cathode do not perform well as an anode when being used for charging purposes. The electrochemical cell is not intended to be limited, however, and it is possible for the oxygen evolution anode  430  and the oxygen reduction cathode  410  to be the same electrode, with the flow of the electrolyte solution  420  modified accordingly. In embodiments where the oxygen evolution anode  430  is separate, it should be permeable to the electrolyte solution  420 , thereby allowing for the flow of the electrolyte solution  420  from the electrolyte path  450  to the metal fuel electrode  440  through the oxygen evolution anode  430 . 
     As illustrated in  FIG. 4 , during recharge mode the electrolyte solution  420  must flow opposite to that of discharge mode, from the metal fuel electrode  440  to the oxygen evolution anode  430 . The reversible flow generator  460  may be configured to switch the flow of the electrolyte solution  420  during operation of the reversible flow electrochemical cell  400 , and may include a means for reversing the flow. Flow reversal can be achieved through any suitable construction, including but not limited to a controllable electrolyte gate system, or the use of multiple pumps facing opposite directions of flow, wherein only one is activated to achieve the desired flow direction. 
     In recharge mode, a power supply  310  is connected to the metal fuel electrode  440  and to the oxygen evolution anode  430 , and the circuit to the external load  300  is intercepted. By supplying an electrical current across the metal fuel electrode  440  and the oxygen evolution anode  430 , the metal fuel of the metal fuel electrode  440 , which was converted from a metal to an oxidized fowl, including but not limited to spent metal fuel oxide, metal fuel hydroxide, and metal fuel hydroxide anions, during discharge mode, is reduced from the oxidized form back to the metal fuel. At the same time, the oxygen evolution anode  430  oxidizes hydroxide ions generated during the reduction of the oxidized metal fuel, and are present in the electrolyte solution  420 . Oxidation of the hydroxide ions evolves oxygen, which travels within the electrolyte solution  420 , before entering the evolved oxygen recovery system  200  as described above. 
     The foregoing illustrated embodiments have been provided solely for illustrating the structural and functional principles of the present invention are not intended to be limited. For example, the electrochemical cell in the present invention maybe practiced using different fuels, oxidizers, electrolytes, and or different overall structural configuration or materials. Thus, the present invention is intended to encompass all modifications, substitutions, alternations, and equivalents within the spirit and scope of the following appended claims.