Patent Publication Number: US-2007116996-A1

Title: Regenerative fuel cell/electrolyzer stack

Description:
This application claims the benefit of U.S. Provisional Application No. 60/738,738, filed on Nov. 22, 2005. 
    
    
     BACKGROUND  
      Fuel cells are clean and efficient sources of electricity used today in applications ranging from cell phones to space vehicles. A fuel cell generates electricity by chemically combining two or more reactant substances to produce an electric current and a chemical product. Many fuel cell designs utilize hydrogen (H 2 ) and oxygen (O 2 ) as reactant substances, with water (H 2 O) as the primary product.  
      Fuel cells are also sometimes used in conjunction with electrolyzers to form regenerative fuel cell systems. In a reaction that is the reverse of such fuel cell reactions, electrolyzers split water into hydrogen and oxygen when an electric current is provided. Regenerative fuel cell systems have the functionality of both fuel cells and electrolyzers, and may produce power or reactants in different operational modes. In an electrolyzer mode, regenerative systems act as an electrolyzers, utilizing an external source of power, such as a power grid or solar cell to split water into hydrogen and oxygen. In a fuel cell mode, the regenerative system acts as a fuel cell, recombining the hydrogen and oxygen generated in the electrolyzer mode to generate electricity.  
      Existing designs for regenerative fuel cell systems have significant disadvantages in size and efficiency. For example, one regenerative fuel cell design requires that the same cells be used for both electrolysis and fuel cell reactions. Because the cells must operate in both electrolyzer and fuel cell modes, it is difficult to optimize them for both. As a result, the overall efficiency of such a regenerative system suffers compared to stand alone electrolyzer and fuel cells. Another common regenerative system design includes a first stack of cells used exclusively for fuel cell operation, and a second stack of cells used exclusively for electrolysis. Although this design allows optimization for the different cell types and provides greater efficiency compared to single cell set designs, such a design adds significant size and bulk to the system as two stacks of cells must be included.  
     SUMMARY  
      According to one general aspect, the present invention is directed to a regenerative fuel cell/electrolyzer stack. The regenerative fuel cell/electrolyzer stack may comprise, according to various embodiments, a fuel cell electrode assembly comprising first and second fuel cell electrodes, as well as a fuel cell electrolyte. The regenerative fuel cell/electrolyzer stack may also comprise an electrolyzer electrode assembly, with the electrolyzer electrode assembly comprising first and second electrolyzer electrodes. A conductive plate may be positioned between the fuel cell electrode assembly and the electrolyzer electrode assembly. The conductive plate may comprise a first surface facing the first fuel cell electrode and a second surface facing the first electrolyzer electrode. The first surface may comprise at least one flow path open to the first fuel cell electrode, and the second surface may comprise at least one flow path open to the first electrolyzer electrode.  
      According to another general aspect, the present invention is directed to a method of operating a regenerative fuel cell/electrolyzer stack. The regenerative fuel cell/electrolyzer stack may comprise a fuel cell electrode assembly. The fuel cell electrode assembly may comprise a fuel cell cathode, a fuel cell anode and a fuel cell electrolyte. The regenerative fuel cell/electrolyzer stack may also comprise an electrolyzer electrode assembly. The electrolyzer electrode assembly may comprise an electrolyzer cathode and an electrolyzer anode. A first conductive plate may be positioned between the fuel cell electrode assembly and the electrolyzer electrode assembly. A second conductive plate may be positioned opposite the fuel cell electrode assembly from the first conductive plate. Also, a third conductive plate may be positioned opposite the electrolyzer electrode assembly from the first conductive plate. The method may comprise, according to various embodiments, the step of providing an electrical connection between the first and third conductive plates. The method may also comprise the steps of providing a hydrogen-containing substance to the fuel cell anode via a fuel cell anode flow path in the first conductive plate, and providing an oxygen-containing substance to the fuel cell cathode via a fuel cell cathode flow path in the second conductive plate. In various embodiments, the method may also comprise the step of providing a coolant via a second flow path in the first conductive plate and a first flow path in the third conductive plate.  
      According to yet another general aspect, the present invention is directed to a regenerative fuel cell system. The system may comprise a plurality of fuel cell electrode assemblies, a plurality of electrolyzer electrode assemblies and a plurality of conductive plates. The plurality of electrolyzer electrode assemblies may be positioned such that at least a portion of the electrolyzer electrode assemblies and at least a portion of the fuel cell electrode assemblies are interleaved. Also, the plurality of conductive plates may be positioned between one of the plurality of fuel cell electrode assemblies and one of the plurality of electrolyzer electrode assemblies. The system may also comprise a switching network comprising a plurality of switches coupled to the plurality of conductive plates and a control circuit in communication with the switching network. The control circuit may be configured for configuring the switching network to electrically short the plurality of conductive plates across the plurality of electrolyzer electrode assemblies when the system is in a fuel cell mode. The control circuit may also be configured for configuring the switching network to electrically short the plurality of conductive plates across the plurality of fuel cell electrode assemblies when the system is in an electrolyzer mode. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIGS. 1 and 2  are exploded block diagrams of regenerative fuel cell/electrolyzer stacks according to various embodiments of the present invention;  
       FIG. 3  is a flow chart showing a process flow for operating a regenerative fuel cell/electrolyzer stack in an electrolyzer mode;  
       FIG. 4  is a flow chart showing a process flow for operating a regenerative fuel cell/electrolyzer stack in a fuel cell mode;  
       FIG. 5  is an exploded three dimensional view of a regenerative fuel cell/electrolyzer stack according to various embodiments of the present invention;  
       FIG. 6  is an exploded three dimensional view of a portion of a regenerative fuel cell/electrolyzer stack according to various embodiments of the present invention;  
       FIG. 7  is an exploded three dimensional view of a portion of a regenerative fuel cell/electrolyzer stack according to various embodiments of the present invention; and  
       FIG. 8  is a block diagram of a regenerative fuel cell/electrolyzer system according to various embodiments of the present invention.  
    
    
     DESCRIPTION  
      As used herein and unless otherwise noted, the term “electrolyte” refers to any substance or material that is a conductor of ions. As used herein, the term “ionomer” refers to an electrolyte that includes a polymer.  
      Referring to the figures,  FIG. 1  shows an exploded block diagram of a regenerative fuel cell/electrolyzer stack  100  having a fuel cell mode and an electrolyzer mode according to various embodiments of the present invention. The stack  100  includes electrolyzer electrode assemblies  108  and fuel cell electrode assemblies  110  with common conductive plates  102 ,  104  positioned between therebetween. According to various non-limiting embodiments, the fuel cell electrode assemblies  110  and electrolyzer electrode assemblies  108  are interleaved. The conductive plates  102 ,  104  in various embodiments may, but need not be bi-polar, and may route reactants, products, coolants, conditioners and/or other substances to and/or from the electrode assemblies  108 ,  110 . In one non-limiting embodiment, each conductive plate  102 ,  104  may route reactants and/or products to and/or from one electrolyzer electrode assembly  108  and one fuel cell electrode assembly  110 .  
      The stack  100  may also include switch units  120  and  122  positioned to short conductive plates  102  and  104  to one another to configure the stack  100  for operation in fuel cell and electrolyzer modes. The switch units  120 ,  122  may be configured according to any suitable switching technology. In various non-limiting embodiments, the switch units  120 ,  122  may be implemented as a solid state circuit, for example, including one or more transistors. For example, the switch units  120 ,  122  may include a semi-conductor switching material that is part of the electrode assemblies  108 ,  110 . Adjacent plates  102 ,  104  may be in physical contact with one another through semi-conductor switching contacts. This may allow switching to take place through the plane of the plates  102 ,  104  rather than in the plane of the plates  102 ,  104 . In still other non-limiting embodiments, the switch units  120 ,  122  may include mechanical switches actuated manually or automatically, for example, by one or more solenoids.  
      The stack  100  may be configured to operate in a fuel cell or electrolyzer mode by shorting conductive plates  102 ,  104  across the set of electrode assemblies  108 , 110  that are not needed for the selected mode. For example, the stack  100  may be configured to operate in a fuel cell mode by closing switch units  120 , which short conductive plates  102 ,  104  across electrolyzer electrode assemblies  108 , rendering them electrically inactive. Conversely, the stack  100  may be configured to operate in an electrolyzer mode by closing switch units  122 , which short conductive plates  102 ,  104  across fuel cell electrode assemblies  110 , rendering them electrically inactive.  
      By utilizing common conductive plates  102 ,  104  for both fuel cell and electrolyzer operation, the stack  100  may avoid the weight and bulk problems associated with having distinct fuel cell and electrolyzer cells. At the same time, having separate fuel cell electrode assemblies  110  and electrolyzer electrode assemblies  108  may allow each of the assemblies  108 ,  110  to be optimized for its respective operation. It will be appreciated that the total number of conductive plates  102 ,  104 , fuel cell electrode assemblies  110  and electrolyzer electrode assemblies  108  included in the stack  100  may vary depending on the power storage and output requirements of the particular application. Also, various embodiments may include unequal numbers of electrolyzer electrode assemblies  108  and fuel cell electrode assemblies  110 .  
       FIG. 2  shows a block diagram of a portion  106  of the regenerative fuel cell/electrolyzer stack  100  according to various embodiments showing one electrolyzer electrode assembly  108 , one fuel cell electrode assembly  110  and surrounding conductive plates  102 ,  104 . The fuel cell electrode assembly  110  may include a fuel cell cathode  208 , a fuel cell anode  212  and a fuel cell electrolyte  210 . When the stack  100  is operated in a fuel cell mode, the fuel cell electrode assembly  110  may drive a load resistance  121  as described in more detail below. It will be appreciated that the fuel cell electrode assembly  110  may be configured according to any suitable fuel cell type including, for example, a proton exchange membrane (PEM) or polymer electrolyte fuel cell, an alkaline fuel cell, a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, etc. The electrolyzer electrode assembly  108  may be configured to match the fuel cell electrode assembly  110 .  
      The fuel cell cathode  208  and anode  212  may be made from any suitable suitable material including, for example, porous plates made of metal or another conductive material. The fuel cell electrolyte  210  may be any electrolyte suitable for use in a fuel cell application, and may be determined based on the type of fuel cell technology being implemented in a particular application. For example, in applications where the fuel cell electrode assembly  110  is configured according to PEM fuel cell technology, the fuel cell electrolyte  210  may be any suitable ionomer including, for example, a fluorinated sulfonic acid copolymer, such as the NAFION product available from DU PONT. In various non-limiting embodiments, the electrolyte  210  may be solid or liquid and in various embodiments, may be retained in a porous matrix material.  
      It will be appreciated that the fuel cell electrode assembly  110  may include various other components (not shown). For example, various gas diffusion media may manage the flow of hydrogen and oxygen at the electrodes  208 ,  212 . One or more catalysts such as, for example, platinum, may be present on the surface of the electrodes  208 ,  212  to promote the fuel cell reaction. In one non-limiting embodiment, the electrodes  208 ,  212  may be made of platinum. Also, various seals, compression limiters, frames and other components may manage compression, thermal and electrical factors within the fuel cell electrode assembly  110 .  
      The electrolyzer electrode assembly  108  may include an electrolyzer anode  202 , an electrolyzer cathode  206  and an electrolyte  204 . The electrolyzer anode  202  and cathode  206  may be made from any suitable conductive material including, for example, porous plates made of metal or other conductive materials. The electrolyte  204  may be any kind of electrolyte suitable for electrolyzer operation, such as an alkaline material or acid. The electrolyte  204  may be solid or liquid, and in various embodiments, may be retained in a porous matrix material. In one non-limiting embodiment, the electrolyte  204  may include a solid ionomer, or proton exchange membrane (PEM), forming a PEM type electrolyzer cell. The ionomer may be a fluorinated sulfonic acid copolymer such as, for example, the NAFION brand product available from DU PONT. When the stack  100  is operated in an electrolyzer mode, the electrolyzer electrode assembly  108  may be biased by a power supply  119  as described in more detail below. Like the fuel cell electrode assembly  110 , the electrolyte electrode assembly  108  may include other components (not shown) including, for example, catalysts, gas diffusion media, seals, compression limiters, frames, etc.  
      Conductive plates  102 ,  104  may be constructed from any suitable electrically conductive material including, for example, carbon, graphite, any suitable metal, etc. It will be appreciated that the material of the conductive plates  102 ,  104  may be determined by the type of fuel cell and electrolyzer cells used. For example, when solid oxide cells are used, the conductive plates  102 ,  104  may not be made of metal. The conductive plates  102 ,  104  may perform various tasks within the stack  100  including, for example, managing the flow of reactants and products to and from the electrode assemblies  108 ,  110 . Accordingly, the plates  102 ,  104  may include one or more flow paths  112 ,  114 ,  116 ,  118  for directing substances towards and away from the electrode assemblies  108 ,  110  including, for example, reactants, products, coolants, conditioners, etc. For example, conductive plates  102  may include an electrolyzer anode flow path  112  open to an electrolyzer anode  202  on a first major surface and a fuel cell anode flow path  118  open to a fuel cell anode  212  on a second major surface. Conductive plates  104  may include an electrolyzer cathode flow path  114  open to an electrolyzer cathode  206  on a first major surface and a fuel cell cathode flow path  116  open to a fuel cell cathode  208  on a second major surface. In various embodiments, and in various mode, reactants, products, coolants, conditioners, etc. may be directed in either direction through flow paths  112 ,  114 ,  116 ,  118 .  
      The various flow paths  112 ,  114 ,  116 ,  118  may take any suitable form. For example, flow paths  112 ,  114 ,  116 ,  118  may take the form of channels or grooves on the surface of the respective conductive plates  102 ,  104  or of the porous diffusion media or electrode. Grooves of the various flow paths  112 ,  114 ,  116 ,  118  may be fed by ducts (not shown in  FIG. 2 ) as described below with reference to  FIG. 6-7 . It will be appreciated that flow paths  112 ,  114 ,  116 ,  118  may be optimized for use with the electrolyzer electrode assembly  108  or the fuel cell electrode assembly. For example, electrolyzer anode and cathode flow paths  112 ,  114  may be made from a hydrophilic material, as it is preferable to maintain adequate hydration of the electrolyzer electrode assembly  108 . In contrast, fuel cell anode and cathode flow paths  118 ,  116  may include a hydrophobic material, as it is preferable to remove water from the fuel cell electrode assembly  110 .  
       FIG. 3  is a flow chart illustrating a process flow  300  for operating the fuel cell/electrolyzer stack  100  in an electrolyzer mode according to various embodiments. At step  302 , the fuel cell electrode assembly  110  may be conditioned for operation in an electrolysis mode. This may involve purging fuel cell reactants and products from the fuel cell anode flow path  118  and the fuel cell cathode flow path  116 . In another non-limiting embodiment, conditioning species may be introduced to the fuel cell electrode assembly  110  via the fuel cell anode and cathode flow paths  118 ,  116 . Exemplary conditioning species include nitrogen, inert gases, reactants, etc.  
      At step  304 , conductive plates  102 ,  104  may be shorted across the fuel cell electrode assembly  110 , for example, by closing switch unit  122 . In various embodiments, the load resistance  121  may also be disconnected. At step  306 , water may be provided at the electrolyzer anode flow path  112 . Because the electrolyzer anode flow path  112  is open to the electrolyzer anode  202 , water provided at the flow path  112  may come into contact with the electrolyzer anode  202 . The water may encounter various intermediate components before reaching the electrolyzer anode  202 , including, for example, diffusion media, catalyst layers, etc. In one non-limiting embodiment, water may also be provided at the electrolyzer cathode flow path  114 . Because the electrolyzer cathode flow path  114  is open to the electrolyzer cathode  206 , the water provided at the flow path  114  may come into contact with the electrolyzer cathode  206 .  
      At step  308 , an electric current may be provided between the electrolyzer anode  202  and the electrolyzer cathode  206 . The current may be generated, for example, by power supply  119  (see  FIG. 2 ). The power supply  119  may be any kind of apparatus for generating an electric current. For example, the power supply  119  may be a power grid, a solar cell, a wind or water turbine, a generator, etc. Although the power supply  119  is shown connected to only one electrolyzer electrode assembly  108 , it will be appreciated that the power supply  119  may be connected to multiple electrolyzer electrode assemblies  108  included in the stack  100 . In various non-limiting embodiments, the electrolyzer electrode assemblies  108  may be arranged in series or in parallel relative to one another, or any combination thereof  
      In response to the electric current, water provided to the electrolyzer anode  202  and or cathode  206  via the electrolyzer anode flow path  112  is split into hydrogen and oxygen. When PEM cells are used, the oxygen may continue to flow through the electrolyzer anode flow path  112  where it is collected at step  310 . The hydrogen may be transported across the electrolyzer electrolyte  204  to the electrolyzer cathode  206  where it may be collected via the electrolyzer cathode flow path  114  at step  312 . It will be appreciated that the steps of the process flow  300  may be performed in any suitable order or simultaneously.  
       FIG. 4  is a flow chart illustrating a process flow  400  for operating the stack  100  in a fuel cell mode according to various embodiments. At step  402 , the electrolyzer anode  202  and electrolyzer cathode  206  may be conditioned for operation in the fuel cell mode. This may involve purging reactants and products from the electrolyzer anode flow path  112  and electrolyzer cathode flow path  114 .  
      At step  404 , the conductive plates  102 ,  104  may be shorted across the electrolyzer electrode assembly  108 , for example, by closing switch unit  120 . In various non-limiting embodiments, the power supply  119  may also be disconnected. At step  406 , a hydrogen containing substance may be provided at fuel cell anode flow path  118 . The hydrogen containing substance may be any substance, compound, or solution including hydrogen such as, for example, hydrogen gas, a hydrogen rich gas, natural gas, etc. Because the fuel cell anode flow path  118  is open to the fuel cell anode  212 , the hydrogen containing substance may come into contact with the fuel cell anode  212 . In various embodiments the hydrogen containing substance may encounter one or more intermediate components between the fuel cell anode flow path  118  and the fuel cell anode  212  including, for example, gas diffusion media, catalyst layers, etc.  
      At step  408 , an oxygen containing substance may be provided at the fuel cell cathode flow path  116 . The oxygen containing substance may be any substance, compound or solution including oxygen, such as, for example, oxygen gas, an oxygen rich gas, air, etc. Because the fuel cell cathode flow path  116  is open to the fuel cell cathode  208 , the oxygen containing substance may come into contact with the fuel cell cathode  208 . In various embodiments, the oxygen containing substance may, like the hydrogen containing substance, encounter one or more intermediate components between the fuel cell cathode flow path  114  and the fuel cell cathode  208  including gas diffusion media, catalyst layers, etc.  
      When the hydrogen containing substance is provided to the fuel cell anode  212  and the oxygen containing substance is provided to the fuel cell cathode  208 , hydrogen and oxygen present may chemically combine in a fuel cell reaction producing electric current, water, and heat. Electric current may be generated between the fuel cell cathode  208  and anode  212 , and may drive load resistance  121  (see  FIG. 2 ). The load resistance  121  may represent any device or system commonly powered by electricity including, a motor, a light, a computer, etc. In will be appreciated that in various embodiments, the load resistance  121  may be driven by a plurality of fuel cell electrode assemblies  110  arranged in series or parallel.  
      Water may be generated at the fuel cell cathode  208  and transported away from the cathode  208  along the fuel cell cathode flow path  116 , where it may be collected at step  410 . At least a portion of the heat generated by the fuel cell reaction may be dissipated, for example, into the conductive plates  102 ,  104 . At step  412 , a coolant substance may be provided to the electrolyzer anode flow path  112  and/or the electrolyzer cathode flow path  114  within conductive plates  102 ,  104 . The coolant substance may be circulated to carry heat away from the stack  100 . The coolant substance may be an aqueous solution, air, refrigerant, or any other suitable substance. It will be appreciated that the steps of the process flow  400  may be performed in any suitable order or simultaneously.  
       FIGS. 5-7  show exemplary physical embodiments of conductive plates  102 ,  104  and electrode assemblies  108 ,  110  that may be included in the stack  100  according to various embodiments.  FIG. 5  shows an exploded stack portion  500  including an electrolyzer electrode assembly  108 , a fuel cell electrode assembly  110 , and conductive plates  102  and  104 . An electrolyzer cathode flow path  114  is shown as a series of grooves on a first face  542  of the conductive plate  104 . Accordingly, flow path  114  may be open to the electrolyzer cathode (not shown) present at electrolyzer electrode assembly  108 . The flow path  114  may terminate at a duct section  524  present in the conductive plate  104 . Each of the other components  102 ,  108 ,  110  of the stack portion  500  may include corresponding duct sections  524 . When the components  102 ,  104 ,  108 ,  110  are assembled, the duct sections  524  of each component may align, forming one duct running along the stack portion  500  and allowing outside access to the electrolyzer cathode flow path  114 .  
      A fuel cell anode flow path  118  is shown in  FIG. 5  as a series of grooves located on a face  540  of conductive plate  102 . The flow path  118  may be open to a fuel cell anode (not shown) present at the fuel cell electrode assembly  110 . Also, the flow path  118  may terminate at duct section  532 . Corresponding duct sections  532  may be present on all of the shown components  102 ,  104 ,  108 ,  110  and may form a duct when the stack portion  500  is assembled, similar to duct sections  524  as described above.  
       FIG. 5  also shows various posts  534 ,  536 ,  538  present on components  108 ,  104  and  102 . Posts  534 , shown in the electrolyzer electrode assembly  108 , may be connected to the anode and cathode (not shown) of the electrolyzer electrode assembly  108 , and may be used to connect a power supply to the electrolyzer electrode assembly  108 . Posts  536  and  538  may be used to short respective conductive plates  104  and  102  to surrounding conductive plates depending on the stack portion&#39;s mode of operation. For example, when the stack portion  500  is configured to operate in a fuel cell mode, plate  104  and  102  may be shorted by posts  536  and  538 , thus rendering electrolysis electrode assembly  110  electrically inactive.  
       FIG. 6  shows an exploded diagram of a stack section  600 , according to various embodiments, including conductive plates  102 ,  104  and electrolyzer electrode assembly  108  including anode  202 , cathode  206  and electrolyte  204 . Conductive plate  102  may include shorting posts  648  and  652 . Shorting posts  648  and  652  may be used to short conductive plate  102  to adjacent conductive plates during the operation of the stack. For example, shorting posts  648 ,  652  may form a portion of switch units  120 ,  122 .  
      A first face  601  of conductive plate  102  may include an electrolyzer anode flow path  112 . The electrolyzer anode flow path  112  is shown as a groove cut in the face  601 . Accordingly, the electrolyzer anode flow path  112  may be open to the electrolyzer anode  202 . Electrolyzer anode flow path  112  is shown terminating at duct sections  620  and  622 . Other electrolyzer anode flow paths (not shown) on other conductive plates (not shown) within the stack may also terminate at duct sections  620  and  622  located in the other conductive plates. Corresponding duct sections  620  and  622  may be included in all of the components of the stack section  600 . When the stack section  600  is assembled, duct sections  620  and  622  may form input/output ducts for all electroyzer anode flow paths in conductive plates within the stack. The conductive plate  102  may also include input/output duct sections  624  and  626  for the electrolyzer cathode flow path  112  as well as duct sections  628 ,  630 ,  632  and  634 , serving as input and outputs for other flow paths discussed in more detail below.  
      A first face  605  of conductive plate  104  is also shown in  FIG. 6 . The first face  605  includes electrolyzer cathode flow path  114 , which may be a groove in the face  605  open to electrolyzer.cathode  206  as shown. The electrolyzer cathode flow path  114  may terminate at duct sections  624  and  626 . In addition, the first face  605  of the conductive plate  104  may include duct sections  620 ,  622 ,  628 ,  630 ,  632  and  634 . Shorting posts  650  and  654  may be used to short conductive plate  104  to adjacent conductive plates during operation of the stack.  
       FIG. 7  shows an exploded diagram of a stack portion  611  including embodiments of conductive plates  102 ,  104  as well as fuel cell electrode assembly  110  including cathode  208 , anode  212  and electrolyte  210 . A second face  607  of conductive plate  104  is shown including fuel cell cathode flow path  116  shown as a groove in the face  607  open to the fuel cell cathode  208 . Fuel cell cathode flow path  116  may terminate at duct sections  628  and  630 . A second face  603  of conductive plate  102  is also shown including a fuel cell anode flow path  118  shown as a groove open to the fuel cell anode  212 . The fuel cell anode flow path  118  may terminate at duct section  632  and  634 .  
       FIG. 8  shows a system  800  for operating the regenerative fuel cell/electrolyzer stack  100  according to various embodiments of the invention. It will be appreciated that the system  800  is but one embodiment of a system utilizing regenerative fuel cell/electrolyzer stacks according to various embodiments of the present invention. Other systems may exclude some of the components shown with system  800 , or include additional components.  
      The system  800  may include a cell stack  100 , a valve assembly  804 , reactant/product storage  808 ,  810 ,  812  and control circuitry  806 . The control circuitry may include any kind of control devices known in the art, including, for example, logic circuitry, a computer system, etc. Control circuitry  806  may operate the valve assembly  804  to provide reactants and collect products from the cell stack  100  during fuel cell and electrolyzer operation, for example, according to the process flows  300 ,  400  described above.  
      The control circuitry  806  may also configure the cell stack  100  for operation alternatively in a fuel cell mode and an electrolyzer mode. When the cell stack  100  is operated in fuel cell mode, the control circuitry  806  may configure the cell stack  100  for fuel cell operation, for example, by shorting common conductive plates across the electrolyzer electrode assemblies. The control circuitry  806  may also configure the valve assembly  804  to provide hydrogen containing substance and oxygen containing substance to the stack  100  from hydrogen storage  810  and oxygen storage  812 , respectively. The valve assembly  804  may be further configured to remove water to water storage  808 . When the system  800  is operated in electrolyzer mode, the control circuitry  806  may configure the cell stack  100  for electrolyzer operation, for example, by shorting common conductive plates across the fuel cell electrode assemblies. The control circuit  806  may also configure the valve assembly  804  to provide water to the stack  100  and remove the products, hydrogen and oxygen.  
      While several embodiments of the invention have been described, it should be apparent that various modifications, alterations and adaptations to those embodiments may occur to persons skilled in the art with the attainment of some or all of the advantages of the present invention. For example, although portions of the disclosure describe elements specific to PEM fuel cell and electrolyzer configurations, it will be appreciated that stacks according to various embodiments may utilize other fuel cell and electrolyzer configurations using other fuels, ions, electrolytes, etc. The present disclosure, therefore, is intended to cover all such modifications, alterations and adaptations without departing from the scope and spirit of the present invention as defined by the appended claims.