Patent Publication Number: US-2009220828-A1

Title: System and method for fuel cell start up

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Fuel cells may be used to supply power in a wide variety of applications. Exemplary transportation applications include hybrid electric vehicles (HEV), electric vehicles (EV), Heavy Duty Vehicles (HDV) and Vehicles with 42-volt electrical systems. Exemplary stationary applications include backup power for telecommunications systems, uninterruptible power supplies (UPS), and distributed power generation applications. 
     Electrochemical fuel cells convert reactants, namely a fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. 
     2. Description of the Related Art 
     One type of electrochemical fuel cell is the proton exchange membrane (PEM) fuel cell. PEM fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two electrodes. 
     In a fuel cell, an MEA is typically interposed between two electrically conductive separator or fluid flow field plates that are substantially impermeable to the reactant fluid streams. The separator plates act as current collectors and may provide mechanical support for the MEA. In addition, the separator plates have channels, trenches, or the like formed therein which serve as paths to provide access for the fuel and the oxidant fluid streams to the anode and the cathode, respectively. Also, the fluid paths provide for the removal of reaction byproducts and depleted gases formed during operation of the fuel cell. 
     In a fuel cell stack, a plurality of fuel cells are connected together, typically in series but sometimes in parallel or a combination of series and parallel, to increase the overall output power of the fuel cell system. In such an arrangement, one side of a given separator plate may be referred to as an anode separator plate for one cell and the other side of the plate may be referred to as the cathode separator plate for the adjacent cell. 
     When a fuel cell has been shut down for a long period of time, the gas composition present at the cathode and anode flow fields of the fuel cell typically consists mainly of air. This may be due for example to air crossover through the membrane as well as air leaks in the seals and valves of the fuel cell system. On starting up such a fuel cell, adding hydrogen fuel to the anode electrode results in a wavefront as the air present at the anode is displaced by the hydrogen. This wavefront causes the cathode potential downstream of the wavefront to rise to a value that may contribute to corrosion of the cathode electrode. 
     Various solutions have been proposed to mitigate the above described problem. Some solutions have proposed purging the flow fields with inert gasses during the shut down operation of the fuel cell, or drawing an electrical load from the fuel cell during startup of the fuel cell to limit the cathode potential. These approaches to dealing with the described problem often give rise to substantially increased complexity and cost of the fuel cell system which is undesirable. US-2002-0076582-A1 proposes using an extremely rapid purging of the anode flow field upon start up with a hydrogen reducing fluid fuel so that air is purged from the anode flow field in no more than 1 second, or as quickly as no more than 0.05 seconds. This solution may reduce the corrosion effects, but has not proved effective in eliminating them. 
     U.S. Pat. No. 6,838,199-B2 proposes a method for starting up a fuel cell including the steps of: purging the cathode flow field with the reducing fluid fuel; then, directing the reducing fluid fuel to flow through the anode flow field; next, terminating flow of the fuel through the cathode flow field and directing an oxygen containing oxidant to flow through the cathode flow field; and connecting a primary load to the fuel cell so that electrical current flows from the fuel cell to the electrical load. This method merely shifts the corrosion problem from the cathode of the fuel cell to the anode of the fuel cell. 
     Solutions to eliminate or further minimize electrode corrosion upon startup of a fuel-cell are therefore desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, a method for starting operation of a fuel cell system comprises supplying a fuel to both the anode electrode and the cathode electrode of the fuel cell system at substantially the same time during a first stage in the startup process, ceasing the supply of the fuel to the cathode electrode during a second stage in the startup process, and supplying an oxidant to the cathode electrode during a third stage in the startup process. 
     In another embodiment, a fuel cell system comprises an anode electrode with an adjacent anode flow field, a cathode electrode with an adjacent cathode flow field, a fuel supply device coupled to the anode flow field and coupleable to the cathode flow field, and a controller configured to control the fuel supply device to supply both the anode flow field and the cathode flow field with a fuel at substantially the same time during a start up of the fuel cell system. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
         FIG. 1  is a schematic diagram illustrating an embodiment of the present invention comprising a valve coupled to both the anode inlet and the cathode inlet of a fuel cell. 
         FIG. 2  is a schematic diagram illustrating an embodiment showing a container coupled to both the anode inlet and the cathode inlet of a fuel cell. 
         FIG. 3  is a schematic diagram illustrating an embodiment using a recirculation system coupled to both the anode and the cathode of a fuel cell. 
         FIG. 4  is a schematic diagram illustrating an embodiment using both a fuel recirculation system and an oxidant recirculation system coupled to the fuel cell. 
         FIG. 5  is a schematic diagram showing a typical fuel distribution through a fuel cell stack during startup. 
         FIG. 6  is a schematic diagram illustrating a possible hydrogen-air wavefront arising from implementation of an embodiment of the present invention. 
         FIG. 7  is a schematic diagram illustrating possible effects of introducing an oxidant into the cathode of a fuel cell after supplying fuel containing fluid into the cathode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description and enclosed drawings, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. One skilled in the art will understand, however, that the invention may be practiced without all of these details. In other instances, well-known structures associated with fuel cell systems have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open sense, that is as “including, but not limited to.” 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Further more, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention. 
       FIG. 1  illustrates a fuel cell system  100  according to one embodiment. A fuel cell  102  comprises an ion-exchange membrane  108  disposed between a cathode electrode  104  and an anode electrode  106 . The assembly comprising the membrane  108 , and the electrodes  104 ,  106  is referred to as a membrane electrode assembly (MEA)  110 . Cathode flow field  112  and anode flow field  114 , adjacent to the cathode electrode  104  and the anode electrode  106  respectively, allow an oxidant and a fuel or reactant to come into fluid contact with the electrodes  104 ,  106 . The flow fields  112 ,  114  may comprise channels, trenches, or the like formed within separator plates (not shown) as described above. 
     The fuel cell system  100  further comprises cathode inlet  116  and anode inlet  118  to enable the introduction of the oxidant and fuel streams into the cathode flow field  112  and the anode flow field  114  respectively. Cathode outlet  120  and anode outlet  122  provide for the removal of reaction byproducts and depleted fluids formed during operation of the fuel cell. 
     Valves  134 ,  136  are coupled to the outlets  120 ,  122  to either regulate the pressure of the fluids within the fuel cell  102 , or as purge valves, to expel the reaction byproducts and depleted fluids formed during operation of the fuel cell  102  from the fuel cell  102 . 
     At a stage in the start up operation of the fuel cell system  100 , a controller  127  operates valves  128 ,  130 , and  132  in concert to supply fuel from the fuel source  126  to both the cathode electrode  104  and the anode electrode  106  at substantially the same time. For the purposes of this invention, this is defined as the first stage in the start up operation of the fuel cell system. It should however be noted that the controller  127  may take other actions during the start up of the fuel cell system either before, after, in-between, or simultaneously with the stages described in this disclosure. These actions may for example comprise: purging the electrodes  104 ,  106  with a passivating fluid, connecting an electrical load to the fuel cell, circulating cooling fluid through the fuel cell, and operating heaters, among other actions. While such activities are not described in detail, these and other start up actions are well known and persons of ordinary skill in the art can readily select suitable start up actions for a given application. The controller  127  may employ information (arrows pointing toward controller  127 ) received from sensors and monitors, and may provide control signals (arrows pointing away from controller  127 ) to various valves, switches, actuators, solenoids, relays, contactors, motors, pumps, fans, blowers, compressors and other equipment. 
     The timing of the actuation of the valves  128 ,  130 , and  132  may depend on factors such as the relative volumes of the cathode flow field  112  and the anode flow field  114 , as well as the volume of the piping leading to the inlets  116 ,  118 , among other factors. Calculating the actual timing and sequence of the valve operations is well within the abilities of an individual of ordinary skill in the art using well established principles. 
     For example, assuming the volume of the cathode flow field  112  is equal to the volume of the anode flow field  114 , and assuming the valves  130 ,  132  are placed close enough to the cathode inlet  116  and the anode inlet  118  that the volume of the piping between the valves  130 ,  132  and the inlets  116 ,  118  is negligible, operating valve  128  first, and then operating valves  130  and  132  at substantially the same time would supply the fuel to both the cathode electrode  104  and the anode electrode  106  at substantially the same time. 
     The presence of a fuel on a cathode electrode and an anode electrode at substantially the same time should provide symmetrical conditions at the cathode electrode and the anode electrode, which in turn should avoid the creation of a high potential region, which in turn contributes to the minimization or elimination of the corrosion problem previously mentioned. This is described in more detail below. 
     At some period after the fuel has been introduced into the cathode flow field  112 , the controller  127  halts the supply of the fuel to the cathode flow field  112 . This is defined as the second stage of the start up of the fuel cell system  100 . This may be accomplished by, for example, closing valve  130 . This period may be predefined, or may be calculated or otherwise determined by the controller  127  during operation of the fuel cell system. 
     In some embodiments, once the hydrogen-air wavefronts have been eliminated from the anode flow field  114  by the passage of the fuel through the anode flow field  114  (i.e., the air has been substantially expelled from the flow field, or has been thoroughly mixed in to the fuel gas so that a wavefront is no longer present), an oxidant may be supplied to the cathode flow field  112 . This is defined as the third stage of the start up of the fuel cell system  100 . It should be appreciated that the amount of time required to eliminate the hydrogen-air front on the anode electrode  106  may be calculated for a given fuel cell system, and therefore the various components described may be actuated for a pre-determined period of time. 
     Oxidant is provided to the cathode electrode  104  of the fuel cell  102  by an oxidant source  124 . In some embodiments the oxidant source  124  may comprise a storage device such as oxygen tanks. In other embodiments the oxidant source  124  may comprise an active device such as an air compressor or an air blower, among others. In some embodiments the oxidant source  124  may further comprise various other components such as filters, two-way valves and/or check valves. In some embodiments the oxidant source  124  may include means to prevent the fuel from escaping to atmosphere or from contaminating the oxidant source  124  during the first stage of the start up of the fuel cell  102 . For example, in embodiments where a compressor is used to supply oxidant to the fuel cell  102 , the compressor might be operated at a low speed to inhibit the fuel from traveling towards the oxidant source  124 , or to dilute the fuel entering the fuel cell  102 . 
     Once the fuel is present at the anode electrode  106 , and an oxidant is present at the cathode electrode  104 , the fuel cell  102  may be ready to supply power to an external load (not shown), and the start up procedure is complete. In some embodiments it may be desirable to connect an electrical load (not shown) to the fuel cell  102  during some or all of the above described stages to further minimize the corrosion, or to produce more rapid heating of the fuel cell  102 . 
       FIG. 2  shows an embodiment of a fuel cell system  200  including an accumulator  240 . In this embodiment, the controller  127  first operates valves  228  and  242  to fill the accumulator  240  with a fuel supplied by the fuel source  226 . Once sufficient fuel is accumulated in the accumulator  240 , valve  242  is closed. On starting up the fuel cell  202 , the controller  127  operates valves  230  and  232  to supply the fuel to both the cathode electrode  204  and to the anode electrode  206  at substantially the same time. The remainder of the start up operations may then duplicate the operations described above. 
     In some embodiments it may be desirable to supply the cathode electrode with a known volume of fuel during the start up process. For example, to prevent the exhaust of fuel from the fuel cell  202  to the atmosphere it may be desirable to cease the supply of the fuel to the cathode electrode  204  before the fuel completely fills the cathode flow field  212 . Using an accumulator  240  as shown, can therefore be useful to supply a known quantity of fuel to the cathode electrode  204 . 
     In some embodiments valve  244  may be used to isolate the oxidant source  224  from the cathode flow field  212  during some stages of the start up process, in order to prevent the fuel from contaminating the oxidant source  224  during the startup process. 
       FIG. 3  illustrates another embodiment of the present invention. As shown in  FIG. 3 , a recirculation system  350  may be used to supply fuel to both the cathode electrode  304  and the anode electrode  306  at substantially the same time. The recirculation system  350  comprises a recirculation pump  352  to circulate fluids through the anode flow field  314  during normal operation. Alternatively, other devices may be used to achieve the same objectives as the recirculation pump  352  shown in  FIG. 3 . For example, in some embodiments the recirculation pump  352  may be replaced by a blower, a jet pump, a combination of these devices, or other suitable devices. 
     On start up, the controller  127  operates valve  328  to supply fuel from the fuel source  326  to the recirculation pump  352 . The controller  127  then operates recirculation pump  352 , and valves  354  and  356  to supply the fuel to both the cathode electrode  304  and the anode electrode  306  at substantially the same time. Three-way valve  356  is operated to direct the fluid exhausted from the cathode outlet  320  into the recirculation system  350  to be circulated through both the cathode flow field  312  and the anode flow field  314 . In some embodiments it may be desirable to begin circulating the fluid already in the flow fields  312 ,  314  before operating valve  328  to supply the fuel to the system. 
     In some embodiments, valve  328  may be operated to only supply a limited amount of the fuel to the recirculation pump  352 . For example, valve  328  may be operated to supply an amount of fuel to the recirculation pump  352  such that the concentration of hydrogen in the air present in flow fields  312 ,  314  remains below a threshold value (for example below a flammable limit of 4% hydrogen in air). 
     Similar to the examples above, once sufficient fuel has been introduced into the flow fields  312 ,  314 , the valve  354  is operated to fluidly isolate the cathode inlet  316  from the anode inlet  318 . Valve  356  is operated to isolate the cathode outlet  320  from the recirculation system  350 , and may be further operated to exhaust any fluids from the cathode flow field  312  to atmosphere. Valve  344  is then operated to supply an oxidant from the oxidant source  324  to the cathode inlet  316 . 
       FIG. 4  illustrates an embodiment comprising an anode recirculation system  450  and a cathode recirculation system  460 . The anode recirculation system  450  comprises a recirculation pump  452  to circulate fluids through the anode flow field  414  during normal operation, and the cathode recirculation system  460  comprises a blower  464  to circulate fluids through the anode flow field  412  during normal operation. Alternatively, other devices may be used to achieve the same objectives as the recirculation pump  452  and the blower  464  shown in  FIG. 4 . For example, in some embodiments the recirculation pump  452  and/or the blower  464  may be replaced by a blower, a jet pump, a combination of these devices, or other suitable devices. 
     On start up, the controller  127  operates valve  428  to supply a fuel from the fuel source  426  to the valves  432 ,  454 . The controller  127  then operates valves  432 ,  454 , recirculation pump  452 , and blower  464  to supply the fuel to both the cathode electrode  404  and the anode electrode  406  at substantially the same time. 
     In some embodiments it may be desirable to begin circulating the fluid already in the flow fields  412 ,  414  before operating valves  432 ,  454  to supply the fuel to the fuel cell  402 . In some embodiments it may be desirable to operate the recirculation pump  452  and the blower  464  at different speeds to vary the rate of recirculation of the fluids in recirculation systems  450 ,  460 . 
     In some embodiments, valves  432 ,  454  may be operated to only supply a limited amount of the fuel to either or both the anode flow field  414  and the cathode flow field  412 . For example, valves  432 ,  454  may be operated to supply an amount of fuel to the flow fields  412 ,  414  such that the concentration of hydrogen in the air present in flow fields  412 ,  414  remains below a threshold value (for example below a flammable limit of 4% hydrogen in air). 
     Similar to the examples above, once sufficient fuel has been introduced into the flow fields  412 ,  414 , the valve  454  may be operated to fluidly isolate the cathode inlet  416  from the anode inlet  418 . 
     Three-way valve  466  is then operated to supply an oxidant from the oxidant source  424  to the blower  464 . 
     Valves  134 ,  136  are operated to exhaust any fluids from the flow fields  412 ,  414  to atmosphere as required. Valves  134 ,  136  may also be used to regulate the pressures of the fluids in the flow fields  412 ,  414 . 
     Three-way valve  466  may be operated to vary the proportions of fluid recirculated through the cathode recirculation system  460 , and the proportion of fluid introduced into the system from an oxidant source  424 . 
       FIG. 5  illustrates a fuel cell stack  560  comprising a number of fuel cells  502 . The fuel cell stack typically comprises a fuel inlet header  562 , a fuel outlet header  564 , and corresponding oxidant inlet and outlet headers (not shown). The fuel inlet header  562  provides fluid to each of the fuel cells  502 . As fuel is typically introduced from an external source into a single section of the fuel inlet header  562  (for example at  566  on  FIG. 5 ) a fuel distribution such as that shown by dotted line  568  might exist. For example, the fuel cell  502  closest to the fuel introduction point  566  might be 25% filled at the time fuel begins entering the fuel cell  502  furthest from the fuel introduction point  566 . The fuel distribution  568  is largely affected by the design of the headers  562 ,  564  as well as the flow fields within the fuel cell stack  560 . In some embodiments it is therefore desirable to design the fuel headers, the oxidant headers, the flow fields, and the control of the various components shown in  FIGS. 1-4  in such a way so that, on start up, the fuel enters the cathode and anode flow fields of an individual fuel cell at substantially the same time. 
       FIG. 6  shows the expected behavior of hydrogen-air wavefronts  670  present in the cathode flow field  612  and the anode flow field  614  of a fuel cell  602 . Region  1  ( 672 ) denotes the region where air is present in the flow fields  612 ,  614  on both sides of the MEA  610 . Region  2  ( 674 ) denotes a region where hydrogen is present on one electrode and air is present on the other electrode (i.e., the region between the wavefronts  670 ). Region  3  ( 676 ) denotes a region where hydrogen is present on both sides of the MEA  610 . Currents established within region  2  ( 674 ), by proton transfer occurring at  678 , should be balanced by reverse currents established within region  3  ( 676 ) due to proton transfer  680  back to the anode electrode  606 , which maintains charge neutrality. This pumping of hydrogen from the cathode electrode  604  to the anode electrode  606  should prevent the buildup of a large cell voltage, which should in turn minimize or eliminate corrosion due to this mechanism. 
     As can be seen in  FIG. 6 , without being bound by theory, it is therefore predicted that electrode corrosion can be minimized by causing hydrogen-air wavefronts to be present in both the cathode flow field  612  and the anode flow field  614  at the same time. Electrode corrosion typically occurs in the electrode opposite the hydrogen-air wavefront, and by causing hydrogen-air wavefronts to be present on both electrodes, reverse currents may be generated that may minimize the electrode corrosion. 
     In some embodiments, the hydrogen-air wavefronts do not progress through the flow fields  612 ,  614  at the same rate. In further embodiments there might be a delay between the formation of a wavefront in one flow field, and the formation of a wavefront in the opposite flow field. 
     Therefore, as used herein and in the appended claims, supplying fuel to both flow fields at substantially the same time is defined as supplying fuel to both flow fields so that at some period of time, hydrogen-air wavefronts exist within both flow fields. 
     A suitable fuel for the purposes of this invention comprises a hydrogen containing fluid. The fuel could for example comprise a substantially pure hydrogen gas, a hydrogen-rich fluid such as reformate, methanol, or other suitable compounds containing hydrogen. 
       FIG. 7  shows the expected behavior of a fuel cell when an oxidant (in this case air) is introduced into the cathode flow field  712  at a stage after the supply of the fuel to the cathode flow field  712  has ceased. Region  4  ( 782 ) denotes a region where hydrogen is present on both sides of the MEA  710 . In region  4  ( 782 ) remaining hydrogen in the cathode flow field  712  is recovered by hydrogen pumping into the anode flow field  714 , as depicted by the arrow  780 . Region  5  ( 784 ) denotes the oxidant (in this case air) in the cathode flow field  712  introduced after the supply of the fuel to the cathode has ceased. In some embodiments the oxidant is only introduced into the cathode flow field  712  after the anode flow field  714  is completely filled with the fuel (i.e., no hydrogen-air wavefront exists in the anode flow field  714 ). In region  5  ( 784 ) protons travel from the anode electrode  706  to the cathode electrode  704 , denoted by the arrow  778 . This represents the normal, power producing, operation of the fuel cell  702 . Once the anode flow field  714  is substantially filled with fuel, and the cathode flow field  712  is substantially filled with the oxidant, the fuel cell  702  may be ready for normal operation, i.e., the fuel cell  702  may be ready to provide power to a load (not shown). 
     The foregoing detailed description has set forth various embodiments of the devices and/or processes via the use of block diagrams, schematics, and examples. Insofar as such block diagrams, schematics, and examples contain one or more functions and/or operations, it will be understood by those skilled in the art that each function and/or operation within such block diagrams, flowcharts, or examples can be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. In one embodiment, parts of the present subject matter may be implemented via Application Specific Integrated Circuits (ASICs). However, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented in standard integrated circuits, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more controllers (e.g., microcontrollers) as one or more programs running on one or more processors (e.g., microprocessors), as firmware, or as virtually any combination thereof, and that designing the circuitry and/or writing the code for the software and or firmware would be well within the skill of one of ordinary skill in the art in light of this disclosure. 
     In addition, those skilled in the art will appreciate that the methods and control mechanisms taught herein are capable of being distributed as a program product in a variety of forms, and that an illustrative embodiment applies equally regardless of the particular type of signal bearing media used to actually carry out the distribution. Examples of signal bearing media include, but are not limited to, the following: recordable type media such as floppy disks, hard disk drives, CD ROMs, digital tape, and computer memory; and transmission type media such as digital and analog communication links using TDM or IP based communication links (e.g., packet links). 
     Although specific embodiments of and examples for a fuel cell system and methods are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in the relevant art. 
     For example, those skilled in the art will recognize that the embodiments disclosed herein, in whole or in part, can be equivalently implemented using a wide variety of standard components and circuits. For example three-way valves may be replaced by two two-way valves. Two-way valves may be replaced by check valves or other devices chosen to fulfill a similar purpose. Designing the circuitry and/or hardware and/or control strategies would be well within the skill of one of ordinary skill in the art in light of this disclosure. 
     The various embodiments described above can be combined to provide further embodiments. 
     These and other changes can be made to the invention in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all fuel cell systems. Accordingly; the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.