Abstract:
A vacuum fuel cell system ( 10 ) and procedure provide for starting up a fuel cell ( 12 ) with a rapid fuel purge of an anode flow field ( 38 ) to minimize corrosion of a carbon catalyst support layer ( 26 ) by a reverse current mechanism produced by movement of a fuel-air front through the anode flow field ( 38 ). A vacuum source ( 90 ) applies a vacuum to the anode flow field ( 38 ) while the fuel cell ( 12 ) is shut down and while a fuel inlet valve ( 70 ) and a fuel exhaust valve ( 74 ) are closed. The resulting vacuum within the anode flow field ( 38 ) produces rapid purge of the fuel through the anode flow field ( 38 ) upon start up, and a strong vacuum will get rid of essentially all of the air within the anode flow field ( 38 ) to virtually eliminate movement of the fuel-air front.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a system and procedure that minimizes performance degradation of fuel cells resulting from starting up the fuel cells.  
       BACKGROUND ART  
       [0002]     Fuel cells are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid and oxygen containing oxidant reactant streams to power electrical apparatus such as motors, and transportation vehicles, etc. In fuel cells of the prior art, it has been discovered that, upon start up of fuel cells, corrosion takes place on catalyst layers of electrodes, and especially on cathode catalyst layers. That corrosion leads to performance loss of the cathode catalyst layers and the fuel cells.  
         [0003]     In starting up known fuel cells that contain air on both anode and cathode catalyst layers and that employ a proton exchange membrane “PEM” as an electrolyte disposed between a cathode and anode catalyst layer, an oxygen containing oxidant is directed to flow through a cathode flow field that directs the oxidant to flow adjacent to the cathode catalyst layer. At about the same time a hydrogen rich reducing fluid fuel stream is directed to flow through an anode flow field that directs the fuel to flow adjacent the anode catalyst layer. As the fuel flows through the anode flow field, a fuel-air front is created moving along the anode catalyst layer until the fuel forces all of the air out of the anode flow field. It has been observed that catalyst layers that are opposite the fuel-air front experience substantial corrosion with each start up of known fuel cells. This problem has come to be characterized as a result of a “reverse current mechanism” resulting from the advance of the fuel-air front through the flow field, which is described in more detail in commonly owned U.S. patent application Ser. No. 10/305,301 that has been published under Publication No. U.S. 2002/0134165 A1.  
         [0004]     It is known that purging the anode and cathode flow fields with inert gases immediately upon shut down of the fuel cell passivates the anode and cathode catalyst layers to minimize such oxidative decay. For example, commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen as the anode side purge gas, and a cathode side purging mixture comprising a very small percentage of oxygen (e.g. less than 1%) with a balance of nitrogen. Both of these patents also discuss the option of connecting a dummy electrical load across the cell during the start of a purging process to lower the cathode potential rapidly to between the acceptable limits of 0.3-0.7 volt. However, the costs and complexity of such stored inert gases are undesirable especially in automotive applications where compactness and low cost are critical, and where the system must be shut down and started up frequently.  
         [0005]     Known improvements to the problem of oxidation and corrosion of electrode catalysts and catalyst support materials have reduced the deleterious consequences of the presence of oxygen on the cathode electrode and a non-equilibrium of reactant fluids between the anode and cathode electrodes that result in unacceptable anode and cathode electrode potentials upon and during shut down and start up of a fuel cell. However, it has been found that even with known solutions, the presence of any oxygen within an anode flow field during start up results in a reverse current leading to unacceptable, localized electrode potentials and corrosion of catalysts and catalyst support materials.  
         [0006]     Consequently, there is a need for a procedure for starting up a fuel cell that minimizes oxidation and corrosion within the fuel cell.  
       DISCLOSURE OF INVENTION  
       [0007]     The invention is a procedure for starting up a fuel cell with a fuel purge using a vacuum to reduce or eliminate oxygen within the shut down fuel cell prior to purging the cell with fuel. The fuel cell includes a cathode secured adjacent one side of an electrolyte layer of the cell and an anode secured adjacent an opposed side of the electrolyte layer, wherein the cathode includes a catalyst supported on carbon. The fuel cell also includes a cathode flow field defined adjacent the cathode and an anode flow field defined adjacent the anode for directing the oxygen containing oxidant and reducing fluid fuel reactant streams to flow through the fuel cell. During shut down of the fuel cell, both the cathode and anode flow fields are filled with air, and the primary electricity using device or load is disconnected from the fuel cell. The procedure includes the steps: of applying a vacuum to the anode flow field; then delivering a continuous flow of fresh hydrogen containing fuel into the anode flow field; then delivering a flow of oxidant to the cathode flow field; and, then connecting the primary load to the fuel cell. The procedure is repeated each time the fuel cell is started up. In an alternative embodiment, a vacuum may also be applied to the cathode flow field.  
         [0008]     The invention also includes a vacuum fuel cell system for starting up a fuel cell that includes a vacuum source such as a vacuum pump secured in fluid communication with the anode flow field, and in an alternative embodiment, the vacuum pump is also secured in fluid communication with the cathode flow field. The vacuum system also includes valves for controlling flow of the fuel and oxidant streams into and through the fuel cell as well as valves for controlling application of the vacuum to the fuel cell.  
         [0009]     In a preferred embodiment, the fuel cell may include a porous water transport plate, that is also known as a cooler plate, for directing flow of a cooling liquid through the fuel cell. Where the water transport plate is a porous plate secured in fluid communication with the anode flow field, the vacuum pump may also apply a vacuum to a coolant accumulator in fluid communication with the cooling fluid in order to minimize a pressure differential across the porous water transport plate while the vacuum is being applied to the anode flow field. The invention includes applying a vacuum to the anode and/or cathode flow field that results in a pressure differential between the flow fields and the cooling fluid that is not greater than a bubble pressure of the porous water transport plate.  
         [0010]     The vacuum level applied to the anode and/or cathode flow field may range from 21 kilo Pascals (“kPa”) (about 3 pounds per square inch (“psi”) to about 95 kPa (about 13.5 psi) below ambient pressure. A fuel inlet pressure of about 10.5 kPa (about 1.5,psi) above ambient pressure results in a pressure differential between the entering fuel and the anode flow field of between about 31.5 kPa (4.6 psi) to about 105.5 kPa (15 psi). Such an enhanced pressure differential greatly decreases an amount of time necessary for the hydrogen fuel to pass through the anode flow field, thereby decreasing oxidation and corrosion resulting from the reverse current mechanism associated with movement of the fuel-air front. More importantly, the vacuum may remove virtually all of the air within the anode and/or cathode flow fields. Removal of the air from the anode flow field essentially eliminates the reverse current mechanism that creates the corrosion.  
         [0011]     Accordingly, it is a general purpose of the present invention to provide a procedure for starting up a fuel cell using a fuel purge that overcomes deficiencies of the prior art.  
         [0012]     It is a more specific purpose to provide a procedure for starting up a fuel cell using a fuel purge that minimizes oxidation and corrosion of catalyst support materials.  
         [0013]     These and other purposes and advantages of the present procedure for starting up a fuel cell using a fuel purge will become more readily apparent when the following description is read in conjunction with the accompanying drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]      FIG. 1  is a simplified schematic representation of a preferred embodiment of a vacuum fuel cell system capable of performing the procedure for starting up a fuel cell using a fuel purge in accordance with the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Referring to the drawings in detail, a vacuum fuel cell system is shown in  FIG. 1 , and is generally designated by the reference numeral  10 . The system includes a fuel cell  12  having an anode  14  a cathode  16  secured to opposed sides of an electrolyte layer  18 . The anode includes an anode substrate  20  having an anode catalyst layer  22  disposed on the substrate  20  on a side adjacent the electrolyte layer  18 . Similarly, the cathode  16  includes a cathode substrate  24  having a cathode catalyst supported on a carbon support  26  disposed on the substrate on a side adjacent the electrolyte layer  18 . The fuel cell  12  also includes an anode flow field plate  28  adjacent the anode substrate  20  and a cathode flow field plate  30  adjacent the cathode substrate  24 .  
         [0016]     The cathode flow field plate  30  defines a plurality of oxidant channels  32  extending across the plate  30  forming a cathode flow field for directing flow of an oxygen containing oxidant, such as air, across the cathode flow field plat  20  from an oxidant inlet  34  to an oxidant outlet  36 . The anode flow field plate  28  also has a plurality of fuel channels  38  extending across the plate  28  forming an anode flow field for directing flow of a hydrogen containing reducing fluid fuel from a fuel inlet  40  to a fuel outlet  42 .  
         [0017]     The fuel cell  12  may also include a cooler plate  44  secured adjacent the cathode flow field plate  20 . The cooler plate  44  may be either a solid plate for removing heat from the cell  12 , or may be a porous plate known in the art for removing heat and fuel cell  12  product water as well as providing for humidification of reactant streams, etc. A coolant pump  46  may be secured to a coolant loop  48  for passing a liquid coolant such as water or an antifreeze solution through the cooler plate  44 , a radiator  50 , a flow control or pressure control valve  52  and a coolant accumulator  54  so that the liquid coolant circulates through the cooler plate  44 . It should be understood that the vacuum fuel cell system  10  would include a plurality of fuel cell similar to the described fuel cell  12  cooperatively arranged in a fuel cell stack assembly well known in the art. In such a cell stack assembly, an additional cooler plate (not shown) would be secured adjacent the anode flow field plate  28  and would receive flow of the liquid coolant from the coolant loop  48 , as is well known. Therefore, the discussion herein will assume that the cooler plate  44  is a water transport plate and is in direct fluid communication with the anode flow field  38  for purposes of efficiency of description of the relationship between the pressure differential between the liquid coolant within the coolant loop and the anode flow field  38 .  
         [0018]     The vacuum fuel cell system  10  also includes an oxidant source  56  in fluid communication with an oxidant inlet line  58  having a oxidant inlet valve  59  and possibly an oxidant blower  60  secured to the line  58  for directing the oxidant reactant stream into and through the cathode flow field  32 . An oxidant exhaust line  62  and an oxidant exhaust valve  64  are also secured in fluid communication with the cathode flow field  32  in a manner known in the art for selectively directing the oxidant out of the fuel cell  12 . The system  10  also includes a fuel source  66  secured in fluid communication through a fuel inlet line  68  having a fuel inlet valve  70  with the anode flow field  38 . A fuel exhaust line  72  and fuel exhaust valve  74  are also secured in fluid communication with the anode flow field  38  to selectively direct the fuel out of the fuel cell  12 .  
         [0019]     As reactant streams are controlled to flow through the fuel cell  12 , electricity is produced in a manner well known in the art and the electricity is directed through a power circuit  76  to a primary load  78 , such as a motor to power an automobile, through a primary load switch  80 . An auxiliary load  82  may be secured across the power circuit, as shown schematically in  FIG. 1 , including a diode  84  between the auxiliary load  82  and an auxiliary load switch  86 , for lowering the cell voltage from its open circuit voltage of about 0.90-1.0 volts per cell to about 0.20 volts per cell or less, as is known in the art.  
         [0020]     The vacuum fuel cell system  10  also includes a vacuum source means for selectively applying a vacuum to the anode flow field  38  and the cathode flow field  32 . By the phrase “selectively applying”, it is meant that the vacuum may be applied at a predetermined level for a predetermined duration at a predetermined time, such as just prior to directing flow of the fuel reactant stream into the anode flow field  38 . The vacuum source means may be a traditional vacuum pump known in the art or any other apparatus known in the art that is capable of generating a vacuum within the anode flow field  38  and cathode flow field  32 . The vacuum pump  90  may be in fluid communication through a vacuum draw line  92  and pump valve  94  with a vacuum receiver  96  for enhancing efficiency of the vacuum pump  90  by permitting a relatively long generation of a vacuum within the receiver  96  by a relatively small pump  90 , so that the receiver may thereafter rapidly apply the vacuum to the flow fields  38 ,  32 .  
         [0021]     An anode vacuum draw line  98  and anode vacuum valve  100  are secured in fluid communication between the anode flow field  38  and the vacuum pump  90  for selectively permitting the vacuum force to draw on the anode flow field  38 . A cathode vacuum draw line  102  and cathode vacuum valve  104  are also secured in fluid communication with the vacuum pump  90  for selectively permitting the vacuum force to draw on the cathode flow field  32 . If the cooler plate  44  of the vacuum fuel cell system  10  is a porous water transport plate  44 , a coolant loop vacuum draw line  106  and coolant loop vacuum valve  108  may also be secured in fluid communication between the vacuum pump  90  and the coolant accumulator  54 . If the cooler plate  44  is solid, then there would be no coolant loop draw line  106 .  
         [0022]     In operation of the vacuum fuel cell system  10 , as the fuel cell  12  is generating electricity to power the primary load  78 , the vacuum pump  90  is not operating and the anode vacuum valve  100 , cathode vacuum valve  104  and any coolant loop vacuum valve  108  are closed so that no fluids pass through them. The fuel cell  12  is shut down in a manner known in the art, as for example disclosed in commonly owned U.S. Pat. No. 6,635,370. As described therein, the fuel cell  12  is shut down essentially as follows: the primary load is removed by opening the primary load switch  80  (as shown in  FIG. 1 .); the flow of oxidant through the cathode flow field  32  is then discontinued by closing the oxidant inlet and exhaust valves  59 ,  64 ; the auxiliary load  82  is connected by closing the auxiliary load switch  86  to consume oxygen in the cathode flow field  32 ; and, the fuel flow is then discontinued by closing the fuel inlet and exhaust valves  70 ,  74 , while the auxiliary load preferably remains connected during shut down of the vacuum fuel cell system  10 .  
         [0023]     By the procedure of the present invention, in starting up the fuel cell  12 , the aforesaid oxidant and fuel inlet and exhaust valves  59 ,  64 ,  70 ,  74  remain closed. In a first embodiment of the procedure, a vacuum is applied to the anode flow field  38  by operating the vacuum pump  90  and opening the anode vacuum valve  100  until a predetermined vacuum level is achieved within the anode flow field  38 . Then, the anode vacuum valve  100  is closed and the vacuum pump  90  is stopped. Next, the fuel inlet valve  70  and fuel exhaust valve  74  are opened to permit a rapid flow or purge of fuel through the anode flow field  38 . Then, the auxiliary load  82  is disconnected; the oxidant inlet and exhaust valves  59 ,  64  are opened to permit flow of the oxidant through the cathode flow field  32 , while any oxidant blower  60  is operated; and then the primary load  78  is connected. The coolant pump  46  would be operated as or shortly after the primary load is connected. If the cooler plate  44  is a porous water transport plate  44 , then the coolant pump  46  will be operated prior to the introduction of the hydrogen fuel to the anode flow field  38 .  
         [0024]     If the cooler plate  44  is a porous water transport plate  44  in direct fluid communication with the anode flow field  38 , while the vacuum is being applied to the anode flow field  38 , the coolant loop vacuum valve  108  is opened to permit a vacuum to be drawn within the coolant accumulator  54 . That effectively decreases any pressure differential between the liquid coolant within the water transport plate  44  and the pressure within the anode flow field  38 , thereby permitting a greater overall vacuum to be applied to the anode flow field  38  without exceeding a bubble pressure of the water transport plate  44  and drawing any liquid coolant into the anode flow field  38 . In an alternative embodiment, a vacuum may also be applied to the cathode flow field by opening the cathode vacuum valve  104 .  
         [0025]     By applying a vacuum to the anode flow field  38 , the rate of movement of any fuel-air front through the anode flow field is significantly enhanced, which effectively minimizes the reverse current mechanism that leads to oxidation and corrosion of the carbon support of the cathode catalyst layer  26  and anode catalyst layer  22 . More importantly, if the vacuum is at a sufficient level to remove virtually all of the air within the anode flow field  38  prior to introduction of the hydrogen fuel, then there is virtually no fuel-air front moving through the anode flow field  38 , which even further minimizes any oxidation or corrosion of the carbon in the catalyst layers  22 ,  26 . By applying a vacuum to both the anode flow field  38  and also to the cathode flow field  32 , more air is removed which further minimizes the occurrence of any possible reverse current mechanism.  
         [0026]     It has been determined that the vacuum level applied to the anode and/or cathode flow field may range from 21 kilo Pascals (“kPa”) (about 3 pounds per square inch (“psi”) to about 95 kPa (about 13.5 psi) below ambient pressure. A fuel inlet pressure of about 10.5 kPa (about 1.5 psi) above ambient pressure is typical and results in a pressure differential between the entering fuel and the anode flow field of between about 31.5 kPa (4.6 psi) to about 105.5 kPa (15 psi). The vacuum level is set by the boiling point of water. The vacuum pump  90  should be capable of drawing a vacuum equal to or greater than the vapor pressure of water at twenty degrees centigrade.  
         [0027]     Data has been established and presented in the following TABLE 1 by the inventors of the present invention regarding the effects of varying vacuums applied to the anode flow field  38 . Table 1 shows the absolute pressure at the fuel inlet  40 , a typical pressure drop, for nominal flow rates, across the anode flow field  38  expressed as a pressure differential, the absolute pressure within the anode flow field  38  after the vacuum is applied and before the hydrogen fuel purge is initiated, and an estimate of the time for the hydrogen front to pass through the anode flow field.  
                                                     TABLE 1                                   Anode Flow   Time for                   Field Pressure   Hydrogen       Fuel           Before   Front to       Inlet   Pressure   Pressure   Hydrogen   Pass Through       Pressure   Drop   Drop   Purge   Flow Field       kPa   psi   kPa   kPa   Seconds                                111.7   1.5   10.3   101.3   0.41       111.7   4.5   31.0   80.6   0.14       111.7   7.5   51.7   60   0.09       111.7   10.5   72.4   39.3   0.06       111.7   16.2   111.7   0   0.04                  
 
         [0028]     As is apparent, achieving a pressure differential of approximately 111.7 kPa between the hydrogen fuel entering the anode flow field  38  and the initial pressure within the anode flow field  38  significantly decreases the time required for the hydrogen to pass through the anode flow field  38 . However, it is stressed again that the benefit achieved in decreased corrosion is significantly greater than a direct comparison to corrosion rates at the slower times to pass through the anode flow field  38 . That is because at the higher vacuum levels, there is virtually no air remaining within the anode flow field  38 , and hence the reverse current mechanism cannot take place resulting in virtually no oxidation or corrosion. The fuel cell  12  and any fuel cell stack of the vacuum fuel cell system  10  must be designed with adequate mechanical integrity to withstand the described pressure differentials. The operation of the present vacuum fuel cell system  10  would be facilitated by controllers and sensors known in the fuel cell art, as fore example described in the Patents referred to above. For purposes herein, the word “about” means plus or minus 10 per cent.  
         [0029]     All of the aforementioned U.S. Patents and U.S. Patent Application are incorporated herein by reference.  
         [0030]     While the present invention has been disclosed with respect to the described and illustrated embodiments of a procedure and system for starting up a fuel cell with a fuel purge, it is to be understood that the invention is not to be limited to those embodiments. Accordingly, reference should be made primarily to the following claims rather than the foregoing description to determine the scope of the invention.